Food Chemistry 138 (2013) Contents lists available at SciVerse ScienceDirect. Food Chemistry

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1 Food Chemistry 138 (2013) Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: Analytical Methods Electrochemical determination of Sudan I in food samples at graphene modified glassy carbon electrode based on the enhancement effect of sodium dodecyl sulphonate Xinying Ma, Mingyong Chao, Zhaoxia Wang Department of Chemistry and Chemical Engineering, Heze University, Heze , PR China article info abstract Article history: Received 15 March 2012 Received in revised form 18 October 2012 Accepted 3 November 2012 Available online 12 November 2012 Keywords: Sudan I Sodium dodecyl sulphonate Graphene Modified electrode This paper describes a novel electrochemical method for the determination of Sudan I in food samples based on the electrochemical catalytic activity of graphene modified glassy carbon electrode (GMGCE) and the enhancement effect of an anionic surfactant: sodium dodecyl sulphonate (SDS). Using ph 6.0 phosphate buffer solution (PBS) as supporting electrolyte and in the presence of mol L 1 SDS, Sudan I yielded a well-defined and sensitive oxidation peak at a GMGCE. The oxidation peak current of Sudan I remarkably increased in the presence of SDS. The experimental parameters, such as supporting electrolyte, concentration of SDS, and accumulation time, were optimised for Sudan I determination. The oxidation peak current showed a linear relationship with the concentrations of Sudan I in the range of mol L 1, with the detection limit of mol L 1. This new voltammetric method was successfully used to determine Sudan I in food products such as ketchup and chili sauce with satisfactory results. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Food safety has attracted increasing public concerns in recent years because illicit chemicals are sometimes added to food products as colourants by some food and beverage manufacturers. 1- Phenylazo-2-naphthol, commonly known as Sudan I, is a synthetic lipophilic azo dye extensively used in many industrial fields, such as engine oil, car wax, shoe polishes etc. Sudan I is a possible carcinogen and can cause tumors, it has been classified as a category 3 carcinogen by the International Agency for Research on Cancer (IARC) (Stiborova, Martinek, Rydlova, Hodek, & Frei, 2002). Due to its potential carcinogenicity, addition of Sudan I to food and beverages has been strictly forbidden by many countries. Unfortunately, some food manufacturers still purposefully add it to foodstuffs such as ketchup, chili sauce to increase their luster due to its low cost. Therefore, it is very necessary to develop convenient, sensitive and reliable methods for the determination of Sudan I in foodstuffs. Various methods have been developed for the determination of Sudan I, such as high performance liquid chromatography (HPLC) (Long et al., 2011; Tateo & Bononi, 2004), high performance liquid chromatography mass spectrometry (HPLC MS) (Calbiani et al., 2004; Zhang, Zhang, & Sun, 2006), liquid Corresponding author. Tel.: ; fax: address: chao@rychem.com (M. Chao). chromatography-photodiode array detection (HPLC-PAD) (Cornet, Govaert, Moens, Van Loco, & Degroodt, 2006), gas chromatography mass spectrometry (GC MS) (He et al., 2007), capillary electrophoresis (Mejia, Ding, Mora, & Garcia, 2007). However, chromatography methods require expensive equipments, large amount of organic solvents and are time-consuming. Capillary electrophoresis method is not very sensitive. Hence, it is still of great significance to develop sensitive and simple detection methods for Sudan I. In recent years, electrochemical sensors have attracted wide attention due to their convenience, fastness, higher sensitivity, selectivity and reproducibility. Electrochemical determination of Sudan I using modified electrodes with various modifiers such as montmorillonite calcium (Lin, Li, & Wu, 2008), gemini surfactant-ionic liquid-multiwalled carbon nanotube composite film (Mo et al., 2010), Fe 3 O 4 nanoparticles (Yin et al., 2011), mesoporous carbon (Yang, Zhu, Jiang, & Guo, 2009), single-walled carbon nanotubes and iron(iii)-porphyrin (Yunhua, 2010), and multiwall carbon nanotubes (Yang, Zhu, & Jiang, 2010) has been reported. For electrochemical determination of Sudan I, the modified material is a key factor which can affect the determination sensitivity and selectivity. To the best of our knowledge, electrochemical determination of Sudan I using graphene modified electrode based on the enhancement effect of sodium dodecyl sulphonate has not been reported. Graphene is an allotrope of carbon, whose structure is oneatom-thick planar sheets of sp 2 -bonded carbon atoms that are /$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.

2 740 X. Ma et al. / Food Chemistry 138 (2013) densely packed in a honeycomb crystal lattice (Geim & Novoselov, 2007). Its outstanding characteristics such as having a large specific surface area, excellent conductivity, and strong mechanical strength, as well as its low-energy and atomic thickness have attracted great attention for both fundamental science and applied researches (Yang et al., 2010). It has been used to prepare a new generation of electrodes for electrochemical studies due to its strong electrocatalytic activity towards many molecules (Guo, Wen, Zhai, Dong, & Wang, 2010; Kang et al., 2010; Kim et al., 2010; Zhou, Zhai, & Dong, 2009). Surfactants are a kind of amphiphilic molecule with a hydrophilic polar head on one side and a long hydrophobic tail on the other. It can adsorb on the hydrophobic surface of an electrode and alter the properties of electrode/ solution interface, which in turn can heavily influence the electrochemical process of electroactive species (Char, Niranjana, Swamy, Sherigara, & Pai, 2008; Kotkar & Srivastava, 2008; Zhang & Wu, 2004; Zheng, Ye, & Zhang, 2008). In this study, a graphene-modified glassy carbon electrode (GMGCE) was prepared using graphene sheets prepared by a redox method. Then, an electrochemical sensor was fabricated using the GMGCE and the electrochemical behaviors of Sudan I at the surface of the GMGCE in the absence and presence of sodium dodecyl sulphonate (SDS) were investigated. It was found that not only graphene exhibited excellent electrocatalytic activity towards the redox reaction of Sudan I, SDS also can remarkably improve detection sensitivity of Sudan I, giving a clear increase of oxidation current of Sudan I. The oxidation peak current showed a linear relationship with the concentrations of Sudan I in the range of mol L 1, with the detection limit of mol L 1. This new method for the determination of Sudan I is of great simplicity, high sensitivity, good accuracy and low cost, and has been demonstrated by the determination of Sudan I in foodstuffs such as ketchup and chili sauce. 2. Materials and methods 2.1. Reagents Graphite powder (<20 lm) was obtained from Qindao Graphite Corporation (Qingdao, China), sodium borohydride was from Tianjin Daofu Chemical New Technique Development Co., Ltd. (China), Sudan I was purchased from Acros (Germany) and mol L 1 Sudan I stock solution was prepared by dissolving it in absolute ethanol. Sodium dodecyl sulphonate was from Shanghai Chemical Factory (China) and mol L 1 was prepared by dissolving it in double distilled water. All other chemical reagents (analytical-reagent grade) were obtained from Beijing Chemical Reagent Company (Beijing China). Phosphate buffer solutions (PBS) were prepared by mixing the stock solutions of 0.2 mol L 1 Na 2 HPO 4 and 0.1 mol L 1 citric acid. All aqueous solutions were prepared in double distilled water Apparatus Electrochemical measurements were conducted on a CHI 660C Electrochemical Workstation (Chen-hua, Shanghai, China). Infrared spectra were recorded using a Varian 660-IR spectrometer (Agilent, America). Raman spectra were obtained using a Lab- RAM-HR Raman Spectrometer (Jobin Yvon, France). Scanning electron microscope (SEM) image was obtained using a field emission SEM Sirion 200 (FEI, America). TEM image was obtained using a JEM-2010 transmission electron microscope (JEOL, Japan). All electro-chemical experiments were carried out using a threeelectrode system consisted of a working electrode (a bare or graphene-modified glassy carbon electrode, 3 mm in diameter) a counter electrode (a platinum wire electrode), and a reference electrode (a Ag/AgCl electrode). Acidity was measured by a PHS-3B Precision ph Meter (Shanghai, China), and all sonication was done using a KQ-100 Ultrasonic Cleaner (Kunshan, China) Preparation of the nano-graphene Nano-graphene platelets were prepared according to a slightly modified literature procedure. Briefly, Graphite powder was oxidised with potassium permanganate in sulphuric acid to give graphite oxide (GO), which was then subjected to thermal exfoliation in water, followed by a further flake separation treatment with ultrasonication. The resulting GO platelets then underwent a chemical reduction treatment with sodium borohydride to give nano-graphene platelets (Fu, Liu, Zou, & Li, 2005; Hummers & Offeman, 1958; Si & Samulski, 2008; Tang et al., 2009) Preparation of the GMGCE A glassy carbon electrode (GCE, Ø = 3 mm) was polished before each experiment with gold sand paper and 0.05 lm alumina powder, respectively, and rinsed thoroughly with doubly distilled water between each polishing step, then washed successively with 50% nitric acid, ethanol and doubly distilled water in ultrasonic bath, and dried in air. The GMGCE was prepared by casting 4 ll of graphene suspension (0.3 g L 1 in water) on the GCE and drying under an infrared lamp Analytical procedure Using a GME as working electrode, a Ag/AgCl electrode as reference electrode and a platinum as counter electrode, cyclic voltammetry was used in the electrochemical measurements ml of PBS (ph 6.0), 2.00 ml of absolute ethanol, 0.30 ml of SDS, 7.70 ml of water and a certain amount of Sudan I sample solution were added into the cell. Firstly, preconcentration was performed at the surface under open-circuit for 180 s with stirring. After that, Cyclic voltammograms (CVs) were obtained by scanning in the potential range from 1.0 to 1.0 V at a scan rate of 100 mv s 1. Upon completion of each scan, the modified electrode was placed in a mixed solution of ethanol and ph 6.0 PBS (ethanol:pbs = 1:1) and cyclic scan was continued until no peak comes out, then the electrode was washed with water and dried with filter paper for reuse Sample preparation Ten grams of chili sauce or ketchup sample was weighed into a ground glass wide mouth bottle, and then ml of absolute ethanol was added. The mixture was placed in an ultrasonic bath for 25 min. After that, the mixture was filtered and the filtrate was collected in a 100 ml volumetric flask. The treatment was repeated three times and the collected filtrate was diluted to volume with absolute ethanol Method evaluation Analytical recovery of the method was determined on six blank samples spiked at three different levels of concentration: , and mol L 1 (n = 6 replicates per concentration level). Reproducibility was determined by the repeatability of responses and was expressed as relative standard deviation (RSD) of the same six spiked blank samples. The limit of detection was estimated using a method of gradually decreasing the concentration of Sudan I.

3 X. Ma et al. / Food Chemistry 138 (2013) Results and discussion 3.1. Characterisation of graphite/graphene/gmgce The obtained graphite, graphene and GMGCE were fully investigated by IR, Raman spectroscopy, SEM and TEM. Fig. S1 shows the IR spectra of the graphite and graphene (see Supplementary data). The IR spectra show the absorption bands of C OH, C@C, phenyl and C O C in the wave range number of 3450, 1558, and cm 1 respectively, which demonstrate that graphene has been successfully prepared and the graphene sheets contain abundant C O C and C OH functional groups. The functionalised and defective graphene sheets are more hydrophilic and can be easily dispersed in solvents with long-term stability. Fig. S2 shows the Raman spectrum of the graphene (see Supplementary data), which shows the D band at 1347 cm 1 that occurs at the edges or in defectives of the graphene sheets as well as the G band at 1597 cm 1 that shows the in-phase vibration of the graphite lattice. The relative intensity ratio of the D and G lines provides a sensitive measure of the disorder and crystallite size of the graphitic layers. Fig. S3 shows the SEM image of the graphene film on the GCE (see Supplementary data), revealing the crumpled and wrinkled structure of the graphene film on the electrode. Fig. S4 shows the TEM image of the graphene nanosheets (see Supplementary data), revealing its mono- or few-layer planar sheetlike morphology Electrochemical behavior of Sudan I Firstly, the catalytic ability of the graphene film towards the oxidation of Sudan I was evaluated by studying the electrochemical behaviors of Sudan I at the bare GCE and the GMGCE using the cyclic voltammetry method. Cyclic voltammograms of Sudan I at the bare GCE and the GMGCE are shown in Fig. 1, which shows that almost no current response of Sudan I was observed at the bare GCE, while the current response of Sudan I remarkably increased at the GMGCE, indicating that the graphene film can significantly catalyse the oxidation process of Sudan I and the electron transfer rate of Sudan I in the graphene film is much faster. This may be attributed to the special chemical and nano-mesh structure of graphene, which has a large specific surface area and a large number of defects. These defects in graphene are resulted from its oxidation reduction preparation process and they serve as highly active reaction sites in the electrochemical reactions at the GMGCE. All these unique physical and chemical properties make the Sudan I reactivity at the modified electrode significantly improved and the response signal greatly increased. From the CVs of Sudan I at the modified electrode, we can know that E pa = 66 mv, E pc = 13 mv, the potential difference is 79 mv, i pa /i pac < 1, which indicates that the reaction process of Sudan I at the modified electrode is a quasi-reversible redox process. The electrochemical behavior of Sudan I at the GMGCE is similar to that at multiwall carbon nanotube modified glassy carbon electrode (Yang, Zhu, & Jiang, 2010). Then, in order to evaluate the enhancement effect of SDS, the electrochemical oxidation of Sudan I at the GMGCE in the absence and presence of SDS were compared by cyclic voltammetry. Fig. 2 shows that, in ph 6.0 PBS, almost no redox peaks were observed in mol L 1 SDS solution (a), and a pair of redox peaks were observed in mol L 1 Sudan I solution with oxidation and reduction currents of i pa = 5.59 la and i pc = la, respectively (b). However, when test was conducted in the presence of mol L 1 SDS, oxidation and reduction currents of mol L 1 Sudan I increased to i pa = la and i pc = la, respectively (c), both oxidation and reduction peak currents were almost doubled. The remarkable peak current enhancement reveals that SDS can facilitate the electron transfer of Sudan I and can significantly improve the sensitivity of Sudan I. This may be attributed to the special structure of SDS, which is a surfactant containing a long hydrophobic C H chain and a hydrophilic sulphonyl group. When SDS is at a low concentration, the surfactant molecules can assemble at the electrode surface as monomers through the strong hydrophobic interaction between their C H chains and the graphene film on the electrode (Hu & Hu, 2004; James, 1997). Hydrophobic Sudan I is induced to the electrode surface by SDS, influencing the electrochemical process of electroactive Sudan I. As a result, peak currents of Sudan I is increased and the sensitivity is improved Choice of supporting electrolyte Because the electrochemical responses of Sudan I show great difference in different supporting electrolytes, choice of suitable supporting electrolyte is of great importance. In this work, the electrochemical oxidation responses of Sudan I in a variety of supporting electrolyte, such as phosphate buffer, HAc-NaAc buffer, and BR buffer, in ph ranges of , and , respectively, were investigated. It was found that better sensitivity and peaks were observed in phosphate buffer. Thus, PBS was chosen as the supporting electrolyte. Using PBS as the supporting electrolyte, various ph values were analysed. Fig. 3 shows the influence of solution ph on Sudan I oxidation peak current. Using PBS of various ph, results show that, in a ph range from 2.2 to 8.0, oxidation peak current firstly increases with increasing ph and reaches maximum at ph 6.0, then decreases as ph continues to increase. Furthermore, oxidation peak of Sudan I shifts negatively with ph increasing and Fig. 1. Cyclic voltammograms of mol L 1 Sudan I at the bare electrode (a) and the GMGCE (b) in ph 6.0 PBS. Scan rate: 100 mv s 1. Fig. 2. Cyclic voltammograms of mol L 1 SDS (a), mol L 1 Sudan I (b), and mol L 1 SDS mol L 1 Sudan I (c) at the GMGCE in ph 6.0 PBS. Scan rate: 100 mv s 1.

4 742 X. Ma et al. / Food Chemistry 138 (2013) Fig. 3. Cyclic voltammograms of mol L 1 Sudan I at different ph values. Each of the letters from a to g corresponds to ph of 2.2, 3.0, 4.0, 5.0, 6.0, 7.0 and 8.0, respectively. Inset is the plot of the peak potential of Sudan I versus ph value of buffer solutions. Sudan I oxidation peak potential changes linearly depending on a ph from 2.2 to 8.0, and the equation is E pa = ph, r = , which indicates that the redox reaction of Sudan I involves the protons. This is consistent with literature reported results (Gan, Li, & Wu, 2008; Yin et al., 2011) Effect of surfactant In order to choose an appropriate surfactant, the influence of different surfactants on the GMGCE function was investigated by cyclic voltammetry. It was found that all investigated surfactants such as sodium dodecyl sulphonate (SDS), sodium dodecyl sulphate (SDSF), sodium dodecyl benzene sulphonate (SDBS), cetyl trimethyl ammonium bromide (CTAB) and cetyl pyridium bromide (CPB) could enhance the redox peak currents of Sudan I at the GMGCE and SDS gave the best effect. It was also found that the peak current enhancement was closely related to the concentration of SDS. The relationship between the oxidation peak current of Sudan I and the concentration of SDS is illustrated in Fig. S5 (see Supplementary data). From the figure we can see that, as the concentration of SDS increases from 0 to mol L 1, there is a sharp increase in the oxidation peak current; when the concentration of SDS is further increased to from to mol L 1, there is a slight increase in the oxidation peak current; when the concentration of SDS is higher than mol L 1, the oxidation peak current conversely decreases. The oxidation peak current of Sudan I reaches maximum when the concentration of SDS is at mol L 1, therefore, mol L 1 is chosen as the concentration of SDS in this work. This may be attributed that, when SDS concentration is low, the adsorption of SDS as the monomer and could effectively affect the charge transfer rate instead of the surface properties of the GMGCE. When SDS concentration became higher, SDS formed a monolayer on the electrode surface and resulted in a change of electrode/solution interface. Fig. 4. Cyclic voltammograms of mol L 1 Sudan I in mol L 1 SDS in ph 6.0 PBS at different scan rates. Each of the letters from a to o corresponds to scan rates of 40, 80, 120, 160, 200, 240, 280, 320, 360, 400, 440, 480, 520, 560, 600, respectively, (in mv s 1 ). Inset is the plot of oxidation Sudan I peak currents versus scan rates. that the electrochemical behaviors of Sudan I at the GMGCE was an adsorption process. Scan rate of 100 mv s 1 gave the best redox peaks of Sudan I, therefore, 100 mv s 1 was chosen as the best scan rate Effect of accumulation time Stirring time exerts great influence on Sudan I oxidation peak current at the modified electrode. Measurements were made with various stirring time at a Sudan I concentration of mol L 1. Results showed that additional stirring could improve the sensitivity of the method, the oxidation peak current increased with increasing stirring time and reached maximum at 180 s. This is because excess potential caused by concentration difference of Sudan I on electrode surface becomes smaller with better mixing, resulting increased response current. Hence, a stirring time of 180 s was chosen in this study Linearity range, limit of detection and reproducibility The relationship between the oxidation peak current and the concentration of Sudan I was examined by cyclic voltammetry, and the results are shown in Fig. 5. It was found that, in ph 6.0 PBS, the oxidation peak current of Sudan I at the GMGCE is linearly proportional to its concentration over the range from to mol L 1, with a correlation coefficient of The linear regression equation can be expressed as 3.5. Effect of scan rate The effect of scan rate on the redox reaction of Sudan I at the GMGCE was investigated at a concentration of mol L 1 in the presence of mol L 1 SDS in ph 6.0 PBS (Fig. 4). It was found that, with the increase of scan rate, the oxidation potential shifted in the positive direction and the reduction potential shifted in the negative direction. The oxidation peak currents linearly increased with the scan rates ranging from 40 to 600 mv s 1 with the linear regression equations expressed as i pa (- A) = v (mv s 1 ), r = , suggesting Fig. 5. Cyclic voltammograms of different concentrations of Sudan I at GMGCE in ph 6.0 PBS in the presence of mol L 1 SDS. Each of the letters from a to h corresponds to concentrations of 0.075, 0.375, 0.75, 1.12, 2.25, 3.75, 5.62, 7.50, respectively, (in lmol L 1 ). Inset is the plot of the oxidation peak currents versus concentrations of Sudan I.

5 X. Ma et al. / Food Chemistry 138 (2013) Table 1 Determination results of Sudan I in chili sauce and ketchup samples (n = 6). Sample i pa (A) = 3.46c The limit of detection was estimated by gradually decreasing the concentration levels of Sudan I. When the concentration of Sudan I was decreased to mol L 1, the redox peaks can still be observed, but the redox peaks almost disappear when the concentration was further decreased. Therefore, the limit of detection was evaluated to be mol L 1. Ten parallel measurements were made when determining mol L 1 Sudan I samples. The relative standard deviation was found to be less than 4.8%, indicating that the modified electrode had good reproducibility Interference studies Potential interference to the oxidation current of Sudan I from many foreign species, especially those commonly contained in chili sauce and ketchup samples, was individually investigated by cyclic voltammetry. If peak current change is no more than ± 5% when measurements were conducted in the absence and presence of a foreign species, we assume no interference occurs. Experimental results showed that no interference was observed for mol L 1 Sudan I in the presence of Na + (1000), Zn 2+ (1000), Fe 3+ (1000), Ca 2+ (1000), K + (1000), Mg 2+ (1000), glucose (100), ascorbic acid (100), caffeine (100), 4-nitrophenol (50), 4-chlorophenol (50), and phenol (50), indicating that the GMGCE is highly selective towards the determination of Sudan I, where the data in the parenthesis are the concentration ratios of those interferents to Sudan I. These results indicated good selectivity of the method for the determination of Sudan I Application The application of the method has been demonstrated by the analysis of Sudan I in chili sauce and ketchup samples. Samples were prepared and analysed in accordance with procedures described in materials and methods section. No peak current of Sudan I was observed in the voltammograms of these samples, indicating that no Sudan I is contained in these samples and the determination is not interfered by other substances contained in the samples. After the determination, three equal samples of each analysed chili sauce and ketchup were spiked separately with 0.2, 0.4, 0.6 ml of mol L 1 Sudan I standard solution and then similarly analysed. The results and the calculated recoveries and RDS of the analysis are shown in Table Conclusions Added (10 6 mol L 1 ) Found (10 6 mol L 1 ) Recovery (%) RSD (%) Chilli 0 0 sauce Ketchup The results of the present work revealed that the graphene modified glassy carbon electrode exhibited excellent electrocatalytic activity towards the electrochemical oxidation of Sudan I and sodium dodecyl sulphonate could significantly enhance the sensitivity of the determination. Therefore, the electrochemical responses of Sudan I were greatly increased at the GMGCE in the presence of SDS. The peak currents obtained by cyclic voltammetry were linearly proportional to Sudan I concentrations in a range from to mol L 1 with a detection limit of mol L 1. Due to its convenience, fastness, high sensitivity and selectivity, the method provides a practicable solution for determining Sudan I in food products. Acknowledgements This work was financially supported by National Natural Science Foundation of Shan Dong Province (No. 2R2009BM003) and Heze University Scientific Research Fund (XY12BS07). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at References Calbiani, F., Careri, M., Elviri, L., Mangia, A., Pistarà, L., & Zagnoni, I. (2004). 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