Based on digital imaging, a microfluidic paper chip for the rapid determination of the chlorogenic acid content in honeysuckle was developed and optimized by using carbon dots, which combines the high accuracy of fluorescence carbon dots and the high selectivity of molecularly imprinted polymers. Using nitrogen and sulfur co-doped carbon dots as a fluorescent sensing material and chlorogenic acid as a template molecule, a carbon dot-molecularly imprinted paper chip was made by synthesizing molecularly imprinted polymers on the hydrophilic surface of filter paper. Based on the redness, greenness, and blueness values of colored spots, the experimental conditions were optimized using single-factor experiments.The optimal preparation conditions were molar ratio of chlorogenic acid to 3-aminopropyltriethoxysilane (functional monomer) to ethyl orthosilicate (cross-linking agent) 1:8:8, addition of 1 mL of anhydrous ethanol and 0.2 mL of amino-modified carbon dot solution to 0.01 mmol of chlorogenic acid, and ultrasound-assisted polymerization for 35 min. The optimal color development conditions were sample volume 4 μL, and reaction time 15 min. Under the optimized conditions, a good linear relationship was obtained between the greenness value and chlorogenic acid concentration in the range of 0.025–0.15 mg/mL, with a determination coefficient of 0.9918, and the detection limit of the method 0.0206 mg/mL. The recoveries of chlorogenic acid spiked into honeysuckle were 90.77%–102.10%. The paper chip method has the advantages of low cost, good selectivity and rapidity (It can be completed within 20 min). It can be used as a supplementary method for the determination of chlorogenic acid in the field or under primitive conditions.

Based on the selective quenching effect of the flavonoid rutin on the fluorescence of carbon dots prepared by asparagine pyrolysis, a method for rapid detection of rutin content in the flower buds of Sophora japonica by using chitosanbased carbon dot paper chip was developed. A filter paper was immersed in the casting solution of carbon dots and chitosan. After drying, a carbon dot chitosan film was formed on the surface of the filter paper, so that the chitosan-based carbon dot paper chip was obtained. After samples were dripped on the chip, chromogenic reaction occurred uniformly within a limited region, and pictures were taken under an ultraviolet (UV) lamp at 365 nm for analysis of the red (R), green (G), and blue (B) values of the colored spots in order to optimize the preparation conditions and detection conditions. The results showed that the optimal conditions were as follows: chitosan concentration of 20 mg/mL, carbon spot concentration of 1.0 mg/mL, 50% ethanol as sample solvent, and reaction time of 20 min. Under these conditions, the difference (ΔG) in G-value between sample and solvent spots had a linear relationship with rutin concentration (C) in the range of 4 to 120 μg/mL. Using this method, the average contents of rutin in the flower buds of S. japonica from three batches were determined to be 23.85%, 22.83% and 20.30%, the average relative standard deviations (RSDs) were 6.1%, 5.6% and 6.7%, and the recoveries were 91.27% to 107.5%, which were close to the results of high performance liquid chromatography (HPLC). The proposed method is simple and rapid, and can be used for rapid on-site determination of rutin content in the flower buds of S. japonica.