Photoresponsive Surface Molecularly Imprinted Poly(ether sulfone

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Photoresponsive Surface Molecularly Imprinted Poly(ether sulfone) Microfibers Dongsheng Wang, Xiaoxue Zhang, Shengqiang Nie, Weifeng Zhao, Yi Lu, Shudong Sun, and Changsheng Zhao* College of Polymer Science and Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, China S Supporting Information *

ABSTRACT: In the present study, photoresponsive surface molecularly imprinted poly(ether sulfone) microfibers are prepared via nitration reaction, the wet-spinning technique, surface nitro reduction reaction, and surface diazotation reaction for the selectively photoregulated uptake and release of 4-hydrobenzoic acid. The prepared molecularly imprinted microfibers show selective binding to 4-HA under irradiation at 450 nm and release under irradiation at 365 nm. The simple, convenient, effective, and productive method for the preparation of azo-containing photoresponsive material is also applied to the modification of polysulfone and poly(ether ether ketone). All three benzene-ring-containing polymers show significant photoresponsibility after the azo modification.

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INTRODUCTION Molecule imprinting is a powerful method to prepare artificial receptors with tailor-made molecular recognition binding sites and thus can be applied in many fields, such as separation,1,2 sensors,3 catalysis,4 water treatment,5 and drug design.6 The molecularly imprinted materials can be easily fabricated by the formation of noncovalent interactions or reversible covalent bonds between template molecules and functional groups.7,8 Furthermore, to improve the imprinted efficiency, the molecularly imprinted system can be formed on the material surface, and the so-called surface molecule imprinting is very attractive for chemical and biological sensing applications.9,10 Most recently, a new photoresponsive molecule imprinting technique has been developed. Some photoresponsive chromophores can be grafted onto the molecularly imprinted materials to provide photoresponsibility to the functional groups, and the photoresponsive molecularly imprinted materials can show selectively photoregulated uptake and release of the template molecules.11,12 Among the photoresponsive chromophores, azobenzene is well-known for its trans−cis photoisomerization under the irradiation of ultraviolet light (λ1 = 365 nm) and blue light (λ2 = 450 nm), and the interconversion between the two photoisomers can induce conformational changes in the polymer chains after linking to macromolecules.13−15 To prepare azo-containing photoresponsive molecularly imprinted materials, azobenzene is grafted onto the functional groups, which can form covalent or noncovalent complexes with the template molecules. In this case, the photoisomerization of the azobenzene can induce conformational changes of the functional groups, followed by changing the recognition ability for © 2012 American Chemical Society

the template molecules of the recognition binding sites; thus, the molecularly imprinted materials show photoregulated uptake and release of the template molecules. Gong et al. and Fang et al. reported photoresponsive molecularly imprinted materials prepared via polymerization of azobenzene-based monomers, and the prepared molecularly imprinted particles showed photoregulated uptake and release of the template molecules caffeine and 2,4-dichlorophenoxyacetic acid under irradiation at 450 and 365 nm, respectively.16,17 Herein, to improve the molecular imprinting efficiency and the UV/vis irradiation efficiency, we decided to prepare photoresponsive molecularly imprinted systems on a material surface. Poly(ether sulfone) (PES) is ideal for the preparation of photoresponsive molecularly imprinted material, since it is convenient and effective in modifying the benzene rings on the PES main chain. The azobenzene structure can be constructed on the PES main chain via nitration reaction, nitro reduction reaction, and diazotation reaction, and the phenyl groups, which are on the PES main chain, can be used as the benzene rings in the formed azobenzene structure. Furthermore, PES shows good hydrolytic stability and mechanical and fiberforming properties.18−20 In the present study, three commercial polymers containing benzene rings on the main chains were investigated, including PES, polysulfone (PSF), and poly(ether ether ketone) (PEEK). PES was investigated in detail as a representative in this work. Received: July 4, 2012 Revised: August 14, 2012 Published: August 15, 2012 13284

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Scheme 1. Preparation of the Photoresponsive Surface Molecularly Imprinted PES Microfibers for the Photoregulated Uptake and Release of 4-HAa

a Key: (a) molecularly imprinted azo-funtionalized PES microfibers are prepared by using 4-HA as the template molecules, and acid−base pairing and hydrogen bonding are the main interactions between 4-HA and azo-functionalized amino groups; (b) after removal of the 4-HA templates by using a 0.13 g/L Na2CO3 solution, photoresponsive surface molecularly imprinted PES microfibers are prepared; (c) 450 nm or visible light irradiation on the microfibers results in the uptake of 4-HA; (c′) 365 nm irradiation on the microfibers results in the release of 4-HA.

5) was used as the solvent and was purchased from Chengdu Kelong Chemical Reagent Co. All the chemical reagents were used without further purification, and double-distilled water was used throughout the study. Preparation of the Photoresponsive Molecularly Imprinted PES Microfibers. PES-NO2 was prepared via the nitration reaction. A typical preparation condition was as follows. HNO3 (30 mL) and H2SO4 (40 mL) were mixed in a 250 mL flask; after cooling, PES (10 g, 0.00025 mol) was added into the flask slowly, and the reaction lasted for 8 h at 65 °C with stirring. Then the product was washed with double-distilled water several times, and then yellow PES-NO2 was obtained. The resulting PES-NO2 was dried in an oven for 24 h and then crushed into a powder for the next step. The prepared PES-NO2 powder and PES were dissolved in DMAC with concentrations of 6 and 10 wt %, respectively. The mixture was stirred at room temperature to get a clear homogeneous solution. After being degassed with a vacuum pump, the polymer solution was extruded into distilled water by using a syringe needle at room temperature to prepare porous microfibers. The injection rate was about 60 cm/min, and the microfibers were continuous long fibers. The prepared microfibers were wrapped around the glass slides.21 All the prepared yellow PES/PES-NO2 microfibers were stored in a water bath for 24 h to remove the residual DMAC for the next step. The third step was the preparation of PES/PES-NH2 microfibers. SnCl2 (80 g, 0.356 mol) was dissolved in HCl solution (80 g, 37%), and then ethanol (200 mL) was poured into the mixture. After the solution was stirred at 65 °C for 15 min, the prepared PES/PES-NO2 microfibers (5−7 reels) were added slightly. The reaction lasted for 4 h at 65 °C. After the nitro reduction reaction, the microfibers changed to white. Then the prepared PES/PES-NH2 microfibers were washed with cold water and saturated in double-distilled water for the next step. Moreover, PES/PES-NH2 microfibers were easily oxidized, even when they were immersed in double-distilled water at room temperature, and the white PES/PES-NH2 microfibers could be

Photoresponsive surface molecularly imprinted PES microfibers (microfibers were prepared to increase the effective surface area and thus to improve the imprinting efficiency) were prepared via nitration reaction, the wet-spinning technique, surface nitro reduction, and surface diazotation reaction (as shown in Scheme 1). Attenuated total reflectance Fourier transform infrared (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) analyses were used to demonstrate the successful fabrication. Simultaneously, the azobenzene structure on the main chains of PSF and PEEK was also demonstrated via Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) (shown in the Supporting Information).

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EXPERIMENTAL SECTION

Materials and Reagents. PES (Ultrason E 6020P, CAS No. 25608-63-3) and PSF (Ultrason S2010, CAS No. 25135-51-7) were purchased from BASF Chemical Co. (Germany). PEEK (450G, CAS No. 31694-16-3) was purchased from Victrex Chemical Co. (United Kingdom). Nitric acid (HNO3; 65−67%, CAS No. 7697-37-2), hydrochloric acid (HCl; 35−37%, CAS No. 7647-01-1), sulfuric acid (H2SO4; 98%, CAS No. 7664-93-9), ethanol (C2H6O; AR, CAS No. 64-17-5), sodium nitrite (NaNO2; AR, CAS No. 7632-00-0), aniline (C6H7N; AR, CAS No. 62-53-3), and salicylic acid (2-HA) (C7H6O3; AR, CAS No. 69-72-7) were purchased from Chengdu Kelong Chemical Reagent Co. 4-Hydroxybenzoic acid (4-HA) (C7H6O3; AR, CAS No. 99-96-7) was used as the template molecule and was purchased from Alfa Aesar. Stannic chloride (SnCl2; AR, CAS No. 10025-69-1) was purchased from Tianjin Chemical Reagent Supply Co. Oxalic acid (C2H2O4 2H2O; AR, CAS No. 6153-61-6), 2-furoic acid (C5H4O3; AR, CAS No. 88-14-2), and benzoic acid (C7H6O2; PT, CAS No. 65-85-0) were purchased from Aladdin Chemistry Co. Ltd. N,N-Dimethylacetamide (DMAC) (C4H9NO; AR, CAS No. 127-1913285

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oxidized to yellow PES/PES-NO2 microfibers after 3−5 days. However, during the nitro reduction reaction, without the protection of nitrogen, the oxygen might not influence the PES/PES-NH2 microfibers a lot, due to the airproof container and excessive reduction reagent. To prepare surface molecularly imprinted PES/PES-N2-NH2 microfibers, 4-HA was used as the template molecule. The prepared PES/PES-NH2 microfibers (5−7 reels) were immersed into HCl solution (300 mL, 18%) at 0−5 °C; then a NaNO2 aqueous solution (100 mL, with a NaNO2 concentration of 0.132 g/mL) was slowly added. The reaction was kept at a temperature between 0 and 5 °C and lasted for 30 min. The microfibers were then washed with cold water three times and then quickly added into the mixture of aniline (30 g, 0.322 mol), 4-HA (0.04 g, 0.0003 mol), and water (150 mL) (pH 5, adjusted with 37% HCl). The reaction lasted for 2 h at 0−5 °C and then stopped until the temperature increased to 40 °C, at which it lasted for 10 h. Then the resulting dark red microfibers were washed three times by using ethanol and three times with double-distilled water. After the washing step, the microfibers were immersed into a 0.13 g/L Na2CO3 aqueous solution for 24 h to remove the 4-HA templates (the Na2CO3 aqueous solution was changed every 8 h). The UV−vis absorbance spectrum for the eluent was used to confirm the complete removal of the 4-HA from the microfibers. As a control, nonimprinted microfibers were prepared by using the same method without adding 4-HA, and the pure PES microfibers were prepared simultaneously. Characterization of the Prepared Microfibers. ATR-FTIR spectra for the PES and modified PES microfibers were collected on an FT-IR Nicolet560 (Nicol, United States). Surface analyses of the microfibers (PES, PES/PES-NO2, PES/PESNH2, PES/PES-N2-NH2) were performed with an XPS instrument (XSAM800, KRATOS, Britain) with Al Kα excitation radiation (1486.6 eV). The measurements were conducted at a takeoff angle of 20°, and the XPS resource was run at a power of 180 W (12 kV × 15 mA). Binding energies were calibrated with the containment carbon (C1s, 284.7 eV). Survey spectra were run in the binding-energy range of 0−1000 eV, and the high-resolution spectra of C1s and N1s were collected. The photoisomerization of the azobenzene chromophores on the PES main chain was examined by using UV/vis absorption spectroscopy (UV-1750, Shimadzu, Japan). The polymer solution with a very low concentration (1.0 mg of PES/PES-N2-NH2 microfibers in 100 mL of DMAC) was irradiated at 365 nm for 30 min and then was examined at a wavelength of 250−500 nm. The cross-section of the prepared microfiber was observed by using a scanning electron microscope (JSM-7500F, JEOL, Japan). After being dried at room temperature, the microfibers were quenched by liquid nitrogenous gas, cut with a single-edged razor blade, attached to the sample supports, and coated with a gold layer. The scanning electron microscope was used for the morphology observation of the microfiber cross-section. ATR-FTIR was used to demonstrate the hydrogen bonds between the template molecule 4-HA and the functional groups on the microfiber surface. The molecularly imprinted microfibers after uptake and release of 4-HA were investigated. Photoregulated Uptake and Release of the Template Molecule 4-HA. Photoregulated uptake and release were studied by binding experiments at room temperature under alternating irradiation at 450 and 365 nm. The surface molecularly imprinted, nonimprinted (the control material), and pure PES microfibers were investigated. The microfibers were cut with an average length of about 7.5 cm, and then the microfibers with a total length of 750 cm were added into 20 mL 4-HA aqueous solutions (the initial concentration of 4-HA was 40 μmol/L). The experiments were performed under irradiation at 450 or 365 nm (30 W UV light) for the photoregulated uptake and/or release of 4-HA. The irradiation was changed every 120 min, and the concentrations were monitored with a UV/vis spectrophotometer every 30 min. The binding amounts of the template molecule 4-HA to the molecularly imprinted, nonimprinted, and pure PES microfibers

were investigated under different irradiations at 450 and 365 nm, as shown in the following section. Binding Experiments for the Template Molecule 4-HA under Irradiation at 450 and 365 nm. The recognition of 4-HA was studied by binding experiments at room temperature under irradiation at 450 and 365 nm. During the experiments, molecularly imprinted, nonimprinted, and pure PES microfibers were investigated. For irradiation at 450 nm, microfibers with a total length of about 750 cm were added into 20 mL 4-HA aqueous solutions with a concentration of 40 μmol/L. The binding experiments were carried out under irradiation at 450 nm, and the concentrations were monitored with a UV/vis spectrophotometer at a wavelength of 249 nm. For the binding experiments under irradiation at 365 nm, the microfibers were pretreated with 365 nm UV light (30 W) for 12 h. Besides this, all the procedures were similar to those at 450 nm irradiation. The binding amount ([S]b) for the target molecule and recognition coefficient (α) could be calculated by using the following equations:22,23

[S]b =

α=

(C0 − Ct )V S

(1)

[S]imprinted [S]nonimprinted

(2)

where C0 and Ct are the template concentrations (μmol/L) in the solutions measured initially and after interval time t, respectively, V is the volume of the bulk solution (L), S (m2) is the surface area of the dry microfibers used, [S]imprinted is the binding amount to the molecularly imprinted microfibers (μmol/m2), and [S]nonimprinted is the binding amount to the nonimprinted microfibers (μmol/m2). Selectivity of the Molecularly Imprinted Microfibers. The selectivity of the imprinted microfibers was also investigated. The molecularly imprinted and nonimprinted microfibers were immersed into 20 mL (40 μmol/L) 4-HA, salicylic acid (2-HA), benzoic acid (BA), 2-furoic acid (2-FA), and oxalic acid (OA) aqueous solutions. The binding experiments were performed under alternating irradiation at 450 and 365 nm, and the irradiation was changed every 120 min. The concentrations were monitored by UV/vis spectrophotometry every 30 min.

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RESULTS AND DISCUSSION ATR-FTIR Analysis. ATR-FTIR was used to investigate the surface compositions of the prepared microfibers, and the spectra for PES, PES/PES-NO2, PES/PES-NH2, and PES/PESN2-NH2 microfibers are shown in Figure 1. As shown in the figure, for the PES/PES-NO2 microfibers, the peak at 1539 cm−1 was attributed to the asymmetric stretching vibration of −NO2, and the peak at 1346 cm−1 was attributed to the symmetric stretching vibration of −NO2. After the surface nitro reduction reaction, the peaks at 1539 and 1346 cm −1 disappeared, and a new peak at 1631 cm−1 appeared, which could be attributed to N−H stretching. There were no peaks between 3200 and 3500 cm−1. The characteristic peaks for the PES/PES-N2-NH2 microfibers were not obvious in the ATRFTIR spectrum. Two small peaks at 1588 and 1474 cm−1 were observed, which could be attributed to NN stretching. It was uncertain whether the azo structure on the PES/PES-N2-NH2 microfiber surface was demonstrated only by the ATR-FTIR spectrum, so more analysis on the microfiber surface should be done. XPS Analysis. To characterize the surface of the prepared microfibers, XPS was employed to examine the surface compositions of the prepared PES, PES/PES-NO2, PES/PESNH2, and PES/PES-N2-NH2 microfibers. There was no significant difference in the XPS spectra for PES, PES/PES13286

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the surface, and more detailed characterization of the surface composition was provided by the high-resolution XPS spectra of the C1s and N1s regions. The C1s XPS spectra for the prepared PES, PES/PES-NO2, PES/PES-NH2, and PES/PES-N2-NH2 microfibers are shown in Figure 2. As shown in Figure 2a, for the PES microfibers, the C1s peak was fitted by two unique carbon moieties: one was for C−C, CC, and C−H (284.76 eV), and the other was for C− O and C−S (286.32 eV).24 However, in Figure 2b−d, for PES/ PES-NO2, PES/PES-NH2, and PES/PES-N2-NH2, respectively, new signals at 287.33, 287.42, and 287.38 eV were observed, which could be attributed to C−N after introduction of −NO2, −NH2, and −N2-NH2 to the microfibers. However, there was no significant difference between the C1s spectra for the surface of the PES/PES-NO2, PES/PES-NH2, and PES/PES-N2-NH2 microfibers. The N1s XPS spectra for the prepared PES/PES-NO2, PES/ PES-NH2, and PES/PES-N2-NH2 microfibers are shown in Figure 3. There was no N1s peak for the PES microfibers; however, two peaks at 405.86 and 401.42 eV were observed for the PES/PES-NO2 microfibers, and the peak at 405.86 eV could be attributed to −NO2 in the prepared PES/PES-NO2 microfibers. For the PES/PES-NH2 microfibers, as shown in Figure 3c, a new peak at 399.69 eV was observed, which could be attributed to −NH2 on the microfiber surface. After the surface diazotation reaction, a peak at 400.23 eV appeared,

Figure 1. ATR-FTIR spectra for the prepared microfibers.

NO2, PES/PES-NH2, and PES/PES-N2-NH2 (shown in the Supporting Information). A small difference was that, in the spectra of PES/PES-NO2, PES/PES-NH2, and PES/PES-N2NH2, the peak for N1s was observed. However, only from the XPS spectra, we could not obtain sufficient information about

Figure 2. XPS spectra of the C1s region for the prepared microfibers: (a) PES, (b) PES/PES-NO2, (c) PES/PES-NH2, and (d) PES/PES-N2-NH2. 13287

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Figure 3. XPS spectra of the N1s region for the prepared microfibers: (a) PES, (b) PES/PES-NO2, (c) PES/PES-NH2, and (d) PES/PES-N2-NH2. Note a: there is no N1s XPS spectrum for the pure PES microfibers.

which could be attributed to NN on the prepared PES/PESN2-NH2 microfiber surface.25 Surface elemental compositions of the prepared microfibers are shown in Table 1. PES microfibers had no nitrogen, which

N1s XPS spectra for the microfibers of PES/PES-NO2, PES/ PES-NH2, and PES/PES-N2-NH2, respectively. The surface contents of −NO2, −NH2, and −NN− of the modified microfibers are shown in Table 2. The content of −NO2 on the

Table 1. Surface Elemental Compositions for the Prepared Microfibers

Table 2. Contents of −NO2, −NH2, and −NN− on the Modified Microfiber Surface

[C] (mol %) PES PES/PES-NO2 PES/PES-NH2 PES/PES-N2NH2

73.49 60.23 63.35 63.62

[N] (mol %)

[S] (mol %)

[O] (mol %)

4.25 4.78 9.40

3.39 6.10 6.53 4.09

23.13 29.43 25.35 22.89

N/C PES/PES-NO2 PES/PES-NH2 PES/PES-N2NH2

0.07 0.07 0.15

[−NO2] (mol %)

[−NH2] (mol %)

[−NN−] (mol %)

42.33a 20.35 30.96

24.92 22.83b

12.02

a The content of −NO2 on the PES/PES-NO2 microfiber surface is calculated by using the N/C molar ratio; in fact, the content of −NO2 is the nitrated yield of the benzene rings on the PES main chain. bThe content of −NH2 on the PES/PES-N2-NH2 microfiber surface is the residual (not azo-modified) −NH2 content on the benzene rings on the PES main chain. The −NH2 grafted on −NN− is not included.

was presented in PES/PES-NO2, PES/PES-NH2, and PES/ PES-N2-NH2 microfibers. The N/C molar ratios were calculated to be 0.07, 0.07, and 0.15 for the PES/PES-NO2, PES/PES-NH2, and PES/PES-N2-NH2 microfibers, respectively. It could be noticed that there was no significant difference between the calculated N/C molar ratios for PES/ PES-NO2 and PES/PES-NH2; however, after the surface diazotation reaction, the N/C molar ratio increased sharply due to the introduction of more nitrogen and −NN−. The contents of −NO2, −NH2, and −NN− on the prepared microfiber surface could be calculated by using the

PES/PES-NO2 microfiber surface was 42.33%, which was lower than the nitrated yield of PES (shown in the Supporting Information), since not all of the PES-NO2 was presented on the microfiber surface.26 After the nitro reduction reaction, the content of −NO2 decreased to 20.35% on the PES/PES-NH2 microfiber surface, and the content of −NH2 was 24.92%, which indicated that more than half of the −NO2 (55.05%) was 13288

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Figure 4. (a) Prepared yellow PES/PES-NO2, white PES/PES-NH2, and dark red PES/PES-N2-NH2 microfibers. (b) UV/vis spectra for the transazo-functionalized (red) and the partially switched (green) PES. A polymer solution with very low concentration (1.0 mg in 100 mL of DMAC) was used. (c, d) Scanning electron micrographs for the cross section of the prepared photoresponsive surface molecularly imprinted microfibers under different magnifications (150× and 3000×, respectively).

reduced to −NH2. The content of −NN− on the PES/PESN2-NH2 microfiber surface was calculated to be 12.02%. UV/Vis Analysis and the Photoisomerization of the Grafted Azobenzene Chromophores. The photoisomerization of the azobenzene chromophores on the PES main chain was examined by using UV/vis absorption spectroscopy. The UV/vis spectra of the azo-functioned PES illustrated lightinduced switching (Figure 4b). It was noticed that the absorbance at 300 nm decreased to 80% after irradiation at 365 nm for 30 min, which indicated that approximately 20% of the trans isomers changed to cis compounds.27 However, the isomerization rate for the azo-modified PES was smaller than those of many azo-contained small molecules, which ranged between 40% and 60%.28 This was caused by the larger steric effect of the azobenzene chromophores grafted onto the PES main chains. After irradiation at 450 nm (visible light), the absorbance at 300 nm increased (shown in the Supporting Information). Furthermore, in the UV/vis spectrum of the trans-azo-modified PES, the absorbances at 400, 300, and 275 nm were attributed to the n−π*, π−π*, and σ−σ* transitions, respectively. Scanning Electron Microscopy (SEM) of the Molecularly Imprinted Microfibers. Scanning electron micrographs of the cross-section of the prepared photoresponsive molecularly imprinted microfiber are shown in Figure 4c,d. As shown in the figure, the diameter of the prepared microfibers was about 250 μm, and the surface area of the prepared microfibers was calculated by using the SEM images directly, which was about 0.0785 cm2 per centimeter of microfibers. Furthermore, the surface area per gram of

microfibers was about 0.0238 m2, which was much larger than that for the same mass of PES membranes or microparticles.29,30 The prepared molecularly imprinted microfibers showed a porous structure: a skin layer was found on the surface of the prepared microfibers, followed by a fingerlike structure, which was regarded as a typical structure for fibers prepared using the liquid−liquid phase separation technique.31 In our former studies, the flat-sheet and hollow fiber membranes were prepared via the liquid−liquid phase separation technique,18−20 and the micropores on the membrane surface were large enough for water molecules passing under pressure; furthermore, some water-soluble polymers (PEG-1000, PEG-2000, and PEG-4000) could also pass the micropores. Due to the porous structure of the prepared microfibers, the inside of the microfibers had the ability to adsorb the template molecules. However, in the present study, the specific recognition sites and the photoresponsive chromophores were mostly distributed on the surface of the molecularly imprinted microfibers; the specific affinity for the template molecules and the photoresponsibility of the molecularly imprinted microfibers existed on the surface. Moreover, the prepared molecularly imprinted, nonimprinted, and pure PES microfibers had about the same surface area, since the skin layer and the porous structure of the microfibers were formed during the liquid−liquid phase separation, which could not be destroyed during the following reactions. Hydrogen Bonds between the Template Molecules and the Functional Groups on the Molecularly 13289

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complex-forming process, since the peak at 1586 cm−1, which was attributed to the phenyl groups, also showed a peak blue shift after uptake. Photoregulated Uptake and Release of the Template Molecule 4-HA. Photoregulated uptake and release were studied by binding experiments at room temperature under alternating irradiation at 365 and 450 nm. The surface molecularly imprinted, nonimprinted (the control material), and pure PES microfibers were investigated, and the results are shown in Figure 6. As shown in the figure, during the first 120 min (0−120 min), under irradiation at 450 nm, about 45.9, 13.2, and 9.0 μmol of 4-HA were bound per square meter of the azofunctionalized molecularly imprinted, nonimprinted, and pure PES microfibers, respectively. During the time between 120 and 240 min, the irradiation at 365 nm caused a steady release of 4-HA for the molecularly imprinted microfibers. A total of about 26.9 μmol of 4-HA (58.7% of the receptor-bound 4-HA) was released from the molecularly imprinted microfibers. However, for the pure PES microfibers, no release of 4-HA was observed. The photoregulated release of the bound 4-HA could be attributed to the photoinduced trans → cis isomerization of the azobenzene chromophores on the surface of the molecularly imprinted microfibers, resulting in a geometry change of the receptors. The host−guest interaction was therefore weakened, and then the bound 4-HA was released. During the time between 240 and 360 min, irradiation at 450 nm caused the uptake of about 23.8 μmol of 4-HA (51.8% of the receptor-bound 4-HA) back onto the molecularly imprinted microfibers, and this photoregulated uptake of 4-HA could be attributed to the photoinduced cis → trans isomerization of the azobenzene chromophores on the surface of the molecularly imprinted microfibers. The uptake−release cycle was investigated for several loops, and no significant attenuation was observed. It is worth pointing out that, for the nonimprinted microfibers, slight photoregulated uptake and release were also observed, but they were in the opposite direction in comparison with those of the molecularly imprinted microfibers.17,32,33 This could also be attributed to the photoinduced

Imprinted Microfiber Surface. The hydrogen bonds between the template molecules and the functional groups on the microfiber surface were demonstrated via the ATR-FTIR spectra for the photoresponsive molecularly imprinted microfibers after uptake and release. Figure 5 shows the spectra of the

Figure 5. ATR-FTIR spectra for the molecularly imprinted microfibers after the processes of uptake and release.

molecularly imprinted microfiber after uptake and release of 4HA. As shown in the figure, after uptake under irradiation at 450 nm, peaks at 1678, 1589, 1482, and 1152 cm−1 were observed; after release under irradiation at 365 nm, the peaks shifted to 1674, 1586, 1476, and 1148 cm−1. After uptake of 4-HA under irradiation at 450 nm, hydrogen bonds between −NH2 on the microfiber surface and −OH or −COOH in 4-HA formed. After release of 4-HA under irradiation at 365 nm, the hydrogen bonds were broken and the 4-HA molecules were released from the microfiber surface; thus, blue shifts of the peaks in the spectrum were observed since the hydrogen bonds were broken.23 Furthermore, the complex of the functional groups and the template molecules might not only be formed via the hydrogen bonds between −NH2 and −OH or −COOH. The π−π stacking effect might also play an important role in the

Figure 6. Photoregulated uptake and release of 4-HA for the azo-functionalized molecularly imprinted (tilted squares, red), nonimprinted (squares, green), and pure (triangles, blue) PES microfibers. The irradiation was changed every 120 min, and the initial 4-HA concentration was 40 μmol/L. 13290

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trans → cis and cis → trans isomerization of the azobenzene chromophores on the microfiber surface. Binding Experiments for the Template Molecule 4-HA under Irradiation at 450 and 365 nm. To investigate the recognition ability for the template molecule 4-HA of the prepared microfibers, binding experiments for the molecularly imprinted, nonimprinted, and pure PES microfibers to 4-HA were performed at room temperature under irradiation at 450 and 365 nm. As shown in Figure 7a, under irradiation at 450 nm, the binding amounts (μmol/m2) increased with time, and the

tively. The calculated recognition coefficient was about 3.5. The results suggested that the adsorption capacity for the molecularly imprinted microfibers was higher than that for the nonimprinted microfibers. The binding of 4-HA to the nonimprinted microfibers was caused by acid−base pairing interaction between 4-HA and the azo-functionalized microfibers and the large porosity of the PES-based microfibers. However, for the molecularly imprinted microfibers, specific adsorption sites formed by the template molecules played an important role in the 4-HA recognition, besides the acid−base pairing interaction and large porosity. The interactions between the templates and substrates were very complex, and in this system, from the molecule structure of 4-HA and azofunctionalized PES, the acid−base pairing and hydrogen bonds were the main interactions between 4-HA and the molecularly imprinted PES microfibers. The results of the recognition experiments under irradiation at 365 nm are shown in Figure 7b. As shown in the figure, the saturated binding amounts were observed after 200 min for all the microfibers and were about 20.9, 16.1, and 9.2 μmol/m2 for the molecularly imprinted, nonimprinted, and pure PES microfibers, respectively. The calculated recognition coefficient was about 1.3. It could be noticed that the molecularly imprinted microfibers had a higher adsorption capacity under 450 nm irradiation than that under irradiation at 365 nm. This could be attributed to the photoinduced trans → cis isomerization of the azobenzene chromophores on the surface of the molecularly imprinted microfibers. As mentioned above, the steady release and uptake of 4-HA for the molecularly imprinted microfibers were caused by irradiation at 365 and 450 nm, respectively; these could be attributed to the different adsorption capacities of the molecularly imprinted microfibers under 365 and 450 nm irradiation. Selectivity of the Molecularly Imprinted Microfibers. To demonstrate the specific affinity of the template molecule 4HA for the molecularly imprinted microfibers, we compared the adsorption of 4-HA, 2-HA, BA, 2-FA, and OA to the molecularly imprinted and nonimprinted PES microfibers, as shown in Figure 8.

Figure 7. Time course of binding to the molecularly imprinted microfibers (tilted squares, red), nonimprinted microfibers (squares, green), and pure PES microfibers (triangles, blue) in 4-HA aqueous solution under irradiation of 450 nm (a) and 365 nm (b).

saturated binding amounts were observed after about 200 min for all the microfibers. The saturated binding amounts of 4-HA were about 46.4, 13.3, and 9.4 μmol/m2 for the molecularly imprinted, nonimprinted, and pure PES microfibers, respec-

Figure 8. Photoregulated uptake and release of 4-HA (blue), 2-HA (green), BA (orange), 2-FA (purple), and OA (red) for the molecularly imprinted microfibers (solid line) and nonimprinted microfibers (dotted line) (drawn on the vice axes). The recognition coefficients for 4-HA, 2HA, BA, 2-FA, and OA after the processes of uptake (solid) and release (hollow) are also shown (drawn on the main axes). 13291

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After uptake under irradiation at 450 nm for 120 min, the calculated recognition coefficient for 4-HA was 3.5, while those for 2-HA, BA, 2-FA, and OA were 1.4, 1.2, 1.2, and 1.0, respectively. After release under irradiation at 365 nm for 120 min, the recognition coefficients for 4-HA, 2-HA, BA, 2-FA, and OA decreased to 1.3, 0.5, 1.1, 1.1, and 1.0, respectively. The recognition coefficient for 4-HA was much larger than those for the analogues after uptake under irradiation at 450 nm, and this was caused by the specific recognition ability for the template molecule 4-HA of the molecularly imprinted microfibers. Moreover, it could be noticed that, for 4-HA, the recognition coefficient decreased sharply after the release process, which indicated that, after release under the irradiation at 365 nm, the molecularly imprinted microfibers lost the specific recognition ability for the template molecules. Furthermore, as shown in the figure, the microfibers (molecularly imprinted and nonimprinted) showed the most obvious photoresponsive uptake and release of 4-HA while slight uptake and release of BA, 2-FA, and OA. These also indicated that the prepared photoresponsive surface molecularly imprinted PES microfibers showed a specific affinity for the template molecule 4-HA.

AUTHOR INFORMATION

Corresponding Author

*Phone: +86-28-85400453. Fax: +86-28-85405402. E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially sponsored by the National Natural Science Foundation of China (Grants 50973070, 51073105, and 51173119), State Education Ministry of China (Doctoral Program for High Education, Grant 20100181110031), and Program for Changjiang Scholars and Innovative Research Team in University (Grant IRT1163)

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ABBREVIATIONS PES poly(ether sulfone) PSF polysulfone PEEK poly(ether ether ketone) 4-HA 4-hydrobenzoic acid ATR-FTIR attenuated total reflectance Fourier transform infrared XPS X-ray photoelectron spectroscopy FTIR Fourier transform infrared NMR nuclear magnetic resonance SEM scanning electron microscopy

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CONCLUSION In this work, photoresponsive molecularly imprinted PES microfibers were prepared for the photoregulated uptake and release of 4-HA via surface azobenzene structure formation. ATR-FTIR and XPS demonstrated the azobenzene structure on the microfiber surface. The photoisomerization of the azobenzene chromophores was demonstrated by using UV/ vis absorption spectroscopy. Binding experiments under alternated irradiation at 450 and 365 nm demonstrated the photoregulated uptake and release of 4-HA for the prepared microfibers: the uptake of 4-HA was observed under 450 nm irradiation, while release was observed under 365 nm irradiation. Recognition experiments indicated that the molecularly imprinted microfibers showed higher recognition ability for 4-HA under irradiation at 450 nm, which highlighted the results of the photoregulated uptake and release experiment. Compared with the adsorption of 4-HA, 2-HA, BA, 2-FA, and OA to the molecularly imprinted and nonimprinted microfibers, the prepared molecularly imprinted microfibers showed specific recognition of the template molecule 4-HA under irradiation at 450 nm, and lost the specific recognition ability under irradiation at 365 nm. Furthermore, the polymers (PSF and PEEK) with molecular structures similar to that of PES were also azo-modified and showed similar photoresponsibilities.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Reagents and materials, preparation of PES-NO2, PSF-NO2, PEEK-NO2, PES-NH2, PSF-NH2, PEEK-NH2, PES-N2-NH2, PSF-N2-NH2, and PEEK-N2-NH2, FTIR and 1H NMR spectra for unmodified and modified PES, PSF, and PEEK, preparation of molecularly imprinted microfibers, nonimprinted microfibers, and pure PES microfibers, XPS spectra for prepared PES and modified microfibers, UV/vis spectra for PES-N2-NH2, PSF-N2-NH2, PEEK-N2-NH2, and target molecules 4-HA, 2HA, BA, 2-FA, and OA, and optical microscopy photograph for the molecularly imprinted microfibers. This material is available free of charge via the Internet at http://pubs.acs.org. 13292

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