A Low-cost Fluorescent Sensor Based on N, S-CQDs for the Ratiometric Detection of Azithromycin.

This study presents a novel ratiometric fluorescent sensor based on nitrogen and sulfur co‑doped carbon quantum dots (N, S‑CQDs) for detection of azithromycin (AZM), a widely used veterinary macrolide antibiotic whose residues in dairy products and aquatic environments pose potential health risks. The N, S‑CQDs were synthesized in a one‑step hydrothermal process using 2,5‑diaminobenzenesulfonic acid as the sole precursor, affording a high quantum yield of 39.7% along with excellent water solubility and fluorescence stability. Mechanistic investigations revealed that AZM modulates the microenvironmental pH, thereby shifting the protonation–deprotonation equilibrium of the N, S‑CQDs and selectively enhancing fluorescence at 445 nm. Concurrently, electrostatic attraction and hydrogen‑bonding interactions between AZM and the N, S‑CQDs induce fluorescence quenching at 550 nm. Based on this dual‑ signal response, a ratiometric fluorescence sensor was constructed. This sensor exhibited high selectivity and sensitivity towards AZM, with a detection limit of 27.7 nM and a quantification limit of 92.4 nM. It was successfully applied to complex real samples. Unlike conventional ratiometric probes that require two fluorescent materials, the present system achieves ratiometric detection using only a single fluorescent material, thereby simplifying probe design and reducing detection costs. This work not only presents a promising sensing platform for the detection of AZM, but also broadens the design strategy for single‑component fluorescent sensor, thus highlighting the wider potential of CQDs in sensing applications.