Direct electrochemical evidence suggests that aqueous microdroplets spontaneously produce hydrogen peroxide.

Recent reports have detailed the striking observation that electroactive molecules, such as hydrogen peroxide (H 2 O 2 ) and radical water species (H 2 O .+ /H 2 O .- ), 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 (H 2 O 2 ) in single aqueous microdroplets suspended in 1,2-dichloroethane. The oxidation of H 2 O 2 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 H 2 O 2 . Further, we show that the stochastic electrochemical signal is highest at the mass transfer limitation of the H 2 O 2 couple and is dampened when the potential nears the formal potential. We validate these results by showing that the addition of a H 2 O 2 selective probe, luminol, decreases the stochastic electrochemical response at alkaline pH (11.5). Our results support the observation that H 2 O 2 is generated in water microdroplets at concentrations of ~100 s of µM.