Simple flow injection for screening of total antioxidant capacity by amperometric detection of DPPH radical on carbon nanotube modified-glassy carbon electrode

An amperometric flow injection (FI) method suitable for evaluation of ‘total antioxidant capacity’ (TAC) is presented. In this method, a carrier stream of a solution of 2,2-diphenyl-1-picrylhydrazyl radical (DPPH•) continuously flows through an electrochemical cell, furnished with a carbon nanotube modified-glassy carbon electrode (CNT/GC) as the working electrode. At the applied voltage of 0.05 V (vs. Ag/AgCl), DPPH• is reduced resulting in a constant electric current. For measurement of the TAC, a sample zone containing antioxidant(s) is injected into the carrier stream therein reduction reaction of DPPH• occurring within the sample zone. The decreased amount of the radical in the sample zone leads to a drop of the amperometric signal at the CNT/GC electrode. We have also compared the performance of the CNT/GC electrode to the unmodified GC electrode using cyclic voltammetry. The sensitivity of the CNT/GC electrode was more than twenty five times greater than the bare GC electrode. The study of the sweep rate dependence showed that the cathodic and anodic current of 0.1 mM DPPH solution varied linearly (r2=0.998) with the square root of the scan rate, from 0.02 to 0.12 Vs−1. These results demonstrated that the CNT/GC electrode is appropriate for the quantitation of antioxidants via amperometric detection of the residual concentration of non-reacted DPPH•. We obtained linear calibrations for all the antioxidants tested including gallic acid, catechin, quercetin, caffeic acid and Trolox. The system offers rapid sample throughput (45 samples h−1) and good precision of 3.2% R.S.D., for 20 μL-injection of 2.5 μM Trolox (n=30). This method was applied to evaluate the TAC of extracts of some Thai indigenous vegetables.