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.