Temperature-Responsive Smart Nanoreactors: Poly(N

Aug 21, 2012 - A nanoreactor with temperature-responsive poly(N-isopopylacrylamide) (PNIPAM) coated on the external pore mouth of mesoporous silica ho...
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Temperature-Responsive Smart Nanoreactors: Poly(N‑isopropylacrylamide)-Coated Au@Mesoporous-SiO2 Hollow Nanospheres Zhe Chen,† Zhi-Min Cui,‡ Chang-Yan Cao,† Wei-Dong He,§ Lei Jiang,‡ and Wei-Guo Song*,† †

Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, Peopleʼs Republic of China ‡ School of Chemistry and Environment, Beihang University, Beijing 100191, Peopleʼs Republic of China § Department of Polymer Science and Engineering, CAS Key Laboratory of Soft Matter Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, Peopleʼs Republic of China S Supporting Information *

ABSTRACT: A nanoreactor with temperature-responsive poly(N-isopopylacrylamide) (PNIPAM) coated on the external pore mouth of mesoporous silica hollow spheres and Au nanoparticles at the internal pore mouth were fabricated. Such spatial separation allows both Au nanoparticles and PNIPAM to function without interfering with each other. Transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Fourier transform infrared (FTIR) spectra, and temperature-dependent optical transmittance curves demonstrate successful grafting of PNIPAM. This nanoreactor shows repeated on/off catalytic activity switched by temperature control. It shows excellent catalytic activity toward 4nitrophenol (4-NP) reduction at 30 °C [below lower critical solution temperature (LCST) of PNIPAM] with a turnover frequency (TOF) of 14.8 h−1. However, when the temperature was 50 °C (above LCST), the TOF dropped to 2.4 h−1. Kinetic studies indicated that diffusion into the mesopores of the catalyst was the key factor, and the temperature-responsive behavior of PNIPAM was able to control this diffusion.

1. INTRODUCTION Smart materials that can change their chemical properties in response to external stimuli (pH, heat, light, magnet, etc.) are attractive in catalysis, as catalytic reactions can be controlled by applying appropriate stimuli.1−6 Poly(N-isopopylacrylamide) (PNIPAM) is a temperature-responsive polymer that exhibits a sharp phase transition at a lower critical solution temperature (LCST).7−9 Several studies have used PNIPAM or its copolymers as catalyst carriers.4,10−13 For example, Wu and co-workers10 fabricated hollow PNIPAM/Ag nanocomposite spheres, which showed typical thermal sensitivity and controllable catalytic activity. Marty and co-workers14 reported the use of macromolecular design via the interchange of xanthates (MADIX) and reversible addition−fragmentation chain transfer (RAFT) polymers to modulate the surface net charge of gold nanoparticles as a crucial parameter to modulate their catalytic properties. Yin and co-workers15 synthesized Fe3O4/SiO2/ PNIPAM/SiO2−Au colloids that can be used as recoverable supports for nanocatalysts, in which PNIPAM acted as a bridge to link Fe3O4/SiO2 core and SiO2−Au shell, but not thermoresponsive carrier. Gao and co-workers16 grafted PNIPAM onto SiO2 and characterized the composite in depth and envisioned potential applications of this PNIPAM© 2012 American Chemical Society

coated composite in controlled release of drugs, smart catalysts, and smart nanoreactors. In the studies cited above, the noble metal catalyst particles were directly wrapped in the polymer matrix; that is, the catalyst nanoparticles were deposited onto the polymer body. Direct contact between the catalyst and PNIPAM allowed the polymer to directly control the access to the catalyst and consequently its activity. However, the full potential of the catalyst was also compromised. The PNIPAM layer may be not dense enough to cover every catalytic site of the catalyst, thus the reaction can still occur to a certain extent even when the polymer is collapsed. Moreover, when the temperature is above LCST, the polymer may interfere with the catalytic reactions (e.g., CO and N−H may participate in certain organic reactions). Below or above the LCST, the PNIPAM densities around Au are different. While this is a potentially beneficial feature in terms of regulating the catalyst efficiency, it is difficult in practice to control in a predictable way. Thus a new catalyst structure with spatial separation of Au and PNIPAM could Received: April 1, 2012 Revised: August 20, 2012 Published: August 21, 2012 13452

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Figure 1. (a) Schematic illustration for the synthesis process of PNIPAM/Au@meso-SiO2; (b, c) TEM image of Au@meso-SiO2 composite with different magnification; (d, e) TEM image of PNIPAM/Au@meso-SiO2 composite with different magnification.

Loading Au Nanoparticles onto the Carbon Nanosphere to Form Au/C. In a typical process, 100 mg of carbon nanospheres was dispersed in 50 mL of distilled water and stirred for 10 min as part A. For part B, 0.1 g of SnCl2 (a linker and reducing agent that can help to load Au nanoparticles onto carbon nanospheres uniformly) was dissolved in 20 mL of 0.02 M HCl solution. Parts A and B were mixed together under stirring for 10 min, and then the suspension was centrifuged. After being washed with distilled water five times, the precipitate was dispersed in 50 mL of distilled water. Then 216 μL of 0.097 M HAuCl4 was added, and 10 min later, 10 mL of 0.15 M sodium formate solution was added, followed by stirring for 5 h. After centrifugation and being washed with distilled water five times, the precipitate was dried at 60 °C for 6 h. Production of Au@Mesoporous SiO2 Nanoreactors. The Au/C composite obtained from the previous step was first dispersed in a solution containing 40 mL of H2O, 30 mL of ethanol, 0.15 g of CTAB, and 568 μL of NH3·H2O with ultrasonication for 20 min. Then 150 μL of TEOS was added and the mixture was vigorously stirred for 6 h. The precipitate was harvested after centrifugation, washed with distilled water and with ethanol three times, and dried at 60 °C for 6 h. The product was then calcined at 400 °C in N2 flow for 2 h and then in air for 6 h to remove carbon spheres, CTAB template, and other organic species. Synthesis of PNIPAM/Au@meso-SiO2 Colloids. Au@ SiO2 colloids (50 mg) were mixed with MPS (0.5 mL) and ethanol (50 mL) for 48 h at room temperature. The colloids were washed with ethanol to remove excess MPS and redispersed in 2 mL of ethanol. The resulting MPS-modified Au@SiO2 ethanol solution (2 mL) was mixed with aqueous solution (100 mL) of NIPAM monomers (0.2 g) and MBA (0.02 g) by mechanical stirring. After being degassed with nitrogen for 40 min, the solution was heated up to 70 °C, and KPS solution (0.02 g/mL, 1 mL) was injected to initiate the polymerization. After 4 h of reaction, the final products were collected by centrifugation and washed with distilled water five times; the precipitate was dried at 60 °C for 6 h.

provide improved temperature control over catalyst performance. In our previous work, a nanoreactor with noble metal nanoparticles loaded in the inner wall of hollow mesoporous SiO2 nanosphere was designed.17 This nanoreactor exhibited high catalytic activity in Suzuki cross-coupling reactions due to its specific nanostructure. Another advantage of the mesoporous SiO2 wall was that it could act as a shield to protect catalyst nanoparticles. The 30 nm thick mesoporous SiO2 wall of the nanoreactor separated the external reaction solution from the noble metal nanoparticles. In this study, the structural advantage of such nanoreactor was exploited to produce a temperature-responsive nanoreactor by binding PNIPAM to the external surface of Au@meso-SiO2 nanospheres. The resulting “smart” nanoreactors exhibited sharply different catalytic activity at temperatures below and above the LCST of PNIPAM. In 4-nitrophenol (4-NP) reduction, these nanoreactors showed excellent catalytic activity at 30 °C (below LCST of PNIPAM) and very low activity at 50 °C (above LCST). Kinetic studies indicated that diffusion into the mesopores of the catalyst was the key factor, and the temperature-responsive behavior of PNIPAM was able to control this diffusion.

2. EXPERIMENTAL SECTION Materials and Reagents. Glucose, ethanol, sodium formate, tetraethyl orthosilicate (TEOS), ammonia solution (25 wt %), HAuCl 4, SnCl2·2H2O, and potassium persulfate (KPS) were purchased from Beijing Chemical Reagent Co. Cetyltrimethylammonium bromide (CTAB), [3-(methacryloyloxy)propyl]trimethoxysilane (MPS, 98%), N-isopropylacrylamide (NIPAM, 99%) and N,N′methylenebisacrylamide (MBA, 98%) were bought from Alfa Aesar. All chemicals were used as received without further purification.

Synthesis of Carbon Nanospheres. Carbon nanospheres were synthesized according to literature procedure.18 Briefly, glucose (4.5 g) was dissolved in 30 mL of water to form a clear solution and then transferred into a 40 mL Teflon-sealed autoclave. The autoclave was maintained at 180 °C for 4 h. The products were separated by centrifugation, followed by washing three times with water and ethanol, and finally oven-dried at 80 °C for further use. 13453

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4-Nitrophenol Reduction Catalyzed by Au Catalyst. The catalytic reduction of 4-nitrophenol with NaBH4 was accomplished as follows: A 0.10 mmol/L 4-nitrophenol aqueous solution (1.50 mL) and 10.0 mmol/L NaBH4 aqueous solution (1.50 mL) in a quartz cell for UV−vis spectroscopy were heated to the desired temperature (30 or 50 °C), and 1.0 mL of the colloidal dispersion containing 0.5 mg of Au@SiO2 or 0.86 mg of PNIPAM/Au@SiO2 catalyst (the contents of Au were kept constant) was heated to the same temperature. The two solutions or suspensions were then added to the quartz cell, and UV−vis absorption spectra were recorded immediately after mixing and then subsequently after given time intervals at the given temperature. Characterization and Measurements. Transmission electron microscopy (TEM) was carried out on a JEOL 1011F electron microscope running at 100 kV. Temperature dependence of optical transmittance of PNIPAM/Au@SiO2 was measured by UV−vis spectroscopy. The temperature of the sample cells was increased at a rate of 1 °C/min from 25 to 50 °C. The conversion of 4-NP was monitored online with a UV− vis spectrophotometer (Shimadzu UV1601). Two different scanning modes were employed in the testing process. For UV−vis absorption spectra, the scan range was 200−600 nm and the scan rate was 1 scan/min, while for time-dependent conversion of 4-NP, the monitoring wavelength was fixed at 400 nm and the scan rate was 60 scans/min. Fourier transform infrared spectroscopy (FTIR) was carried out on a Thermo Nicolet iZ10. Thermogravimetric analysis (TGA) was performed under nitrogen atmosphere at a heating rate of 10 °C/min on a Perkin-Elmer instrument TG/DTA 6300.

Figure 2. UV−vis spectrum of PNIPAM/Au@SiO2 solid powder. (Inset) Digital photo of PNIPAM/Au@SiO2.

the PNIPAM/Au@SiO2 powder. The sample shows the typical purple color of Au nanoparticles. Figure 3a shows TGA curves of Au@SiO2 and PNIPAM/ Au@SiO2. The weight loss below 200 °C was due to physisorbed water and residual organic solvent and corresponded to 5.9% for Au@SiO2 and 5.7% for PNIPAM/Au@ SiO2. At higher temperature, no obvious weight loss was observed for Au@SiO2 sample. However, there was a sharp weight loss of about 36% between 300 and 400 °C for the PNIPAM/Au@SiO2 composite, presumably due to the combustion of PNIPAM layer.22,23 Such a degree of weight loss was consistent with the loading of PNIPAM. After 400 °C, the composite became stable and kept 58% of the original weight. The presence of PNIPAM in the PNIPAM/Au@SiO2 composite was further confirmed by FTIR, as shown in Figure 3b. The peak at 1078 cm−1 was due to the asymmetrical stretching of Si−O−Si. The peaks at 960 and 804 cm−1 were due to Si−OH bending and Si−O−Si bending, respectively. The weak peak at 1620 cm−1 came from hydroxyl groups on SiO2.24,25 After coating PNIPAM on the outside of Au@SiO2, strong new peaks at 1529 and 1630 cm−1 appeared, which were due to the characteristic CO and N−H stretches of PNIPAM,16,26 indicating that PNIPAM was successfully coated onto Au@SiO2. The temperature-responsive behavior of the sample provided visual evidence for successful coating of PNIPAM. The phase transition of PNIPAM is affected by its molecular weight and the polymer concentrations.27 Typically, the LCST of PNIPAM is 32 °C in aqueous solution.28 Figure 4 shows the temperature dependence of optical transmittance at 500 nm for 0.5 g/L aqueous suspension of PNIPAM/Au@meso-SiO2. The mixture was a slightly purple clear solution at 25 °C with nearly 90% transmittance. To obtain temperature-dependent spectra, the initial temperature was kept at 25 °C and increased to 50 °C at a rate of 1 °C/min. After reaching 50 °C, the temperature started to be decreased to 25 °C with a rate of 1 °C/min. The transmittance quickly decreased to about 15% at 37 °C. On the basis of these results, the LCST of this composite was determined to be 34 °C. The inset of Figure 4 shows the digital image of the sample in three cycles at 25 and 50 °C, respectively. The light purple dispersion at 25 °C is nearly transparent and remains very stable below LCST. When heated to 50 °C, the sample became cloudy and a solid deposit appeared at the bottom of the bottle. When the cloudy

3. RESULTS AND DISCUSSION As illustrated in Figure 1, the temperature-responsive nanoreactor was fabricated in two steps (see Experimental Section for details). First, Au@meso-SiO2 nanoreactor was prepared as reported.17 This catalyst was composed of mesoporous silica hollow spheres and Au nanoparticles residing inside the spheres. Hydroxyl groups on the external surface of the meso-SiO2 nanosphere allowed Au@meso-SiO2 nanoreactor to be functionalized with a layer of MPS through siloxane linkages.19 MPS then trigged the copolymerization of NIPAM monomers and MBA at the external surface of the Au@mesoSiO2 nanoreactor to form a layer of PNIPAM.15 Figure 1 shows TEM images of Au@meso-SiO2 and PNIPAM/Au@meso-SiO2 nanospheres. The thickness of the mesoporous silica walls was about 40 nm, although the thickness can be readily controlled by adjusting the concentration of silica precursors. Figure 1b,c shows highly dispersed Au nanoparticles with an average size of about 5 nm residing on the inner wall of hollow SiO2. No Au nanoparticles were found outside the hollow SiO2 spheres. Different from Au@meso-SiO2 spheres in Figure 1c, which had clear boundaries in the TEM image, a blurred coating layer can be observed on the TEM image of the PNIPAM/Au@meso-SiO2 composite (Figure 1d,e). The blurred layer was most likely the PNIPAM shell. Such layers were observed on nearly all spheres, indicating that PNIPAM loading was quite uniform. The diameter of Au nanoparticles residing at the SiO2 inner wall was about 5 nm. The UV−vis spectrum of PNIPAM/Au@ SiO2 in Figure 2 shows a broad peak at 538 nm, which is the characteristic plasmon absorption exhibited by Au nanoparticles.20,21 The inset of Figure 2 shows a digital photo of 13454

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Figure 3. (a) TGA curves and (b) FTIR spectra of Au@SiO2 and PNIPAM/Au@SiO2 catalyst.

295 nm increased gradually, indicating the reduction of 4-NP and formation of 4-aminophenol (4-AP), respectively. The characteristic plasmon peak of Au nanoparticles was not observed in the reaction solution because the concentration of PNIPAM/Au@meso-SiO2 nanoreactor (catalyst = 0.5 g/L) was too small, so the signal from Au nanoparticles was overshadowed by the absorption of 4-NP. With excess NaBH4, the reaction was treated as a first-order reaction.33−35 Blank experiment confirmed that the reduction reaction did not proceed without the Au catalyst. In control experiments, the reduction of 4-NP was catalyzed by Au@SiO2 composite at 30 and 50 °C. These experiments were carried out to ensure that any observed temperature-responsive behavior of the PNIPAM/Au@meso-SiO2 composite can be attributed to the PNIPAM layer, not other factors, such as the self-decomposition of NaBH4 in aqueous solution. In the absence of the PNIPAM coating (i.e., the Au@SiO2 composite) at 30 °C, the reaction was complete in 5 min, while at 50 °C the reduction rate was much faster and the reaction finished in 70 s (data not shown). The turnover frequency (TOF) was 17.8 and 76.1 h−1 at 30 and 50 °C, respectively. The increase of the reduction rate of 4-NP with temperature agreed well with those of other reported Au catalysts.34 From the Arrhenius equation, the activation energy Ea was roughly calculated to be about 60 kJ/ mol, which was close to the data from other reports.34 The PNIPAM/Au@meso-SiO2 composite showed very different temperature-responsive behavior (Figure 5). At 30 °C (below the LCST of PNIPAM), the 4-NP reduction reaction proceeded at a high rate (Figure 5a,c) and was complete in 6 min, resulting in a TOF value of 14.8 h−1, which was similar to that of Au@SiO2 (17.8 h−1), indicating that PNIPAM has little influence on the catalytic property of the Au nanoparticles below the LCST, where the polymer should be expanded. However, when the temperature was increased above LCST of PNIPAM to 50 °C, the reaction rate dropped sharply (Figure 5b,d). After 6 min, the 4-NP conversion was only about 10% and reached 80% after 30 min. The TOF value decreased to 2.4 h−1. As shown in Figure 5a,b, several isosbestic points in the UV−vis absorption spectra of the reaction mixture were observed, as reported in the literature, indicating that side reaction besides catalytic reduction of 4-NP was minimal.30,35 The sharp difference of catalytic activity of PNIPAM/Au@ meso-SiO2 composite at 30 and 50 °C can be attributed to the thermoresponsive behavior of PNIPAM. When the reaction

Figure 4. Temperature-dependent optical transmittance curve of PNIPAM/Au@meso-SiO2 aqueous solutions (0.5 g/L) vs time. The testing temperature increased or decreased at 1 °C/min with the initial temperature of 25 °C.

suspension was cooled to room temperature again, it turned transparent slowly. Repeated change of the temperatures around the LCST resulted in repeated changes of the transmittances, as shown in Figure 4. Although a solid deposit appears at the bottom of the bottle, a majority of PNIPAM/ Au@meso-SiO2 composite is still dispersed in the cloudy solution (as solution is cloudy and purple). We extended this work to include PNIPAM/Pt@meso-SiO2 nanoparticles, which exhibited similar thermoresponsive properties. Figure S1 (Supporting Information) shows a digital image of PNIPAM/ Pt@meso-SiO2 solutions at 25 and 50 °C, respectively. Au catalyzed reduction of 4-nitrophenol (4-NP) in the presence of NaBH4 (Scheme 1) has been a model reaction to test the responsive catalytic property of the PNIPAM/Au@ meso-SiO2.11,29−32 The conversion of 4-NP was monitored by UV−vis spectroscopy. When catalyst was added into the solution, the absorption at 400 nm decreased and absorption at Scheme 1. 4-Nitrophenol Reduction Catalyzed by Au Catalyst

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Figure 5. Successive UV−vis absorption spectra of reduction of 4-nitrophenol catalyzed by PNIPAM/Au@meso-SiO2 at (a) 30 °C and (b) 50 °C. (c, d) Time-dependence conversion curves of 4-nitrophenol runs in panels a and b. (Inset) Linear fitting of time dependence of the absorption of pnitrophenolate ions in first order.

temperature was 30 °C, the PNIPAM chains should adopt an extended conformation, allowing reagents to easily diffuse through the polymer coating to the pore mouth of SiO2 and be converted at the surface of Au nanoparticles. At 50 °C (above LCST), PNIPAM chains should collapse to form a dense layer covering the mesoporous silica surface. The dense and hydrophobic PNIPAM layer is expected to hinder the diffusion of 4-NP into the nanoreactor. As a result, the reduction reaction was largely inhibited, even though the reaction temperature was higher. Similar effect of diffusion control on reaction rate was also reported for catalytic dendrimerstabilized nanoparticles.32 The temperature-responsive PNIPAM/Au@meso-SiO2 catalyst could also be easily recycled by centrifugation. Such temperature-responsive behavior was reversible, as the recycled catalyst showed the same high activity and thermoresponsive behavior. The kinetics of this reaction was investigated to elucidate the role of PNIPAM layer. During the phase transformation period, the catalyst was not in stable form, and the catalysis results are not reliable during such time. We avoided this problem by heating the reactant mixture solution and the catalyst suspension separately (reactant solution in the UV cell and catalyst in a separate vial). After both solutions reached 50 °C, the catalyst suspension was added to the reactant solution. By such experimental design, the PNIPAM aggregation process should have finished before kinetic data were collected. By fixing the detector at 400 nm, a fast scan could be made to obtain near real-time 4-NP conversion data. As shown in Figure 5a,b, the time-dependent conversion of 4-NP showed several

informative features. As the concentration of NaBH4 (3.75 mmol/L) far exceeded that of 4-NP (0.0375 mmol/L) and could be considered as constant, the reaction rate was determined only by 4-NP’s concentration. From the plot in Figure 5c, the kinetics of the reduction could be treated as a first-order reaction for 4-NP at 30 °C, leading to a rate constant Kapp value of 0.01 s−1.34 However, from the nearly linear plot in Figure 5d, it seemed that the reduction was a zero-order reaction at 50 °C. The turnover frequency was 2.4 h−1, compared to 14.8 h−1 at 30 °C. These results are consistent with the temperature-responsive behavior of PNIPAM/Au@ meso-SiO2: as the reaction temperature exceeded LCST, the collapsed PNIPAM polymers blocked most of the diffusion path of the reagent to the nanoreactor. The diffusion of reagents to the catalyst then became the rate-determining step, and the reaction followed pseudo-zero-order kinetics. An important feature of the reduction of 4-NP catalyzed by Au nanoparticles was the induction time (t0). Several studies have proposed that the induction time was due to a substrateinduced surface restructuring to initialize the reaction.30,34 In addition, t0 was found to decrease sharply as temperature increased.30,34 From Figure 5c,d, t0 was estimated to be 11 s at 30 °C and 2.5 min at 50 °C. The much longer induction time at 50 °C provides further evidence that diffusion might play a vital role in reactions catalyzed by PNIPAM/Au@meso-SiO2. In this thermoresponsive nanoreactor, the Au nanoparticles were spatially separated from the PNIPAM polymer by the mesoporous silica wall. Such spatial separation allowed both Au nanoparticles and PNIPAM to function without interfering with 13456

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Langmuir each other. The mesopores that connected the external PNIPAM layer and internal Au nanoparticles were the mass diffusion channels for reactant species to enter the nanoreactor and products to exit the nanoreactor. When the reaction temperature was below LCST of PNIPAM, the hydrogen bond between solvent water and PNIPAM chain dominated, and the PNIPAM chains can adopt extended conformations, as allowing access to the mesopores, as illustrated in Figure 1. In this state, reagents can readily diffuse through the mesopores and be converted by Au clusters residing at the inside pore mouth. However, when the reaction was carried out above the LCST of PNIPAM, intramolecular hydrogen bonds dominated, causing the polymer chains to collapse and form a complete layer covering the SiO2 surface to block the mesopores. Thus the entrance of the nanoreactor was blocked and the nanoreactor was switched off. When the reaction temperature was decreased below LCST again, the nanoreactor was switched back on. In potential applications, such a temperature-responsive nanoreactor could be used as a catalyst in exothermic reactions to prevent the reaction from accelerating out of control, a major concern for scaled-up catalytic reactions. When the reactions start to accelerate, increased solution temperature will trigger the PNIPAM collapse to switch off the catalyst, preventing further temperature increase. In future studies, we plan to explore temperature-responsive polymers that work in organic solutions, as PNIPAM in this study works only in aqueous systems. We will also try to develop a light-triggered smart catalyst, which may have faster on and off speed.

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REFERENCES

(1) Yuan, Y.; Yan, N.; Dyson, P. J. pH-Sensitive gold nanoparticle catalysts for the aerobic oxidation of alcohols. Inorg. Chem. 2011, 50, 11069. (2) Berryman, O. B.; Sather, A. C.; Lledo, A.; Rebek, J. Switchable catalysis with a light-responsive cavitand. Angew. Chem., Int. Ed. 2011, 50, 9400. (3) Zeltner, M.; Schatz, A.; Hefti, M. L.; Stark, W. J. Magnetothermally responsive C/Co@PNIPAM-nanoparticles enable preparation of self-separating phase-switching palladium catalysts. J. Mater. Chem. 2011, 21, 2991. (4) Jiang, X. W.; Xiong, D. A.; An, Y. L.; Zheng, P. W.; Zhang, W. Q.; Shi, L. Q. Thermoresponsive hydrogel of poly(glycidyl methacrylateco-N-isopropylacrylamide) as a nanoreactor of gold nanoparticles. J. Polym. Sci., Polym. Chem. 2007, 45, 2812. (5) Zhao, D. Y.; Chen, X.; Liu, Y.; Wu, C. L.; Ma, R. J.; An, Y. L.; Shi, L. Q. Thermosensitive and pH-sensitive Au-Pd bimetallic nanocomposites. J. Colloid Interface Sci. 2009, 331, 104. (6) Singh, N.; Karambelkar, A.; Gu, L.; Lin, K.; Miller, J. S.; Chen, C. S.; Sailor, M. J.; Bhatia, S. N. Bioresponsive mesoporous silica nanoparticles for triggered drug release. J. Am. Chem. Soc. 2011, 133, 19582. (7) Hirokawa, Y.; Tanaka, T. Volume phase-transition in a nonionic gel. J. Chem. Phys. 1984, 81, 6379. (8) Li, L. Y.; He, W. D.; Li, W. T.; Zhang, K. R.; Pan, T. T.; Ding, Z. L.; Zhang, B. Y. Thermal and pH-sensitive gold nanoparticles from Hshaped block copolymers of (PNIPAM/PDMAEMA)-b-PEG-b(PNIPAM/PDMAEMA). J. Polym. Sci., Polym. Chem. 2010, 48, 5018. (9) Wang, Y.; Yan, R.; Zhang, J. Z.; Zhang, W. Q. Synthesis of efficient and reusable catalyst of size-controlled Au nanoparticles within a porous, chelating and intelligent hydrogel for aerobic alcohol oxidation. J. Mol. Catal. A: Chem. 2010, 317, 81. (10) Xie, L.; Chen, M.; Wu, L. M. Fabrication and properties of hollow poly(N-isopropylacrylamide)-Ag nanocomposite spheres. J. Polym. Sci., Polym. Chem. 2009, 47, 4919. (11) Xiong, D. A.; Li, Z.; Zou, L.; He, Z. P.; Liu, Y.; An, Y. L.; Ma, R. J.; Shi, L. Q. Modulating the catalytic activity of Au/micelles by tunable hydrophilic channels. J. Colloid Interface Sci. 2010, 341, 273. (12) Carregal-Romero, S.; Buurma, N. J.; Perez-Juste, J.; Liz-Marzan, L. M.; Herves, P. Catalysis by Au@pNIPAM nanocomposites: Effect of the cross-linking density. Chem. Mater. 2010, 22, 3051. (13) Kim, J. H.; Lee, T. R. Hydrogel-templated growth of large gold nanoparticles: Synthesis of thermally responsive hydrogel-nanoparticle composites. Langmuir 2007, 23, 6504. (14) Beija, M.; Palleau, E.; Sistach, S.; Zhao, X. G.; Ressier, L.; Mingotaud, C.; Destarac, M.; Marty, J. D. Control of the catalytic properties and directed assembly on surfaces of MADIX/RAFT polymer-coated gold nanoparticles by tuning polymeric shell charge. J. Mater. Chem. 2010, 20, 9433. (15) Ge, J. P.; Huynh, T.; Hu, Y. P.; Yin, Y. D. Hierarchical magnetite/silica nanoassemblies as magnetically recoverable catalystsupports. Nano Lett. 2008, 8, 931. (16) Chen, J. C.; Liu, M. Z.; Chen, C.; Gong, H. H.; Gao, C. M. Synthesis and characterization of silica nanoparticles with well-defined thermoresponsive PNIPAM via a combination of RAFT and click chemistry. ACS Appl. Mater. Interfaces 2011, 3, 3215. (17) Chen, Z.; Cui, Z. M.; Niu, F.; Jiang, L.; Song, W. G. Pd nanoparticles in silica hollow spheres with mesoporous walls: a nanoreactor with extremely high activity. Chem. Commun. 2010, 46, 6524.

ASSOCIATED CONTENT

S Supporting Information *

One figure with a photograph of PNIPAM/Pt@meso-SiO2 solutions at temperatures above and below the PNIPAM LCST, and one table with data demonstrating recyclability of PNIPAM/Au@meso-SiO2 reduction catalyst. This material is available free of charge via the Internet at http://pubs.acs.org/.

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ACKNOWLEDGMENTS

We thank the National Basic Research Program of China (2009CB930400), National Natural Science Foundation of China (21121063), and the Chinese Academy of Sciences (KJCX2-YW-N41) for financial support.

4. CONCLUSION In summary, we produced a temperature-responsive PNIPAM/ Au@meso-SiO2 composite nanoreactor by binding PNIPAM to the external pore mouth of the mesoporous wall of Au/mesoSiO2 hollow spheres. In this thermoresponsive nanoreactor, the Au nanoparticles were spatially separated from the PNIPAM polymer by the mesoporous silica wall. Such spatial separation allowed PNIPAM and Au nanoparticles to function without interfering with each other. As a result, the catalyst showed sharp on and off behavior in catalytic reduction of 4-NP at temperatures below or above the LCST of PNIPAM. Kinetic studies indicated that diffusion into the mesopores of the catalyst was the key factor, and the temperature-response behavior of PNIPAM was able to control the diffusion. Such catalyst design offers a new way to prevent runaway reactions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]. Notes

The authors declare no competing financial interest. 13457

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dx.doi.org/10.1021/la3022535 | Langmuir 2012, 28, 13452−13458