Pitfalls in Probing Singlet Oxygen by Electron Paramagnetic Resonance Spectroscopy in Engineered Environmental Systems
Yang Zong, Long Chen, Jing-Hang Wu, Fuqiang Liu, Xiaohong Guan, Zhendong Lei, Jun Xu, Deli Wu, Wen Liu, Han-Qing Yu
Abstract
Sterically hindered amine (SHA)-based electron paramagnetic resonance spectroscopy (EPR) is still widely used to detect singlet oxygen ( 1 O 2 ) in engineered environmental systems. Nevertheless, the reliability and interpretability of EPR are limited by the insufficient knowledge of the mechanism of SHA transformation to the nitroxide radical (the 1 O 2 indicator). Here, we systematically investigate the pitfalls and limitations of EPR in detecting 1 O 2 . We find that multiple non- 1 O 2 species, including sulfate radical (SO 4 •– ), hydroxyl radical (HO • ), iodate radical (IO 3 • ), high-valent iron, direct electron transfer (DET), etc., could drive the SHA-to-nitroxide radical transformation, hence severely interfering with 1 O 2 detection. The nitroxide radicals are generated via two distinct patterns dependent on the steric hindrance of non- 1 O 2 species. One is SO 4 •– /IO 3 • /DET-driven single electron transfer with H 2 O and ground-state O 2 participation. The other is HO • /high-valent iron-mediated hydrogen atom abstraction and radical coupling/oxygen-rebounding. Unexpectedly, pH, rather than 1 O 2, governs the EPR results by modulating the SHA probe distributions and controlling the deprotonation processes, further contributing to the invalidity of EPR in probing 1 O 2 under environmental-relevant conditions. This study uncovers the fundamental flaws of EPR for 1 O 2 detection, enabling a more accurate interpretation of EPR data for 1 O 2 identification.