Electrochemiluminescence Amplification in Bead-Based Assays Induced by a Freely Diffusing Iridium(III) Complex

Heterogeneous electrochemiluminescence (ECL) assays employing tri-n-propylamine as a co-reactant and a tris(2,2'-bipyridine)ruthenium(II) ([Ru(bpy)₃]²⁺) derivative as an emissive label are integral to the majority of academic and commercial applications of ECL sensing. This model system is an active research area and constitutes the basis of successfully commercialized bead-based ECL immunoassays. Herein, we propose a novel approach to the enhancement of such conventional ECL assays via the incorporation of a second metal coordination complex, [Ir(sppy)₃]^3- (where sppy = 5'-sulfo-2-phenylpyridinato-C²,N), to the experimental system. By employing ECL microscopy, we are able to map the spatial distribution of ECL emission at the surface of the bead, from [Ru(bpy)₃]²⁺ labels, and solution-phase emission, from [Ir(sppy)₃]^3-. The developed [Ir(sppy)₃]^3--mediated enhancement approach elicited a significant improvement (70.9-fold at 0.9 V and 2.9-fold at 1.2 V vs Ag/AgCl) of the ECL signal from [Ru(bpy)₃]²⁺ labels immobilized on the surface of a polystyrene bead. This dramatic enhancement in ECL signal, particularly at low oxidation potentials, has important implications for the improvement of existing heterogeneous ECL assays and ECL-based microscopy, by amplifying the signal, opening new bioanalytical detection schemes, and reducing both electrode surface passivation and deleterious side reactions.