Carbon-Ring Electrodes with 1-μm Tip Diameter

The successful fabrication of ultrasmall ring-shaped electrodes with tip diameters as small as 1 μm is reported. These electrodes are formed by pyrolysis of methane Inside of hot quartz micropipettes. The tip Is filled with epoxy and cleaved to expose a ring-shaped carbon electrode. The small total dimention of these electrodes should make them unique for voltammetric measurements In ultrasmall environments. Voltammograms obtained at these electrodes are sigmoidal at low scan rates (<1 V/s), and the limiting current can be approximated by the equation derived for a spherical correction to a hemispherical electrode (I₁= 2πrnFDC). Apparent charge transfer kinetics for ferricyanide reduction are slowed at these electrodes, and these phenomena are discussed In relation to the carbon film and the effect of electrode dimension. Finally, these electrodes demonstrate a large degree of selectivity for the oxidation of dopamine In the presence of ascorbic acid and are useful for trace determination of dopamine to the micromolar level.