Polymer film-modified electrode surfaces

The redox polymers, both of organic and inorganic origin (such as polyvinylpyridine modified by redox-active complexes of metals Prussian blue and related materials), can be considered as a version of electrodes of the second kind however, the equilibrium is usually estabhshed with respect to cations. Electron conducting polymers (polyanyline, polypyrrol, and so forth) also pertain, in the first approximation, to the electrodes of the second kind, which maintain equilibrium with respect to anion. Ion exchange polymer films on electrode surfaces form a subgroup of membrane electrodes. 

Polymer-film modified electrodes are used for a variety of applications, such as electrocatalysis (5-8). analysis (9.10). and more recently for their permselectivity characteristics (11-1H). These polymer films are formed by casting the film on an electrode surface (12), using radio-frequency plasma (15), or by electropolymerization (13.16.17). Alternatively, a polymer film 1s formed as a discrete membrane Eind subsequently applied to an electrode (18). The use of both cast and discrete membrane films... 

Much work has been undertaken to modify electrode surfaces with films which are themselves conducting. The most promising approaches involve organic charge transfer and radical ion polymers. Coordination chemistry has, to date, played little part in this work (a good recent review is available),67 but one example relating to ferrocene chemistry can be quoted. In this example a well known electron acceptor, 7,7, 8,8 -tetracyanoquinodimethane (TCNQ 27), is modified and incorporated into polymer (28) in which the iron(II) of the ferrocene unit is the electron donor. The electrical conductivity of such a film will depend on partial electron transfer between ion and TCNQ centres as well as on the stacking of the polymer chains. The chemistry of other materials, based on coordination compounds, which have enhanced electrical conductivity is covered in Chapter 61. 

Considerable potential exists to design surface modified electrodes which can mimic the behaviour of electronic components. For example, a rectifying interface can be produced by using two-layer polymer films on electrodes. The electroactive species in the layers have different redox potentials. Thus electron transfer between the electrode (e.g. platinum) and the outer electroactive layer is forced to occur catalytically by electron transfer mediation through the inner electroactive layer. 

The relative fragility and preparative difficulty associated with monolayer-modified electrode surfaces hampered significant analytical progress for some time, and it was not until polymer-film electrodes were developed that the utility of modified electrodes in analysis could be demonstrated. 

Gorton and coworkers have been particularly active in this field and produced an excellent review of the methods and approaches used for the successful chemical modification of electrodes for NADH oxidation [33]. They concentrated mainly on the adsorption onto electrode surfaces of mediators which are known to oxidise NADH in solution. The resulting systems were based on phenazines [34], phenoxazines [35, 36] and pheno-thiazines [32]. To date, this approach has produced some of the most successful electrodes for NADH oxidation. However, attempts to use similar mediators attached to poly(siloxane) films at electrode surfaces have proved less successful. Kinetic analysis of the results indicates that this is because of the slow charge transfer between the redox centres within the film so that the catalytic oxidation of NADH is restricted to a thin layer nearest the electrode surface [37, 38]. This illustrates the importance of a charge transfer between mediator groups in polymer modified electrodes. 

SECM can also be used to study the flux of species produced at a modified electrode surface, such as one with a film of polymer (Section 14.2.3). In one type of experiment, the tip is held at a potential where it can detect an electroactive ion released from the polymer film during a redox process (30-32). For example the SECM was used to detect the release of Br during the reduction of oxidized polypyrrole (PP) in the form, PP" Br . During a reductive cyclic voltammetric scan, Br was found to be released only in a later part of the scan, after an appreciable amount of cathodic charge had passed. This result suggested that during the early phase of the reduction the uptake of cations, rather than the release of anions, maintained charge balance in the film. 

The reality is that surface electrode modification is needed to make the ultramicroelectrode material selective for NO. Therefore, the design of modified electrode surfaces using organized layers is very attractive and provides the ideal strategy. In the general case, the chemical modification of electrode surfaces with polyelectrolytes and metal complex-based polymer films has expanded the scope of appUcation of such designed electrodes and provided a lot of options for then-use in various experimental conditions. In addition to their electrocatalytic applications, such electrodes showed a great promise for electroanalysis. As far as this aspect is concerned, substantial improvements in selectivity, sensitivity, versatiUty and reproducibility can be achieved. 

Conductive polymer films on electrodes have been prepared by electrochemical polymerization of electroactive monomers such as a pyrrole-substituted mediator, or by evaporating solutions containing preformed polymer. Examples of electrocatalyses reported include the oxidation of alcohols by pyrrole-substituted 2,2,5,5-tetramethyl-3-pyrroline-l-oxyl [26] and organohalide de-halogenation by pyrrole-substituted 4,4 -bipyridinium salt [27]. The preparation of mediator-modified electrode by evaporating solutions of preformed polymers was carried out by dip-coating polymers including mediators on electrode surface or by covalent attachment of mediators to dip-coated polymers on electrode surfaces. Examples of the former electrocatalyses are selected from the several reports on the oxidation of NADH by dopamine... 

There are several methods used to modify electrode surfaces adsorption of reagents on the surface, covalent bonding by reaction between the modifier and functional groups on the electrode surface, incorporation of the modifier within a gel or an electrically conducting polymer film, physically coaling the electrode surface with the modifier, e.g., an enzyme, or mixing the modifier with carbon paste. 

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