Magnetic molecularly imprinted microspheres via yeast stabilized Pickering emulsion polymerization for selective recognition of λ-cyhalothrin

Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 27–36

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Magnetic molecularly imprinted microspheres via yeast stabilized Pickering emulsion polymerization for selective recognition of ␭-cyhalothrin Wenjing Zhu a , Wei Ma a,c , Chunxiang Li a,∗ , Jianming Pan a,∗∗ , Xiaohui Dai a , Mengying Gan b , Qin Qu a , Yunlei Zhang b a

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China School of Environment, Jiangsu University, Zhenjiang 212013, China c School of Chemistry and Chemical Engineering, Pingdingshan University, Pingdingshan 467099, China b

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• The magnetic molecularly imprinted polymer microspheres were successfully produced. • The MMIMs were prepared based on yeast microorganism. • The MMIMs possessed excellent selective adsorption capacity for LC. • The MMIMs presented good magnetic sensitivity and stabilization.

a r t i c l e

i n f o

Article history: Received 26 December 2013 Received in revised form 31 March 2014 Accepted 3 April 2014 Available online 13 April 2014 Keywords: ␭-Cyhalothrin Yeast Pickering emulsion polymerization Magnetic molecularly imprinted microspheres Selective separation

a b s t r a c t In our study, magnetic molecularly imprinted microspheres (MMIMs) were prepared via Pickering emulsion polymerization. The template ␭-cyhalothrin (LC), functional monomers and hydrophobic Fe3 O4 nanoparticles, served as magnetic carrier, were contained in oil phase. Modified yeast dispersed in water phase was employed as stabilizer to establish stable oil-in-water Pickering emulsion, and the imprinting process was conducted by thermally initiated radical polymerization of functional and crosslinked monomers. Characterization results indicated that the Pickering emulsion and MMIMs were successfully produced. The MMIMs possessed thermal stability (below 200 ◦ C), magnetic sensitivity (Ms = 0.73 emu g−1 ) and magnetic stability (over the pH range of 2.0–8.0). Then, the MMIMs were used as sorbents for the selective recognition of LC. The Langmuir isotherm model gave a better fit to the experimental data, indicating the monolayer molecular adsorption for LC. The adsorption kinetic data was well described by the pseudo-second-order kinetics model, suggesting that the chemical process could be the rate-limiting step in the adsorption of LC. The selectivity study displayed the excellent selective recognition for LC compared to structural analog and non-analog in unitary and binary solutions. Furthermore, the satisfying regeneration capability of the sorbents was proved by at least three repeated adsorption–desorption cycles. © 2014 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +86 0511 88790683; fax: +86 0511 88791800. ∗∗ Corresponding author. E-mail addresses: [email protected] (C. Li), [email protected] (J. Pan). http://dx.doi.org/10.1016/j.colsurfa.2014.04.011 0927-7757/© 2014 Elsevier B.V. All rights reserved.

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W. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 27–36

1. Introduction Pyrethroids including ␭-cyhalothrin (LC), fenvalerate, cyfluthin, bifenthrin are widely used in agriculture, fishery, forestry and household pest control because of their high capability of pest-killing. Pyrethroids can undergo a relatively rapid biotransformation and excretion in mammals compared to well-known persistent organochlorinated compounds [1]. However, despite their low toxicity to mammalian, pyrethroids still not only cause neurotoxicity and make central nervous system intoxication, but also are suspected to have endocrine-disrupting effects through dermal absorption or ingestion [2,3]. It is necessary to separate pyrethroids from water, soil or other environment. Adsorption is widely applied and considered to be a good technology due to its quickness and easiness. Hence, the key of pyrethroids separation is to find or prepare new class of sorbents prone to be collected and with high mechanical strength, adsorption capacity and selectivity. In order to prepare sorbents with high mechanical strength, the method of Pickering emulsion polymerization was introduced in our system. As we know, according to the pioneering work of Ramsden [4] and Pickering [5], solid colloidal particles are observed to adsorb at fluid interfaces between water and oil to form so-called “Pickering emulsions” [6]. Currently, compared to conventional surfactant systems, Pickering emulsions stabilized by colloidal particles draw high attention thanks to significant advantages of lower toxicity, relatively well controlled size, especially more stabilization and rigidity providing high mechanical strength [7]. The types of emulsions produced by colloidal particles are determined by the contact angles of the particles on interfaces. The contact angles greater than 90◦ mean that the hydrophobic particles tend to be wetted by oil and to form water-in-oil (w/o) emulsions. On the opposite side, the contact angles less than 90◦ suggests the hydrophilic particles are more easily wetted by water and oil-inwater (o/w) emulsions are generated [8]. Common solid stabilizers of Pickering emulsion include inorganic substance, such as silica [9,10], Fe3 O4 nanoparticle [11] or calcium carbonate [12] and clay involving Laponite [13] and montmorillonite [14]. Microorganism, compared with the materials above, has advantages of low cost, easily available source and abundant active biomolecule on the cell wall without further modification process [15]. As we know, Pravit et al. [16] developed an oil-in-water Pickering emulsion stabilized by biobased material based on a bacteria-chitosan network (BCN) and opened up opportunities for the development of an environmental friendly new interface material as well as the novel type of microreactor utilizing bacterial cells network. Yeast, one of the most important and interesting groups of microorganisms with nontoxicity and good biocompatibility [17], has not been reported to be applied in Pickering emulsion so far. Molecular imprinting technology (MIT) has been reported to be used to selectively recognize and separate pyrethroids [3,18]. MIT provides a promising alternative way to create materials with highly selectivity and additional advantages of reusability and lowcost. Molecularly imprinted polymers (MIPs) are synthetic artificial receptors produced by the cross-linking of functional monomers in the presence of the template molecule. Removal of the template yields imprinted cavity capable of recognizing template molecule under appropriate conditions [19]. In order to obtain MIPs prone to be collected, magnetism was introduced in our system. To our knowledge, magnetic separation technology has received considerable attention in recent years for its great potential application in cell separation, drug delivery and enzyme immobilization. This technology used in MIT provides a relatively rapid and easy way for removal of magnetic polymers from sample matrices by employing appropriate magnetic field [20]. Participation of magnetic component in MIPs can build a controllable rebinding process and allow magnetic separation in a convenient way [21,22]. By far, researches

on magnetic MIPs based upon yeast have been rarely studied. Our group Li et al. [15] have successfully prepared imprinted polymers based on magnetic yeast via atom transfer radical polymerization for selective recognition of cefalexin. As we know, Pickering emulsion polymerization introduced in this system can overcome the disadvantages of the MIPs produced by traditional polymerization method, such as low mechanical strength, abnormity in shape and size and poor dispersion [23]. Until now, several articles about MIPs by Pickering emulsion polymerization have been reported by our team. For example, Liu et al. [24] prepared the fluorescent MIPs by Pickering emulsion polymerization for recognition of LC. Pan et al. [25] produced magnetic MIPs by attapulgite and magnetic carrier stabilized Pickering emulsion polymerization. Hang et al. [26] achieved magnetic MIPs through Pickering emulsion polymerization using magnetic halloysite nanotubes as the stabilizer. However, microorganism especially yeast stabilized Pickering emulsion applied in MIT has not been reported up to now. In this work, the magnetic molecularly imprinted microspheres (MMIMs) were acquired through Pickering emulsion polymerization. The yeast particles, as the emulsion stabilizer, were firstly modified by oleic acid, in order to supply magnetism, a few hydrophobic Fe3 O4 nanoparticles served as magnetic carrier were also added into oil phase. Then the MIPs were formed from inner oil phase, and the yeast particles attached onto the outer surface of microsphere. The characterization and the adsorption behaviors of these sorbents for the template LC were investigated. The high mechanical strength, magnetism, biocompatibility and excellent selective adsorption capability provide more opportunities to the novel MMIMs to be applied in many different fields.

2. Experimental 2.1. Materials Yeast powder was purchased from Angel Yeast Co. ␭Cyhalothrin (LC), fenvalerate (FL) were supplied by Jiangsu Huangma Agrochemicals Co., Ltd. Iron (II) chloride tetrahydrate (FeCl2 ·4H2 O), iron (III) chloride hexahydrate (FeCl3 ·6H2 O), sodium hydroxide (NaOH), oleic acid, ethanol, methacrylic acid (MAA), acetic acid, toluene and methanol were received from Sinopharm Chemical Reagent (Shanghai, China). Diethylphthalate (DEP), 4-vinylpyridine (4-VP), ethyl glycol dimethacrylate (EGDMA), dimethyl 2,2 -azobis (2-methylpropionate) (AIBME) and fluorescein isothiocyanate (FITC) were obtained from Aladdin Reagent (Shanghai, China).

2.2. Instruments The morphologies were observed by an optical microscope (OM, Shanghai Peter EM (BM) Optical Instrument Manufacturing, China), a scanning electron microscope (SEM, JEOL, JSM-7001F, Japan) and a transmission electron microscope (TEM, JEOL, JEM-2100, Japan). IR spectra were recorded on a FTIR apparatus (Nicolet, NEXUS470, USA). A confocal laser scanning microscope (CLSM, Leica, TCS SP5II, Germany) was used for fluorescence testing. Magnetic measurements were conducted by VSM (7300, Lakeshore). Thermogravimetric analysis (TGA) was carried out using a DSC/DTA-TG (Netzsch, STA 449C Jupiter, Germany). The identification of crystalline phases was performed using a X-ray diffractometer (Bruker, D8 Advance, Germany). An atomic absorption spectrophotometer (TBS-990, Beijing Purkinge General Instrument Co. Ltd., Beijing, China) was used for magnetite leakage measurement. High-performance liquid chromatography (HPLC) analysis was performed on an Agilent system (Agilent, 1200, Germany) equipped

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with a UV–vis detector. A UV/Vis spectrophotometer (Shimadzu, UV-2450, Japan) was used to acquire UV/vis adsorption spectra. 2.3. Synthesis of hydrophobic Fe3 O4 nanoparticles The preparation of Fe3 O4 Nanoparticles followed the coprecipitation method: 1.35 g of FeCl3 ·6H2 O and 0.6 g of FeCl2 ·4H2 O was dispersed in 50 ml of deionized water. After 15 min of stirring at 30 ◦ C, 50 ml of NaOH (0.5 mol L−1 ) was added rapidly in a N2 atmosphere and the dispersed solution was under vigorous stirring for 20 min. The resultant black precipitation was collected with a Nd–Fe–B permanent magnet and washed with ethanol three times. Then, Fe3 O4 nanoparticles were hydrophobic modified by oleic acid: acquired Fe3 O4 was dispersed in 40 ml of a mixture of oleic acid and ethanol (1:3, v/v) and the compound was stirred for 6 h at 50 ◦ C. The product was collected with a Nd–Fe–B permanent magnet and washed with ethanol six times. The resulting hydrophobic Fe3 O4 nanoparticles were dried at 40 ◦ C under vacuum.

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LC. After adsorption for 12 h at 25 ◦ C, the sorbents were isolated by external magnetic field and the concentrations of LC in the supernatant were measured by UV/vis spectrophotometer at 277.6 nm. Adsorption kinetics was carried out to explore influence of contact time on adsorption of sorbents for LC. 10 mg of sorbents were separately dispersed in 10 ml of solutions containing LC concentration of 200 mg L−1 . After adsorption for different hours, the residual concentrations of LC were detected. The amount of LC adsorbed at time t was calculated by a mass balance relationship: Qt =

1000(C0 − Ct )V WM

(1)

where Qt (␮mol g−1 ) is the amount of LC absorbed, C0 (mg L−1 ) is defined as the initial concentration of LC in the solution, Ct (mg L−1 ) represents the remaining concentration in the solution at time t, V is the solution volume (L), W is the sorbents mass (g), and M stands for the molecular weight of LC. In Eq. (1), when Ct is replaced by Ce , Qe is calculated.

2.4. Modification of yeast

2.8. Selective recognition experiments

1.0 g of yeast was dissolved in 30 ml of a mixture of deionized water and ethanol (1:2, v/v). After stirring for 10 min, 5 ml of oleic acid was added and the compound was stirred for 12 h at 50 ◦ C. The product was filtered and washed by ethanol. The modified yeast was dried at 40 ◦ C under vacuum.

To evaluate the selective recognition of MMIMs, FL introduced as structural analog and DEP, as non-analog were compared to LC. In single-component adsorption experiment, 10 mg sorbents were separately added into three kinds of solution each containing 100 mg L−1 LC, FL or DEP. After adsorption for 12 h at 25 ◦ C, the concentration of LC, FL and DEP were measured by UV/vis spectrophotometry at 277.6 nm, 277.5 nm and 254 nm, respectively. Competitive adsorption was also investigated. The binary solutions (LC/FL, LC/DEP, 100 mg L−1 /100 mg L−1 ) took the place of unitary solutions. The residual concentrations of LC were determined by HPLC at 277.6 nm. The injection loop volume was 20 ␮L, and the mobile phase was consisted of methanol and deionized water with a volume ratio of 88:12. The flow rate was 1.0 ml min−1 and the column temperature was 25 ◦ C.

2.5. Synthesis of MMIMs Firstly, the pre-assembly solution was produced: 0.25 mmol of LC, 1.0 mmol of MAA and 1.0 mmol of 4-VP was dissolved in 0.4 ml of toluene by sonication for 5 min. The mixture was kept still in dark place for 6 h. Secondly, the Pickering emulsion was established to prepare MMIMs: 0.2 g of modified yeast was dispersed in 10 ml of deionized water by stirring for 10 min to form the water phase. To prepare the oil phase, 1 ml of EGDMA as the cross-linker and 0.12 ml of AIBME as the initiator were mixed by ultrasonic bath for 10 min with the pre-assembly solution in which 0.025 g of hydrophobic Fe3 O4 nanoparticles was added. The two phases were mixed by violent hand-shaking for 10 min so that the Pickering emulsion was established. Afterwards, the vessel containing emulsion was sealed up and heated to polymerize at 65 ◦ C for 12 h. The product MMIMs were washed by ethanol and distilled water several times and dried naturally. The template molecules were removed by repeatedly washing with a mixture of methanol/acetic acid (95:5, v/v) until no LC was detected in the eluent. On the opposite, the magnetic non-imprinted microspheres (MNIMs) were prepared without the addition of LC. 2.6. Magnetite leakage experiments Magnetite leakage experiments were performed to detect concentration of magnetite likely leaked out from MMIMs in various environments. 10 mg of MMIMs was separately placed in 10 ml of deionized water with different pH (ranging from 2.0 to 8.0). After 24 h at 25 ◦ C, the MMIMs were isolated by external magnetic field and the amounts of magnetite leaked out from MMIMs in the supernatant were determined by an atomic absorption spectrophotometer. 2.7. Static adsorption experiments Static adsorption experiments included adsorption isotherm and adsorption kinetics. In adsorption isotherm studies, 10 mg of sorbents were separately added into 10 ml of solutions of ethanol and deionized water (1:1, v/v) containing various concentrations of

2.9. Regeneration experiments After 10 mg of MMIMs adsorption in 10 ml of 200 mg L−1 LC solution for 12 h at 25 ◦ C, the amount of LC in the supernatant was determined. The MMIMs which adsorbed template were washed by methanol/acetic acid (95:5, v/v) solution until no LC was detected in eluent. The adsorption–desorption process was repeated at least three times. 3. Results and discussion 3.1. Preparation of Pickering emulsion and MMIMs The process of preparation of Pickering emulsion and MMIMs is presented in Scheme 1. Fe3 O4 nanoparticles and yeast were firstly modified by oleic acid. The contact angles of the two materials are shown in Fig. S1. The modified hydrophobic Fe3 O4 nanoparticles with a large contact angle (Fig. S1a, 137◦ ) added in oil phase were regarded as magnetic carrier. Appropriate hydrophobic modification brought yeast greater contact angle (Fig. S1c, 79◦ ) than unmodified one (Fig. S1b, 40◦ ) and made it more easily to be absorbed on the interface of water and oil so that stable Pickering emulsion was established. Afterward, hydrophobic Fe3 O4 nanoparticles were added into the oil phase containing toluene, LC, MAA, 4-VP, EGDMA and AIBME. Modified yeast employed as stabilizer was dispersed in water to form water phase. The photographs of oil and water phase before and after emulsification are displayed in Scheme 1. Before emulsification, black oil phase dispersion containing Fe3 O4 nanoparticles was floated at the top of the tube while white water phase dispersion with yeast was located at the bottom

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Scheme 1. Synthesis Approach of MMIMs by Pickering Emulsion Polymerization.

of the tube. Then, the brown emulsion was easily formed by handshaking without sonication and turned out to be homogeneous, keeping uniform and stable for quite a long time. In the imprinting process, toluene is regarded as a good solvent for monomer and template and is a good pore forming agent that endows the polymer matrix with a well-developed pore structure and high specific surface area, therefore increases the binding capacity of the sorbents [27]. MAA and 4-VP, chosen as functional monomers, were firstly mixed with template LC to establish self-assembled system through the driving force of ␲–␲ stacking interactions and hydrogen bonding. Then, functional monomers MAA and 4-VP and crosslinker EGDMA were initiated by AIBME to carry on free radical polymerization and the stereochemical polymer architecture was built up. After the removal of template molecular, imprinted cavities with recognition sites were left in polymer network which had selective binding capacity for LC. The contact angle of MMIMs after polymerization shown in Fig. S1d was 68◦ which might be attributed to the addition of hydrophilic functional monomer MAA. And hydrophilicity was propitious to made sorbents dispersed well in water phase and easily recognize and absorb LC from natural environment. 3.2. Characterization Graphs of yeast, emulsion and imprinted microspheres before and after polymerization are exhibited in Fig. 1. Fig. 1a shows the optical photograph of yeast in water phase and the SEM image of oleic oil modified yeast (inset). Unmodified yeast particles presented good property of dispersion in water and its average size was about 3.0–4.0 ␮m. After modified, the size of ellipsoid-like yeast did not change significantly but it could be found that yeast surface was covered with oleic acid layer which suggested that yeast was successfully modified by oleic acid. According to optical photograph of droplets of yeast stabilized Pickering emulsion and the TEM image of Fe3 O4 nanoparticles (inset) in Fig. 1b, the o/w emulsion appeared to be homogeneous and uniform, which was attributed to the violent hand-shaking emulsification and the average diameter was about 52.8 ␮m. From single droplet, it was clearly observed that yeast particles arranged tightly on the surface of emulsion droplets between oil and water phase, and there was very small amount of yeast left behind in water phase indicating the appropriate quantity of yeast addition. Black spots in oil phase revealed the congeries

of Fe3 O4 nanoparticles contained in the polymerization system. As seen in TEM image, the average size of Fe3 O4 nanoparticles was about 10 nm and the homogeneous particles presented a spherelike shape. After polymerization, the optical photograph (Fig. 1c) and the SEM image (Fig. 1d) of MMIMs and its closeup surface (inset) are also displayed in Fig. 1. The average particle diameter was about 42 ␮m a little less than that of emulsion droplets above. When the reaction finished, the modified yeast was fixed on polymer layer surface firmly. The appearance of polymer microspheres coated with modified yeast was similar to the shape of emulsion droplets in Fig. 1b, which suggested the process of polymerization was disimpassioned and mild and had little influence on morphology of the resultant. From SEM image and magnification of MMIMs, some PMAA microsphere particles could be observed lain around or on the surface of yeast indicating imprinted polymer was acquired successfully. A CLSM was employed to detect fluorescence property. Yeast was firstly fluorescence labeled with FITC. The fluorescence images are exhibited in Fig. 2. In Fig. 2b, droplets of Pickering emulsion stabilized by yeast labeled with FITC was prepared. Oil droplets could be seen covered by a tight, dense layer of yeast which was consistent with the phenomenon observed in Fig. 2. The photograph of imprinted polymer microsphere after polymerization was shown in Fig. 2c. Descent of fluorescence intensity and blur of yeast appearance could be due to the coated polymer especially PMAA on yeast particles, which also gave evidence for the achievement of polymerization. The FTIR spectra of yeast, modified yeast and MMIMs are displayed in Fig. 3. According to spectrum of yeast in Fig. 3a, the characteristic absorption peaks of the stretching vibration of O H and asymmetric stretching vibration of C H in aliphatic saturated hydrocarbons from yeast were around 3323 cm−1 and 2927 cm−1 , respectively. The peak at 1654 cm−1 was assigned to amide I, mainly resulted from C O stretching vibration. And peaks of amide II and amide III at 1540 cm−1 and 1240 cm−1 were attributed to bending vibration of N H and stretching vibration of C N [28]. These results indicated the presence of protein in yeast particles. Moreover, the strong characteristic absorption peaks of the stretching vibration of P O observed at 1070 cm−1 suggested the presence of the cell membrane in yeast. Compared with the spectrum of yeast, the modified yeast in Fig. 3b had a new characteristic absorption peak of C O in carboxyl from oleic acid at 1708 cm−1 ensured the

W. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 27–36

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Fig. 1. Before polymerization: optical photograph of yeast in water phase (a) (inset: SEM image of oleic oil modified yeast) and droplets of yeast stabilized Pickering emulsion (b) (inset: TEM image of Fe3 O4 nanoparticles). After polymerization: optical photograph (c) and SEM image (d) of MMIMs and its closeup surface (inset).

yeast was successfully hydrophobic modified which also be proved by the decline and slight excursion of absorption peaks of O H, amide and P O. Furthermore, the peaks of C H at 2927 cm−1 and 2860 cm−1 were intensified through encapsulation of yeast by oleic acid. In Fig. 3c, stretching vibration of O H from hydroxyl groups in MAA molecules was founded at 3440 cm−1 . MMIMs showed a remarkable peak at 1731 cm−1 , which was assigned to C O stretching vibration of EGDMA and MAA. The peaks at 1250 cm−1 and 1153 cm−1 suggested the presence of C O symmetric and asymmetric stretching vibration of ester groups in EGDMA [29]. A new peak at 1640 cm−1 could be imputed to C C vibration of EGDMA, suggesting that the bonds of the EGDMA molecules were not 100% cross-linked in the MMIMs [30]. These consequences showed that the MMIMs were successfully produced. Fig. S2 shows the EDX spectrum of the inside of MMIMs and the inset is the XRD patterns of Fe3 O4 nanoparticles (a) and MMIMs

(b). In the EDX spectrum, after Pickering emulsion polymerization, peaks for the elements of C, O and Fe were observed in MMIMs. A large amount of C and O confirmed the success of synthesis of imprinted polymer and the Fe atom suggested the presence of Fe3 O4 nanoparticles in microspheres. In XRD patterns, six characteristic peaks that corresponded to Fe3 O4 were observed in Fe3 O4 nanoparticles and MMIMs, and the peak positions could be indexed to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) (JCPDS card 190629 for Fe3 O4 ). The results indicated the Fe3 O4 nanoparticles were successfully encapsulated in polymer system. The thermo-gravimetric analysis (TGA) graphs of yeast (a), MNIMs (b) and MMIMs (c) are presented in Fig. 4. As seen in Fig. 4, within the initial temperature range (<200 ◦ C), the weight loss was mainly due to the loss of residual water for each particle, which was 7.21%, 5.38% and 4.90% for yeast, MMIMs and MNIMs, respectively. With the temperature increased to 520 ◦ C, the significant weight

Fig. 2. Fluorescence images of yeast labeled with FITC (a), droplets of yeast stabilized Pickering emulsion (b) and MMIMs (c).

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Fig. 3. FTIR spectra of yeast (a), oleic acid modified yeast (b) and MMIMs (c).

losses of yeast (62.71%), MMIMs (93.04%) and MNIMs (79.38%) could be observed which could be ascribed to the loss of carbohydrate in yeast and the polymer in MMIMs and MNIMs. In MMIMs and MNIMs curves, the difference of the weight loss may be due to the template molecules, which resulted in the different degree of polymerization between MMIMs and MNIMs. Above 520 ◦ C, the remaining mass for MMIMs and MNIMs might be attributed to the thermal resistance of the magnetic nanoparticles and residual carbon by calcining polymers. The magnetic hysteresis loop of Fe3 O4 (a) and MMIMs (b) are exhibited in Fig. 5. There was no hysteresis for the two curves, suggesting that both samples were superparamagnetic. The saturation magnetization (Ms ) values of Fe3 O4 and MMIMs were 53.87 and 0.73 emu g−1 , respectively. The magnetic intensity of MMIMs was much lower than Fe3 O4 in respect that the location of magnetic carrier was inside of microsphere. Fortunately, these microspheres still could be separated from solution quickly (within 2 min) in the presence of an external magnetic field (Fig. 5c). According to Fig. 5d, in magnetite leakage studies, the amount of Fe (III) ions leaked away from MMIMs was minimal in the solution with the pH of 4.0–8.0 and then increased at a lower pH. At pH 2.0, the maximum weight of Fe (III) ions leaked away from 10 mg of MMIMs was only about 2.009 ␮g so that the leakage rate was only about 0.02%, demonstrating that MMIMs successfully prevented magnetite leakage owing to the modification by oleic acid of the Fe3 O4 nanoparticles embedded in oil phase. These consequences indicated that the excellent magnetism and effective prevention of magnetite leakage were in

Fig. 5. Magnetic hysteresis loop of Fe3 O4 (a) and MMIMs (b), photographs of MMIMs suspended in water and in the presence of an externally placed magnet (c) and magnetite leakage graph of MMIMs (d).

favor of the reuse of MMIMs and the recognition and separation of LC from water environment at a neutral pH. 3.3. Adsorption isotherm To study LC adsorption performance of the MMIMs and MNIMs sorbents, it is inevitable to establish adsorption equilibrium curves and compare properties such as adsorption capacity of the two sorbents. The binding properties of MMIMs and MNIMs for LC were evaluated by equilibrium adsorption experiments at 25 ◦ C which are illustrated in Fig. 6. Moreover, adsorption equilibrium data of MMIMs and MNIMs fitting to two isotherm models, Langmuir [31] and Freundlich [32] are also shown in Fig. 6. And the nonlinear equations of the two isotherm models are expressed as follows, respectively: Qe =

KL Qm Ce 1 + KL Ce

Qe = KF Ce 1/n

Fig. 4. TGA curves of yeast (a), MNIMs (b) and MMIMs (c).

(2)

(3)

where Qe (␮mol g−1 ) is the equilibrium adsorption capacity, Ce (mg L−1 ) is the equilibrium concentration of LC and Qm (␮mol g−1 ) is the maximum adsorption capacity of the sorbents. KL (L mg−1 ) is the Langmuir adsorption equilibrium constant while KF (␮mg g−1 ) is the Freundlich adsorption equilibrium constant. To predict the favourability of an adsorption system, the Langmuir equation can

W. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 27–36

Fig. 6. Equilibrium data and modeling for the adsorption of LC onto MMIMs and MNIMs.

also be expressed in terms of the affinity constant RL defined as follows [33]: 1 RL = 1 + Cm KL

(4)

where Cm is the maximal initial concentration of adsorbate. The RL indicates the favourability and the capacity of adsorption system. When 0 < RL < 1.0, it represents good adsorption. 1/n is a measure of the exchange intensity or surface heterogeneity, with a value of 1/n smaller than 1.0 describing a favorable removal condition [34]. All calculated values and the values of R2 are listed in Table 1. From the experimental data of MMIMs and MNIMs in Fig. 6, the adsorption capacity increased along with the augment of LC concentration. The maximum adsorption capacity for MMIMs and MNIMs were 57.15 ␮mol g−1 and 36.90 ␮mol g−1 respectively when Ce was in the range of 0–250 mg L−1 . In the same situation, the efficacy of the sorbents for LC in this passage was better than that in our previous work [25,26] owing to the exposure of yeast out of the imprinted microspheres promoting adsorption capacity for pyrethroids. The adsorption capacity of MMIMs was higher than that of MNIMs under same conditions, suggesting the significant preference of MMIMs for LC [35]. As seen in Fig. 6, the Langmuir isotherm model gave a better fit to the experimental adsorption data (R2 > 0.99), indicating a uniform solid surface of the sorbents and the regular monolayer molecular adsorption for LC molecules. From Table 1, the values of RL for MMIMs and MNIMs were 0.5272 and 0.7379 and 1/n were 0.7263 and 0.8847 respectively, suggesting that the experiment conditions were favorable for LC adsorption and separation process of MMIMs was more favorable than that of MNIMs. 3.4. Adsorption kinetics In order to explore the bonding and the rate-controlling mechanism of adsorption process, the adsorption kinetic experiments of MMIMs and MNIMs were conducted at an initial concentration of 200 mg L−1 . The kinetic data analyzed by the pseudo-first-order and pseudo-second-order kinetic models are illustrated in Fig. 7.

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Fig. 7. Kinetic data and modeling for the adsorption of LC onto MMIMs and MNIMs.

The pseudo-first-order [36] and pseudo-second-order [37] kinetic equations are given by Eqs. (4) and (5): Qt = Qe − Qe e−k1 t Qt =

(5)

k2 Qe2 t 1 + k2 Qe t

(6)

where Qt (␮mol g−1 ) and Qe (␮mol g−1 ) are the adsorption amount for LC at time t and at equilibrium, respectively. k1 (min−1 ) and k2 (g ␮mol−1 min−1 ) are rate constants of the pseudo-first-order and pseudo-second-order models. All the calculated results and the values of R2 are listed in Table 2. Based on the pseudo-second-order kinetic rate constant, the initial adsorption rate h (␮mol g−1 min−1 ) and half equilibrium time t1/2 (min) are also involved in Table 2 according to the following equations [38]: h = k2 Qe2 t1/2 =

(7)

1 k2 Qe

(8)

As seen in Fig. 7, the adsorption kinetic data of MMIMs and MNIMs increased fast within the first 60 min, and achieved 93.75% and 85.26% of the equilibrium capacity respectively, finally reached equilibrium within 4–6 h. MMIMs showed a much higher adsorption equilibrium capacity than MNIMs, suggesting the specificity of MMIMs for LC. In terms of adsorption kinetics models, the pseudosecond-order kinetic model (the greater values of R2 ) yielded a better fit for the adsorption of LC onto both MMIMs and MNIMs, and the calculated Qe values (Qe,c ) of pseudo-second-order kinetic equation were also close to the experimental data (Qe,e ). These consequences indicated that the chemical process could be the rate-limiting step during the adsorption for LC [39]. h and t1/2 are usually applied in the measurement of the adsorption rate. The values of h and t1/2 in Table 2 indicated that the MMIMs had excellent kinetic properties. 3.5. Selectivity analysis The selectivity of imprinted and non-imprinted polymer was assessed with structural analog FL and non-analog DEP both in

Table 1 Adsorption equilibrium constants for Langmuir and Freundlich isotherm equations. Sorbents

Langmuir isotherm equation R2

MMIMs MNIMs

0.9925 0.9905

Freundlich isotherm equation

KL (L mg−1 ) −3

4.04 × 10 1.60 × 10−3

Qm (␮mg g−1 )

RL

R2

KF (␮mg g−1 )

1/n

125.00 138.89

0.5272 0.7379

0.9823 0.9774

1.2241 0.3192

0.7263 0.8847

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Table 2 Kinetic constants for the pseudo-first-order equation and pseudo-second-order equation. Sorbents

MMIMs MNIMs a b c d

Pseudo-first-order equation

Pseudo-second-order equation

Qe,e (␮mol g−1 )a

Qe,c (␮mol g−1 )b

k1 (min−1 )

R2

Qe,c (␮mol g−1 )

k2 (g ␮mol−1 min−1 )

h (␮mol g−1 min−1 )c

t1/2 (min)d

R2

48.01 26.70

46.12 25.56

0.1100 0.0519

0.8031 0.8589

47.83 27.02

4.53 × 10−3 3.19 × 10−3

10.3625 2.3292

4.62 11.60

0.9790 0.9556

Qe,e (␮mol g−1 ) is the experimental value of Qe . Qe,c (␮mol g−1 ) is the calculated value of Qe . h (␮mol g−1 min−1 ) is the initial adsorption rate. t1/2 (min) is the half-equilibrium time.

individual and in mixed solutions of the analytes at concentrations of 100 mg L−1 . In unitary experiment, the adsorption capacity of MMIMs and MNIMs for different targets and the structures of three compounds are described in Fig. 8. It could be seen that the MMIMs possessed the highest adsorption value for LC among the three adsorbates. The adsorption behavior of MMIMs and MNIMs for FL was similar to that for LC but its capacity was lower, which suggested that the memory of specific binding sites played an important role in the conformation memory [40]. However, the capacity for DEP indicated there was nearly no selectivity between MIP and NIP observed. These consequences showed that as-prepared sorbents had no specificity for non-analog, and could recognize template and its analog and especially had the most excellent selective binding ability of template LC. In terms of structures, there were N and F atoms in LC and FL molecules which could form hydrogen bonds. However, despite DEP molecule was small enough to get into the imprinted cavity, it had not enough groups to bind with cross-linked polymer which resulted in poor recognition capability. The distribution coefficients (Kd ), selectivity coefficients (k) and relative selectivity coefficient (k ) of FL and DEP with respect to LC can be obtained according to Eqs. (9)–(10).

Table 3 Adsorption selectivity parameters of MMIPs and MNIPs. Comparative compounds

MMIMs Kd (L g−1 )

LC FL DEP

0.1641 0.1101 0.0331

k

MNIMs k

Kd (L g−1 )

k

1.491 4.962

0.1124 0.0794 0.0309

1.415 3.633

1.054 1.366

X is the comparative compound. A relative selectivity coefficient k can be defined as Eq. (10). kM and kN are the selectivity coefficients of MMIMs and MNIMs, respectively. k =

kM kN

(11)

(10)

Values of Kd , k, and k are summarized in Table 3. The k values of MMIMs for FL and DEP were 1.491 and 4.962, respectively, suggesting that the selective recognition for comparative compounds followed the order LC > FL > DEP. k is an indicator to express the adsorption affinity of recognition sites to the template molecules. The k values for two analytes were 1.054 and 1.366, respectively, more than 1.0, indicating the MMIMs had higher absorption selectivity than the MNIMs. Furthermore, the adsorption selectivity for LC was also investigated in binary solution containing LC/FL and LC/DEP, respectively. Fig. 9 exhibits the adsorption capacity of MMIMs and MNIMs for LC. The MMIMs still displayed a high adsorption value for LC in the presence of competitive compounds, indicating that the specific adsorption of MMIMs for LC was not significantly affected by competitive compounds. It could be explained that LC was the best match to the recognition sites of the imprinted cavities which were

Fig. 8. Adsorption capacity of MMIMs and MNIMs for LC, FL, and DEP. Inset: the structures of the test compounds.

Fig. 9. Adsorption selectivity of the MMIMs and MNIMs for LC in dual-solute solutions.

Kd =

Qe Ce

(9)

In Eq. (8), Kd (L g−1 ) represents the distribution coefficient. The selectivity coefficient (k) for binding of a specific compound can be obtained according to the following equation: k=

Kd(TCP) Kd(X)

W. Zhu et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 453 (2014) 27–36

35

2013M530240), Postdoctoral Science Foundation funded Project of Jiangsu Province (no. 1202002B) and Programs of Senior Talent Foundation of Jiangsu University (no. 12JDG090). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.colsurfa. 2014.04.011. References

Fig. 10. Regeneration of MMIMs for three cycles.

not complementary to the competitive molecules. On the contrary, the adsorption value of MNIMs for LC was influenced by other two compounds seriously because of the non-specificity of the binding sites in non-imprinted polymer structure. 3.6. Regeneration analysis The regeneration capability of imprinted microspheres was evaluated by three sequential cycles of adsorption–desorption. As shown in Fig. 10, after three regeneration cycles, the adsorption capacity of MMIMs for LC was about 87.99% of that after the first cycle, which might be attributed to the reduction of binding sites in imprinted polymer matrix after cycles of regeneration [41]. The result indicated that the satisfying regeneration capability of the as-prepared sorbents had potential application in practice. 4. Conclusions In our investigation, the MMIMs were successfully synthesized by Pickering emulsion polymerization and the sorbents were applied to the selective adsorption for LC from aqueous solutions. The adsorption equilibrium and kinetic data matched Langmuir isothermal model and pseudo-second-order kinetics model, respectively. The MMIMs sorbents were proved to possess selective binding of LC and had satisfying regeneration capability. In addition, MMIMs also exhibited some unique features as follows: (1) the microorganism yeast employed in the preparation of the sorbents especially exposed on the surface of MMIMs was propitious to the increase of the adsorption capacity for LC. (2) The addition of MAA as one of functional monomers in polymer structure not only enhanced the specificity of the binding sites, but also improved the hydrophilicity of MMIMs in favor of adsorption for LC from aqueous system. (3) Magnetic carrier embedded in polymer matrix guaranteed the enough magnetic intensity to meet the separation requirement, and prevented magnetite leakage effectively at the same time. The significant adsorption capacity, remarkable selectivity and excellent magnetism provide more opportunities for the development of this novel material based on microorganism with potential application in recognition and separation of environmental contamination. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (nos. 21107037, and 21176107), Natural Science Foundation of Jiangsu Province (nos. BK2011461, and BK2011514), National Postdoctoral Science Foundation (no.

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