Enhancing Electrochemiluminescence with Two-Component Spontaneously Adsorbed Monolayers and Films

Multicomponent films offer important opportunities in electrochemiluminescence, ECL, including the possibility to coimmobilize an electron transfer mediator to facilitate the generation of radicals from a coreactant and to enhance indirect ECL at potentials that are lower than the direct route. Moreover, the local microenvironment, as well as energy and electron transfer pathways, can be controlled which have important implications for improving ECL assays and sensors. Here, spontaneously adsorbed monolayers and thin films of [Ru-(bpy)₂(Qbpy)]²⁺, RuN₆, and [Ru-(bpy)₂(Qbpy)-Cl]⁺, RuN₅Cl, have been formed on carbon and gold electrodes, where bpy is 2,2'-bipyridyl and Qbpy is 2,2':4,4″:4',4″-quarterpyridyl. The mixed films show well-defined voltammetric responses associated with the Ru^2+/3+ couple within each complex. For dense RuN₆ monolayers, the ECL response is sharply localized around the Ru^2+/3+ oxidation, with a single, well-defined ECL response reflecting the direct Ru³⁺/coreactant radical annihilation pathway. Significantly, in mixed RuN₆:RuN₅Cl 20:80 films, a highly efficient, indirect ECL generating pathway is switched on due to the RuN₅Cl centers acting as electron transfer mediators to the coreactant. This pathway enables ECL generation at lower potentials while the combination of both pathways gives dramatically enhanced ECL generation efficiency across a wide range of potentials. Quantitative Rehm-Weller and spectral overlap analysis shows that neither electron-transfer nor energy-transfer quenching of the excited state of RuN₆ by RuN₅Cl is a significant issue. Significantly, using 2-(dibutylamino)-ethanol, DBAE, as the coreactant delivers substantially brighter ECL from the mixed films compared to tripropyl amine, TPrA, reflecting its lower oxidation potential and more efficient formation of strongly reducing radicals within or near the film. Moreover, the ability to induce ECL at lower potentials with significant intensity is an important advantage for the miniaturization of multianalyte detection platforms. Together, these findings advance a coherent framework for engineering immobilized Ru-based ECL emitters within mixed composition films, and highlights [Ru-(bpy)₂(Qbpy)]²⁺-type systems as strong candidates for next-generation biosensing and wireless ECL devices.