Quantitative Dynamics and Modulation of Reactive Oxygen Species in Aqueous Microdroplets.

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.