Xylan-Derived N,S-Codoped Carbon Dots for Hg2+ Detection and Reusable Paper-Based Sensing

The development of renewable fluorescent nanomaterials for heavy-metal detection provides an opportunity to integrate biomass valorization with functional sensing. Herein, N,S-codoped carbon quantum dots (N,S-CQDs) were prepared by one-step hydrothermal co-carbonization of bagasse-derived xylan, a renewable and hydroxyl-rich hemicellulose selected as the carbon precursor, and L-cysteine. The quasi-spherical N,S-CQDs had a mean diameter of 3.25 ± 0.04 nm and exhibited locally ordered carbon domains and abundant N-, S-, and O-containing surface functionalities. They exhibited excitation-dependent blue fluorescence with optimal excitation/emission wavelengths of 330/420 nm and a relative fluorescence quantum yield of 67.25%. Among the tested metal ions, Hg2+ caused the strongest fluorescence quenching, with a concentration-dependent response over 0–1.0 μM and a detection limit of 4.2 nM. Spectroscopic changes and fluorescence recovery after Ethylenediaminetetraacetic acid (EDTA) treatment were consistent with reversible interactions between Hg2+ and surface functional groups. Immobilization on filter paper yielded a visually responsive Hg2+ sensor with a fluorescence-based detection limit of 6.23 μM and reversible quenching-recovery behavior during repeated Hg2+/EDTA treatments. This work integrates renewable xylan valorization, heteroatom-doped carbon dot synthesis, and Hg2+-responsive fluorescence, broadening the functional utilization of hemicellulosic resources in portable heavy-metal sensing and providing a design strategy for biomass-derived fluorescent carbon materials for environmental detection.