DISPROPORTIONATION DURING ELECTROOXIDATION OF CATECHOLAMINES AT CARBON-FIBER MICROELECTRODES

The effect of following chemical reactions during chronoamperometry and cyclic voltammetry at microelectrodes has been evaluated by digital simulation and the results have been compared to experiments. This study was motivated by the demonstrated utility of microelectrodes to monitor catecholamine secretion from individual biological cells. Since following chemical reactions can increase the total number of coulombs passed, such an occurrence can affect the calibration of the measured response. However, at microelectrodes, products formed by chemical reactions after electron transfer are less likely to return to the electrode because of the divergent diffusion field that can exist at electrodes of small dimensions. The degree to which these effects are apparent has been evaluated quantitatively by digital simulation of the DISP1 scheme for a disk-shaped electrode. The predictions of the simulation are verified in an experimental study of the anodic oxidation of diphenylanthracene in acetonitrile containing pyridine. In contrast, the DISP1 reaction of catecholamines at carbon-fiber microelectrodes exhibits much less enhanced current than predicted by theory. The experimental data suggest this is due to the heterogenous nature of the carbon surface with respect to electron transfer. Thus, for most applications of carbon-fiber microelectrodes as sensors of catecholamine secretion from cells, the effect of the DISP1 reaction can be ignored.