Unexpected Generation of Singlet Oxygen at the Air-Water Interface of Aqueous Microdroplets

Singlet oxygen (¹O₂) is a classical nonradical reactive oxygen species (ROS), which is of great significance in both natural and engineering environments. Here, we found that ¹O₂ can be generated spontaneously at the air-water interface of microdroplets under dark conditions. ¹⁸O isotope labeled O₂ (¹⁸O₂) and water (H₂¹⁸O) were used to identify the source and formation pathway of ¹O₂ and previously reported ROS produced in microdroplets. The results showed that ¹O₂ and superoxide anions (•O₂^-) were sourced from O₂, •OH was sourced from H₂O, while H₂O₂ can be produced from both H₂O and O₂. Moreover, the excited hydrated electron (e^-(aq)) induced indirect energy transfer is responsible for exciting O₂ from the ground state to the singlet state. The findings of this work provide an important complement to the construction of a complete ROS chemistry in microdroplets.