Ionic liquid-based microwave-assisted extraction of verbascoside from Rehmannia root

Rehmannia root is widely used as drug and functional food in East Asia and verbascoside content is the main quality control index of Rehmannia root because of its pharmacological activities; thus a fast, and environmentally-benign extraction method of verbascoside from Rehmannia root is a much-needed project. In this work, an ionic liquid (IL)-based microwave-assisted method for the extraction of verbascoside from Rehmannia root was proposed and the driving forces underlying the extraction processes were also explored. Several ILs with different cations and anions and the parameters of microwave-assisted extraction (MAE), including extraction time, the IL concentration, liquid-solid ratio, and microwave irradiation power, were investigated. Experimental results indicated that hydrogen bonding and hydrophobicity of ILs, π-π stacking between ILs and verbascoside, and steric hindrance effect were the fundamental parameters affecting the extraction efficiency. The IL, 1-benzyl-3-methylimidazolium chloride ([Bemim]Cl) showed the highest extraction ability, thus it was selected as the optimal solvent. Meanwhile the optimized extraction conditions were 30 g g-1 of liquid-solid ratio, 0.3 mol L-1 of [Bemim]Cl, 210 W of microwave irradiation power, and 10 s of extraction time. Under the optimal extraction conditions, the extraction yields of verbascoside from fresh, dried, and processed Rehmannia roots were 0.92, 0.34, and 0.22 mg g-1, respectively. Compared with conventional heating reflux extraction and Soxhlet extraction, the IL-based MAE was an efficient and rapid method (extraction time was only 10 s) with simple sample preparation.