Intelligent Nanomaterials: Processes, Properties, and Applications 9780470938799, 047093879X

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Intelligent Nanomaterials

Scrivener Publishing 3 Winter Street, Suite 3 Salem, MA 01970 Scrivener Publishing Collections Editors James E. R. Couper Richard Erdlac Norman Lieberman W. Kent Muhlbauer S. A. Sherif

Ken Dragoon Rafiq Islam Peter Martin Andrew Y. C. Nee James G. Speight

Publishers at Scrivener Martin Scrivener ([email protected]) Phillip Carmical ([email protected])

Intelligent Nanomaterials Processes, Properties, and Applications

Edited by

Ashutosh Tiwari, A jay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner

4£

Scrivener

WILEY

Copyright © 2012 by Scrivener Publishing LLC. All rights reserved. Co-published by John Wiley & Sons, Inc. Hoboken, New Jersey, and Scrivener Publishing LLC, Salem, Massachusetts. Published simultaneously in Canada. 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, scanning, or otherwise, except as permitted under Section 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, (978) 750-8400, fax (978) 750-4470, or on the web at www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., Ill River Street, Hoboken, NJ 07030, (201) 748-6011, fax (201) 748-6008, or online at http://www.wiley.com/go/permission. Limit of Liability/Disclaimer of Warranty: While the publisher and author have used their best efforts in preparing this book, they make no representations or warranties with respect to the accuracy or completeness of the contents of this book and specifically disclaim any implied warranties of merchantability or fitness for a particular purpose. No warranty may be created or extended by sales representatives or written sales materials. The advice and strategies contained herein may not be suitable for your situation. You should consult with a professional where appropriate. Neither the publisher nor author shall be liable for any loss of profit or any other commercial damages, including but not limited to special, incidental, consequential, or other damages. For general information on our other products and services or for technical support, please contact our Customer Care Department within the United States at (800) 762-2974, outside the United States at (317) 572-3993 or fax (317) 572-4002. Wiley also publishes its books in a variety of electronic formats. Some content that appears in print may not be available in electronic formats. For more information about Wiley products, visit our web site at www.wiley.com. For more information about Scrivener products please visit www.scrivenerpublishing.com. Cover design by Russell Richardson Library of Congress Cataloging-in-Publication ISBN 978-0-470-93879-9

Printed in the United States of America 10

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Data:

Contents Preface

PART I Inorganic Materials 1.

Synthesis, Characterization, and Self-assembly of Colloidal Quantum Dots Saint M. Emin, Alexandre Loukanov, Surya P. Singh, Seiichiro Nakahayashi and Liyuan Han 1.1 Introduction 1.2 Size-dependent Optical Properties of Quantum Dots 1.2.1 Band Gap Energies 1.2.2 Absorption Spectra 1.3 Procedures for Synthesis of Colloidal Quantum Dots 1.3.1 Synthesis of Quantum Dots in Reverse Micelles 1.3.2 Synthesis of Quantum Dots in Aqueous Media 1.3.3 Hot-matrix Synthesis of Quantum Dots 1.4 Types of Semiconductor Quantum Dots 1.4.1 Binary Quantum Dots 1.4.2 Alloyed Quantum Dots 1.4.3 Core/shell Quantum Dots: "Type-I" 1.4.4 Core/shell Quantum Dots: "Type-II" 1.4.5 Quantum Dot/quantum Well Nanocrystals 1.4.6 Transition-element-doped Quantum Dots 1.5 Surface Functionalization of Quantum Dots 1.5.1 Self-assembly of Colloidal Quantum Dots 1.6 Conclusions References

xxi

1 3 3 4 7 9 10 10 12 14 14 15 16 17 18 19 21 23 23 28 30 4.

4.

CONTENTS

4.

One-dimensional Semiconducting Metal Oxides: Synthesis, Characterization and Gas Sensors Application Nguyen Due Hoa 2.1 Introduction 2.2 Synthesis of 1-D Metal Oxide 2.2.1 Vapor Phase Growth 2.2.2 Vapor-liquid-solid Mechanism 2.2.3 Vapor Solid Mechanism 2.3 Solution Phase Growth 2.3.1 Template Assisted Synthesis 2.3.2 Template Free Synthesis 2.4 Gas Sensor Applications 2.4.1 SnO z NWs Based Gas Sensors 2.4.2 W 0 3 NWs Based Gas Sensors 2.4.3 ZnO NWs Based Gas Sensors 2.4.4 Ti0 2 NWs Based Gas Sensor 2.4.5 CuO NWs Based Gas Sensors 2.4.6 ln 2 0 3 NWs Based Gas Sensors 2.5 Conclusions Acknowledgement References

4.

Rare-earth Based Insulating Nanocrystals: Improved Luminescent Nanophosphors for Plasma Display Panels Prashant K. Shartna and Avinash C. Pandey 3.1 What is Plasma Display Panel? An Introduction and Overview 3.2 History of Plasma Display Panel 3.3 Working of Plasma Display Panel 3.3.1 Advantages of Plasma Display Panel 3.3.2 Disadvantages of Plasma Display Panel 3.4 Nanophosphors for Plasma Display Panel 3.4.1 Blue Nanophosphors 3.5 Synthesis of BAM:Eu2+ Nanophosphors by Sol-gel Method 3.5.1 Chemicals Used 3.5.2 Methodology 3.5.3 Characterization of Prepared Nanophosphors 3.5.4 Results and Discussion

39 40 42 42 43 45 49 49 61 63 63 68 73 74 76 79 81 81 82

89 90 91 93 96 97 98 99 101 101 101 101 102

CONTENTS

3.6 3.7

3.8

3.9

3.10

3.11 3.12

Time Evolution Studies and Decay Time Determination Synthesis of BAM:Eu2+ Nanophosphors by Solution Combustion Method 3.7.1 Chemicals Used 3.7.2 Methodology 3.7.3 Characterization of Prepared Nanophosphors 3.7.4 Results and Discussion Green Nanophosphors 3.8.1 Yttrium Aluminum Garnet Y3Al5012:Tb3+ (YAG:Tb3+) Nanophosphors 3.8.2 Synthesis of Y3Al5012:Tb3+ (YAG:Tb3+) Nanophosphors by Sol-gel Method 3.8.3 Chemicals Used 3.8.4 Methodology 3.8.5 Characterization of Prepared Y3Al5C»12:Tb3+ (YAG:Tb3+) Nanophosphors 3.8.6 Results and Discussion Terbium Doped Yttrium Ortho-borate (YB03:Tb3+) Nanophosphors 3.9.1 Synthesis of Terbium Doped Yttrium Ortho-borate (YB03:Tb3+) Nanophosphors 3.9.2 Chemicals Used 3.9.3 Methodology 3.9.4 Characterizations Used 3.9.5 Result and Discussion Red Nanophosphors: Yttrium Aluminum Garnet Y3A1.012:Eu3+ (YAG:Eu3+) Nanophosphors 3.10.1 Synthesis of Yttrium Aluminum Garnet Y3Al5012:Eu3+ (YAG:Eu3+) Nanophosphors by Sol-gel Method 3.10.2 Chemicals Used 3.10.3 Methodology 3.10.4 Characterizations Used 3.10.5 Results and Discussion Time Evolution Studies Europium Doped Yttrium Ortho-borate (YB03:Eu3+) Nanophosphors 3.12.1 Synthesis of Europium Doped Yttrium Ortho-borate (YBÖ3:Eu3+) Nanophosphors by Reverse Micelles Method

vii

105 106 106 106 107 107 113 113 114 114 114 115 115 118 119 119 120 120 120 125 125 125 126 126 127 132 132 133

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CONTENTS

3.12.2 Chemicals Used 3.12.3 Synthesis of YB0 3 :Eu 3+ Nanoparticles 3.12.4 Characterizations Used 3.12.5 Results and Discussion 3.13 Europium Doped Yttrium Oxide (Y203:Eu3+) Nanophosphors 3.13.1 Synthesis of Europium Doped Yttrium Oxide (Y203:Eu3+) Nanophosphors by Solution Combustion Method 3.13.2 Chemicals Used 3.13.3 Methodology 3.13.4 Characterizations Used 3.13.5 Results and Discussion 3.14 Conclusions Acknowledgements References 4.

Amorphous Porous Mixed Oxides: A New and Highly Versatile Class of Materials Sadanand Pandey & Shivant B. Mishra 4.1 Introduction 4.2 Description of a Porous Solid Material 4.2.1 Qualitative Description of a Porous Solid 4.2.2 Origin of Pore Structures 4.2.3 Idealized Systems : Pore Shape and Size 4.3 Sol-gel Method for the Production of Porous Oxides 4.3.1 Synthesis of micro and mesoporous materials 4.3.2 Template-assisted Synthesis 4.4 Characterization of Porous Mixed Oxides 4.5 Application of Porous Mixed Oxide 4.5.1 Catalysts 4.5.2 Other Application of Porous Mixed Oxide 4.6 Conclusions Acknowledgements References

133 133 134 134 138

139 139 139 140 140 144 145 145

149 150 150 150 151 152 154 157 158 161 167 167 171 172 174 174

CONTENTS

5.

Zinc Oxide Nanostructures and their Applications Rizwan Wahab, I.H. Hwang, Hyung-Shik Shin, Young-Soon Kim, Javed Musarrat and M.A. Siddiqui 5.1 Introduction 5.2 Importance of Metal Oxides Nanostructures 5.3 General Introduction of Antibacterial Activity 5.4 Experimental 5.4.1 Material Synthesis 5.4.2 Characterization of Synthesized Materials 5.4.3 Antibacterial Activity of Zinc Oxide Micro-flowers (ZnO-MFs) 5.5 Application of Grown Nanomaterials as an Antibacterial Agent 5.5.1 Nanostructures of ZnO: Fabrication and Characterization 5.5.2 Chemical Reaction Mechanism of Synthesized Zinc Oxide Micro-flowers (ZnO-MFs) 5.5.3 Antibacterial Activity of Synthesized Zinc Oxide Micro-flowers (ZnO-MFs) 5.5.4 Possible Mechanism 5.6 General Introduction of Cancer and the Role of Nanobiotechnology 5.6.1 Experimental 5.6.2 Materials Characterization 5.6.3 Cell Proliferation 5.7 Result and Discussion 5.7.1 X-ray Diffraction Pattern 5.7.2 Morphological or Structural Observation of Fabricated Material 5.7.3 Transmission Electron Microscopy (TEM) Results 5.7.4 FTIR Spectroscopy 5.7.5 Cell Viability via MTT Method and their Observation 5.8 Conclusions and Future Directions Acknowledgements References

ix

183

184 185 185 187 187 188 189 189 189 192 194 198 200 201 202 202 202 202 203 203 204 205 207 208 208

4. CONTENTS 4.

4.

Smart Nanomaterials for Space and Energy Applications Raghvendra S. Yadav,Ravindra P. Singh, Prinsa Verma, Ashutosh Tiwari and Avtnash C. Pandey 6.1 Introduction 6.2 Nanomaterials in Photovoltaic Cells for Space Application 6.2.1 Current Research on Materials and Devices 6.2.2 Crystalline Silicon 6.2.3 Thin Film Processing 6.2.4 Transparent Conductors 6.2.5 Cadmium Telluride Solar Cell 6.2.6 Multifunction Thin Film Photovoltaic Cells 6.2.7 Gallium Arsenide Substrate 6.2.8 Germanium Substrate 6.2.9 Indium Phoshide Substrate 6.2.10 Nanocomposites 6.2.11 Quantum Well Solar Cells 6.2.12 Nanowires and Tubes 6.2.13 Quantum Dots 6.3 Nanomaterials for Hydrogen Storage 6.3.1 Carbon Nanotubes 6.3.2 Boron Nitride Nanotubes 6.3.3 Hydride Materials 6.3.4 Metal-organic Materials 6.4 Nanomaterials in Batteries 6.5 Nanomaterials for Energy Storage in Supercapacitors 6.6 Conclusions and Future Prospects Acknowledgement References Thermochromic Thin Films and Nanocomposites for Smart Glazing Russell Binions 7.1 Introduction 7.2 Principles and Background Theory to Solar Control Coatings 7.2.1 Ambient Radiation 7.2.2 Solar Thermal Surfaces

213 214 215 218 218 219 219 220 220 221 222 222 222 223 223 224 224 225 228 230 233 233 236 238 240 240 251 252 254 254 256

CONTENTS

7.2.3

Thin Films for Window Glazing: Static Properties 7.2.4 Spectrally Selective Thin Films: Heat Mirrors 7.2.5 Thin Films for Window Glazing: Dynamic Properties 7.3 Semiconductor-to-metal Transitions 7.3.1 Vanadium Dioxide 7.3.2 Challenges for V0 2 use in Architectural Glazing 7.4 Synthetic Techniques 7.4.1 Physical Vapour Deposition 7.4.2 Pulsed Laser Deposition 7.4.3 Sol-gel Synthesis 7.4.4 Chemical Vapour Deposition 7.4.5 Atmospheric Pressure Chemical Vapour Deposition 7.4.6 Aerosol Assisted Chemical Vapour Deposition 7.4.7 Hybrid Aerosol Assisted/Atmospheric Pressure Chemical Vapour Deposition 7.4.8 Comparison of Production Methods 7.5 Recent Results 7.5.1 Fluorine Doped V0 2 7.5.2 Nanocomposite Thin Films and Energy Modelling Studies 7.5.3 The Ideal Thermochromic Coating 7.6 Outlook and Conclusions Acknowledgments References

PART II Organic Materials 8.

Polymeric Nano-, Micellar and Core-shell Materials Angel Contreras-Garcta, Guillermina and Emilio Bucio 8.1 Introduction 8.2 Stimuli-responses 8.3 Intelligent Micro- and Nano-materials Synthesis

xi

258 259 260 263 269 273 274 274 276 278 279 283 285 287 288 289 289 294 306 310 311 312

317 319 Burillo, 319 319 321 323

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CONTENTS

8.3.1 Coacervation/ precipitation 8.3.2 Particles by Chemical Crosslinking 8.3.3 Heterogeneous Polymerization 8.3.4 Polymer Adsorption on Nanoparticles 8.3.5 Layer-by-layer Polymeric Shell 8.3.6 Precipitation on Templates 8.3.7 Grafting onto the Surface of Particles 8.3.8 Self-assembly of Micelles 8.3.9 Radiation-grafting of Nano Polymers 8.4 Characterization of Nano Sensitive Polymers 8.4.1 Swelling Measurements 8.4.2 Thermo Sensitive Nano Polymers [194-197] 8.4.3 pH Critical Point 8.4.4 Surface Plasmon Resonance Spectroscopy (SPR) 8.4.5 FTIR Spectroscopic Method for the Determination of the LCST 8.4.6 Thermal Transition of Responsive Materials 8.4.7 Contact Angle 8.4.8 Microscopy References

9.

323 323 325 328 329 329 330 331 332 334 334 335 335 336 336 337 337 338 339

Conjugates of Nanomaterials with Phthalocyanines 347 Edith Antunes, Christian Litwinski and Tebello Nyokong 9.1 Background on Nanomaterials 348 9.1.1 Semiconductor Quantum Dot (QD) Nanoparticles 350 9.2.1 Magnetic Iron Nanoparticles (MNPs) 361 9.3.1 Carbon Nanotubes (CNTs) 365 9.4 Phthalocyanines (Pcs) 372 9.4.1 Background and History 372 9.4.2 Use in Photodynamic Therapy 373 9.4.3 General Synthetic Methods of Phthalocyanines and Pc Nanoparticles 374 9.5 Photophysical and Photochemical Behavior 377 9.5.1 Singlet Oxygen 378 9.4.2 Fluorescence Quantum Yields (Φρ) and Lifetimes (T ) 384

CONTENTS

9.4.3

Triplet State Quantum Yields (Φτ) and Lifetimes (ττ) 9.5 Phthalocyanine and Nanomaterial Conjugates 9.5.1 Synthesis 9.5.2 Fe-NPs Mixed with MPcs 9.5.3 Pc-QDs References 10. Nanostructured Carbon and Polymer Materials- Synthesis and their Application in Energy Conversion Devices Debmalya Roy, B. Shastri, Md. Immamuddin, K. Mukhopadhyay 10.1 Introduction 10.2 Inorganic and Organic Semiconductors for Solar Cell 10.3 Materials for Organic Solar Cell: Donor 10.4 Materials for Organic Solar Cell: Acceptor 10.5 Our Efforts Towards Material Synthesis for OPV 10.6 Conclusions Acknowledgements References 11. Advancement in Cellulose Based Bio-plastics for Biomedicals S. K. Shukla 11.1 Introduction 11.2 Plasticity Modulation 11.2.1 Composite formation 11.2.2 Microbial Bioplastics 11.2.3 Copolymerization 11.2.4 Melt Mixing and Physical Annealing 11.2.5 Nanocomposites 11.3 Applications 11.3.1 Enteric Coatings 11.3.2 Sustained Release 11.3.3 Tissue Engineering 11.3.4 Sensors and Recognition 11.4 Future Challenges 11.5 Conclusions References

xiii 387 389 389 401 402 410

425 426 428 431 440 444 460 461 462 467 467 469 469 472 474 476 478 479 480 480 481 482 483 485 485

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CONTENTS

P A R T III

C o m p o s i t e Materials

12. Intelligent Nanocomposite Hydrogels Mohammad Sirousazar and Mehrdad Kokabi 12.1 Introduction 12.2 Temperature-sensitive Intelligent Nanocomposite Hydrogels 12.3 pH-sensitive Intelligent Nanocomposite Hydrogels 12.4 Magnetic-field-sensitive Intelligent Nanocomposite Hydrogels 12.5 Other Stimuli-sensitive Intelligent Nanocomposite Hydrogels 12.6 Multi-stimuli-sensitive Intelligent Nano Composite Hydrogels 12.7 Conclusions References 13. Polymer/Layered Silicates Nanocomposites for Barrier Technology Philip W Labuschagne, Sean Moolman and Arjun Maity 13.1 Introduction 13.2 Polymer/Layered Silicate (PLS) Nanocomposite 13.3 Gas Permeability 13.4 Permeability of Polymer-Layered Silicate Nanocomposites 13.5 Preparation Method 13.6 Process Conditions 13.7 Clay Loading 13.8 Clay Surfactant 13.9 Compatibilizer and Polymer 13.10 Conclusions References 14. Polymers/Composites Based Intelligent Transducers Ajay Kumar Mishra Shwani B. Mishra, Ashutosh Tiwari 14.1 Introduction 14.2 Polymers and Polymer Nanocomposites for Transducers

487 489 489 495 505 513 519 523 527 527 533 533 535 536 539 553 554 556 559 562 565 566 571 571 573

CONTENTS

XV

14.3 Polymer-carbon Nanotubes-based Nanocomposites for Transducers 14.4 Conclusions References

575 578 579

PART IV Biomaterials and Devices

583

15. Hydrogel Nanoparticles in Drug Delivery Mehrdad Hamidt, Kobra Rostamizadeh and Mohammad-Ali Shahbazi 15.1 Introduction 15.2 Properties of Nanogels 15.3 Characterization of Nanogels 15.4 Preparation of Nanogel Networks 15.4.1 Physical Self-assembly of Interactive Polymers 15.4.2 Chemical Synthesis in Heterogeneous Colloidal Environments 15.4.3 Covalent Crosslinking of Preformed Polymer 15.4.4 Template-assisted Preparation of Nanogel Particles 15.5 Smart Nanogels for Drug Delivery Systems 15.5.1 pH-responsive Nanogels in Drug Delivery 15.5.2 Temperature-responsive Nanogels in Drug Delivery 15.5.3 Glucose-responsive Drug Delivery Nanogels 15.5.4 Photo-responsive-based Drug Delivery Nanogels 15.5.5 Magnetically-responsive Drug Delivery Nanogels 15.5.6 Redox-responsive Drug Delivery Nanogels 15.5.7 Ultrasound-responsive Drug Delivery Nanogels 15.6 Conclusions References

585

586 589 590 593 593 595 596 597 597 597 602 610 612 614 616 617 618 619

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CONTENTS

16 Mode of Growth Mechanism of Nanocrystal Using Biomolecules Sharda Sundaram Sanjay, Ravtndra P. Singh, Ashutosh Tiwari and Avinash C. Pandey 16.1 Introduction 16.2 Mode of Growth Mechanism of Nanocrystals 16.2.1 Biomolecules 16.2.2 Building Blocks 16.2.3 Mechanism of Formation of Inorganic Nanocrystals 16.2.4 The Classical Crystal Growth Kinetics 16.2.5 Factors for Controlling the Subsequent Growth Processes 16.2.6 Properties of Bio-functionalities which Efficiently Influence the Nanoparticles Growth 16.2.7 Types of Interactions between Nanoparticles and Biomolecules 16.2.8 Condition for the Bioconjunction of Nanocrystals and Biomolecules 16.3 Conclusions Acknowledgements References

625 626 629 629 630 631 634 635 635 636 641 644 644 644

17 Quantum Dots for Detection, Identification and Tracking of Single Biomolecules in Tissue and Cells 649 By Alexandre R. Loukanov, Saint Emin 17.1 Introduction 650 17.2 Detection of Membrane Proteins in Tissue Replica 654 17.2.1 TEM versus STEM for visibility 655 17.2.2 EDX analysis has a difficulty with Pt/carbon replica 659 17.2.3 Co-localization of AMPA and NMDA with Qdots and Au-colloids 662 17.3 Subunit Co-localization of Membrane Proteins by Immunolabeled 1 nm Gold Nanoparticles 665 17.3.1 Detection of Immunolabeled 1 nm Nanogold Particles on Sds-Frl 666

CONTENTS

17.3.2 Distribution Pattern Analysis 17.3.3 Energy Dispersive X-Ray Differentiation of Ultrasmall Gold and Semiconductor Nanoparticles 17.3.4 Subunit Labeling 17.4 Labeling and Intracellular Tracking of DNA with Quantum Dots 17.5 Perspectives References 18. Nanofibers-based Biomedical Devices Debasish Mondal and Ashutosh Tiwari 18.1 Introduction 18.2 Nanofibers Fabrication Techniques 18.2.1 Electrospinning 18.2.2 Phase Separation 18.2.3 Self-assembly 18.3 Polymeric Materials for Nanofibers 18.3.1 Natural Polymers 18.3.2 Synthetic Polymers 18.4 Biocompatibility of Nanofibers 18.5 Application of Nanofibers in Biomedical Devices 18.5.1 Nanofibrous Scaffold for Tissue Engineering 18.5.2 Nanofibers for Therapeutic Agents Release 18.5.3 Nanofibers for Biosensors 18.6 Status and Prognosis References 19. Nano-sized Carrier Systems as New Materials for Nuclear Medicine Martin Hruby 19.1 Introduction 19.2 Imaging of Reticuloendothelial System with Radiolabeled Nanoparticles 19.3 Local Applications of Nanoparticles 19.4 Nanoparticles for Cancer Imaging and Therapy 19.5 Minimization of Systemic Radiation Burden

xvii 667 669 669 671 675 676 679 680 681 681 685 685 686 686 687 687 691 691 701 705 706 708 715 715 719 720 722 731

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CONTENTS

19.6 Conclusions Aknowledgements References 20. Biomimetic Materials Toward Application of Nanobiodevices Ravindra P. Singh, Jeong-Woo Choi, Ashutosh Tiwari, and Avinash C. Pandey 20.1 Introduction 20.2 Biomimetic Peptides and Proteins 20.3 Biomimetic DNA 20.4 Biomimetics Metal and Metal Oxides Nanostructures Formation 20.5 Graphene and Carbon Nanotubes 20.6 Biomimetics Smart Polymer 20.7 Conclusions and Future Perspectives Acknowledgements References 21. Lipid Based Nano-biosensors for Medical Diagnostics Georgia-Paraskevi Nikoleli, Dimitrios P. Nikolelis and Nikolaos Tzamtzis 21.1 Introduction 21.2 Methods for Preparation Biosensors Based on Lipid Films 21.2.1 Metal Supported Lipid Layers 21.2.2 BLM Formed on a Surface of Glassy Carbon Electrode 21.2.3 Stabilized Lipid Films Formed on a Glass Fiber Filter 21.2.4 Bilayers Formed on Conducting Polymers, Semiconductors, Carbon Nanotube Surfaces 21.2.5 BLM Formed on Microporous Material 21.3 Applications of Nano Biosensors Based on Lipid Films for Uses in Medical Diagnostics 21.4 Conclusions References

733 734 734 741

742 743 758 762 767 771 774 775 775 783 783 785 785 786 786 788 789 790 797 798

CONTENTS

xix

22. Polymeric Nanofibers and their Applications in Sensors Murugan Ratnalingam, Ashutosh Tiwari 22.1 Introduction 22.2 Polymer Nanofibers 22.3 Electrospinning of Polymer Nanofibers 22.3.1 Overview of Electrospinning 22.3.2 System Configuration 22.3.3 Spinning Process and Mechanism 22.4 Different Types of Fiber Collectors and Fiber Geometry 22.4.1 Types of Fiber Collectors 22.4.2 Fiber Geometry and Dimension 22.5 Applications of Electrospun Nanofibers in Sensors 22.5.1 Chemical Sensors 22.5.2 Biological Sensors 22.6 Conclusions References

814 814 820 822 823

Index

827

801 802 803 807 807 807 808 809 811 811

Preface The creation of new materials is one of the fundamental driving forces of industry and lays the foundation for new products to enhance the wealth and well being of society. The last three decades has seen extraordinary advances in the generation of new materials based on both fundamental elements and composites, driven by advances in synthetic chemistry and often drawing inspiration from nature. The concept of an intelligent material envisions additional functionality built into to the molecular structure, such that a desirable response occurs under defined conditions. The last decade has seen the emergence of particular material properties engineered by exploiting the extraordinary behavior of nanostructures. Nanomaterials are built of components with at least one dimension in the nanometer range. More specifically, dimensions of 1-100 nm are generally considered to fall in this class. At these dimensions, extraordinary physical and chemical properties can be observed, which have formed the basis for a burgeoning nanotechnology industry. While examples can be found of serendipitous use of nanotechnology right back to ancient Egyptian times, the systematic understanding and commercial exploitation of nanomaterials emerged in the early 1990's, as indicated by a burgeoning number of patents at that time covering fullerenes, carbon nanotubes, dedrimers, quantum dots, nanowires etc. These new varieties of low dimensional materials have much larger surface to volume (S/V) ratios as compared to their bulk counterparts. With decrease in the size of nanoparticles, the S/V ratio increases abruptly. It is well known that surface atoms in any material are loosely bound as compared to the interior atoms, hence with increase in surface area the surface free energy increases. Nanomaterials therefore play a very prominent role in physical, chemical and biomedical engineering applications due to their high surface energies. Also, the electronic configuration of atoms within the materials is very xxi

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PREFACE

important since this principally determines the type of bonding and thus electrical, optical, luminescent, mechanical and magnetic properties. At nanoscale dimensions, materials exhibit entirely different properties as compared to their bulk counterpart. Noble metallic nanoparticles/nanostructures exhibit interesting features of localized surface plasmon resonant (LSPR); absorption can be tuned from ultraviolet region to infrared region of electromagnetic spectrum and this field has been developed to deliver potential applications in photonics, optoelectronics, optical-data storage, solar cells, filters, sensors, not to mention the considerable scope in medical engineering, such as DNA labeling, tumor and cancer therapy etc. Study of the propagation of electromagnetic waves through metallic nanostructures with different shapes has become a major field due to fascinating applications in antennas and also left handed materials. Semiconductor nanostructures on the other hand are very promising candidates for applications in luminescent devices such as light emitting diodes, flat screen displays, lasers etc. and especially in electronic devices, due to their extraordinary feature of band gaps ranging from UV-visible to infrared regions. Silicon has reigned supreme amongst materials responsible for miniaturization of the world of electronics in the last century However, recent progresses in the design of materials synthesized from other semiconducting families, such as III-V or II-VI, are showing even more promise for versatile applications and may provide a new generation of materials. For example, nano-micro structures of zinc oxide /sulphide, tin oxide, cadmium sulphide and titanium oxide exhibit interesting electrical, optical and mechanical properties. They can be used in a wide variety of applications ranging from sensors, LEDs, flat panel displays, energy storage/ harvesting and batteries. Similarly, advances in synthesizing nano-micro structures from insulators like silica and polymers, have found interesting applications in biomedical engineering such as drug delivery and implants. Conducting polymers have recently opened an entirely new field of organic field-effect transistors, organic light-emitting diodes, light weight electronics, etc. The current challenges in material engineering demand the fabrication of multicomponent composite materials having multifunctional properties. Inter-mixing of two or

PREFACE

xxiii

several nanostructural components into a composite form will give rise to complementary properties which will enable these materials to exhibit the capability of self-repair under any external damage or perturbation. These kind of materials which exhibit the ability of self-repairing under external cause clearly fall into the category of 'intelligent nanomaterials'. Nanotechnology was first conceptualised by science fiction writers like Robert Heinlen and Eric Frank Rüssel in the 1940s, but it was Richard Feynman's (1918-1988) visionary talk "There's plenty of room at the bottom" in 1959 that is often credited with launching nanotechnology. While much of the early focus was on nanofabrication and nanoelectronics, the application of science and technology at the nano-scale also promises to revolutionise medicine in the 21st century, enabling us to understand many diseases leading to new insights in diagnostics and therapy and contributing to the development of new generations of medicinal products exploiting functional materials, nano-biomaterials and medical nano-devices. Healthcare is one of the largest and most rapidly expanding needs in society today and smart nanomaterials will have application in a diverse arena including drug screening technologies, biocompatible materials and orthopaedic implants, lab-on-a-strip and nanobiosensors, drug delivery, degenerative disease diagnosis and treatment, self-assembled bio-structures, advanced medical imaging and regenerative medicine. On a wider front, intelligent nanomaterials are contributing to new coatings, fabrics, memory and logic chips, contrast media, optical components, superconducting electrical components etc. Some commentators have rather overstated the market size for nanomaterials by referring to the price of finished products such as cars and phones, rather than the intermediate products more properly attributed to the materials themselves. Nevertheless, even conservative estimates place the current market size for the materials alone at around US$4b. Current products have been dominated by simple nanostructures with beneficial properties such as the antimicrobial properties of silver nanoparticles. However, we are now witnessing the emergence of active nanostructures in the form of electronics, sensors, drug release technologies and adaptive structures. The future promises molecular nanosystems with hierarchical functions and evolutionary systems that will power the next generation of industrial development.

xxiv

PREFACE

This book aims to provide an up-to-date introduction to the fascinating field of intelligent nanomaterials. In general description, this large and fairly comprehensive volume includes twenty two chapters divided into four main areas: inorganic materials, organic materials, composite materials, and biomaterials. It covers the latest research and developments in intelligent nanomaterials: processing, properties, and applications. Included are molecular device materials, biomimetic materials, hybrid-type functionalized polymers-composite materials, information-and energy-transfer materials, as well as environmentally friendly materials. The book is written for a large readership including university students and researchers from diverse backgrounds such as chemistry, materials science, physics, biological science and engineering. It can be used not only as a text book for both undergraduate and graduate students, but also as a review and reference book for researchers in the materials science, bioengineering, pharmacy, biotechnology and nanotechnology. Ashutosh Tiwari, Ajay Kumar Mishra, Hisatoshi Kobayashi, Anthony P.F. Turner,

PhD PhD PhD PhD

N o v e m b e r 2011

PARTI INORGANIC MATERIALS

1 Synthesis, Characterization, and Self-assembly of Colloidal Quantum Dots Saim M. Emin1, Alexandre Loukanov2, Surya P. Singh1, Seiichiro Nakabayashi3 and Liyuan Han1 1

Advanced PhotoOoltaics Center, National Institute for Materials Science (NIMS), Tsukuba, Japan 2 Laboratory of Engineering Nanobiotechnology, University of Mining and Geology, Sofia, Bulgaria department of Chemistry, Faculty of Science, Saitama University, Saitama, Japan

Abstract We give a review for the preparation of various types of quantum dots, in which we emphasize general principles through examples that have led to relevant physico-chemical results in this area. Self-assembly procedures of quantum dots are explored because of the unique ways that small objects organize at the nanoscale level. Keywords: Colloidal quantum dot, self-assembly, surface functionalization

1.1

Introduction

During the past two decades, colloidal semiconductor nanocrystals (also known as quantum dots, abbreviated QDs) have been the subject of intensive research. QDs are of great interest owing to their size-dependent optical properties such as long fluorescence lifetimes, high extinction coefficients, broad absorption spectra, and narrow emission spectra [1]. These properties of QDs enable them to be used in bio-imaging [2], solar cells [3], light-emitting diodes Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (3-38) © Scrivener Publishing LLC

3

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INTELLIGENT NANOMATERIALS

(LEDs) [4], and fluorescence sensing [5]. The earliest studies of QDs, in which the QDs were embedded in glass, date back to 1980s [6, 7]. However, fundamental work initiated in the 1990s on colloidal QDs ultimately led to the development of the synthetic procedures that are currently used to produce QDs with well-defined sizes [8]. Since then, many advances have been made in the development of new QD materials with well-defined morphologies [9]. Mapping of the size-dependent optical properties of QDs is a goal that we share with many researchers. In this chapter, we discuss mainly photophysical studies of type II-VI and IV-VI QDs. To contribute to this fast-growing field, we have systematically explored recent advances in the synthesis and self-assembly of QDs. QDs have been prepared by a variety of methods such as molecular beam epitaxy (MBE) [10], metal-organic chemical vapor deposition (MOCVD) [11], electrochemical deposition [12], and colloidal procedures [13]. Among these methods, colloidal procedures produce QDs with good optical properties and relatively narrow size distributions [14]. Currently developed colloidal routes produce QDs with 10% variation in particle radius and with size control at the 2-5 A level [15]. Many physical properties of nanometer-scale QDs differ from those observed for their bulk-crystal analogues. Examples of such physical properties include melting points, charging energies, and bandgap alignment. Nevertheless, the size-dependent shifts observed in QDs' absorption and photoluminescence spectra are what make them attractive for widespread use [16]. In this chapter, we describe methods for preparation of QDs with well-defined sizes and optical properties. Furthermore, we survey techniques for the self-assembly of QDs on liquid and solid surfaces. In particular, the self-assembly of QDs onto solid substrates is discussed as it pertains to the preparation of optoelectronic devices [17].

1.2

Size-dependent Optical Properties of Quantum Dots

Size-dependent fluorescent emission is one of the most important features of colloidal QDs. On the basis of this property, various fluorescent probes have been designed for tissue labeling [18-20], tumor cell detection [21], or fabrication of multicolor LED devices [22]. Figure 1.1a shows a series of luminescent CdSe nanocrystals

SYNTHESIS, CHARACTERIZATION, AND SELF-ASSEMBLY

5

Sizes of quantum dots Samll

Large

Wavelength (nm)

Figure 1.1 (a) Photoluminescence of CdSe QD samples exposed to ultraviolet light, (b) Absorption and PL spectra of CdTe QDs with varying sizes as indicated for each sample, (c) TEM image of a film composed of spherical CdTe QDs. The inset in (c) is a selected area electron diffraction (SAED) pattern from the QD film. Reprinted from refs. [23, 24].

prepared by means of colloidal procedures [23]. As a general rule, the size-dependent emission is attributed to the QDs' band gap, which causes a shift in the absorption and photoluminescence (PL) spectra (Fig. 1.1b) [24]. This phenomenon, which is unique to QDs and is not observed in analogous bulk semiconductor crystals, can be explained with quantum-mechanical models and is discussed further below. Typical sizes of colloidal QDs are on the order of several nanometers as revealed by transmission electron microscopy (TEM; Fig. 1.1c). In a bulk semiconductor, an electron can be excited from the valence band to the conduction band by absorption of a photon with an appropriate energy. The excited electron in the conduction band leaves behind a positive hole in the valence band. This electron-hole pair, called an exciton, has its lowest energy state slightly below the lower edge of the conduction band, and its wavefunction extends over a large distance. The average separation

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distance between the electron and hole in an exciton is referred to as the Bohr radius; the Bohr radius differs among materials [25]. In a bulk semiconductor, the dimensions of the crystal are much larger than the exciton Bohr radius, allowing the exciton to extend to its natural limit. In terms of electron energy levels, this condition corresponds to a continuous band structure. However, if the size of a semiconductor crystal becomes small enough that it approaches the length of the exciton Bohr radius, then the electron energy levels are greatly affected. In that case, the energy levels are treated as discrete, meaning that there is a small and finite separation between the energy levels. This phenomenon is called the "quantum confinement effect" [26], and its theoretical treatment is usually based on the quantum mechanical particle-in-a-box approach [27]. For example, the Bohr exciton radii in bulk CdS and CdSe are approximately 3 and 5 nm, respectively; therefore, the quantum confinement effect is observed in CdS and CdSe QDs with sizes near those Bohr radii [28]. It is useful to compare the dimensions of QDs with their Bohr radii in order to estimate the quantum confinement regime. In general, the Bohr radius of a particle is given as [29]

where ε is the dielectric constant of the material; m is the reduced mass of the electron-hole pair; m is the electron mass, 9.109 x 10~31 kg; and a is the Bohr radius of the hydrogen atom. For nanocrystals, it is convenient to consider three different Bohr radii: one for the electron (ae), one for the hole (ah), and one for the exciton (a ). With these Bohr radii, three different confineexc

'

ment regimes can be considered [30]. The first of these is the strong confinement regime, which occurs when the nanocrystal radius, R, is much smaller than a . a,, and a (i.e., R w

I W

> w H

a

m W

a

I—I 0/1

a

> o r> o

C/)

O

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INTELLIGENT NANOMATERIALS

3

CO

c a>

i

■ i—■—i—■—i—i—r~4P transition of Eu2+ ions in BaMgAl10O17:Eu2+ nanostructure host. It is already observed that the energy states associated with the luminescent center are influenced by the host lattice material. The degree to which they are influenced depends also on the size and shape of nanostructures [34,35]. Recently, Y. Lin et al. [36] reported preparation of the ultrafine SrAl 2 0 4 :Eu, Dy needle-like phosphor and its optical properties. This group observed that the optical absorption edge shifts to the blue as the phosphor particle size decreases. They explained that it may be associated with the quantum size effect of the nanometer phosphor, which increased

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the kinetic energy of the electrons and resulted in a larger band gap, and thus required higher energy to excite the luminescent powders. Simultaneously they observed that the emission maximum shifted to shorter wavelength, they explained that it may be caused by the prepared technology, which resulted in some changes of the crystal field around Eu2+. Although the 4f electrons of Eu2+ are not sensitive to lattice environment due to the shielding function of the electrons in the inner shell, the 5d electron may couple strongly to the lattice. Consequently, the mixed states of 4f and 5d will be split by the crystal field, which may lead to the blue-shift of its emission peak. In our case, the blue-shift in the emission band may be attributed to the changes of the crystal field around Eu2+ arising from the nanosized particles. Since the excited 4f65d1 configuration of Eu2+ ion is extremely sensitive to the change in the lattice environment in contrast to the shielded 4P ground configurations, the 5d electron may couple strongly with the lattice. Therefore, the mixed states of 4f and 5d will be influenced strongly by the crystal field. On the other hand, the particles with nanometer size make the surface energy increase significantly, which causes the change of the crystal field around the local environment of Eu2+. These reasons lead to the blue-shift of PLE and PL emission peak in BaMgAl10O17:Eu2+ nanophosphor with decrease in particle size. Table 3.1 contains information about change in emission peak position, decay time, color coordinate, and relative intensity with change in diameter of BaMgAl10O17:Eu2+ nanostructure. The colorimetric coordinate (x, y) for BAM:Eu2+ nanophosphor were calculated using equidistant wavelength method [37]. Table 3.1 summarizes the comparison of CIE colorimetric coordinates for BaMgAl10O17 nanostructure with variation of diameter size of nanostructure. To study the decay behaviors of Eu2+ luminescence in BaMgAl10O17 nanostructures with varied diameter size, fluorescence decay curve for the 4f65d1 ->4P transition of the Eu2+ were studied [38]. It is found that the life time of BaMgAl10O17 nanostructure is varied as a function of diameter size of nanostructure. Table 3.1 summarizes the change in decay time of BaMgAl10O17:Eu2+ nanostructure with variation in diameter size. From Table 3.1, it is clear that the life time varies with the diameter size of BaMgAl10O17:Eu2+ nanorods and decay rate decreases with increase in diameter size. The PL intensity of the 4f6 5d 1 ->4F transition is strongly related to the decay time of a particular transition. The decrease in decay rate trend is continue to diameter size 62 nm, 85 nm, 115 nm and then 160 nm,

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however the trend of decrease of decay rate with increase of size deviates in case of BaMgAl10O17:Eu2+ nanostructure with diameter size 450 nm. The increment in decay rate at diameter size 450 nm in case of BaMgAl10O17:Eu2+nanorods is due to decrease in surface states, responsible for non-radiative transition, with increase of size. In general, the photoluminescence (PL) decay rate is a sum of the radiative and non-radiative decay rates. Therefore, the origin of the decrease in PL decay rate versus size may be radiative and nonradiative. The possibility may be invoked for the non-radiative process in origin is surface states, responsible for non-radiative decay, which changes as surface-to-volume ratio varies with size. Other possibility for radiative in origin, may be considered for the size dependence as quantum confinement effect may lead to size dependent oscillator strength. The quantum confinement effect in nanoparticles predicts a decreased radiative decay rate as the size increases [39]. As PL and PLE spectra are dependent on the size of diameter of BAM nanostructures indicating that the quantum confinement effect as well as surface effect both is responsible for change in decay rate.

3.8 Green Nanophosphors 3.8.1

Yttrium A l u m i n u m Garnet Y 3 Al 5 0 12 :Tb 3+ (YAG:Tb 3+ ) Nanophosphors

In PDPs, phosphors are excited by vacuum ultra violet (VUV) radiation, especially by 147 and 173 nm radiations, from inert gas plasma, which shows the unique requirement for PDP phosphors that they can only be excited by the higher VUV excitations. So we have to look on such host materials that are excited by VUV radiation and must have low crystal field symmetry around Eu3+. In the same pursuit, the host "nano-sized yttrium aluminum garnet Y3A15012 (YAG)" was found to contain Al3+" O2" bonding, presumably having high bonding energy due to its nano size and high valance(y) of Al3+ ion. Thus we have chosen YAG as host material in the present investigation by assuming that this high bonding energy may match the VUV excitation. It is well known that YAG is very stable under e-bombardment, so it may be stable under the plasma [40], the typical environment in PDP. In display panels, the application of

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nano-sized YAG phosphor offers the potential possibility of higher resolution and better luminescence efficiency as compared to the traditional sub-microsized phosphors. Due to high disorder near the surface, the synthesis of nano-sized YAG:Tb3+ phosphor must be potential answer to achieve good chromaticity. In the present section, we are revealing the sol-gel method to synthesize YAG:Eu3+ nanophosphor with excellent structural and optical properties. In order to check the stability of prepared nanophosphors with time and temperature, time evolution studies along with annealing treatment are also performed for these samples. 3.8.2

Synthesis of Y3Al5012:Tb3+ (YAG:Tb3+) Nanophosphors by Sol-gel Method

3.8.3

Chemicals Used

For the synthesis of YAG:Tb3+ nanohosphors, the metal nitrates i.e. yttrium nitrate Y(N03)2, aluminum nitrate Al(N03)3.9H20, and terbium nitrate Tb(N03)3 were taken. Citric acid and ethylene glycol were used as chelating and polymerizing agents, respectively. All chemicals were of analytical reagent grade and were directly used without any special treatment. In the present work, all the samples were prepared in a clean room of class 1000 under ambient conditions. 3.8.4

Methodology

In a typical Sol-Gel synthesis process nominal composition of metal nitrates were dissolved in double distilled water. Then ethylene glycol and citric acid were added. Citric acid solution was used to chelate the metal ions and to polymerize with ethylene glycol for gel formation. Homogeneous colorless solution was obtained after continuous stirring for more than 5 hours. After adding appropriate amount of ethylene glycol into the clear solution, the mixture was continuously stirred and heated at 100°C. At this stage, excess water evaporates and white gels were obtained. The powder obtained via (by) grinding the gel, were annealed at different temperatures ranging from 700°C-1200°C in the steps of 100°C for four hours in air. Two separate reactions, for 5% doping, one using

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Tb(N0 3 ) 3 and second using Eu(N0 3 ) 3 were performed for synthesis ofYAG:Tb3+. 3.8.5

Characterization of Prepared Y 3 Al 5 0 12 :Tb 3+ (YAG:Tb3+) Nanophosphors

The prepared nanophosphors were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and vacuum ultraviolet photoluminescence spectroscopy (VUV-PL) in order to elaborate structural and optical properties in a precise manner at different annealing temperatures. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using CuK al radiation with wavelength of 1.54 Ä in wide angle region from 10°to 80°on 2Θ scale. The size and morphology of prepared nanostructures were recorded on transmission electron microscope model Tecnai T-30 G 2 S - Twin electron microscope operated at 300 KV accelerating voltage. The VUV-PL and life time studies were performed on McPherson VUV spectrometer system model 2035, which is a Czemy Turner spectrometer having focal length of 360 mm and aperture ratio of f/4.8 and the VUV-PL results were analyzed by Gram suit software. 3.8.6

Results and Discussion

Figure 3.13 show the XRD patterns of the synthesized YAG:Tb3+ nanophosphors after annealing at different temperatures between 700°C-1200°C in the steps of 100°C for four hours. Initially XRD spectra show amorphous nature for the samples heated upto 800°C but later on broad peaks appeared for the samples annealed at 900°C-1200°C/ which were in good agreement with the standard JCPDS file (ICDD file No. 33-0040) for YAG and can be attributed as the monophasic cubic garnet structure for YAG nanophosphors. The broadening of XRD peaks (i.e. Scherrer's broadening) can be attributed to nanosize and the size, d, of the crystallites in samples were estimated by Debye-Scherrer's equation using Scherrer's broadening (FWHM). The average particle sizes were -15 nm, 20 nm, 26 nm and 32 nm for samples annealed at 900°C, 1000°C, 1100°C and 1200°C, respectively for YAG nanophosphors and were found consistent with TEM results (Fig 3.14). The narrower and much intense XRD peaks at higher annealing temperature indicates enhancement

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12Q0C

00 C 1000 C 900 C 800 C 700 C

10

20

30

40

50

60

70

80

90

2Θ (degree)

Figure 3.13 XRD spectra of YAG:Tb3+ nanophosphors annealed at temperatures ~700°C-1200°C in steps of 100°C for four hours.

Figure 3.14 TEM images and morphology of YAG:Tb3+ [(a) annealed at 700°C and (b) annealed at 1200°C] samples.

in crystallinity as compared to those for sample annealed at lower temperatures. TEM images were recorded by dissolving the synthesized powder sample in ethanol and then placing a drop of this dilute ethanolic solution on the surface of copper grid. Particles were initially spherical in nature having diameter nearly 15 nm for sample annealed up to 800°C. Hierarchical nanostructures of YAG:Tb3+ having size -40 nm were produced due to agglomeration of these spherical

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nanostructures on higher annealing (up to 1200°C) temperature. The crystallite size obtained using XRD data and observed TEM results reveal that these phosphors are in nanometer range. The solgel route using citric acid as polymerizing agent led to the formation of tiny gel cages during polymerization process, subsequently restricting the particle growth within the cage, resulted in formation of nanostructures. The representative TEM images and morphology of the YAG:Tb3+ [(a) annealed at 700°C and (b) annealed at 1200°C] samples are shown in Fig 3.14. Since many display devices including plasma display panels utilizes 147 nm and 173 nm VUV excitation, generated through typical Xe discharge, we have investigated the emission characteristics of all the three prepared samples particular by the same excitations. Beside this, since high temperature baking is essential for the screen printing process of phosphors during the manufacturing of PDPs, we have chosen 1200°C annealed sample for luminescence studies, as it is directly related to stability and thermal degradation of prepared nanophosphors. Figure 3.15 shows the obtained VUVPL of YAG:Tb3+ consisting of f-»f transition lines within 4Ρ electron configuration of Tb3+, i.e., 3 D4->7F6 (490 nm) in the blue region and 5 D 4 ^- 7 F 5 (545 nm) in the 20-1-

WF:

18-

Under 147 nm excitation

16 = 14-1 & g· 12 m

B 10 c -i a

Under 173 nm excitation

Q.

i e >

4 2

o-r

400

450

500

550

600

650

700

Wavelength (nm)

Figure 3.15 VUV-PL spectra of YAG:Tb3+ nanophosphors annealed at 1200°C under different VUV excitation.

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green region, as well as 5D4->7F4 (591 n m ) and 5D4-»7F3 (620 nm) in the red region. The strongest one is located at 545 nm, corresponding to the 5 D 4 -> 7 F. transition of Tb3+. Kang et al. [41] reported that the particle size, shape, crystallinity, defects etc significantly affects the luminescence characteristics of YAG:Tb3+. Whereas, Van der Weg et al. [42] studied the effect of doping concentration on the luminescence behavior of YAG:Tb3+. They observed that at low doping concentrations, primarily the absorption of excitation wavelength affects the luminescence characteristics without having any direct effect of the process of cross relaxation favoring emission from 5 D 3 state, whereas, at higher concentration the cross relaxation process favors the emission from 5D4 state. So, in present case, the cross relaxation process dominates. So, beyond 480 nm, the observed green luminescence is assigned to 5D4—»7F transitions (j = 3—6) and could be described by electric dipole-dipole interaction [43]. It is notable that the emission intensity of the sample excited with VUV 147 nm is much higher than that of the sample excited with VUV 173 nm. The samples annealed at 1200°C has better crystallinity and brings in the perfection to the nanophosphor surface as well as the better distribution of europium activator and also the decreases the defects at the surface of nanophosphor, resulting in enhanced emission intensity.

3.9 Terbium Doped Yttrium Ortho-borate (YBOs:Tb3+) Nanophosphors Recently, it is demostrated that with the reduction in particle size of crystalline system, there is remarkable modification in their properties which are different from those of bulk because of high surface to volume ratio and quantum confinement effect of nanometer materials [44-53]. In 1994, Bhargava et al. [54] reported that radiative transition rate of ZnS:Mn nanocrystals increases with decrease of particle size. They reported that the presence of an impurity within a nanocrystal and localization of electron and hole wave function due to quantum confinement leads to faster energy transfer to impurity in smaller particles as compared to transfer rate for band to band transition or surface recombination. Hence, luminescence efficiency increases with decrease in size of the particle. In 2001, Dijken et al. [55] also studied quantum efficiency of nanocrystalline

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ZnO with particle size. They also observed that quantum efficiency of the ZnO nanoparticles increases with the decrease in particle size. The observed quantum efficiency was 20 % for 0.7 nm ZnO particle and decreased to 12% non-linearly for 1.0 nm particle. Further, R.N. Bhargava etal. [56-58] studied the luminescence properties of Tb3+ doped Y 2 0 3 nanocrystal, where the electrons and holes of the host is not considered either weakly or strongly confined, since the Bohr diameter of electrons in Y 2 0 3 is about 1 nm while the particle sizes are in the range of 5 nm. This is incontrast to the Mn2+ doped ZnS nanocrystals where the Bohr diameter is 5 nm and is comparable to the diameter of the ZnS nanoparticle [54]. To explain the enhancement in luminescence in case of Tb3+ doped Y 2 0 3 nanosized particles, a new model of doped nanocrystals is proposed where the change in the oscillator strength is a result of the quantum confinement of a localized atom by the boundary of the host. This system is known as "Quantum Confined Atom" and the Quantum Confined Atom (QCA) is a nanocomposite comprising of few isolated atoms and the properties of the caged atoms are retained, protected and significantly amplified via its interaction with the host. In this particular section, we report enhanced green luminescence in Tb3+ doped YB0 3 nanocrystals as compared to Tb3+ ion in YB0 3 bulk phosphor. YBOs:Tb3+ nanoparticles are synthesized by co-precipitation method while bulk YB03:Tb3+ phosphor by solidstate reaction method. The terbium ion doped YB0 3 nanocrystal host have particle size 9-20 nm, while, YB0 3 : Tb bulk phosphor particles are of size 100-300 nm. The enhanced luminescence in YB0 3 nanocrystal host is explained on the basis of modulation of f-f transition due to quantum confinement of Tb3+ ion in YB0 3 nanocrystal host. The photoluminescence property of Tb3+ doped YB0 3 nanophosphor is also studied at 147 nm excitation for application in plasma display panels. 3.9.1

Synthesis of Terbium D o p e d Yttrium Ortho-borate (YB0 3 :Tb 3+ ) Nanophosphors

3.9.2

Chemicals Used

For the synthesis of YB03:Tb3+ nanohosphors, the metal nitrates i.e. yttrium nitrate Υ(ΝΌ3)2 and terbium nitrate Tb(N0 3 ) 3 were

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purchased from Sigma-Aldrich, Germany. All chemicals were of analytical reagent grade and were directly used without any special treatment. In the present work, all the samples were prepared in a clean room of class 1000 under ambient conditions. 3.9.3

Methodology

To obtain pure phase as well as quantum confined nanocrystals of YB03:Tb3+; low concentration of precursor solution and low annealing temperature 800°C restricted to 120 min only were carried out. In the experiment, 0.01M aqueous solution of yttrium nitrate, boric acid and terbium nitrate were prepared and were kept under stirring for 2 hours. In the above solution, ammonia solution was mixed. The pH of the solution was 9. This solution was again kept stirring for 24 hours at 60°C. The obtained product was filtered and washed with de-ionized water and ethanol and then annealed at 800°C for 120 minutes. For bulk phosphor, same stoichiometric amount of precursors were mixed and sintered at 800°C for 120 minutes. 3.9.4

Characterizations Used

The structural characterization were carried out on X-ray diffraction (XRD), Rigaku D/max-2000 x-ray powder diffractometer using Cu Κα (λ = 1.5405 A) radiation. Scanning electron microscopy (SEM) images were taken on a FEG Quanta 200 (FEI Company). Emission and excitation spectra were carried out on Perkin- Elmer LS 55 spectrometer. Quanta 200 FEG (FEI company) fitted with an energy dispersive X-ray spectroscopy (EDS: Genesis 2000, EDAX), was used for elemental analysis. VUV emission spectra were recorded on VUV spectrometer, McPherson, USA. 3.9.5

Result and Discussion

YB03:Tb3+ nano-sized phosphor and bulk phosphor both have a hexagonal vaterite structure and no second phase was observed (Fig 3.16). The YB03 phase thus obtained shows good agreement with Joint Committee on Powder Diffraction Standards Card No. 16-0277. Moreover, the lower diffraction intensity and broadening of diffraction peaks of nanophosphors synthesized by

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2Θ (degree)

Figure 3.16 XRD pattern of Tb3+ doped YB03 nanophosphor synthesized co-precipitation method, respectively. OptedfromR. S. Yadav and A. C. Pandey, ]. Alloys and Comp., 494 (2010) L15-L19.

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Figure 3.17 Photoluminescence emission spectra of YB03:Tb3+ Nanophosphor. OptedfromR. S. Yadav and A. C. Pandey,}. Alloys and Comp., 494 (2010) L15-L19.

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co-precipitation method as shown in Fig 3.16 indicates the ultrafine nature of the particles. We have studied photoluminescence excitation (PLE) and photoluminescence (PL) emission spectrum of YB03:Tb3+ nanophosphor and bulk phosphor. Fig 3.17 shows the photoluminescence spectra of YB03:Tb3+ nanophosphor under 254 nm (UV) excitation. The obtained emission spectra of YB03:Tb3+ consists of f-»f transition lines within 4F electron configuration of Tb3+, i.e., 5D4->7F6 (488) in the blue region and 5D4-»7F5 (542 nm) in the green region, as well as 5 D4-»7F4 (591 nm) and 5D4-»7F3 (625 nm) in the red region. The strongest one is located at 542 nm, corresponding to the 5D4-»7F5 transition of Tb3+. It is noticable that the photoluminescence efficiency of YB03:Tb3+ nanophosphor at 254 nm excitation is enhanced as compared to bulk. In photoluminescence excitation of YB03:Tb3+, the absorption spectra (not shown here) is associated with the green emission 5D4-»7F5 (542 nm). The spectral features in photoluminescence excitation (PLE) show a transfer mechanism of the excited carriers from 4P-»5d bands to 4P - manifold of the Tb3+ ion. For example, for YB03:Tb3+ nanophosphor, the peak absorption for the most efficient transfer to 5D4-»7F5 (542 nm) occurs at 244 nm, while in bulk phosphor it occur at 262 nm. The peak absorption is quite different from what is observed in case of YB03:Tb3+ nanophosphor and bulk phosphor. From these results it is clear that the peak of the PLE (absorption) moves to higher energy as the size of the particles decreases and the intensity of the absorption peak (PLE) increases as the peak position moves to the higher energy. It means that the product of the absorption and emission intensity (i.e. PLE) is greater for nanophosphor and comparable to bulk one. To explain the results of the quantum efficiency of YB03: Tb3+ nanophosphor and YB03:Tb3+ bulk phosphor, we have drawn the standard configuration coordinate (CC) model, similar to proposed by R.N.Bhargava et al. [57] and A. Daud et al. [59] for Tb3+ in Y 2 0 3 . In the standard configuration coordinate model as shown in Fig 3.18, we have schematically included the effect of the distortion imposed by the quantum confinement on 4f-»5d bands. This is shown as two different configuration coordinate parabolas, named as 'bulk' and 'nanoparticle'. From Fig 3.18, it is clear that as the particle size changes from bulk to nano, the transfer mechanism of excited carriers also changes. In bulk phosphor, the carriers go through directly downwards in a Tb3+-ion energy manifold, resulting in a slow process. However,

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50

40

t u J

30

> o c

111

20

10

0 C o nf i g u ratio n coo r d i n a te

Figure 3.18 Configuration coordinate model of Tb3+ in YB03 nanophosphor and bulk phosphor. Opted from R. S. Yadav and A. C. Pandey, ]. Alloys and Comp., 494 (2010) L15-L19.

as the particle size changes from bulk to nano, the efficiency of phosphor changes primarily because the transfer to energy level 5 D4 takes place directly without going through intermediate levels. This is due to the confinement of Tb ions imposed by the YB0 3 host boundary. In this model the excited state of dopant provide a significant overlap with the host boundary. The overlap of the wavefunctions of the extended excited states and host leads to strong modulation of the excited states of the dopant. This modulation leads to efficient and fast transfer of carriers from host to the dopant atom. Fig 3.19 shows the photoluminescence spectra of YB03:Tb3+ nanophosphor and bulk phosphor at 147 nm excitation. From Figure 3.6, it is clear that under 147 nm (VUV) excitation, YB03:Tb3+ nanophosphor has diminished luminescence efficiency as compared to bulk. This can be explained as follows: The intensity of light can be expressed as l(d) = Le-d

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5

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Excitation wavelength = 147 nm

■? έ.

1 YBO^Tb3* Bulk phosphor

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5!

9

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300

1

400

r-

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- 1

500

(

600

T

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— |

700

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Figure 3.19 Photoluminescence spectrum of YB03:Tb3+ nanophosphor and bulk phosphor at 147 nm excitation. Opted from R. S. Yadav and A. C. Pandey, /. Alloys and Comp., 494 (2010) L15-L19.

where I (d) is the light intensity at a given thickness d, I0 is the light intensity entering the medium and «= is the intensity absorption coefficient of the material (cm-1), which depends on the excitation wavelength and increases with decreasing the wavelength [60]. Therefore, there will be difference in penetration depth of the excitation photons of UV (254 nm) and VUV (147 nm). As the excitation wavelength becomes short, the penetration depth is decreased due to large absorption coefficient [61-63] and the excitation volume from which photoluminescence is observed moves closer to the surface. Since the particle surface can substantially act as a defect or a source of non-radiative recombination routes, the larger surface area due to nanosized particles end result in diminished luminescent efficiency under VUV irradiation. It is also notable that the atomic arrangement on the surface of nanosized phosphor is different from the bulk and consequently nanophosphor particles with large surface to volume ratio will be influenced by VUV irradiation. Due to the low penetration depth of VUV irradiation, the ratio of the dead volume in nanophosphor, into which the excited beam cannot reach, would increase. In bulk phosphor, ratio of the dead volume decreases especially under VUV irradiation.

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That is w h y bulk YB03:Tb3+ phosphors have better luminescence at 147 nm excitation in comparison to nanophosphor. The decrease in luminescence in YB03:Tb3+ at 147 nm is also supported by decay characteristics of YB03:Tb3+. According to the experimental data, the decay time is about 2.7 ms for bulk and 3.9 ms for YB03:Tb3+ nanophosphor, which is shorter than that of commercial Zn2Si04:Mn2+ green phosphor (23 ms) [64]. Since the life time are determined by both the radiative transition rate AR and the nonradiative transition rate ANR, and the life time τ can be expressed by T = 1/(A R +A N R )

(3.3)

and radiative quantum efficiency (η) can be expressed as η = Α κ / ( Α κ + ΑΝΚ)

(3.4)

From above equation it is clear that life-time also play the main role in the luminescent quantum efficiency. The higher decay time and lower quantum efficiency of nanosized YB03:Tb3+ as compare to bulk counterpart revealed that quantum efficiency and decay life time, near or on the surface of nanoparticles are greatly distributed by such non-radiative factors such as surface defects especially in nanophosphor at VUV (147 nm) excitation. The finding of the present study may give insight to printing technology of barrier ribs in PDPs to counts the problem of dead volume by improving the packing fraction and controlling the surface defects with the help of capping agents transparent to VUV photons.

3.10 Red Nanophosphors: Yttrium Aluminum Garnet Y3Al5012:Eu3+ (YAG:Eu3+) Nanophosphors 3.10.1

Synthesis of Yttrium A l u m i n u m Garnet Y 3 Al 5 0 12 :Eu 3+ (YAG:Eu3+) Nanophosphors b y Sol-gel Method

3.10.2

Chemicals Used

For the synthesis of YAG:Eu3+ nanostructures, the metal nitrates i.e. yttrium nitrate Y(N0 3 ) 2 , aluminum nitrate Al(N0 3 ) 3 .9H 2 0 and europium nitrate Eu(N0 3 ) 3 , citric acid and ethylene glycol were procured from E. Merck Limited, Mumbai- 400018, India.

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Citric acid and ethylene glycol were used as chelating and polymerizing agents, respectively. All chemicals were of AR grade and were directly used without any special treatment. In the present work, all the samples were prepared in a clean room of class 1000 under ambient conditions. 3.10.3

Methodology

In a typical Sol-Gel synthesis process nominal composition of analytical grade of metal nitrates were dissolved in double distilled water. Then ethylene glycol and citric acid were added. Citric acid solution was used to chelate the metal ions and to polymerize with ethylene glycol for gel formation. Homogeneous colorless solution was obtained after continuous stirring for more than 5 hours. After adding appropriate amount of ethylene glycol into the clear solution, the mixture was continuously stirred and heated at 100°C. At this stage, excess water evaporates and white gels were obtained. The powder obtained via(by) grinding the gel, were annealed at selected different temperatures ranging from 700°C-1200°C in the steps of 100°C for four hours in air. 3.10.4

Characterizations Used

The prepared YAG:Eu3+ nanostructures were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and photoluminescence spectroscopy (PL) in order to elaborate structural and optical properties in a precise manner at different annealing temperatures. XRD was performed on Rigaku D/max2200 PC diffractometer operated at 40 kV/20 mA, using CuKal radiation with wavelength of 1.54 Ä in wide angle region from 10°to 80°on 2Θ scale. The size and morphology of prepared nanostructures were recorded on transmission electron microscope model Technai 30 G 2 S - Twin electron microscope operated at 200 KV accelerating voltage. PL studies were performed on a Perkin Elmer LS 55 luminescence spectrophotometer using a Xenon discharge lamp, equivalent to 20 kW for 8 microsecond duration as the excitation source at room temperature. The Vacuum Ultra Violet Photo Luminescence (VUV-PL) were recorded on McPherson VUV spectrometer system model 2035, which is a Czemy Turner spectrometer having focal length of 360 mm and aperture ratio of f/4.8 and the VUV-PL results were analyzed by Gram suit software.

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127

Results and Discussion

Figure 3.20 (a) shows the XRD pattern of the YAG:Eu3+ nanostructures after annealing at different temperatures between 700°C-1200°C in the steps of 100°C for four hours. Initially XRD spectra show amorphous nature for the samples but later on broad peaks appeared for the samples annealed at 900°C-1200°C, which were in good agreement with the standard JCPDS file for YAG:Eu3+ (ICDD file No. 33-0040) and can be attributed as the monophasic cubic garnet structure. The broadening of XRD peaks (i.e. Scherrer's broadening) can be attributed to nanosized formation of YAG:Eu3+ and the particle size 'd' of the crystallites in samples were estimated by Debye-Scherrer's equation using Scherrer's broadening (FWHM). The average particle sizes were -15 nm, 20 nm, 26 nm and 32 nm for samples annealed at 900°C, 1000°C, 1100°C and 1200°C, respectively and were found consistent with TEM results (Fig 3.21). Furthermore, it was also observed that as we go on increase the annealing temperature the XRD peaks become narrow and much intense, indicating enhancement in crystallinity as compared to those for sample annealed at lower temperatures. The crystallite sizes, estimated by XRD, were increased, from 10 nm to 32 nm, with annealing temperatures and were found to be consistent with TEM results. TEM images were recorded by dissolving the as synthesized powder sample in ethanol and then placing a drop of this dilute (b)

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Figure 3.20 XRD spectra of YAG:Eu3+ hierarchical nanostructures annealed at temperatures ranging from 700°C-1200°C in steps of 100°C for four hours (a) on the very first day and (b) after 150 days. Opted from P. K. Sharma, M. Kumar, P. K. Singh, A. C. Pandey and V.N. Singh,}. Appl. Phys., 105, 034309 (2009).

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Figure 3.21 TEM images of the samples annealed at (a) 700°C (b) 800°C (c) 900°C (d) 1000°C (e) U00°C and (f) 1200°C for four hours, resulting in YAG:Eu3+ hierarchical nanostructures. Opted from P. K. Sharma, M. Kumar, P. K. Singh, A. C. Pandey and V.N. Singh, J. Appl. Phys., 105,034309 (2009).

ethanolic solution on the surface of copper grid. The morphology of the samples annealed at (a) 700°C (b) 800°C (c) 900°C (d) 1000°C (e) 1100°C and (f) 1200°C for four hours are shown in Fig 3.21. Particles were initially spherical in nature having diameter nearly 15 nm for sample annealed up to 800°C. Hierarchical nanostructures of YAG:Eu3+ having size ~40 nm were produced due to agglomeration of these spherical nanostructures on higher annealing (up to 1200°C) temperature. The crystallite size obtained using XRD data and observed TEM results reveal that these phosphors are in nanometer range. The sol-gel route using citric acid as polymerizing agent led to the formation of tiny gel cages during polymerization process, subsequently restricting the particle growth within the cage, resulted in nanosized YAG:Eu3+ hierarchical structures. In order to study the annealing effect on the luminescence behavior of the synthesized samples, photoluminescence (PL) excitation studies at 225 nm were also carried out. The emission spectra of these YAG:Eu3+ nano-phosphors annealed for four hours at temperatures 700°C, 1000°C and 1200°C, respectively are depicted in Fig 3.22(a). The PL spectra consists of a number of sharp lines (peaks) ranging from 530 nm to 650 nm, associated with forced

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electric dipole transition from excited 5 ϋ 0 ->Τ (J = 0,1,2,3,4) levels of Eu3+ activator ions. The related electric dipole transitions of Eu3+ ion along with the energy level diagram is drawn in the Fig 3.23 for better understanding. The strongest red peak corresponding to 608 nm was due is attributed to 5D0—>7F2 transition that confirmed the YAG crystallization whereas the 585 nm orange emission originates due to magnetic dipole transition 5 D 0 — ^ transition was typically because of. Lower symmetry of crystal field near the activator Eu3+ ion will results in higher ratio of red and orange peak intensity (R/O value). The R / O value strongly depends on local symmetry of activator Eu3+ ion. Due to higher degree of disorder near the surface, nanomaterials generally gives better chromaticity. These were the main reason behind getting better chromaticity in and thus is the case for synthesized YAG:Eu3+ hierarchical nanostructures. From Fig 3.22 it is also clear that the position of emission peaks is unaffected due to annealing temperature, indicating that the environment around the Eu3+ activator ion was impassive after annealing, where as it was found that maximum luminescence intensity extensively depended on annealing temperature. At higher annealing temperature the maxima of maximum luminescence intensity was found to be improved due to better crystallization. The luminescence behavior of the synthesized YAG:Eu3+ hierarchical nanostructures

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Figure 3.23 Room temperature vacuum-ultraviolet photoluminescence spectra of YAG:Eu3+ hierarchical nanostructures annealed at 1200°C for four hours and Energy level diagram of Eu3+ and possible electric dipole transition within energy levels of Eu3+ ions. Opted from P. K. Sharma, M. Kumar, P. K. Singh, A. C. Pandey and V.N. Singh, ]. Appl. Phys., 105,034309 (2009).

is significantly dependent on the particle size, the growth in grain size at higher temperature and the decrease in the surface area of the particle and the increase in emission intensity may be attributed to the same. Thus, the change in surface to volume ratio, removal of surface defects and non-radiative rates led to improvement in luminous efficiency. Since many display devices including plasma display panels utilizes 147 nm and 173 nm VUV excitation, generated through typical Xe discharge, we have investigated the emission characteristics of the sol-gel derived YAG:Eu3+ hierarchical nanostructures in particular by the same excitations. The VUV-PL spectra were noticeably different from the normal photoluminescence spectra. As normal photoluminescence spectra showed maximum luminescence intensity for sample annealed at 1200°C, we recorded the VUV-PL emission spectra only for the sample annealed at 1200°C. The VUV-PL emission spectra for sample annealed at 1200°C are as depicted in Fig 3.24(a). Fig 3.24(a) is the spectra recorded just after annealing, Fig 3.24 [a (A)] shows spectra without applying any filter i.e. for excitation of whole VUV range. While Fig 3.24 [a (B)] and Fig 3.24 [a (C)] represent the spectra for 147 nm and 173 nm excitations, respectively. For whole VUV excitation range, six sharp peaks at the positions of 598 nm, 610 nm, 647 nm, 675 nm, 715 nm and 725 nm, along with

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Figure 3.24 Room temperature vacuum-ultraviolet photoluminescence spectra of YAG:Eu3+ hierarchical nanostructures annealed at 1200°C for four hours. Spectra (A) were recorded without applying any filter, spectra (B) was recorded after applying 147 nm filter and spectra (C) were recorded after applying 173 nm filter. All these spectra were recorded (a) on the very first day and (b) after 150 days. Opted from P. K. Sharma, M. Kumar, P. K. Singh, A. C. Pandey and V.N. Singh, J. Appl. Phys., 105, 034309 (2009).

a broad diffused band centered at 445 nm were observed. In this case the broad diffused band appeared only because of overlapping of different emission wavelengths corresponding to different transitions of Eu3+ ions. On the other hand, for the samples excited by at 147 nm and 173 nm radiations, emission spectra show one sharp intense peak at the position of 610 nm followed by three other weak peaks at 647 nm, 715 nm and 725 nm, respectively. The intense sharp peak at 610 nm originates due to the forced electric dipole transition for 5D0->7F2 of Eu3+ activator. While the peak at 647 nm corresponds to 5D0—>^F3 transition and 715 nm and 725 nm peaks were due to 5D0—>7F4 transition of Eu3+ ion, respectively. In this case, the orange peak intensity almost vanished and only strong red peak appeared. As already discussed in previous paragraph, the ratio of red and orange peak intensity ( R / O value) strongly depends on local symmetry of activator ion Eu3+. Lower symmetry of crystal field near the activator ion Eu3+ will result in higher R / O value. Due to higher degree of disorder near the surface, nanomaterials generally gives better chromaticity. Removal of orange peak in present case suggests that VUV excitation further lowered the crystal field symmetry around the Eu3+ ion as discussed previously.

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3.11 Time Evolution Studies The contrast and brightness of these display devices (including PDPs) are reported to degrade with time. So for Thus from the view point of their use application in display devices, it seemed sounded necessary to check the performance of the prepared synthesized YAG:Eu3+ hierarchical nanophosphors with respect to time. In order to do this, we performed time evolution studies on at regular intervals of time for 150 days. In this section, we are revealing in-depth discussions on the results obtained from of XRD, photoluminescence and VUV-PL results during and after 150 days. Fig 3.20(b) shows the XRD pattern of the YAG:Eu3+ nanostructures after annealing it at different temperatures ranging from between 700°C-1200°C for four hours in the steps of 100°C. These results are in excellent agreement with the XRD results obtained on the very first day. These results showed that the prepared YAG:Eu3+ hierarchical nanophosphors are very stable and the particle size remains almost constant even after 150 days. Similar stable results depicting stability were also obtained for UV irradiated photoluminescence and VUV-PL studies results. The emission spectra after 150 days of these YAG:Eu3+ hierarchical nanostructures annealed at temperatures 700°C, 1000°C and 1200°C for four hours are depicted shown in Fig 3.22(b). The PL spectra obtained after 150 days are similar to that of obtained for the samples synthesized on the very first day (Fig 3.22(a)), and consist of same peaks ranging from 530 nm to 650 nm, associated with forced electric dipole transition from excited 5 D0—>7F (J = 0,1,2,3,4) levels of the activator Eu3+ ions. In the same way Similarly, the VUV-PL results also showed exact similarity between the spectra recorded on the very first day and remained same after 150 days. These results further confirmed that the prepared YAG:Eu3+ hierarchical nanophosphors were very stable with respect to time and temperature variations. Thus these nanophosphors can be seen as one of the most feasible candidate for red phosphor which is necessary and condemnatory constituent for flat panel displays, underlying the importance of the current work.

3.12 Europium Doped Yttrium Ortho-borate (YB03:Eu3+) Nanophosphors In the previous section, we have discussed about the 'YAG:Eu3+ nanoparticles', the luminescence efficiency and VUV absorption of

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which were not encouraging. In contrast to the above work and the work done over the past few years, YB03:Eu3+ still seems to be best possible red phosphor due to its high VUV transparency and exceptional optical damage threshold [65]. Although, YB03:Eu3+ possess strong VUV absorption and high efficiency, yet the practical application/use of YB03:Eu3+ is still limited as the characteristic emission of YB03:Eu3+ is composed of equal contributions of orange (5D0—»^) and red (5D0—»T^ transitions, leading to orangered emission instead of pure red and consequently resulting in poor chromaticity. The red emission (5D0—»T^ transitions) is hypersensitive to the local crystal field symmetry around activator Eu3+ ion [66, 67], and will be relatively strong if the crystal field symmetry around Eu3+ is low [68-73]. In the present section, we have attempted to increase the 5OQ—>rF2 contribution in YB03:Eu3+ phosphor by reducing the crystal field symmetry so as to improve the color purity/chromaticity. The synthesis of "nano-sized YB03:Eu3+" in a "micelles" seems to be a possible answer due to high disorder at the surface in nanosize. Eu3+ (1-40%) doped YB0 3 nanoparticles were synthesized by reverse micelles method. Our result reveals that "nano-sized YBOs:Eu3+" synthesized by "reverse micelles method" possesses very low crystal field symmetry leading to better chromaticity with nonlinear enhancement in luminescence. 3.12.1

Synthesis of Europium D o p e d Yttrium Ortho-borate (YB0 3 :Eu 3+ ) Nanophosphors b y Reverse Micelles Method

3.12.2

Chemicals Used

For the synthesis of YB03:Eu3+ nanostructures, cetyl-tri-methylammonium bromide (CTAB), co-surfactant n-butanol (50 ml) and solvent n-heptane (50 ml), Y(N0 3 ) 3 and Eu(N0 3 ) 3 , acid (H 3 B0 3 ) and NaOH were procured from E. Merck Limited, Mumbai- 400018, India. Citric acid and ethylene glycol were used as chelating and polymerizing agents, respectively. All chemicals were of AR grade and were directly used without any special treatment. In the present work, all the samples were prepared in a clean room of class 1000 under ambient conditions. 3.12.3

Synthesis of YB0 3 :Eu 3+ Nanoparticles

Initially, two separate micelles, PR1 and PR2, were prepared by thoroughly mixing in a homogenizer for two hours. The above micelles

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contained thesurfactantcetyl-tri-methyl-ammoniumbromide(CTAB), co-surfactant n-butanol(50ml) and solvent n-heptane(50ml) common, while PRl contained Y(N03)3 and Eu(N03)3, and PR2 contained boric acid (H3B03) and NaOH aqueous solution, respectively. Now PRl was added drop wise to PR2 while constantly stirring and then again homogenized for 2 h. The obtained white precipitates were washed several times with absolute ethanol and double distilled water and dried in vacuum oven at 50°C giving white YB03:Eu3+ nanoparticles. Thus obtained YB03:Eu3+ nanoparticles were annealed for four hours at higher temperatures (100-1200°C) to get YB03:Eu3+ nanoparticles of different sizes. 3.12.4

Characterizations Used

X-ray diffraction (XRD) was performed on Rigaku D/max-2200 PC diffractometer using CuKal (1=1.54 Ä) radiation. The Tecnai 30 G2 S-Twin electron microscope (300 kV) was used to take transmission electron microscopy (TEM) images. The Vacuum Ultra Violet Photo Luminescence (VUV-PL) were recorded on McPherson VUV spectrometer system model 2035, which is a Czemy Turner spectrometer having focal length of 360 mm and aperture ratio of f/4.8 and the VUV-PL results were analyzed by Gram suit software. 3.12.5

Results and Discussion

Figure 3.25(a) shows the XRD patterns of the prepared YB03:Eu3+ nanoparticles. All the peaks could be indexed to the YB03 standard

2Θ (degree)

Doping % of euion 3+

Figure 3.25 (a) XRD patterns for YB03:Eu samples annealed at different temperatures, (b) Quenching concentration of YB03:Eu3+ for sample prepared at room temperature obtained by monitoring the emissions of 5D0—>7F2. Opted from P. K. Sharma, R. K. Dutta and A. C. Pandey, Opt. Lett. 35,2331-2333 (2020).

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JCPDS file (No 16-277) having hexagonal phase with vaterite-type structure. By applying the Scherrer formula to the full width at half maximum of the diffraction peaks, the mean particle size could be calculated as 5nm for sample prepared at room temperature. The size of annealed YB03:Eu3+ samples increases with annealing temperature and found to be 8,20,30 and 60 nm for annealing at 600°C, 800°C, 1000°C and 1200°C for four hours, respectively. The corresponding TEM images are shown in Fig. 3.26, here (a), (b), (c), (d), (e) represents sample at room temperature, annealed at 600°C, 800°C, 1000°C and 1200°C for four hours, respectively, whereas (f) corresponds to commercial bulk YB03:Eu3+ phosphor of size -1-3 mm. TEM results showed that the prepared YB03:Eu3+ nanoparticles are spherical-like in nature and there is almost no amorphous component in SAED. The grain sizes are distributed in the range of 5-60 nm for different samples and consistent with the mean particle size obtained from XRD. The electron diffraction pattern shown in the inset of Fig. 3.26(a) also indicates a hexagonal structure. In order to study the effect of doping concentration on the luminescence behavior of the samples synthesized at room temperature, quenching studies were carried out and is shown in

Figure 3.26 TEM images and electron diffraction of YB03:Eu3+ nanoparticles, (a) sample prepared at room temperature, (b), (c), (d), (e) represents sample annealed at 600°C, 800°C, 1000°C, 1200°C for four hours, respectively, whereas (f) corresponds to commercial bulk YB03:Eu3+ phosphor. OptedfromP. K. Sharma, R. K. Dutta and A. C. Pandey, Opt. Lett. 35,2331-2333 (2010).

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Fig 3.25 (b). Previous studies have shown that quenching concentration increases as the particle size decreases [74], so we have chosen sample prepared at room temperature (~5 nm) for quenching studies. The quenching concentration was found about 30% Eu3+ ion doping which is far greater than the previously reported results for both bulk and nano YB03:Eu3+ [75, 76]. The concentration quenching effect is due to dominance of non-radiative transitions over the radiative one, because the particle boundary encumbrance causes the resonance energy transfer only within one particle, whereas the enhanced mobility of the excited state within the host matrix increases the non-radiative de-excitation probability via quenching centers (traps) [77]. Therefore, for particles containing few or no traps, as in case of nanoparticles, quenching occurs only at high Eu3+ concentration or do not quench at all. The quenching studies forcefd us to further study the 30% Eu3+ doped YB0 3 nanoparticles for chromaticity and luminescence efficiency as function of particle size under VUV excitation. We annealed the 30% Eu3+ doped samples at different temperature to get different sizes of particles and then studied their optical properties under 147nm VUV excitation for comparing these results with commercial bulk YB03:Eu3+ phosphors. The obtained comparative emission spectra are depicted in Fig 3.27(a). The VUV-PL spectra consists of a number of sharp peaks associated with forced electric dipole transition from excited 5D0->7F (J = 0,1,2,3,4) levels of Eu3+

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activator ions. The major emissions of YB03:Eu3+ are at 595nm corresponding to 5D0—>7Fj, 612 and 628 nm corresponding to 5D0—>7F2 transitions, respectively. The highest luminescence intensity, almost 1.5 times higher than the commercial bulk, is obtained for sample prepared at room temperature (~5 nm in size) with dramatically improved color chromaticity. Although, the major peak positions in the emission spectra are identical to each other, the red to orange (R/O) intensity ratios are quite different. In YB03:Eu3+ nanoparticles of size 7F2 transition (electric dipole transition) robustly depends on the local symmetry of Eu3+ ions. Consequently, the 5 D 0 —»^transition should be relatively strong as the Eu3+ ions occupy the inversion center sites, whereas the 5D0—>7F2 transition is parity forbidden and should be very weak. The uncharacteristic luminescent activity of YB03:Eu3+ significantly depends on the particle size as more atoms are located at the particle surface when the particle size is reduced. Besides the size of nanoparticles, numerous existing surface defects, consequence to the low temperature synthesis, plays an important role. These defects may increase the degree of disorder, lowering the local symmetry of Eu3+ ions located at the surface of the particles. This in turn increases the transition probability of 5D0—>7F2 responsible for enhancement of the red emission, thereby improving the color chromaticity as shown in Fig 3.28(a). The luminescence decay time of YB03:Eu3+ nanoparticles and commercial bulk phosphor was also calculated for the 5D0—»7F2 transition (612 nm) at 147 nm VUV excitation and is shown in Fig 3.28(b). The original experimental decay curve was fitted with

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exponential decay function, 1= A exp(-t/x), where, I is intensity, A is constant, t is time and τ is the decay time. The calculated decay time for YBOs:Eu3+ nanoparticle (-3.62 ms) is found well shorter than that for commercial bulk phosphor (~4.73 ms). The reduced decay time makes it possible that the more and more phosphors are ready to contribute without quenching for sustainable emission efficiency. Thus the interplay of radiative and non-radiative transition rates coupled with considerable reduction in life/decay time towards enhanced luminescence efficiency needs careful inspection for which meticulous study is still underway.

3.13 Europium Doped Yttrium Oxide (Y203:Eu3+) Nanophosphors Recently, rare earth based nanoparticles have engrossed great attention as red light emitter in modern age flat panel display devices and low pressure flourscent lamps [80, 81]. The quality of phosphor is of great importance for the performance of these modern flat panel display devices as they directly influence the brightness and lifetime. From above point of view, the phosphor should have good luminescence efficiency as well as high color purity with long term stability [82]. In the same hunt the host

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"nano-sized Y 2 0 3 " was found to be promising host material for Eu3+, rare earth ions [83-85]. Due to high disorder near the surface, the synthesis of nano-sized Y203:Eu3+ phosphor must be potential answer to achieve good chromaticity whereas the lower site symmetry of Eu3+ in nanosized samples could improve the ratio of red emission to orange emission leading to better chromaticity. Although, several researchers have reported the luminescence characteristics of Y203:Eu3+ nanoparticle under UV excitations [8385], but none of them have explored their luminescence characteristics under higher VUV excitations. In the same pursuit, present section talks about the solution combustion synthesis of spherical nanoparticles of Y203:Eu3+ yielding intense red luminescence under higher VUV excitation with relatively shorter luminescence decay time. 3.13.1

Synthesis of Europium D o p e d Yttrium Oxide (Y 2 0 3 :Eu 3+ ) Nanophosphors b y Solution Combustion Method

3.13.2

Chemicals Used

For the synthesis of Y203:Eu3+ nanoparticles, analytical reagent (AR) grade metal nitrates i.e. yttrium nitrate Y(N0 3 ) 2 , europium nitrate Eu(N0 3 ) 3 and glycine NH 2 CH 2 COOH were procured from Merck India. All chemicals were of AR grade and were directly used without any special treatment. In the present work, all the samples were prepared in a clean room of class 1000 under ambient conditions. 3.13.3

Methodology

In this reaction glycine was used as fuel for the combustion reaction. The europium ion concentration was kept as 20 weight %. All the metal nitrates and glycine were mixed in double distilled water to form the precursor solution. Then the solution was concentrated by direct flame heating until excess free water evaporated and spontaneous ignition occurred. The resultant Y203:Eu3+ nanoparticles were formed after the combustion was finished. Thus obtained fluffy product was grind to obtain fine powders of Y203:Eu3+ nanoparticles and were annealed at 600°C, 800°C, 1000°C and 1200°C for four hours in air.

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3.13.4

Characterizations Used

The prepared Y203:Eu3+ nanoparticles were characterized by X-ray diffraction (XRD, Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using 1.54 Ä CuK al radiation), transmission electron microscopy (TEM, Tecnai 30 G2S-Twin microscope operated at 300 kV), energy dispersive X-Rays (EDX) and photoluminescence spectroscopy (PL, Perkin Elmer LS 55 luminescence spectrophotometer). The Vacuum Ultra Violet Photo Luminescence (VUV-PL) were recorded on McPherson VUV spectrometer system model 2035, which is a Czemy Turner spectrometer having focal length of 360 mm and aperture ratio of f/4.8 and the VUV-PL results were analyzed by Gram suit software. 3.13.5

Results and Discussion

Figure 3.29(a) show the XRD spectra of the Y203:Eu3+ nanoparticles after annealing at 600°C, 800°C, 1000°C and 1200°C for four hours. These XRD spectra are in well accordance with the standard JCPDS file for Y 2 0 3 (JCPDS file No. 86-1326) and can be attributed as the poly crystalline cubic structure having space group la3 (206). The broadening of XRD peaks (i.e. Scherrer's broadening) can be attributed to nanosized formation of Y203:Eu3+ nanoparticles. The average crystallite sizes obtained using Debye-Scherrer's equation were ~30 nm for sample annealed at 600°C, whereas, 1200°C annealed sample shows ~50 nm and were found consistent with TEM results (Fig 3.30). The narrower and much intense XRD peaks at sample annealed at 1200°C indicates enhancement in crystallinity as compared to those for sample annealed at 600°C. No peaks attributed to other phases were observed indicating good quality samples. Figure 3.30 shows representative TEM images of the Y203:Eu3+ nanoparticles annealed at (a) 600°C (b) 1200°C. The average particle sizes were ~30 nm for sample annealed at 600°C; whereas, 1200°C annealed sample shows ~60 nm particles with little agglomeration. Corresponding HRTEM and electron diffraction were shown in Fig 3.30(c) and Fig 3.30(d) respectively. Diffraction rings shows poly crystalline cubic phase of Y203:Eu3+ nanoparticles. Fig 3.29(b) shows the EDX spectrum of Y203:Eu3+ nanoparticles annealed at 1200°C. From the EDX line traces it can be concluded that Eu3+ was successfully substituted into the crystal of Y203:Eu3+ nanoparticles and the Yttrium: Europium ratio is 80:19.1 weight %. This shows

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20 (degree)

1000 2000 3000 4000 5000 6000 7000 8000 9000 10000 Energy (eV)

Figure 3.29 (a) XRD spectra of Y203:Eu3+ nanoparticles annealed at 600°C-1200°C for four hours; (b) EDAX spectra of Y203:Eu3+ nanoparticles annealed at 1200°C; this spectra shows Yttrium:Europium :: 80:19.1 weight % contents.

that almost all the europium ions were incorporated in the Y 2 0 3 host. In order to study the annealing effect on the luminescence behavior of the synthesized samples, PL studies at 225 nm were performed. The emission spectra of these Y203:Eu3+ nanoparticles annealed at 600°C and 1200°C for four hours are depicted in Fig 3.31(a). The PL

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Figure 3.30 Representative TEM images and SAED patterns of the Y203:Eu3+ nanoparticles annealed at (a) 600°C (b) 1200°C. Corresponding HRTEM images are shown in figure (c) and (d) respectively.

(a)

t ■

Annealed® 600 "G Annealed ©i2oa*C

W* CC>W\*V - vKXL·^^

i\ 550

575

600

W-* 625 650

Wavelength (nm)

675

700

350 400 450 500 550 600 650 700 750 Wavelength (nm)

Figure 3.31 (a) Photoluminescence spectra of Y203:Eu3+ nanoparticles, lex=225 nm. (b) Vacuum-ultraviolet photoluminescence spectra of Y203:Eu3+ nanoparticles annealed at 1200°C for four hours. For figure (b) black line corresponds to 147 nm excitation and red line corresponds to 173 nm excitation. Inset shows the luminescence decay curve of Y203:Eu3+ nanoparticle.

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spectra consists of a number of sharp peaks ranging from 550 nm to 650 nm, associated with forced electric dipole transition from excited 5DQ—>7Fj (J = 0, 1, 2, 3, 4) levels of Eu3+ activator ions. These luminescence characteristics are very similar to that of our previous work for YAG:Eu3+ hierarchal nanostructures [86]. The strongest red peak corresponding to 610 nm is attributed' to 5DQ—»7F2 transition, whereas the 585 nm orange emission originates due to magnetic dipole transition 5D0—>7Fr Lower symmetry of crystal field near the activator Eu3+ ion results in higher ratio of red and orange peak intensity (R/O value) [87-92]. The R / O value strongly depends on local symmetry of activator Eu3+ ion and as is seen from figure, it remains almost same for both the samples. Due to higher degree of disorder near the surface, nanomaterials generally give better chromaticity and so is the case for synthesized Y203:Eu3+ nanoparticles. From Fig 3.31 it is also clear that the position of emission peaks are unaffected due to annealing, indicating that the environment around the Eu3+ activator ion is impassive after annealing, where as it is found that maximum luminescence intensity extensively depend on annealing temperature. At higher annealing temperature the maxima of luminescence intensity is improved due to better crystallization. The luminescence behavior of the Y203:Eu3+ nanoparticle significantly depend on the particle size, the growth in grain size at higher temperature and the decrease in the surface area of the particle and the increase in emission intensity may be attributed to the same. Thus, the change in surface to volume ratio, removal of surface defects and non-radiative rates led to improvement in luminous efficiency. In PDPs, phosphors are excited by vacuum ultraviolet (VUV) radiation, especially by 147 and 173 nm, from inert gas plasma, which shows the unique requirement for PDP phosphors that they can only be excited by the higher VUV excitations. So we have explored the luminescence characteristics particularly by the same excitations. We had recorded the VUV-PL emission spectra only for the sample annealed at 1200°C because this sample showed the maximum luminescence intensity under UV excitation. Figure 3.31(b) is the VUVPL spectra recorded for 147 nm and 173 nm excitations. For the samples excited at 147 nm and 173 nm, emission spectra show one sharp intense peak at the position of 610 nm followed by four other weak peaks at 580 nm, 600 nm, 630 nm and 650 nm, respectively. The intense sharp peak at 610 nm originates due to the forced

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electric dipole transition for 5D0—»Τ2 of Eu3+ activator. The peak at 630 nm corresponds to 5D0—>7F3 transition and 580 nm and 600 nm peaks were due to 5D0—»7F0 transition of Eu3+. In this case, the orange peak intensity almost vanished and only strong red peak appeared. Removal of orange peak in present case suggests that VUV excitation further lowered the crystal field symmetry around the Eu3+ ion as discussed previously. The luminescence decay time of the 5D0—»7F2 transition at 610 nm was also calculated against the 147 nm VUV excitation and is shown in the inset of Fig 3.31(b). The original experimental decay curve was fitted with exponential decay function, I = A exp (-t/τ), where, I is intensity, A is constant, t is time and τ is the decay time. The calculated decay time for the decay curve shown in the inset of Fig 3.31(b) is ~4 ms. several authors showed that the phosphors with short luminescence decay time are suited more as compared to the long decay phosphors for application point of view in modern days flat panel display devices as it lowers the possibility of luminescence quenching.

3.14

Conclusions

We have discussed about the rare-earth based insulating nanocrystals as new hope for the improved performance of plasma display panels. Before going into details, we have briefly discussed about the existing science of plasma display panels with state-of-art history of the development of plasma display panels till date. Ahead of main focus of the present chapter, we have briefly described the advantages and disadvantages of existing plasma display panels. It is observed that the luminous efficiency and lifetime of plasma display panels (PDPs) are directly related to the performance of phosphors used in PDPs, thus higher efficiency, higher stability against high temperature processes and a long lifetime along with good color chromaticity against vacuum-ultraviolet (VUV) radiation are major concerns while selecting suitable phosphors for PDPs. In the same pursuit, the present chapter talks about the plasma display phosphor of nanosize, usually termed as "Nanophosphors" as better alternatives to the currently used micron-submicron size commercial phosphors.

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The developed nano-sized Blue (BAM:Eu2+), Green (YAG:Tb3+ and YB03:Tb3+) and Red (YAG:Eu3+, YB03:Eu3+ and Y203:Eu3+) nanophosphors were studied for their luminescence properties under VUV excitations in view of their luminous efficiency, lifetime and higher stability against time and high temperature processes as these properties are directly related to the performance of phosphors used in PDPs. Time evolution studies showed these nanophosphors to be extremely stable, suggesting them to be one of the most feasible candidates for PDP applications and seize great potential in flat panel display application, underlying the importance of the current work. Further, to improvise the performance of PDP's with nanophosphors, the technological processes of printing of phosphors on ribs (for making the pixels) have to be looked into differently as low penetration of VUV due to its heavy absorption poses problems in dealing with high dead volume for non-radiative transitions (degrading the luminous efficiency).

Acknowledgements Authors are thankful to DST and CSIR, India for supporting "Nanotechnology Application Centre" under 'Nano-Mission' and 'NMITLF schemes of projects.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

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4

Amorphous Porous Mixed Oxides: A New And Highly Versatile Class of Materials Sadanand Pandey & Shivani B. Mishra Department of Chemical Technology, University of Johannesburg, South Africa

Abstract

Porous solids are a rapidly developing class of materials with an ever-increasing range of applications. Among these materials, amorphous porous mixed oxides possess the extraordinary properties of fine tuning the chemical composition, microstructure, porosity and surface properties. The tailor made composition of the mixed oxides can be fabricated from the availability of variety of precursor, additives and modifiers. Besides, their performance can be found close to that of crystalline phase. These amorphous mixed oxides can easily be synthesized using acid / base catalyzed one-pot sol-gel procedure. Sol-gel processes allow for a relatively facile tailoring of the morphology of mixed oxides to the desired application in the field of nanotechnology. Pore volume, porosity, specific surface area, and surface acidity are typical parameters that can be tailored. This chapter deals with the preparation methodology, characterization techniques, and application of amorphous porous mixed oxides.

Keywords: Porous solid, microporous, mesoporous, sol-gel process, characterization technique

4.1 Introduction Most of the materials are to some extent porous, indeed, it is quite difficult to find or prepare a truly non-porous solid. Many of the physical properties such as density, strength and thermal conductivity Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (149-182) © Scrivener Publishing LLC

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are known to depend on the pore structure of a solid. For a great industrial application such as in the design of catalysts, industrial adsorbents, membranes and ceramics, the control of porosity is known to be a very important factor. The chemical reactivity of solids and the physical interaction of solids with gases and liquids are influence by the porosity. Thus Porous solids are known to be a rapidly developing class of materials with an ever-increasing range of applications. To provide a comprehensive overview of the area of amorphous porous mixed oxide, this chapter will begin with a brief introduction to porous materials. The basic concepts and description of porous material will be given to understand the context of mesoporous and micropores materials. Following this introduction, a systematic classification of the types and scope of porous materials will be presented.In this chapter, focus on the special group of porous materials i.e. Amorphous mixed oxides prepared by sol-gel procedures, which benefit from very simple synthesis procedures and an ever-increasing degree of versatility are highlighted. The different important parameters which effect the final properties of material will also be discuss. The different important properties and their characterization methods will be briefly described before major applications in various fields are reviewed. Finally in this chapter, in conclusion part, future directions are identified as challenges and opportunities to researchers in this field.

4.2 Description of a Porous Solid Material 4.2.1

Qualitative Description of a Porous Solid

According to definition of porous material, any solid material may be regarded as porous material, if it contains cavities, channels or interstices. Porous material can be instance classify according to their availability to an external fluid by the help of (Figure 4.1). During the classification of porous material, those pores that are totally isolated from the neighbors as in region (I) are known as closed pores. These types of close pores influence macroscopic properties such as mechanical strength, bulk density and thermal conductivity. But these close pores are inactive in adsorption of gases and in fluid flow. The second categories of pores are those

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Figure 4.1 Schematic cross-section of a porous solid.

which have a continuous channel of communication with the external surface of the body, like (II) (III) (IV) (V) and (VI) are known as open pores. Open pores may be subdivided into many types. When pores are open at one end like (Π) and (VI). They are known as blind or dead end or saccafe pores. Other may be open at two ends through pores, like around (V). On the bases of shapes, pores are also be classified as: • Cylindrical (either open(III) or Blind (VI)) • Ink- bottle shaped (II) • Funnel shaped (IV) or slit shaped. Close to, but different from porosity is the roughness of the external surface, represented around (VH). To make the distinction, a convenient and simple convention is to consider that a rough surface is not porous unless it has irregularities that are deeper than they are wide. 4.2.2

Origin of Pore Structures

There are two different types of porous materials i.e. consolidated and unconsolidated. • Consolidated porous materials exist as relatively rigid, macroscopic bodies whose dimensions exceed those

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of the pores by many orders of magnitude. Such types of porous material are also known as Agglomerates. • Unconsolidated porous material being non-rigid more or less loosely packed assemblages of individual particles. Such types of porous material are known as Aggregate. It is found that distinction between two types of materials i.e. consolidated and unconsolidated is not always clear. Though two forms of materials are often interconvertible. Example, by grinding of the former, and by sintering of the latter. Thus, porous materials are known to be formed by several different routes. • A first class of porous material form highly regular networks. Pores are an inherent feature of particular crystalline structures, e.g. zeolites and some clay minerals. • A second class of porous materials is formed by loose packing (i.e. aggregation) and subsequent consolidation (i.e. agglomeration) of small particles as for instance in some inorganic gels and in ceramics. These processes are constitutive, in that the final structure depends mainly on the original arrangement of the primary particles and on their size. • A third route is described as subtractive, in that certain elements of an original structure are selectively removed to create pores. Examples are the formation of porous metal oxides by thermal decomposition of hydroxides, carbonates, nitrates, oxalates and of porous glasses by chemical etching of multiphase solids. Many porous organic polymer membranes are formed in this way. A more complex process, although related to the same mechanisms, is that of the activation of carbons. Finally, the pore structure of plant and animal tissues, which is of literally vital importance and must fulfill stringent conditions, is determined by natural processes of cell division and self organization, which are as yet imperfectly understood. 4.2.3

Idealized Systems : Pore Shape and Size

The shape of pores is preferably described in term of spheres (possible in silica gel, zirconia gel etc), Slits (possible in clay and

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activated carbons), prism (some fibrous zeolite), Cavities and Windows (in other zeolites) or cylinders (possible in activated oxides like alumina or magnesia) The real porous solids description is complicated, due to the existence of : • different shapes of pores in the same material • connections between pores, which may vary in size, shape and location • a distribution in the size of the pores. It was necessary to introduce description based upon the concepts of connectivity", "percolation", "tonuosity" in order to describe there complexity. The sizes of pores play a major role for the use of porous solids. So to carry out a pore size analysis, a number of methods were developed. When the geometrical shape of pores is well defined and known (eg. Cylinder, slit shaped etc), then the pore size has got a precise meaning. In the previous IUPAC documents, the following distinctions and definitions were adopted [1-4] • Micropores have widths smaller than 2 nm. • Mesopores have widths between 2 and 50 nm. • Macropores have widths larger than 50 nm. Now coming to microporous oxides. There are different techniques for the synthesis of microporous oxides which can be used for application in heterogeneous catalysis. But in this chapter we have focused on the production of amorphous microporous oxide by the technique of sol-gel process. The simplest methods by which to prepare amorphous porous oxides are sol-gel processes. The mixed oxide improved the properties of material which can then be used for various applications. For example. Pure ZrO z and Ti0 2 carriers have very small specific surface area, low thermal stability and high price, which make them unsuitable for industrial applications. In order to overcome these drawbacks, increasing attentions has been paid to the development of mixed oxide supports by combining the higher surface areas and thermal stability of silica with the unique acidic properties of ZrQ, and Ti0 2 . Mixed titania-silica and zirconia-silica materials are potentially useful in a number of technological applications, including catalyst.

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4.3 Sol-gel Method for the Production of Porous Oxides During the past two decade, the development of sol gel techniques has led to a fast progress in the synthesis of porous material. The process of sol gel compliment a broad range of conventional procedures such as precipitation or impregnation methods followed by high-temperature treatments which are used for the synthesis of amorphous solids or glasses. In comparison with the conventional method of synthesis, such as ceramic firing, impregnation [5-7], precipitation/peptization [8, 9], or ion exchange on supported oxides [10], the sol gel method is found to be an attractive and easy to tailor. Through the method of sol gel, a different broad range of porous material such as mixed oxides, metal oxides, crackfree or nonshrinking monoliths, fibers, membranes, and highly ordered crystalline materials with pores of uniform size can easily be obtained [11]. The sol gel process represent one of the attractive alternative to conventional method for the synthesis of polynary porous mixed oxides [12-17] or glasses [18]. The significance feature of sol gel process is that it is low cost, requires mild reaction conditions and always provide a homogenous gels with most of the element present in the periodic table which are capable of formation of solid oxide. The single most important characteristics of sol-gel preparation of amorphous mixed catalytic materials are its ease of control that translates into the following advantages: 1. The ability to maintain high purity (because of purity of starting materials); 2. The ability to change physical characteristic such as pore size distribution and pore volume; 3. The ability to vary compositional homogeneity at a molecular level; 4. The ability to prepare samples at low temperatures; 5. The ability to introduce several components in a single step; 6. The ability to produce samples in different physical forms. Generally there are four important key steps in a sol gel process, the purpose of each step and the associated experimental

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parameter that can be varied. Because the properties of product can be potentially be affected by these parameter. The entire four steps are shown in (Table 4.1). The sol-gel is a process in which hydrolysis of precursors follows by polycondensation with the formation of an oxide gel take place. Example. Si0 2 - based amorphous microor mesoporous solids or glasses. There are number of steps involve in process of sol gel, which are as follow: Step 1 Step 2

Hydrolysis and prehydrolysis; Acid-, base-, or fluoride-catalyzed polycondensation for gelation;

Table 4.1 Important parameter in the various steps of a sol-gel process. Step

Purpose

Important Parameter

Solution chemistry

To form a gel

Type of precursor; Type of solvent; pH (acid/ base content); Water content; Precursor concentration Temperature

Aging

To allow a gel to undergo changes in properties

Time; Temperature; Composition of pore liquid(e.g. pH) Aging environment (e.g. humidity)

Drying

To remove solvent from a gel

Drying method (e.g. evaporation Vs supercritical Vs freeze drying); Temperature and pressurization rate

Calcination/ sintering

To change the physical / chemical properties of the solid, often resulting in crystallization and densification

Temperature and heating rate; Time; Gaseous environment (e.g. inert Vs reactive glass)

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Step 3 Step 4 Step 5

Drying; Aging; and Calcination,

In each step of the sol gel process, the different parameter were controlled, so that, the synthesis of final microstructure possess a improve properties of selectivity and functional activity. One of the important oxides that are amorphous oxides is synthesis by process of sol gel in a polycondensation reaction under kinetically controlled reaction conditions. The different properties of amorphous oxides such as pore structure and porosity, composition, surface polarity, surface acidity, and crystallinity can be control by the choice of reagents, additives, reaction and drying conditions. The important structure of Meso- and macroporous are essential for the required mass transport for the catalytic reactions [19]. It is of intense importance to avoid the oxide domains formation of individual elements during the preparation of mixed oxides. Material properties different from those of a purely physical mixture of individual oxides can only be guarantee when all types of metal ion of which mixed oxide is composed must have homogenous distribution. This important property is termed as homogenous mixed oxide or simply mixed oxide. The presence of second or third metal centers result in the modification of many physical and chemical properties of the metal oxides. For the formation of desired mixed oxides, the differences in rates of hydrolysis and polycondensation as well as electronegativity of the central atoms play a very crucial role. Acid catalyzed hydrolysis is essential for the formation of mixed oxide sols. While in case of base catalyzed hydrolysis depending on the differences in the metal centers, electronegativity [20], result in precipitation and formation of inhomogeneous sols. The attacking agent under basic gelation conditions is the hydroxide ion, which accelerate the two important steps of sol-gel process i.e. hydrolysis as well as polycondensation by attacking directly at the metal ion. Sequential formation of pure particulate oxides will dominate the process under basic gel conditions. Except for interparticle voids, base catalyzed silica is known to be nonporous. It is known that fluoride ions not only catalyzed the acid and base catalyzed hydrolysis but it also catalyze the hydrolysis reaction of pure oxides [21]. Accelerated hydrolysis and condensation of oxide precursors by fluoride shorten the time of gelation due to

AMORPHOUS POROUS MIXED OXIDES

157

the high electronegativity of fluoride [22]. The proton will preferentially known to attack the alkoxide or anionic oxygen connected to the metal ions under acidic conditions. Only small effect on the electron density of these anionic oxygen atoms was observed due to difference in electronegativity and thus the proton does not differentiate between metal ions. Thus truly mixed oxides formation is highly enhanced under acidic reaction conditions. A pre-hydrolysis step or chelation is often advisable to compensate for the different reactivity of various precursors for the formation of mixed oxide. For example. Reactivity is found to be much higher for zirconium, titanium, tin or aluminum alkoxides with water as comparison to the reactivity of alkoxysilanes. The increase in the reactivity is due to the lower electronegativity and higher Lewis acidity of the metal ions and the possibility of increasing the coordination number. The gelation time and degree of condensation were strongly affected by water content, acidity and molecular structure of complexing agent such as chain length or steric hindrance. And these often control the three-dimensional metal-oxygen-metal network [23, 24] growth process. In case of rapidly gelling material small pores are often form. During the initial process of hydrolysis, the correlation of pore volume and specific surface area of titanium silicates with pH and water content were studied by Yu & Wang [25]. The narrow pore size distribution were often provided by the condensation reactions for gelation at low temperatures, mild drying of the gel, and a slow temperature increase during calcinations [26]. 4.3.1

Synthesis of Micro and Mesoporous Materials

The drying of gels under normal pressure (microporous xerogels) or supercritical conditions (mesoporous aerogels) are used for density and porosity of the oxides [27-31]. Presently supercritical drying for a high temperature as well as low temperature variations are used [32]. The supercritical drying process of aerogel preparation have important characteristic of high specific surface area, mesoporous structures, and low density. These important characteristics lead to the various applications, such as isolators, batteries, supercapacitors, electrode materials, and others [33-35]. To prepare the porous crystalline materials, the method of sol-gel has also been used. Some materials such as are zeolites (microporous alumosilicates with three-dimensional periodic network

158

INTELLIGENT NANOMATERIALS

structures [36-40]; use of aluminophosphates [41]; zeolite-based membranes with molecular sieving properties [42]; and titanium silicates, such as TS-1, which are important materials for a broad range of applications [43]. Crystalline microporous material have also used for the application as catalysis, separation, and ion exchange. The use of zeolites is limited to molecules small enough to penetrate the well-defined pores. The lower chemical and mechanical stability of MCM-type material having large pores reduces their potential for practical applications. Pores shape selectivity have demonstrated by Thomas and Raja [41] in aluminophosphates with various reactions. The microporous zeolites are mainly used for the shape selectivity processes [44]. A shape selective properties was also shown by Amorphous microporous mixed oxides prepared by solgel methods. It reflect a very versatile, a simple, and largely unexplored alternative to the well-defined and thus limited zeolites. By tailoring of shape selectivity, pore diameter, and surface polarity, an amorphous microporous titanium silicate can be prepared, and thus it represents synthetically simple alternatives to zeolites. Ti-doped X and Y-zeolites have the largest pores among the titanium silicates, with the pore diameters of 0.7 nm. To form micro-, meso-, or macroporous materials through the formation of a primary organic network, ethylene glycol has been used in the process of sol-gel. Pyrolysis steps for the removal of organic backbone are needed. Depending on the conditions chosen, xerogels [45] or aerogels can be obtained. The morphological properties of the porous materials formed can be controlled by a. The synthesis parameters, b. Dilution of the reactive components, or c. Control of the growth mechanism of the particles or clusters through the choice of Reactants (precursors), the pH of the solution, catalyst type, gelation temperature, and calcination protocol [46]. Use of different catalyst and precursor for porous mixed oxide preparation are mention in (Table 4.2). 4.3.2

Template-assisted Synthesis

Mobil developed MCM materials [66] having a highly ordered mesoporous amorphous silicates with tailored pore size, high

AMORPHOUS POROUS MIXED OXIDES

159

symmetry, and very high surface areas (>1000 m 2 g_1). The presence of organic templates, most commonly surfactants [67], result in the well defined porosity in these materials. These surfactants form micelles, preferably rod-like, under appropriate conditions. And the inorganic oxides adhere in only few atomic layers on the surface of these rods. These rod are seen to stacked together, and after the removal of organic surfactants, the inorganic walls still remains which result in the formation of a high porous material. Because of these silicates' well-defined pore size, in the range of 2-20 nm is achieved. These types of synthesis principle has been successfully transfer to a broad range of oxides and mixed oxides. Table 4.2 Used of different catalysts and precursors for porous mixed oxide preparation. Catalyst

Precursor

Example

Reference(s)

Mineral acid

Metal alkoxides Metal salts/ alkoxides

Si-Ti-Ox Ti-Ce-Ox

47 48

Nitric acid

Metal nitrates, ethylenglycole

Ce-Sm-Ox

49

Acetic acid

Metal alkoxides, metal halides

Si-Ti-Ox

Citric acid

Metal salts

Cu-Ce-Ox Zr-Ce-Ox Cu-Co-Al-Ox

52 53 54

Tartaric acid

Metal alkoxides

Si-Ti-Ox

55

Propionic acid

Metal nitrates, complexing agents

Co-Mn-Ni-Ox

56

Polyoles

Metal salts

Zn-Co-Ox Co-Fe-Ox M-Ox

57 58 59

HA

Metal halides Metal oxides

Cu-Ce-Ox V-Sb-Ox

52 60

Epoxides

Metal salts

M-Ox M-Si-Ox Y-Zr-Ox

50,51

61 62-64 65

160

INTELLIGENT NANOMATERIALS

The common applications of these materials are M41S or transition metal-containing redox materials, molecular sieves [68], or mesoporous SBA-15 silica gels [69]. These silicates' potential application as shape-selective catalysts for medium-sized molecules probably has gathered the most attention [39]. For the formation of porous oxides and mixed oxides, template methods have successfully been used [70-72]. The templates size and type play important role in tailored of pore size of these materials. In the synthesis of isotropic, hexagonal, cubic, or laminar mesostructures of mixed oxides [73, 74], a variety of cationic, anionic, or nonionic templates have already been used successfully. The negative charge of many oxides at pH values higher than their isoelectric point is justifying the use of cationic templates. But the availability of cationic templates [ammonium and sulfonium salts [75] is limited. One of the main disadvantages of use of ionic templates is the need for extreme pH values for synthesis and a limited lower pore size. Synthesis of highly stable Zr, Ti, and V oxides with Gemini templates are described by Kim [76, 77] which contain many hydrophilic or hydrophobie groups and form micelles at low concentrations. Synthesis of porous transition metal oxides by neutral method, based on the use of di- or tri-block copolymers is characterized by simplicity, low cost, and high tolerance to a large number of elements [78-80]. For the creation of nanostructures, Self-assembly of block copolymers, [81, 82] surfactants, [83] colloidal suspensions [84] and proteins [85] provides a versatile approach with potential applications in biomaterials, optoelectronics, and nanotechnology [86], To organize a mesoporous silica, the use of surfactant species has been explored over a wide range of conditions, with the discovery of mesoporous silicates based on amphiphilic supramolecular templates, [87, 88]. These condition relied on various interactions including vander Waals forces, hydrogen bonding, covalent bonding and electrostatic that have been balanced to achieve selfassembly and thereby direct mesostructures formation [83, 89-91]. Presently, block copolymers are used increasingly, to organize mesostructural composite solids. For example, using amphiphilic block copolymers as the structure-directing agents, aluminosilicate mesostructures, [92] and mesoporous silica [93-96] with large ordering lengths (>15 nm) and a variety of macroscopic morphologies (e.g., thin film and fiber) have been synthesized. The formation of non-silica mesoporous oxides, for the extension of the surfactant templating procedure has been less widespread,

AMORPHOUS POROUS MIXED OXIDES

161

although these mesoporous metal oxides hold more promise in applications that involve electron transport and transfer or magnetic interactions [97]. Mesoporous Mn0 2 / [98] A1203, [99,100] Ti0 2 , [101,102] Nb 2 O s , [91 ] Ta 2 0 5 , [91] Zr0 2 , [103-107] Hf0 2 , [108] and Sn0 2 [109] and reduced Pt [110] have been synthesized using low molecular weight surfactants over the past few years. A surfactant templating strategy was well developed by Stucky and co-worker, for the synthesis of nonsilica based mesostructures, mainly metal oxides, [83, 89] in which both positively and negatively charged low molecular weight surfactants were used in .the presence of water soluble inorganic species. It was found that for the formation of the organic-inorganic mesophase, charge density matching between the surfactant and the inorganic species is important. Many of these non-silica mesostructures are unfortunately, not thermally stable. Non-ionic surfactants to synthesize mesoporous alumina in neutral media are used by Pinnavaia and co-workers and it was suggested that mesoporous materials with wormlike channels are assembled by hydrogen-bonding interactions of the inorganic species with the surfactant agents [99]. The stable mesoporous transition metal oxide, Ti0 2 , using a modified sol-gel method, in which an organometallic precursor was hydrolyzed in the presence of alkyl phosphate surfactants [101] is prepared by Antonelli and Ying. Long-chain quaternary ammonium, primary amines, or amphoteric cocamidopropyl betaine as structure-directing agents. [105, 106] are used for preparing Mesoporous Zr0 2 . For the preparation of mesoporous ZrO z [104], a scaffolding process was developed by Knowles et al. P. Yang and co-worker [111] has developed a simple and general procedure for the synthesis of ordered large-pore (up to 14 nm) mesoporous metal oxides, including Ti0 2 , Z r 0 2 Nb 2 O s , Ta,05, AL O,, SnO„ W O , , HfO„ and mixed oxides SiAlO , AL TiO , 2

'

2

3'

2'

3'

ZrTiO , SiTiO , and ZrW,0 .

4.4

2'

y'

2

yf

Characterization of Porous Mixed Oxides

The mixed oxides are found to be one of the most complex materials as compare to the pure metals or single oxides due to; • variable oxidation states in the oxide; • variable co-ordination of metal ions in the oxides;

162

INTELLIGENT NANOMATERIALS

• The redox properties of oxide; • The distribution and type of acidic or basic active centers. In order to determine the microscopic or atomic structures of these complex mixed oxide; a combination of different types of characterization method has to be applied which are shown in (Table 4.3). For the porous amorphous oxides or mixed oxides characterization, a number of methods are available [27,112]. But when once study powders or monolithic oxides the method gets differ. For the characterization of the morphology of the porous material, "adsorptive method" in order to determine the specific surface area as well as pore sizes and porosities are commonly used [113]. The micropores and mesopores are determined by the method of adsorption isotherm of nitrogen and argon [114,115]. Whereas these types of pores (particles) has been also characterized through mercury porosimetry [19,116,117]. Materials may contain micropores as well as mesopores. Mesopores, especially those which are in order or uniform can be identified by hysteresis between adsorption and desorption isotherm in which hysteresis form, provide knowledge regarding the pore shape [118]. The BJH model is commonly used for the pore size distribution of mesopores materials [119]. But in case of micropores material in the range of 1-2 nm a especial care is to be taken in the interpretation of pore size distribution. Because the BJH model is based on the Kelvin equation and this model is mainly valid for the pore size in the range of between 2-5 nm to approximately 100 nm, but not below this range. The BET model is used for the determination of specific surface area of material. This model basically used for the type (II) and (IV) isotherm, while using BET model for the other isotherm a great care is to be taken. These types of sorption method cannot be used for determining the porosity and the specific surface of mixed oxide films. Such types of thin films morphology can only be determined microscopically by the technique of electron microscope imaging [120]. Or small- angle XRD spectra from reflections before and after the calcinations of the thin film [121]. Solid State NMR (27A1-, 29Si MAS-NMR) and infrared spectrometry (DRIFTS) in a case of amorphous titanium silicates provide in formations as to the connectivity of the two elements in the mixed oxide network (M-O-M or M-O-H) and with mixed oxides especially the connectivity between identical or different element [122].

AMORPHOUS POROUS MIXED OXIDES

163

Table 4.3 Different types of characteristic methodologies available for complex amorphous materials. Various Characterstic Methodologies For Porous Mixed Oxide Atomic Environment

• Solid state NMR • EXAFS, XANES • UV-VIS • SAXS,SANS • Raman

Surface polarity

• DRIFT • Water adsorption • TGA/DTG • TPSR • 'HCPMASNMR

Morphology

• Adsorptive methods • Density • Composition (XRD, EDX, XPS) • Microscopy (electron, optical)

Gel formation

• Viscosity • Solid state NMR • SAXS

For the characterization of porous system in amorphous silica prepared by method of sol gel, the Pulsed-field gradient NMR (PFG- NMR) techniques are used (Table 4.4) provides an overview of the common method used for the mixed oxides characterization. The catalytically active centers in an oxide matrix are often seen in metal particles. So a big difference in a catalytic performance can be made by isolated metal particles. X-ray absorption edge can be used for providing information on the co-ordination and next neighbor atoms of the active center atom. EXAFS which is known as Fourier transformation of the extended absorption edge is found to be the best method to confirm the atomic distribution of atoms of interest. It also provides information regarding the type of atom and local structure (number, type and distance of the first- and second neighbor atoms). The valence and electronic structure of the atoms of interest can easily obtained by the technique of. narrow absorption edge

164

INTELLIGENT NANOMATERIALS

spectroscopy (XANES). Example. Isolated nature of Ti centers in the amorphous silica framework of microporous mixed oxides (AMM) - Ti Si was confirming by the XANES measurement technique [123]. X-Ray powder diffraction is also a very important technique for providing information regarding the crystallinity of the oxides. By diffraction pattern the embedded nanoparticle or domain in amorphous matrix of oxides can be easily be identified. The first indication for the crystallization and particle sizes are given by the signal height and half width. Detail information related to the microstructure of mixed oxides, (i.e. for detecting particles down to approx. 1 nm in size and to identify the particle size of noble metals on or in oxides), these can be obtained by following the important and one of the most excellent technique known as High-resolution transmission electron microscopy (HR-TEM). These information is possible due to presence of a very good contrast which is lacking in a simple TEM. Information on the state of amorphicity of the material and the formation of crystalline phases and the temperature at which crystallization of phases start are provided by using the technique of Temp-dependent powder X-ray diffractogram. Additional information related to homogeneity, especially that of Si-based binary mixed oxides, and information as to hydrogen-bridged or terminal hydroxyl groups are provided by 29Si cross polarization magic angle spinning (CP MAS) NMR and Ή CP MAS NMR respectively. Now coming to UV-VIS spectroscopy. The homogeneity of the active center in a selected aerogels is determined by UV-VIS spectroscopy. It also gives information of reversible water adsorption at the surface due to reversible shift of the absorption signals at the Ti centers. In addition to the above absorptive method which are described, chemisorptions of alcohols is increasingly used to know information regarding the nature of active centers in porous mixed oxides. The Fourier-transform infrared spectroscopy (FTIR) method for the hydrogen containing group (-OH or -CH) are used for determing the content of silanol group or any organic group present on the surface of solid oxides. X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that are use to determine the different information such as the elemental composition, empirical formula, chemical state and electronic state of the element which is present in a

Table 4.4 Most important analysis methods for mixed oxides. Method

Information

Example of Mixed Oxide

UV-VIS spectroscopy

Coordination, homogeneity

AMM Ti-Si-Ox

XRD

Thermic stability

Me-Si-Ox Ti-Si-Ox

EXAFS/XANES

Local coordination, crystallinity

Y-Fe-Ti-Ox AMM Ti-Si-Ox

Electron micoscopy (SEM, HR-TEM)

Crystallinity, structure of surface

Me-Sn-O Cu/V-Si-Ox

Reference(s) 47 126 112 127 47,128 129 130

ON

00

Z

H

HR solid-state NMR 29Si, 27A1, 3, P, 1 7 0, Ή

Coordination, structure, details

Ti-Si-Ox

Diffuse reflectance FTIR spectroscopy (DRIFT)

Homogeneity, surface acidity

AMM Ti-Si-Ox

N 2 -physisorption

Specific surface, pore/particle size

12, 27,131

132

a1 r ro a Z H

z> zo

Na-Y, Na-ZSM-5, Si0 2 , MCM-41; Me-Si-Ox AM Al-Si-Ox

114,133

% M XJ I—I

>

1X1

(Continued)

>

1X1

Table 4.4 (cont.) Most important analysis methods for mixed oxides. Method

Information

Example of Mixed oxide

ON

00

Reference(s)

H

TGA/DTA

Adsorbed species, organic content, phase transformations

Ti-Si-Ox Al-Ti-Ox Al-Si-Ox

43,112 133 133,134

Raman spectroscopy

MOx species in the bulk or surface area of oxid, phase purity

V-Nb-Ox

135

Chemisorption (TPSR)

Nature of active center

Mo-V-Te-Nb-Ox

136

Low-angle scattering (SAXS)

Structure of network

Ti-Si-Zr-Ox

Photoelectron spectroscopy (XPS)

Composition at the surface and in the bulk

Ti-Si-Ox,

EXAFS/XANES

Local coordination, crystaflinity

Y-Fe-Ti-Ox AMM Ti-Si-Ox

M'ossbauer spectroscopy

Magnetic behavior of Fe3+ centers

AMM Fe-Al-Si-Ox

'AMM (Amorphous Mixed Oxide)

Z

a1 r ro a Z H

z> zo

% 23

M XJ I—I

>

1X1

137

127 47,128 128

AMORPHOUS POROUS MIXED OXIDES

167

material. It is a surface chemical analysis technique that can be used to analyzed the surface chemistry of a material in it's a "received state" or after some treatment. XPS is also known as ESCA (Electron spectroscopy for chemical analysis) [124]. X-Ray fluorescence (XRF) is the emission of characteristic secondary (or fluorescent) X-rays from a material that has been excited by bombarding with high energy X-ray or gamma rays. This method is also widely used for elemental and chemical analysis [125].

4.5 4.5.1

Application of Porous Mixed Oxide Catalysts

There are many industrial and academic applications of porous mixed oxides. Porous mixed oxide use as support material for Nobel metals [138] or transition metals. The different catalytic applications of sol-gel derieved amorphous mixed oxides are provided in a (Table 4.5). Among the various amorphous mixed oxide, Si-Ti mixed oxides have been given special attention. These material is a simple and low cost alternative to Ti- containing zeolites (TS-1, Ti-ß, Ti- MCM-41), when prepared by sol gel process. The Ti- containing zeolites and Si glasses is basically use as a selective epoxidation of cyclic or linear alkenes and allylalcohols and this is found to be one of main application of Ti- containing Zeolite and Si glasses. It was known that if synthesis condition and mild drying and calcination procedure are followed, then the amorphous mixed oxide of narrow pore size distribution and porosity between 10-30% can be easily be formed without using complexing agents and prehydrolysis by a single-step acid catalyzed sol-gel process [139, 47]. Amorphous mixed oxides pore diameter are formed between 0.6- 0.7 nm by this process, and it can easily increased to the mesopore range by varying some of the important reaction conditions, such as change in the nature of the alkoxide anions or increase of acid concentrations or by use of templates [26]. By using CTAB (cetyl trimethyl ammonium bromide) as a template, a microporous as well as mesoporous Zr- Si mixed oxide are prepared [140]. Where the composition of the mixed oxide controlled the micro- or mesoporosity. The morphology of this Amorphous mixed oxide can be

Table 4.5 Application of mixed oxide as heterogeneous catalysts. Application

ON

00

Mixed Oxide

Reference(s)

Oxidation of propylene to acrolein

Fe-Te-Se MO

150

Catalyst in kumada coupling

Ni/Mg-La MO

151

C-C bond forming reaction

Tin-tungsten MO

152

Biodesel production

Ca-Si-Ox MO

153

Transesterification of palm kernel oil

Zn/Fe MO

154

Synthesis of dimethyl carbonate from methyl carbamate and methanol

Zn/Fe MO

155

Oxidation of propane to Acrolein

Mo-Cr-Te MO V-Cr-Sb MO Mo-V-Cr MO

156

Catalyst for Suzuki coupling

Pd- doped MO

157

Synthesis of isobutyraldehyde from methanol and ethanol

V 2 0 5 / Ti0 2 -Si0 2 MO

158

Epoxidation of cycloalkenones

Ti-Si Aerogels

159

Epoxidation of Allylic alcohol

Ti (OSiMe3)4 MO

160

Ammoxidation of propane

Sn/ V / Sb MO

161

Ammoxidation of 3-Picoline

V 2 0 5 -Ti0 2 MO

162

Z

H

a1 r ro a Z H

z> zo

% M XJ I—I

>

1X1

Synthesis of Aniline from cyclohexanol and ammonia

M o 0 3 / A1203 V 2 0 5 -Mo0 3 /Al 2 0, NiO- M o 0 3 / A1203 MO

163

Hydration of alkynes to ketones

Sn- W MO

164

Combustion of Methane

MnOx-Ce0 2 MO

165

Combustion of volatile organic compound

Mn 2 0 3 -Fe 2 0 3 MO

166

Epoxidation of a- isophorone with hydroperoxides

Ti-Si-Ox MO

167

Olefin epoxidation

Ti-Si-O Al-Ti-Ox V-Si-Ox

59 54 184

> o

Water-splitting catalysts

Ni-Ta-Ox Pt/ K-Ta(Zr)Q, Bil2TiO20 N i O / Sr3Ti207

195 170 171 172

X O 1000 nm, so although thermochromism was observed they were less effective than W-doped films with similar switching properties. The rate of change of T was calculated to be -19°C/ at% of fluorine present in the films, which compares reasonably to -23°C/at% for W-doping. Further work [75] yielded considerable success by co-doping V0 2 with fluorine and tungsten simultaneously. The rate of change of Tc for co-doping was found to be -15°C/ at%. It was postulated that this lower than expected value was due to the similar way W6+ and P ions affect the V0 2 lattice. Both ions weaken the V4+-V4+ pairing via charge transfer, thus destabilising the semiconducting phase. It was considered that dopant ions in close

290

INTELLIGENT NANOMATERIALS

proximity could act on the same V4+ pair, negating the effect of the second dopant. Doping V 0 2 separately with tungsten and fluorine, however, led to the expected cumulative lowering effect on Tc. A thermochromic switching temperature of 0°C was achieved with F and W levels of 2.1 and 1.8 at% respectively. This led to revision of the above theory, and the hypothesis that a proportion of the F ions doped into the crystal lattice become inoperative in the presence of tungsten. This, combined with a greater fraction of fluoride ions trapped at grain boundaries (as a result of observed reduction in crystallite size with higher dopant levels), could explain the unexpectedly high transition temperature in co-doped films. In this section we report results from aerosol assisted chemical vapour deposition (AACVD) studies on the production of fluorine doped vanadium dioxide thin films. The AACVD reaction of vanadyl acetylacetonate at 525°C in ethanol onto glass substrates afforded brown/yellow films. The films were adherent to the substrate, they could not be wiped off with a piece of toweling, passed the Scotch tape test and resisted scratching with a brass stylus. They could however be abraded with a steel stylus. The addition of trifluroacetic acid (TFAA) into the reaction mixture (summarised in Table 7.1) led to the production of thin films with identical mechanical properties but with a noticeably lighter colour, albeit the same yellow / brown of the undoped films. The film thickness was found to be comparable between samples of doped and undoped films prepared under similar conditions and we attribute the lightening of the film colour to the incorporation of fluorine into the films. EDAX spot analysis indicated that the films had a uniform composition across the substrate close to V O r Scanning electron microscopy of films prepared from the AACVD route (Figure 7.1) indicated a granular growth morphology typical of AACVD reactions of VO(acac)2 [125,133]. As a larger amount of TFAA is added to the reaction mixture the island size in the deposited film increases, from 50 nm for 0.20 ml of TFAA (Figure 7.16a) to 120 nm for 0.9 and 1.0 ml of TFAA (Figure 7.16c). This changing island size suggests that the TFAA is having an effect in the growth mechanism of the films. It is possible that this is a templating type affect as has been seen elsewhere [134], or that the TFAA is preferentially absorbing onto the substrate surface and providing a lower energy path to island nucleation.

THERMOCHROMIC T H I N FILMS

291

Table 7.1 Reaction flask contents and micro analytical data. Flask Contents

Phase (XRD/ Raman)

Atomic% Fluorine (WD AX/ED AX)

1 [140]

0.1325 gVO(acac) 2 20.0 ml EtOH 0.02 ml TFAA

V0 2 (m)

0.0%

2 [150]

0.1325 g VO(acac)2 19.8 ml EtOH 0.20 ml TFAA

V 0 2 (m)

0.7%

3 [145]

0.1325 g VO(acac)2 19.5 ml EtOH 0.50 ml TFAA

V 0 2 (m)

1.1%

4 [100]

0.1325 g VO(acac)2 19.1 ml EtOH 0.90 ml TFAA

V0 2 (m)

0.8%

5 [135]

0.1325 g VO(acac)2 19.0 ml EtOH 1ml TFFA

V 0 2 (m)

4.0%

Sample [Thickness /nm]

Raman Spectroscopy (Figure 7.17) confirmed the presence of monoclinic VOz and indicated that no other vanadium oxide phases were present as has been observed previously [133] at least to the limit of detection. Raman spectroscopy also indicated the presence of graphitic carbon with a large peak in the 900-1100cm-1 region (not shown). This peak was relatively stronger with larger amounts of fluorine incorporation. This suggests that the decomposition of TFAA on the substrate surface is incomplete. Raman spectroscopy was also used to monitor the thermochromic transition of the vanadium dioxide thin films, in all cases this was found to occur between 55 and 65°C. UV/Vis spectroscopy above and below the transition temperature (Figure 7.18) indicated that the films were indeed thermochromic showing a switch in infrared transmittance and reflectance. Notably the change in both transmittance and reflectance was somewhat smaller than what has been observed for tungsten doped vanadium dioxide thin films previously - a maximum change of

292

INTELLIGENT NANOMATERIALS

Figure 7.16 Scanning electron micrographs of a) Sample 2, b) Sample 3 and c) sample 4.

15% at 2500 nm opposed to a change of 55% at 2500 nm [65]. The observed change in reflectance of 5% is also smaller than what has been seen before. The main effect of increasing the fluorine concentration incorporated into the deposited film was that the films became more transmissive compared to undoped and tungsten doped vanadium dioxide samples made previously [65]. Analysis with side on electron microscopy indicated that 1 at.% fluorine doped vanadium dioxide films that were 140 nm thick were as transmissive as 110 nm films of tungsten doped vanadium dioxide [65] although they still retained the characteristic yellow/brown colour

THERMOCHROMIC T H I N FILMS 3500

225 192

3000

293

621

2500 E

j] 2000 IB

jfi&m/ W

n 1500 c V £

^JWN

^ * ^ « w ^ * ^

-'^^

1000 500 100

xw/vu^ 200

3Q0

4Q0

500

Wave number/cm"

600

700

800

1

Figure 7.17 Variable temperature Raman spectroscopy of sample 3 indicating a thermochromic transition - marked lines indicated monoclinic vanadium dioxide related stretching/vibrational bands.

300

800

1300 1800 Wavelength/nm

2300

Figure 7.18 Variable temperature UV/Vis spectroscopy of sample 2.

of vanadium dioxide. This can be explained if the fluorine is not homogenously dispersed throughout the vanadium dioxide film and some undoped vanadium dioxide remains. Incorporation of higher concentrations of fluorine and more carefully tailored flow conditions will help to improve film colour and homogeneity. The transition temperature of the films was broadly the same (~60°C) irrespective of the level of fluorine incorporation. This is

294

INTELLIGENT NANOMATERIALS

somewhat surprising given previous reports of fluorine-doped films prepared by PVD [67,75]. However these films were co-doped with tungsten and it is possible that fluorine doping had a negligible effect and that the tungsten was responsible entirely for the change in transition temperature. This could also be a consequence of poor fluorine dispersion throughout the film, the area examined by Raman spectroscopy could be fluorine deficient and hence the thermochromic transition temperature was just that of vanadium dioxide. 7.5.2 7.5.2.1

Nanoeomposite Thin Films and Energy M o d e l l i n g Studies Nanoeomposite Thin Film Deposition

Composite V 0 2 / A u nanoparticle films were grown by the use of hybrid aerosol assisted /atmospheric pressure CVD from auric acid, [HAuQ 4 ] and tetraoctylammonium bromide (TOAB) in methanol (the aerosol component) and [VO(acac)2] (the atmospheric pressure component). The films showed good surface coverage, uniformity and reproducibility. Film thickness could be easily varied by increasing or decreasing the time of deposition. In all cases at least the first 75% of the substrate is covered, similar to that observed previously with V 0 2 films produced from the APCVD reaction of vanadyl acetylacetonate. Similarly there are changes in thickness that correlate with the temperature gradient across the substrate surface as seen previously [65] and a highly uniform area 2 cm x 5 cm in the middle of the substrate. The colour of the films (Figure 7.19 & Table 7.2) could be altered substantially with a range of blues and

(a)

(b)

(c)

(d)

(e)

(f)

Figure 7.19 Examples of glass (3 x 5 cm approximate size) with gold and vanadium dioxide nano-composite films. The films had a Au/V ratio determined by ED AX of: a) 0 (W doped V02) b) 0.09 c) 0.15 d) 0.30 e) 0.36 f) oo (gold nanoparticle film).

Table 7.2 Summary of experimental conditions and chemical analysis. Row conditions were constant for all experiments; Plain flow = 2 L.min-1, VO(acac)2 Bubler flow = 4 L.min ' and Aerosol flow = 1 L.min-1. Samples

Contents of AACVD Flask

ED AX/ WDAX/ XRD Phase

Au/V Ratio

λ Max in Visible Transmission/nm

λ Max in Visible Reflection/nm

Thermochromic 1

Not used

V0 2 (m)

0.09- 0.96

570

570

Thermochromic 2

40 mg HAuCl 4 No TOAB

V0 2 (m) + Au

0.12-0.99

570^195

570-495

Thermochromic 3

No HAuCl 4 1.00-0.12 g TOAB

V0 2 (m)

N/A

570

570

40 mg HAuCl 4 1.00-0.12 g TOAB

V0 2 (m) + Au

Thermochromic 4

E? o n

X

N/A

570-495

570-495

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greens being produced in contrast to the yellow/brown colour normally associated with monoclinic vanadium dioxide; this is attributed to the inclusion of gold nano-particles in the V 0 2 matrix. The films became more blue in colour with the incorporation of more gold nano-particles in the films. The films from the hybrid CVD reaction were also more adherent than films of vanadium dioxide or gold produced using AACVD [73]; they passed the Scotch tape test and could not be wiped off with a piece of towelling. The films could however, be marked with a brass or steel stylus similar to films of V 0 2 produced previously [65]. The films were found to be electrically insulating suggesting that the amount of gold in the films was not enough to exceed the percolation limit. Details of experimental conditions are given in Table 7.2. Scanning electron microscopy (Figure 7.20) of the samples indicates an island growth morphology. Typically for samples produced with TOAB (Samples 4) a smaller average island size of 75 nm (Figure 7.20a) is seen compared with 150 nm (Figure 7.20b) for those produced without (Samples 2). Scanning electron microscopy (Figure 7.21) of the samples grown without gold indicates an island growth morphology. Typically for samples produced with TOAB but no gold (Samples 3) a smaller particle size of 50 nm (Figure 7.21a) is seen compared with 100 nm (Figure 7.21b) for those produced without TOAB or gold (Samples 1). Analysis with energy dispersive analysis of X-rays indicated that where appropriate the films contained gold and vanadium

Figure 7.20 Secondary electron scanning electron microscopy pictures of typical samples of a V0 2 /Au nano-particle film produced using the hybrid AA/AP CVD methodology grown (a) with TOAB (b) without TOAB.

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Figure 7.21 Secondary electron scanning electron microscopy pictures of typical samples of a V02 film produced using the hybrid AA/AP CVD methodology grown (a) with TOAB (b) without TOAB.

(summarised in Table 7.2) and no contaminant, at least to the limit of detection of the methodology (around Vi atom% depending on the element). X-ray diffraction analysis (Figure 7.22) indicates that in all cases cubic gold and monoclinic vanadium dioxide are formed, though the films were not particularly crystalline. The films were quite thin and a large broad hump centred at 24 2Θ is seen from the amorphous glass substrate, this is particularly problematic for thermochromic 4. The gold crystallite size was evaluated using Scherrer equation, it was found that for samples produced with TOAB the gold crystallite size was 13 ± 3 nm in diameter, where as crystallites produced without the use of TOAB were smaller, measuring 8 + 2 nm in diameter. Annealing in nitrogen at 500°C for two hours did not improve the crystallinity of the V0 2 phase in the films, however peaks relating to V2Os appeared in the X-ray diffraction pattern. Indeed in some cases there is some evidence of the formation of V 2 0. in the as produced samples. This is not entirely unexpected as previous work on vanadium dioxide by APCVD has seen a similar phenomenon and has shown that surface V20_ does not effect the thermochromic behaviour of the bulk film [65]. X-ray photoelectron analysis of the sample surface suggests a variety of vanadium environments due to the broadness and asymmetry of the peaks, we believe V2Os to be present only at the surface; indeed, on etching only a single vanadium environment (V2 at 516.8 and 523.4 eV) is observed consistent with vanadium dioxide. XPS shows that gold is found in a single metallic environment,

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INTELLIGENT NANOMATERIALS Thermochromic 1 The rmochramie2 - Thermochromic 3 — Thermochromic 4

10

15

20

25 30 35 40 2 theta scale/degrees

55

Figure 7.22 X-ray diffraction patterns of the 4 types of thermochromic coating produced by the hybrid AA/APCVD method under investigation.

Au4f 83.6 and 87.2 eV, this indicates several things: there are no other detectable gold compounds present in the film, there is no un-reacted precursor on the film surface and indicates that the gold is present only as gold nanoparticles on the surface and in the bulk of the host film matrix. XPS and Raman spectroscopy were used to evaluate carbon contamination in the films. In the case of V0 2 films grown with gold, with or without the use of TOAB insignificant amounts of carbon, less than 1 atom%, could be detected in the bulk of the films using XPS. More carbon was detected on the surface of the films using XPS but is most likely from an external source. However V0 2 films grown with TOAB but no gold had detectable levels of carbon throughout the film. Raman spectroscopy indicated the presence of V0 2 (m) with large peaks at 192, 222,388 and 611 cm"1. There were also broad peaks centred at 1371 and 1591 cm 1 which correspond to graphitic carbon for the samples of V0 2 grown with TOAB. 7.5.2.2 Energy Modelling Methodology Energy Plus software developed by the Lawrence Berkeley National Laboratory [135] and US Department of Energy was used to perform energy simulations and analysis. Energy Plus™ is an energy analysis and thermal load simulation program. Based on a user's

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description of a building from the perspective of the building's physical make-up, associated mechanical systems etc. A series of simulations with different configurations and settings were run in order to evaluate the performance of the thermochromic coatings in different climates. The simulation set period is one year, with data points gathered every hour. A very simple model of a room in a building was constructed in Energy Plus™. The room has external dimensions 6 x 5 x 3 m (length x width x height) and it is placed so that the axis of every wall is perpendicular to one of the orientation North, South, West and East. We consider the room to represent the fagade of a generic building so that just one wall is exposed to the external environment (weather, sun, wind, etc.); the remaining three walls are not affected by external conditions. The building is located in the northern hemisphere and the external wall is supposed to be exposed to the southern side. Two different glazing possibilities were considered; one where the window was 1.5 x 2.5 m located in the middle of the southern wall surface (covering 25% of this surface) considered to represent a residential scenario. The other comprised the whole of the southern face (100%) - a glazing wall, representing a modern commercial building. The model is summarised in Figure 7.23. Further details governing the materials used for walls etc, have been previously reported [136]. In both cases the window is double glazed with a 12 mm air cavity, the coating was always modelled on the inside face of the outer pane. The only difference between

Figure 7.23 The two room models: ori the left - window 1.5 m x 2.5 m (25%); on the right - glazing wall (100%).

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each simulation was the glazing or coating used, the 7 examples investigated are summarised in Table 7.3 and their spectra shown in Figure 7.24. The external ground temperatures were taken to be 18°C throughout the year, as this remains relatively constant after a small depth. The internal conditions were chosen to be air-conditioned between 19-26°C to maintain a comfortable working/living environment. The required illuminance level in an office building is taken to be 500 lux, this corresponds to a lighting load of 400 W. The lights are fully dimmable: lowering their output when there is an adequate illuminance from the sun, in order to save energy. It is considered that they can be dimmed in the whole range from 0 to 100%. The dimming control is automatic and zoned. The casual heat gain (persons + equipment) is taken to be 500 W in total and the ventilation rate used is 0.025 m 3 /s. Building occupancy was set as occupied from 8:00 till 18:00, five days a week, as is normal for an office. The simulations were run for a number of different cities in Europe and one in northern Africa in order that a wide range of climatic conditions were covered. The specific cities chosen for the simulations were: Cairo (Egypt), Palermo, Rome and Milan (Italy), Paris (France), London (UK), Helsinki (Finland) and Moscow (Russia). The model is clearly limited because the building is not ideal for all climates. Insulation layers, as well as the materials chosen here, in warmer and cooler climates would be different from that used in the model depending not only on local climate conditions but also on the constructive techniques and materials available in loco. Likewise the assumption that a constant ground temperature of 18°C throughout the year is significant. However, by using the results obtained from the plain glass (Optifloat) simulations as a baseline we aim to isolate the change in energy performance caused by the use of different glazings. The thermochromic properties of the glazing were modelled in version 3.0.0 of Energy Plus by entering the spectral data of the glazing in the hot and cold states. The glazing was switched between the hot and cold states using the shading control feature of EnergyPlus which can "replace" glazing elements in a window, according to environment conditions or set control criteria. The surface temperature of the glazing was correlated against incident solar radiation. The shading control automatically switched the glazing from the cold to hot state when the incident solar radiation exceeded that required for the glazing surface to exceed the

Table 7.3 Summary of glasses and films used in energy simulations and their characteristics. Sample

T/°C

Cold Solar Transmittance /%

Hot Solar Transmittance 1%

Room Temperature Emissivity

Hot State Emissivity

Optifloat Clear (plain float glass)

-

92

92

0.837

0.837

Sputtered silver coating (SB)

-

82

82

0.030

0.030

76

76

0.837

0.837

c

Absorbing glass (AB) (body tinted absorbing glass) Thermochromic 1

(vo2)

E? 59

78

74

0.825

0.795

o n

X

Thermochromic 2 (V0 2 + Gold)

43

Thermochromic 3 (V0 2 + TOAB)

38.5

61

51

0.827

0.789

Thermochromic 4 (V0 2 + Gold + TOAB)

45.5

77

49

0.828

0.797

56

48

0.800

0.752

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500

1000

1500

2000

2500

500

Wavelength {nm)

1000

1500

2000

2500

Wavelength (run)

(d) ioo

(C) 100

500

1000

1500

2000

Ξ500

500

Wavelength (nm)

1000

1500

2000

2500

Wavelength (nm) (0 100

500

1000

1500

2000

Wavelength (nm)

2500

500

1000

1500

2000

2500

Wavelength (nm)

Figure 7.24 Transmittance and reflectance spectra for the thin films used in the study: (a) Thermochromic 1, (b) Thermochromic 2, (c) Thermochromic 3, (d) Thermochromic 4, (e) Sputtered silver coating, (f) Absorbing glass.

transition temperature, switching back to the cold state when solar radiation fell below the trigger value. Note the latest release of Energy Plus, version 3.1.0, now includes a specific thermochromic glazing module, this will facilitate future simulation studies in this area.

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7.5.2.3

Total Energy

303

Consumption

The total energy consumption for our building is taken as the sum of heating, cooling and artificial lighting energy required for the whole year period, these results are presented in Figures 25 & 26. The total energy consumption is higher in the coldest cities. This higher energy consumption may be an experimental artefact as (a)

3000

Figure 7.25 (a) Total annual energy consumption and (b) percentage improvement to clear-clear glazing ίότ the 25% window to wall ratio model; Cairo, Palermo, I Rome, Milan, Paris, i London, I Moscow, ll Helsinki.

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Th1

Th1

Th2

Th2

Th3

Th3

Th4

SB

Th4

AB

SB

C

AB

Figure 7.26 (a) Total annual energy consumption and (b) percentage improvement to clear-clear glazing for the 100% window to wall ratio model; Cairo, Palermo, Rome, : Milan, Paris, I London, Moscow, Helsinki.

previously discussed; however it may also be explained by the low average temperatures, not allowing the thermochromic glazing to switch for significant periods of time into the hot state. As the thermochromic coatings display insignificant low-e behaviour (Table 7.3), we would not expect this sort of glazing to be suitable for colder climates. This is illustrated clearly in Figure 7.25b where the overall performance in cooler climates is not as good as plain float glass.

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In warmer climates however the thermochromic glazings perform more favourably than both plain glass and the commercial glazings evaluated here. The best behaviour is shown for thermochromic 3 in the city of Palermo for the 25% window model and in the city of Rome for the 100% window model. The best overall performance is shown in the 100% window model in cities with warm climates. The thermochromic coatings are a more favourable choice for large glazed areas in these cities. Large glazed surfaces, such as the 100% window analyzed in this work, contribute to a greater percentage to the overall energy balance of the building, potentially minimizing heat losses and as in this case, controlling the incoming solar radiation. It is interesting to note that the thermochromic 1 glazing, even though it never switches in its hot state because of its high switching temperature, still performs well in the warmer environmental conditions, albeit not as well as the other thermochromic coatings. This suggests that the variable heat mirror properties of thermochromic coatings are not the only important features of these coatings for solar control. Examining the spectra for thermochromic 1 on Figure 7.24a, it can be seen that the coating is absorbing in the visible and near infra-red; therefore it is likely that this will make a contribution to the energy performance of this glazing. 7.5.2.4

Building Energy Simulations

Discussion

The best energy saving performance was for thermochromic 3 on a glazing wall in warmer climates (Figure 7.26). In fact the thermochromic glazing is considerably more effective when installed on a glazing wall (Figure 7.26) than in a residential scenario (Figure 7.25.). In both cases, the best energy saving performance is in these warmer climates where the thermochromic products have comparable or better energy performance than the industry benchmarks evaluated here. The thermochromic glasses perform less well in cooler climates. This is in part because the transition temperatures are too high and the coatings do not spend a significant time in the metallic state during summer to prevent heat entering the building. This is also because the semi-conducting states of the thermochromic films have poor low-e properties (Table 7.3). As such re-radiated blackbody radiation is not retained in the building during the winter and energy must be expended in heating the building. The coatings are however, very effective in warmer climates. This is for two reasons;

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firstly because they are coloured they have are intrinsically absorbing in nature. This can be seen from the performance of thermochromic 1, which has comparable energy saving properties to the industry standard absorbing glass benchmark. In this instance the transition is very high (59°C) and the coating does not spend any time in the metallic state, therefore all of its energy saving behaviour derives from its colouration and subsequent absorbing properties. Secondly there is an additional effect caused by the variable heat mirror characteristics of thermochromic glazing that helps to improve their performance beyond that of the existing products evaluated. As one would expect the additional energy benefit is greatest for the film with the lowest transition temperature (thermochromic 3). In warmer climates it is anticipated that the energy saving properties of these films could be improved further by reducing the transition temperature to around 25°C by introducing additional strain into, or tungsten doping the films. 7.5.3

The Ideal Thermochromic Coating

The aim of this section is to consider the performances of "ideal" thermochromic coatings; this was done to assess what will be the energy savings if a thermochromic film with ideal properties was used. We have decided to consider films with particular spectral characteristics chosen on the basis of the previous sections results and what can be considered to be reasonably achievable in practice. Several values of the thermochromic transition temperature are chosen; we have also selected different values for the change in transmission and reflectance and thus different extents of the thermochromic effect. We have chosen to run simulations using the weather file for Palermo, as this provides a hot and sunny environment that thermochromic technology shows most promise for. The simulations were run only for the 100% glazing wall. 7.5.3.1

Spectral Characteristics

The spectral characteristics of an ideal thermochromic films were defined as follows: in the cold state (monoclinic, semi-conducting) the maximum transmission in the visible region (300-700 nm) of the spectrum is set to 65%, whilst in the infra red (800-2500 nm) it is defined as 80%. Conversely the reflectance is set at 17% in the visible and 12% in the infra-red. In the hot state (rutile, metallic)

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the optical properties in the visible region remain the same, i.e. 65% transmission and 17% reflectance. In the infra-red they change; it is important that the change covers the range 800-1200 nm; this is the region where the most significant solar heat energy is to be found, so a larger change in this region is expected to have a more profound effect on the energy saving properties of this glass. Spectra with different changes were considered, between 65% and 0%; the example spectra for transmittance and reflectance are shown in Figures 7. 27 & 7.28 respectively. We have also examined the effect of changing the transition temperature between 20°C and 35°C. We have not looked at temperatures below 20°C as at this temperature the film is always in the hot state. Results of the simulations are shown in Figure 7.29. Several observations can be made from the results of the simulations. Firstly that all of the ideal products perform better than those standard products they are compared to here. Comparing the spectra of the ideal coatings and the standard products reveals that the ideal coatings have a combination of infrared reflective and absorbing behaviour, whereas the standard products have only one mode of operation. Thus the ideal coatings have a clear advantage. Where there is no change in the optical properties the performance of the coatings is identical. This is expected as the ΔΤ = 65% j

ΔΤ = 45% ΔΤ = 20%

?

ΔΤ = 0%

« 50

1

c

s

£

f

i

F V) e

|

I

500

I

I

,

ί

1000 1500 Wavelength (nm)

1 .

2000

.

I

2500

Figure 7.27 Trasmittance spectra for ideal thermochromic coatings, showing a cold-hot decrease of 65, 45, 20 or 0%.

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INTELLIGENT NANOMATERIALS AR = 65% 75

1

AR = 45% AR = 20%

:

J

AR = 0%

8- 50 o o c

tt

t

i

25 1

I

1000

500

1500

2000

2500

Wavelength (nm)

Figure 7.28 Reflectance spectra for a ideal thermochromic coatings, showing a cold-hot increase of 65,45,20 or 0%.

««- J J J J !

~

0

Jl [jilt

30 -

'.Lull Δ=(65

60

55

mil

50 45 40

20

10

0)

SB

AB

Figure 7.29 Energy consumption improvement for ideal thermochromic films with different changes in transmittance and reflectance. I: T. = 35CC,: IT = 30°C, : T = 25°C, : T = 20°C, ; Sputtered Silver Coated Glass, Blue Body Tinted Glass.

film is behaving in a static manner rather than a dynamic one. The larger the change between hot and cold transmission and hot and cold reflection the higher the percentage improvement versus the clear - clear configuration; this is expected. The higher the

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difference in infrared reflectivity the more solar energy is reflected, as such the energy benefit derived from it is proportionally larger. The thermochromic transition temperature, T, plays an important role in the energy performance of the thermochromic coatings. For the example with the largest change in optical properties (A(T,R) = 65%), a change from T = 35°C to T = 20°C leads to a change of 30% in the energy performance relative to the clear - clear configuration. In all the cases the lower T. the lower is the energy consumption and the higher the percentage improvement in energy performance. The lower T the longer the film spends in the hot, metallic state. In warmer climates, such as in Palermo, this significantly enhances the energy performance of the glazing. The best energy saving performance is calculated to derive from a film with a low transition temperature and a large change in the infrared optical properties above and below this temperature. 7.5.3.2

Comparison to Real Films

Comparing the ideal coating spectra and the spectra of the synthesised coatings, several things become clear. Firstly, the ideal spectra have higher values of maximum transmission in the visible region. This means that more light is allowed into the room with the ideal spectra, reducing the lighting cost; also the ideal films are also less absorbing in the near infra-red than the real films. Secondly, the real films do not have such an extreme change in the near infrared (800-1200 nm) as the ideal films; the consequence of this is that the variable heat mirror property inherent in the thermochromism contributes less to the overall energy performance of these films. This is somewhat offset by the fact that there is a resultantly higher absorption character to the film; this means that the real films are less dynamic than they potentially could be. The ideal coating that performs the best is the one which has the largest change in infra-red reflectivity (a 65% change) and the lowest value for T (20°C); this leads to a 50% improvement over the clear - clear configuration. The best performance comes from a sample with the lowest T. subsequently the film is always in the hot state, suggesting that the chromic nature of the films is irrelevant and that the origin of the energy saving effect is a combination of the heat mirror and absorbing properties of the coatings, certainly in warmer climates. Considering these results and comparing them with the ones of real thermochromic samples (in Palermo on a 100% glazing wall), the best performing one is thermochromic 3 which

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has an improvement of 40% compared to the clear - clear configuration. This is the one that replicates more the behaviour of the ideal coating; in fact it also compares well with both Blue Body Tinted Glass (30% improvement) and Sputtered Silver Coated Glass (22% improvement). The results of the simulations based on the ideal spectra (Figure 7.29) suggest that in order to improve the performance of the real thermochromic coatings in building glazing further two approaches ought to be taken. Firstly we must look to maximise the change in infra-red reflectivity with a particular emphasise on the near infra-red region (800-1200 nm). Secondly we must look to lower the transition temperature, Tc, through doping or introducing strain into the film so that the metallic properties of the rutile phase are fully utilised.

7.6 Outlook and Conclusions The use of thermochromic coatings in architectural glazing has been postulated for many years and a great deal of work has been done on film synthesis and characterisation. Little has been published on the energy saving benefit of such films. In recent work we have found that thermochromic films may provide an additional 10% energy benefit when compared with current existing approaches (relative to a clear - clear glazing system). This arises through a combination of absorbing and variable heat mirror behaviour. The emissivity of the films is high in both the cold and hot state suggesting that this technology is not suitable for cooler climates. This is also suggested by the results of the modelling studies. The thin films (produced with gold, thermochromic 2 & 4) are nanocomposites; the percolation limit is not reached and the films are insulators at room temperature. The nanoparticles are discreet and not connected and as such do not alter the conductivity of the films or significantly effect the reflectivity in the far infrared. They do however contribute to a higher near infrared reflectivity in the cold state, leading to more heat mirror type behaviour. The cost of gold may prove to be an issue in the production of these coatings; it is likely that any industrial process using gold will be expensive. It is impossible to produce a meaningful figure based on the experiments conducted in our laboratories; but it is likely (at this stage) that the cost outweighs the additional energy benefit that

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the gold provides. The main function of the gold in these films is to alter the colour from an aesthetically unappealing yellow/brown to a range of pleasant greens and blues. Current approaches to producing coloured glass use body-tinting processes. These can be problematic to produce as the float line may spend a large amount of time off line whilst the melt is bought under control. The hidden benefit that this methodology brings is that it may be incorporated directly onto a float line and started and stopped at will; as such less time would be lost through the float line being unproductive. It may even prove to be possible to change film colour online just by changing the nanoparticle concentration. Additionally we have presented some results on fluorine doping vanadium dioxide thin films, these show that it is possible to make the films more transmissive (less deeeply coloured) in the visible and to shift the position of the absorption peak towards the UV. The building energy use simulation results on the real films clearly show that, for warmer climates, a lower transition temperature leads to more energy saved, reduction of the thermochromic switching temperature to around room temperature will lead to further energy saving behaviour. The modelling studies conducted on the ideal spectra are informative; they show that absorbing behaviour is as important as heat mirror behaviour. In the ideal scenario the transition temperature will be as low as possible in order to maximise the number of hours the coating spends in the hot, infrared reflective state and the change in the infrared optical properties will be as large as possible to maximise the benefit of the hot, reflective state. Overall we hope that we have demonstrated that thermochromic thin film technology provides a useful and viable approach towards the production next generation energy efficient glazing.

Acknowledgments I would like to thank the Royal Society for a Dorothy Hodgkin research fellowship. I would also like to thank the EPSRC for funding (EP/H005803/1). Dr Troy Manning and Mr Bob Harris of Pilkington-NSG are thanked for the provision of glass substrates, commercial glass samples as well as guidance and advice. Mr Kevin Reeves is thanked for his invaluable assistance with electron microscopy. Mr Andreas Kafizas and Dr David Morgan are thanked

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for assistance with X-ray photoelectron spectroscopy. I also thank my co-workers, colleagues and students for their help and participation in this research.

References 1. CG. Granqvist, Thin Solid Films 193-194/Part 2 (1990) 730. 2. C.G. Granqvist, Applied Physics A: Materials Science & Processing 52/2 (1991) 83. 3. A.M. Omer, Renewable and Sustainable Energy Reviews 12/9 (2008) 2265. 4. C.G. Granqvist, Advanced Materials 15/21 (2003) 1789. 5. C.G. Granqvist, Appl. Opt. 20/15 (1981) 2606. 6. C.G. Granqvist, Solar Energy Materials and Solar Cells 91/17 (2007) 1529. 7. A. Andersson, O. Hunderi, C.G. Granqvist, Journal of Applied Physics 51/1 (1980) 754. 8. R.E. Collins, Τ.Μ. Simko, Solar Energy 62/3 (1998) 189. 9. A.C. Fischer-Cripps, R.E. Collins, G.M. Turner, E. Bezzel, Building and Environment 30/1 (1995) 41. 10. J.D. Garrison, R.E. Collins, Solar Energy 55/3 (1995) 151. 11. M. Lenzen, R.E. Collins, Solar Energy 61/1 (1997) 11. 12. Y. Fang, P.C. Eames, B. Norton, T.J. Hyde, J. Zhao, J. Wang, Y. Huang, Solar Energy 81/1 (2007) 8. 13. N. Ng, R.E. Collins, L. So, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films 21/5 (2003) 1776. 14. T. Minaai, M. Kumagai, A. Nara, S. Tanemura, Materials Science and Engineering B 119/3 (2005) 252. 15. N. Ng, R.E. Collins, L. So, Materials Science and Engineering B 119/3 (2005) 258. 16. G.W. Mbise, D.L. Bellac, G.A. Niklasson, C.G. Granqvist, Journal of Physics D: Applied Physics 30/15 (1997) 2103. 17. G.B. Smith, S. Dligatch, R. Sullivan, M.G. Hutchins, Solar Energy 62/3 (1998) 229. 18. A.M. Al-Shukri, Desalination 209/1-3 (2007) 290. 19. H. Köstlin, G. Frank, Thin Solid Films 89/3 (1982) 287. 20. S.M.A. Durrani, E.E. Khawaja, A.M. Al-Shukri, M.F. Al-Kuhaili, Energy and Buildings 36/9 (2004)891. 21. J.K. Fu, G. Atanassov, Y.S. Dai, F.H. Tan, Z.Q. Mo, Journal of Non-Crystalline Solids 218 (1997) 403. 22. H. Kawasaki, T. Ohshima, Y. Yagyu, Y Suda, S.I. Khartsev, A.M. Grishin, Journal of Physics: Conference Series 100/1 (2008) 012038. 23. X. Zhang, S. Yu, M. Ma, Solar Energy Materials and Solar Cells 44/3 (1996) 279. 24. R. Binions, S.S. Kanu, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences 466 (2010) 19. 25. T.D. Manning, I.P. Parkin, R.J.H. Clark, D. Sheel, M.E. Pemble, D. Vernadou, Journal of Materials Chemistry 12/10 (2002) 2936. 26. F.J. Morin, Physical Review Letters 3/1 (1959) 34. 27. I. Bouessay, A. Rougier, P. Poizot, J. Moscovici, A. Michalowicz, J.M. Tarascon, Electrochimica Ada 50/18 (2005) 3737.

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119. T.D. Manning, I.P. Parkin, M.E. Pemble, D. Sheel, D. Vernardou, Chemistry of Materials 16/4 (2004) 744. 120. U. Qureshi, T.D. Manning, I.P. Parkin, Journal of Materials Chemistry 14/7 (2004) 1190. 121. W.B. Cross, I.P. Parkin, Chemical Communications/14 (2003) 1696. 122. R. Binions, C.J. Carmalt, I.P. Parkin, Thin Solid Films 469^170 (2004) 416. 123. M.B, Sahana, G.N. Subbanna, S.A. Shivashankar, Journal of Applied Physics 92/11 (2002) 6495. 124. C. Piccirillp, R. Binions, I.P. Parkin, Chemical Vapor Deposition 13/4 (2007) 145. 125. C. Piccirillo, R. Binions, I.P. Parkin, Thin Solid Films 516/8 (2008) 1992. 126. C. Piccirillo, R. Binions, I.P. Parkin, European Journal of Inorganic Chemistry 2007/25 (2007) 4050. 127. R.G. Palgrave, I.P. Parkin, Journal of the American Chemical Society 128/5 (2006) 1587. 128. M. Saeli, R. Binions, C. Piccirillo, G. Hyett, I.P. Parkin, Polyhedron 28/11 (2009) 2233. 129. M. Saeli, R. Binions, C. Piccirillo, I.P. Parkin, Applied Surface Science 255/16 (2009)7291. 130. M. Saeli, C. Piccirillo, I.P. Parkin, I. Ridley, R. Binions, Solar Energy Materials and Solar Cells 94/2 (2010) 141. 131. S.S. Kanu, R. Binions, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Science 466/2113 (2010) 19. 132. R. Binions, C.J. Carmalt, I.P. Parkin, Measurement Science & Technology 18/1 (2007) 190. 133. C. Piccirillo, R. Binions, I. Parkin, Chemical Vapor Deposition 13/4 (2007) 145. 134. M. Saeli, R. Binions, C. Piccirillo, G. Hyett, I.P. Parkin, Polyhedron 28/11 (2009) 2233. 135. http://appsl.eere.energy.gov/buildings/energyplus/. 136. Saeli. M, Dipartimento di Progetto e Costruzione Edilizia (DPCE), University of Palermo, Palermo, 2008.

PART II ORGANIC MATERIALS

PART II ORGANIC MATERIALS

8

Polymeric Nano-, Micellar and Core-shell Materials Angel Contreras-Garcia1, Guillermina Burillo2, and Emilio Bucio2 department of Engineering Physics, Ecole Poly technique, Montreal, Canada 2 Departamento de Quimica de Radiaciones y radioquimica, Instituto de Ciencias Nucleares, Universidad Nacional Autonoma de Mexico, Ciudad Universitaria, Mexico

Abstract

Recently, the intelligent nano-polymer modified nano-particles have showed much potential in advanced materials. This chapter reviews the synthesis, characterization and properties of smart core-shell nano-composites of intelligent polymers and nano-particles. The stimuli-responsive polymeric and hybrid materials have been synthesized as nano-structured particles in a range of sizes, from nanometers to a few micrometers in the form of homo- and copolymer nano-gels, core-shell structures, micelles, dendrimers, grafted surface nano-particles, nano-composites, thinfilms,and more complex architectures. Furthermore, the nano-materials can be synthesized by different techniques, which include physical or chemical procedures, such as precipitation and polymeric crosslinking, respectively. Ionizing radiation techniques also have been used to obtain grafted nano-materials. The chapter further explores these nano-material synthesis techniques.

8.1

Introduction

The particles in the range of sizes from nanometers to few micrometers which show stimuli-responsive properties were reviewed in this chapter, as well as, the synthesis and characterization techniques. The properties of the nanoparticles undergo change in their aggregation state, dimensions, structure, and interactions as a Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (319-346) © Scrivener Publishing LLC

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response of external stimuli. One of the most important applications of the particles in the nano size range are in the biomedical field. In general, the particle structure can resemble a living cell in its complexity where many particle's compartments interact, exchange chemicals, receive energy, perform mechanical work, alter the chemical and physical properties, all of these in response to environmental stimuli [1]. The term responsive particle is referred as a particle-polymer hybrid structure or, in other words, a core-shell structure with a core and a polymeric shell. The core could be formed of any material: metal, metal oxide, nonmetal oxide, salt, polymer, liquid, or gas (hollow structures). Responsive properties of the particle may originate from the polymeric shell, polymer core, and both the core and the shell. Several protein and other drugs have been designed to target various cellular processes, creating a demand for the development of intelligent drug delivery systems (DDS) that can sense and respond directly to pathophysiogical conditions. Micro- and nano-scale intelligent systems can maximize the efficacy of therapeutic treatments in numerous ways because they have the ability to rapidly detect and respond to disease states directly at the site, sparing physiologically healthy cells and tissues and thereby improving a patient's quality of life. This new class of "intelligent therapeutics" refers to intelligent and responsive delivery systems that are designed to perform various functions like detection, isolation a n d / o r release of therapeutic agent for the treatment of diseased conditions [2]. To meet these requirements, researchers must be able to interface synthetic and hybrid materials with dynamic biological systems on the micro- and nano-length scale. It has been shown that the nature and molecular weight (MW) of the interacting polymers as well as various environmental parameters (the nature of solvent, pH, ionic strength of solution, temperature, and polymer concentration) have significant influence on the complex formation. Numerous studies have been also devoted to the interaction of poly methacrylic acids (PMA) with metal ions [3-6], since knowledge of association phenomena of metal ions with charged macromolecules is of importance for the understanding of their physicochemical behavior in environmental and biological systems. PMA could be successfully used as a component of characteristic 'intelligent' organic-inorganic hybrid materials [7].

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8.2

321

Stimuli-responses

Controlling the wettability of a surface is of considerable importance at biological interfaces, for industrial processes, and in agricultural applications [8-10]. For these applications, stimulusresponsive polymers offer unique properties as they often undergo large changes in surface energy when switching their conformation upon application of an external stimulus. Stimulus-responsive polymers, particularly in combination with micro- or nanostructured surfaces, even allow switching between extremes of hydrophilicity and hydrophobicity. Responsive behavior of polymeric or hybrid nanoparticles could be formally considered as a combination or sequence of several events: reception of an external signal (physical or chemical), chemical reaction of the material or changes of the material's properties, and transduction of the changes into a macro/microscopically significant event such as aggregation-deaggregation of nanoparticles, inversion of o / w (w/o) emulsion, or release of particles' cargo. These events are commonly referred as to a response. For example, microgel particles from a crosslinked weak polyelectrolyte (polybase) dispersed in water are sensitive to changes in pH of the aqueous suspension. Changes (decrease) in pH cause changes in the ionization degree of the polyelectrolyte. The osmotic pressure due to the increased ionization will result in swelling of the particles. The resulting response can be read out as changes of the solution turbidity [11-15]. The reception of an external signal is related to the sensitivity of the polymeric component to various changes in the nanoparticles environment. Different polymers were developed and used for the responsive nanoparticles that can change their properties upon external stimuli such as light, temperature, chemicals, and biomolecules (signaling molecules) [16-23]. The reception mechanism is typically based on one of two scenarios: (1) responsive polymers have functional groups sensitive to one or more of the above listed stimuli or (2) responsive polymers form structures that can be disintegrated (or created) by the external stimuli. Light-responsive polymers are supplied with photoactive groups such as azobenzene, spirobenzopyran, triphenylmethane, or cinnamonyl that can undergo reversible structural changes under UV-vis light. These functional fragments change size and shape, or form ionic or zwitterionic species upon irradiation [24-26].

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In temperature-responsive polymers, the balance between segment-segment interactions and segment-solvent intermolecular interactions can be shifted by temperature changes. Most known examples of such polymer-solvent pairs are poly N-isopropylacrylamide (PNIPAAm), poly ethylene oxide (PEO), poly propylene oxide (PPO), poly lactic acid (PLA) (homo- and copolymers), proteins, and polysaccharides in aqueous solutions [27-31]. Polypeptides and polysaccharides typically undergo the coil-helix transition upon decreasing temperature below the UCST. The polymer-solvent interactions in the case of PNIPAAm decreases upon increasing temperature above the lower critical solution temperature (LCST) (close to 32°C for PNIPAAm) due to the dominating effect of hydrophobic interactions at an elevated temperature [32-36]. Examples of pH-responsive polymers are weak polyelectrolytes with acidic or basic functional groups (carboxylic, phosphoric, or amino functional groups). Changes in pH result in a shift of chemical equilibrium and in a change of the ionization degree of the polymer chains [37-40]. Reception of chemical and biochemical signals can be based on a physical interaction or chemical reaction between functional groups in the polymer and signaling molecules. There are many examples of specific complex formation between synthetic polymer materials and ligands, e.g., glucose-responsive polymers with phenylboronic side groups [41]. The selective interaction of the responsive polymers with signaling molecules relies on selective molecular recognition phenomena using the conjugation of responsive polymers with biological molecules such as DNA [42], enzymes [43,44], antibodies [45], and other proteins [46-49]. The interactions that involve DNA and proteins result in formation or cleavage of junction points between the functional polymers and signaling molecules. Applications of enzymes refer to two possible situations: (1) polymers with immobilized enzymes [50] and (2) materials with fragments that are substrates for enzymes [51]. In the first case, a substrate diffusing to the polymer from the surrounding aqueous medium can be biocatalytically converted into products which interact with the responsive polymer and cause chemical changes in the polymer. For example, catalytic oxidation of glucose by glucose oxidase yields gluconic acid and, consequently, results in a decrease of pH in the local environment. These changes are then used to trigger response of the nanoparticles [52]. In the second case, the enzyme is used as an "external stimulus" that cleaves the chemical bonds in the material.

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Metal and metal oxide nanoparticles (sometimes embedded in larger structures, such as liposomes or microparticles) offer the likely best-known case of 'remotely' responsive materials: external (magnetic) field can drive them to specific macro- or microtargets [53, 54], such as infection sites [55] or tumoral masses, or even sub-micron targets, such as ion channels on cell membranes [56]. Magnetic fields can also be used for producing reversible agglomeration [57] or alignment [58] for separation or processing purposes, which can be effectively utilized in cascade recognition events [59].

8.3 Intelligent Micro- and Nano-materials Synthesis There are several techniques to synthesize t|ie nano-materials, which include physical and chemical methods (Figure 8.1). 8.3.1

Coacervation/precipitation

The coacervation method has been widely employed for the preparation of microcapsules. The term coacervation is commonly used to describe the process according to which colloidal polymer aggregates formed upon the separation of an homogeneous aqueous polymer solution are deposited onto the surface of dispersed liquid droplets, thus, resulting in the production of reservoir-type microcapsules. The separation of the aqueous polymer solution can be induced by the addition of a strongly hydrophilic substance, thus, leading to the formation of two phases, a rich and a poor in colloidal polymer aggregates [60-62]. Though simple coacervation generally involves the use of gelatin [63-65], various other polymers have been successfully employed for the production of microcapsules [66-68]. In practice, any hydrophilic polymer can be utilized as a wall-forming material, provided that it coacervates upon a change of a phase separation inducing variable including pH, temperature, solvent concentration, and electrolyte concentration. 8.3.2

Particles b y Chemical C r o s s l i n k i n g

Aside from the above described coacervation and precipitation methods controlled chemical crosslinking of certain polymers may

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Polymer adsorption on nanoparticles

Graft copolymerwilh water-soluble backbone

Hydrophllic nanDparliclos

Microgel coated by the LbL assembly

PtMlllve charosd mlcrog&l

Polymeric particte

Pemy kiss

formation

) Monomer ΐ

ary s

Figure 8.1 Schematic representation of different types of synthesis to obtain nano-materials.

lead to the formation of micro or nanoparticles. For example, colloidally stable aqueous dispersions of pH-sensitive nanoparticles were prepared from chitosan via condensation reaction of amino groups of chitosan with di- and tricarboxylic acids. Depending on

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the acid used and the crosslinking degree, the particles with the polycation, polyanion, and polyampholyte behavior were produced by Borbely and coworkers [69, 70]. The chitosan nanoparticles crosslinked with poly(ethylene glycol) dicarboxylic acid demonstrated pronounced swelling provided by the length and flexibility of the crosslinker [71]. Zhang and coworkers [72] described the fabrication of temperature-responsive hydrogel NPs from poly(GMA-co-NIPAAm) copolymer. The particles were formed in organic solvent during the crosslinking reaction of epoxy groups of GMA with diamine and then transferred into an aqueous medium. Gold nanoparticles were successfully synthesized inside the hydrogel particles in this study. The authors reported the temperatureinduced swelling-deswelling and aggregation-disaggregation of the resulting nanocomposite colloids. 8.3.3

Heterogeneous Polymerization

Polymerization in heterogeneous media is one of the many strategies for fabrication of colloidal nanoparticles. The technique has been extensively developed and has numerous applications for the synthesis of monodisperse core-shell particles and for control of the particle surface properties. Emulsion, precipitation, and dispersion polymerizations are among the mostly used synthetic approaches for preparing responsive nanogels [73, 74]. Nucleation and growth are two main steps of these polymerization techniques. The principles and possible applications of these techniques were recently reviewed by Matyjaszewski and coworkers [75]. The techniques also allow the synthesis of hybrid particles by inclusion of inorganic materials such as silica [76], alumina [77], zeolite [78], iron oxide [79, 80], and noble metal nanoparticles [81-84]. As recently reviewed [85], various types of temperature, pH, ionic strength, and UV responsive particles can be designed and produced by heterogeneous polymerization processes. Here, we review the most recent representative examples on the synthesis of core-shell particles by heterogeneous polymerizations. An example of other kind synthesis is the fabrications of nanofibres made by Wang et al. They prepared, nano-polyethylene fibers under atmospheric pressure via in situ ethylene extrusion polymerization, with MCM-41 and SBA-15 a family of mesoporous zeolites [86, 87]; supported zirconocene dichloride (Cp2ZrC12) catalytic systems, respectively. The effects of the geometrical structures and

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surface properties of MCM-41 and SBA-15 on the morphology of the resultant polyethylene, catalytic activity and polymerization rate were investigated and compared in various polymerization conditions. The diameters of the resultant fibers prepared with SBA-15 supported catalysts are larger than those prepared with MCM-41 supported catalysts. At the polymerization temperature of 70°C, the fiber morphology of the polyethylene prepared with SBA-15 supported catalytic system disappeared. Furthermore, the resultant polyethylene had higher melting points compared to the homogeneous counterpart, particularly the samples prepared with MCM-41 supported catalysts [88]. Arias et al. [89] describe a reproducible method to prepare polymeric colloidal nanospheres of poly(ethyl-2-cyanoacrylate), poly(butylcyanoacrylate), poly(hexylcyanoacrylate) and poly(octylcyanoacrylate) with a magnetite core. The method is based on the emulsion polymerization procedure, often used in the synthesis of poly(alkylcyanoacrylate) nanospheres for drug delivery. The heterogeneous structure of the particles confer them both magnetic-field responsiveness and potential applicability as drug carriers. The hysteresis cycles of both magnetite and composite particles demonstrate that the polymer shell reduces the magnetic responsiveness of the particles, but keeps their soft ferrimagnetic character unchanged. A detailed investigation of the capabilities of the core/shell particles to load this drug is shown. Lyon and Jones report the synthesis and characterization of temperature and pH responsive hydrogel particles (microgels) with core-shell morphologies. Core particles composed of crosslinked poly(N-isopropylacrylamide) (PNIPAAm) or poly(NIPAAmco-acrylic acid) P(NIPAAm-AAc) were synthesized via precipitation polymerization and then used as nuclei for subsequent polymerization of P(NIPAAm-AAc) and PNIPAAm, respectively [90]. The poly(NIPAAm-co-AA) core particles displayed a strong dependence of particle size on both temperature and pH. Compared to the homogeneous particles, both poly(NIPAAm-co-AA) (core)/PNIPAAm (shell) and PNIPAAm (core)/poly(NIPAAm-co-AA) (shell) particles demonstrated a more complex pH-responsive behavior. It was observed that multistep volume phase transition occurred when the AAc component became highly charged at a high pH that was attributed to localization of PNIPAAm in the particle shell. Berndt et al. [91] present a core-shell microgel consisting of poly-N-isopropylacrylamide (PNIPAAm) in the shell and

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poly(N-isopropylmethacrylamide) (PNIPMAm) in the core. Both polymers display a lower critical solution temperature (LCST) in aqueous solution; in D 2 0 the transition temperatures are 34°C for PNIPAAm and 44°C PNIPMAm, whereas in H 2 0 the transition temperatures are slightly lower. As a consequence, the particle size of the core-shell microgel decreases upon heating in two steps. These doubly temperature-sensitive microgel particles are expected to have a denser shell when the temperature is in between the two LCSTs such that the shell is collapsed but the core is swollen applied surfactant-aided radical precipitation polymerization to synthesize core-shell microgel particles consisting of PNIPAAm in the shell and PNIPMAm in the core. The difference in the LCST for the polymers was of about 10°C leading to doubly temperature-sensitive behavior [91]. The stimuli responsive character of microgels has been subject to extensive studies [92-94] and the investigations have been extended to microgels with more sophisticated structure providing additional functionality [95-99]. Interestingly, when the temperature was in the range between the two LCSTs, these microgel particles had the denser collapsed shell as compared with the swollen core. A dispersion polymerization approach was applied for the synthesis of hybrid poly(NIPAAmco-HEMA-co-MAA) microgels containing magnetic Fe 3 0 4 nanoparticles [100]. According to the reported procedure, first microgels particles were synthesized, and then the magnetic nanoparticles were prepared inside the microgels. The particles were found to be suitable as emulsifiers for o / w emulsions with both polar and unpolar oils because of the particle surface activity. Both thermo and magnetic responsive particles having a magnetic core with crosslinked PNIPAAm shell were synthesized by a co-precipitation method using atom transfer radical polymerization [101]. A special type of responsive nanoparticles is the termed Janus particles; the term Janus is used for the description of particles whose surfaces of both hemispheres are different from a chemical point of view. So, they could be used as building blocks for supraparticular assemblies, as dual-functionalized devices, as particular surfactants if one hemisphere is hydrophilic and the other hydrophobic, they are one of the fruits of the remarkable progress which has been achieved in both polymer and inorganic chemistry yielding welldefined particles from the viewpoint of their size, their shape and their surface chemistry. In particular, Janus particles are now available in the size range of a few nanometers, i.e., five thousand times

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smaller than the pioneering Janus glass beads. Anyway, the dissymmetry checking of these nanoparticles is not a simple task and, even if TEM and high-resolution TEM are now common imaging techniques, they are not usable for evidencing Janus particles whose difference between both hemispheres is at the molecular level. In those last cases, an interesting but largely unanswered problem concerns the surface mobility of ligands, which could lead to ligand exchanges and tend to their isotropic arrangements. If the reported works did not mention the instability of the Januslike character, no systematic study was focused on such aspects [102-104]. Synthesis of the nanogels by ATRP led to increased colloidal stability, higher swelling ratios, and enhanced control of degradability on treatment with tributyl phosphine or glutathione, as compared to analogs prepared by conventional free radical polymerization in inverse miniemulsion. Li and Armes used the disulfide-functional dimethacrylate as a branching agent during the polymerization of N-(2- hydroxypropyDmethacrylamide (HPMA) via ATRP [105]. Disulfide bonds within the branched copolymers were efficiently cleaved either upon exposure to dithiols or by treatment with benzoyl peroxide [106] 8.3.4

Polymer Adsorption on Nanoparticles

Adsorbed to interfaces, polymers can exhibit very different dynamic properties as compared to volume systems. A few investigations of the interfacial phase transition of NIPAAm have been published. Light scattering was employed, where the total thickness of the adsorption layer was determined in dependence of temperature. Grafted interfacial NIPAAm has been studied on silica particles [107] and on latex particles [108, 109] the latter system with detailed investigations of the kinetics of the coil-to-globule transition [110, 111]. In grafted layers, the coil-to-globule transition region was divided into two types of transition, a broad one occurring below 31 °C at better than θ-solvency conditions, and a narrow one at higher temperature. The transition of physisorbed NIPAAm on latex particles was similarly broadened as in grafted layers, as detected by changes of the layer thickness. In a heating-cooling cycle a hysteresis was observed and attributed to an "extended brushlike conformation", which was formed after cooling. This conformation was kinetically unstable and relaxed to loopily adsorbed chains which were kinetically stable [112, 113]. The LCST transition was

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furthermore not reversible with respect to the adsorbed amount, since if excess polymer was present, the adsorbed amount changed in the transition [114]. At very low surface coverage on silica, no transition was detected [115]. NIPAAm copolymers have, according to our knowledge, not yet been studied at interfaces. 8.3.5

Layer-by-layer Polymeric Shell

Polymer-based materials, such as polymer particles, polymer based micelles, polymer-drug conjugates, polymer capsules, and polymersomes are alternative materials that have potential for encapsulating and delivering poorly water-soluble compounds. [116, 117] Among the various polymer-based drug delivery vehicles, layer-by-layer (LbL)-assembled polymer capsules are particularly attractive, [118,119] as they can accommodate high payloads, [120-124] and are compatible with targeting [125, 126] and controlled release of the therapeutics [127, 128]. To date, three main approaches have been proposed for incorporating water-insoluble compounds within LbL capsules. The first method entails encapsulation of the therapeutic through the assembly of polyelectrolyte multilayers on crystal particles (in instances where the therapeutic forms crystals) [129, 130]. The second technique is to load waterinsoluble drugs to the capsule by introducing lipophilic phases inside the capsules, such as back filling of the capsules with an oil-loaded drug [131]. The third approach is to embed liposomes within polymer capsule multilayers, resulting in hybrid carrier capsosomes [132]. However, these methods have limitations associated with sample polydispersity, [130] the necessity for an oil phase, [131] and capsule size (>1 μκι) [130,132]. Thus, developing robust techniques to prepare colloidally stable polymer capsules of submicrometer-size with high payloads of water-insoluble drugs still represents a challenge. This highlights the need for the development of a facile, generic nanocarrier system that combines the advantages of both mesoporous silica materials and polymer capsules to improve the flexibility in their design and application. 8.3.6

Precipitation on Templates

One possible way to assemble various compounds on colloidal particles is the layer-by-layer (LbL) adsorption of oppositely charged macromolecules [133,134] This method of multilayer film assembly

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provides a defined shell composition on a colloidal core. The shell thickness is a function of the number of assembled layers, and it can be tuned in the nanometer range. Up to now a variety of materials, such as synthetic polyelectrolytes, proteins, DNA, inorganic particles, and lipids, have been utilized as building blocks for shells on colloids. The LbL assembly method can be applied to coat various charged particles, such as organic and inorganic colloids, biological cells, or nanocrystals of pharmaceuticals. The size of particles explored for this method can be varied from 0.1 to 10 μπ\ [135-137]. Noncharged macromolecules can also adsorb at the interface under certain conditions. This may happen, for example, due to hydrophilic-hydrophobic interactions between the macromolecules and the particles in the solution. By exploiting this, many different substances may be precipitated on the colloids, and the functional properties of the shells can be significantly extended. The coating of colloidal particles has already been achieved by the precipitation of inorganic salts on the surface of polystyrene latex particles [138, 139]. The controlling of precipitation on colloids implies the search for an adequate window in the possible range of polymer concentration, particles, and coagulation speed for each polymer one wishes to make the shells. 8.3.7

Grafting onto the Surface of Particles

Grafting polymer onto the surface of inorganic particles is a field of great interests. Most of the works were carried out for micron and submicron particles in solutions via various chemical processes, including radical, [140] anionic, [141] and cationic [142] polymerizations. Also, a few reports have been devoted to the grafting of polymers onto the particulate surfaces in emulsion rather than in solution [143-145]. It was suggested that emulsion polymerization offers strong potential advantages for modifying the surface of nanosized particles. This is because the grafting polymers tend to cover the particles as long as suitable reaction conditions are available, for example, in the presence of functionalized particles or using small amounts of surfactant [146] From both theoretical and engineering points of view, however, the grafting reaction mechanism involved has not been elucidated with satisfaction so far, and most of the grafting polymers attached to the particles do not possess any reactive group that can be used to chemically link the particles to the matrix. A grafted polymer shell may consist either

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of chains of a single polymer (homopolymer or random copolymer), of two or more different homopolymers (mixed brushes), or of block-copolymers. In the first case, the responsive properties of the polymer brush are introduced by stimulitriggered changes in the chemical structure of the polymer, or by changes in the balance between polymer-polymer and polymer-solvent interactions. In the latter two cases, the mechanism of response is more complex because the changes in the shell are also due to microphase separation in the mixed [147,148] or block-copolymer brush [149]. 8.3.8

Self-assembly of Micelles

Directed self-assembly of nanoparticles into specific structures can provide controlled fabrication of nanometer-sized building blocks with unique and useful electronic, optical, and magnetic properties [150-153] In general, ligand- or polymer-stabilized nanoparticles can self-assemble into two-dimensional arrays [154-155]. On structured templates, two- or one-dimensional arrays of nanoparticles can be directed with particular arrangements [156, 157]. For example, hexagonal arrays of nanoparticles on a monolayer of diblock copolymer micelles [158] and one-dimensional chains of nanoparticles on ridge-and-valley structured carbon [159] has been demonstrated. Short-range self-assembled alloys of nanoparticles in two-dimensional arrays were also demonstrated [160]. In most cases of directed self-assembly, however, a single kind of nanoparticles was employed. The self-assembly of ionic surfactants into wormlike micelles has been widely investigated [161-163]. Wormlike micelles are typically formed by adding salt to solutions of a cationic surfactant such as cetyltrimethylammonium bromide (C16TAB). The salt facilitates micellar growth by screening the electrostatic repulsions between the charged surfactant headgroups. Much like polymers, wormlike micelles tend to be long and flexible chains (contour lengths of ca. 1 μπ\) that become entangled into a transient network, there by imparting viscoelasticity to the solution. The micelles can thus be exploited for thickening and rheology-control applications in aqueous systems. Molecular self-assembly is mediated by weak, noncovalent bonds—notably hydrogen bonds, ionic bonds (electrostatic interactions), hydrophobic interactions, van der Waals interactions, and water-mediated hydrogen bonds. Although these bonds are

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relatively insignificant in isolation, when combined together as a whole, they govern the structural conformation of all biological macromolecules and influence their interaction with other molecules. The water-mediated hydrogen bond is especially important for living systems, as all biological materials interact with water. Phospholipids readily undergo self-assembly in aqueous solution to form distinct structures that include micelles, vesicles and tubules. This is largely a result of the hydrophobic forces that drive the non-polar region of each molecule away from water and toward one another. The dimensions and shape of the supramolecular lipid structures depend upon various factors, such as the geometry and curvature of the polar head and the shape and length of the nonpolar tails [164]. In this regard, Zhang has reported the synthesis of different types of self-assembly materials; his research produced novel supramolecular architectures as nanofibers, bionanotubes, nanometer-thick coatings on surfaces, nanofiber peptide and protein scaffolds, nanowires using bioscaffolds, bio-optical structures and optical waveguides [165]. 8.3.9

Radiation-grafting of Nano Polymers

Radiation processing has many advantages over other conventional methods. When using radiation for nano-material processing, no catalysts or additives are needed in order to initiate the reaction. Grafting techniques for nano-polymers normally include the preirradiation, as well as the mutual or the simultaneous method; the energy source being gamma ray, UV or electrons. The main advantage of these radiation-assisted methods is that they are relatively simple; surface grafting of polymers has attracted great interest in the past few decades, as it allows the tailoring of surface properties of various polymer materials, and thus specific functionalities useful for many applications can be effectively produced. When polymers are exposed to ionizing irradiation, trapped radicals and peroxides or hydroperoxides are formed as a result; these are capable of initiating grafting copolymerization reactions. Various graft nano-polymerization approaches have been developed, including direct graft polymerization of vinyl monomers already containing desirable functional groups, graft copolymerization of two (or more) different types of monomers onto a polymer backbone, and graft polymerization of a precursor-monomer that can be

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subsequently modified [166]. On the other hand, radiation grafting. is a promising method for the modification of the materials, and is of particular interest for achieving specifically desired chemical properties as well as excellent mechanical properties. The radiation grafting has been used for the preparation of smart materials, such as thermo and pH-responsive nano materials [167-172]. 8.3.9.1

Direct Method

In the direct method, a nano-polymer is irradiated in contact with a monomer, which may be a gas, vapor, liquid or in solution. Irradiation produces active sites in the nano polymer matrix, mainly macroradicals, which can initiate the graft polymerization and homopolymerization. If the polymer has a tendency to crosslinking a grafted polymer is formed, but when a polymer has a tendency to chain scission, the process can result in a block copolymer formation, but because degradation of polymers requires higher absorbed doses than the grafting process, it is possible to perform grafting on these no polymers. This method presents a homopolymerization problem because the irradiation also generates radicals on the monomers that initiate homopolymerization. Radiation grafting predominates if the yield of radicals from the monomer is considerably less than the yield from the polymer matrix [173,174]. 8.3.9.2

Pre-irradiation Method

The nano polymer matrix is irradiated in the absence of air (in vacuo or under an inert atmosphere). Grafting is initiated by macroradicals trapped in the irradiated nano polymer and homopolymerization does not occur. The disadvantages of this method are the possible degradation of the nano polymer matrix, because the dose is higher than in the direct method, there a significant dependence on the reaction temperature and on the crystallinity of the nano polymer because the concentration of trapped macroradicals is higher in a crystalline than in an amorphous polymer, and a comparatively low degree of grafting is obtained [173,174]. 8.3.9.3

Pre-irradation

OxidativeMethod

This method involves pre-irradiation of the nano-polymer, but in the presence of air or oxygen, so that the macroradicals formed are converted to peroxides a n d / o r peroxides, and when the irradiated

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polymer is heated in the presence of monomer (in the absence of air) the peroxides decompose to give the macroradicals, that are the active sites for graft polymerization [173,174]. 8.3.9.4 Radiation Binary Graft Nano Polymers by One and Two Step Method Radiation-induced binary grafting has been developed for preparing newly, highly functional and improved properties from existing materials. Gamma-ray radiation is a well-known method for polymer and nano-polymer materials modification. The radiation induced binary graft can be obtained in one or two steps applying the pre-irradiation or the direct method; the way to achieve the grafted materials depends of the chemical structure of polymer that serve as a substrate and/or the monomers that are going to be grafted [175-179]. When the grafting is carried out in one step with two monomers, random copolymer branches are formed. By contrast, the two steps method leads to branches containing only one kind of monomers. Most surface grafting polymerizations are carried out through the one-step or simultaneous method, in which the grafting proceeds in the presence of monomer under ionizing irradiation [180-183]. The two-step or combinatorial technique first introduces dormant groups on substrates under ionizing radiation and, then, the grafting polymerization is initiated by heating or irradiation [184-187]. Pre-irradiation method is classified into trapped radical method and peroxide method. The nano-polymer is irradiated first in presence of air, which leads to either hydroperoxides or dialkylperoxides. These are stable and can be decomposed at high temperatures. In a second step the polymer put in contact with the monomer to initiate the grafting reaction. In the direct method, the active sites formed during irradiation contact with a reactive monomer simultaneously, initiate the polymerization of the monomer and form graft chains to the polymer substrate.

8.4 Characterization of Nano Sensitive Polymers The main methods of characterization of the pH and thermal sensitivity of the polymeric films are described. 8.4.1

Swelling Measurements

The equilibrium of swollen nano-materials in distilled water is determined by swelling the dried grafted polymeric samples for

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different periods of time and weighing them on an analytical balance until a maximum and constant weight is obtained [188-193]. The degree of swelling of the films (%) is calculated according to the formula:

S(%) =

(s(-s;

xlOO

(8.1)

Where S, and S; are the weights of swollen and dried films, respectively. 8.4.2

Thermo Sensitive Nano Polymers [194-197]

Miscibility in high molecular weight polymer blends is governed mostly by intermolecular interactions because contribution of the combinatorial entropy to mixing is negligibly small. Miscibility for most miscible polymer blends is caused by specific (i.e. attractive) interactions, which leads to lower critical solution temperature (LCST) behavior, LCST, defined as the critical temperature at which a polymer solution undergoes phase transition from a soluble to an insoluble state above the critical temperature [18]. PNIPAAm is the most popular temperature-responsive polymer since it exhibits a sharp phase transition in water at around 32°C [198]. On the other hand, upper critical solution temperature (UCST) type miscibility occurs only in the dispersion force-dominant systems in which exchange interactions on mixing are very small. In spite of such a limited condition, UCST-type miscibility has been observed for some blends of random copolymers as well as of homopolymers similar to each other in chemical structures [199-201]. The LCST of different grafted films are determined by swelling them in water for the equilibrium time at different temperatures and plotting the swelling percentage as a function of temperature. The inflexion point gives the LCST. 8.4.3

pH Critical Point

The pH critical point was determined in the same way from a plot of swelling percentage in different pH solutions with a buffer of citric acid and sodium phosphate. In pH-sensitive nano materials, the

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responsive mechanism lies within the side chain groups, branches, and crosslinks of a polymer's chemical structure. In polymer networks that contain weak acid or base groups, absorption and adsorption of water can occur simultaneously. The movement of water into the polymer network results in ionization of the acid and base pendant groups. This phenomenon is controlled by the solution's pH, ionic composition and ionic strength [202-204]. Acrylic acid (AAc) is one of the most popular monomers that have been grafted onto different polymeric matrices, and its polymer or copolymers with pH sensitive response have a capability to undergo further chemical reaction to produce new functional groups [166,205]. 8.4.4

Surface Plasmon Resonance Spectroscopy (SPR)

Is a surface-sensitive characterization based on the evanescent field of the surface plasmon. The latter is an electromagnetic wave traveling along the interface between a metal and a dielectric. Its electric field decays exponentially into both materials over a distance of a few hundred nanometers and the wave has a finite propagation length due to damping processes in the metal. The resulting data are a direct measure of the local average refractive index of the dielectric close to the surface, and with Fresnel calculations either the thickness or the refractive index of thin films at an interface can be determined. Furthermore, a time-dependent measurement mode enables the detection of changes in the local average dielectric constant due to the adsorption of molecules onto the surface or changes in film properties due to an external trigger [206]. Surface plasmons are the quanta of charge-density waves of free electrons in a metal propagating along the interface of a metal and a dielectric medium such as buffer or air. The electromagnetic field of these surface waves reaches its highest intensity at the metal surface and decreases exponentially into the adjacent phase. Therefore, it is influenced by the optical properties of this phase. The strong dependence of the surface plasmons on the refractive index of the dielectric medium can be used for sensor development purposes [207-212]. 8.4.5

FTIR Spectroscopic Method for the Determination of the LCST

This is a new and simple method for determining the LCST in both linear and crosslinked polymers by FTIR spectroscopy [213-214].

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This technique has been shown to be useful for probing transitions such as a lipid bilayer transition, for the self-association of aqueous surfactants, and for studying changes in hydrogen bonding in polymers. In addition, changes in the infrared spectra provide information on the conformation and bonding state of the functional groups involved in the transition. In this method, the FTIR-ATR spectra of PNIPAAm change dramatically in the vicinity of the coil-globule transition temperature. Although the polymer will aggregate and precipitate at the bottom of ATR crystal above LCST, this exactly indicate the occurrence of phase separation, actually the polymer will also aggregate to some extent above LCST using the IR transmittance measurements. In addition, the generalized 2D IR correlation spectroscopy, proposed by Noda [215,216], could be effectively applied to the examination of IR spectra of polymer solutions [217]. 8.4.6

Thermal Transition of Responsive Materials

Heskins and Gillet were the first to report that an endotherm can be observed at the LCST upon heating an aqueous solution of PNIPAAm [218]. Grinberg [219] studied the volume phase transition in responsive polymers using high-sensitivity differential scanning calorimetry (DSC) and the swelling behavior of the polymers at different scanning rates was considered. It was possible to measure the dependence of the transition parameters on the heating rate. The DSC measurements, by heating at different rates, provide results closely approximating equilibrium. The transition temperature, enthalpy, and entropy of this thermosensitive behavior as well as the transition LCST can be determined. The LCST of the linear polymers (0.5%, w / v ) was determined using an aqueous sample (30-40 mg by weight). The samples were run in a sealed aluminium crucible under a nitrogen purge at 2°C/min unless otherwise stated. Samples were analysed in the range from 0 to 50°C. The transition temperature has been defined in previous studies as either the temperature of onset or the peak temperature. In the current work the phase transition temperature was defined as the maximum of the endothermic transition peak [220]. 8.4.7

Contact Angle

This method consists of the measurement of advancing and receding contact angles by a dynamic method [221], or by the sessile

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drop method using the optical contact angle [222, 223]. The measurements of the contact angles for various samples are carried out at room temperature, 1 minute after water drops are deposited onto the surface of polymeric films, using direct microscopic measurement of the contact angles. The contact angle is an important parameter in surface science. It is a common measure of the hydrophobicity of a solid surface. In the literature, it is well established that meaningful contact angle measurements can be used in the calculation of solid surface tensions. In the past several decades, numerous techniques have been used to measure contact angle which were inspired by the idea of using the equation first derived by Thomas Young in 1805 [224-226]. 8.4.8

Microscopy

Surface morphology investigation and force-curve measurement with atomic force microscope (AFM) was used for determination of stiffness; microscopic infrared imaging is commonly used to found the distribution of specific compunds on the surface. Especially, the use of a confocal laser microscope enables surface morphology measurement of extremely soft surface on the swelled copolymers, which is essential to investigate pH response in buffer solutions. Nano-porous polymeric systems fabricated using fluorescently end-labeled samples of PAAc permitted real-time imaging of changes in internal structure using confocal microscopy (LSCM). The investigation and relationship between the surface morphology and the composition and other condition of the copolymers, and in-situ change of surface morphology induced by the external stimulation such as pH and/or temperature change can be evaluated by the scanning electronic microscopy (SEM), with this technique is possible to visualize the morphology surface by means of a micrograph; furthermore the composition along the surface and at different thickness of the micrometer order of magnitude can be recorded by Raman spectroscopy. Force curve measurement by AFM will be performed using a special home-made tip with a colloidal nano-sphere attached to the cantilever, which is essential to measure an extremely soft surface. These techniques help to investigate the relationship between the surface stiffness and the composition and other condition of the copolymers, and change of surface stiffness induced by the external stimulation such as pH and/or temperature change. Using microscopic infrared imaging, could be

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investigate the change of spatial distribution of specific chemical species induced by the external stimulation. Combination of these results reveal the actual surface behavior as the response to the applied external stimuli (pH a n d / o r temperature change), and it will enable us to optimize the design of the materials [227, 228].

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9 Conjugates of Nanomaterials with Phthalocyanines Edith Antunes, Christian Litwinski and Tebello Nyokong Department of Chemistry, Rhodes University, Grahamstown, South Africa

Abstract

Phthalocyanines and inorganic nanomaterials have garnered a great deal of interest in recent years due to their potential use as sensors, catalysts, and as tools for imaging and drug delivery. This chapter will discuss synthesis, modification of the nanomaterials, characterization (using a multitude of techniques including microscopy, spectroscopy and a variety of photophysical methods), as well as properties and potential applications exhibited by nanomaterials, such as quantum dots, iron nanoparticles and carbon nanotubes, alone or in combination with phthalocyanines. Also addressed will be the use of microscopic techniques, such as atomic force microscopy (AFM), transmission electron microscopy (TEM), scanning electron microscopy (SEM) and spectroscopic techniques, such as x-ray powder diffraction (XRD), x-ray photoelectron spectroscopy (XPS), as well as Raman, fluorescence, and laser flash photolysis/transient absorption spectroscopies. Additionally, the chapter explores thermal analysis methods, including thermogravimetric analysis in the characterization of the nanomaterials. Keywords: Phthalocyanines, quantum dots, carbon nanotubes, magnetic nanoparticles, photophysics photochemistry Abbreviations: A = acceptor AFM = atomic force microscopy CNT = carbon nanotubes D =donor DLS = dynamic light scattering DOS = density of states Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (347-424) © Scrivener Publishing LLC

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DSC ET FID HT IC IR ISC MNP MPc MRI MWCNT NIR NP OCT Pc PDT PET PHT PL PT QD SAMs SEM SWCNT TAS TEM TGA TOP TOPO XRD XPS

= differential scanning calorimetry = energy transfer = free-induction decay = hyperthermia = internal conversion = infra red = intersystem crossing = magnetic nanoparticle = metallophthalocyanine = magnetic resonance imaging = multi walled carbon nanotube = near infrared = nanoparticle = optical coherence tomography = phthalocyanine = photodynamic therapy = positron emission tomography = photohyperthermia = photoluminescence = photothermal = quantum dot = self assembled monolayers = scanning electron microscopy = single walled carbon nanotube = transient absorption spectroscopy = transmission electron microscopy = thermal gravimetric analysis = trioctylphosphine = trioctylphosphine oxide = x-ray diffraction = x-ray photoelectron spectroscopy

9.1 Background on Nanomaterials The aim of today's research in the nanosciences is to develop a basic knowledge of elementary, nanoscale-sized building blocks, in order to form physical, chemical or biological systems which, on a macroscopic scale, will have new properties and new functions. Research in nanoscience, therefore, establishes the basis for new technologies and thus economic development. Nanoparticles (NPs) are 1-100 nm in size and are made from a variety of materials from organic dendrimers and liposomes, to

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gold, carbon, silicon, metal, metal oxides and semiconductors. These materials are found in a variety of shapes such as spheres, nanotubes, and stars etc. and have found a variety of applications in chemistry, materials sciences, physics, medicine and electronics [1-3]. Due to their distinct chemical, physical and biological properties, nanoparticles have generated a great deal of interest over the past few decades. The quantum size effect is responsible for the unique properties (optical and magnetic) of nanomaterials and they differ markedly from their bulk counterparts, prompting researchers to investigate their applications in imaging and sensing which would be essential tools in medical diagnostics. Depending on their size and shape, noble metal NPs for example, absorb light strongly due to surface plasmon resonance effects [4], while semiconductor nanocrystals such as quantum dots possess a large absorption and a narrow emission spectrum which is red shifted as the NP size increases [5], Additionally, the properties of a NP, which have a large surface to volume ratio, may be further fine tuned by functionalization for targeted applications, such as ultrasensitive sensors including chemical sensors or disease markers [6, 7]. Furthermore, with the advancement of imaging technology, multifunctional NPs have found successful application in imaging, drug delivery, and cancer research. Cancer remains to be an immense concern in modern day society, and reliable, sensitive diagnostic tools are essential for the early detection of cancerous cells. Photodynamic therapy (PDT) has played an increasing role in the effort to treat tumors in cancer therapy. The PDT modality combines the selectivity of a fibre optic directed light with the cell destruction properties of singlet oxygen (or other radicals), together with a photosensitizer. It is believed that the photosensitizer generates singlet oxygen which in turn destroys the cancer cells. Progress in evaluating nanomaterials in humans has been slow due to the unknown effects of NPs in vivo, although in vitro tests have been conducted [8]. The potential toxicities of these new materials need to be understood for the safe and continued development of nanotechnology as the range of nanoparticles and applications increases. This includes evaluation of the synthetic methods, physicochemical characteristics and cytotoxic properties.

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9.1.1 9.1.1.1

Semiconductor Quantum Dot (QD) Nanoparticles General and Medical Applications

As a result of the quantum confinement of their electrons in all three physical dimensions [9,10], quantum dots (QDs) are defined as 0-dimensional semiconductor materials (Figure 9.1). With physical dimensions smaller than the exciton Bohr radius; usually less than 10 nm [10-13]; QDs have quantized energy levels giving them unique properties which lie between those of bulk and atomic materials [10-14]. Of particular interest is their optical properties which includes a broad absorption spectrum, allowing for excitation over a broad range of wavelengths, and their size-tunable narrow emission spectra which may span the ultraviolet (UV) to infrared region [12, 15]. QDs also possess excellent photostability and they are therefore well suited to fluorescence imaging applications [15-17], holding characteristics such as high fluorescence quantum yields and low photobleaching rates (which are a problem with other organic based dyes). Fluorescence labeling and staining is sensitive (often approaching the sensitivity afforded by radioisotopes), and it offers a number of important advantages over other methods, such as sensitivity, simplicity, multi-colour detection, stability, and affordability Multiple targets using these fluorescent labels can be spectrally resolved and as such it allows for the detection and resolution of two or more labeled targets in the same sample. Thus quantum dots of different

0D

Quantum dot

1D

Quantum wire

it ■ ΙΪ^. DOS

DOS

2D

3D

u u Quantum well

DOS

Bulk

DOS

Figure 9.1 Quantum confinement structural features and density of states (DOS) effects [10]. Adapted from reference 10.

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sizes (and therefore different colours) could be functionalized with different biomolecules for localization in different target cells. Owing to the ease with which the QD size can be tuned, and subsequently the size-dependent properties thus arising, QDs have also found numerous applications in diverse fields such as high-density data storage, chemical sensing, optics (e.g. NLO)[18], telecommunications, computing, solar cells [19], Light Emitting Diodes [20], and extensively in biomedicine as labels [10,14, 21]. Recently QDs have found focus as a new generation of photosensitizers in photodynamic therapy (PDT) [14, 21-23]. Although limited [22], QDs are capable of transferring energy to ground state molecular oxygen to generate cytotoxic singlet oxygen, but more importantly they may be used as imaging tools, thus enhancing the efficacy of PDT [22, 24, 25]. Conjugation of a QD to a mediating PDT photosensitizer e.g. a metallophthalocyanine (MPc), facilitates the intriguing probability of increased PDT efficiency through energy transfer (ET). 9.1.1.2

Syntheses of Organo and Water-soluble Quantum

Dots

Easy, reproducible synthesis together with a narrow size distribution (within 2%) [26], is desired in the synthesis of high quality QDs. The photophysical properties i.e. photoluminescence (PL) quantum yields and luminescence lifetimes (and thus imaging capabilities) are highly influenced by the solvent, core, shell, and coating characteristics. A variety of methods have been reported for the surface modification of QDs [23, 27, 28], but currently two synthetic strategies are used to prepare them i.e. non-aqueous and aqueous syntheses, bearing in mind that biomedical applications often require water-soluble systems to enable biological compatibility. This brings into play the importance of nanoparticle surface modification. Thus functionalization of the NP surface allows the tuning of the overall properties of particles to fit targeted applications. The surface modification of nanoparticles by functional molecules, other inorganic particles or polymers is necessary to (a) stabilize the nanoparticles in solution and thereby influence the growth and shape of the NPs; (b) provide functional groups at the surface to enable further derivatization; (c) enhance the NP solubilization in a variety of solvents, extending their application potential; (d) modify the electronic, optical, spectroscopic and chemical properties of the particles; (e) modify the NP's capability to assemble in specific

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arrays; (f) give the NPs the ability to target desired chemical, physical, or biological environments; (g) protect the nanoparticle core by preventing oxidation and biodegradation; (h) reduce aggregation tendencies (due to high surface energies present), improving aqueous dispersion, (i) reduce toxicity (e.g. cadmium based QDs) and finally, (j) play a role in molecular recognition in sensing or assays which could allow for the early detection of tumours, in environmental sampling etc. [29]. a) Synthesis of organo soluble QDs In this method, QDs are produced by the pyrolysis (at>300°C) of organometallic precursors which can be metal alkyls, metal oxides or metal halides (depending on the QD needed) and often organophosphine- or bistrimethylsilyl-chalcogenides in high boiling point coordinating organic solvents such as trioctylphosphine (TOP) or trioctylphosphine oxide (TOPO), alkyl phosphites, alkyl phosphates, pyridines, alkylamines, or furans, are used, resulting in hydrophobic QDs [30-32]. Further surface modification would then be necessary to transfer the hydrophobic QDs to the aqueous phase, if desired. This step often results in decreased photoluminescence [33-36]. b) Synthesis of aqueous soluble QDs Water-soluble functional NPs are vital for application in a variety of biomedical applications. However, the synthesis of robust functional NPs generally results in hydrophobic NPs due to the hydrophobic surfactant coating methods employed. Hydrophobic NPs may be converted simultaneously into hydrophilic water-soluble particles, which would then introduce chemical functionalities on the particle surface so that a variety of molecules may be covalently attached. Hydrophobic NPs may be converted into hydrophilic, functional NPs by: 1) ligand exchange of the original surfactant by hydrophilic ligands such as thiols or other functional groups, and 2) bilayer formation between amphiphilic molecules/polymers and then passivating the surfactant layer surrounding NPs [37,38]. The most common aqueous synthetic procedure is the hydrothermal method [39^1]. This method has been used for the synthesis of a variety of nanocrystals i.e. CdS, CdSe,CdTe, ZnS, ZnSe and HgTe. Simply, the method involves dissolution of the salt of interest e.g. Cd (for CdTe QDs), in water followed by the addition of the stabilizer (which also serves as the capping agent) e.g. MPA to produce the Cd precursor. Adjustment of the pH to alkaline

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conditions using NaOH, all done under an inert atmosphere, then follows. Rapid injection of a freshly prepared sodium hydrogen telluride (NaHTe) solution [42-50], follows to form the colloidal CdTe QDs. Ostwald ripening to promote quantum dot growth then takes place upon the introduction of heat (100°C) [51]. Aliquots at the desired emission wavelengths are then transferred and the reaction terminated. As mentioned previously, the stabilizer plays a critical role in controlling the size and shape distribution of the QD during growth, as well as its crystal structure and stability [52]. This direct aqueous synthetic route is cheaper and more reproducible, generating water-soluble, biocompatible QDs directly, thus eliminating the need for further surface modification [39]. Organic bifunctional capping ligands, where a surface anchoring moiety would bind to the inorganic QD surface (for example, thiol) and an opposing hydrophilic end group (for example, hydroxyl or carboxyl functional groups) may be used to achieve water-compatibility. Thiol containing moieties such as thioglycolic acid and mercaptopropionic acid are frequently used to produce water-soluble QDs. These capping agents are attached to the surface of the QD via the thiol moieties, which additionally suppress emission intermittence (blinking), and are terminated by carboxylic acid functional groups. These terminal carboxyl groups facilitate hydrophilic interactions and also provide a point of attachment for biomolecules [33, 53] or other materials of interest such carbon nanotubes or phthalocyanines. Depending on the surface modification accomplished, further molecular interactions can be achieved via covalent attachment, electrostatic forces, adsorption, hydrogen bonding and silanization [14, 54], providing the gateway to a host of applications. Silanization is regarded to be one of the most widely used methods for the surface modification of NPs [55, 56]. Silica acts as a robust, inert layer against the degradation of optical properties and imparts water solubility. Dyes and QDs encapsulated within silica shells have displayed enhanced fluorescence lifetimes and luminescence due to improved chemical stability, photostability, much lower toxicity and non-specific adsorption under aqueous biological conditions [57, 58]. The marked decrease in toxicity is thought to be due to the silica coating acting as a robust barrier against the oxidation of the QD core. In addition, the silica coating may also allow for functionalization of the NPs with other biomolecules and chemicals.

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9.1.1.2

Absorption and Fluorescence Spectra of QDs

The electronic and optical properties of bulk semiconductors are dependent on the bandgap between the valence and conduction band energies. In progressing from bulk material to QDs, the density of states ceases to be continuous resulting in a specific number of discrete states for the electrons to occupy at a given energy (Figures 9.1 and 9.2) [59, 60]. The size of this bandgap changes with size, where a decrease in QD size results in a larger bandgap with a concomitant blue shift in the absorption spectra. This property therefore allows for tuning of the electronic and optical characteristics of the nanoparticle. Figure 9.3 shows the influence of size changes on the emission properties of QDs. The effects of chemical composition on QDs are shown in Figure 9.4. The most convenient and common method for the initial characterization of quantum dots is the acquisition of UV/Vis absorption and emission spectra since these properties are closely related to QD size. A typical absorption spectrum of high quality QDs are characterized by well resolved features (Figure 9.5), while broadening is associated with a broad dispersion in size and shape distribution. Although growth conditions such as temperature, pH and solvent properties, presence of defects and solution concentrations used in obtaining the spectra also play a role [13]. Narrow, distinctive

Bulk semiconductor Conduction band

Quantum dot β

Conduction band

Valence band

Figure 9.2 Simplified diagram illustrating discrete energy levels of a QD compared to the continuous levels in a corresponding bulk semiconductor crystal. Adapted from reference [59].

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Size

500

400

600 Wavelength (nm)

700

Figure 9.3 CdTe QD emission wavelength tuned by changing the nanoparticle size. Adapted from reference [49].

Near UV

UV 200

300

Near IR 400

CdS ZnS

500 600 Wavelength (nm) CdTe

ZnSe CdSe

700 -*■

800

·«-

PbSe/Te

PdS

Figure 9.4 Effects of chemical composition on QD emission.

photoluminescence or fluorescence emission spectra are usually attributed to the radiative recombination [61] of excitons or free carriers in QDs. 9.1.1.3

Characterization

Due to the size dependent properties of QDs (and other nanoparticles), size determination becomes a critical component in the characterization of these nanomaterials. The complete characterization of a nanoparticle is difficult, and a plethora of techniques exist, where most are complimentary and should be used in combination to provide different pieces of information in order to build a consistent picture. Among the tools available for particle size measurement and characterization, two essential techniques are used

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450

550

650

750

Wavelength (nm)

Figure 9.5 Classic absorption and fluorescence emission spectra of high quality QDs. Adapted from reference [49].

including microscopy (imaging) and spectroscopy [62]. These techniques may be applied, not only to QDs, but also to other nanomaterials such as carbon nanotubes and magnetic nanoparticles, and will be briefly described here. Where the technique is only applicable to a particular nanoparticle, it will be discussed in the appropriate section. The size of QDs such as CdTe, CdSe and CdS may also be calculated from the position of the absorption peak in the UV/Vis absorption spectra using a polynomial fitting function (Equation 9.1). D= (9.8127 χ10-7)λ 3 - (1.7147χ10-3)λ 2 + (1.0064)λ- (194.84) (9.1) where λ is the absorption maxima of the QDs. The fitting function is not valid for sizes of quantum dots outside the size range of 1 to 9 nm [63]. a) Spectroscopic Techniques Absorption and fluorescence techniques have already been covered in section 9.1.3. Additional spectroscopic techniques that may be used for the characterization of NPs follow. X-ray diffraction (XRD) is a convenient and reliable tool and is used to obtain structural information relating to the crystal lattice, Figure 9.6 [42, 64-67]. The mean particle size may be determined

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Figure 9.6 Simplified diagram illustrating typical X-ray diffractogram of CdTe QDs. Adapted from reference 65.

[68] from the line broadening of the X-ray pattern using the Scherrer equation (Equation 9.2) for particle sizes less than 9 nm.

d(A) =

kX ßCos6

(9.2)

where k is an empirical constant (0.9), λ is the X-ray source wavelength (1.5405 Ä for CuK al ), β is the full width at half maximum (FWHM) of the diffraction peak in radians, and Θ is the angle of the peak (Figure 9.6). IR and Raman spectroscopy may be used to determine surface modifications and the presence of functional groups on the surface of the nanoparticle, where FT-IR may be additionally used for asymmetrical vibration analysis. X-ray photoelectron spectroscopy (XPS) is a powerful tool that may be used to quantitatively determine the elemental composition, chemical and electronic states of material surfaces. Additionally it may be used for depth profiling. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) may also be used as a quantitative technique for elemental composition of NPs. Dynamic light scattering (DLS) has also been used to determine particle size, as such termed the hydrodynamic diameter. Due to

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the Brownian motion of particles in solution, DLS is able to measure the fluctuations taking place in laser light scattering. Although deviations in size determination, from that determined by for example transmission electron microscope (TEM), are often caused by the organic capping agent of the nanoparticle surface [69]. Solid state NMR has also been used to study and characterize nanoparticles e.g. chemical grafting at the surface of Si0 2 nanoparticles [70], and the doping of lanthanum fluoride nanoparticles [71]. b) Microscopic Techniques Accurate size distributions, shapes and images (providing access to the internal surface structure and fine morphological details) of QD cores may be obtained by high resolution transmission electron microscopy (TEM) and scanning electron microscopy (SEM). These are used extensively, especially for nanoparticles that do not absorb in the UV/Vis regions, with both techniques relying on electrons interacting with atomic nuclei. Sample preparation for these techniques, however, often result in particle agglomeration or aggregation, seriously hampering particle size measurement e.g. aggregation of MPA capped QDs at low pH, which would then be lead to an overestimation of size. Complimentary to TEM and SEM, are the scanning probe methods atomic force microscopy (AFM) and scanning tunneling microscopy (STM). Both these techniques may provide high resolution 3D topological information and imaging of the surface structure, while sample preparation is relatively easier. In STM a sharp conducting tip is moved across the surface, while in AFM a sharp cantilever tip is moved either in contact or non-contact mode across the sample surface. In AFM, the sample (in this case, QDs) should have a greater affinity for the substrate than for the cantilever tip (as the images will then be streaked), where agglomeration will also present a problem in imaging and size determination. STM is widely regarded to be a challenging technique in itself. All these techniques (as well as spectrofluorimetry and UV/Vis spectroscopy) may therefore also be used to describe the nature and strength of the bonding between the nanoparticle surface and the capping, and to understand the influence of the capping on the properties of the nanoparticles. 9.1.1.3

Photophysical Behavior

As described previously, the photophysical properties of semiconductor QDs differ from those of the bulk material. Due to quantum

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size effects, the bandgap energy can be tuned, resulting in the formation of atomic like levels. Many properties of QDs can be systematically described and understood in terms of the quantum confinement effect. QDs show different photophysical behaviour in the QD ensemble compared to single molecule measurements. As already stated, QDs have a broad, steady state absorption band and a narrow emission band with a high luminescence quantum yield. The photoluminescence properties of QDs typically depend on surface structure, chemical environment, type of capping agent and a number of interactions [72]. In time-resolved luminescence experiments with time correlated single photon counting (TCSPC, equipment shown in Figure 9.7) and fluorescence upconversion, QDs show multi-exponential decay curves [66, 67, 73, 74], a typical TCSPC trace is shown in Figure 9.8. Ultrafast lifetimes in the fs to low ps range were assigned to electron relaxation to the bottom of the conduction band and to recombination of trapped electrons and holes [73]. Decay times in the high ps to low ns region, are, according to some researchers [73, 75, 76], caused by the band-edge recombination at the surface; but this is contradicted by others who attribute this short lifetime to intrinsic recombination of initially populated core states [77-79].

Sample Beam splitter Laser

Photo diode

N -if rl ^A

Filter

PMT

Monochromator

>

'Start' 'Stop'

Figure 9.7 A typical TCSPG equipment set-up.

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40 Time (ns)

Figure 9.8 A typical TCSPC trace for CdTe QDs. Adapted from reference 66.

The luminescence lifetimes in the single-digit ns region is a result of radiative electron-hole recombination processes due to surface defects [80]. The origin of the lifetimes in the 20-40 ns range is thought to be due to the involvement of surface states in the carrier recombination process [80], where the increase in radiative lifetime as a result of trapping of carrier states by surface states, is a well established feature [77]. This is clearly observed by large quenching of the QD emission after attachment to other molecules compared to the shorter luminescence lifetimes, and is thought to be due to the presence of a formal bond which enables the interaction of the two species, resulting in the shorter fluorescence lifetime. Single-molecule studies may be contrasted with measurements conducted on a QD assembly of molecules, where individual behavior cannot be distinguished and only average characteristics measured. QDs [81] show, at a single-molecule level [82], emission intensities fluctuating between bright and dark states even under continuous excitation. These fluctuations are known as blinking. Despite 15 years of research on this phenomenon, the origin of the blinking is still not completely understood. It is known that the durations of light and dark periods follow power-law statistics [83, 84] independent from sample temperature [84], QD size or composition [85], nanoparticle shape [86] or excitation intensity

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[87]. There is a consensus that a QD stops emitting photons when it becomes charged, probably when electrons ejected from the core become trapped in defects on the nanocrystal's surface. Charged QDs can still be excited by photons to produce an electron-hole-pair, but this excitation is lost as heat. This process is termed Augerrecombination [88]. Auger recombination in nanocrystals and nanorods occurs due to three-carrier collisions (i.e., electron, hole, and third charge carrier) [89, 90]. Recently Wang etal. succeeded in synthesizing non-blinking QDs [91] by constructing a core of three types of elements (Cd, Se, Zn) and by grading the material of the shell so that core and shell blend into each other. The graded core-shell boundary is thought to make the processes attributed to Auger recombination inefficient. 9.2.1 9.2.1.1

Magnetic Iron Nanoparticles (MNPs) General and Medicinal

Applications

Magnetic nanoparticles (MNPs), due to a reduction in size (5-20 nm in diameter), are excellent magnetic materials (exhibiting superparamagnetism) and have been used in ultra-high density magnetic storage media [92], as versatile probes in biological detection (MNPs are functionalized with fluorescent probes for use in assays), anticancer agents [93] and as magnetic resonance imaging (MRI) contrast agents [94] e.g. FePt and iron oxide (y-Fe203 or Fe 3 0 4 ). These NPs are relatively easy to synthesize, they generally show a narrow size distribution and may be synthesized in a variety of sizes and shapes (depending on the synthetic approach used); resulting in several applications in biology. A variety of contrast agents and optical labels are required for different types of imaging and detection modalities such as MRI, positron emission tomography (PET), optical coherence tomography (OCT) and fluorescence based imaging. MRI and PET are ideal for in vivo imaging, while fluorescence-based imaging is mostly used for in vitro imaging. MNPs are good candidates as MRI contrast agents, possessing attributes such as: i) a narrow size distribution and thus tuneable properties, ii) high chemical stability, iii) superparamagnetism (MNPs < 10 nm in size), iv) excellent mechanical, optical electrical properties and v) ease of synthesis. Since MNPs can interact with induced electromagnetic fields of various frequencies and, owing to their small size, are able to penetrate a wide range of

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materials including bodily tissues, their properties allow them to be tracked, manipulated, imaged and remotely heated, opening up numerous possibilities for cancer treatment such as hyperthermia (HT), e.g. alternating current (AC) magnetic field-assisted cancer therapy (such as HT) [95-97]. These MNPs may also be used for tissue repair [98], magnetofection, immunoassay, detoxification of biological fluids, drug delivery, cell separation and cellular therapy (such as cell labeling or targeting) [99]. In an effort to explain why MNPs are important in imaging applications, a brief description of MRI will follow: Magnetic resonance imaging (MRI) is a powerful, non-invasive, diagnostic imaging tool which facilitates a cross-sectional view of the human body by employing the nuclear magnetic resonance of protons present in a large portion of the human body. In a large magnetic field, the hydrogen nuclei in the body align themselves with the direction of the magnetic field (Larmor precession). A radiofrequency (RF) pulse with a frequency equal to this Larmor frequency is applied perpendicular to the magnetic field, causing the net magnetic moment to tilt away from the magnetic field. Removal of the RF pulse causes the nuclei to return to their equilibrium state, resulting in relaxation of the nuclei, producing a freeinduction decay (FID) response signal. The FID response signal is measured and reconstructed to obtain MR images. Two relaxation processes exist: longitudinal (Ύ^ spin lattice) and transverse (T2, spin-spin) relaxation processes [100]. Superparamagnetic NPs have been used as MRI T 2 -shortening agents in clinical practice, and have played an important role in the detection of small metastatic liver tumors a n d / o r hepatocarcinomas [101-103]. Due to the NPs' magnetic susceptibility effect, T2 is significantly shortened and the signal intensity from normal hepatic tissue decreases, causing a contrast enhancement of malignant metastatic tumors. Magnetite is a common magnetic iron oxide with a cubic inverse spinel structure. Oxygen forms the face-centered cubic (fee) closed packing, while the Fe cations occupy the interstitial tetrahedral and octahedral sites [104], Figure 9.9. In these spinel ferrites, the electrons can hop between Fe2+ and Fe3+ ions in the octahedral sites at room temperature, rendering the magnetites an important class of half-metallic materials [104,105]. With proper surface capping these magnetic iron oxide nanoparticles can be dispersed into suitable solvents, forming homogeneous suspensions, called ferrofluids

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Figure 9.9 Schematic of a partial unit cell and magnetic ordering of spinel ferrite structure [104], A and B are the two cationic sites in the spinel structure through which coordination occurs with oxygen. Fe304 as spinel ferrite is FeFe204 (AB204). Adapted from reference [104].

(FF) or magnetic fluids (MF) [106] with unique physical, chemical, thermal, and mechanical properties [107-111]. As with QDs, the control of the monodisperse size is important as the properties of the nanocrystals are strongly dependent on the dimension of the nanoparticles. NP heterodimers are hybrid inorganic NPs, where the unique properties of each NP alone is combined. These NPs have been actively explored for the enhancement of imaging, targeting, and delivery and show great promise as probes for magnetic-based cell separation, targeting, delivery, MRI and fluorescence-based biolabeling applications. Multifunctional composites of gold with QDs and MNPs have been extensively studied e.g. Fe 3 0 4 -Au [112], CdSe-Au [113], and PbSe-Au-Fe 3 0 4 [114]. 9.2.1.2

Syntheses

Elaborate physical methods such as gas phase deposition and electron beam lithography may be used to synthesize magnetic

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nanoparticles, although size control in the nanometer range is not effectively realized [115]. Numerous chemical methods such as microemulsions [116], sol-gel syntheses [117], hydrothermal reactions [118], hydrolysis and thermolysis of precursors [119], flow injection syntheses [120], electrochemical deposition [121] and electrospray syntheses [122] have also been used to synthesize MNPs. Wet chemical synthetic methods are preferred since the NPs may be produced in large quantities and they are easily dispersable. This dispersivity allows for the opportunity to fabricate nanostructured devices such as sensors through self assembly to form self assembled monolayers (SAMs). SAMs are typically sensitive to aggregation and thus the need for high dispersivity is a necessity. Chemically disordered NPs may additionally be converted to the more ordered states through annealing, which allows for NPs with a higher magnetocrystalline anisotropy and ferromagnetism at room temperature. This process would also remove any stabilizing agents that may still be present. MNPs such as FePt NPs, are usually synthesized in solution via co-reduction of the metal (either organometallic e.g. Fe and Pt(acac)2 or ionic (e.g. FeCl2)) precursors into the metallic state in the presence of surfactant molecules (one that would bind to iron and the other to platinum for example). The surfactant molecules (such as oleylamine and oleic acid) stabilize the metallic nuclei by preventing aggregation, while also controlling the size and shape of the NP. Oxidized metal precursors require the additional use of a reducing agent, although the solvent used (e.g. diphenyl ether) in the reaction is often also used as the reducing agent when organometallic precursors are used [123-125]. To facilitate the syntheses of NPs with reproducible size and shape, inert atmospheres and temperature control are also required. As with the QDs, capping or coating agents containing appropriate functional groups (such as thiols, carboxyl, amino or silyl groups) may then be employed to induce water solubility or to facilitate covalent attachment of other molecules such as phthalocyanines or biomolecules, to form an amide bond for example. The most efficient method [126-129] for the production of magnetic iron oxide nanoparticles is the wet chemical co-precipitation of Fe2+ and Fe3+ (molar ratio of 1:2) aqueous salt solutions by addition of a base [99], resulting in a black precipitate. Control over size, composition and even shape of the nanoparticles may be realized with this method. Complete precipitation of Fe 3 0 4 , with

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a stoichiometric (Fe3+/Fe2+) ratio of 2:1 [130] may be expected at a pH between 8 and 14. However, Fe 3 0 4 may be oxidized and transformed to maghemite (yFe203) due to its instability and sensitivity to oxygen, critically affecting the physical and chemical properties of the MNPs. Thus great care is necessary when the reaction is carried out. Fe 3 0 4 NPs may be stabilized by capping agents such as citric acid or polymer surface complexing agents such as dextran or inorganic materials like silica, gold or gadolinium [131-133] during the precipitation process. This provides stability in solution, size control and also facilitates conjugation with a variety of biological ligands for biomedical applications. The synthesis of high quality MNPs are dependent on the salts used (e.g. chlorides, sulphates, nitrates, perchlorates), the Fe2+ to Fe3+ concentration ratio, pH, temperature and ionic strength of the media [134-136]. MNPs may be characterized by several techniques described for QDs which include XRD, XPS, ICP-OES, IR and Raman spectroscopies, and by microscopic techniques such as AFM, TEM and SEM as characterization by UV/Vis spectroscopy may not always be possible with MNPs. 9.3.1 9.3.1.1

Carbon Nanotubes (CNTs) General and Medical

Applications

Carbon nanotubes (CNTs) have outstanding chemical, mechanical, physical and optical properties. They have a tensile strength 20 times that of steel, carry 1000 times more current density than copper wires and transport charges down the nanotube without significant scattering [137]. CNTs have a simple chemical composition and atomic-bond configuration, but sp 2 hybridized elemental carbon can form an amazing variety of structures, such as graphite (3D), graphene (2D), carbon nanotubes (CNTs, ID) and fullerene (0D) [138]. Due to the spatial confinement of their electrons in two dimensions, CNTs are defined as quasi-1 -dimensional (ID) quantum structures or pseudo-quantum wires (Figure 9.10) [138,139]. CNTs are essentially a single graphite sheet rolled into a seamless hollow tube, with diameters ranging from 0.4-2 nm, and are several microns in length (Figure 9.10) [139-142]. The tubes may be single-wall CNTs (SWCNTs), or with additional graphene tubes around the core of a SWCNT, multi-walled CNTs (MWCNTs), where adjacent graphene sheets are separated by 0.334 nm and diameters

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Figure 9.10 Schematic representation of (a) a 2-dimensional graphite layer with the lattice vectors a, and a2 and the roll-up vector Ch = naj + na2° (n, n) and (b) the resultant SWCNT tubular structure [139]. Adapted from reference 139.

range from 0.4 to 50 nm [143]. The unique physical properties of CNTs, including high electrical conductivity, surface area, tensile strength, resilience, thermal stability and metallic to semiconducting current carrying capacity [140,144-149], are determined by the orientation of the graphite plane relative to the tube axis as well as the tube diameter (Figure 9.10). Known to absorb up to 100 times their own volume of hydrogen, SWCNT have found use in the storage of hydrogen in fuel cells [150]. In vitro medical studies have shown that CNTs may be used to deliver therapeutic drugs and diagnostic molecules into cells [151-154]. In addition, CNT have the ability to absorb light in the near-infrared (NIR) region and are therefore capable of initiating cell death via a photothermal (PT) or photohyperthermia (PHT) effect [151] e.g. in cancerous tissue since such tissue is sensitive to heat, while normal cells are often unaffected by slightly higher temperatures [155]. PHT is also known to improve the effectiveness of other cancer therapies such as chemotherapy, radiotherapy and PDT. However, recent studies on mice show that CNT can cause lung inflammation and the formation of lesions known as granulomas because of asbestos-like, length-dependent, pathogenic behaviour [156]. A lack of solubility limits the application of CNTs in chemical, biochemical and biomedical applications [144, 153, 157, 158], though end and side-wall functionalization is possible enabling solubilization, dispersion, and conjugation with biological ligands and other chemically relevant molecules.

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Two main modification pathways are used to functionalize SWCNT i.e. by covalent (Figures 9.11a and 9.12) and non-covalent (adsorptive) (Figure 9. lib) [153] means. However with covalent modification, the sp 2 -hybridized system is disrupted, since sp3 hybridisation results upon covalent functionalization, a characteristic that is seldom desired, as many of the CNTs optical, electrical and thermal properties are dependent on the extended π-conjugated system. As with the QDs and MNPs, introduction of carboxylic acid groups on CNTs allows for convenient covalent functionalization. This can be readily achieved by reaction under oxidative conditions using strong acids, such as H N 0 3 and or H 2 S0 4 which form the carboxyl terminated groups primarily at the open ends or at defect sides on the CNT (Figure 9.12a) [140, 144,145]. Nitrene cycloaddition (Figure 9.12b), arylation, use of diazonium salts (Figure 9.12c), or 1,3-dipolar cycloadditions (Figure 9.12d) may also be employed for direct sidewall functionalization [141,159]. The π -conjugated backbone of the nanotubes, and thus the electronic and catalytic properties [158, 160, 161] of the CNT, may be

Figure 9.11 Schematic diagram showing the 3 main approaches to obtain modified carbon nanotubes with biomolecules (a) the covalent approach, (b) non-covalent approach and (c) hybrid approach (i) a small anchor molecule is attached noncovalently to the nanotube surface and then (ii) a chemical reaction links the biomolecule of interest to the anchor [153]. Adapted from reference 154.

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COOH

Figure 9.12 Carbon nanotube chemical modification by (a) oxidation using strong acids, (b) nitrene cycloaddition, (c) arylation using diazonium salts and (d) 1,3-dipolar cycloaddition [153].

maintained by non-covalent surface modification where planar molecules may be adsorbed onto the CNT surface via π- π interactions [162,163]. Subsequent functionalization can then be obtained with biomolecules or other entities with the aromatic compounds serving as a platform [153]. 9.3.1.2

Syntheses

A carbon-arc discharge method was used to prepare the first CNTs by Iijima using a cathode [164]. Using solid state carbon precursors, arc-discharge [164, 165], laser ablation [166], pyrolysis of iron (II) phthalocyanines [167-169] have all been used to produce flawless nanotube structures. Catalytic chemical vapor deposition (CCVD) [138,170-173] methods use hydrocarbon gases as the carbon source, while catalyst particles serve as seeds to nucleate the growth of CNTs. These synthetic approaches are limited by the production of impurities such as fullerenes, amorphous carbon, graphite particles and graphitic polyhedrons as well as metallic particles or clusters where catalysts have been used [174].

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Metallophthalocyanines (MPc, where M = Fe, Co, Ni) have been used to prepare CNTs and have the potential to produce CNTs simply and cheaply [175-178] with the added advantage of being amenable to large-scale production [174-177]. Diameter size control remains a drawback however. MWCNTs are formed when excess phthalocyanines (metallated and metal-free) are used [179], while ball milling under an inert, high-pressure environment [180, 181] to first activate the Pc followed by heating at 1000°C under a A r / H 2 (95/5) atmosphere at a flow rate of 50 cm 3 /min [182] produced nanotubes at low growth rates and amorphous carbon residues. 9.3.1.3

Characterization

Thermal analysis techniques such as differential scanning calorimetry (DSC) [154, 183] and thermal gravimetric analysis (TGA), together with IR and Raman may be used to characterize CNTs and any subsequent functionalization. TGA, particularly useful, is a thermodynamic characterization method that records a sample's mass loss or gain versus temperature. The technique may be considered spectroscopic, where different decay profiles point to structurally different materials and provides information related to phase changes, thermal stability and composition. Similarly, DSC measures the difference in the amount of heat required to increase the temperature of a sample, versus that of a reference and is measured as a function of temperature. Thus samples which have undergone a physical transformation will behave differently. FT-IR may be useful here in the characterization of chemically functionalized CNTs, while Raman spectroscopy is often used for characterization of CNTs. Raman spectra of SWCNT, for example may be distinguished from other forms of carbon by two dominant Raman signatures i.e. the radial breathing modes (RBM) between 100-300 cm-1 and the G band, typically found around 1590 cm-1. Any changes in the composition of the CNTs should bring about definitive changes in these two signature bands [184-186]. 9.3.1.4

Photophysical Behaviour

The density of electronic states (DOS) indicates the number of allowed electron states at a particular energy and is useful in understanding the optical transitions taking place in solids (Figure 9.13) [187]. One dimensional (ID) materials like CNTs show sharp peaks

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E

11

3D DOS

2D DOS

1DDOS

0DDOS

Figure 9.13 Diagram showing energy versus density of states for materials of various dimensions. Occupied electron energy levels (valence band) are on the bottom and unoccupied levels (conduction band) are on top of each graph. The first two dipole-allowed absorption transitions (Eu and E22) are labeled for the 0D and ID densities of states (DOS) adapted from reference 197.

in the ID DOS (called van Hove singularities) that are similar to molecular levels. In its simplest form, optical transitions for CNTs takes place between matching points in the ID DOS. These transitions are abbreviated as E.. (En, E22) (Figure 9.13). The energy varies inversely with CNT diameter and is proportional to the nearest neighbor C-C distance and the nearest neighbor interaction energy [188]. As mentioned in section 9.3.1.2, CNT are synthesized as heterogeneous samples with different diameters, chiralities and lengths. They may also be composed of a mixture of semiconductor and conductor material since this depends on the diameter and chirality of the CNT [187]. CNT have the tendency to aggregate when synthesized, limiting the optical studies conducted on CNT bundles. CNT bundles show a strong absorption in the near infrared region (NIR) which is the result of a mixing of energy states for different CNT structures. Fluorescence is not observed with CNT bundles because the photoexcited carriers that are generated in semiconducting CNTs relax along efficient non-radiative channels provided by metallic CNT in the bundle. Raman spectroscopy of CNT bundles shows unique ID behaviour and special characteristics, such as radial breathing modes, the twist acoustic mode and ID phonon sub-bands [189]. Isolation of individual CNTs was a major step in understanding the optical properties exhibited by CNTs. This was accomplished by encapsulating single CNTs in a variety of surfactant-like micelles [190], polymers and DNA [191]. Isolated semiconductor CNTs show fluorescence, with CNT fluorescence emissions showing no indication of blinking on time scales ranging from 20 ms to 100 s. 50% of the emitting CNTs show blinking only at low temperatures

CONJUGATES OF NANOMATERIALS

371

(1.8 K) [192]. It is unclear whether the rate of blinking slows down at low temperatures or if the blinking gets disabled at higher temperatures. The fluorescence pattern of a single CNT revealed resolved optical transitions for distinct nanotube structures [190]. The assignment of a specific structure to each optical feature would require additional Raman resonance measurements as well as a complicated fitting procedure as several assignments are possible [193]. Resonance raman spectroscopy has played an important role in the characterization of CNTs in terms of diameter distribution in bundles and whether a nanotube is metallic or a semiconductor [189]. The observation of raman transitions from a single CNT [194] is possible due to the very large density of electronic states that occur in this ID nanostructure at certain well-specified energies which are dependent on the unique geometric structure of each CNT. When the incident or scattered photons in the raman process are in resonance with an electronic transition between the valence and conduction bands at these special energy states, E.., the Raman signal becomes very large as a result of the strong coupling which occurs between the electrons and phonons of the CNT. These resonance conditions allows for an intense raman signal enhancement, and, thus the possibility to observe each feature of the carbon nanotube Raman spectrum on the single CNT level that is normally observed in CNT bundles [189]. The investigation of excited-state relaxation dynamics of both bundled and isolated CNTs were carried out by fs transient absorption spectroscopy (fs-TAS) [195]. TAS is typically a pump-probe experiment where a short laser pulse (pump) places the molecule in the excited state, a second, delayed pulse or white light (probe) is used to monitor the population of a particular excited state. Such TAS experiments with CNTs display three distinct signal recovery regimes. While fast intraband relaxation (S0 (heavy atom effect).

CONJUGATES OF NANOMATERIALS

379

It is believed that during photosensitization (photocatalysis), the MPc molecule is first excited to the triplet state, and then transfers the energy to ground state oxygen, 0 2 ( 3 Σ ), generating excited state oxygen, 02(1Δ ), the chief cytotoxic species, which subsequently oxidizes the substrate. This is the Type II mechanism as shown in below in Scheme 9.2 [217,262-269].

MPc 3

x

02

+

->

MPc* Subs

'MPc*

->

3

-»

+ -»

3

MPc*

(9.3)

l

(9.4)

Oxidation P r o d u c t s

(9.5)

02

02

Scheme 9.2 Type II mechanism. Here MPc is the metallophthalocyanine, ISC is intersystem crossing and Subs is the substrate. 3 0 2 represents 02(3Δ ) and Ό 2 represents Ο20Δ ).

Singlet oxygen is generated when oxygen in its triplet state (302) interacts with a sensitizer (in its triplet state, 3MPc*). Thus singlet oxygen quantum yields are expected to be comparable to the MPc triplet state quantum yields if quenching of the latter by ground state oxygen is efficient [263]. Metallophthalocynanines (MPc) are well known photosensitizers [215,220, 270-283]. The excited triplet state of the MPc (3MPc*) can also interact with ground state molecular oxygen or substrate molecules (Equations. 9.6 and 9.7) generating Superoxide (Equations. 9.6 and 9.8) and hydroperoxyl radicals (Equation. 9.9), which subsequently afford oxidation of the substrate (Equations. 9.10 and 9.11) by a Type I mechanism (Scheme 9.3) [217, 264]. ZnPc complexes in particular are well known for their photosensitizing abilities [275-283], while unmetallated phthalocyanine complexes show very little PDT effect [217]. It has been reported that the Type II mechanism is more prevalent [263] in photo-initiated oxidation reactions; thus the magnitude of singlet oxygen quantum yield (ΦΔ), which expresses the amount of singlet oxygen generated per quanta of light, is often employed

380

INTELLIGENT NANOMATERIALS

3

MPc* + o 2 -> MPc+* +

3

+ Subs -> MPc" +

MPc*

MPc" + o 2 -> MPc Π"·

H+

KJ2

HO;

+

+

-> HO*2

Subs-H -> H 2 0 2 +

Subs+*,Subs* , H 2 0 2

—»

or

(9.6)

Subs+*

(9.7)

or

(9.8) (9.9)

Subs* (9.10)

Oxidation Products (9.11)

Scheme 9.3 Type I mechanism. Here MPc is the metallophthalocyanine, Subs is the substrate.

as a main criterion in choosing the photosensitizers to be used in photocatalytic reactions. Many factors associated with the sensitizers are responsible for the magnitude of the determined quantum yield of singlet oxygen, including: triplet excited state energy, ability of substituents and solvents to quench the singlet oxygen, the triplet excited state lifetime and the efficiency of the energy transfer between the triplet excited state of the sensitizer and the ground state of oxygen. The singlet oxygen quantum yields for the MPc complexes may be conveniently determined using a singlet oxygen quencher such as 1,3-diphenylisobenzofuran (DPBF), or by using the singlet oxygen luminescence method (SOLM). The two methods give comparable results [279] though side reactions are excluded when SOLM is employed. When DPBF is employed in micellar solutions, chain reactions occur as a result of the endoperoxide initially formed, but this problem can be overcome by adding sodium thiosulfate which destroys endoperoxide [279]. DPBF is an efficient Ό 2 quencher in organic solvents and its disappearance can be readily monitored by following the decrease in its

CONJUGATES OF NANOMATERIALS

3

02

τ

02

Ί

02

+

+ kd

3

MPc* —*^-> 3

> kq

DPBF

Ό2

0 2 Natural decay

381

(9.12) (9.13)

) Oxidation products (chemical quenching) (9.14)

+ DPBF

Ό2

kp(DFBF>

) 3

Q2

+

DPBF*

(physical quenching) (9.15) + MPc

Ό2

Ό2

+ MPc

kp(MFc)

>302

kq(MFc>

+ MPc* (physical quenching) (9.16)

) Chemical reaction (chemical quenching)

(9.17)

Scheme 9.4 Physical and chemical reactions of DPBF in the presence of Ό 2 .

absorption peak at 416 nm (in dimethylsulfoxide (DMSO), for example, Figure 9.17. Anthracene-9,10-bis-methylmalonate (ADM A) maybe employed as a singlet oxygen quencher in aqueous media by monitoring its disappearance at 380 nm in the presence of singlet oxygen. DPBF acts exclusively as a chemical quencher in DMSO and other organic solvents, and thus equation 9.15 may be disregarded. Physical quenching through equation 9.16 may also be ignored since ΦΔ does not depend on the concentration of the sensitizer (MPc). Furthermore, the reaction rate of the sensitizer with Ό 2 according to equation 9.17 is negligible compared to the rate of Ό 2 of reaction with DPBF. Thus, only Equations 9.13

382

INTELLIGENT NANOMATERIALS

ft

o$o"01 350

400

450

500

550 600 650 Wavelength (nm)

700

750

800

Figure 9.17 Change in absorption spectra of diphenylisobenzofuran (DPBF) as singlet oxygen is produced by the phthalocyanine. Inset: Representation of the reaction of singlet oxygen with DPBF. Scheme 9.4 shows the fate of Ό 2 in the presence of DPBF.

and 9.14 are relevant to the decay of singlet oxygen, and these equations may be used in deriving equation 9.18 for the determination of ΦΔ employing a standard such as unsubstituted ZnPc [281, 283, 284]:

d sta w . e ΦΛ=ΦΛ

w

Std

abs

(9.18)

abs

where ^td is the singlet oxygen quantum yield for the standard, W and WStd are the DPBF photobleaching rates in the presence of MPc derivatives under investigation and the standard, respectively. Iabs and Iabs are the rates of light absorption by the MPc derivative and standard, respectively. A similar equation may be derived for the use of ADMA as a quencher. I abs is determined by Equation 9.19.

^abs

—

oSI

(9.19)

where a is the fraction of light absorbed, S is the cell area irradiated, N A is Avogadro's constant and I the light intensity.

CONJUGATES OF NANOMATERIALS

383

In the absence of appropriate standards, DPBF (or ADMA) quantum yields (ΦΟΡΒρ) are calculated using equation 9.20 [279]: Φ,DPBF

(C0-Ct)V tl abs

(9.20)

where C0 and C t are the respective concentrations of DPBF before and after irradiation, t the irradiation time and V is the volume of the sample. Singlet oxygen quantum yields are then calculated using equation 9.21 [279]: 1 ODPBF

ΦΔ

1 1 k, O.kjDPBF]

(9.21)

where kd is the singlet oxygen decay rate constant and k the reaction rate constant of DPBF in the presence of singlet oxygen. 1/Φ Δ is obtained from the intercept of a plot of 1/Et

/ 0 E v

Si

Figure 9.25 Capping agents employed in the Fe MNP synthesis. OEt = ethoxy.

200

300

400 500 600 Wavelength (nm)

700

800

Figure 9.26 Electronic absorption spectra of magnetite capped with (i) hydroxyl groups and (ii) 3-aminopropyltriethoxysilane. Adapted from reference 317.

[313], indicating that MNPs do not affect the absorbance properties of the MPc derivatives, and may therefore be used in conjunction for medical applications. The results obtained demonstrated that the interaction of nanoparticles with MPcs lead to improved photophysical and photochemical properties reinforcing the idea of excellent photodynamic activity of MPc-MNH systems for PDT and local HT. The synthesized magnetic iron oxide nanoparticles (i and ii) did not show any appreciable fluorescence. Very low emission intensities were observed at about 450 and 540 nm upon excitation at 260 nm. The spectra also showed many emission peaks characteristic of iron NPs [318], Figure 9.27. CoPc/Fe nanocomposites with both adjustable electric and magnetic properties were produced by employing unsubstituted cobalt(II)-phthalocyanine (CoPc) and liquid Fe(CO)5 as the starting materials in a one-step thermolysis reaction. The nanocomposites thus produced were found to have distinctive currant-bun particle morphologies where the surface was found to be organicsubstance-rich [319].

398

INTELLIGENT NANOMATERIALS 6-1

>4c o

0300

400

500 600 Wavelength (nm)

700

800

Figure 9.27 Fluorescence emission spectra of magnetite capped 3-aminopropyltriethoxysilane. Adapted from reference 317.

9.5.2A

Characterization (AFM, SEM, TEM, MS, XRD, Raman, Thermal analysis)

The iron oxide NPs synthesized by Idowu and Nyokong [313] were characterized by UV/Vis and IR spectroscopies, XRD and by photophysical measurements such asfluorescencespectroscopy and by transient absorption spectroscopy when combined with the Pc. The triplet decay curves for MPc alone and in the presence of iron oxide NPs were found to be similar. The low-density CoPc/Fe nanocomposites were fully characterized by FT-IR, XRD, SEM, high resolution TEM, TG and differential thermal analysis (DTA). In addition, the magnetic hysteresis loop and microwave electromagnetic parameters of the nanocomposite particles were measured. The results indicated that the CoPc/Fe nanocomposite consisted of micrometer-sized regular spheroids with CoPc coating the Fe NPs. The nanocomposites showed good antioxidative properties, high magnetic susceptibility and could be utilized as a microwave absorber [319]. 9.5.1.5

Photophysical Behavior of Pc-nanomaterial Conjugates

9.5.1.6

Pc-CNTs

The interaction of Pcs (or SubPcs) and CNTs are investigated mostly as artificial photosynthetic systems in the transfer of light energy into other energy sources. Within this perspective, photoinduced electron transfer is the most important process taking place between Pcs and CNT. Pcs are thermally and chemically stable compounds

CONJUGATES OF NANOMATERIALS

399

with intense absorption in the red/near-IR region of the solar spectrum, possessing high extinction coefficients («200 000 M_1 cm"1), making them ideal light harvesting systems. On the other hand, CNTs are known as good electron acceptors and, depending on the structure, good conductors [187]. In the CNT's conduction band, a continuous electronic state is provided for collection of electrons from for e.g. a jc*-state of an organic conjugated molecule like a Pc. These electrons may be transported one by one under nearly ballistic conditions along the nanotube ID axis. Therefore the combination of CNTs with Pcs is expected to lead to novel nano systems that may lead to major breakthroughs in the conversion of solar energy into electricity. The covalent or noncovalent attachment of different electron donors, including ferrocene [320, 321], tetrathiafulvalene [322, 323] and porphyrins [324-333] to the nanotube surface is at the forefront of current investigations. With porphyrins, especially, it is possible to generate long-lived charge-separated states upon photoexcitation. Two general approaches for the functionalization of CNTs have been reported, as described previously; the covalent attachment of molecules to the open ends or sidewalls of CNTs or the noncovalent interaction of aromatic molecules or macromolecules like Pcs to the outer nanotube walls. a) Covalent Attachment Torres and Blau [334] synthesized a ZnPc-SWCNT hybrid which showed through UV/ Vis and IR spectroscopy as well as TEM that the tubes are no longer aggregated into large bundles. Nevertheless the ZnPc-SWCNT system was scarely soluble in most organic solvents, making photophysical investigations impossible. An erbium(III) Bisphthalocynaninato (ErPc2)-CNT system was reported by Xu et al. [335] to show ground state charge transfer from the Pc rings to the CNT. Yang et al. [64] synthesized a tetraamino-MnPc-MWCNT hybrid material, and formed a single layered device which showed a photoconductivity that was higher than that of the pure tetraaminoMnPc alone or a tetraamino-MnPc/MWCNT blended composite. This was attributed to optimized photoinduced electron transfer from the Pcs to the MWCNTs within the covalently linked material. More recently, this same process was used to prepare a hybrid free base Pc-MWCNT system which showed enhanced optical limiting properties [336]. This hybrid system also showed a decrease in the Pc fluorescence intensity, suggesting quenching of the free

400

INTELLIGENT NANOMATERIALS

base Pc singlet excited state by the covalently linked MWCNTs. Surprisingly a CuPc-MWCNT hybrid synthesized by Yang et al. [337] turned out to be soluble in a variety of organic solvents, showing improved photoconductivity than pure CuPc or the Cu(II)Pc/ MWCNT blended composite. Torres et al. synthesized a covalently linked sidewall functionalized ZnPc-SWCNT [338] conjugate, which revealed, in photophysical experiments, that the nanotubes act as the electron acceptor component within this ZnPc-SWCNT material. This was previously observed in a related porphyrin-SWCNT hybrid [325]. Furthermore, Torres et al. reported on an analogous system using a metal free Pc (23) linked to SWCNT [339]. Photophysical studies on this system seemed to indicate that the nature of the spacer did not have a notable effect on the photoinduced electron transfer process in these hybrid systems. Campidelli et al. [307] synthesized, via 'click' chemistry, a ZnPcSWCNT hybrid. This method was employed in order to avoid a stepwise synthesis where not all functionalities attached to the CNT are reacted with the Pc molecules and therefore not exploited. Subsequent photophysical characterization demonstrated the electron injection from the photoexcited ZnPc to the CNT. b) Noncovalent Attachment The immobilization of a Pc onto the CNT sidewalls results from the π-π interaction between the conjugated surface of the CNTs and the aromatic Pc macrocycles. Wang et al. showed that tetraferi-butyl substituted Pcs lose their color after reaction with CNTs [340]. Hatton et al. reported that concentrated dispersion of tetrasulfonated CuPcs and oxidized MWCNTs resulted in hydrogen-bonding-based interactions taking place [341]. Spectroscopic and morphological studies showed evidence that the Pcs were stacked in columns along the MWCNTs. D'Souza and co-workers reported on the noncovalent bond formation between a Zn- Naphthalocyanine (19, ZnNc) and a SWCNT using a pyrene-based derivative as the bridge [330]. Photoinduced electron transfer between the singlet excited ZnNc molecule and the SWCNT acceptor was measured using steady-state and timeresolved emission studies [330]. Enhanced photosensitivity was observed by Cao et al. in TiOPc/ MWCNT devices, where the composite contained 6 wt% CNTs. This complex reached a 5-fold higher photoconductivity compared to

CONJUGATES OF NANOMATERIALS

401

the undoped TiOPc devices. This result was attributed to photoinduced charge transfer (CT) from the TiOPc to the MWCNT component [342]. The same authors reported on the assembly of MWCNTs with the reduced form of a sandwich-type erbium(III) phthalocyaninate (ErPc2) [343]. The photoconductivity of the MWCNT-ErPc 2 hybrid showed enhanced photosensitivity as a result of the formation of a CT complex. UV/Vis experiments showed that, as the MWCNT content was increased, an accompanying, new band at 770 nm arose, which was attributed to the oxidised ErPc2. 9.5.2

Fe-NPs Mixed with MPcs

On addition of Fe-NPs (MF) to MPcs, the Φ τ values increased for all complexes (Table 9.4) as expected by the heavy atom effect and the presence of a paramagnetic species. During laser irradiation, the AlPc derivatives were partly transformed to an anion (Pc 3 ), whilst on addition of Fe NPs, the phototransformation of AlPc derivatives decreased. These observations suggest that the Fe NPs result in the recovery of the Q band absorption by oxidizing the Pc 3 back to the Pc 2 as shown by Equation 9.29. AlPc" 3 + Fe3+ -► AlPc- 2 + Fe 2+

(9.29)

Thus in the presence of Fe NPs, the effects of Pc 3 on the photophysical properties of AlPc will be minimal. Table 9.4 Photophysical behavior of ZnPc and AlPc in the absence and presence of Fe NPs in DMSO. Complexes

a

Φ

τ τ (μβ)

ZnPc

0.65[65]

350

ZnPc-Fe NPs

0.70

310

AlPc

0.20

720

AIPc-Fe NPs

0.28

750

AITSPc3

0.38

1335

AITSPca- Fe NPs

0.41

1295

TSPc = tetrasulfo phthalocyanine

402

9.5.3

INTELLIGENT NANOMATERIALS

Pc-QDs

Energy transfer (ET) processes and, more importantly, Forster Resonance Energy Transfer (FRET) may take place between mixed or linked QDs and Pc molecules. Energy transfer processes between molecules are regarded as the transfer of excitation energy from a donor (D) to an acceptor (A) molecule. The following explanations are restricted to an incoherent radiationless energy transfer starting from an optically excited molecule. An energy transfer can be described by Figure 9.28. Both molecules are firstly in the ground state (Figure 9.28), which, upon irradiation with light, the donor goes into an excited state. Energy transfer to acceptor subsequently takes place. In general, radiative and nonradiative energy transfer is differentiated. In radiative or trivial energy transfer, no direct interaction between D and A takes place. The excited D emits a photon which will be absorbed by A. This process could be dominant in very dilute solutions because this energy transfer has a quadratic distance dependence (when compared to e.g. FRET which has a distance dependence of the power of six, Equation 9.33) acceptor. The radiationless energy transfer can take place via many mechanisms dependent on the strength of the interaction between the involved molecules [344]. The most important energy transfer mechanism in a Pc-QD system is FRET. FRET is a very weak interaction between D and A which leads to a long-range dipole-dipole interaction between D and A. The D typically emits at a shorter wavelength which also overlaps with the absorption spectrum of A (Figure 9.29). The rate of FRET is dependent on the center-to-center distance r between D and A, the extent of spectral overlap of the D's r-O-f

FRET

J-,r Y T

"T

X

1

D*

ä3=*

Donor absorption

Accepter emission

FRET=Forster resonance energy Donor (QD)

Figure 9.28 Representation of FRET.

Acceptor (MPe)

CONJUGATES OF NANOMATERIALS

403

-300

500

600 700 Wavelength (nm)

800

Figure 9.29 Overlap of QD emission and MPc absorption required for FRET to occur.

emission spectrum and the A's absorption spectrum (Figure 9.29), the emission quantum yield of D and the relative orientation of D and A transition dipoles [345,346]. A decrease in the D fluorescence emission accompanied by an increase in the A's fluorescence is an indication that FRET has occurred, Figure 9.30. The FRET efficiency (Effss, Efftr) can be determined experimentally either from the fluorescence quantum yields (Φρ(Ε)), Φ ^ } ) or the fluorescence lifetime ( TD , τΌΑ) of D in the absence and presence of A Equation 9.30 [49, 75,290]. DA

Φ,HD) % = i - ^

(9.30)

'■DA

(9.31)

F(D)

E& = 1 -

However, Equation 9.31 makes the assumption that the decay of the D is a single exponential in the absence (τ0) and presence (τϋΑ) of acceptor and thus holds rigorously only for a homogeneous system (i.e. identical donor-acceptor complexes) in which the donor and donor-acceptor complexes have single exponential decays [290]. Such single-exponential decays are rare in biomolecules where FRET is an important energy transfer mechanism. Thus for D-A systems decaying with multi-exponential lifetimes the energy

404

INTELLIGENT NANOMATERIALS

Stimulated emission of AlPc

500

550

600 650 700 750 Wavelength (nm)

800

Figure 9.30 The decrease in QDsfluorescenceemission and an increase in the MPc fluorescence during FRET.

transfer efficiency must be calculated from the amplitude weighted lifetimes (Equation 9.32)

■ΣΓ—N

Where X = S or O

F

o

21

SR

SR-h

299

| ^ N — Z n - N f l T-SR

0 SR H3CV

+

capped QDs, showing that the capping of the QDs has a strong influence on the FRET. Britton et al. [47, 48] covalently linked and mixed CdTe QDs capped with MPA and TGA to zinc and indium tetraaminophthalocyanines using EDC and NHS as the coupling agents. Both the linked and mixed QDs-ZnTAPc complexes showed FRET, whereas the mixed and linked QD-InTAPc complexes showed none.

Table 9.7 Data on some FRET studies of MPcs/QDs conjugates which are mixed or linked. ZnPc Derivatives

QD capping

J

R

o

(10"14cm6K)

(10-10m)

r (10 10m)

Eff(%)

Ref.

ZnTSPca (mixed)

TGA

1.80

57.0

67.0

30

45

ZnTCPc" (mixed)

TGA

0.72

49.0

63.0

20

45

12 (mixed)

MPA

64

44.68

44.08

52

67

12 (linked)

MPA

64

44.68

34.70

82

67

20 (mixed)

MPA

107

51.59

58.95

31

46

ZnTAPca (mixed)

MPA (3.0 nm)

3.43

30.8

25.9

74

48

3

ZnTAPc (linked)

MPA (3.0 nm)

3.43

30.8

20.0

93

48

3

ZnTAPc (mixed)

TGA (3.0 nm)

2.18

24.8

22.0

68

48

a

ZnTAPc (linked)

TGA (3.0 nm)

2.18

24.8

21.6

70

48

3

ZnTAPc (mixed)

MPA (3.5 nm)

8.90

29.3

39.2

16

48

ZnTAPc3 (linked)

MPA (3.5 nm)

8.90

29.3

26.6

65

48

' TSPc= Tetrasulfophalocyanine, OCPc = Octacarboxy phthalocyanine, TAPc = Tetraamino phthalocyanine.

n o c o w O

> o w I—
4^ O

410

INTELLIGENT NANOMATERIALS

References 1. Y. Gogotsi, Nanomaterials Handbook, ISBN: 9780849323089, CRC Taylor and Francis, Boca Raton, FL, 2006. 2. G. A. Ozin, A.C. Arsenault, Nanochemistry. A chemical approach to nanomaterials, RSC Publishing, Cambridge (UK), 2005. 3. Vincent Rotello, Nanoparticles: building blocks for nanotechnology (Series: Nanostructure Science and Technology), ISBN: 9780306482878, Springer, 2004. 4. K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, Journal of Physical Chemistry B, Vol. 107, p. 668,2003. 5. P. Alivisatos, Pure and Applied Chemistry, Vol. 72, p. 1,2000. 6. J. Gao, H. Gu, B. Xu, Accounts of Chemical Research, Vol. 42, p. 1097,2009. 7. M. Liong, S. Angelos, E. Choi, K. Patel, J.F. Stoddart, J.I. Zink, Journal of Materials Chemistry, Vol. 19, p. 6251,2009. 8. M.A. Albrecht, C.W. Evans, C.L. Raston, Green Chemistry, Vol. 8, p. 417, 2006. 9. G.L. Hornyak, J. Dutta, H.F. Tibbals, A.K. Rao, Introduction to Nanoscience, CRC Press, Taylor and Francis Group, Boca Raton, 2008. 10. B. Rogers, S. Pennathur, J. Adams, Nanotechnology: Understanding small systems, CRC Press, Taylor and Francis Group, Boca Raton, 2008. 11. W.C.W. Chan, D.J. Maxwell, X.H. Gao, R.E. Bailey, M.Y. Han, S.M. Nie, Current Opinions in Biotechnology, Vol. 13, p. 40,2002. 12. T. Jamieson, R. Bakhshi, D. Petrova, R. Pocock, M. Imani, A.M. Seifalian, Biomaterials, Vol. 28, p.4717,2007. 13. T.J. Bukowski, Critical Reviews in Solid State and Materials Sciences, Vol. 27, p. 119,2002. 14. P. Juzenas, W. Chen, Y.-P. Sun, M.A.V.N. Coelho, R. Generalova, N. Generalova, I.L. Christensen, Advanced Drug Delivery Reviews, Vol. 60, p. 1600,2008. 15. C. Seydel, Science, Vol. 300, p. 80,2003. 16. X.H. Gao, S.M. Nie, Journal of Physical Chemistry B, Vol. 107, p. 575,2007. 17. N.Y. Morgan, S. English, W. Chen, V. Chernomordik, A. Russo, P.D. Smith, A. Gandjbakhche, Academy of Radiology, Vol. 12, p. 313, 2005. 18. R.E. Schwerzel, K.B. Spahr, J.P. Kurmer, V.E. Wood, J.A. Jenkins, Journal of Physical Chemistry A, Vol. 102, p. 5622,1998. 19. R. Kniprath, J.P. Rabe, J.T. McKleskey, D. Wang, S. Kirstein, Thin Solid Films, Vol. 518, p. 295, 2009. 20. E. W. Forsythe, M. A. Abkowitz, Yongli Gao, and C. W. Tang, Journal of Vacuum Science and Technology A , Vol. 18, p. 1869,2000. 21. D.K. Chatterjee, L.S. Fong, Y. Zhang, Advanced Drug Delivery Reviews, Vol. 60, p. 1627,2008. 22. A.C.S. Samia, X.B. Chen, C. Burda, Journal of the American Chemistry Society Vol. 125, p. 15736,2003. 23. A.C.S. Samia, S. Dayal, C. Burda, Photochemistry and Photobiology, Vol. 82, p. 617, 2006. 24. R. Bakalova, H. Ohba, Z. Zhelev, M. Ishikawa, Y. Baba, Nature Biotechnology, Vol. 22, p. 1360,2004. 25. Y.N. Konan-Kouakou, R. Boch, R. Gurny, E. Allemann, Journal of Control Release, Vol. 103, p. 83,2005. 26. S. Santra, K.M. Wang, R. Tapec, W H . Tan, Journal of Biomedical Optics, Vol. 6, p. 160,2001.

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10 Nanostructured Carbon and Polymer Materials- Synthesis and their Application in Energy Conversion Devices Debmalya Roy, B. Shastri, Md. Immamuddin, K. Mukhopadhyay Nanoscience and Technology Division, Defence Material and Store Research & Development Establishment (DMSRDE), Kanpur, India

Abstract

In this chapter, we focus on the role of organic and polymer chemistry to provide the much needed support in the development of organic photovoltaic (OPV) devices with the new and fundamental researches on novel materials with tailor-made properties. A good understanding on the excited state reactivity of photoactive materials will help to prepare new materials and molecules capable of absorbing light over a given wavelength range to drive electron transfer. The scientifically and technologically well-equipped chemistry community has explored the possibilities of developing and optimizing the charge separation in the light-harvesting architectures. However, it is yet to bear fruit due to the difficulty of the transportation of electrons and holes to the corresponding electrodes. Modeling charge mobility in organic semiconductors is complicated due to the presence of bulk heterogeneity in the macrostructure. The understanding of the interface between the metal electrode and the organic material, where charge collection takes place, is even more intriguing. In this chapter, we have highlighted the key features that enable people to design new materials for the vast landscape of solar energy conversion and the materials synthesis for OPVs. Keywords: Organic photovoltaics, tailor made donor and acceptor, metallofullerene, polymer nanofibers

Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (425-466) © Scrivener Publishing LLC

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Introduction

More than 30 Nobel Prize winners recently expressed their concern about funding for research on energy and urged U.S. President Mr. Barrack Obama to make good on his pledge to provide increased, stable funding for energy research and development [1]. Prof. F. Sherwood Rowland, who won the Nobel Prize in Chemistry in 1995 said, "The question for chemists is how to get substitutes for energy from fossil fuels, for example, or finding a way to put fossil fuels somewhere other than the atmosphere - those are technologically achievable goals, but they have a longer-range aspect. What we are looking for is energy solutions that are advanced and that can last for centuries or more, and chemists will need to be working on these things for an extended period of time." Our life and civilization without the plentiful and constant supply of energy is beyond imagination and, therefore, subsequently the awareness to preserve and secure adequate supply of energies has become a major concern [2-4]. Due to the very fast socio-economic development of the world, the energy needs are increasing exponentially every year. Academia and industry have joined hands together with spontaneous support from governments of all countries to counter this one of the most formidable challenges the world has ever faced. The magnitude of the challenge has become more demanding in light of the need to maintain an ecological footprint for not to jeopardise the long term sustainability [5-9]. In recent years, we have seen exhaustive campaigning and lobbying from all quarters to make us aware of the inadvertent consequences of enduring fossil fuel consumption which is so far our only major source to supply the huge energy demand. The initiatives which have been pursued extensively to meet the clean energy demand in years to come are carbon natural energy-fossil fuel in conjunction with carbon sequestration, nuclear power and renewable energies [11-12]. The stores of fossil fuels are limited and exhausting at a very fast rate and nuclear power is very expensive and still has to address the many technological, safety and scientific challenges. Hence, efforts are now being made worldwide to use the renewable resources to fulfill the global energy needs. Among the choices of renewable energies, solar energy emerges as the most viable to meet our energy demand. Although solar radiation is ideal to meet the demand, the energy produced from sunlight remains less than 0.01% of the global energy demand because it requires

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new initiatives in our technological and scientific know-how to harvest incident photons with greater efficiency and economical viability [6,8-9]. Unlike the sporadic research on nuclear energy from a united scientific community by internationally supported small number of high-profile projects, such as international thermonuclear experimental reactor (ITER) [13] and international fusion materials irradiation facility (IFMIF) [14], solar energy research is significantly more dispersed. Over a decade, the Swedish-led Solar-H took initiative on artificial photosynthesis by bringing together teams from some 28 nations to promote research on this visionary goal [15]. The US Department of Energy sponsored a broad-based overview of the state and future of solar energy conversion that spanned across numerous scientific boundaries [16]. The European Science Foundation has promoted a similar exercise for European research on photo voltaics [17]. The Asian and African nations are also taking major perspectives for implementing short and medium term targets of solar energy [18] (Fig. 10.1). In 2010, India's Prime Minister Dr. Manmohan Singh launched the Jawaharlal Nehru National Solar Mission under the brand name Solar India to produce ambitious 20,000 MW through solar power by 2022 [19]. Its success has

Figure 10.1 The biggest solar energy production base in the world: China Solar Valley. China aims to get 15 percent of its power from renewable energy sources, including hydroelectric dams, by 2020 [10].

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the potential of transforming India's energy prospects and contributing to national as well as global efforts to combat climate change. On that occasion he said, "If the mission is to become a reality, we will have to create many solar valleys on the lines of Silicon Valley that is spurring our IT industry across the country. I am convinced that solar energy can be the next scientific and industrial frontier in India after atomic energy, space and IT" [20]. There are many reviews and reports available in the literature on the technological aspects and the development of inorganic [21-25] and organic solar cells [26-30]. In this chapter our objective is to focus on the chemistry of materials for organic solar cells and to highlight some of our recent efforts in this direction.

10.2 Inorganic and Organic Semiconductors for Solar Cell Semiconducting inorganic materials have widely been used for the fabrication of solar cells with various device configurations such as single-crystal, polycrystalline, and amorphous thin film structures. These have progressed tremendously over the last fifteen years and commercial silicon PV modules are available for residential as well as industrial applications [23-25]. Multiple-junction cells offer improved conversion efficiency, however at much greater fabrication costs and lengthier energetic payback times (Fig. 10.2). In principle, by matching the solar spectrum with the large number of different multiple band gap materials, the conversion efficiency can reach 67% at 1-sun intensity. However, in practice only two to three band gap materials are used as most of the gain in efficiency is achieved by using three band gap materials [23]. On record, the present efficiency in the laboratory of a multijunction solar cell based on three junctions [GaInP2/GaAs/Ge (or GalnAs)] is 41% (under solar concentrations of 140-240 suns) [31]. The highest reported 1-sun efficiency of single junction silicon PV cells in the laboratory is 25% and the efficiency of commercial PV modules is about 75% of the maximum values measured in the laboratory, which is already close to the theoretically estimated upper limit of thermodynamic efficiency of silicon predicted by Shokley and Queisser in way back at 1961 [32]. Efficiencies of the third generation inorganic solar cells could be significantly enhanced by the judicious use of the "hot carriers"

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Figure 10.2 Efficiency and cost projections for wafer based (first generation), thin films (second generation), and the third generation PV technologies based on multiple exciton generation (MEG) using IV-VI, II-VI and II-V quantum dots (QD).

to the carrier collecting contacts with appropriate work functions and by the multiple excitons generation using impact ionization [33, 34]. By precisely controlling the fabrication of 3-D arrays of IV-VI, II-VI, and III-V quantum dots, multiple excitons could be generated and delocalized electrons could permit the transport and collection of hot carriers to produce a higher photopotential in a PV or photoelectrochemical cell [35]. The various configurations of third generation quantum dot solar cells using the nanocrystalline titanium oxide (Ti02) or cadmium selenide (CdSe) quantum dots in polymer matrix are being investigated in various laboratories for higher quantum yields [36, 37]. Further initiatives are necessary in Auger recombination process of multiexcitons in charge separation dynamics and the better understanding on optoelctronic properties of nanostructured inorganic photovoltaic configurations to produce reliable, reproducible and substantial increment in quantum yield of the third generation inorganic solar cells for technologically and economically competitive [38, 39]. However, in this endeavour the advent of molecular chemistry has been extremely limited, the device physics and the enduring technological developments spearhead all the efforts [40]. Organic semiconductor is a less expensive alternative to the inorganic semiconductor like silicon due to the low processing temperature, solution coating and fast processing on flexible plastic

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substrate [41]. Compared to silicon, they can have higher optical absorption coefficients that offer the possibility for the production of very thin solar cell in nanometre range. Additional attractive feature for organic solar cell is the possibility for thin flexible devices which can be fabricated using high throughput, low temperature approach that employ well established spin coating, casting and printing techniques in a roll to roll process [26-30]. The working principle for the conversion of solar to electrical energy of the inorganic and organic semiconductors has been schematically represented in Fig 10.3. The dielectric constant of the organic semiconductors are typically low and are in the range of 2-4 compared to dielectric constant of silicon, which is 11.8. The photogenerated carriers are, therefore, correlated and form bound electron-hole pairs that are called excitons unlike the uncorrelated electrons and holes in inorganic semiconductors which move freely in the conduction and valence bands. An internal electric field is thus required to separate the geminate electronhole pairs produced in organic semiconductors so that they can be collected at oppositely charged electrodes and utilized for OPVs. The number of reports on OPV has taken off significantly since the beginning of the new millennium. The trend suggests an average growth of ~65% per year which is almost twice the pace of emerging world photovoltaic market [42]. In recent years we have seen the tremendous efforts in optimizing OPV materials and chemistry has been played a large role in designing and developing newer materials with unique properties for organic solar cell [9,40]. Polymer solar cell fabrication techniques are much conducive to volume level production of photovoltaic panels as shown in Type II band offset system Inorganic semiconductors

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Figure 10.3 The difference in working principle of organic and inorganic semiconductor based photovoltaics.

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Fig 10.4. Utilizing these well established fabrication techniques along with the optimized organic photovoltaic materials and the device geometry, high performance commercially viable organic solar cells have been produced and demonstrated [43-45]. These commercially produced polymer solar cells were laminated for making it environmentally safe and operationally robust. Recently a round robin study was initiated at the Ris0 National Laboratory for Sustainable Energy, Technical University of Denmark on the flexible large area roll-to-roll processed solar cell modules [46]. It has been shown that the solar cell module could be efficiently packed by the plastic barrier materials and have been transported by air-mail and surface-mail paths to the eighteen different laboratories in Northern America, Europe and Middle East. It has been clearly demonstrated by the group of Prof. Frederik C. Krebs that it is possible with the current technology to share devices and obtain consistent data even over a long periods of time. This round robin study enabled the possibility of a plastic solar cell to come to the market a step ahead. The development of lightweight, flexible polymeric solar cells utilizing nanostructured materials has attracted a lot of attention. In fact, it gained momentum since the discovery of fullerene molecule [47] followed by the introduction of the bulk hetrojunction (BHJ) concept [48, 49]. Although the BHJ solar cells with the internal quantum efficiency approaching cent percentage has been demonstrated in the laboratory of Prof. Alan J. Heeger [50], however, the power conversion efficiencies and lifetimes of OPVs are still low compared to the competing inorganic technologies. It is likely that further endeavours and concerted efforts at the crossover of several technological and scientific fields are required to increase the longevity of the organic solar cells through device architecture, material stability, performance and degradation lifetime, before the large area OPV modules could compete the production of on grid electrical energy [26-30].

10.3 Materials for Organic Solar Cell: Donor The first report addressing the photovoltaic potential of organic molecules was published dated back to 1959 when Kallman and Pope discovered that anthracene can be used to make a solar cell [51], however, the first OPV device produced a photovoltage of

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Figure 10.4 A flow chart of the six different processing steps involved in the fabrication of the organic photovoltaic modules by role to roll (R2R) coating method.

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only 0.2 V and had an extremely low efficiency. The attempts to improve the efficiency of solar cells based on a single organic material, so-called homojunction, were unsuccessful, mainly because organic semiconductors have low dielectric constant unlike their inorganic counterpart [40, 41]. The photogenerated carriers are therefore not free in the organic materials and the electron hole pair is coulombically bound. A breakthrough was achieved by Tang who used two different materials, stacked in layers, to dissociate the excitons [52]. In this so-called heterojunction (Fig 10.5a), an electron donor material and an electron acceptor material are brought together. By carefully matching the band-offset of these materials, electron transfer from the donor to the acceptor, or hole transfer from the acceptor to the donor, could be made energetically favourable. The exciton diffusion length in most organic semiconductors is typically in the range of 3-10 nm which limits the efficiency of the planar heterojunction PV devices [53-54]. Excitons generated away from a very thin layer close to the interface have the higher probability of recombination before collecting at the electrodes. By restricting the domain boundaries of donor and acceptor in nanosize, an interpenetrating network of bulk heterojunction architectures (Fig. 10.5b) can be fabricated. The highly folded phase segregated morphology of a bulk heterojunction device enables an efficient exciton harvesting and charge separation due to the many orders increment of the area of donor acceptor interfaces. Most of the OPV devices are now fabricated in the bulk heterojunction morphology by spin casting the polymer and an electron acceptor from a common organic solvent [55-57]. In principle, all kinds of organic materials with p-type semiconducting behaviour and absorption in the visible region could be the potential candidate for donor in OPV. These materials can be organic small molecular solids like phthalocyanin or porphyrin or tetrathiafulvalene etc. and conducting polymers such as those based on the thiophene or phenylene vinylene or fluorene derivatives etc. While the fundamental properties of both classes, polymers and small molecules, are essentially the same, the difference mainly relates to the way the thin films are prepared [58-60]. Small molecules are typically thermally evaporated in vacuum and polymers are processed from solution. It has also been found that many small-molecular materials are soluble in organic solvent and the solubility can be increased by the synthetic addition of side chains. However, due to the low molecular weight of small

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molecule organic semiconductors, bulk heterogeneity in the thin film have often been reported which cause reduction in charge mobility and charge separation across the bicontinuous interpenetrating networks of donor acceptor materials [60]. On the other hand, an ultrafast charge transfer has been demonstrated in the conjugated polymer and fullerene interpenetrating networks resulting the efficient conversion of absorbed photons to electrons [47, 61]. Plastic materials, when properly modified, combine the excellent processing advantages and mechanical properties of conventional polymers with the desired electrical, optical, electronic and magnetic properties of metals and semiconductors. Besides, polymers could be prepared by simple chemical or electrochemical methods and can be chemically tailored to fit a wide range of technological applications. Their reversible change from a metallic conductor to a semiconductor material by electrochemical doping /undoping enables the application of these materials in different electronic devices, including photovoltaic and photoelectrochemical cells. A simple coating or printing process would enable roll-to-roll manufacturing of flexible, low-weight and cost-efficient PV modules [26-30]. One of the most investigated topics in organic photovoltaics in the past two decades is the design, synthesis, and the structure-property relationship of small bandgap ( H o > o w S

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Figure 10.21 (a) represents the comparative resistances of the pellets made out of rr-P3HT and rr-P3HT with one weight percentage of Ge endometallofuUerene whereas (b) represents the difference in the resistances over the range of 10 K to 500 K.

endometallo fullerene were carried out by four-probe low-temperature resistance measurement in the temperature range of 500 K to 10 K as shown in Fig 10.21a. Comparative conductance measurement plot (Fig 10.21b) shows the enhancement of conductance of P3HT by addition of one weight percentage of Ge endometallo fullerene. This phenomenon clearly indicates the charge transfer across the interface. MetallofuUerene makes the P3HT matrix more ordered resulting enhancement of electron and hole mobilities in the matrix [113]. The temperature dependent resistances of rr-P3HT and rr-P3HT-Ge endometallofuUerene mixture show semiconducting type behaviour indicating a higher availability of charge carriers at

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high temperature. However, it is evident from Fig 10.21b that the ratio of resistances of rr-P3HT and rr-P3HT-metallofullerene mixture is the highest in the temperature range of 300-350K. At lower and higher temperatures, the differences in the resistance are not that significant in rr-P3HT-metallofullerene materials. At low temperatures, crystallinity of the polymer matrix is low and increasing the temperature helps to align rr-P3HT in the mixture due to lattice vibration of the matrix resulting in higher conductivity [113]. It has been demonstrated by atomic force microscopy (AFM) studies (Fig 10.22) that the increase in temperature make the more ordered heterojunctions between rr-P3HT and the metallofullerene which in turn trigger charge separation across the interfaces. However, after a limiting temperature (~350 K in this case; Fig 10.21b), the alignment of rr-P3HT starts degradation due to the very high thermal energy assisted phonon coupling in the nanocomposite matrix. At very high temperatures (~500 K in this case; Fig 10.21b) the resistance ratio of polymer and the composite is almost one, indicating the complete disruption of alignment in polymer-filler network. Therefore, only in a certain temperature range where the alignment of polymer chains in the nanocomposite matrix is the best, an efficient charge transfer at the interface can be seen [111-113]. The sunlight generally increases the temperature of the photovoltaic cell. A lot of systematic studies have been carried out on the effect of temperature on the photovoltaic cell and module. Studies have shown that there are many pockets of temperature zones in a

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photovoltaic cell, however, the highest temperature reported was ~50°C when the starting operating temperature was 20°C for a photovoltaic cell. Our comparative resistance measurements (Fig 10.21b) show a plateau in the temperature range of 300-350 K. Therefore, the most efficient charge transfer in the metallofullerene-rr-P3HT matrix happens in the temperature range of 300-350 K. AFM images (Fig 10.22) also confirm that at 50°C the more ordered heterojunctions between rr-P3HT and metallofullerene is formed, which in turn could trigger charge separation across interfaces. Therefore, rr-P3HT and metallofullerene has all the potentials to become the complementary donor acceptor materials for OPVs [111-113]. As we have mentioned earlier that to enhance the efficiency of OPVs, the donor material should harvest more photons from solar spectrum [62-64]. rr-P3HT which has been extensively used as donor material for OPVs, could not absorb photons above the 1.9 eV energies and hence showed lower efficiency as explained in Fig 10.8. Another major bottleneck of the P3HT based OPV system is that, the device need to anneal at a high temperature for sufficiently long time as illustrated in Fig 10.27 to make the polymer film more crystalline and compact [75-77]. Device annealing often lead to alter the both physical and chemical properties of the film and resulted lower stability and performance of OPV devices [83-86]. We have synthesized the nanofibers of P3HT using the self assembly technique in a solvent where the solubility is strongly dependant on temperature [112]. The use of nanofibers of rr-P3HT as a donor material for OPV has three folds advantages. The much longer conjugation length in fiber structure compared to the isolated P3HT could be tailored by the self assembly of the monolayers of P3HT into a well-ordered lamellar structure. The fiber structure of rr-P3HT has a lower bandgap due to the coplanar structure and therefore the absorption is red shifted to the near IR region, which help to harvest more photons from sunlight as illustrated in Fig 10.23. The second advantage of using nanofibers of P3HT as donor material in OPV compared to normal P3HT is the much higher packing density [112]. The more compact orientation of polymer microstructure is possible due to the nanometre size diameters of the fiber structures. The higher number of light absorbing chromophores could be accommodated in the active area of the OPV device by using fiber type polymer microstructure which in turn will increase the photocurrent density by accumulation of more photons from sunlight.

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Self assembled rr-P3HT

I c

600 Wavelength (nm)

Figure 10.23 Absorption characteristics of pristine rr-P3HT and self assembled rr-P3HT fibers: higher absorption in P3HT nanofibers help to accumulate more photons from sunlight.

The third and the most important advantage of the use of nanofibers of P3HT as electron donor is to avoid the device annealing step. An extensive research on both the precursor and the thermocleavable side chain routes have been carrying out as the nonconjugated solubilising groups reduce the density of chromophores in the polymer and do not contribute to light harvesting or charge transportation [30]. The side chains of the polymer backbone make the material soft and porous [83-86]. The softness inculcated by alkyl groups leads to the instability of polymer solar cells and hence the nanofibers of P3HT which construct a highly crystalline film without annealing should drastically enhance the stability and performance of OPVs. The Fig 10.24 demonstrates the fact that the absorption of the as grown P3HT nanofiber film is as good as the annealed normal rr-P3HT film after one hour heating at 120°C. The typical recipe for making the nanofibers of P3HT is in principle similar to the fibre formation of polymer and the important ingredient is the judicious selection of the solvent in which the sample shows the strong temperature dependant solubility [112, 114-116]. The higher molecular weight fraction of rr-P3HT polymer was fractionated using soxhlet apparatus. The rr-P3HT was subjected to a series of soxhlet extractions with acetone, n-hexane, dichloromethane and THE The soxhlet was kept under a nitrogen purge to avoid unintentional doping of rr-P3HT. Each solvent was refluxed until the filtrate was colourless. The collected fractions were concentrated by rotavapour and then dried under inert atmosphere at elevated

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[112,114-116]. The slower the cooling rate higher will be the density and the length of nanofibers due the proper orientations of the hydrophobic nonconjugated alkyl side chains. The lighter fractions of rr-P3HT in hexane and dichloromethane did not precipitate as fibers from p-xylene due to the higher saturating concentration compared to higher molecular weight. This explanation is in consistence with the fact that the lower molecular weight fraction of a polymer is much more soluble than the higher molecular weight fraction and thus does not self assemble in the same solvent [114]. Another crucial parameter dominating the nanofiber formation is the regioregularity of polythiophene. Head-Tail coupled P3HT yielded nanofibers whereas non regio regular P3HTs do not self assembled into fiber morphology. By disturbing the chain regularity of P3HT, the crystallinity of the polymer matrix is significantly reduced which render the self-organization of the polymer chain into a well-ordered lamellar sheet. This observation is also corroborating with the fact that the nanofiber formation of the P3HT in a solution is dominated by crystallization of the polymer chain. The solution having a P3HT concentration higher than two weight percentage leads to the formation of a stable gel at lower temperature as the viscosity of the solution strongly decrease under cooling. The nanofibers of P3HT were characterized by the AFM, SEM and UV-VIS-NIR spectroscopic techniques (Fig 10.23-Fig 10.25).

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Figure 10.26 (al) represents the I-V characteristics of the ITO/P3HT-5% Ge metallofullerene/Al while (all) illustrates the same for the ITO/P3HT-10% Ge metallofullerene /Al, whereas the (b) demonstrates the current voltage properties of the ITO/nanofibers of P3HT-10% functionalized Gd metallofullerene /(Al) bulk-hetero-junction photovoltaic device.

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For photovoltaic device fabrication rr-P3HT and rr-P3HT nanofibers were mixed with desired weight percentage metallofullerene composition in 1, 2 dichlorobenzene (for nanofibers p-xylene solvent is used). This solution was spin cast on pre-cleaned ITO substrate at 10000 rpm and an active layer was fabricated. The film was annealed at 80°C in 10~5 torr vacuum for 30 minutes (for nanofibers based films, no heat treatment is given). Top layer of aluminium of 100-150 nm thickness was thermally deposited. Finally, an ITO/ rr-P3HT-Fullerene (Ge/Gd)/Al bulk-hetero-junction photovoltaic devices were formed in which ITO material on glass substrate and aluminium top layer work as electrodes for making electrical connections. The fabricated devices were characterized for short circuit dark current and photo current. 1.5 A.M solar simulator at different light intensities was used for the purpose [111-113]. As expected the conductivity of rr-P3HT nanofibers are much more due to the higher packing density of nanofibers. The fibrous structures of the self assembled rr-P3HT and the better solubility of functionalized Gd metallofullerenes form the ordered bulk heterojuctions and hence more than ten folds enhancement of photocurrent density compared to Ge metallofullerene and rr-P3HT system has been achieved in ambient atmosphere. Short circuit current and the open circuit voltage of the fabricated solar device are shown in Fig 10.26. Dark current for the device is almost nil at no bias condition for Ge metallofullerene and P3HT system. The Fig 10.26al is for ITO/P3HT-5% Ge metallofullerene/Al while Fig 10.26aII is for ITO/P3HT-10% Ge metallofullerene/Al, whereas the Fig 10.26b is for ITO/nanofibers of P3HT-10% functionalized Gd metallofullerene/Al bulk-hetero-junction photovoltaic devices. By choosing a proper functional group on the acceptor material, a fairly good amount of absorption in UV, visible and NIR region could be achieved which leads to harvest more photons from sunlight and makes this material potentially more efficient for photovoltaic application [111-113].

10.6

Conclusions

Improving the charge separation in nanostructured assemblies of donor-acceptor materials could aid in the development of next generation light energy harvesting devices. Development of strategies to organize ordered assemblies of two complimentary components on electrode surfaces will be the key for improving the performance

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of nanostructured organic solar cells. The focus of this chapter is to highlight the role of chemistry by thorough understanding of the structure-property relationships of the tailored made materials for the resurgence of OPV. In true sense OPV is the culmination of many scientific and technological arenas and hence the concerted efforts in the crossover of different professional connoisseurs are required to strive OPV as a technology for surpassing the competing inorganic technologies. The architecture of OPV and device physics remain the most investigated realms in this young scientific field. To gain quick impact within the community, a lack of accuracy has been reported by many experts in measuring, reporting, and reviewing OPV properties and performances. The unrealistic and scientifically questionable claims and the record efficiencies create scepticism and mistrust of the field, leading the community into an endless and dangerous tendency to outbid the last report. A set of protocols have been formulated to measure the efficiency of solar cell under simulated sunlight (air mass 1.5). For authoritative and reliable results, international institute like Fraunhofer Institute for Solar Energy Systems (ISE) in Germany, National Renewable Energy Laboratory (NREL) in USA and National Institute of Advanced Industrial Science and Technology (AIST) in Japan etc. deliver certificates. The coherent and systematic studies of the all facets of OPV will enable this field to grow and emerge as the most promising candidate to solve the planet's energy need.

Acknowledgements The authors gratefully acknowledge the help from The Centre for Genomic Application (TCGA), New Delhi for recording MALDI spectra and Central Drug Research Institute (CDRI), Lucknow for kindly allowing us to record the FAB mass spectra respectively. We also thank Director, National Physical Laboratory (NPL), New Delhi for permitting us to record the XPS and AFM images. We thank Prof. J. N. Moorthy and Prof. Sabyasachi Sarkar, Department of Chemistry, Indian Institute of Technology (ΙΓΤ), Kanpur for helping us to record FT-NMR and FT-IR spectra, respectively. Authors are thankful to Prof. Ashutosh Sharma and Mr. Alok Srivastava, DST Unit of Nanoscience, IIT, Kanpur for the EDX experiments. We acknowledge the help from Mr. Amit Saraiya and the other technical staff members of Nanoscience and Technology Division, DMSRDE for simulation of XPS spectra, resistance measurements and recording the UV-vis spectra. We would like to thank Prof. Asima Pradhan, Department

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of Physics, ΙΓΓ, Kanpur for kindly permitting us to record the photoluminescence spectra in her laboratory. Babita thanks the Defence Research and Development Organization (DRDO), New Delhi for junior research fellowship. Authors thank Prof. Vikram Kumar, Department of Physics, ΠΤ, Delhi, Prof N. Sathyamurthy, Department of Chemistry, IIT, Kanpur and Indian Institute of Science Education and Research (USER) Mohali, Chandigarh, Prof. A. J. Pal, Solid State Physics, Indian Association for the Cultivation of Science (IACS), Kolkata and Dr. Kanik Ram, former group head, Nanoscience and Technology Division, DMSRDE, Kanpur for helpful discussions and suggestions. Authors are grateful to the Director, DMSRDE, Kanpur to permit using our experimental findings.

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11 Advancement in Cellulose Based Bio-plastics for Biomedicals S. K. Shukla Department of Polymer Science, Bhaskaracharya College of Applied Science, University of Delhi, India

Abstract

Bio-plastics have emerged as promising materials due to their smart behavior and range of applications in the diverse fields of science and technology. The wide spectrums of biopolymers and their derivatives are used as bio-plastics with well-focused properties suitable for many tasks. Among them, cellulose-based plastics posses a unique position because of their biodegradability and their wide occurrence around the world. The chemical engineering steps is part of the essential approach to optimize the properties in order to generate the unique characteristics, such as crystallinty, functionality, compatibility, and process ability. However, the specialty of bio-plastics requires a highly optimized chemical structure, as well as dimensions with defined uncertainty. The present compilation is an attempt to synchronize the various engineering routes and their reported and expected properties, with the objective of establishing a bridge among the chemical engineering, properties, and application. Keywords: Bio-plastics, cellulose, co-polymerization, melt mixing, nanocomposite, enteric coating, tissue engineering, organ implant, molecular recognition

11.1 Introduction The exponential g r o w t h of the h u m a n population has caused the accumulation of h u g e amounts of non-degradable w a s t e materials across world a n d caused rigorous environmental challenges [1-2]. This changed the living conditions in the biosphere dramatically, in Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (467-486) © Scrivener Publishing LLC

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such a way that the presence of non-biodegradable residues is affecting the potential survival of many species. For this reason scientists and engineers are compelled to replace petroleum based plastics from biodegradable plastics. It is organic plastics or a form of plastics derived from renewable biomass sources, such as vegetable oil, corn starch, pea starch, etc [3^1]. Bioplastics serves as traditional plastics when in use but completely biodegradable within a composting cycle. Hyatt brothers (USA) opened their first factory to manufacture celluloid in 1869, and open the age of thermobioplastics. However till date only limited bioplastics such as celluloses, fats, lignin etc are in use. While many leading automobile industries has announced to replace petroleum based plastics from bioplastics in scheduled time. The major challenges in the use of bioplastics are optimization in their processing condition, environmental resistance, impact strength etc. These parameters can be optimized either by adding additives or employed chemical engineering routes. The later one is most fundamental and able to modify the intrinsic polymeric structure including chains, crystalline structure and spatial arrangement responsible for development of various properties. In general bioplastics can be classified as Table 11.1. Presently, bioplastics are considered as new generation of materials, able to significantly reduce the environmental impact in terms of energy consumption and green-house effect in specific applications. Today cellulose based bio-plastics are used in some specific industrial applications such as composting bags, fast food service ware (cups, cutlery, plates, straws, etc.), soluble foams for industrial packaging, wrapping film, laminated paper, food containers, nursery pots, plant labels, in medical field. Many new sectors are growing outside biodegradability, driven by improved technical performances of bioplastics versus traditional materials, as in the case of biofillers for tires. Bioplastics from renewable origin constitute a slot of market, which requires high efforts in the material development and economic breakthroughs achieved in the last few years. Which opens new possibilities for products in the mass markets in packaging applications, automobile and biomedical field. The most disadvantageous feature of bio-plastics, need to be modified are self degradation, mechanical properties, thermal stability, and processing conditions. This required to be essentially optimized by suitable chemical engineering, either by adding additives or co-polymerization. The many efforts have been also made through physical treatment such as thermal annealing or exposure to electromagnetic radiation.

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Table 11.1 Classification of bioplastics. Types

Examples

Biosynthetic origin

Natural bioplastics produced by microorganism such as PHBs and aliphatic PHAa

Chemical nature

Synthetic bioplastics obtained by chemical synthesis resembled to natural ones. Bioplastics containing aliphatic fatty acids with double or triple bonds

Monomers size

Containing short chain length (C3-C5 monomers Medium chain length ( C6-C14) Long Chain length > 14

Number of monomers

Homoplastics (single type monomers present Hetroplastics ( t w o or more than two monomers presents)

Types of monomers accumulated by microbes

Unique ( single bioplastics), more than one (mixed bioplastics)

In recent times most of the world population is proffered the use biodegradable materials. A summary about preferential demand of materials is presented in Fig. 11.1 and a representative bio-plastic fabrication scheme in Fig. 11.2. The present article is an attempt to compile the important adopted chemical routes for major modulation of plasticity in bio-polymer in order to develop appropriate properties along with their suitable biomedical applications. The bio-plastic with improved properties are also reported along with some critical challenges for scientific community. The important ones are complied here in following sections: 11.2 11.2.1

Plasticity M o d u l a t i o n Composite Formation

Composing of two polymers leads appropriate plasticity and widely used in developing bio-plastics. Cellulose acetate (CA) bioplastic, are developed by citrate based plasticizer and organically

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^-eferctia consumption

Biodegradable Conpostable

Biodegradable

52%

Recyclable

46%

None

Compcstable

" Consumer preference ndustry preference

Figure 11.1 Current demand of materials.

Plants product Enzymatic degradation

Sugar Fermentation

Lactic acid Purification

Pure lactic acid Polymerization Polylactic acid Processing

Plastic Figure 11.2 Flow chart for the product of plastic from sweet potato.

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modified clay nano fillers. Further used as a substitute the existing petroleum-based polypropylene/thermoplastic olefins (PP/TPO) based composites for automotive applications [5]. The chemical composition of ingredient (amount of additives), and processing conditions required to be thoroughly optimized to make exfoliated a n d / o r intercalated clay/CA composite for use as a matrix in fabricating CA nanocomposites. By adding a modified montmorillonite (MMT-OH) into plasticized CA matrix during high-shear force melt extrusion, the exfoliated a n d / o r intercalated clays inside continuous matrix was formed. The composite formed in optimized processing conditions (temperature, shear force and time), under the processing (extrusion followed by compression molding vs. extrusion followed by injection molding). The thermal (heat deflection temperature - HDT and coefficient of thermal expansion - CTE ), morphological (transmission electron microscopy - TEM and X-ray diffraction - XRD), and mechanical (tensile and impact strength) properties were found improved. The composite is also termed as green composite because the use of renewable and environmentally benign materials such as pure cellulose acetate, citrate based plasticizer, and organically modified clay nanofillers. The improvements in tensile strength by approximately 38%, tensile modulus by approximately 33%, CTE, and HDT is possible after adding (5 wt%) clay to CA plastic matrix. The addition of a small amount of compatibilizer is resulted to enhance miscibility of matrix and clay nanofillers and further resulted nanocomposites with improveed mechanical and thermal properties of the for different industrial applications. Like cellulose, poly lactic acid (PLA) biocomposites with cellulose fibres can be processed by using combined moulding technology ( two-step extrusion coating process and consecutively injection moulding) [6]. The addition of 30 wt% of man-made cellulose, causes the charpy impact strength at ambient temperature increased by 3.60 times, compared to reinforced PLA. Tensile strength also increased by factor 1.45 and stiffness by approx. 1.75. The reinforcing of fibres (30 wt%) enhanced both E-modulus and tensile strength by factor 2.40 and 1.20, respectively. The Charpy -notch impact resistance of PLA could be improved by factor 2.4. Further mechanical properties of PLA composites in contrast to polypropylene made in comparable processing conditions show much better results. However, the exceptions are Charpy impact strength and tensile/bending elongation, where PP composites

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show slightly better values than PLA composites caused by their much higher ductility need to overcome. The dynamic mechanical analysis showed that incorporation of fibrous reinforcement causes a decrease in polymer chains mobility. The storage modulus of PLA-based composites remained much higher than of unreinforced PLA. The glass transition temperature derived from loss modulus characterizes with a slight shift to higher temperatures, if compared to matrix polymer. 11.2.2

Microbial Bioplastics

Bioplastics synthesized by microorganisms (or part of them) under different environmental conditions is an important family of biomaterials. They are polyesters, produced by a range of microbes, cultured under different nutrient and environmental conditions. The formation bioplastics with microbes are special type of biomaterial, till date, more than 160 different polyesters with plastic properties have been synthesized and this number is still growing exponentially by means of genetic and metabolic engineering techniques [7-8]. These polymers are usually built from hydroxy-acyl-CoA derivatives via different metabolic pathways. Depending on their microbial origin, bioplastics differ in their monomer composition, macromolecular structure and physical properties. Most of them are biodegradable and biocompatible, which makes them extremely interesting from the biotechnological biomedical application point of view. The polymers, which are usually lipid in nature, are accumulated as storage materials (in the form of mobile, amorphous, liquid granules), allowing microbial survival under stress conditions. The number and size of the granules, the monomer composition, macromolecular structure and physico-chemical properties vary, depending on the producer organism. They can be observed intracellularly as light-refracting granules or as electronlucent bodies that, in overproducing mutants, cause a striking alteration of the bacterial shape. The large number of bioplastics are enzymatically polymerized by the condensation of the function group present in a monomeric units. Further, the specific properties needed for the applications of bioplastics need to be optimized have been widely discussed by Angelova [9-10]. However, biomaterial design requirements are mostly dependent on the specific device application, with the key to materials selection being an understanding of the functional

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characteristics required of the device. A flow chart of different types of bio plastics with appropriates applicationa are shown in Fig 11.3. Yu et.al [ 11] prepared bioplastic by waste material such as malt wastes (barley), obtained from a beer brewery, employing fermentation method. The details outline of used method with condition details are given in Fig 11.4. Further, the different carbohydrates including sucrose, lactic acid, butyric acid, valeric acid, in various combinations, were utilized as the carbon sources for the production of bioplastics by using microbial routes. As the first step in pursuit of eventual usage of industrial food wastewater as nutrients for microorganisms to synthesize bioplastics, the usage of malt wastes from a beer brewery plant as the carbon sources for the production of bioplastics by microorganisms. The specific polymer production yield by A. Latus DSM 1124

Commercial polymers

'Medical-grade' polymers

New polymer compositions Novel surface-modification and-characterization techniques

Special fabrication techniques

Commercial process techniques

New physical forms Passive implants and devices

~Z~

New designs and fabrication techniques

New biomolecules and recongnition sequences Macromolecular self-assemblies Mammalian cells

Improved healing and functioning of implants New biosensors, diagnostic assays, bioseparations and bioprocesses

New biocompatible and biofunctional implants and devices

Targeted peptide drug-delivery systems

'Stimulus-responsive' drug-delivery systems and artificial organs

Regeneration of tissues and organs using biodegradable and biofunctional scaffolds

Figure 11.3 Representative scheme about importance of bio polymeric materials into the 21st century.

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Prepared mait media + culture

Chemical & physical analysis

Temp. Fermenter pH Stirring D.O.

«- Oven dry

·*- Extraction «- Freeze dry

Growth phase

1 Production phase

Centrifugation

Figure 11.4 Flow chart of fermentation for bioplastics production.

increased to 70% polymer/cell (g/g) and 32 g/L cell dry wt, using malt wastes as the carbon source. The results of these experiments indicated that, with the use of different types of food wastes as the C source, in different polyhydroxyalkanoate copolymers could be produced with distinct polymer properties. 11.2.3

Copolymerization

Copolymerisation is a technique adopted to hybridized the polymer properties. Wide number of biodegradable polymers are obtained from copolymerization of synthetic monomers [12-13], and can be distinguished according to the chemical structures. The some examples are polycaprolactones, (PCL) polyesteramides, aliphatic or aromatic copolyesters. All these polyesters are soft at room temperature. The Poly(e-caprolactone) is obtained by ring opening polymerisation of e-caprolactone in the presence of aluminium isopropoxide which is widely used as a PVC solid plasticizer or for polyurethane technology. But, it also finds also some application based on its biodegradable character in biomedical domains such as controlled release of drugs, soft compostable packaging. The chemical structure and the properties of the polyester, shows a very low glass transition temperature (-61 °C) and low melting point (65°C), which could be a handicap in some applications. Therefore, PCL is generally blended or modified by copolymerisation, and crosslinked to generate requisite properties. Compared to aliphatic copolyesters, aromatic copolyesters are often based on terephtalic diacid show the unique chemical structure and

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the properties [14]. The increase of terephthalic acid content tends to decrease the degradation rate, while aromatic like aliphatic copolyesters degrade totally in microorganisms environment (compost). Biodegradation has been analysed and concluded that there is no indication for an environmental risk (ecotoxicity) when aliphaticaromatic copolyesters are introduced into composting processes. Currently, natural fiber-reinforced based thermoplastics are envisioned as an emerging new class of benign composite materials. The prospective benefits of these composites are derived from the properties of the natural fibers such as renewability, biodegradability, low cost, low density, wide acceptable specific mechanical properties, ease of separation, and carbon dioxide sequestration. The fabrication of green composite from cellulose fibers (mixture of newspaper, magazine, and kraft paper fibers) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) by the melt mixing technique [15]. The composites were processed by extrusion followed by injection molding process. The processing was performed in specially designed microcompounding instrument shown in Fig 11.5. Further, incorporation of recycled cellulose fibers (RCF) had brought considerable improvement in the bioplastic properties such as tensile and storage moduli and heat deflection temperature (HDT). The coefficient of linear thermal expansion (CLTE) value of polyhydroxybutyrate co-hydroxyvalerates (PHBV) was reduced significantly upon reinforcement with RCF. The thermomechanical properties of polyhydroxybutyrate co-hydroxyvalerates (PHBV) Starch granules + water + polyols

Piasticized starch Fragmentation

Melting Disruption r

Plasticization

. ,. n Degradation

Figure 11.5 Schematic diagram of micro-compounding instrument.

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based composite had competed favorably with PP-based composite at similar fiber loadings. The lower processing temperature and the shorter cycle time in processing of PHBV-based composite were beneficial in avoiding the attrition and degradation of the cellulosic fibers. The possibilities of high fiber loading in the PHBV-based composites reduces the amount of expansive PHBV resin in the composite formulations Polyhydroxyalkanoates based bio-plastic produced in different ways, chemically or biologically, or by fermentation from feedstock. This family of bioplastics comprises mainly of homopolymer, polyhydroxybutyrate (PHB), and different copolyesters, polyhydroxybutyrate co-hydroxyalkanoates such as polyhydroxybutyrate co-hydroxyvalerates (PHBV), or polyhydroxybutyrate co-hydroxyhexanoate (PHBHx), polyhydroxybutyrate co-hydroxyoctonoate (PHBO) and polyhydroxybutyrate co-hydroxyoctadecanoate (PHBOd). PHB is a natural and highly crystalline polyester (80%) based polymer with a high melting point, (173-180°C), compared to the other biodegradable polyesters. The glass transition temperature (Tg) is around 5°C. It is an intracellular storage product of bacteria and algae. After fermentation, PHB can be obtained by solvent extraction. Monsanto has developed genetically modification in plants to make them produce small quantities of PHB. The homopolymer shows a narrow window for the process condition. To ease the transformation, PHB can be plasticized with citrate ester, but the corresponding copolymer (PHBV) is more adapted for the process. On the other hand, mixed cultures from activated sludge could utilize soya waste to produce biopolymers of copolymer composition of 79% PHB and 21% PHV (endowed with physical and thermoplastics properties different than the other bioplastics produced). Thus, specific biopolymers with copolymers of desirable physical and mechanical properties (such as flexibility, tensile strength, and melting viscosity) can be formulated in fermentation by appropriate selection of substrates (or combination of substrates), and the type of PHA-producing microorganisms. The usage of less valuable food wastes as carbon source in fermentation would tremendously reduces the cost of the production of bioplastics and minimizes waste production, and at the same time produce environmentally friendly bioplastics. 11.2.4

Melt Mixing and Physical Annealing

Novel "green" composites plastics were successfully fabricated from recycled cellulose fibers (RCF) and a bacterial polyester,

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poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) by melt mixing technique. Different weight contents (15%, 30%, and 40%) of the fibers were incorporated in the PHBV matrix by Bhardwaj et ah [15]. The effect of the fiber weight contents on the thermal, mechanical, and dynamic-mechanical thermal properties of PHBV was found comparative with RCF-reinforced polypropylene (PP) composites. The tensile and storage moduli of the PHBV-based composites were improved by 220% and 190%, respectively, by reinforcement with 40 wt% RCF. Halpin-Tsai and Tsai-Pagano's equations were suits for the theoretical modeling of the tensile modulus of PHBV-based composites. The heat deflection temperature (HDT) of the PHBV based composites was increased from 105 to 131°C, while the coefficient of linear thermal expansion (CLTE) value was reduced by 70% upon reinforcement with 40 wt% RCF. The PHBV-based composites had also shown better tensile and storage moduli and lower CLTE values than PP-based composites. The use of a chemical compatibilizer is an efficient means to achieve adhesion or tackiness. Maleic anhydride-grafted-cellulose acetate butyrate (CAB-g-MA) is one suitable compatibilizer. Which is used in biocomposite fabrication, by utilizing a twin-screw reactive extrusion process in the presence of a free radical initiator like (2,5-dimethyl-2,5-di(tert- butylperoxy)hexane). The unique feature of the process is solvent-free nature for grafting of maleic anhydride onto CAB, without hydroxyl group protection [16]. The results revealed that by adding approximately 10 wt% of CAB-g-MA into a plasticized cellulose acetate butyrate (TEB)-industrial hemp fiber biocomposites system, an improvement in tensile strength by 20% and in tensile modulus by (45%) were reported. These findings are the future promising that they pave the way for future studies involving the use of as a suitable compatibilizer for development of biocomposites. In another report, [17] thermoplastic starch (TPS) materials have been prepared by kneading, extrusion, compression and injection moulding of several native starches with the addition of glycerol as a plasticizer. The two types of crystallinity developed in TPS directly due to processing: (i) residual crystallinity: native A-, B- or C-type crystallinity caused by incomplete melting of starch during processing; (ii) processing-induced crystallinity: amylose VH , VA_ or EH type crystallinity which is formed during thermomechanical processing. The amount of residual crystallinity is related to processing conditions

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like processing temperature or applied shear stress. The composition of the mixture indirectly influences the amount of residual crystallinity. Lower amounts of glycerol caused a reduction in residual crystallinity. This effect is attributed to the increase in melt viscosity at decreasing plasticizer content, which causes an enhancement of shear stress on the melt. It has been found that composition and processing parameters of bioplastics are interrelated. The process induced crystallinity, also influenced by processing parameters, and is caused by the fast recrystallization of amylose into single-helical structures. The other parameters such as increasing the screw speed during extrusion or increasing residence time during kneading caused direct affects on the crystallinity. The addition of complexing agents like calcium stearate or the presence of lysophospholipids caused the crystallization into different types of structures. In waxy starches, containing no amylose, obviously no V- or E-type crystallinity is reported.

11.2.5

Nanocomposites

The nano dimension technology incorporate unique opportunity for bio-plastics for different application. Cellulose whiskers have a strong tendency for aggregation because presence of strongly interacting surface hydroxyl groups. If cellulose whiskers are decorated with a small number of sulfate groups with sulfuric acid. Good dispersion of these negatively charged cellulose whiskers can be achieved when the whisker-whisker hydrogen bonding interactions are "switched off" by competitive binding with a hydrogen-bondforming solvent. Upon solvent removal, the interactions between the whiskers are "switched on" and they assemble into a percolating matrix network. These strong hydrogen bonding whiskers have led to a significant enhancement in the tensile storage modulus (Ε') of tunicate whiskers composite. Further, exposure to water, acts as a chemical regulator and switches the hydrogen bonding between the whiskers within the polymer matrix "off". When these nanocomposites were dried, the original stiffness was restored, which proves the reversibility of the composite [18]. Which has been combined the switching mechanism with a chemically influenced thermal transition and discovered that nanocomposites based on poly(vinyl acetate) (PVAc) and cellulose whiskers display such a "dual" responsive behavior. The exposure to water or artificial cerebrospinal fluid (ACSF) at 37°C, diffusion of water into the nanocomposites not only disengages the whisker matrix, but also plasticization by

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water decrease the glass transition temperature (T) of PVAc and the nanocomposites from -60 to ~20°C, i.e. from above to below physiological temperature. This approach has provided materials, with a dramatic mechanical contrast of three orders of magnitude between the dry state at room temperature and the water (swollen state) at 37°C. The temperature, the water or ACSF take-up was very significant (-70-90% w / w for 16.5% v / v tunicate whiskers). With the objective of lowering the "soft" modulus of nanocomposites, to reduce the level of swelling, and to explore the potential use of cellulose whiskers from a more accessible bio-source than tunicates. The exploration of stimuli responsive nanocomposites using cotton cellulose whiskers (CCW) is reported to exhibit properties for suitable goals. In another attempt an environmentally friendly polymer hybrids ( biodegradable thermoplastic starch (TPS)/clay nanocomposites ) were prepared through melt intercalation method. TPS was taken from natural source (potato starch) by gelatinizing and plasticizing it with water and glycerol. The dispersion of the silicate layers in the TPS hybrids observed and found that the TPS/Cloisite Na+ nanocomposites showed higher tensile strength and thermal stability, better barrier properties to water vapor than the TPS. Which is due to the formation of the intercalated nanostructure [19]. Nanocomposites of Bacterial Cellulose - hydroxyapatite (BC-Hap) was produced by introducing the mineral phase into the bacteria culture medium during the formation of cellulose fibrils. For this purpose, carboxy methyl cellulose(CMC) 1% w / v was used to suspend the Hap nanoparticles by controlling the viscosity of the culture medium. By using CMC, the average diameter of cellulose fibres is almost 50% lower than the average diameter of the unmodified BC fibres. This result was attributed to the effect that some polysaccharides cause in the fibre assembly during the cellulose biosynthesis. The fibre orientation distributions of BC and BC-CMC images demonstrated that freeze-dried BC consists of randomly assembled nanofibres forming a network. Also, the pore size of BC increases 47.8% when CMC is added to the culture media. Image analysis of the Hap particles before and after the synthesis of the nanocomposites suggest that an amount of Hap powders (22%) did not get included in the final nanocomposites.

11.3 Applications The rapid development of sustained and controlled biomedical technology is being driven effectively by extracting maximum benefits

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from potential material in pharmaceutical applications. A variety of biomedical applications, make use of cellulose biodegradable polymers. Some of the applications employed cellulose based materials are components of scaffolds for bone regeneration, artificial blood vessel, temporary skin substitutes (Biofills), haemodialysis membranes, and controlled drug release systems. In chronic wound dressing, it provides a moist environment for optimal healing due to its ability to absorb water directly and more effectively. It has been recomended to be used as a dialysis membrane for bioartificial pancreas and immunoisolation of islets transplantation. The cellulose based materials have also been also projected for incorporation into orthopedic implants, as a bone replacements and for controlled drug delivery. Starch and cellulose composites are better biocompatible, and this potential has been utilized in polymers designed for bone cement and controlled drug delivery [20]. The entire biomedical applications are difficult compile in the present chapter, but an honest attempt have been made to reproduce the significant research findings of researchers as a per suitable data available. 11.3.1

Enteric Coatings

The objective of entering coating is to permit an oral dosages form (tablets or capsule) to survive in appropriate environment including atmosphere to body organs. The reason for this approach include protecting a hydrolytically or enzymetically fragile drug from highly acidic enzyme rich gastric juice in stomach. The polymer chain are fully insoluble in low pH but become soluble at higher pH in small intestine [21]. Many materials have been examined and utilized for enteric coating, like cellulose acetate phthalate (CAP) applied as thin coating from organic solution fulfills ideal coating property. Another cellulose derivatives reported very useful as enteric coating material is Hydroxypropoylmethyl cellulose phthalate (HPMCP). The film forming ability is important aspects of plasticity and is depends on several factors including molecular weight distribution. The chemical treatment like acyl substitution addition of additives plays great role in optimizing film forming ability. 11.3.2

Sustained Release

The material matrix with dispersing or embedding properties are commonly used as very effective media for oral sustained release,

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for active or highly water soluble drugs. The simplest formula for preparing matrix through direct compression of well mixed formula containing ingredients, both hydrophilic and hydrophobic polymeric material have employed for preparation of direct matrix compression method. Cellulose acetate with ~2.5 degree of substitution of acetyl is an effective material for drug release of acetaminophen tablet. The mixed matrix of cellulose acetate and calcium phosphate found appropriate for release theophyline, dyphilline and proxyphylline [22]. The major criteria for control drug release is molecular weight, plasticity level and solubility. Generally a zero order release doses used commercially and is so called elementary osmotic pump (O.P.) delivery system. The O.P. delivery system is important technology due to its function independently of specific properties of drugs but some time active delivery is required in O.P. than for diffusion controlled. Chemical energy drives O.P. deliver system and the basic device is consisting of active salt (osmotic agent) and a single isotropic core separated by movable partition. The core should be fully or partially encapsulated by a semi permeable membrane and the water permeates the membrane and dissolves the osmotic agent resulting the water activity gradient across the membrane. The constant rate of release persist until all solid material in the device has been expelled. The controlled release also used to stabilize polymer and rubber articles. Evan et.al extend the controlled release of cellulose acetate coatings to polymer stabilization [23]. By compounding the microencapsulated antidegradants into rubber articles extends the protection of rubber from oxygen and ozone. 11.3.3

Tissue Engineering

The fundamental premise of tissue engineering is to develop tissue substitutes to restore or improve the function of diseased or damaged human tissues. Many biomaterials have been studied as scaffolds, in which the cells a n d / o r growth factors can be seeded, cultured, and then, implanted to induce and direct the growth of new, healthy tissue. The primary function of a scaffold is tissue conduction. Therefore, it must allow cell attachment, migration onto or within the scaffold, cell proliferation and cell differentiation. It must also provide an environment where the cells conserve their phenotype behavior. The successful development of tissue engineering scaffolds requires proper

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substrates for cell survival and differentiation. The attachment of cells to the biomedical materials can be improved by adhesion. These molecules, present in the extracellular matrix proteins, regulate the adhesion, migration and growth of cells, by binding to the receptors sites located on the outer cellular membranes [24-26]. Bacterial cellulose is a material with excellent biocompatibility and mechanical properties, thus holding great potential for biomedical applications. The functionalize bacterial cellulose, developed through recombinant proteins containing adhesion peptides conjugated with a cellulose binding-module. The use of recombinant proteins containing a cellulose-binding module(CBM) domain, exhibiting high affinity and specificity for cellulose surfaces, allows the control on the interaction of this material with cells. The CBM may virtually be combined to any biologically active protein for the modification of cellulose-based materials, suitable in vitro or in vivo applications. The recombinant proteins containing the RGD9Arg-Gly-Asp sequences were cloned and successfully expressed in fusion with a specific family of CBM (Clostridium thermocellum in E. coli expression system). The recombinant proteins containing the adhesion peptide are able to promote adhesion and spreading of the cells. Thus proteins containing the sequence RGD showed a stronger effect on fibroblast cells. The effect of different adhesive sequences depend on the material where they are adsorbed because of conformation effects [27]. Different type of micro/nano particles have been suspended by varying the viscosity of the culture media in Bacterial cellulose based hydroxyapatite nanocomposite. These nano size solid particles can be added to the medium for the formation of nanocomposites with BC to produce modified form nanocomposite. Furthermore, the biocompatibility of the materials and the bioactivity are factors that make this composite material suitable for potential biomedical applications like tissue engineering [28]. 11.3.4

Sensors and Recognition

Cellulose-based electro-active material [29] has been reported as a smart material having merits of light weight, biodegradability, large displacement output and low actuation voltage under dry conditions. The cellulose actuator has shown limited bending displacement and force compared with the other smart materials.

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Because plant cellulose (PC) synthesized by various chemical reactions, so it has many impurities and less crystallinity. Also, the pure cellulose actuators cannot be activated in wet or hydrated environments due to high water uptake by hydrophilicity of the pure cellulose. But, if we want to use them to implantable active biomedical devices, the activation in hydrated condition should be achieved first. Bacterial cellulose based (an electro-active biopolymer) actuator was developed and activated in the wet environment for biomedical applications. When bacterial cellulose (BC) membranes with biocompatibility and biodegradability were electrically activated under fully hydrated condition, the bending deformations were observed for both step and harmonic electrical input signals. The chemical, thermal, mechanical and electrical properties of the bacterial cellulose were greatly improved through the chemical (Lithium Chloride) treatment. The bending actuations of the LiCl treated bacterial cellulose actuator under both step and harmonic electrical inputs show much larger tip displacements and faster response time than those of the pristine BC actuator because of lower stiffness and higher ionic exchange capacity and proper control of crystallinity. The results showed the potential of the bacterial cellulose as electro-active biopolymers in wet environments for implantable biomedical devices and confirmed that the LiCl treated bacterial cellulose can be adjusted to get better actuation performance by controlling the crystallinity and stiffness of the BC [30]. Further, polysaccharide cross-bridging protein comprising a cellulose-binding domain fused to a starch-binding domain. The protein-based reagent was shown to have cross-bridging ability in different model systems composed of insoluble or soluble starch and cellulose. The cellulose binding domains (CBD ) based reagents have the potential to modify the physical and the chemical properties of cellulose containing materials [31]. The ability to produce proteins that contain different carbohydrate-binding modules paves the way to introduce biological recognition to material science.

11.4 Future Challenges Plastics are very rugged, can be processed in many ways and are also lighter and cheaper than most other materials. They are the first choice in many industrial and commercial applications.

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Correspondingly, consumption is increasing continuously, from 60 million tonnes world-wide in 1980 to an estimated 260 million tonnes in the year 2010. Bioplastics is the designation for innovative plastics manufactured from regenerative raw materials. They can replace the previously used fossil plastics and plastic materials in many applications. Creative scientists and technicians are currently engaged in adapting them to conventional applications. Genetic modification (GM) is also a challenge for the bioplastics industry [32]. None of the currently available bioplastics can be considered first generation products but require the use of GM crops. Thus looking further ahead, some of the second generation bioplastics manufacturing technologies under development employ the "plant factory" model, using genetically modified crops or genetically modified bacteria to optimise efficiency [33]. Currently, fossil fuel is still used as an energy source during the production process. This has raised questions by some regarding how much fossil fuel is actually saved by manufacturing bioplastics. Only a few processes have emerged that actually use less energy in the production process. Therefore, researchers are still working on refining the processes used in order to make bioplastics viable alternatives to petrochemical plastics. Energy use is not the only concern when it comes to biopolymers and bioplastics. There are also concerns about how to balance the need to grow plants for food, and for use as raw materials. Thus, agricultural space needs to be shared. Researchers are looking into creating a plant that can be used for food, but also as feedstock for plastic production. Scientists are attempting to genetically engineer corn to contain the bacterial enzyme responsible for plastic production. Eventually, they are hoping to create the plant in a way which would restrict the plastic production to the stem, and leaves of the plant. This would leave the edible part of the corn plastic free. The edible part of the corn would be used as food, or as livestock feed. The plastic would be removed from the remaining part of the corn plant. Currently, the main limitations for the bulk production of bioplastics are its high production and recovery costs. However, genetic and metabolic engineering has allowed biosynthesis in several recombinant organisms (other bacteria, yeasts or transgenic plants), by improving the yields of production and reducing overall costs. In conclusion there are three important limitations in the bulk production of bioplastics: a. the special growth conditions required for the synthesis of these compounds (usually unbalanced nutrient conditions that cause slow growth).

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b. the difficulty involved in synthesising them from inexpensive precursors. c. the high cost of their recovery.

11.5 Conclusions Cellulose based bioplastics has brought a revolution in the field of material science. These bioplastics actually encompass many advantageous properties over fossil plastic like biodegradability, biocompatibility, solubility, responsiveness suitable for wide range of biomedical applications. The cellulose based bioplastics optimized and prepared by composite formation, copolymerization, microbial process, melt mixing and by using nano tools. There are different types of characterization techniques available for physical and chemical characterisation these new generation materials. There are many applications of bioplastics in the development biomedical field like implant materials, tissue engineering, pharamacy, drug delivery, sensors and molecular recognition. In more specified health care materials, biomaterials for orthopaedic, and cardiovascular applications, tissue repair, biosensors, and controlled drug delivery and many features are likely to be developed and applied in the near future, all of which will depend on the development of new generation of bioplastics. In the science of cellulose based bioplastic, there are many challenges and opportunities ahead of scientific community. For example, improving efficiency is a major concern for the production of plastics and biomaterials, efficiency, cost, recovery and goal to prepare bioplastic with a high selectivity properties to fulfil industrial and scientific demands.

References 1. Bioplastics Online. Available from: http://www.eplastics.pl [accessed 29.01.07] [in Polish] 2. a News on Plastic Industry. Available from: http://www.plastemart.com [accessed 31.01.07]. b. A. P. Gupta, Vijai Kumar, Manjari Sharma, and S. K. Shukla, Polymer-Plastics Technology and Engineering, Vol.48, pp. 587-594,2009. 3. Hong Chua, H.F. Peter, F. Yu, and C K. Ma. Applied Biochemistry and Biotechnology (Humana Press Inc.) 1999,78: 389-399. 4. A. Steinbiichel and B. Fiichstenbusch, Trends Biotechnol. Vol.16, pp 419-427,1998. 5. C. Arief C. Wibowo, Manjusri Misra, Hwan-Man Park, Lawrence T. Drzal, Richard Schalek,l, Amar K. Mohanty, Composites: Part A, Vol. 37, pp. 1428-1433,2006.

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6. A. K. Bledzki, A. Jaszkiewicz, D. Scherzer, Composites: Part A,Vol. 40, pp. 404-412,2009. 7. J. M Luengo, B. Garci, A. Sandoval, G. Naharroy and E. R Olivera, Current Opinion in Microbiology, Vol. 6, pp.251-260,2003. 8. Bioplastics Online. Available from: http://www.eplastics. pi [accessed 23.09.06] [in Polish]. 9. N.Angelova, D.Hunkeler. Trends Biotechnol Vol. 17, pp.409-421,1999. 10. S. Domenek, P. Feuilloley, J. Gratraud, M-H Morel and S. Guilbert, Chemosphere, Vol.54, pp. 551-559,2004. 11. P. H. Yu, H. Chua, A. L. Huang, W. Lo, and G. Q. Chen, Applied Biochemistry and Biotechnology, Vol.70-72, pp. 603-614,1998. 12. A. Tiwari and S.K.Shukla, eXPRESS Polymer Letters, Vol. 3(9), pp.553-559,2003. 13. M. Okada, Progress in Polymer Sei. Vol. 27, pp 87-133,2002. 14. R.J. Muller, U. Witt, E. Rantze, W.D. Deckwer, Polymer Degradation and Stability, Vol. 59, pp.203-208,1998; http://www.biodegnet/bioplastics. 15. R. Bhardwaj, A. K. Mohanty, L. T. Drzal, F. Pourboghrat, and M. Misra, Biomacromolecules, Vol. 7, pp. 2044-2051,2006. 16. A. C. Wibowo, S. M. Desai, A. K. Mohanty, L. T. Drzal, M. Misra, Macromol. Mater. Eng. Vol. 291, pp. 90-95,2006. 17. J. Jeroen. G. van Soest, S. H. D. HuUeman, D. de Wit and J. F. G. Vliegenthart, Industrial Crops and Products, Vol.5, pp. 11-22,1996. 18. K. Shanmuganathan, J. R. Capadona, S. J. Rowan and C. Weder /. Mater. Chem., Vol.20, pp. 180-186,2010. 19. H-M Park,W-K Lee, C-Y Park, W-J Cho, C-S Ha, /. Mater. Sei. Vol. 38, pp. 909- 915,2003. 20. I Levy, T. Paldi, O. Shoseyov, Biomaterials, Vol. 25, pp. 1841-1849,2004. 21. S.H. Wu, D.M. Adams, M.W. Adams, In J.W. McGinity editor, Aqueous Polymerric Coatings For Pharamceutical Dosages Forms, 2nd ed. New York, Dekker, 1997, p 385^18. 22. S.Z. Masih, US patent 4.983.399.1991. b.L. Krowezynski, Extended Release Dosage Forms, Boca Raton H, CRC, Press, 1987, p.131. 23. L.R. Evans, D.A. Benko, J.G. Gillick, W.H. Waddell, Ruber Chem. Technol, Vol. 65, pp. 201,1992. 24. UR Goessler, K. Hormann, F. RiedelF.. IntJMol Med Vol. 13, pp.505-513,2004; 25. E.F. Plow, T.A. Haas TA, L.Zhang J. Loftus, J. W.Smith, / Biol Chem Vol. 275. PP. 21785-21788,2000. 26. A. V Flier, A. Sonnenberg, Cell Tissue Res., Vol 305, PP.285-298,2001. 27. F. K. Andrade, S. M G. Moreira, L. Domingues, F. M. P. Gama J. Biomed. Mater. Res. Part A, PP. 9-17,2009. 28. C. J. Grande, F. G. Torres, C. M. Gomez, M. C. Bano Ada Biomaterialia, vol.5, pp. 1605-1615,2009. 29. J. Kim, J.S. Yun, Z. Ounaies, Macromolecules, Vol. 39, pp. 4202-4206,2006. 30. J-H Jeon, 1-K Oh, C-D Kee, S-J Kim Sensors and Actuators B Vol.146, pp. 307-313,2010. 31. I. Levy, A. Nussinovitch, E. Shpigel, O. Shoseyov, Cellulose, Vol. 9, PP. 91-8,2002. 32. http://en.wikipedia.0rg/wiki/Bioplastic#Genetically modified bioplastics 33. http://biobasics.gc.ca/english/View.asp?x=790

PART III COMPOSITE MATERIALS

12 Intelligent Nanocomposite Hydrogels Mohammad Sirousazar and Mehrdad Kokabi Polymer Engineering Department, Faculty of Chemical Engineering, Tarbiat Modares University, Tehran, Islamic Republic of Iran

Abstract

Development of functional engineered materials possessing intelligence has been one of the most challenging tasks in research over the past decade. The term "intelligent material" refers to materials that possess three main functions, including sensing, processing, and actuating by exerting external stimuli. In recent years, intelligent nanocomposite hydrogels (INCHs), the newly born multifunctional materials of the 21s' century, have gained the serious attention of the scientific community. INCHs are soft and reinforced nanocomposite hydrogels that respond to various stimuli. They exhibit dramatic changes in their properties, e.g., swelling, network structure, or mechanical strength, in response to small variations in external stimuli such as temperature, pH, magnetic or electric fields, light, and presence of molecules. The main goal of this chapter is to review the basic science and technology of INCHs and the recent advances in this area. Additionally, the fundamental concepts of hydrogels, intelligent hydrogels, and nanocomposite hydrogels will be introduced. Keywords: Intelligent hydrogels, nanocomposite hydrogels, stimuli-responsive hydrogels, swelling behavior

12.1 Introduction Hydrogels are solid-liquid porous systems formed from crosslinked hydrophilic homopolymers, copolymers, or macromers to create three-dimensional networks [1,2]. They are insoluble and capable of holding a large amount of water or aqueous solutions ranging from 30-90 % by weight [3]. They are water swollen polymer networks, with Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (489-532) © Scrivener Publishing LLC

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a huge tendency to absorb significant amounts of water while maintaining their structure. They are also inherently soft and elastic due to their thermodynamic compatibility with aqueous solutions [1,4]. The hydrophilic nature of hydrogels is due to the presence of chemical residues such as; hydroxylic (-OH), carboxylic (-COOH), amidic (-CONH-), primary amidic (-CONH2), sulphonic (-S0 3 H), and others that can be found within the polymer backbone or as lateral chains [5]. It is possible to produce hydrogels with hydrophobic properties by blending or copolymerizing hydrophilic and hydrophobic polymers, or by producing interpenetrating polymer networks (IPN) or semi-interpenetrating polymer networks (semi-IPN) of hydrophobic and hydrophilic polymers [6]. Hydrogels resemble natural living tissue due to their high water contents and soft consistency which is similar to natural tissue. Furthermore, the high water content of hydrogels contributes to their biocompatibility [7]. In addition, they can provide stealth properties to the system as well as control over the stability, mechanical properties, and degradation profile making them desirable for biomedical applications [8]. Over the past several decades, hydrogels have attracted much scientific interest and have found diverse applications in many fields including separation and bioseparation processes, molecular filtration, chemical valves, imaging technologies, biosensors, biomedical implants, artificial organs, contact lenses, scaffolds, stem cell culture, artificial skin, wound dressings, and drug delivery devices [4, 7,9-14]. Hydrogels could be synthesized by a crosslinking reaction between polymer molecules or by a crosslinking polymerization, which is simultaneously synthesizing polymer chains and linking them concomitantly [15]. They are mainly prepared by two different crosslinking methods including chemically and physically crosslinking techniques. Chemically crosslinked hydrogels have ionic or covalent bonds produced by the simple reaction of one or more comonomers. Copolymerization, suspension polymerization, polymerization by irradiation, chemical reaction of complementary groups, and crosslinking using enzymes are some common examples. In physically crosslinked hydrogels, the crosslinked network is formed by entanglements, association bonds such as hydrogen bonds or strong van der Waals interactions between chains, or crystallites. Some physically crosslinking methods are ionic interactions, crystallization, and crosslinking by hydrophobic interactions [6,16]. Hydrogels can be classified from different viewpoints. Based on preparation method, they may be classified as; homopolymers,

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491

copolymers, multipolymers, or interpenetrating polymeric hydrogels. According to the nature of the side groups, they can be classified as neutral, anionic, cationic, or ampholytic hydrogels. Based on physical properties, they can be classified as amorphous, semicrystalline, hydrogen-bonded, supermolecular structures and hydrocolloidal aggregates. Finally, according to their mechanical and network characteristics, they can be classified as affine or phantom networks [6, 7,16]. Intelligent or smart hydrogels are important kinds of hydrogels that have the ability to react to changes in their external environment in an intelligent manner. The initial finding about the smartness of polymers was reported by Kuhn on the muscle-like movement of hydrogels and the possibility of transforming chemical energy into mechanical work [17]. The essential development on the intelligent hydrogels was made in 1980 by Tanaka who found the volume phase transition of the partially hydrolyzed polyacrylamide (PAAm) hydrogel [18]. The intelligent hydrogels can exhibit dramatic changes in their swelling behavior, network structure, permeability, or mechanical strength in response to small variations in external stimuli such as temperature, pH, ionic strength of the surrounding fluid, electric or magnetic fields, light, pressure, presence of biomolecules such as glucose and proteins, and so on [7,19-24]. The main response of intelligent hydrogels against exerted stimuli is reversible swelling/des welling volume phase transition. The changes in volume can be more than a hundred fold based on absorption or release of aqueous solution [15]. The most outstanding feature of intelligent hydrogels is that discrete changes in hydrogel volume will occur when the external stimulus reaches a critical point [21]. In recent years, intelligent hydrogels have been examined as promising materials in various fields, as sensors and valves, for example, and also for biomedical applications particularly in drug delivery systems and tissue engineering [19, 25]. In practical applications of intelligent hydrogels, and even conventional hydrogels, high mechanical tenacity is always desired in addition to softness and high water content. However, in most cases, hydrogels do not meet the mechanical requirements and are seldom used as mechanical devices because of their weak mechanical properties such as low strength, poor stability, and low fracture toughness especially in a swollen state [9,12,26,27]. These disadvantages are mainly attributed to the constrained molecular motion of the polymer chains due to the large number of randomly

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arranged crosslinks in the hydrogel network [28]. These problems become very important in the case of intelligent hydrogels where they are prepared in very small sizes in order to achieve fast response against stimuli. Therefore, it is necessary to improve the mechanical properties of hydrogels in order to widen their use, and the design of hydrogels with good mechanical performance is critically important in many existing and potential application areas of soft materials [9, 29,30]. To improve upon this limitation, researchers have focused on using different aspects of nanotechnology to prepare a new class of hydrogels, the so called nanocomposite hydrogels. The nanocomposite hydrogels could be prepared by introducing nanostructured inorganic particles to the hydrogel matrix in order to increase its strength and stiffness properties [31]. In 2002, for the first time Haraguchi and Takehisa reported the creation of a novel nanocomposite hydrogel with a unique organicinorganic network structure which was fabricated by in situ polymerization of N-isopropylacrylamide (NIPAAm) in aqueous suspension of laponite nanoclay [32]. The construction of the nanocomposite hydrogel was achieved, not by the mere incorporation of clay nanoparticles into a chemically crosslinked network, but by allowing the clay platelets to act as multifunctional crosslinkers in the formation of polymer-clay nanocomposite hydrogel [33]. In other words, it was found that the laponite particles act as crosslinkers (instead of traditional organic crosslinkers) and poly(N-isopropylacrylamide) (PNIPAAm) chains form physical crosslinks at the laponite-polymer interface via strong secondary interactions resulting in their incorporation into the polymer network [10,11,34]. The prepared nanocomposite hydrogels exhibited extraordinary mechanical, optical, and swelling/deswelling properties which could overcome the limitations and disadvantages of conventional chemically crosslinked hydrogels. In recent years nanocomposite hydrogels as novel engineered materials have attracted much scientific attention, and they are believed to be a revolutionary type of hydrogels [33,35]. In general, the nanocomposite hydrogels are defined as reinforced hydrophilic three-dimensional networks consisting of at least a crosslinked polymeric matrix and a nanoscale reinforcing agent [36]. They may also be defined as crosslinked polymer networks swollen with water or aqueous solutions in the presence of nanoparticles or nanostructures [36]. Nanocomposite hydrogels mainly utilize

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493

nanoscopic inorganic materials which form crosslinking junctions. In these hybrid hydrogels, the ends of the polymer chains may adsorb strongly on the surfaces of the nanoparticles, and enough of the chains attach to different particles to provide the bridges that constitute a three-dimensional network structure [37]. They can be synthesized through polymerization of hydrophilic monomers in the presence of nanoparticles. Nanoscale integration of both organic and inorganic components offers controllable mechanical strength and swelling [38]. Different kinds of nanoparticles such as gold, silver, iron oxide, carbon nanotubes, and various types of clays, e.g., laponite, montmorillonite, hy drotalcite, and bentonite have been used successfully in production of nanocomposite hydrogels [39-47]. Among the possible nanoparticles, clays have been used extensively in production of nanocomposite hydrogels because they are natural, abundant, and inexpensive minerals that have a unique layered structure, and high mechanical strength as well as high chemical resistance [48]. Clays have sandwich types of structures with one octahedral Al sheet and two tetrahedral Si sheets, the so called philosilicates [49,50]. The properties of clay-based nanocomposite hydrogels are strongly affected by the mixing process of organic and inorganic parts of the system, and also by the morphology of the prepared nanocomposite hydrogel. Like the other kind of polymer-clay nanocomposites, the literature commonly refers to three types of morphology for nanocomposite hydrogels including, immiscible (conventional or microcomposite), intercalated, and exfoliated. Using the X-ray diffractometry (XRD) technique one can easily determine the morphology of nanocomposite hydrogels. Transmission electron microscopy (TEM) is also a complementary method in this respect. The probable morphologies of nanocomposite hydrogels have been shown schematically in Figure 12.1 along with examples of TEM images and the expected XRD profiles [51]. In the case of microcomposite, the platelets of clay or organically modified clay (organoclay) exist in particles comprised of factoids or aggregates of them with no separation of platelets. Thus, the XRD profile of the nanocomposite hydrogel is expected to look essentially the same as that obtained for the clay. In the case of nanocomposite hydrogel with intercalated morphology, the polymer chains diffuse into the galleries of clay layers and basal spacing of clay layers is increased, so a shift in the peak of XRD profile towards lower angles for nanocomposite hydrogel

494

INTELLIGENT NANOMATERIALS Immiscible

26

Intercalated

2fl

Exfoliated

M

Figure 12.1 Morphologies of clay-based nanocomposite hydrogels with corresponding typical XRD and TEM results (Reprinted from (Polymer), vol. 49 Copyright (2008) with permission from (Elsevier) [51]).

is occurred in comparison with the peak of clay. For the nanocomposite hydrogel with exfoliated morphology, no peak is expected since there is no regular spacing of the platelets [51]. In recent years, nanocomposite hydrogels have been created for novel applications in different practical areas such as sensor-actuator and bioactuator systems, oil recovery, biomedical scaffolds, drug delivery devices, and wound dressing systems [52-60]. In early years a combination of technologies of nanocomposite and intelligent hydrogels, a novel class of hydrogels known as intelligent nanocomposite hydrogels (INCHs), were developed. INCHs have the advantages of nanocomposite and intelligent hydrogels contemporaneously. They are nanocomposite hydrogels which can exhibit response to applied external stimuli. They can be classified from different viewpoints, but the suitable method is classification based on the type of stimulus (like the popular classification of conventional intelligent hydrogels). In this chapter the INCHs are categorized based on the type of stimulus such as; temperature, pH, magnetic field, other stimuli (including electric field, light, and

INTELLIGENT NANOCOMPOSITE HYDROGELS

495

specific molecules), and multi-stimuli. The fundamental concepts, preparation methods, properties, and applications of each class of INCHs along with recent advances will be reviewed.

12.2 Temperature-sensitive Intelligent Nanocomposite Hydrogels Temperature-sensitive hydrogels are one of the most widely studied kinds of intelligent hydrogels. They undergo a reversible volume phase transition due to a change in the polymer and swelling agent compatibility by changing the temperatures of swelling medium as an external stimulus [16, 61]. The main response of temperature-sensitive INCHs is swelling /collapse behavior which occurs via increasing or decreasing the temperature of swelling medium up or down a specific temperature, known as volume phase transition temperature (VPTT). In general, temperature-sensitive INCHs are divided into two main classes including positive or negative temperature-sensitive INCHs. Positive temperaturesensitive INCHs uptake swelling agent and swell upon heating, increasing the swelling medium temperature above VPTT. In the case of positive temperature-sensitive INCHs, the VPTT is the same upper critical solution temperature (UCST) of hydrogel. Below this temperature the polymer chains of hydrogel is hydrophobic, and by increasing the temperature above VPTT (or UCST) the nature of chains is changed to hydrophilic. As a result, the hydrogel absorbs the swelling agent and undergoes a volume phase transition. The response of negative temperature-sensitive INCHs is completely different. Negative temperature-sensitive INCHs have a critical temperature known as lower critical solution temperature (LCST), which by increasing the temperature above it, the nature of the chains of hydrogel is changed from hydrophilic to hydrophobic. Therefore, negative temperature-sensitive INCHs can swell and undergo a volume phase transition upon decreasing the swelling medium temperature below LCST. The responses of positive and negative temperature-sensitive INCHs have been schematically presented in Figure 12.2. Temperature-sensitive hydrogels are mostly prepared based on PNIPAAm and its derivatives and have attracted the most attention because of the facile tuning of its properties [23]. PNIPAAm is

496

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Figure 12.3 Preparation methods of (a) polysiloxane nanoparticles and (b) nanocomposite hydrogels (Reprinted from (Biomaterials), vol. 29 Copyright (2008) with permission from (Elsevier) [75]).

503

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INTELLIGENT NANOMATERIALS

photopatterning of INCHs which might extend their utility in applications such as cell coculture and microfluidics. Recently, Gant et al. [76] prepared PNIPAAm/polysiloxane colloidal nanoparticles based INCHs with increased VPTT above body temperature. This was performed by introducing N-vinylpyrrolidone (NVP) as a hydrophilic comonomer to the hydrogel network. By incorporating NVP, the VPTT was increased so that the hydrogel would exist in the swollen state at body temperature. Using these INCHs one can design temperature-sensitive drug delivery devices with an increased diffusion of drug in vivo. As mentioned before, without using NVP, the hydrogel exhibits a VPTT of 33-34°C, which is below body temperature. Thus, the hydrogel would be in a collapsed state in vivo, which would ultimately limit diffusion of the target analyte. They also examined a typical hydrogel as a self-cleaning biosensor membrane to minimize the effect of the host response and its utility for an optical glucose sensor. It was found that the hydrogel exhibits good mechanical strength, glucose diffusion, and in vitro cell release upon thermal cycling. They reached the conclusion that this kind of nanocomposite hydrogel may be useful as biosensor membranes to minimize biofouling and extend the lifetime and efficiency of implantable glucose sensors and other biosensors [76]. Polyhedral oligomeric silsesquioxane (POSS) molecules have also been used to prepare INCHs. A typical POSS molecule having the structure of cube-octameric frameworks represented by the formula (R8Si8012) with an inorganic silica-like core (Si8012) surrounded by eight organic corner groups, one or more of which is reactive or polymerizable, is shown in Figure 12.4 [77]. Mu and Zheng [77] used Octa(propylglycidyl ether) POSS (OpePOSS) as a nano-crosslinking agent to prepare the crosslinked PNIPAAm hydrogels. They prepared organic-inorganic PNIPAAm networks via thermal crosslinking between PNIPAAm and OpePOSS (Figure 12.5). It was shown that by using moderate contents of POSS, the PNIPAAm/POSS hydrogels displayed much faster response rates in swelling, deswelling and reswelling experiments than the PNIPAAm hydrogels prepared via the free radical copolymerization of NIPAAm monomer with conventional crosslinker. It was also found that POSS decreases the VPTT of PNIPAAm hydrogel. They postulated that the structural incorporation of hydrophobic POSS moiety significantly weakens the miscibility of PNIPAM with water, and thus the VPTT shifts to the lower temperatures. In addition, at temperatures below VPTT, the POSScontaining hydrogels had lower equilibrium swelling ratios than

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the POSS-free PNIPAM hydrogels which is attributed to the incorporation of hydrophobic POSS structural units [77].

12.3 pH-sensitive Intelligent Nanocomposite Hydrogels One of the other widely studied types of INCHs is pH-sensitive hydrogels. These hydrogels are polyelectrolytes containing either

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INTELLIGENT NANOMATERIALS

acidic (polyanions) or basic (polycations) pendant groups. They can either accept or release protons in response to appropriate pH and ionic strength changes in aqueous media [78]. Anionic hydrogels deprotonate and swell more when external pH is higher than the pKa of the ionizable groups on polymer chains (Ka is the acid dissociation constant of the anionic hydrogel). On the other side, cationic hydrogels protonate and swell more when external pH is lower than the τρΚ^ of the ionizable groups (Kb is the base dissociation constant of the cationic hydrogel) [6]. The responses of anionic and cationic hydrogels are schematically presented in Figure 12.6. The main driving force for swelling in pH-sensitive hydrogels is the localization of fixed charges on the pendant groups and the creation of a strong electrostatic repulsion force [16]. Typical pH-sensitive polyanions are based on the polymers containing carboxylic or sulphonamide groups [6, 61]. For instance, a typical anionic hydrogel containing carboxylic group could be deprotonated in basic medium as follows: [RCOOH] H y d r o g e l + [ O H ] a q -> [RCOO] H y d r o g e l + H 2 0

(12.1)

Therefore, the density of charged groups within the network strongly increases accompanied by an adequate generation of mobile counterions inside the hydrogel, which induces the phase transition due to electrostatic repulsion. In an acidic medium, the acidic hydrogel could be protonated as follows: [RCOO-L, L

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In this condition, both the charge density and the content of mobile counterions within the hydrogel are decreased leading to hydrogel shrinking [15]. pH-sensitive INCHs are prepared by incorporating nanoparticles into anionic or cationic pH-sensitive hydrogels. Lee and Chen [46] prepared anionic INCHs on the basis of acrylic acid (AA), poly(ethylene glycol) methyl ether acrylate (PEGMEA), and bentonite clay by photopolymerization using UV irradiation. They modified the hydrophilic mineral bentonite with TMAACl and the resultant poly(AA-co-PEGME A)/bentonite INCHs had the exfoliated morphology. The gel strength and crosslinking density of INCHs were enhanced by adding bentonite, by means of formation of crosslinking between the hydrogel matrix and the exfoliated bentonite during the polymerization. The equilibrium swelling results

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showed that by increasing the amount of incorporated bentonite, the swelling ratios of INCHs are decreased. They attributed this to the hydrophobic nature of bentonite modified with TMAAC1 which decreases the hydrophilic nature of the hydrogel matrix. They also investigated the effects of pH on the swelling ability and response of prepared INCHs. The results indicated that the swelling ratios of INCHs increase by increasing the pH of the swelling medium. The volume transition of INCHs in different pH solutions was not obviously affected by the addition of more bentonite in the hydrogel, and they possessed excellent pH response under high clay loadings. The volume transition point for all INCHs was at around the pKa of PAA of about pH=5 (Figure 12.7) [46]. In another work, Lee and Chen [79] prepared the same pH-sensitive poly(AA-co-PEGMEA) INCHs using Hydrotalcite (HT). HT is a type of layered doubled hydroxide which can be synthesized by reacting dilute aqueous solutions of magnesium and aluminum chlorides with sodium carbonate [45]. It has received considerable attention

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Figure 12.7 Swelling behavior of poly(AA-co-PEGMEA)/bentonite INCHs containing 0-7 mol% of bentonite in different pH values (Reprinted from (Journal of Applied Polymer Science), vol. 91 Copyright (2004) with permission from (John Wiley and Sons) [46]).

because of its application as catalysts, ion exchangers, absorbents, ceramic precursors, and organic-inorganic nanocomposites [80]. HT could not easily disperse in water and aqueous solutions, so Lee and Chen [79] modified it with hydrophilic intercalant 2-acryloylamido2-methyl propane sulfonic acid (AMPS). In intercalated HT, AMPS is ionically bonded to the surface of HT by an ion exchange process, which facilities its dispersion in water. Like the poly(AA-co-PEGMEA) /bentonite nanocomposite hydrogels, these anionic INCHs are pH responsive and show volume transition around pH=5. However, the effect of HT content on the equilibrium swelling ratios of poly(AA-co-PEGMEA) hydrogels was completely different from that of bentonite. The results showed that by increasing the content of intercalated HT in nanocomposite hydrogels, the swelling ratios are increased. They suggested that this is because of the hydrophilic nature of HT clay intercalated with hydrophilic AMPS, which makes the nanocomposite hydrogels more hydrophilic [79]. Using the technology of preparation of semi-IPNs, Ma et al. [28] fabricated the pH-sensitive nanocomposite hydrogels on the basis of PNIPAAm crosslinked by laponite using in situ free radical polymerization in the presence of linear sodium carboxymethylcellulose (CMC). The results showed that the laponite particles were substantially exfoliated to form nano-dimension platelets

INTELLIGENT NANOCOMPOSITE HYDROGELS

509

dispersed homogeneously in the hydrogels and that they acted as a multifunctional crosslinker. The pore sizes in CMC/PNLPAAm/ laponite hydrogels decrease with increasing clay content. Because of the function of clay as a crosslinking agent, higher clay contents lead to the formation of a more densely crosslinked network, and thus the pore sizes are decreased. On the other hand, the pore sizes in the CMC/PNIPAAm/laponite hydrogels are larger than those in the PNIPAAm/laponite hydrogels with the same clay contents, which is related to the fact that a more expanded network can be generated by electrostatic repulsions among carboxylate anions (-COO) of CMC during the polymerization process. The C M C / PNIPAAm/laponite nanocomposite hydrogels exhibit good pH sensitivity because of the presence of CMC as a polyanion containing carboxylic groups on its structure. When the pH value of the swelling medium is below or above the pKa of that of the carboxylic groups (about 4.6), the carboxylic groups in the CMC chains are protonated or ionized respectively, which influences the swelling behavior of the CMC/PNIPAAm/laponite hydrogels. The C M C / PNIPAAm/laponite hydrogels swell faster than the corresponding PNIPAAm/laponite hydrogels at pH 7.4, however, they swell slower than the PNIPAAm/laponite hydrogels at pH 1.2. This is due to the fact that at pH 1.2 (below the pKa of CMC), the carboxylic groups in CMC are protonated to -COOH groups, and the hydrogen bonds between -COOH and -CONH- groups are formed, which leads to polymer-polymer interactions predominating over the polymer-water interactions. As a result, the swelling rates of the CMC/PNIPAAm/laponite hydrogels are lower than those of the PNIPAAm/laponite hydrogels under this acidic condition. At pH 7.4 (above the pKa of CMC), the carboxylic groups become ionized, and the hydrogen bonds formed are weakened and destroyed. Moreover, the electrostatic repulsion between -COO" groups leads to the network expanding more (Figure 12.8) [28]. Huang et al. [81] prepared a kind of pH-sensitive, semi-IPN INCH composed of starch-g-AA and modified bentonite with poly(dimethyldiallylammonium chloride) (PDMDAAC). The results showed that the compressive strength of starch-g-AA/ PDMDAAC/bentonite INCHs can be two times greater than corresponding starch-g-AA/PDMDAAC hydrogels. The prepared INCHs also had excellent pH-sensitivity in the range of pH from 2 to 4. At pH 2 to 3, the INCHs had a low swelling ratio as compared with those at pH 4 to 13, with an abrupt jump around pH 4.

510

INTELLIGENT NANOMATERIALS

PNIPA/Clay hydrogel CMC/PNIP/VClay hydrogel under acidic condition (pH = 1.2)

CMC/PNiPA/Clay hydrogel under basic condition (pH = 7.4)

Figure 12.8 Schematic illustrations of CMC/PNIPAAm/laponite and PNIPAAm/laponite hydrogels at pH values of 1.2 and 7.5 (Reprinted from (Journal of Polymer Science Part B: Polymer Physics), vol. 46 Copyright (2008) with permission from (John Wiley and Sons) [28]).

The lower level of swelling ratio at a low pH is due to the higher crosslinking density from the formation of hydrogen bonds and the entanglements between starch-g-AA segments and PDMDAAC chains. At a pH above 4, the COOH groups on the starch-g-AA chains dissociate, which breaks the hydrogen bonds and decreases crosslinking density. The osmotic pressure of the counter ions and the charge repulsion also increases the swelling capacity which correlates to the concentration of carboxylate anions. Additionally, the existence of the quaternary nitrogen ions on the PDMDAAC chains effectively shields the electrostatic attraction between the sodium ions and the carboxylate anions at a higher pH, and the hydrogel shows a high swelling ratio [81]. Silver (Ag) nanoparticle has also been utilized in the production of pH-sensitive INCHs. Ag is an important commercially available metal and its nanoparticles are superior to other nanosized metal particles because of their excellent optical, electrical, and antibacterial properties [82, 83]. A variety of preparation methods such as chemical reduction, thermal decomposition, gas condensation, microwave reduction routes, irradiation, ultrasonication, and actinochemistry techniques can be utilized to prepare the Ag nanoparticles [82, 84]. By utilization of Ag nanoparticles, Xiang and Chen [83] prepared a kind of pH-sensitive INCH on the basis of 2-hydroxyethyl methacrylate (HEMA), poly(ethylene glycol) methyl ether methacrylate (PEGMMA), and methacrylic acid (MAA). The poly(HEMA-PEGMA-MAA) / Ag (or PHPM/Ag) nanocomposite hydrogels were prepared by in situ reducing Ag+ ions

INTELLIGENT NANOCOMPOSITE HYDROGELS

511

anchored in the hydrogel by the deprotonized carboxyl acid groups (Figure 12.9). The structural studies using XRD and TEM techniques showed that high crystalline Ag nanoparticles could form in the nanocomposite hydrogel with small and regular sizes as well as discrete structures. The PHPM/Ag INCHs showed good response to a variation of environmental pH. This response is in the form of swelling/collapse phase transition as well as increasing or decreasing the electrical conductivity. At higher p H values above pKa of MAA, the INCH is at a swollen condition because of deprotonation of -COOH groups on chains of INCH. Due to the swelling, the distance between Ag nanoparticles is large enough, acting as a barrier for Ag nanoparticles to come into contact with one another. The electrical conductivity of PHPM/Ag INCHs depends on the ion transmission in the polyelectrolyte hydrogel network, which results in a lower level of conductivity. As the pH decreases close to the pKa of MAA, the INCH undergoes a change in conformation due to protonation of -COO" groups and as a result, dramatic decrease of the hydrogel volume occurs because of the intrachain collapse. It causes the distance between the Ag nanoparticles to dramatically reduce, and makes the Ag nanoparticles become closepacked or compacted in the nanocomposite hydrogel which leads to a significantly increased conductivity value. They suggested these pH-sensitive INCHs be used as novel intelligent materials to

(Ag* ions loaded at low SR) Ag particle/PHPM composite hydrogel

Figure 12.9 Schematic representation of Ag nanoparticles formation in PHPM hydrogel at low and high swelling ratios (Reprinted from (European Polymer Journal), vol. 43 Copyright (2007) with permission from (Elsevier) [83]).

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INTELLIGENT NANOMATERIALS

be used in different applications including electronics, biosensors, and drug delivery devices [83]. Most pH-sensitive INCHs are based on anionic polymers which swell at a higher p H in basic environments. Nonetheless, a few kinds of cationic INCHs, which swell in acidic conditions, have also been prepared by some research groups. Recently, Zhu et al. [85] prepared cationic INCHs containing AAm and positively chargeable monomer 2-(dimethylamino) ethyl methacrylate (DMAEMA) in an aqueous suspension of laponite XLS by an in situ copolymerization method. The copolymerization was initiated either by redox or UV radiation. Both INCHs exhibited similar pH-responsive swelling behaviors. In INCHs initiated with redox, when the DMAEMA content is higher than 5 mol% in the monomers, the pH-responsive swelling becomes more distinct. The higher the DMAEMA content in the INCH, the higher the equilibrium swelling ratio becomes at a low pH (around 2-3). The obtained results indicate that the INCHs containing DMAEMA are weak polyelectrolytes, which are charged at pH < 4 and neutral at p H > 4 due to the protonation and deprotonation of the amine groups. By contrast, when the DMAEMA content is lower than 5 mol%, the swelling ratio is low and almost pH- independent. On the other hand, the INCHs initiated by UV radiation show similar swelling behavior, with higher swelling ratios than those initiated by redox. From the viewpoint of mechanical properties, ultrahigh tensibility, higher than 1700% of the strain at break, was observed for prepared pH-sensitive INCHs [85]. Mujumdar and Siegel [86] synthesized anionic and cationic pH-sensitive copolymer INCHs on the basis of PNIPAAm and laponite, using acid comonomer MAA and base comonomer DMAEMA, respectively. Nearly all of the prepared INCHs were highly elastic and exhibited elongation strains at break in the range of 600-1000% due to the increased crosslinking densities of INCHs in the presence of nanoclay crosslinkers. Cationic PNIPAAm/ DMAEMA/laponite hydrogels were not sensitive to p H at low DMAEMA substitutions (lower than 5 mol%). With greater than 5 mol% substitution, a significant swelling transition was observed. The PNIPAAm/DMAEMA/laponite hydrogels swell at lower pH values and shrink at a higher pH, as expected from the basic character of DMAEMA. The hydrogels exhibited a sharp swelling transition near pH 8-9. Anionic PNIPAAm/MAA/laponite hydrogels also undergo a transition from a relatively collapsed state at

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513

a low pH to a swollen state at a high pH. With increasing MAA comonomer content in the hydrogel, the transition was magnified (transition pH 4.7-6.8). The prepared INCHs seem to be promising materials for use as drug delivery systems and novel tissue culture substrates because of their excellent response against pH (at a broad pH range), good mechanical properties, and tunable hydrophobicity and ionization states [86]. As mentioned before, nearly all of pH-sensitive hydrogels and INCHs are prepared using ionic polymers or polyelectrolytes containing at least one monomer of an acidic or basic group. Nonetheless, some nonionic nanocomposite hydrogels could show good response against pH, due to the presence of some ionizable groups on the surfaces of nanoparticles incorporated into the network of nanocomposite hydrogel. Recently, Li et al. [78] prepared pH-sensitive INCHs on the basis of PAAm which is a nonionic polymer and cannot be ionized in aqueous solutions. PAAm nanocomposite hydrogels were crosslinked by laponite XLS, in which the surface has a number of anionic groups (OH). The anionic groups of laponite in the network of PAAm/laponite hydrogels can affect the swelling behavior of hydrogel in different pH solutions via hydrolization of the network in either acidic or basic conditions. Measuring the responses of PAAm/laponite INCHs at different pH values showed that the equilibrium water uptake value is high at strong basic condition (pH of 11) and strong acidic condition (pH of 2), while it is low otherwise [78].

12.4 Magnetic-field-sensitive Intelligent Nanocomposite Hydrogels Magnetic-field-sensitive hydrogels are often prepared by incorporating or loading of magnetic nanoparticles into crosslinked polymeric hydrogels. In many cases the magnetic nanoparticles are loaded inside temperature-sensitive hydrogels. By applying the magnetic field the nanoparticles act as heating agents, and by increasing or decreasing the temperature inside the temperaturesensitive hydrogel they endow the smartness to hybrid hydrogel. Among the magnetic nanoparticles, special attention has been drawn to superparamagnetic iron oxide nanoparticles (i.e., Fe 2 0 3 and Fe 3 0 4 ) to produce magnetic- field-sensitive hydrogels [87]. In the case of iron oxide nanoparticles, the nanoparticles can be

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heated through several mechanisms including hysteresis, Neel and Brownian relaxation, and frictional losses in a viscous fluid [8]. Nanosized iron oxide particles can be synthesized by different techniques such as the sol-gel, micro emulsion, sonochemical, ultrasonic spray pyrolysis, and microwave plasma methods [88]. The response of magnetic-field-sensitive hydrogels is dependent on the magnetism of magnetic particles and their geometrical sizes [4]. Unlike the polymer-clay nanocomposite hydrogel, there is little effect on the mechanical properties of the resulting nanocomposite hydrogels by the incorporation of metal nanoparticles as long as the interactions between polymer and nanoparticles are weak. In this case, phase transition, thermosensitivity, and viscoelasticity of the hydrogel remains unchanged [36]. The incorporation of magnetic nanoparticles into hydrogels can create tunable nanocomposite hydrogels that can be remotely controlled by a magnetic field having an excellent potential to be used in diverse applications, for instance as sensors, switches, artificial muscles, membranes, and drug delivery systems [87]. Satarkar and Hilt [57] prepared magnetic-field-sensitive INCHs on the basis of temperature-sensitive PNIPAAm by the incorporation of superparamagnetic Fe 3 0 4 nanoparticles. The PNIPAAm/ Fe 3 0 4 nanocomposite hydrogels were synthesized by UV photopolymerization using poly(ethylene glycol) dimethacrylate (PEGDMA) as the crosslinker with various magnetic nanoparticles loadings, having a spherical shape with a mean diameter of 20-30 nm. Remote-controlled drug delivery systems on the basis of prepared magnetic-field-sensitive INCHs were also prepared by loading pyrocatechol violet dye as a model drug. It was found that the PNIPAAm/Fe 3 0 4 INCHs are responsive to alternating magnetic fields. The application of an alternating high-frequency magnetic field to the INCHs leads to heat generation, which can drive the swelling transition of the temperature-sensitive PNIPAAm hydrogel. Under the external alternating magnetic field, the PNIPAAm/ Fe 3 0 4 hydrogel responds to the increased (or decreased) temperature induced by Fe 3 0 4 nanoparticles and also can exhibit an on-off release of drug. The response of typical magnetic-field-sensitive drug delivery systems containing negative and positive temperature-sensitive INCHs are schematically presented in Figure 12.10 [57]. Li et al. [89] prepared magnetic poly(acrylic acid-acrylamide-butyl methacrylate) (PAAB) nanocomposite hydrogels containing Fe 3 0 4 nanoparticles and used them as adsorbents for the removal and

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515

Particle

Drug Alternating magnetic field Alternating magnetic fieldj

Particle

V

Change in Κ βηΊρθΓ3 , υΓΘ

T>LCST Ο Λ Ι

Change in temperature

Si

Drug

ffTÄX- >UCST

m ^->e>t.T> ^elf, 296K

Hydrogel systems with negativetemperature sensitivity (LOST)

Hydrogel systems with positivetemperature sensitivity (UCST)

Figure 12.10 Schematic responses of typical magnetic-field-sensitive drug delivery systems containing negative and positive temperature-sensitive INCHs (Reprinted from (Journal of Controlled Release), vol.130 Copyright (2008) with permission from (Elsevier) [57]).

separation of cationic dyes from aqueous solutions. PAAB/Fe 3 0 4 nanocomposite hydrogels were synthesized using in situ coprecipitation reaction methods. These INCHs can be easily manipulated in a magnetic field for the removal and separation of cationic dyes, such as crystal violet (CV) and basic magenta (BM), from aqueous solutions due to their electrostatic and hydrophobic interactions with cationic dye molecules under basic conditions. The magnetic-fieldsensitive INCHs which have absorbed cationic dyes can be readily separated from the absorption media with a simple magnetic field due to their magnetic responsive behavior. Figure 12.11a, b shows the photographs of non-responsive PAAB and magnetic-field-sensitive PAAB hydrogels attracted by a magnet. As shown, compared with non-responsive PAAB hydrogel (a) magnetic hydrogel (b) could be easily attracted from a water solution by an external magnetic field, indicating its sensitive magnetic response. Figure 12.11c also shows the magnetization of magnetic PAAB hydrogels prepared using 60 and 85% of iron salt solutions as a function of the applied magnetic

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Field (Oe)

Figure 12.11 Photographs of a swollen (a) non-magnetic and (b) magnetic PAAB hydrogels and (c) magnetization curves of PAAB hydrogels prepared using 60 and 85% of iron salt solutions (Reprinted from (Polymers for Advanced Technologies), DOI: 10.1002/pat.l782 Copyright (2010) with permission from 0ohn Wiley and Sons) [89]).

field at 25°C. Both INCHs prepared by 60 and 85% of iron salt solutions exhibit the superparamagnetic characteristic with saturation magnetization intensity of 4.5, 7.1 emu/g, respectively. The higher saturation magnetization of INCH prepared by 85% of iron salt is due to its higher content of the resultant Fe304 nanoparticles in the hydrogel matrix. The magnetic-field-sensitive PAAB/Fe304 nanocomposite hydrogels have potential applications in the related wastewater treatment industries [89]. Samantha et al. [90] fabricated the magnetic-field-sensitive INCHs on the basis of PEGMMA and PEGDMA with Fe304 nanoparticles via free radical polymerization as implantable biomaterials for thermal cancer therapy applications. It was shown that the thermal response

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of the INCHs can be controlled through the external alternating magnetic field which could be used for thermal cancer therapy by the heating of the tumor tissue. For hyperthermic applications the temperature of cancerous tissue needs to reach 41-45°C for effective therapy, whereas temperatures above 50°C cause damage to cancer cells via thermoablation. It was demonstrated that the temperature of the prepared INCHs can be controlled by changing the external alternating magnetic field strength so that the INCHs either reach hyperthermic or thermoablative temperatures. The heating response of INCHs was shown to be dependent on the Fe 3 0 4 loading in the hydrogels and the strength of the magnetic field. For INCH with a higher Fe 3 0 4 content, lower magnetic field strength is needed to get the desired temperature, and as the field strength is increased for a particular system the temperature also increased [90]. Hawkins et al. [8] synthesized degradable INCHs with temperature sensitive degradation using Fe3Ö4 nanoparticles. The nanocomposite hydrogels were prepared using a special kind of degradable macromer achieved from the mixing of 3-Morpholinopropylamine and Diethylene Glycol Diacrylate (Figure 12.12). Free radical polymerization was used to create the macromer/Fe 3 0 4 magneticfield-sensitive nanocomposite hydrogels. Upon application of an external magnetic field, the nanoparticles heat and increase the degradation rate of the hydrogel. The release of a model drug from the INCHs was modulated by exposing the system to the alternating magnetic field. The drug release mechanism involves remotely heating magnetic nanoparticles to increase the degradation rate of the degradable hydrogel and, therefore, increase the rate of drug release from the system [8]. Magnetic-field-sensitive hydrogels could also be prepared as core-shell hybrid systems by the coating of magnetic nanoparticles (as the core) by crosslinked hydrogel as the shell or vice versa. In cases of using nanoparticles as the core, nanoparticles could possess high magnetization and superparamagnetic behavior along with low toxic properties due to encapsulating by the protective shell, which may make them of use in biomedical applications. In addition, the shell could prevent the nanoparticle core from probable oxidation and aggregation [91]. Liu et al. [92] fabricated magnetic-field-sensitive hollow and coreshell microcapsules by suspension polymerization of NIPAAm and/or AAm using Fe 2 0 3 nanoparticles. The microcapsules were prepared by dissolving monomers in suspended aqueous droplets

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INTELLIGENT NANOMATERIALS 3-Morpholinopropylamine

o

Diethylene glycol diacrylate

o

o

o

Degradable macromer

Figure 12.12 Synthesis of degradable macromer (Reprinted from (Pharmaceutical Research), vol. 26 Copyright (2009) with permission from (Springer) [8]).

and subsequent polymerization at 60°C. The hollow microcapsules were achieved by homopolymerization of NIPAAm. Because of the hydrophobic nature of PNIPAAm at the polymerization temperature (60°C), the resulting PNIPAAm deposits from the interior water phase onto the interface to form PNIPAAm/Fe203 nanocomposite shells. When AAm is homopolymerized, the magnetic core-shell beads with the PAAm hydrogel cores and Fe203 nanoparticle shells can be obtained. This is due to the fact that PAAm is hydrophilic at the polymerization temperature. If NIPAAM and AAm are copolymerized, the magnetic nanocomposite microcapsules with two kinds of supracolloidal structures (hollow or core-shell) will be obtained by varying the NIPAAm/AAm ratio. The fabrication methods of hollow and core-shell microcapsules have been illustrated in Figure 12.13. The prepared INCH microcapsules exhibited a clear magnetic response after which they can readily be moved and collected using an external magnetic field in water. These microcapsule beads may find applications as delivery vehicles for biomolecules, drugs, cosmetics, food supplements, and living cells [92]. In another work, Liu and coworkers [93] prepared magneticfield-sensitive nanocomposite beads with Fe203 magnetic nanoparticles as the shell and alginate as the core. Alginate is a well-known biomaterial obtained from brown algae and is widely used in drug delivery and tissue engineering due to its biocompatibility, low toxicity, relatively low cost, and simple gelation process with divalent cations such as Ca2+ [94]. Liu et al. [93] used the alginate/Fe 2 0 3

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519

Oil phase

^

Cil phase Em u Is if h ion

•

·

·

>Q

Polymerization at 60::C ^

Oil pti^se

Water phase Monomers and Initiators

• Figure 12.13 Preparation methods of magnetic-field-sensitive hollow and core-shell microcapsules (Reprinted from (Ada Biomaterialia), vol. 6 Copyright (2010) with permission from (Elsevier) [92]).

microcapsules in the sustained release of insulin microcrystal as a model drug, and observed a dual-controlled insulin release mechanism in response to an exerted magnetic field.

12.5

Other Stimuli-sensitive Intelligent Nanocomposite Hydrogels

Other kinds of stimuli such as electric field, light, presence of specific molecules and biomolecules, and so forth, could also be applied on INCHs to enforce them to exhibit the appropriate responses. Electricfield-sensitive hydrogels, an important kind of intelligent hydrogels, are normally made up of polyelectrolytes containing ionic groups, such as pH-sensitive hydrogels [95]. Tanaka et al. [96] first discovered the deformation of hydrogels in an electric field. The response of electric-sensitive hydrogels generally is in the form of volume or shape changes like swelling, shrinking, and bending behaviors in the presence of an applied electric field [21]. Electric-field-sensitive hydrogels play an excellent role in areas of energy exchange devices, because they can transform electric energy to mechanical energy. Furthermore, they also have found applications in many domains such as drug delivery systems, chemical convertors, memory component switches, and artificial muscles [21].

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Liu et al. [58] prepared electrical-field-sensitive INCHs on the basis of chitosan and MMT, and systematically studied them for drug release behavior following electrostimulation. Chitosan is a polycationic polysaccharide derived from the deacetylation of chitin, which has been used in a wide variety of applications in the biomedical field [97, 98]. It is an electrical-field-sensitive hydrogel, but its responsiveness and reversibility upon repeated on-off electrostimulation switching operations has major limitations, especially in clinical applications, as it suffers from too much structural instability for the precise control of the release of the drug upon cyclic electrostimulation. While, in the prepared chitosan/MMT INCHs by Liu et al. [58], this problem has been solved due to the presence of nanoclay in the network of chitosan hydrogel which creates drastic improvements in the anti-fatigue property. They loaded vitamin B12 as the model drug in the chitosan/MMT nanocomposite hydrogels, and investigated the variations in the release kinetics and the mechanism of vitamin B12 action under a given electrical field stimulus. Under a constant driving voltage of 5 V, the time-dependent cumulative release of the drug from the hydrogels with different MMT contents revealed that both the release rate and the cumulative drug amount upon electrostimulation were higher than that with no applied electric field. This is due to the ejection of drug from the electrosensitive hydrogels as a result of deswelling and syneresis. A change in the electrical field causes a change in the drug release profile. Therefore, pulsatile patterns of drug release can be created by switching the electrical field off and on. Typical repeated on-off release profiles for pure chitosan (pure CS) and chitosan/MMT nanocomposite hydrogel containing 2% of MMT (CS-MMT2) are shown in Figure 12.14 [58]. Light also can be used as an external stimulus to INCHs. Since light can be imposed instantly and delivered in specific amounts with high accuracy, light-sensitive hydrogels may possess special advantages over others [95]. Light-sensitive hydrogels have been categorized as either UV-sensitive or visible light-sensitive hydrogels. UV-sensitive hydrogels can be synthesized for example by introduction of bis(4-dimethylamino) phenylmethyl leucocyanide into the polymeric matrix in which ionization of the leuco derivative with UV radiation results in discontinuous swelling at a constant temperature [99]. Using visible light as a stimulus has good advantages over UV light because it is readily available, inexpensive, safe, clean, and easily manipulated [95]. Visible light-sensitive hydrogels are mainly

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2

4 6 8 On-off cyclic number

521

10

Figure 12.14 Repeated on-off release profiles of pure chitosan and chitosan/MMT nanocomposite hydrogel under electric field (Reprinted from (Ada Biomaterialia), vol. 4 Copyright (2008) with permission from (Elsevier) [58]).

prepared by the incorporation of light sensitive chromophores into temperature-sensitive hydrogels, in which the absorption of light by the chromophore results in the heating and subsequent swelling or collapse of the hydrogel [100]. Light-sensitive hydrogels can also be prepared using optically active metal nanoshells (e.g., gold nanoshell) capable of absorbing near infrared light [8]. The light-sensitive hydrogels are unique candidates as microfluidic valves. A microfluidic valve involves an intelligent hydrogel anchored by a rigid pillar, in such a manner so that the hydrogel swells in response to a change in the external stimulus, blocking the flow [101]. Microfluidic devices are commonly prepared using pH-sensitive hydrogels [102, 103]. These kinds of microfluidic devices could control the flow in microchannels based on the local environmental pH. Replacing the pH by light as a stimulus, Scott et al. [104] prepared a novel kind of microfluidics device on the basis of light-sensitive INCHs. They prepared the light-sensitive INCHs on the basis of a temperature-sensitive P(NIPAAm-co-AAm) hydrogel loaded with light-sensitive gold nanoparticles (gold colloids and nanoshells). In the prepared microfluidics system, dramatic reversible changes in the size and shape of hydrogel can be induced by exposure to the specific wavelengths of light, depending on the kind of nanoparticles. Gold colloid-based INCH collapsed in response to green light, while the gold nanoshellbased one collapsed in response to near-IR light (Figure 12.15). Using these light-sensitive INCHs, they designed a typical microfluidics

522

INTELLIGENT NANOMATERIALS (a) 120100 1

I

01

I

80 ■ 60· T

40200

(b)

- Colloid composite

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10

1

20

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30 40 Time(mln)

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30

40

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Figure 12.15 The response of gold colloid- and nanoshell-based INCHs against green (shown as green squares) and near-IR (shown as red circles) lights (Reprinted from {Advanced Materials), vol. 17 Copyright (2005) with permission from (John Wiley and Sons) [104]).

device which can respond to the green and IR lights with a very fast response time of less than five seconds [104]. Sometimes, INCHs can be responsive to the presence of specific molecules in the swelling medium. The response of these kinds of INCHs can be achieved by increasing or decreasing the amount of specific molecules in the medium. These INCHs are mainly used in sensor or biosensor applications. Glucose-sensitive INCHs are a main kind of molecule-sensitive hydrogels which have a unique application in the biomedical field. One of the most challenging problems in diabetes mellitus is the development of self-regulated insulin delivery systems. Delivery of

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insulin is different from delivery of other drugs, since insulin has to be delivered in an exact amount at the exact time of need. Thus, self-regulated insulin delivery systems require a glucose sensing ability and an automatic on-off mechanism. Many hydrogels have been developed for modulating insulin delivery, and all of them have a glucose sensing system [95]. For this purpose, a glucose electrochemical sensor has been widely used because it is rapid, accurate, selective, and low cost, and has other potential applications. The glucose electrochemical sensor can be prepared in two distinct types of structures, including enzymatic and non-enzymatic sensors [105]. Using a simple and controllable electrodeposition method, Feng et al. [105] prepared chitosan/gold nanoparticles as a glucose-sensing film on a glassy carbon electrode surface, and further used it for the construction of a glucose non-enzymatic sensor. It was found that co-depositing the chitosan with gold nanoparticles, a very good electrode material for the oxidation of glucose in alkaline solutions, guarantees the homogeneity and stability of the gold nanoparticles film, and that this biocomposite film could provide plenty of active sites for the direct oxidation of glucose. The prepared sensor exhibited high performance including wide linearity, simple operation, and good stability at the optimized conditions. The prepared INCH-based glucose sensor exhibited good linear behavior in the concentration range from 4x10-4 to 1.07x10-2 mol/1 for the quantitative analysis öf glucose with a limit of detection of 3.70xl0~4 mol/1. In general, the constructed nanocomposite hydrogel sensor demonstrated good performance on glucose detection [105].

12.6 Multi-stimuli-sensitive Intelligent Nanocomposite Hydrogels Some INCHs are dual- or multi-responsive, meaning they can respond to two or more stimuli simultaneously. In these systems, the hydrogels can operate with a single stimulus in the absence of the other ones, and when all of the stimuli are exerted, the synergetic effect increases the response rate of the hydrogel. Multi-stimulisensitive hydrogels are mainly prepared by the copolymerization or interpenetration of two or more polymers, with each having its own sensitivity to a special stimulus. Temperature- and pH-sensitive dual responsive hydrogels are the more commonly studied types

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of multi-stimuli-sensitive hydrogels, and are also the favorite ones in biomedical systems because both stimuli have important physiological factors [106]. Song et ah [107] prepared temperature- and pH-sensitive INCHs by in situ free radical polymerization of NIPAAm monomers in the presence of linear PAA chains using laponite as the crosslinker. The PNIPAAm/PAA/laponite nanocomposite hydrogels consisted of two polymer components, temperature-sensitive PNIPAAm, and pH-sensitive PAA, which means that the semi-IPNs should show both temperature and pH-sensitive swelling behavior. Here, since the pKa of AA is around 4.5, the carboxylic acid groups are ionized (COO) and deionized (COOH) above and below this pH, respectively. So, the INCHs show much higher swelling ratios in pH values higher than 4.5. The INCHs also indicate a clear temperature-sensitivity around the LCST of PNIPAAm. Song et ah [107] also conducted the oscillatory swelling/deswelling experiments to check the repeatability of the stimulus-response of prepared temperature- and pH-sensitive INCHs by alternating the pH (pH=2 and 7) and temperature (20 and 40°C). Figure 12.16 shows the oscillatory swelling/deswelling responses against the temperature and pH for typical semi-IPNs on the basis of PNIPAAm/ PAA/laponite nanocomposite hydrogel containing 10 wt% of PAA and 7xl0"2 mol of laponite per 1 1 of water (sI-NC7-PAA10), and also PNIPAAm/PAA conventional hydrogel containing 10 wt% of PAA and 10~2 mol of Ν,Ν-methylenebisacrylamide (BIS) per monomer (sI-ORl-PAA10). As observed, sI-NC7-PAA10 nanocomposite hydrogel exhibits outstanding swelling/deswelling behavior as the temperature was brought repeatedly above and below the LCST. The swelling/deswelling ratio for sI-NC7-PAA10 nanocomposite hydrogel is much larger than that for sI-ORl-PAA10 conventional hydrogel. This may be due to the high crosslinking density in the network of the conventional hydrogel and the highly flexible, thermosensitive PNIPAAm chains in the network of the nanocomposite hydrogel. Furthermore, nanocomposite hydrogel demonstrates remarkable swelling/deswelling behavior in response to repeated pH changes [107]. Similar temperature- and pH-sensitive dual responsive nanocomposite hydrogels were prepared by Hu et ah [108] using temperature sensitive NIPAAm monomer, p H sensitive sodium methacrylate (SMA), and laponite XLS as the crosslinker. The PNIPAAm/SMA/laponite nanocomposite hydrogels

INTELLIGENT NANOCOMPOSITE HYDROGELS

12

-12

24

36

48

60

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Time (hr) (b)

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Figure 12.16 Oscillatory swelling response of typical PNIPAAm/PAA/laponite and PNIPAAm/PAA hydrogels upon (a) temperature and (b) pH changes (Reprinted from {Macromolecular Chemistry and Physics), vol. 209 Copyright (2008) with permission from (John Wiley and Sons) [107]).

demonstrated good volume phase transition with varying temperature and / o r pH of the swelling medium. The results also showed that the response has a strong dependency on the SMA content in the nanocomposite hydrogels [108]. Ma et al. [98] prepared INCHs responsive to temperature and pH using semi-IPNs on the basis of a PNIPAAm network crosslinked by laponite in the presence of linear chains of carboxymethylchitosan (CMCS). CMCS is derived from chitosan by introducing -CH 2 COOH groups onto -OH along the chitosan molecular chain which is an amphoteric polyelectrolyte containing both cationic and anionic charges. The PNIPAAm/CMCS/laponite INCHs exhibit a VPTT around 33°C with no significant deviation from the conventional PNIPAAm hydrogel. The minimum swelling ratios of PNIPAAm/CMCS/laponite hydrogels at different pHs appear

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around the isoelectric point (IEP) of CMCS which is about 3. At the conditions which the pH of the external solution deviates from the IEP, the INCHs behave as either polycations or polyanions and the swelling ratios increase. At pH equals to IEP, the numbers of the anionic and cationic groups on the hydrogel (-COO" and -NH3+) are nearly equal, and the intraionic attraction between opposite charges results in the lowest mobile ionic concentration in the hydrogel. As a result, the osmotic pressure in the swelling medium causes the hydrogel to shrink to a minimum equilibrium volume. When the p H deviates from the IEP, the amount of residual ionic concentration in the INCH increases gradually with the increase or decrease of the pH. It leads to the increase of the hydrogel swelling ratio [98]. Kabiri et al. [109] prepared an INCH on the basis of PAMPS via solution polymerization of AMPS monomer using MMT nanoclay modified with chitosan (chitoMMT), which is responsive to pH and solvents. The swelling studies on PAMPS /chitoMMT nanocomposite hydrogel at different pH levels showed that in a wide range of pH (3-11) it preserves a high swelling capacity that is not significantly effected by the pH. On the other hand, swelling was significantly decreased at a pH lower than 3 and higher than 11. In order to investigate the effect of the solvent, they examined the swelling capability of PAMPS/chitoMMT nanocomposite hydrogel in different water-miscible solvents including methanol, ethanol, acetone, ethylene glycol, polyethylene glycol, N-methyl-2-pyrrolidone (NMP), and dimethylsulfoxide (DMSO). The swelling capacity was found to be in order of ethylene glycol- DMSO>methanol>ethano l>NMP>acetone [109]. Sivudu and Rhee [110] prepared a kind of magnetic-field-sensitive nanocomposite hydrogels which are also responsive to pH. They fabricated these dual-sensitive INCHs by the in situ development of Fe 3 0 4 magnetite nanoparticles in PA Am hydrogel. The PA A m / Fe 3 0 4 nanocomposite hydrogels showed superparamagnetic properties. The saturation magnetization of a typical PAAm/Fe 3 0 4 hydrogel was 4 e m u / g , which is a typical characteristic of superparamagnetic materials. At the same time, the prepared magneticfield-sensitive hydrogels demonstrated a pH dependent swelling. The results showed that the swelling capacity increases as the pH increases from 4 to 7 and decreases at pH 10, while the conventional PAAm hydrogel shows no change in swelling behavior at various pH values. These types of pH- and magnetic-field-sensitive nanocomposite hydrogels could be useful for drug delivery and bioseparation applications [110].

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12.7 Conclusions Intelligent nanocomposite hydrogels are recognized as a revolutionary type of intelligent materials in which both strength and smartness are gathered together in a soft structure. The large amount of recent reports has explained some of the physics and chemistry behind the unique properties of INCHs, and great progress has been made, and is still being done, in this area. Most research has focused on the preparation of new INCHs using new host substances and nanostructured materials. Finding novel applications for prepared INCHs is another main research topic. However, fewer attention has been drawn to the theoretical aspect of these multifunctional materials. Therefore, developing mathematical models describing the properties, behavior, and response of INCHs is another challenging task for the scientific community. Since the main application of INCHs is in the medical and biomedical fields, design and fabrication of non-toxic INCHs with acceptable biocompatibility could be the interesting issue.

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13 Polymer/Layered Silicates Nanocomposites for Barrier Technology Philip W Labuschagne, Sean Moolman and Arjun Maity Polymers and Composites, Materials Science and Manufacturing, Council for Scientific and Industrial Research, Pretoria, South Africa

Abstract

Plastics are used increasingly in packaging applications due to a number of favorable properties, such as versatility, low weight, and low cost. However, their biggest drawback when compared to traditional packaging materials, such as glass and metals, is their relatively high permeability to gases and vapors. Specifically in food and beverage packaging, low permeability to oxygen is critical. This area of research has challenged polymer scientists, materials scientists, physicists, and chemists all over the world to fabricate new nanocomposite materials for specialized applications. They have shown that the incorporation of layered silicates can significantly reduce the permeability of gases through plastics. This chapter is aimed at highlighting the interesting potential aspects of research on selective hybrid nanocomposite materials. The effect of aspect ratio and the degree of clay exfoliation and polymer-clay interaction on the gas permeability will be discussed along with the effect of the types of organic modifier, polymer chemical structure, nature of compatibilizer, blending sequence, processing conditions as well as clay loading on these properties. Keywords: Polymers, nanocomposites, layered silicates, barrier property, surfactant, compatibilizer

13.1

Introduction

For food, beverage, and pharmaceutical packaging applications, p o l y m e r s such as poly(propylene) (PP), poly(ethyelene) (PE), Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (533-570) © Scrivener Publishing LLC

533

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INTELLIGENT NANOMATERIALS

poly(ethyleneterephthalate) (PET), poly(vinylchloride) (PVC), etc., have several benefits over glass and metal packaging. These include environmental benefits from reduced energy use in their manufacture and transport (due to much lower weight). In addition, polymer packaging is versatile, cheap, easy to mold into different shapes, colors, etc. Due to these advantages, polymers have seen tremendous growth as a substitution for glass and metal in many packaging applications over the past few decades. However, further substitution of glass and metal in packaging is limited by a major drawback: the high gas permeability of polymers in general compared to glass and metal. Many food, beverage, agricultural, and pharmaceutical products are sensitive to oxygen - e.g., beer, tomato sauce, mayonnaise, jams, preserves, fruit juices, vitamins, etc. (especially when no preservatives are used). For other products such as carbonated soft drinks, carbon dioxide (C0 2 ) retention is also important. In order to address these shortcomings, a great deal of research has focused on improving the oxygen barrier properties of polymers. Some approaches that have been followed are: blending or laminating high barrier polymers with low barrier polymers [1-6]; forming inorganic-organic hybrids, i.e., crosslinking an inorganic material into an organic material (e.g., a polymer) [7-10]; chemical vapor deposition, where an inorganic carbon or silicon oxide-based coating is deposited onto a plastic substrate [11-16]. Recent research has indicated that the incorporation of nanofillers into the polymer matrix can significantly reduce the permeability of gas through plastics. The basic principle is that the presence of high aspect-ratio, gas-impermeable silica sheets dispersed within a polymer matrix increases the tortuous path length for diffusing gas molecules (Figure 13.1), thereby providing excellent barrier properties. However, achieving optimum reduction in permeability is complex and is dependent on many variables, each of which need to be carefully considered when formulating polymer/nanofiller nanocomposites for improved gas barrier properties. Recently, several nanofillers have been recognized to enhance polymer performance. The packaging industry has focused its attention mainly on layered inorganic solid-like clays and silicates, because they have a number of benefits: their availability, low cost, significant property enhancements, and relatively simple processability. This chapter is aimed at highlighting the impact of a number of variables, such as clay aspect ratio, the degree of clay exfoliation, polymer-clay interactions, types of organic modifier, polymer

PLS NANOCOMPOSITES FOR BARRIER TECHNOLOGY

535

Figure 13.1 Tortuous path of a permeant in a clay nanocomposite. (Reproduced from Adame & Beall, 2009) [17].

chemical structure, nature of compatibilizer, and processing conditions as well as clay loading on barrier properties.

13.2 Polymer/Layered Silicate (PLS) Nanocomposite Over the past decades, polymer-based composites have been widely used in such diverse areas as transportation, construction, electronics, and consumer products because they offer the unusual combination of stiffness, strength, and low weight that is difficult to attain separately from their individual counterparts. On the other hand, polymer-based nanocomposites are a relatively new class of hybrid materials with ultrafine phase dimensions, typically along the order of a few nanometers [18]. Because of their nanometer size feature, nanocomposites possess unique properties typically not shared by their more conventional microcomposite components, and therefore offer a new technology and business opportunity. On the basis of the interaction strength between polymer and layered silicate, one type of conventional composite and two types of nanocomposites are thermodynamically achievable (Figure 13.2). (a) Conventional composite: The clay tactoid structure predominating in microcomposites, in which the polymer and clay factoids remain immiscible, resulting in

536

INTELLIGENT NANOMATERIALS

Layered silicate

(a) Phase-separated (microcomposite)

(b)

Intercalated (nanocomposite)

Polymer

(c)

Exfoliated (nanocomposite)

Figure 13.2 A schematic representation of hybrid materials derived from interaction between clays and polymers:(a) phase-separated microcomposite; (b) intercalated nanocomposite and (c) exfoliated nanocomposite [20].

agglomeration of the clay in the polymer matrix and poor macroscopic properties of the materials. (b) Intercalated nanocomposites: The intercalated nanocomposites result from the penetration of polymer chains into the interlayer region of clay, resulting in an ordered multilayer structure with alternating polymer/inorganic layers at a repeated distance of a few nanometers, regardless of polymer to clay ratio. (c) Exfoliated nanocomposites: The exfoliated nanocomposites involve extensive polymer penetration, with the clay layers delaminated and randomly dispersed in the polymer matrices that totally depend on the clay loading. Therefore, these nanocomposites exhibit the best properties due to the optimal interaction between clay and polymer [19].

13.3

Gas Permeability

Permeation of a gas through a polymer occurs through three basic steps: (a) sorption of the gas molecules onto the polymer surface; (b) diffusion of the gas molecules through the polymer along a concentration gradient; and (c) desorption of the gas molecules from the

PLS NANOCOMPOSITES FOR BARRIER TECHNOLOGY

537

opposite surface of the polymer [21]. The kinetic component of the gas transport is described by the diffusion coefficient D (in m 2 s-1), while the thermodynamic component is described by the solubility coefficient S (in mol m~3 Pa~l), and reflects the gas/polymer affinity. When the gas transport obeys Fick's law for diffusion and Henry's law for solubility, the gas permeability P (in mol Pa -1 nr 1 s_1) may be expressed as: P = DxS

(13.1)

Since very limited interaction occurs between low-polarity gas molecules (such as oxygen and carbon dioxide) and polymers, diffusion coefficients play a greater role in determining permeation rates. Various models have described the diffusion of small molecules through polymers [22, 23]. According to free volume theory, diffusion in amorphous polymers occurs when penetrant molecules jump across temporary gaps in the polymer matrix created by thermally activated motion of the chain segments [24]. It therefore follows that diffusion rates will be reduced in cases where chain mobility is restricted, free-volume is reduced, or impermeable barriers are introduced. For instance, polymers that self-associate strongly through hydrogen-bonds show not only restricted chain mobility but the chains are aligned closer to one another. This results in very low gas permeability rates as is illustrated in polymers such as poly(vinyl alcohol) (PVOH), poly(acrylonitrile) (PAN), and poly(vinylidiene chloride) (PVDC). Also, the presence of impermeable barriers, such as the crystallites in semi-crystalline polymers, increases the tortuous path length for diffusing molecules as they need to circumvent these barriers [25, 26]. Thus, physical attributes that affect the availability of free volume and tortuosity of the permeating gas molecules through a polymer matrix, have the greatest impact on gas permeability rates [27]. Table 13.1 lists the physical and chemical attributes of various polymers and their effect on oxygen permeability rates. Decreasing oxygen permeability is generally accompanied by attributes which are expected to increase the tortuous path length of diffusion molecules, such as: increased density (due to closer chain packing), increased glass-transition temperature (T ) (resulting from reduced chain flexibility), increased solubility parameter (due to strong intra-molecular interactions) and decreasing fractional free volume (less "space" available for diffusing molecules) [28].

oo

Table 13.1 Relationship between various polymer characteristics and oxygen permeability. Polymer

Oxygen Permeability [29]

Low-density poly(ethylene) (LDPE)

2.2

Poly(styrene) (PS)

1.9

Density (g/cm3) [29] 0.92

T

g

(°C) [29] -35

Fractional Free Volume

Solubility Parameter [30]

2

H W

r1

I—I

0.12 [31]

16.4

o M H

1.04

±92

0.176 [32]

19.3

0.10 [33]

18.0

0.10 [31]

18.1

Poly(propylene) (PP)

1.7

0.903

-20

High-density poly(ethylene) (HDPE)

0.3

0.95

-35

Poly(ethyleneterephthalate) (PET)

0.0444

1.4

80

0.10 [34]

20.0

Nylon-6

0.0285

1.13

56

0.12 [32]

20.3

Poly(vinylidiene chloride) (PVDC)

0.00383

1.7

-4

n/a

21.3

Poly(acrylonitrile) (PAN)

0.00015

1.17

90

0.08 [32]

27.4

Poly(vinyl alcohol) (PVOH)

0.00005

1.29

85

0.03 [32]

26.3

z

> o % >

1/1

PLS NANOCOMPOSITES FOR BARRIER TECHNOLOGY

539

13.4 Permeability of Polymer-Layered Silicate Nanocomposites Layered silica, such as montmorillonite (MMT) clay, can generally be described as crystalline materials where octahedral sheets of ~ l n m thick are sandwiched between two tetrahedral sheets of silica (Figure 13.3) [35, 36]. The length-to-width or aspect ratio of these sheets can vary between 30 and 2000 nm depending on the type of clay and the degree of dispersion or exfoliation. The sheets also act as impermeable barriers to diffusing molecules, and when

Qf

1

one clay platelet L: 100-200 nm in case of MMT

Tetrahedral

Octahedral

Tetrahedral

Figure 13.3 The structure of a 2:1 layered silicate [36, 37].

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INTELLIGENT NANOMATERIALS

exfoliated and oriented optimally within a polymeric matrix, they force diffusing molecules to follow longer and more tortuous paths, which can lead to orders of magnitude reduction in gas permeability rates. Table 13.2 summarizes the barrier improvement factors (BIF) of a number of polymerMayered silicate (PLS) nanocomposites and the conditions under which they were prepared. It is also understood that due to interaction between the silica sheets and polymer, mobility of the polymer chains surrounding the sheets are restricted [35]. Thus, the three main factors that affect the permeability of PLS nanocomposites are: the volume fraction of the nanosheets; their orientation relative to the diffusion direction and; their aspect ratio. Several models describe mass transfer within PLS nanocomposites. These models generally assume that the sheets do not absorb or conduct the penetrant, and that interaction between the sheets and polymer are sufficient to prevent void formation at the interface. A simple permeability model, which assumes that the sheets are evenly dispersed and oriented perpendicular to the diffusion direction, was proposed by Nielsen [38]. The solubility coefficient is predicted to be:

S = S o (l-0)

(132)

where S0 is the solubility co-efficient of the pure polymer, and φ is the volume fraction of the nanosheets that are dispersed in the polymer matrix. For approximation of the diffusion coefficient it is necessary to take into consideration the tortuous path length for diffusing molecules in order to circumvent the impermeable sheets. This can be accounted for by the tortuosity factor (τ), which is defined as:

T=y

(13.3)

where V is the actual distance that a permeant must travel to diffuse through the polymer matrix when nanosheets are present, and I is the distance it would have travelled without nanosheets. The diffusion coefficient can then be approximated as follows: D = D0T

(13.4)

Table 13.2 Summary of barrier improvement factors (BIF) of a number of polymerX layered silicate nanocomposites and the conditions under which they were prepared. Polymer

Compatibiliser (wt%)

Clay Type

Surfactant

d-Spacing(A)"

Composite Preparation

Test Specimen Preparation

Permeant

Clay Content (wt%)

BIF"

Ref.

1

1.27

[50]

3

1.45

1

1.20

3

1.68

2.8 vol%

1.22

POLYOLEFINS

13

Low density poly(ethylene) LDPE

None

Cloisite 6A

2M2H-AC

35.1

Melt blending

Film blowing (blow-up ratio 3:1)

Oxygen

LDPE ionomer

None

Cloisite 6A

2M2H-AC

35.1

Melt blending

Film blowing (blow-up ratio 3:1)

Oxygen

Twin-blade kneading & compression moulding

Oxygen

High density poly(ethylene) HDPE

None

r1 >

z

o o o

SS

*t

o

Nanofil 757 modified 1

OD3M

18.2

Nanofil 757 modified 2

20D2M

24.5

Nanofil 757 modified 3

M30D

32.5

Cloisite Na+ modified 2

20D2M

25.1

1.46

Cloisite Na+ modified 3

M30D

34.8

1.74

Optigel CK modified 1

OD3M

18.5

1.23

Optigel CK modified 2

20D2M

26.6

1.64

w

Optigel CK modified 3

M30D

35.8

1.59

n

Melt blending

[51]

1.39

on H W on 11

1.54

O

» h-
2

z

H o W o f o r1 SS o *t

ow H

on H 2; W on

1.69

>

1.17

11

32.8

1.50

O

M30D

38.4

1.64

2M2HT

24.2

w > »

-

Melt extrusion

Film blowing

Oxygen

2

1.37

5

1.90

7

1.80

LDPE

LLDPE-s-MA (MFtl.5)

5

1.10

HDPE

HDPE-s-MA (MFI: 2)

5

0.28

o

[52]

en h-
o

>

Figure 17.15 Preparation of Qdots-labeled plasmid DNA. The first step is incorporation of aminoallyl dUTP into DNA by PCR procedure. The second step is covalently attachment of biotin- succinimidyl ester to the amino group, and third step is labeling with Qdots-streptavidin.

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Figure 17.16 Time-lapse imaging experiments performed using confocal microscopy to track uptake of Qdots-DNA conjugate in cyanobacteria Synechocystis sp. PCC6803. Scale bar = 10 μπι.

The successful transformation of cyanobacteria shows the non toxicity of labeled slr2060. Thus Qdots-DNA conjugates represent an ideal method of tracking plasmid DNA since they produce a highly stable fluorescent signal that can be used for long-term studies (such as the 24-h time-lapse confocal imaging and Photostability experiments) and the conjugation does not affect the DNA functionality. This technique enabled correlation between DNA distribution and protein expression by simultaneous tracking using time-lapse imaging. Qdots-DNA conjugates may be used in future studies to gain a better understanding of the efficiencies of the various processes involved in the cellular and nuclear uptake of plasmid DNA.

17.5 Perspectives The dynamic behavior of cells is a consequence of the coordinated and elaborate interactions between complexes of macromolecules that constitute their formed structures or organelles. Live-cell imaging and high-resolution determination of the location of nucleic acids, proteins, metabolites, and ions can now be made within subcellular domains down to the molecular level, revealing the important information required to understand complex cellular function. Light and electron microscopic methods have proved extremely powerful in providing information about cells, and techniques such as immunolabeling have been applied successfully to identify biological macromolecules.

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The technology reported here provides practical methods to combine LM and EM with correlative multiple immunolabeling using different quantum dots and colloidal metal nanoparticles as labels. The method limits quenching of fluorescent dyes for LM and provides molecular spatial resolutions for EM of the exact same labels. Simultaneous multiple correlative immunolabeling to detect more than two antigens can be achieved using metal nanoparticles of different compositions including Zn, Cd, Te, Au, Pt etc. on an appropriate antibody. This method is not limited by the necessity of using labels of different sizes, and therefore, could open a number of new biological application requiring small labels. Because the only requirement is that labels have different chemical compositions that can be differentiated be analytical EM, this method is not restricted to only two labels. The number of different labels depends on the number of species with sufficiently different X-ray spectra that can be produced.

References 1. Special issue on Biological Imaging, Science 300, 75,2003. 2. M. Howarth, W. Liu, S. Puthenveetil, Y. Zheng, L.F. Marshall, M.M. Schmidt, K. Wittrup, M.G. Bawendi, A.Y. Ting, Nature Methods, Vol. 5, p. 397,2008. 3. X. Michalet, F.F. Pinaud, L.A. Bentolila, J.M. Tsay, S. Doose, J.J. Li, G. Sundaresan, A.M. Wu, S.S. Gambhir, S. Weiss, Science 307, p.538,2005. 4. R.M. Albrecht, O.E. Olorundare, S.R. Simmons, J.S. Loftus, D.F. Mosher, In Methods of Enzymology. New York, Academic Press, p. 456,1992. 5. R.M. Albrecht, S.R. Simmons, J.B. Pawley, Immunochemistry, A practical approach, Chapter 8 (ed. by J.E. Beesley), p. 151,1993. 6. W.P. Faulk and G.M. Taylor, Immunochemistry 8, p. 1081,1971. 7. R. Nisman, G. Dellaire, Y Ren, R. Li, D.P. Bazett-Jones, Journal of Histochemistry & Cytochemistry 52(1), p. 13,2004. 8. W.C. Chan and S. Nie, Science 281, p. 2016,1998. 9. L. Groc, M. Lafourcade, M. Heine, M. Renner, V. Racine, J. Sibarita, B. Lounis, D. Choquet, L. Cognet, The Journal of Neuroscience 27(46), p. 12433,2007. 10. H.I. Yeh, S. Rothery, E. Dupont, S.R. Coppen, N.J. Severs, Circulation Research 83, p. 1246,1998. 11. I. Kandela, R. Bleher, R. Albrecht, Journal of Histochemistry and Cytochemistry 55, p. 983,2007. 12. A. Loukanov, N. Kamasawa, R. Danev, R. Shigemoto, K. Nagayama, Ultramicroscopy 110, p. 366,2010. 13. K. Fujimoto, Journal of Cell Science 108, p. 3443,1995. 14. S. Emin, A. Loukanov, M. Wakasa, S. Nakabayashi, Y. Kaneko, Chem. Lett. 39, p. 654,2010. 15. G. Milligan, M. Bouvier, FEBS J. 272, p. 2914,2005.

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16. R. Vincente, A. Escalada, N. Villalonga, L. Texido, M. Roura-Ferrer, M. MartinSatue, C. Lopez-Iglesias, C. Soler, C. Solsona, M. M. Tamkun, A. Felipe, /. Biol. Chem. 281, p. 37675,2006. 17. F. Plane, R. Johnson, P. Kerr, W. Wiehler, K. Thorneloe, K. Ishii, T. Chen, W. Cole, Circ Res. 96, p. 216,2005. 18. P. M. Kerr, O. Clement-Chomienne, K. S. Thorneloe, T. T. Chen, K. Ishii, D. P. Sontag, M. P. Walsh, W. C. Cole, Circ. Res. 89, p. 1038,2001. 19. G. E. Torres, T. M. Egan, M. M. Voigt, /. Biol. Chem. 274, p. 6653,1999. 20. M. L. Mayer, N. Armstrong, Annu. Rev. Physiol. 66, p. 161,2004. 21. M. Bouvier, Nat. Rev. Neurosci. 2, p. 274,2001. 22. S. Ferre, R. Baler, M. Bouvier, M. G. Caron, L. A. Devi, T. Durroux, K. Fuxe, S. R. George, J. A. Javitch, M. J. Lohse, K. Mackie, G. Milligan, K. D. Pfleger, J. P. Pin, N. D. Volkow, M. Waldhoer, A. S. Woods, R. Franc, Nat. Chem. Biol. 5, p. 131, 2009. 23. M. Chabre, P. Deterre, B. Antonny, Trends Pharmacol. Set. 30, p. 182, 2009. 24. G. L. Collingridge, R. W. Olsen, J. Peters, Neuropharmacol. 56, p. 2, 2009. 25. K. Matsuda, E. Miura, T. Miyazaki, W. Kakegawa, K. Emi, S. Narumi, Y. Fukazawa, A. Ito-Ishida, T. Kondo, R. Shigemoto, M. Watanabe, M. Yuzaki, Science 328, p. 363,2010. 26. W. Kakegawa, T. Miyazaki, K. Kohda, K. Matsuda, K. Emi, J. Motohashi, M. Watanabe, M. Yuzaki, / Neurosci. 29, p. 5738,2009. 27. W. Kakegawa, T. Miyazaki, K. Emi, K. Matsuda, K. Kohda, J. Motohashi, M. Mishina, S. Kawahara, M. Watanabe, M. J. Yuzaki, Neurosci. 28, p. 1460,2008. 28. J. Tanaka, M. Matsuzaki, E. Tarusawa, A. Momiyama, E. Molnar, H. Kasai, R. Shigemoto, }. Neurosci. 25, p. 799,2005. 29. M. Masugi-Tokita, E. Tarusawa, M. Watanabe, E. Molnär, K. Fujimoto, R. Shigemoto, /. Neurosci. 21, p. 2135,2007. 30. M. Yuzaki, Neurosci. 162, p. 633,2009. 31. B. D. Ripley, / R Stat Soc Ser B Stat Methodol 41, p. 368,1979. 32.1. A. Prior, C. Muncke, R. G. Parton, J. F. Hancock, / Cell Biol 160, p. 165, 2003. 33. T. Rizk et al, J. Biomed. Mater. Res. A 68, p. 360,2004. 34. R. Savic et al, Science 300, p. 615,2003. 35. J. Panyam et al, Int. ]. Pharm. 262, p. 1,2003. 36. J. Panyam et al, FASEB J. 16, p. 1217,2002. 37. M. Tanimoto et ah, Bioconjugate Chem. 14, p. 1197, 2003. 38. W.C. Tseng et al, J. Biol. Chem. 272, p. 25641,1997. 39. P. Leahy, G.G. Carmichael, E.F. Rossomando, Bioconjugate Chem. 7, p. 545,1996. 40. M.E. Dowty et al, Proc. Natl. Acad. Sei. USA 92, p. 4572,1995. 41. J.E. Hagstrom et al, Biochim. Biophys. Ada 1284, p. 47,1996. 42. M. Egli, W. Saenger, in Principles of nucleic acid structure, Springer, 1983. 43. A. Loukanov, S. Emin, S. Singh, A. Angelov, K. Nagayama, Adv. Mat. Lett. 1(2), p. 114,2010.

18 Nanofibers-based Biomedical Devices Debasish Mondal 1 and Ashutosh Tiwari2 Hntegrative Regenerative Medicine centre, Department of Clinical and Experimental Medicine, Linköping University, Linköping, Sweden 2 Biosensors and Bioelectronics Centre, Institute of Physics, Chemistry and Biology, Linköping University, Linköping, Sweden

Abstract

Polymer fibers have been employed in many biomedical applications — for example, sutures, tissue engineering matrices, gauzes and bandages, and drug delivery devices. These are made of either non-biodegradable polymers or biodegradable polymers, such as poly (lactide-co-glycolide). Nanofiber-based materials have several advantages compared to conventional fibers. In particular, they represent a very large surface area to volume ratio, high porosity, and variable pore-size distribution. Additionally, the surface functionality is possible to influence, and various morphologies are achievable, including nanotubes. Recently, electrospun nanofiber matrices have garnered a lot of attention and shown great potential in biomedical applications. The three-dimensional synthetic biodegradable scaffolds are designed utilizing nanofibers to serve as an excellent framework for cell adhesion, proliferation, and differentiation. Physiochemical properties of nanofiber scaffolds can be governed by manipulating electrospinning parameters to meet the demands of a specific applications. Various attempts have been made to modify nanofiber surfaces with several bioactive molecules to allow cells with the requisite chemical cues and a more in vivo -like environment. Nanofibers have been used as scaffolds for skin tissue engineering, musculoskeletal tissue engineering (bone, cartilage, ligament, and skeletal muscle), vascular tissue engineering, neural tissue engineering, and as carriers for the controlled

Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (679-714) © Scrivener Publishing LLC

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delivery of therapeutic molecules (drugs, proteins, and DNA). This chapter focuses on recent developments on fabricating biomimetic (ECM-like) nanofibrous scaffolds for tissue engineering, controlled release of bioactive molecules, and biosensors for biomedical devices. Keywords: Electrospinning, nanofibers, core-shell nanofibers, biomaterial, scaffold, drug delivery, tissue engineering, biosensors

18.1 Introduction The essence of nanotechnology is in the creation and utilization of nanomaterials and devices at the level of atoms, molecules, supramolecular structures, in the development of unique properties and phenomena of matters with diameter less than 100 nanometers [1]. Polymer nanofibers, an important class of nanomaterials, have acquired significant interest over the past decade. These nanofibrous substrates represent a unique class of materials due to their three-dimensional features that closely mimic the structure of the natural extracellular matrix (ECM) [2, 3]. Nanofibrous systems may be ideal candidates as direct implantable biomedical devices for tissue regeneration due to their high surface area-to-volume ratio characteristics also enable enhancement in cellular attachment and efficient mass transport from the scaffolds. The usage of nanofibrous scaffolds for biomedical applications has drawn a great deal of attention in the past several years. For examples, nanofibrous scaffolds have been demonstrated as desirable substrates for tissue engineering [4-7], wound dressing [8, 9], artificial blood vessels [10,11], and immobilized enzymes [12-14]. They have also been used as vehicles for controlled drug (gene) delivery [15-22]. The nanofibrous scaffold must exhibit suitable physical and biological properties closely matching the desired requirements for a successful application to a specific target. For example, in tissue engineering, the electrospun scaffold should physically match the nanofibrous features of extracellular matrix (ECM) with desirable mechanical properties. It should also be capable to promote cell adhesion, spreading and proliferation. For tissue engineering, the nanofibrous scaffold should not only serve as a substrate for tissue regeneration, but also used as a carrier for deliver suitable bioactive agents, including drugs (e.g. antibiotic agent), within a controlled manner during healing.

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Various fabrication techniques such as melt-blown [23, 24], phase separation [25, 26], self assembly [27,28], template synthesis [28, 29] and electrospinning [30, 31], have been employed to produce suitable polymer nanofibers for different purposes. Amongst, electrospinning is the most popular, simple, cost-effective and able to produce continuous nanofibers of various materials from polymers to ceramics. Furthermore, electrospinning seems to be the only method which can be further formulated for large scale production of continuous nanofibers for industrial applications. In recent years, electrospinning technique which was invented about 100 years ago has emerged as a popular technique due to its possible biomedical applications [32]. Pure polymers as well as composites and blends of both natural and synthetics polymers have been successfully electrospun into nanofiber scaffolds. The produce fiber's diameter is in the range of ~3 nm-6 μπ\ and several meters in length. Electrospinning technique has several advantages over other fabrication techniques listed in Table 18.1. Recently, electrospun scaffolds have acquired a lot of attention and show great potential in tissue engineering and controlled drug release due to its high surface area to volume ratio, high porosity and variable pore-size distribution and structures mimicking natural extracellular matrix (ECM) [32-36].

18.2 Nanofibers Fabrication Techniques 18.2.1

Electrospinning

Electrospinning was first patented by Cooley [37] and Morton [38] in 1902. An electrospinning device basically consists of a high-voltage power supply, which creates an electric field between a grounded collector and a syringe or capillary tube with a metal spinneret of small diameters filled with polymer solution as presented in Figure 18.1 [39]. In brief, when a sufficiently high voltage is applied to liquid droplet and the applied electric potential overcomes the surface tension of the polymer, then a polymer jet is ejected from the surface of a charged polymer solution. The ejected jet under the influence of applied electrical field travels rapidly to the grounded collector. Before reaching the collector, the solution jet which is applied with electrostatic charges undergoes stretching. With evaporation of solvents, the jet is eventually solidified on the collector in the form of nanofibers (Figure 18.2). The elongation and thinning of the fiber leads to the formation of

Table 18.1 Advantages and disadvantages of different nanofiber fabrication techniques. Fabrication Technique Drawing

Simple equipment

Continuous process Fiber dimensions can be varied using different templates

oo

Disadvantages

Advantages •

ON

• • •

Discontinuous process Not scalable No control on fiber dimensions

•

Not scalable

X

H M I—l

o M H

Template synthesis

• •

Temperature induced phase separation

• Simple equipment • Convenient to process • Mechanical properties of fiber matrices can be varied by changing the polymer composition

• Limited to specific polymers • Not scalable • No control on fiber dimensions

Molecular self-assembly

• Only smaller nanofibers of few • nm in diameter and few micron • in length can be fabricated

• Complex process • Not scalable • No control on fiber dimensions

Electrospinning

• Simple instrument • Continuous process • Cost effective compared to other • methods • Scalable • Ability to fabricate fiber diameters few nm to several microns.

• Jet instability • Toxic solvents • Packaging, shipping, handling

>

oz % a

>

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(b)

Shell solution Core solution

Syringe pump

Figure 18.1 Schematics illustration of electrospinning setup [39]. Reprinted with permission from Elsevier.

a uniform fiber with micro to nanometer scale diameters, which can be collepted in various orientations to create unique structures with different composition and mechanical properties [39,40]. Usually electrospinning produces nonwoven two-dimensional (2-D) sheet because the rate at which the polymer solution is ejected by the orifice must be controlled at a low value if nanoscaled fiber diameter is desired. Nevertheless, 3-D nonwoven fibrous sheet can be obtained if the electrospinning time is prolonged. One efficient method to increase the thickness of the nonwoven sheet is to use more than one orifice simultaneously, in which the electrospinning speed will be proportional to the number of orifices. In future this method may be helpful to produce 3-D nanofibrous scaffold in a short time [40]. Kidoaki et al. investigated the preparation of a multilayer structured nanofiber mat by electrospinning different polymer solutions layer by layer [41]. A wide range of polymers have been used in electrospun nanofibers. Biodegradable polymers such as poly(lactic-co-glycolic acid), PLGA or PCL and water soluble biomaterials such as poly(ethylene oxide) (PEO) and polyvinyl alcohol (PVA) can be easily electrospun into nanofiber, utilizing water or organic solvent [42-45]. Natural polymers such as collagen [46, 47], silk protein [48-50], elastinmimetic peptide [51], fibrinogen [52], casein and lipase enzyme [53], and DNA [54], have also been electrospun into nanofibers.

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Figure 18.2 Electrospinning nanofibers of various morphologies and patterns; (a) random PCL fibers, (b) aligned PCL fibers, (c) core-shell fiber, (d) Porous PLLA fiber [40].

Electrospinning of naturally occurring biomaterials is much more challenging compared with synthetic polymers because of difficulties in finding suitable solvents. l^lAS^-Hexafluoro^-propanol (HFP) is a commonly used solvent for electrospinning of proteins [46, 54]. Silk fibroin electrospun nanofibers have been produced with formic acid as solvent [49, 50]. Silk fibroin can also be mixed with PEO in water and electrospun [51, 52]. Huang et al. utilized a genetic engineering method to synthesize polypeptides containing the repeated elastomeric peptide sequence of elastin, (Val-Pro- Gly-Val-Gly)4(ValPro-Gly-Lys-Gly). The polypeptide was dissolved in water and electrospun into nanofibers with diameters from 200 to 300 nm under appropriate conditions [51]. Fang and Reneker described the electrospinning of calf thymus sodium-DNA aqueous solutions (concentration, 0.3 to 1.5%) into nanofibers with diameters of 50 to 80 nm [54].

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18.2.2

685

Phase Separation

Phase separation is a new technique frequently used for the formation of nanofibrous foam materials. Porous polymer scaffolds are produce by removal of the solvent through freeze-drying or extraction. Phase separation can be induced by changing the temperature, called thermal induced phase separation or by adding nonsolvent to the polymer solution, called non-solventinduced phase separation [55,56]. Gelation is the most critical factor that controlled the porous morphology of the nanofibrous foams. Gelation duration varied with polymer concentration and gelation temperature. Low gelation temperature led to the formation of the nanoscale fiber networks, whereas high gelation temperature led to the formation of a platelet-like structure due to the nucleation of crystals and their growth. This limitation of platelet-like structure formation can be overcome by increased cooling rates. Nevertheless, the average diameter of fibers was not significantly affected by gelation condition or polymer concentration. When process parameters, such as solvent, polymer concentration, gelation temperature, and gelation time are precisely controlled, micro- or nanoscale polymer fibers can be obtained. The 3D porous continuous fibrous network formed by the phase separation process showed high porosity of about 98% within blocks of the material [56]. Yang et al. reported that a 3-D meshwork, made up of PLLA nanofibers with diameters ranging from 50-500 nm has been prepared by the phase separation method, using tetrahydrofuran (THF) as solvent [24]. 18.2.3

Self-assembly

Self-assembly which is an autonomous approach, involves the spontaneous organization of individual molecules into a welldefined and stable hierarchical structure with preprogrammed non-covalent interactions. Nanofibers matrices can be produced by self-assembly approach [58-61]. Hartgerink et al. reported the effect of variations in the molecular structure of the Pas on the selfassembled nanofibers. They found that modifications in the alkyl chain length of the PA alter the pH sensitivity of nanofibers, which affects self-assembly and modification of the C~ terminal region led to changes in length and stiffness of the nanofibers. This study also introduced three different methods of forming self-assembled

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PAs, including pH-controlled self-assembly, drying on surface induced self-assembly, and divalent-ion-induced self-assembly [60] Synthetic polymer nanofibers have been prepared by self assembly of diblock polymers (AnBm) when the two blocks segregate from one another in bulk owing to their incompatibility. The volume fraction of A and B can be controlled to obtain cylinder shaped B domains, with nanoscaled diameter, embedded in a continuous matrix of A [59-61]. Liu et al. reported self-assembly of polyphenylene dendrimers into micrometer-long nanofibers. They found that the morphology of the dendrimer nanofibers highly depended on substrate, solvent, and preparation method. The mechanism for the formation of the dendrimer nanofibers is supposed to be the 3-d interactions between the dendrimer molecules [62, 63]. The diameter of the nanofibers produce by this technique is much thinner compared to the electrospinning technique, but productivity also very less with this method.

18.3 Polymeric Materials for Nanofibers 18.3.1

Natural Polymers

Natural polymers are very similar, often identical, to macromolecular substances present in the human body. Various natural polymers are used as biomaterials, such as gelatin, chitosan, collagen, hyaluronic acid, elastin, silk, and wheat protein [3,48, 68,105]. Collagen nanofibers have been manifested to show compatibility with a number of cell types, including myoblasts and chondrocytes [3]. Huang et al. prepared the blending of type I collagen nanonbers (produced by electrospinning) with poly(ethylene oxide) (PEO). Their results demonstrated that, the mechanical strength of the nanofiber system was significantly increased due to a high number of inter-molecular interactions between collagen and PEO [48]. These studies depicted the promising role of collagen in tissue engineering. Chitosan, a natural biomaterial, has been used to make nanofibers. Nonwoven or aligned chitosan/PEO (90:10) nanofibers have been formulated utilizing the electrospinning technique. They found that the nanofibers possessed structural integrity in water and their cell studies demonstrated enhanced attachment of human osteoblasts and chondrocytes onto the nanofibers. Hyaluronic acid is a most important component of the ECM of tissue and has been

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used as a biomaterial. Um et al. produced nanofibers of hyaluronic acid using the technique of electrospinning [64]. They observed that electrospinning of hyaluronic acid does not allow the uniform production of high-quality nonwoven nanofibers. Another natural biomaterial that has been well studied is gelatin. Zhang et al. prepared gelatin/PCL composite fibrous scaffolds using the electrospinning technique and indicated that the composite nanofibers have improved mechanical strength and wettability compared with gelatin or PCL alone [105]. Moreover, the nanofibrous scaffold of gelatin-PCL showed good cell attachment, growth, and migration of bone marrow stromal cells. Therefore, composite nanofibers of natural and synthetic materials could be a good choice for improving the mechanical properties of natural biomaterials for tissue engineering applications. Silk fibroin is another potential natural biomaterial for nanofibrous scaffolds because of their cytocompatibility, fiber diameter, and high porosity. Therefore, a wide variety of natural polymers have been explored for the formulation of nanofibers as scaffolds for tissue engineering. 18.3.2

Synthetic Polymers

A large variety of synthetic polymers has been used to form nanofibers. These include poly (lactic-coglycolic acid) (PLGA), most important polymer used to fabricate nanofibers for bone and cartilage tissue engineering and controlled delivery of therapeutic molecules [17]; polydactic acid) (PLA) [24]; polyethylene terephthalate) (PET) for blood vessel; PCL in neural and cartilage tissue engineering [40-45]; various copolymers such as poly(L-lactic acid-co-epsilon-caprolactone) (PLLA-CL) as a biomimetic ECM for smooth muscle and endothelial cells; poly(ethylene-co-vinylacetate) (PEVA) nanofibers for controlled drug delivery; and PLGA-poly(ethylene glycol) (PLGAPEG) block copolymeric nanofibrous scaffolds as a matrix for DNA delivery. Therefore, a wide variety of synthetic polymers has been utilized for the nanofibers formulation by electrospinning technique.

18.4

Biocompatibility of Nanofibers

Although a variety of biodegradable and non-biodegradable polymers have been fabricated into nanofibrous scaffolds using different techniques discussed above, many of them have low

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biological compatibility with cells, which has limited their biomedical applications. So, various biodegradable and nonbiodegradable synthetic nanofibers have been surface-modified with bioactive molecules for advanced therapeutic application. Synthetic polymers are much easier to process and more controllable in electrospinning than natural polymers. Water soluble polymers and natural polymers are frequently considered to be difficult to directly process into nanofibers due to their unstable nature to processing conditions and weak mechanical property [66, 67]. A wide variety of natural polymers having unique biological functions can be immobilized onto the nanofibrous surface of synthetic polymers without compromising bulk properties due to the processing benefits of synthetic polymers. Such functionalized synthetic nanofibers can direct enhance cellular interactions and organization, because tissue regeneration process is strongly involved in various biochemical cues on the cell contacting surface. For controlled drug delivery applications, the electrospinning process enables a wide variety of hydrophobic therapeutic agents to be directly incorporated within the bulk phase of nanofibers for controlled release. For example, a biodegradable polymer, PLGA solution containing hydrophobic anti-cancer drugs such as paclitaxel was directly electrospun to formulate the drug releasing nanofibrous mesh [68]. Alternatively, hydrophilic and therapeutic molecules such as proteins and nucleic acids were covalently and physically immobilized onto the modified surface of nanofibrous matrix for modulating cellular functions. Surface modification has the advantage of not altering the scaffold architectures significantly. Several surface modification techniques for applying synthetic polymer nanofibers to tissue engineering and drug delivery are presented here in Figure 18.3. Plasma treatment has been commonly employed to introduce desired functional groups and molecular chains onto the nanofibrous surface of an electrospun matrix. Plasma exposure improves the surface adhesion and wetting properties of the matrix [69-72]. Ideal selection of the plasma source enables the introduction of various functional groups on the target surface to improve biocompatibility or to allow subsequent covalent immobilization of various bioactive molecules. For example, various electrospun nanofibers made of poly (ε-caprolactone) (PCL), PCL/ hydroxyapatite, polystyrene, and silk fibroin were surface-modified by

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Blocompatlble nanofibers

Electrospum nanofibers

Functionalized surface

Biologically or therapeutics ly fu nationalized nanofibers (b) Induced by piasma or radiation

«V

Monomer Electrospum nanofibers

(c)

*

Immobilization of protein, enzyms, growth factor, drug Surface graft polymerization

Biologically or the rape utically functionalized nanofibers

Biologically or the rape utically functional agents Electrospinning

Bland solution

Surface orientation

Figure 18.3 Surface modification of electrospun nanofibers by different techniques; (a) Plasma treatment or wet chemical method, (b) Surface graft polymerization, (c) Co-electrospinning [166]. Reprinted with permission from Elsevier.

air or argon plasma, resulting in an improved cell adhesion and proliferation [70-73]. Poly (ε-caprolactone) (PCL) nanofibers were prepared by electrospinning and type-I collagen was then immobilized on the nanofibers after surface modification by plasma treatment. Collagen immobilization enhanced the attachment, spreading and proliferation of human dermal fibroblasts [74]. Gelatin grafting on to plasma treated PCL nanofibers

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improved endothelial cell spreading and proliferation as well as cell orientation [75]. Most of the synthetic biodegradable polymers retain their hydrophobic surface nature, often requiring hydrophilic surface modification for desired cellular responses. So, surface graft polymerization has been utilized to introduce the surface hydrophilicity and also to introduce the multi-functional groups on the surface for covalent immobilization of bioactive molecules for the purpose of enhanced cell adhesion, proliferation, and differentiation [76-79]. The surface graft polymerization is often initiated with plasma and UV radiation treatment to generate free radicals for the polymerization. Electrospun polyethylene terephthalate (PET) nanofibers were modified with poly (methacrylic acid) by graft polymerization in a mild condition without any structural changes in the bulk phase [80]. Electrospun polyurethane (PU) nanofibers were modified with poly(4-vinyl-N-hexyl pyridinium bromide) on the surface for antibacterial applications [81]. Since most of the biomolecules are charged cationic or anionic polyelectrolytes, the immobilization of these biomacromolecules onto 3D nanofibrous scaffolds can be accomplished by a layer-by-layer self-assembly process [82-84]. For example, nanofibrous PLLA scaffold was activated in an aqueous poly(diallyldimethylammonium chloride) (PDAC) solution to obtain positively charged pore wall surface. The scaffold was subsequently immersed in a solution of negatively charged biomacromolecules (e.g., gelatin). Positive or negative charges are developed on the surface by alternative immersion in the two different solutions. Thus self-assembly approach is carried out in aqueous solutions under mild conditions and offers a controlled way to regulate the surface charge type and the thickness of the surface modification layer [85]. Growth factors and DNA may also be self-assembled on to nanofibrous scaffolds as a gene-delivery strategy to regulate cellular response and gene expression. Nanoparticles and functional polymer segments can be directly exposed on the surface of nanofibers by co-electrospinning with bulk polymers [86-89]. For example, when PLLA solution was blended with hydroxyapatite (HAp) nanocrystals, HAp was exposed on the surface of the resultant electrospun fibers, giving rise to high surface free energy and low water contact angle [90]. These composite fibers exhibited a retarded degradation rate as compared to pure PLLA fibers.

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18.5 Application of Nanofibers in Biomedical Devices 18.5.1

Nanofibrous Scaffold for Tissue Engineering

Biodegradable scaffolds are generally recognized as an essential element in tissue engineering and regenerative medicine strategies. Scaffolds are temporarily used as guides for cell seeding, migration, proliferation and differentiation prior to the regeneration of biologically functional tissue or natural extracellular matrix (ECM). Ideally, to create an artificial tissue engineered construct capable of regenerating a fully functional tissue, morphological similarity with the native tissue is important. Nanofibrous scaffolds with fiber diameter down to nanometer are explored as potential for tissue engineering by virtue of their high surface area, high porosity, and the similarity of their 3D architecture to the natural ECM. Such physical cues enhance cell adhesion, proliferation and differentiation, and consequently neo-tissue formation on nanofibrous meshes of both natural and synthetic polymers [91, 92]. Several nanotopographical features have been created and utilized as new generation tissue engineering scaffolds and biomedical implant surfaces [93]. The noteworthy nanotopographic features of the nanofiber matrices allow cells the necessary physical cues similar to the nanotopographical features of a natural basement membrane. Polymeric nanofiber matrices can also act as carriers for a variety of bioactive agents, such as antibiotics, antifungal, antimicrobial, proteins (enzymes, DNA, etc.), anticancer and other valued drugs [94]. Core-shell nanofibers have been successfully designed to release the desired bioactive agents at therapeutic concentrations in both a spatial and temporal pattern [95]. Recently Jayasinghe et al. have successfully manifested the feasibility of encapsulating living cells within core-shell microfiber matrices [96, 97] and they found that the microfiber matrices, obtained via cell electrospinning showed significant numbers of viable cells over long periods of time. Therefore, nanofiber matrices encapsulated with suitable growth factors, cells or bioactive agents have a great potential for use in tissue regeneration by providing cells with essential physical and chemical cues. Therefore, nanofibrous systems have been pursued as scaffolds for the regenerate various tissues such as bone, skin, blood vessel, tendon/ligament and nerve.

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18.5.1.1

Bone Tissue Engineering

A wide variety of synthetic and natural materials have been explored for bone tissue engineering. Although, nanofibrous scaffolds have been studied well for multiple tissue types, musculoskeletal tissue is most likely well studied. The physical properties of bone tissue such as mechanical strength, pore size, porosity, hardness, and overall 3D architecture are very important factors to design the scaffolds for bone tissue engineering. Scaffolds with a pore size in the range of 100-350 μιη and porosity greater than 90% are preferred for better cell/tissue in-growth and hence enhanced bone regeneration [98, 99]. Yoshimoto et al. developed nonwoven electrospun PCL scaffolds for the purpose of bone tissue engineering [100]. MSCs derived from bone marrow of neonatal rats were seeded on the nanofibrous scaffold to understand the influence of mesenchymal stem cells (MSCs) on nanofibers. The results suggested that the MSCs migrated inside the scaffold and produced abundant extracellular matrix in the scaffold. Shin et al. tested the PCL nanofibers along with MSCs in vivo in a rat model in continuation to this study. They observed the ECM formation throughout the scaffold along with mineralization and type I collagen synthesis [101]. These studies indicated that PCL-based nanofibrous scaffolds are potential candidates for bone tissue engineering. Recently, Araujo et al. developed electrospun PCL nanofibers and surface-modified with calciumphosphate. They found that calciumphosphate layers on nanofibers showed similar characteristics of human bones and the nanofibers strongly enhanced osteoblastic differentiation and proliferation for a prolonged period of cell culture, suggesting potential application in bone tissue engineering [102]. Venugopal et al. investigated that, PCL provided mechanical stability, collagen supported cell adhesion and proliferation, and nanoHA (nHA) provided the mineralization of osteoblasts for bone regeneration. The osteoblast proliferation rate is increased by 35% and mineralization by 55% upon addition of nHA and collagen [103]. In another study, Meng et al. produced electrospun scaffolds of Type I collagen and poly(3-hydroxybutyrate-cohydroxyvalerate) (PHBV). PHBV is well known biodegradable, biocompatible, nontoxic, thermoplastic polyester produced by bacteria. The study indicated that the PHBV-collagen nanofibrous scaffold accelerated the adhesion and growth of NIH3T3 cells more effectively than PHBV nanofibrous scaffold [104]. Zhang et al. observed that incorporating gelatin with PCL, bone marrow stromal cells spread

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better and migrated deeper inside the gelatin/PCL scaffold. The cells infiltrated up to 114 μιη in gelatin/PCL scaffold compared to 48 μιη in only PCL scaffold [105]. 18.5.1.2

Cartilage Tissue Engineering

In the past few years, nanofibers have been explored as new functional structures for cartilage tissue regeneration [106]. Several synthetic and natural materials such as PCL, PLGA, chitosanbased materials, or starch-based materials have been used for cartilage TE [107-110]. Cartilage, an avascular tissue composed of chondrocytes entrapped in an ECM rich in proteoglycans and collagens [111]. Cartilage has low self-regeneration potential, due to the absence of vascular networks and progenitor cells in the tissue [112]. Chondrocytes have been used to generate engineered cartilage tissue, [113, 114] and are usually isolated from articular cartilage tissues. Stem cells are also commonly proposed for cartilage TE. Chondrocytes are developmentally derived from stem cells [114]. Stem cells are easier to obtain and manipulate compared to adult chondrocytes, as they can undergo several passages before loosening their differentiation potential [115]. Selecting the ideal source of cells for cartilage tissue engineering and the choice of the scaffold is a demanding and challenging task. A recent study demonstrated the dissimilar chondrogenic differentiation behavior of MSCs derived from different tissues, namely, human embryonic stem cells, bone marrow, and adipose tissue. Additionally, their behavior also differs among the tested silk and chitosan scaffolds [115]. Realizing the importance of electrical and mechanical properties for cartilage reconstruction, conductive nanofibrous scaffolds were fabricated by electrospinning of biodegradable poly(lactic acid) (PLA) mixed with single wall carbon nanotubes (SWNT) [116]. An in vitro test showed that the SWNT incorporated nanofiber scaffold still allows cells to grow with no hostile influence on cell proliferation [116] Wise et al. produced electrospun oriented polycaprolactone (PCL) scaffolds (500 or 3000 nm fiber diameter). Human mesenchymal stem cells (hMSCs) were cultured on oriented nano and microfibrous electrospun PCL scaffolds as well as a randomporous PCL film to achieve a similar structure to articular cartilage [117]. Cell viability, morphology, and orientation on the fibrous scaffolds were determined as a function of time for biological

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investigations. They reported that engineering an oriented ECM environment to regulate tissue alignment could be optimized by oriented electrospun nanofibers. Creating the superficial zone of articular cartilage, may be significantly improved by a combination of stem cells and nanofibrous scaffolds [117]. Recently electrospun polycaprolactone (PCL) and starch compounded PCL (SPCL) nanofiber webs were used to evaluate extracellular matrix (ECM) formation by bovine articular chondrocytes (BACs) [118]. Da Silva et al. investigated the suitability of PCL and SPCL nanofiber webs in chondrocyte cultures, and their capability to produce ECM when seeded onto these nanofibrous materials. They also evaluated the effect of culture conditions (static vs. dynamic) on ECM formation. They reported that an extensive cell colonization of the entire nanofiber web was found for both materials and some degree of cell infiltration inside the nanofiber webs was noticeable for both materials. PCL and SPCL nanofiber webs are potential for ECM formation and therefore are adequate for cartilage tissue engineering [118]. 18.5.1.3

Ligament Tissue Engineering

Aanterior cruciate ligament (ACL) is very important for knee stabilization. The ACL has inferior healing capability due to poor vascularization, and needs to replace after significant damage has occurred. Currently available treatments, grafts and prostheses have host limitations, such as donor scarcity, donor-site morbidity, tissue rejection, disease transmission and poor long-term performance. Sahoo et al. investigated the supplementation of PLGA nanofibers on knitted PLGA scaffold for tendon/ligament tissue engineering. They observed that knitted PLGA displayed the appropriate mechanical properties and integrity for replacing a tendon/ligament while PLGA nanofibers possess a larger surface area and better hydrophilicity [119]. Thus, depositing nanofibers on a knitted substrate would facilitate cell attachment, new ECM deposition, and tissue formation without compromising mechanical integrity. Porcine bone marrow stromal cells attachment onto the hybrid and control (knitted) scaffolds was found to be comparable, while cell proliferation was faster in the hybrid scaffold. Cells in the hybrid scaffolds have also exhibited a higher expression of collagen I, decorin, and biglycan genes as well as an abundant production of ECM on the hybrid scaffold [119].

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18.5.1.4

695

Skeletal Muscle Tissue Engineering

Skeletal muscle consists approximately 48% of the body mass and control voluntary movement of the body and maintenance of the structural shapes [6, 120]. Riboldi et al. investigated the electrospun scaffolds, made from degradable polyester urethane (PEU), DegraPol®, for skeletal muscle tissue engineering [6]. Adhesion and proliferation of murine myoblast cell line (C2C12) on electrospun scaffolds is comparable with tissue culture plastic, which is indicative of no toxicity and excellent cell compatibility and their results indicated that the electrospun microfibers of PEU showed satisfactory mechanical properties and encouraging cellular response in terms of adhesion and differentiation. Based on these studies, the electrospun polyester urethane (PEU), DegraPol® show potential to be further explored as a scaffolding system for skeletal muscle tissue engineering. Huang et al. seeded C2C12 cells onto a gelatin- or fibronectin-coated non-woven electrospun PLLA fiber mesh (fiber diameter 500 nm) and noted disordered actin filament arrangement on randomly oriented fiber mesh, whereas actin fibers were aligned along the fiber direction on aligned fiber meshes [121]. Myotubes attachement and formation of multinucleated myofibers were found along the nanofiber direction globally after seven days in differentiation media. Myotubes were highly organized on the aligned mesh and significantly longer than those on randomly oriented scaffolds [121]. 18.5.1.5

Skin Tissue Engineering

Tissue engineering is an alternative treatment to traditional autografts and allografts for excessive skin loss. Out of the two layers of skin, epidermis and dermis, the epidermis has less capacity to heal, but the dermis has tremendous capacity tö regenerate. However, when large areas of the epidermis need to be replaced, normal regeneration is lacking. The scar tissue that forms in the absence of dermis lacks elasticity, flexibility, and strength of the normal dermis [122]. Therefore, scar tissue limits movements, causes pain, and is cosmetically undesirable. So, engineered skin tissue would be an excellent alternative, not only for wound dressings but also to stimulate the regeneration of the dermis. Several other natural and synthetic polymers have been explored for skin tissue engineering [122]. Min et al. developed nonwoven silk fibroin nanofibers

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by electrospinning for skin tissue engineering and they found that type I collagen coated fibroin nanofibers promoted keratinocytes/ fibroblast adhesion and spreading due to their high porosity and high surface area to volume ratio. Thus, the silk fibroin nanofibers showed potential to be developed as a scaffold for skin tissue engineering [48]. Khil et al. studied polyurethane nanofiber matrices for wound dressing materials in a rat skin defect model, showed increased rate of epithelialization with well-organized dermis which provided good support for wound healing [8]. Zong et al. observed that PLAGA nanofiber matrices in a rat model depicted an excellent anti-adhesion effect [123]. Several polymers of both natural and synthetic origins, alone or in combination, were successfully electrospun into nanofiber scaffolds, and evaluated as dermal substitutes with cells. Patel et al. investigated in vitro wound healing on poly(L-lactide) nanofibers coated with heparin, which was then used to bind laminin (an ECM protein) and bFGF, both of which encourage growth and migration of dermal fibroblasts [124]. An artificial wound was created in a monolayer of human dermal fibroblasts, with the underlying scaffold either random or oriented parallel or perpendicular to the wound. Random scaffolds provided adequate wound coverage after 48 h. Nevertheless, wound filling was enhanced by perpendicular fiber alignment. In particular, the number of cells near the center of wound was significantly greater. For parallel fibers, cells were hindered in entering the wound region [130]. Powell et al. developed bovine collagen nanofibrous scaffolds using either freeze-dry (FD) or electrospinning (EC) processes [125]. They noted that no significant differences were observed in cell proliferation, surface hydration, or cellular organization between the electrospun collagen skin substitutes (ECSS) and freeze-dry collagen and reported that electrospun scaffold was a better choice for skin substitutes than freeze- dried one [125]. In another study, Rho et al. reported producing of collagen type I nanofibrous matrix by the electrospinning process for the application of wound dressing. They studied three groups of nanofibrous scaffold: uncoated collagen nanofibers, collagen nanofibers treated with collagen type I and treated with laminin [126]. They examined the effects on cytocompatibility, cell behavior, cell and collagen nanofiber interactions, and open wound healing in rats. They observed relatively low cell adhesion on uncoated collagen nanofibers, whereas collagen nanofibrous matrices treated with type I collagen or laminin were functionally active in response in

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normal human keratinocytes. Rho et al. concluded that the treated ones with collagen or laminin showed better results among 3 type scaffolds of electrospun collagen nanofibers [126]. 18.5.1.6

Vascular Tissue Engineering

Efforts to develop a small-diameter vascular graft ( 0.22 micron size is not common nowadays.

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Sulfur colloid is however not a very well defined material, it has relatively wide size distribution and is not biodegradable. Several 99m Tc-labeled alternatives have been proposed for this purpose, such as phytate nanoparticles [25-27], tin colloid [19] or Sb2S3 nanoparticles [21,24]. However, none of these nanoparticles became as popular as sulfur colloid. Nanoparticles may be also used as X-ray contrast agents if they contain heavy atoms with sufficient electron density and thus opacity in X-ray region. The agent Thorotrast, thorium dioxide nanoparticles for RES imaging, widespread in the 1930s and 1940s, is now obsolete. It was shown that α-radiation from thorium contained in the nanoparticles, together with radiation from the decay cascade of 232Th, significantly increases the incidence of malignities. Much safer are iodine-based radioopaque nanoparticles, however the main drawback is relatively high amount of iodine neoessary for sufficient contrast.

19.3 Local Applications of Nanoparticles In some cases it is advantageous to keep radioactivity for therapeutical purposes in the site into which it was injected; such applications include above all radiosynovectomy (application of radioactivity into synovial space of the inflammated joint to suppress inflammation) [28-30] and brachytherapy (local radiotherapy of solid tumor by an implanted emitter) [31,32]. For radiosynovectomies [28-30], nanoparticles posses two significant advantages: they keep the radionuclide in the synovial space because of hindered diffusion due to their size and they are internalized by macrophages. Macrophages are the main cause of symptoms of inflammation, so when cytotoxic radioactivity is concentrated inside them, it has beneficial antiinflammatory effects. The choice of radionuclides is focused on ß-emitters with half-life of several days, mainly 90Y (T 1/2 = 64.00 h) [28, 29]. The ß" radiation has several milimeters effective range in the tissue with high linear energy transfer (LET) within the particle path, which corresponds to high biological activity in situ, but low radiation burden of distant tissues. From other materials, promising are nanoparticle-bound oc-emitters such as 2n At-labeled silver nanoparticles [33] since a radiation has even higher biological effectivity than ß" radiation.

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Emitters suitable for brachytherapy are, except of ß" emitters as Au [31, 32], mostly low-energy γ-emitters with half-attenuation thickness in tissue several centimeters such as 125I (T 1/2 = 59.408 days, 27.47 keV electron capture X-rays, 75.7% + several minor lines) [34-37] or 103Pd (T1/2 = 16.991 days, 20.22 keV electron capture X-rays, 42.3% + several minor lines) [36, 37], which allow therapy of larger tumors. Brachytherapy currently used in clinical medicine is realized by implantation of macroscopic metal closed seeds into the tumor. However, there exists another approach using nanostructured thermoresponsive materials which is currently studied [8-10,38]. Thermoresponsive (sometimes also called thermosensitive) polymers with lower critical solubility temperature (LCST) are soluble in aqueous milieu at low temperature while coil-to-globule transition followed by aggregation into nanoparticles and subsequent macroscopic phase separation occures at higher temperature (above their cloud point temperature, CPT). LCST is the temperature minimum in the polymer concentration vs. CPT plot. It was proven on a model system that if such polymer [poly(I\Msopropyl acrylamide) (PNIPAA) in this case], soluble at room temperature and precipitated at body temperature, is injected into muscle, it remains in the site of application and is only slowly eliminated from the local depo [39]. It should thus be useful for local injectable brachytherapy without the necessity of surgery and with the possibility to eliminate the polymer after it fulfills its task [8-10, 38]. 198

There are few biohydrolyzable thermoresponsive polymers reported - above all elastin-like peptides [40-42] and polyphosphazenes [43]. Polyphosphazenes, inorganic-backbone based materials, have generally too fast hydrolytic degradation times for the intended purpose [43]. Elastin-like poly peptides are produced by introduction of the particular encoding gene into bacteria Escherichia coli, protein expresion and purification [40-42]. Production of polymer of sufficient purity (i.e. nonimmunogenicity given by contaminating bacterial proteins) in necessary quantitiesis considerably difficult and expensive and also biodegradation is realtively fast for the brachytherapy use. Although polyGV-isopropyl acrylamide) is not chemically biodegradable, it is not toxic even at very high concentrations and as we have recently shown it is also slowly washed out from the application site due to physical equilibration between soluble and solid phase [39]. After redissolution is poly(N-isopropyl acrylamide)

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immediately eliminated from the body via both hepatobiliary route and renal excretion depending on its molecular weight.

19.4 Nanoparticles for Cancer Imaging and Therapy The hot topic of the current nanotechnology research in nuclear medicine is the single photon emission computed tomography (SPECT), positron emission tomography (PET) and radiotherapy of solid tumors [5-7], since cancer treatment has still many limitations to be improved and solid tumors rank among main causes of death in developped countries. Nanosized delivery devices may exploit several targeting mechanisms to solid tumors. Solid tumors spontaneusly acumulate biocompatible polymers, polymer micelles, liposomes and nanoparticles of the < 200 nm size due to leaky nature of the newly formed tumor neovasculature and poor or missing lymphatic drainage in the solid tumor tissue. This so-called Enhanced Permeation and Retention (EPR) effect [44-48} is relatively universal for many solid tumors, allows to concentrate nanoparticles more than one order of magnitude compared to surrounding tissue and may be further enhanced by vazodilatators or other means of tumor targeting [44] (see below). Although the EPR effect is known already for a relatively long time, there are still considerations which should be clarified, above all optimal size of the carrier. The use of vazodilatators (e.g. nitrates such as nitroglycerine) is a newly introduced approach [48] to enhances the EPR effect (which itself can concentrate the radionuclide in cancer tissue by ca one order of magnitude compared to surrounding tissue). Vazodilatators increase blood supply into tumor, which acceleates and enhances nanoparticle deposition. Systemic administration of nitroglycerine is the most commonly studied method [48]. Numerous types of cancer cells overexpress on their surface receptors for ligands or antigens which are otherwise present in organism in a limited number only or just in some tissues, or even ligands that are under normal conditions not expressed in adult organism. If a ligand for the particular receptor or an antibody against such antigen is attached to the drug delivery system, this may lead to specific interaction of such system with cancer cells and subsequent accumulation in tumor tissue. If such receptor is

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able to facilite endocytosis, the system may be internalized into the cell in this way. Targeting with specific ligands may be very effective, however, several points are to be addressed [49]. • The ligand must be attached by a spacer, which does not hinder interaction with the receptor and attachment must be done on the site in the ligand molecule which is not responsible for the interaction with receptor. • The ligand-receptor interaction must be sufficiently strong and selective. In many cases, considerable enhancement of the binding constant may be achieved by binding multiple ligands to one nanoparticle which causes multiple interaction of the nanoparticle with the surface of the cell. • The receptor on the target cell must be in compartment accessible for the nanoparticles. In this way, only receptors with extracellular binding sites are suitable targets and the nanoparticle must be smaller than ca 200 nm to enable extravasation. However, ligand targeting for drug delivery systems in general possesses also some disadvantages (although less serious for radionuclide delivery systems than for delivery systems for chemical drugs), above all: • Cancer tissue, especially in advanced stages of the disease, is a phenotypically inhomogenous population of cells and significant fraction of them may not express the target receptor or antigen, especially after previous targeted therapy, when the receptor/antigen positive cells were destroyed and thus receptor/antigen negative cells passed through the selection as resistant [50]. This disadvantage is less serious for radionuclide delivery systems, because ionizing radiation has usually sufficient range to kill not only the cell with which it interacts, but also the surrounding cells so the full cancer cell population is impaired, but it is still a concern. • Most molecular targets hyperexpressed on cancer cells are also, although in lesser extent, present in other tissues, since cancer cells developped from normal cells.

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• Many receptors are present on the surface of the cells in a limited number only (so the targeting is saturable) and are downregulated by feedback after exposure to their ligands [51]. This is more serious for carriers of chemotherapeuticals than for radiopharmaceuticals, where the dose is significantly lower. Production of antibodies is one of the key defences of the organism against foreign structures with very high selectivity. No wonder that they became one of the first targeting moeities agains cancer cells which were studied for targeted radionuclide therapy [52-54]. Currently, chemically homogenous hybridoma-produced monoclonal antibodies are considered for this purpose only and therapy with radiolabeled monoclonal antibodies became widely used in clinic to treat B cell non-Hodgkin's lymphoma (90Y-labeled anti-CD20 antibody Zevalin® [52, 53]) and follicular lymphoma (131I-labeled anti-CD20 antibody Bexxar®[52-54]). Antibodies have been conjugated with nanocarriers such as nanoparticles [55,56] or liposomes [56, 57] to target them against cancer cells. Despite high specifity of antibodies, they possess numerous disadvantages: • Highly expensive production in industrial scale (partly given also by difficult purification from other proteins from the biological source). • High sensitivity to denaturation and loss of activity during chemical modifications and low reproducibility of synthesis of the conjugates. • Slow kinetics of deposition in target tissues. • Relatively strong interactions of he antibodies with tissues they are not targeted to. • Necessity to use the antibody of the same biological species (during development, preclinical testing is done on laboratory animals while clinical testing is done on human beings and the tested substance should be the same) due to immunogenicity. • Problems with internalization into cells (which, however, is more important for Auger electron emitters and chemotherapeuticals than for ß" and γ emitters). The relatively high molecular weight of antibodies (ca 150 kDa) enhances EPR effect, but also decreases the available transport

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capacity and slows down targeting rate. This is why lower molecular weight fragments of antibodies responsible for antigen recognition (ScFv and Fab, respectively) [58,59] became popular, however they also possess limitations, above all they are very expensive and have lower antigen binding constant than native antibody. Glycosylation of tumor cell membranes is different compared to most other tissues, which may be exploited for targeting with saccharide-recognizing proteins of plant origin, lectines [60, 61]. Many lectines possess significantly higher affinity to cancer cells than to other tissues and in addition, many lectines are directly cytotoxic (e.g., viscotoxin from Viscum album or ricin from Ricinus communis) due to proteosynthesis inhibition. However, they also have drawbacks to be considered - above all immunogenicity since they are plant proteins, relatively low saccharide binding constant and the native lectins are noncovalently-bound oligomers which dissociate easily. Production and sensing of growth factors is essential for tissue proliferation and formation of new vessels including tumor neovasculature. Most tumors highly overexpress receptors for such growth factors, which make growth factors highly attractive as targeting moieties. The most commonly studied growth factor receptors hyperexpressed in tumor tissue are vascular endothelial growth factor receptor (VEGFr) [62-65] and endothelial growth factor receptor (EGFr) [66-68]. Recently it was shown on more than 1000 human biopsia samples that receptor for follicle-stimulating hormone (FSHr) [69], in healthy adult expressed only in gonades (testes and ovaria), is largely (in > 90% cases) overexpressed in the vascularization zone of nearly all types of malignant tumors, being another promise for a holy grail of universal tumor targeting approach. The receptor is overexpressed only in the surface layer of the tumor, both several mm to the depth of the tumor tissue and several mm to the surrounding "healthy" tissue. Radionuclides with larger range of radiation (γ-emitters) may hit even cells in the middle of the larger tumor lesions, which would be otherwise left unaffected by nonranged chemotherapeuticals. It is highly probable that this approach will work even for (micro)metastases. As any other method, also this approach has some drawbacks. The main drawbacks are relatively low and saturable transport capacity (which, however, is less serious for radiopharmaceuticals than for chemical drugs), downregulation of receptors upon exposure to their ligands [51] and very high price of the protein ligands

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(although also less expensive synthetic non-protein ligands are curently studied). The newly formed tumor vasculature epithelium expresses integrins in high amount, especially oc^ 3 integrin, which binds the RGD (Arg-Gly-Asp) peptide and its analogues [70-74]. Expression of the RGD peptide-binding integrin could be in addition significantly enhanced by previus γ-irradiation. This targeting approach was extensively studied in last decade [70-73]. It is relatively effective, especially if multiple RGD peptides per carrier are attached and suitable analogs (e.g. cyclic RGD) are used, and epithelium of the vessels is highly accessible for nanoparticulate carriers, however the targeting is not completely seletive for tumor vasculature since integrins are also expressed in all other vessels, although in lower extent. Bone tissue is a natural oriented composite with mechanical support and protection function, which is unique in the body since it contains inorganic matrix hydroxyapatite. This calcium phosphate mineral selectively binds bisphosphonates and hydroxybisphosphonates, which are structural analogs of diphosphate present in the crystal structure of hydroxyapatite. Hydroxybisphosphonates and their metal complexes are very quickly and strongly accumulated in bones after administration [75] and inhibit bone resorption, which is exploited in paliative treatment of bone metastases [75-77], therapy of osteoporosis [75, 77] and of Paget disease [75]. Radiometal complexes of bisphosphonates, especially methylenediphosphonate complex with 99mTc, are widely used in bone scintigraphy [78-80], since bisphosphonates accumulate more in places of bone with high remodelling rate where hydroxyapatite is more exposed to blood, i.e. in bone metastases and broken/damaged areas [78-80]. This is why hydroxybisphosphonate moieties were also proposed for targeting of polymer carriers [81, 82]. Tumor tissue grows rapidly and thus needs more organic and inorganic nutrients and viamins than non-proliferating tissues. This is why it accumulates more such sutances, which may be exploited for drug and radionuclide targeting. Iron is transported within the organism in the form of complex with chelating protein transferrin. It was found that cancer cells uptake several times more iron-loaded transferrin than normal cells and that transferrin may be used as effective tumor targeting for nanosized delivery devices [68, 83-85]. In fact, the use of transferrin in nuclear medicine dates back many years ago because the

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use of 67Ga and 68Ga citrate complexes for the imaging of tumor and inflammation lesions [86-88] is based on the fact that Ga3+ forms stronger complex with transferrin than Fe3+ and is transported into tumor tissue by the transferrin-mediated way. The main drawback of transferin as a targeting moiety is its relatively high normal concentration in blood plasma (for a healthy adult human 2-4 g / L from which 30% is iron-loaded) which competes with the transferrin-targeted delivery device and necessity to isolate this protein from blood plasma (however, since the transferrin receptor binding peptide sequence has been elucidated, transferrin may be substituted by the oligopeptide sequence HAIYPRH [84]). Folic acid is an essential vitamine for the biosynthesis of nucleic acids (and thus for cell division and proliferation) and receptors for its uptake are overexpressed up to two orders of magnitude more on cancer cells than on normal cells. Folate is a higly studied tumor targeting ligand [68,89]. It became a model substance for the studies on tumor targeting in general [90], e.g. how many folate molecules is necessary for the targeting of one liposome and how the density of targeting moieties influences selectivity for tumor cells [89] or how the way of attachment of folate (which one of the two carboxyls is used for conjugation) to polymer influences its targeting potential [91]. Folic acid antagonists (methotrexate, raltitrexed etc.) are commonly used chemotherapeuticals since this vitamin is essential for cell division. Folate first seemed to be an ideal and inexpensive targeting moiety, however accumulation in healthy tissues is not negligible since it is an essential vitamine [92]. Numerous membrane-bound enzymes are also overexpressed in tumor tissue, e.g. carbonic anhydrase IX [93], which offers potential for tumor targeting with their selective inhibitors as ligands. Tumor targeting strategies in general may be exploited in both radiodiagnostics and radiotherapy of solid tumors or may be used just to follow the fate of the delivery system for a chemical drug in every particular patient to evaluate and customize the anticancer care in real time. The choice of suitable radiuonuclide depends on the actual use of the system [5, 7, 94]. For both radioimaging purposes and radiotherapy, physical half-life from severeal hours to few days is optimal, offering both enough time for targeting and sufficiently fast extinction of radioactivity from the organism after the the system fulfills its task. The radionuclides most commonly studied or used in connection with nanomedicines for planar scintigraphy and

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SPECT are γ-emitters 99mTc(23,25,79,83,95), m I n [T1/2 = 2.8047 days, 171.3 keV (90%), 245.4 keV (94%)] [96,97] and radionuclides which may be used both for radiotherapy and imaging since they are primarily ß" emitters, but with accompanying γ-emission, i.e. ml [T1/2 = 8.02 days, 364.5 keV (81.7%) + several minor γ-lines] [54] and 188 Re [T1/2 = 17.01 h, 155.0 keV (15%)] [98, 99]. For PET imaging the choice of radionuclides is much more limited, especially considering that positron (ß+) emitters are mostly short-lived compared to radionuclides with other decay modes. The generally most widespreadly used radionuclide 18F [5-7] (T1/2 = 109.8 min, commonly used in the form of [18F]-2-fluoro-2deoxy-D-glucose, FDG [65], to track metabolic activity) has relatively short half-life for the nanoparticles for cancer applications [100]. This is why longer-lived radionuclides, especially M Cu (T1/2 = 12.70 h, electron capture 43.6%, ß+17.4%, ß"39%) [5-7,59,101] and 124I (T 1/2 = 4.176 days, electron capture 77.2%, ß+ 22.8%)[5-7, 102, 103], are preferred for such applications. Unfortunately 64Cu and 124I are not pure positron emitters, which leads to higher radiation burden if only imaging is claimed (however ^Cu is also very promising for radiotherapy [101] with PET real-time monitoring). For radiotherapy, ß" emitters are radionuclides of choice, 90Y [28,30,38,104], 131I [54], «Cu [5-7,59,101] and 188Re [98,99] being the most frequently considered in connection with nanomedicine. The high energy electron (β' radiation) has several mm range in tissue and high biological effectivity. In the last decade, oc-emitters as 211At (Tm = 7.214 h) [33] and ^ A c (T1/2 = 10.0 days) [105] (which behaves as in situ α-nanogenerator, see below) are quite often studied, since the α-radiation has within its short range (typically less than 0.1 mm in tissue) tremendous biological effectivity independent on eventual hypoxia. Auger electron emitters are another choice (see below). In addition, SPECT and PET may be further combined with other imaging modalities (magnetic resonance imaging [106,107] or fluorescence [72,106, 108-110]) or with therapy (theragnostics) in one system, taking specific advantages of each of the modality in one system. There are several types of nanosized delivery devices studied for nuclear medicine, each of them having certain advantages and certain disadvantages: • Conjugates of molecularly water-soluble polymers [81, 111] and antibodies [54]. Antibodies are biodegradable proteins with both passive and active

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targeting potential on their own. Synthetic polymers may be made biodegradable by introduction of suitable linkages into the main chain and may be actively targeted by conjugation with suitable targeting moieties. However, high molecular weights (necessary for effective EPR effect) with defined structure in the same time is relatively hard to achieve with such synthetic polymers. All components of the conjugate must be chemically attached to the polymer. • Dendrimers are polymers with branched tree-like architecture and have similar advantages and disadvantages as soluble polymers and antibodies, but may be made more defined at higher molecular weights [84,112]. Dendrimers are generally harder to produce in large scale than linear polymers. • Polymer micelles [47, 68, 113] are supramolecular assemblies of amphiphilic block or graft polymers with one type of blocks soluble in water and one type blocks insoluble in it. Self-assembly of block copolymers into micelles is driven by hydrophobic interactions among the water-insoluble blocks. The main advantage of polymer micelles as radionuclide carriers [8, 114] for cancer applications is that micelles have high apparent molecular weight with relatively narrow size distribution while being biodegradable by physical equilibrium with unimers (free polymer chains nonintegrated into micelles present in solution) [45, 47]. The unimers may have molecular weight below renal threshold so the system may be completely eliminated from the body. Active payload may not be only chemically attached to the micelle-forming polymer (although this is preferred due to radiolabeling stability), but may be also only physically dissolved into the micelle core. The main disadvantage of micelles is their principial physical instability, which may be eliminated by biodegradable crosslinking of the nanoparticle core (e.g. by transchelatable metal chelation bond). In such case, a polymer-based nanoparticle is formed. The material for non-water-soluble block must be chosen carefully to avoid membrane toxicity due to amphiphilic character of the whole macromolecule.

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• Perfluorohydrocarbon nanoparticles are stabilized emulsion-like nanoparticles which share most advantages and disadvantages with the micelles [115, 116]; toxicity concerns are lower due to complete biological inertness of perfluorohydrocarbons. • Superparamagnetic particles are polymer-coated iron oxide nanoparticles [74,100,107]. The main advantage originates from their magnetic properties. They may be used for multimodality imaging by radionuclide imaging methods together with MRI (magnetic resonance imaging) [74,107], since they behave as contrast agents, and also additional targeting may be achieved by application of external magnetic filed. After accumulation in tumor tissue, high-frequency magnetic field may be used for thermoablation of the target tissue. The main disadvantage is that magnetic methods require relatively high amounts of magnetic material to be applied and that iron oxide is only slowly biodegradable in the organism. • Metal nanoparticles [31-33], usually polymer-coated against unwanted uptake into RES. They may be tailored in a defined and reproducible way with very narrow size distribution and under some circumstance they may be very easily radiolabeled (e.g. 198Au-labeled gold colloid may be made by neutron irradiation before or after formation of the colloid or silver colloid may serve as m At carrier due to strong coordination between Ag and At) [31-33]. If they are coated with polymer, then they may also be labeled via polymer. The main disadvantage is that they are not biodegradable. • Quantum dots are fluorescent semiconductor-based nanoparticles which have the main advantage that they may combine radioimaging with fluorescence imaging [117-119]. However, they are made of highly toxic materials as cadmium selenide or cadmium telluride, so the eventual biodegradation (which is very slow) would lead to chronic poisoning. • Carbon nanotubes [5, 108] and nanodiamonds are non-biodegradable carbon-based materials. They have spherical (nanodiamonds) or rod-like (nanotubes)

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shape. They may be easily covalently modified on the surface and radiolabeled. In addition, nanodiamonds are nontoxic and may be made fluorescent with suitable spectral properties for in vivo imaging by proton or electron irradiation. The main disadvantage of nanodiamonds and nanotubes is their nonbiodegradability. Carbon nanotubes are also highly carcinogenic due to mechanical stimulation of the cells to proliferation (mechanotoxicity and mechanocarcinogenity similar to azbestos). • Liposomes are vesicles composed of phospholipid bilayer, single-walled or multilamellar, usually coated with polymer to be protected against RES entrapment [57, 85, 89, 92]. The radionuclide may be attached to the surface, dissolved in the phospholipid bilayer (hydrophobic species) or dissolved in the inner aqueous phase (hydrophilic species). They share most advantages with micelles, however they are not easy to produce in lower size optimal for EPR effect. In contrast to micelles, they enable noncovalent radiolabeling by dissolution of suitable complex into the aqueous core.

19.5 Minimization of Systemic Radiation Burden The main drawback of radiopharmaceuticals is the radiation burden of healthy tissues. There exist several approaches to suppress this drawback. The most straightforward is optimization of blood circulation time; too short blood circulation time prevents effective tumor targeting while too long blood circulation time increases systemic radiation burden. For reticuloendothelial system, blood circulation time in tens of minutes is usually sufficient since the targeting is relatively fast, while for cancer applications several hours is currently considered to be optimal [5-7]. Especially in cancer therapy applications, the inherent targeting of the system given by its structure and size may be further enhanced by external stimuli, such as external magnetic field targeting (see above) or neutron flux. Additional targeting with neutron flux is based on nuclear reaction in vivo: relatively biologically harmless epithermal (slow) neutron flux is converted in tissue by the reaction with

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suitable stable nuclide into biologically extremely effective secondary ionizing radiation. The most commonly studied radionuclide for. this purpose is 10B and in such case the therapy is called BNCT (Boron Neutron Capture Therapy) [120, 121]. The suitable carrier (such as liposome [120, 121] or boron nitride nanotubes [122] or nanoparticles [123]) preconcentrates the target nuclide in tumor tissue {first targeting) and then narrow neutron beam is focused to the lesion {second targeting). The method, although undoubtedly elegant, possess some drawbacks. Above all, high concentrations of boron must be achieved (20-35 μg 10 B/g) so high amounts of carrier are necessary; isotopically enriched material must be used (natural abundance of 10B is only 19.9 mole%) which makes the material rather expensive, sufficiently strong neutron source is essential (currently only nuclear fission reactor, which has limited availability, fulfills the criteria) and tissues contain high amounts of sodium. Sodium, natural monoisotope 23Na, is readily activated to radioactive ß" emitter 24Na (T 1/2 = 14.96 h) which causes high whole-body radiation burden due to sodium mobility in the body despite the neutron beam is precisely collimated to the target site. Radioactive decay of many radionuclides {e.g., m In, 125I or 99mTc) is accompanied with emission of several so-called Auger electrons (AEs) with relatively low energy, which are emitted during reorganization of the electron shell of the atom after decay. Since a charged particle, like an electron, looses its energy in material most effectively {i.e. has the highest linear energy transfer, LET) just before it completely stops ("Bragg peak"), AEs are potent ionizing radiation within their very short range in living tissue (typically few nanometers) [124,125]. However, their very short range means that if AE emitting radionuclides are intended to be used as radiotherapeuticals, they must be brought into cell and preferentially directly into DNA [5-7,113, 124-127], as the main ionizing radiation biological target, to be effective. Especially advantageous in such cases are AE emitter-bearing intercalators [124-127] or nucleobase analogs [128], e.g. 125I-iodinated daunorubicin [126], which target and accumulate the radionuclide directly in DNA. Such isotopically labeled intercalators would be extra beneficial in connection with drug delivery system, which would assure selective targeted delivery of the radionuclide into tumor tissue {first targeting by EPR effect or ligand-based targeting). In the tumor tissue the radionuclide on an

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intercalator is released in its active form (second targeting by pH or enzyme-triggered release). The intercalator then brings the radionuclide into the cell nuclei (third targeting). The idea of in situ radionuclide nanogenerators uses radionuclides 230U or 225Ac. Uranium-230 is an α-emitter which has decay half-life 20.8 days [129]. This radionuclide has short-lived a-decay cascade [230U -> 226Th (T1/2 = 30.57 min) -> 222Ra (T1/2 = 38 s) -> 218Rn (TV2 = 35 ms) -> 214Po (T1/2 = 164 μβ) -> 210Pb; 210Pb is also radioactive β" emitter, but long-lived with T1/2 = 22.3 years so its radioactivity is negligible] which produces subsequently total 5 biologically extremely efficient short-ranged (ca 40 microns) α-particles from one uranium atom. Uranium-230 is thus an isotope generator which may be relatively safely delivered to the target tissue where it releases plethora of radionuclides producing high radiation doses [129]. In a similar way, 225Ac produces total 4 α-particles during its decay cascade to stable 209Bi [105,130]. The main disadvantage of the system is very high cost of 230U and also the fact that the intermediate of the decay cascade 226Th has half-life 30.57 min which allows it to be washed off from the site of accumulation of the nanocarrier. Other than target tissues may thus be contaminated with the radionuclides from the rest of the decay cascade. It is not principially possible to attach uranium atom to the carrier so that also rest of the decay cascade remains on the carrier because the α-particle is so heavy that due to the momentum conservation law the recoil delivered to the daughter nuclide is tens of kiloelectronvolts, at least three orders of magnitude higher energy than any chemical bond can stand (typical binding energy of a chemical bonds is maximum ca 10 eV).

19.6

Conclusions

The use of nanocarriers in nuclear medicine brings many benefits which otherwise cannot by achieved by other curently available means. The main challenges and perspectives for current research in this area are in the development of radioimaging and theragnostic agents for cancer applications based on synthetic polymers and their supramolecular assemblies. Despite extensive emerge in the development of polymer conjugates of chemical drugs and obvious advantages of radiopharmaceuticals over chemical drugs, there are

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still not many publications on radionuclide-based nanomedicine applications, so this field still waits to be fully exploited.

Aknowledgements Financial support of the Grant Agency of the Czech Republic (grants # P207/10/P054, # P108/10/1560 and # 203/08/0543), of the Grant Agency of the Academy of Sciences of the Czech Republic (grant IAAX00500803) and of the Ministry of Education,Youth and Sports of the Czech Republic (grant # IM4635608802) is gratefully acknowledged.

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112. A. Almutairi, R. Rossin, M. Shokeen, A. Hagooly, A. Ananth, B. Capoccia, S. Guillaudeu, D. Abendschein, C.J. Anderson, M.J. Welch, J.M.J. Frechet, Proceedings of the National Academy of Sciences of the United States of America, Vol. 106, p. 685, 2009. 113. H. Fonge, H. Lee, R.M. Reilly, C. Allen, Molecular Pharmaceutics, Vol. 7, p. 177, 2010. 114. M. Hruby, S.K. Filippov, J. Panek, M. Novakova, H. Mackova, J. Kucka, D. Vetvicka, K. Ulbrich, Macromolecular Bioscience, Vol. 10, p. 916,2010. 115. M. Hughes, S. Caruthers, T. Tran, J. Marsh, K. Wallace, T. Cyrus, K. Partlow, M. Scott, M. Lijowski, A. Neubauer, P. Winter, G. Hu, H. Zhang, J. McCarthy, B. Maurizi, J. Allen, C. Caradine, R. Neumann, J. Arbeit, G. Lanza, S. Wickline, Proceedings of the Ieee, Vol. 96, p. 397,2008. 116. G. Hu, M. Lijowski, H.Y. Zhang, K.C. Partlow, S.D. Caruthers, G. Kiefer, G. Gulyas, P. Athey, P.; Scott, M. J.; Wickline, S. A.; Lanza, G. M. International Journal of Cancer, Vol. 120, p. 1951,2007. 117. W.B. Cai, K. Chen, Z.B. Li, S.S. Gambhir, X.Y. Chen, Journal of Nuclear Medicine, Vol. 48, p. 1862, 2007. 118. K. Chen, Z.B. Li, H. Wang, W.B. Cai, X.Y. Chen, European Journal of Nuclear Medicine and Molecular Imaging, Vol. 35, p. 2235, 2008. 119. F. Duconge, T. Pons, C. Pestourie, L. Herin, B. Theze, K. Gombert, B. Mahler, F. Hinnen, B. Kuhnast, F. Dolle, B. Dubertret, B. Tavitian, Bioconjugate Chemistry, Vol. 19, p. 1921,2008. 120. N. Duzgunes, Methods in Enzymology: Vol. 465 Liposomes, Part G, Academic Press Inc., San Diego, 2009. 121. S. Altieri, M. Balzi, S. Bortolussi, P. Bruschi, L. Ciani, A.M. Clerici, P. Faraoni, C. Ferrari, M.A. Gadan, L. Panza, D. Pietrangeli, G. Ricciardi, S. Ristori, Journal of Medicinal Chemistry, Vol. 52, p. 7819,2009. 122. G. Ciofani, V. Raffa, A. Menciassi, A. Cuschieri, Nanoscale Research Letters, Vol. 4, p. 113, 2009. 123. W. Chen, S.C. Mehta, D.R. Lu, Advanced Drug Delivery Reviews, Vol. 26, p. 231, 1997.. 124. S. Ghirmai, E. Mume, V. Tolmachev, S. Sjoberg, Carbohydrate Research, Vol. 340, p. 15,2005. 125. D. Murali, O.T. DeJesus, Bioorganic & Medicinal Chemistry Letters, Vol. 8, p. 3419,1998. 126. L.M. Ickenstein, K. Edwards, S. Sjoberg, J. Carlsson, L. Gedda, Nuclear Medicine and Biology, Vol. 33, p. 733,2006. 127. E.B. Kullberg, B. Stenerlow, S. Ghirmai, H. Lundqvist, P.U. Malmstrom, A. Orlova, V. Tolmachev, L. Gedda, International Journal of Oncology, Vol. 27, 1355, 2005. 128. S.N. Reske, S. Deisenhofer, G. Glatting, B.D. Zlatopolskiy, A. Morgenroth, A.T.J. Vogg, A.K. Buck, C. Friesen, Journal of Nuclear Medicine, Vol. 48, p. 1000, 2007. 129. A. Morgenstern, C. Apostolidis, F. Bruchertseifer, R. Capote, T. Gouder, F. Simonelli, M. Sin, K. Abbas, Applied Radiation and Isotopes, Vol. 66, p. 1275,2008. 130. C. Antczak, J.S. Jaggi, C.V. LeFave, M.J. Curcio, M.R. McDevitt, D.A. Scheinberg, Bioconjugate Chemistry, Vol. 17, p. 1551,2006.

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Biomimetic Materials Toward Application of Nanobiodevices Ravindra P. Singh1,3,Jeong-Woo Choi3-4, Ashutosh Tiwari 2 ,and Avinash C. Pandey 1 Wanotechnology Application Centre, Department of Physics University of Allahabad, Allahabad, India 2 Biosensors and Bioelectronics Centre, Institute of Physics, Chemistry and Biology, Linköping University, Linkoping, Sweden interdisciplinary Program of Integrated Biotechnology, Sogang University, Seoul, Korea department of Chemical and Biomolecular Engineering, Sogang University, Seoul, Republic of Korea

Abstract

Biology is inherently nano, with the focus on a cell, which is a warehouse of structures and functional units that are finely harmonized on the nanometer scale. New intelligent nanobiodevices and new nanostructures biomaterials are expected to speed up quantitative biomedical research, boost our diagnostic capabilities, and increase the length and quality of our lives. At the same time, nanostructures inspired by nature or created using biological processes are expected to reduce the production costs of new nanodevices making them accessible for the public. Nanobiotechnology is an emerging field that has made its contribution to all spheres of human life. The biological synthesis of nanoparticles has paved the way for better methodologies and approaches in the fabrication of nanobiodevices. There is an ever-increasing need to develop environmentally benign processes, i.e. biomimetic methods in place of synthetic protocols involving toxic ingredients. This is an economical, efficient, eco-friendly, and simple process. Progress in this area will provide new green paths in the development of controlled shape and size NPs. There is an enormous interest in exploiting biomimetic materials in variety of applications to nanobiodevices, such as nanobios"ensors and nanobioelectronic, as their size scale is similar to that of biological molecules (e.g., proteins, DNA, recombinant Ashutosh Tiwari, Ajay K. Mishra, Hisatoshi Kobayashi and Anthony P.F. Turner (eds.) Intelligent Nanomaterials, (741-782) © Scrivener Publishing LLC

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metalloenzymes) and structures (e.g., viruses and bacteria). This domain of scientific arena is still, at present, in the state of infancy stage of applied research, but is regarded as one of the most promising research fields for the near future. Keywords: Biomimetic approach, biomolecules, nanodevices, self assembled monolayer, nanotechnology, nanoparticles

20.1 Introduction Humans, having always been exposed to tiny particles via dust storms, volcanic ash, and other natural processes, our bodily systems are well adapted to protect us from these potentially harmful intruders by actively neutralizing and eliminating foreign matter in the body, including viruses and nonbiological particles. The manipulation of matter at the scale of atoms, nanotechnology is creating many new materials with characteristics not always easily predicted from current knowledge. Within the nearly limitless diversity of these materials, some happen to be toxic to biological systems; others are relatively benign, while others confer health benefits. Some of these materials have desirable characteristics for industrial applications, as nanostructure materials often exhibit beneficial properties, from UV absorbance in sunscreen to oil-less lubrication of motors. For nanotechnologies with clearly associated health risks, intelligent design of materials and devices is needed to derive the benefits of these new technologies while limiting adverse health impacts [1-3]. Multifunctional nanoparticle systems (MFNPSs) development are very immense and improved systems involving the physical and biological aspects utilizing optical and magnetic resonance imaging capabilities in biomedical research pertaining to health (safety) and ethical concerns [4]. Biomimetic materials are the materials that mimic a biological environment to perform a desired cellular response, facilitating their task. One has to keep in mind that the main task of a biomimetic material is not necessarily the specific interaction with a cell or tissue, but rather the fulfillment of the intended purpose, e.g. the targeting of a certain cell type or providing a scaffold structure for tissue growth; this specific interaction is intended as a tool for the material to achieve these goals. New intelligent drug delivery vehicles, nanobiosensors, nanomedical imaging/nanobiodevices, and new nanostructured biomaterials are expected to speed up

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biological and medical research, boost up our diagnostic capabilities, and increase the length and quality of our lives. At the same time, nanostructures inspired by nature using biological processes are expected to reduce the production costs of novel nanodevices making them accessible for the public. Nature provides a wide range of materials with different functions and which may serve as a source of bioinspiration for the materials scientist. A thorough analysis of structure-unction relations in natural tissues must precede the engineering of new bio-inspired materials. Biomimetic materials research is becoming a rapidly growing and enormously promising field. From the discovery through observation of nature will be gradually replaced by a systematic approach involving the study of natural tissues in materials laboratories [5-9].

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Biomimetic Peptides and Proteins

The developing method is to make nanofunctional structures economically and with great precision and accuracy. Photolithography has been the main technique used to miniaturize structures; this top-down method can produce a smallest line width of around 32 nm and is rapidly approaching its theoretical limit. The top-down technologies, such as electron beam lithography [10] and scanning probe lithography (SPM or AFM lithography [11-13] can not produce nanostrucures economically. The bottom-up nanostructure production methods is now very important to develop nanodevices. The combination of top-down and bottom-up technology is essential to produce more advanced devices with nanofunctional structures. Life built up by biomolecules act as a nanoblock for nanofabrication for nanodevices using both top down and bottomup technology, if we their properties are utilized and controlled properly. There have been established a few strong evidences like S. Banerjee's group that reported nanodot floating gate memory using a heat-shock protein. Y. Yang's group reported a new type memory using tobacco mosaic virus (TMV). M. Ozkan et al. replaced TMV conjugates by cosahedral cowpea mosaic virus (CPMV), a plant virus with semiconducting quantum dots, and successfully produced another type of biomedicated memory. A. Belcher's group utilized filamentous virus Ml 3 to fabricate battery electrodes. D. Kondo et al. synthesized carbon nanotubes utilizing iron oxide core of ferritin as a nanoparticle catalyst, aimed for use as

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electric wire in microelectronic devices. Y. Nakama et al., proposed the fabrication of nanostructures for miniband formation to add a new function to electronic devices utilizing the iron oxide core of ferritin. Yui et al. reported an aligning ferritin molecule with cores in tubular structures [14-26]. A cage-shaped protein is known as apoferritin, exists widely in many biological species and plays an important role in the homeostasis of ferrous ion levels in the body. The ferrihydrite cores in ferritin can be removed fairly easily by dialysis against a solution containing a reducing agent and thus ferritin without a ferrihydrite core is called apoferritin. The outer and inner diameters are approximately 12 and 7 ran, respectively. The native apoferritin consists of 24 polypeptide subunits and there are two types of subunits, the light chain subunit (L-subunit) and the heavy chain subunit (H-subunit). H-subunit has a ferroxidase center which oxidizes ferrous iron to ferric iron whereas L-subunits play an important role in the self-assembly of ferritin molecules into a two-dimensional crystal at the air-water interface via a salt-bridge interaction. Recombinant ferritins are also studied intensively, which includes L-ferritin composed of only L-subunits. A smaller cage-shaped protein, Dps (DNA-binding proteins from starved cells) is sometimes used when smallness is the priority [27-31]. There have now been fabricating many kinds of nanostructures for nanoelectric devices using the cage shaped proteins as biotemplates as shown in Figure 20.1. The biotemplated NP synthesis is a method to produce homogeneous NPs. Fe, Cr, Co and In oxides or Ni hydroxide NPs in horse spleen apoferritin (HsAFr) cavity using a one-pot synthesis method, which is suitable for mass-production due to its simplicity. These metal complex NPs have the potential to be used in nanodevices, such as single electron transistor, catalysis and floating gate memory. Wong et al. for the first time reported the synthesis of CdS NP in the cavity with diameters of 2.5 nm and 4.5 nm by incremental addition of source ions. The synthesis of semiconductor CdSe and ZnSe NPs in the apoferritin cavity are reported by a new chemical reaction system. The chemical reaction between Cd2+ and Se2_ is very fast and induces CdSe aggregation very quickly. Using the biomineralization mechanism in the apoferritin and the slow chemical reaction system, we can produce many kinds of compound semiconductor NP cores with a variety of sizes and photoluminescence. Several investigators are attempting for the synthesis of ZnS, AgS, and Au2S NPs in the apoferritin

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As catalyst for CNT, semiconductor

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As floating gate memory

Apoferritin without core combines biological nofabrication ■up method)

As electron trap

As vertical transistor, nanodisks

Nanostructure fabrication by ferritin biomolecule as nano block

As single electron transistor As bio-layerby layer fabrication

Figure 20.1 Shows the biofabrication of nanostructure using ferritn as nanoblock and its nanodevice applications.

cavity [32-39]. Fan et al. reported that the direct evidence of apoferritin mediated (Pt-Ft) nanoparticles possesses both catalase and peroxidase activities. Apoferritin (apoFt) used as a nucleation substrate, synthesized 1-2 nm platinum nanoparticles (Pt-Ft) which are highly stable. Oxygen gas bubbles were generated from hydrogen peroxide as substrate, decomposed by Pt-Ft; strongly supports Pt-Ft reacts as catalase, other than peroxidase, wheras with organic dyes and hydrogen peroxide as substrates, distinctive color products were formed catalyzed by Pt-Ft, which indicates a peroxidase-like activity. Further, these biomimetic properties showed differential response to pH and temperature for different reaction substrates. Pt-Ft showed a significant increase in catalase activity with increasing pH and temperature. The HRP-like activity of Pt-Ft was optimal at physiological temperature and slightly acidic conditions [40]. These size controlled semiconductor NPs in the protein shell are promising materials for various nanotechnology applications, such as quantum well, fluorescent biological label (Q-dot) and light source for quantum communication. Thus apoferritin, a cage shaped protein can form a variety of homogeneous NPs in the

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cavity and deliver them to specific areas utilizing delivery methods ranging from one-to-one to hexagonally close-packed arrays. The protein shells can be eliminated and the NPs can be used for making key-components of nanodevices [41]. The structural organization within biological cells has many levels. The bottom- u p i.e., from small to large structures, the first three levels are: (i) the level of macromolecules (or copolymers) which have a backbone of monomers connected by covalent bonds; (ii) the level of supramolecular assemblies of many similar molecules, the formation of which is governed by noncovalent forces such as the hydrophilic or hydrophobic interactions with water; (iii) the level of complex architectures which contain different types of building blocks a n d / o r different types of assemblies. The macromolecular components of the cell including proteins, nucleic acids, polysaccharides, and lipids are known. All of these macromolecules are copolymers, which are built up from a certain number of different monomers or building blocks [42]. The new synthetic methods have been developed which allow the construction of hybrid molecules consisting of biopolymers coupled to synthetic ones. Single molecule methods have been established by which one can determine the physical properties of single macromolecules; one can label these molecules by a fluorescent probe and then track their motion both in solution and bound to a sheet-like membrane or rod-like filament. Also one can firmly anchor them at a solid surface and then probe individual macromolecules by various experimental methods. One may also use the tip of an atomic force microscope in order to pull at a single copolymer, which is anchored to a solid surface. The area where single molecule methods have led to much insight is the active movement of molecular motors along filaments. In the cell, these motors are responsible for the directed transport of vesicles and organelles over tens of micrometres or even centimetres [43]. Biomembranes are highly flexible and can easily adapt their shape to external perturbations. In spite of this flexibility, they are rather robust and keep their structural integrity even under strong deformations. This combination of stability and flexibility is a consequence of their internal fluidity. This was first realized in the context of lipid bilayers, which are the simplest biomimetic membranes as shown in Figure 20.2. Fluid membranes have unusual elastic properties, which determine their morphology [44]. The stability of lipid bilayers makes it possible to isolate them

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Biomimetic membrane behave like natural cell membrane

immobilization of proteins, enzymes, carbohydrate DNA/RNA, antibody, receptors etc

nan ode vie es e.g. nanochannels Bioconjugation

???????????????????? Bioinspired interface on the substrate by assembly of phospholipids polar groups.

Figure 20.2 Shows biomimetic membranes for the fabrication of nanodevices.

and to manipulate them in various ways: one can suck them into micropipettes, attach them to other surfaces, and grab them with optical tweezers generated by focused laser beams. These membranes are even self-healing: if one pinches small holes into them, the holes close again spontaneously. In the last couple of years, new types of biomimetic membranes have been constructed. One example is provided by bilayers of amphiphilic diblock copolymers. Both artificial and hybrid copolymers have been found to undergo spontaneous bilayer formation. Another type of biomimetic membrane is provided by polyelectrolyte multilayers. These multilayers are constructed in a layer-by-layer fashion where one alternatively adds negatively and positively charged polyelectrolytes onto solid templates. These multilayers form dense polymer networks, which are reminiscent of the filament networks close to the outer plasma membrane of cells. These new types of biomimetic membranes have a large potential for applications as drug

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delivery systems. In addition to the soft and flexible assemblies, biomimetic research has also produced hard materials in the form of biomimetic minerals. These minerals are typically built up from rather simple building blocks such as hydroxyl apatite or calcium carbonate [45]. Several attempts have been made to build u p complex architectures consisting of rodlike filaments within membrane compartments. It has been demonstrated that both actin filaments and microtubules can be polymerized within lipid vesicles. This can be directly observed in the optical microscope since the growing filaments induce morphological transformations of the vesicles. In the case of actin, two different procedures have been realized. One of these procedures led to shells, which are reminiscent of the cytoskeleton cortex, the other to protrusions, which resemble microvilli. The layer-by-layer construction of polyelectrolyte multilayers makes it possible to incorporate layers of different species of polyelectrolytes a n d / o r of other types of colloids. In this way, one can construct complex multilayers, which represent multifunctional interfaces. Complex architectures may also be constructed using chemically structured surfaces. The multifunctional interfaces of biomembranes arise from the lateral organization of these membranes into specialized membrane domains. New techniques have been developed which make it possible to chemically structure solid surface on the nanometre scale. These structured surfaces can be used to build up supramolecular architectures with a defined lateral organization [46]. Biological cells have amazing nanobiomaterials properties, which are based on their macromolecular and supramolecular building blocks, build u p in the water medium; for examples membranes and biological polymers. Biological membranes are soft and flexible nanostructures display the intracellular architectures and the interactions between cells with multifunctional intelligent nanobiomaterials. Besides this, biological cells are built u p hard materials in the form of biominerals with their control morphology at nanometre scale level. It is well known that all cells contain identical macromolecules, however different types of cells can survive in very adverse environments like live in boiling water, in strong acids, at the low temperatures, and under the various pressures of the deep sea. These different adaptations are due to different supramolecular architectures with a large variety of very efficient transport systems containing nanoscaled proteins or nanoengins, which transform not only chemical energy into mechanical work but also

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responsible for the transmembrane transport of ions/macromolecules, adhesion/fusion of membranes, intracellular transport of vesicles and organelles, cell division and cell locomotion. The biomimetic materials and transport properties of biological cells are very complex. The biomimetic designed model materials systems are biocompatible with defined physical, chemical or biological characteristics and have applications in bioengineering, pharmacology and medicine [47]. Learning from nature has inspired us the fabrication of novel artificial smart materials for example, the development and application of bio-inspired nanochannels. Bio-inspired nanochannels enable many potential approaches to study various biomolecules in confined spaces and in real-time by current measurements with precisely controlled functions. Applications for these systems range from simulating the process of ion transport in living organisms by using biomimetic nanochannels to applying artificial nanochannel systems to investigate the chemistry, structure, size, and conformational states of biomolecules [48]. To developing artificial membranes in the form of lipid bilayers in which specialized and highly efficient transport proteins are incorporated are very challenging aspects. This raises the question: is it possible to mimic biological membranes and create membrane-based sensor a n d / o r separation devices? In the development of biomimetic sensor/separation technology, the ion channels and carriers (transporters) are important. The ion channels are highly efficient membrane pore proteins capable of transporting solutes at very high rates, up to 109 molecules per second. Carrier proteins generally have a lower turnover but are capable of transport against gradients. An ideal sensor/separation device requires the supporting biomimetic matrix to be virtually impermeable to anything but the solute in question. A biomimetic support matrix will generally have finite permeabilities to water, electrolytes, and non-electrolytes. The feasibility of a biomimetic device thus depends on the relative transport contribution from both protein and biomimetic support matrix. Also the stability of the incorporated protein must be addressed and the protein-biomimetic matrix must be encapsulated in order to protect it and make it sufficiently stable in a final application in some current developments of biomimetic sensor/separation devices. Supported membrane nanodevices are based on natural or artificial ion channels embedded in a lipid membrane deposited on a

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chip wafer. Membrane conductance is modulated by biorecognitive events, with the use of intrinsic binding sites of the ion channel or via artificial sites fused to the channel protein. Artificial ion gates are constructed by coupling a specific ligand for the analyte near the channel entrance or a site important to triggering channel conformation. The binding event leads to the closure of the ion channel or induces a conformational change of the channel, reducing the ion flux. The signal transduced from the device is the decrease in the ion flux-induced electron current at a silversilver chloride electrode at ultimate single-molecule sensitivity. The setup of the device needs gel membrane supports that allow membrane formation and contribute to the stability of the bilayer by exposure of functional groups that promote electrostatic interaction and formation of hydrogen bridges and enable the introduction of covalent spacers and anchors. Photo-cross-linked polyvinylpyrrolidone and polyacrylamide, electropolymerized polydiaminobenzene and coated agarose, as well as various chemical modifications of these polymers, were employed as membrane supports. With optimized assemblies, the membrane support did allow the formation of stable bilayer membranes, proved by gigaseal (electrical sealing with giga-ohm resistance) to be free of any point defects in the lipid assembly. The chemically engineered ion channels, amphiphilic lipids, microlithographically designed chip, isolated polymer frames, and a hydrogel membrane support are very useful in the new bionanodevice. Single ion channels within the supported lipid bilayer are provided stable signals at an operational stability to test and screen for membrane receptors but still insufficient to use this device as a sensor for off-site application [49]. The emerging field of biomimetics allows one to mimic biology or nature to develop nanomaterials, nanodevices and processes. Molecular-scale devices, superhydrophobicity, self-cleaning, drag reduction in fluid flow, energy conversion and conservation, high adhesion, reversible adhesion, aerodynamic lift, materials and fibres with high mechanical strength, biological self-assembly, antireflection, structural coloration, thermal insulation, self-healing and sensory-aid mechanisms are some of the examples found in nature that are of commercial interest [50]. He et al. highlighted the assembly of multifunctional biomimetic microcapsules at the molecular level for biophysical research and the biomedical field. Among the available molecular assembly techniques, layer-by-layer assembly has attracted extensive attention for the fabrication of

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biomimetic microcapsules because it possesses engineered features including size, shape, thickness, composition and permeation, and the capability of incorporating different types of biomolecules. They also highlighted how biomimetic microcapsules can be fabricated by directly applying lipids and proteins as assembly pairs and how layer-by-layer assembled polyelectrolyte microcapsules can be interfaced with biological components such as phospholipid membranes and proteins. The applications of these biomimetic microcapsules in drug delivery, biosensors, and hybrid nanodevices are also addressed [51]. Yan and Yu investigated that the dendrimers proved themselves as functional nanodevices for drug delivery because they can carry drug molecules with a greater water solubility, bioavailability, and biocompatibility. They used mesoscopic simulations to investigate the tension-mediated complexes comprising charged dendrimers and lipid bilayer membranes and demonstrated that the permeability of charged dendrimers can be effectively enhanced in the tense membranes, and the permeability in the actual hole is several times higher than that in the lipid-poor section. The findings have implications in tuning intracellular delivery rates and amounts in nanoscale complex and chemotherapeutics [52]. The electronic properties of silicon, such as the conductivity, are largely dependent on the density of the mobile charge carriers, which can be tuned by gating and impurity doping. When the device size scales down to the nanoscale, routine doping becomes problematic due to inhomogeneities. He et at reported that a molecular monolayer, covalently grafted atop a silicon channel, can play a role similar to gating and impurity doping. Charge transfer occurs between the silicon and the molecules upon grafting, which can influence the surface band bending, and makes the molecules act as donors or acceptors. The partly charged end-groups of the grafted molecular layer may act as a top gate. The doping- and gating-like effects together lead to the observed controllable modulation of conductivity in pseudometaloxide-semiconductor field-effect transistors. These results offer a paradigm for controlling electronic characteristics in nanodevices at the future diminutive technology nodes [53]. Song et al. constructed an active biomimetic system by integrating kinesin motor, microtubule, and man-made biomimetic microcapsule by using the layer-by-layer technique and could serve as cargos in this active biomimetic system and can be transported by kinesin motors along microtubules. It may help to create kinesin-powered complex hybrid micro and nanodevices [54]. Microfluidic devices may be

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highly beneficial to the rapid fabrication of small quantities of various nonviral vectors with different functionalities, which is indispensable for effective order-made gene therapy. Kuramoto et ah reported a microfluidic chip-based approach for fabricating small quantities of nonviral vectors in a short time in preparation for order-made gene therapy applications. This approach permitted us to fabricate multifunctional envelope-type nanodevices, composed of a compacted or condensed DNA core and a lipid bilayer membrane shell, which are considered as promising nonviral vectors for gene therapy applications [55]. Sun et al. fabricated superhydrophobic Ti0 2 nano-strawberry rutile films, on which superhydrophobicity and superhydrophilicity can be reversibly switched by alternation of ultraviolet irradiation and dark storage at a large scale in aqueous solution via a seeded growth method at low temperature without any pressure equipment [56]. The interactions between water and biological molecules have the potential to influence the structure, dynamics, and function of biological systems, hence the importance of revealing the nature of these interactions in relation to the local biochemical environment. Fukuma et al. investigated the structuring of water at the interface of supported dipalmitoylphosphatidylcholine bilayers in the gel phase in phosphate buffer solution using frequency modulation atomic force microscopy. They presented experimental results supporting the existence of intrinsic (i.e., surface-induced) hydration layers adjacent to the bilayer. These results demonstrated that the intrinsic hydration layers are stable enough to present multiple energy barriers to approaching nanoscale objects, such as proteins and solvated ions, and are expected to affect membrane permeability and transport [57]. Water among the restricted space of crowded biological macromolecules and at membrane interfaces is essential for cell function, though the structure and function of this biological water itself remains poorly defined. Higgins et al. reported that by using a highly sensitive dynamic atomic force microscope technique in conjunction with a carbon nanotube probe, and revealed a hydration force with an oscillatory profile, reflects the removal of u p to five structured water layers from between the probe and biological membrane surface with fluidity [58]. Skin has the potential to provide an important noninvasive route for diagnostic monitoring of human subjects for a wide range of applications. Dimensions of surface features in skin suggested that nanodevices and microdevices could be utilized to monitor molecules and ions

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extracted from the skin. Methods of enhancing extraction from the skin for diagnostics are being developed including reverse iontophoresis, electroporation and sonophoresis. Connolly et al. reported a model system for the simulation of in vivo extraction of molecules and ions by reverse iontophoresis that displays similar behavior to skin both in terms of molecular flux levels and electrical impedance characteristics. The device has potential for use in the development of complete reverse iontophoresis/sensor systems, tested glucose [59]. The molecularly-targeted contrast agent is a kind of technology image the cardiovascular disease in the near future. This is non-invasive plaque detection and preemptive treatments with nanoparticulate agents within reach for clinical applications. The advent of molecularly-targeted nanoparticle technology may change the way to detect atherosclerotic lesions, determine their clinical significance and even provide non-invasive treatments using so-called intelligent contrast agents that are able to interrogate the vascular wall [60]. Drug delivery systems (DDS) capable of releasing an active molecule at the appropriate site and at a rate that adjusts in response to the progression of the disease or to certain functions/biorhythms of the organism. Light-responsiveness nanomaterials are sensitive to electromagnetic radiation, which can be applied on demand at well delimited sites of the body. Some light-responsive DDS are of a single use while others able to undergo reversible structural changes when cycles of light/dark are applied, behaving as multi-switchable carriers, releasing the drug in a pulsatile manner, regarded as functional components of intelligent DDS [61]. Molecular modeling is a key methodology for research and development and provides nanoscale images at atomic and even electronic resolution, predicts the nanoscale interaction of unfamiliar combinations of biological and inorganic materials, and evaluates strategies for redesigning biopolymers for nanobiotechnological uses. Molecular modeling tested scaffold proteins, redesigned apolipoproteins found in mammalian plasma that hold the discoidal membranes in the proper shape, and predicted the assembly as well as final structure of the nanodiscs [62]. Yinghao et al. attempted direct observation of the light-driven rotation of a FoF(l)ATP motor and observed that the ATP motor is expected to be a promising rotary molecular motor in the development of nanodevices [63]. ATP-driven motor proteins, acts as the actuators of living cells, possess promising characteristics. Protein bodies from sieve elements of higher plants provide a novel example. Sieve elements

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of higher plants contains protein bodies utilized in the microfluidics systems for pressure-driven transport of photo-assimilates throughout the plant. Unique protein bodies in the sieve elements of legumes act as cellular stopcocks, by undergoing a Ca 2+ -dependent conformational switch in which they plug the sieve element. In living cells, this mechanism is probably controlled by Ca 2+ -transporters in the cell membrane. Knoblauch et al. reported the rapid, reversible, anisotropic and ATP-independent contractility in these protein bodies in vitro based on the unique biological function of the legume crystalloid protein bodies and their contractile properties, and gave them the distinctive name forisome (gate-body) [64]. Single nanometre-sized pores (nanopores) embedded in an insulating membrane are an exciting new class of nanosensors for rapid electrical detection and characterization of biomolecules. Notable examples include alpha-hemolysin protein nanopores in lipid membranes and solid-state nanopores in Si3N4. Storm et al. reported a new technique for fabricating silicon oxide nanopores with single-nanometre precision and direct visual feedback, using state-of-the-art silicon technology and transmission electron microscopy They reported a pore of 20 nm is opened in a silicon membrane by using electron-beam lithography and anisotropic etching. After thermal oxidation, the pore can be reduced to a single-nanometre when it is exposed to a high-energy electron beam. This fluidizes the silicon oxide leading to a shrinking of the small hole due to surface tension. When the electron beam is switched off, the material quenches and retains its shape. This technique dramatically increases the level of control in the fabrication of a wide range of nanodevices [65]. The boron nitride nanotube/nanosheet presented the purpose and significance. Briefly, hexagonal boron nitride (h-BN) is a layered material with a graphite-like structure in which planar networks of BN hexagons are regularly stacked. As the structural analogue of a carbon nanotube (CNT), a BN nanotube (BNNT) was first predicted in 1994; since then, it has become one of the most intriguing noncarbon nanotubes. Compared with metallic or semiconducting CNTs, a BNNT is an electrical insulator with a band gap of ca. 5 eV, basically independent of tube geometry. In addition, BNNTs possess a high chemical stability, excellent mechanical properties, and high thermal conductivity. The same advantages are likely applicable to a graphene analogue-a monatomic layer of a hexagonal BN. Such unique properties make BN nanotubes and nanosheets a promising nanomaterial in a variety of potential fields such as

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optoelectronic, nanodevices, functional composites, hydrogen accumulators, electrically insulating substrates perfectly matching the CNT, and graphene lattices [66]. In the in vitro motility assay, actin filaments are propelled by surface-adsorbed myosin motors, or rather, myosin motor fragments such as heavy meromyosin (HMM). Recently, efforts have been made to develop actomyosin powered nanodevices on the basis of this assay but such developments are hampered by limited understanding of the HMM adsorption geometry. Persson et ah investigated new insights not only for the development of motor powered nanodevices but also for the interpretation of fundamental biophysical studies of actomyosin function and for the understanding of surface-protein interactions in general [67]. Atherosclerosis is the disease mechanism responsible for coronary heart disease (CHD), the leading cause of death worldwide. One strategy to combat atherosclerosis is to increase the amount of circulating high-density lipoproteins (HDL), which transport cholesterol from peripheral tissues to the liver for excretion. The process, known as reverse cholesterol transport, is thought to be one of the main reasons for the significant inverse correlation observed between HDL blood levels and the development of CHD. The synthesis of biomimetic HDL nanostructures that replicate the chemical and physical properties of natural HDL provides novel materials for investigating the structure-function relationships of HDL and for potential new therapeutics to combat CHD [68]. The recent advances in the understanding of biosilica production, biomodification of diatom frustules and their subsequent applications in bio/chemical sensors are reported as a model membrane for filtration and separation [69]. Smart nanostructures are sensitive to various environmental or biological parameters and offer great potential for numerous biomedical applications such as monitoring, diagnoses, repair and treatment of human biological systems. The smart nanostructures for biomedical applications especially emphasize drug delivery using smart nanostructures to develop various novel techniques of sensing, imaging, tissue engineering, biofabrication, nanodevices and nanorobots for the improvement of healthcare [70]. Proteins play an important role in the formation of biominerals in the body, which induce or inhibit mineralization of calcium phosphate together with modulation of the mineral phase structure. Many efforts have been made to understand the mechanism of this process and mimic the exquisite structure of biominerals in biomimetic methodologies. The recent advances in

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INTELLIGENT NANOMATERIALS

the synthesis of calcium phosphate materials by taking advantage of protein assemblies along with the examples of templates based on proteins and polypeptides that have been employed to manufacture calcium phosphate nanomaterials [71]. Collagen is a protein that undergoes a multi-step, hierarchical self-assembly which starts from individual peptide chains that assemble into a canonical triple helix. These triple helices then assemble into higher order structures which are often, but not always, fibrous in nature. The collagen mechanism of assembly commonly referred to as collagen mimetic peptides (CMPs), and its interactions of with cell-surface receptors are very important for nanodevices [72]. The self-healing materials are a very important and inspired by natural processes, allow the fabrication of auto-repairing systems. The self-healing systems found in natural processes and others created by manmade activity with special emphasis on the role played in this field by nanostructures. Finally, the self-healing of gold nanoparticles during laser irradiation is considered in more detail since it is a rare example of a functional nanomaterial with self-repairing properties [72]. Enzymes, together with the process of self-assembly, constitute necessary components of the foundation of life on the nanometre scale. The exceedingly high efficiency and selectivity exhibited by enzymes for catalyzing biotransformations naturally lead to the exploration of enzyme mimics and the applications of enzymes in industrial biotransformations. While the mimicking of enzymes aims to preserve the essence of enzymes in a simpler system than proteins, industrial biotransformations demand high activity and stability of enzymes. Recent research suggests that small peptidebased nanofibers in the form of molecular hydrogels can provide a general platform to achieve both important goals, addressing important societal problems in energy, environment, and health [74]. The use of peptide and protein-related materials as smart building blocks can be very practical for new material synthesis and device fabrications. For example, peptides and proteins have superior specificity for target binding as seen in the antibody recognition and this biological recognition function can be used to assemble them into specific structures and shapes in large scale, as observed in the S-layer protein assembly. Collagens are assembled from triple helix peptides in micron-size with precise recognition between peptides and these biological assemblies can undergo smart structural change with pH, ionic strength, temperature, electric/ magnetic fields. In addition, assemblies of peptides can template complex 3D crystallization processes with catalytic function, thus

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enabling to grow various materials in physiological conditions at low temperature in aqueous solution. The biomimetic growth of nanomaterials in aqueous solution is extremely useful when they are applied to therapeutics and medical imaging in vivo since these nanomaterials will be well dispersed in bodies. Peptides also play significant roles in signal transduction pathways in cells. For example, neuropeptides are used as neurotransmitters between synapses and these peptides bind receptors on the surface of cells to cascade the signal transduction. These versatile functions of peptides are extremely practical and here we discuss them with examples of relevant applications such as nanoreactors, sensors, electronics, and stimulus-responsive materials. It should be noted that peptide/ protein assemblies can be applied to build up micron-scale materials that still feature excellent nano-scale ensembles, which essentially bridges the nano-world and the micro-world [75]. The peptides and proteins, can interact and self-assemble into highly ordered supramolecular architectures with functionality. By imitating the processes where biological peptides or proteins are assembled in nature, one can delicately design and synthesize various peptide building blocks composed of several to dozens of amino acids for the creation of biomimetic or bioinspired nanostructured materials. A new kind of peptide building block is the diphenylalanine motif. The application of diphenylalanine-based peptide nanomaterials as nanotubes, spherical vesicles, nanofibrils, nanowires are enormous [76]. Short synthetic peptides is an effective nanobiomaterials in applications ranging from controlled gene and drug release, skin care, nanofabrication, biomineralization, membrane protein stabilization to 3D cell culture and tissue engineering. They have due to their unique nanostructures, remarkable simplicity and biocompatibility, also possess antimicrobial activities. These attractive features are inherently related to their selective affinity to different membrane interfaces, high capacity for interfacial adsorption, nanostructuring and spontaneous formation of nano-assemblies. Apart from sizes, the primary sequences of short peptides are very diverse as they can be either biomimetic or de novo designed. Recent advances in studying peptide amphiphiles, focusing on the formation of different nanostructures and their applications in diverse fields, which will serve as useful guidance for functional materials design and nanobiotechnology [77]. Mumm et al. constructed a biopolymer-based template assembly around the spines of the bristle worm Aphrodita aculeata (sea mouse), and found a very high aspect ratio nanowires and nanotubes.

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INTELLIGENT NANOMATERIALS

The parallel, hexagonal arrangement in the spine constitutes a photonic crystal, which gives the animal its colourful, iridescent appearance. Around the nanochannels, the spines consist of a chitin/protein composite material, which withstand the chemical and thermal conditions needed for established template-assisted nanofabrication strategies. The presented system of parallel nanowires or nanotubes in a biopolymer matrix might be utilized in applications [78]. Biological self-assembly is a very complex and results in highly functional materials (bottom-up approach) using biomolecular building blocks of precisely defined shape, size, hydrophobicity, and spatial distribution of functionality because of many small forces to increase the complexity and functionality of self-assembled nanomaterials. The coiled-coil peptide motif plays in the creation of functional units, assemblies, and systems [79].

20.3 Biomimetic DNA Nanotechnology is found in living systems. DNA is a molecule with physical properties that make it ideal for both nanoscale construction and information storage [80-82]. The use of DNA for biomimetic control systems and advantage of the nanomechanical properties are very interesting achievement due to its twist and extend, supercoiling (DNA strands wrap around each other in a tightly packed manner), to bend DNA coated cantilevers by exposure to DNA of the appropriate sequence and to create a nanoscopic tweezer-like device that can be closed or opened with the addition of auxiliary DNA fragments. The stability of supercoiled DNA depends on the presence of positively charged ions, which balance the close interactions of the negatively charged DNA helices. Atomic force microscopy of self-assembled DNA-protein nanostructures demonstratet that the supercoiling can be regulated by controlling the ionic strength of the solution, thus resulting in an ionic switch. The ionic concentrations within cells are regulated by ion pumps and channels, which are known to be vital for a myriad of physiological functions. Discrete switching of nucleic acid supercoiling might play crucial roles in allowing enzymes to access the DNA, affecting nuclear processes ranging from transcription to replication. Thus DNA is a ubiquitous biomolecule that contains our blueprints focuses on the information contained within its sequence. Currently, electronic properties of DNA and

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nanomechanical properties of DNA are very interesting and appealing. DNA is as nanomechanical devices and axon of a nerve cell as wire-like components of neurons use molecular motors to transport vesicles over distances as long as meters within biological systems [83-88]. Antipina et al. investiged DNA interaction with cationic lipids is of immense importance for the fabrication of biosensors and nanodevices. Lipid/DNA complexes can be applied for direct delivery of DNA-based biopharmaceuticals to damaged cells as non-viral vectors. The synthesis, physical-chemical properties and first transfection and toxicity experiments are reported. Special attention was focused on the capability of CI and CII to complex DNA at low and high subphase pH values. Langmuir monolayers at the air/water interface represent a well-defined model system to study the lipid/DNA complexes. Interactions and ordering of DNA under Langmuir monolayers of the new cationic lipids were studied using film balance measurements, grazing incidence X-ray diffraction (GIXD) and X-ray reflectivity (XR). They demonstrated the ability of these cationic lipids to couple with DNA at low as well as at high pH value and showed good transfection efficacy and low toxicity in the in vitro experiments indicating that lipids with such structures are promising candidates for successful gene delivery systems [89]. The molecular recognition properties of DNA molecules combined with the distinct mechanical properties of single and double strands of DNA which can be utilized for the construction of nanodevices, and can perform ever more tasks with possible applications ranging from nanoconstruction to intelligent drug delivery. With the help of DNA it is possible to construct machinelike devices that are capable of rotational motion, pulling and stretching, or even unidirectional motion. It is possible to devise autonomous nanodevices, to grab and release molecules, and also to perform simple information-processing tasks [90]. A molecular machine or nanomachine is a mechanical device that is measured in nanometers and convers chemical, electrical and optical energy to controlled mechanical function. Nanomachines are responsible for the directed transport of macromolecules, membranes, chromosomes within the cytoplasm and play a crucial role in every biological process including muscles contraction, cell division, intracellular transport, ATP production and genomic transcription and translation. Biological molecular machines are molecular proteins that convert the chemical energy released by hydrolysis of ATP into mechanical energy.

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INTELLIGENT NANOMATERIALS

Myosin, kinesin and ATPase are biological nanomachines that perform locomotive and actuation functions in living organisms such as cargo transporters, propellers and molecular sorters and many more as shown in Figure 20.3. Devices that change state in response to an external trigger might be used for molecular sensing, intelligent drug delivery or programmable chemical synthesis. Biological molecular motors that carry cargoes within cells have inspired the construction of rudimentary DNA walkers that run along self-assembled tracks. It has even proved possible to create DNA motors that move autonomously, obtaining energy by catalysing the reaction of DNA or RNA fuels. Bath and Turberfiel reported DNA nanomachines are made by self-assembly, using techniques that rely on the sequence-specific interactions that bind complementary oligonucleotides together in a double helix. They can be activated by interactions with specific signalling molecules or by changes in their environment [91]. Smart materials able to sense environmental stimuli can be exploited as intelligent carrier systems. Acidic pH-responsive polymers, for instance, exhibit a variation in the ionization state upon lowering the pH, which leads to their swelling. The different permeability of these polymers as a function of the pH could be exploited for the incorporation and subsequent release of previously trapped payload molecules/nanoparticles. Deka et al. reported a proof of concept of a novel use of pH-responsive polymer nanostructures based on 2-vinylpyridine and divinylbenzene, having an overall size below 200 nm, as cargo system for magnetic nanoparticles, for oligonucleotide sequences, as well as for their simultaneous loading and controlled release mediated by the pH [92]. Zhou et al. for the first time reported the controlled power release of biofuel cells (BFCs) by aptamer logic systems processed according to the Boolean logic operations "programmed" into the biocomputing systems. On the basis of the built-in Boolean NAND logic, the fabricated aptamerbased BFCs logically controlled by biochemical signals enabled us to construct self-powered and intelligent logic aptasensors that can determine whether the two specific targets are both present in a sample [93]. Hou et al. reported artificial bioinspired intelligent nanopore machines/system which has an ion concentration effect that provides a nonlinear response to potassium ion at the concentration ranging from 0 to 1500 μΜ. This phenomenon is caused by the G-quadruplex DNA conformational change with a positive correlation with ion concentration [94].

• Prole» coat

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DNA N m m M n

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Iw o > r1 DNA Figure 20.3 Shows DNA nanomachines and nanomotors.

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n

g o 2 o 2

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INTELLIGENT NANOMATERIALS

20.4 Biomimetics Metal and Metal Oxides Nanostructures Formation Noble metal nanoparticles are well known to its core properties in wide array of biomolecular detection and diagnostics, biosensing, antimicrobials, therapeutics, optoelectronics, recording media, solar cells, electrical batteries, optical receptors, catalysts to name a few. During past decade, synthesis of metal nanoparticles via biological materials such as virus, bacteria, fungus, algae, lower and higher plants have shown that templating by these biological materials can yield a variety of nanocrystals under mild reaction conditions. The rapid advancement in metal nanoparticle synthesis by these bioinspired materials provide an additional insights over conventional methods as it is clean, nontoxic, ecologically sound and eco-friendly. The synthesis and characterization of noble metal nanoparticles are an important area of research related to the size and shape of nanoparticles, which provides not only an efficient control over physicochemical properties but also their potential applications in the various sectors including catalysis, sensor technology, biological labeling, optoelectronics recording media and optics [95-102] as shown in Figure 20.4. The size, shape and surface morphology play crucial roles in controlling the physical, chemical, optical and electronic properties of the nanoscopic materials [103-104]. Various methods are available for nanoparticles synthesis based on the physicochemical methods, but the most of these methods are highly reactive and causes potential environmental and biological hazards [105]. Biomimetics have advantages over physicochemical methods because of its clean, non-toxic chemicals, environmentally benign solvents, and user-friendly nature [106]. Shankar et ah reported the synthesis of gold and silver nanoparticles by the reduction of aqueous AuCl4~ and Ag + ions using extracts from geranium, neem and lemongrass plants and they revealed that the gold nanotriangles showed that ketones/ aldehydes present in the extract may play an important role in directing the shape evolution in these nanostructures. Ankamwar et al. reported the synthesis of gold nanotriangles using tamarind leaf extract as the reducing agent and identified that tamarind plant as a potential candidate for shape-controlled synthesis of gold nanoparticles due to tartaric acid (-COOH) [107-111].

Chemical

Synthetic routes

Physical

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Figure 20.4 Shows applications of biomimetics metal and metal oxides nanostructures.

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INTELLIGENT NANOMATERIALS

Shukla et al. reported biomimetic synthesis of silver nanoparticles using crude black pepper (Piper nigrum) extract at room temperature. The average particle size of silver nanoparticles was found in the range of 20-50 nm [112]. Wang et al. reported a very simple, reliable, clean, nontoxic and eco-friendly biological method for the synthesis of semiconductor monoclinic Se nanoparticles using the Bacillus subtilis. The Se nanomaterial crystals with high surfaceto-volume ratio, good adhesive ability and biocompatibility have been employed as enhancing and settled materials for H 2 Ö 2 electrochemical biosensor [113]. Park et al. synthesized sulfur-doped manganese oxide (ID) nanowires nanostructures with controllable crystal structures and crystallite dimensions via one-pot nonhydrothermal solution route and the special importance is that both sulfur-doped manganate nanowires show promising electrode performances for lithium secondary batteries [114]. The potential of inorganic nanoparticles for the drug or gene delivery carriers are due to their high cellular uptake capacity, non immunogenic response, and low toxicity. Layered double hydroxide (LDH), i.e. anionic clay, is one of the most promising candidates for various biological purposes. Choi et al. reported the LDH sizedependent toxicity using cultured human lung cells; and revealed that 50 nm particles were determined to be more toxic than larger particles, while LDHs within the size range of 100 to 200 nm exhibited very low cytotoxicity in terms of cell proliferation, membrane damage, and inflammation response. Thus, LDH is an attractive biocompatible delivery carrier for biological and medical applications [115]. The different polymorphs of vanadium oxide possess electrical switching properties, as a result address energy-efficiency issues; an example of the intelligent regulation of infrared light. The presence of infinite vanadium ion chains in the crystal structure plays a decisive role in determining the electrical properties of vanadium oxides. Wu et al. investigated the synthesis of vanadium oxide materials. The nanostructures not only promotes a mechanistic understanding of the temperature-driven electrical switching properties but also provides the right materials for constructing smart devices that can selectively filter out infrared light [116]. Vanadium oxide/polyaniline nanotubes were produced by cationic exchange between hexadecylamine and polyaniline after the synthesis of vanadium oxide nanotubes by sol-gel method followed by hydrothermal treatment and may be utilized as cathode for ion-Li batteries [117]. Vanadium oxide nanostructures show

BIOMIMETIC MATERIALS TOWARD APPLICATION

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electrochemistry and catalysis property, but little is known about their toxicology. Laura et al. show toxicological aspects of vanadium oxide nanotubes using cultured human colon carcinoma cells (Caco-2) and their viability assessed with the neutral red assay and demonstrated a significant loss in viability after four hours. The physical size and structure of the nanotubes may play an important role not only in their cytotoxic effects, but also use in the safety devices [118]. Samuel et al. reported a new method to produce vanadium oxide nanofibers with dimensions o a I—I

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Carbon namUte Figure 20.5 Shows few applications of graphene and carbon nanotubes.

Btammebc carbon rtanoUbe based nanodevice

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the antibacterial property of graphene oxide nanowalls using the Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus and also found that the graphene oxide nanowalls reduced by hydrazine were more toxic to the bacteria than the unreduced graphene oxide nanowalls. The better antibacterial activity of the reduced nanowalls was assigned to the better charge transfer between the bacteria and the more sharpened edges of the reduced nanowalls, during the contact interaction [130]. The direct dispersion of hydrophobic graphite or graphene sheets in water without dispersing agents is a great challenge. Li et al. reported that chemically converted graphene sheets obtained from graphite can readily form stable aqueous colloids through electrostatic stabilization without the surfactant stabilizers and opening up enormous opportunities to use this unique carbon nanostructure for many technological applications [131]. The DNA intercalative agent is essential for understanding DNA scission, repair, and signal transduction. Ren et al. explored systematically the graphene oxide (GO) interaction with DNA molecules using fluorescence spectroscopic (FL), circular dichroism (CD), gel electrophoresis, DNA thermal denaturation and demonstrated that the GO nanosheets could intercalate efficiently into DNA molecules. They also illustrated that the scission of DNA by GO sheets combining with copper ions could take place pronouncedly. The scission of DNA by the GO/Cu 2 + system is critically dependent on the concentrations of GO and Cu2+ and their ratio. DNA cleavage ability exhibited by the GO with several other metal ions and the fact that GO/Cu 2+ -cleaved DNA fragments can be partially relegated suggest that the mechanism of DNA cleavage by the GO/metal ion system is oxidative and hydrolytic. The result reveals that the GO/Cu 2 + could be used as a DNA cleaving system that should find many practical applications in biotechnology and as therapeutic agents [132]. Graphene and its functionalized derivatives are unique and versatile building blocks for self-assembly to fabricate graphene-based functional materials with hierarchical microstructures. Xu et al. reported a strategy for three-dimensional self-assembly of graphene oxide sheets and DNA to form multifunctional hydrogels. The hydrogels possess high mechanical strength, environmental stability, dye-loading capacity, self-healing property and provides a new insight for the assembly of functionalized graphene with other building blocks, especially biomolecules, which will help rational design and preparation of hierarchical graphene-based materials [133]. Zhang et al. reported the studies

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on surface modification of graphene with 1-octadecanethiol and its application as heavy metal sensors e.g. mercury (II) (Hg2+) detection at 10 ppm [134]. Biomimetic approaches are of great interest for the synthesis of nanomaterials because biological architectures often resist large mechanical stresses much better than man-made ones. The controlled molecular self-assembly of CNTs is possible. Coating CNTs with oligopeptides enables the production of self-assembled composite structures. By varying the factors that influence peptidepeptide interactions (e.g., salt concentration), these structures can assume a wide range of shapes and sizes, such as cylindrical microfibers and flat ribbons [135]. In addition, DNA could be used as an alternative crosslinker [136], as well as synthetic organic molecules [137]. Shim et al. demonstrated a simple process of transforming general commodity cotton threads into intelligent e-textiles using a polyelectrolyte-based coating with carbon nanotubes (CNTs), make them promising materials for many high-knowledge content garments. In addition to this they reported that CNT-cotton threads can be used to detect albumin with high sensitivity and selectivity. They also focus the proof-of-concept for the application of these materials as wearable biomonitoring and telemedicine sensors, which are simple, sensitive, selective, and versatile [138]. Carbon nanotubes are used for the construction of electrodes for electrochemical devices such as batteries, capacitors, and actuators. Electrodes composed entirely of carbon nanotubes (bucky paper) have high surface areas but are typically weak, and have insufficient conductivity for practical macroscopic applications. Whitten et al. established a technique that uses naturally occurring biopolymers to produce electrodes (free standing films) that exhibit conductivities of 300 S/cm. These composites also have considerable mechanical strength (up to 145 MPa) and sufficient specific capacitance of 19-27 F / g to enable them to be used as free standing electrodes for the various potential applications [139]. Singh et al. demonstrated the highly successful capacity for in vivo implantation of a new carbon nanotube-based composite that is, itself, integrated with a hydroxyapatite-polymethyl methacrylate to create a nanocomposite for the critical demands of biological integration applications in a diversity of areas including carbon nanotube, regeneration, chemistry, and engineering research [140]. Kaung et al. reported the rational design of a peptide recognition element (PRE) that is capable of noncovalently attaching to SWNTs as well as binding to trinitrotoluene

BIOMIMETIC MATERIALS TOWARD APPLICATION

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(TNT). Both computational and experimental analyses demonstrate that the peptide retains two functional domains for SWNT and TNT binding for the creation of chemosensors using designed PRE as selective surface coatings for targeted analytes [141].

20.6 Biomimetics Smart Polymer Smart materials that can respond to external stimuli are of widespread interest in biomedical science. Thermal-responsive shape memory polymers, a class of intelligent materials that can be fixed at a temporary shape below their transition temperature (T(trans)) and thermally triggered to resume their original shapes on demand, hold great potential as minimally invasive self-fitting tissue scaffolds or implants. The intrinsic mechanism for shape memory behavior of polymers is the freezing and activation of the longrange motion of polymer chain segments below and above T(trans), respectively. Both T(trans) and the extent of polymer chain participation in effective elastic deformation and recovery are determined by the network composition and structure, which are also defining factors for their mechanical properties, degradability, and bioactivities. Such complexity has made it extremely challenging to achieve the ideal combination of a T(trans) slightly above physiological temperature, rapid and complete recovery, and suitable mechanical and biological properties for clinical applications. Xu and Song reported a shape memory polymer network constructed from a polyhedral oligomeric silsesquioxane nanoparticle core functionalized with eight polyester arms. The cross-linked networks comprising this macromer possessed a gigapascal-storage modulus at body temperature and a T(trans) between 42 and 48 degrees C. The materials could stably hold their temporary shapes for > 1 year at room temperature and achieve full shape recovery O

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Figure 22.3 Two- and three-dimensional structures made of electrospun fibers (A), and different nanofiber morphologies such as (a) beaded, (b) smooth, (c) core-shell, and (d) porous fibers [from Ref.41].

POLYMERIC NANOFIBERS AND APPLICATIONS IN SENSORS

22.4.2

811

Fiber Geometry and D i m e n s i o n

Electrospinning can produce nanofibers with different sizes (micro or nano, for example) and shapes such as beaded fibers, smooth fibers, ribbon-like fibers, branched fibers, porous fibers, and coreshell fibers. Different morphologies of polymer nanofibers are shown in Figure 22.3b [41]. There are multiple spinning conditions affect the fiber geometry and dimension, which includes polymer solution conditions such as viscosity, elasticity, conductivity and surface tension, spinning conditions such as applied voltage, distance between spinneret tip and fiber collector, flow rate, spinneret diameter and hydrostatic pressure in capillary tube, and ambient conditions such as temperature and humidity. The fibers dimension depends primarily on the polymer jet sizes since nanofibers are resulted from evaporation or solidification of the charged polymer jets. It has been recognized that during the traveling of a polymer jet after ejection from the spinneret, the polymer jet split into multiple polymer strands, which results in the deposition of fiber with different diameters onto the fiber collector. Polymer solution property in particular viscosity is the most influential parameter in determining fiber diameter. For example, a high viscous polymer produces larger fiber diameter as compared to low viscous [42,43]. Deitzel et al. noticed that the fiber diameter increased with increasing the polymer concentration.1421 Demir et al. further demonstrated that the fiber diameter is proportional to the cube of the polymer concentration [43]. As discussed earlier, the polymer concentration is directly proportional to the viscosity of polymer solution. Therefore, the higher the polymer viscosity the larger the fiber diameter we get. The spinning conditions also greatly affect the fiber morphology. The most important spinning parameters that affect the morphology of the polymer nanofibers are summarized in Table 22.2. Another solution property which greatly affects the fiber diameter is conductivity of the polymer solution. The high conductivity or dielectric constant of the polymer solution results into the formation of thin fibers. For example, Zong et al. demonstrated that increase the conductivity of the polymer solution, by using salts, greatly reduced the diameter of fibers.[441 Electrical applied voltage is also another cause for the control of fiber diameter. This is because high voltage ejects large amount of polymer solution from the spinneret tip, which results in a larger fiber diameter. In addition to these spinning parameters, solution flow rate

Table 22.2 Effect of changing electrospinning parameters on the resultant fiber morphology [from Ref.25]. Effect on Fiber Morphology

Process Parameter Viscosity/ concentration

Conductivity/solution charge density

Surface tension

•

Low concentrations/viscosities yielded defects in the form of beads and junctions; increasing concentration/ viscosity reduced the defects

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No conclusive link established between surface tension and fiber morphology

Polymer molecular weight

•

Increasing molecular weight reduced the number of beads and droplets

Dipole moment and dielectric constant

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Flow rate

• •

Lower Flow rates yielded fibers with smaller diameters High flow rates produced fibers that were not dry upon reaching the collector

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3 w

Collector Composition and geometry

Ambient Parameters

• Smoother fibers resulted from metal collectors; more porous fiber structure was obtained using porous collectors • Aligned fibers were obtained using a conductive frame, rotating drum, or a wheel-like bobbin collector • Yrans and braided fibers were also obtained • Increased temperature caused a decrease in solution viscosity, resulting in smaller fibers • Increasing humidity resulted in the appearance of circular pores on the fibers

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INTELLIGENT NANOMATERIALS

also affects the size of the fibers. For example, the lower flow rate of the polymer solution results reduction in the fiber diameter. These data suggest that the spinning conditions should be optimized to control the fiber geometry and dimension.

22.5 Applications of Electrospun Nanofibers in Sensors The electrospun fibers find applications in numerous fields such as sensors, tissue engineering and drug delivery. For example, many polymers (e.g. PLLA and PLGA) were electrospun as non-woven membranes and used as a scaffold in tissue repair and regenerative applications, mainly because of their interconnected, three-dimensional porous structure and relatively large surface areas, similar to the morphology of natural extra-cellular matrix [45,46]. However, the research and development of chemical and biosensors based on electrospun polymer nanofibers are very limited. Now the trend is drastically changes toward the development of polymer nanofibers as a sensing element for various sensor applications, in particular sensing chemical and biological entities, due to the nanofiber's functional properties that enhance the sensitivity of the sensor elements. The following section explicates how electrospun polymer fibers can be used as a sensing element in various sensor applications. 22.5.1

Chemical Sensors

Electrospun fibers with controllable membrane thickness, nanostructures, diversity of materials, large specific surface and fiber aspect ratio are expected to be an ideal candidate as the structure of sensing materials for sensing chemicals, including gases. Table 22.3 shows some of the electrospun fiber-based sensing materials used in sensor applications. Ding et al. (2004) reported the fabrication of electrospun polymer nanofiber-based Quartz crystal microbalance (QCM) sensors to detect ammonia (NH3) [47]. The viscous blend solutions of poly(acrylic acid) (PAA) and poly(vinyl alcohol) (PVA) were electrospun on the QCM surface in order to use as a sensing element. The size of nanofibers was in the range of 100-400 nm. The authors tested a series of nanofiber membranes with various weight percentage of PAA to PVA for their sensitivity to detect

POLYMERIC NANOFIBERS AND APPLICATIONS I N SENSORS

815

NH 3 . The nanofiber-based sensors showed fast responses to NH 3 after adding the PAA component in fibers (see Figure 22.4a). The average resonance frequency shifts of fibers coated QCM sensors with 11, 18, 25, and 33 wt% of PAA to PVA were 40, 150, 240, and 380 Hz, respectively. The results showed that the sensing properties were mainly affected by the content of PAA component in the nanofiber membranes, concentration of NH 3 , and relative humidity. Additionally, the sensitivity of nanofiber membranes coated QCM sensor was much higher than that of continuous film coated QCM sensor. This study demonstrated the efficacy of electrospun polymer nanofibers as an effective sensing element better than the conventional thin films. PAA (polyacrylic acid); PVA (polyvinyl alcohol); PEI (polyethyleneimine); HCSA (10-camphorsulfonic acid); PAN! (polyaniline); PEO (polyethylene); PDPA (polydiphenylamine); PMMA (polymethyl methacrylate); POT (polyo-toludine); PS (polytyene); MWCNT (multi-walled carbon nano-tube); CB (carbon black); PECH (polypichlorohydrin); PIB (polyisobutylene); PVP (polyvinylpyrrolidone); PAN (polyacrylonitrile); VOC (volatile organic compound); RT (room temperature). Electrospun nanofibers were also used for sensing other chemicals such as ethanol (C2H5OH). For example, Wang et al. (2008) reported the electrospinning of zinc oxide (ZnO) nanofibers with an average diameter of 150 nm suitable for sensing ethanol [48]. This study demonstrated that the electrospun ZnO nanofibers possess excellent sensing properties against ethanol at an operating temperature of 300°C, including a rapid response (time ~3 s) and recovery (time ~8 s), super sensitivity, and high selectivity. The authors studied in sensing other gases as well and the cross sensitivity is shown in Figure 22.4b. Figure 22.4b depicts the responses of the sensor to C 2 H 5 OH, CO, NO, N 0 2 , CH 4 , H2, and C2H2 at 300°C. These results indicated that the sensor is less sensitive to or totally insensitive other gases expect ethanol. This study revealed that ZnO nanofibers showed high sensitivity to ethanol but very low sensitivity to other gases at an operating temperature of 300°C. This kind of electrospun ZnO nanofiber-based sensor can be used for sensing chemicals like ethanol. Electrospun nanofibers were also used in detecting toxic gases such as carbon monoxide. For example, Lim et al. (2010) reported the preparation of mesoporous indium oxide (ln 2 0 3 ) nanofibers by electrospinning PVA solution with indium acetate, and their suitability in sensing carbon monoxide (CO) gas in air [49]. After calcining the

Figure 22.3 Two- and three-dimensional structures made of electrospun fibers (A), and different nanofiber morphologies such as (a) beaded, (b) smooth, (c) core-shell, and (d) porous fibers [from Ref.41]. Types

Materials

Array of Fibers

Gases Tested

Fiber Diameter

Operating Temperature (°C)

Detection Limit

oo

I—1

Ref.

Acoustic wave

PAA-PVA PAA PEI-PVA

Nonwoven Nonwoven Nonwoven

100-400 nm 1-7 μπι 100-600 nm

NH 3 NH 3 H2S

RT RT RT

50 ppm 130 ppb 500 ppb

[47] [48] [50]

Resistive

HCSAPAN/PEO PDPA-PMMA PANI PMMA-PANI HCSA-PANI HCSA-POT/PS MWCNT/ nylon CB-PECH, PEO, PIB, PVP

Single

100-500 nm

NH 3

RT

500 ppb

[52]

Nonwoven Nonwoven Nonwoven Single Nonwoven

-400 nm 0.3-1.5 μη 250-600 nm 20-150 nm 0.2-1.9 p m

NH 3 Amines (C2H5)3N Alcohols H20

Nonwoven Oriented

110-140 nm ~3pm

RT RT RT RT RT RT RT

1 ppm 100 ppm 20 ppm No data No data No data 1000 ppm 5 ppm 250 ppm 500 ppm

[53] [54] [55] [56] [57] [58] [59]

voc

CH 3 OH s C

5H10C12

C 6 H 5 CH 3 C2HC13

w c r

I—I

o

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o > ►—I

o w > r1

o

f

►
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3

w

w > z

a >

>n

t->

Photoelectric

Co-ZnO

Nonwoven

50-400 nm

o2

RT

0.32 Torr

[70]

Optical

Oxides-PAN

Nonwoven

50-200 nm

Co2

RT

700 ppm

[71]

n 1—1

o z M

z

o w 00 I—>

^1

oo

I—1

2

H M1 f"1 I-

(b) 0„_^ N X

-100-

c -200-

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\\

l\

-300>. o

1

\

/

\

1 \

\\ // \\

-I

c Φ ^400-

u.

/

\l \ . i // \ \l

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σ

/ \

/ \

\\ / / A / /

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\ / \

\

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-a

60 40 H

c

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oc 5 0 -

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/ c

/ \\ \ y / / \\V^^^// / e

/ \ '/ \ /

\/

-500-

/

/

/

^-^_

o

80-

1

■

1

1000

■

1

'

1

2000 3000 Time (s)

'

1

4000

'

1

5000

υ0

1000 2000 3000 4000 Concentration (ppm)

5000

Figure 22.4 Response of nanofiber-based QCM sensors with various weight percentage of PAA to PVA exposed to 50 ppm of NH 3 at the relative humidity of 55%. (a) 0 wt%; (b) 11 wt%; (c) 18 wt%; (d) 25 wt%; (e) 33 wt% (a) [from Ref.47], and cross sensitivity of the electrospun ZnO nanofiber-based sensor to C 2 H 5 OH, CO, NO, N 0 2 , CH,, H2, and C2H2 (b) [from Ref.48].

POLYMERIC NANOFIBERS AND APPLICATIONS I N SENSORS

819

PVA/indium acetate composite nanofiber precursor, In 2 O s nanofibers were formed in the size range of 150-200 nm in diameter. The authors compared the efficacy and sensing properties of electrospun ln 2 0 3 nanofibers with ln 2 0 3 powders. It was found that the ln 2 0 3 nanofibers showed a high sensitive and response to CO in air as compared to their powder counterparts, which further demonstrated the efficacy of electrospun nanofibers for use in sensor applications. In another notable study, Ji et al. (2008) reported the electrospinning of poly(methyl methacrylate) (PMMA) nanofibers suitable for use in sensing gases [50]. They have developed a nanostructured composite based on in situ polymerization growth of poly(aniline) on the surface of PMMA nanofibers, which was then transferred to an interdigitated gold electrode to construct a gas sensor. The electrical responses of the gas sensor based on the electrospun composite nanofiber towards triethylamine (TEA) vapors were investigated. It was revealed that the sensor showed a sensing magnitude as high as 77 towards TEA vapor of 500 ppm. The effects of fiber diameters, spinning time, nature and concentration of doping acids on the gas sensitive properties of the composite have also been investigated. The diameters of the electrospun PMMA fibers had an effect on the sensing magnitude of the gas sensor, which is proposed to relate to the difference in the surface-to-volume ratio of the nanofibers. Furthermore, it was found that the concentration of doping acids only led to changes in resistance of the sensor, but did not affect its sensing characteristics. The gas sensor with toluene sulfonic acid as the doping acid exhibited the highest sensing magnitude, which was explained by taking into account of the sensing mechanism and the interactions of doping acids with TEA vapor. The composite fibers exhibited a high sensing magnitude towards TEA vapor in the range of 20-500 ppm. In addition, the responses were linear, reversible and reproducible, suggesting their potential of electrospun PMMA nanofibers as a sensitive element for the detection of low concentration amine vapor. Electrospun nanofibers were also used in sensing metal ions. For example, Wang et al. (2002) reported the electrospinning of PAA nanofiber in the form of non-woven membranes in conjugation with pyrene methanol (PM), as a fluorescent indicator, suitable for sensing metal ions (Fe3+ and Hg2+) and 2,4-dinitrotoluene (DNT) due to their excellent surface area to volume ratio. [51] This group compared the electrospun polymer nanofiber membranes with electrostatically layer-by-layer (ELBL) assembled polymer

820

INTELLIGENT NANOMATERIALS

films. The experimental data indicated that electrospun nanofiber membranes can offer enhanced sensitivity in sensing metal ions. For instance, the Stern-Volmer plots of electrospun membranes showed good linearity for ferric (Fe3+), mercuric (Hg2+), and DNT, as compared to casting films made by electrostatic layer-by-layer method, which is due to the higher surface area of the electrospun a nanofiber than their casting film counterparts. Due to the quenching effect of these chemicals to the pyrene moieties, the fluorescent intensity of nanofibers had a linear response to the concentration of quenchers, and the nanofibers showed high sensitivities. These experimental examples, and others, are clearly confirmed the efficacy of electrospun nanofibers for use in sensor applications. 22.5.2

Biological Sensors

Electrospun nanofibers were also used for biological sensor applications. For example, Shin et al. (2009) reported the development of an amperometric biosensor utilizing electrospun nanofibers made of polyaniline and polystyrene for measuring glucose concentration.1521 This type of amperometric sensor converts the concentration of an analyte into an electrical signal by integrating biological sensing. The electrospun polymer nanofibers were immobilized with glucose oxidase and the electrical property of the polymer composite nanofibers was investigated. The experimental data showed that the conductive current was increased when increasing the glucose concentration, which suggested the efficacy of electrospun polymer nanofibers in measuring glucose concentration. In another study, Li et al. (2006) reported the electrospun poly (lactic acid) (PL A) nanofibers for sensing biological agents [53]. This article reported the effectiveness of biotin incorporation into electrospun PLA membranes and the biotinylated DNA probe was successfully captured by immobilized streptavidin in a preliminary biosensor assay using an electrospun PLA nanofiber membrane as substrate. Electron probe microanalysis confirmed the presence of biotin in the electrospun fibers and that the final biotin levels are proportional to the amount in the initial dispersions. Preliminary biosensor assays confirmed that streptavidin immobilized on the nanofiber membrane surface could capture a biotinylated DNA probe. Photographs of typical biosensor assay results from both membrane substrates are shown in Figure 22.5, which indicates that streptavidin was successfully immobilized on the PLA membrane through deposition

POLYMERIC NANOFIBERS AND APPLICATIONS IN SENSORS

821

and that it was capable of capturing biotinylated DNA capture probes hybridized to the target sequence-liposome complex, thus proving the validity of the concept of using nanofiber membranebased biosensors. Recently, Ding et at. (2010) reported the electrospun porous hemoglobin (Hb) based biosensor for the detection of hydrogen peroxide (H 2 0 2 ) and nitrite.1541 They have electrospun the Hb onto the surface of glassy carbon (GC) electrode without using immobilization matrix, which offers an excellent electrochemical sensing platform. The electro catalytic property of Hb modified GC electrode was investigated using H 2 0 2 and nitrite as model sensing compounds. This study demonstrated that the Hb modified electrode has enhanced activity in the electrochemical reduction of H 2 0 2 and nitrite, which offers a number of attractive features and is explored to develop an amperometric biosensor. The Hb based amperometric biosensor has fast responses to H 2 0 2 and nitrite, good dynamic response ranges, and excellent detection limits. Based on the experimental data, this study demonstrated the efficacy of electrospun Hb for use in biosensor applications. Electrospun polymer nanofibers were also used as a urea biosensor. For example, Sawicka et al. (2005) reported the use of electrospun nanofibers made of Urease with polyvinylpyrrolidone (PVP) can be

Figure 22.5 Capture of E. coli synthetic target sequence on the PES membrane (top) and electrospun PLA nanofiber membrane (bottom). Circled area is the capture zone where streptavidin is deposited. Squared area is where the background measurement is taken that serves as an internal control of the performance of each membrane, i.e. presenting possible non-specific binding of liposomes on the membrane [from Ref.53].

822

INTELLIGENT NANOMATERIALS

used as a urea sensing detector.[55] Urease encapsulated in electrospun membranes has proven to be excellent urea biosensing material, due to the fact that the large surface area offers an improved adsorption rate and therefore reduces the response time. The immobilized enzyme remained active inside the polymer solution. Most importantly the reactivity was maintained inside the electrospun nonwoven membrane. The electrospun membrane acted as catalyst in the hydrolysis in different concentrations of urea solutions. The successful immobilization of urease implies reproducibility with other enzymes used for biosensing, such as glucose oxidase, horseradish peroxidase, etc. This kind of electrospun nanofiber cam be used as a cost efficient urea sensing material applicable in medical diagnosis, environmental and bioindustrial analysis. All of these experimental examples, and others, are confirmed the efficacy of electrospun polymer nanofibers for potential use in sensor applications.

22.6

Conclusions

This chapter has highlighted the salient features and current status in the development of sensors based on electrospun nanofibers. Most of the recent work on electrospinning has focused either on trying to understand the fundamental aspects of the process in order to gain control of nanofiber morphology, structure, surface functionality, and strategies for assembling them or on determining appropriate conditions for electrospinning of various polymers suitable for sensor applications. On the other hand, as per the literature survey, fewer efforts have been directed in the past few years towards the development of novel sensors based on electrospun nanofibers, in spite of their fabulous functional properties in comparison to their microscale counterparts. The electrospun nanofiber-based sensing devices have a number of key features, including high sensitivity, exquisite selectivity, fast response and recovery, and potential for integration of addressable arrays on a massive scale. In general, polymer nanofiber based sensors show great promise for future applications in sensing chemicals and biological entities. The development of cheap, sensitive, and reliable multi-analyte sensors for the detection of chemicals and biological entities is still required for multiple applications and for the advancement of sensors. The electrospun nanofibers can be utilized for this purpose owing to their functional properties and tunable surface modification features as

POLYMERIC NANOFIBERS AND APPLICATIONS IN SENSORS

823

discussed in this chapter. In the future development of sensors in particular for sensing biological entities, it is necessary to develop various biocompatible materials as a sensing element. Based on the experimental examples illustrated in this chapter, and others, the electrospun polymer nanofibers will provide new opportunities to develop sensors (both chemical and biological) with better sensitivity and long term reliability. This is an exciting time to be involved in the development of electrospun nanofiber-based sensors in order to formulate them as a promising sensing element for a variety of chemical and biological sensing applications, with great challenges and also great expectations ahead.

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35. Landau, O.; Rothschild,A.;Zussman, E. Processing-Microstructure-Properties Correlation of Ultrasensitive Gas Sensors Produced by Electrospinning. Chem. Mater., 2009,22, 9-11. 36. Luoh, R.; Hahn, H.T. Electrospun nanocomposite fiber mats as gas sensors. Compos. Sei. TechnoL, 2006,66,2436-2441. 37. Taylor, G. Electrically Driven Jets. Proceedings of the Royal Society of London Series a-Mathematical and Physical Sciences, 1969,323,453-475. 38. Yarin, A.L.; Koombhongse, S.; Reneker, D.H. Bending instability in electrospinning of nanofibers. /. Appl. Phys., 2001,89, 3018-3026. 39. Reneker, D.H.; Chun, I. Nanometre diameter fibres of polymer, produced by electrospinning. Nanotechnology, 1996, 7,216-223. 40. Huang, J.X.; Virji, S.; Weiller, B.H.; Kaner, R.B. Polyaniline nanofibers: Facile synthesis and chemical sensors. /. Am. Chem. Soc., 2003,125,314-315. 41. Ramakrishna, S.; Fujihara, K.; Teo, W.E.; Yong, T.; Ma, Z.; Ramaseshan, R. Electrospun nanofibers: solving global issues. Mater. Today, 2006, 9,40-50. 42. Deitzel, J.M.; Kleinmeyer, ].; Harris, D.; Beck Tan, N.C. The effect of processing variables on the morphology of electrospun nanofibers and textiles. Polymer, 2001,42, 261-272. 43. Demir, M.M.; Yilger, I.; Yilgor, E.; Ennan, B. Electrospinning of polyurethane fibers. Polymer, 2002,43,3303-3309. 44. Zong, X.H.; Kim, K.S.; Fang, D.; Ran, S.; Hsiao, B.S.; Chu, B. Structure and process relationship of electrospun bioabsorbable nanofiber membranes. Polymer, 2002,43,4403-1412. 45. Liao, S.; Murugan, R.; Chan, C.K.; Ramakrishna, S. Processing nanoengineered scaffolds through electrospinning and mineralization, suitable for biomimetic bone tissue engineering. /. Mech. Behav. Biomed. Mater., 2008, 2, 252-260. 46. Yang, F; Murugan, R,; Wang, S.; Ramakrishna, S. Electrospinning of n a n o / micro scale poly(L-lactic acid) aligned fibers and their potential in neural tissue engineering. Biomaterials, 2005,26,2603-2610. 47. Ding, B.; Kim, ].; Miyazaki, Y; Shiratori, S. Electrospun nanofibrous membranes coated quartz crystal microbalance as gas sensor for NH3 detection. Sensor Actuators B-Chem., 2004,202,373-380. 48. Wang, W.; Huang, H.M.; Li, Z.Y.; Zhagn, H.N.; Wang, Y; Zheng, W ; Wang, C. Zinc Oxide Nanofiber Gas Sensors Via Electrospinning. /. Am. Ceram. Soc, 2008,91,3817-3819. 49. Lim, S.K.; Hwang, S.H.; Chang, D.; Kim. S. Preparation of mesoporous ln 2 0 3 nanofibers by electrospinning and their application as a CO gas sensor. Sensor Actuators B, 2010.149,28-33. 50. Ji, S.Z.; Li, Y; Yang, M.J. Gas sensing properties of a composite composed of electrospun poly(methyl methacrylate) nanofibers and in situ polymerized polyaniline. Sensor Actuators B-Chem., 2008,133,644-649. 51. Wang, X.Y., et al. Electrospinning technology: A novel approach to sensor application. J Macromol Sei Pure Appl Chem, 2002, A39(10), 1251-1258. 52. Shin, Y.J.; Wang, M.; Kameoka, J. Electrospun Nanofiber Biosensor for Measuring Glucose Concentration. /. Photopolym. Sei. TechnoL, 2009, 22, 235-237.

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Index [70] PCBM, 440 [84] PCBM, 441 3-diethylaminopropyl, 601 ABC transporters, 718 Absorption spectra phthalocyanine - Fe NPs, 396-397 Phthalocyanine - CNT conjugate, 399,401 Phthalocyanine - QD conjugates, 390,403 quantum dots, 354-356, 359 size determination, 356 Add atom conditions, 446 Adriamycin, 588, 618 Aerosol assisted chemical vapour deposition, 285-286 AFM, 454, 458 Aggregation, 596, 603 AIST, 461 Albumin, 587, 594, 616 Alginate, 518, 519 Amine, 599, 606 Aminoallyl - dUTP, 672 Amorphous, 149,150,153,154,155, 156,158,162,163,164,166, 167,170,171,172,173 Amphiphilic, 607 Antibody, as targeting moiety, 722, 724, 728 Anticancer, 599,615 Antimony sulfide nanoparticles, 720 magnetic nanoparticles, 361-363

phthalocyanines, 372-373 quantum dots, 350-351 Aqueous medium, 589, 590, 605 Aromatic ring, 611 Aspect ratio, 533, 534 Aspirin, 613 Atmospheric pressure chemical vapour deposition, 283-284, 287 Atomic force microscopy (AFM) magnetic nanoparticles, 365 Phthalocyanine - CNT conjugate, 394 Auger electrons, 718, 724, 732 Auger recombination, carbon nanotubes, 371-372 quantum dots, 360 Azobenzene, 612, 613 Bacterial Cellulose (BC), 479 Band gap, 68, 76, 79, 354, 359 Barrier, 533 Bentonite, 493, 506-509 Bexxa®, 724 BHJ, 431, 433^35, 438, 442,444 Binding Actuation, 483 Binding modulation, 483 Binding site, 671 Bioavailability, 589 Biochips, 797 Biocompatibility, 490, 518,527 Biocompatibility of nanofibers, 687-690 Biocompatible, 823 827

828

INDEX

Biocomposite, 471 Bioimaging, 599 Biological process, 741-743 Biological sensors, 801,803,820 Biomimetic, 763,774-775 approaches, 742, 770 assemblies, 773 biomembranes, 747 DNA, 758-759 materials, 741-743 metals, 762 methods, 741 peptide, 743 properties, 745 protein, 743 smart polymer, 771 synthesis, 764 Biomolecules, 742-743 Biopolymer, 746, 758, 770 Biosensor, 490, 504, 512,522 Biosensors, 751, 759, 764 Bio-sensors, 571, 572, 574,575, 578 Biotemplate, 774 Biotin, 588,630, 631, 642 Biotransformations, 756 Bisphosphonates, 726 Bone scintigraphy, 726 Bone targeting, 726 Boron Neutron Capture Therapy (BNCT), 732 Boron nitride nanoparticles, 732 Boron nitride nanotubes, 228 Boronic acid, 587, 610, 611 Bottom-up, 743, 758 Bovine serum albumin (BAS), 587,594 Bragg peak, 732 Brychytherapy, 720, 721 Bulk production, 484 Cadmium telluride solar cells, 220 Capping, agent, 643 molecule, 643 Characterization of nanopolymers

infrared spectroscopy, 336 surface morphology, 338 surface plasmon resonance spectroscopy (SPR), 336 swelling behavior, 334 thermal transitions, 337 wettability, 337 Carbohydrate, 610 Carbon, 41, 45, 52, 53 Carbon granularity, 656 Carbon nanotubes, 225,573, 575, 730, 731, 743,755, 776, 770 Carbonic anhydrase IX, 727 Carboxylic acid, 599, 600 Carboxymethylcellulose (CMC), 508-510 Carboxymethylchitosan (CMCS),525 Catalyst(s), 39, 40,44 Catecholamines, 792 Cationic, 596,600, 605, 606, 609, 617 Cellulose Whisker, 478-479 Cerebellum, 667 Chains solar valley, 427 Characterization technique, 149,173 Chemical, 39-43,50-52,57,59, 61, 70, 76, 80 Chemical sensors, 814, 823,825 Chemical synthesis of intelligent nanopolymers atom transfer radical polymerization (ATRP), 328 fabrications of nanofibres, 325 Chemical vapour deposition, 252, 279-282,287 Chitosan, 587,588, 594,599, 601 Classification, 469 Clay, 492-494,497-501, 506-510, 512,514,520, 526 Cloud Point Temperature (CPT), 721

INDEX

CNT, 446 Coil-to-globule transition, 721 Colloidal, 589, 594,595, 597,599 Colloids, 99mTc labeled, 716, 719 Co-localization, 663 Colon, 598, 602 Compatibilizer, 562 Compatiliser, 477 Concavalin A (ConA), 611 Confocal microscopy, 592, 608 Control, 46, 54, 70, 73, 74 Conventional TEM, 666 Copolymerisation, 474,476 Copolymerization, 595, 606-608 Core shell, 639 Correlative immunolabeling, 654 Correlative microscopy, 649 Critical Micelle Concentration (CMC), 601 Crosslinker, 492,498,500,502, 504, 509,512, 514, 524 Crosslinking density, 498, 506, 510, 512, 524 Crystalline silicon, 218 Cyanobacteria, 673 Cytochrome c, 638 Cytosine, 628 Dark-field STEM, 656 Daunorubicin, radiolabeled, 732 Dendrimers, 729 Dendritic spines, 666 Dengue virus, 795 Density of States (DOS) carbon nanotubes, 369-370 nanoparticles, 350 Dexamethasone, 587 Dextran, 587,594, 613 Dextrin, 594 Diameter, 43, 44, 46, 47, 50, 52, 53, 61, 62, 64, 66, 67, 70, 75, 77 Differential scanning calorimetry (DSC) carbon nanotubes, 369

829

Distribution pattern analysis, 667 DNA, 593,606, 608, 610 DNA nanomachines, 758, 760 Dopamine, 791 Double-hydrophilic block copolymer (DHBCP), 613 Downreagulation of receptor expression, 725 Doxorubicin, 588, 601 Drug delivery, 490,491, 494,496, 497,504, 512-515, 518, 519, 526 Drug delivery system, 753 Dynamic light scattering (DLS), 357-358 EDX, 448-449,651, 659 Elastin-like peptides, 721 Electrical, 39,40, 63,69, 78 Electrical properties, 589 Electric-field-sensitive INCH, 519-521 Electroactive material, 482-483 Electrochemical, 573, 574, 576,577, 578 Electrochemical impendance microscopy, 796 Electrode, 572,573,574,576, 577,578 Electrode(s), 55,56, 63, 67, 80 Electrodeposition, 47, 50, 54-46, 76 Electrospinning, 681-684,802-809, 815,819, 822 Electrostatic force, 806-809 Electrostatic repulsion, 506,509 Encapsulation, 593,594, 643, 644 Endothelial Growth Factor receptor (EGFr), 725 Energy consumption, 253, 303, 311 Energy diagram, 438 Energy modelling, 252,298-306, 310-311

830

INDEX

Energy transfer, 377, 399 Enhanced Permeation and Retention (EPR) effect, 722, 724, 729, 731, 732 Enteric Coating, 480 Epinephrine, 792 EQE, 441 Estrogen receptor, 792 Exciton diffusion length, 433 Exfoliated morphology, 494,497, 500,501,506 Extracellular face, 655 Extracellular fluid, 598 FAB, 447-448 Ferritin, 744 Fiber collector, 807-809 Flory's theory, 589 Flourine doping, 289-294 Fluorescein, 588 Fluorescence band gap, 354 carbon nanotubes, 370-371 fluorescence lifetimes, 359-360, 371,384-387 fluorescence quantum yields, 359, 384-387 forster resonance energy transfer (FRET), 402-409 phthalocyanine - Fe NPs, 397-398 phthalocyanine-CNT conjugate, 399-^00 phthalocyanine-QD conjugate, 403 quantum dots, 354-356, 359 Fluorescent labels, 716 Folic acid, as targeting moiety, 727 Follicle-stimulating hormone receptor, 725 Forster Resonance Energy Transfer (FRET), 401-409 Four probe resistance, 453-454 Fourier transform infrared, 590,591

Free radical polymerization, 498, 500,501,508,516,517,524 Freely-suspended BLMs, 785 Freeze fractured replica, 654,656 Frist OPV, 431 FT-IR, 447,451-452 FT-NMR, 447 Fullerene derivatives, 442 Gallium citrate scintigraphy, 727 Gas sensors, 39,49,63, 68,70, 73, 76, 79, 81 Gastrointestinal tract, 598,612 Gd@C82,451-452 Ge@C82,445-455 Gelation, 638 Gene expression, 673 Genetic Modification (GM), 484 Genome incorporation, 673 Genotyping biochips, 797 Germanium, 445-446,448-450 Glucose, 587, 610-612 Glucose-responsive, 610, 611 Glucose-sensitive INCH, 522, 523 Glutamate receptor, 665 Glycochips, 797 Glycoprotein, 610 Gold, 252,273-274,287,294-267 Gold nanoparticles, 198Au-labeled, 730 GPC, 457 Graphene, 767-769 Growth, 39,42-49,52, 60-62, 67, 74 Heat mirror, 254,259-260 Heat-shock protein, 743 Heparin, 617 Hepatobiliary route, 722 Heterogeneous, 593,595, 604 Hot carriers, 428-429 HPLC, 447 Hybrid BLM, 785 Hybrid hydrogel, 493, 513 Hydride materials, 230

INDEX

Hydrodynamic diameter, 590, 614 Hydrogel, 585-587, 589,600, 605, 607,610,618 Hydrogen Storage, 224 Hydrophilic, 586, 593-596, 603-609, 613 Hydrophobie, 593-596, 599, 601, 603-609, 613 Hydrotalcite (HT), 507, 508 Hydrothermal, 39,42, 52, 60, 61, 62,70 Hydroxybisphosphonates, 726 Hypoxia, 718 IFMIF, 427 Imaging fluorescence, 350, 361 magnetic resonance imaging (MRI), 361-362 optical coherence tomography (OCT), 361 positron emission tomography (PET), 361 Immunogold, 664 Immunolabeling, 650 In situ polymerization, 492,497, 499 Indole acetic acid, 629 Inductively coupled plasmaoptical emission spectroscopy (ICP-OES), magnetic nanoparticles, 365 quantum dots, 357 Infra-Red spectroscopy carbon nanotubes, 369 magnetic nanoparticles, 365 phthalocyanine-CNT conjugate, 394-395, 399 phthalocyanine-QD conjugate, 390 phtholocyanine - Fe NPs, 396, 398 quantum dots, 357 Inorganic solar cell, 428-430 Insulin, 587, 588,594,610-612

831

Integrin, 726 Intelligent carrier system, 760 e-textile, 770 hybrid materials, 773 nanobiomaterials, 741-742 nanodevices, 741 polymer, 774 Intelligent drug delivery systems emulsion polymerization procedure, 326 intelligent therapeutics, 320 polymer-drug conjugates, 329 Intercalated morphology, 493 Intercalation, 553 Intercalators, radiolabeled, 732 Interpenetrating polymer network (IPN), 490, 508,509,524, 525 Intracellular, 598, 617 Intramembrane receptors, 659,666 Ion channel switch biosensor, 796 Ionic strength, 491,506 Ion-sensitive, 585 IPCE, 437 Iron oxide, 493,513,514 Iron oxide nanoparticles, polymer-coated, 730 ISE, 461 Isomerization, 612, 613 ITER, 427 Jablonski diagram, 377-378 Langmuir-Blodgett films, 796 Laponite, 501,508-510,512,513, 524,525 Laser flash photolysis setup, 388 triplet state lifetimes, 378,389 triplet state quantum yields, 387-389 Layer-by-layer assembly, 819 Layered silicate, 535 L-cysteine, 616

832

INDEX

Lectine, as targeting moiety, 725 Light responsive polymers, 321 Light-sensitive INCH, 520-522 Line scan, 662 Linear Energy Transfer (LET), 720,732 Lipicl films, 583 Liposomes, 731, 732 Low emmissivity, 253,256, 258-259, 304 Lower Critical Solubility Temperature (LCST), 721 Lower critical solution temperature (LCST), 495-497, 499,500, 515,524, 603-609, 614 Lu3N@C80,444 Lu3N@C80-PCBM, 445

Microporous, 149,153,157,158, 164,167,170,171,173 Mimicking power, 773 Mixed oxides, 149,150,154,156, 157,158,159,160,161,162, 163,164,165,166,167,170, 171,172,173 Montmorillonite (MMT), 497, 498, 501,520,521, 526 Multidrug resistance, 717, 718 Multifunctional, 609, 611, 612 Multiple bandgap materials, 428 Multiple exciton generation, 429 Multiple labeling, 651 Multi-stimuli-sensitive INCH, 523-526

Macrophages, 720 Magnetic field, targeting by, 731 Magnetic nanoparticles, 513, 514, 517,518 Magnetic Resonance Imaging (MRI), 730 Magnetic-field-sensitive INCH, 513-519, 526 Magnetic-responsive, 585 MALDI-TOF, 447-448 Mechanism, 39-49,62, 67, 70, 71, 73 Melt mixing, 476 Membrane proteins, 654 Mesoporous, 149,150,155,157, 158,160,161,167,171,173 Metal nanoparticles, 762 Metal oxide nanoparticles, 762 Metal-organic materials, 233 Methacrylic acid (MAA), 510-513 Methotrexate, 587 Methylenedithosphonate complex, 99mTc-labeled, 726 Mica, 501, 502 Micelle, 588,596, 597, 601, 614 Micrbialbioplastics, 472-473 Microcompounding, 475 Microfluidics, 496,504,521

NaCl, 607 Nanobiosensors, 583, 741-742 Nanocomposites, 222, 252,294, 478,482,535, 571-575 Nanocrystals composed, 13 Nanodiamonds, 730, 731 Nanoelectrodes, 773-774 Nanofi-bers for biosensors, 705-706 Nanofibers for therapeutic agents release, 701-705 Nanofibrous scaffold for tissue engineering, 691 Bone tissue engineering, 692 Cartilage tissue engineering, 693 Ligament tissue engineering, 694 Nerve tissue engineering, 700-701 Skeletal muscle tissue engineering, 695 Skin tissue engineering, 695-697 Vascular tissue engineering, 697-700 Nanofibers, 455-460 Nanogel, 585, 589-597, 599-602, 604-618

INDEX

Nanogenerator, 728, 732 Nanogold particles, 657 Nanomaterials, 572, 574, 576,578, 580,215 Nanomaterials in batteries, 233 Nanomechanicla properties, 758 Nanoparticles, 742, 745, 775 Nanostructures biomaterials, 741-742 Nanotechnology, 742, 745, 758, 775 Nanowires and nanotubes, 223 Natural polymers, 686-687 Negative temperature-sensitive INCH, 495, 496, 501, 514, 515 Neovasculature, 722, 725 Nitroglycerine, 722 NREL, 461 Nucleobases, radiolabeled, 732 Nucleotide, 630 N-vinylpyrrolidone (NVP), 504 Organic plastics, 468 Oriented attachment, 634 Osmotic pressure, 510, 526 Osmotic pressure delivery, 481 Osteoporosis, 726 Ostwald ripening, 634 Oxidation, 45-49,52, 55, 70, 76 p type semiconductor, 433 P3HT, 433,435-439,442-444, 451-^60 Paget disease, 726 Passive absorption, 641 PCBM, 435-438,440-445 PCDTBT, 440 PEG, 588, 601, 602, 607, 608,617 Perfluorohydrocarbon nanoparticles, 730 Permeability, 536,539,554 Pharmacokinetic, 590 Phase separation, 685, 721 Phenylboronic acid, 587, 611

833

Phosphors, 8 Photodynamic therapy (PDT) phthalocyanines, 373-374 quantum dots, 351 singlet oxygen, 378-379 Type I mechanism, 379 Type II mechanism, 379-380 Photon correlation spectroscopy, 590,591 Photopolymerization, 502,506,5014 Photoreceptor, 612 Photo-responsive, 612, 613, 618 Photovoltaic cells, 215 pH-responsive, 585,597, 599-602, 609,618 pH-responsive polymers poly (acrylic acid) (PAAc), 326 polyelectrolytes, 322 pH-sensitive INCH, 505-513, 519, 521,523,524 Phthalocyanin, 433 Phthalocyanines, 372-377 Physical annealing, 468, 476 Physical vapour deposition, 274-275,288-289 Phytate nanoparticles, 720 PL, 451,453 Plasma display panel, 5 Plasmid DNA, 672 Plasticity modulation, 469 Pluronic, 588,608, 617 Poly((2-dimethyl amino) ethyl methacrylate) P(DMAEMA), 609 Poly(acrylamide) (PAAm), 600 Poly(diethylaminoethyl methacrylate) (PDEAEMA), 600 Poly(dimethylaminoethyl methacrylate) (PDMAEMA), 600 Poly(dimethyldiallylammonium chloride) (PDMDAAC), 509, 510

834

INDEX

Polyvinyl alcohol, 610 Poly(ethylene glycol) dimethacrylate (PEGDMA), Porous fibers, 811 Porous solid, 149,150,151, 514,516 153,155 Poly(ethylene glycol) methyl Porphyrin, 433 ether acrylate (PEGMEA), 506-508 Positive temperature-sensitive Poly(ethylene glycol) methyl ether INCH, 495,496, 514, 515 Positron Emission Tomography methacrylate (PEGMMA), (PET), 722, 728 510,516 Postsynaptic site, 666 Polyethylene oxide) (PEO), 603, PPV, 433,435,440,441,443 608 Process Conditions, 554 Poly(ethylenimine), 597 Proliferation, 617 Poly(methacrylic acid), 599, 600 Properties Poly(N-isopropyl acrylamide) (PNIPAA), 721 carbon nanotubes, 365 Poly(N-isopropylacrylamide), 587, magnetic nanoparticles, 361 593, 604, 605 phthalocyanines, 372 Poly(N-isopropylacrylamide) quantum dots, 354 (PNIPAAm), 492,495-498, Protein, 587,593,594,607, 499-502, 504,505, 508-510, 610, 616 Protoplasmic face, 655 512, 514, 518, 524,525 Polypropylene oxide) (PPO), 608 Pseudoplastic properties, 605 Polyacrylamide (PAAm), 491, 513, Pulmonary drug delivery, 716 Pulsed laser deposition, 276-277 518,526 Purkinje cell, 659, 665 Polyacrylic acid (PAA), 497,507, 524,525 Polyanion, 506,509,526 Quantum confinement, 350 Quantum dots, 224, 716, 717, Polycation, 506,520, 526 Polyelectrolyte, 505, 511-513,519, 730, 743 525,589,594,596,599, 600, Quantum well solar cells, 223 Quaternary structure, 665 609, 610 Polyhedral oligomeric silsesquioxane (POSS),504,505 Radiation grafting onto Polymer micelles, 729, 731 nanoparticles Polymer nanofiber, 801,802, 805 direct method, 334 Polymerase chain reaction, 672 gamma rays, 332, 334 Polymeric nanoparticles, 772 pre-irradiation methods, 333 Polymerization, 553, 593,595, 597, Radiosynovectomy, 719, 720 601, 606 Raman spectroscopy Polymers, 571, 572, 573,574, 578 carbon nanotubes, 369-371 Polymers, water soluble, 728 magnetic nanoparticles, 365 Polyphosphazenes, 721 phthalocyanine-CNT conjugate, Polysiloxane, 502-504 394-395

INDEX

phthalocyanine-QD conjugate, 390 quantum dots, 357 Receptor, 663, 790 Redox property, 447 Renal excretion, 722, 729 Renewable energy, 426^127 Response, 39,42,67,69-81 Responsive polymeric particles colloidal particles, 330 complexes with biomolecules, 322-323, 325,330 core-shell structure and materials, 320, 327 crosslinking, 319,323-325, 333 hybrid particles with inorganic materials, 325-326 hydrophilic polymers, 321, 323, 327, 330 Reticuloendothelial system (RES), 719, 730, 731 RGD peptide, 726 Rheological, 605, 612 Rhodamine B, 613 Ricin, 725 Ripley's K-function, 667 Roll to roll process, 430-432 sBLMs, 792 Scanning electron microscopy, 590, 591, 796 Scanning electron microscopy (SEM) magnetic nanoparticles, 365 phthalocyanine - Fe NPs, 398 quantum dots, 358 SDS-FRL method, 654 Selectivity, 39, 67 Self assembly, 457,459 Self-assembled monolayers, 786 Self-assembled structure, 593 Self-assembly, 685-686, 805

835

Self-assembly nanoparticles Synthesis and properties, 331-332 Self-assembly of quantum dots, 23 Semiconducting Semiconductor, 68, 76, 79 Semiconductor to metal transition, 251,263-264 Sensor, 801-805,807,814-819 Shokley & Queisser limit, 428 Silica, 588, 606 Silver nanoparticles, 716, 730, 764,766 Single Photon Emission Computed Tomography (SPECT), 719, 722, 728 Singlet oxygen detection, 380, 383-384 mechanism, 379-381 properties, 378-384 quantum yield, 382-383 quenchers, 380-381 siRNA, 587 Small bandgap polymers, 439 Small molecular solid, 433,435 Smart drug delivery systems, 585, 586,589,597,599,618 Smart nanomaterials, 216 Sodium methacrylate (SMA), 524, 525 Solar control, 253-254,257 Solar energy materials, 253, 256 Solar H, 427 Solar India, 427 Sol-gel, 278,288-289 Sol-gel process, 149,153,155,156, 167,173 Sol-Gel synthesis, 14 Solid state NMR quantum dots, 358 Solid tumors, 722 Solventless BLMs, 787 Space application, 215

836

INDEX

Spatial randomness, 668 water-soluble quantum dots, Spinneret, 807-809,811,813,819 351-353 Splaying, 809 Synthesis of quantum dots Status and prognosis, 706-707 aqueous media, 12 Steric hindrance, 665 hot-matrix, 14 Stimuli-responsive nanoparticles, reverse micelles, 10 585 Synthetic Polymers, 687 Streptavidine, 630, 631 Subunit, 665 Taxonomic, 797 Subunit labeling, 669 Taylor cone, 808, 809 Sulfur colloid, 99mTc-labelled, 719, Temperature-responsive polymers 720 Supercapacitors, 236 lower critical solution Superparamagnetic nanoparticles, temperature (LCST) 322, 730 327-328,335-337 Surface area, 801-804,807,809, poly (ethylene oxide) (PEO) 814, 819 322 Surface modification, 820, 822 poly (lactic acid) (PLA) 322 carbon nanotubes, 366-367 poly (N-isopropylacrylamide) quantum dots, 351-353 (NIPAAm) 322, 325-329, Surface tension, 809,811,812 335,337 Surfactant, 559, 60, 61 pol (N-isopropylmethacrylamide) Suspension polymerization, 490,517 (PNIPMAM) 327 Sustained release, 480 poly (propylene oxide) (PPO) Swelling, 589, 590,592, 596, 600, 322 upper critical solution 610-615 Swelling ratio, 498-502, 505, temperature (UCST) 322, 507-512, 522, 524-526 335 Swelling/deswelling, 491, Temperature-sensitive, 585, 605, 492, 524 606, 608 Synthesis Temperature-sensitive carbon nanotubes, 368-369 INCH, 495-505,513-515, magnetic iron nanoparticles, 521,524 Template, 39,42,49-61,67, 68, 70, 363-365 organo-soluble quantum dots, 71, 74-77 Tetraoctylammonium bromide, 351-352 phthalocyanine nanoparticles, 377 287,294-298 phthalocyanine-CNT conjugate, Tetrathiafulvalene, 433 Theranostics, 718 390,399-401 Thermochromism, 251-252,254, phthalocyanine-QD conjugate, 389-390, 392 260,262-263, 306,311 phthalocyanines, 374-377 Thermocleavable polymers, 439, phtholocyanine - Fe NPs, 396 455-156

INDEX

Thermogravimetric analysis (TGA) carbon nanotubes, 369 phthalocyanine-CNT conjugate, 395-396 Thermoplastic starch (TPS), 477 Thermoresponsive polymers, 721 Thin film processing, 219 Thioglycolic acid, 628 Third generation solar cell, 429 Thorium dioxide nanoparticles, 720 Thorotrast, 720 Three-dimensional network, 489, 492,493 Time correlated single photon counting (TCSPC), 359 carbon nanotubes, 371 measurement of fluorescence lifetimes, 359,371, 385 measurement of fluorescence quantum yields, 359 phthalocyanine-CNT conjugate, 399^00 quantum dots, 359 Tin colloid, 720 Tissue engineering, 481-482,491, 518, 602, 609 Top-down, 743 Tortuous path, 535 Transducers, 572,573,575, 576, 577,578 Transfected cells, 673 Transferrin, 726, 727 Transformation, 673 Transgenic plants, 484 Transient absorption spectroscopy carbon nanotubes, 371 energy transfer, 387 phthalocyanine - Fe NPs, 398,401 Transmission Electron Microscopy (TEM), 493,494, 503,511, 590-591 magnetic nanoparticles, 365

837

phthalocyanine - Fe NPs, 398 phthalocyanine-CNT conjugate, 394, 399 quantum dots, 358 size determination, 358 Transparent conductors, 219 Trimethyl (acrylamido propyl) ammonium iodide (TMAAI), 501, 502 Trimethyl acrylamido propyl ammonium chloride (TMAAC1), 497,501, 502, 506,507 Triplet state lifetimes, 378 phthalocyanine - Fe NPs, 401 triplet quantum yields, 387-389 Tumor, 598,599, 601-603, 614-616 Type II band offset system, 430 Ultrasound-responsive, 617, 618 Unimers, 729 Upper critical solution temperature (UCST),495,496,515, 603,614 Urea, 607 US DOE, 427 UV-VIS-NIR, 451-452,456-457 Vanadium dioxide, 251-252,254, 260,269-272,281-284, 286-287,289-294,296-298 Vascular Endothelial Growth Factor receptor (VEGFr), 725 Viscotoxin, 725 Voltage-gated sodium channels, 792 Volume phase transition, 605,608 Volume phase transition temperature (VPTT), 495, 501,502, 504, 505,525

838

INDEX

Volume transition temperature (VTT), 603 Watson-Crick pairing, 673 XPS, 449-450 X-ray diffractometry (XRD), 493, 494, 500,501,511 X-ray photoelectron spectroscopy (XPS), 357, 365

X-ray powder diffraction magnetic nanoparticles, 365 phthalocyanine - Fe NPs, 389-394, 396,398 quantum dots, 356-357 scherrer equation, 357 Zeta potential, 608 Zevalin®, 724

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