Article pubs.acs.org/IECR
Versatile Method To Obtain Homogeneous Imprinted Polymer Thin Film at Surface of Superparamagnetic Nanoparticles for Tetracycline Binding Jiangdong Dai,† Zhiping Zhou,† Chunyan Zhao,§ Xiao Wei,† Xiaohui Dai,‡ Lin Gao,‡ Zhijing Cao,‡ and Yongsheng Yan*,‡ †
School of Material Science and Engineering, Jiangsu University, Zhenjiang 212013, China School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China § School of The Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China ‡
ABSTRACT: We present a simple and versatile approach for the preparation of homogeneous imprinted polymer thin film based onto superparamagnetic nanoparticles to form the well-defined core−shell structure involving in surface modification and the subsequent in situ precipitation polymerization. The as-synthesized core−shell magnetic molecularly imprinted nanoadsorbents (MMINs) were systematically characterized, and binding equilibrium, kinetics, and selectivity property were evaluated by binding experiments. The binding amount of tetracycline (TC) increased with the increase in temperature and Langmuir isotherm described the data well, with the maximum binding capacity of 52.08 μmol g−1 at 318 K. The kinetics rapidly achieved the equilibrium within 20 min, benefiting from imprinted polymer thin film (25 nm), and the data was well-described by the pseudo-second-order rate equation. MMINs showed the highest selective recognition for TC. In addition, enhanced superparamagnetic separability (56.16 emu g−1) and reusability of MMINs provided the potential applications for environmental remediation, biological molecule purification, and drug delivery. Guan et al.17 Surface imprinted core−shell nanomaterials are expected to improve the binding capacity, binding kinetics, and site accessibility to the target molecules. Moreover, because the internal core is independent of the outer layer, multiple functionalities (magnetic or fluorescent) can be feasibly combined into MIPs for environmental, biomedical, and biotechnological applications.19−23 Magnetic nanoparticles (e.g., Fe3O4, Fe2O3, and Co3O4) have recently drawn considerable attention in potential applications such as magnetic separation, biosensors, and drug deliver,18 and could be easily collected and rapidly separated by an external magnetic field without additional centrifugation or tedious filtration. In particular, superparamagnetic nanoparticles can be easily redispersed into solutions by simple ultrasonication and cyclically reused after the adsorbate removal. When magnetic nanoparticles are functionalized with a molecularly imprinted polymer layer, as expected, specific recognition performance can be well combined with the magnetic property. Magnetic molecularly imprinted polymers (MMIPs) have attracted increasing interest in various applications.19−27 Until now, two main methods have been performed to obtain molecular imprinting at surface of magnetic particles, i.e., the grafting of “living” fragments onto the particle surface19−22 and the dispersion of magnetic particles in the solutions containing polymeric precursor. 23−27 Lu et al.19 prepared surface molecularly imprinted core−shell nanoparticles via surface-
1. INTRODUCTION Molecularly imprinting is increasingly being developed technology for the preparation of synthetic antibody mimics with specific molecule recognition ability,1,2 which generally involves self-assembly of templates and functional monomers and the subsequent copolymerization in the presence of crosslinking monomers. Three-dimensional binding sites are generated in the polymer, which match with the template in size, shape, and location of functional groups. After template removal, therefore, the obtained polymers can specifically recognize and bind the same or structurally very similar molecules.3 Unlike natural biological recognition elements such as antibodies and enzymes, which are very unstable, these synthetic receptors, namely molecularly imprinted polymers (MIPs), possess the greater chemical and physical stability, as well as lower cost and rapid and easier preparation. MIPs have been widely used in various fields including sensors,4 solid phase extraction,5 catalysis,6 chromatography separation,7 and so on. Usually, the bulk MIPs are prepared by conventional methods and exhibit high selective recognition but irregular size and shape as well as poor site accessibility to the target molecules.8 Synthetic strategies have been developed for addressing these problems in the past decade.9−13 Surface imprinting has provided a convenient approach for preparing the imprinted thin layer onto the surface of solid supports to form the core−shell structure.14,15 Zhang’s group reported a surface functional monomer-directing strategy for the highly dense imprinting of 2,4,6-trinitrotoluene (TNT) at the surface of silica nanoparticles.16 Molecularly imprinted shells on polystyrene (PS) colloidal spheres were first synthesized by © 2014 American Chemical Society
Received: Revised: Accepted: Published: 7157
December 7, 2013 March 29, 2014 April 3, 2014 April 3, 2014 dx.doi.org/10.1021/ie404140y | Ind. Eng. Chem. Res. 2014, 53, 7157−7166
Industrial & Engineering Chemistry Research
Article
initiated atom transfer radical polymerization (Si-ATRP) from the “living” agent functionalized Fe3O4 support. Haupt’s group described a general protocol to synthesize superparamagnetic molecularly imprinted polymer nanocomposites via a RAFTmediated approach.21 Although the surface grafting approach has been currently applied to obtain core−shell MMIPs, the modification process of surface-initiated or chain-transfer agents normally require a time-consuming multistep reaction. The latter method was relatively simpler, which frequently involved magnetic nanoparticles that were surface premodified with surfactants (e.g., PEG and oleic acid)23−26 or polymerable monomers27,29 and were subsequently embedded in the imprinted polymer matrix. Luo et al.26 prepared magnetic and hydrophilic molecularly imprinted polymers via an inverse emulsion−-suspension polymerization to remove water-soluble acid dyes from contaminated water. Ding and his co-workers27 reported that a core-hell magnetic molecularly imprinted polymer was prepared and used to detect the sildenafil and vardenafil in herbal dietary supplements. In our previous work, the Fe3O4 nanoparticles were modified with 3-(methacryloxyl)propyltrimethoxysilane, and then atom-transfer radical emulsion polymerization was adopted for preparing the imprinted polymer at the surface.28 To the best of our knowledge, however, MMIPs previously prepared generally had low saturation magnetic values and an irregular shape. Thus, Liu et al.29 improved the synthetic approach for MMIPs, in which acrylic acid monomers were first anchored at the surface of magnetic nanoparticles by a simple reaction with unsaturated iron ions, and the uniform imprinted polymer film (38 emu g−1) was then coated onto the particle surface. So far, due to the uncontrollable nature of surface polymerization, the highly homogeneous core−shell MMIPs still has been rarely reported. Herein, the aim of this present study is to provide a versatile approach for obtaining the homogeneous core−shell magnetic molecularly imprinted nanoadsorbents (MMINs), the synthetic route of which is shown in Figure 1. The uniform imprinted polymer thin film was grafted onto the surface of vinyl-modified superparamagnetic nanoparticles via a simple in situ precipitation polymerization through a two-step heating process. Tetracycline (TC) was used as a probe to investigate the rationality and feasibility of this method. Tetracycline antibiotics (TCs), a kind of alkaline broad spectrum antibiotic, are commonly used to prevent or cure several infectious diseases in humans and farm animals or to promote growth as feed additives because of their good activity against acute diseases caused by Gram-positive and Gram-negative bacteria.30 However, the widespread use and abuse of TCs have resulted in unsafe residue levels in water, soil, and animal source food, which could be directly toxic or else cause allergic reactions in some hypersensitive individuals.31 Development of a more effective and sensitive method for the selective recognition and removal antibiotics was thus of great importance, especially for the residue at very low concentration levels. To the best of our knowledge, there are rare reports about the magnetic molecularly imprinted polymers with regular morphology and high saturation magnetic using TC as the template. The structural characteristics, morphological information, and magnetic property of the prepared MMINs were studied in detail. The binding isotherm, binding kinetics, and selective recognition capacity of MMINs were investigated and discussed, as well as stability and reusability.
Figure 1. Synthetic route for core−shell magnetic molecularly imprinted nanoadsorbents.
2. EXPERIMENTAL SECTION 2.1. Materials. Sodium acetate (NaAc), acetic acid (HAC), methacrylic acid (MAA), acetonitrile, methanol, ethanol, 2,2′azobis(isobutyronitrile) (AIBN), ethylene glycol (EG), poly(ethylene glycol) (PEG), anhydrous toluene (99.8%), and iron(III) chloride hexahydrate (FeCl3·6H2O, 99%) were all analytical grade and obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Tetracycline (TC, 98%), chlortetracycline (CTC, 98%), ciprofloxacin (CIP, 98%), cefalexin (CFX, 98%), ethylene glycol dimethacrylate (EGDMA), and 3-(methacryloxyl)propyltrimethoxysilane (MPTMS) (98%) were purchased from Aladdin Reagent Co., Ltd. (Shanghai, China) and used as received. Deionized ultrapure water was purified with a Purelab Ultra (Organo, Tokyo, Japan). 2.2. Instruments. Infrared spectra (4000−400 cm−1) were recorded on a Nicolet NEXUS-470 FTIR apparatus, Madison, WI). The morphology of MIPs was obtained by using a scanning electron microscopy (SEM, S-4800) or transmission electron microscope (TEM, JEOL IEM-200CX). Magnetic measurements were carried out using a VSM (7300, Lakeshore) under a magnetic field up to 10 kOe. The thermogravimetric analysis (TGA) of samples was measured using a Diamond TG/DTA instrument (STA 449C Jupiter, Netzsch, Germany) under N2 up to 800 °C with a heating rate of 10 °C min−1. The identification of the crystallographic structure was performed using a Rigaku D/max-γB X-ray diffractometer (XRD) monochromatized with Cu Kα radiation over the 2θ range of 20−80° at a scanning rate of 0.02 deg s−1. The concentration of ions was tested by using a TBS-990 atomic absorption spectrophotometer (Beijing Purkinge General Instrument Co. Ltd., Beijing, China). A reversed-phase HPLC system (Agilent 1200 series, Santa Clara, CA) was equipped with a UV−vis 7158
dx.doi.org/10.1021/ie404140y | Ind. Eng. Chem. Res. 2014, 53, 7157−7166
Industrial & Engineering Chemistry Research
Article
In the binding kinetics study, 5.0 mg of MMINs was added to the TC solution with an initial concentration of 100 μmol L−1 at the predetermined time intervals at 298, 308, and 318 K, respectively. The rebinding amounts of TC adsorbed (Qt, μmol g−1) were calculated according to the following equation
detector to achieve the simultaneous detection the content of multicomponent solution samples. 2.3. Synthesis and Functionalization of Magnetic Nanoparticles. Monodispersed magnetic nanoparticles were prepared according to the previous method.32 Briefly, FeCl3· 6H2O (1.35 g) was dissolved in 40 mL of ethylene glycol to form a homogeneous solution. NaAc (3.6 g) and PEG (1.0 g) were subsequently added. The mixture solution was stirred vigorously for 30 min and then sealed in a 50 mL Teflon-lined stainless-steel autoclave. The drying and air circulation oven was heated to 200 °C, maintained at 200 °C for 8.0 h, and allowed to cool to room temperature. The magnetic Fe3O4 nanoparticles were collected with the help of an external magnetic field, washed with ethanol and water several times, and dried in a vacuum oven at 60 °C overnight. A portion of the obtained Fe3O4 nanoparticles (0.5 g) and 2.0 mL of MPTMS were dispersed into 100 mL of dry toluene and vigorously stirred under N2 at 90 °C for 12 h. The products were then collected and washed with toluene and ethanol several times. Finally, the vinyl-modified magnetic Fe3O4 nanoparticles (V−Fe3O4) were dried under vacuum for further use. 2.4. Preparation of Core−Shell MMINs. The thin imprinted polymer film was grafted onto the surface of Fe3O4 nanoparticles via in situ precipitation polymerization according to the following procedure: in brief, MAA (1.0 mmol), EGDMA (8.0 mmol), and TC (0.5 mmol) were dissolved into 60 mL of acetonitrile in a one-neck round-bottom flask for prepolymerization at room temperature away from light. V− Fe3O4 (100 mg) was added in to the mixture solution. And the solution was degassed in an ultrasonic bath and exchanged with N2. AIBN (15 mg) was added into the flask under N2 protection. Finally, the flask was placed in a constant temperature bath oscillator with the rotation rate of 200 rpm, and the reaction temperature was 50 °C for 6.0 h and then maintained at 60 °C for an additional 24 h. After the polymerization reaction, the products were collected by magnetic separation and washed with ethanol, distilled water, and acetone to remove the unreacted monomers. The template molecules were removed by Soxhlet extraction with an eluent of methanol/acetic acid (9.0/1.0, v/v) until no TC could be detected. The obtained MMIPs were then dried under vacuum for 12 h for use. Magnetic nonmolecularly imprinted nanoadsorbents (MNINs) were obtained as reference under the same procedure in the absence of template molecule. 2.5. Batch Binding Experiments. The binding property of MMINs toward TC was studied by a batch mode of operation. In the study of binding isotherm, 5.0 mg of MMINs was added into TC solution (10 mL) with the initial concentrations ranging from 10 to 200 μmol L−1 at the three temperatures, respectively (298, 308, and 318 K), and the pH of the solution was set at 5.0. After 12 h of contact, the saturated polymers were separated with an external magnet. The free TC concentration in the aqueous solution was determined by the UV−vis spectrophotometer at 276.8 nm. The equilibrium binding amount of TC was calculated as follows
(C − Ce)V Qe = o m
Qt =
V (Co − C t) m
(2)
−1
where Ct (μmol L ) is the concentration of free TC solution at any time t (min). To investigate the specific recognition property, 5.0 mg of MMINs was added into the test tubes containing 10 mL of antibiotic solution with 100 μmol L−1 of TC, CTC, CIP, and CFX, respectively, the chemical structures of which are shown in Figure 2. The experiments were carried out at 298 K for 12
Figure 2. Chemical structures of TC, CTC, CFX, and CIP.
h. Meanwhile, the double-antibiotic solutions containing TC and one other antibiotic were also studied, and the amounts of TC binding to the MMINs/MNINs were quantified by HPLC, investigating the effect of existence of competitive antibiotics. All the binding experiments above-mentioned were carried out in duplicate, and the mean values were used. 2.6. Magnetite Leakage Study. To estimate the amount of iron ions that is likely to leach from MMIPs, 100 mg of MMINs were suspended in distilled water (10 mL) with different pH ranging from 2.0 to 9.0 and were then shaken for 12 h. Subsequently, the MMINs were collected by an external magnet, and the leaked amount of iron ions was determined by a graphite furnace atomic absorption spectrophotometer. 2.7. Regeneration Ability Study. The cyclic binding capacity of MMINs for TC was tested to investigate its stability and reusable performance, which was a critical element for the practical application in waste treatment. The MMINs (5.0 mg) was added into TC solution (0.5 mM) at 298 K for 0.5 h and were then separated by a magnet. The binding and desorption procedures were repeated eight times using the identical batch of MMINs.
3. RESULTS AND DISCUSSION 3.1. Characterization. The FT-IR spectra of MPTMS, Fe3O4, V−Fe3O4, MMINs, and MNINs are shown in Figure 3. The main functional groups of the predicted structure could be observed with the corresponding infrared absorption peaks. The characteristic peak of Fe−O for Fe3O4 and V−Fe3O4 was observed at 588 cm−1. Compared with the spectra of Fe3O4, V−
(1)
−1
where Qe (μmol g ) is the equilibrium binding capacity of TC, C0 and Ce (μmol L−1) are the initial and equilibrium concentration, respectively, and V (L) and m (g) are the volume of solution and weight of nanoadsorbent, respectively. 7159
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Industrial & Engineering Chemistry Research
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Figure 4. SEM images of the Fe3O4 (a, b) and MMINs (c, d).
polymer film could be clearly distinguished. Therefore, it could be concluded that the proper surface modification of MPTMS could direct the highly selective occurrence of imprinted polymerization at the surface of magnetic nanoparticels, which was also demonstrated by other previous work.17 Figure 6 shows the X-ray diffraction (XRD) pattern of Fe3O4, V−Fe3O4, and MMINs. In the 2θ range of 20−70°, six characteristic peaks were indexed as (220), (311), (400), (422), (511), and (440), respectively (JCPDS card 19-0629 for Fe3O4).The XRD pattern of V−Fe3O4 and MMINs was similar to that of Fe3O4, indicating that the surface modification and grafting of polymer did not change the crystalline structure of magnetic nanoparticels. Thermogravimetric analysis (TGA) of Fe3O4 (a), V−Fe3O4 (b), and MMINs (c) is shown in Figure 7. With the initial temperature range (
