Polymer Trilayer Composite and the

Gold/silica/poly(N,N′-methylenebisacrylamide) (Au/SiO2/polyMBAAm) trilayer composite materials were prepared by distillation precipitation polymeriz...
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Langmuir 2008, 24, 1019-1025

1019

Synthesis of a Au/Silica/Polymer Trilayer Composite and the Corresponding Hollow Polymer Microsphere with a Movable Au Core Guangyu Liu,†,‡ Hongfen Ji,† Xinlin Yang,*,† and Yongmei Wang‡ Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, and Department of Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed August 22, 2007. In Final Form: October 21, 2007 Gold/silica/poly(N,N′-methylenebisacrylamide) (Au/SiO2/polyMBAAm) trilayer composite materials were prepared by distillation precipitation polymerization of N,N′-methylenebisacrylamide (MBAAm) in the presence of Au/SiO2 particles as seeds, in which the seeds were prepared by a combination of gold-complexing and silane coupling agent with a further modified Sto¨ber method. The polymerization of MBAAm was performed in neat acetonitrile with 2,2′-azobisisobutyronitrile as an initiator to encapsulate the Au/SiO2 seeds driven by the hydrogen-bonding interaction between the hydroxyl group on the surface of the seeds and the amide unit of polyMBAAm without modification of the Au/SiO2 surface in the absence of any stabilizer or surfactant. Hollow polyMBAAm microspheres with movable Au cores were further developed by the selective removal of the middle silica layer with hydrofluoric acid. The resultant trilayer Au/SiO2/polyMBAAm composite and hollow polyMBAAm microspheres with movable Au cores were characterized by transmission electron microscopy. The diffusion of chemicals across the polyMBAAm shell was investigated by a catalytic reduction of 4-nitrophenol to 4-aminophenol in the presence of sodium borohydride as a reductant.

Introduction The development of materials with a novel structure has been a fundamental focus of chemical research, which promotes advances in both the academic and industrial fields. In recent years, organic-inorganic composite materials, which combine the properties of inorganic and organic building blocks within a single material, have attracted rapidly expanding interest for materials scientists because of the possibility to combine the various functional groups of organic components with the advantages of a thermally stable and robust inorganic substrate.1,2 These organic-inorganic composite materials exhibit novel and excellent properties, such as mechanical, chemical, electrical, rheological, magnetic, optical, and catalytic, by varying the compositions, dimensions, and structures, and have promising diverse applications in drug delivery systems, diagnostics, coatings, and catalysis.3-7 The silica/polymer composite materials with various interesting morphologies, including a silica core/polymer shell,8 an organic core/silica shell,9 and raspberry-like,10 snowman-like,11 daisy* To whom correspondence should be addressed. Phone: +86-2223502023. Fax: +86-22-23503510. E-mail: [email protected]. † Institute of Polymer Chemistry. ‡ Department of Chemistry. (1) Zhang, Y. D.; Lee, S. H.; Yonnessi, M.; Liang, K. W.; Pittman, C. U. Polymer 2006, 47, 2984-2996. (2) Strachotova, B.; Strachota, A.; Uchman, M.; Slouf, M.; Brus, J.; Plestil, J. et al. Polymer 2007, 48, 1471-1482. (3) Zhu, J.; Morgan, A. B.; Lamelas, F. J.; Wilkie, C. A. Chem. Mater. 2001, 13, 3774-3780. (4) Cho, J. D.; Ju, H. F.; Hong, J. H. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 658-670. (5) Zhou, J.; Zhang, S. W.; Qiao, X. G.; Li, X. Q.; Wu, L. M. J. Polym. Sci., Part A: Polym. Chem. 2006, 44, 3202-3209. (6) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 22102216. (7) Hu, Y. Q.; Wu, H. P.; Gonsadves, K. E.; Merhari, L. Microelectron. Eng. 2001, 56, 289-294. (8) Zhang, K.; Chen, H. T.; Chen, X.; Chen, Z. M.; Cui, Z. C.; Yang, B. Macromol. Mater. Eng. 2003, 288, 380-385. (9) Tissot, I.; Novat, C.; Lefebvre, F.; Bourgeat-Lami, E. Macromolecules 2002, 34, 5737-5739. (10) Reculusa, S.; Poncet-Lagrand, C.; Ravaine, S.; Minogotaud, S.; Duguet, E.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 2354-2359.

shaped and multipod-like,12 and raisinbun-like13 morphologies, have been synthesized by different techniques. The preparation of the silica/polymer composite materials can be generally classified into two categories: the self-assembly of the resultant silica and polymer particles through physical or physicochemical interaction and the direct polymerization of the monomer on the surface of the silica particles. However, it was difficult to control the morphology of the resultant silica/polymer hybrid particles, and the encapsulation efficiencies of the polymer on the silica core were low for both dispersion polymerization14-16 and emulsion polymerization.17,18 Further, the surface-initiated atom transfer radical polymerization (ATRP) has been widely used to afford well-defined silica/polymer hybrids with the initiatormodified silica particles as the macroinitiator,19-21 in which the synthesis needed a long reaction time and conversion of monomer to polymer was low to encapsulate the silica core. Hollow microspheres have attracted increasing attention due to their unique properties, such as low density, high specific area, good flow ability, and surface permeability. They have found wide applications in many fields, such as catalysis, controlled drug delivery systems, artificial cells, fillers, pigments, light weight structural materials, nanoreactors, low dielectric (11) Derro, A.; Reculusa, S.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, S. Colloids Surf., A 2006, 284-285, 78-83. (12) Reculusa, S.; Minogotaud, C.; Bourgeat-Lami, E.; Duguet, E.; Ravaine, R. Nano Lett. 2004, 4, 1677-1682. (13) Barthlet, C.; Hickey, A. J.; Carins, D. B.; Armes, S. P. AdV. Mater. 1999, 11, 408-410. (14) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1998, 197, 293308. (15) Bourgeat-Lami, E.; Lang, J. J. Colloid Interface Sci. 1999, 210, 281289. (16) Corcos, F.; Bourgeat-Lami, E.; Novat, C.; Lang, J. Colloid Polym. Sci. 1999, 277, 1142-1151. (17) Tiarks, F.; Landfester, K.; Antonietti, M. Langmuir 2001, 17, 57755780. (18) Lula-Xavier, J. L.; Guyot, A.; Bourgeat-Lami, E. J. Colloid Interface Sci. 2001, 250, 82-92. (19) von Werne, T.; Patten, T. E. J. Am. Chem. Soc. 1999, 121, 7409-7410. (20) Perruchot, C.; Khan, M. A.; Kamitsi, A.; Armes, S. P. Langmuir 2001, 17, 4479-4481. (21) Harrak, A. E.; Carrot, G.; Oberdisse, J.; Jestin, J.; Boue, F. Polymer 2005, 46, 1095-1104.

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constant materials, acoustic insulation, and photonic crystals.22-26 Various methods, including spray drying, the templating method, emulsion processes, the hydrothermal method, and self-assembly techniques, have been reported to fabricate the hollow spherical characteristics, in which diverse organic and inorganic materials, such as polymers,27 carbon,28,29 metallic oxide,30,31 semiconductors,32 metals,33 and composite materials,34 have been utilized to construct the hollow microspheres. Many efforts have been made to synthesize the hollow polymer microspheres via different physical and chemical methods, such as the encapsulation of a hydrocarbon nonsolvent,35 layer-by-layer (LbL) assembly of polymer electrolytes,36 micelle formation of block copolymers,37 and surface-initiated ATRP.38 Furthermore, monodisperse coreshell spherical colloids with movable gold cores have been prepared by three major steps, which allow the optical sensing of chemical diffusion into the cavity.39 Polypyrrole (PPy) hollow capsules with movable hematite cores were produced through soaking sandwich ellipsoids in a hydrofluoric acid (HF) aqueous solution.40 Tin-encapsulated spherical hollow carbon for the anode material in lithium secondary batteries has been synthesized by a sol-gel polymerization technique with subsequent decomposition of tributylphenyltin (TBPT) at 700 °C under an argon atmosphere.41 An important concern of hollow microspheres is to accommodate guest nanoparticles in their cavity, which results in an interesting structure as hollow microspheres with movable cores and novel properties different from those of host hollow microspheres and the guest nanoparticles. Nanoparticles such as gold,39,42 silver,43 tin,41 silica,44 and iron oxide45 could be incorporated into the interior of the hollow microspheres by various techniques. In our previous work, monodisperse polymer microspheres with different functional groups on their surfaces46-48 and silica/ (22) Jiang, P.; Bertone, J. F.; Colvin, V. L. Science 2001, 291, 453-457. (23) Ding, J.; Liu, G. J. J. Phys. Chem. B 1998, 102, 6107-6113. (24) Shchukin, D. G.; Shutava, T.; Shchukina, E.; Sukhorukov, G. B.; Lvov, Y. M. Chem. Mater. 2004, 16, 3446-3451. (25) Kim, S. W.; Kim, M.; Lee, W. Y.; Hyeon, T. J. Am. Chem. Soc. 2002, 124, 7642-7643. (26) Xu, X.; Asher, S. A. J. Am. Chem. Soc. 2004, 126, 7940-7945. (27) Pavlyuchenko, V. N.; Sorochinskaya, O. V.; Ivanchev, S. S.; Klubin, V. V.; Kreichman, G. S.; Budtov, V. P.; Skrifvars, M.; Halme, E.; Koskinen, J. J. Polym. Sci., Part A: Polym. Chem. 2001, 39, 1435-1449. (28) Jang, J.; Ha, H. Chem. Mater. 2003, 15, 2109-2111. (29) Yoon, S. B.; Sohn, K.; Kim, J. Y.; Shin, C. H.; Yu, J. S.; Hyeon, T. AdV. Mater. 2002, 14, 19-21. (30) Zhong, Z.; Yin, Y.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 206-209. (31) Titirici, M. M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 38083812. (32) Huang, J.; Xie, Y.; Li, B.; Liu, Y.; Qian, Y.; Zhang, S. AdV. Mater. 2000, 12, 808-811. (33) Chen, Z.; Zhan, P.; Wang, Z. L.; Zhang, J. H.; Zhang, W. Y.; Ming, N. B.; Chan, C. T.; Sheng, P. AdV. Mater. 2004, 16, 417-422. (34) Wu, D.; Ge, X.; Zhang, Z.; Wang, M.; Zhang, S. Langmuir 2004, 20, 5192-5195. (35) McDonald, C. J.; Bouck, K. J.; Chaupt, A. B.; Stevens, C. J. Macromolecules 2000, 33, 1593-1605. (36) Park, M. K.; Onishi, K.; Locklin, J.; Caruso, F.; Advincula, R. C. Langmuir 2003, 19, 8550-8554. (37) Stewart, S.; Liu, G. J. Chem. Mater. 1999, 11, 1048-1054. (38) Mandal, T. K.; Fleming, M. S.; Walt, D. R. Chem. Mater. 2000, 12, 3481-3487. (39) Kamata, K.; Lu, Y.; Xia, Y. J. Am. Chem. Soc. 2003, 125, 2384-2385. (40) Hao, L. Y.; Zhu, C. L.; Jiang, W. Q.; Chen, C. N.; Hu, Y.; Chen, Z. Y. J. Mater. Chem. 2004, 14, 2929-2934. (41) Lee, K. T.; Jung, Y. S.; Oh, S. M. J. Am. Chem. Soc. 2003, 125, 56525653. (42) Kim, M.; Sohn, K.; Na, H. B.; Hyeon, T. Nano Lett. 2002, 2, 1383-1387. (43) Cheng, D. M.; Zhou, X. D.; Xia, H. B.; Chan, H. S. O. Chem. Mater. 2005, 17, 3578-3581. (44) Zhang, K.; Zhang, X.; Chen, H.; Chen, X.; Zhang, L.; Zhang, J.; Yang, B. Langmuir 2004, 20, 11312-11314. (45) Zheng, T.; Pang, J.; Tan, G.; He, J.; McPherson, G. L.; Lu, Y.; John, V. T.; Zhan, J. Langmuir 2007, 23, 5143-5147. (46) Bai, F.; Yang, X. L.; Huang, W. Q. Macromolecules 2004, 37, 97469752. (47) Bai, F.; Yang, X. L.; Huang, W. Q. Eur. Polym. J. 2006, 42, 2088-2097.

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poly(N,N′-methylenebisacrylamide) (SiO2/polyMBAAm) coreshell composite materials49 were prepared by distillation precipitation polymerization in acetonitrile with 2,2′-azobisisobutyronitrile (AIBN) as an initiator. Here, we describe the synthesis of a gold/silica/polyMBAAm trilayer heterostructure composite and the corresponding hollow polyMBAAm microspheres with movable Au cores by the subsequent removal of the silica midlayer with hydrofluoric acid. Further, the diffusion of chemical reagents across the shells of hollow polyMBAAm particles was investigated by catalytic reduction of 4-nitrophenol to 4-aminophenol with sodium borohydride as a reductant. Experimental Section Chemicals. Tetrachloroauric acid trihydrate (HAuCl4‚3H2O) was purchased from Shenyang Research Institute of Nonferrous Metals, China. Tetraethyl orthosilicate (Si(OC2H5)4, TEOS) and (3-aminopropyl)trimethoxysilane (APS) were purchased from Aldrich and used without any further purification. Sodium silicate was purchased from Tianjin Guangfu Fine Chemical Engineering Institute and used without purification. N,N′-Methylenebisacrylamide (MBAAm; chemical grade, Tianjin Bodi Chemical Engineering Co.) was recrystallized from acetone. AIBN was available from the Chemical Factory of Nankai University and recrystallized from methanol. HF (containing 40 wt % HF) was provided by Tianjin Chemical Reagent Institute. Acetonitrile (analytical grade, Tianjin Chemical Reagents II Co.) was dried over calcium hydride and purified by distillation before use. Sodium borohydride (NaBH4) was purchased from Tianjin Chemical Reagents III Co. in analytical grade. 4-Nitrophenol (4-NP; Tianjin Chemical Reagent Factory) was recrystallized from petroleum ether and ethyl acetate. All the other reagents were analytical grade and used without any further purification. Synthesis of Au/silica Particles. Au/silica core-shell particles were prepared according to the techniques described by Liz-Marza´n et al.,50 in which the Au sol was synthesized at first by the standard sodium citrate reduction method. A 50 mL portion of 1% sodium citrate solution was added to 950 mL of HAuCl4 aqueous solution containing 50 mg of Au in the boiling state with vigorous stirring, and the reduction was performed further for 30 min. After the Au sol was cooled to room temperature, a freshly prepared aqueous solution of APS (2.5 mL, 1 mM) was added to 500 mL of the Au sol under vigorous magnetic stirring to coat the Au particles with silica. The mixture of APS and Au sol was allowed to stand for 15 min to ensure the complete complexation of the amine groups of APS with the Au particles. A solution of active silica solution was prepared by adjusting the pH of a 0.54 wt % sodium silicate solution to 10-11 by progressive addition of cation exchange resin of poly(divinylbenzene-co-methacrylic acid), which was prepared according to our previous paper.51 A 20 mL portion of active silica was then added to 500 mL of the surfactant-stabilized Au sol under vigorous magnetic stirring, and the resulting dispersion with a pH of 8.5 was allowed to stand at ambient temperature for 3 days. The particles were ultracentrifuged, decanted, and redispersed in 500 mL of water/ ethanol (1:4, v/v). A 0.3 mL sample of TEOS and 2 mL of 25% aqueous ammonium solution were then added to the above prepared Au sol, and the encapsulation was performed further for 12 h with vigorous stirring. Addition of 0.3 mL of TEOS was repeated for the efficient encapsulation of silica onto Au cores to result in mondisperse Au/silica particles. The resultant Au/silica core-shell particles were centrifugated, decanted, and dried in a vacuum oven at 50 °C till constant weight. Preparation of a Au/Silica/PolyMBAAm Trilayer Composite. A typical procedure for the distillation precipitation polymerization is as follows: In a dried 50 mL two-necked flask, 0.04 g of Au/silica (48) Liu, G. Y.; Yang, X. L.; Wang, Y. M. Polym. Int. 2007, 56, 905-913. (49) Liu, G. Y.; Yang, X. L.; Wang, Y. M. Polymer 2007, 48, 4385-4392. (50) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 43294335. (51) Bai, F.; Yang, X. L.; Li, R.; Huang, B.; Huang, W. Q. Polymer 2006, 47, 5775-5784.

Au/Silica/Polymer Trilayer Composite Synthesis core-shell particles was suspended in 40 mL of acetonitrile as a red suspension. Then MBAAm (0.01 g, total as 0.025 wt % of the reaction system) and AIBN (0.0002 g, 2 wt % relative to the monomer) were dissolved in the suspension. The two-necked flask attached to a fractionating column, Liebig condenser, and receiver was submerged in a heating mantle. The reaction mixture was heated from ambient temperature till the boiling state within 20 min, and the reaction system was kept under the refluxing state for a further 10 min. The polymerization was ended after 20 mL of acetonitrile was distilled off the reaction system within 70 min. After the reaction, the resultant Au/silica/polyMBAAm trilayer composite was purified by repeating centrifugation, decantation, and resuspension in acetone with ultrasonic bathing three times. The composite particles were then dried in a vacuum oven at 50 °C till constant weight. The other distillation precipitation polymerizations were very similar to the typical procedure by varying the weight ratio of the MBAAm monomer to Au/silica particles, while the amount of AIBN initiator was maintained at 2 wt % relative to the monomer. The treatment of these composite particles was the same as the typical process. The reproducibility of the polymerizations was confirmed through several duplicate and triplicate experiments. Synthesis of Hollow PolyMBAAm Microspheres with Movable Au Cores. The resultant Au/silica/polyMBAAm trilayer composite particles were immersed in 40 wt % hydrofluoric acid solution for 2 h. The excessive HF and the formed SiF4 were expelled out of the reaction system. The hollow polyMBAAm microspheres with movable Au cores were purified by several centrifugation/washing cycles in water till pH 7. The resultant hollow particles were then dried in a vacuum oven at 50 °C till constant weight. Catalytic Reduction of 4-NP to 4-Aminophenol (4-AnP). The catalytic activity of the hollow polyMBAAm microspheres with movable Au cores was investigated through the reduction of 4-NP to 4-AnP in the presence of NaBH4 as the reductant at ambient temperature in an aqueous solution. A typical reduction was carried out as follows: 1 mL of 4-NP aqueous solution (0.1 mmol/L, 1 × 10-4 mmol) was placed in a standard quartz cell. Then 2 mL of freshly prepared NaBH4 aqueous solution (0.1 mol/L, 2 × 10-4 mol) and a 0.1 mL suspension of hollow polyMBAAm microspheres with movable Au cores (1 g/L, containing 6 × 10-8 mol of Au, with a polyMBAAm shell thickness of 23 nm) were subsequently introduced to the quartz cell with gentle shaking. The catalytic activity was monitored by a UV-vis spectrometer with a decrease of the intensity of the peak at 400 nm attributed to the typical absorption of 4-NP. The catalyst was recovered by centrifugation, decantation, washing, and drying. Then 1 mL of 4-NP aqueous solution (0.1 mmol/L) and 2 mL of freshly prepared NaBH4 aqueous solution (0.1 mol/L) were added to test the recycling activity of the catalyst three times. Characterization. The size and size distribution of the Au/silica core-shell particles, Au/silica/polyMBAAm trilayer composite particles, and hollow polyMBAAm microspheres with movable Au cores were determined by transmission electron microscopy (TEM; Tecnai G2 20 S-TWIN). X-ray photoelectron spectroscopy (XPS) analysis was carried out with a PHI 5300 XPS surface analysis system (Physical Electronics, Eden Prairie, MN) using a Mg KR X-ray source operating at 250 W and 13 kV (hν ) 1253.6 eV). The electronic binding energy of C 1s (284.6 eV) was used as the internal standard. UV-vis spectra were performed on a Shimadzu UV-2101 PC spectrometer ranging from 200 to 600 nm.

Results and Discussion It is difficult to directly perform the polymerization on the surface of gold nanoparticles for the preparation of the composite containing noble metallic Au and polymer due to the lack of an appropriate interaction between Au particles and the monomer. To overcome such a problem, the addition of an interlayer between the Au particles and the polymer layer is necessary, which should have suitable interaction between the Au particles and the polymer layer. Coating the gold nanoparticles with a silica layer

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accomplishes such a requirement. The silica layer with active hydroxyl groups on the surface not only occurs as a stable and homogeneous dispersion in nonaqueous solvent but also behaves as the ideal core for further polymerization to afford the complicated composite. Au/silica core-shell particles were synthesized according to the method described in the literature by Liz-Marza´n et al.50 The TEM micrograph of Au/silica particles in Figure 1a indicates that most of the Au/silica particles had a core-shell structure with spherical shapes, in which the gold nanoparticles were in the middle of the core-shell particles. The mean diameters of the Au core and Au/silica core-shell particles were 13 and 58 nm, respectively. In other words, the thickness of the silica shell was around 23 nm. Such results demonstrated that the silica layer was effectively encapsulated over the Au nanoparticles, which permitted the further development of inorganic/polymer trilayer composites. Preparation of a Au/Silica/PolyMBAAm Trilayer Composite. Scheme 1 illustrates the preparation of Au/silica/ polyMBAAm trilayer composites with Au/silica particles as seeds by the distillation precipitation polymerization of MBAAm in acetonitrile and the further development of hollow polyMBAAm microspheres with movable Au cores via the subsequent removal of the silica midlayer in hydrofluoric acid. Distillation precipitation polymerization has been proven to be a useful and versatile technique for the synthesis of monodisperse polymer microspheres with different functional groups46-48 and core-shell microspheres.49 In the present work, Au/silica particles were used as templates for the preparation of Au/silica/polyMBAAm trilayer composite particles as shown in Scheme 1. The hydrogen-bonding interaction between the hydroxyl groups on the surface of the Au/silica template particles and the amide unit of MBAAm played an active role during the distillation precipitation polymerization and served as a driving force for the efficient encapsulation of polyMBAAm on the Au/ silica particles. The mechanism of the formation of monodisperse silica/polyMBAAm core-shell microspheres driven by the hydrogen-bonding interaction between the hydroxyl group and the amide group has been investigated in detail in our previous paper.49 In the polymerization system, an MBAAm monomer was first adsorbed onto the surface of Au/silica particles via the hydrogen-bonding interaction as illustrated in Scheme 1 to incorporate the reactive vinyl groups. Then the adsorbed vinyl groups on the surface of the Au/silica seeds captured the newly formed polyMBAAm oligomers for the growth of Au/silica/ polyMBAAm trilayer composite particles, which was much similar to the role of the residual vinyl groups on the poly(divinylbenzene) (PDVB) microspheres during the distillation precipitation polymerization.46 Figure 1 shows the TEM micrographs of Au/silica/polyMBAAm trilayer composites, which indicate that the composite microspheres had spherical shapes, in which some particles were slightly aggregated due to the high reactivity of the MBAAm monomer. The absence of the small second initiated particles indicated that the capture ability of the incorporated vinyl groups through the hydrogen-bonding interaction was efficient and enough for the construction of the Au/silica/polyMBAAm trilayer microspheres, in which the thickness of the out-layer polyMBAAm was conveniently controlled by the MBAAm monomer loading for the distillation precipitation polymerization. The experimental conditions for the distillation precipitation polymerizations of MBAAm with Au/silica particles as seeds, the size and yield of the resultant Au/silica/polyMBAAm composite materials, and the thickness of the polyMBAAm shell

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Figure 1. Micrograph of the microspheres: (A) TEM micrograph of Au/silica core-shell particles, (B-E) TEM micrographs of Au/ silica/polyMBAAm trilayer particles with different MBAAm feeds during polymerization as the mass ratio to Au/silica particles [(B) 1/4, (C) 1/2, (D) 3/4]. Scheme 1. Preparation of Au/Silica/PolyMBAAm Composites and the Corresponding Hollow Polymer Microspheres with Au Cores

layer are summarized in Table 1. The low MBAAm monomer loading was employed for the polymerization due to the high reactivity of the MBAAm monomer as the divinyl groups were connected by the flexible methylene and diamide groups, which was discussed in detail in our previous work.48 The size of the trilayer composite particles increased with increasing MBAAm loading during the polymerization. The maximum diameter of

104 nm was afforded when the ratio of MBAAm to Au/silica particles (in mass) was 3/4 as entry D in Table 1. This meant that the thickness of the polyMBAAm shell layer in the trilayer composite was increased from 12 to 23 nm with MBAAm feed enhancement from 1/4 to 3/4 (in mass ratio to the Au/silica particles). The yield of the resultant Au/silica/polyMBAAm trilayer microspheres in Table 1 was as high as 95% irrespective

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Table 1. Size, Polymer Shell Thickness, Yield, and Fraction of the Polymer Component of Au/Silica/Polymer Particles with Different Recipes in the Feed entry

Au/SiO2 mass (g)

MBAAm mass (g)

Dn (nm)

polymer shell thickness (nm)

yieldb (%)

fraction of polymerc (%)

fraction of SiO2d (%)

Aa B C D

0.04 0.04 0.04 0.04

0 0.01 0.02 0.03

58 81 92 104

0 12 17 23

0 96 95 98

0 18 33 41

73 60 53

a Au/silica particles. b Yield ) (MAu/silica/polymer - MAu/silica)/Mmonomer × 100. c Fraction of polymer ) (MAu/silica/polymer - MAu/silica)/MAu/silica/polymer × 100. d Fraction of SiO2 ) (MAu/silica/polymer - MAu/air/polymer)/MAu/silica/polymer × 100.

Figure 2. XPS spectra of the Au/silica/polyMBAAm trilayer composite with a polymer shell thickness of 23 nm.

to the MBAAm feed changing from 1/4 to 3/4 (in mass ratio to the Au/silica core). In other words, polyMBAAm was quantitatively encapsulated onto the surface of the Au/silica seeds with the aid of the hydrogen-bonding interaction between the hydroxyl group on the surface of the silica layer and the amide group of the polyMBAAm component. The high reactivity of MBAAm, which originated from its structure as discussed in our previous work,48,49 led to the rough surface of the Au/silica/polyMBAAm trilayer composite and interparticle connections, though the concentration of MBAAm monomer in the feed was low. On the other hand, PDVB cannot be directly encapsulated over the Au/ silica particles without modification of the surface due to the hydrophobic nature of the DVB monomer, in which the hydrogenbonding interaction was absent between the PDVB component and the Au/silica particles during the polymerization. The fraction of the polyMBAAm component was obtained by the weight increase of the resultant trilayer composites after the polymerization as summarized in Table 1. The fraction of the polyMBAAm component was increased significantly from 18% to 41% when the MBAAm feed was enhanced from 1/4 to 3/4 (mass ratio to the Au/silica core) during the polymerization. The surface components of the Au/silica/polyMBAm trilayer composite with a polymer shell thickness of 23 nm were determined by XPS spectra as shown in Figure 2, in which the electronic binding energy of C 1s (284.6 eV) was used as the internal standard. The peaks at 531.4, 399.7, and 102.1 eV were ascribed to the binding energy of O 1s, N 1s, and Si 2p, respectively. The atom concentration on the surface of the trilayer composite (excluding H) is also illustrated in Figure 2, in which the Si concentration was calculated as low as 0.99%. This result indicated that the silica was slightly impervious to the polymer shell layer during the encapsulation of polyMBAAm. All these

results further confirmed the successful encapsulation of polyMBAAm over the Au/silica particles. Preparation of PolyMBAAm Hollow Microspheres with Movable Au Cores. The silica midlayer of the resultant Au/ silica/polyMBAAm trilayer composites was selectively removed by etching in hydrofluoric acid to afford polyMBAAm hollow microspheres with movable Au cores. The driving force for such removal was due to the formation of SiF4 gas, which was given off from the composites during the etching process. The TEM micrographs of polyMBAAm hollow microspheres with movable Au cores having different polymer shell thicknesses are shown in Figure 3. When the MBAAm monomer feed during the polymerization was as low as 1/4 (mass ratio to the Au/silica particles), the results in Figure 3A with partially collapsed particles indicate that the polyMBAAm shell layer was not thick (around 12 nm) enough to support the cavities formed during the selective etching of silica midlayers. The hollow polyMBAAm microspheres with movable Au cores as shown in Figure 3B,C were obtained with an MBAAm feed in the range of 1/2 to 3/4 (mass ratio to the Au/silica particles), in which the convincing hollow-sphere structures were observed with the presence of circular rings of nonaggregated spheres and a cavity in the interior. The thickness of the polyMBAAm shell layer for the resultant polyMBAAm hollow microspheres with movable Au cores was estimated from TEM characterization increasing from 16 to 23 nm as summarized in Table 1, while the MBAAm loading was increased from 1/2 to 3/4 (mass ratio to the Au/silica particles) during the distillation precipitation polymerization. In short, the shell thickness of the resultant polyMBAAm hollow microspheres with movable Au cores can be conveniently controlled by altering the amount of MBAAm monomer feed during the formation of the shell layer by distillation precipitation polymerization. However, when the loading of MBAAm for the polymerization was increased further, second initiated small polyMBAAm particles with rough surfaces were obtained, in which the capture ability of the composite particles was not enough during the polymerization in the case of high MBAAm loading due to the high reactivity of the MBAAm monomer. The fraction of silica component in the trilayer composites as the mass difference between Au/silica/polyMBAAm and the corresponding hollow polyMBAAm microspheres with movable Au cores after etching was decreased significantly from 73% to 53% when the MBAAm feed during the polymerization was increased from 1/4 to 3/4 (mass ratio to the Au/silica particles). Such results were consistent with the fraction of polyMBAAm component in the Au/silica/ polyMBAAm trilayer composites as discussed above. Further, these results confirmed the successful removal of the silica layer by hydrofluoric acid during the etching process. Catalytic Behavior of Hollow PolyMBAAm Microspheres with Movable Au Cores. Since Au nanoparticles located in the cavity of the cross-linked polyMBAAm hollow microspheres and the hydrophilic nature of the polyMBAAm shell permitted the molecules of reactants and products in water to pass through

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Figure 3. TEM micrographs of polyMBAAm hollow microspheres with movable Au cores and different polyMBAAm shell thicknesses by altering the MBAAm feed during the distillation precipitation polymerization: (A) 1/4, (B) 1/2, (C) 3/4 (in mass ratio to the Au/silica particles).

the polymer shell layer, polyMBAAm hollow microspheres with movable Au cores could be utilized as the catalyst in aqueous solution. Nowadays, metallic gold catalysts with high activity, stability, and selectivity are investigated widely, especially the supported species which can be recovered through a simple procedure.52,53 To study the catalytic property of the hollow polyMBAAm microspheres with movable Au cores, the reduction of 4-NP to 4-AnP with sodium borohydride as the reductant was used as a model reaction. A solution of 4-NP has a yellow color and a distinct UV-vis spectrum profile with the absorption maximum at around 400 nm. When 4-NP solution was mixed with NaBH4 solution, the yellow of the solution did not change, indicating the absence of the reduction. In other words, BH4under the experimental conditions was unable to reduce 4-NP to 4-AnP without polyMBAAm hollow microspheres with movable Au cores. After the hollow polyMBAAm microspheres with movable Au cores were introduced into the mixture, the yellow color faded gradually with a decrease of the peak at 400 nm in the UV-vis spectrum as shown in Figure 4. The catalytic activity can be estimated from the reaction time for the complete disappearance of the UV-vis absorption at 400 nm of 4-NP. The result in Figure 4 indicates that the peak at 400 nm of the UVvis absorption disappeared completely after 20 min. Therefore, the polyMBAAm hollow microspheres with movable Au cores (52) Praharaj, S.; Nah, S.; Ghosh, S. K.; Kundu, S.; Pal, T. Langmuir 2004, 20, 9889-9892. (53) Liu, W.; Yang, X. L.; Xie, L. J. Colloid Interface Sci. 2007, 313, 494502.

Figure 4. UV-vis absorption spectra of the catalytic reduction of 4-nitrophenol to 4-aminophenol developed with different reaction times. Reaction conditions: 1 mL of 4-aminophenol aqueous solution (0.1 mmol/L), 2 mL of fresh prepared aqueous NaBH4 solution (0.1 mol/L), 0.1 mL suspension in water of hollow polyMBAAm microspheres with movable Au cores (1 g/L, polyMBAAm shell thickness 23 nm).

acted as a catalyst to transfer the electron from NaBH4 to 4-NP for the effective reduction to 4-AnP in water. Furthermore, the hollow polyMBAAm microspheres with movable Au cores as a heterogeneous catalyst were isolated from the reaction system by simple ultracentrifugation and decantation. The heterogeneous

Au/Silica/Polymer Trilayer Composite Synthesis Table 2. Reaction Time for the Complete Disappearance of 4-NP in the UV-Vis Spectra during the Catalytic Reaction of Each Recycling recycling no.

reaction time (min)

1 2 3

20 20 20

catalyst was recovered, washed, dried, and then recycled for the reduction of 4-NP to 4-AnP. The catalytic activity of the hollow polyMBAAm microspheres with movable Au cores during the recycling was quantitatively determined by the reaction time for the complete disappearance of the UV-vis absorption at 400 nm corresponding to 4-NP as summarized in Table 2. The results demonstrated that the catalyst retained high activity during the recycling, which was indicated by the reaction time remaining at around 20 min. All these results indicated that the hollow polyMBAAm microspheres with movable Au cores acted as a recyclable nanoreactor and catalyst for the effective reduction of 4-NP to 4-AnP.

Conclusion Au/silica/polyMBAAm trilayer composites with regular shape were prepared by distillation precipitation polymerization of MBAAm in neat acetonitrile with Au/silica particles as seeds and AIBN as the initiator in the absence of any additive, in which the hydrogen-bonding interaction between the amide group of

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the polyMBAAm component and the hydroxyl group on the surface of the Au/silica seeds acted as a driving force for the efficient encapsulation of polyMBAAm over the Au/silica particles. The thickness of the polyMBAAm shell layer and the morphology of the resultant Au/silica/polyMBAAm trilayer composites were conveniently controlled by the MBAAm monomer loading during the polymerization, in which the silica was slightly impervious to the polymer shell layer during the encapsulation of polyMBAAm. The polyMBAAm hollow microspheres with movable Au cores and the polyMBAAm shell thickness in the range of 12-23 nm were further developed by the selective etching of the silica midlayer in hydrofluoric acid from Au/silica/polyMBAAm trilayer composites. The hollow polyMBAAm microspheres with movable Au cores were used as a recyclable nanoreactor and catalyst for the effective reduction of 4-NP to 4-AnP. Acknowledgment. This work was supported in part by the National Science Foundation of China (Project No. 20504015) and the Opening Research Fund from the State Key Laboratory of Polymer Chemistry and Physics, Chinese Academy of Sciences (Project No. 200613). Supporting Information Available: TEM images of Au/silica/ poly(MBAAm-co-MAA) trilayer composites and poly(MBAAmco-MAA) microspheres with movable Au cores. This material is available free of charge via the Internet at http://pubs.acs.org. LA7025957