In vitro comparison of the selectivity of electrodes for in vivo electrochemistry

The properties of 4 types of carbon electrodes designed for use as in vivo sensors of easily oxidized species in the mammalian brain have been evaluated in aqueous solutions at physiological pH. The electrodes are formed from a graphite-epoxy mixture, carbon paste, or carbon fibers, and have the geometries of a disk or a cylinder. The voltammetric properties of several catecholamines, some of their metabolites and precursors, uric acid, and ascorbic acid are reported at unmodified carbon surfaces. The problem of overlap of the voltammetric waves of ascorbate and catechols is addressed, and two different methods which minimize this problem are examined. These are the use of disk-shaped electrodes fabricated from carbon fibers, which facilitate the use of subtracted voltammograms to determine small changes in the concentration of catecholamines in the presence of ascorbic acid, and the use of electrochemically modified, cylindrically-shaped electrodes also fabricated from carbon fibers. Voltammetry at the modified electrodes gives evidence the catechols can be resolved from ascorbate, and that catechols, but not ascorbic acid, adsorb to the electrode surface.