The voltammetry of NADH has been characterized at carbon-fiber microelectrodes at scan rates up to 100 V/s. Electrochemical pretreatment of the electrode dramatically changed the properties of the modified electrode. Anodic pretreatment of the surface resulted in an adsorptive wave for NADH oxidation, while less adsorption was evident under more moderate conditions. The pH of the buffer used for the anodization played a critical role in determining the voltammetric peak shape. Oxidation of NADH at slow scan rates (< 10 V/s) fouled the electrode. In contrast, consistent and reproducible voltammetry of NADH was observed at faster scan rates (100 V/s). This voltammetric measurement was used to monitor NADH generated during the oxidative deamination of glutamate catalyzed by glutamate dehydrogenase. A 150-microns-l.d. microdialysis fiber was used to entrap the enzyme near the microelectrode tip, forming the dehydrogenase-modified carbon-fiber microelectrode.

Two techniques, cyclic voltammetry and the microscopic imaging of electrochemically generated chemiluminescence (ECL), have been used to evaluate the effectiveness of various electrochemical pretreatments on the electron-transfer properties of carbon-fiber microelectrodes. The surfaces of carbon-fiber microelectrodes were electrochemically treated to produce different levels of surface oxides in the following manner: after normal polishing and cleaning in hot toluene and water, the carbon surface was activated by applying a cyclic potential from -0.2 to 2.0 V at a frequency of 50 Hz for 3 s in solutions of varying pH: 1M HCl, pH 7.4 and pH 12.0 phosphate buffers. Cyclic voltammetry was employed to elucidate the effect of the surface pretreatment on the overall voltammetric properties of the different pretreatment methods. The ECL emission intensity was imaged with resolution on the submicrometer scale with a conventional fluorescence microscope equipped with a cooled, slow-scan CCD camera. In addition to investigating pretreatment effects with luminol, we have also examined the ECL properties of a positively charged species, ruthenium tris (2,2'-bipyridine) dichloride hexahydrate, whose luminescent properties are also well documented. Such information is not only invaluable for the rational design of surface-modified ultramicroelectrodes, but it can also yield considerable information concerning the surface interactions influencing organic electron-transfer reactions at carbon surfaces.