Negatively charged molybdate mediated nitrogen-doped graphene quantum dots as a fluorescence turn on probe for phosphate ion in aqueous media and living cells

Since inorganic phosphate ion (PO₄ ³⁻) plays very important roles in environment or living organisms, developing a selective and sensitive detection method for PO₄ ³⁻ is highly desired. Owing to the superior optical properties, graphene quantum dots (GQDs) have been developed as a promising emitting material in fluorescence analysis. Herein, we reported the first example of negatively charged molybdate-mediated nitrogen doped graphene quantum dots (Mo₇O₂₄ ^6--mediated N-GQDs) as a fluorescence “off-on” probe for PO₄ ³⁻ via “anion ion-mediated” strategy. The N-GQDs was firstly modified with Mo₇O₂₄ ⁶⁻ through a complex bonding system containing ionic and hydrogen bonds. The formation of N-GQDs/Mo₇O₂₄ ⁶⁻ complex leaded to photoluminescence (PL) quenching of N-GQDs. In the presence of PO₄ ³⁻, strong affinity between PO₄ ³⁻ and Mo₇O₂₄ ⁶⁻ produced ammonium phosphomolybdate, which destroyed the pre-formed N-GQDs/Mo₇O₂₄ ⁶⁻ structure and detached Mo₇O₂₄ ⁶⁻ from N-GQDs surface. Thus, the PL of N-GQDS was in turn switched on. Under optimal conditions, this probe exhibited a good linear relationship between PL response and PO₄ ³⁻ concentration in the range from 7.0 to 30.0 μM with a limit of detection of 50 nM. Also this probe with high selectivity and sensitivity has been successfully used to sense PO₄ ³⁻ in natural water, biological fluid, and living cells.