pH-Responsive Fluorescence EEM to Titrate the Interaction between Fluorophores and Acid/Base Groups in Water-Soluble Organic Compounds of PM<sub>2.5</sub>
Yuanyuan Qin, Yanrong Yang, Juanjuan Qin, Leiming Zhang, Songjun Guo, Xueming Zhou, Rongzhi Chen, Jihua Tan, Kang Xiao, Xinming Wang
Abstract
This study demonstrates a new approach to examine the chemical interaction between fluorophores and acid/base functional groups in water-soluble organic compounds (WSOC) of particulate matter (PM2.5) by systematically investigating the synchronous variation of fluorescence spectral characteristics with pKa during pH titration. It was found that pH had an important impact on the excitation–emission matrix (EEM) fluorescence characteristics of WSOC. The peak intensity, average apparent quantum yield, and Stokes shift changed markedly at pH of ∼5 and 8, indicating carboxylic and phenolic or amino groups were possibly bound to the π-conjugated fluorophores as substituents. In contrast to the winter WSOC, the summer WSOC exhibited overall lower fluorescence intensity but much higher susceptibility to pH change, suggesting that the π-conjugated systems of fluorophore cores were more thoroughly intervened by the acid/base substituents, possibly due to secondary formation/reactions of aerosols in summer. Since fluorescence involves light adsorption, relaxation, and re-emission, the pH-responsive features of EEM fluorescence may have profound impacts on atmospheric photochemical processes, particularly in pH-specific scenarios. The fluorescence indices examined in this study may usefully serve as indicators of molecular functionality, reaction traces, and source characteristics in future PM2.5 studies.