Deciphering the mechanism of hydrogen peroxide formation in ultrasound-mediated water-in-oil microdroplets.

Microdroplet chemistry has emerged as a fascinating field, demonstrating remarkable reaction acceleration and enabling thermodynamically unfavorable processes. The spontaneous generation of hydrogen peroxide (HO) in water microdroplets presents a particularly intriguing phenomenon with significant implications for green chemistry and prebiotic processes. However, the transient nature of conventional microdroplets has hindered in-depth mechanistic investigations. This study employs ultrasound-mediated water-in-oil microdroplets to elucidate the underlying mechanism of HO generation. Under ultrasound irradiation, the HO concentration increases linearly with a production rate of 0.24 mM min, reaching 14.37 mM after one hour. Notably, 99% of this production occurs at the water-oil interface, corresponding to approximately 0.10 mM m min. Quantification of key intermediates reveals that superoxide radical (·O ) concentrations are approximately tenfold higher than those of HO and thousandfold higher than those of hydroxyl radicals (·OH). Through radical scavenging and isotope labeling experiments, we identify dissolved oxygen as the primary source and ·O as the main intermediate in HO formation, following the pathway: O → ·O → HO. We validate the critical role of the water-oil interface in initiating HO production charge separation reactions and demonstrate the significance of proton availability and surface propensity in facilitating efficient HO generation. These findings not only advance our understanding of microdroplet interfacial chemistry but also offer potential applications in atmospheric chemistry, green disinfection, and origins of life research.