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Langmuir 2003, 19, 9450-9458
Alkyl Selenide- and Alkyl Thiolate-Functionalized Gold Nanoparticles: Chain Packing and Bond Nature Chanel K. Yee,†,⊥,| Abraham Ulman,*,†,⊥ Julia D. Ruiz,‡ Atul Parikh,‡,| Henry White,§,⊥ and Miriam Rafailovich§,⊥ Department of Chemical Engineering and Chemistry, Polytechnic University, Six Metrotech Center, Brooklyn, New York 11201, JDR & ANP, Bioscience Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, Department of Materials Sciences and Engineering, State University of New York at Stony Brook, Stony Brook, New York 11794-2275, and NSF MRSEC for Polymers at Engineered Interfaces Received July 11, 2002. In Final Form: June 16, 2003 We have studied the effects of relative mole ratios of the reactant precursors in the one-phase synthesis of alkaneselenoate- and alkanethiolate-functionalized gold nanoparticles. Specifically, we prepared a series of dodecaneselenoate (DDSe)- and dodecanethiolate (DDT)-functionalized gold nanoparticles using four different Se/Au and S/Au mole ratios in reactant mixtures at two different reaction temperatures employing three different solvents. In all cases, the synthesis relied on the reduction of H[AuCl4], in the presence of dodecanethiol (DDT) and didodecyl diselenide (DD2Se2) using lithium triethylborohydride (superhydride) as the reducing agent. Nanoparticle formation, structure, and bonding characteristics were investigated using a combination of transmission electron microscopy, UV absorption spectroscopy, thermogravimetric analysis, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. Passivation by alkyl selenide was more efficient and was characterized by greater chain density and stronger Au-Se bond strength when high ligand/substrate ratios were employed. Particle size was surprisingly uniform in all cases, independent of mole ratio. By contrast, particle size (2-5 nm) was found to increase with increasing mole ratios when the passivating ligand was alkanethiolate, whose chain grafting density increased with increasing mole ratio, fully coincided with the literature. These results can be reconciled in terms of a simple mechanistic scenario wherein the nanoparticle formation using alkanethiolate ligands proceeds via the formation of a “polymer-like” intermediate between the Au ions and the alkanethiolate ligands prior to reduction whereas such an intermediate is not formed when selenoate is used as the binding ligand.
Introduction Within the growing and increasingly more complex area of nanotechnology,1 nanoparticles have become a significant focal point of activity. This is because the confinement and quantum effects that dominate nanoparticle behavior render their material properties significantly different from those of their bulk counterparts. Furthermore, a wide variety of nanoparticles can be controllably synthesized. As a result, they provide interesting materials for the fundamental explorations of size-dependent and nanoscale phenomena. Their technological importance stems from their novel and useful structural, optical, and functional properties.2 In this regard, gold nanoparticles have emerged as prototypical models of metal nanoparticles * Corresponding author. Phone: (718) 260-3119 (O), (718) 2603125 (F). E-Fax: (810) 277-6217. E-mail:
[email protected]. | Present address: Department of Applied Science, University of California Davis, 1 Shields Avenue, Davis, CA 95616-8254. † Polytechnic University. ‡ Los Alamos National Laboratory. § State University of New York at Stony Brook. ⊥ NSF MRSEC for Polymers at Engineered Interfaces. (1) Murphy, C. J.; Arkin, M. R.; Jenkins, Y. I.; Ghatlia, N. D.; Bossmann, S. H.; Turro, N. J.; Barton, J. K. Science 1997, 262, 1025. (b) Wang, J.; Palecek, E.; Nielsen, P. E.; Rivas, G.; Cai, X.; Shiraishi, H.; Donta, N.; Luo, D.; Farias, P. J. Am. Chem. Soc. 1996, 118, 7667. (c) Wang, J.; Fermandes, J. R.; Kubota, L. T. Anal. Chem. 1998, 70, 3699. (d) Uddin, A. H.; Plunno, P. A.; Hudson, R. H.; Damha, M. J.; Krull, U. J. Nucleic Acids Res. 1997, 25, 4139. (e) Fang, X.; Liu, X.; Schuster, S.; Tan, W. J. Am. Chem. Soc. 1999, 121, 2921. (f) Millan, K. M.; Mikkelsen, S. R. Anal. Chem. 1993, 65, 2317. (2) Wang, J.; Rivas, G.; Cai, X.; Shiraishi, H.; Donta, N.; Luo, D.; Farias, P. Anal. Chem. 1996, 68, 2629. (b) Bohrmann, B.; Kellenberger, E. J. Histochem. Cytochem. 1994, 42, 635. (c) Hall, D. B.; Holinlin, R. E.; Barton, J. K. Nature 1996, 382, 731.
with wide-ranging applications. They have been used as substrates for well-defined, high-density polymer brushes by surface-initiated living radical polymerization,3 as centers in networks,4 as catalysts’ oxide supports,5 and as sensors.6 Gold nanoparticles are also finding new broad applications in bioscience including their use as substrates for DNA attachment,7 as signal amplifiers for biological recognition, and as mediators in transfection of mammalian cells.8 In the synthesis of gold nanoparticles, to date, organosulfur derivatives have provided the most widely used ligands. Specifically, most previous studies have focused on alkanethiolate-functionalized gold nanoparticles, but the use of dialkyl sulfides has also been reported,9 presumably because of their widespread use as selfassembling molecules at gold surfaces. In this vein, organoselenoates provide a useful alternative. The formation of stable, well-defined self-assembled monolayers (3) Jordan, R.; West, N.; Ulman, A.; Chou, Y.-M.; Nuyken, O. Macromolecules 2001, 34, 1606. (b) Ohno, K.; Koh, K.-m.; Tsujii, Y.; Fukuda, T. Macromolecules 2002, 35, 8989. (4) Wang, T.; Zhang, D.; Xu, W.; Li, S.; Zhu, D. Langmuir 2002, 18, 8655. (5) Haruta, M. Catal. Today 1997, 36, 153. (6) Jia, J.; Wang, B.; Wu, A.; Cheng, G.; Li, Z.; Dong, S. Anal. Chem. 2002, 74, 2217. (b) Liu, T.; Tang, J.; Zhao, H.; Deng, Y.; Jiang, L. Langmuir 2002, 18, 5624. (c) Lindblad, M.; Lestelius, M.; Johansson, A.; Tengvall, P.; Thomsen, P. Biomaterials 1997, 18, 1059. (d) Niemeyer, C. M.; Boldt, L.; Ceyhan, B.; Blohm, D. Anal. Biochem. 1999, 268, 54. (7) Storhoff, J. J.; Elghanian, R.; Mirkin, C. A.; Letsinger, R. L. Langmuir 2002, 18, 6666. (8) Sandhu, K. K.; McIntosh, C. M.; Simard, J. M.; Smith, S. W.; Rotello, V. M. Bioconjugate Chem. 2002, 13, 3. (9) Shelley, E. J.; Ryan, D.; Johnson, S. R.; Couillard, M.; Fitzmaurice, D.; Nellist, P. D.; Chen, Y.; Palmer, R. E.; Preece, J. A. Langmuir 2002, 18, 1791.
10.1021/la020628i CCC: $25.00 © 2003 American Chemical Society Published on Web 09/25/2003
Functionalized Gold Nanoparticles
(SAMs) of alkaneselenoates on polycrystalline gold films was first reported by Samant and co-workers10a and has since been characterized by many at polycrystalline and single crystal gold surfaces. Alkaneselenols and dialkyl diselenides are now known to organize at the gold surface to form stable single monolayers in an incommensurate packing wherein the aliphatic chains organize in an ordered conformation at high densities presenting a cant angle of approximately 15° from the surface normal. To the best of our knowledge, however, their use in the synthesis of gold nanoparticles has not been explored. The use of alkaneselenols and particularly dialkyl diselenides in the nanoparticle synthesis is potentially attractive for two reasons. First is the bonding preference. Since the Se-Se is a weaker bond than S-S (42 vs 54 kcal mol-1, respectively), it can be expected that oxidative addition of the Se-Se bond to gold should be more favorable and facile. Moreover, selenium is slightly less electronegative than sulfur (2.55 vs 2.58 Pauling units) and larger in size (1.98 vs 1.84 Å); One would, therefore, expect a stronger selenoate (soft base)-gold(I) (soft acid) bonding. Second are the chain-structural properties. The aliphatic chains of alkaneselenoates have been previously shown to yield a packing assembly considerably denser than that of alkanethiol monolayers on gold.10 While the latter (alkanethiolates) adopt a close packed chain configuration at 30° tilt, alkaneselenoates pack in a distorted hexagonal configuration at a considerably lower tilt of about 15°. It appears that this stronger substrateheadgroup binding and denser chain packing for alkaneselenoates can be exploited to gain some synthetic control of the structure and properties of gold nanoparticles. In this paper, we present the one-phase preparation11 and characterization of alkaneselenoate passivated gold nanoparticles. We also provide a detailed study of the effects of the ligand to substrate precursor ratios on nanoparticle structure and properties and carry out a direct comparison of their morphological, structural, and bonding preferences with their alkanethiolate counterparts. Experimental Section Chemicals. All chemicals were purchased from Aldrich and used as received except didodecyl diselenide, which was synthesized in our laboratory. Solvents were obtained from EM Science or Aldrich. Tetrahydrofuran (THF) used for the synthesis was freshly distilled to remove the stabilizer and further dried by molecular sieves. Preparation of DDSe/Au and DDT/Au Nanoparticles. Four Se/Au (and S/Au) mole ratios have been synthesized: 2:1, 1:1, 1:2, and 1:3. Four samples of 99.2 mg (0.2 mmol) didodecyl diselenide (C12H25Se-SeC12H25, DD2Se2) were each added to vigorously stirred solutions of 0.2, 0.4, 0.8, and 1.2 mmol of hydrogen tetrachloroaureate(III) trihydrate (HAuCl4‚3H2O), in 20 mL of freshly distilled anhydrous THF, at 6 °C, under N2 purge. In a similar fashion, four samples of 80.8 mg (0.4 mmol) of dodecanethiol (C12H25SH, DDT) in 20 mL of freshly distilled anhydrous THF were each mixed with solutions of 0.2, 0.4, 0.8, and 1.2 mmol of HAuCl4‚3H2O, respectively, under vigorous stirring. Each reaction mixture was stirred for 2 h, before 1.0 M of lithium triethylborohyride (superhydride) in THF was added dropwise. The addition rate was adjusted to maintain the reaction (10) Samant, M. G.; Brown, C. A.; Gordon, J. G., II. Langmuir 1992, 8, 1615. (b) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir 1999, 15, 5314. (c) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir 1998, 14, 625. (d) Koji, N.; Takeshi, S.; Masato, T.; Makoto, T. Langmuir 2000, 16, 2225. (e) Huang, F. K.; Horton, R. C.; Myles, D. C.; Garrell, R. L. Langmuir 1998, 14, 4802. (11) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3486. (b) Yee, C.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 4314.
Langmuir, Vol. 19, No. 22, 2003 9451 temperature of 6 ( 1 °C. Gas evolution and color changes were immediately observed in each of the reaction flasks upon addition of the reducing agent. The addition of superhydride (in excess12) was terminated when no more H2 gas evolved and the reaction temperature did not increase. The reaction mixtures were stirred for an additional 2 h to ensure reaction completion. Each resulting solution mixture was centrifuge with ethanol to remove excess thiol or diselenide, as confirmed by thin-layer chromatography (TLC, with hexane as the eluent). After centrifugation, the supernatant was discarded. The purified nanoparticles were dried in a vacuum desiccator overnight. Pure nanoparticles can be resuspended in chloroform. Elemental Analysis. Samples of four mole ratios of Au/DDSe nanoparticles were sent to Galbraith Laboratories, Inc. (Knoxville, TN) for inductively coupled plasma optical emission spectroscopy (ICP-OES). Percentages of carbon, hydrogen, gold, and selenium in 2:1, 1:1, 1:2, and 1:3 mole ratios were obtained respectively as the following: 29.19%, 5.11%, 49.23%, 16.36%; 23.35%, 3.82%, 60.16%, 13.12%; 3.78%, 0.65%, 93.39%, 2.00%; 1.92%,
