On-site detection of tobramycin using low-cost and ultrasensitive ratiometric fluorescence sensing platforms based on S,N-co-doped dual emission carbon quantum dots.

BACKGROUND: Tobramycin (TOB) is widely used in livestock and aquaculture, but its excessive use leads to residues in animal-derived foods and the environment, threatening human health. Rapid on-site detection methods are urgently needed to ensure food safety and environmental surveillance.

RESULTS: We synthesized sulfur and nitrogen co-doped dual-emission carbon quantum dots (S,N-CQDs) with emission at 340 nm (carbon core) and 620 nm (surface state). A polyadenine-extended TOB aptamer selectively quenched the 620 nm emission via electrostatic interaction-induced aggregation, while the 340 nm emission served as stable internal reference. Upon TOB addition, specific aptamer-target binding restored the 620 nm fluorescence, enabling ratiometric detection. Smartphone based dual and single-light-source portable platforms were further developed for visual on-site detection via RGB analysis. The three methods exhibited linear ranges of 10.0 nM-1.0 μM, detection limits of 2.14, 8.02, and 4.87 nM, and recoveries of 95.2%-99.4% in spiked milk and river water.

SIGNIFICANCE: This work presents an integrated "detection-visualization" strategy combining a ratiometric fluorescence aptasensor with smartphone-based portable platforms. The method offers several distinct advantages: (1) the ratiometric design with built-in self-calibration ensures reliable detection in complex matrices; (2) the portable platforms enable rapid, user-friendly, and equipment-free on-site analysis; (3) the entire sensing system is cost-effective and does not require sophisticated instrumentation. These features make our approach a promising tool for routine monitoring of antibiotic residues in food safety and environmental protection.