Deep eutectic solvent-based extraction and recovery of phenolic compounds: From conventional media to switchable recovery strategies.

Phenolic compounds are widely distributed in plant-derived matrices and are important targets in food, pharmaceutical, agricultural, and biomass-utilization fields. Their reliable determination depends strongly on efficient sample preparation, but conventional organic solvent extraction often raises concerns related to solvent consumption, toxicity, selectivity, and sample-cleanup burden. Deep eutectic solvents (DESs) have been widely explored as tunable extraction media for phenolic compounds because their polarity, viscosity, hydrogen-bonding capacity, and solvation behavior can be adjusted by changing the hydrogen-bond acceptor, hydrogen-bond donor, molar ratio, and water content. However, conventional DES systems still face important limitations, including slow mass transfer caused by high viscosity, difficulty in solvent removal due to low volatility, incomplete analyte recovery, and possible interference with chromatographic or mass-spectrometric analysis. This review summarizes conventional DES-based extraction of phenolic compounds as the baseline, and then focuses on responsive switchable DESs (RS-DESs) as recovery-oriented solvent systems. RS-DESs can undergo reversible changes in phase behavior, polarity, miscibility, or solvation ability in response to temperature, pH, CO2/N2, or ionic strength, thereby facilitating extraction, phase separation, analyte recovery, and solvent recycling. Particular attention is paid to extraction mechanisms, structure-property relationships, quantitative comparison with conventional DES systems, and analytical compatibility with HPLC, LC-MS, and related sample-cleanup workflows. Finally, the review discusses current scientific, analytical, and industrial challenges, including switching reproducibility, phenolic stability, DES residues, matrix effects, toxicity, biodegradability, and scale-up feasibility.