Structure, chemically modified electrode, analysis

From a survey of the literature in chemically modified electrodes [13], one can identify simple phenomenological models that have been very successful for the analysis of a particular aspect of the experimental data. Such models are, for instance, the Dorman partition model [24, 122], the Laviron [158], Albery [159] and Anson models [127] to account for the nonideal peak width, the Smith and White model for the interfacial potential distribution [129], and so on. Most of these models contain one or more adjustable parameters that give some partial information about the system. For example, the lateral interaction model proposed by Anson [127] provides a value for the lateral interactions between oxidized and reduced sites, but does not explain the origin of the interactions, neither does it predict how they depend on the experimental conditions or the polymer structure. In addition, none of these models provide information on the interfacial structure. 

To characterize the properties of molecules and polymer films attached to an electrode surface, a wide variety of methods have been used to measure the electroactivity, chemical reactivity, and surface structure of the electrode-immobilized materials [9]. These methods have been primarily electrochemical and spectral as indicated in Table I. Suffice it to say that a multidisciplinary approach is needed to adequately characterize chemically modified electrodes combining electrochemical methods with surface analysis techniques and a variety of other chemical and physical approaches. 

Electrodes doped with mediators are also successful in analyses using NAD (P)-dependent dehydrogenases (86-88). In these cases, the mediator is firmly adsorbed to the electrode. The cofactor is oxidized by the mediator, which becomes reduced. The mediator is reoxidized by an electrochemical process on the electrode. This technology makes it possible to reduce the amount of cofactor needed, for example, in flow injection analysis and also eliminates the need for enzymatic regeneration systems. A further successful development uses a carbon paste chemically modified with a dehydrogenase, the coenzyme, and a phenoxazine mediator. This complex structure is then coated with a polyester sulfonic acid cation exchanger (86). The mediators used are of aromatic polycyclic structure and are firmly bound to graphite or other carbon electrodes (Fig. 2) (89). 

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