Single-Rain Droplet Amperometry Reveals Spontaneous and Regulated H₂O₂ Formation on Leaf Surfaces

Recent studies indicate that both sprayed and condensed water droplets can spontaneously generate hydroxyl radicals (^•OH) via interfacial reactions and contact electrification, leading to the formation of hydrogen peroxide (H₂O₂). However, their behavior under natural conditions, such as in tiny rain droplets, remains poorly understood. Here, we performed on-site electroanalysis of single rain droplets (5 μL) and demonstrated the spontaneous, catalyst-free formation of detectable H₂O₂ levels within tiny droplets on natural leaf surfaces. Notably, the amount of H₂O₂ is influenced by leaf surface topography and hydrophobicity (Rosa vs Buxus leaves as model systems), sunlight exposure versus shade─likely due to temperature rather than UV effects─and the pH of rain droplets. Using a combined electrode (≈1.2 mm diameter), we achieved direct electrochemical detection of H₂O₂ in single droplets with a detection limit of 0.27 ppm (∼8 μM), complemented by parallel colorimetric assays for validation. Controls excluded UV or photosynthetic interference, and repeated analyses on artificial plastic surfaces and behind glass (blocking UV radiation) confirmed that H₂O₂ formation originates solely from the droplet interface (∼5 μL), consistent with previous studies of artificial droplet studies. In contrast, bulk rainwater (0.5 mL) or large droplets showed no detectable oxidants. Importantly, this study focuses not on H₂O₂ sensing but on revealing natural phenomena and their regulating factors on leaves, as probed by miniaturized sensors and optical kits. These findings reveal previously hidden interfacial chemistry, demonstrating that tiny rain droplets can act not only as cleansing agents but also as natural oxidants, potentially providing protection against microbial contamination and localized biocorrosion, although further studies on natural rain droplets are needed. The results further suggest that similar processes may occur in aerosols, dew, and fog, highlighting the need for further investigation of spontaneous reactive oxygen species formation in natural droplets.