Imaging of nonuniform current density at microelectrodes by electrogenerated chemiluminescence

The chemiluminescence arising from reaction of electrogenerated radical cations of 9,10-diphenylanthracene (DPA) and benzonitrile (solvent) radical anions has been used to image microelectrodes with dimensions in the micrometer range. Experimental conditions including supporting electrolyte, DPA concentration, and excitation frequency were optimized to affect high luminescent intensity. In solutions of high resistance, the light was found to be temporally delayed with respect to the applied potential due to the increased time required to charge the double layer. Spatially nonuniform light at disk- and band-shaped microelectrodes was observed under certain conditions, with the highest intensity occurring at the region of the electrode with highest curvature. The optimum condition for observation of the nonuniform light was with very high electrode currents. Under this condition, the current density approaches that of the primary current distribution, a circumstance where spatially nonuniform potentials occur. This phenomenon was also examined at a conical electrode as a method of reducing the emission area. A submicrometer-size light source was obtained at high frequencies with an electrode that had a significantly larger uninsulated area.