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

Singlet oxygen ( 1 O 2 ) is a classical nonradical reactive oxygen species (ROS), which is of great significance in both natural and engineering environments. Here, we found that 1 O 2 can be generated spontaneously at the air–water interface of microdroplets under dark conditions. 18 O isotope labeled O 2 ( 18 O 2 ) and water (H 2 18 O) were used to identify the source and formation pathway of 1 O 2 and previously reported ROS produced in microdroplets. The results showed that 1 O 2 and superoxide anions (•O 2 – ) were sourced from O 2, •OH was sourced from H 2 O, while H 2 O 2 can be produced from both H 2 O and O 2 . Moreover, the excited hydrated electron (e – (aq)) induced indirect energy transfer is responsible for exciting O 2 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.