Beyond Amine Coreactants: Persulfate-Oxalate Cascade Reactions for Low-Overpotential Bead-based Electrochemiluminescence

Abstract

Bead-based electrochemiluminescence (ECL) is inherently limited by the distance between the electrode and the immobilized Ru(bpy)32+ luminophores, requiring sufficiently long-lived reactive diffusive intermediates to enable remote excitation. Tertiary amines, and particularly tri-n-propylamine (TPrA), remain the benchmark coreactant for Ru(bpy)32+-based systems. However, amines are typically used at high concentrations, which, together with their toxicity, volatility, and limited compatibility with aqueous and biological environments, motivates the development of alternative coreactant systems. Here, we introduce an amine-free strategy for the remote excitation of Ru(bpy)32+-labeled beads at unprecedently low onset potential (−0.08 V vs Ag/AgCl) by exploiting a self-sustaining S2O82–-C2O42– radical cascade initiated by the Ru(NH3)63+/2+ redox couple. The cascade generates the complementary SO4•– and CO2•– radicals required for the remote excitation of Ru(bpy)32+, thereby enabling, for the first time, amine-free ECL from bead-bound luminophore. Mechanism investigations further reveal how the incorporation of Ru(bpy)32+ perturbs the reaction network by introducing competitive radical-consuming pathways that divert reactive radicals from propagation toward luminophore excitation. Correlating the ECL intensity with the coulometric descriptor of the cascade radical propagation efficiency, nS, demonstrates that maximum emission arises from a balance between radical propagation and consumption reactions. These results establish an amine-free, low-potential route for bead-based ECL generation and provide mechanistic principles for the rational development of alternative coreactant systems beyond conventional tertiary amines for bead-based ECL.