Rational Control of Interfacial Spontaneous Redox Reactions by Modulating Single-Microdroplet Charging

Microdroplet chemistry has garnered increasing attention due to the discovery that extreme redox reactions can spontaneously occur at droplet interfaces. Most current studies rely on ensemble-averaged aerosol behavior, yet each microdroplet may exhibit vastly different reactivities depending on its size and charge polarity. To address this problem, we developed a dielectric barrier electrospray platform, which treats the entire liquid volume confined within the emitter tip as a single stationary microdroplet reactor. This setup enables precise charge control and in situ monitoring of interfacial reactions in this isolated, picoliter-to-nanoliter droplet. Our results reveal that redox processes at microdroplet interfaces can proceed as spontaneous interfacial electrochemical reactions, driven by the polarity of microdroplet charge. The reaction mechanism can be explained well by a modified microdroplet electric double-layer model. The reaction Faradaic current was measured at 58 pA, corresponding to a current density of 1.7 mA/m²─remarkably consistent with estimations of charged microdroplet activity in atmospheric clouds. With this platform, we demonstrated that the disproportionation of nitrogen at microdroplet interfaces originates from the reaction polarity being defined at the single-droplet level by its charge, each selectively catalyzing the oxidation or reduction of N₂. These findings not only shed light on the atmospheric nitrogen cycle but also offer critical theoretical guidance for future bulk nitrogen chemistry enabled by microdroplet catalysis.