Chapter 16
Extraction of Metal Nanoparticles from within Dendrimer Templates Joaquin C . Garcia-Martinez, Orla M. Wilson, Robert W . J. Scott, and Richard M. Crooks *
Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842-3012
The synthesis and characterization of Pd, A u , A g and bimetallic AuAg dendrimer-encapsulated nanoparticles (DENs), and their subsequent extraction from their dendrimer templates with alkanethiol ligands to yield near-monodisperse metal monolayer-protected clusters (MPCs) are reported. In particular, the factors affecting the extraction experiment ionic strength, thiol concentration, dendrimer generation and D E N size - are examined, and it is conclusively shown that under optimal conditions, the particles remain intact throughout the extraction process. UV-vis spectroscopy and H R T E M are used to characterize the nanoparticles before and after extraction, and FTIR confirms that the dendrimer remains in the aqueous phase after the extraction. Furthermore, the affinity of specific ligands for different metal surfaces can be used to preferentially extract nanoparticles of one metal from a mixture of A u and A g DENs in a process referred to as selective extraction. A n application of the selective extraction process to the chemical characterization of bimetallic A u A g DENs is briefly discussed.
© 2006 American Chemical Society
215
216
Background Here we demonstrate that Pd, A u , A g and bimetallic A u A g nanoparticles can be extracted from within dendrimer templates and transferred to an organic phase using alkanethiol and alkanoic acid surfactants while leaving the dendrimer intact in the aqueous phase (Scheme l). * The extraction of metal nanoparticles from dendrimers is a facile route for preparing monodisperse monolayer-protected clusters (MPCs), and it avoids the need for multi-step purification processes. The results very strongly suggest that individual nanoparticles are extracted from the dendrimer without significant loss of metal or aggregation. FT-IR spectroscopy was used to ascertain the whereabouts of the dendrimer after extraction; the amide bands, characteristic of P A M A M dendrimers, were only present in the aqueous phase. Thus, the dendrimer can be recycled and used for the preparation of another batch of dendrimerencapsulated nanoparticles (DENs). One further benefit of the extraction experiment is that the resulting MPCs can be easily characterized by techniques such as mass spectroscopy or electrochemistry, and thus the extraction experiment can be used to provide information about the original size and shape distributions of the DENs. We will also examine recent successful extractions of A u , A g , and bimetallic A u A g DENs using alkanethiol and alkanoic acid surfactants, and show how the extraction method can be used to gain chemical information of the surface structure of nanoparticles. In 1998 we reported the synthesis of dendrimer-encapsulated Cu nanoparticles, and shortly thereafter found that it was also possible to prepare a number of other types of near-monodisperse dendrimer-encapsulated nanoparticles (DENs), * including metals such as Pd, * A u , and A g . We further showed that the resulting organic/inorganic composites were catalytically active for certain simple types of reactions. " DENs are prepared by a two-step process. First, metal ions are sequestered within the dendrimer, followed by the addition of a reducing agent such as N a B H . This results in the reduction of the metal ions to form a zerovalent metal nanoparticle. Because the synthesis takes place in a well-defined dendrimeric template, the resulting metal nanoparticle (the replica) can be quite monodisperse in size. Among the desirable characteristics of DENs are that they can be solubilized in nearly any solvent, * the nanoparticle surface is unpassivated and therefore catalytically active, and the dendrimer branches can be used as selective gates to control access of small molecules to the encapsulated nanoparticles. This extraction method is important for several reasons. First, it demonstrates that nanometer-scale materials prepared within a molecular 1
5
6,7
3
4,5
8
9
11
12
9
14
15
1 6
11
4
9
11
13
11
217 template can be removed leaving both the replica and template undamaged. Second, it provides a straightforward approach for preparing highly monodisperse metallic and bimetallic MPCs without the need for subsequent purification. Third, it demonstrates that multiple, fairly complex operations, including formation of covalent bonds, electron-transfer, molecular transport, heterogeneous self assembly, and nanoparticle transport, can all be executed within the interior of a dendrimer.
Scheme 1. Reprinted with permission from J. Am. Chem. Soc. 2004[ 126, ΜΠΟ Ι 6178. Copyright 2004 American Chemical Society.
Objective Here, we present a detailed study of the parameters that control the extraction of nanoparticles from dendrimer templates. Some of the fundamental questions to be answered include: how can such large nanoparticles escape the confinement of the dendrimer interior, and what are the driving forces for the extraction? To begin, we have examined the impact of the following parameters on the extraction process: (1) different metals such as Pd, A u , and A g ; ( 2 ) the nanoparticle size; (3) the concentration of the w-alkanethiol extradant; and (4) the generation and the peripheral functionalization of the dendrimer. Furthermore, we will show how the extraction process can be used to separate A u and A g nanoparticles from an aqueous mixture of A u and A g DENs. This selective extraction approach takes advantage of the affinity of alkanethiol and alkanoic acids for A u and A g oxide surfaces, respectively. As an application for this process, we will briefly show how the extraction technique can allow the separation and structural determination of AuAg core/shell nanoparticles.
218
Experimental Fourth-, sixth- and eighth-generation hydroxyl-terminated P A M A M dendrimers (G4-OH, G6-OH, and G8-OH, respectively) having an ethylenediamine core were obtained as 10-25% aqueous solutions from Dendritech, Inc. (Midland, MI). A l l w-alkanethiols, ascorbic acid, w-undecanoic acid, N a B H , H A u C l , and A g N 0 were used as received from the Aldrich Chemical Co. (Milwaukee, WI). H P L C grade ethanol, toluene, and hexane were purchased from E M D Chemicals Inc., and 18 ΜΩ-cm M i l l i - Q water (Millipore, Bedford, M A ) was used to prepare aqueous solutions. Cellulose dialysis sacks having a molecular weight cutoff of 12,000 were purchased from Sigma Diagnostics, Inc. (St. Louis, MO). Synthesis and extraction of metal nanoparticles from the interior of P A M A M dendrimers. The method used to prepare and extract DENs has previously been described in the literature, ' but small deviations from this procedure were necessary in some cases. A short summary of the basic procedure follows, with specific reference to the synthesis of fourth-generation, hydroxyl-terminated P A M A M dendrimers containing 55-atom A u nanoparticles (G4-OH(Au )). To prepare the A u DENs, a 10 mL aqueous solution containing 2.0 μΜ G4-OH and 110 μΜ H A u C l was vigorously stirred for 2 min, followed by the addition of a 5-fold molar excess of N a B H (150 m M in 0.3 M NaOH). The reduction of the intradendrimer A u complex to zerovalent A u can be easily followed as the color changes from yellow to brown upon addition of N a B H . After the synthesis of the A u DENs was complete, a 150-fold molar excess of N a B H was added to the aqueous A u D E N solution, followed by the addition of 10 mL of a 20 m M κ-dodecanethiol (HSCi ) solution in toluene. The vial was shaken for 5 min, and the resulting emulsion was allowed to settle for 5 min, after which phase separation was complete. The toluene layer containing the A u M P C product was purified by first concentrating the solution to 1 mL on a rotary evaporator, and then adding 15 mL of ethanol to precipitate the MPCs. Centrifugation resulted in separation of the A u MPCs from excess free nalkanethiol and others impurities. The M P C s were washed and centrifuged twice with ethanol to ensure complete purification. Similar procedures were followed for the synthesis and extraction of G4-OH(Pd ) DENs. G 6 - O H ( A g ) DENs were synthesized using an intradendrimer displacement reaction, which converts Cu DENs to A g D E N s . Specifically, a 0.55 m M solution of G6-OH(Cu ) DENs was synthesized as follows: 0.28 mL of a 20.0 m M solution of C u ( N 0 ) was added to 9.06 mL of a 11.0 μΜ solution of G6-OH. The solution was purged with N for 15 min prior to reduction with a 3-fold excess of BH *. Excess B H * was removed by addition of acid, and then 1.1 mL of 0.01 M A g N 0 was added to the Cu D E N solution. As a consequence of the enhanced nobility of A g and the stronger oxidizing power of A g compared to Cu, the Ag* 4
4
3
1 5
55
4
4
3 +
4
4
2
40
U0
16
55
3
2
4
4
3
+
2
219 2+
ions oxidize the Cu° nanoparticles to C u and in doing so are themselves reduced to A g . Characterization. UV-vis absorbance spectra were recorded with a Hewlett-Packard H P 8453 UV-vis spectrometer using quartz cells. UV-vis spectra of DENs were collected using deionized water as the reference. UV-vis spectra of metal MPCs were obtained immediately after extraction and without purification, using either toluene or hexane as the reference. H R T E M was performed using a J E O L 2010 transmission electron microscope (JEOL U S A Inc., Peabody, M A ) . Samples were prepared by placing one drop of a solution on a holey-carbon-coated Cu grid ( E M science, Gibbstown, NJ) and allowing the solvent to evaporate in air. 0
1 6
Results & Discussion Extraction of Palladium and Gold Nanoparticles Pd or A u nanoparticles are extracted from their dendrimer templates by first adding a large excess of N a B H to the aqueous solution and then introducing a toluene solution containing an appropriate w-alkanethiol (Scheme 1). After shaking for 5 min, the aqueous phase turned from brown to colorless and the toluene phase turned from colorless to brown, indicating extraction of the nanoparticles from the dendrimer and transport of the resulting MPCs into the toluene phase. Note that Esumi and coworkers have previously reported that n~ alkanethiols adsorb onto the surface of A u D E N s , but they did not observe extraction of the nanoparticles from the dendrimers. This is likely a consequence of our finding that the presence of both an alkanethiol surfactant and a sufficiently high ionic strength is required for the extraction of D E N s . Here, we rely on an excess of N a B H to increase the ionic strength, but identical results were obtained when NaCl, Mg(N0 )2, or N a S 0 were used for this purpose. Figure 1 shows UV-vis absorption spectra of aqueous-phase G4-OH(Pd ) and G8-OH(Au ) (m = 55, 147, 1022) prior to extraction (black lines) and the corresponding toluene-phase, «-alkanethiol-stabilized MPCs after extraction (red lines). In the case of Pd DENs (Figure la), the aqueous G4-OH(Pd ) solution displays one intense band at 200 nm, corresponding to absorption by the dendrimer and B H " , and a gradually increasing absorption towards higher energy, which is characteristic of Pd colloids. A l l the A u D E N aqueous solutions also display an increasing absorbance toward higher energy, which results from interband electronic transitions of the encapsulated zerovalent A u nanoparticles. In addition, a plasmon band at 500-550 nm appears in the G8-OH(Au 2) solution, which is not present for the smaller gold DENs. 4
1 7 1 8
1 3
4
3
2
4
40
ra
40
4
1215,19
l02
220
Plasmon bands arising from A u nanoparticles larger than 2 nm are typically observed in this range. Therefore the broad width and weak absorbance o f the plasmon band appearing for the G 8 - O H ( A U J O 2 ) solution suggests that the particles are in the 2 - 3 nm range. * In addition, all the U V - v i s spectra of the A u DENs (Figure lb-d) contain a small peak at 2 9 0 nm superimposed on the rising background. Although there is controversy in the literature regarding the origin of this band, it is reproducibly observed during A u D E N syntheses and does not seem to affect nanoparticle properties. The U V - v i s spectra of the M P C s in the toluene phase after extraction of the nanoparticles are very similar to those of the aqueous-phase DENs in both form and intensity (the spectra are cut off below 2 8 5 nm due to toluene absorption). It is especially interesting to point out the absence of a plasmon band in the U V - v i s spectra of the A u and A U | nanoparticles after extraction, which indicates that no significant change in particles size occurs during the extraction process. The small variations in the U V - v i s spectra of the aqueous D E N solutions and the extracted M P C solutions probably result from changes in the environment of the metal surface of the nanoparticles after extraction, and it is known that changes in solvent and adsorbed ligands have a profound influence over the U V - v i s spectra of metal nanoparticles. Regardless of these small differences, however, the main point is that Pd and A u DENs can be extracted from dendrimers with no significant increase in the average nanoparticle size. To demonstrate that the dendrimer template remains in the aqueous phase, we examined the toluene phase and remaining aqueous phase after extraction. After extraction of G 4 - O H ( P d ) DENs, the aqueous phase was dialyzed for 4 8 h to remove toluene and B H \ and the resulting absorbance spectrum o f this solution (blue line, Figure la) indicated that only the G 4 - O H dendrimer was present. That is, the characteristic interband absorption signature of Pd DENs was absent. FT-IR spectroscopy of both the toluene and aqueous phases after extraction showed that the dendrimer is only present in die aqueous phase. This means that the dendrimer can be recovered and recycled to prepare additional DENs. This principle has been demonstrated and the results indicate that more than 8 0 % of the original dendrimer can be recovered after two cycles of D E N preparation and subsequent nanoparticle extraction. H R T E M micrographs were obtained to evaluate the average particle size and size distributions of Pd and A u nanoparticles before and after the extraction from the dendrimer templates. The average diameters were obtained by measuring the size of 1 5 0 randomly selected particles. In all cases, the average size of the nanoparticles was identical before and after extraction within experimental error. Specifically, before extraction, the G 4 - O H ( P d ) , G 8 OH(Au 5), G 8 - O H ( A u i ) , and G8-OH(Aui 22) DENs, had average nanoparticle diameters o f 1.7 ± 0 . 4 nm, 1.2 ± 0 . 2 nm, 1.5 ± 0 . 3 nm , and 2 . 1 ± 0 . 7 nm, respectively, which agree with previous results obtained for similarly-prepared 20,21
2
22
24
25,26
15,25
5 5
4 7
22,23
40
4
1,3
3
40
5
47
0
221
Wavelength (nm)
Wavelength (nm)
Figure 1. UV-vis absorbance spectra demonstrating extraction of (a) Ρd nanoparticles from a 6.2 μΜ G4-OH(Pd ) DEN solution, (b) Au and (c) Au nanoparticles from 2.0 μΜ G8-OH(Au ) and G8-OH(Au ) DEN solutions, and (d) Au 2 nanoparticles from a 0.50 pMG8-OH(Au ) DEN solution. The black line corresponds to the aqueous phase before extraction, and the red line was obtainedfrom the organic phase after extraction with a toluene solution containing 20 mM of n-hexanethiol for the extraction of Pd^ and 20 mM of n-dodecanethiolfor the extraction of Au nanoparticles. The blue line in spectrum (a) corresponds to the aqueous phase after extraction. Reprinted with permission from J. Am. Chem. Soc. 2003, 125, 11190-11191 andJ. Am. Chem. Soc. 2004, 126, 16170-16178. Copyright 2003 and 2004 American Chemical Society. 40
i47
55
55
H7
m2
l02
m
222 12
15
materials in our group. * After extraction the average nanoparticle sizes were 1.5 ± 0.3 nm, 1.3 ± 0.4 nm, 1.5 ± 0.3 nm, 2.2 ± 0.5 nm for MPC(Pd4o), M P C ( A u ) , M P C ( A u i ) and M P C ( A u i ) , respectively. The data strongly suggest that the individual nanoparticles are extracted from the dendrimer without significant loss of metal or aggregation. The extraction of the nanoparticles is also nearly independent o f the peripheral functionality or the generation of the dendrimer. A u DENs containing 55 A u atoms were prepared using three different generations of P A M A M dendrimers: G4-OH(Au ), G6-OH(Au )» and G8-OH(Au ). We have previously shown that the particle size only depends on the dendrimer-to-metalion molar ratio and is independent o f the dendrimer generation. Indeed, these three samples have very similar nanoparticle sizes and distributions; specifically, the average nanoparticle diameters were 1.2 ± 0.3 nm, 1.3 ± 0.2 nm, and 1.2 ± 0.2 nm for the G4-OH(Au ), G6-OH(Au ), and G8-OH(Au ) DENs, respectively. These results are very close to the calculated values assuming that these nanoparticles are spherical in shape. After the extraction, the average particle size was 1.2 ± 0.3 nm, 1.3 ± 0.4 nm, and 1.3 ± 0.4 nm for the A u nanoparticles extracted from the interior of the G4-OH, G6-OH, and G8-OH dendrimers, respectively. This indicates that the particle size of the final MPCs depends only on the molar ratio of dendrimer to metal ions used to form the nanoparticles. In addition, similar-sized A u nanoparticles have been prepared and successfully extracted using amine-terminated P A M A M dendrimers instead of hydroxyl-terminated dendrimers. The UV-vis spectra and H R T E M micrographs of the resulting A u MPCs also show that are essentially identical to those of the A u D E N precursors. Figure 2 shows the effect of concentration of the w-alkanethiol ligand during the extraction of the A u nanoparticles from the dendrimers. Specifically, four 5.0 mL aliquots of G4-OH(Au ) ([Au]= 110 μΜ) were extracted using 5.0 mL of dodecanethiol solutions of 110, 330, 550, and 1100 μΜ in toluene. These solutions correspond to H S C / A u molar ratios of 1, 3, 5, and 10, respectively. Note that only 76% of the A u atoms in an ideal A u cluster are on the surface, so in all cases a stoichiometric excess of H S C is present in solution. UV-vis spectra of the toluene phases after extraction are essentially identical to that shown in Figure l b (prepared with a HSCj /Au molar ratio = 182), except for the case where the H S C i / A u molar ratio = 1, for which a plasmon band is observed near 500 nm. This suggests that at low w-alkanethiol ligand concentrations the extraction process results in particle aggregation during extraction. H R T E M data confirms this hypothesis, as shown in Figure 2. Figure 2a-d shows micrographs and particle-size distributions for extractions of A u DENs using HSC) /Au molar ratios of 1, 3, 5, and 10, respectively. The results from these data are summarized in Figure 2e, which shows that both the average particle size and the particle-size distribution decrease as the H S C / A u molar ratio increases from 1 55
47
022
55
55
55
15
55
55
55
27
5 5
3
55
I2
5 5
} 2
2
2
3
2
!2
223 to 10. We rationalize this finding in terms of the following proposed extraction mechanism: when low concentrations of Λ-alkanethiols are used, the nanoparticles undergo relatively slow extraction and the nascent MPCs are only partially passivated for some time following extraction. During this period, the average particle size increases via metal aggregation. It is possible to decrease the period of partial thiol passivation, and thus the likelihood of aggregation, by increasing the concentration of the n-alkanethiol.
Figure 2. HRTEM micrographs and particle-size distributions forAu$$ nanoparticles extractedfrom G4-OH dendrimers using the following HSCj^Au molar ratios: (a) 1, (b) 3, (c) 5, and (d) 10. The horizontal axis of the histograms is the same for each panel to facilitate comparison of the size distributions. Figure 2(e) shows the average particle size (from HRTEM) vs. the HSCi/Au molar ratio. The vertical bars indicate the standard deviation of the particle-size distribution. Reprinted with permission from J. Am. Chem. Soc 2004, 126, 16170-16178. Copyright 2004 American Chemical Society.
224 Selective Extraction of Silver and Gold Nanoparticles Since reporting the extraction of Pd and A u nanoparticles from within dendrimer templates, " we have been investigating the potential o f using the extraction process as a characterization tool. In particular, we were interested in determining whether the extraction method could be used to characterize the surface structure of monometallic and bimetallic DENs. As a proof-of-concept experiment, we reasoned that i f we could find surfactants that had highly selective affinities for different metals, then it would be possible to separate mixtures of nanoparticles. This idea is based on the concept of "orthogonal assembly", which has its roots in a 1989 paper by Wrighton and Whitesides. The authors demonstrated that two different surfactants could selectively interact with each of two different metal or metal-oxide surfaces. More specifically, several groups have shown that κ-alkanoic acids bind to metal-oxide surfaces such as A g oxides, but not pure metallic surfaces such as A u . Here, we report the separation of A u and A g dendrimer-encapsulated nanoparticles from an aqueous mixture of the two using a selective extraction approach, Figure 3. 1
3
4,3
28
2 9 , 3 0
4
Figure 3. Demonstration of the selective extraction ofAg MPCs and simultaneous extraction of both Au andAg MPCs from an aqueous mixture of similarly-sized Au andAg DENs. Reprinted with permission from Chem. Mater. 2004, 16, 4202-4204. Copyright 2004 American Chemical Society.
In this method, an w-alkanoic acid present in hexane and having a high affinity for A g oxide surfaces (but not Au) is added to an aqueous mixture of similarly-sized A g and A u DENs. Through strong ligand-nanoparticle interactions, the A g nanoparticles are selectively extracted into the organic phase resulting in a solution of A g M P C s . Subsequent addition of an organic phase 1 6
1 5
4
225 containing an w-alkanethiol leads to extraction of the remaining A u DENs. It is also possible to extract both metals simultaneously by employing the strong affinity of w-alkanethiol molecules for both A u and A g surfaces (by addition of a strong reducing agent to remove the A g oxide layer), or selectively extract just A u nanoparticles from a mixture of DENs. These results are important, because they demonstrate a simple chemical approach for separating nanoparticles having different chemical compositions. Figure 4 shows UV-vis absorption spectra for an aqueous mixture of G4O H ( A u i ) and G6-OH(Agn ) DENs prior to extraction (black line), the hexane phase after the first extraction with κ-decanoic acid (red line), and the second hexane phase after subsequent extraction with w-dodecanethiol (green line). After the first extraction, the H-decanoic acid/hexane phase reveals a peak at 420 nm, which corresponds to the plasmon absorption of A g . The position and magnitude of this band are nearly identical to those of the original G 6 - O H ( A g ) solution, indicating a quantitative extraction of the A g nanoparticles. Likewise, after the second extraction, the spectrum of A u MPCs is almost identical to that of the G6-OH(Aui ) DENs prior to extraction. The presence of a much larger plasmon band for A g DENs compared to A u DENs is in accordance with their fundamental optical properties and previous literature reports. 47
0
31
no
W 7
47
24,32
H R T E M was used in combination with large-area energy dispersive X-ray spectroscopy (EDS) to analyze the particle size and atomic composition of the samples before and after extraction. Prior to extraction the mixture of DENs have a particle size of 1.6 ± 0.3 nm and large-area EDS analysis of 10 areas on the T E M grid indicates an average composition of 43% A g and 57% A u . Analogous analysis of the hexane phase after the first extraction with w-decanoic acid results in particles with a diameter of 1.7 ± 0.4 nm and an average composition of 95 ± 6% A g and 5 ± 6% Au. This is convincing evidence that nalkanoic acids preferentially extract the A g nanoparticles from the D E N mixture. H R T E M and EDS analysis of the particles resulting from the second extraction with w-dodecanethiol indicate a particle-size distribution of 1.4 ± 0.3 nm and a metal composition of 8 ± 6% A g and 92 ± 8% Au. From these results we conclude that similarly-sized A u and A g DENs can be separated. In the presence of a reducing agent no oxide is present on the A g surface, and therefore it is possible to simultaneously extract both A g and A u DENs using /i-dodecanethiol. Further confirmation of the need for a A g oxide layer for selective extraction with /i-decanoic acid was obtained by adding N a B H to the mixture of A g and A u DENs. Under these reducing conditions, neither of the DENs extract with w-decanoic acid, presumably due to the absence of the A g oxide layer. N M R spectroscopy confirmed that B H * did not reduce or otherwise react with the acid. 1 1 0
4
4
226
QQ1 . . . . ι .. , • ι . . . . ι . . . . ι . . . . 1 '300 400 500 600 700 800
Wavelength (nm) Figure 4. UV-vis absorbance spectra demonstrating the separation of G6OH(AU\4T) and G6-OH(Agn^ DENs by selective extraction. The black line corresponds to the aqueous phase mixture of G6-OH(Auw) and G6-OH(Agu ) DENs before extraction, the red line was obtained from the hexane phase after extraction with a solution containing n-decanoic acid, the green line was obtained from the second hexane phase after extraction with a solution containing n-dodecanethiol. Reprinted with permission from Chem. Mater. 2004, 16, 4202-4204. Copyright 2004 American Chemical Society. 0
Extraction of A u A g Bimetallic Nanoparticles Since the selective extraction experiment indicated that orthogonal assembly of surfactants onto metal nanoparticles having different surface compositions provides a basis for separation of A u and A g DENs, we sought to further test its adaptability as a characterization tool of A u A g bimetallic D E N s . Three structurally unique bimetallic DENs were synthesized and characterized: A u A g alloys, core/shell [Au](Ag) and core/shell [AuAg alloy](Ag) (for structured materials, brackets indicate the core metal and parentheses indicate the shell metal). The bimetallic DENs were characterized using UV-vis spectroscopy, H R T E M , and single-particle X-ray EDS. We have shown that depending on the surface metal and its oxidation state, these nanoparticles can be extracted from the dendrimer into an organic phase with either ^-alkanethiol or w-alkanoie acid molecules. While w-dodecanethiol in the presence of N a B H will quantitatively extract all A u A g nanoparticles into the organic phase regardless of structure, nundecanoic acids will extract only A u A g alloy particles having significant A g 5
4
227 loadings, and A g shell nanoparticles. Thus, this selective extraction strategy provides an important new tool for chemical analysis of the structure of bimetallic nanoparticles in the 1-3 nm diameter size range. We envision that under certain conditions alloy nanoparticles could be separated from core/shell particles or from monometallic nanoparticles that may have formed during the reduction step. We are currently exploring the extension of this selective extraction strategy to other metals, and our aim is to explore new ligands, such as phosphines, w-alkylamines, and n-alkaneisocyanides, that can interact either with Au, A g , or other metals, such as Pd, Pt, Cu, or Fe. This will make it possible to lay out a wet-chemical roadmap for the qualitative analysis of the surface structure and composition of monometallic and bimetallic DENs.
Conclusions To summarize, we have shown that both Pd and A u DENs with diameters of less than 2.2 nm can be extracted intact from within the interior of dendrimer templates using alkanethiol ligands. Extraction proceeds quickly, regardless of the size of the D E N , the dendrimer generation, and the peripheral functionalization of the dendrimer. The success of the extraction experiment depends only on the ionic strength of the dendrimer solution and the thiol concentration. The average particle size and optical properties of the extracted nanoparticles are the same as the precursor DENs as evidenced by H R T E M and UV-vis spectroscopy. The mechanism proposed for the extraction involves penetration of the dendrimer by the alkanethiol, adsorption of the thiol to the nanoparticle surface, and extraction of the resulting M P C into the organic phase. We have further shown that selective extraction, based on the affinity of ligands for different metal surfaces, can be used to separate a mixture of A u and A g DENs. The selective extraction technique can be used as a chemical characterization tool which, we believe, will greatly aid in the structure elucidation of bimetallic nanoparticles with diameters in the 1 - 3 nm range. We are further exploring other metal and surfactant combinations to fully realize the power of this technique for purifying and characterizing the smallest of nanoparticles.
Acknowledgments We gratefully acknowledge the U . S. Department of Energy, D O E - B E S Catalysis Science grant no. DE-FG02-03ER15471, the Robert A . Welch Foundation, and the U . S. National Science Foundation (Grant No. 0211068) for financial support of this work. Dr. Joaquin C. Garcia-Martinez thanks the
228 Ministerio de Educacion, Cultura y Déporte of Spain for postdoctoral fellowship support. We also thank Dr. Zhiping Luo of the T A M U Microscopy and Imaging Center for assistance with H R T E M and EDS measurements.
References (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) N.;
Garcia-Martinez, J. C.; Scott, R. W. J.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 11190-11191. Garcia-Martinez, J. C.; Crooks, R. M. Polym. Preprint 2004, 45, 462-463. Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2004, 126, 16170-16178. Wilson, Ο. M.; Scott, R. W . J.; Garcia-Martinez, J. C.; Crooks, R. M. Chem. Mater. 2004, 16, 4202-4204. Wilson, Ο. M.; Scott, R. W. J.; Garcia-Martinez, J. C.; Crooks, R. M. J. Am. Chem. Soc. 2005, 127, 1015-1024. Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. Whetten, R. L . ; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G . ; Vezmar, I.; Alvarez, M. M.; Wilkinson, A . Acc. Chem. Res. 1999, 32, 397-406. Zhao, M.; Sun, L . ; Crooks, R. M. J. Am. Chem. Soc. 1998, 120, 48774878. Crooks, R. M.; Zhao, M.; Sun, L . ; Chechik, V.; Yeung, L . K . Acc. Chem. Res. 2001, 34, 181-190. Crooks, R. M.; Lemon, Β. I.; Sun, L.; Yeung, L . K . ; Zhao, M. Top. Curr. Chem. 2001, 212, 81-135. N i u , Y.; Crooks, R. M. C. R. Chim. 2003, 6, 1049-1059. Zhao, M.; Crooks, R. M. Angew. Chem. Int. Ed. 1999, 38, 364-366. N i u , Y.; Yeung, L . K . ; Crooks, R. M. J. Am. Chem. Soc. 2001, 123, 68406846. Scott, R. W. J.; Y e , H.; Henriquez, R. R.; Crooks, R. M. Chem. Mater. 2003, 15, 3873-3878. K i m , Y.-G.; Oh, S.-K.; Crooks, R. M. Chem. Mater. 2004, 16, 167-172. Zhao, M.; Crooks, R. M. Chem. Mater. 1999, 11, 3379-3385. Satoh, K . ; Yoshimura, T.; Esumi, K . J. Colloid. Interface. Sci. 2002, 255, 312-322. Esumi, K . ; Satoh, K . ; Torigoe, K . Langmuir 2001, 17, 6860 - 6864. Gröhn, F.; Bauer, Β. J.; Akpalu, Υ. Α.; Jackson, C. L . ; Amis, E. J. Macromolecules 2000, 33, 6042-6050. Rao, C . N. R.; Kulkarni, G . U.; Thomas, P. J.; Edwards, P. P. Chem. Eur. J. 2002, 8, 28-35. Alvarez, M. M.; Khoury, J. T.; Schaaff, T. G . ; Shafigullin, M. N.; Marat, Vezmar, I.; Whetten, R. L . J. Phys. Chem. Β 1997, 101, 3706-3712.
229 (22) Templeton, A . C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem., Β 2000, 104, 564-570. (23) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427-3430. (24) Mulvaney, P. Langmuir 1996, 12, 788-800. (25) Lee, W. I.; Bae, Y.; Bard, A . J. J. Am. Chem. Soc. 2004, 126, 8358-8359. (26) Esumi, K . ; Suzuki, Α.; Aihara, N.; Usui, K.; Torigoe, K . Langmuir 1998, 14, 3157-3159. (27) Leff, D . V.; Ohara, P. C.; Heath, J. R.; Gelbart, W . M. J. Phys. Chem. 1995, 99, 7036-7041. (28) Laibinis, P. E.; Hickman, J. J.; Wrighton, M. S.; Whitesides, G . M . Science 1989, 245, 845-847. (29) Schlotter, Ν . E.; Porter, M. D.; Bright, T. B . ; Allara, D . L . Chem. Phys. Lett. 1986, 132, 93-98. (30) Tao, Y.-T. J. Am. Chem. Soc. 1993, 115, 4350-4358. (31) Creighton, J. Α.; Eadon, D. G . J. Chem. Soc. Faraday Trans. 1991, 87, 3881-3891. (32) Kreibig, U . ; Vollmer, M. Optical Properties of Metal Clusters; Springer, 1995; Vol. 25.
