[Progress in the application of deep eutectic solvents to extraction and separation technology]

With the rapid development of green chemistry, the design and application of the related methods and requisite solvents have received increasing attention in recent years. Deep eutectic solvents (DESs) are mixtures formed from a hydrogen bond acceptor (HBA) and a hydrogen bond donor (HBD). Generally, ionic liquids (ILs) and DESs have similar physical and chemical properties, and hence, find application in the same fields. However, DESs have many advantages over ILs, such as non-toxicity, environmental friendliness, low cost, and biodegradability. Thus, there are many areas where DESs play a key role and act as new, efficient green extraction solvents. DESs can aid the extraction and separation of different target compounds from a variety of samples, thus promoting the rapid development of sample pretreatment technology. As extraction solvents, DESs offer unique advantages. In dispersive liquid-liquid microextraction (DLLME), DESs show incredible ability to extract residual drugs, metal ions, and bioactive components from complex matrices, which would require complicated sample preparation steps when using traditional organic extraction solvents. Compared with traditional organic extraction solvents, DESs have considerable merits of greenness, hypotoxicity, higher extraction efficiency, etc. Moreover, as a dispersant, a DES can accelerate the diffusion of the extractant in the sample solution during DLLME, owing to its benefits of miniaturization and low cost. Traditional dispersants such as methanol and acetonitrile have many disadvantages, including high volatility, flammability, and toxicity, while DESs are environmentally friendly. Therefore, the combination of DES and DLLME has recently gained prominence in the field of sample preparation. Additionally, the combination of DES and solid-phase extraction (SPE) has broad application prospects. By virtue of their diverse functions, DESs have been used as eluents, in combination with a solid-phase extraction column and a stir bar, to elute analytes from the sorbent surface. The molar ratio of the HBA and HBD is one of the important factors influencing the elution efficiency. DESs can be combined with magnetic multiwalled carbon nanotubes, magnetic graphene oxide, and other nanocomposites to specifically adsorb target analytes through hydrogen bonding, π-π forces, and electrostatic forces. In addition, the DES can be used in the synthesis of magnetic nanocomposites and molecularly imprinted polymers when combined with magnetic materials. Magnetic nanocomposites functionalized with DES show excellent performance and high efficiency in the extraction process. The combination of DES and magnetic materials would promote the development of magnetic materials for green chemistry and expand the application of DES to several other fields. However, to the best of our knowledge, research on the microstructure, physical and chemical properties, and extraction mechanism of DESs is still in its nascent stage. Therefore, exploring the theoretical mechanism and applications of new DESs with special functions would be an essential future research direction. This article integrates the research progress of DESs in extraction separation technology; introduces the preparation, properties, and classification of DESs; and summarizes the applications of DESs in DLLME and SPE.