ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2012, том 86, № 2, с. 364-371
КОЛЛОИДНАЯ ХИМИЯ И ЭЛЕКТРОХИМИЯ
y%K 541.13
ELECTROCHEMICAL REDUCTION OF TARTRAZINE AT MULTI-WALLED CARBON NANOTUBE-MODIFIED PYROLYTIC GRAPHITE ELECTRODE
© 2012 Y. Z. Song*, J. M. Xu*, J. S. Lv*, H. Zhong*, Y. Ye**, J. M. Xie***
*Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry & Chemical Engineering, Huaiyin Normal University, Huai An 223300, People's Republic of China **Key Laboratory for the Synthesis and Application of Organic Functional Molecules, Ministry of Education, Hubei University, Wuhan, 430062, People's Republic of China ***School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang, 212013, People's Republic of China
E-mail: songyuanzhi@126.com Received Juni 03, 2010
Abstract — Electrochemical reduction of tartrazine at multi-walled carbon nanotube-modied pyrolytic graphite electrode is investigated. A simple, sensitive and inexpensive method for determination of tartrazine in drinks is proposed. The accuracy and reproducibility of the determination method for various known amounts of tartrazine were evaluated. This method was satisfactorily applied for the determination of tartra-zine in drinks. The reduction peak currents were proportional to tartrazine concentrations over two intervals in the range from 2.0 to 70.0 mg l-1 and from 70.0 to 230.0 mg l-1, and the detection limit for tartrazine is 0.5 mg l-1.
Keywords: tartrazine, multi-walled carbon nanotubes, CV, modified electrode, drinks.
INTRODUCTION
Tartrazine is an azo compound that can be found in common food products such as beverages, candies, dairy products and bakery products. The genetic toxicity of some azo-dyes has been confirmed [1, 2]. For this reason, the accurate and reliable methods for the determination of synthetic dyes in foodstuff are required. Spectrophotometry [3-6], adsorptive voltam-metry [7], thin-layer chromatography [8], reversed -phase liquid chromatography (RPLC) [9, 10], ion-pair RPLC [11, 12] and capillary electrophoresis [13-17] have been used for the determination ofvarious water-soluble synthetic dyes. However, there is no report on study of reduction of tartrazine at multi-walled carbon nanotube-modified pyrolytic graphite electrode.
Since carbon nanotubes (CNTs) were discovered in 1991 [18], CNTs have attracted much attention of researchers. The modifcation of electrode substrates with multi-walled carbon nanotubes (MWCNTs) has been documented to result in high sensitivities, promotion electron-transfer, resistance to surface fouling and reduction of over potentials. It has been reported that MWCNTs modified electrodes were successfully applied to study and determine many organic molecules [19, 20].
In present work, the electrochemical reduction of tartrazine at multi-walled carbon nanotube-modied pyrolytic graphite electrode is investigated, and a simple, rapid and effective cyclic voltammery method for the determination of tartrazine in drinks is developed.
EXPERIMENTAL
Chemicals. Tartrazine, amaranth, ponceau 4R, sunset yellow and brilliant blue FCF were purchased from National Research Center for CRM'S (Beijing, China). Multi-walled carbon nanotubes (MWCNTs) were purchased from Shenzhen Nano-Technologies Port Co. Ltd. (China). All other reagents were analytical grade. Double-distilled water was used throughout. 0.1 M phosphate buffer solution was prepared by dissolving 0.1 M NaCl and 0.1 M Na2HPO4 in the double-distilled water of 1000 ml and adjusted desired pH values with 6 mol l-1 HCl or 1 mol l-1NaOH.
Instrumentation. For all electrochemical experiments a CHI660B Electrochemical Analyzer (CHI, USA) was employed. The electrochemical cells consisted of a three electrodes, a 2 mm diameter pyrolytic graphite electrode (PGE) and MWCNTs composite modified PGE were used as working electrode. A platinum wire served as the counter electrode and a saturated calomel electrode (SCE) was used as the reference electrode. The MWCNT material was characterized by transmission electron microscopy (TEM) (JEM 2100, JEOL, Japan ).
Preparation of multi-walled carbon nanotube and modified PGE. The MWCNTs was purified in boiling concentrated nitric acid for 3 h, followed by rinsing with deionized water and drying under atmosphere. Open-end MWCNTs with hydrophilic surface were thus obtained. Before modification, the PGE was polished with 0.05 ^m alumina slurry on a polishing cloth, rinsed thoroughly with doubly distilled water, and then sonicated in ethanol and doubly distilled water for 10 min, sequentially. The modifier suspension
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I x 105, A
-2
-0.4 E, V
Fig. 1. TEM image of the purified MWCNTs.
Fig. 2. CVs oflmM K4[Fe(CN)6] at (a) the modified electrode and (b) bare PGE. Scan rate: 50 mV s-1.
was prepared by dispersing the MWCNTs in N, N-dimethylformamide under sonication for 30 min. The MWCNT modified PGE was prepared by casting 4 ^l of the mentioned above black suspension on the PGE surface using a micropipette and left to dry at room temperature. Before the voltammetric measurements, the modified electrode was cycled between —1 and 1 V (scan rate 100 mV s-1) in 0.1 M phosphate buffer solution of pH 5.0 for several times until acquiring the reproducible responses.
RESULTS AND DISCUSSION
Geometries of SWNTs suspension. The purified MWCNT particles were characterized by transmission electron microscopy (TEM). The TEM image of SWNTs is shown in Fig. 1. Many nanocarbon tubes with diameters ranging from 20 to 30 nm were observed.
Microscopic area of the MWCNT modified PGE. The cyclic voltammery (CV) of different kinds of electrode in the K3Fe(CN)6-K4Fe(CN)6 system was shown in Fig. 2. Compared with the bare PGE, the MWCNT modified PGE shows higher peak current. The increase in voltammetric response of ferrocyanide is simply due to the intrinsic properties of the MWCNTs, which works to produce a larger peak current than that of the bare PG electrode. The real active surface area of different electrodes will be estimated. In a reversible process, the following Randles-Sevcik formula has being used:
->1/2.
Д/2
ipa = 2.69 x 105n3/2 AcD
where, ipa refers to the anodic peak current, n is the electron transfer number, A is the microscopic surface area of the electrode (cm2), D0 is the diffusion coefficient
(cm2 s-1), c0 is the bulk concentration of K3Fe(CN)6 (mol cm-3) and v is the scan rate (Vs-1). From the slope of the plot of ipa versus v1/2, the microscopic areas can be calculated. The electrode surface area of the MWCNT modified PGE was 0.096 cm2 and for bare PGE was 0.055 cm2, indicating that the microscopic area of the MWCNT modified PGE increased significantly and was about 1.75 times larger than the microscopic area of the bare PGE.
Faradic impedance spectroscopy of different electrode. Faradic impedance spectroscopy would be an effective method to probe the interfacial electron-transfer resistance at the modified electrode. Figire 3 shows the Faradic impedance spectra at the bare PGE and MWCNT/PGE. Curve b is the Nyquist plot of bare PGE, there has a remarkable semicircle portion. Compared with curve b, curve a represent a nearly straight line at higher frequencies, which means the electron-transfer resistance between the modified PGE and the solution is relative small and can be ignored, so the presence of MWCNTs can accelerate the electron transfer.
Electrochemical behavior of tartrazine at MWCNT/PGE. The electrochemical response of MWCNT/PG electrode in purged N2 0.1 M phosphate buffer solution of pH 5.0 is shown in Fig. 4. The reduction peak for tartrazine at bare PGE and MWCNT/PG electrode are observed at -0.809 and —0.563V respectively, and the reduction peak potential of tartrazine at MWCNT/PG electrode shifted to positive direction, the peak current also increased. These results indicated that the MWCNT modified electrode promoted the electrochemical reduction of tartrazine by considerably accelerating the rate of electron transfer. The MWCNT interface has a large surface area, a
4
2
0
-Z\ Ohm 800
600
400
200
200
400
600
800 1000 Z, Ohm
Fig. 3. Electrochemical impedance spectroscopy for (a) MWCNT/PGE, (b) PGE.
great deal of active sites, better conductivity and favorable electrocatalytic power, all of them led to the dissimilar conjugation effect of tartrazine with the bare electrode interface.
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-3
-6
0.2
0.4
0.6
1.0
E, V
Fig. 4. CVs of 50.0 mg l-1 tartrazine at (a) MWCNT-mod-ified PGE and (b) bare PGE, CVs of (c) MWCNT-modi-fied PGE and (d) bare PGE. Scan rate: 100 mV s-1; supporting electrolyte: 0.1 M phosphate buffer with pH 5.0;
4 ц1 accumulation volume of 1.0 mg 1 sion.
-1
MWCNT suspen-
Amount of the modifier. The reduction current of tartrazine at modified electrode can be affected by the amount of MWCNTs on the electrode surface. This can be controlled by using the same volume (4.0 ^l) of the suspensions with the different concentrations of MWCNTs, casted on the surface of PGE. The experiments showed that the reduction peak current for 50.0 mg l-1 tartrazine increased quickly by increasing the concentration of MWCNTs suspension deposited on the surface of PGE up to 1.0 mg ml-1 (Fig. 5). Further increase, caused a gradual decrease in the cathodal current of tartrazine with increase in background current. As a result, 4.0 ^l of 1.0 mg ml-1 MWCNTs suspension was selected as optimum volume for preparation of the modified electrode.
Influence of pH. On the electrochemical behavior of tartrazine was investigated at different pH values in the range of 3.0 to 8.0. Figure 6 (insert) shows the CVs of 50.0 mg l-1 tartrazine on the surface of the modified electrode over the discussed pH range. It was found that the peak potential shifted negatively with pH increasing and a good linear relationship was observed between the E and pH values in the range of 3.0 to 7.0 E, V = -0.0685 pH - 0.1911 (R = 0
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