ELECTROCHEMISTRY AT MICROCYLINDER ELECTRODES (CARBON FIBER, MODIFIED, ULTRAMICROELECTRODES, ANALYSIS)

Interest in ultramicroelectrodes has motivated an examination of microcylinder as analytical sensors. Microcylinder electrodes have a uniform current distribution and are constructed readily from carbon fibers or thin metal wires (5-(mu)m radius). The relationship between the observed current and the radial diffusional flux of electroactive species to the cylindrical electrode surface is verified as proportional to 1/1n(t). True steady-state behavior is never observed at these electrodes; however, it is shown that a quasi-steady-state current is observed with slow scan rates at electrodes with small radii. These features are used to evaluate analogous electrode geometries such as the hemicylinder and band.; Slow electron-transfer properties at the cylindrical surface of carbon fibers, relative to those observed at metal electrodes such as platinum or mercury, has necessitated the activation of the carbon surface before electroanalysis. The nature and effect of the activation treatments has been related to the surface state of the carbon electrode. In this work the electrochemical activation of a single carbon fiber is examined in detail. Voltammograms obtained at this electrode display quasi-reversible electrochemical kinetics; in particular, the overpotential for the oxidation of ascorbic acid is diminished greatly which allows the voltammetric resolution of ascorbate from the catechol neurotransmitters and metabolites. Despite the increase in electrochemical kinetics at this activated electrode, many species exhibit decreased current response, while others show evidence of adsorption at the electrode surface. These features are examined in terms of blockage of the electrode surface and ion-exchange capacity of electrochemically generated surface oxides.; Selectivity and stability of electrochemical sensors is improved by the use of ionomeric polymers such as perfluorosulfonate polymers. Several polymeric coatings with anion-exchange properties are examined for use with ultramicroelectrodes. The analytical potential of these films is evaluated with respect to selective analysis of anionic electroactive species.