pubs.acs.org/Langmuir © 2009 American Chemical Society
Synthesis of pH-Responsive Nanocomposite Microgels with Size-Controlled Gold Nanoparticles from Ion-Doped, Lightly Cross-Linked Poly(vinylpyridine) Kensuke Akamatsu,*,†,‡ Megumi Shimada,† Takaaki Tsuruoka,† Hidemi Nawafune,† Syuji Fujii,§ and Yoshinobu Nakamura§ † Department of Nanobiochemistry, Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojimaminami, Chuo-ku, Kobe 650-0047, Japan, ‡PRESTO, Japan Science and Technology Agency, 3-5 San-ban-cho, Chiyoda-ku, Tokyo 102-0075, Japan, and §Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan
Received July 7, 2009. Revised Manuscript Received September 25, 2009 The synthesis of composite microgels consisting of pH-responsive latexes with gold nanoparticles was investigated along with the optical properties of the products. The gold nanoparticles were deposited by wet chemical reduction from gold ions adsorbed in cross-linked poly(2-vinylpyridine) latexes, by which the mean particle size of the gold nanoparticles could be systematically controlled over a range of 10-30 nm simply by varying the reduction rate. Microscopic analysis showed that the gold nanoparticles were formed only on the surface of the microgels, resulting from diffusion of the gold ions from the interior to the surface of the microgels during reduction treatment. The resulting nanocomposites preserved the pH-responsive properties of the pure latexes. The degree of plasmon coupling, originating from dipole interactions among the gold nanoparticles, was dependent on the size of the nanoparticles and could be reversibly controlled by varying the pH of the aqueous solution. The process allowed independent control of the size and interparticle distance among gold nanoparticles, an ability that is important in increasing the fundamental understanding of the structure-dependent properties of gold nanoparticles and also for biological applications using functionalized composite latexes/microgels.
Introduction Over the last two decades, the synthesis and application of novel stimulus-responsive microgel particles have received increasing amounts of attention.1 The two most common stimuli for such microgels are temperature and pH. Temperature-responsive microgels, especially poly(N-isopropylacrylamide)-based microgels, have been extensively studied over the past 20 years.1,2 There are also many examples of pH-responsive microgels, such as alkaliswellable latexes based on (methyl)acrylic acid, that find widespread use in industrial applications, including thickeners for cosmetic and pharmaceutical formulations.3 However, the number of studies of acid-swellable microgels has been limited.4 Stimuli-responsive composite polymer gels containing inorganic nanoparticles are developing as an important class of materials based on the unique size-dependent properties of nanoparticles in combination with the macroscopic properties of polymer gels. Because the incorporation of nanoparticles into polymer gels *To whom correspondence should be addressed. E-mail: akamatsu@ center.konan-u.ac.jp. (1) (a) Murray, M. J.; Snowden, M. J. Adv. Colloid Interface Sci. 1995, 54, 73–91. (b) Saunders, B. R.; Vincent, B. Adv. Colloid Interface Sci. 1999, 80, 1–25. (c) Pelton, R. H. Adv. Colloid Interface Sci. 2000, 85, 1–33. (d) Kawaguchi, H. Prog. Polym. Sci. 2000, 25, 1171–1210. (e) Lyon, L. A.; Debord, J. D.; Debord, S. B.; Jones, C. D.; McGrath, J. G.; Serpe, M. J. J. Phys. Chem. B 2004, 108, 19099–19108. (2) Pelton, R. H.; Chibante, P. Colloids Surf. 1986, 20, 247–256. (3) (a) Rodriguez, B. E.; Wolfe, M. S.; Fryd, M. Macromolecules 1994, 27, 6642– 6647. (b) Saunders, B. R.; Crowther, H. M.; Vincent, B. Macromolecules 1997, 30, 482–487. (4) (a) Ma, G. H.; Fukutomi, T. Macromolecules 1992, 25, 1870–1875. (b) Loxley, A.; Vincent, B. Colloid Polym. Sci. 1997, 275, 1108–1114. (c) H.; Iijima, M.; Kataoka, K.; Nagasaki, Y. Macromolecules 2004, 37, 5389–5396. (d) Amalvy, J. I.; Wanless, E. J.; Li, Y.; Michailidou, V.; Armes, S. P.; Duccini, Y. Langmuir 2004, 20, 8992–8999. (e) Dupin, D.; Fujii, S.; Armes, S. P.; Reeve, P.; Baxter, S. M. Langmuir 2006, 22, 3381–3387.
1254 DOI: 10.1021/la902450c
provides additional functionality (e.g., optical responsiveness,5 catalytic activity,6 and magnetic properties7), there have been many reports on the synthesis of stimuli-responsive microgels containing metal nanoparticles,8 nanorods,9 quantum dots,10 and magnetic nanoparticles.7 These composite microgels are currently the focus (5) (a) Gang, Y. L.; Liu, L.; Lee, L. P. Nano Lett. 2005, 5, 5–9. (b) Mitsuishi, M.; Koishikawa, Y.; Tanaka, H.; Sato, E.; Mikayama, T.; Matsui, J.; Miyashita, T. Langmuir 2007, 23, 7472–7474. (c) Xu, H.; Xu, J.; Jiang, X.; Zhu, Z.; Rao, J.; Yin, J.; Wu, T.; Liu, H.; Liu, S. Chem. Mater. 2007, 19, 2489–2494. (d) Li, D.; Cui, Y.; Wang, K.; He, Q.; Yan, X.; Li, J. Adv. Funct. Mater. 2007, 17, 3134–3140. (e) Yusa, S.; Fukuda, K.; Yamamoto, T.; Iwasaki, Y.; Watanabe, A.; Akiyoshi, K.; Morishima, Y. Langmuir 2007, 23, 12842–12848. (6) (a) Mei, Y.; Sharma, G.; Lu, Y.; Ballauff, M. Langmuir 2005, 21, 12229– 12234. (b) Mei, Y.; Lu, Y.; Polzer, F.; Ballauff, M. Chem. Mater. 2007, 19, 1062–1069. (7) (a) Kroll, E.; Winnik, F. M. Chem. Mater. 1996, 8, 1594–1596. (b) Pich, A.; Bhattacharya, S.; Lu, Y.; Boyko, V.; Adler, H.-J. P. Langmuir 2004, 20, 10706– 10711. (c) Bhattacharya, S.; Eckert, F.; Boyko, V.; Pich, A. Small 2007, 3, 650–657. (d) Rubio-Retama, J.; Zafeiropoulos, N. E.; Serafinelli, C.; Rojas-Reyna, R.; Voit, B.; Cabarcos, E. L.; Stamm, M. Langmuir 2007, 23, 10280–10285. (e) Mohammadi, Z.; Cole, A.; Berkland, C. J. J. Phys. Chem. C 2009, 113, 7652–7658. (8) (a) Liu, S.; Weaver, J. V. M.; Save, M.; Armes, S. P. Langmuir 2002, 18, 8350–8357. (b) Kim, J.-H.; Lee, T. R. Chem. Mater. 2004, 16, 3647–3651. (c) Zhang, J.; Xu, S.; Kumacheva, E. Adv. Mater. 2005, 17, 2336–2340. (d) Suzuki, D.; Kawaguchi, H. Langmuir 2005, 21, 8175–8179. (e) Lu, Y.; Mei, Y.; Ballauff, M. J. Phys. Chem. B 2006, 110, 3930–3937. (f) Mohan, Y. M.; Premkumar, T.; Lee, K.; Geckeler, K. E. Macromol. Rapid Commun. 2006, 27, 1346–1354. (g) Suzuki, D.; Kawaguchi, H. Langmuir 2006, 22, 3818–3822. (h) Oishi, M.; Hayashi, H.; Uno, T.; Ishii, T.; Iijima, M.; Nagasaki, Y. Macromol. Chem. Phys. 2007, 208, 1176–1182. (i) Palioura, D.; Armes, S. P.; Anastasiadis, S. H.; Vamvakaki, M. Langmuir 2007, 23, 5761–5768. (j) Kim, H.; Daniels, E. S.; Dimonie, V. L.; Klein, A. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 912–925. (k) Kozlovskaya, V.; Kharlampieva, E.; Chang, S.; Muhlbauer, R.; Tsukruk, V. V. Chem. Mater. 2009, 21, 2158–2167. (9) (a) Gorelikov, I.; Field, L. M.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 15938–15939. (b) Das, M.; Sanson, N.; Fava, D.; Kumacheva, E. Langmuir 2007, 23, 196–201. (c) Karg, M.; Pastoriza-Santos, I.; Perez-Juste, J.; Hellweg, T.; Liz-Marzan, L. M. Small 2007, 3, 1222–1229. (10) (a) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908– 7914. (b) Janczewski, D.; Tomczak, N.; Han, M.-Y.; Vancso, G. J. Macromolecules 2009, 42, 1801–1804.
Published on Web 10/09/2009
Langmuir 2010, 26(2), 1254–1259
Akamatsu et al.
Article
of considerable scientific research because of their potential for technological application in industry and in the fields of biology and medicine.11 The properties of metal nanoparticles are generally dependent on several structural parameters, including composition, crystallinity, nanoparticle size, interparticle distance, and the dielectric properties of the supporting medium.12 Because the size and interparticle distance of metal nanoparticles determine quantum size effects and particle-to-particle interactions, respectively, it is very important to achieve precise control of these parameters as well as gel functionality in nanoparticle/gel composite systems. Stimuli-responsive composite polymer gels are suitable systems for the systematic investigation of the distance-dependent properties of metal nanoparticles through the volume control of microgels. Although the synthesis of metal nanoparticles in microgels has been reported,5-10 systematic control of the size, shape, and location of nanoparticles in and/or on microgels remains a considerable challenge. Nanoparticles deposited by in situ synthesis processes are generally polydisperse, and size control is relatively difficult.7a,8d-i In addition, the formation of nanoparticles in microgels occasionally results in a loss of full gel functionality. These drawbacks limit the potential applications of metal/polymer composite microgels. Thus, further study of such microgels requires the development of a reliable synthesis strategy that addresses these issues. To the best of our knowledge, there have been no previous reports on the preparation of composite microgels in which the size of the metal nanoparticles may be systematically controlled and a narrow size distribution obtained. This motivated us to develop a method that allows the effective deposition of metal nanoparticles on stimuli-responsive microgels by a novel, simple, and robust synthesis protocol. In this work, we used cross-linked poly(vinylpyridine) microgels as a matrix for doping gold ions to prepare hybrid microgels containing gold nanoparticles by chemical reduction treatment. Using a relatively weak reducing agent, dimethylamineborane, and diffusion-controlled formation and growth, we demonstrated that the gold nanoparticles were deposited only on the microgel surface, which allowed systematic control of the nanoparticle size. Furthermore, the core-shell morphology of composite microgels fully preserved the pH-responsive properties of the microgels, enabling us to examine the size- and distance-dependent optical properties of the deposited gold nanoparticles.
Experimental Section Materials. 2-Vinylpyridine (2VP, 97%) and divinylbenzene (DVB: 55%, containing 4-ethylvinylbenzene and 3-ethylvinylbenzene) were purchased from Aldrich and treated with basic alumina in order to remove inhibitors (p-tert-butylcatechol). Poly(ethylene glycol) methyl ether methacrylate (PEGMA) macromonomer (Mn = 2080, 50 wt % aqueous solution) and R,R0 -azodiisobutyramidine dihydrochloride (AIBA, 97%) were purchased from Aldrich and used as received. Aliquat 336 was purchased from Sigma-Aldrich and used as received. Chloroauric acid (HAuCl4, 99.3%), sodium hydroxide (NaOH), and dimethylamineborane (11) (a) Mori, T.; Maeda, M. Langmuir 2004, 20, 313–319. (b) Ali, M. M.; Su, S.; Filipe, C. D. M.; Pelton, R.; Li, Y. Chem. Commun. 2007, 4459–4461. (c) Oishi, M.; Hayashi, H.; Iijima, M.; Nagasaki, Y. J. Mater. Chem. 2007, 17, 3720–3725. (12) (a) Mirkin, C. A.; Letsinger, R. E.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607–609. (b) Alivisatos, A. P. J. Phys. Chem. B 1996, 100, 13226–13239. (c) Gross, A. F.; Diehl, M. R.; Beverly, K. C.; Richman, E. K.; Tolbert, S. H. J. Phys. Chem. B 2003, 107, 5475–5482. (d) Teng, X.; Yang, H. J. Am. Chem. Soc. 2003, 125, 14559–14563. (e) Franzl, T.; Klar, T. A.; Schietinger, S.; Rogach, A. L.; Feldmann, J. Nano Lett. 2004, 4, 1599–1603. (f) Tokareva, I.; Minko, S.; Fendler, J. H.; Hutter, E. J. Am. Chem. Soc. 2004, 126, 15950–15951. (g) Tomita, S.; Akamatsu, K.; Shinkai, H.; Ikeda, S.; Nawafune, H.; Mitsumata, C.; Kashiwagi, T.; Hagiwara, M. Phys. Rev. B 2005, 71, 180414–4.
Langmuir 2010, 26(2), 1254–1259
(DMAB) were purchased from Wako Chemical Co. and used as received. Deionized water (RFD240NA: GA25A-0715, Advantec MFS, Inc.) was used for the synthesis and purification of the P2VP latex particles. Synthesis of P2VP Latex. The stimulus-responsive microgels used in this study, poly(2-vinylpyridine) lightly cross-linked with divinylbenzene, were prepared according to the literature.13 Aliquat 336 surfactant (1.00 g) and PEGMA stabilizer (2.00 g) were dissolved in deionized water (80.0 g) in a 250 mL singlenecked round-bottomed flask. A co-monomer mixture of 2VP (9.90 g) and DVB (0.100 g) was then added, causing the solution pH to increase to approximately pH 8.2. The flask was sealed with a rubber septum, and the aqueous solution was degassed at ambient temperature using five vacuum/nitrogen cycles. The degassed solution was stirred at 250 rpm using a magnetic stirrer and heated to 60 °C with the aid of an oil bath; after 20 min, an aqueous azo initiator solution (0.100 g of AIBA dissolved in 9.00 g of water) was injected. The copolymerizing solution turned milky white within 10 min, and stirring was continued for 24 h at 60 °C. The P2VP latex particles were centrifuged at 8000 rpm for 20 min using a centrifuge (Hitachi CF16RXII), followed by careful decantation of the supernatant, replacement with fresh water, and redispersion of the sedimented particles with the aid of an ultrasonic bath. This protocol was carried out to remove residual 2VP monomer, excess Aliquat 336 surfactant, and nongrafted PEGMA stabilizer. Synthesis of Nanocomposite Microgels. P2VP latex (6.0 wt %) was mixed with aqueous HAuCl4 solution (20 mM) in a ratio of 1:19 (v/v) at room temperature, and the mixed solution was stirred for 1 h. The microgels were then purified by dialysis, providing ion-doped microgels as precursors for the composite microgels. Aqueous DMAB solution (10 mM, 11.4 mL) was then gradually mixed with the solution of ion-doped microgels (0.2 wt %, 2 mL) at appropriate time intervals under stirring for 30 min to reduce the gold ions to metallic gold, followed by dialysis using distilled water. Characterization. The chemical structures of the pure and ion-doped microgels were analyzed by Fourier-transform infrared spectroscopy (FTIR) using an FTIR 670 Plus instrument (Japan Spectroscopic Co.) at a resolution of 4 cm-1 with 100 accumulations. The samples were prepared after drying the emulsions at 40 °C for 12 h and were measured using the standard KBr method. The number of gold ions in the microgels was quantified by inductively coupled plasma (ICP) atomic emission spectroscopy (SPS7700 plasma spectrometer, Seiko Instruments). To extract gold ions, the ion-doped gels were immersed in hydrochloric acid solution (1 vol %) at room temperature for 3 h. The morphology of the microgels and the mean size and size distribution of the gold nanoparticles were confirmed by planview and cross-sectional transmission electron microscopy (TEM, JEM-2000EX, JEOL) operating at 200 kV. The samples for crosssectional TEM observation were prepared by embedding the dried composite microgels into epoxy resin, followed by curing and sectioning into slices of about 100 nm thickness using a conventional microtome technique with a diamond knife (Leica, Ultracut R).
Results Synthesis of Ion-Doped Precursor Microgels. Figure 1 is a schematic diagram of the preparation process of the composite microgels. The pure microgels showed swelling-shrinking behavior around pH 3 to 4.13 The microgels were coated with a poly(ethylene glycol) corona that rendered them water-dispersible in any pH range. The microgel core diameters for samples deposited on TEM copper grids from solution at pH 7.0 and (13) Dupin, D.; Fujii, S.; Armes, S. P.; Reeve, P.; Baxter, S. M. Langmuir 2006, 22, 3381–3387.
DOI: 10.1021/la902450c
1255
Article
Akamatsu et al.
Figure 1. Schematic representation of the synthesis process for pH-responsive microgels with surface gold nanoparticles.
Figure 2. TEM images of microgels obtained after the reduction of gold ions using aqueous DMAB solution. The images were taken from samples obtained by dropping the solution at (A-D) pH 8 and (E-H) pH 2 onto a carbon-coated copper grid. The gold nanoparticles were grown by reduction with DMAB at dropping rates of (A, E) 11.4 mL s-1, (B, F) 1 mL s-1, (C, G) 500 μL s-1, and (D, H) 190 μL s-1. Scale bar: 200 nm. (I-L) Size histograms obtained for samples E-H.
2.0 were ca. 250 and 500 nm, respectively (results not shown). Because the synthesized microgels were protonated at pH
