Nitrogen and sulfur doped graphene quantum dots as a fluorometric paper-based sensor for highly selective and sensitive detection of mercury ions in aqueous samples

Mercury ions (Hg²⁺) are toxic contaminants in the food industry and environment, posing severe health risks, including Minamata disease. Rapid and reliable on-site detection methods for Hg²⁺ in food and environmental samples are crucial. This study presents nitrogen and sulfur doped graphene quantum dots (N, S-GQDs) as fluorescent sensors for the detection of Hg²⁺. The N, S-GQDs were synthesized in only 5 min from citric acid and L-cysteine through rapid heating. The identification of carboxyl, hydroxyl, thiol, and amine groups, along with specific elemental compositions, was achieved through Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) techniques. These analyses verified the successful doping of nitrogen and sulfur into the GQD framework and highlighted their strong interaction capabilities with Hg²⁺ ions. The paper-based sensor exhibits high selectivity and sensitivity, with a linear range of 0.1–10.0 μg/L and a limit of detection of 0.048 μg/L, yielding strong fluorescence responses to Hg²⁺. Testing in real water and food samples further validates its effectiveness, yielding results consistent with those obtained using inductively coupled plasma optical emission spectroscopy (ICP-OES) validation. Furthermore, the sensor demonstrated high selectivity for Hg²⁺ over twelve metal cations and six anions. These paper-based sensors, modified with polycyclic aromatic hydrocarbons (PAHs), are cost-effective, biodegradable, biocompatible, and environmentally friendly due to the abundance of paper material. PAHs reduce the paper's pore size and facilitate π-π stacking interactions between the hexagonal rings of PAHs and the graphene sheet of N, S-GQDs. This improves particle dispersion, minimizes aggregation, and helps maintain the fluorescence stability of N, S-GQDs. Therefore, it is highly promising for practical trace Hg²⁺ analysis in water and food.