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Since inorganic phosphate ion (PO 4 3−) plays very important roles in environment or living organisms, developing a selective and sensitive detection method for PO 4 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 (Mo 7 O 24 6--mediated N-GQDs) as a fluorescence "off-on" probe for PO 4 3− via "anion ion-mediated" strategy. The N-GQDs was firstly modified with Mo 7 O 24 6− through a complex bonding system containing ionic and hydrogen bonds. The formation of N-GQDs/Mo 7 O 24 6− complex leaded to photoluminescence (PL) quenching of N-GQDs. In the presence of PO 4 3−, strong affinity between PO 4 3− and Mo 7 O 24 6− produced ammonium phosphomolybdate, which destroyed the pre-formed N-GQDs/Mo 7 O 24 6− structure and detached Mo 7 O 24 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 PO 4 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 PO 4 3− in natural water, biological fluid, and living cells. A negatively charged molybdate mediated nitrogen-doped graphene quantum dots (Mo 7 O 24 6--mediated N-GQDs) as a fluorescent off-on probe for PO 4 3− has been developed and successfully utilized in real samples and live cells imaging. Image 1 • A fluorescence "turn on" N-doped graphene quantum dots nanocomposite for inorganic phosphate ion. • The nanocomposite working in "anion ion-mediated" principle. • The probe exhibiting the excellent PO 4 3− detection performance in biological fluids and living cells. • Presenting the possible detection mechanism. [ABSTRACT FROM AUTHOR]