Choline chloride-urea-based natural deep eutectic solvent for highly efficient extraction of polyphenolic antioxidants from Pluchea indica (L.) Less leaves

Pluchea indica leaves (PIL) is a polyphenolic-rich plant. Polyphenols, such as phenolics and flavonoids, are natural antioxidants. This study aims to evaluate the ability of natural deep eutectic solvent (NADES) based on ultrasonic-assisted extraction (UAE) for highly efficient extraction of the polyphenolic antioxidant compounds from PIL. Choline chloride-urea (ChCl-U) was selected based on screening of 17 types of NADES. This study uses Response Surface Methodology (RSM) to help determine optimal extraction conditions. The selected variables include molar ratio, temperature, and the water addition in the NADES. The responses used for the RSM study were total phenolic content (TPC), total flavonoid content (TFC), and antioxidant activity using 2,2-diphenyl-1-picrylhydrazyl (DPPH), ferric reducing antioxidant power (FRAP), 2,2′-azinobis (3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), and reducing power (RP) methods with microtiter analysis. The best condition for polyphenolic antioxidants extraction using ChCl-U NADES was a 1:3 molar ratio, temperature of 52 °C, and 26% of water addition in NADES. Under these conditions, the extraction of total phenolics and flavonoids from PIL with antioxidant activity increased up to 1.2 folds and 3.1 folds compared with 50%-ethanol and methanol, respectively. The morphology of the leaves before and after extraction was also evaluated with a scanning electron microscope (SEM) to observe the effects of ultrasonic vibrations with different solvents. Furthermore, the compound di-caffeoylquinic acid (DCQA) in PIL was selectively found in ChCl-U extract through quadrupole time-of-flight mass spectrometry (UPLC-QToF-MS/MS) analysis. Thus, ChCl-U combined with UAE can be developed for polyphenolic extraction from PIL for extract production as an antioxidant.