Converting waste Chrysanthemi Flos residues into high-value fluorescent carbon dots for rapid and selective detection of mercury (II) ions in aqueous environments

Chrysanthemi Flos (CF) is widely used as an edible and medicinal crop, resulting in large amounts of residue production annually. The discarded CF residues (CFR) cause environmental pollution and wastage of resources. Converting CFR into functional carbon dots (CDs) is an important strategy to achieve its “waste-to-wealth” goal. Here, a CFR-derived CDs (CFR-CDs) based fluorescent sensor was successfully developed for the rapid and selective detection of mercury (Ⅱ) ions in aqueous environments, and its sensing mechanism for detecting Hg²⁺ was elucidated. First, the CFR was used as a precursor to extract fluorescent CDs via a simple and green hydrothermal method. The prepared CFR-CDs possessed a typical graphite-like structure with an average size of 2.23 nm, and exhibited good hydrophilicity, excellent photostability, and excitation-dependence fluorescence behavior, while emitting blue fluorescence. Interestingly, the CFR-CDs were highly selective towards Hg²⁺, and their fluorescence was rapidly quenched by Hg²⁺ (within 1 min). The prepared CFR-CDs were then used as a fluorescent sensor for efficient detection of Hg²⁺ in the range of 0.3–10 μmol/L with a low limit of detection (LOD) of 0.129 μmol/L. The recovery of the Hg²⁺ in actual water environments ranged between 98.60 % and 104.00 % with a relative standard deviation < 5.0 %, demonstrating the reliability of the developed sensor. Additionally, the detection mechanism was revealed to be the synergistic action of the static quenching effect (SQE), inner filter effect (IFE) and photo-induced electron transfer (PET), generating CFR-CDs/Hg²⁺ complexes through the formation of surface chemical bonds and promoting the rapid shutdown of the sensor signal. This study demonstrates, for the first time, the application of CFR towards producing green and low-cost CDs as promising fluorescent sensor for the detection of Hg²⁺ in actual environmental samples, and is therefore helpful for improving the utilization efficiency of CFR and promoting the realization of carbon peak and carbon neutrality goals.