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Nitrogen and sulphur co-doped carbon quantum dots (N,S-CQDs) were obtained using a facile hydrothermal approach from a mixture of o-phenylenediamine and methionine. As developed N,S-CQDs displayed blue fluorescence, with a quantum yield of 21%. The characterization of N,S-CQDs was completed with UV-vis., FT-IR, PL, XRD, Raman, DLS, XPS, TEM, and electrochemical examinations. In this paper, we first illustrate the development of N,S-CQDs as a fluorescent sensor for detecting heavy metal ion Hg(II) in the linear range of 0–18 μM with a detection limit of 120 nM. The PL quenching of N,S-CQDs was primarily ascribed to the photoinduced electron transfer (PET) between N,S-CQDs and Hg(II) and dynamic quenching process. Secondly, we have developed N,S-CQD-modified platinum electrode (PtE/N,S-CQD) to serve as an electrochemical sensor for the efficient and selective recognition of Hg(II) ions with a wide linearity range of 10–60 nM and a low limit of detection of 2 nM. Both the developed sensors exhibit excellent recovery results for Hg(II) in drinking water and milk samples. Additionally, we have used the agar well diffusion method to examine the bactericidal efficacy of N,S-CQDs against both Gram-negative and Gram-positive bacterial strains.

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