DMF-mediated synthesis of N,S-co-doped carbon quantum dots and their fluorescence quenching mechanism toward Hg ²⁺ <i>via</i> surface complexation

Nitrogen and sulfur co-doped carbon quantum dots (N,S-CQDs) were synthesized <i>via</i> a green, one-step hydrothermal method using waste lemon peel as the carbon source and glutathione (GSH) as the N,S dopant in a dimethylformamide (DMF) medium. The reaction was conducted at 180 °C for 10 hours to achieve efficient synthesis. The resulting N,S-CQDs showed excellent water dispersibility, strong photostability, and bright fluorescence, with optimal excitation and emission wavelengths of 340 nm and 420 nm, respectively. High-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT-IR) confirmed their uniform morphology and successful incorporation of nitrogen and sulfur heteroatoms. Notably, the fluorescence of the N,S-CQDs could be rapidly and selectively quenched by Hg²⁺ within 10 minutes at room temperature, without requiring any additional surface modification of the CQDs or auxiliary reagents in the sensing procedure. Under optimized conditions, the sensor exhibited a linear fluorescence quenching response to Hg²⁺ over the concentration range of 17.6 nM to 20 µM (<i>R</i> ² = 0.9899), with a low detection limit of 17.6 nM. Mechanistic studies suggested that the quenching primarily resulted from the formation of nonfluorescent complexes between Hg²⁺ and functional groups on the CQD surface. The developed sensor was successfully applied to detect trace levels of Hg²⁺ in real lake water samples collected near textile manufacturing sites, demonstrating its potential as a sustainable, rapid, and cost-effective tool for practical environmental mercury monitoring.