Nitrogen-Doped Graphene Quantum Dots as “Off–On” Fluorescent Probes in Paper-Based Test Kits for Selective Monitoring of Cyanide in Food

We successfully developed a fluorometric paper-based test kit for the selective and sensitive determination of cyanide using nitrogen-doped graphene quantum dots (N-GQDs) as the fluorescent probe. Citric acid and tris(hydroxymethyl)aminomethane were precursors for the one-step synthesis of N-GQDs via in situ hydrothermal methods, providing a high quantum yield of 57.9%. The proposed mechanism uses a fluorescence turn-on approach. Specifically, the fluorescence of N-GQDs is quenched by the incorporation of Ag+ via a photoinduced electron transfer (PET). During the detection step, sulfuric acid converts cyanide (CN–) into hydrogen cyanide (HCN). The Ag+ species on the N-GQD surface then react with the evolved HCN via a coordination bond to form a silver cyanide complex, resulting in the fluorescence emission of the N-GQDs being turned back on. As a result, the fluorescence emission intensity of N-GQDs linearly increased with increasing CN– concentrations in the range of 0.5–25 mg L–1, with a limit of detection (LOD) of 0.08 mg L–1. Notably, the developed sensor has advantages in terms of simplicity, rapidity, low cost, and high selectivity toward CN–. The analytical performance of the test kit was also validated the performance of the test kit against a conventional precipitation titration method.