Sensitive Detection\nof Hg<sup>2+</sup> and l‑Cysteine through Optical\nAsymmetry-Tuned Fluorescence Switch\nOff-On Behavior in N‑Doped Chiral Carbon Dot

Blue-emissive nitrogen-doped chiral carbon dots (d-NCD230\nand l-NCD230) exhibiting antipodal chiroptical activity,\nsynthesized from the thermal pyrolysis of citric acid and d/l-aspartic acid in 1:2 molar ratios, have been explored\nas chirality-based fluorescent turn-off/on probes for the detection\nof Hg<sup>2+</sup> and l-cysteine (l-Cys). Circular\ndichroism (CD) spectroscopy revealed that the chiroptical activity\noriginates from a synergy among intrinsic chirality, chiral precursors\non the NCD surface, and hybridization of lower energy levels within\nthe embedded chiral chromophore. Quantitative analysis of optical\nasymmetry using the Kuhn asymmetry factor (<i>g</i>) at\nthe CD signal of 312 nm showed a higher value for d-NCD230\n(1.03 × 10<sup>–4</sup>) compared to l-NCD230\n(1.13 × 10<sup>–5</sup>). Moreover, we have demonstrated\nchirality transfer and chiral inversion phenomena in d/l-NCDs by preparing carbon dots with different precursor ratios\nat different temperatures and probing them through CD spectroscopy.\nThe NCDs exhibited selective fluorescence quenching in the presence\nof Hg<sup>2+</sup>, demonstrating linearity in the Stern–Volmer\nplot. Limits of detection (LODs) for Hg<sup>2+</sup> were calculated\nto be 129 and 192 nM for d-NCD230 and l-NCD230,\nrespectively, in the 0–150 μM concentration range. The\nquenching mechanism involves nonradiative electron transfer due to\nHg<sup>2+</sup> binding to oxygen-rich functional groups on the d/l-NCD230 surface. The slight variation in LOD values\nbetween d-NCD230 and l-NCD230 indicates the negligible\neffect of the chirality on Hg<sup>2+</sup> sensing. Notably, the fluorescence\nintensity of d/l-NCD230 could be restored upon adding l-cysteine, with d-NCD230 showing a more pronounced\nenhancement than l-NCD230. This differential response is\nattributed to a preferential stereoselective interaction arising from\nthe homochirality of d-NCD230/Hg<sup>2+</sup> and l-cysteine. These findings demonstrate the potential of chiral nitrogen-doped\ncarbon dots as sensitive and selective probes for Hg<sup>2+</sup> and l-cysteine, with implications for environmental monitoring and\nbiological sensing applications.