Inorganic

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A Novel and Universal Route to SiO2-Supported Organic/Inorganic Hybrid Noble Metal Nanomaterials via Surface RAFT Polymerization Jiliang Liu, Lifen Zhang, Suping Shi, Shuai Chen, Nianchen Zhou, Zhengbiao Zhang, Zhenping Cheng,* and Xiulin Zhu* Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China Received December 26, 2009. Revised Manuscript Received August 15, 2010 Polymer-encapsulated gold or silver nanoparticles were synthesized and sterically stabilized by a shell layer of poly(4-vinylpyridine) (P4VP) grafted on SiO2 nanoparticles that acts as a scaffold for the synthesis of hybrid noble metal nanomaterials. The grafting P4VP shell was synthesized via surface reversible addition-fragmentation chain transfer (RAFT) polymerization of 4-vinylpyridine (4VP) using SiO2-supported benzyl 9H-carbazole-9-carbodithioate (SiO2BCBD) as the RAFT agent. The covalently tethered P4VP shell can coordinate with various transition metal ions such as Au3þ or Agþ and therefore stabilize the corresponding Au or Ag nanoparticles reduced in situ by sodium borohydride (NaBH4) or trisodium citrate. The SiO2-supported RAFT agent and the Au or Ag nanoparticles embedded in the P4VP shell layer were characterized by UV-vis spectrophotometer, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and surface-enhanced Raman spectroscopy (SERS).

Introduction Modification of inorganic nanoparticles with organic polymer layers has received increasing interest in the past few years because of the tunable and unique properties of the resulting hybrid nanomaterials stemming from their small size and large specific surface area. These organic/inorganic hybrids have already led to novel applications in electronics, optics, biosciences, and engineering.1 Inorganic silica nanoparticles turn out to be perfectly suited as a solid substrate because of its chemical resistance, mechanical stability, relatively low costs, high specific surface area, the great variety of available particle sizes, and its numerous applications.2 There are a number of reports on the synthesis of hybrid silica nanoparticles both by “grafting-to” and “grafting-from” techniques. Higher grafting densities and thicker polymer layers can be obtained using the “grafting-from” approach as compared with “grafting-to” technique.3 Procedures for the “grafting-from” technique commonly involve covalent attachment of a suitable atom transfer radical polymerization (ATRP) initiator or reversible addition-fragmentation transfer (RAFT) agent to silica. RAFT polymerization, a recently developed controlled radical polymerization (CRP) technique, has been widely used to prepare different functional polymer materials with predetermined molecular weights, narrow polydispersities, and advanced architectures because this technique can adapt to a wide range of reaction conditions, easily get various block copolymers, and have the *Corresponding authors. E-mail: [email protected] (Z.P.C.), [email protected] (X.L.Z.); Fax 86-512-65882787, 65112796. (1) (a) Kickelbick, G. Prog. Polym. Sci. 2003, 28, 83–114. (b) Gravano, S. M.; Dumas, R.; Liu, K.; Patten, T. E. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 3675– 3688. (c) Vatutsina, O. M.; Soldatov, V. S.; Sokolova, V. I.; Johann, J.; Bissen, M.; Weissenbacher, A. React. Funct. Polym. 2007, 67, 184–201. (d) Hamouda, S. B.; Nguyen, Q. T.; Langevin, D.; Chappey, C.; Roudesli, S. React. Funct. Polym. 2007, 67, 893–904. (2) (a) Radhakrishnan, B.; Ranjan, R.; Brittain, W. J. Soft Matter 2006, 2, 386– 396. (b) Takamura, M.; Yamauchi, T.; Tsubokawa, N. React. Funct. Polym. 2008, 68, 1113–1118. (3) (a) Zhao, B.; Brittain, W. J. Prog. Polym. Sci. 2000, 25, 677–710. (b) Huang, C. L.; Tassone, T.; Woodberry, K.; Sunday, D.; L. Green, D. L. Langmuir 2009, 25, 13351–13360.

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compatibility with a wide range of functional monomers.4 On the other hand, in contrast to the large number of reports using ATRP to prepare polymer grafted substrates, there are surprisingly few reports on the application of RAFT techniques to the synthesis of polymer grafted substrates, probably due to the difficulty to covalently attach RAFT agent to a substrate.5 Perrier et al. successfully grafted methyl acrylate to silica via a Z-supported RAFT polymerization.6 Following a similar principle, they designed different silica-supported RAFT agents to mediate the polymerization of various monomers, such as styrene, methyl methacrylate, vinyl acetate, butyl acrylate, N,N-dimethylacrylamide, and N-isopropylacrylamide.7 Benicewicz et al. reported the synthesis of 20 nm silica nanoparticles grafted with methyl methacrylate and the probable mechanism during the polymerization.8 Hong et al. reported poly(N-isopropylacrylamide) grafted to the silica nanoparticles and studied their thermal properties.9 Brittain et al. reported a novel way to immobilize the RAFT agent onto silica nanoparticles by click chemistry and then obtained welldefined polymer brushes of PS, PAM homopolymer, and PS-bPMA block copolymer brushes by surface RAFT polymerization.10 Brittain et al. first immobilized an azo-initiator on silicon wafers, and well-defined different polymer brushes of PS, PMMA, and PDMA were covalently tethered onto the silicon (4) (a) Chiefari, J.; Chong, Y. K.; Ercole, F.; Krstina, J.; Jeffery, J.; Le, T. P. T.; Mayadunne, R. T. A.; Meijs, G. F.; Moad, C. L.; Moad, G.; Rizzardo, E.; Thang, S. H. Macromolecules 1998, 31, 5559–5562. (b) Lowe, A. B.; McCormick, C. L. Aust. J. Chem. 2002, 55, 367–379. (c) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2005, 58, 379–410. (d) Moad, G.; Rizzardo, E.; Thang, S. H. Aust. J. Chem. 2006, 59, 669–692. (5) Pei, X. W.; Hao, J. C.; Liu, W. M. J. Phys. Chem. C 2007, 111, 2947–2952. (6) Zhao, Y. L.; Perrier, S. Macromolecules 2006, 39, 8603–8608. (7) (a) Zhao, Y. L.; Perrier, S. Macromolecules 2007, 40, 9116–9124. (b) Nguyen, D. H.; Wood, M. R.; Zhao, Y. L.; Perrier, S.; Vana, P. Macromolecules 2008, 41, 7071– 7078. (8) Li, C. H.; Han, J. W.; Ryu, C. Y.; Benicewicz, B. C. Macromolecules 2006, 39, 3175–3183. (9) Hong, C. Y.; Li, X.; Pan, C. Y. J. Phys. Chem. C 2008, 112, 15320–15324. (10) (a) Ranjan, R.; Brittain, W. J. Macromol. Rapid Commun. 2007, 28, 2084– 2089. (b) Ranjan, R.; Brittain, W. J. Macromol. Rapid Commun. 2008, 29, 1104–1110. (c) Ranjan, R.; Brittain, W. J. Macromolecules 2007, 40, 6217–6223.

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wafers by RAFT polymerizations in the presence of a free RAFT agent.11 Poly(4-vinylpyridine) (P4VP) has been well-known as a typical coordinative polymer, which contains the pyridine group and can coordinate with various transition metal ions.12 To the best of our knowledge, there are few reports involved the P4VP synthesis via RAFT polymerization,13 and only one document reported the P4VP-functionalized silica.14 Metal nanoparticles, especially gold and silver nanoparticles, have been extensively investigated over the past decade due to their unique electronic, optical, antimicrobial, surface-enhanced Raman spectroscopy, and catalytic properties.15 These properties are neither those of the bulk metal nor that of individual atoms but strongly depend on the size and shape of the nanoparticles and interparticle distance. The progress in polymer-stabilized nanoparticles is one of the most salient approaches for the metal nanoparticles stability problem and controlling the interparticle distance.16 The aim of this work is to develop a novel method for fabricating silica-supported and polymer-stabilized metal nanoparticles via the RAFT technique. Here, the P4VP was selected as the polymer stabilizer, and two noble metals, gold and silver, were used as the model metal nanoparticles. In this work, about 100 nm SiO2 nanoparticles were synthesized, and the RAFT agent, benzyl 9H-carbazole-9-carbodithioate (BCBD), was immobilized onto the surfaces of the SiO2 nanoparticles. The functionalized silica was then used as a silica-supported RAFT agent to mediate the polymerization of 4-vinylpyridine (4VP) from the silica nanoparticle surface to produce a well-defined and covalently tethered P4VP shell (SiO2-g-P4VP). By surface activation with Au3þ or Agþ aqueous solution and then reduction by sodium borohydride (NaBH4) or trisodium citrate, the Au or Ag nanoparticles embedded in the P4VP shell layer of core-shell silica nanoparticles were obtained. To the best of our knowledge, this is the first example for the synthesis SiO2-supported organic/inorganic hybrid noble metal nanomaterials via surface RAFT polymerization.

Experimental Section Materials. Tetraethyl orthosilicate (TEOS, 99%) was purchased from Aldrich. 4-(Chloromethyl)phenyltrimethoxysilane (90%) was supplied from Alfa Aesar. 4-Vinylpyridine (4VP, 95%) was obtained from Acros, distilled under reduced pressure, and then stored at -15 °C. Azobis(isobutyronitrile) (AIBN, 99%, Shanghai Chemical Reagent Co., Ltd., China) was recrystallized (11) Baum, M.; Brittain, W. J. Macromolecules 2002, 35, 610–615. (12) (a) El-Hamshary, H.; El-Garawany, M.; Assubaie, F. N.; Al-Eed, M. J. Appl. Polym. Sci. 2003, 89, 2522–2526. (b) Rivas, B. L.; Quilodran, B.; Quiroz, E. J. Appl. Polym. Sci. 2004, 92, 2908–2916. (c) Vidts, K. R. M.; Du Prez, F. E. Eur. Polym. J. 2005, 42, 43–50. (d) Kim, S. M.; Kim, G. S.; Lee, S. Y. Mater. Lett. 2008, 62, 4354–4356. (e) Gniewek, A.; Trzeciak, A. M.; Tylus, W. J. Catal. 2005, 229, 332–343. (f ) Sidorov, S. N.; Bronsyein, L. M.; Colfen, H.; Antonietti, M. J. Colloid Interface Sci. 1999, 212, 197–211. (13) (a) Zhao, D. Y.; Chen, X.; Liu, Y.; Wu, C. L.; Ma, R. J.; An, Y. L.; Shi, L. Q. J. Colloid Interface Sci. 2009, 331, 104–112. (b) Shi, S. P.; Zhang, L. F.; Zhu, J.; Zhang, W.; Cheng, Z. P.; Zhu, X. L. eXPRESS Polym. Lett. 2009, 3, 401–412. (c) Bo
zovic-Vukic, J.; Ma
non, H. T.; Meuldijk, J.; Koning, C.; Klumperman, B. Macromolecules 2007, 40, 7132–7139. (d) Zheng, G. H.; Pan, C. Y. Macromolecules 2006, 39, 95–102. (e) Convertine, A. J.; Sumerlin, B. S.; Thomas, D. B.; Lowe, A. B.; McCormick, C. L. Macromolecules 2003, 36, 4679–4681. (14) Lu, C. H.; Zhou, W. H.; Han, B.; Yang, H. H.; Chen, X.; Wang, X. R. Anal. Chem. 2007, 79, 5457–5461. (15) (a) Kickelbick, G. Prog. Polym. Sci. 2002, 28, 83–114. (b) Alivisatos, A. P. Science 1996, 271, 933–937. (c) Zhang, Y. W.; Peng, H. H.; Huanga, W.; Zhou, Y. F.; Yan, D. Y. J. Colloid Interface Sci. 2008, 325, 371–376. (d) Dubas, S. T.; Kumlangdudsana, P.; Potiyaraj, P. Colloids Surf., A 2006, 289, 105–109. (e) Rivas, L.; Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. Langmuir 2001, 17, 574–577. (f ) Kim, Y. N.; Yoo, S. H.; Cho, S. O. J. Phys. Chem. C 2009, 113, 618–623. (g) Ji, N.; Ruan, W. D.; Wang, C. X.; Lu, Z. C.; Zhao, B. Langmuir 2009, 25, 11869–11873. (16) (a) Macanas, J.; Farre, M.; Munoz, M.; Alegret, S.; Muraviev, D. N. Phys. Status Solidi A 2006, 203, 1194–1200. (b) Chatterjee, U.; Jewrajka, S. K. J. Colloid Interface Sci. 2007, 313, 717–723.

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twice from ethanol. Ammonium hydroxide (28%), silver nitrate (AgNO3, AR grade), hydrochloroauric acid (HAuCl4, AR grade), sodium borohydride (NaBH4, AR grade), trisodium citrate dihydrate (AR grade), thiophenol (TP), carbon disulfide (CS2, AR grade), carbazole (AR grade), and toluene (AR grade) were purchased from Shanghai Chemical Reagent Co., Ltd., China. Toluene was dried with sodium and distilled before use. All other reagents were analytical grade and used as received without mentioned.

Synthesis of Benzyl 9H-Carbazole-9-carbodithioate (BCBD). The synthesis of benzyl 9H-carbazole-9-carbodithioate was carried out according to the method in the literature.17 BCBD: 1 H NMR (CDCl3): δ 4.73 (s, 2H), 7.32-7.47 (m, 9H), 8.00, 7.98 (d, 2H), 8.44, 8.46 (d, 2H). EA. C20H15NS2, calculated: C 72.03, H 4.53, N 4.20, S 19.23; found: C 72.15, H 4.53, N 4.18, S 19.14. HPLC, 99%. Synthesis of Bare Silica Nanoparticles. Ammonium hydroxide (28 wt % in water, 15 mL) and ethanol (250 mL) were added into a three-necked round-bottom flask at 30 °C. A mixture of tetraethyl orthosilicate (TEOS, 10 mL) and ethanol (10 mL) was added into a flask via a dropping funnel under stirring. After stirring for 24 h at 30 °C, the nanoparticles were isolated by centrifugation. The sediments were redispersed in ethanol and centrifugation again. This purification cycle was repeated for three times. The obtained nanoparticles were then dried under vacuum at 50 °C.

Synthesis of Silica-Supported Phenylmethyl Chloride (SiO2-Cl). The obtained above silica (2.50 g) was added into toluene (30 mL) in a flask, and the flask was degassed with argon for 30 min. Then 4-(chloromethyl)phenyltrimethoxysilane (0.30 g, 1.21 mmol) was added and heated to 95 °C for 2.5 h. After cooled to room temperature, the solid was filtered off, washed with toluene and diethyl ether, and then dried under vacuum at room temperature.

Synthesis of Silica-Supported Benzyl 9H-Carbazole-9carbodithioate (SiO2-BCBD). A suspension of potassium hydroxide (0.12 g, 2.1 mmol) in DMSO (20 mL) was prepared, and carbazole (0.36 g, 2.1 mmol) was added under vigorous stirring. The solution was stirred for 2 h at room temperature, and then carbon sulfide (0.19 g, 2.5 mmol) was added dropwise. The resultant reddish solution was stirred for 5 h at room temperature, and then the benzyl chloride-functionalized silica (SiO2-Cl) (2.50 g) was added. After stirring at room temperature for 36 h, the nanospheres were isolated by centrifugation at 7200 rpm and purified by subjecting them to successive centrifugation/redispersion cycles in DMSO, 2-propanol, and finally in diethyl ether. The obtained yellow product (SiO2-BCBD) was dried under vacuum. Elemental analysis: S 1.20% (CTA loading: 0.188 mmol/g).

Synthesis of Core-Shell SiO2-g-P4VP Nanoparticles via RAFT Polymerization (SiO2-g-P4VP). In a dry ampule, silica-supported RAFT agent (SiO2-BCBD) (0.10 g, 0.0188 mmol), 4VP (1.0 mL, 9.3 mmol), AIBN (0.10 mL, 0.02 mol/L in DMF), DMF (4.0 mL), and BCBD (0.006 g, 0.0188 mmol) were added and dispersed under ultrasonic. The mixture was degassed with argon for 15 min. Polymerization was carried out under vigorous stirring for a predetermined time at 60 °C. The polymerization was stopped by quenching the ampule in ice water, and the product was filtered off and washed with DMF for several times. The obtained P4VP-grafted nanoparticle (SiO2-g-P4VP) was dried in a vacuum overnight at 50 °C.

Synthesis of Au Nanoparticles Embedded in the Shell Layer of Core-Shell SiO2-g-P4VP Nanoparticles. To a 50 mL glass tube, a given volume of 5.0 mmol/L HAuCl4 aqueous solution and SiO2-g-P4VP were added, where the molar ratio of 4VP to HAuCl4 was 2:1. The SiO2-g-P4VP nanoparticles were dispersed under ultrasonic. After stirring for 2 h at room temperature, (17) Zhou, D.; Zhu, X. L.; Zhu, J.; Cheng, Z. P. J. Appl. Polym. Sci. 2007, 103, 982–988.

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Article the nanospheres were purified by subjecting them to successive centrifugation/redispersion cycles in deionized water. The purified nanospheres were finally dispersed in 20 mL of deionized water, and then excess volume of 0.1 mol/L NaBH4 aqueous solution was added to the above suspension with vigorous stirring. The mixture immediately turned into reddish-brown and carried out at 50 °C for 4 h to enable the Au3þ to be converted into gold metal nanoparticles. The resultant nanospheres were purified by subjecting them to successive centrifugation/redispersion cycles in deionized water. The final reddish-brown product (SiO2-g-P4VP/ Au) was dried under reduced pressure at room temperature. In order to obtain Au nanoparticles with larger size (∼15 nm), the method18 reduced Au3þ by citrate in boiling water was used. 1.0 mL of 1% trisodium citrate aqueous solution was instead of the NaBH4 aqueous solution above, and the reduction was carried out in boiling water. The other procedure was similar to that in the case of NaBH4 aqueous solution.

Liu et al. Scheme 1. Schematic Diagram Illustrating the Process for the Preparation of Metal Nanoparticles Embedded in the Shell Layer of Core-Shell SiO2-g-P4VP Nanoparticles

Synthesis of Ag Nanoparticles Embedded in the Shell Layer of Core-Shell SiO2-g-P4VP Nanoparticles. Accord-

ing to the method reported by Lee and Meisel:19 To a 50 mL glass tube, a given volume of 1.0 mmol/L AgNO3 aqueous solution and SiO2-g-P4VP were added, where the molar ratio of 4VP to AgNO3 was 2:1. The SiO2-g-P4VP nanoparticles were dispersed under ultrasonic. After stirring for 2 h at room temperature, the nanospheres were purified by subjecting them to successive centrifugation/ redispersion cycles in deionized water. The purified nanospheres were finally dispersed in 100 mL of deionized water. The aqueous solution was heated to boiling with vigorous strring and then to which 2.0 mL of 1% sodium citrate aqueous solution was added. The mixture was kept boiling for 20 min to enable the Agþ to be converted into silver metal nanoparticles. The resultant nanospheres were purified by subjecting them to successive centrifugation/redispersion cycles in deionized water. The final black product (SiO2-g-P4VP/Ag) was dried under reduced pressure at room temperature. Characterization. Elemental analysis of C, H, N, and S was measured with an EA1110 instrument. Transmission electron microscopy (TEM) images were recorded on FEI TecnaiG220 transmission electron microscopy at 200 kV. The sample was prepared by mounting a drop of the nanosphere dispersion on a carbon-coated Cu grid and allowing the sample to dry in air. UV-vis absorption spectra were taken on a U-3900 spectrophotometer. Fourier transform infrared (FT-IR) spectra were recorded on a Nicolet Magna 550 IR spectrometer. The spectra were collected at 32 scans with a spectral resolution of 4 cm-1. Thermogravimetric analysis (TGA) was performed in air at a heating rate of 10 °C/min from room temperature to 600 °C with a SDT-2960 TG/DTA TA Instruments. Powder X-ray diffraction (XRD) measurement was performed on an Xpert-PRO MPO. The XRD patterns were recorded by using Cu KR irradiation (λ = 1.541 78 A˚). Surface compositions were measured by X-ray photoelectron spectroscopy (XPS). XP spectra were collected on an XSAM800 spectrometer at a pressure of about 2  10-8 Torr using Mg KR radiation as the exciting source and which was operated at 12 kV and 11 mA. All binding energies (BEs) were referenced to the C 1s hydrocarbon peak at 284.6 eV. In peak synthesis, the line width (full width at half-maximum) of the Gaussian peaks was maintained constant for all components in a particular spectrum. The number-average molecular weight (Mn) and molecular weight distribution (Mw/Mn) of the resulting polymers were determined using a Waters 1515 gel permeation chromatograph (GPC) equipped with a refractive index detector (Waters 2414); DMF þ 0.05 mol/L LiBr was used as the eluent at a flow rate of 0.8 mL/min and 30 °C. Raman spectra were obtained using a confocal microprobe Raman system (HR800, Jobin Yvon). It is a single spectrograph instrument equipped with (18) Ji, X. H.; Song, X. N.; Li, J.; Bai, Y. B.; Yang, W. S.; Peng, X. G. J. Am. Chem. Soc. 2007, 129, 13939–13948. (19) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391–3395.

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a holographic notch filter and a CCD detector. The excitation wavelength was 632.8 nm from a He-Ne laser, and the greatest laser power was 10 MW. To prepare Raman reporter-labeled SiO2-g-P4VP/Au nanoparticles, a total of 10 μL of 1 mM probe molecule (TP) in ethanol was added to 1.0 mL of SiO2-g-P4VP/Au nanoparticles, and the mixture was allowed to shake for 2 h to let it have a well reaction. The resultant mixture above was subjected to surface-enhanced Raman (SER) measurement.

Results and Discussion Synthetic Pathway of SiO2-Supported Organic/Inorganic Hybrid Noble Metal Nanomaterials. Scheme 1 exhibits the synthetic pathway to prepare the noble metal nanoparticles embedded in the shell layer of core-shell SiO2-g-P4VP nanoparticles. Bare silica nanoparticles were prepared using the St€ ober process.20 In this study, silica nanoparticles with an average diameter of 100 ( 5 nm, as determined by TEM (Figure 1a), were used as the inorganic core. The surface functional groups of bare silica were converted to the benzyl chloride groups by reaction of the 4-(chloromethyl)phenyltrimethoxysilane.21 The benzyl chloride groups on the silica nanoparticles surfaces were further reacted with carbazole and carbon sulfide, leading to the silica-supported chain transfer agent (CTA) (SiO2-BCBD). In the structure of this silica-supported CTA, the silica support is a part of the leaving R groups of the RAFT agent SiO2-BCBD, so high molecular weight of grafted polymers and grafting density can be achieved.21,22 The average diameter of the SiO2-Cl and SiO2BCBD is 100 ( 5 nm (Figure 1b,c), which is identical to the bare silica nanoparticles. After surface RAFT polymerization of 4VP, the obtained P4VP shell grafted on the silica nanoparticles can coordinate with various metal ions such as Au3þ and Agþ to form metal ion complexes. The metal nanoparticles was obtained by the in-situ reduction of these metal ions by a reducing agent NaBH4 or trisodium citrate, and the SiO2-supported Au or Ag hybrid nanomaterials stabilized by P4VP can be obtained. UV-vis Spectra of the Nanoparticles. The attachment of the RAFT agent BCBD onto silica nanoparticles was confirmed by UV-vis spectra. Figure 2 shows the UV-vis spectra of the bare silica, SiO2-Cl, SiO2-BCBD, and BCBD. The RAFT agent BCBD showed absorption at 240, 285, and 312 nm due to the (20) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1981, 81, 354–368. (21) (a) Pittaya, T.; Craig, A.; Perrier, S. Macromolecules 2005, 38, 6770–6774. (b) Zhao, Y. L.; Perrier, S. Macromol. Symp. 2007, 248, 94–103. (c) Zhao, Y. L.; Perrier, S. Macromolecules 2007, 40, 9116–9124. (22) Li, C. Z.; Han, J. W.; Chang, Y.; Benicewicz, B. C. Macromolecules 2006, 39, 3175–3183.

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Figure 1. TEM images of (a) the bare silica nanoparticle, (b) benzyl chloride-functionalized silica nanoparticle (SiO2-Cl), and (c) the silicasupported BCBD nanoparticle (SiO2-BCBD).

Figure 2. UV-vis spectra of the RAFT agent BCBD, silica-supported BCBD (SiO2-BCBD) nanoparticle, benzyl chloride-functionalized silica nanoparticle (SiO2-Cl), and the bare silica nanoparticle.

phenyl rings and carbazole group. The absorption at 365 nm is ascribed to the thiocarbonyl group. After the attachment, the SiO2-BCBD nanoparticles showed very similar absorption bands at 240, 285, 312, and 365 nm. No typical absorption was found for the bare silica and SiO2-Cl. Elemental analysis of the SiO2-BCBD showed that it contains 1.2% of S element; namely, the CTA loading on the SiO2-BCBD nanoparticles was 0.188 mmol/g. If the density of the SiO2 is assumed to be identical to that of bulk silica (2.07 g/cm3), we can roughly estimate the CTA loading density of ATRP initiators at the surface of SiO2 (100 nm in diameter) is 3.9 CTAs/nm2. The CTA loading density (LD) (1/ nm2) can be estimated using the following formula as reported by our previous document:23 LD ¼

NBCBD NA Sparticle =ðFsilica Vparticle Þ

where NBCBD is the mole number of 1 g of SiO2@BCBD, Sparticle and Vparticle are the surface area and volume of one particle, respectively, Fsilica is the density of the SiO2, and NA is Avogadro’s number. Surface RAFT Polymerization of 4VP. The SiO2-BCBD was used as a CTA to mediate RAFT polymerization of 4VP to (23) Li, Q.; Zhang, L. F.; Zhang, Z. B.; Zhou, N. C.; Cheng, Z. P.; Zhu, X. L. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 2006–2015.

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Figure 3. TEM images of the SiO2-g-P4VP core-shell nanoparticle with different thickness by adjusting polymerization time: (a) 6, (b) 9, (c) 18, and (d) 24 h. Table 1. Thickness of the P4VP Shell Grafted on the Silica Nanoparticles entry 1 2 3 4

polymerization time (h)

6 9 18 24 a Determined by TEM results.

shell thicknessa (nm) 10 13 16 18

produce the core-shell SiO2-g-P4VP nanoparticles. In this work, higher molecular weight of grafted polymers and grafting density were needed to form the shell. The polymerizations ([4VP]:[SiO2BCBD]:[BCBD]:[AIBN] = 500:1:1:0.1) were conducted in the presence of a free RAFT agent BCBD in DMF at 60 °C for a different time. Figure 3a-d shows the TEM images of the SiO2-gP4VP nanoparticles with different polymerization times. It is observed that core-shell structure nanoparticles can be discernible from Figure 3a-d. In addition, the thickness of the P4VP increases with the polymerization time, as shown in Table 1, from 10 to 18 nm when the polymerization time increases from 6 to 24 h, DOI: 10.1021/la102994g

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Figure 4. ln([M]0/[M]) and grafting percentage as a function of polymerization time for the surface RAFT polymerization of 4VP using SiO2-BCBD as the RAFT agent in DMF in the presence of free RAFT agent BCBD at 60 °C. Polymerization conditions: [4VP]0/ [SiO2-BCBD]0/[free BCBD]0/[AIBN]0 = 5000/5/5/1; weight of SiO2@BCBD = 50 mg, diameter of SiO2@BCBD = 100 nm.

Figure 6. Fourier transform infrared (FT-IR) spectra of (a) the bare silica nanoparticle and (b) the SiO2-g-P4VP nanoparticle.

Figure 5. Evolution of molecular weight (Mn,GPC) and molecular

Figure 7. Thermogravimetric analysis (TGA) of (a) the bare silica nanoparticle, (b) SiO2-BCBD, and (c) the SiO2-g-P4VP obtained by the polymerization of 4VP for 24 h.

weight distribution (Mw/Mn) of the free P4VP with monomer conversion for the surface RAFT polymerization of 4VP using SiO2-BCBD as the RAFT agent in DMF in the presence of free RAFT agent BCBD at 60 °C. Polymerization conditions: [4VP]0/ [SiO2-BCBD]0/[free BCBD]0/[AIBN]0 = 5000/5/5/1; weight of SiO2@BCBD = 50 mg, diameter of SiO2@BCBD = 100 nm.

indicating the “living” feature of the surface RAFT polymerization. In order to further confirm the “living” feature, the kinetic experiments in the presence of a free RAFT agent BCBD were investigated. It is noted that there were a lot of free P4VPs produced due to the presence of the free RAFT agent BCBD. The percentage of free P4VPs in the total polymer was about 60 wt %. The results are shown in Figures 4 and 5. From Figure 4, a first-order kinetic plot for the solution RAFT polymerization of 4VP in the presence of the free RAFT agent BCBD was observed; at the same time, the graft percentage increased with polymerization time, indicating constant propagating radical concentration throughout the polymerization. Figure 5 shows the evolution of the number-average molecular weight (Mn,GPC) and molecular weight distribution (Mw/Mn) values of the obtained free P4VPs on the conversion for the surface RAFT polymerization of 4VP in DMF at 60 °C. As shown in Figure 5, the Mn;GPC values of the polymers increased linearly with monomer conversion while keeping low Mw/Mn values (Mw/Mn = 1.14-1.29). The experimental molecular weights were close to their corresponding theoretical ones. These results demonstrated the “living” feature of the surface RAFT polymerization. 14810 DOI: 10.1021/la102994g

In Figure 6, the FT-IR spectrum of the SiO2-g-P4VP nanoparticles shows that the characteristic absorption peaks of the pyridine rings occur at 1603 and 823 cm-1.24 Figure 7 shows the TGA analysis of the bare silica, SiO2-BCBD, and SiO2-g-P4VP. After grafting P4VP, the sample of the SiO2-g-P4VP nanoparticles showed about 37% of weight loss while only about 7% of weight loss for bare silica and 12% of weight loss for SiO2-BCBD at 550 °C, indicating that the larger percent of the weight loss resulted from the decomposition of the grafted P4VP on the surface of the silica nanoparticles. Morphology and XRD Spectra of the SiO2-g-P4VP/ Metal Nanoparticles. The core-shell nanoparticles with a P4VP shell provide a facile way to synthesize different hybrid metal nanomaterials. In this work, the SiO2-g-P4VP nanoparticles obtained by surface RAFT polymerization were first activated by HAuCl4 or AgNO3 solution and then reduced to form the SiO2-g-P4VP/Au by NaBH4 aqueous solution or SiO2-gP4VP/Ag hybrid nanoparticles by trisodium citrate aqueous solution. As shown in Figure 8a,b, the synthesized Au with an average diameter of 2.5 ( 1.0 nm and Ag with an average diameter of 3.0 ( 1.5 nm nanoparticles were embedded in the P4VP shell layer of the SiO2-supported core-shell nanoparticles. The SiO2-g-P4VP/Au and SiO2-g-P4VP/Ag hybrid nanoparticles were examined by XRD, shown in Figure 9. Four diffraction (24) Wong, K. N.; Colson, S. D. J. Mol. Spectrosc. 1984, 104, 129–151.

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Figure 8. TEM images of (a) the SiO2-g-P4VP/Au and (b) the SiO2-g-P4VP/Ag nannoparticles as well as the corresponding histograms of the gold and silver nanoparticles embedded in P4VP shell.

peaks at 2θ of 38.1°, 44.3°, 64.6°, and 77.5° (curve b) and 38.1°, 44.4°, 64.4°, and 77.6° (curve c) correspond to the (111), (200), (220), and (311) planes of the cubic phase gold and silver, respectively, indicating that Au and Ag nanoparticles formed in the P4VP shell are high crystallinity.25 These data are in excellent agreement with JPCDS date (File No.: PDF# 00-002-1095) and (File No.: PDF# 00-001-1164). No diffraction peaks corresponding to metal oxides are observed, which confirms that the metal nanoparticles on the surfaces of the P4VP layer are successfully formed. Surface Analysis by X-ray Photoelectron Spectroscopy (XPS). The chemical composition of the nanosphere surfaces at

various stages was determined by XPS. Figure 10a shows the XPS wide scan spectrum of the silica-supported phenylmethyl chloride nanoaprticles SiO2-Cl. It shows five peak components at binding energies (BEs) of about 103, 154, 201, 284, and 533 eV, attributable to Si 2p, Si 1s, Cl 2p, C 1s, and O 1s species from the silica and the corresponding phenylmethyl chloride attached on the nanoparticles, respectively.26 Especially, the presence of the signal of Cl 2p, which is further confirmed by the Cl 2p core-level spectrum (Figure 10b), indicated the successful immobilization of the phenylmethyl chloride group on the silica nanoparticles. After the reaction between the phenylmethyl chloride group and carbazole, the RAFT agent (BCBD) was immobilized on the sufaces of the nanoparticles, which is confirmed by the XPS wide scan and S 2p core-level spectra of the SiO2-BCBD nanoparticles, as shown in Figure 10c,d. By comparison with Figure 10a, it can be seen that, from Figure 10c, the Cl 2p signal (201 eV) disappears and a new signal (163 eV) of S 2p appears, which is also confirmed by the S 2p core-level spectrum as shown in Figure 10d. The BEs of 162.9 and 164.2 eV are assigned to the CdS and C-S species of the RAFT agent BCBD, respectively.13b,26,27 After grafting of P4VP via RAFT polymerization for 9 h, the XPS wide scan and N 1s core-level spectra of the SiO2-g-P4VP nanoparticles are shown in Figure 10e,f. From Figure 10e, it can be seen that some signals about silica (i.e., Si 2p, Si 2s and O 1s) almost disappear in the wide scan spectrum; however, a new signal at the BE of about 399 eV both in wide scan (Figure 10e) and core-level (Figure 10f) spectra, attributable to the N 1s species of the imine moiety (-N=) of the pyridine rings in the grafted P4VP, can be observed. The disappearance of the signals of silica indicates that whole the surfaces of the silica were coated by the P4VP with a thickness more than the detected depth (∼8 nm for the organic

(25) (a) Lu, Y.; Mei, Y.; Schrinner, M.; M€oller, M. W.; Breu, J. J. Phys. Chem. C 2007, 111, 7676–7681. (b) Cheng, D.; Zhou, X.; Xia, H.; Chan, H. S. O. Chem. Mater. 2005, 17, 3578–3581.

(26) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1992. (27) Peng, Q.; Lai, D. M. Y.; Kang, E. T.; Neoh, K. G. Macromolecules 2006, 39, 5577–5582.

Figure 9. XRD patterns of (a) the SiO2-g-P4VP, (b) SiO2-g-P4VP/ Au, and (c) the SiO2-g-P4VP/Ag core-shell nanoparticles.

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Figure 11. TEM image of the SiO2-g-P4VP/Au nannoparticles with an average Au diameter of 15 nm reduced by trisodium citrate aqueous solution.

Figure 12. SERS spectra of (a) bare SiO2-g-P4VP nanoparticles, (b) bare SiO2-g-P4VP/Au nanoparticles with an average Au diameter of 15 nm, and (c) SiO2-g-P4VP/Au nanoparticles with an average Au diameter of 15 nm for thiophenol adsorbed.

Figure 10. XPS (a) wide scan and (b) Cl 2p core-level of SiO2-Cl; XPS (c) wide scan and (d) S 2p core-level of SiO2-BCBD; XPS (e) wide scan and (f) N 1s core-level of SiO2-g-P4VP; XPS (g) wide scan and (h) Ag 3d core-level of SiO2-g-P4VP/Ag; XPS (i) wide scan and ( j) Au 4f core-level of SiO2-g-P4VP/Au. The SiO2-g-P4VP nanoparticles were obtained by the RAFT polymerization of 4VP for 9 h.

matrix) for the XPS technique,28 which is consistent with the thickness (13 nm obtained from the 9 h RAFT polymerization of 4VP) from the TEM results, as shown in Table 1. Figure 10g-j shows the XPS wide scan, Ag 3d and Au 4f core-level spectra of both the SiO2-g-P4VP/Ag and SiO2-g-P4VP/Au hybrid nanoparticles, respectively. In Figure 10g, a new signal attributable to Ag 3d (∼368 eV) can be observed besides the signals of C 1s and N 1s. The doublet with a BE at 368.3 eV (Ag 3d5/2) and 374.3 eV (Ag 3d3/2) can be indexed to the silver nanoparticles.29 Similarly, a new signal corresponding to the Au 4f (∼85 eV) appears in Figure 10i, and the doublet with a BE at 83.6 eV (Au 2f7/2) and 87.3 eV (Au 2f5/2), as shown in Figure 10j, can be assigned to the gold nanoparticles.30 SERS Activity of SiO2-g-P4VP/Au Nanoparticles. It is well-known that Au nanopaticles can be used as SERS activity materials, but small Au nanoparticles have very weak SERS (28) Briggs, D. Surface Analysis of Polymers by XPS and Static SIMS; Cambridge University Press: New York, 1998. (29) Zhu, J. T.; Jiang, W. Mater. Chem. Phys. 2007, 101, 56–62. (30) Rabbania, M. M.; Kob, C. H.; Baec, J. S.; Yeumd, J. H.; Kima, I. S.; Oh, W. Colloids Surf., A 2009, 336, 183–186.

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activity. In order to study the SERS activity of the obtained hybrid nanoparticles, the SiO2-g-P4VP/Au nanoparticles with an average Au diameter of 15 nm (Figure 11) was used to analyze the characteristics of SERS activity, as shown in Figure 12. Strong SERS signals at 1012, 1209, and 1603 cm-1 (Figure 12b) were observed by comparison with that in the case of bare SiO2-g-P4VP (Figure 12a). This is due to the binding interaction between Au nanoparticles and P4VP (the pyridine group) on SiO2 as reported by Wu et al.31 In addition, thiophenol (TP) was used as the probing molecules for the SiO2-g-P4VP/Au nanoparticles because of its well-established Raman spectral data and large Raman scattering cross section.32 As can be seen from Figure 12c, besides the signals at 1012, 1209, and 1603 cm-1 mentioned above, new strong SERS signals at 419, 998, 1022, 1072, and 1571 cm-1 from TP were observed,33 indicating that this kind of hybrid nanoparticle can be used as SERS activity materials.

Conclusions The core-shell SiO2 nanoparticles with P4VP shell were synthesized via surface RAFT polymerization of 4VP. The resultant P4VP with pyridine groups can easily be employed for the in situ preparation of organic/inorganic hybrid noble metal nanoparticles by the method of coordination with various transition metal ions first and then reduction in situ. The obtained Au (31) Wu, D. Y.; Liu, X. M.; Duan, S.; Xu, X.; Ren, B.; Lin, S. H.; Tian, Z. Q. J. Phys. Chem. C 2008, 112, 4195–4204. (32) Cui, Y.; Ren, B.; Yao, J. L.; Gu, R. A.; Tian, Z. Q. J. Phys. Chem. B 2006, 110, 4002–4006. (33) Bao, F.; Yao, J. L.; Gu, R. A. Langmuir 2009, 25, 10782–10787.

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and Ag nanoparticles were embedded in the P4VP shell layer. Furthermore, this method allows the control over shell thickness by simply adjusting the polymerization time, and it will be able to synthesize various hybrid metal nanoparticles where the corresponding metal ions can coordinate with P4VP. Thus, it is a novel and universal method for fabricating this kind of noble metal hybrid nanomaterials.

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Acknowledgment. The financial support from the National Nature Science Foundation of China (Nos. 20874069, 50803044, 20974071, and 20904036), the Specialized Research Fund for the Doctoral Program of Higher Education contract grant (No. 200802850005), the Qing Lan Project, and the Program of Innovative Research Team of Soochow University is gratefully acknowledged.

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