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Langmuir 2008, 24, 310-315
Gold Nanoparticles with Perfluorothiolate Ligands Amala Dass, Rui Guo,† Joseph B. Tracy,‡ Ramjee Balasubramanian,§ Alicia D. Douglas,| and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed August 28, 2007. In Final Form: October 15, 2007 Two syntheses of gold nanoparticles with fluorinated alkyl and aryl thiolate ligands are reported. The fluorous Au nanoparticles are smaller than previous gold fluor-capped examples, and are in the 44-75 Au atom size range. Fluoroalkyl thiolate-protected (1H,1H,2H,2H-perfluorodecanethiolate) nanoparticles synthesized by a Brust reaction are a mixture of (mainly) ∼8.5 kDa (ca. 44 core atoms) and ∼14 kDa (ca. 75 core atoms) species, by MALDI-mass spectrometry. This composition is consistent with thermogravimetric analysis (TGA) results of the ligand shell composition. 19F NMR spectra display a progressive line broadening of resonances for fluorine sites closer to the Au core. A second synthetic route used a (ligand replacement) reaction of pentafluorobenzenethiol with Au55(PPh3)12Cl6. The exchange is (as previously observed for nonfluorinated thiols) accompanied by nanoparticle core size changes to produce a polydisperse mixture within which a Au75 core species could be electrochemically discerned by its characteristic 0.74 V electrochemical energy gap. Further characterization of the polydisperse nanoparticle product was done by HPLC, TEM, TGA, optical spectroscopy, and NMR data. Both varieties of fluorous nanoparticles exhibit solubilities typical of perfluorinated materials, as opposed to proteo versions.
Introduction Gold monolayer-protected clusters (MPCs) with cores in the nanometer size regime have intriguing optical and electronic properties.1 A dramatic core-size evolutionary property of these nanoparticles has been observed,2 with a transition from bulklike to molecule-like behavior as the core size falls below ∼2 nm.3 The capping monolayers have also been varied; their influence on nanoparticle solubilities yields solubilities ranging from aqueous and nonaqueous (organic) phases to fluorous phases. Fluorous chemicals, where hydrogen is replaced with fluorine, generally have distinctively changed solubility propertiess differing from the usual common organic and aqueous phasess that can be useful in separation, purification, and synthetic chemistry.4,5 A number of capped nanoparticles4b,7-12 and selfassembled monolayers (SAM)6 on planar surfaces based on fluorous ligands or stabilizing coatings have been reported, including Au,10a Ag,7b Ir,7b Pt,7b Pd,10b and CdSe11 nanoparticles, * Corresponding author. E-mail:
[email protected]. † Present address: Wilson Greatbatch, Buffalo, NY. ‡ Present address: Department of Materials Science, North Carolina State University, Raleigh, NC. § Present address: Department of Chemistry, Old Dominium University, Norfork, VA. | Present address: CEM Corp., Matthews, NC. (1) (a) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293-346. (b) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27-36. (2) (a) Chen, S.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2099-2101. (b) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428-433. (c) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261-5270. (3) (a) Lee, D.; Donkers, R. L.; Wang, G.; Harper, A. S.; Murray, R. W. J. Am. Chem. Soc. 2004, 126, 6193-6199. (b) Price, R. C.; Whetten, R. L. J. Am. Chem. Soc. 2005, 127, 13750-13751. (c) Shichibu, Y.; Negishi, Y.; Tsukuda, T.; Teranishi, T. J. Am. Chem. Soc. 2005, 127, 13464-13465. (4) (a) Gladyz, J. A.; Curran, D. P.; Horvath, I. T. Handbook of Fluorous Chemistry; Wiley-VCH: Weinheim, Germany, 2004. (b) ref 4a, Chapter 12, pp 491-506. (5) Howe-Grant, M. Fluorine Chemistry: A ComprehensiVe Treatment; John Wiley, New York, 1995 (6) Barriet, D.; Lee, T. R. Curr. Opin. Colloid Interface Sci. 2003, 8, 236242.
carbon nanotubes, and ZnO nanorods.11 The nanoparticle products are sometimes soluble in fluorous and insoluble in nonfluorous solvents. Some have been prepared by reaction in supercritical CO2 (scCO2) media7,9 and others in fluorous solvents.11 The protecting ligands and coatings range from 1H,1H,2H,2Hperfluorodecanethiol to the fluorinated surfactant FOMBLIN to perfluoroamines,8-10 and generally the nanoparticle core diameters are several nanometers or larger. Exceptions are the small nanoparticles reported by Korgel et al.,7 who demonstrated a dependence of Ag nanoparticle sizes prepared in scCO2 on the concentrations of precursors7b and scCO2 density (e.g., CO2 pressure). Ag nanoparticles capped with 1H,1H,2H,2H-perfluorodecanethiol down to 2 nm diameter are produced at higher pressures. The only previous example of stabilization of Au nanoparticles with fluorous thiolate ligands is that of Yonezawa et al.8b The present study of Au nanoparticles protected by fluorous thiolate ligands was prompted by development of the synthetic, analytical, and property chemistry of Au nanoparticles protected (7) (a) Shah, P. S.; Holmes, J. D.; Doty, R. C.; Johnston, K. P.; Korge, B. A. J. Am. Chem. Soc. 2000, 122, 4245-4246. (b) Shah, P. S.; Husain, S.; Johnston, K. P.; Korge, B. A. J. Phys. Chem. B 2001, 105, 9433-9440. (c) Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2002, 106, 1217812185. (8) (a) Yonezawa, T.; Onoue, S-Y.; Kimizuka, N. AdV. Mater. 2001, 13, 140142. (b) Yonezawa, T.; Onoue, S-Y.; Kimizuka, N. Langmuir 2001, 17, 22912293 (9) (a) Kameo, A.; Yoshimura, T.; Esumi, K. Colloids Surf. A 2003, 215, 181-189. (b) Esumi, K.; Sarashina, S.; Yoshimura, T. Langmuir 2004, 20, 51895191. (10) (a) Moreno-Manas, M.; Pleixats, R.; Tristany, M. J. Fluorine Chem. 2005, 126, 1435-1438. (b) Tristany, M.; Courmarcel, J.; Dieudonne, P.; Moreno-Mans, M.; Pleixats, R.; Rimola, A.; Sodupe, M.; Villarroya, S. Chem. Mater. 2006, 128, 716-722. (c) Tristany, M.; Chaudret, B.; Dieudonne, P.; Guari, Y.; Lecante, P.; Matsura, V.; Moreno-Manas, M.; Philippot, K.; Pleixats, R. AdV. Funct. Mater. 2006, 16, 2008-2015. (11) Voggu, R.; Biswas, K.; Govindaraj, A.; Rao, C. N. R. J. Phys. Chem. B 2006, 110, 20752-20755. (12) (a) Wang, G.; Huang, T.; Murray, R. W.; Menard, L.; Nuzzo, R. G. J. Am. Chem. Soc. 2005, 127, 812-813.(b) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604-3612 (c) Hicks, J. F.; Miles, D. T.; Murray, R. W. J. Am. Chem. Soc. 2002, 124, 13322-13328. (d) Terrill, R. H.; Postlethwaite, T. A.; Chen, C-H.; Poon, C-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537-12548.
10.1021/la702651y CCC: $40.75 © 2008 American Chemical Society Published on Web 12/05/2007
Gold Nanoparticles with Perfluorothiolate Ligands
by monolayers of nonfluorous thiolate ligands.3,12,14-37 There has been an emphasis on careful analytical assessment of the composition and dimensions of these MPCs, in order that associated studies of MPC properties (solubility, electrochemistry, spectroscopy, stability, etc.) can lead to connections between Au core size, the chemical nature of the protecting monolayer, and MPC properties. While size is not unimportant, MPC solubilities are mostly controlled by their protecting monolayers, so much (13) Handbook of PreparatiVe Inorganic Chemistry; Brauer, G., Ed.; Academic Press: New York, 1965; p 1054. (14) (a) Schmid, G.; Pfeil, F.; Boese, R.; Bandermann, F.; Meyer, S.; Calis, G. H. M.; van der Velden, J. W. A. Chem. Ber. 1981, 114, 3634-3642. (b) Schmid, G. Inorg. Synth. 1990, 27, 214-218. (15) Brust, M.; Walker, M.; Bethell, D.; Schriffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801-802 (16) McNeal, C. J.; Winpenny, R. E. P.; Hughes, J. M.; Macfarlane, R. 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G.; White, R. J.; Harper, A. S.; Mills, A. J.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 6864-6870. (c) Franceschetti, A.; Zunger, A. Phys. ReV. B 2000, 62, 26142623. (19) (a) Schaaff, T. G. Rapid Commun. Mass Spectrom. 2003, 17, 25672570. (b) Schaaff, T. G. Anal. Chem. 2004, 76, 6187-6196. (20) (a) Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2003, 125, 4046-4047. (b) Negishi, Y.; Tsukuda, T. Chem. Phys. Lett. 2004, 383, 161-165. (c) Tsunoyama, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2006, 128, 6036-6037. (d) Tsunoyama, H.; Nickut, P.; Negishi, Y.; Al-Shamery, K.; Matsumoto, Y.; Tsukuda, T. J. Phys. Chem. C 2007, 111, 4153-4158. (21) (a) Schaaff, T. G.; Knight, G.; Shafigullin, M. N.; Borkman, R. F.; Whetten, R. L. J. Phys. Chem. B 1998, 102, 10643-10646. (b) Gutierrez, E.; Powell, R. D.; Furuya, F. R.; Hainfeld, J. F.; Schaff, T. G.; Shafigullin, M. N.; Stephens, P. W.; Whetten, R. L. Eur. Phys. J. D 1999, 9, 647-651. (22) (a) Schaaff, T. G.; Whetten, R. L. J. Phys. Chem. 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Am. Chem. Soc. 2003, 125, 18-19. (c) Lica, G. C.; Zelakiewicz, B. S.; Tong, Y. Y. J. Electroanal. Chem. 2003, 554-555, 127-132 (27) (a) Donkers, R. L.; Lee, D.; Murray, R. W. Langmuir 2004, 20, 19451952. (b) Song, Y.; Harper, A. S.; Murray, R. W. Langmuir 2005, 21, 54925500. (28) Frey, S.; Heister, K.; Zharnikov, M.; Grunze, M. Phys. Chem. Chem. Phys. 2000, 2, 1979-1987. (29) Guo, R.; Murray, R. W. J. Am. Chem. Soc. 2005, 127, 12140-12143. (30) (a) Warner, M. G.; Reed, S. M.; Hutchison, J. E. Chem. Mater 2000, 12, 3316-3320. (b) Woehrle, G. H.; Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 2005, 127, 2172-2183. (c) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1997, 119, 12384-12385. (31) Hostetler, M. J.; Wingate, J. E.; Zhong, C. J.; Harris, J. E.; Vachet, R. W.; Clark, M. R.; Londono, J. D.; Green, S. J.; Stokes, J. J.; Wignall, G. D.; Glish, G. L.; Murray, R. W. Langmuir 1998, 14, 17-30. (32) Huang, T.; Murray, R. W. J. Phys. Chem. B 2001, 105, 12498-12502. (33) (a) Wei, G. T.; Liu, F. J. Chromatogr. A 1999, 836, 253-260. (b) Wei, G.; Liu, F.; Wang, C. R. C. Anal. Chem. 1999, 71, 2085-2091. (c) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. Langmuir 2000, 16, 9912-9920. (d) Wilcoxon, J. P.; Martin, J. E.; Provencio, P. J. Chem. Phys. 2001, 115, 998-1008. (e) Siebrands, T.; Giersig, M.; Mulvaney, P.; Fischer, C. -H. Langmuir 1993, 9, 2297-2300. (f) Fischer, C.-H.; Weller, H.; Katsikas, L.; Henglein, A. Langmuir 1989, 5, 429-432. (34) Bos, W.; Steggerda, J. J.; Yan, S.; Casalnuovo, J. A.; Mueting, A. M.; Pignolet, L. H. Inorg. Chem. 1988, 27, 948-951.
Langmuir, Vol. 24, No. 1, 2008 311
so that chromatographic separations can be designed on the basis of solvation concepts.36 This report deals with preparation of very small MPCs protected by fluorous thiolate monolayers and with extending the analytical characterization of fluorous nanoparticles. MPCs bearing monolayers of fluorous thiolate ligands were prepared by two synthetic approaches, both using highly fluorinated or perfluorothiols. One synthesis using the Brust reaction15 and a fluoroalkyl thiol (1H,1H,2H,2H-perfluorodecanethiolate) produced a mixture of nanoparticle sizes within which Au cores of ∼8.5 kDa (ca. 44 core atoms) and ∼14 kDa (ca. 75 core atoms) mass are discernibly present, using MALDImass spectrometry, TEM, TGA, and XPS. The 19F NMR spectra display a progressive line broadening of resonances for fluorine sites closer to the Au core, analogous to that known25-27 for proton spectra. The second synthesis uses the small nanoparticle Au55(PPh3)12Cl6 as a template for ligand exchange with pentafluorobenzenethiol, as in a previous18a case for a nonfluorous thiol. The exchange yields a mixture of nanoparticle sizes but with a sufficient content of a 75 Au atom core MPC to be electrochemically detected by its energy gap (0.74 V). This MPC mixture was also examined by a range of analytical methodologies. Experimental Section Chemicals. Pentafluorothiophenol (Aldrich, 97%), triphenylphosphine (Aldrich, 99%), boron trifluoride diethyletherate (Aldrich), trifluorotoluene(Aldrich), tetraoctylammonium bromide (Oct4NBr, Aldrich), 2-(4-hydroxyphenylazo)benzoic acid (HABA) (Aldrich), trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) (Aldrich), Celite (Aldrich, high purity analytical grade), 1H,1H,2H,2H-perfluorodecanethiol (Fluka, g99.0%), tetrabutylammonium perchlorate (Bu4NClO4, Fluka, g99%), perfluoro(2-butyltetrahydrofuran) (Oakwood products), acetonitrile (Fisher, g99.9%), Solkane 365mfc (1,1,1,3,3-pentafluorobutane, Micro Care Corp., New Britain, CT), and dichloromethane (Fisher, 99.9%) were used as received. HAuCl4·3H2O was synthesized according to literature procedures.13 Schmid’s protocol14 was used for the synthesis of Au55(PPh3)12Cl6. Au MPC Synthesis. Synthesis by Brust Reaction. One millimole of each HAuCl4 and tetraoctylammonium bromide (TOABr) was codissolved in 200 mL of trifluorotoluene, 3 mmol of 1H,1H,2H,2Hperfluorodecane thiol was added, and the reaction was stirred for 10 min as the deep red Au(III) salt becomes reduced to a colorless Au(I) species. The mixture was cooled in an ice bath to 0 °C (30 min) and 10 mmol of NaBH4 dissolved in 20 mL of ice-cooled NANOpure water was added and the reaction stirred for 3 h in an ice bath. The reaction mixture was washed three times with 150 mL portions of water and the organic phase removed by rotary evaporation at room temperature. The product was covered with 20 mL of toluene, sonicated, and centrifuged to recover the black nanoparticles; this step was repeated until no Oct4NBr or disulfide was observed in the toluene wash liquid (1H NMR). The nanoparticles were soluble in hydrofluorocarbons like trifluorotoluene and Solkane 365 mfc and in fluorocarbon solvents like perfluorohexane and perfluorobenzene and were insoluble in toluene, acetone, CH2Cl2, and liquid CO2. Synthesis by Ligand Exchange. In a typical reaction, 7 mg of Au55(PPh3)12Cl6 in 2 mL of CH2Cl2 was mixed with 3.7 µL of pentafluorobenzenethiol (a 3:1 thiol/ligand ratio) and stirred for ∼22 h. After removing the CH2Cl2 solvent, the exchange product was washed copiously with methanol, and the reaction product (∼4-5 (35) Choi, M. M. F.; Douglas, A. D.; Murray, R. W. Anal. Chem. 2006, 78, 2779-2785. (36) (a) Jimenez, V. L.; Leopold, M. C.; Mazzitelli, C.; Jorgenson, J. W.; Murray, R. W. Anal. Chem. 2003, 75, 199-206. (b) Song, Y.; Jimenez, V. L.; Mckinney, C.; Donkers, R. L.; Murray, R. W. Anal. Chem. 2003, 75, 5088-5096. (c) Song, Y.; Heien, M. L.; Jimenez, V.; Wightman, R. M.; Murray, R. W. Anal. Chem. 2004, 76, 4911-4919. (37) Tracy, J. B.; Kalyuzhny, G.; Crowe, M. C.; Balasubramanian, R.; Choi, J.-P.; Murray, R. W. J. Am. Chem. Soc. 2007, 129, 6706-6707.
312 Langmuir, Vol. 24, No. 1, 2008 mg) redissolved in 2 mL of CH2Cl2 was rereacted with an additional 5 µL of pentafluorobenzenethiol for another 22 h. Removing the CH2Cl2 solvent, the final exchange product (∼2 mg) was recovered by the addition of methanol, precipitating the black product, which was thoroughly washed with further methanol. The final product was soluble in CH2Cl2, THF, and DMF and insoluble in the fluorous solvent perfluoro(2-butyltetrahydrofuran). Measurements. Mass Spectrometry. MALDI mass spectra were collected on a Voyager DE Biospectrometry Workstation (Perspective Biosystems, Inc., Framingham, MA) in the linear mode using a nitrogen laser (337 nm). 2-(4-Hydroxyphenylazo)benzoic acid (HABA) and trans-2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB) were used as MALDI matrices. Spectra were obtained in negative ion mode using an acceleration voltage of 25 kV and a delay time of 350 ns. Electrochemistry. Cyclic voltammetry and Osteryoung square wave voltammetry (OSWV) of Au MPCs were performed using a Bioanalytical Systems (BAS) Model 100B at 11 °C in 0.1 M Bu4NClO4/CH2Cl2. The 1.6 mm diameter Pt working electrode was polished, rinsed, and sonicated in NANOpure water, rinsed with absolute ethanol and acetone, and cleaned by potential-cycling in 0.5 M H2SO4 for 15 min. A Pt coil counter electrode and Ag wire quasireference electrode were used. HPLC. Separations were done with an instrument equipped with a Waters 600 controller pump, a photodiode array detector (Waters 996 PDA, detected at 400 nm), a Rheodyne 7725 injection valve with a 50 µL sample loop, and a fluorophase WP stainless steel column (250 × 4.6 mm i.d. column, 5 µm particle size, 300 Å pore size, Thermo Electron Corp.). The mobile phase was 90% CH2Cl2 (10 mM Bu4NClO4)/10% CH3CN at a flow rate of 0.7 mL/min. The sample was filtered through a 0.45 µm Nalgene syringe filter with Teflon membrane prior to injection. NMR. 1H and 19F NMR spectra of Au MPCs and free thiols were collected with Bruker AC 400 MHz, Bruker AC 500 MHz, and Varian 600 MHz spectrometers. Hexafluorobenzene and methylene chloride-d2 were used as solvents for spectra of the Brust synthesis and ligand exchange products, respectively. A coaxial tube insert with acetone-d6 was used for lock purposes when hexafluorobenzene was used. 19F NMR was referenced to CFCl3. UV-Vis spectra of Au MPCs were taken using a Shimadzu UV1601 UV-visible spectrophotometer. ThermograVimetric analysis (TGA) was carried out in a PerkinElmer Pyris 1 thermogravimetric analyzer with 2-3 mg of MPCs in an Al pan at a heating rate of 10 °C/min. Transmission electron microscopy phase contrast images were obtained with a side-entry Phillips CM 12 microscope operating at 120 keV of Au MPCs prepared by spreading a droplet of diluted MPC solution (∼1 mg/10 mL CH2Cl2) and drying in air for 20 min on standard carbon-coated (20-30 nm) Formvar films on copper grids (400 mesh). XPS measurements were taken on a Kratos Axis Ultra X-ray photoemission spectrometer. Au MPCs were either placed as a powder on a Cu substrate or drop-cast from a CH2Cl2 solution onto a clean glass slide that was held on the sample holder with double-sided sticky tape. The X-ray target was Al and a charge neutralizer was used.
Results and Discussion Fluorous MPCs by Brust Reaction. In the two-phase Brust15 approach, trifluorotoluene was used as the solvent into which AuCl4- was phase-transferred and reacted first with the perfluorodecanethiol and then with borohydride reductant, producing a black solution of dissolved nanoparticles. (Initial experiments used CH2Cl2 as the solvent, but the formed nanoparticles precipitated, a circumstance that usually promotes size polydispersity.) The as-prepared nanoparticles were soluble in hydrofluorocarbons like trifluorotoluene and Solkane 365 mfc and in fluorocarbon solvents like perfluorohexane and perfluorobenzene but were insoluble in toluene, acetone, and CH2Cl2.
Dass et al.
Figure 1. MALDI-mass spectra (negative mode) of the fluorinated gold nanoparticle showing peaks at 8.5 kDa (∼Au44) and 14 kDa (∼Au75) in (a) DCTB matrix (inset shows peaks spaced ∼200 mass units denoting that it represents the Au core; a third peak around ∼10 kDa is observed); the individual peaks contain no further details (see Supporting Information Figure S-1) and (b) HABA matrices. No spectral signals were observed in the positive ion mode.
Residual phase transfer agent (Oct4NBr) in the nanoparticle product was removed by repeated ultrasonication of nanoparticle suspensions in tolueneswith recovery by centrifugation. Removal of the Oct4N+ surfactant gradually causes the purified nanoparticles to become insoluble in trifluorotoluene, although other fore-mentioned solubilities remained unchanged. Such a solubility change on purification has been noticed before.8 Mass Spectrometry. Mass spectrometry has been an important tool in identifying the core mass of Au nanoparticles, as seen in work with 252Cf-PDMS,16 LDI-MS,17-20 MALDI-MS,21 and ESI-MS.22,23,37 We used MALDI-MS, which although accompanied by extensive fragmentation effects, gives estimates of core counts of Au atoms.21,22,24 Negative ion mode MALDI-mass spectra (Figure 1) with HABA and DCTB matrices show maxima at ∼8.5 and ∼14 kDa masses, with the species giving the 8.5 kDa maximum being somewhat more populous. There is a hint of a third maximum at 10 kDa in the inset of Figure 1A. The spectral peaks obtained using the HABA matrix (Figure 1B) were broad and lacked fine structure, whereas those obtained in the (nonpolar, aprotic) DCTB matrix39 show a series of peaks spaced by an average of 210220 mass units (Figure 1A and inset), suggesting progressive losses of Au and S atoms. Simple progressive loss of perfluorodecane thiolate ligands (479 Da each) would give a much wider peak spacing, so the fluorous chains are apparently largely dissociated from the nanoparticle during the ionization event. Indeed, loss of alkyl chains by fragmentation is common in
Gold Nanoparticles with Perfluorothiolate Ligands
previous19,21,22,24 MS observations. The 8.5 kDa maximum observed in Figure 1A cannot be taken as representing the actual nanoparticle core present in the sample prior to ligand loss, given the series of Au and S loss peaks flanking it at higher mass. The loss peaks can be discerned to continue to about 9.2 kDa which would be consistent with a ∼44 Au atom core, which has been observed previously3b by MALDI-MS. (Also, previously observed Au38L24 nanoparticles have recently37 been reassigned as Au25L18.) The 10 and 14 kDa maxima are correspondingly likely to originate from ∼50 and ∼7518a,17c atom cores, respectively. ESI is for thiolate-protected nanoparticles a “softer” ionization method than MALDI and has produced spectra displaying molecular ions in a few cases.3b,23,37 In future experiments it may produce more definitive core compositions. Transmission Electron Microscopy (TEM) and Optical Spectroscopy. TEM of the perfluorodecanethiolate-protected MPCs adds little to the information gained by MALDI-MS. Images (see Supporting Information Figure S-3) confirm that a mixture of core sizes is present, but the indistinct contrast boundaries between the dark nanoparticle core and the lighter perfluorodecane thiolate/grid background make a TEM size determination not very useful. Optical absorbance spectra of thiolated gold nanoparticles exhibit a surface plasmon resonance band at 520 nm for core sizes >∼2 nm diameter, while core sizes 2 nm in diameter show a surface plasmon resonance band at ∼520 nm,31 which is damped in largely featureless spectra31 of 1.6 nm Au140 nanoparticles and evolves into step-like fine structure for smaller, molecule-like nanoparticles. The UV-vis spectrum of peak 5 shows a step-like fine structure, which is an indication of a very small, molecule-like nanoparticle. None of the UVvis spectra of peaks 1-4 show a distinct 520 surface plasmon resonance (peak 1 shows a broadened suggestion of one), suggesting that the nanoparticles all have core diameters < 2 nm. The spectrum of the previous18a (polydisperse) nonfluorous Au75 product was also featureless.
Figure 6. (a) Reversed-phase HPLC of polydisperse Au75(SC6F5)32. The mobile phase was 90% CH2Cl2 (10 mM Bu4NClO4)/10% CH3CN at a flow rate of 0.7 mL/min, and the stationary phase was a fluorophase WP stainless steel column. (b) Overlay of UV-vis spectra of peaks 1-5 from reversed-phase HPLC of polydisperse Au75(SC6F5)32.
of core sizes. On this basis, the current peaks in Figure 5 correspond to only about 10% of the nanoparticle sample. Correspondingly, the cyclic voltammetry of the pentafluorobenzenethiolate-protected nanoparticles does not show well-resolved current peaks (see Supporting Information Figure S-6). Thermogravimetric Analysis. TGA of the pentafluorobenzenethiolate exchange product (see Supporting Information Figure S-7) shows a two step, ∼30% weight loss, which, assuming a Au75 core, would correspond to a 32 pentafluorobenzenethiolate ligand monolayer shell, i.e., Au75(SC6F5)32.This is a rough estimate, given the polydispersity of the nanoparticle product according to the voltammetric results and TEM images (see Supporting Information Figure S-8). The previously reported (nonfluorous ligands)18a Au75 was of notably higher purity and its TGA indicated 40 ligands. The smaller number of ligands suggested by the present result is consistent with the greater steric bulkiness of the pentafluorobenzenethiolate ligands. XPS. Spectra revealed no phosphorus or chlorine content in the pentafluorobenzenethiolate-protected nanoparticles, which supports complete exchange of the original triphenylphosphine and chloride ligands.18a,30 Reversed-Phase HPLC and Optical Spectrosopy. Asprepared nanoparticles are usually polydisperse and require further purification in order to characterize their size-dependent properties, such as electrochemical and optical properties. Different chromatographic separations have been employed to separate polydisperse metal nanoparticles, such as size exclusion chromatography (SEC),33 ion-exchange chromatography (IEC),34 ionpair chromatography,35 and reversed-phase chromatography.36 Many parameters can affect the separation of the gold nanoparticles, such as core size, types of ligands, ligand heterogeneity, core charge state, and stationary and mobile phase compositions. Figure 6a shows the reversed-phase HPLC separation of the
Very weak luminescence was observed for the exchanged fluorous product (see Supporting Information Figure S-9). The solution was excited at 400 nm, where its absorbance was ca. 0.12. Usually, smaller core size nanoparticles with polar or watersoluble ligands show stronger luminescence intensity than large ones with relatively nonpolar organic soluble ligands;3,32 that does not appear to be the case with these nanoparticles. 1H and 19F NMR. 1H NMR of the parent Au (PPh ) Cl 55 3 12 6 shows only a broadened resonance at ca. 7 ppm attributable to the triphenylphosphine proton. This peak is absent in the spectrum of the ligand exchange product Au75(SC6F5)32, indicating that most or all of the initial triphenylphosphine ligands were replaced by the incoming pentafluorobenzene thiolate (see Supporting Information Figure S-10). The 19F NMR spectra of the exchange product Au75(SC6F5)32 were not of high quality, showing only a flattened peak at ca. -164 ppm, probably corresponding to the overlapped m- and p-fluorines. No resonance could be seen for the fluorine in the ortho-position on the ring (see Supporting Information Figure S-11).
Conclusions Gold nanoparticles with fluorinated alkyl and aryl thiolate ligands have been prepared by two routes. MALDI-MS, thermogravimetric analysis, and XPS results show that the Brust synthesis product is a mixture of 8.5 kDa (Au44) and 14 kDa (Au75) species. Ligand replacement reaction of pentafluorobenzenethiol with Au55(PPh3)12Cl6 gave a polydisperse mixture of MPCs that includes a Au75 core species, as signaled by its characteristic 0.74 V electrochemical energy gap. HPLC chromatography of this material gave five well-defined peaks and offers a potential pathway to isolation of purer samples of this perfluoro-MPC. Acknowledgment. This research was supported in part by grants from the National Science Foundation and the Office of Naval Research. Supporting Information Available: Analytical results ancillary to those presented above. This material is available free of charge via the Internet at http://pubs.acs.org. LA702651Y
