The Air–Water Interface of Water Microdroplets\nFormed by Ultrasonication or Condensation Does Not Produce H<sub>2</sub>O<sub>2</sub>

Recent reports on\nthe production of hydrogen peroxide (H<sub>2</sub>O<sub>2</sub>) on\nthe surface of condensed water microdroplets without\nthe addition of catalysts or additives have sparked significant interest.\nThe underlying mechanism is thought to be ultrahigh electric fields\nat the air–water interface; smaller droplets present larger\ninterfacial areas and produce higher (detectable) H<sub>2</sub>O<sub>2</sub> yields. To gain insights into this phenomenon, we performed\ncondensation experiments and quantified H<sub>2</sub>O<sub>2</sub> formation as a function of the vapor source. Specifically, we compared\nthe H<sub>2</sub>O<sub>2</sub> concentration in water microdroplets\ncondensed from the vapor realized via (i) heating water in the range\nof 50–70 °C and (ii) ultrasonic humidification (as exploited\nin the original report). Experimental results revealed that the H<sub>2</sub>O<sub>2</sub> level inside water microdroplets condensed via\nheating water was below our detection limit (≥0.25 μM),\nregardless of the droplet size or the substrate wettability. In contrast,\nwater droplets condensed via ultrasonic humidification contained significantly\nhigher (∼1 μM) H<sub>2</sub>O<sub>2</sub> concentrations.\nWe conclude that the ultrasonic humidifiers contribute to H<sub>2</sub>O<sub>2</sub> production, not droplet interfacial effects.