Langmuir 2003, 19, 7679-7681
Multilayer Formation Using Oppositely Charged Gold- and Silver-Dendrimer Nanocomposites Kunio Esumi,* Shintaro Akiyama, and Tomokazu Yoshimura Department of Applied Chemistry and Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received May 6, 2003. In Final Form: July 3, 2003
Introduction Recently, the electrostatic layer-by-layer deposition of oppositely charged polyelectrolytes and other species such as proteins,1 DNA,2 conducting polymers,3 metal, or semiconductive nanoparticles4 is of interest as a facile means of creating ordered, functional films. By using suitably charged polyions as counterions, oppositely charged materials can also be deposited to create ultrathin films with a range of structure and important electronic and optical properties.5 This approach has been further extended to grow multilayers with multifunctional nanostructured materials.6 Dendrimers represent a new architecture of synthetic macromolecules. Unlike linear polymers, dendrimers are composed of a core molecule and hyperbranches that regularly extend from the core to the terminal groups and have a definite molecular weight and size.7 In addition, dendrimers with higher generations take a spherical shape, and they can encapsulate metal complexes, nanoparticles, or other inorganic and organic molecules.8,9 Actually, metal-dendrimer nanocomposites have been prepared using dendrimers with various terminal groups. Using hydroxyl-terminated poly(amidoamine) PAMAM, dendrimer nanocomposites encapsulated with metal nanoparticles such as Cu, Pt, and Pd have been obtained.10 On the other hand, in amino-terminated PAMAM systems, gold nanoparticles are formed at the surface of the dendrimer or encapsulated in the dendrimer depending on the generation of the dendrimer, as was demonstrated by Gro¨hn et al.11 and our group. 12,13 For another system, nanoparticles are observed at both the interior and exterior of the dendrimer molecules. The formation mechanisms for these three different types of metal-dendrimer nanocomposites have been explained by Balogh et al.14 * Author to whom correspondence should be addressed. (1) Lvov, Y.; Ariga, K.; Ichinohe, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117. (2) Lvov, Y.; Decher, G.; Sukhorukov, G. Macromolecules 1993, 26, 5396. (3) Cheung, J. H.; Fou, A. F.; Rubner, M. F. Thin Solid Films 1994, 244, 985. (4) Feldheim, D. L.; Crabar, K. C.; Natan, M. J.; Mallouk, T. E. J. Am. Chem. Soc. 1996, 118, 7640. (5) Decher, G. Science 1997, 277, 1232. (6) Watanabe, S.; Regen, S. L. J. Am. Chem. Soc. 1994, 116, 8855. (7) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and Other Dendritic Polymers; Wiley: Chichester, 2001. (8) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (9) Astruc, D.; Chardac, F.; Chem. Rev. 2001, 101, 2991. (10) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (11) Gro¨hn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E. J. Macromolecules 2000, 13, 6042. (12) Esumi, K.; Suzuki, A.; Yamahira, A.; Torigoe, K. Langmuir 2000, 16, 2604. (13) Esumi, K.; Torigoe, K. Prog. Colloid Polym Sci. 2001, 117, 80.
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Because fabrication of ordered semiconducting and metallic nanoparticles on solid substrates has been a topic for substantial research because of their functional properties, it is important to explore a method to assemble these metal-dendrimer nanocomposites to form welldefined films. Recently, multilayer formation has been reported using poly(sodium-4-styrenesulfonate) and goldPAMAM dendrimer nanocomposites.15 Atomic force microscopy indicates that the nanoclusters are arrayed with high uniformity at the nanometer scale. Further, PAMAM films have been fabricated by the self-assembly of PAMAM dendrimers using PAMAM dendrimers with positively charged surface amino groups and negatively charged carboxylic groups.16 It is found that the average thickness of the molecular layer in multiplayer films is much smaller than the diameter of ideal spherical dendritic macromolecules. The studies described previously motivate us to fabricate layer-by-layer formation using oppositely charged golddendrimer and silver-dendrimer nanocomposites. Such layer-by-layer films will provide a new development of thin conducting or catalytic films. In this study, golddendrimer and silver-dendrimer nanocomposites have been prepared and their multilayer formations have been examined. Experimental Section Materials. PAMAM dendrimers (generation ) 5.0 and 5.5) were synthesized according to previous literature.17 The purity of the dendrimers was confirmed by 1H and 13C NMR. AgNO3 and HAuCl4 were obtained from Kanto Chemical Co. Milli-Q water (Millipore Co.) was used in all experiments. The other chemicals were of analytical grade. Preparation of Metal-Dendrimer Nanocomposites. The preparation of gold-dendrimer nanocomposites in an aqueous solution was conducted by the chemical reduction of a HAuCl4PAMAM (generation ) 5.0) mixture with sodium borohydride. For a typical experiment, 20 cm3 of a freshly prepared 20 mmol dm-3 HAuCl4 solution and 4 cm3 of water were added to 25 cm3 of an aqueous PAMAM solution, and the solution was stirred for 1 h. Then, 1 cm3 of 1 mol dm-3 freshly prepared ice-cold sodium borohydride was quickly added to the solution under stirring and left for 30 min. The final HAuCl4 concentration was 2 mmol dm-3. Similarly, silver-PAMAM (generation ) 5.5) dendrimer nanocomposites were prepared using AgNO3. The ζ potentials of the metal-dendrimer nanocomposites were measured using a Laser-ζ-potential apparatus (Otsuka Electronics Co., ELS-8000). Multilayer Formation. Glass plates cleaned by piranha solution were coated with a priming layer of 3-aminopropyltrimethoxysilane by immersing them in a 10% amino silanemethanol solution for 30 min, followed by dip-washing and nitrogen-drying. The pHs of the gold-dendrimer and silverdendrimer nanocomposites in aqueous solutions were adjusted to pH 3 and 11 by using HCl or NaOH. The ionic strength of the solutions was not adjusted. The metal-dendrimer nanocomposites coating was performed as follows: the glass plates were dipped in the aqueous solution of the silver-dendrimer nanocomposites for 30 min, after which time they were rinsed with water of pH 11 and finally nitrogen-dried. Then, the glass plates were dipped (14) Balogh, L.; Valluzzi, R.; Laverdure, K. S.; Gido, S. P.; Hagnauer, G. L.; Tomalia, D. A. J. Nanopart. Res. 1999, 1, 353. (15) He, J.-A.; Valluzzi, R.; Yang, K.; Dolikhanyan, T.; Sung, C.; Kumar, J.; Tripathy, S. K.; Samuelson, L.; Balogh, L.; Tomalia, D. A. Chem. Mater. 1999, 11, 3268. (16) Tsukruk, V. V.; Rinderspacher, F.; Bliznyuk, V. N. Langmuir 1997, 13, 2171. (17) Tomalia, D. A.; Baker, H.; Dewald, J. R.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117.
10.1021/la034777s CCC: $25.00 © 2003 American Chemical Society Published on Web 07/26/2003
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Langmuir, Vol. 19, No. 18, 2003
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Figure 1. TEM images of metal-dendrimer nanocomposites: (a) gold-dendrimer nanocomposites; (b) silver-dendrimer nanocomposites. in the aqueous solution of the gold-dendrimer nanocomposites, followed by washing with water at pH 3 and nitrogen-drying. This procedure was repeated 10 times. It should be mentioned that the dipping time is taken as 30 min in this study because the spectra change is hardly observed at above a 20-min dipping time. UV-Visible Spectrophotometry. UV-vis spectra of the glass plates were recorded with an Agilent 8453 spectrophotometer. The glass plates were placed perpendicular to the beam using a slide holder to maintain the same positioning during each measurement. Quartz Crystal Microbalance (QCM). A 27-MHz QCM is available from Intium Inc. (Japan). The diameter of its quartz plate is 8 mm, and Au electrodes are deposited on both sides (diameter, 2.5 mm; area, 4.9 mm2). The QCM crystals were immersed in an aqueous solution of poly(ethylene imine) and then rinsed with water. Then, the positively charged gold QCM was immersed in the silver-dendrimer nanocomposites in an aqueous solution, rinsed with water, and dried. The gold QCM was immersed in the gold-dendrimer nanocomposites in an aqueous solution, rinsed with water, and then dried. This procedure was repeated eight times for the formation of multilayers. The difference in the QCM resonance frequency before and after adsorption was used to evaluate the multilayer film growth.
Results and Discussion Before discussing the formation of the multilayers, it is essentially important to characterize each gold-dendrimer and silver-dendrimer nanocomposite. When metal ions such as Au3+ or Ag+ are reduced with NaBH4 in the presence of dendrimer, a typical plasmon band appears for gold nanoparticles at 510 nm and silver nanoparticles at 450 nm. These bands indicate the formation of gold or silver nanoparticles. Figure 1 shows the transmission electron microscopy (TEM) images of gold- and silverdendrimer nanocomposites. Because TEM measurements are sensitive only to the electron-dense metal particles, the sizes of the gold/silver nanoparticles alone can be evaluated by TEM measurements. The sizes of the gold and silver nanoparticles are 2-4 and 5-10 nm, respectively, where the standard deviation of the size distribution
Figure 2. UV-vis spectra of the layer-by-layer growth of silverdendrimer (generation ) 5.5) nanocomposites/gold-dendrimer (generation ) 5.0) nanocomposites on glass plates: (a) odd layer numbers; (b) even layer numbers.
is much smaller for the gold particles compared to that of the silver particles. In addition, the structure of the metal-dendrimer nanocomposites can be estimated by using dynamic light scattering, which provides the hydrodynamic radius of the nanocomposites. From the data18 that shows that the hydrodynamic diameter of the metal-dendrimer nanocomposites is almost equal to 2 times the dendrimer diameter plus the diameter of the metal particles, it is conceivable that the metal-dendrimer nanocomposites are formed by dendrimer molecules adsorbing on the metal particles. Furthermore, the result that the ζ potentials of metal-dendrimer nanocomposites are +40 mV at pH 3 for the gold-dendrimer (generation ) 5.0) and -45 mV at pH 11 for the silver-dendrimer (generation ) 5.5) also supports the structure of the nanocomposites described previously. The metal-dendrimer nanocomposites have been deposited onto the positively charged aminated silane film surface, and a multilayer of silver- and gold-dendrimer nanocomposites has been obtained. Here, silver-dendrimer and gold-dendrimer nanocomposites are deposited from pH 11 and 3 because their nanocomposites exhibit appropriate ζ-potential values. Figure 2 shows the UV-vis spectra of the silver- and gold-dendrimer nano(18) Hayakawa, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19, 5517.
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Figure 4. Multilayer film growth of silver-dendrimer nanocomposites/gold-dendrimer nanocomposites on gold-coated QCM electrodes as a function of the layer number.
Figure 3. Layer-by-layer adsorption of silver-dendrimer (generation ) 5.5) nanocomposites/gold-dendrimer (generation ) 5.0) nanocomposites on glass plates, plotted as the film absorbances at (a) 440 and (b) 560 nm as a function of the layer number.
composite multilayers where (a) spectra of odd-numbered depositions from silver-dendrimer nanocomposites and (b) spectra of even-numbered depositions from golddendrimer nanocomposites are given. It is seen that the absorbances of both spectra increase with the deposition number, indicating the multilayer formation of silver- and gold-dendrimer nanocomposites. The change in the absorbance by a multilayer formation is plotted in Figure 3. Although the absorbances at both 440 and 560 nm increase with the layer number for silver- and gold-
nanocomposites, the absorbance after each deposition of the gold-dendrimer nanocomposites is always reduced compared to the absorbance of the previous silverdendrimer nanocomposite deposited. This suggests that the desorption of the nanocomposites takes place to some extent after each deposition of the gold-dendrimer nanocomposites. Such a desorption has also been observed using the layer-by-layer deposition of PAMAM dendrimer and poly(styrene sulfonate).19 To check the multilayer formation of metal nanocomposites, QCM measurements were also conducted. Figure 4 shows the change in the frequency shift with the formation of a multilayer. Because the change in the shift at the deposition of each even number (2, 4, 6, and 8) is decreased compared to that at the deposition of each odd number (1, 3, 5, and 7), each deposition of the gold nanocomposites renders the desorption of the nanocomposites to some extent. This result is in good agreement with that obtained with the UV-vis measurements. Although the reason some desorption occurs during the deposition is not clearly understood, the size of the metaldendrimer nanocomposites may be important. Because the size of the silver nanoparticles is considerably larger than that of the gold nanoparticles, as is shown in Figure 1, the interaction between the silver-dendrimer nanocomposites may be smaller than that between the golddendrimer nanocomposites, so that the silver-dendrimer nancomposites deposited will desorb to some extent by the deposition of the gold-dendrimer nanocomposites. Also, there is a possibility that because the particle size and standard deviation of the silver nanoparticles are relatively large, the adsorption of the dendrimer molecules on the silver nanoparticles is not uniformly that the silverdendrimer nanocomposites with a lower adsorption of dendrimer might be desorbed by the mulitilayer formation. LA034777S (19) Khopade, A. J.; Caruso, F. Langmuir 2002, 18, 7669.
