Nitrogen and sulfur co-doped carbon dots derived from granatums and ammonium persulfate to detect tetracyclines in milk

Abstract

Highly photoluminescent nitrogen and sulfur co-doped carbon dots (N,S-CDs) derived from the granatum as carbon source and ammonium persulfate as passivator was developed as a probe for the detection of tetracyclines (DOC, TC, OTC, and CTC) in milk products. Transmission electron microscopy (TEM) exhibited that the as-prepared N,S-CDs were quasi-spherical particles with an average diameter of ∼5.5 nm. X-ray diffraction (XRD) showed that the N,S-CDs possessed a graphitic-like structure. Fourier-transform infrared (FT-IR) and X-ray photoelectron spectroscopy (XPS) confirmed the existence of -OH, -COOH, R-SO3H and -NH2 on the surface of N,S-CDs. The amidation reaction between the -COOH of N,S-CDs and -CO-NH2 of TCs combined with the inner-filter effect resulted in the fluorescence (FL) quenching. The FL quenching efficiencies reached 98.5%, 51.6%, 46.5%, and 39.4% after introducing DOC, TC, OTC, and CTC. Highly fluoresce quenched for DOC was ascribed to the existence of H on C6 of DOC induced weak electrostatic repulsion between N,S-CDs and DOC. The nanosensor allowed the detecting TCs in the range of 0.08∼3.05 µmol L-1 for DOC with the detection limit of 2.87 nmol L-1, 0.32∼5.85 µmol L-1 for TC with 14.57 nmol L-1, 0.31∼6.29 µmol L-1 for OTC with 16.99 nmol L-1, and 0.30∼6.06 µmol L-1 for CTC with 17.11 nmol L-1 (S/N=3). Recoveries of 83.93∼126.22% and RSDs of 1.11%∼5.83% were achieved for TCs detection in milk product samples, indicating the optical sensor provided an alternative strategy for real applications in food safety control.