CoreâShell Magnetic Molecularly Imprinted ... - ACS Publications

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Core−Shell Magnetic Molecularly Imprinted Polymers as Sorbent for Sulfonylurea Herbicide Residues Shan Shan Miao,†,‡ Mei Sheng Wu,† Hai Gen Zuo,†,‡,§ Chen Jiang,†,‡ She Feng Jin,† Yi Chen Lu,†,‡ and Hong Yang*,† †

Jiangsu Key Laboratory of Pesticide Science, College of Sciences, and ‡Key Laboratory of Monitoring and Management of Crop Diseases and Pest Insects, Ministry of Agriculture Nanjing Agricultural University, Nanjing 210095, China § Jiangxi Entry and Exit Inspection and Quarantine Bureau, Nanchang 330002, China

J. Agric. Food Chem. 2015.63:3634-3645. Downloaded from pubs.acs.org by DURHAM UNIV on 08/07/18. For personal use only.

S Supporting Information *

ABSTRACT: Sulfonylurea herbicides are widely used at lower dosage for controlling broad-leaf weeds and some grasses in cereals and economic crops. It is important to develop a highly efficient and selective pretreatment method for analyzing sulfonylurea herbicide residues in environments and samples from agricultural products based on magnetic molecularly imprinted polymers (MIPs). The MIPs were prepared by a surface molecular imprinting technique especially using the vinyl-modified Fe3O4@SiO2 nanoparticle as the supporting matrix, bensulfuron-methyl (BSM) as the template molecule, methacrylic acid (MAA) as a functional monomer, trimethylolpropane trimethacrylate (TRIM) as a cross-linker, and azodiisobutyronitrile (AIBN) as an initiator. The MIPs show high affinity, recognition specificity, fast mass transfer rate, and efficient adsorption performance toward BSM with the adsorption capacity reaching up to 37.32 mg g−1. Furthermore, the MIPs also showed crossselectivity for herbicides triasulfuron (TS), prosulfuron (PS), and pyrazosulfuron-ethyl (PSE). The MIP solid phase extraction (SPE) column was easier to operate, regenerate, and retrieve compared to those of C18 SPE column. The developed method showed highly selective separation and enrichment of sulfonylurea herbicide residues, which enable its application in the pretreatment of multisulfonylurea herbicide residues. KEYWORDS: vinyl-modified Fe3O4@SiO2, magnetic MIPs, adsorption, sulfonylurea herbicides residues, complex environmental media

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chemical sensors.18,19 Although the molecular imprinting for sulfonylurea recognition has been reported before,20−23 most of the MIPs were prepared by bulk/precipitation polymerization methods, which exhibited some limitations including incomplete template removal, slow mass transfer, small binding capacity, and irregular polymers shape. To address the issues, the surface molecular-imprinting strategy by immobilization of template molecules at the surface of solid substrates has been recently introduced.24 MIPs prepared by this method have small dimensions with an extremely high surface-to-volume index, providing complete removal of the templates, excellent accessibility to target species, and low resistance to mass transfer.25−27 Nanoparticles serve as a promising support for surface imprinting owing to their large external surface area to volume ratio. They improve the physical and chemical properties of sorbents such as uniform spherical geometry as well as stability and facile dispersion.28 Magnetic nanoparticles are widely used in many areas including target drug delivery,29 catalysis,30 and adsorption processes.31 When magnetic nanoparticles are coated with MIPs, the magnetic imprinted microspheres are capable of selectively recognizing the template molecule in complex matrix and are readily isolated from sample solutions by an external magnetic

INTRODUCTION Sulfonylurea is a class of highly selective herbicides used for controlling broad-leaf weeds and some grasses in cereals and economic crops. Because of its widespread usage and high mobility, sulfonylurea is often detected in soil, water, and other ecological systems.1 Although the dosage (10−40 g of active ingredient/ha) of sulfonylurea used in agriculture is much lower as compared to that of other herbicides,2 it can be toxic to plants, even though 1% or less of the originally applied amount was left in fields.3−5 Thus, it is important to develop a highly efficient and sensitive technique for detecting sulfonylurea herbicide residues in environments and samples from agricultural products. Previously, most of the sample pretreatments for sulfonylurea analysis used solid-phase extraction (SPE) with C18 columns for purification and concentration,6,7 but the C18−SPE column is nonrenewable. Furthermore, such a method cannot fully eliminate interferences coextracted with sulfonylurea. The molecular imprinting technique is a recently developed method for producing polymeric macromolecules with specific recognition sites complementary in shape, size, and functional groups to template molecules.8,9 Since the synthesized molecularly imprinted polymers (MIPs) display high mechanical and chemical stability and utilization for most organic solvents,10 MIPs are used as artificial receptors to selectively target molecules from a mixture of chemical species.11−13 In this regard, MIPs can be used in many aspects such as a sorbent in SPE,9,14 new drug delivery systems,14,15 capillary electro chromatography,16 and membrane separation,17 as well as a receptor layer in © 2015 American Chemical Society

Received: Revised: Accepted: Published: 3634

December March 17, March 22, March 23,

25, 2014 2015 2015 2015 DOI: 10.1021/jf506239b J. Agric. Food Chem. 2015, 63, 3634−3645

Article

Journal of Agricultural and Food Chemistry field.32 Recently, a magnetic molecularly imprinted polymer using bare Fe3O4 as supporting matrix was applied to extraction of sulfonylurea herbicides.33 However, the bare Fe3O4 magnetic nanoparticles is easy to agglomerate. If a protective layer is coated on the surface of bare Fe3O4 magnetic nanoparticles, the stability of Fe3O4 magnetic nanoparticles would have been largely improved. To address the question, we developed a new type of core−shell magnetic MIPs with SiO2-coated Fe3O4 magnetic particles as supporting matrix and bensulfuron-methyl as the template molecule. The core−shell magnetic molecularly imprinted polymers have been synthesized as the sorbent materials of SPE and successfully applied to assessing sulfonylurea herbicide residues in food and environmental media. The objective of the study was to provide an efficient, selective, and accurate method for analyzing sulfonylurea herbicide at trace abundance in multiple media.

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double-distilled water and acetone, isolated with a magnet, and dried under vacuum overnight. The citric acid-modified Fe3O4 magnetic core was well protected by a silica film by Stö ber method with a slight modification.35 Fe3O4 magnetic nanoparticles (1 g) were mixed with 100 mL of 2-propanol and 20 mL of double-distilled water and sonicated for 30 min. Four milliliters of tetraethylorthosilicate (TEOS) and 2.5 mL of ammonium hydroxide were added to the mixture drop by drop. The mixture was stirred under a nitrogen atmosphere at room temperature for 20 h. The resultant product (Fe3O4@SiO2) was separated with a magnet and rinsed with ethanol and double-distilled water several times, and dried under vacuum. The Fe3O4@SiO2 particles were allowed to react with MPS to introduce double bonds for polymerization. Briefly, 300 mg of Fe3O4@ SiO2 was added in 90 mL of toluene, sonicated for 10 min, and stirred vigorously. The mixing solution of 5 mL of MPS and 10 mL of toluene was then dropped into the resulting solution for 30 min. The stirring continued under a nitrogen atmosphere at 110 °C for 24 h. The product (vinyl-modified Fe3O4@SiO2) was collected by a magnet, rinsed with methanol several times thoroughly, and dried under vacuum. The synthesis procedure for bensulfuron-methyl (BSM)-imprinted microspheres with a molar ratio of BSM/methacrylic acid (MAA)/ TRIM = 1:4:10 is described as follows. Prior to polymerization, the prearranged solution of bensulfuron-methyl (0.625 mM) and functional monomer (MAA, 2.5 mM) in 30 mL of anhydrous DMF was placed in a refrigerator and kept for 12 h to form the template−monomer complex. Vinyl-modified Fe3O4@SiO2 (0.1 g), TRIM (6.25 mM), AIBN (50 mg), and 30 mL of DMF were mixed by ultrasonication for 15 min. The mixture and template−monomer complex were added into the flask and purged with high purity nitrogen for 10 min. To obtain a high crosslinking density, prepolymerization was undertaken at 50 °C for 6 h and then stirred at 60 °C for 24 h. After polymerization, the polymers (MIP1) were separated and washed with methanol/acetic acid (9:1, v/v) several times in a Soxhlet extraction apparatus until no template molecule (BSM) was detected by HPLC. The polymers were washed with doubledistilled water several times until neutral and dried under vacuum. In parallel, nonimprinted microspheres (NIP1) were prepared in a similar way with no addition of template BSM. Meanwhile, in MIP2 (or NIP2) and MIP3 (or NIP3) preparation, vinyl-modified Fe3O4@SiO2 was substituted by citric acid-modified Fe3O4 and Fe3O4@SiO2, successively (Supporting Information, Table S1). Molecular Simulation. Interactions between template BSM and functional monomer MAA were simulated using the Hyperchem 7.5 software package (Hypercube, Inc., USA). The structures of BSM, MAA, and their complexes were drawn using Chembiodraw Ultra 12.0 software. The three-dimensional structure of BSM and MAA were optimized. The minimum energies of the structures were analyzed iteratively until the convergence value was less than 0.01 kcal mol−1 and calculated through semiempirical quantum methods (PM3). The dimers of BSM and MAA were optimized, and the binding energies of BSM and MAA were calculated at the same level. The binding energy (ΔE) was defined as

MATERIALS AND METHODS

Materials and Chemicals. The pesticides BSM (methy α-(4,6dimethoxypryrimidin-2-yl-carbamoyl sulfamoyl)-O-toluate, 96.1%), triasulfuron (TS, N-(6-methoxy-4-methyl-1,3,5-triazin-2-yl-aminocarbonyl)-2-(2-chloroethoxy)-benzenesulfonamide, 95%), prosulfuron (PS, N-((3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl) amino)carbonyl)2-(3,3,3-trifluoropropyl)benzone sulfonamide, 95%), pyrazosulfuron-ethyl (PSE, ethyl-5-(4,6-dimethoxypyrimidin-2-yl-carbamoylsulfamoyl)-1methylpyrazole-4-ca, 98%), and propazine (PPZ, 6-chloro-n,n′-bis(1-methylethyl)-1,3,5-triazine-2,4-diamine, 97%) were obtained from Syngenta Nantong Crop Protection Co., Ltd. Fe3O4 (200 nm), trimethylolpropane trimethacrylate (TRIM) were purchased from Aladdin Co., Ltd. 3-Methacryloyloxypropyltrimethoxysilane (MPS) was purchased from Shanghai Jiachen Chemical Co., Ltd. HPLC grade acetonitrile was from ROE scientific INC. All other chemicals were of analytical grade provided by Nanjing Chemical Reagent Co., Ltd. Azodiisobutyronitrile (AIBN) was recrystallized by methanol. N,N-Dimethylformamide (DMF) was dried over 3 Å molecular sieves. Instruments and Analytical Methods. The interaction between BSM and functional monomer MAA was studied by UV-2600 spectrophotometer (Shimadzu, Japan) at 190−280 nm. A series of solutions were prepared in acetonitrile in which the molar ratio of BSM and MAA was set at 0:1, 1:0, 1:1, 1:2, 1:4, 1:8, 1:10, 1:20, 1:30, 1:40, 1:50, and 1:60. The background UV absorbance of only BSM or MAA was taken out by the absorbance of solvent acetonitrile, and the background UV absorbance of other solutions was taken out by the absorbance of corresponding MAA. A TENSOR-27 FT-IR spectrometer (Bruker, Germany) with a 2 cm−1 resolution and a spectral range of 400−4000 cm−1 was employed to examine the FT-IR spectra of nanopaticle samples by a pressed tablet (sample: KBr = 1:100, w/w). The size and morphology of nanoparticle samples were analyzed using an S-4800 field emission scanning electron microscope (SEM, Hitachi High-Technologies Corporation, Japan) and a Tecnai 12 transmission electron microscope (TEM, Philips, Holland). Phase identification was done using X-ray polycrystal diffraction (XRD, Bruker-AXS Corporation, Germany) patterns, using a D8 Advance X-ray diffractometer with Cu Kα1 irradiation at γ = 0.15418 nm. The magnetic properties were studied using an EV7 vibrating sample magnetometer (VSM, ADE, US) operating at room temperature. All chromatographic measurements were performed using a HPLC system (Waters 2489, Waters Technologies Co. Ltd.), equipped with a 515 pump and a UV−vis detector. A C18 column (250 mm × 4.6 mm i.d., 5 μm) was used, with the mobile phase of acetonitrile−water (acidified with glacial acetic acid to pH 3.0) (60:40, v/v), detection at 235 nm, and a flow rate of 0.3 mL min−1. The injection volume was 20 μL. Preparation of Magnetic and Microsphere MIPs. Prior to use, the bare Fe3O4 magnetic nanoparticles were modified with negatively charged citrate groups.34 Fe3O4 magnetic nanoparticles (2 g) were mixed in 150 mL (20 mM) of trisodium citrate and sonicated for 0.5 h, followed by magnetic stirring at 30 °C for 24 h. The obtained citric acidmodified Fe3O4 magnetic nanoparticles were washed several times with

ΔE = Ecomplex − (Etemplate + Emonomer ) Measurement of Adsorption Isotherm and Kinetic Adsorption Curve. Static equilibrium adsorption experiments were conducted: 10 mg of MIP1 or NIP1 was added into the centrifuge tubes containing 5 mL of methanol/water (3/7, v/v) solution with BSM concentration varying from 0.02 to 0.40 mM and shaken for 24 h at room temperature. The supernatant and polymer were separated by a permanent magnet. The BSM concentration in the supernatant was measured by HPLC. MIP1 or NIP1 (10 mg) was mixed with 5 mL of methanol/water (3/7, v/v) solution with BSM at 0.2 mM and shaken at regular time intervals. The supernatant and polymer were separated by a permanent magnet. Concentrations of BSM in the supernatant were determined by HPLC. According to the BSM concentration before and after adsorption, the equilibrium amount of substrate bound to the polymer (Q, mg g−1) was calculated by following eq 1: 3635

DOI: 10.1021/jf506239b J. Agric. Food Chem. 2015, 63, 3634−3645

Article

Journal of Agricultural and Food Chemistry Q=

MV (C0 − C1) m

MIP1 was used as sorbents to prepare the SPE cartridge. MIP1 particles (100 mg) were packed in an empty 3 mL solid phase extraction (SPE) cartridge. The SPE cartridge was capped with PTFE frits at the top and bottom, respectively. The MIP-SPE cartridge was washed with 10 mL of methanol−acetic acid (9:1, v/v) to remove residue and preconditioned with 5 mL of methanol and 5 mL of ultrapure water. Standard solution or sample solution was loaded onto the preconditioned MIP-SPE cartridge at a rate of 0.5 mL min−1. After sample loading, the MIP-SPE cartridge was washed with 2 mL of methanol/water (3/7, v/v). The elution step was performed using 2 mL of methanol. The eluate was collected and filtered through a 0.45 μm filter before HPLC analysis. For comparison, C18 SPE (Supelco, 3 mL, 500 mg) was used with a similar protocol to that of the MIP-SPE columns. Determination of Environmental Samples. Rice water (paddy soil water), soil, and rice grain were collected for analysis. Water sample was directly added into the centrifuge tube containing magnetic MIP or loaded onto the preconditioned MIP-SPE column. The soil was collected from a rice field, and the plant debris was removed from the soil. The soil was dried under natural conditions and passed through a 2 mm sieve. The soil sample was extracted ultrasonically with 20 mL of acetone−water (3:1, v/v) for 30 min and centrifuged. The extraction procedure was performed in triplicate. The supernatant was concentrated to remove acetone at 40 °C by a rotary evaporator. The residual water was loaded onto the preconditioned MIP-SPE column. The rice was ground. The rice grains were rinded in a grinder (FW177, Taisite Instrument Co. Ltd., China) and passed through a 0.38 mm sieve. The rice grain was soaked and ultrasonicated with dichloromethane three times, each time with 20 mL for 30 min. The extracting solution was centrifuged, and the supernatant was combined and concentrated by a rotary evaporator at 40 °C. The residue was redissolved in 1 mL of methanol and then diluted with 20 mL of water. The diluting solution was loaded onto the preconditioned MIP-SPE column. The MIP-SPE column was washed. The eluent was collected and analyzed using the methods indicated above. Each sample was repeated in triplicate. To evaluate accuracy of the method, the spiked recoveries of samples were investigated. Test samples of soils (10.0 g) and ground rice grains (10.0 g) were prepared using an appropriate volume of standard mixture solution (each pesticide 1.0 mg·L−1) to obtain the final concentrations of 0.1, 0.25, and 0.5 mg kg−1, respectively. The spiked samples were incubated and left to stand for about an hour. The water samples were spiked with each herbicide at the concentrations of 0.05, 0.25, and 0.5 mg L−1. Extraction, purification, and concentration were performed in the same way indicated above. Statistic Analysis. All of the experiments were performed at least three repetitive independent treatments. The values were expressed as the means ± standard deviation. ANOVA was conducted using the mixed model procedure in SPSS statistics 20. The significance of the differences among the means was calculated by Turkey’s test. Statistical significance was set at p < 0.05.

(1)

where C0 and C1 represent the initial solution and final solution concentration (mM) of BSM, respectively; V is the solution volume (mL); m (mg) is the weight of the polymer particles; and M (g mol−1) is the molar mass of the template. All average results from three independent tests were used for the following discussion. In order to further study the binding properties of magnetic MIP1, experimental data were fitted to the Langmuir isotherms (eq 2), Freundlich isotherms (eq 3), and Langmuir−Freundlich isotherms (eq 4).

Ce C 1 = e + Qe Qm Q mKL

(2)

log Q e = (1/n)log Ce + log KF

(3)

m

B=

Nt aF 1 + aF m −1

(4) −1

where Qe (mg g ) and Ce (mg L ) are the amount adsorbed on magnetic MIP1 and concentration of BSM in the solution at equilibrium, respectively; Qm (mg g−1) is the theoretical maximum adsorption capacity; KL (L mg−1) is the Langmuir constant related to the affinity of adsorption sites; KF (L mg−1) and n are determined from a linear plot of log Qe versus log Ce, demonstrating adsorption capacity and intensity, respectively; Nt is the total number of binding sites; a is related to the binding affinity (K0) via K0 = a1/m, and m is the heterogeneity index that varies from 0 to 1 (m = 1 means the media is homogeneous). Selectivity Experiment. To evaluate the specific recognition ability of MIP1 for BSM, 10 mg of MIP1 (or NIP1) was added into 10 mL centrifuge tubes containing 5 mL of methanol and water (3/7, v/v) solution with 0.2 mM of BSM, TS, PS, PSE, or PPZ. Meanwhile, 10 mg of MIP1 (or NIP1) was added into the mixed solution of these five pesticides with initial concentrations of 0.2 mM. Each mixture with MIP1 (or NIP1) was shaken at room temperature for 24 h. The supernatant was separated with a permanent magnet. Concentrations of BSM, TS, PS, PSE, and PPZ were determined by HPLC. The interrelated absorbed coefficient was evaluated by the following equation: Static distribution coefficient Kd =

Cp Cs

(5)

where Cp is the concentration on the absorbed medium, and Cs is the final free concentration of the solution. The selectivity coefficient of MIP1 to BSM with respect to the competitor species can be obtained according to eq 6.

Selectivity coefficient k =

Kd(SUs) Kd(PPZ)

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(6)

RESULTS AND DISCUSSION Preparation of Imprinted Magnetic Microspheres. The schematic representation of MIP preparation is illustrated in Figure 1. The synthetic approach included the following procedures: (1) obtaining the shell−core structure of vinyl-modified Fe3O4@SiO2; (2) self-assembly of the template and functional monomers; (3) polymerization in the presence of cross-linker, initiator and porogen; and (4) achieving recognition cavities in the polymer matrix by eluting template BSM. To obtain highly stable and well-dispersed magnetic nanoparticles, silica was selected to encapsulate Fe3O4. Silica has a small dimension with extremely high surface-to-volume ratio.25 It can screen the magnetic dipolar attraction between the magnetic Fe3O4 molecules in liquid media. Also, most of the template molecules are situated at the surface or are in the proximity of surface. Furthermore, since the silanol group stands on the Fe3O4 surface coated by silica, the modified Fe3O4 could increase

where Kd (SUs) stands for static distribution coefficient of the template or the structural analogues; and Kd (PPZ) is the static distribution coefficient of pesticide PPZ. The value of the relative selectivity coefficient k′ defined in eq 7 indicates the enhanced extent of adsorption affinity and selectivity of MIP1 to the template BSM. k′ =

kM I P kN I P

(7)

where kMIP and kNIP are the selectivity coefficients of MIP1 and NIP1, respectively. Separation Enrichment of Sulfonylurea Herbicides. MIP1 was used directly as sorbents for sulfonylurea herbicides. Water samples (20 mL) and 100 mg of magnetic MIP1 particles were mixed in a centrifuge tube. After being shaken at room temperature for 3 h, MIP1 was removed by a permanent magnet, first washed with water, and then washed with 4 mL of methanol. The eluent solution was collected and filtered through a 0.45 μm filter before HPLC analysis. 3636


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Journal of Agricultural and Food Chemistry

Figure 1. Schematic representation of the possible process of BSM imprinted polymers.

the noncovalent MIP.36 When BSM concentration is kept constant and the ratio of BSM/MAA increased from 1:0 to 1:60 (BSM/MAA), the maximum absorption wavelengths of BSM presented obviously red shifts, and the maximum absorbance intensity of BSM also largely decreased with the increasing molar ratio of MAA (Figure 2). The result indicated that hydrogen bonding existed between BSM and MAA, and affected the π−π* absorption band of BSM. To confirm the hydrogen bonding between BSM and MAA, we employed a molecular simulation technique that is widely adopted to study molecular interactions between the template and functional monomer at the molecular level.15,37−39 Simulated results showed that the binding energies (ΔE) of BSM with MAA were −11.14 and −17.03 kcal mol−1, respectively, when BSM interacted with MAA at mole ratios of 1:1 and 1:2, indicating that the binding energy was lowest with −17.03 kcal mol−1 and that the complex with a mole ratio of 1:2 was most stable. Stable hydrogen bonding interactions between BSM and MAA were observed involving two intermolecular hydrogen bonds (at each O in the −SO2-group) and an intramolecular hydrogen bond at O27 and H38 (Supporting Information, Figure S1). Although molecular simulation showed the theoretic binding sites and ratio between BSM and MAA, the

biocompatibility and bioconjugation with various groups. The silanol group on the Fe3O4@SiO2 surface further reacted with MPS to introduce vinyl groups, which subsequently reacted with the functional monomer MAA. The copolymerization of terminal CC double bonds of MPS with MAA can directly produce the network structures and form MIP nanospheres. To investigate the modification of Fe3O4, a series of molecularly imprinted and nonimprinted polymers were prepared (Supporting Information, Table S1). All BSM-imprinted polymers (MIP) had much greater adsorption capacity for the template than the corresponding referenced polymers (NIP) (Supporting Information, Table S1). Among the three MIPs, MIP1 showed the greatest binding capacity for BSM, about twice as much as MIP2 and MIP3, suggesting that more recognition sites existed in MIP1 than in MIP2 and MIP3. MIP3 showed the lowest binding capacity for BSM among three MIPs. MIP1 was therefore selected for further investigation. Interaction between BSM and MAA in Solution. In order to acquire the high adsorption capacity of MIP, interactions between the functional monomer and template were investigated. Since hydrogen-bonding can provide precise molecular recognition depending on both distance and direction between the monomer and template, it is commonly used for assembling 3637


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Fe3O4@SiO2 with silica coating was extended to about 620 nm, corresponding to a 420 nm thickness of SiO2 layers coated on the Fe3O4 particles (Figure 3B). The silica shell of Fe3O4@SiO2 enhanced the biocompatibility and stability of Fe3O4 magnetic particles and prevented oxidation and aggregation of Fe3O4 magnetic particles. However, Fe3O4@SiO2 provided the sites to react with MPS, which could offer vinyl end groups favorable for further grafting.41 Furthermore, the uniform core−shell MIP was successfully constructed, and the MIP1 had a polymer layer with about 20 nm thickness (Figure 3C), revealing that the binding sites almost existed at the surface of Fe3O4@SiO2-MIP. The MIP1 spheres appear more irregular, presumably due to the presence of the imprint. In contrast, NIP1 spheres appear smoother, showing more uniform appearance (Figure 3E). However, the MIP1 had a three-dimensional structure and appeared more porous and looser in comparison with that of NIP1. The result demonstrated the fixing of template BSM and functional monomer MAA on the Fe3O4@SiO2 surface in the presence of the cross-linker, porogen, and initiator in the polymerization system (Figure 3D). This core−shell structure of MIP1 should be effective in mass transport for rebinding and releasing the template. In FT-IR spectra, the peak at 580.54 cm−1 was attributed to the stretching vibrations of Fe−O (Figure 4A). In comparison with the infrared data of Fe3O4, the peaks at 804.41, 948.85, and 1089.67 cm−1 correspond to the stretching vibrations of Si−O, Si−O−H, and Si−O−Si, respectively (Figure 4B−D), pointing to the formation of silica coating on the surface of Fe3O4. The vibration at 1696.29 cm−1 in vinyl-modified Fe3O4@SiO2 was the peak of carbonylic groups, indicating the successful modification using MPS on the Fe3O4@SiO2 surface (Figure 4B). Meanwhile, the strong peaks of the CO group at 1736.01 cm−1 and C−H

Figure 2. UV spectra of BSM with different molar ratios of MAA in acetonitrile. Concentration of BSM, 0.05 mmol L−1; concentration of MAA for lines 1−11, 0, 0.05, 0.1, 0.2, 0.4, 0.5, 1, 1.5, 2, 2.5, and 3 mmol L−1 respectively. Corresponding pure MAA solutions as blank.

experimental approach affected the choice of the most appropriate ratio between them. Since the association between the monomer and template is governed by an equilibrium in the noncovalent approach, the functional monomers normally have to be added in excess relative to the number of moles of the template to favor the formation of the template−monomer complex. Therefore, the working molar ratio of the template to functional monomer was typically set with 1:4.40 Characterization of Core−Shell Magnetic Molecularly Imprinted Polymers. The TEM images showed that the size of Fe3O4 magnetic particles was around 200 nm with an irregular shape (Figure 3A). The diameter of the uniform spherical

Figure 3. TEM images Fe3O4 (A), Fe3O4@SiO2 (B), and MIP1 (C) with scale bars of 0.2 μm. SEM images of MIP1 (D) and NIP1 (E) with scale bars of 500 nm. 3638


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paramagnetism at room temperature. We were sure that all of the nanoparticle samples with good superparamagnetism can be redispersed rapidly as soon as the external magnetic field was removed. Furthermore, the saturation magnetization of Fe3O4@SiO2 (54.3 emu g−1) and vinyl-modified Fe3O4@SiO2 (33.4 emu g−1) were lower than that of Fe3O4 (119.5 emu g−1) (Figure 5) because the small particle surface was inactive and the magnetic layer was not collinear with the magnetic field.43 The saturation magnetization of MIP1 with an additional polymer layer was significantly decreased to 13.4 emu g−1. However, the MIP1 remained strongly magnetic and could be isolated using an external magnet in a few seconds. Adsorption Isotherms. The adsorption isotherm experiments were conducted with different initial concentrations of BSM ranging from 0.02 to 0.4 mM. The adsorbability of MIP1 continuously increased with the increment of BSM ranging from 0.02 to 0.35 mM and reached equilibrium at 0.35 mM (Figure 6A). The adsorption capacity of MIP1 (37.32 mg g−1) was about 2.02-fold over that of NIP1 (18.45 mg g−1) at 0.4 mM of BSM. The result confirmed that the arrangement of MAA in MIP1 was inherently different from that of NIP1. The corresponding parameters fitting Langmuir, Freundlich, and Langmuir−Freundlich isotherm models to the experimental data were obtained (Figure 6B−D). The values of Qm and KL were 43.48 mg g−1 and 0.0083 L mg−1, respectively; the values of n and KF were 1.247 and 1.0007 L mg−1, respectively; and the values of Nt and m were 98.67 mg g−1 and 1, respectively. K0 is equal to a (3.517M−1). Although the Freundlich model has been shown to be generally applicable to most noncovalently imprinted polymers,28,44 it did not fit the experimental data well. The Langmuir isothermal equation with an R2 value of 0.9267 seemed more desirable to describe the adsorption of BSM on magnetic MIP1 than the Freundlich isothermal equation (R2 = 0.7532) and the Langmuir−Freundlich equation (R2 = 0.7527). However, the isotherms (MIP1) showed 3 distinct regions: region 1, a low slope linear relationship at low concentration demonstrating weak adsorption; region 2, a higher slope linear relationship at intermediate concentrations; and region 3, a low slope linear relationship at high concentration, suggesting that sites were nearly saturated (Figure 6A). Adsorption Kinetics. The adsorption kinetics of magnetic MIP1 was investigated at 0.2 mM BSM. An overall 5-fold increase was observed in adsorption capacity with magnetic MIP1 versus NIP1 (Figure 7). For MIP1 there was a fast rising adsorption during the first 60 min before the adsorption equilibrium was reached. In the first 60 min, the magnetic MIP1 attained up to 86% of equilibrium absorption amounts of BSM. These results indicated that most of the template was sorbed to the imprinted sites at a fast rate, suggesting that the binding sites of the magnetic MIP1 were on or near the surface. In contrast, the magnetic NIP1 showed a low affinity but similar equilibration time to that of MIP1 as there was a clear increase in amount sorbed from time 0 to 60 min. Magnetic MIP Binding Specificity. In order to investigate the selectivity of magnetic MIP1 microspheres for sulfonylurea herbicides, binding experiments were conducted by choosing triasulfuron (TS), prosulfuron (PS), and pyrazosulfuron-ethyl (PSE) as the structural analogues of BSM template and propazine (PPZ) as a representative of different classes of compounds of dissimilar structure. NIP1 was used for comparison. The binding amounts of BSM for magnetic MIP1 were higher than those of the other three sulfonylurea herbicides and PPZ,

Figure 4. FT-IR spectra of Fe3O4 (A), vinyl-modified Fe3O4@SiO2 (B), MIP1 (C), and NIP1 (D).

group of methyl at 2970.91 cm−1 (Figure 4C and D) indicated the successful grafting of a polymer layer on the vinyl-modified Fe3O4@SiO2. In addition, MIP1 had the same characteristic peaks with NIP1, showing the complete removal of the template from MIP1. XRD is a powerful tool for crystal structure characterization. The XRD patterns for the magnetic particles with and without coating were displayed in Supporting Information, Figure S2. These samples with specific diffraction peaks were highly crystalline materials, and the four particles contain Fe3O4 with a spinel structure.42 The peak positions of the four samples remained unchanged, indicating that the step-by-step procedure of coating and polymerization made no crystalline structure change of Fe3O4. The intensities of the characteristic peaks in MIP1 slightly decreased owing to the encapsulation by the polymer layer. VSM was applied to the characterization of the magnetic properties of the synthesized particles. The remnant magnetization (Mr) and coercivity (Hc) can be determined from the VSM data (Figure 5). The values of Mr and Hc were 5.44 emu g−1

Figure 5. Magnetization curves at 298 K of Fe3O4 (a), Fe3O4@SiO2 (b), vinyl-modified Fe3O4@SiO2 (c), and MIP1 (d).

and 34.25 Oe for Fe3O4, 1.17 emu g−1 and 27.68 Oe for Fe3O4@ SiO2, 1.10 emu g−1 and 41.77 Oe for vinyl-modified Fe3O4@SiO2 and 0.57 emu g−1 and 12.92 Oe for MIP1, respectively. The very weak hysteresis indicated that four samples had good super3639


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Figure 6. Adsorption isotherms (A) of BSM on MIP1 and NIP1, Langmuir plot (B), Freundlich plot (C), and Langmuir−Freundlich plot (D) to estimate the binding nature of MIP1. Experimental conditions: 0.02−0.4 mmol L−1 BSM with 10.0 mg of the polymers for 24 h, and the binding medium was 5 mL of methanol−water (30:70, v/v). The measurements were repeated three times.

suggesting that the template molecule BSM had a relatively higher affinity for the imprinted polymer than others (Figure 8). It was possible that the specific sites existing in magnetic MIP1 were complementary in shape, size, and spatial distribution to template BSM. The template fitted the cavities well in the imprinted polymer.45 Moreover, the absorption amounts of magnetic MIP1 toward the five pesticides were lower in the mixture (Figure 8B) than in individual solutions (Figure 8A). The phenomenon demonstrated that the binding sites were fixed in magnetic MIP1 and that the five compounds competed with each other in the mixed solution, leading to the reduced adsorption on magnetic MIP1. There were interactions of hydrogen bonding between the hydroxyl group in MAA and sulfonyl group in BSM (Supporting Information, Figure S1). Three analogous pesticides (TS, PS, and PSE) also have the group (−SO2−) that is the same in BSM (Supporting Information, Figure S3). Therefore, the binding capacities of the analogues were much higher than that of PPZ, indicating that magnetic MIP1 also possessed high selectivity to the structural analogues of BSM. This was supported by the fact that the relative binding affinity was a function of compound structure (Supporting Information, Figure S1). PSE is most similar to BSM in molecular dimensions and

Figure 7. Adsorption kinetic curves of BSM on MIP1 and NIP1. Experimental conditions: 0.2 mmol L−1 BSM in the binding medium of 5 mL of methanol−water (3:7, v/v) with 10 mg of MIP1 or NIP1 for a certain time period. The measurements were repeated three times. 3640


Article


Figure 8. Adsorption selectivity of BSM onto MIP1 and NIP1 in single solution (A) and mixed solution (B). Ten milligrams of the polymers was suspended in 5 mL of methanol−water (3:7, v/v), in which the concentration of BSM, TS, PS, PSE, and PPZ was 0.2 mmol L−1 (A). Ten milligram polymers were suspended into 5 mL of mixed solution containing a mixture of BSM, TS, PS, PSE, and PPZ with a concentration of 0.2 mmol L−1 for each compound (B). The measurements were repeated three times. BSM, bensulfuron-methyl; TS, triasulfuron; PS, prosulfuron; PSE, pyrazosulfuron-ethyl; PPZ, propazine.

Table 1. Specific Recognition Properties of Magnetic MIP1 and NIP1 MIP1

NIP1

analytes

Cs (mM)

Kd

k

Cs (mM)

Kd

k

k′

BSM TS PS PSE PPZ

0.127 0.174 0.157 0.151 0.195

0.573 0.148 0.278 0.327 0.026

22.038 5.692 10.692 12.577 1

0.190 0.196 0.186 0.185 0.195

0.053 0.019 0.077 0.083 0.024

2.208 0.792 3.208 3.458 1

9.981 7.187 3.333 3.637 1

was no obvious difference of the NIP1 binding capacity for five herbicides, which likely depended on the same mechanism of nonspecific absorption. To further assess the molecular recognition property for MIP1, the static distribution coefficient (Kd), selectivity coefficient (k), and relative selectivity coefficient (k′) were calculated based on the data in Figure 8B by eqs 5−7. The k values of magnetic MIP1 showed a significant increase compared to the values of NIP1 due to the imprinting effect (Table 1). The k′ for PPZ was much lower than that for TS, PS, and PSE, indicating that the magnetic MIP1 had a high binding affinity for these

functional groups, and it had the highest binding affinity among the competing herbicides tested. The binding of PSE on magnetic MIP1 was greater than those of TS and PS. Compared with BSM, the mismatch of the structure and size with specific cavities might hinder TS entering the imprinted cavities and lead to the lowest binding capacity of TS among the four analogues. Since the three F atoms in PS bind to hydroxyl hydrogen in MAA, the binding affinity of magnetic MIP1 to PS was larger than that of TS. In addition, PPZ, with the least structural similarity to BSM, showed the lowest binding capacity (3 μmol g−1) (Figure 8B). In contrast, there 3641


Article

Journal of Agricultural and Food Chemistry Table 2. Results of the Determination and Recoveries of Water Spiked with Four Sulfonylurea Herbicides (n = 3) MIP as sorbents compound bensulfuronmethyl

triasulfuron

prosulfuron

pyrazosulfuronethyl

a

spiked (μmol L−1)

observed (μmol L−1)

0 0.05 0.25 0.5 0 0.05 0.25 0.5 0 0.05 0.25 0.5 0 0.05 0.25 0.5

nda 0.044 0.215 0.456 nda 0.038 0.169 0.413 nda 0.036 0.178 0.417 nda 0.040 0.195 0.423

MIP-SPE cartridge

recovery (%)

RSD (%)

87.81 85.95 91.15

3.20 4.14 1.13

76.83 67.46 82.53

2.81 2.48 1.49

72.47 71.34 83.39

1.76 1.59 2.01

80.55 77.96 84.66

2.47 7.22 1.18

observed (μmol L−1) nda 0.051 0.229 0.438 nda 0.050 0.209 0.404 nda 0.042 0.224 0.417 nda 0.046 0.206 0.391

C18 cartridge

recovery (%)

RSD (%)

102.00 91.38 87.62

0.83 3.40 2.50

100.80 83.42 80.75

2.70 2.09 1.69

83.73 89.45 83.34

1.40 4.53 7.60

92.06 82.18 78.14

0.22 8.74 3.52

observed (μmol L−1)

recovery (%)

RSD (%)

76.20 65.92 65.22

8.18 7.06 7.04

67.15 63.08 75.93

7.48 5.35 2.79

59.90 59.88 60.13

3.86 8.09 7.68

77.49 78.94 73.52

6.51 4.14 6.70

nda 0.038 0.165 0.326 nda 0.034 0.159 0.380 nda 0.030 0.150 0.301 nda 0.039 0.197 0.368

nd: not detected.

Table 3. Results of the Determination and Recoveries of Rice Soil and Grain Spiked with Four Sulfonylurea Herbicides (n = 3) MIP-SPE cartridge sample soil

compound bensulfuron-methyl

triasulfuron

prosulfuron

pyrazosulfuron-ethyl

grain

bensulfuron-methyl

triasulfuron

prosulfuron

pyrazosulfuron-ethyl

a

−1

−1

spiked (μmol kg )

observed (μmol kg )

0 0.1 0.25 0.5 0 0.1 0.25 0.5 0 0.1 0.25 0.5 0 0.1 0.25 0.5 0 0.1 0.25 0.5 0 0.1 0.25 0.5 0 0.1 0.25 0.5 0 0.1 0.25 0.5

a

recovery (%)

C18 cartridge RSD (%)

observed (μmol kg−1)

recovery (%)

RSDc(%)

73.31 72.95 70.93

4.77 1.37 2.09

46.76 42.31 33.88

3.22 3.25 6.00

47.74 65.95 32.79

3.23 8.71 6.55

76.54 73.99 51.06

0.74 3.17 7.92

62.72 65.70 71.86

6.41 4.01 4.70

61.64 60.28 71.67

8.56 5.48 7.37

51.83 32.12 49.38

4.89 7.45 5.35

a

nd 0.090 0.240 0.473 nda 0.052 0.128 0.330 nda 0.084 0.222 0.409 nda 0.084 0.202 0.429 nda 0.086 0.229 0.434 nda NDQb 0.127 0.266 nda 0.073 0.204 0.407 nda 0.075 0.206 0.428

89.63 95.78 94.67

3.41 2.24 3.71

52.26 51.18 65.99

2.44 2.91 7.54

84.17 88.65 81.77

2.57 2.34 0.81

83.63 80.77 85.72

2.95 4.55 2.52

85.67 91.50 86.72

5.15 5.12 3.53

50.81 53.16

5.77 0.93

73.21 81.51 81.42

7.08 6.61 1.61

75.24 82.20 85.50

4.28 1.79 6.19

nd 0.073 0.182 0.355 nda 0.047 0.106 0.169 nda 0.048 0.165 0.164 nda 0.077 0.185 0.255 nda 0.063 0.164 0.359 nda NDQb NDQb NDQb nda 0.062 0.151 0.358 nda 0.052 0.080 0.247

nd: not detected. bNDQ could be detected but could not be determined quantitatively. cRSD relative standard deviation.

analogues but not for the different class compound of dissimilar structure (PPZ). Therefore, the magnetic MIP1 can be used as

group-recognition materials because of the similar adsorption properties for these structural analogues. 3642


Article

Journal of Agricultural and Food Chemistry Methodological Validation and Application to Real Samples. The magnetic MIP-SPE cartridges were applied for purifying and concentrating the selected sulfonylurea herbicides in real samples. The washing step for the MIP-SPE is the key parameter for the method of sample pretreatment.46 To obtain maximum selectivity and recovery of sulfonylurea herbicides in the washing step, water containing different percentages of methanol was tested. Two milliliters of methanol/water (45/55 or 35/65, v/v) led to lower recoveries among all ratios of methanol and water (Supporting Information, Figure S4). The recovery of BSM showed no significant difference at 30:70, 25:75, and 15:85 (MeOH/H2O). For the recovery of PS, the use of 30:70 (MeOH/H2O) showed no significant difference compared to 25:75, as well as 25:75 compared to 15:85. The recoveries of TS and PSE using 30:70 (MeOH/H2O) were lower than 25:75 and 15:85 but were also reached or greater than 80%. As more interference in paddy soil and grain occurred, an appropriately high ratio of methanol in washing solution could provide high elution strength. Thus, 2 mL of methanol/water (30/70) was chosen as the washing solution in the experiment in order to eliminate the impurities and keep the recoveries of the four sulfonylurea herbicides. Under the optimized conditions of magnetic MIP-SPE coupled with HPLC, a good linearity was achieved for standards in the range of 0.04−0.8 μM for BSM, TS, PS, and PSE with correlation coefficients of 0.9996, 0.9954, 0.9936, and 0.9941, respectively (Supporting Information, Table S2). Their LODs were in the range of 6.4−9.5 nM. The created methodology was applied to the rice water, paddy soil, and grain samples. Initial analyses confirmed that the three samples were free of sulfonylurea herbicides. The detecting signals of four sulfonylurea herbicides with MIP-SPE procedure (Supporting Information, Figure S5-Ac, Bc, and Cc) were stronger than that without the MIP-SPE procedure (Supporting Information, Figure S5-Ab, Bb, and Cb) in the water, soil, and grain samples. The analysis of blank samples showed that most of the interference substances were washed during the methanol/ water (3/7, v/v) cleaning up process (Supporting Information, Figure S5-Aa, Ba, and Ca). Although there were some impurities after the soil sample was treated by magnetic MIP-SPE, they had the least impact on the quantitative determination of sulfonylurea herbicides (Supporting Information, Figure S5B). The accuracy of the method was evaluated by the recovery test conducted with spiked samples. The relative recoveries of BSM, PS, and PSE with magnetic MIP-SPE as columns for the spiked rice water, soil, and grain were 78.14−102.00%, 80.77−95.78%, and 73.21−91.50%, respectively, and the relative standard deviations (RSDs) were 0.22−8.74%, 0.81−4.55%, and 1.61− 7.08%, respectively (Tables 2 and 3). The recoveries of four herbicides for rice water using magnetic MIP as sorbents were slightly lower than those with the MIP-SPE column. The recoveries of TS in the three spiked samples were 50.81− 100.80%, with RSDs ranging from 0.93% to 7.54% (Tables 2 and 3). The C18 column only had the nonspecific adsorption, resulting in much lower recoveries of sulfonylurea herbicides in the spiked samples. The results demonstrated that the magnetic MIP-SPE coupled with HPLC was an efficient and effective methodology for selective extraction and sensitive assessment of multiple sulfonylurea herbicides at a very low concentration. Reusing MIPs. To verify the recyclability of MIP-SPE, regeneration (adsorption/desorption) cycles were conducted with four sulfonylurea herbicides. After MIP-SPE columns were used to analyzed samples in food or environmental media, the columns could be regenerated by washing with 5 mL of acetic

acid, 5 mL of methanol, and 5 mL of water. The result showed that MIP-SPE columns could be used for six cycles at least, and the average recovery had only a loss of less than 5.5%, indicating that the selectivity of the MIP-SPE was specific and stable for four sulfonylurea herbicides in food and environmental media.

■

ASSOCIATED CONTENT

S Supporting Information *

Preparation composition of different polymers; linear range, method limit of detection (LOD) of MIP-SPE coupled with HPLC for four sulfonylurea herbicides; molecular simulation of interactions between BSM and MAA; XRD patterns at 298K of Fe3O4 (a), Fe3O4@SiO2 (b), vinyl-modified Fe3O4@SiO2 (c), and MIP1 (d); structures of four sulfonylurea herbicides (BSM, PSE, PS, and TS) and PPZ; recoveries of BSM, TS, PS, and PSE from the washing step using different ratios of methanol/ water (v/v); and chromatograms of spiked sulfonylurea herbicides. This material is available free of charge via the Internet at http:// pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Weigang No.1, Chemistry Building, College of Sciences, Nanjing Agricultural University, Nanjing 210095, China. Tel: +86-25-84395207. E-mail: [email protected]. Funding

We acknowledge the Special Fund for Agro-scientific Research in the Public Interest (No. 201203022) from the Ministry of Agriculture of China. This work was supported by funding from Jiangsu Innovation Program for Graduate Education (KYLX_0580). Notes

The authors declare no competing financial interest.

■

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