Graphene quantum dots as fluorescence probes for turn-off sensing of melamine in the presence of Hg(2+).

A rapid and sensitive fluorescence sensing system for melamine based on charge transfer quenching of the fluorescence of graphene quantum dots (GQDs) in the presence of Hg(2+) is proposed. The synthesized GQDs were strongly luminescent with predominantly aromatic sp(2) domains. Melamine could coordinate with mercury through nitrogen atoms in both its amine and triazine groups and bring more Hg(2+) to the surface of GQDs through π-π stacking, thus leading to quenching of the GQDs' fluorescence. The quenching mechanism was investigated in detail and ascribed to charge transfer from the GQDs to Hg(2+) with melamine acting as the linkage agent. The melamine demonstrated a linear range 0.15-20 µM and a detection limit of 0.12 µM, which was far below the regulatory level, suggesting the promising practical usage of this sensing system. This sensing system also possessed high selectivity for melamine in the presence of possible interferences. Finally, this novel sensor was successfully applied for melamine detection in raw milk and satisfactory recovery was achieved.