Insight into the Deep Eutectic Solvent Extraction Mechanism of Flavonoids from Natural Plant

Deep eutectic solvents (DESs) are a new class of green solvents with potential applications for the extraction of target compounds from both liquid and solid samples. However, current research in the field has focused on demonstrating the advantages in extraction efficiency in terms of more extracted material and shortened time, while the mechanism of the increased efficiency has not been systematically studied. Herein, we explored the solvent and solute interaction mechanisms with the use of three extraction methods (heating, microwave-assisted, and mechanochemical extraction) and different types of DESs. Choline chloride (ChCl) was used as the hydrogen-bond acceptor, while hydrogen-bond donors used are malonic acid, methylurea, and glycerin. Flavonoids from Flos Sophorae were extracted. H-1 nuclear magnetic resonance (NMR) spectrometry, ultraviolet-visible (UV-vis) spectrometry, scanning electron microscopy (SEM), and ultrahigh-performance liquid chromatography (UPLC) analyses were performed to investigate the interactions between the flavonoids and the plant cell walls with DESs, and chemical reactions between the DESs and flavonoids. We also systematically evaluated the influence of several key factors on the extraction efficiency, which was consistent with the experimental results. The influence of DES in the sample on qualitative and quantitative UPLC analyses was systematically studied, and conditions were optimized. This study should provide insights into the interactions of specific DESs with various target compounds and help design more efficient extraction methods.