2.7. Determination of Antioxidant Activity Using the ABTS Free Radical Scavenging Method

The antioxidant activity of the study plant extracts against ABTS was determined by the method described by [39]. Radical ABTS^•+ was prepared through oxidation of ABTS by potassium persulfate. A mixture (1:1; v/v) of ABTS (7 mM) and potassium persulfate (4.95 mM) was prepared and kept in the dark for 16 h at room temperature.

Then, the mixture was diluted with methanol until it reached absorbance values of 1–1.5 at 734 nm. Aliquots of 0.1 mL of methanolic extract of each sample (at 4 different concentrations: 0.1, 0.5, 1, and 2 mg/mL; two replicates per sample and concentration) had 3.9 mL of the ABTS^•+ dilution added. The absorbance decrease was measured at 734 nm in a UV-30 spectrophotometer. The blank was prepared with ABTS^•+.

The results were expressed in milligram equivalents of quercetin per milligram of dry weight. The calibration line was established using the following concentrations of quercetin: 0.00062, 0.00125, 0.0025, 0.005, 0.01, and 0.032 mg/mL.