Copper and Nitrogen Co-doped Fluorescent Carbon Quantum Dots as a Fluorescent ''turn-off" Sensor for Determination of Tetracyclines Antibiotics.

Copper and nitrogen co-doped fluorescent carbon quantum dots (Cu-NCDs) were synthesized via a one-pot hydrothermal synthesis method using L-tyrosine and copper chloride as precursors. The as-prepared Cu-NCDs exhibited bright blue fluorescence (emission peak at 427 nm) with high photostability (10.5% intensity loss after 1 h irradiation at 365 nm). Upon introduction of tetracyclines antibiotics (TCs), include oxytetracycline (OTC), chlortetracycline (CTC) and tetracycline (TC), the fluorescence intensity of Cu-NCDs was selectively quenched through fluorescence resonance energy transfer (FRET), as confirmed by time-resolved fluorescence decay analysis. This phenomenon established a linear relationship between fluorescence quenching efficiency and TCs concentration. The calculated limits of detection (S/N = 3) for three target analytes were determined to be 0.24 µM (OTC), 0.56 µM (CTC), and 0.19 µM (TC), respectively. Accurate quantification of TCs residues in tap water and dairy products demonstrated recovery rates of 97.94 - 112.20% (OTC), 94.93 - 110.00% (CTC), and 97.58 - 107.67% (TC), with relative standard deviations (RSDs) below 4.16% (n = 3). Based on the paper sensor platform, on-site detection of TCs has been achieved, and visual semi quantitative analysis can be completed within 1 min.