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Preparation of Gold-Dendrimer Nanocomposites by Laser Irradiation and Their Catalytic Reduction of 4-Nitrophenol Kazutaka Hayakawa, Tomokazu Yoshimura, and Kunio Esumi* Department of Applied Chemistry and Institute of Colloid and Interface Science, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received February 26, 2003. In Final Form: April 23, 2003 Gold nanoparticles were prepared in the presence of poly(amidoamine) (PAMAM) dendrimers (generations 3.0 and 5.0) or poly(propyleneimine) (PPI) dendrimers (generations 3.0 and 4.0) with surface amino groups by laser irradiation. UV-visible absorption, dynamic light scattering, and transmission electron microscopy have been used to study the formation and structure of the nanocomposites. The reduction of Au3+ ions proceeded with an increase of the irradiation time, and the average diameters of the gold nanoparticles obtained tended to decrease with an increase of the dendrimer concentration. It is suggested that the dendrimers are adsorbed on the nanoparticles as a monolayer. Studies of the reduction reaction of 4-nitrophenol by the gold nanoparticles with NaBH4 reveal that the rate constants for PPI dendrimers are higher than those for PAMAM dendrimers, whereas the size of the PPI dendrimer of the same generation is considerably smaller than that of the PAMAM dendrimer.
Introduction Noble metal nanoparticles exhibit unique characteristics that are not observed in bulk metals.1,2 A distinct absorption band in the ultraviolet-visible region, which is known as the surface plasmon band, is a typical example of the characteristics that come from their small sizes and high surface/volume ratios for gold, silver, and copper. Such unique optical properties have materialized extended applications of the nanoparticles to photochemical catalysts,3,4 sensors,5,6 and nonlinear optical materials,7,8 where it is very important to control the particle size and size distribution as well as the surface composition. Several preparation methods of metal nanoparticles, including wet and dry processes, have been developed.9 In the wet methods, the particle sizes can be controlled by choosing many parameters, such as the kinds of metal ions, stabilizers, and reducing agents, their concentrations, temperatures, etc. In recent years, the direct use of the laser has been proposed for the chemical preparation of nanoparticles in solution. For example, when an aqueous solution containing a silver salt and a surfactant is irradiated by a laser, silver nanoparticles with well-defined sizes and a welldefined shape distribution are obtained.10 In addition, gold and silver nanoparticles have been prepared by the direct ablation of metal plates in a solution containing a * Author to whom correspondence should be addressed. (1) Hayat, M. A. Colloidal Gold; Academic Press: New York, 1989. (2) Schmidt, G. Chem. Rev. 1992, 92, 1709. (3) Dawson, A.; Kamat, P. V. J. Phys. Chem. B 2001, 105, 960. (4) Fendler, J. H. Chem. Rev. 1987, 87, 877. (5) (a) 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. Nature 1992, 267, 1629. (b) Grabar, K. C.; Freeman, R. G.; Hommer, M. B.; Natan, M. J. Anal. Chem. 1995, 67, 735. (6) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lehnhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (7) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 5334. (8) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remacle, F.; Levine, P. D. Acc. Chem. Res. 1999, 32, 415. (9) Schmid, G. Clusters and Colloids from Theory to Applications; VCH: Weinheim, Germany, 1994. (10) Abid, J. P.; Wark, A. W.; Brevet, P. F.; Girault, H. H. Chem. Commun. 2002, 792.
surfactant.11 However, this ablation provides the formation of nanoparticles of a controlled size but with a significant polydispersity. Furthermore, laser irradiation has also been applied for the controlled reshaping and resizing of nanoparticles.12,13 For the preparation of metal nanoparticles, stabilizers play an important role to control the formation of nanoparticles as well as their dispersion stability. Recently, dendrimers have attracted increasing attention for interesting stabilizers and templates. Dendrimers are composed of a core molecule, hyperbranches that regularly extend from the core, and terminal groups having a defined molecular weight and size.14 Additionally, dendrimers of higher generations take a spherical shape, and they can encapsulate metal complexes, nanoparticles, or other inorganic and organic guest molecules.15,16 Many metaldendrimer composites have been synthesized such as copper,17,18 gold,19-22 silver,20,22-24 platinum,20,25,26 and palladium.25,27 The sizes of gold in the composites decrease (11) Mafune, F.; Kohno, J.; Takeda, Y.; Kondow, T.; Sawabe, H. J. Phys. Chem. B 2000, 104, 9111; 2001, 105, 5114. (12) Abid, J. P.; Girault, H. H.; Brevet, P. F. Chem. Commun. 2001, 829. (13) Kamat, P. V.; Flumiani, M.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 3123. (14) Fre´chet, J. M. J.; Tomalia, D. A. Dendrimers and Other Dendritic Polymers; Wiley: Chichester, U.K., 2001. (15) Bosman, A. W.; Janssen, H. M.; Meijer, E. W. Chem. Rev. 1999, 99, 1665. (16) Astruc, D.; Chardac, F. Chem. Rev. 2001, 101, 2991. (17) Zhao, M.; Sun, L.; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 4877. (18) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (19) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157. (20) Esumi, K.; Suzuki, A.; Yamahira, A.; Torigoe, K. Langmuir 2000, 16, 2604. (21) Gro¨hn, F.; Kim, G.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 2179. (22) Manna, A.; Imae, T.; Aoi, K.; Okada, M.; Yogo, T. Chem. Mater. 2001, 13, 1674. (23) Balogh, L.; Valluzzi, R.; Laverduer, K. S.; Gido, S. P.; Hagnauer, G. L.; Tomalia, D. A. J. Nanopart. Res. 1999, 1, 353. (24) Zeng, J.; Stevenson, M. S.; Hikida, R. S.; Van Patten, P. G. J. Phys. Chem. B 2002, 106, 1252. (25) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364. (26) Esumi, K.; Nakamura, R.; Suzuki, A.; Torigoe, K. Langmuir 2000, 16, 7842. (27) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938.
10.1021/la034339l CCC: $25.00 © 2003 American Chemical Society Published on Web 05/16/2003
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Hayakawa et al. Chart 1. Structures of the Dendrimers
Table 1. Physical Properties of PAMAM and PPI Dendrimers Used molecular weight generation
surface group
3 4 5
32 64 128
PAMAM 6909
diameter, nm
PPI
PAMAM
PPI
3514 7168
3.6
2.4 2.8
28 826
5.4
with an increase of the dendrimer generation as well as the concentration of dendrimer having surface amino groups.20 In the higher generations of dendrimers with surface amino groups, the gold nanoparticles are encapsulated21 in the dendrimers. In addition, several catalytic reactions have been investigated using metal-dendrimer nanocomposites.25,27-29 For gold-dendrimer nanocomposites, the reduction reaction from 4-nitrophenol to 4-aminophenol is accelerated by decreasing the generation of dendrimers as well as the concentration of the dendrimers.28 In the present study, we investigated preparation of gold nanoparticles in the presence of poly(amidoamine) (PAMAM) dendrimers or poly(propylene imine) (PPI) dendrimers by laser irradiation. The catalytic activity in the reduction of 4-nitrophenol using thus obtained golddendrimer nanocomposites was also studied. Experimental Section Materials. PAMAM dendrimers with surface amino groups (generations 3.0 and 5.0) were prepared according to the literature.30 Their purity was confirmed by 1H and 13C NMR. PPI dendrimers with surface amino groups (generations 3.0 and 4.0) were obtained from Aldrich Chemical Co. The physical characteristics of PAMAM and PPI dendrimers are given in Table 1.31 The structures of the dendrimers (PAMAM G3 and PPI G3) are shown in Chart 1. Tetrachloroauric acid tetrahydrate (HAuCl4‚4H2O) was obtained from Tokyo Kasei Kogyo Co., Ltd., and used without further purification. The other chemicals were of reagent grade. The water used was purified through a Milli-Q plus system. (28) Esumi, K.; Miyamoto, K.; Yoshimura, T. J. Colloid Interface Sci. 2002, 254, 402. (29) Chechik, V.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 1243. (30) 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. (31) Crooks, R. M.; Chechik, V.; Lemon, B. I.; Sun, Li.; Yeung, L. K.; Zhao, M. In Metal Nanoparticles, Synthesis, Characterization, and Applications; Feldheim, D. L., Foss, C. A., Eds.; Marcel Dekker: New York, 2002; Chapter 11.
Methods and Measurements. A freshly prepared HAuCl4 aqueous solution (0.5 cm3, 2 mmol dm-3) was added to 4.5 cm3 of PAMAM or PPI dendrimer dissolved in an aqueous solution (0.14, 0.2, and 0.6 mmol dm-3 as the surface amino group concentrations). The final molar ratios of [dendrimer]/[Au3+] were 0.7, 1, and 3, respectively. The mixed solution was left stirring by bubbling pure nitrogen to complete the formation of the Au3+dendrimer complex. Then, a volume of 2 cm3 of the solution was placed in a quartz cell and irradiated with the second harmonic (532 nm) output of the LOTIS TII LS2106 Nd:YAG laser focused on a spot (0.196 cm2) on the solution surface by a lens. A typical pulse width and repetition rate were 10 ns and 10 Hz, respectively. The output of the 532 nm was 80 mJ/pulse. The absorption spectrum of the irradiated solution was measured by a Hewlett-Packard 8452 A diode array UV-visible spectrophotometer. The dynamic light scattering (DLS) measurements for the irradiated solution were performed using a Photal DLS spectrometer DLS-9000 (Otsuka Electronics Co., Ltd.) equipped with an argon ion laser. Each solution was filtered over a 0.45-µm filter before the measurements. A transmission electron microscope observation was performed for the samples dried on carbon-coated copper grids. A Hitachi H-9000 NAR transmission electron microscope was operated at an accelerating voltage of 300 kV and a direct magnification of ×200 000. The size distribution of the gold nanoparticles was determined from about 200 particles. The catalytic reduction of 4-nitrophenol was studied as follows. In the standard quartz cuvette with a 1-cm path length, 2.65 cm3 of water, 0.15 cm3 of the irradiated solution, and 0.1 cm3 of 3 mmol dm-3 4-nitrophenol were added. Then, the addition and proper mixing of 0.1 cm3 of aqueous 0.3 mol dm-3 NaBH4 to the reaction mixture caused the decrease in the intensity of the peak of 4-nitrophenol. The absorption spectra were recorded every 5 s in the range of 190-820 nm at 15 °C. The control experiment was also carried out using mixtures of NaBH4 and 4-nitrophenol; the absorption spectrum of 4-nitrophenol was unaltered.
Results and Discussion Variations of UV-visible absorption spectra of the aqueous solution containing PAMAM G5 dendrimer and HAuCl4 with laser irradiation are shown in Figure 1. Here, the molar ratio of [PAMAM/[Au3+] is 0.7. The absorption band of colloidal gold appeared at around 520 nm, and its intensity increased with an increase of the irradiation time. The increment of the intensity in the absorption band almost leveled off at 60 min of irradiation. In Figure 1b,c, a transmission electron microscopy (TEM) micrograph and the size distribution of gold nanoparticles obtained at 40 min of irradiation are also given; the average particle size is relatively large with a large standard
Gold-Dendrimer Nanocomposites
Figure 1. (a) Variation of UV-visible absorption spectra of the solution containing PAMAM G5 dendrimer and HAuCl4 as a function of the irradiation time: 1, before irradiation; 2, 10 min; 3, 20 min; 4, 30 min; 5, 40 min; and 6, 60 min after irradiation. [PAMAM]/[Au3+] ) 0.7. (b) TEM micrograph and (c) size distribution of gold nanoparticles after laser irradiation of 40 min.
deviation. When the molar ratio of [PAMAM G5]/[Au3+] increased, the irradiation time when the increment of the leveled-off intensity in the absorption became long and the average size at the corresponding irradiation time became small. On the other hand, a different behavior in the UV-visible spectra was observed when the solution containing PAMAM G3 dendrimer and Au3+ was irradiated (Figure 2; molar ratio of [dendrimer]/[Au3+] ) 0.7); the intensity in the absorption at around 520 nm increased with an increase of the irradiation time, but the band shifted to a longer wavelength at 130 min of irradiation.
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Figure 2. (a) Variation of UV-visible absorption spectra of the solution containing PAMAM G3 dendrimer and HAuCl4 as a function of the irradiation time: 1, before irradiation; 2, 10 min; 3, 20 min; 4, 40 min; 5, 60 min; 7, 100 min; and 8, 130 min after irradiation. [PAMAM]/[Au3+] ) 0.7. (b) TEM micrograph and (c) size distribution of gold nanoparticles after laser irradiation of 80 min.
This suggests that some aggregation of gold nanoparticles occurs at a longer irradiation time. However, an increase in the molar ratio of [PAMAM G3]/[Au3+] also rendered the average size of gold nanoparticles small and the size distribution narrow. Thus, it is found that PAMAM dendrimers operate as a stabilizer as well as a size controller. Similarly, laser irradiation was performed for aqueous solutions containing Au3+ ions and PPI dendrimer. A
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Hayakawa et al. Table 2. Average Diameter (nm) of Gold Nanoparticles Prepared by Laser Irradiation [dendrimer]/ [Au3+]
PAMAM G3
PAMAM G5
PPI G3
PPI G4
0.7 1.0 3.0
14.0 ( 5.0 11.2 ( 3.7 6.0 ( 1.3
8.3 ( 4.4 9.7 ( 3.5 6.8 ( 1.9
16.4 ( 5.9 8.9 ( 2.2 12.6 ( 3.7
12.7 ( 7.5 7.0 ( 2.0 8.1 ( 1.5
Table 3. Hydrodynamic Diameters (nm) of Gold-Dendrimer Nanocomposites Prepared by Laser Irradiation
Figure 3. (a) Variation of UV-visible absorption spectra of the solution containing PPI G4 dendrimer and HAuCl4 as a function of the irradiation time: 1, before irradiation; 2, 10 min; 3, 20 min; 4, 30 min; 5, 40 min; 6, 60 min; and 7, 80 min after irradiation. [PPI]/[Au3+] ) 0.7. (b) TEM micrograph and (c) size distribution of gold nanoparticles after laser irradiation of 80 min.
typical result for an aqueous solution of Au3+ ions and PPI G4 dendrimer is shown in Figure 3 at the molar ratio of [PPI G4]/[Au3+] ) 0.7. With an increasing laser irradiation time, the intensity in the absorption band at around 520 nm increased and leveled off at above 80 min. From a TEM micrograph, it is also seen that the sizes of the particles are relatively large with a large standard deviation. In addition, the average size had a tendency to decrease with an increasing molar ratio of [PPI G4]/[Au3+]. A similar result was obtained for aqueous solutions con-
[dendrimer]/ [Au3+]
PAMAM G3
PAMAM G5
PPI G3
PPI G4
0.7 1.0 3.0
25.5 ( 6.1 18.9 ( 4.2 18.7 ( 5.0
21.0 ( 4.5 13.0 ( 1.7 18.1 ( 5.0
25.5 ( 6.1 18.9 ( 4.2 18.7 ( 5.0
21.0 ( 4.5 13.0 ( 1.7 18.1 ( 5.0
taining Au3+ ions and PPI G3 dendrimer. It is interesting to note here that the standard deviation decreases with an increase of the concentration for both PAMAM and PPI dendrimers. Table 2 shows the average diameters of gold nanoparticles prepared by laser irradiation at the irradiation time corresponding to the leveled-off intensity in the plasmon band. To investigate the structures of gold-dendrimer nanocomposites, the average hydrodynamic diameter of gold nanoparticles obtained by laser irradiation was measured by DLS, where the diameter was obtained as the numberweighted average. The data for the DLS measurements are given in Table 3, where the irradiation time for gold nanoparticles is the same as that in Table 2. The TEM measurements should be sensitive only to the electrondense metal particle, whereas the DLS measurements are sensitive to the size of the whole nanocomposite. If the metal particles are covered with dendrimers as a monolayer, the difference in the measurements will correspond to two times the hydrodynamic diameter of the dendrimer. From the hydrodynamic diameters of the dendrimers given in Table 1 as 3.6 nm for PAMAM G3, 5.4 nm for PAMAM G5, 2.4 nm for PPI G3, and 2.8 nm for PPI G4, it is found that the TEM and DLS data are in good agreement, indicating that the gold-dendrimer nanocomposites prepared by laser irradiation consist of gold nanoparticles covered with dendrimer as a monolayer. However, at the present time, it is not clear whether a fully covered monolayer is formed or not. The formation of gold nanoparticles by the laser irradiation of Au3+ ions may occur through the photolysis of water and formation of radicals in the solution.32 Indeed, near the focal point, which defines the reaction volume, the energy density may be high enough for the multiphoton process to take place. With the multiphoton process, the productions of e-(aq), OH(aq), and H(aq) radicals are created by the photodecomposition of water.33 The gold nuclei formed by the water photolysis are able to grow either through the addition of already reduced Au3+ ions or by the addition of Au3+ ions followed by reduction. The growth reaction is in competition with the termination reaction, which is the adsorption of the dendrimer onto the particles. In addition, because it is considered that complex formation19,20 occurs between AuCl4- and the amino groups of the dendrimers, the nucleation of gold occurs from the free Au3+ ions and the complex of AuCl4and dendrimers, where the rate of nucleation for the former will be faster than that for the latter. Thus, the growth process may be affected by the complex. Such a reduction (32) Belloni, J.; Mostafavi, M.; Remita, H.; Marignier, J.-L.; Delcourt, M. O. New J. Chem. 1998, 1239. (33) Thomsen, C. L.; Madsen, D.; Keiding, R.; Thogersen, J.; Christiansen, O. J. Chem. Phys. 1999, 110, 3453.
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Figure 4. Successive UV-visible absorption spectra of the reduction of 4-nitrophenol by gold nanoparticles prepared with laser irradiation of 90 min: [PAMAM G5]/[Au3+] ) 3; [4-nitrophenol] ) 0.2 mmol dm-3; and [NaBH4] ) 10 mmol dm-3.
Figure 5. Plots of ln A versus time for the reduction of 4-nitrophenol by gold nanoparticles prepared with laser irradiation: (a) [PAMAM G3]/[Au3+] ) 3, irradiation time ) 60 min and (b) [PPI G3]/[Au3+] ) 3, irradiation time ) 140 min.
reaction may be supported by the result that the time required for the complete reduction by laser irradiation increases with the molar ratio of [dendrimer]/[Au3+], and no reduction of Au3+ ions by laser irradiation occurs when the molar ratio of [dendrimer]/[Au3+] is more than 5 at this experimental condition. It would be expected that the particle sizes become smaller as a result of the faster termination process when the molar ratio of [dendrimer]/ [Au3+] increases. Actually, the average particle sizes and their distributions of gold nanoparticles for both PAMAM and PPI dendrimers decreased with an increase of the molar ratio of [dendrimer]/[Au3+]. Figure 4 shows successive UV-visible absorption spectra of the reduction of 4-nitrophenol by the goldPAMAM G5 dendrimer formed by laser irradiation. Similar spectra changes were also obtained for the other systems. In the absence of any catalysis, the peak due to 4-nitrophenol at 400 nm remains unaltered. The addition of one aliquot of gold nanoparticles to the reaction mixture causes the fading and ultimate bleaching of the yellow color of the 4-nitrophenol in an aqueous solution. Because the amount of the gold nanoparticles added is very small, the absorption spectra of 4-nitrophenol are hardly interfered with by the gold nanoparticles. The reduction can be visualized by the disapperance of the 400-nm peak with the concomitant appearance of a new peak at 300 nm. This peak has been attributed to 4-aminophenol.34 The concentration of the BH4- ions greatly exceeds those of 4-nitrophenol and the catalysis nanoparticles. The excess of NaBH4 used increased the pH of the reacting system, but by that means retarded the degradation of BH4-, and the liberated hydrogen purged out air, thereby checking the aerial oxidation of the reduced product of 4-nitrophenol. Furthermore, because the concentration of BH4- is very high, it remains essentially constant during the reaction. Therefore, pseudo-first-order kinetics with respect to 4-nitrophenol could be used in this case to evaluate the catalytic rate. A good linear correlation with time, that is, ln A vs time plot, was obtained for all the systems studied (Figure 5). Here, A stands for the absorbance at any time. The values of the pseudo-firstorder rate constants determined from these plots are given in Table 4. The apparent first-order rate law fitted reasonably well to ∼90% of the reaction. Although a distinct difference was not seen in the rate constant when compared with the generation and molar
Table 4. Apparent Rate Constant for the Reduction of 4-Nitrophenol (10-3 sec-1)
(34) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196, 247.
[dendrimer]/[Au3+]
PAMAM G3
PAMAM G5
PPI G3
PPI G4
0.7 1.0 3.0
2.13 2.81 3.70
1.78 2.74 2.41
9.23 9.01 9.49
13.2 11.1 10.6
ratio of the respective dendrimers, a distinct difference between PAMAM and PPI dendrimers was observed; the rate constants for PPI dendrimers are considerably higher than those for PAMAM dendrimers. It is supposed that the diffusion of 4-nitrophenol mainly controls the rate of reduction.28 Because PAMAM and PPI dendrimers with surface amino groups are considered to adsorb on the gold nanoparticles, the adsorbing dendrimers would affect the diffusion of 4-nitrophenol to the gold nanoparticle surfaces. Indeed, the DLS results suggest that the gold nanoparticles are covered with PAMAM/PPI dendrimers to some extent as a monolayer, where the dynamic diameters of PPI dendrimers alone are smaller than those of PAMAM dendrimers. However, one finds that the density of PPI is greater than that of PAMAM using the molecular weight and diameter. Accordingly, it can be concluded that the rate constant of the reduction reaction is predominantly controlled by the sizes of the dendrimers adsorbing on the gold nanoparticles. Conclusions In the present study, gold nanoparticles are prepared in the presence of the PAMAM or PPI dendrimers with surface amino groups by laser irradiation, and the catalytic activity by the gold nanoparticles thus prepared is investigated in the reduction reaction of 4-nitrophenol in an aqueous solution. It is found that the particle sizes of gold nanoparticles prepared using dendrimers decreases with an increase of the molar ratio of [dendrimer]/[Au3+], and their standard deviation also becomes small. However, a clear difference in the particle size with two kinds of the dendrimers is not observed. Stable gold nanoparticles are obtained by the dendrimers adsorbing on the gold surfaces as a monolayer. A distinct difference in the catalytic activity is not observed with respect to the generation as well as the molar ratio of [dendrimer]/[Au3+] for both the dendrimers, but PPI dendrimers show a higher activity than PAMAM dendrimers. This suggests that the rate constant is significantly affected by the size of the dendrimer adsorbing on the nanoparticles. LA034339L
