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

Abstract

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.