PANI-CNT nanocomposites

CHAPTER 9

PANI-CNT nanocomposites Mohsen Khodadadi Yazdia, Hoda Saeidia, Payam Zarrintajb,c,d, Mohammad Reza Saebe, Masoud Mozafarif,g,h a

School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran Polymer Engineering Department, Faculty of Engineering, Urmia University, Urmia, Iran c Color and Polymer Research Center (CPRC), Amirkabir University of Technology, Tehran, Iran d Advanced Materials Group, Iranian Color Society (ICS), Tehran, Iran e Department of Resin and Additives, Institute for Color Science and Technology, Tehran, Iran f Bioengineering Research Group, Nanotechnology and Advanced Materials Department, Materials and Energy Research Center (MERC), Tehran, Iran g Cellular and Molecular Research Center, Iran University of Medical Sciences, Tehran, Iran h Department of Tissue Engineering & Regenerative Medicine, Faculty of Advanced Technologies in Medicine, Iran University of Medical Sciences (IUMS), Tehran, Iran b

1. Introduction PANI is the most studied conducting polymer because of the low-cost monomer, easy fabrication methods, chemical and environmental stability, fair processing, good electrical conductivity, biocompatibility, and redox switching [1]. Based on oxidation level, PANI can exist in three main forms: leucoemeraldine base (LB, fully reduced), emeraldine base (EB, half-oxidized), and pernigraniline base (PNB, fully oxidized). The emeraldine is the most stable form of PANI; upon doping, it changes into conductive emeraldine salt (ES). On the other hand, carbon nanotubes are an allotrope of carbon that can be classified into single-walled nanotubes (SWNTs) and multiwalled nanotubes (MWCNTs) [2]. SWNTs can be considered as one wrapped sheet of graphene, while in MWCNTs multiple wrapped graphene sheets are stacked; this implies that SWCNTs are usually more flexible than MWCNTs. CNTs have extraordinary electrical, thermal, and mechanical properties [3]. Thanks to these fascinating properties and high specific surface area, CNTs have been widely utilized in manufacturing polymer nanocomposites for various applications [4, 5]. Polyaniline/CNT nanocomposites that inherit the fascinating properties of PANI and CNT moieties can be used in a variety of diverse applications, as illustrated in Fig. 1. These applications are summarized in this chapter.

2. Synthesis and properties PANI is usually synthesized through chemical or electrochemical methods, but there are other synthesis methods as well [1]. Generally speaking, the chemical synthesis is more applicable for mass production of electroactive PANI-based materials [6, 7]. On the other Fundamentals and Emerging Applications of Polyaniline https://doi.org/10.1016/B978-0-12-817915-4.00009-9

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Fig. 1 Various applications of PANI/CNT nanocomposites.

hand, electrochemical synthesis provides fewer control parameters, including applied potential, applied current, potential scan rate, and current scan rate [6]. But these parameters significantly affect the electrochemical performance of PANI [6]. The electrochemical method provides fine control of the polymerization reaction; besides, the oxidant- and surfactant-free nature of the method is more environmentally preferable and fewer impurities will be present in the final products. However, more production cost relative to chemical methods is expected. In the electrochemical method, PANI thin films can deposit on a conductive substrate such as ITO glass and platinum; this process combines synthesis and processing steps in a single easy step. Chemical methods, on the other hand, can be carried out under different conditions (e.g., temperature, sonication, plasma, templates, stirring, solvents) and using different additives (e.g., surfactants, initiator, enzymes, dopants/codopants, aminodiphenylamine/p-phenylenediamine, catalysts) which provide huge versatility in the morphology/properties of PANI [1, 8–10]. PANI is usually produced in powder form, which does not melt and has limited solubility. Thus, its processing is relatively difficult, limiting its applications. The conductivity of as-prepared PANI powder is roughly on the order of 1–10 S/cm but much higher/lower quantities are also reported [1]. In fact, the conductivity is highly dependent on the dopant type; molecular weight, morphology, crystallinity, and doping level are also important factors affecting the electrical conductivity of PANI. Similarly, PANI/CNT nanocomposites can also be prepared through various methods, such as in situ chemical polymerization, electrochemical polymerization, Interfacial polymerization, solution mixing, and electrophoretic methods [11, 12]. As the PANI does not

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melt, the melt mixing procedure is not applicable for PANI/CNT nanocomposites. Aniline molecules can adsorb on the CNT surface, through π-π interactions, prior to in situ polymerization. The adsorbed molecules act as the nucleation sites where polymerization begins and a PANI layer forms, wrapping the CNT surface [13]. CNTs usually affect the polymerization, morphology, crystallization, and electrical properties of the PANI/CNT nanocomposites [13]. The morphology of PANI/CNT nanocomposites synthesized through a simple in situ polymerization is dependent on the CNT loading [14]. Functionalization of CNTs may increase the dispersibility in organic solvents and the interaction with PANI chains. For instance, amino-functionalized CNTs act as reactive sites in aniline polymerization where PANI chains can be chemically grafted to the CNT surface [15]. In electrochemical polymerization, PANI/CNT films may be created through codeposition of PANI and CNTs on the working electrode surface or deposition of PANI on a CNT-coated electrode [16, 17]. Higher conductivity enhanced mechanical integrity, and improved electrochemical capacitance was observed for PANI/CNT films compared with pure PANI [18]. Electrophoretic deposition (EPD) is a colloidal process usually used in the ceramic production industries [19]. In this process, suspended charged particles move toward conductive electrodes with opposite surface charges and deposit onto them, forming a film of controlled thickness [19]. The electrophoretic method can be used for manufacturing a uniform, dense, and nanostructured film of PANI or PANI/CNT nanocomposites [20]. For example, both PANI and CNTs can be suspended in an appropriate solvent (e.g., acetonitrile) where they dissociate into ions. CNTs are wrapped by the positively charged PANI chains, making colloidal spheres. These positively charged spheres are attracted toward the electrode that is connected to the negative terminal of the battery. Solution mixing is a simple method in which PANI and CNT are dispersed in similar or different solvents, usually with the aid of ultrasonic waves in order to disintegrate CNT bundles/agglomerates [21]. After evaporation of the solvent/solvents, the PANI/CNT nanocomposites are obtained. Moreover, CNT functionalization could enhance its dispersion in the solvent and CNT interaction with PANI chains improves as well. Accordingly, more homogeneous, well-dispersed nanocomposites are usually obtained when CNTs are functionalized. Interfacial polymerization usually occurs at the interface of inorganic/organic solvents in which aniline is usually dissolved in the organic solvent while the oxidation agent is dissolved in the aqueous phase. In general, PANI nanofibers are created in interfacial polymerization of aniline, while granular morphology dominates in oxidative polymerization in an acidic aqueous medium [11]. Although pristine CNT is almost insoluble in all solvents, functionalized CNTs are dispersible in many organic solvents; organic solvents containing aniline monomer and dispersed functionalized CNTs polymerize at the interface of the organic phase with an aqueous phase that contains initiator molecules. For

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example, MWCNTs immensely functionalized with 4-aminobenzoic acid were dissolved in dichloromethane containing aniline monomer, followed by the addition of acidic aqueous solution of APS [22]. The results showed that PANI-g-MWCNT is created at the interface and gradually diffuses into the top aqueous phase. Without CNTs, PANI nanofibers, 20 nm in diameter and several tens of microns in length, were obtained. On the other hand, PANI nanofibers are also created accompanied by PANI/CNT coreshell nanostructures in which the average diameter is significantly higher than pristine functionalized PANI/CNT nanofibers. When dynamic interfacial polymerization was carried out in this research, the morphology changed profoundly compared to static interfacial polymerization discussed previously. In fact, less ordered PANI nanofibers are created through dynamic interfacial polymerization. Thanks to the high specific area of CNTs, PANI-g-MWNT showed higher surface areas, i.e., 52.56 and 85.20 m2/g for PANI and PANI-g-MWNT nanocomposites, respectively. It is worth mentioning that other polymerization methods (e.g., plasma) utilized for pure PANI are also applicable for the synthesis of PANI/CNT nanocomposites [14]. CNTs show high thermal stability [23]. In fact, thermal decomposition of MWCNTs begins around 650°C in the air, which is significantly higher than that for PANI, which is below 400°C. Thus, incorporation of CNTs usually increases the heat stability of polymer/CNT nanocomposites through increasing the temperature of decomposition onset and the temperature where maximum weight loss occurs [24]. CNT incorporation usually increases the electrical conductivity of PANI because of the higher electrical conductivity of CNTs [13]. Like other insulating polymers, dedoped PANI shows a percolation threshold while the conductivity of doped PANI gradually increases with CNT loading [14]. CNT incorporation could also induce crystallinity in the PANI and affect its molecular weight [13]. These improved properties can result in more widespread applications of PANI in diverse fields.

3. Applications 3.1 Sensors and biosensors A sensor is a device that provides information about the concentration of chemical species in a liquid or gas. As shown in Fig.2, sensors are usually composed of two major elements: the sensing element and transducer. There are receptors in the sensing elements on which only a special analyte can be adsorbed, resulting in a change in the physical properties of the sensing element. The transducer produces an output signal proportional to this change [25]. The produced signal is then filtered and amplified in the preprocessor followed by a signal processing system, and finally it may be displayed. The exiting signal from preprocessor or processor may be sent directly to other devices, such as controllers. Sensors with a natural derived or synthetic biomimetic sensing element are known as biosensors. Biosensors are usually used for detection of biological analytes such as cells,

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Fig. 2 Schematic representation of a typical sensor.

nucleic acids, proteins, etc.; for instance, glucose biosensors are widely used for glucose monitoring in the blood for the management of diabetes [26]. Sensors may be classified according to output signals [27]. Electrochemical, metal oxide semiconductor, catalytic bead (pellistor), photoionization detectors (PIDs), infrared point, and optical sensors are well-known chemical sensors. More interestingly, wireless communication technology can be combined with different sensors as well [28]. Sensitivity, selectivity, and response and recovery time are the main performance parameters of sensors [29]. Both carbon nanotubes [30] and PANI [31] have been used in sensing applications. CNT-based chemical sensors have been widely investigated for detection of different species in different applications, such as monitoring of air pollutants, food/agricultural applications, biological, and security applications [30]. A highly delocalized π-electron system in pristine CNTs is uniformly distributed along the tube and is responsible for the high electrical conductivity of these nanotubes. However, adsorption of different species disturbs this uniform electron cloud, resulting in an altered electronic structure that may be used in sensing elements of sensors. This intriguing feature accompanied by high specific surface area has made CNTs appealing nanomaterials for manufacturing different types of sensors [30, 32]. However, due to the weak interaction of the CNT surface with the analyte, surface or end functionalization is usually applied when designing CNT-based sensors [33]. Modified CNTs (e.g., with amino and carboxyl) have also been used in sensor/biosensor applications [34]. Several sensors based on various

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fundamental physical principles, including chemiresistors, chemicapacitors, and fieldeffect transistors (FETs), have been developed using CNTs. In addition, conducting polymers have many characteristics that make them good candidates for sensor manufacturing [31]. The electrical conductivity of PANI depends on the charge mobility along the polymer chain, which can be altered through doping. Furthermore, interaction with absorbed molecules can also change the bandgap and electrical conductivity as well. This concept is the basis of chemiresistors in which the resistivity change is proportional to gas concentration [31]. Nanostructured PANI with higher surface area can interact more effectively with the analyte, resulting in the development of more effective sensors [35]. Chemiresistive gas sensors based on CNTs suffer from low selectivity and sensitivity (due to poor interaction with analyte molecules), which may be improved through incorporation of intrinsically conducting polymers (ICPs). PANI/CNT hybrid nanocomposites that combine chemical structure, electronic properties, and surface area of these materials could potentially enhance the sensing ability of sensors based on pure CNT or PANI. A much faster response was observed for PANI/SWCNT nanocomposites compared to pure PANI nanofibers [36]. It was observed that these PANI/SWCNT nanocomposites with nanofibrillar morphology and tunable conductivities are sensitive to very low concentrations of hydrochloric acid and ammonia gases. Excellent gas sensing performance was also observed for PANI/SWCNT nanocomposites for NH3, NO2, and H2S gases [37]. Superior chemical stability of PANI/SWCNT core-shell nanocomposites compared to bare SWCNT and PANI was also observed [38]. It was mentioned that special electron interactions between SWCNT and PANI improve sensitivity and stability of hydrazine sensors. PANI/CNT nanocomposites have also been in biosensor applications. For example, a high sensitivity cholesterol biosensor with fast response was fabricated using PANI/MWCNT nanocomposite films deposited on ITO glass [39]. PANI/CNT-based biosensors have also been used for development of microbial biosensors [40].

3.2 Batteries and supercapacitors Energy storage devices are tremendously important for both mature technologies such as conventional vehicles and emerging technologies such as electric vehicles and portable electronics. Different types of batteries, including lead acid, Li-ion, nickel metal hydride (NiMH), and Ni-Cd have been developed and used in different applications, including automotive, industrial, and portable electronic devices. These batteries are generally subdivided into rechargeable (or secondary) batteries and primary batteries that cannot be recharged. Li-ion rechargeable batteries are the fastest growing battery technology, which is widely used in electric vehicles and portable electronic devices.

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Although the energy density of Li-ion batteries is relatively high, they suffer from low power density because of electrochemical limitations. In contrast, supercapacitors have high power density, while their energy density is much lower than Li-ion batteries. Double layer supercapacitors, pseudocapacitors, and hybrid capacitors are the major types of supercapacitors used nowadays. For electric vehicle applications, high power is needed for better acceleration along with high energy density in order to increase the range. Accordingly, a hybrid combination of battery and supercapacitors are usually used, which is shown as hybrid storage in Fig. 3 [41]. According to this figure, hybrid storage is similar to traditional combustion engines, which implies applicability of hybrid storage in real electric vehicles. In Fig. 3, different energy storage devices are compared according to energy and power density and discharge time as well. Because of their redox nature, ICPs began to be considered for energy storage applications soon after their birth. In fact, ICPs, especially PANI, thanks to high capacitance, proper electrical conductivity, low monomer cost, and easy synthesis, have attracted much interest as an electrode material [42]. While the theoretical capacity of polypyrrole, polythiophene, polyparaphenylene, and polyacetylene are <150 mA h g 1, the theoretical capacity of PANI is 295 mA h g 1 [43]. Higher capacity, along with the aforementioned advantages of PANI, has resulted in the intensive application of PANI in batteries and capacitors. For example, the electrochemical performance of molybdenum disulfide (MoS2) anode material can be effectively improved through PANI incorporation [44]. PANI/MWCNT nanocomposite-based hybrid electrodes have been used as the electrode for rechargeable batteries [45–48]. In these electrodes, improved mechanical

Fig. 3 Efficiency landscapes patterned by data collected from literature on batteries and supercapacitors.

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integrity and electrical conductivity, increased capacity, enhanced cycle stability, and increased coulombic efficiency are usually observed. Conducting polymers, including PANI, have been used for the development of electrode materials for supercapacitors [49]. In fact, PANI is intrinsically a supercapacitor material rather than a battery material [50]. Morphology and doping level significantly affect the specific capacitance of the PANI supercapacitors, which could be adjusted by controlling the synthesis method or postsynthesis modifications [6]. Unfortunately, the specific capacity of most PANI-based supercapacitors is <1000 F/g with a relatively poor cyclic stability, which can limit the applications. However, developing nanocomposites of PANI with carbon nanomaterials has been widely investigated in order to increase the specific capacitance and electrochemical behavior of supercapacitors. For example, very high specific capacitance (2200 F/g) has been reached for electrodes manufactured via PANI deposition on porous carbon [51]. PANI/MWCNT with high specific capacitance (1065 F/g) and excellent cyclic stability (92.2% capacity retention after 1000 cycles) has been reported by Yang et al. [52]. Besides, excellent cyclic stability (more than 92%) and good specific capacitance (328–560 F/g) have also been reported by other researchers [53–57].

3.3 Solar cells Solar energy harvesting is critical for growing electricity demand. Although silicon-based solar cells are the most widely used solar cell technology nowadays, other emerging technologies such as dye-sensitized solar cells (DSSCs) and perovskite solar cells are under rapid development and commercialization. The power conversion efficiency and other performance factors of the DSSCs are significantly dependent on the selection of structural materials. PANI and its nanocomposites have been mostly used for manufacturing the photoanode and counter electrode of DSSCs. For example, PANI/CNT films have been used as the transparent conductive electrode for manufacturing a flexible ITO-free photoanode [58]. The flexible natures of PANI/CNT films and their good transparency are very interesting for flexible solar cells, organic LEDs (OLEDs), e-papers, and display applications. However, most of the studies dealing with the application of PANI in DSSCs are devoted to the development of PANI-based counter electrodes [59–63]. For instance, Tai et al. manufactured bifacial transparent DSSCs that are able to produce electrical power upon light illumination on each side of the solar cell [61]. Power conversion efficiency of this DSSC was comparable to DSSCs with a Pt-based counter electrode. Higher conversion efficiency was observed (9.24% vs. 8.08%) for counter electrodes based on PANI/MWCNTs compared to PANI [64]. Besides, electrocatalytic activity of PANI/CNT electrodes is higher than pure PANI counter electrodes. Improved

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power conversion and electrocatalysis activity were also observed for DSSCs with a counter electrode made of PANI/SWCNT nanocomposites [65]. Improved charge transfer between PANI and SWCNT is proposed to be responsible for enhanced power conversion efficiency.

3.4 EMI shielding It is well known that when an alternating current flow through a conductor, electromagnetic waves (EMs) can be produced around the conductor. Thus, it is possible for the EMs of neighboring conductors/wires to interfere, like other waves, which may lead to difficulties, especially in microelectronics. Electromagnetic interference (EMI) can be considered as a type of environmental pollution that can significantly affect the performance of electronic devices and may bear health and security concerns. For example, in coaxial cables, the inner wire is surrounded by a tubular conducting shield in order to prevent EMI. Miniaturization in electronic industries has resulted in many more EMI problems. Thus developing strategies for preventing this type of pollution (i.e., EMI shielding) is very important for fast-growing microelectronics. Effective EMI shields should effectively reflect or attenuate the incident waves. Two groups of materials are usually used in manufacturing EMI shields: 1. magnetic materials including magnetic metals (such as Fe, Co, Ni) and ferrites (such as Fe2O3) 2. dielectric materials including carbon-based materials (such as carbon fibers, CNTs, graphene, carbon black), SiC fibers, ZnO, SrTiO3, and ICPs Metals are most frequently used in EMI shielding applications because of high reflectance. For example, a shielding room against radiofrequency (e.g., cellphone or wifi radiation) using a fine metallic mesh or thin metallic film is a conventional application of metal-based shields. For example, Scotchtint is a commercial reflective shield that not only blocks the RF but also harmful UV radiation; in addition, it has energysaving performance through heat gain/loss reduction in summer/winter. Thus, EMI shields based on metals are usually in the form of coatings and thin films. However, metal coatings suffer from some disadvantages such as high density, relatively high cost, and low corrosion/scratch resistance; besides, the reflection may be problematic in some cases, such as with stealth aircraft where wave absorption is highly preferred. Carbon-based materials (e.g., carbon nanofibers, CNTs, graphene, carbon black) have also been used as conductive fillers in EMI shielding applications [66, 67]. Electrical conductivity, chemical resistance, and low weight are the major advantages of carbonbased fillers, which are normally used in polymer matrices. Conductivity of the carbon/ polymer composites, as a major attenuation mechanism, depends on the conductivity and morphology of the carbon materials; i.e., percolation threshold and conductivity level

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can be greatly influenced by these factors. In fact, one-dimensional nanostructures with high aspect ratios such as carbon nanotubes with extraordinary electrical conductivities could create percolation at extremely low loadings [67, 68]. Generally speaking, the shielding effectiveness order of carbon-based fillers is: carbon black < carbon fiber < carbon nanotube < graphene sheet [66]. Despite the fact that the intrinsic properties of materials such as electrical conductivity, magnetic properties, and dielectric constant are very important in designing effective EMI shields, the architecture of such materials is also of vital importance. In other words, foams, multilayers, woven structures, and patterned structures significantly affect the shielding effectiveness (SE) of the shields. For example, extraordinary EM wave absorbers have been recently fabricated using graphene foams [69]. ICPs can also be used in EMI shielding applications because they can effectively absorb electromagnetic waves [70]. Apart from relatively high conductivities, they also benefit from high dielectric constants, which are an important requirement for effective wave absorption [71]. In fact, unlike metals and carbon-based shields, ICPs absorb EM waves rather than reflect them [72]. Corrosion resistance, light weight, affordable cost, versatility, processability, and adjustable conductivities are some of the advantages of ICPs that make them promising for EMI shielding applications. PANI, due to the monomer cost, environmental stability, easy polymerization, and tunable conductivity, has gained much attention in EMI shielding applications. Tantawy et al. synthesized a PANI nanopowder with solid state polymerization and used it for shielding in the X-band [73]. They found that wave absorption is the dominant mechanism, which increases with PANI loading. In addition, the versatile morphology and chemical modifications of PANI make it more interesting for EMI applications. More recently, different composites of PANI with other polymers [74] and multifunctional hybrid particles containing PANI and ferrite, Ti3SiC2 [75], graphite [76], graphene, graphene oxide [77], and CNT, graphite nanosheets/SWNT [78], graphite oxide/γ-Fe2O3/BaTiO3 [79], have been used in EMI applications. Synergistic effects in PANI@MWCNT core-shell nanostructures have resulted in high total shielding effectiveness (SE) of 27.5 to 39.2 dB in the Ku-band [80]. Flexible films of PANI/ CNT with very high specific shielding effectiveness (SSE) of 7.5 * 104 dB cm2 g 1 was reported by Li et al. in which PANI provides mechanical integrity and alignment for CNTs [81]. This outstanding SSE is attributed to high electrical conductivity (3009 S/cm) and low density of the manufactured films. Better impedance matching of PANI/CNT nanocomposites relative to pure CNTs, polarization losses especially at the CNT-PANI interface, multiple reflections, and higher surface area are the most important factors that enhance the absorption of incident waves, just like polymer nanocomposites based on graphene or graphene oxide [82].

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3.5 Anticorrosive coatings It was estimated that annual corrosion costs exceed $2500 billion globally [83]. In the conventional methods, coatings based on chromium, copper, and zinc may be used in anticorrosive coatings, which are environmentally problematic and may bear high costs. Some coatings, such as epoxy, provide a barrier to moisture and/or oxygen. Lack of these species prevent redox reactions at the substrate and inhibit corrosion. In corrosioninhibiting coatings, incorporation of a metal with more negative reduction potential, such as zinc-rich primers, protect the substrate from corrosion. Due to the electrochemical properties of ICPs, they can be used in manufacturing anticorrosive coatings. All-polymer coatings based on ICPs have been used in corrosion protection in different substrates [84–86]. The redox potential of metal substrates significantly affects the oxidation state of PANI, which implies galvanic coupling between metal and polyaniline [85]. Besides, PANI can release anions on the damaged surface, which results in a barrier layer that prevents ion diffusion inward [85]. Along with a barrier to aggressive anions, ICPs act as strong oxidants for metals and shift their potential toward noble metals, i.e., passivate them, resulting in significant reduction of corrosion rate [87]. PANI films may be used as the corrosion protection layer or PANI may be used as a filler in other resins such as epoxy, polyurethane, alkyd, etc. [88, 89]. Regardless of the anticorrosive characteristics, these coatings may possess other properties, such as antistatic and self-healing [90, 91]. The passivating effect of PANI that originates from oxidation ability in the ES form of PANI is the main mechanism of PANI anticorrosion capabilities [92]. ICPs can form a compact layer on the metal surface such that they inhibit corrosion. Electrodeposited PANI/MWCNT films significantly increase the corrosion resistance of mild steel substrate even at medium to high corrosiveness [93]. Brush-like nanostructures of PANI/CNT are synthesized using in situ chemical polymerization [94]. These hierarchically structured nanocomposites show high electroactivity in a wide pH range from acidic to basic environments. When incorporated into epoxy-modified acrylic resins, excellent anticorrosion performance was observed. Passivation of the surface in acidic pH and a barrier effect under neutral and basic conditions was reported. In another work, corrosion protection of mild steel in NaCl solution was significantly improved using CNT core-PANI shell nanostructures [95]. In addition, it was observed that hardness of PANI films greatly improved when introducing carboxylated CNTs, and the hydrophobicity was also increased. Generally speaking, the incorporation of CNTs, especially functionalized CNTs, can greatly affect the anticorrosion properties of PANI-based coatings.

3.6 Biomedical applications CNT-based nanomaterials have been widely investigated in diverse biomedical applications including biosensors, drug/gene delivery, tissue engineering, bio-integrated

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electronics, and actuators in which the electroactive nature of CNTs is very important [96–98]. Although concentrated dispersion of carbon nanotubes may show minimal cytotoxicity, functionalization of CNTs improves this effect [99]. Some of the studies have revealed some toxicity level for CNTs used in biological model systems, which are attributed to metal catalyst impurities, functionalization, size, and shape of CNTs [96]. CNTs are usually incorporated into biocompatible polymers, both natural and synthetic, for biomedical applications. For instance, CNTs have been used in cardiac and neural tissue engineering where conductivity of the hydrogel is very important because of the conductive nature of tissue [100, 101]. On the other hand, ICPs have also been widely investigated in biomedical applications, including tissue engineering, drug/gene delivery, neural interfaces, biosensors, and artificial muscles [102, 103]. But, biodegradation, biostability, processability, and mechanical/electrical properties of these polymers cannot fulfill the requirements, so they are usually mixed with carbon nanomaterials, metallic nanoparticles, and biodegradable polymers. For example, ICP-based polymers have been widely used in tissue engineering applications [103, 104]. Both EB and ES (HCl dopant) forms of PANI did not induce skin sensitization/irritation when tested in vivo while both forms showed cytotoxicity when examined in vitro [83]. But this study showed that the cytotoxicity mainly originates from byproducts that present in the PANI structure. Interestingly, conductivity and especially biocompatibility of the PANI can be greatly improved by removing impurities using a solvent extraction method [105]. The problems associated with the hydrophobicity, solubility, biodegradation, and mechanical/structural properties of PANI can be improved through combination with biodegradable biopolymers such as PEG, PCL, chitosan, alginate, etc. Unfortunately, PANI is not biodegradable and may accumulate in the body. On the other hand, aniline oligomers or oligoanilines have attracted much interest in biomedical applications because of good solubility, biocompatibility, and fair conductivity [106]. Moreover, oligoanilines degrade in biological systems and could be removed through the renal system [106]. Furthermore, the cytotoxicity of aniline oligomers depends on the number of monomers and end functional groups, which implies that it can be greatly improved [107]. Agarose-based hydrogels containing carboxylcapped aniline pentamer have been successfully used in neural tissue engineering [108]. The aniline oligomer not only modifies the conductivity of the hydrogel, but also improves the adhesion of PC12 cells. Hydrogels have grabbed great attention in tissue engineering because they mimic the extracellular matrix (ECM) of human body tissue [109]. Injectable hydrogels can be used as a minimally invasive method for cardiac repair after myocardial infarction (MI), as shown in Fig. 4 [110]. Cell encapsulation can significantly enhance the viability of myocardiocyte and improve cardiac repair after MI [111]. Besides, self-healing hydrogels can provide mechanical integrity after injection and electrical conductive hydrogels provide cell communication that resembles native myocardium [112]. Conductive hydrogels are

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Fig. 4 Conductive hydrogels as a minimally invasive therapy for MI.

frequently manufactured through the incorporation of conductive moieties such as CNTs or ICPs [113]. Conductive PANI- or CNT-based hydrogels are also widely investigated for bioelectronics applications [114]. More interestingly, PANI can also act as a photothermal agent for drug/gene delivery and cancer therapy (based on the hyperthermia effect) as well, because it absorbs well in the near-infrared (NIR) region [115, 116]. These imply the fabrication of multifunctional hydrogels based on CNT and ICP nanostructures. For example, self-healing, flexible, elastic, thermosensitive, and conductive hydrogels of poly(N-isopropylacrylamide) loaded with CNT and polypyrrole were synthesized by Deng et al., which absorb NIR light [117]. Similarly, hydrogel-based CNT/ PANI nanocomposites can potentially be used as multifunctional hydrogels for nerve, cardiac, and skeletal muscle tissue engineering. PANI/CNT nanocomposites also show good biocompatibility that depends on the dose [118]. Poly(N-isopropylacrylamide)-CNT-polyaniline (PNIPAM-CNT-PANI) microfabric scaffolds show excellent results for growth and proliferation of different cells [119]. Nanofibers based on poly(N-isopropyl acrylamide-co-methacrylic acid) (PNIPAM-co-MAA) containing PANI/CNT were manufactured using electrospinning [120]. Cell viability and growth is higher for the 3D scaffold manufactured of nanocomposite fibers compared to neat (PNIPAM-co-MAA) fibers. Thus, these conducting nanocomposite fibers can be used for manufacturing scaffolds for cell culture applications, especially with 3D structures.

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Fig. 5 Schematic representation of PANI/CNT-based actuators for biomedical applications.

Actuators can be used in different applications such as microelectromechanical systems (MEMS) and biomedical applications [121]. Both PANI and CNT have been applied in manufacturing actuators and artificial muscles because they are low-voltage electrochemical systems [122–124]. Unfortunately, actuation strains of ICPs and CNTs are <15% and 1%, respectively, which is much less compared to skeletal muscles [125]. Thus one can expect that PANI/CNT nanocomposites would improve the actuation strength, as depicted in Fig. 5. PANI-coated CNT mats show superior actuation strain compared to pure CNT mats [126]. In fact, approaching the natural skeletal muscles in terms of force, speed, and displacement is the final goal in manufacturing synthetic actuators.

3.7 Heavy metal and dye removal Heavy metal ions that present in the wastewater of many industrial activities have numerous adverse environmental impacts on all kinds of life, especially aquatic ecosystems. Many methods have been developed for the removal of heavy metal ions, such as electrochemical treatments, chemical precipitation, ion exchange, membrane filtration, photocatalysis, and adsorption [127]. Adsorption is the most favorable technique because of its simplicity, flexibility, and cost features. Adsorption is a successful physicochemical method for heavy metal removal in which mass transfer from liquid bulk to the adsorbent surface accompanied by ions/adsorbent interactions is important for high removal efficiency. Activated carbon [128], CNTs [129], and clay minerals [130] are important

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adsorbent materials used in heavy metal ion removal [128]. Moreover, conjugated PANI and polypyrrole have also been used in adsorption [131–133]. In fact, PANI and its derivatives can be used as an effective adsorbent both for dyes and heavy metal ions [131]. A combination of adoption and electrochemical reactions are considered as the contributing mechanism of PANI-based adsorbents [131]. In some cases, PANI excels at adsorption compared to activated carbon [134]. PANI-based nanostructures with various morphologies that benefit from higher specific surface area are more focused in wastewater treatment [131]. More recently, PANI-CNT nanocomposites have been used as an adoption platform [135]. PANI/CNT nanocomposites show good selectivity for removing Cr(VI) from aqueous solution [136]. It was revealed that a combination of adsorption of ions through chelation and ion reduction are the contributing mechanism of ion removal with these nanocomposites. On the other hand, MWCNTs/Fe3O4/PANI nanocomposites have been used for removing the ionic dyes from aqueous solutions [137]. High adsorption capacity was observed for methyl orange and Congo red dyes, which was attributed to amine and imine groups in PANI structure. After adsorption, these magnetic nanoparticles can be easily separated from wastewater using a magnet. PANI/MWCNT nanocomposites for removal of alizarin yellow R show higher adsorption capacity compared to neat PANI or MWCNTs [138]. The combination of high specific surface area of MWCNTs and nitrogen-containing groups of PANI is an important factor for efficient removal. Furthermore, the environmental stability of CNT and PANI species is advantageous for designing adsorbents.

3.8 Catalysis Very small nanoparticles of noble metals, especially below 10 nm, are widely applied in catalysis applications. For example, Pt is the most frequently used catalyst for proton exchange membrane fuel cells (PEMFCs), usually deposited on a support material such as carbon black, which suffers from several drawbacks [139, 140]. CNT supports are newly emerged alternatives in which Pt immobilization on the inert surface of CNTs is challenging [141]. One innovative solution in order to strongly attach Pt nanoparticles on the CNT surface with controlled size and distribution is using a PANI coating on the CNTs. Conjugated PANI can attach to the CNT surface through π-π interactions while its nitrogen atoms can create covalent bonds with Pt atoms [142]. Accordingly, PANI act as a conductive bridge between Pt nanoparticles and CNT, where electrochemical stability of these catalysts is reported to be excellent [142]. In fact, the electron pair of the nitrogen atoms in PANI chains can form coordination complexes with metals. This feature along with good adherence to the unfunctionalized CNT surface through π-π interactions has made PANI a promising electron conductive bridge between the inert CNT surface and metal particles.

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On the other hand, Pd nanoparticles are a well-known catalyst for Heck-Mizoroki coupling reactions. Both PANI [143] and PANI@CNT [144] core-shell nanostructures have been used as support for Pd nanoparticles. Controlling the thickness of the PANI coating is very important to catalyst stability [144]. In fact, when the thickness of the PANI coating is high, the PANI layer acts as bulk PANI and causes the CNT effect to be wiped out. Besides, PANI chains in thick layers are not strongly affected by π-π interactions with CNTs such that they can deform or detach from the surface of the CNTs, which adversely affects the stability of catalysts [144].

4. Conclusion Carbon nanotubes have extraordinary electrical, mechanical, and thermal properties. Highly delocalized electrons results in high in-plane conductivity. In addition, they possess a unique one-dimensional structure with good flexibility. On the other hand, PANI is an organic based polymer with aromatic monomers connected through nitrogen-containing bridges. Alternating single and double bonds in PANI give it light-absorbing properties that are tunable through doping. On the other hand, the conductivity of PANI is also adjustable through a doping process. Furthermore, the morphology of PANI can be greatly varied via tuning the polymerization parameters. All these fascinating characteristics of CNT and PANI have resulted in numerous applications of them, either separately or in combination with each other. Some of these applications are discussed in this chapter. Sensors and biosensors, energy storage devices, DSSCs, and biomedical applications are among the fastest growing applications of PANI, CNT, and their nanocomposites. PANI-CNT nanocomposites that inherit the major properties of pure PANI and CNTs can compensate for the weakness of each pure material. Thus we can predict that the applications of PANI-CNT nanocomposites will exceed those of pure PANI or CNTs.

References [1] S. Bhadra, et al., Progress in preparation, processing and applications of polyaniline. 34 (8) (2009) 783–810. [2] B.K. Kaushik, M.K. Majumder, Carbon nanotube: properties and applications, in: Carbon Nanotube Based VLSI Interconnects, Springer, 2015, pp. 17–37. [3] P.R. Bandaru, Electrical properties and applications of carbon nanotube structures. J. Nanosci. Nanotechnol. 7 (4–5) (2007) 1239–1267. [4] J. Chen, et al., A review of the interfacial characteristics of polymer nanocomposites containing carbon nanotubes. 8 (49) (2018) 28048–28085. [5] W. Khan, R. Sharma, P. Saini, Carbon nanotube-based polymer composites: synthesis, properties and applications, in: Carbon Nanotubes-Current Progress of their Polymer Composites, InTech, 2016. [6] A. Eftekhari, L. Li, Y. Yang, Polyaniline supercapacitors. J. Power Sources 347 (2017) 86–107. [7] M.A.C. Mazzeu, et al., Structural and morphological characteristics of polyaniline synthesized in pilot scale. 9 (1) (2017) 39–47.

PANI-CNT nanocomposites

[8] Y. Zhao, et al., Effect of additives on the properties of polyaniline nanofibers prepared by high gravity chemical oxidative polymerization. 31 (18) (2015) 5155–5163. [9] J. Stejskal, et al., Polyaniline and polypyrrole prepared in the presence of surfactants: a comparative conductivity study. 44 (5) (2003) 1353–1358. [10] Z.D. Zujovic, et al., Structure of ultralong polyaniline nanofibers using initiators. 44 (8) (2011) 2735–2742. [11] A. Kumar, et al., Polyaniline–carbon nanotube composites: preparation methods, properties, and applications. 57 (2) (2018) 70–97. [12] P. Gajendran, R.J.P. Saraswathi, A. Chemistry, Polyaniline-carbon nanotube composites. 80 (11) (2008) 2377–2395. [13] M.K. Yazdi, et al., Effects of multiwall carbon nanotubes on the polymerization model of aniline. 25 (12) (2018) 265. [14] C. Oueiny, S. Berlioz, F.-X. Perrin, Carbon nanotube–polyaniline composites. Prog. Polym. Sci. 39 (4) (2014) 707–748. [15] I.Y. Jeon, et al., Grafting of polyaniline onto the surface of 4-aminobenzoyl-functionalized multiwalled carbon nanotube and its electrochemical properties. 48 (14) (2010) 3103–3112. [16] J.-E. Huang, et al., Well-dispersed single-walled carbon nanotube/polyaniline composite films. 41 (14) (2003) 2731–2736. [17] C. Downs, et al., Efficient polymerization of aniline at carbon nanotube electrodes. 11 (12) (1999) 1028–1031. [18] M. Wu, et al., Electrochemical fabrication and capacitance of composite films of carbon nanotubes and polyaniline. 15 (23) (2005) 2297–2303. [19] L. Besra, M. Liu, A review on fundamentals and applications of electrophoretic deposition (EPD). Prog. Mater. Sci. 52 (1) (2007) 1–61. [20] C. Dhand, et al., Preparation of polyaniline/multiwalled carbon nanotube composite by novel electrophoretic route. 46 (13) (2008) 1727–1735. [21] S. Srivastava, et al., Study of chemiresistor type CNT doped polyaniline gas sensor. 160 (5–6) (2010) 529–534. [22] I.Y. Jeon, L.S. Tan, J.B. Baek, Synthesis and electrical properties of polyaniline/polyaniline grafted multiwalled carbon nanotube mixture via in situ static interfacial polymerization. J. Polym. Sci. A: Polym. Chem. 48 (9) (2010) 1962–1972. [23] F. Xu, et al., Thermal stability of carbon nanotubes. 102 (2) (2010) 785–791. [24] M. Moniruzzaman, K.I.J.M. Winey, Polymer nanocomposites containing carbon nanotubes. 39 (16) (2006) 5194–5205. [25] A. Zribi, J. Fortin, Functional Thin Films and Nanostructures for Sensors: Synthesis, Physics and Applications. Springer Science & Business Media, 2009. [26] D. Bruen, et al., Glucose sensing for diabetes monitoring: recent developments. 17 (8) (2017) 1866. [27] S. Abdul, et al., Functional thin films and nanostructures for sensors, in: Fundamentals of Nanoparticles, Elsevier, 2018, pp. 485–519. [28] P. Kassal, et al., Wireless chemical sensors and biosensors: a review. (2018). [29] X. Liu, et al., A survey on gas sensing technology. 12 (7) (2012) 9635–9665. [30] V. Schroeder, et al., Carbon nanotube chemical sensors. Chem. Rev 119 (1) (2019) 599–663. [31] I. Fratoddi, et al., Chemiresistive polyaniline-based gas sensors: a mini review. 220 (2015) 534–548. [32] I.V. Zaporotskova, et al., Carbon nanotubes: sensor properties. A review. 2 (4) (2016) 95–105. [33] R. Tang, et al., Carbon nanotube-based chemiresistive sensors. 17 (4) (2017) 882. [34] T.A. Freitas, et al., Amino-functionalization of carbon nanotubes by using a factorial design: human cardiac troponin T immunosensing application. 2014 (2014). [35] J. Huang, et al., Nanostructured polyaniline sensors. 10 (6) (2004) 1314–1319. [36] Y. Liao, et al., Carbon nanotube/polyaniline composite nanofibers: facile synthesis and chemosensors. 11 (3) (2011) 954–959. [37] J.-H. Lim, et al., Electrical and gas sensing properties of polyaniline functionalized single-walled carbon nanotubes. 21 (7) (2010) 075502.

159

160

Fundamentals and emerging applications of polyaniline

[38] M. Ding, et al., Chemical sensing with polyaniline coated single-walled carbon nanotubes. 23 (4) (2011) 536–540. [39] C. Dhand, et al., Polyaniline–carbon nanotube composite film for cholesterol biosensor. 383 (2) (2008) 194–199. [40] E. Bayram, E. Akyilmaz, Development of a new microbial biosensor based on conductive polymer/ multiwalled carbon nanotube and its application to paracetamol determination. Sens. Actuators B: Chem. 233 (2016) 409–418. [41] L. Kouchachvili, W. Yaı¨ci, E. Entchev, Hybrid battery/supercapacitor energy storage system for the electric vehicles. J. Power Sources 374 (2018) 237–248. [42] H. Wang, et al., Polyaniline (PANi) based electrode materials for energy storage and conversion. 1 (3) (2016) 225–255. [43] M.E. Bhosale, et al., Organic small molecules and polymers as an electrode material for rechargeable lithium ion batteries. (2018). [44] H. Liu, et al., Porous tremella-like MoS2/polyaniline hybrid composite with enhanced performance for lithium-ion battery anodes. 167 (2015) 132–138. [45] L. Yu-Jeong, et al., Polyaniline and multi-walled carbon nanotube composite electrode for rechargeable battery. 22 (2012) s717–s721. [46] Z. Zhao, et al., Chloride ion-doped polyaniline/carbon nanotube nanocomposite materials as new cathodes for chloride ion battery. 270 (2018) 30–36. [47] J.Y. Kim, Y.J. Park, Carbon nanotube/Co3O4 nanocomposites selectively coated by polyaniline for high performance air electrodes. Sci. Rep. 7 (1) (2017) 8610. [48] M.-S. Wang, et al., One dimensional and coaxial polyaniline@tin dioxide@multi-wall carbon nanotube as advanced conductive additive free anode for lithium ion battery. 334 (2018) 162–171. [49] Q. Meng, et al., Research progress on conducting polymer based supercapacitor electrode materials. 36 (2017) 268–285. [50] A. Rudge, et al., A study of the electrochemical properties of conducting polymers for application in electrochemical capacitors. 39 (2) (1994) 273–287. [51] L.Z. Fan, et al., High electroactivity of polyaniline in supercapacitors by using a hierarchically porous carbon monolith as a support. 17 (16) (2007) 3083–3087. [52] Y. Yang, et al., In situ preparation of caterpillar-like polyaniline/carbon nanotube hybrids with core shell structure for high performance supercapacitors. 78 (2014) 279–287. [53] V. Gupta, N. Miura, Polyaniline/single-wall carbon nanotube (PANI/SWCNT) composites for high performance supercapacitors. Electrochim. Acta 52 (4) (2006) 1721–1726. [54] C. Meng, et al., Highly flexible and all-solid-state paperlike polymer supercapacitors. 10 (10) (2010) 4025–4031. [55] B. Dong, et al., Preparation and electrochemical characterization of polyaniline/multi-walled carbon nanotubes composites for supercapacitor. 143 (1–3) (2007) 7–13. [56] G. Otrokhov, et al., Enzymatic synthesis of polyaniline/multi-walled carbon nanotube composite with core shell structure and its electrochemical characterization for supercapacitor application. 123 (2014) 151–157. [57] Y. Zhou, et al., Polyaniline/multi-walled carbon nanotube composites with core–shell structures as supercapacitor electrode materials. 55 (12) (2010) 3904–3908. [58] R.V. Salvatierra, et al., ITO-free and flexible organic photovoltaic device based on high transparent and conductive polyaniline/carbon nanotube thin films. 23 (12) (2013) 1490–1499. [59] W. Hou, et al., Serrated, flexible and ultrathin polyaniline nanoribbons: an efficient counter electrode for the dye-sensitized solar cell. 322 (2016) 155–162. [60] K.-H. Park, et al., High performance dye-sensitized solar cell by using porous polyaniline nanotubes as counter electrode. 260 (2015) 393–398. [61] Q. Tai, et al., In situ prepared transparent polyaniline electrode and its application in bifacial dye-sensitized solar cells. 5 (5) (2011) 3795–3799. [62] J. Wu, et al., Bifacial dye-sensitized solar cells: A strategy to enhance overall efficiency based on transparent polyaniline electrode. 4 (2014) 4028. [63] S. Yun, A. Hagfeldt, T. Ma, Pt-free counter electrode for dye-sensitized solar cells with high efficiency. Adv. Mater. 26 (36) (2014) 6210–6237.

PANI-CNT nanocomposites

[64] H. Zhang, et al., Bifacial dye-sensitized solar cells from covalent-bonded polyaniline-multiwalled carbon nanotube complex counter electrodes. 275 (2015) 489–497. [65] B. He, et al., Efficient dye-sensitized solar cells from polyaniline–single wall carbon nanotube complex counter electrodes. 2 (9) (2014) 3119–3126. [66] J.-M. Thomassin, et al., Polymer/carbon based composites as electromagnetic interference (EMI) shielding materials. 74 (7) (2013) 211–232. [67] M.H. Al-Saleh, W.H. Saadeh, U.J.C. Sundararaj, EMI shielding effectiveness of carbon based nanostructured polymeric materials: a comparative study. 60 (2013) 146–156. [68] S. Mondal, et al., Low percolation threshold and electromagnetic shielding effectiveness of nanostructured carbon based ethylene methyl acrylate nanocomposites. 119 (2017) 41–56. [69] Y. Zhang, et al., Broadband and tunable high-performance microwave absorption of an ultralight and highly compressible graphene foam. 27 (12) (2015) 2049–2053. [70] Y. Wang, X. Jing, Intrinsically conducting polymers for electromagnetic interference shielding. Polym. Adv. Technol. 16 (4) (2005) 344–351. [71] J. Joo, A.J. Epstein, Electromagnetic radiation shielding by intrinsically conducting polymers. Appl. Phys. Lett. 65 (18) (1994) 2278–2280. [72] T.A. Skotheim, J. Reynolds, Conjugated Polymers: Processing and Applications. CRC Press, 2006. [73] H.R. Tantawy, et al., Comparison of electromagnetic shielding with polyaniline nanopowders produced in solvent-limited conditions. 5 (11) (2013) 4648–4658. [74] N.C. Das, et al., Electrical conductivity and electromagnetic interference shielding effectiveness of polyaniline–ethylene vinyl acetate composites. 54 (2) (2005) 256–259. [75] S. Shi, L. Zhang, J. Li, Complex permittivity and electromagnetic interference shielding properties of Ti3SiC2/polyaniline composites. J. Mater. Sci. 44 (4) (2009) 945–948. [76] P. Saini, et al., Electromagnetic interference shielding behavior of polyaniline/graphite composites prepared by in situ emulsion pathway. 113 (5) (2009) 3146–3155. [77] C. Basavaraja, W.J. Kim, Y.D. Kim, D.S. Huh, Synthesis of polyaniline-gold/graphene oxide composite and microwave absorption characteristics of the composite films. Mater. Lett. 65 (19–20) (2011) 3120–3123. [78] Y.-J. Chen, et al., Investigation of the electric conductivity and the electromagnetic interference shielding efficiency of SWCNTs/GNS/PAni nanocomposites. 20 (8) (2011) 1183–1187. [79] Y.-E. Moon, et al., Synergetic improvement in electromagnetic interference shielding characteristics of polyaniline-coated graphite oxide/γ-Fe2O3/BaTiO3 nanocomposites. 19 (2) (2013) 493–497. [80] P. Saini, et al., Polyaniline–MWCNT nanocomposites for microwave absorption and EMI shielding. 113 (2–3) (2009) 919–926. [81] H. Li, et al., Lightweight flexible carbon nanotube/polyaniline films with outstanding EMI shielding properties. 5 (34) (2017) 8694–8698. [82] T. Wang, et al., Preparation of flexible reduced graphene oxide/poly(vinyl alcohol) film with superior microwave absorption properties. RSC Adv. 5 (2015) 88958–88964. [83] P. Humpolicek, et al., Biocompatibility of polyaniline. 162 (7–8) (2012) 722–727. [84] M. Ates, A review on conducting polymer coatings for corrosion protection. J. Adhes. Sci. Technol. 30 (14) (2016) 1510–1536. [85] R.M. Torresi, et al., Galvanic coupling between metal substrate and polyaniline acrylic blends: corrosion protection mechanism. 50 (11) (2005) 2213–2218. [86] P.P. Deshpande, et al., Conducting polymers for corrosion protection: a review. 11 (4) (2014) 473–494. [87] T. Ohtsuka, Corrosion protection of steels by conducting polymer coating. Int. J. Corros. 2012 (2012). [88] J.-W. Wu, et al., Anti-corrosion characteristics of electrodeposited self-doped polyaniline films on mild steel in low acidity. 8 (5) (2018) 155. [89] H. Wang, et al., Waterborne polyaniline-graft-alkyd for anticorrosion coating and comparison study with physical blend. 126 (2019) 187–195. [90] Z. Li, et al., Self-healing active anticorrosion coatings with polyaniline/cerium nitrate hollow microspheres. 341 (2018) 64–70. [91] A. Zhu, et al., The synthesis and antistatic, anticorrosive properties of polyaniline composite coating. 122 (2018) 270–279.

161

162

Fundamentals and emerging applications of polyaniline

[92] J. Fang, et al., A study on mechanism of corrosion protection of polyaniline coating and its failure. 49 (11) (2007) 4232–4242. [93] W. Li, Electrodeposition of PANI/MWCNT coatings on stainless steel and their corrosion protection performances. Int. J. Electrochem. Sci. 13 (2) (2018) 1367–1375. [94] G. Qiu, A. Zhu, C. Zhang, Hierarchically structured carbon nanotube–polyaniline nanobrushes for corrosion protection over a wide pH range. RSC Adv. 7 (56) (2017) 35330–35339. [95] A.M. Kumar, Z.M. Gasem, In situ electrochemical synthesis of polyaniline/f-MWCNT nanocomposite coatings on mild steel for corrosion protection in 3.5% NaCl solution. Prog. Org. Coat. 78 (2015) 387–394. [96] R. Alshehri, et al., Carbon nanotubes in biomedical applications: factors, mechanisms, and remedies of toxicity: miniperspective. 59 (18) (2016) 8149–8167. [97] T. Kim, M. Cho, K.J. Yu, Flexible and stretchable bio-integrated electronics based on carbon nanotube and graphene. Materials (Basel) 11 (7) (2018) 1163. [98] N. Sinha, J.-W. Yeow, Carbon nanotubes for biomedical applications. IEEE Trans. Nanobioscience 4 (2) (2005) 180–195. [99] S. Smart, et al., The biocompatibility of carbon nanotubes. 44 (6) (2006) 1034–1047. [100] A.M. Martins, et al., Electrically conductive chitosan/carbon scaffolds for cardiac tissue engineering. 15 (2) (2014) 635–643. [101] X. Liu, et al., Functionalized carbon nanotube and graphene oxide embedded electrically conductive hydrogel synergistically stimulates nerve cell differentiation. 9 (17) (2017) 14677–14690. [102] B. Guo, P.X. Ma, Conducting polymers for tissue engineering. Biomacromolecules 19 (6) (2018) 1764–1782. [103] T.H. Qazi, R. Rai, A.R. Boccaccini, Tissue engineering of electrically responsive tissues using polyaniline based polymers: a review. Biomaterials 35 (33) (2014) 9068–9086. [104] R. Balint, N.J. Cassidy, S.H. Cartmell, Conductive polymers: towards a smart biomaterial for tissue engineering. Acta Biomater. 10 (6) (2014) 2341–2353. [105] V. Kasˇpa´rkova´, et al., Conductivity, impurity profile, and cytotoxicity of solvent-extracted polyaniline. 27 (2) (2016) 156–161. [106] P. Zarrintaj, et al., Oligoaniline-based conductive biomaterials for tissue engineering. (2018). [107] H. Qi, et al., Biocompatibility evaluation of aniline oligomers with different end-functional groups. 2 (6) (2013) 427–433. [108] P. Zarrintaj, et al., A novel electroactive agarose-aniline pentamer platform as a potential candidate for neural tissue engineering. 7 (1) (2017) 17187. [109] H. Geckil, et al., Engineering hydrogels as extracellular matrix mimics. 5 (3) (2010) 469–484. [110] A. Hasan, et al., Injectable hydrogels for cardiac tissue repair after myocardial infarction. 2 (11) (2015) 1500122. [111] A. Chow, et al., Human induced pluripotent stem cell-derived cardiomyocyte encapsulating bioactive hydrogels improve rat heart function post myocardial infarction. 9 (5) (2017) 1415–1422. [112] R. Dong, et al., Self-healing conductive injectable hydrogels with antibacterial activity as cell delivery carrier for cardiac cell therapy. 8 (27) (2016) 17138–17150. [113] D. Mawad, A. Lauto, G.G. Wallace, Conductive polymer hydrogels, in: Polymeric Hydrogels as Smart Biomaterials, Springer, 2016, pp. 19–44. [114] H. Yuk, B. Lu, X. Zhao, Hydrogel bioelectronics. Chem. Soc. Rev. 48 (6) (2019) 1642–1667. [115] E.I. Yslas, et al., Polyaniline nanoparticles for near-infrared photothermal destruction of cancer cells. 17 (10) (2015) 389. [116] S. Maleki Dizaj, et al., Calcium carbonate nanoparticles as cancer drug delivery system. 12 (10) (2015) 1649–1660. [117] Z. Deng, et al., Multifunctional stimuli-responsive hydrogels with self-healing, high conductivity, and rapid recovery through host–guest interactions. 30 (5) (2018) 1729–1742. [118] S. Ben-Valid, et al., Polyaniline-coated single-walled carbon nanotubes: synthesis, characterization and impact on primary immune cells. 20 (12) (2010) 2408–2417. [119] A. Tiwari, et al., Influence of poly (n-isopropylacrylamide)-CNT-polyaniline three-dimensional electrospun microfabric scaffolds on cell growth and viability. 99 (5) (2013) 334–341.

PANI-CNT nanocomposites

[120] Y. Sharma, et al., Fabrication of conducting electrospun nanofibers scaffold for three-dimensional cells culture. 51 (4) (2012) 627–631. [121] R. Agrawal, et al., Smart actuators for innovative biomedical applications: an interactive overview, in: Applied Microbiology and Bioengineering, Elsevier, 2019, pp. 101–119. [122] M. Beregoi, et al., Polyaniline based microtubes as building-blocks for artificial muscle applications. 253 (2017) 576–583. [123] J.A. Lee, et al., All-solid-state carbon nanotube torsional and tensile artificial muscles. 14 (5) (2014) 2664–2669. [124] L. Chen, et al., Large-deformation curling actuators based on carbon nanotube composite: advancedstructure design and biomimetic application. 9 (12) (2015) 12189–12196. [125] G.M. Spinks, et al., Carbon-nanotube-reinforced polyaniline fibers for high-strength artificial muscles. 18 (5) (2006) 637–640. [126] M. Tahhan, et al., Carbon nanotube and polyaniline composite actuators. 12 (4) (2003) 626. [127] A. Azimi, et al., Removal of heavy metals from industrial wastewaters: a review. 4 (1) (2017) 37–59. [128] F. Fu, Q. Wang, Removal of heavy metal ions from wastewaters: a review. J. Environ. Manage. 92 (3) (2011) 407–418. [129] A. Abbas, et al., Heavy metal removal from aqueous solution by advanced carbon nanotubes: critical review of adsorption applications. 157 (2016) 141–161. [130] M.K. Uddin, A review on the adsorption of heavy metals by clay minerals, with special focus on the past decade. Chem. Eng. J. 308 (2017) 438–462. [131] E.N. Zare, A. Motahari, M. Sillanp€a€a, Nanoadsorbents based on conducting polymer nanocomposites with main focus on polyaniline and its derivatives for removal of heavy metal ions/dyes: a review. Environ. Res. 162 (2018) 173–195. [132] H.N.M.E. Mahmud, A.O. Huq, R. Yahya, The removal of heavy metal ions from wastewater/aqueous solution using polypyrrole-based adsorbents: a review. RSC Adv. 6 (18) (2016) 14778–14791. [133] Y. Zheng, et al., Enhanced selectivity for heavy metals using polyaniline-modified hydrogel. 52 (13) (2013) 4957–4961. [134] P. Baruah, D. Mahanta, Adsorption and reduction: combined effect of polyaniline emeraldine salt for removal of Cr (VI) from aqueous medium. Bull. Mater. Sci. 39 (3) (2016) 875–882. [135] R. Kumar, et al., Adsorption modeling and mechanistic insight of hazardous chromium on para toluene sulfonic acid immobilized-polyaniline@ CNTs nanocomposites. (2018). [136] J. Wang, et al., Combined adsorption and reduction of Cr (VI) from aqueous solution on polyaniline/ multiwalled carbon nanotubes composite. 32 (9) (2015) 1889–1895. [137] Y. Zhao, et al., Hierarchical MWCNTs/Fe3O4/PANI magnetic composite as adsorbent for methyl orange removal. 450 (2015) 189–195. [138] K. Wu, et al., Multi-walled carbon nanotubes modified by polyaniline for the removal of alizarin yellow R from aqueous solutions. 36 (1–2) (2018) 198–214. [139] D. Stevens, J.R. Dahn, Thermal degradation of the support in carbon-supported platinum electrocatalysts for PEM fuel cells. Carbon 43 (1) (2005) 179–188. [140] H.A. Gasteiger, et al., Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs. 56 (1–2) (2005) 9–35. [141] N. Soin, et al., Sputter deposition of highly dispersed platinum nanoparticles on carbon nanotube arrays for fuel cell electrode material. 19 (5–6) (2010) 595–598. [142] D. He, et al., Polyaniline-functionalized carbon nanotube supported platinum catalysts. Langmuir 27 (9) (2011) 5582–5588. [143] L. Yu, et al., Heck reactions catalyzed by ultrasmall and uniform Pd nanoparticles supported on polyaniline. J. Org. Chem. 80 (17) (2015) 8677–8683. [144] R. Yu, et al., Pd nanoparticles immobilized on carbon nanotubes with a polyaniline coaxial coating for the Heck reaction: coating thickness as the key factor influencing the efficiency and stability of the catalyst. 8 (5) (2018) 1423–1434.

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