Application of Cu2+-based electron spin resonance spectroscopy in measurement of antioxidant capacity of fruits

The antioxidant capacity of 22 kinds of fruits was measured by the developed electron spin resonance (ESR) method based on Cu2+ sensor. Cu2+ is reduced to Cu+ by the antioxidants in the fruits, and the remaining Cu2+ was determined by ESR and UV-Vis spectroscopy. Cu2+ can give an ESR signal whereas Cu+ cannot, and the loss of the ESR signal was used to quantify the antioxidant capacity of various fruits. The results were shown as vitamin C equivalent antioxidant capacity (VCEAC). The VCEAC values obtained by ESR and UV-Vis methods ranged from 24.23 to 688.61 mg/100 g and from 24.12 to 677.79 mg/100 g, respectively. Cupric ion reducing antioxidant capacity (CUPRAC) and 1,1-diphenyl-2-picryl-hydrazyl (DPPH) methods were employed for comparison. Based on Pearson’s correlation test, the results obtained by CUPRAC and DPPH methods were both significantly correlated with these obtained by the present method, which indicated that the novel method was reliable. Total phenolic content for all kinds of fruits was measured with the Folin–Ciocalteu reagent, and VCEAC values obtained by the ESR method were significantly correlated with total phenolic contents.