Rubrene-Oxalate Electrochemiluminescence Reveals Physicochemical Properties of Triphasic Boundaries

Multiphase chemical systems are abundant in nature and widespread in industrial and pharmaceutical settings. Moreover, multiphase, microconfined systems have been implicated in unique and fascinating chemistries, such as reaction acceleration, unexpected chemical transformations, and reactive species production. Despite the importance and prevalence of multiphase environments, they remain challenging to study. Hence, alternative strategies are necessary to investigate these systems. In this study, we present an electrochemiluminescence (ECL) system that selectively illuminates triphasic boundaries (liquid|liquid|electrode interfaces) within multiphase systems. This triphasic boundary-confined ECL was achieved using rubrene as the luminophore, which is only soluble in organic solvents, and oxalate as the coreactant, whose solubility is limited to the aqueous phase. With this imaging platform, we interrogated single microdroplets, microdroplet arrays, and a bulk liquid|liquid|electrode interface. By restricting the ECL signal to the triphasic boundaries present in each of these systems, we mapped these boundaries within microdroplet arrays and tracked and imaged the dynamic movement of droplets, specifically a reorganization event and a coalescence event, in real time. Moreover, this system revealed differences in a 3D space by showing distinct ECL emission layer diffusion patterns at two unique imaging planes of a bulk liquid|liquid|electrode interface. Our results demonstrate ECL microscopy's ability to interrogate complex, multiphase systems in real time.