Deciphering the Kinetics of Spontaneous Generation of H 2 O 2 in Individual Water Microdroplets.

Spontaneous generation of H 2 O 2 in sub-10 μm-sized water microdroplets has received increasing interest since its first discovery in 2019. On the other hand, due to the short lifetime of these microdroplets (rapid evaporation) and lack of suitable tools to real-time monitor the generation of H 2 O 2 in individual microdroplets, such a seemingly thermodynamically unfavorable process has also raised vigorous debates on the origin of H 2 O 2 and the underlying mechanism. Herein, we prepared water microdroplets with a long lifetime (>1 h) by virtue of microwell confinement and dynamically monitored the spontaneous generation of H 2 O 2 in individual microdroplets via time-lapsed fluorescence imaging. It was unveiled that H 2 O 2 was continuously generated in the as-prepared water microdroplets and an apparent equilibrium concentration of ∼3 μM of H 2 O 2 in the presence of a H 2 O 2 -consuming reaction can be obtained. Through engineering the geometry of these microdroplets, we further revealed that the generation rates of H 2 O 2 in individual microdroplets were positively proportional to their surface-to-volume ratios. This also allowed us to extract a maximal H 2 O 2 generation rate of 7.7 nmol m -2 min -1 in the presence of a H 2 O 2 -consuming reaction and derive the corresponding probability of spontaneous conversion of interfacial H 2 O into H 2 O 2 for the first time, that is, ∼1 of 65,000 water molecules in 1 s. These findings delivered strong evidence that the spontaneous generation of H 2 O 2 indeed occurs at the surface of microdroplets and provided us with an important starting point to further enhance the yield of H 2 O 2 in water microdroplets for future applications.