Articles Fabrication of Boron-Doped CVD Diamond Microelectrodes John B. Cooper,* ,† Song Pang, † Sacharia Albin, ‡ Jianli Zheng, ‡ and Robert M. Johnson † Department of Chemistry and Department of Electrical and Computer Engineering, Old Dominion University, Norfolk, Virginia 23529 Diamond microelectrodes are fabricated using microwave plasma CVD for the growth of electrically conducting single microcrystallite diamonds as well as diamond films on etched tungsten wires which are subsequently sealed in glass. The electroactive diamond is exposed by either mechanical polishing or by chemical etching of the glass. The resulting microelectrodes yield steady-state cyclic voltammograms at low scan rates. Diamond exhibits a unique combination of characteristics including high thermal conductivity, low coefficient of friction, chemical inertness, optical transparency from the UV to the IR, high mechanical stability, and high corrosion resistance. In an undoped state, diamond exhibits high electrical resistivity (typi- cally >10 8 Ω-cm). However, when diamond is p-doped, it becomes conductive and is suitable for use as an electrode. Typical resistivities for boron-doped diamond films are on the order of 10 Ω-cm. It has been demonstrated that boron-doped polycrys- talline diamond films possess a low double-layer capacitance and a relatively high polarization resistance toward surface oxidation, suggesting their feasible use in electroanalytical applications. 1 Currently, it is common for noble metals such as platinum or gold to be used when chemical inertness of the electrode surface is a primary consideration. However, in aqueous solutions, detection of analytes is often not possible at negative potentials using such electrodes due to the high Faradaic currents produced by the hydrogen evolution reaction (HER). Although mercury electrodes eliminate this problem, these electrodes are not mechanically stable and require the mercury drop electrode to be replenished, resulting in the generation of hazardous waste. A common alternative is the use of graphite and glassy carbon electrodes, which also offer the benefits of a large overpotential for the HER. Unfortunately, these electrodes are susceptible to fouling and surface oxidation, making them unsuitable for long-term monitor- ing applications. For example, it has recently been reported in a comparative study that while boron-doped diamond electrodes do not suffer from microstructural damage or surface oxidation when potential cycled in acidic fluoride solutions, both graphite and glassy carbon electrodes exhibit significant corrosion in the forms of pitting, cavitation, and surface oxidation. 2 Like carbon and mercury electrodes, diamond also exhibits a large overpotential for the reduction of aqueous media. 3,4 In addition to their resistance to corrosion, diamond electrodes also exhibit a resistance to surface fouling. As an example, it has been demonstrated that diamond electrodes can be voltammetri- cally cycled in aqueous solutions of ferri-/ ferrocyanide for two weeks without degradation of the analytical response of the electrode, while for glassy carbon electrodes, the response begins to decrease only after several minutes of cycling due to surface fouling. 5 Several investigations have demonstrated that diamond electrodes exhibit a useful analytical response for a wide range of redox reactions in solution. 6-10 In addition, recent investigations have shown that diamond exhibits an enhanced response for the commercially important reduction of nitrate. 11 This finding suggests that diamond may have additional advantages over conventional electrodes which are not yet realized. As the size of an electrode is decreased, radial diffusion becomes the dominate form of mass transport to and away from the electrode surface. This enables a steady-state concentration of analyte to be maintained at the electrode surface even when the electrode is polarized. 12-15 Such electrodes are often referred to as microelectrodes and offer many benefits. One obvious benefit is the steady-state response, which allows simple continu- ous monitoring of solutions or streams for changes in analyte † Department of Chemistry. ‡ Department of Electrical and Computer Engineering. (1) Swain, G. M.; Ramesham, R. Anal. Chem. 1993 , 65, 345-351. (2) Swain, G. M. J. Electrochem. Soc. 1994 , 141, 3382-3393. (3) Martin, H.; Argoitia, A.; Landau, U.; Anderson, A.; Angus, J. J. Electrochem. Soc. 1996 , 143, L133-L136. (4) Vinokur, N.; Miller, B.; Avyigal, Y.; Kalish, R. J. Electrochem. Soc. 1996 , 143, L238-L240. (5) Swain, G. M. Adv. Mater. 1994 , 6, 388-392. (6) DeClements, R.; Hirsche, B.; Granger, M.; Xu, J.; Swain,G. J. Electrochem. Soc. 1996 , 143, L150-L153. (7) Alehashem, S.; Chambers, F.; Strojek, J.; Swain, G. Anal. 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