‘Off–On’ determination of lead (Pb²⁺) and fluoride (F⁻) ion in fish and wastewater samples using N, S co-doped carbon quantum dots (N, S-CQDs)

Lead is a global priority pollutant. Its presence in aquatic systems is harmful to the human health. Fluoride is essential to the human body, especially in dental health. However, excess fluoride in the body can lead to serious health concerns. Therefore, a simple approach to monitoring lead and fluoride in environmental samples is paramount. In this study, we synthesized N and S co-doped carbon quantum dots under the hydrothermal method by employing citric acid, glutamine, and sodium sulphide (Na₂S) as precursors. Characterization of the developed nanosensor was carried out using Fourier transform infrared spectroscopy (FTIR), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL) spectrophotometer, ultraviolet–visible spectroscopy (UV–Vis), and X-ray diffraction (XRD). The as-prepared nanosensor is spherical with an average particle diameter of 3.45 ± 0.86 nm and emits light in the green region of the spectrum. This material was employed as an ‘on–off’ and ‘off–on’ fluorescent sensor to determine Pb²⁺ and F⁻ rapidly and selectively. The fluorescence was quenched (turned off) in the presence of Pb²⁺ because of the strong interaction between Pb²⁺ ions and the surface functional groups of the as-synthesized material. Subsequently, the quenched fluorescence of the N, S-CQDs + Pb²⁺ system was restored (turned on) upon the introduction of F⁻ ions, owing to the formation of ionic bonds between Pb²⁺ and F⁻. The N S-CQDs were selective towards Pb²⁺. At the same time, the N, S-CQDs + Pb²⁺ system exhibited selectivity towards F⁻ ions amidst other ions with low detection limits (LODs) of 13.35 nM and 43.17 nM for Pb²⁺ and F⁻, respectively. The dynamic quenching mechanism was suggested based on the absorption spectra and lifetime results. Satisfied recoveries of 89.30–116.40% for Pb²⁺ and 90.22–115.05% for F- (RSD < 5) were obtained in practical samples of wastewater and fish. We believe that the as-synthesized N S-CQDs can effectively serve as reliable, accurate, and swift nanosensor for detecting Pb²⁺ and F⁻ in environmental samples.