A Novel Fluorescence Spectroscopy Approach to Characterization of Interaction Between Humic Substances and Pyrene: Determination of Environmental Polarity

Abstract

This study proposes a novel fluorescence spectroscopy approach that opens up additional possibilities for using pyrene as a polarity probe. In contrast to the standard approach that is based on measuring the ratio of pyrene vibronic peak intensities, our approach uses the ratio of pyrene integral fluorescence intensity upon excitation into vibronically coupled first and second singlet states: transition probabilities from the ground to those excited electronic states demonstrate opposite trends with environmental polarity variations. In our work, the existence of this effect has been demonstrated for aqueous solutions of pyrene with varying concentrations of humic substances. This provides independent evidence of the non-zero fluorescence quantum yield of pyrene molecules bound by humic substances and indicates their lower environmental polarity. Moreover, the obtained results lay the groundwork for development of a reliable approach to quantitative characterization of pyrene interaction with humic substances.