Food Chemistry 145 (2014) 619–624

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Analytical Methods

Electrochemical determination of toxic ractopamine at an ordered mesoporous carbon modified electrode Xiao Yang a, Bo Feng b, Peng Yang a, Yonglan Ding a,c, Yi Chen a, Junjie Fei a,⇑ a

Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, College of Chemistry, Xiangtan University, Xiangtan 411105, PR China College of Chemical Engineering, Xiangtan University, Xiangtan 411105, PR China c Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Hunan Normal University), Ministry of Education, Changsha, Hunan 410081, PR China b

a r t i c l e

i n f o

Article history: Received 2 February 2013 Received in revised form 28 July 2013 Accepted 22 August 2013 Available online 3 September 2013 Keywords: Ordered mesoporus carbon Ractopamine Chemical modified electrode Electrochemical sensor Pork samples

a b s t r a c t A sensitive electrochemical sensor was developed to detect toxic ractopamine using ordered mesoporus carbon (OMC) modified glass carbon electrode (OMC/GCE). Cyclic voltammetry was used to investigate the electrochemical behaviours of ractopamine on OMC/GCE. The results indicated that the OMC modified electrode can remarkably enhance electrocatalytic activity towards the oxidation of ractopamine with a great increase of peak current. The oxidation mechanism was studied and the results showed that the oxidation of ractopamine involved two protons and two electrons of its two phenolic hydroxyl groups. The signal for the determination of ractopamine was recorded using differential pulse voltammetry (DPV) and the optimisation for the experimental conditions was also conducted. The results showed that the response of the sensor to concentration of ractopamine displayed a linear correlation over a range from 0.085 lM to 8.0 lM with a detection limit of 0.06 lM, demonstrating favourable sensitivity and selectivity for the detection of ractopamine. Finally, the method was successfully applied for the determination of ractopamine in pork samples with satisfying recoveries in the range of 96.6–104.5% and excellent RSD of less than 5%. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Ractopamine is a phenethanolamine with b2-adrenergic agonist properties, and has been originally used as tocolytics, bronchodilators, and heart tonics in human and veterinary medicine (Lu et al., 2012). Meanwhile, this compound is illegally applied as a nutrient repartitioning agent in the livestock industry to divert nutrients from fat deposition to muscle production in China (Dong et al., 2012). However, the drug residues accumulated in animal tissues may pose a potential risk for consumer health, such as muscular tremors, vomiting, nervousness, and cardiac palpitations (Bolera et al., 2012; Halsey et al., 2011). Hence, ractopamine is not licensed for animal production in many countries. Up-to-now, different analytical methods have been developed for the detection of ractopamine in animal feeds, animal tissues and body liquids. These methods include liquid chromatography– mass spectrometry (Blanca et al., 2005), gas chromatography–mass spectrometry (He, Su, Zeng, Liu, & Huang, 2007), immunoassay (Pleadin, Perši, Vulic´, Milic´, & Vahcˇic´, 2012; Shen & He, 2007), high-performance liquid chromatography (Turberg, Rodewald, & Coleman, 1996), and capillary electrophoresis (Wang, Zhang, Wang, Shi, & Ye, 2010). Since ractopamine contains phenolic ⇑ Corresponding author. Tel.: +86 (731) 58298876; fax: +86 (731) 58292251. E-mail addresses: [email protected], [email protected] (J. Fei). 0308-8146/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2013.08.093

hydroxyl group, it should be electrochemical active and could be oxidised at electrode surface. Therefore, electrochemical methods may be preferred methods for the detection of ractopamine due to its advantages of low instrumental cost and fast analysis. Ni, Wang, and Kokot (2010) investigated the oxidation of ractopamine at GCE surface and the results showed that the oxidation of ractopamine involved two electrons. Other voltammetric methods based on multi-walled carbon nanotube film-modified GCE (Liu, Zhou, Wang, Cheng, & Wu, 2012) and graphene oxide film-modified GCE (Wu, Sun, Li, & Wu, 2012) were developed for the detection of ractopamine in recent years. However, the direct electrochemical detection of ractopamine is also limited, maybe due to the poor response activity on ordinary electrode surface (Liu et al., 2012). Ordered mesoporous carbon (OMC) is a kind of novel advanced carbon material, which was initially synthesised in 1999 (Ryoo, Joo, & Jun, 1999). Since then, OMC has attracted great attention due to its uniform and tailored pore structure, high specific surface area, large pore volume, chemical inertness and good conductivity. Applications of OMC as catalysts (Ding, Chan, Ren, & Xiao, 2005), adsorbents (Hartmann, Vinu, & Chandrasekar, 2005), energy storages (Zhou, Zhu, Hibino, & Honma, 2003), capacitor devices (Li, Song, & Chen, 2006) and lithium-ion batteries (Zeng et al., 2012) have been developed. In addition, the electrochemical sensors based on OMC have been reported for the determination of L-cysteine (Zhou, Ding, Guo, & Shang, 2007), Sudan I (Yang, Zhu, Jiang,

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& Guo, 2009), morphine (Li et al., 2010), ultratrace nitroaromatic explosives (Zang, Guo, Hu, Yu, & Li, 2011). To the best of our knowledge OMC has never been explored for electrochemical detection of ractopamine up to now. In this paper, the electrochemical oxidation of ractopamine was achieved using a GCE modified OMC, and the catalytic mechanisms of ractopamine oxidation at OMC/GCE was investigated. The oxidation signal of ractopamine was obviously increased on the surface of OMC film-modified GCE, compared with that on the bare GCE surface. Moreover, the analytical performances of the electrochemical sensor were evaluated by the determination of ractopamine in pork samples.

2. Experimental 2.1. Reagents SBA-15 with pore diameter of 8 nm and BET of 650 m2/g was purchased from Nanjing XFNANO Materials Tech Co., Ltd (China). Ractopamine, as the hydrochloride salt, was obtained from Sigma and dissolved into doubly distilled water to prepare 1.0  103 M standard solution. The 0.1 M phosphate buffer solution (PBS), which was made up from Na2HPO4 and NaH2PO4, was employed as the supporting electrolyte. All other chemicals were of analytical reagent grade and all the solutions were prepared from doubly distilled water. 2.2. Apparatus All electrochemical measurements were performed with a computer-controlled model CHI 630C Electrochemical Workstation (Shanghai Chenhua Instrument Corporation, China). A three-electrode configuration was employed, consisting of a glassy carbon electrode (GCE) or a modified GCE serving as the working electrode, while saturated calomel electrode (SCE) and platinum wire served as the reference electrode and counter electrode, respectively. All potentials were referred to the SCE. Small angle X-ray diffraction (XRD) patterns were obtained on an XRD D/max-2500 VK/PC (Rigaku, Japan) operating at 40 kV and 50 mA and using Cu Ka radiation (k = 0.15406 nm). The TEM image was recorded on transmission electron microscope (HRTEM, JEOL, JEM-2100F). Nitrogen adsorption and desorption isotherms were measured on NOVA 2200e Surface Area & Pore Size Analyzer (Micromeritics, USA). The pore size distributions (PSD) were calculated by the BJH method. 2.3. Synthesis of ordered mesoporous carbon Ordered mesoporous carbon (OMC) was synthesised by using SBA-15 as the template and sucrose as the carbon source according to the method reported by Jun et al. (2000). In a typical process, 1 g SBA-15 was added to a solution obtained by dissolving 1.25 g sucrose and 0.14 g H2SO4 in 5 g H2O. The mixture was placed in a drying oven for 6 h at 100 °C, and subsequently the oven temperature was increased to 160 °C and maintained for 6 h. The silica sample, containing partially polymerised and carbonised sucrose at the present step, was treated again at 100 °C and 160 °C using the same oven after the addition of 0.8 g sucrose, 0.09 g H2SO4 and 5 g H2O. The resulting dark brown sample was completely pyrolysed at 900 °C for 6 h under Ar flow at a heating rate of 2 °C min1. Finally, the mesoporous carbons were obtained after dissolution of the silica framework in 5% hydrofluoric acid, by filtration, washed several times with ethanol, and dried at 120 °C.

2.4. Preparation of modified electrodes The glass carbon electrode (GCE) with 3 mm in diameter was polished with 1, 0.3 and 0.05 lm alumina slurry on a polishing cloth, respectively, followed by sonication in nitric acid/water (1:1), acetone/water (1:1) and doubly distilled water successively. The obtained OMC (5 mg) was dispersed in 10 mL N, N0 -dimethyl formamide and the mixture was sonicated for 30 min to obtain a homogeneous black suspension. Then 6 lL of the obtained suspension was dropped on the GCE surface and allowed to dry for about 15 min under an infrared lamp. The thickness of the modified layer was estimated to be 0.15 lm. 2.5. Sample preparation Pork samples were purchased from a local supermarket and pretreated according to reported method (Liu et al., 2012; Lu et al., 2012). Briefly, 2.0 g smashed sample was homogenised using 4 mL 0.1 M HClO4, then ultrasonicated for 20 min, and then heated at 80 °C for 30 min. After cooling and 15-min centrifugation at 10,000 rpm, the clear liquid phase was collected. After that, the pH value of collected liquid was adjusted to 10 using 10% Na2CO3, and then 1.6 g NaCl was added. Subsequently, ractopamine was extracted twice using 5 mL ethyl acetate. Finally, ractopamine was reversely extracted to 2 mL 0.1 M HCl solution. The reverse extraction was repeated, and the sample solution was diluted to 10 mL using pH 7.0 phosphate buffer. Spiked samples were prepared as follows: 2.0 g pork sample was spiked with a known amount of ractopamine standard solution using micro syringe and then pretreated according to the above-mentioned method. 2.6. Analytical procedure The pH 7.0 phosphate buffer (0.1 M) was used as the supporting electrolyte for the detection of ractopamine. After 3-min accumulation at 0.2 V, the differential pulse voltammograms were recorded from 0.3 V to 0.9 V. The oxidation peak current of ractopamine was measured. The pulse amplitude and the pulse width of DPV method were 50 mV and 40 ms, respectively. Comparison method for the detect of ractopamine in real samples was performed on an Agilent 1100 LC system equipped with a G1312A FLD fluorescence detector. Chromatographic conditions (Shishani, Chai, Jamokha, Aznar, & Hoffman, 2003) are as follows: Column: SB-C18 (250 mm  4.6 mm, 5 lm particle size); flow rate: 1 mL min1; Wavelength Excitation at 226 nm, emission at 306 nm; Mobile phase: 320 mL of acetonitrile, 680 mL of deionised water, 20 mL of glacial acetic acid and 0.87 g of 1-pentanesulfonic acid, mixed well and degassed; injection volume 50 lL. 3. Results and discussion 3.1. Characterisation of OMC The morphologies and the structures of OMC were characterised using TEM. It can be seen from Fig. 1A that the synthesised materials show highly ordered array of carbon nanorods with the pore size of ca. 3–4 nm. The structure of OMC was further investigated by small-angle XRD, as shown in lower right corner of Fig. 1A, the ordered arrangement of the carbon materials exhibit two well-resolved XRD peaks, which can be assigned to (1 0 0), (1 1 0) diffractions of the 2-D hexagonal space group (p6 mm). In addition, Fig. 1B demonstrates the nitrogen adsorption–desorption isotherms of OMC. The isotherms clearly show that the OMC has type IV with type H1 hysteresis loops for a typical mesoporous structure. High specific surface area of 1092 m2 g1 with a average

X. Yang et al. / Food Chemistry 145 (2014) 619–624

pore diameter of ca. 3.9 nm and a specific pore volume of 1.1 cm3 g1 were also calculated from the isotherms. The electrochemical properties of OMC/GCE were investigated using potassium ferricyanide as an electrochemical probe. Cyclic voltammograms obtained at the OMC modified GCE and a bare GCE in the presence of 5 mM K3Fe(CN)6/0.1 M KCl solution are given in Supplemental Data (Fig. 1s). The peak separations are found to be 78 mV and 102 mV at OMC/GCE (curve a) and the bare GCE (curve b), respectively. As DEp is a function of the rate of electron transfer, the lower DEp indicates a higher electron transfer rate. It can be deduced that a higher electron transfer rate was occurred at OMC modified GCE compared with that at bare GCE. According to Randles–Sevcik equation (Bard & Faulkner, 1980): Ip = 2.69  105AD1/2n3/2v1/2C, where Ip is the peak current (A), A is the area of the electroactive surface area (cm2), D is the diffusion coefficient of the molecule in solution (cm2 s1), n is the number of electrons transferred, v is the scan rate (V s1) and C corresponds to the bulk concentration of the probe (mol cm3). According to the experimental data and above equation, the electroactive surface area of OMC modified electrode surface was calculated to be 0.0753 cm2, which is more than that of GCE (0.0583 cm2). The obtained results indicate that OMC could greatly improve the electrochemical properties of the modified electrode and exhibit faster electron rate.

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3.2. Electrochemical behaviours of ractopamine at OMC/GCE Fig. 2 shows the cyclic voltammograms of ractopamine at bare GCE and OMC/GCE. Inset of Fig. 2 is the magnified vision of curve a and b. Fig. 2a demonstrates the cyclic voltammogram of 5.0 lM ractopamine at a bare GCE and the result shows that only a small and broad oxidation peak of ractopamine occurs at 0.730 V and no corresponding reduction peak of ractopamine is observed in the reverse scan, which indicates that the electrochemical behaviour of ractopamine at the bare GCE is an irreversible and sluggish process. It can be distinctly seen that the oxidation of ractopamine at the bare GCE requires a large overpotential. Compared with that of the bare GCE, the oxidation peak current of ractopamine increases significantly and the oxidation peak potential shifts negatively to 0.612 V at the OMC/GCE (Fig. 2, curve f). The obviously increased peak current and the decrease in the anodic overpotential of 118 mV for ractopamine clearly demonstrate that OMC/GCE exhibits a good electrochemical response and a faster electron transfer kinetics for the oxidation of ractopamine. This is most likely to be attributed to the large surface area of OMC and a large number of edge plane defect sites at the surface of the OMC/GCE, which may provide many favourable sites for electron transfer to molecules (Zhou, Guo, Hou, & Peng, 2008; Zhu, Tian, Yang, Jiang, & Yang, 2008). In addition, with addition of ractopamine, the oxidation currents gradually increase at OMC/GCE (Fig. 2, curve c–f). 3.3. The effect of the scan rate on the oxidation of ractopamine at OMC/GCE Fig. 3A shows cyclic voltammograms of 5.0 lM ractopamine at different scan rates ranging from 10 to 175 mV s1. The oxidation peak current increases linearly with the scan rate (inset of Fig. 3A) and the calibration equation is ipa (lA) = 1.637 + 268.773 v (V s1) (R = 0.9984), which indicates that oxidation of ractopamine occurred at the OMC/GCE is a adsorption controlled process. Additionally, it can be seen that the oxidation peak potentials (Epa) shift positively along with the increase of the scan rate, also revealing an irreversible oxidation process of ractopamine. For an irreversible and adsorption – controlled oxidation process, the relationship between Epa and lgt is defined by Eq. (1) according to Laviron (1979)’s theory:

Fig. 1. (A) TEM image of OMC viewed from [1 0 0] direction. Inset: XRD diffraction patterns of OMC. (B) Nitrogen adsorption–desorption isotherms for OMC. Inset: pore size distribution for OMC.

Fig. 2. Cyclic voltammograms obtained for oxidation of ractopamine in 0.1 M PBS (pH 7.0) at bare GCE with (a) and without (b) 5.0 lM ractopamine and at OMC/GCE containing (c) 0.0 M, (d) 0.8 lM, (e) 3.0 lM, (f) 5.0 lM ractopamine. Inset shows the enlarged view of (a) and (b). Scan rate: 50 mV s1.

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0

Epa ¼ E0 þ ð2:303RT=ana FÞ lgðRTk =ana FÞ þ ð2:303RT=ana FÞ lg t

3.5. The effect of accumulation potential and time

ð1Þ

00

Here, E is the formal potential (V), T is the temperature (K), a is the electron transfer coefficient, na is the number of transferred electron, k0 is the standard heterogeneous electron transfer rate constant, and F is the Faraday constant. In this work, the value of Epa also increases linearly with lgt (Eq. (2)) as can be seen from Fig. 3B.

Epa ¼ 0:5132 þ 0:06138lg tðR ¼ 0:9988Þ

ð2Þ

Combining Eq. (1) with Eq. (2), the value of ana was calculated to be 0.9634. Generally, a is assumed as 0.4–0.6 for a totally irreversible electrode process. Hence two electrons are involved in the oxidation of ractopamine, which is consistent with the reported results (Liu et al., 2012; Ni et al., 2010).

3.4. The effect of pH on the oxidation of ractopamine at OMC/GCE The pH of the supporting electrolyte has a significant influence on the ractopamine electrooxidation at the OMC modified GCE, by altering both peak currents and peak potentials. The effect of pH on the oxidation of ractopamine was invistigated by cyclic voltammetry and the curves are given in Supplemental Data (Fig. 2s). The anodic peak potentials of ractopamine shift negatively along with the increasing pH, which is a consequence of the deprotonation involved in the oxidation of ractopamine that is facilitated at higher pH values. Furthermore, the peak potentials shift with the pH values following the linear equation of Epa = 1.142–0.070 pH (R = 0.999). The slope of 70 mV/pH is close to the theoretical value (59 mV/pH), so it can be deduced that the proton number is equal to that of the number of transferred electrons. Since two electrons are involved in the oxidation of ractopamine, it can be concluded that the oxidation of ractopamine involves two protons and two electrons, which is illustrated with Scheme 1. The peak currents of ractopamine increase from pH 5.0 and reach maximum value at pH 7.0, then the responses decrease slightly. Hence, the optimum pH value of 7.0 was employed in the following experiments.

The accumulation potential and time, which are two crucial parameters for accumulation, were investigated by differential pulse voltammetry (DPV) and the results are given in Supplemental Data (Fig. 3s) .When the accumulation potential changed positively from 0.1 V to 0.2 V, the adsorption of ractopamine at the electrode surface become more efficient, and consequently the DPV peak currents increase. When the accumulation potentials are more positive than 0.2 V, the DPV peak currents decrease slightly, and at the same time the background currents increase. Therefore, 0.2 V was fixed as the optimal accumulation potential. The DPV peak currents increase rapidly with the increase of accumulation time from 0 to 180 s. Rapid adsorption of ractopamine on the surface of the OMC modified GCE is responsible for this phenomenon. However, for a longer accumulation time, the plot levels off, indicating, presumably, that a limiting amount of ractopamine on the electrode surface has been adsorbed. Further increase of the accumulation time leads to only slight increase of the amount of ractopamine adsorbed on the electrode owing to surface saturation. The sensitivity for lower ractopamine concentrations could be increased by increasing the accumulation time, but the linear concentration range was diminished. 3.6. Calibration plot The calibration plot for the ractopamine determination was constructed based on results of anodic differential pulse voltammetry as shown in Fig. 4. The oxidation peak current of ractopamine is linear to its concentration in the range from 0.085 lM to 8.0 lM with a regression equation of ip (lA) = 1.768 C (lM) + 0.505 (R = 0.997). For the concentration of ractopamine higher than 8.0 lM, the peak current increased only slightly and reached a constant value due to saturation of the OMC modified GCE surface. The detection limit was 0.06 lM at a 3:1 signal-tonoise ratio for accumulation time of 180 s. 3.7. Reproducibility, stability and interference Under the optimised conditions, the OMC/GCE was used to determine 3 lM ractopamine for eight times by DPV. The relative

Fig. 3. (A) Cyclic voltammograms of 5.0 lM ractopamine in 0.1 M PBS (pH = 7.0) at OMC/GCE at various scan rates, from inner to outer 10, 25, 50, 75, 100, 125, 150, 175 mV/s. (B) Calibration plot of E vs. lgt from 5.0 lM ractopamine in 0.1 M PBS (pH = 7.0).

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OH

CHCH2NHCHCH2CH2 OH

OH

Table 1 Measurement results of ractopamine in pork samples (n = 3).

-2e-2H +

CH3 O

CHCH2NHCHCH2CH2 OH

Sample

Spiked (lg g1)

Pork A

0.00 1.50 0.00 3.00 0.00 4.50

O

CH3

Scheme 1. Oxidation mechanism of ractopamine at OMC/GCE.

Pork B Pork C

standard deviation (RSD) was 2.8%, which indicates that the electrochemical response of ractopamine at the OMC/GCE was highly reproducible. Six different OMC/GCEs were fabricated and the RSD for the detection of 3 lM ractopamine is 3.7%, revealing excellent repeatability of OMC/GCE. When the electrode was kept at room temperature for two weeks, the oxidation peak current of ractopamine only decreased 5.2%, suggesting good stability. The potential interferences for the detection of ractopamine were studied. Under the optimised conditions, the oxidation peak current of 3 lM ractopamine was individually measured in the presence of different interferents and the peak current change was then checked. It was found that at least 3 mM glucose; 1.5 mM glycine and uric acid; 0.75 mM ascorbic acid; 0.3 mM guanine; 0.15 mM dopamine have little effect on the current response of 3 lM ractopamine (the peak current changes were 1.6%, 3.2%, 4.8%, 3.7%, 2.9%, 4.6%, respectively).

3.8. Detection of ractopamine in pork samples In order to confirm the sensitivity and generality of the proposed method, the developed method was applied for the determination of ractopamine in pork samples. The pork samples were treated with the procedures described in Section 2.5, and then detected by DPV under the optimal conditions using standard addition method. Fortunately, no ractopamine was detected in the real pork samples. So ractopamine standards with different concentrations were spiked into the sample, and then analysed under the same conditions. The measurement results are listed in Table 1. The recoveries are from 96.6% to 104.5% and the RSD is less than 5% (n = 3). Finally, in order to testify the reliability and accuracy of this method, LC was used to detect ractopamine in real water samples.

Expected (lg g1) 1.50 3.00 4.50

Found (lg g1) 0.00 1.567 0.00 2.898 0.00 4.578

RSD (%)

Recovery (%)

Detected by LC (lg g1)

3.57

104.5

1.487

4.28

96.6

3.067

3.95

101.7

4.569

From the comparisons listed in Table 1, the similarity of results of this method and LC method indicates the reliability of the results obtained in this study. Its sound results showed that the method is quite valuable and seems to be of great utility for further sensor development. 4. Conclusion Electrochemical detection of toxic ractopamine has been successfully achieved by using OMC modified GCE. The detection conditions were optimised and a electrochemical sensing platform was developed for the sensitive and rapid detection of ractopamine. The electrochemical behaviours of ractopamine at OMC/ GCE were investigated, and the oxidation of ractopamine involved two electrons and two protons. Furthermore, the electrochemical sensor was applied for the detection of ractopamine in pork samples and results showed that the recoveries are from 96.6% to 104.5% and the RSD is less than 5% (n = 3). Its sound results showed that the method is quite valuable and seems to be of great utility for further sensor development. Acknowledgements This work was supported by the National Natural Science Foundation of China (21275123, 20975088, 21105085, 31270988), Project of Hunan Provincial Natural Science Foundation of China (12JJ7002), the Key Project of Chinese Ministry of Education (210152) and Opening Fund of Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Hunan Normal University), Ministry of Education (KLCBTCMR2011-6) and the Construct Program of the Key Discipline in Hunan Province. 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.2013.08.093. Reference

Fig. 4. DPVs of ractopamine at various concentrations: 0.8, 1.2, 1.6, 2.0, 2.5, 3.0 and 4.0 lM from inner to outer. Inset is the calibration curve of current response vs. ractopamine concentration. Accumulation potential: 0.2 V, accumulation time: 180 s.

Bard, A., & Faulkner, L. R. (1980). Electrochemical method, fundamentals and applications. New York: Wiley. 226–236. Blanca, J., Muñoz, P., Morgado, M., Méndez, N., Aranda, A., Reuvers, T., & Hooghuis, H. (2005). Determination of clenbuterol, ractopamine and zilpaterol in liver and urine by liquid chromatography tandem mass spectrometry. Analytica Chimica Acta, 529, 199–205. Bolera, D. D., Shrecka, A. L., Faulknera, D. B., Killefera, J., McKeitha, F. K., Hommb, J. W., & Scanga, J. A. (2012). Effect of ractopamine hydrochloride (Optaflexx) dose on live animal performance, carcass characteristics and tenderness in early weaned beef steers. Meat Science, 92, 458–463. Ding, J., Chan, K. Y., Ren, J. W., & Xiao, F. S. (2005). Platinum and platinum– ruthenium nanoparticles supported on ordered mesoporous carbon and their electrocatalytic performance for fuel cell reactions. Electrochimica Acta, 50, 3131–3141. Dong, J. X., Li, Z. F., Lei, H. T., Sun, Y. M., Ducancel, F., Xu, Z. L., Boulain, J. C., Yang, J. Y., Shen, Y. D., & Wang, H. (2012). Development of a single-chain variable fragment-alkaline phosphatase fusion protein and a sensitive direct

624

X. Yang et al. / Food Chemistry 145 (2014) 619–624

competitive chemiluminescent enzyme immunoassay for detection of ractopamine in pork. Analytica Chimica Acta, 736, 85–91. Halsey, C. H. C., Weber, P. S., Reiter, S. S., Stronach, B. N., Bartosh, J. L., & Bergen, W. G. (2011). The effect of ractopamine hydrochloride on gene expression in adipose tissues of finishing pigs. Journal of Animal Science, 89, 1011–1019. Hartmann, M., Vinu, A., & Chandrasekar, G. (2005). Adsorption of vitamin E on mesoporous carbon molecular sieves. Chemistry of Materials, 17, 829–833. He, L. M., Su, Y. J., Zeng, Z. L., Liu, Y. H., & Huang, X. H. (2007). Determination of ractopamine and clenbuterol in feeds by gas chromatography–mass spectrometry. Animal Feed Science and Technology, 132, 316–323. Jun, S., Joo, S. H., Ryoo, R., Kruk, M., Jaroniec, M., Liu, Z., Ohsuna, T., & Terasaki, O. (2000). Synthesis of new, nanoporous carbon with hexagonally ordered mesostructure. Journal of the American Chemical Society, 122, 10712–10713. Laviron, E. (1979). General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems. Journal of Electroanalytical Chemistry, 101, 19–28. Li, L. X., Song, H. H., & Chen, X. H. (2006). Pore characteristics and electrochemical performance of ordered mesoporous carbons for electric double-layer capacitors. Electrochimica Acta, 51, 5715–5720. Li, F., Song, J. X., Shan, C. S., Gao, D. M., Xu, X. Y., & Niu, L. (2010). Electrochemical determination of morphine at ordered mesoporous carbon modified glassy carbon electrode. Biosensors & Bioelectronics, 25, 1408–1413. Liu, Z., Zhou, Y. K., Wang, Y. Y., Cheng, Q., & Wu, K. B. (2012). Enhanced oxidation and detection of toxic ractopamine using carbon nanotube film-modified electrode. Electrochimica Acta, 74, 139–144. Lu, X., Zheng, H., Li, X. Q., Yuan, X. X., Li, H., Deng, L. G., Zhang, H., Wang, W. Z., Yang, G. S., Meng, M., Xi, R. M., & Aboul-Enein, H. Y. (2012). Detection of ractopamine residues in pork by surface plasmon resonance-based biosensor inhibition immunoassay. Food Chemistry, 130, 1061–1065. Ni, Y. N., Wang, Y. X., & Kokot, S. (2010). Voltammetric, UV–Vis Spectrometric and fluorescence study of the interaction of ractopamine and DNA with the aid of multivariate curve resolution-alternating least squares. Electroanalysis, 22, 2216–2224. Pleadin, J., Perši, N., Vulic´, A., Milic´, D., & Vahcˇic´, N. (2012). Determination of residual ractopamine concentrations by enzyme immunoassay in treated pig’s tissues on days after withdrawal. Meat Science, 90, 755–758. Ryoo, R., Joo, S. H., & Jun, S. (1999). Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation. Journal of Physical Chemistry B, 103, 7743–7746.

Shen, L., & He, P. L. (2007). An electrochemical immunosensor based on agarose hydrogel films for rapid determination of ractopamine. Electrochemistry Communications, 9, 657–662. Shishani, E., Chai, S. C., Jamokha, S., Aznar, G., & Hoffman, M. K. (2003). Determination of ractopamine in animal tissues by liquid chromatographyfluorescence and liquid chromatography/tandem mass spectrometry. Analytica Chimica Acta, 483, 137–145. Turberg, M. P., Rodewald, J. M., & Coleman, M. R. (1996). Determination of ractopamine in monkey plasma and swine serum by high-performance liquid chromatography with electrochemical detection. Journal of Chromatography B, 675, 279–285. Wang, W. Y., Zhang, Y. L., Wang, J. Y., Shi, X. S., & Ye, J. N. (2010). Determination of bagonists in pig feed, pig urine and pig liver using capillary electrophoresis with electrochemical detection. Meat Science, 85, 302–305. Wu, C., Sun, D., Li, Q., & Wu, K. B. (2012). Electrochemical sensor for toxic ractopamine and clenbuterol based on the enhancement effect of graphene oxide. Sensors and Actuators B, 168, 178–184. Yang, D. X., Zhu, L. D., Jiang, X. Y., & Guo, L. P. (2009). Sensitive determination of Sudan I at an ordered mesoporous carbon modified glassy carbon electrode. Sensors and Actuators B, 141, 124–129. Zang, J. F., Guo, C. X., Hu, F. P., Yu, L., & Li, C. M. (2011). Electrochemical detection of ultratrace nitroaromatic explosives using ordered mesoporous carbon. Analytica Chimica Acta, 683, 187–191. Zeng, F. Y., Hou, Z. H., He, B. H., Ge, C. Y., Cao, J. G., & Kuang, Y. F. (2012). Influence of heat-treatment on lithium ion anode properties of mesoporous carbons with nanosheet-like walls. Materials Research Bulletin, 47(8), 2104–2107. Zhou, M., Ding, J., Guo, L. P., & Shang, Q. K. (2007). Electrochemical behavior of Lcysteine and its detection at ordered mesoporous carbon-modified glassy carbon electrode. Analytical Chemistry, 79, 5328–5335. Zhou, M., Guo, L. P., Hou, Y., & Peng, X. J. (2008). Immobilization of Nafion-ordered mesoporous carbon on a glassy carbon electrode: Application to the detection of epinephrine. Electrochimica Acta, 53(12), 4176–4184. Zhou, H. S., Zhu, S. M., Hibino, M., & Honma, I. (2003). Electrochemical capacitance of self-ordered mesoporous carbon. Journal of Power Sources, 122(2), 219–223. Zhu, L. D., Tian, C. Y., Yang, D. X., Jiang, X. Y., & Yang, R. L. (2008). Bioanalytical application of the ordered mesoporous carbon modified electrodes. Electroanalysis, 20, 2518–2525.