Core−Shell−Corona Au−Micelle Composites with a Tunable Smart

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Core-Shell-Corona Au-Micelle Composites with a Tunable Smart Hybrid Shell Xi Chen, Yingli An, Dongyun Zhao, Zhenping He, Yan Zhang, Jing Cheng, and Linqi Shi* Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed January 24, 2008. ReVised Manuscript ReceiVed March 29, 2008 Micelles having a core of polystyrene and a mixed shell of poly(ethylene glycol) and poly(4-vinylpyridine) were formed through self-assembly of a triblock copolymer poly(ethylene glycol)-block-polystyrene-block-poly(4vinylpyridine) in acidic water (pH 2). Reducing the HAuCl4-treated micelle solution leads to the formation of the Au-micelle composites with a core of polystyrene, a hybrid shell of poly(4-vinylpyridine)/Au/poly(ethylene glycol), and a corona of poly(ethylene glycol). The gold nanoparticles with controlled sizes were anchored to poly(4-vinylpyridine) to form the physically cross-linked hybrid shell. In aqueous solution, the hybrid shell is swollen and the swollen degree is sensitive to the pH condition. Under basic conditions, the channel in the hybrid shells of the composite is produced, which renders the composites a good catalytic activity. In addition, the composites also show good stability, unchanged hydrodynamic diameter, and surface plasmon absorption under different pH conditions.

Introduction Gold nanoparticles (GNs) have been extensively studied due to their special size effects compared to the bulk metal,1,2 and their perspective in many applications such as catalysts,3 sensors,4 electronics,5 and photonics.6 The properties of GNs are related to their size, shape, and space distribution, which are strongly dependent on the formation system of the GNs. Dendrimers,7 latex particles,8 microgels9 or other polymers, etc.10 have been widely used as “nanoreactors” or carriers for the GNs, because they can serve as good stabilizers for preventing the GNs from aggregating. Fabrication of GNs using block copolymers is particularly focused because block copolymers can offer powerful capabilities for synthesizing GNs11 and effective means for controlling particle location and pattern based on their rich diversity of structures.12 The properties of the formed composites consisting of GNs and block copolymers can be modulated not only through the nature of their constituting units but also through the distances of the neighboring particles or the morphology of the whole system.13 In general, one block of the block copolymer is soluble in the solvent, while the other one forms the core, which is regarded * Corresponding author. Fax: +86 22 23503510. E-mail: Shilinqi@nankai. edu.cn. (1) Gangopadhyay, R.; De, A. Chem. Mater. 2000, 12, 608. (2) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (3) Luo, J.; Jones, V. W.; Maye, M. M.; Han, L.; Kariuki, N. N.; Zhong, C. J. J. Am. Chem. Soc. 2002, 124, 13988. (4) Siwy, Z.; Trofin, L.; Kohli, P.; Baker, L. A.; Trautmann, C.; Martin, C. R. J. Am. Chem. Soc. 2005, 127, 5000. (5) Hassenkam, T.; Moth-Poulsen, K.; Stuhr-Hansen, N.; Norgaard, K.; Kabir, M. S.; Bjornholm, T. Nano Lett. 2004, 4, 19. (6) Tan, Y.; Qian, W.; Ding, S.; Wang, Y. Chem. Mater. 2006, 18, 3385. (7) Hong, M.-Y.; Yoon, H. C.; Kim, H.-S. Langmuir 2003, 19, 4866. (8) Shi, W.; Sahoo, Y.; Swihart, M. T.; Prasad, P. N. Langmuir 2005, 21, 1610. (9) Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 201. (10) Lei, L.; Gohy, J.-F.; Willet, N.; Zhang, J.-X.; Varshney, S.; Jerome, R. Macromolecules 2004, 37, 1089. (11) (a) Fustin, C.-A.; Colard, C.; Filali, M.; Guillet, P.; Duwez, A.-S.; Meier, M. A. R.; Schubert, U. S.; Gohy, J.-F. Langmuir 2006, 22, 6690. (b) Jaramillo, T. F.; Baeck, S.-H.; Cuenya, B. R.; McFarland, E. W. J. Am. Chem. Soc. 2003, 125, 7148. (12) Chiu, J. J.; Kim, B. J.; Kramer, E. J.; Pine, D. J. J. Am. Chem. Soc. 2005, 127, 5036. (13) Lu, L.; Zhang, H.; Sun, G. Y.; Xi, S. Q.; Wang, H. S.; Li, X. L.; Wang, X.; Zhao, B. Langmuir 2003, 19, 9490.

as the nanoreactor for reduction nucleation and growth of GNs, and the formed composites possess a core-shell structure with Au located in the core.14 Recently, the composites with a thin metal shell are of immense interest because of their unique application in many areas, such as nonlinear optics;15 catalysis;16 chemical, electronic, and optical sensors;17 and surface-enhanced Raman scattering.13 Until now, these structured composites have been formed mainly by an aggregation-based method using silica spheres or a templatebased method using functionalized polymer microspheres (also called latexes or latex beards).13,16 However, micelle-based methods for these structured composites are seldom reported.18 In a previous work, we showed that a three-layered structured composite with a polystyrene core, a hybrid shell of Au/poly(4vinylpyridine), and a poly(ethylene glycol) corona could be formed by reducing a mixed aqueous solution of polystyreneblock-poly(4-vinylpyridine) micelle/poly(ethylene glycol)-blockpoly((4-vinylpyridine) chain/HAuCl4 with NaBH4.18c In that work, the shell was formed by the physical interaction of GNs with the poly(4-vinylpyridine) blocks of polystyrene-blockpoly(4-vinylpyridine) micelle and poly(ethylene glycol)-blockpoly(4-vinylpyridine) chains.18c Herein, a triblock copolymer micelle template with a polystyrene core and a mixed shell composed of poly(4-vinylpyridine) and poly(ethylene glycol) is used to prepare a new three-layered composite comprising a polystyrene core, a hybrid shell of poly(4vinylpyridine)/Au/poly(ethylene glycol), and a poly(ethylene glycol) corona. The hybrid shell is swollen and the degree of swelling is sensitive to the pH condition of the aqueous solution: (14) (a) Antonietti, M.; Wenz, E.; Bronstein, L. M.; Seregina, M. S. AdV. Mater. 1995, 7, 1000. (b) Kang, Y.; Taton, T. A. Angew. Chem., Int. Ed. 2005, ¨ ller, M. AdV. Mater. 1996, 8, 337. (d) 44, 409. (c) Spatz, J. P.; Roescher, A.; MO Roescher, A.; Mo¨ller, M. AdV. Mater. 1995, 7, 151. (e) Li, J.; Shi, L.; An, Y.; Li, Y.; Chen, X.; Dong, H. Polymer 2006, 47, 8480. (f) Bronstein, L.; Chernyshov, D.; Valetsky, P.; Tkachenko, N.; Lemmetyinen, H.; Hartmann, J.; Fo¨rster, S. Langmuir 1999, 15, 83. (15) Twardowski, M.; Nuzzo, R. G. Langmuir 2002, 18, 5529. (16) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M. Chem. Mater. 2007, 19, 1062. (17) Armelao, L.; Bertoncello, R.; Dominicis, M. D. AdV. Mater. 1997, 9, 736. (18) (a) Hou, G.; Zhu, L.; Chen, D.; Jiang, M. Macromolecules 2007, 40, 2134. (b) Mayer, A. B. R.; Mark, J. E. Colloid Polym. Sci. 1997, 275, 333. (c) Chen, X.; Liu, Y.; An, Y.; Lu¨, J.; Li, J.; Xiong, D.; Shi, L. Macromol. Rapid Commun. 2007, 28, 1350.

10.1021/la800244g CCC: $40.75  2008 American Chemical Society Published on Web 06/25/2008

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Scheme 1. Formation of Polymer Micelles with Mixed Shells and as Templates for Synthesizing Core-Shell-Corona Composites with Tunable Smart Hybrid Shells

At low pH value (pH 2), poly(4-vinylpyridine) and poly(ethylene glycol) are hydrophilic, and the hybrid shell of poly(4vinylpyridine)/Au/poly(ethylene glycol) is well-swollen; upon increasing the pH value (pH g7), poly(4-vinylpyridine)/Au becomes hydrophobic while poly(ethylene glycol) is hydrophilic, and the hybrid shell of poly(4-vinylpyridine)/Au/poly(ethylene glycol) is still swollen, but the swelling decreases. Under a high pH condition (pH g7), the hydrophilic poly(ethylene glycol) chains connect the composite core with their one chain end and pass through the hydrophobic shell of poly(4-vinylpyridine)/ Au; thus, the interspaces between the hybrid poly(4-vinylpyridine)/Au shell and the hybrid poly(4-vinylpyridine)/Au shellsurrounded poly(ethylene glycol) chains can be used as channels to connect the core and the outer milieu.19 This smart behavior of the hybrid shell may endow the composite certain important functions, such as controllable ion and/or matter transport. In addition, these structured composites show the same hydrodynamic diameter under different pH conditions and good catalytic activity under basic conditions.

Experimental Section Materials. Poly(ethylene glycol) monomethyl ether (CH3OPEG114-OH) (Mw ) 5000, PDI ) 1.05, the subscript indicates the number of the repeating units) was purchased from Fluka. Styrene, 4-vinylpyridine, and CuCl were purchased from Aldrich and purified according to ref 20. Tris[2-(dimethylamino)ethyl]amine (Me6TREN) was synthesized according to ref 21. Other solvents were of analytical grade and were used as received without further purification. The triblock copolymer poly(ethylene glycol)-blockpolystyrene-block-poly(4-vinylpyridine) (PEG114-b-PS83-b-P4VP38, the subscript indicates the number of the repeating units) used in this paper was synthesized by sequential atom transfer radical polymerization (ATRP) of styrene and 4-vinylpyridine using bromidetailed CH3O-PEG114-Br (PEG114-Br) as macroinitiator. The macroinitiator was synthesized according to ref 22 The composition of the block copolymer PEG114-b-PS83-b-P4VP38 was determined by 1H NMR spectroscopy and the PDI of the triblock copolymer measured by GPC analysis was 1.27. In the controlled experiments, the used diblock copolymers are PEO114-b-P4VP52, with the subscript indicating the number of the repeating units. Fabrication of Hybrid Composites. The micelle with a PS core and a mixed shell of PEG/P4VP was formed by continually adding acidic water (pH 2) into PEG-b-PS-b-P4VP/DMF solution (0.2 mg/ mL) with a rate of 10 s/d until turbidity appearing in the solution. Standing for 24 h, about 5-fold acidic water (pH 2) was rapidly added to the solution and then the solution was dialyzed in a dialysis bag against acidic water (pH 2) for 7 days to remove solvent DMF. The resultant micelle solution with a concentrate of 0.02 mg/mL was used to prepare Au-micelle composites. The in situ formation of GNs in the shell of micelles was carried out by the addition of HAuCl4 into a micelle aqueous solution and subsequent reduction with NaBH4. For a typical example, 1.0 mL of HAuCl4 acidic aqueous solution (pH 2, 8.9 mg/mL) was added into 18 mL of micelle acidic

aqueous solution (pH 2, 0.02 mg/mL). After gently stirring for 24 h at room temperature, a 10-fold excess of NaBH4 was then quickly added, and stirring was continued for 24 h. The Au/micelle composite solution was obtained after being dialyzed against acidic water (pH 2) for another 24 h. If not specified, all the experiments were carried out at room temperature. Catalytic Reduction of p-Nitrophenol. The catalytic reduction was conducted in a standard quartz cell with a path length of 1 cm. The NaBH4 aqueous solution was first mixed together with p-nitrophenol aqueous solution (pH 10, adjusted by 1 mol/L NaOH aqueous solution) and then the mixture was fixed to a given temperature. Immediately after addition at the same temperature of a colloidal dispersion of micelle-supported gold nanoparticles with a molar ratio of 1/4 of gold to 4-VP, the absorption spectra were recorded by a TU-8110 UV-vis spectrophotometer. Characterization. Dynamic laser scattering (DLS) measurements were performed on a laser light scattering spectrometer (BI-200SM) equipped with a digital correlator (BI-9000AT) at 532 nm at room temperature. The strength-average hydrodynamic diameter and the hydrodynamic diameter distribution of the micelle or composite were obtained at a measurement angle of 90°. The detailed method of DLS can be found elsewhere.23 Transmission electron microscopy (TEM) measurement was conducted by using a Philips T20ST electron microscopy at an acceleration voltage of 200 kV, whereby a small drop of micelle/composite solution was deposited onto a carbon-coated copper EM grid and dried at room temperature at atmospheric pressure. UV-vis spectra were recorded on a TU-8110 UV-visible spectrophotometer equipped with two silicon diode detectors and a xenon flash lamp. 1H NMR spectroscopy was recorded on a Bruker AV300 spectrometer at room temperature.

Results and Discussion As shown in Scheme 1, Au-micelle composites are generated via GNs in situ forming in a polymer micelle with a mixed shell (PMMS). This PMMS, comprising a polystyrene core, a mixed shell composed of a short P4VP chain, and a long PEG chain, is obtained by the self-assembly of a triblock copolymer PEGb-PS-b-P4VP in acidic water (pH 2). Reducing the HAuCl4treated PMMS solution with excess NaBH4 yields the threelayered composites with a PS core, a swollen hybrid shell composed of P4VP/GNs/PEG, and a PEG corona. Upon adjusting the pH value from 2 to 7 with NaOH, the hybrid Au/P4VP becomes hydrophobic and collapses to the PS core, while the (19) (a) Li, G.; Shi, L.; Ma, R.; An, Y.; Huang, N. Angew. Chem., Int. Ed. 2006, 45, 4959. (b) Lu, J.; An, Y.; Huang, N.; Chen, X.; Li, J.; Shi, L. Chem. J. Chin. UniV. 2007, 28, 982. (c) Ma, R.; Wang, B.; Xu, Y.; An, Y.; Zhang, W.; Li, G.; Shi, L. Macromol. Rapid Commun. 2007, 28, 1062. (20) Xia, J.; Zhang, X.; Matyjaszewski, K. Macromolecules 1999, 32, 3531. (21) Ciampocini, M.; Nardi, N. Inorg. Chem. 1966, 5, 41. (22) (a) Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18, 8350. (b) Zhang, W.; Shi, L.; Wu, K.; An, Y. Macromolecules 2005, 38, 5743. (23) (a) Zhang, W.; Shi, L.; An, Y.; Wu, K.; Gao, L.; Liu, Z.; Ma, R.; Meng, Q.; Zhao, C.; He, B. Macromolecules 2004, 37, 2924. (b) Zhang, G.; Wu, C. J. Am. Chem. Soc. 2000, 122, 10201.

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Figure 1. 1H NMR spectra of the following: (a) unimers of PEG-bPS-b-P4VP in DMF, (b) micelle of the same copolymers obtained by adding acidic D2O (pH 2) into the copolymer/DMF solution, and (c) micelle of the same block copolymers obtained by adjusting the pH value from 2 to 7 using NaOD.

hydrophilic PEG chain connects the PS core with its one chain end and passes through the collapsed hybrid shell; thus, the channels, i.e., the interspaces between the hybrid shell and the hybrid shell-surrounded PEG chains, are formed.19 In an aqueous solution, the hybrid shell is swollen and the swollen degree could be adjusted by changing the pH value. In the following sections, we will discuss the formation and characters of the template PMMS, as well as the formation and smart properties of the corresponding composites Formation of Template Micelle. The PMMS is formed through micellization of PEG-b-PS-b-P4VP in acidic water (pH 2). Since acidic water is a good solvent for PEG and P4VP but a precipitant for PS, PEG-b-PS-b-P4VP would self-assemble into micelles with PS as core and PEG/P4VP as shell. The transition of PEG-b-PS-b-P4VP from single chains to micelles is confirmed through 1H NMR, DLS, and TEM studies. As shown in Figure 1, when acidic D2O (pH 2) is added to the PEG-bPS-b-P4VP/DMF solution, the signal A at δ 6.4-7.0 (Figure 1a) ascribed to the PS blocks disappears while the signals B and C, corresponding to P4VP and PEG, respectively, remain unchanged, which indicates that PS is hydrophobic and forms the core, while PEG and P4VP are hydrophilic and form the shell. The micelles are characterized by TEM and DLS and the results are shown in Figure 2. At pH 2, the micelles show an average hydrodynamic diameter of 65.5 nm with a distribution range from 25 to 125 nm (Figure 2A). Increasing pH to 7, the average hydrodynamic diameter is 66.0 nm and the distribution ranges from 25 to 125 nm (insert in Figure 2A). The TEM images from the two aqueous solutions show uniformly spherical morphologies with the same size of 35 nm (Figure 2B,C).It is well-known that when the shells of the micelles are composed of two types of polymer chains, the micelles could be PMMSs or Janus micelles.24 The TEM and DLS results shown in Figure 2 indicate that PEG-b-PS-b-P4VP in acidic water (pH 2) forms PMMSs instead of Janus micelles. Because if Janus micelles are formed, they would form larger aggregates as the pH changes from 2 to 7, and thus the sizes from DLS and TEM will become larger.24 The sizes of the micelles at two different pH values are not changed, indicating that the micelles are PMMSs. At pH 7, since the P4VP deprotonates and becomes hydrophobic and thus (24) (a) Erhardt, R.; Zhang, M. F.; Bo¨ker, A.; Zettl, H.; Abetz, C.; Frederik, P.; Krausch, G.; Abetz, V.; Mu¨ller, A. H. E. J. Am. Chem. Soc. 2003, 125, 3260. (b) Erhardt, R.; Bo¨ker, A.; Zettl, H.; Kaya, H.; Pyckhout-Hintzen, W.; Krausch, G.; Abetz, V.; Mu¨ller, A. H. E. Macromolecules 2001, 34, 1069.

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collapses to the PS core while PEG is soluble (as shown in Figure 1c, the signal B ascribed to the P4VP disappears, while the signal C ascribed to the PEG retains), the PMMS transforms to a three-layered structure with PS as core, P4VP/PEG as mixed shell, and PEG as corona. The pH change inducing the P4VP chains from hydrophilic to hydrophobic only leads to the micelle transforming from a core-shell to a core-shell-corona structure, but the size and the stability of the micelle are not affected (Figure 2). The similar experimental results for PMMSs were also reported by our group19a and Liu et al.25 Due to the different chains in the shell, PMMS is thought to have some special functions and could meet many requirements in applications.19,26 In previous reports, PMMS has been used to improve the solubility of the micelle in different polarity solvents,26b used as a desirable precursor for the preparation of targeting drug delivery system,26a and used as a candidate for simulating a biomembrane channel.19 In the present case, we use the PMMS as a template or a reactor for the synthesis of GNs. And the obtained composites are expected to have unique properties integrating the PMMS and GNs. Formation of Core-Shell-Corona Composites with Tunable Smart Shells. The HAuCl4 is added into the PMMS aqueous solution to complex with the P4VP, leading to the formation of the hybrid PMMS possessing a PS core and a hybrid shell of P4VP/Au3+/PEG. The composites with different gold content are formed by directly adding reductant NaBH4 to three hybrid PMMS solutions in which the molar ratio (MR) of HAuCl4 to 4-VP is respectively 1/2, 1/4, and 1/8. The onset of the reaction could be seen from the wine-red color of the solution. No precipitates are found in the reaction and at the subsequent stored period (about six months) for all the systems, indicating the good stability of the formed composites solutions. DLS is used to measure the hydrodynamic diameters, and the typical distribution results are shown in Figure 3A,B. Clearly, all the composites in an acidic or neutral aqueous solution show a single distribution, indicating that the aggregation or association of the composites has not taken place. At pH 2 and MR ) 1/8 (Figure 3A), the composites show an average hydrodynamic diameter of 65 nm with a distribution range from 30 to 120 nm; upon changing the MR to 1/2 (Figure 3B), the composites show an average hydrodynamic diameter of 66 nm with a distribution range from 30 to 110 nm. The hydrodynamic diameter of the composite is the same with the PMMSs (Figure 2A) and hardly affected by the loaded gold nanoparticles. This result could be interpreted by the structure of the PMMS template in an acidic solution. Under the forming conditions of the composites, the GNs complex with the short P4VP chains of the mixed shell, while the PS core and the long PEG chains of the mixed shell are kept well; thus, the hydrodynamic diameter of the formed composite is hardly changed compared to the PMMS (Figure 2A). The hydrodynamic diameters of the composites are also hardly influenced by the pH conditions. When MR ) 1/8 and pH 2 (Figure 3A), the average hydrodynamic diameter is about 65 nm with a range of 30-120 nm; upon increasing the pH value to 7 (insert in Figure 3A), the average hydrodynamic diameter is about 66 nm with a distribution range from 40 to 90 nm. When MR ) 1/2 and pH 2 (Figure 3B), the composites show an average hydrodynamic diameter of 66 nm with a distribution range from 30 to 110 nm; upon changing the pH value from 2 to 7 (inset in Figure 3B), the average hydrodynamic diameter is about 65 nm with a distribution range from 50 to 80 nm. The DLS behavior (25) Hoppenbrouwers, E.; Li, Z.; Liu, G. Macromolecules 2003, 36, 876. (26) (a) Kataoka, K.; Harada, A.; Nagasaki, Y. AdV. Drug DeliVery ReV. 2001, 47, 113. (b) Hui, T.; Chen, D.; Jiang, M. Macromolecules 2003, 38, 5834.

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Figure 2. (A) Hydrodynamic diameter distributions of PEG-b-PS-b-P4VP micelles aqueous solutions at different pH values (pH 2 and 7) and TEM images of the micelles formed from the aqueous solution with pH 2 (B) and pH 7 (C).

Figure 3. Hydrodynamic diameter distribution of the composites under acidic and/or neutral conditions (A, B) and the corresponding TEM images (C, D), where the molar ratio of Au to 4-VP is 1/8 for the cases of A and C and 1/2 for the cases of B and D, respectively.

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is very different from the reported core-shell-corona spherical micelles with a PS core, a P2VP shell, and a PEG corona created by an ABC polystyrene-block-poly(2-inylpyridine)-block-poly(ethylene oxide) copolymer (PS-b-P2VP-b-PEG) in water;27 i.e., the hydrodynamic diameter of the later shows a dependence on the pH value due to the whole length change of the P2VP and PEG chains induced by the (de)protonation of the P2VP blocks. The DLS results can be interpreted by two facts. In the first one, the association and aggregation of the composites in both acidic and neutral aqueous solution are prevented because of the good solubility and sterical repulsion of the PEG chains. The other one is related to the structures of the composites under different pH conditions. At pH 2, since the GNs complex with the P4VP, the resultant composites have a PS core and a hybrid shell of P4VP/Au/PEG; upon increasing the pH to 7, the hybrid P4VP/ Au shell collapses, but the PS core and long PEG chains are kept. Therefore, the pH change only induces the stretch and collapse of the P4VP chains in the composites, while the hydrodynamic diameters of the composites are hardly changed. The typical TEM images of the composites (the MR values of Au to 4-VP is 1/8 and 1/2) are shown in Figure 3C,D. The well-defined core-shell structure is readily seen, possibly due to the more electron-dense of the P4VP/Au/PEG hybrid shell than the PS core. It should be noted that the TEM images (Figure 3C,D) only show the two-dimensional morphologies of the composites in the dried state. Indeed, these formed composites in aqueous solution have a PS core, a hybrid shell of PEG/Au/ P4VP, and a PEG corona in three dimensions. Because of the large contrast between the PEG and the GNs, the corona PEG chains could not be discerned under TEM observation. The composites have a number-average diameter of about 35 nm, which is in reasonable agreement with that of the PMMSs (Figure 2B,C). The sizes of the GNs in the composites are changed with the content of the added HAuCl4. When the MR of HAuCl4 to 4-VP is low (1/8), the gold particles show a number-average diameter of about 2.5 nm (Figure 3C); upon increasing the ratio to 1/2, the sizes of the GNs increase to about 4 nm (Figure 3D). The results indicate the sizes of the GNs could be controlled through the triblock copolymer PMMS. In Figure 3 C,D, GNs are found to be uniformly distributed in the composites shells, which in turn indicates that the P4VP and PEG chains are mixed in the shell of the composites. The hybrid shells of the composites are physically cross-linked through the complexing of GNs with P4VP and their smart property could be triggered by changing pH conditions. Under a low pH condition (pH 2), since the PEG and P4VP chains are hydrophilic, the hybrid shells are swollen (Scheme 1). Under a high pH condition (pH g7), the PEG is still hydrophilic, but the P4VP/Au becomes hydrophobic and collapses to the PS core, so the degree of swelling of the hybrid shell decreases. Under this condition (pH g7), the interspaces between the hybrid P4VP/Au shell and the hybrid P4VP/Au shell-surrounded PEG chains connect the PS core and the outer milieu and could serve as channels for small molecules to pass through (Scheme 1). Therefore, the resultant composites possess a PS core, a swollen hybrid shell of P4VP/Au/PEG with channels, and a PEG corona (Scheme 1).19 The channels in the hybrid shell could not be directly discerned under the TEM observation (Figure 3), but their existence and functions could be confirmed by our previously reported experimental results.19b,c In one experiment, ibuprofen was first loaded in the core of the complex micelles with a poly(tert-butyl

acrylate)(PtBA) core and a mixed shell of PEG/P4VP at pH 2.5. Increasing the pH value to 12 led to the P4VP collapse and thus the channels were formed by hydrophilic PEG chains passing through the collapsed P4VP shell of the complex micelles.19b The results of the controlled release of ibuprofen indicated that the release velocity was directly proportional to the percentage of channels in the P4VP shell of the micelles.19b More evidence for the channels in the hydrophobic shell was discovered by studying the complex micelles with a P4VP core surrounded by a mixed poly(N-isopropylacrylamide)/poly(ethylene glycol) (PNIPAM/PEG) shell prepared by comicellization of PNIPAM93b-P4VP58 and PEG114-b-P4VP58 in aqueous solutions using the laser light scattering (LLS) technique.19c Increasing the temperature above the lower critical solution temperature (LCST) of the PNIPAM induced the collapse of the PNIPAM block, but the hydrophilic PEG chains, which were tethered on the P4VP core, penetrated through the hydrophobic PNIPAM shell into the aqueous milieu to stabilize the micelles. The interspaces between the PNIPAMs and the PNIPAMs-surrounded PEG chains connected the core and the outer milieu, serving as channels for water and H+ to pass through. In the case of high content of PEG chains, a large amount of channels resulted in an easier uptake of water and H+ by P4VP core, which in turn led to disintegration of the complex micelles into free PEG-b-P4VP chains soon after the addition of dilute HCl; however, for the case of the low content of PEG chains, the fewer channels only made the complex swell more slowly during acidification.19c Properties of Composites with Tunable Smart Shells. Surface plasmon absorption of the composites with different gold content was studied by UV-vis absorption spectrum, and the results are shown in Figure 4. When the MR of Au to 4-VP is 1/8 (part a in Figure 4A), no obvious peak is observed within the UV-vis range, which suggests the sizes of the formed GNs are less than 3 nm.28 This is also consistent with the TEM observation shown in Figure 3C. The absorbance intensity is increased by increasing the amount of HAuCl4 in solution, while the characteristic absorbance peaks appears at a same position of 523 nm, irrespective of HAuCl4 content or gold particle size (parts b and c in Figure 4A). This can be explained by a classic Mie theory for spherical particles with diameters of 10 nm and

(27) Gohy, J. F.; Willet, N.; Varshney, S.; Zhang, J. X.; Je´roˆme, R. Angew. Chem., Int. Ed. 2001, 40, 3214.

(28) Zheng, P.; Jiang, X.; Zhang, X.; Zhang, W.; Shi, L. Langmuir 2006, 22, 9393.

Figure 4. UV-vis spectra of three Au-micelle composite solutions with initial molar ratio of 1/8 (a), 1/4 (b), and 1/2 (c) of Au to 4-VP at pH 2 (A) and the same three composite solutions at pH 7 (B). The experiments were carried out after the composites had been formed for a period of 1 month.

Core-Shell-Corona Au-Micelle Composites

Figure 5. 1H NMR spectra of single PEG-b-P4VP chains in D2O at pH 2 and composites of PEG-b-P4VP/Au in D2O at pH 2 and 7. The composites were formed by reducing the mixture of HAuCl4/PEG-bP4VP using NaBH4. The concentration of PEG-b-P4VP is 2.5 mg/mL and the mole ratio of gold to 4-VP is 1/2.

smaller,29 where the plasmon band position becomes independent of the particle size. The similar results were also reported by Youk30 and Sidorov et al.31 The surface plasmon resonance band of the individual GNs (no coupling) is still able to be observed when increasing the pH value from 2 to 7 (Figure 4B), which indicates that the gold particles are still separated from each other. Almost the same intensities and absorbance peaks are obtained when the pH value changed from 2 to 7 (Figure 4B), indicating that the pH change has no effect on the surface plasmon absorption of the Au-micelle composites. The phenomenon is different from the thermosensitive composites comprising GNs and PNIPAM chains, which exhibited temperature-dependent change in surface plasmon resonance absorbance as the distance of the neighboring gold particles was changed by changing temprature.32 In the present case, the shell of the composites comprises pH-responsible P4VP chains, but the position of the UV-vis absorbance is not shifted when the pH value of the composites solution is changed, which indicates that the distance between the neighboring gold particles is not affected by the swelling or shrinking of P4VP chains. The behavior is possibly due to the weak pH-responsiveness of the composites; i.e., the motion of P4VP is prohibited due to the formation of the P4VP/ Au hybrid structure in the shell by gold particles complexing with the short P4VP chains. In order to confirm the above deduction, a 1H NMR study for the pH-dependent characteristic of the P4VP/Au was carried out. The pH-dependent characteristic of the hybrid shell of the composites of PEG-b-PS-b-P4VP/Au in aqueous solution may be recognized by the 1H NMR study, but it was difficult for us to give direct 1H NMR proof because it is complicated to exclude the DMF in the preparation process of the H NMR sample (DMF can complex with gold nanoparticles). In order to simplify the present analysis, the 1H NMR studies of PEG-b-P4VP/Au composites in D2O (pH 2 and 7) were carried out because the change of the P4VP in the composites of PEG-b-P4VP/Au under different pH conditions is similar to that of the PEG-b-PS-bP4VP/Au composites. The 1H NMR results shown in Figure 5 indicate that the PEG-b-P4VP/Au in the aqueous solution displays (29) Wilcoxon, J. P.; Martin, J. E.; Provincio, P. J. Chem. Phys. 2001, 115, 998. (30) Youk, J. H. Polymer 2003, 44, 5053. (31) Sidorov, S. N.; Bronstein, L. M.; Kabachii, Y. A.; Valetsky, P. M.; Soo, P. L.; Maysinger, D.; Eisenberg, A. Langmuir 2004, 20, 3543. (32) (a) Zhu, M.-Q.; Wang, L.-Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656. (b) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175.

Langmuir, Vol. 24, No. 15, 2008 8203

Figure 6. Spectra of p-nitrophenol in the presence of the Au composite. The absorbance A measured at different time t (in seconds) in the graph was plotted against the wavelength. The measurement conditions of the reduction reactions were as follows: [p-nitrophenol] ) 0.015 g/L, [NaBH4] ) 3.76 g/L, [composites] ) 4.75 × 10-3g/L, T ) 20 °C, pH 10.

a pH-sensitive property; i.e., when the pH value is 2, the composites of PEG-b-P4VP/Au show two characteristic peaks with positions at δ 7.0-9.0, indicating the P4VP chains retain a soluble state due to their protonation; upon increasing the pH value from 2 to 7, the disappearance of the above two characteristic peaks indicates that the P4VP chains of the composites collapse due to their deprotonattion. In acidic D2O (pH 2), the P4VP peak intensity of the composite greatly decreases compared with the single PEG-b-P4VP chains, indicating that the stretch of the P4VP in the composites is partly prohibited. The amount of the soluble P4VP in the composites can be approximately estimated by calculating the area ratio of the characteristic peaks of P4VP to PEG. The results in Figure 5 show that the soluble P4VP of the composites is only about one-fifth of the total P4VP chains, which clearly indicates that the amount of P4VP with stretched state has decreased due to its complexing to gold particles. So the stretch of the P4VP chains in the hybrid shell of the P4VPb-PS-b-PEG/Au composites should be also partly prohibited at pH 2 and thus their pH-dependent characteristics are weak. The 1H NMR results also confirmed the swollen structures of the P4VP/Au/PEG shell of the triblock copolymer composites under both acidic and neutral conditions and their adjustable behavior induced by the pH variation. As one of the important properties of the noble metal composites, catalytic activity are often studied.16,33,35 In the present study, the catalysis property of the composites of PEGb-PS-b-P4VP/Au is also tested by using reduction of p-nitrophenol by sodium borohydride. The composites concentration used in the system is so low that the adsorption of the composites can be disregarded and the reduction can be followed by monitoring the UV-vis spectra as a function of time. The results are shown in Figure 6. The peak at 400 nm is attributed to the presence of p-nitrophenate ions, which are formed immediately after addition of NaBH4 to the system. The peak height (absorbance intensity) at 400 nm is found to gradually decrease with time. At the same time, a new peak at 312 nm appears, indicating that p-aminophenol is produced.33a The four points that intersect all the spectra at 230, 258, 280, and 330 nm shown in the UV-vis spectra indicate (33) (a) Liu, J.; Qin, G.; Raveendran, P.; Ikushima, Y. Chem. Eur. J. 2006, 12, 2131. (b) Liu, W.; Yang, X.; Xie, L. J. Colloid Interface Sci. 2006, 304, 160. (c) Zhang, M.; Liu, L.; Wu, C.; Fu, G.; Zhao, H.; He, B. Polymer 2007, 48, 1989. (34) Ghosh, S. K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Appl. Catal., A 2004, 268, 61. (35) Hayakawa, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 5517.

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Figure 7. Experimental results of the apparent kinetic rate constant (Kapp) of different composites. The reaction conditions are as follows: [p-nitrophenol] ) 0.015 g/L, [NaBH4] ) 3.76 g/L, [composites] ) 4.75 × 10-4g/L, T ) 20 °C, pH 10.

that p-aminophenol is the unique product of the reduction.16 Additional experiments demonstrates that no reduction takes place in the absence of the composites, indicating that the composites are necessary for the conversion of the p-nitrophenol. The catalytic activity of the composite was further studied by comparing its catalytic reduction reaction rate of p-nitrophenol with that of the composite of PEG-b-P4VP/Au (the gold content as well as the preparation of PEG-b-P4VP/Au are same with that of PEG-b-PS-b-P4VP/Au and the composites are composed of a hybrid P4VP/Au core with spherical GNs of about 3 nm and a PEG shell). The sodium borohydride in the reactive system greatly exceeded the content of p-nitrophenol so that the rates of the reduction are assumed to be independent of the concentration of p-nitrophenol; thus, the kinetics of the reduction can be treated as first-order in p-nitrophenol concentration.34 The ratio of absorbance At of p-nitrophenol at time t to its value A0 measured at t ) 0 directly gives the corresponding concentration ratio Ct/C0 of p-nitrophenol. Thus, the kinetic equation of the reduction could be shown as follows:

dCt/dt ) Kappt

or

ln(C0/Ct) ) ln(A0/At) ) Kappt

(1)

where Ct is the concentration of p-nitrophenol at time t and Kapp is the apparent rate constant, which can be obtained from the decrease of peak intensity at 400 nm with time. As shown in Figure 7, a linear relation between ln(C0/Ct) and reactive time t has been obtained for composites of PEG-b-PS-b-P4VP/Au and PEG-b-P4VP/Au, indicating that the catalytic reduction reaction follows first-order rate kinetics. The apparent rate constant Kapp can be calculated from the slope of the fitting line shown in Figure 7. The composites of PEG-b-PS-b-P4VP/Au demonstrate a larger Kapp(2.65 × 10-3 s-1) than the composites of PEG-b-P4VP/Au (1.32 × 10-3 s-1), which indicates that the PEG-b-PS-b-P4VP/Au shows a higher catalytic activity. The catalytic activities of the noble metal nanoparticles are strongly affected by the carrier structures.16,33b,35 Yu et al. demonstrated that palladium nanoparticles embedded in spherical polyelectrolyte brushes exhibited a higher catalytic activity than palladium encapsulated in the spherical thermosensitive microgel systems, since the reactant molecules could diffusion and reach to the metal nanoparticles more quickly in the former system.16 Esumi et al. found that the reduction rate constant of gold was well-affected by the dendrimer structure when gold/dendrimer

was used as a catalyst for the reduction of p-nitrophenol in aqueous solution.35 It could be explained that the rate constant of the reduction reaction is predominantly controlled by the sizes of the dendrimers adsorbing on the gold nanoparticles. When the noble metal composites are applied to the catalytic activity of the reduction reaction of p-nitrophenol, the reactants are needed to contact the noble metal surface.16,35 In principle, the noble metal nanoparticles, which possess a larger surface to be contacted by the reductants, generally show better catalytic activity.16 In our case, the sizes and the shapes of the GNs in both the composites of PEG-b-P4VP/Au and of PEG-b-PS-b-P4VP/Au are similar, so their catalytic activities are directly related to their different structures. For the composites of PEG-b-P4VP/Au, since the GNs are embedded in the P4VP core, the speed for the reactants to diffusion and reach to the GNs is slow, and the catalytic activity is low. In contrast, two aspects of the composites of PEG-b-PS-b-P4VP/Au profit their higher catalytic activity. On the one hand, the GNs are located in the shell, which is expected to be easier for the reactants to reach and for the products to leave the GNs; on the other hand, since the hydrated PEG chains connect the PS core and pass through the collapsed shell of P4VP/Au (Scheme 1), channels are formed in the hybrid shell.19 These existing channels not only provide an effective way for the reactants to reach and for the products to leave the hydrophobic hybrid shell but also make much more GNs appear in the channels. Thus, more gold particles that are embedded in the hybrid shell can effectively participate in the catalytic activity and the reactants contact the GNs at a faster speed. Just as the above-mentioned, the composites of PEG-b-PS-b-P4VP/Au show better catalytic activity.

Conclusions We have used a pH-sensitive triblock copolymer micelle with a mixed shell as a template to synthesize gold nanoparticles. Since the P4VP chain of the shell has good ability to complex gold, gold nanoparticles with controllable sizes are in situ anchored onto the shell of the composites with a PS core, a hybrid shell of P4VP/Au/PEG, and a PEG corona. At a low pH value (pH 2), P4VP and PEG are hydrophilic, and the hybrid shell of P4VP/ Au/PEG is swollen; at a higher value (pH g7), P4VP/Au become hydrophobic while PEG is hydrophilic, and the hybrid shell of P4VP/Au/PEG is still swollen, but the degree of swelling decreases. Under a high pH condition (pH g7), the hydrophilic PEG chains connect the composite core with their one chain end and pass through the hydrophobic shell of P4VP/Au; thus, the interspaces between the hybrid poly(4-vinylpyridine)/Au shell and the hybrid poly(4-vinylpyridine)/GNs shell-surrounded PEG chains produce channels in the shell of the formed composites. The composites show good catalytic activity due to their novel hybrid shell structures. The component PEG in the corona will provide enough stability for the composites in aqueous solution in a wide environment change such as pH, temperature, and ion strength. Besides, since the shell and corona of the composite and the adjustable structures in the hybrid shell have good biocompatibility, these composites may have important application in biological processes such as ion and/or matter transport and ion regulation. Acknowledgment. We thank the National Natural Science Foundation of China (No. 20474032), the Program for New Century Excellent Talents in Universities, and the Outstanding Youth Fund (No. 50625310) for financial support. LA800244G