Electrically Conductive Polymers and Polymer Composites

Electrically Conductive Polymers and Polymer Composites

From Synthesis to Biomedical Applications

Edited by Anish Khan, Mohammad Jawaid, Aftab Aslam Parwaz Khan, and Abdullah M. Asiri

The Editors

Dr. Anish KhanKing Abdulaziz UniversityCenter of Excellence for Advanced Materials Research,Chemistry DepartmentP.O. Box 8020321589 JeddahSaudi Arabia

Dr. Mohammad JawaidUniversiti Putra MalaysiaBiocomposite Technology Lab, INTROP43400 SerdangSelangorMalaysia

Dr. Aftab Aslam Parwaz KhanKing Abdulaziz UniversityCenter of Excellence for Advanced Materials ResearchChemistry DepartmentP.O. Box 8020321589 JeddahSaudi Arabia

Prof. Abdullah M. AsiriKing Abdulaziz UniversityCenter of Excellence for Advanced Materials ResearchChemistry DepartmentP.O. Box 8020321589 JeddahSaudi Arabia

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About the Editors xiii Preface xvii

1 Bioinspired Polydopamine and Composites for Biomedical Applications 1Ziyauddin Khan, Ravi Shanker, Dooseung Um, Amit Jaiswal, and Hyunhyub Ko

1.1 Introduction 11.2 Synthesis of Polydopamine 21.2.1 Polymerization of Polydopamine 21.2.2 Synthesis of Polydopamine Nanostructures 31.3 Properties of Polydopamine 51.3.1 General Properties of Polydopamine 51.3.2 Electrical Properties of Polydopamine 61.3.2.1 Amorphous Semiconductor Model (ASM) of Melanin Conductivity 71.3.2.2 Spin Muon Resonance Model (SMRM) of Melanin Conductivity 81.4 Applications of Polydopamine 101.4.1 Biomedical Applications of Polydopamine 111.4.1.1 Drug Delivery 111.4.1.2 Tissue Engineering 121.4.1.3 Antimicrobial Applications 121.4.1.4 Bioimaging 151.4.1.5 Cell Adhesion and Proliferation 161.4.1.6 Cancer Therapy 161.5 Conclusion and Future Prospectives 21 References 23

2 Multifunctional Polymer-Dilute Magnetic Conductor and Bio-Devices 31Imran Khan, Weqar A. Siddiqui, Shahid P. Ansari, Shakeel Khan, Mohammad Mujahid Ali khan, Anish Khan, and Salem A. Hamid

2.1 Introduction 312.2 Magnetic Semiconductor-Nanoparticle-Based Polymer

Nanocomposites 342.3 Types of Magnetic Semiconductor Nanoparticles 342.3.1 Metal and Metal Oxide Nanoparticles 34

Contents

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Contentsviii

2.3.2 Ferrites 352.3.3 Dilute Magnetic Semiconductors 362.3.4 Manganites 372.4 Synthetic Strategies for Composite Materials 372.4.1 Physical Methods 382.4.2 Chemical Methods 402.4.2.1 In Situ Synthesis of Magnetic Nanoparticles and Polymer

Nanocomposites 402.4.2.2 In Situ Polymerization in the Presence of Magnetic Nanoparticles 412.5 Biocompatibility of Polymer/Semiconductor-Particle-Based

Nanocomposites and Their Products for Biomedical Applications 422.5.1 Biocompatibility 422.6 Biomedical Applications 43 References 43

3 Polymer–Inorganic Nanocomposite and Biosensors 47Anish Khan, Aftab Aslam Parwaz Khan, Abdullah M. Asiri, Salman A. Khan, Imran Khan, and Mohammad Mujahid Ali Khan

3.1 Introduction 473.2 Nanocomposite Synthesis 483.3 Properties of Polymer-Based Nanocomposites 483.3.1 Mechanical Properties 483.3.2 Thermal Properties 513.4 Electrical Properties 523.5 Optical Properties 533.6 Magnetic Properties 543.7 Application of Polymer–Inorganic Nanocomposite in Biosensors 543.7.1 DNA Biosensors 543.7.2 Immunosensors 583.7.3 Aptamer Sensors 613.8 Conclusions 62 References 63

4 Carbon Nanomaterial-Based Conducting Polymer Composites for Biosensing Applications 69Mohammad O. Ansari

4.1 Introduction 694.2 Biosensor: Features, Principle, Types, and Its Need in Modern-Day

Life 704.2.1 Important Features of a Successful Biosensor 714.2.2 Types of Biosensors 714.2.2.1 Calorimetric Biosensors 714.2.2.2 Potentiometric Biosensors 724.2.2.3 Acoustic Wave Biosensors 724.2.2.4 Amperometric Biosensors 724.2.2.5 Optical Biosensors 72

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4.2.3 Need for Biosensors 724.3 Common Carbon Nanomaterials and Conducting

Polymers 734.3.1 Carbon Nanotubes (CNTs) and Graphene (GN) 734.3.2 Conducting Polymers 734.4 Processability of CNTs and GN with Conducting Polymers, Chemical

Interactions, and Mode of Detection for Biosensing 744.5 PANI Composites with CNT and GN for Biosensing Applications 754.5.1 Hydrogen Peroxide (H2O2) Sensors 754.5.2 Glucose Biosensors 764.5.3 Cholesterol Biosensors 774.5.4 Nucleic Acid Biosensors 784.6 PPy and PTh Composites with CNT and GN for Biosensing

Applications 794.7 Conducting Polymer Composites with CNT and GN for the Detection

of Organic Molecules 804.8 Conducting Polymer Composites with CNT and GN for Microbial

Biosensing 834.9 Conclusion and Future Research 83 References 84

5 Graphene and Graphene Oxide Polymer Composite for Biosensors Applications 93Aftab Aslam Parwaz Khan, Anish Khan, and Abdullah M. Asiri

5.1 Introduction 935.2 Polymer–Graphene Nanocomposites and Their Applications 965.2.1 Polyaniline 975.2.2 Polypyrrole 1025.3 Conclusions, Challenges, and Future Scope 106 References 108

6 Polyaniline Nanocomposite Materials for Biosensor Designing 113Mohammad Oves, Mohammad Shahadat, Shakeel A. Ansari, Mohammad Aslam, and Iqbal IM Ismail

6.1 Introduction 1136.2 Importance of PANI-Based Biosensors 1186.3 Polyaniline-Based Glucose Biosensors 1186.4 Polyaniline-Based Peroxide Biosensors 1206.5 Polyaniline-Based Genetic Material Biosensors 1216.6 Immunosensors 1226.7 Biosensors of Phenolic Compounds 1236.8 Polyaniline-Based Biosensor for Water Quality Assessment 1236.9 Scientific Concerns and Future Prospects of Polyaniline-Based

Biosensors 1246.10 Conclusion 126 References 126

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7 Recent Advances in Chitosan-Based Films for Novel Biosensor 137Akil Ahmad, Jamal A. Siddique, Siti H. M. Setapar, David Lokhat, Ajij Golandaj, and Deresh Ramjugernath

7.1 Introduction 1377.2 Chitosan as Novel Biosensor 1397.3 Application 1517.4 Conclusion and Future Perspectives 152 Acknowledgment 153 References 153

8 Self Healing Materials and Conductivity 163Jamal A. Siddique, Akil Ahmad, and Ayaz Mohd

8.1 Introduction 1638.1.1 What Is Self-Healing? 1638.1.2 History of Self-Healing Materials 1638.1.3 What Can We Use Self-Healing Materials for? 1648.1.4 Biomimetic Materials 1648.2 Classification of Self-Healing Materials 1648.2.1 Capsule-Based Self-Healing Materials 1658.2.2 Vascular Self-Healing Materials 1658.2.3 Intrinsic Self-Healing Materials 1678.3 Conductivity in Self-Healing Materials 1698.3.1 Applications and Advantages 1708.3.2 Aspects of Conductive Self-Healing Materials 1718.4 Current and Future Prospects 1718.5 Conclusions 172 References 173

9 Electrical Conductivity and Biological Efficacy of Ethyl Cellulose and Polyaniline-Based Composites 181Faruq Mohammad, Tanvir Arfin, Naheed Saba, Mohammad Jawaid, and Hamad A. Al-Lohedan

9.1 Introduction 1819.2 Conductivity of EC Polymers 1839.2.1 Synthesis of EC–Inorganic Composites 1839.2.2 Conductivity of EC-Based Composites 1849.3 Conductivity of PANI Polymer 1879.3.1 Synthesis of PANI-Based Composites 1899.3.2 Conductivity of PANI-Based Composites 1909.4 Biological Efficacy of EC and PANI-Based Composites 1929.5 Summary and Conclusion 194 Acknowledgments 195 References 195

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10 Synthesis of Polyaniline-Based Nanocomposite Materials and Their Biomedical Applications 199Mohammad Shahadat, Shaikh Z. Ahammad, Syed A. Wazed, and Suzylawati Ismail

10.1 Introduction 19910.2 Biomedical Applications of PANI-Supported Nanohybrid

Materials 20110.2.1 Biocompatibility 20110.2.2 Antimicrobial Activity 20210.2.3 Tissue Engineering 20410.3 Conclusion 211 Acknowledgment 211 References 211

11 Electrically Conductive Polymers and Composites for Biomedical Applications 219Haryanto and Mohammad Mansoob Khan

11.1 Introduction 21911.2 Conducting Polymers 21911.2.1 Conducting Polymer Synthesis 22111.2.1.1 Electrochemical Synthesis 22111.2.1.2 Chemical Synthesis 22111.2.2 Types of Conducting Polymer Used for Biomedical

Applications 22111.2.2.1 Polypyrrole 22111.2.2.2 Polyaniline 22211.2.2.3 Polythiophene and Its Derivatives 22211.3 Conductive Polymer Composite 22311.3.1 Types of Conductive Polymer Composite 22311.3.1.1 Composites or Blends Based on Conjugated Conducting

Polymers 22311.3.1.2 Composites or Blends Based on Non-Conjugated Conducting

Polymers 22411.3.2 Methods for the Synthesis of Conductive Polymer Composites 22511.3.2.1 Melt Processing 22511.3.2.2 Mixing 22511.3.2.3 Latex Technology 22511.3.2.4 In Situ Polymerization Method 22511.4 Biomedical Applications of Conductive Polymers 22611.4.1 Electrically Conductive Polymer Systems (ECPs) for Drug Targeting

and Delivery 22611.4.2 Electrically Conductive Polymer System (ECPs) for Tissue

Engineering and Regenerative Medicine 227

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Contentsxii

11.4.3 Electrically Conductive Polymer Systems (ECPs) as Sensors of Biologically Important Molecules 227

11.5 Future Prospects 22811.6 Conclusions 228 References 228

Index 237

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Anish Khan is currently working as Assistant Professor at the Chemistry Department, Centre of Excellence for Advanced Materials Research, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia. He completed his PhD from the Aligarh Muslim University, India in 2010. He has 13 years research experience working in the field of organic–inorganic electrically conducting nanocomposites and its applications in making chemical sensors. He completed his Postdoctoral from the School of Chemical Sciences, University Sains Malaysia (USM) in electroanalytical chemistry within 1 year. More than 100 research arti-cles have been published in referred international journals. He has attended more than 10 international conferences/workshops and published two books and seven book chapters. He has also completed around 20 research projects. Managerial Editor of Chemical and Environmental Research (CER) Journal and Member of the American Nano Society, his field of specialization is polymer nanocomposite/cation exchangers/chemical sensors/microbiosensors/nano-technology, applications of nanomaterials in electroanalytical chemistry, materi-als chemistry, ion‐exchange chromatography, and electroanalytical chemistry, dealing with the synthesis, characterization (using different analytical techniques), and derivatization of inorganic ion exchanger by the incorporation of electrically conducting polymers. Preparation and characterization of hybrid nanocomposite materials and their applications, polymeric inorganic cation exchange materials, electrically conducting polymeric materials, composite material used as sensors, green chemistry by remediation of pollution, heavy metal ion‐selective membrane electrodes, biosensors for neurotransmitters.

Dr Mohammad Jawaid is currently working as Senior Fellow Researcher (Associate Professor), at Biocomposite Technology Laboratory, Institute of Tropical Forestry and Forest Products (INTROP), Universiti Putra Malaysia, Serdang, Selangor, Malaysia and is also Visiting Professor at the Department of Chemical Engineering, College of Engineering, King Saud University, Riyadh, Saudi Arabia since June 2013. He is also Visiting Scientist to TEMAG Laboratory, Faculty of Textile Technologies and Design at Istanbul Technical University, Turkey. Previously he worked as Visiting Lecturer, Faculty of Chemical Engineering, Universiti Teknologi Malaysia (UTM) and also worked as Expatriate Lecturer under the UNDP project with the Ministry of Education of Ethiopia at Adama University, Ethiopia. He received his Ph.D. from Universiti Sains Malaysia,

About the Editors

About the Editorsxiv

Malaysia. He has more than 10 years of experience in teaching, research, and industries. His area of research interests includes hybrid reinforced/filled poly-mer composites, advance materials: graphene/nanoclay/fire retardant, lignocel-lulosic‐reinforced/filled polymer composites, modification and treatment of lignocellulosic fibers and solid wood, nanocomposites, and nanocellulose fibers, polymer blends. So far he has published 13 books, 27 book chapters, and more than 195 international journal papers, and five published review papers under top 25 hot articles in ScienceDirect during 2013–2015. He is also the Deputy Editor‐in‐Chief of Malaysian Polymer Journal, Guest Editor of Special issue‐Current Organic Synthesis, Current Analytical Chemistry, International Journal of Polymer Science, and Editorial board member‐Journal of Asian Science Technology and Innovation. Beside that he is also reviewer of several high‐impact ISI journals of Elsevier, Springer, Wiley, Saga, and so on. Presently he is supervising 20 PhD students and 8 Master’s students in the field of hybrid composites, green composites, nanocomposites, natural‐fiber‐reinforced com-posites, nanocellulose, and so on. Seven PhD and four Master students gradu-ated under his supervision in 2015–2017. He has several research grants at the university, national and international level on polymer composites of around RM 3 million (USD 700 000). He also delivered the Plenary and Invited Talk in International Conferences related to composites in India, Turkey, Malaysia, Thailand, and China. Beside that he is also a member of the technical committee of several national and international conferences on composites and materials science.

Aftab Aslam Parwaz Khan is currently working as Assistant Professor, Chemistry Department, Centre of Excellence for Advanced Materials Research (CEAMR), Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia. He has a PhD from the Aligarh Muslim University, India, on the topic prepara-tion and characterization of nanomaterials and their applications in drug deliv-ery systems. Major fields are Materials Science, Medicinal Chemistry. He has published two books and more than 100 research papers. His research encom-passes all aspects of polymer composites, homogenous catalysis, doped metal nan-oparticle synthesis, and characterization as well as novel application in environmental studies, chemical sensing, drug delivery systems for mechanistic and interaction studies using a wide range of spectroscopic techniques and ther-modynamic parameters.

Abdullah M. Asiri is Professor in the Chemistry Department,  –  Faculty of Science, King Abdulaziz University. A PhD (1995) from the University of Wales, College of Cardiff, UK on Tribochromic compounds and their applications. He has published more than 1000 research articles and 20 books. Currently the chairman of the Chemistry Department, King Abdulaziz University, he also serves as the director of the Center of Excellence for Advanced Materials Research. Director of Education Affairs Unit–Deanship of Community services. Member of Advisory committee for advancing materials, National Technology Plan (King Abdulaziz City of Science and Technology, Riyadh, Saudi Arabia). Color chemistry, Synthesis of novel photochromic, thermochromic systems,

About the Editors xv

novel colorants, coloration of textiles, plastics, Molecular modeling, Applications of organic materials into optics such as OEDS, High‐performance organic dyes and pigments. New applications of organic photochromic compounds in new novelty. Organic synthesis of heterocyclic compounds as precursor for dyes. Synthesis of polymers functionalized with organic dyes. Preparation of some coating formulations for different applications. Photodynamic thereby using Organic Dyes and Pigments Virtual Labs and Experimental Simulations. He is member of the Editorial board of Journal of Saudi Chemical Society, Journal of King Abdulaziz University, Pigment and Resin Technology Journal, Organic Chemistry Insights, Libertas Academica, Recent Patents on Materials Science, Bentham Science Publishers Ltd. Beside that he has professional membership of the International and National Society and professional bodies.

xvii

The current book deals about the conductive polymer nanocomposite about the device advancement. The current focus of the book is the preparation and applications of the polymer conductive nanocomposite for biological applicability. The conducting polymer composites are the material of current era and are in demand. The polymer conductive nanocomposites are the field of the multidisciplinary use in science and technology that’s why this composite are different from rest of the material currently in the market. The special characteristic of the book is that it presents a unified knowledge of conductive polymer composite on the basis of characterization, design, manufacture, and applications. This book has collective information about the conducting polymer nanocomposite special attention to the bio devices applications. This book benefits to the lecturers, students, researchers, and industrialist who are working in the field of material science with special attention to conducting polymer based composites. Present book on polymer conducting composite for electronic devices is a valuable reference book, hand book, and text book for teaching, learning, and research in both academic and industrial interest.

This book cover a wide range of the topics on the conducting polymer composite particularly multifunctional polymer‐dilute magnetic conductor, polymer‐inorganic nanocomposite, carbon nanomaterials based conducting polymer composites, synthesis of polyaniline‐based nanocomposite, and self‐healing conductive materials.

We are highly thankful to contributors of book chapters who provided us their valuable innovative ideas and knowledge in this edited book. We attempt to gather information related to conducting polymer composites bio‐device application from diverse fields around the world (Malaysia, India, Korea, USA, Saudi Arabia, South Africa and so on) and finally complete this venture in a fruit-ful way. We greatly appreciate contributor’s commitment for their support to compile our ideas in reality. We are highly thankful to Wiley team for their gener-ous cooperation at every stage of the book production.

30th Nov, 2017 Anish Khan, Aftab Aslam Parwaz Khan, and Abdullah M. Asiri

Saudi ArabiaMohammad Jawaid

Malaysia

Preface

1

Electrically Conductive Polymers and Polymer Composites: From Synthesis to Biomedical Applications, First Edition. Edited by Anish Khan, Mohammad Jawaid, Aftab Aslam Parwaz Khan, and Abdullah M. Asiri.© 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

1

1.1 Introduction

Understanding the systems and functions existing in nature and mimicking them led researchers to discover novel materials and systems useful in all disciplines of science, whether it is chemistry, biology, electronics, or materials science [1, 2]. Numerous biopolymers (carbohydrates and proteins) such as cellulose, starch, collagen, casein, and so on, are naturally occurring polymers and have vast appli-cation in the biomedical research field. In recent years, PDA, a bioinspired poly-mer having a molecular structure similar to that of 3,4‐dihydroxy‐l‐phenylalanine (DOPA), which is a naturally occurring chemical in mussels responsible for their strong adhesion to various substrates, has been regarded as a promising polymer, with applications in energy, electronics, and biomedical fields, due to its chemi-cal, optical, electrical, and magnetic properties [3, 4]. For example, PDA can be easily deposited or coated with any substrate type of one’s choice, including superhydrophobic surfaces, making it a highly beneficial material for coating and strong adhesive applications [3]. PDA also has various functional groups such as amine, imine, and catechol in its structure, which opens up the possibility for it to be integrated covalently with different molecules and various transition metal ions, thus making it a prerequisite in many bio‐related applications.

Herein, this chapter describes the general synthetic route, polymerization mechanism, key properties, and biomedical applications of PDA. PDA can be synthesized by oxidation and self‐polymerization of dopamine under ambient conditions; however, it can also be synthesized by enzymatic oxidation and elec-tropolymerization processes, which are discussed in detail. Furthermore, this chapter also gives a brief idea about the characteristic properties of PDA such as optical, electrical, adhesive, and so on, followed by an extensive discussion of its applications in drug delivery, bioimaging, tissue engineering, cell adhesion and proliferation, and so on, with a special focus on its conductivity.

Bioinspired Polydopamine and Composites for Biomedical ApplicationsZiyauddin Khan1, Ravi Shanker1, Dooseung Um1, Amit Jaiswal2, and Hyunhyub Ko1

1 Ulsan National Institute of Science and Technology (UNIST), School of Energy & Chemical Engineering, UNIST-gil 50, Ulsan 44919, Republic of Korea2 BioX centre, School of Basic Sciences, Indian Institute of Technology Mandi, Kamand, Mandi 175005, Himachal Pradesh, India

1 Bioinspired Polydopamine and Composites for Biomedical Applications2

1.2 Synthesis of Polydopamine

1.2.1 Polymerization of Polydopamine

In the general synthesis of PDA, the dopamine monomer undergoes oxidation and self‐polymerization in an alkaline medium (pH > 7.5) with air as an oxygen source for oxidation. This self‐polymerization of the oxidative product of dopa-mine reaction is extremely facile and does not require any complicated steps. Although the polymerization of dopamine looks simple, the synthesis mechanism has not yet been investigated comprehensively [3, 5]. As shown in Figure 1.1, it is believed that in an alkaline solution dopamine is first oxidized by oxygen to dopa-mine quinone, followed by intramolecular cyclization to leucodopaminechrome through Michael addition. The formed intermediate leucodopaminechrome undergoes further oxidation and rearrangement to form 5,6‐dihydroxyindole, which may yield 5,6‐indolequinone by further oxidation [6]. Both these indole derivatives can undergo branching reactions at a different position (2, 3, 4, and 7), which can yield various isomers of dimers and finally higher oligomers. These oligomers can self‐assemble by dismutation reaction between catechol and o‐qui-none to form a cross‐linked polymer [3, 6]. Furthermore, there have been various other reports in which the authors have tried to investigate the exact mechanism of PDA formation, but this aspect is still unclear [7–10].

Along with the oxidation and self‐polymerization of dopamine in an alkali solution, PDA can also be synthesized by enzymatic oxidation and electropolym-erization processes [11–13]. Enzymatic polymerization has attracted considera-ble interest owing to its environment‐friendly characteristics. Inspired by the formation of melanin in a living organism, dopamine has been enzymatically polymerized using laccase enzyme into PDA at pH 6 (Figure 1.2) [1]. In laccase‐ catalyzed polymerization, laccase gets entrapped into the PDA matrix, which offers great advantages in biosensing and biofuel cell applications. In contrast to the enzymatic process, dopamine can also be electropolymerized and deposited

HO

HO

NH2 NH2O

O

HO

HO NH

NH

NH N

H

HO

ONH

HO

HO

HO

HOn

HO

HO

n

Oxidation Cyclization

Rearrangement

Oxidation

Polymerization

or

2

35

6

4

7

9

8

Figure 1.1 Formation mechanism of PDA in an alkali solution. (Reprinted with permission from Refs [5] and [3] Copyright 2011 and 2014 American Chemical Society.)

1.2 Synthesis of Polydopamine 3

on the substrate at a given potential in a deoxygenated solution. However, the electropolymerization process requires highly conductive materials, which is one of the main disadvantages of this process of dopamine polymerization.

1.2.2 Synthesis of Polydopamine Nanostructures

A great deal of attention has been paid of late toward the synthesis of monodis-perse PDA nanoparticles and PDAs with different morphologies, which can be used for other applications such as chemical sensors, energy storage, and so on. The size of the PDA particles can be tuned using a different ratio of solvents and base [14, 15]. Usually, after the self‐polymerization reaction, PDA tends to form uniform spherical particles after prolonged reaction up to 30 h. Ai et al. have demonstrated that the size of PDA spheres can be controlled by varying the ratio of ammonia to dopamine and thereby synthesize various sizes of PDA nanopar-ticles (Figure 1.3a–e) [14]. In another study, Jiang et al. reported that varying the amount of ethanol and ammonia can also tune the size of PDA particles (Figure 1.3f ) [15].

Recently, PDA with some unique morphology, for example, PDA nanotubes, have also been reported using a template‐based method. Yan et al. coated a PDA layer on ZnO nanorods as a template by self‐polymerization reaction of dopa-mine; and later the ZnO nanorod template was etched by ammonium chloride solution, leaving behind hollow PDA nanotubes (Figure 1.4a) [16]. Xue et al. reported the scalable synthesis of PDA nanotubes using curcumin crystal as a template [17], as shown in Figure 1.4b. These PDA nanotubes are several tens of micrometers long with 40‐nm wall thickness and 200‐ to 400‐nm tube diameter, which can be tuned by stirring rate and curcumin crystal size. Further to nano-tubes, freestanding films of PDA and hybrid PDA films have also been prepared for their use in structural color, by layer‐by‐layer assembly [18–20]. In one of the reports, Yang et al. have reported composite freestanding films of PDA with pol-yethyleneimine (PEI), which was grown on air/water interface [20]. The prepared film was a freestanding transparent film, more than 1 cm in diameter, 80 nm in thickness, and without any visual defects on the film surface as proved by field

Lac DA

Polymerization Cast coating

DA

ElectrodeE

lectrode

O2

H2O DAox

PDA

HQ O2

BQ H2Oe–

MWCNTs PDA

Figure 1.2 Graphical representation of the formation of PDA–laccase–MWCNT nanocomposite film on GCE for hydroquinone biosensing. (Reprinted with permission from Ref. [1] Copyright 2010 American Chemical Society.)

1 Bioinspired Polydopamine and Composites for Biomedical Applications4

emission scanning electron microscopy (FESEM). The film size can be tuned by the container which holds the dopamine and PEI solution.

Although there has been excellent progress in preparing different shapes and sizes of PDA nanoparticles, producing monodisperse nanoparticles is still a chal-lenge, which is an essential parameter in biological science to ensure consistency in experiments. In the near future we can expect that this field will make further progress in producing highly monodisperse nanoparticles.

= Carbon atom = Carbon precursor

= Nitrogen atom

= Dopamine

= Oxygen atom

Water and ethanol

NH4OH

Room temperature

200 nm

(iii)

0%

0%W

ater

100%

Eth

anol

100%

Qualitative change

Quantitative change

Qualitative change

(ii)

(i)

(f)

(b)

(e)(d)(c) 380 nm520 nm780 nm

Dopamine

Polydopamine

NH4OH

H2O+ROH

(a)

Figure 1.3 (a–e) Schematic representation of sub-micron size PDA particles and their morphological study. (Redrawn and reprinted with permission from Ref. [14] Copyright 2013 Wiley-VCH.) (f ) Study of EtOH and ammonia concentration on PDA morphology. (Redrawn and reprinted with permission from Ref. [15] Copyright 2014 Nature Publishing Group.)

1.3 Properties of Polydopamine 5

1.3 Properties of Polydopamine

1.3.1 General Properties of Polydopamine

PDA is an analog of eumelanin (a type of natural melanin) due to the similarity in chemical structure/component, which leads to the resemblance in physical properties [3, 21, 22]. Therefore, PDA has been regarded as a natural biopolymer, which has been utilized as a coating material in various applications. PDA is most commonly known for its inherent adhesive property; but functionalities of PDA have not been limited to adhesion as it possesses various properties, which are listed and discussed here.

1) Optical properties: PDA shows broadband absorption ranging from ultravio-let (UV) to visible region, which increases exponentially toward the UV spec-trum as in the case of the naturally occurring analog eumelanin. The absorption in the UV region originates from oxidation of dopamine to dopachrome and dopaindole; however, the absorption in the visible and near‐infrared (NIR) region is due to the subsequent self‐polymerization process [23, 24].

ZnO nanorod

(3) ZnO removal

(a)

(b)

Polydopamine (PDA)

Curcumin crystals

CurcuminDopamine

Water

Tris-HCl

Ethanol

PDA nanotubes

PDA@Curcumin

1. Crystallization

HOHO HO

O O

OHOCH3

OCH3NH2

+

2. Polymerization

3. Puri�cation

Ethanol/acetone

n

n

NHNH

OHHOOHHO

or

(2) Self-polymerization

Dopamine

ZnO

200 nm200 nm

200 nm

10 μm

200 nm

(1) Dopamine adsorption

24 min

13 nm 24 nm

53 nm

75 nm96 min

120 min

48 min

PDA nanotube

Figure 1.4 (a) Graphical representation of PDA nanotube synthesis and its high-resolution TEM images. (Reprinted with permission from Ref. [16] Copyright 2016 Royal Society of Chemistry.) (b) PDA nanotube synthesis by curcumin crystals and its morphology. (Reprinted with permission from Ref. [17] Copyright 2016 American Chemical Society.)

1 Bioinspired Polydopamine and Composites for Biomedical Applications6

2) Electrical conductivity: In 1974, McGinness et al. observed the electrical switching properties of eumelanin, and since then it was assumed that eumelanin has organic semiconductive properties [25, 26]. It was suggested that highest occupied molecular orbital (HOMO) and lowest occupied molecular orbital (LUMO) levels of eumelanin act as valence and conduction bands as in the case of the semiconductor. Eumelanin is an aromatic com-pound that results in HOMO and LUMO levels composed of π‐system and the charges move through this π‐system, leading to the electrical conductivity of eumelanin. See the electrical properties (Section  1.3.2) for a detailed description.

3) Adhesive property: PDA displays a strong adhesive property to all kinds of surfaces and it is believed that this property arises due to the presence of the catechol group. However, it is not well understood yet how PDA diffuses to a different kind of surface, but based on literature it can be stated that PDA interacts with the substrate by a covalent or noncovalent binding mechanism [27, 28].

4) Biocompatibility and biodegradation property: Biocompatibility and biodeg-radation are the key parameters for any material to have an application in the biomedical field. PDA, a major component of melanin, shows exceptional biocompatibility even at high doses when its cytotoxicity was studied with mouse 4T1 breast cancer cells and human cervical cancer cells (HeLa cells) [29]. However, melanin can be degraded in vitro in the presence of oxidizing agents such as hydrogen peroxide, which is also the case for PDA [30]. The color fading was observed in PDA when incubated with hydrogen peroxide, which suggests its degradation [29]. Bettinger et al. in an in vivo study also suggests complete degradation of implanted PDA in 8 weeks [31].

1.3.2 Electrical Properties of Polydopamine

Organic semiconductors possess structural similarity to biological compounds, which opens up the possibility of their use in biomedical science [32]. A few of  the most used organic semiconductors in biomedical science are poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulphonate) (PEDOT:PSS) and poly(3‐hexylthiophene) (P3HT) due their excellent ion and electron mobility, and higher tissue integration ability [33, 34]. PEDOT:PSS is one of the first and widely used active channels in biomedical devices such as organic electrochemical transistors (OECTs) [33]. The performance of these devices can be improved by making a thinner film of the active channel below 100 nm [35, 36]. However, past litera-tures for such devices are mainly based on four transducing materials: P3HT, polypyrrole, PEDOT:PSS, and polyaniline [37]. This opens up the possibility of searching for alternative materials to be used in bioelectronic devices, in particular for edible electronics.

Interest in melanin, both natural and synthetic, has bloomed since the seminal study by McGinness et al. [25]. In recent years, PDA, also called synthetic mela-nin similar to natural melanin, has emerged as an additional candidate to be used in bioelectronics for transduction purposes. Since major research work in the context of the conductivity studies has been done on natural melanin, from

1.3 Properties of Polydopamine 7

hereon we use the word melanin as PDA’s properties are essentially similar to those of melanin. Melanin has some very interesting properties for biomedical application, such as broad monotonic optical absorption [38, 39], free radical population state [24, 40], and the possibility of making thin films less than 100 nm, thus offering device integration with neurons [4, 24, 41], hydration‐dependent electrical and photoconductivity ranging from 10−8 to 10−4 S cm−1, depending on the hydrated state [39, 42], and the ability to link electronic and protonic/ionic signals in a common mechanism through comproportionation reaction (CRR) [43].

To describe each point is beyond the scope of this chapter, so we have mainly focused on its charge transport/electrical properties based on two different charge transport models available in the literature. Of the two models available to explain melanin charge transport properties, the first is based on an amor-phous semiconductor model (ASM) and the other is hydration‐dependent muon spin resonance (μSR) described by a CRR.

1.3.2.1 Amorphous Semiconductor Model (ASM) of Melanin ConductivityThis model is based on the four observations and considers melanin to be an amorphous semiconductor because it shows the following:

● Semiconductor‐type Arrhenius temperature dependence on its conductivity [42, 44]

● Bistable switching behavior [25, 45] ● Broadband optical absorbance [39] ● Stable free radical: unpaired electrons at the Fermi energy level [46].

However, there are a few shortcomings in this model, the first one being its broad absorbance (Figure 1.5a), which can also be described by the oligomer structure, that is, the spectrum is made up of multiple individual chemical chromophores [39, 49]. It cannot describe the delocalized electronic state for which large 2D sheet‐type structures are required; this is not true for oligomers, which are fairly small. The second is that only wet melanin samples display hydration‐dependent switching behavior.

To observe the conductivity of hydration‐dependent melanin, Mostert et al. measured the water–melanin adsorption isotherm on melanin pallet samples and the result is shown in Figure 1.5b, which exhibits the significant presence of water in melanin [47]. Mostert et al. also measured the hydration‐dependent conductivity using two different contact geometries, that is, sandwich and van der Pauw, and the results are shown in Figure 1.5c,d [48].

It can be seen from Figure 1.5c that the conductivity increases by orders of magnitude in a sub‐exponential manner. However, the specimen was found to be at nonequilibrium in sandwich geometry due to low exposure of the surface area by the presence of the contacts. Therefore, an open‐contact arrangement, van der Pauw geometry (Figure 1.5d inset), has been used where ~71% of surface area can be exposed than to ~37% in sandwich geometry; and the result is shown in Figure 1.5d [48]. Interestingly, these data were found not to be in agreement with previous literature and also could not be explained by the existing ASM theory [42, 50].