Analytical Sciences

Abstract − Analytical Sciences, 35(5), 529 (2019).

Polyphenol Analysis in Black Tea with a Carbon Nanotube Electrode
Satoshi MURAKAMI,* Shota TAKAHASHI,* Hitoshi MUGURUMA,* Naomi OSAKABE,** Hitoshi INOUE,*** and Tatsuya OHSAWA***
*Graduate School of Engineering and Science, Shibaura Institute of Technology, 3-7-5 Toyosu, Koto, Tokyo 135-8548, Japan
**Department of Bioscience and Engineering, College of Systems Engineering and Science, Shibaura Institute of Technology, 307 Fukasaku, Minuma, Saitama 337-8570, Japan
***Nippon Shizai Co., Ltd., 1-5-6 Bakurou-machi, Chuo, Osaka 541-0059, Japan
An electrochemical analysis of polyphenols (theaflavin (TF1), theaflavin-3-gallate (TF2A), theaflavin-3′-gallate (TF2B), theaflavin-3,3′-digallate (TF3), and epigallocatechingallate (EGCG)) in a black tea infusion is demonstrated. The characterization of each polyphenol in a solution containing only a single type of polyphenol for a redox reaction at the CNT electrode with cyclic voltammetry (CV) was conducted. The oxidation peak at around +0.30 V for TF1 is assigned to catechol group in a benzotropolone ring. The oxidation peak at around +0.35 V for TF2A, TF2B, and TF3 is assigned to both of the catechol groups in the benzotropolone ring and the pyrogallol group in the gallate ring. The oxidation peak at around +0.35 V for EGCG is assigned to a pyrogallol group in the gallate ring. Current changes of those individual polyphenols at the peak potential are proportional to their concentrations (linear range 0.28 – 94 μM; detection limit 0.11 μM). The CV curve for real black tea, which is mainly composed of a mixture of the mentioned five compounds, is produced by the sum of those. The current change of the mixture solution of polyphenols is also proportional to the mass concentration of the total polyphenols and the sensitivity defined as the slope of current vs. concentration plot is independent of the ratio of the individual polyphenols. This indicates that the peak current at around +0.35 V can quantify the total amount of polyphenols in a black tea. Additionally, the shape of the CV curve can roughly estimate the ratio of [catechins]/[theaflavins]. The values for real samples determined from CVs show good agreement with that obtained by high-performance liquid chromatography.