Dissolved Oxygen Redox as the Source of Hydrogen Peroxide and Hydroxyl Radical in Sonicated Emulsive Water Microdroplets

Sonicated emulsive water microdroplets (SEWMs) accelerate and enable a variety of catalyst-free chemical transformations. However, significant unanswered questions remain regarding the chemical intermediates they form and their possible redox origin. In this study, we identified dissolved O₂ as the primary originator of reactive oxygen species (ROS) such as OH^• and H₂O₂. We uncovered the role of dissolved O₂ redox by using a combination of microelectrochemical methods to detect H₂O₂, isotopic methods to identify the source of H₂O₂, and a combination of electron spin resonance and the DMPO spin trap to detect radicals such as OH^•. Notably, we found that H₂O₂ production is correlated with O₂ content via a reduction pathway enabled by a sufficiently large reducing power that can additionally generate H₂ and even perform Pb electroless deposition on Au and Cu metal substrates. Building on our findings, continuous O₂ bubbling of SEWMs showed accumulation of H₂O₂ up to ∼88 mM in the aqueous phase within 1 h of sonication, demonstrating the scale-up promise of this method. Distinct to sonochemistry of a single phase, this study advances our understanding of the confluence of redox and chemical reaction mechanisms within SEWMs as a biphasic system. This insight paves the way for improving their reaction kinetics, yield, and selectivity, positioning these attractive redox microreactors as alternatives to traditional electrolyzers.