Ratiometric fluorescent sensing of phosphate ion in environmental water samples using flavin mononucleotide-functionalized Fe3O4 particles

Phosphate ion (PO43-) serves as an important nutrient carrier to support the growth of aquatic animals and plants in aquatic systems. However, excess concentrations of PO43- are the key factor responsible for eutrophication, resulting in rapid deterioration of water quality. Therefore, accurate determination of PO43- 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 PO43-, and showed excellent fluorescence response and good selectivity for PO43- detection. With the addition of PO43-, the fluorescence intensity restored is proportional to PO43- 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 PO43- and FMN@Fe3O4 is proposed. FMN is first adsorbed by its terminal phosphate group on Fe3O4 particles to quench fluorescence. Free PO43- replaces the adsorbed FMN and restores the quenched fluorescence to achieve the aim of PO43- 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.