A study of electrochemiluminescence mechanism of Rubrene/TPrA system in an aqueous solution via stochastic collision electrochemistry in an emulsion reactor

Background: Electrochemiluminescence (ECL) is an electrochemically induced process in which radicals generated at the electrode surface undergo exergonic electron transfer reaction to form excited states and luminesce. ECL, with high sensitivity and superior spatiotemporal control, has been widely applied in bioanalysis and light-emitting devices. The ECL signal of rubrene (Rub) was observed in Rub/TPrA oil-in-water (o/w) emulsions, which was inconsistent with the theory of ion-transfer coupled electron-transfer in Rub emulsion droplets, and the conventional ECL mechanism in Rub/TPrA system couldn't explain this phenomenon. Results: Choosing the toluene oil-in-water emulsion droplets as reactors, we put forward and verified a new ECL mechanism of Rub in aqueous solution on the basis of cyclic voltammograms, single-entity electrochemistry, ECL detection techniques and COMSOL finite element simulation. The new mechanism involved that Rub was reduced to Rub^•- by the highly reducing species TPrA^•, Rub^•- was then oxidized to the excited state Rub∗ by the stable intermediate TPrA^•+, and finally Rub∗ was inactivated by emitting photons. In simulation, the standard oxidation rate constant of TPrA in the emulsion droplets was deduced as 1.5 × 10⁻⁴ cm/s, and the deprotonation rate of TPrA^•+ was about 200 s⁻¹. Besides, emulsion droplets colliding on Au ultramicroelectrode (UME) were found to be able to amplify ECL signals. Significance: The newly proposed ECL mechanism of Rub/TPrA emulsion reactors broke the solubility problem of Rub in an aqueous, promoting the research and application of the organic luminophore Rub in bioanalysis. With the ECL signals amplification capability, the o/w emulsion reactors are promising to future design of ECL biosensors with higher sensitivity.