Determination of Chlorogenic Acid in Honeysuckle Using MnO₂-Based “On-Off ” Fluorescent Carbon Dots Paper-Based Microfluidic Chip

MnO₂ nanoparticles can quench the fluorescence of carbon dots（CDs） synthesized from citric acid and urea, thereby turning the system "off". Upon the additon of chlorogenic acid（CGA）, the oxidation-reduction reaction between CGA and MnO₂ occurred, reducing MnO₂ and restoring CDs fluorescence, thus switching the system "on". The recovered fluorescence intensity correlated positively with CGA mass concentration. A paper-based microfluidic chip was fabricated using filter paper as the substrate, with hydrophilic/hydrophobic patterns created by a cut-and-paste method. CDs and MnO₂ nanoparticle solutions were deposited onto the hydrophilic zones and dried to form a MnO₂-based "on-off" fluorescent sensing platform. After direct sample application, the paper-based microfluidic chip was imaged under the UV light of 365 nm, and the red（R）, green（G）, and blue（B） values of the coloring spots were quantitatively analyzed. The preparation and detection conditions of the above paper-based microfluidic chip were optimized. As shown by the results, under these optimized conditions of CDs mass concentration of 0.50 g·L⁻¹, MnO₂ mass concentration of 0.087 g·L⁻¹, spotting volume of 4 μL, and the color development time of 25 min, linear relationship between values of B/（R+G+B） of sample coloring spots and mass concentration of CGA was kept in the range of 0.06‒0.70 g·L⁻¹, with detection limit（3s/k） of 0.059 mg·L^-1. Test for recovery was made by the standard addition method, giving results in the range of 90.8%‒99.6%. This method was applied for parallel analysis of actual honeysuckle samples 5 times, with RSDs of the determined values not greater than 6.5%, and the determined results were close to those obtained by high performance liquid chromatography.