Food Chemistry 169 (2015) 114–119
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Analytical Methods
High catalytic activity of electrochemically reduced graphene composite toward electrochemical sensing of Orange II Mira Yun a, Ju Eun Choe a, Jung-Min You a, Mohammad Shamsuddin Ahmed a, Kyungmi Lee a, Zafer Üstündag˘ b,⇑, Seungwon Jeon a,⇑ a b
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Chemistry, Faculty of Arts and Science, Dumlupınar University, 43100 Kutahya, Turkey
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 5 June 2014 Accepted 30 July 2014 Available online 7 August 2014 Keywords: Azo dye Electrochemically reduced GO Electrochemical sensor Orange II Platinum nanoparticles
a b s t r a c t Orange II, an azo dye, is sometimes illegally used as a red dye in food products despite its adverse health effects if consumed. Therefore, the determination of low concentrations of Orange II is an important target. An Orange II sensor was prepared using electrochemically reduced graphene oxide grafted with 5-amino-1,3,4-thiadiazole-2-thiol-Pt nanoparticles (denoted as ERGO-ATDT-Pt) onto a glassy carbon electrode (GCE) and investigated for Orange II detection in 0.1 M acetate buffer solution (ABS at pH 4.5) with prominent reversible redox peaks. A wide linear range of 1 108–6 107 M with a low detection limit of 3.4 1010 M (s/n = 3) was found for Orange II detection. This developed ERGOATDT-Pt/GCE sensor showed good selectivity, excellent stability and better response to the real sample analysis with excellent recovery. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Orange II (sodium 4-[2-(2-oxonaphthalen-1-ylidene)hydrazinyl]benzenesulfonate) is an azo dye containing one or more azo groups (R1–N = N–R2) tinged with colour through visible-light absorption. It is used in many industries, such as for organic lightemitting diodes (OLEDs), inks, soaps, wood preservatives, textiles, hair dyes, leather materials, shoe polishes, cosmetics, wood stains, and foodstuffs (Yadav, Kumar, Dwivedi, Tripathi, & Das, 2012). Many countries, however, have regulated the use of Orange II in foodstuffs because it poses a risk to human health as carcinogenic (Guo, Pan, & Jing, 2004; Pourreza & Zareian, 2009), and reduces the number of red blood cells, accompanied by the lowering of hemoglobin and packed cell volume (Gan, Sun, Lin, & Li, 2013). Hence, the determination of low concentrations of Orange II is an important target. Methods of determining the concentration of Orange II were recently reported, including photocatalysis (Cetinkaya, Neuwirthová, Kutláková, Tomášek, & Akbulut, 2013; Divya, Bansal, & Jana, 2013), capillary electrophoresis (PérezUrquiza, Ferrer, & Beltrán, 2000; Takeda et al., 1999), spectroscopy (Pourreza & Zareian, 2009), HPLC–MS (Fang et al., 2013; Zou, He, Yasen, & Li, 2013), and polarography (Guo et al., 2004). Although ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Üstündag˘), [email protected] (S. Jeon). http://dx.doi.org/10.1016/j.foodchem.2014.07.143 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.
these techniques, they are time-consuming and require complex instrumentation (You et al., 2011). The electrochemical (i.e. voltammetric) method, on the other hand, has high sensitivity, fast response speed, short analysis time, excellent selectivity, low cost, and handling convenience (Wang et al., 2013; You et al., 2011). Graphene oxide (GO), a single layer of graphite oxide, has abundant functional groups, such as epoxy and hydroxyl group. Therefore, a reaction between the functional groups of GO and the amine groups of an amine-terminated ionic liquid should easily occur (Yang et al., 2009). The introduction of a heterocyclic compound in the graphene sheets yielded a large surface-active groups-to-volume ratio, superb thermal stability, good electrical and mechanical properties (Ahmed, Han, & Jeon, 2013; Yang, Li, Rana, & Zhu, 2013). On the other hand, noble metals, such as Au, Pd, and Pt nanoparticles (NPs), have been used for electrocatalysis (Ahmed & Jeon, 2013; You et al., 2013). Among them, the Pt NPs have attracted much attention owing to its unique effects in electrocatalysis and in the enhancement of the electron transfer (Xu, He, Sun, & Wang, 2013). The loading of Pt NPs on the heterocycle-doped GO layers showed the synergy effects of a narrower particle size distribution and improved catalytic performance (Zhao, Zhou, Xiong, Wang, Chen, O’Hayre, & Shao, 2013; Zhang, Liang, Song, & Wu, 2010). In this work, we synthesise an electrochemical sensor for orange II detection with better selectivity, sensitivity and low detection limit. The synthesis of GO with 5-amino-1,3,4-thiadiazole-2-thiol
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(ATDT) and covalently bonded with Pt NPs (GO-ATDT-Pt) were pursued, and glassy carbon electrode (GCE) coated with GO-ATDT-Pt was electrochemically reduced (ERGO-ATDT-Pt). The electrochemical reduction efficiently removes the oxygen-containing functional groups from GO and exposes the electro-conductive (Filik et al., 2013). The interactions between ERGO and Pt NPs can enhance the catalytic activity for the electrochemical determination of Orange II. The as synthesised ERGO-ATDT-Pt was characterised via several instrumental techniques. This catalyst was then used to modify the GCE (ERGO-ATDT-Pt/GCE), and applied as an Orange II sensor using cyclic voltammetry (CV) and chronoamperometry (CA) techniques in 0.1 M ABS at pH 4.5. The ERGO-ATDT-Pt/GCE showed improved sensitivity to the electrocatalytic determination of Orange II at low concentration.
a BAS 100B/W voltammetric analyzer (Bioanalytical Systems, West Lafayette, IN, USA) at room temperature (RT), under an argon atmosphere. The pH measurements were performed using a pH glass electrode with a JENCO meter. XPS was performed using a VG Multilab 2000 spectrometer (ThermoVG Scientific, Southend on Sea, Essex, UK) in an ultra-high vacuum chamber. The survey scan data were collected at 50 eV pass energy. The morphologies of the materials were examined using a JSM-7500F JEOL field emission scanning electron microscope (FE-SEM) and a JEM-2100F JEOL field emission transmission electron microscope (FE-TEM). 2.3. Synthesis of the GO-ATDT-Pt NPs The synthesis procedure of GO-ATDT-Pt is shown in the hypothesised Scheme 1. GO was obtained through graphite oxidation, using an improved version of Hummer’s method (Marcano et al., 2010). A 9:1 mixture of concentrated H2SO4/H3PO4 was added to a 1:6 mixture of graphite and KMnO4 at 50 °C, and the resulting mixture was stirred for 12 h. After cooling at RT, the mixture was transferred onto ice with 30% H2O2 (3 mL). The resulting solution was centrifuged and then filtered. The filtered solid material was washed several times with water and was finally rinsed with 30% HCl. The GO was dispersed in tetrahydrofuran (THF) with ATDT into roundbottom flasks and was then stirred at 50 °C for 22 h. Thereafter, the materials were filtered, washed with THF and ethanol, and dried in a vacuum oven at 40 °C for 24 h. A mixture of 30 mg GO-ATDT and 10 mM H2PtCl6 was suspended by sonication in 30 mL deionised water. Then 0.1% NaBH3 solution as a reducing agent was slowly dropped into the mixture. The mixture was filtered and washed several times with deionised water. Finally, GO-ATDT-Pt NPs were obtained after drying in a vacuum oven at 40 °C for 1 day.
2. Experiment 2.1. Materials and chemicals The graphite power (325 mesh, 99.999%), Orange II sodium salt, ATDT, hydrogen hexachloroplatinate (IV) hydrate, and sodium borohydride (NaBH4) were from Aldrich, South Korea. The KMnO4, H2SO4, and H3PO4 were purchased from Daejung Co., South Korea. All the other reagents were of analytical grade and were used without further purification. A 0.1 M acetate buffer solution (ABS, at pH 4.5) was prepared with 0.1 M CH3COOH and 0.1 M CH3COONa. Double-distilled water was used in the preparation of the aqueous electrolyte solutions. 2.2. Equipment A three-electrode assembled cell consisting of a modified GCE working electrode (3.0 mm in diameter), a platinum-wire counter electrode, and an Ag/AgCl (3.0 M NaCl) reference electrode was employed. The electrochemical experiments, EIS, CV, and CA, were carried out using a VersaSTAT3 (Potentiostat, AT frontier Inc.), a CHI electrochemical workstation (CH Instruments, Inc., USA), and O
O
2.4. Preparation of ERGO-ATDT-Pt/GC electrode The surface of the GCE (3.0 mm diameter) was carefully polished with 0.05 lm alumina paste, was washed several times with distilled water, and was finally rinsed with methanol. The GCE OH
HO
OH
OH
OH
O
O OH
KMnO4 OH O
OO
H2SO4 / H3PO4
O
HO
H2 N OH
OH
OH
O
5-amino-1,3,4-thiodiazole-2-thiol (ATDT)
O
OH
O OH
OH
O
N
S
HS N
O
O
SH
S N
THF
N
(a) graphite
N H O
O
O
OH
H N
OH
OH
H N
(b) GO HS Pt
S
S N
N
N H
H N H N
Pt
S
S N
N
N H
O
O
OH OH
H N OH
S N
OH
O
O
O S N N H N O S N N
S N
N
N H O
Pt
O
O
OH OH
O
S
O
OH
H N
OH H N O
N
S
S N
(c) GO-ATDT Pt Pt
S
(e) ERGO-ATDT-Pt Electrochemical reduction
Pt
S N
Pt
S
N
N H O
S N
N
N H O
OH
O
O
OH
O
O
H N OH
OH
O
OH
H N
OH
OH
OO
H2PtCl6
O
O
OH H N O
O
S
S N N H N O S N N
S N
N
S
Pt
Pt S N
N
S
S
Pt
Pt
(d) GO-ATDT-Pt Scheme 1. The schematic electrochemical route of ERGO-ATDT-Pt synthesis.
S N
N
O
SH
N SH
S N
N
O
Pt
S N N S
S
N
OH
O O
S N
O
SH
SH
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surface was coated with a 10.0 lL black GO-ATDT-Pt suspension (1 mg/mL H2O). After drying at RT, GO-ATDT-Pt/GCE was reduced electrochemically through 30 successive cycles of CVs in an electrochemical cell containing 0.05 M PBS solution (pH 5) over a potential range of 0 to 1.5 V at a 50 mV s1 scan rate. The ERGO-ATDT-Pt/GCE was washed with distilled water before and after each experiment. All the experiments were carried out in an argon atmosphere, at RT. 2.5. Real sample preparation 2.5 g of chili sauce or ketchup samples were weighed into a beaker, and then 10 mL of acetonitrile was added. After they were mixed sufficiently, the beaker was placed in an ultrasonic bath for 25 min. The extract was filtered and filtrate was collected in a 50 mL centrifuge tube. The residual solid was repeatedly extracted three times with 10 mL acetonitrile. 1.0 g of NaCl and 2.0 mL of distilled water were added to the extract, and then the mixture was kept in the refrigerator for 3 h to remove lipids and some natural pigments (Gan et al., 2013). Finally, the upper acetonitrile layer was collected in a 50 mL volumetric flask. 3. Results and discussion 3.1. Characterization of the catalysts The surface morphology of ERGO-ATDT-Pt on the GCE plate is shown as SEM Image in Fig. 1(A). It can be seen that the ERGOATDT-Pt was slightly stretched and separated, indicated that its surface area was broadened. Fig. 1(B) shows the TEM image of the ERGO-ATDT-Pt. In the TEM image, the spherical-like Pt NPs are well dispersed and exhibited diameter of about 5–10 nm. Lattice lines can be observed, with a spacing of 0.23 nm, corresponding to the Pt (1 1 1) lattice spacing of face-centered cubic Pt (Zhou, Kang, Song, & Chen, 2012). The XPS spectra of the GO, GO-ATDT, GO-ATDT-Pt, and ERGOATDT-Pt are shown in Fig. 1(C). The as prepared GO showed distinct C and O 1s peaks at around 285 and 532.5 eV, respectively, with no other element detected (Ahmed & Jeon, 2012). The combination of GO and ATDT resulted in the emergence of S 2p (165.62 eV) and N 1s (399.97 eV) signals in the spectra. The Pt 4f peaks were evident for the GO-ATDT-Pt and ERGO-ATDT-Pt samples, indicating the growth of numerous Pt NPs covalently bonding from the Pt precursors on the GO-ATDT. The GO-ATDT-Pt and ERGO-ATDT-Pt spectrum presents a doublet corresponding to the Pt 4f7/2 (73.17 eV) and 4f5/2 (76.4 eV) peaks. The Pt 4f7/2 peaks shifted to higher binding energies compared to the bulk Pt0 peaks (71.2 eV) (You, Kim, & Jeon, 2012). The increased binding energy corresponds to a slightly charged state (d+) of the Pt NPs originating from the charge transfer from the Pt NPs to the S atoms (RSH þ M0n ! RS Mþ M0n1 þ 1=2H2 ) (Fu, Wang, Wu, Gui, & Tang, 2001; Kwon et al., 2011). The relative surface atomic compositions were estimated from the corresponding peak areas. The Pt contents were 0.91 and 0.78 at% for GO-ATDT-Pt and ERGO-ATDT-Pt, respectively. Fig. 1(C) shows the high resolution of the C 1s XPS spectra of the GO and functional GO products. Three different types of GO peaks at 285, 287.3, and 288.5 eV are presented, corresponding to C–C/C@C, C–O, and C@O, respectively (Ahmed & Jeon, 2014; Ahmed, Kim, & Jeon, 2013). The spectrum of GO-ATDT shows a significant decrease of the oxygen-containing groups. GO-ATDT-Pt shows a greater decrease of the oxygen-containing groups due to the Pt supported onto the GO-ATDT using NaBH4 as reducing agent. Finally, the oxygen content in the XPS spectra of the synthesised ERGO-ATDT-Pt was more decreased, and simultaneously, the C–C/C@C bonds dramatically increased.
EIS was used to probe the electrical impedance features of the modified electrode. Fig. 1(D) shows the Nyquist plots of the bare, GO-ATDT-Pt, and the ERGO-ATDT-Pt modified GCE. The typical Nyquist plot includes a semi-circle portion corresponding to the electron-transfer-limited process, with a diameter equal to the electron transfer resistance, and a linear portion representing the diffusion-limited process (Kim, Kim, You, Han, & Jeon, 2012). The bare GCE showed a semi-circle portion and a linear portion. The GO-ATDT-Pt/GCE appears a comparatively bigger semi-circle than GCE, indicating a high resistance surface at GO-ATDT-Pt/GCE due to resistant GO surface. On the other hand, almost linear plot can be observed on ERGO-ATDT-Pt/GCE due to highly conductive ERGO surface and Pt NPs, indicating a highly electro-conductive surface at ERGO-ATDT-Pt/GCE. This indicates that these electrodes showed controlled reaction through diffusion. The measured electrical-resistance values of the three modified electrodes are as follows: ERGO-ATDT-Pt/GCE < bare GCE < GO-ATDT-Pt/GCE. It was concluded that the electrical conductivity of ERGO-ATDT-Pt/GCE was enhanced due to electrochemical reduction.
3.2. Electrochemical application of ERGO-ATDT-Pt/GCE The voltammetric behaviour of Orange II at various modified electrodes was investigated via CV. The Fig. 2A shows the CVs of the bare GCE, the GO-ATDT-Pt/GCE, and the ERGO-ATDT-Pt/GCE in a 0.1 M ABS at pH 4.5 containing 50 lM Orange II at a 100 mV s1 scan rate. The oxidation peak potentials of Orange II were 0.144 V (GO-ATDT-Pt) and 0.163 V (ERGO-ATDT-Pt), respectively. The current response of ERGO-ATDT-Pt/GCE (125 lA) is much higher compared to GO-ATDT-Pt/GCE and bare GCE (inset Fig. 2A). Fig. 2(B) shows the scan rate dependent experiment on Orange II at ERGO-ATDT-Pt/GCE via CV. The redox peak currents of the modified electrode in the Orange II containing solution increased linearly with the increasing scan rate in Fig. 2(B), indicating that Orange II was adsorbed on the surfaces of the ERGO-ATDT-Pt modified electrode. As shown, the linearity of the plots is good with a 0.9985 correlation coefficient. The CVs curves were nicely separated from each other and redox peaks were stronger with the increasing of scan rare, as shown in the inset of Fig. 2(B). The influence of the accumulation time on the oxidation peak current of Orange II was investigated. The current increased rapidly within the accumulation time of 90 s, and then increased gently. The oxidation peak current slightly increased when the accumulation time was too long (longer than 120 s), indicating that the adsorption on the ERGO-ATDT-Pt/GCE surface was nearly finished after 120 s. An accumulation time of 120 s was selected considering both sensitivity and working efficiency. Fig. 2(C) shows the CV recording at various concentrations (20–70 lM) of Orange II at the ERGO-ATDT-Pt/GCE at the 100 mV s1 scan rate. A reversible redox peaks were observed for Orange II and the peak currents increased with the increasing of Orange II concentrations. Fig. 2(C) (inset) shows that the oxidation current was proportional to the concentration of Orange II, based on the following linear regression equation: ipa (lA) = 1.075 [COrange II] (lM) 1.337. The plots showed good linearity with a 0.9967 correlation coefficient. The amperometric responses at the ERGO-ATDT-Pt/GCE to the subsequent additions of Orange II in the 0.1 M ABS at pH 4.5 at 100 mV s1 are illustrated in Fig. 2(D). Fig. 2(D) (inset) provides the plot of the response currents vs. the concentrations of Orange II. In the 10–600 nM concentration region, the linear regression equation was ip (nA) = 0.3536 [COrange II] (nM) + 8.1558, with a 0.9977 correlation coefficient and a 3.4 1010 M (s/n = 3) detection limit.
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Fig. 1. (A) FE-SEM image and (B) FE-TEM image of ERGO-ATDT-Pt. (C) XPS survey spectra of GO, GO-ATDT, GO-ATDT-Pt and ERGO-ATDT-Pt, and core-level XPS spectra of C 1s. (D) Nyquist plots of EIS for bare GCE, GO-ATDT-Pt/GCE and ERGO-ATDT-Pt/GCE in a 0.1 M KCl solution containing 5 mM [Fe(CN)6]3/[Fe(CN)6]4.
The Table S1 displays a comparison of previous results with our result for the determination of Orange II with various methods. Studies of Orange II dye in areas such as HPLC (Lim et al., 2013; Zou et al., 2013) and photocatalytic degradation (Chen et al., 2013; Moualkia, Rekhila, Izerrouken, Mahdjoub, & Trari, 2014; Riaz, Chong, Man, Khan, & Dutta, 2013) are very recent and have compared; however, the determination by using voltammetry has been hardly studied yet. The detection limit of Orange II on the ERGO-ATDT-Pt/GCE was calculated at very low concentration as 3.4 1010 M (s/n = 3) using CA, with a wide range of linear range (1 108–6 107 M), suggesting that Orange II could be easily detected with amperometric method even at very low
concentration. Therefore, the developed electrode contributes to superior performance for the determination of Orange II dye. 3.3. Repeatability, precise and robustness of ERGO-ATDT-Pt/GCE To evaluate of the repeatability and precise on the ERGOATDT-Pt/GCE tested with the seven modified electrodes and compared the oxidation peak current of each electrodes in 50 lM Orange II. As a result, the relative standard deviation (RSD) is 5.06%, indicating that this electrode had good repeatability and precise. Also, in order to determine the robustness of sensor for repeated electrochemical performance, the original buffer current
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Fig. 2. (A) The CVs on bare GCE, GO-ATDT-Pt/GCE, and ERGO-ATDT-Pt/GCE in 50 lM at scan rate of 100 mV s1; (B) the plot of redox peak current vs. scan rate, inset: the scan rate dependent CVs in presence of 20 lM at 30–300 mV s1 scan rates; (C) the concentration dependent CVs in 0, 2, 5, 10, 20, 30, 50, and 70 lM, inset: the plot of oxidation peak current vs. concentration; (D) amperometric responses of ERGO-ATDT-Pt/GCE in various concentrations (10–600 nM) at 100 mV applied potential. Inset: plot of amperometric currents vs. concentration; in Orange II containing 0.1 M ABS (pH 4.5).
Fig. 3. (A) Comparison with the original buffer current (the absence of Orange II) after 50 times CV scans of ERGO-ATDT-Pt/GCE in 0.1 M ABS at pH 4.5. (B) The effect of interferents (Sudan I, Methyl orange, Methyl red and Congo red of each 200 nM) for 200 nM Orange II of ERGO-ATDT-Pt/GCE at a constant potential of 0.1 V.
was compared to the measured currents after 50 times scans using CV in the absence of Orange II. As in Fig. 3(A), the original signaling current of ERGO-ATDT-Pt/GCE was substantially maintained after measuring 50 times.
II by Sudan I 9.66%, Methyl orange 6.50%, Methyl red 7.49%, Congo red 7.38%, respectively. The results demonstrated that the developed electrochemical sensor has a good selectivity to the Orange II detection.
3.4. Interference
3.5. Analysis of real samples
Interferences for several other compounds with similar structure with Orange II were investigated by CA (Fig. 3(B)). Sudan I, Methyl orange, Methyl red and Congo red are an azo dye containing one or more azo groups (R1–N = N–R2) like Orange II. The interfering responses to the subsequent addition of Orange II, Sudan I, Methyl orange, Methyl red, Congo red and Orange II of each 200 nM concentrations were measured at a constant potential of 100 mV. The interfering substances showed much smaller currents than Orange II; their currents were increased compared to Orange
In order to prove the validity of this method, Orange II in some chili sauce and ketchup samples was measured. Samples were prepared in accordance with procedures described in materials and chemicals section. No peak current of Orange II was observed in these samples and other peaks by interfering substances were not appeared. Further, each of the samples was measured by adding Orange II standard solution, and then the recoveries and RSD were calculated as shown in Table 1. Almost all recoveries were more than 97% and the RSD was calculated as below 5%, indicating
M. Yun et al. / Food Chemistry 169 (2015) 114–119 Table 1 Real sample analysis results of Orange II in chili sauce and ketchup samples. Sample
Added (nM)
Found (nM)
RSD (%)
Recovery (%)
Chili sauce
50 100 200
48.8 97.7 201.2
3.9 2.8 3.7
97.6 97.7 100.6
Ketchup
50 100 200
49.2 96.8 202.5
3.3 4.7 4.1
98.4 96.8 101.25
that no Orange II is contained in these samples and the precision of this method is good. It is determined that the measured samples did not contain the Orange II dye, in accordance with the prohibition for the addition of the Orange II dye in food. 4. Conclusions In conclusion, electrochemically reduced GO grafted with 5-amino-1,3,4-thiadiazole-2-thiol-Pt NPs was prepared with GO functionalised with ATDT, covalent bonding with Pt NPs, and electrochemical reduction of GO-ATDT-Pt. The developed ERGOATDT-Pt/GCE sensor displayed a reversible redox peaks for Orange II in 0.1 M ABS at pH 4.5. ERGO-ATDT-Pt/GCE exhibited outstanding electrocatalytic activity towards the determination of Orange II by improving the redox peak currents and lowering the oxidation overpotential. The detection limit on ERGO-ATDT-Pt/GCE for Orange II was 3.4 1010 M (s/n = 3). The ERGO-ATDT-Pt/GCE showed good selectivity and sensitivity for the determination of Orange II. Acknowledgements This research was supported by the Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (20100007864). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.foodchem.2014. 07.143. Reference Ahmed, M. S., Han, H. S., & Jeon, S. (2013a). One-step chemical reduction of graphene oxide with oligothiophene for improved electrocatalytic oxygen reduction reactions. Carbon, 61, 164–172. Ahmed, M. S., & Jeon, S. (2012). New functionalized graphene sheets for enhanced oxygen reduction as metal-free cathode electrocatalysts. Journal of Power Sources, 218, 168–173. Ahmed, M. S., & Jeon, S. (2013). The nanostructure of nitrogen atom linked carbon nanotubes with platinum employed to the electrocatalytic oxygen reduction. Journal of Nanoscience and Nanotechnology, 13, 306–314. Ahmed, M. S., & Jeon, S. (2014). Highly active graphene-supported NixPd100x binary alloyed catalysts for electro-oxidation of ethanol in an alkaline media. ACS Catalysis, 4, 1830–1837. Ahmed, M. S., Kim, D., & Jeon, S. (2013b). Covalently grafted platinum nanoparticles to multi walled carbon nanotubes for enhanced electrocatalytic oxygen reduction. Electrochimica Acta, 92, 168–175. Cetinkaya, T., Neuwirthová, L., Kutláková, K. M., Tomášek, V., & Akbulut, H. (2013). Synthesis of nanostructured TiO2/SiO2 as an effective photocatalyst for degradation of acid orange. Applied Surface Science, 279, 384–390. Chen, P. K., Lee, G. J., Davies, S. H., Masten, S. J., Amutha, R., & Wu, J. J. (2013). Hydrothermal synthesis of coral-like Au/ZnO catalyst and photocatalytic degradation of Orange II dye. Materials Research Bulletin, 48, 2375–2382. Divya, N., Bansal, A., & Jana, A. K. (2013). Photocatalytic degradation of azo dye Orange II in aqueous solutions using copper-impregnated titania. International Journal of Environmental Science and Technology, 10, 1265–1274.
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Food Chemistry journal homepage: www.elsevier.com/locate/foodchem
Analytical Methods
High catalytic activity of electrochemically reduced graphene composite toward electrochemical sensing of Orange II Mira Yun a, Ju Eun Choe a, Jung-Min You a, Mohammad Shamsuddin Ahmed a, Kyungmi Lee a, Zafer Üstündag˘ b,⇑, Seungwon Jeon a,⇑ a b
Department of Chemistry and Institute of Basic Science, Chonnam National University, Gwangju 500-757, Republic of Korea Department of Chemistry, Faculty of Arts and Science, Dumlupınar University, 43100 Kutahya, Turkey
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 5 June 2014 Accepted 30 July 2014 Available online 7 August 2014 Keywords: Azo dye Electrochemically reduced GO Electrochemical sensor Orange II Platinum nanoparticles
a b s t r a c t Orange II, an azo dye, is sometimes illegally used as a red dye in food products despite its adverse health effects if consumed. Therefore, the determination of low concentrations of Orange II is an important target. An Orange II sensor was prepared using electrochemically reduced graphene oxide grafted with 5-amino-1,3,4-thiadiazole-2-thiol-Pt nanoparticles (denoted as ERGO-ATDT-Pt) onto a glassy carbon electrode (GCE) and investigated for Orange II detection in 0.1 M acetate buffer solution (ABS at pH 4.5) with prominent reversible redox peaks. A wide linear range of 1 108–6 107 M with a low detection limit of 3.4 1010 M (s/n = 3) was found for Orange II detection. This developed ERGOATDT-Pt/GCE sensor showed good selectivity, excellent stability and better response to the real sample analysis with excellent recovery. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction Orange II (sodium 4-[2-(2-oxonaphthalen-1-ylidene)hydrazinyl]benzenesulfonate) is an azo dye containing one or more azo groups (R1–N = N–R2) tinged with colour through visible-light absorption. It is used in many industries, such as for organic lightemitting diodes (OLEDs), inks, soaps, wood preservatives, textiles, hair dyes, leather materials, shoe polishes, cosmetics, wood stains, and foodstuffs (Yadav, Kumar, Dwivedi, Tripathi, & Das, 2012). Many countries, however, have regulated the use of Orange II in foodstuffs because it poses a risk to human health as carcinogenic (Guo, Pan, & Jing, 2004; Pourreza & Zareian, 2009), and reduces the number of red blood cells, accompanied by the lowering of hemoglobin and packed cell volume (Gan, Sun, Lin, & Li, 2013). Hence, the determination of low concentrations of Orange II is an important target. Methods of determining the concentration of Orange II were recently reported, including photocatalysis (Cetinkaya, Neuwirthová, Kutláková, Tomášek, & Akbulut, 2013; Divya, Bansal, & Jana, 2013), capillary electrophoresis (PérezUrquiza, Ferrer, & Beltrán, 2000; Takeda et al., 1999), spectroscopy (Pourreza & Zareian, 2009), HPLC–MS (Fang et al., 2013; Zou, He, Yasen, & Li, 2013), and polarography (Guo et al., 2004). Although ⇑ Corresponding authors. E-mail addresses: [email protected] (Z. Üstündag˘), [email protected] (S. Jeon). http://dx.doi.org/10.1016/j.foodchem.2014.07.143 0308-8146/Ó 2014 Elsevier Ltd. All rights reserved.
these techniques, they are time-consuming and require complex instrumentation (You et al., 2011). The electrochemical (i.e. voltammetric) method, on the other hand, has high sensitivity, fast response speed, short analysis time, excellent selectivity, low cost, and handling convenience (Wang et al., 2013; You et al., 2011). Graphene oxide (GO), a single layer of graphite oxide, has abundant functional groups, such as epoxy and hydroxyl group. Therefore, a reaction between the functional groups of GO and the amine groups of an amine-terminated ionic liquid should easily occur (Yang et al., 2009). The introduction of a heterocyclic compound in the graphene sheets yielded a large surface-active groups-to-volume ratio, superb thermal stability, good electrical and mechanical properties (Ahmed, Han, & Jeon, 2013; Yang, Li, Rana, & Zhu, 2013). On the other hand, noble metals, such as Au, Pd, and Pt nanoparticles (NPs), have been used for electrocatalysis (Ahmed & Jeon, 2013; You et al., 2013). Among them, the Pt NPs have attracted much attention owing to its unique effects in electrocatalysis and in the enhancement of the electron transfer (Xu, He, Sun, & Wang, 2013). The loading of Pt NPs on the heterocycle-doped GO layers showed the synergy effects of a narrower particle size distribution and improved catalytic performance (Zhao, Zhou, Xiong, Wang, Chen, O’Hayre, & Shao, 2013; Zhang, Liang, Song, & Wu, 2010). In this work, we synthesise an electrochemical sensor for orange II detection with better selectivity, sensitivity and low detection limit. The synthesis of GO with 5-amino-1,3,4-thiadiazole-2-thiol
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(ATDT) and covalently bonded with Pt NPs (GO-ATDT-Pt) were pursued, and glassy carbon electrode (GCE) coated with GO-ATDT-Pt was electrochemically reduced (ERGO-ATDT-Pt). The electrochemical reduction efficiently removes the oxygen-containing functional groups from GO and exposes the electro-conductive (Filik et al., 2013). The interactions between ERGO and Pt NPs can enhance the catalytic activity for the electrochemical determination of Orange II. The as synthesised ERGO-ATDT-Pt was characterised via several instrumental techniques. This catalyst was then used to modify the GCE (ERGO-ATDT-Pt/GCE), and applied as an Orange II sensor using cyclic voltammetry (CV) and chronoamperometry (CA) techniques in 0.1 M ABS at pH 4.5. The ERGO-ATDT-Pt/GCE showed improved sensitivity to the electrocatalytic determination of Orange II at low concentration.
a BAS 100B/W voltammetric analyzer (Bioanalytical Systems, West Lafayette, IN, USA) at room temperature (RT), under an argon atmosphere. The pH measurements were performed using a pH glass electrode with a JENCO meter. XPS was performed using a VG Multilab 2000 spectrometer (ThermoVG Scientific, Southend on Sea, Essex, UK) in an ultra-high vacuum chamber. The survey scan data were collected at 50 eV pass energy. The morphologies of the materials were examined using a JSM-7500F JEOL field emission scanning electron microscope (FE-SEM) and a JEM-2100F JEOL field emission transmission electron microscope (FE-TEM). 2.3. Synthesis of the GO-ATDT-Pt NPs The synthesis procedure of GO-ATDT-Pt is shown in the hypothesised Scheme 1. GO was obtained through graphite oxidation, using an improved version of Hummer’s method (Marcano et al., 2010). A 9:1 mixture of concentrated H2SO4/H3PO4 was added to a 1:6 mixture of graphite and KMnO4 at 50 °C, and the resulting mixture was stirred for 12 h. After cooling at RT, the mixture was transferred onto ice with 30% H2O2 (3 mL). The resulting solution was centrifuged and then filtered. The filtered solid material was washed several times with water and was finally rinsed with 30% HCl. The GO was dispersed in tetrahydrofuran (THF) with ATDT into roundbottom flasks and was then stirred at 50 °C for 22 h. Thereafter, the materials were filtered, washed with THF and ethanol, and dried in a vacuum oven at 40 °C for 24 h. A mixture of 30 mg GO-ATDT and 10 mM H2PtCl6 was suspended by sonication in 30 mL deionised water. Then 0.1% NaBH3 solution as a reducing agent was slowly dropped into the mixture. The mixture was filtered and washed several times with deionised water. Finally, GO-ATDT-Pt NPs were obtained after drying in a vacuum oven at 40 °C for 1 day.
2. Experiment 2.1. Materials and chemicals The graphite power (325 mesh, 99.999%), Orange II sodium salt, ATDT, hydrogen hexachloroplatinate (IV) hydrate, and sodium borohydride (NaBH4) were from Aldrich, South Korea. The KMnO4, H2SO4, and H3PO4 were purchased from Daejung Co., South Korea. All the other reagents were of analytical grade and were used without further purification. A 0.1 M acetate buffer solution (ABS, at pH 4.5) was prepared with 0.1 M CH3COOH and 0.1 M CH3COONa. Double-distilled water was used in the preparation of the aqueous electrolyte solutions. 2.2. Equipment A three-electrode assembled cell consisting of a modified GCE working electrode (3.0 mm in diameter), a platinum-wire counter electrode, and an Ag/AgCl (3.0 M NaCl) reference electrode was employed. The electrochemical experiments, EIS, CV, and CA, were carried out using a VersaSTAT3 (Potentiostat, AT frontier Inc.), a CHI electrochemical workstation (CH Instruments, Inc., USA), and O
O
2.4. Preparation of ERGO-ATDT-Pt/GC electrode The surface of the GCE (3.0 mm diameter) was carefully polished with 0.05 lm alumina paste, was washed several times with distilled water, and was finally rinsed with methanol. The GCE OH
HO
OH
OH
OH
O
O OH
KMnO4 OH O
OO
H2SO4 / H3PO4
O
HO
H2 N OH
OH
OH
O
5-amino-1,3,4-thiodiazole-2-thiol (ATDT)
O
OH
O OH
OH
O
N
S
HS N
O
O
SH
S N
THF
N
(a) graphite
N H O
O
O
OH
H N
OH
OH
H N
(b) GO HS Pt
S
S N
N
N H
H N H N
Pt
S
S N
N
N H
O
O
OH OH
H N OH
S N
OH
O
O
O S N N H N O S N N
S N
N
N H O
Pt
O
O
OH OH
O
S
O
OH
H N
OH H N O
N
S
S N
(c) GO-ATDT Pt Pt
S
(e) ERGO-ATDT-Pt Electrochemical reduction
Pt
S N
Pt
S
N
N H O
S N
N
N H O
OH
O
O
OH
O
O
H N OH
OH
O
OH
H N
OH
OH
OO
H2PtCl6
O
O
OH H N O
O
S
S N N H N O S N N
S N
N
S
Pt
Pt S N
N
S
S
Pt
Pt
(d) GO-ATDT-Pt Scheme 1. The schematic electrochemical route of ERGO-ATDT-Pt synthesis.
S N
N
O
SH
N SH
S N
N
O
Pt
S N N S
S
N
OH
O O
S N
O
SH
SH
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surface was coated with a 10.0 lL black GO-ATDT-Pt suspension (1 mg/mL H2O). After drying at RT, GO-ATDT-Pt/GCE was reduced electrochemically through 30 successive cycles of CVs in an electrochemical cell containing 0.05 M PBS solution (pH 5) over a potential range of 0 to 1.5 V at a 50 mV s1 scan rate. The ERGO-ATDT-Pt/GCE was washed with distilled water before and after each experiment. All the experiments were carried out in an argon atmosphere, at RT. 2.5. Real sample preparation 2.5 g of chili sauce or ketchup samples were weighed into a beaker, and then 10 mL of acetonitrile was added. After they were mixed sufficiently, the beaker was placed in an ultrasonic bath for 25 min. The extract was filtered and filtrate was collected in a 50 mL centrifuge tube. The residual solid was repeatedly extracted three times with 10 mL acetonitrile. 1.0 g of NaCl and 2.0 mL of distilled water were added to the extract, and then the mixture was kept in the refrigerator for 3 h to remove lipids and some natural pigments (Gan et al., 2013). Finally, the upper acetonitrile layer was collected in a 50 mL volumetric flask. 3. Results and discussion 3.1. Characterization of the catalysts The surface morphology of ERGO-ATDT-Pt on the GCE plate is shown as SEM Image in Fig. 1(A). It can be seen that the ERGOATDT-Pt was slightly stretched and separated, indicated that its surface area was broadened. Fig. 1(B) shows the TEM image of the ERGO-ATDT-Pt. In the TEM image, the spherical-like Pt NPs are well dispersed and exhibited diameter of about 5–10 nm. Lattice lines can be observed, with a spacing of 0.23 nm, corresponding to the Pt (1 1 1) lattice spacing of face-centered cubic Pt (Zhou, Kang, Song, & Chen, 2012). The XPS spectra of the GO, GO-ATDT, GO-ATDT-Pt, and ERGOATDT-Pt are shown in Fig. 1(C). The as prepared GO showed distinct C and O 1s peaks at around 285 and 532.5 eV, respectively, with no other element detected (Ahmed & Jeon, 2012). The combination of GO and ATDT resulted in the emergence of S 2p (165.62 eV) and N 1s (399.97 eV) signals in the spectra. The Pt 4f peaks were evident for the GO-ATDT-Pt and ERGO-ATDT-Pt samples, indicating the growth of numerous Pt NPs covalently bonding from the Pt precursors on the GO-ATDT. The GO-ATDT-Pt and ERGO-ATDT-Pt spectrum presents a doublet corresponding to the Pt 4f7/2 (73.17 eV) and 4f5/2 (76.4 eV) peaks. The Pt 4f7/2 peaks shifted to higher binding energies compared to the bulk Pt0 peaks (71.2 eV) (You, Kim, & Jeon, 2012). The increased binding energy corresponds to a slightly charged state (d+) of the Pt NPs originating from the charge transfer from the Pt NPs to the S atoms (RSH þ M0n ! RS Mþ M0n1 þ 1=2H2 ) (Fu, Wang, Wu, Gui, & Tang, 2001; Kwon et al., 2011). The relative surface atomic compositions were estimated from the corresponding peak areas. The Pt contents were 0.91 and 0.78 at% for GO-ATDT-Pt and ERGO-ATDT-Pt, respectively. Fig. 1(C) shows the high resolution of the C 1s XPS spectra of the GO and functional GO products. Three different types of GO peaks at 285, 287.3, and 288.5 eV are presented, corresponding to C–C/C@C, C–O, and C@O, respectively (Ahmed & Jeon, 2014; Ahmed, Kim, & Jeon, 2013). The spectrum of GO-ATDT shows a significant decrease of the oxygen-containing groups. GO-ATDT-Pt shows a greater decrease of the oxygen-containing groups due to the Pt supported onto the GO-ATDT using NaBH4 as reducing agent. Finally, the oxygen content in the XPS spectra of the synthesised ERGO-ATDT-Pt was more decreased, and simultaneously, the C–C/C@C bonds dramatically increased.
EIS was used to probe the electrical impedance features of the modified electrode. Fig. 1(D) shows the Nyquist plots of the bare, GO-ATDT-Pt, and the ERGO-ATDT-Pt modified GCE. The typical Nyquist plot includes a semi-circle portion corresponding to the electron-transfer-limited process, with a diameter equal to the electron transfer resistance, and a linear portion representing the diffusion-limited process (Kim, Kim, You, Han, & Jeon, 2012). The bare GCE showed a semi-circle portion and a linear portion. The GO-ATDT-Pt/GCE appears a comparatively bigger semi-circle than GCE, indicating a high resistance surface at GO-ATDT-Pt/GCE due to resistant GO surface. On the other hand, almost linear plot can be observed on ERGO-ATDT-Pt/GCE due to highly conductive ERGO surface and Pt NPs, indicating a highly electro-conductive surface at ERGO-ATDT-Pt/GCE. This indicates that these electrodes showed controlled reaction through diffusion. The measured electrical-resistance values of the three modified electrodes are as follows: ERGO-ATDT-Pt/GCE < bare GCE < GO-ATDT-Pt/GCE. It was concluded that the electrical conductivity of ERGO-ATDT-Pt/GCE was enhanced due to electrochemical reduction.
3.2. Electrochemical application of ERGO-ATDT-Pt/GCE The voltammetric behaviour of Orange II at various modified electrodes was investigated via CV. The Fig. 2A shows the CVs of the bare GCE, the GO-ATDT-Pt/GCE, and the ERGO-ATDT-Pt/GCE in a 0.1 M ABS at pH 4.5 containing 50 lM Orange II at a 100 mV s1 scan rate. The oxidation peak potentials of Orange II were 0.144 V (GO-ATDT-Pt) and 0.163 V (ERGO-ATDT-Pt), respectively. The current response of ERGO-ATDT-Pt/GCE (125 lA) is much higher compared to GO-ATDT-Pt/GCE and bare GCE (inset Fig. 2A). Fig. 2(B) shows the scan rate dependent experiment on Orange II at ERGO-ATDT-Pt/GCE via CV. The redox peak currents of the modified electrode in the Orange II containing solution increased linearly with the increasing scan rate in Fig. 2(B), indicating that Orange II was adsorbed on the surfaces of the ERGO-ATDT-Pt modified electrode. As shown, the linearity of the plots is good with a 0.9985 correlation coefficient. The CVs curves were nicely separated from each other and redox peaks were stronger with the increasing of scan rare, as shown in the inset of Fig. 2(B). The influence of the accumulation time on the oxidation peak current of Orange II was investigated. The current increased rapidly within the accumulation time of 90 s, and then increased gently. The oxidation peak current slightly increased when the accumulation time was too long (longer than 120 s), indicating that the adsorption on the ERGO-ATDT-Pt/GCE surface was nearly finished after 120 s. An accumulation time of 120 s was selected considering both sensitivity and working efficiency. Fig. 2(C) shows the CV recording at various concentrations (20–70 lM) of Orange II at the ERGO-ATDT-Pt/GCE at the 100 mV s1 scan rate. A reversible redox peaks were observed for Orange II and the peak currents increased with the increasing of Orange II concentrations. Fig. 2(C) (inset) shows that the oxidation current was proportional to the concentration of Orange II, based on the following linear regression equation: ipa (lA) = 1.075 [COrange II] (lM) 1.337. The plots showed good linearity with a 0.9967 correlation coefficient. The amperometric responses at the ERGO-ATDT-Pt/GCE to the subsequent additions of Orange II in the 0.1 M ABS at pH 4.5 at 100 mV s1 are illustrated in Fig. 2(D). Fig. 2(D) (inset) provides the plot of the response currents vs. the concentrations of Orange II. In the 10–600 nM concentration region, the linear regression equation was ip (nA) = 0.3536 [COrange II] (nM) + 8.1558, with a 0.9977 correlation coefficient and a 3.4 1010 M (s/n = 3) detection limit.
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Fig. 1. (A) FE-SEM image and (B) FE-TEM image of ERGO-ATDT-Pt. (C) XPS survey spectra of GO, GO-ATDT, GO-ATDT-Pt and ERGO-ATDT-Pt, and core-level XPS spectra of C 1s. (D) Nyquist plots of EIS for bare GCE, GO-ATDT-Pt/GCE and ERGO-ATDT-Pt/GCE in a 0.1 M KCl solution containing 5 mM [Fe(CN)6]3/[Fe(CN)6]4.
The Table S1 displays a comparison of previous results with our result for the determination of Orange II with various methods. Studies of Orange II dye in areas such as HPLC (Lim et al., 2013; Zou et al., 2013) and photocatalytic degradation (Chen et al., 2013; Moualkia, Rekhila, Izerrouken, Mahdjoub, & Trari, 2014; Riaz, Chong, Man, Khan, & Dutta, 2013) are very recent and have compared; however, the determination by using voltammetry has been hardly studied yet. The detection limit of Orange II on the ERGO-ATDT-Pt/GCE was calculated at very low concentration as 3.4 1010 M (s/n = 3) using CA, with a wide range of linear range (1 108–6 107 M), suggesting that Orange II could be easily detected with amperometric method even at very low
concentration. Therefore, the developed electrode contributes to superior performance for the determination of Orange II dye. 3.3. Repeatability, precise and robustness of ERGO-ATDT-Pt/GCE To evaluate of the repeatability and precise on the ERGOATDT-Pt/GCE tested with the seven modified electrodes and compared the oxidation peak current of each electrodes in 50 lM Orange II. As a result, the relative standard deviation (RSD) is 5.06%, indicating that this electrode had good repeatability and precise. Also, in order to determine the robustness of sensor for repeated electrochemical performance, the original buffer current
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Fig. 2. (A) The CVs on bare GCE, GO-ATDT-Pt/GCE, and ERGO-ATDT-Pt/GCE in 50 lM at scan rate of 100 mV s1; (B) the plot of redox peak current vs. scan rate, inset: the scan rate dependent CVs in presence of 20 lM at 30–300 mV s1 scan rates; (C) the concentration dependent CVs in 0, 2, 5, 10, 20, 30, 50, and 70 lM, inset: the plot of oxidation peak current vs. concentration; (D) amperometric responses of ERGO-ATDT-Pt/GCE in various concentrations (10–600 nM) at 100 mV applied potential. Inset: plot of amperometric currents vs. concentration; in Orange II containing 0.1 M ABS (pH 4.5).
Fig. 3. (A) Comparison with the original buffer current (the absence of Orange II) after 50 times CV scans of ERGO-ATDT-Pt/GCE in 0.1 M ABS at pH 4.5. (B) The effect of interferents (Sudan I, Methyl orange, Methyl red and Congo red of each 200 nM) for 200 nM Orange II of ERGO-ATDT-Pt/GCE at a constant potential of 0.1 V.
was compared to the measured currents after 50 times scans using CV in the absence of Orange II. As in Fig. 3(A), the original signaling current of ERGO-ATDT-Pt/GCE was substantially maintained after measuring 50 times.
II by Sudan I 9.66%, Methyl orange 6.50%, Methyl red 7.49%, Congo red 7.38%, respectively. The results demonstrated that the developed electrochemical sensor has a good selectivity to the Orange II detection.
3.4. Interference
3.5. Analysis of real samples
Interferences for several other compounds with similar structure with Orange II were investigated by CA (Fig. 3(B)). Sudan I, Methyl orange, Methyl red and Congo red are an azo dye containing one or more azo groups (R1–N = N–R2) like Orange II. The interfering responses to the subsequent addition of Orange II, Sudan I, Methyl orange, Methyl red, Congo red and Orange II of each 200 nM concentrations were measured at a constant potential of 100 mV. The interfering substances showed much smaller currents than Orange II; their currents were increased compared to Orange
In order to prove the validity of this method, Orange II in some chili sauce and ketchup samples was measured. Samples were prepared in accordance with procedures described in materials and chemicals section. No peak current of Orange II was observed in these samples and other peaks by interfering substances were not appeared. Further, each of the samples was measured by adding Orange II standard solution, and then the recoveries and RSD were calculated as shown in Table 1. Almost all recoveries were more than 97% and the RSD was calculated as below 5%, indicating
M. Yun et al. / Food Chemistry 169 (2015) 114–119 Table 1 Real sample analysis results of Orange II in chili sauce and ketchup samples. Sample
Added (nM)
Found (nM)
RSD (%)
Recovery (%)
Chili sauce
50 100 200
48.8 97.7 201.2
3.9 2.8 3.7
97.6 97.7 100.6
Ketchup
50 100 200
49.2 96.8 202.5
3.3 4.7 4.1
98.4 96.8 101.25
that no Orange II is contained in these samples and the precision of this method is good. It is determined that the measured samples did not contain the Orange II dye, in accordance with the prohibition for the addition of the Orange II dye in food. 4. Conclusions In conclusion, electrochemically reduced GO grafted with 5-amino-1,3,4-thiadiazole-2-thiol-Pt NPs was prepared with GO functionalised with ATDT, covalent bonding with Pt NPs, and electrochemical reduction of GO-ATDT-Pt. The developed ERGOATDT-Pt/GCE sensor displayed a reversible redox peaks for Orange II in 0.1 M ABS at pH 4.5. ERGO-ATDT-Pt/GCE exhibited outstanding electrocatalytic activity towards the determination of Orange II by improving the redox peak currents and lowering the oxidation overpotential. The detection limit on ERGO-ATDT-Pt/GCE for Orange II was 3.4 1010 M (s/n = 3). The ERGO-ATDT-Pt/GCE showed good selectivity and sensitivity for the determination of Orange II. Acknowledgements This research was supported by the Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Education, Science and Technology (20100007864). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.foodchem.2014. 07.143. Reference Ahmed, M. S., Han, H. S., & Jeon, S. (2013a). One-step chemical reduction of graphene oxide with oligothiophene for improved electrocatalytic oxygen reduction reactions. Carbon, 61, 164–172. Ahmed, M. S., & Jeon, S. (2012). New functionalized graphene sheets for enhanced oxygen reduction as metal-free cathode electrocatalysts. Journal of Power Sources, 218, 168–173. Ahmed, M. S., & Jeon, S. (2013). The nanostructure of nitrogen atom linked carbon nanotubes with platinum employed to the electrocatalytic oxygen reduction. Journal of Nanoscience and Nanotechnology, 13, 306–314. Ahmed, M. S., & Jeon, S. (2014). Highly active graphene-supported NixPd100x binary alloyed catalysts for electro-oxidation of ethanol in an alkaline media. ACS Catalysis, 4, 1830–1837. Ahmed, M. S., Kim, D., & Jeon, S. (2013b). Covalently grafted platinum nanoparticles to multi walled carbon nanotubes for enhanced electrocatalytic oxygen reduction. Electrochimica Acta, 92, 168–175. Cetinkaya, T., Neuwirthová, L., Kutláková, K. M., Tomášek, V., & Akbulut, H. (2013). Synthesis of nanostructured TiO2/SiO2 as an effective photocatalyst for degradation of acid orange. Applied Surface Science, 279, 384–390. Chen, P. K., Lee, G. J., Davies, S. H., Masten, S. J., Amutha, R., & Wu, J. J. (2013). Hydrothermal synthesis of coral-like Au/ZnO catalyst and photocatalytic degradation of Orange II dye. Materials Research Bulletin, 48, 2375–2382. Divya, N., Bansal, A., & Jana, A. K. (2013). Photocatalytic degradation of azo dye Orange II in aqueous solutions using copper-impregnated titania. International Journal of Environmental Science and Technology, 10, 1265–1274.
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