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Role of Poly(amidoamine) Dendrimers for Preparing Nanoparticles of Gold, Platinum, and Silver Kunio Esumi,* Akihiro Suzuki, Atsushi Yamahira, 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 September 29, 1999. In Final Form: December 13, 1999 Nanoparticles of gold, platinum, and silver were prepared by reduction of their metal salts with NaBH4 in the presence of poly(amidoamine) dendrimers. The dendrimers used were generation 3, 4, and 5 having surface amino group (G3, G4, and G5) as well as generation 3.5, 4.5, and 5.5 having surface carboxyl group (G3.5, G4.5, and G5.5). The metal nanoparticles obtained were characterized by UV-vis spectroscopy and transmission electron microscopy. By using G3-G5, gold nanoparticles in the 1.5-4.0 nm size regime were obtained where their size decreased with increasing concentration of the dendrimers as well as the generation of the dendrimers. Similarly, platinum nanoparticles with a diameter of 2.4-3.0 nm were obtained, and their size was insensitive against the concentration as well as the generation of G3-G5. In the case of silver nanoparticles, very small silver particles using G3.5-G5.5 were obtained. In addition, the dendrimer concentration required for obtaining stable nanoparticles was found to be dependent on the interactions between metal nanoparticles and the dendrimers: when the ratio of [surface group of dendrimer]/[metal salt] was above unity, nanoparticles of gold and silver were obtained, while stable platinum nanoparticles were only obtained above the ratio of 40/1. From infrared spectroscopy it is suggested that the metal nanoparticles adsorb on the exteriors of the dendrimers.
Introduction Many preparation methods of metal nanoparticles including wet and dry atmospheres have been developed 1 because metal nanoparticles have been applied in electrooptical devices, electronic devices, imaging materials, and so on. Fabrication of nanoparticles becomes one of important topics in nanotechnology. 2 Accordingly, for that purpose it is important to control the particle size, shape, and size distribution of metal particles. In the wet method, various metal nanoparticles have been prepared in the presence of polymer or surfactant as a protective colloid.1 In particular, poly(vinylpyrrolidone) has been extensively used in aqueous solution or waterethanol solution.3-6 Until now the usage of protective colloids has been limited to linear polymers. Recently, dendrimers, also known as cascade, cauliflower, or starburst molecules, are attracting increasing attention because of their unique structures and properties.7-11 Generally, 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 (1) Bradley, J. S. In Clusters and Colloids from Theory to Applications; Schmid, G., Ed.; VCH: Weinheim, 1994; Chapter 6 and references therein. (2) Chow, G.-M.; Gonsalves, K. E. Nanotechnology; ACS Symposium Series 622; American Chemical Society: Washington, DC, 1996. (3) Yonezawa, T.; Toshima, N. J. Chem. Soc., Faraday Trans. 1995, 91, 4111. (4) Toshima, N.; Harada, M.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (5) Esumi, K.; Wakabayashi, M.; Torigoe, K. Colloids Surf. 1996, 55, 109. (6) Esumi, K.; Sato, N.; Torigoe, K.; Meguro, K. J. Colloid Interface Sci. 1992, 295, 49. (7) Tomalia, D. A. Angew. Chem., Int. Ed. Engl. 1990, 29, 138. (8) Frechet, J. M. Science 1994, 263, 1710. (9) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (10) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (11) Newkome, G. R.; Moorefield, C. N.; Vogtle, F. Dendritic Molecules: Concepts, Synthesis, Perspectives; VCH: Weinheim, 1996.
random-coil structures. Further, dendrimers might provide reaction sites including their interior or periphery. In fact, many interesting interactions with dendrimers have been reported.9,11 For example, gold nanoparticles have been prepared by reduction of HAuCl4 with UV irradiation in the presence of poly(amidoamine) dendrimers with surface amino groups.12 It was found that stable gold nanoparticles with a diameter less than 1 nm are prepared in the presence of the later generation dendrimers (generations 3-5), and the dendrimer concentrations required for obtaining stable gold nanoparticles are extremely low compared to those of other linear polymers. Thus, the results suggest that poly(amidoamine) dendrimers act as a very effective protective colloid for preparing gold nanoparticles. Also, gold nanoparticles in the 2-3 nm size regime have been prepared by reduction of HAuCl4 with a chemical reductant in the presence of poly(amidoamine) dendrimers.13 In addition, preparation of dendrimer-encapsulated platinum nanoparticles has been reported.14 As an extension of the previous study with UV irradiation, the present report focuses on the preparation of gold, platinum, and silver nanoparticles in aqueous solution by reduction of respective metal salts with NaBH4 in the presence of poly(amidoamine) dendrimers. For this purpose, various generations of poly(amidoamine) dendrimers with surface amino groups as well as carboxyl groups were used. Experimental Section Materials. Various generations of poly(amidoamine) dendrimers were synthesized, involving exhaustive Michael addition to ethylenediamine core with methyl acrylate and exhaustive amidation of the resulting esters with large excesses of ethyl(12) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157. (13) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256. (14) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217.
10.1021/la991291w CCC: $19.00 © 2000 American Chemical Society Published on Web 02/03/2000
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enediamine.15 The ester-terminated dendrimers were hydrolyzed by addition of NaOH to obtain dendrimers with surface carboxyl groups. The dendrimers with surface amino groups are referred as G(3-5) and those with surface carboxyl groups as G(3.5-5.5). HAuCl4, H2PtCl6, and AgNO3 were kindly supplied by Tanaka Kikinzoku Kogyo K.K. The water used in this study was purified through a Milli-Q Plus system. The other chemicals were of analytical grade. The structures of the dendrimers are shown in Scheme 1. Methods and Measurements. Metal ions and dendrimers were dissolved in deionized water, separately. Then both solutions were mixed with deionized water to make desired concentrations of metal ions and dendrimers. The solutions were vigorously stirred during addition of excess amount of ice-chilled aqueous NaBH4 at room temperature for 0.5 h. The concentration of NaBH4 added was 10 times that of metal ions. Here, the concentration of dendrimers refers to the surface functional groups. UV-vis spectra of solutions before and after the reduction of metal ions were measured with a UV spectrophotometer (HewlettPackard 8452A). Average particle sizes of metals obtained were determined from several negative films (approximately 100 particles) taken using a transmission electron microscope (Hitachi H-800). The samples were prepared by mounting a drop of the solution on carbon-coated Cu grids and allowed to dry in air. FT-IR spectra for dendrimers and metal particles were obtained by forming thin transparent KBr pellets containing the samples.
Results and Discussion Au Colloids. We have reduced 0.2 mM solutions of HAuCl4 by addition of NaBH4 in the presence of dendrimers. Absorption spectra of the solutions before and after chemical reduction with NaBH4 are shown in Figure 1. In the aqueous solution before reduction, HAuCl4 shows a strong absorption band at λ ) 220 nm and a shoulder at 290 nm due to charge transfer between the metal and chloro ligands. The absorbance at 290 nm increased by adding G5 dendrimer, probably due to the formation of (15) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck, J.; Ryder, J.; Smith, P. Macromolecules 1986, 19, 2466.
Figure 1. Absorption spectra of HAuCl4 in the presence of dendrimer, G5, before (a) and after (b) reduction with NaBH4.
ion pairs between G5 and AuCl4-. Since the estimated pKNH2 is about 9.7 and the aqueous solution before reduction is an acidic one, it is expected that the amino groups of the dendrimer are proton donated, resulting in the electrostatic attraction forces between NH3+ of the dendrimer and AuCl4-. A similar ion-pair formation between chitosan and HAuCl4 has been suggested.16 After reduction with NaBH4, the 220 nm band of AuCl4-
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Figure 3. Change in average particle diameter of gold as a function of [dendrimer]/[Au3+].
Figure 2. TEM micrographs and particle size distribution of gold particles obtained in the presence of dendrimer: (a) [HAuCl4] ) 0.2 mM, [G3] ) 0.4 mM, [NaBH4] ) 2 mM; (b) [HAuCl4] ) 0.2 mM, [G5] ) 0.4 mM, [NaBH4] ) 2 mM.
vanished, indicating that AuCl4- is completely reduced. Instead, one can see an absorption band at 280 nm and a broad band at around λ ) 520 nm which is assigned to a plasmon band of gold particles. The absorption band at 280 nm would correspond to certain gold cluster, although Mosseri et al.17have noted a peak of some gold cluster around λ ) 320 nm. It should be noted that the solution after reduction changes from acidic solution to a basic one. In this situation, it seems that the surface of gold particles obtained interacts strongly with the amino groups of the dendrimer because the lone pair of the terminal primary amines is available to interact with the gold surface.18 In the absence of the dendrimers, gold particles obtained were easily flocculated and sedimented. Figure 2 shows the TEM micrographs and particle size distribution of gold particles. It is seen that the average particle size of gold using G5 is smaller than that using G3. To obtain the relationship between the particle size of gold and the generation of the dendrimer, the average diameter of gold particles is plotted against the ratio of [dendrimer]/[Au3+] for G3, G4, and G5 in Figure 3. The average diameter decreased rapidly and then gradually with increasing ratio of [dendrimer]/[Au3+] for G3-G5, where the average diameter for G3 was greater than those for G4 and G5 at the same ratio of [dendrimer]/[Au3+]. Thus, it is demonstrated that the dendrimers operate as a very effective protective colloid for the preparation of gold particles since only a very small amount of the (16) Yonezawa, Y.; Kawabata, I.; Sato, T. Ber. Bunsen-Ges. Phys. Chem. 1996, 100, 39. (17) Mosseri, S.; Henglein, A.; Janata, E. J. Phys. Chem. 1989, 93, 6791. (18) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phan, V. T.; Dermody, D. L.; Garcia, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, T. M. J. Am. Chem. Soc. 1998, 120, 4492.
Figure 4. FT-IR spectra of (a) dendrimer, G5, and (b) dendrimer, G5/gold particles.
dendrimers is required to obtain nanometer size of gold particles compared to other linear polymers. In addition, it is conceivable that the size and shape of the dendrimers play an important role for the preparation of gold nanoparticle. Actually, Garcia et al.13 have pointed out that higher generation dendrimers have the capacity to envelop the growing colloids and serve as a template for particle growth. To confirm the interaction between gold nanoparticles and the dendrimers, FT-IR spectra of dendrimer (G5) alone and gold nanoparticles obtained from dendrimer solution were measured (Figure 4). The amide bands at 1630 and 1540 cm-1, which are characteristic of the dendrimer branches, are very similar for both (a) and (b). This may suggest that gold nanoparticles adsorb on the exterior of the dendrimers. A similar discussion has been reported for the G4 dendrimer/gold nanoparticles.13 Pt Colloids. The optical spectra of H2PtCl6 aqueous solution before and after reduction with NaBH4 in the presence of dendrimer, G3, are shown in Figure 5. Before reduction, a shoulder band appeared at about 270 nm which can be assigned to charge transfer between the dendrimer and PtCl62-. After reduction, a broad absorption band was observed in a wide wavelength region which is a typical band for platinum colloids. Further, a shoulder at about 280 nm appeared at above the ratio of [G3]/[Pt4+] ) 50/1. This band may be derived from the interaction between platinum colloids and the dendrimer. Below the ratio of [G3]/[Pt4+] ) 40/1, many aggregates of platinum
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Figure 6. TEM micrographs and size distribution of platinum particles obtained in the presence of dendrimer: (a) [H2PtCl6] ) 0.1 mM, [G4] ) 8.0 mM, [NaBH4] ) 1 mM; (b) [H2PtCl6] ) 0.1 mM, [G5] ) 8.0 mM, [NaBH4] ) 1 mM. Figure 5. Absorption spectra of H2PtCl6 in the presence of dendrimer, G3, before (a) and after (b) reduction with NaBH4.
colloids were observed, and stable platinum colloids could not be obtained. The change in the optical spectra of H2PtCl6 before and after reduction in the presence of G3 was very similar to those in the presence of G4 and G5. As one example, TEM micrographs of platinum colloids obtained in the presence of G4 and G5 and their size distributions are given in Figure 6. It is apparent that the size distributions are relatively small, and the average particle sizes are about 2.4 nm for both systems. In the case of G3, the average size was about 3.0 nm at the ratio of [dendrimer]/[Pt4+] ) 80/1, whereas platinum particles were aggregated at the ratio of [dendrimer]/[Pt4+] ) 40/1. These results indicate that the particle sizes of platinum are insensitive against the size and shape of the dendrimers. It is apparent that the protective action by three dendrimers is very similar when the ratio of [dendrimer]/ [Pt4+] is at above 40/1. However, a comparison of the dendrimer concentration required for stable gold and platinum colloids indicates that the interaction between platinum colloids and the dendrimers is weaker than that between gold colloids and the dendrimers. Actually, FTIR spectra of the platinum colloids prepared in the presence of G5 show that the absorption band due to the interaction between platinum colloids and amino groups of the dendrimer is very weak. Accordingly, a much greater concentration of the dendrimers is required to stabilize platinum colloids compared to that for gold colloids. In this case, it is also conceivable that platinum colloids adsorb on the exterior of the dendrimers. On the other hand, by using fourth-generation poly(amidoamine) dendrimer with hydroxyl-terminated groups, it has been reported14 that platinum nanoparticles can be encapsulated inside the dendrimer This is probably due to different interactions of platinum nanoparticles with the functional groups of the dendrimers.
Figure 7. Absorption spectra of AgNO3 in the presence of dendrimer, G5.5, after reduction with NaBH4.
Ag Colloids. Since it is expected that Ag+ ions strongly adsorb on the dendrimers having surface carboxyl groups through electrostatic attractive forces, the reduction of AgNO3 by addition of NaBH4 in the presence of G3.5, G4.5, or G5.5 was carried out. Without the dendrimers, we only obtained very unstable silver particles which sedimented rapidly. Figure 7 shows the change in optical spectra of silver colloids obtained in the presence of G5.5 with various concentrations. Although a typical plasmon band of silver colloids was observed at 380 nm at the ratio of [G5.5]/ [Ag+] ) 1/1, another band at around 450 nm appeared with increasing concentration of G5.5. Further, the absorbances at both bands considerably decreased. The color of silver colloids also changed with increasing concentration of G5.5; yellow at the ratio of [G5.5]/[Ag+] ) 1/1 turned to orange at the higher G5.5 concentrations.
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Figure 8. FT-IR spectra of (a) dendrimer, G4.5, and (b) dendrimer, G4.5/silver particles.
In the case of G3.5 and G4.5, a similar result was observed as that of G5.5. It is interesting to note that the time required for the color change to yellow or orange after addition of NaBH4 increased with increasing concentration of the dendrimers because of the repulsive action of BH4with the negatively charged dendrimers that carry a reacting Ag+ ions. Such a delay of reduction of Ag+ ions has been observed in the presence of polyacrylate.19 The long-wavelength absorption can be found in the dielectric properties of very small particles (
