Preparation of Polydopamine Coated Magnetic Nanoparticles and Its Application of Determination for Chlorogenic Acid in Lonicera Japonica Thunb.

The polydopamine coated magnetic nanoparticles （Fe₃O₄NPs@SiO₂@PDA） were prepared using ferroferric oxide as the magnetic core through in-situ oxidation self-polymerization technology. The nanoparticles were characterized by Fourier transform infrared spectrometer, field emission transmission electron microscope and vibrating sample magnetometer. The adsorption performance of Fe₃O₄NPs@SiO₂@PDA for chlorogenic acid was investigated by adsorption kinetics and adsorption isotherms. It was showed by the results that the adsorption process of chlorogenic acid by Fe₃O₄NPs@SiO₂@PDA was conformed to the pseudo-second-order kinetic model and Langmuir adsorption isotherm model, with the maximum adsorption capacity of 61.179 8 mg·g⁻¹. The nanoparticles could be reused for 5 times. 0.5 g of Lonicera japonica Thunb. sample was taken and ground into powder, and 50 mL of 50% （volume fraction, the same below） methanol solution was added. The mixture was ultrasonicated for 40 min. After cooling, the solution was filtered. 5 mL of the filtrate was taken and made its volume up to 50 mL with 50% methanol solution. Then 3 mL of the diluted solution was taken, and 3 mg of Fe₃O₄NPs@SiO₂@PDA were added. The pH of the solution was adjusted to 5 with 0.5 mol·L⁻¹ hydrochloric acid solution, and the mixture was ultrasonically oscillated at 40 ℃ for 20 min. The Fe₃O₄NPs@SiO₂@PDA were separated by a magnet and washed with water, and 2 mL of 5% （volume fraction） formic acid solution was added. After ultrasonic elution for 10 min, the Fe₃O₄NPs@SiO₂@PDA were separated by a magnet, and the supernatant was filtered through a 0.22 μm membrane. Chlorogenic acid in the filtrate was determined by high performance liquid chromatography. As shown by the results, linear relationship between values of peak area and mass concentration of chlorogenic acid was found in the range of 20.0−120.0 mg·L⁻¹, with the lower limit of determination （10S/N） of 2.0 mg·L⁻¹. 100.0 mg·L⁻¹ chlorogenic acid standard solution was determined 6 times, and RSDs of the determined values was less than 2.0%. Test for recovery was made by standard addition method on the actual sample, giving results in the range of 98.2%−99.1%. The method was applied to the analysis of actual Lonicera japonica Thunb. samples, and the detection amounts of chlorogenic acid were in the range of 13.51−16.45 mg·g⁻¹.