From vineyard waste to smart sensors: Nitrogen-doped carbon quantum dot prepared from grape leaves and applications in high-sensitivity visual detection of gallic acid.

At present, with the rapid development of nanotechnology, the development of nanomaterials and how to use their fluorescence properties in the rapid and efficient determination of organic matter are major opportunities and challenges. Fortunately, nitrogen-doped carbon quantum dots (N-CQDs) are prepared by the hydrothermal method using grape leaves as the carbon source and isosorbitol-urea deep eutectic solvent (DES) as the modified nitrogen dopant. N-CQDs are adopted as the fluorescent probe to explore its application in the detection of gallic acid (GA) in wine. TEM characterization shows that the average particle size of N-CQDs is 4 nm. FTIR and XPS characterizations indicate that the surface of the quantum dot material is rich in functional groups, such as -OH, -C=O, -COOH and -NH. UV-vis and fluorescence spectroscopic analyses reveal that under the condition of an optimal excitation wavelength of 350 nm, the optimal emission wavelength is 410 nm. The detection of GA is within the detection range of 0.1-100 μmol·L, the detection limit is as low as 0.038 μmol·L. Applied to wine samples, the recovery rate of GA is between 88.9 % and 102.5 %. Through spectral characterization and quantum chemical calculation, the structure and the detection process of N-CQDs are simulated and analyzed respectively. The results show that under the static quenching effect, a non-fluorescent complex is formed between GA and N-CQDs, resulting in the fluorescence quenching effect. N-CQDs have excellent water solubility, stability, anti-interference ability and other advantages, expectly efficient and sensitive detection of GA. This study provides a highly potential scheme for the detection of GA. Meanwhile, it provides data support and research basis for the subsequent application of carbon quantum dots in detection.