A systematic study of the principles of deep eutectic solvent-based microwave-assisted extraction (DES-MAE) was performed. It was found that the heating rates of most DESs decreased (heat capacity increased) under microwave irradiation with increasing water content, allowing high-efficiency extraction for thermally sensitive compounds. In addition, DESs containing carboxylic acids reacted with hydroxyl groups of sugar and choline chloride, resulting in cell wall destruction and inhibition of cellulose, hemicellulose, and lignin reconnection in cell walls through hydrogen bonds, thus leading to better extraction performance. This was verified by extracting anthraquinones from Rheum palmatum using DES-MAE and optimizing extraction conditions. DES with citric acid as the hydrogen bonding donor gave the highest extraction efficiency under the optimized conditions. In addition, anthraquinones in the DES extract were recovered using three kinds of silica modified by different functional groups. The results showed that material containing a phenyl group is beneficial to the recovery of anthraquinones in acid-based DESs because it can facilitate strong hydrophobic and π−π interactions. This study showcases the green chemistry applications of DES-MAE in a laboratory and industry alike, and demonstrates the recovery of natural products from DES extracts. The findings also provide valuable information for green extraction, modification, and application of cellulose, hemicellulose, and lignin.

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. 1 H 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.

A strategy was developed to recover natural products from deep eutectic solvents based on denaturation of the solvents. A method similar to nucleic acid denaturation was used to disrupt the intermolecular forces in the deep eutectic solvents to reduce their ability to solubilize and interact with the flavonoids at recovery. MIL-100 (Cr) was added to the system for sorptive separation and recovery of the target compounds from the denaturized deep eutectic solvents. The factors affecting denaturation of the deep eutectic solvents and adsorption of the target compounds were systematically investigated and optimized. The recovery method was further combined with mechanochemical extraction to establish a convenient and efficient protocol for obtaining natural products from plants. Moreover, due to the conjugated structure of MIL-100 (Cr), the adsorbed natural products could be directly analyzed by atmosphere matrix assisted laser desorption ionization-mass spectrometry (AP/MALDI-MS) without elution. This study provided a strategy for improving the efficiency of separation and recovery of natural products using deep eutectic solvents and also established a protocol for obtaining or analyzing natural products in a green, efficient, and convenient manner.