Polymer and biopolymer brushes: for materials science and biotechnology 9781119455028, 1119455022, 9781119455042, 1119455049

Table of contents :
Volume 1 Preface xxi List of Contributors xxiii 1 Functionalization of Surfaces Using Polymer Brushes: An Overview of Techniques, Strategies, and Approaches 1Juan M. Giussi,M. Lorena Cortez,Waldemar A. Marmisolle, and Omar Azzaroni 1.1 Introduction: Fundamental Notions and Concepts 1 1.2 Preparation of Polymer Brushes on Solid Substrates 4 1.3 Preparation of Polymer Brushes by the "Grafting-To" Method 5 1.4 Polymer Brushes by the "Grafting-From" Method 9 1.4.1 Surface-Initiated Atom Transfer Radical Polymerization 9 1.4.2 Surface-Initiated Reversible-Addition Fragmentation Chain Transfer Polymerization 10 1.4.3 Surface-Initiated Nitroxide-Mediated Polymerization 13 1.4.4 Surface-Initiated Photoiniferter-Mediated Polymerization 13 1.4.5 Surface-Initiated Living Ring-Opening Polymerization 15 1.4.6 Surface-Initiated Ring-Opening Metathesis Polymerization 17 1.4.7 Surface-Initiated Anionic Polymerization 18 1.5 Conclusions 20 Acknowledgments 21 References 21 2 Polymer Brushes by AtomTransfer Radical Polymerization 29Guojun Xie, Amir Khabibullin, Joanna Pietrasik, Jiajun Yan, and KrzysztofMatyjaszewski 2.1 Structure of Brushes 29 2.2 Synthesis of Polymer Brushes 31 2.2.1 Grafting through 31 2.2.2 Grafting to 32 2.2.3 Grafting from 32 2.3 ATRP Fundamentals 33 2.4 Molecular Bottlebrushes by ATRP 38 2.4.1 Introduction 38 2.4.2 Star-Like Brushes 40 2.4.3 Blockwise Brushes 42 2.4.4 Brushes with Tunable Grafting Density 45 2.4.5 Brushes with Block Copolymer Side Chains 46 2.4.6 Functionalities and Properties of Brushes 50 2.5 ATRP and Flat Surfaces 55 2.5.1 Chemistry at Surface 55 2.5.2 Grafting Density 55 2.5.3 Architecture 56 2.5.4 Applications 57 2.6 ATRP and Nanoparticles 58 2.6.1 Chemistry 58 2.6.2 Architecture 59 2.6.3 Applications 61 2.7 ATRP and Concave Surfaces 63 2.8 ATRP and Templates 63 2.8.1 Templates from Networks 63 2.8.2 Templates from Brushes 64 2.9 Templates from Stars 65 2.10 Bio-Related Polymer Brushes 66 2.11 Stimuli-Responsive Polymer Brushes 74 2.11.1 Stimuli-Responsive Solutions 76 2.11.2 Stimuli-Responsive Surfaces 78 2.12 Conclusion 79 Acknowledgments 80 References 80 3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 97Tuncer Caykara 3.1 Introduction 97 3.2 Polymer Brushes via the Surface-Initiated RAFT Polymerization Process 99 3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process 101 3.3.1 pH-Responsive Brushes 102 3.3.2 Temperature-Responsive Brushes 106 3.3.3 Polymer Brushes on Gold Surface 110 3.3.4 Polymer Brushes on Nanoparticles 114 3.3.5 Micropatterned Polymer Brushes 115 3.4 Summary 117 References 119 4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush 123Bin Li and Feng Zhou 4.1 Introduction 123 4.2 "Electro-Click" Chemistry 124 4.3 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization 129 4.4 Possible Combination of eATRP and "e-Click" Chemistry on Surface 136 4.5 Surface Functionality 136 4.6 Summary 137 Acknowledgments 138 References 138 5 Polymer Brushes on Flat and Curved Substrates:What Can be Learned fromMolecular Dynamics Simulations 141K. Binder, S.A. Egorov, and A.Milchev 5.1 Introduction 141 5.2 Molecular Dynamics Methods: A Short "Primer" 144 5.3 The Standard Bead Spring Model for Polymer Chains 148 5.4 Cylindrical and Spherical Polymer Brushes 150 5.5 Interaction of Brushes with Free Chains 152 5.6 Summary 153 Acknowledgments 156 References 157 6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes 161Rikkert J. Nap,Mario Tagliazucchi, Estefania Gonzalez Solveyra, Chun-lai Ren, Mark J. Uline, and Igal Szleifer 6.1 Introduction 161 6.2 Theoretical Approach 163 6.3 Applications of the Molecular Theory 177 6.3.1 Acid-Base Equilibrium in Polyelectrolyte Brushes 178 6.3.1.1 Effect of Salt Concentration and pH 178 6.3.1.2 Effect of Polymer Density and Geometry 184 6.3.2 Competition between Chemical Equilibria and Physical Interactions 186 6.3.2.1 Brushes of Strong Polyelectrolytes 186 6.3.2.2 Brushes ofWeak Polyelectrolytes: Self-Assembly in Charge-Regulating Systems 189 6.3.2.3 Redox-Active Polyelectrolyte Brushes 193 6.3.3 End-Tethered Single Stranded DNA in Aqueous Solutions 195 6.3.4 Ligand-Receptor Binding and Protein Adsorption to Polymer Brushes 201 6.3.5 Adsorption Equilibrium of Polymer Chains through Terminal Segments: Grafting-to Formation of Polymer Brushes 207 6.4 Summary and Conclusion 212 Acknowledgments 216 References 216 7 Brushes of Linear and Dendritically Branched Polyelectrolytes 223E. B. Zhulina, F. A. M. Leermakers, and O. V. Borisov 7.1 Introduction 223 7.2 Analytical SCF Theory of Brushes Formed by Linear and Branched Polyions 224 7.2.1 Dendron Architecture and System Parameters 225 7.2.2 Analytical SCF Formalism 226 7.3 Planar Brush of PE Dendrons with an Arbitrary Architecture 229 7.3.1 Asymptotic Dependences for Brush Thickness H 231 7.4 Planar Brush of Star-Like Polyelectrolytes 232 7.5 Threshold of Dendron Gaussian Elasticity 234 7.6 Scaling-Type Diagrams of States for Brushes of Linear and Branched Polyions 235 7.7 Numerical SF-SCF Model of Dendron Brush 236 7.8 Conclusions 238 References 239 8 Vapor Swelling of Hydrophilic Polymer Brushes 243Casey J. Galvin and Jan Genzer 8.1 Introduction 243 8.2 Experimental 245 8.2.1 General Methods 245 8.2.2 Synthesis of Poly((2-dimethylamino)ethyl methacrylate) Brushes with a Gradient in Grafting Density 245 8.2.3 Synthesis of Poly(2-(diethylamino)ethyl methacrylate) Brushes 245 8.2.4 Chemical Modification of Poly((2-dimethylamino)ethyl methacrylate) Brushes 246 8.2.5 Bulk Synthesis of PDMAEMA 246 8.2.6 Preparation of Spuncast PDMAEMA Films 246 8.2.7 Chemical Modification of Spuncast PDMAEMA Film 247 8.2.8 Spectroscopic EllipsometryMeasurements under Controlled Humidity Conditions 247 8.2.9 Spectroscopic EllipsometryMeasurements of Alcohol Exposure 247 8.2.10 Fitting Spectroscopic Ellipsometry Data 248 8.2.11 Infrared Variable Angle Spectroscopic Ellipsometry 248 8.3 Results and Discussion 248 8.3.1 Comparing Polymer Brush and Spuncast Polymer Film Swelling 250 8.3.2 Influence of Side Chain Chemistry on Polymer Brush Vapor Swelling 252 8.3.3 Influence of Solvent Vapor Chemistry on Polymer Brush Vapor Swelling 256 8.3.4 Influence of Grafting Density on Polymer Brush Vapor Swelling 259 8.4 Conclusion 262 8.A.1 Appendix 263 8.A.1.1 Mole Fraction Calculation 263 8.A.1.2 Water Cluster Number Calculation 264 Acknowledgments 265 References 265 9 Temperature Dependence of the Swelling and Surface Wettability of Dense Polymer Brushes 267Pengyu Zhuang, Ali Dirani, Karine Glinel, and AlainM. Jonas 9.1 Introduction 267 9.2 The Swelling Coefficient of a Polymer Brush Mirrors Its Volume Hydrophilicity 269 9.3 The Cosine of the Contact Angle ofWater on aWater-Equilibrated Polymer Brush Defines Its Surface Hydrophilicity 270 9.4 Case Study: Temperature-Dependent Surface hydrophilicity of Dense PNIPAM Brushes 272 9.5 Case Study: Temperature-Dependent Swelling and Volume Hydrophilicity of Dense PNIPAMBrushes 274 9.6 Thermoresponsive Poly(oligo(ethylene oxide)methacrylate) Copolymer Brushes: Versatile Functional Alternatives to PNIPAM 277 9.7 Surface and Volume Hydrophilicity of Nonthermoresponsive Poly(oligo(ethylene oxide)methacrylate) Copolymer Brushes 279 9.8 Conclusions 282 Acknowledgments 283 References 283 10 Functional Biointerfaces Tailored by "Grafting-To"Brushes 287Eva Bittrich, Manfred Stamm, and Petra Uhlmann 10.1 Introduction 287 10.2 Part I: Polymer Brush Architectures 288 10.2.1 Design of Physicochemical Interfaces by Polymer Brushes 288 10.2.1.1 Stimuli-Responsive Homopolymer Brushes 288 10.2.1.2 Combination of Responses Using Mixed Polymer Brushes 290 10.2.1.3 Stimuli-Responsive Gradient Brushes 293 10.2.2 Modification of Polymer Brushes by Click Chemistry 293 10.2.2.1 Definition of Click Chemistry 293 10.2.2.2 Modification of End Groups of Grafted PNIPAAm Chains 295 10.2.3 Hybrid Brush Nanostructures 297 10.2.3.1 Nanoparticles Immobilized at Polymer Brushes 298 10.2.3.2 Sculptured Thin Films Grafted with Polymer Brushes 300 10.3 Part II: Actuating Biomolecule Interactions with Surfaces 303 10.3.1 Adsorption of Proteins to Polymer Brush Surfaces 303 10.3.1.1 Calculation of the Adsorbed Amount of Protein from Ellipsometric Experiments 305 10.3.1.2 Preventing Protein Adsorption 306 10.3.1.3 Adsorption at Polyelectrolyte Brushes 310 10.3.2 Polymer Brushes as Interfaces for Cell Adhesion and Interaction 313 10.3.2.1 Cell Adhesion on Stimuli-Responsive Polymer Surfaces Based on PNIPAAm Brushes 315 10.3.2.2 Growth Factors on Polymer Brushes 318 10.4 Conclusion and Outlook 320 Acknowledgments 321 References 321 11 Glycopolymer Brushes Presenting Sugars in Their Natural Form: Synthesis and Applications 333Kai Yu and Jayachandran N. Kizhakkedathu 11.1 Introduction and Background 333 11.2 Results and Discussion 334 11.2.1 Synthesis of Glycopolymer Brushes 334 11.2.1.1 Synthesis of N-Substituted Acrylamide Derivatives of Glycomonomers 334 11.2.1.2 Synthesis and Characterization of Glycopolymer Brushes on Gold Chip and SiliconWafer 334 11.2.1.3 Synthesis and Characterization of Glycopolymer Brushes on Polystyrene Particles 335 11.2.1.4 Synthesis and Characterization of Glycopolymer Brushes with Variation in the Composition of Carbohydrate Residues on SPR Chip 338 11.2.1.5 Preparation of Glycopolymer Brushes-Modified Particles with Different Grafting Density (Conformation) 338 11.2.2 Applications of Glycopolymer Brushes 341 11.2.2.1 Antithrombotic Surfaces Based on Glycopolymer Brushes 341 11.2.2.2 Glycopolymer Brushes Based Carbohydrate Arrays to Modulate Multivalent Protein Binding on Surfaces 345 11.2.2.3 Modulation of Innate Immune Response by the Conformation and Chemistry of Glycopolymer Brushes 351 11.3 Conclusions 356 Acknowledgments 357 References 357 12 Thermoresponsive Polymer Brushes for Thermally Modulated Cell Adhesion and Detachment 361Kenichi Nagase and Teruo Okano 12.1 Introduction 361 12.2 Thermoresponsive Polymer Hydrogel-Modified Surfaces for Cell Adhesion and Detachment 362 12.3 Thermoresponsive Polymer Brushes Prepared Using ATRP 363 12.4 Thermoresponsive Polymer Brushes Prepared by RAFT Polymerization 368 12.5 Conclusions 372 Acknowledgments 372 References 372 Volume 2 Preface xxi List of Contributors xxiii 13 Biomimetic Anchors for Antifouling Polymer Brush Coatings 377Dicky Pranantyo, Li Qun Xu, En-Tang Kang, Koon-Gee Neoh, and Serena Lay-Ming Teo 13.1 Introduction to Biofouling Management 377 13.2 Polymer Brushes for Surface Functionalization 378 13.3 Biomimetic Anchors for Antifouling Polymer Brushes 379 13.3.1 Mussel Adhesive-Inspired Dopamine Anchors 379 13.3.1.1 Antifouling Polymer Brushes Prepared via the "Grafting-To" Approach on (poly)Dopamine Anchor 383 13.3.1.2 Antifouling Polymer Brushes Prepared via the "Grafting-From" Approach on (poly)Dopamine Anchor 386 13.3.1.3 Direct Grafting of Antifouling Polymer Brushes Containing Anchorable Dopamine-Derived Functionalities 389 13.3.2 (Poly)phenolic Anchors for Antifouling Polymer Brushes 391 13.3.3 Biomolecular Anchors for Antifouling Polymer Brushes 393 13.4 Barnacle Cement as Anchor for Antifouling Polymer Brushes 397 13.5 Conclusion and Outlooks 399 References 400 14 Protein Adsorption Process Based on Molecular Interactions at Well-Defined Polymer Brush Surfaces 405Sho Sakata, Yuuki Inoue, and Kazuhiko Ishihara 14.1 Introduction 405 14.2 Utility of Polymer Brush Layers as Highly Controllable Polymer Surfaces 406 14.3 Performance of Polymer Brush Surfaces as Antifouling Biointerfaces 408 14.4 Elucidation of Protein Adsorption Based on Molecular Interaction Forces 412 14.5 Concluding Remarks 416 References 417 15 Are Lubricious Polymer Brushes Antifouling? Are Antifouling Polymer Brushes Lubricious? 421Edmondo M. Benetti and Nicholas D. Spencer 15.1 Introduction 421 15.2 Poly(ethylene glycol) Brushes 422 15.3 Beyond Simple PEG Brushes 424 15.4 Conclusion 429 References 429 16 Biofunctionalized Brush Surfaces for Biomolecular Sensing 433Shuaidi Zhang and Vladimir V. Tsukruk 16.1 Introduction 433 16.2 Biorecognition Units 435 16.2.1 Antibodies 435 16.2.2 Antibody Fragments 435 16.2.3 Aptamers 437 16.2.4 Peptide Aptamers 438 16.2.5 Enzymes 438 16.2.6 Peptide Nucleic Acid, Lectin, and Molecular Imprinted Polymers 439 16.3 Immobilization Strategy 439 16.3.1 Through Direct Covalent Linkage 440 16.3.1.1 Thiolated Aptamers on Noble Metal 440 16.3.1.2 General Activated Surface Chemistry 442 16.3.1.3 Diels-Alder Cycloaddition 444 16.3.1.4 Staudinger Ligation 444 16.3.1.5 1,3-Dipolar Cycloaddition 446 16.3.2 Through Affinity Tags 447 16.3.2.1 Biotin-Avidin/Streptavidin Pairing 447 16.3.2.2 NTA-Ni2+-Histidine Pairing 448 16.3.2.3 Protein A/Protein G - Fc Pairing 449 16.3.2.4 Oligonucleotide Hybridization 450 16.4 Microstructure and Morphology of Biobrush Layers 451 16.4.1 Grafting Density Control 451 16.4.2 Conformation and Orientation of Recognition Units 453 16.5 Transduction Schemes Based upon Grafted Biomolecules 462 16.5.1 Electrochemical-Based Sensors 462 16.5.2 Field Effect Transistor Based Sensors 463 16.5.3 SPR-Based Sensors 465 16.5.4 Photoluminescence-Based Sensors 466 16.5.5 SERS Sensors 468 16.5.6 Microcantilever Sensors 469 16.6 Conclusions 471 Acknowledgments 472 References 472 17 Phenylboronic Acid and Polymer Brushes: An Attractive Combination with Many Possibilities 479Solmaz Hajizadeh and Bo Mattiasson 17.1 Introduction: Polymer Brushes and Synthesis 479 17.2 Boronic Acid Brushes 481 17.3 Affinity Separation 483 17.4 Sensors 487 17.5 Biomedical Applications 492 17.6 Conclusions 494 References 494 18 Smart Surfaces Modified with Phenylboronic Acid Containing Polymer Brushes 497Hongliang Liu, ShutaoWang, and Lei Jiang 18.1 Introduction 497 18.2 Molecular Mechanism of PBA-Based Smart Surfaces 498 18.3 pH-Responsive Surfaces Modified with PBA Polymer Brush and Their Applications 501 18.4 Sugar-Responsive SurfacesModified with PBA Polymer Brush and Their Applications 503 18.5 PBA Polymer Brush-Based pH/Sugar Dual-Responsive OR Logic Gates and Their Applications 504 18.6 PBA Polymer Brush-Based pH/Sugar Dual-Responsive AND Logic Gates and Their Applications 506 18.7 PBA-Based Smart Systems beyond Polymer Brush and Their Applications 509 18.8 Conclusion and Perspective 511 References 512 19 Polymer Brushes andMicroorganisms 515Madeleine Ramstedt 19.1 Introduction 515 19.1.1 Societal Relevance for Surfaces Interacting with Microbes 515 19.1.2 Microorganisms 516 19.2 Brushes and Microbes 519 19.2.1 Adhesive Surfaces 529 19.2.2 Antifouling Surfaces 530 19.2.2.1 PEG-Based Brushes 531 19.2.2.2 Zwitterionic Brushes 533 19.2.2.3 Brush Density 533 19.2.2.4 Interactive Forces 535 19.2.2.5 Mechanical Interactions 537 19.2.3 Killing Surfaces 537 19.2.3.1 Antimicrobial Peptides 540 19.2.4 Brushes and Fungi 543 19.2.5 Brushes and Algae 546 19.3 Conclusions and Future Perspectives 549 Acknowledgments 551 References 552 20 Design of Polymer Brushes for Cell Culture and Cellular Delivery 557Danyang Li and Julien E. Gautrot Abbreviations 557 20.1 Introduction 559 20.2 Protein-Resistant Polymer Brushes for Tissue Engineering and In Vitro Assays 561 20.2.1 Design of Protein-Resistant Polymer Brushes 561 20.2.2 Cell-Resistant Polymer Brushes 565 20.2.3 Patterned Antifouling Brushes for the Development of Cell-Based Assays 567 20.3 Designing Brush Chemistry to Control Cell Adhesion and Proliferation 570 20.3.1 Polyelectrolyte Brushes for Cell Adhesion and Culture 570 20.3.2 Control of Surface Hydrophilicity 573 20.3.3 Surfaces with Controlled Stereochemistry 574 20.3.4 Switchable Brushes Displaying Responsive Behavior for Cell Harvesting and Detachment 576 20.4 Biofunctionalized Polymer Brushes to Regulate Cell Phenotype 581 20.4.1 Protein Coupling to Polymer Brushes to Control Cell Adhesion 581 20.4.2 Peptide-Functionalized Polymer Brushes to Regulate Cell Adhesion, Proliferation, Differentiation, and Migration 583 20.5 Polymer Brushes for Drug and Gene Delivery Applications 586 20.5.1 Polymer Brushes in Drug Delivery 586 20.5.2 Polymer Brushes in Gene Delivery 590 20.6 Summary 593 Acknowledgments 593 References 593 21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications 605Ursula Koniges, Sade Ruffin, and Rastislav Levicky 21.1 Introduction 605 21.2 Applications 605 21.3 Preparation 607 21.4 Physicochemical Properties of DNA Brushes 610 21.5 Hybridization in DNA Brushes 613 21.6 Other Bioprocesses in DNA Brushes 618 21.7 Perspective 619 Acknowledgments 620 References 621 22 DNA Brushes: Advances in Synthesis and Applications 627Renpeng Gu, Lei Tang, Isao Aritome, and Stefan Zauscher 22.1 Introduction 627 22.2 Synthesis of DNA Brushes 628 22.2.1 Grafting-to Approaches 628 22.2.1.1 Immobilization on Gold Thin Films 628 22.2.1.2 Immobilization on Silicon-Based Substrates 632 22.2.2 Grafting-from Approaches 634 22.2.2.1 Surface-Initiated Enzymatic Polymerization 634 22.2.2.2 Surface-Initiated Rolling Circle Amplification 634 22.2.2.3 Surface-Initiated Hybridization Chain Reaction 634 22.2.3 Synthesis of DNA Brushes on Curved Surfaces 637 22.3 Properties and Applications of DNA Brushes 637 22.3.1 The Effect of DNA-Modifying Enzymes on the DNA Brush Structure 637 22.3.2 Stimulus-Responsive Conformational Changes of DNA Brushes 639 22.3.3 DNA Brush for Cell-Free Surface Protein Expression 643 22.3.4 DNA Brush-Modified Nanoparticles for Biomedical Applications 645 22.4 Conclusion and Outlook 649 References 649 23 Membrane Materials Form Polymer Brush Nanoparticles 655Erica Green, Emily Fullwood, Julieann Selden, and Ilya Zharov 23.1 Introduction 655 23.2 Colloidal Membranes Pore-Filled with Polymer Brushes 657 23.2.1 Preparation of Silica Colloidal Membranes 657 23.2.2 PAAM Brush-Filled Silica Colloidal Membranes 658 23.2.3 PDMAEMA Brush-Filled Silica Colloidal Membranes 659 23.2.4 PNIPAAM brush-filled silica colloidal membranes 664 23.2.5 Polyalanine Brush-Filled Silica Colloidal Membranes 666 23.2.6 PMMA Brush-Filled SiO2@Au Colloidal Membranes 670 23.2.7 Colloidal Membranes Filled with Polymers Brushes Carrying Chiral Groups 672 23.2.8 pSPM and pSSA Brush-Filled Colloidal Nanopores 673 23.3 Self-Assembled PBNPs Membranes 676 23.3.1 PDMAEMA/PSPM Membranes 676 23.3.2 PHEMA Membranes 678 23.3.3 pSPM and pSSA Membranes 680 23.4 Summary 683 References 683 24 Responsive Polymer Networks and Brushes for Active Plasmonics 687Nestor Gisbert Quilis, Nityanand Sharma, Stefan Fossati,Wolfgang Knoll, and Jakub Dostalek 24.1 Introduction 687 24.2 Tuning Spectrum of Surface Plasmon Modes 688 24.3 Polymers Used for Actuating of Plasmonic Structures 692 24.3.1 Temperature-Responsive Polymers 692 24.3.2 Optical Stimulus 694 24.3.3 Electrochemical Stimulus 695 24.3.4 Chemical Stimulus 696 24.4 Imprinted Thermoresponsive Hydrogel Nanopillars 697 24.5 Thermoresponsive Hydrogel Nanogratings Fabricated by UV Laser Interference Lithography 699 24.6 Electrochemically Responsive Hydrogel Microgratings Prepared by UV Photolithography 702 24.7 Conclusions 705 Acknowledgments 706 References 706 25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics 709Casey Yan and Zijian Zheng 25.1 Introduction 709 25.2 Mechanisms of Polymer-Assisted Metal Deposition 712 25.3 Role of Polymer Brushes 716 25.4 Selection Criterion of Polymer Brushes Enabling PAMD 716 25.5 Strategies to Fabricate Patterned Metal Conductors 717 25.6 PAMD on Different Substrates and Their Applications in Soft Electronics 720 25.6.1 On Textiles 720 25.6.2 On Plastic Thin films 721 25.6.3 On Elastomers 724 25.6.4 On Sponges 728 25.7 Conclusion, Prospects, and Challenges 731 References 732 26 Nanoarchitectonic Design of Complex Materials Using Polymer Brushes as Structural and Functional Units 735M. Lorena Cortez, Gisela Dyaz,Waldemar A. Marmisolle, Juan M. Giussi, and Omar Azzaroni 26.1 Introduction 735 26.2 Nanoparticles at Spherical Polymer Brushes: Hierarchical Nanoarchitectonic Construction of Complex Functional Materials 736 26.3 Nanotube and Nanowire Forests Bearing Polymer Brushes 737 26.3.1 Polymer Brushes on Surfaces DisplayingMicrotopographical Hierarchical Arrays 738 26.3.2 Environmentally Responsive Electrospun Nanofibers 740 26.4 Fabrication of Free-Standing "Soft" Micro- and Nanoobjects Using Polymer Brushes 741 26.5 Solid-State Polymer Electrolytes Based on Polymer Brush-Modified Colloidal Crystals 743 26.6 Proton-Conducting Membranes with Enhanced Properties Using Polymer Brushes 745 26.7 Hybrid Architectures Combining Mesoporous Materials and Responsive Polymer Brushes: Gated Molecular Transport Systems and Controlled Delivery Vehicles 747 26.8 Ensembles of Metal NanoparticlesModified with Polymer Brushes 750 26.9 Conclusions 754 Acknowledgments 755 References 755 Index 759

Citation preview

Polymer and Biopolymer Brushes

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Polymer and Biopolymer Brushes for Materials Science and Biotechnology Volume 1

Edited by

Omar Azzaroni Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), CONICET, Universidad Nacional de La Plata, La Plata, Argentina

Igal Szleifer Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, Evanston, IL, USA

This edition first published 2018 © 2018 by John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. The right of Omar Azzaroni and Igal Szleifer to be identified as the editors of this work has been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA Editorial Office 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging-in-Publication Data applied for. Hardback ISBN: 9781119455011 Cover design by Wiley Set in 10/12pt WarnockPro by Aptara Inc., New Delhi, India 10 9 8 7 6 5 4 3 2 1

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This book is dedicated to our families. Igal Szleifer also wants to dedicate this book to his coauthor Omar Azzaroni, who took over the bulk of the work after Prof. Szleifer suffered a stroke in October 2015. If not for Omar’s work and dedication, this book would not have materialized. Igal is truly grateful.

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Contents Volume  Preface xxi List of Contributors xxiii 

Functionalization of Surfaces Using Polymer Brushes: An Overview of Techniques, Strategies, and Approaches 1 Juan M. Giussi, M. Lorena Cortez, Waldemar A. Marmisoll´e, and Omar Azzaroni

1.1 1.2 1.3 1.4 1.4.1 1.4.2

Introduction: Fundamental Notions and Concepts 1 Preparation of Polymer Brushes on Solid Substrates 4 Preparation of Polymer Brushes by the “Grafting-To” Method 5 Polymer Brushes by the “Grafting-From” Method 9 Surface-Initiated Atom Transfer Radical Polymerization 9 Surface-Initiated Reversible-Addition Fragmentation Chain Transfer Polymerization 10 Surface-Initiated Nitroxide-Mediated Polymerization 13 Surface-Initiated Photoiniferter-Mediated Polymerization 13 Surface-Initiated Living Ring-Opening Polymerization 15 Surface-Initiated Ring-Opening Metathesis Polymerization 17 Surface-Initiated Anionic Polymerization 18 Conclusions 20 Acknowledgments 21 References 21

1.4.3 1.4.4 1.4.5 1.4.6 1.4.7 1.5



Polymer Brushes by Atom Transfer Radical Polymerization 29 Guojun Xie, Amir Khabibullin, Joanna Pietrasik, Jiajun Yan, and Krzysztof Matyjaszewski

2.1 2.2 2.2.1

Structure of Brushes 29 Synthesis of Polymer Brushes Grafting through 31

31

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2.2.2 2.2.3 2.3 2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.5 2.5.1 2.5.2 2.5.3 2.5.4 2.6 2.6.1 2.6.2 2.6.3 2.7 2.8 2.8.1 2.8.2 2.9 2.10 2.11 2.11.1 2.11.2 2.12

Grafting to 32 Grafting from 32 ATRP Fundamentals 33 Molecular Bottlebrushes by ATRP 38 Introduction 38 Star-Like Brushes 40 Blockwise Brushes 42 Brushes with Tunable Grafting Density 45 Brushes with Block Copolymer Side Chains 46 Functionalities and Properties of Brushes 50 ATRP and Flat Surfaces 55 Chemistry at Surface 55 Grafting Density 55 Architecture 56 Applications 57 ATRP and Nanoparticles 58 Chemistry 58 Architecture 59 Applications 61 ATRP and Concave Surfaces 63 ATRP and Templates 63 Templates from Networks 63 Templates from Brushes 64 Templates from Stars 65 Bio-Related Polymer Brushes 66 Stimuli-Responsive Polymer Brushes 74 Stimuli-Responsive Solutions 76 Stimuli-Responsive Surfaces 78 Conclusion 79 Acknowledgments 80 References 80



Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 97 Tuncer Caykara

3.1 3.2

Introduction 97 Polymer Brushes via the Surface-Initiated RAFT Polymerization Process 99 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process 101 pH-Responsive Brushes 102 Temperature-Responsive Brushes 106 Polymer Brushes on Gold Surface 110

3.3 3.3.1 3.3.2 3.3.3

Contents

3.3.4 3.3.5 3.4

Polymer Brushes on Nanoparticles 114 Micropatterned Polymer Brushes 115 Summary 117 References 119



Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush 123 Bin Li and Feng Zhou

4.1 4.2 4.3

Introduction 123 “Electro-Click” Chemistry 124 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization 129 Possible Combination of eATRP and “e-Click” Chemistry on Surface 136 Surface Functionality 136 Summary 137 Acknowledgments 138 References 138

4.4 4.5 4.6



Polymer Brushes on Flat and Curved Substrates: What Can be Learned from Molecular Dynamics Simulations 141 K. Binder, S.A. Egorov, and A. Milchev

5.1 5.2 5.3 5.4 5.5 5.6

Introduction 141 Molecular Dynamics Methods: A Short “Primer” 144 The Standard Bead Spring Model for Polymer Chains 148 Cylindrical and Spherical Polymer Brushes 150 Interaction of Brushes with Free Chains 152 Summary 153 Acknowledgments 156 References 157



Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes 161 Rikkert J. Nap, Mario Tagliazucchi, Estefania Gonzalez Solveyra, Chun-lai Ren, Mark J. Uline, and Igal Szleifer

6.1 6.2 6.3 6.3.1 6.3.1.1 6.3.1.2 6.3.2

Introduction 161 Theoretical Approach 163 Applications of the Molecular Theory 177 Acid–Base Equilibrium in Polyelectrolyte Brushes 178 Effect of Salt Concentration and pH 178 Effect of Polymer Density and Geometry 184 Competition between Chemical Equilibria and Physical Interactions 186

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6.3.2.1 Brushes of Strong Polyelectrolytes 186 6.3.2.2 Brushes of Weak Polyelectrolytes: Self-Assembly in Charge-Regulating Systems 189 6.3.2.3 Redox-Active Polyelectrolyte Brushes 193 6.3.3 End-Tethered Single Stranded DNA in Aqueous Solutions 195 6.3.4 Ligand–Receptor Binding and Protein Adsorption to Polymer Brushes 201 6.3.5 Adsorption Equilibrium of Polymer Chains through Terminal Segments: Grafting-to Formation of Polymer Brushes 207 6.4 Summary and Conclusion 212 Acknowledgments 216 References 216 

Brushes of Linear and Dendritically Branched Polyelectrolytes 223 E. B. Zhulina, F. A. M. Leermakers, and O. V. Borisov

7.1 7.2

Introduction 223 Analytical SCF Theory of Brushes Formed by Linear and Branched Polyions 224 Dendron Architecture and System Parameters 225 Analytical SCF Formalism 226 Planar Brush of PE Dendrons with an Arbitrary Architecture Asymptotic Dependences for Brush Thickness H 231 Planar Brush of Star-Like Polyelectrolytes 232 Threshold of Dendron Gaussian Elasticity 234 Scaling-Type Diagrams of States for Brushes of Linear and Branched Polyions 235 Numerical SF-SCF Model of Dendron Brush 236 Conclusions 238 References 239

7.2.1 7.2.2 7.3 7.3.1 7.4 7.5 7.6 7.7 7.8

229



Vapor Swelling of Hydrophilic Polymer Brushes 243 Casey J. Galvin and Jan Genzer

8.1 8.2 8.2.1 8.2.2

Introduction 243 Experimental 245 General Methods 245 Synthesis of Poly((2-dimethylamino)ethyl methacrylate) Brushes with a Gradient in Grafting Density 245 Synthesis of Poly(2-(diethylamino)ethyl methacrylate) Brushes 245 Chemical Modification of Poly((2-dimethylamino)ethyl methacrylate) Brushes 246 Bulk Synthesis of PDMAEMA 246 Preparation of Spuncast PDMAEMA Films 246 Chemical Modification of Spuncast PDMAEMA Film 247

8.2.3 8.2.4 8.2.5 8.2.6 8.2.7

Contents

8.2.8

Spectroscopic Ellipsometry Measurements under Controlled Humidity Conditions 247 8.2.9 Spectroscopic Ellipsometry Measurements of Alcohol Exposure 247 8.2.10 Fitting Spectroscopic Ellipsometry Data 248 8.2.11 Infrared Variable Angle Spectroscopic Ellipsometry 248 8.3 Results and Discussion 248 8.3.1 Comparing Polymer Brush and Spuncast Polymer Film Swelling 250 8.3.2 Influence of Side Chain Chemistry on Polymer Brush Vapor Swelling 252 8.3.3 Influence of Solvent Vapor Chemistry on Polymer Brush Vapor Swelling 256 8.3.4 Influence of Grafting Density on Polymer Brush Vapor Swelling 259 8.4 Conclusion 262 8.A.1 Appendix 263 8.A.1.1 Mole Fraction Calculation 263 8.A.1.2 Water Cluster Number Calculation 264 Acknowledgments 265 References 265 

Temperature Dependence of the Swelling and Surface Wettability of Dense Polymer Brushes 267 Pengyu Zhuang, Ali Dirani, Karine Glinel, and Alain M. Jonas

9.1 9.2

Introduction 267 The Swelling Coefficient of a Polymer Brush Mirrors Its Volume Hydrophilicity 269 The Cosine of the Contact Angle of Water on a Water-Equilibrated Polymer Brush Defines Its Surface Hydrophilicity 270 Case Study: Temperature-Dependent Surface hydrophilicity of Dense PNIPAM Brushes 272 Case Study: Temperature-Dependent Swelling and Volume Hydrophilicity of Dense PNIPAM Brushes 274 Thermoresponsive Poly(oligo(ethylene oxide)methacrylate) Copolymer Brushes: Versatile Functional Alternatives to PNIPAM 277 Surface and Volume Hydrophilicity of Nonthermoresponsive Poly(oligo(ethylene oxide)methacrylate) Copolymer Brushes 279 Conclusions 282 Acknowledgments 283 References 283

9.3

9.4 9.5 9.6

9.7 9.8

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

Functional Biointerfaces Tailored by “Grafting-To” Brushes Eva Bittrich, Manfred Stamm, and Petra Uhlmann

10.1 10.2 10.2.1

Introduction 287 Part I: Polymer Brush Architectures 288 Design of Physicochemical Interfaces by Polymer Brushes 288 Stimuli-Responsive Homopolymer Brushes 288 Combination of Responses Using Mixed Polymer Brushes 290 Stimuli-Responsive Gradient Brushes 293 Modification of Polymer Brushes by Click Chemistry 293 Definition of Click Chemistry 293 Modification of End Groups of Grafted PNIPAAm Chains 295 Hybrid Brush Nanostructures 297 Nanoparticles Immobilized at Polymer Brushes 298 Sculptured Thin Films Grafted with Polymer Brushes 300 Part II: Actuating Biomolecule Interactions with Surfaces 303 Adsorption of Proteins to Polymer Brush Surfaces 303 Calculation of the Adsorbed Amount of Protein from Ellipsometric Experiments 305 Preventing Protein Adsorption 306 Adsorption at Polyelectrolyte Brushes 310 Polymer Brushes as Interfaces for Cell Adhesion and Interaction 313 Cell Adhesion on Stimuli-Responsive Polymer Surfaces Based on PNIPAAm Brushes 315 Growth Factors on Polymer Brushes 318 Conclusion and Outlook 320 Acknowledgments 321 References 321

10.2.1.1 10.2.1.2 10.2.1.3 10.2.2 10.2.2.1 10.2.2.2 10.2.3 10.2.3.1 10.2.3.2 10.3 10.3.1 10.3.1.1 10.3.1.2 10.3.1.3 10.3.2 10.3.2.1 10.3.2.2 10.4



11.1 11.2 11.2.1 11.2.1.1

Glycopolymer Brushes Presenting Sugars in Their Natural Form: Synthesis and Applications 333 Kai Yu and Jayachandran N. Kizhakkedathu

Introduction and Background 333 Results and Discussion 334 Synthesis of Glycopolymer Brushes 334 Synthesis of N-Substituted Acrylamide Derivatives of Glycomonomers 334 11.2.1.2 Synthesis and Characterization of Glycopolymer Brushes on Gold Chip and Silicon Wafer 334 11.2.1.3 Synthesis and Characterization of Glycopolymer Brushes on Polystyrene Particles 335

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11.2.1.4 Synthesis and Characterization of Glycopolymer Brushes with Variation in the Composition of Carbohydrate Residues on SPR Chip 338 11.2.1.5 Preparation of Glycopolymer Brushes-Modified Particles with Different Grafting Density (Conformation) 338 11.2.2 Applications of Glycopolymer Brushes 341 11.2.2.1 Antithrombotic Surfaces Based on Glycopolymer Brushes 341 11.2.2.2 Glycopolymer Brushes Based Carbohydrate Arrays to Modulate Multivalent Protein Binding on Surfaces 345 11.2.2.3 Modulation of Innate Immune Response by the Conformation and Chemistry of Glycopolymer Brushes 351 11.3 Conclusions 356 Acknowledgments 357 References 357 

Thermoresponsive Polymer Brushes for Thermally Modulated Cell Adhesion and Detachment 361 Kenichi Nagase and Teruo Okano

12.1 12.2

Introduction 361 Thermoresponsive Polymer Hydrogel-Modified Surfaces for Cell Adhesion and Detachment 362 Thermoresponsive Polymer Brushes Prepared Using ATRP 363 Thermoresponsive Polymer Brushes Prepared by RAFT Polymerization 368 Conclusions 372 Acknowledgments 372 References 372

12.3 12.4 12.5

Volume  Preface xxi List of Contributors xxiii 

Biomimetic Anchors for Antifouling Polymer Brush Coatings 377 Dicky Pranantyo, Li Qun Xu, En-Tang Kang, Koon-Gee Neoh, and Serena Lay-Ming Teo

13.1 13.2 13.3 13.3.1 13.3.1.1

Introduction to Biofouling Management 377 Polymer Brushes for Surface Functionalization 378 Biomimetic Anchors for Antifouling Polymer Brushes 379 Mussel Adhesive-Inspired Dopamine Anchors 379 Antifouling Polymer Brushes Prepared via the “Grafting-To” Approach on (poly)Dopamine Anchor 383

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13.3.1.2 Antifouling Polymer Brushes Prepared via the “Grafting-From” Approach on (poly)Dopamine Anchor 386 13.3.1.3 Direct Grafting of Antifouling Polymer Brushes Containing Anchorable Dopamine-Derived Functionalities 389 13.3.2 (Poly)phenolic Anchors for Antifouling Polymer Brushes 391 13.3.3 Biomolecular Anchors for Antifouling Polymer Brushes 393 13.4 Barnacle Cement as Anchor for Antifouling Polymer Brushes 397 13.5 Conclusion and Outlooks 399 References 400 

Protein Adsorption Process Based on Molecular Interactions at Well-Defined Polymer Brush Surfaces 405 Sho Sakata, Yuuki Inoue, and Kazuhiko Ishihara

14.1 14.2

Introduction 405 Utility of Polymer Brush Layers as Highly Controllable Polymer Surfaces 406 Performance of Polymer Brush Surfaces as Antifouling Biointerfaces 408 Elucidation of Protein Adsorption Based on Molecular Interaction Forces 412 Concluding Remarks 416 References 417

14.3 14.4 14.5



Are Lubricious Polymer Brushes Antifouling? Are Antifouling Polymer Brushes Lubricious? 421 Edmondo M. Benetti and Nicholas D. Spencer

15.1 15.2 15.3 15.4

Introduction 421 Poly(ethylene glycol) Brushes 422 Beyond Simple PEG Brushes 424 Conclusion 429 References 429



Biofunctionalized Brush Surfaces for Biomolecular Sensing 433 Shuaidi Zhang and Vladimir V. Tsukruk

16.1 16.2 16.2.1 16.2.2 16.2.3 16.2.4 16.2.5 16.2.6

Introduction 433 Biorecognition Units 435 Antibodies 435 Antibody Fragments 435 Aptamers 437 Peptide Aptamers 438 Enzymes 438 Peptide Nucleic Acid, Lectin, and Molecular Imprinted Polymers 439

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16.3 16.3.1 16.3.1.1 16.3.1.2 16.3.1.3 16.3.1.4 16.3.1.5 16.3.2 16.3.2.1 16.3.2.2 16.3.2.3 16.3.2.4 16.4 16.4.1 16.4.2 16.5 16.5.1 16.5.2 16.5.3 16.5.4 16.5.5 16.5.6 16.6

Immobilization Strategy 439 Through Direct Covalent Linkage 440 Thiolated Aptamers on Noble Metal 440 General Activated Surface Chemistry 442 Diels–Alder Cycloaddition 444 Staudinger Ligation 444 1,3-Dipolar Cycloaddition 446 Through Affinity Tags 447 Biotin–Avidin/Streptavidin Pairing 447 NTA–Ni2+ –Histidine Pairing 448 Protein A/Protein G – Fc Pairing 449 Oligonucleotide Hybridization 450 Microstructure and Morphology of Biobrush Layers 451 Grafting Density Control 451 Conformation and Orientation of Recognition Units 453 Transduction Schemes Based upon Grafted Biomolecules 462 Electrochemical-Based Sensors 462 Field Effect Transistor Based Sensors 463 SPR-Based Sensors 465 Photoluminescence-Based Sensors 466 SERS Sensors 468 Microcantilever Sensors 469 Conclusions 471 Acknowledgments 472 References 472



Phenylboronic Acid and Polymer Brushes: An Attractive Combination with Many Possibilities 479 Solmaz Hajizadeh and Bo Mattiasson

17.1 17.2 17.3 17.4 17.5 17.6

Introduction: Polymer Brushes and Synthesis Boronic Acid Brushes 481 Affinity Separation 483 Sensors 487 Biomedical Applications 492 Conclusions 494 References 494



Smart Surfaces Modified with Phenylboronic Acid Containing Polymer Brushes 497 Hongliang Liu, Shutao Wang, and Lei Jiang

18.1 18.2

Introduction 497 Molecular Mechanism of PBA-Based Smart Surfaces 498

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18.3 18.4 18.5 18.6 18.7 18.8

pH-Responsive Surfaces Modified with PBA Polymer Brush and Their Applications 501 Sugar-Responsive Surfaces Modified with PBA Polymer Brush and Their Applications 503 PBA Polymer Brush–Based pH/Sugar Dual-Responsive OR Logic Gates and Their Applications 504 PBA Polymer Brush-Based pH/Sugar Dual-Responsive AND Logic Gates and Their Applications 506 PBA-Based Smart Systems beyond Polymer Brush and Their Applications 509 Conclusion and Perspective 511 References 512



Polymer Brushes and Microorganisms 515 Madeleine Ramstedt

19.1 19.1.1 19.1.2 19.2 19.2.1 19.2.2 19.2.2.1 19.2.2.2 19.2.2.3 19.2.2.4 19.2.2.5 19.2.3 19.2.3.1 19.2.4 19.2.5 19.3

Introduction 515 Societal Relevance for Surfaces Interacting with Microbes 515 Microorganisms 516 Brushes and Microbes 519 Adhesive Surfaces 529 Antifouling Surfaces 530 PEG-Based Brushes 531 Zwitterionic Brushes 533 Brush Density 533 Interactive Forces 535 Mechanical Interactions 537 Killing Surfaces 537 Antimicrobial Peptides 540 Brushes and Fungi 543 Brushes and Algae 546 Conclusions and Future Perspectives 549 Acknowledgments 551 References 552



Design of Polymer Brushes for Cell Culture and Cellular Delivery Danyang Li and Julien E. Gautrot

20.1 20.2 20.2.1 20.2.2

Abbreviations 557 Introduction 559 Protein-Resistant Polymer Brushes for Tissue Engineering and In Vitro Assays 561 Design of Protein-Resistant Polymer Brushes 561 Cell-Resistant Polymer Brushes 565

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20.2.3 20.3 20.3.1 20.3.2 20.3.3 20.3.4 20.4 20.4.1 20.4.2 20.5 20.5.1 20.5.2 20.6

Patterned Antifouling Brushes for the Development of Cell-Based Assays 567 Designing Brush Chemistry to Control Cell Adhesion and Proliferation 570 Polyelectrolyte Brushes for Cell Adhesion and Culture 570 Control of Surface Hydrophilicity 573 Surfaces with Controlled Stereochemistry 574 Switchable Brushes Displaying Responsive Behavior for Cell Harvesting and Detachment 576 Biofunctionalized Polymer Brushes to Regulate Cell Phenotype 581 Protein Coupling to Polymer Brushes to Control Cell Adhesion 581 Peptide-Functionalized Polymer Brushes to Regulate Cell Adhesion, Proliferation, Differentiation, and Migration 583 Polymer Brushes for Drug and Gene Delivery Applications 586 Polymer Brushes in Drug Delivery 586 Polymer Brushes in Gene Delivery 590 Summary 593 Acknowledgments 593 References 593



DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications 605 Ursula Koniges, Sade Ruffin, and Rastislav Levicky

21.1 21.2 21.3 21.4 21.5 21.6 21.7

Introduction 605 Applications 605 Preparation 607 Physicochemical Properties of DNA Brushes Hybridization in DNA Brushes 613 Other Bioprocesses in DNA Brushes 618 Perspective 619 Acknowledgments 620 References 621



DNA Brushes: Advances in Synthesis and Applications 627 Renpeng Gu, Lei Tang, Isao Aritome, and Stefan Zauscher

22.1 22.2 22.2.1 22.2.1.1 22.2.1.2 22.2.2 22.2.2.1 22.2.2.2

Introduction 627 Synthesis of DNA Brushes 628 Grafting-to Approaches 628 Immobilization on Gold Thin Films 628 Immobilization on Silicon-Based Substrates 632 Grafting-from Approaches 634 Surface-Initiated Enzymatic Polymerization 634 Surface-Initiated Rolling Circle Amplification 634

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22.2.2.3 22.2.3 22.3 22.3.1 22.3.2 22.3.3 22.3.4 22.4

Surface-Initiated Hybridization Chain Reaction 634 Synthesis of DNA Brushes on Curved Surfaces 637 Properties and Applications of DNA Brushes 637 The Effect of DNA-Modifying Enzymes on the DNA Brush Structure 637 Stimulus-Responsive Conformational Changes of DNA Brushes 639 DNA Brush for Cell-Free Surface Protein Expression 643 DNA Brush-Modified Nanoparticles for Biomedical Applications 645 Conclusion and Outlook 649 References 649



Membrane Materials Form Polymer Brush Nanoparticles 655 Erica Green, Emily Fullwood, Julieann Selden, and Ilya Zharov

23.1 23.2 23.2.1 23.2.2 23.2.3 23.2.4 23.2.5 23.2.6 23.2.7

Introduction 655 Colloidal Membranes Pore-Filled with Polymer Brushes 657 Preparation of Silica Colloidal Membranes 657 PAAM Brush-Filled Silica Colloidal Membranes 658 PDMAEMA Brush-Filled Silica Colloidal Membranes 659 PNIPAAM brush-filled silica colloidal membranes 664 Polyalanine Brush-Filled Silica Colloidal Membranes 666 PMMA Brush-Filled SiO2 @Au Colloidal Membranes 670 Colloidal Membranes Filled with Polymers Brushes Carrying Chiral Groups 672 pSPM and pSSA Brush-Filled Colloidal Nanopores 673 Self-Assembled PBNPs Membranes 676 PDMAEMA/PSPM Membranes 676 PHEMA Membranes 678 pSPM and pSSA Membranes 680 Summary 683 References 683

23.2.8 23.3 23.3.1 23.3.2 23.3.3 23.4



Responsive Polymer Networks and Brushes for Active Plasmonics 687 Nestor Gisbert Quilis, Nityanand Sharma, Stefan Fossati, Wolfgang Knoll, and Jakub Dostalek

24.1 24.2 24.3 24.3.1 24.3.2 24.3.3

Introduction 687 Tuning Spectrum of Surface Plasmon Modes 688 Polymers Used for Actuating of Plasmonic Structures 692 Temperature-Responsive Polymers 692 Optical Stimulus 694 Electrochemical Stimulus 695

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24.3.4 24.4 24.5 24.6 24.7

Chemical Stimulus 696 Imprinted Thermoresponsive Hydrogel Nanopillars 697 Thermoresponsive Hydrogel Nanogratings Fabricated by UV Laser Interference Lithography 699 Electrochemically Responsive Hydrogel Microgratings Prepared by UV Photolithography 702 Conclusions 705 Acknowledgments 706 References 706



Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics 709 Casey Yan and Zijian Zheng

25.1 25.2 25.3 25.4 25.5 25.6

Introduction 709 Mechanisms of Polymer-Assisted Metal Deposition 712 Role of Polymer Brushes 716 Selection Criterion of Polymer Brushes Enabling PAMD 716 Strategies to Fabricate Patterned Metal Conductors 717 PAMD on Different Substrates and Their Applications in Soft Electronics 720 On Textiles 720 On Plastic Thin films 721 On Elastomers 724 On Sponges 728 Conclusion, Prospects, and Challenges 731 References 732

25.6.1 25.6.2 25.6.3 25.6.4 25.7



Nanoarchitectonic Design of Complex Materials Using Polymer Brushes as Structural and Functional Units 735 M. Lorena Cortez, Gisela D´ıaz, Waldemar A. Marmisoll´e, Juan M. Giussi, and Omar Azzaroni

26.1 26.2

Introduction 735 Nanoparticles at Spherical Polymer Brushes: Hierarchical Nanoarchitectonic Construction of Complex Functional Materials 736 Nanotube and Nanowire Forests Bearing Polymer Brushes 737 Polymer Brushes on Surfaces Displaying Microtopographical Hierarchical Arrays 738 Environmentally Responsive Electrospun Nanofibers 740 Fabrication of Free-Standing “Soft” Micro- and Nanoobjects Using Polymer Brushes 741 Solid-State Polymer Electrolytes Based on Polymer Brush–Modified Colloidal Crystals 743

26.3 26.3.1 26.3.2 26.4 26.5

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26.6 26.7

26.8 26.9

Proton-Conducting Membranes with Enhanced Properties Using Polymer Brushes 745 Hybrid Architectures Combining Mesoporous Materials and Responsive Polymer Brushes: Gated Molecular Transport Systems and Controlled Delivery Vehicles 747 Ensembles of Metal Nanoparticles Modified with Polymer Brushes 750 Conclusions 754 Acknowledgments 755 References 755 Index 759

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Preface Polymers at interfaces is a field which has fascinated physicists and chemists now for nearly half a century, with respect to both basic and applied research. Polymer brushes refer to polymeric assemblies tethered at one end to a solid substrate either through covalent attachment or physical adsorption. At sufficiently high grafting density, due to repulsive interactions, the tethered chains stretch away from the surface into the solvent creating polymer brush structure. The conceptual origins of polymer brushes can be traced back to the 1950s, when it was discovered that flocculation could be prevented by grafting polymer chains onto colloidal particles. Over the past decades, developments in this field led to its valuation as a premier technique for chemical modification of solid substrates—polymer brushes offer a macromolecular perspective on the modification of interfacial properties of materials. The creativity of chemists provided a means for developing a wide variety of polymer brushes with unprecedented interfacial properties. Most of this progress stemmed from interdisciplinary work exploiting polymer chemistry as a key enabler to rationally design polymer interfaces and macromolecular assemblies entirely from scratch. Properties such as biocompatibility, wettability, corrosion resistance, friction, affinity to a specific target molecule, or even electroactivity can be manipulated by modifying a substrate with polymer brushes. Owing to their flexibility to create macromolecular interfaces in which chemical composition, thickness, and film architecture can be controlled and even addressed with nanoscale precision, polymer brushes have found applications in multiple areas concerning new adhesive materials, protein-resistant or protein adhesive biosurfaces, chemical gates, microfluidic devices, and drug delivery platforms, among other examples. Polymer brushes constitute indeed a remarkable and growing category within the world of polymer science. A research field where engineering the integration and combination of macromolecular building blocks at the nanometer and molecular level leads to new opportunities for the development of novel and improved interfacial architectures.

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Preface

For several years now, innovative research in polymer brushes is no longer circumscribed to the realm of polymer science but has begun to enter the domain of physical chemistry, nanoscience materials science, and biotechnology as well. This transformation was catalyzed by the development of new polymerization techniques, which introduced less demanding synthetic strategies facilitating scientific community-wide access to an expertise so far believed to be exclusive domain of polymer chemists. Nowadays, polymer brushes represent a fertile ground to harness the chemical, physical, or biological activity of a myriad of macromolecular components and put them to work onto a broad variety of surfaces with specific purposes in mind. This book covers the most relevant topics in basic research and those having potential technological applications. We acknowledge the considerable effort of each of the authors who has made excellent contributions to this book. We believe they have done a splendid job, and that their work will make this book a valuable reference and teaching resource. Last, but not least, we hope this book will contribute to give the reader a feeling of the enormous potential, the multiple applications, and the many up-andcoming trends behind the development of macromolecular interfaces based on the use of polymer brushes. Omar Azzaroni and Igal Szleifer

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List of Contributors Isao Aritome Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Omar Azzaroni Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Edmondo M. Benetti Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093-CH Zurich, Switzerland K. Binder Institute f¨ur Physik, Johannes Gutenberg-Universit¨at Mainz, Staudinger Weg 9, D-55099 Mainz, Germany Eva Bittrich Leibniz-Institut f¨ur Polymerforschung Dresden e.V., 01069 Dresden, Germany

O. V. Borisov Institute of Macromolecular Compounds RAS, Saint Petersburg, 199004, Russian Federation ITMO University, Saint Petersburg, 197101, Russian Federation Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Mat´eriaus (IPREM), UMR 5254 CNRS UPPA, 64053 Pau, France Tuncer Caykara Department of Chemistry, Faculty of Science, Gazi University, 06500 Besevler, Ankara, Turkey M. Lorena Cortez Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Ali Dirani Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium

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List of Contributors

Gisela D´ıaz Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Jakub Dostalek Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria S.A. Egorov Department of Chemistry, University of Virginia, Charlottesville, VA 22901, USA Stefan Fossati Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Emily Fullwood Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA Casey J. Galvin Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA Julien E. Gautrot Institute of Bioengineering, School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK

Jan Genzer Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA Juan M. Giussi Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Karine Glinel Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium Erica Green Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA Renpeng Gu Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Solmaz Hajizadeh Division of Pure and Applied Biochemistry, Department of Chemistry, Lund University, Sweden Yuuki Inoue Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

List of Contributors

Kazuhiko Ishihara Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Lei Jiang CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Alain M. Jonas Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium En-Tang Kang Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Amir Khabibullin Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Jayachandran N. Kizhakkedathu Centre for Blood Research and Department of Pathology & Laboratory Medicine, Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z3, Canada

Wolfgang Knoll Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Ursula Koniges New York University Tandon School of Engineering, Brooklyn, NY 11201, USA F. A. M. Leermakers Physical Chemistry and Soft Matter, Wageningen University, Wageningen, 6703 HB, The Netherlands Rastislav Levicky New York University Tandon School of Engineering, Brooklyn, NY 11201, USA Bin Li State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Danyang Li Institute of Bioengineering, School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK Hongliang Liu CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

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Waldemar A. Marmisoll´e Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Bo Mattiasson Indienz AB, Annebergs G˚ard, Billeberga, Sweden Division of Biotechnology, Department of Chemistry, Lund University, 221 00 Lund Sweden Krzysztof Matyjaszewski Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA A. Milchev Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Kenichi Nagase Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University (TWIns), 8-1 Kawadacho, Shinjuku, Tokyo 162-8666, Japan Rikkert J. Nap Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, 2145 Sheridan Road Evanston, IL 60208-3100, USA

Koon-Gee Neoh Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Teruo Okano Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University (TWIns), 8-1 Kawadacho, Shinjuku, Tokyo 162-8666, Japan Joanna Pietrasik Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland Dicky Pranantyo Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Nestor Gisbert Quilis Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Madeleine Ramstedt Department of Chemistry, Ume˚a University, 901 87 Ume˚a, Sweden

List of Contributors

Chun-lai Ren National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

Nicholas D. Spencer Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093-CH Zurich, Switzerland

Sade Ruffin New York University Tandon School of Engineering, Brooklyn, NY 11201, USA

Manfred Stamm Leibniz-Institut f¨ur Polymerforschung Dresden e.V., 01069 Dresden, Germany

Sho Sakata Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Julieann Selden Department Chemistry, University of Utah, Salt Lake City, UT 84112, USA Nityanand Sharma Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Estefania Gonzalez Solveyra Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, 2145 Sheridan Road Evanston, IL 60208-3100, USA

Igal Szleifer Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, 2145 Sheridan Road Evanston, IL 60208-3100, USA Mario Tagliazucchi INQUIMAE-CONICET, Ciudad Unversitaria, Pabell´on 2, and Ciudad Aut´onoma de Buenos Aires, Buenos Aires C1428EHA, Argentina Lei Tang Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Serena Lay-Ming Teo Tropical Marine Science Institute, National University of Singapore, Singapore 117585, Singapore

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Vladimir V Tsukruk School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

Jiajun Yan Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA

Petra Uhlmann Leibniz-Institut f¨ur Polymerforschung Dresden e.V., 01069 Dresden, Germany

Casey Yan Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA

Mark J. Uline Department of Chemical Engineering, University of South Carolina, SC 29208, USA

Kai Yu Centre for Blood Research and Department of Pathology & Laboratory Medicine, University of British Columbia, Vancouver, BC V6T 1Z3, Canada

Shutao Wang CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Guojun Xie Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Li Qun Xu Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore

Stefan Zauscher Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Shuaidi Zhang School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Ilya Zharov Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA Zijian Zheng The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

List of Contributors

Feng Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China

E. B. Zhulina Institute of Macromolecular Compounds RAS, Saint Petersburg, 199004, Russian Federation ITMO University, Saint Petersburg, 197101, Russian Federation

Pengyu Zhuang Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium

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Polymer and Biopolymer Brushes

Polymer and Biopolymer Brushes for Materials Science and Biotechnology Volume 2

Edited by

Omar Azzaroni Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), CONICET, Universidad Nacional de La Plata, La Plata, Argentina

Igal Szleifer Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, Evanston, IL, USA

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This edition first published 2018 © 2018 by John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, except as permitted by law. Advice on how to obtain permission to reuse material from this title is available at http://www.wiley.com/go/permissions. The right of Omar Azzaroni and Igal Szleifer to be identified as the editors of this work has been asserted in accordance with law. Registered Office John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030, USA Editorial Office 111 River Street, Hoboken, NJ 07030, USA For details of our global editorial offices, customer services, and more information about Wiley products visit us at www.wiley.com. Wiley also publishes its books in a variety of electronic formats and by print-on-demand. Some content that appears in standard print versions of this book may not be available in other formats. Limit of Liability/Disclaimer of Warranty In view of ongoing research, equipment modifications, changes in governmental regulations, and the constant flow of information relating to the use of experimental reagents, equipment, and devices, the reader is urged to review and evaluate the information provided in the package insert or instructions for each chemical, piece of equipment, reagent, or device for, among other things, any changes in the instructions or indication of usage and for added warnings and precautions. While the publisher and authors have used their best efforts in preparing this work, they make no representations or warranties with respect to the accuracy or completeness of the contents of this work and specifically disclaim all warranties, including without limitation any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives, written sales materials or promotional statements for this work. The fact that an organization, website, or product is referred to in this work as a citation and/or potential source of further information does not mean that the publisher and authors endorse the information or services the organization, website, or product may provide or recommendations it may make. This work is sold with the understanding that the publisher is not engaged in rendering professional services. The advice and strategies contained herein may not be suitable for your situation. You should consult with a specialist where appropriate. Further, readers should be aware that websites listed in this work may have changed or disappeared between when this work was written and when it is read. Neither the publisher nor authors shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. Library of Congress Cataloging-in-Publication Data applied for. Hardback ISBN: 9781119455011 Cover design by Wiley Set in 10/12pt WarnockPro by Aptara Inc., New Delhi, India 10 9 8 7 6 5 4 3 2 1

This book is dedicated to our families. Igal Szleifer also wants to dedicate this book to his coauthor Omar Azzaroni, who took over the bulk of the work after Prof. Szleifer suffered a stroke in October 2015. If not for Omar’s work and dedication, this book would not have materialized. Igal is truly grateful.

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Contents Volume  Preface xxi List of Contributors xxiii 

Functionalization of Surfaces Using Polymer Brushes: An Overview of Techniques, Strategies, and Approaches 1 Juan M. Giussi, M. Lorena Cortez, Waldemar A. Marmisoll´e, and Omar Azzaroni

1.1 1.2 1.3 1.4 1.4.1 1.4.2

Introduction: Fundamental Notions and Concepts 1 Preparation of Polymer Brushes on Solid Substrates 4 Preparation of Polymer Brushes by the “Grafting-To” Method 5 Polymer Brushes by the “Grafting-From” Method 9 Surface-Initiated Atom Transfer Radical Polymerization 9 Surface-Initiated Reversible-Addition Fragmentation Chain Transfer Polymerization 10 Surface-Initiated Nitroxide-Mediated Polymerization 13 Surface-Initiated Photoiniferter-Mediated Polymerization 13 Surface-Initiated Living Ring-Opening Polymerization 15 Surface-Initiated Ring-Opening Metathesis Polymerization 17 Surface-Initiated Anionic Polymerization 18 Conclusions 20 Acknowledgments 21 References 21

1.4.3 1.4.4 1.4.5 1.4.6 1.4.7 1.5



Polymer Brushes by Atom Transfer Radical Polymerization 29 Guojun Xie, Amir Khabibullin, Joanna Pietrasik, Jiajun Yan, and Krzysztof Matyjaszewski

2.1 2.2 2.2.1

Structure of Brushes 29 Synthesis of Polymer Brushes Grafting through 31

31

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2.2.2 2.2.3 2.3 2.4 2.4.1 2.4.2 2.4.3 2.4.4 2.4.5 2.4.6 2.5 2.5.1 2.5.2 2.5.3 2.5.4 2.6 2.6.1 2.6.2 2.6.3 2.7 2.8 2.8.1 2.8.2 2.9 2.10 2.11 2.11.1 2.11.2 2.12

Grafting to 32 Grafting from 32 ATRP Fundamentals 33 Molecular Bottlebrushes by ATRP 38 Introduction 38 Star-Like Brushes 40 Blockwise Brushes 42 Brushes with Tunable Grafting Density 45 Brushes with Block Copolymer Side Chains 46 Functionalities and Properties of Brushes 50 ATRP and Flat Surfaces 55 Chemistry at Surface 55 Grafting Density 55 Architecture 56 Applications 57 ATRP and Nanoparticles 58 Chemistry 58 Architecture 59 Applications 61 ATRP and Concave Surfaces 63 ATRP and Templates 63 Templates from Networks 63 Templates from Brushes 64 Templates from Stars 65 Bio-Related Polymer Brushes 66 Stimuli-Responsive Polymer Brushes 74 Stimuli-Responsive Solutions 76 Stimuli-Responsive Surfaces 78 Conclusion 79 Acknowledgments 80 References 80



Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 97 Tuncer Caykara

3.1 3.2

Introduction 97 Polymer Brushes via the Surface-Initiated RAFT Polymerization Process 99 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process 101 pH-Responsive Brushes 102 Temperature-Responsive Brushes 106 Polymer Brushes on Gold Surface 110

3.3 3.3.1 3.3.2 3.3.3

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3.3.4 3.3.5 3.4

Polymer Brushes on Nanoparticles 114 Micropatterned Polymer Brushes 115 Summary 117 References 119



Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush 123 Bin Li and Feng Zhou

4.1 4.2 4.3

Introduction 123 “Electro-Click” Chemistry 124 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization 129 Possible Combination of eATRP and “e-Click” Chemistry on Surface 136 Surface Functionality 136 Summary 137 Acknowledgments 138 References 138

4.4 4.5 4.6



Polymer Brushes on Flat and Curved Substrates: What Can be Learned from Molecular Dynamics Simulations 141 K. Binder, S.A. Egorov, and A. Milchev

5.1 5.2 5.3 5.4 5.5 5.6

Introduction 141 Molecular Dynamics Methods: A Short “Primer” 144 The Standard Bead Spring Model for Polymer Chains 148 Cylindrical and Spherical Polymer Brushes 150 Interaction of Brushes with Free Chains 152 Summary 153 Acknowledgments 156 References 157



Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes 161 Rikkert J. Nap, Mario Tagliazucchi, Estefania Gonzalez Solveyra, Chun-lai Ren, Mark J. Uline, and Igal Szleifer

6.1 6.2 6.3 6.3.1 6.3.1.1 6.3.1.2 6.3.2

Introduction 161 Theoretical Approach 163 Applications of the Molecular Theory 177 Acid–Base Equilibrium in Polyelectrolyte Brushes 178 Effect of Salt Concentration and pH 178 Effect of Polymer Density and Geometry 184 Competition between Chemical Equilibria and Physical Interactions 186

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6.3.2.1 Brushes of Strong Polyelectrolytes 186 6.3.2.2 Brushes of Weak Polyelectrolytes: Self-Assembly in Charge-Regulating Systems 189 6.3.2.3 Redox-Active Polyelectrolyte Brushes 193 6.3.3 End-Tethered Single Stranded DNA in Aqueous Solutions 195 6.3.4 Ligand–Receptor Binding and Protein Adsorption to Polymer Brushes 201 6.3.5 Adsorption Equilibrium of Polymer Chains through Terminal Segments: Grafting-to Formation of Polymer Brushes 207 6.4 Summary and Conclusion 212 Acknowledgments 216 References 216 

Brushes of Linear and Dendritically Branched Polyelectrolytes 223 E. B. Zhulina, F. A. M. Leermakers, and O. V. Borisov

7.1 7.2

Introduction 223 Analytical SCF Theory of Brushes Formed by Linear and Branched Polyions 224 Dendron Architecture and System Parameters 225 Analytical SCF Formalism 226 Planar Brush of PE Dendrons with an Arbitrary Architecture Asymptotic Dependences for Brush Thickness H 231 Planar Brush of Star-Like Polyelectrolytes 232 Threshold of Dendron Gaussian Elasticity 234 Scaling-Type Diagrams of States for Brushes of Linear and Branched Polyions 235 Numerical SF-SCF Model of Dendron Brush 236 Conclusions 238 References 239

7.2.1 7.2.2 7.3 7.3.1 7.4 7.5 7.6 7.7 7.8

229



Vapor Swelling of Hydrophilic Polymer Brushes 243 Casey J. Galvin and Jan Genzer

8.1 8.2 8.2.1 8.2.2

Introduction 243 Experimental 245 General Methods 245 Synthesis of Poly((2-dimethylamino)ethyl methacrylate) Brushes with a Gradient in Grafting Density 245 Synthesis of Poly(2-(diethylamino)ethyl methacrylate) Brushes 245 Chemical Modification of Poly((2-dimethylamino)ethyl methacrylate) Brushes 246 Bulk Synthesis of PDMAEMA 246 Preparation of Spuncast PDMAEMA Films 246 Chemical Modification of Spuncast PDMAEMA Film 247

8.2.3 8.2.4 8.2.5 8.2.6 8.2.7

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8.2.8

Spectroscopic Ellipsometry Measurements under Controlled Humidity Conditions 247 8.2.9 Spectroscopic Ellipsometry Measurements of Alcohol Exposure 247 8.2.10 Fitting Spectroscopic Ellipsometry Data 248 8.2.11 Infrared Variable Angle Spectroscopic Ellipsometry 248 8.3 Results and Discussion 248 8.3.1 Comparing Polymer Brush and Spuncast Polymer Film Swelling 250 8.3.2 Influence of Side Chain Chemistry on Polymer Brush Vapor Swelling 252 8.3.3 Influence of Solvent Vapor Chemistry on Polymer Brush Vapor Swelling 256 8.3.4 Influence of Grafting Density on Polymer Brush Vapor Swelling 259 8.4 Conclusion 262 8.A.1 Appendix 263 8.A.1.1 Mole Fraction Calculation 263 8.A.1.2 Water Cluster Number Calculation 264 Acknowledgments 265 References 265 

Temperature Dependence of the Swelling and Surface Wettability of Dense Polymer Brushes 267 Pengyu Zhuang, Ali Dirani, Karine Glinel, and Alain M. Jonas

9.1 9.2

Introduction 267 The Swelling Coefficient of a Polymer Brush Mirrors Its Volume Hydrophilicity 269 The Cosine of the Contact Angle of Water on a Water-Equilibrated Polymer Brush Defines Its Surface Hydrophilicity 270 Case Study: Temperature-Dependent Surface hydrophilicity of Dense PNIPAM Brushes 272 Case Study: Temperature-Dependent Swelling and Volume Hydrophilicity of Dense PNIPAM Brushes 274 Thermoresponsive Poly(oligo(ethylene oxide)methacrylate) Copolymer Brushes: Versatile Functional Alternatives to PNIPAM 277 Surface and Volume Hydrophilicity of Nonthermoresponsive Poly(oligo(ethylene oxide)methacrylate) Copolymer Brushes 279 Conclusions 282 Acknowledgments 283 References 283

9.3

9.4 9.5 9.6

9.7 9.8

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

Functional Biointerfaces Tailored by “Grafting-To” Brushes Eva Bittrich, Manfred Stamm, and Petra Uhlmann

10.1 10.2 10.2.1

Introduction 287 Part I: Polymer Brush Architectures 288 Design of Physicochemical Interfaces by Polymer Brushes 288 Stimuli-Responsive Homopolymer Brushes 288 Combination of Responses Using Mixed Polymer Brushes 290 Stimuli-Responsive Gradient Brushes 293 Modification of Polymer Brushes by Click Chemistry 293 Definition of Click Chemistry 293 Modification of End Groups of Grafted PNIPAAm Chains 295 Hybrid Brush Nanostructures 297 Nanoparticles Immobilized at Polymer Brushes 298 Sculptured Thin Films Grafted with Polymer Brushes 300 Part II: Actuating Biomolecule Interactions with Surfaces 303 Adsorption of Proteins to Polymer Brush Surfaces 303 Calculation of the Adsorbed Amount of Protein from Ellipsometric Experiments 305 Preventing Protein Adsorption 306 Adsorption at Polyelectrolyte Brushes 310 Polymer Brushes as Interfaces for Cell Adhesion and Interaction 313 Cell Adhesion on Stimuli-Responsive Polymer Surfaces Based on PNIPAAm Brushes 315 Growth Factors on Polymer Brushes 318 Conclusion and Outlook 320 Acknowledgments 321 References 321

10.2.1.1 10.2.1.2 10.2.1.3 10.2.2 10.2.2.1 10.2.2.2 10.2.3 10.2.3.1 10.2.3.2 10.3 10.3.1 10.3.1.1 10.3.1.2 10.3.1.3 10.3.2 10.3.2.1 10.3.2.2 10.4



Glycopolymer Brushes Presenting Sugars in Their Natural Form: Synthesis and Applications 333 Kai Yu and Jayachandran N. Kizhakkedathu

11.1 11.2 11.2.1 11.2.1.1

Introduction and Background 333 Results and Discussion 334 Synthesis of Glycopolymer Brushes 334 Synthesis of N-Substituted Acrylamide Derivatives of Glycomonomers 334 11.2.1.2 Synthesis and Characterization of Glycopolymer Brushes on Gold Chip and Silicon Wafer 334 11.2.1.3 Synthesis and Characterization of Glycopolymer Brushes on Polystyrene Particles 335

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11.2.1.4 Synthesis and Characterization of Glycopolymer Brushes with Variation in the Composition of Carbohydrate Residues on SPR Chip 338 11.2.1.5 Preparation of Glycopolymer Brushes-Modified Particles with Different Grafting Density (Conformation) 338 11.2.2 Applications of Glycopolymer Brushes 341 11.2.2.1 Antithrombotic Surfaces Based on Glycopolymer Brushes 341 11.2.2.2 Glycopolymer Brushes Based Carbohydrate Arrays to Modulate Multivalent Protein Binding on Surfaces 345 11.2.2.3 Modulation of Innate Immune Response by the Conformation and Chemistry of Glycopolymer Brushes 351 11.3 Conclusions 356 Acknowledgments 357 References 357 

Thermoresponsive Polymer Brushes for Thermally Modulated Cell Adhesion and Detachment 361 Kenichi Nagase and Teruo Okano

12.1 12.2

Introduction 361 Thermoresponsive Polymer Hydrogel-Modified Surfaces for Cell Adhesion and Detachment 362 Thermoresponsive Polymer Brushes Prepared Using ATRP 363 Thermoresponsive Polymer Brushes Prepared by RAFT Polymerization 368 Conclusions 372 Acknowledgments 372 References 372

12.3 12.4 12.5

Volume  Preface xxi List of Contributors xxiii 

Biomimetic Anchors for Antifouling Polymer Brush Coatings 377 Dicky Pranantyo, Li Qun Xu, En-Tang Kang, Koon-Gee Neoh, and Serena Lay-Ming Teo

13.1 13.2 13.3 13.3.1 13.3.1.1

Introduction to Biofouling Management 377 Polymer Brushes for Surface Functionalization 378 Biomimetic Anchors for Antifouling Polymer Brushes 379 Mussel Adhesive-Inspired Dopamine Anchors 379 Antifouling Polymer Brushes Prepared via the “Grafting-To” Approach on (poly)Dopamine Anchor 383

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13.3.1.2 Antifouling Polymer Brushes Prepared via the “Grafting-From” Approach on (poly)Dopamine Anchor 386 13.3.1.3 Direct Grafting of Antifouling Polymer Brushes Containing Anchorable Dopamine-Derived Functionalities 389 13.3.2 (Poly)phenolic Anchors for Antifouling Polymer Brushes 391 13.3.3 Biomolecular Anchors for Antifouling Polymer Brushes 393 13.4 Barnacle Cement as Anchor for Antifouling Polymer Brushes 397 13.5 Conclusion and Outlooks 399 References 400 

Protein Adsorption Process Based on Molecular Interactions at Well-Defined Polymer Brush Surfaces 405 Sho Sakata, Yuuki Inoue, and Kazuhiko Ishihara

14.1 14.2

Introduction 405 Utility of Polymer Brush Layers as Highly Controllable Polymer Surfaces 406 Performance of Polymer Brush Surfaces as Antifouling Biointerfaces 408 Elucidation of Protein Adsorption Based on Molecular Interaction Forces 412 Concluding Remarks 416 References 417

14.3 14.4 14.5



Are Lubricious Polymer Brushes Antifouling? Are Antifouling Polymer Brushes Lubricious? 421 Edmondo M. Benetti and Nicholas D. Spencer

15.1 15.2 15.3 15.4

Introduction 421 Poly(ethylene glycol) Brushes 422 Beyond Simple PEG Brushes 424 Conclusion 429 References 429



Biofunctionalized Brush Surfaces for Biomolecular Sensing 433 Shuaidi Zhang and Vladimir V. Tsukruk

16.1 16.2 16.2.1 16.2.2 16.2.3 16.2.4 16.2.5 16.2.6

Introduction 433 Biorecognition Units 435 Antibodies 435 Antibody Fragments 435 Aptamers 437 Peptide Aptamers 438 Enzymes 438 Peptide Nucleic Acid, Lectin, and Molecular Imprinted Polymers 439

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16.3 16.3.1 16.3.1.1 16.3.1.2 16.3.1.3 16.3.1.4 16.3.1.5 16.3.2 16.3.2.1 16.3.2.2 16.3.2.3 16.3.2.4 16.4 16.4.1 16.4.2 16.5 16.5.1 16.5.2 16.5.3 16.5.4 16.5.5 16.5.6 16.6

Immobilization Strategy 439 Through Direct Covalent Linkage 440 Thiolated Aptamers on Noble Metal 440 General Activated Surface Chemistry 442 Diels–Alder Cycloaddition 444 Staudinger Ligation 444 1,3-Dipolar Cycloaddition 446 Through Affinity Tags 447 Biotin–Avidin/Streptavidin Pairing 447 NTA–Ni2+ –Histidine Pairing 448 Protein A/Protein G – Fc Pairing 449 Oligonucleotide Hybridization 450 Microstructure and Morphology of Biobrush Layers 451 Grafting Density Control 451 Conformation and Orientation of Recognition Units 453 Transduction Schemes Based upon Grafted Biomolecules 462 Electrochemical-Based Sensors 462 Field Effect Transistor Based Sensors 463 SPR-Based Sensors 465 Photoluminescence-Based Sensors 466 SERS Sensors 468 Microcantilever Sensors 469 Conclusions 471 Acknowledgments 472 References 472



Phenylboronic Acid and Polymer Brushes: An Attractive Combination with Many Possibilities 479 Solmaz Hajizadeh and Bo Mattiasson

17.1 17.2 17.3 17.4 17.5 17.6

Introduction: Polymer Brushes and Synthesis Boronic Acid Brushes 481 Affinity Separation 483 Sensors 487 Biomedical Applications 492 Conclusions 494 References 494



Smart Surfaces Modified with Phenylboronic Acid Containing Polymer Brushes 497 Hongliang Liu, Shutao Wang, and Lei Jiang

18.1 18.2

Introduction 497 Molecular Mechanism of PBA-Based Smart Surfaces 498

479

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18.3 18.4 18.5 18.6 18.7 18.8

pH-Responsive Surfaces Modified with PBA Polymer Brush and Their Applications 501 Sugar-Responsive Surfaces Modified with PBA Polymer Brush and Their Applications 503 PBA Polymer Brush–Based pH/Sugar Dual-Responsive OR Logic Gates and Their Applications 504 PBA Polymer Brush-Based pH/Sugar Dual-Responsive AND Logic Gates and Their Applications 506 PBA-Based Smart Systems beyond Polymer Brush and Their Applications 509 Conclusion and Perspective 511 References 512



Polymer Brushes and Microorganisms 515 Madeleine Ramstedt

19.1 19.1.1 19.1.2 19.2 19.2.1 19.2.2 19.2.2.1 19.2.2.2 19.2.2.3 19.2.2.4 19.2.2.5 19.2.3 19.2.3.1 19.2.4 19.2.5 19.3

Introduction 515 Societal Relevance for Surfaces Interacting with Microbes 515 Microorganisms 516 Brushes and Microbes 519 Adhesive Surfaces 529 Antifouling Surfaces 530 PEG-Based Brushes 531 Zwitterionic Brushes 533 Brush Density 533 Interactive Forces 535 Mechanical Interactions 537 Killing Surfaces 537 Antimicrobial Peptides 540 Brushes and Fungi 543 Brushes and Algae 546 Conclusions and Future Perspectives 549 Acknowledgments 551 References 552



Design of Polymer Brushes for Cell Culture and Cellular Delivery Danyang Li and Julien E. Gautrot

20.1 20.2 20.2.1 20.2.2

Abbreviations 557 Introduction 559 Protein-Resistant Polymer Brushes for Tissue Engineering and In Vitro Assays 561 Design of Protein-Resistant Polymer Brushes 561 Cell-Resistant Polymer Brushes 565

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Contents

20.2.3 20.3 20.3.1 20.3.2 20.3.3 20.3.4 20.4 20.4.1 20.4.2 20.5 20.5.1 20.5.2 20.6

Patterned Antifouling Brushes for the Development of Cell-Based Assays 567 Designing Brush Chemistry to Control Cell Adhesion and Proliferation 570 Polyelectrolyte Brushes for Cell Adhesion and Culture 570 Control of Surface Hydrophilicity 573 Surfaces with Controlled Stereochemistry 574 Switchable Brushes Displaying Responsive Behavior for Cell Harvesting and Detachment 576 Biofunctionalized Polymer Brushes to Regulate Cell Phenotype 581 Protein Coupling to Polymer Brushes to Control Cell Adhesion 581 Peptide-Functionalized Polymer Brushes to Regulate Cell Adhesion, Proliferation, Differentiation, and Migration 583 Polymer Brushes for Drug and Gene Delivery Applications 586 Polymer Brushes in Drug Delivery 586 Polymer Brushes in Gene Delivery 590 Summary 593 Acknowledgments 593 References 593



DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications 605 Ursula Koniges, Sade Ruffin, and Rastislav Levicky

21.1 21.2 21.3 21.4 21.5 21.6 21.7

Introduction 605 Applications 605 Preparation 607 Physicochemical Properties of DNA Brushes Hybridization in DNA Brushes 613 Other Bioprocesses in DNA Brushes 618 Perspective 619 Acknowledgments 620 References 621



DNA Brushes: Advances in Synthesis and Applications 627 Renpeng Gu, Lei Tang, Isao Aritome, and Stefan Zauscher

22.1 22.2 22.2.1 22.2.1.1 22.2.1.2 22.2.2 22.2.2.1 22.2.2.2

Introduction 627 Synthesis of DNA Brushes 628 Grafting-to Approaches 628 Immobilization on Gold Thin Films 628 Immobilization on Silicon-Based Substrates 632 Grafting-from Approaches 634 Surface-Initiated Enzymatic Polymerization 634 Surface-Initiated Rolling Circle Amplification 634

610

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22.2.2.3 22.2.3 22.3 22.3.1 22.3.2 22.3.3 22.3.4 22.4

Surface-Initiated Hybridization Chain Reaction 634 Synthesis of DNA Brushes on Curved Surfaces 637 Properties and Applications of DNA Brushes 637 The Effect of DNA-Modifying Enzymes on the DNA Brush Structure 637 Stimulus-Responsive Conformational Changes of DNA Brushes 639 DNA Brush for Cell-Free Surface Protein Expression 643 DNA Brush-Modified Nanoparticles for Biomedical Applications 645 Conclusion and Outlook 649 References 649



Membrane Materials Form Polymer Brush Nanoparticles 655 Erica Green, Emily Fullwood, Julieann Selden, and Ilya Zharov

23.1 23.2 23.2.1 23.2.2 23.2.3 23.2.4 23.2.5 23.2.6 23.2.7

Introduction 655 Colloidal Membranes Pore-Filled with Polymer Brushes 657 Preparation of Silica Colloidal Membranes 657 PAAM Brush-Filled Silica Colloidal Membranes 658 PDMAEMA Brush-Filled Silica Colloidal Membranes 659 PNIPAAM brush-filled silica colloidal membranes 664 Polyalanine Brush-Filled Silica Colloidal Membranes 666 PMMA Brush-Filled SiO2 @Au Colloidal Membranes 670 Colloidal Membranes Filled with Polymers Brushes Carrying Chiral Groups 672 pSPM and pSSA Brush-Filled Colloidal Nanopores 673 Self-Assembled PBNPs Membranes 676 PDMAEMA/PSPM Membranes 676 PHEMA Membranes 678 pSPM and pSSA Membranes 680 Summary 683 References 683

23.2.8 23.3 23.3.1 23.3.2 23.3.3 23.4



Responsive Polymer Networks and Brushes for Active Plasmonics 687 Nestor Gisbert Quilis, Nityanand Sharma, Stefan Fossati, Wolfgang Knoll, and Jakub Dostalek

24.1 24.2 24.3 24.3.1 24.3.2 24.3.3

Introduction 687 Tuning Spectrum of Surface Plasmon Modes 688 Polymers Used for Actuating of Plasmonic Structures 692 Temperature-Responsive Polymers 692 Optical Stimulus 694 Electrochemical Stimulus 695

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24.3.4 24.4 24.5 24.6 24.7

Chemical Stimulus 696 Imprinted Thermoresponsive Hydrogel Nanopillars 697 Thermoresponsive Hydrogel Nanogratings Fabricated by UV Laser Interference Lithography 699 Electrochemically Responsive Hydrogel Microgratings Prepared by UV Photolithography 702 Conclusions 705 Acknowledgments 706 References 706



Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics 709 Casey Yan and Zijian Zheng

25.1 25.2 25.3 25.4 25.5 25.6

Introduction 709 Mechanisms of Polymer-Assisted Metal Deposition 712 Role of Polymer Brushes 716 Selection Criterion of Polymer Brushes Enabling PAMD 716 Strategies to Fabricate Patterned Metal Conductors 717 PAMD on Different Substrates and Their Applications in Soft Electronics 720 On Textiles 720 On Plastic Thin films 721 On Elastomers 724 On Sponges 728 Conclusion, Prospects, and Challenges 731 References 732

25.6.1 25.6.2 25.6.3 25.6.4 25.7



Nanoarchitectonic Design of Complex Materials Using Polymer Brushes as Structural and Functional Units 735 M. Lorena Cortez, Gisela D´ıaz, Waldemar A. Marmisoll´e, Juan M. Giussi, and Omar Azzaroni

26.1 26.2

Introduction 735 Nanoparticles at Spherical Polymer Brushes: Hierarchical Nanoarchitectonic Construction of Complex Functional Materials 736 Nanotube and Nanowire Forests Bearing Polymer Brushes 737 Polymer Brushes on Surfaces Displaying Microtopographical Hierarchical Arrays 738 Environmentally Responsive Electrospun Nanofibers 740 Fabrication of Free-Standing “Soft” Micro- and Nanoobjects Using Polymer Brushes 741 Solid-State Polymer Electrolytes Based on Polymer Brush–Modified Colloidal Crystals 743

26.3 26.3.1 26.3.2 26.4 26.5

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26.6 26.7

26.8 26.9

Proton-Conducting Membranes with Enhanced Properties Using Polymer Brushes 745 Hybrid Architectures Combining Mesoporous Materials and Responsive Polymer Brushes: Gated Molecular Transport Systems and Controlled Delivery Vehicles 747 Ensembles of Metal Nanoparticles Modified with Polymer Brushes 750 Conclusions 754 Acknowledgments 755 References 755 Index 759

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Preface Polymers at interfaces is a field which has fascinated physicists and chemists now for nearly half a century, with respect to both basic and applied research. Polymer brushes refer to polymeric assemblies tethered at one end to a solid substrate either through covalent attachment or physical adsorption. At sufficiently high grafting density, due to repulsive interactions, the tethered chains stretch away from the surface into the solvent creating polymer brush structure. The conceptual origins of polymer brushes can be traced back to the 1950s, when it was discovered that flocculation could be prevented by grafting polymer chains onto colloidal particles. Over the past decades, developments in this field led to its valuation as a premier technique for chemical modification of solid substrates—polymer brushes offer a macromolecular perspective on the modification of interfacial properties of materials. The creativity of chemists provided a means for developing a wide variety of polymer brushes with unprecedented interfacial properties. Most of this progress stemmed from interdisciplinary work exploiting polymer chemistry as a key enabler to rationally design polymer interfaces and macromolecular assemblies entirely from scratch. Properties such as biocompatibility, wettability, corrosion resistance, friction, affinity to a specific target molecule, or even electroactivity can be manipulated by modifying a substrate with polymer brushes. Owing to their flexibility to create macromolecular interfaces in which chemical composition, thickness, and film architecture can be controlled and even addressed with nanoscale precision, polymer brushes have found applications in multiple areas concerning new adhesive materials, protein-resistant or protein adhesive biosurfaces, chemical gates, microfluidic devices, and drug delivery platforms, among other examples. Polymer brushes constitute indeed a remarkable and growing category within the world of polymer science. A research field where engineering the integration and combination of macromolecular building blocks at the nanometer and molecular level leads to new opportunities for the development of novel and improved interfacial architectures.

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Preface

For several years now, innovative research in polymer brushes is no longer circumscribed to the realm of polymer science but has begun to enter the domain of physical chemistry, nanoscience materials science, and biotechnology as well. This transformation was catalyzed by the development of new polymerization techniques, which introduced less demanding synthetic strategies facilitating scientific community-wide access to an expertise so far believed to be exclusive domain of polymer chemists. Nowadays, polymer brushes represent a fertile ground to harness the chemical, physical, or biological activity of a myriad of macromolecular components and put them to work onto a broad variety of surfaces with specific purposes in mind. This book covers the most relevant topics in basic research and those having potential technological applications. We acknowledge the considerable effort of each of the authors who has made excellent contributions to this book. We believe they have done a splendid job, and that their work will make this book a valuable reference and teaching resource. Last, but not least, we hope this book will contribute to give the reader a feeling of the enormous potential, the multiple applications, and the many up-andcoming trends behind the development of macromolecular interfaces based on the use of polymer brushes. Omar Azzaroni and Igal Szleifer

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List of Contributors Isao Aritome Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Omar Azzaroni Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Edmondo M. Benetti Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093-CH Zurich, Switzerland K. Binder Institute f¨ur Physik, Johannes Gutenberg-Universit¨at Mainz, Staudinger Weg 9, D-55099 Mainz, Germany Eva Bittrich Leibniz-Institut f¨ur Polymerforschung Dresden e.V., 01069 Dresden, Germany

O. V. Borisov Institute of Macromolecular Compounds RAS, Saint Petersburg, 199004, Russian Federation ITMO University, Saint Petersburg, 197101, Russian Federation Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Mat´eriaus (IPREM), UMR 5254 CNRS UPPA, 64053 Pau, France Tuncer Caykara Department of Chemistry, Faculty of Science, Gazi University, 06500 Besevler, Ankara, Turkey M. Lorena Cortez Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Ali Dirani Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium

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Gisela D´ıaz Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Jakub Dostalek Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria S.A. Egorov Department of Chemistry, University of Virginia, Charlottesville, VA 22901, USA Stefan Fossati Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Emily Fullwood Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA Casey J. Galvin Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA Julien E. Gautrot Institute of Bioengineering, School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK

Jan Genzer Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA Juan M. Giussi Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Karine Glinel Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium Erica Green Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA Renpeng Gu Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Solmaz Hajizadeh Division of Pure and Applied Biochemistry, Department of Chemistry, Lund University, Sweden Yuuki Inoue Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan

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Kazuhiko Ishihara Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Lei Jiang CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Alain M. Jonas Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium En-Tang Kang Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Amir Khabibullin Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Jayachandran N. Kizhakkedathu Centre for Blood Research and Department of Pathology & Laboratory Medicine, Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z3, Canada

Wolfgang Knoll Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Ursula Koniges New York University Tandon School of Engineering, Brooklyn, NY 11201, USA F. A. M. Leermakers Physical Chemistry and Soft Matter, Wageningen University, Wageningen, 6703 HB, The Netherlands Rastislav Levicky New York University Tandon School of Engineering, Brooklyn, NY 11201, USA Bin Li State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Danyang Li Institute of Bioengineering, School of Engineering and Materials Science, Queen Mary, University of London, Mile End Road, London, E1 4NS, UK Hongliang Liu CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

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Waldemar A. Marmisoll´e Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, CC. 16 Suc. 4, 1900 La Plata, Argentina Bo Mattiasson Indienz AB, Annebergs G˚ard, Billeberga, Sweden Division of Biotechnology, Department of Chemistry, Lund University, 221 00 Lund Sweden Krzysztof Matyjaszewski Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA A. Milchev Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria Kenichi Nagase Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University (TWIns), 8-1 Kawadacho, Shinjuku, Tokyo 162-8666, Japan Rikkert J. Nap Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, 2145 Sheridan Road Evanston, IL 60208-3100, USA

Koon-Gee Neoh Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Teruo Okano Institute of Advanced Biomedical Engineering and Science, Tokyo Women’s Medical University (TWIns), 8-1 Kawadacho, Shinjuku, Tokyo 162-8666, Japan Joanna Pietrasik Institute of Polymer and Dye Technology, Lodz University of Technology, Stefanowskiego 12/16, 90-924 Lodz, Poland Dicky Pranantyo Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore Nestor Gisbert Quilis Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Madeleine Ramstedt Department of Chemistry, Ume˚a University, 901 87 Ume˚a, Sweden

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Chun-lai Ren National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, People’s Republic of China

Nicholas D. Spencer Laboratory for Surface Science and Technology, Department of Materials, ETH Zurich, Vladimir-Prelog-Weg 5, 8093-CH Zurich, Switzerland

Sade Ruffin New York University Tandon School of Engineering, Brooklyn, NY 11201, USA

Manfred Stamm Leibniz-Institut f¨ur Polymerforschung Dresden e.V., 01069 Dresden, Germany

Sho Sakata Department of Materials Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Julieann Selden Department Chemistry, University of Utah, Salt Lake City, UT 84112, USA Nityanand Sharma Biosensor Technologies, AIT Austrian Institute of Technology GmbH, Muthgasse 11, Vienna 1190, Austria Estefania Gonzalez Solveyra Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, 2145 Sheridan Road Evanston, IL 60208-3100, USA

Igal Szleifer Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, 2145 Sheridan Road Evanston, IL 60208-3100, USA Mario Tagliazucchi INQUIMAE-CONICET, Ciudad Unversitaria, Pabell´on 2, and Ciudad Aut´onoma de Buenos Aires, Buenos Aires C1428EHA, Argentina Lei Tang Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Serena Lay-Ming Teo Tropical Marine Science Institute, National University of Singapore, Singapore 117585, Singapore

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Vladimir V Tsukruk School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA

Jiajun Yan Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA

Petra Uhlmann Leibniz-Institut f¨ur Polymerforschung Dresden e.V., 01069 Dresden, Germany

Casey Yan Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC 27695-7905, USA

Mark J. Uline Department of Chemical Engineering, University of South Carolina, SC 29208, USA

Kai Yu Centre for Blood Research and Department of Pathology & Laboratory Medicine, University of British Columbia, Vancouver, BC V6T 1Z3, Canada

Shutao Wang CAS Key Laboratory of Bio-inspired Materials and Interfacial Science, CAS Center for Excellence in Nanoscience Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China Guojun Xie Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, PA 15213, USA Li Qun Xu Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore

Stefan Zauscher Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA Shuaidi Zhang School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Ilya Zharov Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA Zijian Zheng The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong

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Feng Zhou State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China Pengyu Zhuang Universit´e catholique de Louvain, Institute of Condensed Matter and Nanoscience, Bio & Soft Matter, Louvain-la-Neuve, Belgium

E. B. Zhulina Institute of Macromolecular Compounds RAS, Saint Petersburg, 199004, Russian Federation ITMO University, Saint Petersburg, 197101, Russian Federation

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 Functionalization of Surfaces Using Polymer Brushes: An Overview of Techniques, Strategies, and Approaches Juan M. Giussi, M. Lorena Cortez, Waldemar A. Marmisoll´e, and Omar Azzaroni Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, La Plata, Argentina

. Introduction: Fundamental Notions and Concepts From an historical perspective, much of the interest in chemical modification of surfaces originated from their importance to different technologies, namely wetting, adhesion, catalysis, lubrication, detergency, biocompatibility, corrosion, or colloidal stabilization, among other examples.1–3 During the early years, when sophisticated spectroscopies working under ultrahigh vacuum conditions were developed, the central interest of surface science was especially focused on unraveling the atomic and electronic structures of metals, metal oxides, and semiconducting surfaces (usually as single crystals). However, by the end of past century, with the advent of new chemical techniques to integrate functions on solid substrates, surface science started to look at organic surfaces as ideal partners to address emerging and challenging issues on the technological agenda, for example, in relation to the development of antifouling biocompatible coatings for biomedical devices or the rational design of antifogging coatings with frost-resisting capabilities for the automotive industry. The controlled transfer of organized monolayers of amphiphilic molecules from the air–water interface to a solid substrate was the first molecular-scale technology for the rational design of organic surfaces, this being a technology designed by Langmuir and Blodgett in the 1930s.4,5 Later on, Levine and Zisman studied the physical properties of monolayers adsorbed at the solid– air interface and their effects on friction and wettability.6 However, a new element came into play in the early 1980s with the discovery of self-assembled Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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1 Functionalization of Surfaces Using Polymer Brushes

monolayers (SAMs).7,8 SAMs provided a method to create organic surfaces with known, reproducible structures. Alkanethiolate SAMs definitively shifted the focus of surface science from metals and metal oxides to surfaces constituted of organic molecules and allowed studies of biologically relevant surfaces. The ability to control the composition of the surface made it possible not only to examine structure–property relationships, but, which is more important, to design and prepare surfaces displaying functions relevant in materials science, nanoscience, and biology. Despite the versatility of these systems, one important limitation of SAMs is that functional groups can only be introduced at the surface. This means that SAMs by themselves cannot generate functional three-dimensional (3D) interfacial architectures—this being a decisive factor in multiple contemporary applications of organic thin films. In light of this context, polymer brushes gradually emerged as major players in different technological areas demanding new approaches for surface modification.9–12 Polymer brushes refer to assemblies of macromolecules that are tethered by one end to a surface or interface and can be generated through different strategies. Bringing polymeric building blocks— in the form of polymer brushes—into the game opens a new dimension: Chemical groups can be carried all along the polymer backbone and can be placed in different pseudo-3D spatial arrangements when multiblock polymers are used. The most prominent difference between SAMs and polymer brushes is the dimension of the building blocks themselves: SAMs are constituted of assemblies of small molecules, whereas polymer brushes are constituted of polymeric chains. The use of macromolecular building blocks brings in functional versatility but also introduces structural complexity that depends on the grafting density of the tethered polymer chains. For example, the entropic cost for polymer brushes to stretch out to their maximum length is very high and, consequently, polymer brushes are disordered at the molecular level. At very low grafting density, the so-called “mushroom regime,” the polymers adapt a more or less random coil conformation (see Figure 1.1).13 In good solvents, the thickness of the anchored polymer, H, in the mushroom regime (low grafting density) scales as H ∝ N𝜎 0 , where N is the degree of polymerization of the polymer and 𝜎 is the grafting density. The grafting density (𝜎) is defined by 𝜎 = (H𝜌NA )/Mn (where H, brush thickness; 𝜌, bulk density of the brush composition; NA , Avogadro’s number; and Mn molecular weight of the tethered polymer chains). Upon increasing the grafting density, polymer chains interact with each other and there will be a degree of distortion from the random coil. At sufficiently high grafting density, the so-called brush regime is reached (Figure 1.1). In this regime, the brush height scales as H ∝ N𝜎 1/3 . These simple scaling laws connecting grafting density and molecular weight (or degree of polymerization) with brush height were first derived by Alexander14 and de Gennes15 and then corroborated experimentally by different authors. The overlap between the chains and the degree of stretching is heavily dependent on the grafting density, the chain length (or the degree of

1.1 Introduction: Fundamental Notions and Concepts

3.5

H ∼ σ1/3

H ∼ σ0

3.0

H (nm)

2.5 brush 2.0

H/Rg

30

CMPE:PO ratio 1:1 (S1) 1:2 (S2) 1:5 (S5)

mushroom 1.5

10

1.0 0.01

0.1 PAAm grafting density (nm–2)

Figure . Wet thickness of polyacrylamide (PAAm) films as a function of the PAAm grafting density. Samples prepared on substrates containing the initiator gradients made of 1-trichlorosilyl-2-(m/p-chloromethyl phenyl) ethane: octadecyltrichlorosilane (CMPE: OTS) mixtures (w/w) 1:1 (squares), 1:2 (circles), and 1:5 (triangles). The inset shows a cartoon illustrating different polymer states with increasing grafting density. Source: Wu et al. 2003.13 Reproduced with permission of American Chemical Society.

polymerization), and the solvent quality. In good solvents, the chains swell forcing them to stretch away from the surface. The extent of stretching is governed by a competition or interplay between the entropic loss due to chain stretching and the excluded volume interactions between different segments of the tethered polymer chains. If the excluded volume interaction can be altered, then the conformation of the chains will, in turn, change. As a result, polymer brushes experience swelling and collapse transitions in good and poor solvents, associated with large conformational changes of the polymer backbones. The conformational behavior of polymer brushes can become even richer when charges are introduced into the monomer units of the polymer backbone, that is, polyelectrolyte brushes. Pioneering works by Pincus16 and Zhulina et al.17 revealed the existence of different regimes for polyelectrolyte brushes in which the brush height was correlated with different parameters such as salt concentration, length of the polymer chains, grafting density, and degree of charging. The above-mentioned notions reveal that the grafting density plays an important role in establishing the regime in which the macromolecular system operates, that is, “mushroom” or “brush” regime (Figure 1.1).13 In many cases, for simplicity of expression, the term “polymer brush” is used as a synonym of the terms “tethered polymer chains” or “end-grafted polymers.” However,

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1 Functionalization of Surfaces Using Polymer Brushes

strictly speaking, the term “polymer brush” should be associated with a layer of tethered polymer chains under specific conditions—when the behavior of the tethered layer is dictated by strong interactions between densely grafted polymer chains. A more detailed discussion of the definition of “polymer brushes” can be found in Brittain and Minko.18

. Preparation of Polymer Brushes on Solid Substrates The chemical and structural properties of thin polymer films are determined through the choice of monomer, surface attachment method, and polymerization conditions. Two general approaches are commonly employed to fabricate thin polymer films bound to a chosen substrate: “grafting to” and “grafting from” as shown in Figure 1.2. In the “grafting-to” technique, presynthesized polymers “GRAFTING-TO” APPROACH

“GRAFTING-FROM” APPROACH

Surface-binding sites

Polymer-initiating moieties

PHYSISORPTION AND/OR CHEMISORPTION OF PREFORMED BUILDING BLOCKS

POLYMER BRUSHES POLYMERS

SURFACE-INITIATED POLYMERIZATION MONOMERS

Polymerization Techniques ATRP, RAFT, ROMP, NMP,

etc

Macromolecular Multifunctionality by Design Organic, Polymeric and Inorganic Substrates

Figure . Conceptual illustration of the chemical strategies (grafting-to and grafting-from approaches) used to tether functional polymer brushes on a wide variety of substrates. The figure also includes an atomic force microscopy image (500 × 500 nm2 ) of poly(2-(methacryloyloxy)-ethyl-trimethyl-ammonium chloride) brushes grown on silicon substrates via SI-ATRP. Source: Azzaroni 2012.9 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA.

1.3 Preparation of Polymer Brushes by the “Grafting-To” Method

are anchored to a surface from solution. On the other hand, the “grafting-from” method involves sequential growth of polymer chains from a surface.19 The ability of low molecular weight monomers to diffuse to surface-active sites more readily than preformed macromolecules is the key concept, which differentiates the two approaches that we will discuss in the following sections.

. Preparation of Polymer Brushes by the “Grafting-To” Method The “grafting-to” approach is based on the chemical reaction between presynthesized polymers and reactive groups on the substrate, usually via an end functionality designed into the macromolecule. This chemical reaction can take place in a solution or in a melt. One of the attractive features of the “graftingto” method is that it does not involve elaborate synthetic procedures. However, typical thickness values obtained by this technique are rather low, which is often considered a significant disadvantage of the method. During a grafting-to procedure, macromolecular diffusion through a developing film rapidly encounters significant steric hindrance. As a result, many surface-active sites remain uncoupled and only thin, low-density polymer films can be achieved. In other words, “excluded volume” effects become more pronounced as the thickness of the polymer layer increases.20 Karim et al.21 argued that homogeneous, dense layers can be obtained readily in a poor solvent, because the steric interchain repulsion is diminished and the volume occupied by a polymer chain is smaller. This would allow other chains to reach the substrate more easily. This concept then led to the crucial task of devising new approaches for producing thicker grafted layers by minimizing the excluded volume interactions. These new strategies were based on the use of polymer melts,22 or grafting from a concentrated polymer solution23,24 and enabled the modification of flat25–27 and porous28 substrates, fibers,29–31 and nanoparticles32,33 using “grafting-to” techniques. The “grafting-to” approach has also been used for creating polymer brushes with variations in grafting density along a substrate—the so-called “gradient polymer brushes.” This type of film architecture can be obtained either by inducing a gradient in the grafting temperature in order to exploit the temperature dependence of grafting kinetics34–36 or by methods based on gradual controlled immersion of the substrate into a solution of the reactive polymer in order to control the time of the grafting reaction.37 One of the most important aspects of the “grafting-to” process is its direct link to the surface chemistry of the employed substrate, as it governs four key elements: (i) the compatibility of the method with the surface that we are intending to modify, (ii) the processing conditions, (iii) the maximum grafting

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1 Functionalization of Surfaces Using Polymer Brushes

density of the layers that can be obtained, and (iv) the chemical stability of the tethered polymer layers. In many cases, this method demands the preconditioning of the substrates in order to introduce the desired complementary functional groups for the grafting reaction. To this end, several groups explored a variety of strategies for surface preparation and priming, including plasma treatments,38 chemisorption of SAMs,39 and deposition of reactive polymer layers,40 among others. Among different approaches for surface preparation, the use of SAMs represents one of the most popular strategies to introduce predesigned reactive functional groups onto the most frequently used working substrates, Au and SiO2 /Si. In the case of gold substrates, thiol chemistry has been extensively used for attaching a broad variety compounds with the thiol (–SH) terminal groups to gold substrates.41 On the other hand, the use of silane chemistry is the preferred strategy to modify not only SiO2 /Si surfaces but also oxide surfaces.42 The introduction of thiol groups in the polymer structure permits the facile modification of gold substrates via a “grafting-to” approach. The strategy has been used to prepare polystyrene (PS),43,44 polyethylene oxide,45,46 poly(Nisopropylacrylamide),47,48 poly[(2-dimethylamino)ethylmethacrylate],49 and xyloglucan50 brushes by simple chemisorption of thiol-terminated polymer chains on the gold substrate. Moreover, an alternative and complementary approach was also envisaged making use of the reduction of the dithioester end group of a polymer synthesized via reversible addition–fragmentation chain transfer (RAFT) polymerization. This strategy has been employed to generate mixed brush layers on gold nanoparticles by reducing gold precursors in the presence of RAFT-synthesized PS and poly(N-isopropyl acrylamide) (PNIPAM) polymers.48 The “grafting-to” modification of SiO2 /Si substrates using silane chemistry can be accomplished through different strategies. Rafailovich and co-workers26 resorted to Si(OH)3 -terminated PS for the preparation of polymer brushes via a “grafting-to” method. End-functionalized polymers were spun onto Si substrates, and samples were then vacuum annealed at 170◦ C for 3 days in order to ensure enough mobility for the chain ends to reach the grafting surface, where they were chemically reacted via silane chemistry to form 5-nm-thick PS brushes. Different grafting densities were obtained by dipping the samples in toluene for different times subsequent to film deposition. As a result, the grafting density decreased upon increasing the duration of the immersion. In a similar vein, the grafting of tri-ethoxysilane-terminated PS onto silicon substrates from a melt was thoroughly investigated by Jones and co-workers.20 These authors observed that the initial film thickness of the spun-cast layer and the polymer molecular weight have a strong influence on the properties of the final grafted layer. In the case of PS brushes grafted via very reactive trichlorosilane end groups, Karim et al.21 have shown that the morphology of

1.3 Preparation of Polymer Brushes by the “Grafting-To” Method

the grafted layer was dependent on the grafting time. Short reaction times gave rise to a grafted layer exhibiting an inhomogeneous island-like structure. Upon increasing the reaction time, the island-like structures increased in size and for sufficiently long grafting times homogeneous films were obtained. According to these authors, the random deposition of the reactive polymer chains on the substrate was the main mechanism responsible for the morphological evolution of the film. Another particularly interesting approach to tether polymer layers using a “grafting-to” method is to take advantage of the chemical reactivity of epoxy groups. The high reactivity of this functional group is due to the high tension in the three-membered epoxide ring as well as to the polarity caused by the oxygen atom. Through this strategy, different authors reported the preparation of grafted polymer layers by reacting epoxy groups with (i) carboxyl groups,51–58 (ii) amino groups,37,59–63 (iii) thiols groups,64 and (iv) maleic anhydride.65,66 The formation of covalent bonds between surface-confined moieties and functional groups in the polymer chains has also been extended to the exploration of quaternization reactions as synthetic routes. In this regard, alkylation of polymers bearing pyridine rings in the presence of surfaces exposing halogen atoms has been investigated by Minko and co-workers as a method to prepare stable polymer layers.67,68 In some cases, substrates are preconditioned by simple adsorption of an anchoring polymer layer (“primer”) onto the working surface. As an example, Nnebe and Schneider reported the use of physisorbed poly(ethyleneimine) as a strategy to functionalize silica surfaces with amino groups which would then be employed to graft succinimidylpropionic acid-derivatized polymers.69 However, when using physisorbed “primers,” the stability of the grafted layer could be compromised leading to film desorption in the presence of different environmental variables such as solvent, ionic strength, or temperature. The stability of the priming layer can be enhanced by resorting to chemisorption instead of physisorption. In this configuration, after the chemisorption process, the priming layer exposes reactive segments located in the “loop” and “tail” domains of the anchored polymer chains that are not linked to the substrate.70 One typical example of chemisorbed polymeric primers is the case of polymers bearing epoxy groups. An interesting investigation carried out by K¨othe et al.40 revealed that surface hydroxyl groups exposed on oxide surfaces are reactive enough to form chemical bonds with adsorbed epoxidized polybutadiene layers. This experimental observation permitted further substrate modification by diisocyanate- and amino-terminated polymers. With regard to this subject, special attention has been given to the use of poly(glycidyl methacrylate) (PGMA), where an epoxy group is present in each monomer unit, as a macromolecular primer compatible with different “grafting-to” chemistries (Figure 1.3). Extensive work by Luzinov and his collaborators on this topic demonstrated that uniform and homogeneous

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1 Functionalization of Surfaces Using Polymer Brushes

Figure . Schematic depiction of the macromolecular anchoring layer constituted of PGMA.

epoxy-containing polymer layer can be deposited onto various surfaces by adsorption, spin coating, or dip coating.71–74 By way of example, Iyer et al.75 demonstrated that carboxylic acid- and anhydride group-terminated PS polymer chains can be grafted onto PGMAmodified silicon wafers yielding dense and homogeneous layers. PS films were dip-coated from toluene solution onto wafers modified with a PGMA anchoring layer. Samples were then annealed during 18 h in a vacuum oven preheated to 150◦ C to enable the end groups to react with the epoxy-modified substrate. The same group also demonstrated the effective tethering of polyethylene glycol (PEG) layers from a melt onto PGMA primers using a similar “grafting-to” approach.71,73 These authors observed that the maximum grafting amount was limited by the concentration and availability of epoxy groups in the PGMA layer chemisorbed to the substrate. As expected, the grafting density of the tethered PEG layers was dependent on the molecular weight of the polymer chains. The use of a macromolecular PGMA platform also provides an interesting avenue to create mixed polymer brushes consisting of two or more incompatible polymers grafted onto the same substrate. One of the key advantages of this approach relies on the thermal stability of epoxy groups that enables the sequential grafting of different end-functionalized polymer onto the same substrate. In this way, a number of research groups devised the construction of “responsive” surfaces exposing either hydrophobic/hydrophilic or anionic/cationic mixed brushes through the sequential grafting onto PGMA layers.76–79 Another interesting strategy to graft polymers on PGMA layers is by using the technique called “solvent-assisted grafting.”80 This technique enables the tethering of polymer layers at relatively low temperatures (20–40◦ C) and is based on the saturation of the polymer film to be grafted with solvent vapor. In this way, the solvent present in the deposited polymer layer not only acts as a plasticizer reducing the glass transition temperature but also decreases the polymer layer viscosity which, in turn, enhances the mobility of the polymer chains at the interface. As a result, due to the enhanced interfacial mobility the “solvent-assisted grafting” technique can yield, under mild conditions, grafting densities comparable to those obtained during melt grafting.

1.4 Polymer Brushes by the “Grafting-From” Method

. Polymer Brushes by the “Grafting-From” Method ..

Surface-Initiated Atom Transfer Radical Polymerization

Atom transfer radical polymerization (ATRP) represents one of the most popular techniques for the formation of polymer brushes through surface-initiated polymerization. One of the most appealing aspects of ATRP is its chemical versatility and robustness to grow a variety of monomers. The technique was developed in the mid-1990s,81–83 and since then it has been employed to grow different types of polyelectrolyte and polymer brushes. This polymerization technique relies on the reversible redox activation of a “dormant” alkyl halide terminated polymer chain end by a halogen transfer to a transition metal complex (Figure 1.4). This process leads to the homolytic rupture of the carbon– halogen bond, thus generating free radical species at the polymer chain end. This activation step involves an electron transfer from the transition metal complex to the halogen atom, which, in turn, leads to the oxidation of the transition metal complex. Concomitantly, upon increasing the concentration of the oxidized form of the catalyst, the equilibrium is displaced toward the formation of halogen-capped dormant species. Due to the complex interplay between the different species involved in the polymerization process, the rate and the extent of the ATRP reaction are highly dependent on different parameters, such as catalyst concentration, type of ligand, solvent, and initiator.84 One of the first attempts to grow polymer brushes via surface-initiated atom transfer radical polymerization (SI-ATRP) was reported by Huang et al. involving the grafting of poly(acrylamide) (PAM) brushes from a self-assembled benzyl chloride monolayer on silica gel using Cu(bpy)2 Cl to control the radical population.85 Later on, Fukuda and co-workers employed 2-(4-chlorosulfonylphenyl)ethyl silane SAMs deposited via the Langmuir–Blodgett technique to grow poly(methyl methacrylate) (PMMA) brushes.86 In order to control the

Figure . Scheme describing the preparation of poly(2-hydroxyethyl methacrylate)-blockpoly(dimethylaminoethyl methacrylate) (PHEMA-b-PDMAEMA) diblock copolymer brushes via SI-ATRP from 2-bromoisobutyrate-terminated SAMs chemisorbed on gold surfaces.

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polymerization, these authors added p-toluenesulfonyl chloride as a sacrificial initiator. In many cases, an alternative strategy to achieve a controlled polymerization is to add CuII —or the metal ion in the oxidized form—directly to the polymerization solution. Using this versatile strategy, Matyjaszewski and co-workers demonstrated the controlled living polymerization of PS brushes from silicon substrates bearing bromoisobutyrate moieties.87 The solvent also plays an important role in tuning the performance of SIATRP. Different authors reported a marked increase in the polymerization rate in the presence of polar solvent, being this effect more drastic in the case of aqueous solvents.88–90 By way of example, we can mention that Jones and Huck were able to grow 50-nm-thick PMMA brushes in a controlled manner within 4 h of reaction time using a water/methanol mixture as a polymerization medium.91 In a similar fashion, the combination of different catalyst and deactivator species in the presence of aqueous solvents can facilitate the tuning of the polymerization reaction. Bruening and co-workers reported the synthesis of 700-nm-thick poly(2-hydroxyethyl methacrylate) (PHEMA) brushes using a mixed halide CuI Cl/CuII Br2 /bpy catalyst into aqueous medium.92 The success of this strategy based on mixed halide systems relies on the higher free energy of dissociation of the C–Cl bond as compared to the C–Br bond. As a consequence, this difference between halide systems is translated into a displacement of the equilibrium between dormant and propagating radical species toward the formation of dormant species, thus increasing the control over the polymerization reaction.93 In order to reduce the amount of catalyst used in the ATRP reaction, Matyjaszewski and his collaborators have introduced an interesting variation to the traditional ATRP that allows not only to reduce the concentration of the copper catalyst to a few parts per million but also to increase the tolerance toward the presence of oxygen in the polymerization solution. This technique is known as “activators (re)generated by electron transfer” ATRP or Activators ReGenerated by Electron Transfer (ARGET) ATRP94,95 and involves the use of reducing agents such as ascorbic acid, or even metallic Cu(0), to reconstitute CuI from CuII in solution and activate the surface-initiated polymerization.96–101 .. Surface-Initiated Reversible-Addition Fragmentation Chain Transfer Polymerization RAFT polymerization is a controlled/living polymerization technique in which chain growth is initiated using a free radical initiator, for example, azobisisobutyronitrile (AIBN), and mediated by a chain transfer agent (CTA) constituted of dithioester, dithiocarbamate, or trithiocarbonate compounds. In this technique, radical transfer between growing chains is responsible for providing good control over the polymerization process. Concomitantly, the “capping”

1.4 Polymer Brushes by the “Grafting-From” Method

Figure . Scheme describing the preparation of polymer brushes through RAFT polymerization: (a) poly(methylmethacrylate) and (b) polystyrene.

of growing chains by the dithioester moiety confers good living characteristics to the polymerization reaction (Figure 1.5). In most cases, the generation of polymer brushes via RAFT polymerization involved the use of either surface-immobilized conventional free radical initiators or surface-immobilized RAFT agents. One of the first attempts to grow brushes using surface-initiated reversibleaddition fragmentation chain transfer polymerization (SI-RAFT) was reported by Baum and Brittain using silicon substrates modified with SAMs bearing azo initiator groups in the presence of a dithiobenzoate CTA.102 These studies revealed that small amounts of untethered radical initiator, for example, AIBN, dissolved in solution were necessary for surface-initiated polymerization to be accomplished. The authors hypothesized that the addition of initiator in solution was required to scavenge impurities that quickly terminate growing chains. It is worth noting that the presence of free initiator in solution also increases the amount of radicals in the systems, which are necessary to avoid early termination by CTA capping. However, the drawback of this approach relies on the fact that the surface reaction takes place in parallel with polymer growth in solution and consequently brush-modified samples must be extensively washed with a solvent before performing any characterization. One of the interesting features of this technique is that, even though the polymerization is rather slow, it is highly living. In this context, the same authors demonstrated that the reinitiation of the polymer chains permitted the growth of block copolymer brushes of PS-b-PDMA (polystyrene-block-poly(N,Ndimethylacrylamide)) and PDMA-b-PMMA (poly(N,N-dimethylacrylamide)block-poly(methyl methacrylate)), and the sequential reinitiation of chains with the same monomer multiple times. Thereafter, different groups employed this strategy to grow poly(chloromethylstyrene) (PCMS),103 poly(pentafluorostyrene),103 poly(sulfobetaine methacrylate),104 poly(sodium 4-styrenesulfonate) (PSS-(Na)),104 PMMA,105

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poly(poly(ethylene glycol) methyl ether methacrylate),105 and poly(2(dimethylamino) ethyl methacrylate) (PDMAEMA)105 brushes from azofunctionalized substrates. On the other hand, instead of performing SI-RAFT using free radical initiator-modified substrates, several research groups explored the synthesis of polymer brushes using surface-immobilized RAFT agents. In general, RAFT agents can be immobilized according to two synthetic approaches: the R-group and Z-group approaches (Figure 1.6). The R-group approach refers to a configuration in which the RAFT agent is tethered to the surface through the leaving and reinitiating R group. This approach has been successfully employed to grow polymer brushes from a variety of dithiobenzoateor trithiocarbonate-modified substrates, including silicon wafers,106–108 silica particles,109–112 CdSe,113 and gold114 nanoparticles, and multiwalled carbon nanotubes.115–117 The Z-group approach (Figure 1.6) refers to the anchoring of the RAFT agent via the stabilizing Z group and has been employed to prepare a variety of

Figure . (a) Scheme describing the SI-RAFT polymerization of poly(butyl acrylate) brushes from dithiobenzoate-modified silica surfaces via a R-group approach. (b) Scheme describing the SI-RAFT polymerization of poly(methyl acrylate) brushes from silica surfaces modified with trithiocarbonate derivatives via a Z-group approach.

1.4 Polymer Brushes by the “Grafting-From” Method

Figure . Scheme describing the synthesis of PS brushes nitroxide-mediated polymerization.

polymer brushes constituted of methacrylic, acrylic, styrenic, and acrylamidebased monomer units.118–124 ..

Surface-Initiated Nitroxide-Mediated Polymerization

Nitroxide-mediated polymerization constitutes a living polymerization technique based on the reversible capping of an active chain-end radical with a nitroxide leaving group. The first successful implementation of this technique to synthesize polymer brushes was reported by Husseman et al.125 These authors employed silicon wafers modified with bound alkoxyamine initiator molecules to grow PS brushes—ca. 100 nm in thickness after 16 h of polymerization. The strategy to grow the polymer brushes relies on heating the initiatorfunctionalized wafer to 120◦ C, in order to cleave off the alkoxyamine moiety with the subsequent release of an alkyl radical and the stable nitroxide radical, (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) (Figure 1.7). The propagation is controlled by the reversible “capping” of the growing chain by the TEMPO radical, thus conferring a “living character” to the polymerization (Figure 1.7). In most cases the use of surface-bound initiators alone is not sufficient to attain a controlled polymerization. The very small number of growing polymer chains, as compared to the monomer concentration, gives a very low overall concentration of free TEMPO, which in turn leads to inefficient capping of chain ends. One of the most common strategies to solve this problem is to add a “free” alkoxyamine initiator to the polymerization solution. However, this route can also lead to the formation of polymer in solution which must be removed from the brushes by extensive rinsing with solvent before further characterization. Since then, surface-initiated nitroxide-mediated polymerization was employed by different research groups for growing different polymer brush systems such as poly(3-vinylpyridine),126,127 poly(4-vinylpyridine),128 PSS(Na),128 and poly(4-(poly(ethylene glycol) methyl ether)styrene) brushes,129 from a variety of TEMPO-functionalized substrates. ..

Surface-Initiated Photoiniferter-Mediated Polymerization

Surface-initiated iniferter-mediated polymerization is a technique based on the use of a special type of initiators called “iniferters.” These iniferters are

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Figure . Scheme describing the preparation of polystyrene-block- poly(methyl methacrylate) (PS-b-PMMA) brushes through SI-PIMP from a benzyl-N,Ndiethyldithiocarbamate-derivatized SiO2 /Si substrate.

molecules displaying singular characteristics provided that they can simultaneously act as initiators, transfer agents, and terminators. In general, dithiocarbamate derivative molecules are capable of acting as photoiniferters as they can initiate upon exposure to light and act as transfer agents or terminators during polymerization. Upon exposure to UV light, the photoiniferter molecules undergo photolysis, yielding a carbon radical and a dithiocarbamate radical. While the carbon radical is reactive and can initiate polymerization by reacting with the monomers, the dithiocarbamate radical is stable and reacts weakly, also with the monomers.130 However, and more important, the dithiocarbamate radical can reversibly terminate the propagating chains, thereby imparting the “living” characteristics to photoiniferter-mediated photopolymerization (Figure 1.8). In other words, upon photolysis the carbon radical undergoes addition of monomers to initiate the chain propagation and concomitantly the dithiocarbamate radical act as a transfer agent inducing reversible termination of the growing polymer chains (Figure 1.8). This interesting strategy was first proposed by Otsu et al. in the early 1980s, demonstrating that photoinifertermediated polymerization of methyl methacrylate (MMA) exhibits living characteristics, that is the molecular weight of the PMMA chains increased linearly with monomer conversion.131,132 The same group then demonstrated that the living characteristics of this technique allowed for block copolymers of PS and PMMA to be synthesized, both in solution and on the surface of particles.133 Due to the photosensitive nature of the iniferter molecules, this polymerization technique is heavily reliant on the intensity of the irradiating light. As such, surface-initiated photoiniferter-mediated polymerization (SI-PIMP) can be spatially and temporally controlled by manipulating the location, intensity, and duration of UV irradiation.134,135

1.4 Polymer Brushes by the “Grafting-From” Method

Along these lines, Matsuda and his collaborators extensively explored the use of SI-PIMP to synthesize different polymer brush systems, including poly(acrylic acid), PNIPAM, PCMS, poly(poly(ethylene glycol) methacrylate), poly(sodium methacrylate) (PMAA-(Na)), and poly(methacrylic acid) (PMAA), from substrates modified with benzyl-N,N-diethyldithiocarbamatemoieties.136–140 Using a similar strategy, Hadziioannou and co-workers resorted to the use of a trimethoxysilane-appended benzyl-N,Ndiethyldithiocarbamate iniferter to derivatize silicon substrates and then grow PS brushes.141 It is important to mention that some studies on the growth of MMA brushes revealed a pseudo-living behavior due to irreversible termination reactions. This effect ultimately leads to the loss of surface free radicals upon increasing exposure (or polymerization) time.142 The nonlinear growth of the brush layer as a function of irradiation time was ascribed to bimolecular termination reactions, rather than chain transfer to monomer. To overcome this limitation, a strategy to increase the amount of deactivating species was proposed in order to attain a controlled radical polymerization behavior.143,144 This strategy was based on the addition of tetraethylthiuram disulfide to the polymerization solution with the aim of deactivating the generated dithiocarbamyl radicals. ..

Surface-Initiated Living Ring-Opening Polymerization

The application of surface-initiated ring-opening polymerization (SI-ROP) as a strategy to graft polymer layers on modified substrates was first proposed by Jordan and Ulman in the late 1990s.145 These authors exploited the capabilities of the living cationic ROP of 2-ethyl-2-oxazoline to produce linear poly(N-propionylethyleneimine) (PPEI) (Figure 1.9). The synthetic protocol to attain the polymer layers consisted of several steps. First, gold-coated substrates were modified with trifluoromethane sulfonate (triflate) moieties through the chemisorption of 11-hydroxyundecanethiol SAMs and their subsequent vapor-phase functionalization. Thereafter, reaction with 2-ethyl-2oxazoline in refluxing chloroform for 7 days resulted in the formation of PPEI brushes of about 9 nm in thickness. Termination of the polymerization process was accomplished by the addition of N,N-dioctylamine, which in turn also generated amphiphilic brushes. This surface-initiated polymerization reaction does not require the presence of catalyst to produce well-defined brush layers; however, the brush growth is extremely slow as compared to catalyzed ringopening polymerization (ROP). Within this framework, Hawker and co-workers have used aluminum alkoxide catalyzed ROP to synthesize poly(𝜀-caprolactone) (PCL) brushes grafted from gold surfaces (Figure 1.9).146 Di(ethylene glycol)-terminated SAMs were employed to expose OH groups for initiation, which resulted in good polymer growth reproducibility and long-term stability of the polymer layer. The

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Figure . Scheme describing the preparation of different polymer brushes by living ROP: (a) poly(N-propionylethyleneimine), (b) PCL, and (c) PLA.

synthesis of the PCL brush layer was accomplished through organometallic catalysis in the presence of diethylaluminum alkoxides. The catalyzed SI-ROP process conducted at room temperature led to the formation of PCL brushes up to 70 nm thick in a few hours. It was observed that, in order to attain a good outcome of the surface-initiated process, it was necessary to add a free initiator (benzyl alcohol) to the polymerization solution. In this way, the brush thickness was controlled by the initial alcohol: 𝜀-caprolactone ratio. The addition of free initiator in solution facilitates the exchange of the active site between bound and free polymer chains, thus promoting the establishment of adequate molecular weight control. However, this strategy leads to the generation of free polymer in solution that must be removed from the bound polymer brushes by rigorous solvent rinsing before proceeding to further work with the brush samples. Later on, Choi and Langer reported the formation of chiral poly(lactic acid) (PLA) brushes grafted from gold and silicon substrates by ROP of l-lactide using tin(II) octoate as a catalyst (Figure 1.9). Depending on the nature of the substrate, these authors employed platforms of different chemical nature to

1.4 Polymer Brushes by the “Grafting-From” Method

grow the PLA brushes. In the case of gold substrates, the chosen platform was an oligo(ethylene glycol) terminated SAM that led to PLA brushes up to 12 nm thick after 3 days of reaction at 40◦ C. According to these authors, the thermal instability of thiol SAMs on gold precluded the use of higher reaction temperatures which could have led to the optimization of the polymerization reaction. However, the use of thermally stable amine-terminated SAMs on Si/SiO2 surfaces enabled the use of much higher reaction temperatures, which ultimately permitted the formation of PLA brushes up to 70 nm thick after 3 days of reaction at 80◦ C without requiring free initiator in solution. On the other hand, Wieringa et al. reported the growth of poly(l-glutamate) brushes from silicon and glass substrates modified with amine groups.147 The proposed strategy involved the use of N-carboxy anhydrides of l-glutamates as monomers. These species are cyclized amino acids that undergo ROP in the presence of amine groups, thus leading to the formation of polyaminoacid brushes up to several tens of nanometers in thickness in only a few hours. The “living” nature of the polymerization technique was demonstrated by the reinitiation of the polymer chains, and the subsequent formation of diblock copolymer brushes. ..

Surface-Initiated Ring-Opening Metathesis Polymerization

Ring-opening metathesis polymerization (ROMP) is a variant of olefin metathesis chain-growth polymerization in which the driving force of the reaction is the relief of ring strain in cyclic olefins, for example, functionalized norbornenes. This polymerization technique takes place in the presence of metathesis catalysts to generate polymers from cyclic olefins. One of the first successful attempts to grow polymer brushes via surface-initiated ring-opening metathesis polymerization (SI-ROMP) was reported by Whitesides and coworkers in 2000.148 The synthetic route relied on the use surface-grafted ruthenium catalysts to grow brushes from norbornene-based monomers on silicon substrates (Figure 1.10). The active surface bearing the catalytic sites was prepared by exposing norbornene-terminated trichlorosilane SAMs to a solution of Grubbs-type ROMP catalyst. Then, exposure to solutions of norbornenebased monomers prompted the rapid brush growth under controlled conditions, forming polymer layers up to 90 nm in thickness within 30 min. Further exposure of these brushes to a solution of a second monomer led the formation of diblock copolymer brushes, as revealed by infrared spectroscopy and ellipsometry. A similar approach was also explored by Grubbs and co-workers to grow polymer brushes from silicon substrates using norbornene as a monomer.149 Moon and Swager150 used SI-ROMP to prepare poly(p-phenylene ethynylene) (PPE) brushes employing a norbornene-capped PPE macromonomer (Figure 1.10). The reaction conditions employed for the synthesis led to the formation

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Figure . Polymer brushes grown by ring-opening metathesis polymerization of norbornene-derived monomers.

of 10 nm-thick PPE brushes and subsequent reinitiation in the presence of a mixture of 5-(bicycloheptenyl)-triethoxysilane, and the PPE macromonomer gave rise to 16 nm-thick diblock copolymer brushes. ..

Surface-Initiated Anionic Polymerization

Anionic polymerization is a polymerization technique that involves the polymerization of vinyl monomers in the presence of strong electronegative groups, and it is carried out through a carbanion-active species.151,152 The initiation is triggered by species that undergo nucleophilic addition to the monomer. In most of the cases, the strength of the base used to initiate the polymerization depends on the monomer structure. The polymerization mechanism is based on the propagation of an ionic-active species, and consequently it is sensitive to the nature of the counterions in the reaction medium. The technique is very selective to the type of monomers that can be polymerized. This is due to the experimental fact that substituent groups should be able to stabilize the carbanions that are formed in the polymerization reaction. In general, monomers bearing substituents capable of stabilizing the carbanion through resonance or induction, for example, nitro, cyano, vinyl, phenyl, are compatible with anionic polymerization. In addition, the nature and purity of the solvent also play a critical role. In general, aprotic solvents can prevent transfer to solvent and termination. However, the presence of electrophilic impurities in the solvent can react with ionic sites and dramatically affect the polymerization. In this context, Schouten and co-workers153 explored the modification of silica surfaces with styrene groups and initiated the polymerization by activating the styrene units in the presence of tert-butyllithium. This strategy was extended to the formation of block copolymers of poly(styrene-block-isoprene)

1.4 Polymer Brushes by the “Grafting-From” Method

Figure . Scheme describing the preparation of different polymer brushes by living anionic polymerization: (a) polystyrene and (b) polystyrene-block-polyisoprene.

onto silica microparticles and glass slides (Figure 1.11). Several authors pointed out that a major limitation of this approach was the use of tert-butyllithium (t-BuLi) as an initiator for surface-initiated polymerization styrene in toluene.154 This has been ascribed to the fact that t-BuLi initiation is very slow in nonpolar solvents, yielding broad molecular weight distributions. Ulman and co-workers explored the use of SAMs exposing biphenyllithium groups to initiate the anionic polymerization of styrene on gold substrates (Figure 1.11).155 The bromobiphenyl groups were converted into initiating species by reaction with sec-butyllithium and subsequent addition of styrene led to the slow formation of uniform PS films reaching 18 nm in thickness after 3 days of polymerization. Then, Ingall et al.156 demonstrated that anionic initiation of polymerization is also feasible using a bromopropyl trichlorosilane coupling agent to form the initial monolayer, followed by lithiation with lithium di-tert-butylbiphenyl. Using this approach, these authors were able to synthesize poly(acrylonitrile) brushes of ∼245 nm in thickness. On the other hand, Advincula and his collaborators explored an interesting alternative route based on the use of SAMs bearing 1,1-diphenylethylene (DPE) terminal groups as initiating sites for growing polymer brushes via surface-initiated anionic polymerization.157–159 One of the most attractive advantages of using DPE is that it can react quantitatively with simple alkyllithiums to form a monoaddition product, a 1,1-diphenylalkyllithium initiating species.160 These species are very reactive and serve as initiators for polymerization not only of styrenes and dienes in organic solvents but

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1 Functionalization of Surfaces Using Polymer Brushes

methacrylates and vinylpyridines at low temperatures in polar solvents such as tetrahydrofuran.160 The same route has also been exploited by Quirk and co-workers to grow different polymer brush systems.161,162 The preparation of block copolymer brushes is also plausible through the use of DPE-terminated silane or thiol SAMs and the sequential addition of monomers. In a typical setting, the first reaction is allowed to reach completion and then the second monomer is added to the living chains, thus leading to the growth of the second block. This strategy has been employed by Advincula and co-workers to grow polystyrene-b-polyisoprene (PS-b-PI) and polybutadieneb-polystyrene) (PBd-b-PS) block copolymer brushes on Au and SiO2 /Si substrates.163

. Conclusions The purpose of this chapter is to bring the reader up to date with the most recent experimental developments in relation to the preparation of polymer brushes—the chapter was designed to give the reader the big picture. We presented a general description of the synthetic approaches, focusing on some of the most relevant examples of the different synthetic strategies. It was our intention to lead the researcher through the vast literature in such a way that he or she will be able to pursue particular investigation with suitable guidance. In this context, we believe that carefully chosen references serve to guide the reader through the extensive literature, which makes the field accessible to a wide and varied audience including scientists, students, postdoctoral fellows, engineers, and industrial researchers. We hope that graduate students will find the chapter useful in their research and understanding of polymer brushes and beyond. As summarized in Figure 1.12, a wide range of synthetic strategies have been employed for the preparation of polymer brushes on solid surfaces. Along the

Synthetic methods "Grafting-to" methods

"Grafting-from" methods

+

Surface-initiated polymer

Polymer melts Concentrated Polymer Solution Gradient Polymer Brushes Solvent-assisted grafting

Reactive surface group

Self-Assembled Monolayers (SAMs) Thiol Chemistry Silane Chemistry Polymers Physisorption Chemisorption

Polymerization type

Presynthesized polymer

Strategy



Atom Transfer Radical Polymerization (SI-ATRP) Reversible-Addition Fragmentation Chain Transfer (SI-RAFT) Nitroxide-Mediated Polymerization (SI-NMP) Photoiniferter-Mediated Polymerization (SI-PIMP) Ring-opening Polymerization (SI-ROP) Anionic Polymerization (SI-AP)

Figure . Synopsis of the main synthetic approaches for preparing polymer brushes.

References

different chapters of this book, we will see the importance of having at hand the most current techniques and procedures to tether polymer layers on different substrates. Polymer brushes offer an enormous infrastructure for a highly interdisciplinary integration of inorganic, organic, biological, and supramolecular systems on surfaces.

Acknowledgments The authors acknowledge financial support from ANPCyT (PICT 2010-2554, PICT-2013-0905), Fundaci´on Petruzza, Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas (CONICET) (PIP 0370), and the Austrian Institute of Technology GmbH (AIT–CONICET Partner Lab: “Exploratory Research for Advanced Technologies in Supramolecular Materials Science” – Exp. 4947/11, Res. No. 3911, 28-12-2011). J.M.G., M.L.C., W.A.M., and O.A. are CONICET fellows.

References  Leyden, D. E., Collins, W. T., Eds.; Chemically Modified Oxide Surfaces; Gordon and Breach: New York, 1990.  Alkire, R. C., Kolb, D. M., Lipkowski, J., Ross, P. N., Eds.; Chemically Modified Electrodes; Wiley-VCH: Weinheim, 2009.  Decher, G., Schlenoff, J. B., Eds.; Multilayer Thin Films: Sequential Assembly of Nanocomposite Materials; Wiley-VCH: Weinheim, 2012.  Blodgett, K. B. J. Am. Chem. Soc. 1935, 57 (6), 1007–1022.  Blodgett, K. B.; Langmuir, I. Phys. Rev. 1937, 51 (11), 964–982.  Levine, O.; Zisman, W. A. J. Phys. Chem. 1957, 61 (8), 1068–1077.  Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105 (13), 4481–4483.  Sagiv, J. J. Am. Chem. Soc. 1980, 102 (1), 92–98.  Azzaroni, O. J. Polym. Sci., Part A: Polym. Chem. 2012, 50 (16), 3225–3258.  Barbey, R.; Lavanant, L.; Paripovic, D.; Sch¨uwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Chem. Rev. 2009, 109 (11), 5437–5527.  Chen, T.; Ferris, R.; Zhang, J.; Ducker, R.; Zauscher, S. Prog. Polym. Sci. 2010, 35 (1), 94–112.  Luzinov, I.; Minko, S.; Tsukruk, V. V. Soft Matter 2008, 4 (4), 714–725. ˇ  Wu, T.; Efimenko, K.; Vlˇcek, P.; Subr, V.; Genzer, J. Macromolecules 2003, 36 (7), 2448–2453.  Alexander, S. J. J. Phys 1977, 38, 983.  de Gennes, P. Macromolecules 1980, 13 (5), 1069–1075.  Pincus, P. Macromolecules 1991, 24 (10), 2912–2919.  Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28 (5), 1491–1499.

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 Brittain, W. J.; Minko, S. J. Polym. Sci., Part A: Polym. Chem. 2007, 45 (16), 3505–3512.  Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. Rev. 2004, 33 (1), 14–22.  Jones, R. A. L.; Lehnert, R. J.; Schonherr, H.; Vancso, J. Polymer (Guildf ). 1999, 40 (2), 525–530.  Karim, A.; Tsukruk, V. V.; Douglas, J. F.; Satija, S. K.; Fetters, L. J.; Reneker, D. H.; Foster, M. D. J. Phys. II 1995, 5 (10), 1441–1456.  Iyer, K. S.; Luzinov, I. Macromolecules 2004, 37 (25), 9538–9545.  Zdyrko, B.; Hoy, O.; Kinnan, M. K.; Chumanov, G.; Luzinov, I. Soft Matter 2008, 4 (11), 2213–2219.  Taylor, W.; Jones, R. A. L. Langmuir 2010, 26 (17), 13954–13958.  Lee, H.-S.; Penn, L. S. Macromolecules 2008, 41 (21), 8124–8129.  Zhao, W.; Krausch, G.; Rafailovich, M. H.; Sokolov, J. Macromolecules 1994, 27 (11), 2933–2935.  Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, J.; Tsukruk, V. V. Macromolecules 2000, 33 (3), 1043–1048.  Lee, H.-S.; Penn, L. S. Macromolecules 2010, 43 (1), 565.  Michielsen, S.; Lee, H. J. Langmuir 2007, 23 (11), 6004–6010.  Tsyalkovsky, V.; Klep, V.; Ramaratnam, K.; Lupitskyy, R.; Minko, S.; Luzinov, I. Chem. Mater. 2008, 20 (1), 317–325.  Ramaratnam, K.; Tsyalkovsky, V.; Klep, V.; Luzinov, I. Chem. Commun. (Camb). 2007, (43), 4510–4512.  Motornov, M.; Sheparovych, R.; Lupitskyy, R.; MacWilliams, E.; Hoy, O.; Luzinov, I.; Minko, S. Adv. Funct. Mater. 2007, 17 (14), 2307–2314.  Tsyalkovsky, V.; Burtovyy, R.; Klep, V.; Lupitskyy, R.; Motornov, M.; Minko, S.; Luzinov, I. Langmuir 2010, 26 (13), 10684–10692.  Ionov, L.; Zdyrko, B.; Sidorenko, A.; Minko, S.; Klep, V.; Luzinov, I.; Stamm, M. Macromol. Rapid Commun. 2004, 25 (1), 360–365.  Zdyrko, B.; Luzinov, I. Polym. Mater. Sci. Eng. Div. 2003, 89, 293.  Zdyrko, B.; Klep, V.; Luzinov, I.; Sidorenko, A.; Ionov, L.; Minko, S.; Stamm, M. Polym. Prepr. 2003, 44 (1), 522.  Motornov, M.; Sheparovych, R.; Tokarev, I.; Roiter, Y.; Minko, S. Langmuir 2007, 23 (1), 13–19.  Minko, S.; M¨uller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125 (13), 3896–3900.  Luzinov, I.; Julthongpiput, D.; Liebmann-Vinson, A.; Cregger, T.; Foster, M. D.; Tsukruk, V. V. Langmuir 2000, 16 (2), 504–516.  K¨othe, M.; M¨uller, M.; Simon, F.; Komber, H.; Jacobasch, H.-J.; Adler, H.-J. Colloids Surf., A. 1999, 154 (1), 75–85.  Vericat, C.; Vela, M. E.; Benitez, G.; Carro, P.; Salvarezza, R. C. Chem. Soc. Rev. 2010, 39 (5), 1805–1834.

References

 Onclin, S.; Ravoo, B. J.; Reinhoudt, D. N. Angew. Chem., Int. Ed. 2005, 44 (39), 6282–6304.  Corbierre, M. K.; Cameron, N. S.; Lennox, R. B. Langmuir 2004, 20 (7), 2867–2873.  Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Laaziri, K.; Lennox, R. B. Langmuir 2005, 21 (13), 6063–6072.  Cha, S.-H.; Kim, J.-U.; Lee, J.-C. Macromol. Res. 2008, 16 (8), 711–716.  Himmelhaus, M.; Bastuck, T.; Tokumitsu, S.; Grunze, M.; Livadaru, L.; Kreuzer, H. J. EPL (Europhysics Lett). 2003, 64 (3), 378.  Liu, G.; Cheng, H.; Yan, L.; Zhang, G. J. Phys. Chem. B 2005, 109 (47), 22603–22607.  Shan, J.; Nuopponen, M.; Jiang, H.; Viitala, T.; Kauppinen, E.; Kontturi, K.; Tenhu, H. Macromolecules 2005, 38 (7), 2918–2926.  Liu, G.; Yan, L.; Chen, X.; Zhang, G. Polymer (Guildf ). 2006, 47 (9), 3157–3163.  Nordgren, N.; Ekl¨of, J.; Zhou, Q.; Brumer Iii, H.; Rutland, M. W. Biomacromolecules 2008, 9 (3), 942–948.  Hoffmann, F.; Wolff, T.; Minko, S.; Stamm, M. J. Colloid Interface Sci. 2005, 282 (2), 349–358.  Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. Langmuir 2003, 19 (19), 7832–7836.  Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125 (51), 15912–15921.  LeMieux, M. C.; Julthongpiput, D.; Bergman, K. N.; Cuong, P. D.; Ahn, H.-S.; Lin, Y.-H.; Tsukruk, V. V. Langmuir 2004, 20 (23), 10046–10054.  Minko, S.; Patil, S.; Datsyuk, V.; Simon, F.; Eichhorn, K.-J.; Motornov, M.; Usov, D.; Tokarev, I.; Stamm, M. Langmuir 2002, 18 (1), 289–296.  Luzinov, I.; Tsukruk, V. V. Macromolecules 2002, 35 (15), 5963–5973.  Vyas, M. K.; Nandan, B.; Schneider, K.; Stamm, M. Eur. Polym. J. 2009, 45 (5), 1367–1376.  Vyas, M. K.; Schneider, K.; Nandan, B.; Stamm, M. Soft Matter 2008, 4 (5), 1024–1032.  Huang, H.; Penn, L. S. Macromolecules 2005, 38 (11), 4837–4843.  Huang, H.; Cammers, A.; Penn, L. S. Macromolecules 2006, 39 (20), 7064–7070.  Piehler, J.; Brecht, A.; Valiokas, R.; Liedberg, B.; Gauglitz, G. Biosens. Bioelectron. 2000, 15 (9), 473–481.  Motornov, M.; Sheparovych, R.; Katz, E.; Minko, S. ACS Nano 2008, 2, 41.  Penn, L. S.; Hunter, T. F.; Quirk, R. P.; Lee, Y. Macromolecules 2002, 35 (7), 2859–2860.  De Vos, K.; Girones, J.; Popelka, S.; Schacht, E.; Baets, R.; Bienstman, P. Biosens. Bioelectron. 2009, 24 (8), 2528–2533.

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1 Functionalization of Surfaces Using Polymer Brushes

 Luzinov, I.; Julthongpiput, D.; Tsukruk, V. V. Polymer (Guildf ). 2001, 42 (5), 2267–2273.  Luzinov, I.; Julthongpiput, D.; Tsukruk, V. V. Macromolecules 2000, 33 (20), 7629–7638.  Tam, T. K.; Ornatska, M.; Pita, M.; Minko, S.; Katz, E. J. Phys. Chem. C 2008, 112 (22), 8438–8445.  Lupitskyy, R.; Motornov, M.; Minko, S. Langmuir 2008, 24 (16), 8976– 8980.  Nnebe, I. M.; Schneider, J. W. Macromolecules 2006, 39 (10), 3616–3621.  Fleer, G. J., Cohen Stuart, M. A., Scheutjens, J. M. H., Cosgrove, T., Vincent, B., Eds.; Polymers at Interfaces; Chapman & Hall: New York, 1993.  Zdyrko, B.; Klep, V.; Luzinov, I. Langmuir 2003, 19 (24), 10179–10187.  Liu, Y.; Klep, V.; Zdyrko, B.; Luzinov, I. Langmuir 2004, 20 (16), 6710–6718.  Zdyrko, B.; Varshney, S. K.; Luzinov, I. Langmuir 2004, 20 (16), 6727–6735.  Ionov, L.; Sidorenko, A.; Stamm, M.; Minko, S.; Zdyrko, B.; Klep, V.; Luzinov, I. Macromolecules 2004, 37 (19), 7421–7423.  Iyer, K. S.; Zdyrko, B.; Malz, H.; Pionteck, J.; Luzinov, I. Macromolecules 2003, 36 (17), 6519–6526.  Draper, J.; Luzinov, I.; Minko, S.; Tokarev, I.; Stamm, M. Langmuir 2004, 20 (10), 4064–4075.  Ionov, L.; Sidorenko, A.; Eichhorn, K.-J.; Stamm, M.; Minko, S.; Hinrichs, K. Langmuir 2005, 21 (19), 8711–8716.  Ionov, L.; Stamm, M.; Minko, S.; Hoffmann, F.; Wolff, T. Macromol. Symp. 2004, 210, 229–235.  Ionov, L.; Houbenov, N.; Sidorenko, A.; Stamm, M.; Luzinov, I.; Minko, S. Langmuir 2004, 20 (23), 9916–9919.  Zdyrko, B.; Luzinov, J. Polym. Prep. 2007, 48, 773.  Wang, J.-S.; Matyjaszewski, K. J. Am. Chem. Soc. 1995, 117 (20), 5614–5615.  Kato, M.; Kamigaito, M.; Sawamoto, M.; Higashimura, T. Macromolecules 1995, 28 (5), 1721–1723.  Percec, V.; Barboiu, B. Macromolecules 1995, 28 (23), 7970–7972.  Patten, T. E.; Matyjaszewski, K. Acc. Chem. Res. 1999, 32 (10), 895–903.  Huang, X.; Wirth, M. J. Anal. Chem. 1997, 69 (22), 4577–4580.  Ejaz, M.; Yamamoto, S.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 1998, 31 (17), 5934–5936.  Matyjaszewski, K.; Miller, P. J.; Shukla, N.; Immaraporn, B.; Gelman, A.; Luokala, B. B.; Siclovan, T. M.; Kickelbick, G.; Vallant, T.; Hoffmann, H. Macromolecules 1999, 32 (26), 8716–8724.  Nanda, A. K.; Matyjaszewski, K. Macromolecules 2003, 36 (3), 599–604.  Wang, X. S.; Lascelles, S. F.; Jackson, R. A.; Armes, S. P. Chem. Commun. 1999, 130, 1817.  Wang, X.-S.; Armes, S. P. Macromolecules 2000, 33 (18), 6640–6647.  Jones, D. M.; Huck, W. T. S. Adv. Mater. 2001, 13 (16), 1256–1259.

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 Huang, W.; Kim, J.-B.; Bruening, M. L.; Baker, G. L. Macromolecules 2002, 35 (4), 1175–1179.  Matyjaszewski, K.; Shipp, D. A.; Wang, J.-L.; Grimaud, T.; Patten, T. E. Macromolecules 1998, 31 (20), 6836–6840.  Min, K.; Gao, H.; Matyjaszewski, K. J. Am. Chem. Soc. 2005, 127 (11), 3825–3830.  Jakubowski, W.; Matyjaszewski, K. Angew. Chem., Int. Ed. 2006, 45 (27), 4482–4486.  Zhao, H.; Kang, X.; Liu, L. Macromolecules 2005, 38 (26), 10619–10622.  Bombalski, L.; Min, K.; Dong, H.; Tang, C.; Matyjaszewski, K. Macromolecules 2007, 40 (21), 7429–7432.  Esteves, A. C. C.; Bombalski, L.; Trindade, T.; Matyjaszewski, K.; Barros-Timmons, A. Small 2007, 3 (7), 1230–1236.  He, J.; Wu, Y.; Wu, J.; Mao, X.; Fu, L.; Qian, T.; Fang, J.; Xiong, C.; Xie, J.; Ma, H. Macromolecules 2007, 40 (9), 3090–3096.  Matyjaszewski, K.; Dong, H.; Jakubowski, W.; Pietrasik, J.; Kusumo, A. Langmuir 2007, 23 (8), 4528–4531.  Wischerhoff, E.; Uhlig, K.; Lankenau, A.; B¨orner, H. G.; Laschewsky, A.; Duschl, C.; Lutz, J. Angew. Chem., Int. Ed. 2008, 47 (30), 5666–5668.  Baum, M.; Brittain, W. J. Macromolecules 2002, 35 (3), 610–615.  Yu, W. H.; Kang, E. T.; Neoh, K. G. Ind. Eng. Chem. Res. 2004, 43 (17), 5194–5202.  Zhai, G.; Yu, W. H.; Kang, E. T.; Neoh, K. G.; Huang, C. C.; Liaw, D. J. Ind. Eng. Chem. Res. 2004, 43 (7), 1673–1680.  Chen, Y.; Sun, W.; Deng, Q.; Chen, L. J. Polym. Sci., Part A: Polym. Chem. 2006, 44 (9), 3071–3082.  Rowe-Konopacki, M. D.; Boyes, S. G. Macromolecules 2007, 40 (4), 879–888. ¨ Shi, X.-N. Mater. Lett. 2007, 61 (10),  Yuan, K.; Li, Z.-F.; Ling-Ling, L. U.; 2033–2036.  Li, D.; Luo, Y.; Li, B.; Zhu, S. J. Polym. Sci., Part A: Polym. Chem. 2008, 46 (3), 970–978.  Ranjan, R.; Brittain, W. J. Macromol. Rapid Commun. 2007, 28 (21), 2084–2089.  Lu, C.-H.; Zhou, W.-H.; Han, B.; Yang, H.-H.; Chen, X.; Wang, X.-R. Anal. Chem. 2007, 79 (14), 5457–5461.  Hong, C.-Y.; Li, X.; Pan, C.-Y. Eur. Polym. J. 2007, 43 (10), 4114–4122.  Tsujii, Y.; Ejaz, M.; Sato, K.; Goto, A.; Fukuda, T. Macromolecules 2001, 34 (26), 8872–8878.  Skaff, H.; Emrick, T. Angew. Chem., Int. Ed. 2004, 43 (40), 5383–5386.  Raula, J.; Shan, J.; Nuopponen, M.; Niskanen, A.; Jiang, H.; Kauppinen, E. I.; Tenhu, H. Langmuir 2003, 19 (8), 3499–3504.  Xu, G.; Wu, W.-T.; Wang, Y.; Pang, W.; Zhu, Q.; Wang, P. Nanotechnology 2007, 18 (14), 145606.

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1 Functionalization of Surfaces Using Polymer Brushes

 Hong, C.; You, Y.; Pan, C. J. Polym. Sci., Part A: Polym. Chem. 2006, 44 (8), 2419–2427.  Xu, G.; Wu, W.-T.; Wang, Y.; Pang, W.; Zhu, Q.; Wang, P.; You, Y. Polymer (Guildf ). 2006, 47 (16), 5909–5918.  Peng, Q.; Lai, D. M. Y.; Kang, E. T.; Neoh, K. G. Macromolecules 2006, 39 (16), 5577–5582.  Zhao, Y.; Perrier, S. Macromolecules 2006, 39 (25), 8603–8608.  Wang, G.-J.; Huang, S.-Z.; Wang, Y.; Liu, L.; Qiu, J.; Li, Y. Polymer (Guildf ). 2007, 48 (3), 728–733.  Zhao, Y.; Perrier, S. Macromolecules 2007, 40 (25), 9116–9124.  Stenzel, M. H.; Zhang, L.; Huck, W. T. S. Macromol. Rapid Commun. 2006, 27 (14), 1121–1126.  Takolpuckdee, P.; Mars, C. A.; Perrier, S. Org. Lett. 2005, 7 (16), 3449–3452.  Perrier, S.; Takolpuckdee, P.; Mars, C. A. Macromolecules 2005, 38 (16), 6770–6774.  Husseman, M.; Malmstr¨om, E. E.; McNamara, M.; Mate, M.; Mecerreyes, D.; Benoit, D. G.; Hedrick, J. L.; Mansky, P.; Huang, E.; Russell, T. P. Macromolecules 1999, 32 (5), 1424–1431.  Matsuno, R.; Yamamoto, K.; Otsuka, H.; Takahara, A. Macromolecules 2004, 37 (6), 2203–2209.  Kobayashi, M.; Matsuno, R.; Otsuka, H.; Takahara, A. Sci. Technol. Adv. Mater. 2006, 7 (7), 617–628.  Zhao, X.; Lin, W.; Song, N.; Chen, X.; Fan, X.; Zhou, Q. J. Mater. Chem. 2006, 16 (47), 4619–4625.  Andruzzi, L.; Senaratne, W.; Hexemer, A.; Sheets, E. D.; Ilic, B.; Kramer, E. J.; Baird, B.; Ober, C. K. Langmuir 2005, 21 (6), 2495–2504.  Lambrinos, P.; Tardi, M.; Polton, A.; Sigwalt, P. Eur. Polym. J. 1990, 26 (10), 1125–1135.  Otsu, T.; Yoshida, M. Macromol. Rapid Commun. 1982, 3 (2), 127–132.  Otsu, T.; Yoshida, M.; Tazaki, T. Macromol. Rapid Commun. 1982, 3 (2), 133–140.  Otsu, T.; Ogawa, T.; Yamamoto, T. Macromolecules 1986, 19 (7), 2087–2089.  Nakayama, Y.; Matsuda, T. Macromolecules 1996, 29 (27), 8622–8630.  Higashi, J.; Nakayama, Y.; Marchant, R. E.; Matsuda, T. Langmuir 1999, 15 (6), 2080–2088.  Lee, H. J.; Nakayama, Y.; Matsuda, T. Macromolecules 1999, 32 (21), 6989–6995.  Kidoaki, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17 (4), 1080–1087.  Kidoaki, S.; Ohya, S.; Nakayama, Y.; Matsuda, T. Langmuir 2001, 17 (8), 2402–2407.  Matsuda, T.; Kaneko, M.; Ge, S. Biomaterials 2003, 24 (24), 4507–4515.  Matsuda, T.; Ohya, S. Langmuir 2005, 21 (21), 9660–9665.

References

 De Boer, B.; Simon, H. K.; Werts, M. P. L.; Van der Vegte, E. W.; Hadziioannou, G. Macromolecules 2000, 33 (2), 349–356.  Rahane, S. B.; Kilbey, S. M.; Metters, A. T. Macromolecules 2005, 38 (20), 8202–8210.  Luo, N.; Hutchison, J. B.; Anseth, K. S.; Bowman, C. N. Macromolecules 2002, 35 (7), 2487–2493.  Rahane, S. B.; Metters, A. T.; Kilbey, S. M., II. Macromolecules 2006, 39 (26), 8987–8991.  Jordan, R.; Ulman, A. J. Am. Chem. Soc. 1998, 120 (2), 243–247.  Husemann, M.; Mecerreyes, D.; Hawker, C. J.; Hedrick, J. L.; Shah, R.; Abbott, N. L. Angew. Chem., Int. Ed. 1999, 38 (5), 647–649.  Wieringa, R. H.; Siesling, E. A.; Geurts, P. F. M.; Werkman, P. J.; Vorenkamp, E. J.; Erb, V.; Stamm, M.; Schouten, A. J. Langmuir 2001, 17 (21), 6477–6484.  Kim, N. Y.; Jeon, N. L.; Choi, I. S.; Takami, S.; Harada, Y.; Finnie, K. R.; Girolami, G. S.; Nuzzo, R. G.; Whitesides, G. M.; Laibinis, P. E. Macromolecules 2000, 33 (8), 2793–2795.  Juang, A.; Scherman, O. A.; Grubbs, R. H.; Lewis, N. S. Langmuir 2001, 17 (5), 1321–1323.  Moon, J. H.; Swager, T. M. Macromolecules 2002, 35 (16), 6086–6089.  Hsieh, H.; Quirk, R. P. Anionic Polymerization: Principles and Practical Applications; CRC Press: Boca Raton, FL, 1996.  Quirk, R. P. In Encyclopedia of Polymer Science and Technology; Kroschwitz, J. I., Mark, H. F., Herman F., Eds.; Wiley-Interscience, 2003; Vol 5. pp. 111–163.  Oosterling, M. L. C. M.; Sein, A.; Schouten, A. J. Polymer (Guildf ). 1992, 33 (20), 4394–4400.  Advincula, R. In Surface-Initiated Polymerization I; Springer, 2006; pp 107–136.  Jordan, R.; Ulman, A.; Kang, J. F.; Rafailovich, M. H.; Sokolov, J. J. Am. Chem. Soc. 1999, 121 (5), 1016–1022.  Ingall, M. D. K.; Honeyman, C. H.; Mercure, J. V; Bianconi, P. A.; Kunz, R. R. J. Am. Chem. Soc. 1999, 121 (15), 3607–3613.  Zhou, Q.; Nakamura, Y.; Inaoka, S.; Park, M. K.; Wang, Y.; Mays, J.; Advincula, R. Polym. Mater. Sci. Eng. 2000, 82, 290.  Advincula, R.; Zhou, Q.; Mays, J. Polym. Mater. Sci. Eng. Div. 2001, 84, 875.  Zhou, Q.; Fan, X.; Xia, C.; Mays, J.; Advincula, R. Polym. Mater. Sci. Eng. Div. 2001, 84, 835.  Quirk, R. P.; Yoo, T.; Lee, Y.; Kim, J.; Lee, B. Adv. Polym. Sci. 2000, 153, 67.  Quirk, R. P.; Mathers, R. T. Polym. Mater. Sci. Eng. Div. 2001, 84, 873.  Quirk, R. P.; Mathers, R. T. Polym. Mater. Sci. Eng. Div. 2001, 85, 198.  Advincula, R.; Zhou, Q.; Park, M.; Wang, S.; Mays, J.; Sakellariou, G.; Pispas, S.; Hadjichristidis, N. Langmuir 2002, 18 (22), 8672–8684.

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 Polymer Brushes by Atom Transfer Radical Polymerization Guojun Xie,1 Amir Khabibullin,1 Joanna Pietrasik,2 Jiajun Yan,1 and Krzysztof Matyjaszewski1 1 2

Department of Chemistry, Carnegie Mellon University, Pittsburgh, PA, USA Institute of Polymer and Dye Technology, Lodz University of Technology, Lodz, Poland

. Structure of Brushes Polymer brushes are densely grafted polymer chains with one end attached to the substrate that can be either a polymer, a surface or an interface.1–3 Molecular bottlebrushes (cylindrical macromolecular brushes, molecular brushes, bottlebrush copolymers) are a class of graft copolymers with polymeric branches densely grafted from a polymeric backbone. In addition to linear polymers, polymer networks can also serve as backbones. Due to the steric overcrowding caused by the side chains, bottlebrushes are forced to stretch and adopt an extended, cylindrical conformation (Scheme 2.1).1 The rigid nature of molecular bottlebrushes is the primary contributor to their unusual physical properties, which are often dramatically different from their linear counterparts with similar overall composition. The high molecular weight and extended conformations of molecular brushes facilitates their visualization as individual molecules on surfaces by atom force microscopy (AFM).4 This unique property allows more extended confirmation of complex architectures5 and calculation of size, molecular weight, and molecular weight distribution.6 Polymer brushes are also referred to an array of polymer chains tethered by one end to a surface or an interface.2,3 The conformations of polymer brushes are strongly affected by the distance between the tethered chains.2,7 If polymer chains are tethered to a flat surface, two regimes are distinguished, depending on the respective conformation of the grafted chains (Scheme 2.2): (1) in the sparse grafting regime with D > 2RG , chains are assumed to adapt a relaxed conformation (also called “mushroom” structure); (2) in the dense grafting regime with D < 2RG , excluded volume interactions give rise to stretched chain Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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2 Polymer Brushes by Atom Transfer Radical Polymerization

Scheme . Schematic illustration of the extended cylindrical conformation of molecular bottlebrushes. Source: Matyjaszewski and Tsarevsky 2014.73 Reproduced with permission of American Chemical Society.

conformations (“brushes”). Here, D denotes the distance between tethered chains and RG is the chains’ radius of gyration.3 For nanostructured surfaces with topographical features comparable to the polymer chain dimension, the chain conformation is also controlled by the curvature of the surface that affects local polymer segment density. The relation between surface curvature and chain conformation is illustrated in Scheme

convex

nano D

RG

D > RG macro/sparse

concave D < RG macro/dense

Scheme . Illustration of the effect of surface curvature on conformation of grafted chains. Source: Hui et al. 2014.3 Reproduced with permission of American Chemical Society.

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2.2 Synthesis of Polymer Brushes

Figure . Three main strategies for preparing molecular bottlebrushes: grafting through, grafting to, and grafting from.

2.2. Chain crowding decreases on convex surfaces but increases on concave surfaces.3

. Synthesis of Polymer Brushes Synthetic strategies for molecular bottlebrushes can be divided into three categories (Figure 2.1): “grafting through” (polymerization of macromonomers), “grafting to” (attachment of side chains to the backbone), and “grafting from” (grafting the side chains from the backbone).1 ..

Grafting through

Due to the compatibility toward a wide range of monomers and reaction conditions, conventional free radical polymerization of macromonomers was extensively reported to afford grafted copolymers.8–10 In addition, core–shell and heterografted structures were created using block and doubly substituted macromonomers, respectively.11,12 However, the poor control and chain end functionality precludes the well-defined structure of afforded brushes. Hence, various methods of controlled/living polymerization were introduced to solve this problem. Anionic polymerization of macromonomers including polyisoprene (PI), polybutadiene (PB), and polystyrene (PS) with terminal styrenic and methacrylic functionalities was reported.13–16 In some cases, significant steric hindrance and insufficiently pure macromonomers compromised the realization of high molecular weight and control over molecular weight distribution. Due to the larger spacing between side chains and ring strain as the driving force, ring-opening metathesis polymerization (ROMP) of





2 Polymer Brushes by Atom Transfer Radical Polymerization

norbornenyl macromonomers became an attractive alternative synthetic method for polymerization of unsaturated macromonomers.17 Versatility of this method was demonstrated by the synthesis of brushes with PS,18 poly(ethylene oxide) (PEO),19 poly(𝜀-caprolactone) (PCL),20 polylactide (PLA), and poly(n-butyl acrylate) (PnBA).21 Reversible-deactivation radical polymerization (RDRP) techniques such as atom transfer radical polymerization (ATRP) were also reported for preparation polymer brushes via the “grafting-through” method.22,23 Copolymerization of macromonomers was described to afford heterografted molecular brushes that were expected to show interesting solution, bulk, and surface properties.24,25 The grafting density of molecular bottlebrushes afforded by the “graftingthrough” method is 100%. However, limited by the inherently low concentration of polymerizable groups and the steric hindrance of side chains, it is challenging to synthesize molecular brushes with a high degree of polymerization (DP) and low dispersity. Also, unreacted macromonomers are difficult to be removed by regular purification techniques such as dialysis and distillation. ..

Grafting to

The attractive feature of the “grafting-to” approach is that both the backbone and side chains are prepared and characterized independently.1 However, only the most efficient coupling reactions such as nucleophilic substitutions and click-type coupling reactions can be used, otherwise grafting density is limited. In “grafting-to” methods, involving nucleophilic substitution, well-defined side chains were usually prepared by living anionic polymerization and then reacted with a backbone units containing a functional group such as benzylic halides that are susceptible to nucleophilic attack.26,27 The highly efficient copper(I)-catalyzed azide–alkyne coupling (CuAAC) reaction is commonly known as a form of “click chemistry” suitable for postpolymerization modification.28,29 Synthesis of molecular brushes with grafting density ∼97% was reported via the “click” reaction between azido-terminated polymer side chains and poly(2-hydroxyethyl methacrylate) modified with 4pentynoic acid.30,31 In the “grafting-to” method, both backbones and side chains are prepared separately. This strategy involves the reaction of chain-end functional groups of side chain polymers with complimentary functional groups on the backbone precursors. However, grafting density is limited by slow diffusion of the bulky side chains. Similar to the “grafting-through” method, the removal of unreacted polymer side chains is also challenging. ..

Grafting from

In the “grafting-from” method, a polymer backbone (macroinitiator) with a predetermined number of initiation sites is prepared and side chains are

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2.3 ATRP Fundamentals Visible Light monomer catalyst

Visible Light monomer catalyst

Figure . Patterning of polymer brushes from uniformly functionalized substrates using (a) a photomask for patterns or (b) a neutral density filter for gradient structures. Source: Braunecker et al. 2007.48 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA.

grafted from it. RDRP including ATRP,32 stable free radial polymerization,33 and reversible addition-fragmentation chain transfer (RAFT) polymerization34 are suitable for synthesis of molecular brushes via the “grafting-from” method. Similar to conventional radical polymerization, excellent functionalgroup-tolerance of RDRP methods allows large flexibility in the choice of monomers, whereas low radical concentration suppresses the inter- and intramolecular radical termination resulting in macroscopic gelation and pendant macrocycles, respectively.35 Synthesis of molecular brushes via RDRP was reported using acrylates,4 methacrylate,22,36 acrylamide,37 acrylonitrile,38 vinyl acetate,39 styrene.40 Furthermore, precise control of the polymerization process allows the fabrication of molecular brushes with more complex structures including branched,5 multilayered (i.e., core–shell),41 and gradient42 structures. In the “grafting-from” method, although side chains are not perfectly identical, high grafting density and long backbones were attained. However, initiation efficiency requires optimization of reaction conditions in order to assure high grafting density. Similar to molecular bottlebrushes, polymer brushes tethered to surfaces can be synthesized via either “grafting-from” or “grafting-to” method.43,44 In addition to the two methods, polymer-grafted nanoparticles were also prepared from star polymer template and replacement of small molecule stabilizers with chain-end functionalized polymeric ligands.45,46 Additionally, introduction of templates allows the fabrication of two-dimensional (2D) and threedimensional (3D) patterns during surface modification (Figure 2.2).47

. ATRP Fundamentals ATRP is a versatile method for the preparation of well-defined polymers.48 It is one of the most robust and widely empolyed RDRP techniques for





2 Polymer Brushes by Atom Transfer Radical Polymerization

polymerization of a broad range of commercially available functional monomers.49–51 While conventional radical polymerization proceeds with slow continuous initiation, fast propagation, and inevitable radical termination, ATRP creates and exploits a dynamic equilibrium between growing radicals and dormant species.52 The active radicals are deactivated after adding one or several monomer units and converted back to the dormant state. This approach allows for preparation of polymers with precisely controlled molecular weight (MW), molecular weight distribution, polymer composition, topology, and functionality. ATRP is attractive due to simple experimental setup with readily available initiators and catalysts that can be used in a range of solvents under a broad spectrum of reaction conditions, allowing for precisely controlled molecular engineering.53 In ATRP, the dormant species are either low MW initiating alkyl halides or a macromolecular species (Pn − X). The activators are typically ligand-stabilized transition metal complexes in their lower oxidation states (Mm /L). When dormant species intermittently react with activators, active radicals (Pn ∙ ) and the deactivator: transition metal complexes in their higher oxidation state, coordinated with the transferred halide ligands (X – Mm+1 /L) are reversibly formed. After adding a few monomer units, the growing radical then reacts with a deactivator to reform a dormant species and regenerates the activator. As in any radical polymerization, radical termination is also present. Scheme 2.3 illustrates a typical ATRP equilibrium. The rate of an ATRP (Rp ) is a function of the propagation rate constant and the concentration of monomer and growing radical. The concentration of the growing radicals depends on the ATRP equilibrium constant, as well as on the concentration of the dormant species, activators, and deactivators, as shown in Equation (2.1) KATRP , = kact /kdeact , which depends on the strength of both Scheme . ATRP equilibrium (top) and low catalyst ATRP techniques (bottom).

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2.3 ATRP Fundamentals

the C–X and the CuII –X bonds. The equilibrium constant increases with the strength of the CuII –X bond, or the halogenophilicity of the CuI complex, and decreases with the strength of the C–X bonds. [ ] Rp = kp [M] P∗n = kp KATRP

(

[Pn X][CuI L][M] [X − CuII L]

) (2.1)

A catalytic ATRP process can be mediated by many redox-active transition metal complexes. Copper complexes are most often used, but ATRP has also been successfully carried out using Ru, Fe, Mo, Os, and other metals.54 “Normal” ATRP (as it was originally defined) suffers from one drawback: high catalyst loading in reaction mixture—up to ca. 1 mol% versus monomer. This residual metal compounds make purification of the final product difficult.55 Also, in ATRP, as in any radical process, radical termination occurs. It involves only ca. 1–10% of all chains. Radical termination leads to irreversible transformation of a fraction of the activator to deactivator, leading to a decrease of the ATRP rate. However, according to Equation (2.1), the ATRP rate is a function of the ratio of the concentrations of the activator to the deactivator, but it does not depend on the absolute catalyst concentration. Taking this into account, several novel ATRP techniques were developed to resolve the problem of a large amount of needed catalyst and to reduce a slowdown in the rate of polymerization due to radical termination (Scheme 2.3, bottom).56 These novel low catalyst concentration procedures include ARGET (activators regenerated by electron transfer) ATRP,57 ICAR (initiators for continuous activator regeneration) ATRP,58 SARA (supplemental activator and reducing agent) ATRP,59–61 photochemically mediated ATRP,62–66 eATRP, where the activator/deactivator ratio is controlled electrochemically,67,68 and metal-free ATRP, which moderated by an organic photocatalyst.69–72 These recent developments are represented in Scheme 2.4, which also summarizes the possibilities for engineering macromolecular architecture provided by ATRP, as well as a few of the targeted applications for the resulting materials.73 One of the key points for preparation of the desired product in a controlled manner is the appropriate choice of initiator and ligand and their amounts.74,75 For efficient initiation of the polymerization of selected monomer, the alkyl halide initiator should possess sufficient reactivity. The reactivity depends on the structure of the alkyl group and transferable halogen, or pseudohalogen.76 The reactivity of alkyl halides follows the order of 3◦ > 2◦ > 1◦ , the radical stabilizing group: CN > ester ∼ aryl, and the order of halides: I ∼ Br > Cl, which are more reactive than the pseudohalides. The ATRP activation rate constants for various initiators are shown in Scheme 2.5. The effect of the selected ligands on the ATRP rate constant is profound, and the range of activity of the formed copper-based ATRP catalyst complexes, covers over six orders of magnitude.75,77,78 Generally, Cu complex activity in ATRP for ligands follows the order of tetradentate (cyclic bridged) > tetradentate



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block

statistical

cyclic

gradient

POROUS MEMBRANES

Pn-Pm + Cu''Br2/L

APPLICATIONS

side chain block copolymer brush

CARBON NANOFIBERS CO2 CAPTURE

BIOCONJUGATES

hybrid particles

COMPLEX ARCHITECTURE

multi functional star

multi functional

PARTICLE DISPERSANT

macromonomer

PHOTONICS

telechelic

side-functional

FUNCTIONALITY

REDUCING AGENT

kp

kt

Pm + Cu''Br2/L +M

end-functional

kd

ka

Scheme . Overview of recent advances in ATRP that allow a reduction in catalyst loading, down to parts per million levels, engineering macromolecular architecture, and applications for some of the resulting materials. Source: Matyjaszewski and Tsarevsky 2014.73 Reproduced with permission of American Chemical Society.

DRUG DELIVERY SYSTEMS

network

(hyper) branched

star

ORGANIC NANOTUBES

graft

linear

TOPOLOGY

ARGET ATRP ICAR ATRP SARA ATRP eATRP

Pm-Br + Cu'Br/L

OXIDIZED AGENT

ATRP

SOUND AND VIBRATION DAMPENING

THERMOPLASTIC ELASTOMER

periodic

homopolymer

COMPOSITION

CI

CI O

O

Br Br O

O

O

10–2

O

Br

CN

I MIP (0.53)

O

O

BrAN (7.1)

Br

CN O O

104

EBPA (5.3×10–3)

Br

BrPN (23)

Br

101

Scheme . ATRP activation rate constants for various initiators with CuI X/PMDETA (X = Br or Cl) in MeCN at 35◦ C. 3◦ initiators are in red; 2◦ : blue; 1◦ : black; with isothiocyanate/thiocyanate: left half-filled; chloride: open; bromide: filled; iodide: bottom half-filled; amide: ▾; benzyl: ▴; ester: □; nitrile: ◦; phenyl ester: ♢. Source: Tang and Matyjaszewski 2007.76 Reproduced with permission of American Chemical Society.

MBrP (0.33)

tBBrP (0.12)

O

O Br

O

O

100

Br

DEBrPA (0.040)

N

O

10–1

O CI O O Br CI O MCIAc (1.6×10–3) MCIP (0.015) MBrAc (0.030) BzSCN (1.2×10–5)

SCN

10–5 10–3

kact (M–1 s–1) Extrapolated Values

O CI CN O CI CN CI Br Br CIAN (0.14) CIPN (0.46) EtCliB (0.022) EtBriB (2.7) –3 BzCI (5.5×10 ) PECI (0.010) MBriB (2.6) PEBr (0.17) BzBr (0.10) –6 BzNCS (8.9×10 )

NCS



2 Polymer Brushes by Atom Transfer Radical Polymerization

(branched) > tetradentate (cyclic) > tridentate > tetradentate (linear) > bidentate. The activity of ligands for ATRP also depends on the nature of the nitrogen atom and follows the order of aliphatic amine > imine > aromatic amines. Steric effects are also very important. Electron-donating substituents in pyridines also make the strong effect on bipyridine, picolylamine, and tris(pyridylmethylamine) ligands.79–81 The ATRP activation rate constants for Cu complexes with various ligands are shown in Scheme 2.6. There are several other factors in ATRP that affect polymerization control and properties of final product. The polymerization media play a significant role in the process. ATRP can be conducted in bulk, solution, or in a variety of heterogeneous media including microemulsion, miniemulsion, emulsion, suspension, dispersion, and inverse miniemulsion.82 The choice of media primarily depends on solubility or heat transfer considerations, for example, conditions have to be selected so that the catalyst complex and the products are at least partially soluble in the reaction medium. ATRP is strongly accelerated in the presence of more polar solvents,83 at higher temperatures84 and pressures.85

. Molecular Bottlebrushes by ATRP ..

Introduction

The main chain of a molecular brush is commonly referred as the backbone whereas the branches as the side chains. The conformation of bottlebrushes is closely related to the structural parameters. If the length of the backbone is significantly longer than that of the densely grafted side chains, intramolecular excluded volume effects generated by side chains causes stretching of the backbone. In this case, the entire bottlebrush adopts an extended, cylindrical conformation.86 Alternatively, if molecular brushes have side chains on the order of the length of backbones, the polymers generally resemble star polymers with the side chains pointing out radially.87–89 ATRP and other controlled radical polymerization (CRP) methods are suitable for synthesis of molecular brushes especially via the “grafting-from” method (Scheme 2.7) because they provide concurrent growth of all chains and low radical concentrations, suppressing the inter- and intramolecular radical termination. Based on variation of chemical composition of both backbones and side chains, molecular bottlebrushes reported in the literature can be divided into several categories (Scheme 2.8): (1) brushes with homopolymer side chains: linear brushes,4,90–105 gradient brushes with a gradient in grafting density along the copolymer backbone,106 brush–block–coil copolymers,107 and branched brushes with branches along the backbone5 and (2) brushes with copolymer side chains: brushes with block/random copolymer side chains,108–115 heterografted brushes,22,116 and brush–block–brush copolymers.117–123

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N

N

N

N

N

N

N

N

N

N

N

ET6 TREN (0.044)

N

N

O

O

10–1

N

N

N

N

O

N

O

O

N

N O O

N

O O

MA6 TREN (1.2)

O

N[2,2,2,] (0.14)

N

N

O

N

O

N N

N

N

But O O

N

O

N

N

N N

tNtpy (8.2)

N

N N dNbpy (0.6)

N

O O O tBu

N

N N

N

N

102

TPMA (62)

N

BPED (4.5)

N

O tBu

tBu O O

101

Extrapolated Values

BA6 TREN (4.1)

But

O

N

tBu O O

N[2,2,3,] (0.23)

N

N

N

PMDET A (2.7)

N

n-C17H35

BPMODA (1.1)

N

C18H35

dHDbpy (0.20)

N

100

Me4 Cyclam (0.67)

N

N

n-C17H35

N

N

N

N N N

N N Me6 TREN (4.5 × 102)

N

N

TPEDA (10.8)

N N N

103 kact (M–1 s–1)

Cyclam-B (7.1 × 102)

N

N

Active

Scheme . ATRP activation rate constants for various ligands with EtBriB in the presence of CuX (X = Br, Cl) in MeCN at 35◦ C. N2: red; N3: black; N4: blue; amine/imine: solid; pyridine: open; mixed: left-half solid; linear: □; branched: ▴; cyclic: ◦. Source: Tang and Matyjaszewski 2006.78 Reproduced with permission of American Chemical Society.

CN

N

N

N N bpy (0.066)

Me3 TAN (0.38)

N

N[3,2,3,] (5.0 × 10–3)

N

N

10–2

CN

NC CN AN6 TREN (0.012)

N

CN

N[2,3,2,] (1.2 × 10–3)

NC

–3

N[2,3] (9.2 × 10 )

N

N

N

NPPMI (R = n-C3H7] N N NOPMI [R = n-C8H17) TMEDA (0.015) –3 [2.4 × 10 ) R

10–3

N

N



2 Polymer Brushes by Atom Transfer Radical Polymerization

Scheme . Synthesis of a backbone macroinitiator and subsequent side chain growth.

..

Star-Like Brushes

During the synthesis of molecular brushes via “grafting from” by ATRP, the macroinitiator is essentially the template from which the brush structure is derived.1 Macroinitiators can be obtained via the polymerization of a monomer containing an ATRP initiator moiety or a monomer carrying a precursor, which can be subsequently converted into an ATRP-initiating group. Therefore, the ability to tailor the structure of macroinitiators allows the preparation of advanced molecular architectures brushes. The preparation of nonlinear brushes with a shape of three-, four-, and six-arm stars was reported.124,125 As shown in Scheme 2.9, star-like backbones

Scheme . Classification of molecular brushes according to branching topologies and chemical composition of side chains.

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TMSO

O

O

O

O

O

O

O O

Br

I = 4BriBu

O

O

O

i

O

O

Br

I = 3Br Bu

Br

O

O

Br

Br

O

(CH3)3SiO

O

( n

1.5 Br PrBr, r.t

i

KF/THF/1% TBAF

)

1.5 BriPrBr, r.t

( O

KF/THF/1% TBAF

Br

O

O

n

O

)

ATRP of nBuA

O

n

O

)

m Br

O

nBuA: pBPEM: CuBr:CUBr2:dNbpy 500: 1: 0.5: 0.025: 1

ATRP of nBuA

nBuO2C

O

(

nBuA: pBPEM: CuBr:CUBr2:dNbpy 500: 1: 0.5: 0.025: 1

Scheme . Synthesis of three- and four-arm star molecular brushes. Source: Matyjaszewski et al. 2003.124 Reproduced with permission of American Chemical Society. Reprinted with permission124 . Copyright 2003 American Chemical Society.

M: I: CuCl: dNbpy 2400: 1 : 3 : 6

Br

O

Br

ATRP

M: I: CuCl: dNbpy 1800: 1 : 2.25 : 4.5

2-trimethylsilyloxyethyl methacrylate (HEMA-TMS)

n

O



2 Polymer Brushes by Atom Transfer Radical Polymerization

Figure . AFM images (1 × 1 𝜇m) of (a) three-arm star PBPEM-g-PnBA brushes. AFM images, and (b) four-arm star PBPEM-g-PnBA brushes. Source: Matyjaszewski et al. 2003.124 Reproduced with permission of American Chemical Society.

were prepared by ATRP of HEMA-TMS (2-(trimethylsilyloxy)ethyl methacrylate, precursor of ATRP initiator) with tri- or tetrafunctional initiators. Side chains were grafted from these macroinitiators by the subsequent ATRP of n-butyl acrylate. These nonlinear structures can be clearly observed in AFM images (Figure 2.3). More structurally complex molecular brushes prepared by ATRP that composed of bottlebrush arms and a molecular spoked wheel (MSW) core were also reported.125 Linear chain arms were grafted from a sixfold ATRP initiator (MSW6-Br ) and subsequently functionalized with ATRP moieties to form sixarm macroinitiators. Grafting of side chains from the macroinitiators yielded star-shaped bottlebrushes (Scheme 2.10). ..

Blockwise Brushes

A blockwise backbone is another architectural possibility for a molecular brush. The simplest example is a brush–coil block copolymer in which one block is a cylindrical brush, whereas the other is composed of a linear polymer, as shown in Figure 2.4.126 Upon UV irradiation, hydrogen bonding moiety—ureidopyrimidinone (UPy) group—was released due to leaving of the UV-labile protecting group and the single-chain polymeric nanoparticle block folded due to strong intramolecular hydrogen bonds. Brush–brush block copolymers, in which both blocks contain side chains, can be viewed as the brush-like analogs of linear block copolymers. The most straightforward method to synthesize brush–brush block copolymers is by

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1 – n = 450 4 – n = 300 2 – n = 450 5 – n = 300

3a – n = 450 m = 20 3b – n = 450 m = 40 6a – n = 300 m = 60 6b – n = 300 m = 150

Scheme . The synthesis of star-like bottlebrushes with hexa-ATRP initiator (MSW6-Br ) via the double “grafting-from” approach. Source: ´ Burdynska et al. 2014.125 Reproduced with permission of American Chemical Society.

MSW6-Br



2 Polymer Brushes by Atom Transfer Radical Polymerization

hv

O

O

S x O S

105 O

1165 O

400 O

NC

O

O

O

O

O

O

O

O

O

O NH

NH O

x Br

4 x = c) 22 d) 44

O

O

O

O

O

S x O S

105 O

1165 O

400 O

NC

O

x Br

5 x = c) 22 d) 44

O

NH

N H O

O

O H N

NH N N

N HN NH N

O

OH N

NO2

N H O

O O

O

O

N H

Figure . Structures of brush-block-coil copolymer (4c,d and 5c,d) including representative AFM height micrographs of 4d (top left) and 5d (top right) (scale bar = 50 nm) and schematic representation of the polymer structures on the mica surface (bottom). Source: Stals et al. 2013.126 Reproduced with permission of American Chemical Society.

sequential polymerization of two types of macromonomers.21 However, versatility of the “grafting-through” method is limited by issues related to large steric hindrance between grafts, low concentration, and poor accessibility of the terminal functional groups. In order to prepare brush–brush block copolymers, the combination of “grafting through” and “grafting from” was employed. Sequential homopolymerization of poly(ethylene glycol) methacrylate (PEGMA) macromonomer and 2-hydroxyethyl methacrylate (HEMA)

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2.4 Molecular Bottlebrushes by ATRP

PEG-MC(1)

)

44

(

CH3O

CH3 CH3 C )n (CH2 C )m O C O O O CH2 OH CH2

)

CuCl/BPy in H2O

(

O CH2 CH2

H3C CHBr C O OCH3

CH3 H3C CH ( CH2 C )n Br (Cl) H3C CH ( CH2 C O C O C O OCH3 O OCH3 HEMA CH2 CH2 CuCl/BPy in H2O/MeOH O CH2 CH2 (

)

CH3 C CH2 C O O CH2 CH2

O CH2 CH2

CH3O

CH3O

44

PEG brush macroinitiator(2)

PEG brush-block-PHEMA(3) Et3N in THF

HEMA

O Br

Br

CH3 CH3 CH3 H3C CH ( CH2 C )n (CH2 C CH2 C ) 1 O O C O C O O O OCH3 O CH2 O OH CH2 O O Br CH2 CH2 )

CuCl/BPy in H2O/MeOH

44

(

Block-Type Brush(5)

CH3O

44

Scheme . Synthesis of brush-brush block copolymer via ATRP. Source: Ishizu et al. 2004.127 Reproduced with permission of Elsevier Inc.

resulted in a polymer that could be functionalized with ATRP-initiating sites, from which the PHEMA side chain of second block were grafted (Scheme 2.11).127 Similarly, brush-like block copolymers were prepared by growing PnBA side chains from AB or ABA macroinitiators synthesized from octadecyl methacrylate (ODMA) and HEMA-TMS.128 Due to the crystallization-induced aggregation of poly(octadecyl methacrylate) segments, self-assembly of the AB and ABA block copolymer brushes was observed by AFM, giving rise to a new class of supersoft elastomers. An alternative way to prepare brush block copolymers is to grow two types of side chains sequentially from different initiating sites in two blocks of macroinitiators. Scheme 2.12 shows the synthesis of heterografted block brushes with PCL and PnBA side chains using only the “grafting-from” technique.117 ..

Brushes with Tunable Grafting Density

Molecular bottlebrushes are a unique class of dense graft copolymers, whose physical properties are strongly affected by steric congestion between the side chains, which can be systematically tuned by changing the spacing between initiating sites. For example, non-initiating sites can be introduced into the





2 Polymer Brushes by Atom Transfer Radical Polymerization

Scheme . Synthesis of the heterografted block brushes containing PCL and PBA side chains. Source: Lee et al. 2008.117 Reproduced with permission of American Chemical Society.

backbone by copolymerization of a protected monomer, 2-(trimethylsilyloxy) ethyl methacrylate (HEMA-TMS), with methyl methacrylate (MMA).106 By continuous feeding of HEMA-TMS during the course of the polymerization, gradient composition in the backbone was created, because the reactivity ratios of MMA and HEMA-TMS were both close to unity (Scheme 2.13).129 Also, a gradient backbone can be spontaneously obtained by copolymerizing monomers with significant different reactivity ratios, that is acrylates and methacrylates (Scheme 2.14).25,42,130 After growth of side chains, the asymmetric structure of brushes with gradient grafting densities was confirmed by AFM. When gradient brushes adsorbed on a surface were compressed, the rod– globule transition occurred at the end where the brush was densely grafted, leaving a molecule with a globular “head” and an extended “tail”.131 ..

Brushes with Block Copolymer Side Chains

Preparation of molecular brushes via the “grafting-from” method by ATRP allows effective retention of chain end functionality. This unique advantage can be further taken to create the core–shell structure by performing the chain extension.

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O

O

ATRP polymerization constant addition of HEMA-TMS

O

(MMA)

Transformation to macroinitiator p(MMA-grad-BPEM) (II)

Brush synthesis by grafting from approach

Scheme . Subsequent synthesis of the macroinitiator precursor (I), the macroinitiator (II), and macromolecular brush copolymer (III). Source: ¨ Borner et al. 2002.106 Reproduced with permission of American Chemical Society.

p(MMA-grad-HEMA-TMS) (I)

TMSO

(HEMA-TMS)

O



2 Polymer Brushes by Atom Transfer Radical Polymerization

Scheme . Synthetic scheme for the gradient copolymer, macroinitiator, and molecular brush: (a) MMA/HEA-TMS system and (b) HEMA-TMS/n-BA system. Source: Lee et al. 2005.130 Reproduced with permission of American Chemical Society.

Brushes with PnBA cores and PS shells as well as the inverted structures were prepared by ATRP (Scheme 2.15).40 Different properties of two types of brushes were characterized by visualization of the conformation and microstructure of the brush macromolecules on a mica surface. The core– shell brushes with the PnBA (soft) core were almost fully stretched, whereas the ones with PS core exhibited longitudinal contraction compared to the contour length of the main chain. Such different behavior can be attributed to the relative hardness/softness of two blocks at the ambient temperature (soft block: Tg,PnBA = −50◦ C; hard block Tg,PS = 100◦ C) and the entropic elasticity of the tightly adsorbed PnBA side chains extended the backbone. Similarly, brushes with PS-b-PtBA (polystyrene-block-poly(tert-butyl acrylate)), PtBAb-PS, PtBA-b-PnBA, PCL-b-PnBA (poly(𝜀-caprolactone)-block-poly(n-butyl

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2.4 Molecular Bottlebrushes by ATRP (a)

(b)

Scheme . (a) General approaches to brushes with block copolymer side chains of ¨ different sequences. (b) Synthesis of brush with PnBA-block-PS side chains. Source: Borner et al. 2001.40 Reproduced with permission of American Chemical Society.

acrylate)), PNVP-b-PVOAc (poly(N-vinylpyrrolidone)-block-poly(vinyl acetate) side chains were prepared.39,132–134 In addition to core–shell structures afforded by molecular brushes with diblock side chains, the core–shell–corona structure was also prepared with triblock side chains.38 Micelle-like structures of molecular brushes with block copolymer side chains are not susceptible to dissociation on dilution compared to conventional multimolecular micelles. These unique properties allow the application of brushes as templates to prepare novel nanocomposites (Scheme 2.16), which will be further discussed with in more details in following parts of this chapter.

Stabilization

Carbonization

PBA PAN PAA Crosslinker Cross-section view

Scheme . Shell cross-linked molecular brushes and their conversion into nanostructured carbons through thermal treatment. Source: Tang et al. 2007.38 Reproduced with permission of American Chemical Society.





2 Polymer Brushes by Atom Transfer Radical Polymerization

One major advantage of ATRP is the possible post-polymerization modification of the terminal halogen end groups.135,136 Molecular brushes with both short- and long-side chains were prepared by partial removal of the bromine chain ends via selective capping with 4-butoxy-TEMPO (2,2,6,6tetramethylpiperidine 1-oxyl) and subsequent chain extension of remaining active chains-forming longer PnBA grafts.137 Similar systems with bimodal brushes grafted from nanoparticles have been described in the literature, where these particles showed enhanced particles dispersion as well as improved thermal and mechanical properties in comparison to particles with a high density of monomodal grafts (Scheme 2.17).138–140

..

Functionalities and Properties of Brushes

Due to excellent functional group tolerance of ATRP, functionality can be easily introduced by incorporation of functional monomers during polymerization. As shown in Figure 2.5, fluorescein O-methacrylate was copolymerized with n-butyl acrylate by ATRP in a “grafting-from” reaction with a multifunctional linear macroinitiator to form pH-responsive fluorescent bottlebrushes.141 Similarly, solubility in water can be tuned by simply adding water-soluble poly((oligo(ethylene glycol)) methacrylate) (POEGMA) segments (Scheme 2.18).142 More details about stimuli responsive polymer brushes will be introduced in Section 2.10. The properties of a molecular brush are strongly affected by the nature of side chains due to their large weight fraction. For example, molecular brushes with PEO,22,25 long-chain alkyl (i.e., octadecyl),25 and PCL143 were partially crystalline. DSC results showed that PEO brushes with short PEO side chains (MWPEOMA monomer = 300 g/mol, DPPEO = 5) were amorphous whereas those with long PEO side chains (MWPEOMA monomer = 300 g/mol, DPPEO = 23) were crystalline.144 In contrast, molecular brushes with PnBA side chains whose glass transition is far below room temperature behaved as very soft material, showing modulus at the level of 102 –104 Pa.145–147 Due to the high density of grafts, physical properties of molecular brushes are primarily dictated by high steric congestion between the side chains. As Figure 2.6a shows, when molecular brushes were deposited onto substrate, they spread to increase the number of monomeric contacts with the substrate. The brush-like architecture imposed constraints on the spreading process, making it anisotropic and leading to extension of the backbone, which resulted in spontaneous scission of the covalent backbone.148,149 The tension depends on the branching (or grafting) density, length of the branches, and interaction with the surrounding environment (e.g., solvent molecules and substrate).150,151 Such accurate tension control allowed the fabrication of tensile machines at the molecular level. For example,

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´ Scheme . Synthetic approaches toward bimodal bottlebrushes. Source: Burdynska et al. 2015.137 Reproduced with permission of American Chemical Society.



2 Polymer Brushes by Atom Transfer Radical Polymerization

415 O O

O

415 O O

O O

O O

OO

O

40

O

OO

O

O

–

O

O

NaOH

40

HCl

NaOH

O

O

HCl

O

O

Neutral

Basic

O

(a)

(b)

Figure . (a) Schematic representation of the bottlebrushes under neutral and basic conditions. (b) Solutions of bottlebrushes under neutral (left) and basic (right) conditions. Source: Nese et al. 2011.141 Reproduced with permission of American Chemical Society.

tension-induced activation of S–S bond was studied employing molecular brushes containing S–S bond in the backbone.149,152,153 The steric congestion between the side chains in molecular brushes can be utilized to achieve some unique properties comparing to linear analogs. Long-range patterning of thin films on sub-100 nm length scales was reported using mixture of chemically incompatible brushes without the need of encoding specific complementarity (Figure 2.7b).154 Upon compression, the hairy architecture of molecular brushes enhanced steric repulsion and the penalty of conformational entropy due to the stretch of polymeric side chains. Therefore,

1 Sequential ATRP 1

2

3

Crosslinking - MeOH

O

(

O

2

O

3

O

tBA O

APTS O

(

) O

O Si O O O) n

O

O Si O O

OEGMA ATRP initiation site

(a)

(b)

(c)

Scheme . Synthetic route to obtain water-soluble organo-silica hybrid nanotubes ¨ templated by core–shell–corona molecular bottlebrushes. Source: Mullner et al. 2010.142 Reproduced with permission of American Chemical Society.

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2.4 Molecular Bottlebrushes by ATRP f ⎢⎢ = S.d

(a)

δ Spreading

f⊥ = S. δ

d (b)

Time

Figure . (a) Schematic of the spreading of a brush-like macromolecule on an attractive substrate. (b) Schematics of an adsorbed macromolecule (left), which undergoes spontaneous scission of the covalent backbone (right). Source: Sheiko et al. 2006.148 Reproduced with permission of Nature Publishing Group.

the formation of intermixed brushes was unexpectedly entropically favored (Figure 2.7c). Molecular brushes may change their shapes upon lateral compression on a substrate, which allows them to be used as pressure sensors. If the film pressure increases, the number of side chains adsorbed to the surface decreases allowing the backbone to coil.155,156 This causes the macromolecules to become more compact and occupy less area on the substrate. Therefore, the area per molecule can be used as a pressure-sensitive parameter to gauge the variations of film pressure within flowing monolayers.157 Significant enhancement of molecular alignment was observed during spreading of brush-like macromolecules on the surface of highly oriented pyrolytic graphite.158 Unlike conventional flow-induced orientation of anisotropic objects such as rod-like particles, the observed orientation of molecular brush was not coupled with the direction of flow (Figures 2.8a– 2.8c). The orientation of the brush-like macromolecules on the graphite was attributed to the epitaxial adsorption of alkyl side chains on graphite along one of the three crystallographic axes of the (0001) surface (Figure 2.8d). The role of flow was merely to enhance diffusion and thus facilitate epitaxial ordering of the large macromolecules.





2 Polymer Brushes by Atom Transfer Radical Polymerization

(a)

D0

(c) PDMA

PBA

Compression D1

500 nm Spacers (b)

Stabilizer ΔSC >0 D2

150 nm

Figure . AFM height imaging of phase separation of a mixture of linear PBA and PDMA deposited on a mica substrate (a) and a uniform mixture of brush-like PBA and PDMA (b). Mechanism of uniformly mixing: whereas the hydrophilic PBA brushes play the role of mixture stabilizers, the hydrophobic species play the role of spacers (diluents) reducing the steric repulsion between the brushes (c). Source: Sheiko et al. 2013.154 Reproduced with permission of Macmillan Publishers Limited.

(a)

(b)

(c)

500 nm (d)

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Figure . (a)–(c) Higher magnification height images were measured in different areas of the precursor film on graphite. The images reveal a lack of correlation between the flow direction and orientation of the flowing molecules. (d) Epitaxial adsorption of side chains leads to uniaxial alignment of polymer backbones along a particular crystallographic axis within the (0001) plane. Source: Xu et al. 2006.158 Reproduced with permission of American Chemical Society.

2.5 ATRP and Flat Surfaces

. ATRP and Flat Surfaces ..

Chemistry at Surface

In general, the chemistry of surface-initiated atom transfer radical polymerization (SI-ATRP) has several special features distinct from that ATRP in solution.159 For example, surface curvature plays an important role in determining initiation efficiency. The initiation efficiency of SI-ATRP from a flat substrate has been estimated to be around 10%.160–162 In SI-ATRP from a flat surface, one end of each individual polymer chain is fixed onto a substrate. This forces the polymer chains to grow in close proximity to one another, creating crowding of polymer chains and forcing them to assume a chain extended brush conformation. The brush conformation is evident from the greater thickness of a grafted polymer layer on a flat substrate than the radius of gyration of the free polymer. The calculation of grafting density shows that each polymer chain occupies a smaller projected area than that predicted by its radius of gyration, further confirming the chain-extended brush conformation. The termination of living chains during SI-ATRP could offer an explanation to some of the experimentally observed phenomena. For example, termination provides plausible explanation for the experimentally observed decrease in the growth rate of the grafted polymer layer, even when no significant monomer depletion is expected in the bulk contacting solution. This experimental trend was repeatedly reported for various types of substrates and monomers163–167 and was also supported by an experimentally measured decrease in the concentration of halide groups on the surface.168 On flat substrates, the possible termination modes depend on the polymerization locus. For SI-ATRP accompanied with simultaneous polymerization in the contacting solution, termination could occur between two surface radicals, two solution radicals, or between a surface and a solution radical. On the other hand, the termination could only occur between two surface radicals for surface-confined SI-ATRP. These possible termination modes are illustrated in Figure 2.9.

..

Grafting Density

Grafting density is an important parameter for hybrid materials. The properties imparted by the grafted polymer layer depend not only on the type of polymer chemistry but also on uniformity, grafting density, and thickness of the grafted polymer layer. The grafted polymers with moderate-to-high grafting densities gave different properties than observed for low grafting density.169,170 The grafted chain length was controlled using ATRP. In general, the grafting density that was obtained by SI-ATRP was higher than those attained by the grafting-to method or by free radical polymerization. On the





2 Polymer Brushes by Atom Transfer Radical Polymerization

×

C

× ×

×

×

×

×

×

× ×

×

×

×

100 nm

×

×

×

×

×

×

× ×

×

× ×

×

×

1n m

×

× ×

× ×

×

×

×C

×

X

×

×C

×

×

010

00

nm

10

= Radical × = Dormant

Figure . (a) Possible termination modes involved in SI-ATRP on a flat substrate. The estimated distances shown are calculated based on the assumption of a high grafting density (1 chain/nm2 ), a typical ratio of radical to dormant chains in ATRP ([P∙ ]/[PX] = 10–4 to 10–6 ), and typical brush thickness of 100 nm. (b) Migration of surface radicals through activation/deactivation in SI-ATRP promotes termination between surface radicals. Source: Zhou et al. 2012.353 Reproduced with permission of American Chemical Society.

other hand, predicting and tailoring the surface to possess a certain grafting density remains one of the major challenges in SI-ATRP, since not all factors affecting the grafting density are fully understood.171 On a flat substrate, the measurement of grafting density is challenging due to the limited amount of grafted polymer. To estimate the grafting density, it was assumed that grafted chains have the same properties as free chains in polymerizations conducted simultaneously in both phases166,172–175 or an accurate relationship between swollen and dry thicknesses of polymer layers.176 The mechanistic studies of grafting methacrylate polymer chains through the monomer-modified surfaces were reported.177 Polymer chains in a self-limiting process formed a polymer layer on the surface during the grafting-through process. In general, the grafting density and polymer layer thickness were largely independent of many reaction conditions, such as concentrations of monomer, initiator, and surface concentration of monomer, reaction time, or temperature. This makes the grafting-through technique a robust method for preparation of hybrid materials.

..

Architecture

One significant advantage of using ATRP for introducing polymer brushes to the substrate is that it allows preparation of more complex polymer chain

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2.5 ATRP and Flat Surfaces

microstructures and topologies and better control over polymer brush properties, which translates to generating more precise surface properties. ATRP allows for preparation of complex polymer brush structures from block copolymer brushes, to linear chains with gradient composition and gradient molecular weight brushes, to binary brush types, etc., which creates various possible applications of a hybrid substrate with highly tunable and unique properties.172 Miktoarm hybrid systems, that is systems with types of polymer brushes, were prepared via ATRP by grafting onto178 and grafting from;179 however, these methods provided limited grafting density. Another approach to prepare miktoarm systems included using asymmetric Y-shaped difunctional initiators and sequential grafting two types of brushes using SI-ATRP and surfaceinitiated nitroxide-mediated radical polymerization.180 ..

Applications

Due to its versatility, SI-ATRP is a common method for the preparation of functional flat surfaces for a wide range of applications. Quite promising are biomedical applications: antifouling, antimicrobial, biocompatible surfaces.43,181–186 For biomaterial applications such as biosensors and implants, where the material is in contact with various proteins in a complex biological environment, it must possess antifouling surface properties. The antifouling and antimicrobial properties are important in development of biosensors, implants, for separation and filtration of biological species. All these applications require minimization of surface adsorption of biorelevant species and bacteria. These properties are introduced by grafting biocompatible polymer brushes via SI-ATRP from the surface, for example, by grafting hydrophilic neutral or zwitterionic polymers (e.g., POEGMA, poly(2-hydroxyethyl methacrylate) (PHEMA), poly(acrylic acid) (PAA), poly(2-methacryloyloxyethyl phosphorylcholine), poly(carboxybetaine methacrylate), poly(sulfobetaine methacrylate) (PSBMA)). The biocompatibility of silicon,165,173,187–194 gold,195 and polymeric196–199 surfaces grafted with hydrophilic and zwitterionic polymer brushes was investigated. The zwitterionic polymer brush was grafted from titanium and stainless steel via SI-ATRP, and the surfaces with high resistance against cell, bacterial, and protein adhesion were prepared.200,201 The introduction of quaternary ammonium groups into the backbone of polymer brushes killed bacteria.202,203 Also, polyionic brushes were grafted using SI-ATRP to introduce surface lubrication properties204–206 for creation of low friction systems, such as biological surfaces in artificial joints. The friction properties heavily depended on external environment, for example, humidity. Similar behavior was observed for the swelling characteristics of a PS brush, controlled by the solvent composition, for tribological properties of surfaces modified with various alkyl methacrylates.207,208





2 Polymer Brushes by Atom Transfer Radical Polymerization

Another interesting feature that was introduced to a surface using SI-ATRP was the self-healing properties. The driving force for such behavior was the interaction between positively and negatively charged polyionic brushes209 or the dipole–dipole interactions between zwitterionic polymer brushes.210 The adhesion reversibility of the surfaces grafted with zwitterionic polymer brushes was special better than those with oppositely charged polyelectrolyte brushes. Moreover, the debonding reaction was achieved simply by placing the substrate in water at an elevated temperature, 50◦ C. Finally, SI-ATRP was applied to modify the surface properties of metals to introduce corrosion resistance.211–214

. ATRP and Nanoparticles ..

Chemistry

SI-ATRP from the surface of spherical nanoparticles has several features in initiation and termination due to positive curvature. Nanoparticles have much larger surface-to-volume ratio than flat systems, allowing a sufficient amount of polymer to be collected for further characterization. In fact, the properties of polymers obtained from SI-ATRP conducted on particles showed an excellent agreement with the properties of the polymers formed in the solution phase, indicating similar kinetic patterns.215 The surface curvature plays an important role in initiation efficiency. The initiation efficiency from a convex substrate exceeds 30%216–224 and can increase with time216,219 and monomer conversion.221,223,224 In SI-ATRP from nanoparticles, there are two possible termination modes: inter- and intraparticle termination. The intraparticle termination is similar to termination between two surface radicals observed for flat substrates, with the additional constraint of a curvature effect. On the other hand, interparticle termination may lead to gelation, as illustrated in Figure 2.10. For spherical nanoparticles with d = 20 nm, interparticle termination via coupling/combination of only 0.125% of the chains (ca. 1600 chains per particle) can result in macroscopic gelation, according to Flory’s gelation theory.222 Macroscopic gelation was reported even when no bimodality was observed in the molecular weight distribution of the cleaved grafted chains.224 There are several ways to prevent the macroscopic gelation, including using a lower concentration of particles, or stopping the reaction at low monomer conversion.221,223,224 A “sacrificial” initiator was added in solution to form unbound polymer chains and prevent network formation.225,226 The macroscopic gelation was avoided by conducting SI-ATRP in a miniemulsion system, due to particles compartmentalization.222 Another effective way to avoid macroscopic gelation is conducting the polymerization in a high-pressure system.218 Under high pressure, the

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2.6 ATRP and Nanoparticles

Figure . Inter- and intraparticle termination modes between two surface radicals in SI-ATRP on nanoparticles. Source: Vana 2006.159 Reproduced with permission of Springer.

× ×

× ×

×

×

× ×

polymerization proceeds with enhanced propagation rate constants and diminished termination rate constants. This leads to a faster polymerization with better a living characteristic. Successful synthesis of PMMA-grafted chains with molecular weight above 1 million and low dispersity (Mw /Mn < 1.3) on silica nanoparticles was reported using activator generated by electron transfer atom transfer radical polymerization in a vessel pressurized to 6 kbar.218 The retention of high chain end functionality was confirmed by conducting SI-ATRP of methyl acrylate (MA) using the PMMA-grafted nanoparticles as the surface macroinitiator.

..

Architecture

SI-ATRP allows for precise control of polymer brush architecture on the convex surfaces. Polymer brushes grafted from spherical nanoparticles can be divided into two regimes, according to Daoud and Cotton scaling model.3 The first one is concentrated particle brush (CPB) regime, in which due to excluded volume interactions the polymer chains stretch. The second is semidilute particle brush (SDPB) regime, which assumes relaxed chain conformations. The transition between two regimes depends on surface grafting density, chain length, surface curative, and other parameters (Figure 2.11).3 Polymer nanocapsules were prepared using SI-ATRP by growing polymer brushes from particles, cross-linking of the tethered polymer brush, followed by etching the particles.227,228 Silica nanoparticles acted as templates for the preparation of polymeric nanocapsules. As an extension of this concept, formation of nanonetwork polymers was demonstrated.229 The membranes



2 Polymer Brushes by Atom Transfer Radical Polymerization

(a)

rc

r hSDPB

hCPB

RO

~ N 0.8 (b)

d/dc



101

~ N 0.5

CPB

SDPB

d ~ N0.5 100 d ~ N0.8 8SiO2-MMA-N 8SiO2-S-N 30SiO2-S-N 60SiO2-S-N

10–1 10–2

10–1

100

101

102

N/Nc (c)

CPB N > Nc

Figure . Panel a: Illustration of the particle brushes, according to the extended Daoud–Cotton model. In the vicinity of the particle surface, chains assume stretched conformations (CPB regime, hCPB ∼ N0. 8); as particle brush size exceeds the critical radius (r > rc), chains assume relaxed conformations (SDPB regime, hSDPB ∼ N0.5 ). (continued)

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2.6 ATRP and Nanoparticles

of carbon materials were obtained by carbonizing the polyacrylonitrile (PAN) chains and etching of core of the nanoparticles.230 A cross-linked brush architecture was introduced to the surface of nanoparticles via ATRP.231 A difunctional methacrylate-based cross-linker was copolymerized with a monofunctional monomer to introduce polymer brushes on the surface of gold nanoparticles. Due to higher reactivity of difunctional crosslinker, the cross-linked polymer shell formed near the surface of the nanoparticle. The monofunctional polymer brushes continued to grow from the shell. The cross-linked shell protected the gold surface, prevented the detachment of the polymers from the surface, and provided excellent thermal stability of hybrid material. Recently, a particle brush with bimodal molecular weight distribution was prepared via SI-ATRP and the matrix-free hybrid materials displayed enhanced overall toughness at higher inorganic content.140 ..

Applications

ATRP was applied to create polymer-coated magnetite nanoparticles which combined magnetic properties of the core with the functional groups of polymer brush shell,232,233 providing better dispersion.234–237 Another potential application demonstrated for polymer-coated magnetite nanoparticles was in oil–water separation, as shown in Figure 2.12.238 The negatively charged grafted polymer, poly(sulfobetaine methacrylate), allowed the nanoparticles to absorb water, which was separated from the oil phase by applying an external magnetic field. Due to magnetic properties of the core, the particles were easily separated from the oil phase. Antimicrobial properties were introduced to magnetite nanoparticles by grafting poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMAEMA) brushes via SI-ATRP and further quaternization of amines.239 Another possible application of SI-ATRP included the preparation of magnetic inks for the greener paper-recycling process. In such systems, the polymer brushes provided the stability of carbon-coated magnetite nanoparticles in water.235,240 ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Figure . (Continued) Panel b: Dependence of the reduced brush height (d/dc) on the reduced DP of surface-grafted chains (N/Nc ) determined from analysis of electron micrographs of particle monolayers (PS- and PMMA-grafted SiO2 particle systems; R0 ≅ 7.7 (abbreviated as 8), 29.6 (30), and 56.9 (60) nm; grafting density 𝜌s ≅ 0.5 nm−2 ). The CPB → SDPB transition is determined from the depicted trend as Nc = 250, 1280, and 1850 for particles with radius 7.7, 29.6, and 56.9 nm, respectively. Different colored regions indicate the predicted CPB and SDPB regimes (DC model). Good agreement between experimental and predicted scaling relation was observed for all particle systems. Panel c: Illustration of “effective interaction models.” Particle brushes within the CPB regime interact by hard-sphere type potentials; particles with chains in the SDPB interact in a similar manner to star polymer systems. Source: Jin, et al. 2009.196 Reproduced with permission of American Chemical Society.





2 Polymer Brushes by Atom Transfer Radical Polymerization

TEOS

ATRP initiator

Ammonia

Br

Br

SPMA

H H O

O H H

O H H

O H H

Oil/water separation

H H O

PSPMA brushes

SI-ATRP

Br Br

O H H H H O

Fe3O4

Br

H H O

O H H H H O

O H H

Br

H H O

O H H

Hydration

Br Br

in water

Fe3O4@SiO2

Harvest water

Hydrated layer

Figure . Zwitterionic polymer brushes on magnetite nanoparticles as a tool to separate oil and water. Source: Liu et al. 2014.238 Reproduced with permission of American Chemical Society.

Moreover, magnetic inks from the recycling process were recovered for subsequent reuse. Magnetic nanoparticles grafted with a block copolymer of polyhedral oligomeric silsesquioxanes and PMMA were used as smart fillers, as their location in the solution was controlled using magnetic field, allowing localization of fillers.237 A wide variety of polymer architectures on the nanoparticle surface is possible by postgrafting modification of brushes. For example, polymeric nanocapsules were obtained by cross-linking the polymer brushes on nanoparticles with further etching the nanoparticles.227 Carbonization prior to etching allowed formation of nanonetwork carbon materials.229 The polymer brushes were acidified to form acidic brush layer and used as a reusable catalyst in the dehydration of fructose.241 Quantum dots often suffer from lack of stability and poor dispersability in composite materials. Thus, the quantum dot surfaces were modified by SIATRP. For example, PMMA brushes were grafted from CdS/SiO2 core–shell nanoparticles.242 A film formed from the resulting grafted nanoparticles exhibited the same luminescence properties as the CdS core. The inorganic cores were uniformly distributed throughout the polymer matrix. PnBA was grafted from CdS quantum dots via miniemulsion ATRP, and materials showed an even dispersion of CdS cores in the polymer matrix.243 SI-ATRP was also used to prepare Janus particles with unique properties arising from their asymmetric structure.244,245 Due to the pH-responsive nature of the polymer brushes, the resulting Janus particles exhibited pH-responsive

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2.8 ATRP and Templates

aggregation. The synthesis of Janus particles through biphasic SI-ATRP at a Pickering emulsion interface was successful.246 Janus particles, with PCL and poly(N-isopropylacrylamide) (PNIPAm), were prepared by a combination of polymer single-crystal templating and SI-ATRP.247

. ATRP and Concave Surfaces The confinement effect is more important for concave systems than for flat surfaces. Under similar conditions, lower molecular weights and broader distributions were reported for polymers grafted from concave surfaces as compared to flat surfaces or to solution.248–250 This is especially evident for very small pore size, ranging from 1.8 to 2.3 nm.248,249 For ordered mesoporous silica with larger, ca. 15 nm cylindrical pores, an excellent control was observed for SI-ATRP of acrylonitrile until polymer filled all pores.251 SI-ATRP was employed to graft polymers from porous materials to improve antifouling properties,252 biocompatibility, and/or to introduce surface responsiveness to external stimuli,253–255 and for drug delivery carriers.256 Surfaceresponsive mesoporous silica nanoparticles were prepared by modification of the surface with PNIPAm brushes.257 The drug was released after the change in brush conformation triggered by temperature change. A similar approach was applied to induce drug release by other external stimuli, such as temperature, pH, etc. SI-ATRP was also used to modify carbon black surface. Carbon black has excellent bulk properties; however, its surface is not compatible with most polymer matrices. This reduces the application of carbon black in nanocomposites. The modification of carbon black improved the dispersability and introduced various other functional groups to the surface.258–260

. ATRP and Templates ..

Templates from Networks

Well-defined inorganic networks are important hard templates for complexed structures. Polymer brushes grafted from the interior surface fills the pores with no discontinuation or voids. One such example was successful preparation of mesoporous carbon material using PAN brushes grafted from mesoporous silica as a precursor and template, respectively.261–263 Mesoporous silica was surface-modified with tetherable ATRP initiators and grafted with PAN or PAN-based copolymer brushes. Carbonization of PAN resulted in a welldefined carbon material after removal of silica template (Figure 2.13).





2 Polymer Brushes by Atom Transfer Radical Polymerization

(a)

(c)

1. Br

O

Si

Cl

O toluene/pyridine, 110°C 2.

SiO2

Si Cl toluene/pyridine, 80°C

Br

11 nm

SiO2

O O Si

Si O N

Si

50 nm stabilization of PAN, carbonization

CuCl N N

(c)

SiO2 removal

PAN

carbon nanorod

50 nm

Figure . Preparation of mesoporous carbon using mesoporous silica as a template. (a) Synthesis of mesoporous carbon via SI-ATRP; (b)–(c) transmission electron microscopy (TEM) images of carbon templated by SBA-15 silica. Source: Kruk et al. 2005.261 Reproduced with permission of American Chemical Society.

..

Templates from Brushes

Polymer brushes were used as templates for zero-dimensional,264–266 onedimensional (1D),108,109,267–270 or 3D271,272 structures based on their morphologies and assemblies. Typical zero-dimensional structures are hollow particles templated by inorganic or polymer particle brushes. Hollow TiO2 particles have been prepared using a PDMAEMA grafted from a PS sphere. Void size and shell thickness can be simply adjusted by changing the ionic monomer fraction in the PS sphere and DP of PDMAEMA brushes.264 Similarly, hollow polymeric spheres were achieved by grafting polymer brushes from inorganic particles followed crosslinking of the polymer and removal of inorganic content.265 Preparation of 1D structures templated by molecular bottlebrushes was extensively investigated.108,109,267–269,273 For instance, from a poly(2bromoisobutyryloxyethyl methacrylate) backbone, diblock copolymer poly((3-acryloxypropyl)trialkoxylsilane-b-oligo(ethylene glycol) methacrylate) (PAPTS-b-POEGMA) was synthesized. Hydrolyzed by aqueous ammonium hydroxide, the trimethoxysilane formed silsesquioxane network, which was further converted into silica by calcination.109,267,268 The study was further extended to TiO2 -based 1D structures.109,269 3D mesoporous carbons were prepared using polymer brushes as a template. Similar to a mesoporous silica template, silica particles grafted with a

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2.9 Templates from Stars (a)

(b)

(c)

ATRP

ATRP

APTS

OEGMA

ATRP initiating site

(e)

(d)

Crosslinking of the core

700 °C

O

Si O O Si

Si O O Si O O

Si

O

Si

O

Si

Si

O

O O Si

O

Si O

O Si

Si

Silsesquioxane network

Figure . Templated synthesis of silica nanowire. (a) ATRP macroinitiator PBIEM with DP = 3,200; (b) CPB with side chains of 20 APTS units; (c) core–shell CPB with an additional 57 OEGMA units; (d) soluble organo-silica hybrid nanowires with a cross-linked silsesquioxane network in the core; (e) inorganic silica nanowires after pyrolysis. Source: Yuan et al. 2008.267 Reproduced with permission of Nature Publishing Group.

PAN-based polymer can serve as a hard template for porous carbon. Both PAN and poly(styrene-co-acrylonitrile) were capable precursors for the carbon material.272 The inorganic–polymer hybrid nanoparticles pack into the ordered structure in bulk material. Upon carbonization at elevated temperatures in the absence of oxygen, the polymer brushes were converted into nitrogen-enriched carbon material. Finally, silica nanoparticles were removed by hydrofluoric acid etching. Brunaur–Emmett–Teller (BET) adsorption experiment demonstrated that the specific surface area was 450 m2 /g. In contrast to strategies using nanoparticles as hard templates, core–shell-structured PBA-b-PAN bottlebrushes were used as soft templates to form porous nitrogen-doped carbons via a two-step thermal treatment, where intermolecular cross-links were formed in the PAN block followed by decomposition of the PBA block yielding the mesopores.274

. Templates from Stars Star polymers prepared via ATRP have well-defined spherical shape and controllable size. They were used as effective templates for preparation





2 Polymer Brushes by Atom Transfer Radical Polymerization

of spherical inorganic/hybrid nanoparticles. A universal methodology for preparation of various types of well-defined spherical nanoparticles with solid/core–shell/hollow topologies was developed.46 The “core-first” strategy275 was applied to synthesize the star template. 2-Bromoisobutyryl bromide (2BiB) modified 𝛽-cyclodextrin (𝛽-CD) was used to graft diblock copolymer poly(tert-butyl acrylate-b-styrene) (PtBA-b-PS). The PtBA block was converted to PAA by hydrolysis and used to load various inorganic precursors. Similarly, hollow nanoparticles were prepared from star templates with PSb-PtBA-b-PS arms. A poly(4-vinylpyridine) (P4VP) was used as a secondary functional block in the preparation of core–shell nanoparticles, that is, star templates with P4VP-b-PtBA-b-PS arms were used. Well-defined metal (Au, Ag), oxides (TiO2 , Fe3 O4 , ZnO, Cu2 O), titanates (PbTiO3 , BaTiO3 ), and CdSe nanoparticles were prepared from these star polymer templates.

. Bio-Related Polymer Brushes Synthesis of some polymer brushes was inspired by natural products. Proteoglycans are brush-like polyelectrolytes that consist of a protein backbone with carbohydrate side chains (Figure 2.15). These molecules are found in different places in the human body performing functions including cell signaling and joint lubrication.276,277 Inspired by natural lubricating protein lubricin, a system showing extremely low friction coefficients (∼10−3 ) was designed (Figure 2.16a). In such systems, two mica surfaces were fully covered by the polymer. The polymer adopted a loop conformation giving rise to a weak and long-range repulsive interaction force between the surfaces. Under high compression, stronger repulsive forces appear due to the strong compression of the grafted pendant chains of the polymer.278 Similarly, the solution containing molecular brushes and linear (a)

(b)

(c)

–

COO H H OH

500 nm

H

CH2OH SO3– O H H β H O H H H H NHCOCH3 n=10-50 O OH O

100 nm

Figure . Structural hierarchy of proteoglycan aggregate in cartilage9 . (a) TEM micrograph of the aggregate, (b) protein backbone with glucosaminoglycan side chains, and (c) chemical structure of the disaccharide repeating unit of the side chains. Source: Sheiko et al. 2008.1 Reproduced with permission of Elsevier.

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2.10 Bio-Related Polymer Brushes LUBRICIN

(a)

SMB-like domain

Mucin-like domain

PEX-like domain

COOH

H2N

~2-3 nm

~200 nm

~2-3 nm

ADHESIVE

REPULSIVE

ADHESIVE

(b)

ABA Bottlebrush DP~800

DP~185

Cationic domain (qDMAEMA) (c)

O

O

O

95 O O Br–

N

200 O O

N+ O– P O O

O

Cationic domain (qDMAEMA)

Bottlebrush domain (PMPC)

H 90 OO

DP~185

200 O O

O

O O

45 Cl

O

H

O

200 200 O O O O

O

O

Cl 45

O– O P O

O O

90 O

N+ Br–

O

O

95 OO

N

Figure . Schematic representations of the protein lubricin found in mammalian synovial fluids (a) and the bottlebrush polymer mimicking lubricin (b) and (c). Source: Banquy et al. 2014.278 Reproduced with permission of American Chemical Society.

polymers was used to provide wear protection between surfaces via forming the boundary film due to entanglements between both polymers.279 In addition to 1D brushes (molecular bottlebrushes), bio-related application based on 2D brushes on surfaces has also been studied. The permanent and nonleaching antibacterial glass, paper, or polymer surfaces were prepared by growing an antimicrobial polymer directly from the surfaces using anchored ATRP initiators.202,203 As shown in Schemes 2.19 and 2.20, after functionalized with ATRP initiators, surfaces of different chemical composition were modified with PDMAEMA with a cationic nature permitting them to serve as permanent highly effective biocides.202,203





2 Polymer Brushes by Atom Transfer Radical Polymerization O OH

O

O

Br

Br

CH2CH3

Br

Br

O O

Triethylamine, Chloroform, rt.

OH

CH2CH3

O

DMAEMA, Cu(I)Br / bpy 1,2-Dichlorobenzene, 80°C, 48 hr O O

(

CH2

CH3(CH2)mBr

O

O O

O

)n

O

( CH

)n

2

O

O Nitromethane

CH2CH3 O

O

CH2CH3

O

(CH2)mCH3 N+

Br –

N

Scheme . Modification of surfaces with PDMAEMA. Source: Lee et al., 2004.202 Reproduced with permission of American Chemical Society.

Immobilization of proteins with high binding capacities on surfaces while maintaining their activity is highly desirable for fabrication protein microarrays and other biotechnological applications.280 Immobilization of ribonuclease A (RNase A, commonly used to remove RNA from plasmid DNA) using PAA brushes on silicon surfaces has been reported. Such modification was achieved via both covalent esterification and a high binding capacity of metal–ion complexation method to PAA brushes (Scheme 2.21). The polymer brushes immobilized 30 times more enzyme, as compared to self-assembled

Scheme . Synthesis of 2-benzophenonyl bromoisobutyrate and benzophenone chemistry (a) and a schematic drawing of surface-initiated ATRP of DMAEMA (b). Source: Huang et al. 2007.203 Reproduced with permission of American Chemical Society.

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0.1 M EDC 0.1 M NHS H2O, 30 min (CH2-CH)n Br O O O N

Protein Immobilization (Method A or B)

Method A: R=

SiOx

O Cl Si (CH2)2 O Cl

Br

t-BA/CuBr PMDETA/e-2-bib acetone, 80°C, 24 h

O Cl Si (CH2)2 O Cl

SiOx

SiOx

O Cl Si (CH2)2 O Cl

SiOx

2.10 Bio-Related Polymer Brushes

O Cl Si (CH2)2 O Cl

NH

(CH2-CH)n Br O O (CH3)3 200°C, 30 min

(CH2-CH)n Br O OH O

Method B: R= NH

O

O

N

SiOx

O

Cl Si (CH2)2 O Cl

O

RNase A (CH2-CH)n Br O R

O

Cu N NH

RNase A

O N NH

RNase A

Scheme . Immobilization of RNase A by metal–ion complexation and covalent immobilization. Source: Cullen et al. 2008.281 Reproduced with permission of American Chemical Society.

monolayers, which could potentially lead to a decrease in detection limits of protein microarrays.281 Since the selective detection of analytes over a wide range of concentrations is critical for biosensors, their performances are significantly affected by the strength of interaction between the analyte and sensor surface and the absence of unwanted nonspecific binding. Therefore, the specific architecture of polymer brush provides an important solution to “screen” such adsorption of proteins and other molecules on the sensor surface.182 Anti-Salmonella antibodies were immobilized onto poly(sulfobetaine methacrylate) (SBMA)-grafted Six N4 surfaces through these (N-hydroxysuccinimide) linkers (Scheme 2.22). The fluorescent images showed that antibody-coated poly-SBMA efficiently captured Salmonella with only low background noise as compared to antibodycoated monolayers lacking the polymer brush. Upon exposure to a mixture of Alexa Fluor 647-labeled fibrinogen (FIB) and Salmonella, the fluorescent images showed the capture of Salmonella with no adsorption of FIB, demonstrating the prevention of nonspecific adsorption toward proteins by the zwitterionic layer during the detection of bacteria in complex matrices.282

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2 Polymer Brushes by Atom Transfer Radical Polymerization

Scheme . Procedure for the attachment of anti-Salmonella antibodies to poly-SBMA-coated Six N4 surfaces. Source: Nguyen et al. 2012.282 Reproduced with permission of American Chemical Society.

Due to excellent properties (load bearing, low density, and biocompatibility), titanium and titanium-based alloys have been widely used as artificial implants in dental, bone replacement, and orthopedic surgery.283 Furthermore, capability of inducing osteogenesis is essential for better performance in medical application. By grafting poly(OEGMA-r-HEMA) brushes via SI-ATRP as a film on a Ti surface, resistance against cell adhesion was achieved. After immobilization of fibronectin (FN) and recombinant human bone morphogenetic protein-2 (rhBMP-2) onto p(OEGMA-r-HEMA) matrices, polymer surfaces could induce the adhesion of MC3T3 cells on Ti surfaces (Scheme 2.23). MC3T3 is an osteoblast precursor cell line derived from Mouse musculus calvaria. Moreover, the protein-tethered surface exhibited enhanced cell differentiation in terms of alkaline phosphatase activity compared to that of the pristine Ti surface at similar cell proliferation rates.284 SI-ATRP provides valuable solutions to improving performance of biomedical devices/techniques by utilizing the properties of inorganic nanoparticles. Among the techniques for phosphopeptide enrichment, immobilized metal affinity chromatography (IMAC) is a powerful method. However, the lack of high specificity and large binding capacity of conventional IMAC materials still limit sensitive detection of phosphopeptide.285 Significant improved enrichment specificity for the phosphopeptide was reported using Fe3 O4 @SiO2 core– shell nanoparticle decorated with PEGMA brushes bearing titanium phosphate moieties (Scheme 2.24). While the inorganic core allowed enrichment with magnetic field, the PEG brushes bearing both nonfouling property and the enhanced binding capacity enabled a high phosphopeptide recovery (over 70%) and low limit of detection (0.5 fmol).286

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Ti HS(CH2)10CH3

(

O

OEGMA/HEMA

O

CuCl/Asca/Bipy

Ti

O(CH2CH2O)n-H O O OH O

S(CH2)11O

Br

S(CH2)11O

O

HS(CH2)11O

S(CH2)10CH3

Br

S(CH2)10CH3

)x(

)y

2.10 Bio-Related Polymer Brushes

Ti

Protein

(

)x(

HO N Protein O

Protein

NHS/EDC

O O(CH2CH2O)n COOH O O O COOH O

S(CH2)10CH3

H N

S(CH2)11O

S(CH2)10CH3

O

S(CH2)11O

(

)x(

)y

O O(CH2CH2O)n O O O O

)y

Ti

Succinic Anhydride DMAP

Ti

Scheme . A protein-decorated Ti surface was prepared in four steps, and the formation of an initiator SAM and SIP as well as carboxylation led to a carboxylated poly(OEGMA-r-HEMA) coating, to which proteins were immobilized. Source: Ren et al. 2011.284 Reproduced with permission of American Chemical Society.

In addition to inorganic materials, the substrates for modification with polymer brushes via ATRP can also be proteins. Polymer-based protein engineering (PBPE) offers an attractive method to add novel functions to protein by predictably influencing their activity and stability.287 Biopharmaceuticals increasingly rely on uniform protein–polymer conjugates for consistent and sustainable therapeutic effects.288,289 By initiating ATRP of poly(ethylene oxide) methacrylate from a chymotrypsin (CT, protein) –initiator, preparation of near-uniform protein–polymer has been reported with the −D of polymer chain per protein molecule as low as 1.05.290 Such a protein-engineering process is expected to be highly versatile because the only major requirement is accessible functional groups (i.e., –NH2 ) on the surface to couple the initiator molecules. The functional-group tolerance of ATRP allows people to simply utilize responsiveness of polymer to tune the property of protein. For example, the lower critical solution temperature (LCST)- or upper critical solution temperature (UCST)-induced collapse of polymer chains on enzyme bioconjugates resulted in the suppression of their biocatalytic activity (Figure 2.17).287 Also, good chain-end functionality afforded by ATRP also allowed the combination of LCST and UCST in one bioconjugate by grafting diblock copolymers from



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4–

Br

(b)

PEG brushes shell

Fe3O4@SiO2-Br

Fe3O4@SiO2@PEG

ATRP of PEG

Br Br Br Br Br

Nano LC-MS/MS analysis

MALDI-TOF-MS analysis

phosphopeptides nonphosphopeptides 4– Fe3O4@SiO2@PEG-Ti

Incubation

Add 12.5% NH3 . H2O Elution Lyophilized to dryness and redissolved

Discard the supernatant

Collect the supernatant

Apply a magnet

Discard the supernatant

Washing

Scheme . (a) Synthesis of the poly(ethylene glycol) (PEG) brush-decorated magnetic Fe3 O4 @SiO2 nanoparticle and the immobilization of Ti4+ ion to form Fe3 O4 @SiO2 @PEG–Ti4+ ; (b) workflow of phosphopeptides enrichment from a biological sample using Fe3 O4 @SiO2 @PEG–Ti4+ IMAC nanoparticle. Source: Zhao et al. 2012.286 Reproduced with permission of American Chemical Society.

Fe3O4@SiO2@PEG-Ti

1.CDI,NH2(CH2)2NH2 2.POCL3. priddine 3.Ti(SO4)2

Derivation

Fe3O4@SiO2 core/shell

and initiator Br

Modify with amine

Br Br Br

Br

PEG-NH-(CH2)2-NH-PO3-Ti4+ brushes shell

Fe3O4core

Coat with SiO2

Br

Br Br Br

Br Br Br Br Br

(a) Br

2.10 Bio-Related Polymer Brushes Above UCST

Below UCST

O

O

Above UCST

+

N

N O S – O O

5 °C

O

+

N

(kcat/KM)x : 0.79 (kcat/KM)CT

O

O

O

O S – O O

25 °C (kcat/KM)x : 1.08 (kcat/KM)CT

+

O S – O O

40 °C (kcat/KM)x : 0.96 (kcat/KM)CT

CT-pNIPAm conjugate: LCST at ~30°C Below LCST

Below LCST

O

Above LCST

NH

O

NH O

5 °C (kcat/KM)x : 0.77 (kcat/KM)CT

25 °C (kcat/KM)x : 0.53 (kcat/KM)CT

NH

40 °C (kcat/KM)x : 0.12 (kcat/KM)CT

Figure . Schematic of bioconjugate conformation and its impact on kcat /KM as a function of temperature. At 5◦ C, the polymer component of CT–PDMAPS was in a collapsed, hydrophobic state. At 40◦ C, the polymer component of CT-PNIPAm was also collapsed and hydrophobic. At 25◦ C, both CT-PDMAPS and CT-PNIPAm polymers were in their extended and hydrophilic state. Source: Cummings et al. 2013.287 Reproduced with permission of Elsevier.

enzyme surface (Figure 2.18). The LCST temperature of PNIPAm is between room temperature and body temperature. Thus, it is a good candidate to incorporate into materials that need to be synthesized in aqueous solution at room temperature, but then change behavior once in the body. Attaching poly[N,N’dimethyl(methacryloylethyl) ammonium propane sulfonate] (PDMAPS) with UCST to an enzyme can potentially increase stability at low temperature as well as long-term storage time before use.291 In addition to temperature, the conformation of PDMAEMA was also responsive toward the change of pH due to protonation or deprotonation. At lower pH, protonation of the amino group leads to the stretching of polymer chains. The chymotrypsin–PDMAEMA conjugates have higher relative enzyme activities as compared to native CT below pH 8. Especially, the conjugates had a 10-fold higher enzyme activity than native enzyme at pH 5.0.287 Similarly, enzyme activity of chymotrypsin–quaternized poly(2(dimethylamino)ethyl methacrylate) conjugates at low pH was increased compared to native CT due to charge stabilization of the active site catalytic triad.292 Also, selective binding of positively charged polypeptide protease inhibitors was achieved utilizing electrostatic attraction the cationic shell and the inhibitor while the negatively charged inhibitors were repelled.292





2 Polymer Brushes by Atom Transfer Radical Polymerization Above UCST and below LCST

Below UCST

Above LCST

CT

CT

CT

)

)

O

NH

pSBAm

) O

)

pNIPAm

Suc-AAPF-pNA (CT substrate)

NH

N+ O S

O

O

Figure . Schematic of the hypothesized effect of poly(sulfobetaine methacrylamide) (PSBAm) and PNIPAm polymer collapse on substrate affinity (KM ). At 25◦ C, both PSBAm and PNIPAm were in their extended conformation and allowed N-succinyl-L-alanine-L-alanineL -proline- L -phenylalanine-p-nitroanilide) (Suc-AAPF-pNA) access to a CT-active site. At temperatures below PSBAm UCST and above PNIPAm LCST, polymer collapse inhibited access to the active site for Suc-AAPF-pNA due to steric blocking. At temperatures below PSBAm UCST, this effect is hypothesized to be more pronounced than at temperatures above pNIPAm LCST, because the PSBAm block was closer to the enzyme core than the pNIPAm block. Source: Cummings et al. 2014.291 Reproduced with permission of American Chemical Society.

Although PEGylation has been the benchmark for achieving protein stabilization and increased body circulation times,293,294 binding affinity is often compromised and overall bioactivity is reduced.295,296 In contrast, poly(carboxybetaine) (PCB) conjugation has been shown to improve stability in a manner similar to PEGylation, whereas the new conjugates retain or even improve the binding affinity as a result of enhanced protein–substrate hydrophobic interactions.297 Unlike PEG that impeded enzyme–substrate hydrophobic–hydrophobic interactions as a result of its amphiphilic characteristics, PCB created a local environment that increased enzyme–substrate hydrophobic–hydrophobic interactions (Figure 2.19). As a result, substrate’s affinity for the binding pocket was increased.

. Stimuli-Responsive Polymer Brushes Stimuli-responsiveness was successfully incorporated into polymer brushes (Figure 2.20). Several types of stimuli were efficiently used, that is temperature,298,299 light,111 pH,298 ionic strength,300,301 or electric field.302 In some cases, polymers could respond to two or more different types of stimuli,

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2.11 Stimuli-Responsive Polymer Brushes (a)

(b)

PEG O O H 2O

CT H2O

H 2O H2O

H2O

O O

H2O O

O

H2O H2O Inhibition

(c)

Zwitterions H2O – + + – HO 2

H2O + –

– +

Promotion

Figure . Mechanism of how PEG and PCB polymers influence binding affinity. (a) Relationship between enzyme and substrate without polymer. (b) PEG impedes affinity by reducing enzyme–substrate hydrophobic–hydrophobic interactions as a result of its amphiphilic characteristics. (c) Super-hydrophilic PCB has a strong effect on the structure of water, creating a local environment that increases enzyme–substrate hydrophobic– hydrophobic interactions, thereby increasing the substrate’s affinity for the binding pocket. Source: Keefe and Jiang 2012.297 Reproduced with permission of Nature Publishing Group.

Figure . Stimuli, response, and applications of stimuli-responsive polymer brushes.



2 Polymer Brushes by Atom Transfer Radical Polymerization P(BPEM486-graft-(DMA17-stat-BA5)) Mn = 1.13 x 106 g/mol 110 100 Rh(nm)

1.0 w/v %

90 80

Δ

70 60 50 10

Intermolecular 20

30 40 T(°C)

50

60 60 58

Δ

0.1 w/v %

56 Rh(nm)



54 52 50 48 46 10

Intramolecular

20

30 40 T(°C)

50

60

Figure . Apparent hydrodynamic average diameters as a function of temperature for poly(BPEM-graft-DMA-stat-BA), (Mn,th = 1,130,000), molecular brush, 0.1 and 1.0 wt% solutions concentrations. Source: Lee et al. 2010.35 Reproduced with permission of Elsevier.

depending on their composition, that is temperature and light111 or pH.298 The main types of stimuli applied in biomedical field are temperature, pH, and ionic strength. Variation of the environment of stimuli-responsive brushes affects both chemical and physical material properties. Some relevant examples of stimuli-responsive polymer brushes are discussed below, and more information could be found in several reviews.303–307 ..

Stimuli-Responsive Solutions

Molecular brushes (bottlebrushes) are unique materials because their reversible conformational changes, in response to external stimuli, can be limited to and furthermore observed within the single molecule (Figure 2.21).35,37 Due to extremely high congestion between the side chains and constraint structure, molecular brushes have unique properties, not achievable in other systems.1,308 Stimuli-responsive molecular brushes can act as unimolecular micelles and be used for the controlled encapsulation and release of therapeutic agents.309,310 Since the side chains are covalently linked to a backbone, they do not exhibit any critical micelle concentration.132 Thermoresponsive side chains in molecular brushes include poly(Nisopropylacrylamide) (PNIPAm),311 poly(2-(dimethylamino)ethyl methacrylate)) (PDMAEMA),312 poly(oligo(ethylene glycol) methyl ether methacrylate

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2.11 Stimuli-Responsive Polymer Brushes

(POEGMA)),313 or their copolymers. Polymers with tunable LCST are of increasing interests for biological applications, such as cell patterning,314 smart drug release,315 or DNA sequencing.316,317 Temperature of phase transition can be tuned by variation of molecular weight,318 chain end groups,319,320 or the polymer architecture.321,322 The first temperature-responsive molecular brushes employed poly(Nisopropylacrylamide) side chains.323 They showed a transition from single macromolecules to aggregates within a relatively small temperature range (20–38◦ C). In another example, 2-(dimethylamino)ethyl methacrylate) (DMAEMA) or N,N-dimethylacrylamide was copolymerized with nbutyl acrylate units in the side chains and demonstrated the concentrationdependent thermal transition.37 Polymers with oligo(ethylene glycol) side chains can be considered as an alternative for PNIPAm as temperature-responsive segments in biomaterials area because PEO is a nontoxic polymer used in many biomaterials applications, that is biosensors or drug delivery systems. Molecular brushes with statistical or block copolymers of di(ethylene glycol) methyl ether methacrylate (MEO2 MA) and tri(ethylene glycol) methyl ether methacrylate (MEO3 MA) were prepared via ATRP.324,325 For brushes with statistical copolymer side chains, the LCST increased with the molar fraction of MEO3 MA in the side chain. However, for brushes with block copolymer side chains, the cloud point displayed two stages of aggregation during heating, resulting from both intermolecular and intramolecular aggregation. This behavior was strongly dependent on the sequence of the side chain segments, and it was not possible to observe a similar effect for linear analogs. Molecular brushes with pH responsiveness can contain ionic or ionizable side chains such as polyacids326,327 [i.e., poly(styrene sulfonate), PAA] and polybases328,329 (i.e., PDMAEMA, poly(4-vinyl pyridine)) donating or accepting protons upon pH changes. A change in the degree of ionization, due to pH variation, leads to conformational changes of single molecular brush molecule. Polyelectrolyte brushes can mimic proteoglycans molecules present in the human body.276,330 Synthetic models of proteoglycans were based on cylindrical polyelectrolyte brushes with poly(styrenesulfonate) side chains.331,332 Loosely grafted PAA with different grafting densities can act in a similar way.327 A globule-to-extended conformation transition on the mica surface for these brushes was monitored and quantified by AFM. Molecular brushes with PDMAEMA side chains also changed structures upon variation of pH. Salt-responsive molecular brushes were less explored. Quaternized PDMAEMA brushes exhibited conformational switch in response to concentrations of mono-, di-, and trivalent salts through electrostatic screening or through ionic complexion.333 Cationic molecular brushes with polynorbornene backbones and PCL side chains containing quaternary ammonium groups showed ionic-strength-dependent stimuli responsiveness.300

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2 Polymer Brushes by Atom Transfer Radical Polymerization

Dual-responsive brushes may be used in drug or protein delivery. They can be prepared by copolymerization of monomers with different stimuli responsiveness, such as thermal and pH,333 thermal and photo,111 pH and redox,333 or ionic strength and photo.334 ..

Stimuli-Responsive Surfaces

Several stimuli-responsive surfaces were prepared by chemical grafting,335,336 which include grafting reversibly swollen/collapsed polymers,335,337 binary (mixed) brushes,338,339 or block copolymers.336,339 The collapsed or expanded chains affect surface morphologies, hardness, friction, and adhesion properties.335 In a case of particles, some irreversible aggregation can be generated upon the changes in the environment.231 The behavior and stimuli-responsive properties of polymer brushes attached to the surface depend mostly on the polymer composition, grafting density, and molecular weight.340 Due to the anchoring of one end of polymer chains at the surface, the freedom of the chain movement is partially limited. Computer simulations showed that high graft density homopolymer brushes reversibly switch surface properties more efficiently than low graft density polymers.341 For low grafting densities, the nature of the substrate also plays an important role and affects properties. By grafting two incompatible polymer chains onto the surface, heterogeneous binary brushes were created. The surface chemistry together with surface morphology could affect phase separation governed by solvent quality.338,342 Mixed polyelectrolyte brushes consisting of PAA and poly(2-vinylpyridine) (P2VP) showed a bipolar permselective behavior.343 Such brushes underwent transition upon variation of pH from the positively charged to the negatively charged state. These systems differ principally from homopolymer brushes or grafted homopolymers with selected stimuliresponsive groups, that is, temperature and pH. PNIPAm, POEGMA, poly(2-alkyloxazoline), and their copolymers are well suited as temperature-responsive polymer brushes. Adjusting polymer compositions can fine-tune phase transitions. Laser light scattering done for PNIPAm grafted on silica nanoparticles showed a two-stage decrease of the intensityaverage hydrodynamic radius and average radius of gyration of nanoparticles upon heating.231,344 The first transition occurred in a temperature range of 20– 30◦ C, which can be assigned to the n-cluster induced collapse of the inner zone of PNIPAm brushes. N-cluster concept was developed by de Gennes and coworkers.345 The collapse of the outer zone of the PNIPAm brushes showed a second phase transition above 30◦ C. Temperature-responsive polymer brushes are appropriate for temperaturemodulated cell adhesion and detachment. The key advantage of those systems

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2.12 Conclusion

is that they do not require digestion with enzymes that may harm and disintegrate tissue cells.346 The brush thickness, grafting density, and position of cell adhesive sites within the brush are parameters that should be also considered.347 Thermally modulated wettability property, which is observed for such systems, enabled temperature-dependent interactions with other bioactive compounds. For example, PNIPAm-modified silica beads could be an effective packing material for high-performance liquid chromatography.348 Similar to molecular brushes, pH-responsive polymer brushes contain polyacids349,350 or polybase units,329,351 such as PAA, PMAA, PDMAEMA, or P4VP. Due to strong repulsive forces between charged polymer chains and associated osmotic pressure, they can achieve high degree of swelling. This is one of the hallmarks of such systems. At high ionic strength, swelling decreases with increasing salt concentration. Proteins can be regarded as very special bionanoparticles. As discussed in the preceding section, the enzyme chymotrypsin was modified at its lysine residues with an ATRP initiator. Stimuli-responsive polymers such as PNIPAm, PDMAPS, and PDMAEMA were grafted from the protein.287,291,292 Also, block copolymers with different UCST or LCST values, respectively, were attached to chymotrypsin. The polymer shell P(SBAM-b-NIPAM) was created by the grafting-from approach and responded to both lower and higher temperatures. Stimuli-responsive polymer brushes represent one of the great challenges concerning their synthesis and design process. Polymer brush conformation is modulated by a variety of parameters, and understanding their role is essential to control the polymer brushes stimuli responsiveness.

. Conclusion Driven by the development of CRP techniques such as ATRP, the control over molecular weights and architectures has significantly facilitated the preparation of well-defined polymer brushes. Realization of many expected desirable properties in polymer brushes requires the congested environment created by high grafting density. ATRP and other CRP methods are particularly adaptable for the fabrication of polymer brushes, especially via “grafting-from” approaches. While many applications for polymer brushes have been proposed and realized, further investigation of surface, bulk, and solution behavior as well as new synthetic pathways should be carried out in order to fully explore the potential of these materials. For example, typical synthesis of molecular brushes via the “grafting-from” method requires both limited monomer conversion and slow polymerization to prevent macroscopic gelation arising from intermolecular

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2 Polymer Brushes by Atom Transfer Radical Polymerization

termination between growing side chains. However, this limitation was circumvented by confining different growing brushes into miniemulsion droplets352 or plausibly could be extended to high-pressure chemistry. Although impressive progress has been achieved in the preparation polymer brushes with unique properties that conventional linear polymer cannot provide, new possibilities are still waiting for further exploration in this field of material science.

Acknowledgments Support from the National Science Foundation (via grants DMR 1501324, DMR 1436219, and DMR-1410845) as well as the Department of Energy (via grant DE-EE0006702) is acknowledged. J.P. acknowledges the financial support from National Science Center (via grants DEC-2012/04/M/ST5/00805, UMO2011/03/B/ST5/01084, UMO-2014/14/A/ST5/00204).

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 Kaneider, N. C.; Dunzendorfer, S.; Wiedermann, C. J. Biochemistry 2004, 43, 237–244.  Swann, D. A.; Sotman, S.; Dixon, M.; Brooks, C. Biochem. J. 1977, 161, 473–485.  Banquy, X.; Burdy´nska, J.; Lee, D. W.; Matyjaszewski, K.; Israelachvili, J. J. Am. Chem. Soc. 2014, 136, 6199–6202.  Faivre, J.; Shrestha, B. R.; Burdynska, J.; Xie, G.; Moldovan, F.; Delair, T.; Benayoun, S.; David, L.; Matyjaszewski, K.; Banquy, X. ACS Nano 2017, 11, 1762–1769.  Xu, F. J.; Cai, Q. J.; Li, Y. L.; Kang, E. T.; Neoh, K. G. Biomacromolecules 2005, 6, 1012–1020.  Cullen, S. P.; Liu, X.; Mandel, I. C.; Himpsel, F. J.; Gopalan, P. Langmuir 2008, 24, 913–920.  Nguyen, A. T.; Baggerman, J.; Paulusse, J. M. J.; Zuilhof, H.; van Rijn, C. J. M. Langmuir 2012, 28, 604–610.  Geetha, M.; Singh, A. K.; Asokamani, R.; Gogia, A. K. Prog. Mater. Sci. 2009, 54, 397–425.  Ren, X.; Wu, Y.; Cheng, Y.; Ma, H.; Wei, S. Langmuir 2011, 27, 12069– 12073.  Chen, H.; Deng, C.; Li, Y.; Dai, Y.; Yang, P.; Zhang, X. Adv. Mater. 2009, 21, 2200–2205.  Zhao, L.; Qin, H.; Hu, Z.; Zhang, Y.; Wu, R. a.; Zou, H. Chem. Sci. 2012, 3, 2828–2838.  Cummings, C.; Murata, H.; Koepsel, R.; Russell, A. J. Biomaterials 2013, 34, 7437–7443.  Rosendahl, M. S.; Doherty, D. H.; Smith, D. J.; Carlson, S. J.; Chlipala, E. A.; Cox, G. N. Bioconjugate Chem. 2005, 16, 200–207.  Grace, M. J.; Lee, S.; Bradshaw, S.; Chapman, J.; Spond, J.; Cox, S.; DeLorenzo, M.; Brassard, D.; Wylie, D.; Cannon-Carlson, S.; Cullen, C.; Indelicato, S.; Voloch, M.; Bordens, R. J. Biol. Chem. 2005, 280, 6327–6336.  Lele, B. S.; Murata, H.; Matyjaszewski, K.; Russell, A. J. Biomacromolecules 2005, 6, 3380–3387.  Cummings, C.; Murata, H.; Koepsel, R.; Russell, A. J. Biomacromolecules 2014, 15, 763–771.  Murata, H.; Cummings, C. S.; Koepsel, R. R.; Russell, A. J. Biomacromolecules 2014, 15, 2817–2823.  Jevˇsevar, S.; Kunstelj, M.; Porekar, V. G. Biotechnol. J. 2010, 5, 113–128.  Harris, J. M.; Chess, R. B. Nat. Rev. Drug Discov. 2003, 2, 214–221.  Fishburn, C. S. J. Pharm. Sci. 2008, 97, 4167–4183.  Veronese, F. M. Biomaterials 2001, 22, 405–417.  Keefe, A. J.; Jiang, S. Nat. Chem. 2012, 4, 59–63.  Yamamoto, S.-i.; Pietrasik, J.; Matyjaszewski, K. Macromolecules 2008, 41, 7013–7020.

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References

 Nomura, K.; Makino, H.; Nakaji-Hirabayashi, T.; Kitano, H.; Ohno, K. Colloid Polym. Sci. 2015, 293, 851–859.  Yao, K.; Chen, Y.; Zhang, J.; Bunyard, C.; Tang, C. Macromol. Rapid Commun. 2013, 34, 645–651.  Willott, J. D.; Murdoch, T. J.; Humphreys, B. A.; Edmondson, S.; Webber, G. B.; Wanless, E. J. Langmuir 2014, 30, 1827–1836.  Kutnyanszky, E.; Hempenius, M. A.; Vancso, G. J. Polym. Chem. 2014, 5, 771–783.  Guo, J.; Peng, L.; Yuan, J. Eur. Polym. J. 2015, 69, 449–459.  Hadasha, W.; Klumperman, B. Polym. Int. 2014, 63, 824–834.  Christau, S.; Genzer, J.; von Klitzing, R. Z. Phys. Chem. 2015, 229, 1089–1117.  Hadjesfandiari, N.; Yu, K.; Mei, Y.; Kizhakkedathu, J. N. J. Mater. Chem. B 2014, 2, 4968–4978.  Wu, L.; Glebe, U.; Boeker, A. Polym. Chem. 2015, 6, 5143–5184.  Sumerlin, B. S.; Matyjaszewski, K. Macromol. Eng. 2007, 2, 1103–1135.  Zhao, P.; Liu, L.; Feng, X.; Wang, C.; Shuai, X.; Chen, Y. Macromol. Rapid Commun. 2012, 33, 1351–1355.  Bajpai, A. K.; Shukla, S. K.; Bhanu, S.; Kankane, S. Prog. Polym. Sci. 2008, 33, 1088–1118.  Skvarla, J.; Zednik, J.; Slouf, M.; Pispas, S.; Stepanek, M. Eur. Polym. J. 2014, 61, 124–132.  Han, X.; Zhang, X.; Zhu, H.; Yin, Q.; Liu, H.; Hu, Y. Langmuir 2013, 29, 1024–1034.  Du, Z.; Sun, X.; Tai, X.; Wang, G.; Liu, X. RSC Adv. 2015, 5, 17194–17201.  Yamato, M.; Akiyama, Y.; Kobayashi, J.; Yang, J.; Kikuchi, A.; Okano, T. Prog. Polym. Sci. 2007, 32, 1123–1133.  Bae, Y. H.; Okano, T.; Hsu, R.; Kim, S. W. Makromol. Chem., Rapid Comm. 1987, 8, 481–485.  Nagase, K.; Kobayashi, J.; Kikuchi, A.; Akiyama, Y.; Kanazawa, H.; Okano, T. RSC Adv. 2015, 5, 66155–66167.  Albarghouthi, M. N.; Barron, A. E. Electrophoresis 2000, 21, 4096–4111.  Otake, K.; Inomata, H.; Konno, M.; Saito, S. Macromolecules 1990, 23, 283–289.  Furyk, S.; Zhang, Y.; Ortiz-Acosta, D.; Cremer, P. S.; Bergbreiter, D. E. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 1492–1501.  Huber, S.; Hutter, N.; Jordan, R. Colloid Polym. Sci. 2008, 286, 1653–1661.  Baltes, T.; Garret-Flaudy, F.; Freitag, R. J. Polym. Sci., Part A: Polym. Chem. 1999, 37, 2977–2989.  Satokawa, Y.; Shikata, T.; Tanaka, F.; Qiu, X.-p.; Winnik, F. M. Macromolecules 2009, 42, 1400–1403.  Li, C.; Gunari, N.; Fischer, K.; Janshoff, A.; Schmidt, M. Angew. Chem., Int. Ed. 2004, 43, 1101–1104.

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 Yamamoto, S.-i.; Pietrasik, J.; Matyjaszewski, K. Macromolecules 2007, 40, 9348–9353.  Yamamoto, S.-I.; Pietrasik, J.; Matyjaszewski, K. J. Polym. Sci., Part A: Polym. Chem. 2007, 46, 194–202.  Schuwer, N.; Klok, H.-A. Langmuir 2011, 27, 4789–4796.  Lee, H.-i.; Boyce, J. R.; Nese, A.; Sheiko, S. S.; Matyjaszewski, K. Polymer 2008, 49, 5490–5496.  Pietrasik, J.; Tsarevsky, N. V. Eur. Polym. J. 2010, 46, 2333–2340.  Fielding, L. A.; Edmondson, S.; Armes, S. P. J. Mater. Chem. 2011, 21, 11773–11780.  Muir, H. Biochem. Soc. Trans. 1983, 11, 613–622.  Lienkamp, K.; Noe, L.; Breniaux, M.-H.; Lieberwirth, I.; Groehn, F.; Wegner, G. Macromolecules 2007, 40, 2486–2502.  Lienkamp, K.; Ruthard, C.; Lieser, G.; Berger, R.; Groehn, F.; Wegner, G. Macromol. Chem. Phys. 2006, 207, 2050–2065.  Xu, Y.; Bolisetty, S.; Drechsler, M.; Fang, B.; Yuan, J.; Ballauff, M.; Mueller, A. H. E. Polymer 2008, 49, 3957–3964.  Xu, Y.; Bolisetty, S.; Drechsler, M.; Fang, B.; Yuan, J.; Harnau, L.; Ballauff, M.; Muller, A. H. E. Soft Matter 2009, 5, 379–384.  Jones, D. M.; Smith, J. R.; Huck, W. T. S.; Alexander, C. Adv. Mater. 2002, 14, 1130–1134.  LeMieux, M. C.; Lin, Y.-H.; Cuong, P. D.; Ahn, H.-S.; Zubarev, E. R.; Tsukruk, V. V. Adv. Funct. Mater. 2005, 15, 1529–1540.  Takei, Y. G.; Aoki, T.; Sanui, K.; Ogata, N.; Sakurai, Y.; Okano, T. Macromolecules 1994, 27, 6163–6166.  Minko, S.; Muller, M.; Usov, D.; Scholl, A.; Froeck, C.; Stamm, M. Phys. Rev. Lett. 2002, 88, 035502.  Julthongpiput, D.; Lin, Y.-H.; Teng, J.; Zubarev, E. R.; Tsukruk, V. V. J. Am. Chem. Soc. 2003, 125, 15912–15921.  Aumsuwan, N.; Danyus, R. C.; Heinhorst, S.; Urban, M. W. Biomacromolecules 2008, 9, 1712–1718.  Merlitz, H.; He, G.-L.; Wu, C.-X.; Sommer, J.-U. Phys. Rev. Lett. 2009, 102, 115702.  Minko, S.; Mueller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896–3900.  Motornov, M.; Tam, T. K.; Pita, M.; Tokarev, I.; Katz, E.; Minko, S. Nanotechnology 2009, 20, 434006.  Wu, T.; Zhang, Y.; Wang, X.; Liu, S. Chem. Mater. 2008, 20, 101–109.  Wagner, M.; Brochard-Wyart, F.; Hervet, H.; de Gennes, P. G. Colloid Polym. Sci. 1993, 271, 621–628.  Canavan, H. E.; Cheng, X.; Graham, D. J.; Ratner, B. D.; Castner, D. G. J. Biomed. Mater. Res., Part A 2005, 75A, 1–13.  Halperin, A.; Kroger, M. Biomaterials 2012, 33, 4975–4987.

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 Z. J. Liu, Y. L. L.; F. F. Geng, F. Lv, R. J. Dai, Deng, Y. K. Z. a. Y. L. Front. Mater. Sci. 2012, 6, 60–68.  Dong, R.; Lindau, M.; Ober, C. K. Langmuir 2009, 25, 4774–4779.  Israels, R.; Gersappe, D.; Fasolka, M.; Roberts, V. A.; Balazs, A. C. Macromolecules 1994, 27, 6679–6682.  Weir, M. P.; Heriot, S. Y.; Martin, S. J.; Parnell, A. J.; Holt, S. A.; Webster, J. R. P.; Jones, R. A. L. Langmuir 2011, 27, 11000–11007.  Min, K.; Yu, S.; Lee, H.-i.; Mueller, L.; Sheiko, S. S.; Matyjaszewski, K. Macromolecules 2007, 40, 6557–6563.  Zhou, D.; Gao, X.; Wang, W.-J.; Zhu, S. Macromolecules 2012, 45, 1198–1207.

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 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions Tuncer Caykara Department of Chemistry, Faculty of Science, Gazi University, Besevler, Ankara, Turkey

. Introduction Functionalization of solid substrates is central to many areas of research, ranging from microelectronics to biomaterials.1,2 Functional polymer brushes on solid substrates are desired for proving control of reversible surface properties with external stimuli (e.g., pH, temperature, solvent, and ionic strength). Different conformations of polymer chains on solid substrates including mushroom and brush are possible with end-anchored functional chains by varying grafting density and media.3,4 The control of conformational characteristics is allowed for advances in areas such as surface friction modulation,5 antifouling,6–8 lubricity,9–11 switchable wettability,12,13 and autophobicity14 to name but a few. Synthesis of functional polymer brushes is especially carried out by one of surface-mediated reversible addition-fragmentation chain transfer (RAFT) polymerization techniques, namely surface-initiated RAFT polymerization and interface-mediated RAFT polymerization. In both cases, prior functionalization of surfaces with a self-assembled monolayer (SAM) of the interlinker molecule bearing a long chain alkyl thiol (for a gold surface) or siloxane/chlorosilane (for a silicon/glass/indium–tin oxide surface) with a suitable end functional group is necessary. Especially, a SAM with an epoxy/amine/hydroxyl end group is useful to directly immobilize carboxylic acid bearing initiator or RAFT agent molecules. In the surface-initiated RAFT polymerization (Scheme 3.1a), the polymer brush layer is generated in situ from a suitable surface-immobilized

Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions

Scheme . Different methods for surface-mediated RAFT polymerization process (I, M, and ∙ indicate initiator, monomer, and radical, respectively).

initiator.15,16 The surface-initiated RAFT polymerization yields a low grafting density, mainly caused by recombination of radicals, low initiator/RAFT agent ratio, low initiation efficiency, and long half-time of initiator.17 Chains anchored to the substrate formed at later stages of polymerization would be shorter than those produced at the beginning. This difference in the chain length is not only caused by the late initiation, but also caused by a steric hindrance effect of early chains on the propagation of new chains. This leads to a broader molecular weight distribution.18

3.2 Polymer Brushes via the Surface-Initiated RAFT Polymerization Process

Interface-mediated RAFT polymerization is another versatile technique (Scheme 3.1b). There are two approaches to conduct the interface-mediated RAFT polymerization depending on the type of species anchored to the substrate; (i) RAFT agent via the Z group and (ii) RAFT agent via the R group. The Z-group approach requires radical chains to diffuse through the polymer layer in order to react with the RAFT agent immobilized at the surface. In the Z-group approach, no propagation occurs on the surface, which eliminates surface radical termination and allows the truly living chains to be grafted onto the surface.19 As the polymerization proceeds, the accessibility of the RAFT agent decreases, resulting in similar limitations as in a typical “grafting-to” method, known as the autophobic effect for polymer brushes.19,21 However, the R-group approach stands as the most useful way to prepare thick polymer brushes with a narrow molecular weight distribution.22–25 This approach is characterized by the attachment of the RAFT agent via its R group to a surface, while the polymerization is initiated in solution with or without a free RAFT agent. In the absence of an additional free RAFT agent, the theoretical grafting efficiency is close to 50%. However, this efficiency is decreased by addition of free RAFT agent in solution.19 The decrease in the grafting efficiency directly reflects on the grafting density and thus in the thickness of the polymer brush. Moreover, it has been shown experimentally and theoretically that in the R-group approach termination occurs more frequently on the surface compared to solution.19 Thus, conditions for controlling polymerization on the surface are not readily achievable. However, the living character of the RAFT technique and lack of use of any toxic catalyst resulted in a novel avenue for the development of novel biomaterials. Here, we focus on developments from our own investigations to discuss key steps in preparing brushes on solid substrates by both surface-initiated RAFT polymerization and interface-mediated RAFT polymerization for biological functions.

. Polymer Brushes via the Surface-Initiated RAFT Polymerization Process Surface-initiated RAFT polymerization from the initiator-containing SAMs is attractive, since a high density of initiators can be immobilized on the solid surface and the initiation mechanism is more well-defined. Especially, covalent attachment of the interlinker molecules directly silicon surface via Si−C bonds, in the absence of the native oxide layer, has been attracted a great deal of attention, because of the thermal and chemical stability (against to solvent or acid/base treatments) of the resulting Si−C bonds.26–28 Yu et al.29 utilized the surface-initiated RAFT polymerization technique to graft poly(4-vinylbenzyl chloride) [poly(VBC)] from a silicon substrate using a surface-anchored azo

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions

H (1) t-BOC protected allylamine, UV Irradition (2) 25% TFA, 10% NH4OH

Scheme . Schematic diagram illustrating the process of the surface-initiated RAFT polymerization of 4PV from the Si-Azo surface. Source: Demirci et al. 2013.30 Reproduced with permission of Elsevier.

NH2 ACVC, rt, 10 h O

CN N

NH

Cl

N

NC 4VP, ACVA, RAFT agent 70°C, 20 h

O

O NH X : Se (RAFT-Se) S (RAFT-S)

)nX

( NC

X N

initiator. An ω-unsaturated alkyl ester was used to immobilize the azo initiator on the silicon surface. Cumyl dithiobenzoate and cumyl phenyldithioacetate were used as free chain transfer agents (CTA) in solution to control the graft polymerization. The free polymer generated in solution was analyzed to estimate the molecular weight of the surface-grafted polymer. A linear relationship between the film thickness and number-average molecular weight (Mn ) of the free polymer was observed, and the polydispersity (PDI) of the free polymer was approximately 1.2–1.3. The surface-grafted poly(VBC) was further functionalized to give the Si-g-viologen surface with redox-reponsive properties. We also used a similar strategy to graft polymer brushes of poly(4vinylpyridine) [poly(4VP)] on silicon surface (Scheme 3.2).30 In this study, a new selenium-based RAF agent (4-cyanopentanoic acid diselenobenzoate, RAFT-Se) was synthesized and used in the surface- initiated RAFT polymerization of 4-vinylpyridine. RAFT polymerization involves a reversible additionfragmentation cycle, in which transfer of the dithioester groups between the activated and dormant polymer chains mains the controlled character of the polymerization process. Because of the analog of selenium (Se) with sulfur (S) in an electronic structure, RAFT-Se is also useful for RAFT polymerization. The surface-initiated RAFT polymerization with the RAFT-Se was the same polymerization mechanism as its analog, 4-cyanopentanoic acid

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process 24

RAFT-Se RAFT-S

20 Thickness (mm)

Figure . Evolution of the thickness (h, nm) of the poly(4VP) brushes synthesized in the presence of RAFT-Se and RAFT-S as a function of Mn . Source: Demirci et al. 2013.30 Reproduced with permission of Elsevier.

16 12 8 σRAFT-Se = 0.51 chain/nm2 σRAFT-S = 0.66 chain/nm2

4 0

0

4

8

12 16 Mn / 1000 (g/mol)

20

24

dithiobenzoate (RAFT-S). The relationship between the thickness (h, nm) of the poly(4VP) brushes and Mn of the corresponding free polymers formed in the presence of RAFT-Se and RAFT-S is given in Figure 3.1. A direct proportion relationship was observed between the Mn of the polymer (it was assumed that the free polymer and grafted polymer with the same Mn are formed in the same polymerization conditions31,32 ) and brush thickness (h = σMn ∕ρNA 10−21 , where σ (chains/nm2 ) is the grafting density, ρ (1.15 g/cm3 ) is the density of polymer, and NA (6.02 × 1023 mol−1 ) is Avogadro‘s number).33 From the slope of the thickness-versus-Mn curve, grafting density, σ (chains/nm2 ) of the polymer brushes prepared in the presence of RAFT-Se (σRAFT-Se ) and RAFT-S (σRAFT-S ) was estimated to be 0.51 and 0.66 chains/nm2 , respectively.

. Polymer Brushes via the Interface-Mediated RAFT Polymerization Process Interface-mediated RAFT polymerization has been widely explored as a technique to modify the solid substrates due to its ability to precisely control the structure of the grafted polymer chains with a low-to-high range of grafting densities. As mentioned in the Introduction section, in this technique, there are two general routes to prepare polymer brushes, including using (i) Z-group and (ii) R-group approaches. In the Z-group approach, the RAFT agent is attached to the solid surface via its stabilizing Z group. The macroradicals always propagate in solution before they attach to the solid surface via the chain transfer reactions with attached RAFT agents. In the R-group approach, the RAFT agent is attached to the solid substrate via its leaving and reinitiating R group. The solid substrate acts as part of the leaving R group, and thus the macroradicals are located on the terminal end of the surface-attached polymer

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions

chain, which facilitates the growth of grafted polymer chains. Therefore, the interface-mediated RAFT polymerization based R group is preferred to obtain the polymer brushes with high grafting density. Some examples of the polymeric brushes prepared by the interface-mediated RAFT polymerization based R group were given in this chapter. ..

pH-Responsive Brushes

Polyelectrolyte brushes constitute very important class of “smart” surface and nanoactuators that are widely employed in many biomedical applications.34,35 The structures and properties of polyelectrolyte brushes are strongly dependent on the Coulombic interactions between charged polymer chains. The use of a variety of environmental triggers, including pH and salt concentration, to control the conformation of polyelectrolyte brushes showed very pronounced responsive properties of the polymer chains.36–41 We reported the synthesis of pH-responsive mixed-charge polyelectrolyte brushes composed of negatively charged carboxylic acid (4-vinylbenzoic acid, VBA) and positively charged quaternary amine ((ar-vinylbenzyl)trimethylammonium chloride, VBTAC) monomers via the interface-mediated RAFT polymerization process, whose properties can be readily varied by changing pH (Scheme 3.3).42 The RAFT agent was immobilized on the Si–H surface in three steps involving (i) coupling of t-butyloxycarbonyl (t-BOC) protected allyamine to the Si–H surface under UV light, (ii) conversion of the t-BOC groups into the free amine groups by trifluoroacetic acid, and (iii) the amide reaction of allylamine with the RAFT agent. The interface-mediated RAFT polymerization of negatively charged VBA and positively charged VBTAC monomers from the RAFT agent immobilized silicon substrate was conducted in a buffer (pH 7.5) at 70◦ C. A certain amount of the RAFT agent was also added into polymerization solution as the sacrificial CTA to increase the control over the polymerization and help evaluate the molecular weight and PDI of the grafted polymers.31,43 The formation of poly(VBA-co-VBTAC) brushes was confirmed by grazing angle-Fourier transform infrared (GA-FTIR) and x-ray photoelectron spectroscopy (XPS) measurements. The water contact angle decreased from 46.5 ± 0.6◦ at pH 2.0– 21.9 ± 0.9◦ at pH 12.0, and it stayed a constant value with further increasing the pH (Figure 3.2). The phase transition pH of poly(VBA-co-VBTAC) brushes was detected as 7.65 (this value is almost close to the pKa value of poly(VBA) in aqueous solution at 25◦ C,44 ) from the derivative of the curve of the water contact angle (dθ/dpH) versus pH (Figure 3.2). The pH-responsive wettability of the poly(VBA-co-VBTAC) brushes was reversible for repeated cycle of alternating treatment by aqueous solution with pH 2 and 12 (Figure 3.3). At lower pH, the VBTAC units are positively charged, whereas the VBA units remain as neutral and the surface is slightly hydrophobic (water contact angle

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process

Scheme . Schematic diagram illustrating the process of interface-mediated RAFT polymerization of VBA and VBTAC from the Si-RAFT agent. Source: Adapted from Demirci et al. 2013.42 Reproduced with permission of Wiley Periodicals, Inc.

H H + H

O NH

O

t-BOC protected allylamine

Si-H

(1) UV Irradiation (2) 25% TFA, 10%NH4OH

NH2 Si-NH2 CPAD RT, 60 h O NH

S NC

S

Si-NH-CPADB VBA / VBTAC / CPAD / ACPA 70°60C, 4 h

O NH

(

)m (

)nS

NC

S

Si-g-poly(VBA-co-VBTAC) O

OH

+ O

is 46.5 ± 0.6◦ ). As the pH increases, the VBA units convert into the carboxylate form and the surface is completely hydrophilic (water contact angle is 21.9 ± 0.6◦ ). The root-mean-square (rms) roughness values determined by AFM (Figure 3.3) after treatment with pH 2 (top) and 12 (bottom) are 10.8 and 6.7 nm, respectively. The reversible properties of the copolymer brushes can be employed to regulate the immobilization of charged biomolecules such as DNA and proteins.

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 50 0

45

–1

40

–2

35

–3

30

–4

25

Figure . The water contact angle as a function of pH for poly(VBA-co-VBTAC) brushes. Source: Demirci et al. 2013.42 Reproduced with permission of Wiley Periodicals, Inc.

–5

pKa = 7.65

–6

20 15

dθ/dpH

Contact angle (θ)



–7 2

4

6

8

10

12

–8

pH

For DNA immobilization, we prepared the cationic poly(ar-vinylbenzyl) trimethylammonium chloride) [poly(VBTAC)] brushes by the interfacemediated RAFT polymerization.45 Initially, silicon surfaces were modified with the RAFT agent by using an amide reaction involving a silicon wafer modified with allylamine and 4-cyanopentanoic acid dithiobenzoate (CPAD). Poly(VBTAC) brushes were then prepared via the interface-mediated RAFT polymerization from the CPAD immobilized surface. The cationic poly(VBTAC) brushes were used for quantitative DNA immobilization by adjusting the solution pH. It is well known that DNA molecules are likely immobilized onto the cationic surfaces by electrostatic interaction between negatively charged oligonucleotides and positively charged polymer brushes. The cationic surfaces were first incubated in DNA solutions (pH 7.0). The surfaces were then taken out from the solution, and washed with pH 7.0 buffer solution, and dried with a stream of nitrogen. The film thickness was measured by ellipsometry. Adsorption capacity Γ (mg/m2 ) was calculated from the ellipsometric thickness (h), and refractive index (n) values using Eq. (3.1)46 Γ=

h(n1 − n2 ) dn∕dc

(3.1)

where n2 is the refractive index of DNA solution, n2 is the refractive index of buff solution, dn/dc standard for the change of refractive index against solution concentration, and a value of 0.185 mL/g was used in this work.47–49 The immobilization behavior of the cationic poly(VBTAC) brushes was evaluated −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→ Figure . Reversible change of the water contact angles for the poly(VBA-co-VBTAC) brushes under the alternating treatment by aqueous solution with pH 2 and 12, respectively. Two-dimensional (2D) and three-dimwnsional (3D) Atomic force microscopy (AFM) images at pH 2 (top) and 12 (bottom), and photographs of 4 μL water droplets (top) on the polymer brush. Source: Demirci et al. 2013.42 Reproduced with permission of Wiley Periodicals, Inc.

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process

pH 2 5

46.2 ± 0.4

60 40

0

1

2

5

20 0

0

0

3 2 m μ

4

1

1

μm 3

μm 4 5 3 2

4

40 nm

nm

0

nm 40

1

2

3 μm

4

5

–20

0 0

2

50

μm

4

6

40 35 30 25 20 15

2

12

2

12

2

12

pH pH 12

2

12

nm 45

5

22.8 ± 0.9 4

30 nm

30

5

0

1

4

0

nm 40

0

1

3 2 m μ

15

5

0

0

1

μm 2 3

μm 3 4 2

Contact angle (θ)

45

1

0 0

2

4 μm

6

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3 μm

4

5

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 5

4 Ci / Γm × 102



3

2 Ci / Γm = 0.127.Ci + 1.049.10–2

1

R2 = 0.952

0

5

10

15

Ci (mg/mL) ×

20

Figure . The adsorption isoterm of the cationic poly(VBTAC) brushes. Sample size: 10 mm × 10 mm. The points are the average of data from three batches, and the limits of the error bars are the calculated adsorption capacities from the three batches. Source: Demirci and Caykara 2013.45 Reproduced with permission of Elsevier.

25

102

by the Langmuir equation (Eq. 3.2, Figure 3.4) adopted for the equilibrium adsorption45 : C Ci 1 = + i Γe K m Γm Γm

(3.2)

where Ci (mg/mL) is the initial concentration of DNA solution, Γe (mg/m2 ) is the adsorption capacity at equilibrium, Γm (mg/m2 ) is the maximum adsorption capacity at equilibrium, and Km (mL/mg) is a constant related to the adsorption. Kinetic parameters were calculated as 12.05 mg/mL for Km and 7.91 g/m2 for Γm . .. Temperature-Responsive Brushes Some of polymer brushes exhibit temperature-responsive property and have wide applications in water treatment,50 drug delivery,51,52 and biomedicals.53–55 Poly(N-isopropylacrylamide) [poly(NIPAM)] is one of the most studied synthetic responsive polymer for polymer brush application, which undergoes a transition from a phase transition from a hydrophilic state to a hydrophobic state in water at 32◦ C.56–59 Recently, the predominance of poly(NIPAM) as the paradigmatic example of temperature-responsive polymer challenged by the discovery the monomers composed of a methacylate moiety connected to a short poly(ethylene glycol) (PEG) chain, because their polymers exhibit a lower critical solution temperature (LCST) in water, which can be finely tuned anywhere between 26 and 90◦ C depending on length of the PEG side chain,60–63 and which is almost independent of molecular weight, concentration, and ionic strength, contrarily

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process OH Si

OH OH

Si-OH

APTS 25°C, 2 h

Si

O O Si O

NH2

Si-APTS CTA-NHS / ethyl acetate 48 h, 50°C

Si

O O Si O

O S

N H

NC

S

C12H25

S

Si-CTA MEO2 MA, free CTA, AIBN DMF, 60°C

Si

O O Si O

O N H

S NC

Si-g-poly(MEO2MA)

O

O

S

C12H25

S

O H3CO

Scheme . Schematic diagram illustrating the processes of immobilization of CTA, and the interface-mediated RAFT polymerization from the Si-CTA surface. Source: Adapted from Zengin et al. 2013.64 Reproduced with permission of Wiley Periodicals, Inc.

to poly(NIPAM).63 We have shown that finely tunable temperature-responsive poly(2-(2-methoxyethoxy)ethyl methacrylate [poly(MEO2 MA)] brushes can be easily obtained by interface-mediated RAFT polymerization (Scheme 3.4). They demonstrated how water contact angle measurements allow us to obtain thermodynamically relevant phase transition temperature.64 In this study, RAFT agent molecules were immobilized on the Si–OH surface via a two-step process involving (i) self-assembly of 3-aminopropyltrimetoxy silane (APTS) on the surface, and (ii) an amide reaction of APTS with RAFT agent molecules. The interface-mediated RAFT polymerization of MEO2 MA from the RAFT agent immobilized surface was accomplished in the presence of free RAFT agent to better control the polymerization in DMF at 60◦ C. The

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 4

3 In ([M]0/ [M]t)



2

1

0

0

2

4

8 6 Time (h)

10

12

14

Figure . Relationship between ln([M0 ]/[Mt ]) and polymerization time. Source: Zengin et al. 2013.64 Reproduced with permission of Wiley Periodicals, Inc.

formation of poly(MEO2 MA) brushes was confirmed by GA-FTIR and XPS measurements. Evidence on the controlled polymerization was provided by the “free” poly(MEO2 MA) formed from the free RAFT agent molecules. Figure 3.5 shows the linear relationship between ln([M0 ]/[Mt ]) and time, where [M0 ] is the initial monomer concentration and [Mt ] is the monomer concentration. The results indicated that the concentration of the growing species remains constant, and first-order kinetic was obtained. The time evolution of Mn was obtained for the free poly(MEO2 MA) by gel permeation chromatography (GPC) analysis. Typical GPC traces of free polymers as a function of polymerization time are shown in Figure 3.6. The GPC traces are monomodal Figure . Evolution of the GPC traces as a function of polymerization time for the polymers in solution. Source: Zengin et al. 2013.64 Reproduced with permission of Wiley Periodicals, Inc.

Polymerization time (h) 12 9 5 3

18

20

2 1 0.5

22 24 26 Elution time (min)

28

30

32

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process 120 (a) 100

Contact angle (θ)

Figure . (a) Temperature dependence of water contact angles of poly(MEO2 MA) brushes and (b) derivative curve of (dθ/dT) versus T. Data points are average of three measurements taken at different points along the sample surface. Source: Zengin et al. 2013.64 Reproduced with permission of Wiley Periodicals, Inc.

80 60 40

dθ/dT (θ/°C)

15

(b)

27.6°C

10 5 0 15

20

25

30 T/°C

35

40

45

and have narrow molecular weight distributions, which are characteristics of well-defined polymers prepared via controlled/living radical polymerization techniques. It can be clearly shown that the elution peaks shifted to higher molecular weight and also decreased in PDI values as the time increased. To explore the temperature-responsive behavior of the poly(MEO2 MA) brushes, the water contact angles were measured at different temperatures. Figure 3.7a shows the temperature (T) dependence of water contact angle (θ) of the polymer brush. As shown in this figure, the water contact angle curve exhibits a sigmoidal with two plateau regions at low and high temperatures and one smooth transition in between. The data for the poly(MEO2 MA) brush show no sharp changes at any point throughout the transition. Hence, the phase transition temperature of the polymer brush was detected as 27.6◦ C from the derivative of the curve of the water contact angle (dθ/dT) versus T (Figure 3.7b). It should be noted that this value is almost close to the LCST of poly(MEO2 MA) in water.61 The permanence of temperature-responsive properties of the polymer brushes was also demonstrated by several repeated temperature-changing cycles from 20 to 40◦ C (Figure 3.8). A switch character from hydrophilic to hydrophobic indicates a good reversibility of the surface wettability of the poly(MEO2 MA) brushes. Moreover, at 20◦ C, the AFM image

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions 40°C T < LCST

nm

5

nm 20

5

20°C

15

0

1

2

μm

3

4

5

4

60

nm

15 5 –5

0 –5

80

0

0

–10

0

2

4

μm

5

100

nm

Contact angle (θ)

μm 2 1

0

20°C

6

μm 2 3

10 40°C

1

4

120

3

10

40

0

1

2

3

4

5

0 8 4 0

1

2

0

6

2

μm

μm

3

4

4

5

6

5

T < LCST

0

1

4

0

0

1

1

3 2 μm

μm 3 4 2

5

5

Cycles

μm 4 3 2



1

3 2 μm

4

5

0

Figure . Reversibility of the surface wettability of the poly(MEO2 MA) brushes. 2D and 3D AFM images at 20◦ C (left, the z scale is 20 nm) and 40◦ C (right, the z scale is 15 nm), and photographs of 4 μL water droplets (top) on the polymer brushes. The cross sections corresponding to the red line shown in the AFM images are given below each image. Source: Zengin et al. 2013.64 Reproduced with permission of Wiley Periodicals, Inc.

of the poly(MEO2 MA) brush is almost flat (rms = 2.13 nm) (Figure 3.8). This means that the poly(MEO2 MA) chains are distributed tightly and extend well out at this temperature. When the temperature is increased to 40◦ C, a large number of aggregates became clearly visible on the surface (rms = 2.68 nm). The AFM images also indicated that the poly(MEO2 MA) brushes undergo a significant structural change around the LCST, which is a result of the phase transition. ..

Polymer Brushes on Gold Surface

The high affinity of thiols for the metal surfaces, in particular gold surface, makes it possible to modify various metal substrates with well-defined polymer chains prepared via interface-mediated RAFT polymerization. We reported the synthesis of poly[(oligoethylene glycol) methyl ether acrylate] [poly(OEGA)] brushes by interface-mediated RAFT polymerization and used to selectively immobilize streptavidin proteins (Scheme 3.5).65 Initially, 11mercapto-1-undecanol molecules were immobilized on gold surface by the SAM process. The resulting SAM-OH thickness was measured by ellipsometry at 1.2 ± 0.6 nm, a value consistent with the literature data.66 The formation of SAM-OH was also confirmed by the presence of the peaks at about 2920–2851 cm−1 , which are assigned to the aliphatic CH2 stretching vibrations

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process

Scheme . Stepwise fabrication process for creating streptavidin-immobilized poly(OEGA) brushes. Source: Adapted from Zengin et al. 2012.65 Reproduced with permission of Wiley Periodicals, Inc.

(Figure 3.9a). Moreover, C 1s and O 1s core-level XPS spectra displayed in Figure 3.10a unambiguously attested the signature of hydroxyl groups with typical binding energies at 287.0 eV (C 1s, C–O/C–S) and 533.0 eV (O 1s, C–O). Establishment of the Au−S bond was also evidenced by the S 2p1/2 level at 163.1 eV and the S 2p3/2 level at 161.8 eV. Free thiol groups (higher binding energies) were not observed.67 In the next modification step, the CTA molecules were attached to the reactive SAM by an ester reaction. The CTA immobilization was apparent from the appearance of terminal carboxylic acid bands at 3500– 3200 cm−1 (OH stretching vibration) and 1650 cm−1 (carboxylic acid C=O), ester bands at 1758 cm−1 (C=O stretching vibration) and 1250 cm−1 (C−O stretching vibration), respectively, in the GA-FTIR spectrum ([Figure 3.9b). The C=S stretching band at 1057 cm−1 further confirmed successful covalent attachment of the CTA on the reactive SAM. The core-level XPS spectra

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions

Absorbance

1057 cm–1

1758 cm–1 1671 cm–1

2920 cm–1 2851 cm–1

1103 cm–1

(c)

(b) ×25

(a)

×25 4000

3500

3000 2500 2000 Wavenumbers (cm–1)

1500

1000

Figure . GA-FTIR spectra of (a) SAM-OH, (b) CTA layer, and (c) poly(OEGA) brushes. Source: Zengin et al. 2012.65 Reproduced with permission of Wiley Periodicals, Inc. O 1s

C 1s

(a)

O _ -H

S 2p

538

534

530

Au-_ S2p

294

290

286

C=_ O C-_ O 534

530

(c)

C _ =O

294

C _ -O/_ C-S

290

C=_ O C-_ O

C _ -O/_ C-S

286

168 166 164 162 160 158 C=_ S C-_ S-C

282

1/2

1/2

C-_ S-C

O _ -H

C-_ S-Au

168 166 164 162 160 158

C _ -O/_ C-H

C _ =O

O _ -H 538

282

C _ -O/_ C-H

(b)

538

Au-_ S2p

C _ -C/_ C-H C _ -O/_ C-S

C/s



534

530

294 290 286 282 Binding energy (eV)

168 166 164 162 160 158

Figure . O 1s, C 1s, and S 2p core-level XPS spectra of (a) SAM-OH, (b) CTA layer, and (c) poly(OEGA) brushes. Source: Zengin et al. 2012.65 Reproduced with permission of Wiley Periodicals, Inc.

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process

6

nm

(b)

μm

4

5 –3

0

4 1

2

μm

3

4

5

2

μm

4

6

3 μm 0

12 4 –4

1

2

μm

3

4

5

nm

nm

nm

4 2 –2

0

2 1

3

0 –4

0

2

1

1

4

0

0

1

0

0

3 1 –1

8

2

2

3

μm

μm

3

3

12

0 –1

(c)

16

2 1

5

5

nm (a)

4

3

42.2 ± 0.3°

29.5 ± 0.6°

4

nm

5

34.1 ± 0.8°

0

2

μm

4

6

0

2

μm

4

6

Figure . 2D AFM images (5 μm × 5 μm) in ambient conditions, surface cross-section analysis, and photographs of 4 μL water droplets for (a) SAM-OH, (b) CTA layer, and (c) poly(OEGA) brushes. Source: Zengin et al. 2012.65 Reproduced with permission of Wiley Periodicals, Inc.

of CTA overlayer consist of O 1s and C 1s peaks curve fitted into components with binding energies at about 533.1 eV (O−H), 532.0 eV (C=O), and 531.6 eV (C−O) for O 1s and 289.6 eV (C=O), 286.9 eV (C−S/C−C−O), and 285.1 eV (C−C/C−H) for C 1s (Figure 3.10b). The attachment of CTA onto the reactive SAM was also confirmed from the appearance of a S 2p peak curve fitted into four components at about 164.5 and 163.8 eV (C−S−C), 162.6 eV (C=S), and 162.6 and 161.6 eV (C−S−Au). The thickness of CTA layer was measured by ellipsometry at 1.44 ± 0.11 nm; this value is consistent with the theoretical value (1.2 nm) obtained by ab initio calculations. Surface morphology and cross section of SAM-OH and CTA-modified gold substrates are shown in Figures 3.11a and 3.11b. Analysis of the SAM-OH showed a rms roughness of 1.46 ± 0.22 nm, whereas a slight rms increase (rms = 1.78 ± 0.40 nm) could be measured after CTA attachment. Finally, as illustrated by inset images in Figure 3.11, the attachment of CTA induced a change in surface wettability characterized by a slight decrease of water contact angle from 34.1 ± 0.8◦ (SAMOH) to 29.5 ± 0.6◦ (CTA layer). The interface-mediated RAFT polymerization of OEGA on gold surface was performed in the presence of free CTA to better control the polymerization. The presence of the free CTA slows down the propagation of free chains in solution and favors the addition-fragmentation reactions on gold surface. The formation of poly(OEGA) brushes was confirmed by GA-FTIR (Figure 3.9c), XPS (Figure 3.10c), AFM (Figure 1.11c), and water contact angle measurements. The poly(OEGA) brushes with carboxylic end group were then functionalized with biotin molecules in the presence of 1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide and N-hydroxysuccinimide

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



3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions O TEOS

O O Si O

MPS

O O O Si O

CTA/AIBN

O

γ–Fe2O3 Nanoparticles

O O O Si O

CH3 C C CN H2 CH

O

O CH2 HO C C O C CH3 H2 H2 n

O

CH3 C C CN H 2 CH S 3 S

HEMA AIBN CTA

3

S S

Scheme . Schematic representation of the synthesis of hybrid nanoparticles. Source: Adapted from Zengin et al. 2013.69 Reproduced with permission of American Chemical Society.

molecules. Subsequently, fluorescein isothiocyanate (FITC) streptavidin units were immobilized on the poly(OEGA) brushes by biotin-mediated specific intreractions.68 The immobilization of FITC-streptavidin on the polymer brush surface was confirmed by fluorescence microscopy measurements.65 ..

Polymer Brushes on Nanoparticles

Many other substrate surfaces in addition to magnetic silica particles can be modified using the interface-mediated RAFT polymerization process by attaching the appropriate RAFT agent (Scheme 3.6).69 The first step is the formation of a silica coating on the maghemite nanoparticles to prevent oxidation and dissolution of the γ-Fe2 O3 . To coat γ-Fe2 O3 nanoparticles with silica, a microemulsion method70 was employed. The transmission electron microscopy (TEM) images showed unaggregated particles with a dark γ-Fe2 O3 core, which may consistent of one γ-Fe2 O3 nanoparticle (∼14 nm), outside a silica shell (∼50 nm) (Figure 3.12). In this case, the thickness of the silica shell is about 25 nm. The silica-coated magnetic nanoparticles (MNPs) were then transformed into double-bond-bearing spheres that are suitable for immobilization RAFT agent by condensation of 3-methacryloxpropyltrimethoxysilane (MPS) onto the surface of the particle. Coupling of the RAFT agent was achieved by a one-step process in bearing a terminal double bond region of the MPS-modified MNPs from reactions with a mixture of azo initiator and RAFT agent. RAFT-agent modified MNPs were then used as a RAFT agent for the interface-mediated RAFT polymerization of 2-hydroxyethyl methacrylate (HEMA), where the polymer chains were grafted directly from the nanoparticle surface to give magnetic and reactive core–shell hybrid nanoparticles (Figure 3.12).

3.3 Polymer Brushes via the Interface-Mediated RAFT Polymerization Process

Figure . TEM images of (a) oleic acid-stabilized MNPs, (b) silica-coated MNPs, and (c) hybrid MNPs. Source: Zengin et al. 2013.69 Reproduced with permission of American Chemical Society.

The dithiobenzoate end groups were removed via the azo initiator due to their toxicity and potential aminolysis reaction (Scheme 3.7). The pedant –OH groups of poly(HEMA) on the hybrid MNPs were reacted with monoclonal antitau. After separating tau protein from the sample matrix, they were sandwiched with the surface-enhanced Raman scattering (SERS) substrate composed of polyclonal antitau and Raman reporter on gold NPs. The limit of detection for the sandwich assay is less than 25 fM, which is comparable to the sensitivity of conventional optical biosensing methods. An additional clear advantage of this highly sensitive SERS method on an engineered substrate is the speed of detection and simple substrate preparation. Moreover, this method does not require several binding and purification steps an in enzyme-linked immunosorbent assay (ELISA), which can take hours to complete.

..

Micropatterned Polymer Brushes

The fabrication of micropatterned polymer architectures on solid substrates is of remarkable importance in biotechnology as these surfaces are more lasting compared to conventional polymer films. Production of micropatterned polymer brushes is conventionally achieved by using some lithography techniques such as e-beam lithography,71,72 scanning probe lithography,73,74 capillary force lithography,75,76 and photolithography.77,78 Some of the conventional techniques involve the attachment of initiators on surfaces micropatterned using photolithography. In another technique, a micropatterned initiator layer is formed by patterning an initiator containing a SAM. In order to produce micropatterned polymer brushes, a suitable monomer is polymerized on these surfaces by surface-initiated polymerization. These techniques are quite complicated and can increase the probability of surface contamination. Therefore, we prepared the micropatterning of poly(6-azidohexylmethacrylate)

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions O

CH3 C C CN H2 O CH2 CH3 HO C C O C C CH3 H2 H2 n

O O Si O

O

O

CH3 C C CN H2 O CH2 CH3 HO C C O C C CH3 H2 H2 n

O O Si O

AIBN/DMF

O

S

NC C CH3 CH3

S

(1) DSC/DMAP (2) anti-Tau (3) Tau

O

CH3 C C CN H2 O CH2 CH3 O C C O C C CH3 H2 H2 n O NC C CH3 CH3

O O Si O N H

Raman Tag Au

LASER

O

O

CH3 C C CN H2 O CH2 CH3 O C C O C C CH3 H2 H2 n O NC C CH3 CH3

O O Si O N H

O

SERS Au

Intensity



800

1000

1200

1400

1600

1800

2000

Raman Shift (cm–1)

Scheme . Representative presentation of the preparation of sandwich assay. Source: Adapted from Zengin et al. 2013.69 Reproduced with permission of American Chemical Society.

[poly(AHMA)] brushes by a combination of photolithography and interfacemediated RAFT polymerization for DNA hybridization (Scheme 3.8).79 First, a thin coating of phtoresist was spin-coated on the RAFT agent immobilized surface and patterned using photolithography. UV light and oxygen plasma were then used to destroy the organic molecules on the unexposed regions. The poly(AHMA) brushes were then grown on the micropatterned regions via interface-mediated RAFT polymerization. By this way, highly resolved micropatterned polymer brush structures down to ∼2.0 μm lines were obtained. The micropatterned brush structures were then used for the immobilization and hybridization of single-stranded DNA molecules. Comparison of AFM images before and after hybridization showed that the height of the DNA hybridized lines (16.7 ± 0.7 nm) is almost the same as the probe DNA immobilized surface (17.7 ± 0.3 nm) (Figure 3.13). From the

3.4 Summary O Si

(

HN

)nS O

Biotin-ended Cy3-labelled Prob DNA

S

O S

OH O

N N N NH2

O Si

(

HN

)nS O

N N N

O S

S

OH O

Cy5-labelled comp-DNA (%64 hybridization)

O Si

(

HN

)nS O

O S

S

OH O

N N N

Scheme . Stepwise fabrication process for creating DNA-hybridized poly(AHMA) brush micropatterns. Source: Adapted from Cimen and Caykara 2015.79 Reproduced with permission of Royal Society of Chemistry.

fluorescence intensity measurements, the hybridization ratios for complementary and noncomplementary DNA were found to be 64% and 8%, respectively. This reasonably straightforward approach offers a potential platform for the immobilization of complicated structures on silicon surfaces for approaches in biosensing and production of novel devices and new materials.

. Summary In summary, a number of recent advances in the field of polymer brush synthesis via surface-mediated RAFT polymerization (surface-initiated RAFT and interface-mediated RAFT) techniques have been described in this chapter. Whether utilizing a surface-initiated RAFT polymerization or an interfacemediated RAFT polymerization, polymer brushes are being formed of various sizes, structures, compositions, and topology (micropattering). Although they are formed predominantly on either silicon or gold substrates, this field is

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions

Figure . Fluorescence microscopy image (left) and 2D and 3D AFM images (right) of complementary DNA hybridized on the poly(AHMA) micropatterns under ambient conditions: (a) 45 × 45 μm2 , (b) 20 × 20 μm2 , and (c) 5 × 5 μm2 . The cross sections corresponding to the red line shown in the AFM images are given below each image. Source: Cimen and Caykara 2015.79 Reproduced with permission of Royal Society of Chemistry.

constantly expanding to introduce such brushes onto MNPs, carbon nanotubes, polymer films, and a wealth of other materials. With the ability to polymerize, these materials by using any RAFT process, the possible combinations of substrate, synthesis approach, and polymerization route are providing numerous opportunities. Moreover, due to the high functional group tolerance of this technique, RAFT agent groups at the end of polymer chains have been incorporated to a variety of substrates, thereby enabling the modification of copolymer or surface properties.

References

References                           

Wolkow, R. A. Annu. Rev. Phys. Chem. 1999, 50, 413–441. Buriak, J. M. Chem. Rev. 2002, 102, 1272–1306. Li, J.; Chen, X.; Chang, Y. C. Langmuir 2005, 21, 9562–9567. Wu, T.; Gong, P.; Szleifer, I.; Vlcek, P.; Subr, V.; Genzer, J. Macromolecules 2007, 40, 8756–8764. Yang, J.; Chen, H.; Xiao, S.; Shen, M.; Chen, F.; Fan, P.; Zhong, M.; Zheng, J. Langmuir, 2015, 31 (33), 9125–9133 Giglio, E. D.; Cometa, S.; Cioffi, N.; Torsi, L.; Sabbatini, L. Anal. Bioanal. Chem. 2007, 389, 2055–2063. Hollman, O.; Gutberlet, T.; Czeslik, C. Langmuir 2007, 23, 1347–1353. Xu, F. J.; Neoh, K. G.; Kang, E. T. Prog. Polym. Sci. 2009, 34, 719–761. Drummond, C.; Rodriguez-Hernandez, J.; Lecommandoux, S.; Richetti, P. J. Chem. Phys. 2007, 126, 184906–184912. Vyas, M. K.; Nandan, B.; Schneider, K.; Stamm, M. J. Colloid Interface Sci. 2008, 328, 58–66. Zeng, H. B.; Tian, Y.; Zhao, B. X.; Tirrell, M.; Israelachvili, J. Langmuir 2009, 25, 4594–4564. Demirci, S.; Demirci, S. K.; Caykara, T. J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 2677–2685. Zengin G. Karakose, A.; Caykara, T. Eur. Polym J. 2013, 49 (10), 3350–3358. Epps, T. H.; Longchamp, D. M. D.; Fasolka, M. J.; Fischer, D. A.; Lablonski, E. L. Langmuir 2007, 23, 3355–3362. Zaho, B.; Brittain, W. J. Prog. Polym. Sci. 2003, 25, 667–710. Kato, K.; Uchida, E.; Kang, E. T.; Uyama, Y.; Ikada, Y. Prog. Polym. Sci. 2003, 28, 209–259. Zhou, D.; Mastan, E.; Zhu, S. Macromol. Theory Simul. 2012, 21, 602–614. Beija, M.; Martty, J.-D.; Destarac, M. Prog. Polym. Sci. 2011, 36, 845–886. Perrier, S.; Takolpuckdee, P.; Mars, C. A. Macromolecules 2005, 38, 6770–6774. Rotzoll, R.; Vana, P. J. Polym. Sci. Part A: Polym. Chem. 2008, 46, 7656–7666. Stenzel, M. H.; Zhang, L.; Huck, W. T. S. Macromol. Rapid Commun. 2006, 27, 1121–1150. Tsujii, Y.; Ejaz, M.; Sato, K.; Goto, A.; Fukuda, T. Macromolecules 2001, 34, 8872–8878. Titirici, M.-M.; Sellergren, B. Chem. Mater. 2006, 18, 1773–1779. Zhou, D.; Gao, X.; Wang, W.-J.; Zhu, S. Macromolecules 2011, 45, 1198–1207. Olivier, A.; Meyer, F.; Raquez, J.-M.; Damman, P.; Dubois, P. Prog. Polym. Sci. 2012, 37, 157–181. Buriak, J. M. Chem. Rev. 2002, 102, 1272–1308. Buriak, J. M. Chem. Commun, 1999, 1051–1060.

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3 Polymer Brushes by Surface-Mediated RAFT Polymerization for Biological Functions

        

                   

Wayner, D. D. M.; Wolkow, R. A. J. Chem. Soc. Perkin Trans. 2002, 2, 23–34. Yu, W. H.; Kang, E. T.; Neoh, K. G. Ind. Eng. Chem. Res. 2004, 43, 5194–5202. Demirci, S.; Demirci, S. K.; Caykara, T. Polymer 2013, 54, 5345–5350. Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610–615. Turan, E.; Demirci, S.; Caykara, T. Thin Solid Films 2010, 518, 5950–5954. Luzinov, I.; Julthongpiput, D.; Malz, H.; Pionteck, V. V.; J. Macromolecules 2000, 33, 1043–1048. Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950–15951. Moya, S.; Azzaroni, O.; Farhan, T.; Osborne, V. L.; Huck, W. L. S. Angew, Chem. Int. Ed. 2004, 44, 4578–4581. R¨uhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Grohn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Adv. Polym. Sci. 2004, 165, 79–150. Biesalski, M.; Johannsmann, D.; R¨uhe, J. J. Chem. Phys. 2002, 117, 4988–4994. Biesalski, M.; Johannsmann, D.; R¨uhe, J. J. Chem. Phys. 2004, 120, 8807–8814. Farhan, T.; Azzaroni, O.; Huck, W. T. S. Soft Matter. 2005, 1, 66–68. Biesalski, M.; R¨uhe, J. J. Chem. Phys. 1999, 32, 2309–2316. Biesalski, M.; R¨uhe, J.; Johannsmann, D. J. Chem. Phys. 2000, 111, 7029–7037. Demirci, S.; Demirci, S. K.; Caykara, T. J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 1612–1619. Yuan, K.; Li, Z.; Lu, L.; Shi, X. Mater. Lett. 2007, 61, 2033–2036. Gabaston, L. I.; Furlong, S. A.; Jackson, R. A.; Armes, S. P. Polymer 1999, 40, 4505–4514. Demirci, S.; Caykara, T. Mater. Sci. Eng. C 2013, 33, 111–120. Kim, Y. B.; Hong, M. Y.; Shon, H. K.; Chegal, W.; Cho, H. M.; Moon, D. W.; Kim, H. S.; Lee, T. G. Appl. Surf. Sci. 2008, 255, 1110–1112. Zhao, X. B.; Pan, F.; Coffey, P.; Lu, J. R. Langmuir 2008, 24, 13556–13564. Reschel, T.; Konak, C.; Qupicky, D.; Seymour, L. W.; Ulbrich, K. J. Controlled Release 2002, 81, 201–217 Tumolo, T.; Angnes, L.; Baptista, M. S. Anal. Biochem. 2004, 333, 273–279. Zhou, F.; Huck, W. T. S. Phys. Chem. Chem. Phys. 2006, 8, 3815–3823. Miyata, T.; Asami, N.; Uragami, T. Nature, 1999, 399, 766–769. Aksay, I.; Trau, M.; Manne, S.; Honnma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P.; Gruner, S. Science, 1996, 273, 892–898. Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950–15951. Cimen, D.; Caykara, T. J. Matter. Chem. 2012, 22, 13231–13238. Zengin, A.; Caykara, T. J. Polym. Sci. Part A: Polym. Chem. 2012, 50, 4443–4450. Dimitrov, I.; Trzebicka, B.; M¨uller, A. H. E.; Dworak, A.; Tsvetanov, C. B. Prog. Polym. Sci. 2007, 32, 1275–1343.

References

 Schild, H. G. Prog. Polym. Sci. 1992, 17, 163–249.  Gil, E. S.; Hudson, S. M. Prog. Polym. Sci. 2004, 29, 1173–1222.  Turan, E.; Zengin, A.; Caykara, T. J. Polym. Sci. Part A: Polym. Chem. 2011, 49, 5116–5123.  Lutz, J. F. J. Polym. Sci. Part A: Polym. Chem. 2008, 46, 3459–3470.  Han, S.; Hagiware, M.; Ishizone, T. Macromolecules 2003, 36, 8312–8319.  Lutz, J. F.; Hoth, A. Macromolecules 2006, 39, 893–896.  Lutz, J. F.; Akdemir, O.; Hoth, A. J . Am. Chem. Soc. 2006, 128, 13046–13047.  Zengin, A.; Yildirim, E.; Caykara, T. J. Polym. Sci. Part A: Polym. Chem. 2013, 51, 954–962.  Zengin, A.; Caykara, T J. Polym. Sci. Part A: Polym. Chem. 2012, 50, 4443–4450.  Kim, J.-B.; Bruening, M. L.; Baker, G. L. J. Am. Chem. Soc. 2000, 112, 558–569.  Olivier, A.; Raquez, J.-M.; Dubois, P.; Damman, P. Eur. Polym. J. 2011, 47, 31–39.  Stafford, L.; Tian, X.; Weiss, G. A. Chem. Biochem. 2002, 3, 1229–1234.  Zengin, A.; Tamer, U.; Caykara, T. Biomacromolecules 2013, 14, 3001–3009.  Yi, D. K.; Selvan, S. T.; Lee, S. S.; Papaefthymiou, G. C.; Kundaliya, D.; Ying, J. Y. J. Am. Chem. Soc. 2005, 127, 4990–4991.  Zhang, G. J.; Tanii, T.; Zako, T.; Hosaka, T.; Miyake, T.; Kanari, Y.; Funatsu, T. W.; Ohdomari, I. Small, 2005, 1, 833–837.  Lussi, J. W.; Tang, C.; Kuenzi, P. A.; Staufer, U.; Csucs, G.; Voros, J.; Danuser, G.; Hubbell, J. A.; Textor, M. Nanotechnology, 2005, 16, 1781–1786.  Binnig, G.; Rohrer, H. Helv. Phys. Acta. 1982, 55, 726–735.  Binnig, G.; Quate, C. F.; Gerber, C. Phys. Rev. Lett. 1986, 56, 930–933.  Liu, Y.; Klep, V.; Luzinov, I. J. Am. Chem. Soc. 2006, 128, 8106–8107.  Prucker, O.; Schimmel, M.; Tovar, G.; Knoll, W.; Ruhe, J. Adv. Mater. 1998, 10, 1073–1077.  Husemann, M.; Morrson, M.; Benoit, D.; Frommer, K. J.; Mate, C. M.; Hinsberg, W. D.; Hedrick, J. L.; Hawker, C. J. J. Am. Chem. Soc. 2000, 122, 1844–1845.  Prucker, O.; Habicht, J.; Park, I. J.; Ruhe, J. Mater. Sci. Eng. C, 1999, 8–9, 291–297.  Cimen, D.; Caykara, T. Polym. Chem. 2015, 6, 6812–6818.

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 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush Bin Li and Feng Zhou∗ State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, People’s Republic of China

. Introduction There is an increasing demand for polymer-based thin films with well-defined spatial control in the field of biology and modern chemistry.1,2 Polymers at surfaces can be designed with many interesting properties that are a consequence of conformation changes of polymer chains, which have potential applications as functional materials.3–6 In the past few decades, chemists have made great progress with regard to the assembly of an impressive wide range of molecules onto surfaces. For example, Langmuir–Blodgett deposition, layerby-layer assembly, and electrostatic or hydrophobic adsorption are commonly used and well established. However, most of these technologies require specific chemical or physical substrate properties. The two methods named “grafting to” and “grafting from” are mostly used to assemble small molecules or polymers onto surfaces, either though physisorption or chemisorption techniques.7,8 However, covalent immobilizations yield polymers on surfaces with higher stability than physisorbed polymers and are less prone to degradation. The grafting-to technique consists of adsorbing polymers that is induced by various anchor groups to form a covalent bond with a complementary surface functionality, but the film thickness and graft density are quite limited due to the steric effect of polymer chains. The grafting-from approach involves in situ initiation of surface polymerization from an initiatordecorated substrate and has been widely used to generate polymeric films with high grafting densities and various functional groups, however, disadvantages of this technique include such as rigorous synthetic protocols and limited controllability to the polymerization process. Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush

There is a continuous search for new, versatile, and more controllable mediating means for surface-induced reactions. Electrochemistry offers many benefits including the generation of a dynamic solution, and its ability to provide temporal and spatial control over the reaction process and also compatibility with automation. The objective of this chapter is to provide a status report on synthetic strategies and mechanistic aspects of electrochemically induced Cu(I)-catalyzed chemical reactions for surface modifications from reactive selfassembled monolayers (SAMs). The main focus is in particular on electrochemically induced Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC or “e-click” chemistry) and surface-initiated atom transfer radical polymerization (SI-eATRP), for the two different chemical reactions are both Cu(I)-catalyzed reactions that have the potential to provide refined control of the reaction parameters and surface structures electrochemically. eATRP is also applicable to a wide variety of material surfaces and compatible well with “click” chemistry and photolithography techniques that can be used as a new route to surface functionalization in aqueous solution.

. “Electro-Click” Chemistry SAMs are playing a key role in many surface modifications methods. As constitute a very important class of two-dimensional (2D) materials which are versatile in tuning surface/interface properties.9–11 Recently, a range of new approaches to surface reactions have been applied to precision surface functionalization, with the aim of increasing efficiency and controllability in space and time in a high-throughput manner at surfaces. In particular, CuAAC or “click” chemistry reactions between azides and terminal alkynes have become the most popular “click” reactions in the selective modification of reactive surfaces, and “electro-click” reactions offer new opportunities to expand the function and to tailor the properties of surfaces.12–14 “e-Click” Chemistry Directed by Scanning Electrochemical Microscopy. Scanning electrochemical microscopy (SECM) is a powerful tool to surface modifications and surface patterning, allows both local mediating of surface-initiated systems from reactive moieties, and the analysis of the surface morphology at micrometer scales, as well as the tuning and optimization of experimental parameters in situ.15 Bard and co-workers demonstrated an electrochemically induced “click” chemistry to immobilize fluorescent molecules onto azido moieties coated glass substrate (Figure 4.1).16 A gold microelectrode directed by SECM was brought close to the azido-terminated monolayer on a glass substrate, the catalytically active CuI was locally generated via electrochemical reduction of a CuII salt and was employed to immobilize alkyne-functionalized fluorescent molecules via the “e-click” reaction. This study offered a feasible surface

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4.2 “Electro-Click” Chemistry

Cu(II) Cu(I) + R˙≡

Click chemistry by SECM

[R˙≡–Cu(I) [ N3

N3

( )

10

N3

N3

N3

10

10

N3

N3

( ) ( ) ( ) ( ) 10

R˙ N N

10

( )

Si Si Si Si Si Si O O OO O OO OO O O

10

N

R˙ N N

N

N3

( )

10

hv

N3

( ) ( ) ( ) ( ) 10

10

10

10

N3

( )

Si Si Si Si Si Si O O OO O OO OO O O

10

Figure . Electrochemically reduction of CuII to CuI at a gold microelectrode to catalyze the CuAAC reaction via the SECM technique. Source: Ku et al. 2008.16 Reproduced with permission of American Chemical Society.

patterning method via SECM; it was believed that the featured pattern size and shape could be controllable by selecting the tip size and the relative distance between the tip and the substrate. Stenciled “e-Click” Chemistry. Larsen et al. demonstrated a method for fast production of gradients on an electrically conductive surface (poly-3,4(1-azidomethylethylene)-dioxythiophene, PEDOT-N3 ) using stenciled electroclick chemistry (Figure 4.2). Catalytic CuI was generated locally through electrochemical reduction of the CuII complex, and the amount of catalyst generated was determined by the spatial confinement of the active electrodes. Surface gradients were formed by covalently bound alkyne-bearing molecules onto azide-terminated conductive polymers, a stencil on the counter-electrode defined the shape and multiplicity of the gradient on the conducting polymer substrate, the reaction areas and the gradient patterns were controlled by the geometry of the exposed counter electrode (Figures 4.2d and 4.2e). Furthermore, it was proved that the catalyst concentration, geometry of the setup, electrical potential, and reaction time had a strong effect on the gradient formation. “e-Click” Reaction on a Bipolar Electrode. A bipolar electrode (BPE) is widely used to electro-synthesize novel materials for a wide variety of applications. Electrochemical generation of CuI species at or near the electrode surface can be utilized immediately to catalyze surface reactions. BPE, with a potential gradient across the surface of the electrode, the CuI catalyst should be generated at the cathodic side of the BPE with a concentration gradient across the reactive SAMs surface, which would affect the outcome of the surface reaction. Inagi

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Cu(II)

Cu(II)

O

Cu(II)

n

R

Cu(I)

Cu(I)

O

Cu(II)

S

O

N N N

R

n

Cu(II)

–0.5V

0V + –

(e)

(d)

Copper electrode Insulating layer

100 μm

Figure . Schematic of the stenciled electro-click chemistry. (a) The azide/alkyne “click” reaction. (b) A copper counterelectrode with a patterned insulating coating forms a “stencil” to form gradient distribution the catalytic CuI complex. (c) Gradient formation between alkynes and azides moieties. Schematic of a stenciled copper counterelectrode (d) and fluorescence microscopy image of 2D gradients (e). Source: Hansen et al. 2010.17 Reproduced with permission of American Chemical Society.

Cu(I)

Cu(I)

Cu(ll), –0.5V vs Cu(0) DMSO, RT

PEDOT-N3 electrode with gradient

(c)

Cu(I)

S

O

N+ N

–N

PEDOT-N3 electrode

(b)

Insulating layer

Copper electrode

(a)

4.2 “Electro-Click” Chemistry (a)

(b) U-type cell

S n

Bipolar electrode

O

PEDOT-N3 film

Cu(II) Cu(I)

Cu(II) N3

R

Cathodic surface

Anodic surface

O

R R

R

“Electro-click” reaction

S

Anode

Cathode

(Driving electrodes)

C8F17

S m

O

R

O

N N N

n-m O

O R

N3 R

R

Figure . Schematic illustration of (a) the U-type cell used for bipolar electrolysis and (b) the “electro-click” reaction of PEDOT-N3 film and terminal alkyne using cathodically generated Cu(I) species. Source: Shida et al. 2012.18 Reproduced with permission of American Chemical Society.

and co-workers reported the “e-click” reaction of azide-functionalized PEDOTN3 and a terminal alkyne using electrochemically generated CuI species on a BPE in a gradient manner.18 PEDOT-N3 was fixed on the indium tin oxide (ITO) electrode through the electro-polymerization and was subjected into a U-type electrolytic cell and acted as a BPE (Figure 4.3). The U-type cell was equipped with a pair of driving anodes and cathodes and was filled with copper sulfate and alkyne. On the BPE in this U-type cell, one side facing the cathode acted as an anodic surface and another side as a cathodic surface. A triazole ring formed at the cathodic electrode indicated the successful proceeding of the “eclick” reaction. Besides the full homogeneous functionalization, SAMs could be modified with complex controlled composition and architecture, length-scale, shape, and functionality, and various gradients could be created with multiple functionalities. “e-Click” by Using Microelectrode Arrays. The “e-click” technique on electrode may offer a way to the local modulation of the concentration of the CuII /CuI catalyst, a popular strategy to selectively modify surface is the using of microelectrode arrays.19–21 The possibility of the combination of the “e-click” reaction with other lithography techniques allows spatially selective functionalization of surfaces by electrochemical activation and deactivation of

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush (a)

(b) V[Cu(I)]

Cu+

Cu2+ Cu+

Cu2+

+ e–

ΔV

e– 1. e-click 2. Base

(c)

Figure . Micrometer-scale surface "clicked" gradient formation via a [Cu(I)] solution gradient. (a) Schematic representation of the electrochemical generation of a [Cu(I)] gradient in solution between the electrodes of an interdigitated electrode array via the reduction of Cu(II) to Cu(I) and oxidation of Cu(I) to Cu(II) at the cathode and anode, respectively. (b) Fluorescence microscopy image of the resulting surface gradient. Scale bar, 100 mm. Source: Krabbenborg et al. 2013.23 Reproduced with permission of Nature Publishing Group.

CuII /CuI complexes, which could find potential applications in biochemistry.22 Huskens and co-workers reported an electrochemically induced Cu(I)catalyzed azide-alkyne 1,3-dipolar cycloaddition, Cu(I) catalyst was generated through the electro-reduction of Cu(II) at the cathode, a stable concentration of [Cu(II)/Cu(I)] was maintained and a gradient concentration of [Cu(I)] formed by the application of a potential difference on the electrode (Figure 4.4). Subsequently, a surface-bound fluorescein dye gradient was prepared by means of the “e-click” reaction of a fluorescein-labeled alkyne to an azide-terminated monolayer due to the catalyst concentration gradient. Gradient features such as steepness and surface density could be easily controlled through the electroparameters in a high sensitive and controlled manner.23 Moreover, bicomponent gradients have been fabricated via a two-step procedure of the “e-click” reaction of two different alkyne-modified dyes and followed by switching of the polarity of the electrodes after the first step. Meanwhile, by means of the transfer patterned “e-click” method, bi-directional surface gradients can be obtained.24 Multiple “Click” Reaction in One Pot. Polymer-based thin films bearing reactive groups offer significant advantages for the intricate design of surface functionalities. Schaaf et al. reported a one-pot strategy to prepare polymer films at the substrate through the CuI -catalyzed “click” reaction between two polymers bearing either azide or alkyne groups (Figure 4.5). All the reactants present simultaneously in the reaction solution, CuI was generated electrochemically from CuII ions present in the solution by the application of a multiple potential cycled between –350 and +600 mV (vs. Ag/AgCl) on the polymer surface.

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4.3 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization

Figure . One-pot CuI -driven formation of films using electrochemically controlled “click” reaction. Source: Rydzek et al. 2011.25 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The CuI catalyst then diffused from the surface toward the solution and started the click reaction. Resulted in the continuous buildup of a film through the formation of triazole molecules between the azide- and alkyne-bearing units at the film/solution interface. Meanwhile, this concept was extended to other systems of the film formation, for example, supramolecular coordination-driven assembly.25 This “electro-click” method makes it possible to impart a variety of functionalities such as hydrophobicity/hydrophilicity or “smart” units onto surface very easily. Multicomponent surfaces can also be generated either through sequential “click” reactions or in a self-sorting manner, and multiple modifications are available in one pot. Moreover, efficient “click” reactions conducted in aqueous conditions are crucial to the end-functionalization of bioconjugated polymers.

. Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization Polymer brushes, a forest of polymer chains attached on the surface, with the polymer chains stretching away from the surface because of the high grafting density of polymer chains, which can be easily obtained with the advent of controlled/“living” radical polymerization techniques, for example, atom transfer radical polymerization (ATRP),26 reversible addition-fragmentation transfer,27 and nitroxide-mediated polymerization.28 The controlled/“living” characteristics of radical polymerizations allow polymers to be synthesized with predetermined molecular weight, low dispersity, and well-defined architectures, which

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush

have largely contributed to the development and implementation of polymer synthesis, biotechnology, materials science, and surface science.7,29 ATRP in aqueous media is of particular interest for the preparation of watersoluble polymers in biorelevant research fields, for example, polymer−protein conjugates and other biohybrids, have excellent potential for biomedical applications. However, ATRP in water provides several intriguing challenges, such as rigorous experimental conditions and a limited control over polymer growth and unavoidable radical termination reactions. Several hypotheses have been advanced to explain the difficulty of gaining a fine-controlled ATRP in aqueous conditions, for example, a high ATRP equilibrium (KATRP ), defined as the ratio of activation (kact ) and deactivation (kdeact ) rate coefficients, will lead to high radical concentration and high probability of termination reactions. Moreover, the halidophilicity of CuII L2+ , the disproportionation of CuI L+ , may also contribute to the outcome of the polymerization under aqueous conditions.30,31 Electrochemically Mediated Atom Transfer Radical Polymerization (eATRP). The use of electrochemistry as an external stimulus for surface-initiated polymerization has gained great interest recently, because it is easy to control the polymerization process by adjusting electrochemical parameters such as the applied potential, current, and the total charge passed, furthermore, these various system parameters can be used to modulate and optimize the polymerization process in situ as well as the quality of the resultant polymers. Matyjaszewski et al. were the first to report an electrochemically mediated atom transfer radical polymerization (eATRP) in 2011.31,32 The applied electrochemical potentials were used to reversibly activate the copper catalyst through the one-electron reduction of an initially added air-stable CuII /L (Figure 4.6). Polymerization kinetics was modulated in a real-time manner by varying the magnitude of applied potentials, which substantially enhanced the level of control of an ATRP process. eATRP on Conducting Substrate. A conventional SI-ATRP process always suffers from drawbacks in terms of rigorous synthetic protocols, limited controllability, heavy consumption and waste of unreacted monomers. With the aim of expanding the scope of the SI-ATRP and attaining a good control in

Figure . Mechanism of conventional (delimited by the dashed line) and aqueous eATRP. Source: Bortolamei et al. 2011.31 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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4.3 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization

Figure . (a) Mechanism of electrochemically induced SI-ATRP. (b) Relationship between the applied potentials and the dry thickness of poly(3-sulfopropyl methacrylate potassium salt) (PSPMA) brushes (reaction time 1 h). (c) AFM image of binary patterned brushes, PSPMA (vertical line) and poly(2-hydroxyethyl methacrylate) (PHEMA) (horizontal line). (d) Recycling of the SPMA monomer solution. Source: Li et al. 2012.33 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

aqueous media, we developed an electrochemical method to perform surface polymerization on initiator-modified gold substrate (Figure 4.7a), the ATRP initiator-decorated gold substrate acted as the working electrode, CuI /L was (re)generated electrochemically by a one-electron reduction of an initially added CuII /L in the vicinity of the initiator layer under a cathodic current, reacted with initiator (R−Br) generating radicals and initiated the polymerization.33 It has been demonstrated that the applied potentials have a strong effect on the kinetics of the brush growth (Figure 4.7b), that is a more negative potential value corresponds to a higher current value and is accompanied by a faster reduction rate of CuII /L and provides a higher concentration of local CuI /L catalyst around the initiator layer and thus a faster polymerization rate. Two different monomers, 3-sulfopropyl methacrylate potassium salt (SPMA) and 2-hydroxyethyl methacrylate (HEMA), were successfully polymerized on a thiol initiator-decorated gold working electrode. Furthermore, the successful

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush

copolymerization of the two different monomers of HEMA and SPMA indicated the controlled/“living” nature of eATRP and the preserved end chain functionality, which would offer the possibility of postmodification of polymer brushes via further orthogonal electro-induced chemical reactions, for example, “e-click” chemistries. The very small amount of oxygen in the polymerization solution was scavenged by the lower oxidation state of CuI /L catalyst, whereas the CuI /L could be (re)generated electrochemically after each reduction/oxidation cycle and hence deoxygenation of the polymerization solution seems to be no longer needed, which is very attractive in practical applications. Since electrochemisty permits a dynamic equilibrium between the two oxidation states of CuI /L and CuII /L, the monomer/catalyst solution can be reused many times (Figure 4.7d). Surface Patterning through eATRP. Surface patterning is always the method of choice for precise functionalization of surfaces at the micro- or nanoscale with strict arrangement and control of surface functionalities. The so-called “top– down” methods, for example, microcontact printing (μCP), scanning probes, UV, and e-beam lithographies, are commonly employed to generate patterns on surfaces.4,34 μCP has been widely used as a simple and flexible method to replicate patterns from a microstructured poly(dimethylsiloxane) (PDMS) stamp, reactive molecules of the “ink” (e.g., thiols) on the PDMS patterns is then transferred to the surface of the substrate (e.g., gold) by conformal contact. Different features of patterned surfaces can be achieved by simply varying the pattern profiles on the PDMS stamp, and more complex patterns could be obtained by multistep printing.35 For example, by a two-step printing of a thiol initiator on gold and followed by SI-ATRP, binary polymer brushes (i.e., surfaces containing two different polymers) were prepared (Figure 4.7c). eATRP on Other Substrates. This electrochemical method for SI-ATRP is highly versatile and compatible with both conducting and nonconducting substrates. As shown in Figure 4.7a, CuI was generated in situ from the initiator layer and thus the working electrode should have sufficient conductivity.33 However, a relative high concentration of [CuI /L]/[CuII /L] around the initiator layer accelerated the polymerization rate with heavy termination reactions, and further linear growth of the polymer brush was quite limited. In particular, this effect will be exacerbated by the fact that when the activation rate coefficient (kact ) in aqueous media is very high.30,33,36 In order to explore the limits of control of aqueous ATRP and define a set of guidelines for conducting a successful ATRP in water, we reported eATRP to control the polymerization in a confined microscale gap through catalyst diffusion, CuI activators generated at the platinum working electrode and diffused to the ATRP initiator modified substrate, which was positioned close to the working electrode (Figure 4.8). The concentration ratio of [CuI ]/[CuII ] was adjusted and maintained throughout the polymerization by simply adjusting applied potentials and arrangements of the initiator-covered substrate, and the

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4.3 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization

Figure . Schematic illustration of using diffusion to control eATRP for surface grafting of polymer brushes. Source: Li et al. 2013.37 Reproduced with permission of American Chemical Society.

utility of eATRP has been substantially introduced to any flat substrate (e.g., Si, Ti, PDMS) if one can immobilize the ATRP initiator on it.37 It is noteworthy that the applied potentials and d values (distance between the initiator and the working electrode) of the microgap have a strong effect on the polymerization rate. A less negative potential and an increased d value lead to an increase of the [CuII /L]/[CuI /L] concentration ratio in the small gap. The polymer brush growth can be tuned by changing the applied voltage and the gap size. A longer duration of living chain growth was observed under a higher concentration ratio of [CuII /L]/[CuI /L] due to an increased rate of deactivation within each activation/deactivation cycle and decreased termination reactions (Figure 4.9a). Since it allows a reversible switching between the higher and lower oxidation state of CuII /CuI complexes, CuI /L activators were generated

Figure . Electrochemically controlled brush growth. (a) Confined eATRP of SPMA on initiator-covered surfaces with respect to time, inset is SPMA polymerization in which CuI catalyst was generated in situ from the initiator surface. (b) Reversible switching of the polymerization between its “on” and “off” states by the application of multistep intermittent potentials. Source: Li et al. 2013.37 Reproduced with permission of American Chemical Society.

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush

under a cathodic current and initiated the polymerization, and the oxidation of the CuI /L was achieved via an anodic current yielding inactive CuII /L species for the cessation of polymerization; the SI-ATRP can be started, stopped, and restarted in a highly responsive manner by the application of multistep intermittent potentials (Figure 4.9b). The controllability over the polymer growth was dramatically enhanced. Gradients Created by Cu(I) Catalyst Diffusion. Gradient polymer brushes, with gradual variation of physicochemical properties along the surface, are in great demand for biochemical applications, the exploration of new classes of polymer brushes is of importance both in the fields of surface science and materials chemistry. They are especially promising in controlling of dynamic phenomena, for example, directional motion of molecules, droplets, and cells. A higher concentration ratio of [CuI /L]/[CuII /L] generated at the initiator layer resulted in a fast polymerization rate, whereas a lower concentration ratio of [CuI /L]/[CuII /L] led to a slow polymerization rate. If an ATRP initiator coated substrate was placed at a tilt angle with respect to the working electrode (Figure 4.8), the [CuI ]/[CuII ] concentration ratio would vary along the initiator surface; this concentration difference would result in different polymerization rates along the surface and lead to a three-dimensional (3D) gradient polymer brush with a gradual in-plane variation in height. The shape, steepness of the gradient could be easily controlled by the topology feature of the initiator, the arrangement of the working electrode in combination with the variation of the applied potentials (Figure 4.10). This approach using the catalyst concentration variation at a surface for gradient buildup is quite promising to create novel functional surfaces. (a)

(b)

z y

x

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34 2

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20. Y[μm] 0.0 –20.0 –150.0 –100.0 –50.0

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200

0.0 X[μm]

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30 min y = 16.7x + 90 1h

160 120

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–0.2

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150.0

–0.6 –50.0 –25.0 0.0 25.0 50.0 75.0 Z[nm]

Figure . Gradient polymer brushes formation on homogeneous (a) and patterned (b) surfaces through Cu(I) catalyst diffusion. Source: Li et al. 2013.37 Reproduced with permission of American Chemical Society.

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4.3 Electrochemically Induced Surface-Initiated Atom Transfer Radical Polymerization

(a)

(b) D [μm]

2H2O→O2+4H+

Cu(II)

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Glass plate 0 10 mm Cu(I)

GC plate as BPE, 25 mm E

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200 180 160 140 120 100 80 60 40 20 0

10 mm 0

HN

O

NH

= Si O n O

0 L

Br m

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10

glass

Figure . (a) Schematic view of fabrication of gradient and patterned polymer brushes by bipolar electrolysis assisted eATRP. (b) Film thickness profile at various positions along the poly(NIPAM)-tethered substrate. Source: Shida et al. 2015.38 Reproduced with permission of John Wiley and Sons.

Patterning and Gradient Formation by Bipolar Electrolysis Assisted eATRP. Inagi et al. reported a bipolar electrochemical method for the fabrication of both gradient and patterned polymer brushes via eATRP (Figure 4.11). A potential gradient generated on a BPE allowed the formation of a concentration gradient of a [CuI ] catalyst at the BPE and produced a reaction field over which the concentration ratio of [CuI ]/[CuII ] varied gradually and hence offered a means to control the polymerization rate during the eATRP process on an initiatormodified surface, resulting in 3D gradient growth of polymer brushes. NIsopropylacrylamide (NIPAM) was successfully polymerized from an initiatormodified substrate surface set close to the BPE.38 The chain end functionality was preserved during the eATRP process and could be reinitiated after the first step of polymerization; the height of the polymer brush increased in a linear manner with increasing the polymerization time. This new method can be used to polymerize a wide range of monomers to fabricate complex gradient with controlled thickness, steepness, and modified area by varying the electrolytic conditions. Moreover, patterned polymer brushes were prepared by using an electrolytic system with an insulating cylinder and BPE that was mounted below the cylinder, generating a circular potential distribution and a circular cathodic surface under the cylinder, the CuI catalyst generated electrochemically around the cathodic area of the BPE mesh (Pt) and diffused to the initiator surface and initiated the polymerization, resulted in the site-selective formation of a circular pattern of polymer brush on the initiator surface. Overall, electrochemically induced ATRP for surface modifications with polymers has a number of attractive features, including improved livingness and controllability over the whole polymerization process, less catalyst loading,

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush

facile synthesis with various functionalities, and the compatibility with automation either in potentiostatic or galvanostatic mode.37,39,40 It would be applied to any copper-catalyzed reactions and provided a robust and effective way for accurately determining, for example, redox behavior of catalyst and reaction kinetics.41,42

. Possible Combination of eATRP and “e-Click” Chemistry on Surface This new concept of electro-induced surface-initiated film buildup and the generality of the electro-induced surface reactions make it applicable to any substrate presenting reactive moieties. The CuI -catalyzed azide–alkyne cycloaddition is utilized particularly compatible with ATRP, due to the ease of incorporating clickable functionality into polymers either through the use of functional monomers or postpolymerization reactions; the halogen groups of polymer chain end can be substituted by azido moieties for the “click” reaction. Moreover, ATRP and azide-alkyne cycloaddition use the same catalyst in each process and allow for the one-pot synthesis, and the choice of SI-ATRP together with “click” chemistry has allowed various functional surfaces for novel applications. For example, a wide range of azide-containing biomolecules such as biotin, carbohydrates, and proteins can be immobilized at polymeric surfaces without losing their biological functionalities.43,44 The “click” chemistry can be employed as a potential candidate of coupling reactions between polymeric surfaces and incoming functional molecules of interest. Vermonden and co-workers reported the synthesis of polymers through ATRP and the subsequent azide substitution and ‘click’ chemistry, three reactions using only one catalyst in one pot.45 The reaction was catalyzed by the ATRP catalyst (Cu) in aqueous solution at ambient condition and followed by a “click” reaction using the same catalyst after the substitution of the living chain end of the polymer chains. This electrochemical means for surface modifications is highly versatile, compatible with many functional groups, solvents, and substrates. The application of “click” chemistry together with ATRP will contribute to rapid development in the available range of polymeric architectures, functional materials, and bioconjugates.46

. Surface Functionality Control over the surface functionality and interaction with the local environment are key to the performance of materials used in practical applications. The recently developed electrochemically mediated reactions, in particular

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4.6 Summary

“e-click” and eATRP reactions, have contributed greatly to the control over surface profiles in terms of length-scale, structure, and functionality. SI-ATRP has been successfully used to synthesize polymers with various functionalities that respond to external stimuli such as temperature, light, pH, or ionic strength, which could find application in sensor and “smart surface.” A polymeric thin film bearing reactive groups offers the possibility to modify such film via further chemical reactions. ATRP is tolerant to many functional groups, and these functional groups might be capable of robust orthogonal “click” chemistry, which are highly efficient and selective, and can be incorporated into hybrids with novel properties. Multiple functional groups can be incorporated into polymers by using either direct polymerization of functional monomers or postmodification reactions. A few nanometers of polymers grown from nanoparticles or flat surfaces, dramatically change surface properties such as dispersibility and stabilization,47 wettability,48 lubricity,49 antifouling and antimicrobial properties,50,51 membrane science,52 actuation,53 and biorelated applications.11,54 Novel organic/inorganic hybrids which combine the best of each constituent synergistically, indicating many new applications. Biomaterials and bioconjugates composing hydrophilic segments and especially stimuli-responsive polymers have largely contributed to the development in the fields of biotechnology, drug and gene delivery, and tissue engineering.44,55 Nowadays, chemists and engineers have abilities to design and fabricate functional surfaces in terms of both structure and morphology by taking advantages of functional materials, and the chemical complexity on 2D and 3D surfaces with well-defined spatial control has significantly increased. However, the complexity of these surfaces is typically limited to one or two different functionalities, and the challenge of controlling over the spatial distribution of the physical and chemical properties still remains and hence new methodologies should be developed to fabricate more complex structures and surfaces with high fidelity and functionalities.

. Summary In this chapter, a number of recent advances in the field of electrochemically induced copper-catalyzed surface reactions have been described. With the ability to polymerize these materials by means of either “e-click,” electrochemically mediated surface-initiated polymerization (eSIP) process, or the possible combination of the two techniques, polymers are formed with controlled size, composition, and architecture. Such exciting and varied possibilities are set to find applications in many research fields, such as organic electronics, biotechnology, and tissue engineering. Nevertheless, detailed mechanistic understanding and the optimization of the synthetic process are required to design and

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4 Electro-Induced Copper-Catalyzed Surface Modification with Monolayer and Polymer Brush

prepare functional materials for targeted applications; it remains a big challenge to further develop novel coatings for our daily life.

Acknowledgments Supports from the National Science Foundation (grants 21434009) and Key Research Program of CAS (grant KJZD-EW-M01) are gratefully acknowledged.

References  Ye, Q.; Zhou, F.; Liu, W. Chem. Soc. Rev. 2011, 40, 4244–4258.  Krishnamoorthy, M.; Hakobyan, S.; Ramstedt, M.; Gautrot, J. E. Chem. Rev. 2014, 114, 10976–11026.  Alswieleh, A. M.; Cheng, N.; Canton, I.; Ustbas, B.; Xue, X.; Ladmiral, V.; Xia, S.; Ducker, R. E.; El Zubir, O.; Cartron, M. L.; Hunter, C. N.; Leggett, G. J.; Armes, S. P. J. Am. Chem. Soc. 2014, 136, 9404–9413.  Welch, M. E.; Ober, C. K. J. Polym. Sci. Part B: Polym. Phys. 2013, 51, 1457–1472.  Azzaroni, O. J. Polym. Sci. Part A: Polym. Chem. 2012, 50, 3225–3258.  Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; M¨uller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010, 9, 101–113.  Barbey, R.; Lavanant, L.; Paripovic, D.; Sch¨uwer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Chem. Rev. 2009, 109, 5437–5527.  Advincula, R. C.; Brittain, W. J.; Caster, K. C.; R¨uhe, J. Polymer Brushes: Synthesis, Characterization, Applications. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2004.  Herrwerth, S.; Eck, W.; Reinhardt, S.; Grunze, M. J. Am. Chem. Soc. 2003, 125, 9359–9366.  Li, B.; Ye, Q. Antifouling Surfaces of Self-Assembled Thin Layer. Springer: Berlin, 2015.  Wei, Q.; Becherer, T.; Angioletti-Uberti, S.; Dzubiella, J.; Wischke, C.; Neffe, A. T.; Lendlein, A.; Ballauff, M.; Haag, R. Angew. Chem. Int. Ed. 2014, 53, 8004–8031.  Arnold, R. M.; Patton, D. L.; Popik, V. V.; Locklin, J. Acc. Chem. Res. 2014, 47, 2999–3008.  Nicosia, C.; Huskens, J. Mater. Horiz. 2014, 1, 32–45.  Hong, V.; Udit, A. K.; Evans, R. A.; Finn, M. G. Chembiochem 2008, 9, 1481–1486.

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References

 Lhenry, S.; Leroux, Y. R.; Orain, C.; Conan, F.; Cosquer, N.; Le Poul, N.; Reinaud, O.; Le Mest, Y.; Hapiot, P. Langmuir 2014, 30, 4501–4508.  Ku, S.-Y.; Wong, K.-T.; Bard, A. J. J. Am. Chem. Soc. 2008, 130, 2392–2393.  Hansen, T. S.; Lind, J. U.; Daugaard, A. E.; Hvilsted, S.; Andresen, T. L.; Larsen, N. B. Langmuir 2010, 26, 16171–16177.  Shida, N.; Ishiguro, Y.; Atobe, M.; Fuchigami, T.; Inagi, S. ACS Macro Lett. 2012, 1, 656–659.  Hansen, T. S.; Daugaard, A. E.; Hvilsted, S. r.; Larsen, N. B. Adv. Mater. 2009, 21, 4483–4486.  Devaraj, N. K.; Dinolfo, P. H.; Chidsey, C. E. D.; Collman, J. P. J. Am. Chem. Soc. 2006, 128, 1794–1795.  Bartels, J.; Lu, P.; Maurer, K.; Walker, A. V.; Moeller, K. D. Langmuir 2011, 27, 11199–11205.  Lind, J. U.; Acikgoz, C.; Daugaard, A. E.; Andresen, T. L.; Hvilsted, S.; Textor, M.; Larsen, N. B. Langmuir 2012, 28, 6502–6511.  Krabbenborg, S. O.; Nicosia, C.; Chen, P.; Huskens, J. Nat. Commun. 2013, 4, 1667.  Nicosia, C.; Krabbenborg, S. O.; Chen, P.; Huskens, J. J. Mater. Chem. B 2013, 1, 5417–5428.  Rydzek, G.; Jierry, L.; Parat, A.; Thomann, J. S.; Voegel, J. C.; Senger, B.; Hemmerle, J.; Ponche, A.; Frisch, B.; Schaaf, P.; Boulmedais, F. Angew. Chem. Int. Ed. 2011, 50, 4374–4377.  Hui, C. M.; Pietrasik, J.; Schmitt, M.; Mahoney, C.; Choi, J.; Bockstaller, M. R.; Matyjaszewski, K. Chem. Mater. 2014, 26, 745–762.  Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610–615.  Nicolas, J.; Guillaneuf, Y.; Lefay, C.; Bertin, D.; Gigmes, D.; Charleux, B. Prog. Polym. Sci. 2013, 38, 63–235.  Li, B.; Yu, B.; Ye, Q.; Zhou, F. Acc. Chem. Res. 2015, 48, 229–237.  Fantin, M.; Isse, A. A.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2015, 48, 6862–6875.  Bortolamei, N.; Isse, A. A.; Magenau, A. J. D.; Gennaro, A.; Matyjaszewski, K. Angew. Chem. Int. Ed. 2011, 50, 11391–11394.  Magenau, A. J. D.; Strandwitz, N. C.; Gennaro, A.; Matyjaszewski, K. Science 2011, 332, 81–84.  Li, B.; Yu, B.; Huck, W. T. S.; Zhou, F.; Liu, W. Angew. Chem. Int. Ed. 2012, 51, 5092–5095.  Nie, Z.; Kumacheva, E. Nat. Mater. 2008, 7, 277–290.  Zhou, F.; Zheng, Z.; Yu, B.; Liu, W.; Huck, W. T. S. J. Am. Chem. Soc. 2006, 128, 16253–16258.  Li, B.; Yu, B.; Zhou, F. Macromol. Rapid Commun. 2013, 34, 246–250.  Li, B.; Yu, B.; Huck, W. T.; Liu, W.; Zhou, F. J. Am. Chem. Soc. 2013, 135, 1708–1710.

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 Shida, N.; Koizumi, Y.; Nishiyama, H.; Tomita, I.; Inagi, S. Angew. Chem. Int. Ed. 2015, 54, 3922–3926.  Magenau, A. J. D.; Bortolamei, N.; Frick, E.; Park, S.; Gennaro, A.; Matyjaszewski, K. Macromolecules 2013, 46, 4346–4353.  Wang, Y.; Soerensen, N.; Zhong, M.; Schroeder, H.; Buback, M.; Matyjaszewski, K. Macromolecules 2013, 46, 683–691.  Bell, C. A.; Bernhardt, P. V.; Monteiro, M. J. J. Am. Chem. Soc. 2011, 133, 11944–11947.  Qiu, J.; Matyjaszewski, K.; Thouin, L.; Amatore, C. Macromol. Chem. Phys. 2000, 201, 1625–1631.  Lee, B. S.; Lee, J. K.; Kim, W.-J.; Jung, Y. H.; Sim, S. J.; Lee, J.; Choi, I. S. Biomacromolecules 2007, 8, 744–749.  Nicolas, J.; Mantovani, G.; Haddleton, D. M. Macromol. Rapid Commun. 2007, 28, 1083–1111.  de Graaf, A. J.; Mastrobattista, E.; van Nostrum, C. F.; Rijkers, D. T. S.; Hennink, W. E.; Vermonden, T.; Abruna, H. D. Chem. Commun. 2011, 47, 6972–6974.  Lutz, J.-F.; B¨orner, H. G.; Weichenhan, K. Macromolecules 2006, 39, 6376–6383.  Mai, W.; Sun, B.; Chen, L.; Xu, F.; Liu, H.; Liang, Y.; Fu, R.; Wu, D.; Matyjaszewski, K. J. Am. Chem. Soc. 2015, 137, 13256–13259.  Azzaroni, O.; Brown, A. A.; Huck, W. T. S. Adv. Mater. 2007, 19, 151–154.  Wei, Q.; Cai, M.; Zhou, F.; Liu, W. Macromolecules 2013, 46, 9368–9379.  Yang, W. J.; Neoh, K.-G.; Kang, E.-T.; Teo, S. L.-M.; Rittschof, D. Prog. Polym. Sci. 2014, 39, 1017–1042.  Ye, Q.; Gao, T.; Wan, F.; Yu, B.; Pei, X.; Zhou, F.; Xue, Q. J. Mater. Chem. 2012, 22, 13123–13131.  Ulbricht, M.; Yang, H. Chem. Mater. 2005, 17, 2622–2631.  Li, B.; Du, T.; Yu, B.; van der Gucht, J.; Zhou, F. Small 2015, 11, 3494–3501.  Jiang, H.; Xu, F. J. Chem. Soc. Rev. 2013, 42, 3394–3426.  Such, G. K.; Johnston, A. P. R.; Liang, K.; Caruso, F. Prog. Polym. Sci. 2012, 37, 985–1003.

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

 Polymer Brushes on Flat and Curved Substrates: What Can be Learned from Molecular Dynamics Simulations K. Binder,1 S.A. Egorov,2 and A. Milchev3 1

Institute f¨ur Physik, Johannes Gutenberg-Universit¨at Mainz, Mainz, Germany Department of Chemistry, University of Virginia, Charlottesville, VA, USA 3 Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria 2

. Introduction As discussed elsewhere in this book, polymer brushes can find numerous applications to tune various surface properties of a material (wettability of a surface, adhesive properties, lubrication, preventing protein adsorption in a biological environment (“nonfouling surfaces”), use of polymer brush coated nanoparticles as drug carriers, and many others). In order to control such properties to obtain the optimal function, a detailed theoretical understanding of the macromolecular conformation in the brush and the overall structure of the brush and its dynamical properties would be useful. This is a difficult problem: Apart from local intramolecular interactions in a macromolecule (that control its intrinsic stiffness, for instance), the monomers interact with the solvent molecules and the substrate atoms, as well as with the monomers of the other endgrafted chains. Even in the simplest case, namely substrates with a short-range repulsion, electrically neutral polymers, no hydrogen bonds or other effects of a specific chemistry, and using macromolecules that are monodisperse linear homopolymers, this is a very rich problem (see, e.g., Ref. 1 for a review). Depending on parameters such as grafting density 𝜎g , chain length N (i.e., the number of effective monomeric units along the chain), persistence length 𝓁p , and the solvent quality, there is a delicate interplay between the entropic elasticity of the more or less stretched chain molecules in the brush and the various enthalpic interactions. Analytical theories can only provide a rather approximate and crude description of such problems, and hence the use of atomistic computer simulation techniques, such as Monte Carlo2,3 and Molecular Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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5 Polymer Brushes on Flat and Curved Substrates

Dynamics (MD)4,5 methods, is very desirable. However, it is easy to see that this is a challenging problem, and trying to use a chemically realistic all-atom model easily leads to an unreasonably large effort with regard to the necessary computational resources. We can see this easily by considering a classical experiment,6 where by neutron reflectometry methods the monomer density profile of chemically endgrafted almost monodisperse polystyrene (PS-SiCl3 ) on a silicon slab was studied, with a molecular weight of Mw ≈ 105,000, for various solvent conditions. The dimensionless grafting density 𝜎g was 𝜎g = (a∕D)2 = ˚ is the statistical segment size and D is the average dis0.027, where a = 6.7 A tance between grafting sites. Depending on solvent quality, the observed profile varies over a distance z from 30 to 100 nm above the (flat) substrate. So it would be desirable then to simulate a cubic box with a linear dimension 100 nm, containing both the grafted chains and explicit solvent: but even for the fastest supercomputers available today, this is an enormous effort. For example, a simulation of (nongrafted) polybutadiene chains (with 1440 chains each containing Np = 116 “united atoms” representing CH2 or CH3 groups) confined between graphite walls could use only linear dimensions of about 15 × 15 × 20 nm, needing substantial computer resources at a JUGENE supercomputer.7 Consequently, computer simulations of polymer brushes almost exclusively restrict attention to coarse-grained models, where a chain molecule is described by effective monomeric units connected by effective bonds, such that three to five covalent bonds along the chain backbone are lumped into one effective bond, chemical details such as torsional potentials are not explicitly considered, and the effective beads are taken as spherically symmetric objects, their diameter being of the order of the statistical segment length, which is taken as an effective length unit. Often the solvent molecules are not included explicitly in the simulation, and thus the solvent quality is only described implicitly by the choice of appropriate interactions between the effective monomeric units. Then it becomes possible to simulate long enough chains (with a number N of effective monomeric units of a few hundreds) and large enough simulation boxes (containing of the order of 105 such monomeric units) to obtain sufficiently accurate data on both static and dynamic properties of the brushes (e.g., Ref. 8). However, when one uses such coarse-grained models, the explicit connection between the model and a specific material is lost. Constructing coarse-grained models from a suitable mapping to a chemically realistic model still is an active area of research9 and out of our focus here; we only remark that such a mapping for polymer brushes would be dependent on the thermodynamic state (temperature, pressure, nature of the solvent, grafting density, etc.). Thus, the main value of simulation is to provide a qualitative understanding of the properties of polymer brushes and to provide a general insight into their structure–property relationships, rather than to predict specific numbers for actual materials. A prominent use of simulations hence is to test the

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5.1 Introduction

theoretical predictions of approximate theories, since the latter are often based on the same model as the simulation (or a simplified version thereof ), but involve additional very severe and questionable assumptions. Common to most theories are mean-field assumptions. For instance, the single-chain mean field theory (SCMF) of Szleifer and Carignano35 treats the intramolecular interactions exactly (numerically) whereas interactions of this polymer chain with other polymers and solvent molecules are treated with a mean field approximation. The mean field interactions depend on the density of the polymer in the z direction (perpendicular to the grafting plane) and affect the polymer configuration: This problem needs to be solved iteratively to reach self-consistency. In practice, this method is used for not too long chains (typically N ≤ 100). Since the reference state for SCMF is a single chain in good solvent, the method is excellent for very low grafting density; if we could neglect excluded volume interactions, and consider the single chain problem as that of a Gaussian chain, SCMF reduces to the self-consistent field theory (SCFT). This is the most popular theory, since it can be solved analytically explicitly in the strong stretching limit (leading to the well-known parabolic density profile of the brush), and a more general numerical version on the lattice (due to Scheutjens and Fleer57 ) is most widely used. None of these theories describe local effects on the scale of individual monomers (such as the density oscillations, the so-called “layering,” in a dense brush near the substrate); such effects can be captured by density functional theories (DFT), however. But the latter have problems to deal with the conformational statistics of long-chain molecules in various conditions. In order to give an example for the spirit of the present chapter (and whet the appetite of the reader for what follows), Figure 5.1 shows the probability distributions pi (zi ) for special monomers (labeled by index i from the monomer i = 1 next to the grafting site up to the free chain end i = N, for N = 64) together with the corresponding local relaxation times 𝜏i . Scaling arguments10,11 predict 𝜏i ∝ [⟨zi 2 ⟩ − ⟨zi ⟩2 ]∕Deff , where Deff is some effective short-time diffusion constant. Since the mean square fluctuation of the coordinate zi of the ith monomer (in the z-direction normal to the flat grafting surface) increases monotonously with i, as is obvious from Figure 5.1a, one would expect a monotonous increase of 𝜏i with i as well, but this is not the case (Figure 5.1b). Note that also a more refined calculation, based on the well-known Rouse model of polymer dynamics12 where one considers the dynamics of monomer i in a local potential due to the neighboring chains, as provided by SCFT13 confirms the scaling argument. However, both this theory and the scaling theory fail because they do not take into account that the local monomeric mobility also depends rather distinctly on the position i along the contour, and should not be taken as a constant (∝ Deff , as done above). This is an example how one can examine specific steps in theoretical arguments. Note also that the enhanced mobility of monomers in the outer region of a brush also is of relevance for various

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5 Polymer Brushes on Flat and Curved Substrates (a)

(b) 0.25 3500

i = 16 i = 32 i = 48 i = 64

0.20

3000 Relaxation times τi

Probability distribution function P(z)



0.15

0.10

2500 2000 1500 1000

N = 64 x N = 64 z N = 64 xyz

0.05 500 0.00

0

5

10

15

20

25

30

35

z-postion

40

0 0

10

20

30

40

50

60

70

Monomer position i along chain contour

Figure . (a) Probability distribution Pi (zi ) of a monomeric unit labeled by the index i (i = 1 is next to the grafting site, i = N = 64 the free chain end) plotted versus the coordinate zi , in the z-direction perpendicular to the planar grafting surface, for a reduced grafting 𝜎g = 0.125 (see Section 5.3 for a detailed definition of the model). Four cases (i = 16, 32, 48, and 64) are included, as indicated. (b) Relaxation times 𝜏i plotted versus monomer index i. Both the times referring to lateral motions (x) and to transverse motions (z) as well as the total times are shown, as indicated. Source: Reith et al. 2012.8 Reproduced with permission of American Chemical Society.

applications, such as lubrication properties when weakly interpenetrating brushes are sheared against each other,14,15 a topic that is still strongly debated (e.g. Refs. 16–19). The outline of this chapter is now as follows: in Section 5.2 we give a short “primer” on the technique of MD simulations; Section 5.3 briefly describes the standard bead-spring model15 and a few extensions. Section 5.4 then presents selected examples of recent applications where chains are grafted on spherical and cylindrical surfaces, whereas Section 5.5 discusses the interaction of brushes with free chains. Section 5.6 concludes the chapter with a brief summary.

. Molecular Dynamics Methods: A Short “Primer” MD essentially means that one deals with the classical mechanics of N interacting particles by numerical integration of the Newton’s equation of motion. ⃗ = {⃗rj } = (xj , yj, zj ), j = 1,…, N , these equations Using Cartesian coordinates X are mj d2 ⃗rj ∕dt 2 = −𝜕Upot ∕𝜕⃗rj = f⃗j

(5.1)

with mj being the mass of the jth particle and f⃗j is the force acting on it. In this section, we assume for simplicity that the forces arise from the gradient 𝜕∕𝜕⃗rj

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5.2 Molecular Dynamics Methods: A Short “Primer”

⃗ and that we just have only pairwise interactions of the potential Upot (X) ⃗ = Upot (X)

N −1 ∑ N ∑

U(|⃗rj − ⃗rk |)

(5.2)

j=1 k>j

although this needs to be generalized when we wish to allow, for example, for a bond-angle potential in a polymer, which depends on the coordinates of three successive monomers along the chain contour. In classical mechanics, the total energy ⃗ = E = Ekin + Upot (X)

N ∑ 1 i=1

2

⃗ mi (d⃗ri ∕dt)2 + Upot (X)

(5.3)

is a conserved quantity (constant of motion), so MD realizes the NVE ensemble of statistical mechanics (the volume of the simulation box is also held fixed, and boundary conditions need to be specified, e.g., periodic boundary conditions in the x,y directions parallel to the grafting plane of a flat polymer brush). However, in order to solve Eq. (5.1) by numerical integration, the time t needs to be advanced in discrete steps 𝛿t and this inevitably amounts to an integration error. To make this error as small as possible, one needs an efficient algorithm. We mention here only the Verlet algorithm4,5 : ⃗rj (t + 𝛿t) = 2⃗rj (t) − ⃗rj (t − 𝛿t) +

1 (𝛿t)2 f⃗j (t) + O((𝛿t)4 ) mj

(5.4)

whereas the velocity 𝜐⃗j (t) = d⃗rj ∕dt of the jth particle becomes 𝜐⃗j (t) =

1 [⃗r (t + 𝛿t) − ⃗rj (t − 𝛿t)] + O((𝛿t)3 ) 2(𝛿t) j

(5.5)

This algorithm is manifestly time reversible (exchange of ⃗rj (t + 𝛿t) and ⃗rj (t − 𝛿t) yields the propagator for the time evolution going backwards in time, and the sign of the velocity then is opposite). This algorithm is particularly stable over very long times. Of course, we need to clarify what is the scale for the time step 𝛿t. Let us assume that nonbonded monomers interact with the well-known Lennard– Jones (LJ) potential, with r = |⃗rjk | = |⃗rj − ⃗rk | U(r) = 4𝜀[(a∕r)12 − (a∕r)6 ]

(5.6)

and we take the parameters for the scales of energy and monomer diameter ˚ 𝜀∕kB ≈ 120K, and also we may take the the same as for argon atoms, a ≈ 3.4A, −23 corresponding mass, m ≈ 6.6 × 10 g. Rescaling the coordinates as ⃗rj∗ = ⃗r∕a

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5 Polymer Brushes on Flat and Curved Substrates

yields, from Eq. (5.4), ⃗rj∗ (t + 𝛿t) = 2⃗rj∗ (t) − ⃗rj∗ (t − 𝛿t) −(𝛿t)2

⃗r∗ 48𝜀 jk ∑ ∗ −13 ∗ −7 [(rjk ) − (rjk ) ∕2] ∗| ma2 |⃗rjk k(≠j)

(5.7)

From Eq. (5.7), we see that the natural time unit 𝜏0 is 𝜏0 = (ma2 ∕𝜀)1∕2

(5.8)

which becomes 𝜏o ≈ 2.1 × 10−12 s for argon. The time step 𝛿t has to be much smaller than 𝜏0 , for example, 𝛿t = 10−14 s, to reach a very good accuracy. So even with a million of time steps one only gets a real time of the order of 10ns! Since good equilibration of a model system requires that the total length of a MD run exceeds the largest relaxation time of the system, we readily recognize why the computer simulation of polymeric systems is such a challenging task (see, e.g., Ref. 20 for a more complete discussion). This problem is particularly severe for polymer brushes, since the largest relaxation time is already predicted to scale as N3 even if entanglement effects are ignored,11 and even larger powers of N were empirically found from simulations.8 If one would just use Eqs. (5.1)–(5.8), one would realize the so-called “microcanonical ensemble” of statistical mechanics, where E is strictly conserved and the conjugate thermodynamic variable, the absolute temperature T, is a fluctuating quantity. However, since in classical statistical mechanics the distributions of positions and velocities factorize, one simply has a Maxwell– Boltzmann distribution for the velocities. Therefore, T can be extracted from the kinetic energy of the particles observed in the simulation. We can define a “kinetic temperature” T as follows: 1 ∑ 2 Ekin = mj 𝜐⃗2j , 𝜏= 3kB N 3k B N j=1 N

T = ⟨𝜏⟩

(5.9)

and from the fluctuations (which are down by a factor of N ) one can infer the specific heat Cv of the system (⟨…⟩ stands for an average taken in the appropriate ensemble), [⟨𝜏 2 ⟩ − ⟨𝜏⟩2 ]∕⟨𝜏⟩2 =

2 (1 − 3kB ∕2Cv ) 3N

(5.10)

Similarly, since V is also fixed, the conjugate intensive thermodynamic variable, the pressure p, also is a fluctuating variable. Defining the appropriate dynamical variable as ( ) N ∑ 1 ⃗rj ⋅ f⃗j P= 2Ekin + (5.11) 3V j=1

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5.2 Molecular Dynamics Methods: A Short “Primer”

the virial theorem states p = ⟨P⟩ =

N NkB T 1 ∑ ⟨⃗r ⋅ f⃗ ⟩ + V 3V j=1 j j

(5.12)

Since for very long runs due to the accumulation of integration errors, the total energy is not really conserved, the microcanonical ensemble often is impractical, however, and one carries out simulations using a “thermostat,” so that one realizes a strict canonical NVT ensemble. For polymeric systems, the most popular thermostat is the “Langevin thermostat”: One adds a friction term plus a random force to the equation of motion,20 mj

d2 ⃗rj dt 2

=−

𝜕Upot 𝜕⃗rj

−𝛾

d ⃗rj dt

+ f⃗jR (t)

(5.13)

where 𝛾 is the friction coefficient and the random force f⃗jR (t) satisfies the fluctuation dissipation theorem ⟨ ⟩ f⃗jR (t) ⋅ f⃗kR (t ′ ) = 6kB T𝛾𝛿jk 𝛿(t − t ′ ) (5.14) Using again Eq. (5.9), one can verify that the temperature is actually kept constant, while now the total energy E fluctuates, ⟨E2 ⟩ − ⟨E⟩2 = Nk B T 2 C v

(5.15)

since ⟨E⟩∝N , the relative mean square fluctuations are down by a factor 1/N , of course. We emphasize, however, that adding these two extra terms in Eq. (5.13) in comparison with Eq. (5.1) will distort the intrinsic dynamics of the system. For example, when one simulates a dilute polymer solution, one should be able to verify the Zimm model,12 which predicts that due to long-range hydrodynamic interactions the selfdiffusion constant Dself of the macromolecule scales inversely with its gyration radius Rg (Rg ∝ aN 𝜈 with 𝜈 ≈ 0.59 for good solvent conditions10 ), and this is also found experimentally. However, using Eqs. (5.13) and (5.14), one finds21 that the friction and random forces (which violate momentum conservation that holds for strictly Newtonian dynamics) screen the hydrodynamic interactions among the monomers, and hence Dself ∝ 1∕N results,21 as in the Rouse model.12 Note that due to the difficulty of carrying out long enough strictly microcanonical runs that conserve energy in practice well enough, D¨unweg and Kremer21 generated an ensemble of well-equilibrated chain configurations with Langevin dynamics (Eqs. 5.13 and 5.14), which were used as starting configurations for the microcanonical runs from which the dynamic structure factor S(q,t) was sampled (where q is the momentum transfer in a scattering experiment).21 In recent work on sheared polymer brushes,17,18 momentum conservation was implemented by replacing the “Langevin thermostat” (i.e., Eqs. 5.14 and





5 Polymer Brushes on Flat and Curved Substrates

5.15) by the “dissipative particle dynamics”22 thermostat: There a friction force acts on the relative velocity d(⃗rj − ⃗rk )∕dt between two particles only. Note that for “nonequilibrium molecular dynamics”23 simulations, where external forces create shear deformations, the resulting entropy production would heat up the simulated system, if no thermostat is applied to remove the created heat (see Ref. 24 for a discussion of computational aspects of thermostats for sheared polymer brushes).

. The Standard Bead Spring Model for Polymer Chains Following the pioneering work of Grest et al.,25,26 we only describe here in detail the implicit solvent model where for good solvent conditions the effective interaction between the monomeric units in the system is repulsive, [ ] (5.16) Umm (r) = 4𝜀 (a∕r)12 − (a∕r)6 + 1∕4 , r < rc = 21∕6 a while Umm (r ≥ rc ) = 0. Equation (5.16) is essentially the LJ-potential (Eq. 5.6), cut off and shifted to zero in the minimum. Umm (r) acts between all pairs of monomers, both bonded and nonbonded ones. Bonded pairs experience in addition the finitely extensible nonlinear elastic (FENE) potential, [ )2 ] ( , r < R0 (5.17) UFENE (r) = −0.5kR20 ln 1 − r∕R0 while UFENE (r ≥ R0 ) = ∞. Parameters typically are chosen as 𝜀 = 1, temperature T = 1.0𝜀∕kB , and a = 1 (unit of length), k = 30, R0 = 1.5, and the particle mass m = 1, so 𝜏0 = 1 (Eq. 5.8). The friction coefficient can be chosen as, for example, 𝛾 = 0.5 in Eq. (5.13). If one includes a real solvent in the simulation (see, e.g., Refs. 17 and 18), one may simply use the same model (Eqs. 5.16 and 5.17), but reduce the “chain length” N to “N = 2” (dimers) to describe the solvent molecules. Almost incompressible solvents then are obtained choosing a (reduced) density 𝜚 = N/V = 0.9. The model equations (5.16) and (5.17) define a fully flexible macromolecule, where the persistence length 𝓁p (describing local chain stiffness27 ) is of the same order as the size a of the effective monomer. We define here the persistence length in terms of the correlation function between subsequent bond vectors b⃗ i = ⃗ri − ⃗ri−1 along the chain contour as28 𝓁p = −a∕ ln(⟨cos 𝜃⟩)

⟨cos 𝜃⟩ = ⟨b⃗ i+1 ⋅ b⃗ i ⟩∕⟨b⃗ 2i ⟩.

(5.18)

Recalling that the textbook definition27 in terms of the exponential decay of bond correlations s steps apart along the chain backbones, ⟨b⃗ i+s ⋅ b⃗ i ⟩ ∝ exp[−s|b⃗ i |∕𝓁p ] makes sense only for phantom chains for which interactions

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5.3 The Standard Bead Spring Model for Polymer Chains

among the monomers are completely ignored.28 Now it is well known that most ˚ polymers do exhibit some stiffness, for example, for polystyrene 𝓁p ≈ 10−12 A, and some other polymers (polyelectrolytes, double-stranded DNA, etc.) are much stiffer. In the framework of a coarse-grained model, we may control stiffness by a bond angle potential, Ubond (𝜃) = 𝜀b [1 − cos 𝜃]

(5.19)

where 𝜃 is the angle formed between the two subsequent unit vectors b⃗ i ∕|b⃗ i | and b⃗ i+1 ∕|b⃗ i+1 |. The energy parameter 𝜀b controls the bending stiffness and hence 𝓁p (note 𝓁p ≈ 𝜀b ∕kB T for 𝜀b > 2). For planar brushes, the impenetrable repulsive surface is represented by a rigid square lattice with lattice spacing a, and the atoms placed on the lattice sites interact with the monomers with the same potential (Eq. 5.16). Of course, different assumptions are made when one deals with an attractive substrate, using then an attractive LJ potential for the interaction between wall’s atoms and monomers, for instance.29 The grafting sites at this surface then typically also are chosen as a square array, the spacing being an integer multiple of a, and periodic boundary conditions in the x and y directions parallel to the substrate are used. We also note that Eq. (5.16) may be replaced by a LJ potential truncated (and shifted to zero) at a larger value of rc (e.g., rc = 2.5a) to include an attractive part in the monomer–monomer potential, and thus vary the solvent quality of this implicit solvent model.30 However, a slightly more realistic variation of solvent quality can be achieved by using explicit solvent models, where one then complements Umm (r) by potentials for Ums (r) and Uss (r) for monomer–solvent and solvent–solvent interactions (e.g., Ref. 31). However, the drawback of such explicit solvent models is that for semidilute concentrations the computational effort due to the large number of solvent molecules becomes huge. Thus, often then only chain lengths N ≤ 100 are accessible (e.g., Ref. 31). While such chain lengths are useful for studying either very dense (melt-like) brushes or the mushroom to brush crossover,32 they can hardly probe the strong stretching limit in the semidilute solution case. According to the “Alexander picture”33 (which is a crude simplification but nevertheless provides a useful first orientation), a chain in such a brush is described as a linear string of nb spherical blobs of diameter D, D being the distance between the grafting sites. If excluded volume interactions are not very strong, chain statistics inside a blob is still ideal, that is, the number n of monomers in the part of the backbone corresponding to one blob is n = (D∕a)2 = 𝜎g−1 , and then nb = N∕n. For N = 1000 and 𝜎g = 0.027 (as in a typical experiment6 ) one finds nb ≈ 27 and the brush height h becomes h = nb D, that is about 100 nm, roughly compatible with observation. If N < 100, however, it is hardly possible to have both n very large (to maintain semidilute conditions) and nb (to reach the strong stretching limit).





5 Polymer Brushes on Flat and Curved Substrates

. Cylindrical and Spherical Polymer Brushes When polymers under good solvent conditions are grafted on the outer surface of a cylinder, it matters how the cylinder radius R compares to the brush height h.1,26 For R ≫ h, the structure of the brush is not essentially different from the structure of a brush at a strictly planar substrate; note that the box-like density profile 𝜚(z ≤ h) = 𝜚0 and 𝜚(z > h) = 0,33 is found in reality only for very dense brushes, when 𝜚0 is close to the melt density, whereas for semidilute conditions the parabolic density profile 𝜚(z) = 𝜚0 (1 − z2 /h2 ) predicted by the strong stretching limit of the SCFT34 is a better approximation.26,35 For R ≪ h, however, a power law for the density profile 𝜚(r) results, 𝜚(R < r < R + h) ∝ r−(3v−1)∕2v ≈ r−0.65 (using 𝜈 ≈ 0.59, from Ref. 10); with the Flory value10 𝜈 = 3/5 the power law would be r−2∕3 . While this behavior was verified by simulations and SCFT decades ago (see Refs. 1 and 26 for reviews), cylindrical brushes with R ≪ h under bad solvent conditions have found attention only recently.36,37 Depending on solvent quality, grafting density, and chain length, the structure of the brush then can either be a fairly compact cylinder or a pearl-necklace type structure; however, the “pearls” are not spherical objects, but rather ellipsoids elongated along the cylinder axis. We note that the details of this behavior are not fully understood yet: Simulations of laterally inhomogeneous brushes under bad solvent conditions suffer from equilibration problems, and analytical theories like SCFT also are difficult for such spatially inhomogeneous situations. An interesting related case results when the grafting of the polymers occurs on a more or less flexible polymer backbone rather than on a rigid rod: These so-called “bottle brush” polymers have found enormous interest recently (see, e.g., Refs. 38–41). For good solvents or theta solvent conditions, the structure of the bottle brush is cylindrical for length scales less than the persistence length 𝓁p along the backbone. An interesting question then is to understand how 𝓁p depends on the chain length N of the side chains and their grafting density. Simulations have shown that for typical conditions 𝓁p is of the same order as the cross-sectional radius Rc of the bottle brush,39,40 , whereas for N → ∞ one expects a much stronger backbone stiffening. Also understanding precisely how the effective contour length Lcc of the cylinder depends on the backbone molecular weight is a nontrivial issue.39,40 Bottle brushes can also serve as building blocks to form complex supermolecular aggregates.41 Another variation on this theme is the grafting of chains on the inner surface of a cylinder.42–44 If solvent conditions are such that h < R in the center of the cylinder, there is an inner region (of radius R − h) through which transport of fluid or even macromolecules is still possible; however, if the chains are more strongly stretched, not enough free space to allow transport near the cylinder axis may be left.44 Thus, this situation may be of interest to control flow in nanocylindrical channels and related applications. As an example,

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5.4 Cylindrical and Spherical Polymer Brushes (a)

(b)

20

10

15

10

σg = 0.02 N = 8, N = 50f σg = 0.05 σg = 0.1 σg = 0.2 σg = 0.3

9 8 Reez

σg = 0.05 σg = 0.08 σg = 0.10 σg = 0.12

7 6

5

5 2

4

6 R

8

2

3

4

5

6 7 R

8

9

10

(c)

ξg

ξ

2R

Figure . (a) End-to-end distance of a free chain (Nf = 50) along the cylinder axis for a nanotube where chains of length N = 8 are grafted at the inner cylinder surface plotted versus the cylinder radius R. MD results for four choices of the grafting density 𝜎g are shown. (b) Same as (a), but for a SCFT calculation using a cylindrical lattice version of SCFT (c) Change in polymer conformations on decreasing nanopore radius R in the polymer brush coating, the blob size 𝜉g is determined by the grafting density of the brush, whereas the size of the blobs of the free chain (𝜉) is given by 2(R − h) as long as 𝜉 > 𝜉g . When 𝜉 would become less than 𝜉g , the free chain penetrates into the brush. Source: Wang et al. 2012.44 Reproduced with permission of American Chemical Society.

Figure 5.2 shows how the combination of MD and SCFT can elucidate the rather complex behavior that may occur: when a free chain of length Nf is brought in such a brush-coated cylinder, one finds that its linear dimension Reez along the cylinder axis may exhibit a nonmonotonic variation with the cylinder radius, rather than the expected monotonic decrease with R. This SCFT prediction is qualitatively supported by MD, and the latter method also validates the physical interpretation: only when R − h is large enough, will the free chain form a string of blobs of diameter 𝜉 = 2(R − h), whereas if 𝜉 would become too small, the free energy cost when the chain overlaps the “blobs” of the brush is less than the free energy penalty resulting from strong stretching, and the free chain rather forms blobs of diameter 2R. While the MD simulation is practical for rather small N and Nf , however, SCFT can be worked out for much larger chains, and hence it is useful to combine both methods to draw a complete picture. Finally we turn attention to polymers grafted to spherical nanoparticles; Such objects are of particular importance to produce polymer nanocomposites with





5 Polymer Brushes on Flat and Curved Substrates

desired mechanical and rheological properties, and hence have been studied intensively (see, e.g., Refs. 1, 26, and 45). In the “star polymer”46 limit where the brush height h exceeds the nanoparticle radius Rc , one expects for good solvent conditions a power law for the density profile 𝜚(Rc < r < Rc + h) ∝ r(1∕v−2) ≈ r−1∕3 , and the brush height should exhibit a scaling behavior (where h0 describes the brush height of a planar brush) )1∕3 ( h = h0 F(h0 ∕Rc ), h0 = const 𝜎g a2 aN (5.20) with F (𝜍) ∝ 𝜍 −2∕5 [1,26] for large 𝜍 = h0 ∕Rc . The complete crossover scaling function F(𝜁 ) has been estimated by SCFT methods.47 An accurate estimation of F(𝜁 ) by MD has been attempted recently,48 but it turned out that one needs to use a number f of grafted chains of order f ≈ 102 , and then Rc ∕a ≈ 10 is needed to avoid crowding of monomers near the grafting sites. Then very large N is needed to realize the limit h0 ≫ Rc . It was also found48 that DFT provides a very accurate description of the local structure of the spherical brush for distances r − Rc ≪ h, but fails to reproduce the star polymer scaling. SCFT, on the other hand, does not describe the local packing of the monomers near the grafting surface well. Thus, there is still need for more work on rather basic aspects of spherical polymer brushes. Similar problems are encountered for spherical brushes under theta conditions.49 Particularly difficult situations are encountered for poor solvent conditions, where the brush-coated nanoparticles tend to aggregate into nonspherical clusters.50

. Interaction of Brushes with Free Chains When a polymer brush interacts with a polymer solution, the number of cases that can be considered amounts to a large parameter space: In addition to the grafting density 𝜎g and chain length Ng of the grafted chains and the solvent quality for the grafted chains (described, e.g., by some energy parameter 𝜀g ), we need to consider the chain length Nf of the free chains, their concentration in the solution, the solvent quality for the free chain may differ (𝜀f ), and in addition, an interaction between monomers and grafted chains (𝜀fg ) may arise. Only special cases of this rich subject have been considered; and in this section we address just one example, namely, the critical adsorption of a single macromolecule in polymer brushes.51 It has been predicted that the adsorbing brush can be treated like a pure surface on which an adsorption potential of range W = h and strength U acts.52 Theory then predicts that adsorption occurs if U exceeds a critical value Uc given by53 Uc = a2 𝜋 2 ∕(24W 2 ), Gaussian chains

(5.21a)

Uc ∝ W −5∕3 , self-avoiding chains

(5.21b)

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5.6 Summary

In a mean field theory, U is related to the monomer density 𝜚brush ≈ 𝜎g Ng ∕h as U = 𝜀fg 𝜚brush a3 = 𝜀fg 𝜎g Ng a3 ∕h and hence the critical value 𝜀cfg of adsorption 1∕3

would scale as (h ∝ a(𝜎g a2 ) Ng , in the strong stretching limit) )−4∕3 −2 ( Ng , Gaussian chains 𝜀cfg ∝ 𝜎g a2 )−11∕9 −5∕3 ( Ng , excluded volume chains 𝜀cfg ∝ 𝜎g a2

(5.22a) (5.22b)

Thus the simple mean field theory predicts a monotonic decrease of 𝜀cfg with

increasing 𝜎g . However, already a numerical SCF approach51 shows that for small 𝜎g this decrease stops and a minimum value of 𝜀cfg is reached, while for dense brushes a rather strong increase of 𝜀cfg is found again (Figure 5.3a). The interpretation of this effect is most nicely seen from the MD simulation (Figures 5.3b and 5.3c). Only for sufficiently small 𝜎g gets the free chain “sucked into” the brush if 𝜀fg is sufficiently strong; for large 𝜎g the brush–solvent interface rather acts like a hard wall, on which the free chain gets adsorbed, and there always is only little overlap of the density profiles of free and grafted chains in this case (Figure 5.3c). Clearly, this situation is outside of the scope of the theory (Eqs 5.21a, 5.21b, 5.22a, and 5.22b). Figures 5.3b and 5.3c illustrate nicely also the pronounced layering that one expects for the dense brushes (this can be described by DFT but not by SCFT, of course). One might think that the problem of critical adsorption of polymers in a brush is a rather academic problem; however, we like to draw attention to the fact that this phenomenon actually is utilized in “reversed phase liquid chromatography”54 for the separation of macromolecules in solution. In the above discussion, we have restricted attention to the case Nf ≫ Ng . Of course, it also is of interest to consider the opposite limit Nf ≪ Ng ,55 or the interaction of brushes with nanoparticles56 (for a brief review of pertinent simulations, see Ref. 1).

. Summary In this chapter, we have attempted to give the reader the flavor of what a MD simulation is and what it is useful for, discussing a few selected examples, drawn from the research group of the authors for the sake of simplicity only (we apologize to all colleagues in the field, whose excellent work has been disregarded here simply due to lack of space). We have emphasized that MD simulations have some intrinsic limitations (due to the large relaxation times that appear particularly when the polymer chain length gets very long). But when one is aware of the limitations, one can advantageously use them to test various theories, both on static and dynamic



5 Polymer Brushes on Flat and Curved Substrates (a)

5

1/Nf --> 0 Nf = 4096 Nf = 2048 Nf = 1024

χfg

4

3 Adsorbed 2

1 Desorbed 0 −3 10

10

−2

−1 σg 10

(b)

10

0

(c) 0.03

0.03 g

0.02

εc= 0.40

0.01

0

0

10

20

z /σ

ϕb(Z) εfg = 0.34 εfg = 0.38 εfg = 0.42 εfg = 0.46 εfg = 0.50 εfg = 0.56 εfg = 0.58

30

N = 16 g

0.02

ϕp(Z)

N = 16

ϕp(Z)



0.01

40

0 0

10

20

ϕb(Z) εfg = 0.40 εfg = 0.44 εfg = 0.48 εfg = 0.52 εfg = 0.56 εfg = 0.60 εfg = 0.64

30

40

Z

Figure . (a) Phase diagram for the adsorption transition of free chains of length Nf attracted by a brush with chain length Ng = 32 and grafting density 𝜎g (note a = 1 was chosen in the figure), and the Flory Huggins parameter 𝜒fg is the dimensionless analog of 𝜀fg . The “transition” is a sharp phase transition only for Nf → ∞ (broken curves for finite Nf show the location where the derivative |dΓads ∕d𝜒fg | of the adsorbance Γads has a rounded maximum). (b). Density profile of grafted chains (Ng = 16, 𝜎g a2 = 0.25) and of a free chain (Nf = 400) at various values of 𝜀fg . For 𝜀fg > 𝜀cfg = 0.40, the majority of the monomers of the free chain is absorbed into the brush. (c) Same as (b) but for 𝜎g a2 = 0.75. Increasing 𝜀fg leads to a localization of the free chain at the brush. Source: Milchev et al. 2014.51 Reproduced with permission of Royal Society of Chemistry.

properties of polymer brushes, and the detailed insight that they give makes MD a useful tool also for the discovery of new phenomena. An example has been given in Section 5.5, namely the crossover from adsorption inside a brush to adsorption on the brush–solution interface when the grafting density increases. With respect to testing theories, we have argued that a useful application can also be the examination of specific assumptions in an analytical derivation (see the example regarding local monomer mobility and local monomeric relaxation time, quoted already in the introduction). The theories that we have emphasized here are DFT and SCFT. We have argued that DFT is valuable on small scales (e.g., describing the packing of monomeric units near

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5.6 Summary

the grafting substrate in very good agreement with MD), but sometimes fails to describe the large-scale structure of the brush and the associated power laws.48 DFT also requires a nonnegligible numerical effort, such as MD, but unlike MD it is not plagued by statistical errors, and so is useful in certain cases. Of course, the standard theoretical tool is SCFT, most popular due to its explicit analytic solution (parabolic brush density profile, etc.) in the strong stretching limit.34 However, in experiments6 and simulations1,8,15,26,43 one always sees clear deviations of 𝜚(z) near z = h from the parabolic density profile, there is no sharp vanishing of 𝜚(z) at z = h but rather a smooth decay of 𝜚(z) extending beyond z = h (For the end monomer distribution 𝜚e (z) for i = N in Figure 5.1a, a singularity at z = h is implied). This smooth nonsingular decay is captured by both SCMF35 and the Scheutjens–Fleer57,58 numerical lattice versions of SCFT. However, no version of SCFT describes the local packing effects, and SCFT also has problems to describe the brush–mushroom crossover at low grafting density, because SCFT cannot correctly describe the nontrivial scaling of the structure of a single chain under good solvent conditions (like for a free chain in ∞ solution, the height he of a mushroom, defined as he = ∫0 zN pN (zN )dzN from the distribution of the free chain end, cf. Figure 5.1a, scales as he ∝ N 𝜈 with 𝜈 ≈ 0.59,10 ). Such nontrivial values of exponents are only understood by the renormalization group methods of the theory of critical phenomena.10 As an example for the subtleties of the brush–mushroom crossover, Figure 5.4 shows some recent results59 for the effect of varying persistence length 𝓁p . While for fully flexible chains one immediately crosses from swollen mushrooms (he ∝ N 0.59 ) over to brushes (he ∝ N) as soon as mushrooms “touch”, for semiflexible polymers there is an additional regime for not too large N, where the mushrooms are still Gaussian, he ∝ (𝓁p N)1∕2 . This Gaussian regime occurs59 since in the (larger) volume that a semiflexible mushroom takes excluded volume interactions are not yet effective as long as N < N ∗ ∝ (𝓁p ∕a)3 . Eventually for N ≫ N ∗ we expect one more crossover in Figure 5.4, where excluded volume becomes dominant also in semiflexible brushes, so also for 𝓁p = 3 or 𝓁p = 5 the asymptotic slope for large N is 1− 𝜈 = 0.41 again, but obviously even for N = 500 there is no indication of this asymptotic slope yet. We emphasize this finding as a warning message against the premature use of scaling arguments: 𝓁p = 3 does not mean large chain stiffness, and nevertheless the asymptotic scaling is not at all reached. Note also that SCFT does not describe correctly how the data settle down for small N∕Nmb on an 𝓁p -independent horizontal plateau (h(0) = he was taken from Monte Carlo simulation here59 since SCFT cannot describe mushrooms). Again, this mushroom to brush crossover may sound as a rather academic problem, but we mention that in a biophysical context it has been suggested as an intracelluar regulation mechanism60 ; also semiflexible brushes formed from biopolymers find recent attention.61 In this context, we draw attention to dense brushes formed from rather short and stiff chains (“polymer bristles”62 ), which show unconventional

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5 Polymer Brushes on Flat and Curved Substrates 101 MD

1

1

10 N/Nmb

5 0. slo p

sl op

e

e

=

=

slo p

e

=

0. 4

0.

5

h(σg)/h(0)

sl op

e

=

0. 4

SCFT

h(σg)/h(0)



100 lp = 1 lp = 3 lp = 5

100 10−2

10−1

100 N/Nmb

101

102

Figure . Log-log plot of the brush height h(𝜎g ), normalized by its value h(𝜎g = 0) = he for the corresponding single mushroom versus the scaled chain length N∕Nmb for three different choices of the persistence length 𝓁p , as indicated in the figure, namely 𝓁p = 1 (flexible chains, 𝜀b = 0) and semiflexible chains with 𝓁p = 3 or 𝓁p = 5, respectively. Full straight line indicates(the expected slope for flexible chains (1 − 𝜈 ≈ 0.4), broken straight ) lines show the slope 1 − 1∕2 = 1∕2 for Gaussian chains. The chain length Nmb where for semiflexible polymers the crossover from mushrooms to brushes occurs is estimated from the condition 𝜋⟨R2gxy ⟩𝜎g = 1 using ⟨R2gxy ⟩ = 𝓁p N∕9. The main panel shows MD data; the insert shows SCFT results. All data refer to 𝜎g a2 = 0.015625, and chain lengths N = 10, 20, 30, 50, 70, 100, 200, 300, 400, and 500 are included. Source: Egorov et al. 2015.59 Reproduced with permission of Royal Society of Chemistry.

ordering behavior under compression.63 This is another example showing how MD simulations are useful for discovering unexpected new phenomena; and hence serving as a guide for both theory and experiments.

Acknowledgments S.A.E. acknowledges partial financial support from the ACSPRF grant 53934ND6 and from the “Schwerpunkt f¨ur Rechnergest¨utzte Forschung in den Naturwissenschaften (SRFN)” at the University of Mainz and from the Alexander von Humboldt foundation A.M. thanks the Deutsche Forschungsgemeinschaft (DFG) for partial support (grant no BI 314/24). The examples

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References

of research results shown in the figures are taken from original research done jointly with H.-P. Hsu, D. Reith, P. Virnau and R. Wang; it is a pleasure to thank them for their fruitful collaboration.

References  Binder, K.; Milchev, A. J. Polym. Sci., B: Polym. Phys. 2012, 50, 1515–1555.  Frenkel, D.; Smit, B. Understanding Molecular Simulation. From Algorithms to Applications, 2nd ed.; Academic Press, San Diego, CA, 2002.  Landau, D. P.; Binder, K. A Guide to Monte Carlo Simulation in Statistical Physics, 4th ed.; Cambridge University Press: Cambridge, UK, 2015.  Allen, M. P.; Tildesley, D. J. Computer Simulation of Liquids; Clarendon Press: Oxford, UK, 1987.  Rapaport, D. C. The Art of Molecular Dynamics Simulation, 2nd ed.; Cambridge University Press: Cambridge, UK, 2004.  Karim, A.; Satiya, S. K.; Douglas, J. F.; Ankner, J. F., Fetters, L.-J. Phys. Rev. Lett. 1994, 73, 3407–3410.  Solar, M.; Yelash, L.; Virnau, P.; Binder, K.; Paul, W. Soft Mater. 2014, 12, 880–889.  Reith, D.; Milchev, A.; Virnau, P.; Binder; K. Macromolecules 2012, 45, 4381–4393.  Voth, G.A., Ed. Coarse-Graining of Condensed Phase and Biomolecular Systems; CRC Press: Boca Raton, FL, 2009.  De Gennes, P. C. Scaling Principles in Polymer Physics; Cornell University Press: Ithaca, NY, 1979.  Klushin, L. I.; Skvortsov, A. M. Macromolecules 1991, 24, 1549–1553.  Doi, M. S.; Edwards, S. E. The Theory of Polymer Dynamics; Oxford University Press: Oxford, UK, 1986.  Johner, A.; Joanny, J. F. J. Chem. Phys. 1993, 98, 1647–1658.  Klein, J. Annu. Rev. Mater. Sci. 1996, 26, 581–612.  Grest, G. S. Adv. Polym. Sci. 1999, 138, 149–183.  Binder, K.; Kreer, T.; Milchev, A. Soft Matter 2011, 7, 7159–7172.  Spirin, L.; Galuschko, A.; Kreer, F.; Binder, K.; Baschnagel, J. Phys. Rev. Lett. 2011, 106, 168301.  Spirin, L.; Kreer, T. ACS Macro Lett. 2013, 2, 63–66  De Beer, S.; Kutnuyanszky, E.; Sch¨on, P. M.; Vansco, G. J.; M¨user, M. H. Nat. Commun. 2014, 5, 3781.  Binder, K., Ed. Monte Carlo and Molecular Dynamics Simulations in Polymer Science; Oxford University Press: New York, 1995.  D¨unweg, B.; Kremer, K. J. Chem. Phys. 1993, 99, 6983–6997.  Hoogerbrugge, P. J; Koelman, J. M. H. V. Europhys. Lett. 1992, 19, 155–160.

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 Evans, D. J.; Morris, J. P. Statistical Mechanics of Nonequilibrium Liquids; Academic Press: London, 1990.  Pastorino, C.; Kreer, T.; M¨uller, M.; Binder, K. Phys. Rev. E 2007, 76, 026706.  Kremer, K.; Grest, G. S. J. Chem. Phys. 1990, 22, 5057–5086  Grest, G. S., Murat, M. In Ref. 20, Chapter 9.  Grosberg, A. Yu., Khokhlov, A. R. Statistical Physics of Macromolecules; AIP Press: New York, 1994.  Hsu, H.-P.; Paul, W.; Binder, K. Macromolecules 2010, 43, 3094–3102.  Grest, G. S. Macromolecules 1994, 27, 418–426.  Grest, G. S.; Murat, M. Macromolecules 1993, 16, 3108–3117.  Dimitrov, D. I.; Milchev, A.; Binder, K. J. Chem. Phys. 2007, 127, 084905.  Wittmer, J.; Johner, A.; Joanny, J. F.; Binder, K. J. Chem. Phys. 1994, 101, 4379–4390.  Alexander, S. J. Phys. (Paris) 1977, 38, 983–987.  Milner, S. T.; Witten, T. A; Cates, M. E. Macromolecules 1988, 21, 2610–2619.  Szleifer, T.; Carignano; M. A. Adv. Chem. Phys. 1996, 94, 165–260.  Sheiko, S. S.; Borisov, O. V; Prokhorova, S. A.; M¨oller, M. Eur. Phys. E 2004, 19, 125–131.  Theodorakis, P. E.; Paul, W.; Binder, K. EPL, 2009, 88, 63002.  Sheiko, S. S.; Sumerlin, B. S.; Matyarzewski, K. Prog. Polym. Sci. 2008, 33, 759–785.  Hsu, H.-P.; Paul, W.; Binder; K. Macromol. Theory Simul. 2011, 20, 510–525.  Theodorakis, P. E.; Hsu, H.-P., Paul, W.; Binder, K. J. Chem. Phys. 2011, 135, 164903.  Basch´e, T.; M¨ullen, K.; Schmidt, M., Eds. From Single Molecules to Nanoscopically Structured Materials. Springer: Heidelberg, Germany, 2014.  Manghi, M.; Aubouy, M.; Gay, C.; Ligoure, C. Eur. Phys. J. E 2001, 5, 519–530.  Dimitrov, D. I.; Milchev, A.; Binder, K. J. Chem. Phys. 2006, 125, 034905.  Wang, R.; Egorov, S. A.; Milchev, A.; Binder, K. Macromolecules 2012, 45, 2580–2587.  Kalb, J.; Dukes, D.; Kumar, S. K.; Hoy, R. S.; Grest, G. S. Soft Matter 2011, 7, 1418–1425.  Daoud, M.; Cotton, J.-P. J.Phys. (Paris) 1982, 43, 531–538.  Wijmans, C. M.; Zhulina, E. B. Macromolecules 1993, 26, 7214–7224.  LoVerso, F.; Egorov, S. A.: Milchev, A.; Binder, K. J. Chem. Phys. 2010, 133, 184901.  LoVerso, F.; Yelash, L.; Egorov, S. A.; Binder, K. Soft Matter 2012, 8, 4185–4196.  LoVerso, F.; Egorov, S. A.; Binder, K. Macromolecules 2012, 45, 8892–8902.  Milchev, A.; Egorov, S. A.; Binder, K. Soft Matter 2014, 10, 5974–5990.  Skvortsov, A. M.; Klushin, L. I.; Polotsky, A. A.; Binder, K. EPL, 2013, 104, 18003.

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References

 Klushin, L. I.; Polotsky A. A.; Hsu, H.-P.; Markelov, D. A.; Binder, K.; Skvortsov, A. M. Phys. Rev. E 2013, 87, 022604.  Andersen, S. I.; Birdi, K. S. Prog. Colloid Polym. Sci. 1990, 82, 52–61.  Milchev, A.; Egorov, S. A.; Binder, K. J. Chem. Phys. 2010, 132, 18405.  Yaneva, J.; Dimitrov, D. I.; Milchev, A.; Binder, K. J. Colloid Interface Sci. 2009, 336, 51–58.  Scheutjens, J. M. H.M.; Fleer, G. J. Chem. Phys. 1979, 83, 1619–1635.  Cosgrove, T.; Heath, I.; van Lent, B.; Leermakers, F.; Scheutjens, J. M. H. M. Macromolecules 1987, 20, 1692–1635.  Egorov, S. A.; Hsu, H.-P.; Milchev, A.; Binder, K. Soft Matter 2015, 11, 2604–2616.  Dumont, E. L. P.; Belmas, H.; Hesse, H. Langmuir 2013, 29, 15142.  Higaki, Y.; Okazaki, R:, Takahara, A. ACS Macro Lett. 2012, 1, 1124–1127.  Milchev, A.; Binder, K. J. Chem. Phys. 2012, 136, 194901.  Milchev, A.; Binder, K. Soft Matter 2014, 10, 3783–3797.

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 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes Rikkert J. Nap1* , Mario Tagliazucchi2* , Estefania Gonzalez Solveyra1 , Chun-lai Ren3 , Mark J. Uline4 , and Igal Szleifer1 1 Department of Biomedical Engineering, Department of Chemistry, and Chemistry of Life Processes Institute, Northwestern University, Evanston, IL, USA 2 INQUIMAE-CONICET, Ciudad Unversitaria, Pabell´on 2, and Ciudad Aut´onoma de Buenos Aires, Buenos Aires, Argentina 3 National Laboratory of Solid State Microstructures and Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing, People’s Republic of China 4 Department of Chemical Engineering, University of South Carolina, SC, USA

. Introduction End-grafted polymer layers have been the subject of extensive theoretical investigations in the past.1–4 These studies were aimed to understand different aspects of polymer brushes such as their structure,5–8 their ability to modulate the interactions between planar surfaces9,10 or colloids,11 and the formation of self-assembled aggregates.12,13 These topics have also been addressed for endgrafted layers of strong polyelectrolytes, that is polyelectrolytes with a fixed and environment-independent charge, whose analysis requires the additional complexity of considering electrostatic interactions and the presence of counterions and coions. The structure of neutral end-grafted polymers can be modulated by changing the quality of the solvent (i.e., the effective strength of Van der Waals interactions between monomers), whereas the structure of strong polyelectrolyte brushes also depends on the salt concentration. In both cases, however, the nature of the interaction forces is physical, rather than chemical, since there is no formation or rupture of chemical bonds. In many cases, the molecular organization of polymer and polyelectrolyte brushes is dictated not only by physical interactions but also influenced by the presence of coupled chemical reactions. Arguably, the most common example of polyelectrolyte brushes affected by chemical reactions is that of layers of end-grafted weak polyacids or polybases. In these systems, each polyelectrolyte Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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

6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

segment is a weak acid or basic group that can be protonated or deprotonated, according to the chemical reactions: − + AH → ←A +H

for polyacids and + − B + H2 O → ← BH + OH

for polybases. Due to their ability to respond to changes of solution pH, weak polyelectrolyte brushes have been extensively studied both experimentally and theoretically. This ability confers pH responsiveness to the surfaces coated by the brush, an effect that have found applications in pH-triggered nanochannels,14,15 electrochemical devices,16 smart colloids,17 plasmonic sensors,18 and so on. Polymer and polyelectrolyte brushes incorporating chemical reactions besides acid–base chemistry have also been described. For example, redox polyelectrolyte brushes (where the segments can be oxidized or reduced), brushes with ion binding or ion complexation capabilities, brushes with ligands that specifically recognize biological molecules in solution, DNA brushes that can hybridize with DNA oligomers in solution, polymers that can reversibly bound/unbound to the substrate, etc. In most of these cases (including weak polyelectrolytes), the chemical reactions are reversible. Chemical reversibility has two important consequences: (i) chemical changes in the film can be reverted, which allows the creation of responsive devices that can be activated or deactivated in response to an external stimulus (e.g., pH, electrode potential, concentration of biomolecules, or ions), (ii) it is possible to describe and understand the degree of progress of the reactions in terms of a chemical equilibrium. In other words, for fast and reversible reactions, we can study the properties of polyelectrolyte or polymer brushes using thermodynamics, without the need to resort to kinetic arguments. Acid–base chemical reactions in weak polyelectrolyte brushes have been extensively studied with different theoretical methodologies, including numerical and analytical self-consistent field approaches (SCF),19–21 scaling arguments,22,23 molecular theories,24 and Monte Carlo simulations.25 Other types of chemical reactions were also studied, although less systematically than acid– base reactions. However, an important finding valid for all different types of chemical reactions is that chemical equilibria in end-grafted brushes can behave very differently from those of isolated molecules in solution. These differences are traced back to the confined molecular environment within the brush, which arises from the high local density of polymer molecules in the layer. This high molecular density is fixed—even in the presence of strong polymer–polymer repulsions—by the grafting of the chains to the substrate. A high local density of segments creates a chemical and physical environment within the brush that is different from the bulk, unconfined solution.

6.2 Theoretical Approach

For example, the electrostatic potential within a negatively charged brush will be more negative than in solution. Therefore, the concentration of positively charged protons inside the brush will be larger than in bulk solution. As a consequence of Le Chatelier’s principle, for a fixed bulk pH, the fraction of protonated acid–base species inside the brush will be larger than in bulk solution. This effect, known as charge regulation, has profound consequences on the acid–base properties of surface-confined polyacids and polybases, as we shall discuss in this chapter. Similar effects for other types of chemistry will be discussed as well. Note also that salt in solution screens electrostatic interactions and thus the electrostatic potential and proton concentration within a polyelectrolyte brush will depend on ionic strength. Therefore, the degree of protonation of acid–base species in the brush will depend on salt concentration as well, which shows that chemical reactions in confined environments can be tuned by modulating the strength of physical interactions in the system. This effect is particularly interesting in systems with competing physical interactions, for example, repulsive electrostatic forces that tend to swell the brush versus attractive hydrophobic interactions that tend to collapse it. As we will show in this chapter, chemical equilibria can couple to such competing physical interactions, leading to interesting and unexpected forms of molecular organization. In this chapter, we will present theoretical insights on the behavior of chemical equilibria in polymer and polyelectrolyte brushes gained through the use of a mean-field molecular theory. This theory captures the coupling that exists between different physical and chemical interactions and explicitly includes molecular details on the system. In the first section, we present the formulation of the theory for weak polyelectrolyte brushes and discuss how it can be modified to study other types of chemical equilibria. In the following sections, we discuss the applications of the theory to polyelectrolyte brushes involving different types of chemical reactions: weak polyelectrolyte brushes, redox brushes, brushes that bind ions and biological molecules and, finally, polymers in solution that reversibly adsorb onto surfaces by a terminal segment, thus forming polymer brushes.

. Theoretical Approach The theoretical approach presented in this chapter is a molecular theory that is based upon a generalization of a theory originally developed to describe the thermodynamic and structural properties of surfactant micelles and lipid bilayers.26–28 Subsequently, it has been generalized and modified to predict the properties of polymer-tethered layers.8,29–32 The molecular theory explicitly incorporates the molecular details of the system under consideration, such as size, shape, charge distribution, and polymer conformations.8,33

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

The theory has been applied to a large variety of polymer-tethered systems. The predictions of the molecular theory have been found in good quantitative agreement with both experimental observations and computer simulations. For example, the prediction of the thickness of end-tethered polyacrylic acid polymers was found in very good agreement with experimental observations.34–36 Likewise, the predictions of the apparent pKa or effective charge of acid–ligand coated gold nanoparticles,37 and the conductivity of polymertethered nanopores15 were in good agreement with experiments. Similar predictions on the behavior of electrodes modified by redox polyelectrolytes were found in good agreement with experimental observations.38 The molecular theory is a density functional theory whose basic idea consists of writing the free energy of the end-tethered polymer layer as a functional of the probability distribution of the conformations and the spatial distribution of all molecular species in the system, including the tethered polymers, the solvent molecules, and the ions. Explicitly, a large set of different polymer conformations is inputted into the theory and the free-energy minimization determines the probability of each of the polymer conformations. The probability distribution depends critically on external parameters such as temperature, pH, salt concentration, grafting density, and curvature of grafting surface. This probability distribution function (pdf ) of the chain conformations is the central quantity in the molecular theory. From knowledge of the pdf, any average structural and thermodynamic property related to the tethered layer can be calculated. As aforementioned, the pdf is obtained by minimization of the free energy. Thus, by specification of the different entropic and energetic contributions of the endtethered polyelectrolyte layer, we can obtain the pdf and calculate equilibrium conformational and thermodynamic properties of end-tethered polyelectrolyte layers. In this chapter, we present a theoretical description for end-tethered weak polyelectrolytes within the molecular theory approach.24,34 To do so, we need to introduce a number of definitions and parameters to describe the system of interest. We assume that the tethered polyelectrolyte layer consists of Np polymer chain molecules that are all end-tethered to a surface. The surface area is denoted A for a planar surface or A(R) for a cylindrical or spherical surface with a radius of curvature R. The number of tethered chain molecules per unit area of the surface is given by 𝜎 = Np ∕A(R). This surface density is often referred to as the surface coverage. We will assume that all chains are irreversible endtethered to the surface (this requirement will be elaborated in the last part of this chapter, where we will study the adsorption of polymer chains through their terminal segment). For simplicity, we assume that all chains have nseg segments and that each segment has a volume vp and is chargeable. The description of tethered block copolymers in which not all segments have the same volume or are not all chargeable is straightforward and will be discussed further on in this section. Finally, we assume that the end-tethered polyelectrolyte layer is immersed in an aqueous solution that is in contact with a reservoir

6.2 Theoretical Approach

containing a monovalent salt at a fixed concentration and pH. In the present case, we will consider the salt to be sodium chloride, which is assumed to be completely dissociated, unless stated otherwise. We now specify the various contributions of the free energy. The total Helmholtz free energy describing an end-tethered polyelectrolyte layer has the following contributions: F = −TSconf − TSmix + EVdW + Fchem + Felect .

(6.1)

The first term in Eq. (6.1) describes the contribution to the free energy that arises from the conformational entropy of the polymers. The second term represents the translational or mixing entropy arising from the mobility of the coions, counterions, and solvent molecules. The variable T corresponds to the absolute temperature. The third term denotes the nonelectrostatic Van der Waals (VdW) interaction of the polymers. The fourth term is related to the chemical equilibrium of the chargeable acid groups and contains both the enthalpic and entropic costs associated with it. The last term accounts for the electrostatic interactions. In the next paragraphs, we will describe in detail the content of each of the contributions. The conformational entropy of the end-tethered polymer is described by8,24 ∑ Sconf = −kB Np P(𝛼) ln P(𝛼). (6.2) 𝛼

Here P(𝛼) is the probability of finding an end-tethered polymer chain in conformation 𝛼. A polymer conformation corresponds to the set of coordinates that denotes the positions of all the segments that constitute the polymer chain. In principle, the sum in the conformational entropy needs to be extended over all possible conformations. In practice, unless the polymer chain is very short, that is, the number of polymer segments is less than nseg < ∼ 15, the number of conformations is very large and their enumeration is beyond the computation power of current computers. Therefore, instead of considering all possible conformations, we use a Monte Carlo8,39 or molecular dynamic simulation40,41 to generate a large representative set of polymer conformations. Usually we employ a Monte Carlo scheme where our chain model is based on the rotational isomeric model (RIS),39 where the dihedral angles in the chain can take three different values and the bond length is fixed. Here, it is very important to emphasize that for a given number of segments per polymer and surface geometry, we generate the set of conformations only once and use the same set for all calculations. For all different control variables such as temperature, salt concentration, pH, and surface coverage, we use the same set of conformations. However, the probability of each conformation (given by P(𝛼), where 𝛼 denotes a conformation) strongly depends on external control variables, such as pH, salt concentration, or temperature. This results in different equilibrium conformational and thermodynamic properties as a function of the external conditions.

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

Given the pdf, we can compute various structural and thermodynamic quantities related to the polymers. Foremost of the structural quantities is the polymer volume fraction, which is given by Np ∑ ⟩ ⟨ P(𝛼)n(𝛼; ⃗r )vp . 𝜙p (⃗r) = A(⃗r) 𝛼

(6.3)

Here n(𝛼; ⃗r)d⃗r is the number of polymer segments that can be found in the volume element [⃗r, ⃗r + d⃗r ] and belongs to polymer conformation 𝛼, ⃗r represents a point in space, and vp corresponds to the volume of one polymer segment. The volume of element [⃗r, ⃗r + d⃗r] is equal to A(⃗r)d⃗r. The second term in the free-energy expression represents the mixing or translational entropy of the solvent and other mobile species, such as the coand counterions. It is given by Smix = −

∑ i

kB

∫

d⃗r𝜌i (⃗r)[ln(𝜌i (⃗r)vw ) − 1],

(6.4)

where 𝜌i (⃗r) corresponds to the number density of mobile species i and vw is the volume of water. Given the number density, we can compute the volume fraction of the mobile species: 𝜙i (⃗r) = 𝜌i (⃗r)vi , with vi being the volume of species i. The third contribution, EVdW , describes the solvent quality and is represented in the free energy by a quadratic term in the polymer density. This nonelectrostatic Van der Waals interaction is given by EVdW = −

1 d⃗rd⃗r′ 𝜒g(|⃗r − ⃗r′ |)⟨𝜌p (⃗r)⟩⟨𝜌p (⃗r′ )⟩. 2∬

(6.5)

Here, 𝜒 is the strength of the Van der Waals interaction and the function g(|⃗r − ⃗r′ |) = −𝓁 6 ∕(|⃗r − ⃗r′ |)6 is an attractive Van der Waals potential, where 𝓁 corresponds to the segment length. For good solvent conditions,𝜒 = 0. The repulsive interactions in the theory are modeled as excluded volume interactions. The intramolecular or intrachain excluded volume interactions are exactly accounted for during the generation of the chain conformations. Each chain conformation is tested for overlap in space for every pair of segments, and only those chain conformations that do not overlap are retained. Meaning all chain conformations are self-avoiding. Not only is self-avoidance tested internally for every chain conformation, but all chain conformations are also tested for avoidance with respect to the surface. Hence, only self-avoiding conformations that do not overlap with the surface are considered. The intermolecular excluded forces are accounted for by assuming that the system is incompressible at every position. This implies that the system satisfies the following packing constraints: ⟨𝜙p (⃗r)⟩ + 𝜙w (⃗r) + 𝜙Na+ (⃗r) + 𝜙Cl− (⃗r) + 𝜙H+ (⃗r) + 𝜙OH− (⃗r) = 1 ∀⃗r

(6.6)

6.2 Theoretical Approach

or ⟨𝜙p (⃗r)⟩ +

∑

𝜙i (⃗r) = 1.

(6.7)

i

Here, the index i runs over all mobile species that include the solvent, coions, counterions, protons, and hydroxyl molecules. These packing constraints are enforced by introduction of Lagrange multipliers 𝜋(⃗r) in the free energy functional. The fourth contribution to the free energy represents the free energy contribution arising from the chemical equilibrium. This chemical equilibrium can be, for example, an acid–base equilibrium − + AH → ←A +H ,

(6.8)

a ligand–receptor binding equilibrium, L+R→ ← LR,

(6.9)

an oxidation/reduction equilibrium, z−1 oxz + e− → ← red ,

(6.10)

an ion-binding equilibrium, Mz+ + zA− → ← MAz ,

(6.11)

etc. We will describe here the formulation of the theory for the case of the acid– base equilibrium (6.8). However, the following treatment of chemical equilibrium is general and can be applied to other types of chemical reaction equilibria as well. The reader is referred to the literature for the formulation of the molecular theory for ligand–receptor interactions,42,43 and redox reactions.38,44 It is also worthwhile to mention that the same general methodology can be used to describe hydrogen bond formation in tethered polymers.46 Before presenting the free-energy contribution related to acid–base equilibrium of the protonation and deprotonation of the acid groups, it is useful to make a small digression to describe a single monomeric acid in dilute solution. In a dilute solution, the amount of deprotonation or amount of charge on the acid group is determined by the chemical equilibrium constant Ka of the acid– base equilibrium and given by Ka =

[A− ][H+ ] . [AH]

(6.12)

Formally, [i] corresponds to the activity of species i. Provided that the solution is sufficiently dilute, the activities are usually approximated by the molar concentration. The chemical equilibrium is related to the free-energy change 0 of the reaction Ka = Ce−ΔGa ∕kB T . The constant C is introduced for consistency

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

of units, because Ka is conventional expressed in molar concentration. Hence C is equals to C = 1∕Na vw , where Na is Avogadro’s number. The free-energy change of the reaction is given as the sum over the standard free energy of formation or chemical potential of the products, the deprotonated acid (A− ) and the proton (H+ ), minus the standard free energy of formation of the protonated acid (AH): 0 0 0 ΔGa0 = 𝜇A − + 𝜇 + − 𝜇AH . H

(6.13)

To characterize the acid–base equilibrium, it is convenient to introduce the fraction of deprotonation or fraction of charged acids: f =

[A − ] . [A− ] + [AH]

(6.14)

Isolating f from the expression of the chemical equilibrium constant then yields f =

1 1 1 = . = pKa ) −(pH− [AH] [H+ ] 1 + 10 1+ − 1+ [A ] Ka

(6.15)

Hence, the fraction of charged acid groups in bulk solution is solely determined by the equilibrium constant Ka and the pH = − log[H+ ] of the solution (where we approximated the activity of protons by its concentration). This expression only holds for low-molecular-weight acid groups in dilute solution. However, the acid groups of a polyacid are connected. Due to the connectivity of the monomers, the charged monomers are correlated and the distribution and amount of deprotonation of the acid group is influenced by both its position along the polyacid chain and the local environment it experiences. Hence, the degree of deprotonation will change and deviate from the ideal dilute solution behavior as expressed by Eq. (6.15). For tethered polyacids, the monomers are not only connected, but they are also end-tethered to a surface. The anchoring of a polyacid molecule to a surface will further change the degree of deprotonation because of the interactions the monomers experience within the polymer layer. The polymer layer is inhomogeneous in the direction away from the grafting surface. Therefore, different monomers experience different local environments depending on their location with respect to the surface. Thus, the degree of deprotonation becomes explicitly position dependent. We shall describe the free energy contribution arising from the acid–base equilibrium with the following position-dependent functional[24,47] : 𝛽Fchem =

∫

[ d⃗r⟨𝜌p (⃗r)⟩ f (⃗r) ln f (⃗r) + (1 − f (⃗r)) ln(1 − f (⃗r)) ]

0 (1 − f (⃗ r)) +𝛽𝜇A0 − f (⃗r) + 𝛽𝜇AH

+ 𝛽𝜇 0 + H

∫

0 d⃗r𝜌H + (⃗r) + 𝛽𝜇OH −

∫

(6.16) d⃗r𝜌OH− (⃗r).

6.2 Theoretical Approach

Here, ⟨𝜌p (⃗r)⟩ = ⟨𝜙p (⃗r)⟩∕vp is the polymer number density, f (⃗r) is the positiondependent degree of deprotonation, and 𝛽 = 1∕kB T equals the inverse temperature. Here, we have assumed that all segments are chargeable; therefore, the total density associated with the charged polymer segments is equal to ⟨𝜌A− (⃗r)⟩ = f (r)⟨𝜌p (⃗r)⟩. Similarly, the density of the uncharged polymer segments is equal to ⟨𝜌AH (⃗r)⟩ = (1 − f (r))⟨𝜌p (⃗r)⟩. The first and second terms in Eq. (6.16) describe the entropy of the deprotonated charged state (A− ) and protonated state (AH). The third and fourth terms within the integral correspond to the standard chemical potential of the deprotonated and protonated state of the acid, respectively. Similarly, the last two contributions in Eq. (6.16) describe the standard chemical potentials associated with the protons and the hydroxyl molecules. A distance-dependent charge fraction was first used to describe tethered polyelectrolytes layers in the pioneering work by Isra¨el and co-workers.19,22,48 Their seminal approach was based upon a generalization of a lattice SCF theory, also referred to as Scheutjens–Fleer theory.49 They assumed that the acid– base equilibrium constant in the tethered polymer layer was the same as in the bulk solution. This approximation means that one assumes that Eq. (6.15) still holds, and consequently one can replace the degree of protonation and proton concentration (f and [H+ ]) with their local position-dependent values: f (⃗r) and [H+ (⃗r)]. The approach presented here is different because the local degree of deprotonation is not imposed but results as an outcome from the minimization of the total free-energy functional. The resulting expression for the degree of deprotonation is presented below. So far, we have assumed that all monomers or segments are chargeable. Considering polyelectrolyte chain molecules that have segments that are not all chargeable simply entails replacing in the above equations, the polymer density with the density of those segments that are chargeable. Considering polyelectrolyte chains containing more than one type of acid unit is equally straightforward.46,50,51 For every acid type, we write down a contribution to the free energy that is much like Eq. (6.16), which leads to 𝛽Fchem =

∑ k

{ ∫

⟩[ ⟨ d⃗r 𝜌k (⃗r) fk (⃗r) ln fk (⃗r) + (1 − fk (⃗r)) ln(1 − fk (⃗r)) P

]} 0 0 f (⃗ r ) + 𝛽𝜇 (1 − f (⃗ r )) + 𝛽𝜇k, − k,AH A + 𝛽𝜇0 + H

∫

0 d⃗r𝜌H+ (⃗r) + 𝛽𝜇OH −

∫

(6.17)

d⃗r𝜌OH− (⃗r).

Here, the index k refers to the different acid types present in the polyelectrolyte chain, ⟨𝜌kp (⃗r)⟩ denotes the number density of polymer segment of type k, and fk (⃗r) corresponds to the degree of deprotonation of acid group of type k.

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

The above formalism can also be applied to more complex acid–base equilibrium reactions. For example, recently the molecular theory was extended to describe the acid–base chemical equilibrium of diprotic acids.52 − + AH2 → ← AH + H

2− + AH− → ←A +H .

(6.18)

A diprotic acid can be found in three distinct states, namely neutral, single, and double deprotonated, as compared to only the two states of a “simple” acid. In analogy to our treatment of the “simple acid,” we can introduce a positiondependent fraction for each of the three states in which a diprotic acid can be found: fAH2 (⃗r), fAH− (⃗r), and fA2− (⃗r). The total number of acid molecules group is conserved; hence, the three fractions are not all independent. The sum of the three fractions of the diprotic monomer adds up to one: fAH2 (⃗r) + fAH− (⃗r) + fA2− (⃗r) = 1. Now we can write, in a completely analogue fashion to Eq. (6.16), the chemical free energy contribution as follows: 𝛽Fchem =

∫

[ d⃗r⟨𝜌p (⃗r)⟩ fAH2 (⃗r) ln fAH2 (⃗r) + fAH− (⃗r) ln fAH− (⃗r)+ fA2− (⃗r) ln fA2− (⃗r ) ]

0 0 fAH2 (⃗r) + 𝛽𝜇AH r) + 𝛽𝜇 0 −2 fA−2 (⃗r) + 𝛽𝜇AH − fAH− (⃗ 0 + 𝛽𝜇H +

∫

(6.19)

A

2

0 d⃗r𝜌H+ (⃗r) + 𝛽𝜇OH −

∫

d⃗r𝜌OH− (⃗r).

The first, second, and third terms in the chemical free energy of Eq. (6.19) describe the entropy of the neutral state (AH2 ), the single deprotonated charged state (AH− ), and the double protonated state (A2− ). The fourth through sixth terms correspond to the standard chemical potential of the three different states the monomer can be found in. The last term in the free energy functional described in Eq. (6.1) accounts for the electrostatic interactions, that need to satisfy the Maxwell equations, that is, the Poisson equation. We employ a variational approach to treat the electrostatic interactions, meaning we will use an electrostatic energy functional whose stationary solution leads to the Poisson equation. The following functional represents this contribution53–55 : ) (⟨ ⟩ 2 1 ⃗ r 𝜓(⃗r)) + d⃗r 𝜌q (⃗r) 𝜓(⃗r) − 𝜀0 𝜀r (⃗r)(∇ dS 𝜎q (S)𝜓(S). Felect = ∫ ∫ 2 S

(6.20) Here, 𝜓(⃗r) is the electrostatic potential, ⟨𝜌q (⃗r)⟩ corresponds to the charge number density, and 𝜎q (S) is equal to the surface charge density present at the interface or boundary. The total number charge density is given by ∑ qi 𝜌i (⃗r). (6.21) ⟨𝜌q (⃗r)⟩ = qp f (⃗r)⟨𝜌p (⃗r)⟩ + i

6.2 Theoretical Approach

The first term corresponds to the charge arising from the charged polymer segments, whereas the second term describes all the net charge arising from all charged mobile species. Here qi is the amount of charge of mobile species i, that is, qNa+ = qH+ = −qCl− = −qOH− = −qp = e, where e denotes the elementary electric charge. In the electrostatic energy functional, 𝜀0 and 𝜀r (⃗r) correspond to the dielectric permittivity of vacuum and the position-dependent relative dielectric permittivity of the material. Because we employ a variational approach to describe the electrostatic interactions, the charge density and the electrostatic potential are independent variables in the above energy functional. This has the great advantage that we can take the functional derivative with respect of the number density of the mobile ions and 𝜓(r) independently. Only upon variation of the energy functional with respect to 𝜓(r), one can connect the charge density with the electrostatic potential. Variation of the energy functional with respect to 𝜓(r) results in the Poisson equation and its boundary conditions54 : 𝛿Felect [𝜓] ⃗ r)) = ⟨𝜌q (⃗r)⟩∧ ⃗ ⋅ (𝜀0 𝜀r (⃗r)∇𝜓(⃗ = 0 ⇒ −∇ 𝛿𝜓 ⃗ r)|S ⋅ n̂ s = 𝜎q (S). −𝜀0 𝜀r (⃗r)∇𝜓(⃗

(6.22)

Also, the value of the energy functional at its stationary point becomes equal to the electrostatic energy54,56 : Felect =

1 1 dS 𝜎q (S)𝜓(S). d⃗r ⟨𝜌q (⃗r)⟩𝜓(⃗r) + 2∫ 2∫

(6.23)

S

Application and use of the energy functional (Eq. 6.20) requires the formulation of a constitutive equation for the dielectric properties of the system. The above energy functional implies that we have assumed that the material obeys linear dielectric material properties, meaning that the polarization of the material is linearly proportional to an applied electric field. This is a very common assumption. Furthermore when the dispersed media is very dilute, one can assume that the relative dielectric permittivity is constant and equal to that of the dispersing medium, that is, that of water: 𝜀r (⃗r) ≈ 𝜀w . This is an assumption frequently made in polymeric systems. Another possible assumption that is frequently employed is that the position-dependent relative dielectric permittivity is equal to the weighted average of the volume fraction of the various components, ∑ 𝜀i 𝜙i (⃗r), (6.24) 𝜀r (⃗r) = i

where 𝜀i corresponds to the relative dielectric permittivity of species i. In Refs. 24 and 57, we have considered in detail various aspects and consequences of a spatial varying dielectric permittivity, including the effect of a spatially varying solvation or Born self-energy. We found that even for relative densely tethered

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

polymer layers, the influence of a spatially varying dielectric permittivity as compared to a constant background dielectric permittivity is small. Thus, to a good degree, we can assume that the dielectric permittivity is equal to that of water. Putting all the free energy contributions together results in the following Helmholtz free energy per unit area,24,58 f =

∑ 𝛽F P(𝛼) ln P(𝛼) + dr G(r)𝜌w (r)(ln(𝜌w (r)vw ) − 1) =𝜎 ∫ A(R) 𝛼 +

∫

dr G(r)𝜌Na+ (r)(ln(𝜌Na+ (r)vw ) − 1)

+

∫

dr G(r)𝜌Cl− (r)(ln(𝜌Cl− (r)vw ) − 1)

+

∫

+

∫

) ( 0 dr G(r)𝜌H + (r) ln(𝜌H + (r)vw ) − 1 + 𝛽𝜇H + ) ( 0 dr G(r)𝜌OH − (r) ln(𝜌OH − (r)vw ) − 1 + 𝛽𝜇OH −

(6.25)

) [ ( dr G(r)⟨𝜌p (r)⟩ f (r) ln f (r) + 𝛽𝜇A0 − ∫ )] ( 0 + (1 − f (r)) ln(1 − f (r)) + 𝛽𝜇AH ) ( 1 +𝛽 dr G(r) ⟨𝜌q (r)⟩𝜓(r) − 𝜀0 𝜀r (r)(∇r 𝜓(r))2 + 𝛽𝜎q 𝜓(R) ∫ 2 1 ′ + 𝛽 drdr G(r)⟨𝜌p (r)⟩𝜒g(r, r′ )⟨𝜌p (r′ )⟩. 2 ∬ +

So far, we have discussed the various free-energy contributions in relative general terms, we did not explicitly specify the geometry of the grafting surface nor did we make any assumptions about the spatial dependence of the density distributions. However, for simplicity, in Eq. (6.25), we assumed that the system has only inhomogeneities in one coordinate, r, which describes the distance to a planar, cylindrical, or spherical surface. Considering only the radial direction explicitly implies that the system is homogeneous in the angular or lateral directions. Extension to more complex coordinate systems is straightforward (see Refs. 52 and 59–63). The function dr G(r) = drA(r)∕A(R) is the volume element in either the planar, cylindrical, or spherical coordinates divided by the grafting area. The geometrical factor is proportional to the Jacobian determinant and describes the changes in volume as a function of the distance from the surface. The geometrical factor G(r) is equal to 1, r∕R, and (r∕R)2 for planar, cylindrical, and spherical geometries, respectively. The free energy is minimized with respect to P(𝛼), 𝜌i (r), and f (r), and varied with respect to the electrostatic potential, 𝜓(r), under the constraints of incompressibility (Eq. 6.6), and the fact that the system is in contact with a bath

6.2 Theoretical Approach

of cations, anions, protons, and hydroxyl ions. Therefore, the proper thermodynamic potential is the semigrand potential and it is given by 𝛽F 𝛽W = − 𝛽𝜇Na+ dr G(r)𝜌Na+ (r) − 𝛽𝜇Cl− dr G(r)𝜌Cl− (r) ∫ ∫ A(R) A(R) − 𝛽𝜇H +

∫

dr G(r)(𝜌H+ (r) + (1 − f (r))⟨𝜌p (r)⟩)

(6.26)

− 𝛽𝜇OH− +𝛽

∫

dr G(r)𝜌OH− (r) ∫ ) ( ) ( ∑ ⟩ ∑ ⟨ 𝜙i (r) − 1 + 𝜆 P(𝛼) − 1 . dr G(r)𝜋(r) 𝜙p (r) + i

𝛼

Here, 𝜇i is the chemical potential of molecular species i that are also found in the bath. The first and second integrals correspond to the total number of sodium and chloride ions per unit area. The third integral accounts for the total number of protons in the system, which is the sum of the free protons and those protons that are in the protonated state of the acidic monomers. The fourth integral is similar to the third integral and represents the total number of hydroxyl molecules. The fifth integral enforces the incompressibility constraint through the use of the Lagrange multipliers 𝜋(r). The final constraint ensures ∑ that the pdf is properly normalized: P(𝛼) = 1. 𝛼

Minimization of the free energy with respect to P(𝛼) yields )] [ ( 1 P(𝛼) = exp − dr n(r; 𝛼) 𝛽𝜋(r)vp − 𝛽 dr′ G(r′ )𝜒g(r, r′ )⟨𝜙(r′ )⟩ ∫ ∫ q [ ] × exp − dr n(r; 𝛼)(𝛽𝜓(r)qp + ln f (r)) . ∫ (6.27) Minimization of the free energy with respect to the solvent density yields an expression for the solvent density or the local volume fraction that is given by 𝜙w (r) = 𝜌w (r)vw = exp(−𝛽𝜋(r)vw ).

(6.28)

Similarly, the density of the sodium and chloride ions reads 𝜌i (r) =

1 exp(𝛽𝜇i − 𝛽𝜋(r)vi − 𝛽𝜓(r)qi ), vw

(6.29)

whereas the density of the protons and hydroxyl ions is given by 𝜌i (r) =

1 exp(𝛽𝜇i − 𝛽𝜇i0 − 𝛽𝜋(r)vi − 𝛽𝜓(r)qi ). vw

(6.30)

For the degree of protonation, we obtain 𝜌 (r) f (r) = Ka0 w . 1 − f (r) 𝜌H + (r)

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(6.31)

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes 0

Here, Ka0 = e−ΔGa ∕kB T is the dimensionless chemical equilibrium constant, given as the exponent of the free energy of the reaction ΔGa0 (Eq. 6.13). Finally, by variation with respect to the electrostatic potential, we obtain the Poisson equation and its boundary conditions, discussed previously in Eq. (6.22). In the pdf P(𝛼) (Eq. 6.27), the factor q ensures that the probability distribution is properly normalized. It is related to the Lagrange multiplier 𝜆 introduced in Eq. (6.26) : q = exp(𝜆). We prefer to write q instead of 𝜆 because q can be identified as the single-chain partition function in the (mean) field of 𝜋(r) and 𝜓(r), because q is the sum over all energetic Boltzmann factors: ∑ ∑ q = Pu (𝛼) = exp(...). Here Pu (𝛼) denotes to the unnormalized pdf, which 𝛼

𝛼

is equal to the exponential term in P(𝛼) (Eq. 6.27). The first term in the exponential in P(𝛼) results from the repulsive excluded volume interactions, as it involves the Lagrange multipliers 𝜋(r) that are related to the packing or incompressibility constraint. From the equations for the solvent density and ion density, we see that these Lagrange multipliers can be interpreted as the local osmotic pressures.8 This should have been apparent because the Lagrange multipliers are associated with the excluded volume interactions. Although the 𝜋 field is referred to as the local osmotic pressure that ensures incompressibility, it should be stressed that it has an important nonlocal component. Namely, the local osmotic pressure in the exponential of the pdf is multiplied by n(𝛼; r) and via n(𝛼; r) the polymer conformation 𝛼 links different locations, which results in a nonlocal coupling of 𝜋(r) at various locations. The second term in the exponential is related to the Van der Waals interactions. The third term in the exponential includes a purely electrostatic contribution associated with all chargeable monomers, instead of the expected energetic Boltzmann factor exp(−𝛽 ∫ drn(𝛼, r)f (r)𝜓(r)qp ). Similarly, the fourth term in the exponential of the pdf contains an entropy-like term involving the degree of protonation. It is important to emphasize that these terms in the pdf occur after algebraic rearrangement. The terms arising from the electrostatic interactions and chemical equilibrium are combined with the equation for the fraction of charged monomers (Eq. 6.31) to yield the third and fourth terms in the pdf. Prior to algebraic rearrangement, the probability distribution reads: P(𝛼) =

)] [ ( 1 dr′ G(r′ )𝜒g(r, r ′ )⟨𝜙(r′ )⟩ exp − drn(r; 𝛼) 𝛽𝜋(r)vp − 𝛽 ∫ ∫ q [ ] × exp − drn(r; 𝛼)(𝛽f (r)𝜓(r)qp + f (r) ln f (r) + (1 − f (r)) ln(1 − f (r))) ∫ [ ] ( ) 0 × exp − drn(r; 𝛼) 𝛽𝜇A0 − f (r) + 𝛽(𝜇AH − 𝜇H + )(1 − f (r)) . ∫

(6.32)

6.2 Theoretical Approach

The terms in the exponential of Eq. (6.32) have the “conventional” Boltzmannlike contributions. However, the last line involves the standard chemical potentials of the deprotonated and protonated states and their values are not known. We only know the change in standard Gibbs free energy of the reaction. Hence, we use the equation for the degree of deprotonated monomers (Eq. 6.31) to rearrange the exponential and obtain the expression for the pdf as presented in Eq. (6.27). The expression for the position-dependent degree of protonation can be rearranged to yield: f (r) = 1+

1 . 𝜌H+ (r)

(6.33)

Ka0 𝜌w (r)

Comparison of this equation with the equation for the degree of deprotonation of a monomeric acid (Eq. 6.15) shows that Ka0 is the true equilibrium constant controlling the chemical equilibrium (instead of the bulk chemical equilibrium constant in molar units, Ka ). In complete analogy to the equation of the bulk equilibrium constant (Eq. 6.12), we can define a position-dependent equilibrium constant by taking the ratio of the reactants and the product Ka (r) ≡

𝜙 (r) [A− (r)][H+ (r)] = Ka exp(−𝛽𝜋(r)vw ). = Ka0 w [AH(r)] Na vw

(6.34)

The position dependence of the above “apparent” equilibrium constant emphasizes the fact that changes in the local environment influence the local degree of deprotonation. The occurrence of the position-dependent solvent density or volume fraction directly alters the value of the degree of protonation. The degree of deprotonation is indirectly changed because it is determined by the local osmotic pressure and electrostatic potential, which are in turn determined by the local volume and charge densities of the polymer, solvent, and ionic species. The degree of deprotonation is also coupled in a nontrivial and nonlinear fashion to the P(𝛼) and therefore to the local polymer density, which in turn affects the local osmotic pressure and local electrostatic potential. This will also be discussed in the Applications section. In infinite dilute solution where 𝜙w → 1 and 𝜋 → 0, the experimental equilibrium constant Ka is related to Ka0 by Ka = CKa0 = Ka0 ∕(Na vw ) and we can use Eq. (6.34) to obtain the value of constant C = 1∕(Na vw ) as mentioned previously. As discussed before, the variation of the free-energy functional with respect to the electrostatic potential leads to the Poisson equation and its boundary conditions (Eq. 6.22). The Poisson equation plus boundary conditions determine uniquely the electrostatic potential up to an arbitrary constant. We use this gauge invariance to set the value of the electrostatic potential of the reservoir or bulk solution, with which the polyelectrolyte system is in thermodynamic equilibrium, to zero: 𝜓 bulk = 0. The dielectric properties of the substrate

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

are assumed to satisfy: 𝜀0 𝜀s d𝜓(r) | = 0, where the subscript s refers to the dr r=R substrate. This assumption implies that the relative permittivity of the substrate is zero or so small that its value can be neglected. Finally, we observe that the electrostatic boundary conditions imply that the system is charge neutral. To be precise, the value of the integral of the free total charge density is equal and opposite to the bound surface charges. The general expression for the number density of the small mobile ionic species is presented in Eq. (6.29). The explicit expressions for the number density of the sodium and chloride ions are 𝜌Na+ (r) =

1 exp(𝛽𝜇Na+ − 𝛽𝜋(r)vNa+ − 𝛽𝜓(r)e), vw

𝜌Cl− (r) =

1 exp(𝛽𝜇Cl− − 𝛽𝜋(r)vCl− + 𝛽𝜓(r)e), vw

(6.35)

whereas the explicit volume fractions for the protons, hydroxyl molecules, and water molecules are given by ( ) 𝜙H+ (r) = exp 𝛽𝜇H+ − 𝛽𝜇0 + − 𝛽𝜋(r)vH+ − 𝛽𝜓(r)e , H ( ) 0 (6.36) 𝜙OH− (r) = exp 𝛽𝜇OH− − 𝛽𝜇OH − − 𝛽𝜋(r)vOH− + 𝛽𝜓(r)e , 𝜙w (r) = exp(−𝛽𝜋(r)vw ). Observe that we assumed that the protons, hydroxyl, and water molecules have the same volume. Here, it is important to point out that the incompressibility constraint reduces the number of thermodynamically independent variables. Hence, the chemical potential of the water molecules needs not be specified explicitly. Therefore, the chemical potentials, 𝜇i , are in reality exchange chemical potentials, which are defined as the difference between the chemical potential of species i and the chemical potential of water. Likewise, the global charge neutrality constraint and the water self-dissociation equilibrium reduce the number of thermodynamic independent variables further. The values of the exchange chemical potential of the remaining ionic species can be expressed by relating them to their bulk concentrations: ) ( (6.37) vw = exp 𝛽𝜇i − 𝛽𝜇i0 − 𝛽𝜋 bulk vi − 𝛽𝜓 bulk qi . 𝜌bulk i Further discussions of exchange chemical potential and its applications within the molecular theory can be found in Refs. 8, 24, and 64. The concentration of the protons and hydroxyl ions are related through the water-self-dissociation equilibrium reaction. In other words, the chemical potentials of the proton and hydroxyl molecules are not independent, but related. We will assume that in bulk water the bulk proton and hydroxyl molecules obey the following relation: Kw = [H+ ][OH− ], where Kw is the water dissociation constant in bulk solution, so that pKw = − log Kw = 14 at 298.15 K.

6.3 Applications of the Molecular Theory

In analogy to the position-dependent acid–base equilibrium constant, we can define an apparent position-dependent water dissociation constant. Namely Kw (r) ≡ [H+ (r)][OH− (r)] = Kw exp[−2𝛽(𝜋(r) − 𝜋 bulk )].

(6.38)

The bulk number density of the ions (the left-hand side of Eq. 6.37) can be obtained from knowledge of the salt concentration and the bulk pH. For pH 7, we have 𝜌Na+ = 𝜌Cl− = Na csalt . When the pH is not equal to 7, we add either NaOH or HCl to bring the solution to the desired bulk pH. For pH < 7, we add HCl to make the solution acidic. To raise the pH above 7, we add NaOH, making the solution basic. The addition of either NaOH or HCl increases also the concentration of either Na+ or Cl− ions respectively. The extra amount of positive or negative ions is |[H+ ] − [OH− ]|. The unknowns in Eqs. (6.3), (6.27), and (6.33)–(6.36) are the Lagrange multipliers or lateral pressures, 𝜋(r), and the electrostatic potential, 𝜓(r). Solutions for the lateral pressure and the electrostatic potential can be obtained numerically in the following way: The expressions for the volume fractions of all components are substituted into the incompressibility constraint and the Poisson equation, resulting in a set of integro-differential equations. The solution of this set of equation determines the lateral pressures and the electrostatic potential. (Observe that for the case 𝜒 ≠ 0, the polymer density is another “unknown” variable and we also solve self-consistently for the polymer volume fraction). The degree of protonation of the acidic groups and the density of the ions and the solvent and polymers are known once the lateral pressures and the electrostatic potential are determined. To numerically implement the molecular theory for each specific system, we discretize space. This converts the nonlinear integro-differential equations into a set of coupled nonlinear algebraic equations that can be solved by standard numerical techniques.65 Details on the discretization procedure and numerical methods can be found in Refs. 8 and 24. The inputs required to solve the nonlinear equations are the bulk pH, the bulk salt concentration (Csalt ), the molecular volumes of the all species (vi with i = pol, H+ , OH− , Na+ , Cl− , water), the surface densities of the end-tethered polymers (𝜎), the geometry of the anchoring surface, the dissociation equilibrium constants pKa of the acid groups (equilibrium constants in molar concentrations), the strength of the polymer–polymer segment interaction (𝜒), and a large set of representative polymer conformations that we generated as described above.

. Applications of the Molecular Theory In the following sections, we present results of the molecular theory for different polymer and polyelectrolyte brush systems that involve chemical equilibria. These systems highlight the versatility of the molecular theory, and they were

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

chosen to emphasize the coupling between the excluded volume, electrostatic, and other molecular interactions with chemical equilibria. The effects of the salt concentration, pH, geometry, local polymer density, and local chemical equilibrium are highly coupled and influence each other. It is important to understand this coupling on a fundamental level as well as a practical level. Quite a considerable experimental effort has been devoted to design responsive polymeric materials,4 for example, pH-sensitive polymer-coated nanopores to control ionic conduction, antifouling surfaces comprising tethered polymers, and nanoparticles and colloids steric stabilized by grafted polymers. Therefore, understanding the coupling between the molecular interactions is useful for the rational design of responsive polymer and polyelectrolyte layers for these specific as well as other applications.

..

Acid–Base Equilibrium in Polyelectrolyte Brushes

In the first application, we will focus on the effect of acidity and salt concentration on the structural properties of tethered polymer layers. Subsequently, we will discuss the effect of geometry by comparing polymers tethered to planar and curved surfaces; that is, spherical and cylindrical surfaces. Applications of polymers end-tethered to curved surfaces involve polymer-coated nanoparticles, polymer-coated carbon nanotubes, and polymer-coated nanopores. ... Effect of Salt Concentration and pH

Figure 6.1a shows a schematic representation of the tethered polymer layer. In Figure 6.1b, we present the volume fractions of the protonated and unprotonated groups of polyelectrolyte chains end-tethered to a planar surface for two different salt concentrations, namely Csalt = 0.1 M and Csalt = 0.01 M. The acid groups have a pKa = 5, which corresponds to the pKa of a carboxylic group in polyacrilic acid. The inset of Figure 6.1b shows the degree of deprotonation, also known as the degree of dissociation, as a function of the distance to the surface. The bulk solution has a pH 5 and, because the pKa 5, the degree of deprotonation of a single acid in solution would be f = 1/2. However, the degree of deprotonation within the polyelectrolyte brush is less than a half: The number of charged acid groups is less than the number of uncharged or protonated acid groups. The degree of protonation with the polymer layer is less than that of a single similar acid group in dilute solution because the acid–base equilibrium of the acid group shifts toward the uncharged protonated species to lower the degree of protonation within the polyelectrolyte layer. Moreover, the acid–base equilibrium shifts further toward the neutral state of the acid groups as the salt concentration is reduced. The reason that the degree of deprotonation of the acidic monomers within a polymer layer is reduced compared to the degree of deprotonation (charge)

6.3 Applications of the Molecular Theory

(a)

(b)

0.20 0.5

〈ϕi (z)〉

10 mM

0.3 0.2

10 mM

0.1

0.10 0.05 0.00

f (z)

0.4

0.15

z

100 mM

0

100 mM

0.0

100 mM

10 mM 5

0

10

10 20 z (nm)

15

30

20

z (nm) Figure . (a) Schematic representation of a polyacid brush tethered to a planar surface. (b) Volume fraction of protonated (full lines) and deprotonated (dashed lines) polymer monomers as a function of the distance from the surface. The inset shows the fraction of deprotonated (charged) acid groups as function of distance from the surface. Two salt concentrations are considered Csalt = 100 mM and Csalt = 10 mM, as indicated next to each curve. Calculation conditions correspond to a surface coverage of 𝜎 = 0.2 chains/nm2 , polymer chain length of Np = 50 segments, segment length l = 0.3 nm, bulk pH = 5, and pKa = 5. Source: Adapted from Nap et al. 200624 (with the difference that here the segment length equals l = 0.3 nm). Reproduced with permission of Wiley Periodicals, Inc.

of a single isolated acid is the following. The large local density of the polyelectrolyte in the layer would imply a large concentration of charged monomers if the acidic monomers would have the same level of deprotonation as an acid monomer in solution. This high concentration of charges would result in much larger electrostatic repulsions than the same deprotonated acid (charged) molecule would have in a dilute solution. To reduce these large unfavorable energetic repulsions, the system responds with a combination of three different mechanisms. First, it stretches the polymer chains, thereby increasing the average distance between the charged acid monomers. However, chain stretching will result in a large loss of conformational entropy of the polymers. Also, the polymer chain has a maximum extension: There is a limit on how far the chain can be extended. Hence, chain stretching provides only a limited possibility to compensate the unfavorable electrostatic repulsions. A second, more efficient, way to reduce the electrostatic repulsions is to bring counterions from the reservoir solution into the polyelectrolyte layer. This results in an increase in the number of the counterions within the layer that leads to an increase of the electrostatic screening, which results in a reduction of the electrostatic repulsions. But confining counterions can only occur at the cost of reducing their translational entropy. A third possibility to lower the electrostatic repulsions is

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

to decrease the charge of the acid monomers. The free-energy cost of reducing the charge of a weak acid is equal to the Gibbs free energy of the reaction (ΔGa0 ). For weak polyelectrolytes, the free-energy cost associated with shifting the chemical equilibrium toward it uncharged state is usually less costly than that of counterion confinement or chain stretching. Therefore, in a weak polyacid layer, the degree of deprotonation is shifted toward the protonated or uncharged state as compared to the degree of deprotonation of the same acid molecule in dilute solution. It should be stressed that although an end-tethered weak polyelectrolyte layer regulates its degree of charge down, it still confines counterions and stretches its chains. However, because of the system’s ability to regulate the amount of charge, the occurrence of counterion confinement and chain stretching is much smaller than for a noncharge-regulating (i.e., a strong) polyacid. The charge-regulation mechanism allows a weak polyelectrolyte brush, at least partially, to reduce the unfavorable electrostatic interactions within the layer and adopt a less unfavorable frustrated state. On a final note, charge regulation, counterion confinement, as well as chain stretching are all unfavorable from a free-energy perspective. Hence, the application of the various mechanisms by the system to compensate for unfavorable electrostatic repulsions can best be viewed as an attempt to reduce the amount of frustration and find the least, albeit still, frustrated thermodynamic state of the system. An alternative way of understanding the charging of the weak tethered polyacids is in terms of a reduced local pH that exists within the polymer layer. As the salt concentration is reduced, the proton concentration within the polymer layer increases. The proton concentration increases or equivalently the local pH (defined as –log10 ([H+ ](r)), where [H+ ](r) is the molar concentration of protons at r) decreases in order to compensate for the reduction of counterions. Figure 6.2 shows the local pH as a function of the distance from the surface for aqueous solutions with different salt concentrations. In the low pH environment, that is, within the polymer layer, the acid–base chemical equilibrium that will be established is shifted toward its uncharged state as compared to the equilibrium that exist in the bulk phase, which has a higher pH value. Furthermore, there is a large concentration of chargeable monomers within the layer. The effect of the increased polymer density also favors the uncharged protonated state of the chargeable monomers, following the mechanisms discussed above. The density effect reduces the local position-dependent “equilibrium” constant, causing a further shift toward the protonated or neutral state of the acid monomers. Effectively, we have invoked a local Le Chatelier principle, where the increase in the local proton concentration leads to a reduction in the concentration of charge acid monomers. It needs to be stressed that the local pH is a consequence of both the polymer density, ion concentration, and the degree of deprotonation of the polyelectrolyte chains and vice versa; that is, the degree of deprotonation or extent of the acid–base equilibrium of the acids on the polyelectrolyte also determines the local pH changes.

6.3 Applications of the Molecular Theory

5.00

4.50 pH (z)

100 mM 4.00 10 mM 3.50 1 mM 3.00

0

10

20

30

40

50

z (nm) Figure . Local pH, defined as –log10 [H+ ](z), where [H+ ](z) is the proton concentration at z as a function of the distance from the planar surface. The lines correspond to different salt concentrations, which are from top to bottom: 100, 10, and 1 mM respectively. The surface coverage is 𝜎 = 0.2 chains/nm2 , the polymer chain length is Np = 50 segments, segment length equals l = 0.3 nm, bulk pH = 5 and pKa = 5. Source: Adapted from Nap et al. 200624 (with the difference that here the segment length equals l = 0.3 nm). Reproduced with permission of Wiley Periodicals, Inc.

To summarize the behavior of weak polyelectrolyte layers, we computed the average degree of deprotonation (or charge) in the polyacid layer. For a weak polyelectrolyte end-tethered to a planar surface, this quantity is given by ⟨f ⟩ =

∫

f (z)⟨𝜌p (z)⟩dz ∫

.

(6.39)

⟨𝜌p (z)⟩dz

Figure 6.3 shows the average degree of charge as a function of solution pH for a weak tethered polyelectrolyte layer in aqueous solutions having different salt concentrations and for polyelectrolyte layers with different surface coverages. For comparison, the solid line labeled “bulk” shows the degree of deprotonation that an isolated acid monomer in dilute solution would have, which follows the ideal acid–base (Henderson–Hasselbalch) relation f = 1∕(1 + [H+ ]∕Ka ). The “titration” curves of Figure 6.3 show that under all conditions considered the average degree of protonation is lower for the tethered polyelectrolyte layer than for a single isolated acid. The “titration curves” are all shifted toward the neutral state. The magnitude of the shift increases with decreasing amount of salt and increasing surface coverage of the polymer layer. The pH value

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

(a)

1.0 0.8

〈f〉

0.6 0.4 0.2 0.0

(b)

2

4

6 pH

8

10

2

4

6 pH

8

10

1.0 0.8 0.6

〈f〉



0.4 0.2 0.0

Figure . Average degree of deprotonation as a function of bulk pH for (a) different salt concentration and (b) various polymer surface coverages. In (a), the surface coverage is equal to 𝜎 = 0.3 chains/nm2 and in (b) the salt concentration is equal to Csalt =100 mM. The number of segments is equal to Np = 50 and pKa = 5. The dashed line corresponds to the ideal degree of deprotonation of an isolated acid group in dilute solution. The solid lines show the calculations for the brush for varying Csalt (panel a, from top to bottom: Csalt = 200, 100, 50, 10, 1 mM) and 𝜎 (panel b, from top to bottom: 𝜎 = 0.05, 0.1, 0.2, 0.4 nm–2 ). Source: Adapted from Nap et al. 201457 (with the difference that here the segment length equals l = 0.3 nm). Reproduced with permission of American Institute of Physics.

6.3 Applications of the Molecular Theory

for which the degree of protonation is equal to 0.5 is called the apparent pKa and has special significance. The apparent pKa can be obtained experimentally through titration experiments,37,66,67 which allows comparison between experimental results and theoretical predictions. For pH values around the apparent pKa the shift in the degree of charge is largest, while for pH values that are much lower or much higher than the apparent pKa the shift is much smaller. For pH values that are much lower or much higher than the apparent pKa , the acid is either completely uncharged or completed charged and for those pH values, the charge–regulation mechanism, as explained above, is unable to shift the acid–base equilibrium in a significant way. The effect of charge regulation on the degree of deprotonation of the polyelectrolyte is most prominent for pH values around the apparent pKa . The shape of the curve of the average degree of deprotonation as a function of pH for a tethered polyelectrolyte layer is similar to that of an isolated acid. However, closer examination of the curves shows quantitative differences. First, the apparent pKa of a tethered polyelectrolyte can shift as much as 1 or 2 pH units from the bulk pKa , depending on the salt concentration of the aqueous solution and the surface coverage of the tethered layer. The titration curves are not only shifted, but they are also much broader and less symmetric than the titration curve of an isolated acid monomer. For example, an isolated acid monomer has a degree of deprotonation of ⟨f ⟩ = 1∕11 when the pH is one unit below the pKa and a degree of deprotonation of ⟨f ⟩ = 10∕11 when the pH is one unit above the pKa . Thus, an isolated acid monomer can change from almost uncharged to almost completely charged within two pH units. On the other hand, an acid group in a tethered polyelectrolyte layer requires a larger change of pH to change its average degree of deprotonation by the same amount. For example, a polyelectrolyte brush with a surface coverage of 𝜎 p = 0.4 nm−2 in an aqueous solution with a salt concentration of CNaCl = 0.1 M has an apparent pKa = 6.24 and requires a pH increase of 2.95 units to change its average degree of deprotonation from 1/11 to 10/11. Figure 6.4 quantifies this broadening of the “titration curve” of polyelectrolyte layers. The figure shows the width of the titration curve as a function of salt concentration and surface coverage. The width is defined as the change of pH needed to change the average degree of deprotonation from ⟨f ⟩ = 1∕11 to ⟨f ⟩ = 10∕11. Figure 6.4 shows that the width can increase quite significantly, and for lower salt concentrations the width increases by as much as 50%. Figure 6.4 also emphasizes that while the apparent pKa allows us to determine the degree of deprotonation of an isolated acid monomer for any pH value, it does not univocally determine the degree deprotonation of a polyacid layer. In other words, the apparent pKa contains information about the pH where the protonation transition occurs, but it does not contain information on the width of this transition. Figures 6.3 and 6.4 demonstrate that polyacid brushes behave quantitatively and qualitatively different from isolated acid molecule. Katchalsky and

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

ΔpH 3.2 3 2.8 2.6 2.4 2.2 2 0.5

3.2 3 2.8 2.6 2.4 2.2 2

0.4 0.3 σ(nm–2)

0.2

0.1 0

0.3

0.25

0.2

0.1 0.15 Csalt(M)

0.05

Figure . The pH width of “titration curves” as a function of the salt concentration and surface coverage. The pH width is defined as the pH interval required for the average degree of charge to change form 1/11 to 10/11. An ideal isolated acid in solution has a pH width of 2. The number of segments is Np = 50 and the pKa = 5.

co-workers already recognized this fact in their seminal work on the behavior of polyacid solutions.68–70 They already introduced and discussed concepts such as apparent pKa and the shift in the titration curves induced by salt concentration (for more discussions on these subjects, see their works Refs. 68–73 and 1,74, and 75). ... Effect of Polymer Density and Geometry

Figures 6.3 and 6.4 show the large effect that the polymer density has on the degree of charge in the polyacid layer. With increasing surface coverage, the apparent pKa shifts to higher values, and likewise the pH interval over which the brush acquires charge widens. These observations can be understood in terms of the above sketched charge–regulation mechanism: With increasing polymer density, the amount of potential charges increases and so it does the strength of the energetic unfavorable repulsions. In order to alleviate those repulsions, the acid–base equilibrium shifts to lower levels of deprotonation as the polymer density increase. Hence, the apparent pKa shifts to higher values, thus higher pH values are required to charge the polyelectrolyte layer. The effect of density is also closely linked to the geometry of the anchoring surface. To illustrate this, we compared results for planar surfaces with those obtained for cylindrical and spherical surfaces. Figure 6.5 shows the volume fractions of charged and neutral monomers as a function of the distance from the surface for a cylindrical and a spherical surface. The surface coverage, 𝜎, the bulk pH, the salt concentrations, as well as all other conditions are identical to the planar case depicted in Figure 6.1. Therefore, any difference between these

6.3 Applications of the Molecular Theory (a)

(b)

r

r

R R

0.06 0.5

0.05

0.2 10 mM

0.02

f (r)

f (r)

10 mM 0

10 20 r (nm)

100 mM

〈ϕi(r)〉

〈ϕi(r)〉

0.3 0.1

0.5

0.03

0.4

0.04 0.03

100 mM

0.02

100 mM

0.4

10 mM

0.3

100 mM 0.2

0

5

10 15 20 r (nm)

6

8

10 mM

0.01

0.01 0.00 10 mM 0

5

10

15

20

0.00

10 mM 0

r (nm)

2

4

10

r (nm)

Figure . Volume fraction of protonated (solid lines) and deprotonated (dashed lines) polymer monomers as a function of the distance from the surface for cylindrical (a) and spherical (b) surfaces. The inset shows the fraction of deprotonation of the chargeable acid groups as a function of distance from the surface. Two salt concentration are considered Csalt = 100 mM and Csalt = 10 mM, as indicated next to each curve. The other conditions correspond to a surface coverage of 𝜎 = 0.2 chains/nm2 , the polymer chain length is Np = 50 segments, the bulk pH = 5 and the pKa = 5. The radius of curvature is R = 0.5 nm. The value of the curvature was chosen to maximize contrast with the planar case presented in Figure 6.1. Source: Adapted from Nap et al. 200624 (with the difference that here the segment length equals l = 0.3 nm). Reproduced with permission of Wiley Periodicals, Inc.

results has to be attributed to the different surface geometry. The volume fraction profiles are very different in both shape and value for the planar, cylindrical, and spherical geometries. This is a consequence of the fact that for the different geometries considered the volume element as a function of the distance from the surface is different. Namely, the available volume is proportional to G(r), which is equal to 1, r, and r2 for planar, cylindrical, and spherical surfaces, respectively. This geometry effect also holds for neutral polymer brushes. However, for weak polyelectrolytes, the geometry effect not only affects the distribution of the polymer but also influences the distribution of charge. As we move away from the surface in the cylindrical and spherical geometries, the amount of available volume as function of distance from the surface increases. Thus, the polymer density profile changes from the “parabolic” profile obtained for a planar surface to an “exponential” decaying profile. This also affects the charge distribution. Because the density within a cylindrical and spherical brush is

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

lower than in a planar brush, the degree of deprotonation for a curved brush is higher. The degree of deprotonation of the acid groups of end-tethered polyelectrolyte increases for increasing surface curvature. The spherical geometry has a degree of deprotonation that is closest to that of an isolated acid in solution as compared to the planar and cylindrical geometries. Albeit, the degree of deprotonation is higher for curved geometries, the overall charge density is actually lower as compared to the planar case, due to the geometry effect. The coupling between acid–base equilibrium, local polymer density, and geometry has not only structural effects but also thermodynamics effects. For instance, nanoparticles can be stabilized, that is, prevented from aggregating, by coating them with a polymer brush. The stabilization is size dependent. The smaller the nanoparticle, that is, the more curved the nanoparticle surface, the higher the surface coverage required to prevent aggregation. For nanoparticles coated with weak polyelectrolytes, the stability is also strongly pH- and salt-dependent as pH and ionic strength control the amount of charge in the polymer coating and, hence, determine the electrostatic repulsions between the nanoparticles. Other examples where coupling between geometry and local acid–base equilibrium is important and can lead to novel behavior involve the conduction of nanopores coated with polyelectrolytes76 and the binding of nanoparticles coated with polyelectrolytes to lipid membranes.61 In summary, in this section we showed that excluded volume interactions, electrostatic repulsions, and chemical interactions are intricately coupled together and influence each other. In the next sections, we are going to explore various aspects of the coupling between the physical and chemical interactions within polyelectrolyte and polymer brushes in further detail. ..

Competition between Chemical Equilibria and Physical Interactions

... Brushes of Strong Polyelectrolytes

Self-assembly due to the competition of physical interactions is a recurring and intriguing theme in soft materials. For example, simulation and theory have shown that neutral, single-component polymer brushes aggregate in poorsolvent conditions into different self-organized nanostructures, such as pinned micelles, stripes, and layers with solvent-filled holes.12,13,60,77–80 These structures were observed by AFM,81–84 thus confirming theoretical predictions. Brushes made of mixtures of two types of neutral polymers (binary brushes) also self-assemble into different morphologies, as shown by both theoretical85–87 and experimental2,84,88,90 studies. The tendency of a neutral polymer brush to self-assemble into nanostructures of different morphology can be explained by the competition between segment–segment attraction forces, the conformational entropy of the chains, and osmotic forces. This competition occurs under the constraint that chains are irreversible end-grafted to the substrate.12,13,59 In poor-solvent conditions,

6.3 Applications of the Molecular Theory

a bulk solution of polymer chains would phase separate into solvent-rich and polymer-rich phases.39 On the other hand, macroscopic phase separation cannot occur for a polymer brush because the chains lack translational freedom due to the irreversible grafting of the chains to the substrate. Frustration of macroscopic phase separation leads to local formation of aggregates, in a process termed microphase separation. The shape and size of these aggregates are determined by a number of factors, including the surface density of the chains (𝜎), their length (Np ), and the quality of the solvent (𝜒 parameter).12,13,78 Let us now consider the process of microphase separation in a polyelectrolyte brush (see Fig. 6.6) rather than in a neutral polymer end-grafted layer. In the polyelectrolyte system, the electrostatic repulsion among segments is an additional interaction that competes with those described in the preceding paragraph. More specifically, the formation of self-assembled structures leads to a local increase in the density of the polyelectrolyte, which in turn leads to an increase in the electrostatic repulsions among polyelectrolyte segments. These electrostatic repulsions hinder the formation of self-assembled structures, as it is shown in the morphology diagram of Figure 6.6b. The diagram shows the different morphologies of the system as a function of the average charge per segment, fq , and the quality of the solvent, as predicted by the molecular theory.59 Note that in this particular implementation of the molecular theory, we performed fully three-dimensional calculations in order to allow the formation of inhomogeneities in planes parallel to the substrate. The average fraction of charge, fq , indicates the fraction of charged segments in the polyelectrolyte chain (one can imagine, e.g., a random copolymer where some segments are charged and others are neutral). The quality of the solvent is given by the parameter 𝜒 c /𝜒, where 𝜒 is the strength of the VdW attractive interactions (see the section Theoretical Approach) and 𝜒 c is the critical value of 𝜒 that leads to microphase separation of a neutral polymer brush in the limit of vanishing surface coverage. The smaller 𝜒 c /𝜒, the poorer the quality of the solvent and the stronger the effective Van der Waals interactions.34 The morphologies observed in Figure 6.6 (shown as isodensity surfaces in the diagrams) are hemispherical micelles (M), stripes (S), holes in a continuous layer (H), and the homogeneous brush (HB). The predicted ability of polyelectrolyte brushes to self-assemble into diverse structures may be interesting for applications in nanopattering and stimuli-responsive surfaces. Interestingly, in a strong-polyelectrolyte brush, electrostatic and VdW interactions play a similar effect on the morphology behavior of the system: Increasing either fq or 𝜒 c /𝜒 leads to the same sequence of morphology changes, M → S → H → HB (see Fig. 6.6b). In order to understand this sequence, we can argue that, for a given set of conditions, the polyelectrolyte chains will attempt to achieve an optimal density, which will increase for decreasing fq and 𝜒 c /𝜒.91 On the other hand, the self-assembled aggregates will also tend to minimize their area exposed to the solvent due to their hydrophobic nature. For very

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segment-segment vdW interactions

x

z

y

les

elle

s

es

Ho

ip Str

Mic

(b)

M

S

H

0.50 0.0099 0.031

0.55

0.60

0.65

0.70

0.75

0.80

0.85

𝜒c /𝜒

0.091

0.24

0.50

0.76

HB

0.91

Not-regulating Polyacid

fq

us neo oge h m Ho Brus

Figure . (a) Scheme of an end-grafted layer of strong polyelectrolyte chains with negatively charged segments in poor-solvent conditions. (b) Morphology diagram showing the structure of the system as a function of the average charge per monomer (fq ) and the strength of the effective Van der Waals interactions between polymer segments, 𝜒 c /𝜒 (higher 𝜒 c /𝜒 corresponds to weaker interactions and better solvent quality). The symbols in the diagram indicate the morphology of the system: M, micelles; S, stripes; H, holes; HB, homogeneous polyelectrolyte brushes. Examples of these morphologies are shown as isodensity surfaces (surfaces of constant density of the polymer). The dashed line indicates the onset of stability of the homogeneous brush (see the text; the system is predicted to be a homogeneous brush above this line). Calculation conditions: 𝜎 = 0.111 chains/nm2 , N (segments per chain) = 50, Csalt = 0.1 M. Source: Adapted from Tagliazucchi et al. 2010.15,59 Reproduced with permission of National Academy of Sciences and American Institute of Chemical Engineers (AIChE).

Fixed Charge

(a)

6.3 Applications of the Molecular Theory

low charge and strong VdW attractions, the optimal density will be high and, since the amount of polyelectrolyte chains contributing to the structures is fixed by the grafting density, the self-assembled structures will be small and nonextended, that is, they will be isolated aggregates rather than extended aggregates. The structure that minimizes the area of nonextended aggregates is the hemisphere, thus micelles are the equilibrium structure for small fq and 𝜒 c /𝜒. As the quality of the solvent and the fraction of charge increase, the optimal density of the aggregates decreases and the aggregates grow. Thus, micelles fuse together forming aggregates that are now extended in one dimension, that is, the stripes. A further increase of fq and/or 𝜒 c /𝜒 leads to aggregates extended in two dimensions, namely the holes. Finally, increasing fq and/or 𝜒 c /𝜒 beyond the holes morphology destabilizes the self-assembled aggregates with respect to the homogeneous layer. The reader is referred to Ref. 91 for a more quantitative argument based on this idea to explain the location of the morphologies in the diagram. The transition between self-assembled structures and a homogeneous brush can be also described in the context of the thermodynamics of mixtures. A twocomponent bulk mixture can exist either as a homogeneous solution or separate into two phases (a polymer-rich and a polymer-poor phase39 ). If one calculates the chemical potential of one of the components as a function of their concentration under the assumption of a homogeneous solution, then it may occur that—in some conditions—this chemical potential becomes a decreasing function of the concentration, which is thermodynamically inconsistent. This inconsistency emerges from the assumption of homogeneity, thus the condition 𝜕𝜇i /𝜕ci ≤ 0 indicates that the system must phase separate. In the case of polyelectrolyte brushes, one can propose a similar criterion for microphase separation, namely 𝜕𝜇p /𝜕𝜎 ≤ 0, where 𝜇 p is the chemical potential of the polyelectrolyte chains and 𝜎 is the surface coverage.34 Note that this criterion is not exact because (i) polyelectrolyte chains are grafted and not free in solution and (ii) polyelectrolyte brushes are multicomponent systems, rather than binary systems, due to the presence of salt ions. However, despite being an approximate criterion, the condition 𝜕𝜇p /𝜕𝜎 ≤ 0 captures the transition between self-assembled aggregates and homogeneous polyelectrolyte brushes (see the dashed line in Figure 6B), although it overestimates the range of stability of the self-assembled aggregates. ... Brushes of Weak Polyelectrolytes: Self-Assembly in Charge-Regulating Systems

In the preceding section, we described how polyelectrolyte brushes selfassemble into a rich variety of nanostructures due to the competition between electrostatic repulsions and VdW attractions. The properties of these physical interactions arise from the chemical nature of the polyelectrolyte chains, which can be changed by chemical reactions. Let us consider, for example, a weak

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

polyacid brush (Figure 6.7). In this system, all segments in each polyelectrolyte chain bear a weak acid group, which can be either negatively charged (high pH) or neutral (low pH), according to the following chemical equilibrium: Ka

−−−−−− → ← − H+ + P-Ac− . P-HAc −

(6.40)

For an isolated group in the bulk, the pH for which the fractions of Ac− and HAc are equal is the pKa of the acid. However, the apparent pKa within the brush (i.e., the pH for which the fractions of Ac– and HAc in the brush become equal) can be much higher than the pKa due to the charge-regulation effect described in Section 3.1.1.24 Figure 6.7b shows the morphology diagram for a weak polyelectrolyte brush as a function of the solution pH and 𝜒 c /𝜒. As expected, increasing the pH leads to a transition from self-assembled structures to the homogeneous brush due to the increase in the average charge per monomer at higher pH. The thermodynamic criteria for microphase separation discussed in the preceding section also works well for weak polyelectrolyte brushes (see the dashed line in Figure 6.7b). Furthermore, the morphology diagram contains all the morphologies already discussed for strong polyelectrolytes: micelles, stripes, and holes. However, the morphology diagram for weak polyelectrolyte brushes also has a new morphology that is absent for strong polyelectrolyte brushes: micelles plus nonaggregated chains (the M + NA morphology). This finding is conceptually interesting because it shows that adding a chemical equilibrium to the competing physical interactions may result in new strategies of self-organization that can lead to novel self-assembled structures. In Figure 6.8a, we analyze the M + NA morphology. The figure shows density maps along a plane parallel to the surface for increasing values of pH. We observe that upon increasing the solution pH, nonaggregated chains form at the expense of chains that were part of a micelle at lower pH. In other words, the M + NA morphology is a coexistence between the micelles and the nonaggregated chains. In Ref. 59, it is also shown that the nonaggregated chains are more charged and extended than those within micelles. Upon increasing the solution pH, chains within the micelles increase their charge and thus experience an increase in electrostatic repulsions. There are two possible strategies to decrease this repulsion: (i) to displace the acid–base equilibrium reaction toward the uncharged acid at the cost of increasing the chemical equilibrium contribution to the free energy, that is, the charge regulation mechanism; or (ii) to separate from the micelle and form a nonaggregated chain, at the cost of losing energetically favorable segment–segment contacts. Notably, some chains in the system choose to follow the first strategy and remain in the micelle and others choose to follow the second one and separate as nonaggregated chains. We stress that this interesting behavior is enabled by the presence of a chemical equilibria, which allows different chains to have different degrees of charge. In

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segment-segment vdW interactions

x

z

y

ipe

s

les

Ho

les icel

Str

M

(b)

3.0

0.50

0.55

0.60

0.65

0.70

0.75

0.80

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𝜒c /𝜒

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S

4.0

M

H

4.5

pH

5.0

5.5

6.0

M+NA

HB

No

s + hains C elle Mic gated e r g g n-a

s eou gen o Hom Brush

z x

y

Figure . Same as Figure 6.6 but for a weak polyelectrolyte, whose charge depends on the solution pH. The morphology diagram is shown as a function of 𝜒 c /𝜒 and the solution pH. The pKa of the segments is 5. Note the presence of the micelles + nonaggregated chains (M + NA) morphology in the diagram, which is absent for a system of strong noncharge-regulating polyelectrolytes (diagram in Figure 1.6b). All calculation parameters are as in Figure 6.6. Source: Adapted from Tagliazucchi et al. 2010.59 Reproduced with permission of American Institute of Chemical Engineers (AIChE).

P-COOH

P-COO– + H+

(a)

lles

(b)

10 nm

Mi ce

ipe

0

〈ϕP(r)〉

0.2 0.4 0.65

〈ϕP(r)〉

y z

x

(d)

(e)

)

10 nm

0

0.2 0.4 0.65

〈ϕP(r)〉

Pil lar (pH s + N 5.0 A )

Figure . Effect of pH on morphologies exhibiting the coexistence between nonaggregated chains and self-assembled nanoaggregates for weak polyelectrolytes in poor-solvent conditions. The acid segments of the weak polyelectrolyte may be either deprotonated (−1 charge) or protonated (neutral), depending on the pH and their local environment. Panels a, b, and d show the density of the polymer in a plane parallel to the substrate for the coexistence of nonaggregated chains with micelles (a), stripes (b), and pillars formed due to the interaction with an opposing attractive wall (d). Panels c and e show lateral schemes of the pH evolution of the one-wall (c) and two-wall (e) systems. Calculation conditions: 𝜎 = 0.111 chains/nm2 (panels a and d) or 0.16 chains/nm2 (panel b); Np = 50; Csalt = 0.1 M; 𝜒 c /𝜒 = 0.53 (panels a and b) or 0.47 (panel d); pKa = 5; separation between opposing surface = 7.5 nm (panel d only), energy of attraction of segments to opposite surface = 3.0 kB T (panel d only). Source: Adapted from Tagliazucchi et al. 201015 and 201491 . Reproduced with permission of American Institute of Chemical Engineers (AIChE) and American Chemical Society.

(c)

10 nm

Str

Mi ce (pH lles + 5.5 NA 0)

Str ip ( es + p H 5.4 NA 5)

(p H 5.2 5)

)

3.0

s( pH

Pi

Mi ce (p lles + H 5.6 NA 5) Str ip ( es + p H 5.6 NA 5)

4.4 pH llar s(

(a)

H o mo ( ge e n p H 5.6 ous )

6.3 Applications of the Molecular Theory

the case of the strong polyelectrolyte, the charge of the segments is fixed and equal for all segments, therefore the M + NA morphology is not observed. Interestingly, the coexistence between nonaggregated chains and selfassembled nanostructures is not limited to the M + NA morphology. We have observed stripes coexisting with nonaggregated chains (see Figure 6.8b) for surfaces coverages higher than those of Figure 6.7.91 We have also investigated the effect of having a second surface opposing the substrate (see Figure 6.8e). This geometry is found in AFM-colloidal tip92–95 or surface-force-apparatus experiments.96–98 In the case, where the opposing surface and the polyelectrolyte segments interact attractively,95 chains extend in order to locate as many segments as possible in contact with the opposing surface (note that we always model the substrate where chains are grafted as a purely repulsive surface). In poorsolvent conditions, the combination of strong segment–segment and segment– opposing surface attractions leads to the formation of bundles of chains that bridge both surfaces, which we name pillars (see Figures 6.8d and 6.8e).91 For low pH values (corresponding to neutral polymers), we observe only pillars, but as the pH increases some polyelectrolyte chains separate from the pillars, which leads to pillars coexisting with nonaggregated chains. In other words, the coexistence between nonaggregated chains and nanostructures is a rather general strategy for the self-organization of weak polyelectrolyte brushes in poorsolvent conditions. Given the ubiquitous nature of acid–base equilibrium in natural and man-made materials, one can speculate that novel self-organization strategies may emerge from the coupling of this equilibrium with competing physical interactions in other biological and soft-matter systems. ... Redox-Active Polyelectrolyte Brushes

In this section, we discuss polyelectrolyte brushes whose charge can be modified by a reduction/oxidation (redox) reaction according to the following chemical equilibrium: E0

−−−−−− → ← − P − Redn−1 . P − Ox + e (M) − n

−

(6.41)

In this equation, P-Ox and P-Red denote the oxidized and reduced state of the monomers, which have charges n and n – 1, respectively. The redox polyelectrolytes to be studied are grafted on the surface of a metal electrode (see Figure 6.9), which acts as a source or sink of electrons for the redox reaction, depending on its electrode potential, E.99 In the equilibrium equation, e– (M) represents an electron in the metal electrode. The standard reduction potential E0 is the standard redox potential of the redox group in bulk solution, that is, if we have isolated redox groups in solution at E = E0 , half of them will be reduced and the other half will be oxidized. In our system, the redox groups are not free in solution but they are located in the end-grafted polyelectrolyte chains. Like the acid–base equilibrium, the molecular theory incorporates the effect of the

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

Electrode

segment-segment vdW interactions

x

y

Red (q=0)

Ox (q=+1)

z

us

Mic

s elle

o ne ge h o m s Ho Bru (b)

𝜒c /𝜒

–0.3

0.55

0.60

0.65

0.70

0.75

0.80

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–0.2

–0.1

M 0.1

(E–E0) / V

0.0

S

0.2

HB

0.3

s

ipe

ipe

Str

Str

s

Figure . (a) Diagram of an end-grafted layer of redox-active polyelectrolyte in poor solvent conditions. The layer is grafted on the surface of an electrode, whose potential can be externally controlled. One every five segments in the polyelectrolyte is redox active. Redox-active segments exist either in oxidized (charge +1) or reduced (charge 0) states depending on the potential of the electrode. (b) Morphology diagram for the system in A, showing the morphology of the system as a function of the difference between the electrode potential (E) and the standard potential of the redox segments in the polymer (E0 ) and the effective Van der Waals interactions between polymer segments, 𝜒 c /𝜒 (higher 𝜒 c /𝜒 corresponds to weaker interactions and better solvent quality). The dashed line indicates stability of the homogeneous brush (see the text; the system is predicted to be a homogeneous brush above this line). Calculation conditions: 𝜎 = 0.082 chains/nm2 , Csalt = 0.1 M, Np = 50. Source: Adapted from Tagliazucchi et al. 2010.59 Reproduced with permission of American Institute of Chemical Engineers (AIChE).

(a)

6.3 Applications of the Molecular Theory

local environment into E0 to yield an apparent redox potential, E0,app . For E ≫ E0,app , the redox groups in the polyelectrolyte brush will be fully oxidized and for E ≪ E0,app they will be fully reduced. Redox polyelectrolyte brushes have been prepared and experimentally studied in Refs. 16 and 100–102, and a comprehensive discussion of the molecular theory applied to redox polyelectrolytes can be found in Refs. 38, 44, and 103. Figure 6.9 shows an example of a redox brush where the oxidized groups have a charge of +1 (n = 1), and the reduced groups are, therefore, neutral. Figure 6.9b shows the morphology diagram for this system as a function of 𝜒 c /𝜒 and E – E0 . We observe that, as expected, the homogeneous brush is favored when increasing 𝜒 c /𝜒 (i.e., good solvent) and E – E0 (increasing charge). The thermodynamic criterion for microphase separation discussed above (the dashed line in Figure 6.9b) qualitatively captures the transition between self-assembly aggregates and the homogeneous brushes. The morphology diagram for redox polyelectrolytes in Figure 6.9b shows micelles and stripes morphologies, although morphologies with coexisting nanostructures and nonaggregated chains are not present in this case. The reason why acid–base equilibrium leads to the M + NA and similar morphologies while the redox equilibrium does not is still unclear, but it may be related to the fact that the acid–base equilibrium depends on the local concentration of H+ (see the equilibrium equation), which is heterogeneous across the system and coupled to the local charge density, whereas the redox equilibrium depends on the electrode potential, which is fixed externally and is the same for all segments. ..

End-Tethered Single Stranded DNA in Aqueous Solutions

The interactions between metal ions and nucleic acids play fundamental roles in determining the physical properties of nucleic acids in aqueous environments.104 Divalent metal cations are known to be essential to nucleic acid structure,105–109 replication, and transcription,110–113 and even serve as the target molecules for some biodiagnostic sensors.114 Short sequences of singlestranded DNA (ssDNA) or RNA, commonly referred to as oligonucleotides, are being increasingly incorporated into surface-based biosensors115 and nanotemplates116 based upon their propensity to selectively and strongly bind to a wide range of target analytes. In recent years, great progress has been made in developing aptamers for use as surface-based sensors.117 While it is well established that interactions between metal ions and oligonucleotides involve the negatively charged phosphate backbone and the nucleophilic heteroatoms on the nitrogenous bases when they take place in aqueous solution,118,119 surprisingly little is known about metal cation–DNA interactions at interfaces.120 To further elucidate the local structure of end-grafted ss-DNA strands in various aqueous metal ion environments, we employed a molecular theoretical

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

study.121 In order to characterize the spatial variation of pH and salt concentration from the brush interior to bulk solution, we use the molecular meanfield theory as described in the theory section. While the previous sections, we applied the molecular theory to weak polyacids carrying carboxylic or acetate groups, here, we are considering polyacid brushes of ssDNA. Thus we shall consider the size, shape, charge, and charge distribution of the ssDNA, cations, anions, and solvent molecules, as well as the different configurations of the ssDNA chains. As noted before, the molecular theory explicitly considers the acid–base equilibrium of the chargeable species on the polyacid. Our model has been shown to properly account for the coupling between acid–base equilibrium, electrostatic, and steric interactions that determines the equilibrium behavior of systems with these intermolecular interactions. We will apply it here to model ssDNA tethered to a planar surface in contact with either a NaCl or a MgCl2 salt solution. There are two major reasons for choosing these two salts (a) magnesium is divalent whereas sodium is monovalent and (b) the volume of solvated magnesium is larger than that of sodium. We also present the effects of the protonation states of the ssDNA polyacid on the equilibrium density profiles of the ions in solution. We will explicitly consider the inhomogeneity of the system only in the direction perpendicular to the surface (see the Theoretical Approach). Our model ssDNA polyacid has 12 monomers that represent 12 bases. We use a volume of 0.4 nm3 per monomer unit in order to mimic the size of the nucleotide unit in ssDNA that consists of the nucleobase along with a five carbon sugar and phosphate group that comprise the backbone. The segment length of the chain is 0.362 nm. The volumes of the solvated Na+ (aq) and Cl– (aq) are both 0.12 nm3 . We take the volume of solvated divalent magnesium ion to be 0.18 nm3 . As before, the water is treated as dissociable from the neutral H2 O into H+ (aq) and OH– (aq), each of the three with a volume of 0.03 nm3 . We take the pKa of the polyacid groups to be 3 to try to capture the electrostatic properties of DNA. This value of the pKa is an estimate based on literature values for the nucleobases adenine (A), guanine (G), and cytosine (C), which are in the range of 2.5–4.5.122–124 The measured pKa of the phosphate backbone is about 0–1,122 so that neutralization of a nucleotide takes place through the binding of a proton by the nucleobase, rather than by the negatively charged phosphate. Our results are applicable to a ssDNA chain that does not include a substantial amount of thymine (T), since the pKa value for T to gain a proton is less than –2. The bulk pH in all calculations is 7. As we described in Section 3.1, a weak polyacid has three mechanisms to reduce the electrostatic repulsions between charged segments: stretching of the chains, bringing in bulky counterions to screen the charge, and shifting the acid–base equilibrium; thus increasing the protonation of the polyacid and reducing the polymer charge at the expense of the chemical free energy. Since

6.3 Applications of the Molecular Theory

there are only 12 monomer units comprising the chain, stretching of the chain to reduce electrostatic repulsions is not going to be a major factor. The competition between the other two mechanisms for reducing repulsions changes with the bulk environment, and this is the major focus of the next section. Let us begin by looking at the polyacid and sodium ion number density profiles in different bulk salt environments and surface coverages of ssDNA polyacid. We are taking a grafting density of 0.06 chains/nm2 to represent the lower surface coverage and 0.44 chains/nm2 to represent the higher surface coverage. A surface coverage of 0.06 chains/nm2 corresponds to an average volume fraction of the polyacid in the layer of 0.09, and a surface coverage of 0.44 chains/nm2 corresponds to an average volume fraction in the layer of 0.60. Figures 6.10a and 6.10b show the density profiles at a bulk salt concentration of 0.1 M. In Figure 6.10a, the surface coverage of the polyacid is at

z (nm)

z (nm) Na+

z (nm)

Na+

z (nm)

z (nm)

(d)

(molec/nm3)

Polyacid

Polyacid

z (nm)

(c)

(molec/nm3)

Na+

(molec/nm3)

(b) Polyacid

(molec/nm3)

(a)

Polyacid

z (nm)

Na+

z (nm)

Figure . The average number density profile of polyacid and Na+ (aq) ions as a function of distance from the wall for the polyacid layer in contact with a NaCl bulk solution. The insets show the degree of protonation as a function of distance from the wall. (a) The surface coverage of polyacid is 0.06 chains/nm2 , and the bulk salt concentration is 0.1 M. (b) The surface coverage of polyacid is 0.44 chains/nm2 ,and the bulk salt concentration is 0.1 M. (c) The surface coverage of polyacid is 0.06 chains/nm2 , and the bulk salt concentration is 1 × 10−5 M. (d) The surface coverage of polyacid is 0.44 chains/nm2 , and the bulk salt concentration is 1 × 10−5 M. Note the very different scales for the graphs and the insets. Source: Uline 2011.121 Reproduced with permission of American Chemical Society.

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

0.06 chains/nm2 . The insets to Figure 6.10 show the ensemble average of the local degree of protonation as a function of the distance from the wall, ⟨fH (z)⟩. ⟨fH (z)⟩ represents the fraction of acid groups that are protonated at distance z from the wall. In Figure 6.10a, the equilibrium protonation fraction is around 5 × 10–4 , which means that the charged ssDNA layer is neutralized by the counterions present in solution. This is largely due to the high salt concentration in comparison to that of the free protons in the bulk solution (106 more Na+ (aq) ions relative to H+ (aq) molecules in the bulk). Figure 6.10b shows the higher surface coverage of the polyacid, and we see that there is a considerably higher fraction of protonated segments (roughly three orders of magnitude), so less sodium cations are needed to help neutralize the brush. Figure 6.10b clearly shows that the density of sodium ions is consistently below the polyacid density profile. The peak in the sodium ion concentration at the outer boundary of the brush is a consequence of the fact that the polymer concentration in this region falls rapidly from a large value in the bulk monolayer to zero, and the bulky hydrated counterions, which neutralize the charged brush, are sterically repelled from the interior of the brush and accumulate at its outer periphery in an attempt to keep the monolayer charge neutral. The first conclusion that we can draw from Figure 6.10b is that the local density of polyacid results in a dramatic shift in the acid–base equilibrium in order to minimize the electrostatic repulsions. This is the result of paying in chemical free energy to reduce the electrostatic repulsions and to avoid the localization of a very high concentration of ions, that is counterion confinement. The acid–base chemical equilibrium is shifted by bringing in the smaller protons from the bulk and therefore increasing the local number density of protons and shifting the protonation of the layer and ultimately decreasing the electrostatic repulsions. Observe that the charge–regulation mechanism for ssDNA brushes is similar to that in weak polyacid brushes with carboxylic chargeable groups considered in Sections 3.1 and 3.2. However, because the DNA segments are more bulky than the carboxylic groups the effect of excluded volume interactions on the shift in the acid–base equilibrium is more pronounced. Comparison between Figures 6.10a and 6.10b shows a much higher protonation fraction for the higher surface coverage system. Due to a combination of excluded volume interactions and the entropy penalty of confining many counterions in the polyacid layer, the system shifts the mechanism of lowering the electrostatic repulsions from counterion confinement within the layer to charge regulation by shifting the acid–base equilibrium. Figures 6.10c and 6.10d depict the polyacid and sodium ion number density profiles for bulk salt concentration of 1 × 10–5 M at which the bulk concentrations of sodium cations and of protons differ by only 2 orders of magnitude. The polyacid is approximately 56% protonated at the low surface coverage and almost completely protonated in the high surface coverage case. As a result, there is almost no need for the sodium cations to penetrate into the layer to neutralize the brush, as can be seen in Figures 6.10c and 6.10d. Figure 6.10c

6.3 Applications of the Molecular Theory

shows the density distributions and the local protonation fraction at a low surface coverage of the polyacid, and we see that there is still a small enhancement of sodium cations in the layer compared to the bulk. In Figure 6.10d, we see that the concentration of sodium cations in the dense layer is close to the bulk value, except for a peak localized at the outer boundary of the layer. The acid–base equilibrium is shifted following Le Chatelier’s principle to increase the amount of protons bound to the polyacid, and so the total charge in the polyacid layer decreases. Next we look to quantify the effects of changing the salt from NaCl to MgCl2 . The differences between the magnesium and the sodium ions are the valency and the size of the ions (Mg2+ (aq) is 50% larger in the volume). Figures 6.11a and 6.11b show the polyacid and magnesium number density profiles at a bulk salt concentration of 0.1 M. In Figure 6.11a, the surface coverage is 0.06 chains/nm2 . The equilibrium protonation fraction remains low

Mg2+

z (nm)

Polyacid

z (nm)

Mg2+

z (nm)

z (nm)

z (nm)

Mg2+

z (nm)

Polyacid

Polyacid

(molec/nm3)

(d)

(c)

(molec/nm3)

Polyacid

(molec/nm3)

(b)

(molec/nm3)

(a)

z (nm) 2+

Mg

z (nm)

Figure . The average number density profile of polyacid and Mg2+ (aq) ions as a function of distance from the wall for the polyacid layer in contact with a MgCl2 bulk solution. The insets show the degree of protonation as a function of distance from the wall. (a) The surface coverage of polyacid is 0.06 chains/nm2 , and the bulk salt concentration is 0.1 M. (b) The surface coverage of polyacid is 0.44 chains/nm2 ,and the bulk salt concentration is 0.1 M. (c) The surface coverage of polyacid is 0.06 chains/nm2 , and the bulk salt concentration is 1 × 10−5 M. (d) The surface coverage of polyacid is 0.44 chains/nm2 , and the bulk salt concentration is 1 × 10−5 M. Note the very different scales for the graphs and the insets. Source: Uline 2011.121 Reproduced with permission of American Chemical Society.

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

(around 2 × 10–4 ), which means that the layer is neutralized by the magnesium cations in the layer. We can see in Figure 6.11a that the density profile of the cation follows the polyacid profile (recall that the magnesium cations carry two positive charges so if the number density is almost half that of the polymer number density there is an almost equal amount of positive and negative charge in that region). Figure 6.11b shows the high surface coverage, and since the fraction of protonated segments is still very small (around 1%), even at high coverage the brush is neutralized mainly by magnesium cations. This means that while there is still an effect of excluded volume and counterion confinement and some protons enter the layer to shift the acid–base equilibrium and reduce the electrostatic repulsions, this effect is greatly reduced in the MgCl2 salt case compared to the NaCl salt. Namely, the Mg2+ (aq) confinement is a better choice for the system to neutralize the charge of the polymer. The free energy cost of confinement, both excluded volume and entropy, is less than the chemical free energy cost of protonation. Figures 6.11a and 6.11b show that the magnesium cations have no trouble getting into the monolayer even though they have a larger volume compared to the sodium ions. This effect is easily understood when one considers the fact that one doubly charged magnesium ion does the job of two sodium ions in neutralizing the layer, and therefore the number of magnesium ions needed to neutralize the brush is half that of a sodium one. Since we took the volume of the hydrated magnesium ion to be 50% larger than that of the hydrated sodium ion, whereas the number of magnesium ions inside the brush is about two times lower than in the case of sodium, the total volume occupied by magnesium in the brush is much lower than the corresponding volume occupied by sodium ions. This reduction in steric repulsive interactions, together with the lower loss of entropy by confinement, for magnesium compared to sodium, aids in the enhanced retention of magnesium ions in the dense brush, relative to the sodium case (compare Figures 6.10 and 6.11). Figures 6.11c and 6.11d also show that the system reduces its electrostatic repulsions by enhancing the bulky salt cation penetration into the layer to neutralize the polymer charge even for the very low salt concentration of 1 × 10–5 M. This is very different from the monovalent cation case where the system prefers to pay acid–base chemical reaction free energy to shift the chemical equilibrium to reduce the electrostatic repulsions for the low bulk salt case. The degree of protonation seen in the insets of Figures 6.11c and 6.11d shows that acid–base equilibrium is not shifted as much with the divalent cation: The maximum degree of protonation is half of what was seen with the sodium salt. In this section, we have employed the molecular mean-field theory to study the effects of bulk salt concentration on the charge regulation of polyacid layers, designed to mimic an ssDNA layer, tethered to a planar surface. In general, we find that by lowering the bulk salt concentration we are increasing the amount of protons binding to the polyelectrolyte layer. We examined

6.3 Applications of the Molecular Theory

both the low and the high surface coverages of the polyacid in order to quantify the effect of increasing the total amount of steric repulsions, as well the total amount of negative charge in the layer. We started with the NaCl solution and observed at the low surface coverage that decreasing the bulk salt concentration increased the protonation, but only to a point where the sodium cations are still needed to neutralize the charge in the layer. At the high surface coverage, decreasing the bulk salt concentration increased the protonation to a point where the sodium is no longer needed to neutralize the layer and nearly all the sodium ions are expelled from the layer. We also found that replacing the sodium salt by that of magnesium decreased the protonation of the polyacid and reduced the cation expulsion from the layer (because of the twice higher valency of magnesium ions compared to sodium ions). The main conclusion from this study is to point out the nontrivial coupling that exists between chemical equilibrium, physical interactions, and molecular organization with inhomogeneous polymer layers. The dual role of the salt to screen and regulate charge is highly nonlinear and clearly depends on the valency of the counterions. The main point is that the free energy treatment should contain all of the contributions and that the optimization should be done in a coupled way in order to obtain the proper behavior. The quality of the approximations in the theory for multivalent ions is yet to be tested; however, we believe that the main qualitative effects are captured in this approach. However, both explicit counterion binding and the degree that electrostatic coupling of divalent ions is captured needs to be thoroughly tested. .. Ligand–Receptor Binding and Protein Adsorption to Polymer Brushes In previous sections of this chapter, we focused on chemical reactions occurring in polyelectrolyte brushes. We demonstrated how chemical reactions such as acid–base equilibrium, redox reactions, and ion condensation are coupled with the physical interactions of the polymer brush, and that they are able to alter the molecular organization and affect properties such as the state of charge of the polyelectrolyte brushes. The relevance of chemical reactions is not limited to polyelectrolyte brushes. In this section and following, we are going to change perspective and focus on a different class of chemical reactions: namely ligand–receptor binding. Ligand–receptor binding is very important in a number of biological applications. For example, surface-grafted water-soluble polymer spacers are of scientific and technological interest because they avert the nonspecific adsorption of biomolecules on biologically relevant materials.125–127 Also, end-functionalized polymers can improve specific binding of proteins through ligand–receptor-mediated interactions.42,43,128 For example, in the field of targeted drug delivery,129–131 liposomes are coated with a protective layer

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

of end-grafted poly(ethylene glycol) (PEG), which serves the dual purpose of reducing nonspecific adsorption of proteins to the liposome surface due to the presence of the polymer brush, which act as a steric barrier to the proteins, and improving the binding between ligands found at the terminal ends of the polymer and cell-bound receptors. Both effects can be tuned depending on the polymer molecular weight, surface coverage, and type of chemical structure. Films composed of polymer brushes and bound proteins form a dense and crowded environment. Consequently, the structure of the polymer brush and the amount of adsorbed proteins, that is, the extent of ligand–receptor binding between protein and polymer, will be influenced by a multitude of physical and chemical interactions. This is analog to the coupling between molecular organization, physical interaction, and chemical state as described previously for weak polyelectrolyte brushes. Both experiment and theory are needed for a comprehensive understanding of these complex interactions between polymer brushes and adsorbing proteins. The molecular theory has the advantages of explicitly including different interactions and molecular conformations, yielding accurate predictions, as confirmed by experimental observations126–128 and providing insights that could not have been obtained from experiments only. Nonspecific protein adsorption on biomedical surfaces initiates a range of adverse responses that can compromise function and, in some cases, lead to biomedical device failure. Polymer-based surfaces have been proposed for preventing such protein adsorption, as schematically shown in Figure 6.12. However, prevention of protein adsorption by grafted polymer chains is a complex process that involves the competition between strong attractive and repulsive forces. Therefore, it is important to consider all the interactions together as the changes in molecular organization lead to a lack of additivity of the individual forces. The effective interactions between a protein and Z

Umfbulk

Umfmax D

Umf(z=0)

Umf

Figure . Schematic representation of a polymer-grafted surface to prevent protein adsorption and the potential of mean-force(Umf ) experienced by a protein.

6.3 Applications of the Molecular Theory

a polymer-modified surface can be expressed in terms of the potential of mean force of the protein, Umf. The potential of mean force is the free energy that a single protein experiences averaged over the degrees of freedom of all the other molecules in the system.125,127 According to the potential of mean force shown in Figure 6.12, protein adsorption may take a long time to occur, since proteins need to cross the repulsive barrier presented by the polymer layer. During this process, the driving force of protein adsorption is described by a negative value of Umf (z = 0), mainly resulting from the protein–surface attraction. Adjacent to the surface, there is a barrier due to the steric repulsions between proteins and the polymer layer. Whether the prevention is successful or not depends on the height (Umf max ) and width (D) of this barrier, which are the two important factors determining the thermodynamics and kinetics of the process. Protein adsorption as described in the last paragraph is kinetically inhibited, but thermodynamically allowed. However, it is possible to make protein adsorption an unfavorable thermodynamic process, and thus to complete inhibition of protein adsorption, by using a chain surface density larger than a critical value. Protein isotherms, reflecting the relationship between the amount of adsorbed protein and the polymer surface coverage, are studied to get these critical densities for different conditions. Results from the molecular theory on protein (fibrinogen) adsorption isotherms on a peptoid antifouling polymer brush quantitatively agree with experimental data and provide critical densities for different polymer spacers.126 Kinetic control sets different criteria for avoiding protein adsorption. For instance, drug carriers need to survive in the blood stream for the time necessary to deliver the drug to its target. In this case, kinetically delaying protein adsorption beyond the timescale for drug delivery is the necessary design criteria. Theoretical results125,132,133 show that the kinetic process slows by orders of magnitude, with both chain length and surface coverage. Particularly, the time for initial protein adsorption can be prolonged by increasing chain length or surface coverage. If the time for delivering drug carriers to targets is shorter than the time for initial adsorption, drug carriers can not only effectively prevent nonspecific protein adsorption but also accurately reach targets for binding. Ligand–receptor binding is an important specific interaction in many biological systems, such as drug delivery systems mentioned above. The binding is basically a chemical equilibrium that can be shifted toward the direction of dissociated species or the bound pair depending upon the conditions of the environment and the intrinsic binding constant (Kb ). Consider the general case of a ligand (L), a receptor (R), and their bound state (LR). The chemical equilibrium can be expressed by L + R ↼ ⇁ LR. The standard free energy of binding is defined by ΔG0 = −RT ln Kb , where R is the gas constant and T is the temperature. Therefore, the binding constant determines the binding efficiency in the ideal case. In fact, the binding constant of biological relevant ligand–receptor pairs has a very wide range of values. One of the strongest known binding constants is

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

that of biotin–avidin/streptavidin134 that can be of the order 1015 M−1 . Examples of lower binding constants include that of nitrilotriacetic acid–histidine,135 whose value is of the order of Kb = 106 M−1 . Many biomimetic systems take advantage of a wide range of binding strength to control the binding of different species. Many practical binding events happen on a surface, such as a cell surface. The existence of a surface breaks the symmetry of the system and leads to dramatic deviations from the ideal binding in the dilute solution case. The question then becomes, what are the new binding constants? Furthermore, in a crowed environment, binding events are highly coupled. For example, the binding efficiency decreases greatly when more and more surface binding events happen due to the existence of steric repulsions between bound proteins. The next question is then, how can we increase the binding efficiency effectively? In order to illustrate the above questions, let us consider the binding of small proteins to ligands that are attached to the free ends of polymers tethered to a planar surface.42 In this case, it is convenient to describe the efficiency of ligand–receptor binding by the fraction of bound proteins, fLR (this quantity is analogous to f, the fraction of charged weak acid groups in an acid–base equilibrium; see the section Theoretical Approach). If the polymer surface coverage is very low and receptors are in large excess, fLR is given by 1∕(1 + K 1[R] ), where b Kb is a constant and [R] is the receptor (protein) concentration, that is, we are in the ideal solution case. As the number of ligand-functionalized polymer chains grafted to the surface increases, the fraction of the bound proteins changes. According to the molecular theory, fLR is expressed as ( ) q q 1 fLR = 1∕ 1 + L R , qLR Kb [R] where qL , qR , and qLR are the partition functions of a polymer chain endfunctionalized with a ligand, a protein with its receptor and the bound pair, respectively. The presence of the partition functions of each species in the expression fLR accounts for the nonideal interactions arising from the crowed environment. Now, we can define an apparent binding constant as app

Kb

=

qLR K , qL qR b

which could be larger or smaller than the intrinsic binding constant depending on the values of partition functions of the different species. Note that polymer chains are flexible, both in the dissociated state and bound pair state. So qL and qLR are very sensitive to the conformations of the polymer. An in-depth discussion of the molecular theory applied to ligand–receptor binding to tethered polymers can be found in Ref. 42. In Figure 6.13, we show the ligand–receptor binding results from Ref. 42. The figure shows the amount of bound proteins as a function of the surface

6.3 Applications of the Molecular Theory

ρLR (nm–2)

0.03

0.02

0 0

0.02

0.02

0.01 PEG-4400 PEG-2200 no PEG

0

0.25

0

0.5

σ

0.75

1

(nm–2)

Figure . Number of bound proteins per unit area (𝜌LR ) as a function of the surface coverage of polymers with a ligand attached to their end-group (𝜎). Calculations are for pKb = 7, proteins of radius 1.5 nm, and different polymer lengths, as shown in the figure. The inset shows the plot for small values of 𝜎. Source: Longo and Szleifer 2005.42 Reproduced with permission of American Chemical Society.

coverage for different molecular weight polymers. The results obtained with the molecular theory indicate that the binding efficiency can be increased with the presence of polymer spacers that have low and intermediate polymer surface coverages compared to the bare surface case. In Figure 6.14, we present three cartoons to illustrate the observed binding behavior. Figure 6.14a shows a schematic representation of the ligand–receptor mediated protein binding

(a)

(b)

(c)

Figure . Schematic representation of small proteins bound to (a) ligands on a bare surface, (b) ligands in the free-end group of polymers grafted to a surface with a low surface coverage, and (c) ligands in the free-end group of polymers grafted to a surface with a high surface coverage.

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

to the bare surface (A). Figures 6.14b and 6.14c show binding of proteins mediated by to surfaces with polymer spacers with low surface coverage (B) and high surface coverage (C). In general, the longer the polymer chain, the larger fLR is for the same surface coverage of ligands. This is because the flexible polymer chains make the distribution of ligands wider than those on a bare surface, effectively diminishing the steric repulsions between the bound proteins. The introduction of polymer spacers effectively increases the space available for ligand–receptor binding to occur. On the other hand, the number of bound proteins as a function of the polymer surface coverage goes through a maximum. Such an optimal surface coverage with a maximum number of bound proteins on the polymer spacers was found theoretically not only for planar surfaces42 but also for nanopores and nanochannels.43 For a better understanding of the appearance of the optimal surface coverage, information on the molecular level is needed. The analysis of the distribution of ligands reveals that the most homogeneous distribution of ligands is observed for the surface coverage that corresponds to the maximal binding, demonstrating that a homogeneous distribution of the bound proteins is optimal for minimization of repulsions and maximization of binding, as Figure 6.14b shows. For the high surface coverage case, the binding efficiency decreases for the same reason as for a bare surface case. For high surface coverages, all polymer chains are strongly stretched, which causes all the ligands that are attached to the end segments to be located on the sharp interface of the polymer brush and the solution. Hence, in these conditions strong repulsions between bound proteins cannot be avoided, leading to a lower number of bound proteins, as schematically depicted in Figure 6.14c. As one of the strongest binding interactions, streptavidin–biotin binding has many important applications. Because of the high affinity between biotin and streptavidin, the binding is almost irreversible. In consideration of the binding events happening on the surface, the main effect leading to the decrease of the binding efficiency is the occurrence of steric repulsions among bound streptavidin proteins. Biotin-functionalized polymer spacers are an effective way to avoid strong steric repulsions and increase the streptavidin–biotin binding efficiency in a crowed environment. The binding of streptavidin to biotins located at the terminal ends of poly(ethylene oxide) (PEO) spacers was studied both experimentally136 and theoretically with the molecular theory.128 Quantitative agreements between experiment and theory on the surface tension of the layers were obtained.136 The distribution of streptavidin in PEO–biotin layer is predicted to form multiple protein layers, which allows to greatly increase the binding efficiency compared to the bare surface case.128 The multilayer structure forms when grafted polymers are long enough, and ligand–receptor binding is strong enough, so that the strong binding interaction can compete against the entropic loss of polymers, resulting from distorted or stretched conformations, to optimize binding.

6.3 Applications of the Molecular Theory

.. Adsorption Equilibrium of Polymer Chains through Terminal Segments: Grafting-to Formation of Polymer Brushes As stressed in the previous sections, chemical modification of nanomaterials’ surface is key in a wide range of applications, from electronics and technology,137,138 to biomedical therapeutics and diagnosis.139,140 Engineering the surface has been crucial to stabilize colloidal suspensions,141 impart specific electronic properties, improve immunomodulation and biocompatibility,140 prevent biofouling (uncontrolled adsorption of cells and proteins),126,142,143 and enhance biodistribution and site-specific recognition.144,145 Commonly used methods resort to adding end-functionalized polymers to the surface via adsorption from a solution in contact with it. So far we have discussed examples of the molecular theory in which the polymer surface density was a fixed quantity, determined externally. In this final section, we discuss the process of polymer adsorption on surfaces of different curvature from aqueous solutions comprising a polymer binary mixture. In such cases, the surface density of the polymers is not an input (as the preceding examples) but an output to the theory, and the governing variable (among others that we shall discuss in the following) is the chemical potential of the polymers in the bulk solution. The equilibrium amount of end-adsorbed chains on the surface is determined by the equality of chemical potentials between the grafted polymers and the free chains in the bulk solution. The functionalized chain ends adsorb to the particle surface forming a layer that provides the desired effect. The thermodynamic behavior of polymers adsorbing on surfaces is intimately related to the molecular organization of the chains at the grafting surface. Seminal work by Carignano and Szleifer when developing the molecular theory for grafted polymeric layers addressed the conformational and thermodynamic behavior of linear and branched polymers grafted on planar, cylindrical, and spherical surfaces.8,29,31 Immobilizing a molecule onto a surface reduces its translational and conformational entropy with respect to the free molecule in solution in a way that greatly depends on the geometric constraint imposed by the surface.146 The surface curvature plays a central role, as it determines the available volume to the molecules grafted to its surface. The geometry of the system defines whether this amount increases (curved convex surfaces, like spheres and cylinders), decreases (curved concave objects, like nanotubes), or remains constant (planar surfaces) as a function of the distance from the surface, as discussed previously in Section 3.1.2. The constraint imposed by the surface determines the molecular organization of the grafted species and with that it governs the adsorption process. In a recent publication, the molecular theory was applied to investigate the effect of local curvature and molecular architecture (intramolecular connectivity of the monomers) on the adsorption of polymer mixtures on cylindrical and spherical surfaces.147 The derivation of the theory is slightly different from the

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

one outlined in Section 1, but it retains the key concepts explained previously (for further details, we refer the interested reader to Ref. 147). In adsorption processes from polymer solutions consisting of only one type of polymer in a good solvent, the equilibrium amount of adsorbed chains results from the balance between competing processes: the attractive forces between the polymer functionalized end segment and the surface, the loss of conformational entropy induced by packing of the chains at the surface, and the excluded volume repulsions between the monomers that are confined to the surface vicinity. Figure 6.15a shows isotherms for the surface binding of polymers of different architecture (schematically shown in Fig. 6.15) through their terminal segment. These isotherms reflect the importance of the shape and size of the polymer molecules, along with the surface curvature and morphology. Increasing the molecular weight of the polymers (i.e., the total number of segments) leads to an increase in the steric repulsion within the adsorbed layer and, therefore, to a decrease of the adsorbed density (solution of 35l vs. solution of 20l, see structures in the scheme of Figure 6.15). Regarding molecular topology, rearranging the same number of monomers in a branched chain (352b) rather than a linear one (35l) leads to bulkier molecules that experience stronger intramolecular repulsions and reach a lower grafting density at equilibrium than the linear polymer (in the good-solvent regime). The equilibrium amount of end-adsorbed chains increases as the radius of the curvature of the surface decreases, irrespective of the molecular topology or

Figure . Curvature and molecular topology effects on polymer adsorption from one-polymer solutions. (a) Equilibrium surface coverage of adsorbed polymer as a function of the curvature for spherical (full lines and solid symbols) and cylindrical (dashed lines and empty symbols) surfaces. Results correspond to the adsorption from aqueous solutions containing one type of polymer of different architecture, as shown in scheme (circle symbols: linear polymer of 20 monomers, 20l; symbols: linear polymer of 35 monomers, 35l; diamond symbols: branched polymer of 35 total monomers, 25 segments in the backbone, and 2 branches of 5 segments each, 35-2b). (b) Partition between spheres and cylinders as a function of radius, for the same mixtures as in panel a. Source: Gonzalez Solveyra et al. 2016.147 Reproduced with permission of Royal Society of Chemistry.

6.3 Applications of the Molecular Theory

the morphology of the surface, but it is always higher on spheres than on cylindrical surfaces (Figure 6.15a). This is due to the fact that the available volume at a given distance from the surface is larger for spherical surfaces than for cylindrical ones, scaling as (r/R)2 and (r/R), respectively (where r is the radial direction and R the radius of the surface). This decreases the steric repulsions within the layer on the spheres, allowing for a greater adsorption than on a cylindrical geometry with the same radius. In nanomaterials that combine different curvatures and morphologies, such as nanorods (NRs), combining spherical caps with a cylindrical body, curvature leads to denser films on the caps than on the body. This translates into a curvature-driven partition between the caps and the body (quantified as the ratio of the equilibrium surface densities on the sphere and the cylinder, 𝜎 sph /𝜎 cyl ) that is always larger than one (Figure 6.15b). Polymer chains with a larger number of segments adsorb preferentially to regions with large available volume and have larger sphere/cylinder partition than short polymers (solution of 20l vs. solution of 35l; Figure 6.15b). When introducing a second polymer type to the bulk solution, the overall adsorption process is determined by the competition between the polymer molecules for the available surface area, modulated by the underlying curvature of the surface. In view of the sphere-to-cylinder partitioning observed for one-polymer solutions, the following question raised: Can said partition be improved by resorting to adsorption from polymer mixtures containing linear and branched chains in solution? And, if so, what are the optimal conditions that would improve the partition of the branched polymers in the mixture toward surfaces of higher curvature, such as the spherical ends of NRs? For a bulk mixture comprising linear polymers of 20 monomers (20l) and branched polymers of 35 total monomers, 25 segments in the backbone and 2 branches (35-2b), a monotonic decrease (increase) in the equilibrium adsorbed amount of the linear (branched) polymer, as the solution becomes more concentrated in the branched polymer was observed (Figure 6.16a). The linear polymer 20l preferentially adsorbs until the composition of the solution reaches 80% of the branched polymer, since it can pack better in the proximity of the surface than the branched chain. Increasing the fraction of branched polymer in solution (xb bulk ) increases the amount of the branched polymer on the surface and the steric repulsions in the layer. In order to lower those repulsions, the system combines two mechanisms: (i) stretching both the linear and branched chains (at the cost of conformational entropy) and (ii) decreasing the total amount of adsorbed polymer, 𝜎 TOT = 𝜎 l + 𝜎 b (at the cost of decreasing the total adsorption energy; Figure 6.16a). The bulkier nature of the branched chains gives rise to sphere-to-cylinder surface density ratios always larger than for the linear one (Figure 6.16b). Also, the partition for the branched polymer is larger when adsorbing from solutions containing binary polymer mixtures rather than only the branched polymer and solvent solutions. The balance between the attraction forces of the end segment of the polymers and the surface, the repulsive interactions between adsorbed molecules and the

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

Figure . (a) Density of surface-adsorbed polymers as a function of the molar fraction of the branched polymer in the bulk for a mixture comprising a linear polymer of 20 monomers (20l, dashed line) and a branched polymer of 35 total monomers, 25 segments in the backbone and 2 branches (35-2b, solid line) on a sphere of R = 1 nm (see scheme on the left). (b) Partition of the linear, the branched and the total amount of polymer between a sphere and a cylinder of 1 nm radius. The style of lines is the same as in panel a. (c) Composition of the surface-adsorbed mixture as a function of the molar fraction of the branched polymer (35-2b) in the bulk for different geometries and curvatures. Full, dashed, and dotted lines correspond to a spherical, cylindrical, and planar surfaces, respectively. The dotted line labeled “xbsurf = xbbulk ” corresponds to a surface composition equal to that of the bulk solution. Source: Adapted from Gonzalez Solveyra et al. 2016.147 Reproduced with permission of Royal Society of Chemistry.

6.3 Applications of the Molecular Theory

entropy of mixing of polymer molecules determines the adsorption process. This balance is in turn modulated by the architecture of the polymers, the curvature of the surface, and the competition between the different polymers in the mixture for the available area. As a result, the equilibrium composition of the surface mixture departs strongly from that of the bulk solution, and it depends on the curvature and morphology of the surface, and the specific details of the polymers in the mixture (Figure 6.16c). The positions of the branches along the polymer backbone are crucial in the adsorption process, as shown in Figure 6.17. Short branches near the surface favor the adsorption of the linear polymer in almost the whole range of bulk solution composition, due to increased steric repulsions near the surface. To gain further insights about the effects of the molecular architecture on the adsorption process, a systematic study was carried out, varying the number of branches, the branching positions, the backbone length, and the topology of the polymer molecules in the mixtures. In all cases studied, the linear polymer in the mixture was the same. It was found that increasing the spacing between the position of the first branch and the surface strongly favors adsorption, despite the increase in the total number of monomers (Figure 6.17). Results from the

Figure . Effect of the position of the branches with respect to the surface on the adsorption for polymer mixtures comprising a linear polymer of 20 monomers (see structure 20l in scheme) and the branched polymers depicted in the scheme, on a spherical surface of R = 5 nm. The surface density of the adsorbed linear polymer as a function of the bulk molar fraction of the branched polymer is shown in dash-dot lines for the different branched polymers (from top to bottom: 35b-3b∗ , 35-3b, and 55-3b). The surface density for the branched polymer for each case is shown with full line (from top to bottom: 55-3b, 35-3b, and 35b-3b∗ ). Source: Gonzalez Solveyra et al. 2016.147 Reproduced with permission of Royal Society of Chemistry.

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

Figure . Partition of the branched polymer between a sphere and a cylinder of radius 5 nm (panel a) and 1 nm (panel b). Polymer mixtures comprise a linear polymer of 20 monomers (20l) and a branched polymer with one, two, or three branches (in blue, green and red, respectively, as shown in scheme) and backbone lengths of 20, 30, and 40 monomers (in circle, square, and star symbols, respectively, see scheme on the left). All the cases are for a fixed bulk composition with xbbulk = 0.9. Source: Adapted from Gonzalez Solveyra et al. 2016.147 Reproduced with permission of Royal Society of Chemistry.

calculations also showed that curvature-induced partition between spherical and cylindrical surfaces can be adjusted by tailoring the molecular topology of the chains in the mixture. Figure 6.18 summarizes the effect of the number of branches and the length of the backbone of the branched polymer on the partition between spheres and cylinders of radius 5 nm at a constant bulk com= 0.9). In general, the position consisting of 90% of the branched polymer (xbulk b partition between spheres and cylinders increases when increasing the number of branches or the backbone length. In this last example, we showed how the molecular theory can be used to perform systematic analysis of the relevant parameters on polymer adsorption from binary mixtures. Resorting to this theoretical methodology also allowed outlining useful design rules that would lead to a partition of the branched polymer toward regions of higher curvature. The calculations suggest that by adjusting the surface curvature, the molecular architecture of the polymer chains in the mixture, and the concentration of the bulk solution, it would be possible to engineer anisotropic nanoparticles with a spatially resolved modified surface.

. Summary and Conclusion Advances in material science during the past decades enabled the synthesis of nanosized materials with tailored morphologies, shapes, and sizes, ranging from a few to hundreds of nanometers, examples of which include spherical and

6.4 Summary and Conclusion

nonspherical nanoparticles, nanotubes, and nanopores. Further surface modification with soft materials involving polymer molecules and targeting ligands, such as proteins, antibodies, or nucleic acids, confers specific functionalities to the nanomaterials. Functionalization with soft materials has, therefore, become a key step in the preparation of nanoconstructs for applications ranging from electronics and technology, new sources of energy to biomedical therapeutics and diagnosis. Soft materials confined to a surface behave very differently from that in solution due to the competition between physical and chemical interactions acting on different length scales, modulated by the underlying surface. A fundamental understanding of such interactions is crucial for the rational design of engineered nanomaterials for any given application. However, the development of an accurate description of the interfacial behavior of these materials requires addressing directly the coupling between molecular organization, physical interactions, and chemical equilibria present in these systems and this poses serious challenges to existing theoretical models. In this chapter, we described a molecular theory that allows explicit treatment of this coupling. This theoretical approach explicitly considers the size and shape of the molecular species, the charge state, the conformations, and molecular volume of all molecules involved. It accounts for the physical interactions among them such as electrostatics, excluded volume repulsions, and Van der Waals interactions and coupled chemical equilibria involving acid–base equilibrium, ligand–receptor binding, and redox reactions. In this chapter, we discussed results obtained from the molecular theory for polymer and polyelectrolyte brushes coupled to different types of chemical equilibria in very diverse systems and geometries. We started our discussion with a description of the properties of weak polyelectrolyte brushes grafted on surfaces of different geometries in the good solvent regime. The high local density of chargeable polymer segments within the layer creates a local chemical and physical environment that is quantitatively and qualitatively very different from that of the bulk solution. The confinement of the polyacid molecules to the surface results in strong electrostatic repulsions between the negatively charged monomers within the layer. The system tries to compensate for these unfavorable repulsions by (i) increasing the amount of positively charged counterions and protons inside the brush and (ii) by shifting the chemical acid–base equilibrium toward the uncharged and protonated state.24,35.36 The latter mechanism of charge regulation has profound consequences on the acid–base properties of surface-confined polyacids and polybases, and it is strongly dependent on the curvature and geometry of the surface.24,37,146 Charge regulation is also strongly influenced by the size and valency of the counterion.37,121 The results presented here emphasize the importance of explicitly considering charge regulation as a key mechanism for the responsiveness of modified nanomaterials to local changes in electrostatic potential and/or pH. Decreasing the quality of the solvent in polyelectrolyte brushes results in qualitatively different behaviors arising from the balance between highly

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

nonadditive interactions. The addition of VdW attractive interactions leads to the possibility of microphase separation and self-assemble into various structures.15,59,81,83 In the case of weak polyelectrolytes, their charge regulating ability allows the formation of different morphologies that can be controlled by external variables such as pH and salt concentration.15,59,81,83 This demonstrates that the coupling between chemical state and different physical interactions can lead to novel molecular organization, making weak polyelectrolyte brushes of potential interest for the fabrication of stimuli responsive materials and nanopatterned surfaces. Another strength of the molecular theory is its ability to include very detailed molecular features of the species in the system. In this context, we discussed the effect of local curvature and polymer molecular architecture on the adsorption from polymer binary mixtures on surfaces of different geometry.147 The balance between repulsive forces and polymer–surface and polymer–polymer attractions governs the amount and composition of the adsorbed layer that can be modulated by the architecture or shape of the polymers, the curvature of the surface and the competition between the different polymers in the mixture for the available area. These results suggest that the combination of nanoscale curvature and tailored molecular architecture can result in anisotropic nanoparticles with spatially enriched domains, which ultimately could lead to nanoconstructs with directional chemical interactions. In the framework of the molecular theory, the description of chemical reactions as binding reactions also enables the study of counterion condensation on polyelectrolyte brushes,148 ion-pairing,45 and ligand–receptor binding.40,42,128 Regarding the latter, our results on the binding of small proteins to receptors that are attached to the free ends of surface-grafted polymers show that there exists an optimal polymer surface coverage that optimizes the number density of protein binding.42 This finding contrasts with the intuitive picture that more receptors on the surface would lead to more binding of proteins. The observed nonmonotonic behavior is a direct consequence of the competition between ligand–receptor binding and the molecular reorganization of the polymer layer.42 Ligand–receptor binding can be further optimized by combining multiple physical and chemical interactions. An example of this strategy can be found in Ref. 61, which investigated the binding of a nanoparticle coated with an mixture of neutral polymers and weak polyelectrolytes end-functionalized with ligands with a membrane of oppositely chargeable lipid molecules and receptors sites. Combing charge regulation and ligand–receptor binding resulted in a nonadditive and synergistic increase of the binding strength of the nanoparticle with the lipid membrane. The combination of charge regulation with ligand–receptor binding is a powerful tool to design and engineer multivalent nanoparticles with enhanced selectivity and binding. Another example where the competition of different competing physical and chemical interactions can result in

6.4 Summary and Conclusion

unusual and unexpected behavior is the translocation of nanoparticles through a nuclear pore complex. The combination of electrostatic and hydrophobic interactions can result into a complex translocation potential that has both wells and barriers, in contrast to the simple barrier potential observed for a hydrophilic/neutral translocating nanoparticles.50 The aforementioned examples also illustrate that coupling of physical interaction with chemical reactions is not limited to the examples mentioned in this chapter. This concept of coupling between chemical state, molecular organization, and physical interactions is also relevant beyond polymer brushes. The concept is in particular relevant to understand many biological systems as they involve a plethora of different chemical reactions and physical interactions. The effect of macromolecular crowding of gene transcription is just one of many biologically relevant examples. It is also important to emphasize the limitations of the approach presented here. While the theoretical approach outlined explicitly considers the molecular details of the species in the system and includes the coupling between physical interactions and chemical equilibria, it is still an approximate (mean-field) theory, with a number of limitations. For example, for highly charged systems, we expect charge–charge correlations to become important. Hence, the occurrence of ion–ion pairs as well as (multivalent) ion condensation needs to be considered. Ion–ion pairing and counterion condensation have recently been included into the theory by modeling these electrostatic phenomena as chemical reactions.45,148–150 These were attempts to include short-range charge– charge correlation into the theory. Electrostatic interactions also involve a macroscopic dielectric response function, that is a constitutive equation that we usually assume to be constant. To improve that we have considered in the past different dielectric functions into the theory.24 Although the different dielectric response functions lead to essential similar behavior of the end-tethered layers for the conditions considered, it is fundamentally not clear how a macroscopic dielectric response function fits into a microscopic theory. However, from a practical point view, we have found good agreement between experiment and theory for a number of relevant systems, meaning for most practical applications a constant dielectric background provides a reasonable description. For example, theoretical predictions of the layer thickness of polyacrylic acid layers,34 the ionic conduction of coated nanopores,76 and the apparent pKa of gold nanoparticles functionalized with acid functionalized ligands37 were in good agreement with experimental observations. Therefore, we believe that the molecular theory is able to capture the essential physics and chemistry of the systems considered here. The results discussed in this chapter reflect the importance of a theoretical framework that allows an explicit consideration of the coupling between molecular organization, physical interaction, and chemical state. Given the growing interplay between theory and experiments, an adequate molecular modeling

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6 Modeling of Chemical Equilibria in Polymer and Polyelectrolyte Brushes

strategy that allows understanding the interplay of the interactions acting at different levels would translate into rational design rules toward engineering stimuli responsive nanomaterials with controlled spatial functionalization for a wide range of applications.

Acknowledgments This work was supported as part of the Center for Bio-Inspired Energy Science, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award no. DE-SC0000989 and by the grant no. EB005772 from the National Institute of Biomedical Imaging and Bioengineering (NIBIB) at the National Institutes of Health (NIH). This research was supported in part through the computational resources and staff contributions provided by the Quest High Performance Computing Facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology. M.T. is a fellow of CONICET. E.G.S. acknowledges support from grant no. EB005772 from the NIBIB at the NIH. C.-l.R. acknowledges support from National Natural Science Foundation of China under grant no. 21274062.

References  R¨uhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gr¨ohn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Adv. Polym. Sci. 2004, 165, 79–150.  Santer, S.; Kopyshev, A.; Yang, H.-K.; R¨uhe, J. Macromolecules 2006, 39, 3056–3064.  Ballauff, M.; Borisov, O. Curr. Opin. Colloid Interface Sci. 2006, 11, 316–323.  Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; M¨uller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010, 9, 101–113.  Alexander, S. J. Phys. France 1977, 38, 983–987.  de Gennes, P. G. Macromolecules 1980, 13, 1069–1075.  Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988, 21, 2610–2619.  Szleifer, I.; Carignano, M. A. Adv. Chem. Phys. 1996, 94, 165–260.  de Beer, S.; Muser, M. H. Soft Matter 2013, 9, 7234–7241.  Grest, G. S. Phys. Rev. Lett. 1996, 76, 4979–4982.  Zhulina, E. B.; Borisov, O. V.; Priamitsyn, V. A. J. Colloid Interface Sci. 1990, 137, 495–511.

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 Bard, A. J.; Faulkner, L. R. Electrochemical Methods, 2nd ed.; John Wiley and Sons, Inc.: New York, 2001.  Lillethorup, M.; Torbensen, K.; Ceccato, M.; Pedersen, S. U.; Daasbjerg, K. Langmuir 2013, 29, 13595–13604.  Yu, B.; Hu, H.; Wang, D.; Huck, W. T. S.; Zhou, F.; Liu, W. J. Mater. Chem. 2009, 19, 8129–8134.  Kim, B. Y.; Ratcliff, E. L.; Armstrong, N. R.; Kowalewski, T.; Pyun, J. Langmuir 2010, 26, 2083–2092.  Tagliazucchi, M.; Calvo, E. J.; Szleifer, I. Langmuir 2008, 24, 2869–2877.  Holland, J. G.; Geiger, F. M. J. Phys. Chem. B 2012, 116, 6302–6310.  Grilley, D.; Soto, A. M.; Draper, D. E. Methods Enzymol. 2009, 455, 71–94.  Record, M. T. Biopolymers 1975, 14, 2137–2158.  Sines, C. C.; McFail-Isom, L., Howerton, S. B.; VanDerveer, D.; Williams, L. D. J. Am. Chem. Soc. 2000, 122, 11048–11056.  Davey, C. A.; Richmond, T. J. Proc. Natl. Acad. Sci. U. S. A. 2002, 99, 11169–11174.  Dobi, A.; Agoston, D. v. Proc. Natl. Acad. Sci. U. S. A. 1998, 95, 5981–5986.  Wu, F. Y. H.; Wu, C. W. Annu. Rev. Nutr. 1987, 7, 251–272.  Ghosh, A.; Ginty, D. D.; Bading, H.; Greenberg, M. E. J. Neurobiol. 1994, 25, 294–303.  Kunkel, T. A.; Loeb, L. A. J. Biol. Chem. 1979, 254, 5718–5725.  DeHaseth, P. L.; Lohman, T. M.; Record, M. T. Biochemistry 1977, 16, 4783–4790.  Liu, J.; Cao, Z.; Lu, Y. Chem. Rev. 2009, 109, 1948–1998.  Cho, E. J.; Lee, J.-W.; Ellington, A. D. Annu. Rev. Anal. Chem. 2009, 2, 241–264.  Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609.  Stoltenburg, R.; Reinemann, C.; Strehlitz, B. Biomol. Eng. 2007, 24, 381–403.  Subirana, J. A.; Soler-L´opez, M. Annu. Rev. Biophys. Biomol. Struct. 2003, 32, 27–45.  Ahmad, R.; Arakawa, H.; Tajmir-Riahi, H. A. Biophys. J. 2003, 84, 2460–2466.  Holland, J. G.; Malin, J. N.; Jordan, D. S.; Geiger, F. M. J. Am. Chem. Soc. 2011, 133, 2567–2570.  Uline, M. J.; Rabin, Y.; Szleifer, I. Langmuir 2011, 27, 4679–4689.  Bloomfield, V. A.; Crothers, D. M.; Tinoco Jr, I. Nucleic Acids: Structures, Properties, and Functions; University Science Books: Herndon, VA, 2000.  Verdolino, V.; Cammi, R.; Munk, B. H.; Schlegel, H. B. J. Phys. Chem. B 2008, 112, 16860–16873.  Lippert, B. Chem. Biodiversity 2008, 5, 1455–1474.  Fang, F.; Satulovsky, J.; Szleifer, I. Biophys. J. 2005, 89, 1516–1533.  Lau, K. H. A.; Ren, C.; Sileika, T. S.; Park, S. H.; Szleifer, I.; Messersmith, P. B. Langmuir 2012, 28, 16099–16107.

References

 Ren, C.-l.; Schlapak, R.; Hager, R.; Szleifer, I.; Howorka, S. Langmuir 2015, 31, 11491–11501.  Ren, C.-l.; Carvajal, D.; Shull, K. R.; Szleifer, I. Langmuir 2009, 25, 12283–12292.  Maruyama, K.; Ishida, O.; Takizawa, T.; Moribe, K. Adv. Drug Delivery Rev. 1999, 40, 89–102.  Gabizon, A. A. Clin. Cancer Res. 2001, 7, 223–225.  Gabizon, A.; Horowitz, A. T.; Goren, D.; Tzemach, D.; Shmeeda, H.; Zalipsky, S. Clin. Cancer Res. 2003, 9, 6551–6559.  Satulovsky, J.; Carignano, M. A.; Szleifer, I. Proc. Natl. Acad. Sci. U. S. A. 2000, 97, 9037–9041.  Fang, F.; Szleifer, I. Biophys. J. 2001, 80, 2568–2589.  Merkel, R.; Nassoy, P.; Leung, A.; Ritchie, K.; Evans, E. Nature 1999, 397, 50–53.  Nieba, L.; Nieba-Axmann, S. E.; Persson, A.; H¨am¨al¨ainen, M.; Edebratt, F.; ˚ F.; Pl¨uckthun, A. Anal. Hansson, A.; Lidholm, J.; Magnusson, K.; Karlsson, A. Biochem. 1997, 252, 217–228.  Chao, C.-Y.; Carvajal, D.; Szleifer, I.; Shull, K. R. Langmuir 2008, 24, 2472–2478.  Huang, J.; Momenzadeh, M.; Lombardi, F. IEEE Des. Test. Comput. 2007, 24, 304–311.  Maruccio, G.; Cingolani, R.; Rinaldi, R. J. Mater. Chem. 2004, 14, 542.  Petros, R. A.; DeSimone, J. M. Nat. Rev. Drug Discovery 2010, 9, 615–627.  Sapsford, K. E.; Algar, W. R.; Berti, L.; Gemmill, K. B.; Casey, B. J.; Oh, E.; Stewart, M. H.; Medintz, I. L. Chem. Rev. 2013, 113, 1904–2074.  Sperling, R. A.; Parak, W. J. Philos. Trans. R. Soc., A 2010, 368, 1333–1383.  Ham, H. O.; Park, S. H.; Kurutz, J. W.; Szleifer, I. G.; Messersmith, P. B. J. Am. Chem. Soc. 2013, 135, 13015–13022.  Palui, G.; Aldeek, F.; Wang, W.; Mattoussi, H. Chem. Soc. Rev. 2014, 1–35.  Bertrand, N.; Wu, J.; Xu, X.; Kamaly, N.; Farokhzad, O. C. Adv. Drug Delivery Rev. 2014, 66, 2–25.  Mout, R.; Moyano, D. F.; Rana, S.; Rotello, V. M. Chem. Soc. Rev. 2012, 41, 2539.  Tagliazucchi, M.; Szleifer, I. Soft Matter 2012, 8, 7292.  Gonzalez Solveyra, E.; Tagliazucchi, M.; Szleifer, I. Faraday Discussions 2016, 191, 351–372.  Hehmeyer, O. J.; Arya, G.; Panagiotopoulos, A. Z.; Szleifer, I. J. Chem. Phys. 2007, 126, 244902.  Antypov, D.; Holm, C.; Barbosa, M. C. Phys. Rev. E 2005, 71, 061106.  Cheng, H.; Zhang, K.; Libera, J. A.; Olvera de la Cruz, M.; Bedzyk, M. Biophys. J. 2006, 90, 1164.

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 Brushes of Linear and Dendritically Branched Polyelectrolytes E. B. Zhulina,1,2 F. A. M. Leermakers,3 and O. V. Borisov1,2,4 1

Institute of Macromolecular Compounds RAS, Saint Petersburg, Russian Federation ITMO University, Saint Petersburg, Russian Federation 3 Physical Chemistry and Soft Matter, Wageningen University, Wageningen, The Netherlands 4 Institut des Sciences Analytiques et de Physico-Chimie pour l’Environnement et les Mat´eriaus (IPREM), UMR 5254 CNRS UPPA, Pau, France 2

. Introduction Polyelectrolyte (PE) brushes exemplify a rare situation in materials science that theoretical concepts formulated in the late 1980s and early 1990s of the previous century,1–4 preceded and later initiated extensive experimental research in the field. The experimental investigations of PE brushes—layers of ionic macromolecules end-tethered to substrates of various nature—have exploded in the mid-1990s due to the invention of “grafting-from” techniques based on surfaceinitiated controlled radical polymerization.5 In contrast to more traditional approaches of anchoring presynthesized polymers to a surface, the “graftingfrom” method leads to rather dense and fairly monodisperse PE monolayers. In addition to many synthetic PE brushes described in the literature, monolayers of end-tethered charged biomacromolecules are also found as functional motives in biological systems.6,7 Over the past two decades significant progress has been made in understanding the equilibrium properties of brushes formed by linear polyions. The theoretical work has advanced along the following lines: (1) From a generic simplified box-like model3,4,8 to refined analytical and numerical self-consistent field (SCF) theories that described the intrinsic structure,9–13 elastic,11,14,15 and tribological,16–18 properties of PE brushes; (2) from planar to curved (spherical, cylindrical) colloidal PE brushes, with further extensions to the solution properties of branched PEs19–25 ; (3) from strong PE brushes with a constant degree of ionization, to weak PE brushes with chain ionization adapting to the salinity Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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7 Brushes of Linear and Dendritically Branched Polyelectrolytes

and pH in the solution26–29 ; (4) from individual PE monolayers to complexes of PE brushes with oppositely charged species and globular proteins.30–33 Recently, the analytical SCF theory of linear PE brushes11,12 has been extended to brushes formed by ionic macromolecules with more complex, branched architectures (ionic dendrons).34 There are a number of specific features of dendron brushes (charged and neutral) that sets them apart from the monolayers of linear chains. These are (i) stronger fluctuations in the extension of individual dendrons (particularly in nonplanar geometries) leading to a more flat overall polymer density profile; (ii) a pronounced tendency to a layered (stratified) structure at high grafting densities at which the tethered macromolecules exhibit nonlinear elasticity; (iii) sharper repulsive force versus distance profiles in interacting opposing brushes; (iv) weaker mutual interpenetration and, as a consequence, projected reduction in frictional forces between sliding brush-decorated surfaces. Therefore, the combination of ionic charges with branched topologies of the surface-decorating macromolecules endows a strong impact on the stabilizing and lubrication properties of polymer brushes. This might put biological brush-like layers of branched polysaccharides and glycoproteins that decorate bacterial,35,36 epithelial,6 and endothelial37 cell surfaces in a new perspective, especially because these layers are thought to control the adhesive properties of (bio)interfaces. In this chapter, we outline the general approach and review key results of the SCF theory of planar PE brushes composed of ionic macromolecules with branched architectures. We focus here on flexible polymers with long spacers comprising of many monomer units. The dendrons with short spacers have been extensively studied by computer simulations during the past decade, and they are out of scope of this chapter. Similarly to our earlier studies on neutral systems,38 we aim to emphasize differences between PE brushes formed by branched polyions and linear ones. Therefore, we also include in consideration planar brushes of linear polyions. It must be stressed, however, that for many features of dendron brushes there is no consensus yet, as these continue to be actively discussed in the literature.

. Analytical SCF Theory of Brushes Formed by Linear and Branched Polyions Let us consider a planar brush of charged branched macromolecules (dendrons) tethered to an impermeable inert surface. The dendrons are attached to the surface by the terminal monomer of the root segment with grafting area s per macromolecule. This value is small enough to ensure that the intermolecular interactions dominate over the intramolecular ones. Let us assume that the

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7.2 Analytical SCF Theory of Brushes Formed by Linear and Branched Polyions

brush is immersed in a polar solvent (e.g., water), which penetrates the brush and increases its thickness compared to a solvent-free case. ..

Dendron Architecture and System Parameters

We term a branched macromolecule a symmetric dendron if all its spacers and branching functionalities are the same in all its generations. A selection of the possible architectures of these macromolecules is shown schematically in Figures 7.1a–7.1c. Each symmetric dendron, with a number of generations g = 0, 1, 2, … and functionality of the branching points q = 1, 2, 3, … is composed of N=

n(qg+1 − 1) q−1

(7.1)

monomer units. Here n ≫ 1 is the number of monomer units per spacer, and dendrons with g = 0 or with q = 1 are linear chains. We also introduce the number of monomer units P = n(g + 1) in the longest elastic path in the dendron. In a symmetric comb-like macromolecule with number p = 2 of repeat units (Figure 7.1d), the number of monomer units in the main chain P = 3n and N = n(2q + 3). The spacers in branched macromolecules are assumed to be intrinsically flexible with the Kuhn segment length on the order of monomer size a. Moreover, in water a ≃ lB , where lB = e2 /(𝜀kB T) is the Bjerrum length (e is elementary charge, 𝜀 is the dielectric permittivity, kB is the Boltzmann constant, and T is the temperature).

(a)

(b)

(c)

(d)

Figure . Schematic representation of selected architectures of branched polyions: symmetric dendrons of first (a), second (b), and third (c) generations, symmetric comb (d). Branching points are marked by filled circles. Mobile co- and counterions are not shown.

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7 Brushes of Linear and Dendritically Branched Polyelectrolytes

A fraction 𝛼 < 1 of the monomer residues in the polyion is permanently (e.g., positively) charged. In experimentally relevant situations, 𝛼 ≥ n−1 . The PE brush is in contact with a bulk solution of monovalent salt with fixed concentrations c− = c+ = cs of counter- and coions that specify the Debye screening length rD = (8𝜋lB cs )−1/2 . The degree of ionization of the polyion, 𝛼, is assumed to be independent of the pH and the salt concentration cs in the solution. The thermodynamic quality of the solvent is close to theta conditions. We introduce the “bare” Gouy−Chapman length Λ, which is related to the charge number density per unit area 𝛼N/s of the tethered polyions as s (7.2) Λ= 2𝜋lB 𝛼N and is independent of branching of the macromolecule. ..

Analytical SCF Formalism

The analytical SCF model of polymer brushes is built upon: (i) the approximation that the macromolecules have Gaussian elasticity on all length scales and (ii) the strong stretching approximation that paves the way to use chain trajectories to describe the conformations of the tethered macromolecules. One target of the SCF theory is to find a self-consistent molecular potential U(z) acting on the monomers of the tethered chains at a distance z from the grafting surface. This problem has been originally solved by A. Semenov for neutral solvent-free planar and concave monolayers in microsegregated block copolymer melts.39 He predicted a parabolic shape of the molecular potential U(z) in brushes of linear end-tethered chains with N ≫1 monomers, ) ( U(z) 𝜋 2 2 3 (H − z2 ). (7.3) = 2 kB T 2a 2N The latter specifies free energy penalty for insertion of a probe monomer with volume a3 at height z < H above the grafting surface. In Semenov’s theory,39 the chain trajectory is specified by the most probable position z(m) of a monomer with ranking number m in a polymer chain with end point located at a height z1 above the grafting surface. The chain trajectory is characterized by the nonnegative stretching function √ dz(m) 𝜋 z12 − z2 . (7.4) E(z1 , z) = = dm 2N In the case of linear (Gaussian) elasticity for all chain segments, the chain tension t(z1 ,z) at a distance z from the surface is related to the stretching function E(z1 ,z) as t(z1 ,z) = 3kB Ta−2 E(z1 ,z). It was demonstrated a few years later that the expression for the molecular potential in Equation (7.3) is valid not only in solvent-free monolayers, but also in nonionic brushes swollen in solvents with arbitrary strength.40,41 In these

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7.2 Analytical SCF Theory of Brushes Formed by Linear and Branched Polyions

brushes, the monomer volume fraction 𝜙(z) is related to molecular potential U(z) in Equation (7.3) as U(z) 𝛿fconc [𝜙(z)] = + B. kB T 𝛿𝜙(z)

(7.5)

Here, fconc [𝜙(z)]a−3 is the concentration-dependent free energy density of interactions between monomers and B is the solvent-dependent calibration constant. At low volume fractions of monomers, 𝜙(z) ≪ 1, fconc [𝜙(z)] can be presented in a virial expansion, fconc [𝜙(z)] = v𝜙(z)2 + w𝜙(z)3 , with second and third virial coefficients of monomer–monomer interactions, va3 and wa6 . In good and theta solvents with v ≥ 0, B = 0, whereas in poor solvents with v < 0, B > 0.42 Remarkably, Equation (7.3) is also applicable to brushes of strong and weak linear PEs. In a strong PE brush, tethered polyions have a fixed degree of ionization, 𝛼 = const. In contrast, weak PE brushes are composed of macromolecules that regulate their local degree of ionization 𝛼(z) adapting to the pH and salinity in the solution. In the case of a strong PE brush of linear polyions with nonionic interactions negligible with respect to electrostatic ones, the self-consistent molecular potential U(z) in Equation (7.3) is linked to the electrostatic potential 𝛹 in (z) in the brush as11,12 U(z) = 𝛼e𝛹in (z).

(7.6)

In the case of a weak PE brush, the relationship between U(z) and 𝛹 in (z) has been formulated in Ref. 29. An important step in the development of analytical SCF models of dendron brushes was done by G. Pickett.43 He demonstrated that regular branching of tethered nonionic macromolecules does not change the parabolic shape of the molecular potential U(z), but merely modifies the numerical prefactor 𝜋/(2N) in Equation (7.3). That is, the self-consistent molecular potential U(z) in a PE brush of branched polyions adopts the form U(z) 3 = 2 k 2 (H 2 − z2 ). kB T 2a

(7.7)

The stretching function Ej = dz/dm for a spacer in generation j with positions √ zj and zj+1 > zj of end-points adopts functional form Ej = k 𝜆2j − z2 . Con-

stant 𝜆j = 𝜆j (zj ,zj+1 ,n) is determined by conservation of the total number n of monomers in the spacer. The full set of constants {𝜆} for the macromolecule of given architecture is uniquely specified by positions of its branching points. For a spacer in the last generation (free branch) of a dendron or a side chain in a comb-like polymer, the {𝜆} set ensures zero tension (t = 0) at the chain free ends. For a spacer connecting two branching points (including the stem

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7 Brushes of Linear and Dendritically Branched Polyelectrolytes

end-attached to the surface), the {𝜆} set ensures conservation of the spacer length and force balance in the branching points. For example, in the brush √ of star-like polyions with q free branches, the

stretching function E1 = k z12 − z2 of a free branch ensures zero tension at the branch end-point located at height z1 above the surface. Conservation of number n of monomers in a free branch provides the relationship between position z0 of the branching point and position z1 of the branch end-point as z0 = z1 cos(kn). Conservation of number n of monomers in the stem and force balance in the branching point lead to the expression for the stem stretching function √ √ E0 = k qz12 − z2 = k (q + 1)z02 − z2 with k = n−1 arctan(q−1∕2 ). (7.8)

We term k a topological coefficient as it depends on the architecture of the branched macromolecules. Therefore, if the topological coefficient k for a particular architecture of dendrons is established, Equations (7.3)–(7.6) allow for a straightforward extension of the existing analytical SCF theory of linear PE brushes11,12,29 to the brushes formed by branched polyions. In order to compare the properties of brushes formed by polyions with the same molecular weight N, fraction 𝛼 of charged monomers, and grafting area s, but having different architectures, we introduce the branching parameter 𝜂=

2Nk . 𝜋

(7.9)

Table . Analytical expressions for topological coefficient k for symmetric branched macromolecules depicted in Figure 7.1.

Architecture

Topological coefficient k

Symmetric dendron

Symmetric comb Letters in brackets correspond to the corresponding schematics in Figure 7.1.

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7.3 Planar Brush of PE Dendrons with an Arbitrary Architecture

The value of 𝜂 is equal to unity for linear polyions and increases upon PE branching. G. Pickett evaluated numerically values of the topological coefficient k for brushes formed by symmetric regularly branched dendrons with branching functionality q = 2 and number g of generations up to g = 6.43 More recent studies44–46 demonstrated that values of the topological coefficient k can be found analytically for a variety of dendron architectures. Some of these results are summarized in Table 7.1.

. Planar Brush of PE Dendrons with an Arbitrary Architecture In the linear elasticity regime of dendron brushes, the architecture of branched macromolecule (charged or neutral) is characterized by the value of its topological coefficient k. According to Equations (7.6) and (7.7), the reduced electrostatic potential 𝜓in (z) = e𝛹in (z)∕kB T in the interior of a strong PE brush with fixed degree of ionization 𝛼 is specified as 𝜓in (z) ≈

U(z) (H 2 − z2 ) 3k 2 2 2 (H − z ) ≡ = 𝛼kB T 2𝛼a2 H02 (k)

(7.10)

with calibration 𝜓(z = H) = 0. The characteristic length, which governs the decay in electrostatic potential 𝜓in (z), √ 2a2 𝛼 1∕2 (7.11) H0 (k) = 3 k depends on the degree of polyion branching via the topological coefficient k. For linear PE chains with N monomers and k = 𝜋/(2N), Equation (7.11) reduces to the expression in Ref. 11, √ 8 1∕2 𝛼 N. (7.12) H0 = a 3𝜋 2 The reduced electrostatic potential outside of the brush 𝜓out (z) = e𝛹out (z)∕kB T is governed by the net charge per unit area, eQ(k), associated with the interior of PE brush. The net number of charges Q(k) arises due to ionized groups distributed with concentration 𝛼c(z) = 𝛼𝜙(z)a−3 within the brush, and mobile coand counterions with respective concentrations c+ (z) and c− (z). By using the Poisson equation d2 𝜓in (z) = −4𝜋lB 𝜌(z), dz2

(7.13)

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7 Brushes of Linear and Dendritically Branched Polyelectrolytes

one finds the net concentration of charges 𝜌(z) = 𝛼c(z) + c+ (z) − c− (z) to be invarient with respect to the distance z to the surface: 𝜌(z) =

1 . 2𝜋lB H02 (k)

(7.14)

The net number of charges per unit area Q(k) = 𝜌(z)H =

H 2𝜋lB H02 (k)

(7.15)

sets the “external” Gouy–Chapman length ̃ Λ(k) =

H 2 (k) 1 = 0 , 2𝜋lB Q(k) H

(7.16)

which together with the Debye length, rD , govern the distributions of mobile ions in the solution above the brush. The electric field above the brush edge (i.e., at z > H) coincides with that created by a planar surface with surface charge density Q(k). The corresponding expression for 𝜓out (z) can be found, for example, in Ref. 12. At equilibrium, the mobile ions are distributed according to the Boltzmann law as c± (z) = c± (H) exp[∓𝜓(z)] with

{ 𝜓(z) =

𝜓in (z)

0≤z≤H

𝜓out (z)

z≥H

(7.17)

.

(7.18)

The polymer density distribution in the brush is then specified as c(z) =

1 + c− (H) exp[+𝜓in (z)] − c+ (H) exp[−𝜓in (z)]. 2𝜋lB H02 (k)

(7.19)

By using the conservation condition, H

s

∫

(7.20)

c(z)dz = N

0

one finds the brush thickness H. More details of the analytical SCF model can be found in the original publication.34 From collecting all of the above, it follows that the reduced inverse Gouy– Chapman length 𝜁0 = H0 ∕Λ, the reduced inverse Debye screening length 𝜅0 = H0 ∕rD and the reduced brush thickness h0 = H/H0 are interrelated with the

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7.3 Planar Brush of PE Dendrons with an Arbitrary Architecture

branching parameter 𝜂 (Eq. (7.9)) as 2 √ ⎛√ 2 ⎞ 𝜅0 𝜋 ) ( 𝜁0 2 ⎜ + h0 𝜂 2 + h0 𝜂 ⎟ exp h0 2 𝜂 2 ⋅ erf(h0 𝜂) = h0 𝜂 + 2 ⎟ 𝜂 8 ⎜ 𝜂 ⎠ ⎝ 2 √ ⎛√ 2 ⎞ 𝜅0 i 𝜋 ) ( 2 2 ⎟ exp − h 2 𝜂 2 ⋅ erf(ih 𝜂), ⎜ + + h 𝜂 − h 𝜂 0 0 0 0 2 ⎟ 8 ⎜ 𝜂 ⎠ ⎝

wherein erf(x) =

2 √ 𝜋

(7.21)

x

∫0 exp(−t 2 )dt is the error function and i2 = −1. Note

that Equation (7.21) was derived in Ref. 34 in a slightly different form, and we formulate it here in variables convenient for comparison between linear and branched polyions. The solution of Equation (7.21) h0 = h0 (𝜉0 , 𝜅0 , 𝜂) can be obtained numerically. However, the asymptotic dependences for the brush thickness H = h0 ⋅ H0 in various brush regimes can be extracted from Equation (7.21) in an analytical form.

..

Asymptotic Dependences for Brush Thickness H

According to the established nomenclature in the literature, PE brushes can be found in the so-called osmotic (OB), charged (CB), salt dominated (SB), and quasi-neutral (QnB) regimes. In the low-salt osmotic regime OB, the majority of mobile counterions are confined inside the brush. As a result, a PE brush is swollen by osmotic pressure of ions, and its thickness H exceeds by far the Gouy–Chapman length, H ≫ Λ. In the charged regime CB (also known in the literature as the Pincus brush regime), a PE brush is almost completely charged due to release of counterions in the surrounding solution and the brush thickness H ≪ Λ. In the salt-dominated regime SB, electrostatic interactions are partially screened by the ions of added salt that penetrate the brush. In this regime, the ionic interactions are described through an effective second virial coefficient of monomer–monomer interactions, veff = 𝛼 2 /(4cs ). Finally, in the quasineutral regime, QnB, the electrostatic interactions between charged monomers are much weaker than the short-range monomer–monomer repulsions, and the PE brush demonstrates a quasi-neutral behavior, that is, its thickness H obeys the same power law dependence as the thickness of a nonionic polymer brush. In the osmotic and charged brush regimes, the effect of salt is negligible and in order to find the power law asymptotes for H in these regimes, one can put 𝜅0 = 0 in Equation (7.21). The latter then reduces to √ 𝜋 2 2 ) ( 𝜁0 = h0 𝜂 + h0 𝜂 exp h0 2 𝜂 2 ⋅ erf(h0 𝜂) 𝜂 2

(7.22)

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

7 Brushes of Linear and Dendritically Branched Polyelectrolytes

In the osmotic regime OB h0 𝜂 ≫ 1, and the first term in the right-hand side (r.h.s.) of Equation (7.22) can be omitted. The corresponding expression for H is presented in the first line in Equation (7.23). In the regime of a charged brush CB, the second term in r.h.s. of Equation (7.22) is negligible. The brush thickness in this regime is obtained from the equality 𝜁0 ≈ h0 𝜂 2 and presented in the second line in Equation (7.23). Finally, in the salt-dominated SB regime an expansion of Equation (7.21) with respect to the small parameters h0 𝜂 ≪ 1, and h0 𝜂 2 ∕𝜅0 ≪ 1, leads to the third line in Equation (7.23), √ ( ) ⎧√ √ 1∕2 H0 ⎪ 8 a𝛼 N √ 2 √ ⋅ osmotic brush (OB) ⋅ ln ⋅√ ⎪ 2 𝜂 𝜂Λ 𝜋 ⎪ 3𝜋 ⎪ ⎪ 2 2 3 16 𝛼 lB a N H≈⎨ ⋅ charged brush (CB) . ⎪ 𝜋2 s𝜂 2 ⎪ )1∕3 ( 2 )1∕3 ( 2 )1∕3 ⎪ ( 𝛼 a 8 ⎪ ⋅ ⋅ salted brush (SB) ⎪ N 𝜋2𝜂2 4c s s ⎩

(7.23)

In the quasi-neutral regime QnB, the molecular potential U(z) is linked to the interaction free energy density under theta solvent conditions, fconc = w𝜙(z)3 , via Equation (7.5). In this limit, the√volume fraction profile 𝜙(z) has an elliptic shape42 and is specified as 𝜙(z) = U(z)∕(3wkB T). Normalizing the polymer concentration profile c(z) = a−3 𝜙(z) according to Equation (7.20), provides the brush thickness H in a theta solvent, (

8 H𝜃 ≈ aN 𝜋2𝜂

(

)1∕2 1∕4

⋅ (2w)

⋅

a2 s

)1∕2 (7.24)

For specified topologies of PE dendron brushes with known values of the topological coefficient k, one finds the brush thicknesses H in the respective regimes by substituting 𝜂 = 2Nk/𝜋 in Equations (7.23) and (7.24).

. Planar Brush of Star-Like Polyelectrolytes To illustrate the general SCF formalism described above, we consider a PE brush of symmetric star-like polyions with degree of polymerization N = fn, which are end-tethered by the terminal monomer of one of its branches while keeping q = f − 1 free branches. By using the expression for the topological coefficient k presented in the first row of Table 7.1, and substituting it in

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7.4 Planar Brush of Star-Like Polyelectrolytes

Equations (7.23), we find the asymptotic scaling dependences for the brush thickness H in subsequent ionic regimes: √ ( ) √ ⎧ √2 1∕2 H0 𝛼 2 √ ⎪ ⋅ ln ⋅√ ≅ 𝛼 1∕2 f 1∕2 osmotic brush (OB) ⎪ arctan(q−1∕2 ) 3 𝜂Λ 𝜋 ⎪ 2 2 2 ⎪ 𝛼 lB aN 2 2𝜋𝛼 lB a(q + 1)n H∕an ≈ ⎨ ≅ charged brush (CB) . s ⎪ s[arctan(q−1∕2 )]2 ⎪ ⎪ 𝛼 2∕3 f 2∕3 𝛼 2∕3 (q + 1)1∕3 ≅ salted brush (SB) ⎪ ⎩ (2sacs )1∕3 (2sacs )1∕3 [arctan(q−1∕2 )]2∕3 (7.25) The first expression in each of the lines in Equation (7.25) reduces to the corresponding expressions for a linear PE brush (f = 2) with degree of chain polymerization N = 2n. The second expression (power law asymptotic dependence with omitted logarithmic prefactor and/or numerical coefficient) holds in the limit f ≫ 1. In the QnB regime of quasi-neutral behavior, the analytical SCF model predicts the following asymptotic expression for the thickness H of star-like polymer brushes under theta solvent conditions38 : ( 2 )1∕2 ( 2 )1∕2 2(q + 1)1∕2 a 1∕4 3∕4 a ⋅ (2w) ⋅ ≅f , if f ≫ 1. H𝜃 ∕an ≈ √ s s 𝜋 ⋅ arctan(q−1∕2 ) (7.26) In Figure 7.2, we present the reduced brush thickness h0 = H/H0 as a function of the reduced salt concentration 𝜅02 = (H0 ∕rD )2 ∼ cs , calculated according to Equation (7.21), with the topological coefficient k specified for arm-tethered PE 10

f=2 H/H0

Figure . Dependence of the reduced thickness h0 = H/H0 of the brush formed by arm-tethered star-like PEs with the same total number of monomer units N, degree of ionization 𝛼, grafting density 𝜎, and varied number of branches: f = 2 (linear chains), 4, and 8 as a function of the reduced salt concentration 𝜅02 = (H0 ∕rD )2 ∼ cs . 𝜁0 = H0 ∕Λ = 15.

f=4

1

− 1/3 f=8 10−1 10−2

102

1

κ02

102





7 Brushes of Linear and Dendritically Branched Polyelectrolytes

stars (g = 1 in Table 7.1) with the same values of total degree of polymerization N, fraction 𝛼 of charged monomers, and grafting area s, but different number of arms, f. By using these coordinates, we can directly compare the features of PE dendron brushes with the same mass per unit area (N/s) and different branching functionalities, q = f − 1, of star-like polyions. Recall that f = 2 corresponds to a linear PE with N = 2n monomers. As one can see from Figure 7.2, the effect of polyion branching on the brush thickness H is more pronounced in the low-salt osmotic regime OB. Here, the decrease in the brush thickness H = h0 ⋅ H0 is stronger than in the salt-dominated regime SB located at higher salt concentrations cs . Moreover, the transition to the SB regime is shifted to larger values of cs . This is because in a more compact PE dendron brush the concentration of counterions is larger than in an equivalent PE brush of linear chains, and therefore a larger salt concentration cs is required to reach the onset of the salt-dominated regime.

. Threshold of Dendron Gaussian Elasticity The analytical SCF model presumes that the brush thickness H remains noticeably smaller than the contour length of the longest path, aP. The onset of nonlinear chain elasticity can be quantified by monitoring the tension in the root segment of the dendron at the grafting surface (z = 0) for strongly stretched macromolecules with free end located at z1 = H. In particular, in the case of star-like polymers, the requirement t(z1 = H, z = 0) ≤ kB Ta−1 , or equivalently, E0 (z1 = H, z = 0) ≤ a must be obeyed. Therefore, according to Equation (7.8), the condition 1 H ≅√ an q ⋅ arctan(q−1∕2 )

(7.27)

determines the threshold of the Gaussian chain elasticity (i.e., the boundary between the linear and nonlinear elasticity regimes). By substituting the corresponding expressions for H as collected in Equations (7.25) and (7.26), one finds the threshold of linear elasticity in the respective OB, CB, SB, and QnB regimes. For example, in the regime of an osmotic brush (OB), inequality (7.27) determines the upper boundary of the OB regime as 𝛼≅

3 ( /√ ) ≅ f −1 , if f ≫ 1 2q ⋅ ln 2𝜁 𝜋

(7.28)

with 𝜁 = H0 (k)∕Λ = H0 ∕(𝜂Λ) ≅ 𝛼 3∕2 f 3∕2 n2 alB ∕s. Thereby smaller fractions 𝛼 of the charged monomers are required to ensure linear elasticity of highly branched tethered stars with f = (q + 1) ≫ 1.

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7.6 Scaling-Type Diagrams of States for Brushes of Linear and Branched Polyions

Figure . Diagram of states for a brush of linear polyions in contact with salt-free solution in double logarithmic coordinates (𝛼, sa−2 ). The ratio of Bjerrum length lB to monomer size a is set to unity, third virial coefficient w = 1. The diagram shows the charged brush regime CB, the osmotic brush regime OB, and the regime QnB (all with linear elasticity for tethered polyions).

1

OB

−1

α 2/3

QnB

CB 1/4

N −4/5 1

N 4/5

sa−2

By substituting H𝜃 from Equation (7.26) in inequality (7.27), one finds the threshold of linear chain elasticity in the QnB regime as sa−2 ≅

(q + 1) ≅ f 3∕2 , if f ≫ 1. 𝜋 arctan(q−1∕2 )

(7.29)

At smaller grafting areas s, a neutral brush of star-like polymers undergoes vertical segregation into macromolecules with strongly and weakly stretched stems.47,48

. Scaling-Type Diagrams of States for Brushes of Linear and Branched Polyions In Figures 7.3 and 7.4, we present the respective scaling-type diagrams of states for PE brushes of linear and star-like polyions in contact with salt-free solutions in double logarithmic (𝛼, sa−2 ) coordinates. Figure . Diagram of states of tethered star-like PEs with f > 2 branches in contact with salt-free solution in double logarithmic coordinates (𝛼, sa−2 ). The ratio of Bjerrum length lB to the monomer size a is set to unity, third virial coefficient w = 1. The diagram shows the osmotic brush regime OB, the charged brush regime CB, the regime QnB of a quasi-neutral brush, and the regime SSP with strongly stretched longest elastic path.

1 −2 f

SSP

−1

SSP

−1

α

OB

2/3

CB 1/4

f

2/5

N −4/5 0

QnB

1

f

3/2

f

1/10 N 4/5

sa−2





7 Brushes of Linear and Dendritically Branched Polyelectrolytes

The diagrams comprise the ionic and quasi-neutral brush regimes OB, CB, and QnB, all with linear (Gaussian) chain elasticity, and in case of star-like polyions an additional regime SSP with strongly stretched longest elastic path. The latter regime in Figure 7.4 is divided by a dashed line 𝛼 ≅ wf 2 (a−2 s)−2 into subregimes in which the strong stretching of the longest path is caused by ternary interactions between monomers (to the left of the dashed line) or by electrostatic interactions between the charged monomers (to the right of the dashed line). The boundaries between neighboring regimes are obtained by equating the corresponding expressions for the brush thickness H in Equations (7.25) and (7.26). The OB–SSP and QnB–SSP boundaries in Figure 7.4 are determined by respective Equations (7.28) and (7.29). All the brush regimes are located at grafting areas s much smaller than the threshold area s∗ ≅ R2star , where Rstar is the average size of an individual star-like polyion in a dilute solution.25 As it is seen from Figures 7.3 and 7.4, branching of polyions leads to a significant contraction of both the osmotic and quasi-neutral brush regimes in favor of the SSP regime with nonlinear chain elasticity. The effect of polyion branching on the location and width of the CB regime is relatively small. Below we use the numerical Scheutjens–Fleer self-consistent field (SF-SCF) model to check and complement the predictions of the analytical SCF theory.

. Numerical SF-SCF Model of Dendron Brush The method of Scheutjens and Fleer (SF-SCF) maps the SCF equations onto a lattice,49 where the lattice site is chosen to fit the segment size a. The chains are modeled as strings of amorphous beads with segment numbers j = 1, 2, …, N, where consecutive segments along the chain sit on adjacent lattice sites. The socalled freely jointed chain (FJC) model is used to account for the chain statistics. There exists a computationally efficient propagator formalism to evaluate the partition function, which is easily adopted to deal with the dendron-like chain architectures in the brush.50,51 The FJC model has a finite extensibility, and this makes the numerical SCF also applicable for the cases where the chains, or some chain parts, become stretched comparable to their contour length. The numerical SF-SCF approach deals (on the mean field level) with all interactions between segments including the long-ranged ionic ones. The latter are treated in the Poisson–Boltzmann framework,52 whereas the nonelectrostatic interactions between monomers are accounted for as in the full Flory–Huggins equation of state.53 The SF-SCF method requires an iterative procedure to optimize the free energy of the system, and after this optimization thermodynamic and structural

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7.7 Numerical SF-SCF Model of Dendron Brush (a) 3

(b) 3

s = 103

2 αψ(z) 2

(Hk)

s = 103

2 αψ(z)

g=2 1 p=2 g=1 g=0 0

g=3

0

0.5

(z/H)2

2

(Hk)

g=3

5.103

1

104 1

0

0

0.5

1

(z/H)2

Figure . Numerical SF-SCF results for (a) the reduced electrostatic potential 𝛼𝜓∕(kH)2 as a function of the reduced squared distance (z/H)2 in PE brushes of symmetric dendrons with g = 0 (linear chain), g = 1, 2, and 3, and comb-like polymer with p = 2, all with q = 3. Reduced values were obtained using the heights as found from the numerical results, that is, H/a = 79.5, 128.5, 290, and 510 for g = 0, 1, 2, 3 dendrons, respectively, and H/a = 243.3 for the comb brush. Other parameters: n = 200, sa−2 = 1000, Flory interaction parameter 𝜒 = 0.5, 𝛼 = 0.2, volume fraction of salt ions 𝜙s = cs a3 = 10−3 . (b) Variation in reduced electrostatic potential 𝛼𝜓∕(kH)2 in the brush of dendrons with g = 3 upon an increase in grafting area sa−2 (shown near the curves). The heights used to reduce the plotted quantities are H/a = 510, 451, 314 for s/a2 = 103 , 5 × 103 , 104 , respectively. All other parameters are the same as in (a). The values of the topological coefficient k were computed by the equations given in Table 1. The straight solid line is plotted as predicted by Equation (7.10).

information is typically obtained with an accuracy of seven significant digits.54 The details of the SF-SCF method can be found elsewhere (see, e.g., Ref. 49). In Figure 7.5a, we present the dimensionless electrostatic potential 𝜓 = e𝛹 ∕kB T as calculated numerically by the SF-SCF method for planar PE brushes of linear chains (g = 0), regularly branched dendrons with a number of generations g = 1, 2, 3, and comb-like polymers with p = 2 repeat units, all in contact with salt-added solution. The data are presented in the reduced coordinates prompted by Equation (7.10). That is, we plot 𝛼𝜓∕(kH)2 as a function of reduced distance (z/H)2 . As it is seen from Figure 7.5a, the data for linear (g = 0), star-like (g = 1) and comb-like (p = 2) polyions are consistent with the analytical predictions. These data collapse on the straight line intersecting the y-axis at 𝛼𝜓(0)∕(kH)2 = 3∕2 and the x-axis at (z∕H)2 = 1. However, the data for dendrons with larger number of generations g = 2 and 3 deviate from this straight line due to the onset of nonlinear chain elasticity. In the nonlinear elasticity regime, brush thickness H is smaller than predicted by Equation (7.21), and the net charge distribution in the brush is more compact, which gives rise to increased values of the electrostatic potential compared to Equation (7.10). A larger grafting area s is required to ensure the Gaussian elasticity of these dendrons on all length scales.





7 Brushes of Linear and Dendritically Branched Polyelectrolytes

0.01 0.005

0.005

φ 0.01

0.015 0

0

200

z

400

g =0

(a)

g =1

(b)

g =2

(c)

g =3

(d)

600

4 × 10−5 2 × 10−5 2 × 10−5 φe 4 × 10

0

200

400

z

600

−5

4 × 10−5 0

Figure . The overall volume fraction profiles 𝜑(z) (left panels) and the corresponding (overall) end point distributions 𝜑e (z) (right panels) calculated with the SF-SCF method for the brushes presented in Figure 7.1a. In each of the panels, the less extended along the z-axis line corresponds to 𝜙s = cs a3 = 10−3 whereas the more extended line is for the zero salt limit. (a) Linear chains with g = 0, and N = n = 200, (b)–(c)–(d) g = 1, 2, 3, respectively. The arrows point to the local indentations in the end-point distributions. Other parameters are the same as in Figure 7.6a.

In Figure 7.5b, we demonstrate the change in shape of the electrostatic potential 𝜓 = e𝛹 ∕kB T upon an increase in grafting area s (values of s/a2 are shown near the curves). As is seen from Figure 7.5b, an increase in s makes the reduced potential 𝛼𝜓∕(kH 2 ) closer to the straight line predicted by Equation (7.10). In Figures 7.6, we present the numerically calculated polymer density profiles 𝜑(z) (left panels) and end-point distributions 𝜑e (z) (right panels) for the same values of the parameters as in Figure 7.5a and also in salt-free solution. As it follows from Figures 7.6, PE dendron brushes with Gaussian elasticity exhibit smooth polymer density profiles 𝜑(z) and end-point distributions 𝜑e (z). The stratification of the free ends due to onset of nonlinear chain elasticity is indicated by indentations shown by arrows in Figures 7.6c and 7.6d. In contrast to well-defined peaks in stratified brushes of neutral star-like polymers,48 free end distributions in strongly stretched PE brushes (Figures 7.6c and 7.6d) do not demonstrate dramatic peculiarities and remain rather smooth. However, the corresponding distributions of the branching points (not shown) exhibit sharp peaks indicative of the layered structure in strongly stretched PE brushes.

. Conclusions The presented analytical SCF model provides a unified description of planar PE brushes formed by linear and regularly branched flexible polyions with the

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References

Gaussian (linear) elasticity in salt-free and salt-added solutions. The architecture of branched polyions is accounted for explicitly via the topological coefficient k. Under the conditions of the linear chain elasticity, the topological coefficient k serves as a universal parameter of a dendron brush independent of thermodynamic quality of the solvent, fraction of charged monomers, concentration of added salt ions, chain grafting density, brush compression, etc. The predictions of the analytical SCF model are in good agreement with the numerical SF-SCF calculations.

References     

                

Miklavic, S. J.; Marcelja, S. J. Phys. Chem. 1988, 92, 6718–6722. Misra, S.; Varanasi, S.; Varanasi, P. P. Macromolecules 1989, 22, 4173–4179. Pincus, P. A. Macromolecules 1991, 24, 2912–2919. Borisov, O. V.; Birshtein, T. M.; Zhulina, E. B. J. Phys. II 1991, 1, 521–526. Ruhe, J.; Ballauff, M.; Biesalski, M.; Dziezok, P.; Gruhn, F.; Johannsmann, D.; Houbenov, N.; Hugenberg, N.; Konradi, R.; Minko, S.; Motornov, M.; Netz, R. R.; Schmidt, M.; Seidel, C.; Stamm, M.; Stephan, T.; Usov, D.; Zhang, H. Adv. Polym. Sci. 2004, 165, 79–150. Button, B.; Cai, L. H.; Ehre, C.; Kesimer, H.; Hill, D. B.; Sheehan, J. K.; Boucher, R. C.; Rubinstein, M. Science 2012, 337, 937–941. Fuchs, E.; Cleveland, D. Science 1998, 279, 514–519. Wittmer, J.; Joanny, J.-F. Macromolecules 1993, 26, 2691–2697. Zhulina, E. B.; Borisov, O. V.; Birshtein, T. M. J Phys. II 1992, 1, 63–74. Biesheuvel, P. M.; de Vos, W. M.; Amoskov, V. M. Macromolecules 2008, 41, 6254–6259. Zhulina, E. B.; Borisov, O. V. J. Chem. Phys. 1997, 107, 5952–5967. Zhulina, E. B.; Klein Walterink, J.; Borisov, O. V. Macromolecules 2000, 33, 4945–4953. Matsen, M. W. Eur. Phys. J. E 2011, 34, 45. Zhulina, E.B; Boulakh, A.B.; Borisov, O. V. Z. Phys. Chem. 2012, 226, 624–643. Attili, S.; Borisov, O. V.; Richter, R. P. Biomacromolecules 2012, 13, 1466–1477. Zhulina, E. B.; Rubinstein, M. Macromolecules 2014, 47, 5825–5838. Sokoloff, J. B. J. Chem. Phys. 2008, 129, 014901. Sirchabsan, M.; Giasson, S. Langmuir 2007, 23, 9713–9721. Ross, R.; Pincus, P. Macromolecules 1992, 25, 2177–2183. Zhulina, E.B; Borisov, O. V. Macromolecules 1996, 29, 2618–2626. Misra, S.; Mattice, W. L.; Napper, D.H. Macromolecules 1994, 27, 7090–7098. Jusufi, A.; Likos, C. N. Rev. Modern Phys. 2009, 81, 1753–1772.

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7 Brushes of Linear and Dendritically Branched Polyelectrolytes

 Klein Wolterink, J.; van Male, J.; Cohen Stuart, M. A.; Koopal, L. K.; Zhulina, E. B.; Borisov, O. V. Macromolecules 2002, 35, 9176–9190.  Leermakers, F. A. M; Ballauff, M.; Borisov, O. V. Langmuir 2008, 24, 10026–10034.  Borisov, O. V.; Zhulina, E.B.; Leermakers, F. A. M; Ballauff, M.; M¨uller, A.H.E. Adv. Polym. Sci. 2011, 241, 1–55.  Israels, R.; Leermarkers, F. A. M.; Fleer, G. J. Macromolecules 1994, 27, 3087–3093.  Zhulina, E. B.; Birshtein, T. M.; Borisov, O. V. Macromolecules 1995, 28, 1491–1499.  Lyatskaya, Yu. V.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1995, 28, 3562–3569.  Zhulina, E. B.; Borisov, O. V. Langmuir 2011, 27, 10615–10633.  Leermakers, F. A. M.; Ballauff, M.; Borisov, O. V. Langmuir 2007, 23, 3937–3946.  Henzler, K.; Haupt, B.; Lauretbach, K.; Wittemann, A.; Borisov, O. V.; Ballauff, M. J. Am. Chem. Soc. 2010, 132, 3159–3163  Biesheuvel, P. M.; Leermakers, F. A. M.; Cohen Stuart, M. A. Phys. Rev. E 2007, 7, 011802.  Pergushov, D. V.; Borisov, O. V.; Zezin, A. B.; M¨uller, A. H. E. Adv. Polym. Sci. 2011, 241, 131–161.  Zhulina, E. B.; Borisov, O. V. Macromolecules 2015, 48, 1499–1508.  Camesano, T. A.; Abu-Lail, N. I. Biomacromolecules 2002, 3, 4, 661–667.  Abu-Lail, N. I.; Camesano, T. A. Biomacromolecules 2003, 4, 4, 1000–1012.  Weinbaum, S.; Tarbell, J. M.; Damiano, E. R. Annu. Rev. Biomed. Eng. 2007, 9, 121–167.  Borisov, O. V.; Polotsky, A. A.; Rud, O. V.; Zhulina, E. B.; Leermakers, F. A. M.; Birshtein, T. M. Soft Matter 2014, 10, 2093–2101.  Semenov, A. N. Sov. Phys. JETP 1985, 61, 733–742.  Skvortsov, A. M.; Gorbunov, A. A.; Pavlushkov, I. V.; Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A. Polym. Sci. U.S.S.R. 1988, 30, 1706–1715.  Milner, S. T.; Witten, T. A.; Cates, M. E. Macromolecules 1988, 21, 2610–2619.  Zhulina, E. B.; Borisov, O. V.; Priamitsyn, V. A. J. Colloid Interface Sci. 1990, 137, 495–511.  Pickett, G. T. Macromolecules 2001, 34, 8784–8791.  Zhulina, E. B.; Leermakers, F. A. M.; Borisov, O. V. Sci. Tech. J. Inf. Technol. Mech. Opt. 2015, 15, 493–499.  Zhulina, E. B.; Borisov, O. V. Macromolecules 2015, 48, 8025–8035.  Zhulina, E. B.; Leermakers, F. A. M.; Borisov, O. V. Langmuir 2015, 31, 6514–6522.  Merlitz, H.; Wu, C.-X.; Sommer, J.-U. Macromolecules 2011, 44, 7043–7049.  Polotsky, A. A.; Leermakers, F. A. M.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 2012, 45, 7260–7273.

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References

 Fleer, G. J.; Cohen Stuart, M. A.; Scheutjens, J. M. H. M.; Cosgrove, T.; Vincent, B. Polymers at Interfaces; Chapman & Hall: London, 1993  Leermakers, F. A. M.; Scheutjens, J. M. H. M. J. Chem. Phys. 1988, 89, 3264–3274.  Meijer, L. A.; Leermakers, F. A. M.; Lyklema, J. J. Chem. Phys. 1999, 110, 6560–6579.  Isra¨els, R.; Leermakers, F. A. M.; Fleer, G. J.; Zhulina, E. B. Macromolecules 1994, 27, 3249–3261.  Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: New York, 1953  Evers, O. A.; Scheutjens, J. M. H. M.; Fleer, G. J. Macromolecules 1990, 23, 5221–5232.

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 Vapor Swelling of Hydrophilic Polymer Brushes Casey J. Galvin and Jan Genzer Department of Chemical & Biomolecular Engineering, NC State University, Raleigh, NC, USA

. Introduction Vapors play important roles across all scales in nature. Relative humidity (RH) level, in particular, is a central factor influencing the function and properties of many natural systems. For instance, the interior of the camel’s nose allows the animal to exhale subsaturated air in a dehydrated state in order to conserve water, instead of the typically saturated air respired in a hydrated state.1 On the microscale, the physical properties of spider silk depend strongly on the ambient RH level.2 On the macroscale, condensation of water vapor in cloud forests creates unique ecological habitats.3 Vapors and RH are important in industrial settings and technologies as well. Controlling and minimizing RH level in packaged foods is critical to prevent spoilage and ensure good shelf life.4 Water and organic vapors have also found extensive use in lithography applications, such as in the vapor-annealing process of diblock copolymers to produce patterned thin films.5 Maintaining sufficient RH level is also vital in forming the water meniscus in the dip pen nanolithography technique.6 On a fundamental level, vapor pressure and RH are major factors in controlling the adhesion and friction behavior between surfaces.7 The past three decades have witnessed the emergence of thin polymer films, with thicknesses smaller than 100 nm, as systems of practical and fundamental interest. Films of this range of thicknesses are typically spuncast films8 or polymer brushes.9,10 Exposing thin polymer films to solvent vapor can result in extensive swelling of the polymer film. As explored in this chapter, the extent of swelling depends on the topological structure of polymers in such films, such that spuncast thin polymer films and polymer brushes comprising the same

Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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

8 Vapor Swelling of Hydrophilic Polymer Brushes

Figure . Thin polymer films (curved lines) swell in the presence of solvent vapor (circles). The structure of spuncast polymer films (left) and polymer brushes (middle) at low and high solvent vapor pressure are depicted schematically. Varying the functional groups contained within the polymer side chain and solvent molecules (right) also modulates the swelling behavior of the films. Figure are not drawn to scale.

polymer species exhibit differences in swelling behavior upon exposure to compatible solvent vapors. The differences in the structure of spuncast polymer films and polymer brushes are depicted in Figure 8.1. In a spuncast film of sufficient thickness, such as those considered here, the chains comprising the film will possess different mobilities11 and conformations,12 depending on their location relative to the polymer/air and substrate/polymer interfaces. Polymer chains in polymer brushes are grafted covalently to the substrate by one end of the polymer chains, as defined for the purposes of this chapter. As the areal grafting density of polymer chains in the brush increases (i.e., the distance between neighboring chains decreases), the chains stretch away from the substrate.13 In addition to the structure of the thin polymer film, the interactions between the side chain chemistry of the polymer chain and the solvent molecule affect swelling behavior. The compatibility of the solvent vapor and polymer species can be tuned by modifying the side chain chemistry of the polymer chains. These concepts are explored in this chapter for a library of hydrophilic polymer brushes exposed to water and alcohol vapors. The side chain chemical function groups represented in this library and of the alcohols used in this study are shown in Figure 8.1.

8.2 Experimental

. Experimental ..

General Methods

Acetonitrile, 2-(dimethylamino)ethyl methacrylate (DMAEMA), 2(diethylamino)ethyl methacrylate (DEAEMA), methyl iodide (MeI), propyl iodide (PrI), 1,3-propanesultone (PS), dimethylsulfoxide (DMSO), methanol, ethanol, tetramethylene ethylenediamine, 2,2′ -bipyridyl, CuCl, and inhibitor remover packing were purchased from Sigma-Aldrich and used as received. n-Octyltrichlorosilane (OTS) was purchased from Gelest and used as received. The atom transfer radical polymerization initiator, [11-(2-bromo2-methyl)propionyloxy] undecyltrichlorosilane (BMPUS), was synthesized following a previously published procedure.14 Silicon wafers (0.5 mm thick, 100 mm diameter, p-doped, orientation [100]) were purchased from Silicon Valley Microelectronics. .. Synthesis of Poly((-dimethylamino)ethyl methacrylate) Brushes with a Gradient in Grafting Density A silicon wafer measuring 4.5 cm × 5 cm was sonicated in methanol, dried with a stream of N2 gas, and treated in a UV-ozone chamber for 20 min. This wafer was then placed horizontally next to a reservoir containing a 4:1 mixture of mineral oil:OTS for 7 min in an enclosed plastic Petri dish to generate a gradient in OTS along the length of the wafer. After vapor OTS deposition, the wafer was placed immediately into a solution of 30 μL of 5 vol% BMPUS in anhydrous toluene and 30 mL of anhydrous toluene and incubated at −20◦ C overnight. The wafer was then removed from solution, rinsed with ethanol, dried with a stream of N2 gas, sonicated in ethanol for 20 min and dried with a stream of N2 gas. The sample was immediately analyzed by contact angle measurement using deionized water as a probing liquid, then dried with a stream of N2 gas before immersion into a custom-built glass reactor containing the polymerization solution. The polymerization proceeded with molar ratios of [DMAEMA]:[CuCl]:[Bpy] of [1]:[0.067]:[0.030]. Specifically, the polymerization solution comprised 50 mL of DMAEMA (0.297 mol; purified by passing through a column containing inhibitor remover), 50 mL of DMSO (0.582 mol), 3.1251 g 2,2′ -bipyridyl (0.020 mol), and 0.9339 g of CuCl (0.009 mol). The reaction was allowed to proceed for 3 h. Following polymerization, the sample was removed, rinsed extensively with ethanol, and sonicated in ethanol for 20 min before being dried under a stream of N2 gas. ..

Synthesis of Poly(-(diethylamino)ethyl methacrylate) Brushes

An identical procedure was employed for poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) brush synthesis, except without the OTS

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



8 Vapor Swelling of Hydrophilic Polymer Brushes

deposition step in order to produce a polymer brush with homogeneous grafting density (𝜎). The polymerization proceeded with molar ratios of [DEAEMA]:[CuCl]:[Bpy] of [1]:[0.080]:[0.036]. Specifically, the PDEAEMA polymerization solutions contained 45 mL of DMSO (0.634 mol), 45 mL of DEAEMA (0.224 mol) purified by passing through a column containing inhibitor remover, 2.813 g 2,2′ -bipyridyl (0.018 mol), and 0.8394 g CuCl (0.008 mol). The polymerization lasted for 4 h, after which point the sample was removed, rinsed extensively with ethanol, sonicated in ethanol for 20 min, and finally dried with a stream of N2 gas. .. Chemical Modification of Poly((-dimethylamino)ethyl methacrylate) Brushes The postpolymerization modification (PPM) reactions on poly((2dimethylamino)ethyl methacrylate) (PDMAEMA) were carried out using 0.1 M solutions of MeI, PrI, or PS in acetonitrile at 40◦ C for 48 h in an orbital shaker.15,16 Modified samples were rinsed extensively with acetonitrile and THF and then dried under a stream of N2 gas. ..

Bulk Synthesis of PDMAEMA

Ten milliliters of DMAEMA monomer (0.059 mol) was purified by passing through a column with inhibitor remover. and 9.2 mg of AIBN (5.6 × 10−5 mol) was dissolved into the bulk monomer while degassing solution by bubbling with N2 for 1 h in a Schlenk flask. The molar ratio of [DMAEMA]:[AIBN] was 1050. The flask was sealed and immersed in an oil bath at 90◦ C. The reaction was allowed to proceed for 4 h, after which the flask was opened and the contents dissolved in THF and precipitated in hexanes. Repeated precipitation from THF into hexanes recovered a white solid. ..

Preparation of Spuncast PDMAEMA Films

1.5 cm × 1.5 cm silicon wafer segments were sonicated in ethanol, dried by a stream of N2 , and treated in a UV-ozone chamber for 20 min. Bulk PDMAEMA was dissolved in THF to create either 1 or 2 wt% solutions. PDMAEMA solution was pipetted onto clean wafers and spuncast at 2000 rpm for 60 s with a 500 rev/s ramp. For OTS-based samples, clean wafers were placed horizontally above a drop of pure OTS in an enclosed plastic vessel and kept for 10 min at ambient conditions. These OTS-modified substrates were sonicated in ethanol for 20 min, then dried under a stream of N2 gas, and characterized by ellipsometry. PDMAEMA solution was pipetted onto the OTS-modified substrate and spuncast at 2000 rpm for 60 s with a 250 rev/s ramp.

8.2 Experimental

..

Chemical Modification of Spuncast PDMAEMA Film

A silicon wafer supporting a PDMAEMA spuncast film was incubated in an enclosed glass vial containing 100 μL of solution for 48 h. The sample was then removed and exposed to a stream of N2 gas prior to characterization by infrared variable angle spectroscopic ellipsometry (VASE).

.. Spectroscopic Ellipsometry Measurements under Controlled Humidity Conditions Measurements were performed on a variable angle spectroscopic ellipsometer (J. A. Woollam) controlled by WVASE32 software (J. A. Woollam) using a liquid cell with windows fixed at an incidence angle of 70◦ (relative to the sample normal). Contained within the cell were two inverted vial caps (1 cm diameter) holding either pure KOH or a saturated aqueous solution of K2 SO4 . Spectroscopic ellipsometry (SE) measurements were performed every 5 min using the “dynamic scan” option in the WVASE32 software at an incidence angle of 70◦ from 400 to 1000 nm. The duration of each measurement was 3 min. A custom poly(methyl methacrylate) lid with a single opening was used to allow access for the RH-temperature probe (Omega Engineering). The probe was connected via a universal serial bus to a computer and recorded temperature and RH level every 5 min at the start of each measurement. The final RH level inside the cell during a measurement was calculated as RHt=0 + (3/5)∗ (RHt=1 – RHt=0 ), using the assumption that the increase in RH was linear over the measurement period. The approximation was found to be accurate within 1% RH. Note that above ≈85% RH, the change in RH during a measurement is 95%), regioselective, and very fast. This type of click chemistry was applied in the development of bioconjugates,107,108 and was used to functionalize or cross-link polymers in solution,109,110 or in a film.102,111,112 Since there is a broad spectrum of click chemical reactions with a wide scope of applications, we refer to the literature for more information.113,114 ... Modification of End Groups of Grafted PNIPAAm Chains

Click chemistry is widely used in the preparation and functionalization of soft materials,113 and especially polymer brushes.64,67,115,116,117 In the preparation of polymer brushes, click chemistry was used to react an alkyne-terminated chain transfer agent for reversible addition chain fragmentation transfer (RAFT) polymerization with azide-modified silica nanoparticles to generate polymer brushes on nanoparticles with intermediate grafting densities.64,115 It was used to click azide-terminated polymer chains to an alkyne-functionalized silicon substrate, or to prepare a pseudobrush intermediate layer in a “grafting-to” process.67,116 Very recently click chemistry was combined with microcontact printing to generate rewritable polymer brush micropatterns.118 Next to the modification of flat substrates and nanoparticles with brushes so-called molecular bottle brushes can be generated by clicking side chains to a linear polymer backbone.66,119,120 To add complex functionality to a polymer brush surface, a variety of click reactions were used to modify the brushes after the grafting process.117 Among them are the azide–alkyne cycloadditions, thermally induced,65 strainpromoted,121 or catalyzed by Cu(I),32,121 as well as click reactions based on thiol groups,117,122 activated ester groups and azlactones.117 As an example of postfunctionalization of “grafting-to” brushes, we present the chain prolongation of PNIPAAm brushes by TAAC.65 Bifunctionalized PNIPAAm chains were grafted with tert-butyl protected carboxylic groups at one end of the chains to a PGMA layer while protected alkyne groups at the other chain ends stayed active for the postfunctionalization process of the brush (Figure 10.5). At each PNIPAAm chain (Mn = 48,300 g/mol determined by 1 H NMR), three alkyne groups were present, which were protected with trimethylsilyl groups. In the “grafting-to” process, 60% of the protecting groups are cleaved.32



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NH O

N H

X=Cl or Br

O

m

O

X

NH

n

N3

TMS

TAAC

I.

Grafting to

TAAC

NAlk = 3

N3

Spin coating

Alkyne-functionalized PNiPAAm brush

II.

N3

TAAC-condition High enough chain mobility at the brush/polymer interface optimal for T > Tg

N3

III.

TAAC

O

NH N N N

HN

O

O

All chain extended PNiPAAm brush

NHO

m

X n

NH

X o

Figure . (I, II) Preparation of alkyne-functionalized PNIPAAm brushes and (III) chain extension via TAAC. Alkyne groups are drawn at the surface of the PNIPAAm brush, but can have a depth distribution. Source: Rauch, et al. 2013.65 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA.

Covalent linkage to PGMA

tBuO

Alk-PNiPAAm (PN4)

O

o

O

Az-PNiPAAm (PN2)

HN

X

10.2 Part I: Polymer Brush Architectures (b) Before TAAC After TAAC

80

+ 80 %

2O

60 40 + 74 %

20

5 0

/dT (nm/°C)

100

ddH

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2O

(nm)

(a) 120

–5 –10

–20 Tc ~ 31 °C

–25 15

18

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30

33

36

39

Increased sensitivity (2.9x)

Before TAAC After TAAC

–15

15

20

25

30

35

40

Temperature (°C)

Figure . (a) Temperature-dependent swollen brush layer thickness and (b) the first derivative of the layer thickness of a PNIPAAm brush with chain extension of every grafted chain. Source: Rauch et al. 2013.65 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA.

Afterwards, azide-functionalized PNIPAAm (Mn = 35,000 g/mol determined by 1 H NMR) was spin-coated and reacted with the alkyne-functionalized PNIPAAm via TAAC in the melt at a temperature of 150◦ C above the Tg of the polymer. From the change in the dry polymer brush thickness the grafting density of the click-immobilized azide-PNIPAAm could be calculated. It was the same as the initial grafting density of 0.17–0.18 nm−2 of the alkyne–PNIPAAm brush. Thus all chains have been prolonged. Temperature-sensitive swelling of the PNIPAAm brushes was investigated before and after chain prolongation. The swollen brush thickness was higher below as well as above the critical temperature Tc as compared to the original brush, and a 2.9 × higher sensitivity was observed for the extended PNIPAAm brush. Tc stayed constant at 31◦ C (Figures 10.6a and 10.6b). The temperature-dependent refractive index of the swollen layer, and the water content varied only slightly before and after chain extension. By grafting binary brushes of alkyne–PNIPAAm and nonfunctionalized PNIPAAm to the surface, the density of alkyne groups at the surface can be changed and brushes with selective chain extension can be generated. The strategy of chain extension of “grafting-to” brushes can lead to novel responsive and biointeractive interfaces and should be adaptable for other binary brush systems. Similarly, diblock copolymer brushes can be obtained in this way. ..

Hybrid Brush Nanostructures

The combination of polymer brushes with inorganic nanostructures, such as metal nanoparticles and sculptured thin films (STF), or with organic “nanoparticles,” such as viruses, proteins, drugs, and polymer particles, provides novel systems for applications in sensors, drug delivery, catalysis, or medical diagnostics. We discuss here two surface-bound hybrid structures:

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10 Functional Biointerfaces Tailored by “Grafting-To” Brushes

inorganic nanoparticles immobilized in or at the interface of polymer brushes,30,123,124–127 and inorganic slanted thin films (STF) grafted with polymer brushes.31,128 (For an overview on mobile organic/inorganic nanoparticles grafted with polymer brushes, we refer to the literature.77,129–135 )

... Nanoparticles Immobilized at Polymer Brushes

Immobilization of nanoparticles (NP) at polymer brush surfaces can lead to nanocomposite materials with novel optical and mechanical properties, or catalytic activity. NPs were formed by reduction of metal salts inside the brush,30,123 or preformed NPs were inserted into the brush and immobilized by hydrogen bonding136 and hydrophobic interaction.137 Preformed NPs were also physisorbed at the polymer–solution interface.127,138 For the in situ creation of particles by metal salt reduction, the density of the brush matrix limits the particle size (limiting the maximal concentration of the precursor salt) and also effectively prevents agglomeration of the formed NP.123 One important application of NP incorporated in polymer brushes is the tuning of the catalytic activity on a polymer surface.30,124,125,139,140 Catalytic reactions by gold, platinum, and palladium particles in polymer brushes were investigated, and stimuli-responsive catalytic activity was achieved by mixed PNIPAAm–P2VP “grafting-to” brushes.125 Here, additionally, the NP content could be tuned by the composition of this binary brush, since the precursor metal salt only associated with the P2VP chains and not with PNIPAAm. These catalytically active hybrid systems provide models for more complex bioactive systems, such as enzymes immobilized in polymer brushes.141,142 The interaction of NP and grafted polymer chains as well as the infiltration behavior was theoretically studied, with focus on the effect of solvent quality,143–145 polydispersity, grafting density, and particle size.145–147 For a model system of single particles approaching a polymer brush, √ the higher the polydispersity the easier a small particle (radius R < 1∕(2 𝜎), with 𝜎 the grafting density) can penetrate deep into the brush. Large particles adsorb better at low polydispersity but do not penetrate deeply.146 For bad solvent quality, the brushes collapse, which reduces the free energy barrier and favors particle penetration into the brush and adsorption,143 while polydispersity reduces the free energy for NP immersion as well.147 NPs in this context also serve as model objects to study and understand the interaction of biological nanoscale objects like proteins and viruses with an artificial surface. Stimuli-responsive polymer brushes loaded with noble metal particles or fluorescence nanocrystals are highly interesting as chemical and biological nanosensors.123,136,138,148,149 The principle of these sensing systems is based on the stimuli-responsive swelling of polymer chains in response to their respective trigger, for example, pH, temperature, or solvent. Due to the different swelling at different solution conditions, the distance between NP and

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10.2 Part I: Polymer Brush Architectures

0.08

0.09 547 nm

0.07

> LCST

0.08

< LCST

0.07

Absorbance

0.09

Absorbance

the substrate as well as the distance between neighboring NP changes, which affects the measurement signal from the collective NP ensemble, respectively. For CdSeS nanocrystals, the fluorescence interference contrast near the reflecting silicon surface was monitored, utilizing changes in the distance between NP and surface,149 while for noble particles the change in the surface plasmon resonance (SPR) absorbance band due to swelling-induced changes in the distance between NP was evaluated.136,138,148 Localized surface plasmons are oscillations of charge densities in confined geometries such as NPs or nanoislands. A surface plasmon absorption band occurs at a selected wavelength in the case of resonance excitation. Intensity and position of the band are characteristic for material, size, shape, and distribution of the nanodomains. Changes in the environmental conditions highly influence the resonance conditions and lead to wavelength shifts in the SPR band as well as to changes in intensity. Silver NPs were formed in situ in pH-sensitive polycationic P2VP brushes by reduction of AgNO3 , where the Ag+ ions replace counterions along the P2VP chains in the brush before reduction. pH-sensitive changes in the absorbance spectrum were observed between the fully stretched state of P2VP at pH 2 (absorption maximum at 410 nm) and the fully collapsed state at pH 6 (430 nm).148 Gold NPs were immobilized at temperature-sensitive PNIPAAm brushes,136 at pH-sensitive P2VP brushes,138 and reduced in situ inside poly-METAC (2-(methacryloyloxy) ethyltrimethylammonium chloride) brushes.123 For the Au NP–PNIPAAm brush system, the shift in absorbance maximum of the Au NP with a diameter of 5–6 nm upon temperature-sensitive swelling of PNIPAAm below and above the LCST was 12 nm (Figure 10.7). Drying the system increased the absorbance wavelength further.136 Due to the collapse of PNIPAAm chains above the LCST, the nanocomposite layer is dehydrated and the interparticle distance reduced, changing the dielectric

Wavelength (nm)

Wavelength (nm) 480

540

600

559 nm

480

540

600

Figure . Absorbance spectrum at 23◦ C below the LCST of PNIPAAm (left) and 40◦ C above the LCST (right) of Au-NP–PNIPAAm brush nanocomposites in water. Source: Gupta et al. 2010.136 Reproduced with permission of American Chemical Society.



10 Functional Biointerfaces Tailored by “Grafting-To” Brushes (a)

(b) pH 5.0

Absorbance



0.35

pH 5.0

0.20

pH 2.0 pH 2.0

400

500

600

700

Wavelength, nm

500

600

700

800

Wavelength, nm

Figure . T-SPR spectra of Au nanoislands grafted with the PGMA polymer layer and P2VP brush at pH 2 and 5 (a), and with additional citrate-capped Au NP (d = 11.7 ± 1.9 nm) immobilized at the P2VP brush (b). Source: Tokareva et al. 2004.138 Reproduced with permission of American Chemical Society.

environment of the NP. These changes could be reproduced over several heating and cooling cycles with an error of ca. 2 nm in band position. Tokareva et al. generated pH-sensitive Au NP brush composites on glass substrates with gold nanoislands (thickness 4 nm) to utilize and enhance transmission surface plasmon resonance (T-SPR) spectroscopy.138 The sensitivity of changes in the absorbance of the gold nanoislands on the substrate could be greatly increased by immobilizing 12 nm large Au NP at the P2VP brushes. In Figure 10.8a, the pH-sensitive shift of the absorbance maximum between pH 2 and 5 without Au NP is 6 nm, whereas it is increased to 50 nm in the presence of immobilized Au NP at the P2VP brush (Figure 10.8b). Thus with this setup very strong pH sensitivity of the nanosensor could be demonstrated. The combination of localized surface plasmon resonance (LSPR), based on regular nanodomains, and the stimuli-responsivity of polymer brushes has potential in biosensing by combining changes in the stimuli-responsive swelling of the polymer layer and specific or unspecific adsorption of biomolecules to the brush or the nanoparticles.150,151 Next to surface-bound brushes, single nanoparticle LSPR is very promising since NP can move freely in solution, less sample volume is required as for the ensemble methods, and multiplexing by multiple NP with different wavelengths of the SPR band maximum is easy to achieve.150 ... Sculptured Thin Films Grafted with Polymer Brushes

Hybrid brush nanostructures based on sculptured thin films (STF) are an example of highly ordered, regular, inorganic, surface-bound, three-dimensional (3D) nanostructures modified with polymer brushes.31,128,152 STFs are

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10.2 Part I: Polymer Brush Architectures

prepared by glancing angle deposition with different architectures and from a variety of materials.153 3D nanostructures, for example, slanted or straight columns, helices, or chevrons, are generated by electron beam evaporation at an oblique angle, while the deposition is influenced by ballistic shadowing at the atomic scale and diffusion of atoms on the surface. PAA “pseudo” brushes and end-grafted PNIPAAm brushes were prepared by “grafting-to” on STF substrates with slanted silicon columns to generate pH- or temperature-sensitive 3D nanostructures.31,128 STF thickness is 80–100 nm before brush preparation with an angle of the slanted columns of 60–63◦ relative to the surface normal, a volume fraction of the STF of 17–20%, and an intercolumnar space of ca. 20 nm. These structural parameters of the STF were obtained from model fits to generalized ellipsometry data by using an anisotropic Bruggeman effective medium approach,128,154 and were confirmed by scanning electron microscopy (SEM). With both methods, the samples were monitored again after preparation of polymer brushes. For PAA “pseudo” brushes, the penetration of PAA into the intercolumnar space of slanted columnar films of silicon could be deduced from SEM (Figure 10.9). Before spin coating of the polymers void spaces are clearly visible (Figure 10.9a), while after spin coating (and grafting) distinguishing between

Figure . Images obtained by SEM of a slanted columnar film of silicon columns before brush preparation (a), after spin coating and grafting of PGMA as well as PAA (b) and after extraction of ungrafted excessive polymer (c). Source: Kasputis et al. 2013.128 Reproduced with permission of American Chemical Society.

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10 Functional Biointerfaces Tailored by “Grafting-To” Brushes

individual silicon columns is difficult (Figure 10.9b). Furthermore, STF thickness as well as the slanting angle of the columns was changed after spin coating and after the reaction of the PGMA anchoring layer with the PAA. For the given sample (STF parameters: t = 92 ± 2 nm, 𝜃 = 60.4 ± 1.6◦ ), the thickness t decreased to 66 ± 2 nm, and the slanting angle 𝜃 increased to 66.6 ± 1.8◦ , as obtained by SEM. After extraction of nongrafted excess polymer with ethanol, individual columns can be distinguished once more but with a rougher surface than before brush preparation. The PAA brush fraction inside the silicon columns was 17– 33% calculated from generalized ellipsometry data, and was dependent on the angle of the vapor flux in the STF deposition process. The polymer brush fraction decreased with higher deposition angle and thus higher slanting angle of the columns with respect to the surface normal. pH-sensitive swelling of these PAA “pseudo” brushes inside the STF columns was monitored simultaneously by generalized ellipsometry and quartz crystal microbalance with dissipation (QCM-D). With this coupled setup, changes of the polymer brush fraction inside the columns and of the layer thickness of a top layer (from the optical method: ellipsometry) can be coupled to changes in the surface-bound mass and viscosity of the system (from the mechanical method: QCM-D). Swelling was compared between pH 3.7, where the PAA brush is collapsed, and pH 7.3, where the PAA brush is swollen due to the dissociation of COOH groups to COO− and subsequent counterion localization as well as an increase of the osmotic pressure inside the brush. PAA brushes swell out of the slanted silicon columns, which could be deduced by an increase of the top layer thickness at pH 7.3 (Figure 10.10). Also inside the slanted columns, the PAA swells pH sensitive because the volume fraction of the polymer changes with pH. At pH 7.3, the volume fraction is smaller than at pH 3.7, indicating uptake of buffer in the intercolumnar space. Additionally, swelling of the PAA brush leads to an increase of associated mass and viscosity at pH 7.3 compared to pH 3.7, as obtained by modeling the QCM-D data (not shown). Simultaneous generalized ellipsometry and QCM-D measurements were again utilized to monitor temperature-sensitive swelling of PNIPAAm brushes on STF and protein adsorption at both brush systems at pH 5.31 While on PAA brushes, a considerable amount of protein can be adsorbed due to electrostatic attraction, the modification of STF with PNIPAAm led to nonfouling 3D nanostructures. Recently, bare titanium STF films were used as model biomaterial substrates to study the influence of the highly ordered, regular nanotopography on the adsorption of proteins and on cell adhesion.152 Surface area and ambient pore space of the STF affected protein adsorption considerably, and cell–material interaction can be enhanced depending on the structure of the STF.

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10.3 Part II: Actuating Biomolecule Interactions with Surfaces

Figure . Scheme of a slanted columnar thin film grafted with a swellable polymer brush (top), and pH-sensitive changes in top-layer thickness and brush volume fraction inside the columns (bottom). The brushes were swollen in 0.1 M acetate buffer solution with pH 3.7 (gray background) and pH 7.3 (white background). Source: Christau et al. 2014.127 Reproduced with permission of American Chemical Society.

. Part II: Actuating Biomolecule Interactions with Surfaces ..

Adsorption of Proteins to Polymer Brush Surfaces

The process of protein adsorption is characterized by individual steps including the transport to the surface during adsorption and from the surface during the desorption process, the deposition, and structural relaxation at the surface as well as the detachment process and a restructuring of the desorbed protein in solution.155 Driving forces of adsorption are dehydration of apolar surfaces, structural rearrangements of the protein, interaction between electrical double layers, and van der Waals dispersion forces.156 Adsorption of the protein molecules on brushes is classified as primary, secondary, or ternary adsorption

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10 Functional Biointerfaces Tailored by “Grafting-To” Brushes

Figure . Modes of protein adsorption on polymer brushes: Primary adsorption is due to interaction with the substrate, if the brush parameters allow for a diffusion of the proteins to the substrate. Secondary adsorption at the brush–solution interface occurs for very dense brushes and/or large protein molecules, and ternary adsorption is due to attractive interaction between functional groups along the polymer chains and on the protein molecules.

based on the prevailing interactions sites (Figure 10.11). These are the substrate, the brush–solution interface, or individual chemical groups on the polymer chains, respectively. The measurement of the adsorbed amount Γ for long adsorption times (t∞ ) and different supply rates (concentration of the protein in solution) leads to the adsorption isotherm Γ (ceq ).155 Since the protein molecules undergo relaxation at the sorbent–surface the plateau-adsorbed amount depends on the ratio of relaxation rate and supply rate. Thus the mode of supply (e.g., convection/concentration gradient) affects Γ (ceq ). If hysteresis between adsorption and desorption occurs, the adsorption process is not fully reversible. This implies that physical changes, like a structural rearrangement within the protein molecules, have occurred during the adsorption process that are not fully reversed after desorption. At a solid or hydrophobic surface, it was found that the plateau value of the adsorption isotherm is mostly lower or comparable to a monolayer of closely packed protein molecules with protein layer thicknesses in the range of the dimension of the molecules.155,157 The latter is taken as indication that only comparably small structural rearrangements in the protein molecule take place, and no expanded loop and tail structure is formed. Protein adsorption at highly swellable surfaces like hydrogels158–160 or polyelectrolyte brushes73,161,162 was investigated because of their large potential for applications in biological environments, especially in drug release systems. Novel effects of these flexible surfaces like adsorption enhancement due to large water accessible charged surfaces and entropic effects (polyelectrolytemediated adsorption)162 as well as protein resistance could be found.163,164

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10.3 Part II: Actuating Biomolecule Interactions with Surfaces

... Calculation of the Adsorbed Amount of Protein from Ellipsometric Experiments

Protein adsorption at soft polymer surfaces grafted to silicon substrates can be quantified, for example, by in situ ellipsometry,165 reflectometry,60 or radio assays of labeled proteins.27 In the ellipsometric experiment, the ellipsometric angles of a polymer brush in buffer or protein solution are recorded and modeled to a box layer with thickness d using a Cauchy dispersion for the transparent polymer or polymer– protein layer to model the in situ refractive index.165 For the calculation of the adsorbed amount of protein from ellipsometry, the swelling state of the polymer brush is important. If the brush is collapsed, that means refractive index and layer thickness are close to the values of the dry polymer brush, an individual protein layer on top of the dense brush layer can be modeled, and the original de Feijter equation can be used to calculate the adsorbed amount of protein.73,166 For the protein adsorption at swollen polymer brushes, modified models based on the de Feijter equation were proposed.27,165 For high as well as low adsorbed amounts of protein, it is difficult to model an individual protein layer on top of swollen polymer brushes. An interface contrast in the refractive index between brush and protein is either not present (penetration of protein into, e.g., polyelectrolyte brushes161 ), or it is too small, with respect to the inhomogeneity of a possible brush–protein interface, to be modeled. The refractive index of, for example, a swollen PNIPAAm brush below the LCST is n (633 nm) = 1.35 ± 0.01,38 whereas the refractive index for adsorbed protein in situ at a hydrophobized silica surface was found to be between 1.365 and 1.38 depending on the type of protein.157 Thus, a composite polymer–protein layer is modeled from the ellipsometric data.165 For this composite layer, the adsorbed amount of the protein is calculated from a modified de Feijter approach, according to nads − npol n − namb + (dads − dpol ) ads Γprot = dpol dn∕dc dn∕dc where dpol and npol are the thickness and refractive index of the swollen polymer brush before adsorption. dads and nads are modeled for the composite protein–polymer layer after adsorption. namb is the refractive index of the ambient and dn/dc the refractive index increment of the protein. The approach considers the increase of concentration Δc of adsorbed protein molecules in the composite layer and its effect on the layer refractive index n. It is based on the assumption that n depends linearly on Δc. It does not distinguish, whether protein is adsorbed inside the brush or at the solution interface, or if the brush conformation changes due to adsorption. For the limit of virtually no adsorption at protein-resistant polymer brush surfaces, Xue et al. suggested to calculate the adsorbed amount according to the formula above with dads = dpol .27

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10 Functional Biointerfaces Tailored by “Grafting-To” Brushes

For the adsorption of relatively small Arg-Gly-Asp tripeptides (RGD) (as compared to protein molecules), good agreement was found between amounts of RGD peptide that were calculated in this way and values obtained from reversephase HPLC analysis.167 Brushes functionalized with RGD peptides were subjected to acidic hydrolysis and extracted amino acids were chromatographically separated and detected. Quantification of amino acids was done using external standards. Since ellipsometry provides changes in the ellipsometric angles Δ and Ψ due to adsorption processes at the brush surface, and the protein amount is calculated from parameters of a layer modeling, significant changes in the ellipsometric angles are necessary to model changing layer parameters and define the detection limit for protein adsorption. Xue et al. simulated changes in Δ and Ψ for lysozyme adsorption at PNIPAAm brushes and 2[methoxy(polyethyleneoxy)propyl]-trimethoxysilane self-assembled monolayers (SAM-OEG).27 Depending on the system, they found a detection limit of the adsorbed amount of protein of 0.1 mg/m2 for the 1.8-nm thick SAM layers, while for the PNIPAAm brushes (wet thickness > 49 nm) theoretical shifts in Δ and Ψ for 0.5 mg/m2 protein amount were barely detectable and ellipsometric angles were particularly sensitive to adsorbed amounts of protein higher than 1 mg/m2 . ... Preventing Protein Adsorption

Interfaces of great interest for biorelated applications are protein-resistant brushes which prevent nonspecific protein adsorption as well as cell/bacteria adhesion, representing novel nonfouling ( cc , the counterion osmotic pressure no longer suffices to maintain the chains fully extended and thus the brush chains can contract. Quasi-neutral behavior has been confirmed experimentally for dsDNA

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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications

brushes.67 Further increases in A lead to sparse coverages for which interactions between brush chains disappear, and the chains exist as individual entities whose configuration depends on their intrinsic stiffness and extent of charging.66 Under typical conditions, the osmotic and quasi-neutral brush regimes are those most applicable to experimental studies of DNA brushes. For ssDNA chains, lp0 ≈ lB because of greater backbone flexibility. While osmotic and quasi-neutral brush phases are still predicted for ssDNA chains, the degree of chain extension in the osmotic regime is expected to be less than the maximal value H = L due to the higher flexibility, and thus tendency to contract, of ssDNA compared to dsDNA. Experimental verification of these expectations for ssDNA brushes remains incomplete. By monitoring the intrabrush salt concentration with electrochemical capacitance measurements, Shen et al. deduced that H depended on cs with an exponent of −0.28,68 close to −1/3 predicted for quasi-neutral brushes.66 By using atomic force microscopy to measure brush heights under different salt concentrations, Ngavouka et al. found a weaker dependence of H ∼ cs −0.17 .69 The two studies mentioned above used different sequences and, therefore, effectively different polymers: oligodeoxythymine was used in Ref. 68 whereas a mixed sequence containing all four bases was used in Ref. 69. These different results could reflect sequencedependent effects such as base pairing between ssDNA chains whose variation with salt and chain coverage is not accounted for by the scaling laws developed for simpler polyelectrolyte brushes. The above results, summarized for the case of monovalent counterions, can change significantly if multivalent counterions are present in the solution in which the brush is bathed. Multivalent counterions can act as electrostatic cross-links by interacting with more than one DNA backbone charge. In solution, such associations can trigger DNA condensation.70 Exposure of ssDNA brushes to Mg2+ counterions created a brush state that was unresponsive to salt concentration, a finding that was interpreted to indicate that the brush was collapsed, or precipitated, by Mg2+ .71 In the case of dsDNA brushes, exposure to spermidine3+ , an organic counterion, condensed brush chains into fibers that formed dendritic-like patterns when imaged on the solid support.72 The charged nature of DNA enables these molecules to respond to surface electric fields. This susceptibility has been exploited by various investigators to trigger structural modulation of DNA brushes (Figure 21.5).73–75 Both dsDNA and ssDNA chains were readily attracted to or repelled from conductive supports, such as a gold electrode, by application of an attractive or repulsive surface potential. Realization of efficient, large amplitude modulation requires that the brush be not too dense, that the ionic strength be sufficiently low (below about 0.1 M) so the fields are sufficiently long ranged to interact with the brush chains, and that applied fields are not switched too rapidly to allow adequate time for the chains to respond.74 Application of attractive (positive) biases causes the DNA chains to adsorb to or reorient toward the surface, with

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Fluorescence Electrode bias

21.5 Hybridization in DNA Brushes

Negatively charged surface Passivation layer

Positively charged surface

Surface-DNA linker

Fluorescent tag

Time

Figure . Electric field switching of brush chains between adsorbed (under positive bias) and repelled (under negative bias) orientations, illustrated for dsDNA brushes. Fluorescence is quenched in the adsorbed orientation. Source: Rant et al. 2004.74 Reproduced with permission of American Chemical Society.

the chains effectively providing countercharge to the positive surface charges. Repulsive (negative) biases cause the chains to orient away from the surface. DNA brushes could be switched a million times between such states using modest biases (∼ 0.2 V).74 Significantly, the presence of defects on the solid support affects the local field distributions which, in turn, can alter conformations of immobilized DNA chains.76

. Hybridization in DNA Brushes The primary application of DNA brushes has been analysis of nucleic acid samples with microarray technologies. In these experiments, nucleic acid chains (“targets”) in a solution sample diffuse to and hybridize with ssDNA chains (“probes”) of the brush. It is common practice to first denature the targets to increase fraction of single-stranded regions available for hybridization, as well as to fragment them into smaller sizes that can more easily approach and hybridize to the probes. The widespread use of DNA microarrays has mobilized extensive interest in basic understanding of the underlying surface hybridization reaction.77–79 Hybridization at a solid/liquid interface differs significantly from the solution process because of interactions unique to the interfacial environment. The thermodynamic state of an immobilized ssDNA probe includes interactions with surrounding chains in the brush and possibly with the solid support. Similarly, for a duplex, consisting of a hybridized probe/target pair, there are interactions with surrounding duplexes, unhybridized probes, and between unhybridized target tails (Figure 21.6). Consideration of equivalent path diagrams80 indicates that surface and solution hybridization free energies differ because the interaction of an unhybridized probe with the interfacial environment is not



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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications Target–target

Probe–duplex Probe–probe Duplex–duplex

Figure . Illustration of intermolecular interactions arising in a partly hybridized DNA brush.

equivalent to that of the duplex; therefore, the conversion of an unhybridized probe to a duplex within a DNA brush introduces free energy offsets that do not arise for solution hybridization. These offsets can be determined by comparing free energies at the surface, ΔG◦ sur , with their solution counterparts, ΔG◦ sol . One way to estimate these free energies is to extract them from melting curves for solution and surface hybridization, which provide the enthalpy ΔH◦ , entropy ΔS◦ , and ΔG◦ = ΔH◦ – TΔS◦ of hybridization.70 Figure 21.7 compares the ratio of surface-to-solution hybridization enthalpy, ΔH◦ sur /ΔH◦ sol , and hybridization entropy ΔS◦ sur /ΔS◦ sol , as determined from melting curve analysis for brushes of 25mer DNA probes hybridizing to fully Figure . Ratio of surface to solution thermodynamic functions at two different ionic strengths. Error bars represent variation observed over different 25mer DNA sequences, as reported in Ref. 80. Source: Qiao et al. 2015.80

ΔH°sur /ΔH°sol 0.037 M ΔS°sur /ΔS°sol Ionic strength ΔH°sur /ΔH°sol 0.11 M ΔS°sur /ΔS°sol 0.0

0.2

0.4 0.6 Ratio

0.8

1.0

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21.5 Hybridization in DNA Brushes

complementary targets. The surface values are lower than those in solution. The lower values may indicate existence of competing processes, such as base pairing between probes in a brush, that on average reduce the enthalpic and entropic changes associated with binding to target strands, as well as barriers arising from the elevated charge density in a probe layer.80 Such barriers are expected to vary with the extent of hybridization of the probe layer; for example, later arriving targets may experience decreased affinity due to additional repulsive interactions with already bound targets. For the case of electrostatic corrections, this dependence has been incorporated into the classical Langmuir isotherm81 in the theoretical models by Vainrub and Pettitt82 and by Halperin and co-workers.83 For conditions typical of hybridization experiments (akin to a quasi-neutral brush), these theories lead to isotherms of the form b

o o = ΔGNE + RTb1 rD2 𝜎 ΔGsur

where ΔG◦ NE includes all deviations from ΔG◦ sol other than deviations from electrostatics of target partitioning into the probe layer, rD is the Debye screening length, and 𝜎the immobilized charge per area that includes contributions from probes as well as hybridized targets. The exponent b2 = 1 if the charge of the probe layer is viewed as confined to a two-dimensional plane82 or b2 = 2 if it is viewed as a three-dimensional layer,83 whereas b1 is a given combination of parameters.83 The last term on the right accounts for electrostatic corrections to hybridization stemming from surface partitioning of target molecules. Experimental data on surface hybridization as a function of the salt concentration have lent support to b2 = 2,84 consistent with the brush charge being distributed within a three-dimensional space. The theoretical considerations summarized above predict that sensitivity of hybridization to salt concentration increases at surfaces. Experiments have reported both stronger80 and comparable or weaker84–87 dependencies than in solution. A weaker dependence could reflect the presence of doublestranded associations in the ssDNA brush facilitated by partial complementarity between probes and the high nucleotide concentration. Such associations between probes would decrease salt dependence of hybridization because of a lower net formation of new base pairs from target binding. At low salt concentrations, approaching osmotic conditions cs < cc , hybridization is effectively halted (Figure 21.8).88 By varying these two parameters, where the counterion concentration in the brush cc is controlled by the surface coverage 1/A of brush chains, such nonhybridizing (NH regime) conditions were mapped out experimentally in cs -A space (Figure 21.9). Other regimes were also identified, including conditions in which hybridization occurred but was limited by predominantly electrostatic (SH-E regime) or packing (SH-P regime) penalties.84 In the pseudo-Langmuir (PL) regime, typical of hybridization on microarrays, neighboring brush chains were close

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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications

1.0 Fraction of hybridized probes

Na+ counterion K+ counterion

0.8 0.6 0.4 0.2 0.0 0.01

0.1

1

10

100

Cc /Cs

Figure . Observed fraction of hybridized probes as a function of the ratio of counterion concentration cc , needed to balance the charge density of the unhybridized DNA brush, to the solution concentration of cations cs . When cs is too low, hybridization is below detection (right of the dashed line). Data are shown for experiments involving two types of monovalent counterions, Na+ and K+ . Source: Adapted from Gong and Levicky 2008.88 Reproduced with permission of National Academy of Sciences.

Investigated regimes

1 PL Cs (M)



0.1

SH-P

SH-E

L

NH 0.01

1

10

Figure . An experimentally derived diagram illustrating different types of hybridization behavior as a function of salt concentration cs and surface density 1/A of 20mer brush chains. The shaded regions of PL, SH-P, SH-E, and NH regimes represent conditions studied experimentally; the Langmuir (L) regime is expected in the limit of no interactions between probes but was not investigated (see the text for discussion). Source: Irving et al. 2010.84 Reproduced with permission of American Chemical Society.

1/A (1012 cm–2)

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21.5 Hybridization in DNA Brushes

enough to interact but, counterintuitively, the interaction did not depend strongly on the separation between chains and as such did not affect target binding.88 This unique behavior may reflect tendency of the unhybridized probe chains to associate,84,89,90 with PL regime spanning separations for which flexibility of the ssDNA probes makes these associations tolerant of changes in interchain distance. Temperature also affects the difference between surface and solution hybridization. For the conventional approximation that hybridization does not cause significant heat capacity changes, the rate of change of ΔG◦ of hybridization with temperature is equal to −ΔS◦ . As noted in Figure 21.7, surface hybridization tends to exhibit more modest entropic changes; therefore, a lowering of temperature is less stabilizing at the surface than in solution.80 For this reason, at lower temperatures the difference ΔΔG◦ = ΔG◦ sur - ΔG◦ sol increases in favor of solution hybridization, consistent with the observation made a decade ago that surface hybridization tends to be less favorable.79 Conversely, one benefit of the elevated temperatures common to commercial assays is that surface hybridization can then more effectively compete with base pairing associations present in the solution sample. Surface hybridization is also subject to factors unique to the local environment. One such factor is chemistry of the solid support. For example, on gold-coated supports the propensity of DNA bases to adsorb to the support leads to destabilization of double-stranded regions, as evident in lowering of the melting point.91 Lack of adequate chemical passivation (e.g., by backfilling with alkanethiols37,41,92 ) to block interactions between ssDNA brush chains and the support can thus suppress hybridization,41,91 an effect also observed on gold nanoparticles.93 On the other hand, comparison of hybridization thermodynamics to otherwise similar DNA brushes prepared on aldehydeand isothiocyanate-modified (Figure 21.3) glass slides showed only modest differences.80 Simulations of unhybridized oligodeoxycytosine ssDNA brushes on hydrophilic and hydrophobic supports found only minor differences in organization of the chains.94 Taken together, these reports highlight that sequence- as well as surface-chemical factors determine how a particular support affects organization and function of DNA brushes. Another surface-specific effect arises from asymmetry introduced by tethering of the brush chains. Gradients in composition, electrostatic potential, and other properties are expected across an ssDNA brush. These variations should, in principle, cause hybridization to similarly acquire a dependence on position within the brush. In support of this expectation, some studies found that the penalty due to a single base mismatch was asymmetric about the middle of a DNA brush chain59,95 ; other studies reported more symmetric distributions.96,97 As demonstrated through Monte Carlo simulations, asymmetry can also arise from differences in interactions of the solid support with base pairs depending on their position98 ; thus surface collisions stabilized base pairs at the free end of a duplex by suppressing end-fraying. Others showed that kinetics of

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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications

hybridization were markedly slowed when a short, incoming target had to penetrate deeper into a brush in order to bind than when its binding site was at the free ends of brush chains.99,100 These results indicate that an activation barrier is associated with target entry and suggest that the most accessible initiation site for hybridization is at the free end of a brush chain.

. Other Bioprocesses in DNA Brushes While hybridization is the most studied bioprocess in DNA brushes, DNA brushes are much more versatile in their functionality and can also interact, for example, with various DNA-processing enzymes and small molecules. A number of studies have investigated such processes, as illustrated by the following examples. Transcription from brushes of dsDNA genes revealed that the brush tended to exclude RNA polymerase enzyme, but that successful synthesis of messenger RNA (mRNA) was still achieved.101 Transcription was facilitated by placing the promoter near the solution boundary of the brush and orienting it toward the solid support, with an approximately twofold greater rate of transcription in this orientation than from the tethered toward the free end (Figure 21.10). Transcription was also demonstrated from brushes on beads102 as well as used in a diagnostic setting as a mechanism of signal amplification.103 In this strategy, hybridization to a ssDNA brush was followed by transcription of the hybridized brush and detection of the generated transcripts by a second DNA brush. Transcription from DNA brushes has also led to production of (a) Rate [RNA gene–1 min–1]



6

(b) IN OUT

5

OUT

IN

OUT

IN

OUT

IN

4 3 2 1 0 Bottom

Middle

Top

Bottom

Middle

Top

Figure . Transcription from dsDNA brushes. (a) Experimentally determined rates and (b) schematic illustration. Arrows in (b) indicate start and direction of transcription. Source: Daube et al. 2010.101 Reproduced with permission of National Academy of Sciences of the USA.

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21.7 Perspective

RNA aptamers, which could then diffuse and attach to neighboring brushes to bestow capability for detection of protein biomarkers.104 Interactions of various DNA-binding proteins with DNA brushes has been explored by several groups.105–114 In addition to transcription and previously mentioned use of ligases and polymerases for fabrication, DNA brushes can be enzymatically trimmed or degraded. Restriction enzymes can be used to trim dsDNA brushes at restriction sites that consist of specific, enzyme-recognized short sequences that can be incorporated at a desired location along a brush chain.115 Nucleases can be used to degrade DNA brushes in various ways; for example, Exonuclease I was used to digest ssDNA brushes from the 3′ to the 5′ end,8 DNase I digests ssDNA and dsDNA at internal bonds, whereas Exonuclease III can degrade just those strands in a dsDNA brush with a blunt ended or recessed 3′ terminus.116

. Perspective The basic understanding of DNA brushes has greatly progressed over the past two decades, driven in part by desire to better understand commercial technologies such as DNA microarrays. Nevertheless, significant gaps in this understanding exist, especially with regard to how organization of DNA brushes impacts their hybridization behavior. Probe–probe associations have been implicated to alter the functionality of ssDNA brushes in commercial diagnostic applications.117 Therefore, an important area for future inquiry is to clarify how base-directed interactions in single-stranded brushes alter the organization and/or function of the brush. For example, neighboring chains in an ssDNA brush can forge associations due to partial sequence complementarity. How are such physical interchain “crosslinks” affected by constraints imposed by tethering of the chains and the location and strength of associating regions along the backbone? Can their thermodynamic and kinetic impacts on hybridization be predicted? Although interactions of DNA with solid supports have been examined in detail for some materials such as gold, their significance for most supports of commercial importance is not established. Interactions that lead to adsorption of DNA chains91,118–120 can strongly affect hybridization, and it would therefore be useful to rank surface chemistries in their affinity toward ssDNA chains of a specific sequence. Moreover, surface interactions need not be just passive adsorption; for example, application of repulsive electric potentials is known to accelerate dehybridization of dsDNA brushes.121,122 Studies of how passive and actively modulated brush/surface interactions influence surface hybridization, or other brush functions, would help clarify the role of the solid support in biotechnologies and materials chemistry applications incorporating DNA brushes. Here, single-molecule imaging methods can provide detailed

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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications

insights into processes such as adsorption and surface diffusion that can affect hybridization between solution strands and end-immobilized ssDNA molecules.123 In the case of surface hybridization, it is important to note that molecular characteristics of the solution partner, including its length and secondary structure, are as important as those of the brush. While studies have demonstrated that secondary structure124,125 and length126,127 of the analyte species can exert strong influence on the outcome of a hybridization experiment, the existing understanding is not sufficient to determine how to optimally process samples for analysis. How will DNA brushes continue to be used in the future? The unmatched programmability of DNA hybridization is expected to continue to facilitate academic discoveries in material chemistry, as well as underpin new biosensor designs and concepts. At the same time, the commercial applications of DNA brushes are evolving rapidly. Dramatic reductions in costs of sequencing over the past decade128 made sequencing approaches a viable alternative to sequence identification by microarrays, reducing their research use. Concurrently, microarrays are experiencing growth in clinical diagnostics, though again in competition with sequencing approaches. Both sequencing and microarrays have unique advantages. Sequencing provides direct determination of sequence, and, if costs are acceptable, can identify concentrations of sequences without a hard limit on dynamic range on the low or high end. In contrast, microarrays identify sequences through hybridization and, as such, can detect only those sequences included on the array. This becomes a limitation if the goal is to discover unknown sequences. Also, once a DNA brush is fully hybridized an upper limit on detectable material is reached, whereas a lower limit is defined by extent of cross-hybridization and background against which the fully complementary signal must be resolved. Typically, microarray dynamic range is limited to three to five decades in analyte concentration. Advantages of microarrays include their more straightforward operation. They do not rely as extensively on enzymatic reactions as most sequencing approaches do, and the hardware tends to be simpler. This makes microarrays more convenient for small laboratories or for in-the-field applications. Also, restricting measurements to just the sequences on the array make for more efficient data management and analysis. Future applications are expected to see both sequencing and microarray technologies adapted to their best, usespecific advantages.

Acknowledgments This work was supported by the National Science Foundation of the United States under award numbers DMR 12-06754 and CBET 16-00584.

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References

References  M¨uller, H. J.; R¨oder, T. Microarrays; Elsevier Academic Press: Burlington, MA, 2006.  Mardis, E. R. Trends Genetics 2008, 24, 133–141.  Macfarlane, R. J.; Lee, B.; Jones, M. R.; Harris, N.; Schatz, G. C.; Mirkin, C. A. Science 2011, 334, 204–208.  Tan, S. J.; Campolongo, M. J.; Luo, D.; Cheng, W. Nat. Nanotechnol. 2011, 6, 268–276.  Seeman, N. C. Ann. Rev. Biochem. 2010, 79, 65–87.  Sassolas, A.; Leca-Bouvier, B. D.; Blum, L. J. Chem. Rev. 2008, 108, 109–139.  Weng, X.; Jiang, H.; Li, D. Microfluid. Nanofluid. 2011, 11, 367–383.  Liu, Q. H.; Wang, L. M.; Frutos, A. G.; Condon, A. E.; Corn, R. M.; Smith, L. M. Nature 2000, 403, 175–179.  Rao, A. N.; Grainger, D. W. Biomater. Sci. 2014, 2, 436–471.  Ravan, H.; Kashanian, S.; Sanadgol, N.; Badoei-Dalfard, A.; Karami, Z. Anal. Biochem. 2014, 444, 41–46.  Li, N. K.; Kim, H. S.; Nash, J. A.; Lim, M.; Yingling, Y. G. Molec. Simul. 2014, 40, 777–783.  Pirrung, M. C.; Southern, E. M. Biochem. Mol. Biol. Edu. 2014, 42, 106–113.  Bulyk, M. L. Methods Enzymol. 2006, 410, 279–299.  Hauschild, K. E.; Stover, J. S.; Boger, D. L.; Ansari, A. Z. Bioorg. Med. Chem. Lett. 2009, 19, 3779–3782.  Cagnin, S.; Caraballo, M.; Guiducci, C.; Martini, P.; Ross, M.; SantaAna, M.; Danley, D.; West, T.; Lanfranchi, G. Sensors 2009, 9, 3122–3148.  Gooding, J. J. Electroanalysis 2002, 14, 1149–1156.  Katz, E.; Willner, I. Electroanalysis 2003, 15, 913–947.  Lucarelli, F.; Tombelli, S.; Minunni, M.; Marrazza, G.; Mascini, M. Anal. Chim. Acta 2008, 609, 139–159.  Palecek, E.; Fojta, M.; Tomschik, M.; Wang, J. Biosensors Bioelectron. 1998, 13, 621–628.  Vercoutere, W.; Akeson, M. Curr. Opin. Chem. Biol. 2002, 6, 816–822.  Lu, F.; Yager, K. G.; Zhang, Y.; Xin, H.; Gang, O. Nat. Commun. 2015, 6, 6912.  Srinivasan, B.; Vo, T.; Zhang, Y.; Gang, O.; Kumar, S.; Venkatasubramanian, V. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 18431–18435.  Beaucage, S. L.; Iyer, R. P. Tetrahedron 1992, 48, 2223–2311.  Beier, M.; Hoheisel, J. D. Nucleic Acids Res. 1999, 27, 1970–1977.  Lee, P. H.; Sawan, S. P.; Modrusan, Z.; Arnold, L. J.; Reynolds, M. A. Bioconjugate Chem. 2002, 13, 97–103.  Timofeev, E. N.; Kochetkova, S. V.; Mirzabekov, A. D.; Florentiev, V. L. Nucleic Acids Res. 1996, 24, 3142–3148.  Yang, M. S.; Kong, R. Y. C.; Kazmi, N.; Leung, A. K. C. Chem. Lett. 1998, 257–258.

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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications

 Zammatteo, N.; Jeanmart, L.; Hamels, S.; Courtois, S.; Louette, P.; Hevesi, L.; Remacle, J. Anal. Biochem. 2000, 280, 143–150.  Beattie, W. G.; Meng, L.; Turner, S. L.; Varma, R. S.; Dao, D. D.; Beattie, K. L. Mol. Biotechnol. 1995, 4, 213–225.  Guo, Z.; Guilfoyle, R. A.; Thiel, A. J.; Wang, R.; Smith, L. M. Nucleic Acids Res. 1994, 22, 5456–5465.  Walsh, M. K.; Wang, X.; Weimer, B. C. J. Biochem. Biophys. Methods 2001, 47, 221–231.  Buxboim, A.; Bar-Dagan, M.; Frydman, V.; Zbaida, D.; Morpurgo, M.; Bar-Ziv, R. Small 2007, 3, 500–510.  Rogers, Y.-H.; Jiang-Baucom, P.; Huang, Z.-J.; Bogdanov, V.; Anderson, S.; Boyce-Jacino, M. T. Anal. Biochem. 1999, 266, 23–30.  Chrisey, L. A.; Lee, G. U.; O‘Ferrall, C. E. Nucleic Acids Res. 1996, 24, 3031–3039.  O’Donnell, M. J.; Tang, K.; Koster, H.; Smith, C. L.; Cantor, C. R. Anal. Chem. 1997, 69, 2438–2443.  Uszczynska, B.; Ratajczak, T.; Frydrych, E.; Maciejewski, H.; Figlerowicz, M.; Markiewicz, W. T.; Chmielewski, M. K. Lab Chip 2012, 12, 1151–1156.  Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 8916–8920.  Ulman, A. Chem. Rev. 1996, 96, 1533–1554.  Aqua, T.; Naaman, R.; Daube, S. S. Langmuir 2003, 19, 10573–10580.  Satjapipat, M.; Sanedrin, R.; Zhou, F. M., Langmuir 2001, 17, 7637–7644.  Levicky, R.; Herne, T. M.; Tarlov, M. J.; Satija, S. K. J. Am. Chem. Soc. 1998, 120, 9787–9792.  Kimura-Suda, H.; Petrovykh, D. Y.; Tarlov, M. J.; Whitman, L. J. J. Am. Chem. Soc. 2003, 125, 9014–9015.  Wolf, L. K.; Gao, Y.; Georgiadis, R. M. Langmuir 2004, 20, 3357–3361.  Opdahl, A.; Petrovykh, D. Y.; Kimura-Suda, H.; Tarlov, M. J.; Whitman, L. J. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 9–14.  Bodrossy, L.; Sessitsch, A. Curr. Opin. Microbiol. 2004, 7, 245–254.  Forman, J. E.; Walton, I. D.; Stern, D.; Rava, R. P.; Trulson, M. O. ACS Symp. Ser. 1998, 682, 206–228.  Fiche, J. B.; Buhot, A.; Calemczuk, R.; Livache, T. Biophys. J. 2007, 92, 935–946.  Ge, D.; Wang, X.; Williams, K.; Levicky, R. Langmuir 2012, 28, 8446–8455.  Phares, N.; White, R. J.; Plaxco, K. W. Anal. Chem. 2009, 81, 1095–1100.  Livache, T.; Roget, A.; Dejean; Earthet, C.; Bidan, G.; Teoule, R. Nucleic Acids Res. 1994, 22, 2915–2921.  Johnson, P. A.; Levicky, R. Langmuir 2003, 19, 10288–10294.  Lee, H. J.; Wark, A. W.; Li, Y.; Corn, R. M. Anal. Chem. 2005, 77, 7832–7837.  Fang, S. P.; Lee, H. J.; Wark, A. W.; Corn, R. M. J. Am. Chem. Soc. 2006, 128, 14044–14046.  Bamdad, C. Biophys. J. 1998, 75, 1997–2003.

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References

 Southern, E. M.; Case-Green, S. C.; Elder, J. K.; Johnson, M.; Mir, K. U.; Wang, L.; Williams, J. C. Nucleic Acids Res. 1994, 22, 1368–1373.  Southern, E. M.; Maskos, U.; Elder, J. K. Genomics 1992, 13, 1008–1017.  Pease, A. C.; Solas, D.; Sullivan, E. J.; Cronin, M. T.; Holmes, C. P.; Fodor, S. P. A. Proc. Natl. Acad. Sci. U. S. A. 1994, 91, 5022–5026.  Blanchard, A. P.; Kaiser, R. J.; Hood, L. E. Biosensors Bioelectron. 1996, 11, 687–690.  Hughes, T. R.; Mao, M.; Jones, A. R.; Burchard, J.; Marton, M. J.; Shannon, K. W.; Lefkowitz, S. M.; Ziman, M.; Schelter, J. M.; Meyer, M. R.; Kobayashi, S.; Davis, C.; Dai, H.; He, Y. D.; Stephaniants, S. B.; Cavet, G.; Walker, W. L.; West, A.; Coffey, E.; Shoemaker, D. D.; Stoughton, R.; Blanchard, A. P.; Friend, S. H.; Linsley, P. S. Nat. Biotechnol. 2001, 19, 342–347.  Schreiner, S. M.; Hatch, A. L.; Shudy, D. F.; Howard, D. R.; Howell, C.; Zhao, J.; Koelsch, P.; Zharnikov, M.; Petrovykh, D. Y.; Opdahl, A. Anal. Chem. 2011, 83, 4288–4295.  Pincus, P. Macromolecules 1991, 24, 2912–2919.  Borisov, O. V.; Zhulina, E. B.; Birshtein, T. M. Macromolecules 1994, 27, 4795–4803.  Manning, G. S. Quart. Rev. Biophys. 1978, 11, 179–246.  Hagerman, P. J. Annu. Rev. Biophys. Biophys. Chem. 1988, 17, 265–268.  Bosco, A.; Camunas-Soler, J.; Ritort, F. Nucl. Acid. Res. 2014, 42, 2064–2074.  O’Shaughnessy, B.; Yang, Q. Europhys. Lett. 2006, 75, 427–433.  Bracha, D.; Karzbrun, E.; Shemer, G.; Pincus, P. A.; Bar-Ziv, R. H. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 4534–4538.  Shen, G.; Tercero, N.; Gaspar, M. A.; Varughese, B.; Shepard, K.; Levicky, R. J. Am. Chem. Soc. 2006, 128, 8427–8433.  Ngavouka, M. D. N.; Bosco, A.; Casalis, L.; Parrise, P. Macromolecules 2014, 47, 8748–8753.  Bloomfield, V. A. Biopolymers 1997, 44, 269–282.  Wang, K.; Zangmeister, R. A.; Levicky, R. J. Am. Chem. Soc. 2009, 131, 318–326.  Bracha, D.; Bar-Ziv, R. H. J. Am. Chem. Soc. 2014, 136, 4945–4953.  Kelley, S. O.; Barton, J. K.; Jackson, N. M.; McPherson, L. D.; Potter, A. B.; Spain, E. M.; Allen, M. J.; Hill, M. G. Langmuir 1998, 14, 6781–6784.  Rant, U.; Arinaga, K.; Fujita, S.; Yokoyama, N.; Abstreiter, G.; Tornow, M. Nano Lett. 2004, 4, 2441–2445.  Josephs, E. A.; Ye, T. J. Am. Chem. Soc. 2012, 134, 10021–10030.  Josephs, E. A.; Ye, T. Nano Lett. 2012, 12, 5255–5261.  Graves, D. J. Trends Biotechnol. 1999, 17, 127–134.  Harrison, A.; Binder, H.; Buhot, A.; Burden, C. J.; Carlon, E.; Gibas, C.; Gamble, L. J.; Halperin, A.; Hooyberghs, J.; Kreil, D. P.; Levicky, R.; Noble, P. A.; Ott, A.; Pettitt, B. M.; Tautz, D.; Pozhitkov, A. E. Nucleic Acids Res. 2013, 41, 2779–2796.

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21 DNA Brushes: Self-Assembly, Physicochemical Properties, and Applications

 Levicky, R.; Horgan, A. Trends Biotechnol. 2005, 23, 143–149.  Qiao, W.; Chiang, H.-C.; Xie, H.; Levicky, R. Chem. Commun. 2015, 51, 17245–17248.  Langmuir, I. J. Am. Chem. Soc. 1918, 40, 1361–1403.  Vainrub, A.; Pettitt, B. M. Phys. Rev. E 2002, 66, 041905.  Halperin, A.; Buhot, A.; Zhulina, E. B. Biophys. J. 2004, 86, 718–730.  Irving, D.; Gong, P.; Levicky, R. J. Phys. Chem. B 2010, 114, 7631–7640.  Peterlinz, K. A.; Georgiadis, R. M.; Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 3401–3402.  Watterson, J. H.; Piunno, P. A. E.; Wust, C. C.; Krull, U. J. Langmuir 2000, 16, 4984–4992.  Azam, M. S.; Gibbs-Davis, J. M. Anal. Chem. 2013, 85, 8031–8038.  Gong, P.; Levicky, R. Proc. Natl. Acad. Sci. U. S. A. 2008, 105, 5301–5306.  Lee, O. S.; Schatz, G. C. J. Phys. Chem. C 2009, 113, 15941–15947.  Welling, R. C.; Knotts, T. A. J. Chem. Phys. 2015, 142, 015102.  Petty, T. J.; Wagner, C. E.; Opdahl, A. Langmuir 2014, 30, 15277–15284.  Lee, C. Y.; Gong, P.; Harbers, G. M.; Grainger, D. W.; Castner, D. G.; Gamble, L. J. Anal. Chem. 2006, 78, 3316–3325.  Brown, K. A.; Park, S.; Hamad-Schifferli, K. J. Phys. Chem. C 2008, 112, 7517–7521.  Elder, R. M.; Jayaraman, A. J. Chem. Phys. 2014, 140, 155103.  Wick, L. M.; Rouillard, J. M.; Whittam, T. S.; Gulari, E.; Tiedje, J. M.; Hashsham, S. A. Nucleic Acids Res. 2006, 34, e26.  Naiser, T.; Ehler, O.; Kayser, J.; Mai, T.; Michel, W.; Ott, A. BMC Biotechnol. 2008, 8, 48.  Pozhitkov, A.; Noble, P. A.; Domazet-Loso, T.; Nolte, A. W.; Sonnenberg, R.; Staehler, P.; Beier, M.; Tautz, D. Nucleic Acids Res. 2006, 34, e66.  Allen, J. H.; Schoch, E. T.; Stubbs, J. M. J. Phys. Chem. B 2011, 115, 1720–1726.  Peterson, A. W.; Wolf, L. K.; Georgiadis, R. M. J. Am. Chem. Soc. 2002, 124, 14601–14607.  Hagan, M. F.; Chakraborty, A. K. J. Chem. Phys. 2004, 120, 4958–4968.  Daube, S. S.; Bracha, D.; Buxboim, A.; Bar-Ziv, R. H. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 2836–2841.  Andreadis, J. D.; Chrisey, L. A. Nucleic Acids Res. 2000, 28, e5.  Sendroiu, I. E.; Gifford, L. K.; Luptak, A.; Corn, R. M. J. Am. Chem. Soc. 2011, 133, 4271–4273.  Chen, Y.; Nakamoto, K.; Niwa, O.; Corn, R. M. Langmuir 2012, 28, 8281–8285.  Bonham, A. J.; Hsieh, K.; Ferguson, B. S.; Vallee-Belisle, A.; Ricci, F.; Soh, H. T.; Plaxco, K. W. J. Am. Chem. Soc. 2012, 134, 3346–3348.  Bonham, A. J.; Neumann, T.; Tirrell, M.; Reich, N. O. Nucl. Acids. Res. 2009, 37, e94.

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References

 Neumann, T.; Bonham, A. J.; Dame, G.; Berchtold, B.; Brandstetter, T.; Ruhe, J. Anal. Chem. 2010, 82, 6124–6131.  Ricci, F.; Bonham, A. J.; Mason, A. C.; Reich, N. O.; Plaxco, K. W. Anal. Chem. 2009, 81, 1608–1614.  Bulyk, M. L. Adv. Biochem. Eng. Biotechnol. 2007, 104, 65–85.  Boal, A. K.; Yavin, E.; Lukianova, O. A.; O’Shea, V. L.; David, S. S.; Barton, J. K. Biochemistry 2005, 44, 8397–8407.  Gorodetsky, A. A.; Ebrahim, A.; Barton, J. K. J. Am. Chem. Soc. 2008, 130, 2924–2925.  Shumaker-Parry, J. S.; Aebersold, R.; Campbell, C. T. Anal. Chem. 2004, 76, 2071–2082.  Smith, E. A.; Erickson, M. G.; Ulijasz, A. T.; Weisblum, B.; Corn, R. M. Langmuir 2003, 19, 1486–1492.  Williams, K.; Kim, C. S.; Kim, J. R.; Levicky, R. Analyst 2014, 139, 1463–1471.  Tison, C. K.; Milam, V. T. Biomacromolecules 2008, 9, 2468–2476.  Lee, H. J.; Li, Y.; Wark, A. W.; Corn, R. M. Anal. Chem. 2005, 77, 5096–5100.  Langdon, W. B.; Upton, G. J. G.; Harrison, A. P. Brief. Bioinform. 2009, 10, 259–277.  Kastantin, M.; Schwartz, D. K. Small 2013, 9, 933–941.  Chan, V.; Graves, D. J.; McKenzie, S. E. Biophys. J. 1995, 69, 2243–2255.  Erickson, D.; Dongqing, L.; Krull, U. J. Anal. Biochem. 2003, 317, 186–200.  Heaton, R. J.; Peterson, A. W.; Georgiadis, R. M. Proc. Natl. Acad. Sci. U. S. A. 2001, 98, 3701–3704.  Johnson, R. P.; Richardson, J. A.; Brown, T.; Bartlett, P. N. J. Am. Chem. Soc. 2012, 134, 14099–14107.  Monserud, J. H.; Schwartz, D. K. ACS Nano 2014, 8, 4488–4499.  Mir, K. U.; Southern, E. M. Nat. Biotechnol. 1999, 17, 788–792.  Gao, Y.; Wolf, L. K.; Georgiadis, R. M. Nucleic Acids. Res. 2006, 34, 3370–3377.  Halperin, A.; Buhot, A.; Zhulina, E. B. Biophys. J. 2005, 89, 796–811.  Yao, D. F.; Kim, J.; Yu, F.; Nielsen, P. E.; Sinner, E. K.; Knoll, W. Biophys. J. 2005, 88, 2745–2751.  Wetterstrand, K. A. DNA sequencing costs: Data from the NHGRI Genome Sequencing Program (GSP), Available at www.genome.gov/sequencingcosts. Accessed on January 28, 2016.

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 DNA Brushes: Advances in Synthesis and Applications Renpeng Gu, Lei Tang, Isao Aritome, and Stefan Zauscher Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC, USA

. Introduction Polymer brushes have been of great interest for a wide range of applications in nanotechnology, for example, functional membranes, biosensors, and cell culture platforms,1 because of their versatility in polymer chain length, grafting density, and chemical identity.2 Among biopolymeric brushes, end-grafted DNA brushes has increasingly attracted attention due to its unique biomolecular properties. Solid-phase oligonucleotide synthesis, developed in the 1960s, can be considered as the first example of a method to synthesize DNA brushes.3 In the 1970s, the first functional DNA brushes on a flat substrate were created in the form of DNA microarrays, to detect DNA or RNA in general.4 The first example of DNA brushes on a curved substrate was introduced in 1996, as an approach to modify nanoparticles.5 In this chapter, we review recent developments in DNA brushes synthesis and its emerging applications in bionanotechnology. In the first part, we categorize the synthesis methods into “grafting-to” and “grafting-from” approaches. In the “grafting-to” approach, we focus on the choice of the substrate and its associated chemical reactions to enable a DNA-surface bond. In the “graftingfrom” approach, we discuss the DNA synthesis from the perspective of surfaceinitiated polymerization. In the second part, we highlight the latest applications enabled by DNA brushes, ranging from nanopatterning, biosensing, tuning surface wettability, cell-free protein expression, to drug delivery.

Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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22 DNA Brushes: Advances in Synthesis and Applications

. Synthesis of DNA Brushes In analogy with synthetic polymer brushes, synthesis approaches for DNA brushes can be classified into two categories: (i) the “grafting-to” approach, in which preformed DNA chains react with the modified substrate surface and (ii) the “grafting-from” approach, which involves the polymerization of nucleotides from surface-grafted initiators. To date, the “grafting-to” approach has been widely explored for the fabrication of DNA microarrays and many studies have been reported for DNA immobilization on a range of diverse, solid substrates.6 The “grafting-from” approach has been developed only recently, and harnesses DNA synthesis strategies including surface-initiated enzymatic polymerization (SIEP), surface-initiated rolling circle amplification (SI-RCA), and surfaceinitiated DNA hybridization chain reaction (SI-HCR). ..

Grafting-to Approaches

DNA has been grafted onto a diverse range of surfaces, including gold thin films, silicon oxide/glass, synthetic polymer supports, and carbon-based materials such as carbon nanotubes. The choice of substrate generally depends on the downstream applications and thus determines the strategies for DNA immobilizations. ... Immobilization on Gold Thin Films

Gold thin films are one of the most common substrate for DNA immobilization for the following reasons: First, gold can be easily modified with a range of functional groups by the self-assembly of alkanethiols that bond through the relatively robust Au–S bond (∼126 kJ/mol).7 Second, gold can be considered as a signal transducer to report DNA hybridization, in which gold functions as an electrode in electrochemical sensing, as a conducting surface in field effect transistor devices, or as an optical transducer in surface plasmon resonance (SPR) sensors. Immobilization strategies on gold thin films fall into three categories: (i) direct anchoring of thiolated DNA onto the substrate; (ii) direct anchoring of block polynucleotides, where specific nucleobases adsorb onto gold; and (iii) the attachment of end-functionalized DNA to alkane thiol selfassembled monolayer (SAM) formed on the gold surface. Direct anchoring of thiolated DNA is typically achieved by immersing a pristine gold surface into an aqueous buffer solution of DNA molecules that are end-functionalized with alkanethiol at either the 5′ or 3′ end. Grafting density6a and molecular orientation6b of the immobilized DNA are important factors by which to adjust the chemical and functional properties of the immobilized DNA molecules. The grafting density of DNA brushes can be determined by fluorescent imaging,8 electrochemical methods,9 radio labeling,6b,10 SPR,6a,11 and X-ray

22.2 Synthesis of DNA Brushes

photoelectron spectroscopy (XPS).6b,12 Recent studies show that ionic strength and the nature of the salt in the immobilization buffer strongly affect the grafting density. For example, Herne and Tarlov investigated the role of buffer concentration on immobilization of thiolated single-stranded DNA (ssDNA) using KH2 PO4 buffer and showed that the amount of surface-grafted DNA increased with increasing buffer concentration, reaching a maximum at a salt concentration of around 0.4 M.6b Kinetic studies with different ionic strengths were reported by Peterson et al.6a They found that immobilization in either 1 M KH2 PO4 (pH 4) or 1 M NaCl-Tris-EDTA NaCl-TE) (pH 7.2) shows similar kinetics of film formation and similar, final DNA grafting densities. They also noted that the rate of thiolated DNA (HS-DNA) adsorption in the initial stage is consistently slower at lower ionic strengths compared with adsorption at high ionic strengths, which suggests that the electrostatic repulsion between DNA strands dominates, and that a high salt concentration can compensate for such electrostatic interactions. A significant effect, caused by the identity of salts was shown by Petrovykh et al.12a They investigated DNA densities of HS-(dT)25 brushes, prepared in various buffer solutions, using FT-IR and XPS, and found that the cation valency has a large effect on the immobilization. The densities of DNA layers obtained with divalent Ca2+ and Mg2+ buffers reached over 5 × 1013 /cm2 , which corresponds to an average spacing between the individual ssDNA chains of ∼1 nm, a distance comparable to the radius of a DNA double helix. On the other hand, the densities of DNA layers prepared in monovalent buffers are (2–3) × 1013 /cm2 . They speculate that divalent cations may cause intermolecular or intramolecular electrostatic cross-linking of the negatively charged ssDNA, leading to more compact molecular conformations at the surface (Figure 22.1a). Another dominant factor for the density of surface grafted DNA is the length of the tethered oligonucleotides (ODNs). Steel et al. investigated the relationship between the length and the density of surface-grafted ssDNA by a 32 P radio labeling approach, and found that the densities of longer ssDNA brushes is lower than that of shorter ones.10a They postulate that as the ssDNA length increases, the effect of a single thiol group on ssDNA immobilization becomes less significant when compared to the large number of adsorptive nucleotide– gold interactions that arise between each DNA chain and the substrate. This relationship between molecular orientation of DNA brushes and their length was revealed experimentally by Petrovykh et al.13 Using a combination of X-ray absorption near edge structure (NEXAFS), XPS, and FT-IR, they studied the orientation and ordering of HS-(dT)5 and HS-(dT)25 and found that short HS-(dT)5 chains exhibit a primarily upright orientation with less disorder compared with HS-(dT)25 strands that formed mostly disordered films (Figure 22.1b). Disordered molecular orientation, which would hamper some applications, can be avoided by dense packing or by formation of mixed SAMs, using

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22 DNA Brushes: Advances in Synthesis and Applications

Figure . Schematic illustration of DNA brush grafting density and molecular orientation under different conditions: (a) divalent buffer vs. monovalent buffer, (b) short ssDNA vs. long ssDNA, (c) backfilling vs. nonbackfilling. (d) Schematic illustration of a (dT)m –(dA)n oligonucleotide brush on a gold surface. The (dA) blocks (dark grey) adsorb flat on the gold surface, whereas the (dT) blocks (light grey) extend away from the surface. Increasing the length of the (dA) block decreases the grafting density of the oligonucleotides. Source: (d) Opdahl et al. 2007.16 Reproduced with permission of National Academy of Sciences.

backfilling with alkane thiols. For the immobilization of HS-(dT)25 , for example, Howell et al. applied a CaCl2 containing buffer which led to an extremely dense and upright ssDNA layer, as shown by NEXAFS.14 Herne and Tarlov used 6-mercapto-1-hexanol as a spacer molecule to prevent the interaction of surface-attached ssDNA molecules with the gold substrate through the nitrogen-containing nucleobases.6b Lee et al. also showed that the molecular orientation of thiolated 21mer ODNs changed into a more upright orientation after backfilling with 11-mercapto-1-undecanol (Figure 22.1c).12c Another strategy to directly anchor DNA onto gold surfaces is to use an oligo(dA) block as the anchoring group, which has the strongest affinity to gold surfaces compared with the other nucleobases.15 Opdahl et al. used block-(dT)m (dA)n to fabricate a (dT)m brush and showed that the length of the (dA)n chain affects DNA grafting density critically (Figure 22.1d).16 Schreiner et al. applied additional oligo-(dA) molecules as lateral spacers in this system to further control the DNA brush density.17 Although the binding of oligo-(dA) blocks to gold surfaces is not as robust as that of thiolated DNA, this approach provides promising control over the grafting density by selecting the length of the oligo(dA) anchoring group. Attachment of end-functionalized DNA to alkane thiol SAMs is another well-known strategy to fabricate DNA brushes (Figure 22.2a). This approach

22.2 Synthesis of DNA Brushes (a)

F O F

F

DNA

DNA

DNA

F

O

N O

O O O

O

DNA

–NH2 O N

O

HN O

–SH

O

(b)

Target DNA Biotin-DNA Target DNA Streptavidin

Biotin-thiol method

O=C O= O= O= O= O=C O= C C C C O=C C

–NH -R

–NH -R

–NH -

–NH -R

–NH -

–NH -

–NH -R

–NH -

Streptavidin OH OHOH OHOHOH OHOH OHOHOH OHOH OH

–NH -R

Biotin-DNA

O=C

MUA (or amine coupling) method

Figure . Schematic illustration of (a) the fabrication of DNA brushes on SAM modified gold-based substrates and (b) the streptavidin-biotin coupling reaction to immobilize biotinylated ssDNA with subsequent target DNA hybridization. Streptavidin is immobilized on the surface either through biotin-streptavidin molecular recognition binding, using a mixed, SAM, or through covalent attachment on a carboxyl-containing surface using amine coupling. Source: Su et al. 2005.24 Reproduced with permission of American Chemical Society.

has the advantage over the direct anchoring approaches that alkane SAMs can work as spacers to prevent nonspecific adsorption of biomolecules. One widely used spacer is 11-mercaptoundecylamine (MUAM), which is used in combination with hetero-bifunctional cross-linkers. For example, Brockman et al. applied 4-(N-maleimidomethyl) cyclohexane-1-carboxylate to immobilize 5′ thiol modified ssDNA onto a MUAM SAM.18 Smith et al. used N-succinimidyl S-acetylthiopropionate to create a protected sulfhydryl-terminated layer, which decomposes to form thiol groups on the SAM surface.19 Maleimide functionalized ssDNA, or thiolated ssDNA, can directly interact with this SAM surface. In another approach, alkanethiols modified with terminal aldehyde groups are used to immobilize amine-modified ssDNA.20 While the reaction of aldehyde with primary amine gives an unstable Schiff base (imine), it can, however, be converted easily into a stable secondary amine, using reducing agents such as NaBH3 CN or NaBH4 . The approach for immobilization of aminefunctionalized ssDNA was reported by Lockett et al.21 They used tetrafluorophenyl ester SAMs which were designed to react effectively with aminessDNA and showed that it permitted higher ssDNA coupling efficiency than

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22 DNA Brushes: Advances in Synthesis and Applications

that for N-hydroxysuccinimide (NHS)-ester SAMs. To prevent nonspecific adsorption, thiol-SAMs with oligo(ethylene glycol) spacers can be used.22 The prototypical approach for affinity coupling is to use the avidin/streptavidin– biotin protein–ligand pair, as shown in Figure 22.2b. The (strept)avidin–biotin recognition is one of the strongest noncovalent, biological interactions, with a dissociation constant (Kd ) of 10−15 M for avidin and 10−14 M for streptavidin.23 In addition to high affinity, the tetrameric biotin-binding sites in avidin/ streptavidin allow for a “sandwich-like” affinity coupling approach, that links biotin-modified DNA and a biotin-modified surface via a (strept)avidin bridge.24 ... Immobilization on Silicon-Based Substrates

Silicon-based substrates are another widely used substrate.25 Glass slides, for example, are ubiquitous in microarray technology, where they function as a solid support for the immobilization of DNA with high spatial density. DNA has also been grafted onto optical glass fibers, where the fibers function not only as solid supports but also as signal transduction elements. Furthermore, silicon wafers are used as supports in miniaturized sensor systems, capitalizing on the semiconducting properties of silicon. Immobilization of DNA on silicon oxide surfaces is typically achieved by surface functionalization with organosilanes, followed by covalent attachment of DNA onto the functionalized surface with or without a spacer (Figure 22.3). It is generally believed that the reaction of organosilane reagents on silicon oxide surfaces proceeds by a three-step process: (i) formation of an organosilane SAM, (ii) hydrolysis of hydrolyzable groups such as –Si(OR)x or –SiClx groups, and (iii) a condensation reaction between silanol groups (Si-OH) of the hydrolyzed organosilane and that of the silicon oxide surface.26 To achieve

Figure . Schematic illustration of the fabrication of DNA brushes on silicon-based substrates.

22.2 Synthesis of DNA Brushes

reproducible results, these reactions require precise control over the reaction conditions such as humidity, quality and nature of the solvents, temperature, and time, because horizontal and vertical polycondensation reactions lead to two- and three-dimensional Si–O–Si networks formation.27 Several reviews include more details on these issues.25b,28 ....

Attachment on Amine-Functionalized Surface

Amine-functionalized surfaces are frequently used for the immobilization of ODNs. This is often achieved with (3-aminopropyl)-triethoxysilane, whereas monofunctional (3-aminopropyl)dimethyl-ethoxysilane or bifunctional (3aminopropyl)diethoxymethylsilane are used to control surface amine density.29 To control amine density, strategies that use mixed SAMs with inert organosilanes are also reported.29a,30 The coupling between the modified ODN and the activated surface is carried out directly6c or through a homo-/heterolinker. The latter is generally employed because the linker, as a physical spacer, can minimize the steric hindrance experienced by surface-tethered ODNs. As homofunctional linkers, succinic anhydride,31 glutaraldehyde,32 PEG-dicarboxymethyl,31 or dendrimers with aldehyde groups,33 have been reported for the immobilization of amine-functionalized ODNs. As heterofunctional linkers, a compound with maleimide groups and NHS ester on the termini29–30,34 or one with maleimide groups and an isocyanate group34 have been used for the attachment of thiol-functionalized ONDs. Furthermore, the heterofunctional linker succinimidyl N-propargyl glutariamidate has been used for the attachment of azidefunctionalized ONDs.35 ....

Attachment to Epoxy- and Thiol-Functionalized Surfaces

Surface functionalization of thiol or epoxy groups is typically achieved by silanization using (3-glycidyloxypropyl)-trimethoxy silane or (3mercaptopropyl)trimethoxysilane. On epoxy-functionalized surfaces, commercially available amine- or thiol-functionalized ODNs can be applied for direct attachment.36 Alkyne-amine compounds can be used as heterofunctional linkers to immobilized thiolated ODNs via a thiol-yne reaction.37 On thiol-functionalized surfaces, Escorihuela et al. reported directly coupling of alkene-functionalized ODN using thiol-ene click chemistry.38 A heterofunctional linker with terminal NHS ester and maleimide moiety is also generally used for ODN immobilization.39 ....

Attachment on Other Functionalized Surfaces

Aldehyde-functionalized surfaces are available for direct coupling with aminefunctionalized ODNs.6c,40 ODN attachments on the carboxylated surface can be achieved by direct coupling with amine-functionalized ODN.6c ODN

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22 DNA Brushes: Advances in Synthesis and Applications

immobilization on azide-functionalized surfaces via alkyne-azide click chemistry is also available.41 Affinity coupling can also be used for immobilization of functionalized ssDNA onto alkanethiol SAMs. ..

Grafting-from Approaches

Although the solid phase, chemical synthesis of ODNs42 is the classical “grafting-from” approach to build up DNA brushes, the length of the synthesized ODN is typically less than 150 nucleotides due to a decline in coupling efficiency and increased cost. To overcome some of these limitations, several strategies have been developed to obtain long DNA brushes, such as SIEP,43 SI-RCA,44 and SI-HCR.45 ... Surface-Initiated Enzymatic Polymerization

Different DNA polymerases have been used for the surface-initiated enzymatic polymerization. One example to grow a dsDNA brush uses Taq-polymerase, which does not require a primer and catalyzes the template-free polymerization of dATP and dTTP into poly d(A-T) (Figure 22.4a).46 The other method uses the template-independent terminal deoxynucleotidyl transferase (TdT), which repetitively adds mononucleotides to the 3′ end of ODN “initiators” to extend a ssDNA brush.47 This method not only provides an approach for high-density ssDNA brushes but also offers a strategy to incorporate unnatural deoxynucleotides with a variety of functional groups (Figure 22.4b).43,48 ... Surface-Initiated Rolling Circle Amplification

Another “grafting-from” approach to fabricate DNA brushes uses rolling circle amplification (RCA) (Figure 22.5a).44,49 First, the ODN primers are covalently attached to the surface and allowed to hybridize to circular DNA templates. DNA polymerases are then used to synthesize long, linear DNA polymers by replicating the circular DNA molecules. Given the nature of the templatedependent polymerase, the ssDNA brushes synthesized by SI-RCA are potentially long (∼9 kb), tandemly repeated, and complimentary to the circular templates. ... Surface-Initiated Hybridization Chain Reaction

Another “grafting-from” approach that does not require a DNA polymerase is SI-HCR. The concept of HCR was first reported by Dirks and Pierce.50 As shown in Figure 22.5b, two stable species of DNA hairpins coexist in solution until the introduction of initiator strands triggers a cascade of hybridization events and a long dsDNA chain is formed as a product. To use photoirradiation as a trigger, Huang et al. have modified this concept by introducing photoreactive groups into the hairpin structure,51 and then applied this modified system to perform SI-HCR and achieved DNA brushes with ∼10 um lateral dimensions.45

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: d(T-A)9-NH2

A T A T

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A25-SH

TdT + Co2+ + dATP

TdT extension

Silicon

Au

TdT

Figure . Schematic illustration of (a) the surface-initiated poly d(A-T) growth by Taq DNA polymerase and (b) the growth of ssDNA brushes through SIEP reaction. Source: (a) Chi et al. 2005.46 Reproduced with permission of American Chemical Society. (b) Khan et al. 2013.43 Reproduced with permission of American Chemical Society.

S

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b b*

y

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y

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x a* FAM-H1

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Synthesize linear DNA polymers by rolling circle amplification

Plasma etch to destroy unprotected primers then remove resist

Derivatize coverslip then covalently attach DNA primers

n n

UV light

n

Assemble chip then hybridize circular DNS to primers

Coat with photoresist and pattern array of posts via photolithography

Figure . (a) Fabrication of DNA brushes arrays using a destructive micropatterning technique and RCA. (b) Fabrication of photocontrolled DNA brushes patterns using SI-HCR. Source: (a) Barbee et al., 2011.44 Reproduced with permission of John Wiley and Sons. (b) Huang et al. 2015.45 Reproduced with permission of John Wiley and Sons.

x a*

x a*

(b)

(a)

22.3 Properties and Applications of DNA Brushes

..

Synthesis of DNA Brushes on Curved Surfaces

In general, the approaches to fabricate DNA brushes on curved surfaces, for example, nanoparticles, are similar to those employed on flat surfaces (see 22.2.1 and 22.2.2). For example, Mirkin et al. used Au-SH chemistry to selfassemble thiolated ODNs on the surface of AuNPs, to build spherical nucleic acids (SNAs) with gold cores and densely and highly oriented nucleic acids shells (Figure 22.6a).5,52 This strategy has also been used to functionalize gold nanorods53 and gold nanowires.54 Pei et al. proposed a strategy to functionalize AuNPs with diblock DNA ODNs using oligo-(dA) as an anchoring block and to regulate the surface density of the DNA brush by adjusting the length of the oligo-(dA) block (Figure 22.6b).55 Beyond Au nanostructures, Zhang et al. developed a general approach to immobilize DNA brushes on nanoparticles protected with aliphatic ligands.56 As shown in Figure 22.6c, the particle surface was first coated with an azide-bearing amphiphilic polymer, and then functionalized with dibenzocyclootyle-terminated DNA strands, using copper-free azide-alkyne click chemistry. Additionally, Zhao et al. and Jeong et al. demonstrated that using SI-RCA can extend ODNs anchored on AuNPs or gold nanowires to build long DNA brushes (Figure 22.6d).57

. Properties and Applications of DNA Brushes In this section, we provide selective, recent examples that harness the exceptional properties of DNA brushes for applications ranging from nanolithography, biosensors, surface modifications, and cell-free surface transcription to drug delivery. Based on the use of the DNA brushes, we discuss applications from the following four perspectives: (i) the use of DNA-modifying enzymes for nanopatterning and biosensing, (ii) the use of exogenously triggered, conformational changes of DNA brushes, (iii) the use of directional coding of DNA brushes for cell-free surface transcription and translation, and (iv) the use of DNA brushes–modified nanoparticles in drug delivery and biosensing applications. ..

The Effect of DNA-Modifying Enzymes on the DNA Brush Structure

In solution, molecular biologists have applied DNA-modifying enzymes for decades to manipulate DNA structure on the nucleobase level. Given the advantages provided by a solid support (e.g., in microarrays), the use of DNAmodifying enzymes has become quite attractive also for the manipulation of DNA structures on surfaces. Different types of DNA-modifying enzymes, such as polymerases, transferases, ligases, restriction endonucleases, and nucleases, can be used to elongate, cut, digest, or modify a DNA brush on surfaces. In contrast to chemical modifications on synthetic polymer brushes, the specificity of

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Thiol DNA primer

Primer-Au

Circular DNA template

N3-NP

Salt Aging

Covalently attach DNA

DNA polymerase

(b)

PAE

AuNP

–N3

Pt

Au

Fe3O4

CdSe/ZnS

Rolling circle amplification

Anchoring Recognition

N

S

O

NH2

O

C

NH2 O

N

N N

N

O

Figure . (a) Synthesis of SNA-AuNP conjugates. (b) Conjugation of polyA-DNA and AuNPs and spatial control by varying the length of the polyA blocks. (c) Synthesis scheme for nanoparticle-based programmable atom equivalents. (d) Scheme of RCA on AuNPs. Source: (a) Cutler et al. 2012.52 Reproduced with permission of American Chemical Society. (b) Pei et al. 2012.55 Reproduced with permission of American Chemical Society. (c) Zhang et al. 2013.56 Reproduced with permission of American Chemical Society. (d) Zhao et al. 2006.57 Reproduced with permission of John Wiley and Sons.

SH

Coat with amphiphilic polymer

= Citrate = Alkylthiol-DNA

Initial DNA Adsorption

Hydrophobic NP

AuNPs

(d)

(c)

(a)

22.3 Properties and Applications of DNA Brushes

the enzymes allows for the manipulation of DNA brushes with a molecularlevel precision. In a demonstration of bottom-up nanolithography, a thiol-terminated ODN “initiator” was anchored to gold islands that were prepatterned on a silicon substrate, and then a template independent polymerase (i.e., TdT) catalyzed in situ polymerization of mononucleotides to yield ssDNA brushes (Figure 22.7a).58 In an example of top-down DNA brush nanolithography, DNase I was adhered to an AFM cantilever tip, which thus allowed for the locally confined, enzymatic digestion of specific regions of the DNA brushes, resulting in DNA brush patterns with nanoscale lateral dimensions (Figure 22.7b).59 In addition to elongation and digestion of DNA brushes, restriction enzymes have also been used to selectively digest certain regions of DNA brushes.60 The access of enzyme to the DNA brushes largely depends on the grafting density of the DNA molecules on the surface. For example, the restriction enzyme DpnII is inhibited when the spatial separation between DNA molecules is smaller than the size of the enzyme dimers. Taking advantage of the ability to control spatial separations, an engineered dsDNA matrix with a high-density “fence” sidewall can provide a barrier to the enzyme’s lateral access, and thus protect the adjacent low density DNA brush from digestion.60b Beyond nanopattering on surfaces, the growth of DNA brushes has been used as a means to amplify the signal in the detection of DNA, RNA, and protein targets on surfaces.61 In one DNA detection example, the target analyte hybridizes to a capture strand that is immobilized on the substrate surface such that the 3′ -OH group of the target is accessible on the surface. TdT-based SIEP is then used to amplify the hybridization events by incorporating multiple fluorescent nucleotides in the DNA chain to allow for a fluorescent readout of the probetarget hybridization events (Figure 22.7c).62 This innovative, TdT-based technique provides an on-chip, isothermal, and label-free detection/amplification method for DNA and RNA detection. Another example, shown in Figure 22.7d, uses RCA amplification for DNA detection. A Y-shaped structure is formed in the presence of target DNA and can be cleaved by the nicking endonuclease to generate a primer for the RCA reaction. Using quantum dot-modified ODNs as a signal tag, an ultrasensitive electrochemical detection of DNA with attomolar limit of detection and a linear range of 6 orders of magnitude (from 11 × 10−18 to 1 × 10−11 M) was achieved.63 ..

Stimulus-Responsive Conformational Changes of DNA Brushes

The exceptional intra- and intermolecular interactions between nucleobases distinguish DNA brushes from other synthetic polymer brushes. The secondary structures originating from molecular interactions among DNA brushes can also be stimulus responsive. This property has been widely

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Potential

+

(d) Polymerase-based biosensors

Au

DNAse I

Height (nm)

2) dNTPs Phi29 polymerase

Figure . The use of DNA-modifying enzymes. (a) Bottom-up method, polynucleotides grow from prepatterned ODN initiators on a gold substrate. (b) Top-down nanopatterning of DNA via enzymatic lithography. (c) A template-independent TdT catalyzed polymerization of DNA from the 3′ -OH end of a hybridized target. (d) Polymerase amplified hybridization event with ultrasensitive detection of DNA. Source: (a) Chow et al. 2005.47a Reproduced with permission of American Chemical Society. (b) Hyun et al. 2004.59 Reproduced with permission of American Chemical Society. (c) Tjong et al. 201148 (left) and 201362b (right). Reproduced with permission of American Chemical Society. (d) Ji et al. 2012.63 Reproduced with permission of American Chemical Society.

3ʹ

Probe 5ʹ

TdT incorporates fluorescent days

DNA detection

(c) TdT-based biosensors

Au

TdT + Co2+ + dNTP

P Pa

(b) Subtractive nanopatterning with DNase I

Current

(a) Additive nanopatterning

1) T4 ligase Circular template

22.3 Properties and Applications of DNA Brushes

employed to transduce probe–target binding events into a measureable readout, such as fluorescent, colorimetric, chemiluminescent, and electrochemical signals. One transformative application is DNA microarray technology, which is used for DNA detection.61,64 Considering the comprehensive reviews available on the subject, we illustrate this property by providing examples that exclusively cover surface property changes that arise from controlled, conformational changes in the DNA brushes. In solution, multistranded DNA complexes have been exploited as molecular motors.65 More recently, the immobilization of DNA strands onto microfabricated cantilevers has inspired the development of molecular motors on surface. These surface-based molecular motors enable “label-free” cantilever biosensors, which directly transduce the nanomechanical forces exerted by surface-tethered, stimulus-responsive ODNs.66 The reversible, conformational changes of DNA brushes present an exciting approach to translate biochemical reactions into micromechanical motion. For example, the proton-fueled C-rich i-motif is responsive to pH changes.67 Protonation of C-bases and their noncanonical base pairing with deprotonated C-bases, generates an interdigitated, quadruple helix that is stable at pH 4.5–5.0. As shown in Figure 22.8a, at first, strand X adopts the i-motif conformation. When the pH rises above 6.5, cytosine bases become deprotonated and therefore strand X unfolds and hybridizes with strand Y. This hybridization event generates an extended double helix and induces a micromechanical bending motion of the cantilever beam. Another example utilizes the high packing density of ssDNA on surfaces to create intermolecular channels with sub-nanometer diameters to confine water molecules. The surface stress thereby becomes a function of the relative humidity.68 Interestingly, upon hybridization, the attractive force at the initial hydration stage (low humidity) of ssDNA molecules turns into a repulsive force. Thus, the large difference in the humidity-induced stress becomes an identifier for the DNA hybridization event and leads to a biosensor with fM sensitivity. Beyond mechanical property changes, the pH sensitivity of the C-rich i-motif has been used to tune surface wettability.69 A C-rich DNA brush, with a hydrophobic boron-dipyrromethene (BODIPY) fluorophore attached to the extended end of the ODN, was immobilized on a gold surface via thiol-Au bonds (Figure 22.8b). At low pH, the immobilized DNA exposes the hydrophilic phosphate backbones on the surface. When the pH is raised, the i-motif is destabilized and the DNA chains adopt an extended conformation, which exposes the hydrophobic BODIPY on the surface, switching the surface from hydrophilic to hydrophobic. Another example that harnesses the reversible i-motif conformational change is the creation of a nanometer-height container that can switch between closed and open states in response to a change in pH.70 At pH 4.5, the i-motif domain of the DNA brush folds to form a closed nanocompartment structure.

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N

H

+

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deoxyribose

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OH– Y

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H+

duplex (X-Y)

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II pH 8.5

(b) Surface wettability

H

Figure . Harnessing DNA secondary structures: (a) label-free micromechanical motion for DNA detection and (b) i-motif conformational change to control the wettability of a surface. Source: (a) Shu et al., 2005.67 Reproduced with permission of American Chemical Society. (b) Wang et al., 2007.69 Reproduced with permission of John Wiley and Sons.

H

N

deoxyribose

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–

H O

22.3 Properties and Applications of DNA Brushes

At pH 8.0, the i-motif domain unfolds into a single-stranded form, and becomes an open and loose structure. ..

DNA Brush for Cell-Free Surface Protein Expression

Beyond traditional, in vivo protein expression, cell-free DNA translation using cell extracts has become an important tool for molecular biologists.71 In this section, we highlight the application of surface-bound, linear double-stranded DNA templates (dense and ordered DNA brushes) for the use as a cell-free protein expression platform. Compared to DNA in homogeneous solution, the spatially controlled packing density of DNA brushes provides an innovative approach to modulate the transcription and translation of DNA to produce a protein. In the concept of translation from DNA array to protein array, the advantages include the repeated use of the same DNA array and the elimination of extensive protein purification steps.72 Considering that brush density can be used to sterically exclude macromolecules, the cell extract concentration (e.g., enzymes, ribosome) is thus a function of brush density and the relative distance to the substrate surface.73 To optimize the transcription rate of T7 RNA polymerase, the design for the interchain distance in the DNA brush should be on the order of the dimension of the biomacromolecules required for transcription and translation. With respect to the promoter sequence orientation, the transcription rate is approximately twofold higher when the promoter sequence is facing inwards toward surface rather than outwards from the surface.74 To demonstrate the concept of cell-free protein expression, a green fluorescent protein (GFP) was expressed by assembling a digestion–ligation cascade. In this case, two types of DNA brushes are used: one coding for GFP but lacking the promoter sequence, and the other including the promoter but no coding sequence.75 As illustrated in Figure 22.9a, a cut of the T7 promoter from the B–D fragment and a following paste to the A–C fragment turns this into an A–D fragment containing a T7 promoter upstream of the GFP gene. The cut-and-paste process was followed by a washing step, and then an E. coli cellfree transcription/translation mix was added for the expression of GFP. Both the digestion–ligation process and the expression of GFP were monitored by fluorescent microscopy. Toward the goal of developing an artificial cell with the ability to synthesize proteins, a microfluidic device with multiple compartments was assembled and perfused with cell extract in the main channel.76 Reaction components were diffused into the DNA compartments as shown in Figure 22.9b. Measurements showed that a linear concentration gradient of synthesized protein formed along the capillary wall. An activator–repressor gene circuit was implemented to achieve oscillatory gene expression dynamics, where the geometry

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usio

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Figure . Cell-free surface protein expression system. (a) A cut-and-paste process for GFP expression (T7 promoter is marked as P). (b) Microfluidic device for dynamic GFP expression, the bottom plots are GFP intensity profiles of an unregulated construct, and an emergent oscillation of an activator–repressor network. Source: (a) Bar et al. 2009.75 Reproduced with permission of American Chemical Society. (b) Karzbrun et al. 2014.76 Reproduced with permission of The American Association for the Advancement of Science.

A

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22.3 Properties and Applications of DNA Brushes

of the microfluidic device enables the control over the frequency and the amplitude of the oscillation. ..

DNA Brush-Modified Nanoparticles for Biomedical Applications

End-functionalized DNA not only enables the construction of DNA brushes on flat substrates, but also on curved substrates, such as nanoparticles (see Section 22.2.3). The base-pair recognition ability endows DNA-modified nanoparticles with unique features for biomedical applications. In this section, we highlight the applications of DNA-functionalized nanoparticles in controlled drug delivery and gene diagnosis. As mentioned in 22.3.2, DNA brushes that contain an i-motif domain can change conformation in response to a change in pH. Using DNA i-motif brushes as caps, situated on the pores of mesoporous silica nanoparticles (MSNs), these DNA brushes can control the release of cargo from the pores in response to a change in pH.77 As shown in Figure 22.10a, at pH 4.5, the i-motif domain of DNA brushes folds and blocks the pores, keeping the dye molecules inside. At pH 8.0, the i-motif domain unfolds into its single-stranded form, opening the pores, and thus allows for the release of dye molecules from the MSNs. Moreover, adding malachite green carbinol base (MGCB) can transform the system from pH responsive to photoresponsive, which can be used for remote-controlled drug release (Figure 22.10b).78 Under irradiation with UV light (365 nm), immobilized MGCB molecules dissociate into malachite green (MG) cations and OH− ions, which increases the local pH and leads to the unfolding of the DNA i-motif. This process is reversible when the light is turned off. Since UV light is not a favorable stimulus for in vivo controlled drug release, researchers designed and fabricated more convenient and efficient, remotely controlled drug delivery systems that are responsive to alternating magnetic field or near-IR (NIR) light. For example, Ruiz-Hernandez et al. immobilized ssDNA on MSN surfaces, and capped the pores by hybridization with complementary DNA chains that were functionalized with magnetic nanoparticles (Figure 22.10c).79 Remote control in this system is achieved by applying an alternating magnetic field, which increases the local temperature around the magnetic nanoparticles. The temperature increase induces DNA dehybridzation, which subsequently allows for the release of cargo from the pores. Alternatively, Xiao et al. built a DNA-based platform that responds to NIR light. In this system, CG base pairs of hybridized DNA strands are used for drug (e.g., Doxorubicin) loading, whereas gold nanorods serve as NIR light-to-heat transducers.53 Upon NIR irradiation, the heat produced by the gold nanorods dehybridizes the DNA double helix, which in turn triggers the release of the loaded drugs (Figure 22.10d).

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Figure . Schematic illustration of smart drug release from DNA brushes-based platforms. The i-motif capped MSNs respond to changes in (a) pH value and (b) UV light (c) The magnetic nanoparticles capped MSNs respond to the alternating magnetic fields. (d) DNA functionalized gold nanorods respond to near-infrared light. Source: (a) Chen et al. 2011.77 Reproduced with permission of Oxford University Press. (b) He et al. 2012.78 Reproduced with permission of John Wiley and Sons. (c) Ruiz-Hernandez et al. 2011.79 Reproduced with permission of American Chemical Society. (d) Xiao et al. 2012.53 Reproduced with permission of John Wiley and Sons.

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TLR antagonist oligonucleotides Core Gold: Au-SNA Liposome: L-SNA

Immunostimulatory (IS-SNA)

TLR agonist oligonucleotides

Immunomodulatory SNA

(5′ or 3′)-Recognition Sequence-Spacer-Attachment Group Spacer: A10, T10, PEG Attachment Group: SH, N3

Core: Gold (Au), Silver (Ag), Iron Oxide (Fe3O4), polymers, Quantum Dots (CdSe), Silica (SiO2), Core-shell (Au@SiO2, nanoshells), hollow cross-linked SNAs, pure DNA

(b)

NF-kB

MyD88

Inflammatory response

cytoplasm

endocytosis

TLR9

Figure . (a) The anatomy of SNA nanostructures. (b) Endocytosis of SNA through the cell membrane via interaction with scavenger receptors. (c) Structure and endocytosis of immunomodulatory SNA that modulate toll-like receptor activity in endosomes for immune regulation. Source: (a) Cutler et al. 2012.52 Reproduced with permission of American Chemical Society. (b) Cutler, et al. 2012.52 Reproduced with permission of American Chemical Society. (c) Cutler et al. 2012.52 Reproduced with permission of American Chemical Society; Radovic-Moreno et al. 2015.83 Reproduced with permission of National Academy of Sciences.

(c)

(a)

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22 DNA Brushes: Advances in Synthesis and Applications

The SNAs developed by Mirkin’s group,80 provide an exciting alternative, as they have high nuclease resistance and can rapidly enter the cells by endocytosis without the use of transfection agents (Figures 22.11a and 22.11b).81 These unique properties make SNAs powerful vehicles for delivering nucleic acid-based therapeutics for intracellular gene82 and immune regulation (Figure 22.11c).83 Furthermore, by modifying the structure of DNA brushes, SNAs can be adapted to attach drugs, antibodies, or small molecules for drug delivery,84 cellular targeting,85 and cellular imaging.86 In addition to their use as delivery vehicles, SNAs can be used to detect intracellular mRNA in living cells.87 As shown in Figure 22.12a, the anchored ODN with a specific recognition sequence is complementary to the target mRNA.

(a)

Target mRNA

Cy5 Reporter Flare 3′-

-5′

Recognition sequence 5′(Antisense DNA)

-SH-3′

(b)

Target Binding

Au NPs

Aptamer- reporter duplexes

Aptamer nanoflare

Released reporter “flares”

Figure . Schematic illustration of the nanoflare structures and detections of target (a) mRNA and (b) adenosine triphosphate. Source: (a) Halo et al. 2014.88 Reproduced with permission of National Acadamy of Sciences. (b) Zheng et al. 2009.89 Reproduced with permission of American Chemical Society.

References

The ODN is prehybridized to a short ODN that contains a fluorophore (“flare”). The fluorescence, however, is quenched due to the close proximity to the gold surface. The “flare” is displaced when the targeted mRNA binds to the recognition sequence, which can be detected by fluorescence imaging. When coupled with flow cytometry, this “NanoFlare” system can be used to detect and isolate living, circulating tumor cells from blood for cancer diagnosis.88 By further engineering the DNA brushes, the “NanoFlare” can also be applied to detect other analytes in a living cell, such as adenosine triphosphate (Figure 22.12b).89

. Conclusion and Outlook Beyond the prominent role of DNA as a genetic material, DNA has increasingly been used as a versatile biosynthetic material for the construction and manipulation of nanoscale structures and devices. In this chapter, we reviewed the recent developments in syntheses and applications of DNA brushes. Advances in DNA synthesis, including chemical and enzymatic methods, combined with the growing availability of unnatural nucleotides, allow for the synthesis of nucleic acid-based polymers with a broad range of chemical and physical functionalities. Compared with synthetic polymer brushes, DNA-based brushes thus offer an alternative way for biocompatible surface modifications and, more importantly, open up opportunities for creating versatile, responsive, and functional surfaces that transcend biomedical applications.

References  (a) Krishnamoorthy, M.; Hakobyan, S.; Ramstedt, M.; Gautrot, J. E. Chem. Rev. 2014, 114, 10976–11026; (b) Azzaroni, O. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 3225–3258.  Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677–710.  Letsinge, R. L.; Mahadeva, V. J. Am. Chem. Soc. 1965, 87, 3526–3527.  (a) Michael Grunstein, D. S. H. Proc. Natl. Acad. Sci. U. S. A. 1975, 72, 3961–3965; (b) Gergen, J. P.; Stern, R. H.; Wensink, P. C. Nucleic Acids Res. 1979, 7, 2115–2136.  Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609.  (a) Peterson, A. W.; Heaton, R. J.; Georgiadis, R. M. Nucleic Acids Res. 2001, 29, 5163–5168; (b) Herne, T. M.; Tarlov, M. J. J. Am. Chem. Soc. 1997, 119, 8916–8920; (c) Zammatteo, N.; Jeanmart, L.; Hamels, S.; Courtois, S.; Louette, P.; Hevesi, L.; Remacle, J. Anal. Biochem. 2000, 280, 143–150; (d) Proudnikov, D.; Timofeev, E.; Mirzabekov, A. Anal. Biochem. 1998, 259, 34–41;

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22 DNA Brushes: Advances in Synthesis and Applications

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            

(e) Rasmussen, S. R.; Larsen, M. R.; Rasmussen, S. E. Anal. Biochem. 1991, 198, 138–142; (f ) Canete-Rosales, P.; Gonzalez, M.; Anson, A.; Martinez, M.; Yanez, C.; Bollo, S. Electrochim, Acta 2014, 140, 489–496. Schreiber, F. Prog, Surf, Sci, 2000, 65, 151–256. (a) Demers, L. M.; Mirkin, C. A.; Mucic, R. C.; Reynolds, R. A.; Letsinger, R. L.; Elghanian, R.; Viswanadham, G. Anal. Chem. 2000, 72, 5535–5541; (b) Castelino, K.; Kannan, B.; Majumdar, A. Langmuir 2005, 21, 1956–1961; (c) Choi, S.; Murphy, W. L. Langmuir 2008, 24, 6873–6880. Steel, A. B.; Herne, T. M.; Tarlov, M. J. Anal. Chem. 1998, 70, 4670–4677. (a) Steel, A. B.; Levicky, R. L.; Herne, T. M.; Tarlov, M. J. Biophys. J. 2000, 79, 975–981; (b) Gong, P.; Lee, C. Y.; Gamble, L. J.; Castner, D. G.; Grainger, D. W. Anal. Chem. 2006, 78, 3326–3334. Georgiadis, R.; Peterlinz, K. P.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166–3173. (a) Petrovykh, D. Y.; Kimura-Suda, H.; Whitman, L. J.; Tarlov, M. J. J. Am. Chem. Soc. 2003, 125, 5219–5226; (b) Petrovykh, D. Y.; Kimura-Suda, H.; Tarlov, M. J.; Whitman, L. J. Langmuir 2004, 20, 429–440; (c) Lee, C. Y.; Gong, P.; Harbers, G. M.; Grainger, D. W.; Castner, D. G.; Gamble, L. J. Anal. Chem. 2006, 78, 3316–3325. Petrovykh, D. Y.; Perez-Dieste, V.; Opdahl, A.; Kimura-Suda, H.; Sullivan, J. M.; Tarlov, M. J.; Himpsel, F. J.; Whitman, L. J. J. Am. Chem. Soc. 2006, 128, 2–3. Howell, C.; Jeyachandran, Y. L.; Koelsch, P.; Zharnikov, M. J. Phys. Chem. C 2012, 116, 11133–11140. Kimura-Suda, H.; Petrovykh, D. Y.; Tarlov, M. J.; Whitman, L. J. J. Am. Chem. Soc. 2003, 125, 9014–9015. Opdahl, A.; Petrovykh, D. Y.; Kimura-Suda, H.; Tarlov, M. J.; Whitman, L. J. Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 9–14. Schreiner, S. M.; Shudy, D. F.; Hatch, A. L.; Opdahl, A.; Whitman, L. J.; Petrovykh, D. Y. Anal. Chem. 2010, 82, 2803–2810. Brockman, J. M.; Frutos, A. G.; Corn, R. M. J. Am. Chem. Soc. 1999, 121, 8044–8051. Smith, E. A.; Wanat, M. J.; Cheng, Y. F.; Barreira, S. V. P.; Frutos, A. G.; Corn, R. M. Langmuir 2001, 17, 2502–2507. Peelen, D.; Smith, L. M. Langmuir 2005, 21, 266–271. Lockett, M. R.; Phillips, M. F.; Jarecki, J. L.; Peelen, D.; Smith, L. M. Langmuir 2008, 24, 69–75. Lee, C.-Y.; Nguyen, P.-C. T.; Grainger, D. W.; Gamble, L. J.; Castner, D. G. Anal. Chem. 2007, 79, 4390–4400. Green, N. M. Adv. Protein Chem. 1975, 29, 85–133. Su, X. D.; Wu, Y. J.; Robelek, R.; Knoll, W. Langmuir 2005, 21, 348–353. (a) Schena, M.; Shalon, D.; Davis, R. W.; Brown, P. O. Science 1995, 270, 467–470; (b) Luderer, F.; Walschus, U. In Wittmann, C.; Ed.; Immobilisation of DNA on Chips I; Berlin, Heidelberg: Springer-Verlag Berlin Heidelberg, 2005; Vol. 260, pp. 37–56.

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 Sagiv, J. J. Am. Chem. Soc. 1980, 102, 92–98.  (a) Zhao, X.; Kopelman, R. J. Phys. Chem. 1996, 100, 11014–11018; (b) Schwartz, D. K. Ann. Rev. Phys. Chem. 2001, 52, 107–137.  (a) Pirrung, M. C. Angew. Chem., Int. Ed. Engl. 2002, 41, 1276–1289; (b) Barbey, R.; Lavanant, L.; Paripovic, D.; Schuewer, N.; Sugnaux, C.; Tugulu, S.; Klok, H.-A. Chem. Rev. 2009, 109, 5437–5527.  (a) Oh, S. J.; Cho, S. J.; Kim, C. O.; Park, J. W. Langmuir 2002, 18, 1764–1769; (b) Shircliff, R. A.; Stradins, P.; Moutinho, H.; Fennell, J.; Ghirardi, M. L.; Cowley, S. W.; Branz, H. M.; Martin, I. T. Langmuir 2013, 29, 4057–4067.  Shircliff, R. A.; Martin, I. T.; Pankow, J. W.; Fennell, J.; Stradins, P.; Ghirardi, M. L.; Cowley, S. W.; Branz, H. M. ACS Appl. Mater. Interfaces 2011, 3, 3285–3292.  Walsh, M. K.; Wang, X. W.; Weimer, B. C. J. Biochem. Biophys. Methods 2001, 47, 221–231.  Razumovitch, J.; Meier, W.; Vebert, C. Biophys. Chem. 2009, 139, 70–74.  Le Berre, V.; Trevisiol, E.; Dagkessamanskaia, A.; Sokol, S.; Caminade, A. M.; Majoral, J. P.; Meunier, B.; Francois, J. Nucleic Acids Res. 2003, 31, e88.  Jin, L.; Horgan, A.; Levicky, R. Langmuir 2003, 19, 6968–6975.  Seo, T. S.; Bai, X. P.; Ruparel, H.; Li, Z. M.; Turro, N. J.; Ju, J. Y. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 5488–5493.  (a) Liu, X. Z.; Krull, U. J. Anal. Chim. Acta 2006, 562, 1–8; (b) Mahajan, S.; Sethi, D.; Seth, S.; Kumar, A.; Kumar, P.; Gupta, K. C. Bioconjugate Chem. 2009, 20, 1703–1710.  Escorihuela, J.; Banuls, M.-J.; Puchades, R.; Maquieira, A. J. Mater. Chem. B 2014, 2, 8510–8517.  Escorihuela, J.; Banuls, M.-J.; Grijalvo, S.; Eritja, R.; Puchades, R.; Maquieira, A. Bioconjugate Chem. 2014, 25, 618–627.  Halliwell, C. M.; Cass, A. E. G. Anal. Chem. 2001, 73, 2476–2483.  Schlapak, R.; Pammer, P.; Armitage, D.; Zhu, R.; Hinterdorfer, P.; Vaupel, M.; Fruhwirth, T.; Howorka, S. Langmuir 2006, 22, 277–285.  (a) Rozkiewicz, D. I.; Gierlich, J.; Burley, G. A.; Gutsmiedl, K.; Carell, T.; Ravoo, B. J.; Reinhoudt, D. N. Chembiochem 2007, 8, 1997–2002; (b) Uszczynska, B.; Ratajczak, T.; Frydrych, E.; Maciejewski, H.; Figlerowicz, M.; Markiewicz, W. T.; Chmielewski, M. K. Lab Chip 2012, 12, 1151–1156.  Tian, J.; Ma, K.; Saaem, I. Mol. Biosyst. 2009, 5, 714–722.  Khan, M. N.; Tjong, V.; Chilkoti, A.; Zharnikov, M. J, Phys. Chem. B 2013, 117, 9929–9938.  Barbee, K. D.; Chandrangsu, M.; Huang, X. Macromol. Bioscience 2011, 11, 607–617.  Huang, F.; Zhou, X.; Yao, D.; Xiao, S.; Liang, H. Small 2015, 11, 5800–5806.  Chi, Y. S.; Jung, Y. H.; Choi, I. S.; Kim, Y. G. Langmuir 2005, 21, 4669–4673.  (a) Chow, D. C.; Lee, W. K.; Zauscher, S.; Chilkoti, A. J. Am. Chem. Soc. 2005, 127, 14122–14123; (b) Chow, D. C.; Chilkoti, A. Langmuir 2007, 23, 11712–11717.

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 Tjong, V.; Yu, H.; Hucknall, A.; Rangarajan, S.; Chilkoti, A. Anal. Chem. 2011, 83, 5153–5159.  (a) Fire, A.; Xu, S. Q. Proc. Natl. Acad. Sci. U. S. A. 1995, 92, 4641–4645; (b) Liu, D. Y.; Daubendiek, S. L.; Zillman, M. A.; Ryan, K.; Kool, E. T. J. Am. Chem. Soc. 1996, 118, 1587–1594; (c) Paul, X. H.; Lizardi, M.; Zhu, Z.; Bray-Ward, P.; Thomas, D. C.; Ward, D. C. Nat. Genet. 1998, 19, 225–232; (d) Schweitzer, B.; Wiltshire, S.; Lambert, J.; S. O’Malley, Kukanskis, K.; Zhu, Z.; Kingsmore, S. F.; Lizardi, P. M.; Ward, D. C. Proc Natl. Acad. Sci. U. S. A. 2000, 97, 10113–10119; (e) Girish Nallur, C. L., Fang, L.; Cooley, S.; Dave, V.; Lambert, J.; Kukanskis, K.; Kingsmore, S.; Lasken, R.; Schweitzer, B. Nucleic Acids Res. 2001, 29, e118.  Dirks, R. M.; Pierce, N. A. Proc. Natl. Acad. Sci. U. S. A. 2004, 101, 15275–15278.  Huang, F.; You, M.; Han, D.; Xiong, X.; Liang, H.; Tan, W. J. Am. Chem. Soc. 2013, 135, 7967–7973.  Cutler, J. I.; Auyeung, E.; Mirkin, C. A. J. Am. Chem. Soc. 2012, 134, 1376–1391.  Xiao, Z. Y.; Ji, C. W.; Shi, J. J.; Pridgen, E. M.; Frieder, J.; Wu, J.; Farokhzad, O. C. Angew. Chem., Int. Ed. 2012, 51, 11853–11857.  Jeong, J.; Kim, H.; Lee, J. B. Int. J. Mol. Sci. 2015, 16, 13653–13660.  Pei, H.; Li, F.; Wan, Y.; Wei, M.; Liu, H. J.; Su, Y.; Chen, N.; Huang, Q.; Fan, C. H. J. Am. Chem. Soc. 2012, 134, 11876–11879.  Zhang, C.; Macfarlane, R. J.; Young, K. L.; Choi, C. H. J.; Hao, L. L.; Auyeung, E.; Liu, G. L.; Zhou, X. Z.; Mirkin, C. A. Nat. Mater. 2013, 12, 741–746.  Zhao, W. A.; Gao, Y.; Kandadai, S. A.; Brook, M. A.; Li, Y. F. Angew/Chem., Int. Ed. 2006, 45, 2409–2413.  (a) Chow, D. C.; Lee, W. K.; Zauscher, S.; Chilkoti, A. J. Am. Chem. Soc. 2005, 127, 14122–14123; (b) Khan, M. N.; Tjong, V.; Chilkoti, A.; Zharnikov, M. Angew. Chem., Int. Ed. Engl. 2012, 51, 10303–10306.  Hyun, J.; Kim, J.; Craig, S. L.; Chilkoti, A. J. Am. Chem. Soc. 2004, 126, 4770–4771.  (a) Castronovo, M.; Radovic, S.; Grunwald, C.; Casalis, L.; Morgante, M.; Scoles, G. Nano Lett. 2008, 8, 4140–4145; (b) Castronovo, M.; Lucesoli, A.; Parisse, P.; Kurnikova, A.; Malhotra, A.; Grassi, M.; Grassi, G.; Scaggiante, B.; Casalis, L.; Scoles, G. Nat. Commun. 2011, 2, 297.  Tjong, V.; Tang, L.; Zauscher, S.; Chilkoti, A. Chem. Soc. Rev. 2014, 43, 1612–1626.  (a) Tjong, V.; Yu, H.; Hucknall, A.; Rangarajan, S.; Chilkoti, A. Anal. Chem. 2011, 83, 5153–5159; (b) Tjong, V.; Yu, H.; Hucknall, A.; Chilkoti, A. Anal. Chem. 2013, 85, 426–433.  Ji, H.; Yan, F.; Lei, J.; Ju, H. Anal. Chem. 2012, 84, 7166–7171.  Sassolas, A.; Leca-Bouvier, B. D.; Blum, L. J. Chem. Rev. 2008, 108, 109–139.  Shin, J. S.; Pierce, N. A. J. Am. Chem. Soc. 2004, 126, 10834–10835.

References

 (a) Zhang, J.; Lang, H. P.; Huber, F.; Bietsch, A.; Grange, W.; Certa, U.; McKendry, R.; Guntherodt, H. J.; Hegner, M.; Gerber, C. Nat. Nanotechnol. 2006, 1, 214–220; (b) Mukhopadhyay, R.; Lorentzen, M.; Kjems, J.; Besenbacher, F. Langmuir 2005, 21, 8400–8408.  Shu, W.; Liu, D.; Watari, M.; Riener, C. K.; Strunz, T.; Welland, M. E.; Balasubramanian, S.; McKendry, R. A. J. Am. Chem. Soc. 2005, 127, 17054–17060.  Mertens, J.; Rogero, C.; Calleja, M.; Ramos, D.; Martin-Gago, J. A.; Briones, C.; Tamayo, J. Nat. Nanotechnol. 2008, 3, 301–307.  Wang, S.; Liu, H.; Liu, D.; Ma, X.; Fang, X.; Jiang, L. Angew. Chem., Int. Ed. Engl. 2007, 46, 3915–3917.  Mao, Y.; Liu, D.; Wang, S.; Luo, S.; Wang, W.; Yang, Y.; Ouyang, Q.; Jiang, L. Nucleic Acids Res. 2007, 35, e33.  Ramachandran, N.; Hainsworth, E.; Bhullar, B.; Eisenstein, S.; Rosen, B.; Lau, A. Y.; Walter, J. C.; LaBaer, J. Science 2004, 305, 86–90.  He, M.; Stoevesandt, O.; Palmer, E. A.; Khan, F.; Ericsson, O.; Taussig, M. J. Nat. Methods 2008, 5, 175–177.  Buxboim, A.; Daube, S. S.; Bar-Ziv, R. Mol. Syst. Biol. 2008, 4, 181.  Daube, S. S.; Bracha, D.; Buxboim, A.; Bar-Ziv, R. H. Proc. Natl. Acad. Sci. U. S. A. 2010, 107, 2836–2841.  Bar, M.; Bar-Ziv, R. H. Nano Lett. 2009, 9, 4462–4466.  Karzbrun, E.; Tayar, A. M.; Noireaux, V.; Bar-Ziv, R. H. Science 2014, 345, 829–832.  Chen, C. E.; Pu, F.; Huang, Z. Z.; Liu, Z.; Ren, J. S.; Qu, X. G. Nucleic Acids Res. 2011, 39, 1638–1644.  He, D. G.; He, X. X.; Wang, K. M.; Cao, J.; Zhao, Y. X. Adv. Funct. Mater. 2012, 22, 4704–4710.  Ruiz-Hernandez, E.; Baeza, A.; Vallet-Regi, M. ACS Nano 2011, 5, 1259– 1266.  Choi, C. H. J.; Hao, L. L.; Narayan, S. P.; Auyeung, E.; Mirkin, C. A. Proc. Natl. Acad. Sci. U. S. A. 2013, 110, 7625–7630.  Seferos, D. S.; Prigodich, A. E.; Giljohann, D. A.; Patel, P. C.; Mirkin, C. A. Nano Lett. 2009, 9, 308–311.  Rosi, N. L.; Giljohann, D. A.; Thaxton, C. S.; Lytton-Jean, A. K. R.; Han, M. S.; Mirkin, C. A. Science 2006, 312, 1027–1030.  Radovic-Moreno, A. F.; Chernyak, N.; Mader, C. C.; Nallagatla, S.; Kang, R. S.; Hao, L. L.; Walker, D. A.; Halo, T. L.; Merkel, T. J.; Rische, C. H.; Anantatmula, S.; Burkhart, M.; Mirkin, C. A.; Gryaznov, S. M. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 3892–3897.  Zhang, X. Q.; Xu, X. Y.; Lam, R.; Giljohann, D.; Ho, D.; Mirkin, C. A. ACS. Nano 2011, 5, 6962–6970.  Zhang, K.; Hao, L. L.; Hurst, S. J.; Mirkin, C. A. J. Am. Chem. Soc. 2012, 134, 16488–16491.

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22 DNA Brushes: Advances in Synthesis and Applications

 Song, Y.; Xu, X. Y.; MacRenaris, K. W.; Zhang, X. Q.; Mirkin, C. A.; Meade, T. J. Angew. Chem., Int. Ed. 2009, 48, 9143–9147.  Seferos, D. S.; Giljohann, D. A.; Hill, H. D.; Prigodich, A. E.; Mirkin, C. A. J. Am. Chem. Soc. 2007, 129, 15477–15479.  Halo, T. L.; McMahon, K. M.; Angeloni, N. L.; Xu, Y. L.; Wang, W.; Chinen, A. B.; Malin, D.; Strekalova, E.; Cryns, V. L.; Cheng, C. H.; Mirkin, C. A.; Thaxton, C. S. Proc. Natl. Acad. Sci. U. S. A. 2014, 111, 17104–17109.  Zheng, D.; Seferos, D. S.; Giljohann, D. A.; Patel, P. C.; Mirkin, C. A. Nano Lett. 2009, 9, 3258–3261.

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 Membrane Materials Form Polymer Brush Nanoparticles Erica Green, Emily Fullwood, Julieann Selden, and Ilya Zharov Department of Chemistry, Salt Lake City, UT, USA

. Introduction Nanoporous membranes are important in fundamental studies of molecular transport and molecule–surface interactions in confined spaces,1 and are used in separations2,3 and sensing.4,5 They have been prepared using a variety of methods including lithography,6 anodic oxidation of aluminum membranes,7 track etching of polymers,8 surfactant-directed self-assembly,9 and by templating colloidal crystals.10,11 Responsive nanoporous membranes are produced by modifying the nanopore surfaces with a molecular layer containing chargeable moieties that are capable of electrostatic interactions12 with the diffusing species. Alternatively, controlled molecular transport has been achieved by modifying the nanopore surfaces with polymer molecules that respond to environmental stimuli.13 A number of responsive nanoporous membranes have been prepared using self-assembled polymeric membranes,9 zeolites,14 silicon nitride membranes,6 and nanotubes.15,16 Such membranes can be used in separations of biomacromolecules17 and drug molecules,18 in controlled release and drug delivery systems,19,20 and in chemical sensors.4 These applications result from the ability to control the molecular transport through the nanopores. An ideal responsive membrane should contain pores whose size and surface chemistry can be easily controlled to impart the controlled transport for a wide range of species, while simultaneously maintaining high transport rates of desired molecules. Therefore, silica colloidal crystals are promising materials for nanoporous membranes because they form through self-assembly, allow straightforward control of nanopore size in a broad range, possess exceptionally high molecular throughput, and offer a large variety of surface chemistries.

Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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23 Membrane Materials Form Polymer Brush Nanoparticles (a)

(b)

Figure . SEM images of silica colloidal membranes prepared on glass from 440 nm diameter silica spheres deposited (a) top view (size bar 4 μm) and (b) side view (size bar 2 μm). The geometric projection of a pore observed from the (111) plane is outlined in the inset in (a). Source: Newton et al. 2005.21 Reproduced with permission of American Chemical Society.

Silica colloidal crystals comprise a close-packed face-centered cubic (fcc) lattice of amorphous nonporous silica spheres of a submicrometer diameter (Figure 23.1) with ordered arrays of interconnected three-dimensional nanoscale voids.21,22 The preparation of silica spheres is straightforward,23 self-assembly of the spheres is well developed,24 and pore size in the crystals can be readily controlled by selecting the sphere size (the distance from the center of the nanopore projection to the nearest silica sphere surface is ca. 15% of the sphere radius). Surface silanol groups can be directly modified by nucleophilic silylation to introduce a variety of functional groups.25 Alternatively, silica surface can be first modified with 3-aminopropyltriethoxysilane, followed by treatment with organic molecules carrying electrophilic moieties such as acyl chloride, isocyanate, isothiocyanate, carboxylic acid, sulfonyl chloride,26 or succinimidyl ester.27 3-Aminopropyltriethoxysilane-treated silica can also be modified with 2-bromoisobutyrylbromide,28 which can serve as atom transfer radical polymerization initiator.29 This provides the possibility of growing various polymer brushes on the silica surface.30–33 In this chapter, we summarize our work on the preparation and studies of polymer brush-filled colloidal nanopores using polyacrylamide,34 temperature-responsive poly(N-isopropylacryl amide) (PNIPAAM),35 pH- and ion-responsive poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA),36,37 and temperature-responsive poly(l-alanine).38 We also describe the prepared and properties of proton-conducting silica colloidal membranes by modifying the surface of the nanopores with sulfonated polymer brushes.39 This approach includes the formation of silica colloidal crystals followed by pore filling by surface-initiated atom transfer radical or and ring-opening polymerization (SI-ATRP).

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes

We also summarize our new approach to the preparation of porous40 and proton conducting39 membranes, by self-assembly of PBNPs. In this approach, polymer brushes are first grown on the silica nanoparticle surface, followed by the assembly of the corresponding membrane materials from the colloidal solution via casting or precipitation.

. Colloidal Membranes Pore-Filled with Polymer Brushes ..

Preparation of Silica Colloidal Membranes

Silica colloidal membranes can be assembled on a hydrophilic solid support (glass, oxidized silicon, metal, etc.) using the vertical deposition technique.24 In this technique, silica colloidal membranes are deposited onto the support, which is placed vertically into colloidal solution of silica spheres in ethanol. The solutions are allowed to evaporate for 2–3 days in a vibration-free environment. The thickness of the colloidal membrane is controlled by varying the weight percent of silica spheres in the colloidal solution. Suspended silica colloidal membranes can be prepared in silicon41 and glass42 supports. For example, silica colloidal membranes suspended in silicon have been prepared using a 0.3-mm thick Si(100) wafer, with an array of openings shaped as a truncated square pyramid with 50 × 50 μm smaller dimension or a single 40 × 40, 100 × 100, 250 × 250, or 500 × 500 μm opening. Silica colloidal membranes suspended in glass can be prepared in a similar way42 using a 150-μm-thick glass slide. The free-standing silica colloidal membranes43 can be prepared as a material that is mechanically durable and possesses large area (Figures 23.2 and 23.3). To prepare such membranes, the silica spheres assembled into a colloidal crystal (a)

(b)

Figure . SEM images of sintered colloidal crystals comprised of 180 nm silica spheres: (a) SEM image showing no major cracks over a large area (size bar = 50 μm); (b) magnified image displaying the close-packed fcc lattice (size bar = 2.5 μm). Source: Bohaty et al. 2009.43 Reproduced with permission of American Chemical Society.

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23 Membrane Materials Form Polymer Brush Nanoparticles

Figure . Photographs of sintered silica colloidal membranes. Photographs of nanofrits. Top: without PTFE washers showing the sintered colloidal frit in the epoxy. Bottom: with PTFE washers. Source: Bohaty et al. 2009.43 Reproduced with permission of American Chemical Society.

are physically bonded together by sintering at 1050◦ C, at which temperature silica flows at the surface. The preparation process is time- and cost-effective. The sintering process can be followed by rehydroxylation with tetrabutylammonium hydroxide in water to restore the surface hydroxyl groups.44 The diffusional flux of the Fe(bpy)3 2+ ion through free-standing silica colloidal membranes in acetonitrile is on the order of 3.6 × 10−10 mol/s/cm2 and is in good agreement with the calculated flux, confirming that the membranes are crack-free and have no major defects.45 Finally, we found that gold-coated silica spheres can be self-assembled into colloidal crystals using the same techniques.45 Moreover, these colloidal crystals can be sintered to obtain free-standing membranes (Figure 23.4) with gold-coated nanopores, suitable for further surface modifications using thiol chemistry.

..

PAAM Brush-Filled Silica Colloidal Membranes

We modified the surface of nanopores in colloidal films assembled from 205 nm silica spheres, with poly(acrylamide) (PAAM) brushes using SI-ATRP. First,

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes

(a)

(b)

Figure . SEM images of (a) self-assembled and (b) sintered SiO2 @Au colloidal membranes. Scale bar is 2 μm in (a) and 5 μm in (b). Source: Ignacio-de Leon and Zharov 2013.45 Reproduced with permission of American Chemical Society.

silica surface was modified with initiator moieties, followed by the polymerization of acrylamide in the presence of copper(I) chloride and bipyridine. We found that the colloidal crystal lattice remained unperturbed by the polymerization. The polymer brush thickness could be controlled by polymerization time and was monitored by measuring the flux of redox species across the nanopores using cyclic voltammetry. The nanopore size and polymer brush thickness were calculated based on the limiting current change. Polymer brush thickness increased in the course of 26 h of polymerization in a logarithmic manner from 1.3 to 8.5 nm leading to nanopores as small as 7.5 nm.46 ..

PDMAEMA Brush-Filled Silica Colloidal Membranes

We performed SI-ATRP of 2-(dimethylamino)ethyl methacrylate (DMAEMA) inside the nanopores of the silica colloidal membranes composed of 255 nm silica spheres (Figure 23.5) and studied the molecular transport across the resulting PDMAEMA-modified nanoporous membranes as a function of pH and salt concentration for positively charged (Ru(NH3 )6 3+ ) and neutral (Fc(CH2 OH)2 ) redox-active species.36 The polymer brush thickness inside the nanopores after 20 h of polymerization was estimated as 10–15 nm, thus filling most of the void inside the colloidal crystal.36 PDMAEMA is a well-studied environmentally responsive polymer47 whose behavior is governed by both electrostatic and hydrophobic interactions. It has been found that at low pH electrostatic repulsions between the protonated tertiary amine groups lead to PDMAEMA swelling.48 In contrast, at high pH most of the amine groups are deprotonated and neutral. As a result, interactions between nonpolar groups in a polar solvent (hydrophobic interactions) are increased, which leads to a more compact PDMAEMA conformation. In addition, PDMAEMA exhibits ion-responsive behavior. Increasing ionic strength

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23 Membrane Materials Form Polymer Brush Nanoparticles

A

Figure . Modification of silica spheres with PDMAEMA brushes. Source: Schepelina and Zharov 2008.36 Reproduced with permission of American Chemical Society.

of the solution leads to a collapsed conformation of PDMAEMA at acidic pH as a result of charge screening in the protonated polymer.49 First, we studied the pH response for PDMAEMA-modified colloidal membranes using positively charged Ru(NH3 )6 3+ . As can be seen in Figure 23.6a, the limiting current for PDMAEMA-modified colloidal membrane electrodes is highly dependent on pH. This effect is summarized in Figure 23.7. The limiting current increases with increasing pH by ∼80% with an abrupt change at pH ∼4–5. For polyelectrolyte brushes, the chain conformation is governed by the electrostatic interactions between the charged monomer units.50 At high pH, the amine groups of the polymer chains are deprotonated, and the polymer is considered to be neutral. As a result, the polymer chains tend to attain a collapsed (a)

(b) 6

–5

ilim, nA



4 –15 2 –25 –0.45

–0.25

–0.05

0.15

0

0

0.25

0.5

E, V vs Ag/AgCI

Figure . Representative voltammetric responses for PDMAEMA-colloidal film Pt electrodes (20 h polymerization) at different pH for Ru(NH3 )6 3+ (a) and for Fc(CH2 OH)2 (b). Voltammograms recorded above pH 5 are at the bottom in (a) and at the top in (b). Voltammograms recorded below pH 4 are at the top in (a) and at the bottom in (b). Source: Schepelina and Zharov 2008.36 Reproduced with permission of American Chemical Society.

ilim, nA

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes –30 (a)

–25

–20

–15

–10

–5 0

1

2

3

4

5

6

7

8

9 10

(b)

0

1

2

3

4

5

6

7

8

9 10

pH

Figure . Limiting current (Ru(NH3 )6 3+ ) as a function of increasing pH for PDMAEMA-colloidal membrane Pt electrodes for 5 h (a) and for 20 h (b) polymerizations. Source: Schepelina and Zharov 2008.36 Reproduced with permission of American Chemical Society.

conformation due to the hydrophobic interactions, and the diffusion of the positively charged Ru(NH3 )6 3+ is not sterically hindered by the polymer brush, nor it is repelled electrostatically. In contrast, at low pH, the polymer brush becomes protonated and stretches away from the surface as a result of electrostatic repulsions between the charged monomer units and between the polymer chains. The diffusion of Ru(NH3 )6 3+ in this case is blocked as a result of both electrostatic repulsion from the positively charged polymer chains and as a result of the steric hindrance. To isolate the pH effect on the polymer conformation, we examined the diffusion of a neutral redox-active molecule, Fc(CH2 OH)2 , assuming that it does not electrostatically interact with the polymer chains. The limiting current of Fc(CH2 OH)2 decreased only by ∼30% (Figure 23.6b) with no abrupt change when pH was lowed from neutral to acidic, which should result exclusively from the conformational changes in the PDMAEMA chains. The influence of the solution ionic strength on the diffusion across the protonated PDMAEMA-modified colloidal membranes was studied by measuring the limiting current of Ru(NH3 )6 3+ and Fc(CH2 OH)2 as a function of KCl concentration. Figure 23.8 shows the dependence of the limiting current of Ru(NH3 )6 3+ on the salt concentration at pH 2.4 where the PDMAEMA chains are protonated. The limiting current increases with increasing KCl concentration. The addition of KCl progressively screens the charge within the polymer brush. As a result, the diffusion of Ru(NH3 )6 3+ becomes easier. Similar results were observed for the colloidal membranes obtained after both 5 and 20 h polymerization. These results are in good agreement with previously reported studies for PDMAEMA-modified surfaces.50–53 No significant effect of the salt concentration on the diffusion of Fc(CH2 OH)2 at low pH was observed, which suggests that, under these conditions, the conformation of the PDMAEMA

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23 Membrane Materials Form Polymer Brush Nanoparticles

ilim, nA

–5

0M 0.005 M 0.05 M 0.1 M 0.5 M

–15

–25 –0.6

–0.3

0

E, V vs. Ag/AgCI Figure . Voltammetric responses of the protonated PDMAEMA-modified colloidal film Pt electrodes for Ru(NH3 )6 3+ reduction in water as a function of KCl concentration. KCl concentrations are shown adjacent to each voltammogram. Source: Schepelina and Zharov 2008.36 Reproduced with permission of American Chemical Society.

chains is not significantly affected by the salt concentration.52,53 Both pHand ion-dependent changes in limiting current were reversible (Figure 23.9). The nanoporous PDMAEMA-modified colloidal membranes can be cycled between the low and high pH and between the low and high ionic strength regimes without apparent loss of responsiveness. To demonstrate that the “weak” polyelectrolyte PDMAEMA can be converted into a “strong” polyelectrolyte brush with a fixed, pH-independent number of charges, the quaternization of the polymer by treating the PDMAEMA–colloidal membrane electrodes with ethyl bromide was performed.54 Figure 23.10 shows the limiting current of Ru(NH3 )6 3+ and Fc(CH2 OH)2 of these quaternized electrodes at neutral pH. 0 (a)

(b) –7

ilim, nA



–10 –12

–20 –30

–17 0

1

2

3

4

5

6

7

8

0

1

2

3

4

5

6

7

Cycle number

Figure . (a) Limiting current (Ru(NH3 )6 3+ ) at pH 2 (squares) and at pH 7 (diamonds). (b) Limiting current (Ru(NH3 )6 3+ ) at 0.05 M KCl (squares) and at 0 M KCl (diamonds) at pH 2 for PDMAEMA-colloidal film Pt electrodes after polymerization for 5 h. Source: Schepelina and Zharov, 2008.36 Reproduced with permission of American Chemical Society.

8

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes (a)

(b)

ilim, nA

–5

4

–15 2 –25 –35 –0.55

0 –0.2

0.15

0

0.3

0.6

E, V vs Ag/AgCI

Figure . Representative voltammetric responses of Ru(NH3 )6 3+ (a) and Fc(CH2 OH)2 (b) of PDMAEMA-colloidal film Pt electrodes (5 h polymerization). Before quaternization (bottom), after quaternization (top), and after quaternization in the presence of 0.5 M KCl (top) in (a). Before quaternization (top), after quaternization (bottom), and after quaternization in the presence of 0.5 M KCl (middle) in (b). Source: Schepelina and Zharov 2008.36 Reproduced with permission of American Chemical Society.

The limiting current of both neutral and positively charged species decreased significantly after the quaternization. This suggests that the polymer brush carrying a large number of quaternary ammonium ions blocks the diffusion of ions and molecules almost completely regardless of pH as a result of both strong electrostatic repulsion and steric hindrance. Next, the limiting current for Ru(NH3 )6 3+ and Fc(CH2 OH)2 at 0.5 M concentration of KCl was measured. No significant change in the limiting current for Ru(NH3 )6 3+ with increased ionic strength (Figure 23.10a) was observed, indicating that the positive charge of the quaternized polymer brush was not significantly screened under these conditions. At the same time, for Fc(CH2 OH)2 a ∼10% increase in the limiting current was observed (Figure 23.10b), suggesting small conformational changes in the polymer chains. This result is in agreement with the previously reported data,50 which demonstrated that at moderate salt concentration the thickness of strong polyelectrolyte brushes does not strongly depend on the salt concentration due to the osmotic pressure of the counterions trapped inside the brush (osmotic brush regime). PDMAEMA brushes were also prepared on the surface of nanopores in freestanding silica colloidal membranes,37 which showed similar pH-responsive transport behavior.37 The silica surface was functionalized using the reaction of the surface amines with 2-bromoisobutyryl bromide, in order to attach the ATRP initiator moieties to the nanopore surface, followed by the ATRP of DMAEMA inside the nanopores of the free-standing membranes, as described above. In order to estimate the molecular weight of the polymer chains formed on the nanopore surface, we carried out the polymerization of DMAEMA in the presence of a small amount of sacrificial initiator in solution. This resulted in

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23 Membrane Materials Form Polymer Brush Nanoparticles 6.E-05 M / mol L–1



2.E-05

(a)

3.E-05

0.E+00 0.E+00

(b)

1.E-05

5.E+03

1.E+04

0.E+00 0.E+00

5.E+03

1.E+04

Time / s

Figure . Diffusion rates of Fe(bpy)3 2+ through PDMAEMA-modified colloidal membranes (180 nm silica spheres) with (black) and without (gray) 50 mM trifluoroacetic acid after (a) 16 h and (b) 22 h of polymerization. Source: Schepelina et al. 2010.37 Reproduced with permission of John Wiley & Sons, Inc.

the formation of a free polymer, which we isolated and analyzed by gel permeation chromatography. We assumed that the chains formed at the surface and those formed in solution have similar molecular weights. The Mw PDMAEMA after 16 h of polymerization was 19.9 kDa, after 22 h was 23.3 kDa, and after 44 h was 30.5 kDa. We used Fe(bpy)3 2+ diffusion measurements to determine the responsive behavior of the PDMAEMA-modified membranes with and without trifluoroacetic acid present in the diffusing solution. Figure 23.11 shows the diffusion rate of Fe(bpy)3 2+ with and without trifluoroacetic acid present in solution through the membranes polymer-modified at different polymerization times. For the membrane modified with the polymer for 16 h, the diffusion rate of Fe(bpy)3 2+ was by 42% (2.4 times) lower in the presence of 50 mM trifluoroacetic acid than in the absence of the acid. After a longer polymerization time (22 h), the acid-controlled response of the membrane increased greatly (Figure 23.11) and the diffusion blockage reached 95%. The switching behavior of the polymer-modified colloidal membranes was reversible as was shown by cycling between “on” and “off ” states. In addition to the polymer length, increasing the membrane thickness as well as decreasing the nanopore size enhanced the gating behavior and allowed to achieve the complete acid-controlled gating. ..

PNIPAAM brush-filled silica colloidal membranes

Poly(N-isopropylacrylamide), PNIPAAM, is a well-known temperature responsive polymer55 that has been used in the preparation of thermoresponsive membranes.56,57 We modified colloidal membrane nanopores of 15 nm “radius” with PNIPAAM brushes (Figure 23.12)58 of 9.3 nm estimated thickness and measured the temperature response for the membranes modified

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes

Figure . Preparation of PAAM and PNIPAAM brushes on silica sphere surface.

at different polymerization times. As can be seen in Figure 23.13, two types of response are observed. For nanoporous membranes modified with a thin polymer brush, the limiting current increases with temperature, with a gradual change at ca. 29◦ C (Figure 23.13a). For colloidal membranes modified with thick polymer brush, a reverse change is observed, where the limiting current decreases with increasing temperature, with an abrupt change at ca. 29◦ C (Figure 23.13b). These results are consistent with two types of PNIPAAM morphologies56,57 inside the nanopores, which lead to two types of molecular transport mechanisms through these nanopores. We believe that when N-isopropylacrylamide is polymerized for a short period of time, it forms a dense brush (Figure 23.14a). Transport through such nanopores mainly happens in the polymer-free volume of the nanopores. With rising temperature, the conformation of the polymer chains inside the brush changes in such a way that the brush shrinks, providing a larger volume for diffusion (Figure 23.14a), which results in the observed increase in molecular transport. When polymerization is conducted for a sufficiently long time, polymer chains in the brushes growing from the opposite nanopore wall become long enough to meet and interpenetrate. This leads to a highly porous and permeable hydrogel structure (Figure 23.14b). When

ilim, nA

–6.5 (a)

–12.0

–4.0

(b)

–6.0

–1.5

0.0 19

23

27

31

35

39

22

26

30

34

38

42

Temperature, °C

Figure . Limiting current (Ru(NH3 )6 3+ ) as a function of increasing temperature for PNIPAAM-opal membrane Pt electrodes after polymerization for (a) 15 min and (b) 90 min. Source: Schepelina and Zharov 2007.35 Reproduced with permission of American Chemical Society.

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23 Membrane Materials Form Polymer Brush Nanoparticles

(a)

(b)

Figure . Schematic representation of the processes that occur upon heating and cooling of a (a) PNIPAAM brush (15 min polymerization) and (b) PNIPAAM gel (90 min polymerization) inside a colloidal nanopore. Source: Schepelina and Zharov 2007.35 Reproduced with permission of American Chemical Society.

the temperature is increased, the hydrogel becomes dehydrated and impermeable to aqueous permeants (Figure 23.14b), but it does not shrink to open the nanopores.56,57

..

Polyalanine Brush-Filled Silica Colloidal Membranes

Poly(l-alanine) brushes, which been shown to respond to solvent polarity, ionic strength, and temperature,59 were grown with 12 nm thickness on the surface of the 19-nm “radius” nanopores in silica colloidal membranes assembled on Pt electrodes (opal electrodes) according to Figure 23.15.38 Polymerization time was varied from 7 min to 6 h to create polymer brushes of various lengths. We investigated the temperature-response for poly(l-alanine)-modified colloidal membranes using cyclic voltammetry. To exclude the possibility that the observed changes in the molecular transport would result from electrostatic effects [60], the temperature response of colloidal membrane electrodes for a neutral molecule, Fc(CH2 OH)2 , was examined. As can be seen in Figure 23.16, the limiting current for the modified colloidal membrane electrodes is indeed affected by temperature. For all colloidal membrane electrodes, the limiting current slightly increased with increasing temperature as a result of

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Figure . Modification of silica spheres by poly(L-alanine) brushes. Source: Abelow and Zharov 2009.38 Reproduced with permission of Royal Society of Chemistry.

23 Membrane Materials Form Polymer Brush Nanoparticles

10

8

(a)

8

(b)

6

6 4

4

2

2

ilim, nA



0

0 0 1

0.2

0.4

0 1

(c)

0.8

20

40

60

80 100

(d)

0.8

0.6

0.6

0.4

0.4

0.2

0.2

0

0

–0.2 0

0.2

0.4

E vs. Ag/AgCI, V

0

20

40 60 80 Temperature, °C

100

Figure . (a–c) Representative voltammetric responses, and (b–d) plots of Fc(CH2 OH)2 limiting current as a function of temperature for poly(L-alanine)-colloidal membrane Pt electrodes after polymerization for 1 and 3 h, respectively. Source: Abelow and Zharov 2009.38 Reproduced with permission of Royal Society of Chemistry.

increasing diffusion coefficient. We observed the effect of temperature for both thin and thick polymer brushes. For nanoporous membranes modified with a thinner polymer brush (polymerization time of 1 h), the limiting current increased with increasing temperature, with a transition temperature of ca. 65◦ C (Figures 23.16a and 23.16b). For colloidal membranes modified with thick polymer brushes (polymerization time of 3 h), a temperature response was observed at a higher temperature of ca. 75◦ C (Figures 23.16c and 23.16d). The temperature-dependent change in the limiting current was reversible for both types of polymer brush. The poly(l-alanine)-modified membranes could be cycled between low and high temperatures without apparent loss of responsiveness (Figure 23.17). pH-responsive behavior of poly(l-alanine)-modified silica colloidal membranes was also investigated. As can be seen in Figure 23.18, the limiting current for the poly(l-alanine)-modified colloidal membrane electrodes is affected by pH, where the limiting current increases with increasing pH. This suggests that as pH is increased the polymer brush shrinks, increasing the nanopore size. This reverse is also true, as the limiting current decreases for decreasing pH indicating an elongation of the polymer brush. This trend, while smaller for

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes

ilim, nA

10

100

(a)

5

(b)

50

0

0 0

1

2

3

4

0

1

2

3

4

Cycle number

Figure . Limiting current at low (diamonds) and high (squares) temperature for poly(L-alaniny)-colloidal membrane Pt electrodes (Fc(CH2 OH)2 ) after polymerization for (a) 1 h and (b) 3 h, as a function of temperature cycling. Source: Abelow and Zharov 2009.38 Reproduced with permission of Royal Society of Chemistry.

colloidal membranes carrying thinner polymer brushes, is consistent for the entire range of polymer brush sizes studied. The pH-dependent change in the limiting current is reversible for polymer brushes of various lengths. The nanoporous polymer–modified colloidal membranes could be cycled between low and high pH. The observed pH-responsive behavior may originate from the electrostatic interaction between the amine end groups of the poly(l-alanine) brush and the residual amines on the silica surface in a manner similar to that described earlier for other types of surface-immobilized polymer brushes.61 To confirm this hypothesis, we estimated the grafting density of poly(l-alanine) 8

5

(a)

(b)

ilim, nA

6 3

4 2

1

0 –2

–1 0

0.2

0.4

0 E, V vs. Ag/ACI

0.2

0.4

Figure . (Fc(CH2 OH)2 ) voltammetric responses for colloidal film Pt electrodes surface-modified with poly-L-alanine for 3 h (a) and 6 h (b) at pH 3 (bottom line), pH 6 (middle line), and pH 8 (top line). Source: Abelow and Zharov 2009.38 Reproduced with permission of Royal Society of Chemistry.

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23 Membrane Materials Form Polymer Brush Nanoparticles

O S

( )9 O

Br

+

ONa O

CuBr, CuBr2 2,2′-bipy pH 9

O S

( )9 O

(

)

n COOH

Figure . Reaction scheme for pore filling of the Au-coated SiO2 membrane with PMAA by ATRP.

chains on the silica spheres using thermogravimetric analysis (TGA). Based on its results, we estimated the grafting density as 0.61 chains per nm2 , well below the maximum grafting density that can be achieved for the silica surface, thus supporting our explanation of the pH-responsive behavior observed for the poly(l-alanine)-modified colloidal nanopores. ..

PMMA Brush-Filled SiO @Au Colloidal Membranes

We prepared PMAA brush-filled gold-coated colloidal membranes by ATRP (Figure 23.19).45 The sintered membranes prepared from SiO2 @Au nanoparticles were first modified with a SAM of the initiator then soaked in a mixture of the methacrylate monomer and copper(I) catalyst for various reaction times to prepare polymer brush-filled membranes (Figure 23.20). To determine if PMAA polymer brushes inside the nanopores respond to variations in the pH, we studied the diffusion of a neutral dye so that any changes in flux could be attributed solely to changes in polymer conformation. Diffusion experiments of ferrocenecarboxaldehyde, Fc(CHO), through PMAA brush-filled SiO2 @Au membranes (Figure 23.20) showed that there was a general trend of decreasing rate of diffusion with increasing length of polymer chains within the nanopores (Figure 23.21). The incorporation of polymers inside the nanopores resulted in a decrease in the effective size of the Figure . SEM image of sintered Au-coated SiO2 membrane with PMAA brush grown for 30 min (scale bar = 2.5 μm). Source: Ignacio-de Leon and Zharov 2013.45 Reproduced with permission American Chemical Society.

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes 3E-06

Moles transported/cm2

3E-06 2E-06 2E-06 1E-06 5E-07 0E+00 0

20000

40000

60000

80000

Time (s)

Figure . Comparison of the flux of ferrocenecarboxaldehyde through Au-coated membranes filled via ATRP for 10 min (with acid, 1; without acid, 2) and 30 min (with acid, 3; without acid, 4). Source: Ignacio-de Leon and Zharov 2013.45 Reproduced with permission American Chemical Society.

nanopores as they became partially blocked. Comparison of the calculated Deff values showed that diffusion was hindered by a factor of 7.6 as the polymer brushes inside the nanopores were allowed to grow for 30 min. We did not observe complete blockage of the nanopores even as ATRP time was extended to 45 min, as the calculated Deff values for Fc(CHO) in 30- and 40-min ATRPmodified membranes were similar. Upon addition of 50 mM trifluoroacetic acid to the diffusion solution, we observed an increased flux for Fc(CHO) through the membranes (Table 23.1). This was consistent with the known behavior of PMAA in response to pH changes. In the absence of TFA, the polymer brushes are extended as a result Table . Summary of calculated average diffusion coefficients for ferrocenecarboxaldehyde through PMAA brush-filled gold-coated membranes. Selectivity was calculated as the ratio of coefficients with and without acid. Deff (cm /s × − ) Filled with PMAA brush for

Without acid

With acid

Selectivity

10 min

6.7 ± 0.6

16 ± 2

2.4

30 min

0.9 ± 0.2

12 ± 2

14

45 min

0.9 ± 0.3

10 ± 3

12

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23 Membrane Materials Form Polymer Brush Nanoparticles

of a significant degree of dissociation of the –COOH groups. Indeed, the solution pH ∼7 was higher than the pKa ∼5 of PMAA.62 In the presence of TFA, deprotonation along the chain is suppressed and thus the polymers undergo deswelling as electrostatic repulsions are minimized. The PMAA chains fold and collapse closer to the surface, thus leading to larger nanopore openings. The calculated selectivities, obtained as the ratio of Deff in the presence and absence of acid (Table 23.1), demonstrated that the PMAA brushes within the nanopores were indeed pH responsive. pH permselectivity was only slightly higher than 2 for the Au-coated membrane after 10 min ATRP, but increased to 14 after the 30-min ATRP. .. Colloidal Membranes Filled with Polymers Brushes Carrying Chiral Groups We prepared nanoporous silica colloidal films whose nanopores were filled with polymer brushes containing chiral selector moieties in the side chains.63 These polymer brushes were grown in ∼50 nm length on the surface of the nanopores using surface-initiated polymerization of monomers 1–4 (Figure 23.22). We used cyclic voltammetry to investigate the enantioselectivity of the resulting pore-filled films by measuring the limiting current of the redox-active chiral for the electrodes coated with the colloidal films. The enantioselectivity of the poly(1R)-filled colloidal films, defined as the ratio of the limiting currents for the S and R enantiomers of chiral probe 2, was 2.2. This is similar to the enantioselectivity found for colloidal films modified with a monolayer of chiral selectors analogous to 1R.64 We observed a reversal of enantioselectivity when the chirality of the selector moieties attached to the polymer was changed from R to S. A similar behavior has been observed previously for colloidal films modified with monolayers of chiral selector moieties,64 which confirmed that

Figure . Chiral monomers used in ATRP modification of colloidal films and chiral probe used for cyclic voltammetry measurements.

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes

similar phenomena are taking place once the pores are filled with polymers carrying chiral selectors in their side chains. Poly(2R)-filled colloidal films, most similar in structure to poly(1R)-filled films, showed a similar enantioselectivity (1.80), whereas poly(3S)- and poly(4S)-filled films showed a lower enantioselectivity (1.20 and 1.69, respectively), likely due to their reduced ability for noncovalent interactions. Overall, these permselectivity values are comparable to those observed in monolayer-modified colloidal films,64,65 antibody-modified nanotubes membranes,66 and compare favorably to the results obtained for optically active polyelectrolyte membranes.67 We also prepared free-standing nanoporous silica colloidal membranes porefilled with chiral polymer brushes using monomer 1R,68 using ATRP. We measured the diffusion rates of enantiomers through these membranes and calculated the corresponding permselectivity, which was ∼1.3. We studied the diffusion of both enantiomers of a chiral dye through the membranes pore-filled with poly(1R) grown for 40 min. The resulting selectivity of 1.3 was lower than the 1.8 selectivity obtained for the corresponding thin colloidal films, which may be attributed to the lower grafting density. The higher number of chiral selectors inside the nanopores filled with polymer brushes compared to those modified with chiral selector monolayer, as well as preventing through-solution diffusion by pore-filling with polymers did not lead to increased enantioselectivity. This suggests that the enantioselectivity of chiral colloidal nanopores depends solely on the energy difference between the enantiomer/selector complexes and that through-solution diffusion plays a minor role in the transport of enantiomers, which occurs predominantly through site hopping. ..

pSPM and pSSA Brush-Filled Colloidal Nanopores

We prepared sintered silica colloidal membranes pore-filled with poly(3sulfopropyl-methacrylate) (pSPM) and poly(stryrenesulfonic acid) (pSSA) brushes (Figure 23.23) covalently attached to the nanopore surface.39 The sintered membranes ∼300 μm thick and ∼1 cm across were modified with pSPM and pSSA sulfonated polymer brushes by surface-initiated ATRP (Figure 23.23). TGA was used to characterize the polymer-modified sintered membranes. The polymer weight % for pSPM membranes is ∼8 wt% and the for pSSA membranes is ∼6 wt% after 10 h of polymerization. Water uptake of the sintered membranes (Figure 23.24) was measured by soaking the membranes in water at room temperature for 24 h. Given that the void fraction of the colloidal crystal is 26%, it appears that both polymers are significantly hydrated, with pSPM being hydrated almost completely, and pSSA falling slightly below. The water uptake of pSPM and pSSA sintered silica membranes is lower than that of Nafion® (∼38% water uptake measured under the same conditions69 ). The sintered polymer-modified membranes did not swell in the course of these experiments.

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23 Membrane Materials Form Polymer Brush Nanoparticles

Figure . Preparation of pSPM and pSSA brushes on silica surface.

We measured the proton conductivity of the polymer brush-filled colloidal membranes using electrochemical impedance spectroscopy. Figure 23.25a shows the proton conductivity of sintered pSPM and pSSA membranes at 30 and 98◦ C as a function of relative humidity. It is clear that both membranes have similar proton conductivities, which increase with relative humidity, with a maximum value of ∼2 × 10−2 S/cm achieved at 30◦ C and 94% RH. The proton conductivity of the polymer-modified sintered colloidal membranes is affected by temperature, as shown in Figure 23.25b. The proton conductivity for both membranes at 60% RH increases gradually with increasing temperature up to 85◦ C and then decreases. 30 25 Water uptake, %



20 15 10 5 0 Unmodified

pSPM- 7.8%

pSSA- 5.8%

Figure . Water uptake for sintered silica membranes after soaking in water at room temperature for 24 h. Source: Smith and Zharov 2009.39 Reproduced with permission of American Chemical Society.

23.2 Colloidal Membranes Pore-Filled with Polymer Brushes

Proton conductivity, S cm–1

Relative humidity, % 10 20 30 40 50

60 70 80 90 100 20 2E-02

(a)

40

Temperature, °C 60 80

100

(b)

1E-02

pSPM- 7.8 wt%- 98 °C

1E-03

pSPM- 7.8 wt%- 30 °C pSSA- 5.8 wt%- 98 °C pSSA- 5.8 wt%- 30 °C

2E-03

1E-04

pSPM- 7.8 wt%

pSSA- 5.8 wt%

Figure . Proton conductivity of pSPM- and pSSA-sintered colloidal membranes as a function of (a) relative humidity at 30 and 98◦ C and (b) temperature at 60% relative humidity. Source: Khabibullin et al., 2014.71 Reproduced with permission of The Royal Chemical Society.

Proton conductivity, S cm–1

Overall, the proton conductivity values obtained for the sintered polymermodified colloidal membranes are comparable to those of Nafion (2 × 10−3 – 7 × 10−2 S/cm). The most extensively sulfonated polystyrene membranes reported (20 mol% sulfonation) had the proton conductivity of 5 × 10−2 S/cm at room temperature in their fully hydrated state.70 We also studied the dependence of proton conductivity in polymer brush-filled sintered silica colloidal crystal on the degree of sulfonation of the polymer.71 The concentration of sulfonic groups was varied by copolymerizing different ratios of 3-sulfopropylmethacrylate (SPM) and 2-ethoxyethyl-methacrylate (EEMA). Three regions are clearly seen in the proton conductivity plot (Figure 23.26). First, a flat low conductivity region corresponds to the 20–40% content of SPM in the copolymer. Increasing SPM content to 50% leads to a dramatic increase in proton conductivity, which 0.012

0.006

0

0

25

50 75 SPM content, mol%

100

Figure . Proton conductivity of pSPM/pEEMA copolymer brush-filled colloidal membranes as a function of SPM monomer fraction.

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23 Membrane Materials Form Polymer Brush Nanoparticles

grows by the factor of 4. In the range of SPM concentration from 50% to 60%, every minor change in monomers ratio causes a significant increase in proton conductivity. Finally, the third region can be roughly characterized as “saturation,” where increasing of SPM content from 60% to 100% causes conductivity growth by only ∼20%. In most PEMs, acidic groups form ion-rich clusters that need to be connected with each other in order to provide high proton conductivity. It can be concluded that in this membrane the ion-rich clusters effectively connect to each other in the region of 50–60 mol% SPM content.

. Self-Assembled PBNPs Membranes ..

PDMAEMA/PSPM Membranes

To form these membranes,40 we prepared PBNPs using SI-ATRP of 3-sulfopropylmethacrylate (SPM) and N-dimethyl-aminoethylmethacrylate (DMAEMA), as shown in Figure 23.27, and varied the length of the polymer brushes using the polymerization time to find the optimal ratio of this length to the silica sphere diameter. We discovered that upon mixing two ethanol colloidal solutions containing 390 nm silica spheres modified with short PSPM and PDMAEMA brushes (10 and 40 nm, respectively), a gel was rapidly formed and after complete evaporation of ethanol, irregular cracked pieces of a solid material were formed. To improve the mechanical properties of the assembled material, 2-ethoxyethyl methacrylate (EEMA) and methyl methacrylate (MMA), were added to PSPM and PDMAEMA brushes, respectively. The molar ratio of 1:1 was optimal for the formation of durable, flexible and large area (∼1.5 cm2 ) crack-free membranes. The SEM images (Figure 23.28) showed closely packed yet disordered silica spheres with interstitial spaces. The materials were stable for days in organic solvents. However, they softened in 5–10 min and completely dispersed within ∼5 min of sonication in water. The materials reassembled after complete water evaporation and remained durable and flexible. They could withstand multiple cycles of assembly–disassembly without losing their properties. The flexibility of the material depended on the thickness and the length/thickness ratio. For instance, a 10-mm long and 0.2-mm thick sample prepared from 4 wt% solution of PBNPs showed significant flexibility (Figure 23.28). We also prepared films on a porous support and found that they possess ethanol flux of 380 L/m2 /h (1.6 gpm) at 1.45 bar, which is comparable or exceeds the flux of commercially available ultrafiltration membranes. We used ethanol solutions of G5 PAMAM dendrimer and polystyrene nanoparticles to determine the filtration cutoff of the supported films. The 6-nm dendrimer molecules passed through the film, whereas 39 nm polystyrene beads were retained completely. Thus, the filtration cutoff of the films was between 25 and 39 nm.

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Figure . Preparation of PSPM and PDMAEMA copolymer brushes on the surface of silica spheres.



23 Membrane Materials Form Polymer Brush Nanoparticles

Figure . Representative SEM image of PDMAEMA/PSPM membrane and photograph showing its flexibility. Source: Khabibullin et al., 2014.40 Reproduced with permission of American Chemical Society.

.. PHEMA Membranes We discovered that nanoporous materials can be prepared by the assembly of PBNPs carrying poly(2-hydroxyethyl methacrylate) (PHEMA) brushes (Figure 23.29) from their ethanol solutions. The length of PHEMA brushes on 330 nm silica spheres required to form the free-standing porous materials was ∼15 nm with the average molecular weight of ∼6000 g/mol (approximately 48 HEMA monomers per brush), as determined by TGA with the assumption of 0.5 HEMA chains per nm2 . After ethanol evaporation, a solid material formed as smooth and evenly thick flat pieces of ∼2 cm2 area. SEM images of the materials (Figure 23.30) showed closely packed yet disordered silica spheres. The SEM images of the material cross section (Figure 23.30) demonstrate that PBNPs form a continuous assembly without mechanical defects and with a smooth surface.

Figure . Preparation of PHEMA PBNPs.

23.3 Self-Assembled PBNPs Membranes

Figure . Representative SEM images of the top and cross-section view of PHEMA PBNP membrane. Source: Khabibullin et al. 2014.40 Reproduced with permission of American Chemical Society.

We measured the flexural strength of the PHEMA PBNP membranes using the four-point bending test, and found it to be 0.5 ± 0.1 MPa. Despite the low flexural strength, the “neutral” materials can be handled, sonicated, sandwiched between plastic or metal rings, and even dropped from 1 m height without breaking or cracking. We found that PHEMA PBNP membranes were stable in water for at least 72 h, but softened in ethanol and acetonitrile within ∼30 min and completely dispersed in 24 h. Sonication accelerated this process, and the materials dispersed completely after 15 min of sonication. We prepared supported PHEMA PBNP films on regenerated cellulose. The flux of water through the 1.3-mm thick film made of 460 nm PHEMA-modified silica spheres deposited on regenerated cellulose support with on 0.2 μm pores under 0.35 bar (5 psi) pressure was 18 L/m2 /h (0.08 gpm). This flux is comparable to that of much thinner nanoporous polymeric ultrafiltration membranes with similar porosity.72 To demonstrate the tunability of the pore size in PHEMA PBNP membranes, we deposited silica spheres of two different diameters (280 and 460 nm) modified with PHEMA brushes from ethanol solution on top of nylon filters with 0.2 μm pore size (Figures 23.31a–23.31c). We found that 6 nm dendrimer molecules passed through the films made of 280 nm PBNOs, whereas 20 nm gold nanoparticles were completely retained (Figures 23.31d–23.31f ). The films made of PHEMA-modified 460 nm PBNPs possessed a larger cutoff: They were permeable to 20 nm gold nanoparticles, whereas 40 nm gold nanoparticles were completely retained. These results demonstrate that reversible “neutral” materials are capable of size-selective ultrafiltration and that their pore size can be easily tuned by changing the silica spheres size, and potentially polymer brush length.

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23 Membrane Materials Form Polymer Brush Nanoparticles

(a)

(b)

(d)

(c)

(e)

(f)

Figure . Preparation of supported PHEMA PBNP membrane and isolation of Au nanoparticles. (a) Formation of “neutral” membrane on cellulose support inside stirred cell. (b) Disassembled stirred cell with “neutral” membrane on support. (c) Supported membrane. (d) Ultrafiltration of 20 nm Au nanoparticles through “neutral” membrane made of 280 nm “hairy” silica spheres. (e) Disassembled stirred cell with Au nanoparticles trapped inside the “neutral” membrane. (f ) Dispersed “neutral” membrane with Au nanoparticles in solution. Source: Khabibullin et al. 2014.40 Reproduced with permission of American Chemical Society.

..

pSPM and pSSA Membranes

Sulfonated linear polymer brushes, pSPM and pSSA, were grown on the initiator-grafted silica nanoparticles via ATRP (Figure 23.23). The length of these bushes was controlled by the polymerization time in the range of 40– 400 nm and monitored by dynamic light scattering (DLS). TGA was also performed for the resulting PBNPs and showed that longer polymerization times lead to the growth of longer polymer chains. The membranes prepared by casting of the PBNPs carrying short polymer brushes were stiff. Their SEM showed the individual silica nanoparticles and nanopores (Figures 23.32a and 23.32c). Casting of the silica spheres carrying longer polymer chains (Figures 23.32b and 23.32d) resulted in polymer-like films. These membranes were flexible, compared to the stiff membranes made with shorter polymer brushes. The membranes were kept in a constant humidity-temperature chamber at 97% RH and room temperature for 24 h, and their average water uptake was calculated using Equation (23.1). Membranes prepared using PBNPs with longer polymer brushes incorporated more water, with the membranes made using the longest polymer brushes being saturated with water (Figure 23.33).

23.3 Self-Assembled PBNPs Membranes (a)

(b)

(c)

(d)

Figure . SEM images of (a) pSPM-39.5 nm, (b) pSPM 361.5 nm, (c) pSSA-41.5 nm, and (d) pSSA-486 nm PBNP membranes. Size bars: A, C = 5 μm, B, D = 2.5 μm. Source: Smith and Zharov 2009.39 Reproduced with permission of American Chemical Society.

Water uptake, %

For both types of self-assembled PBNP membranes, pSPM and pSSA, there is an increase in proton conductivity with longer polymers (Figures 23.34a and 23.35a), due to an increase in the number of sulfonic acid groups. The membranes possess comparable proton conductivities, with a maximum value of

100 80 60 40 20 0 pSSA- 486

pSSA- 41.5

pSPM- 361.5

pSPM- 111.5

pSPM- 39.5

Figure . Water uptake of polymer-modified self-assembled silica membranes after keeping membranes in a humidity–temperature controlled chamber at 97% RH and room temperature for 24 h. Source: Smith and Zharov 2009.39 Reproduced with permission of American Chemical Society.

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23 Membrane Materials Form Polymer Brush Nanoparticles

10

Proton conductivity, S cm–1

1E-01

Relative humidity, % 30 50 70 90

20 1E-01

(a)

1E-02

Temperature, °C 40 60 80

100

(b)

1E-02

1E-03 1E-03

1E-04 1E-05

1E-04

1E-06 1E-07 1E-08

360 nm, 98 °C

360 nm, 30 °C

112 nm, 98 °C

112 nm, 30 °C

40 nm, 98 °C

40 nm, 30 °C

1E-05 1E-06

360 nm

112 nm

40 nm

Figure . Proton conductivity of pSPM membranes as a function of (a) relative humidity at 30◦ C and 98◦ C and (b) temperature at 60% relative humidity. Source: Smith and Zharov 2009.39 Reproduced with permission of American Chemical Society.

∼6 × 10−2 S/cm for both pSPM 362 nm and pSSA 486 nm at 98◦ C and 70% RH. As expected, the proton conductivity of all the membranes increases with increasing relative humidity. The proton conductivity for most of the membranes increases with temperature (Figures 23.34b and 23.35b), except for the pSPM 40 nm and pSSA 42 nm membranes. These membranes possess lower conductivity at higher temperature. We speculate that this is the result of a short length of the polymer chains, which makes it hard to retain water molecules at Relative humidity, %

Temperature, °C

10 20 30 40 50 60 70 80 90 100 20 1E-01 1E-01 (a) (b) Proton conductivity, S cm–1



40

60

80

100

1E-02

1E-02

1E-03

1E-04

1E-05

486 nm, 98 °C

486 nm, 30 °C

42 nm, 98 °C

42 nm, 30 °C

1E-03

486 nm

42 nm

Figure . Proton conductivity of pSSA membranes as a function of (a) relative humidity at 30 and 98◦ C and (b) temperature at 60% RH. Source: Smith and Zharov 2009.39 Reproduced with permission of American Chemical Society.

References

higher temperatures. The highest conductivity values were 4 × 10−2 S/cm at 105◦ C for pSPM 362 nm and 4.7 × 10−2 S/cm at 105◦ C for the pSSA 486 nm membranes at 60% RH.

. Summary In this chapter, we described the preparation and transport properties of silica colloidal membranes whose nanopores carry polymer brushes. These membranes have been prepared using two different approaches. In the first approach, silica colloidal membranes comprising of silica spheres in closepacked fcc lattice with ordered arrays of nanopores are prepared by selfassembly. These membranes are surface-modified in a well-defined and controlled manner with polymer brushes. We formed polymer brushes with narrow molecular mass distribution and controlled length on the nanopore surface using ATRP and ring-opening polymerization. The transport in the resulting nanoporous membranes can be controlled by pH, ionic strength, and temperature. We also prepared proton conducting silica colloidal membranes by surface grafting with sulfonated polymer brushes and studied the properties of these membranes. There are various potential applications for polymer brush membranes. Due to their ordered nanoporous structure and mechanical durability, they can be applied in nanofluidics, nanofiltration, and as catalyst support. Reversible control of transport via external stimuli may be useful in drug-release devices, in size-, charge-, and structure-selective separations, and in microfluidic and sensing devices. These membranes broaden the growing field of inorganic membrane materials containing ordered arrays of nanopores and prepared via self-assembly, such as anodized alumina,73 containing highly ordered pores with columnar structure, and mesoporous silica, containing multiple cylindrical channels (mesopores) arranged in a two-dimensional network of honeycomb-like structure.74–76 In the second approach, nanoporous and proton conducting membranes are formed by self-assembly of PBNPs. The resulting membranes are formed reversibly and their pore size can be controlled by the size of the PBNP-building blocks. Self-assembled PBNP membranes possess several advantages over the traditional polymeric membranes and are promising candidates for ultra- and nanofiltration and for applications in fuel cells and lithium batteries.

References  Tanev, P. T.; Butruille, J.-R.; Pinnavaia, T. J. In Chem. Adv. Mater.; Interrante, L. V., Hampden-Smith, M. J. Eds.; Wiley-VCH: New York, 1998; p. 329.  Davis, M. E. Nature 2002, 417, 813–821.

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 Kim, B.; Lim, S.H.; Ryoo, W. J. Biomater. Sci. 2009, 20, 427.  Ignacio-de Leon, P. A.; Cichelli, J. A.; Abelow, A. E.; Zharov, I. J. Inorg. Gen. Chem. 2014, 640, 649–654.  Cichelli, J. A.; Zharov, I. J. Am. Chem. Soc. 2006, 128, 8130–8131.  Cichelli, J. A.; Zharov, I. J. Mater. Chem. 2007, 17, 1870–1875.  Lee, S. B.; Mitchell, D. T.; Trofin, L.; Nevanen, T. K.; S¨oderlund, H.; Martin, C. R. Science 2002, 296, 2198–2200.  Rmaile, H. H.; Schlenoff, J. B. J. Am. Chem. Soc. 2003, 125, 6602–6603.  Ignacio-de Leon, P. A.; Cichelli, J.; Abelow, A.; Zhukov, A.; Stoikov, I. I.; Zharov, I. Isr. J. Chem. 2014, 54, 767–773.  Pereira, F.; Vall´e, K.; Belleville, P.; Morin, A.; Lambert, S.; Sanchez, C. Chem. Mater. 2008, 20, 1710–1718.  Carreta, N.; Tricoli, V.; Picchioni, P. J. Membr. Sci. 2000, 166, 189–197.  Khabibullin, A.; Minteer, S. D.; Zharov, I. J. Mater. Chem. A. 2014, 2, 12761–12769.  Yang, S. Y.; Ryu, I.; Kim, H. Y.; Kim, J. K.; Jang, S. K.; Russell, T. P. Adv. Mater. 2006, 18, 709–712.  Kipke, S.; Schmid, G. Adv. Funct. Mater. 2004, 14, 1184–1188.  Vivero-Escoto, J.; Slowing, I.; Trewyn, B. G.; Lin, V. S.-Y. Small 2010, 6, 1952–1967.  Vallet-Regi, M.; Ramila, A.; del Real, R. P.; Perez-Pariente, J. Chem. Mater. 2001, 13, 308–311.  Soler-Illia, G. J. A. A.; Azzaroni, O. Chem. Soc. Rev. 2011, 40, 1107–1150.

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 Responsive Polymer Networks and Brushes for Active Plasmonics Nestor Gisbert Quilis, Nityanand Sharma, Stefan Fossati, Wolfgang Knoll, and Jakub Dostalek Biosensor Technologies, AIT – Austrian Institute of Technology GmbH, Vienna, Austria

. Introduction Plasmonics represents a rapidly developing research field that concerns nanoscale manipulating of light by using metallic nanostructures. Plasmonics is pursued to impact a wide range of applications including optical spectroscopy,1,2 optical communication technologies,3,4 photovoltaics,5 and chemical analytical technologies.6–9 The majority of plasmonic structures that have been developed up to now are passive, and their optical properties are fixed after they are prepared. Plasmonic structures that can be reversibly tuned are attractive materials that can advance implementation of plasmonics in areas such as miniaturized photonic circuits10 and sensors.11,12 Such activities lead to the establishing of a new research branch that is referred to as active plasmonics. Active plasmonics was recently subject to several reviews.13–15 This chapter focuses at hybrid plasmonic structures that utilize responsive polymer brushes and networks. These materials can be designed to change their refractive index, absorption, or volume in response to stimuli including temperature, pH, light intensity, or current.15–17 The chapter briefly introduces design rules of plasmonic structures and discusses the control of resonant coupling of light to collective charge oscillations at the metal (Section 24.2). Afterwards, Section 24.3 provides an overview of stimuli-responsive polymer architectures that can be employed for actuating plasmonic structures. Finally, the chapter presents three examples of surface plasmon actuating by responsive hydrogel gratings that were carried out at our laboratory. Sections 24.4–24.6 illustrate the preparation of hydrogel gratings by laser interference lithography, nanoimprint lithography, and contact lithography. Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

. Tuning Spectrum of Surface Plasmon Modes Surface plasmons are optical waves that allow tightly confining light energy at a metallic surface. They originate from collective oscillation of electron density coupled with associated electromagnetic field. Along the surface of a metallic film, propagating surface plasmons (PSPs) can travel with the propagation constant kPSP : √ √ 2 2 n n 2𝜋 √ √ m d , kPSP = (24.1) 𝜆 n2m + n2d where k0 = 2𝜋∕𝜆 is the wavenumber of light in vacuum, nm is complex refractive index of metal, and nd is the refractive index of the adjacent dielectric. The coupling of a light beam to PSPs travelling on the metallic surface can be utilized by using diffraction or prism couplers. These couplers increase the momentum of a light beam in order to phase match it with PSPs at the interface. In the diffraction coupler, the metal surface is periodically corrugated (see Figure 24.1a) and the propagation constant of incident light is enhanced by the grating momentum 2𝜋/Λ so it matches that of PSP: 2𝜋 2𝜋 (24.2) nd sin(𝜃) ± m = ±Re {kPSP (𝜆)}, 𝜆 Λ where Λ is the period, 𝜃 is the angle of incidence, integer m is a diffraction order, and Re{} states for the real part of a complex number. When the condition (24.2) is fulfilled, the energy of a light beam propagating in the far field can be transferred to PSPs which typically manifests itself as a decrease in the reflectivity at resonant angle 𝜃 and wavelength 𝜆. Another widely used means for the excitation of PSPs is based on the Kretschmann configuration of attenuated total reflection (ATR) method, which is depicted in Figure 24.1b. It shows a

Figure . Resonant excitation of PSPs by using (a) diffraction grating and (b) by the ATR method with the Kretschmann configuration.

24.2 Tuning Spectrum of Surface Plasmon Modes

light beam that totally internally reflects at the interface between a high refractive index (np ) glass and a thin metal film (exhibiting refractive index nm ) with a lower refractive index (nd ) dielectric on the top. Similarly to the grating coupler, PSPs are resonantly excited and the intensity of reflected beam decreases when the following resonant condition is fulfilled at a certain angle of incidence 𝜃 and wavelength 𝜆: 2𝜋 n sin(𝜃) = Re{kPSP (𝜆)}. 𝜆 p

(24.3)

As can be seen from the phase-matching conditions (24.2) and (24.3), the resonant coupling to PSPs can be tuned by changing the propagation constant kPSP , which depends on the refractive index nd of the dielectric adjacent to the metal. Figure 24.1 illustrates that this dielectric is probed by the evanescent field of PSPs that penetrates to a distance Lp . For instance, a gold surface supports PSPs that probe the adjacent dielectric to a distance of about ∼100 nm in the red part of spectrum. When changing the refractive index nd within this depth, Re{kPSP } is altered, which leads to detuning of the resonant coupling to evanescent PSP. Figure 24.2 shows an example of the resonant coupling to PSPs that is manifested as a dip in the wavelength reflectivity spectrum. The sensitivity of the surface plasmon resonance (SPR) wavelength at which the reflectivity dip minimum occurs was investigated for both prism and grating couplers. The reflectivity spectra were simulated for a gold surface and adjacent dielectric refractive index of nd = 1.33, 1.345, and 1.36. The data in Figure 24.2 reveal that a change of refractive index of 𝛿nd = 0.03 shifts the SPR wavelength 𝛿𝜆 by about the full width of the half minimum (FWHM) of the resonance dip, which yields Δ𝜆 = 15 nm for the grating coupled SPR and Δ𝜆 = 50 for the Kretschmann

Figure . Sensitivity of surface plasmons to refractive index changes: (a) grating coupling on a sinusoidal modulated Au surface with the period Λ = 440 nm, modulation amplitude of 12.5 nm, and angle of incidence 𝜃 = 0◦ . (b) The coupling to surface PSPs by using a prism with np = 1.845, 50 nm thick Au layer, and angle of incidence of 𝜃 = 51.7◦ .

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

configuration. It is worth of noting that these values correspond to the figure of merit (FOM; defined as (𝛿𝜆/Δnd )/Δ𝜆) of about 30. An alternative option to modulate SPR offers the interaction between multiple PSP modes. This can be utilized by, for example, a dielectric grating with a period Λ = 𝜋∕Re{kPSP } that diffraction couples counterpropagating PSPs. This effect relates to Bragg scattering of PSPs, and it leads to opening of a bandgap in their dispersion relation. The occurrence of the bandgap manifests itself as splitting in the SPR resonance to two branches that are associated with the excitation of 𝜔+ and 𝜔− PSP modes. These modes exhibit standing wave nature, and the field intensity of 𝜔− mode is confined in the grating maxima whereas that of 𝜔+ in the grating minima. The occurrence of these modes can be seen as two distinct resonances in the wavelength reflectivity spectrum as illustrated in Figure 24.3. The splitting of the resonance 𝛿𝜆 is increasing with the modulation depth of the grating as showed by series of curves (1)–(5). These data were obtained for a dielectric grating with a refractive index nh = 1.48 and a dielectric with nd = 1.33 on the top. They suggest that there is possible opening

Figure . Bragg scattering of PSPs manifested as a split of SPR. (a) Assumed configuration with the refractive index of the grating of nh = 1.48, np = 1.845, nd = 1.33, and period of Λ = 280 nm. (b) Simulated reflectivity spectra for varied grating modulation depth dh1 , residual layer thickness dh2 , and angle of incidence 𝜃 as specified in the table (c). Source: Sharma et al. 2016.7 Reproduced with permission of Optical Society of America.

24.2 Tuning Spectrum of Surface Plasmon Modes

Figure . LSPs supported by (a) an individual metallic nanoparticle and (b) near-field coupling of LSPs when approaching two identical nanoparticles to close vicinity.

the bandgap that is wider than the spectral width of individual SPR Δ𝜆 if the grating modulation depth dh1 is comparable to the penetration depth Lp . Another type of surface plasmons can be observed on metallic nanoparticles (see Figure 24.4). These modes are referred to as localized surface plasmons (LSPs), and they allow for tighter confinement of electromagnetic field, which penetrates to a distance Lp comparable with the nanoparticle size (typically few tens of nanometers). LSPs with the dipole moment can be excited directly by an incident optical wave. This interaction manifests itself by the enhanced extinction cross-section 𝜎 ext associated with increased scattering and absorption at resonant wavelength. For spherical nanoparticles with a diameter D ≪ 𝜆, the resonant wavelength can be determined analytically from Eq. (24.4). For more complex shape of nanoparticles, the spectrum of LSP modes needs to be typically simulated numerically: Re{nm }2 − Im{nm }2 = −2n2d .

(24.4)

Similarly to PSPs, the resonant excitation of LSPs is sensitive to variations in the refractive index nd in the vicinity to the metallic nanoparticle surface. As shown in Figure 24.5a, an order of magnitude stronger change in the refractive index of 𝛿n = 0.4 is needed in order to shift the resonance wavelength by the resonance FWHM Δ𝜆. This translates to a lower FOM (of about FOM ∼3) of typically used LSP-based plasmonic structures. However, localized surface plasmon resonance (LSPR) wavelength can also be efficiently modulated by the near-field coupling. As seen in Figure 24.5b, the LSPR wavelength redshifts when two identical Au nanoparticles approach to distances d that are shorter than the dimeter D. For Au nanoparticles with a diameter of D = 40 nm, the magnitude of the shift in the LSPR wavelength is similar to the resonance width when the gap reaches d = 4 nm. It is worth of mentioning that the coupling of LSP modes supported by individual nanoparticles leads to the strong confinement of light energy in the gap as indicated in Figure 24.4b.

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

Figure . (a) LSPR shift on a spherical Au nanoparticle of a diameter of D = 30 nm due to a change in nd . (b) Near-field coupling between two metallic nanoparticles as a function of a distance d (oblate nanoparticles with radii 20 and 15 nm were assumed with electric field aligned perpendicular to the axis of symmetry).

. Polymers Used for Actuating of Plasmonic Structures Responsive polymers can be grafted to a metallic surface in order to actuate PSP and LSP modes by refractive index changes 𝛿nd . Such systems can be designed to exploit changes in the real part of refractive index Re{nd } by, for example, modulating the polymer layer density. In addition, the imaginary part of the refractive index Im{nd } allows to strongly alter the spectrum of surface plasmon modes by, for example, the photochromic effect. Moreover, responsive polymer layers can be used as a spacer between metallic nanoparticles and their swelling and collapsing allows controlling the distance d between them. If the distance is comparable to the size of metallic nanoparticles, such modulation enables to strongly actuate LSP modes by the near-field coupling (see Figure 24.5b). ..

Temperature-Responsive Polymers

Poly(N-isopropylacrylamide) (pNIPAAm) is arguably the mostly used thermoresponsive polymer. It exhibits a lower critical solution temperature (LCST) of 32◦ C. pNIPAAm shows a water-swollen structure below its LCST, and above its LCST it undergoes an abrupt phase transition, which is associated with expelling of water. Highly swollen pNIPAAm-based polymer networks were investigated by the combined probing with PSPs and optical waveguide spectroscopy.8,9 In these works, polymer network layers with a swelling ratio (SR) above 10 and a thickness up to several micrometers were used. This material was postmodified by protein molecules in evanescent wave optical biosensors.18 As the refractive index of the pNIPAAm-based brush or networks nd

24.3 Polymers Used for Actuating of Plasmonic Structures

Figure . (a) Dependence of the refractive index nd of the pNIPAAm-based hydrogel film attached to the Au surface. (b) Structure of the polymer that can be photo-cross-linked by UV light. Source: Toma et al. 2013.19 Reproduced with permission of American Chemical Society.

is proportional to the density of polymer chains, it exhibits higher refractive index nd in the collapsed state than in the swollen state. The temperaturecontrolled swelling and collapsing of the hydrogel layer in contact with water allows changing its refractive between nd = 1.46 and 1.36 (see Figure 24.6). This material was employed in plasmonic amplification of fluorescence assays where the pNIPAAm-based hydrogel three-dimensional binding matrix backbone was used for modulating the excitation of long-range surface plasmons that probed the interface.19 This work utilized a Kretschmann configuration of ATR and temperature stimulus allowed for the complete switching of the plasmonically enhanced excitation. The response time of the pNIPAAm brush or hydrogel depends on its thickness as it scales with diffusion of water in and out of the structure. About millisecond time was reported for about micrometer thick pNIPAAm-based layer.19 For about 30-nm thick PNIPAAm-based brush, the response time of 150 μs was measured.20 Nguyen et al. grafted pNIPAAm brushes on arrays of Au nanoparticles. They optimized the brush density and thickness in order to maximize the SPR wavelength shift 𝛿𝜆 associated with its swelling and collapsing.21 This work utilized a brush exhibiting the SR of about 2 and thickness up to 50 nm. Refractometric changes associated with the swelling and collapsing allowed shifting the LSPR band by 𝛿𝜆 = 20 nm (the spectral width of the band was about Δ𝜆 ∼ 75 nm). The same group employed a similar brush for surface-enhanced Raman spectroscopy (SERS).22 They used a structure that consisted of triangular Au nanoparticles prepared by nanosphere colloidal lithography with grafted pNIPAAm brushes and Au nanorods attached to the top (see Figure 24.7a). By collapsing the brush that contained the investigated analyte, the Au nanorods

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24 Responsive Polymer Networks and Brushes for Active Plasmonics (a)

(b) Gold tringles Gold nanorods

1 kcount (I) 18°C

(II) 40°C

LCST (32 °C)

(II)

Swollen PNIPAM brush Collapsed PNIPAM brush

(I)

600

800 1000 1200 1400 1600 Raman shift/cm–1

Figure . (a) Schematic of thermo-responsive polymer brush system that allows tuning of near field coupling between the gold nanorods and larger gold triangular nanonoparticles. (b) Example of SERS spectra of the Nile Blue A recorded in the polymer brush swollen (I) and collapsed (II) state. Source: Nguyen et al. 2015.22 Reproduced with permission of American Chemical Society.

were dragged closer to Au triangular nanoparticles, which results in near-field coupling and strong field enhancement in the gap (see Figure 24.4b). This effect enables increasing the SERS signal originating from the Nile Blue analyte (see Figure 24.7b). ..

Optical Stimulus

Thermoresponsive polymers can be optically triggered by incorporating metallic nanoparticles. Upon the optical excitation of LSPs, increased temperature occurs in their vicinity due to the damping of LSPs. Such materials allow controlling LSPR, and they are pursued for thermal imaging and cancer therapy.23 Another means for light-triggered response can be utilized by incorporating molecular switches such as azobenzene. These functional groups undergo light-induced conformational change between the cis and the trans isomerization. Exposure to UV light (𝜆 = 300–400 nm) induces a transition to trans isomer, whereas irradiation at higher wavelengths (𝜆 > 400 nm) triggers the inverse transition to cis isomer. These transitions are associated with a change in the distance between the para-carbon atoms from about 6 to ˚ This mechanical change has been exploited for the design of responsive 10 A. materials.24 For instance, Joshi et al. have demonstrated a photoreversible plasmonic structure with azobenzene conjugates attached to Au nanoparticles by monitoring LSPR shifts and vibrational bands by SERS (see Figure 24.8a). Conformational changes originating from the transition between cis to trans isomerization alter the thickness of a dielectric shell capping the nanoparticles, and thus they shift the LSPR peak (see Figure 24.8b). It is worth of mentioning that also other factors associated with energy transfer between the azobenzene and the gold are probably involved in the plasmonic shift.25 In addition,

24.3 Polymers Used for Actuating of Plasmonic Structures (a)

(b) N

cis

O

S

S S S S S

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Si Si Si O O O O OO O O O (Glass)

trans O

S S S S S

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N

Blue UV

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Si Si Si O O O O OO O O O (Glass)

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N

t = 0 min, blue t = 60 min, black

0.16 0.12 0.08 0.04 0 400

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Wavelength (nm)

Figure . (a) Structure of the photoreversible conformational changes of the azobenzene-containing SAMs attached to a gold nanoprism and (b) time-dependent optical response of the monitored LSPR peaks upon constant irradiation of blue light. Source: Joshi et al. 2014.25 Reproduced with permission of American Chemical Society.

azobenzene-based layers can be used for the modulation of near-field coupling between Au nanoparticles. The switching in isomerization allowed for changing distance between Au nanoparticles with an average size of 10 nm and shift in LSPR wavelength of 90 nm.26 Shiraishi et al. reported that spiropyran moieties provide more stable photoswitchable system at room temperature than azobenzene.27 This organic photochromic molecule exhibits two isomerizations (merocyanine and spirocyclic forms) that are associated with ring opening and closing upon UV–visible light exposure. The merocyanine isomer exhibits larger dipole moment. If a conjugate of spiropyran was attached to Au nanoparticles, its switching to merocyanine isomer was accompanied with reversible aggregation of the colloid and a redshift of the LSPR peak of about 50 nm.

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Electrochemical Stimulus

Conducting polymers have been explored for actuating plasmonic response of metallic nanoparticles. Leroux et al. have reported a large blueshift of LSPR from 𝜆 = 608 to 571 nm for Au nanoparticles capped with a polyaniline (PANI) film that was switched between reduced and oxidized state.28 The observed LSPR shift was associated with pronounced change in both Im{nd } and Re{nd } of the polymer.29 In addition, poly(3,4-ethylenedioxythiophene) (PEDOT)based capping layers were used for actuating LSPR.30 The profound shift of the LSPR peak from 𝜆 = 685 to 877 nm was ascribed to a large change in nd associated with oxidation and reduction of PEDOT. In a similar way, 3,4ethylenedioxythiophene (EDOT) conjugated to polynorbornene brushes has been employed for active control of LSPR (see Figure 24.9).31

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

Figure . Extinction spectra of the EDOT conducting polymer when switched from the reduced state (dot curve) to the oxidized state (dash–dot curve) for networks with embedded Au NPs with a diameter of (a) D = 2 nm and (b) D = 7 nm. Source: Yavuz et al. 2009.31 Reproduced with permission of American Chemical Society.

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Chemical Stimulus

Joshi et al. reported a sensing platform capable to detect glucose in physiological samples by using triangular Au nanoparticles and poly(allylamine) polymer. The protonation and deprotonation of this pH-responsive polymer causes a change of the polymer conformation inducing a shift in the LSPR peak due to associated refractive index changes.32 In addition, poly(2-vinylpyridine) polymer brushes have been employed in a multilayer structure that was sandwiched between Au nanoislands and chemically synthesized Au nanoparticles. A 50-nm blueshift was measured upon changing the pH from 5.0 to 2.0.33 Moreover, a nanometer thick layer composed of poly(4-vinylpyridine) was deposited on Au nanoparticles. This material showed dramatic redshift of the LSPR wavelength of ∼ 90 nm upon changing the pH from 2.0 to 7.0. At increased pH, the Au nanoparticles were pushed closer to each other resulting in strong near-field plasmonic coupling.34 Recently, Jiang et al. have prepared Au nanorod particles capped with the PANI shell. The LSPR was actuated by changing the refractive index of the doped and undoped states of the PANI shell. The proton-doping state of the PANI polymer was controlled by using hydrochloric acid solution, which leads to the proton-doped emeraldine form with larger electrical conductivity. The reversible switching to the undoped form of PANI was achieved by neutralizing the acid with NaOH solution. As seen in Figure 24.10, this “on/off ” switching from the doped to the undoped state results in a large redshift of the resonance plasmon scattering peak.35

24.4 Imprinted Thermoresponsive Hydrogel Nanopillars

Figure . (a) Plasmonic shift and (b) switching kinetics between the doped and the undoped form of the pH responsive PANI capping a metallic nanoparticle (shell thickness of 39 nm).35 Source: Jiang et al., 2014.35 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

. Imprinted Thermoresponsive Hydrogel Nanopillars Among various lithography methods, nanoimprint lithography (NIL) represents an attractive technique for the preparation of nanostructures due to its high-throughput enabled by, for example, a roll-to-roll technique. The UVNIL patterning of polymers is realized by pressing a mold against a softened or fluid polymer layer and trapping the pattern in the solid state by UV curing. This approach was used to structure the pNIPAAm-based hydrogel (see Figure 24.6b) with arrays of nanopillars. The polymer was spin-coated on top of an Au film. The pNIPAAm-based layer was dried and subsequently imprinted by using a polydimethylsiloxane (PDMS) working stamp that was soaked with ethanol. As seen in Figure 24.11a, upon the contact with a working stamp, the ethanol diffused from the PDMS working stamp and dissolved the pNIPAAm-based polymer that filled working stamp pores. Then, the surface was dried again in contact with the working stamp. Finally, the working stamp was detached and the prepared structure was UV cross-linked by irradiation dose of 10 J/cm2 at wavelength 𝜆 = 365 nm. The imprinted structure was observed by AFM ex situ. Figure 24.12a shows the pristine imprinted pattern of rectangular arrays of nanopillars with a period of Λ = 460 nm, height dhp ∼208 nm, and diameter of D = 130 nm (measured at half of the maximum height). Then, the surface was brought into contact with water and the pNIPAAm structure was allowed to swell at temperature T = 22◦ C. Interestingly, the structure was completely erased after subsequent drying at the same temperature (see Figure 24.12b). However, when the surface was swollen again in water at T = 22◦ C and water evaporated at higher temperature

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

Figure . (a) Schematics of the imprinting procedure. (b) Optical system that was employed for the probing of swelling and collapsing of pNIPAAm-based interface by optical waveguide-enhanced diffraction.

of T = 38◦ C, the arrays of nanopillars were partially recovered on the surface as seen in Figure 24.12c. The erasure of the structure at T = 22◦ C was probably caused by the surface tension occurring upon the water evaporation on the top of the flexible hydrogel structure. However, at elevated temperature above the LCST of the polymer (see Figure 24.6a), the structure first collapsed and become a sufficiently rigid structure to withstand the subsequent water evaporation. The shape of the pillars that swelled and dried at elevated temperature (see Figure 24.12(c) changed compared to those freshly prepared, but their volume stayed approximately the same. The height of the nanopillars decreased by a factor of ∼3, and they exhibit pronged shape that indicates their bending. In order to observe temperature-induced changes in pNIPAAm nanostructures in situ, diffraction measurements were carried out by using the setup in Figure 24.11b. In these measurements, the intrinsically low diffraction intensity originating from highly swollen hydrogel nanopillars was amplified by the (a)

227 nm 200 180 160 140 120 100 80 60 40 0

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16.3 nm 14.0 12.0 10.0 8.0 6.0 4.0 2.0 0.0

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87 nm 80 70 60 50 40 30 20 10 0

Figure . Ex situ AFM observation of (a) arrays of freshly prepared pNIPAAm nanopillars in air compared to the structure that was swollen in water and dried at temperature (b) of T = 22◦ C below LCST and (c) of T = 38◦ C. Source: Pirani et al. 2017.36 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

24.5 Thermoresponsive Hydrogel Nanogratings Fabricated by UV Laser Interference Lithography

Figure . Peak diffraction intensity T−1 measured upon the resonant excitation of TE0 and TM1 modes (lines a guide for eye) for arrays of pNIPAAm-based nanopillars that are in contact with water at temperature 22–50◦ C. Source: Pirani et al. 2017.36 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

resonantly enhanced surface plasmon and optical waveguide waves travelling along the Au surface. Their excitation enhances the electromagnetic field strength, which translates to increased diffraction efficiency. As Figure 24.13 summarizes, diffraction efficiency gradually increases with the temperature approaching LCST of pNIPAAm. It reaches its maximum at temperature of T ∼38◦ C, and above this value it decreases and it drops by a factor of about 6 at temperature T = 50◦ C. The increase of the diffraction efficiency in the temperature 20–38◦ C can be ascribed to the collapse of the polymer and formation of denser pillars. It is worth of mentioning that in situ measurements by AFM did not allow observing the structure. Based on a respective model, the decrease in the diffraction signal at higher temperature can be explained by the bending of nanopillars.36

. Thermoresponsive Hydrogel Nanogratings Fabricated by UV Laser Interference Lithography Another means for facile structuring of polymer layers is laser interference lithography. It was employed to prepare crossed gratings by using an interference setup utilizing Llyod’s mirror configuration.7 Photo-cross-linkable pNIPAAm-based polymer layer of thickness ∼80 nm was spin coated on top of a glass substrate with 47 nm of gold film. A crossed grating with a period varying from Λ = 280 to 450 nm was recorded by double exposure to the UV interference field when the sample was rotated by 0◦ and 90◦ . The areas of the pNIPAAm-based polymer that were exposed to maximum UV interference field intensity are cross-linked, whereas those where the interference field exhibits its minimum not. Therefore, after subsequent rinsing the structure with ethanol and water, the polymer that was not exposed to UV light is washed away.

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

Figure . AFM images of a pNIPAAm grating with Λ = 310 nm that was dried at (a) room temperature T = 22◦ C and (b) at elevated temperature T > LCST. Comparison of the (c) denser grating with Λ = 290 nm and sparser grating with Λ = 450 nm observed after drying at elevated temperature. Scale bar corresponds to the length of 1 μm. Source: Sharma et al. 2016.7 Reproduced with permission of Optical Society of America.

Ex situ AFM observation of prepared gratings shows similar features as for the imprinted nanopillars (see Section 24.4). As seen in Figure 24.14a, the recorded polymer layer exhibits flat surface after washing in water and drying at room temperature. However, when the drying is performed at elevated temperature above the pNIPAAm LCST, the structure is recovered (see Figure 24.14b). In general, periodic regularity of prepared gratings that were dried at higher temperature is perturbed. This can be attributed to the process of collapsing and drying similar as discussed in Section 24.4. This effect contributed to the distortion of the periodic nanostructures when the thickness in the swollen state was comparable with the period Λ. This was confirmed when two submicrometer grating geometries with the same thickness, but a shorter period of Λ = 290 nm and a longer period Λ = 450 nm were examined where shorter (Figure 24.14c) is more perturbed than the longer period (Figure 24.14d) structure.

24.5 Thermoresponsive Hydrogel Nanogratings Fabricated by UV Laser Interference Lithography

The prepared grating structures were used for actuating of a plasmonic bandgap. The used structure resembles the geometry depicted in Figure 24.3a. The period Λ was tuned to 280 nm as was determined by a series of numerical simulations presented in Figure 24.3b. The swelling and collapsing of the hydrogel grating changes its refractive index which alters the Bragg scattering of counter-propagating PSPs. This is associated with reversible opening and closing of a plasmonic bandgap as observed by measuring angular and wavelength reflectivity at temperature T = 22–45◦ C. Figure 24.15 shows that

Figure . Measured reflectivity for a pNIPAAm grating with the period Λ = 280 nm and temperature (a) T = 22◦ C, (b) T = 34◦ C, (c) T = 37◦ C, (d) T = 40◦ C, and (e) T = 45◦ C. (f ) Cross section of the reflectivity at each temperature for the indicated angle of incidence 𝜃 res at which the bandgap occurs. Subsequent reflectivity curves are offset by 0.3 along the reflectivity axis. Source: Sharma et al. 2016.7 Reproduced with permission of Optical Society of America.

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

the resonant excitation of PSPs manifests itself as a dark band in the reflectivity spectrum, which shifts to higher angles 𝜃 when decreasing the wavelength 𝜆. As the temperature is raised above the pNIPAAm LCST (T > 32◦ C), the average refractive index of the polymer structure increases and the collapse of the hydrogel leads to the occurrence of a periodic modulating on the surface. This induces splitting in the PSP dispersion relation due to the diffraction coupling of PSPs at the wavelength 𝜆 ∼800 nm. The width of the bandgap was evaluated by plotting the cross section of SPR reflectivity as seen in Figure 24.15f. It shows that gradual collapse of the grating leads to the opening of the plasmonic bandgap and reaches its maximum of Δ𝜆 = 12 nm at around T = 37◦ C.

. Electrochemically Responsive Hydrogel Microgratings Prepared by UV Photolithography An example of the redox-responsive linear polymer is given in Figure 24.16a. Its chains are composed of thermoresponsive isopropyl acryl amide, acrylic acid conjugated with the cross-linker unit benzophenone, and vinyl-ferrocene. The copolymerization of these monomers resulted in a redox switchable system that can be cross-linked with UV light. Upon oxidation of the ferrocenes

Figure . (a) Preparing of a hydrogel grating by using contact photolithography. (b) Structure a redox-responsive polymer and (c) a microscope image of the grating structure grafted to Au surface.

24.6 Electrochemically Responsive Hydrogel Microgratings Prepared by UV Photolithography

to the charged ferrocenium ions, the enhanced Coulombic repulsion stabilizes the swollen state of the gel with the collapse being shifted to higher temperatures as has been shown for bulk gels with chemical control of the redox state of the ferrocene units.37 The presented example concerns a thin redox-responsive hydrogel grating structure (with periodicities in the several 10-μm range). The linear copolymer that contains all functional groups needed in the final product was spin coated onto the Au layer. Prior to the polymer deposition, the Au surface was modified with a thiol conjugated with benzophenone. Grafting of the polymer and its cross-linking was done via UV irradiation of the copolymer through a Cu grid mask with stripes generating a periodic illumination pattern on the polymer film (see Figure 24.16b). The unexposed polymer is still soluble and can easily be washed away. A light microscopic picture of such a resulting structure is shown in Figure 24.16c. The hydrogel grating was prepared on a glass substrate with a 50-nm thick Au film. This substrate was optically matched to a glass prism in order to probe the grating with resonantly excited PSPs by using the Kretschmann configuration of ATR (see Figure 24.17). Against the Au surface with the polymer grating, a flow cell with the integrated reference electrode and counterelectrode was attached. The angle of incidence 𝜃 of an optical wave hitting the sensor surface was tuned so PSPs were resonantly excited at the Au surface at area coated with the hydrogel. Upon a flow of phosphate-buffered saline, the potential between the Au and reference electrode was modulated in order to oxidize and reduce the ferrocene conjugate in the polymer grating. The induced variations of net charge density translate to changes in the SR and of the refractive index of the hydrogel. Since the hydrogel was prepared in a periodically patterned structure, the swelling and collapsing of the system can be observed from detuning of SPR as well as from changes in the diffraction intensity in series of reflected diffraction orders.23,29 As there is shown in Figure 24.18, increasing of the potential leads to the collapse of the hydrogel which detunes the SPR and increases the zero-order reflectivity. The detuning of the SPR excitation leads to lower field strength probing the surface and accompanied decreased intensity of a diffracted light beam. This result confirms the concept of an electrically responsive gel in a grating format as a sensitive technique, complementing the classical surface plasmon spectroscopy. It can not only be used to study bioaffinity reactions38 with the demonstrated advantages, for example, a reduced sensitivity to refractive index fluctuations caused by temperature or ionic strength variations39 ; it is also very well suited to study polymeric systems per se. The fact that the periodic structure alternating between sample and reference areas in the preparation of the thin film structure offers the possibility of permanently referencing the sample signal against an internal inert standard.

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

–2

ti o ac f fr Di

θ

–1

n de or

0

rs

1 2 LaSFN9 prism LaSFN9 glass Au Λ

Outlet Teflon cell

Inlet BK7 glass

O-ring

PSP Polymer field grating

Electrochemical instrument

Figure . Optical setup for SPR-enhanced diffraction measurement of the responsive hydrogel grating.

This “Fourier-space” approach offers additional advantages similar to holographic techniques. Although not demonstrated here explicitly, one can easily imagine to multiplex the readout scheme of this optical technique. Different grating periodicities would lead to diffraction intensities in different directions but in the same plane of incidence, easily detected by the angular discrimination of their coupling to the optical far-field. Moreover, the corresponding grating vectors do not need to be necessarily collinear: different gratings could be simply be rotated in the plane of the sample thus offering the detection of diffraction intensities propagating into different directions outside the plane of incidence. This could offer a certain “multiplexing” as it might be useful, for example, for the sensing of a variety of bioanalytes: The surface-attached ligands for various bioanalytes could be arranged in periodic structures that differ in their periodicity and/or in the direction of the grating vector. The binding events thus would lead to changes in the different diffraction intensities that can be monitored cross-talk free and completely independent from one another.

24.7 Conclusions

Potential E (mV)

800 600 400 200 0

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18.0 17.5 17.0 16.5 16.0 15.5 15.0 14.5 14.0 -5

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Figure . Variation of the cross-linked hydrogel grating upon potential change observed from SPR reflectivity changes (upper curve) and diffraction intensity (bottom curve).

. Conclusions Hybrid materials that comprise responsive polymer brushes and hydrogels grafted to metallic nanostructures hold particular potential in areas of sensing, optical spectroscopy, drug delivery, and therapy. These materials can be designed to provide multiple functionalities. Externally triggered change in their optical properties allows for active control of plasmonic response of the nanostructures and control of the electromagnetic field confinement. In addition, they may be postmodified with biomolecules in order to specifically respond to biological analytes. Moreover, they can protect the surface of the metallic structure from fouling in contact with complex biological fluids such as blood serum and saliva. This book chapter provides a brief introduction to plasmonic nanostructures that can be actuated by polymers responsive to wide range of stimuli including temperature, light, current, as well as specific chemicals. It discusses means of their preparation and key performance characteristics that are essential for their design in order to serve in specific application.

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24 Responsive Polymer Networks and Brushes for Active Plasmonics

Acknowledgments This work was partially supported by Austrian Science Fund (FWF) through the project ACTIPLAS (P244920-N20) and Small – Sized Analytes Biosensing on Resonant Nanostructures (I 2647). In addition, this project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no 633937, project ULTRAPLACAD, under grant agreement no 642787, Marie Sklodowska-Curie Innovative Training Network BIOGEL.

References  Bauch, M.; Toma, K.; Toma, M.; Zhang, Q.; Dostalek, J. Plasmonics 2013, 9 (4), 781–799.  Lee, S. J.; Guan, Z.; Xu, H.; Moskovits, M. J. Phys. Chem. C 2007, 111 (49), 17985–17988.  Ozbay, E. Science 2006, 311 (5758), 189–193.  Engheta, N. Science 2007, 317 (5845), 1698–1702.  H¨agglund, C.; Z¨ach, M.; Petersson, G. r.; Kasemo, B. Appl. Phys. Lett. 2008, 92 (5), 053110.  Santos, G. M.; Zhao, F.; Zeng, J.; Li, M.; Shih, W. C. J. Biophoton. 2015, 8 (10), 855–863.  Sharma, N.; Petri, C.; Jonas, U.; Dostalek, J. Opt. Exp. 2016, 24 (3), 2457.  Harmon, M. E.; Jakob, T. A. M.; Knoll, W.; Frank, C. W. Macromolecules 2002, 35 (15), 5999-6004.  Beines, P. W.; Klosterkamp, I.; Menges, B.; Jonas, U.; Knoll, W. Langmuir 2007, 23 (4), 2231–2238.  MacDonald, K. F.; Zheludev, N. I. Laser Photon. Rev. 2009, 4 (4), 562–567.  Gehan, H. l. n.; Fillaud, L.; Chehimi, M. M.; Aubard, J.; Hohenau, A.; Felidj, N.; Mangeney, C. ACS Nano 2010, 4 (11), 6491–6500.  Tokarev, I.; Minko, S. Soft Matter 2012, 8 (22), 5980.  Dong, L.; Agarwal, A. K.; Beebe, D. J.; Jiang, H. Nature 2006, 442 (7102), 551–554.  Gao, W.; Vecchio, D.; Li, J.; Zhu, J.; Zhang, Q.; Fu, V.; Li, J.; Thamphiwatana, S.; Lu, D.; Zhang, L. ACS Nano 2014, 8 (3), 2900–2907.  Chen, K.; Leong, E. S. P.; Rukavina, M.; Nagao, T.; Liu, Y. J.; Zheng, Y. Nanophotonics 2015, 4 (1), 186–197.  Kozlovskaya, V.; Kharlampieva, E.; Khanal, B. P.; Manna, P.; Zubarev, E. R.; Tsukruk, V. V. Chem. Mater, 2008, 20 (24), 7474–7485.  Sun, Y.; Jiang, L.; Zhong, L.; Jiang, Y.; Chen, X. Nano Res. 2015, 8 (2), 406–417.  Aulasevich, A.; Roskamp, R. F.; Jonas, U.; Menges, B.; Dostalek, J.; Knoll, W. Macromol. Rapid Commun. 2009, 30, 872–877.

References

 Toma, M.; Jonas, U.; Mateescu, A.; Knoll, W.; Dostalek, J. J. Phys. Chem. C 2013, 117 (22), 11705–11712.  Winkler, P.; Belitsch, M.; Tischler, A.; Hafele, V.; Ditlbacher, H.; Krenn, J. R.; Hohenau, A.; Nguyen, M.; Felidj, N.; Mangeney, C. Appl. Phys. Lett. 2015, 107 (14).  Nguyen, M.; Sun, X.; Lacaze, E.; Winkler, P. M.; Hohenau, A.; Krenn, J. R.; Bourdillon, C. l.; Lamouri, A.; Grand, J.; Levi, G.; Boubekeur-Lecaque, L. l.; Mangeney, C.; Felidj, N. ACS Photon. 2015, 2 (8), 1199–1208.  Nguyen, M.; Kanaev, A.; Sun, X.; Lacaze, E.; Lau-Truong, S.; Lamouri, A.; Aubard, J.; Felidj, N.; Mangeney, C. Langmuir 2015, 31 (46), 12830–12837.  Rothenhaeusler, B.; Knoll, W. Appl. Phys. Lett. 1987, 51, 783–785.  Zhang, N.; Schweiss, R.; Zong, Y.; Knoll, W. Electrochim. Acta 2007, 52, 2869–2875.  Joshi, G. K.; Blodgett, K. N.; Muhoberac, B. B.; Johnson, M. A.; Smith, K. A.; Sardar, R. Nano Lett. 2014, 14 (2), 532–540.  Sidhaye, D. S.; Kashyap, S.; Sastry, M.; Hotha, S.; Prasad, B. L. Langmuir 2005, 21 (17), 7979-84.  Shiraishi, Y.; Shirakawa, E.; Tanaka, K.; Sakamoto, H.; Ichikawa, S.; Hirai, T. ACS Appl. Mater. Interfaces 2014, 6 (10), 7554-62.  Leroux, Y. R.; Lacroix, J. C.; Chane-Ching, K. I.; Fave, C.; F´elidj, N.; L´evi, G.; Aubard, J.; Krenn, J. R.; Hohenau, A. J. Am. Chem. Soc. 2005, 127 (46), 2–16023.  Rothenh¨ausler, B.; Knoll, W. Opt. Commun. 1987, 63, 301–304.  Stockhausen, V.; Martin, P.; Ghilane, J.; Leroux, Y.; Randriamahazaka, H.; Grand, J.; Felidj, N.; Lacroix, J. C. J. Am. Chem. Soc. 2010, 132 (30), 10224–10226.  Yavuz, M. S.; Jensen, G. C.; Penaloza, D. P.; Seery, T. A.; Pendergraph, S. A.; Rusling, J. F.; Sotzing, G. A. Langmuir 2009, 25 (22), 13120–13124.  Joshi, G. K.; Johnson, M. A.; Sardar, R. RSC Adv. 2014, 4 (30), 15807.  Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126 (49), 15950–15951.  Nergiz, S. Z.; Singamaneni, S. ACS Appl. Mater. Interfaces 2011, 3 (4), 945–951.  Jiang, N.; Shao, L.; Wang, J. Adv. Mater 2014, 26 (20), 3282–3289.  Pirani, F.; Sharma, N.; Moreno-Cencerrado, A.; Fossati, S.; Petri, C.; Descrovi, E.; Toca-Herrera, J. L.; Jonas, U.; Dostalek, J. Macromol. Chem. Phys. 2017, 218 (6), 1600400.  Saleem, M.; Yu, H.; Wang, L.; Zain-ul-Abdin; Khalid, H.; Akram, M.; Abbasi, N. M.; Huang, J. Anal. Chim. Acta 2015, 876, 9–25.  Yu, F.; Tian, S.; Yao, D.; Knoll, W. Anal. Chem. 2004, 76, 3530–3535.  Yu, F.; Yao, D.; Liu, J.; Knoll, W. Anal. Chem. 2004, 76, 1971–1975.

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 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics Casey Yan and Zijian Zheng The Hong Kong Polytechnic University, Hung Hom, Hong Kong, People’s Republic of China

. Introduction Recent advances in soft electronics open up many fascinating ideas toward a more futuristic lifestyle that we long for.1–3 Applications of soft electronics including robotic skins,4 wearable displays,5 flexible sensors,6,7 flexible solar cells8,9 supercapacitors (SCs),10,11 smart textiles,12,13 etc. give revolutionary changes to the way on how we interact with the surrounding environments, communicate with other people, and also perform physiological monitoring and diagnosis on individuals.14 As its recognition is ever increasing, soft electronics is predicted to be one of the top technologies and demanding markets with estimated revenue of US$26.54 billion reached by 2016 and is even anticipated to have a market share increased up to US$69.03 billion by 2026.15 In the fabrication of these soft devices, how to fabricate high-performance conductors as interconnects, electrodes, and contacts still encounter several obstacles. First of all, it is the stable performance provided by these conductors even when they are repeatedly stretched, compressed, twisted, or deformed into arbitrary shapes. Second, there is still a lack of scalable and compatible fabrication method for high-performance and robust soft conductors in the industry. To address these challenges, the first idea pioneering soft conductors is to borrow the conventional technology that has long been established in the semiconductor industry. Precise conductive circuits are manipulated on soft polymeric substrates instead of brittle and rigid silicon wafers of merely planar surface.16 However, there are a few disadvantages regarding this technology migration. First, most of the electronic fabrication processes on silicon wafers heavily rely on an expensive vacuum disposition technique such as chemical Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

vapor deposition. A high-temperature reaction condition (∼900◦ C) is always required where majority of the polymeric substrates cannot survive. Second, the etching process requires strong organic solvents, which can easily degrade polymeric substrates. Degraded polymeric substrate becomes misshaped, brittle, and no longer flexible and stretchable, or otherwise the as-deposited conductive pathways are readily delaminated from the polymeric substrates. Third, the conventional microelectronic fabrication is usually carried out in a clean room to avoid contamination of the conductive species. As a result, the whole migration on flexible polymeric substrates is still expensive, material incompatible, and processing unfavorable. Recently, these challenges are partially solved. Rather than adopting old technologies in the semiconductor industry, two practical strategies to fabricate soft conductors regarded as “structures that stretch” and “materials that stretch” are developed and recently reviewed by Roger and co-workers.17–19 In the former strategy, it is proposed that materials of high flexibility and stretchability can be, respectively, achieved by varying their physical sizes and structures, which gives rise to two ideas in materials engineering: (1) once the material is sufficiently thin, it is flexible; (2) materials can be structured into all sorts of spring-like configurations to allow moderate stretch–release deformation. Further insights are therefore given into engineering conductive nanomaterials of various wavy shapes such as buckling,20–23 serpentine,24 arm,25 and percolated/mesh configurations26,27 that enable a stretch–release system. Still, the patterning of these wavy features generally involves sophisticated designs, complicated mechanical modeling, and extremely precise alignment. In the latter strategy, new materials possessing both conductivity and flexibility are explored. Conductive materials such as intrinsically conducting polymers, and carbon materials are filled into28–30 or mixed up with31 flexible rubbery matrix to yield soft conductive composites. Among all of the conductive materials, metal is still the best choice in most applications because of its high stability, high conductance, well-understood properties, and low cost. Metal conductors are originally fabricated by conventional microfabrication technologies, with incorporation of the abovementioned softening strategies. Recently, fueled by the rapidly increasing interest in direct write technologies that allow extremely low-cost and highthroughput fabrication in electronics, printing technologies such as transfer printing,32–36 inkjet printing,37–40 and screen printing41 emerge as alternative approaches to patterning metal thin films on soft substrates.42 The most reported is to print metal precursors (mostly metal nanoparticles and nanowires), which has successfully demonstrated a roll-to-roll patterning possibility on soft substrates. However, after printing the metal precursors, hightemperature annealing (∼250◦ C) is usually required, which limits the use of broader range of plastic substrates as most of the plastics have a Tg < 250◦ C. Even though many local heating methods such as infra-red, pulse laser, pulse photon heating have been developed to significantly decrease the substrate

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25.1 Introduction

temperature, this precursor printing strategy has only been successful in expensive materials such as Au and Ag. Printing materials that are easily oxidized such as Cu and Ni nanoparticles still face remarkable challenges. In later developments, ELD stands out as an alternative strategy for conformal metal deposition on flexible substrates. ELD is an autocatalytic redox reaction in which metal ions in solution are reduced to form metal thin films on substrate surface from the bottom up. As ELD is an all-solution process and can be done in ambient conditions, it is compatible to the use of most plastic substrates such as poly(ethylene terephthalate) (PET), polyimide (PI), acrylonitrile butadiene styrene (ABS), polyvinylidene fluoride (PVDF), polyacrylonitrile, and elastomers such as rubber and polydimethylsiloxane (PDMS). However, the adhesion of these metal thin films on the polymeric substrates is not assured owning to the significant mismatch in mechanical properties between the rigid metal layer and the soft substrates at the metal–substrate interface. Until very recently, polymer brushes introduced on the substrate surface to modify surface architecture bring some new insights. Polymer brushes are assemblies of macromolecular chains chemically tethered one end on a substrate surface. Once with sufficiently high grafting density, polymer chains are forced to induce significant stretch-away from the substrate surface to form a brush-like configuration. Meantime, a significant increase in the amount of functional groups on polymer brushes can be further utilized in the subsequent chemical reactions. Taking these advantages of polymer brushes, an allsolution strategy, namely polymer-assisted metal deposition (PAMD) has been recently developed to fabricate highly durable, flexible, stretchable metal conductors of various substrates including plastics, elastomers, papers and textiles.43 In this strategy, polymer brushes grafted on the substrate enable subsequent ELD, and at the same time act as a bridging layer between the substrate and the as-deposited metal. Remarkably, an interpenetrating matrix of the brush and the as-deposited metal is established which qualifies the asdeposited metal for excellent durability to withstand repeated cycles of bending and stretching without significant conductance failure. Importantly, such a fabrication principle is compatible with many different kinds of printing technologies that span the resolutions from nanometer to millimeter scales. PAMD therefore provides a promising fabrication of high-performance, durable, and flexible soft metal conductors for soft electronics. In this chapter, we first briefly explore the mechanisms of PAMD, the reaction chemistry, as well as the selection criterion of polymer brush enabling PAMD. In later sections, we focus on how PAMDs realize soft metal conductors of various soft substrates (textiles, plastic thin films, elastomers, and sponges) and also the use of these soft metal conductors in soft electronics applications. We also look into the prospects of polymer brush in the field of soft electronics, as well as the challenges of PAMD realizing soft metal conductors in the foreseeable future.

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

. Mechanisms of Polymer-Assisted Metal Deposition PAMD is a universal strategy to deposit robust metal thin films on any substrates. The robustness of the deposited metal heavily relies on the introduction of the functional brush layer, which bridges the substrates and the metals. Typically, PAMD involves three steps as illustrated in Scheme 25.1, which includes (1) grafting of polymer brushes, (2) loading of catalytic moieties or metal seeds, and (3) ELD on the catalyzed or metal seeds loaded areas. In the first step, pristine substrate surface is first modified with a layer of functional self-assembled organosilane as an initiator for the subsequent surfaceinitiated polymer brush grafting. Silane deposition as the first step to modify

Scheme . Typical PAMD procedures on a substrate surface. (1) Grafting of polymer brushes. (2) Loading of catalytic moieties or metal seeds. (3) ELD on the catalyzed or metal seeds loaded areas.

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25.2 Mechanisms of Polymer-Assisted Metal Deposition

the substrate surface is of great importance because of the following reasons. First, silane can completely modify the substrates surface with specific functional groups that allows functional polymer brush grafting in the next step. Second, silane deposition is relatively easy to be achieved on broad ranges of substrates. Generally, any surface with hydroxyl or oxide groups can be spontaneously reacted with silane. In a typical silane deposition process, silane molecules, such as trichlorosilanes or trialkoxysilanes, hydrolyze and condense on any substrate surface with hydroxyl or oxide groups by self-assembly. It is worthy of note that most of the plastic substrates do not possess any hydroxyl or oxide groups on the surface. Therefore, prior to the silane deposition, the substrate has to be either exposed to an oxygen plasma or immersed into concentrated sodium hydroxide solution to render the substrate surface hydrophilic. The type of silane determines the subsequent polymerization strategy to be adopted. For example, typical silanes used are Br- and vinyl-terminating, respectively, suitable for the brush grafting strategies such as atom transfer radical polymerization (ATRP) and free radical polymerization (FRP).44,45 The most reported Br-terminating silane for ATRP is 3-(trichlorosilyl)propyl 2-bromo-2-methylpropanoate whereas vinyl-terminating silanes for FRP are octenyltrichlorosilane (OTS), vinyltrimethoxysilane (VTMS), and [3(methacryloyloxy)propyl]trimethoxysilane (MPTS). The surface chemistry of silane deposition and the corresponding brush grafting via ATRP or FRP are illustrated in Figures 25.1a–25.1d. After silane modification of the substrate surface, a surface-initiated polymerization strategy—“grafting from”—is commonly adopted to ensure a high brush grafting density. Mostly reported polymer brush in PAMD is the cationic poly[2-(methacryloyloxy)ethyl]trimethylammonium chloride (PMETAC) brush, which can be grafted by using either ATRP or FRP. To decide which polymerization strategy for brush grafting, we have to identify the type of silane deposited in the first step. When Br-terminating silane is deposited on the substrate, ATRP is used to grow the polymer chains. Typical ATRP involves free radicals (R∙ ) generated from the oxidation of the metal ligand complex that concomitantly abstracts the halogen atom X from the initiator (R–X). The free radical species (R∙ ) generated then attack monomers and allow polymer chain to propagate. One supreme advantage of ATRP is that it is capable of preparing well-defined polymer brushes of desired molecular weight.46,47 Huck and co-workers first reported the grafting of PMETAC brushes via ATRP on Au and Si/SiO2 substrates.48 The PMETAC brushes of well-defined thickness of ∼16–20 nm were obtained within 2–3 h polymerization time. When vinyl-terminating silane is deposited on the substrate, FRP is used to grow the polymer chains. In typical FRP, free radical species (R∙ ) generated by thermal decomposition or photolysis of an initiator attack monomers and subsequently trigger polymer chain propagation. FRP of PMETAC brushes was first reported by Zheng and co-workers, in which PMETAC-grafted PET, PI, PDMS, cellulose

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

Figure . Surface chemistry of silane deposition and brush grafting via ATRP or FRP. (a) Br-terminating ATRP initiator 3-(trichlorosilyl)propyl 2-bromo-2-methylpropanoate; (b) vinyl-terminating OTS; (c) vinyl-terminating VTMS; and (d) vinyl-terminating MPTS for subsequent grafting of PMETAC. (e) PMETAC brush grafting can be first carried out by copolymerizing METAC with MPTS to form a copolymer P(METAC-co-MPTS) via FRP. Subsequent hydrolysis and condensation of the silane part in the copolymer can yield surface wrapping of the copolymers on the substrate.

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25.2 Mechanisms of Polymer-Assisted Metal Deposition

paper, and cotton fabrics were prepared.49 The as-grafted PMETAC brushes with 1 h polymerization time were of thickness ∼24 nm with surface roughness of ∼4 nm, which are highly comparable to the brushes grafted using ATRP. While concerning the scalability of the brush grafting, ATRP is not the first choice as it is time consuming and requires nitrogen protection throughout the entire polymerization process. Conversely, FRP is always preferred in scale production as the whole process can be done in ambient conditions. Efficient brush grafting can also be achieved by first copolymerizing the monomer of the polymer brush with the silane via FRP to form a copolymer (Figure 25.1e). Subsequent hydrolysis and condensation of the silane part in the copolymer can yield surface wrapping of the copolymers on the substrate, with functional polymer brushes exposing to the surrounding environment for further chemical reactions. After grafting of polymer brushes on the substrate surface, catalytic moieties or metal seeds for subsequent ELD are immobilized onto the polymer brushes. The selection of catalytic moieties or metal seeds is highly specific, which depends on the chemical properties of the polymer brushes grafted in the previous step. For example, the cationic PMETAC brushes are restricted to couple with anionic catalytic moieties such as [PdCl4 ]2− , which have high affinity to quaternary ammonium group (QA+ ) in the PMETAC.48 Note that the ion exchange process in PMETAC can be carried out either by immersing the polymer-grafted substrate into the [PdCl4 ]2− solution or printing the [PdCl4 ]2− ink on the polymer-grafted substrate with various printing methods such as inkjet printing, screen-printing, or dip-pen nanolithography.49 Other brush such as anionic poly(acrylic acid) (PAA) requires the coupling of metal cations followed by a reduction process to obtain metal seeds adsorbed on the brush surface for the subsequent ELD (will be discussed briefly in the next section). In the final step of PAMD, the catalytic moieties or metal seeds loaded substrates are immersed into the ELD plating bath for metal deposition, where the areas immobilized with catalytic moieties or metal seeds act as effective sites for ELD of metal. Mostly reported metals deposited by PAMD are Cu and Ni due to their relatively low cost comparing to Ag and Au. In the ELD process, metal ions in the ELD plating bath are reduced by a reducing agent (formaldehyde and dimethylamine borane for Cu and Ni plating, respectively) to form conformal metal thin films on the substrate surface. It should be noted that the loading amount of catalytic moieties or metal seeds in the previous step plays an important role in the ELD process as it can highly affect the quality and thickness of the deposited metal.48 Typically, the thickness of the metal film is directly proportional to the ELD time. In addition, thicker polymer brush layer can result in thicker metal layer and higher rate of ELD, as longer polymer chain possesses more functional groups to accommodate more catalytic sites or metal seeds for subsequent ELD reaction.

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

. Role of Polymer Brushes In principle, the functional polymer brush layer grafted on the substrate surface can assist ELD in two aspects. First, it numerously increases the number of immobilization sites for catalytic moieties or metal seeds attachment for metal ELD. With an increased amount of catalysts or metal seeds loaded onto the polymer brushes, the rate of ELD can be significantly increased. Second, it allows an interpenetrating matrix structure to be established which enables a remarkable adhesion between the as-deposited metal and the substrate. In this context, polymer brush as an interfacial layer can be regarded as ‘glue’ that bridges the deposited metal and the substrate surface. Thus, the metal layer shows remarkable adhesion on the substrate surface and robustness against repeated cycles of mechanical actions.

. Selection Criterion of Polymer Brushes Enabling PAMD To further understand the polymer brush assisted ELD system, the selection of polymer brushes enabling PAMD is based on one critical criterion, in which the polymer brush itself should be capable of capturing catalytic moieties (Pd, Pt) or adsorbing metal seeds (Cu, Ni, Ag and Au) in order to enable subsequent ELD to occur. Different types of brushes for PAMD were systematically reviewed by Liu et al.,50 and some of the polymer brushes reported in the literatures for PAMD are also shown in Figure 25.2. The most reported polymer brush for ELD Figure . Some of the reported polymer brushes enabling PAMD.

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25.5 Strategies to Fabricate Patterned Metal Conductors

is the cationic PMETAC brush. Due to the high affinity of the anionic [PdCl4 ]2− moieties to the QA+ in the PMETAC, PMETAC can couple with [PdCl4 ]2− for the subsequent ELD. Another polymer brush, the anionic poly(methacryloyl ethyl phosphate) (PMEP), performs similar ion exchange process in which PMEP can couple with cationic [Pd(NH3 )4 ]2+ moieties due to the strong affinity of [Pd(NH3 )4 ]2+ moieties to the phosphate groups in the PMEP. Grafting of PMEP was demonstrated by Liu et al. in 2009.51 They fabricated two oppositely charged cationic PMETAC and the anionic PEMP brushes, respectively, on a Ag-coated Si substrate. Respective immobilization of catalytic anionic [PdCl4 ]2− moieties for PMETAC and cationic [Pd(NH3 )4 ]2+ moieties for PEMP were carried out for copper and nickel ELD to yield bimetallic patterns. Other brushes that exhibit the ability to carry out ELD include, but not limit to, poly[1,1′ -bis(4-vinylbenzyl)-4,4′ -bipyridinium dinitrate] (PVBVN) and PAA. However, they adopt different approaches to ELD comparing to PMETAC and PMEP. PVBVN allows subsequent ELD upon the radiation of UV. The viologen derivatives in PVBVN carry out a redox reaction, and the metal ions in the solution are reduced to form a very thin layer of metal seeds adsorbed on the brush surface. Therefore, these adsorbed metal seeds serve as nucleation sites for further growth of metal in the subsequent ELD step.52 Other polymer brush such as PAA assists ELD in a similar way like PVBVN. PAA relies on the loading of metal seeds where metal can grow in the subsequent ELD process. However, metal seed loading on PAA brush cannot be achieved by UV radiation, but requires manual metal ion loading by providing copious metal ion source to the brushes with an extra step of metal ion reduction to yield metal seeds adsorbed on the brushes. Usually, such a reduction step is accomplished by a reducing agent, for instance, sodium borohydride. Reduction of metal ions can form a thin layer of metal seeds for the subsequent ELD to yield a conformal metal coating. In 2010, Garcia et al. reported the electroless plating of Cu on the ABS-polycarbonate and polyamide (PA) polymers by grafting of PAA via a GraftFast process.53 Later, they used the same strategy to fabricate patterned localized copper tracks onto flexible polymers such as PET and PVDF sheets via photolithography and direct printing using a commercial laser printer.54

. Strategies to Fabricate Patterned Metal Conductors Theoretically, infinitely large soft metal conductors can be prepared by PAMD for scale applications such as electromagnetic shielding and electrostatic discharge. However, for microelectronics applications, metal patterns on substrate (instead of entirely coated with metal thin film) are always demanded. To control the patterning of metal thin film in PAMD, there are two approaches to go for. One is to pattern the polymer brushes, for example, by patterning the

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

Scheme . Strategy to obtain patterned metal conductors by patterning the silane initiators on the substrate. (1) Deposition of patterned functional silanes. (2) Grafting of polymer brushes from the patterned silanes. (3) Loading of catalytic moieties or metal seeds. (4) ELD to obtain patterned metal conductors.

underlying initiators. The schematic illustration of this approach is shown in Scheme 25.2. In this approach, the silane initiators are patterned on the substrate surface by printing (usually microcontact printing). Polymer brushes are then confined to grow from the silanes to form patterned polymer brushes. Subsequent ion exchange and ELD result in patterned metal thin films on the substrate. On the contrary, areas without initiator deposition are free from brush grafting and ELD therefore no metals are deposited. Another approach is to control the catalytic moieties loading on the specific brush-grafted surface via a matrix-assisted catalytic printing (MACP) method, which is recently reported by Guo et al.49 The schematic illustration of this approach is shown in Scheme 25.3. In this approach, the underlying principle to achieve patterned metal is to confine the ELD areas by patterning the catalytic moieties on the brushes. To do that, the catalytic moieties must first be prepared as printable ink. To prepare the catalytic moieties as printable ink suitable for different printing techniques, a delivering matrix polymer, poly(ethylene glycol) is used as the carrier of the catalytic moieties as well as a thickener to

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25.5 Strategies to Fabricate Patterned Metal Conductors

Scheme . Strategy to obtain patterned metal conductors by patterning catalytic ink on the substrate. (1) Deposition of functional silanes. (2) Grafting of polymer brushes. (3) Printing of ink containing catalytic moieties. Catalytic moieties then diffuse from the ink to the polymer brushes (inset). (4) After diffusion, localized brush areas loaded with catalytic moieties. (5) ELD to obtain patterned metal conductors.

tune the viscosity of the ink. The ink is then printed on the substrate through scanning probe printing, inkjet printing, or screen printing, depending on the feature size of the metal pattern required. After printing, the catalytic moieties diffuse from the ink to the brush layer to carry out ion exchange for subsequent ELD. Guo et al. first reported the use of MACP on various substrates such as

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

PET and PI thin films, cellulose papers, cotton fabrics, and PDMS,49 with feature size of the metal patterns ranging from nanometer to meter scales.

. PAMD on Different Substrates and Their Applications in Soft Electronics ..

On Textiles

Textiles inheritably of great flexibility and wearability are very suitable substrates for soft electronics. One advantage is that textiles can be flexibly integrated into many fabric structures for textile electronics once they are conductive. Liu et al. first demonstrated the PAMD process on cotton fibers.44 They modified the cotton fiber surface with Br-terminating silane for the grafting of PMETAC brushes via SI-ATRP. Subsequent ELD of Cu on the cotton fiber surface resulted in Cu-coated cotton yarns (Figure 25.3a) with outstanding mechanical durability and electrical stability upon repeated cycles of bending, stretching, and washing. Application of these metallic cotton textiles was demonstrated as interconnects in textile electronics by connecting a 9-V battery and a LED bulb (Figures 25.3b and 25.3c). However, concerning the feasibility in scales production, demonstration from Liu et al. is not appropriate as ATRP requires long polymerization time as well as an inert nitrogen atmosphere. Therefore, further modification had been made for a more practical and scalable approach, in which the polymerization of PMETAC was demonstrated via in situ FRP instead of SI-ATRP.55 In the fabrication, vinyl-terminating silane was deposited on the cotton fibers to allow FRP to graft PMETAC brushes on the fiber surface. Since the whole fabrication is aqueous and air compatible, the fabrication can be scaled up by incorporating with pad-dry-cure technology— a textile finishing technique commonly used in dyeing and functional treatment on fabrics. Importantly, PAMD can be applied on wide varieties of natural and synthetic fibers such as nylon (Figure 25.3d), polyester (Figure 25.3e), Kevlar (Figure 25.3f ), and spandex (Figure 25.3g), showing a great versatility of PAMD on different textile substrates that well suits for different end purposes. In 2015, Liu et al. first demonstrated a flexible and wearable yarn-based SC utilizing Ni-coated cotton yarns synthesized by PAMD (Figure 25.3h).56 The Ni-coated cotton yarns as current collectors in the SC, processed high conductivity and robustness in repeated bending with only 2% increase in electrical resistance after 5000 bending cycles. With subsequent electrochemical deposition of reduced graphene oxide (RGO) on the Ni-coated cotton fiber surface and the deposition of electrolyte gel, a solid-state yarn SC was obtained. Owning to the robustness of the nickel metal and also the hierarchical structures offered by the cotton fibers, the solid-state yarn SC not only achieved remarkable performances with volumetric energy density and power density of 6.1

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25.6 PAMD on Different Substrates and Their Applications in Soft Electronics

(a)

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Figure . (a) SEM images of the cross-section morphologies of PMETAC-modified cotton fiber that was immersed in ELD plating bath for 30 min. (b) Conductive yarn or (c) fabric is used as an electrical wire or substrate for powering a blue LED. The yarn or fabric was placed in contact with the battery and LED without using extra conductive glue or paste.44 SEM images of Cu-coated (d) nylon fabric, (e) polyester fabric, (f ) Kevlar yarns, and (g) spandex monofilaments. The insets are SEM images of fibers in high magnification.55 . (h) Digital image of a 500-m long Ni-coated cotton yarn wound on a spinning cone.56 . Source: (a)–(c) Liu et al. 2010.44 Reproduced with permission of American Chemical Society. (d)–(g) Wang et al. 2014.55 Reproduced with permission of John Wiley & Sons, Inc. (h) Liu, https://www.nature.com/articles/ncomms8260. Used under CC by 4.0 https://creativecomm ons.org/licenses/by/4.0/

and 1400 mW/cm3 , respectively, but also maintained high capacitance retention (82%) even after 10,000 cycles of charging and discharging. The schematic structure and the performances of the SC yarns are shown in Figure 25.4. ..

On Plastic Thin films

PAMD can be easily realized on plastic thin films to obtain uniform and patterned metal using roll-to-roll printing. Recently, direct-write technologies

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

Figure . Performances of solid-state SC yarns. (a) Schematic illustration of the structure of one SC yarn. (b) Cyclic voltammetry curves of the device at scan rates ranging from 5 to 100 mv/s. (c) Galvanostatic charge/discharge (GCD) curves of the device at different current densities. (d) Cycle life of the device. The inset is the GCD curve from the 9990th to 10,000th cycle. (e) Device capacitance as a function of the device length.56 Source: Liu, https://www. nature.com/articles/ncomms8260. Used under CC by 4.0 https://creativecommons.org/ licenses/by/4.0/

become inevitable for fast and versatile printing of conductive traces on arbitrary substrates. First demonstrated by Guo et al., PAMD can be integrated into various printing techniques such as scanning probe printing, inkjet printing, and screen printing on PET thin films to yield metal conductors of sizes spanning from nanometer to meter scales.49 The multiscale Cu structures enabled by various printing methods on PET are shown in Figure 25.5. With such an advance in printable PAMD, Li et al. first reported the fabrication of flexible organic solar cells (OSC) using printable Cu electrodes, which

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25.6 PAMD on Different Substrates and Their Applications in Soft Electronics

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Figure . Multiscale Cu structures on PET fabricated by MACP using different printing methods. (a)–(d) Nanometer to micrometer Cu-dot arrays fabricated by dip pen nanolithography (DPN): optical images of DPN-printed ink arrays (a), the corresponding Cu dot arrays (b); (c) SEM image of a 5 × 5 Cu dot array with an average dot diameter of 1.7 ± 0.3 μm, and (d) optical image of a 4 × 4 array of nanometer Cu dots (the insert shows an SEM image of a 380-nm dot). (e, f ) Sub-100 μm Cu dots with different spacings fabricated by inkjet printing: optical images of the inkjet-printed ink patterns (e) and the corresponding Cu patterns (f ). Centimeter-scale Cu interconnectors printed by screen printing: digital image of the printed ink (g) and the corresponding Cu patterns (h). (i) The AFM height profile of Cu on PET; the inset shows the magnified optical image in (h).49 Source: Guo et al. 2013.49 Reproduced with permission of John Wiley & Sons, Inc.

served as the bottom back electrodes in the flexible OSC.57 The printed Cu electrodes were fabricated by following the same PAMD procedures (Figure 25.6). Remarkably, the sheet resistance of the printed Cu electrodes could achieve ∼750 mΩ/sq using four-probe method and easily pass the “Scotch-tape” adhesion test. After spin-coating electron transporting layer, active layer and the

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics (c)

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Figure . Printed Cu electrodes for solar cell applications. (a) 36 printed Cu electrodes on a 50-mm × 60 mm PET sheet. (b) Optical microscope image of printed Cu electrodes. (c) AFM topography (10 μm × 10 μm) and (d) cross-sectional analysis of printed Cu electrodes on PET.57 Source: Li K et al. 2014.57 Reproduced with permission of John Wiley & Sons, Inc.

top transparent electrode consecutively (Figure 25.7a), the performance of the device could reach a power conversion efficiency of 2.77%. This is the highest value among all the full-solution processed OSCs reported to date (using P3HT:PCBM as active layer). The robustness of these Cu electrodes in OSC was also proven with 1000 repeated cycles of the bending test (Figures 25.7d and 25.7e). ..

On Elastomers

Ultrahigh strain enabled in soft conductors is of paramount importance as many applications such as electronic skins and medical implants demand high strain values up to ∼100–200%. Elastomers of high deformability and stretchability are therefore very suitable for being used as the substrates for soft conductors. However, one critical issue is that the elastomer and the deposited metal cannot be stretched to the same extent, leading to performance failure.

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25.6 PAMD on Different Substrates and Their Applications in Soft Electronics Catalytic printing

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Figure . (a) Schematic illustration of the fabrication process of the OSCs based on printed Cu electrodes. (b) Electrical resistance measurement of the printed and evaporated Cu electrodes during 1000 cycles of bending test (bending radius = 6.5 mm, speed = 0.5 Hz). The inset shows that one complete bending test cycle includes one stretch and one compression, that is, bending from +180◦ to −180◦ . (c) Normalized photovoltaic parameters of OSCs with printed and evaporated Cu electrodes during 1000 cycles of bending test. The inset shows a photograph of three bent devices. (d) Optical images of the surfaces of Cu electrodes after 1000 bending cycles. (e) Optical images of the surfaces of OSCs after 1000 bending cycles.57 Source: Li K et al. 2014.57 Reproduced with permission of John Wiley & Sons, Inc.

Addressing this challenge, in 2011, Wang et al. first demonstrated a highly promising fabrication of stretchable metal conductors using PAMD with a prestrain method on elastomers such as PDMS and rubber.45 The SI-ATRP brush grafting strategy was used to graft PMETAC on the elastomer surface. At this stage, the elastomer was uni- or biaxially stretched to a desired strain and

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

the entire prestrained sample was ion-exchanged with [PdCl4 ]2− moieties and subsequently immersed into ELD solution to grow a layer of Cu. Due to the mechanical mismatch between the Cu and the elastomer, buckles of Cu immediately formed once the prestrain was released (Figure 25.8b). Upon stretching, these buckles flattened again and allowed high strain of the elastomers without cracking the metal on top (Figure 25.8e). As-reported, the stretchable metal conductor on PDMS and rubber band could, respectively, achieve 100% and 300% tensile strain while maintaining stable conductance. Meanwhile, no observable cracks and peel-offs were found after repeated stretching and releasing cycles, indicating the excellent adhesion of Cu on the elastomer, which was bridged by the interfacial PMETAC brushes. Although such a prestrain strategy allows large tensile strain, the handling procedure is still tedious and is difficult for large-size application. Further efforts need to be made to eliminate the prestrain procedures. Inspired by the microstructures on rose petals, in 2014, Guo et al. first reported the biomimicking fabrication of omnidirectionally stretchable metal conductors by PAMD without using the prestrain method. In that report, it was found that the microstructured topography on natural rose petal can enable very high stretchability of rigid materials deposited on top. To do that, Guo et al. poured PDMS prepolymer (Sylgard 184) on natural rose petals, which were used as replication molds. After curing the PDMS and peeling off from the rose petals, the elastomeric PDMS petals (E-petals) were fabricated. The surface features of these E-petals consisted of crater-like structures, which were separated by sharp ridges (Figure 25.9g), a complementary topography to the nature rose petal mold. A thin layer of Cu was then made conformably on the E-petals by PAMD, similar to that on PET. The Cu on the E-petals showed a significantly outstanding adhesion, and the whole soft metal conductor allows omnidirectional tensile strain with stable conductivity, which maintained unchanged even after 1000 stretching cycles. Such an excellent performance of the metal thin film is actually contributed by two major factors. First, it is the special microstructures and the sharp ridges that the rose petal offers to the PDMS, where the propagation of the cracks in metal could be effectively inhibited. Second, the −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→ Figure . (a) SEM images of the surface morphology of the as-made Cu layer at the prestrain stage. The magnified image in the inset shows the particles are ≈100–200 nm in diameter. (b) SEM images of a buckled Cu layer when the prestrain (30%) is released. The inset in (b) shows the cross-sectional view. (c) Change in conductivity (P/P0 ) of the stretchable conductor fabricated at 70% prestrain as a function of tensile strain. (d) Conductivity stability of the stretchable conductor fabricated at 70% prestrain under repetitive stretching (70% strain) and relaxing (0 strain) cycles. (e) Optical microscopy images of the surface morphologies of the buckled Cu layer on PDMS obtained at 70% prestrain at various tensile strains.45 . Source: Wang et al. 2011.45 Reproduced with permission of John Wiley & Sons, Inc.

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25.6 PAMD on Different Substrates and Their Applications in Soft Electronics

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

polymer brush layer between the metal and the PDMS acts as an outstanding interfacial layer to provide strong adhesion to Cu on the PDMS. Demonstration of these Cu-coated E-petals in soft electronics was also reported. A new type of strain sensor was fabricated by using metal electrodes on E-petal. With consecutive deposition of Cu and Ag, followed by spin coating of graphene oxide (GO) and subsequent reduction to RGO, the strain sensors were capable of detecting human finger motions at different levels of bending (Figures 25.9k and 25.9l). ..

On Sponges

With the increasing interest in the large-scale applications of soft metal conductors, three-dimensional (3D) metal conductors are very suitable candidates as they can provide a mechanically durable structure with stable conductance due to their extensive 3D network. However, most of the fabrications of 3D metal conductors usually involve expensive conductive materials such as single-wall carbon nanotubes, graphene, and silver nanowires infiltrated into the matrix structures. Moreover, most of the fabrication procedures are found to be difficult for industrialization. In 2013, Yu et al. first reported the fabrication of 3D metal conductors (Cu, Cu/Ag, and Au) using PAMD with polyurethane (PU) sponges as the substrate.59 Importantly, they simplified the silane deposition and the polymerization process into one simple dip-cure procedure for a high-throughput fabrication. In the experiment, the precleaned PU sponges were dipped into an ethanol solution of poly[2-(methacryloyloxy)ethyl trimethylammonium chloride-co-3-(methacryloyloxy)propyl]trimethoxysilane] [P(METAC-co-MPTS)]. Followed by hydrolysis and curing steps, P(METACco-MPTS) was self-cross-linked and formed a dense and thin interfacial layer on top of the PU scaffolds. With subsequent ion exchange and ELD, 3D interconnected metal was made. For example, a large-size metal-coated PU brick (9 cm × 10 cm × 4 cm) was fabricated (Figure 25.10e) with sheet resistance of ∼0.57 Ω/sq. Infiltrated with PDMS, the conductive PU composite acted as a durable and compressible/stretchable interconnect for LED arrays −−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−→ Figure . Preparation and characterization of E-petals. (a) Schematic illustration of the fabrication of E-petals from natural rose petals. Digital images of (b) a fresh yellow rose petal, (c) rose petals taped on bottom of a petri dish, and (d) as-made E-petals. SEM images of (e) natural rose petal, (f ) topography, and (g) cross section of E-petals, respectively. (h) Digital image of ELD-Cu/E-petal. (i) SEM image showing the topographic structure of ELD-Cu/E-petal. (j) Cross-sectional SEM image of ELD-Cu/E-petal, showing the thickness of Cu. (k) Demonstration of the printed strain senor as electronic skin. The device was attached on a finger, which was bent at four different gestures. (l) Typical current change of the strain sensor at the four different states shown in (k).58 Source: Guo R. S, http://onlinelibrary.wiley. com/doi/10.1002/advs.201400021/full. Used under CC BY 4.0 https://creativecommons.org/ licenses/by/4.0/

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25.6 PAMD on Different Substrates and Their Applications in Soft Electronics

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

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25.7 Conclusion, Prospects, and Challenges

(Figures 25.10i and 25.10j), thanks to the outstanding adhesion of the metal layer provided by the bridging of the PMETAC brushes.

. Conclusion, Prospects, and Challenges This book chapter focuses on the application of polymer brushes as functional interfacial materials for making soft metal conductors. A universal strategy namely PAMD to fabricate soft metal conductors of various substrates including textiles, plastic thin films, elastomers, and sponges, is discussed. By introducing a thin layer of polymer brushes as an interfacial material between the metal and the substrate, excellent adhesion of the metal layer on the substrate is resulted to withstand many cycles of mechanical deformations without conductance failure. Since robustness and the stable conductance of the soft metal conductors are always crucial and challenging in enabling high-performance soft electronics, PAMD provides a powerful solution to address these issues and puts forth a strategy for scale fabrications of soft metal conductors. Importantly, PAMD is demonstrated to be compatible with the textiles and printable electronics industries to fabricate soft metal conductors for interconnects, SCs, solar cells, and strain sensors. In the future prospects, it is expected that more soft electronics applications such as flexible and wearable displays, energy harvesting devices, and robotic skins will be demonstrated by using the soft metal conductors fabricated by PAMD. These devices can be further incorporated into many wearable systems and pave the way for the internet of things with soft electronics. However, there are still some limitations and challenges in PAMD. First, the type of metal deposited by PAMD is always limited to Cu, Ni, Ag, or Au due to the restriction of ELD chemistry. Second, encapsulation methods of these devices for full integration in many soft electronics are still remained to investigate. Third, many efforts are still required from multidisciplinary expertise to integrate the whole PAMD process into many industrial procedures, as many machines are needed to redesign to accommodate PAMD for the best fabrications. ←−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−− Figure . Optical images of PU-metal (a) and PU-metal-PDMS (b). (c) SEM image of PU-Cu/Ag. The insets show the cross section of Cu/Ag film on PU sponges. (d) SEM image of PU-Cu/Ag-PDMS. The insets show the cross section of PU-Cu/Ag-PDMS. (e) Optical image of PU-Cu bricks before (top left) and after (bottom right) cutting into half, showing that the metal deposition was even all through this thick sample. (f ) Optical image of a 15.5-in. PU-Cu/Ag-PDMS. The resistivity is 10 ± 5 mΩ/cm (averaged at six locations). Fatigue tests of bendability (g), compressibility (h), and stretchability (i) of PU-Cu/Ag-PDMS. The specific values of R0 in (g), (h), and (i) are 4.0, 4.3, and 1.5 Ω, respectively. (j) Top: schematic diagram of the circuits with four LEDs. Bottom: optical images of the as-made LED arrays in twisting, folding, rolling, and stretching states. (k) Optical images of two-dimensional LED arrays using patterned interconnects at 30% tensile strain.59 Source: Yu et al. 2014.59 Reproduced with permission of John Wiley & Sons, Inc.

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25 Polymer Brushes as Interfacial Materials for Soft Metal Conductors and Electronics

References  Sekitan, I. T.; Someya, T. Adv. Mater. 2010, 22, 2228–2246.  Sekitani, T.; Zschieschang, U.; Klauk, H.; Someya, T. Nat. Mater. 2010, 9, 1015–1022.  Someya, T., Ed. Stretcable Electronics; Wiley-VCH, Weinheim, Germany, 2013.  Kim, D. H.; Lu, N. S.; Ma, R.; Kim, Y. S.; Kim, R. H.; Wang, S. D.; Wu, J.; Won, S. M.; Tao, H.; Islam, A.; Yu, K. J.; Kim, T. I.; Chowdhury, R.; Ying, M.; Xu, L. Z.; Li, M.; Chung, H. J.; Keum, H.; McCormick, M.; Liu, P.; Zhang, Y. W.; Omenetto, F. G.; Huang, Y. G.; Coleman, T.; Rogers, J. A. Science 2011, 333, 838–843.  White, M. S.; Kaltenbrunner, M.; Glowacki, E. D.; Gutnichenko, K.; Kettlgruber, G.; Graz, I.; Aazou, S.; Ulbricht, C.; Egbe, D. A. M.; Miron, M. C.; Major, Z., Scharber, M. C.; Sekitani, T.; Someya, T.; Bauer, S.; Sariciftci, N. S. Nat. Photon. 2013, 7, 811–816.  Someya, T.; Sekitani, T.; Iba, S.; Kato, Y.; Kawaguchi, H.; Sakurai, T. Proc. Nat. Acad. Sci. U. S. A. 2004, 101, 9966–9970.  Someya, T.; Kato, Y.; Sekitani, T.; Iba, S.; Noguchi, Y.; Murase, Y.; Kawaguchi, H.; Sakurai T. Proc. Nat. Acad. Sci. U. S. A. 2005, 102, 12321–12325.  Lipomi, D. J.; Bao, Z. Energy Environ. Sci. 2011, 4, 3314–3328.  Kaltenbrunner, M.; White, M. S.; Glowacki, E. D.; Sekitani, T.; Someya, T.; Sariciftci, N. S.; Bauer, S. Nat. Commun. 2012, 3, 770.  Futaba, D. N.; Hata, K.; Yamada, T.; Hiraoka, T.; Hayamizu, Y.; Kakudate, Y.; Tanaike, O.; Hatori, H.; Yumura, M.; Iijima, S. Nat. Mater. 2006, 5, 987–994.  Meng, C. Z.; Liu, C. H.; Chen, L. Z.; Hu, C. H.; Fan, S. S. Nano Lett. 2010, 10, 4025–4031.  Hu, L. B.; Pasta, M.; La Mantia, F.; Cui, L. F.; Jeong, S.; Deshazer, H. D.; Choi, J. W.; Han, S. M.; Cui, Y. Nano Lett. 2010, 10, 708–714.  Cherenack, K.; Zysset, C.; Kinkeldei, T.; Munzenrieder, N.; Troster, G. Adv. Mater. 2010, 22, 5178–5182.  Webb, R. C.; Bonifas, A. P.; Behnaz, A.; Zhang, Y. H.; Yu, K. J.; Cheng, H. Y.; Shi, M. X.; Bian, Z. G.; Liu, Z. J.; Kim, Y. S.; Yeo, W. H.; Park, J. S.; Song, J. Z.; Li, Y. H.; Huang, Y. G.; Gorbach, A. M.; Rogers J. A. Nat. Mater. 2013, 12, 938–944.  Das, R.; Harrop, P. Organic & Flexible Electronics Forecasts, Players & Opportunities 2016–2026; IDTechEx, Cambridge, UK, 2015.  Gates, B. D. Science 2009, 323, 1566–1567.  Kim, D. H.; Rogers, J. A. Adv. Mater. 2008, 20, 4887–4892.  Rogers, J. A.; Someya, T.; Huang, Y. G. Science 2010, 327, 1603–1607.  Kim, D. H.; Xiao, J. L.; Song, J. Z.; Huang, Y. G.; Rogers, J. A. Adv. Mater. 2010, 22, 2108–2124.  Khang, D. Y.; Jiang, H. Q.; Huang, Y.; Rogers, J. A. Science 2006, 311, 208– 212.

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References

 Sun, Y. G.; Choi, W. M.; Jiang, H. Q.; Huang, Y. G. Y.; Rogers, J. A. Nat. Nanotechnol. 2006, 1, 201–207.  Kim, D. H.; Ahn, J. H.; Choi, W. M.; Kim, H. S.; Kim, T. H.; Song, J. Z.; Huang, Y. G. Y.; Liu, Z. J.; Lu, C.; Rogers, J. A. Science 2008, 320, 507–511.  Choi, W. M.; Song, J. Z.; Khang, D. Y.; Jiang, H. Q.; Huang, Y. G. Y.; Rogers, J. A. Nano Lett. 2007, 7, 1655–1663.  Kim, D. H.; Song, J. Z.; Choi, W. M.; Kim, H. S.; Kim, R. H.; Liu, Z. J.; Huang, Y. G. Y.; Hwang, K. C.; Zhang, Y. W.; Rogers J. A. Proc. Nat. Acad. Sci. U. S. A. 2008, 105, 18675–18680.  Hung, P. J.; Jeong, K. H.; Liu, G. L.; Lee, L. P. Appl. Phys. Lett. 2004, 85, 6051–6053.  Ko, H. C.; Stoykovich, M. P.; Song, J. Z.; Malyarchuk, V.; Choi, W. M.; Yu, C. J.; Geddes, J. B.; Xiao, J. L.; Wang, S. D.; Huang, Y. G.; Rogers, J. A. Nature 2008, 454, 748–753.  Park, S. I.; Xiong, Y. J.; Kim, R. H.; Elvikis, P.; Meitl, M.; Kim, D. H.; Wu, J.; Yoon J.; Yu, C. J.; Liu, Z. J.; Huang, Y. G.; Hwang, K.; Ferreira, P.; Li, X. L.; Choquette, K.; Rogers, J. A. Science 2009, 325, 977–981.  Sekitani, T.; Noguchi, Y.; Hata, K.; Fukushima, T.; Aida, T.; Someya, T. Science 2008, 321, 1468–1472.  Sekitani, T.; Nakajima, H.; Maeda, H.; Fukushima, T.; Aida, T.; Hata, K.; Someya, T. Nat. Mater. 2009, 8, 494–499.  Chun, K. Y.; Oh, Y.; Rho, J.; Ahn, J. H.; Kim, Y. J.; Choi, H. R.; Baik, S. Nat. Nanotechnol. 2010, 5, 853–857.  Wu, Q.; Xu, Y. X.; Yao, Z. Y.; Liu, A. R.; Shi, G. Q. ACS Nano 2010, 4, 1963–1970.  Loo, Y. L.; Willett, R. L.; Baldwin, K. W.; Rogers, J. A. J. Am. Chem. Soc. 2002, 124, 7654–7655.  Schmid, H.; Wolf, H.; Allenspach, R.; Riel, H.; Karg, S.; Michel, B.; Delamarche, E. Adv. Funct. Mater. 2003, 13, 145–153.  Felmet, K.; Loo, Y. L.; Sun, Y. M. Appl. Phys. Lett. 2004, 85, 3316–3318.  Smythe, E. J.; Dickey, M. D.; Whitesides, G. M., Capasso, F. ACS Nano 2009, 3, 59–65.  Carlson, A.; Bowen, A. M.; Huang, Y. G.; Nuzzo, R. G.; Rogers, J. A. Adv. Mater. 2012, 24, 5284–5318.  Kamyshny, A.; Ben-Moshe, M.; Aviezer, S.; Magdassi, S. Macromol. Rapid Commun. 2005, 26, 281–288.  Cheng, K.; Yang, M. H.; Chiu, W. W. W.; Huang, C. Y.; Chang, J.; Ying, T. F.; Yang, Y. Macromol. Rapid Commun. 2005, 26, 247–264.  Van Osch, T. H. J.; Perelaer, J.; De Laat, A. W. M.; Schubert, U. S. Adv. Mater. 2008, 20, 343–345.  Ahn, B. Y.; Duoss, E. B.; Motala, M. J.; Guo, X. Y.; Park, S. I.; Xiong, Y. J.; Yoon, J.; Nuzzo, R. G.; Rogers, J. A.; Lewis, J. A. Science 2009, 323, 1590– 1593.

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 Krebs, F. C.; Alstrup, J.; Spanggaard, H.; Larsen, K.; Kold, E. Solar Energy Mater. Solar Cells 2004, 83, 293–300.  Moonen, P. F.; Yakimets, I.; Huskens, J. Adv. Mater. 2012, 24, 5526–5541.  Yu, Y.; Yan, C.; Zheng, Z. J. Adv. Mater. 2014, 26, 5508–5516.  Liu, X. Q.; Chang H. X.; Li, Y.; Huck, W. T. S.; Zheng, Z. J. ACS Appl. Mater. Interfaces 2010, 2, 529–535.  Wang, X. L.; Hu, H.; Shen, Y. D.; Zhou, X. C.; Zheng, Z. J. Adv. Mater. 2011, 23, 3090–3094.  Patten, T. E.; Matyjaszewski, K. Adv. Mater. 1998, 10, 901–915.  Matyjaszewski, K.; Xia, J. H. Chem. Rev. 2001, 101, 2921–2990.  Azzaroni, O.; Zheng, Z. J.; Yang, Z. Q.; Huck, W. T. S. Langmuir 2006, 22, 6730–.  Guo, R. S.; Yu, Y.; Xie, Z.; Liu, X. Q.; Zhou, X. C.; Gao, Y. F.; Liu, Z. L.; Zhou, F.; Yang, Y.; Zheng, Z. J. Adv. Mater. 2013, 25, 3343–3350.  Liu, X. Q.; Zhou, X. C.; Li, Y.; Zheng Z. J. Chem.–Asian J. 2012, 7, 862–870.  Liu, Z. L.; Hu, H. Y.; Yu, B.; Chen, M. A.; Zheng, Z. J.; Zhou, F. Electrochem. Commun. 2009, 11, 492–495.  Cui, W. S.; Wu, D. Z.; Wang, W. C.; Zhang, L. Q.; Cao, B.; Jin, R. G. Surf. Coat. Technol. 2009, 203, 1885–1890.  Garcia, A.; Berthelot, T.; Viel, P.; Polesel-Maris, J.; Palacin, S. ACS Appl. Mater. Interfaces 2010, 2, 3043–3051.  Garcia, A.; Polesel-Maris, J.; Viel, P.; Palacin, S.; Berthelot, T. Adv. Funct. Mater. 2011, 21, 2096–2102.  Wang, X. L.; Yan, C.; Hu, H.; Zhou, X. C.; Guo, R. S.; Liu, X. Q.; Xie, Z.; Huang, Z. F.; Zheng, Z. J. Chem. – Asian J. 2014, 9, 2170–2177.  Liu, L. B.; Yu, Y.; Yan, C.; Li, K.; Zheng, Z. J. Nat. Commun. 2015, 6, 7260.  Li, K.; Zhen, H. Y.; Niu, L. Y.; Fang, X.; Zhang, Y. K.; Guo, R. S.; Yu, Y.; Yan, F.; Li, H. F.; Zheng, Z. J. Adv. Mater. 2014, 26, 7271–7278.  Guo, R. S.; Yu, Y.; Zeng, J. F.; Liu, X. Q.; Zhou, X. C.; Niu, L. Y.; Gao, T. T.; Li, K.; Yang, Y.; Zhou, F.; Zheng, Z. J. Adv. Sci. 2015, 2, 1400021.  Yu, Y.; Zeng, J. F.; Chen, C. J.; Xie, Z.; Guo, R. S.; Liu, Z. L.; Zhou, X. C.; Yang, Y.; Zheng, Z. J. Adv. Mater. 2014, 26, 810–815.

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 Nanoarchitectonic Design of Complex Materials Using Polymer Brushes as Structural and Functional Units M. Lorena Cortez, Gisela D´ıaz, Waldemar A. Marmisoll´e, Juan M. Giussi, and Omar Azzaroni Instituto de Investigaciones Fisicoqu´ımicas Te´oricas y Aplicadas (INIFTA), Universidad Nacional de La Plata, CONICET, La Plata, Argentina

. Introduction Many materials exhibit structures on multiple length scales—from nano to macro; indeed, in some materials, the structural and functional units themselves have structure.1 This structural hierarchy can play a major part in determining their macroscopic properties and concomitantly guide the synthesis of new materials for specific applications. The science behind hierarchical and hybrid materials spans over multiple approaches. Let us think for a moment of how Nature deals with hierarchical and complex structures. For biomaterials involved in interfacial processes, common geometries include capillaries, dendrites, globules, hair-like, or fin-like attachments supported on larger substrates.2 From many years now, significant efforts have been directed toward the fabrication of functional materials involving multiple length scales and functionalities in close resemblance to Nature. For example, porous fibrous structures can behave like lightweight solids. Depending on what is attached on their surfaces, these core structures can act as multishape-active composites. If nanobuilding blocks can be linked to other nanomaterials, then the resulting material could act synergistically to express enhanced functions from different counterparts. This concept integrating chemistry and form opens up the possibility of taking a functional material of any shape and size to afford new materials with concerted functions. From the synthetic viewpoint, nanotubes, nanofibers, thin films, and composite nanoparticles facilitate the required nanochemical control over different levels of space organization to obtain a variety of powerful materials in which a delicate interplay between Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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26 Nanoarchitectonic Design of Complex Materials

chemical identity, shape, and morphology dictates function and utility. It seems evident that the construction of hierarchical, hybrid, and complex architectures integrating a myriad of nanobuilding blocks and polymer brushes offers diverse chemical and physical properties that make these hybrid systems promising candidates for active components in multiple applications. However, it is important to consider that such complex nanoarchitectures require the tailored production and organization of complex matter displaying spatially addressed chemistry using different wet chemical processes and self-organization pathways. Therefore, to achieve this goal it is important to design strategies for the controlled preparation of multicomponent nanostructures on different settings. Research efforts on this matter are often referred to as “nanoarchitectonics,” a term popularized by Ariga and co-workers.3,4 Within this framework, the ample functional and structural versatility of polymer brushes make them “ideal” building blocks for “soft nanoarchitectonics.” To this end, this work is entirely dedicated to bringing together the latest research on nanoarchitectonics applications of polymer brushes in multiple research areas. It is therefore hoped that the juxtaposition of different molecular and supramolecular approaches will contribute to the further evolution of this fascinating area of materials science.

. Nanoparticles at Spherical Polymer Brushes: Hierarchical Nanoarchitectonic Construction of Complex Functional Materials One of the first explorations combining spherical brushes and different nanomaterials was reported by Ballauff ’s group. These researchers extensively studied the construction of supramolecular hierarchical structures for catalysis through the integration of nanoparticles on the surface of spherical polyelectrolyte brushes.5 Spherical polymer brushes represented by a solid core with dimensions in the 100-nm range onto which long polymer brushes are densely grafted were employed as platforms to grow metal nanoparticles. The strategy relied on the confinement/complexation of counterions that can be used to generate metal nanoparticles within the polyelectrolyte environment. One of attractive features of these hierarchical hybrid systems is their ease of manipulation that makes them excellent nanosystems for applications in catalysis,6,7 including the use of platinum nanoparticles in heterogeneous hydrogenation reactions8 and bimetallic Au–Pt nanoparticles for the oxidation of alcohols.9 Furthermore, the combination of different chemistries in different length scales also facilitated the preparation of composites of metal nanoparticles and TiO2 immobilized in spherical polyelectrolyte brushes.10 These composite hierarchical systems have been synthesized by reduction of the respective metal (Au, Pt, Pd) ions adsorbed on the surface of as-prepared TiO2

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26.3 Nanotube and Nanowire Forests Bearing Polymer Brushes

nanoparticles that were previously confined into poly(styrene sodium sulfonate) brushes grafted on a polystyrene core. The nanoarchitectonic combination of nanomaterials in the form of very stable colloidal systems represents a most promising strategy to attain efficient heterogeneous photocatalysts. For instance, the photocatalytic activity of the hierarchical hybrid particles was two to five times higher than that of the pure TiO2 particles. This finding was ascribed to the enhanced adsorption of the probe molecule at the metal NP@TiO2 @polyelectrolyte brush hierarchical interface. A rather similar approach was proposed by Minko and co-workers11 to fabricate hierarchically organized single-nanoparticle structures employing 200 nm silica cores modified with pH-responsive poly(2-vinylpyridine) brushes, into which 15 nm gold nanoparticles were synthesized. These authors showed that the pH-driven actuation (swelling and collapse) of the polymer brush resulted in the modulation of the interparticle distance and the concomitant shift in the maximum wavelength of the surface plasmon absorption peak. Such hierarchically assembled nanostructures present potential capabilities to be used as free-standing single-particle sensors in various miniaturized analytical systems. More recently, Puretskiy and Ionov have elegantly demonstrated the hierarchical construction of nanoparticle@microparticle systems employing polymer brushes as structural and functional units to synthesize raspberry-like particles.12 The raspberry-like particles were prepared by immobilization of silica nanoparticles on the surface of silica microparticles previously modified with poly(glycidyl methacrylate) brushes. The raspberry-type microparticles were grafted with poly(pentafluorostyrene) chains and used as building blocks to create ultrahydrophobic surfaces.

. Nanotube and Nanowire Forests Bearing Polymer Brushes Seminal contribution from Jiang’s lab described the modification of aligned carbon nanotubes (ACNT) films13–16 with poly(N-isopropylacrylamide) (PNIPAM) brushes in order to confer thermoresponsive properties to the hierarchical architecture constituted of “soft” and “hard” counterparts.17 Their results demonstrated that both the macroscopic and the microscopic properties of the modified ACNTs exhibit remarkable responsiveness, revealing that the polymer brush operates across the multiple length scales. On the macroscopic scale, the ACNT film showed temperature-dependent wettability, whereas on the microscopic scale atomic force microscopy imaging indicated that every single carbon nanotube constituting the hierarchical architecture is also thermoresponsive. In a similar vein and inspired by the nanofibrillar structures of gecko adhesives, Javey et al. explored the construction of chemical connectors using Ge/parylene nanowire forests modified with PNIPAM brushes.18

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26 Nanoarchitectonic Design of Complex Materials

Figure . Scanning electron microscopy (SEM) micrographs of Ge/parylene nanowire (NW) forests (a) before and (b) after modification with PNIPAM brushes. Wetting properties of PNIPAM-modified NW forests at (c) 20◦ C and (d) 35◦ C. (e) Thermoresponsive shear adhesion strength of hybrid NW fasteners. Source: Ko et al. 2010.18 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Several practical applications require the use of programmable fasteners that can change their adhesion properties on command. In this context, these authors designed hierarchical core/multishell hybrid forests with an outer shell of PNIPAM brushes capable of reversibly changing their wet adhesion strength in response to thermal changes of the environment. The distinctive thermoresponsive features of the PNIPAM brush outer layer in combination with the three-dimensional (3D) geometric configuration of the high aspect nanowire forest confers unique binding properties that arise from the tunable contribution of van der Waals interactions along with hydrophobic surface effects (Figure 26.1). .. Polymer Brushes on Surfaces Displaying Microtopographical Hierarchical Arrays It is a well-known fact that substrate geometry can amplify contact angle changes of responsive polymer brushes. In 2004, L´opez and co-workers19 demonstrated that changes in macroscopic surface hydrophobicity of polymer brushes can be quantitatively related to changes in surface nanostructure. These experiments were based on the modification of anodized nanoporous aluminum oxide with PNIPAM and conclusively showed that it is possible

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26.3 Nanotube and Nanowire Forests Bearing Polymer Brushes

to dynamically change the effective interfacial energy using surface-grafted stimuli-responsive polymers on roughened surfaces. In fact, this effect is much more significant when a substrate displaying microtopographical arrays is used. One of the first reports on this matter was contributed by Jiang et al.20 showing the roughness-enhanced thermoresponsive wettability of PNIPAM brushes. Using a silicon substrate exhibiting a regular array of square silicon microconvexes that was modified with thin PNIPAM brushes (∼45 nm), these authors demonstrated that despite of being a challenging subject in surface chemistry reversible switching between superhydrophilicity (contact angle (CA) ∼ 0o ) and superhydrophobicity (CA ∼ 150o ) is fully feasible. A similar concept was later used by Sun et al.21 to create structured surfaces with solvent-responsive wettability. Well-aligned square micropillars modified with copolymer PNIPAM brushes bearing specially designed double amino acid units (Figure 26.2) displayed reversible wettability switching between superhydrophobicity and high hydrophilicity. The wettability switching was amplified due to the hierarchical nature of the substrate, a structured substrate composed of well-aligned square micropillars with a side length of about 10 μm and separation of about 12 μm, displaying nanofibrous structures on each pillar. According to the molecular design of the polymer brush, water treatment induces a dramatic (a)

(b) O

O

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after etching

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ca. 156°

(c) Water Methanol Structured Si at 20°C

Structured Si at 20°C

Figure . (a) Chemical structure of the copolymer brush containing double amino acid units. (b) SEM images of structured Si substrates. The insets correspond to single silicon pillars before (upper) and after chemical etching. Solvent responsive wetting properties of copolymer brush films containing double amino acid units: (a) after methanol treatment, (b) after water treatment. Source: Wang et al. 2009.21 Reproduced with permission of Royal Society of Chemistry.

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26 Nanoarchitectonic Design of Complex Materials

increase of hydrophobicity, whereas the hydrophilic state can be restored by the methanol–alkali treatment. This interesting phenomenon has been attributed to the strong but tunable hydrogen bonding, hydrophobic and electrostatic interactions interplay induced by the double amino acid units. ..

Environmentally Responsive Electrospun Nanofibers

If we look at forms of Nature, we observe that nanofibers are very attractive building blocks in the construction of hierarchically organized nanostructures. However, any successful design of multifunctional hierarchical materials using these elements demands a broad repertoire of nanofibers from a variety of materials to become available. During the past decade, electrospinning gained sound reputation as a versatile nanofiber fabrication technique enabling the preparation of superfine fibers with diameters ranging from 10 to 100 nm. In recent years, some groups started to explore the incorporation of polymer brushes on the surface of electrospun nanofibers with the aim of generating multilevel nanostructures on hierarchically organized composites. The size of the electrospun fibers can be in the nanoscale, and the fibers may possess different surface textures. The incorporation of polymer brushes to the fiber surface might lead to different modes of interaction with other materials compared with typical materials displaying no hierarchical organization. Brandl et al.22 developed a rapid strategy to graft PNIPAM brushes from electrospun nanofibers using surface-initiated atom transfer radical polymerization (SI-ATRP) (Figure 26.3). The procedure simply involved electrospinning of an atom transfer radical polymerization (ATRP) macroinitiator and subsequent PNIPAM grafting using a “grafting-from” approach. The electrospun ATRP macroinitiator was based on a copolymer of methyl methacrylate (MMA) and 2-hydroxyethyl methacrylate, which was subsequently modified with

Figure . SEM micrographs of macroinitiator-modified nanofibers prior to (a) and after (b) surface-initiated polymerization of PNIPAM brushes. Source: Brandl et al. 2011.22 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

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26.4 Fabrication of Free-Standing “Soft” Micro- and Nanoobjects Using Polymer Brushes

2-bromo-2-methylpropionyl bromide to introduce the ATRP initiators. The thermoresponsive properties of the poly(NIPAAm)-grafted films and fibers were corroborated by contact angle measurements. On the other hand, Kobayashi and co-workers23 prepared electrospun nanofibers from a random copolymer of styrene and 4-vinylbenzyl 2-bromopropionate. Poly(styrene sulfonate) brushes were grown on the surface of the hydrophobic nanofibers, and the composite nanomaterials exhibited excellent water dispersibility owing to the hydrophilic brush layer.

. Fabrication of Free-Standing “Soft” Micro- and Nanoobjects Using Polymer Brushes Free-standing soft nanostructures are playing a more and more important role in the design of complex composite materials. Looking at Nature’s complete technological design, we observe that many organisms are able to synthesize intricate free-standing architectures, for example, particles, capsules, tubes, wires, and so on, with meaningful nanostructures that cannot be reproduced using conventional synthetic strategies. Bottom-up construction of architectures as complex as those present in Nature is, of course, an impossible task. In view of this situation, several research groups considered particularly valuable to devise flexible methods to control structure and topology of polymer brushes to afford “soft” free-standing low-dimensional systems with different purposes. Pioneering work by Edmondson and Huck24 demonstrated the preparation of quasi-two-dimensional (2D) polymer films by delaminating cross-linked poly(glycidyl methacrylate) brushes grafted onto gold substrates through controlled electrolysis (Figure 26.4). Free-standing, molecularly thin films based on cross-linked polymer brushes have the potential to act as tailorable scaffolds for ligands to promote polyvalent interactions at synthetic and biological surfaces or even as anisotropic elements in complex or liquid crystalline fluids. In very much the same way, the same group succeeded in controlling not only the formation of hybrid 2D polymer–metal composite microobjects25 but also the folding of these 2D hybrid composites into 3D microstructures, like microcages or microcontainers.26 One of the first reports referring to the fabrication of polymeric nanospheres using polymer brushes was contributed by Walt and co-workers,27 employing PBzMA-co-PEGDMA (poly(benzyl methacrylate-co-poly(ethylene glycol) dimethacrylate) brushes via SI-ATRP from SiO2 particles to obtain hollow polymer particles after HF etching of the sacrificial silica cores. Nanocrystal templating has been also used by M¨ohwald and collaborators28 to create capsules whose envelope was constituted of polymer brushes. In this approach, polymer capsules were prepared by SI-ATRP of 2-(dimethylamino)ethyl methacrylate brushes and its copolymers with

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26 Nanoarchitectonic Design of Complex Materials Br O

Br O O

Br O O

O

(a) 1–10 μm

Backfill With Inert Thiol

PDMS Stamp Au (200 nm) Cr (15 nm) Si

(i) Grow PGMA (ii) Cross-link (NaOH/MeOH) (iii) Dye (Rhodamine B/EDC)

s

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25 μm

Figure . (a) Schematic describing the synthesis of quasi-2D polymer objects by micropatterning a gold surface using microcontact printing followed by growth of polymer brushes, cross-linking and electrochemical liftoff. (b) Fluorescence microscope image of 30 nm thick, 2 mm diameter polymer sheets, and (c) 3 mm wide lines obtained after electrochemical liftoff. Source: Edmondson and Huck 2004.24 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

2-(diethylamino)ethyl methacrylate and poly(ethylene glycol) methyl ether methacrylate from gold nanoparticles. Polymer brushes were cross-linked with 1,2-bis(2-iodoethoxy) ethane, and then KCN aqueous solutions were used to etch Au cores to yield the hollow capsules. These capsules showed swelling at low pH and shrinking at high pH as well as their ability to encapsulate and release rhodamine 6G. In line with these results, Kim et al. reported the “grafting-from” polymerization inside polyelectrolyte capsules and demonstrated that the interior of hollow “soft” particles can be locally modified with polymer brushes.29 This clever strategy relies on the preparation of polyelectrolyte hollow capsules whose inner layer is coated with a water-soluble initiator, followed by polymerization of a desired monomer on the inner walls.

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26.5 Solid-State Polymer Electrolytes Based on Polymer Brush–Modified Colloidal Crystals

Monodisperse silica particles grafted with oxetane-bearing polymer brushes were also used to engineer and fabricate hollow spheres.30 SI-ATRP of MMA with 3-ethyl-3-(methacryloyloxy)methyloxetane (EMO) from silica particles led to the formation core–shell systems exhibiting a block copolymer shell of the type poly(3-ethyl-3-(methacryloyloxy)methyloxetane)-block-poly(methyl methacrylate) (PEMO-b-PMMA). The inner PEMO block located between the PMMA outer shell and silica core was cross-linked by the cationic ring-opening reaction of the oxetane groups and subsequent removal of the inorganic core by HF etching gave polymeric hollow spheres (Figure 26.5). On the other hand, the synergistic combination of surface-initiated polymerization techniques and nanoporous membrane templating31 has greatly facilitated the creation of complex and functional nanotubular structures. The approach takes advantage of both the new properties conferred by polymerizing diverse monomer units and the tight dimensional control offered by nanotemplating to enable new functionalities that arise from the highly anisotropic “one-dimensional” nanotube format. Li’s group reported the synthesis of poly(N-isopropylacrylamide)-co(N,N′ -methylenebisacrylamide) (PNIPAM-co-MBAA) nanotubes with different composite ratios through SI-ATRP into nanoporous alumina templates.32 These authors found that the dimension and shape of the nanotubes, especially the wall thickness, were highly dependent on the brush composition and, however, in all the cases, PNIPAM-co-MBAA nanotubes exhibited high flexibility and stability. A rather similar strategy was recently approached by Zou and co-workers to synthesize polymer nanograss and nanotubes by surfaceinitiated photopolymerization in cylindrical alumina nanopores.33 In this case, they used pore-confined surface-initiated photopolymerization the mixture of glycidyl methacrylate (GMA) monomer and ethylene glycol dimethacrylate (EGDMA) as a cross-linker to obtain a variety of low-dimensional structures ranging from nanotubes to nanoneedles.

. Solid-State Polymer Electrolytes Based on Polymer Brush–Modified Colloidal Crystals Solid electrolytes represent a new class of solid-state ionic materials with potential applications in several technological areas as they exhibit an exceptionally high ionic conduction at room temperature. These properties make them ideal candidates for minimizing or even eliminating the shortcomings of liquid/aqueous electrolytes.34 Very recently, Sato and co-workers35 developed an interesting nanoarchitectonic approach for fabricating a leak- and vapor-free, nonflammable, and solid electrolytes with a highly ion-conductive network channel (Figure 26.6). The conceptual framework to develop such platforms was based on the 3D self-assembly of silica particles modified with ionic liquid-type polymer brushes. In this manner, the 3D scaffold facilitates





26 Nanoarchitectonic Design of Complex Materials (a)

PEMO Layer

SiO2

EMO CuBr/dNbipy

MMA CuCl/dNbipy

Anisole 60°C

70°C

O =

O

PMMA Outer Layer

Si(CH2)6OCOC(CH3)2Br O

BF3OEt2 in dichloromethane

Cross-linked PEMO Layer SiO2 removal by HF etching

(b)

2 μm

2 μm Figure . (a) Description of the synthetic steps involved in the fabrication of polymeric hollow spheres. (b) Transmission electron microscopic image of the hollow spheres. The inset shows a scanning electron micrograph of the polymeric hollow spheres. Source: Morinaga et al. 2007.30 Reproduced with permission of American Chemical Society.

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26.6 Proton-Conducting Membranes with Enhanced Properties Using Polymer Brushes O

(a)

EtO EtO Si EtO

SiO2

(d)

O

O Br O

BHE

N + – N(CF SO ) 3 22

PSiP

N + – N(CF SO ) 3 22

O

DEMM-TFSI

lonic Liquid

LRP

Cast

(b)

fcc structure (c)

tp-plane

cp-plane

Figure . (a) Schematic representation of the preparation process of solid-state conductors via 3D self-assembly of silica particles modified with ionic liquid-type polymer brushes. (b) Photograph of produced hybrid solid film and (c) analyzed structure of hybrid array in solid state. (d) SEM image (15 000×) of a fractured hybrid solid film. Source: Sato et al. 2011.35 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

the formation of polymer brush domains continuously connected, forming network channels displaying enhanced ion conduction on the nanometer scale. As a proof of concept, a Li4 Ti5 O12 /hybrid solid electrolyte/LiMn2 O4 unit cell was fabricated through the integration of the solid-state polymer electrolyte into a bipolar lithium-ion rechargeable battery and connected with a bipolar electrode of Li4 Ti5 O12 (anode) and LiMn2 O4 (cathode) layers. The Coulombic efficiency of this unit cell after 50 cycles was 98%, demonstrating that the nanoarchitectonic design of ion-conducting pathways using polymer brushes can improve the performance of lithium-ion rechargeable batteries.

. Proton-Conducting Membranes with Enhanced Properties Using Polymer Brushes Proton exchange membranes (PEMs) represent essential elements in energy conversion technologies.36,37 In this context, Azzaroni and co-workers developed a nanoarchitectonic approach to create artificial proton-conducting channels based on the molecular design of well-oriented hydrophilic domains using polymer brushes (Figure 26.7).38 Ordered 2D macroporous silicon membranes were modified with sulfonate-bearing polyelectrolyte brushes using poreconfined SI-ATRP in order to confer them proton-conducting properties.39



26 Nanoarchitectonic Design of Complex Materials Initiator-coated pore

800 nm

Humidifying agent

Proton source

180 μm

(a)

Br O Surface-confined ATRP initiator

Surface-initiated atom transfer radical polymerization (SI-ATRP) n

NH O PEL Brush-Coated Pore

Si

m

OO

O

Proton Conduction O x

HO3S

O O O

SPM

Si/SiO2

MeOEGMA

–1

(b) log [Conductivity / S cm–1]



–2

polySPM-co-MeOEGMA

–3 –4 Nafion 117 –5

polySPM

–6

(a)

–7 20

30

40

50

60

70

80

90

100

Relative humidity / %

Figure . (a) Schematic representation of the pore-filling surface polymerization process used to create the proton-conducting channels. (b) Scanning electron micrograph corresponding to the macroporous silicon scaffold modified with the polymer brushes (scale bar: 3 μm). Conductivity versus relative humidity plots corresponding to the silicon membrane modified with polySPM-co-MeOEGMA brushes (black dots), the silicon membrane modified with polySPM brushes (white dots), and a Nafion 117 membrane (gray dots). Source: Yameen et al. 2009.40 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

The fabricated silicon-poly(sulfopropyl methacrylate) hybrid membranes exhibited proton conductivity as well as temperature- and humidity-dependent functional properties similar to that exhibited by typical Nafion membranes. These nanoarchitectured hybrid membranes displayed proton conductivities in the range of 10−2 S/cm. In another set of experiments, the same authors explored the integration of comonomers into the molecular design of the proton channel to improve the hydration of the polyelectrolyte bearing sulfonate groups. To this end, a small fraction of monomethoxy oligo-(ethylene glycol) methacrylate (MeOEGMA) was copolymerized with the sulfonate-bearing monomer. Poly(ethylene glycol) (PEG) derivatives exhibit excellent hydroscopic properties; hence, in the presence of PEGylated macromolecular architectures,

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26.7 Hybrid Architectures Combining Mesoporous Materials and Responsive Polymer Brushes

water molecules are able to form hydrogen bonds with the ethylene oxide units of the polymer chains. In this sense, proton conduction characterization revealed that the incorporation of a small fraction of MeOEGMA monomer units in the polyelectrolyte brush architecture promoted a five orders of magnitude increase in the proton conductivity measured at low relative humidities.40 In addition, the nanoarchitectured platforms displayed high conductivity values (∼10−2 S/cm1 ) regardless of the humidity, thus surpassing the performance of Nafion. These experiments illustrate the possibilities offered by “soft” nanoarchitectonics to fabricate tailorable proton-conducting membranes with highly optimized physical and chemical characteristics.

. Hybrid Architectures Combining Mesoporous Materials and Responsive Polymer Brushes: Gated Molecular Transport Systems and Controlled Delivery Vehicles The combination of mesoporous silica and polymer brushes has found an incredible resonance and a vast number of applications during the past decade, especially as drug delivery platforms. Due to its high surface area, large pore volume, and tunable pore size, mesoporous silica has been widely investigated as a potential candidate for drug delivery.41 As a result of this situation, their surface modification with polymer brushes gained wide acceptance as a strategy to tailor the chemical properties of the porous materials. In this context, surface modification involving homogeneous derivatization of inner and outer surfaces using conventional “grafting-to” and “grafting-from” approaches has been extensively explored. One of the first attempts to manipulate the transport properties of mesoporous materials using responsive polymer brushes was reported by L´opez et al.42 SI-ATRP was employed to graft PNIPAM brushes onto the inner and outer surfaces of mesoporous silica particles. These authors showed that below the lower critical solution temperature (LCST) swollen, hydrated PNIPAM brushes preclude the transport of solutes through the mesopores, whereas at higher temperatures (above LCST) brushes collapse on the pore walls, making the pores permeable to solutes. The same concept was then extended to the use of surface-initiated reversible addition fragmentation chain-transfer (SIRAFT) to prepare PNIPAM-coated mesoporous silica nanospheres with controlled transport properties.43 Later on, Oupicky and co-workers44 demonstrated that selective modification of the outer surface of silica nanoparticles with PNIPAM brushes via a “grafting-to” approach was a plausible strategy to improve the thermo-triggered transport properties of the nanoarchitectured composite material. According to these authors, the selective modification of





26 Nanoarchitectonic Design of Complex Materials

the outer surface permitted the nanoconstruction of mesoporous delivery platforms exhibiting a low level of leakage upon switching the conformational state of the grafted chains. It should be noted that this mode of operation is opposite to that typically observed in mesoporous systems in which the PNIPAM is grown from the surface of the porous silica, that is diffusion occurs when the brush is swollen and retarded when it is collapsed. On the other hand, Mart´ınez-M´an˜ ez and co-workers reported for the first time the use of brush-like architectures to control the gating properties of mesoporous material by ion and pH modulation.45 This by grafting of polyamines on the external surface of mesoporous silica scaffolds, the opening and closure of the mesopores was controlled via either hydrogen-bonding interactions between deprotonated amines (open pores) or Coulombic repulsions between protonated amino groups (closed pores). Under acidic conditions, the amines are fully protonated, the gate is closed, and access to the inner pores is precluded. In contrast, under neutral conditions the amines are deprotonated, the gate is open, and probe molecules can enter the pores. Concomitantly, an anion-controlled effect was also observed. In the neutral pH region, the gate is only open in the presence of small anions such as Cl− , whereas bulky anions such as adenosine 5′ -triphosphate (ATP) close the gate through formation of strong complexes with the amines at the pore entrance. More recently, several groups explored similar strategies to synthesize pHmodulable brush-coated mesoporous particles. SI-ATRP has been employed to prepare poly(2-(diethylamino)ethyl methacrylate)-coated mesoporous silica nanoparticles resulting in hybrid nanoparticles with a pH-sensitive polymer shell and mesoporous core.46 poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) brushes act as good “gatekeepers” to control access to the pores via a pH-dependent open-close mechanism, which was confirmed by the well-controlled release of rhodamine B from the mesopores by adjusting the solution pH. Poly(acrylic acid) brushes grafted onto the pore outlets of mesoporous silica via SI-RAFT polymerization were also employed to create “smart” nanogates sensitive to pH variations.47 Feng and co-workers used a “grafting-to” strategy to anchor poly(vinylpyridine) (PVP) brushes onto mesoporous silica surfaces, resulting in a proton-gated macromolecular barrier suitable for drug delivery purposes.48 Under neutral or slightly alkaline conditions, PVP brushes are collapsed on the pore entrance, thus blocking the passage of species trapped in the interior of the mesoporous particle. Upon lowering the pH conditions, the protonated brushes become swollen and permeable to the trapped molecule. On the other hand, the use of cross-linked polymer brushes as “gatekeepers” on the surface of mesoporous silica-based nanomaterials enables the gating operation in the presence of redox stimuli.49 Poly(N-acryloxysuccinimide) brushes were grafted at the pore entrance of mesoporous particles. After loading the dye molecules into the particles, the openings are blocked by the addition of cystamine, a disulfide-based

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26.7 Hybrid Architectures Combining Mesoporous Materials and Responsive Polymer Brushes

bifunctional primary amine, which allows polymer chains to be cross-linked through the reaction between cystamine and N-oxysuccimide groups along the polymer chain. The cross-linked macromolecular barrier formed around the pore opening can be reopened by cleaving the disulfide bond of cystamine in the presence of dithiothreitol, a disulfide reducing agent. In this setting, the gating mechanism relies on redox reactions in which the cross-linked polymer brush works as an off–on switch in response to redox signals. Modification of mesoporous silica thin films with polyelectrolyte brushes has also been explored as a strategy to manipulate the permselective properties of the membrane-like interfaces.50 In this nanoarchitectured films, the solvent and the mobile ionic species are confined into the nanodomains of the polyelectrolyte brushes hosted in the mesoporous matrix. Depending on the nature of the interaction between the polyelectrolyte brushes and the ionic species (attractive or repulsive), diffusion through the pore can be feasible or not. In a similar vein, the integration of poly[2-(methacryloyloxy) ethyl phosphate] brushes into and onto mesoporous silica thin films permitted the creation of mesostructured interfaces with reversible gate-like transport properties that can be controlled not only by protons but also by calcium ions.51 The ion-gate response/operation was based on the protonation and/or chelation of phosphate monomer units in which the polyelectrolyte brush works as an off–on switch in response to the presence of protons or Ca2+ ions. On the other hand, the creation of photoactive hybrid polymer brush– inorganic assemblies displaying light-activated gating and permselective transport of ionic species was described by Brunsen et al.52 The combination of “caged” poly2-[(4,5-dimethoxy-2-nitrobenzoxy) carbonyl] aminoethyl methacrylate (PNVOCAMA) brushes and mesoporous oxide thin films gave rise to the nanoarchitectonic design of photoactive interfaces. Owing to the hydrophobic and bulky nature of the monomers that precludes their free diffusion into the hydrophilic inner environment of the nanoporous framework, surface-initiated polymerization of NVOCAMA monomers on initiatorfunctionalized mesoporous films resulted in the selective growth of the photolabile brush atop the mesoporous film. Selective modification of the “outer” layer of the hybrid-mesostructured film proved decisive to control the gating properties. The hydrophobic nature of the outer PNVOCAMA layer precludes hydrated ions from entering into the mesopores and diffusing across the interfacial nanoarchitecture. Then, light exposure resulted in the cleavage of the chromophore from the polymer layer generating poly(2-aminoethyl methacrylate) brushes that upon protonation under acidic solutions confer permselective properties to the nanoarchitectured mesoporous film. Another interesting example of “soft” nanoarchitectonics is the synergistic integration of polymer brushes and mesoporous materials to attain hybrid materials with unusual properties. In this sense, Calvo et al.53 have





26 Nanoarchitectonic Design of Complex Materials

demonstrated that the interplay between the intrinsic acid–base properties of silica mesoporous frameworks and the pH-responsive properties of weak zwitterionic brushes can gave rise to new proton-gated cation-selective membranes with properties observed neither in mesoporous films nor in brushes. Mesoporous silica thin films modified with zwitterionic poly(methacryloyl-llysine) brushes act as gateable ionic filters modulating the passage of cations while inhibiting the passage of anions over a wide pH range. This behavior closely resembles the gating properties of biological acid-sensing ion channels. According to these authors, a “bipolar” Donnan exclusion phenomenon is responsible for building up, in a reversible manner, a chemically actuated ionic barrier under acidic conditions. The bipolar environment arises from the synergistic coexistence of negative charges due to the silanolate groups on the silica pore walls and positive charges due to the protonated methacryloyl-l-lysine monomer units on the brush layer. Results discussed in this section reveals that the combination of polymer brushes and mesoporous materials offers a wide range of opportunities for materials nanoarchitectonics54–56 provided that these hybrid systems can exhibit functional domains ordered in space. In fact, the integration of polymer brushes into mesoporous scaffolds bring about the possibility to create phase-separated regions (functional domains) within the pores that can behave as “gatekeepers” of nanoscale size.

. Ensembles of Metal Nanoparticles Modified with Polymer Brushes Nowadays, it is widely accepted that the control of order and periodicity of 3D arrangements of inorganic nanoparticles can be exploited at a whole new level to design a broad variety of new materials and devices.57 Ensembles of nanoparticles can display new electronic, magnetic, and optical properties as a result of interactions between the excitons, magnetic moments, or surface plasmons of individual nanoobjects.58 However, before reaching this level of synthetic control and design, it is first necessary to finely tune the interactions between neighboring nanoparticles as well as between the nanoparticles and their surrounding environment. In this regard, polymer brushes grafted to the surface of the colloidal particles59 can play a decisive role in attaining highly tunable conditions leading to the stable formation of ensembles of nanoparticles in a controllable manner. Seminal work of Fukuda’s group described early attempts to build up ordered arrays of gold nanoparticles coated with high-density PMMA brushes through the implementation of the Langmuir–Blodgett technique (Figure 26.8).60 Owing to the high surface density and the controlled chain length, grafted PMMA chains at the air–water interface exert interparticle interactions of an

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26.8 Ensembles of Metal Nanoparticles Modified with Polymer Brushes

(e)

(a)

0 10 [nm]

200 400 600

250 nm 200 (b)

400

600

800 800 [nm]

(f)

200 400 600

0

(c)

20 [nm]

0 250 nm

200

400

600

800 800 [nm]

(g) 250 nm (d) 20 [nm]

0 200 400

0

600 200

250 nm

400

600 800

800 [nm]

Figure . TEM images of the transferred nanoparticle films corresponding to (a) initiator-coated Au nanoparticles and (b–d) Au-nanoparticle–PMMA hybrids: Mn of the PMMA graft (b) 1,2000, (c) 28,000, and (d) 62,000. (e–g) AFM images of the transferred films of Au-nanoparticle–PMMA hybrids: Mn of the PMMA graft (e) 12,000, (f ) 28,000, and (g) 62,000. Source: Ohno et al. 2003.60 Reproduced with permission of Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.





26 Nanoarchitectonic Design of Complex Materials

extremely long range and, consequently, the Au-nanoparticle–PMMA hybrids form a 2D array with a high degree of structural order that translates into an exceptional controllability of the interparticle distance. 2D self-organization was also demonstrated for polystyrene-capped gold nanoparticles.61 The polystyrene brush-modified nanoparticles self-organize into hexagonally ordered monolayers when cast onto solid substrates from chloroform solutions, and the distance between the gold cores in the dried monolayer can be easily controlled by the molecular weight of the grafted polystyrene chains. Controlling the spatial distribution of the macromolecular units located at the nanoparticle surface offers a broad range of alternatives to nanoparticle systems with controlled anisotropy which can be used in a number of applications. Wang et al.62 synthesized Janus-type gold nanoparticles using bicompartment polymer brushes (PMMA and poly(ethylene oxide) (PEO)) leading to the formation of hybrid nanobuilding blocks with the following stoichiometry: PEO114 -Au6 -PMMA208 . Mixed brush-coated nanoparticles (NPs) are soluble in good solvents for PMMA and PEO, such as acetone, chloroform, tetrahydrofuran, and dimethylformamide, but they have a well-defined tendency to form particle clusters and wormlike aggregates in dioxane and dioxane/acetone mixed solvents. This phenomenon was attributed to PEO-mediated particle assembly. Note, however, that Janus-type configurations are not the only scenarios allowing two different types of chemistry to occur on the same particle. By exploiting the amphiphilic properties of gold nanoparticles coated with mixed polymer brushes of PEG and PMMA, the assembly of plasmonic dimers was recently demonstrated.63 Formation of dimeric nanoparticle assemblies is of considerable interest for applications requiring strong near-field coupling of surface plasmon resonances. In this case, Duan and co-workers showed that Au NPs functionalized with amphiphilic brush-type coatings preferentially form dimers in water due to the collapse and aggregation of hydrophobic grafts and reorganization of hydrophilic grafts (Figure 26.9). As a result, the interparticle plasmonic coupling can be reversibly controlled by modulating the assembly/disassembly of the amphiphilic nanoparticles in different environmental conditions. Using a similar approach, the same group created responsive plasmonic assemblies at oil–water interfaces in which the plasmon coupling of gold nanoparticle and nanorod ensembles was reversibly modulated by conformational changes in the polymeric coating.64 Nanocrystals modified with mixed PEG and PMMA brushes displayed collective plasmonic properties that could be tailored by changing solvent quality, whereas those modified with mixed PEG and poly(2-(diethylamino)ethyl methacrylate) brushes display pHdependent assembling properties. Edwards et al.65 demonstrated that increasing the solution temperature of poly(poly(ethylene glycol) methyl ether monomethacrylate) (PPEGMA)coated gold nanoparticles above the LCST of the polymer triggers reversible transport of the nanoparticles across an oil/water interface. Boyer et al.66 have

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26.8 Ensembles of Metal Nanoparticles Modified with Polymer Brushes (a)

Au

Au

Au

Au H2O DMF

Au

Au

Hydrophilic brush

Au Au

Au

Hydrophobic brush

80

(b)

(c)

f (%)

60

40

20

50 nm

0 Monomer

Dimer

Multimer

Figure . (a) Schematic representation of the solvent-induced self-assembly of amphiphilic gold nanocrystals modified with mixed PEG and PMMA polymer brushes. (b) TEM image of the self-assembled dimmers. (c) Statistical fractions of monomer, dimer, and multimeric structures. Source: Cheng et al. 2011.63 Reproduced with permission of American Chemical Society.

coated 20 nm gold nanoparticles with copoly(oligo ethylene glycol)acrylates with LCSTs between 15 and 92◦ C and demonstrated that the thermoresponsive properties of the polymer–gold hybrids can be tuned by adjusting the composition of the polymer coating. In a similar vein, Gehan et al.67 also described the combination of solidsupported lithographically fabricated gold NPs and thermoresponsive PNIPAM brushes to generate calibrated and dynamically controlled hybrid gold/polymer architectures for real-time nanosensing based on the phase transition of the polymer layer. Rezende and co-workers68 further explored the formation of stable Langmuir monolayers of gold nanoparticles grafted with thermosensitive PNIPAM brushes. At low surface concentration, the gold





26 Nanoarchitectonic Design of Complex Materials

nanoparticles are attached to the surface by PNIPAM chains adsorbed in a pancake conformation yielding a thin and compact layer. Upon isothermal compression, the chains are reorganized to form brush-type structures immersed in the bulk phase. Upon increasing the working temperature PNIPAM chains collapse with the corresponding decrease in the interparticle distance. The reversibility of the macromolecular reorganization at the air–liquid interface was used as a reversible thermoswitch to modulate the interparticle distance and the dielectric environment of gold nanoparticle monolayers. A recent work by Klok’s group69 investigated the thermoresponsive properties of PPEGMA-coated gold nanoparticles with gold core diameters in the 5– 47 nm range. According to their experimental evidence, these systems not only display size-dependent LCST transitions but also show cooperative thermosensitive behavior. This indicates that a PPEGMA-coated particle with a relatively high LCST can cooperatively assemble with another PPEGMA-coated particle with a lower LCST. This interesting cooperative aggregation phenomenon was utilized to manipulate the assembly of nanoparticles onto surfaces through the capture and binding of PPEGMA-coated nanoparticles on substrates modified with complementary thermoresponsive PPEGMA brushes. These results open a new perspective in the practical applications on thermoresponsive coatings provided that these findings could be exploited for enzyme recovery, protein purification, or even the thermotriggered creation of structured nanoparticle thin films. Very recently, Duan and co-workers70 reported on an interesting approach leading to a new class of turn-on surface enhanced Raman scattering (SERS) sensors for the sensitive and selective detection of cadmium ion (Cd2+ ) by taking advantage of the interparticle plasmonic coupling generated in the process of Cd2+ -selective nanoparticle self-aggregation. Their approach is based on the use of gold nanoparticles modified with a Raman-active dye and a polymer brush layer displaying Cd2+ -chelating properties. Addition of Cd2+ to the test solution promotes interparticle self-aggregation and immediately turns on the SERS fingerprint signal with up to 90-fold of signal enhancement. Control experiments with various metal ions confirmed that the platform is specific to Cd2+ ions, and, in contrast to other nanoparticle-based colorimetric assays, the method proved capable of detecting Cd2+ in heavily colored samples. This strategy clearly reveals the possibility for developing hybrid materials with integrated and collective functionalities.

. Conclusions Nanoarchitectonics is aimed at arranging nanoscale structural units into a configuration that creates a novel functionality through mutual interactions among those units. In this chapter, we have shown that polymer brushes can be used

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References

as structural and functional units to create novel nanoarchitectures displaying new functionalities arising from the delicate interplay of their nanoscale building blocks. Examples described in this chapter clearly reveal that chemists and material scientists have learnt many ways to combine concepts and tools from selfassembly and supramolecular chemistry to construct complex, hybrid, and hierarchical materials. However, it seems evident that a higher degree of structural sophistication and functionality probably cannot yet be obtained by sole reliance on the combination of self-assembly and supramolecular interactions. Controlled incorporation of polymer brushes into the nanoarchitectured systems can yield unique nanomaterials that have neither inorganic nor polymeric analogues. While most inorganic nanoarchitectures constitute a topologically well-defined but rigid structure, polymer brushes exhibit a flexible nature. In this context, the new horizons provided by “soft” nanoarchitectonics appear very wide and the future offers the prospect of many developments as chemists and material scientists show an increased mastery in construction of nanoarchitectures combining structural features and functional properties of inorganic materials and polymer brushes. The evolution of nanoarchitectonics at the beginning of this century was accompanied by applications of these developments in different technological areas. Yet, there is a need to keep exploring new avenues to attain hybrid, complex, and hierarchical nanoarchitectures exhibiting strictly controlled structure, topology, and function. We believe that this is the cornerstone to convert molecular functions into macroscopic properties expressed at the level of the architectured assemblies, thus leading to the nanoarchitectonic design and production of addressable molecular materials.

Acknowledgments The authors acknowledge financial support from ANPCyT (PICT 2010-2554, PICT-2013-0905), Fundaci´on Petruzza, Consejo Nacional de Investigaciones Cient´ıficas y T´ecnicas (CONICET) (PIP 0370) and the Austrian Institute of Technology GmbH (AIT–CONICET Partner Lab: “Exploratory Research for Advanced Technologies in Supramolecular Materials Science” – Exp. 4947/11, Res. No. 3911, 28-12-2011). J.M.G., M.L.C., W.A.M., and O.A. are CONICET fellows.

References  Pagliaro, M.; Ciriminna, R.; Palmisano, G. Chem. Rec. 2010, 10 (1), 17–28.  Ante, A.; Budimir, M. In Engineering the Future; Dudas, L.; INTECH: Vienna, 2010; pp. 26–46.

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26 Nanoarchitectonic Design of Complex Materials

 Aono, M.; Ariga, K. Adv. Mater. 2016, 28 (6), 989–992.  Ariga, K.; Li, J.; Fei, J.; Ji, Q.; Hill, J. P. Adv. Mater. 2016, 28 (6), 1251–1286.  Lu, Y.; Wittemann, A.; Ballauff, M. Macromol. Rapid Commun. 2009, 30 (9–10), 806–815.  Wunder, S.; Polzer, F.; Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. C 2010, 114 (19), 8814–8820.  Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M.; Drechsler, M. Chem. Mater. 2007, 19 (5), 1062–1069.  Sharma, G.; Mei, Y.; Lu, Y.; Ballauff, M.; Irrgang, T.; Proch, S.; Kempe, R. J. Catal. 2007, 246 (1), 10–14.  Schrinner, M.; Proch, S.; Mei, Y.; Kempe, R.; Miyajima, N.; Ballauff, M. Adv. Mater. 2008, 20 (10), 1928–1933.  Lu, Y.; Lunkenbein, T.; Preussner, J.; Proch, S.; Breu, J.; Kempe, R.; Ballauff, M. Langmuir 2010, 26 (6), 4176–4183.  Lupitskyy, R.; Motornov, M.; Minko, S. Langmuir 2008, 24 (16), 8976–8980.  Puretskiy, N.; Ionov, L. Langmuir 2011, 27 (6), 3006–3011.  Ren, Z. F.; Huang, Z. P.; Xu, J. W.; Wang, J. H.; Bush, P.; Siegal, M. P.; Prevencio, P. N. Science 1998, 282 (5391), 1105–1107.  Wei, B. Q.; Vajtai, R.; Jung, Y.; Ward, J.; Zhang, R.; Ramanath, G.; Ajayan, P. M. Nature 2002, 416 (6880), 495–496.  Liu, H.; Li, S.; Zhai, J.; Li, H.; Zheng, Q.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43 (9), 1146–1149.  Xia, F.; Zhu, Y.; Feng, L.; Jiang, L. Soft Matter 2009, 5 (2), 275–281.  Sun, T.; Liu, H.; Song, W.; Wang, X.; Jiang, L.; Li, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43 (35), 4663–4666.  Ko, H.; Zhang, Z.; Chueh, Y.-L.; Saiz, E.; Javey, A. Angew. Chem., Int. Ed. 2010, 49 (3), 616–619.  Fu, Q.; Rama Rao, G. V.; Basame, S. B.; Keller, D. J.; Artyushkova, K.; Fulghum, J. E.; L´opez, G. P. J. Am. Chem. Soc. 2004, 126 (29), 8904–8905.  Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem. 2004, 116 (3), 361–364.  Wang, X.; Qing, G.; Jiang, L.; Fuchs, H.; Sun, T. Chem. Commun. 2009, No. 19, 2658.  Brandl, C.; Greiner, A.; Agarwal, S. Macromol. Mater. Eng. 2011, 296 (9), 858–864.  Yoshikawa, C.; Zhang, K.; Zawadzak, E.; Kobayashi, H. Sci. Technol. Adv. Mater. 2011, 12 (1), 15003.  Edmondson, S.; Huck, W. T. S. Adv. Mater. 2004, 16 (15), 1327–1331.  Comrie, J. E.; Huck, W. T. S. Langmuir 2007, 23 (3), 1569–1576.  Kelby, T. S.; Wang, M.; Huck, W. T. S. Adv. Funct. Mater. 2011, 21 (4), 652–657.  Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12 (11), 3481–3487.

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References

 Duan, H.; Kuang, M.; Zhang, G.; Wang, D.; Kurth, D. G.; M¨ohwald, H. Langmuir 2005, 21 (24), 11495–11499.  Choi, W. S.; Park, J.-H.; Koo, H. Y.; Kim, J.-Y.; Cho, B. K.; Kim, D.-Y. Angew. Chem., Int. Ed. 2005, 44 (7), 1096–1101.  Morinaga, T.; Ohkura, M.; Ohno, K.; Tsujii, Y.; Fukuda, T. Macromolecules 2007, 40 (4), 1159–1164.  Azzaroni, O.; Lau, K. H. A. Soft Matter 2011, 7 (19), 8709–8724.  Cui, Y.; Tao, C.; Tian, Y.; He, Q.; Li, J. Langmuir 2006, 22 (19), 8205–8208.  Zou, C.; Luo, Z.; Le, D. H.; Dessources, K.; Robles, A.; Chen, G. J. Mater. Chem. 2011, 21 (38), 14543.  Agrawal, R. C.; Pandey, G. P. J. Phys. D. Appl. Phys. 2008, 41 (22), 223001.  Sato, T.; Morinaga, T.; Marukane, S.; Narutomi, T.; Igarashi, T.; Kawano, Y.; Ohno, K.; Fukuda, T.; Tsujii, Y. Adv. Mater. 2011, 23 (42), 4868–4872.  Srinivasan, S. In Fuel Cells: From Fundamentals to Applications; Springer: Berlin, 2006; pp. 575–605.  Bussian, D. A.; O’Dea, J. R.; Metiu, H.; Buratto, S. K. Nano Lett. 2007, 7 (2), 227–232.  Yameen, B.; Kaltbeitzel, A.; Langner, A.; Duran, H.; M¨uller, F.; G¨osele, U.; Azzaroni, O.; Knoll, W. J. Am. Chem. Soc. 2008, 130 (39), 13140–13144.  Yameen, B.; Kaltbeitzel, A.; Glasser, G.; Langner, A.; Mu¨uller, F.; Go¨usele, U.; Knoll, W.; Azzaroni, O. ACS Appl. Mater. Interfaces 2010, 2 (1), 279–287.  Yameen, B.; Kaltbeitzel, A.; Langer, A.; M¨uller, F.; G¨osele, U.; Knoll, W.; Azzaroni, O. Angew. Chem., Int. Ed. 2009, 48 (17), 3124–3128.  Vallet-Reg´ı, M.; Balas, F.; Arcos, D. Angew. Chem., Int. Ed. 2007, 46 (40), 7548–7558.  Fu, Q.; Rao, G. V. R.; Ista, L. K.; Wu, Y.; Andrzejewski, B. P.; Sklar, L. A.; Ward, T. L.; L´opez, G. P. Adv. Mater. 2003, 15 (15), 1262–1266.  Chung, P.-W.; Kumar, R.; Pruski, M.; Lin, V. S.-Y. Adv. Funct. Mater. 2008, 18 (9), 1390–1398.  You, Y.-Z.; Kalebaila, K. K.; Brock, S. L.; Oupicky D. Chem. Mater. 2008, 20 (10), 3354–3359.  Casas´us, R.; Marcos, M. D.; Mart´ınez-M´an˜ ez, R.; Ros-Lis, J. V.; Soto, J.; Villaescusa, L. A.; Amor´os, P.; Beltr´an, D.; Guillem, C.; Latorre, J. J. Am. Chem. Soc. 2004, 126 (28), 8612–8613.  Sun, J.-T.; Hong, C.-Y.; Pan, C.-Y. J. Phys. Chem. C 2010, 114 (29), 12481–12486.  Hong, C.-Y.; Li, X.; Pan, C.-Y. J. Mater. Chem. 2009, 19 (29), 5155.  Liu, R.; Liao, P.; Liu, J.; Feng, P. Langmuir 2011, 27 (6), 3095–3099.  Liu, R.; Zhao, X.; Wu, T.; Feng, P. J. Am. Chem. Soc. 2008, 130 (44), 14418–14419.  Calvo, A.; Yameen, B.; Williams, F. J.; Azzaroni, O.; Soler-Illia, G. J. A. A. Chem. Commun. 2009, 18, 2553–2555.

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 Brunsen, A.; D´ıaz, C.; Pietrasanta, L. I.; Yameen, B.; Ceol´ın, M.; Soler-Illia, G. J. A. A.; Azzaroni, O. Langmuir 2012, 28 (7), 3583–3592.  Brunsen, A.; Cui, J.; Ceol´ın, M.; Campo, A. del; Soler-Illia, G. J. A. A.; Azzaroni, O. Chem. Commun. 2012, 48 (10), 1422–1424.  Calvo, A.; Yameen, B.; Williams, F. J.; Soler-Illia, G. J. A. A.; Azzaroni, O. J. Am. Chem. Soc. 2009, 131 (31), 10866–10868.  Ariga, K.; Vinu, A.; Yamauchi, Y.; Ji, Q.; Hill, J. P. Bull. Chem. Soc. Jpn. 2012, 85 (1), 1–32.  Ariga, K.; Hill, J. P.; Lee, M. V; Vinu, A.; Charvet, R.; Acharya, S. Sci. Technol. Adv. Mater. 2008, 9 (1), 14109.  Soler-Illia, G. J. A. A.; Azzaroni, O. Chem. Soc. Rev. 2011, 40 (2), 1107–1150.  Brust, M. Nat. Mater. 2005, 4 (5), 364–365.  Nie, Z.; Petukhova, A.; Kumacheva, E. Nat. Nanotechnol. 2010, 5 (1), 15–25.  Dong, H.; Zhu, M.; Yoon, J. A.; Gao, H.; Jin, R.; Matyjaszewski, K. J. Am. Chem. Soc. 2008, 130 (39), 12852–12853.  Ohno, K.; Koh, K.; Tsujii, Y.; Fukuda, T. Angew. Chem., Int. Ed. 2003, 42 (24), 2751–2754.  Yockell-Leli`evre, H.; Desbiens, J.; Ritcey, A. M. Langmuir 2007, 23 (5), 2843–2850.  Wang, B.; Li, B.; Dong, B.; Zhao, B.; Li, C. Y. Macromolecules 2010, 43 (22), 9234–9238.  Cheng, L.; Song, J.; Yin, J.; Duan, H. J. Phys. Chem. Lett. 2011, 2 (17), 2258–2262.  Cheng, L.; Liu, A.; Peng, S.; Duan, H. ACS Nano 2010, 4 (10), 6098–6104.  Edwards, E. W.; Chanana, M.; Wang, D.; M¨ohwald, H. Angew. Chem., Int. Ed. 2008, 47 (2), 320–323.  Boyer, C.; Whittaker, M. R.; Luzon, M.; Davis, T. P. Macromolecules 2009, 42 (18), 6917–6926.  Gehan, H.; Mangeney, C.; Aubard, J.; L´evi, G.; Hohenau, A.; Krenn, J. R.; Lacaze, E.; F´elidj, N. J. Phys. Chem. Lett. 2011, 2 (8), 926–931.  Rezende, C. A.; Shan, J.; Lee, L.-T.; Zalczer, G.; Tenhu, H. J. Phys. Chem. B 2009, 113 (29), 9786–9794.  Gibson, M. I.; Paripovic, D.; Klok, H.-A. Adv. Mater. 2010, 22 (42), 4721–4725.  Yin, J.; Wu, T.; Song, J.; Zhang, Q.; Liu, S.; Xu, R.; Duan, H. Chem. Mater. 2011, 23 (21), 4756–4764.

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

Index Note: Page numbers followed by “f ” indicate figures and those followed by “t” indicate tables.

a AAEM, see 2′ -acrylamidoethyl-𝛼-dmannopyranoside acid–base equilibrium, in polyelectrolyte brushes 178 geometry, effect of 184–186, 185f pH, effect of 178–184, 181f–182f, 184f polymer density, effect of 184–186 salt concentration, effect of 178–184, 179f AcMo, see N-acryloylmorpholine 2′ -acrylamidoethyl-𝛼-dmannopyranoside (AAEM) 334, 335f 2′ -acrylamidoethyl-𝛽-dglucopyranoside 334, 335f 2-acrylamidophenylboronate 489 3-acrylamidopropyl trimethylammonium chloride (APTAC) 367 acrylonitrile butadiene styrene (ABS) 711 Activators ReGenerated by Electron Transfer (ARGET) ATRP 10

adhesive extracellular matrix 560 adsorption 271 of proteins 405–417 affinity separation 483–487 AFM, see atomic force microscopy AIBN, see azobisisobutyronitrile aldehyde-functionalized surfaces 633–634 Alexander picture 149 algae 518, 518f aligned carbon nanotubes (ACNT) films 737 alkanethiolate self-assembled monolayers 2 alkanethiol compounds 609 amine-functionalized surfaces 633 amino acids, quantification of 488 Amphora coffeaformis 546 AMPs, see antimicrobial peptides anionic polymerization 18–20, 19f of macromonomers 31–32 antibodies 435, 436f antibody fragments 435–437, 436f antifouling biointerfaces, polymer brushes surfaces as 408–412

Polymer and Biopolymer Brushes: for Materials Science and Biotechnology, First Edition. Edited by Omar Azzaroni and Igal Szleifer. © 2018 John Wiley & Sons, Inc. Published 2018 by John Wiley & Sons, Inc.

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Index

antifouling polymer brushes barnacle cement as anchor for 397–399 biomimetic anchors for 379–399 biomolecular anchors for 393–396 density 533–535, 534f–535f examples of 545t, 547t grafting of 389–391, 390f–391f interactive forces 535, 536f, 537 mechanical interactions 537 microbes and 530–537 mussel adhesive-inspired dopamine anchors for 379–391 PEG-based 531–532 (poly)phenolic anchors for 391–393 zwitterionic 533 antigens 435 antimicrobial peptides (AMPs) 540, 542–543, 543f antithrombotic surfaces, glycopolymer brushes on 341–345 adhesion 342–343, 343f–344f, 345 platelet activation 342–343, 343f–344f, 345 protein binding from human plasma 341–342, 342f surface-induced blood coagulation 345 APBA, see m-aminophenylboronic acid APTAC, see 3-acrylamidopropyl trimethylammonium chloride aptamers 437–438 atomic force microscopy (AFM) 29, 275 height profiling of 455–456, 456f imaging 455, 458f–459f protein adsorption and 412–416, 413f study on lectin binding to glycopolymer brushes with variation in composition 349–351, 350f

atom transfer radical polymerization (ATRP) 9, 130, 560 activation rate constants 35, 37f, 38, 39f in aqueous conditions 130 and concave surfaces 63 and controllable polymer surfaces 406–408, 407f drawback of 35 electrochemically mediated 130 equilibrium 130 and flat surfaces applications 57–58 architecture of 56–57 chemistry at surface 55, 56f grafting density 55–56 functional-group tolerance of 71 fundamentals 33–38, 36f–37f ligands for 38 low catalyst concentration procedures 34–35 molecular bottlebrushes by 38–54 with block copolymer side chains 46–50 blockwise 42–45 functionalities 50–54 properties of 50–54 star-like 40–42 with tunable grafting density 45–46 and nanoparticles applications 61–63, 62f architecture of 59, 60f–61f, 61 chemistry 58–59, 59f rate of 34–35 and templates from brushes 64–65, 64f from networks 63, 64f thermoresponsive polymer brushes preparation by 363–368, 365f–366f, 368f atom transfer radical polymerization (ATRP) macroinitiator 740

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ATRP, see atom transfer radical polymerization ATRP initiators 591 attenuated total reflection (ATR) method 688 autophobic effect, for polymer brushes 99 azobisisobutyronitrile (AIBN) 10

b BAECs, see bovine carotid artery endothelial cells Balanus amphitrite 385, 397 barnacle cement, for antifouling polymer brushes 397–399, 398f bead spring model for polymer chains 148–149 biobrush layers grafting density control 451–453, 453f recognition units, conformation and orientation of 453–461, 454f–456f, 458f–459f, 461f biofilms 515 biofouling 377 management 377 mitigation of 377 bioinspired poly(dopamine) (PDA) 568 biomimetic anchors, for antifouling polymer brushes 379–399 biomolecular anchors, for antifouling polymer brushes 393–396 bioorthogonality 439 biorecognition units antibodies 435, 436f antibody fragments 435–437, 436f aptamers 437–438 conformation and orientation of 453–461, 454f–456f, 458f–459f, 461f enzymes 438–439

lectins 439 molecular imprinted polymers 439 peptide aptamers 438 peptide nucleic acid 439 bio-related polymer brushes 66–74, 66f–67f biosensors 487–492 biotin–avidin/streptavidin pairing 447–448 bipolar electrode (BPE) 125, 127, 127f patterning and gradient formation by 135–136, 135f Bjerrum length 225 block copolymerization, of PIPAAm brushes 369–370 block copolymer side chains, brushes with 46–50 blockwise brushes 42–45 BMP-2, see bone morphogenetic 2 BMPUS, see [11-(2-bromo-2methyl)propionyloxy] undecyltrichlorosilane Boltzmann law 230 bone morphogenetic 2 (BMP-2) 383 boronate 483–486 boronic acid brushes 481–483, 481f bovine carotid artery endothelial cells (BAECs) 364 bovine serum albumin (BSA) 383, 467 BPE, see bipolar electrode branched polyions 224–226, 225f [11-(2-bromo-2-methyl)propionyloxy] undecyltrichlorosilane (BMPUS) 245 Brunaur–Emmett–Teller (BET) adsorption 65 brush–brush block copolymers 42, 44, 44f brush-filled colloidal nanopores 656 brush regime 2, 3f BSA, see bovine serum albumin

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Index

c CAG-peptide functionalized PCU-film surfaces schematic illustration of 584 Candida albicans 517, 544 Candida tropicalis 544 carbon–heteroatom (C–X–C) bonds 293 carbon radical 14 carboxymethyl chitosan (CMCS) 383 cationic polymers 590 Cauchy film 248 cell adhesion 565 protein coupling to polymer brushes 581 cell adhesion and culture 570–573 cell adhesion-mediating proteins 565 cell-free surface protein expression system 644f cell-released enzymes 560 cell-resistant polymer brushes 565 chain transfer agent (CTA) 10–11 charge-regulating systems, self-assembly in 189–190, 191f–192f, 193 charge regulation 163 chemical equilibria in polymer/ polyelectrolyte brushes, theoretical approach 163–177 chiral monomers 672, 672f chiral selectors 673 Chlorella 546, 548 choline phosphate (CP) 567 click chemistry 32 definition 293–295 end groups of grafted PNIPAAm chains, modification of 295, 296f–297f, 297 CMCS, see carboxymethyl chitosan coarse-grained models 142 colloidal lithography 570 colorimetric sugar sensors 487

complementary density gradient of PDMAPS schematic illustration of 586 Con A 347 concentrated particle brush (CPB) regime 59 conducting polymers 695 conductive materials 710 contact angle of water on water-equilibrated polymer brush 270–271 controllable polymer surfaces, polymer brushes layers as 406–408, 407f chain length and layer thickness, control of 408 conversion of terminal groups 408 graft density, control of 408 monomer selection 407 controlled radical polymerization (CRP) 38 controlled stereochemistry 574–576 copolymer brushes 587 copolymerization of macromonomers 32 CPB, see concentrated particle brush Craspedostaurus 546, 548 CRP, see controlled radical polymerization CTA, see chain transfer agent CuAAC, see Cu(I)-catalyzed azide–alkyne cycloaddition Cu(I) catalyst diffusion 134, 134f Cu(I)-catalyzed azide–alkyne cycloaddition (CuAAC) 124, 295, 487 curved substrates, polymer brushes on 141–157, 144f cyclopentadiene 444 cylindrical polymer brushes 150–152, 151f cysteine methacrylate (CysMA) 564

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Index

d DEAEMA, see 2-(diethylamino)ethyl methacrylate Debye screening length 226 de Feijter equation, for adsorbed amount of protein 305 degraded polymeric substrate 710 degree of deprotonation 178, 179f density functional theories (DFT) 143 DFT, see density functional theories DHI, see dihydroxyindole dielectric-modulated FET (DMFET) 464–465 Diels–Alder cycloaddition 444, 445f 2-(diethylamino)ethyl methacrylate (DEAEMA) 245 dihydroxyindole (DHI) 383 2-(dimethylamino)ethyl methacrylate (DMAEMA) 245 1,1-diphenylethylene (DPE) 19–20 1,3-dipolar cycloaddition 446–447 dip-pen nanolithography 449 diprotic acid 169 5,5-dithiobis-(2-nitrobenzoic acid) (DTNB) 469 dithiobis(sulfosuccinimidylpropionate) (DTSSP) 470 dithiocarbamate radical 14 divalent metal cations 195 DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory 518 DMAEMA, see 2-(dimethylamino)ethyl methacrylate DMAPAAm, see N,Ndimethylaminopropylacrylamide DMFET, see dielectric-modulated FET DNA brushes 162, 605–606 bioprocesses in 618–619 fabrication of 632 hybridization in 613–618 perspective 619–620

physicochemical properties of 610–613 properties 637–643 synthesis of 628, 637 DNA brush-modified nanoparticles 645 DNA grafting density 630 DNA hybridization 620 DNA microarrays 606 DNA-modifying enzymes 640 DNA oligomers 607 dopamine anchors, mussel adhesive-inspired 379–391, 381f–382f DPE, see 1,1-diphenylethylene DPI, see dual polarization interferometry drug delivery polymer brushes 586–589 DTNB, see 5,5-dithiobis(2-nitrobenzoic acid) DTSSP, see dithiobis (sulfosuccinimidylpropionate) dual polarization interferometry (DPI) 454

e eATRP, see electrochemically mediated atom transfer radical polymerization ECG, see epicatechin gallate ECM-derived peptides 583 EC-STM, see electrochemical scanning tunneling microscopy EDC, see 1-ethyl-3-(3dimethylaminopropyl) carbodiimide EDTA, see ethylenediaminetetraacetic acid effective medium approximation (EMA) 248 EGCG, see epigallocatechin gallate

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Index

EG3OMe system 424 EG7OMe system 424 elastomers 724 ELD system 716 electrochemical-based sensors 462–463, 463f electrochemically controlled brush growth 133–134, 133f electrochemically mediated atom transfer radical polymerization (eATRP) 130, 130f bipolar electrolysis assisted 135–136, 135f on conducting substrate 130–132, 131f “e-click” chemistry on surface 136 gradients by Cu(I) catalyst diffusion 134, 134f on other substrates 132–134, 133f surface functionality and 136–137 surface patterning and 132 electrochemical scanning tunneling microscopy (EC-STM) 459–460 electro-click chemistry 124–129, 125f–129f on bipolar electrode 125, 127, 127f eATRP and 136 in one pot 128–129, 129f by SECM 124–125, 125f stenciled 125, 126f surface functionality and 136–137 by using microelectrode arrays 127–128, 128f electrophilic moieties 656 electrostatic interactions 570 electrostatic repulsion 179–180, 186, 661 ELISA, see enzyme linked immunosorbent assay ellipsometry 275–276, 276f, 454, 455f EMA, see effective medium approximation

end-grafted polymers 3–4. See also polymer brushes structure of 161 end groups of grafted PNIPAAm chains, modification of 295, 296f–297f, 297 end-tethered single stranded DNA in aqueous solutions 195–201, 197f magnesium and 196 pH, variation of 196 polyacid and magnesium 199–201, 199f polyacid and sodium ion 197–199, 197f salt concentration, variation of 196 structure of 195–196 end-tethered weak polyelectrolytes 164–177 enzyme linked immunosorbent assay (ELISA) 410, 438–439 enzymes 438–439 epicatechin gallate (ECG) 392 epigallocatechin 392 epigallocatechin gallate (EGCG) 392 epoxy-functionalized surfaces 633 Escherichia coli 383, 386–387, 389, 533 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) 383 ethylenediaminetetraacetic acid (EDTA) 458 eukaryotes microorganisms 516

f Fab region 435, 436f Fc region 435, 436f FETs, see field effect transistors field effect transistors (FETs) 463–465, 464f finitely extensible nonlinear elastic (FENE) potential 148

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flat substrates, polymer brushes on 141–157, 144f Flory–Huggins polymer 269 fluorescence sugar sensors 487 “Fourier-space” approach 704 freely jointed chain (FJC) model 236 functional polymer brushes 97

g GA, see gallic acid galactose 338 gallic acid (GA) 392 Gaussian elasticity, threshold of dendron 234–235, 235f gene delivery polymer brushes 590–593 Geobacillus thermocatenulatus 393 glycidyl methacrylate (GMA) monomer 743 glycocalyx 333, 351 glycopolymer brushes 333–334 antithrombotic surfaces based 341–345 adhesion 342–343, 343f–344f, 345 platelet activation 342–343, 343f–344f, 345 protein binding from human plasma 341–342, 342f surface-induced blood coagulation 345 applications of 341–344 carbohydrate arrays to multivalent protein binding on surfaces 345–351 AFM force spectroscopy study on lectin binding 349–351, 350f SPR analysis, interaction of Con A with glycopolymer layers 347–349, 347f, 348t grafted NPs 340t innate immune response, modulation of 351–356

carbohydrate structure on complement activation 351–354 complement activation by grafted glycopolymer chain 354–356, 355f glycopolymer conformation (grafting density), influence of 351–354, 352f–353f synthesis of gold chip 334–335 modified particles with different grafting density (conformation) 338, 339t–340t, 341 N-substituted acrylamide derivatives 334 polystyrene particles 335–336, 336f silicon wafer 334–335 with variation in composition of carbohydrate residues on SPR chip 338, 339t gold surface, polymer brushes on 110–114, 112f–113f Gouy−Chapman length 226, 231 gradient polymer brushes 5, 134, 134f, 293 grafting density 2–3, 3f, 287–288 biobrush layers and 451–453, 453f “grafting-from” approach 123, 378, 480–481 for molecular bottlebrushes synthesis 32–33, 33f on mussel adhesive-inspired dopamine anchors 386–389, 388f on (poly)dopamine anchor 386–389, 388f for polymer brushes preparation 4f, 5, 288 SI-ATRP 9–10, 9f SI-PIMP 13–15, 14f SI-RAFT 10–13, 11f–12f

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Index

“grafting-from” approach (Continued) SI-ROMP 17–18, 18f SI-ROP 15–17, 16f surface-initiated anionic polymerization 18–20, 19f surface-initiated nitroxidemediated polymerization 13 “grafting-through” approach, for molecular bottlebrushes synthesis 31–32 “grafting-to” approach 123, 378 functional biointerfaces tailored brushes 287–320 hMSC on 316 for molecular bottlebrushes synthesis 32 on mussel adhesive-inspired dopamine anchors 383–386, 386f PEO coating 384–385 for polymer brushes preparation 4, 4f, 5–8, 8f, 288 Gram-negative bacteria 516–517, 517f, 529 Gram-positive bacteria 517, 517f, 529 green fluorescent protein (GFP) 643

h harvesting and detachment responsive behavior 576–580 hbmMSCs, see human bone-marrow-derived mesenchymal stem cells HCCHM3 cells 589 Helmholtz free energy 172 HEMA, see 2-hydroxyethyl methacrylate HEMA-TMS 46 hGG, see human chorionic gonadotropin high-intensity focused ultrasound (HIFU) 587 Hill equation 348

hMSC, see human mesenchymal stem cells hollow silica nanoparticles (HSNPs) 587 horseradish peroxidase (HRP) 311, 501 HRP, see horseradish peroxidase HSA, see human serum albumin HSMMs, see human skeletal muscle myoblast cells human aortic smooth muscle cells 366 human bone-marrow-derived mesenchymal stem cells (hbmMSCs) 367 human chorionic gonadotropin (hGG) 457 human mesenchymal stem cells (hMSC) 316 human serum albumin (HSA) 311, 461 human skeletal muscle myoblast cells (HSMMs) 366 human umbilical vein endothelial cells (HUVECs) 366–367 HUVECs, see human umbilical vein endothelial cells hybrid brush nanostructures 297–298 nanoparticles, immobilization of 298–300, 299f–300f sculptured thin films 300–303, 301f, 303f hydrogel glucose sensors 487 hydrogel structure 698 hydrophilicity of polymer 269–270 hydrophilic polymer brushes, vapor swelling of 243–244, 267 experimental bulk synthesis of PDMAEMA 246 chemical modification of spuncast PDMAEMA films 247

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Index

fitting spectroscopic ellipsometry data 248 general methods 245 infrared variable angle spectroscopic ellipsometry 248 PDMAEMA brushes, chemical modification of 246 poly((2-dimethylamino) ethylmethacrylate) brushes with gradient in grafting density 245 spectroscopic ellipsometry measurements 247 spuncast PDMAEMA films, preparation of 246 synthesis of poly(2-(diethylamino) ethylmethacrylate) brushes 245–246 mole fraction calculation 263 results and discussion 248–250, 249f grafting density on 259–262, 259f, 261f polymer brush versus spuncast polymer film swelling 250–252, 251f side chain chemistry 252–256, 253f, 255f solvent vapor chemistry 256–258, 257f–258f water cluster number calculation 264–265, 264f 2-hydroxyethyl methacrylate (HEMA) 44–45, 131–132, 268, 279–282, 411–412

i IMAC, see immobilized metal affinity chromatography immobilized metal affinity chromatography (IMAC) 70 immunoglobulins (Ig), see antibodies

iniferters 13–14 interface-mediated RAFT polymerization 99 polymer brushes via 101–117 on gold surface 110–114, 112f–113f micropatterned 115–117, 118f on nanoparticles 114–115, 115f pH-responsive 102–106 temperature-responsive 106–110, 110f isopropyl acryl amide 702

k killing surfaces, microbes brushes and 537–543, 540f antimicrobial peptides 540, 542–543, 543f Kretschmann configuration 465, 703 Kuhn segment length 225

l Langevin thermostat 147–148 lanthanide-doped upconversion nanoparticles (UCNPs) 589 layered double hydroxides (LDHs) 591 LCST, see lower critical solution temperature Le Chatelier principle 180 lectins 439, 483 Lewis acids 481 ligand–receptor binding 201–206 ligands 483 boronic 485 m-aminophenylboronic acid (APBA) 484, 484t phenylboronic acid 489, 490f lithography methods 697 localized SPR (LSPR) sensors 466 localized surface plasmon resonance (LSPR) 300

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Index

localized surface plasmon resonance (LSPR) wavelength 691 local osmotic pressure 174 lower critical solution temperature (LCST) 267–268, 289, 747 LSPR, see localized surface plasmon resonance LSPR shifts 694 lysozyme 467

m malachite green carbinol base (MGCB) 645 maleimide–thiol direct addition 443, 443f m-aminophenylboronic acid (APBA) 484 ligand 484, 484t Marinobacter hydrocarbonoclasticus 537 matrix-assisted catalytic printing (MACP) method 718 MC3T3, 70 MeOEGMA monomer 747 mercaptoethanol 452 2-methacryloyloxyethyl phosphorylcholine (MPC) 409 3-methylisoxanthopterin (3MI) 468 methyl methacrylate (MMA) 14 3MI, see 3-methylisoxanthopterin microbes brushes and 519, 520t–528t, 529 adhesive surfaces 529–530, 531f algae 546, 547t, 548–549, 549f–550f antifouling surfaces 530–537 fungi 543–546, 545t, 546f killing surfaces 537–543, 540f societal relevance for surfaces interacting with 515–516 microcantilever sensors 469–471, 470f

microcontact printing (μCP) 132 microelectrode arrays, electro-click chemistry by 127–128, 128f microorganisms 516–519, 517f–518f eukaryotes 516 interactions with polymer brushes 515–519 prokaryotes 516 micropatterned polymer brushes 115–117, 118f miktoarm hybrid systems 57 Milner-Witten-Cates (MWC) model 422 MIP, see molecular imprinted polymers mixed polymer brushes 290–292, 292f MMA, see methyl methacrylate molecular bottlebrushes by ATRP 38–54 block copolymer side chains, brushes with 46–50 blockwise brushes 42–45 functionalities 50–54 properties of 50–54 star-like brushes 40–42 tunable grafting density, brushes with 45–46 classification of 40 micelle-like structures of 49 stimuli-responsive 74–79 structure of 29–31, 30f synthesis of 31–33, 31f “grafting-from” approach 32–33, 32f “grafting-through” approach 31–32 “grafting-to” approach 32 molecular dynamics (MD) methods 144–148 and SCFT 150–151 molecular imprinted polymers (MIP) 439 molecular spoked wheel (MSW) 42

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Index

molecular theory 164–177 applications of 177–178 acid–base equilibrium in polyelectrolyte brushes 178–186 chemical equilibria versus physical interactions 186–195 end-tethered single stranded DNA in aqueous solutions 195–201 grafting-to formation of polymer brushes 207–212 ligand–receptor binding 201–206 protein adsorption to polymer brushes 201–206 mole fraction calculation 263 monoclonal antibodies 435 Monte Carlo scheme 165 MPC, see 2-methacryloyloxyethyl phosphorylcholine MSW, see molecular spoked wheel mushroom regime 2, 3f mussel adhesive-inspired dopamine anchors 379–391, 381f–382f direct grafting of antifouling polymer brushes 389–391, 390f–391f “grafting-from” approach on 386–389, 388f “grafting-to” approach on 383–386, 386f

n N-acryloylmorpholine (AcMo) 371–372 N-acylimidazole 445 nanoFET sensor 463–464, 464f nanoflare structures schematic illustration of 647f “NanoFlare” system 649 nanoparticles ATRP and applications 61–63, 62f architecture of 59, 60f–61f, 61

chemistry 58–59, 59f immobilization of 298–300, 299f–300f polymer brushes on 114–115, 115f nanoporous membranes 655 nanoporous silica colloidal films 672 Navicula perminuta 546, 548 N-dimethyl-aminoethylmethacrylate (DMAEMA) 676 near-edge X-ray absorption fine structure (NEXAFS) spectroscopy 461 neutron reflectometry 454, 455f NEXAFS, see near-edge X-ray absorption fine structure NHDFs, see normal human dermal fibroblasts NHS, see N-hydroxysuccinimide N-hydroxysuccinimide (NHS) 383 NIPAM, see N-isopropylacrylamide NIR irradiation 645 N-isopropylacrylamide (NIPAM) 135 nitrile imine-mediated tetrazole-ene cycloadditions (NITEC) 568 nitrilotriacetic acids (NTA)–Ni2+ – histidine pairing 448–449 nitroxide-mediated polymerization 13 N,N-dimethylaminopropylacrylamide (DMAPAAm) 367 n-octyltrichlorosilane (OTS) 245 nonthermoresponsive poly(oligo(ethylene oxide)methacrylate) copolymer brushes 279–282, 280f normal human dermal fibroblasts (NHDFs) 366–367 nucleases 619 nucleic acid 195

o octadecyl methacrylate(ODMA) 45 ODMA, see octadecyl methacrylate

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Index

OEGMA, see oligo(ethylene oxide)methacrylate oligo(ethylene oxide)methacrylate (OEGMA) 268, 410–411 oligonucleotides 195 hybridization 450–451, 451f one pot, electro-click reaction in 128–129, 129f optical sensors 487 OTS, see n-octyltrichlorosilane

p PAA-poly(2 vinylpyridine) (P2VP) brushes 291–292, 298–300 PAMD process 720 PAN, see polyacrylonitrile paratope 435, 436f Parkinson’s disease 590 PBA, see phenylboronic acid PBA-based smart systems beyond polymer brush 509, 510f, 511 PBNP membranes 679 PBNPs membranes 676 PBPE, see polymer-based protein engineering PCA, see principal component analysis PCL films 581 PDA, see polydopamine PDMAEMA, see poly((2-dimethylamino)ethyl methacrylate); poly(2-(N,Ndimethylamino)ethyl methacrylate) PDMAEMA brushes 663 PDMAEMA copolymer brushes preparation of 677f PDMAEMA-MeI brush 250–258, 251f, 253f, 255f, 257f–258f PDMAEMA-modified colloidal membranes 661–662 PDMAEMA-PrI brush 253–258, 253f, 255f, 257f–258f

PDMS, see poly(dimethylsiloxane) PEG-based antifouling polymer brushes 531–532 PEGMA, see poly(ethylene glycol) methacrylate PEGylated surfaces 561 PEI, see polyethyleneimine PEOXA, see poly-2-ethyl-2-oxazoline peptide aptamers 438 peptide nucleic acid (PNA) 439 PG, see pyrogallol PHEMA brushes 565, 679 PHEMA membranes 678 phenol ester 445 phenylboronic acid (PBA) affinity separation and 483–487 biomedical applications 492–493, 493t ligands 489–490 optical sensors 487 polymer brushes and 479–494, 480f sensors 487–492, 491t smart surfaces modification with 497–498, 498f–499f molecular mechanism 498, 500, 500f pH-responsive surfaces 501, 502f, 503 pH/sugar dual-responsive AND logic gate 506–509, 507f–508f pH/sugar dual-responsive OR logic gate 504–506, 505f–506f sugar-responsive surfaces 503–504, 504f photochromic effect 692 photoluminescence (PL)-based sensors 466–468, 467f–468f pH-responsive polymer brushes 102–106 pH-responsive surfaces modification, with PBA polymer brushes 501, 502f, 503

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Index

pH/sugar dual-responsive AND logic gate modification, with PBA polymer brushes 506–509, 507f–508f pH/sugar dual-responsive OR logic gate modification, with PBA polymer brushes 504–506, 505f–506f pH/sugar dual-responsive surfaces 500 p-hydroxyphenylboronic acid 486 PLA films 581 planar polyelectrolyte (PE) brushes dendrons with arbitrary architecture 229–231 star-like 232–234, 233f plasmonics 687 PMbA brushes 449–450, 450f PNA, see peptide nucleic acid PNIPAAM brush-filled silica colloidal membranes 664–666 PNIPAM, see poly(N-isopropyl acrylamide) PNIPAM brushes 578, 737 POEGMA, see poly((oligo(ethylene glycol)) methacrylate) POEGMA brushes 565 polyacrylamide brushes 9 grafting 3f poly(acrylamidophenylboronic acid) (PAAPBA) 580 polyacrylonitrile (PAN) 61 brushes 63, 64f polycationic brush surfaces 529–530 polyclonal antibodies 435 poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA) 246 bulk synthesis of 246 pH-responsive surface and 501, 503

spuncast films chemical modification of 247 preparation of 246 poly(2-(diethylamino)ethyl methacrylate) (PDEAEMA) brush synthesis 245–246 poly(dimethylsiloxane) (PDMS) 132, 449 polydopamine (PDA) anchor 380, 382–389, 382f poly(𝜀-caprolactone) (PCL) brushes 15, 16f polyelectrolyte (PE) brushes 161, 564, 660 acid–base chemical reactions in 162 acid−base equilibrium in 178 geometry, effect of 184–186, 185f pH, effect of 178–184, 181f–182f, 184f polymer density, effect of 184–186 salt concentration, effect of 178–184, 179f chemical equilibria in versus physical interactions 186–195 theoretical approach 163–177 chemical reactions 161–162 electrostatic potential in 163 linear/branched polyions scaling-type diagrams of states for 235–236, 235f SCF theory of 224–229 microphase separation in, process of 187, 188f, 189 pH responsiveness to 162 planar, dendrons with arbitrary architecture 229–231 asymptotic dependences for brush thickness H 231–232

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Index

polyelectrolyte (PE) brushes (Continued) protein adsorption to surfaces 310–313, 312f–313f redox 162 redox-active 193, 194f, 195 SCF theory of linear/branched polyions 224–225 analytical formalism 226–229 dendron architecture 225–226, 225f system parameters 225–226 self-assemble 186–189 star-like 232–234, 233f strong 186–189, 188f weak 189–195 polyelectrolyte poly(acrylic acid) (PAA) brushes 289–290 polyelectrolytes 161 poly(ethylene glycol) (PEG) 291 brushes 422, 423f, 424 poly(ethylene glycol) (PEG) derivatives 746 poly(ethylene glycol) methacrylate (PEGMA) 44–45 polyethyleneimine (PEI) 385 poly-2-ethyl-2-oxazoline (PEOXA) films 427, 428f poly(glycidyl methacrylate) (PGMA) 7–8, 8f poly(2-hydroxyethyl methacrylate) (PHEMA) brushes 10, 268, 424–429, 425f–428f poly(hydroxyl propyl methacrylamide) brushes 565 poly(hydroxypropyl methacrylamide) (PHPMA) brushes 562 poly(lactic acid) (PLA) brushes 16–17, 17f poly(l-glutamate) brushes 17 poly(l-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) 422, 423f, 424 polymer adsorption 208–209, 208f

polymerase chain reaction (PCR) 608 primers 608 polymer-assisted metal deposition 712–715 polymer-based protein engineering (PBPE) 71 polymer brushes 2, 560, 590, 627, 747, 750–751 applications 421–422 in biomedical applications 492–493, 493t bio-related 66–74, 66f–67f cell adhesion and interaction, as interfaces for 313–314 growth factors 318–320, 319f on stimuli-responsive surfaces based on PNIPAAm brushes 315–316, 317f, 318 chemical equilibria in, theoretical approach 163–177 by click chemistry definition 293–295 end groups of grafted PNIPAAm chains, modification of 295, 296f–297f, 297 computer simulations of 142 on curved substrates 141–157, 144f cylindrical 150–152, 151f on flat substrates 141–157, 144f with free chains 152–153, 154f, 156f functional 97 fundamental notions of 1–4 on gold surface 110–114, 112f–113f gradient 5, 134, 134f grafting density of 2–3, 3f, 287–288 grafting-to formation of 207–212, 210f–212f hybrid brush nanostructures 297–298 nanoparticles, immobilization of 298–300, 299f–300f

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Index

sculptured thin films 300–303, 301f, 303f via interface-mediated RAFT 101–117 layers as controllable polymer surfaces 406–408, 407f chain length and layer thickness, control of 408 conversion of terminal groups 408 graft density, control of 408 monomer selection 407 lubrication 421–422, 422f microbes and 519, 520t–528t, 529 adhesive surfaces 529–530, 531f algae 546, 547t, 548–549, 549f–550f antifouling surfaces 530–537 fungi 543–546, 545t, 546f killing surfaces 537–543, 540f and microorganisms 515–551 micropatterned 115–117, 118f mirrors, swelling coefficient of 269–270 on nanoparticles 114–115, 115f phenylboronic acid and 479–494, 480f physicochemical interfaces by, design of mixed polymer brushes, combination of responses using 290–292, 292f stimuli-responsive gradient brushes 293 stimuli-responsive homopolymer brushes 288–290, 290f–291f preparation “grafting-from” approach for 9–20 “grafting-to” approach for 5–8, 8f properties of 421–422 protein adsorption to surfaces 201–206, 303–304, 304f, 409t

calculation, from ellipsometric experiment 305–306 at polyelectrolyte brushes 310–313, 312f–313f prevention 306–309, 307f–309f role of 712–715 SAMs versus 2 via SI-RAFT 99–101 on solid substrates 4–5, 4f spherical 150–152, 151f versus spuncast polymer film swelling 250–252, 251f stimuli-responsive 74–79 structure of 29–31, 30f for surface functionalization 378–379 surface hydrophilicity of 270–271 surfaces as antifouling biointerfaces 408–412 synthesis of 31–33, 31f, 33f, 479–481, 480f tribological behavior of 421 vapor swelling grafting density on 259–262, 259f, 261f side chain chemistry on 252–256, 253f, 255f solvent vapor chemistry on 256–258, 257f–258f volume hydrophilicity 269–270 polymer brushes, selection criterion of 716 polymer brush-filled colloidal membranes 674 polymer brush films 576 poly(mercaptopropyl) methyl siloxane (PMPMS) 609 polymer chains 660 polymer films 243–244, 244f polymeric hollow spheres 744 polymer-modified colloidal membranes 669 polymer surface coatings 287

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Index

poly(methyl methacrylate) (PMMA) brushes 9–10 poly(MPC) brush 409–410 poly(N-isopropyl acrylamide) (PNIPAAm) brushes 289, 361 ATRP technique for grafting and 364 block copolymerization of 369–370 brushes 363–368 end groups of grafted chains, modification of 295, 296f–297f, 297 fabrication of cell sheets 362–363, 363f growth factors 318–320, 319f modified surface 362–363, 362f–363f RAFT polymerization for 368–372 stimuli-responsive surfaces based on 315–316, 317f, 318 terminal functionalization of 369 thermally modulated cell adhesion and detachment on 362–363, 362f poly(N-isopropyl acrylamide) (PNIPAM) polymers 6, 267–268 brushes 289 temperature-dependent surface hydrophilicity of dense brushes 272, 273f, 274 temperature-dependent swelling and volume hydrophilicity of dense brushes 274–277, 275f–276f poly(2-(N,N-dimethylamino)ethyl methacrylate) (PDMAEMA) 61 brushes 65 modification of surfaces with 68 poly(N-propionylethyleneimine) (PPEI) 15

poly(OEGMA-r-HEMA) brushes 70 poly((oligo(ethylene glycol)) methacrylate) (POEGMA) 50 poly(oligo(ethylene glycol) methacrylate) (POEGMA) brushes 562 (poly)phenolic anchors 391–393 polyphenols 392 poly(p-phenylene ethynylene) (PPE) brushes 17–18 polystyrene particles, glycopolymer brushes synthesis of 335–336, 336f, 337t poly(4-vinylbenzyl chloride) (PVBC) 364 poly(4-vinylpyridine) (P4VP) 66 postpolymerization modification (PPM) 246 PPEI, see poly(N-propionylethyleneimine) PPM, see postpolymerization modification principal component analysis (PCA) 460 probability distribution function (pdf ) 164–165 prokaryotes microorganisms 516 proline-histidine-serinearginineasparagine (PHSRN) 583 protein adsorption 561 protein adsorption to polymer brushes 201–206, 202f, 205f, 303–304, 304f, 405–417 analysis of 409t calculation, from ellipsometric experiment 305–306 elucidation of 412–416, 413f–414f, 416f–417f at polyelectrolyte brushes 310–313, 312f–313f prevention 306–309, 307f–309f primary 411

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Index

secondary 411 ternary 411 protein A/protein G – Fc pairing 449–450, 450f protein isotherms 203 Proteus mirabilis 386 proton exchange membranes (PEMs) 745 Pseudomonas aeruginosa 386, 519 pSSA membranes 680–681 proton conductivity of 682 pSSA-sintered colloidal membranes proton conductivity of 675 PVBC, see poly(4-vinylbenzyl chloride) P4VP, see poly(4-vinylpyridine) pyridyil disulfide–thiol exchange reaction 442, 443f pyrogallol (PG) 392

q QCM, see quartz crystal microbalance QCM-D, see quartz crystal microbalance measurements with dissipation monitoring QDs, see quantum dots quantum dots (QDs) 466 quartz crystal microbalance (QCM) 410, 562 quartz crystal microbalance measurements with dissipation monitoring (QCM-D) 274, 275f, 277

r rabbit anti-human prostate-specific antigen (RAH-PSA) 470–471 RAH-PSA, see rabbit anti-human prostate-specific antigen raspberry-like particles 737 RCA amplification 639 RDRP, see reversible-deactivation radical polymerization reaction scheme 670f

redox-active polyelectrolyte brushes 193, 194f, 195 redox polyelectrolyte brushes 162 reduced graphene oxide (RGO) 720 relative humidity (RH) 243 responsive polymers 692 reversible addition–fragmentation chain transfer (RAFT) polymerization 6, 10–11, 97–99 interface-mediated 99 thermoresponsive polymer brushes preparation by 368–372, 370f–371f reversible-deactivation radical polymerization (RDRP) techniques 32 RGD peptides 306, 585 R group approach, interface-mediated RAFT polymerization 99 RH, see relative humidity Rhizomucor miehei 393 ring-opening metathesis polymerization (ROMP) 17–18, 18f of norbornenyl macromonomers 31–32 RIS, see rotational isomeric model ROMP, see ring-opening metathesis polymerization rotational isomeric model (RIS) 165 Rouse model of polymer dynamics 143

s SA, see sialic acid SAMs, see self-assembled monolayers scanning electrochemical microscopy (SECM) 124–125, 125f scattering length density (SLD) profiles 248–250, 249f SCFT, see self-consistent field theory

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Index

Scheutjens and Fleer (SF-SCF) model of dendron brush 236–238, 237f–238f Scheutjens–Fleer theory 169 SCMF, see single-chain mean field theory sculptured thin films (STF) 297–298, 300–303, 301f, 303f SDPB, see semidilute particle brush SECM, see scanning electrochemical microscopy SELEX, see Systematic Evolution of Ligands by EXponential self assembled monolayer (SAM) 628 self-assembled monolayers (SAMs) 1–2 alkanethiolate 2 of DTNB 469 limitation 2 NTA 449 versus polymer brushes 2 sulfhydryl-terminated 448–449 self-assemble polymer brush 186–189 self-consistent field theory (SCFT) 143 of linear/branched polyions PE brushes 224–225 analytical formalism 226–229 dendron architecture 225–226, 225f system parameters 225–226 MD and 150–151 threshold of dendron Gaussian elasticity and 234–235, 235f self-organized nanostructures 186 Semenov’s theory 226 semidilute particle brush (SDPB) regime 59 sensors biosensors 487–492 colorimetric sugar 487

electrochemical-based 462–463, 463f fluorescence sugar 487 hydrogel glucose 487 LSPR 466 microcantilever 469–471, 470f phenylboronic acid 491t, 492 photoluminescence-based 466–468, 467f–468f SERS 468–469, 469f SPR-based 465–466, 465f SPRi 466 SERS, see surface enhanced Raman spectroscopy short sequences of single-stranded DNA (ssDNA) 195, 452–453, 453f sialic acid (SA) 500 SI-ATRP, see surface-initiated atom transfer radical polymerization silica colloidal crystals 655–656 silica colloidal membranes 657 silica spheres modification of 667 silicon-based substrates 632 immobilization on 632–633 silicon nanowire (SiNW) arrays 580 silicon nanowire (SiNW) arrays grafted with poly(acrylamidophenylboronic acid) (PAAPBA) 579 SI-LRP, see surface-initiated living radical polymerization single-chain mean field theory (SCMF) 143 single-walled carbon nanotube (SWNT) 463–464 sintered colloidal crystals 657 SIP, see surface-initiated polymerization SI-PIMP, see surface-initiated photoiniferter-mediated polymerization

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Index

SI-RAFT, see surface-initiated reversible addition fragmentation chain transfer polymerization SI-RAFT polymerization 748 SI-ROMP, see surface-initiated ring-opening metathesis polymerization SI-ROP, see surface-initiated ring-opening polymerization SLD, see scattering length density smart surfaces modification with PBA cross-linked network 498f with PBA monolayer 498f with PBA beyond polymer brush 509, 510f, 511 with PBA polymer brushes 497–498, 498f–499f molecular mechanism 498, 500, 500f pH-responsive surfaces 501, 502f, 503 pH/sugar dual-responsive AND logic gate 506–509, 507f–508f pH/sugar dual-responsive OR logic gate 504–506, 505f–506f sugar-responsive surfaces 503–504, 504f SNA nanostructures, anatomy of 647f solid phase organic synthesis (SPOS) 444 solid substrates, polymer brushes preparation on 4–5, 4f solvent-assisted grafting technique 8 SpA, see staphylococcal protein A spectroscopic ellipsometry measurements of alcohol exposure 247 under controlled humidity conditions 247 fitting data 248 infrared variable angle 248

spherical polymer brushes 150–152, 151f, 736 SPMA, see 3-sulfopropylmethacrylate potassium salt SPOS, see solid phase organic synthesis SPP, see surface plasmon polaritons SPR, see surface plasmon resonance SPR-based sensors 465–466, 465f SPR excitation 703 SPR imaging (SPRi) sensors 466 spuncast PDMAEMA films chemical modification of 247 versus polymer brushes 250–252, 251f preparation of 246 spuncast polymers 243–244, 244f ssDNA, see short sequences of singlestranded DNA staphylococcal protein A (SpA) 449–450, 450f Staphylococcus aureus 383, 389–390, 530 star-like brushes 40–42 star-like polyelectrolyte (PE), planar brushes of 232–234, 233f star polymers 65–66 state-of-the-art of polymer brushes 561 Staudinger ligation 445, 446f stenciled “e-click” chemistry 125, 126f STF, see sculptured thin films stimuli-responsive gradient brushes 293 stimuli-responsive homopolymer brushes 288–290, 290f–291f stimuli-responsive polymer brushes 74–79, 75f–76f solutions 76–78 surfaces 78–79 streptavidin 447–448 streptavidin–biotin binding 206 Streptococcus mutans 392





Index

sugar-responsive surfaces modification, with PBA polymer brushes 503–504, 504f 3-sulfopropylmethacrylate potassium salt (SPMA) 131–132 surface enhanced Raman spectroscopy (SERS) sensors 468–469, 469f surface functionalization, polymer brushes for 378–379 surface hydrophilicity 573–574 surface hydrophilicity, of polymer brushes 270–271 surface immobilization strategy 439–440 of biorecognition 442 biotin–avidin/streptavidin pairing 447–448 Diels–Alder cycloaddition 444, 445f 1,3-dipolar cycloaddition 446–447 general activated surface chemistry 442–444, 443f nitrilotriacetic acids–Ni2+ –histidine pairing 448–449 oligonucleotide hybridization 450–451, 451f of protein 443, 443f protein A/protein G – Fc pairing 449–450, 450f thiolated aptamers on noble metal 440–442, 441f surface-initiated anionic polymerization 18–20, 19f surface-initiated atom transfer radical polymerization (SI-ATRP) 9–10, 9f, 124 applications 57–58 architecture of 56–57 chemistry of 55, 56f and concave surfaces 63 electrochemically induced 129–136 mechanism of 131, 131f grafting density of 55–56

from nanoparticles applications 61–63, 62f architecture of 59, 60f–61f, 61 chemistry 58–59, 59f surface-initiated DNA hybridization chain reaction (SI-HCR) 628 surface-initiated enzymatic polymerization 634 surface-initiated enzymatic polymerization (SIEP) 628 surface-initiated living radical polymerization (SI-LRP) 406 surface-initiated nitroxide-mediated polymerization 13 surface-initiated photoiniferter-mediated polymerization (SI-PIMP) 13–15, 14f surface-initiated polymerization (SIP) 378–379 surface-initiated reversible addition fragmentation chain transfer polymerization (SI-RAFT) 10–13, 11f–12f polymer brushes via 99–101 surface-initiated ring-opening metathesis polymerization (SI-ROMP) 17–18, 18f surface-initiated ring-opening polymerization (SI-ROP) 15–17, 16f surface-initiated rolling circle amplification (SI-RCA) 628 surface plasmon polaritons (SPP) 465–466 surface plasmon resonance (SPR) 410, 562 surface plasmons 688–692 swelling coefficient, of polymer brush mirrors 269–270 SWNT, see single-walled carbon nanotube Systematic Evolution of Ligands by EXponential (SELEX) 437

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Index

t TA, see tannic acid TAAC, see thermal azide-alkyne cycloaddition TAMRA-labeled vasopressin 469 tannic acid (TA) 392 TBA, see thrombin-binding aptamer t-BuLi, see tert-butyllithium TCPS, see tissue culture polystyrene TEG, see thromboelastography temperature-responsive polymer brushes 106–110, 110f temperature-responsive polymers 692–694 templates from brushes 64–65, 64f from networks 63, 64f from stars 65–66 TEMPO, see 2,2,6,6(tetramethylpiperidin1-yl)oxidanyl terminal functionalization, of PIPAAm brushes 369 tert-butyllithium (t-BuLi) 19 tethered polymer chains 3–4. See also polymer brushes 2,2,6,6-(tetramethylpiperidin-1yl)oxidanyl (TEMPO) 13 thermal azide–alkyne cycloaddition (TAAC) 294–295 thermoresponsive polymer brushes 361 hydrogel-modified surfaces for cell adhesion and detachment 362–363, 362f preparation by RAFT polymerization 368–372, 370f–371f using ATRP 363–368, 365f–366f, 368f thermoresponsive polymers 694

thermoresponsive poly(oligo(ethylene oxide)methacrylate) copolymer brushes 277–279, 278f thiolated aptamers, on noble metal 440–442, 441f thiol ester 445 thiol-functionalized surfaces 633 thrombin-binding aptamer (TBA) 466–467 thromboelastography (TEG) 345, 346f time-of-flight secondary ion mass spectrometry (ToF-SIMS) 460–461, 461f tissue culture polystyrene (TCPS) 315, 362–363 ToF-SIMS, see time-of-flight secondary ion mass spectrometry transduction schemes, based upon grafted biomolecules 462 electrochemical-based sensors 462–463, 463f field effect transistor based sensors 463–465, 464f microcantilever sensors 469–471, 470f photoluminescence-based sensors 466–468, 467f–468f SERS sensors 468–469, 469f SPR-based sensors 465–466, 465f transmission surface plasmon resonance (T-SPR) spectroscopy 300 trifluoroacetic acid 671 tunable grafting density, brushes with 45–46

u ultrahigh strain 724 Ulva linza 546, 548 U-type cell 127 UV laser interference lithography 699–701





Index

v

x

Van der Waals interactions 161, 165–166, 174, 188f, 194f, 213 vapors 243 variable angle spectroscopic ellipsometry (VASE) 247–248 VASE, see variable angle spectroscopic ellipsometry vinyl sulfone 443, 443f vitronectin peptide 583

XPS, see X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy (XPS) 460 X-ray reflectometry 275

y Yersinia pseudotuberculosis 396, 535 Young–Dupr´e’s equation 270

w water cluster number calculation 264–265, 264f water-equilibrated polymer brush 270–271 water-soluble polymers 289 WVASE32 software 247 WVASE-IR software 248

z Z-group approach, interfacemediated RAFT polymerization 99 zwitterionic brushes 564 zwitterionic polymer brushes 62f, 308–309, 309f, 533, 566

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