Rapid Colorimetric Detection of Inorganic Phosphate in Eutrophic Waters via Proton-Coupled Electron Transfer in Quinhydrone

Excess inorganic phosphate is a leading cause of eutrophication in aquatic ecosystems, contributing to harmful algal blooms, oxygen depletion, and an overall decline in the water quality. Accurate determination of the phosphate is essential for effective water quality monitoring and environmental management. Traditional methods involve multiple reagents, incubation steps, and long assay times, making them labor-intensive and impractical for large-scale monitoring. Here, we report a simple spectroscopic assay for determining phosphate in eutrophic waters using the unique reactivity and fast proton-coupled electron transfer (PCET) kinetics of quinhydrone (QH) for phosphate. The detection mechanism, supported by density functional theory calculations, involves PCET and hydrogen bonding between QH and dihydrogen phosphate, forming a stable hydrogen-bonded complex (QH-H₂PO₄^-), triggering π → π* transitions and an immediate color change from light yellow to dark brown. The method achieves a low limit of detection (374 nM), a broad linear range (1-350 μM), and selectivity over common interferences, with a total analysis time of 30 s. When applied to phosphate measurements in eutrophic lake water, it demonstrates robust performance in excellent agreement with standard approaches while offering superior speed and simplicity. Requiring only one reagent, no incubation steps, and operating efficiently across a broad pH range (4-9), the method holds promise as an easy-to-use tool for the rapid monitoring of phosphate and nutrient pollution in water systems.