Langmuir 2001, 17, 7735-7741
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Articles Unsymmetrical Disulfides and Thiol Mixtures Produce Different Mixed Monolayer-Protected Gold Clusters Young-Seok Shon,† Carolyn Mazzitelli, and Royce W. Murray* Kenan Laboratories of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290 Received August 22, 2001. In Final Form: October 3, 2001 Synthesis of mixed monolayer-protected Au clusters (MPCs) from unsymmetrical disulfides (HO(CH2)11SS(CH2)11CH3) yields clusters with a 1:1 composition of HO(CH2)11S- and CH3(CH2)11S- thiolates, whereas products from reaction of a 1:1 mixture of thiols (HO(CH2)11SH and CH3(CH2)11SH) contain mostly (97%) HO(CH2)11S- ligands in the monolayer. The results suggest a thermodynamically preferred ligation of HO(CH2)11SH over CH3(CH2)11SH that is circumvented in the disulfide reaction. The core sizes of MPCs synthesized from alkanethiol and dialkyl disulfides are similar, suggesting similar MPC growth dynamics for thiols and disulfides. The compositions of MPCs synthesized starting from gold(I)-thiolate polymer intermediates suggest that the polymer undergoes immediate breakup upon addition of reductant.
Monolayer-protected clusters (MPCs) consist of metal (or semiconductor) cores stabilized by a dense monolayer of ligands. The case of MPCs having Au cores and thiolate ligands is a three-dimensional analogue of self-assembled monolayers (SAMs) prepared by ligation to flat metal (usually Au) surfaces.1-4 Two-dimensional SAMs are formed from surface ligation reactions of alkanethiols, dialkyl disulfides, and alkyl sulfides.5,6 MPCs have important enabling features of stability in dried forms1 and of readily monitored synthetic variations of their monolayers. Our laboratory has regarded MPCs as novel spherical molecular entities and has studied the reaction chemistry of their monolayers.1,4 MPCs of sufficiently small dimensions exhibit properties that straddle the chemical and physics domains.7 The versatility of MPC properties suggests potential applications in catalysis,8 microelectronics,9,10 optics,11 magnetics,12 and chemical recognition.13 † Present address: Department of Chemistry, Western Kentucky University, Bowling Green, KY 42101.
(1) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (2) (a) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (b) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. J. Chem. Soc., Chem. Commun. 1995, 1655. (c) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (d) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. J. Electroanal. Chem. 1996, 409, 137. (3) (a) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104. (b) Badia, A.; Cuccia, L.; Demers, L.; Morin, F.; Lennox, R. B. J. Am. Chem. Soc. 1997, 119, 2682. (4) 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. (5) Whitesides, G. M. Sci. Am. 1995, 9, 146. (6) (a) Ulman, A. Chem. Rev. 1996, 96, 1533. (b) Ulman, A. Thin Films - Self-Assembled Monolayers of Thiols; Academic: Boston, 1998. (7) (a) Heath, J. R.; Shiang, J. J. Chem. Soc. Rev. 1998, 27, 65. (b) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Science, 1995, 270, 1335. (8) (a) Li, H.; Luk, Y.-Y.; Mrksich, M. Langmuir 1999, 15, 4957. (b) Pietron, J. J.; Murray, R. W. J. Phys. Chem. B 1999, 103, 4440. (9) (a) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101. (b) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (10) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. G. J. Appl. Phys. 1997, 82 (2), 696.
Like most research in nanoparticle chemistry and physics, a key ingredient to basic understanding of MPCs is an appreciation1 of the synthetic routes to them and the subtleties of those routes. We have reported on the relation between thiol to gold reaction ratios and core size of MPCs14 and the synthesis of mixed monolayer MPCs using place (ligand) exchange reactions.15 Because mixed monolayer MPCs will likely assume a prominent position in MPC applications, additional synthetic routes to them are a useful research direction. MPC synthesis from symmetrical dialkyl disulfides has been reported;16 the Au core sizes produced are indistinct from those prepared from analogous alkanethiols.16 The objective of this work is an expansion of the synthetic framework for unsymmetrical reactions and a better understanding of the intrinsic factors governing formation of MPC nanoparticles. There is an extensive literature on forming mixed monolayer 2D SAMs on gold surfaces,17-27 including coligation of alkanethiol mixtures with different chain (11) (a) Mulvaney, P. Langmuir 1996, 12, 788. (b) Templeton, A. C.; Pietron, J. J.; Murray, R. W.; Mulvaney, P. J. Phys. Chem. B 2000, 104, 564. (12) (a) Yamada, Y.; Van Drent, W. P.; Abarra, E. N.; Suzuki, T. J. Appl. Phys. 1998, 83, 6527. (b) Kahng, S.-J.; Choi, Y. J.; Park, J.-Y.; Kuk, Y. Appl. Phys. Lett. 1999, 74, 1087. (c) Vernon, S. M. Appl. Phys. Lett. 1999, 74, 1382. (13) (a) Storhoff, J. J.; Mirkin, C. A. Chem. Rev. 1999, 99, 1849. (b) Alivastos, A. P.; Johnson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P., Jr; Schultz, P. G. Nature 1996, 382, 609. (c) Liu, J.; Mendoza, S.; Roman, R.; Lynn, M. J.; Xu, R.; Kaifer, A. E. J. Am. Chem. Soc. 1999, 121, 4304. (d) Aherne, D.; Rao, S. N.; Fitzmaurice, D. J. Phys. Chem. B 1999, 103, 1821. (14) 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.; Porter, M. D.; Evans, N. D.; Murray, R. W. Langmuir 1998, 14, 17. (15) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (b) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (c) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (d) Templeton, A. C.; Cliffel, D. E.; Murray, R. W. J. Am. Chem. Soc. 1999, 121, 7081. (16) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378.
10.1021/la015546t CCC: $20.00 © 2001 American Chemical Society Published on Web 11/10/2001
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lengths and terminal groups,17-19 using tethered bifunctional adsorbates (unsymmetrical sulfides,20,21 disulfides,22-26 and spiroalkanedithiols27), insertion or displacement into preassembled alkanethiolate monolayers,28-30 and chemical modification of terminal groups.31,32 Coligation results have been explained by solubility differences as well as kinetic or thermodynamic preferences.17-19 The constituents of an unsymmetrical disulfide are generally found in a 1:1 proportion in the resulting 2D SAM on Au,25,33,34 but there are recent reports23,24 of deviations from 1:1 compositions when using unsymmetrical disulfides, and dissociative adsorption of unsymmetrical disulfide with S-S bond cleavage followed by thiolate displacement has been suggested.24 In nanoparticle synthesis, changes in the protecting monolayer composition can dramatically change both macroscopic and microscopic properties of the nanoparticle, as has been seen in work on MPC ligand place exchange reactions and amide/ester coupling reactions.15,35 To our knowledge, comparative syntheses of multicomponent MPCs from unsymmetrical disulfides and thiol mixtures have not been reported. We report here such a comparison based on reactions of mixtures of alkanethiols and ω-hydroxy-alkanethiols (e.g., (HO(CH2)11SH plus CH3(CH2)11SH)) and reactions of the unsymmetrical disulfide, HO(CH2)11SS(CH2)11CH3. Since the MPC-forming reaction with thiols involves intermediate formation of a Au(I)-thiolate polymer,36-38 (17) (a) Ostuni, E.; Yan, L.; Whitesides, G. M. Colloids Surf., B 1999, 15, 3. (b) Mrksich, M.; Whitesides, G. M. Annu. Rev. Biophys. Biomol. Struct. 1996, 25, 55. (c) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1994, 98, 563. (d) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (e) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 6560. (f) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665. (18) (a) Lestelius, M.; Liedberg, B.; Tengvall, P. Langmuir 1997, 13, 5900. (b) Kidoaki, S.; Matsuda, T. Langmuir 1989, 5, 7639. (19) (a) Delamarche, E.; Michel, B. Thin Solid Films 1996, 273, 54. (b) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8 (9), 719. (20) Troughton, E. B.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 365. (21) Zhang, M.; Anderson, M. Langmuir 1994, 10, 2807. (22) Chen, S.; Li, L.; Boozer, C. L.; Jiang, S. J. Phys. Chem. B 2001, 105, 2975. (23) Noh, J.; Hara, M. Langmuir 2000, 16, 2045. (24) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440. (25) Takami, T.; Delamarche, E.; Michel, B.; Gerber, Ch.; Wolf, H.; Ringsdorf, H. Langmuir 1995, 11, 3876. (26) Shon, Y.-S.; Kelly, K. F.; Halas, N. J.; Lee, T. R. Langmuir 1999, 15, 5329. (27) (a) Shon, Y.-S.; Lee, S.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 1278. (b) Shon, Y.-S.; Lee, S.; Colorado, R., Jr.; Perry, S. S.; Lee, T. R. J. Am. Chem. Soc. 2000, 122, 7556. (28) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L.; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (29) (a) Kelly, K. F.; Shon, Y.-S.; Lee, T. R.; Halas, N. J. J. Phys. Chem. B 1999, 103, 8639. (b) Shon, Y.-S.; Lee, T. R. J. Phys. Chem. B 2000, 104, 8192. (30) Allara, D. L.; Dunbar, T. D.; Weiss, P. S.; Bumm, L. A.; Cygan, M. T.; Tour, J. M.; Reinerth, W. A.; Yao, Y.-T.; Kozaki, M.; Jones, L., II Ann. N.Y. Acad. Sci. 1998, 852, 349. (31) Yan, L.; Marzolin, C.; Terfort, A.; Whitesides, G. M. Langmuir 1997, 13, 6704. (32) Pan, S.; Castner, D. V.; Ratner, B. D. Langmuir 1998, 14, 3545. (33) (a) Scho¨nherr, H.; Ringsdorf, H.; Jaschke, M.; Butt, H.-J.; Bamberg, E.; Allinson, H.; Evans, S. D. Langmuir 1996, 12, 3898. (b) Scho¨nherr, H.; Ringsdorf, H. Langmuir 1996, 12, 3891. (34) Ishida, T.; Yamamoto, S.; Mizutani, W.; Motomatsu, M.; Tokumoto, H.; Hokari, H.; Azehara, H.; Fujihira, M. Langmuir 1997, 13, 3261. (35) Templeton, A. C.; Hostetler, M. J.; Warmoth, E. K.; Chen, S.; Hartshorn, C. M.; Krishnamurthy, V. M.; Forbes, M. D. E.; Murray, R. W. J. Am. Chem. Soc. 1998, 120, 4845. (36) Al-Sa’ady, A. K. H.; Moss, K.; McAuliffe, C. A.; Parish, R. V. J. Chem. Soc., Dalton Trans. 1984, 1609. (37) Bachman, R. E.; Bodolosky-Bettis, S. A.; Glennol, S. C.; Sirchio, S. A. J. Am. Chem. Soc. 2000, 122, 7146.
Shon et al.
it was of interest to examine the mechanistic and selectivity role of this intermediate. We report the synthesis and reactivity of relevant gold(I)-thiolate polymers. Experimental Section Materials. Hydrogen tetrachloroaurate (HAuCl4‚xH2O) was synthesized according to the literature.39 Used as received were dodecanethiol, 11-mercaptoundecanol, tetraoctylammonium bromide, sodium borohydride, and iodine crystals (all Aldrich), ethyl alcohol (AAPER Alcohol and Chemical Co.), toluene and dichloromethane (EM Sciences), acetonitrile, acetone, sodium bisulfite, and magnesium sulfate (all Mallinckrodt). Water was purified by a Millipore Nanopure water system model 4754. Synthesis of Dodecyl Disulfide (CH3(CH2)11S-S(CH2)11CH3). A 300 mL round-bottom flask containing 11.97 mL (50 mmol) of dodecanethiol in 50 mL of ethanol was slowly heated to 50 °C with stirring. Solid I2 crystals were gradually added until the reaction mixture remained brown, whence the solution was allowed to react for 30 min, afterward removing the ethanol solvent with a rotary evaporator. The product solid was dissolved in CH2Cl2 (ca. 25 mL), washed thrice with 50 mL of saturated sodium bisulfite solution and twice with 50 mL of a brine solution, dried over MgSO4, and then filtered and concentrated under vacuum to obtain the disulfide. 1H NMR (200 MHz, CD2Cl2): δ 2.65 (t, 4 H, CH2SS), 1.54 (m, 4 H, CH2CH2S), 1.45-1.25 (m, 36 H, CH2), 0.87 (t, 6 H, CH3). Synthesis of Dodecyl-11-hydroxyundecyl Disulfide (CH3(CH2)11S-S(CH2)11OH). Dodecanethiol (2.93 mL, 12.5 mmol) and 2.50 g (12.5 mmol) of 11-mercaptoundecanol in 50 mL of ethanol were reacted in a 300 mL round-bottom flask in an oil bath, raising the temperature gradually to 50 °C. Iodine crystals were slowly added until the solution became uniformly brown, and the stirred solution was allowed to react for 1 h, after which solvents were removed with a rotary evaporator and the solid product was dissolved in 25 mL of CH2Cl2. This solution was washed thrice with 50 mL of saturated sodium bisulfite solution and twice with 50 mL of a brine solution, dried over MgSO4, filtered, and concentrated under vacuum to obtain the crude products. Gradient column chromatography of the crude products (v/v ) 8:2 f 2:8; hexane/CH2Cl2) yields dodecyl-11hydroxyundecyl disulfide at (>98%) purity. 1H NMR (200 MHz, CD2Cl2): δ 3.58 (t, 2H, CH2O), 2.65 (t, 4 H, CH2SS), 1.68 (m, 2H, 1.54, CH2CH2O), 1.54 (m, 4 H, CH2CH2S), 1.45-1.25 (m, 32 H, CH2), 0.87 (t, 3 H, CH3). General Procedure for MPC Synthesis. The following standard procedure was used to synthesize MPCs from dialkyl disulfides and alkanethiols, systematically varying (i) the particular organic moiety (dodecyl disulfide, dodecyl-11-hydroxyundecyl disulfide, mixtures of dodecanthiol and 11-mercaptoundecanol) and (ii) the mole ratio of organic moiety to AuCl4-. For the disulfide reaction, to a stirred solution of 1.64 g (3 mmol) of tetraoctylammonium bromide in 80 mL of toluene was added 0.31 g (0.8 mmol) of HAuCl4‚xH2O in 25 mL of Nanopure water. The dark red-brown solution was stirred until all of the AuCl4- was transferred to the toluene phase and the water phase became clear. The organic phase was isolated, 0.32 g (0.8 mmol) of dodecyl disulfide was added, and the solution was stirred for 10 min at room temperature; there is no color change in this step (whereas in the case of thiols, the red-brown toluene solution turns clear). This is followed by addition over a period of 10 s of 0.30 g (8.0 mmol) of NaBH4 in 15 mL of Nanopure water. The solution became dark brown-red and, after stirring at room temperature for 3 h, dark purple-black. The organic phase was collected, the solvents were removed with a rotary evaporator (keeping the temperature below 50 °C), and the black precipitate was suspended in 50 mL of acetonitrile, briefly sonicating it to promote dissolution of undesired reaction constituents. The black precipitate was collected on a glass frit and washed with 80 mL of ethanol, 200 mL of acetonitrile, and 150 mL of acetone. (38) Isab, A. A.; Sadler, P. J. J. Chem. Soc., Dalton Trans. 1982, 135. (39) (a) Handbook of Preparative Inorganic Chemistry; Brauer, G., Ed.; Academic: New York, 1965; pp 1054-1059. (b) Block, B. P. Inorg. Synth. 1953, 4, 14.
Mixed Monolayer-Protected Gold Clusters MPCs generated by BH4- reduction using mixtures of dodecanethiol and 11-mercaptoundecanol in the above procedure were insoluble in toluene, precipitating out of solution during the reduction process. The black precipitate was collected on a glass frit and washed with 100 mL of Nanopure water, 100 mL of toluene, and 100 mL of acetonitrile. Synthesis of Gold(I)-Alkanethiolate Complexes. The synthesis of gold(I)-alkanethiolates was analogous to the previous reaction of MPC synthesis, except the procedure was stopped before adding NaBH4 and the solvents were removed under vacuum. The solid reaction product was dispersed in 50 mL of acetonitrile; the dispersed solid was collected on a glass frit and washed with 80 mL of ethanol, 100 mL of acetonitrile, and 100 mL of acetone. Prior to NMR measurements, the gold(I)alkanethiolate complex was decomposed by addition of a crystal of I2 as described previously,40 which quantitatively liberates ligands as mixed dialkyl disulfides. Measurements. Proton NMR spectra were recorded on a Brucker AC200 spectrometer operating at 200 MHz in CD2Cl2 solutions and internally referenced to δ 5.32 ppm. Typically, a line broadening factor of 1 Hz was used to improve NMR signalto-noise (S/N) and a relaxation delay of 5 s was used to allow adequate signal decay between pulses. Infrared spectra of films of MPCs pressed into a KBr plate were obtained using a Bio-Rad 6000 spectrometer, recording the spectra from 4000 to 500 cm-1. UV-vis spectra of CH2Cl2 solutions in quartz cells were acquired on an ATI UNICAM spectrometer. Transmission electron microscopy (TEM) images of nanoparticles were obtained with a side-entry Phillips CM12 electron microscope operating at 120 keV. Samples were prepared for HRTEM by casting a single drop of a ∼1 mg/mL solution (either hexane or ethanol) onto standard carbon-coated (200-300 Å) Formvar films on nickel grids (600 mesh) and drying in air for more than 30 min. Three typical regions were imaged at 340 000×. Size distributions of the gold cores were obtained from digitized photographic enlargements with Scion Image Beta Release 2. Thermogravimetric analysis (TGA) was performed on 15 mg of dried materials with a Seiko RTG 220 robotic instrument under N2 (flow rate of 50 mL/min), recording data from 25 to 600 °C at a heating rate of 20 °C/min.
Results and Discussion A. MPC Synthesis Reactions. We first consider some general aspects of MPC synthesis from thiols and disulfides, without regard to synthesis of mixed monolayers. The Brust synthesis of MPCs leads to alkanethiolateprotected gold clusters with average core diameters of 1.1-5.2 nm.14,41 The nucleation-growth-passivation reaction14,42,43 responds to various experimental parameters. The average MPC core size decreases at larger thiol/ gold mole ratios,14 when reductant is added rapidly (
