The role of direct oxalate oxidation in electrogenerated chemiluminescence of poly(4-vinylpyridine)-bound Ru(bpy)2Cl+/oxalate system on indium tin oxide electrodes

The electrochemical and electrogenerated chemiluminescence (ECL) properties of indium tin oxide (ITO) electrodes modified with poly(4-vinylpyridine) (PVP)-bound Ru(bpy)(2)Cl+ (where bpy = 2,2'-bipyridine) have been studied. In a sodium oxalate solution, two irreversible oxidation waves as well as two ECL emission waves were observed during the potential scan in the range 0.4-1.4 V (versus Ag/AgCl/saturated KCl reference). The first ECL wave appeared at ca. 0.8 V, which was caused by the excited-state Ru2+* generated through a bimolecular redox reaction between electrogenerated Ru3+ and the strong reducing agent, CO2-.. The latter was formed via a Ru3+-mediated oxidation of oxalate. Direct oxidation of oxalate was not involved in the first ECL process. The second ECL wave started at ca. 1.1 V, which was also from the excited-state Ru2+* generated via the redox reaction between Ru3+ and CO2-.. However, both direct and Ru3+-mediated oxidation of oxalate contributed to the formation of CO2-.. The important role of the direct oxidation of oxalate in the ECL mechanism of PVP-bound Ru(bpy)(2)Cl+/oxalate system was demonstrated. The relative contribution of direct oxidation of oxalate to the observed ECL depended upon the surface concentration of PVP-bound Ru2+, the concentration of oxalate and the electrode potential applied. (C) 2004 Elsevier B.V. All rights reserved.