Research on the Method and Sensor for Fast Measurement of Green Tea Polyphenols by Opto-electrochemical Technique
|Keywords||Tea polyphenols optics electrochemistry fast measurement concentration sensor|
Tea Polyphenols (TP) are characterized by the presence of more than one phenol unit or building block per molecule in tea plants. The polyphenols content in tea infusion or related products is one of the most important quality indicators during the manufacture. Traditional methods for the determination of total polyphenols content rely on colorimetric methods using Folin-Ciocalteu (F-C) reagent or HPLC, which need be operated by trained researchers and cost much time, reagent and labor. These factors have created demands for a simple, rapid and low-cost analytical tool which could be easily used in the test for factory. In this research, a new compact opto-electrochemical sensor (OES) which was achieved by combining cyclic voltammetry (CV) on glassy carbon electrode and UV transmission at one precise wavelength (λ= 275nm), was designed to determine polyphenols contents in green tea.The absorption of TP at 275nm showed a linear response with concentration, y=0.0214x+0.0374, R2=0.9961. The result obtained from the optical method was slightly higher than the Folin-Ciocalteu Assay through the assessment study of 7 different kinds of green tea. The result got from NIR indicated that it was not easy to find the light source at the dedicated wavelength for the determination of TP.This article discussed the possibility of electrochemistry method CV applied on the fast measurement of green tea polyphenols content. The results revealed that the silver electrode was not suitable for the content determination. A linear relationship between the anodic current and the polyphenols concentration was obtained using the carbon paste electrode (CPE) or the glassy carbon electrode (GCE). Test results of green tea samples indicated that the repeatability of the GCE was better than the CPE. In the accuracy analysis, the deviation for GCE and CPE was less than±5%and±7% respectively compared with national standard with F-C reagent. The electrochemistry method cost 40s one time. The CPE was disposable, but the GCE could be used for many times and fixed on the sensor.The sensor was developed by combining CV on glassy carbon electrode and UV transmission at one wavelength with LED light source (λ,=275±5nm). This combination offered the possibility to improve the measurement range and obtain the value with greater certainty. The new sensor could achieve the measurement range from 1 to 1000μg mL-1, and the calibration curve was established for 3 parts, ranging from 1-100,100-450, and 450-1000μg mL-1 respectively. Before the measurement of OES, a suitable calibration curve needed to be chosen for the quantification.15 different kinds of green tea were used in the assessment study. The recoveries of OES were in the range between 80 and 105%, and the RSD got from the measurement was 0.71%,6.94%and 1.63%for optical (calibration curve 1), electrochemical (calibration curve 2 and 3) respectively. The result obtained from the optical system was 5%higher than the Folin-Ciocalteu Assay due to the influence of caffeine or other components, but it was acceptable for the fast and cheap measurement; the electrochemistry method has no significant differences with Folin-Ciocalteu Assay.The presented OES is a rapid and inexpensive way based on optical and electrochemical technique for evaluating the amount of total green tea polyphenols. This preliminary research gave a new idea to detect polyphenols in tea conveniently, which is useful to the development of quality control of in process and standardization of tea products.