Toward mechanistic understanding of electrochemiluminescence generation by tris(2,2'-bipyridyl)ruthenium(II) and peroxydisulfate

Electrochemiluminescence (ECL) generation by tris(2,2'-bipyridyl)ruthenium (Ru(bpy)(3)(2+)) and S2O82- is one of most frequently studied cathodic systems. However, the reaction mechanism is yet to be fully resolved, since the reaction involving highly reactive intermediates occurs at the negative potential limit of aqueous solutions. In this work we used bismuth microwire (BiMW) as the working electrode and ECL microscopy (ECLM) as the imaging tool to study the mechanism of cathodic ECL generation by Ru(bpy)(3)(2+)/ S2O82. Thanks to its high hydrogen evolution overpotential, BiMW electrode could provide us with a wide negative potential window in aqueous solutions. While ECLM allowed us to determine the thickness of ECL layer (TEL), which was found to vary from ca. 3.4 mu m to 3.0 mu m with increasing the concentration of S2O82-. Given the chemical reduction of S2O82- by Ru(bpy)(3)(+) is essential for extending the concentration profile of SO4 circle-, the slightly changed TEL is likely related to the instability of Ru(bpy)(3)(+) in aqueous solutions. Finally, no ECL was detected for Ru(bpy)(3)(2+)-labelled microbeads, implying that S2O82- could not work as the cathodic coreactant like tri-n-propylamine (TPrA) as the anodic one in bead-based immunoassays.