Preparation of a Magnetic Molecularly Imprinted Graphene Composite

Research Article pubs.acs.org/journal/ascecg

Preparation of a Magnetic Molecularly Imprinted Graphene Composite Highly Adsorbent for 4‑Nitrophenol in Aqueous Medium Jing Luo,* Yahan Gao, Kan Tan, Wei Wei, and Xiaoya Liu The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Lihu Street 1800, Wuxi 214122, Jiangsu, China S Supporting Information *

ABSTRACT: In this article, a type of magnetic molecularly imprinted graphene composite as a highly efficient adsorbent was prepared by forming a molecularly imprinted sol−gel polymer on the surface of magnetic graphene. The magnetic Fe3O4 nanoparticles were first deposited on a graphene sheet to prepare the magnetic graphene (MGR). Using the obtained magnetic graphene as a supporting matrix, 4-nitrophenol (4NP) as template, phenyltriethoxysilane and tetramethoxysilane as functional monomers, a magnetic molecularly imprinted graphene composite (MGR@MIPs) was subsequently formed after the sol−gel polymerization and extraction of 4-NP. The preparation conditions (concentrations of monomer and template, and reaction time) were optimized. The as-prepared MGR@MIPs was characterized by FTIR, VSM, SEM, and TEM images. Under the optimized conditions, the obtained MGR@ MIPs exhibited ultrafast adsorption kinetics (2 min to achieve the equilibrium state), large binding capacity (142 mg/g), and high selectivity toward 4-NP (the imprinting factor α is 4.25). In addition, a high saturation magnetization of MGR@MIPs was demonstrated, which allows easy separation from solution by applying an external magnetic field. Meanwhile, MGR@MIPs can be regenerated and reused in successive six cycles with slight loss in adsorption capacity. Finally, MGR@MIPs was successfully used as a highly adsorbent material for the determination and separation of 4-NP in real samples combining with highperformance liquid chromatography (HPLC). KEYWORDS: Magnetic graphene, Molecular imprinting, 4-Nitrophenol, Efficient adsorbent, Easy separation

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INTRODUCTION

Molecular imprinting technique (MIP) has been demonstrated as a powerful technique which creates artificial recognition sites in a synthetic polymer matrix.2−5 In recent years, a surface imprinting technique has aroused great interest, which involves fabricating a thin MIP layer on the surface of supporting materials.6,7 With the recognition cavities located at or approximate to the surface of the MIPs, it is thus easier and quicker to bind and remove template molecules, which lead to higher adsorption capacity and faster binding kinetics in surface imprinting materials. Nanomaterials are the most promising supporting materials for surface imprinting owing to their large external surface-to-volume ratio and well-defined material shape. Up to now, various nanomaterials have been reported for the preparation of surface MIPs composites, which include silica nanoparticles,8,9 magnetic NPs,10−13 nanotubes,14−16 quantum dots,17 and polymeric colloids.18 Graphene, a molecular sheet of graphite, has been considered to be an ideal candidate as supporting material for preparing surface MIP composites owing to its extremely large specific

4-Nitrophenol (4-NP) is a commonly used phenol derivative for the manufacture of dyes, drugs, insecticides, and fungicides and darkens leather, which could be widely found in wastewater from the textile industry and various industries such as pharmaceutical synthesis, iron and steel manufacturing, and electrical/electronic components production. However, owing to its high toxicity and persistence to organisms even at low concentrations in aqueous solution (20−100 μg/L), 4-NP poses a threat to animals and plants in the biosphere, which thus has been classified as Priority Pollutants by the U.S. Environmental Protection Agency (U.S. EPA).1 Therefore, the removal and determination of 4-NP from the aquatic environment have become an important research topic in the fields of environmental monitoring and remediation. Because of the extremely low concentration of NPs and much interference in the complex environmental water samples, the sample pretreatment steps for the determination of trace amounts of 4NP are frequently required to improve the method accuracy and sensitivity (Table S1). In addition, the fast and efficient separation of trace amounts of 4-NP from complex matrices also needs to be urgently solved. © 2016 American Chemical Society

Received: February 21, 2016 Revised: April 21, 2016 Published: May 18, 2016 3316

DOI: 10.1021/acssuschemeng.6b00367 ACS Sustainable Chem. Eng. 2016, 4, 3316−3326

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ACS Sustainable Chemistry & Engineering surface area (2630 m2/g) and good mechanical properties (110 GPa). In addition, its large conjugation system endows graphene with a strong affinity toward carbon-based ring molecules, which are widely found in pollutants, drugs, and biomolecules. In fact, various graphene based MIP composites have been successfully produced in recent years.19−30 The resulting graphene/MIP composites exhibited not only faster adsorption and desorption dynamics but also higher binding capacity. However, the obtained graphene/MIP composites are generally dispersed in solution, and their recovery after the adsorption process still remains a concern. Additional steps, such as filtration and high-speed centrifugation, are usually required to collect the highly dispersed graphene/MIP composites, which raises the operational cost in their practical applications. Magnetic nanoparticles (MNPs) have been widely used as adsorbent due to their strong superparamagnetism. In order to avoid the aggregation of MNPs and obtain a high specific surface area, the combination of advanced carbon materials (carbon nanotube and graphene) with magnetic nanoparticles has recently attracted great interest to prepare magnetic carbon materials, which possess both the easy separation ability and outstanding mechanical properties.31−34 Combining the advantages of imprinted polymer and magnetic carbon materials, efficient adsorbent materials with specific recognition ability, large adsorption capacity, and magnetic separation would be fabricated. Xiao et al.35 prepared molecularly imprinted polymers on the surface of magnetic carbon nanotubes, which could not only be collected rapidly by applying an external magnetic field but also had a high specific surface area, outstanding mechanical properties, and specific recognition for template molecule. It is quite expected that magnetic graphene would also be a promising support for preparing surface imprinted materials. To the best of our knowledge, few works have been published on the employment of magnetic graphene as supporting matrix to prepare surface MIP composites.36−39 On the other hand, most of the presently developed MIPs were prepared using organic acrylate or acrylic type monomers in organic solvent, which were organic solvent-compatible and mostly failed to show specific binding in aqueous system.40 Sol−gel technology provides a facile and green way to prepare three-dimensional silica networks with desirable properties. The sol−gel polymerization begins by simply mixing metal alkoxide or siloxane with water and a mutual solvent (mostly alcohol) in the presence of acid or base catalyst at room temperature. In addition, the sol−gel derived silicon network is both chemically and thermally stable. As a result, the sol−gel based MIPs possess a number of advantages over conventional MIPs, including the simple fabrication process, chemical inertness of the matrix, eco-friendly reaction solvent (aqueous solution), and mild reaction conditions (room temperature).41,42 This work aims to develop a kind of molecular imprinting composite combining the advantages of surface imprinting, magnetic graphene, and sol−gel technology for adsorption and detection of 4-NP. As far as we are aware, although some researchers prepared MIPs using 4-NP as the template,43−45 the preparation of molecularly imprinted polymers on magnetic graphene surface through sol−gel technology has not been reported. Herein, a magnetic graphene/molecularly imprinted polymer (MGR/MIPs) composite is fabricated via a simple and efficient way from Fe3O4 nanoparticle deposited graphene (MGR, supporting matrix), 4-NP (template molecules), phenyltriethoxysilane, and tetramethoxysilane (monomers)

through a one-pot sol−gel polymerization. The resulting MGR@MIPs was characterized by various techniques. The adsorption isotherms, adsorption kinetics, and selective recognition of MGR@MIPs were investigated in detail.

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EXPERIMENTAL SECTION

Chemicals. Graphite powder, iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) chloride tetrahydrate (FeCl2·4H2O), tetramethoxysilane (TMOS), phenyltriethoxysilane (PTEOS), 4-NP, 2nitrophenol (2-NP), 1,4-dinitrophenol (2,4-DNP), and 2,4-dichlorophenol (2,4-DCP) were obtained from Shanghai Aladdin Chemical Reagent. All other chemicals were of analytical grade and used as received without further purification except for special statement. Doubly distilled water was used throughout the work. Instruments. Fourier transform infrared (FTIR) spectra were recorded with an FTLA 2000-104 FTIR spectrometer in reflection configuration. Raman spectrum was recorded with a Renishaw in Via Raman Microscope operating at 514 nm with a charge-coupled device detector. Transmission electron microscope (TEM) images were obtained from a Tecnai G2 F20 microscope operating at 200 kV (FEI, United States). Scanning electron microscopy (SEM) measurements were performed on a field-emission scanning electron microscope. The energy dispersive spectrum (EDS) was measured by using a TEM equipped with an energy spectrum probe. The magnetization measurements of samples were performed by a vibrating sample magnetometer (VSM, MPMS-XL-7, Quantum Design Company). The high performance liquid chromatography (HPLC) analysis was performed on a Waters HPLC system including a binary pump and a variable wavelength UV detector. Chromatography analysis was achieved on a Spherigel C18 column (5 μm, 250 mm × 4.6 mm) for analyte isolation with the mobile phase of methanol/water (60:40, PBS buffer pH 4.5) at the flow rate of 1.0 mL/min. Under these conditions, the retention time for 4-NP was 6.6 min. The wavelength for the UV detector is 280 nm. Preparation of Magnetic Graphene (MGR). Graphene oxide (GO) was synthesized from graphite powder by the modified Hummers method.46 Magnetic graphene (MGR) was prepared according to the literature.47 Briefly, 0.4 g of GO was dispersed in 100 mL of ultrapure water under ultrasonic vibration for 1 h. Then, 8 mmol of FeCl3·6H2O and 4 mmol FeCl2·7H2O were added to the solution with mechanical stirring under a nitrogen atmosphere. After complete dissolution, 10 mL of ammonia (25%) was rapidly added into the solution under fierce stirring, and the pH value of the above mixture was adjusted to 9. Then the mixture solution was heated to 80 °C, and 4 mL of hydrazine hydrate was added with continuous stirring. After 5 h, the solution was cooled down to room temperature. The resulting Fe3O4 deposited graphene, which was designated as magnetic graphene (MGR), was separated from the reaction solution by a magnet, washed with ultrapure water and anhydrous ethanol, and finally dried in vacuum at 40 °C for 12 h. Preparation of MGR@MIPs Composite. In a typical procedure, 50 mg of MGR and 75 mg of 4-NP were dispersed in 30 mL of double-distilled water and subjected to ultrasonication for 30 min. Sixty milligrams of PTEOS and 20 mg of TMOS were then added to the above dispersion, and the pH value of the mixture was adjusted to ∼9.3 with NH3·H2O (28 wt %). The obtained reaction solution was subsequently stirred for 12 h at room temperature. The resulting product was collected by a magnet and washed with methanol/acetic acid (4:1, v/v) for several times to extract the template, until the 4-NP molecules could not be detected. The resulted MGR@MIPs composite was washed with deionized water three times and dried in vacuum. For comparison, nonmolecular imprinted composite (MGR/NIPs) was prepared and processed under the same conditions but without adding the template molecule. As another control sample, pure MIPs without graphene as supporting material were also prepared under the same conditions but without adding the MGR. Binding Experiments. For the kinetics adsorption experiments, 10.0 mg of MGR@MIPs or control MGR@NIPs composite was mixed with 20 mL of 4-NP solution (0.2 mg mL−1) and incubated for a 3317


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ACS Sustainable Chemistry & Engineering

Figure 1. Schematic illustration of the synthetic procedure for MGR@MIPs. certain amount of time from 0.5 to 6 min. For the isothermal adsorption experiments, 10 mg of MGR@MIPs or control MGR@ NIPs composite was added to 20 mL of 4-NP solutions of different concentrations from 0.02 to 0.8 mg mL−1, while employing the adsorption time of 6 min. Then, the supernatant liquid was separated using a permanent magnet. The concentration of 4-NP in the supernatant was measured by UV analysis. The reproducibility was investigated by five batches of samples prepared on different days. According to the differences in the 4-NP concentration before and after adsorption, the amount of 4-NP adsorbed by the MGR@MIPs or control MGR@NIPs composite (Q, mg/g) was calculated using the following formula: Q = (C0 − Ce)V /W

or MGR@NIPs composite, and the separation procedure was then conducted as described earlier for the isothermal adsorption experiments. Regeneration/Reuse of MGR@MIPs. The recovered MGR@ MIPs separated by the magnet was washed with 25 mL of methanol/ acetic acid (4:1, v/v) followed by 5 × 1 mL of methanol, dried in vacuum, and reused in the next cycle of adsorption experiments. Determination of 4-NP in Real Samples. The real aqueous sample was from the lake water of Tai Hu in Jiangsu Province which was spiked with different levels of 4-NP. Twenty milligrams of MGR@ MIPs composite was added into 100 mL of the real aqueous samples. After incubating at room temperature for 5 min, the MGR@MIPs was separated by a permanent magnet, and the supernatant solution was discarded. The MGR@MIPs which absorbed the target molecules (4NP) was washed with 5 mL of methanol/acetic acid (4:1, v/v). This procedure resulted in a preconcentration factor of 20. Finally, the eluted solution was submitted to HPLC for quantitative analysis of target analyte with the mobile phase of methanol/water (60:40, PBS buffer pH 4.5) As a further application, MGR@MIPs was used for the removal and determination of 4-NP in a real wastewater taken from Changzhou Pharmaceutical Factory Co., Ltd. The sample was centrifuged to remove the solids and finally filtered through 0.45 μm nylon membrane filters prior to injection on the HPLC.

(1)

where C0 and Ce represent the initial and final solution concentrations after adsorption (mg mL−1) of 4-NP, respectively. V (mL) is the volume of the solution, and W (g) is the weight of the MGR@MIPs or MGR@NIPs composite. Adsorption Selectivity. The selectivity properties of the MGR@ MIPs composite was evaluated by the imprinting factor (IF). 2Nitrophenol (2-NP), 1,4-dinitrophenol (2,4-DNP), and 2,4-dichlorophenol (2,4-DCP) were employed as competitive molecules. The imprinting efficiency was evaluated by the imprinting factor (IF) calculated from the following formula:

IF = Q MIP/Q NIP

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RESULTS AND DISCUSSION Preparation of MGR@MIPs Composite. The synthetic route of MGR@MIPs composite is illustrated in Figure 1,

(2)

A standard solution of 4-NP, 2-NP, 2,4-DNP, and 2,4-DCP with initial concentrations of 0.2 mg mL−1 was incubated with MGR@MIPs 3318


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Figure 2. Influence of the amount of monomer (PTEOS + TMOS) (A), template (4-NP) (B), and reaction time (C) on the adsorption capacity of MGR@MIPs.

Optimization of the Preparation Conditions. To determine the most favorable conditions for preparing the MGR@MIPs composite, the influence of the amount of monomer (PTEOS and TMOS) and template molecules (4NP) as well as the reaction time were investigated and discussed below in detail. In the preliminary experiments, the adsorption capacities of MGR@MIPs and MGR@NIPs were measured in some selected conditions, and the adsorption capacity of MGR@MIPs was larger than that of MGR@NIPs in all the conditions. The larger adsorption capacity suggests more imprinting sites in the MIP layer. The adsorption capacity of MGR@MIPs was therefore selected as the target of optimization. The influence of monomer was first evaluated. It is wellknown that the type and amount of functional monomer have a great influence on the molecular recognition ability of the obtained MIP materials. In this work, PTEOS and TMOS were selected as monomers on the basis of our previous work.22 Then, 50 mg of MGR, 60 mg of 4-NP, and different amounts of monomer (PTEOS + TMOS) were selected in the preparation of MGR@MIPs, and their adsorption capacities are shown in Figure 2. It has been observed in our previous experiment that an intact and stable sol−gel film could be formed when the mass ratio of PTEOS/TMOS was about 3:1. So in this work, we set the mass ratio of PTEOS/TMOS as 3:1 in preparing MGR@MIPs composite. As shown in Figure 2A, when the total amount of monomer (PTEOS + TMOS) was below 80 mg, the adsorption capacity (Q) increased with increasing amounts of the monomer. It is presumed that the thickness of the inorganic sol−gel MIP film on the surface of MGR increases with increasing amount of the monomer, which thus can accommodate more template molecules (4-NP) and result in more recognition cavities in the MIP layer. However, with the monomer amount above 80 mg, a slight decrease in the adsorption capacity was observed. This can be ascribed to the

which combines the surface imprinting technique and sol−gel polymerization. First, Fe3O4 deposited graphene, which was designated as magnetic graphene (MGR), was prepared via one-pot simultaneous reduction of GO and Fe3O4 NPs deposition. Next, template 4-NP was immobilized on the surface of MGR through π−π stacking interaction and hydrophobic interaction. After the addition of the silica precursor (PTEOS and TMOS), the protic functional groups (e.g., −OH and −COOH) on the surface of MGR could condensate with the silica precursor,48 giving rise to graphenesupported SiO2 sol seeds, which gradually condensed and grew into bigger nanoparticles covered on the graphene sheets with the constant supply of free silica precursor in solvent, leading to a silica layer on the surface of MGR. During this process, the template molecules (4-NP) were embedded in the silicon polymeric network through hydrogen bonds (formed between Si−OH groups in the inorganic matrix and the nitro group as well as the phenol group in 4-NP), constructing threedimensional structures around 4-NP molecules and producing numerous binding sites. Finally, after the removal of the template 4-NP, a thin layer with imprinted cavities complementary to 4-NP in shape, size, and functional group was obtained on the surface of MGR, leading to the formation of the MGR@MIPs composite. It should be noted that there is a competition between surface and bulk polymerization which could influence the homogeneity and thickness of the MIP layer on the surface of MGR. Therefore, it is quite important to optimize the related preparation parameters to produce homogeneous and thin MIP layer on the surface of MGR. Through this optimization, the bulk polymerization effect has been minimized, and the thickness of the MIP layer on the MGR surface was reasonable (16−30 nm), which was measured by the cross-section view of SEM images. The detailed optimization of the preparation conditions is presented in the following segment. 3319


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ACS Sustainable Chemistry & Engineering fact that the sol−gel MIP layer on MGR becomes too thick with the overdose of the added monomer and the overthickness of the MIP film which blocked site accessibility. Thus, the optimum monomer content of 80 mg was used in the reminder of this work. The influence of the amount of 4-NP was similarly investigated, and the results are given in Figure 2B. Fifty milligrams of MGR, 80 mg of monomer, and different amounts of 4-NP (30 to 100 mg) were added in the preparation of MGR@MIPs. It can be observed that the adsorption capacity increased with the increasing amount of template molecule added. This is quite reasonable considering that increasing template amount will lead to more imprinted cavities in the sol−gel film on MGR surface. It should be noted that overdose of 4-NP would lead to a decrease in adsorption capacity as shown in Figure 2B. The amount of 4-NP added in the preparation process was optimized at 75 mg, at which point maximum adsorption capacity was achieved. In addition to the monomer amount, the reaction time also affects the thickness of the sol−gel MIP film on MGR surface and thus the number of imprinting cavities it could accommodate. So the effect of polymerization time was also investigated. As shown in Figure 2C, with the increasing polymerization time, the adsorption capacity initially increased, reached maximum at 12 h, and then significantly decreased. The possible reason is that longer reaction time might lead to the overthickness of the sol−gel MIP film, and 4-NP molecules are perhaps buried too deep and thus difficult to extract to form effective recognition sites. Characterization of Synthesized Magnetic Nanomaterials. The FTIR spectra of the synthesized magnetic nanomaterials are illustrated in Figure 3, which provided direct

completely disappeared, corresponding to the conversion of the carbonyl groups to Si−O−C bands, which has been reported in the silica-graphene oxide (Si-GO) hybrid composite.50 Energy dispersive X-ray (EDX) spectra were conducted to further confirm the existence of Fe3O4 particles and silicon layer on the surface of graphene. As shown in Figure S1A, the spectrum of MGR shows the presence of Fe element peaks (the signal of Cu elements come from the supporting TEM grid), confirming the existence of iron-oxide nanoparticles. For MGR@MIPs (Figure S1B), the emergence of the Si signal in Figure 4B provides direct evidence for the presence of silicon

Figure 4. (A) VSM curves of GO, MGR, and MGR@MIPs. (B) Magnetic separation behavior of MGR@MIPs.

layer on the surface of MGR. In addition, MGR@MIPs exhibits a decrease in the intensity of Fe elements compared with that of MGR. This was expected considering the shielding effect of the imprinted silicon coating on the magnetic component, thus somewhat reducing the signal of Fe. Magnetism is the most important characteristic of magnetic materials. Superior magnetic properties of imprinted materials make the separation process easier, faster, and more efficient from the solution by an external magnetic field, being free of additional filtration or repeated centrifugation in practical application. VSM was employed to investigate the magnetic properties of MGR and MGR@MIPs. Figure 4 shows the hysteresis loops of MGR and MGR@MIPs recorded at room temperature. It is obvious that there was no hysteresis, and both coercivity and remanence were negligible, which indicated that MGR and MGR@MIPs samples had supermagnetic properties. The saturation magnetic values are 43.82 and 30.49 emu g−1 for the MGR and MGR@MIPs, respectively, which made them sensitive to magnetic fields. Compared with MGR, the saturated magnetization value of MGR@MIPs (as shown in Figure 4A) was slightly decreased due to the shielding effect of the sol−gel MIP layers on the surface of MGR.51 However, the MGR@MIPs material remained strongly magnetic and could be isolated within 20 s by use of an external magnet. The separation process of MGR@MIPs in solution by a magnet was shown in Figure 4B. A black homogeneous dispersion of

Figure 3. FTIR spectrum of GO, MGR, and MGR@MIPs.

evidence for the successful preparation of MGR/MIPs. As shown in Figure 3, GO shows an intensive peak at about 1725 cm−1, which is attributed to the stretch vibration of CO. The broad peak from 3600 to 3050 cm−1 corresponds to the stretching vibration of O−H. In comparison with GO, the peak at 1725 cm−1 became a little weak for MGR, indicating the reduction of GO to some extent.49 In addition, the characteristic peak of Fe−O stretch vibration of Fe3O4 at 593 cm−1 appeared in the spectrum of the MGR, indicating that Fe3O4 was successfully deposited on the surface of graphene.32 In comparison with MGR, a new intensive band at around 1120 cm−1 was observed for MGR/MIPs, which is characteristic of the Si−O−Si and Si−O−C asymmetric vibrations, providing the evidence of the presence of silicate network on the surface of MGR.22 In addition, the carbonyl group band at 1725 cm−1 3320


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Figure 5. SEM images of GO (A), MGR (B), and MGR@MIPs (C). TEM images of GO (D), MGR (E), and MGR@MIPs (F).

surface area for MGR@MIPs gives good evidence of the presence of imprinted cavities in the sol−gel SiO2 layer on the surface of MGR. Binding Characteristics of MGR@MIPs. In order to investigate the rebinding ability of MGR@MIPs toward 4-NP, static absorption experiments were performed with different initial 4-NP concentrations ranging from 0.001 to 0.8 mg/mL. The binding isotherm of MGR@MIPs is shown in Figure 6. It

MGR@MIPs in a vial was obtained in the absence of a magnet (left). Once an extra magnetic field was applied outside the wall of the vial, the black MGR@MIPs was quickly attached to the vial wall within 20 s, and the dispersion became clear and transparent (right). Therefore, the resultant MGR@MIPs is highly superparamagnetic and could be collected easily and quickly from the adsorption solution, which is very advantageous for the magnetic separation on a large scale. The morphological structures of MGR and MGR@MIPs were examined by SEM and TEM. The SEM images of GO, MGR, and MGR@MIPs composite are shown in Figure 5. GO shows a typically curved, layer-like structure with a fairly smooth surface (Figure 5A). For MGR (Figure 5B), it can be clearly seen that a large number of uniform nanospheres appeared on the basal planes of graphene, which slightly gathered together owing to the high loading degree of Fe3O4 particles,37 suggesting that the Fe3O4 nanoparticles were successfully deposited onto the surface of graphene. In comparison to MGR, MGR/MIPs showed a rather rough and dense surface (Figure 5C), indicating the existence of imprinting layer on the surface of magnetic graphene sheets. The morphologies of GO, MGR, and MGR@MIPs were further investigated by transmission electron microscopy (TEM). As shown in Figure 5D, GO has a nearly transparent flake-like shape with characteristic crumpled silk waves. The TEM images of MGR revealed that the Fe3O4 NPs were successfully attached on the surface of graphene sheets and the aggregation of some Fe3O4 NPs is also observed (Figure 5E). For MGR@MIPs (Figure 5F), more small nanoparticles in aggregated form were observed on the surface of the graphene sheet, which should be attributed to the cover of imprinting the sol−gel SiO2 layer. To support the presence of imprinted sites created in the polymer, nitrogen adsorption experiments of the MGR@MIPs and MGR@NIPs were carried out. The N2 adsorption and desorption isotherms of MGR@MIPs and MGR@NIPs are shown in Figure S2. The specific surface area of MGR@MIPs was calculated to be 72.5 m2 g−1, which is about three times that of MGR@NIPs (25.4 m2 g−1). MIP materials normally have larger specific surface area than the corresponding NIP material owing to the fact that extraction of template molecules leaves many imprinted cavities. The enhancement in specific

Figure 6. Adsorption isotherms of 4-NP on MGR@MIPs and MGR@ NIPs. Amount of adsorbent, 10 mg; volume, 20 mL; incubation time, 5 min. Data are represented as the mean ± standard deviation (SD) (n = 3).

can be seen that the adsorption capacity of MGR@MIPs increased rapidly with the increasing initial concentrations of 4NP and reached a saturation value of 145 mg/g when the initial concentration of 4-NP was 0.6 mg/mL. The absorption experiments of MGR@NIPs were also carried out, and only a small amount of 4-NP was bound to MGR@NIPs (curve b), which should be attributed to surface nonspecific adsorption. In the whole concentration range, MGR@MIPs displayed significantly higher adsorption capacity than MGR@NIPs, indicating the favorable imprinting effect of the imprinted nanomaterials. The results confirmed the successful formation of molecular recognition sites in MGR@MIPs, which could specifically recognize the template molecules. By contrast, MGR@NIPs had no imprinted sites, and nonspecific 3321


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site in MIPs. It should be noted that heterogeneity is a key attribute of MIPs and an excellent measure for the determination of imprinting effect.55,56 Because of the heterogeneous distribution of imprinted sites, MIPs ensure a higher population of high-affinity binding sites. The adsorption kinetics of 4-NP onto MGR@MIPs and MGR@NIPs were investigated by varying the adsorption time from 0.5 to 6 min and presented in Figure 7. The initial

adsorption played a dominant role, thus resulting in a low binding capacity. The adsorption capacity of MGR@MIPs in our work was compared with other reported imprinted materials using 4-NP as template molecules. Two kinds of bulk MIPs were prepared by Ersoz et al. for the removal of 4-NP using methacrylic acid (MAA) and methacrylamidoantipyrine (MAAP) as functional monomers.52 The maximum adsorption capacities of MAAMIP and MAAP-MIP for 4-NP were only 13.5 mg/g and 24.1 mg/g, respectively. Employing the surface molecular imprinting technique, Guan et al. prepared three kinds of molecularly imprinted polymers (MIPs) on functionalized potassium tetratitanate whisker (F-PTW), which showed a maximum adsorption capacity of 76.05 mg/g.43 In a recent work, Mehdinia developed magnetic molecularly imprinted nanoparticles by copolymerization of MAA and ethylene glycol dimethacrylate on the surface of vinyl modified Fe 3 O 4 nanoparticles for selective detection of 4-NP.53 The maximum adsorption capacity of the magnetic MIP was calculated to be 57.8 mg/g, which has been greatly enhanced to 129.1 mg/g using a “Grafting from” method by the same group very recently.54 However, this value was still less than the value obtained in our work (145 mg/g), implying that MGR is a quite promising supporting material for the preparation of surface imprinted materials. The above results clearly showed that the adsorption capacities of the MGR@MIPs in our work were superior to those of 4-NP imprinted materials reported in previous literature. In addition, the adsorption capacities of commercially available activated carbon toward 4-NP have also been investigated with similar adsorption conditions, and the values were in the range of 10−40 mg/g, which were much lower than that of MGR@MIPs in our work. This remarkably large binding capacity of MGR@MIPs should be attributed to a combination of the large surface area of magnetic graphene and the thin MIP layer on its surface. Graphene has a small dimension with large specific surface, and more recognition sites could be created at its surface, whereas the thin MIP layer ensures the complete extraction of 4-NP molecules. To analyze the interaction kinetics between MGR@MIPs and the template molecules, three different equilibrium isotherm models, Freundlich, Langmuir, and Scatchard models, were used to fit the experimental data. These models can be expressed by eqs 3, 4, and 5, respectively. Freundlich: log(Q e) = log(KF) + 1/ n log(Ce)

(3)

Langmuir: Ce/Q e = Ce/Q max + 1/Q maxKL

(4)

Scatchard: Q e/Ce = (Q max − Q e)/Kd

(5)

Figure 7. Adsorption kinetics of 4-NP on MGR@MIPs, MGR@NIPs, and pure MIPs. Concentration of 4-NP, 0.2 mg/mL; amount of adsorbent, 10 mg; volume, 20 mL. Data are represented as the mean ± standard deviation (SD) (n = 3).

concentration of 4-NP was kept at 0.2 mg/mL. As shown in Figure 7, the adsorption capacity increased rapidly in the first 1 min and almost reached equilibrium after 2 min, revealing a remarkably rapid adsorption dynamics of 4-NP molecules onto MGR@MIPs. Such a fast response feature is quite advantageous in real applications. As far as we are aware, traditionally bulk imprinted materials normally need 1−10 h to reach adsorption equilibrium, whereas for surface imprinted materials, it generally takes 10−60 min to reach adsorption equilibrium for various templates.37,45 This fast dynamics adsorption of MGR@MIPs should be attributed to the combination of surface-imprinted sites, large surface-to-volume ratio of MGR, and complete removal of the 4-NP templates, which ensured efficient mass transfer and allowed 4-NP to reach the surface imprinting cavities in the MGR@MIPs easily and reach adsorption saturation in such a short time. It was also noted that the adsorption capacity of the MGR@MIPs was much higher than that of the MGR@NIPs during the total investigation time. As a control sample, pure MIPs without graphene as supporting material were also prepared under the same conditions but without adding MGR. The adsorption capacity of MIPs was also investigated and provided in Figure 7. It can be observed that the adsorption amount of MIPs increased with the increasing incubation time, and the adsorption equilibrium was not achieved even after 5 min. So, MGR@MIPs revealed a faster adsorption dynamics of 4-NP than pure MIPs, which should originate from the surface imprinting effect. Compared with pure MIPs, 4-NP molecules can be easily removed from or recombined with MGR@MIPs in a shorter time, leading to a faster adsorption dynamics. In addition, in all investigated incubation time, the adsorption capacity of MGR@MIPs is obviously larger than that of MIPs. Both the faster binding kinetics and larger adsorption capacity of MGR@MIPs confirm the advantages of MGR as supporting material in the preparation of molecular imprinted materials.

where Qe, Ce, KF, 1/n, KL, and Kd are the absorbed amount of 4-NP, equilibrium concentration, Freundlich constant, the heterogeneity factor (with values from 0 to 1), Langmuir constant, and Scatchard constant, respectively. The fitting plots with the above three different isotherm models were exhibited in Figure S3. The resulting parameters are summarized in Table S2. Through comparison of correlation coefficient and linearity data, the Freundlich isotherm model is found to better fit the experimental data of 4-NP on MGR@MIPs than others. While the Langmuir isotherm is suitable for the monolayer adsorption and homogeneous distribution of energy over the entire coverage surface, the Freundlich isotherm model well fits the heterogeneous distribution. So the Freundlich isotherm is widely used for determining the heterogeneity of the binding 3322


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ACS Sustainable Chemistry & Engineering Adsorption Selectivity. In order to verify the selectivity of the MGR@MIPs to 4-NP, 2-nitrophenol (2-NP), 1,4dinitrophenol (2,4-DNP) and 2,4-dichlorophenol (2,4-DCP) were used as competitive compounds considering their structural similarities to 4-NP. Figure 8 shows the adsorption

Regeneration and Stability. Regeneration is one of the most important properties for the application of MIP materials. To investigate the stability and regeneration of the MGR@ MIPs, the adsorption−regeneration cycle was repeated six times using the same MGR@MIPs sample (Figure 9. After

Figure 9. Adsorption−desorption cycles of MGR/MIPs. Amount of adsorbent, 10 mg; volume, 20 mL; incubation time, 5 min; and concentration of 4-NP, 0.2 mg/mL. Data are represented as the mean ± standard deviation (SD) (n = 3).

Figure 8. Binding capacities of 4-nitrophenol (4-NP), 4-nitrophenol (2-NP), 2,4-dinitrophenol (2,4-DNP), and 2,4-dichlorophenol (2,4DCP) on MGR@MIPs and MGR@NIPs. Amount of adsorbent, 10 mg; volume, 20 mL; incubation time, 5 min; concentration of all phenols, 0.2 mg/mL. Data are represented as the mean ± standard deviation (SD) (n = 3).

adsorption of 4-NP onto the MGR@MIPs, the MGR@MIPs was regenerated by washing with methanol/acetic acid (4:1, v/ v). The MGR@MIPs was quite stable, and the adsorption capacity kept 95.3% of the initial capacity after six adsorption− regeneration cycles, which demonstrated the physical robustness and mechanically durable imprinted network of MGR@ MIPs. The possible reason for the minor loss in the absorption capacity is that that some recognition sites in MGR@MIPs were jammed after regeneration or destroyed by rewashing. The results showed that the reusability of MGR@MIPs was satisfactory. Adsorption and Determination of 4-NP in Real Samples by the MGR/MIPs. The accuracy and application of MGR/MIPs were evaluated by spiking lake water samples with 4-NP at concentration levels of 10, 50, and 100 μg L−1. For each concentration, three measurements were carried out. Table 1 shows the results obtained for the lake water samples

capacities of the MGR@MIPs and MGR@NIPs for these four compounds with a feeding concentration of 0.2 mg/mL. The imprinting effect of a molecular imprinted material is often evaluated by specific adsorption capacity and imprinting factor. It is obvious that the adsorption capacity of MGR@MIPs for 4NP is much higher than that of the other three analogues, suggesting that MGR@MIPs have a higher affinity for the template 4-NP. In addition, the IF, taken as the ratio of the adsorption capacity of MGR@MIPs and MGR@NIPs, was as high as 4.25 for binding 4-NP, whereas the α values for binding 2-NP, 2,4-DNP, and 2,4-DCP were calculated to be 1.58, 1.39, and 1.67, respectively. The IF value of 4-NP is significantly higher than those of nontemplate molecules, indicating that the cavities in the MGR@MIPs are in good match with 4-NP. The IF values of 2-NP, 2,4-DNP, and 2,4-DCP are quite close to 1, which suggest that both MGR@MIPs and MGR@NIPs could adsorb these molecules but that the adsorbed amounts did not have significant differences between MGR@MIPs and MGR@ NIPs. The weak adsorption of 2-NP, 2,4-DNP, and 2,4-DCP to MGR@MIPs and MGR@NIPs should be attributed to nonspecific adsorption which mainly relies on physical adsorption. From the above data, it is quite clear that MGR@MIPs exhibit both larger rebinding capacity and higher imprinting factor toward 4-NP than toward the nontemplate compounds (2-NP, 2,4-DNP, and 2,4-DCP), confirming the specific recognition of MGR@MIPs toward the template molecule. The good selectivity should originate from the imprinting effect. There are numerous imprinting cavities in the MGR/MIP composite which matched the size, shape, and functional groups of 4-NP, leading to the outstanding recognition behavior toward the target molecules. As for the other three analogues, the microenvironment and the steric complementarity of the imprinted cavity are not suitable, so they could not bind with MGR/MIPs as tightly as 4-NP.

Table 1. Results of Extraction and Determination of 4-NP in Lake Water Samples by MGR@MIPs and MGR@NIPs (n = 3) sample lake water

materials MGR@ MIPs MGR@ NIPs

added (μg L−1)

found (μg L−1)

recovery (%)

RSD (%)

10 50 100 10 50 100

9.47 49.1 101.2 2.31 11.2 25.2

94.7 98.2 101.2 23.1 22.4 25.2

3.4 2.6 2.1 5.7 6.5 4.8

extracted by MGR@MIPs and MGR@NIPs using HPLC-UV spectrometry as the analytical method (the HPLC chromatograms of nonspiked and spiked lake water samples as well as the standard HPLC curve of 4-NP are shown in Figure S5). Good recoveries ranging from 94.7% to 101.2% were obtained for MGR@MIPs, suggesting the accuracy of this method. The criterion for 4-NP to protect aquatic life is 53 μg L−1 according to the guideline levels of the EPA,53 which means the proposed method based on MGR/MIPs is applicable for the extraction 3323


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for the separation and determination of 4-NP in real water samples with a relatively high recovery, demonstrating the potential application of the MGR@MIPs as adsorbent for rapid, highly efficient, and cost-effective sample analysis.

and determination of 4-NP in lake water. It is observed that the recovery obtained for MGR@MIPs was much higher than that for MGR@NIPs, proving the effective recognition property and selectivity of MGR@MIPs. As a further application, MGR@MIPs was used for the removal and determination of 4-NP for real wastewater taken from Changzhou Pharmaceutical Factory Co., Ltd. The sample was centrifuged to remove the solids and finally filtered through 0.45 μm nylon membrane filters prior to injection on the HPLC. The chromatograms of the initial wastewater, extracted by MGR@MIPs and the solution recovered by MGR@MIPs, are exhibited in Figure 10. Paracetamol (1), aminophenol (2),

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b00367. EDX spectra of MGR and MGR@MIPs, N2 adsorption and desorption isotherms of MGR@MIPs and MGR@ NIPs, the fitting plots of adsorption isotherm with Freundlich isotherm model, Langmuir model, and Scatchard model, the adsorption isotherms of 4-NP (lower concentration form 0.001 to 0.1 mg/mL) on MGR@MIPs and MGR@NIPs, HPLC chromatograms of nonspiked, and spiked lake water samples as well as the standard HPLC curve of 4-NP (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-510-85917763. Fax: 86-510-85917763. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

Figure 10. Chromatograms of (a) original wastewater sample, (b) wastewater sample after treatment by MGR@MIPs, and (c) solution recovered by MGR@MIPs. Amount of adsorbent, 20 mg; volume, 100 mL; incubation time, 5 min; the mobile phase, methanol/water (60:40, v/v, PBS buffer pH 4.5); wash solution, 5 mL of methanol−acetic acid (4:1, v/v).

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ACKNOWLEDGMENTS We acknowledge financial support from the National Natural Science Foundation of China (under Grant Nos. 51503064), the Enterprise-University-Research Prospective Program Jiangsu Province (BY2013015-08), the Fundamental Research Funds for the Central Universities (JUSRP 51305A), and MOE & SAFEA for the 111 Project (B13025).

and a small amount of 4-NP (3) were found in the wastewater (curve a). After treatment by MGR@MIPs (curve b), the corresponding peak of 4-NP almost disappeared in the chromatogram of the treated wastewater, indicating that 4-NP was extracted effectively with MGR@MIPs. In contrast, the other peaks related to paracetamol and aminophenol were not significantly reduced, demonstrating the selective adsorption of 4-NP by MGR@MIPs. As can be seen from the chromatogram of extracting solutions (curve c), MGR@MIPs can achieve a very significant preconcentration of 4-NP as indicated by the much stronger peak of 4-NP. The concentration of 4-NP in the wastewater is calculated to be about 1 mg/L. These results demonstrated that the prepared MGR@MIPs exhibited good selectivity and enrichment ability toward 4-NP in the real wastewater sample, which could be employed for the separation and determination of 4-NP at low concentration in real samples.

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REFERENCES

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CONCLUSIONS A magnetic imprinted graphene composite as highly efficient adsorbent was prepared through the combination of surface imprinting technique with magnetic separation using magnetic graphene as the supporting material. The obtained MGR@ MIPs exhibited strong magnetic responsiveness, large adsorption capacity of 142 mg/g, quick equilibrium time of 2 min, and good recognition toward target molecule, which could be easily collected by applying an external magnetic field. The reusability and stability of MGR@MIPs are also satisfactory. Furthermore, the MGR@MIPs have been successfully applied as adsorbents 3324


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