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A highly efficient phase transfer method for preparing alkylamine-stabilized Ru, Pt, and Au nanoparticles
Journal of Colloid and Interface Science 277 (2004) 95–99 www.elsevier.com/locate/jcis
A highly efficient phase transfer method for preparing alkylamine-stabilized Ru, Pt, and Au nanoparticles J. Yang a , Jim Yang Lee a,b,∗ , T.C. Deivaraj b , Heng-Phon Too b,c a Department of Chemical and Bimolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 b Singapore-MIT Alliance, 4 Engineering Drive 3, National University of Singapore, Singapore 117576 c Department of Biochemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Received 6 October 2003; accepted 14 April 2004 Available online 18 May 2004
Abstract A highly efficient phase-transfer method was developed to prepare alkylamine-stabilized nanoparticles of several noble metals. This method involved first mixing the metal hydrosols and an ethanol solution of dodecylamine and then extracting the dodecylamine-stabilized metal nanoparticles into toluene. The efficiency of this phase-transfer method was nearly 100%. Alkylamine-stabilized Ru, Pt, and Au nanoparticles 3.45, 4.33, and 7.89 nm in diameter, respectively, could be prepared this way. The self-assembly of dodecylamine-stabilized Pt and Au particles was also detected by transmission electron microscopy (TEM). 2004 Elsevier Inc. All rights reserved. Keywords: Phase transfer; Alkylamine; Metal nanoparticles; Self-assembly
1. Introduction The phase-transfer method is commonly used to prepare organosols of metals including ruthenium, platinum, and gold [1–6]. The metal nanoparticles need to be stabilized and alkanethiols are often used for that purpose [2,3,5]. Initially, the common practice was to first extract the metal ions from an aqueous solution to a hydrocarbon (toluene) layer with the help of a phase-transfer agent such as tetraoctylammonium bromide and then to carry out the reduction with NaBH4 in the presence of an alkanethiol. Here the nucleation and growth of the metal particles and the attachment of the thiol molecules to the nanoparticles would occur simultaneously in a single step. More recently, Sarathy et al. [2,3] and Zhao et al. [5] adopted a different approach in which metal nanoparticles, instead of metal ions, were directly transferred to the organic phase for thiolation. A metal hydrosol was prepared in advance using NaBH4 as the reducing agent and mixed with a toluene solution of 1-dodecanethiol. Concentrated HCl was then added to the biphasic mixture under strong agitation to enable the transfer. The thiol* Corresponding author. Fax: +65-6779-1936.
E-mail address: [email protected] (J.Y. Lee). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.03.074
stabilized metal nanoparticles prepared this way had uniform size and often self-assembled on the TEM grid. The phase-transfer technique was also used by Fu et al. [4] to prepare alkylamine-stabilized Ru nanoparticles. In that work, Ru nanoparticles were first prepared in a liquid polyol and then extracted into toluene by mixing the Ru sol with a toluene solution of dodecylamine. Viau et al. [1] extended the procedure to prepare thiol-stabilized Ru nanoparticles by replacing dodecylamine with 1-dodecanethiol. The phase transfer in this case was not complete and there were many Ru particles left in the polyol phase after the transfer. To the best of our knowledge, there is currently no reported work on transferring ruthenium nanoparticles from an aqueous environment to a hydrocarbon layer. In addition, the procedures described in the current literature could not be used to transfer citrate-stabilized metal nanoparticles, which are often used as precursors for further assembly [7–13] into a hydrocarbon layer. We herein report a simple method, which can be used to transfer citrate-stabilized Ru, Pt, and Au nanoparticles from an aqueous solution to toluene with high efficiency (∼100%). This method is primarily based on the formation of uniform alkylamine-stabilized metal nanoparticles of Ru, Pt, and Au through a stabilizer exchange process.
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2. Materials and methods RuCl3 ·3H2 O, H2 PtCl6 ·6H2 O, HAuCl4 ·3H2 O, and dodecylamine (98%) purchased from Aldrich, sodium borohydride (98%) from Fluka, sodium citrate (98%) and ethanol (99%) from Merck, and toluene from J.T. Baker, Inc., were used as received. Deionized water was distilled by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven. In a typical experiment, 0.2 ml of 38.8 mM aqueous sodium citrate solution was added to 10 ml of 2 mM aqueous RuCl3 solution. Under vigorous stirring, 1.5 ml of 112 mM aqueous NaBH4 solution was introduced dropwise to prepare a Ru hydrosol in which sodium citrate served as the stabilizer. The molar ratio of NaBH4 to RuCl3 was kept above 8 to ensure the reduction of Ru to the zerovalent state. The Ru hydrosol was also left to stand for 4 h after the addition to complete the reduction reaction. The hydrosol was then mixed with 10 ml of ethanol containing 100 µl of dodecylamine and the mixture was stirred for 2 min. A 5-ml volume of toluene was added and stirring continued for three more minutes. Dodecylamine-stabilized Ru nanoparticles were extracted into the toluene layer rapidly, leaving behind a colorless aqueous solution. The preparations of dodecylamine-stabilized Pt and Au nanoparticles followed the same procedures, except that RuCl3 was replaced by H2 PtCl6 and HAuCl4 , respectively. A JEOL JEM2010 microscope was used to obtain TEM images of the nanoparticles. For TEM measurements a drop of the nanoparticle solution was placed on a 3-mm copper grid covered with a continuous carbon film. Excess solution was imbibed with an adsorbent paper. The average particle size and particle size distribution were obtained from a few randomly chosen areas in the TEM image containing approximately 200 nanoparticles each.
3. Results and discussion Citrate-stabilized metal nanoparticles (Ru, Pt, and Au) could not be transferred directly to toluene by mixing the metal hydrosol together with a toluene solution of dodecylamine. Prolonged stirring only produced a milky mixture of metal hydrosol and toluene, but no particle transfer took place after the mixture was settled down into two immiscible layers in a separating funnel. The metal hydrosol would retain its characteristic color and the toluene layer was completely colorless. As the exchange between alkylamine and citrate ions could only occur at the interface between water and toluene, the failure to transfer the nanoparticles was the result of poor contact between the two phases because of their lack of mutual solubility. Based on this consideration, ethanol, which is water-miscible and a good solvent for dodecylamine, was used in lieu of toluene to increase the inter-
Fig. 1. TEM image and size distribution of alkylamine-stabilized Ru nanoparticles, d = 3.45 nm, σ = 0.68 nm.
facial contact between citrate-stabilized metal nanoparticles and alkylamine. After the ethanol–metal hydrosol mixture was stirred for about 2 min, the initially transparent hydrosol turned turbid, and a deep brown-colored liquid began to appear as suspending droplets near the top of the mixture or adhered to the container walls. This indicated that dodecylamine had displaced citrate from the surface of the metal nanoparticles. The brown-colored liquid could be easily extracted into toluene by adding toluene and stirring the mixture briefly. Figs. 1, 2, and 3 show the TEM images of Ru, Pt, and Au nanoparticles in toluene, respectively. The histogram of particle size distribution, together with the average particle size and the standard deviation is shown below each micrograph in these figures. The particle size distribution was fairly narrow, as shown by the histograms. The standard deviations for Ru, Pt, and Au nanoparticles were 0.68, 0.86, and 0.73 nm, respectively, or 10–20% of the average particle size. The larger size of the Au nanoparticles after phase transfer compared to Pt and Ru nanoparticles could be traced
J. Yang et al. / Journal of Colloid and Interface Science 277 (2004) 95–99
Fig. 2. TEM image and size distribution of alkylamine-stabilized Pt nanoparticles, d = 4.43 nm, σ = 0.86 nm.
to the large particles initially formed in the Au hydrosol (the average particle size of Au, Pt, and Ru in the hydrosol is 5.0, 3.5, and 2.2 nm, respectively, data not shown). The slight growth in the particle size after the phase transfer was most probably caused by particle agglomeration, which occurred during the stabilizer exchange, where citrate was progressively displaced to form the dodecylamine-stabilized metal nanoparticles. The self-assembly of alkylamine-stabilized Pt and Au nanoparticles is also evident from Figs. 2 and 3. The center-to-center nearest-neighbor distance between the particles was nearly constant, at ca. 7.8 nm for platinum (Fig. 2) and at ca. 10 nm for gold (Fig. 3). The yields of the metal nanoparticles were estimated as follows: the toluene solution of metallic nanoparticles was concentrated to 0.5 ml using flowing Ar. A 10-ml volume of ethanol was then added and the mixture was stored at −20 ◦ C for 24 h to precipitate the metal nanoparticles. The product nanoparticles were recovered by centrifugation and washed with ethanol several times to remove nonspecifically-bonded 1-dodecanethiol or dodecylamine. The
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Fig. 3. TEM image and size distribution of alkylamine-stabilized Au nanoparticles, d = 7.89 nm, σ = 0.73 nm.
nanoparticles were then dried at room temperature in vacuum. The yields of all alkylamine-stabilized metal nanoparticles were estimated to be more than 90%. The losses were likely caused by centrifugation and nanoparticle attachment to the container walls. On the other hand, the actual yields could also be lower due to surface oxidation of the metal particles during drying and the residual presence of dodecylamine; both of these would add to the measured product weights. For platinum and gold, the ethanol-mediated phasetransfer method could also be extended to the preparation of alkanethiol-stabilized metal nanoparticles. The detailed procedures followed nearly every step in the alkylaminestabilized Pt and Au nanoparticle preparation, except that 1-dodecanethiol was used instead of dodecylamine. The TEM images of alkanethiol-stabilized Pt and Au nanoparticles thus obtained are shown in Figs. 4 and 5. The selfassembly of the nanoparticles is again evident in these figures. On the other hand, the alkanethiol stabilization of Ru nanoparticles was not as successful by this method.
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Fig. 4. TEM image of alkanethiol-stabilized Pt nanoparticles.
Scheme 1. The proposed process of citrate displacement from nanoparticle surface by −SH (or −NH2 ). (!): citrate ions; (P): SH– (or NH2 –).
Fig. 5. TEM image of alkanethiol-stabilized Au nanoparticles.
These experimental findings show that amine or thiol could displace citrate ions from the surface of metal nanoparticles in a hydrosol provided that there was close contact between the amine molecules and the metal nanoparticles. Experimentally the citrate-stabilized metal nanoparticles (Ru, Pt, Au) could not be redispersed in water after several rounds of centrifugation. This could be easily explained by the progressive loss of the citrate ions as fresh solvent (water) was used in each redispersion attempt. The need to reestablish equilibrium between free and adsorbed citrate ions would slowly but eventually deplete the adsorbed citrate ions to a level inadequate to maintain the particles in suspension. The process of displacing citrate from the metal nanoparticle surface by dodecylamine or 1-dodecanethiol can be depicted as in Scheme 1. There are four steps in this process. Step A is representative of adsorbed citrate ions in equilibrium with their surroundings. The progressive displacement of citrate by SH– (or NH2 –) groups is depicted in steps B through D, where it
is inherently assumed that the binding of SH or NH to the metal nanoparticle surface is more irreversible than the adsorptive interaction between the citrate ions and the metal surface. The failure to transfer citrate-stabilized metal nanoparticles directly from the hydrosol to the toluene solution of dodecylamine (or 1-dodecanethiol) could also be understood from simple adsorption principles. Dissolved dodecylamine (or 1-dodecanethiol) in toluene was unable to exchange sufficiently with the citrate ions because of inadequate contact between the metal nanoparticles and these molecules. The preparation of thiolated oligonucleotide-functionalized Au nanoparticles from initially citrate-stabilized Au nanoparticles [7–13] can also be understood likewise. The transfer method described here can also be used to transfer Au, Pt, and Ru nanoparticles prepared by the NaBH4 reduction of metal precursors without a stabilizer (data not shown). In comparison with the procedure of Sarathy et al. [2,3], where concentrated HCl was used to facilitate the nanoparticle transfer, the current method is a gentler approach. It offers ease of operation and is compatible with the transfer of metals, which are reactive towards concentrated HCl (e.g., Ru).
4. Conclusions A highly efficient phase-transfer method was developed here to transfer citrate-stabilized Ru, Pt, and Au nanoparticles from an aqueous solution to a hydrocarbon layer (toluene). The method relies on the use of ethanol as a mediator to provide and maintain adequate contact between dodecylamine and metal nanoparticles during the transfer process. The efficiency of the transfer was nearly 100% un-
J. Yang et al. / Journal of Colloid and Interface Science 277 (2004) 95–99
der the experimental conditions. The particle size distributions of dodecylamine-stabilized Ru, Pt, and Au nanoparticles thus obtained were fairly narrow, with average particle sizes 3.45, 4.43, and 7.89 nm for the three noble metals, respectively. Alkanethiol-stabilized Pt and Au nanoparticles could similarly be prepared. TEM examinations also revealed the self-assembly of alkylamine- and alkanethiolstabilized platinum and gold nanoparticles on the TEM grids. Acknowledgments The authors acknowledge general financial support from the Singapore–MIT Alliance. Y.J. acknowledges the National University of Singapore for his research scholarship. References [1] G. Viau, R. Brayner, L. Poul, N. Chakroune, E. Lacaze, F. FievetVincent, F. Fievet, Chem. Mater. 15 (2003) 486.
99
[2] K.V. Sarathy, G.U. Kulkarni, C.N.R. Rao, Chem. Commun. (1997) 537. [3] K.V. Sarathy, G. Raina, R.T. Yadav, G.U. Kulkarni, C.N.R. Rao, J. Phys. Chem. B 101 (1997) 9876. [4] X. Fu, Y. Wang, N. Wu, L. Gui, Y. Tang, J. Colloid Interface Sci. 243 (2001) 326. [5] S.-Y. Zhao, S.-H. Chen, S.-Y. Wang, D.-G. Li, H.-Y. Ma, Langmuir 18 (2002) 3315. [6] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) 801. [7] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [8] J.J. Storhoff, R. Elghanian, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 1959. [9] R.C. Mucic, J.J. Storhoff, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 12,674. [10] L. He, M.D. Musick, S.R. Nicewarner, F.G. Salinas, S.J. Benkovic, M.J. Natan, C.D. Keating, J. Am. Chem. Soc. 122 (2000) 9071. [11] S.-J. Park, A.A. Lazarides, C.A. Mirkin, P.W. Brazis, C.R. Kannewurf, R.L. Letsinger, Angew. Chem. Int. Ed. 39 (2000) 3845. [12] J.J. Storhoff, A.A. Lazarides, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, G.C. Schatz, J. Am. Chem. Soc. 122 (2000) 4640. [13] J.J. Storhoff, R. Elghanian, C.A. Mirkin, R.L. Letsinger, Langmuir 18 (2002) 6666.
A highly efficient phase transfer method for preparing alkylamine-stabilized Ru, Pt, and Au nanoparticles J. Yang a , Jim Yang Lee a,b,∗ , T.C. Deivaraj b , Heng-Phon Too b,c a Department of Chemical and Bimolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260 b Singapore-MIT Alliance, 4 Engineering Drive 3, National University of Singapore, Singapore 117576 c Department of Biochemistry, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260
Received 6 October 2003; accepted 14 April 2004 Available online 18 May 2004
Abstract A highly efficient phase-transfer method was developed to prepare alkylamine-stabilized nanoparticles of several noble metals. This method involved first mixing the metal hydrosols and an ethanol solution of dodecylamine and then extracting the dodecylamine-stabilized metal nanoparticles into toluene. The efficiency of this phase-transfer method was nearly 100%. Alkylamine-stabilized Ru, Pt, and Au nanoparticles 3.45, 4.33, and 7.89 nm in diameter, respectively, could be prepared this way. The self-assembly of dodecylamine-stabilized Pt and Au particles was also detected by transmission electron microscopy (TEM). 2004 Elsevier Inc. All rights reserved. Keywords: Phase transfer; Alkylamine; Metal nanoparticles; Self-assembly
1. Introduction The phase-transfer method is commonly used to prepare organosols of metals including ruthenium, platinum, and gold [1–6]. The metal nanoparticles need to be stabilized and alkanethiols are often used for that purpose [2,3,5]. Initially, the common practice was to first extract the metal ions from an aqueous solution to a hydrocarbon (toluene) layer with the help of a phase-transfer agent such as tetraoctylammonium bromide and then to carry out the reduction with NaBH4 in the presence of an alkanethiol. Here the nucleation and growth of the metal particles and the attachment of the thiol molecules to the nanoparticles would occur simultaneously in a single step. More recently, Sarathy et al. [2,3] and Zhao et al. [5] adopted a different approach in which metal nanoparticles, instead of metal ions, were directly transferred to the organic phase for thiolation. A metal hydrosol was prepared in advance using NaBH4 as the reducing agent and mixed with a toluene solution of 1-dodecanethiol. Concentrated HCl was then added to the biphasic mixture under strong agitation to enable the transfer. The thiol* Corresponding author. Fax: +65-6779-1936.
E-mail address: [email protected] (J.Y. Lee). 0021-9797/$ – see front matter 2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.03.074
stabilized metal nanoparticles prepared this way had uniform size and often self-assembled on the TEM grid. The phase-transfer technique was also used by Fu et al. [4] to prepare alkylamine-stabilized Ru nanoparticles. In that work, Ru nanoparticles were first prepared in a liquid polyol and then extracted into toluene by mixing the Ru sol with a toluene solution of dodecylamine. Viau et al. [1] extended the procedure to prepare thiol-stabilized Ru nanoparticles by replacing dodecylamine with 1-dodecanethiol. The phase transfer in this case was not complete and there were many Ru particles left in the polyol phase after the transfer. To the best of our knowledge, there is currently no reported work on transferring ruthenium nanoparticles from an aqueous environment to a hydrocarbon layer. In addition, the procedures described in the current literature could not be used to transfer citrate-stabilized metal nanoparticles, which are often used as precursors for further assembly [7–13] into a hydrocarbon layer. We herein report a simple method, which can be used to transfer citrate-stabilized Ru, Pt, and Au nanoparticles from an aqueous solution to toluene with high efficiency (∼100%). This method is primarily based on the formation of uniform alkylamine-stabilized metal nanoparticles of Ru, Pt, and Au through a stabilizer exchange process.
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2. Materials and methods RuCl3 ·3H2 O, H2 PtCl6 ·6H2 O, HAuCl4 ·3H2 O, and dodecylamine (98%) purchased from Aldrich, sodium borohydride (98%) from Fluka, sodium citrate (98%) and ethanol (99%) from Merck, and toluene from J.T. Baker, Inc., were used as received. Deionized water was distilled by a Milli-Q water purification system. All glassware and Teflon-coated magnetic stir bars were cleaned with aqua regia, followed by copious rinsing with distilled water before drying in an oven. In a typical experiment, 0.2 ml of 38.8 mM aqueous sodium citrate solution was added to 10 ml of 2 mM aqueous RuCl3 solution. Under vigorous stirring, 1.5 ml of 112 mM aqueous NaBH4 solution was introduced dropwise to prepare a Ru hydrosol in which sodium citrate served as the stabilizer. The molar ratio of NaBH4 to RuCl3 was kept above 8 to ensure the reduction of Ru to the zerovalent state. The Ru hydrosol was also left to stand for 4 h after the addition to complete the reduction reaction. The hydrosol was then mixed with 10 ml of ethanol containing 100 µl of dodecylamine and the mixture was stirred for 2 min. A 5-ml volume of toluene was added and stirring continued for three more minutes. Dodecylamine-stabilized Ru nanoparticles were extracted into the toluene layer rapidly, leaving behind a colorless aqueous solution. The preparations of dodecylamine-stabilized Pt and Au nanoparticles followed the same procedures, except that RuCl3 was replaced by H2 PtCl6 and HAuCl4 , respectively. A JEOL JEM2010 microscope was used to obtain TEM images of the nanoparticles. For TEM measurements a drop of the nanoparticle solution was placed on a 3-mm copper grid covered with a continuous carbon film. Excess solution was imbibed with an adsorbent paper. The average particle size and particle size distribution were obtained from a few randomly chosen areas in the TEM image containing approximately 200 nanoparticles each.
3. Results and discussion Citrate-stabilized metal nanoparticles (Ru, Pt, and Au) could not be transferred directly to toluene by mixing the metal hydrosol together with a toluene solution of dodecylamine. Prolonged stirring only produced a milky mixture of metal hydrosol and toluene, but no particle transfer took place after the mixture was settled down into two immiscible layers in a separating funnel. The metal hydrosol would retain its characteristic color and the toluene layer was completely colorless. As the exchange between alkylamine and citrate ions could only occur at the interface between water and toluene, the failure to transfer the nanoparticles was the result of poor contact between the two phases because of their lack of mutual solubility. Based on this consideration, ethanol, which is water-miscible and a good solvent for dodecylamine, was used in lieu of toluene to increase the inter-
Fig. 1. TEM image and size distribution of alkylamine-stabilized Ru nanoparticles, d = 3.45 nm, σ = 0.68 nm.
facial contact between citrate-stabilized metal nanoparticles and alkylamine. After the ethanol–metal hydrosol mixture was stirred for about 2 min, the initially transparent hydrosol turned turbid, and a deep brown-colored liquid began to appear as suspending droplets near the top of the mixture or adhered to the container walls. This indicated that dodecylamine had displaced citrate from the surface of the metal nanoparticles. The brown-colored liquid could be easily extracted into toluene by adding toluene and stirring the mixture briefly. Figs. 1, 2, and 3 show the TEM images of Ru, Pt, and Au nanoparticles in toluene, respectively. The histogram of particle size distribution, together with the average particle size and the standard deviation is shown below each micrograph in these figures. The particle size distribution was fairly narrow, as shown by the histograms. The standard deviations for Ru, Pt, and Au nanoparticles were 0.68, 0.86, and 0.73 nm, respectively, or 10–20% of the average particle size. The larger size of the Au nanoparticles after phase transfer compared to Pt and Ru nanoparticles could be traced
J. Yang et al. / Journal of Colloid and Interface Science 277 (2004) 95–99
Fig. 2. TEM image and size distribution of alkylamine-stabilized Pt nanoparticles, d = 4.43 nm, σ = 0.86 nm.
to the large particles initially formed in the Au hydrosol (the average particle size of Au, Pt, and Ru in the hydrosol is 5.0, 3.5, and 2.2 nm, respectively, data not shown). The slight growth in the particle size after the phase transfer was most probably caused by particle agglomeration, which occurred during the stabilizer exchange, where citrate was progressively displaced to form the dodecylamine-stabilized metal nanoparticles. The self-assembly of alkylamine-stabilized Pt and Au nanoparticles is also evident from Figs. 2 and 3. The center-to-center nearest-neighbor distance between the particles was nearly constant, at ca. 7.8 nm for platinum (Fig. 2) and at ca. 10 nm for gold (Fig. 3). The yields of the metal nanoparticles were estimated as follows: the toluene solution of metallic nanoparticles was concentrated to 0.5 ml using flowing Ar. A 10-ml volume of ethanol was then added and the mixture was stored at −20 ◦ C for 24 h to precipitate the metal nanoparticles. The product nanoparticles were recovered by centrifugation and washed with ethanol several times to remove nonspecifically-bonded 1-dodecanethiol or dodecylamine. The
97
Fig. 3. TEM image and size distribution of alkylamine-stabilized Au nanoparticles, d = 7.89 nm, σ = 0.73 nm.
nanoparticles were then dried at room temperature in vacuum. The yields of all alkylamine-stabilized metal nanoparticles were estimated to be more than 90%. The losses were likely caused by centrifugation and nanoparticle attachment to the container walls. On the other hand, the actual yields could also be lower due to surface oxidation of the metal particles during drying and the residual presence of dodecylamine; both of these would add to the measured product weights. For platinum and gold, the ethanol-mediated phasetransfer method could also be extended to the preparation of alkanethiol-stabilized metal nanoparticles. The detailed procedures followed nearly every step in the alkylaminestabilized Pt and Au nanoparticle preparation, except that 1-dodecanethiol was used instead of dodecylamine. The TEM images of alkanethiol-stabilized Pt and Au nanoparticles thus obtained are shown in Figs. 4 and 5. The selfassembly of the nanoparticles is again evident in these figures. On the other hand, the alkanethiol stabilization of Ru nanoparticles was not as successful by this method.
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Fig. 4. TEM image of alkanethiol-stabilized Pt nanoparticles.
Scheme 1. The proposed process of citrate displacement from nanoparticle surface by −SH (or −NH2 ). (!): citrate ions; (P): SH– (or NH2 –).
Fig. 5. TEM image of alkanethiol-stabilized Au nanoparticles.
These experimental findings show that amine or thiol could displace citrate ions from the surface of metal nanoparticles in a hydrosol provided that there was close contact between the amine molecules and the metal nanoparticles. Experimentally the citrate-stabilized metal nanoparticles (Ru, Pt, Au) could not be redispersed in water after several rounds of centrifugation. This could be easily explained by the progressive loss of the citrate ions as fresh solvent (water) was used in each redispersion attempt. The need to reestablish equilibrium between free and adsorbed citrate ions would slowly but eventually deplete the adsorbed citrate ions to a level inadequate to maintain the particles in suspension. The process of displacing citrate from the metal nanoparticle surface by dodecylamine or 1-dodecanethiol can be depicted as in Scheme 1. There are four steps in this process. Step A is representative of adsorbed citrate ions in equilibrium with their surroundings. The progressive displacement of citrate by SH– (or NH2 –) groups is depicted in steps B through D, where it
is inherently assumed that the binding of SH or NH to the metal nanoparticle surface is more irreversible than the adsorptive interaction between the citrate ions and the metal surface. The failure to transfer citrate-stabilized metal nanoparticles directly from the hydrosol to the toluene solution of dodecylamine (or 1-dodecanethiol) could also be understood from simple adsorption principles. Dissolved dodecylamine (or 1-dodecanethiol) in toluene was unable to exchange sufficiently with the citrate ions because of inadequate contact between the metal nanoparticles and these molecules. The preparation of thiolated oligonucleotide-functionalized Au nanoparticles from initially citrate-stabilized Au nanoparticles [7–13] can also be understood likewise. The transfer method described here can also be used to transfer Au, Pt, and Ru nanoparticles prepared by the NaBH4 reduction of metal precursors without a stabilizer (data not shown). In comparison with the procedure of Sarathy et al. [2,3], where concentrated HCl was used to facilitate the nanoparticle transfer, the current method is a gentler approach. It offers ease of operation and is compatible with the transfer of metals, which are reactive towards concentrated HCl (e.g., Ru).
4. Conclusions A highly efficient phase-transfer method was developed here to transfer citrate-stabilized Ru, Pt, and Au nanoparticles from an aqueous solution to a hydrocarbon layer (toluene). The method relies on the use of ethanol as a mediator to provide and maintain adequate contact between dodecylamine and metal nanoparticles during the transfer process. The efficiency of the transfer was nearly 100% un-
J. Yang et al. / Journal of Colloid and Interface Science 277 (2004) 95–99
der the experimental conditions. The particle size distributions of dodecylamine-stabilized Ru, Pt, and Au nanoparticles thus obtained were fairly narrow, with average particle sizes 3.45, 4.43, and 7.89 nm for the three noble metals, respectively. Alkanethiol-stabilized Pt and Au nanoparticles could similarly be prepared. TEM examinations also revealed the self-assembly of alkylamine- and alkanethiolstabilized platinum and gold nanoparticles on the TEM grids. Acknowledgments The authors acknowledge general financial support from the Singapore–MIT Alliance. Y.J. acknowledges the National University of Singapore for his research scholarship. References [1] G. Viau, R. Brayner, L. Poul, N. Chakroune, E. Lacaze, F. FievetVincent, F. Fievet, Chem. Mater. 15 (2003) 486.
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[2] K.V. Sarathy, G.U. Kulkarni, C.N.R. Rao, Chem. Commun. (1997) 537. [3] K.V. Sarathy, G. Raina, R.T. Yadav, G.U. Kulkarni, C.N.R. Rao, J. Phys. Chem. B 101 (1997) 9876. [4] X. Fu, Y. Wang, N. Wu, L. Gui, Y. Tang, J. Colloid Interface Sci. 243 (2001) 326. [5] S.-Y. Zhao, S.-H. Chen, S.-Y. Wang, D.-G. Li, H.-Y. Ma, Langmuir 18 (2002) 3315. [6] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) 801. [7] C.A. Mirkin, R.L. Letsinger, R.C. Mucic, J.J. Storhoff, Nature 382 (1996) 607. [8] J.J. Storhoff, R. Elghanian, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 1959. [9] R.C. Mucic, J.J. Storhoff, C.A. Mirkin, R.L. Letsinger, J. Am. Chem. Soc. 120 (1998) 12,674. [10] L. He, M.D. Musick, S.R. Nicewarner, F.G. Salinas, S.J. Benkovic, M.J. Natan, C.D. Keating, J. Am. Chem. Soc. 122 (2000) 9071. [11] S.-J. Park, A.A. Lazarides, C.A. Mirkin, P.W. Brazis, C.R. Kannewurf, R.L. Letsinger, Angew. Chem. Int. Ed. 39 (2000) 3845. [12] J.J. Storhoff, A.A. Lazarides, R.C. Mucic, C.A. Mirkin, R.L. Letsinger, G.C. Schatz, J. Am. Chem. Soc. 122 (2000) 4640. [13] J.J. Storhoff, R. Elghanian, C.A. Mirkin, R.L. Letsinger, Langmuir 18 (2002) 6666.