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Size-Dependent Solubility of Thiol-Derivatized Gold Nanoparticles in Supercritical Ethane Nicola Z. Clarke, Cecilia Waters, Kathleen A. Johnson,* John Satherley, and David J. Schiffrin Department of Chemistry, University of Liverpool, Liverpool, L69 7ZD, United Kingdom Received May 29, 2001. In Final Form: July 23, 2001 It is shown that alkane thiol-capped gold nanoparticles can be dissolved in supercritical ethane. Importantly, the solubility was found to be dependent on the core diameter thus allowing separation by size. Nanoparticles with a core diameter less than 1.7 nm were soluble in the supercritical fluid.
Introduction A central question in the use of capped nanoparticles for the construction of organized and crystalline superlattices is their preparation with a very narrow size distribution. Capped gold nanoparticles can be readily synthesized by the two-phase reduction of gold chloroaurate (AuCl4-) in toluene in the presence of alkane thiols.1 The material thus produced differs from other aqueous colloidal gold preparations2 by the presence of well-defined particle sizes corresponding to the “magic numbers” observed in gas-phase clusters.3 Monodisperse nanoparticles of diameter less than 2 nm clearly exhibit coulombic blockade and quantum size effects, even in solution,4 but the possibility of using these materials in electronic components requires, however, accurate separations by size. Fractionation of nanoparticles in solvent mixtures and by electrophoresis has been previously reported;5 other attempts to obtain single-sized dispersions have been based on the control of the synthesis conditions, but these methods lead to polydisperse materials. There has been recent interest in the preparation of nanoparticles in supercritical fluids,6 and it has been observed that fluorinated capped nanocrystals dispersed in scCO2 can provide appropriate media for synthesis.7 It is desirable to have a technique that, while relying on size-dependent solvation, allows a simple and accurate control of nanoparticle-solvent interactions; supercritical solvents appear to be ideally suited for this. Since these solvents can provide a high degree of control of solvation properties, their use in nanoparticle separations was * Corresponding author:
[email protected]. (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) Turkevich, J.; Stevenson, P. J.; Hillier, M. Discuss. Faraday Soc. 1951, 11, 55. (3) Knight, W. D.; Clemenger, K.; de Heer, W. A.; Saunders, W. A.; Chou, M. Y.; Cohen, M. L. Phys. Rev. Lett. 1984, 52, 2141. (4) (a) Chen, S.; Murray, R. W. J. Phys. Chem. B 1999, 103, 9996. (b) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaf, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (5) (a) Schaaf, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N. J. Phys. Chem. B 1997, 101, 7885. (b) Schaaf, T. G.; Whetten, R. L. J. Phys. Chem. B 2000, 104, 2630. (6) (a) Cason, J. P.; Khambaswadkar, K.; Roberts, C. B. Ind. Eng. Chem. Res. 2000, 39, 4749. (b) Cason, J. P.; Roberts, C. B. J. Phys. Chem. B 2000, 104, 1217. (c) Ohde, H.; Rodriguez, J. M.; Ye, X.-R.; Wai, C. M. J. Chem. Soc., Chem. Commun. 2000, 2353. (d) Dimitrijevic, N. M.; Bartels, D. M.; Jonah, C. D.; Takahashi, K.; Rajh, T. J. Phys. Chem. B 2001, 105, 954. (e) Sun, Y. P.; Guduru, R.; Lin, F.; Whiteside, T. Ind. Eng. Chem. Res. 2000, 39, 4663. (7) Shah, P. S.; Holmes, J. D.; Doty, R. C.; Johnston, K. P.; Korgel, B. A. J. Am. Chem. Soc. 2000, 122, 4245.
Figure 1. Schematic diagram of the apparatus employed for the separation of nanoparticles by supercritical extraction. E: Ethane gas cylinder (Linde Gas UK Limited); V1, V2, V3, V4, and V5: high-pressure valves used for pressure control in the extraction rig; P: screw-operated piston pump for pressurizing the rig; PG: pressure gauges; T1 and T2: temperaturecontrolled heating tape; PV: pressure vessel where the extraction was carried out (rated at 100 MPa maximum pressure); S: glass sample container; C: conical flask that received the expanded supercritical fluid containing the dissolved nanoparticles.
investigated in the present work. The strategy developed consisted in dissolving hydrophobic dodecane-thiol passivated nanoparticles in supercritical ethane inside a pressure vessel and measuring the properties of the extracted material. Experimental Section The samples employed were a 50:50 mixture of particles of average core diameter of 1.7 nm1 and those having an average core diameter of 5 nm prepared according to the procedure described by Maye and Zhong.8 The purpose of these experiments was to demonstrate a technique rather than achieve a detailed analysis of polydispersity in the samples obtained; the latter was outside the scope of this work. The equipment used in these experiments is schematically shown in Figure 1. Ethane was used as the supercritical fluid. It was introduced in the pressure vessel (volume ) 18.3 cm3) by means of the screw piston pump P. Ethane has a critical temperature of 305.4 K and a critical pressure of 4.884 MPa,9 and for this reason, the pressure line from the screw piston pump where liquid ethane was stored to the solubilization pressure vessel PV was kept at 313 K. The (8) Maye, M. M.; Zhong, C.-J. J. Mater. Chem. 2000, 10, 1895. (9) Lide, D. R.; Kehiaian, H. V. CRC Handbook of Thermophysical and Thermochemical Data; CRC Press: Boca Raton, FL, 1994.
10.1021/la010794a CCC: $20.00 © 2001 American Chemical Society Published on Web 08/29/2001
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Figure 2. UV-visible spectra of gold nanoparticles for (a) original sample, (b) residual fraction, and (c) fraction dissolved in the supercritical gas.
Figure 3. Time-of-flight mass spectroscopy results of nanoparticles extracted by supercritical ethane at 313 K and 17 MPa.
nanoparticle mixture (approximately 200 mg) was placed inside the pressure vessel in a small open glass container S. Pressure was applied to this vessel by first transferring ethane from the stock gas cylinder E to the screw piston pump while keeping the rest of the system isolated. This was achieved by cooling P with ice while keeping valves V1 and V2 opened and the screw piston fully extended. These valves were then closed, and ethane was allowed into the pressure vessel by opening V3 and V4. The pressure of ethane was increased by gently forcing gas into the pressure vessel using the screw-driven piston. When the pressure reached 17 MPa, the pressure vessel was isolated from the line by closing V3 and V4 and the sample of nanoparticles in it was allowed to equilibrate with the fluid for 12 h at a temperature of 313 K. At the end of the experiment, valve V5 was opened and the nanoparticles dissolved in the supercritical fluid were extracted from the pressure vessel by rapid expansion of the gas through a 5 cm layer of acetone contained in a 1 dm3 conical flask. Acetone (AnalaR grade) was used as the receiving phase since dodecanethiol passivated gold nanoparticles are insoluble in this solvent. Some of the material dissolved in the supercritical fluid also precipitated on the walls of the tube leading from the pressure vessel to the condensation conical flask during expansion. This was dissolved in small amounts of toluene, and the resulting solution was added to the contents of the conical flask, whereupon the nanoparticles in the toluene solution precipitated immediately. The whole precipitate was separated from the acetone phase and dissolved in toluene for characterization by UV-visible spectroscopy and mass spectroscopy using a commercial timeof-flight spectrometer (Micromass, TofSpect, U.K.).
selective extraction of alkane-thiol capped nanoparticles in supercritical ethane. Although this letter aims to show the feasibility of a new and powerful separation technique in nanotechnology, the origin of the dependence of solubility on size should be briefly and qualitatively considered. The solubility of the nanoparticles depends on the standard Gibbs free energy of solubilization in the supercritical fluid, which is a function of particle radius (R). For dilute solutions in the supercritical fluid, when particle-particle interactions are negligible, this quantity is the balance between particle-particle interactions in the solid, particle-solvent interactions in the solution, and solvent-solvent interactions, the latter two being strongly pressure dependent.12 For the reasons outlined below, we propose that the observed decrease in solubility with increasing radius is mainly related to interactions in the solid phase. For a conducting sphere of radius R, its polarizability (RS) is given by13
Results and Discussion A comparison of the spectra of the original material, the particles extracted by the supercritical fluid, and the final residual material is shown in Figure 2. The spectral characteristics of the original and residual samples are dominated by nanoparticles with a diameter greater than 4 nm, displaying a pronounced gold plasmon band at 520 nm.10,11 In contrast, this band is almost absent in the nanoparticles extracted by the supercritical solvent showing that their average size is less than 1.7 nm.5 These results show that significant differences of solubility in the supercritical fluid are observed, dependent on the core radius of the nanoparticles. Figure 3 shows the mass spectra of the extracted nanoparticles. A clear mass peak at 30 kDa is observed, corresponding to the cluster of 147 gold atoms. These results clearly demonstrate the size(10) Alvarez, M. M.; Khoury, J. T.; Schaaf, T. G.; Shafigullin, M. N.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3706. (11) Baum, T.; Bethell, D.; Brust, M.; Schiffrin, D. J. Langmuir 1999, 15, 866.
R S ) R3
(1)
The total polarizability of a thiol-capped nanoparticle also contains a contribution from the alkane chains, which scales with the total number of alkane groups present, that is, approximately with R2. The interaction energy in the solid due to the metal cores scales approximately with RS2,14 or with R6 if the cores are considered as perfectly conducting spheres. Therefore, as the larger size of the nanoparticles increases, the main contribution to the total interaction energy between particles in the solid is due to the cores. For small particles, the model is more complicated. As core size decreases, the relative contribution to the total polarizability by the alkane groups becomes more important compared with that of the core since the former scales with R2 whereas the latter scales with R3. In addition, for particles of sizes less than 1.7 nm, the free electron model breaks down5 and the cores exhibit discrete energy levels. Therefore, their polarizability is expected to decrease more rapidly with R compared with that (12) McHugh, M.; Krukonis, V. Supercritical Fluid Extraction: Principles and Practice, 2nd ed.; Butterworth-Heinemann Series in Chemical Engineering; Butterworth-Heinemann: Boston, 1994; Chapter 5, p 101. (13) Bottcher, C. J. F. Theory of Electric Polarisation; Elsevier Scientific Publishing: Amsterdam, 1973; p 86. (14) Atkins, P. W. Physical Chemistry, 6th ed.; Oxford University Press: Oxford, 1999; p 665.
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predicted from a free electron model, and for small nanoparticles, the particle-particle interactions in the solid are dominated by interactions between the alkyl chains and therefore solubility should increase for decreasing radius. Further work is underway to investigate and quantify these effects and the degree of monodispersity that can be achieved by the method described. The interest in the experiments described here is threefold. First, they demonstrate a technique that has the potential for the large-scale production of single-sized functionalized nanoparticles. Second, the many supercritical solvents that are currently available make this a very versatile method for the separation by size and
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functionality of different nanoparticulate materials. Finally, the fine-tuning of the properties of supercritical fluids that can easily and conveniently be achieved makes this a promising technique for the investigation of interactions between nanoparticles. Acknowledgment. Support by the EPSRC, U.K., for a quota studentship (N.C.) and for the purchase of a timeof-flight mass spectrometer (Grant No. GR/L 24441/01) is gratefully acknowledged. LA010794A
