Evaluation of the Antioxidant and Antimicrobial Activity of Natural Deep Eutectic Solvents (NADESs) Based on Primary and Specialized Plant Metabolites

NADESs represent a modern class of extraction media that align with the principles of green chemistry. They are considered non-toxic and biodegradable, but relatively little is known about their biological activity. This study investigated the antioxidant, antibacterial, and antifungal properties of 40 NADESs. The systems were developed from primary (PRIM) based on choline chloride (ChCl), and specialized (HEVO) plant-derived metabolites, particularly based on thymol and menthol. Their antioxidant activity was evaluated using spectrophotometric tests. The antimicrobial activity was evaluated by the disk diffusion method. The data obtained were analyzed using principal component analysis (PCA) and partial least squares discriminant analysis (PLS-DA). NADESs based on PRIM exhibited negligible antioxidant activity and relatively low antimicrobial activity. By contrast, NADESs containing HEVO, particularly thymol-based systems, indicated significant antioxidant activity, with stronger activity observed at higher molar proportions of thymol. In the 1,8-cineole:thymol system, ABTS activity ranged from 167.37 ± 24.17 to 861.25 ± 33.03 mg Trolox equivalents/mL NADES (molar ratios 9:1 and 1:9, respectively). The 1,8-cineole:thymol system (1:9) also showed strong antimicrobial activity, with a maximum inhibition zone of 39.33 ± 2.52 mm against Staphylococcus aureus. In summary, NADESs based on HEVO exhibit significantly stronger biological activity than those containing only PRIM.