High fluorescence quantum yield of methionine-doped carbon quantum dots for achieving rapid assay of tetracyclines in foodstuffs

The trace-level detection of tetracyclines (TCs) in food products is essential to ensure food safety and public health. Herein, we prepared the methionine-doped carbon quantum dots (Met-CQDs) using citric acid as the precursor. Met-CQDs exhibited a Gaussian unimodal peak centered at 440 nm in the fluorescent excitation spectrum, along with a remarkable greenish-blue emission and a fluorescent quantum yield of 33.5 %. Furthermore, the presence of TC (the quencher) caused a rapid quenching of the fluorescence of Met-CQDs, accompanying with a color transition from light blue to dark bule as TC concentrations increased. The coloring variation was also detected by the images captured by smartphones and RGB analysis software, facilitating portable detection of TC utilizing Met-CQDs as a fluoroprobe. The findings indicate that the Met-CQDs based fluoroprobe exhibits high selectivity, rapid response (only ∼1 min) according to an “ON-OFF” sensing model. This fluorescence sensing method gave a low detection limit (LOD) of 0.032 μM and excellent linearity for TC in the concentration range of 0.1–500 μM. Also, the smartphone-based fluorescence-visualizing approach displayed good linearity with a LOD of 0.33 μM. The interactions between this fluoroprobe and TC occurred by virtue of both inner filter effect (IFE) and static-quenching principle. The average recovery for TC in the milk, honey, and tap water samples was determined to be 98.46 ± 1.71 % by a fluorometric method. Overall, both fluorometric and RGB approaches demonstrate strong correlation with conventional LC-MS/MS, and thus the as-fabricated Met-CQDs are promising for the preliminary screening of TCs’ residues in food products.