Conducting polymers SECM studies

Although most amperometric SECM experiments involved ET reactions at the tip and/or substrate, interfacial IT processes can also be probed. Historically, the first IT reactions studied by SECM were ion-exchange processes at ionically and electronically conductive polymer films (48). The ions of interest were electrochemically active (e.g., Ec(CN)f or Br ) to enable amperometric detection at the tip. It was shown more recently that the tip process can be an IT reaction rather than an ET process if a micropipet electrode is used as an amperometric probe (49). In this section we consider two different types of IT reactions employed in SECM studies, i.e., facilitated IT and simple IT. 

SECM has been applied to the investigation of various technologically important materials and interfaces, for example, metallic corrosion [91-96], fuel cell electrocatalysts [97], semiconductor photocatalysts [12, 60-63, 98], conducting polymers [49, 50, 85, 86, 99-103], liquid-liquid and liquid-gas interfaces [29, 30, 68]. The SECM may be used to image the substrate topography and/or reactivity, or with the tip at a fixed location, to study the local kinetics of the interfacial reactions of interest. 

Deposition of condncting polymers can be accomplished by either direct or feedback modes of SECM. In an earlier study, SECM was used to deposit polyaniline on a Pt substrate by scanning a UME tip in thin, ionically conductive Nafion films coated on the Pt surface (127). Anilinium ions were incorporated into Nafion films and the Pt substrate was biased positively, which caused the electropolymerization of aniline. The resolution of the polyaniline deposition is controlled by the tip size, the thickness of Nafion films, and the electric field distribution. 

An SECM feedback response to an electroactive polymer film can be controlled either by ET kinetics at the film-solution interface or by film conductivity. The contribution of lateral film conductivity to the effective ET rate measured by SECM was addressed in the recent study of polyaryl multilayers attached with ferrocenes [65] or ferrocene-terminated dendrimers [69] on unbiased carbon electrodes. In the latter study, the dependence of apparent ET rate constant, feei. on the generation of dendrimers (Table 6.2) was ascribed to the different efficiencies of electron transport inside and between dendrimers (Figure 6.22). Interestingly, the theory of the aforementioned triple potential step method predicts its potential to separately determine heterogeneous ET rate and lateral conductivity for a thin polymer film coated on an insulating surface [70]. 

A general approach to fabricating solid-state ion-selective microelectrodes has been described whereby a conducting electroactive polymer, which is both an electronic and an ionic conductor (e.g., polypyrrole, polythiophene, or polyaniline), is used to mediate charge exchange between an ion-selective membrane (an ion conductor) and a metal substrate (an electronic conductor) [28]. These electrodes are reported to be robust and mechanically flexible while exhibiting good potential stability with no redox sensitivity. While applications to potentiometric SECM have been described [28], these electrodes have yet to find use in corrosion studies. 

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