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The fabrication of ultramicroelectrodes (UMEs) for analytical electrochemical applications has been explored, using boron-doped diamond as the active electrode material in an insulating coating formed by deposition of electrophoretic paint. Because of the rough nature of the diamond film, the property of such coatings that is normally exploited in the fabrication of UMEs, namely the tendency to retract automatically from sharp protrusions, cannot be used in the present instance. Instead focused ion beam (FIB) sputtering was employed to controllably produce UMEs with well-defined geometry, critical dimension of a few micrometers, and very thin insulating coatings. If the FIB machining is carried out at normal incidence to the diamond electrode surface, significant ion beam damage reduces the yield of successful electrodes. However, if a parallel machining geometry is employed, high yields of ultramicroelectrodes with a flat disk geometry can be obtained very reliably. The electrochemical properties of diamond UMEs are characterized. They show much lower background currents than the equivalent Pt or carbon fiber electrodes but more varied electrochemical response than macroscopic diamond electrodes.

Boron-doped diamond (BDD) ultramicroelectrode (UME) tips were fabricated by the growth of BDD films by chemical vapor deposition onto sharpened tungsten wires. Both nanocrystalline and microcrystalline forms of diamond coatings were examined. The diamond-coated wires were selectively insulated with nail varnish, electrophoretic paint, or fast-setting epoxy to form UME tips of critical dimensions of 1-25 [micro]m. The geometry of the exposed electrode area was disk or hemispherical in most cases. Cyclic voltammetry and chronoamperometry were used to assess exposed electrode area and integrity of the insulation. BDD UMEs were used to obtain SECM approach curves to an insulating and a conducting subslrate, which were fitted to the theory appropriate for the observed tip geometry. The tips were used to obtain SECM images of immobilized respiring E. coli, illustrating the suitability of BDD UMEs for electrochemical imaging in biological media.

The use of an ultramicroelectrode tip as a basis for a Clark membrane electrode allows the sensing of oxygen concentration using the oxygen reduction signal and the application of scanning electrochemical microscopy (SECM) to pure gas-phase measurements. A 25-[micro]m-diameter Pt wire was sealed in glass and the glass coated with silver paint near the tip to produce a Ag ring, Pt disk electrode. A drop of electrolyte provided the contact between the Pt cathode indicator electrode and the Ag anode counter/reference electrode, while the liquid phase was maintained by a high-density polyethylene membrane pulled over the tip and fastened with an O-ring. The membrane isolated the electrode both electrically and chemically from the sample environment. The electrode was tested with both solution- and gas-phase samples, and SECM approach curves were recorded. The presence of a membrane caused a deviation in electrode behavior from theory in both solution and gas phases, due in part to an increase in the electrode response time. However, the approach curves obtained could be used to determine the distance of the membrane from a surface and the thickness of the electrolyte layer within the membrane. SECM images in the gas phase, of oxygen flux through an array of 100-[micro]m-diameter holes in a silicon wafer, were obtained by measuring the reduction current of oxygen while scanning in the x-y plane over the surface.