Boosting the Solubility and Bioactivity of Flavonoids in Nontoxic Betaine-Based Natural Eutectic Solvents

This study presents a green and efficient method for solubilizing hydrophobic bioactive flavonoids using natural deep eutectic solvents (NaDES), emphasizing the use of the complete NaDES–flavonoid systems for enhanced functionality. Among the investigated NaDES, betaine:glycerol (1:2 molar ratio) with 20 wt % water achieved the highest solubility for quercetin (3.36 mg/mL), rutin (5.95 mg/mL), naringenin (5.82 mg/mL), and naringin (13.32 mg/mL). NaDES-containing flavonoids were structurally characterized using 1H NMR and FTIR spectroscopies and thermally analyzed by DSC, revealing a high thermal stability. The NaDES–quercetin system exhibited not only the highest antioxidant activity but also cooperative and significant antibacterial effects against multidrug-resistant Staphylococcus aureus and Pseudomonas aeruginosa, achieving 91.3% and 94.8% reduction, respectively. Both the neat NaDES and the flavonoid-containing systems were noncytotoxic to 3T3-L1 fibroblasts and HaCaT keratinocytes at concentrations up to 100 μg/mL. NADES-containing flavonoids inhibited reactive oxygen species (ROS) production in a time- and dose-dependent manner, while the NADES–quercetin system promoted wound healing in fibroblasts in vitro at lower concentrations. The complete NaDES–flavonoid systems, exemplified by hydrated betaine-glycerol NaDES with quercetin solubility increased by 23,850-fold compared to water, exhibit markedly enhanced bioactivities and hold significant promise for sustainable pharmaceutical applications, based on AGREE and AGREEprep scores.