Voltammetric determination of tannic acid in medicinal plants using MWCNTs-modified electrode

Tannins are a class of natural phenolic antioxidants represented by oligomers and polymers. Tannic acid (gallotannin) is one of the most typical compound widely distributed in plants. It posseses a wide application area in food technology and as a part of traditional herbal medicine in the treatment of various diseases. Tannic acid is one of the standards used for the medicinal plants quality control, for the plants rich of tannins. Therefore, development of sensitive and simple methods for tannic acid quantification is of practical interest. Glassy carbon electrode modified with multi-walled carbon nanotubes (GCE/MWCNTs) has been developed for the determination of tannic acid in medicinal plants. The improvement of tannic acid voltammetric response has been achieved using modified electrode due to the high electroactive surface area and electron transfer rate vs. bare electrode. Tannic acid electrooxidation at the GCE/MWCNTs is irreversible surface-controlled process involving transfer of two electrons and two protons on the first step. In differential pulse mode using Britton-Robinson buffer pH 2.0 as supporting electrolyte, the linear dynamic range of 0.10-7.5 μM with the detection limit of 0.038 μM have been obtained. Method has been applied for the analysis of infusions and decoctions of tannin-containing medicinal plants and compared to the spectrophotometric method. A positive correlations have been observed with ferric reducing power reflecting total content of phenolic compounds in the sample.