N-doped carbon quantum dots as fluorescent probes for high-sensitivity detection of selected azo dyes

In this study, a fluorescent probe was developed to detect azo dyes in wastewater. One-step synthesis of nitrogen-modified carbon quantum dots (N-CQDs) using citric acid and urea as precursors was conducted by using the microwave method. The structure and surface morphology of the as-prepared N-CQDs were characterized using Fourier transform-infrared spectrometer (FT-IR), UV–vis (UV), X-ray photoelectron spectroscopy (XPS), and transmission electron microscope (TEM). Results demonstrated the existence of N in the N-CQDs in addition to the oxygen-containing functional groups on the surface, and the N content of N-CQDs was 61.5% higher than that of CQDs. The N-CQDs particles were spherical with uniform size and distribution, approximately 5 nm. The N-CQDs were yellow-green and exhibited the strongest emission at 550 nm at an excitation light of 290 nm. As the concentration of the selected azo dye Eriochrome Black T (EBT) gradually increased, the fluorescence intensity decreased sharply. The fluorescence quenching of N-CQDs had a good linear relationship with EBT concentration in the range of 0–50μM, and the calculated fluorescence lifetime confirmed that N-CQDs can be reused. The fluorescence intensity test showed that the optimum conditions for the N-CQD as a fluorescence probe are room temperature and a pH value of 8. In a word, the detection of azo dyes with N-CQDs as the fluorescent probe is cost-effective, simple to operate, highly sensitive, and yields satisfactory results. • One-step synthesis of N-doped carbon quantum dots based on microwave method. • N-CQDs showed high efficiency and sensitivity in the detection of the azo dye chrome black T. • N-doped carbon quantum dots can be widely used as fluorescent probes in wastewater treatment.