A stable stock solution of 2,2′-Azino-bis-3-ethylbenzthiazoline-6-sulfonic acid) (ABTS•+) was generated by blending 7.0 mM ABTS solution and 2.45 mM potassium persulfate solution at a ratio of 1:1. After undergoing 12 h incubation time at room temperature with no light exposure, the mixture was regulated to attain an absorbance of 0.70 ± 0.05 at 734 nm by adding an appropriate amount of 95% ethanol.