pH-Responsive Fluorescence EEM to Titrate the Interaction between Fluorophores and Acid/Base Groups in Water-Soluble Organic Compounds of PM2.5

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 pK(a) 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 similar to 5 and 8, indicating carboxylic and phenolic or amino groups were possibly bound to the pi-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 pi-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.

Automated summary

This study investigates the chemical interaction between fluorophores and acid/base functional groups in water-soluble organic compounds in particulate matter, and found that pH significantly affects the fluorescence characteristics of WSOC. Summer WSOC showed higher susceptibility to pH change compared to winter WSOC, possibly due to secondary formation/reactions of aerosols in summer. The findings suggest that the pH-responsive features of EEM fluorescence may have profound impacts on atmospheric photochemical processes.