No abstract

The rates and mechanism of coreactant electrogenerated chemiluminescence (ECL) from tris(2,2‘-bipyridyl)ruthenium(II) (Ru(bpy)3 2+) and the tertiary amines, tripropylamine (TPrA) and trimethylamine (TMeA), in aqueous solution were investigated. Transient (0.5 ms) potential steps were used with microelectrodes to investigate the emission time course under a variety of solution conditions. With amine concentrations that are low with respect to Ru(bpy)3 2+, the emission rises continually during the transient potential step and decays slowly after its termination. In contrast, the emission approaches a plateau during the potential step and is rapidly extinguished afterward with concentrations of Ru(bpy)3 2+ that are much lower than the amine concentration. At intermediate pH values, the emission intensity increases approximately linearly with pH. The emission after the potential step is unaffected by the rest potential. To simulate these temporal characteristics by finite difference methods, a mechanism employing 15 discrete chemical and electrochemical steps was employed, using literature-based thermodynamic values and electron-transfer rate constants evaluated from Marcus theory. The rate-limiting step was found to be the deprotonation of the amine radical cation. In addition, the simulations required a rate constant for the homogeneous oxidation of the tertiary amine by electrogenerated Ru(bpy)3 3+ value much below its Marcusian-calculated value to match the experimental data.

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.

The use of electrogenerated chemiluminescence (ECL) at microelectrodes as a light source for scanning optical microscopy is demonstrated. Cone-shaped microelectrodes were constructed by flame etching carbon fibers to a fine point. ECL generated in solution at such electrodes was forced to the apex of the conducting surface by using high-frequency (20-kHz) potential pulses and high concentrations of ECL reagents in the solution. ECL arose from the reaction of 9,10-diphenylanthracene radical cation with the radical anion of benzonitrile, the solvent. The electrode/light source was raster-scanned a finite distance above the sample surface, and images were generated with standard scanning probe software by collecting the transmitted light with a microscope objective. These images compared favorably to optical images of the same sample. A resolution of approximately 600 nm was achieved with this arrangement even though a feedback loop was not employed to control the tip distance from the sample. The source was sufficiently bright (1.82 pW) that well-defined transmittance spectra could be obtained at individual locations on the sample.