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Ultrasonication-assisted synthesis of molecularly imprinted polymer-encapsulated magnetic nanoparticles for rapid and selective removal of 17[beta]-estradiol from aqueous environment.

INTRODUCTION

Low doses of endocrine disrupting compounds (EDCs), pharmaceuticals, and personal care products in water have been reported (1), (2). There is growing concern about the ecological and human health impact of these emerging compounds. EDCs can have adverse effects on the human endocrine system at trace concentration levels and pose a disproportionate threat to fetal development. Their effects on aquatic organisms have been documented both in the laboratory and in the field, and the feminization of male fish has been linked to wastewater treatment plant discharges (3).

Conventional pollution control technologies are limited by poor performance and high costs of existing sorbent materials. Hence, considerable research is directed toward the development of new types of sorbents (4). The past few decades have seen enormous progress in molecular imprinting techniques, including the development of polymer design protocols and their compatibility for use in aqueous environment. Molecularly imprinted polymers (MIPs) have been used in an increasing number of applications where molecular-binding events are of interest. The stability and low cost of MIPs make them advantageous for use in industrial scale production (5), (6). MIPs are attracting more interest in environmental, food, and pharmaceutical applications for the extraction or clean-up of different classes of compounds from various complex matrices. However, in the use of MIP particles as sorbent for the removal of micropollutants, such as EDCs, the separation of polymer particles from aqueous medium appears to be a challenging issue from the enginerring perspective.

In the recent years, magnetic nanoparticles (MNPs) have received considerable attention due to their specific magnetic properties (7). The environmental applications of MNPs are particularly significant. There is growing interest to combine MIPs with MNPs, based on the formation of magnetic responsive core-shell hybrids. Rapid, affordable, and selective removal of target contaminants can be attained by making use of magnetic response of MNPs and tailor-made selectivity of MIPs.

Among the estrogen-imprinted MIPs, 17[beta]-estradiol (E2) has been templated to develop MIPs as synthetic estrogen receptors (8-11).17[alpha]-Estradiol (a natural epimer of E2) (12) and 6-ketoecradiol (a pseudo E2) (13) were also reported for MIPs preparation to target E2. The researches had specific focus on optimizing the formulation of MIPs, including choice of monomers, cross-linkers, and porogens. The establishment of optimal ratio among the polymerization components and rational design of synthetic routes were of interest for the desired morphology, porosity, cavity distribution, and accessibility. In more recent works, inorganic materials--silica (14) and silica-modified magnetic nanoparticles (15)--were introduced for generating core-shell polymeric, particles for surface imprinting to enhance rebinding performance. New synthetic technique such as microwave irradiation has been applied to reduce polymerization time and improve imprinting efficiency (16). In brief, E2-imprinted polymers have been attempted as an attractive alternative to the conventional stationary phase for separation and determination in high-performance liquid chromatography and solid-phase extraction, and a class of promising sorbent for purification, enrichment, and separation. Highly efficient removal of E2 from water was achieved using macroporous cryogel adsorption medium with embedded MIP particles (17) and combination of liquid membranes and MIPs in extraction of E2 from aqueous samples (18). The effect of environmental factors such as humic acid, pH, and the competing substances on the adsorption of typical steroidal and phenolic estrogens by E2-imprinted MIPs was examined in water and wastewater samples (19-21). The adsorption kinetics of MIPs for water treat-ment was also investigated (22).

For potential environmental applications, the particles with captured estrogens must be removed from aqueous matrix after treatment. Ideally, this could be achieved in a simple, quick, and energy-saving manner. If magnetic materials can be incorporated into the imprinted sorbents, magnetic separation can be performed with applied magnetic field. Consequently, a rapid and selective purification and separation process can he enabled. This proposed approach has specific advantages for estrogenic effect reduction from water streams. The magnetic particles will be capable of replacing the conventional separation methods such as coagulation and flocculation in which addition of chemicals is often required. It is also a preferred technique over filtration in a cost-effective way. The regeneration and reuse of the MIP particles (23) will be facilitated. In fact, magnetic separation has been widely used for in-lab template removal and clean-up step instead of centrifugation and the application of magnetic MIPs for the removal of estrone (24) and bisphenol A (BPA) (25) from water samples has been tested in laboratory scale.

In this work, MIP/NIP-encapsulated MNPs (M-MIPs/M-NIPs) were prepared using ultrasonication-assisted synthesis in an organic polymerization medium with high-aqueous content. The particles were characterized with respect to their structure, sizes, and size distribution. The removal rate of the magnetic MIPs in the presence of a magnetic field was determined in a quantitative manner. The molecular recognition and removal efficiency of the particles toward estrogenic compounds, as represented by E2, was compared to the binding performance of MIPs traditionally formulated in organic polymerization media. The proposed protocol for the removal of estrogenic compounds from aqueous environment was verified using tap water samples spiked with E2.

EXPERIMENTAL

Materials

Oleic acid (OA) and iron (II) chloride tetrahydrate ([FeCl.sub.2]*[4H.sub.2]0) were purchased from Sigma-Aldrich (Oak-ville, ON, Canada), and sodium dodecyl sulfate (SDS) was obtained from Pierce (Rockford, IL). 17[beta]-Estradiol (E2), BPA, methacrylic acid (MAA), and ethylene glycol dimethacrylate (EGDMA) were purchased from Aldrich (St. Louis, MO). 2,2'-Azobis (2-methylpropionitrile) (AIBN) was purchased from Pfaltz & Bauer (Waterbury. CT). Iron (III) chloride anhydrous (Fe[Cl.sub.3]) was obtained from Riedel-de Haen (Seelze, Germany). Ammonium hydroxide was supplied by Fisher Scientific (Pittsburgh, PA). All chemicals and reagents were used as purchased without further purification. For liquid chromatography--tandem mass spectrometry (LC-MS/MS) analysis, stock standard solutions of E2 and BPA at 1000 leg/ml were prepared in methanol and stored at -20[degrees]C. Individual standard solutions were prepared at different concentrations in 50% water-methanol solvent diluted from stock solutions.

Preparation of MNPs

Chemical precipitation was used to synthesize [Fe.sub.3][0.sub.4] MNPs by mixing Fe (11) and Fe (111) chlorides under alkaline condition and inert atmosphere. [Fe.sub.3][0.sub.4] (2.54 g) and [FeCl.sub.2]*[4H.sub.2]0 (1.73 g) were dissolved in 80 mL of deionized distilled water (DDW) contained in a three-necked flask with vigorous stirring under helium. About 10 m1, of ammonium hydroxide was added dropwise after temperature reached 80[degrees]C. The resultant [Fe.sub.3][0.sub.4] was maintained for 30 min at 80[degrees]C after the addition of ammonium hydroxide for aging and enhanced magnetization. The MNPs precipitate was collected by applying an external magnetic field and dried in the vacuum after DDW washing.

Preparation of Aqueous Ferrofluid

Aqueous ferrofluid was prepared through surface modification of [Fe.sub.3][0.sub.4] MNPs with OA and SDS surfactants. About 200 mg of [Fe.sub.3][0.sub.4] was dissolved in a volumetric flask-containing 20 mL of DDW and sonicated for 5 min at 60[degrees]C. A total of 139 [micro]L. OA was added in three equal volumes, followed by 5 min sonication after each addition. About 20 mL of DDW containing 134 mg of SDS was introduced. This was followed by 10 min ultrasonication at ambient temperature. The prepared ultrasonica-0A-SDS aqueous ferrofluid was at 5 mg/ml (in [Fe.sub.3][O.sub.4]).

Preparation of Prepolymerization Solution

The prepolymerization solution was prepared in organic solvent with polymer material composition referring to a reported method (11). The template, 68.1 mg of E2, was first added into 10 mL of acetone/acetonitrile (1:3, v/v). The functional monomer, 172 [micro]L of MAA, was then added and well mixed. The mixture was put in dark for 2 h to allow for hydrogen bonding (H-bonding). The cross-linker, 317 [micro]L of EGDMA, was introduced shortly before final polymerization.

Synthesis of M-MIPs

The encapsulation of MNPs with MIPs was performed using ultrasonication-assisted synthesis in a ferrolluid and prepolymerization solution medium (10:10, v/v). About 10 mL of aqueous ferrolluid was suspended into 10 mL of prepolymerization solution followed by 1 min of sonication degassing. The initiator, 11.5 mg of AIBN, was then added. This was followed by 7 min purging with nitrogen gas for deoxygenating. The reaction vial was capped and well-sealed with paratilm. The vial was mounted with self-assembled holder and placed in ultrasonication bath (Bransonic ultrasonicator, 2510R-DTH, 100 W, 42 KHz, USA) at 65[degrees]C for 2-h polymerization. The resultant particles were separated by an externally applied magnetic field and followed by intensive template extraction with acetic acid in methanol (10:90, v/v). DDW and acetonitrile washing were performed before vacuum drying. As a reference, nonimprinted polymer-encapsulated MNPs were prepared in the absence of template using the same synthetic route.

SEM and FTIR Scan

The morphology and structure of the synthesized M-MIPs and M-NIPs were examined by scanning electron microscope (Tescan-Vega II-XMU, Czech Republic). Fourier transform infrared (FTIR) spectroscopy analysis was carried out on a FTIR spectrometer (Varian 1000 FTIR, USA). About 2.0 mg of sample (MNPs or M-MIPs) powder was pulverized and mixed with 20 mg of KBr. The mixture was dried and pressed to form pellets for recording FTIR spectrum.

Magnetic Transferability

Magnetic transferability was examined on an Abraxis 60-tube magnetic separator and UV-vis spectrophotometer (Varian Cary 3, USA). Stock solutions were prepared by dispersing 1.0 mg of MNPs, M-MIPs, and M-NIPs in 10.0mL of DDW with 10-min sonication, from which a series of working solutions ranging from 0.001 to 0.1 mg/ mL were prepared. About 2.0 mL of the standard solutions was transferred to UV-vis cells for measurement. The wavelength was set at 380 nm for setting up calibration curves. About 1.0 mL of 0.1 mg/mL sample was transferred to test tube and placed in magnetic separator for a period ranging from 0.25 to 30 min. The amount of remained particles in solution was measured by UV-vis absorption. All samples ran in duplicates.

Optimization of LC-MS/MS

Analyses were conducted on an AB Sciex API 2000 triple quadrupole tandem mass spectrometer coupled with a Shimadzu Prominence liquid chromatograph using an analytical column (Phenomenex, 50 X 2.00 mm, 4 [micro]m, USA) at room temperature. The flow rate was set at 0.2 mL/min, using water/acetonitrile mobile phase (30:70, v/v) containing 0.1 % ammonia. The mass spectrometry detection was performed using electrospray ionization probe in negative polarity. The source temperature was set to 350[degrees]C. The main MS/MS parameters were optimized by direct infusion of 0.10 pg/mL standard analytes dissolved in water/acetonitrile-containing 0.1% ammonia.

Adsorption Performance

Sorption binding of E2 to M-MIPs was assessed by suspending a known amount of particles in aqueous solution-containing E2 for incubation. After magnetic separation, the supernatant was sampled for LC-MS/MS analysis to determine the residual concentration. M-NIPs were assessed similarly to compare the binding efficiency. About 1.0 mg of M-MIPs was suspended into 2.0 mL of water-containing 0.5 [micro]g/mL of E2. The prepared solution was then sonicated for 10 min to disperse the particles into solution, followed by 30-min shaking. The mixture was evenly transferred to two glass test tubes and placed on a magnetic separator. After 30 min of magnetic separation, the supernatant was transferred to vial for LC-MS/ MS determination. M-NIPs were treated in the same way. Similarly, 1.0 mg M-MIPs was suspended to 2.0 mL of water-containing 0.5 [micro]g/mL of BPA for comparison study.

RESULTS AND DISCUSSION

Synthesis of MIP-Encapsulated MNPs

Preparation of Aqueous Ferrofluid. Figure 1 presents the aqueous ferrotluid prepared from MNPs and surfactants. OA was used as the primary surfactant and SDS as the secondary surfactant for surface modification to create a colloidal suspension. It is known that MNPs tend to form clusters to reduce surface energy. The surfactants work as dispersion agents by adhering to MNPs and creating a net repulsion, raising the energy required for MNPs to agglomerate, and stabilizing the colloid (26).

[FIGURE 1 OMITTED]

As amphiphilic surfactant, OA possesses strong binding capability toward [Fe.sub.3][O.sub.4] through the carboxyl group. In the weakly acidified solution at pH slightly over 5.0, positively charged [Fe.sub.3][O.sub.4] particles attracted OA to form a layer with outward chains. With sonication assistance, OA was able to be dispersed in water and coated on MNPs at 60[degrees]C. However, the resulting [Fe.sub.3][O.sub.4]-OA exhibited a poor dispersion in aqueous phase attributed to the hydrophobic surface of [Fe.sub.3][O.sub.4]-OA. The dispersion worsened with increasing amount of OA. Introduction of SDS significantly improved the suspension of OA-coated particles. SDS consists of a 12-carbon tail attached to a sulfate group. Addition of SDS resulted in a hydrophobic-hydro-philic change by the formation of a SDS outer layer through interactions between the alkyl chains of SDS and the unsaturated OA chains (27). An examination of various OA/SDS molar ratios, ranging from 1:0.1 to 1:1.5, revealed the solubilizing action of SDS on poorly dispersed OA-coated MNPs. At higher OA/SDS ratios (>1: 0.75), aggregation was observed. Aggregation increased with decreased SDS content. At a molar ratio of 1:1, an even-dispersed [Fe.sub.3][O.sub.4]-0A-SDS aqueous magnetic fluid was realized with the formation of surfactant-modified dual layer. The formed colloidal suspension was capable of dispersing 5.0 mg/mL of [Fe.sub.3][O.sub.4] MNPs in water with subsequent introduction of 12.5 mM of OA and SDS under sonication assistance to generate aqueous ferrofluid.

Polymerization in the Presence of Water. E2-imprinted MIPs were originally prepared using MAA/EGDMA in an organic porogen mixture of acetone and acetonitrile (1:3, v/v) (11). To enable the encapsulation of MIPs on MNPs with aqueous ferrofluid, the introduction of water was examined in a water bath without MNPs to investigate the feasibility of polymerization in the presence of water.

The effect of initiator was first examined by adding various amounts of AIBN, from 1 to 10% (in wt % relative to MAA), into a polymerization solution containing 50% water. In an 18-h cycle, MIP particles were formed at all initiator levels above 2%, but 1% initiator was not sufficient to initiate the polymerization. Formation of nuclei during the nucleation stage responded to the increasing initiator concentration although it was not linearly proportional to the initiator concentration owing to the complexity of free radical polymerization. Compared to the polymerization in organic solvents, MIP particles' formation rate was much accelerated. More rapid diffusion and effectiveness in reducing the activation energy of the reaction between radical and polymerization components in water may explain the acceleration (28). At 6.5% of initiator, a range of temperatures, 60-70[degrees]C, appeared to reach the same % conversion but at different formation rates. It showed that polymerization time strongly depends on temperature, as previously reported (29). It was noted that E2-MAA-EGDMA polymerization in the presence of water resulted in the formation of a jelly-like mixture. Addition of surfactants (OA and SDS) or application of magnetic stirring did not provide satisfactory solution. After dryness in vacuum, white flakes were produced, and further grinding was required. It suggests the complexity of polymerization with high-aqueous content.

Ultrasonication-Assisted Synthesis. Ultrasonication was found to be effective in eliminating the formation of jelly-like mixture. It was further used to develop a synthetic route for the preparation of core-shell E2-imprinted magnetic MIP particles, as illustrated in Fig. 2. The ultrasound enhanced the polymerization rate and spherical nanoparticles Ultrasonication caused an increase in the number of free radicals and resulted in the formation of more nuclei, thus facilitating the MIP growth around the MNPs.

[FIGURE 2 OMITTED]

To minimize the formation of core-free MIP nuclei. the optimal ratio between seed MNPs and MIPs was established by changing from 2.0 to 10.0 mg/mL MNPs in the polymerization mixture. At concentrations < 4.0 mg/mL, many core-free MIP particles were produced, as confirmed by checking their magnetic response. At concentrations > 6.0 mg/mL, unencapsulated magnetic particles were abundant. The amount of seed MNPs was optimized at 5.0 mg/mL. The aqueous content had significant effect on MIP encapsulation. At a high-aqueous content (60%), polymerization slowed down due to the dilution of the polymerization components. At a low-aqueous content (40%), a mixture of unencapsulated MNPs, core-free MIP particles, and magnetic MIP particles was produced. A 50% aqueous content-containing 50 mg MNPs enabled the formation of fully MIP-encapsulated MNPs in a 2-h ultrasonication-assisted synthesis route. A parallel study on NIP encapsulation produced similar findings but showed a quicker nucleation and particle growth than MIP encapsulation.

[FIGURE 3 OMITTED]

In comparison with the conventional procedures for the preparation of core-shell magnetic MIPs, the preparation duration with ultrasonication-assisted synthesis was reduced to 2 h. With proper deoxygenation, purging with inert gas was not required through the reaction medium during polymerization. Most importantly, ultrasonication provides a powerful means to evenly disperse magnetic seeds in the polymerization medium, replacing mechanical stirring that is usually required by the conventional methods and microwave irradiation techniques (31), (32).

Characterization

Particle Size and Morphology. A study on the physical features of the formulated particles can produce information important to the characteristics of rebinding. Figure 3 displays the SEM scans of MNPs, MNPs-OA-SDS, M-NIPs, and M-MIPs. It can be seen from Fig. 3a that MNPs have an average diameter of 20 nm. Figure 3b shows the sizes of 0A-SDS-modified MNPs, ranging from 26 to 28 nm. Surface modification did not cause a significant increase in size. An average thickness of 3-4 nm suggests the formation of an OA-SDS surfactants layer. As shown in Fig. 3c and d, the average size of M-NIPs and M-MIPs was at 200 and 300 nm, respectively. It reveals that there was an increase in diameter by 100 nm from M-NIPs to M-MIPs. This can be attributed to the existence of template-shaped cavities created through hydrogen bonding during polymerization.

Figure 3 clearly demonstrates the spherical morphology of M-MIPs and M-NIPs. The formulated particles exhibit fine sizes and narrow size distribution. The employment of ultrasonication dramatically increases the uniforrnity of the resulting particles and facilitates the polymerization around the magnetic seeds, leading to a steady growth to the finished M-MIPs and M-NIPs. It is possible that ultrasonication also promotes an even binding sites population distribution.

Magnetic Transferability. Vibrating sample magnetometry (33), (34) has been commonly used to evaluate parameters of magnetic particles. In this work, a quantitative recycle of M-MIPs and M-NIPs from aqueous medium on a timely basis was studied by combining magnetic separator and UV-vis spectrometer to investigate magnetic transferability (35). UV-vis spectra of MNPs, M-MIPs, and M-NIPs were obtained in a scan range from 800 to 200 nm at 600 nm/min. The optimal scan wavelength for all the particles was 380 nm. Satisfactory linearity was achieved for all the particles in a concentration range of 0.001-0.1 mg/mL. The results enabled the adoption of UV-vis analysis for quantitative measurement of M-MIPs and M-NIPs in water. Magnetic transferability was examined by comparison of concentration before and after magnetic separation with the established calibration curves prepared from known dispersed amounts.

Figure 4 presents the % recovery of the particles versus duration of applied magnetic field (at 0.1 mg/mL). Dynamically, magnetic-based separation could be phased into two stages, a quick stage and a slow stage, characterized as quickness of magnetic response to the magnetic field. In the first stage, the recovery of magnetic particles was fast. The duration of reaching >80% recovery was 5 min, representing a quick magnetically responsive equilibrium between the particles and the applied magnetic source. In the second stage ranging from 5 to 25 min, magnetically responsive equilibrium was slowly reached. In 25 min, >98% recovery was achieved for M-MIPs and M-NIPS, showing satisfactory magnetic transferability. A 30 min of magnetic separation was able to completely recover M-MIPs and M-NIPs from water. Apparently, after surface modification and encapsulation, M-MIPs and M-NIPs remained strong enough magnetic response to allow for effective and rapid magnetic separation to replace the centrifugation process in a convenient and economical way.

[FIGURE 4 OMITTED]

FTIR Spectra. Figure 5a shows the Fourier transform infrared (FTIR) spectrum of [Fe.sub.3][O.sub.4] nanoparticles. The bands in the FTIR spectrum of [Fe.sub.3][O.sub.4] at 586 [cm.sup.-1] correspond to the Fe--O bond of [Fe.sub.3][O.sub.4]. No other peaks with significant intensity were found. After OA coating, Fe-O peak of [Fe.sub.3][O.sub.4] red shifted to 561 [cm.sup.-1]. The vibrations at 2924 and 2854 [cm.sup.-1] are attributed to the alkyl chains. The peak 1425 [cm.sup.-1], resulting from the carboxylate unit vibration, suggests that OA is bound through the carboxylate anion. The peak at 1394 [cm.sup.-1] and weak band around 1045 [cm.sup.-1] are the C--H and S=0 vibration peaks, respectively, representing the SDS presence on the OA-modified MNPs surface. The FTIR spectrum of M-MIPs (Fig. 6) reveals the nature of the bond formed between MNPs and MIPs. The band at 1697 [cm.sup.-1] indicates C=0 stretch vibration of MAA, and 1637 [cm.sup.-1] indicates the C=C bond of MAA and EGDMA. The peak at 1729 [cm.sup.-1] is attributed to the carboxylate vibration modes of EGDMA. These peaks show the presence of functional monomer and cross linker in the synthesized M-MIPs.

[FIGURE 5 OMITTED]

Adsorption Performance

Quantitative Measurement. An analytical method was developed using LC-MS/MS system to obtain a sensitive and selective analysis of the considered analyies. Quantitative MS/MS analysis was applied in multiple reaction monitoring (MRM) to maximize sensitivity. Water/acetonitrile mobile phase was chosen after comparing chromatographic resolution performance between water/methanol and water/acetonitrile. Ammonia was used as additive to promote deprotonation of free estrogenic molecules (36). For E2 and BPA, the precursor ion used for MRM was the corresponding deprotonated molecular ion. The most intense product ion was used for quantification and the second most intense ion as qualifier for confirmation. The obtained calibration curves exhibited good linearity in a wide range of concentrations, 1. 2, 5, 10, 50, 100, 500, and 1000 ng/mL. The E2 residual in the supernatant was measured at the end of rebinding. Binding efficiency was calculated as the ratio of (initial concentration - final concentration)/initial concentration, using the LC-MS/MS data acquired with optimized LC-MS/MS conditions.

Template Leakage. The potential risk of template leakage from M-MIPs was assessed before proceeding to rebinding process for aqueous samples. High purity of water obtained from a Milli-Q system was used for template leakage experiment. The test was carried out at 0.5 mg/mL of M-MIPs using the procedure described in Experimental section. E2 was not detected in water. Because M-MIPs were designed for the removal of estrogenic compounds from aqueous environment, the application of M-MIPs as sorbent must meet the background check, and there will be no further release of template into water to produce a secondary pollution.

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

Adsorption Analysis. The recognition ability of E2-imprinted M-MIPs toward the target compound was investigated and compared to BPA. The binding efficiency between M-MIPs and M-NIPs for E2 and M-MIPs for E2 and BPA are presented in Figs. 7 and 8, respectively.

Figure 7 shows that E2 can be completely removed by M-MIPs, showing 100.0% binding efficiency at the studied concentration. The result suggests that 1.0 mg of M-MIPs was sufficient to quantitatively rebind 1.0 jig E2 from water. M-NIPs were able to remove ~0.9 [micro]g of E2. M-MIPs demonstrated ~10% higher binding capacity toward E2 than M-NIPs. The magnetic hybrids demonstrated improved binding efficiency compared to the MIP submicron particles prepared by precipitation polymerization using same polymer materials in organic solvents, in which 5.0-10.0 mg of polymer was in need to completely remove 1.0 lig of E2 (22). The M-MIPs possessed binding capacity comparable to the MIPs prepared by UV light-induced irradiation with MAA--trimethylolpropane trime-thacrylate composite, a difference of ~10% in adsorption capacity between MIP and NIP particles was obtained at E2 concentration of 2.9 [micro]M (37). Similar binding performance of MIPs over NIPs prepared with 4-vinylpyridine-EGDMA was observed both in aqueous phase and in organic solvent (38).

In this study, the difference in binding capacity between M-MIPs and M-NIPs was assessed by dispersing same amount (1 mg) of M-NIPs or M-MIPs to the experimental solution containing same amount of E2 molecules. M-NIPs have an average size of 200 nm that is 100 nm smaller in size and larger in volume to size ratio than M-MIPs. The greater specific surface area offers more nonspecific-binding sites for M-NIPs. Approximately 10% difference in binding capacity represents multiple factors determined hydrophobic interactions between the sorbents and E2 and may not reflect the actual binding interactions resulting from specific affinity. It can be expected that in complicated matrices such as natural aqueous environment, the predetermined specific interactions will become dominant.

[FIGURE 8 OMITTED]

Taking into account the size effect, the difference in binding performance between E2 and BPA toward E2 tailor-made M-MIPs provides significant binding characteristics with respect to selectivity and specificity. As shown in Fig. 8, M-MIPs demonstrated as high as 44.0% binding efficiency toward E2 than BPA. E2 and BPA both contain two hydroxyl groups (--OH) capable of multiple hydrogen bonding. However, they are distinct from each other in size, mass, and molecular configuration. Despite the smaller molecular mass, BPA exhibited poor binding interactions with E2-imprinted M-MIPs. When BPA was used as interfering or competing compound, same pattern of results was observed (14), (37), (39). The rebinding performance of M-MIPs further suggests the applicability of E2-imprinted M-MIPs for group-specific removal of steroidal estrogens from aqueous environment.
TABLE 1. Recovery of spiked E2 from tap water.

Spiked           Spiked       Detected     Calculated
sample    concentration  concentration   recovery (%)
                (ng/mL)  after binding
                               (ng/mL)

Sample 1          500.0           20.8           95.9
Sample 2          500.0           21.3           95.8


To assess the applicability and practicality, tap water samples were spiked with 0.5 pg/mL of E2. M-MIPs were used as sorbent at a concentration of 0.5 mg/mL for recovery test by the established procedure. Tap water blanks were also analyzed for background check. The calculated % recovery is summarized in Table 1. A satisfactory recovery of >95% was achieved. The obtained results indicated that E2-imprinted M-MIPs were able to produce good recovery and could be applied effectively for the removal of E2 from aqueous environment.

CONCLUSIONS

Estrogen-imprinted MIPs were encapsulated on surfactant-modified magnetic nanoparticles for the generation of core-shell MIPs by ultrasonication-assisted synthesis. The resultant particles, featuring fine sizes and narrow size distribution, possessed strong magnetic transferability. The recognition ability and adsorption capacity of the imprinted layer toward the target molecule were satisfactory. The incorporation of magnetic properties with imprinted effect by ultrasonication process presented a novel approach for the synthesis of magnetic MIPs without significantly compromising the magnetic responsive ability of MNPs and the selectivity of MIPs. The research revealed that it is essential to modify magnetic particles in a compatible medium to facilitate polymer growth around the magnetic cores and promote an even-imprinted sites distribution on shell.

The binding performance suggested the applicability and practicality of the magnetic hybrids in aqueous environments at trace concentrations. The proposed protocol can be used for rapid and selective removal of EDCs in an environmentally friendly separation, which has the potential for water treatment to address the increasing public concerns arising from these emerging micropollutants. The research work may be further extended by incorporating imprinted surfaces for designing MIPs for the various indented applications.

ACKNOWLEDGMENTS

The authors thank Natural Sciences and Engineering Research Council (NSERC) Canada for continuous financial support of the many operating lab facilities at Carleton University.

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Xinlong Xia, (1) Edward P.C. Lai, (1) Banu Ormeci (2)

(1.) Department of Chemistry, Carleton University, Ottawa, Ontario, K1S 586 Canada

(2.) Department of Civil and Environmental Engineering, Carleton University, Ottawa, Ontario, K1S 5B6 Canada

Correspondence to Edward P.C. Lai; e-mail: edwardlai@carleton.ca

Contract grant sponsor: Canadian Water Network as Innovative Treatment Technologies.

DOI 10.1002/pen.23126

Published online in Wiley Online Library (wileyonlinelibrary.com).

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Author:Xia, Xinlong; Lai, Edward P.C.; Ormeci, Banu
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:1CANA
Date:Aug 1, 2012
Words:5405
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