Long-lived electrochemiluminescence of ruthenium (II) complexes/tri-n-propylamine in aqueous solutions

Although the clinical use of immunoassays based on the oxidative-reduction electrochemiluminescence (ECL) of tris(2,2'-bipyridine)ruthenium (II)/tri-n-propylamine has been a great success, elucidation of the ECL generation mechanism still remains unsatisfactory. We report here our experimental observations of long-lived luminescence that remains detectable for several seconds after termination of electrochemical heterogeneous oxidation. Long-lived luminescence was observed in both a surfactant-free buffer and a surfactant-containing broadly used commercial buffer under different conditions. The slow decay of emission seems to have been unnoticed in previous ECL mechanistic studies. Within the frame of the reaction schemes so far proposed, its origin is inconclusively ascribed to the reductive-oxidation process of ruthenium (II) complex, that is Ru(bpy)₃ ²⁺  → Ru(bpy)₃ ¹⁺  → Ru(bpy)₃ ²⁺ * → Ru(bpy)₃ ²⁺ with the involvement of the tri-n-propylamine-derived radical cation. It is anticipated that long-lived ECL will suggest a research approach to separate some homogeneous reactions from the complicated reaction system and therefore help to resolve the mechanistic mystery.