Ratiometric fluorescent sensing of phosphate ion in environmental water samples using flavin...

Phosphate ion (PO₄^3-) serves as an important nutrient carrier to support the growth of aquatic animals and plants in aquatic systems. However, excess concentrations of PO₄^3- are the key factor responsible for eutrophication, resulting in rapid deterioration of water quality. Therefore, accurate determination of PO₄^3- is of great significance in water quality and security. In this study, flavin mononucleotide (FMN), an intracellular form of vitamin B₂, was used as fluorophore. A novel "off-on" fluorescent sensing platform (FMN@Fe₃O₄) was fabricated for selective and sensitive detection of PO₄^3-, and showed excellent fluorescence response and good selectivity for PO₄^3- detection. With the addition of PO₄^3-, the fluorescence intensity restored is proportional to PO₄^3- concentration in the quantification range of 50 nM-0.75 μM with a limit of detection as low as 20 nM (0.62 μg.L^-1, calculated by P element). An adsorption/desorption sensing mechanism via an in-depth analysis of the interfacial interaction between PO₄^3- and FMN@Fe₃O₄ is proposed. FMN is first adsorbed by its terminal phosphate group on Fe₃O₄ particles to quench fluorescence. Free PO₄^3- replaces the adsorbed FMN and restores the quenched fluorescence to achieve the aim of PO₄^3- detection. In addition, this sensing system has been successfully validated in real water sample analysis and all reagents involved are nontoxic, environmentally benign, and easily-available. Therefore, this assay has great applicability in water quality monitoring.