Quantitative Dynamicsand Modulation of Reactive OxygenSpecies in Aqueous Microdroplets

Abstract Reactive oxygen species (ROS) such as hydroxyl radical (·OH), superoxide radical (O2·–), and hydrogen peroxide (H2O2) differ strongly in reactivity and function. This diversity makes it challenging to regulate ROS-driven chemistry while minimizing undesired side reactions. In conventional ROS-generation platforms, rapid interconversion among these species couples their formation and decay, limiting control over which ROS dominate under a given set of conditions. Aqueous microdroplets have emerged as a distinct reaction environment in which ROS are spontaneously generated at the air–water interface, yet how environmental parameters quantitatively shape ROS speciation and interconversion remains poorly understood. Here, we quantitatively measure ·OH, O2·–, and H2O2 as functions of droplet size, reaction time, and pH in aqueous microdroplets. These measurements reveal a characteristic temporal sequence in which ·OH dominates at early stages, followed by O2·– and ultimately H2O2. This sequence is strongly modulated by pH, which shifts the system between ·OH-rich acidic regimes and O2·– /H2O2-rich alkaline regimes. To rationalize these observations, we develop a minimal nonstationary kinetic framework that captures the coupled interconversion dynamics among ·OH, O2·–, and H2O2 and reproduces their redistribution across experimental conditions. Together, these results establish a quantitative description of ROS dynamics in a nonstationary microdroplet interfacial system and provide a mechanistic basis for rational modulation of ROS speciation in microdroplet redox chemistry.