Spontaneous generation of hydrogen peroxide from aqueous microdroplets

We show H(2)O(2) is spontaneously produced from pure water by atomizing bulk water into microdroplets (1 μm to 20 µm in diameter). Production of H(2)O(2), as assayed by H(2)O(2)-sensitve fluorescence dye peroxyfluor-1, increased with decreasing microdroplet size. Cleavage of 4-carboxyphenylboronic acid and conversion of phenylboronic acid to phenols in microdroplets further confirmed the generation of H(2)O(2). The generated H(2)O(2) concentration was ∼30 µM (∼1 part per million) as determined by titration with potassium titanium oxalate. Changing the spray gas to O(2) or bubbling O(2) decreased the yield of H(2)O(2) in microdroplets, indicating that pure water microdroplets directly generate H(2)O(2) without help from O(2) either in air surrounding the droplet or dissolved in water. We consider various possible mechanisms for H(2)O(2) formation and report a number of different experiments exploring this issue. We suggest that hydroxyl radical (OH) recombination is the most likely source, in which OH is generated by loss of an electron from OH(−) at or near the surface of the water microdroplet. This catalyst-free and voltage-free H(2)O(2) production method provides innovative opportunities for green production of hydrogen peroxide.