2978
Langmuir 2000, 16, 2978-2980
Spontaneous Formation of Gold Nanoparticles in Aqueous Solution of Sugar-Persubstituted Poly(amidoamine)dendrimers Kunio Esumi,* Tomoyuki Hosoya, Akihiro Suzuki, 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 July 30, 1999. In Final Form: September 20, 1999
Introduction Recently, dendrimers, known as highly and regularly branched oligomers, are attracting increasing attention because of their unique structure and properties.1-3 Generally, it has been recognized4,5 that dendrimers of lower generation tend to exist in relatively open forms, while high generation dendrimers take a spherical threedimensional structure, which is very different from linear conventional polymers, which adopt random-coil structures. Accordingly, many interesting reactions using dendrimers have been reported because they can provide reaction sites including the interior or the periphery of the dendrimers and are very useful as model systems.3 In the presence of poly(amidoamine)dendrimers with surface amino groups we obtained gold colloids with a diameter of less than 1 nm through UV photoreduction,6 demonstrating that the dendrimers act as a very effective protective agent for the preparation of gold nanoparticles compared to conventional linear polymers. Since then, many metal particles have been prepared in the presence of dendrimers by the addition of chemical reductants.7-10 The preparation of dendrimer-encapsulated metal nanoparticles in the presence of poly(amidoamine)dendrimers with surface hydroxyl groups by the addition of a chemical reductant, e.g., NaBH4, has also been reported.8-10 As one of the interesting dendrimers, sugar-persubstituted poly(amidoamine)dendrimers (sugar balls) have been synthesized by Aoi et al.11 Such sugar-containing polymers are of considerable interest in a wide variety of fields such as biochemical and medical applications. Since sugar balls have a highly ordered structure with arranged saccharides12 on the peripheries of dendrimers, and the terminal carbohydrate residues can operate as reductants, it can be expected that sugar balls act as protective agents or capsules one as well as reductants for the preparation of (1) Jansen, J. F. G. A.; de Brabander-van der Berg, E. E. M.; Meijer, E. W. Science 1994, 266, 1226. (2) Cooper, A. I.; London, J. D.; Wignall, G.; McClain, J. B.; Samulski, E. T.; Lin, J. S.; Dobrynin, A.; Rubinstein, M.; Burke, A. L. C.; Frechet, J. M. J.; DeSimone, J. M. Nature 1997, 389, 368. (3) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (4) Caminati, G.; Turro, N. J.; Tomalia, D. A. J. Am. Chem. Soc. 1990, 112, 8515. (5) Pistolis, G.; Malliaris, A.; Paleos, C. M.; Tsiourvas, D. Langmuir 1997, 13, 5870. (6) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157. (7) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (8) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (9) Garcia, M. E.; Baker, L. A.; Crooks, R. M. Anal. Chem. 1999, 71, 256. (10) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217. (11) Aoi, K.; Itoh, K.; Okada, M. Macromolecules 1995, 28, 5391. (12) Schmitzer, A.; Perez, E.; Rico-Lattes, I.; Lattes, A.; Rosca, S. Langmuir 1999, 15, 4397.
metal particles. In addition, since terminal groups of dendrimers are considered to affect the formation of metal particles, it is still interesting to study the role of carbohydrate residues of sugar balls. In this work, we discuss the preparation of gold particles in an aqueous sugar ball solution without the addition of any reductant and we investigate the role of the sugar balls for the gold particles preparation. Experimental Section Materials. Poly(amidoamine)dendrimers (generation 3.0 and 5.0) were prepared by using ethylenediamine as an initiator core according to the previous paper.13 Sugar balls (SBn, n ) generation 3.0 and 5.0) were synthesized11 by the reaction of the amine-terminated dendrimers with an excess amount of aldonolactone. To obtain lactobionate, lactobionic acid was evaporated several times from methanol in a vacuum at 50 °C. Poly(amidoamine)dendrimer was dissolved in dry dimethyl sulfoxide under a nitrogen atmosphere. Then, an excess amount of lactobionate in dimethyl sulfoxide was added to the solution by a dropping funnel with stirring and the mixture was reacted at 40 °C for 9 h. When the solution was poured into a large amount of ethanol, precipitation occurred. The precipitate was purified by using a cellulose tube to remove unreacted lactobionate and finally white powdery sugar balls were obtained. The purity of these samples was confirmed by 1H and 13C NMR. In addition, gel permeation chromatography analysis suggested that each sugar ball consists of a single component. The molecular weights and numbers of terminal sugar residues of SBn are 17 765.8, 32 for SB3 and 72 252.7, 128 for SB5. The structure of sugar ball (SB3) is shown in Scheme 1. Tetrachloroauric acid tetrahydrate (HAuCl4‚4H2O) was kindly supplied by Tanaka Kikinzoku Kogyo Co. and used without further purification. Water used in this study was purified through a Milli-Q Plus system until the specific conductivity fell below 0.1 µS cm-1. Methods and Measurements. HAuCl4 and sugar balls were dissolved in water, separately. Then, both solutions were mixed in a quartz cell to form the desired concentrations of HAuCl4 and sugar balls to measure the time course of UV-visible absorption spectra. The UV-visible spectra of the samples were recorded for the wavelength range 190-820 nm at 2 min interval at 25 °C on a Hawlett-Packard 8452A diode array spectrophotometer. The gold particles obtained were characterized by transmission electron microscopy (TEM). The samples were prepared by mounting a drop of the solutions on a carbon-coated Cu grid and allowing the drop to dry in air. They were observed with a Hitachi H-9000 NAR operating at 200 kV and with a direct magnification of 100 000×. Fourier transform infrared spectroscopy for the gold particles and the sugar balls was obtained by forming thin transparent KBr pellets containing the samples.
Results and Discussion Figure 1 shows the evolution of UV-visible spectra by the interaction of AuCl4- and the sugar balls (SB3) after mixing their solutions. Here, the final concentration of HAuCl4 was 0.2 mmol dm-3. It can be seen that the optical density of the absorption band at λ ) 220 nm decreases due to charge transfer between the metal and chloro ligands, while an absorption band at around 550 nm appears and the absorption band shifts smoothly to a constant value of 520 nm with increasing reaction time as well as an increase of the optical intensity. From the Mie’s theory,14 the average gold particle diameter can be estimated from the maximum absorption band; the gold (13) Tomalia, D. A.; Baker, H.; Dewald, J.; Hall, M.; Kallos, G.; Martin, S.; Roeck. J.; Ryder, J.; Smith, P. Polym. J. 1985, 17, 117. (14) Mie, G. Ann. Phys. 1908, 25, 377.
10.1021/la991040n CCC: $19.00 © 2000 American Chemical Society Published on Web 01/20/2000
Notes
Langmuir, Vol. 16, No. 6, 2000 2979 Scheme 1
Figure 1. Evolution of UV-vis spectra after mixing of sugar balls (SB3) and HAuCl4 at 25 °C: [HAuCl4] ) 0.2 mmol dm-3; [SB3] ) 0.5 g dm-3.
particle diameter decreases when the maximum absorption bands shifts from 550 to 520 nm. The evolution of the absorption spectra was almost completed in about 90 min. Upon mixing the two solutions, the color of the solution changed from purple to a dark reddish color. From these results, it is suggested that the reduction of Au3+ ions to Au takes place by the sugar balls (SB3) in which the sugar balls operate as a reductant. Similar results were obtained for the HAuCl4-SB5 system. It is conceivable that at the reductant sites, the hydroxyl groups of the sugar balls play an important role (discussed later). The changes in the optical density in this study are very similar to the nucleation curves calculated from particle size for the
reaction of auric acid and citrate solutions.15 It has been proposed16 by Chow and Zukoski that although in the initial stage of the reaction fluffy large particles which appear to be composed of smaller particles are formed, the large agglomerate falls apart, giving rise to a continual increase in the number density of small particles as the reaction proceeds. These ideas have been confirmed by the shift of the absorption band as well as the observation of TEM micrographs by Chow and Zukoski.16 To obtain the relationship between the formation of gold particles and the reaction time, the evolution of the optical density at 520 nm is presented for the HAuCl4SB3 system (Figure 2). It is apparent that the optical density increases very slowly, indicating an induction time, then sharply increases and finally saturates at a constant value which corresponds to a complete reduction stage. The induction time became shorter when the concentration of sugar balls increased. In the case of the HAuCl4-SB5 system, a similar trend was observed as that of the SB3 system except that the induction time is rather longer compared to that of the SB3 system because the number density of sugar ball molecules at the same weight concentration is less for the AuCl4-SB5 system than that for the AuCl4-SB3 system. In addition, since the sigmoidal shape of the curves shown in Figure 2 suggests that autocatalysis is involved, ln[a/(1 - a)] is expected to change linearly with time in the case of simple autocatalysis,17 where a ) O. D.t / O. D.∞ and O. D.t and O. D.∞ are the optical densities at times t and ∞, respectively. Actually, since the straight line was obtained by plotting ln[a/(1 (15) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (16) Chow, M. K.; Zukoski, C. F. J. Colloid Interface Sci. 1994, 165, 97. (17) Huang, Z.-Y.; Mills, G.; Hajek, B. J. Phys. Chem. 1993, 97, 11 542.
2980
Langmuir, Vol. 16, No. 6, 2000
Figure 2. Change in the absorbance of sugar balls (SB3) with different concentrations and HAuCl4 mixtures at 520 nm as a function of elapsed time at 25 °C: [ HAuCl4 ] ) 0.2 mmol dm-3.
Figure 3. TEM micrographs and size distribution of gold particles: (a) [SB3] ) 0.5 g dm-3; (b) [SB5] ) 0.5 g dm-3. Elapsed time ) 90 min.
a)] vs time (not shown in figure) up to about 50% conversion, it suggests an autocatalytic reaction. From the slopes of these plots the observed rate constant of kobs at 25 °C were calculated: 2.1 × 10-3 s-1 for 0.3 g dm-3 SB3; 3.3 × 10-3 s-1 for 0.4 g dm-3 SB3; 5.1 × 10-3 s-1 for 0.5 g dm-3 SB3. A similar rate constant was obtained for the HAuCl4-SB5 system. Figure 3 shows TEM micrographs and particle size distributions of the gold particles obtained. The figure reveals that the average diameter of gold particles in the
Notes
Figure 4. FTIR spectra of sugar balls (SB3) and sugar balls (SB3)/gold particles.
presence of SB3 and SB5 (0.5 g dm-3) is about 2.3 and 3.3 nm, respectively, although the size distributions are relatively wide. This suggests that sugar balls act as a very effective protective agent for the preparation of gold particles. A relatively wide particle distribution might be attributed to a moderate interaction of gold nanoparticles with the surface of the sugar balls. The difference by two sugar balls on the particle size is due to a different number density of sugar balls at the same weight concentration; the number density of SB3 is about four times that of SB5 and the lower the number density of sugar balls, the larger the average diameter of the gold particles. It is likely that Au3+ ions are reduced on the saccharide groups and the gold particles formed are stabilized on the periphery of the sugar balls. To confirm the interaction of the gold particles obtained with the sugar balls, FTIR spectra of gold particles/sugar balls (SB3) were measured (Figure 4). Since there are no distinct differences in the amide bands between the sugar balls and gold particles/sugar balls, it seems that gold particles are formed and present on the external surface of the sugar balls. Interestingly, it is noteworthy that the sample of gold particles/sugar balls shows a new band near 1732 cm-1, which corresponds to carbonyl groups. A similar result was also obtained for gold particles/sugar balls (SB5). That is, when Au3+ ions are reduced with the hydroxyl groups of the sugar balls, the hydroxyl groups are oxidized to carbonyl groups. Thus, it is found that sugar balls act a protective agent as well as a reductant for the preparation of gold nanoparticles. Finally, it should be noted that Au3+ ions are reduced very slowly in the presence of lactobionic acid which is used for the preparation of sugar balls, but only large aggregated gold particles are obtained. This suggests that sugar balls provide strong reduction power and very high protective action for the formation of gold particles compared to lactobionic acid. It can be concluded from the above results that Au3+ ions can be reduced by the hydroxyl groups of the sugar balls, resulting in the formation of gold nanoparticles which are very stable. In addition, the particle sizes of gold obtained are affected by the concentration and generation of the sugar balls. LA991040N
