Talanta 138 (2015) 40–45

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Rapid analysis of ractopamine in pig tissues by dummy-template imprinted solid-phase extraction coupling with surface-enhanced Raman spectroscopy Xiaohua Xiao n, Kuanglin Yan, Xianfang Xu, Gongke Li n School of Chemistry and Chemical Engineering, Sun Yat-sen University, Guangzhou 510275, China

art ic l e i nf o

a b s t r a c t

Article history: Received 12 December 2014 Received in revised form 27 January 2015 Accepted 1 February 2015 Available online 9 February 2015

Ritodrine has similar skeleton structure to ractopamine and it was selected as the dummy-template molecule to synthesize the molecular imprinted polymers (MIPs). The MIPs exhibited better selectivity to ractopamine than to the dummy-template molecule: the imprint factor for ractopamine was 8.9, while 7.6 for ritodrine. The MIPs were used as sorbents in solid-phase extraction for selective enrichment of ractopamine, and some key parameters were optimized. After that, a rapid surface-enhanced Raman spectroscopy method was developed for analysis of ractopamine and isoxuprine in pig tissue samples. Under the optimal conditions, good linearity was achieved in the range of 20.0–200.0 μg/L for ractopamine and isoxsuprine at 842 cm  1 and 993 cm  1, respectively. The limits of detection were 3.1–4.3 μg/L, which were lower than the maximum allowed by U. S. Food and Drug Administration. The recoveries of ractopamine and isoxsuprine were 72.4–79.7% and 71.0–78.2% for the spiked pork and pig liver, respectively, while the relative standard deviations ranging from 7.4% to 13.0%. The results suggest that the proposed method is sensitive and selective, and it has good potential on the quantitative analysis of trace amounts of β-agonists in complex samples. & 2015 Elsevier B.V. All rights reserved.

Keywords: Dummy-template Molecularly imprinted polymers Solid-phase extraction Surface-enhanced Raman spectroscopy Ractopamine Pig tissues

1. Introduction The demand for faster, more sensitive and cost-effective analytical methods is rapidly increasing during the last decades, especially on the determination of trace analytes in complex matrix such as foods, environmental or biological samples [1]. Normally, an analytical process has several steps including sample preparation, sampling, separation, determination, data handling and treatment, etc. Of these steps, selective sample preparation and sensitive analytical methods might be the most important. Chromatographic methods such as liquid chromatography–mass spectroscopy, gas chromatography– mass spectroscopy and immunoassay such as enzyme-linked immunosorbent assay (ELISA) were typically used in the most cases [2]. However, the chromatographic methods are relatively expensive and complex operation while ELISA is likely to be influenced by matrix interferences and so lead false-positive results. Surface-enhanced Raman spectroscopy (SERS) is a special surface-enhanced optical phenomenon on the nano-scale metal surfaces, which has overcome the low sensitivity of normal Raman spectroscopy via adsorption of analytes on rough nano-scale surfaces typically made from gold and

n

Corresponding authors. Tel.: þ 86 20 84110922; fax: þ 86 20 84115107. E-mail addresses: [email protected] (X. Xiao), [email protected] (G. Li). http://dx.doi.org/10.1016/j.talanta.2015.02.003 0039-9140/& 2015 Elsevier B.V. All rights reserved.

silver [3]. The magnitude of Raman signals in SERS can be enhanced to 104–107 due to the effects of electromagnetic field and chemical enhancement [4]. Hence, the ultrasensitive SERS has been used to characterize and detect organic chemicals and microorganisms such as melamine [5–7], malachite green [8–10], pesticides [11–13], antibiotics [14–16], β-agonists [17,18], etc. Although the limits of detection (LODs) of these methods were acceptable, their further application to detect specific analytes in complex matrices, especially for the quantitative analysis of trace analytes, were greatly limited by the serious matrix interferences and poor selectivity. Therefore, sample pretreatment to reduce or remove undesirable interferences is required before SERS detection. Conventional liquid–liquid extraction method is considered the most time-consuming and error-prone part of the analytical scheme. Some new extraction techniques such as solid-phase extraction (SPE), solid-phase microextraction (SPME) [19] or liquid-phase microextraction (LPME) [20], were developed to reduce the initial sample sizes, and to minimize the amount of hazardous organic solvents. Among them, both SPE and SPME were well established in analytical laboratories. However, the main drawback associated to them is the lack of selectivity of the sorbents. Molecularly imprinted polymers (MIPs), the synthetic materials with artificially generated recognition sites which are able to specifically rebind target molecule, have attracted more and more attention for their high affinity and selectivity for target analyte and its analogues. Molecular imprinted solid-phase extraction (MISPE)

X. Xiao et al. / Talanta 138 (2015) 40–45

was also developed as a relatively new concept in the pretreatment of biological sample, it had been successfully used for enrichment of ractopamine from complex matrices [21]. MISPE had also been combined with SERS for the detection of chemical compounds from complicated samples. Feng [18] reported a MISPE-SERS biosensor system for the detection of α-tocopherol from four different types of vegetable oils based on using a dendritic silver nanostructure as SERS substrate. This biosensing system demonstrated good sensitivity and selectivity for the quantitative detection of different spiking levels of α-tocopherol in vegetable oils. Meanwhile, the further applications of traditional MIPs were greatly limited owing to the unavoidable template leaking, which may influence the accuracy of identification and quantization of the analytes. The structural analogues of the analyte itself, namely dummy-template, can avoid the leakage of the template. To guarantee the selectivity and capacity of the sorbents, the selection of the dummy template is a key factor, while the available dummy template molecules are limited especially for simultaneous determination of homologue compounds. In most case, the imprinting factor or selectivity for dummy template was significantly higher than that for the cross-recognition targets in these molecular imprinted polymers [22]. Ractopamine was a typical β-agonists, which can promote bronchodilation, vasodilatation and increase heart rate. It is also used in animal feeding, effects seen in the skeletal muscle include speed up feeding efficiency and carcass leanness. However, the residual of ractopamine in pork products may pose health risks, particularly to those with asthma or cardiovascular disease. The use of ractopamine in swine has been banned by the majority of countries and areas in the world, such as China, Japan, and European Union, the maximum residue limit of ractopamine in swine liver is strictly limited to 0.15 μg/g or lower in the United States. In recent years, monitoring the residues of β-agonists in swine liver or pork products had been attracted great interest to the government regulatory agencies and the food industry [18]. Since the residue of ractopamine in complex samples is lower than μg/g, efficient sample pretreatments as well as sensitive analytical methods are significantly important. Solid-phase extraction (SPE) [23] or solid-phase microextraction (SPME) [24] with chromatographic methods were commonly used to analysis β-agonists in different matrices. Recently, Du et al. [25] reported a dummytemplate MISPE method for selective analysis of ractopamine in pork, in which salbutamol was used as the dummy-template, the selectivity factors of dummy-template MIPs for salbutamol was significantly higher than that for ractopamine. In the present work, ritodrine was selected as the dummytemplate molecule to synthesize the molecular imprinted polymers due to its highly similar structure skeleton to ractopamine. The MIPs were used as sorbents of SPE for the selective extraction of β-agonists, then a rapid SERS method for analysis of ractopamine in pig tissue samples was developed.

2. Experimental 2.1. Chemicals and materials Ritodrine hydrochloride (RTD) was purchased from Ruihe Chemical Plant (Taiyuan, China); Ractopamine hydrochloride (RAT), isoxsuprine hydrochloride (ISP) and clenbuterol hydrochloride (CLB) were purchased from Sigma (Shanghai, China); 4-nitrophenol (4NP) and trisodium citrate was purchased from Guangzhou Reagent Plant (Guangzhou, China). Methacrylic acid (MAA) and azo-bis(isobutyronitrile) (AIBN) were purchased from Damao Reagent Plant (Tianjin, China). Ethylene glycol dimethacrylate (EGDMA) was purchased from Rohm-Haas AG (USA). Chloroauric acid was purchased from Guangfu Chemical Research Institute (Tianjin, China). Methanol

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(MeOH) and acetonitrile (ACN) of HPLC grade were purchased from Merck (Darmstadt, Germany). Deionized water was used thoroughly. All other reagents were of analytical grade. The individual stock solutions of standards were prepared at the concentration of 100.0 mg/L in acetonitrile, and further solutions of lower concentration were prepared by serial dilution of the stock solutions. The mixed standard solution was prepared with iso-proportional mass of each solution. Au nanoparticles (Au NPs) was prepared according to the method reported previously [3]. Briefly, 200 mL deionized water consist of 5.0 mL 10 mmol/L HAuCl4 was heated to boiling under vigorous stirring, then 1.2 mL 1% (w/v) trisodium citrate was injected rapidly and kept boiling for 40 min. The red solution was detected by Cary-100 UV–vis spectrophotometer (Varian, American) and its absorption wavelength was 530 nm, the average diameter of Au NPs was about 55 nm. 2.2. Instruments An S-4300 scanning electron microscope (Hitachi, Japan) was used to investigate the surface of the MIP polymer. A Nicolet Avatar 330 Fourier transform infrared (FT-IR) spectrometer with a scanning range from 400 to 4000 cm  1 was applied to investigate the composition of the polymer. An LC-20A HPLC system (Shimadzu, Japan) consists of a RF-10AXL fluorescence detector and a diode-array detector was utilized to analyze ractopamine and its analogues. Aspirator A-3S circulating pump was applied to promote the solid-phase extraction. The solid-phase extraction tube and frits were purchased from Anpel company (Shanghai, China). A battery-powered Raman spectrometer with 785 nm laser excitation wavelength (Delta Nu Inspector Raman, Laramie, WY) was used to perform SERS detection. This system consists of a CCD detector (ModelSpec-10:400B, Roper Scientific, Trenton, NJ) with a spectral resolution of 8 cm  1, and a data acquisition system (Photometrics, Tucson, AZ). 2.3. Synthesis of the dummy-template MIPs The dummy-template MIPs were prepared according to our reported work [26] with some modifications. Briefly, 0.33 g RTD, 347 μL MAA and 3.1 mL EGDMA were dissolved in 6.8 mL methanol to prepare the pre-polymer solution. This solution was stirred for 12 h at room temperature, and then 30 mg AIBN was added and dissolved adequately. After purging with nitrogen stream for 5 min, the bottle was sealed immediately and allowed to perform polymerization at 60 1C for 24 h. Non-imprinted polymers (NIPs) were prepared by the same way with the imprinted polymers except without the addition of RTD. The polymers were ground and sieved with a mesh gauge. The template molecule was removed by soxhlet extraction with methanol-acetic acid (9:1, v/v) until no template molecule was detected by HPLC. The polymers were heated at 120 1C for 12 h before use. 2.4. Optimization of molecular imprinted solid-phase extraction procedures 200 mg of MIP particles were packed into a 3.0 mL empty polypropylene cartridge with glasswool frit at each end. The cartridge was activated with 3.0 mL of methanol and equilibrated with 3.0 mL of acetonitrile. Then, the loading solutions in different solvent including deionized water, methanol, acetonitrile and their corresponding aqueous solutions were investigated. 5.0 mL of the pre-treated sample was added into the cartridge at 0.5 mL/min. After that, the cartridge was washed with methanol and eluted with methanol containing 20% (v/v) acetic acid. The eluent was

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evaporated to dryness and the residue was re-dissolved with 100 μL of methanol for further analysis. 2.5. Sample preparation Five grams of pork and liver samples were accurately weighed into a 50 mL beaker flask. After adding 5.0 mL of acetonitrile, the sample was ultrasound for 15 min and centrifuged for 10 min at 3000 rpm, repeated twice and combined the extracts. The extracts were reextracted with 10 mL hexane, 6 mL acetonitrile, 4 mL methylene chloride and 20 mL water in a separator funnel. Then, the supernatant was evaporated to dryness and the residue was re-dissolved with 5.0 mL of acetonitrile before loading onto the MISPE cartridge. 2.6. SERS and HPLC analysis In a typical experiment for detecting RAT measured by SERS, 1.5 mL of the prepared Au NPs was concentrated to 30 μL, then 2.0 μL of the concentrated Au NPs was mixed with 2.0 μL of the sample solution in a silicon substrate (0.5  0.5 cm2) treated in Piranha solution (98% H2SO4/ 30% H2O2, 3:1, v/v) and rinsed thoroughly with ultrapure water. A portable Raman spectrometer with laser wavelength of 785 nm, and a power of 60 mW was used for Raman detection. The typical exposure time was 1 s with three accumulations. HPLC measurement was performed on a Shimadzu LC-20A system and the analytical column was a C18 column (250 mm  4.6 mm id, 5 mm, Dikma). RF-10AxL detector was used to analyze RAT, RTD and ISP. The excitation wavelength was 226 nm while the emission wavelength was 305 nm. The mobile phase was acetonitrile and 0.1% acetic acid solution. The ratio of acetonitrile was changed from 10% (v/v) to 48% (v/v) within 8 min at a flow rate of 1.0 mL/min.

ture and time were optimized. Non-imprinted polymers (NIPs) were prepared by the same way with the imprinted polymers without the addition of RTD. FT-IR spectra of the MIPs before and after removal of the template molecule, as well as that of the NIPs and the template molecule of RTD are illustrated in Fig. 2. When RTD was removed from the MIPs, the MIPs (curve b) had the similar characteristic bands with that of the NIPs (curve c). The bands at 3440 cm  1, 2970 cm  1, 1728 cm  1 and 1156 cm  1 were corresponding to the stretching vibration of – OH, –CH3, –C¼ O and –C–O–C, respectively. The absorption peak at 1515 cm  1, which was corresponding to C¼ C stretching vibration in the benzene ring of RTD (curve a), was not shown in MIPs and NIPs, indicating the leakage of the template can be neglected. The SEM images of the MIPs are shown in Fig. 3. Rough and porous structure was obtained, indicating that the presence of recognition sites in the dummy-template MIPs, which could be ascribed to the removal of template molecules. After removing the template and other substances, 200 mg of MIPs particles were packed into a 3.0 mL empty polypropylene cartridge with glasswool frit at each end. The NISPE was prepared with the same procedure.

3.2. Evaluation of the dummy-template MIP as SPE sorbents The MISPE requires the same processes used in a common SPE procedure, including conditioning, sample loading, washing, and eluting. Factors that probably influenced the extraction process, such as loading solvent, washing solvent, eluting solvent, amount of the washing and eluting solvent, should be evaluated to achieve the highest extraction efficiency. In this study, 5.0 mL 100 μg/L RAT

3. Results and discussion 3.1. Preparation of dummy-template MISPE To avoid the undesirable RAT template interferences in the MIPs, salbutamol was used as the dummy-template, but the selectivity factor of salbutamol was significantly higher than that ractopamine [25]. In this work, RTD was used as the dummy-template because the chemical structure of RTD is highly similar to RAT except the position of methyl group and the carbon number of the main chain in the molecule. The MIPs were prepared according to reference [26] with some modification (Fig. 1). The conditions such as the amount of monomer (MAA) and cross linker (EGDMA), the reaction tempera-

Fig. 2. The infrared spectra of the MIPs and NIPs. (a) RTD; (b) MIPs; (c) NIPs.

Fig. 1. The preparation procedure of dummy-template MISPE.

X. Xiao et al. / Talanta 138 (2015) 40–45

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Fig. 3. The SEM images of the MIPs (A)  500; (B)  3000.

Fig. 4. The binding isotherm of MIPs and NIPs for RTD (A), the Scatchard plot of MIPs and NIPs toward RTD (B). The binding isotherm of MIPs and NIPs for RAT (C), the Scatchard plot of MIPs and NIPs toward RAT (D). Chemical structure of the studied target compounds (E) and extraction amount of target compounds with MISPE and NISPE (F).

standard solution was employed to optimize the loading, washing and elution steps of MISPE. First, the loading solutions in different solvent including demonized water, methanol, acetonitrile and their corresponding aqueous

solutions were investigated. The results showed that the recovery of RAT was more than 90% when using acetonitrile as loading solution, suggesting that acetonitrile effectively promotes the rebinding of analytes to the specific sites. Generally, the MIPs exhibit best extraction

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performance and molecular recognition ability when the polarity of extraction solvent is similar to the polymerization solvent. [26] Therefore the synthesized MIPs were more compatible with polar environment, especially applying acetonitrile as loading solution. The washing step was optimized to reduce the matrix interference and maximize the special interactions between RAT and MIP sorbents. The washing solutions, such as methanol, acetonitrile, water, 50% methanol/water (v/v), 50% acetonitrile/water (v/v), 50% methanol/acetonitrile (v/v) were investigated. When 4.0 mL methanol was used, the impurities in samples could be mostly cleaned up and the recovery of RAT reached a maximum of 92.2% in MISPE. Therefore 4.0 mL methanol was selected as the optimal washing solution for the following experiments. In eluting step, different ratios of acetic acid/methanol (10%, 20%, v/v) and acetic acid/acetonitrile (10%, 20%, v/v) were investigated. Acetic acid/methanol was better than acetic acid/acetonitrile according to the recovery of RAT and 20% acetic acid/methanol (v/v) offered the highest recovery. Additionally, 2.0 mL of 20% acetic acid/methanol (v/v) provided the best elution efficiency. Overall, the optimized MISPE procedures included acetonitrile as loading solution, 4.0 mL methanol as washing solution and 2.0 mL 20% acetic acid/methanol (v/v) as the eluting solution. 3.3. Evaluation of the adsorption properties for the dummy-template MIPs 3.3.1. Extraction capability of the MISPE The saturated adsorption capacity of RTD and RAT on MISPE and NISPE were investigated with 100.0 μg/L individual standard solution. The saturated adsorption capacities of MISPE for RAT, RTD were 47.5 and 45.9 μg/g, while that of NISPE were 8.8 and

8.6 μg/g, respectively. There are 5.4 and 5.3 times of MISPE over the NISPE. The results revealed that the saturated adsorption capacity of MISPE was significantly higher than NISPE, suggesting the resultant MISPE showed a higher affinity for RTD and its analogues than NISPE for the specific adsorption. Notably, the saturated adsorption amount of RAT in MISPE was slightly higher than RTD-the dummy-template molecule. Moreover, Scatchard relationship was usually determined to assess the affinity of MIPs and NIPs. Here, we constructed a Scatchard plot using the expression Q/C¼  Q/Kd þ Qmax/Kd, where Q is the adsorption capacity at adsorption equilibrium, C is the initial concentration of RTD, Kd is the distribution coefficient, and Qmax is the saturated adsorption capacity [27]. Both of the adsorption isotherm of MIPs for RTD and RAT (Fig. 4) showed a good linear regression (R2 ¼0.9759 and 0.9761, respectively). The Qmax of MIPs were 86.3 μg/g for RTD and 87.7 for RAT, which was in keeping with that obtained from the saturated adsorption capacity experiments. By contrast, NIPs showed nonlinearity, indicating no selective adsorption sites are present for RTD and RAT. Since the hydrogen bond was formed between the template and monomers and an ordered arrangement was kept by the polymerization of MIPs, those binding sites came from the template could recognize some compounds with similar chemical structure to the template. Although the characterization of FT-IR indicated the MISPE and NISPE had the same chemical composition, their extraction capabilities were significantly different, which would be due to the imprinting effect of the template. 3.3.2. Selectivity of the MISPE The selectivity of the MISPE and the NISPE for RAT, RTD, ISP, CLB and 4-NP were investigated with 100 μg/L of mixed standard solution and the molecular structures of the analytes are shown in

Fig. 5. SERS spectra of different concentrations of RAT (A), ISP (B) and their linear curves. (The concentrations from bottom to top were 0, 20.0, 50.0, 80.0, 100.0, 150.0 and 200.0 μg/L, respectively).

Fig. 6. SERS spectra of the pig tissue samples spiked with different amount of RAT (A) and ISP (B). ((a) 0 μg/kg; (b) 20.0 μg/kg; (c) 50.0 μg/kg.).

X. Xiao et al. / Talanta 138 (2015) 40–45

4. Conclusion

Table 1 Recoveries of RAT and ISP in spiked pork and pig liver samples (n¼ 3). Samples Compounds Spiked level (μg/kg) 50.0

Pork

RAT ISP Pig liver RAT ISP

100.0

45

150.0

Recovery (%)

RSD (%)

Recovery (%)

RSD (%)

Recovery (%)

RSD (%)

73.4 72.8 72.4 71.0

11.6 10.5 13.0 10.3

76.0 74.5 74.1 74.0

9.7 7.9 9.7 9.0

79.7 78.2 78.6 78.0

11.0 7.4 9.4 7.6

Fig. 4E. Fig. 4F illustrates the binding amounts of these targets on the MISPE and the NISPE. The extraction amounts of RAT and RTD in MISPE was 1.51 and 1.17 nmol, which was 8.9 and 7.8 times over that in NISPE, suggesting satisfactory selectivity of the MISPE for RTD and cross-recognition for RAT. Notably, best selectivity for RAT was also obtained than that for the dummy-template of RTD. The reason could be the relatively higher steric hindrance of RTD since its methyl group was closed to the hydroxyl group in the alkyl chain. And then, better recognition for RAT in MISPE was obtained for the molecular interaction [28] and molecular structure difference [29]. On the other hand, the MISPE had moderate selectivity for another analogue of CLB, its extraction amount was 0.69 nmol, which was 4.6 times over that in NISPE. The reference compound of 4-NP, with different structure and chemical properties, was less adsorbed by both of MISPE and NISPE. 3.4. Analysis of ractopamine in pig samples by MISPE coupling with SERS method Since RAT and ISP have similar chemical structures of similar to RTD, the trace RAT and ISP in pig samples was extracted with the MISPE and directly determined by a SERS method. The aromatic C-H out-of plane bending vibration at 840 cm  1 was selected to analyze RAT, and the benzene asymmetric C-OH stretching vibration at 993 cm  1 was selected to analyze ISP. Good linearity was achieved in the range of 20.0–200 μg/L for RAT and ISP and the results are shown in Fig. 5. The LODs were 3.1, 4.3 μg/L for RAT and ISP, respectively. The precision of the method was investigated with 100 μg/L RAT and ISP standard solution, the intra and inter accuracy were varied from 8.1 to 11.5% and 11.3 to 13.1%, respectively. The established SERS method was applied to analysis of RAT and ISP in pork and pig liver samples. No RAT or ISP was found in the samples, and then spiked samples were further determined. Fig. 6 shows the SERS spectra of the pig muscle samples spiked with different concentrations of RAT and ISP, respectively. Owing to the special recognition to the template molecule and its structural analogues, the established method could be applied for selective and sensitive determination of trace RAT and ISP in pig samples. The recoveries of the pork and pig liver samples spiked with 50.0, 100.0 and 150.0 μg/kg standards were 71.0–73.4%, 74.0–76.0% and 78.0– 79.7%, respectively, with the RSD from 7.4 to 13.0% (Table 1). The quantitative detection limit of the proposed MISPE coupling with SERS method were 10.0 μg/kg for RAT and 20.0 μg/kg for ISP, which were lower than the maximum allowed limits of FDA for ractopamine, suggested that this method is applicable to detect trace amounts of β-agonists in practical samples.

In the present work, a selective and sensitive MIP-SERS method was developed for ractopamine in complex samples by combining the selective enrichments of molecular imprinting polymers (MIPs), the higher capacity of solid phase extraction and the sensitive detection of SERS. The quantitative detection limits of 10.0 μg/L was less than the maximum allowed limits of FDA for ractopamine in pig samples. These results suggested that the MISPE and SERS method is sensitive and accurate, it is of great significance to improve the selectivity and sensitivity for detecting trace amounts of β-agonists in complex samples.

Acknowledgments This work was supported by the Major National Scientific Instrument and Equipment Development Project (2011YQ03012409), the Special Funds of the National Natural Science Foundation of China (Grant no. 21127008), the National Natural Science Foundation of China (Grant nos. 91232703 and 21375155) and Specialized Research Fund for the Doctoral Program of Higher Education (20120171110001), respectively.

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Rapid analysis of ractopamine in pig tissues by dummy-template imprinted solid-phase extraction coupling with surface-enhanced Raman spectroscopy.

Ritodrine has similar skeleton structure to ractopamine and it was selected as the dummy-template molecule to synthesize the molecular imprinted polym...
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