Coordination and Reduction Processes in the Synthesis of Dendrimer

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Coordination and Reduction Processes in the Synthesis of Dendrimer-Encapsulated Pt Nanoparticles Daigo Yamamoto, Satoshi Watanabe, and Minoru T. Miyahara* Department of Chemical Engineering, Kyoto University, Katsura, Nishikyo, Kyoto 615-8510, Japan Received July 28, 2009. Revised Manuscript Received September 13, 2009 We synthesized Pt nanoparticles encapsulated in poly(amidoamine) (PAMAM) dendrimers by Pt2+ coordination and subsequent reduction by NaBH4. To optimize the experimental conditions for the Pt nanoparticle synthesis, we systematically examined the effects of pH, temperature, coordination time, and surface functional groups of the dendrimers on coordination and NaBH4 reduction by UV-vis spectroscopy and transmission electron microscopy (TEM) measurements. We used generation-4 dendrimers (hydroxyl-terminated PAMAM dendrimers; G4-OH) and generation-4.5 dendrimers (carboxyl-terminated PAMAM dendrimers; G4.5-COO-). According to our results, dendrimer-encapsulated Pt nanoparticles with a narrow size distribution were obtained at high Pt2+ coordination ratios (R), while nonencapsulated Pt nanoparticles were formed at low R values. To enhance R, it was necessary to use a neutral G4-OH solution or an acidic G4.5-COO- solution. Temperature had a marked effect on the coordination rate, with an increase in the temperature from room temperature to 50 C, and the coordination time decreased from 10 days to 1-2 days.

Introduction Nanoparticles have unique physical and chemical properties because of their large specific surface area, and hence, they can be used as catalysts1 as well as in optical devices2 and biotechnological applications.3 However, it is important to prevent the aggregation of nanoparticles in solution to ensure that their properties do not change; this is achieved by using protecting agents such as polymers and surfactants.4,5 Dendrimers, which are spherical hyperbranched polymers, are promising candidates for protecting agents. In 1998, Crooks et al. and Balogh and Tomalia independently succeeded in encapsulating monodispersed Cu nanoparticles in poly(amidoamine) (PAMAM) dendrimers having an ethylenediamine core.6,7 Dendrimers are commonly represented as Gn-X, where Gn and X represent the nth generation and the surface functional group, respectively. Encapsulation of metal nanoparticles in dendrimers involves two *Corresponding author. E-mail: [email protected].

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steps: metal ion coordination and reduction by NaBH4 (Scheme 1). In the coordination step, metal ions in solution are transferred to the interior of the dendrimers via coordinate bond formation with the tertiary amine groups present at the branching sites of the dendrimers. Subsequent reduction with NaBH4 affords encapsulated, zerovalent Pt nanoparticles, whose size can be controlled by controlling the metal ion/dendrimer ratio. Thus, the dendrimer acts as a protecting agent and as a reaction field during nanoparticle formation. Following the successful synthesis of dendrimer-encapsulated Cu nanoparticles by Crooks et al. and Balogh and Tomalia, several other research groups attempted to synthesize various types of nanoparticles. Some of these include Au,8-21 Ag,9,22 Cu,7,23 Pd,15,19,20,23-34 Pt,9,24,29,32,35-53 Fe,54 Ni,55 Ru,51,56,57 and Ir58 nanoparticles, as well as involving the (18) Perignon, N.; Mingotaud, A. F.; Marty, J. D.; Rico-Lattes, I.; Mingotaud, C. Chem. Mater. 2004, 16, 4856–4858. (19) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. Chem. Mater. 2004, 16, 5682– 5688. (20) Kim, Y. G.; Garcia-Martinez, J. C.; Crooks, R. M. Langmuir 2005, 21, 5485–5491. (21) Shi, X. G.; Wang, S. H.; Meshinchi, S.; Van Antwerp, M. E.; Bi, X. D.; Lee, I. H.; Baker, J. R. Small 2007, 3, 1245–1252. (22) Zheng, J.; Stevenson, M. S.; Hikida, R. S.; Van Patten, P. G. J. Phys. Chem. B 2002, 106, 1252–1255. (23) Niu, Y. H.; Crooks, R. M. Chem. Mater. 2003, 15, 3463–3467. (24) Zhao, M. Q.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364–366. (25) Li, Y.; El-Sayed, M. A. J. Phys. Chem. B 2001, 105, 8938–8943. (26) Rahim, E. H.; Kamounah, F. S.; Frederiksen, J.; Christensen, J. B. Nano Lett. 2001, 1, 499–501. (27) Sun, L.; Crooks, R. M. Langmuir 2002, 18, 8231–8236. (28) Garcia-Martinez, J. C.; Scott, R. W. J.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 11190–11191. (29) Oh, S. K.; Kim, Y. G.; Ye, H. C.; Crooks, R. M. Langmuir 2003, 19, 10420– 10425. (30) Scott, R. W. J.; Ye, H. C.; Henriquez, R. R.; Crooks, R. M. Chem. Mater. 2003, 15, 3873–3878. (31) Narayanan, R.; El-Sayed, M. A. J. Phys. Chem. B 2004, 108, 8572–8580. (32) Ye, H. C.; Scott, R. W. J.; Crooks, R. M. Langmuir 2004, 20, 2915–2920. (33) Garcia-Martinez, J. C.; Lezutekong, R.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 5097–5103. (34) Pittelkow, M.; Moth-Poulsen, K.; Boas, U.; Christensen, J. B. Langmuir 2003, 19, 7682–7684. (35) Esumi, K.; Nakamura, R.; Suzuki, A.; Torigoe, K. Langmuir 2000, 16, 7842–7846.

Published on Web 10/07/2009

DOI: 10.1021/la902770p

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Article Scheme 1. Encapsulation of Pt Nanoparticles and Structures of PAMAM Dendrimers

Yamamoto et al. Table 1. Summary of Coordination Conditions and Diameters of the Resulting G4-X(Pt40) Particles ref 30 33 39 40

dendrimer G4-Q32a G4-NH2 G4-OH G4-OH

period 2 days 76 h 3 days 10 days

environment pH 5 -

diameter (nm) 1.7 ( 0.3 1.4 ( 0.3 3.4 ( 1b 80%, 0.7c 20%, 1-3c 41 G4-OH 10 days 0.98c 44 G4-OH 10 days stored in refrigerator 1.4 ( 0.2 0.81 ( 0.48c 45 G4-OH 10 days 80%, >1.2c 46 G4-OH 4 days 47 G4-OH 2 days 1.4 ( 0.3 48 G4-OH 3 days under nitrogen flow 52 G4-OH 66 h a G4-Q32 is an amine-terminated dendrimer that is partially (in this case, the number is 32) quaternized to Q. Q = -NHCH2CH(OH)CH2Nþ(CH)3Cl-. b Diameters of Pt particles reduced by hydrogen flow instead of NaBH4. Diameters are measured from TEM or high-resolution TEM (HRTEM) images, except in the case of c. c Values are measured from AFM images.

coordination process. For example, the maximum number bimetallic59-66 and semiconductor13,67-69 nanoparticles, which have good magnetic properties and show photoluminescence and (36) Zhao, M. Q.; Crooks, R. M. Adv. Mater. 1999, 11, 217–220. (37) Kim, J. W.; Choi, E. A.; Park, S. M. J. Electrochem. Soc. 2003, 150, E202– E206. (38) Deutsch, D. S.; Lafaye, G.; Liu, D. X.; Chandler, B.; Williams, C. T.; Amiridis, M. D. Catal. Lett. 2004, 97, 139–143. (39) Liu, D. X.; Gao, J. X.; Murphy, C. J.; Williams, C. T. J. Phys. Chem. B 2004, 108, 12911–12916. (40) Pellechia, P. J.; Gao, J. X.; Gu, Y. L.; Ploehn, H. J.; Murphy, C. J. Inorg. Chem. 2004, 43, 1421–1428. (41) Yang, L.; Luo, Y. F.; Jia, X. R.; Ji, Y.; You, L. P.; Zhou, Q. F.; Wei, Y. J. Phys. Chem. B 2004, 108, 1176–1178. (42) Beakley, L. W.; Yost, S. E.; Cheng, R.; Chandler, B. D. Appl. Catal., A 2005, 292, 124–129. (43) Gu, Y. L.; Xie, H.; Gao, J. X.; Liu, D. X.; Williams, C. T.; Murphy, C. J.; Ploehn, H. J. Langmuir 2005, 21, 3122–3131. (44) Ozturk, O.; Black, T. J.; Perrine, K.; Pizzolato, K.; Williams, C. T.; Parsons, F. W.; Ratliff, J. S.; Gao, J.; Murphy, C. J.; Xie, H.; Ploehn, H. J.; Chen, D. A. Langmuir 2005, 21, 3998–4006. (45) Sun, L. S.; Ca, D. V.; Cox, J. A. J. Solid State Electrochem. 2005, 816–822. (46) Ye, H. C.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 4930–4934. (47) Alexeev, O. S.; Siani, A.; Lafaye, G.; Williams, C. T.; Ploehn, H. J.; Amiridis, M. D. J. Phys. Chem. B 2006, 110, 24903–24914. (48) Raghu, S.; Nirmal, R. G.; Mathiyarasu, J.; Berchmans, S.; Phani, K. L. N.; Yegnaraman, V. Catal. Lett. 2007, 119, 40–49. (49) Tang, L. H.; Zhu, Y. H.; Xu, L. H.; Yang, X. L.; Li, C. Z. Electroanalysis 2007, 19, 1677–1682. (50) Ye, H.; Crooks, J. A.; Crooks, R. M. Langmuir 2007, 23, 11901–11906. (51) Huang, W.; Kuhn, J. N.; Tsung, C. K.; Zhang, Y.; Habas, S. E.; Yang, P.; Somorjai, G. A. Nano Lett. 2008, 8, 2027–2034. (52) Knecht, M. R.; Weir, M. G.; Myers, V. S.; Pyrz, W. D.; Ye, H. C.; Petkov, V.; Buttrey, D. J.; Frenkel, A. I.; Crooks, R. M. Chem. Mater. 2008, 20, 5218–5228. (53) Mark, S. S.; Bergkvist, M.; Yang, X.; Angert, E. R.; Batt, C. A. Biomacromolecules 2006, 7, 1884–1897. (54) Knecht, M. R.; Crooks, R. M. New J. Chem. 2007, 31, 1349–1353. (55) Knecht, M. R.; Garcia-Martinez, J. C.; Crooks, R. M. Chem. Mater. 2006, 18, 5039–5044. (56) Lafaye, G.; Williams, C. T.; Amiridis, M. D. Catal. Lett. 2004, 96, 43–47. (57) Lafaye, G.; Siani, A.; Marecot, P.; Amiridis, M. D.; Williams, C. T. J. Phys. Chem. B 2006, 110, 7725–7731. (58) Jesus, Y.; Vicente, A.; Lafaye, G.; Marecot, P.; Williams, C. T. J. Phys. Chem. C 2008, 112, 13837–13845. (59) Chung, Y. M.; Rhee, H. K. Catal. Lett. 2003, 85, 159–164. (60) Scott, R. W. J.; Datye, A. K.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 3708–3709. (61) Chung, Y. M.; Rhee, H. K. J. Colloid Interface Sci. 2004, 271, 131–135. (62) Endo, T.; Yoshimura, T.; Esumi, K. J. Colloid Interface Sci. 2005, 286, 602– 609. (63) Wilson, O. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 1015–1024. (64) Ye, H. C.; Crooks, R. M. J. Am. Chem. Soc. 2007, 129, 3627–3633. (65) Knecht, M. R.; Weir, M. G.; Frenkel, A. I.; Crooks, R. M. Chem. Mater. 2008, 20, 1019–1028.

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Figure 1. Time-dependent UV-vis spectroscopic data obtained after mixing PtCl42- with (a) G4-OH and (b) G4.5-COO- without pH adjustment. Air was used to obtain the background spectrum.

unique catalytic activities. In addition, it is demonstrated that different types of dendrimers such as a functionalized poly(propyleneimine) dendrimer with a smaller diameter70,71 and a phenylazomethine dendrimer with more rigid structure72 than those of PAMAM yield dendrimer-encapsulated nanoparticles, although a PAMAM dendrimer has been so far most widely used. (66) Li, G. P.; Luo, Y. J. Inorg. Chem. 2008, 47, 360–364. (67) Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 12886–12887. (68) Hanus, L. H.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Langmuir 2000, 16, 2621–2626. (69) Yu-Juan, J.; Yun-Jun, L.; Guo-Ping, L.; Jie, L.; Yuan-Feng, W.; Rui-Qin, Y.; Wen-Ting, L. Forensic Sci. Int. 2008, 179, 34–38. (70) Ooe, M.; Murata, M.; Mizugaki, T.; Ebitani, K.; Kaneda, K. Nano Lett. 2002, 2, 999–1002. (71) Yeung, L. K.; Crooks, R. M. Nano Lett. 2001, 1, 14–17. (72) Yamamoto, K.; Imaoka, T.; Chun, W. J.; Enoki, O.; Katoh, H.; Takenaga, M.; Sonoi, A. Nat. Chem. 2009, 1, 397–402.

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Figure 2. UV-vis spectroscopic data for Pt2þ coordination with (a) G4-OH and (b) G4.5-COO- at various pH after 0 h (solid curves) and 10 days (dotted curves). (c) Relation between pH and R after 10 days.

The key process for the formation of dendrimer-encapsulated monodispersed nanoparticles is the coordination of metal ions with the dendrimers; this is because the number of metal ions coordinated with tertiary amine groups determines the size of the resultant nanoparticles.36 Previous studies have revealed the static mechanism of metal ions that can be accommodated in the dendrimer cavity is found to depend on the type of metal species. With generation-4 PAMAM dendrimers, which have 62 tertiary amine sites, the maximum numbers of Cu, Pt, and Au ions that can be coordinated with a single dendrimer molecule are 16, 62, and >140, respectively.6,17,36 However, the kinetics of the coordination process remains unclear, and the time required to reach coordination equilibrium has not been calculated thus far. Table 1 lists the previously reported experimental data; in these studies, Pt2+ and generation-4 PAMAM dendrimers having several surface functional groups have been used, with the Pt2+-dendrimer ratio set to 40:1. The coordination time varies from 2 to 10 days, and different pH and temperature conditions are used. Nuclear magnetic resonance (NMR) measurements show that at least 6.5 days are required to reach coordination equilibrium.40 Thus, it is apparent that coordination equilibrium could not have been achieved in some of the systems listed in Table 1. In addition, the coordination rate of Pt2+ stored at low temperatures may Langmuir 2010, 26(4), 2339–2345

be slow, although no study has examined the effect of temperature on the coordination rate. However, it should be noted that the diameters of the resulting nanoparticles are almost the same.33,44,47 The reduction process is thought to depend on the preceding coordination process. It is hence reasonable to expect the reduction rates of Pt2þ inside and outside the dendrimer cavity to be different. On the basis of extended X-ray absorption fine structure (EXAFS) measurements, Alexeev et al. concluded that reduction of Pt2þ with NaBH4 does not occur completely in G4-OH(Pt2þ)40 solutions.47 Subsequently, Crooks et al. proposed a new model52 to show the bimodal distribution of fully reduced and unreduced intradendrimer Pt2þ after the addition of NaBH4 and demonstrated that only 5% Pt2þ is reduced during the formation of G6OH(Pt2þ)55. On the other hand, Pt2þ reduction in the dendrimers proceeded to a greater extent in Gu et al.’s experiment, in which they distinguished between empty PAMAM complexes and Pt-PAMAM complexes by atomic force microscopy (AFM) measurements; the height of the PAMAM increased after chemical reduction.43 Nevertheless, the exact mechanism underlying the reduction process remains controversial. The main reason for this is thought to be the difference in the coordination conditions adopted by different research groups as the chemical state of Pt2þ before reduction can affect the resultant nanoparticles. Thus, it is DOI: 10.1021/la902770p

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necessary to carry out a systematic study of the coordination of Pt2þ with PAMAM dendrimers. Our final goal is to determine the optimum conditions required for the synthesis of dendrimer-encapsulated Pt nanoparticles and clarify the coordination mechanism. As shown in Scheme 1, we use carboxyl-terminated PAMAM dendrimers (G4.5-COO-), in addition to the commonly used G4-OH. The surface charge of G4.5-COO- enables better handling of the nanoparticles because of electrostatic interactions, which can lead to the formation of ordered structures via controlled nanoparticle deposition. However, in previous studies, it has been found that the nanoparticles obtained using G4.5-COO- dendrimers were larger than those obtained using G4-OH dendrimers.9,41,48 The effect of the surface groups of the dendrimer on the nanoparticle size is thus worth discussing. In the present study, we systematically examine the effects of pH, temperature, coordination time, and surface functional groups of dendrimers on the coordination process and the resultant Pt nanoparticles.

Experimental Section Chemicals. Two types of PAMAM dendrimers, G4-OH and G4.5-COONa dendrimers (10 and 5 wt % solutions in methyl alcohol, respectively), were obtained from Aldrich Chemical Co. and dried under vacuum prior to use for removal of any solvent. Potassium tetrachloroplatinate(II) (K2PtCl4, 99.99%) and NaBH4 (99%) were also purchased from Aldrich Chemical Co. and used as received. HCl (0.1 M in water, Wako Pure Chemical Industries, Ltd.) and NaOH (98%, Kishida Chemical Co.) were used for pH control. Ultrapure deionized (DI) water (18 MΩ 3 cm, Millipore) was used for sample preparation. Pt2þ Coordination. Aqueous PAMAM (G4-OH or G4.5COONa) solutions (100 μM) were first prepared. The pHs of these PAMAM solutions were adjusted to the desired value by adding 0.1 M HCl, NaOH or not. K2PtCl4 (4.0 mM, 1.0 mL) and DI water were then added to the pH-adjusted solutions to obtain 5 μM [dendrimer (G4-OH or G4.5-COO-) þ 40 Pt2þ] solutions, which were then vigorously stirred for a few minutes. Pt2þ has several complex species due to the hydrolysis of PtCl42- at different pH conditions.73,74 However, we represent them as Pt2þ for simplicity. The UV-vis absorbance spectra of the solutions were measured at regular intervals over a period of 10 days at various temperatures (50 C, room temperature, and 4 C) on a Shimadzu UV-1700 spectrophotometer with quartz cells (path length: 10 mm). The solutions were maintained at 50 and 4 C by storing them in an IS-400 incubator (Yamato Co.) and a refrigerator (SHARP), respectively. Preparation of Pt Nanoparticles. Pt nanoparticles were synthesized according to the method proposed by Crooks et al.,36 which is briefly explained below. Aqueous NaBH4 solution (0.8 mL, 20 mM) was added to the 5 μM [dendrimer þ 40 Pt2þ] solutions (10 mL) stored at room temperature for 0 h, 3 h, 2 days, 5 days, and 10 days. Zerovalent Pt nanoparticles were obtained after chemical reduction with vigorous stirring for 2 h. The obtained nanoparticles were observed under a transmission electron microscope (JEM-1010, JEOL). Samples for transmission electron microscopy (TEM) observation were prepared by placing a drop of the Pt nanoparticle suspension on a carbon-coated copper grid (Okenshoji Co.), which was then dried in vacuum.

Results and Discussion 2þ

Pt Coordination. Figure 1 shows the UV-vis absorbance spectra of 5 μM [dendrimer (G4-OH or G4.5-COO-) þ 40 Pt2þ] (73) Wu, L.; Schwederski, B. E.; Margerum, D. W. Inorg. Chem. 1990, 29, 3578– 3584. (74) Grantham, L. F.; Elleman, T. S.; Martin, D. S. J. Am. Chem. Soc. 1955, 77, 2965–2971.

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Figure 3. Time course of absorbance difference at 250 nm in (a) G4-OH and (b) G4.5-COO- solutions at 50 C, room temperature, and 4 C.

solutions without pH adjustment after various elapsed times. The measured pHs of the G4-OH and G4.5-COO- solutions are 7 (neutral) and 9 (weakly basic), respectively.The absorbance of both the solutions at 250 nm increases gradually up to 10 days because of ligand-to-metal charge transfer (LMCT), that is, charge transfer from the tertiary amine to Pt2þ, and remains almost constant thereafter. Thus, it is apparent that coordination takes a long time (several days) to proceed to completion, as reported previously.36,40 The difference in the absorbance at 250 nm is assumed to be proportional to the concentration of the Pt2þ-tertiary amine complexes,36 according to Beer’s law. Herein, we define the coordination ratio R as the ratio of the tertiaryamine-coordinated Pt2þ content to the initial Pt2þ content. The value of R at an arbitrary time t, R(t), is calculated using the following equation: RðtÞ ¼

AðtÞ - Að0Þ Að¥Þ - Að0Þ

where A(t) is the absorbance at 250 nm at time t, while A(0) and A(¥) represent the absorbance at the initial and equilibrium states, respectively. A(¥) is set to be A(10 days) of G4-OH solution at the neutral pH condition. Although the coordination ratio R can reasonably be quantitative for a comparison of the degree of the coordination under a common pH,36 the comparison between different pH conditions is not truly quantitative because plural complexes of Pt2þ in solution would have an effect on the absorbance at 250 nm. However, we assume that their molar absorption coefficients do not significantly differ. Figure 2 shows the effect of pH on the coordination. In our experiments, the pH of the dendrimer solutions was adjusted to the desired value using HCl/NaOH before the addition of K2PtCl4. In G4-OH solutions (Figure 2a), the coordination reaction proceeded only at neutral pH but not under acidic (pH 2) or basic (pH 12) conditions. However, there was a marked difference in the absorbance at 220 nm after 10 days between the acidic and basic conditions. The absorbance is attributed to Langmuir 2010, 26(4), 2339–2345

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Figure 4. TEM images of Pt nanoparticles coordinated with G4-OH at pH 7 after coordination times of (a) 0 h (R=0.0) and (b) 10 days (R= 1.0). (c) TEM image of the particles described in (a) after drying at 50 C and redispersion in DI water. (d) TEM image of nanoparticles shown in (b) after drying at 50 C and redispersion in DI water.

the LMCT transition band of PtCl42-.36 In our study, absorbance under basic conditions decreased, suggesting that the Cl- ligands were replaced by OH- ligands.73 On the other hand, the absorbance remained unchanged under acidic conditions, indicating that the Cl- ligands were not replaced by other ligands. The low R value probably resulted from the prevention of Pt2þ coordination to the tertiary amines by the competing Hþ species. This result is in accordance with the calculation results by Sun and Crooks which demonstrate that approximately 70% of tertiary amine groups were protonated and metal ion coordination was hindered under the pH 2 condition.75 In the G4.5-COO- solutions (Figure 2b), R was the highest under acidic conditions, which would be due to less surface negative charges of the dendrimer than those at pH 9 because Hþ can react with not only the tertiary amines but also the surface functional groups to form COOH moieties. In addition, the larger number of tertiary amine groups of G4.5-COO- than that of G4-OH as shown in Scheme 1 would contribute to the higher R value of 4.5-COO- than that of G4-OH at the pH 2 condition. Thanks to these two factors, Pt2þ can be coordinated with remaining deprotonated tertiary amines under acidic conditions which is not ideal for the coordination in the G4OH solutions, resulting in high R. Figure 2c shows the pH dependence of R after 10 days in the G4-OH and G4.5-COO- solutions. Under strongly basic conditions, coordination did not occur (R = 0.0) in either of the dendrimer solutions because of the existence of OH-, which probably coordinated with Pt2þ. On the other hand, Pt2þ coordination was favored in solutions whose pH was not controlled, although R in the G4.5-COO- solutions (0.4 at pH 9) was lower than that in the G4-OH solutions (1.0 at pH 7); this was because the surface negative charge of G4.5-COO- hindered the coordination of Pt2þ with the tertiary amine sites. Adjustment of pH to the acidic range resulted in a decrease in the surface charge of G4.5-COO- and a consequent increase in R from 0.4 to 0.6. However, R in acidic G4.5-COO- solutions is still lower than that (75) Sun, L.; Crooks, R. M. J. Phys. Chem. B 2002, 106, 5864–5872.

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in neutral G4-OH (1.0) solutions, suggesting that some of the tertiary amine sites of the dendrimer are coordinated with Hþ. An excessive increase in the Hþ concentration would hinder Pt2þ coordination. Thus, we concluded that the pH of the solution should be adjusted to the optimal value depending on the nature of surface functional groups of the dendrimer. Even under optimal pH conditions, a long time would be taken to reach coordination equilibrium because of the slow coordination rate. To enhance the Pt2þ coordination rate, we examined the effect of temperature on R (Figure 3). For optimum coordination, we adjusted the pH of the G4-OH and G4.5-COO- solutions to 7 and 2, respectively. In both solutions, the Pt2þ coordination rate was dramatically higher at 50 C than at 4 C. At present, the detailed mechanism is unclear for the rate enhancement, but we suppose that an appreciable energy barrier exists for the coordination reaction, which can be overcome at higher temperature. The coordination rate was more sensitive to temperature in the G4.5-COO- solution than in the G4-OH solution; this was attributable to the higher energy barrier for the extraction of Pt2þ from the solution to the interior of the dendrimers in the former case. Pt2þ penetration of the G4.5-COO- surface was difficult because the surface of this dendrimer was denser than that of G4-OH. Thus, we concluded that, by increasing the temperature to 50 C, the coordination time could be decreased from 10 days to 1-2 days in the G4-OH and G4.5-COOsolutions. Preparation of Zerovalent Pt. We reduced Pt2þ solutions by NaBH4 after coordination with G4.5-COO- for 0 h (R=0.0), 3 h (R=0.0), 2 days (R=0.1), 5 days (R=0.5), and 10 days (R=0.6) at pH 2 (see the Supporting Information, Figure S1). Black precipitates are observed at the bottom of the glass vials after a short coordination time (Supporting Information Figure S1a-c), suggesting that the Pt particles have not been encapsulated in the dendrimers because of the low R values. On the other hand, the amount of the Pt precipitate decreases with an increase in the coordination time (Figure S1d-e), indicating the encapsulation of the Pt particles in the dendrimers at high R values. In G4-OH DOI: 10.1021/la902770p

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Figure 5. (a) TEM image of Pt nanoparticles at pH 7; (b) corresponding size-distribution histogram obtained from the TEM data. The histogram was determined by considering more than 200 particles.

Figure 6. TEM images of reduced Pt coordinated with G4.5COO- after 10 days at (a) pH 12, (b) pH 9, and (c) pH 2. (d) Size-distribution histogram obtained for particles shown in (b).

suspensions at pH 7, however, no Pt precipitate is formed after coordination times of 0 h (R = 0.0) and 10 days (R = 1.0). Figure 4a, b shows the TEM images of these two suspensions. Monodispersed nanoparticles are observed after long coordination times (Figure 4b) as well as after short coordination times (Figure 4a). The dispersibility and stability of the reduced Pt particles are confirmed by observing them under a transmission electron microscope after drying at 50 C and redispersion in DI water. The Pt particles are highly aggregated after a coordination time of 0 h (Figure 4c) and are monodispersed after coordination for 10 days (Figure 4d). This observation suggests that, at low R values, the resulting Pt particles are not encapsulated but are surrounded by some dendrimer molecules, whereas at high R values, dendrimer-encapsulated Pt particles are formed. Thus, the suspension formed in the G4-OH solution becomes stable even after a short coordination time, while precipitation of the Pt particles occurs in the G4.5-COO- solution. The reason for the different phenomena observed in the G4-OH and G4.5-COOsolutions at low R values is presently unclear. The electrostatic repulsion among the dendrimers does not play any significant role 2344 DOI: 10.1021/la902770p

Yamamoto et al.

Figure 7. (a) TEM image of Pt nanoparticles which are coordinated with G4.5-COO- at pH 2 and reduced at pH 12. (b) Corresponding size-distribution histogram.

in Pt2þ coordination because the surface charge of G4.5-COO- is almost zero under acidic conditions. G4-OH would interact more strongly with the Pt particles than G4.5-COO-, and hence, a stable dispersion is obtained in G4-OH solutions, wherein the nanoparticles are surrounded by G4-OH molecules. The above observations indicate that R has a marked effect on the Pt particles, and thus, favorable results are obtained if R is increased to the maximum possible extent. In the solutions of both dendrimers, a coordination time of 10 days is preferred for Pt2þ coordination at room temperature. Figure 5a shows the TEM image of the reduced Pt particles coordinated with G4-OH for 10 days at pH 7 (R=1.0). At pH 2 (R=0.1) and 12 (R=0.0), precipitates similar to those shown in Figure S1 in the Supporting Information are formed. Monodispersed Pt nanoparticles are formed only at neutral pH. The number mean diameter of more than 200 randomly selected particles is calculated to be 1.4 ( 0.3 nm using image analysis software (Figure 5b). This result agrees with that reported in other previous studies.33,44,47 However, no monodispersed nanoparticles are obtained in the G4.5-COO- suspensions (Figure 6), and particle aggregation occurs at pH 12 (R=0.0) (Figure 6a). At pH 9, where R is 0.4, the diameters of the obtained nanoparticles are slightly larger (the mean diameter: 1.5 ( 0.4 nm) than those obtained in G4-OH solutions (Figure 6b, d). At acidic pH (R= 0.6), some of the obtained nanoparticles have diameters greater than 10 nm (Figure 6c), although we expect the monodispersity to be high because of the high R value. The formation of slightly large nanoparticles is attributed to the decomposition of NaBH4 at acidic pH, because of which Pt2þ reduction is retarded, and the number of Pt nanoparticles formed is lesser than that at pH 9. In fact, the diameters of the Pt particles reduced after increasing the pH (using NaOH) were 1.4 ( 0.3 nm (Figure 7), although a few large-sized nanoparticles and aggregates still existed. We confirmed that R does not decrease after pH adjustment from the favorable pH to the unfavorable pH for the coordination, which is shown in the Supporting Information (Figure S2). By adjusting the pH after coordination, Pt nanoparticles encapsulated in negatively charged dendrimers were obtained; the structures of the encapsulated particles could be easily controlled, since the electrostatic interaction among them could be manipulated by pH control. Langmuir 2010, 26(4), 2339–2345

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Article

Conclusions We attempted the synthesis of Pt nanoparticles using a dendrimer as a template and investigated the effects of pH, temperature, coordination time, and surface functional groups of the dendrimers on Pt2þ coordination. Then we investigated the nanoparticle formation process at different coordination ratios (R) and arrived at the following conclusions: (1) In the coordination process, the control of temperature and pH is quite important to enhance coordination ratio. In G4-OH and G4.5-COO- solutions, pH should be adjusted to neutral and acidic conditions, respectively, and we succeeded in shortening the coordination time from 10 days to 1-2 days by increasing temperature to 50 C. (2) A high R value favored the formation of monodispersed dendrimer-encapsulated nanoparticles in both the G4-OH and G4.5-COO- solutions. In contrast, at low R values, nucleation was assumed to occur outside the dendrimers, and this resulted in the formation of Pt particles surrounded by dendrimers in the G4-OH solution and precipitate formation in the G4.5-COO- solution. The high coordination ratio can provide monodispersed dendrimer-encapsulated nanopar-

Langmuir 2010, 26(4), 2339–2345

(3)

ticles for both G4-OH and G4.5-COO- dendrimers. In contrast, nucleation is assumed to occur out of dendrimers at low coordination ratios, resulting in the formation of particles which would be surrounded by some dendrimers in the G4-OH solution and precipitates in the G4.5-COO- solution. By appropriate control of pH in the coordination and reduction steps, we successfully prepared monodispersed Pt nanoparticles encapsulated in dendrimers with a negative surface charge.

Acknowledgment. This study was partly supported by the Grant-in-Aid for Scientific Research (B), the Global Centers of Excellence (G-COE) program, and the Core-to-Core (CTC) Program, from Japan Society for the Promotion of Science (JSPS). Supporting Information Available: Images of suspensions in G4.5-COO- (pH 2) after NaBH4 reduction for various coordination times, and time-dependent UV-vis spectroscopic data after mixing NaOH with a G4.5-COO--Pt2þ solution (pH 9). This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la902770p

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