1. Introduction

Electronic devices based on inorganic conductors and semiconductors (e.g., Au, Cu, Pt, and Si) often require numerous etching and lithographic development steps during fabrication leading to longer processing times, costly equipment development, and higher priced devices [1]. In the middle of the twentieth century, these limitations spurred on extensive engineering and research efforts focused on optimizing fabrication methodologies; however, despite significant improvements, these devices are still limited by the physical properties of the inorganic materials they employ. Serendipitously, the discovery of trans-polyacetylene (PA) by Shirakawa [2, 3, 4] resulted in a collaboration with MacDiarmid and Heeger whose efforts led to the discovery of metallic-type electrical conductivity in halogen doped (i.e., p-doped) trans-polyacetylene in the late 1970s (Figure 1a) [4, 5]. In this way, the p-doping of PA (an electrical insulator with a non-zero energy gap (E_g)) induces uniform bond length for both the anti-bonding and bonding domains of the C∙C (π* and π orbitals, respectively). Doping also reduces the energy gap between PA’s valance band (VB) and conducting band (CB) and increases charge carrier density and electrical conductivity (Figure 1b) [5]. Shortly after their discovery, numerous other organic polymers having similar characteristics where identified or “re-discovered” (e.g., polyaniline and polypyrrole); leading to a new family of materials aptly named conducting polymers (CPs) (Figure 1c) [6]. Similar to metals and semiconductors, CPs are characterized as being malleable and electronically conductive (with electrical conductivities ranging from semiconductive to metallic), as well as, having magnetic and optical properties similar to inorganic substances [5, 7]. Therefore, CPs are often referred to as “intrinsically conducting polymers” (ICP) or synthetic metals [1, 5, 8]. Similar to PA, ICPs have an innate inclination to conduct electricity since their structures contain a conjugated π-electron system, which forms uniform bond-lengths throughout the polymer backbone when doped [9].

Figure 1.
(a) Illustration of the p-doping effect on PA, (b) various band gaps (E_g) of different material types and, (c) representative CPs and their chemical structures.

The discovery of the properties of CPs has paved the way for creating new devices and novel applications where the benefits of plastics (i.e., scalability, processability, low-cost, etc.) and retention of high electrical conductivity are desired [10]. In this regard, solution and melt processable polymers could offer a facile approach to applying CP coatings and fabricating devices on large dimensional scales [11]. Furthermore, unlike their inorganic counterparts, CPs can potentially be processed using more modern solution-based processing techniques (e.g., inkjet printing), which could dramatically reduce the complexity, fabrication time, and cost when used to develop integrated electronic devices. Nevertheless, early development of CPs has lacked focus on developing truly “processable CPs” which has hindered their implementation to date.

A non-comprehensive list containing some classical CPs is shown in Figure 1c. Among them, polyaniline (PANI), polypyrrole (PPy), polythiophene (PT), and (poly-(3,4 ethylene dioxythiophene) (PEDOT) have been studied and employed to the greatest extent to date. Interestingly, although not fully appreciated at the time, the conductive nature of PANI was first observed more than 150 years ago after being prepared electrochemically from an acidic solution of aniline monomer [4, 6, 12]. Since then, PANI has become one of the most extensively studied CPs, especially over the last 20 years [12]. This interest in PANI is in large part due to its superior properties that impart high electrical conductivity, redox and ion-exchange properties, and high operational stability (i.e., is stable under a variety of environmental, thermal, and chemical conditions) (Figure 2) [13]. In addition, PANI’s straightforward synthesis, tunable electrical conductivity, potential for chemical functionalization, and low synthetic cost make it an economically viable option compared to other CPs [8, 13, 14, 15, 16, 17, 18]. PANI is therefore being investigated for use in various disciplines, including sensors [19], energy storage devices [20], corrosion inhibitors [21], solar cells [22], and electromagnetic shielding materials [23].

Doping of CPs can dramatically improve their electrical conductivity by many orders of magnitude and is required to obtain highly conductive materials. Doping usually occurs through an oxidation or reduction process and the polymers are considered p- and n-type, respectively [1, 7]. The oxidative/reductive doping process results in the alteration of the electronic configuration of the polymers making them unstable under environmental conditions. Another form of doping is protonic or acid doping. Unlike the oxidative/reductive doping, protonic doping (acid) does not alter the polymer’s electron configuration; consequently, the polymer is environmentally stable in the doped state. It was shown that polyaniline could be doped with acid thus maintaining the same number of π-electrons in the backbone. Inorganic acids such as hydrochloric (HCl), sulfuric (H₂SO₄), nitric acid (HNO₃) and chloric acid (HClO₃) have traditionally been utilized as dopants in conductive PANI production. Polystyrene sulphonic acid (PSS), para-toluene sulfonic acid (p-TSA), camphor sulfonic acid (CSA), and dodecyl benzene sulfonic acid (DBSA) are all organic dopants that can also be used to dope PANI [24].

MacDiarmid proposed three forms for the structure of PANI based on the oxidation states [1, 25]. The leucoemeraldine base (LB) (colorless) is the completely reduced state of PANI, the emeraldine base (EB) (blue) is half-reduced and half-oxidized, and the pernigraniline base (PB) (blue/violet) is the fully oxidized form [1, 26, 27]. The leucoemeraldine base form has only benzenoid rings, the emeraldine base form has both benzenoid and quinonoid rings, while the pernigraniline base form has only the quinoid rings [18]. Upon doping, the emeraldine base is converted to the emeraldine salt (ES) (green) and becomes conducting. The transformation of the emeraldine base form of PANI from an insulating state to the conductive emeraldine salt form is due to the formation of polaronic carriers, i.e., the protonated species in the emeraldine state. The protonation occurs on the imine N rather than the amine N because imines are more basic than amines [24, 28, 29]. Consequently, protonic doping of the ES form produces a bipolaronic species (protonation of both imine Ns) that undergoes an internal relaxation to form the highly conductive polarons [29, 30]. Polaron separations results in charge flow. Due to its excellent stability at ambient temperature and the fact that after doping with acid, the resulting emeraldine salt form of polyaniline is highly electrically conductive, EB is regarded as the most practical form of polyaniline. Unfortunately, even after being doped with an acid, leucoemeraldine and pernigraniline still have low conductivity [1]. Figure 3 shows the different oxidation states of PANI before and after protonic doping.

Figure 3.
Oxidation states and doping of various forms of polyaniline.

Despite the excellent properties and potential applications of PANI, it has some disadvantages that prevent its wide-spread applications. Actually PANI has been used in limited large scale commercial applications [31]. Of the major challenges to commercializing PANI, poor long term electrochemical stability and manufacturing costs due to low solubility in most solvents making processing difficult, are the two major issues [32]. The most successful improvements in PANI’s processability have been achieved by using suitable dopants to increase the solubility and modifying the monomers to incorporate solubilizing alkyl chains. The stability of PANI is improved by forming composite materials [33, 34, 35, 36]. In this chapter, we will focus on modifications of PANI (PANI derivatives) that address the limitations of poor solubility and electrochemical instability. The synthesis, characterization, and applications of these derivatives are the major focus here.

1.1 Synthesis of polyaniline

The three primary methods for producing PANI and its derivatives are chemical (i.e., oxidative polymerization), electrochemical (i.e., anodization), and more recently, palladium-catalyzed C-N cross-coupling (i.e., Buchwald-Hartwig reaction). Historically, chemical oxidative polymerization and electrochemical polymerization were the main methods of producing PANI due to their simplicity and availability of the monomers. Chemical oxidative polymerization involves an acidic aqueous media (pH < 2) with strong oxidants (e.g., potassium dichromate, hydrogen peroxide, ferric chloride, ceric nitrate, sulfate, and ammonium persulfate (APS)) in order to create the electrically conductive emeraldine salt form of PANI (Figure 4) [37, 38]. Unfortunately, the oxidative polymerization conditions produce PANI and PANI-analogs with variation in their structures and properties due to the many side reactions taking place simultaneously. While the overall mechanism is likely quite complex, a simplified mechanism for aniline polymerization in acidic media (pH < 2) is typically agreed to involve the initial formation of aniline radical cations (Figure 5a) that can undergo para-coupling forming a dimer (4ADA), due to the ortho/para (o, p) directing nature of the amine functional group. These dimers can likewise be oxidized to their cation radical form and couple with an additional aniline cation radical to form a trimer. This coupling process repeats until all monomer and/or oxidant expire, and the chain-growth process is terminated (Figure 5b–d) [13, 41]. The utility of this method enables easy scale-up to produce large quantities of PANI powders. As mentioned, the numerous side reactions and alternatively couplings likely occur simultaneously, which produces non-regiospecific PANI product [40, 42, 43]. However, PANI produced in this way does preferentially form p-substituted chains (despite equal likelihood for o-substitution), which has led to some authors suggesting an alternative initiation mechanism involving a phenazine initiation pathway (Figure 6) [39, 40].

Figure 4.
Oxidative polymerization of aniline in acidic medium.

Figure 5.
Mechanism of polymerization for polyaniline. Adapted from Refs. [39, 40].

Figure 6.
Proposed alternative initiation pathway for polyaniline polymerization. Adapted from Ref. [39].

Similar to the chemical-based oxidative polymerization route, electrochemical polymerization also involves the oxidation of aniline monomers, and its mechanism closely resembles that of oxidative polymerization (Figure 5) [39, 40]. Likewise, low pH (pH < 2) is preferred to achieve an electrically conductive PANI film. However, this process utilizes an applied potential at the electrode surface (e.g., using potentiostatic, galvanostatic, or cyclic voltammetry techniques) as opposed to an oxidizer in the bulk solution. In this way, the oxidation of aniline monomers to cation radicals occurs only at the electrode surface. These radicals then undergo coupling with one another to form dimers and ultimately polymers that becomes sufficiently large that it is no longer soluble in the electrolyte. Subsequently, the polymer forms a film at the electrode surface. The process can be terminated by removing the bias or exhausting the monomer [13, 39, 40]. This process typically produces insoluble and very high molecular weight PANI films with a fair amount of regio-irregularity (i.e., cross-linking).

Due to the presence of side reactions for both the chemical and electrochemical polymerization methods and the difficulty of obtaining a soluble PANI product from either methods, a new approach has recently been employed. This approach is based on the palladium-catalyzed C-N cross-coupling reaction developed by Buchwald and Hartwig that utilizes a Pd-catalyst, amine, aryl- halide, and hindered base in a suitable solvent [44, 45, 46]. The Buchwald/Hartwig reaction mechanism (Figure 7a) involves an oxidative insertion of the Pd(0) species I into an aryl halide substrate forming a Pd(II) species II. The increased acidity of the coordinated amine (III) at this point then allows for deprotonation with a hindered base (e.g., HN(SIMe₃)₂) and formation of a palladium-amido complex IV. Reductive elimination regenerates the Pd(0) catalyst I and produces an arylated-amine product. Significant improvements in ligand architecture (Figure 7b) and overall catalyst optimization have led to higher efficiency and broader application of the C-N coupling reaction. Consequently, coupling between numerous hetero(aryl) (pseudo)halides and amines at low temperature and low catalyst loading (ppm) is enabled. Buchwald and co-workers utilized this reaction to perform an efficient synthesis of a tert-butyloxycarbonyl (BOC) protected PANI (Figure 8) [47]. The BOC group, which can be removed during processing, was bulky enough to render the polymer soluble in common organic solvents. The Meyer group incorporated ortho-substituted repeat units into the backbone of PANI chain using the Buchwald/Hartwig reaction [48]. Due to the stepwise mechanism of the polymerization, alternative co-polymers can be produced. The o,p-PANI derivatives demonstrated similar properties (spectroscopic, oxidation states, colors, and conductivity) associated with the classical PANI. Furthermore, meta-substituted PANI can be prepared using this method [49, 50, 51], whereas the oxidative coupling or electrochemical reaction are unable to produce meta-substituted PANI.

Figure 7.
Mechanism of the Buchwald-Hartwig palladium catalyzed C-N cross-coupling reaction and several optimized ligands.

Figure 8.
PANI and a derivative by Buchwald/Hartwig reaction.

2. Polyaniline derivatives

Although conductive polymers, particularly PANI, have numerous distinct benefits and possibilities as previously stated, PANI also has significant downsides. The fundamental disadvantage of PANI’s conductive ES form is its low solubility in common organic solvents due to its stiff backbone, which makes it difficult to process. This insolubility hampers the practical application of PANI. Over the last few years, a lot has been done to make it easier to process PANI [11, 26, 52, 53]. Another, less studied problem of PANI is its electrochemical instability, which is mainly due to the defects in the polymer during the synthesis that resulted from the inherent instability of the radical species produced during the reaction [54, 55, 56]. The main approach to address the solubility, processability, and other aspects of PANI is to prepare PANI derivatives, which is discussed in this chapter. PANI derivatives can be broken down into three groups; (i) substitution on the benzene ring, (ii) substitution on the amino nitrogen, and (iii) fused ring systems (Figure 9) [26, 57].

Figure 9.
Three types of PANI derivatives.

2.1 PANI derivatives with benzene ring substitution

Traditionally, increasing PANI’s solubility was obtained by including substituents on the benzene rings, which weakens the chain’s stiffness and interchain interaction [26, 58, 59, 60, 61]. When substituents such as alkyl, alkoxy, and amino groups are attached to the aromatic rings, the steric barrier between the polymeric chains of the polymer increases, reducing the intermolecular force and increasing the dissolution of the polymer (Figure 10). For example, poly(2,3-dimethylaniline) (P(2,3-DMA)) was synthesized by Ma et al. using oxidative polymerization and was shown to have improved anticorrosion, electrochemical, and crystal properties compared to PANI [62, 63]. Unfortunately, no information was given by the authors pertaining to the solubility, conductivity, or electrochemical stability of the pristine polymer. Patil et al. prepared poly(o-anisidine) (POA) as coatings on copper by the electrochemical polymerization method [64]. Interestingly, the polymer film contained a mixed phase of both the pernigraniline base (PB) and emeraldine salt (ES) form of POA according to the optical absorption spectroscopy data. This polymer also lowered the corrosion rate of copper metal ~100 times more compared to the uncoated copper metal. However, due to the nature of the synthetic method that formed the film on the copper surface, there is no information about the solubility of the polymer. Sathyanarayana et al. prepared an ortho substituted poly ethoxy aniline (POEA) with excellent solubility and corrosion inhibition of iron in the presence of acid chloride [65]. A co-polymer, poly(o−/m-toluidine-co-o-nitroaniline), prepared by emulsion polymerization under oxidative conditions was also reported by the same authors. Interestingly, while the homopolymer of o-nitroaniline was not obtainable under the reaction conditions, the co-polymer with o-/m-toluidine occurred in low to moderate yields. The co-polymers showed improved conductivity, solubility, and thermal stability compared to the homopolymer of o-/m-toluidine [66].

Figure 10.
Examples aryl substituted PANI derivatives.

To achieve water solubility of PANI, derivatives were prepared with water solubilizing hydrophilic groups such as a sulfonate [67, 68], carboxylate [69, 70], phosphonate, boronate [71], and hydroxyl groups) to the aromatic backbone (Figure 11). These groups preform dual function, i.e., they provide water solubility and self-doping of the polymer. Sulfonic acid ring-substituted polyaniline (SPAN), first reported by Epstein and coworkers, was obtained by the sulfonation of the emeraldine base with fuming sulfuric acid [68, 72]. The transport, magnetic, and electrochemical properties of SPAN was later determined through a collaboration with the MacDiarmid group that provided insight to the effect of the sulfonate group on the solubility, doping mechanism, and charge transport of SPAN [73]. The polymer was shown to be self-doping as there was no pH effect on the conductivity for pH < 7.5. Interestingly, the polymer can be casted into a film from aqueous alkaline solvents (0.1 M NH₄OH) forming the blue-violet solution, which when casted onto a substrate and dried, reverts to the shiny green ES form that is highly conductive (0.1 S/cm).

Figure 11.
Hydrophilic self-doped substituted derivatives of PANI.

Carboxylate substituted PANI were synthesized from anthranilic acid (o-aminobenzoic acid) and m-aminobenzoic acid by electropolymerization [70]. The sulphonated PANI, when compared to the carboxylate substituted PANI, was more electrochemically stable in the self-doped state than either the m or o-substituted carboxylate PANI. Another synthesis of copolymers from aniline and 2-substituted anilines with carboxyl groups (anthranilic acid, methyl anthranilate, and ethyl anthranilate) were described by Sá and co-workers [67]. The chemical oxidative method was used to prepare the polymers with a variety of acid dopants. Although the polymers contained variable amounts of phenazine-like by-products, the anthranilic ester polymers had high to moderate solubility in a variety of solvents and were more soluble than the other polymers. Moreover, the conductivities correlated well with the solubility and the methyl anthranilic ester polymer doped with HCl (PANMEA-HCl) gave the highest conductivity, which is comparable to PANI. These results lead to a conclusion that these polymers could be used for a variety of application that are relevant to PANI. A recent example revealed an interested thio-substituted lactic acid (TLA) PANI derivative that was conjugated to the benzene ring of PANI through a thiol bond [69]. The PANI derivative (PANI-TLA) was shown to be 4 orders of magnitude more conductive in alkaline media compared to PANI-base.

Some other interesting PANI derivative structures containing fused aryl and benzoquinone rings at the 2,3-positions of the PANI aryl group, have also been prepared. Poly(1-naphthylamine) [74, 75], poly(1-amino anthracene) [76, 77, 78], and copolymers from aminoquinoline(AQ) and anisidine (AS) [79] were prepared by both oxidative polymerization and electrochemical polymerization. The polymers have PANI-like features. They exist in several different oxidation states with conductivities ranging from 0.055–0.083 S/cm for the naphthalene and anthracene analogs. The conductivity was much less for the AQ/AS copolymer. PANI derivatives containing benzoquinone units fused to the 2,3-positions on the aniline moiety were also developed and their electrochemical properties with respect to cathode electrodes were studied. Some examples are shown in Figure 12. Poly(1,5-diaminoanthraquinone) [poly(DAAQ)] were prepared from 1,5-diaminoanthraquinone (DAAQ) by electrochemical polymerization and the conductivity values at different potential was measured [80]. It was shown that the conductivity values of the polymer ranged from 0.3 to 2.0 S/cm when the voltage range from −2.0 to 0.8 V, which are much higher than PANI at the lower potential. Poly(5-amino-1,4-naphthoquinone) (PANQ), reported by Pham et al., was prepared by electrochemical polymerization [81, 82]. The films had properties similar to PANI and the structure contained one quinone group per ANQ moiety. The polymer had a conductivity of 10⁻¹ S/cm. Poly(5-amino-1,4-dyhydroxy anthraquinone) (PADAQ) was synthesized by oxidative polymerization for cathode material and was determined to behave like PANI [83]. Likewise, poly(1-aminoanthraquinone) (PAAQ) films were prepared by the electrochemical polymerization and tested as efficient cathode material [84]. These polymers were studied for energy storage properties, which is discussed later.

Figure 12.
Fused-aryl substituted PANI derivatives.

2.2 Substituted derivatives of PANI on amino nitrogen atom

Another approach for increasing the solubility of PANI is to prepare N-substituted PANI derivatives (Figure 13) [85, 86]. While N-substitution has been demonstrated to have little effect on the synthesis of the polymers, it has significant effect on the properties of the final polymer due to the steric component of the substituent. The substitutions ranged from phenyl, substituted phenyl, to substituted alkyl chains. PANI N-substituted derivatives have improved solubility and processability; however, the final electronic properties are usually altered owing to the addition of the substituent on the N atom. One of the first studies of N-substituted PANI was reported by Chen and co-workers who prepared a series of N-alkylated PANI, where the alkyl chains ranged from butyl (C4) to the hexadecyl (C16) [86]. The solubility of the PANI derivatives in common organic solvents (tetrahydrofuran (THF), dichloromethane (DCM), and chloroform (CHCl₃)) increased with the size of the alkyl chain, especially above C-6. Lowering the polarity and stiffness of the polymer chains through integration of the flexible alkyl substituent is responsible for the improved solubility. On the other hand, N-alkylated EB become insoluble in highly polar solvents like N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO) due to lack the amine hydrogen necessary for hydrogen bonding. Importantly, the polymer existed in the EB form and can be effectively doped to produced soluble ES. The conductivity ranged from 10⁻⁴ to 10⁻² S/cm with the C-6 alkyl chain having the highest conductivity. Other examples are poly-(aniline-co-N-(4-sulfophenyl)aniline) (PAPSA) [87] and its homo polymer, poly-(N-(4-sulfophenyl)aniline) [88], which were synthesized from aniline and sodium diphenylamine-4-sulfonate by the chemical oxidative polymerization. The polymer is soluble in aqueous basic media. While the conductivity is 10²–10⁶ times higher than other N-alkylsulfonate substituted PANI [89], it is 3 orders of magnitude lower than PANI. In general, N-substitution significantly lowers the conductivity of the polymer.

Figure 13.
N-substituted PANI derivatives.

2.3 Fused-ring PANI derivatives

Fused-ring PANI derivatives have been prepared and studied since the 1960s [90]. These rigid ladder-type PANI derivatives are based on phenazine, phenoxazine, phenothiazine, and their co-polymers (Figure 14). The PANI derivatives maintain the PANI backbone with π-extended structures, which leads to mostly insoluble polymers [91, 92]. Fors and coworkers prepared PANI derivative poly(N-methylphenothiazine dimethylphenylenediamine) (PT-DMPD), from N-methylphenothiazine and dimethylphenylenediamine (Figure 14a), and poly(N-methylphenothiazine benzidine) (PT-BZ) from N-methylphenothiazine and tetramethylbenzidine using Buchwald/Hartwig reaction as redox active materials [93, 94]. Our group also synthesized PANI derivatives containing phenoxazine, carbazole, and phenothiazine heterocycles by the Buchwald/Hartwig cross coupling reaction to study their processability and electrochemical stability (Figure 14b–d) [95, 96, 97]. Unlike classical PANI, these polymers were electrochemically quite stable over 100 CV cycles without noticeable degradation while maintaining processibility by attaching long alkyl chain pendant. Overall, these PANI-analogs offer a unique insight into alternative ways of tuning PANI’s structural, chemical, and physical properties. The electrical conductivities of the undoped (pristine) structures ranges from 10⁻⁵ to 10⁻⁶ S/cm and increase 2–4 orders of magnitude upon doping. More recent work has described additional side chain modification (Figure 14b and d), which can impart greater solubility for processable polymers. Orlov et al. describe the chemical oxidative polymerization of 2,5-dianiline-3,6-dichloro-1,4-benzoquinone (DADCB) to give a PANI derivative (Poly-DADCB) containing an N-substituted 5-aniline-3,6-dichloro-1,4-benzoquinone [98]. It was discovered that Poly-DADCB forms at a faster rate that PANI. However, the electrical conductivity was on the same order of magnitude as PANI.

3. Applications of PANI derivatives

PANI has been explored for many different types of applications such as biological sensors, anti-static wrappings, and electronic devices. Unfortunately, many of these applications are limited by its insolubility and electrochemical instability. However, PANI derivatives have been successful in achieving adequate solubility and electrochemical stability that they have become preferred to PANI in these applications. This section focuses on the applications of PANI derivatives in a variety of fields.

3.1 Antimicrobial activity

PANI and its derivatives have been shown to have antimicrobial properties where the mode of action is different for each derivative. For example, Andriianova et al. demonstrated the substituents effect of PANI derivatives on bactericidal properties of the polymer [99]. It was shown that PANI derivatives with substituents on the benzene ring, regardless of the bulkiness of the substituents, (POT, POA, PPA, PMA, and PClMA) had a negative or no effect on antibacterial activity compared to PANI. Alternatively, N-substituted PANI derivatives (PNClA and PNMA) increased the antibacterial properties against gram-positive (Bacillus subtilis) and gram-negative (Pseudomonas aureofaciens) bacteria (Figure 15). Additionally, these N-substituted.

Figure 15.
Structures of PANI-derivatives with antimicrobial activity. Structures drawn from Ref. [99].

PANI derivatives also showed bacteriostatic properties towards the microorganisms. Here it was determined that the mode of bactericidal activity is due to the positive charges on PANI and its derivatives that disrupt the nature of the bacteria’s cell membrane that ultimately leads to cell lysis. In another example, PANI and poly(3-aminobenzoic acid) (P3ABA) (Figure 16a and b) were investigated as antimicrobial agents and found that the mechanism of action is different for each polymer. While both polymers adversely affected the growth of bacteria, the mechanism of antimicrobial activity for PANI is believed to be a result of the production of hydrogen peroxide (H₂O₂) that leads to hydroxyl radicals. On the other hand, the antimicrobial activity of P3ABA is due to the disruption of the metabolic and respiratory machinery of the bacteria by targeting the ATP synthase, thus causing acid stress [100]. PANI-based materials have also been investigated in tissue engineering applications, which was recently reviewed [102]. Benzyl-substituted PANI (BP), bromine-benzyl-disubstituted PANI (BBP), (Figure 16c) and PANI nanoparticles were tested against both Gram-negative E. coil and Gram-positive B. subtilis; however, it was sown that only BBP had high antibacterial activity where as PANI and BP did not [101].

Figure 16.
(a) PANI (b) P3ABA, and (c) BBP. Structures drawn from Refs. [100, 101].

3.2 Bio- and chemo-sensors

3.2.1 Biosensors

PANI and its derivatives are explored as biosensors. PANI derivative with Au/Pd composites were prepared and analyzed as biosensors for cancer biomarkers [103]. Specifically, the polymers, poly(N-methyl-o-benzenediamine), poly(N-phenyl-o-phenylenediamine), poly(N-phenyl-p-phenylenediamine), and poly(3,3′,5,5′-tetramethylbenzidine) all with Au/Pd nanocomposite were used to simultaneously detect four tumor biomarkers, carcinoembryonic antigen (CEA), carbohydrate antigen 19-9 (CA199), carbohydrate antigen 72-4 (CA724), and alpha fetoprotein (AFP) (Figure 17). Interestingly, these biomarkers can be found in different cancer cells and as such the PANI dervatives/Au/Pd nanocomposities are capable of detecting multiple cancers simultaneously. For example, biomarkers CEA, CA199, CA724 are found in gastric cancer where as liver cancer can be diagnosed from biomarkers CEA, CA199, and AFP. As such, these biosensors can be used to detect gastric and liver cancer simultaneously. In another example, polyaniline (PA), poly(o-anisidine) (POA) and their co-polymer poly(aniline-co-o-anisidine) (PA-co-POA) thin films were used as glucose sensor by trapping glucose oxidase (GOD) in the polymers’ films [104]. Amperometric measurements show that the PA GOD electrode responded the fastest to glucose followed by PA-co-POA, and the least responsive was POA GOD electrodes.

Figure 17.
SWV responses (A) and calibration curves for different concentration of CEA (B), CA199 (C), CA724 (D), and AFP (E) in PBS (pH 5.5) with 1.5 mM H₂O₂. Reproduced from Ref. [103].

3.2.2 Chemosensors

PANI has also been widely explored as chemosensors due to its high conductivity and electrochromic properties. However, there are only few examples of PANI derivatives in sensor applications. PANI derivatives containing ortho-substituted methyl and methoxy groups, ortho,meta-disubstituted methyl, N-substituted methyl, ethyl, and phenyl groups were investigated for their sensory properties towards C-1 to C-5 alcohols [60]. The results revealed that resistance in the polymer increased (conductivity decreased) for all the polymers in the presence of C-1 to C-3 alcohols, but decreased (conductivity increased) for butanol and heptanol. A change in the crytallynity of the polymers due to the incorporation of alcohol between the chains is deemed responsible for the results. Consequently, the chain length of the alcohol and their structures can make a big difference in their interaction with the polymers’ inter and intra chain packing. Poly(o-alkylanilines) such as poly[2-(1-methylbut-1-en-1-yl)aniline] (P-PA), poly[2-(1-methylbutyl)aniline] (P-MB), and poly[2-(2-aminophenyl)pentan- 2-ol] (P-AP) have also shown to have high sensitivity to moisture and ammonia (Figure 18) [19]. P-PA had the largest response to moisture, while P-MB was the most sensitive to ammonia. The results were explained based on the roughness of the films. For example, P-PA had the smoothest surface and gave the highest conductivity in the presence of moisture, while P-MB was the roughest film but had the best interaction with ammonia vapors. Recently, two-dimensional (2D) fluorescent PANI derivatives (PPED and PPBA) were used to demonstrate high selectivity and sensitivity in the detection of iron ion (Fe³⁺), copper ion (Cu²⁺), and hydrochloric acid (HCl) [105], while PANI derivatives from carbazole and fluorene were used as pH sensors in both solution and gas phase [106].

Figure 18.
Humidity and Gas sensors. (a) Plots of the conductivity of P-PA, P-AP and P-MB films on the air humidity. (b) Plots of the conductivity of P-PA, P-AP and P-MB films on the concentration of ammonia vapor. Reproduced from Ref. [14] with permission from the Royal Society of Chemistry.

3.3 Electronic devices: electrochromic glass, solar cells, and LEDs

PANI is well known for its electrochromic properties as the polymer changes color reversibly with an electric field [107]. These materials are usually desirable for smart glass. A similar behavior is also demonstrated for its derivatives. For example, poly(2-methoxyaniline) and its copolymer, poly(2-methoxyaniline-co-3-aminobenzene sulphonic acid), were shown to have electrochromic behavior in a display device by Saharan et al. [108]. The response time was determined to be about 6 s going between the oxidized and reduced states (Figure 19). Additionally, the cycling behavior of the device was deemed to be stable. A PANI derivative containing phenoxazine unit was shown by Almtiri et al. to undergo similar color changes based on the oxidation state of the polymer [96]. Between 0 and 0.4 V, the polymer is light yellow, which changes to dark green between 0.4 and 0.6 V. Further oxidation renders the film deep blue at voltages greater than 0.6 V (Figure 20). The cycling stability was measured by cyclic voltammetry (CV) and was shown to be very stable over 100 cycles. Electrochromic conducting copolymers from aniline and different feed ratio of 4,4′-diaminodiphenyl sulfone (DDS) were deposited on ITO glass plates for spectrochemical analysis [109]. The copolymers displayed multiple colors similar to PANI when the applied potential was switched between −0.2 and 1.2 V, where the color changed from yellow to green and then blue. Upon changing the voltage from −0.2 to 0.0 V, a neutral yellow color formed. Further oxidation from 0.1 to 0.8 V, resulted in the conductive green color known to PANI and its derivatives. From 1.0 to 1.2 V, the polymer formed the blue color known to the fully oxidized pernigraniline form.

Figure 19.
Chronoamperometry of poly(OMA) and electrochromic devices of poly(OMA). Reproduced from Ref. [102] with permission under Creative Commons License.

Figure 20.
Electrochemical and electrochromic behavior of carbazole PANI. Reprinted with permission from Ref. [96]. Copyright 2022 American Chemical Society.

PANI and its derivatives are utilized as hole transport layers in Perovskite solar cells (PSC). For example, poly(o-methoxyaniline) (PoMA) doped with 4-dodecylbenzenesulfonic acid (DBSA) was used to replace spiro-OMeTAD, which was the most explored hole transport material (HTM) in PSC. The best power conversion efficiency (10.05%) was obtained using doped PANI-DBSA with Au as contact, which is comparable with the device using Spiro-OMeTAD. Unfortunately, the device of PoMA with silver contact had the best performance of 5.5%, which is significantly lower than the pristine PANI [110]. This outcome is probable due to the close alignment of PANI and perovskite’s HOMO levels versus PoMA and perovskite’s HOMO levels (Figure 21). Our group have also prepared three PANI derivatives, PANI–carbazole (P1), PANI–phenoxazine (P2), and PANI–phenothiazine (P3), with different energy levels that were used to form an interface between PEDOT:PSS and the perovskite layer [97]. While all the devices from all the polymers performed better than the device without them, P2 exhibited the highest V_oc of 1.13 V and power conversion efficiency of 21.06%. The result is also explained by the close overlap of the perovskite’s HOMO to P2 versus the other polymers (Figure 22).

Figure 21.
Energy diagram of the different materials used in perovskite solar cells assembled in this work. Reprinted from Ref. [110].

Figure 22.
(a) MAPbI3-based PSC device configuration and the chemical structure of three polymers. (b) Energy level alignment of the polymers in the MAPbI3-based PSC devices. Reproduced from Refs. [91, 97] with permission from John Wiley and Sons.

3.4 Anti-corrosion and anti-fouling

PANI and its derivatives have been widely explored as anti-corrosion materials [111]. Trimethylsilyl-substituted polyaniline (PSiAn) was synthesized and used as anticorrosion materials [112]. The fabricated filler was highly hydrophobic due to the trimethylsilyl group. It was incorporated into the epoxy matrix to provide a low porous material with high barrier to aggressive corrosive substances. Anticorrosion coatings prepared from poly (aniline-co-2-ethylaniline) (PEA) micro/nanostructures displayed good anticorrosion performance with corrosion protection efficiency of 87.29% [113]. The hydrophobic property (CA = 145°), low conductivity, and low porosity was attributed to the anticorrosion performance of the composite material. Chloro- and bromo-substituted PANI derivatives have been shown to have excellent anti-corrosion and anti-fouling properties. Aluminum alloy 3105 (AA3105) was coated with poly(2-chloroaniline) (PClAni) by electropolymerization and the polymer was investigated for its anticorrosion properties. Potentiodynamic polarization technique and electrochemical impedance spectroscopy data showed that the metal coated with PClAni had outstanding performance against corrosion versus the untreated metal when subjected to 3.5% NaCl solution [114]. A similar result was obtained with bromo-substituted PANI derivatives (Br-PANI). De-doped Br-PANI in epoxy resin with varying amounts of Br substituents (EBP coatings) were prepared and investigated for anti-fouling and anti-corrosion properties by the accelerated immersion test, electrochemical impedance spectroscopy (EIS), XPS, antibacterial test, and field test [115]. It was shown that the polymers possessed excellent anticorrosion properties after immersion in 12.0 wt% NaCl solution at 95°C for 100 days. Furthermore, the EBP coatings also demonstrated better antibacterial and antifouling performance when compared to pure epoxy coating or de-doped PANI composite coatings. Poly(1-naphthylamine) (PNA) composites with polyvinyl alcohol (PVA) and polyvinylchloride (PVC) have also shown both anticorrosion and antifouling properties. PNA/PVA coatings have shown good corrosion protective efficiency and resistance in acid, alkaline, and saline media [116]. The same polymer composite also demonstrated antibacterial activity against Escherichia coli and Staphylococcus aureus [117]. Examples of the structures for anti-corrosion and anti-fouling are shown if Figure 23.

Figure 23.
Structures of anti-corrosion/anti-fouling PANI derivatives.

3.5 Energy storage devices: supercapacitors and batteries

Supercapacitors are energy storage devices that are characteristically high in energy and power density, as evidenced by a more extended battery life and fast charge/discharge. They are the future of portable consumer electronics that have excellent operational lifetimes. Supercapacitor energy storage devices have 10 times more operational lifetimes than lithium-ion batteries. Some studies have focused on enhancing the capacity of the materials and PANI derivatives are among the materials being studied for increased capacitance. PANI derivative, poly(DAAQ), was investigated as an electrochemical capacitor. The constructed symmetrical (poly(DAAQ)/poly(DAAQ)) electrochemical capacitor was shown to exhibit high specific energy (25–46 Wh/kg) and high specific power (10,200–30,500 W/kg) at discharge rates from 30 to 90 C [80]. Our group also developed supercapacitor from PANI derivative containing the carbazole unit. Cbz-PANI was used to construct electrodes for a supercapacitor device that showed a maximum areal capacitance of 64.8 mF cm⁻² and a specific capacitance of 319 F g⁻¹ at a current density of 0.2 mA cm⁻² in a symmetrical device (Figure 24) [96]. Moreover, the electrode showed excellent cyclic stability (≈ 83% of capacitance retention) over 1000 CV cycles and a 91% capacitance retention after 1000 cycles of charge and discharge in a device.

Figure 24.
Areal capacitance, specific capacitance, and capacitance retention of Cbz-PANI-1/AU@PET. Reprinted with permission from Ref. [96]. Copyright 2022 American Chemical Society.

On the other hand, batteries provide high energy densities that can be delivered over time. Poly(5-amino-1,4-dyhydroxy anthraquinone) (PADAQ) was used as the cathode material in a lithium ion battery [83]. The initial discharge capacity was 101mAhg⁻¹ at the current density of 400 mAhg⁻¹ with the cutoff voltage of 1.5–3.7 V. At a high current density of 1400 mAhg⁻¹, the capacity of the polymer was 95 mAhg⁻¹. Poly(5-amino-1,4-naphthoquinone) (PANQ) recorded an experimental charge storage capacity of 220–290 Ah/kg in nonaqueous electrolytes. The specific energy of the battery is estimated to be about 100 Wh/kg with a 1 h discharge rate in a thin-layer cell where LiC₆ is the negative electrode and a PANQ as the positive electrode [81]. The mean redox potential was determined to be about 2.6 V more positive than Li/Li⁺ couple, which makes it a potential positive electrode for lithium metal or lithium-ion batteries. The PANI derivatives, PT-DMPD and PT-BZ, developed by Fors and coworkers, were investigated as cathode materials in Lithium-ion battery (LiB) technology [93]. Li-Coin cells devices with PT-DMPD or PT-BZ as the cathode delivered discharge capacities of 128 mAh/g and 97 mAh/g, which are 82% and 76% of the theoretical capacity for PT-DMPD and PT-BZ, respectively. However, the Coulombic efficiency (35% for PT-DMPD and 44% for PT-BZ) were relatively low. Additionally, the cycling performances were also poor with the discharge capacities for PT-DMPD and PT-BZ after 50 cycles being 82 mAh/g (64% retention) and 64 mAh/g (66% retention), respectively. To improve the performance of the polymers, Pt-DMPD was copolymerized with a second phenothiazine unit that was crosslinked to another one through a N-atom. The amount of cross-linked monomer was varied. Improved Coulombic efficiencies (150 mAh/g) at very positive operating voltages (2.8–4.3 V vs. Li+/Li) were obtained, which yielded high energy densities. Also, a greater retention capacity (82%) was observed at ultrafast discharge rates (120 C) for the crosslinked co-polymers.

4. Conclusion

To conclude, PANI derivatives are being widely developed in an attempted to address the limitations of PANI, which are insolubility and electrochemical instability. This short summary outlined both older but includes many new developments of PANI derivatives that are able to sufficiently overcome PANI’s challenges and in some cases outperform PANI in several applications. Interestingly, with the developments of new synthetic approach by the Buchwald/Hartwig reaction to synthesize PANI derivatives, many new structures can be achieved that are not possible with the previous synthetic methods of oxidative and electrochemical polymerization. Consequently, it serves to continue developing new PANI derivatives by expanding the substrate scopes to continue to move the field forward in improving the current applications and merge into newer fields.

Acknowledgments

The authors are grateful for the financial support from the National Science Foundation for an award (CHE-1945503).

Conflict of interest

“The authors declare no conflict of interest.”

Deepak Ipe

Chapters authored

Related collaborators

Salvatore Siracusano

Suresh Antony

Glen Ulett

Chee Keong Tan

Alison Carey

Antonella Agodi

Martina Barchitta

Marjanca Starčič Erjavec

Darja Žgur-Bertok

Stefano Ciciliato