Using hydrophobic deep eutectic solvents in liquid–liquid phase extraction of antioxidants coupled with electrochemical detection

Hydrophobic deep eutectic solvents (HDESs) offer a sustainable and cost-effective alternative to conventional organic solvents and ionic liquids for electroanalytical applications following liquid-liquid phase extraction (LLPE). While type III HDESs are inherently ionically conductive and immiscible with water, water uptake and ionic partitioning significantly alter their physicochemical properties, with limited studies on the resulting electrochemical implications, especially for selective sensing. In this study, a model type III HDES composed of 2:1 (n/n) decanoic acid (DecA) and tetrabutylammonium chloride (TBACl) is used to elucidate the influence of water-induced compositional changes. Increasing water content enhances ionic conductivity, viscosity, and ionicity, while the eutectic melting range remains largely unaffected. Cyclic voltammetry shows that peak currents of both electron transfer (ET, e.g., with decamethylferrocene) and proton-coupled electron transfer (PCET, e.g., with naphthoquinone) reactions increase with fluidity, highlighting the role of diffusional mass transport. Overpotentials in PCET reactions are governed by restricted proton availability, due to the strong hydrogen-bonding network, whereas ET reactions are unaffected. 1H NMR analysis confirms increased TBACl leaching with higher volume of the aqueous phase, simultaneously reducing the water uptake by the HDES. The practical utility of HDES is demonstrated through the extraction and voltammetric detection of phytochemical antioxidants. Compared to UV–Vis spectroscopy, HDES-based electrochemical sensing offers superior specificity, effectively minimizing interferences from co-extracted hydrophilic macromolecules immobilized in the HDES phase. These findings provide fundamental insights into the electrochemical behavior of HDESs and underscore their potential in selective electroanalytical sensing applications.