Synthesis and Self-Assembly of Cetyltrimethylammonium Bromide

Publication Date (Web): October 3, 2003 ... In this article, cetyltrimethylammonium bromide (CTAB)-capped gold nanoparticles were synthesized successf...
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Synthesis and Self-Assembly of Cetyltrimethylammonium Bromide-Capped Gold Nanoparticles Wenlong Cheng, Shaojun Dong,* and Erkang Wang* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China Received May 13, 2003. In Final Form: August 14, 2003 In this article, cetyltrimethylammonium bromide (CTAB)-capped gold nanoparticles were synthesized successfully by using CTAB as a phase-transfer catalyst and stabilizer simultaneously in a two-phase toluene/water system. The as-prepared gold nanoparticles were characterized and analyzed by virtue of X-ray photoelectron spectroscopy, UV-visible absorbance spectroscopy, and infrared spectroscopy. The particle size information and collective self-assembling properties of the CTAB-capped gold nanoparticles on carbon-coated copper grid and mica were evaluated by transmission electron microscopy and atomic force microscopy, respectively. As a result, it is demonstrated that the 3-D CTAB monolayers on a gold cluster are in the disordered liquid state. The interparticle spacing can be controlled either physically by the inherent particle-to-particle interactions or chemically by molecular linker. The assembly of both nanoparticles and linker-bridged nanonetworks on mica follows a hydrophobic interaction mechanism.

Introduction Currently, it is widely accepted that metal nanoparticles would serve as fundamental building blocks for future nanoscience and nanotechnology. Especially, these nanoscale metallic blocks are thought to be indispensable components in future nanoelectronic circuits1,2 and nanoscale chemical/biological sensor devices.3,4 Therefore, the controlled preparation of metallic materials in the nanometer range is attracting increasing research efforts.2-14 * To whom correspondence should be addressed. Fax: +86-4315689711. E-mail: [email protected]. (1) Feyman, R. P. In Miniaturization; Gilbert, H. D., Ed.; Reinhold: New York, 1961; pp 282-296. (2) Articles in the Nanoscale Special Issue: Acc. Chem. Res. 1999, 32, 5. (3) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (4) Katz, E.; Willner, I. J. Am. Chem. Soc. 2002, 124, 10290. (5) (a) Frens, G. Nature Phys. Sci. 1973, 241, 20. (b) Alivisatos, A. P. Science 1996, 271, 933. (c) Landes, C. F.; Link, S.; Mohamed, M. B.; Nikoobakht, B.; El-Sayed, M. A. Pure. Appl. Chem. 2002, 74, 1675. (6) (a) El-sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (b) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (7) (a) Kreibig, U.; Volmer, M. Optical Properties of Metal Clusters; Springer-Verlag: Heidelberg, Germany, 1995. (b) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (c) Nath, N.; Chilkoti, A. Anal. Chem. 2002, 74, 504. (d) Olddenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (e) Henglein, A. Langmuir 1999, 15, 6738. (f) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (g) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (h) Malinsky, M. D.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. J. Am. Chem. Soc. 2001, 123, 1471. (8) Boal, A. K.; llhan, F.; DeRouchey, J. E.; Thurn-Albrecht, T.; Russell, T. P.; Rotello, V. M. Nature 2000, 404, 746. (9) Andres, R. P.; Bielefeld, J. D.; Henderson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. R.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (10) (a) Alivisatos, A. P.; Johnsson, K. P.; Peng, X. G.; Wilson, T. E.; Loweth, C. J.; Bruchez, Jr, M. P.; Schultz, P. G. Nature 1996, 382, 609. (b) Novak, J. P. Feldheim, D. L. J. Am. Chem. Soc. 2000, 122, 3979. (11) (a) Mayer, C. R.; Neveu, S.; Cabuil, V. Adv. Mater. 2002, 14, 595. (b) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Adv. Mater. 1995, 7, 795. (12) (a) Shipway, A. N.; Katz, E.; Willner, I. ChemPhysChem 2000, 1, 18. (b) Cheng, W. L.; Dong, S. J.; Wang, E. Angew. Chem., Int. Ed. 2003, 42, 449; Angew. Chem. 2003, 115, 465. (c) Cheng, W. L.; Jiang, J. G.; Dong, S. J.; Wang, E. Chem. Commun. 2002, 1706. (13) (a) Mitchell, G. P.; Mirkin, C. A.; Letsinger, R. L. J. Am. Chem. Soc. 1999, 121, 8122. (b) Storhoff, J. J.; Lazarides, A. A.; Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Schatz, G. C. J. Am. Chem. Soc. 2000, 122, 4640.

Previous studies have shown that some essential properties of metallic nanoparticles, such as quantum-size effects, unusual optics, electronics, magnetics, and colorics, etc., are highly tunable by particles size,5 particle geometry,6 particle surface chemistry,7 interparticle spacing,8-16 and self-assembly process of nanoparticles.12-16 Remarkably, many of these studies of the metallic nanomaterials4-16 are relevant to gold nanoparticles, due to their easy preparation, good stability, and some interesting properties. Passivation of gold nanoparticles by thiol-derivative monolayers makes them able to be handled as a simple organic compound, and the “nanocompound” can be precipitated and isolated in solid form, redissolved, made to participate in chemical reactions, and also be characterizable by NMR, electrochemistry, and mass spectrum characterization, etc.7f,g Further studies show that the size-purified thiol-capped gold nanoparticles tend to form spontaneously 2-D and 3-D superlattices upon solvent removal.14-16 Ordered superlattices were also reported for pentadecylamine17 and quaternary ammonium bromide18 stabilized gold nanoparticles; however, these superlattices are thought to be determined by the balance between local electrostatic repulsion and dispersion forces between the particles, whereas the alkanetelluride-capped gold nanoparticles19 exhibit a different mechanical stability from thiol-capped gold nanoparticles at the air-water interface. In addition, the barrier height for electron transfer was found to be tunable by the redox state of the ligand shell.20 These previous studies show that many essential properties of gold nanoparticles, such as chemical reactivity, (14) Markovich, G.; Collier, C. P.; Henrichs, S. E.; Remalcle, F.; Levine, R. D.; Heath, J. R. Acc. Chem. Res. 1999, 32, 415. (15) (a) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. T.; Schaaff, T. G.; Vezmar, I.; Alvarez, M. N.; Wilkinson, A. Acc. Chem. Res. 1999, 32, 397. (b) Collier, C. P.; Vossmeyer, T.; Heath, J. R. Annu. Rev. Phys. Chem. 1998, 49, 371. (c) Weller, H. Angew. Chem., Int. Ed. Engl. 1996, 35, 1079. (16) (a) Taleb, A.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 2214. (b) Pileni, M.-P. J. Phys. Chem. B 2001, 105, 3358. (17) Brown, L. O.; Hutchison, J. E. J. Am. Chem. Soc. 1999, 121, 882. (18) Fink, J.; Kiely, C. J.; Bethell, D.; Schiffrin, D. J. Chem. Mater. 1998, 10, 922. (19) Brust, M.; Stuhr-Hansen, N.; Nørgaard, K.; Christensen, J. B.; Nielsen, L. K.; Bjφrnholm, T. Nano Lett. 2001, 1, 189. (20) Gittins, D. L.; Bethell, D.; Schiffrin, D. J.; Nichols, R. J. Nature 2000, 408, 67.

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interparticle electron transfer, their optical reactivity, and their ability to partake in self-assembling processes, seem to depend on the nature of the ligand itself or the chemical bond between the ligand and the gold core. Therefore, exploitation of ligand chemistry21 of gold nanoparticles and their self-assembly properties is still challenging. In this work, we report a new synthesis of gold nanoparticles in a two-phase toluene/water system by using cetyltrimethylammonium bromide (CTAB) as a phase-transfer catalyst and a stabilizer simultaneously. IR spectral studies of the resulted CTAB-capped nanoparticles demonstrate strongly that the alkyl chains in the 3-D surface-bound CTAB monolayer are in the disordered liquid state, which is different from the 3-D alkanethiol monoalyer in the solid state.7f,g,22 The CTABcapped gold nanoparticles tend to form spontaneously into 2-D and 3-D superclusters upon toluene evaporation.14-16 The more close-packed particle aggregates can be fabricated by introduction of dithiol molecular linker. In addition, the preliminary AFM studies show that both gold nanoparticles and their nanonetworks exhibit preferential hydrophobic organization on CTAB-coated surfaces, indicating the template effects of surfactants in selfassembly of nanoparticles. Experimental Section Chemicals. HAuCl4‚3H2O (Shanghai Chemicals), toluene (ACROS), sodium borohydride (ACROS), and cetyltrimethylammonium bromide (Beijing Chemicals) were used as received. All aqueous solutions were prepared with high-purity deionized water. Synthesis of CTAB-Capped Gold Nanoparticles and Preparation of Samples for Various Characterizations. The synthesis of CTAB-capped gold nanoparticles was done on a two-phase tolulene/water system detailed as follows: 60 mL of saturated CTAB toluene solution (∼2 mM) was mixed with 20 mL of 1% aqueous solution of hydrogen tetrachloroaurate under vigorous stirring. Then, the yellow toluene phase was separated, and freshly prepared aqueous sodium borohydride solution was added dropwise into the stirred toluene solution until the yellow color of the organic phase changed to clear ruby red. For X-ray photoelectron spectroscopy (XPS) characterization, the sample was prepared by drop-casting one drop of the as-prepared gold nanoparticles in toluene onto a clean glass slide (1 × 1 cm2) and then drying in air. The specimens for transmission electron microscopy (TEM) were prepared by drop-casting one drop of the as-prepared gold colloid onto standard carbon-coated Formvar films on copper grids. For Fourier transform infrared (FT-IR) characterizations, the potassium bromide solid was ground via mortar and pestle carefully into a fine powder and then pressed into a transparent disk. Afterward, the as-prepared gold colloid in toluene or CTAB toluene solution were cast on a freshly prepared KBr disk and dried for 10 min under an infared lamp. The specimens for atomic force microscopy (AFM) characterization were prepared by incubating freshly cleaved mica in the solution containing gold nanopartices or their nanonetwork for ∼3 min, and then toluene was allowed to evaporate at room temperature in air before AFM scanning. Measurements. The UV-visible absorbance spectra were acquired using a Cary 500 UV-visible NTR spectrometer (Varian). Photographs were taken by a FinePixViewer 6900 camera made by Fuji Photo Film Co., Ltd. XPS measurements were conducted with an ESCLAB MK II spectrometer (VG Co.) with Mg KR radiation as the X-ray source. The charging calibration was performed by referring the C1s to the binding energy at 284.6 eV. Transmission infrared spectra were collected in the transmission mode on a Nicolet 520 FT-IR spectrometer. The spectra were obtained over 100 scans from which a (21) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (22) (a) Hostetler, M. J.; Stokes, J. J.; Murray, R. W. Langmuir 1996, 12, 2, 3604. (b) Badia, A.; Singh, S.; Demers, L.; Cuccia, L.; Brown, G. R.; Lennox, R. B. Chem. Eur. J. 1996, 2, 359.

Figure 1. XPS spectrum of Au 4f of the as-prepared CTABcapped gold nanoparticles deposited on a glass slide. background spectrum (empty cell) was automatically subtracted. The electronic images were made on a JEOL 2010 transmission electron microscope operating at 200 KV. AFM images were taken by using a Nanoscope IIIa instrument operating in the tapping mode with standard silicon nitride tips. Typically, the surface was scanned at 2 Hz with 256 lines/image resolution and 1.02.0 V setpoint.

Results and Discussion Synthesis and Characterization of CTAB-Capped Gold Nanoparticles. Tetraoctylammonium bromide (TOAB) is usually used as a phase-transfer reagent of HAuCl4 from aqueous media to nonaqueous media for preparation of gold nanoparticles in a two-phase liquid/ liquid system.7f,g,14-16,18,22 Similarly, it was found here that CTAB can also serve as a phase-transfer reagent of HAuCl4 from water into toluene. Although the gold salts are solvophobic with respect to toluene, they can be transferred into toluene by complexation with CTAB cations (this process is called ion-pair extraction and can be interpretated by solvophobic theory23). The extracted gold salts are incorporated into CTAB inverse micelles in toluene, which can be reduced further to Au0 by NaBH4. This results in generation of gold nanoparticles in toluene as characterized by the following experimental techniques. The oxidation state of gold in the nanoparticles was determined by X-ray photoelectron spectroscopy as shown in Figure 1. The XPS spectrum of the as-prepared gold nanoparticles shows the Au 4f7/2 and 4f5/2 doublet with the binding energies of 84.0 and 87.8 eV, respectively. These are typical values for Au0. The inset A of Figure 2 shows the photograph of the as-prepared gold colloid. The gold colloid exhibits a well-defined surface plasmon band with a maximum absorbance at ∼527 nm (Figure 2, curve a), which is similar to these previously reported gold nanoparticles (>3 nm).2,5,7a-e,11-13,18 The observed strong surface plasmon band shows that surface-bound CTAB molecules on a gold cluster are physically associated and do not dampen strongly the surface electron resonance as chemically bound alkanethiol.7f-h,14-16,22 (23) Miyabe, K.; Taguchi, S.; Kasahara, I.; Goto, K. J. Phys. Chem. B 2000, 104, 8481.

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Figure 2. UV-visible spectra of CTAB-stabilized gold nanoparticles (a, red line) and 1,8-octanedithiol linked nanonetwork (b, blue line) in toluene. Inset: photographs of the as-prepared gold colloid before (A) and after (B) addition of 1,8-octanedithiol.

IR spectral studies of CTAB-capped gold nanoparticles were performed as shown in Figure 3. The FT-IR spectrum of CTAB-capped gold nanoparticles was collected over the range of 400-3200 cm-1 in the transmission mode. The spectrum of CTAB-capped gold nanoparticles is similar to pure CTAB molecules, and characteristic CH2 vibration peaks as well as that of CH3 were clearly observed. This indicates CTAB association with the gold cluster. However, some differences between CTAB tethered to the gold cluster and pure CTAB were also noted at the C-H stretching region and the C-C stretching region. It has been known previously that the symmetric and antisymmetric CH2 stretching vibrations can be used as a sensitive indicator of the ordering of the alkyl chains, and the higher energies for the CH2 stretching vibrations indicate greater incidence of gauche defects.22,24,25 The graph in the bottom left of Figure 3 shows the comparison between free CTAB and CTAB-capped gold nanoparticles in the C-H stretching vibration region. The symmetric and antisymmetric CH2 stretching vibrations of pure CTAB lie at 2849 and 2917 cm-1, respectively, whereas the symmetric and antisymmetric CH2 stretching vibrations of the CTABcapped gold nanoparticles appear at 2858 and 2929 cm-1, respectively. The energetic shift of CH2 stretching vibrations indicates a higher density of gauche defects for CTABcapped gold nanoparticles than pure CTAB molecules. An obvious spectral change was also observed at the C-C stretching vibration region (bottom right of Figure 3). Compared with pure CTAB molecules, both the RT (trans C-C bonds) and RG (gauche C-C bonds) modes were clearly observed in two broad regions centered at 1122 (24) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (b) Ulman, A. An Introduction to Ultrathin Organic Films; Academic: New York, 1991. (c) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506. (25) Snyder, R. G.; Maroncelli, M.; Strauss, H. L.; Hallmark, V. M. J. Phys. Chem. 1986, 90, 5623.

Figure 3. Transmission infrared spectra of the CTAB-capped gold nanoparticles (a). The graphs on the bottom show the comparison of the CTAB-capped gold nanoparticles in regions aI and aII and free CTAB molecules (b) in the corresponding regions.

and 1072 cm-1, respectively (absence of RT and RG modes for pure CTAB molecules might indicate that pure CTAB is in the crystalline state in KBr disks upon solvent evaporation). The presence of a gauche C-C stretching vibration in the spectra of the CTAB-capped gold nanoparticles is indicative of either a near-surface defect or an internal kink in the CTAB capping layer. Another weak chain end-gauche band was also observed at ∼1164 cm-1, which is assigned as methylene rock vibration. In general, the above FT-IR spectral studies of C-H and C-C vibrations show clearly that there is a higher density of gauche defects when CTAB molecules are tethered to a gold cluster. Therefore, it is thought that these CTAB molecules adsorbed on the gold cluster might be in the disordered liquid state,22,24,25 which is different from alkanethiol with comparable chain length on a gold cluster (in Murray’s study, alkanethiolates with comparable chain length are demonstrated to be in the solid state, predominantly, in the all-trans zigzag conformation22a). The higher disorder of 3-D CTAB monolayers than 3-D alkanethiol21a with comparable chain length might be due to a larger volume of “amino head” (i.e., -N+(CH3)3) than “mercapto head” (i.e., -SH). When the amino head of CTAB is attached to a gold nanocluster, it would occupy a larger surface area than the mercapto head. As a result, a larger liberty of movement of the alkane chain would be expected for CTAB monolayers on gold cluster. Thus, it can be imagined reasonably that the CTAB acts as “a tadpole” attached to a spherical subject with comparable dimensions and the free swinging of its flexible “tail” (i.e., alkane

CTAB-Capped Gold Nanoparticles Chart 1. Schematic Model Used for Explaining the Selective Anchoring Mechanism of Gold Nanoparticles on “Patterned” CTAB Monolayersa

a When the as-prepared gold colloid was deposited to mica surfaces, the free CTAB molecules would form into monolayers with hydrophobic alkyl chains exposed. The CTAB layers serve as energetically favorable sites for location of CTAB-capped gold nanoparticles by hydrophobic interactions. As for 1,8octanedithiol-linked nanonetworks, CTAB molecules are thought to cover still outer surfaces of nanonetworks. Therefore, anchoring of the nanonetworks on CTAB monolayers should follow a similar hydrophobic mechanism too.

chain) would not be constricted by its proximate flexible CTAB “tails” when its “large head” is tethered to a gold cluster. Based on the above spectral studies, a possible structure of CTAB-stabilized gold nanoparticles is illustrated in Chart 1. CTAB molecules specifically adsorbed on the gold clusters might form surface ion pairs with Br- ions attached to the Au surfaces, and the cationic CTAB headgroups surround the Br- layer by electrostatic interactions. Thus, the hydrophobic CTAB tail chains would point outward, which results in stability of gold nanoparticles in toluene. In fact, the proposed scheme is experimentally supported by previous studies on TOABcapped gold nanoprticles.18 Self-Assembly of the CTAB-Capped Gold Nanoparticles. Similar to the alkanethiol-capped gold nanoparticles,14-16,21 the CTAB-capped gold nanoparticles organize spontaneously into 3-D or 2-D arrays upon solvent evaporation. Figure 4 shows several representative TEM micrographs of particle arrays prepared by drop-casting the as-prepared gold colloid on a carbon-coated copper grid in air. Both 3-D (Figure 4a-c) and 2-D (Figure 4d,e) particle arrays were observed clearly on the grid. The morphologies of these particle superstructures are complex, showing not regular triangular particle flakes as observed previously for size-selected Ag nanocrystals.26 Further insight (Figure 4c,e) into these superstructures in higher magnification shows that the particles are not orderly arrayed, which might be due to low monodispersity of CTAB-capped gold nanoparticles. The average particleto-particle distance measured from Figure 4e is ∼2 nm, smaller than double the length of CTAB molecule. This demonstrates the interdigitation of alkyl chains between the two adjacent gold clusters, consistent with the previous studies.9,14-16,26-27 Also, the particles in these arrays are still discrete and not fused into larger particles, showing that CTAB-capped gold nanoparticles have comparable stability with alkanethiol-capped particles14-16,26 upon solvent evaporation. (26) Wang, Z. L.; Whetten, R. L.; Vezmar, I.; Alvarez, M. M. Adv. Mater. 1997, 9, 817. (27) Collier, C. P.; Saykally, R. J.; Shiang, J. J.; Henrichs, S. E.; Heath, J. R. Science 1997, 277, 1978.

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The as-prepared nanoparticles can react readily with 1,8-octanedithiol and form into a chemically interconnected nanonetwork. Addition of a low amount of dithiol into the as-prepared gold colloid changes the solution color from red to light blue (inset B of Figure 2), which results in a red-shift of the maximum absorbance peak and widening of the absorbance band (Figure 2, curve b), indicative of aggregation of gold nanoparticles.11-14,28 Parts a and b of Figure 5 show TEM micrographs of 1,8octanedithiol linked nanonetworks of gold nanoparticles. The average particle-to-particle distance was estimated to be ∼1 nm. In fact, the particle-to-particle distance in the nanonetwork is dictated by the octanedithiol molecular linker length, consistent with the previous studies of molecularly bridged particle aggregation.8-13 It is also noted that these gold nanoparticles in these nanonetworks are again discrete and do not fuse into larger particles, still keeping the physical properties of single particles. Although particle aggregates were observed on the copper grid for both the as-prepared gold colloid (case I) and the colloid with dithiol addition (case II), the selfassembling process was thought to follow different mechanisms. CTAB-capped gold nanoparticles are solvophilic with respect to toluene, and it is energetically favorable for them to keep their respective solvation shells to stop particle aggregation in toluene,29 whereas CTAB-capped gold nanoparticles are solvophobic with respect to air,29 and it is energetically favorable for particle aggregation due to strong “solvophobic attraction”.29 The drying of nanoparticle solution on the copper grid would change the solvophilicity of gold nanoparticles to solvophobicity upon exposure to air, as theorectically explained by Rabani and Egorov.29b Thus, the aggregation in case I happens actually on the grid surfaces, and the final structure of the formed aggregates might depend on the balance between dispersion attraction forces15 among gold cores and electrostatic repulsion forces18 among CTAB heads on gold cores. Differently, the aggregation in case II might happen predominantly in toluene solvent. Dithiol has been demonstrated to be a very strong ligand, which would replace CTAB ligands and link nanoparticles into interconnected networks in solution.11 The drying process would not affect the structure of the networks due to an extremely low concentration of nanonetworks.11 Self-assembly of CTAB-stabilized gold nanoparticles and their nanonetworks on mica was investigated by atomic force microscopy. It has been observed previously that self-assembly of octadecylamine on mica using hydrophobic solvents is different from alkanethiol on gold, and only partially covered molecularly thin films were obtained.30 Similarly, it was also found here that only partially covered mica surfaces were obtained after immersion of freshly cleaved mica in the hydrophobic toluene solvent containing the CTAB molecules. The CTAB films are always in the form of continuous or semicontinuous or discrete islands depending on preparation conditions. Therefore, self-assembly of CTAB using hydrophobic toluene solvent actually generates spontaneously “patterned” mica surfaces. Interestingly, both CTABstabilized gold nanoparticles and the dithiol-linked nanonetwork show selective organization on the CTAB “patterned” mica surfaces. Parts A and B of Figure 6 show (28) (a) Tripp, S. L.; Pusztay, S. V.; Ribbe, A. E.; Wei, A. J. Am. Chem. Soc. 2002, 124, 7914. (b) Park, S.-J.; Lazarides, A. A.; Mirkin, C. A.; Lesinger, R. L. Angew. Chem., Int. Ed. 2001, 40, 2909. (c) Hutchinson, T. O.; Liu, Y.-P.; Kiely, C.; Brust, M. Adv. Mater. 2001, 13, 1800. (29) (a) Kokkoi, E.; Van Swol, F. J. Chem. Phys. 1998, 108, 4675. (b) Rabani, E.; Egorov, S. A. Nano Lett. 2002, 2, 69. (30) Benitez, J. J.; Kopta, S.; Ogletree, D. F.; Salmeron, M. Langmuir 2002, 18, 6096.

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Figure 4. Typical TEM micrographs of the CTAB-capped gold nanoparticles deposited on the carbon-coated copper grids. b and c correspond to the magnifying domains selected in a; e corresponds to the magnifying domain selected in d.

Figure 5. Typical TEM micrographs of 1,8-octanedithiol linked nanonetworks at different magnifications.

typical images of gold nanoparticles and their nanonetworks pinning to CTAB monolayers, respectively. From the section analysis, the dark yellow domains were identified as bare mica surfaces; the light yellow domains were identified as CTAB monolayers; the nearly white

patches above CTAB films were identified as gold nanoparticles and their nanonetworks. These gold nanoparticles in nanonetworks are well-resolved in the vertical direction, but individual particles in close contact cannot be distinguished due to AFM tip convolution effects31 in

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spontaneously on mica and not aided by LangmuirBlodgett (LB) techniques.32 Chart 1 illustrates a possible mechanism of preferential anchoring of gold nanoparticles on CTAB-coated surfaces. As for CTAB monolayers, the polar amino (-N+(CH3)3) head is facing the negatively charged mica, and the terminal methyl (-CH3) group is exposed outmost. As for gold nanoparticles, the polar amino (-N+(CH3)3) head is facing gold core surfaces by forming surface ion pairs with Br- ions, and the terminal methyl (-CH3) groups are exposed. Thus, both gold nanoparticles and CTAB-covered surfaces are hydrophobic (toluene-philic), whereas mica surfaces are hydrophilic (toluene-phobic). Thus, the gold colloidal solution would wet favorably CTAB monolayers and “lay down” nanoparticles on them rather than bare mica surfaces. In addition, it is thought that the flexible alkyl chains tethered to gold nanoparticles would tend to insert into underlying CTAB monolayers to form interdigited structure upon toluene evaporation. The interdigitated alkyl chains are thought to keep these nanoparticles from desorption by hydrophobic interactions (similar nanoparticle organization based on hydrophobic forces has also been reported previously33). In fact, the interdigitation of alkyl chains is universal in alkylnanoparticle systems due to the high curvature of the nanoparticle surface,9,14-16,26,27,34,35 which would result in a great free volume and a large liberty of movement of the outmost functional groups. These characteristic properties would favor interdigitation of alkyl chains. Preferential anchoring of 1,8-octanedithiol-linked nanonetworks on CTAB-coated surfaces was thought to follow a hydrophobic mechanism similar to that described above. These nanonetworks are also energetically favorable to locate on hydrophobic CTAB monolayers rather than hydrophilic mica surfaces. Conclusions

Figure 6. Typical AFM images of CTAB-capped gold nanoparticles (A) and 1,8-octanedithiol linked nanonetwork (B) on mica.

According to our knowledge, this is the first report of synthesis of CTAB-capped gold nanoparticles in a twophase liquid/liquid system using CTAB as a phase-transfer catalyst and a stabilizer simultaneously. The IR spectral studies demonstrated that the CTAB monolayer surrounding a gold nanocluster is highly disordered. The interparticle spacing can be changed from ∼2 to ∼1 nm by introduction of a molecular linker. The self-assembly processes of nanoparticles were analyzed by solvophilic and solvophobic effects of nanoparticles. Selective loading of gold nanoparticles on CTAB monolayers rather than bare mica is interesting, which might help us create conductive metallic domains on some confined sites in an insulating mica surfaces.

nanoparticle characterization. Although discrete nanoparticles existing in the dithiol-linked nanonetworks were not resolved by AFM, TEM has demonstrated clearly that these nanoparticles do not fuse into larger nanostructures (Figure 5). These observed nanostructures are very similar to aggregation of alkanethiol-capped gold nanoparticles on DPPC templates at air-liquid interfaces.32 The difference is that the structure obtained here is formed

Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant No. 20275037, Grant No. 29975028) and the National Key Basic Program 2002 Grant No. CB513110. We are also thankful for one of the reviewers for the careful corrections of grammatical errors in the original manuscript.

(31) (a) Freeman, R. G.; Grabar, K. C.; Allison, K. J.; Bright, R. M.; Davis, J. A.; Guthrie, A. P.; Hommer, M. B.; Jackson, M. A.; Smith, P. C.; Walter, D. G.; Natan, M. J. Science 1995, 267, 1629. (b) Grabar, K. C.; Smith, P. C.; Musick, M. D.; Davis, J. A.; Walter, D. G.; Jackson, M. A.; Guthrie, A. P.; Natan, M. J. J. Am. Chem. Soc. 1996, 118, 1148. (c) Cheng, W. L.; Dong, S. J.; Wang, E. Langmuir 2002, 18, 9947. (32) Hassenkam, T.; Nørgaard, K.; Iversen, L.; Kiely, C. J.; Brust, M.; Bjørnholm, T. Adv. Mater. 2002, 14, 1126.

(33) (a) Ellis, A. V.; Vijayamohanan, K.; Goswami, R.; Chakrapani, N.; Ramanathan, L. S.; Ajayan, P. M.; Ramanath, G. Nano Lett. 2003, 3, 279. (b) Aslam, M.; Mulla, I. S.; Vijayamohanan, K. Langmuir 2001, 17, 7487. (34) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281. (35) Cheng W. L.; Dong S. J.; Wang, E. Electrochem. Commun. 2002, 4, 412.

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