DISPROPORTIONATION DURING ELECTROOXIDATION OF CATECHOLAMINES AT CARBON-FIBER MICROELECTRODES

The effect of following chemical reactions during chronoamperometry an d cyclic voltammetry at microelectrodes has been evaluated by digital simulation and the results have been compared to experiments. This stu dy was motivated by the demonstrated utility of microelectrodes to mon itor catecholamine secretion from individual biological cells. Since f ollowing 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 rea ctions after electron transfer are less likely to return to the electr ode because of the divergent diffusion field that can exist at electro des of small dimensions. The degree to which these effects are apparen t 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 diph enylanthracene in acetonitrile containing pyridine. In contrast, the D ISP1 reaction of catecholamines at carbon-fiber microelectrodes exhibi ts much less enhanced current than predicted by theory. The experiment al data suggest this is due to the heterogenous nature of the carbon s urface 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.