Upcycling Citrus aurantium leaf by‑products into bioactive phenolics: Ionic liquid-ultrasound synergy for green extraction, UPLC-QTOF-MS/MS profiling, and hypoglycemic activity

Large quantities of agri-food by-products arise from citrus processing, creating disposal challenges but also opportunities for valorization. To address this, this study aimed to recover bioactive phenolics from Citrus aurantium leaves (CAL) and to define their composition and bioactivity. First, was screened 16 ionic liquids and identified 1-hexyl-3-methylimidazolium chloride ([C₆mim]Cl) as the optimal-polarity solvent for phenolic extraction. Building on this, an ionic liquid–ultrasound synergistic extraction (IL–USE) process was developed and optimized using single-factor experiments combined with response surface methodology. Under the optimal conditions (0.35 mol/L [C₆mim]Cl, liquid-to-solid ratio 27 mL/g, ultrasonic power 280 W, 50 min, 55 °C), the total phenolics yield reached 80.57 ± 0.60 mg/g. Compared with ultrasound-assisted extraction and conventional heat-reflux extraction, IL-USE delivered higher yields. This enhancement was macroscopically reflected in the scanning electron microscopy (SEM) analysis, which revealed more pronounced cell-wall disruption. For downstream enrichment, among five macroporous resins evaluated, HPD-826 performed best, and subsequent optimization of breakthrough and elution parameters afforded a CAL total phenolics (CAL-TP) fraction with 77.45 ± 1.63% purity. Furthermore, UPLC-QTOF-MS/MS tentatively identified 47 constituents, establishing a robust chemical profile of CAL-TP. Functionally, in insulin-resistant HepG2 cells, CAL-TP increased glucose consumption and glycogen synthesis and enhanced hexokinase and pyruvate kinase activities, thereby indicating hypoglycemic potential. Collectively, this study provides a green and efficient process that yields phenolic-rich fractions from CAL with defined composition and promising antidiabetic activity.