Fluorescent Molecular Probes for Detection of One-Electron Oxidants Photochemically Generated by Dissolved Organic Matter

Abstract

We report a dual probe system based on 4'-substituted biphenyl-2-carboxylic acids (BPAs) for analysis of photooxidants generated by dissolved organic matter. The BPA probes are converted to the corresponding benzocoumarins (BZCs) at different rates depending on the mechanism of oxidation; thus, two probes used simultaneously can differentiate strong triplet excited state sensitizers from hydroxylating species such as hydroxyl radical (^•OH) present in dissolved organic matter (DOM). Comparison of the ratios of BZC-CH₃ and BZC-CF₃ product formation using model photooxidants such as NaNO₂, a ^•OH precursor, and model triplet sensitizer lumichrome gave a range of 2 to 250. Application of these probes to DOM isolates and whole natural waters afforded intermediate ratios. Although the oxidation potential of BPAs (>ca. 1.80 V SHE) is significantly higher than the estimated average reduction potential of typical ³CDOM* samples, these results have demonstrated the presence of a small pool of oxidants in the selected DOM isolates and whole water samples that is capable of oxidizing aromatic carboxylates. As an analytical tool, this probe pair can be used between pH 4-6 without affecting the product formation ratio and may find applications in various systems involving complex mixtures of photochemically produced oxidants of differing natures.