Multifunctional sulfur-doped carbon dots for tetracycline residue analysis in food and intracellular lysosomal pH tracking

The overuse of tetracycline (TC) antibiotics has raised significant environmental and health concerns, while traditional detection methods face challenges in real-time monitoring and intracellular analysis. In this study, sulfur-doped carbon dots (S-CDs) were synthesized through a one-step hydrothermal method, using biotin and o -phenylenediamine as precursors. The resulting S-CDs exhibit three key functional properties: (1) a highly sensitive fluorescent quenching response to TC, with a detection limit of 76.6 nM and a linear range of 0–200 μM, driven by specific molecular interactions (inner filter effect and static quenching); (2) pH-dependent fluorescence signaling (pH 2.0–8.0) attributed to protonation of surface acid-base groups; and (3) selective lysosomal targeting, with a Pearson correlation coefficient of 0.95, confirmed by confocal microscopy in HeLa cells and zebrafish models. The S-CDs demonstrated excellent recoveries (97.07–107.99 %) in real food samples (milk, beef, and pork) and exhibited negligible cytotoxicity. This work effectively integrates environmental pollutant detection with intracellular microenvironment analysis, providing a versatile tool for investigating TC-induced lysosomal dysfunction and advancing food safety monitoring and biomedical imaging.