Polyaniline redox activity

Fig. 15.8 Redox-active polymers for heterogeneous dehydrogenation catalysis, (a) trimerized phenanthrene quinone [42], (b) benzoquinone biphenyl copolymer [41], (c) polynaphthoquinone [40], (d) polyaniline [43], (e) pyrolyzed polyacrylonitrile [44].

Interesting supports are the polymeric materials, notwithstanding their thermal instability at high temperatures. In the electrocatalysis field, the use of polypyrrole, polythiophene and polyaniline as heteropolyanion supports was reported [2]. The catalytically active species were introduced, in this case, via electrochemical polymerization. Hasik et al. [3] studied the behavior of polyaniline supported tungstophosphoric acid in the isopropanol decomposition reaction. The authors established that a HPA molecular dispersion can be attained via a protonation reaction. The different behavior of the supported catalysts with respect to bulk acid, namely, predominantly redox activity versus acid-base activity, was attributed to that effect. 

It is now well-established that some enzyme families, including various peroxidases and laccases, catalyze the polymerization of vinyl monomers and other redox active species such as phenol-type structures. Vinyl polymerization by these redox catalysts has recently been reviewed 93). These catalysts have been used to prepare polyanilines 94) and polyphenols 95,96). A few examples of related research are included in this book. For example. Smith et al (57) described a novel reaction catalyzed by horseradish peroxidase (HRP). In the presence of HRP and oxygen, D-glucuronic acid was polymerized to a high molecular weight (60,000) polyether. However, the authors have not yet illucidated the polyether structure. Two other oxidative biotransformations were discussed above i) the sono-enzymatic polymerization of catechol via laccase 31), and ii) the oxidation of aryl silanes via aromatic dioxygenases 30). 

Ellipsometry at noble metal electrode/solution interfaces has been used to test theoretically predicted microscopic parameters of the interface [937]. Investigated systems include numerous oxide layer systems [934-943], metal deposition processes [934], adsorption processes [934, 944] and polymer films on electrodes [945-947]. Submonolayer sensitivity has been claimed. Expansion and contraction of polyaniline films was monitored with ellipsometry by Kim et al. [948]. Film thickness as a function of the state of oxidation of redox active polyelectrolyte layers has been measured with ellipsometry [949]. The deposition and electroreduction of Mn02 films has been studied [950] below a thickness of 150 nm, the anodically formed film behaved like an isotropic single layer with optical constants independent of thickness. Beyond this limit, anisotropic film properties had to be assumed. Reduction was accompanied by an increase in thickness, which started at the ox-ide/solution interface. 

Furthermore, organic redox mediators, such as hydroquinone (HQ) [920-923], methylene blue (MB) [924], indigo carmine [925], p-phenylenediamine (PPD) [926,927], m-phenylenediamine (MPD) [928], lignosulfonates [929], suUbnated polyaniline (SPAni) [930], and humic acids [931], have also been investigated. Table 2.12 shows some typical aqueous electrolytes with organic redox-active molecules, the corresponding redox processes, and the performance of the assembled... 

Chen, W., R. B. Rakhi, and H. N. Alshareef. 2013. Capacitance enhancement of polyaniline coated curved-graphene supercapacitors in a redox-active electrolyte. Nanoscale 5 4134-4138. 

The redox interaction between transition metals and redox-active ligands is likely to permit a smooth reversible redox cycle in the transition metal-catalyzed oxidation reactions. Actually, the Wacker oxidation reaction of a terminal olefin proceeds catalytically only in the presence of a catalytic amount of polyaniline or polypyrrole derivative as a cocatalyst in acetonitrile-water under oxygen atmosphere to give 2-alkanone (Scheme Copper-free catalytic systems are... 

Yamamoto, K., et al. 2000. Doping reaction of redox-active dopants into polyaniline. Polym Adv Technol 11 710. 

Polyaniline is the conducting polymer most commonly used as an electrocatalyst and immobilizer for biomolecules [258-260]. However, for biosensor applications, a nearly neutral pH environment is required, since most biocatalysts (enzymes) operate only in neutral or slightly acidic or alkaline solutions. Therefore, it has been difficult or impossible to couple enzyme catalyzed electron transfer processes involving solution species with electron transport or electrochemical redox reactions of mostly polyaniline and its derivatives. Polyaniline is conducting and electroactive only in its protonated (proton doped) form i.e., at low pH valnes. At pH values above 3 or 4, polyaniline is insulating and electrochemically inactive. Self-doped polyaniline exhibits redox activity and electronic conductivity over an extended pH range, which greatly expands its applicability toward biosensors [209, 210, 261]. Therefore, the use of self-doped polyaniline and its derivatives could, in principle. 

The discovery of self-doped polyanilines was a major breakthrough in the field of conducting polymers due to their desirable properties such as water solubility, pH and temperature independent conductivity, redox activity over a wider pH range and thermal and environmental stability. These polymers can be prepared chemically and electrochemically by various methods discussed in the above sections 2.2-2A. The distinctive properties of self-doped sulfonated polyanilines are discussed in this section. 

In addition to sulfonic acid groups, carboxylic acid groups as ring substituents results in self-doping of polyaniline and influence properties such as solubility, pH dependent redox activity, conductivity, thermal stability, etc. Sulfonated polyanilines are typically obtained by postpolymerization modifications such as electrophilic and nucleophilic substitution reactions. However, carboxylic-acid-functionalized polyanilines are typically synthesized directly by chemical and electrochemical polymerization of monomer in the form of homopolymer or copolymer with aniline. In contrast to sulfonated polyaniline, very few monomers are available for the synthesis of carboxyl acid functionalized polyaniline. Anthranilic acid (2-aminobenzoic acid) is an important monomer and is often used for the synthesis of carboxyl acid functionalized polyanilines. 

Self-doped polyanilines are advantageous due to properties such as solubility, pH independence, redox activity and conductivity. These properties make them more promising in various applications such as energy conversion devices, sensors, electrochromic devices, etc. (see Chapter 1, section 1.6). Several studies have focused on the preparation of self-doped polyaniline nanostructures (i.e., nanoparticles, nanofibers, nanofilms, nanocomposites, etc.) and their applications. Buttry and Tor-resi et al. [51, 244, 245] prepared the nanocomposites from self-doped polyaniline, poly(N-propane sulfonic acid, aniline) and V2O5 for Li secondary battery cathodes. The self-doped polyaniline was used instead of conventional polyaniline to minimize the anion participation in the charge-discharge process and maximize the transport number of Li". In lithium batteries, it is desirable that only lithium cations intercalate into the cathode, because this leads to the use of small amounts of electrolyte... 

Figure 3.24 shows the redox behavior of PABA thin films observed at neutral pH in the presence of NADH and NAD" ". The PABA film was redox inactive at neutral pH (Figure 3.24,a) due to deprotonation and loss of dopant as with polyaniline [150,151). However, in the presence of NADH (Figure 3.24, b) and NAD" " (Figure 3.24, c), PABA films became redox active due to complexation of boronic acid with cis-2,3-ribose diols and subsequent formation of self-doped polymer. In the presence of NADH, the cyclic voltammogram of PABA thin film exhibited a single redox couple at pa 0.05 and pc —0.10 V. In contrast, a second redox couple was observed in the presence of NAD+ at pa 0.34 and pc... 

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