Artificially Intelligent Nanomaterials For Environmental Engineering [1. Auflage] 9783527344949, 3527344942, 9783527816385, 9783527816361, 9783527816354

Table of contents :
Content: Preface xi1 Introduction 11.1 Global Challenges 11.2 Conventional Technologies in Environmental Science and Engineering 21.3 Nanotechnology 31.3.1 History of Nanotechnology Evolution 31.3.2 Concept and Definition 41.3.3 Fields of Current Applications 41.3.4 Nanotechnology in Environmental Engineering 51.4 Artificially Intelligent Materials 71.4.1 Artificial Intelligence (AI) and Nanotechnology 71.4.2 Examples of Artificially Intelligent Nanomaterials 71.4.2.1 Energy Nanogenerator/Nanosensor (Piezoelectric/Triboelectric Materials) 71.4.2.2 Shape-Memory Materials 81.4.2.3 Actuator 91.5 Intelligent Environmental Nanomaterials 111.5.1 Overview 111.5.2 Self-Propelled Nanomotors 121.5.3 Intelligent Gating Membrane 121.5.4 Switchable Oil/Water Separation 121.5.5 Self-Healing Environmental Materials 121.5.6 Molecular Imprinting 131.5.7 Nanofibrous Membrane Air Filters 131.6 Introduction to the Book Chapters 13References 142 Fundamental Mechanisms of Intelligent Responsiveness 272.1 Overview of Intelligent Responsiveness 272.2 Responsiveness in the Polymer System 282.3 Thermoresponsiveness 292.3.1 LCST Thermoresponsiveness 302.3.2 UCST Thermoresponsiveness 322.3.2.1 Coulomb Interaction-Induced UCST Polymers (Polyzwitterions) 322.3.2.2 Hydrogen Bonding-Induced UCST Polymers 332.4 pH Responsiveness 342.4.1 pH-Responsive Basic Polymers 362.4.2 pH-Responsive Acidic Polymers 382.5 Photo-responsiveness 392.5.1 Azobenzene and Its Derivatives 412.5.2 Spiropyran-Based Polymers 422.5.3 Inorganic Photo-Responsive Materials 422.6 Metalic Ion Responsiveness 432.6.1 Poly(NIPAM-co-AAB18C6) 442.6.2 Poly(NIPAM-co-AAB15C5) (15-Crown-5) 452.7 Ion Strength Responsiveness 452.8 Redox Responsiveness 472.9 Multi-responsiveness 492.9.1 Dual Stimuli-Responsive Polymers 492.9.1.1 Thermo- and Photo-Responsive Polymers 492.9.1.2 Thermo- and pH-Responsive Polymers 492.9.1.3 Thermo- and Redox-Responsive Polymers 502.9.2 Multi-Stimuli-Responsive Polymers 502.9.2.1 Thermo-, Photo-, and pH-Responsive Polymers 502.9.2.2 Thermo-, Photo-, and Redox-Responsive Polymers 512.10 Conclusion 52References 523 Filtration Membranes with Responsive Gates 693.1 Membrane Separation for Water Purification and Desalination 693.2 Emerging Design and Concept of Filtration Membranes with Responsive and Intelligent Gates 703.3 Fabrication Methods of Intelligent Gating Membranes 713.3.1 Post-Modification Method 723.3.2 One-Step Formation Method 733.4 Application of Intelligent Gating Membranes to Environmental Separation 733.5 Thermoresponsiveness 743.6 pH Responsiveness 803.6.1 Polybase Gating Membranes 803.6.2 Polyacid Gating Membrane 833.7 Photo-responsiveness 853.7.1 Azobenzene-Based Gating Membranes 853.7.2 Spiropyran-Based Gating Membranes 873.8 Metallic Ion Responsiveness 893.9 Redox Responsiveness 913.10 Ion Strength Responsiveness 923.11 Dual and Multi-Stimuli Responsiveness 953.11.1 pH and Temperature Dual Responsiveness 953.11.2 Temperature and Ion Strength Dual Responsiveness 973.11.3 pH and Ion Strength Dual Responsiveness 973.11.4 Temperature, pH, and Ion Strength Multi-responsiveness 993.12 Conclusions 99References 1004 Switchable Wettability Materials for Controllable Oil/Water Separation 1134.1 Oil Spill Treatment 1134.2 Fundamentals of Special Wettability 1144.2.1 Surface Wetting Properties 1144.2.2 Liquid Wettability in Air 1154.2.3 Oil Wettability Underwater 1174.3 Special Wettable Materials for Oil/Water Separation 1184.4 Switchable Oil/Water Separation 1204.5 Surface Chemistry Behind Stimuli-Responsive and Switchable Wettability 1214.6 Temperature Responsiveness 1214.7 pH Responsiveness 1264.7.1 Pyridine-Based System 1264.7.2 Carboxyl-Based System 1294.7.3 Tertiary Amine-Based pH-Responsive Systems 1314.8 Photo-responsiveness 1324.8.1 Inorganic Photo-responsive Materials 1334.8.2 Organic Photo-responsive Materials 1354.9 Gas, Solvent, Ion, and Electric Field Responsiveness 1364.9.1 Gas Responsiveness 1364.9.2 Solvent Responsiveness 1394.9.3 Ion Responsiveness 1404.9.4 Electric Field Responsiveness 1414.10 Dual/Multi-stimuli 1434.11 Conclusion 143References 1455 Self-Healing Materials for Environmental Applications 1575.1 Biomimetic Self-Healing Materials 1575.2 Overview of Self-Healing Materials 1585.3 Extrinsic and Intrinsic Self-Healing Materials 1595.3.1 Extrinsic Self-Healing Materials 1595.3.2 Intrinsic Self-Healing Materials 1605.4 Self-Healing Materials in Environmental Applications 1625.4.1 Self-Healing of Physical Cracks 1635.4.2 Self-Restoring of Surface Functional Components 1715.4.2.1 Chemical Mechanism 1715.4.2.2 Hydrophobic Self-Healing 1725.4.2.3 Hydrophilic Self-Healing 1745.4.3 Self-Cleaning of Contaminated Surfaces 1765.4.3.1 Superhydrophobicity-Induced Self-Cleaning 1765.4.3.2 Superhydrophilicity-Induced Self-Cleaning 1785.4.3.3 Photocatalytic Self-Cleaning 1815.5 Conclusion 183References 1856 Emerging Nanofibrous Air Filters for PM2.5 Removal 1976.1 Particulate Matter 1976.2 Traditional Technology 1986.3 Nanofibrous Membrane Air Filters 1996.3.1 Filtration Mechanism 1996.3.2 Fabrication Methods 2006.4 Applications 2016.4.1 Transparent Air Filter 2026.4.2 Air Filter for High Thermal Stability 2036.4.3 Air Filter for Thermal Management 2046.4.4 Air Filter for Mass Production 2066.4.5 Self-Powered Air Filter 2066.4.6 Nanofibrous Air Filter for the Simultaneous Removal of PM and Toxic Gases 2086.4.7 Nanofibrous Air Filter with Antibacterial Functions 2126.4.8 Air Filtration and Oil Removal 2146.5 Conclusion 216References 2167 IntelligentMicro/Nanomotors in Environmental Sensing and Remediation 2277.1 Self-Propelling Mechanism of Micro/Nanomotors 2287.1.1 Self-Electrophoretic Mechanism 2287.1.2 Microbubble Propulsion Mechanism 2297.1.3 Self-Diffusiophoresis Propulsion Mechanism 2307.1.4 External Field-Driven Micro/Nanomotors 2317.2 Self-Propelled Micro/Nanomotors as Environmental Sensors 2337.3 Self-Propelled Micro/Nanomotors for Enhanced Organic Contamination Degradation 2417.4 Self-Propelled Micro/Nanomotors as Efficient Antibacterial Agents 2457.5 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent 2487.5.1 Self-Propelled Micro/Nanomotors for the Removal of Oil Droplets 2487.5.2 Self-Propelled Micro/Nanomotors for the Removal of Molecules or Ions 2517.6 Conclusions 257References 2578 Molecular Imprinting Materials in Environmental Application 2658.1 Introduction 2658.2 Fundamental of MIT 2668.2.1 Covalent and Noncovalent Imprinting 2668.2.2 Essential Elements of Molecular Imprinting 2688.2.2.1 Target Templates 2688.2.2.2 Functional Monomers 2698.2.2.3 Cross-Linkers 2718.2.2.4 Porogenic Solvents 2718.2.2.5 Initiators 2718.2.3 Synthesis Methods of MIPs 2718.2.3.1 Free Radical Polymerization 2728.2.3.2 Sol-Gel Processes 2738.3 Molecular Imprinting in Environmental Applications 2738.3.1 Natural and Synthetic Dyes 2738.3.2 Endocrine-Disrupting Compounds 2748.3.3 Polycyclic Aromatic Hydrocarbons (PAHs) 2778.3.4 Pharmaceuticals and Pesticide 2778.3.5 Metal 2818.4 Conclusion 285References 2859 Emerging Synergistically Multifunctional and All-in-One Nanomaterials and Nanodevices in Advanced Environmental Applications 2999.1 Introduction 2999.2 An All-in-One, Point-of-Use Water Desalination Cell 2999.3 3D-Printed, All-in-One Evaporator for Solar Steam Generation 3009.4 All-in-One Photothermic Driven Catalysis and Desalination of Seawater Under Natural Sunlight 3019.5 All-in-One Design of Water Harvesting from Air Powered by Natural Sunlight 3029.6 All-in-One Textile for Personal Thermal Management 3049.7 Conclusion 305References 305Index 309

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Artificially Intelligent Nanomaterials for Environmental Engineering

Artificially Intelligent Nanomaterials for Environmental Engineering Peng Wang, Jian Chang, and Lianbin Zhang

Authors Prof. Peng Wang

KAUST Environmental Nanotechnology Lab 23955 Thuwal Saudi Arabia Dr. Jian Chang

KAUST Environmental Nanotechnology Lab 23955 Thuwal Saudi Arabia Prof. Lianbin Zhang

Huazhong University of Science and Technology School of Chemistry and Chemical Engineering 430074 Wuhan China

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.:

applied for British Library Cataloguing-in-Publication Data

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Germany Typesetting SPi Global, Chennai, India Printing and Binding

Printed on acid-free paper 10 9 8 7 6 5 4 3 2 1

v

Contents Preface xi 1 1.1 1.2

1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.4 1.4.1 1.4.2 1.4.2.1 1.4.2.2 1.4.2.3 1.5 1.5.1 1.5.2 1.5.3 1.5.4 1.5.5 1.5.6 1.5.7 1.6

Introduction 1

Global Challenges 1 Conventional Technologies in Environmental Science and Engineering 2 Nanotechnology 3 History of Nanotechnology Evolution 3 Concept and Definition 4 Fields of Current Applications 4 Nanotechnology in Environmental Engineering 5 Artificially Intelligent Materials 7 Artificial Intelligence (AI) and Nanotechnology 7 Examples of Artificially Intelligent Nanomaterials 7 Energy Nanogenerator/Nanosensor (Piezoelectric/Triboelectric Materials) 7 Shape-Memory Materials 8 Actuator 9 Intelligent Environmental Nanomaterials 11 Overview 11 Self-Propelled Nanomotors 12 Intelligent Gating Membrane 12 Switchable Oil/Water Separation 12 Self-Healing Environmental Materials 12 Molecular Imprinting 13 Nanofibrous Membrane Air Filters 13 Introduction to the Book Chapters 13 References 14

2

Fundamental Mechanisms of Intelligent Responsiveness 27

2.1 2.2 2.3 2.3.1 2.3.2

Overview of Intelligent Responsiveness 27 Responsiveness in the Polymer System 28 Thermoresponsiveness 29 LCST Thermoresponsiveness 30 UCST Thermoresponsiveness 32

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Contents

2.3.2.1 2.3.2.2 2.4 2.4.1 2.4.2 2.5 2.5.1 2.5.2 2.5.3 2.6 2.6.1 2.6.2 2.7 2.8 2.9 2.9.1 2.9.1.1 2.9.1.2 2.9.1.3 2.9.2 2.9.2.1 2.9.2.2 2.10

Coulomb Interaction-Induced UCST Polymers (Polyzwitterions) 32 Hydrogen Bonding-Induced UCST Polymers 33 pH Responsiveness 34 pH-Responsive Basic Polymers 36 pH-Responsive Acidic Polymers 38 Photo-responsiveness 39 Azobenzene and Its Derivatives 41 Spiropyran-Based Polymers 42 Inorganic Photo-Responsive Materials 42 Metalic Ion Responsiveness 43 Poly(NIPAM-co-AAB18 C6 ) 44 Poly(NIPAM-co-AAB15 C5 ) (15-Crown-5) 45 Ion Strength Responsiveness 45 Redox Responsiveness 47 Multi-responsiveness 49 Dual Stimuli-Responsive Polymers 49 Thermo- and Photo-Responsive Polymers 49 Thermo- and pH-Responsive Polymers 49 Thermo- and Redox-Responsive Polymers 50 Multi-Stimuli-Responsive Polymers 50 Thermo-, Photo-, and pH-Responsive Polymers 50 Thermo-, Photo-, and Redox-Responsive Polymers 51 Conclusion 52 References 52

3

Filtration Membranes with Responsive Gates 69

3.1 3.2

Membrane Separation for Water Purification and Desalination 69 Emerging Design and Concept of Filtration Membranes with Responsive and Intelligent Gates 70 Fabrication Methods of Intelligent Gating Membranes 71 Post-Modification Method 72 One-Step Formation Method 73 Application of Intelligent Gating Membranes to Environmental Separation 73 Thermoresponsiveness 74 pH Responsiveness 80 Polybase Gating Membranes 80 Polyacid Gating Membrane 83 Photo-responsiveness 85 Azobenzene-Based Gating Membranes 85 Spiropyran-Based Gating Membranes 87 Metallic Ion Responsiveness 89 Redox Responsiveness 91 Ion Strength Responsiveness 92 Dual and Multi-Stimuli Responsiveness 95 pH and Temperature Dual Responsiveness 95 Temperature and Ion Strength Dual Responsiveness 97

3.3 3.3.1 3.3.2 3.4 3.5 3.6 3.6.1 3.6.2 3.7 3.7.1 3.7.2 3.8 3.9 3.10 3.11 3.11.1 3.11.2

Contents

3.11.3 3.11.4 3.12

pH and Ion Strength Dual Responsiveness 97 Temperature, pH, and Ion Strength Multi-responsiveness 99 Conclusions 99 References 100

4

Switchable Wettability Materials for Controllable Oil/Water Separation 113

4.1 4.2 4.2.1 4.2.2 4.2.3 4.3 4.4 4.5

Oil Spill Treatment 113 Fundamentals of Special Wettability 114 Surface Wetting Properties 114 Liquid Wettability in Air 115 Oil Wettability Underwater 117 Special Wettable Materials for Oil/Water Separation 118 Switchable Oil/Water Separation 120 Surface Chemistry Behind Stimuli-Responsive and Switchable Wettability 121 Temperature Responsiveness 121 pH Responsiveness 126 Pyridine-Based System 126 Carboxyl-Based System 129 Tertiary Amine-Based pH-Responsive Systems 131 Photo-responsiveness 132 Inorganic Photo-responsive Materials 133 Organic Photo-responsive Materials 135 Gas, Solvent, Ion, and Electric Field Responsiveness 136 Gas Responsiveness 136 Solvent Responsiveness 139 Ion Responsiveness 140 Electric Field Responsiveness 141 Dual/Multi-stimuli 143 Conclusion 143 References 145

4.6 4.7 4.7.1 4.7.2 4.7.3 4.8 4.8.1 4.8.2 4.9 4.9.1 4.9.2 4.9.3 4.9.4 4.10 4.11

5

5.1 5.2 5.3 5.3.1 5.3.2 5.4 5.4.1 5.4.2 5.4.2.1 5.4.2.2 5.4.2.3 5.4.3 5.4.3.1

157 Biomimetic Self-Healing Materials 157 Overview of Self-Healing Materials 158 Extrinsic and Intrinsic Self-Healing Materials 159 Extrinsic Self-Healing Materials 159 Intrinsic Self-Healing Materials 160 Self-Healing Materials in Environmental Applications 162 Self-Healing of Physical Cracks 163 Self-Restoring of Surface Functional Components 171 Chemical Mechanism 171 Hydrophobic Self-Healing 172 Hydrophilic Self-Healing 174 Self-Cleaning of Contaminated Surfaces 176 Superhydrophobicity-Induced Self-Cleaning 176

Self-Healing Materials for Environmental Applications

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Contents

5.4.3.2 5.4.3.3 5.5

Superhydrophilicity-Induced Self-Cleaning 178 Photocatalytic Self-Cleaning 181 Conclusion 183 References 185

6

Emerging Nanofibrous Air Filters for PM2.5 Removal 197

6.1 6.2 6.3 6.3.1 6.3.2 6.4 6.4.1 6.4.2 6.4.3 6.4.4 6.4.5 6.4.6

Particulate Matter 197 Traditional Technology 198 Nanofibrous Membrane Air Filters 199 Filtration Mechanism 199 Fabrication Methods 200 Applications 201 Transparent Air Filter 202 Air Filter for High Thermal Stability 203 Air Filter for Thermal Management 204 Air Filter for Mass Production 206 Self-Powered Air Filter 206 Nanofibrous Air Filter for the Simultaneous Removal of PM and Toxic Gases 208 Nanofibrous Air Filter with Antibacterial Functions 212 Air Filtration and Oil Removal 214 Conclusion 216 References 216

6.4.7 6.4.8 6.5

7

Intelligent Micro/Nanomotors in Environmental Sensing and Remediation 227

7.1 7.1.1 7.1.2 7.1.3 7.1.4 7.2 7.3

Self-Propelling Mechanism of Micro/Nanomotors 228 Self-Electrophoretic Mechanism 228 Microbubble Propulsion Mechanism 229 Self-Diffusiophoresis Propulsion Mechanism 230 External Field-Driven Micro/Nanomotors 231 Self-Propelled Micro/Nanomotors as Environmental Sensors 233 Self-Propelled Micro/Nanomotors for Enhanced Organic Contamination Degradation 241 Self-Propelled Micro/Nanomotors as Efficient Antibacterial Agents 245 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent 248 Self-Propelled Micro/Nanomotors for the Removal of Oil Droplets 248 Self-Propelled Micro/Nanomotors for the Removal of Molecules or Ions 251 Conclusions 257 References 257

7.4 7.5 7.5.1 7.5.2 7.6

Contents

8

Molecular Imprinting Materials in Environmental Application 265

8.1 8.2 8.2.1 8.2.2 8.2.2.1 8.2.2.2 8.2.2.3 8.2.2.4 8.2.2.5 8.2.3 8.2.3.1 8.2.3.2 8.3 8.3.1 8.3.2 8.3.3 8.3.4 8.3.5 8.4

Introduction 265 Fundamental of MIT 266 Covalent and Noncovalent Imprinting 266 Essential Elements of Molecular Imprinting 268 Target Templates 268 Functional Monomers 269 Cross-Linkers 271 Porogenic Solvents 271 Initiators 271 Synthesis Methods of MIPs 271 Free Radical Polymerization 272 Sol–Gel Processes 273 Molecular Imprinting in Environmental Applications 273 Natural and Synthetic Dyes 273 Endocrine-Disrupting Compounds 274 Polycyclic Aromatic Hydrocarbons (PAHs) 277 Pharmaceuticals and Pesticide 277 Metal 281 Conclusion 285 References 285

9

Emerging Synergistically Multifunctional and All-in-One Nanomaterials and Nanodevices in Advanced Environmental Applications 299

9.1 9.2 9.3 9.4

Introduction 299 An All-in-One, Point-of-Use Water Desalination Cell 299 3D-Printed, All-in-One Evaporator for Solar Steam Generation 300 All-in-One Photothermic Driven Catalysis and Desalination of Seawater Under Natural Sunlight 301 All-in-One Design of Water Harvesting from Air Powered by Natural Sunlight 302 All-in-One Textile for Personal Thermal Management 304 Conclusion 305 References 305

9.5 9.6 9.7

Index 309

ix

xi

Preface Nowadays, artificial intelligence (AI) is gaining attention in general science and engineering. AI emphasizes the capability of a machine to imitate intelligent humanlike behavior to perform tasks normally requiring human intelligence. As a matter of fact, the concept of AI has been introduced into the field of environmental engineering in the past decade to design and fabricate intelligent environmental nanomaterials toward beneficial uses. Given the inherent complexity and stochastic nature of environmental problems, environmental nanomaterials can greatly benefit from an “intelligent” design, for instance, the ability to change their properties depending on the environmental conditions. The key to the design of an environmental intelligent nanomaterial is entrusting the nanomaterials with a proactive functionality instead of a reactive functionality, which endows them with anticipatory, change-oriented, and self-initiated behavior. As a consequence, these intelligent nanomaterials could perceive their surroundings and subsequently take automated actions or make self-adjustments for the purpose of maximizing their possibility to achieve their desired goal. Nevertheless, the artificially intelligent environmental nanomaterials still remain at their early infant stage, but undoubtedly, they are gaining fast popularity and expected to offer disruptive technologies for next-generation environmental treatment processes. Despite the fact that the concept of intelligent materials has been existing for a while, there is still a big gap between what artificially intelligent materials are perceived and what they can truly offer in practical applications, especially in environmental problem solving. There is thus an urgent need of a book to bridge the gap. This book focuses on the design and application of various artificially intelligent nanomaterials to solving environmental problems, especially water and air pollution. It aims to help the readers who are passionate at the environmental quality and the futuristic ways of improving it. It would certainly help to illustrate to the readers the convergence between AI and nanotechnology that can shape the path for many technological developments in the field of environmental engineering. This book demonstrates the design concepts, majorly chemical principles, of intelligent environmental nanomaterials and provides eye-opening proof-of-concept examples in relevant and significant applications. The book includes the following chapters: introduction, describing the background of environmental nanotechnology, the rise of AI, and the current status

xii

Preface

quo of AI in environmental engineering (Chapter 1), intelligently functional materials and responsive mechanisms (Chapter 2), designing filtration membranes with responsive gates (Chapter 3) , switchable wettability materials for controllable oil/water separation (Chapter 4), self-healing materials for environmental applications (Chapter 5), emerging nanofibrous air filters for PM2.5 removal (Chapter 6), self-propelled nanomotors for environmental applications (Chapter 7), molecular imprinting in wastewater treatment (Chapter 8), and emerging synergistically multifunctional and all-in-one nanomaterials and nanodevices in advanced environmental applications (Chapter 9). This book is intended for undergraduates, graduates, scientists, and professionals in the fields of environmental science, materials science, chemistry, chemistry engineering, etc. It provides coherent and good materials for teaching, research, and professional reference. It is our sincere hope that this book would inspire further research efforts especially from younger generation to develop advanced artificially intelligent nanomaterials for the enhancement of the overall quality of the environment and human health. Peng Wang Thuwal, Saudi Arabia

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1 Introduction 1.1 Global Challenges Water, food, and energy security represent major challenges to the stability and continuity of human populations. However, rapid population growth and steadily improving living standards place enormous pressures on already stressed water resource and agricultural systems. Large amounts of energy are consumed to produce clean water and to treat wastewaters prior to their return to the environment, which inevitably leads to a considerable amount of carbon dioxide (CO2 ) emissions as well as releasing other environmental pollutants. At the global scale, about 2600 km3 of water are withdrawn to supply fooddriven irrigation needs every year. Viewed another way, agriculture consumes nearly 70% of total human freshwater withdrawals. This number is to increase to more than 83% by 2050 to meet the growing food demand by the rapidly growing population. In the last 25 years, access to water with potable quality has gone up from 75% to 90% of the world population, and, nevertheless, 884 million people nowadays still lack access to adequate drinking water in many geographical regions [1]. Thus, ensuring a stable and sustainable water, food, and energy supply into the future is a priority for all nations. Adding to an already dreadful situation, water pollution is becoming a major global challenge [2, 3]. From the United Nations World Water Development Report in 2018, it is said that more than 2 billion people lack access to safe drinking water and more than double that number lack access to safe sanitation. With a rapidly growing global population, demand for water is expected to increase by nearly one-third by 2050 [4]. In addition, WHO estimated that 361 000 deaths in children under five years due to diarrhea, representing more than 5% of all deaths in this age group in low- and middle-income countries, could have been prevented through reduction of exposure to inadequate drinking water [5]. Thus, the ability to remove contaminants from these environments to a safe level and do it rapidly, efficiently, and with reasonable costs is important. With the nonrenewable and pollutant-laden fossil fuels dominating the global energy supply, representing 78% of the world’s primary energy, air pollution is worsening in many parts of the world especially where the economy is heavily dominated by low-tech manufacturing. Millions of people die every year from Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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1 Introduction

diseases caused by exposure to outdoor air pollution [6]. Ninety two percent of the global population, including billions of children, is exposed to hazardous effects of air pollution at which levels exceed WHO limits. Air pollution causes approximately 600 000 deaths in children under five years annually and increases the risk of respiratory infections, asthma, adverse neonatal conditions, and congenital anomalies.

1.2 Conventional Technologies in Environmental Science and Engineering In the past century, the development in water and air treatment technologies by environmental engineers has significantly improved the quality of water and air. The research and relevant design of conventional water and air protection systems experienced its golden age in the first half of twentieth century and has gradually reached their steady states. At the same time, with the ever-growing population and ever-increasing life quality expectation, the demand for safe and clean water and air has never dwindled in the course of human existence and is gradually pushing existing technologies to their limits. Conventional technologies, such as chemical coagulation [7], adsorptions [8, 9], chemical treatment (e.g. advanced oxidation process [AOP]) [10–12], membrane-based separation [13–15], and biological treatment [16, 17], are based on bulk water chemistry. Coagulation, which involves adding chemical coagulants into bulk source water, is commonly used in drinking water plants. The particles in source water that cause turbidity (e.g. silt, clay) are generally negatively charged, while coagulant particles are positively charged. In coagulation and subsequent flocculation, the formed particles in the form of flocs are settled out or later removed by filtration. The effectiveness of the coagulation is controlled by bulk water chemistry, such as dose of the coagulants added and pH, among others. AOP, as a type of chemical treatment, involves accelerated production of highly reactive hydroxyl free radical to degrade the organic pollutants [18–20]. The degradation rates can be affected by several factors from the bulk water chemistry [21]. Adsorption is a process in which pollutants are adsorbed on solid surfaces [9, 22]. Adsorption is a proven and much used water purification technique due to its low energy consumption and maintenance cost, as well as its simplicity and reliability. However, its performance relies on the concentration of the to-be-removed substances, the presence of other competing species, temperature, and pH of the bulk water. Biological treatments rely on bacteria, nematodes, or other small organisms to break down organic wastes using normal cellular processes [23]. Biological wastewater treatment is often a secondary treatment process, used to remove remaining biodegradable organics after primary treatment. These processes can be either anaerobic or aerobic. “Aerobic” refers to the condition where oxygen is present, while “anaerobic” describes a biological process in which oxygen is absent. To obtain an aerobic condition, huge amount of electricity is

1.3 Nanotechnology

typically consumed to re-aerate the bulk wastewater, which can be completely oxygen-depleted. Membrane separation is a technology in which membrane acts as a selective barrier allowing water flowing through while it catches suspended solids and other substances. Membrane separation technology is commonly used for the creation of process water from groundwater, surface water, or wastewater, and it works without the addition of chemicals, with a relatively low energy use and experiencing simple bulk water separation process [24]. Although these conventional technologies are crucial at providing quality water especially at heavily populated areas, conventional water treatment and its infrastructure systems allow little flexibility in response to the changing demand for water quality or quantity, leading to significant energy consumption, water loss, and secondary contamination. For instance, coagulation itself results in the formation of flocs, and thus additional treatment process is required to help the floc to further aggregate and settle. Biological treatment method is at the cost of a long time due to the slow biodegradation process [10]. On the other hand, impurities and pollutants build up on the surface and clog the filtration membranes over extended periods of use, and thus the flux of the wastewater across the filters decreases, leading to higher energy requirements. From air quality point of view, many prevention measures have been taken in addressing air pollution problems: source control, development of clean energy, filtration technologies, etc. [25] Among them, air filtration technology is of great interest due to low equipment cost and low energy consumption. The conventional fibrous membrane (e.g. glass, polyethylene [PE], polypropylene [PP], polyester, and aramid fibers), as a kind of porous media, has been widely applied in different filtration scenes, including disposable respirators, industrial gas cleaning equipment, cleanroom air purification systems, automotive cabin air filters, and indoor air purifiers [26]. Such fibrous media still suffer from some structural and performance disadvantages, such as large fiber diameter, nonuniform fiber diameter and pore size, relatively low filtration efficiency, high basis weight, and poor high-temperature resistance [27]. While the conventional technologies are being pushed toward their capacity limits, innovations in nanomaterials and more broadly nanotechnology have been fueling advances in environmental science and engineering [28].

1.3 Nanotechnology 1.3.1

History of Nanotechnology Evolution

The term “nanotechnology” can be traced back in 1959 when it was first used by Richard Feynman in his famous lecture entitled “There’s Plenty of Room at the Bottom,” which is hailed by many as the herald of the era of nano [29]. Starting 1980s, two major breakthroughs sparked the growth of nanotechnology in the modern era. First, in 1981, the invention of the scanning tunneling microscope provided unprecedented visualization of individual atoms and bonds. Second, fullerenes were discovered in 1985 by Harry Kroto, Richard Smalley, and Robert

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1 Introduction

Curl, who together won the 1996 Nobel Prize in Chemistry [30, 31]. Initially, C60 was not described as nanotechnology while the term was used regarding subsequent work with related graphene tubes (called carbon nanotubes), which suggested potential applications for nanoscale electronics and devices. In the beginning of 2000s, there were commercial applications of nanotechnology, although these were limited to the bulk application of nanomaterials and do not involve atomic control of matter, such as using silver nanoparticles as antibacterial agent, nanoparticle-based transparent sunscreens, and carbon nanotubes for stain-resistant textiles [32–34]. Nanotechnology is developing at a very fast rate, and its development is regarded as another industrial revolution. It is anticipated that increasing integration of nanoscale science and engineering knowledge promises mass applications of nanotechnology in all fields of the industry [35]. 1.3.2

Concept and Definition

Overall, nanotechnology is the manipulation of matter at an atomic, molecular, and supramolecular scale. It is naturally very broad, including fields of science as diverse as surface science, organic chemistry, molecular biology, semiconductor physics, energy storage, microfabrication, molecular engineering, etc. [36–39] The associated research and applications are equally diverse, ranging from extensions of conventional device physics to completely new approaches based upon molecular self-assembly [40], from developing new materials with dimensions on the nanoscale to direct control of matter on the atomic scale. Nanomaterials are defined as the structures that can be produced in a controlled manner in a size ranging from 1 to 100 nm in one, two, or three dimensions [41]. Materials reduced to the nanoscale can show different properties compared with what they exhibit on a macroscale, enabling unique applications. For instance, opaque substances can become transparent (copper); stable materials, combustible (aluminum); and insoluble materials, soluble (gold). A material such as gold, which is chemically inert at normal scales, can serve as a potent chemical catalyst at nanoscales. 1.3.3

Fields of Current Applications

Nanotechnology is widely regarded as a powerful enabling platform, and it has created many new materials and devices with a vast range of applications [42, 43]. The nanotechnology research has produced many scientific breakthroughs and is fostering potentially endless possibilities. Some applications of nanotechnology in the fields of nanomedicine, energy, and environment are briefed as follows. Nanotechnology provides new options for drug delivery and disease therapies. Nanosized drug carrier enables drugs to be precisely delivered to the right location in the body and release drug doses on a predetermined schedule for optimal treatment. The surgical nanorobot, programmed or guided by a human surgeon, can act as a semiautonomous on-site surgeon inside the human body

1.3 Nanotechnology

when introduced into the body through vascular system or cavities [44, 45]. Moreover, the integration of nanotechnology with molecular imaging provides a versatile platform for novel design of nano-probes that have tremendous potential to enhance the sensitivity, specificity, and signaling capabilities of various biomarkers in human diseases. Nanotechnology has potential in securing new sustainable energy sources and in effective use of existing energy resources. It has reduced cost both of solar cells and the equipment needed to produce and deploy them, making solar power economical and hence a more useable alternative to fossil fuels. There is a potential for nanotechnology to cut down on energy consumption through lighter materials for vehicles, smart materials that lead to more effective temperature control, advanced materials that increase the efficiency of electrical components and transmission lines, and materials that could contribute to a new generation of fuel cells and a step closer toward a hydrogen economy, among numerous others [46, 47]. From an environmental engineering point of view, nanotechnology presents new opportunities to improve how contaminants in the environment are measured, monitored, managed, and minimized, which will be discussed heavily in the rest of the chapters. Overall, nanomaterials have two primary advantages over conventional bulk materials: (i) they have small size and thus big specific surface area, which are beneficial to many interface-related applications, and (ii) their properties, including chemical, physical, optical, electronic, mechanical, and magnetic properties, can be judiciously tuned by controlling their size, surface morphology, shape and crystal orientation, etc. As a result, going to nanoscale has opened up numerous new avenues that would otherwise be impossible with conventional bulk materials. 1.3.4

Nanotechnology in Environmental Engineering

Applications of nanotechnology in environmental science and engineering mainly include a high surface area for adsorption (nanoadsorbents), unique surface functionalization properties, high activity for (photo)catalysis (environmental catalytic materials), nanofiltration for wastewater treatment, nanofibrous air filter, water purification and desalination membranes, and sensors for water quality monitoring (Figure 1.1) [15, 48–52]. For example, nanoadsorbents offer significant improvements over conventional adsorbents with their extremely high specific surface area and tunable pore size and surface chemistry. The high surface area and size-dependent surface structure at the nanoscale could create highly active adsorption sites [53], resulting in higher adsorption capacity. Meanwhile, the surface of many nanomaterials can be functionalized to target specific contaminants, achieving high selectivity. As for environmental sensors, the integration of nanomaterials and recognition agents could yield fast, sensitive, and selective sensors for water quality

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Figure 1.1 Nanotechnology applications involving nanophotocatalysts, membrane nanotechnology, and nanoadsorbents for a safe and sustainable water supply. Source: Qu et al. 2012 [48]. Reprinted with permission of American Chemical Society.

monitoring [54]. Effective nanomaterials can improve sensor sensitivity and speed and achieve multiplex target detection by utilizing their unique electrochemical, optical, or magnetic properties. Membrane technology is a key component of an integrated water treatment and reuse paradigm. However, current materials and fabrication methods for membranes are largely based on empirical approaches and lack molecular-level design, thus hampering membrane performance and increasing the cost of water treatment [55]. A nanoscale membrane with molecular-level design can potentially overcome some limitations in conventional membranes especially permeability–selectivity trade-off and high fouling propensity [56]. Photocatalysis is the phenomenon of overcoming the activation energy or temperature of a chemical reaction by light. AOPs paired with sunlight present an attractive option for water treatment by the generation of OH radical [57]. Nanophotocatalysts can be used to break down a wide variety of organic materials, organic acids, estrogens, pesticides, dyes, crude oil, microbes, and inorganic molecules such as nitrous oxides. In combination with precipitation or filtration, photocatalysis can also remove metals like mercury [58]. Photocatalytic oxidation used in water treatment has an obvious advantage of high reaction rates due to high specific surface areas and low mass-transfer restrictions unmatched by other conventional methods, especially when there are high concentrations of organic pollutants in water [59]. Overall, nanotechnology is actively pursued to both enhance the performance of existing treatment processes and develop new processes. It is now a popular belief that many of the solutions to the existing and even future environmental challenges are most likely to come from nanotechnology and especially novel nanomaterials with increased affinity, capacity, and selectivity for environmental contaminants. The field of rational design of nanomaterials for environmental engineering has grown significantly in the past two decades and is poised to make its contribution to creating next-generation environmental technologies in the years to come [7, 50, 60–64].

1.4 Artificially Intelligent Materials

1.4 Artificially Intelligent Materials 1.4.1

Artificial Intelligence (AI) and Nanotechnology

The first work that is now generally recognized as artificial intelligence (AI) was McCullouch and Pitts’ 1943 formal design for Turing-complete “artificial neurons” [65] The AI concept emphasizes the capability of manmade machines to imitate intelligent human behavior to perform tasks normally requiring human intelligence but without humanlike intervention [66]. Thus, the design of AI machine necessitates proactive, instead of reactive, functionality, which endows the machine with anticipatory, change-oriented, and self-initiated behavior. As a matter of fact, AI entered the general field of nanotechnology in the 1990s. Nanomaterials with certain level of AI are entrusted with multiple, synergistic, and proactive functionalities so that these “nanomachines” perceive their environment and subsequently take automated actions or make self-adjustments for the purpose of maximizing their possibility to achieve their desired goal [67]. For practical applications, it is desired to rationally integrate multiple synergistic and advanced functions into one single material and to design the responsive functions that can switch to a desirable function in a controlled fashion in response to the external environmental stimuli. Following this line of thought, the AI materials could provide unprecedented advantages over traditional materials. Recently, there have been significant developments in the materials that are integrated with “artificial intelligence.” These intelligent nanomaterials typically have one or more of their properties (e.g. mechanical, thermal, optical, or electromagnetic properties) able to vary in a predictable or controllable way in response to external stimuli, such as stress, light, temperature, moisture, pH, electric or magnetic fields, etc. 1.4.2

Examples of Artificially Intelligent Nanomaterials

Generally, the response mechanism of intelligent nanomaterials lies in the change in molecular movement in response to external stimuli, which brings about the macroscopic property change of the materials. Some artificially intelligent nanomaterials in engineering fields including energy nanogenerator/nanosensor, shape-memory materials, and artificial muscles are presented as follows. 1.4.2.1 Energy Nanogenerator/Nanosensor (Piezoelectric/Triboelectric Materials)

Piezoelectric materials are crystalline materials exhibiting piezoelectric effect [68–70] and mainly include inorganic semiconducting piezoelectric ZnO nanowires, GaN nanowires, and lead-based and lead-free perovskite materials (e.g. Pb(Zr,Ti)O3 , NaNbO3 , KNbO3 , BaTiO3 , and ZnSnO3 [71–77]) and piezoelectric polymer (e.g. polyvinylidene difluoride [PVDF] and poly(vinylidenefluoride-co-trifluoroethylene) [P(VDF-TrFE)]) [78–80]. Piezoelectric materials have been integrated along with sensors and actuators to

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Figure 1.2 (a) Climate sensor is situated onto the window outward. Inset: Enlarged illustration of the rain sensor. (b) The output voltage generated from different types of sensors with respect to patterns on the surface as dropping water droplet. Inset: Water droplets staying at the sensor surface (scale bar: 10 mm). (c) Output voltage generated from sensors with respect to patterns on the surface as stirring up the wind using an air gun. Inset: Wind speed was recorded by an anemometer. Source: Lee et al. 2015 [85]. Reprinted with permission of John Wiley and Sons.

make intelligent materials. For example, piezoelectric materials have been used to capture and harvest mechanical energy wasted in nature (e.g. airflow, raindrop, sound, human motion, and ocean waves) that can later be used as portable, lightweight, and sustainable power sources [81–84]. Figure 1.2 presents a self-powered pressure sensor based on piezoelectric effect to detect water droplet and wind flow [85]. Triboelectric nanogenerator (TENG) has been produced to collect energy from common environmental sources [86–88]. When contacting and separating two different materials with oppositely charged surfaces, there is surface electron transfer, which in turn creates an electric potential difference. By repeating the contact and separation in a cyclic manner, electrons can be driven to flow through external load, generating a continuous output. Wang’s group pioneered and demonstrated many TENG designs that harvested multiple types of environmental energy [89]. For example, a superhydrophobic and self-cleaning PTFE-based TENG could harvest the water-related energy in the environment [90]. The power generated from water drop could power 20 light-emitting diodes (LEDs) (Figure 1.3). Such water-TENG can also serve as a sensor to detect water/liquid leakage from a container/pipe [91, 92]. 1.4.2.2

Shape-Memory Materials

Shape-memory materials are featured by their ability to recover their original shape from a significant and seemingly plastic deformation when a particular stimulus is applied [93–95]. Shape-memory materials can be inorganic or organic materials [96]. Shape-memory metal alloys can change their shape through microstructural transformation induced by temperature or magnetic fields. On the other hand, shape-memory polymers are intelligent as they have the ability to return from a temporary deformed shape to a memorized permanent shape upon external stimuli, including heat [97–100], light irradiation [101, 102], solvent [103, 104], electrical current [105], and magnetic fields [106]. Representative shape-memory polymers contain polyurethanes [107], cellulose [103, 104], block copolymer of polyethylene terephthalate (PET) and

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1.4 Artificially Intelligent Materials

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Figure 1.3 (a) Output current density of the water-TENG generated from flowing tap water. (b) The alternating current (ac) output transformed to unidirectional pulse output by a full-wave rectifying bridge. (c) The image of the water-TENG used as a power source to light up 20 LEDs. (d) The rectified output used to charge a commercial capacitor of 33 μF. Insets of (b) and (d) are the sketches of the corresponding circuit connection polarities. Source: Lin et al. 2014 [90]. Reprinted with permission of John Wiley and Sons.

polyethylene oxide (PEO) [108], block copolymers containing polystyrene and poly(butadiene) [101, 109], polynorbornene or hybrid polymers consisting of polynorbornene units substituted by polyhedral oligosilsesquioxane (POSS) [110], etc. Shape-memory polymers have several advantages over inorganic materials. They have higher deformation strain, lower stiffness, density and manufacturing cost, potential biodegradability and healability, and the capability to be activated by various stimuli [111, 112]. Therefore, they have diverse promising applications in areas of biomedical devices, the aerospace industry, textiles, flexible electronics, and so forth [113–117]. Figure 1.4 presents a healable shape-memory polymeric films that can heal the mechanical damage and the fatigued shape-memory function [118]. 1.4.2.3

Actuator

Intelligent actuators is a generic term for a class of materials and devices that can offer controllable mechanical responses (contract, expand, or rotate) toward external stimuli, such as electric fields [119, 120], temperature [121, 122], solvent [123], humidity [124–126], and light [127, 128],, and convert those input energies into 2D or 3D movements. As energy transducers, actuators have numerous promising applications, involving switches [129], microrobotics [126], artificial muscles [130, 131], etc. Therefore, the fabrication of various actuators with

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Figure 1.4 (a) Polymeric films are fabricated by layer-by-layer assembly of branched poly(ethylenimine) (bPEI)-graphene oxide (GO) complexes with poly(acrylic acid) (PAA). As-prepared films exhibited the shape-memory function. The small piece of paper stuck on the right side of the film was used to stretch the film during the shape-memory process. (b) Schematic illustration of the shape-memory mechanism of the PAA/bPEI-GO film. The humidity-induced healing and shape-memory behavior are due to the electrostatic and hydrogen-bonding interactions induced by water between PAA and bPEI-GO complexes. Source: Xiang et al. 2017 [118]. Reprinted with permission of American Chemical Society.

intelligent response has become a heated topic in scientific and engineering fields. In 2013, Ma et al. prepared a water-responsive artificial actuator that combined both a rigid matrix (polypyrrole) and a dynamic network (polyol–borate). The actuator could exchange water with the environment to induce its structural expansion and contraction, resulting in rapid and continuous locomotion [125]. The film actuator of this type as an artificial muscle could generate contractile stress up to 27 MPa, lift objects 380 times heavier than itself, and transport cargo 10 times heavier than itself. Meanwhile, by associating with a piezoelectric element, this film can be used as an energy generator driven by water gradients, capable of outputting alternating electricity with a peak voltage of ∼1.0 V. Actuator driven by light possesses distinctive advantages, involving remote control, non-contact actuation, and high-level integration with other components as no

1.5 Intelligent Environmental Nanomaterials

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Figure 1.5 The bilayer actuator was fabricated by exploiting the photothermal conversion and humidity-sensitive properties of polydopamine-modified reduced graphene oxide (PDA-RGO). Therefore, an NIR light-driven walking device capable of performing quick wormlike motion on a ratchet substrate was built by connecting two polyethylene terephthalate plates as claws on opposite ends of the bilayer actuator. Locomotion of a walking device on a ratchet substrate when NIR light is periodically turned on (1–4) and off (5–8). The walking device moves from right to left. Source: Ji et al. 2014 [134]. Reprinted with permission of John Wiley and Sons.

wires or connections are required [132, 133]. Figure 1.5 presents a near-infrared (NIR) light-driven bilayer actuator capable of reversible bending/unbending motions [134].

1.5 Intelligent Environmental Nanomaterials 1.5.1

Overview

Given that there are inherent complexity and unpredictability and more particularly varying and even quite contrasting application scenarios in environmental problems, an ideal design of environmental nanomaterials should be proactive with AI. These nanomaterials work as “nanomachines” that, based on their environmental conditions, make self-adjustments to maximize their possibility to achieve their desired goals [67]. Thus, these nanomaterials are “intelligent” based on the previous definition. The key to a successful design of intelligent nanomaterials is endowing the nanomaterials with proactive functionality that would lead to their change-oriented and self-initiated behaviors during their applications. Given the inherent complexity and stochastic nature of environmental problems, environmental nanomaterials can greatly benefit from an intelligent design, i.e. the ability to change its properties depending on the environmental conditions. However, the development and application of intelligent nanomaterials in the environmental field is comparatively sluggish and still at a very nascent stage,

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although its popularity is growing. However, over the years, there are indeed some exciting exploratory works done in the intelligent environmental nanomaterials, many of which seemingly offer innovative and disruptive technologies. 1.5.2

Self-Propelled Nanomotors

Self-propulsion at nanoscale always represents a challenge. The latest selfpropelled nanomotors can draw in fuel from surrounding medium and generate remarkable thrust and force through the ejection of gas bubbles from chemical reactions or fuel-free stimuli response like light [135–137], magnetic fields [138–140], electric fields [141–143], ultrasound [144, 145], etc. The self-propelled nanomotors that are able to autonomously transport remediation agent throughout polluted samples/media with ultrahigh speed and to penetrate inaccessible locations [146–148] have potential such environmental applications as water quality screening [149–154], removal and degradation of pollutants [155–158], removal of spilled oil [159–161], CO2 scrubbing [162], etc. 1.5.3

Intelligent Gating Membrane

Conventional filtration membranes can be imparted with responsive gates that could self-regulate their permeation and species selectivity by intelligently switching their on/off states, which offer certain hope toward differential water quality or fit-for-purpose separation using the same separation membranes [163]. A number of photothermal materials, when combined with membrane distillation (MD), can harvest solar energy, generate heat locally only at the membrane and bulk water interface, and thus lead to considerably improved energy efficiency when compared with the conventional bulk water heating scheme of the conventional MD processes [164–168]. 1.5.4

Switchable Oil/Water Separation

In the field of oil spill cleanup, intelligent materials show tremendous advantages over conventional methods. A bio-inspired intelligent membrane with superwetting behavior can easily realize gravity-driven oil/water separation, which is of great importance to facilitate the oil spill cleanup, contributing to reduced response time and operation cost [169–171]. Moreover, the intelligent materials could be made to switch their oil and water wettability between two opposite sides in response to external stimuli and offer self-controlled, on-demand, and selective oil/water separation. The intelligent materials would allow for the recovery of the collected oils as well as the reuse of the separating materials, which the conventional materials largely fail to [170, 171]. 1.5.5

Self-Healing Environmental Materials

Self-healing materials can self-recover their physical damages, self-restore their lost functions, and self-clean their contaminated surfaces. The healing property effectively expands the lifetime of the materials and reduces the overall

1.6 Introduction to the Book Chapters

operational cost. Recently, the self-healing materials have been preliminarily extended into environmental areas of water filtration membranes and to fouling resistance of oil/water separation materials with confirmed results at lab scales [172–175]. 1.5.6

Molecular Imprinting

Imprinting has always been seen as the nature of some intelligent animals, which are capable to learn or “imprint” the characteristics of some external stimulus. However, the artificial materials can be also imparted into such an exciting gift to selectively recognize specific molecule, which is called “molecular imprinting.” Molecular imprinting technique is to create the tailor-made and template-shaped binding sites with the memory of the shape, size, and functional groups of the specific template molecules. Molecularly imprinted materials can be prepared by self-assembly of the functional monomers around the template, followed by cross-linking them in the presence of template molecules. After removing the template molecules, the formed cavities complementary in size, shape, and chemical functionality to the template can selectively rebind the template molecules, just like the model of key and lock. In the environmental area, molecularly imprinted materials can selectively recognize and remove specific pollutants from contaminated water [176–184]. 1.5.7

Nanofibrous Membrane Air Filters

The nanofibrous membranes offer a multitude of attractive features such as high specific surface area, high porosity, interconnected porous structure, more active sites, easy functionalization ability, and good mechanical behavior [26, 185–189]. Therefore, the nanofibrous membranes have great potential in air filters that are capable of PM2.5 removal. The intelligently designed nanofibrous membrane air filters can have multifunctions, such as high filtration capacity, high transparency, large-scale production, high thermal stability, toxic gases removal, and even self-powering capability, which have an eminent application as personal protective equipment. From these examples, it is clear that the design of intelligent environmental nanomaterials is meant to create things. Therefore, it is expected that new designs of intelligent environmental nanomaterials will continue to be produced.

1.6 Introduction to the Book Chapters The purpose of this book is to provide a comprehensive review of the stateof-the-art intelligent environmental nanomaterials, with a particular focus on the design concepts and responsiveness of the materials. We will present a broad collection of artificially intelligent materials and systems that are used in environmental problem solving. The book covers the following topics: (i) intelligent functional materials and responsive mechanisms (Chapter 2), (ii) designing filtration membranes with responsive gates

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(Chapter 3), (iii) switchable wettability materials for controllable oil/water separation (Chapter 4), (iv) self-healing materials for environmental applications (Chapter 5), (v) emerging nanofibrous air filters for PM2.5 removal (Chapter 6), (vi) self-propelled nanomotor for environmental applications (Chapter 7), (vii) molecular imprinting in wastewater treatment (Chapter 8), and (viii) emerging synergistically multifunctional and all-in-one nanomaterials and nanodevices in advanced environmental applications (Chapter 9). We hope this book would provide an inspiration for readers to further explore intelligent materials to solve environmental problems.

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2 Fundamental Mechanisms of Intelligent Responsiveness 2.1 Overview of Intelligent Responsiveness Many living creatures in nature have perfected the art of responding to various adverse external stimuli. To name a few examples of naturally existing stimuliresponsive creatures (Figure 2.1), sea cucumbers can rapidly change their stiffness by several orders of magnitude in the face of danger; a Venus flytrap can close its leaf fast enough to catch the insect that touches its trigger hairs; chameleons, octopuses, squids, and cuttlefish change color for camouflage or warning potential predators; Mimosa pudica collapses immediately when its leaves are being touched; and sunflowers follow the movement of the sun for great absorption of solar energy [1, 2]. Nature is a source of inspiration for the design and development of new materials that are capable of responding to stimuli in a controllable and predictable fashion. Many fascinating biological systems exhibit motor functions that perform mechanical transformations triggered by a fuel input. Diverse environmental triggers, involving adenosine triphosphate (ATP) fuel, a pH gradient, and light signals, are common activators of biological motors [3]. For example, the myosin–actin couple represents an ATP-driven motor that is translated to macroscopic mechanical functions of muscles [4, 5]. Following this line of thought, the origin of all intelligent materials is from bio-inspiration, and they operate by responding to specific stimuli [6]. The stimuli responsiveness is often manifested by nature’s ability to reverse and to regenerate in response to the external environment. The most important substances with intelligence in living systems are macromolecules with structures and behaviors that vary according to the conditions in their surrounding environment. This is because numerous biological processes mainly rely on feedback-controlled communication; for example, nucleic acids, proteins, and polypeptides are able to adopt conformations being specific to their surroundings [7]. Likewise, similar adaptive behavior can be imparted to synthetic polymers such that their utility goes beyond providing structural support to instead allow active participation in a dynamic sense. By incorporating multiple copies of functional groups, these polymers are readily amenable to a change in character (e.g. charge, polarity, and solvency) along with a polymer backbone, thus causing molecular changes in chemical structure Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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(c)

Figure 2.1 Pictures of (a) a sea cucumber (yellow) nestling between coral, (b) a Venus flytrap, and (c) a cuttlefish. Source: Zhai 2013 [1]. Reprinted with permission of Royal Society of Chemistry.

to be synergistically amplified to bring about dramatic transformations in macroscopic material properties. There are some special inorganic metal oxides (e.g. TiO2 and ZnO) that exhibit the transition of surface wettability under irradiation of light with suitable wavelength due to their photocatalytic property. However, these hardly realize the structure conformation transitions as similar as stimuli-responsive polymers, most likely due to their rigidity and size inflexibility in response to environmental stimuli.

2.2 Responsiveness in the Polymer System In essence, the response mechanism of stimuli-responsive materials lies on the change in molecular movement in response to external stimuli, correspondingly bringing about the macroscopic property change of the materials. Building responsiveness into intelligent materials is to exploit the changes in the conformation/polarity/reactivity of responsive polymers or functional groups in the material interior or on its surfaces in response to changes in the local environment. Such responsiveness in specially designed polymer or organic molecular system occurs by triggering specific conformational transitions on a microscopic level and amplifying these conformational transitions into macroscopically measurable changes [8]. In inducing the conformation change of the polymers, a suitable environmental stimulus, for instance, heat, light, and pH change, is applied to upset an ongoing equilibrium among polymer–solvent and polymer–polymer interactions and pushes it to a new equilibrium where polymer conformation differs. To better examine how polymer chains behave in solution, the root-meansquare end-to-end distance of a polymer chain is normally expressed as [9] ⟨r2 ⟩1∕2 = 𝛼(nCN )1∕2 l

(2.1)

where 𝛼 is the chain expansion factor, which is a measure of the effect of excluded volume, n is the number of freely jointed links in a hypothetical polymer chain of

2.3 Thermoresponsiveness

equal length (l), and C N is the characteristic ratio, which contains contributions from fixed valence angles and restricted chain rotation [9]. In a poor solvent (𝛼 < 1), the dimensions of the polymer chain are smaller than those in the unperturbed state (𝛼 = 1), and thus polymer chains collapse. While in a good solvent (𝛼 > 1), where polymer–solvent interactions are stronger than polymer–polymer or solvent–solvent interactions, the dimensions of the polymer chain are larger than those in the unperturbed state (𝛼 = 1), leading to an expansion conformation of polymer chains [8]. The responsiveness can be also described by Gibbs–Helmholtz equation, which is the underpinning principle governing the polymer conformation transitions: ΔGmix = ΔH − T ⋅ ΔS

(2.2) −1

where ΔG is Gibbs free energy, ΔH is the enthalpy of mixing (J mol ), ΔS is the entropy of mixing (J mol−1 l−1 ), and T is the temperature of the system (K). The breakage and formation of polymer–polymer and polymer–solvent interactions that changes ΔH, and/or tuning temperature of the system corresponded to the influence of TΔS, can induce the change of ΔGmix , as a consequence of a conformation change of a polymer in response to an environmental stimulus. For example, an extended polymer conformation occurs when polymer–solvent intermolecular interactions are thermodynamically favored, while a contracted polymer conformation is a result of favored polymer–polymer inter- and/or intramolecular interactions and at the same time depressed polymer–solvent interactions. In both cases, ΔGmix of the conformation transitions is negative so the processes are spontaneous. According to the type of the stimuli that change the system’s Gibbs free energy, in this chapter, we will mainly introduce several most investigated stimuliresponsive modes, including temperature, pH, light, ion, ion strength, and redox, as well as multi-stimuli response.

2.3 Thermoresponsiveness Temperature-responsive polymers exhibit a drastic and discontinuous change of their physical properties with temperature. Generally, thermoresponsiveness of the polymer is based on its conformation changes below and above critical solution temperature at which the polymer–solvent system undergoes a miscibility gap and phase separation within a small temperature range. There are two types of critical solution temperatures: lower critical solution temperature (LCST) and upper critical solution temperature (UCST). Water-soluble polymers contain polar groups that can interact with water by dipole–dipole interactions and hydrogen-bonding (enthalpy), thereby avoiding the strong hydrophobic effect in water (entropy). Whether UCST or LCST miscibility gaps occur depends on the free enthalpy of mixing that comprises of enthalpic and entropic contributions. Therefore, LCST or UCST behavior is called enthalpy driven [10]. For LCST response mode, the polymer–solvent system remains monophasic with the polymer taking an extended and stretched conformation in solution at

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temperatures below its LCST, while a phase-separated and contracted polymeric conformation occurs above the LCST [11, 12]. On the contrary, UCST response mode polymers conform to the opposite temperature dependence relationship. Some polymers show LCST as well as UCST behavior, whereas the UCST is found outside the 0–100 ∘ C region and can only be observed under extreme experimental conditions, such as high ionic strength and low pH [10]. Although there exist some polymers that exhibit UCST behavior between 0 and 100 ∘ C, they are too sensitive to electrolytes and concentration (e.g. in case of zwitterionic polymers), which is unsuitable for a broad applications [13–15]. 2.3.1

LCST Thermoresponsiveness

Polymers with typical LCST thermoresponsiveness include poly(N-isopropylacrylamide) (PNIPAM) [16], poly(N-vinylcaprolactam) (PVCL) [17–21], poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) [22–24], poly(MEO2 MA-co-OEGMA) [25], poly(l-lactic acid)–poly(ethylene glycol)–poly(l-lactic acid) (PLLA–PEG–PLLA) triblock copolymers [26], poly(vinylalcohol-covinylacetal) [27], and poly(ethylene oxide)–poly(-propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) copolymers [28]. The similarity among these thermoresponsive polymers is that they contain C—O, C—O, and N—H bonds, which enable them to form hydrogen bonding with solution water molecules or within different parts of single or different polymeric molecules (intraand intermolecular interactions). Upon a change of temperature, the ratio of polymer–water and polymer–polymer hydrogen bonding is changed, leading to an overall conformation change of the LCST response mode polymers. In addition to water, the LCST responsiveness has been observed in highly polar media such as alcohols [29]. Without any doubt, PNIPAM is the most popularly investigated thermoresponsive polymer due to its easy synthesis, low cost, and appropriate LCST (typically 32 ∘ C) [30]. PNIPAM carries hydrophilic amide and hydrophobic isopropyl groups. Below its LCST, water is a good solvent to PNIPAM, and the polymer chains and solvent molecules are in one homogeneous mixing phase and exhibit favorable free energy (ΔG < 0), which is facilitated by the abundant water–PNIPAM hydrogen bonds. Above the LCST, water becomes a poor solvent, and PNIPAM breaks its hydrogen bonds with water and instead enhances hydrogen bonds within and among its own polymeric chains, taking on a contracted and coiled conformation (Figure 2.2) [12]. It is noteworthy that changes in characteristic size between good and poor solvents are normally much more pronounced for surface-confined polymer chains than for polymer chains in solution. Thus, grafting PNIPAAm chains to a membrane surface imparts a temperature responsiveness to that membrane.

H O

O

H

H

O

O

N

H

H

H

H

O H

N

H O

H

H

H O

O O

Heating up H O

O

H

H

O

O

N

H

O

H

O

H

N

N

N

Cooling dowm

N

LCST

Figure 2.2 The structural scheme of PNIPAM at different temperatures. PNIPAM forms hydrogen bonds with water and presents an expanded state at a temperature below LCST. At a temperature above LCST, PNIPAM forms hydrogen bonds among itself and shows a collapsed state. Source: Chang et al. 2018 [31]. Reprinted with permission of Royal Society of Chemistry.

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The LCST of a polymer is further dependent on different parameters such as chain length [32, 33], tacticity [34, 35], pressure [36, 37], and the chemical nature of the end group [38, 39]. In addition, molecular designs of a polymer backbone can tune the responsiveness and control its transition temperature at which a given system is responsive. Since the LCST phase transition is a nanometer-scale event [40], the conformation transitions of polymer chains can be thermodynamically controlled by incorporation of co-monomers [41]. For example, copolymerized with hydrophilic or hydrophobic moieties into PNIPAM copolymers, LCST transitions may shift to higher or lower temperatures, ranging from 40 to 17 ∘ C [42]. 2.3.2

UCST Thermoresponsiveness

In contrast to the polymers with LCST responsiveness, considerably fewer polymers with UCST response mode in water have been identified and investigated [43]. Generally, as increasing the temperature above UCST, the polymer intermolecular or/and intramolecular interactions (hydrogen-bonding or Coulomb interactions) would be broken, and thus polymer–polymer and solvent–solvent interactions are stronger than polymer–solvent interactions [10]. 2.3.2.1

Coulomb Interaction-Induced UCST Polymers (Polyzwitterions)

Below their UCST, the polyzwitterions typically have strong inter- and intramolecular Coulomb attraction between the opposite charges along the polymer backbone and thus are insoluble in water (Figure 2.3) [10, 44, 45]. However, when the temperature is above UCST or with addition of small amount of salt in water, these inter- or intrapolymeric molecular interactions break by the polymer thermal motions or are screened, and the polyzwitterion–water interaction is thus thermodynamically favored under the elevated temperature, leading to an extended conformation of the polyzwitterionic polymers. As can be seen, the UCST mode polyzwitterions exclusively exist in water with very low ionic strengths, which makes it unsuitable under many environmentally relevant conditions. n O

R

N+

5 R = O(CH2)2

O S O O PDMAPS

6 R = NH(CH2)3 PSPP 7 R = O(CH2)11 (a)

(b)

(c)

Figure 2.3 The UCST of zwitterionic polymers relies on intra- and intermolecular Coulomb interaction, including intragroup (a), intrachain (b), and interchain (c). Source: Seuring and Agarwal 2012 [10]. Reprinted with permission of John Wiley and Sons.

2.3 Thermoresponsiveness

T < UCST

T > UCST

n

HN O H

N

H H

N

O

Thermally reversible hydrogen bonding

H

O O

NH n

Figure 2.4 The UCST of polymers is based on intermolecular hydrogen-bonding interaction. Source: Seuring et al. 2011 [43]. Reprinted with permission of American Chemical Society.

Polyzwitterionic polymers can dramatically tune their transition temperature. For example, Roth and coworkers reported polysulfobutylbetaine copolymers with zwitterionic species bearing ammonium and sulfonate groups that displayed the UCST response behaviors with an increase in UCST from 27 to 77 ∘ C as increasing degrees of polymerization from 66 to 186 [46]. 2.3.2.2

Hydrogen Bonding-Induced UCST Polymers

Some polymers, such as poly(ethylene oxide) (PEO) [47–50], poly(vinylmethylether) (PVME) [51], poly(N-acryloyl glycinamide) (poly(NAGA)), and poly (hydroxyethyl methacrylate) (PHEMA) [52], exhibit stable aggregation state due to polymer intermolecular hydrogen bonding under UCST, and high temperature breaks hydrogen bonds and brings polymer chains into solution. However, different from the PNIPAM that breaks water–polymer hydrogen bonds and builds hydrogen bonding from single chain itself above LCST, the conformation transition of UCST polymers is dependent on the breakage and construction of intermolecular hydrogen bonding (Figure 2.4) [43]. Moreover, these UCST polymers generally suffer from unsuitable UCST range (>100 or 400 nm

N

H

Figure 2.14 Synthesis of triple responsive PNIPAM-based copolymers containing azobenzene and TEMPO moieties. Source: Schattling et al. 2011 [243]. Reprinted with permission of Royal Society of Chemistry.

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During the reduction process, the TEMPO moiety can be reduced to the corresponding hydroxylamine. This causes a shift of the hydrophilic/hydrophobic balance to an increased hydrophilicity, thereby leading to a rise in LCST. Meanwhile, azobenzene chromophore can be stimulated by UV light irradiation, resulting in a further increase of LCST.

2.10 Conclusion In this chapter, we discussed the major chemical mechanism behind intelligent materials’ responsiveness to such stimuli as temperature, pH, light, metalic ion, ion strength, and redox, as well as dual or triple stimuli. In the recent past, numerous studies have explored the feasibility of employing stimuli-responsive materials in smart coatings (self-cleaning, self-healing, switchable wettability, adsorption, adhesion, optical properties, etc.), miniaturized devices (actuators, drug delivery systems, microfluidic devices, and controlled release systems), and sensors [2, 8, 98, 244, 245]. In the future, a better comprehensive understanding of the responsive polymeric moieties interaction and responsiveness mechanism is necessary. More importantly, exploring novel polymeric systems that can be controlled by employing more than one stimulus would be focused in future research.

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well-defined thermo-and light-responsive diblock copolymers by atom transfer radical polymerization and click chemistry. Polymer Chemistry 2 (9): 2068–2073. Dirani, A., Laloyaux, X., Fernandes, A.E. et al. (2012). Reversible photomodulation of the swelling of poly(oligo(ethylene glycol) methacrylate) thermoresponsive polymer brushes. Macromolecules 45 (23): 9400–9408. Mu, B. and Liu, P. (2012). Temperature and pH dual responsive crosslinked polymeric nanocapsules via surface-initiated atom transfer radical polymerization. Reactive and Functional Polymers 72 (12): 983–989. Schilli, C.M., Zhang, M., Rizzardo, E. et al. (2004). A new doubleresponsive block copolymer synthesized via RAFT polymerization: poly(N-isopropylacrylamide)-block-poly(acrylic acid). Macromolecules 37 (21): 7861–7866. Kulkarni, S., Schilli, C., Grin, B. et al. (2006). Controlling the aggregation of conjugates of streptavidin with smart block copolymers prepared via the RAFT copolymerization technique. Biomacromolecules 7 (10): 2736–2741. Zhou, J., Wang, G., Hu, J. et al. (2006). Temperature, ionic strength and pH induced electrochemical switching of smart polymer interfaces. Chemical Communications (46): 4820–4822. Tauer, K., Khrenov, V., Shirshova, N., and Nassif, N. (2005). Preparation and application of double hydrophilic block copolymer particles. Macromolecular Symposia 226 (1): 187–202. Xiong, D.a., He, Z., An, Y. et al. (2008). Temperature-responsive multilayered micelles formed from the complexation of PNIPAM-b-P4VP blockcopolymer and PS-b-PAA core-shell micelles. Polymer 49 (10): 2548–2552. Tuncer, C., Samav, Y., Ülker, D. et al. (2015). Multi-responsive microgel of a water-soluble monomer via emulsion polymerization. Journal of Applied Polymer Science 132 (24): 42072. Bütün, V., Armes, S., and Billingham, N. (2001). Synthesis and aqueous solution properties of near-monodisperse tertiary amine methacrylate homopolymers and diblock copolymers. Polymer 42 (14): 5993–6008. Kuramoto, N., Shishido, Y., and Nagai, K. (1997). Thermosensitive and redox-active polymers: Preparation and properties of poly(N-ethylacrylamide-co-vinylferrocene) and poly(N,N-diethylacrylamideco-vinylferrocene). Journal of Polymer Science Part A: Polymer Chemistry 35 (10): 1967–1972. Tang, X., Liang, X., Gao, L. et al. (2010). Water-soluble triply-responsive homopolymers of N,N-dimethylaminoethyl methacrylate with a terminal azobenzene moiety. Journal of Polymer Science Part A: Polymer Chemistry 48 (12): 2564–2570. Akiyama, H. and Tamaoki, N. (2007). Synthesis and photoinduced phase transitions of poly(N-isopropylacrylamide) derivative functionalized with terminal azobenzene units. Macromolecules 40 (14): 5129–5132. Sumaru, K., Kameda, M., Kanamori, T., and Shinbo, T. (2004). Characteristic phase transition of aqueous solution of poly(N-isopropylacrylamide) functionalized with spirobenzopyran. Macromolecules 37 (13): 4949–4955.

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copolymers synthesized by atom transfer radical polymerization. Macromolecules 43 (17): 7073–7081. Zhang, J., Liu, H.-J., Yuan, Y. et al. (2012). Thermo-, pH-, and light-responsive supramolecular complexes based on a thermoresponsive hyperbranched polymer. ACS Macro Letters 2 (1): 67–71. Yu, Y.Y., Tian, F., Wei, C., and Wang, C.C. (2009). Facile synthesis of triple-stimuli (photo/pH/thermo) responsive copolymers of 2-diazo-1, 2-naphthoquinone-mediated poly(N-isopropylacrylamide-co-Nhydroxymethylacrylamide). Journal of Polymer Science Part A: Polymer Chemistry 47 (11): 2763–2773. Schattling, P., Jochum, F.D., and Theato, P. (2011). Multi-responsive copolymers: using thermo-, light-and redox stimuli as three independent inputs towards polymeric information processing. Chemical Communications 47 (31): 8859–8861. Tokarev, I., Motornov, M., and Minko, S. (2009). Molecular-engineered stimuli-responsive thin polymer film: a platform for the development of integrated multifunctional intelligent materials. Journal of Materials Chemistry 19 (38): 6932–6948. Stuart, M.A.C., Huck, W.T., Genzer, J. et al. (2010). Emerging applications of stimuli-responsive polymer materials. Nature Materials 9 (2): 101–113.

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3 Filtration Membranes with Responsive Gates 3.1 Membrane Separation for Water Purification and Desalination In the last century, with the booming of food, biotechnology, and pharmaceutical industries, the conventional separation technologies, such as distillation, sublimation, crystallization, and adsorption, became either less efficient or impossible to meet some special needs from these industries. The membrane-based separation, in which porous membrane serves as a barrier to separate undesired species out of a mixture, came into being first in the early 1900s to aid the development in these industries and has since dominated and expanded its presence in many aspects of major industrial processes. Membrane technology is a key component of an integrated water treatment and reuse paradigm [1]. It is safe to state that the membrane technology has contributed significantly to the steadily improved living standards of mankind in the past. Particularly, in the water treatment sector, membrane-based separation has its niche benefits: (i) Membrane separation is largely physical, and both permeate and retentate can be collected and utilized, which has a special meaning nowadays in wastewater treatment as there is a growing interest in recovering valuable resources, including water, nutrient, energy, etc., from municipal and industrial wastewaters. As resource recovery will be an integral part of wastewater treatment in the coming decades, the importance of membrane in water treatment cannot be overstated. (ii) By employing membranes with different pore sizes or separation mechanisms, membrane separation provides fit-for-purpose products, which offers flexibility and precise separation at the lowest energy cost and thus vastly boosts the separation energy efficiency. (iii) From the engineering point of view, membrane-based separation system requires less space than most of the conventional technologies for the same purpose of separation [2]. All these advantages make membranes essential tools to the current and future water treatment industry. The performance of a membrane largely depends on the membrane material, which bears an inherent trade-off between solvent permeability and solute selectivity or rejection, both of which are unalterable in conventional membranes and cannot be tuned during their operations [3]. Based on the cutoff size of membrane separation, filtration membrane can be classified into microfiltration Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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0.1 nm

10 nm

100 nm

1 μm

10 μm

Viruses

Hydrated ions

Bacteria

Micropollutants Natural organic matter

Protozoa (for example, Cryptosporidium, Giardia)

Algal toxins Reverse osmosis, forward osmosis

Ultrafiltration

Nanofiltration

Solution–diffusion

Microfiltration

Size exclusion

Figure 3.1 Several common membrane processes for water purification and desalination separate largely on the basis of solute size. Source: Werber et al. 2016 [1]. Reprinted with permission of Springer Nature.

(MF), ultrafiltration (UF), nanofiltration (NF), reverse osmosis (RO), and forward osmosis (FO) (Figure 3.1). MF membranes are used to sieve suspended particles and microbial pathogens, while UF membranes are designed to separate macromolecules with smaller scales than MF, such as natural organic matter and smaller pathogens (viruses). In general, the molecular weight cutoff of UF membranes ranges from approximately 5 to 500 kDa [1]. NF membranes can remove scale-forming ions, like calcium and magnesium, as a function of reducing salinity. Typically, molecular weight cutoff of NF membranes is between 100 and 300 Da. Filtration in NF is based on a combination of sieving and solution–diffusion mechanisms. RO and FO membranes are designed for desalination to remove nearly all ions in addition to uncharged solutes of molecular weight greater than around 100 Da [1]. Furthermore, the filtration process of FO is driven by an osmotic pressure difference between the feedwater and draw solution of high osmotic pressure [4, 5]. Over the years, membrane separation has been an important playground for innovative nano-designs, which enable many conventional membranes to steadily improve their separation performances [6, 7].

3.2 Emerging Design and Concept of Filtration Membranes with Responsive and Intelligent Gates Generally, the separation performance of a membrane is determined by trans-membrane flux and the rejection of specific substrate, and such permeability and selectivity are in turn influenced by the membrane pore size, surface properties, and the interactions between the permeating substances and the pore surface. Unfortunately, membrane performance is largely pre-fixated in

3.3 Fabrication Methods of Intelligent Gating Membranes

conventional membranes owing to the lack of molecular-level design [8]. The structural control of the selecting layer is limited, thus hampering dispersing medium and substrate selectivity and increasing high fouling propensity [3]. Novel materials and scalable molecular-level design approach for membrane fabrication would be imperative for overcoming these limitations and further endow the capability of adjusting pore size, pore channel chemistry, etc. in response to environmental conditions. Thus, membranes with tunable permeability and selectivity would clearly outperform the conventional ones and offer considerable promise for performing complex tasks and substantially advancing water purification and desalination technologies. In looking for inspiration to make better members, nature offers us a wonderful model of intelligent gating: cell membrane. The cell membrane possesses extremely selective ion channels that allow only targeted ions to pass through with very high rate. More importantly, these ion channels can be switched on and off on demand by modulation charge/concentration gradient between the sides of the cell membrane, which alters the conformation of the channel proteins. In a sense, cell membrane works as stimuli-responsive intelligent “gates,” regulating their opening and closing in response to chemical or electrical signals, temperature, or mechanical force [9, 10]. The cell membrane is always a great inspiration to scientist to develop artificial intelligent gating membranes toward more versatile and effective separation, but the stimuli-responsive membrane field is still at its infantry stage and is vastly far from being close to the delicacy and precision nature offers us. Valuable but limited efforts have been made in making membranes with stimuli responsiveness and tunable pore size/pore chemistry at laboratory scale, and there has not been any successful application of these membranes at large scale. Synthetic intelligent gating membranes have emerged since the 1960s when the stimuli-responsive behaviors of some polymers were first revealed by Heskins and Guillet [11]. As we discussed in Chapter 2, polymers are the most commonly investigated stimuli-responsive materials. The general design principle of artificial intelligent gating membranes is to incorporate stimuli-responsive materials, dominantly polymers, into the pores of membranes and these polymers, in response to appropriate stimuli, such as temperature, pH, and light, ion, ion strength, etc. The change in conformations and chemistry would in turn adjust the pore sizes and/or the surface properties of the membranes, leading to modulation of permeability and selectivity of the membranes (Figure 3.2) [12–14]. The artificial intelligent gating membranes combine the advantages of porous substrates and intelligent gates for distinct performances responsive to environmental triggers and thus perform more complex tasks [15–19].

3.3 Fabrication Methods of Intelligent Gating Membranes Methods to produce intelligent gating membranes can be generally divided into two categories: (i) post-modification of existing porous membranes, which is

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Open state

Closed state

Triggers Temperature pH Light Swollen state

Shrunken state

Figure 3.2 Schematic representation of the different gating states of intelligent gating membrane in response to appropriate triggers. Source: Chang et al. 2018 [2]. Reprinted with permission of Royal Society of Chemistry.

the most popular method to fabricate polymer-based stimuli-responsive gating membranes and where the polymeric species are bound onto the pore surface via covalent bonding [20–23], van der Waals forces [24], electrostatic interaction [25, 26], and so on, and (ii) one-step formation of stimuli-responsive gating membranes. 3.3.1

Post-Modification Method

The post-modification method means incorporating the gates on existing porous membrane substrates using covalently or noncovalently bonding. Covalent grafting techniques, namely, grafting-to and grafting-from methods, are that functional polymeric materials or responsive small molecules are covalently modified onto the pore surface of an existing membrane. Both methods allow the fabrication of gating membranes with steady gating structures and highly efficient gating performances. For grafting-from method, gating membranes are fabricated by first inducing active sites on the pore surface and then polymerizing functional monomers from the active sites to constitute linear polymers or cross-linked networks in the pores as the intelligent gates. The typical grafting polymerization methods include atom transfer radical polymerization (ATRP) [27–37], reversible addition–fragmentation chain transfer (RAFT) [23, 38], UV-induced grafting [21, 39–43], and plasma-induced grafting [22, 44–60]. With various grafting polymerization methods, the functional gates can be incorporated into a wide range of membrane substrates for forming the intelligent gating membranes. For the grafting-to method, the membrane surface can be functionalized by chemically grafting preformed functional polymeric materials or responsive small molecules based on covalent bonding between the functional group of membrane materials and a reactive group from polymers [20]. Therefore, the intelligent gating membranes are fabricated by incorporating preformed functional gates onto the pore surfaces. Moreover, since polymer chains with well-controlled length or size can be pre-synthesized using well-established methods, the grafting-to method possesses improved controllability and flexibility for the gate microstructures. However, in comparison, the grafting-from method can lead to higher grafting

3.4 Application of Intelligent Gating Membranes to Environmental Separation

densities than grafting-to method due to the existence of steric hindrance by surrounding bonded chains during the grafting-to process. Comparing with the bonding between the gates and pore surface through non-covalent interactions, such as van der Waals forces [24], and electrostatic interaction [25, 26], covalent bonding surface modification is more robust for application. In addition, various existing membranes, organic and inorganic ones, have been utilized as a matrix to fabricate intelligent gating membranes. The organic ones include polypropylene (PP) [52, 61], polycarbonate (PC) [25, 44, 51, 55], polyethylene (PE) [21, 22, 41], polytetrafluoroethylene (PTFE) [62, 63], Nylon-6 [32, 49, 64], polyvinylidene fluoride (PVDF) [23, 33, 49, 50, 57, 65], polyimide (PI) [30], polyamide (PA) [66–68], poly(ethylene terephthalate) (PET) [28, 37, 40], polysulfone (PSF) [42], poly(viny1 chloride) (PVC) [69], and polyethersulfone (PES) [43, 70, 71], while the inorganic membranes mainly include anodic aluminum oxide (AAO) [34, 72], nanoporous silica [73], and nanoporous silicon nitride membranes [74]. Generally, inorganic membranes possess great performance of high thermal and chemical stability, inertness to microbiological degradation, and ease of cleaning after fouling compared with organic membranes, while they have high cost. 3.3.2

One-Step Formation Method

Generally, polymeric membranes can be produced by a phase inversion method, in which solvent is removed from a liquid-polymer solution, leaving a porous, solid membrane due to the polymer transformation from a liquid phase to solid phase [75]. Although the one-step formation of stimuli-responsive gating membranes by the phase inversion method have also been reported in the literature, it is applicable to only a small group of responsive polymers [71, 75, 76].

3.4 Application of Intelligent Gating Membranes to Environmental Separation Generally, membrane pore size modulation by external triggers heavily dominates the intelligent membrane research thus far [24, 75, 77]. However, new designs and concepts are emerging in combining responsive chemistry with membranes toward better membrane performance. For example, for membrane fouling, the membrane surface properties can be switched by changing the wettability of the gates under specific stimuli; thus the affinity between the contaminants and the membrane surface can be weakened. On the other hand, stimuli-responsive materials can be combined with RO membranes to improve their antifouling properties and be incorporated into the membranes to endow the membrane with self-healing capability [66, 68, 78–82]. In addition, there is increasing interest in combining membrane distillation (MD) with photothermal materials, which, in response to solar light, generates heat locally with high energy efficiency [83]. All-in-one membrane has been reported to

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integrate chemical reactions and physical separation in one system, where the trigger-initiated sequential reactions selectively and on demand degraded and separated water pollutants [65]. Overall, this chapter presents the state of the art of the intelligent gating membranes for environmental separation and is organized according to the type of environmental triggers, namely, temperature (i.e. heat), pH, light, ions, ion strength, redox, etc., or multiple stimuli.

3.5 Thermoresponsiveness Filtration membranes responsive to temperature is the main source of intelligent gating membranes that have found wide interest among researchers. As mentioned in Chapter 2, thermoresponsiveness of polymers is mainly based on its conformation changes below and above critical solution temperature (LCST or UCST). PNIPAM has been extensively investigated as a thermoresponsive pore controller in the field of intelligent gating for water filtration [84–86]. Given that the typical LCST of PNIPAM is around 32 ∘ C, the temperature range of the feedwater was usually 20–40 ∘ C for PNIPAM-based membranes in literature. At temperatures below the LCST, PNIPAM chains exhibit swollen state and hydrophilicity owing to the hydrogen bonding between the amide groups from PNIPAM and water molecules, and the gates of the membrane close. With an increase in the temperature above the LCST, the PNIPAM chains switch to shrunk conformation and exhibit hydrophobicity due to hydrogen-bonding cleavage, which leads to the gates open (Figure 3.3) [50, 88]. It was in 1986 when the first intelligent thermoresponsive gating membrane was constructed. The membrane was made by grafting PNIPAM onto a nylon membrane, and the grafted polymer acted as permeation valve regulating water flux by changing water temperature below and above LCST [89]. Ever Polymer network

OFF

Temp > LCST

Temp < LCST

Microporous support

ON

Figure 3.3 Schematic illustration of thermoresponsiveness mechanism of PNIPAM polymer grafted onto porous membranes. Source: Lue et al. 2007 [87]. Reprinted with permission of Elsevier.

3.5 Thermoresponsiveness

since, there had been an explosive growth of using PNIPAM in gating membranes [30, 37, 40, 50, 51, 90]. Moreover, the gating membranes with PNIPAM block copolymers and PNIPAM-based inorganic/organic composites, such as PNIPAM-g-PET [28, 91], PNIPAM-g-PE [21, 22], PNIPAM-g-PP [52], PNIPAM-g-nylon [90], PNIPAM-g-PC [87], PNIPAM-modified SiO2 sphere [31, 45], and PNIPAM-co-glycidyl methacrylate [92], were fabricated by UV or plasma-induced graft polymerization, ATRP, etc. In addition, valuable efforts were made to systematically increase and decrease the LCST by introducing hydrophilic or hydrophobic monomers into N-isopropylacrylamide (NIPAM) monomer solution in the fabrication of thermoresponsive gating membranes, which offers more flexibility in applying intelligent gating membranes to environmental separations. In 2007, Chu et al. grafted thermoresponsive polymers, poly(N-isopropyl-acrylamide-co-acrylamide) (PNA) and poly(N-isopropylacrylamide-co-butyl methacrylate) (PNB), as functional gates onto porous PVDF or nylon membranes via plasma-induced grafting polymerization method (Figure 3.4) [49]. The response temperature of copolymers was raised to 40 ∘ C when 7 mol% of hydrophilic acrylamide was added into the NIPAM co-monomer solution but was reduced to 17.5 ∘ C as 10 mol% of hydrophobic butyl methacrylate was added into the same solution. However, directly changing the temperature of a bulk water during continuous separation is not trivial and more importantly is considered as energy inefficient. Instead of changing the bulk water temperature to induce the membrane pore size change, in 2014, Gajda and Ulbricht reported an in situ local heat generation scheme using an external magnetic field to excite Fe3 O4 nanoparticles co-imbedded into the membrane pore along with PNIPAM by pre-adsorbing CH2

CH

H2C

CH2

O

C

C

+

NH

CH x O

C O

NH

NH2

CH H3C

CH

CH2

CH y C

O

NH2

CH H3C

CH3

CH3

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CH C

CH2

NH CH H3C (b)

C

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CH3

+

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C

C

O

O

NH

O (CH2)3 CH3

CH x

CH H3C

CH3

CH2 C y C

O

O (CH2)3 CH3

Figure 3.4 The polymerization reactions for grafting thermoresponsive copolymers (PNA and PNB) onto membrane substrates. Source: Xie et al. 2007 [49]. Reprinted with permission of Elsevier.

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ΔH on ΔH off

(a) O

(b)

N H

H N

N H

H N

O N H

Fe3O4

Figure 3.5 (a) Schematic illustration of pore size control through the stimulation of superparamagnetic Fe3 O4 nanoparticles in PNIPAAM-functionalized membrane pores under an external magnetic field and (b) the peptide bond between carboxyl groups on the Fe3 O4 nanoparticles surface and prefunctionalized membrane surface. Source: Gajda and Ulbricht 2014 [93]. Reprinted with permission of Royal Society of Chemistry.

a cationic macroinitiator and subsequent photo-initiated graft copolymerization (Figure 3.5) [93]. Such thermoresponsive polymer–nanoparticle hybrid membranes were endowed with remote control in the water flux via electromagnetic field “on” and “off.” The pore size and water flux of the membrane could be tuned between 290 nm, 42 l m−2 h−1 and >400 nm, 240 l m−2 h−1 in the absence and presence of the magnetic field, respectively. Apart from PNIPAM, other thermoresponsive polymers with suitable LCST were also introduced in intelligent gating membranes recently. In 2005, Petrov et al. utilized thermoresponsive poly(vinyl alcohol-co-vinyl acetal) copolymer to modify asymmetric porous poly(acrylonitrile) UF membrane [94]. This modification resulted in the reversible opening and closing of the membrane pores due to the thermoresponsive conformational switch from shrinking to swelling of the copolymer; thus the membrane permeability and selectivity can be intelligently regulated by varying the working temperature from 25 to 45 ∘ C. In the same year, poly(N-vinylcaprolactam) (PVCL), with an LCST around 32∼35 ∘ C, was also involved in the application of thermoresponsive gating membranes. Prez and coworkers fabricated a PVCL-modified PET track-etched membranes via the photochemical immobilization method [95]. The permeability of a mixture of dextrane molecules was investigated as a function of temperature. The concentration of the dextrane molecules with Mn = 100 000 in the filtrate increased with increasing temperature above its LCST, owing to the increase in pore diameter for temperatures above its LCST. In addition, the concentration of the lower molecular weight dextranes (Mn = 6000) remained constant during temperature change between 20 and 45∘ C, indicating that the original pore size of the membranes was too high to hinder their permeability during the filtration process.

3.5 Thermoresponsiveness

In 2014, Wessling and coworkers also prepared PVCL microgel-modified membranes, which exhibited a reversible thermoresponsive behavior [24]. The membrane’s permeability can be alternatively controlled with water at 20 and 45 ∘ C. Similarly, in 2017, Wessling and coworkers also fabricated an electrically conductive SiC–C hollow fiber membrane on which PVCL microgels were immobilized via filtration coating [96]. Differing conventional thermoresponsive behavior of gating membrane through externally tuning the feed stream temperature, the concept they proposed was to adjust the permeability and selectivity of the membrane by controlling the applied electrical power to heat membrane (Figure 3.6a). Therefore, the temperature of the membrane itself directly enabled to initiate a response of the membrane surface. Direct electric heating was more energy efficient compared with heating of the whole feed stream, saving 14% of the consumed energy. The electrical heating caused the microgels to collapse, increasing the pore size for water permeation. Permeability was around 5–10 times greater in the heated state than without the applied voltage (Figure 3.6b).

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0

0

10

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Figure 3.6 (a) Schematic illustration of the electrically heated SiC–C hollow fiber membrane modified by PVCL microgels. (b) Permeability over time for each 10 min cycles of heating with 30 W m−1 (upper) and 100 W m−1 (bottom) and no applied voltage. (c) Selectivity of 200 kDa dextrane as a function of applied temperature on the membrane. The temperature was increased to 50 ∘ C and then cooled back to room temperature. Source: Lohaus et al. 2017 [96]. Reprinted with permission of Elsevier.

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The selectivity can be reversibly tuned in a range of 10–80% for a 200 kDa dextrane by heating the membrane (Figure 3.6c). In 2016, Jin and coworkers grafted pyrene-terminated poly-(MEO2 MA-coOEGMA) (LCST ∼ 32 ∘ C) onto single-walled carbon nanotube (SWCNT) membrane via π−π interaction [15]. The membrane’s pore size varied between 12 and 14 nm when the water temperature was changed between 25 and 40 ∘ C, leading to a stable flux variation between 3730 and 6430 l m−2 h−1 over several cycles. In comparison, the use of UCST response mode polymers in intelligent gating membrane is very limited. Thus, it is not a surprise that the first UCST behavior of the gating membrane with practical responsive temperature was not reported until 2005 when Chu et al. demonstrated interpenetrating polymer networks with poly(acrylamide) (PAAM) and poly(acrylic acid) (PAAC) as negatively thermoresponsive gates [97]. As shown in Figure 3.7, the intelligent Pore Substrate

PAAM/PAAC IPN

Grafted PAAM

Substrate

(a)

(c)

(b) T > UCST

T < UCST

CH2 CH

CH2 CH CH2 CH

C

C O

O

O

O

O

H

H

H

H

H

H

O

HN

O

HN

C

C

O

O

OH

O H2N C

PAAM

C

C OH

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O C

CH2 CH CH2 CH CH2 CH

(d)

CH2 CH CH2 CH

C

O

HN

CH2 CH

PAAC

C

CH2 CH

O

OH

O H 2N C

O C

CH2 CH CH2 CH

(e)

Figure 3.7 A schematic illustration of the functioning of the negative thermoresponsive membrane. The functional gates of the membrane are thermoresponsive interpenetrating polymer networks consisting of poly-acrylamide (PAAM) and poly(acrylic acid) (PAAC), in which the volume phase transition is driven by the hydrogen bonding interactions between the molecules. (a) Porous membrane substrates. (b) Membranes with grafted PAAM gates. (c) Membranes with PAAM/PAAC-based gates. (d) The pores of the membranes are open since the functional gates are under their shrunken state at temperatures below the UCST as a result of PAAM/PAAC complex formation by hydrogen-bonding interactions. (e) The membrane pores are closed because the gates are under their swollen state at temperatures above the UCST as a result of complex dissociation by the breakage of hydrogen bonds. Source: Chu et al. 2005 [97]. Reprinted with permission of John Wiley and Sons.

3.5 Thermoresponsiveness

gates exhibited open state due to the shrinkage of PAAM/PAAC complex via hydrogen-bonding formation at temperatures below UCST of the complex. While at temperatures above the UCST, the PAAM/PAAC complex would swell owing to its dissociation through the breakage of hydrogen bonds, leading to pore “closing.” Thus, the membrane pores can switch from an “open” to a “closed” state once the temperature increases above the UCST. The synthesized membrane exhibited a sharp transition of water permeability in a practical temperature range from 20 to 25 ∘ C. Inspired by the stomatal closure feature of plant leaves at relatively high temperature, in 2017, Zhao and coworkers constructed a negative temperature-response nano-gating membrane by covalently grafting PNIPAM chains on GO sheets via free radical polymerization (Figure 3.8a) [98]. By virtue of the temperature tunable lamellar spaces of GO sheets, the water permeation of this membrane was 12.4 l m−2 h−1 bar−1 at 25 ∘ C and 1.8 l m−2 h−1 bar−1 at 50 ∘ C. Moreover, such membrane was capable of separating multiple molecules with different sizes by regulating the temperature. The rejection rates of the five ions/molecules, Cu2+ (0.8 nm), [Fe(CN)6 ]3− (0.9 × 0.9 nm), rhodamine B (RB, 1.8 × 1.4 nm), coomassie brilliant blue (CBB, 2.7 × 1.8 nm), and cytochrome c (Cyt. c, 2.5 × 2.5 × 3.7 nm), through the gating membrane from 25 to 50 ∘ C were measured (Figure 3.8b). Cu2+ only existed in the filtrate obtained at 50 ∘ C, while trace RB and Cyt. c were obtained in the filtrate at 50 ∘ C. For RB, it enriched in the filtrate obtained at

CH2 CH C N

x O

Intramolecular/intermolecular hydrogen bonding

PNIPAM and water molecule hydrogen bonding

CH2 CH

H

C N

x O

H O

H O

H N C CH CH2

H

y

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PNIPAM

< LCST

Open

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(a) Rejection rate (%)

100

Cu2+ [Fe(CN)6]3–

80

RB CBB Cyt. c

60

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Filtrate at 25 °C

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Mixed

RB + Cyt. c

Cu2+

RB

Cyt. c

40 20 0 25

(b)

30 35 40 45 Temperature (°C)

50

(c)

Figure 3.8 (a) Fabrication process of the thermoresponsive membrane and its water gating property. The water permeance of membrane could be regulated by changing temperature (T). (b) Temperature-dependent rejection rates of membrane to Cu2+ , [Fe(CN)6 ]3− , RB, CBB, and Cyt. c, implying smaller channel size of the membrane with the increasing temperature. (c) Separation of mixed molecules solution, containing Cu2+ , RB, and Cyt. c. Source: Liu et al. 2017 [98]. Reprinted with permission of Springer Nature.

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25 ∘ C, and only the largest Cyt. c remained in the retentate obtained at 25 ∘ C (Figure 3.8c). This nano-gating membrane expands the scope of intelligent gating systems and molecular separation.

3.6 pH Responsiveness Among the various categories of intelligent responsive materials, pH-responsive polymeric materials are one of the most widely investigated. The polyelectrolytes with weak acidic or weak basic groups are typically pH-responsive polymers. Depending on solution pHs, the weak acidic and basic groups undergo reversible protonation and deprotonation, leading to a reversible swollen and shrunken conformation transition due to on-and-off switch of electrostatic repulsion between these functional groups. The pH-dependent conformation changes of the polymers have been widely used as functional gates in the intelligent gating membranes for controllable separation. Such gating membranes have been used toward adjustable water flux and molecular size selectivity for a variety of substances, including proteins [99], fluorescein isothiocyanate–dextran (FITC–dextran) [71], macromolecules [25, 100], vitamin B12 [41, 101], riboflavin [42], Au nanoparticles [102], etc. 3.6.1

Polybase Gating Membranes

The polymers with weak basic groups that have been applied to intelligent gating membranes include poly(4-vinylpyridine) (P4VP) [16, 17], polystyreneb-poly(4-vinylpyridine) (PS-b-P4VP) [99, 102–104], poly(methyl methylacrylateco-4-vinyl pyridine) (P(MMA-4VPy)) [105], poly(2-vinylpyridine) (P2VP) [106], PDMAEMA [33, 107, 108], and poly(allylamine hydrochloride) (PAH) [25]. Under suitable and generally acidic pH conditions, the weak basic groups located in the side chains of these polymers accept protons leading to the electrostatic repulsion among positively charged basic groups, exhibit a swollen state in an acidic environment, and thus result in pore size reduction. In a basic condition, the same basic groups deprotonate and are charge neutral, and the polymers go back to their shrunken state, leading to the increased pore size of the membranes. In 1984, Okahata et al. fabricated the first pH-responsive gating membrane, and in this work, P4VP was grafted onto a porous nylon membrane to act as NaCl permeation valve between pH 2 and 9 [17]. In 1995, Childs and coworkers further grafted P4VP onto microporous PP and PE substrates for pH-responsive gating membranes. The P4VP was stabilized on the PE or PP porous substrate by UV-initiated grafting. Such membrane showed the pH valve effect and a large permeability change, as well as the selectivity of small inorganic ions, which moderately rejected NaCl (40–50%) at pH < 3 and showed no salt rejection in neutral or basic conditions [16]. Although the surface graft polymerization provides a useful method to create pH-responsive gating membranes, its imprecise control of the density of grafted

3.6 pH Responsiveness

1 Cfiltrate/Cfeed

Pore diameter (nm)

800 600 400

0.8 0.6 0.4 0.2

200

0 Feed solution

0 0

(a)

14.5 20.5 8.5 Number of bilayers

(b)

PEO PEO Acidic PEO solution solution solution filtered in filtered in filtered in closed closed state open state state

Figure 3.9 (a) Changes in the average pore diameters in the pores of LbL membrane. (b) Filtration of high molecular weight PEO (0.01 g dl−1 ) using 18.5 bilayer (PAH/PSS) LbL membrane in different conditions. Open and closed states were attained by the pretreatment of multilayer-modified membranes at pH 10.5 and 2.5, respectively. Source: Lee et al. 2006 [25]. Reprinted with permission of American Chemical Society.

chains and inability to control the pore size in the confined geometries of the existing membranes are still challenges [109, 110]. In 2006, Rubner and coworkers used layer-by-layer (LbL) method and assembled multilayers of PAH and poly(sodium 4-styrenesulfonate) (PSS) as intelligent gates in a nanoporous PC membrane. The method of LbL offered an advantageous capability of easy and precise control over the pore diameters of the modified porous membrane [25] (Figure 3.9a), and the functionalized membrane showed approximately 80% poly(ethylene oxide) (PEO) rejection at pH 2.5 and no rejection at all at pH 10.5 (Figure 3.9b). In 2007, Peinemann’s group reported a fast one-step procedure to prepare a block copolymeric (PS-b-P4VP) membrane with nanometer-sized ordered pores by using non-solvent-induced phase separation [75]. In 2011, the same group developed metal-block PS-b-P4VP NF membrane with monodisperse asymmetric pH-responsive nanochannels. This NF membrane possessed high pore density (greater than 2 × 1014 per m2 ) and large-scale reproducibility in m2 scale. The reported pH-sensitive range of the polymer nanochannels with synthetic pores in the nm scale was from 2 to 8 (Figure 3.10), leading to more than 2 orders of magnitude water flux increase and selectivity of the mixture of PEG with different molecular weights [103]. Later, some follow-up works on PS-b-P4VP-based UF membranes were conducted for controllable separation of protein and inorganic/organic molecules [99, 104, 111, 112]. In 2017, Walker and coworkers fabricated phosphotriesterase-functionalized poly(isoprene-b-styrene-b-4-vinylpyridine) nanoporous membranes via self-assembly and non-solvent-induced phase separation [113]. This gating membrane showed changeable permeation from 1522 l m−2 h−1 bar−1 (at pH = 7) to 11 l m−2 h−1 bar−1 (at pH = 3) as a result of conformation transition of the P4VP chains between a collapsed state and swollen state (Figure 3.11). In addition, integrating enzymatic recognition capability into pH-responsive gating membranes may offer an intriguing outlook

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pH 2

pH 10

300 nm

300 nm

(a)

pH 10

pH 2

1 μm

1 μm

Dry

(b)

Figure 3.10 (a) Cryo-field emission scanning electron microscopy and (b) environmental scanning electron microscopy of PS-b-P4VP membranes at pH ∼ 2 and 10; (b, right) dry membrane was observed by environmental scanning electron microscopy. Source: Nunes et al. 2011 [103]. Reprinted with permission of American Chemical Society.

+

+H

N

2500

z

z

–H

+

N H

Collapsed neutral P4VP Swollen charged P4VP

Average permeability (lm–2 h–1 bar–1)

82

Supported ISV117 2000

Neat ISV119 Supported ISV119

1500

1000

500

0

Pl-b-PS membrane matrix (a)

Neat ISV117

2 (b)

4

6

8

pH

Figure 3.11 (a) pH-responsive behavior of the P4VP-based membranes. (b) The permeability of membranes in a buffer solution as a function of pH. Source: Poole et al. 2017 [113]. Reprinted with permission of John Wiley and Sons.

3.6 pH Responsiveness

for engineered biomimetic materials responding to a complex range of external parameters, providing sensing, protection, and remediation capabilities. 3.6.2

Polyacid Gating Membrane

The typical weak acidic polymers that have been reported in the intelligent gating membranes include PAA [42, 55, 57, 65, 100], poly(methacry1ic acid) (PMAA) [17, 41, 108, 114], poly(glutamic acid) (PGA) [62, 115], poly(l-glutamic acid) (PLGA) [116], polystyrene-block-poly(acrylic acid) (PS-b-PAA) [71], and poly(methyl methacrylate-co-acrylic acid) (P(MMA-AA)) [105], among others. The weak acidic polymers have a pH-responsive behavior opposite to the weak basic polymers. The intermolecular hydrogen bonding among the carboxylic groups of the polymers leads to the shrunken conformation of polymer chains under acidic conditions, while under basic conditions, the carboxylic groups deprotonate and become charged, and the electrostatic repulsion among deprotonated carboxylic groups swells the polymer chains, leading to the reducing membrane pore size. The first weak acidic polymeric (i.e. PMAA) gating membrane was reported in 1984 by Okahata et al. [17]. Between 1996 and 1999, Lee et al. fabricated PAA-grafted polymeric membranes by plasma [57] and UV-irradiated graft polymerization method [42], respectively, all showing a decreased riboflavin permeability in pH 4–5 compared with lower pH values. Since 1992, Ito et al. had demonstrated several methods in making weak basic polymer-based intelligent gating membranes by self-assembly or surface graft polymerization of weak polyacids, including PMAA, PAA, and PLGA onto PC, PTFE, and gold-coated membranes [62, 100, 109, 114, 115, 117]. In 2001, Zhang and Ito assembled PAA weak polyelectrolyte with thiol-modified chains on a gold-coated PC porous membrane [100], and the water flux of the membrane with surface densities of self-assembled PAA-SH of 18 pmol cm−2 decreased approximately from 5 to 1 ml cm−2 h−1 when pH increased from 2 to 7. The transport of PEG (Mw ∼ 8000) was also dependent on pH value (polymer retention: 0% at pH 2 and 33% at pH 7). In 2006, Qu et al. designed a pH-responsive intelligently controlled release system that contained weak polyacid, PMAA-g-PVDF as pH-responsive valve, and a cross-linked PDMAEMA hydrogels as a pump to pump the substances out [108]. As pH was decreased from 7 to 2, the membrane gates opened and the hydrogels expanded, leading to a rapid release of a substance into the membrane system by pumping out effect (Figure 3.12), which has potentials to be used as chemical carriers and environmental sensors as well as environmental separation. In 2014, Chu and coworkers reported the fabrication of pH-responsive PES composite membranes blended with PS-b-PAA copolymers, and the membrane pores opened at pH = 3 (pK a ), which showed controlled separation of FITC–dextran [71]. In 2009, Li and coworkers reported pH-dependent MF membranes that were cast by PES-grafted poly(methacrylic acid) (PES-g-PMAA) via phase inversion method. The pore sizes and filtration ability of the membranes can be tuned by changing pH [118]. In the most extreme case, the flux of aqueous acidic solution

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3 Filtration Membranes with Responsive Gates

CH3 CH2

CH3

PDM

C

CH2

n C

O

O

CH2

CH2

N

CH3

PMAA

C n C

O

OH

Swelling ratio

CH3

Swelling ratio

84

pH

pH

pH < min(pKPMAA,pKPDM)

pH > max(pKPMAA,pKPDM)

Figure 3.12 Pumping systems with gating membranes containing pH-responsive gates for enhanced controlled release. Source: Qu et al. 2006 [108]. Reprinted with permission of John Wiley and Sons.

(pH = 1) was about four times as that of aqueous basic solution (pH = 9) through the PES-g-PMAA membrane. Similarly, PES-g-PMAA was also cast into UF membranes via the phase inversion process, showing reversible pH-sensitive permeability as the pH value of feed solution was varied between 2.0 and 10.0 (Figure 3.13a) [119]. Furthermore, by using the same fabrication method, PS-b-PAA was blended with PES membranes with nanoscale pores [71]. Such membranes enabled selectively sieving of FITC–dextran molecule mixtures with different molecular weights of 10, 40, and 70 kDa, at ambient pH values of 3 and 8 (Figure 3.13b). In 2011, Lewis et al. applied a pH-responsive intelligent gating membrane in a multilayered and all-in-one Fenton-reaction-active filtration system for advanced oxidation [65]. The top layer of the membrane contained glucose oxidase (GOx) for in situ H2 O2 generation by reacting with deliberately added glucose in the raw water, which allowed for the flexibility of on-demand initiation of the Fenton reaction. The bottom porous PVDF layer was functionalized with a pH-responsive PAA network, and iron species was immobilized in the PAA layer as catalysis for Fenton reaction. The H2 O2 generated in the top layer decomposed

3.7 Photo-responsiveness 100

100 pH 2.0

90

Solute rejection (%)

70 60 50 40 30

pH = 3 pH = 8

70 60 50 40 30 20

20

10

pH 10.0

10 0

(a)

PSA280-1.5 membrane

80

80 Flux (I (m2h–1))

90

1

2

3

4

5 6 7 8 Cycle time

9

0

10 11 12

(b)

10 40 70 Molecular weight of FITC-dextran (kDa)

Figure 3.13 (a) Reversible change of water permeation through PES-g-PMAA membrane as a function of pH. Source: Shi et al. 2010 [119]. Reprinted with permission of Elsevier. (b) FITC-dextran rejection of pH-responsive membrane under pH = 3 and pH = 8 conditions. Source: Luo et al. 2014 [71]. Reprinted with permission of Elsevier.

and generated free radical oxidants with the help of iron species to oxidize the organic containments in the feedwater and the degradation-generated alkali ions as by-products. The alkali ions, in turn, increased the pH and stimulated the expansion of PAA, leading to a decrease in water flux and thus a longer residence time for pollutant degradation. On the other hand, in the case with the feedwater being free of organic contaminants, the water passed through the membrane with a large flux. Therefore, this all-in-one reactive filtration system possesses responsive and self-initiated intelligent behaviors.

3.7 Photo-responsiveness In photo-responsive gating membrane system, photo-responsive polymers act as functional gates, tuning membrane permeability and selectivity mainly through the reversible conformational and/or structural changes of the polymers in response to photoexcitation. Among various photoresponsive polymers, azobenzene and spiropyran derivatives have been highly investigated as photosensitive gates. For azobenzene-based intelligent membrane, upon UV irradiation, the distance between the para carbon atoms from the azobenzene functional groups decreases, leading to increased membrane pore size, while the membrane pore size decreases under visible light irradiation. On the other hand, the spiropyran groups undergo a transition from nonpolarity to polarity upon exposure to UV light, leading to changes from shrunken to swollen state and corresponding to a reduced pore size of the intelligent membrane. The polar state returns to the nonpolar and hydrophobic state via either a thermal or visible light treatment [43]. 3.7.1

Azobenzene-Based Gating Membranes

Depending on the trans–cis isomerization transition under UV or visible light irradiation, the synthetic azobenzene-based photo-responsive membrane can

85

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Responsive molecules + inorganic hosts

Figure 3.14 Organization of stimuli-responsive ligands within a 3D porous framework imparts is useful for opening and closing a nanoscale valve. Source: Liu et al. 2003 [73]. Reprinted with permission of John Wiley and Sons.

Stimuli

effectively tune its pore size (commonly nanoscale size), which has been applied in the controllable separation of ions [19, 26, 120], dye molecules [121], and macromolecules [18]. In 1983, Anzai et al. pioneered azobenzene-functionalized PVC membrane for the photo-controlled K+ ion permeation [69]. In 2003, Liu et al. organized azobenzene-containing ligands, 4-(3-triethoxysilylpropylureido)azobenzene (TSUA), into an ordered and rigid inorganic silica framework by an evaporation-induced self-assembly procedure (Figure 3.14). The photo-induced trans–cis conformation change of azobenzene ligands transduces solar energy into mechanical work to further promote the operational stability, enabling photo-control over its pore size [73]. In 2014, Shi et al. designed a photo-responsive system based on the host–guest complex between azobenzene (Azo) and 𝛽-cyclodextrin (𝛽-CD) [18]. Under irradiation at 450 nm, trans-azobenzene inserted into 𝛽-CD and formed 1 : 1 inclusion complex, resulting in stably closed pores. Under irradiation at 365 nm, collapsed cis-Azo slid out of the cavity of 𝛽-CD, and thus the pores were opened, showing high permeability for pure water and PEG solutions (Figure 3.15). In 2015, Fujiwara and Imura reported a special responsive gating membrane for water desalination by grafting azobenzene on an AAO membrane and using UV and visible light as a gate switch [19]. The membrane blocked the water passage in darkness but allowed water vapor passing through when simultaneously exposed under UV and visible light. The simultaneous irradiation of UV and visible lights onto the azobenzene induced its consecutive motion between the trans and cis isomers, which promoted the water vapor permeation. Since only water permeated through the membrane, the membrane was utilized for water treatment to remove dye and protein. This water membrane separation process could be also

3.7 Photo-responsiveness

= PEG-600 365 nm = PEG-4000

450 nm

= PEG-10 000

Figure 3.15 Schematic of PEG molecules (Mn = 600, 4000, 10 000) permeated through the membranes. Source: Shi et al. 2014 [18]. Reprinted with permission of Elsevier. Figure 3.16 A seawater desalination system using azobenzene-modified AAO membrane and solar light energy. Source: Fujiwara and Imura 2015 [19]. Reprinted with permission of American Chemical Society.

Direct use of solar light Solar light Seawater pool

Seawater Azobenzenemodified anodized alumina membrane

Drain

Freshwater

applied to seawater desalination (Figure 3.16). As 3.5% sodium chloride solution was employed as model seawater, the salt content of the permeated water through the membranes was less than 0.01%, substantially reaching the level of drinkable freshwater. In 2017, Fujiwara further improved the membrane by using a visible lightresponsive dye, disperse red 1 (DR1), to replace azobenzene, which permitted solely visible light responsiveness [122]. By simultaneously grafting DR1 and blue 14 (DB14) on a PTFE membrane, the membrane was responsive to a wider range of light spectrum [123]. The flux of the double-dye-modified PTFE membrane was higher than the single-dye-modified PTFE membrane. 3.7.2

Spiropyran-Based Gating Membranes

Compared with azobenzene, spiropyran-based polymer chains exhibit greater volume change upon UV irradiation and thus are investigated more with intelligent gating membrane for controllable permeation of water [124], organic solvent [63, 125], protein molecules [43], and caffeine [59, 60].

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In the mid-1920s, Fisher and Hirshberg observed the photochromic characteristics and the reversible reaction of spiropyrans for the first time and thus set in motion the research on spiropyrans [126]. In 1994, Ito and coworkers pioneered a spiropyran-containing methacrylate and acrylamide-functionalized PTFE membrane, whose pore size and permeability for H2 O/CH3 OH were tuned by UV and visible light irradiation [63]. Four years later in 1998, the same group fabricated spiropyran-containing PMMA-grafted glass filter for controllable permeation of toluene liquid, which showed increased flux by UV irradiation and decreased flux under visible irradiation because the copolymer chains in toluene shrank under UV irradiation but swelled under visible irradiation [125]. In 2006, Belfort and coworkers prepared spiropyran-grafted PES UF membrane through a UV-induced graft polymerization method (Figure 3.17a). As-prepared membrane exhibited an optically reversible switching behavior due to the switchable conformation of spiropyrans in response to light (Figure 3.17b). The membrane had 17% lower permeation flux of phosphate-buffered saline (PBS) solution under the visible light as compared with UV light (254 nm) irradiation [43]. Very recently, Padeste and coworkers demonstrated a two-step approach to prepare a photo-responsive gating membrane: grafting PMAA polymer brushes onto porous PP membrane, followed by covalently attaching spiropyran moieties to the grafted PMMA polymer brushes with the lowest grafting level of PMMA Vinyl monomer

Visible light

UV300

UV254

UV254 Vis Radical sites Poly(ether sulfone) membrane (a)

Graft polymer (closed)

Graft polymer (open) NO2

UV N O

O

NO2

Vis

N +

NH O

–O

NH

NH O (b)

Spiropyran (closed)

NH O Merocyanine (open)

Figure 3.17 (a) Schematic of graft polymerization and the switch of spiropyran. (b) The chemical structure of the vinyl spiropyrans in two configurations as a function of UV–Vis irradiation. Source: Nayak et al. 2006 [43]. Reprinted with permission of John Wiley and Sons.

3.8 Metallic Ion Responsiveness

polymer brushes (17% ± 6%). The water flux of photo-responsive membrane decreased by 40% from 405 (visible light with 30 minutes) to 289 l m−2 h−1 (UV light with 30 seconds) [124].

3.8 Metallic Ion Responsiveness Since the crown ethers are able to recognize their matching ions, and amazingly, this selective ion capture process causes a positive and negative shift in the LCST of poly(NIPAM-co-AAB18 C6 ) and poly(NIPAM-co-AAB15 C5 ), respectively, this type of copolymers could be integrated into porous membranes for controllable “closed” or “open” pore switching, which has been employed in intelligent gating membranes for controllable separation [127–131]. In 1993, poly(NIPAM-co-AAB18 C6 ) was first synthesized [132], which, in response to K+ in aqueous solution, underwent conformation transition at 32 ∘ C. In 1999, Yamaguchi et al. reported Ba2+ -responsive gating membrane by grafting poly(NIPAM-co-AAB18 C6 ) onto PE membrane [133]. Two years later, the same group further demonstrated that the pore size of the membrane could be changed from 5 to 27 nm with varying Ba2+ concentrations between 0 and 0.014 M, leading to the tunable rejection of dextran molecules with size ranging between 2.7 and 27.2 nm by the membrane [129]. In 2013, Chu and coworkers developed an ion-responsive membrane that selectively detected and removed Pb2+ from wastewater [46]. Poly(NIPAM-coAAB18 C6 ) copolymer chains as functional gates were grafted onto Nylon-6 membrane substrate via a two-step method combining plasma-induced pore-filling grafting polymerization and chemical modification (Figure 3.18a–c). By simply tuning the operation temperature, the effective removal of Pb2+ and membrane regeneration could be realized (Figure 3.18d–i). It was found that, in the presence of 10−5 M Pb2+ , the pore size of the membrane decreased from 159 to 94 nm, leading to a reduced permeation as low as 0.35 g min−1 cm−2 . The phenomenon of an isothermal reduction of solution flux could be used as an informational signal for detection of trace Pb2+ ions in the environment. This gating membrane offers a promising opportunity in industrial and agricultural applications, such as online detection and timely treatment of trace Pb2+ ions in wastewater discharge, analysis for water quality, and remediation and protection of soil. The pendent 15-crown-5 moieties recognize and capture majorly K+ ions, which leads to a negative shift in the LCST values of the copolymers. Namely, once K+ ions are removed from the crown ether receptors, the copolymer chains swell and the functional gates close. Before 2008, the K+ -responsive gating membranes were dominantly constructed from poly(NIPAM-co-AAB18 C6 ) as discussed earlier [128–130, 133–137]. In 2008, poly(NIPAM-co-AAB15 C5 ) was developed as a new candidate material for ion-responsive intelligent membrane system [127]. Later, Chu et al. grafted poly(NIPAM-co-AAB15 C5 ) onto Nylon-6 membranes via plasma-induced pore-filling grafting polymerization and chemical modification and found that the grafted gates in the membrane pores spontaneously tuned the solution flux

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3 Filtration Membranes with Responsive Gates

(a)

(b)

(c) H2C CH x H2C CH y O C

O C NH

OH

H2C CH x H2C CH y O C

O C NH

NH

CH

CH

H3C CH3

H3C CH3

O O

O O

O O

(d)

T = T1 Pb2+ detection

(f)

(e)

T = T2

Pb2+ presence

Pb2+ separation

Swelling ratio

90

(h)

T = T1

Pb2+ removal

(g)

(i) T1 LCSTa T2 LCSTb T3 Temperature

T = T2

T = T3

Figure 3.18 Schematic illustration of the preparation process (a–c) and the proposed concept of the smart membrane with functional gates for detection and removal of trace Pb2+ ions (d–i). Source: Liu et al. 2013 [46]. Reprinted with permission of Royal Society of Chemistry.

in response to K+ ion in permeate. As shown in Figure 3.19, in the presence of 0.1 M K+ in permeate at 25 ∘ C, the flux increased to 245.4 kg m−2 h−1 (pore size ∼ 118 nm) in comparison with the pure water flux of 4.5 kg m−2 h−1 (pore size ∼ 43 nm) [138]. Other coexisting ions, such as Na+ , Ca2+ , and Mg2+ , did not impose any influence on the membrane flux.

3.9 Redox Responsiveness

Mn+:

300

K+

Na+

Ca2+

Mg2+

Water 0.1M Mn+ Water 0.1M Mn+ Water

0.1M Mn+

100 Pore diameter (nm)

J (kg m–2 h–1)

250 200 150 100 50 0

(a)

d = 118 nm

120

d

80 60

d = 58 nm d = 59 nm d = 58 nm d = 43 nm

40 20

0

60

120 180 240 Time (min)

300

0

360

(b)

Water

Na+

Ca2+

Mg2+

K+

Figure 3.19 (a) Isothermally dynamic changes in solution flux of the poly(NIPAM-co-AAB15 C5 )grafted membrane in pure water and aqueous solutions containing different metal ions and (b) the changes of estimated pore size of the grafted membrane in pure water and aqueous solutions containing different metal ions. Source: Liu et al. 2012 [138]. Reprinted with permission of John Wiley and Sons.

3.9 Redox Responsiveness Redox-sensitive groups such as dithienylethenes [139], ferrocene [140], pyridine [141], or disulfides [142] could be incorporated into polymer structures, initiating the redox reactions and thus resulting in swelling/shrinking conformation transition of the polymers. In the oxidized state, the functional molecules are ionized and water soluble and exhibit a swollen conformation due to their charges, while, in the reduced state, they are deionized and insoluble, corresponding to a shrunken conformation. The first redox-responsive gating membrane was reported in 1997 by Ito et al. [141], who fabricated a poly(3-carbamoyl-1-(p-vinylbenzyl)pyridinium chloride) (PCVPC)-grafted PE/PTFE microporous membrane for controlling the water permeation. In the oxidized state, pyridine groups were ionized, and thus the grafted polymer chains stretched inside the membrane pores, leading to decreased water permeation. In 2014, Brunsen and coworkers reported a hybrid membrane with redoxcontrolled gates by combining a mesoporous silica template and two different redox-responsive ferrocene-containing polymers, polyvinylferrocene (PVFc) and poly(2-(methacryloyloxy)ethyl ferrocenecarboxylate) (PFcMA), via grafting-to and grafting-from method, respectively [143]. In the same year, Vancso and coworkers developed the polyelectrolyte membrane containing redox-active PFS-based poly(ionic liquid) (PIL) and PAA [144]. The porous membrane, in the presence of Fe(ClO4 )3 or ascorbic acid, showed significantly porous structure changes in the oxidized and reduced states, leading to a reversible switch of permeability. The average flow rate of pure water was

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0.10 0.08 Flux (ml cm–2 s–1)

92

0.06 0.04 Re

0.02

Ox 0.00 1

2

3

4

5

6

Number of cycles

Figure 3.20 The reversible switching of the flow of pure water for the oxidized and reduced porous membranes. Source: Zhang et al. 2014 [144]. Reprinted with permission of John Wiley and Sons.

0.092 ± 0.004 ml cm−2 s−1 in the oxidized state and 0.064 ± 0.005 ml cm−2 s−1 in the reduced states, respectively (Figure 3.20).

3.10 Ion Strength Responsiveness Among the categories of responsive gating membranes, the design of intelligent materials that respond to the nature and strength of the salt ions in their surrounding environment is promising as filtration membranes for water desalination and wastewater treatment. Zwitterionic polymers are popularly used as intelligent responsive gates due to the conformational changes of their structures on the pore surface depending on the strength of ions such as NaCl. As shown in Figure 3.21, the “open” and “close” of the membrane pore can be effectively controlled through changing the ion strength. At a low ion strength, responsive membrane possesses an open pore state owing to inter- and/or intrachain association of oppositely charged groups, leading to a collapsed chain conformation. Upon addition of sodium chloride salt, sodium and chloride ions disrupt these electrostatic interactions, and thus the membrane pore changes into closed state as the polymeric chains expand [145]. In 1984, Okahata et al. firstly reported the ion strength-responsive ultrathin nylon capsule membranes by using zwitterionic polymer [146]. In addition, Kang and coworkers synthesized a new graft zwitterionic copolymer, through the graft copolymerization of the zwitterionic N,N ′ -dimethyl(methylmethacryloyl ethyl)ammonium propanesulfonate (DMAPS), with PVDF backbone [147]. Then the responsive MF membranes were cast by phase inversion in aqueous media of different electrolyte concentration and temperature. The permeability of aqueous solutions through the PDMAPS-g-PVDF-based MF membranes

3.10 Ion Strength Responsiveness

Salt solution

Water

+

–

+

+ – +

+

– – –

+

Interchain association

–

+ – + – Intrachain association

“Open” pore

Na+ Cl– + – Na+ Cl– – +

– + Cl–Na+ – + – Cl – Na+ Cl – +

Cl– + Cl– +

Na+ – Na+ –

“Closed” pore

Figure 3.21 Schematic illustration of the conformational states of zwitterionic polymeric chains with increasing salt ion strength. Source: Zhao et al. 2016 [145]. Reprinted with permission of Elsevier.

exhibited a dependence on the strength of salt ion, which decreased with increasing the aqueous NaCl solution concentration from 10−7 to 10−1 mol l−1 . Furthermore, ion strength-responsive membranes also exhibited antifouling and self-cleaning properties in Meng’s research (2014). A commercial RO membrane was modified by zwitterionic polymer, poly(4-(2-sulfoethyl)-1-(4vinylbenzyl) pyridinium betaine) (PSVBP), and this sodium chloride-responsive membrane could reject the protein foulants with a 90% flux restoration by the cleaned membrane [148]. In 2016, Zhu and coworkers fabricated a block copolymeric blend membrane consisting of PES and amphiphilic PES-block-poly(sulfobetainemethacrylate) (PES-b-PSBMA) via non-solvent-induced phase separation process [145]. In addition, the water permeability and molecule sieving of such blend membranes could be tuned by changing the salt concentration in aqueous solution. The BSA rejection increased from 75% to 92% with the increasing NaCl concentration from 0 to 0.1 M. Poly(ionic liquids) (PILs) have also been applied in an ion strength-responsive gating membranes. In 2017, Zhao and coworkers fabricated a PILs-modified PES membrane via an in situ cross-linking copolymerization method [149]. The flux of the aqueous NaCl solution (0.6 M) through the membrane was 26 times higher than the flux of pure water through the membrane. Meanwhile, this membrane exhibited responsiveness toward both anion species, including PF6− , BF4− , and SCN− , and their strength. In 2018, Huang and coworkers constructed intelligent supramolecular mesoporous materials with salt ion strength-induced functional gates for

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selective adsorption and release of organic pollutants from water [150]. Hence, propeller-shaped aromatic amphiphiles that aggregated into hollow spheres were obtained by a self-assembly method. These pores from propeller-shaped aromatic assembly provide an excellent hydrophobic characteristic with the high surface area to remove organic micro-pollutants from wastewater. Correspondingly, the removal efficiency was 92% and 90% for ethinyl estradiol (EE) and bisphenol A (BPA), respectively. Additionally, as shown in Figure 3.22, the folded architecture of propeller tends to be flattened by the salt addition, resulting in a transition from porous to nonporous behavior. Accordingly, most of the removed pollutants (EE and BPA) are able to be released by the dynamic

OR

RO

N

N

OR

1

RO

N

OR

RO

RO

OR

OR

RO N

N N

OR

(a)

2

RO

N

OR

3

RO

R=

NaCl

(b)

Figure 3.22 (a) Molecular structures of 1, 2, and 3. (b) Schematic of porous and nonporous materials from conformation-tunable propeller assembly. Source: Xie et al. 2018 [150]. Reprinted with permission of John Wiley and Sons.

3.11 Dual and Multi-Stimuli Responsiveness

porous assembly, and subsequently, dialysis triggers the porous materials to be recovered.

3.11 Dual and Multi-Stimuli Responsiveness So far, the discussed intelligent gating membranes only respond to a single stimulus. Actually, there have been intelligent materials that can respond to more than one stimulus either simultaneously or independently. 3.11.1

pH and Temperature Dual Responsiveness

Among the multi-stimuli-responsive materials, the dual stimuli mode of pH and temperature has been investigated and introduced into intelligent gating membranes. Commonly, PNIPAM was utilized as the temperature-sensitive component of dual pH- and temperature-responsive membranes [38, 151–158]. In 2009, Yu et al. fabricated thermo- and pH-responsive PP microporous membrane, which was modified by PNIPAM and PAA via RAFT graft polymerization [38]. It was found that PAA and PNIPAM grafting chains exhibited both pHand temperature-dependent water flux, owing to pH responsivity around the pK a value of PAA (4.72) and temperature sensitivity between 20 and 55 ∘ C. Similarly, in 2010, the copolymer PAA-b-PNIPAM was grafted onto the surface of regenerated cellulose membranes [151]. In 2014, the copolymer microgels (PAA-b-PNIPAM) were blended with PVDF membrane [152], which acted as a sensor of temperature and pH to reversibly regulate the permeation of aqueous solution. Meanwhile, the membrane was superior due to its good fouling resistance for bovine serum albumin (BSA) and self-cleaning property (Figure 3.23). In a separate study, poly(N-vinylcaprolactam-co-acrylic acid) was modified onto PSF membranes to have pH- and temperature-induced separation of BSA and self-cleaning [159].

T > LCST or pH < pKa

T < LCST or pH > pKa

PVDF/PNA microgel membranes

PVDF/PNA microgel membranes

Poly(N-isopropylacrylamide) chain Poly(acylic acid) chain

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Poly(N-isopropylacrylamide-co-acrylic acid) (PNA) microgels H2O

Direction of water flow

Figure 3.23 Filtration mechanism of microgel membranes under different temperature and pH conditions. Source: Chen et al. 2014 [152]. Reprinted with permission of Elsevier.

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In 2013, Zhao and coworkers blended a terpolymer of poly(N-isopropylacrylamide-co-methacrylic acid-co-methyl methacrylate) (P(NIPAM-MAA-MMA)) with PES hollow fiber membranes [153]. The modified membranes exhibited pH sensitivity at the pH value between 7.0 and 10.0, while the thermoresponsiveness induced significant changes in pore sizes [154]. When pH was decreased from 10.0 to 2.0, the membrane flux increased by 70%, while the flux got enhanced by 150% with increasing temperature from 20 to 45 ∘ C. In 2017, Hu and coworkers synthesized a binary graft copolymer composed of PAA-b-PNIPAM and PMMA and grafted it on the surface of commercial PSf membrane to serve as thermo- and pH-responsive on/off switches [155]. The water flux of the membranes increased with increasing temperature from 20 to 50 ∘ C and decreased when the pH increased from 2.0 to 8.5. In 2018, Shen and coworkers prepared dual thermo/pH-responsive intelligent gating membranes by in situ assembly of stimuli-responsive poly(N-isopropylacrylaminde-co-methylacrylic acid) P(NIPAM-co-MAA) microgels as gates [160]. As presented in Figure 3.24, the intelligent gates would open as the microgels contract at 70 ∘ C or pH 3, since the NIPAM groups and MAA groups are shrunken and hydrophobic due to the formation of intermolecular hydrogen bonding between amide/carboxyl and carboxyl. On the contrary, the intelligent gates would close due to the expanding volume of microgels with decreasing temperature (30 ∘ C) or increasing pH (11). Correspondingly, the NIPAM groups and MAA groups become swollen and hydrophilic owing to the hydrogen bonding between amide groups and water molecules and the electrostatic repulsion between the protonated carboxylic groups. Moreover, such intelligent gating membranes showed self-cleaning property as the volume of microgels changed in response to pH stimulus and thus removed the contaminants adhered on the membrane surface. Another pH- and temperature-responsive polymer, polystyrene-blockpoly(N,N-dimethylaminoethyl methacrylate) (PS-b-PDMAEMA), also served as intelligent gates for dual stimuli-responsive MF and UF membranes via the non-solvent-induced phase separation process [156, 157]. In 2013, a nylon membrane was modified with PDMAEMA, PNIPAM, and their diblock brushes,

Microgel C H O N C

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Figure 3.24 The stimuli-responsive mechanism of intelligent gates and microgels in response to temperature and pH. Source: Liu et al. 2018 [160]. Reprinted with permission of Royal Society of Chemistry.

3.11 Dual and Multi-Stimuli Responsiveness

which exhibited tunable permeation flux in response to temperature between 30 and 35 ∘ C and pH between 6 and 8 [158]. In Abetz’s study, PS-b-P4VP membranes were first functionalized with polydopamine coating and then reacted with an amine-terminated PNIPAM-NH2 via Michael addition [20]. The modified membranes exhibited pH and thermo double sensitivities, which was proven by measuring the water flux under different temperature (30–35 ∘ C) and pH conditions (pH 3.8–3.4). Similarly, in 2017, Chu and coworkers designed a dual layer dual thermoand pH-responsive composite membrane, consisting of PS-b-P4VP copolymer and PS bended with PNIPAM nanogels as top layer and bottom layer of composite membranes, respectively [161]. Under different temperatures and pH values, the water fluxes of the composite membranes were changed by several orders of magnitude from 778.03 kg m−2 h−1 bar−1 (45 ∘ C, pH 6.8) to 1.13 kg m−2 h−1 bar−1 (20 ∘ C, pH 2.5), and the dual thermo- and pH-responsive permeation performances of the composite membranes were satisfactorily reversible and reproducible (Figure 3.25). 3.11.2

Temperature and Ion Strength Dual Responsiveness

In 2016, PNIPAM and the ion strength-responsive poly-N,N-dimethyl-Nmethacryloyloxyethyl-N-(3-sulfopropyl) ammonium betaine (PSPE) were grafted onto the pore walls of PET track-etched membranes by sequential surface-initiated ATRP process [162]. This membrane exhibited switchable modulations of pore sizes and had controlled UF permeability in response to temperature (between 25 and 40 ∘ C) and salt ion strength (KClO4 ). Similarly, in the same year, Meng and coworkers demonstrated ion strength and temperature dual responsive gating membrane, which was fabricated by a commercial PA-based RO membrane modified with PNIPAM, poly(4-(2-sulfoethyl)-1-(4-vinylbenzyl)pyridinium betain) (PSVBP), and poly(sulfobetaine methacrylate) (PSBMA) [163]. With CaCO3 as the foulant, the modified membranes possessed fouling resistance in the whole operations as long as 320 hours, while with BSA as the foulant, the antifouling performance existed in short term. 3.11.3

pH and Ion Strength Dual Responsiveness

Furthermore, cross-linked BSA-modified track-etched PC membranes was prepared for pH- and ion strength-responsive gating membrane [164]. This is because the repulsive electrostatic interactions between amine groups and carbonyl groups, and the attractive dipole electrostatic interactions among zwitterionic amino acid residue groups among BSA molecules, could be controlled by tuning pH values and ionic concentration in feed solutions. Figure 3.26a demonstrated reversible permeability as pH value was alternated between 2.0 and 10.0 in feed solutions, which changed from 1100 ± 20 to 1566 ± 20 l (m−2 h−1 MPa−1 ). Meanwhile, the water permeability was also controlled by the concentration of NaCl/CaCl2 in aqueous solutions (Figure 3.26b).

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Figure 3.25 (a) The water flux contour map of the composite membrane and four schemes near the four vertices of the contour map correspond to four switching states of the composite membrane at different pH values and temperatures. (b) Water fluxes of the composite membrane at four switching states. (c) Effect of trans-membrane pressure on the water flux of the composite membrane. Source: Ma et al. 2017 [161]. Reprinted with permission of American Chemical Society. 1800 pH = 10.0 1600

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Figure 3.26 (a) Reversible change of water permeability of the composite membrane under pH = 2.0 and 10.0 in feed solutions. (b) The effect of inorganic salt concentration in feed solution on water permeability of the composite membrane. Source: Zhao et al. 2013 [164]. Reprinted with permission of Elsevier.

3.12 Conclusions

3.11.4

Temperature, pH, and Ion Strength Multi-responsiveness

In 2013, Chu and coworkers developed PNIPAM-b-PMAA-based responsive gates in the membrane pores, which were able to respond to temperature, pH, salt ion strength, and anion species [32]. The pore size of membrane and the hydraulic permeability of buffer solution through membranes changed reversibly in response to environmental temperature from 25 to 40 ∘ C, pH stimuli (pH = 3 or pH = 8), sodium chloride concentration between 0.1 and 0.3 M, and changing the salt from sodium chloride to sodium sulfate in the buffer solution.

3.12 Conclusions In conclusion, valuable efforts have been made in making intelligent gating membranes for water treatment applications. The responsive pore size modulation has been the major focus in the past, while trigger-initiated responsiveness of other membrane performance parameters is emerging. Some of the challenges regarding the state of the art of the intelligent membranes are summarized as follows: (1) All of the intelligent membranes in the literature were proven; their utilities at bench scales with simplified testing conditions to provide proofs of concept and so far efforts in scaling up these membranes and challenging them with more realistic testing conditions are rare. Appropriate steps toward scaling up of intelligent gating materials need to realize real-life applicability and commercial viability in large-scale environmental applications [86]. (2) The size modulation of these intelligent membranes is far from being precise. This is so also partially due to the fact the current fabrication methods for filtration membranes largely lack molecular-level design [8], which limits the value of the intelligent gating in the overall improvement of membrane performances, especially selectivity. The precise pore size modulation of the intelligent membranes at nanometer or even sub-nanometer range would be a significant target in future development. (3) The responsiveness of the intelligent membranes is typically induced by changes in the bulk water chemistry, such as pH and temperature, which involves high chemical and/or energy consumption [86]. Generally, pH switching in a scaled-up operation is always associated with the cost of consumption of chemicals. Despite no external chemicals are required for temperature responsiveness, it is no doubt that heat exchangers of fluid (heat generation or cooling) require the cost of fuel. (4) The previous research on the intelligent membranes was dominantly focused on the chemistry and conformation changes of the responsive materials, and little attention was paid to the detailed mechanisms for mass transfer and separation within the intelligent gating membranes [12]. While important efforts have been made to utilize responsive chemistry to improve performance parameters of membranes other than pore size, their importance should be further strengthened.

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Intelligent membranes have the potential to make some difference in the following areas: (1) Membrane fouling is always a major challenge in all kinds of membrane-based separation, and it worsens along with increasing water flux. The self-initiated conformations or chemistry changes in response to changing environmental conditions by stimuli-responsive materials can be a good platform to design membranes with improved antifouling and fouling-resistant performance. Stimuli-responsive materials have been combined with and helped produce a number of filtration membranes with self-healing performance, and more efforts should be invested in this interesting topic. (2) Light, especially UV light, as a remote and clean trigger, has been used to induce performance adjustment of the intelligent membranes. However, direct utilization of solar light to induce the same performance has been rare. Solar light is the most renewable energy source, and thus the integration of photothermal component into conventional and intelligent membranes would result in more energy-efficient membrane separation with better performance. (3) Synergistically multifunctional and all-in-one membranes in the format of point-of-use devices can be a niche area for the intelligent membrane to thrive. Therefore, addressing these critical issues from design, scalability, efficiency, reproducibility, and self-cleaning, intelligent gating membranes would provide the promising potential for a paradigm shift in the field of water treatment.

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4 Switchable Wettability Materials for Controllable Oil/Water Separation 4.1 Oil Spill Treatment Oil is the major energy source in the world, and the oil-related industries play a critical role in the development of modern industry and economy. However, these oil-related products also pose potential environmental concerns owing to the risk of the spillage into the environment. Oily wastewater, stemming from industrial processes such as metallurgy, food, textile, leather, and petrochemical, has become a common pollution source all over the world [1, 2]. Oily wastewater is commonly produced in every major step of the lifetime of petroleum: exploration, transportation, storage, refining, application, and disposal. Moreover, accidental oil release and spill into the unintended environment, including sea, soil, groundwater, river, lake, etc., often occur, which would carry many toxic compounds into the ocean and thus threaten every species along with the marine food chain [3, 4]. Historical oil spill accidents have frequently occurred and never ended, from the 1967 Torrey Canyon oil spill to the latest 2018 East China Sea oil spill, which generated environmental and ecological impacts [5]. On 6 January 2018, an oil spill of 136 000 tons from an Iranian tanker that sank in the East China Sea exposed the marine life to the extremely toxic substance. As the International Tanker Owners Pollution Federation (ITOPF) estimated, between 1970 and 2016, approximately 5.73 million tons of oil were lost as a result of tanker incidents, and in the 2010s the average number of large oil spills reached at 1.7 per year [6]. Especially, these incidents of the heavy oil spill from the offshore oil field, bunker fuels used by ships and cargos, cause the death of sea birds and marine mammals and lead to long-term contamination of sediments [4]. The oily wastewater and oil spill, once produced, demand timely actions to separate oil out of generally bulk water since the dissolution of oil in water as well as spreading of the oil slick is a strong function of time under these scenarios. Thus, petroleum industry, environmental protection agencies, and even nongovernmental organizations (NGOs) have been investing heavily in technologies that can efficiently and effectively separate oil/water mixture [5]. The considerations in selecting treatment options include oil density, oil viscosity, surface tension, etc. The most types of spilled oil with environmental concerns are typically lighter than water with density between 0.90 and 0.98 g cm−3 , Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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although there are oil phases that are heavier than water (i.e. density > 1.0 g cm−3 ), including chlorinated solvents (e.g. carbon tetrachloride [CCl4 ], trichloroethylene [TCE], perchloroethylene [PCE], methylene chloride) that are legacy dense nonaqueous phase liquid (DNAPL) contaminants especially in groundwater in many places worldwide. The viscosity of the light oil is similar to that of water (0.89 mPa s−1 ), while crude oils can have a viscosity up to 100 mPa s−1 due to the high internal friction of long and complex heavy oil molecules. Furthermore, the surface tension of oil (20–30 mN m−1 ) is significantly lower than that of water (72.7 mN m−1 ), which makes oil able to spread out on low surface energy solid. Depending on the amount of the oil spilled or present and the timing of response, the traditional oil contamination treatment technologies include direct burning, physical collection by skimmer and pumping, physical confinement by floating boom, air flotation, gravity separation (e.g. centrifugation), microorganism-based oil digestion, and so forth [7–12]. Each technology has its own niche application scenario, out of which becomes ineffective. With the increasingly stringent environmental regulations, the development of more efficient and cost-effective oil/water separation approaches is imperative to improve the quality of the oil spill cleanup and of treated oily wastewater effluent. The last decade has experienced remarkable progress in fundamental understanding to special and superwettability, which has contributed significantly to oil/water separation [5, 13–15]. Given the background of the majority of the readership of this book, we feel obliged to provide a brief introduction to some essential concepts regarding surface wettability to help better understand and appreciate the content of this chapter.

4.2 Fundamentals of Special Wettability 4.2.1

Surface Wetting Properties

Wettability is considered as an intrinsic solid surface property to define the contact between a liquid and a solid surface and is commonly characterized by liquid droplet contact angle (CA). The surface wettability states of materials are defined based on apparent contact angels (Figure 4.1a), which are physically measured, not calculated. It is measured by a liquid droplet of a small volume sitting on a solid surface, and the whole system is immersed in a bulk phase, with air being the typical bulk phase in conventional wettability studies and water and even oil emerging as new bulk phases recently. The apparent CAs range from 0∘ to 180∘ [17]. Typically, a surface is considered hydrophilic if water apparent CA is lower than 90∘ , while hydrophobic if water CA is higher than 90∘ [18]. In general, a superhydrophobic surface is defined as the state with a very high water CA (>150∘ ) and a low contact angle hysteresis (CAH) or sliding angle (to be defined in Section 4.2.2) (less than 5∘ or 10∘ ) [19, 20]. On the other hand, a superhydrophilic surface possesses a maximum water CA of 5∘ as the upper limit,

4.2 Fundamentals of Special Wettability

Figure 4.1 (a) Definition of CA, 𝜃, based on a sessile liquid drop on a solid surface. (b) Advancing CA, 𝜃 A . (c) Receding CA, 𝜃 R ( 90∘ Water CA < 90∘ Oil CA > 90∘ Oil CA < 90∘

Water CA > 150∘ ; water SA < 10∘ Water CA < 5∘ within 0.5 s Oil CA > 150∘ ; oil SA < 10∘ Oil CA < 5∘ within 0.5 s

Underwater

Superoleophobicity Superoleophilicity Superaerophobicity Superaerophilicity

Oil CA > 150∘ ; SA < 10∘ Oil CA < 5∘ within 0.5 s Air bubble CA > 150∘ ; SA < 10∘ Air bubble CA < 5∘ within 0.5 s

Under oil

Superhydrophobicity Superhydrophilicity Superaerophobicity Superaerophilicity

Water CA > 150∘ ; SA < 10∘ Water CA < 5∘ within 0.5 s Air bubble CA > 150∘ ; SA < 10∘ Air bubble CA < 5∘ within 0.5 s

γLV γSV (a)

θ

γSL (b)

(c)

Figure 4.2 Schematic illustration of a droplet on the different substrates. (a) On a flat substrate and (b and c) on rough substrates. Depending on the structure of the surface, the droplet is either in the Wenzel model (b) or the Cassie–Baxter model (c).

the liquid–vapor–solid three-phase contact line. However, 𝜃 Y in Young’s equation is only for an ideally smooth (i.e. nontextured) and chemically homogeneous solid surface, which is rarely seen in reality [16]. To date, the highest intrinsic water CA of all materials is about 130∘ , measured from a closely packed and self-assembled monolayer with —CF3 functional groups with a surface energy of 6 mJ m−2 [36, 37]. In reality, most solid surfaces are not flat at all and they have a roughness of varying scales. The Young’s relation cannot explain a plethora of hydrophobic surfaces with CA higher than 150∘ (superhydrophobicity) or oleophobic surfaces with repelling oil liquids with very low surface tensions (superoleophobicity). The missing piece here is surface texture, and it is an indispensable signature of attaining special wetting properties in many biological and engineered surfaces [16]. In general, there exist two models: Wenzel and Cassie–Baxter models, both of which are well-known theories to help understand the wetting behaviors of

4.2 Fundamentals of Special Wettability

liquid on rough solid surfaces. In Wenzel state (Figure 4.2b), the liquid droplet is in contact with the entire solid surface and completely fills all voids in the rough surface under the liquid droplet. Wenzel model was proposed in 1936 (Eq. (4.2)): cos 𝜃W = r cos 𝜃Y

(4.2)

where 𝜃 W is the apparent CA in the Wenzel model and r is the surface roughness factor, defined as the ratio of real surface area to planar surface area (r > 1) [38]. The Wenzel model can explain the wettability when a droplet is pinned into a rough surface (Figure 4.2b) [38]. As a result, increasing roughness is able to amplify the intrinsic wettability of the solid surface. That is to say, when 𝜃 Y is less than 90∘ , 𝜃 W will be reduced by an increase in r, and when 𝜃 Y > 90∘ , 𝜃 W will be increased by an increase in roughness. Following this trend, if r ≫ 1, an extreme and special wettability will be obtained (CA > 150∘ or 90∘ ) at the solid/water/oil interface and thus prefer to be strongly hydrated underwater. On the other hand, an oleophilic surface at a solid/water/oil interface can be created with hydrophobicity, and an oleophilicity at a solid/air/oil interface. For the same reason as discussed previously, surface roughness enhances intrinsic under-liquid wettability of a surface.

4.3 Special Wettable Materials for Oil/Water Separation Guided by the principles discussed above, the materials with superwetting states are generally made by combining proper surface roughness and intrinsic surface chemistry. These advanced interface materials with superwetting properties emerge in a wide range of research directions, such as antifouling, self-cleaning, anti-icing, water collection, liquid transfer, corrosion resistance, oil/water separation, etc. [44, 45]. Especially, superhydrophobic/superoleophilic or superhydrophilic/superoleophobic interfacial materials possess an exciting performance for selectively attracting or repelling oil or water. With high separating speed and efficiency as well as reusability, such special wetting separation exhibits great advantages over conventional oily water treatment [13, 46]. It is worth pointing out that the wettability-based oil/water separation has its own limitations and is not meant to compete with, but complementary to, most of the conventional processes. The conventional processes generally work satisfactorily as the first-step treatment of high oil content mixtures while the wettability-based separation can be a beneficial follow-up step after many conventional processes when the composition of the treated mixtures is simpler and more amicable. Typically, the wettability-based oil/water separation systems work in two major ways: filtration-based separation by using modified mesh, textile, membrane, etc. and adsorption-based oil capture by using modified foam, sponge, etc. [5, 13, 15, 33]. Two milestones in the field of superwetting materials for oil/water separation are both from Professor Lei Jiang’s group, which are classified as “oil-removing” and “water-removing” processes. In 2004, Jiang and coworkers pioneered the use of a superhydrophobic and superoleophilic mesh for gravity-driven oil/water separation by filtration [47], which permitted oil and repelled water, namely, oil-removing type [48–52]. However, the membranes’ oleophilicity led to their inevitable fouling and blockage by heavy oil. Inspired by some underwater superoleophobic biomaterials [42], in 2011, Jiang and coworkers further developed a superhydrophilic and underwater superoleophobic hydrogel-coated mesh [53], which, when wetted, percolated only water and repelled oil, namely, water-removing mode (Figure 4.3). The

4.3 Special Wettable Materials for Oil/Water Separation

Crude oil/ water mixture Oil The coated mesh

(a)

Water

(b) Intrusion pressure (kPa)

100.0

99.5

99.0

2.0 1.5 1.0 0.5

(d)

er eth um

He xa ne

Pe tro le

oil

il eta

ble

eo ud

se l

Ve g

Cr

Die

oli ne

er eth um

Pe tro le

ble

oil He xa ne

il eo ud

eta Ve g

se l

Cr

Die

oli Ga s

(c)

2.5

0.0

ne

98.5

Theoretical value Experimental value

3.0

Ga s

Separation efficiency (%)

3.5

Figure 4.3 Oil/water separation systems of the PAM hydrogel-coated mesh. (a) The coated mesh was fixed between two glass tubes, and then the mixture of crude oil and water was put into the upper glass tube. (b) Only water permeated through the coated mesh, while crude oil was repelled and remained in the upper glass tube. (c) The separation efficiency of the coated mesh for a selection of oils. (d) The theoretical and experimental values of intrusion pressures for various oils. Source: Xue et al. 2011 [53]. Reprinted with permission of John Wiley and Sons.

water-removing membranes eliminate the possibility of membrane clogging by viscous oil and thus overcome the oil fouling problem existing in the oil-removing mode. In addition, they avoid the formation of a water barrier between the oil phase and the interfacial materials and achieve natural gravity-driven oil/water separation given the fact that water is generally heavier than oil. Even since this work, various hydrophilic and underwater superoleophobic membrane materials have been fabricated by following similar scientific principle [54–63]. Furthermore, oil/water mixtures typically exist in three forms, for example, free oil with droplets with a diameter larger than 150 μm, dispersed oil of 20–150 μm, and emulsified oil with dispersed phase less than 20 μm [64]. Besides the free oil droplets we mentioned above, the emulsified oil/water mixtures, especially surfactant-stabilized oil-in-water or water-in-oil emulsions, are more difficult to be separated effectively, which is still a big challenge. Recently, the porous materials with special wettability have been explored for oil/water and water/oil emulsion separation with the emergences of microfiltration, ultrafiltration, and nanofiltration membranes [63, 65–67]. In general, the

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emulsion separation mechanism relies on the “wetting effect” and “sieve effect” [56, 57, 68]. Namely, it is needed that special wettable filtration/sponge system have valid pore size to match the emulsion droplet size for optimum separation of emulsions. As classified by special wettability, the recent emulsion separation materials with special wettability include superhydrophobic and superoleophilic materials for water-in-oil emulsions [69–72], superhydrophilic and underwater superoleophobic materials for oil-in-water emulsions [57, 62, 73], and superhydrophilic/superoleophobic [74] and underwater superoleophobic/under-oil superhydrophobic materials [56] for both oil-in-water and water-in-oil emulsions due to their intrinsic properties.

4.4 Switchable Oil/Water Separation In the past few years, considerable progress has been made in making materials with switchable wettability, especially switchable superwettability, toward oil/water separation [5, 13, 14]. Compared with conventional materials with nonresponsive and prefixed wettability, these materials switch their wettability between two opposite sides in response to external triggers and thus can be considered having intelligence (Figure 4.4). These intelligent oil/water separation materials can operate in either oil-removing mode or water-removing mode, suitable for oil removal from wastewater with oil density either higher or lower than water, and thus possess superiority in developing versatile separation process, easy recycling and regeneration, and enhanced antifouling performance, all of which would potentially lead to reduced operation cost toward oil contamination treatment. To obtain a switchable wettability, especially between superoleophobicity and superoleophilicity in aqueous media, the surface chemical composition should be delicately designed such that it comprises both hydrophilic and oleophilic/hydrophobic characteristics, with either characteristic becoming dominantly exposed over the other in response to external stimuli or some chemicals treatment. The demonstration of the switchable oil/water separation was first reported by Wang’s and Tuteja’s separate works in 2012 [74, 76]. Wang and coworkers Superhydrophilicity/ underwater superoleophobicity

Superhydrophobicity/superoleophilicity Triggers

Oil-removing mode

Temperature pH Light

Water-removing mode

Figure 4.4 Scheme of intelligent materials with stimuli-responsive and switchable wettability for controllable oil/water separation. Source: Chang et al. 2018 [75]. Reprinted with permission of Royal Society of Chemistry.

4.6 Temperature Responsiveness

prepared an intelligent surface with switchable superoleophilicity and superoleophobicity in aqueous media by grafting a pH-responsive block copolymer onto nonwoven textiles and polyurethane (PU) sponges, thus realizing controllable oil/water separation and capture/release of oil droplets [76]. In the same year, Tuteja and coworkers developed a hygro-responsive membrane with both superhydrophilicity and superoleophobicity in the air and underwater. This membrane was effectively applied for gravity-driven separation of oil/water mixtures [74]. So far, various external stimuli such as pH, temperature, electric potential, light, solvent, and gas have been developed for controllable surface wettability switch [77–81].

4.5 Surface Chemistry Behind Stimuli-Responsive and Switchable Wettability When it comes to designing an intelligent surface with switchable oil or water wettability, surface roughness (i.e. micro/nanostructure) design is not as important as surface chemistry design. Mostly, these stimuli-responsive surfaces can switch between hydrophobic (underwater oleophilic) and hydrophilic (underwater oleophobic) states and thus are favorable for tunable oil/water separation. Generally speaking, the chemical components for building responsiveness into surface wettability transitions are largely organic materials and more specifically polymeric materials due to their reversible conformation changes and polarity transition in response to environmental stimuli [80]. The chemical configuration transitions of polymer chains can induce the reorientation of polar functional groups, change surface free energy, and thus modulate adhesive forces between the surface and liquid phases in question. The chemical mechanism behind the responsiveness of typical and relevant polymeric materials to environmental stimuli, including heat, pH, light, etc., has been discussed in Chapter 2. Among the diverse wettability switching stimuli, temperature, pH, and light have received the most attentions in oil/water separation, while other unconventional conditions, such as solvent, ion, gas, and electric field, are emerging. Nevertheless, a variety of inorganic oxide materials are used as stimuli-sensitive materials, especially photo-responsive semiconductor metal oxides that have been discussed in Chapter 2. Despite various organic compounds, inorganic oxides offer clear advantages over organic molecules in terms of structural and photochemical stability, low toxicity, and larger wettability changes in some cases. While it is expected that a composite organic/inorganic material-based stimuliresponsive wettability possesses certain advantages, the example of such is seldom seen in literature. This chapter reviews the recent development in polymeric and photocatalytic inorganic materials with stimuli-responsive and switchable wettability for controllable and on-demand oil/water separation.

4.6 Temperature Responsiveness The thermal response has received great attentions in artificial intelligent systems owing to its facile operation and free of chemical addition [82–84]. Among the

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thermoresponsive polymers, PNIPAM is the most widely applied one in this field of switchable and controllable oil/water separation [85–92]. In 2004, Jiang and coworkers [86] first reported the switchable wettability transition on the PNIPAM brush grafted surface. The water CA of the PNIPAM-modified flat surface changed from 63∘ at 25 ∘ C to 93∘ at 40 ∘ C. In 2010, Chen and coworkers prepared a thermoresponsive PNIPAM hydrogel, which can reversibly control the wettability and adhesion to oil at water/solid interface [93]. The hydrogel surface became superoleophobic and low adhesive at 23 ∘ C but oleophobic and high adhesive at 40 ∘ C. In 2013, the temperature-controlled wettability transition was first applied to oil/water separation by Jiang and coworkers who employed PMMA-b-PNIPAM copolymer-coated steel mesh as a separation membrane [87]. This material reversibly switched its wettability in response to temperature change. Below the LCST, the modified mesh membrane was hydrophilic and thus was the water-removing mode, while above the LCST, it switched to oil-removing mode because of its hydrophobicity. The water CA switched between 42∘ and 107∘ , and oil CA switched between 137∘ and 36∘ , in response to temperature change between 10 and 40 ∘ C (Figure 4.5). Below the LCST (20∼25 ∘ C), the membrane was water-removing type, while above the LCST, it became an oil-removing type. In 2016, Wang and coworkers deposited PNIPAM hydrogel into an elastic PU microfiber web structure and obtained a highly flexible, mechanically tough, and thermoresponsive oil/water separation membrane. This composite membrane exhibited a 1 wt% oil-in-water emulsion (at 25 ∘ C) and 1 wt% water-in-oil emulsion (at 45 ∘ C) separation efficiency of >99% [92]. In 2015 and 2016, electrospinning methods were reported by Xin and coworkers [88] and Luo and coworkers [85] to make PNIPAM-based nanofibrous membranes for thermoresponsive and switchable oil/water separation. PNIPAM was grafted to an electrospun regenerated cellulose nanofibrous membrane via the surface-initiated atom transfer radical polymerization (SI-ATRP) method, which exhibited temperature-responsive surface wettability and controllable separation of oil/water mixture [88]. A PMMA-b-PNIPAM-based electrospun fibrous membrane was prepared for controllable water/oil separation with high efficiency (>98%) and high fluxes (∼9400 l h−1 m−2 for water and ∼4200 l h−1 m−2 for oil) through in situ temperature regulation [85]. In 2015, Zhou et al. modified stainless metal meshes with poly(2,2,3,4,4,4-hexafluorobutyl methacrylate)-block-poly(N-isopropylacrylamide) via dip-coating method [94]. The on-demand separation of water/hexane mixtures had high penetration fluxes of 2.50 l s−1 m−2 for water and 2.78 l s−1 m−2 for hexane through adjustment of the temperature while exhibiting a good separation efficiency above 98%. Besides, Jiang and coworkers reported an underwater thermoresponsive surface by grafting underwater oleophilic heptadecafluorodecyltrimethoxysilane and PNIPAM [89]. It exhibited a low adhesive underwater superoleophobicity at 20 ∘ C and high adhesive underwater superoleophobicity at 60 ∘ C. However, directly changing the temperature of a bulk oil/water mixture during continuous separation is not trivial and more importantly considered as energy inefficient given the fact that only a local heat is needed to induce

Contact angle (°)

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>LCST

99%). In addition, it showed desired antifouling and recyclability properties. In addition to filtration-based oil/water separation, in 2017, Wang and coworkers grafted octadecyltrichlorosilane (OTS) and PNIPAM onto the surface of melamine sponge skeletons and prepared a thermoresponsive sponge with reversible superwettability between superhydrophilicity and superhydrophobicity (Figure 4.7a,b) [91]. The membrane, when placed on an oil-spilled water zone, absorbed oil at water temperature at 37 ∘ C and released the absorbed oil at 20 ∘ C (Figure 4.7c). Rapid cleanup of heavy oil spills is always considered as a great challenge because the conventional porous oil sorbents cannot efficiently remove them due to the high viscosity of the oil. Compared with light oils, heavy oils are more viscous (103 –105 mPa s at room temperature) and are thus more difficult to clean up. The high viscosity of heavy oils prevents them to diffuse into the inner pores of sorbents, leading to an ineffective oil capture. An effective way to reduce the viscosity of heavy oils is by increasing their temperature. Noteworthy are recent developments in applying external energy to oil/water separation to improve separation performance. Yu and coworkers designed and prepared a joule-heated sponge for fast cleanup of the viscous crude oil spill, by utilizing electricity to generate heat [95]. The heat being generated

4.6 Temperature Responsiveness

120 90 0°

60

150°

30 0 15

(a)

20

25 30 35 40 Temperature (°)

Water contact angle (°)

Water contact angle (°)

180 150

150

45°C

120

45 (b)

90 60 30 0

25°C 0 2 4 6 8 10 12 14 16 18 20 Cycle

(c)

Figure 4.7 (a) The water contact angle of an OTS/PNIPAM modified sponge switched between 0∘ and 150∘ at temperatures of 25 and 40 ∘ C. (b) The switch between superhydrophilicity and superhydrophobicity was reversible for more than 20 cycles. (c) Fast oil (dichloromethane dyed with Sudan I) absorption at 37 ∘ C (upper) and slow oil desorption at 20 ∘ C of the OTS/PNIPAAM modified sponge (bottom). Source: Lei et al. 2017 [91]. Reprinted with permission of American Chemical Society.

by other energy sources, especially solar light, would have a place to provide assistance to the wettability-based separation of oil/water mixtures deemed difficult otherwise, such as crude oil spill cleanup. In 2018, Wang’s group fabricated a photothermal-assisted solar-driven heavy oil removal, which is promising for highly viscous crude oil spill cleanup [96]. In the same year, Wang and coworkers further integrated the solar-assisted light-to-heat conversion effect of polypyrrole (PPy) and thermoresponsive property of PNIPAM into the melamine sponge [97]. Utilizing the photothermal effect of PPy, PNIPAM/PPy modified sponges locally heated up contacting heavy oil under solar irradiation and significantly reduced its viscosity with the aim at the oil voluntarily flowing into the pores of the sponge. As a result, 2.05 g heavy oil with viscosity as high as 1.60 × 105 mPa was able to be rapidly absorbed under light irradiation for 23 minutes (Figure 4.8), while the sorbed oil was passively forced out the sponge underwater at room temperature due to the hydrophilicity of PNIPAM.

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

Light source Sponge

Heavy oil Water

Weight (g)

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Weight sensor Electronic balance

(a)

2.5 2.0 1.5 1.0 0.5 0.0

Seperating from oil

Touching oil surface

0 (b)

Δm = 2.05 g Light on

Δm = 0.35 g

10

20

30

40

50

60

70

Time (min)

Figure 4.8 (a) Schematic illustration of the lab-made device for the measurement of the heavy oil absorption. (b) The stage-wise process of the heavy oil absorption with original melamine sponge (red line) and PNIPAM/PPy modified sponges (black line) under one sun irradiation. The insets (right) are the digital images of PNIPAM/PPy modified sponges and melamine sponge after bitumen absorption test, respectively. Source: Wu et al. 2018 [97]. Reprinted with permission of John Wiley and Sons.

Without exception, only PNIPAM or PNIPAM-based copolymers have been used for thermoresponsive oil/water separation. It is believed that in the future other novel temperature-responsive polymers would be developed in this regard [98, 99].

4.7 pH Responsiveness As one of the most attractive stimuli, pH-induced reversible special wettability has been widely investigated owing to its easy operation, rapid responsiveness, and no need for special facilities. Overall, polyelectrolytes with weak acidic or weak basic groups, typically involving carboxyl, pyridine, and tertiary amine groups, are the most commonly used pH-responsive polymers for switchable wettability surfaces. These functional groups are able to accept or donate protons with switchable conformation changes upon the pH changes [100–102]. Among all pH-responsive polymers, PMAA [103], poly(2-vinylpyridine)-bpolydimethylsiloxane (P2VP-b-PDMS) [76, 104], 11-mercaptoundecanoic acid (SH(CH2 )10 COOH) [105–109], poly(methyl methacrylate)-block-poly(4-vinylpyridine) (PMMA-b-P4VP) [110], poly(vinylidene fluoride)-graft-poly(acrylic acid) (PVDF-g-PAA) [111], PDMAEMA [112, 113], etc. have been employed in pH-responsive and switchable wettability for oil/water separation. To be convenient, we classified pH-responsive oil/water separation materials into the pyridine-based system, carboxyl-based system, and tertiary amine-based system. 4.7.1

Pyridine-Based System

Pyridine-based polymers, such as P4VP and P2VP, with pK a of approximately 3.5–4.5, have been the most widely used pH-responsive polymers in the

4.7 pH Responsiveness

(a) Water pH 6.5

Oil Water

Oil

pH 2

pH down b m

Si O

pH up n

N

Deprotonated–Oleophilic

b m + NH

Si O

n

Protonated–Oleophilic

(b)

Water

Oil

Oil

Water

Figure 4.9 (a) Schematic diagrams for the controllable oil wettability of the P2VP-b-PDMS grafted surface in response to pH 6.5 and 2. (b) Dry P2VP-b-PDMS-modified textile selectively permeated oil (left), while it only selectively permeated water (right) once the membrane was wetted with acidic water (pH = 2.0). Source: Zhang et al. 2012 [76]. Reprinted with permission of Springer Nature.

application of oil/water separation [114–117]. In 2012, Wang and coworkers, for the first time, revealed a surface with switchable underwater super-oil wettability for highly controllable oil/water separation. The surface was fabricated by grafting the rationally selected copolymer comprising pH-responsive block, P2VP, and oleophilic/hydrophobic PDMS block, onto many commonly available substrates [76]. P2VP block altered its conformation and surface wettability in response to pH changes (from 6.5 to 2), while oleophilic PDMS block on the surface provided controllable and switchable access by oil (Figure 4.9a). The surface modification strategy and oil/water separation selectivity and efficacy were successfully demonstrated via filtration-based oil/water separation systems (Figure 4.9b) and sponge-based oil capture. This surface is the first of its kind that can switch its superoleophilicity and underwater superoleophobicity only under room temperature and without any organic solvent involved. In 2016, the same chemical approach was applied to producing electrospun PDMS-b-P4VP fibers for pH-responsive oil/water separation [104].

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

m

O

b

Syringe pump 5 μm

n

Fiber

O

V

N pH-responsive PMMA-b-P4VP

Steel mesh

100 μm

Collector

Oil

Water Wetted with pH = 3 water Rinsed with pH = 7 water As-prepared membrane

Figure 4.10 The fabrication, morphology, and pH-responsive wettability of PMMA-b-P4VP fibrous membrane. Source: Li et al. 2015 [110]. Reprinted with permission of American Chemical Society.

Similarly, Zhu et al. reported a 3D porous graphene foam with pH-switchable oil wetting property by surface modification using a block of P2VP and polyhexadecyl acrylate (P2VP-b-PHA) [118]. Such foam can effectively absorb oil or organic solvents from the aqueous media at pH of 7.0 and release the adsorbates when the pH is switched to 3.0. Besides grafting pyridine-based block copolymers onto substrates, some efforts were made to engineer these polymers directly into macroscopic structures. For instance, in 2015, Li et al. prepared PMMA-b-P4VP fibrous membranes via an electrospinning process and applied them to pH-controllable oil/water separation (Figure 4.10) [110]. Later, the same group also prepared pH-responsive PDMS-b-P4VP/SiO2 nanoparticle (NP) hybrid fibrous membranes by electrospinning method [104]. The electrospun composite fibrous membrane with 1.0 wt% SiO2 loading exhibited a pH-switchable wettability and separation performance with approximately 9000 l h−1 m−2 for hexane and 32 000 l h−1 m−2 under pH 4 water. Recently, P4VP-grafted melamine sponge was fabricated for intelligent oil absorption and desorption, which was reported by Lei et al. [119]. The as-prepared sponge exhibited underwater superhydrophilic/oleophobic property at pH = 1.0 and hydrophobic/superoleophilic property in neutral solution at pH = 7.0. Correspondingly, oil was absorbed with pH 7 and released underwater at pH = 1.0 without leaving behind any residues in the sponge. Similarly, PDMS sponge was combined with picolinic acid (or 2-pyridinecarboxylic acid), which showed a decrease in water CAs from 138∘ (pH = 7.0) to 10∘ (pH = 2.0) but an increase in underwater oil CAs from 0∘ (pH = 7.0) to 140∘ (pH = 2.0) [120]. The switchable surface wettability endowed the sponge with pH-triggered oil capture and release in aqueous media.

4.7 pH Responsiveness

4.7.2

Carboxyl-Based System

In contrast to the acid-induced swollen pyridine group, the carboxylic group is an alkali-induced swollen group. PAA and PMAA, with pK a of around 4–6 as the commonly reported pH-responsive polyacids [121, 122], are popularly applicable for pH-responsive oil/water separation. In 2014, Shi and coworkers developed a pH-responsive material to realize continuous in situ separation of light oil (hexane)/water/dense oil (dichloromethane) ternary mixtures [106]. Such pH-responsive device was fabricated by modifying 1-decanethiol (SH(CH2 )9 CH3 ) and SH(CH2 )10 COOH mixture on porous copper foam. Based on the density difference, the device separated three liquid phases sequentially, although requiring an alkaline aqueous phase in the mixture. In air, the superhydrophobic/superoleophilic surface of the device allowed dichloromethane to permeate through but repelling water. After an alkaline treatment, the device surface became superhydrophilic and underwater superoleophobic to prevent the passage of hexane while allowing water to penetrate. Since then, the modification of weak polyacid with the carboxylic group has been followed by a number of groups in switchable oil/water separation [105, 107, 108, 123]. For example, Sun et al. assembled the mixed responsive thiol molecules on the surface of Cu(OH)2 nanorod-covered copper mesh and showed an on-demand separation of both the immiscible oil/water mixture and oil-in-water emulsions triggered by pH changes [107, 108, 123]. In 2017, Liu et al. fabricated a pH-responsive copper foam by modification with HS(CH2 )11 CH3 and HS(CH2 )10 COOH for controllable oil/water separation as well [124]. In 2017, Cheng and coworkers prepared multifunctional Janus membranes (JMs) [125] by adding magnetic Fe3 O4 NPs or modifying the pH-responsive molecules (carboxyl-terminated mercaptan, 11-mercaptoundecanoic acid) during the coating process (Figure 4.11). Such membranes can achieve integrated magnetic-driven water droplet collection, in situ decontamination of wastewater, and responsive gating and release of water droplets by adjusting the pH value of the external water. In 2015, Luo and coworkers prepared poly(2,2,3,4,4,4-hexafluorobutyl methacrylate)-block-poly(acrylic acid) (PHFBMA-b-PAA) block copolymers [126]. The obtained three block copolymers was coated onto the stainless steel meshes and showed a pH-responsive wetting behavior based on incorporating the PAA block. However, the separation performance was not influenced significantly by switching pH owing to the abundant carboxyl groups along the chains. In 2017, PVDF-g-PAA were also fabricated into a treelike nanofiber membrane by electrospinning technology, and its pH-triggered oil/water separation was demonstrated [111]. In order to efficiently separate oil/water emulsions and improve antifouling properties, in 2017, Wang and coworkers designed a core–shell fiber-constructed pH-responsive electrospun nanofibrous hydrogel membrane. Such membrane was prepared by a uniaxial electrospinning method, in which PMAA was used

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or Water

Hydrophobic Hydrophilic

PDA/PEI coatings

PDA/PEI-coated Fe3O4

pH-responsive coating

Janus boat + waste water

Janus boat + purified water

Figure 4.11 Schematic illustration of the preparation and operations of a Janus membrane (JM), magnetic Janus membrane (MJM), and pH-responsive Janus membrane (pH-rJM). These Janus membranes were prepared via a coating and peeling method using a commercial PET/PTFE composite membrane (PDA, polydopamine; PEI, polyethyleneimine). Source: Wang et al. 2017 [125]. Reprinted with permission of Royal Society of Chemistry.

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Figure 4.12 Results of the separation of SFE and SDS-SE with different pH values. (a) Separation fluxes and (b) oil concentrations in the corresponding filtrates. Source: Zang et al. 2017 [127]. Reprinted with permission of Royal Society of Chemistry.

as a hydrophilic shell and cellulose acetate (CA) acted as a supporting core to construct functional nanofibers [127]. As-designed nanofibrous membrane possessed superhydrophilic and underwater superoleophobic properties and can effectively separate oil-in-water emulsions in acid, neutral or alkali environments, driven solely by gravity. Taking advantage of the pH-triggered changes in the micromorphology of the nanofibers and the pore sizes of the membranes, the nanofibrous membrane can effectively adjust the permeating flux without affecting separation efficiency of more than 99%, varying from 17 (pH = 3) to 1019 l m−2 h−1 (pH = 11) (Figure 4.12a). Meanwhile, under different pH values, oil contents in the filtrates of both surfactant-free emulsions (SFE) and SDS-stabilized emulsions (SDS-SE) remained below 10 and 45 ppm, respectively (Figure 4.12b). 4.7.3

Tertiary Amine-Based pH-Responsive Systems

PDMAEMA (pK a ∼ 7.0) and PDEAEMA (pK a ∼ 7.3) containing tertiary amine groups are another commonly used pH-responsive weak polybase [128–132]. In 2015, Xue et al. constructed PDMAEMA hydrogel-functionalized pH-responsive PVDF membrane using a combination of in situ polymerization and conventional phase separation [133]. With the help of the deprotonated or protonated tertiary amine groups in PDMAEMA in pure (pH 7.4) or acidic water (pH 2.0), the wettability of the membrane could be modified, and thus the membrane exhibited pH-induced separation of surfactant-stabilized water-in-toluene and toluene-in-water emulsions with high flux (3300 and 1600 l m−2 h−1 , respectively), separation efficiency (above 99.9%), and antifouling capability. Similarly, in 2016, Liu and coworkers modified copolymer poly(dodeyl methacrylate-co-3-trimethoxysilylpropyl methacrylate-co-2-dimethylaminoethyl methacrylate) (PDMA-co-PTMSPMA-co-PDMAEMA) together with silica on the different substrates including cotton fabric, filter paper, and PU foam via dipcoating method [134]. The coated materials exhibited a pH-induced superwettability switch in water and realized a continuous separation of oil/acidic water/oil three-phase mixtures, different surfactant-stabilized emulsion (oil-in-water,

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

(a)

(b) Hexane Water pH = 2 Dichloromethane

After 5 min

Figure 4.13 (a) Continuous separation process of hexane (red)/acidic water (blue)/dichloromethane three-phase systems by the coated cotton fabric. (b) Photograph of absorbing oil process by coated PU foam (upper), repelling oil process by acid-treated PU foam (middle), and releasing oil process in aqueous acidic media (bottom). Source: Danget al. 2016 [134]. Reprinted with permission of American Chemical Society.

water-in-oil, and oil-in-acidic water), and oil uptake and release through in situ and ex situ pH change. As shown in Figure 4.13a, the copolymer/silica-coated cotton fabrics could separate a mixture of hexane, water (pH = 2), and CH2 Cl2 that were poured into the tube. The bottom oil (CH2 Cl2 ) first penetrated through the fabric owing to its superhydrophobicity/superoleophilicity. Then the retained acidic water in situ acidified the fabrics after several minutes, and the fabrics became superhydrophilic, allowing water to permeate through the fabrics. After that, the remaining upmost layer of hexane was collected in the glass tube. On the other hand, the copolymer/SiO2 -coated PU foam could absorb CHCl3 without any residues left behind (Figure 4.13b). With ex situ acid treatment, the surface of the foam changed to be superhydrophilic, and no oil was absorbed. When the oil-loaded PU foam was acidified, water could occupy the pores of the foam, and the oil could be released from PU foam. In 2017, Luo and coworkers synthesized triblock copolymer, polydimethylsiloxane-block-poly(2-hydroxyethyl methacrylate)-block-poly(2-(dimethylamino) ethyl methacrylate) (PDMS-b-PHEMA-b-PDMAEMA) [135], and Zhang and coworkers prepared the quaternary ammonium salts (QAS)-functionalized fluorinated copolymer containing PDMAEMA segments [136]. Both copolymers were functionalized onto the stainless steel mesh, PU sponge, or cotton fabric, which realized controllable separation of water/oil mixtures and reversible oil capture and release in aqueous medium.

4.8 Photo-responsiveness Unlike thermal and pH responses that require the change of the properties of bulk water, photo-responsive wettability is designed to efficiently target only the functionalized separation membrane and has its advantage of being contactless and remote. The materials with photo-responsive and switchable wettability that can be used in oil/water separation can be classified into two types: inorganic and organic materials.

4.8 Photo-responsiveness

4.8.1

Inorganic Photo-responsive Materials

Among the inorganic photo-responsive materials with switchable wettability, ZnO and TiO2 , both responsive only to UV, have been widely investigated and applied in controllable oil/water separation [137–140]. In 2012, Zhai and coworkers demonstrated photo-triggered switchable oil/water separation on aligned ZnO nanorod array-coated mesh (Figure 4.14a), which switched to superhydrophilicity and underwater superoleophobicity under UV irradiation and returned to superhydrophobicity after being stored in darkness for seven days (Figure 4.14b,c) [141]. Later, ZnO-based photo-responsive oil/water separation devices were facilely fabricated by spraying method to modify ZnO NPs/PU mixtures on stainless steel mesh [142] and by a one-step thermal evaporation method to synthesize aligned ZnO array onto stainless steel mesh [143]. In both cases the materials were able to switch between oil-removing and water-removing modes in response to light illumination.

10 μm

1 μm

(a) Air

Air

Air UV Water

Dark

Water

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Air

Water

UV Oil

Dark

Oil

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(c)

Figure 4.14 (a) SEM images of the aligned ZnO nanorod array-coated mesh films. (b) Photographs of a water droplet on the coated mesh film with hydrophobic surface after dark storage (left), with hydrophilic surface under UV irradiation (middle) in air, and with water passing through the hydrophilic mesh film (right). (c) Photographs of an oil droplet (1,2-dichloroethane) on the pristine ZnO-coated mesh film with oleophilic surface in air (left), and it turned into underwater oleophobic surface after UV irradiation (middle and right). Source: Tian et al. 2012 [141]. Reprinted with permission of Royal Society of Chemistry.

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

TiO2 coating SWCNT/TiO2 nanocomposite film

SWCNT network film

Water

Oil

UV light

Oil-in-water emulsion separation Photo-induced superhydrophilic and underwater superoleophobic

Figure 4.15 Schematic illustration of the preparation process of the SWCNT/TiO2 nanocomposite film and the separation process of an oil-in-water emulsion under UV irradiation. Source: Gao et al. 2014 [63]. Reprinted with permission of American Chemical Society.

TiO2 has a similar light-responsive behavior to ZnO, and by integrating TiO2 into membrane materials, photo-triggered switchable oil/water separation was also widely reported [63, 144–146]. In 2013, Feng and coworkers fabricated a double-layer TiO2 -based mesh film for both successive oil/water separation and photodegradation of soluble pollutants in water under UV light irradiation with the aid of the photocatalytic property of TiO2 [147]. In 2014, Jin and coworkers fabricated a UV-responsive SWCNT/TiO2 ultrathin film for rapid separation of oil-in-water emulsions (Figure 4.15) [63]. This ultrathin film, with an initial water drop CA of ∼82∘ , under UV irradiation, turned superhydrophilic and underwater superoleophobic and reverted to its initial water drop CA while standing in dark for seven days. Meanwhile, it separated both SFE and surfactant-stabilized oil-in-water emulsions with ultrahigh water flux (up to 30 000 l m−2 h−1 bar−1 ) and high separation efficiency (99.99%). In addition, the films possessed antifouling and self-cleaning performance up to four operation cycles. In 2015, Kim et al. fabricated a TiO2 -based and UV-responsive PDMS nano-sponge for oil capture and release [144]. Such sponge absorbed oil in water and released more than 98% of the absorbed crude oil under UV irradiation and air bubbling. It could be recycled without sacrificing oil adsorption capacity. In 2016, Xin and coworkers incorporated TiO2 into PVDF fibrous mat by electrospinning and obtained beads-on-string fiber structure [145]. Under UV (or sunlight) irradiation, the material’s surface wettability changed

4.8 Photo-responsiveness

from superhydrophobicity/superoleophility to superhydrophility/underwater superoleophobicity after 90 minutes UV irradiation where the highest water flux of around 70 000 l m−2 h−1 bar−1 was reported. The material recovered ∘ its superhydrophobicity with a water CA of 152∘ after annealing at 110 C for around 70 minutes [146]. Besides, in 2017, an underwater superoleophobic BiVO4 -coated mesh was reported to exhibit sunlight-driven self-cleaning property for oil/water separation, as well as excellent durability against abrasion, cavitation erosion, and ultralow temperature [148]. In addition, Ag-coated mesh has been also demonstrated to possess a UV light-induced oil/water separation [149]. However, these inorganic separation materials have rarely been further investigated. Inorganic photosensitive materials possess lower toxicity and superior photostability and chemical and thermal stability in comparison to limited choices of photo-responsive polymers [150–157]. However, inorganic materials suffer relatively long hydrophobicity recovery time (e.g. several days or weeks) upon dark storage, although gentle heating can speed up the recovery process. 4.8.2

Organic Photo-responsive Materials

As for photo-responsive organic materials for oil/water separation, in 2010, Kulawardana and Neckers reported a photo-sensitive oil sorbent consisting of hydrophobic/oleophilic acrylic and tert-butylstyrene as well as azobenzene

Oil/water mixture

Water

Oil

(c) Separation efficiency

(a)

Chloroform Water

1.1 1.0 0.9 0.8 0.7 0.6 0.5 1

Oil/water mixture

Oil

Water

3 4 Cycle number

5 Hexane Water

1.1 Separation efficiency

(b)

2

(d) 1.0 0.9 0.8 0.7 0.6 0.5 1

2

3 4 Cycle number

5

Figure 4.16 Images of the separation process of oil/water mixture. (a) The membrane was not exposed to UV light, and the oil (chloroform) has a higher density than water. (b) The membrane was exposed to UV illumination for 60 minutes, and the oil (hexane) has a lower density than water. The water and oil phases were dyed blue and red, respectively. (c) The separation efficiency of the membrane before UV irradiation for an oil (chloroform). (d) The separation efficiency of the membrane after 60 minutes UV irradiation for oil (hexane). Source: Zhu et al. 2017 [159]. Reprinted with permission of John Wiley and Sons.

135

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

compositions [158]. It absorbed 15, 19, and 16 times its dry weight in toluene, chloroform, and dichloromethane, respectively. Upon the UV light irradiation, the oil sorbent possessed a rapid and photo-responsive desorption of solvent (86% of solvent expulsed in 30 minutes). In 2017, Lu and coworkers introduced azobenzene moiety into poly(4-vinylphenol sulfofluoridate) through the sulfur (VI) fluoride exchange reaction [159]. Subsequently, a photo-responsive electrospun membrane was fabricated and showed switchable hydrophobicity and hydrophilicity in response to visible or UV light irradiation. This membrane can controllably separate both heavy and light oils from contaminated wastewater with separation efficiency up to 97.9% upon exposure to UV illumination for 60 minutes and be recycled more than five times (Figure 4.16). Recently, a photo-responsive spiropyran-containing polymer-modified melamine formaldehyde sponge was prepared via a radical copolymerization process [160]. The obtained sponges showed high absorption capacity for oils and organic solvents of 70–154 times its own weight and the switchable oil absorption and desorption property under light illumination.

4.9 Gas, Solvent, Ion, and Electric Field Responsiveness 4.9.1

Gas Responsiveness

In the past couple of years, gases, especially, ammonia and CO2 as triggers for the wettability transition, have been applied in controllable oil/water separation [161]. In 2015, Zhao and coworkers fabricated an ammonia-responsive superamphiphobic coating by dip coating of a mixture of silica NPs and heptadecafluorononanoic acid (HFA)-modified TiO2 (Figure 4.17a–c) [162]. When exposed to ammonia gas, the coating became superhydrophilic, which was ascribed to breakage of titanium carboxylate coordination bonding and the subsequent formation of ammonium carboxylate ions. The superhydrophobicity could be recovered by heating. As a result, the superamphiphobicity membrane repelled both hexadecane and water at the beginning, while it permeated water but retained hexadecane upon exposure to ammonia gas, leading to a controllable oil/water separation (Figure 4.17d). Similarly, in 2016, Lin and coworkers fabricated a fluorine-free, environmentally friendly coating with pH-induced wettability transition [163]. The cotton fabric was modified with a mixture of silica NPs and decanoic acid (DA)-modified TiO2 . As a consequence, the fabric was superhydrophobic in air but superoleophilic in neutral aqueous environment, and thus it was permeable to oils but impermeable to water. On the other hand, when the coated fabric was exposed to ammonia vapor, it turned hydrophilic and underwater superoleophobic, thus allowing water to penetrate through but blocking oil. Therefore, this intelligent oil/water separator had the capability to separate either oil or water from a water/oil mixture.

4.9 Gas, Solvent, Ion, and Electric Field Responsiveness F3C F3C F3C CF2 CF CF F2C 2 F2C 2 F2C CF2 CF2 CF2 F2C F2C F2C CF2 CF2 CF2 F2C F2C F2C CF2 CF2 CF2 O O O O O O O O Ti Ti Ti Ti Ti Ti O OO OO OO O O OO O O O TiO TiOTi O Ti Ti OTi

(a) H3 C H3C

O

O

Ti O O

CH3 O + CF3(CF2)6CF2 OH CH3

Ti(OBu)4 Stirring

Ethanol, water

HFA

HFA-TiO2 sol

+

HFA-TiO2 on glass

Dip coating Curing Silica NPs

(b)

Substrate

(c)

100 μm

5 μm

200 nm

(d)

Hexadecane

Ammonia vapor

Water

Needle

Figure 4.17 (a) Schematic illustration of the preparation process for superamphiphobic coating. Right: HFA-TiO2 sol and its coating on a glass slide were transparent. (b) SEM image of the polyester fibers coated with silica NPs/HFA-TiO2 . Inset: Large-area view of the treated polyester fabric. (c) Enlarged SEM image of the coated fiber surface (green arrows indicate HFA-TiO2 thin layer). (d) Controllable oil/water separation process of the functionalized polyester fabric. Source: Xu et al. 2015 [162]. Reprinted with permission of John Wiley and Sons.

Considering that other metal oxides are also available for ammonia-triggered separation of oil/water mixtures, in 2017, Chen and coworkers prepared a superhydrophobic zirconium film with structure of mastoids like on the carbon fiber fabrics by an electrochemical technique [164]. When the superhydrophobic fabric was treated by ammonia vapor, it became hydrophilic and underwater

137

4 Switchable Wettability Materials for Controllable Oil/Water Separation

(a)

CO2

N2

= Water

= Oil

= SNMs

16 000 12 000 8000 Oil Water

4000 1

2

3

4

Number of cycles

5

Water content (ppm)

(c) 90

(b) 20 000 Flux (l m2 h–1)

138

Hexane Petroleum ether Heptane

60

30

0 1

2

3

4

5

Number of cycles

Figure 4.18 (a) Schematic illustration of the CO2 responsive oil/water separation. (b) The flux of oil and water in the absence and presence of CO2 , respectively. (c) Water content in oil in the filtrate after permeating oil/water mixtures. Source: Che et al. 2015 [169]. Reprinted with permission of John Wiley and Sons.

superoleophobic, which led to controllable separation for immiscible oil/water mixtures. In addition, carbon dioxide (CO2 ), as inexpensive, abundant, and nontoxic gas, has also been utilized as a gas trigger. Typically, PDEAEMA and PDMAEMA are commonly used CO2 -responsive polymers. The interaction between tertiary amine groups and CO2 in water induces the exposure on the extended hydrophilic polymer chain, while it could turn to hydrophobic aggregate state with CO2 being removed [165–168] (Figure 4.18a). For example, Che et al. electrospun the copolymer of polymethylmethacrylateco-poly(N,N-dimethylaminoethyl methacrylate) (PMMA-co-PDEAEMA) to make nanostructured fibrous membrane, which, in the absence of CO2 , possessed underwater superoleophilicity, allowing oil to pass with an oil flux of 17 000 l m−2 h−1 [169]. Upon exposure to CO2 in water, the reduced water pH led to protonation of PDEAEMA, which switched the membrane’s wettability to underwater superoleophobicity and resulted in a water permeation with a water flux of 9554 l m−2 h−1 (Figure 4.18b). Furthermore, as shown in Figure 4.18c, the water content in hexane was below 30 ppm for five cycles, and the corresponding

4.9 Gas, Solvent, Ion, and Electric Field Responsiveness

results for petroleum ether and n-heptane were 40 and 60 ppm, respectively, demonstrating that this fibrous membrane is well applicable for industrial oil spill cleanup and oil-polluted water disposal. In 2017, Montemagno and coworkers fabricated a CNT/PDEAEMA hybrid membrane by grafting PDEAEMA on carbon nanotube (CNT), which exhibited gas-modulated (CO2 and N2 ) separation of layered oil/water mixture and oil-inwater emulsions, with >92% demulsification efficiency [170]. In the same year, Zhu’s group reported highly porous poly(styrene-co-N,N-(diethylamino)ethyl methacrylate) (i.e. poly(St-co-DEA)) membranes with “open-cell” structure and CO2 -switchable wettability prepared from water-in-oil high internal phase emulsion (HIPE) templates [171]. This membrane displayed CO2 -controlled oil/water separation and allowed either oil or water to selectively penetrate through the membrane. 4.9.2

Solvent Responsiveness

Through a conventional solvent treatment and/or ion-exchange interaction, surface chemical constituents of a separation membrane can be changed, leading to change in surface free energy and surface wettability. In 2012, Tuteja and coworkers reported hygro-responsive membranes with superhydrophilicity and superoleophobicity both in air and water (Figure 4.19a,b) [74]. Intelligent membranes were prepared by using a blend of 20 wt% fluorodecyl polyhedral oligomeric silsesquioxane (POSS) and cross-linked poly(ethylene glycol) diacrylate (x-PEGDA) as the coating material. The special wettability of the surface was due to the water-induced surface molecular reconfiguration. As shown in Figure 4.19c,d, the surface was relatively rough in the air with several fluorodecyl POSS aggregates, while, underwater, fluorodecyl POSS aggregates disappeared to reveal a smoother surface. This is because PEGDA chains reconfigure to increase their interfacial area with water and facilitate enthalpic gains through hydrogen bonding. Hence, the obtained membranes have realized continuous, solely gravity-driven separation of oil/water emulsions (e.g. surfactant-stabilized oil-in-water and water-in-oil emulsions of water and hexadecane and saline emulsion systems), with a high efficiency (≥99.9%) (Figure 4.19e). Meanwhile, the fluxes of water-rich and hexadecane-rich permeates through the membranes were measured to be 90 and 210 l m−2 h−1 , respectively, and the membranes remained stable over 100 hours (Figure 4.19f ), indicating its high oil fouling resistant. In 2014, Feng and coworkers prepared solvent-responsive oil/water separation copper mesh membrane [172], where tetrahydrofuran (THF) and stearic acid were the solvent species to induce superwettability switch due to the formation of self-assembled monolayer of stearic acid molecules with low surface energy and the removal of the stearic acid by immersing in THF solvent. Additionally, his group prepared a PDMAEMA hydrogel-coated oil/water separation mesh via photoinduced free radical polymerization [173]. As-prepared mesh can switch between superhydrophilicity/underwater superoleophobicity and superhydrophobicity/superoleophilicity through monolayer electrostatic self-assembly and disassembly of stearic acid, as shown in Figure 4.20. As a

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

A*water = 0

(a)

0° (b)

A*water = 0

0°

A*oil = 8.6

(c)

(d)

125° A*oil = 4.3

152°

(e)

(f) 250 200

500 ml 300 ml 100 ml

300 ml 100 ml

Flux (l m–2 h–1)

140

150 100 50

Water-rich permeate Hexadecane-rich permeate

0

0

25

50 75 Time (h)

100

Figure 4.19 Droplets of water (dyed blue) and oil (dyed red) on stainless steel mesh (a) and polyester fabric (b). Insets: Morphologies of the respective dip-coated mesh and fabric surfaces. Optical microscopy images of a surface coated with a 20 wt% fluorodecyl POSS + x-PEGDA blend in air (c) and underwater (d). (e) A scaled-up apparatus used for the continuous separation of water-in-hexadecane emulsions. Water is dyed blue and hexadecane is dyed red. (f ) The fluxes for water-rich and hexadecane-rich permeates as a function of time. Source: Kota et al. 2012 [74]. Reprinted with permission of Springer Nature.

result, oil and water can permeate consecutively through the mesh based on the addition of NaOH aqueous solution, dealing with both water-rich and oil-rich oil/water mixture separation with high separation efficiency (>99.3%). Recently, in 2017, Han and coworkers fabricated a poly(ionic liquid)-based porous hybrid membrane, which showed controlled surface wettability triggered by ethanol wetting and ethanol removal by harsh drying condition. The hybrid showed successful and intelligent oil/water separation [174].

4.9.3

Ion Responsiveness

Typically, ion responsiveness exists in polymers with charged groups, for example, poly(acrylic acid), poly(ionic liquid), polyelectrolyte, etc. [83, 84]. The regulation of surface wettability induced by ionic strength or counterions stems from cation/anion exchange [78]. Feng and coworkers reported a mercury ion-responsive PAA hydrogel-coated oil/water separation mesh. The mesh switched its wettability in response to an increase of Hg2+ concentration due to the strong chelation between mercury ion

N

H3 ) 16C (CH 2

CH 3 H 2) 16

C OO

N

OO

N

OO

C(C

(a)

C(C

CH 3 H 2) 16

4.9 Gas, Solvent, Ion, and Electric Field Responsiveness

NH

NH

NH

NaOOC(CH2)16CH3 + H2O N

C18H36O2 O

O O

O O

O

O

N

N

NaOH O O

O

O

O O

O O

O O

O

(b) Electrostatic self-assembly

DMAEMA BIS,PAM hv, DEOP

Electrodeposition of Cu (c)

NaOH

PDMAEMA hydrogel-coated mesh

Stearic acid modified mesh

lf-assembly Se D i s a s s e m bly

Figure 4.20 Schematic illustration of the as-prepared mesh and the solution-controlled wettability transition. (a) Stearic acid molecules can be modified on PDMAEMA molecules and removed using NaOH solution. (b) The preparation processes of the mesh. (c) Wettability transition of the PDMAEMA hydrogel-coated mesh. Source: Zhang et al. 2014 [173]. Reprinted with permission of Royal Society of Chemistry.

and PAA and the cleavage of the interaction between carboxylic acid groups and water molecules (Figure 4.21) [175]. In 2014, Su and coworkers coated poly(methyl sodium silicate) (PMSS)/poly (allylamine hydrochloride) (PAH) polyelectrolyte multilayers (PEM) onto stainless steel mesh, which enabled on-demand separation of oil from water or vice versa [176]. Switchable wettability was attained by the ion exchange of PEM counterions in NaCl aqueous solution. As a consequence, oil or water can selectively penetrate through the filter with a separation efficiency of above 99.5%. 4.9.4

Electric Field Responsiveness

Generally, electric field responsiveness has advantages over other stimuli, such as remote control, easy operation, and less time consumption (normally within a few seconds). An electric field between a liquid and an underlying conducting solid can induce rearrangement of charges and dipoles, leading to a reduction in interfacial energy and wettability transition from hydrophobicity to hydrophilicity, which is known as electrowetting [177–181]. In 2012, Kwon et al. extended the electrowetting concept into switchable oil/water separation and fabricated a fluorodecyl POSS/PDMS-coated nylon membrane. The membrane, under a voltage, with interfacial energy decreasing, turned into the hydrophilic state from its original hydrophobicity, leading to

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

(a)

(b)

R

O

O–

O

O–

O

O–

O O–

R

R O

O

(c)

O–

O–

O

O

O O

O–

O

O

O O

O–

R

R

O Hg O

O–

O–

O–

O

R O

O Hg O O

O

O–

O Hg O

Figure 4.21 Oil/water separation behavior of PAA hydrogel-coated meshes in different conditions: (a) the pristine mesh with superhydrophilic and underwater oleophobic property, (b) the mesh soaked in 1 mg ml−1 Hg2+ solution for five minutes turned into hydrophobic and underwater oleophobic; (c) the mesh became hydrophobic and underwater oleophilic after soaking in a saturated Hg2+ solution for five minutes. Red solution was petroleum ether and colorless solution was water. Source: Xu et al. 2014 [175]. Reprinted with permission of American Chemical Society.

V=0V

(a)

72°

HD

Water 115°

(e)

V = 0 V (f)

V ≈ 2.0 kV

72° V=0V

(c)

V = 1.5 kV

(b)

V = 1.5 kV

(d)

56° 2 cm

Figure 4.22 (a and b) The macroscopic contact angle for hexadecane on the surface of fluorodecyl POSS/PDMS-coated nylon membrane remained unchangeable with or without charge. (c and d) The macroscopic contact angle for water decreased from 115∘ to 56∘ as the potential of the membrane increased to 1.5 kV. (e) An apparatus with a liquid column of oil (dyed red) and water (dyed blue) above the membrane before applying an electric field. The inset shows a schematic of the membrane module. (f ) Water permeated through while hexadecane was retained above the membrane when a voltage of 2.0 kV applied. Source: Kwon et al. 2012 [182]. Reprinted with permission of John Wiley and Sons.

water permeation and hexadecane repellence (Figure 4.22) [182]. When applying a voltage of 2.0 kV, the composite membrane turned into hydrophilic from its original hydrophobic state and delivered a controlled separation of oil-in-water and water-in-oil emulsions with high separation efficiency (≥99.9%). In 2016, Jiang and coworkers coated rootlike polyaniline nanofibers with micro/nano-hierarchical structure on stainless steel mesh, and the modified mesh became gradually hydrophilic and underwater superoleophobic

4.11 Conclusion

(CA of ∼166∘ ) under an increasing voltage with water CA of 70∘ at 160 V and 40∘ at 170 V, which performed a water-moving type of oil/water separation under proper voltages [183].

4.10 Dual/Multi-stimuli In contrast to the single responsive systems, dual or multi-responsive materials are able to intelligently respond to more than one trigger either simultaneously or independently, providing more intelligence for adapting diverse and complex conditions in practical applications. In 2014, Feng and coworkers coated PDMAEMA hydrogel on stainless steel mesh and obtained thermal and pH dual stimuli-responsive material. The obtained mesh had 55 ∘ C and pH of 13 as the critical points for the wettability switch and for selective oil/water separation (Figure 4.23) [112]. Water could permeate the mesh under 55 ∘ C (pH 7) and pH less than 13 (T = 25 ∘ C), while oil was repelled on the mesh. With the temperature >55 ∘ C or pH > 13, the water retention capacity and the swelling volume of PDMAEMA hydrogel coating decreased, and thus oil was penetrable and collected in situ. Similarly, in 2016, Wang and coworkers grafted PDMAEMA onto cotton fabric by surface-initiated ATRP approach [113]. The transition temperature was about 45 ∘ C and the transition pH was around 7. The fabric reached superhydrophilicity at low pH or temperature and high hydrophobicity at high temperature or pH. The fabric could adsorb oil nearly four times of its own weight, and the absorbed oil could almost all be released automatically in acid and cold water. In 2017, a dual pH- and ammonia-vapor-responsive oil/water separation membrane was reported by Ma et al. [184]. This membrane was fabricated by successively dip-coating electrospun PI in DA-TiO2 and silica NPs. The obtained nanofibrous membrane exhibited dual pH- and ammonia-vapor-responsive wettability and excellent separation efficiency of oil/water mixtures (>99%), as well as thermal stability and abrasion resistance. In this section, we mainly introduced a dual responsive system for controllable oil/water separation. It is no doubt that the specific interactions between diverse responses could provide more possibilities to further increase materials’ intelligence.

4.11 Conclusion In conclusion, switchable wettability materials offer certain promise in controllable oil/water separation without external energy input. The accessibility of the associated stimuli-responsive polymers undoubtedly offers a great promise in practical application. Especially, photo- and electrical-induced polymers have been designed to realize fast wettability switch and remote-controlled separation, possibly placing in some rough or unsafe areas. However, one has to be very cautious in predicting their applicability to real-world oil contamination problems, as, under the state of the art, almost all

143

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4 Switchable Wettability Materials for Controllable Oil/Water Separation

(a) DMAEMA BIS, PAM hv, DEOP

n

O O N

(b) T > 55 °C or pH > 13

T < 55 °C or pH < 13

Figure 4.23 (a) Schematic illustration of the preparation of PDMAEMA hydrogel-coated mesh and the opposite wettability of the mesh when contacted with oil (b). Source: Cao et al. 2014 [112]. Reprinted with permission of American Chemical Society.

of these materials were tested at bench scales with surrogate model water and oils with much simpler compositions and smaller viscosity. Thus, more research efforts need to be directed toward using these materials for real-world applications, such as separation of crude oil with high viscosity, emulsion separation, oil/water mixture with high salinity, industrial oily wastewaters, etc. In doing so, the efficacy, stability, longevity, and fouling propensity of the switchable materials toward practical applications need to be systematically investigated. In practical oil/water separation, the adsorption of dissolved species, including dissolved oil ingredient species, surfactant monomers, dissolved natural organic matter, and salt species, onto the separating materials can be a concern for long-term separation but unfortunately has not been looked at thoroughly with switchable wettability materials. Additionally, the designed surface structure of oil/water separation materials always face unavoidable mechanical damage, involving mechanical stress, flow liquid impact, and high oxidation in water and air. In this regard, inorganic/polymer composite materials may provide more mechanical strength, while the introduction of self-healing could also endow these materials with long-term reliability. In the future, intelligent material-based oil/water separation is promising to deliver high separation efficiency and high flux and antifouling performance through rational design of the nanoscaled structure of the separation layer and surface chemical and adhesive properties [185].

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ity and oil/water separation. ACS Applied Materials & Interfaces 6 (16): 13324–13329. Zhang, G., Li, M., Zhang, B. et al. (2014). A switchable mesh for on-demand oil-water separation. Journal of Materials Chemistry A 2 (37): 15284–15287. Badre, C. and Pauporté, T. (2009). Nanostructured ZnO-based surface with reversible electrochemically adjustable wettability. Advanced Materials 21 (6): 697–701. Mugele, F. and Baret, J.-C. (2005). Electrowetting: from basics to applications. Journal of Physics: Condensed Matter 17 (28): 705–774. Kakade, B., Mehta, R., Durge, A. et al. (2008). Electric field induced, superhydrophobic to superhydrophilic switching in multiwalled carbon nanotube papers. Nano Letters 8 (9): 2693–2696. Chen, J., Kutana, A., Collier, C., and Giapis, K. (2005). Electrowetting in carbon nanotubes. Science 310 (5753): 1480–1483. Krupenkin, T.N., Taylor, J.A., Wang, E.N. et al. (2007). Reversible wetting-dewetting transitions on electrically tunable superhydrophobic nanostructured surfaces. Langmuir 23 (18): 9128–9133. Kwon, G., Kota, A., Li, Y. et al. (2012). On-demand separation of oil–water mixtures. Advanced Materials 24 (27): 3666–3671. Zheng, X., Guo, Z., Tian, D. et al. (2016). Electric field induced switchable wettability to water on the polyaniline membrane and oil/water separation. Advanced Materials Interfaces 3 (18): 1600461. Ma, W., Samal, S.K., Liu, Z. et al. (2017). Dual pH-and ammonia-vaporresponsive electrospun nanofibrous membranes for oil–water separations. Journal of Membrane Science 537: 128–139. Zhu, Y., Wang, D., Jiang, L., and Jin, J. (2014). Recent progress in developing advanced membranes for emulsified oil/water separation. NPG Asia Materials 6 (5): e101.

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5 Self-Healing Materials for Environmental Applications 5.1 Biomimetic Self-Healing Materials Biological materials from plants and animals always exhibit their optimized functional systems during the process of evolution. One of the most amazing performances is self-healing capability of restoring health and soundness of a system and regenerating the tissue structure and functions when damaged by external forces. In all plants and animals, firstly, a self-sealing phase and, secondly, a self-healing phase can be identified. The rapid self-sealing prevents from infection by germs and gives time for the subsequent self-healing of the injury, resulting in wound closure and the organ restoration. The scale of biological self-healing can range from molecule level such as the restore of DNA to macroscopic level like the union of the fractured bones. In addition, for the biological materials, they can heal the damaged surface functional components by re-secretion of relevant chemical substances. Biological materials provide a great inspiration for scientists and engineers to make bio-inspired artificial materials that can heal themselves when damaged [1]. Recently, inspired by inherent regeneration capability of biological materials under external damage, more and more artificial materials have been designed by integrating self-healing properties with mechanical/electrical/chemical functions, which are beneficial for long-term use and improving maintenance, reliability, and durability in their practical application. The introduction of self-healing ability can effectively cut down the replacement cost of some easily worn materials with high price by healing the materials instead of replacing them. Self-healing materials avoid the original degradation through the initiation of a repair mechanism in response to the micro-damage. It is thus undisputed that self-healing capability provides the man-made materials enormous possibilities, in particular for applications where long-term reliability in poorly accessible areas is important. In such a concept, a wide range of intelligent self-healing materials with multifunctions has been developed and explored in diverse scientific areas such as biomedical materials, smart wearable electric devices, environmental application, etc. [1–6].

Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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5.2 Overview of Self-Healing Materials Early in ancient time, the Romans used a form of lime mortar that was found to possess self-healing performance. Self-healing materials can be traced in the 1970s. A hard elastic polypropylene was observed to be capable of healing interlamellar microvoids, which were formed after stretching in the perpendicular direction [7, 8]. However, self-healing materials were widely recognized and studied in the scientific area until the twenty-first century. The initial overview of self-healing materials gradually covered the entire spectrum of materials from polymers to metals and ceramics. In general, self-healing materials could be widely defined as an artificially or synthetically created intelligent substance, which can spontaneously repair damage by themselves without any external diagnosis of the problem or human intervention. Hence the occurrence of damage can be also seen as a trigger to initiate the generation of healing agents toward damage sites, leading to the directed mass transport for filling the defect and the subsequent mending reaction by physical interaction or chemical bonding. In the self-healing process, the healing agents filled into damage sites and gradually became stable and immobilized to guarantee perfect recover of original properties with the healed structures or chemical composition. Resembling biological systems, an ideal self-healing is automatically triggered only by damage. Namely, the damage itself can be applied as a stimulus for the self-healing process. It is mainly based on the intrinsic structure and composition of materials with the ability of detecting the damage and synchronous self-healing. However, in most of the self-healing systems, external stimuli other than the trigger of damage are generally applied to initiate the movement of the healing agent toward damage sites. The typical external stimuli include heat [9–12], pH [13–15], light [16–22], redox [23, 24], electrical field [25, 26], magnetic field [27], etc. In terms of self-healing materials, although inorganic materials such as metals and ceramics have been also used in the construction industry as a healing agent to heal physically structural cracks of materials [28], their diffusive mass transport toward damage sites is always activated by high temperatures (>600 ∘ C for metals and >800 ∘ C for ceramics) [1], due to the fact that their solute atoms as healing agents are relatively tiny and possess a relatively low mobility at the prevailing operating temperatures, which limits their application breadth. In contrast, polymer chains can be more readily mobile for self-healing at low temperature (99.5%) at the temperatures ranging from 25 to 370 ∘ C for more than 120 hours [39]. As shown in Figure 6.6, PI filter can effectively remove all kinds of particles (PM sizes, 0.3–10 μm) from the car exhaust gas with high temperature ranging from 50 to 80 ∘ C. The PM concentrations after filtration were decreased to almost the same as that of clean ambient air. Following this concept, Zhao and coworkers utilized PI nanofibers as building blocks to construct hierarchically porous aerogels through freeze drying and thermally induced cross-linking (Figure 6.7) [97]. The as-prepared polyimide nanofiber aerogels possess outstanding flexibility and toughness, ultralow density (4.6–13.1 mg cm−3 ), high porosity (99.0–99.6%), low thermal conductivity

Removal efficiency (%)

100 80 60 40 20 0

(a)

0.3

0.5

1.0

2.5

PM size (μm)

5.0

10

(b)

Figure 6.6 (a) Removal efficiency of PM particles from car exhaust gas. (b) PM number concentration measurement of car exhaust with air filter. The inset shows a stainless steel pipe coated with a PI filter. Source: Zhang et al. 2016 [39]. Reprinted with permission of American Chemical Society.

203

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6 Emerging Nanofibrous Air Filters for PM2.5 Removal

PMDA

Homogenizing

1) Electrospinning 2) Thermal imidization

ODA

Poly(amic acid) Polyimide nanofiber membrane

Nanofiber dispersion Freezing

Freeze drying

Thermal treatment Chemical cross-linking PINFAs

Physical entanglement Uncross-linked PINFAs

Frozen dispersion

Figure 6.7 Schematic illustration of the fabrication process of PI nanofiber membranes (PINFMs). Yellow fibers represent PI nanofibers. A plausible chemical structure after intermolecular condensation in PINFAs is presented. Source: Qian et al. 2018 [97]. Reprinted with permission of Royal Society of Chemistry.

(29.7–31.8 mW m−1 K−1 ), and high-temperature stability (300 ∘ C for 240 hours in air) in different dimensions. With the combination of the structural features of the aerogels and the specific physicochemical property of PI, the filtration efficiency remained above 99.9% under the mass concentration of PM2.5 above 200 μg m−3 for 22 hours at a velocity of 0.25 m s−1 . The calculated sorption dynamics was about 4.3 μg g−1 s−1 . In addition, to mitigate fire and explosion risk in operation with combustible filtrates, in 2018, Cui et al. further developed a multifunctional nanofibrous air filter that not only efficiently captured PM but also possessed a flame retardant design [98]. This multifunctionality design was achieved through fabricating an electrospun core–shell nanofibrous membrane consisting of the polar polymer Nylon-6 as the shell and the flame retardant triphenyl phosphate (TPP) as the core (Figure 6.8a). The Nylon-6 polymeric shell ensured the high capture efficiency of 99.00% for PM2.5 owing to its large dipole moment initiating strong binding between the fiber surface and polar PM. In terms of the flame retardant core, TPP, it could be released through the melted Nylon-6 shell during combustion of the flammable filtrate and simultaneously suppress the fire by scavenging • H and • OH free radicals generated from combustion (Figure 6.8b–e) [99]. As a result, the self-extinguishing time of the filtrate-attached air filter was nearly instantaneous of 0 s g−1 compared with 150 s g−1 for Nylon-6 alone. 6.4.3

Air Filter for Thermal Management

In 2017, for the first time, thermal management was introduced into nanofibrous air filter face mask by Cui and coworkers for personal cooling/warming purposes [100]. In this design, the nanoPE was chosen as a supporting substrate due to its transparency to the mid-infrared (IR) radiation, and electrospun nylon nanofibers were modified on the PE substrate. As-prepared PE/nylon composite

6.4 Applications Shell: Nylon-6:

Syringe

O

TPP:

O P O O

TPP

Heat Phosphorus-containing free radicals (·PO)

PO

H

PO

OH

HPO

O

Core: TPP

(a)

Airflow

(d)

Nylon-6:

Collec

N H

n

(b)

tor

Ignition sources

Flame and explosions

HPO2

(c) Ignition sources

Airflow

Nylon-6 fiber

Flame retarded

TPP @Nylon-6 fiber

(e)

Figure 6.8 (a) Schematic illustration for the fabrication of the nanofibers by electrospinning. (b) Molecular structure of TPP and Nylon-6. (c) The TPP exhibits flame retardancy by scavenging • H and • OH free radicals during combustion. (d) The air filtration material composed of Nylon-6 nanofibers. Combustive dusts accumulated on the filter are easily ignited by ignition sources, leading to dust explosions. (e) The working mechanism of the multifunctional air filter. Source: Liu et al. 2018 [98]. Reprinted with permission of American Chemical Society.

membrane mask exhibited high PM capture efficiency (99.6% for PM2.5 ) with low pressure drop. As shown in Figure 6.9, it showed excellent heat dissipation and high IR transparency (92.1%), generating the radiative cooling effect. Separately, if the nanoPE substrate was coated by a thin layer of Ag before the nylon fiber deposition, it gave rise to a high IR reflectance (87.0%) for personal warming purpose. These two type of face masks are desirable for personal thermal comfort under hot and cold weather, respectively. Cooling:

Nylon-6 fi

ber Fiber/ nanoPE

E

noP

d na

che Pun

Warming: °C

Nanopore

37.5 Fiber/Ag /nanoPE

Nanofiber

23.5 (a)

(b)

Figure 6.9 Face mask consisting of Nylon-6 nanofibers on top of needle-punched nanoporous-polyethylene (nanoPE) substrate (a) and thermal imaging of faces covered with the sample (b). Source: Yang et al. 2017 [100]. Reprinted with permission of American Chemical Society.

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6 Emerging Nanofibrous Air Filters for PM2.5 Removal

6.4.4

Air Filter for Mass Production

For the purpose of fast and large-scale commercial production of air filter, Cui and coworkers proposed and tested a roll-to-roll transfer fabrication method of nanofibrous air filter (Figure 6.10a) [40]. Accordingly, an ameliorated electrospinning design was built based on fast transfer of electrospun Nylon-6 nanofiber film from rough copper foil to a rolling plastic mesh substrate by laminating and peeling method. Compared with the conventional electrospinning method, the transfer method is 10 times faster and has better filtration performance at the same transmittance (>99.97% removal of PM2.5 at ∼73% of transmittance). Later, a roll-to-roll blow-spinning technique was developed by Cui and Wu et al. and applied to the mass production of transparent air filters (Figure 6.10b) [47]. By sequential multi-needle blow spinning, such transparent nanofibrous air filter films could be coated rapidly on regular window screens (Figure 6.10c), which acquired 90.6% PM2.5 removal efficiency over 12 hours under hazy air conditions (PM2.5 mass concentration > 708 μg m−3 ). Moreover, PM can be easily removed by gentle wiping (Figure 6.10d). 6.4.5

Self-Powered Air Filter

To endow the nanofibrous air filters with more intelligence, Wang et al. demonstrated a concept of self-powered air filter for capturing PM from automobile exhaust using triboelectrification effect. The triboelectric nanogenerator (TENG) in this work harvested electric energy from mechanical movements. In 2015, they fabricated a TENG based on the collision between PTFE pellets and electrode plates that formed a space electric field as high as 12 MV m−1 , and thus Airflow

(b)

(a)

(d)

(c)

Figure 6.10 (a) Photograph of a roll-to-roll process for the transferring of electrospun nanofiber film onto a plastic mesh in a continuous fabrication process for PM2.5 filter. Source: Xu et al. 2016 [40]. Reprinted with permission of American Chemical Society. (b) Schematic illustration of the blow-spinning method of the window screen coating for indoor protection. (c) Real window consisting of a removable metallic screen coated with PAN blow-spun fibers. (d) Successful wiping of nanofibers from the window screen using a tissue paper. Source: Khalid et al. 2017 [47]. Reprinted with permission of American Chemical Society.

6.4 Applications

PM could be absorbed under the electrostatic force, generating 95.5% removal of PM2.5 [101]. In 2017, they developed a TENG-assisted positively charged PI electrospun nanofibrous air filter to enhance the removal of especially superfine PM with a diameter smaller than 100 nm [102]. As shown in Figure 6.11a,b, an electric field forms around the stainless steel meshes and PI nanofiber film, and thus triboelectrification-induced electrostatic absorption effect can work on a larger particle size span, from nanoparticles to microparticles. As a result, the greatest enhancement of PM removal efficiency was 207.8% at the particle diameter of 76.4 nm, and the highest removal efficiency was 90.6% at the diameter of 33.4 nm. This technology with zero ozone release and low pressure drop offers a great promise in air cleaning and haze treatment. In the same year, Ko and coworkers fabricated a percolation network of Ag nanowire on nylon mesh as a transparent, reusable, and active PM2.5 air filter [103]. As shown in Figure 6.11c, by applying a low voltage on the Ag nanowire network, the membrane exhibited a high PM2.5 removal efficiency (>99.99%) due to voltage-induced strong electrostatic force. Meanwhile, the obtained air filter was transparent, low power consumption, antibacterial, and reusable after simple washing. In 2018, Wang and coworkers prepared a washable high-efficiency triboelectric air filter (TAF) for efficiently removing the PMs [104]. The TAF consists Electrostatic zone

Dust air

Dust air

Pl nanofibers +R-TENG

Pl nanofibers

(a)

Clean air

(b) Transparent, reusable, active PM filter

PM

(c)

Ag NW percolation network

Filtrated air

Polluted air

+ Voltage

Figure 6.11 Schematic illustration of the filtration mechanism. (a) Without R-TENG. (b) With R-TENG. Source: Gu et al. 2017 [102]. Reprinted with permission of American Chemical Society. (c) Schematic illustration of the filtration mechanism of the Ag nanowire-coated filter. Source: Jeong et al. 2017 [103]. Reprinted with permission of American Chemical Society.

207

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6 Emerging Nanofibrous Air Filters for PM2.5 Removal

Severely polluted

μg m−3

Heavily polluted

μg m−3

Good

μg m−3

Excellent

μg m−3

Figure 6.12 The schematic diagram of the measurement of the removal efficiency of the face mask made of the TAF. The face mask was worn by a man for four hours. The concentration of PM2.5 changed from severely polluted to good and from heavily polluted to excellent. Source: Bai et al. 2018 [104]. Reprinted with permission of John Wiley and Sons.

of multilayers of the PTFE and nylon fabrics, which can be charged by simply rubbing the PTFE and nylon fabrics against each other. The removal efficiency of the charged TAF can be greatly enhanced by introducing the electrostatic attraction in removing the PMs. After charging, the removal efficiencies of PM0.5 and PM2.5 were increased from 26.3% to 84.7% (3.22 times increase) and 69.1% to 96.0% (1.39 times increase), respectively. As shown in Figure 6.12, the PM2.5 filtering through the worn face mask decreased from 266.71 μg m−3 (severely polluted condition) to 54.71 μg m−3 (good air condition), and correspondingly, PM2.5 in heavily polluted condition with the concentration of 192.32 μg m−3 could be decreased to 27.93 μg m−3 , representing an excellent air condition. In addition, the TAF can be easily cleaned with commercial detergent, and the removal efficiency was maintained after five washing cycles. 6.4.6 Nanofibrous Air Filter for the Simultaneous Removal of PM and Toxic Gases In reality, the pollution of PM particles is always concomitant with gaseous chemicals, such as formaldehyde (HCHO), sulfur dioxide (SO2 ), nitrogen dioxide (NO2 ), benzene, dioxin, ozone, etc. [3, 22, 59, 105, 106]. Taking consideration of these issues, MOFs have offered some help in the capture of harmful gas [107–113] and adsorption and degradation of chemical toxic agents [114, 115]. In 2016, for the first time, Wang and coworkers prepared MOF-based nanofibrous air filter for the simultaneous removal of PM and toxic gases [23]. In this work, MOFs (ZIF-8, UiO-66-NH2 , MOF-199, Mg-MOF-74) were embedded within polymers (PAN, PS, PVP) to prepare nanofibrous air filters by the electrospinning method, and among all, UiO-66-NH2 /PAN and MOF-199/PAN hybrid nanofibrous air filters showed the best PM removal performance and SO2 adsorption (Figure 6.13). This is because the polar functional groups and positive

6.4 Applications

PM2.5 PM10

88 84 80 76

(a)

0.020

SO2 dynamic adsorption capacity Pressure drop

0.015

18

0.010

12

0.005

6

0.000

N N N N N N PA /PA 9/PA /PA 4/PA 8/PA 9 H 2 -7 O3 ZIF Al 2 OF-1 -66-N MOF M iO Mg U

(b)

24 Pressure drop (Pa)

SO2 dynamic adsorption capacity (g/g)

Removal efficiency (%)

92

0

N N N N N PA 8/PA 4/PA 9/PA /PA 7 9 H2 ZIF OF- OF-1 66-N -M M Mg UiO

Figure 6.13 (a) PM removal efficiency of PAN filter, Al2 O3 /PAN filter, and PAN/MOF filters tested on hazy days in Beijing (T = 23.4 ∘ C, RH = 58.6%, PM2.5 = 350 μg m−3 , PM10 = 720 μg m−3 ). (b) The dynamic adsorption capacities of SO2 on PAN filter and PAN filters with different MOF materials at 25 ∘ C with a 100 ppm of SO2 /N2 flow at the rate of 50 ml min−1 . Source: Zhang et al. 2016 [23]. Reprinted with permission of American Chemical Society.

charges on the surface of MOFs can generate electrostatic and polarity-induced interaction with PMs to bond them tightly, while functionalities such as amines and open metal sites of hybrid fibers are crucial for kinetic adsorption of acidic polar gas species. Furthermore, Wang and coworkers used a roll-to-roll hot-pressing method and fabricated MOF-based air filters [91]. In this method, the MOF nanocrystals were generated and stably immobilized onto the flexible substrates via continuously roll-to-roll pressing between two rollers with high temperature for several times (Figure 6.14). The MOF-based nanofibrous air filters showed long-term

Catalyst pulverization MOF Substrate

MOF

Particulate matter

Power plant

MOF@Plastic mesh (80 ~ 100 °C)

Refinery

MOF@Melamine foam (~150 °C)

Traffic

Biomass combustion Roll-to-roll production

MOF@Nonwoven fabric (150–250 °C)

MOFilters

MOF@Metal mesh (>300 °C)

MOF@Glass cloth (>300 °C)

Figure 6.14 The schematic representation of the roll-to-roll production of various MOF-based filters for PM removal. Source: Chen et al. 2017 [91]. Reprinted with permission of John Wiley and Sons.

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6 Emerging Nanofibrous Air Filters for PM2.5 Removal

(i.e. 30 consecutive days) and consistently high PM removal (>90%) under a wide temperature range (80–300 ∘ C). Moreover, such air filter can be easily cleaned by washing with water and ethanol and reused for three times without any change of structure, morphology, and PM removal efficiency. This air filter can be applied in the waste gas treatment in existing piping systems, vehicle or aircraft engine pipes, and reaction vessels. Further, Kim and coworkers proposed heterogeneous nucleation-assisted hierarchical growth of flowerlike MOFs (Zn-based zeolite imidazole frameworks, ZIF-L) (Figure 6.15a) on several substrates (e.g. glass, PU foam, nylon microfibers, and PP microfibers), as air filters for efficient PM removal [116]. MOFs assembled on PP microfibers showed superior properties in terms of PM2.5 removal efficiency (92.5 ± 0.8% for PM2.5 and 99.5 ± 0.2% for PM10 ) (Figure 6.15b), pressure drop (10.5 Pa at 25 l min−1 ), long-term stability, and recyclability even after washing. The PM filtering performances are mainly ascribed to the unique 2D-shaped structure anchored on individual fibers and electrostatic PM screening imparted by the partially charged surface of MOFs. In 2018, Zhang and coworkers presented an immersion method for embedding the MOFs into electrospun nanofibers, which served to effectively remove both PM2.5 and formaldehyde (Figure 6.16a) [117]. The PM2.5 filtration efficiency was raised from 74.5% to 87.2%, after integrating ZIF-67 nanocrystals into the electrospun PAN nanofibers. Moreover, the PM2.5 filtration efficiency remained at more than 99% and dropped by only 0.25% during one-month continuous PM2.5 filtration test (Figure 6.16b). The air filter obtained a formaldehyde removal efficiency of 84%. In addition to MOFs, proteins have great promise for environmentally friendly air filtration with capability of capturing PM and toxic gases like CO and HCHO. For instance, in 2016, Zhong and coworkers employed soy [58] and gelatin protein [59] and fabricated electrospun nanofibrous air filter for the PM and toxic gas removal. The main mechanism is demonstrated in Figure 6.17a. The amine groups from gelatin and soy protein can interact with carbon oxide

100 PM removal (%)

210

PM2.5

PM10

90 80 70 60 0

2 μm (a)

PP

ZIF-8_PP H-ZIF-L_PP

(b)

Figure 6.15 (a) SEM images of hierarchical flowerlike MOFs. (b) PM removal efficiencies of PP microfibers, ZIF-8_PP microfibers, and H-ZIF-L_PP microfibers. Source: Koo et al. 2018 [116]. Reprinted with permission of American Chemical Society.

6.4 Applications ZIF-67@PAN

PM2.5

Filtration efficiency (%)

100

Diffusion Co(AC)2/PAN

99 98 97 96 95

(a)

(b)

3 6 9 12 15 18 21 24 27 30 Date (d)

Figure 6.16 (a) Schematic representation of the fabrication procedures for MOF-based PAN filters. (b) Long-term PM2.5 filtration test results for MOF-based PAN filters. Inset image in (b) is the photograph of a large-scale MOF-based PAN filters (length: 36 cm). Source: Bian et al. 2018 [117]. Reprinted with permission of Royal Society of Chemistry.

rface

s on su

al group

ction Rich fun

COOH R R: Hydrophobic

COOH R (hydrophobic)

OH

NH2

Polar PM

O

Charged/Ionic

H

Hydrophobic

(a)

C

OH

N H H

C

H

Hydrogen bonding Charge–charge interaction Hydrophobic interaction ... Polar

Aldimine linkage PM

Formaldehyde

Charged/Ionic Hydrophobic

100%

(b)

20%

164 g m–2

50%

3.80 g m–2

(c)

O Carbon monoxide

3.43 g m–2

Removal efficiency (%)

HEPA

5%

C

80%

2.80 g m–2

164 g m–2

3.80 g m–2

20%

3.43 g m–2

50%

2.80 g m–2

C Formaldehyde H 80% H

2.25 g m–2

O

2.25 g m–2

Removal efficiency (%)

100%

5% Gelatin fibers

Gelatin fibers

HEPA

Figure 6.17 (a) Schematic illustration of the interaction-based filtration mechanism for soy-protein-based nanofabrics. Source: Souzandeh et al. 2016 [58]. Reprinted with permission of American Chemical Society. (b) Formaldehyde (HCHO) removal efficiency comparison between gelatin-based filters with different areal density and that of the commercial filter. (c) Carbon monoxide (CO) removal efficiency comparison between gelatin-based filters with different areal density and that of commercial filter. Source: Souzandeh et al. 2016 [59]. Reprinted with permission of Royal Society of Chemistry.

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6 Emerging Nanofibrous Air Filters for PM2.5 Removal

(CO) and aldehyde group in formaldehyde (HCHO) to form aldimine bonds with the purpose of the toxic gas removal, and simultaneously PM and other toxic chemicals can be captured via hydrogen bonding and charge–charge, and polar–polar interactions with protein nanofibers. Consequently, the soy protein/PVA electrospun nanofibers possessed HCHO removal efficiency ranging from 30.0% to 62.5% and CO removal efficiency between 76.9% and 90.9%. For gelation fibers, the HCHO and CO removal efficiency could reach as high as 80% and 76%, respectively, in comparison with less than 5% and 3% HCHO and CO removal efficiency on commercial air filters (Figure 6.17b,c). Furthermore, in 2018, Ke and coworkers constructed multifunctional fibrous filter for passive room air purification [118]. As-synthesized nanocrystalline MnO2 catalysts showed excellent HCHO catalytic activity at low temperatures [119, 120], and further was chosen to fabricate an MnO2 /PE/PP composite filter with PM2.5 filtration, HCHO adsorption, and catalytic abilities in air pollutant abatement. As a consequence, as-prepared MnO2 /PE/PP composite filter exhibited complete oxidation for HCHO within 60 minutes and acceptable reversibility over five cycles. After corona charging, MnO2 /PE/PP composite filter kept reasonable filtration efficiency and pressure drop while at the same time possessed fast catalytic degradation of 150 ppm HCHO within 15 minutes. 6.4.7

Nanofibrous Air Filter with Antibacterial Functions

Owing to the presence of bacteria and viruses in air pollutant, research efforts have been also made to provide nanofibrous air filters with antibacterial functions by using Ag [56], ZnO [77], and TiO2 [74] nanoparticles. Ag nanoparticles, as well-known broad-spectrum antibacterial agents, can induce the degeneration of protein [121, 122]. ZnO and TiO2 nanoparticles could generate reactive hydroxyl radicals under UV irradiation, and the hydroxyl radicals, once generated, are highly photooxidative and effectively inhibit bacterial growth by penetrating the cell wall of the bacteria [123–127]. Therefore, these inorganic nanoparticles have been employed to inhibit bacterial growth along with PM removal. In 2015, Wang and coworkers used atomic layer deposition to uniformly seed ZnO on the surface of expanded PTFE matrix and then synthesized wellaligned ZnO nanorods onto the PTFE substrate under hydrothermal conditions (Figure 6.18) [77]. ZnO nanorod modified nanofibrous air filter possessed high efficiency of PM removal (>99.9999%) and high antibacterial activity (>99.0% sterilization rates against both Gram-positive and Gram-negative bacteria). In 2016, Zhao and coworkers fabricated hybrid PLA/TiO2 electrospun nanofibrous air filter by the methods of CVD and hydrothermal synthesis, which can effectively inhibit the propagation of bacteria [74]. In this system, the introduction of TiO2 nanoparticles endows the obtained air filter with antibacterial properties. The reactive hydroxyl radicals generated by the photocatalysis of TiO2 nanoparticles with light irradiation cause the peroxidation of the polyunsaturated phospholipid of the bacteria cell membrane, leading to a loss of respiratory activity and thus killing the bacteria [127, 128]. As a result, the obtained hybrid PLA/TiO2 electrospun nanofibrous membrane loading with 1.75 wt% TiO2

6.4 Applications

Dust

Clean air

Polluted air

PTFE

PTFE

Airflow ZnO nanorods

Functionalized air filter with ZnO nanorods

Figure 6.18 Schematic of the filtration process of the ZnO-functionalized PTFE filters and the SEM images of the filters modified with ZnO nanorods. Source: Zhong et al. 2015 [77]. Reprinted with permission of American Chemical Society.

nanoparticles exhibited a high PM removal efficiency (99.996%) with a relatively low pressure drop (128.7 Pa), as well as a high antibacterial activity of 99.5%. In 2015, Singh and coworkers fixed Ag into air filter to explore its antibacterial and detoxification ability [70]. Hybrid electrospun PVDF-Ag-Al2 O3 nanofibrous air filter was highly efficient to filter 0.36 μm particles, leading to 99.17% filtration efficiency. Silver incorporation enabled the air filter to kill the pathogens and detoxify the chemical compounds that come into contact with them, which provided suitable disinfection with more than 99.5% antibacterial efficiency. In 2016, Zhang and coworkers reported a silk nanofiber air filter, which showed a filtration efficiency of 98.8% for PM2.5 and 96.2% for 300 nm particles with a low pressure drop [56]. In addition, Ag nanoparticles could be easily incorporated into silk nanofibers, enabling antibacterial activity into the air filter, against Escherichia coli, a typical Gram-negative bacterium, and Staphylococcus aureus, a typical Gram-positive bacterium [129–131]. Recently, Xiong and coworkers introduced silver nanoparticles into nanofibers to prepare soy protein isolate (SPI)/polymide-6 (PA6)/Ag nanofibrous air filter, which exhibited over 95% filtration efficiency of PM (PM size less than 0.3 μm) and prevented growth of microorganisms (E. coli and Bacillus) over the filter media [132]. In addition to these metal nanoparticles, carbon nanotubes (CNTs) are also antibacterial candidates to cause damages of bacterial cells, which penetrate into the interior of the bacteria cell and affect the cell division process [133–135]. In 2015, Zhong and coworkers created multiwalled carbon nanotubes (MWCNTs) on a porous alumina ceramic membrane via CVD method [75]. The hybrid filters exhibited high PM filtration efficiency and antibacterial property. As shown in Figure 6.19, the presence of CNTs strongly inhibited the propagation of bacteria on the filters with the antibacterial rate at each test time of 61.90% (40 minutes), 88.57% (80 minutes), and 97.86% (120 minutes), respectively.

213

6 Emerging Nanofibrous Air Filters for PM2.5 Removal

(a)

(c)

(b)

(e) 100

(d)

Antibacterial rate (%)

214

80 60 40 20 0

40

80 Time (min)

120

Figure 6.19 (a) The antibacterial results of the pristine filter for 40, 80, and 120 minutes. (b), (c), and (d) were the antibacterial results of the composite filter for 40, 80, and 120 minutes, respectively. (e) Antibacterial rate of the composite filter at different test time (40, 80, and 120 minutes). Source: Zhao et al. 2015 [75]. Reprinted with permission of Royal Society of Chemistry.

6.4.8

Air Filtration and Oil Removal

In 2018, Chen and coworkers designed a multifunctional inorganic aerogels with the combination of PM removal and oil/water separation [136]. Hydroxyapatite (HAP) nanowire-based inorganic aerogels were fabricated in large quantity using HAP nanowires by freeze drying, forming three-dimensional interconnected highly porous meshwork structure in aerogels. The as-prepared HAP nanowire aerogel showed ultralow density (8.54 mg cm−3 ), high porosity (∼99.7%), high elasticity, and ultralow thermal conductivity (0.0387 W m−1 K−1 ). The as-prepared HAP nanowire aerogel can be used as the highly efficient air filter with high PM2.5 filtration efficiency and low airflow resistance. As shown in Figure 6.20a, the removal efficiency for PM2.5 increased with the increasing thickness of the aerogel filter, reaching at 99% with the filter thickness of 9 mm at high PM concentrations up to 1800 μg m−3 . The aerogel remained stable after standing 120 hours for continuous air purification (Figure 6.20b). Additionally, the hydrophobic HAP nanowire aerogel was applicable for continuous oil/water separation (Figure 6.20c). In 2018, Wang and coworkers demonstrated a superoleophobic surface that can improve the filtration efficiency for the separation of small oil mists from the air with low airflow resistance [137]. Such superoleophobic surface was prepared by using perfluoroalkyl acrylic copolymer-coated commercial glass fibrous filter via the dip-coating method. As illustrated in Figure 6.21a,b, oil droplets tended to spread along oleophilic fibers and accumulated in the intersection region. In contrast, when small oil mists reached the superoleophobic filters, they would easily bounce back and forth, became larger by colliding with each other, and eventually drained away along the fibers. As a consequence, the superoleophobic treatment showed a significant increase in oil mist filtration. A 1.12 mm thick superoleophobic filter showed a filtration efficiency of 99.44% for small oil mists and almost 100% for large oil mists.

100 80 60

3 mm, 2.51 cm s−1 3 mm, 3.35 cm s−1 6 mm, 2.51 cm s−1 6 mm, 3.35 cm s−1 9 mm, 2.51 cm s−1 9 mm, 3.35 cm s−1

40 20 0 0

(a)

300 600 900 1200 1500 1800 PM2.5 concentration (μg m–3)

Removal efficiency (%)

Removal efficiency (%)

6.4 Applications 100 80 60

PM2.5 concentration: 300–600 (μg m–3) Velocity: 3.35 cm s–1

40 20 0 0

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(b)

60 80 Time (h)

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Oil absorption Pump

Hydrophobic HAP aerogel Oil phase Water phase (c)

Figure 6.20 (a) PM2.5 removal efficiencies of the hydrophobic HAP nanowire aerogel filter with different thicknesses at different PM2.5 concentrations and airflow velocities. (b) PM2.5 removal efficiencies of the hydrophobic HAP nanowire aerogel filter (thickness 9 mm) for a long period of time (120 hours). (c) Schematic illustration of the continuous oil/water separation device prepared using the hydrophobic HAP nanowire aerogel. Source: Zhang et al. 2018 [136]. Reprinted with permission of American Chemical Society.

Upstream

Downstream

Filter

Drainage

(a)

Re-entrainment

Superoleophobic

Bounce

Collision

Drainage

Fiber 2 1

Coalescence

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Oil mist

Oleophilic

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(b)

Figure 6.21 (a) Oil disposal routes during oil mist filtration. (b) Schematic illustration of oil mist interaction with fibers with different surface wettabilities. Source: Wei et al. 2018 [137]. Reprinted with permission of Royal Society of Chemistry.

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6.5 Conclusion In conclusion, nanofibrous air filters have evolved rapidly in the past 10 years and have shown great PM2.5 removal performance along with other desirable features (e.g. high transparency, large-scale production, high thermal stability, thermal management, electricity assistance, the removal of toxic gases chemicals, antibacterial property, and oil removal). However, there are challenges lying ahead as summarized in the following. In the current designs, the majority of the nanofibers were deposited on nonwoven substrates to construct composite filter media, and the active nanofibrous layers could not stand alone due to their low mechanical strength. Thus, increasing mechanical strength and the filtration performances of the nanofibrous layers deserve more attention. Continuous and inexpensive production procedure of nanofibrous air filters must be developed to further reduce their production cost and affordability. Many of the current nanofibrous filter designs have multiple functions, but they are put together plainly without synergy. Thus one of the future directions of nanofibrous air filters is to have more intelligence with multifunctions being smartly integrated into one device with feedback communication cycle to maximize its performance. The self-cleaning and/or antifouling capability would improve the filters’ longevity, and better thermal management can further increase their comfort during their use [100]. The incorporation of energy harvesting and generating materials (e.g. piezoelectric or triboelectric materials) would make possible some unprecedented applications, such as air filter with self-powered environmental sensors and air filters with their own lighting systems. Last but not the least, the interaction mechanisms between PM particles and nanofibers have been paid little attention in the past and are largely unclear, so more fundamental and detailed experimental investigations are warranted. With a clearer understanding to the interaction mechanisms, more effective air filter can thus be rationally designed and fabricated in the future.

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chemical-warfare agents. Angewandte Chemie International Edition 54 (23): 6790–6794. Mondloch, J.E., Katz, M.J., Isley, W.C. III et al. (2015). Destruction of chemical warfare agents using metal-organic frameworks. Nature Materials 14 (5): 512–516. Koo, W.-T., Jang, J.-S., Qiao, S. et al. (2018). Hierarchical metal-organic framework assembled membrane filter for efficient removal of particulate matter. ACS Applied Materials & Interfaces 10 (23): 19957–19963. Bian, Y., Wang, R., Wang, S. et al. (2018). Metal-organic framework-based nanofiber filters for effective indoor air quality control. Journal of Materials Chemistry A 6 (32): 15807–15814. Dai, Z., Su, J., Zhu, X. et al. (2018). Multifunctional polyethylene (PE)/polypropylene (PP) bicomponent fiber filter with anchored nanocrystalline MnO2 for effective air purification. Journal of Materials Chemistry A 6 (30): 14856–14866. Rong, S., Zhang, P., Liu, F., and Yang, Y. (2018). Engineering crystal facet of α-MnO2 nanowire for highly efficient catalytic oxidation of carcinogenic airborne formaldehyde. ACS Catalysis 8 (4): 3435–3446. Sekine, Y. (2002). Oxidative decomposition of formaldehyde by metal oxides at room temperature. Atmospheric Environment 36 (35): 5543–5547. Xiu, Z.-m., Zhang, Q.-b., Puppala, H. L., Colvin, V. L. and Alvarez, P. J. (2012). Negligible particle-specific antibacterial activity of silver nanoparticles. Nano Letters 12 (8): 4271–4275. Quadros, M.E. and Marr, L.C. (2011). Silver nanoparticles and total aerosols emitted by nanotechnology-related consumer spray products. Environmental Science & Technology 45 (24): 10713–10719. Wang, X., Yang, F., Yang, W., and Yang, X. (2007). A study on the antibacterial activity of one-dimensional ZnO nanowire arrays: effects of the orientation and plane surface. Chemical Communications (42): 4419–4421. Applerot, G., Lipovsky, A., Dror, R. et al. (2009). Enhanced antibacterial activity of nanocrystalline ZnO due to increased ROS-mediated cell injury. Advanced Functional Materials 19 (6): 842–852. Ghule, K., Ghule, A.V., Chen, B.-J., and Ling, Y.-C. (2006). Preparation and characterization of ZnO nanoparticles coated paper and its antibacterial activity study. Green Chemistry 8 (12): 1034–1041. Okyay, T.O., Bala, R.K., Nguyen, H.N. et al. (2015). Antibacterial properties and mechanisms of toxicity of sonochemically grown ZnO nanorods. RSC Advances 5 (4): 2568–2575. Chen, X. and Mao, S.S. (2007). Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. Chemical Reviews 107 (7): 2891–2959. Lee, W.S., Park, Y.-S., and Cho, Y.-K. (2015). Significantly enhanced antibacterial activity of TiO2 nanofibers with hierarchical nanostructures and controlled crystallinity. Analyst 140 (2): 616–622. Alboofetileh, M., Rezaei, M., Hosseini, H., and Abdollahi, M. (2014). Antimicrobial activity of alginate/clay nanocomposite films enriched with essential oils against three common foodborne pathogens. Food Control 36 (1): 1–7.

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7 Intelligent Micro/Nanomotors in Environmental Sensing and Remediation Micro/nanomotors (or rockets, robots) are microscale or nanoscale devices that self-propel in an aqueous media by converting physical and (bio)chemical energy into motion, similar to their macroscopic counterparts. The concept of micro/nanomotors was first proposed by Sen and Mallouk et al. in the year 2004 [1] (Figure 7.1). They reported for the first time rod-shaped particles, 370 nm in diameter and consisting of 1 μm long Pt and Au segments. These rod-shaped particles moved autonomously in aqueous hydrogen peroxide solutions by catalyzing decomposition of hydrogen peroxide at the Pt end. Almost at the same time, Manners and Ozin prepared gold–nickel nanorods having the gold end anchored to the surface of a silicon wafer, and they produced circular movements in the presence of the hydrogen peroxide fuel [2]. Since then, there has been a rapid development on the self-propelled micro/nanomotors, and a variety of propulsion mechanisms have been employed, mainly including microbubble generation, concentration gradient-induced self-diffusiophoresis, external fields, and so on [3–9]. For instance, the generation of gas bubble by the catalytic or noncatalytic decomposition of chemical fuels, such as H2 O2 , Br2 , or I2 solutions and hydrazine, acidic, and alkaline solutions, has been widely employed for the propulsion of bimetal nanowire motors [10]. On the other hand, some chemical reactions that occur at surfaces of micro/nanomotors can consume reactants and generate products, leading to concentration gradients that in turn power the motion of micro/nanomotors and pumps. Although autonomous self-propulsion is very attractive, the requirement of chemical fuels might be problematic for some special applications of chemically powered micro/nanomotors, because most of the chemical fuels are not compatible with living systems. Therefore, propulsion by an external field provides possibilities for the application of micro/nanomotors in vivo, which mainly include the use of light, electric, magnetic, and ultrasonic sources [5, 11, 12]. Currently, micro/nanomotors can be obtained by various strategies mainly including electrochemical/electroless deposition, physical vapor deposition, strain engineering, and self-assembly [10], which have been well summarized in some recent review articles and thus will not be further covered in this chapter.

Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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2H2O2

370 nm

Au

2H2O + O2

Pt

Figure 7.1 Schematic diagram of a Pt/Au nanorod showing the dimensions used in the calculation of interfacial forces. Source: Paxton et al. 2004 [1]. Reprinted with permission of American Chemical Society.

~2 μm

Self-propelled micro/nanomotors provide unique mixing mechanism without external stirring because of their capability of continuous movement with high speed, thus overcoming the diffusion limit and enhancing mixing efficiencies. In addition, these micro/nanomotors usually have high specific surface area, and after surface functionalization, they can exhibit enhanced catalytic properties. Due to these unique merits, micro/nanomotors have been widely exploited in many areas, such as targeted drug delivery and biosensors [9, 13–16]. In particular, when the chemical fuels are involved in the movement of the catalytic motors, the self-propelled catalytic micro/nanomotors provide great promise for the current focus of environmental problems, ranging from the environmental sensing to decontamination, because the catalytic products would be helpful to the degradation reaction. It is for this reason catalytic micro/nanomotors have been attracting increasing attention in the multidisciplinary environmental area. In this chapter, we will briefly discuss the development of micro/nanomotors in terms of their propelling mechanism and review the recent progress of micro/nanomotors in environmental applications, including water quality sensors, miniature absorbents, and self-propelled catalyst or antibacterial agents.

7.1 Self-Propelling Mechanism of Micro/Nanomotors 7.1.1

Self-Electrophoretic Mechanism

Electrophoresis is the movement of charged objects under an electric field. Typically, an electric field is generated across the motor in a fluid. This electric field drives the motion of the charges on the surface of the motor creating a slip velocity [2, 17–21]. The first example of such self-propelling micro/nanomotor was reported by Sen and Mallouk et al. in the year 2004 [1], although at that time the mechanism of propulsion was not fully understood. In this work, the authors created a bimetallic microrod, approximately 2 mm in length and 400 nm in diameter, with a catalytically active platinum end and an inert gold end (Pt/Au micromotors). The decomposition of H2 O2 generated a proton gradient along the axis of the microrod in solution, and therefore the negatively charged microrod responded by moving toward the proton-rich solution by the self-electrophoretic mechanism. A similar mechanism was then employed by Mano and Heller for the movement of carbon fibers that were impregnated with glucose oxidase on one end and an oxygen reductase at the other [22], and a bioelectrochemical reaction was responsible for the propulsion.

7.1 Self-Propelling Mechanism of Micro/Nanomotors

H2O2

2H+ + 2e– + H2O2

Fluid flow Pt

e–

Au

H+ 2H+

–

+ 2e + O2

2H2O

Figure 7.2 A schematic illustrating self-electrophoresis. H2 O2 is oxidized to generate protons in solution and electrons in the wire on the Pt end. The protons and electrons are then consumed with the reduction of H2 O2 on the Au end. The resulting ion flux induces motion of the particle relative to the fluid, propelling the particle toward the platinum end with respect to the stationary fluid. Source: Paxton et al. 2006 [23]. Reprinted with permission of American Chemical Society.

Then, in the year 2006, Mallouk, Sen, and coworkers explored the role of electrokinetics in the spontaneous motion of platinum–gold nanorods suspended in H2 O2 solutions (Figure 7.2) [23]. The electrochemical decomposition pathway was confirmed by measuring the steady-state short-circuit current between platinum and gold interdigitated microelectrodes (IMEs) in the presence of H2 O2 . The resulting ion flux from platinum to gold implied an electric field in the surrounding solution that can be estimated from Ohm’s law. This catalytically generated electric field could in principle bring about electrokinetic effects that scale with the Helmholtz–Smoluchowski equation. Accordingly, the authors observed a linear relationship between bimetallic rod speed and the resistivity of the bulk solution. Furthermore, they found that the catalytically generated electric field in the solution near a Pt/Au IME in the presence of H2 O2 was capable of inducing electroosmotic fluid flow that could be switched on and off externally. They experimentally demonstrated that the velocity of the fluid flow in the plane of the IME was a function of the electric field, whether catalytically generated or applied from an external current source. These findings indicated that the motion of Pt/Au nanorods in H2 O2 was primarily due to a catalytically induced electrokinetic phenomenon and that other mechanisms, such as those related to interfacial tension gradients, played at best a minor role. 7.1.2

Microbubble Propulsion Mechanism

Bubble propulsion is another important motion mechanism. Micro/nanomotors that utilize this type of motion can generate bubbles on their catalytic side, and the force from the release of the bubbles causes the motion [8, 24–29]. This is a gradient-like mechanism since bubbles need to be asymmetrically generated on one side and not the other, and thus there is a difference in bubble concentration with distance. Usually, larger, hollow, rod-shaped micromotors are driven by this kind of bubble propulsion mechanism. Sanchez and coworkers prepared rolled-up thin Ti/Au film microtube motor, and the inner Au layer was functionalized with catalase, which is one of the most efficient enzymes to decompose hydrogen peroxide to generate oxygen. The hybrid microtubes (Ti/Au-catalase)

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2H2O2

(a)

2H2O + O2

(b)

Figure 7.3 (a) Open view of the hybrid biocatalytic microengine; (b) surface modification of inner Au layer and enzymatic decomposition of peroxide fuel. Source: Sanchez et al. 2010 [30]. Reprinted with permission of American Chemical Society.

showed autonomous motion when immersed in peroxide solution, whereby the enzyme decomposes peroxide into water and molecular oxygen, which, in turn, propels the microengine (Figure 7.3) [30]. In order to avoid the use of H2 O2 and to develop more green bubble propulsion mechanism, Wang and coworkers provided the first example of a water-driven bubble-propelled micromotor that eliminates the requirement for the common hydrogen peroxide fuel. Such water-driven Janus micromotor was obtained by partially coating aluminum microparticles via microcontact of liquid gallium, which resulted in Al/Ga binary alloy microspheres. Hydrogen gas bubbles can be generated from the exposed Al/Ga alloy hemisphere side upon its contact with water, providing a powerful directional propulsion. Such spontaneous generation of hydrogen bubbles relies on the rapid reaction between the aluminum alloy and water. The resulting water-driven spherical motors can move at remarkable speeds of 3 mm s−1 (i.e. 150 body length per second) while exerting large forces exceeding 500 pN. Factors influencing the efficiency of the aluminum–water reaction and the resulting propulsion behavior and motor lifetime, including the ionic strength and environmental pH, were also investigated in this study. The resulting water-propelled Al/Ga/Ti motors moved efficiently in different biological media (e.g. human serum) and hold considerable promise for diverse biomedical or industrial applications [29]. 7.1.3

Self-Diffusiophoresis Propulsion Mechanism

Diffusiophoresis is a phenomenon in which the motion of particles is driven by a concentration gradient of solutes [31–38]. Both the concentration of electrolyte and nonelectrolyte can contribute to this self-diffusiophoresis. In the cases of micro/nanomotors, chemical reactions taking place at surfaces consume

7.1 Self-Propelling Mechanism of Micro/Nanomotors

reactants and generate products, leading to concentration gradients that in turn power the motion of synthetic motors. The phenomenon of electrolyte diffusiophoresis was first experimentally demonstrated by Ebel et al. in the year 1988 [39], although at that time the concept of micro/nanomotors was not introduced. In terms of powering the micro/nanomotors, the most commonly employed self-diffusiophoresis mechanism relies on gradients of H+ or OH− ions, because of their fast diffusion nature. Mallouk and Sen et al. first reported the micromotors propelled by the electrolyte self-diffusiophoresis mechanism. In this work, they exploited silver chloride particles as the motors in the presence of UV light. These silver chloride particles can react with water to produce protons, chloride ions, and hypochlorous acid at their surfaces. Since the diffusion of the protons was much faster than that of the chloride ions, inward electrical fields were generated, leading to electrophoresis of the particles and electroosmosis along the wall [40]. In the year 2011, Sen and coworkers reported a polymerization reaction-driven micromotor by using the nonelectrolyte self-diffusiophoresis mechanism [41]. Such micromotors were prepared based on gold–silica Janus particles, which were obtained by partially depositing Au on 0.96 μm silica particles. The particles were then chemically modified with the Grubbs’ catalyst on the silica side. The authors showed that the motors could move in the presence of norbornene monomer, which was powered by ring-opening metathesis polymerization (ROMP) of norbornene. These motors show increased diffusion of up to 70% when placed in solutions of the monomer. 7.1.4

External Field-Driven Micro/Nanomotors

In some cases of biological or environmental applications, the use of chemical fuels can be problematic for the motors. For example, the consumption of chemical fuels might result in a limited lifetime of the micro/nanomotors. In addition, the insufficient degradation of the chemical fuels could cause the toxicity. Therefore, it is highly desired to develop fuel-free micro/nanomotors [12]. In this context, external fields can be readily employed to power the motion of the motors. Light is one of the most versatile power sources and provides very convenient control of energy input for the micro/nanomotors together with the wide availability of optical techniques. The mechanism for light-driven micro/nanomotors can be attributed to photocatalytic propulsion, photothermal propulsion, photoinduced deformation propulsion, and so on. The photocatalytic propulsion can be found in the micro/nanomotors constructed with photocatalytic components (e.g. TiO2 materials). Sen and coworkers presented the first example of TiO2 micromotors and micropumps. The authors employed commercially available anatase TiO2 particles (size range 0.2–2.5 μm), which were co-dispersed with nonreactive tracer silica particles (SiO2 , 2.34 μm) at a density of about 7 × 109 m−3 , to verify this concept. They found that the TiO2 particles moved at 10 ± 3 μm s−1 upon UV exposure, and the movement stopped immediately after the UV source was removed. Upon the exposure to UV light, the neighboring colloidal particles immediately moved away from the TiO2 particle in the center,

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0s

UV on 2 s

UV on 5 s

UV on 30 s

UV on

2s

UV off 5 s

UV off 15 s

UV off 30 s

UV off

50 μ

Figure 7.4 The SiO2 –TiO2 Janus particles in deionized water aggregate in the absence of UV and repelled each other when UV is on. The process is reversible. Source: Hong et al. 2010 [42]. Reprinted with permission of John Wiley and Sons.

whether they were negatively or positively charged. Nearby TiO2 particles moved away from each other as well (Figure 7.4). Such motion of a TiO2 particle under UV irradiation can be attributed to the diffusiophoresis induced by the photocatalytic reaction, which produces more product molecules than the reactants consumed, making it possible to propel a TiO2 particle [42]. The photothermal effect for self-propulsion, also known as thermophoresis, refers to the generation of a temperature gradient around the Janus particle when it is irradiated with light. The asymmetric distribution of temperature across the particle leads to the circulation of the surrounding liquid and a resulting forward motion of the particle. Sano and coworkers studied self-propulsion of Janus colloidal particle under laser irradiation, which was obtained by thermal evaporation of gold to create a 25 nm thick coating on the half-side of microscale silica or polystyrene particles [43]. The authors successfully demonstrated that the motion of the Janus motors was caused by self-thermophoresis: i.e. absorption of a laser at the metal-coated side of the particle created local temperature gradient, which in turn drove the particle by thermophoresis. To clarify the mechanism, temperature distribution and a thermal slip flow field around a microscale Janus particle were measured for the first time in this work. With measured temperature drop across the particle, the speed of self-propulsion was corroborated with the prediction based on accessible parameters. Structural deformation of materials induced light irradiation can be used for the propulsion of micro/nanomotors. Fischer and coworkers showed that soft micromotors consisting of photoactive liquid-crystal elastomers could be driven by structured monochromatic light to perform sophisticated biomimetic motions. The authors realized continuum yet selectively addressable artificial microswimmers that generated traveling wave motions to self-propel without external forces or torques, as well as microbots capable of versatile locomotion behaviors on demand. The principle of using structured light can be extended

7.2 Self-Propelled Micro/Nanomotors as Environmental Sensors

to other applications that require microscale actuation with sophisticated spatiotemporal coordination for advanced microrobotic technologies [44]. In the meanwhile, other external fields such as magnetic, ultrasonic, and electrical fields can be also used for powering of the motion of micro/nanomotors [45–47]. In addition, these external fields can be also employed to achieve the locomotion. With these powering and locomotion mechanism, various micro/nanomotors have been prepared in the recent decade, providing foundations for the applications in environmental sensing and remediation.

7.2 Self-Propelled Micro/Nanomotors as Environmental Sensors

Speed (μm s−1)

Environmental monitoring or sensing is required to protect the public and the environment from toxic contaminants and pathogens that can be released into a variety of media including air, soil, and water. The surroundings of the self-propelled micro/nanomotors play a critical role for the motion behavior of the catalytic micro/nanomotors, and therefore the observation of the motion of micro/nanomotors directly provides information on the water parameter, which constitutes the first water quality sensor. In the year 2009 [48], Wang and coworkers for the first time reported the pollutant effect on the micromotor speed for motion-based detection of silver ions. In this study, they prepared bi-segment Au/Pt nanowire-based nanomotors exhibiting autonomous self-propulsion due to electrocatalytic decomposition of hydrogen peroxide fuel. In the presence of trace Ag ions, dramatic and specific acceleration of bimetal nanowire motors were observed, which could be exploited as the Ag ion sensor and offers highly selective, sensitive, and simple measurements of trace silver based on direct visualization (Figure 7.5).

Ag+

+

2+

K Pd

(a)

2+

Ni

In

3+

2+

+

2+

2+

Mn Ag Cd Ca Cu

2+

2+

3+

Pb Bi

(b)

Figure 7.5 Motion-based sensing of trace silver ions using catalytic Au/Pt nanowire motors. (a) Selectivity: bar graph comparing the nanomotor speed in 11 different 100 μM metal nitrate salt solutions (of K+ , Pd2+ , Ni2+ , In3+ , Mn2+ , Ag+ , Cd2+ , Ca2+ , Cu2+ , Pb2+ , and Bi3+ ). (b) Accelerated propulsion of catalytic nanomotors in the presence of silver ions. Source: Kagan et al. 2009 [48]. Reprinted with permission of American Chemical Society.

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7 Intelligent Micro/Nanomotors in Environmental Sensing and Remediation

PEDOT SSS

Au S S S OH

O2 H2O

OH

SSS OH OH

OH OH

Catalase H2O2

SSS OH OH

SSS OH OH

0.9 Normalized speed

234

0.6

0.0 (a)

0 min

0.3

0

4 min

3 6 Time (min)

9

(b)

Figure 7.6 (a) Scheme illustrating the pollutant effect on the microfish locomotion speed through inhibition of the catalase biocatalytic layer (bottom) along with the protocol used for immobilizing the enzyme at the inner gold surface of the tubular microengine through a mixed self-assembled binary monolayer of 11-mercaptoundecanoic acid (MUA)/6-mercaptohexanol (MCH) alkanethiols (top). (b) Changes in the swimming behavior of the artificial microfish as a function of time upon exposure to 100 μMHg (black square), 0.6 mM Cu (purple stars), 25 μM sodium azide (red circle), 625 mM aminotriazole (blue triangle), and a control experiment without the toxins (green diamond). Inset: Time-lapse images of the microfish recorded after zero and four minutes swimming in a 100 μMHg solution. Scale bar, 6.0 μm. Source: Orozco et al. 2012 [49]. Reprinted with permission of American Chemical Society.

In addition, by mimicking the live-fish water testing, the same group also reported a biocatalytic nanomotor-based water quality sensor. In this work, they delicately designed and prepared enzyme-powered biocompatible polymeric (PEDOT)/Au-catalase tubular nanomotors. The sensing mechanism of this design relied on the toxin-induced inhibition of the enzyme catalase, responsible for the biocatalytic bubble propulsion of tubular microengines (Figure 7.6a). The locomotion and survival of the artificial microfish are thus impaired by exposure to a broad range of contaminants that lead to distinct time-dependent irreversible losses in the catalase activity and hence of the propulsion behavior (Figure 7.6b). Such use of microfish offers highly sensitive and directly optical visualization of changes in the swimming behavior in the presence of common contaminants and hence can be used as a real-time assessment of the water quality. Quantitative data on the adverse effects of the various toxins upon the swimming behavior of the enzyme-powered artificial swimmer were obtained by estimating common ecotoxicological parameters, including the EC50 (exposure concentration causing 50% attenuation of the microfish locomotion) and the swimmer survival time (lifetime expectancy). Such novel use of artificial microfish addresses major standardization and reproducibility problems as well as ethical concerns associated with live-fish toxicity assays and hence offers an attractive alternative to the common use of aquatic organisms for water quality testing [49]. Similarly, Pumera and coworkers smartly utilized the poisoning effects of catalytic Pt to prepare bubble-propelled nonenzyme Cu/Pt-based microfish motors for the detection of Pb ions in water. Platinum has been demonstrated to show tunable catalytic ability in the presence of a poisoning agent because

7.2 Self-Propelled Micro/Nanomotors as Environmental Sensors

minute concentrations of heavy metals are known to retard the efficiency of catalytic convertors. To this end, the authors used Cu/Pt concentric bimetallic microtubes that were synthesized with a modified electrochemical deposition procedure on a cyclopore polycarbonate template. The microfish motors were powered by the bubble propulsion caused by the catalytic decomposition of hydrogen peroxide at its inner surface of platinum to oxygen. The expulsion of the bubble at the ends propelled the microfish robots forward. In this study, the influence of heavy metals over the viability of microfish motors was investigated. Two principal representative ions of Pb2+ and Cd2+ , commonly present in polluted water, were used as poisons to cripple the inorganic Pt microfish motors. The authors found that a greater decrease in the activity and the velocity of inorganic Pt microfish motors was observed during poisoning with Pb2+ compared with Cd2+ , which can be attributed to the inherent properties of the Pb ion adsorption behavior with platinum. The distinct behavior of Pb2+ with Cu/Pt microfish motors allowed the selective detection of Pb2+ over Cd2+ . This study provided a potential pathway for a continuous monitoring of pollutants by an optical visualization of the activity and mobility of the inorganic Pt microfish robots [50]. Matsui and coworkers employed a different monitoring mechanism to develop sensing micromotor [51]. In their study, the authors developed a new hybrid peptide-metal–organic framework (MOF) motor system that could create motion by releasing diphenylalanine (DPA) peptides from highly organized pores, and most importantly they could sense toxic heavy metals in solution and swim toward the targets. The reconfiguration of peptide self-assembly at the MOF–water interface is a driving force for the motion by creating the surface tension gradient via the asymmetric hydrophobic domain distribution around MOFs (Figure 7.7a). The sensing and directional motion of the peptide-MOF motor was programmed by the pH-sensitive assembly of DPA peptide; the pH gradient around targets generated by Pb-binding enzymes disassembled peptides on the MOF when the peptide-MOF motor moved across this targeted area (Figure 7.7b). Then, the motion of MOF motor was slowed as the MOF moved closer to the area, and eventually it stopped at the highly Pb-concentrated location. This peptide-MOF motor system is one of the fine mimetic examples of chemotaxis that can direct the motion by sensing the location of the target. Beside the cations, micro/nanomotors can be used for the detection of extremely dangerous nerve agents [52]. Wang and coworkers first demonstrated that biocatalytic nanomotors can be exploited for the sensing nerve-agent vapor plumes and were capable of detecting chemical threats present in their surrounding atmosphere [53]. Such biocatalytic nanomotors were fabricated by a membrane template electrodeposition method, which involved the electropolymerization of the EDOT monomer on the walls of the membrane micropores, followed by electrodeposition of the inner Au layer. After dissolving the membrane and release of the resulting PEDOT/Au micromotors, catalase was immobilized onto the Au inner layer of the tubular microengine. The immobilized catalase on the inner Au surface was extremely useful for the breakdown of the hydrogen peroxide fuel and generation of the oxygen bubble. The propulsion efficiency of these biocatalytic micromotor probes thus depends on the influence

235

Pb-binding enzyme

Disassembled DPA peptides

Peptide-MOF swimmer

Ted Cu2+

Motion stopped PbSe QDs

L = 1,4-benzenedicarboxyl Ted = triethylenediamine

L

PDMS

Targeted post (PbSe QDs)

Step (i)

Acidic post (gel: pH 4)

Step (ii)

Metal–organic framework (MOF)

(a)

CH2

CH2

H2N CH C N

CH

O H

C OH O

Diphenylalanine (DPA) (b)

Step (iv)

Step (iii)

Figure 7.7 (a) Design of the peptide-MOF motor to swim toward high pH. (Left) The robust reassembly of released hydrophobic DPA peptides at the edge of MOF creates the asymmetric surface tension distribution that can power the motion toward high surface tension side. (Right) The change of pH gradient in environment triggers the completion of motion because higher pH condition disassembles DPA peptides on the MOF. (b) Scheme of directed motion of peptide-MOF motors by sensing Pb with the aid of Pb-binding enzymes. After urease-conjugated Pb-binding peptide are dropped in the solution, urease is bound to PbSe quantum dots (QDs) (step i). Urea in the solution generates NH3 around the target (step ii), and then the dropped peptide-MOF motor starts swimming (step iii). After the motion is directed toward Pb target, the movement is completed at the target due to high pH gradient (step iv). Source: Ikezoe et al. 2015 [51]. Reprinted with permission of American Chemical Society.

7.2 Self-Propelled Micro/Nanomotors as Environmental Sensors

of the toxic vapor, partitioned into the probe solution, upon enzyme activity. In the presence of chemical vapor, the authors found that the biocatalytic catalase activity was inhibited, resulting in a deceased bubble-generation frequency and a greatly impaired locomotion of the biocatalytic nanomotors. These biocatalytic nanomotors thus offered a simple and direct real-time visualization of changes in the swimming behavior induced by toxic chemical vapor. This leads to a new approach for remote detection of toxic chemical plume based on autonomously moving micromotors that are responsive to the presence of volatile species in their surrounding atmosphere. Based on the similar consideration, Pumera and coworkers showed that the self-propulsion catalytic micromotors could be used for monitoring of organic molecules. In this work, they prepared Cu/Pt concentric bimetallic micromotors and found that the motion of the catalytic micromotors could be used to monitor dimethyl sulfoxide (DMSO). The authors successfully demonstrated that organic molecules containing sulfur moieties significantly inhibited the motions of the catalytic micromotors. The inhibition of such motions can take place in two ways: (i) quenching of • OH radicals generated by the Pt-catalyzed disproportionation of H2 O2 and (ii) the poisoning of the Pt catalyst surface [54]. By combining molecularly imprinted polymers (MIPs), Chen and coworkers developed an attractive magnetic micromotor-based sensor for selective recognition, enrichment, and transport of label-free fluorescent phycocyanin. The MIP-based catalytic micromotor was fabricated through the template electrochemical deposition method using phycocyanin as the imprinting molecule, Ni (0.55%) as the magnetic navigation material, and Pt (24.55%) as the solid support/catalyst to facilitate free movement in solutions. An additional magnetic field was employed for trajectory control. In addition, highly efficient targeted identification and enrichment abilities were demonstrated based on the magnetically imprinted layer. More excitingly, no obvious interference was found from complicated matrices such as seawater samples, along with real-time visualization of phycocyanin loading and transport. The sensing strategy would not only provide potential applications for rapid microscale monitoring of algae blooms but also enrich the research connotations of protein imprinting [55]. Persistent organic pollutants (POPs) are toxic chemicals that can accumulate to hazardous levels in living organisms and in the food chain [56], among which phthalate esters (PAEs) are commonly used as plasticizers in many food packaging materials and are considered a new class of “indirect food” additives. The exposure for PAEs in humans might bring the potentially carcinogenic risks, and it is thus highly desirable for the development of fast and novel methodologies for routine analysis of such compounds in foodstuff and biological samples. Jurado-Sánchez, Escarpa, and coworkers reported “shoot and sense” Janus micromotors for the simultaneous detection and degradation of PAEs in food and biological samples [57]. In this study, Mg/Au Janus micromotors were employed as disposable analytical platforms for the degradation of the non-electroactive diphenyl phthalate (DPP) into phenol, which can be directly measured by differential pulse voltammetry (DPV). Upon contact with the chloride-enriched samples, the Mg surface was readily oxidized, resulting in the generation of hydrogen microbubbles and hydroxyl ions for DPP

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(a)

(b)

108 ± 18 μm s−1

(c)

296 ± 40 μm s−1

t=0s

t=0s t=1s

t=1s

(d)

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Figure 7.8 Time-lapse images showing the efficient motor movement in milk (a), water (b), whiskey (c), and (d) human serum. Top part shows tracking line images illustrating the motor’s propulsion over a one second period in the different samples. Scale bars, 20 μm. Source: Rojas et al. 2016 [57]. Reprinted with permission of American Chemical Society.

degradation into phenol. The authors showed that the bubbles generated and micromotors’ movement greatly improved the analytical signal, increasing the sensitivity while lowering the detection potential. Such increased fluid transport imparted by the micromotors allows for the fast, direct determination of DPP in viscous samples. Samples were directly dropped into the navigating Janus Mg/Au electrode solution, thus avoiding any potential contamination from laboratory equipment. The authors successfully demonstrated the applicability of the developed platform in the determination of DPP in water, milk, whiskey, and raw human serum samples with fast analysis time, good reproducibility, and excellent recoveries (Figure 7.8). Functionalization of micro/nanomotors with fluorescent molecules or quantum dots (QDs) provides additional and most importantly specific methods for environmental sensing. By a change in the fluorescent properties of the micro/nanomotors, one can readily identify contaminates in the surrounding environments of the micro/nanomotors [52, 58, 59]. Wang et al. reported a nanomotor-based fluorescent “on/off” strategy for the rapid “on-the-fly” screening of sarin and soman by coating catalytic microspheres with the fluorophore fluoresceinamine (FLA), which reacts quickly with phosphoryl halides. Such micromotors were prepared by impregnation of FLA into silica microparticles followed by asymmetric deposition of Pt layer by a sputtering process. The continuous mixing induced by the motion of multiple micromotors across a contaminated sample results in a greatly enhanced mass transport and hence leads to increased rates of reaction between contaminated solution and nanomotors when compared with static micromotor counterparts. The rapid nanomotor-based screening can be coupled to more elaborate fluorescence enhancement strategies to identify nerve agents [52]. Wang et al. incorporated fluorescence CdTe QDs on the surface of self-propelled tubular micromotors and realized the smart chemical sensing. In this study, such micromotors were prepared by using PEDOT poly(sodium 4-styrene sulfonate) (PSS)/Pt bilayer tubular microtubes as templates, onto which highly luminescence CdTe QDs were immobilized through a layer-by-layer (LbL) assembly modification protocol via for the electrostatic interaction (Figure 7.9).

7.2 Self-Propelled Micro/Nanomotors as Environmental Sensors

(a) PDDA

PSS

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(c)

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Figure 7.9 Preparation and characterization of QD-based microsensors. (a) Schematic of the layer-by-layer electrostatic self-assembly of COOH–CdTe QDs on PEDOT microrockets: micromotors are sequentially incubated with poly(diallyldimethylammonium chloride) (PDDA) (1) and PSS (2) electrolyte solutions followed by the adsorption of the negatively charged QDs onto the outer positively charged surface of the microrockets (3). (b and c) SEM images showing the conical morphology and rough QD surface monolayer of the resulting motors. (d) Energy-dispersive X-ray (EDX) spectroscopy images illustrating the distribution of Pt, Cd, and Te in the QD micromotors. Scale bars, 500 nm. Source: Jurado-Sánchez et al. 2015 [58]. Reprinted with permission of Royal Society of Chemistry.

The authors showed that the motion-accelerated binding of trace Hg to the QDs selectively quenched the fluorescence emission and led to an effective discrimination between different mercury species and other coexisting ions [58]. In addition, considering the potential toxicity of metallic micro/nanomotor after the use, Dong, Li, and coworkers reported an all-polymer micromotor consisting of a biodegradable polycaprolactone single crystal and catalase for the sensing of the HCl or NH3 gas molecules in the atmosphere with a low detection limit [60]. Due to the biodegradability of polycaprolactone, this micromotor was capable of slowly degrading in solution. The features shown in this study, such as the metal-free structure and the gas-sensing capability, make such all-polymer micromotor potentially attractive for environmental monitoring applications. Besides the chemicals, micro/nanomotors can be also used to bind bacteria after proper modification, providing alternative means for microbial pathogen monitoring [61]. Wang and coworkers prepared self-propelled gold/nickel/ polyaniline/platinum (Au/Ni/PANI/Pt) microtubular nanomotors, functionalized with the Concanavalin A (ConA) lectin bioreceptor, which were shown to be extremely useful for the rapid, real-time isolation of Escherichia coli bacteria from fuel-enhanced environmental, food, and clinical samples (Figure 7.10). These multifunctional nanomotors combined the selective capture of E. coli with the uptake of polymeric drug-carrier particles to provide an attractive motion-based theranostic strategy. Triggered release of the captured bacteria was demonstrated by movement through a low-pH glycine-based dissociation solution. The smaller size of the new polymer–metal microengines offers

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(a) E. coil ConA Release

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Apple juice

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Figure 7.10 Schemes depicting (a) the selective pickup, transport, and release of the target bacteria by a ConA-modified microengine. (b) Surface chemistry involved on the microengines functionalization with the lectin receptor: (1) self-assembling of 11-mercatoundecanoic acid (MUA) and 6-mercaptohexanol (MCH) binary monolayer, (2) activation of the carboxylic terminal groups of the MUA to amine-reactive esters by the 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC)/N-hydroxysuccinimide (NHS) coupling agents, and (3) reaction of NHS ester groups with the primary amines of the ConA to yield stable amide bonds. (c) Images demonstrate the E. coli pickup and transport in peroxide-fuel-containing samples: drinking water, apple juice, and seawater samples. Source: Campuzano et al. 2011 [61]. Reprinted with permission of American Chemical Society.

convenient, direct, and label-free optical visualization of the captured bacteria and discrimination against nontarget cells. In another study by Wang and coworkers, they developed a micromotor-based approach for screening, capturing, isolating, and destroying anthrax simulant spores [62]. In this work, the multilayer microtubular motors, consisting of carboxy polypyrrole, i.e. (COOH-PPy): PEDOT/Ni/Pt multilayers, were prepared using a common template-directed electrodeposition approach, where the exposed carboxy moieties of the new COOH-PPy/metal tubular motors were used to covalently anchoring antibodies, and the intermediate Ni layer provided the motor with a magnetic guidance capability, while the inner catalytic Pt layer was used for the oxidation of the H2 O2 fuel and generating the O2 bubbles thrust essential for the self-propulsion. The authors showed that the new assay

7.3 Self-Propelled Micro/Nanomotors for Enhanced Organic Contamination Degradation

comprising anti-Bacillus globigii antibody-functionalized micromotors was able to navigate in a contaminated solution to recognize, capture, transport, and isolate single and multiple B. globigii spores. Effective discrimination against the excess of nontarget Staphylococcus aureus and E. coli species was demonstrated in this study, along with a successful operation in common environments where spores can be found (e.g. lake or tap water). Subsequently, the authors also demonstrated the accelerated damage (destruction) of anthrax simulant spores through the greatly enhanced mixing of mild quiescent oxidizing solutions imparted by nonfunctionalized micromotors. Similarly, the micromotor-induced mixing accelerates immunoreactions, while the similar size of both micromotors and spores allows for a convenient label-free visualization of the presence of the threat. The new micromotor strategy thus represents an effective approach for detecting the presence of biological threats and mitigating their potential harm.

7.3 Self-Propelled Micro/Nanomotors for Enhanced Organic Contamination Degradation Usually, chemical fuel (mostly hydrogen peroxide) is required for the motion of the self-propelled nanomotors [63]. At the same time, the peroxide also works as a strong oxidizing agent, which has been widely used by the environmental area for the degradation of harmful organic substances. Furthermore, the fast movement of the micro/nanomotor allows for improved contact and mixing between the active reactants and the pollutant, resulting in more efficient degradation processes. Therefore, the chemical-fuel-based catalytic micro/nanomotors can be exploited for the efficient degradation of organic pollutants [63–66]. In terms of the degradation of organic pollutants, the Fenton’s method, one of the most popular advanced oxidation processes (AOP), has been widely employed [67]. Usually, the Fenton’s reactions utilize the highly reactive hydroxyl radical as their main oxidizing agent. The generation of HO• in the Fenton method occurs by the decomposition of H2 O2 in the presence of Fe(II). Fe (II) can be regenerated in the catalytic cycle, and consequently, only a small catalytic amount of Fe(II) is required in the reaction [68]. Sanchez and coworkers for the first time employed micromotors for degrading organic pollutants in water through a Fenton process [69]. In this work, the Fe/Pt-based self-propelled micromotors were prepared by rolling up thin films of evaporated metals, with the length and diameter of the microtube motor being 500 and 40 μm, respectively. The motion of micromotors was driven by the generation of O2 bubbles in the internal Pt layer, while the external Fe layer enabled the degradation of organic pollutants. Rhodamine 6G (Rh6G) was used as a model pollutant in this work to study the water remediation efficiency of a Fenton process performed by Fe/Pt-based self-propelled micromotors. The mechanism of degradation was based on Fenton reactions relying on spontaneous acidic corrosion of the iron metal surface of the microtubes in the presence of H2 O2 , which acts both as a reagent for the Fenton reaction and as main fuel to propel the

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micromotors. Factors influencing the efficiency of the Fenton oxidation process, including the thickness of the Fe layer, pH, and concentration of hydrogen peroxide, were investigated in details. The authors showed that the ability of these catalytically self-propelled micromotors to improve intermixing in liquids resulted in the removal of organic pollutants 12 times faster than when the Fenton oxidation process is carried out without catalytically active micromotors. The synergy between the internal and external functionalities of the micromotors, without the need of further functionalization, results in an enhanced degradation of nonbiodegradable and dangerous organic pollutants at small-scale environments and holds considerable promise for the remediation of contaminated water. Very recently, Chang and coworkers also reported zerovalent iron/platinum (ZVI/Pt) Janus nanomotors with high decontamination efficiency and efficient self-propulsion properties [70]. The ZVI/Pt Janus nanomotors were fabricated by the asymmetric deposition of catalytic platinum (Pt) in one hemisphere of ZVI microspheres. In the ZVI/Pt micromotors–H2 O2 system, ZVI acts as a heterogeneous Fenton-like catalyst for the degradation of organic pollutants, while simultaneously the hemispheric Pt layer catalytically decomposes H2 O2 into water and oxygen, thereby resulting in an oxygen-bubble propulsion system. The ZVI/Pt Janus micromotors were bubble-propelled at a high speed of over 200 μm s−1 in the presence of 5% H2 O2 . In addition, complete oxidative degradation of methylene blue (MB) occurred in the presence of 5% H2 O2 after 60 minutes of treatment, whereas ZVI microspheres removed only 12% of MB in 60 minutes. The magnetic controllable and reusable properties of the ZVI/Pt nanomotors make the water purification process more attractive and feasible. Plant tissues offer attractive features for bioremediation processes owing to their high level of enzymatic activity, high thermal stability, negligible environmental impact, and extremely low costs. The application of plant tissues to treat contaminated water can be a good strategy for environmental remediation. Wang and coworker employed the unmodified natural tissue to construct self-propelled dual-function biocatalytic motors, which realized the in-motion bioremediation of contaminates. These enzyme-rich tissue motors rely on the catalase and peroxidase activities of their radish (Raphanus sativus) body for their propulsion and remediation actions, respectively. The continuous movement of the biocatalytic tissue motors through the contaminated sample facilitates the dynamic removal of phenolic pollutants. Hydrogen peroxide plays a dual role in the propulsion and decontamination processes, as the motor fuel and as co-substrate for the phenol transformation, respectively. Localized fluid transport and mixing, associated with the movement of the radish motors and corresponding generation of microbubbles, greatly improve the remediation efficiency, resulting in the maximal removal of pollutants within three minutes. The new “on-the-fly” remediation process is cost effective as it obviates the need for expensive isolated enzymes and relies on environment-friendly plant tissues [71]. On the other hand, by introducing photocatalytic components in micro/ nanomotors, photocatalytic degradation of organic contaminates can be

7.3 Self-Propelled Micro/Nanomotors for Enhanced Organic Contamination Degradation

achieved [72–74]. Especially, for some light-driven micro/nanomotors, whose motion is based on diffusiophoretic effects, the existence of organic components would accelerate the motion of the photocatalytic micro/nanomotors by enhancing the diffusiophoretic effects due to the photocatalytic degradation of the organics. This kind of photocatalytic micro/nanomotors holds great promise for the removal of the organic contaminates [75]. Wang and coworkers developed highly effective micromotor for photocatalytic degradation of chemical and biological warfare agents (CBWA) based on light-activated TiO2 /Au/Mg microspheres (Figure 7.11a–e) that propel autonomously in natural water and obviate the need for external fuel, decontaminating reagent, or mechanical agitation [72]. These motors generated highly oxidative species (such as superoxide anions, peroxide radicals, hydroxyl radicals, and hydroxyl anions) on their UV-activated TiO2 surface during their autonomous propulsion (Figure 7.11f ), which are responsible for the efficient destruction of the cell membranes of the anthrax simulant B. globigii spore, as well as rapid and complete in situ mineralization of the highly persistent organophosphate nerve agents into nonharmful products. The water-driven propulsion of the TiO2 /Au/Mg micromotors facilitated efficient fluid transport and dispersion of the photogenerated reactive oxidative species and their interaction with the CBWA. Coupling of the photocatalytic surface of the micromotors and their autonomous water-driven propulsion thus leads to a reagent-free operation that holds a considerable promise for diverse “green” defense and environmental applications. In another study, Guan and coworkers prepared water-fueled TiO2 /Pt Janus submicromotors with light-controlled motions by utilizing the asymmetrical photocatalytic water redox reaction over TiO2 /Pt Janus submicrospheres under UV irradiation [76]. The authors showed that the motion state, speed, TiO2 Mg

CWA UV

Au NP

(a)

(b) Ti

UV

Mg

BWA TiO2 Mg

(c)

(d)

(e)

(f)

Figure 7.11 (a) Schematic image of a photoactive TiO2 /Au/Mg motor showing the Mg core, Au nanoparticles, and TiO2 shell. (b) Transmission electron microscopy image of Au nanoparticles embedded in the surface of TiO2 . (c) Scanning electron microscopy image of TiO2 /Au/Mg motor. (d and e) Energy-dispersive X-ray spectroscopy images illustrating the distribution of the Ti shell and the Mg inner core, respectively. (f ) Schematic representation of the self-propulsion and photocatalytic degradation of biological warfare agents (BWA) and chemical warfare agents (CWA) by water-driven spherical TiO2 /Au/Mg micromotors. Source: Li et al. 2014 [72]. Reprinted with permission of American Chemical Society.

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aggregation, and separation behaviors of the TiO2 /Pt Janus submicromotor can be reversibly, wirelessly, and remotely controlled at will by regulating the “on/off” switch, intensity, and pulsed/continuous irradiation mode of UV light. The motion of the water-fueled TiO2 /Pt Janus submicromotor was governed by light-induced self-electrophoresis under the local electrical field generated by the asymmetrical water oxidation and reduction reactions on its surface. The TiO2 /Pt Janus micromotors can interact with each other through the light-switchable electrostatic forces, and hence continuous and pulsed UV irradiation can make the TiO2 /Pt Janus submicromotors aggregate and separate on demand, respectively. Because of the enhanced mass exchange between the environment and active submicromotors, the separated TiO2 /Pt Janus submicromotors powered by the pulsed UV irradiation showed a much higher activity for the photocatalytic degradation of the organic dye than the aggregated TiO2 /Pt submicromotors. To avoid the use of UV light and extend the working wavelength of the photocatalytic micro/nanomotors to the solar spectrum, Qu and coworkers demonstrated a motor plasmonic photocatalyst (MPP) for solar photocatalytic degradation of organic pollutants under the anaerobic stagnant conditions [77]. Such photocatalytic micromotors were prepared by a template method. First, polystyrene spheres (PS) (∼5 μm) were used as the template, onto which polydopamine (PDA) layers were coated to facilitate in situ growth of Pt nanoparticles (NPs). After deposition of PtNPs, a TiO2 layer was coated through ammonia-assisted hydrolysis of tetrabutyl titanate (TBOT). Subsequently, gold nanoparticles (AuNPs) were decorated on the outer surface of the TiO2 layer. Finally, the template was removed through calcination. The micromotors had a two surface asymmetrically functionalized TiO2 semi-shell. The outer surface of the TiO2 shell was decorated with AuNPs to form the plasmonic photocatalyst, while the inner surface of the TiO2 shell was embedded with PtNPs to form the motor structure. Under the anaerobic stagnant conditions, with the help of the fuel (H2 O2 ), the photocatalyst will be propelled by the O2 bubbles to accelerate the mass transfer. In addition, the generated O2 will support the photochemical reaction and finally cause a great increase of the photocatalytic degradation efficiency (Figure 7.12). Similarly, Ren and coworkers reported light-driven Au-WO3 @C Janus nanomotor based on colloidal carbon WO3 nanoparticle composite spheres (WO3 @C) prepared by hydrothermal treatment. The Janus nanomotors can move in aqueous media at a speed of 16 μm s−1 under 40 mW cm−2 UV light due to diffusiophoretic effects. The propulsion of such Au-WO3 @C Janus micromotors (diameter ∼1.0 μm) can be generated by UV light in pure water without any external chemical fuels and readily modulated by light intensity. After depositing a paramagnetic Ni layer between the Au layer and WO3 , the motion direction of the micromotor can be precisely controlled by an external magnetic field. Such magnetic micromotors not only facilitate recycling of motors but also promise more possibility of practical applications in the future. Moreover, the Au-WO3 @C Janus micromotors show high sensitivity toward

7.4 Self-Propelled Micro/Nanomotors as Efficient Antibacterial Agents 1.0

1.0 0.8

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Figure 7.12 Photocatalytic degradation of RB (a) and MO (b) by 0.5 mg ml−1 MPP with 5% H2 O2 under solar irradiation. (c) The photographic images of the super organic mixture (1), the mixture after photocatalytic degradation by TiO2 for 1.5 hours (2), the mixture after photocatalytic degradation by the MPP–H2 O2 system for 1.5 hours (3), and the pure water (4). (d) The corresponding absorption spectra of (3). Source: Zhang et al. 2016 [77]. Reprinted with permission of Royal Society of Chemistry.

extremely low concentrations of sodium-2,6-dichloroindophenol (DCIP) and rhodamine B (RhB). The moving speed of motors can be significantly accelerated to 26 and 29 μm s−1 in 5 × 10−4 wt% DCIP and 5 × 10−7 wt% RhB aqueous solutions (Figure 7.13), respectively, due to the enhanced diffusiophoretic effect, which results from the rapid photocatalytic degradation of DCIP and RhB by WO3 . This photocatalytic acceleration of the Au-WO3 @C Janus nanomotors holds great potential for detection and rapid photodegradation of dye pollutants in water [75].

7.4 Self-Propelled Micro/Nanomotors as Efficient Antibacterial Agents Through the introduction of antibacterial components, micro/nanomotors can be also readily endowed with bacteria-killing capability, which provides an effective route for water remediation [26, 78–82]. Wang group has made many progresses in the field of antibacterial nanomotors. They have coupled antibacterial chitosan with the efficient water-powered

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Figure 7.13 (a) The speed dependence of Au-WO3 @C Janus micromotors upon the DCIP concentration. The inset shows the scheme of degradation reaction of DCIP by WO3 @C particles. (b) Tracklines of micromotors in 5 × 10−13 , 5 × 10−11 , 5 × 10−7 , 5 × 10−4 , 3 × 10−3 , and 5 × 10−3 wt % DCIP solution with 40 mW cm−2 UV intensity, respectively, over one second. Scale bar, 10 μm. (c) The speed change of Au-WO3 @C Janus micromotors in different concentrations of DCIP and RhB solutions in contrast to pure water. (d) The photodegradation efficiency of WO3 @C toward 25 μM DCIP and 6.25 μM RhB upon the exposure to a 400 W Xe lamp for different times. Source: Zhang et al. 2017 [75]. Reprinted with permission of American Chemical Society.

propulsion of magnesium (Mg) micromotors and successfully prepared chitosan-based water-propelled micromotors with strong antibacterial activity. Such micromotors consist of Mg microparticles coated with the biodegradable and biocompatible polymers poly(lactic-co-glycolic acid) (PLGA), alginate (Alg), and chitosan (Chi), with the latter responsible for the antibacterial properties. The authors showed that the distinct speed and efficiency advantages are critically important for the environmentally friendly antibacterial capability in various control experiments by treating drinking water contaminated with model E. coli bacteria. The dynamic antibacterial strategy offers dramatic improvements in the antibacterial efficiency, compared to static chitosan-coated

7.4 Self-Propelled Micro/Nanomotors as Efficient Antibacterial Agents

microparticles (e.g. 27-fold enhancement), with a 96% killing efficiency within 10 minutes. Potential real-life applications of these chitosan-based micromotors for environmental remediation have been demonstrated by the efficient treatment of seawater and freshwater samples contaminated with unknown bacteria. Coupling the efficient water-driven propulsion of such biodegradable and biocompatible micromotors with the antibacterial properties of chitosan holds great promise for advanced antimicrobial water treatment operation [78]. In another study, Wang and coworkers smartly introduced lysozyme into nanomotors system and successfully developed lysozyme-modified fuel-free nanomotors with good bacteria-killing capability [79]. The efficient antibacterial property of lysozyme, associated with the cleavage of glycosidic bonds of peptidoglycans present in the bacteria cell wall, has been combined with ultrasound (US)-propelled porous gold nanowire (p-AuNW) motors as biocompatible dynamic bacteria nanofighters. Coupling the antibacterial activity of the enzyme with the rapid movement of these p-AuNWs, along with the corresponding fluid dynamics, promotes enzyme–bacteria interactions and prevents surface aggregation of dead bacteria, resulting in a greatly enhanced bacteria-killing capability. The large active surface area of these nanoporous motors offers a significantly higher enzyme loading capacity compared with nonporous AuNWs, which results in a higher antimicrobial activity against Gram-positive and Gram-negative bacteria. Ag is also an effective and commonly used antibacterial agent, and therefore it is reasonable and very convenient of introducing Ag into nanomotor systems to obtain nanomotors with the antibacterial property. Sánchez and coworkers described water self-propelled Janus micromotors decorated with silver nanoparticles (AgNPs) for disinfecting E. coli and removing the bacteria from contaminated water. The structure of water self-propelled Janus micromotors consists of a magnesium (Mg) microparticle as a template that also functions as propulsion source by producing hydrogen bubbles when in contact with water, an inner iron (Fe) magnetic layer for their remote guidance and collection, and an outer AgNP-coated gold (Au) layer for bacterial adhesion and improving bactericidal properties. The active motion of microbots increases the chances of the contact of AgNPs on the micromotors surface with bacteria, which provokes the selective Ag+ release in their cytoplasm, and the micromotors self-propulsion increases the diffusion of the released Ag+ ions. In addition, the AgNP-coated Au cap of the micromotors has a dual capability of capturing bacteria and then killing them. The AgNP-coated Janus micromotors were capable of efficiently killing more than 80% of E. coli compared with colloidal AgNPs that killed only less than 35% of E. coli in contaminated water solutions in 15 minutes (Figure 7.14). After capture and extermination of bacteria, magnetic properties of the cap allow collection of Janus micromotors from water along with the captured dead bacteria, leaving water with no biological contaminants. Such water self-propelled Janus micromotors offer an encouraging method for rapid disinfection of water [80]. Wang and coworkers also utilized Ag to prepare antibacterial zeolite-based micromotors by incorporation of silver ions (Ag+ ) into the aluminosilicate zeolite framework. The high antibacterial activity of Ag ions along with the

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Figure 7.14 Bactericidal assay results using AgNP-coated Janus microbots. (a) Optical microscope images of bacteria using fluorescence lamp after the bactericidal assay for control solution, AgNPs, and AgNP-coated Janus microbots (AgNPs-JP). (b) Experimental parameters and controls are compared in water and PBS. E. coli, as model bacteria, are in contact with 0.5 mg of Mg microparticles (Mg), Janus microparticles (JP), cysteamine-modified JP (JP-Cyt), and AgNP-coated microbots (AgNPs-JP) for 15 minutes in water at pH 6 (cyan) and PBS at pH 6 (blue). Source: Vilela et al. 2017 [80]. Reprinted with permission of American Chemical Society.

rapid nanomotor movement enhances the contact between bacteria and reactive Ag+ , leading to a powerful “on-the-fly” bacteria-killing capacity. These attractive adsorptive/catalytic features of the self-propelled zeolite micromotors eliminate secondary environmental contamination compared with adsorptive micromotors. The distinct cubic geometry of the zeolite micromotors leads to enhanced bubble generation and faster movement, in unique movement trajectories, which increases the fluid convection and highly efficient detoxification of CWA and killing of bacteria [26].

7.5 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent Adsorption is defined as the deposition of molecular species onto the surface. The molecular species that gets adsorbed on the surface is known as adsorbate, and the surface on which adsorption occurs is known as an adsorbent. In the environmental treatment, adsorption is probably one of the most commonly used strategies for the removal of contaminates. Due to its unique dynamic property, micro/nanomotors are employed as the microabsorbent and exhibit improved adsorption kinetics. 7.5.1 Self-Propelled Micro/Nanomotors for the Removal of Oil Droplets Oily industrial wastewater treatment and oil spill cleanup are attracting more and more attention in recent years because of the increased use of oil in daily life and frequently occurred oil spill incidents. To treat oil-polluted waters, many strategies have been widely adopted, including the direct burning of the oils, mechanical separation, wettability-based separation, chemical

7.5 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent

dispersant-based approach, microorganism-based selective digestion of oil, etc. [83–94]. However, most of these methods lack the desired selectivity and efficiency and are not cost effective or environmentally friendly. Accordingly, the development of new highly effective oil/water separation methods is highly desired. Micro/nanomotors have the ability of fast moving, and therefore when appropriately modified, they can be exploited as a high-efficient absorbent for the removal of oil from water. Especially, these micro/nanomotor-based microabsorbents are particularly suitable for the removal of small oil droplets, which is difficult to remove through traditional methods. In this regard, Wang and coworkers prepared a self-assembly monolayer (SAM)-modified tubular micromotor and for the first time reported the oil droplet removal by micromotors because of its strong interaction with oily liquids via adhesion and permeation onto its long alkanethiol coating [95]. The catalytic micromotor used in this study was prepared by electroplating poly(3,4-ethylenedioxythiophene) (PEDOT)/Pt bilayer followed by e-beam deposition of Ni/Au and subsequent functionalization with the SAM (Figure 7.15a). In particular, dodecanethiol-coated Au/Ni/PEDOT/Pt micromotors were shown to offer an effective capture and transport of oil droplets from aqueous media. The influence of the alkanethiol chain length upon the oil–nanomotor interaction and the collection efficiency was also discussed by using SAMs of different chain lengths, i.e. hexanethiol (C6), dodecanethiol (C12), and octadecanethiol (C18). The authors found that the optimal C12 superhydrophobic SAM-coated micromotors showed a strong and prolonged interaction with large oil droplets (attached to the glass-slide surface) along (b)

PEDOT/Pt layers

(c)

Pt

SSS SSSS

PEDOT

Superhydrophobic layer Au Ni

Au/Ni layers

(d)

8 μm

(a)

Figure 7.15 Self-propelled micromachines for the removal of oil droplets. (a) Fabrication and modification of the superhydrophobic SAM-Au/Ni/PEDOT/Pt. Au and Ni layers are deposited onto the PEDOT/Pt microengines by e-beam, and a superhydrophobic layer is formed on the Au surface by incubation in a 0.5 mM n-dodecanethiol ethanolic solution. Hexanethiolmodified microsubmarine transporting a payload of multiple oil droplets. Time-lapse images at different navigation times: 11, 50, and 73 seconds for b, c, and d, respectively. Arrows indicate the direction of the microsubmarine movement. Source: Guix et al. 2012 [95]. Reprinted with permission of American Chemical Society.

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with the effective pickup and transport of multiple small oil droplets present in an oil-contaminated water sample (Figure 7.15b–d). Unmodified micromotors did not show such affinity to oil droplets. These results demonstrate that SAM-functionalized micromotors can be useful for facile, rapid, and highly efficient collection of oils in water samples. To simplify the preparation of the oil-removal micro/nanomotors, Guan and coworkers reported a facile growing-bubble-templated NP assembly approach to prepare magnetically modulated MnFe2 O4 pot-like hollow microparticles micromotor. In this approach, the hydrophobic MnFe2 O4 @oleic acid NPs in an oil droplet of chloroform and hexane assembled into a dense NP shell layer due to the hydrophobic interactions between the NP surfaces. With the encapsulated oil continuously vaporizing into high-pressure gas bubbles, the dense MnFe2 O4 NP shell layer then bursts, forming an asymmetric pot-like MnFe2 O4 micromotor by creating a single hole in it. For the as-developed simple pot-like MnFe2 O4 micromotor, the catalytically generated O2 molecules nucleate and grow into bubbles preferentially on the inner concave surface rather than on the outer convex surface, resulting in the continuous ejection of O2 bubbles from the open hole to propel it. The MnFe2 O4 pot-like micromotor can autonomously move in water media with both velocity and direction modulated by external magnetic field, and more importantly because of the inherent hydrophobicity, they can be used for the environmental oil removal without any further surface modification [96]. Recently, AMerkoçi reported a paper-based graphene oxide (GO) rolled-up process for the fabrication of on-demand engineered micromotors. The resultant reduced graphene oxide (RGO) rolled-up tubes are further modified to show magnetic and catalytic movement. The reported micromotors have the ability to open and close reversibly as bubbles are formed/ejected from their internal cavities. The authors have also shown that the as-prepared GO rolled-up micromotors can be successfully exploited for oil removal from water due to the hydrophobicity of RGO [97]. To avoid the use of hydrogen peroxide during the removal of oils, Wang and coworkers reported hydrogen-bubble-propelled Janus micromotor, based on the magnesium–water reaction, which can be self-propelled in seawater without an external fuel. Such Janus micromotors were prepared by asymmetrically coating Mg particles with an average size of ∼30 μm through an e-beam evaporation with Ti, Ni, and Au layers to form the Janus micromotors (with the Ti layer providing a good contact between the Ni/Au layers and the Mg surface) (Figure 7.16a). Upon immersion into seawater, a spontaneous redox reaction occurs, involving the oxidation of the Mg surface to reduce water to hydrogen bubbles. The seawater-driven micromotors can be guided magnetically and be functionalized with long-chain alkanethiols, leading to a superhydrophobic surface that can be used for environmental oil remediation (Figure 7.16b–d) [28]. In addition, the Marangoni effect can be also employed to drive micro/nanomotors for the oil droplet removal. Pumera and coworkers prepared polysulfone (PSf ) capsule by a phase inversion process. The porous PSf polymer capsule releases DMF asymmetrically, resulting in a difference in surface tension, which culminates in propulsion – a phenomenon otherwise termed as the Marangoni effect. Additionally, these PSf capsules can be loaded with the surfactant of

7.5 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent

Au Ni Ti Mg

H2

Cl– Na+ (a)

Mg+ 2H2O

Seawater

H2 + Mg(OH)2

Transport

Approach

Capture

(b)

(c)

(d)

Figure 7.16 (a) Schematic of the Mg-based seawater-driven Janus micromotors. (b) Time-lapse images of a Mg Janus micromotor approach (b), capture (c), and transport (d) the oil droplet in seawater. Scale bar, 50 mm. Source: Gao et al. 2013 [28]. Reprinted with permission of Royal Society of Chemistry.

sodium dodecyl sulfate (SDS) and actively spread them. The authors have shown that the SDS-loaded PSf capsule can be used for the cleaning of water surface contaminated with oil [98]. 7.5.2 Self-Propelled Micro/Nanomotors for the Removal of Molecules or Ions Besides the oil contamination, micro/nanomotors are also useful as adsorbent for removal of heavy metal ions or other organic compounds. As mentioned above, the SDS-loaded PSf capsule prepared by Zhao and Pumera also showed efficacy in the removal of heavy metals, particularly Fe, through a precipitation [99]. The surfactant released by the PSf capsule converts the Fe(III) in the aqueous phase to solid precipitates. Such a motion-induced mixing with controlled release of surfactant is another important mechanism of remediation, wherein contaminants could be flocculated and separated easily. Similarly, Wang and coworkers prepared tubular motors by filling commercial pipette tips with a mixture of laccase solution (10%) as catalytic remediation agent and SDS (90%) as “propeller.” The self-propelled motor could release the biocatalyst, along with the co-release of SDS. The enzyme was released and was rapidly dispersed into the polluted solution while catalytically transforming the phenolic contaminants. The motors were allowed to navigate in a contaminated solution for up to 30 minutes, during which the pollutant is transformed into an innocuous (or less toxic) product. The authors also loaded a complexing agent

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C C C

C

E E E E E

Contaminant +

C

E P

C

E

Enzyme

C

E

P C

P E

P

P P

C

P

Product

Figure 7.17 Scheme illustrating motor-based biocatalytic pollutant remediation involving gradual release and mixing of the enzyme laccase during the motor movement. Pipette tips are filled with a mixture of the enzyme solution (10%) and SDS (90%) as remediation agent and propeller, respectively, and are allowed to navigate in a contaminated solution. During this process, the pollutant is biocatalytically transformed into an innocuous product. Inset: Closer look at the motor. Source: Orozco et al. 2014 [100]. Reprinted with permission of John Wiley and Sons.

ethylenediaminetetraacetic acid (EDTA) into the motors, and such motors can be used for chelating lead ions (Figure 7.17). The concept can be potentially extended in the future to the release of multiple (cocktail) enzymes (from a single motor or from different motors) to provide the means to decontaminate a wide range of chemical and biological toxins [100]. When combining adsorptive components, micro/nanomotors can be directly employed as the active microadsorbent with enhanced adsorptive kinetics because of their continuous moving capability. Carbon-based absorbents have been widely used in the water remediation for its excellent adsorptive ability, and therefore it is not a surprise that micro/nanomotor research communities first exploited carbon allotropes to develop adsorptive micro/nanomotors. Jurado-Sanchez et al. explored activated carbon-based Janus micromotors for adsorptive removal of the varying contaminates in water [101]. Such micromotors were prepared based on commercially available carbon microparticles (SupelTM Sphere Carbon) possessing a remarkably high surface area (100 m2 g−1 ) and micropore density, which played critical roles for extremely fast adsorption kinetics and to the efficient propulsion of the resulting mobile adsorption platforms. To obtain autonomous propulsion, these activated carbon particles were sputter-coated with a 60 nm thick catalytic Pt hemispheric layer. The as-prepared micromotors had an average diameter of 60 μm and can propel at a fast speed of 550 ± 120 μm s−1 , corresponding to a relative speed of nearly 10 body lengths per second. Such fast movement in the presence of chemical fuel reflects the net momentum associated with the detachment of oxygen microbubbles from the catalytic Pt patch, which leads to a directional propulsion thrust. Moreover, the authors have shown that the activated carbon-based Janus

7.5 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent

micromotors have the capability of removing a broad spectrum of pollutants of organic (nerve agents, explosives, dyes) and inorganic (heavy metals) nature and revealed that the continuous movement of multiple activated carbon/Pt micromotors across, along with the high density tail of microbubbles, results in a greatly enhanced fluid dynamics that lead to a significantly higher water purification efficiency and short cleanup times compared with static activated carbon particles. Graphene as a rising star in materials science was also used in the micro/nanomotors system for the removal of the pollutants. When being used as the adsorbent, graphene exhibits good chemical stability, reduced cytotoxicity, large surface area (low density), high hydrophobic surface, large delocalized π electrons, and large-scale production possibilities, which are beneficial to the adsorption of contaminants, including dyes, organic pollutants, and even metals. Merkoçi and coworkers reported the preparation of graphene-based Janus micromotors and their use in the dynamic removal of POPs, such as polybrominated diphenyl ethers (PBDEs) and 5-chloro-2-(2,4-dichlorophenoxy) phenol (triclosan) [81]. The rGO-coated micromotors were obtained by coating the core–shell-structured (𝛾-Fe2 O3 NPs core and silica shell) silica microspheres with a GO layer, followed by the reduction of the resulting GO-coated microparticles and the covering of the as-prepared rGO-coated silica particle with a Pt catalytic layer. Loading of GO onto the microparticles was achieved through electrostatic interactions from the opposite charge of the NH2 –SiO2 substrate and GO nanosheets at neutral pH. The reduction of the GO-coated microparticles was achieved chemically with an environmentally friendly ascorbic acid-based method. Such reduction again imposes a hydrophobic character to the coating, which allows for the recovery of the π electron structure while keeping their adsorption sites completely exposed on the whole surface. The as-prepared rGO-coated micromotors demonstrated superior adsorbent properties with respect to their concomitant GO-coated micromotors, static rGO-coated particles, and dynamic silica micromotor counterparts. The extent of decontamination was studied over the number of micromotors, whose magnetic properties were used for their collection from environmental samples. The adsorption properties were maintained for four cycles of micromotors reuse. The authors have shown that the new rGO-coated micromotors exhibited outstanding capabilities toward the removal of POPs and their further disposition, opening up new possibilities for efficient environmental remediation of these hazardous compounds. Wang and coworkers reported a template electrodeposition protocol for the fabrication of ZrO2 –graphene/Pt bilayer micromotors [102]. To this end, the GO and ZrOCl2 were simultaneously electrodeposited in one step on the inner wall of a polycarbonate (PC) membrane by using CV technique. The simultaneous electrochemical deposition of zirconia and reduced graphene oxide leads to a high surface area with a needlelike zirconia microstructure. The attractive surface properties of graphene sheets are used as growth directing template for the electrochemical synthesis of the high surface area of zirconia nanostructures for effective and selective binding of nerve agents (i.e. organophosphate compounds). Such selective binding is dramatically enhanced by the rapid

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60 40 20 1

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0 (a)

Lead decontamination (%)

movement of the motors and the corresponding bubble-induced solution mixing. The greatly increased fluid transport led to a 15-fold faster remediation compared to the use of the static counterpart structures. The unique material properties allow the convenient alkaline regeneration of the micromotor surface, improving the cost-effectiveness of this methodology. Such a strategy provides an opportunity to develop reusable micromotors for high-affinity capture-based separation of nerve agents and can be extended to verification analysis of chemical weapon convention. Sánchez and coworkers prepared micromotors consisting of nanosized multilayers of graphene oxide, nickel, and platinum, providing different functionalities [103]. The outer layer of graphene oxide captures lead on the surface, and the inner layer of platinum functions as the engine decomposing hydrogen peroxide fuel for self-propulsion, while the middle layer of nickel enables external magnetic control of the micromotors. Graphene oxide-based micromotors can remove lead 10 times more efficiently than static ones, cleaning water from 1000 ppb down to below 50 ppb in 60 minutes (Figure 7.18). Furthermore, after chemical detachment of lead from the surface of micromotors, they can readily be reused. Besides the carbon-based micro–micro/nanomotors, a zeolite with highly porous structures is another important component for constructing adsorptive micro/nanomotors. Zeolites are microporous materials that consist of a 3D arrangement of [SiO4 ]4− and [AlO4 ]5− polyhedra connected through their oxygen atoms to form large negative lattices. Zeolite materials have been widely used for environmental remediation due to their selective sorption capacities, nontoxic nature, availability, and low cost [104]. As has been mentioned above, the silver-exchanged zeolite micromotors developed by Wang and coworkers could be employed to absorb chemical warfare agents (CWA) due to its strong binding to these chemicals [26]. Lead decontamination (%)

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200 1000.0

Figure 7.18 (a) Decontamination of Pb(II) ions in different systems in the presence of (1) H2 O2 (1.5% v/v) and of SDS (0.1% w/v) after 24 hours; (2) SDS (0.1% w/v) and nonmotile GOx-microbots after one hour; (3) SDS (0.1% w/v) and GOx-microbots stirred by external magnets after one hour; (4) H2 O2 (1.5% v/v), SDS (0.1% w/v), and docked GOx-microbots after one hour (immobilized by stationary magnetic field); and (5) in the presence of H2 O2 (1.5% v/v) and SDS (0.1% w/v) motile GOx-microbots after one hour. (b) Decontamination of Pb(II) ions for different concentrations of GOx-microbots after one hour in the presence of H2 O2 (1.5% v/v) and of SDS (0.1% w/v). (Inset: ICP-OES signal of lead concentration after one hour of decontamination process for increasing amount of motors.) Source: Vilela et al. 2016 [103]. Reprinted with permission of American Chemical Society.

7.5 Self-Propelled Micro/Nanomotors as Efficient Miniature Absorbent

On the other hand, specific recognition of absorbate is an important aspect to develop selective absorbent. Pumera and coworkers utilized the highly specific interactions between Hg(II) and T-T base pairs of single-stranded DNA to develop micromotors for the selective removal of Hg(II) in an aqueous environment, which is one of the most toxic pollutants and accumulates in both wildlife species and humans [105]. The DNA-functionalized micromotors were fabricated in a typical tubular shape with the use of membrane template-assisted electrodeposition. The tubular micromotors possess a number of advantages, such as high mobility, high power output, and ease of functionalization. A layer of gold was coated on the outer surface of the microtubes to ensure a large surface area for subsequent functionalization. A platinum layer was electrodeposited subsequently to form the inner surface of the microtubes, resulting in Au/Pt bilayer microtubes. Then, polyT30-SH oligonucleotides were attached to the Au outer surface of the microtubes via sulfur–gold bonds. The authors have shown that the DNA-functionalized microtube motors exhibited increased Hg removal efficiency due to the continuous generation of bubbles that lead to enhanced motion and mixing of the fluid. Because the adsorption of mercury has a positive correlation with the concentration of DNA, the remediation efficiency increases with higher concentrations of DNA-functionalized micromachines. Given the very small amount of DNA used in the process, the micromotor-based mercury removal approach exhibits higher removal efficiency than in previous studies using DNA condensation. The LbL-assembled polyelectrolyte polymer multilayer materials have been demonstrated effective in removing various contaminates including the organics. The LbL deposition strategy allows for versatile deposition of functionalized coatings on various types of supports, thus providing great promise in pollution separation and decontamination. He and coworkers utilized the LbL deposition method to construct a catalytic multilayer shell motor through embedding PtNPs and polymer multilayers on half-trapped templates [106]. With bubbles formed on the Pt layer inside the shell, the shell motors were able to move forward at a top speed of 260 mm s−1 . Meanwhile, the authors showed that such micromotors can selectively adsorb and responsively release anion dye molecules due to the positively charged polymer multilayers. The combination of high propulsion of the catalytic shell motor and its capacity to adsorb anionic molecules in microfluidic devices indicates potential applications in water remediation and as a promising platform for water analysis. To avoid the use of external fuels during the adsorptive process of the micro/nanomotors, Wang and coworkers introduced water-driven Janus micromotors for efficient removal of toxic metals, which were prepared by functionalizing Mg/Au microsphere motors with a self-assembled monolayer of meso-2,3-dimercaptosuccinic acid (DMSA) [107]. The resulting micromotors propel autonomously in complex environmental and biological matrices, containing chloride and surfactant, obviating the need for external (peroxide) fuel or expensive Pt catalysts. Such self-propelled micromotors act as highly efficient dynamic chelation platforms that offer significantly shorter and more efficient water remediation processes compared with the common use of static remediation agents. The authors showed that DMSA-based Janus micromotors had effective decontamination capability as exemplified by the rapid removal of Zn(II), Cd(II), and Pb(II) ions in aqueous conditions (Figure 7.19). Factors

255

Zn Cd Pb DMSA

(a) HO

HO

SH OH O

O S

Zn

S O

O S

O

HO

HO

Zn, Cd, Pb O

O

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O

HO

S S O

O

OH

HO

S

SH

O

O S SH

OH

O

Cd

(b)

Pb

4 min (c)

3 min (d)

(e)

Figure 7.19 Self-propelled metal chelation platforms. Schematics (a) of the Mg/Ti/Au/meso-2,3-dimercaptosuccinic acid micromotor for “on-the-fly” removal of Zn, Cd, and Pb and of (b) the binding of metal ions by the free thiols groups of the DMSA layer on the micromotor surface. Time-lapse images illustrating the motor lifetime in (c) 0.5 M NaHCO3 , (d) 70% seawater, and (e) propulsion of the motor in lake water before (top) and after (bottom) addition of 0.1 M NaCl. Source: Uygun et al. 2016 [107]. Reprinted with permission of Royal Society of Chemistry.

References

influencing the micromachine-enhanced metal chelation process, such as the navigation time and a number of motors, were investigated. High removal efficiencies of ∼100% were obtained for all target metals following two minutes treatment of serum, seawater, or lake water samples spiked with 500 μg l−1 of each heavy metal. The chelation mechanism was characterized using the Langmuir model, indicating strong interaction and monolayer-type adsorption of the target heavy metals onto the DMSA-binding layer. The new nanomotor concept holds considerable promise toward future metal remediation applications.

7.6 Conclusions In this chapter, we have reviewed recent progress of micro/nanomotors in the areas of environmental sensing and remediation. In addition, some of the self-propulsion mechanisms for these micro/nanomotors are introduced. From these research examples, we can see that there has been a great advance in the development of micro/nanomotors given the fact that the concept of the micro/nanomotor only appeared a decade ago. Most importantly, these micro/nanomotors have been shown with great promise in dealing with the environmental issues. We believe that with the further development of materials science, there will be a bright future for the micro/nanomotors. However, it should be noted that there exist several challenges for the practical applications of the micro/nanomotors. (i) For the nano or submicron motors, the large-scale fabrication is still a limiting factor. It is highly desired to develop methods or strategies with easy scale-up ability for the production of the motor devices. (ii) The recycling of the large-scale applied micro/nanomotors should be carefully considered because of not only the expense but also the potential impact or toxicity on the environment. It is definitely not a good idea to produce the secondary contamination due to the use of the micro/nanomotors. (iii) Although there is a rapid development in the fuels for the motors, greener and cheaper fuels are still highly desirable. In addition, how to avoid the use of the noble metal materials and how to develop efficient catalytic micro/nanomotor still need to be considered. The overall cost of the micro/nanomotors currently is still quite high, as a result of complicated fabrication process. We hope that this review chapter on the micro/nanomotors could help the research communities to better understand and think about the future of this area, and hopefully, more researchers from diverse background would join in this field.

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8 Molecular Imprinting Materials in Environmental Application 8.1 Introduction Generally, molecular imprinting is defined as a technique for creating tailor-made and template-shaped binding sites with the memory of the shape, size, and functional groups of the template molecules [1]. The goal of molecular imprinting is to select or sense either a specific species or a molecular fragment for selectively adsorption, sensing, removal, etc. [2]. In the 1930s, the pioneering concept of molecular imprinting was firstly reported by M. Polyakov [3]. In his early research, silica particles were prepared from sodium silicate in the presence of organic additives (benzene, toluene, or xylene). These particles possessed an increased uptake capacity for the associated additive over the other two structural analogs due to the templating effect from the additive used. Currently, molecular imprinting technology (MIT) is intensively utilized for synthesizing synthetic molecularly imprinted polymers (MIPs) with tailor-made recognition sites that are complementary in shape, size, and functional groups to a specific chemical compound known as the templates (target molecules) [4]. The three-dimensional networks of MIPs are able to efficiently adapt to physical and structural properties of the targeted single or multiple molecules including shape, size, and bonding ability, as well as easily bind and rebind the target molecules among closely related molecules in complex media, just like the model of key and lock [1, 2, 5–8]. As recognition systems, MIPs possess three major features, namely, structure predictability, recognition specificity, and application universality. Therefore, they have increasingly drawn attention in many scientific communities, including environmental engineering, agricultural, biological, and medical fields. Particularly, MIPs are promising sorbents for the detection of specific molecules with various applications in the separation of analytes in a complex matrix coupled with chemical sensors [9–16] and also have been used directly and indirectly with solid-phase extraction (SPE) [17–20] and chromatographic separation techniques [21]. In the latest decade, molecularly imprinted materials have been intensively investigated in environmental science engineering because they can selectively recognize and remove specific pollutant substances in wastewater treatment Artificially Intelligent Nanomaterials for Environmental Engineering, First Edition. Peng Wang, Jian Chang, and Lianbin Zhang. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2019 by Wiley-VCH Verlag GmbH & Co. KGaA.

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[1, 5, 22–28]. Although there exist many conventional wastewater treatment strategies, such as physical adsorption [29–31], oxidation [32], and bioremediation [33], MIPs have advantageous characteristics over them especially due to their high selectivity and sensitivity [1, 34–40]. In this chapter, we will focus on the recent advances in molecular imprinting, and a brief overview of the fundamental aspects of molecular imprinting, including molecular templates, functional monomers, and cross-linkers, and general synthesis methods are provided. More importantly, we highlight the recent progresses on MIT in the environmental applications. Challenges and future perspectives regarding MIT are discussed in the end.

8.2 Fundamental of MIT In general, in order to construct a selective recognition site in MIPs, it is usually synthesized by self-assembly of the corresponding functional monomers around a template and then fixed by a cross-linker in the presence of the template molecules. With subsequent removal of some or all of the template molecules from the MIPs with an appropriate extraction solution, the recognition cavities complementary in size, shape, and chemical functionality are formed in the highly cross-linked polymer matrix, which can selectively rebind the target molecules through any combination of size, shape, and functional group matching (Figure 8.1) [2, 7, 41, 42]. 8.2.1

Covalent and Noncovalent Imprinting

There are two main methods to form molecular imprinting between a chosen imprint molecule and complementary functional monomers, one relying on reversible covalent bonds introduced by Wulff [42] and the other involving noncovalent interactions proposed by Mosbach [43] (Figure 8.1). Covalent imprinting is a typical method to realize highly specific imprinting and robust target binding. The imprinted materials only exist as residues of the functional monomer in the pre-polymerization stage. After polymerization, the covalent bonds are cleaved, and thereafter the template is removed from the functional monomer. With rebinding of the target molecules by this kind of MIPs, the same covalent linkage can be reformed. Meanwhile, the high stability of covalent bonds ensures that covalent imprinting yields a more homogeneous binding site distribution and reduces nonspecific interactions. However, covalent imprinting method is less flexible owing to the limited reversible reactions. Only a limited number of functional groups could be imprinted successfully by using the covalent imprinting, and, consequently, this method is less versatile overall [7]. In contrast, noncovalent imprinting does not have such restrictions. In the presence of an appropriate solvent, template–monomer complexes can be formed by electrostatic interactions, hydrogen bonding, van der Waals forces, π–π interactions, etc. Among them, the most commonly used interaction is

Y

Y +

–

iii

i

iv

II +

I

Covalent modification

Ligand exchange

V Y

M Y

Polymerize with Y cross-linker

IV

M

M

Y

L

Imprint

+ –

III

–

Y Y

Y v

Y

Noncovalent association

ii

Y

IC

Y

+

+ –

Wash Cleave covalent bonds

+ Target

L Ligand exchange

M

– Target

M

Figure 8.1 Schematic illustration of MIPs: (i) noncovalent, (ii) electrostatic/ionic, (iii) covalent, (iv) semicovalent, and (v) coordination to a metal center. An imprint molecule is combined with an appropriately chosen functional monomer, through noncovalent, covalent, or ligand (L) to metal (M) interactions with complementary functional groups on the imprint. A complex of the imprint and functional monomer (IC) is formed, in which the functional monomer is bound to the imprint molecule (I) by hydrogen bonding or van der Waals interactions, (II) by electrostatic or ionic interactions (the charges on the imprint and functional monomer may be reversed), (III) through a covalent bond, (IV) through a covalent bond with a spacer (orange), or (V) by ligand–metal or metal–ligand coordination. The functional monomer contains a functional group, Y, which is able to undergo a cross-linking reaction with an appropriate cross-linker. After polymerization of the complex with a cross-linker to form the solid polymer matrix (gray), the imprint–functional monomer interactions are intact. The imprint is removed through washing, cleavage of chemical bonds, or ligand exchange and leaves behind an imprint cavity with functional groups on the walls. Subsequent uptake of a target molecule is achieved by noncovalent interactions (in types i, ii, and iv), by the formation of a covalent bond (in type iii), or by ligand exchange (in type v) with target molecules that fit into the cavity and possess the correct structure. The matrix may also participate in target recognition and binding through nonspecific surface interactions that results from surface features created around the imprint molecule during cross-linking. Source: Lofgreen and Ozin 2014 [2]. Reprinted with permission of Royal Society of Chemistry.

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hydrogen bonding, which usually occurs between methacrylic acid (MAA) groups and primary amines in nonpolar solvents [44]. After polymerization and removal of the template, the functionalized polymeric matrix can rebind the target through the same noncovalent interactions, thus greatly expanding the range of applicative compounds being imprinted. Currently, noncovalent imprinting has been considered as the most popular strategy owing to its simplicity of operation and rapidity of binding and removal [45]. However, noncovalent imprinting does not always give robust results, because it is sensitive to even slight disruption of the interactions holding the complex together (e.g. the presence of water). In order to integrate the durability of covalent imprinting and the rapid target uptake of noncovalent imprinting, semicovalent imprinting has emerged and offers an intermediate alternative, in which the template is bound covalently to functional monomer, but the template rebinding is based on noncovalent approach [1, 46, 47]. Furthermore, metal ions can also participate in imprinting as the template and target species. They can act as the actual imprint to create an imprint cavity that can only interact with the appropriate target metal ion. In this case, the ligands are persistently bound to the matrix by covalent bonds. The selectivity typically relies on the charge that exists in the cavity and the strength of the interactions between the target metal ion and the ligands in the cavity [48]. Therefore, it may cause low selectivity when the metal ion itself acts as an imprinting template, since generally the metal ions have the same charges and similar ionic radii and properties [49]. Alternatively, a metal ion and a ligand complex can both act as the actual imprinting template. The metal ion and ligand complex can easily pre-polymerize with functional monomers. After the polymerization reaction, the metal ion is removed by elution, which leaves specific recognition sites for the metal ion. This method can offer greater adsorption efficiency and selectivity for metal ions. 8.2.2

Essential Elements of Molecular Imprinting

The synthesis of MIPs generally requires five essential elements, which should be taken into consideration in their preparation: (i) a target template, (ii) a functional monomer, (iii) a cross-linker, (iv) a porogenic solvent, and (v) an initiator. 8.2.2.1

Target Templates

Generally, template molecules are target compounds in analytical processes. After the removal of template molecules, the surface of the imprinted cavity contains functional groups that can interact, either covalently or noncovalently, with complementary moieties on an appropriately sized target molecule. An ideal template molecule should satisfy three requirements: (i) functional groups from template molecules should not prevent polymerization; (ii) these functional groups should be well adapted to assemble with functional monomers; and (iii) template molecules should possess excellent chemical stability during the polymerization reaction [1]. So far, a wide range of target templates have been used in molecular imprinting, involving ions (Pb(II), Sr(II), CH3 Hg(II), Hg(I), Cd(II), Cu(II),

8.2 Fundamental of MIT

Cr(III), Fe(III), Ni(II), UO2 2+ , Th(IV), Eu(III), As(III), PO43− ), organic molecules (pesticides [atrazine, 2,4-dichlorophenoxyacetic acid, benzimidazole fungicides], endocrine-disrupting chemicals [bisphenol A (BPA), estradiol, estrone, polycyclic aromatic hydrocarbon (PAH)], explosive [2,4,6,-trinitrotoluene], pharmaceuticals [tetracycline, quinolones, propranolol, digoxin, sulfonamides], amino acids and peptides [tyrosine, alanine, tripeptides, helical peptides, cinchona alkaloids, N-terminal histidine sequence of dipeptides], sugars [d-fructose, d-glucose, d-galactose]), biomacromolecules (lysozyme, adenosine, 3,5-cyclic monophosphate, bovine serum albumin), and cells and viruses (tobacco mosaic virus, bovine leukemia virus, dengue virus, gut-homing T) [50]. 8.2.2.2

Functional Monomers

The role of the functional monomer is to provide functional groups that can form a pre-polymerization complex with the template by covalent or noncovalent interactions. The strength of the interactions between template and monomer directly affects the affinity of MIPs and determines the accuracy and selectivity of recognition sites [51, 52]. Generally, the stronger the interaction is, the more stable the complex, which will thereafter lead to a higher binding capacity of the MIPs. Therefore, a reasonable selection of the functional monomers is very important. In order to select a suitable monomer, unavoidably tedious trial-and-error tests are required. Furthermore, the molar ratios between template and monomer also affect the affinity and imprinting efficiency of MIPs. Namely, lower molar ratios induce less binding sites in polymers owing to fewer template–monomer complexes, while ultrahigh molar ratios would cause higher nonspecific binding capacity, weakening the binding selectivity. In order to increase the imprinting efficiency of MIPs, various strategies have been intensively explored, including spectroscopic measurement (nuclear magnetic resonance [27, 53–55], UV/Vis [56, 57], Fourier-transform infrared spectroscopy) [58, 59], computer simulation [60], and isothermal titration calorimetry [61], to select optimal functional monomers that are capable of forming more stable complexes with templates. Commonly used monomers in molecular imprinting contain MAA, acrylic acid (AA), 2-or 4-vinylpyridine (2- or 4-VP), acrylamide (AM), trifluoromethacrylic acid, and 2-hydroxyethyl methacrylate (HEMA) [1]. Some structures of typical functional monomers are shown in Figure 8.2. Among them, MAA has been used as a versatile functional monomer for molecular imprinting owing to its hydrogen bond donor and acceptor, which show good suitability for ionic interactions [62, 63]. The limited number of functional monomers used in molecular imprinting restricts the selectivity and the further applications of MIPs to some extent. Attempts have been made to devise and synthesize new functional monomers being capable of forming strong interactions with templates. For example, cyclodextrins (CDs), a series of cyclic oligosaccharides with a hydrophilic exterior and a hydrophobic cavity, have aroused extensive interest as attractive functional monomers or co-monomers to imprint various compounds, such as cholesterol [64], tryptophan [65], ursolic acid [66], bilirubin [67],

269

O OH

Methacrylic acid (MAA)

O

O

O

OH

NH2

Acrylamide (AAm)

Acrylic acid (AA)

O O

OH OCH3

2-Hydroxyethyl methacrylate (HEMA)

O

N

O

O

O

2-Vinylpyridine (2-VP) O

O

O N H

O

N H

N,N-methylenebisacrylamide (MBAA) OC2H5 C2H5O Si CH2CH2CH2NH2 OC2H5

3-Aminopropyltriethoxysilane (APTES)

O

Ethylene glycol dimethacrylate (EGDMA)

Trifluoromethacrylic acid (TFMAA)

O O

O

OH

4-Vinylpyridine (4-VP)

O O

O F3C

N

Methyl methacrylate (MMA)

H N

Trimethylolpropane trimethacrylate (TRIM) O O

O

OC2H5 C2H5O Si CH2CH2CH2NCO OC2H5

O Si O

N,O-bismethacryloyl ethanolamine Tetraethoxysilane (NOBE) (TEOS)

Divinylbenzene (DVB)

3-Isocyanatopropyltriethoxysilane (IPTS)

O O

O

O

Glycidilmethacrylate (GMA)

OCH3 H3CO Si OCH3

O

3-Methylacryloxyprolyl trimethoxysilane (3-MPTS)

O

N

O

2-(Diethylamino) ethyl methacrylate (DEM)

Figure 8.2 Structures of commonly used functional monomers and cross-linkers. Source: Chen et al. 2011 [1]. Reprinted with permission of Royal Society of Chemistry.

8.2 Fundamental of MIT

dextromethorphan [68], and cyclobarbital [69]. CDs have some unique superior features over traditional functional monomers [70]. They can form complexes with the template through various interactions, such as van der Waals forces, hydrogen bonding interaction, hydrophobic interaction, electrostatic interactions, and host–guest interactions. The hydroxyl group on CDs can act as a polymerization terminal to form a stable polymer matrix by virtue of a suitable cross-linker. 8.2.2.3

Cross-Linkers

The role of the cross-linker is to fix the functional monomer around the template molecule and thereby form a highly cross-linked rigid framework. The type and the amount of cross-linker can influence on the selectivity and binding capacity of MIPs. In general, a low amount of cross-linkers cannot maintain stable cavity configurations due to the low cross-linking degree, while ultrahigh amounts of cross-linkers will reduce the number of recognition sites per unit mass of MIPs. Commonly used cross-linkers include ethylene glycol dimethacrylate (EGDMA), trimethylolpropane trimethacrylate (TRIM), N,N-methylenebisacrylamide (MBAA) and divinylbenzene (DVB), and so forth (Figure 8.2) [1]. 8.2.2.4

Porogenic Solvents

Porogenic solvent provides a medium for polymerization and assists in pore formation. It acts as dispersion media and pore-forming agents in the preparation process. The polarity of solvent can significantly affect the bonding strength between the template molecule and the functional monomer, the property and morphology of the polymers, and thus the adsorption properties of MIPs. Commonly used solvents for MIP synthesis involve dimethyl sulfoxide (DMSO), 2-methoxyethanol, methanol, tetrahydrofuran (THF), acetonitrile, dichloroethane, chloroform, N,N-dimethylformamide (DMF), and toluene [71, 72]. 8.2.2.5

Initiators

The majority of MIPs are commonly prepared by free radical polymerization (FRP), photopolymerization, and electropolymerization. FRP can be initiated either thermally or photochemically for a wide range of functional groups and template structures. Commonly used initiators in molecular imprinting include azobisisobutyronitrile (AIBN), azobisdimethylvaleronitrile (ADVN), trifluoroacetic acid (TFA), 4,4′ -azo(4-cyanovaleric acid) (ACID), benzoyl peroxide (BPO), dimethylacetal of benzyl (BDK), and potassium persulfate (KPS) [50, 73]. Among them, azo compounds are extensively used as initiators, and AIBN is most conveniently used at the decomposition temperatures of 50–70 ∘ C [50]. 8.2.3

Synthesis Methods of MIPs

In general, the synthesis methods of MIP mainly include FRP and sol–gel processes. The major techniques are described in the succeeding text.

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8.2.3.1

Free Radical Polymerization

Bulk polymerization is the most popular and general FRP in the preparation of MIPs owing to its rapidity and simplicity in preparation and high purity in the produced MIPs. However, the polymers obtained by the bulk polymerization process are in monolithic block form and thus have to be mechanically crushed, ground, and sieved to an appropriate size, leading to a time-consuming process and low polymer yield (only 30–40% of polymer recovered as usable material). Moreover, grinding operation causes irregular particles in shape and size. Some high-affinity binding sites are destroyed and changed into low-affinity sites. Bulk polymerization yields polymers with a heterogeneous binding site distribution, thus greatly confining the use of MIPs as chromatographic adsorbents [74]. In order to overcome these drawbacks, other polymerization strategies have been proposed, such as suspension polymerization [75], emulsion polymerization [76, 77], seed polymerization [69], and precipitation polymerization [75]. These polymerization strategies avoid the posttreatment steps required in bulk polymerization, and more homogeneous binding site distributions are obtained [78]. The conventional suspension polymerization was first introduced by Mayes and Mosbach [79]. In this method, functional monomers or pre-polymerization mixtures (e.g. monomers and templates) used in polymerization are dissolved in a suitable solvent using mechanical agitation, while the monomers polymerize, forming polymer spheres. In general, the traditional water medium, perfluorocarbon liquid, and mineral oil can be used as suitable solvents [80–83]. Emulsion polymerization is an effective method to produce monodispersed MIP particles with a uniform size distribution with high yield. It uses an oil/water biphasic system [45, 76]. The functional monomer, cross-linker, and template are first emulsified in water, and then stabilizers are added to the disperse phase to avoid diffusion in the continuous phase. However, emulsion polymerization is difficult and expensive and suffers from the disturbance of remnants of surfactants, causing low imprinting capacity [45, 84, 85]. The presence of water can also adversely affect the efficiency of this method. Precipitation polymerization is a promising approach to produce high-quality, uniform-sized, and spherical imprinted particles since each polymer chain can grow individually in the dilute reaction system without overlap or coalescence. In precipitation polymerization, the imprinted materials are formed in a large amount of solvent, and then the resulting polymers are transferred into a precipitation medium in which the polymers are insoluble and precipitate at the bottom. In this method, some factors, such as the polarity of solvents, polymerization temperature, and stirring speed, have great effects on the polymer particle size, but their shape remains irregular [74, 86, 87]. In addition to the major polymerization techniques mentioned above, currently, some other polymerization methods are being adopted to further improve the performance of the MIPs, including surface imprinting technique, electrochemical polymerization, ultrasound-assisted polymerization, electrodeposition, seed polymerization, microwave-assisted polymerization, etc. [88–95].

8.3 Molecular Imprinting in Environmental Applications

8.2.3.2

Sol–Gel Processes

In a sol–gel process, an oxide network can be created by polycondensation reactions of molecular precursors in a liquid medium [96]. For example, tetraalkoxysilane precursors, such as tetramethoxysilane (TMOS) and tetraethoxysilane (TEOS), first hydrolyze to form a colloidal suspension and then perform polycondensation reaction to construct highly cross-linked silica materials [2]. The sol–gel process is usually adopted for making water-compatible MIPs in an aqueous medium at room temperature [36]. In terms of sol–gel materials, the template can be even removed by forceful techniques such as combustion. Also, the solvents (e.g. ultrapure water and ethanol) are ecofriendly for these sol–gel process, which is an advantage compared with those used for FRP (e.g. chloroform, acetonitrile, and toluene) [1].

8.3 Molecular Imprinting in Environmental Applications In the past several decades, a wide range of chemical contaminants have been released to the natural water from industries and agriculture [97]. Because of the high selectivity and strong affinity, it has been widely reported in recent years to use MIPs as efficient sorbents for selective recognition, separation, determination, and purification of pollutants in wastewater, including natural and synthetic dyes, endocrine-disrupting compounds (EDCs), PAHs, pharmaceuticals, toxic metal ions, and so forth [98, 99], from tap water, river water, well water, lake water, wastewater, pond water samples, and coastal sediments [5, 22–26, 28, 100–107]. 8.3.1

Natural and Synthetic Dyes

Natural and synthetic dyes are widely used in food, pharmaceutical, cosmetic, textile, and leather industries [108]. It was estimated that about 10–15% of these dyes are lost during the dyeing process and released with the effluent [109]. Many of them are hard to be degraded in the natural environment due to their complex structures. Furthermore, some of them are toxic, mutagenic, and carcinogenic [110]. Therefore, removing dyes from wastewater before discharging them into the environment is very important. In 2009, Ibrahim et al. fabricated dye-imprinted polymers by using MAA as monomers, EGDMA as a cross-linker, chloroform as solvent, AIBN as an initiator for polymerization, and Cibacron reactive red dye as template molecule [111]. The structure of the MIP was robust and resisted dissolution up to 260 ∘ C. The obtained product possessed greater dye adsorption capacity (79.3 mg g−1 ) compared with the nonimprinted polymer (24.0 mg g−1 ), and the significant increase in adsorption capacity demonstrated the effect of molecular imprinting. In addition, the dye-imprinted polymer showed selective extraction of Cibacron reactive red dye even in the presence of Cibacron reactive blue and Cibacron reactive yellow dyes being similar chemical natures to the red dye (Figure 8.3). In 2011, Luo et al. prepared magnetic and hydrophilic MIPs by an inverse emulsion–suspension polymerization method for removing water-soluble acid

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O

NH2 SO3H Cl N

O

N H

N

N H

N N H

SO3H

HO3S

(a) HO3S

O

H C N

OH

H N

N N

H N

N N

SO3H

N

SO3H

HO3S

Cl

(b) SO3H N

HN HO3S

N

N N H

N

HO Cl

CH3

N

N N

Cl Cl

(c)

SO3H

Figure 8.3 The chemical structure of reactive dyes. (a) Cibacron reactive blue. (b) Cibacron reactive red (the imprinted molecule). (c) Cibacron reactive yellow. Source: Al-Degs et al. 2009 [111]. Reprinted with permission of Springer Nature.

dyes from wastewater [112]. As shown in Figure 8.4, a PEG-Fe3 O4 magnetic fluid was firstly synthesized, and its surface was covered by MIPs. After the water-soluble acid dyes were eluted, the recognition sites were thus generated. The removal efficiency toward acid dyes can reach above 95% in distilled water, tap water, river water, and wastewater. More importantly, this material realized five times removal–regeneration cycles without an obvious decrease in the removal efficiency. One of the additional advantages of this material is that it can be controlled by an external magnetic field due to the encapsulated Fe3 O4 , which makes it be easily separated and collected during operation. 8.3.2

Endocrine-Disrupting Compounds

EDCs are one type of serious environmental contaminants, which disturb normal endocrine system of humans and animals [113]. EDCs include natural compounds, such as estrogens, progestogens, and phytoestrogens, synthetic estrogens, and a wide variety of organic pollutants, including pesticides,

8.3 Molecular Imprinting in Environmental Applications

Imprinting

Fe3O4

PEG-Fe3O4 Elution

Separation

N

Binding

S mag-MIP Target molecule Impurities

Figure 8.4 Synthesis route of magnetic and hydrophilic MIPs and the removal of water-soluble acid dyes under an external magnetic field. Source: Luo et al. 2011 [112]. Reprinted with permission of Elsevier.

surfactants, and plasticizers. Estrone, as one of the most potent estrogenic hormones, has the potential to cause serious risks on wildlife and human health [114, 115]. In order to prevent the uncontrolled effects on human health and the deleterious effects on the aquatic environment, it is required to develop accurate and reliable analytical method for the determination of estrone in the environment. In 2009, Wang et al. used estrone as template molecule, 3-aminopropyltriethoxysilane as function monomer, and tetraethoxysilicane as cross-linker to synthesize a highly selective molecularly imprinted silica gel microsphere by combining a surface molecular imprinting technique with a sol–gel process [116]. The as-prepared imprinted material exhibited fast adsorption–desorption dynamics, high affinity, and good recognition and selectivity for estrone in environmental water samples. As a result, the recoveries of estrone in well and lake water samples spiked at two levels (0.5 and 1.0 μg l−1 ) were 86% and 95%, respectively. Similarly, an imprinted organic–inorganic hybrid material was synthesized by the molecular imprinting technique combined with a non-hydrolytic sol–gel process [26]. It used estrone as a template, methacryloxypropyltrimethoxysilane as a cross-linker, and AM as a functional monomer in a mixed solvent by DMSO and toluene. The obtained polymer was used as a sorbent for the separation and quantitative determination of estrone at low concentration levels in river, lake, and tap water with recoveries ranging from 83.38% to 98.12%. Such an imprinted material can be applicable to evaluating environmental risk and monitoring the presence of estrone in drinking water. Ethynylestradiol (EE) is a commonly used estrogenic compound in oral contraceptives. It cannot be sufficiently eliminated during biological treatment of the wastewater, leading to adverse biological responses in fish or other wildlife [117].

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CH3

OH

CH3 O OH

HO

HO Estrone (E1)

Estriol (E3) CH3 OH

OH 17β-Estradiol (E2)

CH3 OH C

CH

HO Ethynylestradiol (EE2)

Figure 8.5 Chemical structures of the estrogens. Source: Bravo et al. 2009 [100]. Reprinted with permission of Springer Nature.

Durand et al. synthesized two EE-imprinted polymers using MAA as functional monomer, EGDMA as a cross-linker, and EE2 as the template in two different polymerization solvents (acetonitrile or toluene) via the noncovalent molecular imprinting protocol [100]. The MIPs were used as selective sorbents for the determination of EE2 in river water samples. Meanwhile, their selectivity was evaluated using several related estrogens, estrone, estriol, and estradiol, as model pollutants (Figure 8.5). The MIP exhibited a stronger binding affinity and selectivity for EE2 than other pollutants. The best sample showed a recovery of 75% when it was applied to the extraction of EE2 from 50 ml spiked river water sample. In addition, xenobiotic compounds such as nonylphenol and BPA have also been proven to be an endocrine disruptor with adverse consequences to the reproductive systems of aquatic organisms and humans [118–121]. Piletsky et al. fabricated an MIP by using nonylphenol as the template for the removal and preconcentration of nonylphenol from contaminated water samples. The MIP showed a recovery efficiency of 99% [28]. Zhang et al. prepared a magnetic MIP of BPA via miniemulsion polymerization, which showed much higher adsorption capacity of 390 mg g−1 than that of nonimprinted polymer (270 mg g−1 ) [122]. The recoveries of spiked water samples ranged from 89% to 106% with a limit of detection (LOD) of 14 ng l−1 . Wu et al. investigated the MIP submicron particles for the selective removal of trace 17β-estradiol (E2) in water [123]. When 1 ml water samples with different E2 concentrations were treated with 20 mg of MIP particles for two minutes, the recovery percentages could reach as high as 97% ± 3%. In 2013, Fe3 O4 microparticles were used as the core support to fabricate MIP core–shell-structured beads with superparamagnetic property for environmental estrogens determination in water samples [124]. These beads were used as dispersed SPE adsorbents in water samples. 17β-Estradiol (E2) was used as template in this synthesis. The test results showed the recoveries rate of this material for estriol (E3), BPA, E2, and

8.3 Molecular Imprinting in Environmental Applications

EE being 72.2–92.1%, 89.3–96.0%, 93.3–102%, and 89.7–95.9%, respectively, with relative standard deviation (RSD) lower than 7.0%. In the same year, Xia et al. also reported a similar magnetic duo-MIP with removal efficiencies of 90%, 90%, 88%, and 98% for estrone (E1), 17β-estradiol (E2), BPA, and diethylstilbestrol (DES), respectively [125]. 8.3.3

Polycyclic Aromatic Hydrocarbons (PAHs)

PAHs, a large group of persistent organic pollutants (POPs), refer to complex mixtures consisting of two or more condensed benzene rings. PAHs are identified as potent carcinogens and mutagens. Researches show that these PAHs are mainly from the diesel engine exhaust, cigarette and wood smoke, natural gas, oil-fired burner emissions, etc. In 2009, Krupadam et al. synthesized an MIP adsorbent via template-directed molecular imprinting method, which used a PAH as a template, MAA as a functional monomer, EGDMA as a cross-linker, and acetonitrile as a porogen [102]. This MIP material can be used as a solid-phase adsorbent for the quantitative enrichment of PAHs from coastal sediments, industrially originated atmospheric particulates, and industrial wastewater. Since the chemical oxygen demand (COD) and total dissolved solids (TDS) could interfere with the trace-level detection of specific environmental pollutants (Figure 8.6), the MIP selective adsorption capacity for PAHs started reducing when COD and TDS were more than 800 mg l−1 in the targeted environmental samples. In addition, the MIP exhibited stable adsorption capability in at least 10 enrichments and desorption cycles. Meanwhile, recoveries of eight PAH compounds, extracted from 10 g of coastal sediments and 1 l of industrial effluent spiked with 10 μl of standard PAHs, were between 85% and 96%. In the next year, Wate et al. prepared an MIP adsorbent for PAHs using a mixture of six PAHs as the template via noncovalent templating synthesis technique [126]. In this synthesis, acetonitrile, MAA, and EGDMA were used as the solvent, the functional monomer, and the cross-linker, respectively. The MIP material showed an excellent affinity toward PAHs in the presence of dissolved organic matters and dissolved inorganic solids, suggesting that this material may be applicable for carcinogenic PAH removal. In addition, the MIP can be reused at least ten repeated cycles without any deterioration in performance. 8.3.4

Pharmaceuticals and Pesticide

In recent years, the occurrence of pharmaceuticals and their metabolites in the surface water, groundwater, and drinking water has been recognized as one of the emerging issues in the environmental field due to their high persistence and low biodegradability [127, 128]. A typical example is diclofenac (DFC), which is an important nonsteroidal anti-inflammatory drug and widely used to reduce inflammation and as an analgesic in conditions such as in arthritis or acute injury [129]. It is one of the most frequently detected pharmaceutical residues in water bodies. In 2011, an MIP material was synthesized by precipitation polymerization using DFC as the

277

After AC adsorption After MIP adsorption Before MIP adsorption

Coastal sediments

8 Molecular Imprinting Materials in Environmental Application

TDS PAH COD TDS PAH COD TDS PAH COD

After AC adsorption Before MIP After MIP adsorption adsorption

Industrial effluents

0

After AC adsorption After MIP adsorption Before MIP adsorption

200

400

600

800

1000

1200

TDS PAH COD TDS PAH COD TDS PAH COD

0

Atmospheric particulates

278

200

400

600

800

1000

1200

1400

TDS PAH COD TDS PAH COD TDS PAH COD

0

100

200

300

400

500

600

700

800

900

Concentration of COD and TDS in mg l–1; and PAHs in ηg l–1 e, 50% diluted d

d, 50% diluted c

c, 50% diluted b

b, 50% diluted a

a, real

Figure 8.6 Interference of TDS and COD in the adsorption of PAHs onto MIP. Source: Krupadam et al. 2009 [102]. Reprinted with permission of American Chemical Society.

template, which showed better selectivity and higher adsorption efficiency for DFC, compared to the powdered activated carbon (PAC) [130]. In addition, the MIP can be reused at least 12 times without significant loss in performance. Besides DFC, carbamazepine (CBZ), an antiepileptic drug, and clofibric acid (CA), a bioactive metabolite of the fibrates drugs, are also frequently detected

8.3 Molecular Imprinting in Environmental Applications

pharmaceutical residues in water bodies [131, 132]. Zhou et al. prepared double-template MIP by precipitation polymerization using CBZ and CA as the double templates and 2-vinylpyridine as the functional monomer [133]. This MIP can selectively remove CBZ and CA from tap water, lake water, and river water in the presence of competitive compounds. It possessed much better performance than the nonimprinted polymer, a commercial PAC, and the C18 adsorbents. Benzimidazole compounds (Figure 8.7) have been widely used both as anthelmintic drugs in the treatment and prevention of parasitic infections and as fungicides to prevent crop spoilage. However, they also exist potential impact on both the environment and public health, especially causing several toxic effects with a chronic exposure to benzimidazole compounds, including teratogenicity, congenic malformations, polyploidy, diarrhea, anemia, pulmonary edemas, or necrotic lymphoadenopathy [134]. The European Water Framework Directive has established a maximum allowable concentration level of 0.1 μg l−1 for most benzimidazole compounds in natural waters and a total concentration of all pesticides of 0.5 μg l−1 [135]. In 2009, Cacho et al. synthesized an MIP material for selective removal of benzimidazole compounds by using thiabendazole (TBZ) as a template, MAA as functional monomer, EDMA and DVB as cross-linkers, and a mixture of acetonitrile and toluene as a porogen [25]. Besides the template molecule itself, this MIP material also showed a strong affinity to a wide range of benzimidazole compounds, such as albendazole, benomyl, carbendazim, fenbendazole, flubendazole, and fuberidazole, which makes it usable for the screening of benzimidazole compounds present in tap water, river water, and well water H N

H N

O

NH

N S

H N

Fuberidazole (FuBZ)

NH N

H N

CH3

Fenbendazole (FenBZ)

CH3

Carbendazim (MBC)

N

O

S

N

N

O

NH

H N

NH N

NH O

O

Albendazole (ABZ)

N

O

H N

F

CH3 O

NH

Thiabendazole (TBZ)

CH3

O O

O O

S

N

N

O

Benomyl (BEN)

CH3

O CH3

Flubendazole (FluBZ)

CH3

Figure 8.7 Chemical structure of some benzimidazole compounds. Source: Cacho et al. 2009 [25]. Reprinted with permission of Elsevier.

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samples at concentration levels below the legislated maximum concentration levels with quantitative recoveries. Similarly, a benzimidazole-imprinted polymer was synthesized by precipitation polymerization using TBZ as template molecule, MAA as functional monomer, and DVB as cross-linker [22]. The prepared polymer exhibited a clear imprint effect for TBZ, as well as the ability of selectively rebinding other benzimidazole compounds (benomyl, carbendazim) in river water, tap water, and well water samples. In this sense, recoveries for the determination of benzimidazole fungicides in spiked samples ranged from 87% to 95%. 2,4-Dichlorophenoxyacetic acid (2,4-D) is a common phenoxy herbicide, while the continuous use of 2,4-D will lead to soil percolation and groundwater contamination, because of their potential toxic, carcinogenic, and mutagenic effects [136]. In 2013, Jiang et al. grafted MIPs on carbon nanotubes (CNTs) for 2,4-D analysis [137]. As shown in Figure 8.8, CNTs were coated with vinyl group-modified silica, followed by a radical precipitation polymerization using AM and styrene as mixed monomers, 2,4-D as the template, EGDMA as the cross-linker, and 2,2-azobisisobutyronitrile (AIBN) as the initiator. The MIP product possessed good selectivity, quick mass transfer, and high recoveries ranging from 74.6% to 81.2% with RSD below 7.0%. In 2014, Shahtaheri et al. synthesized an MIP nanoparticle (MIP-NP) for the selective preconcentration of 2,4-D [138]. This polymer was obtained by CTAB

TEOS NH4OH

Si

CTAB

VTES

Si

Si

Si

removal

Si Si

2,4-D, AM, styrene, EGDMA

Elution Rebinding

Acetonitrile, 55 °C

Cl

OH O

C

Si

H2N O

O

O H2N

Cl

Figure 8.8 Schematic illustration of preparing MIPs for 2,4-D molecules. Source: Yang et al. 2013 [137]. Reprinted with permission of Elsevier.

100 100 Recovery (%)

Adsorption efficiency (%)

8.3 Molecular Imprinting in Environmental Applications

80 60

60 40 20

2,4-D

2,4-D

40

0 0

(a)

80

5 10 15 20 Adsorption time (min)

25

0 (b)

5

10 15 20 25 Desorption time (min)

30

Figure 8.9 The effect of adsorption time (a) and desorption time (b) on the adsorption efficiency and recovery of 2,4-D from MIP-NPs [138].

precipitation polymerization from MAA as the functional monomer, EGDMA as the cross-linker, 2,2′ -azobisisobutyronitrile as the initiator, and 2,4-D as the template molecule in acetonitrile solution. The maximum adsorption capacity of the 2,4-D MIP was 89.2 mg g−1 , and 2,4-D molecules can be separated from the polymeric structure by using acetic acid in methanol (15 : 85 v/v%) as the eluting solvent. The sorption and desorption process finished within 10 and 15 minutes, respectively (Figure 8.9). In addition, the imprinted polymer can be used for seven times without significant decrease in its binding affinities. Highly neurotoxic organophosphates with high toxicity, such as paraoxon, are commonly used as persistent pesticides and nerve agents. It can disrupt the cholinesterase enzyme, which catalyzes the breakdown of acetylcholine, a neurotransmitter needed for proper nervous system function, and thereafter leads to convulsions, coma, respiratory paralysis, and death [139]. In 2017, a molecularly imprinted, polymer-based disposable electrochemical sensor for paraoxon with high recognition ability was prepared by Li et al. [140]. As shown in Figure 8.10, this sensor was based on a screen-printed carbon electrode modified with a surface molecularly imprinted poly(p-aminothiophenol) (PATP)/gold nanoparticle (AuNP) composite film, consisting of a PATP outer layer and an AuNP inner layer. Acetic acid buffer solution was used as solvent. The imprinted film showed greater selectivity and affinity toward PO over the nonimprinted sensor and realized these performances with an extraction time of 10 minutes. Meanwhile, such a sensor possessed high reproducibility and stability. It remained stable for 10 days at 4 ∘ C in a nitrogen-filled plastic bag. The efficiency only decreased 10% and 16% after one month and three months, respectively. Therefore, these MIPs are able to effectively detect the pesticide residuals and other environmentally deleterious chemicals. 8.3.5

Metal

With the rapid increase in industrial use, heavy metals including Pb, Cu, Cd, Ni, and Cr are continuously released into the natural water by various industrial and human activities, which have been considered to be a great threat to the living environment of human and aquatic organisms [141].

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(a) (iv) +

Extraction

Electroploymerization

Rebinding

(iii)

(v) S

(ii)

O2N

= ATP

O

C2H5O C2H5O

HN

P

O

O

= PO

NH2

S NO2

(i)

π-acceptor

NH2

S

OC2H5 OC2H5 P O

H-bond S

N H

π-donor

(b)

SPC E

Figure 8.10 (a) Preparation of the electrochemical sensor and the principle for recognition of paraoxon (PO): (i) electrodeposition of an AuNPs inner layer on the surface of the screenprinted carbon electrode (SPCE); (ii) ATP assembled onto the AuNPs and following PO assembled onto the formed ATP–AuNPs, forming a basis of surface molecular imprinting; (iii) electropolymerization of PATP film on the surface of the electrode in the presence of PO molecules; and (iv)/(v) removal/rebinding of PO on the imprinted sites of the imprinted PATP/AuNPs/SPCE. (b) Schematic illustration of the adsorption of the ATP molecules at the AuNPs surface and the further self-assembly of PO at the ATP–AuNPs [140].

Although the conventional treatment methods, like redox [142], biological flocculation [143], ion exchange [144], membrane separation [145], etc., have been adopted for the removal of heavy metals from various water bodies, most of them are nonspecific method due to their low selectivity toward a particular heavy metal. In recent years, molecular imprinting adsorbents have been increasingly used for the removal and recovery of heavy metals from wastewater. In 2014, Luo et al. prepared a Pb(II)-imprinted thiol-functionalized silica gel sorbent for the selective removal of Pb(II) ions from aqueous samples [146]. As shown in Figure 8.11, it was synthesized through surface imprinting method combined with a sol–gel process using 3-mercaptopropyl trimethoxysilane as a monomer, tetraethyl orthosilicate as a cross-linking agent, and Pb(II) ion as a template. With the inclusion of Fe3 O4 nanoparticles, the sorbent possesses magnetic property. This sorbent possessed the maximum Pb(II) sorption capacity of 32.58 mg g−1 and maintained around 90% of its initial adsorption capacity during the five cycles of the adsorption–desorption process. Furthermore, this sorbent exhibited higher selectivity toward Pb(II) despite Cu(II), Zn(II), or Co(II) interference. Compared with Cu(II), Zn(II), and Co(II), the Pb(II) fits better into the imprinted cavities and possesses higher affinity with the thiol ligands. With cavity geometry suitably oriented, the sorbent provides sites with ligand groups

8.3 Molecular Imprinting in Environmental Applications

TEOS

MPTS Pb2+

Fe3O4

Fe3O4@SiO2

2+

Pb

o

m

Re

ve

2+

ng

Pb

di

n bi

Re HS

SH

HS

SH

Imprinted cavities

SH SH

Fe3O4@SiO2@IIP

Pb2+

SH SH

Template

Figure 8.11 Synthesis route for Pb(II)-imprinted thiol-functionalized silica gel sorbent. Source: Guo et al. 2014 [146]. Reprinted with permission of Elsevier.

for Pb(II) coordination, and hence, it is able to extract Pb(II) selectively in the presence of the other metal ions. Since the imprinting hole of MIPs may be blocked in actual wastewater that contains solid particles or floccules, its adsorption performance may be affected. In 2017, Luo et al. synthesized Pb(II) ion-imprinted polymers (IIPs) with bicomponent polymer brushes [147]. The introduction of bicomponent polymer brushes brought great anti-interference and anti-clogging ability for flocculation and solid particles. The bicomponent brushes played an indispensable role for anti-interference ability using Al–O coordination bond, hydrogen bond, or hydrophobic interactions. As a result, the grafted MIPs could retain more than 80.5% adsorption capacity of the maximum adsorption capacity for copper mine wastewater and can be reused at least six times. In 2015, Yuan et al. prepared a Cu(II)-imprinted microgel by using MAA as function monomer, attapulgite as the support material, EGDMA as a crosslinker, and 2,2′ -azobisisobutyronitrile (AIBN) as an initiator [148]. The Cu(II)-imprinted microgel possessed a higher affinity to Cu2+ than Ca2+ and Mg2+ , and it can be reused for five times with only 15% regeneration loss. In 2018, Ren et al. developed an ion-imprinted technology in conjunction with the sol–gel process under mild conditions for the selective removal of Cu(II) ions from aqueous solution with an adsorption capacity of 39.82 mg g−1 [149]. The IIP was prepared by using Cu(II) ion as a template, N-[3-(2-aminoethylamino) propyl] trimethoxysilane (AAPTMS) as functional monomer, and tetraethyl orthosilicate (TEOS) as cross-linker. Cu content in water can be greatly reduced after selectively removal of Cu(II) by this adsorbent. Cadmium can be enriched in organisms and converted into more toxic ethyl cadmium or other metal–organic compounds. In 2010, in order to selectively absorb Cd2+ ions in aqueous, Buhani et al. prepared Cd(II) IIP [150]. The

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N

OCH3 H3CO Si

HN Cd2+ NH

HN

NH2

+ Cd2+

N

H3CO + CT A rem B/C ov d 2+ al

tion

riza lyme

CTAB

Po

+ g

din

in eb

CH3

R

O H3C

O

Si

O

CH3

NH H

NH

2N

2

NH

O

2+

Cd

H3C

Figure 8.12 Schematic illustration of synthesizing Cd2+ -imprinted mesoporous silica. Source: Li et al. 2016 [151]. Reprinted with permission of Springer Nature.

adsorption capacity of Cd(II) IIP was 83.89 mg g−1 , much higher than that of nonimprinted polymer (35.91 mg g−1 ). The imprinted Cu(II) sorbents can be used several times without noticeable decrease in adsorption capacity. In 2016, Tang et al. designed and synthesized a Cd2+ -imprinted mesoporous silica through a one-step hydrothermal process [151]. Cd2+ was used as the template ion, while 3-(𝛾-aminoethylamino)-propyltrimethoxysilane and tetraethoxysilane were chosen as the functional monomer and cross-linker, respectively, as shown in Figure 8.12. The Cd2+ -imprinted material possessed a highly ordered hexagonal mesostructure, nanosized pore diameters, and a large surface area, resulting in a large adsorption capacity of 40 mg g−1 . This Cd2+ -imprinted material showed a highly specific recognition ability with maximum imprinting factor of 3.0 and a rapid adsorption kinetics (equilibration within five minutes). This material can successfully separate and remove Cd2+ from mineral wastewater samples with high Cd2+ recovery rates. In addition, the recovery remained stable at around 89.3% during six cycles of Cd2+ adsorption. Ni, as an important strategic resource, is widely used in industrial manufacturing and development of national defense. However, its toxicity causes a serious threat to the fauna and flora of receiving water bodies when discharged into environment as industrial wastewater. In 2015, Zhang et al. prepared a magnetic chitosan/poly(vinyl alcohol)/Ni(II) beads by blending CTS, PVA, Ni(II), and Fe3 O4 , and Ni(II)-imprinted beads were obtained by eluting Ni(II) ions from the beads. The Ni(II)-imprinted beads showed the maximum adsorption capacity for Ni(II) ions of 500.0 mg g−1 [152]. The obtained material had great durability and selectivity for Ni(II) ions. In 2018, a Ni(II) IIP was synthesized by bulk polymerization for rapid removal of Ni(II) ions from aqueous solution [153]. Diphenylcarbazide

References

(DPC) and N,N-azobisisobutyronitrile (AIBN) were used as ligand and initiator, respectively. Ni(II) ions could be easily eluted from IIPs with HCl solution. The obtained IIP sorbent had a maximum adsorption capacity of 86.3 mg g−1 at pH 7.0 with initial Ni(II) concentration of 500 mg l−1 . Chromium (Cr) is widely used in electroplating, leather making, metal processing, nuclear power plants, textile industry, and chromium preparation. Cr is usually in the form of Cr(III) and Cr(VI), while the toxicity of Cr(VI) is more than 500 times as much as Cr(III) [154]. Differing from the traditional heavy metal ion pollution being generally cationic, Cr(VI) is in the presence of anionic groups and has toxicity on microbes, plants, animals, and humans. Recently, Periyasamy et al. focused on Cr(VI) removal using ecofriendly materials like cellulose, hydrotalcite, hydroxyapatite, and their composite forms [155]. To enhance the absorption capacity of Cr(VI), cellulose-supported magnetic composites iron oxide-coated cellulose/hydrotalcite and cellulose/hydroxyapatite were synthesized by in situ fabrication method. These magnetic cellulose-supported composites showed the suitability when testing at field conditions by collecting chromium-contaminated water.

8.4 Conclusion As demonstrated in hundreds of researches, MIPs show higher selectivity and strong affinity for target chemicals, compared with nonimprinted sorbents. All these works suggest that MIPs have a particularly high potential for pollutant removal and analysis applications. However, opportunities and challenges are still existing: (i) the fundamental mechanism in molecular imprinting and recognition at the molecular level is still not well understood; (ii) metal ion-imprinted polymers are mainly based on metal cations, and less research has been done with anionic ions; (iii) new and novel monomers need to be designed and synthesized, which would broaden the application breadth of MIT; and (iv) new polymerization methods need to be explored for molecular imprinting to further improve binding capacity and imprinting efficiency. As advanced knowledge from polymer science, nanotechnology, analytical chemistry, environmental science, biotechnology, etc. is being brought into the MIT field, significant breakthroughs can be expected in the years to come.

References 1 Chen, L., Xu, S., and Li, J. (2011). Recent advances in molecular imprinting

technology: current status, challenges and highlighted applications. Chemical Society Reviews 40 (5): 2922–2942. 2 Lofgreen, J.E. and Ozin, G.A. (2014). Controlling morphology and porosity to improve performance of molecularly imprinted sol-gel silica. Chemical Society Reviews 43 (3): 911–933. 3 Polyakov, M. (1931). Adsorption properties and structure of silica gel. Russian Journal of Physical Chemistry 2: 799–805.

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4 Ansari, S. (2017). Application of magnetic molecularly imprinted polymer

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9 Emerging Synergistically Multifunctional and All-in-One Nanomaterials and Nanodevices in Advanced Environmental Applications 9.1 Introduction The rapid progresses on multifunctional nanostructures and their interfacial interactions have encouraged many advanced applications. Multifunctionalities endow nanomaterials with enhanced properties and enable them to adapt to more complex conditions and practical scenarios. In general, these multifunctional materials could be made of multicomponents with improved properties [1–13], and, especially by rational and elaborate design, their multifunctional elements are integrated into all-in-one nanomaterials or nanodevices to target a desired application, which is achieved by the multiple components working in a synergistic or order fashion. Thus, in this sense, they can be seen as another type of “artificial intelligence.” In recent years, some enlightening synergistically multifunctional all-in-one nanomaterials have been proposed, prepared, and successfully tested, many of which represent proof-of-concept of some groundbreaking and next-generation concepts in environment and energy fields. The following are several examples we selected among many interesting and inspirational ones.

9.2 An All-in-One, Point-of-Use Water Desalination Cell In 2010, Han et al. employed the concept of ion concentration polarization (ICP) within nanofluidics channels and created an external pressure-free, fouling-free, all-in-one direct seawater desalination device (Figure 9.1) [14]. In this device, a continuous stream of seawater was divided into desalted and concentrated flows by ICP. As a result, the seawater with salinity ∼500 mM or ∼30 000 mg l−1 can be converted to freshwater (salinity