N-Doped Carbon Nanodots as Temperature Sensors and Fluorescent Probes for the Detection of Tinidazole in Milk

In this study, nitrogen-doped carbon nanodots (N-CDs) with temperature and fluorescence sensing were prepared via hydrothermal method using L-lysine and ethylenediamine as precursors. The synthesized N-CDs exhibited spherical morphology with sizes ranging from 2.8 to 5.2 nm, with an average diameter of 4.03 nm. Maximum fluorescence emission was observed at 390 nm upon excitation at 320 nm, with the excitation spectrum closely overlapping the absorption spectrum of tinidazole (TNZ). In the temperature range of 20 ~ 50 °C, the fluorescence intensity of N-CDs decreased linearly with the increase of temperature. TNZ was detected based on inner filter effect (IFE) using N-CDs as a fluorescent probe. The fluorescence quenching degree had a good linear correlation with the TNZ concentration in the range of 1~100 µM (r = 0.9970), and the detection limit was 0.362 µM. In addition, the detection limits of other nitroimidazole antibiotics, including metronidazole (MNZ), Ornidazole (OMZ) and Seknidazole (SNZ), were 0.324 µM, 0.345 µM and 0.341 µM, respectively. Importantly, this method exhibits minimal interference from ions present in milk and has been validated in real milk samples, with recovery rates ranging from 92.56 to 107.27%. These results highlight the method's strong potential for application in food analysis.