pubs.acs.org/Langmuir © 2009 American Chemical Society
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Metal-Polymer Hybrid Colloidal Particles with an Eccentric Structure† Akira Ohnuma,‡, Eun Chul Cho,‡ Majiong Jiang,§ Bunsho Ohtani,
and Younan Xia*,‡
Department of Biomedical Engineering and §Department of Chemistry, Washington University, St. Louis, Missouri 63130, Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan , and ^Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan )
‡
Received April 28, 2009. Revised Manuscript Received June 17, 2009 We have synthesized metal-polymer hybrid colloidal particles characterized by an eccentric structure by precipitation polymerization in the presence of metal colloids. The key to the formation of an eccentric core-shell structure was to introduce metal colloids a few minutes after (rather than before) starting the polymerization. The hybrid particles were uniform in size, and each one of them contained only one metal nanoparticle at its surface after the experimental procedures had been optimized. This method could be extended to a number of different metal colloids stabilized by small molecules, and the yield was found to be more or less independent of the size of the metal nanoparticles. In addition, the position of the metal nanoparticle in the hybrid particle could be controlled by changing the concentration of cross-linker, and the overall size of the hybrid particles could be altered by solvent treatment. Because of the simplicity of this procedure, it should be possible to use it for the large-scale production of colloidal particles having a hybrid, complex structure.
1. Introduction Increasing attention has been paid to colloidal particles with an anisotropic shape, complex, structure, and/or tunable composition because they can have potential applications such as in the stabilization of oil-in-water or water-in-oil emulsions, as building blocks for “bottom up” or self-assembly approaches to microfabrication, and for fabricating multifunctional optical, electronic, and sensing devises.1 Much effort has been devoted to the synthesis of anisotropic colloidal particles. Most of the methods are based on toposelective surface modification,2 template-assisted self-assembly,3 † Part of the “Langmuir 25th Year: Nanoparticles synthesis, properties, and assemblies” special issue. *Corresponding author. E-mail:
[email protected].
(1) (a) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. J. Mater. Chem. 2005, 15, 3745. (b) Glotzer, S.; Solomon, M. Nat. Mater. 2007, 6, 557. (c) Yang, S.; Kim, S.; Lim, J.; Yi, G. J. Mater. Chem. 2008, 18, 2177. (2) (a) Takei, H.; Shimizu, N. Langmuir 1997, 13, 1865. (b) Fujimoto, K.; Nakahama, K.; Shidara, M.; Kawaguchi, H. Langmuir 1999, 15, 4630. (c) Love, J.; Gates, B.; Wolfe, D.; Paul, K.; Whitesides, G. Nano Lett. 2002, 2, 891. (d) Hugonnot, E.; Carles, A.; Delville, M.; Panizza, P.; Delville, J. Langmuir 2003, 19, 226. (e) Cayre, O.; Paunov, V.; Velev, O. Chem. Commun. 2003, 2296. (f) Cayre, O.; Paunov, V.; Velev, O. J. Mater. Chem. 2003, 13, 2445. (g) Paunov, V.; Cayre, O. Adv. Mater. 2004, 16, 788. (h) Lu, Y.; Xiong, H.; Jiang, X.; Xia, Y.; Prentiss, M.; Whitesides, G. J. Am. Chem. Soc. 2003, 125, 12724. (i) Correa-Duarte, M.; Salgueirino-Maceira, V.; RodriguezGonzalez, B.; Liz-Marzan, L.; Kosiorek, A.; Kandulski, W.; Giersig, M. Adv. Mater. 2005, 17, 2014. (j) Takahara, Y.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. J. Am. Chem. Soc. 2005, 127, 6271. (k) Ohnuma, A.; Abe, R.; Shibayama, T.; Ohtani, B. Chem. Commun. 2007, 3491. (l) Chastek, T.; Hudson, S.; Hackley, V. Langmuir 2008, 24, 13897. (3) (a) Yin, Y.; Lu, Y.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 771. (b) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718. (c) Xia, Y.; Yin, Y.; Lu, Y.; McLellan, J. Adv. Funct. Mater. 2003, 13, 907. (4) (a) Gu, H.; Zheng, R.; Zhang, X.; Xu, B. J. Am. Chem. Soc. 2004, 126, 5664. (b) Teranishi, T.; Inoue, Y.; Nakaya, M.; Oumi, Y.; Sano, T. J. Am. Chem. Soc. 2004, 126, 9914. (c) Akiva, U.; Margel, S. Colloids Surf., A 2005, 253, 9. (d) Kim, J.; Larsen, R.; Weitz, D. J. Am. Chem. Soc. 2006, 128, 14374. (5) (a) Reculusa, S.; Poncet-Legrand, C.; Ravaine, S.; Mingotaud, C.; Duguet, E.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 2354. (b) Yu, H.; Chen, M.; Rice, P.; Wang, S.; White, R.; Sun, S. Nano Lett. 2005, 5, 379. (c) Ge, J.; Hu, Y.; Zhang, T.; Yin, Y. J. Am. Chem. Soc. 2007, 129, 8974. (d) Camargo, P.; Xiong, Y.; Ji, L.; Zuo, J.; Xia, Y. J. Am. Chem. Soc. 2007, 129, 15452. (e) Nagao, D.; Hashimoto, M.; Hayasaka, K.; Konno, M. Macromol. Rapid Commun. 2008, 29, 1484. (6) (a) Roh, K.; Martin, D.; Lahann, J. Nat. Mater. 2005, 4, 759. (b) Nisisako, T.; Torii, T.; Higuchi, T. Chem. Eng. J. 2004, 101, 23. (c) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408.
13880 DOI: 10.1021/la9015146
phase separation,4 surface-controlled nucleation and growth,5 and microfluidic techniques.6 In spite of their impressive success, it is worth pointing out that most of these methods cannot be readily extended to large-scale production because of the 2D strategy or multiple steps involved in a typical fabrication approach. Still, a facile, robust procedure is desired for the preparation of colloidal particles with an anisotropic structure. Inorganic-polymeric hybrid colloidal particles have also attracted considerable attention in recent years because the intimate combination of polymeric and inorganic components offers the promise of interesting properties that are not possible from either material alone. For example, gold allows for the covalent attachment of thiolate ligands to the particle surface,7 generating unique surface functional groups for an array of applications,8 whereas the polymeric coating could aid in the biocompatibility of colloidal particles.9 A number of methods have been demonstrated for generating polymeric shells around inorganic nanoparticles, including the synthesis of particles in the presence of a polymeric ligand,10 polymerization from a particle-bound initiator,11 layer-by-layer deposition,12 and direct attachment of functionalized polymers to the particle surface.13 In these cases, the specific chemical interaction between the particle surface and the surface-bound polymer must be tailored to form a complete, controllable shell. (7) (a) Cao, Y.; Jin, R.; Mirkin, C. J. Am. Chem. Soc. 2001, 123, 7961. (b) Lyon, J.; Fleming, D.; Stone, M.; Schiffer, P.; Williams, M. Nano Lett. 2004, 4, 719. (8) Averitt, R.; Sarkar, D.; Halas, N. Phys. Rev. Lett. 1997, 78, 4217. (9) (a) Corbierre, M. K.; Cameron, N. S.; Sutton, M.; Mochrie, S. G. J.; Lurio, L. B.; Ruhm, A.; Lennox, R. B. J. Am. Chem. Soc. 2001, 123, 10411. (b) Kim, S.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 14652. (10) (a) Wuelfing, W.; Gross, S.; Miles, D.; Murray, R. J. Am. Chem. Soc. 1998, 120, 12696. (b) Lowe, A.; Sumerlin, B.; Donovan, M.; McCormick, C. J. Am. Chem. Soc. 2002, 124, 11562. (11) (a) Nuss, S.; Bottcher, H.; Wurm, H.; Hallensleben, M. Angew. Chem., Int. Ed. 2001, 40, 4016. (b) von Werne, T.; Patten, T. J. Am. Chem. Soc. 2001, 123, 7497. (c) Vestal, C.; Zhang, Z. J. Am. Chem. Soc. 2002, 124, 14312. (d) Sill, K.; Emrick, T. Chem. Mater. 2004, 16, 1240. (12) (a) Gittins, D.; Caruso, F. Adv. Mater. 2000, 12, 1947. (b) Gittins, D.; Caruso, F. J. Phys. Chem. B 2001, 105, 6846. (c) Mayya, K. S.; Schoeler, B.; Caruso, F. Adv. Funct. Mater. 2003, 13, 183. (13) (a) Zhu, M.; Wang, L.; Exarhos, G.; Li, A. J. Am. Chem. Soc. 2004, 126, 2656. (b) Corbierre, M.; Cameron, N.; Lennox, R. Langmuir 2004, 20, 2867.
Published on Web 07/21/2009
Langmuir 2009, 25(24), 13880–13887
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Figure 1. Schematic illustration of the procedure for generating eccentric Au-PS hybrid particles. TEM images indicate (A) Au nanoparticles that we used for the synthesis and reaction intermediates obtained at (B) 5 min (the black arrows indicate PS) and (C) 10 min, respectively, after the initiation of polymerization. The average diameter of the Au particles was 50 nm.
Recently, Yu et al. reported a solvothermal route to the synthesis of silver nanoparticles encapsulated with phenol formaldehyde shells in a concentric or eccentric structure.14 We have developed a simple, versatile procedure for generating eccentric, hybrid particles consisting of Au nanoparticles and polymer beads without using a primer.15 We modified the precipitation polymerization procedure for polystyrene (PS) by adding Au colloids to the system after the polymerization had proceeded for a few minutes. The eccentric particles were uniform in size and morphology, with each PS bead containing only one Au nanoparticle at its surface. In the present study, we analyzed the preparation procedure and the hybrid colloidal particles in more detail and also applied the method to Au nanoparticles of different sizes and to different types of metal nanoparticles. In addition, we found that the position of the metal nanoparticle in a hybrid particle could be controlled by varying the concentration of the cross-linker, and the overall sizes of the PS beads could be altered using a solvent treatment procedure.
2. Experimental Section Chemicals and Materials. Sodium citrate (Na-CA, meets USP testing specifications, Sigma-Aldrich), citric acid (CA, 99.8%, Fisher Scientific), gold(III) chloride trihydrate (HAuCl4 3 3H2O, g99.9%, Aldrich), sodium tetrachloropalladate(II) (Na2PdCl4, 99.998%, Aldrich), chloroplatinic acid hydrate (H2PtCl6, 99.995%, Aldrich), iron(III) chloride anhydrous (FeCl3, g98%, Riedel-de Haen), poly(vinyl pyrrolidone) (PVP, Mw ≈ 55 000, Sigma-Aldrich), potassium persulphate (KPS, 99.99%, Sigma-Aldrich), gold colloids (50 and 100 nm in diameter, Ted Pella), 4-styrenesulfonic acid sodium salt hydrate (NaSS, Aldrich), styrene (g99%, Sigma-Aldrich), divinylbenzene (DVB, 80%, Aldrich), hexadecyltrimethylammonium bromide (CTAB, ∼99%, Sigma), poly(acrylic acid) (PAAc, Mw ≈ 1800, Aldrich), poly(allylamine hydrochloride) (PAA, Mw ≈ 15 000, Aldrich), 3-mercaptopropionic acid (MPA, g99%, Aldrich), ethanol (100%, AAPER Alcohol and Chemical), and tetrahydrofuran (THF, g99%, Sigma-Aldrich) were all used as received. Deionized water with a resistivity of 18 MΩ cm was prepared using an ultrapure water system (Aqua Solutions) and is referred to as water throughout the article. (14) Guo, S.; Gong, J.; Jiang, P.; Wu, M.; Lu, Y.; Yu, S. Adv. Funct. Mater. 2008, 18, 872. (15) Ohnuma, A.; Cho, E. C.; Camargo, P. H. C.; Au, L.; Ohtani, B.; Xia, Y. J. Am. Chem. Soc. 2009, 131, 1352.
Langmuir 2009, 25(24), 13880–13887
Figure 2. (A) SEM and (B-E) TEM images of the eccentric Au-PS particles prepared from Au colloids of different sizes. The average diameters of the Au nanoparticles were (A-C) 50 nm, (D) 10 nm, and (E) 100 nm, respectively. DOI: 10.1021/la9015146
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Preparation of 10 nm Au Colloids. Gold nanoparticles ∼10 nm in size were prepared using the method reported by Frens.16 To 75 mL of an aqueous HAuCl4 solution (0.340 g L-1) heated to boiling was added 9 mL of an aqueous solution containing Na-CA (58.2 mM). After the change in color from deep blue to deep red, the solution was boiled for another 30 min. Preparation of 30 nm Pd Colloids. Palladium nanoparticles ∼30 nm in szie were prepared by heating 11 mL of an aqueous solution containing 57 mg of Na2PdCl4, 105 mg of PVP, and 60 mg of CA at 90 C for 26 h. The solution was washed with ethanol two times (centrifugation at 13 200 rpm for 3 min) to remove small Pd crumbs. Preparation of 10 nm Pt Colloids. Platinum nanoparticles ∼10 nm in size were synthesized by heating 11.2 mL of an aqueous solution containing 33 mg of H2PtCl6, 45 mg of PVP, 60 mg of CA, and 0.3 mg of FeCl3 at 90 C for 26 h. Changing the Stabilizer for Au Colloids from Citrate to CTAB. Three milliliters of the commercial 50 nm Au colloids was centrifuged at 7000 rpm for 10 min, and the supernatant was removed. The precipitate was redispersed in 3 mL of an aqueous solution of CTAB (1 mM). This dispersion was stored at room temperature for 15 min and centrifuged down (7000 rpm for 10 min), collected, and then redispersed in 3 mL of water.
Changing the Stabilizer for Au Colloids from Citrate to PAAc, PAA, or MPA. Three milliliters of the commercial 50 nm Au colloids was centrifuged at 7000 rpm for 10 min, and the supernatant was removed. The precipitate was then redispersed in 3 mL of water, followed by adding 0.03 mL of an aqueous polymer solution (PAAc or PAA, 1% by wt) or
Ohnuma et al. an MPA aqueous solution (1 mM). The mixture was shaken for 1 s with a vortex mixer and then kept at room temperature for 3 h.
Synthesis of Eccentric Metal-Polystyrene Colloidal Particles. In a typical synthesis, 50 mg of KPS, 4.5 mL of water, 6 mg of NaSS, and 16.5 mL of ethanol were added to a 25 mL three-necked flask equipped with a reflux condenser and a Tefloncoated magnetic stirring bar. This system was heated to 70 C under magnetic stirring. When the temperature reached 70 C, 0.2 mL of a styrene-DVB mixture (99:1 w/w) was added to the flask, and 2 min later, 3.0 mL of the aqueous suspension of metal colloids (containing ∼1.4 1011 metal particles) was introduced. The Table 1. Reaction Conditions for Experiments and Remarks on Products time point for concentration stabilizer of adding Au of EtOH colloids (min)a (%, wt/wt)b Au colloids
results
63 NA-CA aggregation (Figure 3A)c 63 NA-CA 99% yield (Figure 2A-E) 63 NA-CA 80% yield (Figure 3B) 0 NA-CA separated (Figure 3C)d 30 NA-CA separated (Figure 3D) 63 CTAB 70% yield (Figure 4A) 63 PAAc aggregation (Figure 4B) 63 PAA aggregation (Figure 4C) 63 MPA aggregation (Figure 4D) a After starting the polymerization. b In the mixture of water and EtOH after the addition of Au colloids. c Au nanoparticles tended to aggregate. d Au nanoparticles and PS beads were separated. 1 2 3 2 2 2 2 2 2
Figure 3. (A, B) TEM images of the products prepared when Au colloids were introduced at (A) 1 min and (B) 3 min, respectively, after starting the polymerization. (C, D) TEM images of the products obtained with two different compositions of the mixture of ethanol and water. The concentration of ethanol was (C) 0% and (D) 30% by weight. The Au nanoparticles were (A, B) 100 nm and (C, D) 50 nm in diameter. 13882 DOI: 10.1021/la9015146
Langmuir 2009, 25(24), 13880–13887
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Figure 4. TEM images of the products prepared with Au colloids whose surfaces were stabilized by (A) CTAB, (B) PAAc, (C) PAA, and (D) MPA, respectively. In all cases, the citrate layer was replaced with the stabilizer shown in each panel. The eccentric Au-PS particles could be obtained only when the strength of the interaction between the Au surface and the stabilizer was relatively weak so that it could be displaced by PS oligomers during nucleation. The Au nanoparticles were 50 nm in size. reaction system was continued with heating at 70 C for another 4 h. The product was collected by centrifugation and washed with a mixture of ethanol and water (60:40 w/w) three times and with ethanol one time and was finally dispersed in 20 mL of ethanol. Etching of the Eccentric Au-PS Particles. The as-prepared dispersion of eccentric Au-PS particles (0.5 mL) was centrifuged (7000 rpm, 10 min), and 0.4 mL of the supernatant was removed. The remaining dispersion was added to 0.4 mL of a mixture of ethanol and THF (the final THF concentration was 8-10% by wt) and shaken for 10 s with a vortex mixer and then washed with ethanol two times. Instrumentation. Scanning electron microscopy (SEM) studies were performed with a FEI Nova NanoSEM 2300 operated at 10 kV. Transmission electron microscopy (TEM) studies were conducted with a FEI Tecnai G2 Spirit operated at 120 kV. Highresolution TEM (HRTEM) images were taken using a JEOL 2100F microscope operated at 200 kV. The samples for SEM studies were prepared by drying droplets of the particle suspensions on silicon substrates and then drying under ambient conditions. The samples for TEM or HRTEM studies were prepared by drying droplets of the particle suspensions on copper grids coated with Formvar carbon (Ted Pella). The UV-vis spectra of the particle suspensions (in water) were obtained using a Varian Cary 50 UV-vis spectrophotometer.
(16) Frens, G. Nature (London) Phys. Sci. 1973, 241, 20.
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3. Results and Discussion Figure 1 shows a schematic illustration of the procedure that we used to generate the eccentric Au-PS hybrid particles and TEM images of samples obtained at different stages of the synthesis. We first added styrene and divinylbenzene (DVB) to a mixture of ethanol and water in which 4-styrenesulfonic acid sodium salt (NaSS) and potassium persulphate (KPS) had been dissolved. Because NaSS and KPS could stabilize the growing PS beads, we did not add other surfactants to the reaction system. After 2 min of synthesis at 70 C, we introduced Au colloids (e.g., 50 nm in size) into the reaction mixture. At this point, PS oligomers and/or monomers started to nucleate by adsorbing onto the surface of the Au nanoparticles, which then grew in size as a result of the polymerization and eventually evolved into spheres as confined by surface tension. As indicated by the TEM image in Figure 1A, the Au nanoparticles that we used for the synthesis were characterized by a range of different morphologies, although they were relatively uniform in size, with a coefficient of variation (CV)
