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Tailored-Control of Gold Nanoparticle Adsorption onto Polymer Nanosheets Hiroyuki Tanaka, Masaya Mitsuishi, and Tokuji Miyashita* Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan Received December 26, 2002. In Final Form: February 19, 2003 This paper focuses on controlling adsorption of gold nanoparticles onto polymer nanosheets. Amphiphilic copolymers, poly(N-dodecyl acrylamide-co-4-vinyl pyridine)s (p(DDA/VPy)s), were synthesized and the ultrathin p(DDA/VPy) films were prepared by the Langmuir-Blodgett (LB) technique. Gold nanoparticles were immobilized onto p(DDA/VPy) LB films through the dipping method and took a uniformly distributed monolayer formation. The amount of gold nanopariticle in the monolayer strongly depended on the VPy content. A patterned gold nanoparticle monolayer was obtained with photopatterned p(DDA/VPy) LB films. These findings suggest that p(DDA/VPy) LB films act as a good template for gold nanoparticle arrays.
Introduction Recently, nanostructured materials have attracted both academic and industrial interest.1-4 In particular, much effort has been devoted to directing nanosized particles, such as gold, silver, semiconductor, and silica nanoparticles, onto dielectric surfaces in a tailored manner. For example, immobilization of nanoparticles by electrostatic interaction, hydrogen bonding, and covalent bonding has been carried out onto a self-assembled monolayer5 and alternative adsorption multilayers.6,7 To date, selective adsorption of nanoparticles and close packing of nanoparticle monolayers have been achieved by modification of the nanoparticle surface.8-10 Previously, we reported that N-alkylacrylamide polymers and copolymers with various functional monomers form a highly oriented polymer monolayer on the water surface; also, monolayers can be transferred onto a solid support by the Langmuir-Blodgett (LB) method.11,12 This yields a polymer nanosheet with a two-dimensional hydrogen bonding network through which amide groups function effectively. Polymer nanosheets with a thickness of 1-2 nm can be used for solid-surface modification. Herein, we found that a gold nanoparticle is adsorbed * To whom correspondence should be addressed. (1) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735-743. (2) Malynych, S.; Luzinov I.; Chumanov, G. J. Phys. Chem. B 2002, 106, 1280-1285. (3) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter D. G.; Natan, M. J. Science 1995, 267. 1629-1632. (4) Cassagneau, T. P.; Sweryda-Krawiec, B.; Fendler, J. H. MRS Bull. 2000, 25, 40-46. (5) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18-52. (6) Schmitt, J.; Decher, G.; Dressick, W. J.; Brandow, S. L.; Geer, R. E.; Shashidhar, R.; Calvert, J. M. Adv. Mater. 1997, 9, 61-65. (7) Masuhara, A.; Kasai, H.; Kato, T.; Okada, S.; Oikawa, H.; Nozue, Y.; Tripathy, S. K.; Nakanishi, H. J. Macromol. Sci. Pure Appl. Chem. 2001, 38, 1371-1382. (8) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148-1153. (9) Peschel, S.; Schmid, G. Angew. Chem., Int. Ed. Engl. 1995, 34, 1442-1443. (10) Sato, T.; Brown, D.; Johnson, B. F. G. Chem. Commun. 1997, 1007-1008. (11) Miyashita, T. Prog. Polym. Sci. 1993, 18, 263-294. (12) Taniguchi, T.; Yokoyama, Y.; Miyashita, T. Macromolecules 1997, 30, 3646-3649.
Figure 1. Schematic illustration of preparation for gold nanoparticle monolayers with p(DDA/VPy) nanosheets.
effectively onto a polymer nanosheet containing 4-vinylpyridine, yielding an adsorbed gold nanoparticle monolayer. The polymer nanosheet showed efficient adhesion forces to immobilize gold nanoparticles. Moreover, the patterned polymer sheet by UV irradiation worked as a template to direct gold nanoparticles. This technique has great potential for applications for controlling nanoparticle monolayer ordering. Experimental Section N-Dodecylacrylamide (DDA) was synthesized according to a published process.13 4-Vinylpyridine was purchased (Aldrich) and vacuum-distilled before use. Amphiphilic copolymers, poly(N-dodecyl acrylamide-co-4-vinyl pyridine)s (p(DDA/VPy)s), with different VPy contents were prepared in toluene by the radical copolymerization method. The VPy contents used were 5.9, 11, 29, and 56 mol %, determined by UV spectroscopy. Monodispersed (13) Arisumi, K.; Feng, F.; Miyashita, T.; Ninomiya, H. Langmuir 1998, 14, 5555-5558.
10.1021/la027076o CCC: $25.00 © 2003 American Chemical Society Published on Web 03/19/2003
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Figure 2. Surface pressure-area isotherms for p(DDA/VPy) monolayers at 15.0 °C. gold nanoparticles were prepared by an aqueous reduction method of HAuCl4 by sodium citrate at reflux, according to refs 1 and 14. The particle diameter was determined to be ∼30 nm by TEM observation. The experimental procedure for preparation of gold nanoparticle arrays is outlined schematically in Figure 1. Three-layer p(DDA/VPy) LB films were transferred onto silicon substrates by the vertical dipping method. Substrates were immersed in gold nanoparticle solution (pH ) 6.0) for several hours. After immersion, the substrate was rinsed in pure water and then finally dried with nitrogen gas. For patterning of gold nanoparticle monolayers, the p(DDA/VPy) LB films were photoirradiated through a photomask with a deep UV lamp (UXM-501MA, USHIO, 50 mW/cm2) for 20 min before immersion.
Results and Discussion The p(DDA/VPy) polymer was spread onto the water surface from a chloroform solution. Monolayers on the water show a steep rise in surface pressure and high collapse pressure, indicating densely packed monolayer formation at the water surface (Figure 2). Monolayers can be transferred onto solid supports as Y-type LB films
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at 30 mN/m and 15.0 °C. The transfer ratios of p(DDA/ VPy) monolayers with 5.9, 11, and 29 mol % contents were 0.9 and 1.0 for up- and downstrokes, respectively.15 From the π-A isotherm in Figure 2, the averaged occupied surface area was determined by extrapolation of the linear portion at the condensed state in Figure 2 to zero surface pressure. The averaged surface limiting areas for polymer monolayers with 5.9, 11, 29, and 56 mol % contents were 0.29, 0.27. 0.22, and 0.20 nm2/unit, respectively. Assuming that the surface limiting area of the DDA group is 0.28 nm2, the surface density of the VPy group is calculated to be 0.21, 0.40, 1.32, and 2.79 (nm2)-1, respectively. Figure 3 shows scanning electron microscopy (SEM) images of gold nanoparticles adsorbed onto p(DDA/VPy) nanosheets. As shown clearly in Figure 3b-e, gold nanoparticles were adsorbed onto p(DDA/VPy) LB films and took a uniformly distributed monolayer over a large area. The adsorption behavior of gold nanoparticles was monitored with surface plasmon spectroscopy in situ, and no significant changes in reflectivity due to desorption of gold nanoparticles were observed during rinsing in pure water. It is noteworthy that the amount of adsorbed gold particles depended on VPy contents. Since gold nanoparticles are negatively charged and the p(DDA/VPy) LB films have positively charged VPy sites, the primary driving force for immobilization is attributable to electrostatic interaction. In addition, repulsive forces exist between gold nanoparticles. This affects gold nanoparticle layer formation, leading to a uniformly distributed monolayer. It must be mentioned that no aggregation is observed in Figure 3, indicating that VPy groups and gold nanoparticles are uniformly distributed in polymer nanosheets. In other words, uniform distribution of VPy groups in polymer nanosheets can control surface charge density and, consequently, the extent of adsorption for gold nanoparticles. A three-layer PDDA LB film containing no VPy group was also utilized as a polymer nanosheet (Figure 3a). No significant adsorption of gold nanoparticles
Figure 3. SEM images of gold nanoparticle monolayers as a function of VPy contents: (a) 0, (b) 5.9, (c) 11, (d) 29, and (e) 56 mol %.
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Figure 4. SEM images of gold nanoparticle arrays with p(DDA/ VPy) nanotemplates. The VPy content is 29 mol %.
with 29 mol % were irradiated under air atmosphere, and photodecomposition occurred at the irradiated portion (Figure 4). Patterned p(DDA/VPy) LB films were immersed into a gold nanoparticle solution. Gold nanoparticles were selectively directed onto the unirradiated portion of p(DDA/VPy) LB films, producing clearly configured gold nanoparticle monolayer patterns with 2.0 µm resolution (Figure 4). In other words, p(DDA/VPy) nanosheets can act as a patterned template for nanoparticle arrays. Further development of pattern processing with higher resolution is expected to yield a nanoparticle wire. In conclusion, tailor-made gold nanoparticle monolayer arrays are achieved using polymer nanosheets composed of p(DDA/VPy) LB films. Predominant factors to immobilize gold nanoparticles are based on electrostatic interaction between nanoparticles and p(DDA/VPy) LB films. The VPy moieties are uniformly distributed in the polymer nanosheet; consequently, a two-dimensional homogeneous gold nanoparticle monolayer can be formed onto the polymer nanosheets. In addition, the polymer nanosheet acts as a template. This technique presents new avenues for advanced nanohybrid materials, in particular, those for surface plasmon and photonic band gap crystal applications.17
was observed, indicating that secondary interaction can be negligible between gold nanoparticles and polymer backbones. The present polymer nanosheets, p(DDA/VPy) LB films, have another useful feature: photopatterns can be drawn.16 As reported previously, deep UV irradiation on LB films through a photomask gives a fine photopattern of p(DDA/VPy) LB films. Three-layer p(DDA/VPy) LB films
Acknowledgment. The authors would like to thank Mr. Y. Hayasaka and Mr. E. Aoyagi of Tohoku University for TEM and SEM observations. This study was supported by a Grant-in-Aid for Scientific Research on Priority Areas (Fundamental Science and Technology of Photofunctional Interfaces, No. 417) and Scientific Research (No. 14205130) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.
(14) Hostetler, M. J.; Wingate, J. E.; Zhong, C.-J.; Harris, J. E.; Zashet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17-30. (15) The transfer ratios of a p(DDA/VPy) monolayer with 56 mol % VPy content were 0.8 and 0.8 for up- and downstrokes, respectively.
LA027076O (16) Aoki, A.; Nakaya, M.; Miyashita, T. Macromolecules 1998, 31, 7321-7327. (17) Linden, S.; Kuhl, J.; Giessen, H. Phys. Rev. Lett. 2001, 86, 46884691.
