Continuous Flow Synthesis of N-Doped Carbon Quantum Dots for Total Phenol Content Detection

The carbon quantum dot (CQD) paper-based analytical device (PAD) has drawn great attention and is being intensively explored. However, the construction of a continuous flow CQD synthesis device remains challenging. In this work, a continuous flow reaction apparatus was constructed to synthesize nitrogen-doped CQDs using a mixed-solvent system of tetraethylene glycol and water. The optical properties of the CQDs were characterized. The CQDs were found to be quenched by phenolics such as chlorogenic acid, salvianolic acid B, and rutin. The CQD PAD was prepared for the determination of the total phenolic content of honeysuckle extracts. A smartphone was used to test the analytical performance of the CQD PAD. The results demonstrated that the degree of fluorescence quenching of the CQDs showed a linear relationship with the concentration of the added chlorogenic acid solution. This method was compared with the total phenolic assay in the Chinese Pharmacopoeia, and the statistical test showed no significant difference between their results. With aqueous tetraethylene glycol as the solvent for the synthesis, the continuous flow reactor for CQD preparation could be easily set up. The CQD PAD is convenient, cheap, and expected to be used for the rapid quality detection of traditional Chinese medicines.