Direct electrochemical evidence suggests that aqueous microdroplets spontaneously produce hydrogen peroxide

Abstract

Recent reports have detailed the striking observation that electroactive molecules, such as hydrogen peroxide (H2O2) and radical water species (H2O.+/H2O.-), are spontaneously produced in aqueous microdroplets. Stochastic electrochemistry allows one to study reactions in real-time occurring inside subfemtoliter droplets, one droplet at a time, when a microdroplet irreversibly adsorbs to an ultramicroelectrode surface (radius ~ 5 µm). Here, we use stochastic electrochemistry to probe the formation of hydrogen peroxide (H2O2) in single aqueous microdroplets suspended in 1,2-dichloroethane. The oxidation of H2O2 at alkaline pH (11.5) differs from near-neutral conditions (6.4), allowing us to create a digital, turn-off sensing modality for the presence of H2O2. Further, we show that the stochastic electrochemical signal is highest at the mass transfer limitation of the H2O2 couple and is dampened when the potential nears the formal potential. We validate these results by showing that the addition of a H2O2 selective probe, luminol, decreases the stochastic electrochemical response at alkaline pH (11.5). Our results support the observation that H2O2 is generated in water microdroplets at concentrations of ~100 s of µM.