Synthesis of nitrogen-doped carbon quantum dots (N-CQDs) using microwaveassisted hydrothermal method for tyramine determination

In this study, we develop a rapid, efficient, low-cost and environmentally friendly method to synthesize nitrogen-doped carbon quantum dots (N-CQDs). With simple input materials as citric acid and urea, N-CQDs were sucessfully synthesized in only 5 minutes using a domestic microwave oven. Characterisation results using TEM, UV–VIS, IR and fluorescence methods have demonstrated the successful doping of N into CQDs. The obtained N-CQDs material has a particle size of less than 10 nm, and the fluorescence quantum efficiency (36.6%), which is significantly higher than that of undoped CQDs (17.2%). The N-CQDs material also exhibits an on-off fluorescence effect in the presence of Au nanoparticles (AuNPs) and in the presence of tyramine, a biological amine commonly found in food products such as cheese, fish sauce, soy sauce, kimchi, etc. The fluorescence recovery of N-CQDs/AuNPs is linearly proportional to the tyramine concentration in the range from 0.02 ppm to 1 ppm, showing the possibility of using this material to detect and quantify tyramine in food samples.