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Article Cite This: ACS Appl. Nano Mater. 2019, 2, 89−99

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SiO2‑Coated Molecularly Imprinted Copolymer Nanostructures for the Adsorption of Bisphenol A Kae-Zheng Chin and Sue-min Chang* Institute of Environmental Engineering, National Chiao Tung University, 1001 University Road, Hsinchu 30068, Taiwan

ACS Appl. Nano Mater. 2019.2:89-99. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/27/19. For personal use only.

S Supporting Information *

ABSTRACT: Molecularly imprinted hybrids (MIHs) were prepared by imprinting bisphenol A (BPA) molecules with linear polystyrene-co-methyl methacrylate chains followed by anchoring SiO2 clusters onto the imprinted organic moieties. While the interior copolymer was responsible for BPA rebinding, the external SiO2 layer confined the conformation of the copolymer, suppressed the nonspecific hydrophobic interactions, and enhanced the water compatibility of copolymer. Styrene and methyl methacrylate (MMA) were used as the dual functional monomers to interact with the BPA templates via π−π stacking interaction and formation of hydrophobic backbones, respectively. The optimal composition molar ratio of BPA/styrene/MMA/3-methacryloxylpropyltrimethoxysilane/tetraethyl orthosilicate with respect to the most satisfied adsorption ability was 1/2/5/5/15. The resulting MIH compound exhibited a superior adsorption performance, including a high imprinting factor of 14, a high adsorption capacity of 40.3 mg/g, a large association constant of 3.8 × 104 M−1, a short equilibrium time of 20 min, and high selectivity factors of 2−24 for BPA target over the other three endocrine-disrupting chemicals according to their sizes and geometries. KEYWORDS: molecular imprinting, organic−inorganic hybrid, BPA adsorption, imprinting factor, selectivity

1. INTRODUCTION Bisphenol A (BPA), a raw material for plastic-related products and variety of surface coating, has been identified as a kind of endocrine disrupting chemical (EDC) which could trigger hormone-related deceases in living organisms. Because of mass production and intensive usage of BPA-derived products, the release of BPA into water medium has been a serious environmental issue.1 BPA compound is difficult to be decomposed by traditional biotreatment. To well control BPA distribution and prevent human exposure to the hazardous compound, development of a highly adsorptive and selective adsorbent for BPA removal has gained a large interest.2−4 Molecular imprinting is a nanomanipulating technique which can create cavities in a polymeric matrix by taking molecules as templates. Because the tailored-made cavities have similar sizes, conformations, and stereochemical features as the template, molecularly imprinted polymers (MIPs) have been demonstrated to show high recognition and adsorption ability for the template compounds.5,6 MIPs basically included organic, inorganic, and hybrid types. Flexibility of organic structures is the important feature that benefits the organically based MIPs to have high adsorption ability. However, flexible structures and hydrophobic interactions between organic substances challenge their recognition ability and selective binding performance at the same time.7,8 In contrast to organic polymers, imprinted oxides have been demonstrated to show a © 2019 American Chemical Society

high imprinting factor and high selectivity to target compounds because of short cross-linking lengths and high cross-linking densities.3,9−12 Nevertheless, the dense and rigid frameworks hinder mass diffusion and thus cause inefficient adsorptions.13 To compromise the advantages and disadvantages of organic and inorganic polymers, the preparation of imprinted hybrids by a combination of the two moieties has gained much interest.14,15 In the early ages, hybrids were just prepared as a kind of organically modified oxide.16−18 In the presence of organic branches in the inorganic matrix, the oxides become more flexible and exhibit a higher affinity for the target compounds. Later, development of coupling agents gives a larger space to adjust the microstructural and physicochemical properties of hybrids.19−21 Surface imprinting by grafting imprinted organic films on inorganic substrates has been employed for hybrid preparation.22−24 Although reduced diffusion length improves adsorption kinetics, limited binding sites of the surface imprinted materials cause low adsorption capacity.25−28 Recently, Lv et al.19 prepared a doxycycline-imprinted hybrid by coupling preformed poly(methacrylic acid) colloids with SiO2 nanoparticles. Through inhibition of swelling of the organic colloids by the SiO2 moieties, this hybrid showed a Received: September 28, 2018 Accepted: January 3, 2019 Published: January 4, 2019 89

DOI: 10.1021/acsanm.8b01706 ACS Appl. Nano Mater. 2019, 2, 89−99

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ACS Applied Nano Materials Scheme 1. Preparation Procedures and Proposed Structure of the SiO2-Coated BPA-Imprinted Hybrid

In this study, a SiO2-coated BPA-imprinted copolymer nanostructure was prepared via free radical polymerization followed by the sol−gel process. Scheme 1 illustrates the preparation concept and the proposed structure for the imprinted hybrid. To prevent formation of nanopores other than imprinted cavities, linear polymer was prepared as the imprinted substrate. Styrene and methyl methacrylate (MMA) were selected as the dual functional monomers to bind the BPA template with π−π interaction and to form the linear chains by linking the monomers, respectively. In contrast to conventional designs in which functional monomers are responsible for both binding the template and forming the polymeric frameworks, the dual monomers with separated functions can improve the selectivity with specific stereointeraction and also create a hydrophobic environmental to enhance the affinity toward the target. To maintain the conformation, enable the water-compatible ability, and reduce the nonspecific bindings of the organic polymer, SiO2 clusters were coated onto the imprinted organic colloids to form a thin layer in the following sol−gel reactions. Under optimal conditions, the hybrid has been demonstrated to exhibit an extraordinarily high imprinting factor (14), high adsorption capacity (40.3 mg/g), a short equilibrium time (20 min), and considerable recognition ability. The influences of each component on the adsorption ability were thoroughly clarified. Moreover, the adsorption character of the hybrid was

high adsorption capacity of 58.2 mg/g for the target and a high selectivity of 4.6−4.9 with respect to the other three analogues. A similar manner was adopted by Clausen et al.29 to fabricate a cholesterol-imprinted hybrid. Such organic-in-inorganic configuration contributed to a maximal target adsorption of 29.51 mg/g and 5.08−6.08 times higher selectivity with respect to 5α-cholestane and 7-dehydrocholesterol. Organic components control the adsorption ability of hybrids mostly. Although the SiO2-coated polymers improved the adsorption performance with a suitable hybrid configuration, the composition of the organic component was less concerning. In a selectivity study of a styrene-containing hybrid, Kim et al.30 found 8.8 times higher adsorption ability of the adsorbent for BPA than for phenol. Duan et al.31 copolymerized hydrophilic 2-acrylamido-2-methylpropanesulfonic acids and hydrophobic styrene monomers to prepare a BPA-imprinted polymer and demonstrated that the sulfonic groups and phenyl groups synergistically improved the adsorption ability for BPA capture. These findings suggest that hydrophobic binding sites and multifunctional groups are beneficial to BPA adsorption. To prepare an advanced imprinted polymer with superiority in adsorption capacity, kinetics, and selectivity, both configuration and chemical compositions have to be taken into consideration. However, the appropriate design for such a hybrid has still not been addressed. 90


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ACS Applied Nano Materials

Figure 1. (a) Adsorption ability of BPA by different adsorbents. Adsorption ability of MIH and NIH on BPA prepared with different (b) MMA/ BPA molar ratios, (c) MPS/BPA molar ratios, (d) TEOS/BPA molar ratios, and (e) recognition ability of MIH on BPA and 2,2′-BPF prepared by different styrene/BPA molar ratios. (methacryloxyl)propyltrimethoxysilane (MPS, 97%, Alfa Aesar), and tetraethyl orthosilicate (TEOS, 98%, Seedchem) were used as the template, major functional monomer, auxiliary functional monomer, coupling agent, and the precursor of SiO2, respectively. First, 1 mmol of BPA and 2 mmol of styrene were added into 5.8 mL of anhydrous ethanol (99.5%, ECHO) under vigorous stirring, and 5 mmol of MMA and 5 mmol of MPS were then added into the mixture in 20 min. Polymerization between the monomers was initiated by addition of 16 mg of potassium persulfate (KPS, 99%, Sigma-Aldrich) into the solution at 70 °C and proceeded for 6 h. After formation of colloids,

examined and insightfully elucidated from thermodynamic points of view.

2. EXPERIMENTAL DETAILS 2.1. Synthesis of Molecularly Imprinted Hybrids (MIHs). MIHs were synthesized by free radical polymerization followed by sol−gel reactions to coat imprinted organic polymers with a thin SiO2 layer. Bisphenol A (BPA, 99%, Tokyo Chemical Industry Co., LTD), styrene (>99%, Sigma-Aldrich), MMA (99%, Alfa Aesar), 391


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ACS Applied Nano Materials 15 mmol of TEOS, 0.9 mL of DI water, and 1 mL of 1 M HCl solution were added into the suspension to grow SiO2 moieties onto the organic colloids by sol−gel reactions. After 2 h of reactions, the colloidal solution was then poured into a crucible, dried at 50 °C, and ground into fine powders. The templates were removed from the hybrid powders with methanol by using a microwave-assisted extraction method. The extraction was performed at 75 °C for 20 min and repeated for six times. For comparisons, nonimprinted hybrids were prepared with the same procedure except for in the absence of BPA templates, and the corresponding samples are called NIHs hereafter. 2.2. Characterizations. Functional groups of MIHs and their interactions with the template were characterized by using a Fouriertransform infrared spectrometer (FTIR, Thermo Scientific Nicolet iS10) equipped with a DTSG detector. Transmitted FTIR spectra were scanned in the range of 4000−500 cm−1 at a step of 1.929 cm−1 for 32 times. Microstructures of hybrids were characterized using a transmission electron microscope (TEM, JEOL, JEM-3000F) operated at an acceleration voltage of 200 kV. The morphology of hybrids was observed using a field-emission scanning electron microscope (FE-SEM, LEO-1530) operated at an acceleration voltage of 5 kV, and the corresponding elemental mapping of the hybrids was performed using an energy-dispersive spectrometer (EDS) equipped in the FE-SEM. Organic and inorganic fractions in the MIHs were determined using a thermogravimetric analyzer (Mettler Toledo TGA/DSC 3+) operated at a heating rate of 10 °C/min under an air flow of 40 mL/min. Nitrogen adsorption−desorption isotherms were measured at 77 K using a gas sorption analyzer (Micromeritics, Tristar 3000). Surface areas of the hybrid powders were estimated using the Brunauer−Emmett−Teller (BET) model according to the adsorption data. 2.3. Adsorption Performance. A BPA stock solution (150 mg/ L) was prepared by dissolving BPA powder in DI water and further diluted to the desired concentrations for following adsorption experiments. Adsorption studies of MIHs and NIHs were performed in batch systems at a fixed temperature of 25 °C. Briefly, 20 mg of powders was dispersed in 12 mL of BPA solutions. After agitation for 2 h, the particles were removed by centrifugation, and the remaining BPA concentrations in the supernatant were measured by using a UV spectrometer (HITACHI 3010) based on the absorbance at 276 nm. The adsorption amount (Q, mg/g) was calculated according to the differences between the initial (Ci, mg/L) and the final concentrations (Cf, mg/L) using the following equation:

Q=

(Ci − Cf )V m

compound. In addition to the isotherms at 298 K, adsorption isotherms at 308 and 318 K were also further measured individually. According to the adsorption results at the different temperatures, the thermodynamic parameters, including ΔG°, ΔH°, and ΔS°, were calculated.

3. RESULTS AND DISCUSSION 3.1. Roles of Each Component of MIHs in the Adsorptions. A SiO2-coated copolymer nanostructure was designed for BPA-imprinted hybrids in this study to create an adsorbent that exhibits superiority in all adsorption capacity, selectivity, and kinetics. To verify the appropriateness of the structural design and chemical selections, the adsorption ability of the adsorbents prepared with different compositions were measured, and the corresponding results are shown in Figure 1. The roles of the SiO2 coating in the adsorption ability of the MIH particles were first identified. While sol−gel-derived SiO2 powders exhibited insignificant BPA adsorption (0.60 mg/g), a bare styrene-co-MMA polymer prepared at the styrene-toMMA molar ratio of 2:5 showed a high BPA adsorption of 4.67 mg/g due to hydrophobic interactions. Such nonspecific interaction could be inhibited by coating the copolymer with a SiO2 layer, and the low adsorption of the SiO2-coated copolymer (1.34 mg/g) supported the hypothesis. Suppression of the nonspecific interactions magnified the imprinting effect on the adsorption ability. As the result, the imprinted hybrid exhibited 19.4 mg/g of BPA adsorption, which was 14 times higher than the adsorption of the NIH powder. According to an adsorption test performed at different pH values, the BPA adsorption of the MIH powder remained similar over pH 2−9 (18.2−19.4 mg/g) even though the surface charge decreased from −0.9 to −41.1 mV with the increasing pH values. This finding reveals that the interior imprinted copolymer dominated the adsorption ability. Moreover, hydrophobic and π−π interactions within the imprinted cavities were the main driving force of the MIH compound for the BPA binding (see the Supporting Information, Figure S1). The hard SiO2 coating not only reduced the nonspecific interactions but also prevented the copolymer from dissolving in the organic solvent during template extraction and assisted the MIH to well disperse in the water medium with the terminal Si−OH groups (see Figure S2). The superiority of the styrene-co-MMA polymer over the polystyrene and polyMMA in the adsorption and recognition ability of the corresponding MIH compounds was further examined. The copolymer led the MIH to exhibit a 1.3−1.4 times higher BPA adsorption (19.4 mg/g) than the other two types of homopolymer (13.8−15.3 mg/g). In addition, with respect to the adsorption of a structural analogue, 2,2′-BPF, the copolymer-based MIH powder exhibited a higher selectivity for BPA (SF = 2.0) compared to the other two types of homopolymer-based powders (SF = 1.2−1.3). The more BPA-compatible microstructure of the imprinted cavities resulting from the π−π stacking interactions of the limited styrene monomers with the template enhanced the adsorption and recognition ability. To optimize the compositions of the hybrids for the highest adsorption and recognition ability, a series of MIHs were prepared by sequentially adjusting the contents of the organic monomer (MMA), the coupling agent (MPS), the inorganic precursor (TEOS), and the template (BPA), and their adsorption ability and those of the corresponding NIHs were measured and compared. Figures 1b−d show adsorption ability of MIHs and NIHs prepared by different chemical

(1)

where V (mL) is the volume of the BPA solutions and m (mg) is the weight of MIH or NIH powders. The imprinting factor (IF, α) was calculated as the ratios between the adsorption amounts of MIHs (QMIH) and those of the corresponding NIHs (QNIH). This factor is an indicative of the imprinting effect on the adsorption performance of the hybrids. The recognition ability of MIHs was evaluated by the selectivity factor (β), which is defined as the adsorption ability of the MIHs for the BPA target (QBPA, mg/g) relative to those of reference EDCs (QEDCs, mg/g). Three endocrine disrupting chemicals, including 2,2′-dihydroxyldiphenylmethane (2,2′-BPF), 17β-estradiol (E2), and phenol, were used as the reference compounds. The concentrations of the 2,2′-BPF, E2, and phenol were measured based on their UV absorbance at the wavelength of 291, 280, and 278 nm, respectively. The adsorption kinetics study was performed by determining the adsorbed BPA amounts at different time intervals with an initial BPA concentration of 50 mg/L, and the corresponding adsorption results were fitted with the pseudo-first-order and pseudo-second-order kinetic models to study the adsorption mechanism. Adsorption isotherms were obtained by measuring the BPA adsorptions at different initial concentrations (10−150 mg/L) at 298 K. The isotherms were fitted with Langmuir and Freundlich models to determine the homogeneity and quantity of the binding sites and also analyzed by Scatchard analysis to identify their affinity for the target 92


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Figure 2. (a) FTIR spectra of as-prepared MIH (green), extracted MIH (red), and NIH (blue) powders. (b) TEM images of the MIH particles; inset is the high magnification of single MIH particle. SEM and EDX mapping images of the imprinted copolymer (c) before SiO2 coating and (d) after SiO2 coating (MIH). (e) N2 adsorption−desorption curves of the MIH (●) and NIH (■) powders. (f) TG analysis of the MIH powder conducted under an oxygen flow of 40 mL/min and a heating rate of 10 °C/min.

respectively. Because the MMA/BPA ratio at 5.0 resulted in the highest IF value (14), this ratio was taken for the following adjustment. MPS was used to covalently link the copolymers to inorganic SiO2 substances. When the BPA/styrene/MMA/TEOS ratio was controlled at 1/2/5/15, increasing the MPS molar ratio

compositions. The molar ratios among the different constituents were defined by taking the BPA amounts as unity. When the BPA/styrene/MPS/TEOS molar ratio was fixed at 1/2/5/15, increasing the MMA/BPA molar ratio from 2.5 to 15 just slightly increased the adsorption ability of the MIHs and NIHs from 18.0 to 20.5 mg/g and from 1.5 to 2.7 mg/g, 93


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3.2. Characterizations of MIH. Figure 2 shows the FTIR spectra, TEM image, SEM mappings, N2-adsorption−desorption curves, and TGA profiles of the optimal MIH and the corresponding NIH particles. All the MIH and NIH compounds contained the CC stretching of aromatics at 1638 cm−1, the CO stretching and the C−O−R of ester groups at 1730 and 1260 cm−1, respectively, the −CH3 bending at 1450 cm−1, and the −CH2 stretching at 2990 cm−1.26,31,33,34 In addition, it can be observed that the Si−O− Si stretching and bending appeared at 1085 and 806 cm−1, respectively, the Si−OH stretching at 950 cm−1, and a broad −OH stretching absorption band at around 3463 cm−1 in the spectra.35 These features confirmed the incorporation of styrene, MMA, MPS, and SiO2 species in the hybrids. In the as-prepared MIH, an additional aromatic CC stretching at 1515 cm−1 was assigned to the phenyl group of BPA molecules.36 The disappearance of this absorption band in the extracted MIH sample verified the successful removal of the template from the imprinted matrix. According to the TEM image (Figure 2b), the primary MIH particles had irregular shapes and particle sizes of ca. 110 nm. These sizes were close to the those (122 nm) measured by the DLS method (Figure S4a). In addition, a smaller particle size (ca. 95 nm) was obtained for the NIH colloids (Figure S4b). Discontinuous morphology and dark spots on the particles suggested different SiO2 nucleation sites and heterogeneous growth of the inorganic clusters around the organic moieties. Aggregation of the copolymer before and after SiO2 coating was observed in the SEM images (see Figure S3a−c). Without specific particle growth control, the aggregates were irregular in shape. The intensive hydrophobic interaction between the copolymer chains led to continuous morphology of the aggregates. In addition, linear growth of the copolymer extruded rods in the aggregates. Introducing SiO2 moieties onto the copolymer hindered the interactions between the copolymer chains, thus resulting in nanoflakes, the aggregates of which contained discontinuous morphology and interparticle porous features. The additional appearance of the Si elements over the MIH sample confirmed the loading of SiO2 moieties onto the copolymers (Figure 2c,d). However, in contrast to even elemental distribution in the copolymer, the C element was less abundant in the central region of a selected MIH particle. This finding indicates heterogeneous incorporation of the MPS coupling agent in the copolymer chains, which resulted in uneven distribution of the SiO2 moieties over the composite. The microtextures of the MIH and NIH were further analyzed with a N2 adsorption−desorption method. Both the MIH and the NIH powders exhibited the type IV N2 adsorption isotherm and type H3 hysteresis loop, indicating slitlike mesopores formed between aggregated plates (Figure 2e). In addition, their unclosed loop reflected the presence of flexible organic substances in the composites.37 These features are in accordance with the flake features observed in the SEM images (Figure S3a). Irrespective of imprinting, linear polymerization retained the similar textures for the composites. Because the microporosity resulting from the imprinting was unavailable in the current measurement, the similar mesoporous textures led the MIH (23 ± 5 m2/g) and NIH powders (18 ± 9 m2/g) to exhibit moderate and close surface areas. The organic and inorganic fractions of the MIH were analyzed by TGA (Figure 2f). Because of desorption of physically adsorbed water molecules, about 3% weight loss was

from 3 to 10 increased the adsorption ability of the corresponding MIHs and NIHs from 17.5 to 20.8 mg/g and from 1.3 to 4.6 mg/g, respectively. Because the increased extent of the NIH-based system was close to that of the MIHbased system, the extra adsorptions of the MIH powder were primarily due to nonspecific interactions. Formation of nanomeshes in between the organic and the inorganic moieties with the increasing content of the coupling agent was considered to be responsible for the increased nonspecific interactions.32 The nonspecific interactions diluted the contribution of imprinting in the adsorptions and thus dramatically decreased the IF value of the MIHs from 13.0 to 14.0 to 4.5 when the MPS/BPA ratio increased from 3 to 5 to 10. To prevent the adverse effect, the MPS/BPA ratio of 5 was selected for the following sample preparations. Rigid SiO2 shells were introduced into the hybrids to maintain the texture of the organic moieties. However, overloadings of the hard moiety would restrict mass diffusion and in turn inhibit the adsorptions. When TEOS/BPA ratios raised from 10 to 20 and the BPA/styrene/MMA/MPS molar ratio was controlled at 1/2/5/5, adsorptions of the corresponding MIHs declined from 18.5 to 19.6 mg/g to 15.2 mg/g, whereas adsorptions of the NIHs were nearly independent of the increased SiO2 content. This result reveals that the SiO2 coating not only suppressed nonspecific matrix interactions but also shielded the affinity of imprinted cavities toward BPA molecules. After compromising the mechanical stability and the shielding effect, the optimal styrene/MMA/ MPS/TEOS molar ratio for the high adsorption was 2/5/5/15. Template addition, which determines the quantity and quality of the imprinted cavities, was the last optimized factor. When increasing the styrene/BPA molar ratio from 1 to 4 by reducing the BPA addition, the MIHs showed decreased BPA adsorption from 23.1 to 16.4 mg/g (Figure 1e). Obviously, more templates introduced into the hybrids contributed to higher adsorptions. The recognition ability of the MIHs was evaluated based on their adsorptions for the BPA compound with respect to those of a structural analogue, 2,2′-BPF. Both compounds contained two phenyl groups arranged in linear but rotated with different angles. The highest selectivity factor (β) of 2.0 was obtained when the styrene/BPA ratio was 2, at which the phenyl groups from the styrene monomers and those of the BPA templates were equal in number. This result suggests that two phenyl groups are required for one imprinted cavity to create the stereospecificity to the BPA template. Below the optimal styrene/BPA ratio, the phenyl groups within the cavities were insufficient to identify the structural difference between the BPA and 2,2′-BPF. Over the optimal ratio, the styrene monomers, which did not complexed with the templates during the imprinting processes, resulted in nonspecific binding sites in the matrix, thus degrading the selectivity toward the target as well. Actually, this deduction was supported by the following two findings. First, the decreased adsorptions were not proportional to the template addition. Second, the adsorption coefficients of the MIHs decreased with increasing styrene/BPA ratios (see the Supporting Information, Table S1). Because the MIH compound prepared with the optimal BPA/styrene/MMA/ MPS/TEOS ratio at 1/2/5/5/15 exhibited the highest IF value (14) and the highest selectivity factor (2.0), its material properties and adsorption behaviors were further characterized to insightfully elucidate the interactions of the MIH with the target compound. 94


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second-order kinetic model. These findings revealed that functional-group-controlled interactions determine the adsorption. Because the rate constant of the MIH-based system (k2 = 2.41 × 10−2 mg/g·min) was lower than that of the NIH-based system (k2 = 7.20 × 10−2 mg/g·min), the target molecules in the imprinted system were driven to diffuse through the SiO2 shell where the molecules experienced a high resistance before they bind with the interior imprinted cavities. On the other hand, in the absence of strong driving force induced by the imprinted cavities, the adsorption in the nonimprinted system just took place on the exterior region, which was rapid but with low capacity. 3.3.2. Adsorption Isotherms. Figure 4 shows BPA adsorption isotherms in the MIH- and NIH-based systems.

observed in the temperature range 50−200 °C.38 Oxidative removal of the organic moieties at 250−600 °C resulted in a weight loss of 37%. It corresponds to 37 wt % of the dried sample and was close to the theoretical organic fraction in the hybrid (47 wt %). A similar phenomenon was also found in the NIH compound (Figure S3e). These results indicate complete reactions among the organic and inorganic monomers in the polymerization and sol−gel processes. 3.3. Adsorption Behaviors of the MIH Powders. 3.3.1. Adsorption Kinetics. Figure 3 shows adsorption kinetic

Figure 3. Adsorption kinetics of BPA by the MIH (●) and NIH (▲) powders. The dashed lines indicate pseudo-first-order model simulation; the solid lines indicate pseudo-second-order model simulation. Figure 4. Adsorption isotherms of MIH (■) and NIH powders (▲). The solid lines indicate Langmuir model simulation; the dashed lines indicate Freundlich model simulation.

curves of the MIH and the corresponding NIH powders. The thin and porous SiO2 shell enabled the MIH powders to rapidly reach an adsorption equilibrium within 20 min. To explore the adsorption mechanism, the time-dependent adsorption was further fitted with the pseudo-first-order and pseudo-second-order kinetic models shown in eqs 2 and 3, respectively.39,40 ln(qe − qt) = ln qe − k1t

(2)

t 1 t = + 2 qt qe k 2qe

(3)

When the initial BPA concentration increased to 150 mg/L, BPA adsorption amounts in the MIH and NIH powders increased to 33.2 and 2.4 mg/g, respectively. Participation of BPA molecules in the imprinting processes remarkably enhanced the adsorption capacity by 14 times. The improvement ascertains the contribution of aligned phenyl groups and customized cavities to the adsorption ability. The adsorption isotherms were further fitted with the Langmuir and the Freundlich model, which are shown in eqs 4 and 5, respectively.41,42

where qe (mg/g) and qt (mg/g) are the adsorbed BPA amounts at equilibrium and at various times (t, min), respectively, and k1 (L/min) and k2 (g/mg·min) are the rate constants of the pseudo-first-order and the pseudo-secondorder kinetics, respectively. With the linear plots of ln(qe − qt) and t/qt against t, the k1, k2, and the corresponding regression coefficients (R2) are obtained. Table 1 lists the kinetic parameters in the MIH- and NIH-based systems. Rather than the pseudo-first-order kinetics, both the imprinted and nonimprinted systems were better fitted with the pseudo-

Ce C 1 = e + qe qm qmKL ln qe =

adsorbents MIH NIH

qe,exp (mg/g) 19.4 1.3

k1 (L/min) −2

5.82 × 10 3.62 × 10−2

R2 0.918 0.622

pseudo-second-order model k2 (g/mg· min) −2

2.41 × 10 7.20 × 10−2

(5)

where Ce (mg/L) is the BPA equilibrium concentration in the solution, qm (mg/g) is the maximum adsorption capacity of the adsorbents, KL (L/g) is the Langmuir constant, and KF and n are the Freundlich constants which are associated with adsorption capacity and adsorption favorability, respectively. According to the linear regression of Ce/qe versus Ce and ln qe versus ln Ce, the qm, KL, KF, and n were estimated. Table 2 lists the fitted data and corresponding regression coefficients. Rather than the Freundlich model, the adsorption of the MIH powders was better fitted with the Langmuir model. This result indicates that most of the binding sites were independent and identical. In contrast, neither the Langmuir nor the Freundlich model can well fit the adsorption in the

Table 1. Kinetic Parameters for the BPA Adsorptions by the MIH and NIH Powders pseudo-first-order model

1 ln Ce + ln KF n

(4)

R2 0.999 0.941 95


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ACS Applied Nano Materials Table 2. Isotherm Parameters for the BPA Adsorption by MIH and NIH Particles Langmuir model

Freundlich model

adsorbents

qe,exp (mg/g)

qm,calc (mg/g)

KL (L/mg)

R2

Kf

n

R2

MIH NIH

33.2 2.4

40.3 3.0

5.1 × 10−2 1.8 × 10−2

0.992 0.619

3.67 0.17

1.932 1.899

0.975 0.739

respectively, for the weak binding sites in the MIH compound. In contrast to the low Kd values in the MIH-based system, the Kd value in the NIH-based system (90.9 mg/L) was 3.3−15.2 times higher. These results again supported that the MIH powders contained a high imprinting degree and insignificant nonspecific binding. 3.3.4. Thermodynamic Adsorption of the MIH Compound. To insightfully understand the adsorption mechanism and corresponding energies, temperature-dependent adsorption isotherms were carried out, and thermodynamic parameters, including the standard free-energy change (ΔG°), the standard enthalpy change (ΔH°), and the standard entropy change (ΔS°), were derived accordingly using the following equations:44

NIH-based system, indicating nonspecific interactions between the powders and the target. From the Langmuir model, the maximum adsorption capacity of the MIH and NIH powders was 40.3 and 3.0 mg/g, respectively. In addition, the adsorption coefficient of the MIH powders (KL = 5.1 × 10−2 L/mg) was 2.8 times higher than that of the NIH powders (KL = 1.8 × 10−2 L/mg). Obviously, imprinting not only created effective binding sites to increase the adsorption capacity but also enhanced the driving force to favor the high adsorption efficiency. 3.3.3. Scatchard Analysis. The affinity of the imprinted and nonimprinted powders toward the target was analyzed by the Scatchard plot, and its equation is as follows:43 Qe Ce

=

Q max − Q e Kd

(6)

ln KL = −

where Qe and Qmax are the equilibrium adsorption amounts and the maximum capacity, respectively, and Kd (mg/L) is the dissociation constant. Figure 5 shows the plot of Qe/Ce against

ΔH ° ΔS° + RT R

(7) (8)

ΔG° = ΔH ° − T ΔS°

where R is the universal gas constant (8.314 J/K·mol), T (K) is the absolute temperature, and KL (L/mol) is the Langmuir constant obtained at various temperatures. Table 3 summarizes Table 3. Thermodynamic Parameters of BPA Adsorption in the MIH Compound T (K) thermodynamic constant

298

308

318

ln KL ΔG° (kJ/mol) ΔH° (kJ/mol) ΔS° (J/mol·K)

9.32 −23.01

9.02 −23.26 −15.5 25.2

8.92 −23.51

the thermodynamic parameters. The negative ΔG° values increased from −23.01 to −23.51 kJ/mol when the temperature increased from 298 to 318 K, indicating a spontaneous and low-temperature-favored adsorption process. In addition, the negative ΔH° (−15.5 kJ/mol) and positive ΔS° (25.2 J/ mol·K) values reveal the high affinity of the MIH compound toward the BPA molecules. These results support hydrophobic interactions as the main driving force for the BPA adsorption. Compared to −NH2 or −COOH functional groups which are mostly adopted to bind the BPA compound,31,40 the hydrophobic interaction was more beneficial for the MIH compound to adsorb the target with the high log Kow value (3.4) because it greatly inhibited water competition. 3.4. Selectivity Tests. The recognition ability of the MIH compound was evaluated by comparing its adsorption ability for BPA over those for other EDCs, including 2,2′-BPF, E2, and phenol. Figure 6 shows the adsorption ability of the MIH and NIH powders for those compounds. The adsorbed amounts of BPA, 2,2′-BPF, E2, and phenol by MIH powders were 19.4, 9.5, 1.4, and 0.7 mg/g, respectively, whereas the nonimprinted powders showed similar adsorptions for all the testing compounds. This phenomenon revealed that the imprinting effect led MIH powders to have different

Figure 5. Scatchard analysis of BPA in the MIH-based system (■). Inset is the plot of the NIH-based system (▲).

Qe in the MIH- and NIH-based systems. Even though the adsorption of the MIH powder was fitted with the Langmuir model, the two types of the binding sites introduced a small deviation from the perfect fitting. On the other hand, van der Waals and weak electrostatic interactions, which were responsible for the adsorption of the NIH powder, were in the same class. Because of large variation of the nonspecific interactions from one site to another, the NIH powder showed poor regressions in both the Scatchard and the Langmuir model fitting. Two distinct slopes, which correspond to strong and weak binding sites, were observed in MIH-based system, whereas there was only one flat slope in the NIH-based system. With the linear plot of Qe/Ce versus Qe, the Kd and Qmax of the MIH- and NIH-based system were estimated for their respective high and low affinity regions (Table S2). According to the slopes and the intercepts of the linear plots, the Kd and Qmax calculated were 6.0 mg/L and 18.9 mg/g, respectively, for the strong binding sites and 27.1 mg/L and 44.6 mg/g, 96


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compound also exhibited an extraordinarily high imprinting factor (α = 14) and high selectivity factors (β = 2−24). These results strongly agreed that the hybrid combination emphasizes the advantages of organic and inorganic moieties in which the MIH can retain a high target affinity and inhibit nonspecific interactions at the same time. Furthermore, the phenyl group in styrene further enhances the recognition ability of MIH with specific planar interactions to identify the BPA according to the rotated angle between their two phenyl groups. Synergistically, the imprinted hybrid exhibits high adsorption ability and selectivity.

4. CONCLUSION A SiO2-coated imprinted copolymer nanostructure has been demonstrated to exhibit high adsorption capacity, efficient adsorption kinetics, and considerable selectivity. The dual monomers, in which one was responsible for template binding and the other one formed the hydrophobic back-bones, played the key roles in the selectivity and adsorption ability. To stabilize the texture of the interior copolymer moieties against solvent dissolution, confining the copolymer moieties with a rigid SiO2 coating was essential in this hybrid-based system. In addition, the hydrophilic oxide shell assisted the hydrophobic copolymers to well disperse in the aqueous solution and reduced the nonspecific hydrophobic interaction, so that the hybrids were able to exhibit high adsorption kinetics and an outstanding imprinting effect. With the optimal BPA/styrene/ MMA/MPS/TEOS molar ratio of 1/2/5/5/15, the imprinted hybrid performed a high imprinting factor of 14, a high adsorption capacity of 40.3 mg/g, a short equilibrium time of 20 min, and high selectivity factors of 2−24 for BPA target over the other three EDCs according to their different sizes and geometries. The results in this study not only validate the advantages of the SiO2-coated copolymer nanostructure in adsorptions but also provide a preparation concept that can be based on to develop other advanced adsorbents.

Figure 6. Selective adsorptions of MIH and NIH compounds for BPA and its structural analogues.

adsorption abilities for different compounds. The selectivity factor (β) for the target with respect to those of 2,2′-BPF, E2, and phenol compounds was 2, 14, and 28, respectively. The high selectivity factors (14−28) for the target over the E2 and phenol compounds revealed that the imprinted cavities exclude the molecules with incompatible sizes and geometries to bind. In contrast, they were less capable of repelling 2,2′-BPF molecules which have a similar size and linear structure as BPA molecules. The binding selectivity of imprinted cavities is often controlled by their conformation and arrangement of functional groups.13,45 In the MIH system, some low quality imprinted cavities with inadequate arrangement of the phenyl groups might accommodate more 2,2′-BPF molecules, and hence this type of analogue is more competitive in binding.2,40 Table 4 summarizes adsorption amounts and parameters of different imprinted compounds for BPA molecules. Compared to the reported data, the MIH compound prepared in this study showed considerably high adsorption capacity (40.3 mg/ g), fast adsorption kinetics (k2 = 2.41 × 10−2 g/mg·min), and strong target affinity (Kd = 6.0 mg/L). In addition, the MIH

Table 4. Adsorption Performance and Related Parameters of Reported BPA-Imprinted Polymers and the MIH Compound adsorbents

Qmax (mg/g)

teqa (min)

k2 (g/mg· min)

KL (L/mg)

AMPS-St/MIP

85.7

60

5.73 × 10−3

5.37 × 10−2

P-MIP Fe3O4-MIP AMPS/MIP

6.2 17.98 5.41

150 40 120

14.6 × 10−2

1.77 × 10−2

NP@MIP

3.94

30

BPA-MIP WC-TMMIP

6.9 8.29

180 450

2.54 × 10−2

1.30

T-MIP

5.03

60

7.7 × 10−3

2.2 × 10−2

61.7

BPA-MIP-3

82.4

30

3.08 × 10−1

2.17 × 10−1

4.8

MI-SBA-15 H-MIP

27.9 40.3

60 20

3.1 × 10−3 2.41 × 10−2

ΔH = 16.9 kJ/mol ΔS = 146 J/mol

14.4 5.1 × 10−2

6.0 18.9

ΔH = −15.5 kJ/mol ΔS = 25.2 J/mol

7.23 × 10−3

Kd (mg/L)

thermodynamic parameters ΔH = −24.06 kJ/mol ΔS = −56.68 J/mol

7.2 × 10−3 15.3 48.5

ΔH = −82.53 kJ/mol ΔS = −238 J/mol ΔH = −8.8 kJ/mol

α

β

2.1

ref 31

4.2 3.9 1.5

BPA/phenol = 3.2 BPA/4,4′-BPFb = 1.7

26 23 40

3.3

BPA/4,4′-BPF = 2.7

43

3.0 3.6

BPA/phenol = 4.6 BPA/phenol = 2.2 BPA/E2 = 2.9 BPA/phenol = 2.4 BPA/E2 = 3.9

46 47

4.1 1.1 3.5 14

48 2

BPA/phenol = 24 BPA/E2 = 11 BPA/2,2′-BPF = 2.0

49 this study

a

Equilibrium time adsorption. bStructural analogue: 4,4′-dihydroxydiphenylmethane. 97


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(10) Yu, D.; Hu, X.; Wei, S.; Wang, Q.; He, C.; Liu, S. Dummy Molecularly Imprinted Mesoporous Silica Prepared by Hybrid Imprinting Method for Solid-Phase Extraction Of Bisphenol A. Journal of Chromatography A 2015, 1396, 17−24. (11) Lee, S.-W.; Ichinose, I.; Kunitake, T. Molecular Imprinting of Azobenzene Carboxylic Acid on a TiO2 Ultrathin Film by the Surface Sol-Gel Process. Langmuir 1998, 14 (10), 2857−2863. (12) Li, T.-J.; Chen, P.-Y.; Nien, P.-C.; Lin, C.-Y.; Vittal, R.; Ling, T.-R.; Ho, K.-C. Preparation of A Novel Molecularly Imprinted Polymer by The Sol−Gel Process for Sensing Creatinine. Anal. Chim. Acta 2012, 711, 83−90. (13) Lofgreen, J. E.; Ozin, G. A. Controlling Morphology and Porosity to Improve Performance of Molecularly Imprinted Sol-Gel Silica. Chem. Soc. Rev. 2014, 43 (3), 911−933. (14) Lin, C. I.; Joseph, A. K.; Chang, C. K.; Wang, Y. C.; Lee, Y. D. Synthesis of Molecular Imprinted Organic−Inorganic Hybrid Polymer Binding Caffeine. Anal. Chim. Acta 2003, 481 (2), 175−180. (15) Fan, H.-T.; Liu, M.-X.; Na, S.-B.; Sun, X.-T. Preparation, Characterization, and Selective Adsorption For Lead(II) of Imprinted Silica-Supported Organic−Inorganic Hybrid Sorbent Functionalized With Chelating S,N-Donor Atoms. Monatsh. Chem. 2015, 146 (3), 459−463. (16) Yan, H.; Wang, M.; Han, Y.; Qiao, F.; Row, K. H. Hybrid Molecularly Imprinted Polymers Synthesized with 3-Aminopropyltriethoxysilane-Methacrylic Acid Monomer for Miniaturized SolidPhase Extraction: A New and Economical Sample Preparation Strategy for Determination of Acyclovir In Urine. J. Chromatogr A 2014, 1346, 16−24. (17) He, H. B.; Dong, C.; Li, B.; Dong, J. P.; Bo, T. Y.; Wang, T. L.; Yu, Q. W.; Feng, Y. Q. Fabrication of Enrofloxacin Imprinted Organic-Inorganic Hybrid Mesoporous Sorbent from Nanomagnetic Polyhedral Oligomeric Silsesquioxanes for the Selective Extraction of Fluoroquinolones in Milk Samples. J. Chromatogr A 2014, 1361, 23− 33. (18) Chang, Y.-S.; Ko, T.-H.; Hsu, T.-J.; Syu, M.-J. Synthesis of an Imprinted Hybrid Organic−Inorganic Polymeric Sol−Gel Matrix Toward the Specific Binding and Isotherm Kinetics Investigation of Creatinine. Anal. Chem. 2009, 81 (6), 2098−2105. (19) Lv, Y.-K.; Zhang, J.-Q.; He, Y.-D.; Zhang, J.; Sun, H.-W. Adsorption-Controlled Preparation of Molecularly Imprinted Hybrid Composites for Selective Extraction of Tetracycline Residues from Honey and Milk. New J. Chem. 2014, 38 (2), 802. (20) Tarley, C. R. T.; Andrade, F. N.; Santana, H. d.; Zaia, D. A. M.; Beijo, L. A.; Segatelli, M. G. Ion-Imprinted Polyvinylimidazole-Silica Hybrid Copolymer for Selective Extraction of Pb(II): Characterization and Metal Adsorption Kinetic and Thermodynamic Studies. React. Funct. Polym. 2012, 72 (1), 83−91. (21) Moraes, J.; Ohno, K.; Maschmeyer, T.; Perrier, S. Synthesis of Silica-Polymer Core-Shell Nanoparticles by Reversible AdditionFragmentation Chain Transfer Polymerization. Chem. Commun. (Cambridge, U. K.) 2013, 49 (80), 9077−88. (22) Mehdinia, A.; Dadkhah, S.; Baradaran Kayyal, T.; Jabbari, A. Design of A Surface-Immobilized 4-Nitrophenol Molecularly Imprinted Polymer Via Pre-Grafting Amino Functional Materials on Magnetic Nanoparticles. J. Chromatogr A 2014, 1364, 12−9. (23) Yuan, Y.; Liu, Y.; Teng, W.; Tan, J.; Liang, Y.; Tang, Y. Preparation of Core-Shell Magnetic Molecular Imprinted Polymer with Binary Monomer for the Fast and Selective Extraction of Bisphenol A From Milk. J. Chromatogr A 2016, 1462, 2−7. (24) Hiratsuka, Y.; Funaya, N.; Matsunaga, H.; Haginaka, J. Preparation of Magnetic Molecularly Imprinted Polymers for Bisphenol A and Its Analogues and Their Application to the Assay of Bisphenol A in River Water. J. Pharm. Biomed. Anal. 2013, 75, 180−5. (25) Anene, A.; Kalfat, R.; Chevalier, Y.; Hbaieb, S. Molecularly Imprinted Polymer-Based Materials as Thin Films On Silica Supports for Efficient Adsorption of Patulin. Colloids Surf., A 2016, 497, 293− 303.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01706.

■

BPA adsorption ability and zeta potentials of the MIH compound in the aqueous phase under different pH values (Figure S1); digital image of styrene-co-MMA polymer and MIH suspensions in DI water (Figure S2); SEM images of styrene-co-MMA polymer (a) before and (b) after coating with SiO2, SEM images of (c) NIH and (d) elemental mapping of NIH particle, and (e) TG analysis of NIH powder (Figure S3); DLS size distributions of (a) MIH and (b) NIH particles in methanol (Figure S4); isotherm parameters of MIH prepared with different styrene/BPA molar ratio (Table S1); Scatchard plot parameters of MIH and NIH (Table S2) (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +886-3-5712121 ext 55506; Fax +886-3-5725958. ORCID

Kae-Zheng Chin: 0000-0003-3853-4975 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology (MOST), Taiwan, for financial support under Grant MOST 106-2628-E-009-005-MY3.

■

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