Aniline, electropolymerization

Illustration of this effect in the case of a two-step process for aniline electropolymerization on mild steel and zinc from an aqueous electrolyte was reported by Lacroix et In acidic medium (the usual conditions for the electrodeposition of PANI) the direct electropolymerization of aniline on zinc or steel fails, because large amounts of metal dissolve before the aniline polymerizes. This can be avoided if the surface is first treated by depositing a thin polypyrrole film in neutral salicylate medium. This PPy layer behaves as a thin noble-metal layer and can be used for the electrodeposition of a PANI film of controllable thickness in an acidic medium. Using this pretreatment, no or very little metal dissolution occurs in this second step. The films exhibit very stable electroactivity in acidic electrolytes, similar to that of PANI deposited on platinum, which indicates that the underlying oxidizable metal is fully protected (Figure 16.1). 

Figure 16.1 Electrochemical cycling in 2 M p-toluene sulfonic solution of a mild-steel electrode coated by a PPy/PANI bilayer (a) first cycle (b) after cycling for 2 h. The bilayer system exhibits the same stability under potentiodynamic cycling as a platinum electrode coated by a PANI film in acidic medium. (Reprinted with permission from Journal of Electroanalytical Chemistry, Aniline electropolymerization on mild steel and zinc in a two-step process by J.-C. Lacroix, J.L. Camalet, P.C. Lacaze et al., 481, 1, 76-81. Copyright (2000) Elsevier Ltd)...

Lacroix, J.C., et al. 2000. Aniline electropolymerization on mild steel and zinc in a two-step process. 

In the case of aniline electropolymerization, the radical cation of ani-Une monomer is formed on the electrode surface by oxidation of the monomer [ 100,105-114]. This process is considered to be the rate-determining step. Radical coupUng and elimination of two protons make mainly para-formed dimers. Chain propagation proceeds with oxidation of the dimer and aniUne monomer on the electrode surface. In this step, the radical cation of the ohgomer couples with a radical cation of aniline monomer. In the final step, PANI is doped by the acid (HA) present in solution. The growth of PANI has been considered to be self-catalyzed. This means that the polymers are formed at the higher rate as the more monomers are deposited onto the polymer smface. It involves the adsorption of the aniUnium ion onto the oxi zed form of PANI, followed by electron transfer to form the radical cation and subsequent reoxidation of the polymer to its most oxidized state. 

Lacroix J.-C., Camalet J. L., Aeiyach S., Chane-Ching K. L, Petitjean J. et al. (2000), Aniline electropolymerization on mild steel and zinc in a two-step process , J. Electroanal. Chem., 481, 76-81. 

Oxidative electropolymerization has been described for iron(II) and ruthenium(II) complexes containing amino-20 and pendant aniline-21 substituted 2,2 -bipyridyl ligands, and amino- and... 

Electrochemistry is one of the most promising areas in the research of conducting polymers. Thus, the method of choice for preparing conducting polymers, with the exception of PA, is the anodic oxidation of suitable monomeric species such as pyrrole [3], thiophene [4], or aniline [5]. Several aspects of electrosynthesis are of relevance for electrochemists. First, there is the deposition process of the polymers at the electrode surface, which involves nucleation-and-growth steps [6]. Second, to analyze these phenomena correctly, one has to know the mechanism of electropolymerization [7, 8]. And thirdly, there is the problem of the optimization of the mechanical, electrical, and optical material properties produced by the special parameters of electropolymerization. 

Electropolymerization in acidic media affords free-standing films that are believed to contain varying degrees of cross-linking [267,292,304]. The miscibility of aniline with water allows for a variety of aqueous oxidants, such as ammonium peroxydisulfate, to be used [305]. Chemical polymerization of aniline can also be performed in chloroform through the use of tetrabutyl ammonium periodate [306]. Accordingly, a number of alkyl [301] and alkoxy-substituted [307] aniline derivatives have been chemically polymerized. Unfortunately, functionalization of the aniline nucleus often leads to a decrease in performance in the resulting polymers [308,309]. 

Perform galvanostatic electropolymerization of aniline on the GC disk electrode by applying a constant current of + 0.014 mA (0.2mA/cm2) for 714 s. The electropolymerization conditions are similar to those used for poly(3,4-ethylenedioxythiophene) (see Procedure 4) and were found to give well-functioning sensors for this particular application, so therefore no further optimization was done. 

Co(0-NH2)TPP] was polymerized onto glassy-carbon electrodes from an electrolytic solution containing the monomer and tetraethylammonium perchlorate as the electrolyte by cycling the electrode potential in the oxidative way (26). The polymerization of this monomer appears to proceed through a radical cation of the porphyrin with a mechanism similar to that of the oxidative electropolymerization of aniline (27). 

In oxidative electropolymerization, monomers such as pyrrole, thiophene, alkylth-iophenes or aniline are dissolved in an appropriate solvent containing an electrolyte that can act as a source for the anions needed to neutralize the cations formed during the oxidation process [33,34]. The nature of this electrolyte is of great importance to the structural features obtained for the electropolymerized layer since the dopants become an intrinsic part of the polymer layer structure. A general outline of a mechanism describing the electropolymerization process is shown in Figure 4.21. 

Li et al. [93] have used l-ethylimidazolium trifluoroacetate, which is a Bronsted acidic ionic liquid, as a medium for the electropolymerization of aniline. They report that in this ionic liquid the oxidation potential of aniline is lower (0.58 V compared to 0.83 V in 0.5 M H2SO4) and that the growth rate of the polymer is increased. Further, the resultant films are smooth, strongly adhered to the Pt working electrode and are very electrochemically stable. Similar results have been reported by Liu et al. [92], who found that this was the best ionic liquid for the polymerization of aniline, compared to the unsatisfactory results observed in other protic ionic liquids 1-butylimidazolium tetrafluoroborate, 1-butylimidazolium nitrate and 1-butylimidazolium p-toluenesulfonate, as well as the l-butyl-3-methylimidazolium hydrogen sulfate and l-butyl-3-methyimidazolium dihydrogen phosphate. 

Electropolymerization — Figure. Cyclic voltammograms and the simultaneously detected EQCM frequency changes during the electropolymerization of aniline at a platinum electrode. Sweep rate 100 mV s-1. Solution composition 0.2 mol dm-3 aniline in... 

Perchlorate salts are used as electrolytes in - electropolymerization reactions involving monomers such as aniline, benzidine, azulene, biphenyl, di-vinylbenzene, and indole. 

A number of desirable properties were exhibited in this work, which include ease of monomer synthesis, mild positive electropolymerization potential, polymer stability to continuous potential cycling, and stability to storage under ambient conditions. Unfortunately, the nature of the polymer backbone could not be definitely assigned. Nevertheless, the utility of pendant phenol and aniline groups for anchoring metal complexes to an electrode surface is a method worth further investigation. 

Instead of chemical oxidative polymerization, electropolymerization can also be considered. A recent study shows that slow but efficient electropolymerization is possible if anilinium-exchanged zeolite Y is subjected to oxidative treatment at the electrode-electrolyte interface. Cyclic voltammetric signatures of the polymerization suggest that it occurs mostly through one dimer (p-aminodiphenylamine) which imdergoes oxidative polymerization. Electrochemical polymerization of aniline in zeolite molecular sieves was studied. A zeolite-modified electrode showed shape-selectivity for 12-molybdophosphoric acid. 

The development of fast spectrometers has opened the possibility of studying, for instance, the spectral changes (UV-Vis and IR) associated with a single CV scan [334-336] or spectra of short-lived intermediates [337,338]. The first approach was taken in a study of the initial steps of the electropolymerization of aniline in aqueous sulfuric acid [335]. 

Of particular interest is the recent development of an electrochemical cell (Fig. 57) that allows for the simultaneous recording of both ESR and UV-Vis spectra of electrogenerated intermediates [374]. This type of cell was used in a study of the initial steps of the electropolymerization of aniline in acidic DMSO [379]. 

The formation of -aminodiphenylamine is supposed to be the key intermediate in the formation of a dark green precipitate at the electrode surface during continued electrolysis of acidic aniline solutions. This has been characterized as an oligomer of aniline, for example, as the octamer emeraldine formed by a cascade of head-to-tail condensations [38,39]. Nelson, however, explained it as a mixture of mainly quinhydrone with a small amount of benzidine salt [37]. Today the electropolymerization of aniline under strongly acidic conditions is intensively studied as an important way to form the conducting polymer polyaniline [40] (see Chapters 31 and 32). 

In contrast, a very stable and efficient electrode modification has been obtained by Fletcher and Bartlett [23]. In this case, a high-surface-area reticulated vitreous carbon electrode has been modified by electropolymerization of aniline, thereby further increasing the surface area. Horseradish peroxidase (HRP) was immobilized on this electrode by cross-linking with glutaldehyde and 1,2-diaminobenzene. The immobilization of the enzyme led to a strongly enhanced lifetime of the enzyme. 

Modified electrodes for this analytical purpose have mostly been formed by electrode adsorption of the mediator systems on the electrode surface or by electropolymerization [24,116]. Recently, for example, NAD(P)H oxidations have been performed on platinum or gold electrodes modified with a monolayer of pyrroloquinoline quinone (PQQ) [117] or on poly(methylene blue)-modified electrodes with different dehydrogenases entrapped in a Nafion film for the amperometric detection of glucose, lactate, malate, or ethanol [118]. In another approach, carbon paste electrodes doped with methylene green or meldola blue together with diaphorase were used for the NADH oxidation [119]. A poly(3-methylthio-phene) conducting polymer electrode was efficient for the oxidation of NADH [120]. By electropolymerization of poly(aniline) in the presence of poly(vinylsulfonate) counterions. 

Figure 1 Cyclic voltammograms recorded at 50mV/s during the electropolymerization on a gold surface of 0.1 M aniline in 0.5 M sulphuric acid (a), and of the polyaniline film in the supporting electrolyte (0.5 M sulphuric acid) alone after polymerization (b).

This section discusses the reactivities and biosensor applications of functionalized electrosynthetic polyanilines prepared by (i) electropolymerization and (ii) chemical polymerization of aniline on the surfaces of Au, glassy carbon or Pt disk electrodes. Four such polyaniline or substituted composite polyaniline films developed by our research group are (a) nanofibrillar polyaniline-polyvinyl sulfonate (PANI-PVS), (b) poly(2,5-... 

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