Chronocoulometric method for the evaluation of antioxidant capacity of medicinal plant tinctures

摘要

The oxidation potentials of medicinal plant tinctures have been studied on glassy carbon electrode, modified layer-by-layer with multi-walled carbon nanotubes and poly(gallic acid) (PGA/MWNT/GCE) in phosphate buffer solution of pH 7.4. PGA-modified electrode has shown significantly higher sensitivity towards antioxidants in tinctures in comparison to bare GCE and MWNT/GCE. A novel approach for the evaluation of antioxidant capacity of medicinal plant tinctures using one-step chronocoulometry at 1.0?V has been developed. A steady state is achieved at 100 s of electrolysis. The antioxidant capacity has been expressed in quercetin equivalents per 1 mL of tincture. Linear dynamic ranges of 0.010–0.25 and 0.25–250 μmol L?1 quercetin with the limits of detection and determination of 2.9 and 9.8 nmol L?1, respectively, have been obtained. Eleven medicinal plant tinctures have been investigated. The antioxidant capacity decreases in the following order: Rhodiola rosea L. Paeonia anomala L. Aralia elata var. mandshurica (Rupr. & Maxim.) J. Wen Echinacea purpurea (L.) Moench ≈ Schisandra chinensis (Turcz.) Baill. Valeriana officinalis L. ≈ Leonurus cardiaca L. Mentha piperita L. Calendula officinalis L. Crataegus spp. Panax ginseng C. A. Meyer. Strong positive correlations of the antioxidant capacity of tinctures with the antioxidant activity and total phenolics (r = 0.9715 and 0.9738, respectively, at rcrit = 0.602, α = 0.05 and n = 11) confirm the accuracy of the developed chronocoulometric method.