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A simple graphene quantum dots (GQDs)-based fluorescence sensing system was developed for rapid, sensitive and low-cost detection of ascorbic acid (AA) in the presence of copper ions. Due to the efficient electron-transfer between GQDs and Cu 2+ ions, the fluorescence of GQDs was greatly quenched by Cu 2+ ions. Upon addition of ascorbic acid, Cu 2+ was reduced to Cu + and the interaction of Cu 2+ -GQDs was destroyed, which inhibit the quenching and thus leading to fluorescence recovery of GQDs. The detection process is ultrafast and can be completed within 5 min. The linear response range of AA was obtained from 0.3 μM to 10 μM. The detection limit was as low as 94 nM, which was comparable to or even better than other reported assays. In addition, this sensing system also showed good selectivity for detecting AA in the presence of different physiological molecules and inorganic ions. Finally, the proposed sensing system was successfully applied to assay AA in real samples with satisfactory results. [ABSTRACT FROM AUTHOR]