Food Chemistry 151 (2014) 53–57
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Sensitive voltammetric determination of vanillin with an AuPd nanoparticles graphene composite modified electrode Lei Shang, Faqiong Zhao, Baizhao Zeng ⇑ The Key Laboratory of Analytical Chemistry for Biology and Medicine, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, PR China
a r t i c l e
i n f o
Article history: Received 22 June 2013 Received in revised form 1 November 2013 Accepted 10 November 2013 Available online 18 November 2013 Keywords: AuPd nanoparticles Graphene Vanillin Voltammetry Electrochemical sensor
a b s t r a c t In this work, graphene oxide was reduced to graphene with an endogenous reducing agent from dimethylformamide, and then AuPd alloy nanoparticles were electrodeposited on the graphene film. The obtained AuPd–graphene hybrid film was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and voltammetry. The electrochemical behavior of vanillin was studied using the AuPd–graphene hybrid based electrode. It presented high electrocatalytic activity and vanillin could produce a sensitive oxidation peak at it. Under the optimal conditions, the peak current was linear to the concentration of vanillin in the ranges of 0.1–7 and 10–40 lM. The sensitivities were 1.60 and 0.170 mA mM 1 cm 2, respectively; the detection limit was 20 nM. The electrode was successfully applied to the detection of vanillin in vanilla bean, vanilla tea and biscuit samples. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Vanillin (i.e. 4-hydroxy-3-methoxybenzaldehyde) has unique aroma and good antioxidative and antimicrobial property (Teissedre & Waterhouse, 2000). It is widely used in producing confectionery, beverage and medicine (Anklam, Gaglione, & Muller, 1997; Burri, Graf, Lambelet, & Loliger, 1989; Walton, Mayer, & Narbad, 2003). However, excessive ingestion of vanillin can cause headaches, nausea and vomiting, and it can also affect the function of liver and kidney (Han, 2002). Hence, it is necessary to detect and control the content of vanillin. As fast, simple and sensitive methods, electroanalysis methods have been exploited for the purpose (Agui, Lopez-Guzman, Gonzalez-Cortes, Yanez-Sedeno, & Pingarron, 1999; Bettazzi, Palchetti, Sisalli, & Mascini, 2006; Hardcastle, Paterson, & Compton, 2001; Luo & Liu, 2012; Luque, Luque-Perez, Rios, & Valcarcel, 2000; Peng, Hou, & Hu, 2012; Zheng, Hu, Gan, Dang, & Hu, 2010). For example, Zheng et al. (2010) fabricated an AuAg alloy nanoparticles based electrochemical sensor for vanillin. They found that AuAg alloy nanoparticles made the electrochemical response of vanillin to increase for five times. Peng et al. (2012) constructed a graphene film coated glassy carbon electrode (GCE) for vanillin detection. The electrode exhibited electrocatalysis to the oxidation of vanillin and it was applied to the determination of biscuit samples. But ⇑ Corresponding author. Tel.: +86 27 68752701; fax: +86 27 68754067. E-mail address: [email protected] (B. Zeng). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.11.044
the performance of these electrodes was still not enough good, partly due to the weak electrocatalysis of the modifying materials. Thus, it is significant to develop new modified electrodes for vanillin detection. Recently, AuPd alloy nanocrystal has attracted much attention due to its excellent catalytic performance in many applications, such as electrocatalytic oxidation of formic acid (Zhang et al., 2011), ethanol (Enache et al., 2006; Lee, Kim, Kang, & Han, 2011) and CO (Xu et al., 2010). The alloy nanocrystal has also been used to prepare electrochemical sensors. For example, Yang, Deng, Lei, Ju, and Gunasekaran (2011) reported the electrodeposition of AuPd alloy nanoparticles on graphene, and found that the resulted AuPd–graphene composite exhibited high electrocatalytic activity to O2. Then they prepared a sensitive glucose sensor by immobilizing glucose oxidase with the composite. Huang et al. (2012) fabricated a 3Au–1Pd alloy nanoparticle graphene film based bisphenol A (BPA) sensor. The sensor showed a linear range of 10 nM 5.0 lM and a detection limit of 4.0 nM. However, as far as we know, AuPd alloy nanoparticle graphene hybrid film has not been used for the voltammetric determination of vanillin. In this work, graphene was prepared by chemically reducing graphene oxide (GO) with an endogenous reducing agent from dimethylformamide (DMF). Then, AuPd alloy nanoparticles were electrodeposited on the graphene film. The electrochemical behavior of vanillin on the as-prepared hybrid film was studied, and a sensitive method for the determination of vanillin was proposed.
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2. Experimental
2.5. Pretreatment of samples
2.1. Reagents
Biscuit, vanilla bean and vanilla tea were pretreated according to references (Avila, Gonzalez, Zougagh, Escarpa, & Rios, 2007; Avila, Zougagh, Escarpa, & Rios, 2009; Peng et al., 2012). Briefly, the solid biscuit sample was ground into powder in a mortar with a pestle, and then about 0.40 g biscuit powder was taken and dispersed in 10 mL ethanol. The mixture was sonicated for 1 h with a supersonic cleaner. After centrifugating (3000 rpm) for 10 min, 1 mL of the supernatant was diluted to 10 mL with 0.1 M PBS (pH 7.0) for voltammetric determination. A 0.2 g vanilla bean sample was finely chopped with a blade and then macerated for 12 h with 5 mL 45 °C water in a closed vessel. Afterwards, 5 mL ethanol was added and it was macerated again for 3 days at room temperature. Then, 1 mL extract was diluted to 1000 mL with 0.1 M PBS (pH 7.0) for electroananlysis. A 0.6 g vanilla tea sample was extracted with 200 mL hot water for 1 h. Afterwards, the mixture was filtered with a nylon membrane (U = 0.22 lm). The remaining solid was dissolved in a final volume of 300 mL PBS (0.1 M, pH 7.0) for voltammetric determination.
Dimethylformamide, HAuCl4, PdCl2, NaNO3, Na2HPO412H2O, NaH2PO42H2O and vanillin were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The stock solution (0.01 M) of vanillin was prepared with redistilled water and stored at 4 °C. When used, it was diluted with 0.1 M phosphate buffer solution (PBS, pH = 7.0). Graphene oxide came from Xianfeng Reagent Co. Ltd. (Nanjing, China). Biscuit, vanilla bean and vanilla tea were purchased from a local supermarket. Other reagents were of analytical grade and the water used was redistilled. 2.2. Apparatus Electrodeposition, linear sweep voltammetry and differential pulse voltammetry were performed with a CHI 832C electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was adopted. The working electrode was a modified glassy carbon electrode (diameter: 3 mm), and the auxiliary and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. Electrochemical impedance spectroscopy was obtained through a CHI 660D electrochemical workstation (CH Instrument Company, Shanghai, China). Fourier transform infrared (FTIR) absorption spectra were recorded with a Nexus-670 spectrometer (Nicolet, USA). The scanning electron microscope (SEM) images and energy dispersive X-ray spectroscopy (EDX) were obtained using a Hitachi X-650 SEM (Hitachi Co., Japan). X-ray diffraction data (XRD) were recorded with a Bruke D8 diffractometer (Germany) using Cu Ka radiation (40 kV, 40 mA) with a Ni filter. X-ray photoelectron spectroscopy (XPS) was performed on an Escalab MKII spectrometer (VG Co., UK), using Mg KR radiation (1253.6 eV) at a pressure of 2.0 10 10 mbar. The peak positions were internally referenced to the C 1s peak at 284.6 eV. All measurements were conducted at room temperature.
2.6. Experimental procedure For electrochemical measurements, 10 mL 0.10 M PBS (pH 7.0) and certain volume of vanillin solution were transferred to a 10 mL cell, and then the three-electrode system was installed on it. After accumulation for 200 s at 0 V, the potential scan was ignited. The linear sweep voltammograms were recorded from 0 to 0.8 V, and the differential pulse voltammograms were recorded from 0 to 0.9 V. After every measurement the electrode was regenerated by cycling the potential between 0 and 0.8 V for several times in a blank solution (i.e. 0.10 M PBS, pH 7.0). The recovery was obtained by determining the concentrations of vanillin added in the sample solutions and then calculating the rate of detected concentration to the real concentration. To evaluate the reproducibility of the modified electrode, seven electrodes were prepared by the same way and 1 lM vanillin solutions were determined. As to the detection limit, it was obtained by decreasing vanillin concentration until the peak current reduced to three times of the noise.
2.3. Preparation of graphene Graphene was prepared by a simple one-step reduction approach (Ai, Liu, Lu, Cheng, & Huo, 2011). Briefly, GO was firstly dispersed in DMF (0.5 mg/mL) with the aid of ultrasonication, and then the mixture was heated in an oil bath (153 °C) for 1 h. Thus, a stable well-dispersed graphene solution was obtained. 2.4. Fabrication of vanillin sensor A clean GCE was modified with 4 lL above-mentioned graphene suspension and then let the solvent to evaporate in air. Following this, AuPd alloy nanoparticles were electrodeposited on it from a deaerated solution containing 1.5 mM HAuCl4, 0.50 mM PdCl2 and 0.1 M NaNO3. The deposition potential was 0.3 V and deposition time was 200 s. The obtained electrode was washed carefully with redistilled water and dried at room temperature. It was denoted as Au3Pd1–graphene/GCE. Similarly, Au1Pd1–graphene/GCE, Au1Pd3–graphene/GCE, Au–graphene/ GCE and Pd–graphene/GCE were fabricated. For preparing these electrodes the electrodeposition solutions were 1.0 mM HAuCl4 + 1.0 mM PdCl2 + 0.1 M NaNO3, 0.50 mM HAuCl4 + 1.5 mM PdCl2 + 0.1 M NaNO3, 2.0 mM HAuCl4 + 0.1 M NaNO3 and 2.0 mM PdCl2 + 0.1 M NaNO3, respectively. For comparison, AuPd/GCE was also fabricated through a similar way.
3. Results and discussion 3.1. Characterization of graphene In this work, GO was reduced to graphene by the carbon monoxide released from DMF (Ai et al., 2011), and the obtained graphene suspension showed excellent dispersibility and longterm stability. FTIR, XRD and XPS were used to characterize the formation of graphene (Figs. S1–S4). The FTIR spectra of GO showed several absorption bands, corresponding to carboxylic acid (C@O stretching vibration at 1630–1750 cm 1), O–H (vO–H at 3400 and 1395 cm 1), C@C (vC@C at 1620 cm 1) and C–O (vC–O at 1060 cm 1), respectively. In contrast, the FTIR spectrum of graphene was featureless. Two absorption bands at about 1050 and 1300 cm 1 belonged to the C–N stretching vibration of DMF adsorbed (Ai et al., 2011). The feature diffraction peak of GO was at 10.4° (0 0 2) (Jeong et al., 2008). After reduction, the diffraction peak shifted to 26.6° (Fig. S2), which was in line with that reported (Guo, Wang, Qian, Wang, & Xia, 2009). According to the XPS the C/O ratio of the graphene was relatively higher than that of the GO, and the peaks assigned to epoxide, hydroxyl and carboxyl groups were significantly weakened (Fig. S3), indicating that most of the groups were eliminated. In addition, the peak located at 284.4 eV (sp2 carbon) was greatly enhanced in comparison with
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that at 285.3 eV (sp3 carbon), indicating that the GO was fully reduced (Li et al., 2008). Additionally, a small peak assigned to N was observed at 400 eV (Fig. S4), which should be produced by the DMF molecules absorbed (Ai et al., 2011). The adsorption of DMF benefited the dispersion of graphene (Paredes, Villar-Rodil, Martinez-Alonso, & Tascon, 2008). 3.2. Morphological analysis As shown in Fig. 1, the obtained graphene presents typical wrinkled sheet structure (Chen, Tang, Wang, Liu, & Luo, 2011), which can provide large rough surface for further modification. The AuPd nanoparticles electrodeposited on the graphene are dense and well dispersed, and their diameters are 30–50 nm (Fig. 1B). At the same time, some black deposit is observable, which must be amorphous oxide/hydroxide. In contrast, on a bare electrode the electrodeposited AuPd nanoparticles are bigger and sparse. The reason is that the bare electrode has less sites for nucleation. 3.3. Structure and composition analysis The composition of AuPd nanoparticles was determined by EDX (Fig. S5), and the atomic ratio of Au/Pd was about 1:1. The XRD patterns of the electrodeposited Au and Pd nanoparticles were consistent with their standard patterns (JCPDS 04-0784 and JCPDS 05-0681) (Fig. S6). The diffraction peaks of the AuPd nanoparticles located between the corresponding diffraction peaks of Au and Pd nanoparticles, indicating that the AuPd nanoparticles were alloy rather than the mixture of monometallic nanoparticles. On the basis of Vegard’s Law (the linear lattice constant–concentration relation) (Jenkins & Snyder, 1996; Klug & Alexander, 1974), the
atomic ratio of Au/Pd in the AuPd alloy nanoparticles was estimated and it was about 1:1, which was in line with the result of EDX analysis. 3.4. Electrochemical impedance spectroscopy The electrochemical impedance spectroscopy of bare GCE, AuPd/GCE, graphene/GCE and AuPd–graphene/GCE were compared (Fig. S7). The electron transfer resistance (Rct) of AuPd/GCE and graphene/GCE was smaller than that of GCE, indicating that graphene and alloy nanoparticles could enhance the electron transfer rate. When AuPd alloy nanoparticles were electrodeposited on the graphene/GCE the Rct decreased further, meaning that the AuPd–graphene hybrid film had higher electron transport rate. 3.5. Electrochemical oxidation of vanillin at different electrodes The linear sweep voltammograms of vanillin at different electrodes are shown in Fig. 2. Vanillin produces a bigger anodic peak at the graphene/GCE (Fig. 2b) and AuPd/GCE (Fig. 2c) than at GCE, indicating that graphene and AuPd nanoparticles can promote the oxidation of vanillin. Furthermore, the AuPd–graphene/GCE presents higher electrocatalytic activity due to the synergetic effect of AuPd and graphene. At the electrode the peak current of vanillin is about twenty times as large as that at the bare GCE. Therefore, the AuPd–graphene hybrid film is considered a better catalyst for the electrochemical oxidation of vanillin. As shown in Fig. 3, vanillin produces similar peaks at the Au–graphene/GCE and graphene/GCE, meaning that Au nanopartilces have little catalysis to the oxidation of vanillin. At the Pd– graphene/GCE vanillin exhibits a bigger anodic peak, confirming the high catalysis of Pd nanoparticles. However, the AuPd alloy nanoparticles show enhanced catalytic activity. This should be ascribed to the synergetic effect of Au and Pd. In AuPd alloy nanoparticles Pd is catalytic element to vanillin and Au offers conducting wire or electron-conducting tunnel for electron transfer (Huang et al., 2012). As can be seen in Fig. 3, the oxidation peak of vanillin is higher at the Au1Pd1–graphene/GCE, meaning that the Au1Pd1 has higher catalytic activity than the Au3Pd1 and Au1Pd3, thus AuPd is selected for the following experiments. For comparison, other bimetallic alloy nanoparticles, such as PdPt, PdCu, PdNi, AuCu, AuNi and AuPt, have also been tested. Unfortunately, they show weak catalysis in this case. For example,
50 5
Current/µA
30
Current/µA
4
40
3 2
d
1 0
-1 -2
20
0.0
0.2
0.4
0.6
c
0.8
Potential/V
b
10
a 0
0.0
0.2
0.4
0.6
0.8
Potential/V
Fig. 1. SEM images of graphene/ITO (A) and AuPd–graphene/ITO (B). The solution for electrodeposition: 0.1 M NaNO3 containing 1.0 mM HAuCl4 and 1.0 mM PdCl2; electrodeposition potential: 0.3 V; electrodeposition time: 200 s.
Fig. 2. Linear sweep voltammograms of bare GCE (a), graphene/GCE (b), AuPd/GCE (c) and AuPd–graphene/GCE (d) in 0.1 M PBS (pH 7.0) solution containing 10 lM vanillin. Inset: the enlarged linear sweep voltammogram of bare GCE. Accumulation potential: 0 V; accumulation time: 200 s.
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L. Shang et al. / Food Chemistry 151 (2014) 53–57 Table 2 Measurement results of vanillin in sample solutions.
30
f e d c b
Current/µA
20
10
a
0
Samples
Added (lM)
Found (lM)
Recovery (%)
RSD (%) (n = 4)
Biscuit
0 0.50 1.50 2.50
2.13 2.66 3.62 4.73
– 106 99.3 104
5.4 3.0 4.9 2.3
Vanilla bean
0 0.50 1.50 2.50
1.77 2.21 3.35 4.41
– 88 105 106
3.7 4.7 4.7 3.1
Vanillin tea
0 0.50 1.50 2.50
0.77 1.19 2.22 3.13
– 84 96.7 94.4
3.4 2.9 3.7 4.6
-10 0.0
0.2
0.4
0.6
0.8
Potential/V Fig. 3. Linear sweep voltammograms of graphene/GCE (a), Au–graphene/GCE (b), Pd–graphene/GCE (c), Au3Pd1–graphene/GCE (d), Au1Pd3–graphene/GCE (e) and Au1Pd1–graphene/GCE (f) in 0.1 M PBS (pH 7.0) solution containing 10 lM vanillin. Accumulation potential: 0 V; accumulation time: 200 s.
2.0
1.0
Peak current/ µA
Current/µA
1.5
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
0.5
0
10
20
30
40
j
50
Concentration/µM
0.0 -0.5
a
increased with increasing electrodeposition time until it was up to 200 s, and then the peak current kept almost unchanged in the time range studied (i.e. 200–400 s) (Fig. S9). Hence, the electrodeposition time 200 s was adopted. The effect of solution pH was also investigated. In the pH range from 6.0 to 9.0 the peak current of vanillin was dependent on it, and the maximum peak current occurred around pH 7.0. In addition, the peak potential (Ep) shifted negatively with pH rising. Furthermore, they showed a good linear relationship as: Ep (V) = 0.98–0.062 pH (R = 0.998) (Fig. S10). The slope 0.062 V/pH indicated that the numbers of electron and proton transferred were same in the electrochemical process. Next, the influence of accumulation potential and accumulation time was examined (Fig. S11). When the accumulation potential was changed from 0.1 to 0.1 V the peak current kept almost unchanged; when it exceeded 0.1 V the peak current decreased. The peak current increased with increasing accumulation time until it was up to 200 s, then the peak current kept almost unchanged. 3.6. Variation of peak current with vanillin concentration
-1.0 0.0
0.2
0.4
0.6
0.8
1.0
Potential/V Fig. 4. Differential pulse voltammograms of vanillin at AuPd–graphene/GCE. Vanillin concentration (from a–j): 0.1, 0.4, 0.7, 1.0, 4.0, 7.0, 10, 20, 30, 40 and 50 lM. Pulse amplitude: 50 mV; pulse width: 0.2 s; accumulation potential: 0 V; accumulation time: 200 s. Inset: the calibration curves.
vanillin produces a smaller peak at the PtPd–graphene/GCE than at the AuPd–graphene/GCE (Fig. S8). The amount of AuPd nanoparticles was related to the electrodeposition time, so electrodeposition time affected the electrochemical response of the resulting AuPd–graphene/GCE to vanillin. Experimental result showed that the peak current of vanillin
Under the optimized conditions, differential pulse voltammetry was used to determine vanillin. Fig. 4 presents the differential pulse voltammograms of vanillin. As can be seen, the peak current (Ip) of vanillin increases with its concentration going up. Moreover, they show linear relationship in the ranges of 0.1–7 and 10–40 lM. The linear equations are Ip (A) = 0.113 c (mol L 1) + 1.43 10 7 (R = 0.996) and Ip (A) = 0.0125 c (mol L 1) + 8.48 10 8 (R = 0.999), respectively. The sensitivities are 1.60 and 0.170 mA mM 1 cm 2, respectively; the detection limit is estimated to be 0.02 lM (S/N = 3). Compared with other modified electrodes (Agui et al., 1999; Bettazzi et al., 2006; Hardcastle et al., 2001; Luque et al., 2000; Peng et al., 2012; Zheng et al., 2010), the AuPd–graphene/GCE shows lower detection limit and higher sensitivity (Table 1).
Table 1 Comparison of different electrodes for vanillin determination. Electrodes a
Cylindrical CFME Graphite electrode GCE PVC/graphite electrode CDAb/Au–AgNPs/GCE Graphene/GCE AuPd–graphene/GCE a b
Carbon fiber microelectrodes. Cellulose diacetate.
Linear range
Detection limit
References
10–700 lM 5–400 lM 50–300 lM 0.66–9.2 mM 0.2–50 lM 0.6–48 lM 0.1–7 lM 10–40 lM
4.2 lM 0.4 lM 0.16 lM 0.29 mM 0.04 lM 0.056 lM 0.02 lM
Agui, Lopez-Guzman, Gonzalez-Cortes, Yanez-Sedeno, and Pingarron (1999) Bettazzi, Palchetti, Sisalli, and Mascini (2006) Hardcastle, Paterson, and Compton (2001) Luque, Luque-Perez, Rios, and Valcarcel (2000) Zheng, Hu, Gan, Dang, and Hu (2010) Peng, Hou, and Hu (2012) This work
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3.7. Stability, reproducibility and selectivity of AuPd–graphene/GCE
References
To test the reproducibility of the modified electrode seven AuPd–graphene/GCEs were prepared by the same way and 1 lM vanillin solutions were determined. As a result, the relative standard deviation (RSD) of the peak currents was 6.3% (n = 7). This indicates that the electrode had good reproducibility. In addition, a 1 lM vanillin solution was determined successively for ten times with an AuPd–graphene/GCE, and the RSD was found to be 4.8%, which reflected the good repeatability. The storage stability of the electrode was also examined. It was found that after stored in air for 1 week the electrode retained 92% of its initial current response. After stored for 1 month the current response decreased by 17%. In addition, experimental results showed when 1000-fold amount of glucose, fructose or sucrose, 500-fold amount of citric acid, 80-fold amount of ascorbic acid, caffeine or theophylline, 2-fold amount of hypoxanthine or xanthine was present the change of peak current of 5 lM vanillin was less than 5%. However, 1-fold amount of ethyl vanillin exhibited serious interference because ethyl vanillin and vanillin are very similar. Hence, the selectivity of the electrode is similar to that reported by Peng et al. (2012) and Zheng et al. (2010)).
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3.8. Application Biscuit, vanilla bean and vanilla tea samples were determined by using the proposed method and the results were summarized in Table 2. The contents of vanillin in these samples were calculated and they were 0.08 mg g 1 (for biscuit), 13.5 mg g 1 (for vanilla bean) and 0.06 mg g 1 (for vanilla tea), respectively. The recoveries for standards added were 84–106%, indicating that the method is reliable.
4. Conclusion Graphene oxide can be reduced by DMF at about 150 °C to produce stable graphene suspension. Well-dispersed AuPd alloy nanoparticles can be prepared on the graphene modified GCE by electrodeposition. The obtained AuPd–graphene composite film has strong electrocatalysis toward the oxidation of vanillin, thus vanillin produces a sensitive anodic peak at the AuPd–graphene modified GCE. The electrode also exhibits good stability and reproducibility, and it can be applied to the determination of vanillin in real samples. This work provides a example for making use of alloy nanoparticles to construct new sensors. Acknowledgements The Authors appreciate the financial support from the National Natural Science Foundation of China (Grant No.: 21075092) and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, Wuhan, China, Grant No. 2010-KF-12).
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. 2013.11.044.
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Sensitive voltammetric determination of vanillin with an AuPd nanoparticles graphene composite modified electrode Lei Shang, Faqiong Zhao, Baizhao Zeng ⇑ The Key Laboratory of Analytical Chemistry for Biology and Medicine, Ministry of Education, College of Chemistry and Molecular Sciences, Wuhan University, Wuhan 430072, Hubei Province, PR China
a r t i c l e
i n f o
Article history: Received 22 June 2013 Received in revised form 1 November 2013 Accepted 10 November 2013 Available online 18 November 2013 Keywords: AuPd nanoparticles Graphene Vanillin Voltammetry Electrochemical sensor
a b s t r a c t In this work, graphene oxide was reduced to graphene with an endogenous reducing agent from dimethylformamide, and then AuPd alloy nanoparticles were electrodeposited on the graphene film. The obtained AuPd–graphene hybrid film was characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and voltammetry. The electrochemical behavior of vanillin was studied using the AuPd–graphene hybrid based electrode. It presented high electrocatalytic activity and vanillin could produce a sensitive oxidation peak at it. Under the optimal conditions, the peak current was linear to the concentration of vanillin in the ranges of 0.1–7 and 10–40 lM. The sensitivities were 1.60 and 0.170 mA mM 1 cm 2, respectively; the detection limit was 20 nM. The electrode was successfully applied to the detection of vanillin in vanilla bean, vanilla tea and biscuit samples. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction Vanillin (i.e. 4-hydroxy-3-methoxybenzaldehyde) has unique aroma and good antioxidative and antimicrobial property (Teissedre & Waterhouse, 2000). It is widely used in producing confectionery, beverage and medicine (Anklam, Gaglione, & Muller, 1997; Burri, Graf, Lambelet, & Loliger, 1989; Walton, Mayer, & Narbad, 2003). However, excessive ingestion of vanillin can cause headaches, nausea and vomiting, and it can also affect the function of liver and kidney (Han, 2002). Hence, it is necessary to detect and control the content of vanillin. As fast, simple and sensitive methods, electroanalysis methods have been exploited for the purpose (Agui, Lopez-Guzman, Gonzalez-Cortes, Yanez-Sedeno, & Pingarron, 1999; Bettazzi, Palchetti, Sisalli, & Mascini, 2006; Hardcastle, Paterson, & Compton, 2001; Luo & Liu, 2012; Luque, Luque-Perez, Rios, & Valcarcel, 2000; Peng, Hou, & Hu, 2012; Zheng, Hu, Gan, Dang, & Hu, 2010). For example, Zheng et al. (2010) fabricated an AuAg alloy nanoparticles based electrochemical sensor for vanillin. They found that AuAg alloy nanoparticles made the electrochemical response of vanillin to increase for five times. Peng et al. (2012) constructed a graphene film coated glassy carbon electrode (GCE) for vanillin detection. The electrode exhibited electrocatalysis to the oxidation of vanillin and it was applied to the determination of biscuit samples. But ⇑ Corresponding author. Tel.: +86 27 68752701; fax: +86 27 68754067. E-mail address: [email protected] (B. Zeng). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.11.044
the performance of these electrodes was still not enough good, partly due to the weak electrocatalysis of the modifying materials. Thus, it is significant to develop new modified electrodes for vanillin detection. Recently, AuPd alloy nanocrystal has attracted much attention due to its excellent catalytic performance in many applications, such as electrocatalytic oxidation of formic acid (Zhang et al., 2011), ethanol (Enache et al., 2006; Lee, Kim, Kang, & Han, 2011) and CO (Xu et al., 2010). The alloy nanocrystal has also been used to prepare electrochemical sensors. For example, Yang, Deng, Lei, Ju, and Gunasekaran (2011) reported the electrodeposition of AuPd alloy nanoparticles on graphene, and found that the resulted AuPd–graphene composite exhibited high electrocatalytic activity to O2. Then they prepared a sensitive glucose sensor by immobilizing glucose oxidase with the composite. Huang et al. (2012) fabricated a 3Au–1Pd alloy nanoparticle graphene film based bisphenol A (BPA) sensor. The sensor showed a linear range of 10 nM 5.0 lM and a detection limit of 4.0 nM. However, as far as we know, AuPd alloy nanoparticle graphene hybrid film has not been used for the voltammetric determination of vanillin. In this work, graphene was prepared by chemically reducing graphene oxide (GO) with an endogenous reducing agent from dimethylformamide (DMF). Then, AuPd alloy nanoparticles were electrodeposited on the graphene film. The electrochemical behavior of vanillin on the as-prepared hybrid film was studied, and a sensitive method for the determination of vanillin was proposed.
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2. Experimental
2.5. Pretreatment of samples
2.1. Reagents
Biscuit, vanilla bean and vanilla tea were pretreated according to references (Avila, Gonzalez, Zougagh, Escarpa, & Rios, 2007; Avila, Zougagh, Escarpa, & Rios, 2009; Peng et al., 2012). Briefly, the solid biscuit sample was ground into powder in a mortar with a pestle, and then about 0.40 g biscuit powder was taken and dispersed in 10 mL ethanol. The mixture was sonicated for 1 h with a supersonic cleaner. After centrifugating (3000 rpm) for 10 min, 1 mL of the supernatant was diluted to 10 mL with 0.1 M PBS (pH 7.0) for voltammetric determination. A 0.2 g vanilla bean sample was finely chopped with a blade and then macerated for 12 h with 5 mL 45 °C water in a closed vessel. Afterwards, 5 mL ethanol was added and it was macerated again for 3 days at room temperature. Then, 1 mL extract was diluted to 1000 mL with 0.1 M PBS (pH 7.0) for electroananlysis. A 0.6 g vanilla tea sample was extracted with 200 mL hot water for 1 h. Afterwards, the mixture was filtered with a nylon membrane (U = 0.22 lm). The remaining solid was dissolved in a final volume of 300 mL PBS (0.1 M, pH 7.0) for voltammetric determination.
Dimethylformamide, HAuCl4, PdCl2, NaNO3, Na2HPO412H2O, NaH2PO42H2O and vanillin were purchased from Sinopharm Chemical Reagent Co. Ltd. (Shanghai, China). The stock solution (0.01 M) of vanillin was prepared with redistilled water and stored at 4 °C. When used, it was diluted with 0.1 M phosphate buffer solution (PBS, pH = 7.0). Graphene oxide came from Xianfeng Reagent Co. Ltd. (Nanjing, China). Biscuit, vanilla bean and vanilla tea were purchased from a local supermarket. Other reagents were of analytical grade and the water used was redistilled. 2.2. Apparatus Electrodeposition, linear sweep voltammetry and differential pulse voltammetry were performed with a CHI 832C electrochemical workstation (CH Instrument Company, Shanghai, China). A conventional three-electrode system was adopted. The working electrode was a modified glassy carbon electrode (diameter: 3 mm), and the auxiliary and reference electrodes were a platinum wire and a saturated calomel electrode (SCE), respectively. Electrochemical impedance spectroscopy was obtained through a CHI 660D electrochemical workstation (CH Instrument Company, Shanghai, China). Fourier transform infrared (FTIR) absorption spectra were recorded with a Nexus-670 spectrometer (Nicolet, USA). The scanning electron microscope (SEM) images and energy dispersive X-ray spectroscopy (EDX) were obtained using a Hitachi X-650 SEM (Hitachi Co., Japan). X-ray diffraction data (XRD) were recorded with a Bruke D8 diffractometer (Germany) using Cu Ka radiation (40 kV, 40 mA) with a Ni filter. X-ray photoelectron spectroscopy (XPS) was performed on an Escalab MKII spectrometer (VG Co., UK), using Mg KR radiation (1253.6 eV) at a pressure of 2.0 10 10 mbar. The peak positions were internally referenced to the C 1s peak at 284.6 eV. All measurements were conducted at room temperature.
2.6. Experimental procedure For electrochemical measurements, 10 mL 0.10 M PBS (pH 7.0) and certain volume of vanillin solution were transferred to a 10 mL cell, and then the three-electrode system was installed on it. After accumulation for 200 s at 0 V, the potential scan was ignited. The linear sweep voltammograms were recorded from 0 to 0.8 V, and the differential pulse voltammograms were recorded from 0 to 0.9 V. After every measurement the electrode was regenerated by cycling the potential between 0 and 0.8 V for several times in a blank solution (i.e. 0.10 M PBS, pH 7.0). The recovery was obtained by determining the concentrations of vanillin added in the sample solutions and then calculating the rate of detected concentration to the real concentration. To evaluate the reproducibility of the modified electrode, seven electrodes were prepared by the same way and 1 lM vanillin solutions were determined. As to the detection limit, it was obtained by decreasing vanillin concentration until the peak current reduced to three times of the noise.
2.3. Preparation of graphene Graphene was prepared by a simple one-step reduction approach (Ai, Liu, Lu, Cheng, & Huo, 2011). Briefly, GO was firstly dispersed in DMF (0.5 mg/mL) with the aid of ultrasonication, and then the mixture was heated in an oil bath (153 °C) for 1 h. Thus, a stable well-dispersed graphene solution was obtained. 2.4. Fabrication of vanillin sensor A clean GCE was modified with 4 lL above-mentioned graphene suspension and then let the solvent to evaporate in air. Following this, AuPd alloy nanoparticles were electrodeposited on it from a deaerated solution containing 1.5 mM HAuCl4, 0.50 mM PdCl2 and 0.1 M NaNO3. The deposition potential was 0.3 V and deposition time was 200 s. The obtained electrode was washed carefully with redistilled water and dried at room temperature. It was denoted as Au3Pd1–graphene/GCE. Similarly, Au1Pd1–graphene/GCE, Au1Pd3–graphene/GCE, Au–graphene/ GCE and Pd–graphene/GCE were fabricated. For preparing these electrodes the electrodeposition solutions were 1.0 mM HAuCl4 + 1.0 mM PdCl2 + 0.1 M NaNO3, 0.50 mM HAuCl4 + 1.5 mM PdCl2 + 0.1 M NaNO3, 2.0 mM HAuCl4 + 0.1 M NaNO3 and 2.0 mM PdCl2 + 0.1 M NaNO3, respectively. For comparison, AuPd/GCE was also fabricated through a similar way.
3. Results and discussion 3.1. Characterization of graphene In this work, GO was reduced to graphene by the carbon monoxide released from DMF (Ai et al., 2011), and the obtained graphene suspension showed excellent dispersibility and longterm stability. FTIR, XRD and XPS were used to characterize the formation of graphene (Figs. S1–S4). The FTIR spectra of GO showed several absorption bands, corresponding to carboxylic acid (C@O stretching vibration at 1630–1750 cm 1), O–H (vO–H at 3400 and 1395 cm 1), C@C (vC@C at 1620 cm 1) and C–O (vC–O at 1060 cm 1), respectively. In contrast, the FTIR spectrum of graphene was featureless. Two absorption bands at about 1050 and 1300 cm 1 belonged to the C–N stretching vibration of DMF adsorbed (Ai et al., 2011). The feature diffraction peak of GO was at 10.4° (0 0 2) (Jeong et al., 2008). After reduction, the diffraction peak shifted to 26.6° (Fig. S2), which was in line with that reported (Guo, Wang, Qian, Wang, & Xia, 2009). According to the XPS the C/O ratio of the graphene was relatively higher than that of the GO, and the peaks assigned to epoxide, hydroxyl and carboxyl groups were significantly weakened (Fig. S3), indicating that most of the groups were eliminated. In addition, the peak located at 284.4 eV (sp2 carbon) was greatly enhanced in comparison with
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that at 285.3 eV (sp3 carbon), indicating that the GO was fully reduced (Li et al., 2008). Additionally, a small peak assigned to N was observed at 400 eV (Fig. S4), which should be produced by the DMF molecules absorbed (Ai et al., 2011). The adsorption of DMF benefited the dispersion of graphene (Paredes, Villar-Rodil, Martinez-Alonso, & Tascon, 2008). 3.2. Morphological analysis As shown in Fig. 1, the obtained graphene presents typical wrinkled sheet structure (Chen, Tang, Wang, Liu, & Luo, 2011), which can provide large rough surface for further modification. The AuPd nanoparticles electrodeposited on the graphene are dense and well dispersed, and their diameters are 30–50 nm (Fig. 1B). At the same time, some black deposit is observable, which must be amorphous oxide/hydroxide. In contrast, on a bare electrode the electrodeposited AuPd nanoparticles are bigger and sparse. The reason is that the bare electrode has less sites for nucleation. 3.3. Structure and composition analysis The composition of AuPd nanoparticles was determined by EDX (Fig. S5), and the atomic ratio of Au/Pd was about 1:1. The XRD patterns of the electrodeposited Au and Pd nanoparticles were consistent with their standard patterns (JCPDS 04-0784 and JCPDS 05-0681) (Fig. S6). The diffraction peaks of the AuPd nanoparticles located between the corresponding diffraction peaks of Au and Pd nanoparticles, indicating that the AuPd nanoparticles were alloy rather than the mixture of monometallic nanoparticles. On the basis of Vegard’s Law (the linear lattice constant–concentration relation) (Jenkins & Snyder, 1996; Klug & Alexander, 1974), the
atomic ratio of Au/Pd in the AuPd alloy nanoparticles was estimated and it was about 1:1, which was in line with the result of EDX analysis. 3.4. Electrochemical impedance spectroscopy The electrochemical impedance spectroscopy of bare GCE, AuPd/GCE, graphene/GCE and AuPd–graphene/GCE were compared (Fig. S7). The electron transfer resistance (Rct) of AuPd/GCE and graphene/GCE was smaller than that of GCE, indicating that graphene and alloy nanoparticles could enhance the electron transfer rate. When AuPd alloy nanoparticles were electrodeposited on the graphene/GCE the Rct decreased further, meaning that the AuPd–graphene hybrid film had higher electron transport rate. 3.5. Electrochemical oxidation of vanillin at different electrodes The linear sweep voltammograms of vanillin at different electrodes are shown in Fig. 2. Vanillin produces a bigger anodic peak at the graphene/GCE (Fig. 2b) and AuPd/GCE (Fig. 2c) than at GCE, indicating that graphene and AuPd nanoparticles can promote the oxidation of vanillin. Furthermore, the AuPd–graphene/GCE presents higher electrocatalytic activity due to the synergetic effect of AuPd and graphene. At the electrode the peak current of vanillin is about twenty times as large as that at the bare GCE. Therefore, the AuPd–graphene hybrid film is considered a better catalyst for the electrochemical oxidation of vanillin. As shown in Fig. 3, vanillin produces similar peaks at the Au–graphene/GCE and graphene/GCE, meaning that Au nanopartilces have little catalysis to the oxidation of vanillin. At the Pd– graphene/GCE vanillin exhibits a bigger anodic peak, confirming the high catalysis of Pd nanoparticles. However, the AuPd alloy nanoparticles show enhanced catalytic activity. This should be ascribed to the synergetic effect of Au and Pd. In AuPd alloy nanoparticles Pd is catalytic element to vanillin and Au offers conducting wire or electron-conducting tunnel for electron transfer (Huang et al., 2012). As can be seen in Fig. 3, the oxidation peak of vanillin is higher at the Au1Pd1–graphene/GCE, meaning that the Au1Pd1 has higher catalytic activity than the Au3Pd1 and Au1Pd3, thus AuPd is selected for the following experiments. For comparison, other bimetallic alloy nanoparticles, such as PdPt, PdCu, PdNi, AuCu, AuNi and AuPt, have also been tested. Unfortunately, they show weak catalysis in this case. For example,
50 5
Current/µA
30
Current/µA
4
40
3 2
d
1 0
-1 -2
20
0.0
0.2
0.4
0.6
c
0.8
Potential/V
b
10
a 0
0.0
0.2
0.4
0.6
0.8
Potential/V
Fig. 1. SEM images of graphene/ITO (A) and AuPd–graphene/ITO (B). The solution for electrodeposition: 0.1 M NaNO3 containing 1.0 mM HAuCl4 and 1.0 mM PdCl2; electrodeposition potential: 0.3 V; electrodeposition time: 200 s.
Fig. 2. Linear sweep voltammograms of bare GCE (a), graphene/GCE (b), AuPd/GCE (c) and AuPd–graphene/GCE (d) in 0.1 M PBS (pH 7.0) solution containing 10 lM vanillin. Inset: the enlarged linear sweep voltammogram of bare GCE. Accumulation potential: 0 V; accumulation time: 200 s.
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L. Shang et al. / Food Chemistry 151 (2014) 53–57 Table 2 Measurement results of vanillin in sample solutions.
30
f e d c b
Current/µA
20
10
a
0
Samples
Added (lM)
Found (lM)
Recovery (%)
RSD (%) (n = 4)
Biscuit
0 0.50 1.50 2.50
2.13 2.66 3.62 4.73
– 106 99.3 104
5.4 3.0 4.9 2.3
Vanilla bean
0 0.50 1.50 2.50
1.77 2.21 3.35 4.41
– 88 105 106
3.7 4.7 4.7 3.1
Vanillin tea
0 0.50 1.50 2.50
0.77 1.19 2.22 3.13
– 84 96.7 94.4
3.4 2.9 3.7 4.6
-10 0.0
0.2
0.4
0.6
0.8
Potential/V Fig. 3. Linear sweep voltammograms of graphene/GCE (a), Au–graphene/GCE (b), Pd–graphene/GCE (c), Au3Pd1–graphene/GCE (d), Au1Pd3–graphene/GCE (e) and Au1Pd1–graphene/GCE (f) in 0.1 M PBS (pH 7.0) solution containing 10 lM vanillin. Accumulation potential: 0 V; accumulation time: 200 s.
2.0
1.0
Peak current/ µA
Current/µA
1.5
1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
0.5
0
10
20
30
40
j
50
Concentration/µM
0.0 -0.5
a
increased with increasing electrodeposition time until it was up to 200 s, and then the peak current kept almost unchanged in the time range studied (i.e. 200–400 s) (Fig. S9). Hence, the electrodeposition time 200 s was adopted. The effect of solution pH was also investigated. In the pH range from 6.0 to 9.0 the peak current of vanillin was dependent on it, and the maximum peak current occurred around pH 7.0. In addition, the peak potential (Ep) shifted negatively with pH rising. Furthermore, they showed a good linear relationship as: Ep (V) = 0.98–0.062 pH (R = 0.998) (Fig. S10). The slope 0.062 V/pH indicated that the numbers of electron and proton transferred were same in the electrochemical process. Next, the influence of accumulation potential and accumulation time was examined (Fig. S11). When the accumulation potential was changed from 0.1 to 0.1 V the peak current kept almost unchanged; when it exceeded 0.1 V the peak current decreased. The peak current increased with increasing accumulation time until it was up to 200 s, then the peak current kept almost unchanged. 3.6. Variation of peak current with vanillin concentration
-1.0 0.0
0.2
0.4
0.6
0.8
1.0
Potential/V Fig. 4. Differential pulse voltammograms of vanillin at AuPd–graphene/GCE. Vanillin concentration (from a–j): 0.1, 0.4, 0.7, 1.0, 4.0, 7.0, 10, 20, 30, 40 and 50 lM. Pulse amplitude: 50 mV; pulse width: 0.2 s; accumulation potential: 0 V; accumulation time: 200 s. Inset: the calibration curves.
vanillin produces a smaller peak at the PtPd–graphene/GCE than at the AuPd–graphene/GCE (Fig. S8). The amount of AuPd nanoparticles was related to the electrodeposition time, so electrodeposition time affected the electrochemical response of the resulting AuPd–graphene/GCE to vanillin. Experimental result showed that the peak current of vanillin
Under the optimized conditions, differential pulse voltammetry was used to determine vanillin. Fig. 4 presents the differential pulse voltammograms of vanillin. As can be seen, the peak current (Ip) of vanillin increases with its concentration going up. Moreover, they show linear relationship in the ranges of 0.1–7 and 10–40 lM. The linear equations are Ip (A) = 0.113 c (mol L 1) + 1.43 10 7 (R = 0.996) and Ip (A) = 0.0125 c (mol L 1) + 8.48 10 8 (R = 0.999), respectively. The sensitivities are 1.60 and 0.170 mA mM 1 cm 2, respectively; the detection limit is estimated to be 0.02 lM (S/N = 3). Compared with other modified electrodes (Agui et al., 1999; Bettazzi et al., 2006; Hardcastle et al., 2001; Luque et al., 2000; Peng et al., 2012; Zheng et al., 2010), the AuPd–graphene/GCE shows lower detection limit and higher sensitivity (Table 1).
Table 1 Comparison of different electrodes for vanillin determination. Electrodes a
Cylindrical CFME Graphite electrode GCE PVC/graphite electrode CDAb/Au–AgNPs/GCE Graphene/GCE AuPd–graphene/GCE a b
Carbon fiber microelectrodes. Cellulose diacetate.
Linear range
Detection limit
References
10–700 lM 5–400 lM 50–300 lM 0.66–9.2 mM 0.2–50 lM 0.6–48 lM 0.1–7 lM 10–40 lM
4.2 lM 0.4 lM 0.16 lM 0.29 mM 0.04 lM 0.056 lM 0.02 lM
Agui, Lopez-Guzman, Gonzalez-Cortes, Yanez-Sedeno, and Pingarron (1999) Bettazzi, Palchetti, Sisalli, and Mascini (2006) Hardcastle, Paterson, and Compton (2001) Luque, Luque-Perez, Rios, and Valcarcel (2000) Zheng, Hu, Gan, Dang, and Hu (2010) Peng, Hou, and Hu (2012) This work
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3.7. Stability, reproducibility and selectivity of AuPd–graphene/GCE
References
To test the reproducibility of the modified electrode seven AuPd–graphene/GCEs were prepared by the same way and 1 lM vanillin solutions were determined. As a result, the relative standard deviation (RSD) of the peak currents was 6.3% (n = 7). This indicates that the electrode had good reproducibility. In addition, a 1 lM vanillin solution was determined successively for ten times with an AuPd–graphene/GCE, and the RSD was found to be 4.8%, which reflected the good repeatability. The storage stability of the electrode was also examined. It was found that after stored in air for 1 week the electrode retained 92% of its initial current response. After stored for 1 month the current response decreased by 17%. In addition, experimental results showed when 1000-fold amount of glucose, fructose or sucrose, 500-fold amount of citric acid, 80-fold amount of ascorbic acid, caffeine or theophylline, 2-fold amount of hypoxanthine or xanthine was present the change of peak current of 5 lM vanillin was less than 5%. However, 1-fold amount of ethyl vanillin exhibited serious interference because ethyl vanillin and vanillin are very similar. Hence, the selectivity of the electrode is similar to that reported by Peng et al. (2012) and Zheng et al. (2010)).
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3.8. Application Biscuit, vanilla bean and vanilla tea samples were determined by using the proposed method and the results were summarized in Table 2. The contents of vanillin in these samples were calculated and they were 0.08 mg g 1 (for biscuit), 13.5 mg g 1 (for vanilla bean) and 0.06 mg g 1 (for vanilla tea), respectively. The recoveries for standards added were 84–106%, indicating that the method is reliable.
4. Conclusion Graphene oxide can be reduced by DMF at about 150 °C to produce stable graphene suspension. Well-dispersed AuPd alloy nanoparticles can be prepared on the graphene modified GCE by electrodeposition. The obtained AuPd–graphene composite film has strong electrocatalysis toward the oxidation of vanillin, thus vanillin produces a sensitive anodic peak at the AuPd–graphene modified GCE. The electrode also exhibits good stability and reproducibility, and it can be applied to the determination of vanillin in real samples. This work provides a example for making use of alloy nanoparticles to construct new sensors. Acknowledgements The Authors appreciate the financial support from the National Natural Science Foundation of China (Grant No.: 21075092) and the State Key Laboratory of Advanced Technology for Materials Synthesis and Processing (Wuhan University of Technology, Wuhan, China, Grant No. 2010-KF-12).
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. 2013.11.044.
