A flow injection system using an unmodified gold screen-printed electrode was employed for total phenol determination in black and green teas. In order to avoid passivation of the electrode surface due to the redox reaction, preoxidation of the sample was realized by hexacyanoferrate(III) followed by addition of an EDTA solution. The complex formed in the presence of EDTA minimizes or avoids polymerization of the oxidized phenols. The previously filtered tea sample and hexacyanoferrate(III) reagent were introduced simultaneously into two-carrier streams producing two reproducible zones. At confluence point, the pre-oxidation of the phenolic compounds occurs while this zone flows through the coiled reactor and receives the EDTA solution before phenol detection. The consumption of ferricyanide was monitorized at 360？mV versus Ag/AgCl and reflected the total amount of phenolic compounds present in the sample. Results were reported as gallic acid equivalents (GAEs). The proposed system is robust, versatile, environmentally-friendly (since the reactive is used only in the presence of the sample), and allows the analysis of about 35–40 samples per hour with detection limit = 1？mg/L without the necessity for surface cleaning after each measurement. Precise results are in agreement with those obtained by the Folin-Ciocalteu method. 1. Introduction Phenolic compounds are a class of chemicals that have a hydroxyl functional group attached to an aromatic hydrocarbon group. The simplest of the class is phenol (C6H5OH) [1]. The term “polyphenols” refers to a group of chemical substances found in plants which are characterized by the presence of more than one phenol unit, for example, hydrolysable tannins (gallic acid) and phenylpropanoids (flavonoid, lignins, and condensed tannins) [1, 2]. The largest and most studied polyphenols are the flavonoids, as catechins, which correspond to the main phenolic compound found in green and black teas [3]. Gallic acid was identified to be main free phenolic acids; the four major catechins are (？)-epicatechin gallate (ECG), (？)-epigallocatechin (EGC), (？)-epigallocatechin gallate (EGCG), and (？)-epicatechin (EC). These species can be associated to a reduction in the risk of cardiovascular disease and some forms of cancer, as well as the improvement of oral health and other physiological functions such as anti-hypertensive effect, body weight control, antibacterial and antivirasic activity, and so forth [1–4]. Therefore, the development of analytical methods for polyphenols represents an important and exciting topic for analytical
References
[1]
V. L. Singleton, “Naturally occurring food toxicants: phenolic substances of plant origin common in foods,” Advances in Food Research, vol. 27, pp. 149–242, 1981.
[2]
H. F. Stich and M. P. Rosin, “Naturally occurring phenolics as antimutagenic and anticarcinogenic agents,” Advances in Experimental Medicine and Biology, vol. 177, pp. 1–29, 1984.
[3]
Y. Zuo, H. Chen, and Y. Deng, “Simultaneous determination of catechins, caffeine and gallic acids in green, oolong, black and pu-erh teas using HPLC with a photodiode array detector,” Talanta, vol. 57, no. 2, pp. 307–316, 2002.
[4]
M. Shiraki, Y. Hara, T. Osawa, H. Kumon, T. Nakayama, and S. Kawakishi, “Antioxidative and antimutagenic effects of theaflavins from black tea,” Mutation Research, vol. 323, no. 1-2, pp. 29–34, 1994.
[5]
Q. Chen, J. Zhao, X. Huang, H. Zhang, and M. Liu, “Simultaneous determination of total polyphenols and caffeine contents of green tea by near-infrared reflectance spectroscopy,” Microchemical Journal, vol. 83, no. 1, pp. 42–47, 2006.
[6]
K. F. Pirker, J. F. Severino, T. G. Reichenauer, and B. A. Goodman, “Free radical processes in green tea polyphenols (GTP) investigated by electron paramagnetic resonance (EPR) spectroscopy,” Biotechnology Annual Review, vol. 14, pp. 349–401, 2008.
[7]
X. Wang, J. Wang, and N. Yang, “Flow injection chemiluminescent detection of gallic acid in olive fruits,” Food Chemistry, vol. 105, no. 1, pp. 340–345, 2007.
[8]
V. L. Singleton, R. Orthofer, and R. M. Lamuela-Raventós, “Analysis of total phenols and other oxidation substrates and antioxidants by means of folin-ciocalteu reagent,” Methods in Enzymology, vol. 299, pp. 152–178, 1998.
[9]
K. Leamsomrong, M. Suttajit, and P. Chantiratikul, “Flow injection analysis system for the determination of total phenolic compounds by using Folin-Ciocalteu assay,” Asian Journal of Applied Sciences, vol. 2, no. 2, pp. 184–190, 2009.
[10]
I. Da Cruz Vieira and O. Fatibello-Filho, “Flow injection spectrophotometric determination of total phenols using a crude extract of sweet potato root (Ipomoea batatas (L.) Lam.) as enzymatic source,” Analytica Chimica Acta, vol. 366, no. 1-3, pp. 111–118, 1998.
[11]
L. K. Ng, P. Lafontaine, and J. Harnois, “Gas chromatographic-mass spectrometric analysis of acids and phenols in distilled alcohol beverages: application of anion-exchange disk extraction combined with in-vial elution and silylation,” Journal of Chromatography A, vol. 873, no. 1, pp. 29–38, 2000.
[12]
I. Novak, M. ？eruga, and ？. Komorsky-Lovri？, “Square-wave and cyclic voltammetry of epicatechin gallate on glassy carbon electrode,” Journal of Electroanalytical Chemistry, vol. 631, no. 1-2, pp. 71–75, 2009.
[13]
M. Goldbach, H. Axthelm, and M. Keusgen, “LTCC-based microchips for the electrochemical detection of phenolic compounds,” Sensors and Actuators B, vol. 120, no. 1, pp. 346–351, 2006.
[14]
S. A. S. S. Gomes, J. M. F. Nogueira, and M. J. F. Rebelo, “An amperometric biosensor for polyphenolic compounds in red wine,” Biosensors and Bioelectronics, vol. 20, no. 6, pp. 1211–1216, 2004.
[15]
B. Prieto-Simón, M. Cortina, M. Campàs, and C. Calas-Blanchard, “Electrochemical biosensors as a tool for antioxidant capacity assessment,” Sensors and Actuators B, vol. 129, no. 1, pp. 459–466, 2008.
[16]
L. D. Mello and L. T. Kubota, “Biosensors as a tool for the antioxidant status evaluation,” Talanta, vol. 72, no. 2, pp. 335–348, 2007.
[17]
M. Trojanowicz, Flow Injection Analysis: Instrumentation and Applications, Book News, Portland, Ore, USA, 2000.
[18]
I. L. De Mattos, L. Gorton, and T. Ruzgas, “Sensor and biosensor based on Prussian Blue modified gold and platinum screen printed electrodes,” Biosensors and Bioelectronics, vol. 18, no. 2-3, pp. 193–200, 2002.
[19]
C.-H. Nieh, B.-C. Hsieh, P.-C. Chen, H.-Y. Hsiao, T.-J. Cheng, and R. L. C. Chen, “Potentiometric flow-injection estimation of tea fermentation degree,” Sensors and Actuators B, vol. 136, no. 2, pp. 541–545, 2009.
[20]
Y.-T. Hung, P.-C. Chen, R. L. C. Chen, and T.-J. Cheng, “Determining the levels of tannin in tea by amperometry of ferricyanide pre-reaction with a sample in a flow-injection system,” Sensors and Actuators B, vol. 130, no. 1, pp. 135–140, 2008.
[21]
R. M. De Carvalho, L. T. Kubota, and S. Rath, “Influence of EDTA on the electrochemical behavior of phenols,” Journal of Electroanalytical Chemistry, vol. 548, pp. 19–26, 2003.
[22]
A. J. Bard and L. R. Faulkner, Electrochemical Methods: Fundaments and Applications, John Wiley & Sons, New York, NY, USA, 1980.
[23]
S. Laschi, I. Palchetti, G. Marrazza, and M. Mascini, “Development of disposable low density screen-printed electrode arrays for simultaneous electrochemical measurements of the hybridisation reaction,” Journal of Electroanalytical Chemistry, vol. 593, no. 1-2, pp. 211–218, 2006.
