Langmuir 1998, 14, 3157-3159
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Preparation of Gold Colloids with UV Irradiation Using Dendrimers as Stabilizer Kunio Esumi,* Akihiro Suzuki, Nariaki Aihara, Kiyoshi Usui, and Kanjiro Torigoe Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received February 10, 1998 Au colloids have been prepared by reduction of metal salt with UV irradiation in the presence of dendrimers. Poly(amidoamine) dendrimers with surface amino groups for generations 0, 1, 2, 3, 4, and 5 (G0-5) have been used. The average particle sizes decrease with an increase of concentration of surface amino group of the dendrimers. In particular, Au colloids with a diameter less than 1 nm are obtained in the presence of the later generation dendrimers(G3-5) at above the molar ratio of surface amino group of dendrimer/ HAuCl4 (4:1). These concentrations are extremely small compared with that of other polymers, indicating that the dendrimers have an effective protective action for the formation of Au colloids.
Introduction Small metal particles have unique properties that result from size effects. Intensive research has been carried out on the synthesis of these small particles and their applications in electrooptical devices, electronic devices, imaging materials, and so on.1-5 The particle size, shape, and size distributions are very important in determining their applications. Metal colloids have been prepared mainly in both aqueous and nonaqueous solvents. The formation of monodisperse metal colloids usually requires the presence of a protective agent. Many small metal and bimetallic particles have been prepared in water-ethanol solutions using poly(N-vinyl-2-pyrrolidone) as a protective agent.6-9 Anionic polymers and surfactants have also been used to prepare stable metal colloidal dispersions.10-14 It is suggested that protective polymers can coordinate to metal ions before reduction forming a polymer-metal ion complex, and then such a complex can be reduced under mild conditions, resulting in a smaller size and a narrower size distribution than those without protective polymers. Using chitosan with amino groups as protective colloid, fine metal particles have also been obtained.15 Recently, dendrimers, also known as cascade, cauliflower, or starburst molecules, are attracting increasing (1) Hache, F.; Ricard, D.; Flytzanis, C.; Kreibig, U. Appl. Phys. 1988, A47, 347. (2) Abeles, B. Appl. Solid State Sci. 1976, 6, 1. (3) Yonezawa, Y.; Konishi, Y.; Hada, H.; Yamamoto, K.; Ishida, H. Thin Solid Films 1992, 218, 109. (4) Ware, M. J. J. Photogr. Sci. 1994, 42, 157. (5) Haruta, M. Catal. Today 1997, 36, 153. (6) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (7) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (8) Esumi, K.; Wakabayashi, M.; Torigoe, K. Colloids Surf. 1996, 109, 55. (9) Bradley, J. S. In Clusters and Colloids From Theory to Applications; Schmid, G., Eds.; VCH: Weinheim, 1994; Chapter 6. (10) Esumi, K.; Sato, N.; Torigoe, K.; Meguro, K. J. Colloid Interface Sci. 1992, 149, 295. (11) Schmid, G.; Lehnert, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 780. (12) Meguro, K.; Nakamura, Y.; Hayashi, Y.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 347. (13) Larpent, C.; Patin, H. J. Mol. Catal. 1988, 44, 191. (14) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1986, 110, 82. (15) Ishizuki, N.; Torigoe, K.; Esumi, K.; Meguro, K. Colloids Surf. 1991, 55, 15.
Figure 1. Variation of UV spectra of the reduction of HAuCl4 with UV irradiation in the presence of dendrimer G5. Concentration of G5 is 0.8 mmol dm-3 based on surface amino group.
attention because of their unique structures and properties.16,17 Dendrimers of lower generation tend to exist in relative open forms, while higher generation dendrimers take a spherical three-dimensional structure, which is very different from linear polymers adopting random-coil structures. In addition, dendrimers might provide reaction sites including the interior or the periphery of the dendrimers. Actually, many interesting interactions with dendrimers have been reported.18 Accordingly it is very interesting to study the role of dendrimers as protective agents for preparation of metal colloids from the standpoint of their unique structures. In this work, gold colloids were prepared by reduction of metal salt with UV irradiation in the presence of dendrimers. For this purpose, poly(amidoamine) dendrimers with surface amino groups having different generations were used. Experimental Section Materials. Poly(amidoamine)dendrimers with different generations (G0, G1, G2, G3, G4, and G5) were synthesized18 (16) Tomalia, D.A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (17) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (18) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466.
S0743-7463(98)00162-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/13/1998
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Figure 2. Electron micrographs of gold colloids and particle size distribution, molar ratio of surface amino group of G5 and HAuCl4: (a) 1:1; (b) 4:1. involving exhaustive Michael addition to ethylenediamine initiator core with methyl acrylate and exhaustive amidation of the resulting esters with large excesses of ethylenediamine. Their purity was examined by NMR. HAuCl4 was kindly supplied by Tanaka Kikinzoku Kogyo K.K. The water used in this study was purified through a Milli-Q system. Methods and Measurements. Since poly(amidoamine) dendrimers G(0-5) have surface amino groups, various mixtures of HAuCl4 and dendrimers (molar ratio of surface amino group of dendrimer and HAuCl4 from 1:1 to 8:1) were prepared under a constant feed concentration of HAuCl4 (0.2 × 10-3 mol dm-3) and they were transferred to rectangular quartz vessels. UV irradiation was carried out with 50 W low-pressure mercury lamps (λmax ) 253.7 nm). The solutions were irradiated in a dark box at ambient temperature. The UV-vis spectra of the colloidal solutions were measured with a Hewlett-Packard 8452A diode array spectrophotometer at 1-cm path length. Electron micrographs of the metal colloids obtained after 5 h of irradiation were taken with a Hitachi H-800 transmission electron microscopy, operating at 200 kV. Their particle sizes and distributions were determined by counting about 200 particles.
Results and Discussion Variations of the absorption spectrum of dendrimer G5HAuCl4 aqueous solution (4:1 in mole of surface amino group and HAuCl4) with irradiation of a 50 W mercury lamp are given in Figure 1. The absorption band of colloidal gold appeared at around 520 nm, and its intensity increased with an increase of the irradiation time. A similar change in the absorption spectrum was also observed for the other dendrimer-HAuCl4 systems except that in the case of the early generations at low concentration the plasmon band is shifted to longer wavelength with increasing UV irradiation time due to coagulation of
gold particles. In addition, a peak at around 290 nm appeared that is assigned to certain carbonyl compounds formed presumably by photodecomposition of the dendrimers. The absorption band at around 290 nm was observed by UV irradiation of dendrimers in aqueous solution without HAuCl4. Although the ligand-to-metal charge-transfer absorption band of AuCl4- at around 310 nm has been observed for AuCl4- and chitosan solution19 having amino groups, a similar band in this study was not observed. Figure 2 shows the electron micrographs of gold colloids obtained in the presence of G5 after UV irradiation. At a surface amino group and HAuCl4 molar ratio of 1:1, the particle size of gold colloids was in 2-18 nm range, having a broad distribution. However,when the ratio is increased to 4:1, relative monodispersed gold particles with a diameters less than 1 nm were obtained. The average particle sizes with the surface amino group concentration of G(0-5) are shown in Figure 3. It is seen that the average particle sizes decrease with increasing concentration of surface amino groups for G(0-5). At a molar ratio of 1:1 of surface amino group and HAuCl4, the average particle size was not dependent on the generation of the dendrimers, but at above 4:1 ratios that became smaller with increasing generation of the dendrimers. In particular, it is interesting to note that ultrafine Au colloids with a diameter less than 1 nm are formed in the presence of higher surface NH2 group concentration of later generation G(3-5). These results suggest that the dendrimers of earlier generation G0 do not operate an effective protective colloid, but the dendrimers of later generations (19) Yonezawa, Y.; Kawabata, I.; Sato, T. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 39.
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Figure 3. Variation of average particle size of gold colloids with concentration of surface amino group of dendrimers G(0-5).
provide sites for complex formation as well as an effective protective action. This protective action can be correlated with microenvironmetal properties estimated by pyrene; the micropolarities of G0 and G1 are very similar and larger than that of G2 due to relatively open structure of G0 and G1 compared with G2.20 Further, the dendrimers of G(3-5) are expected to have closed, increasing compact structure. Thus, it is suggested that the dendrimers of later generations with surface amino groups play an effective protective action for preparation of stable gold particles, probably due to their three-dimensional struc(20) Pistolis, G.; Malliaris, A.; Paleos, C. M.; Tsiourvas, D. Langmuir 1997, 13, 5870.
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tures. There is also a possibility that the internal tertiary amine groups of dendrimers can involve the formation of complex with HAuCl4. When the ratio of amino group and metal salt is more than 2.6, very tiny particles of metals with a diameter at around 5∼6 nm have also been obtained using chitosan as a protective agent.15 In the case of poly(N-vinyl-2-pyrrolidone), gold particles with a diameter around 6 nm have been obtained at the molar ratio of 40-41 of monomer/HAuCl4.6 These results also suggest that the dendrimers with surface amino groups are much more effective protective colloids than other colloids including poly(N-vinyl-2-pyrrolidone). Recently, we have reported preparation of colloidal gold in solutions by photoreduction of the complexes of HAuCl4 with cationic surfactants such as dodecyltrimethylammonium chloride and hexadecyltrimethylammonium chloride.21 Although mechanisms for formation of gold particles with UV irradiation using the dendrimers seem very complex accompanying decomposition of the dendrimers, primary reaction products are assumed as Au0, quaternary ammonium chloride of dendrimers and chloride. The conversion of AuCl4- ions to Au0 may proceed through complex reactions involving AuCl3- ions and AuCl2- ions.22 When the generation of the dendrimers increases, particle growth would be prevented by threedimensional structure of the dendrimer, resulting in a smaller particle size. The preparation of the other metal colloids such as Pt, Pd, and bimetallic colloids protected by dendrimers is now in progress. LA980162X (21) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59. (22) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574.
