<p>Heavy metal ion pollutants pose serious threats to human health and the environment due to their toxicity. An intuitive and convenient technique for detecting heavy metal ions is vital for qualitative monitoring and food safety. In this work, carbon dots (L-CDs) are prepared from a green source (lemon leaves) using a hydrothermal process, yielding a quantum of approximately 12.4%. These L-CDs exhibited excellent selectivity and sensitivity towards Hg²⁺ ions, with a limit of detection (LOD) of 0.23 nM and a wide linear range of 0–8 nM. This detection system may follow a dynamic quenching mechanism, characterized by fluorescent lifetime decay and zeta potential analysis. In addition, it was applied to detect Hg²⁺ ions in real water samples (coal water and industrial water), with recovery rates ranging from 98 to 102%, and relative standard deviations were 0.2–0.6%. Furthermore, the low cytotoxicity and small particle size of the L-CDs also make them suitable for use in antioxidant activity as well as HeLa cell imaging. Overall, this research represents a promising candidate in the field of environmental monitoring, food safety, and biomedical research. </p>