Electrochemical Determination and Antioxidant Capacity Modulation of Polyphenols in Deep Eutectic Solvents

Deep eutectic solvents (DES) are currently developed as ecofriendly and efficient solvents for polyphenol extraction. However, a challenge remains in these processes concerning polyphenol determination that is routinely performed through colorimetric Folin–Ciocalteu assays. Besides the fact that these assays use toxic reagents, they may have serious limitations in some DES media due to possible side reactions or interferences with the DES components as found herein with ethaline (choline chloride/ethylene glycol (1:2)) or betaine/citric acid (2:3). An alternative method based on simple electrochemical analyses directly performed in DES-based media is proposed. Since the direct electrochemistry of polyphenols in DES has been hardly explored, nine polyphenolic standards including seven flavonoids are examined and their electrochemical behaviors are found closely related to those classically obtained in traditional solvents, notably in aqueous solutions. A linear relationship is obtained between concentration and peak current intensity for quercetin in the range 10^–5–10^–4 mol·L^–1, demonstrating the potential of the method for polyphenol content quantitation. More importantly, their antioxidant capacity is evaluated by differential pulse voltammetry through the measurements of the redox potentials. An interesting variation is observed according to the nature of the DES components, suggesting that the antioxidant activity of polyphenols can be modulated through the choice of the DES. Moreover, a good stability of the polyphenol samples in the DES–water mixture under study is obtained after storage for 10 months at 30 ° C. These results open appealing perspectives in polyphenol extraction processes using eutectic solvents as extracting media that can be part of formulation for cosmetic or pharmaceutical applications, resulting in high-added-value extracts.