Composites of Silica and Molecularly Imprinted Polymers for

Nov 7, 2012 - sulfadiazine (SD) was used as a probe to evaluate the effect of coated TMIPs on the PC .... sulfadiazine (SD), and the catalytic activit...
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Article pubs.acs.org/JPCC

Composites of Silica and Molecularly Imprinted Polymers for Degradation of Sulfadiazine Longcheng Xu,† Jianming, Pan,† Qianfang Xia,§ Fenfen Shi,† Jiangdong Dai,† Xiao Wei,† and Yongsheng Yan*,†,‡ †

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang, 212013, P. R. China State Key Laboratory of Natural and Biomimetic Drugs, Peking University, Beijing, 100191, P.R. China § School of Chemical and Material Engineering, Jiangnan University, Wuxi, 214122, P.R. China ‡

S Supporting Information *

ABSTRACT: We report on the fabrication of silica/zinc oxide/zinc sulfide nanoparticles (SiO2/ZnO/ZnS NPs) wrapped with thermoresponsive molecularly imprinted polymers (TMIPs) for photocatalysis (PC) applications. TMIPs were prepared via surface-initiated reversible addition− fragmentation chain transfer (SI-RAFT) polymerization of N-isopropylacrylamide (NIPAm) and ethylene glycol dimethacrylate (EGDMA), rendering the material solution accessible and temperature sensitive. Photodegradation of sulfadiazine (SD) was used as a probe to evaluate the effect of coated TMIPs on the PC performance of SiO2/ZnO/ZnS NPs. The results showed that TMIPs made SiO2/ZnO/ZnS NPs have an outstanding specific affinity PC activity toward template SD. Modification of SiO2/ZnO/ZnS NPs with Ag2S resulted in a tunable PC ability of the prepared material. For SI-RAFT conducted in a controlled manner, a thin layer of polymers (∼100 nm) formed around NPs was measured by a transmission electron microscopy (TEM). Also the polymers were characterized by Fourier transform infrared spectrometer and thermogravimetric analysis. Due to the specific binding of imprinted polymers, thermoresponsiveness of poly-NIPAm shells, and tunable PC ability of NPs cores, the obtained material catalyzed the original template SD with an appreciable selectivity over other structurally related antibiotics and the PC ability could be tunable by changing thte environmental temperature or NPs cores.

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INTRODUCTION Molecular imprinting technique was first proposed by Wulff and Sarhan to obtain molecularly imprinted polymers (MIPs) that were capable of capturing target molecule.1 Because of their high specific selectivity and robustness, MIPs are being increasingly used in some significant application areas (e.g., fluorescence polarization assay,2 chemical sensor,3 recognition and separation,4 drug delivery and controlled release,5 and catalysis6). In the imprinting process, a self-assembled architecture is fabricated for the cross-linking interaction of the template and monomers. However, MIPs prepared by the conventional technique have some disadvantages such as high diffusion barrier, low affinity binding, and low rate mass transfer. In recent years, surface imprinting (2D imprinting) has been used to prepare surface imprinted polymers (SIPs) with better accessibility to specific binding sites.7,8 At present, most SIPs are prepared by radical polymerization; however, the rate of chain propagation cannot be well controlled and polymers generally have a broad size distribution in conventional radical polymerization. Reversible addition−fragmentation chain transfer (RAFT) polymerization is one of the most powerful controlled/“living” radical polymerization (CRP)9,10 techniques because of its relative simplicity and versatility. Its controll© 2012 American Chemical Society

ability relies on the use of a reversible chain transfer agent and the fast and dynamic equilibrium between active radical species and dormant species. Very recently, surface-initiated RAFT (SIRAFT) polymerization was been successfully utilized to synthesize MIPs microspheres.11 Therefore, the binding sites that focused on the surface of supports via SI-RAFT polymerization can make the prepared MIPs capable of recognizing the template molecules more easier. Smart imprinting systems that are at the forefront of MIPs technology have attracted interests in recent years due to their remarkable responsive behaviors.12 A thermosensitive and saltsensitive molecularly imprinted hydrogel for selective protein recognition has been reported, and N-[3-(dimethylamino) propyl] methacrylamide (DMAPMA) and N-isopropylacrylamide (NIPAm) were selected as functional monomers.13 A catalytic active thermosensitive molecularly imprinted polymer based on poly(2-trifluoromethylacrylic acid) (PTFMA) and poly(1-vinylimidazole) (PVI) was prepared by Turner’s group,14 and the imprinted polymer exhibits positively Received: July 5, 2012 Revised: November 1, 2012 Published: November 7, 2012 25309

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efficiently and selectively catalyze the degradation of template sulfadiazine (SD), and the catalytic activity exhibited thermosensitivity, indicating the potential use for smart catalysis strategy. Additionally, MIPs showed good stability, and no obvious reusability deterioration was observed, demonstrating a nice potential use of the MIPs.

thermosensitive molecular recognition and catalysis ability. PNIPAm has been well-known as a thermoresponsive polymer that is more hydrophobic at higher temperatures, exhibiting a lower critical solution temperature (LCST) at ∼32 °C in aqueous solution.15 Thus, the selective molecule recognition ability varies when environmental temperature is changed. Nevertheless, no work on the employment of core templates coated with thermoresponsive molecularly imprinted polymers shells has yet been reported. The controlled fabrication of stimuli-responsive imprinted polymers of two- or threedimensional nanoscale structures containing nanoparticles (NPs) is of great scientific importance for the development of smart MIPs.16 As is known, zinc oxide (ZnO), with a wide band gap of 3.35 eV and a large excitation binding energy (60 meV), is an excellent semiconductor candidate for photocatalysis (PC) applications.17 Especially, ZnO is indeed of particular interest in the photodegradation of organic compounds due to its high surface-to-volume ratio, nontoxicity, and high photonutilizing efficiency.18 However the high degree of photogenerated charge recombination of ZnO impedes its applications. Zinc sulfide (ZnS) (3.66 eV) is a photocatalyst due to the rapid generation of electron−hole pairs by photoexcitation and the highly negative reduction potentials of excited electrons, which is indeed of great importance for PC of various organic substrates.19 However, owing to its relatively wide band gap, ZnS can solely absorb UV light, which accounts for only 4% of the total sunlight, and thus greatly restricts its practical applications. It is highly desirable to develop new photocatalysts with high activities under visible-light illumination. In recent years, Lin et al. have developed an improved photocatalytic activity of ZnO nanocables by surface modification with ZnS.20 This formation was believed to make the system more photosensitive. Ag2S is a kind of fast ion conductor, and Ag cations in Ag2S behave like a “fluid”; therefore, although Ag2S is a stoichiometric compound, there are a lot of cation vacancies in Ag2S NPs.21 This unique feature would enable Ag2S NPs potentially to be an excellent host mediator for expanding semiconductor PC applications. So, the preparation of semiconductor−semiconductor acting as a catalyst for construction of ZnO/ZnS NPs or ZnO/ZnS/Ag2S NPs is quite different from the metal NPs catalyst, and a new avenue is developed for the construction of devices with novel functions and properties. As work in these areas progresses, the need to create new NP systems will be paramount to realize the true potential of semiconductor NPs. Thus, an alternative approach is advocated to create a new selective catalytic generation of smart MIPs. It should be noted that thermoresponsive molecular imprinted polymers (TMIPs) by the facile grafting of poly(ethylene glycol dimethacrylate)-co-poly(N-isopropylacrylamide) (PEGDMAPNIPAm) from SiO2/ZnO/ZnS NPs core templates have not, to the best of our knowledge, been fabricated via SI-RAFT polymerization, which provide the polymers (SiO2/ZnO/ZnS/ MIPs, and MIPs-I) additional advantages such as unique catalytic ability and thermal-sensitive shell. PNIPAm can protect inorganic nanoparticles to prevent their aggregation, and furthermore, its responsiveness allows for tunable adsorption ability. Moreover, in order to investigate the tunable catalytic property of polymers, SiO2/ZnO/ZnS/Ag2S NPs coated with PEGDMA-PNIPAm (SiO2/ZnO/ZnS/Ag2S/ MIPs, MIPs-II) were also fabricated as a comparison. Batch catalysis demonstrated that the type of prepared MIPs could

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EXPERIMENTAL SECTION Materials. Sulfadiazine (SD, 99%), sulfamethazine (SMZ, 99%), sulfamethizol (SMI, 99%), ethylene glycol dimethacrylate (EGDMA, 98%), tetraethyl orthosilicate (TEOS), and phenylmagnesium bromide (20 wt % in tetrahydrofuran (THF) solution) were purchased from Aladdin Reagent Co., Ltd. NIsopropylacrylamide (NIPAm, 97%, Aldrich Reagent Co., Ltd.) was purified by recrystallization from the mixture of benzene and N-hexane (1/3, v/v). Azobisisobutyronitrile (AIBN, AR; Shanghai No. 4 Reagent and H.V. Chemical Co. Ltd., Shanghai, China) was recrystallized from methanol before use. 3Glycidoxypropyltrimethoxysilane (GPTMS, 97%), carbon disulfide, ethanol, methanol, ammonia solution (25.0%28.0%), trolamine (TEA), zinc acetate, thioacetamide (TAA), and silver nitrate were purchased from Sinopharm Reagent Co., Ltd. Preparation of SiO2 NPs. Monodisperse spherical SiO2 nanoparticles were employed as templates, which were prepared by the well-known Stöber method.22 Briefly, 32.5 mL of ethanol, 49.5 mL of water, and 18 mL of ammonia solution were mixed in a 500 mL glass beaker and agitated using a magnetic stirrer (1100 rpm). A mixture of 91 mL of ethanol and 9 mL of TEOS was quickly added to the above solution. Then the mixture was stirred (400 rpm) at room temperature for 2.0 h. The obtained SiO2 NPs were isolated by centrifugation, washed with ethanol, and dried in a vacuum chamber at room temperature overnight. Synthesis of SiO2/ZnO/ZnS NPs and SiO2/ZnO/ZnS/ Ag2S NPs. SiO2/ZnO/ZnS NPs and SiO2/ZnO/ZnS/Ag2S NPs were synthesized according to a prior report23 with some modifications. Typically, 2.0 g of SiO2 NPs were dispersed into a 100 mL ethanol/water (v/v = 2/3) solution and then the mixture was heated to 90 °C. After 10 min, a 20 mL ethanol/ water (v/v = 2/3) solution containing 1.1 or 2.2 g of Zn(Ac)2, 5.32 mL of TEA, and 0.75 g of TAA was dropped simultaneously into the mixture. The system was then continuously stirred for 1.0 h at 90 °C. The resulting powders were centrifugated, washed with ethanol several times, and dried in a vacuum chamber at room temperature overnight. Finally, the powders were sintered in a furnace at 700 °C for 3.0 h. The obtained buff powders were SiO2/ZnO/ZnS NPs. SiO2/ ZnO/ZnS/Ag2S NPs were synthesized in the same method but 0.34 g of AgNO3 were added additionally and the weight of TAA was 0.825 g. Synthesis of 3-Glycidoxypropyltrimethoxysilane Functionalized NPs. A total of 2.0 g of SiO2/ZnO/ZnS NPs (prepared by 1.1 g Zn(Ac)2) or SiO2/ZnO/ZnS/Ag2S NPs (prepared by 1.1 g Zn(Ac)2) and 2 mL of GPTMS were dispersed in a mixture of 150 mL ethanol, 15 mL water and 3.0 mL ammonia solution. The mixture was purged with nitrogen for 5.0 min and then stirred at room temperature for 6.0 h. The GPTMS functioned NPs were isolated by centrifugation, washed with ethanol and dried in a vacuum chamber at room temperature overnight. Synthesis of RAFT Agent. RAFT agent was synthesized by previous reports24 with some modification. Briefly, the 25310

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Scheme 1. Schematic Illustration of the Preparation of SiO2/ZnO/ZnS/MIPs (MIPs-I) and SiO2/ZnO/ZnS/Ag2S/MIPs (MIPsII) via RAFT Polymerization

metric analysis (TGA) was carried out on a STA-449C TGA (Jupiter, Netzsch). X-ray diffractometer (XRD) analysis was recorded on a XRD-D8 X-ray diffractometer (Advance, Bruker) with Cu Kα radiation over the 2θ range of 10−80°. Fluorescence spectra were recorded using QuantaMaster C60/2000 (Photon Technology International, Lawrenceville), and the UV−vis adsorption spectra were carried out on a UV− vis spectrophotometer (UV-2450, Shimadzu, Japan). Batch Catalysis Test. In order to evaluate the photocatalysis abilities of the prepared polymers, we first carried out equilibrium binding experiments using a constant concentration of polymers (1.0 mg mL−1) and varied the concentrations of SD from 0.001 to 0.1 mmol L−1. The binding experiments were kept at 15 and 45 °C in a water bath for 2.0 h to reach adsorption equilibrium. The catalytic properties of these polymer composites were evaluated in a batch format using a thermostatted apparatus irradiated with UV light (λ = 365 nm) or visible light (λ = 500 nm). The initial concentration of SD was 0.1 mmol L−1 (totally 200 mL, pH 7 or 8). The content of the tested polymer composites was 1.0 mg mL−1 in each test. For comparison, the experimental temperature was set 15 and 45 °C. The experimental results of photodegradation of SD were monitored by UV−vis spectra of SD. The self-hydrolysis of SD without polymers was also performed under comparable conditions. The PC efficiency of SD was calculated by the following equation:

phenylmagnesium bromide solution (0.6 M in dry THF (7.25 g), 0.008 mol) was warmed in an oil bath to 40 °C, next carbon disulfide (0.609 g, 0.008 mol) was added over 10 min, and finally the reaction mixture was kept at 40 °C for 1.0 h. SiO2/ ZnO/ZnS-GPTMS (2.0 g) was added to the resultant brown mixture solution, and the reaction was kept at 70 °C for 2 days. Ice hydrochloric acid (1.0 M, 50 mL) was then added. The product was washed with THF, ethanol, and distilled water several times. Then the RAFT agent functionalized SiO2/ZnO/ ZnS NPs (SiO2/ZnO/ZnS-SC(S)Ph) were dried in a vacuum chamber at room temperature overnight. The RAFT agent functionalized SiO2/ZnO/ZnS/Ag2S NPs (SiO2/ZnO/ZnS/ Ag2S-SC(S)Ph) were synthesized by the same method. Preparation of SiO2/ZnO/ZnS/MIPs (MIPs-I) and SiO2/ ZnO/ZnS/Ag2S/MIPs (MIPs-II). The schematic procedures of preparing MIPs-I and MIPs-II were depicted in Scheme 1. In a three neck flask equipped with a magnetic stirring bar, 0.2503 g of SD (1.0 mmol), 0.4570 g of NIPAm (4.0 mmol), 0.37 mL of EGDMA (2.0 mmol), and 1.0 g of RAFT agent (SiO2/ZnSSC(S)Ph) were dispersed in 48 mL of methanol and 12 mL of water. The mixture was sonicated for 10 min to remove the dissolved oxygen. Then 4.0 mg of AIBN was added to the mixture quickly. The polymerization was conducted at 60 °C under the protection of nitrogen for 12 h and the obtained polymers (MIPs-I) were washed with a mixture of methanol/ acetic acid (95:5, v/v) using Soxhlet extraction to remove the template molecules and the unreacted reagents. Finally, the obtained MIPs-I were dried under vacuum at 60 °C overnight. For comparison, SiO2/ZnS/NIPs (NIPs-I) were prepared by using the same synthetic protocol but without the addition of a template molecule SD. Moreover, in the same way, the RAFT agent functionalized SiO2/ZnO/ZnS/Ag2S NPs (SiO2/ZnO/ ZnS/Ag2S-SC(S)Ph) were used to fabricate SiO2/ZnO/ZnS/ Ag2S/MIPs (MIPs-II) and SiO2/ZnO/ZnS/Ag2S/NIPs (NIPsII). Characterization of the Polymers. The morphologies of prepared polymers and substrate materials were observed by a transmission electron microscope (TEM, JEOL, JEM-200CX). Infrared spectra were collected on a NEXUS-470 FT-IR apparatus (4000−400 cm−1; Nicolet, USA). All 1H NMR spectra were recorded using a Bruker 400 MHz spectrometer. BI-200SM (BrookHaven Instruments) was used to determine the size and size distribution of polymers. A Vario EL elemental analyzer (Elementar, Germany) was used to investigate the elemental composition of the prepared materials. Thermogravi-

PC efficiency (%) =

C0 − C A −A 100% = 0 100% C0 A0

(1)

where C0 is the initial concentration of SD, C is the changed concentration of SD, and A0 and A represents the initial and changed absorbance of SD at maximum characteristic absorbance wavelength. Selective Catalysis Studies. In order to estimate the selective PC ability of MIPs-I and MIPs-II, we chose SD, SMZ, and SMI as competitive objects (the structures of SD, SMZ, and SMI were shown in Figure S1). The initial concentration of analytes (SD, SMZ, or SMI) was all 0.1 mmol L−1 (totally 200 mL). The polymer content of the test imprinted polymer was 1.0 mg mL−1 in each test. The final concentrations of analytes were determined by UV−vis spectrophotometer.

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RESULTS AND DISCUSSION Synthesis and Characterization of Polymers. The schematic procedures of preparing of preparation of SiO2/ 25311

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ZnO/ZnS/MIPs (MIPs-I) and SiO2/ZnO/ZnS/Ag2S/MIPs (MIPs-II) were depicted in Scheme 1. The initial silica particles prepared via the well-known Stöber method22 were used as substrate materials. These uniform particles have a mean diameter of about 200 nm (Figure 1a). After coating with

Figure 2. Fourier transform infrared (FT-IR) spectra of bare SiO2 NPs (a), SiO2/ZnO/ZnS NPs (b), RAFT agent functionalized SiO2/ZnO/ ZnS NPs (SiO2/ZnO/ZnS-SC(S)Ph) (c), and SiO2/ZnO/ZnS/MIPs (MIPs-I) (d).

nanoparticles, which may be attributed to the water phase synthesis in the experiment and a similar phenomenon was observed by our a previous report.25 The substrate materials were successfully functionalized with RAFT agent. As shown in Figure 2a, the unmodified SiO2 particles showed the typical stretching of Si−O−Si at 1098 cm−1, symmetric stretching of Si−O−Si at 808 cm−1, and stretching vibrations of Si−OH groups at 946 cm−1.26 In Figure 2b−d, the strong peaks at 460 cm−1 were assigned to the characteristic peak of ZnO.27 In Figure 2c, the peaks at 1495, 1644 (CC bond stretching vibration), and 3431 cm−1 (H− O) indicated that RAFT agent had been grafted on the surfaces of SiO2 particles. The characteristic peaks of SiO2/ZnO/ZnS/ MIPs were as follows: 1729 cm−1 could be attributed to the CO vibration of EGDMA and 2874 cm−1 was from the C−H stretching vibration in the −N(CH3)2 group (Figure 2d). In addition, the coexistence of peaks at 1445, 1644, and 2975 cm−1 in Figure 2d demonstrated the successful PNIPAm-coPEGDMA polymerization.28 We can also know the success of the preparation of RAFT agent from Table 1 and Figure S2.

Figure 1. TEM images of SiO2 NPs (a), SiO2/ZnO/ZnS NPs (1.1 g of Zn(Ac)2) (b), SiO2/ZnO/ZnS NPs (2.2 g of Zn(Ac)2) (c), SiO2/ ZnO/ZnS/Ag2S (1.1 g of Zn(Ac)2) (d), SiO2/ZnO/ZnS/Ag2S (2.2 g of Zn(Ac)2) (e), and SiO2/ZnO/ZnS/MIPs (MIPs-I) (f). The inset in (b) is the edge image of SiO2/ZnO/ZnS NPs at a higher magnification; the inset in (c) is the single SiO2/ZnO/ZnS NPs at a higher magnification; and the inset in (f) is the single MIPs-I at a higher magnification.

Table 1. Elemental Composition of the Materials

ZnO/ZnS layer, the SiO2/ZnO/ZnS NPs were obtained with a layer of 5.0 nm (Figure 1b) (10 nm (Figure 1c)) in thickness. From Figure 1d, some Ag2S NPs attached on the surface of SiO2/ZnO/ZnS NPs could be observed. After modified SiO2/ ZnO/ZnS NPs with GPTMS, we got the RAFT agent by the successful reaction between SiO2/ZnO/ZnS-GPTMS, phenylmagnesium bromide, and carbon disulfide, which could be observed from Figures 2c and 4b. Previously, it was reported that the usage of (4-chloromethylphenyl) silane to form chloromethylphenyl functionalized silica and then reaction with PhC(S)SMgBr to get RAFT agent for polymerization was successful.23 GPTMS is more environmentally friendly than trichloro(4-chloromethylphenyl) silane used in the previous report, and the reaction condition is gentle, making the RAFT agent more useful. Lastly, MIPs-I were fabricated via SI-RAFT polymerization by immersing RAFT agent in a one-pot mixture of ethanol, water, SD, NIPAm and EGDMA. The TEM image of the obtained MIPs-I was shown in Figure 1f. For the reason that the surface-initiated RAFT can be conducted in a controlled manner, a thin layer of polymers (∼ 100 nm) formed around NPs. And what can be observed form inset of Figure 1f that the layer of polymers composed of nanoscale

material type

C (%)

H (%)

SiO2/ZnS SiO2/ZnS/GPTMS RAFT agent-I MIPs-I NIPs-I

0 2.68 5.57 26.11 25.29

0 0.51 0.97 2.55 2.41

From elemental analysis it can be concluded that, compared to bare SiO2/ZnO, the carbon and hydrogen composition in GPTMS modified NPs increased from 0 to 2.68% and 0.51%, respectively. After functioned with RAFT agent, the composition of carbon and hydrogen were increased to 5.57% and 0.97%, respectively, suggesting the successful synthesis of RAFT agent. After polymerization, it could be observed that carbon and hydrogen composition increased significantly. These results could suggest the polymerization of EGDMA and NIPAm on the surface of the support. Figure S2 showed the characteristic peaks of GPTMS and RAFT agent. GPTMS: 1 H NMR (CDCl3, ppm, TMS): 3.4 ppm (a, CH3−O−Si−), 0.5 ppm (b, −Si−CH2−), 1.5 ppm (c, the later −CH2 in −SiCH2−CH2−), 3.3 ppm (d, −CH2−O−), 3.2 ppm and 3.6 ppm (e and e′, −CH2 in −O−CH2−CH−), 3.0 ppm (f, −CH−), 2.6 25312

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Figure 3. Powder X-ray diffraction patterns (XRD) of SiO2/ZnO/ZnS NPs and SiO2/ZnO/ZnS/Ag2S NPs (a) and MIPs-I and MIPs-II (b).

ppm and 2.4 ppm (g and g′, −CH2 in −CH−CH2−O−). RAFT agent: 1H NMR (SOCl2, ppm, TMS): 3.4 ppm (a, CH3−O−Si−), 0.5 ppm (b, −Si−CH2−), 1.5 ppm (c, the later −CH2 in −Si−CH2−CH2−), 3.3 ppm (d, −CH2−O−), 3.6 ppm (e, −CH2 in −O−CH2−CH−), 5.3 ppm (f, −CH in −CH−OH), 2.6 ppm and 2.4 ppm (g and g′, −CH2 in −CH− CH2−S−), 7.5 ppm (h, −CH in −C(S)−CCH−), 7.3 ppm (i, the latter −CH in −CCH−CH), 1.2 ppm (j, the last −CH in −CCH−CHCH−), 8.3 ppm (k, −OH in −CH− OH). The XRD patterns of SiO2/ZnO/ZnS NPs, SiO2/ZnO/ZnS/ Ag2S NPs, MIPs-I and MIPs-II were displayed in Figure 3. The peaks located at 2θ = 31.7°, 34.4°, 36.1°, 47.5°, 56.6°, 62.8°, 67.8°, and 68.9° are indexed to (100), (002), (101), (102), (110), (103), (112), and (201) diffractions of ZnO, respectively (JCPDS No. 36−1451). The peaks located at 2θ = 21.8° and 25.1° are corresponding to the diffraction peaks of ZnS. We could also observe that peaks at 2θ = 28.0°, 38.7°, 48.9°, 53.6°, and 65.4° are corresponding to the diffraction peaks Ag2S (JCPDS No. 14-0072). As shown in Figure 3b, most of characteristic peaks also existed in MIPs-I and MIPs-II, indicating the style of NPs did not change after polymerization and some characteristic peaks mentioned above declined or vanished owing to the encapsulation by the polymer layer. Dynamic layer light scattering studies were carried out to characterize the thermo-modulated changes in sizes and size distributions for MIPs-I, NIPs-I, MIPs-II, and NIPs-II in aqueous dispersion at varying temperatures. Dynamic light scattering (DLS) results indicated the thermosensitivity of the prepared polymers. DLS curves of the thermoresponsive polymers measured at 25 and 45 °C were shown in Figure 4. At 25 °C, the diameter is in the range of 280−400 nm for MIPs-I and 250−400 nm for NIPs-I, with an average value at about 320 and 325 nm, respectively. For MIPs-II and NIPs-II, the diameter is in the range of 300−420 and 260−410 nm and with an average value at 335 and 345 nm, respectively. Upon heating to 45 °C, diameter of MIPs-I and NIPs-I ranges from 270 to 375 and 230−390 nm, with an mean value at 290 and 310 nm, respectively. For MIPs-II and NIPs-II, the diameter is in the range of 285−395 and 247−410 nm and with an average value at 310 and 315 nm, respectively. The decrease of composites size at elevated can be ascribed to the collapse of polymers upon heating. From TGA results (Figure 5a) what can be observed that stability of SiO2/ZnO/ZnS NPs is good and there are rarely weight loss at text temperatures. TGA revealed ∼5.1 wt % difference in weight retentions at 500 °C between SiO2/ZnO/

Figure 4. Size distributions obtained at 25 and 45 °C for 8.0 × 10−2 g L−1 of MIPs-I (a), NIPs-I (b), MIPs-II (c), and NIPs-II (d).

Figure 5. Thermogravimetric analysis (TGA) of SiO2/ZnO/ZnS NPs (a), SiO2/ZnO/ZnS-SC(S)Ph (b), and SiO2/ZnO/ZnS/MIPs (MIPsI) (c).

ZnS NPs and SiO2/ZnO/ZnS-SC(S)Ph (RAFT agent-I). If the mass retention of RAFT agent-I at 500 °C is assumed to be identical to that of SiO2/ZnO/ZnS NPs, the grafting density of RAFT agent at the surface of SiO2/ZnO/ZnS NPs was roughly estimated to be 9.09 × 1014 molecule/nm2 calculated according a previous report.29 For SiO2/ZnO/ZnS/MIPs (MIPs-I) (Figure 5c), at temperature from 25 to 90 °C, the weight 25313

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Figure 6. (A) Room temperature photoluminescence spectra of SiO2/ZnO/ZnS NPs (a) and SiO2/ZnO/ZnS/Ag2S NPs (b); (B) UV−vis absorption spectra of SiO2/ZnO/ZnS NPs (a), MIPs-I (b), SiO2/ZnO/ZnS/Ag2S NPs (c), and MIPs-II (d).

loss was mainly due to the loss of physical-absorbed water, and the weight loss from 90 to 160 °C could be attributed to some unreacted monomers or low molecular weight polymers.30 The weight loss of MIPs-I increased rapidly from 350 to 450 °C and could be attributed to the thermal decomposition of the imprinted polymers. The weight retention at 450 °C obtained for MIPs-I was ∼61.68%; thus, the weight fraction of imprinted polymers was calculated to be ∼21.02%. In Figure 6, photoluminescence (PL) spectroscopy and UV− vis absorption spectra were used to characterize the optical properties of SiO2/ZnO/ZnS NPs and SiO2/ZnO/ZnS/Ag2S NPs. As displayed in Figure 6A, peak deconvolution revealed five emissions centered at 448, 465, 472, 481, and 491 nm for SiO2/ZnO/ZnS NPs. The observed red shift of the peak at 465 to 469 nm for SiO2/ZnO/ZnS/Ag2S NPs may be because of the presence of Ag2S. More importantly, a significant quenching in the PL emission was observed for SiO2/ZnO/ZnS/Ag2S NPs as compared to SiO2/ZnO/ZnS NPs, which could be attributed to the addition of Ag, implying the successful electron transfer from ZnO/ZnS to Ag2S.31 Figure 6B showed UV−vis absorption spectra of SiO2/ZnO/ZnS NPs, SiO2/ZnO/ZnS/ Ag2S NPs, MIPs-I, and MIPs-II. The SiO2/ZnO/ZnS NPs and MIPs showed an absorption edge at around 364 nm, consistent with the bulk bandgap energy of ZnO or ZnS.27,32 Moreover, an additional absorption band at about 439 nm was observed for SiO2/ZnO/ZnS/Ag2S NPs and MIPs-II. This band could be attributed to the typical surface plasmon resonance (SPR) absorption that originated from Ag2S.33 Thermosensitive Dependence Behaviors. It is worthy of noting that for thermosensitive polymers, the thermoinduced swelling/shrinking transition can spontaneously modulate the spatial distribution of polymer matrix layer,34 making the specific binding sites change. In order to determine whether the prepared polymers exhibit a thermal response, we measured the UV−vis absorption of polymers in distilled water.35 Taking MIPs-I as an example, Figure 7 showed temperature-dependent UV−vis spectra of MIPs-I (8.0 × 10−2 g L−1) in aqueous solutions at different temperatures and Figure S3 showed the MIPs-I dispersed in aqueous solution at different temperatures. It can be observed that MIPs-I exhibited an absorbance decrease upon heating solutions from 25 to 60 °C and the inset in Figure 7 indicated the LCST of MIPs-I was 36.1 °C, which was higher than that of pure PNIPAm.14 According to previous studies, the introduction of hydrophobic monomer as the cross-linking agent into PNIPAm chains could slightly change the LCST of PNIPAm.36 In this work, the crosslinking monomer (EGDMA), which is hydrophobic, making

Figure 7. Temperature dependence behaviors of UV−vis absorption spectra obtained for 8.0 × 10−2 g L−1 aqueous dispersions of SiO2/ ZnO/ZnS/MIPs (MIPs-I). The inset: thermosensitive behaviors of MIPs-I in aqueous (8.0 × 10−2 g L−1) depending on temperature from 15 to 60 °C, and the LCST is 36.1 °C.

the LCST of MIPs-I litter higher that that of pure PNIPAm. As MIPs-I is thermoresponsive, the hydrodynamic radius of MIPsI decrease when environmental temperature increase, which could be concluded from the changing trendline of UV−vis absorbance spectra of MIPs-I. When the temperature spans across LCST of MIPs-I, the shrinkage of the polymer layer can greatly decrease the relative template holes volume shaped in imprinting process, leading the lower absorbance and a similar phenomenon was observer by Liu’s group,37 in which the silica/ PNIPAm hybrid nanoparticles are not water-soluble. The same point of the prepared polymer composites and silica/PNIPAm hybrid nanoparticles is that they could only be suspended in water and when temperature increased the hydrodynamic radius of them decreased. The good dispersed of polymer composites at high temperatures leaded a lower absorbance. However, in case of pure polymers, different phenomena were observed.34,35 The absorbance of polymers increased with the temperature increasing, which can be ascribed to the decreasing of hydrophilic properties at high temperatures. Switched Catalysis. All imprinted polymer composites showed a higher binding compared to the corresponding nonimprinted reference. In Figure S4, what can be observed that the equilibrium adsorption capacity (Qe) of SD increased sharply at first, then increased slightly, and finally reached to the binding equilibrium. The Qe of SD for MIPs-I at 15 and 45 °C was 3.98 and 2.45 mg g−1, respectively. The Qe of SD for MIPsII at 15 and 45 °C was 3.65 and 2.27 mg g−1, respectively. Moreover, the Qe of SD for NIPs-I at 15 and 45 °C was 1.65 25314

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Figure 8. Absorption spectra of SD solutions undergoing photodegradation in the presence of prepared polymers. MIPs-I at 15 °C, pH 7, λ = 365 nm (inset: NIPs-I at 15 °C, pH 7, λ=365 nm) (a); MIPs-II at 15 °C, pH 7, λ = 365 nm (inset: NIPs-II at 15 °C, pH 7, λ = 365 nm) (b); MIPs-I at 15 °C, pH 7, λ = 500 nm (inset: NIPs-I at 15 °C, pH 7, λ = 500 nm) (c); MIPs-II at 15 °C, pH 7, λ = 500 nm (inset: NIPs-II at 15 °C, pH 7, λ = 500 nm) (d); MIPs-I at 45 °C, pH 8, λ = 365 nm (inset: NIPs-I at 45 °C, pH 8, λ = 365 nm) (e); MIPs-II at 45 °C, pH 8, λ = 365 nm (inset: NIPs-II at 45 °C, pH 8, λ = 365 nm) (f); MIPs-I at 45 °C, pH 8, λ = 500 nm (inset: NIPs-I at 45 °C, pH 8, λ = 500 nm) (g); MIPs-II at 45 °C, pH 8, λ = 500 nm (inset: NIPs-II at 45 °C, pH 8, λ = 500 nm) (h).

and 1.11 mg g−1, respectively. The Qe of SD for NIPs-II at 15 and 45 °C was 1.56 and 1.02 mg g−1, respectively. The prepared polymers showed temperature responsive binding properties. The Qe was higher for both imprinted and nonimprinted polymers at 15 °C than it at 45 °C. Theses results could be reasoned that at low temperature the swelling of polymers made them more hydrophilic and more lacunose, so the SD molecules could reach the surface of polymers easily. Figure 8 presented the PC behaviors of MIPs-I, NIPs-I, MIPs-II, and NIPs-II and the PC capacities of polymers were monitored by measuring photodegradation behaviors of SD (The initial

concentration of SD was all 0.1 mmol L−1). We also conducted photodegradation of SD itself where there were no polymers added (Figure 9) and SD showed a nice stability within 1.0 h. In view of the temperature responsiveness during the PC process, which can affect the access of template SD to polymers, the binding temperature plays a key role in PC process. For comparison, herein two representative temperatures, i.e., 45 and 15 °C (either higher or lower than LCST of polymers) were selected for contrastive studies. In Figure 8a, b, c and d, the time-dependent absorption spectra of SD solutions under UV light (λ = 365 nm) and visible light (λ = 500 nm) 25315

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Figure 9. C/C0 vs irradiation time plot for SD photodegradation without polymer and in the presence of MIPs-I, NIPs-I, MIPs-II, and NIPs-II at 15 °C, pH 7 (a), C/C0 vs irradiation time plot for SD photodegradation without polymer and in the presence of MIPs-I, NIPs-I, MIPs-II, and NIPs-II at 45 °C, pH 8 (b).

Figure 10. Degradation percentage of SD, SMZ, and SMI in the presence of MIPs-I and NIPs-I for 1.0 h, respectively (15 °C, pH 7, λ = 365 nm) (a), degradation percentage of SD, SMZ, and SMI in the presence of MIPs-II and NIPs-II for 1.0 h, respectively. (15 °C, pH 7, λ = 500 nm) (b).

illumination at 15 °C with pH 7.0 were depicted. First, we put the SD solutions mixed with MIPs-I, NIPs-I, MIPs-II, and NIPs-II under UV light illumination for 1.0 h. The absorption peak corresponding to SD (264 nm) diminished gradually as the exposure time increased, indicating the degradation of SD. For MIPs-I and NIPs-I, about 46.30% and 16.83% of SD were degraded after 1.0 h of irradiation, respectively (Figure 7a). A lower extent of SD photodegradation, to about 30.80% and 7.94% at the same irradiation time for MIPs-II and NIPs-II were achieved, respectively (Figure 8b). This may be a result of Ag2S could not facilitate charge separation under UV light illumination. A contrary result was observed when SD solution was exposed under visible light illumination. The degradation percent of SD for MIPs-I, NIPs-I, MIPs-II, and NIPs-II were 59.23%, 39.43%, 71.65%, and 57.03% (Figure 8c,d). This mainly a result of the Ag2S that can facilitate charge separation by attracting the photoexcited electrons of ZnO and ZnS,38 providing more electrons for reduction of SD, giving a nicer PC activity for MIPs-II and NIPs-II. Moreover, the similar PC experiments were conducted while temperatures were set at 45 °C and pH values were 8.0, the experiment results were shown in Figure 8e−h. For the pH values of SD solutions were 8.0, the absorption peak of SD changed compared that of pH at 7.0 and this could be attributed to the structure change of SD at different pH values. When exposed to UV light, the degradation percent of SD for MIPs-I, NIPs-I, MIPs-II, and NIPs-II were 21.55%, 14.26%, 15.65%, and 11.88%, respectively (Figure 8e, f). When exposed to visible light, the degradation percent of SD for MIPs-I, NIPs-I, MIPs-II, and NIPs-II were 23.87%, 8.46%, 39.96%, and 25.92%, respectively (Figure 8g,h). From Figure 7

what can be concluded that the photodegradation behaviors of SD for MIPs-I, NIPs-I, MIPs-II, and NIPs-II were similar at 15 and 45 °C. The MIPs-I had a higher PC activity under UV light illumination and MIPs-II had a higher PC activity under visible light illumination. For the thermoresponsiveness of prepared polymers, the polymers performed an excellent PC activity at 15 °C that they at 45 °C. We can attribute the results to the higher binding ability of polymers at lower temperature, which had been discussed in the section of “Thermosensitive Dependence Behaviors”. Most of all of the imprinted polymers performed a relative nice PC activity at all experiments, which illustrated that specific binding sites formed in imprinted polymers.39 The photocatalytic efficiency was much lower than pure inorganic materials such as ZnS-Au nanoassemblies,38a which could be surface-initiated RAFT polymerization technique and the imprinted polymer layer was relatively wafery. The imprinted polymers have a nice binding ability of SD for the imprinted hole among them. The SD molecules could reach to the surfaces of the prepared materials easier than pure inorganic semiconductors. Accordingly, although the ZnO, ZnS, and Ag2S NPs in our prepared composites were relatively low-percentage, they could have a close and permanent contact with SD molecules. The template SD had a privilege to access to imprinted polymers, thus, exhibiting an excellent SD molecules photocatalysis activity. To quantitatively understand the photodegradation of SD in the presence of prepared polymers, we analyzed the normalized concentration of SD (C/C0) as a function of irradiation time and the results were shown in Figure 9. Experiments in the absence of polymers had almost no degradation of SD, 25316

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Figure 11. PC activity recycling test on MIPs-II for SD photodegradation (a), degradation percentage of MIPs-II for SD in three times (b).

45 °C. Moreover, MIPs-II retained comparable PC activity after repeated uses for three times, revealing that this kind of polymers could be promisingly utilized in practical selective catalytic field.

indicating a minor extent of self-degradation for SD under both UV and visible light illumination within 1.0 h. The extent of SD photodegradation in the presence of prepared polymers has already illustrated in above discussions. Since the PC activity of polymers is low, it can be significantly enhanced by using other high degree of photogenerated charge nanoparticles as cores.40 So, many other high selective PC activity polymers can be developed based on our this novel strategy in the long term course of PC applications. Selective Catalysis Studies. To study the selectivity of prepared polymers for SD, we performed photodegradation experiments with two other antibiotics, namely, sulfamethazine (SMZ) and sulfamethizol (SMI) (the initial concentrations for the three antibiotics were all 0.1 mmol L−1). Taking the experiments at 15 °C, pH 7.0 as examples, we observed specific PC ability for MIPs-I at λ = 365 nm and MIPs-II at λ = 500 nm to template SD, respectively, and results were shown in Figure 10. The other two antibiotics of similar chemical structure but had a lower degradation amount in the presence of the prepared polymers. We could find the catalysis core of polymers has the same PC activity, thus, the reason could be attributed to the specific binding imprinted polymer shell. During PC process, SD has a preferential access to the bowel of imprinted polymers than nonimprinted polymers, giving more chances for SD assembling to the neighborhood of catalytic core NPs. Reusability of Polymers. To evaluate the reusability of the prepared polymers, we further performed a recycling test by using MIPs-II as the representative polymer. The used polymers were extracted by methanol to remove the absorbed SD for recycling test. As shown in Figure 11a, no appreciable decay of PC activity was found for MIPs-II after they were repeatedly performed and recycled in SD photodegradation for three times. In Figure 11b, it can be observed that there was a small decrease in photodegradation efficiency of SD after three cycles, suggesting good retention of MIPs-II.

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ASSOCIATED CONTENT

* Supporting Information S

Chemical structures of SD, SMZ, and SMI, 1H NMR spectra of GPTMS and RAFT agent, photographs of MIP-I dispersed in water and binding of SD to MIPs and NIPs composite particles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86 511 8890683. Fax: +86 511 88791800. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No. 21077046, No. 21107037, No. 21176107, No. 21174057, No. 21004031 and No. 21207051), National key basic research development program (973 Program, No.2012CBB21500), Ph.D. Programs Foundation of Ministry of Education of China (No. 20093227110015) and Natural Science Foundation of Jiangsu Province (BK2011461, SBK2011459, and BK2011514) and Foundation of State Key Laboratory of Natural and Biomimetic Drugs (K20110105).

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CONCLUSIONS The concept of tunable catalytic molecularly imprinted polymers has been advanced for the first time; thus, the novel tunable selective catalytic molecularly imprinted polymers have been fabricated via surface-initiated reversible addition−fragmentation chain transfer (SI-RAFT) polymerization. This sulfadiazine imprinted polymers composed of PNIPAm matrix that exhibited reversible thermo-induced swelling/shrinking transition and the PC activity could accordingly be modulated by temperature-depended binding behaviors. The photodegradation of sulfadiazine at lower temperature (15 °C) presented a significant catalysis than at

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