Preparation of Silica-Coated Ultrathin Gold Nanowires with High

Feb 4, 2014 - Yoshiro Imura , Shiori Koizumi , Ryota Akiyama , Clara Morita-Imura , and ... Yoshiro Imura , Takuya Mori , Clara Morita-Imura , Hiroki ...
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Preparation of Silica-Coated Ultrathin Gold Nanowires with High Morphological Stability Yoshiro Imura, Satoshi Hojo, Clara Morita, and Takeshi Kawai* Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan S Supporting Information *

ABSTRACT: We demonstrated a preparation method of silicacoated straight ultrathin Au nanowires (NWs). Water-dispersive ultrathin Au NWs capped with a long-chain amidoamine derivative (C18AA) were used for silica coating. The Au NWs were partially covered with 3-mercaptopropanoic acid by the ligand exchange method, and silica coating of the Au NWs was carried out by the hydrolysis of tetraethoxysilane (TEOS) at pH > 6.7 because the shape of the Au NWs was changed under acidic conditions. The thickness of the silica layer depended on the concentration of TEOS, and the layer was able to decrease to 6−10 nm thick. We also demonstrated that the silica-coated Au NWs had high morphological stabilities against external stimuli such as a TEM electron beam, heat, and pH compared with the bare Au NWs.

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INTRODUCTION During the past few decades, concentrated efforts have been made to achieve size and shape control of metal nanomaterials.1−6 In particular, gold nanocrystals have been intensely studied because of their characteristic size- and shapedependent optical properties, which make them interesting for a wide variety of applications.1 Various wet chemical approaches have been developed for the synthesis of spherical nanoparticles (NPs),7,8 nanoplates,9 nanorods,10−12 and nanowires (NWs).6,13−21 Recently, the Xia,13,14 Yang,15 Ravishankar,16,17 and other18−20 research groups have successfully fabricated ultrathin and straight single-crystal Au NWs with diameters of ca. 2 nm using oleylamine as a soft template. Ultrathin Au NWs are expected to be applied as nanoconnectors and nanocatalysts. Xia13 and Giersig18 showed that ultrathin Au NWs were easily broken by external stimuli such as electron beam irradiation. When exposed to a TEM electron beam with accelerating voltage of 120 kV for 10 s, all ultrathin Au NWs were observed to break down into fragments (10−100 nm in length) due to the relatively high localized temperature on the NWs.18,20 Therefore, preparation of ultrathin Au NWs having high thermal stability is desired for nanocatalysis applications under high temperature. Additionally, silica coating of Au nanocrystals (NCs) appears to be an attractive alternative for providing enhanced colloidal stability.1 Although several procedures have been developed for silica-coating of colloidal particles,22−24 the selection of a suitable method is mainly determined by the chemical affinity of the surface material for silica. While some materials such as magnetite,25 titania,26 or zirconia27 can be directly coated with silica, Au NCs require the presence of poly(vinylpyrrolidone) (PVP), aminopropyltrimethoxysilane (APS), or 3-mercaptopropionic acid (3-MPA) on the surface of Au.22,23,28 PVP, APS, © 2014 American Chemical Society

and 3-MPA not only facilitate transfer into ethanol but also promote silica coating. Furthermore, Somorjai et al. showed that mesoporous silica-coated nanocrystals could be used as catalysts under high temperature due to the improvement of their thermal stability.29 Therefore, we prepared silica-coated ultrathin Au NWs for improving the thermal stability of NWs. Previous work has demonstrated that ultrathin Au NWs with diameters of 2 nm and lengths of a few micrometers can be prepared on a lamellar structure in an organogel of a long-chain amidoamine derivative (C18AA, Figure 1).30,31 In this paper, we first examined the pH resistance of ultrathin Au NWs and showed that the morphology of NWs was not changed above pH 6.7. Furthermore, the ultrathin Au NWs were coated with silica to improve their thermal stability above pH 6.7 where the shape is retained. The stability toward external stimuli such as TEM electron beams and temperature was improved by coating

Figure 1. (a) Molecular structure of C18AA. (b) Lamellar structure of C18AA in toluene. Received: September 23, 2013 Revised: January 15, 2014 Published: February 4, 2014 1888

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the ultrathin Au NWs with silica. The silica-coated ultrathin Au NWs are expected to be applied to nanocatalysis under high temperature.

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EXPERIMENTAL SECTION

Materials. 3 N tetraethoxysilane (TEOS), 1 M hydrochloric acid, 1 M sodium hydroxide, ethanol, and toluene were obtained from Kanto Chemicals and used as received without further purification. 28 wt % ammonia aqueous solution was obtained from Wako Pure Chemical Industries. 3-Mercaptopropanoic acid (3-MPA) was obtained from Aldrich. Hydrogen tetrachloroaurate tetrahydrate (HAuCl4·4H2O) was obtained from Nacalai Tesque. Preparation of Water-Soluble Ultrathin Au NWs.30,31 C18AA was synthesized according to a previous report.30−35 Ultrathin Au NWs were prepared by the following method. HAuCl4·4H2O (20 mg) was added to 2 wt % C18AA-toluene gel (10 g) and heated to 55 °C. A 1 M THF solution of LiEt3BH (0.1 mL) as a reducing agent was added to the C18AA toluene and then left for 8 h at 55 °C without stirring. Water-soluble ultrathin Au NWs were obtained by the phase transfer method. Water (10 mL) was gently poured into the ultrathin Au NWs in toluene (10 mL), and the biphasic mixture was left to stand for 24 h. pH Resistance of Water-Soluble Ultrathin Au NWs. The original pH value of water-soluble ultrathin Au NWs was 8.3 after transferring from the toluene phase to the water phase. For evaluation of the stability against pH, 1 M HCl and 1 M NaOH were used to adjust the pH. The morphological stability of Au NWs was checked after 1 day. Ligand Exchange of Ultrathin Au NWs. 0.2, 0.7, 2.0, or 20 mM 3-MPA aqueous solution (1.5 mL) was added into water-soluble ultrathin Au NWs (2.0 mL) at pH 8.3, and the mixture was left for 24 h at room temperature without stirring. Synthesis of Silica-Coated Ultrathin Au NWs. Water-soluble Au NWs capped with 3-MPA and without 3-MPA were employed for the silica-coating procedure. The pH values of Au NWs dispersions were adjusted to pH 6.7 by 1 M HCl. TEOS in ethanol solution (5 mL) and 1.0 wt % NH3 in ethanol solution (10.3 mL) were added into a water-soluble ultrathin Au NWs dispersion (2.0 mL), and the mixture was left for 24 h at room temperature without stirring. Evaluation of the Morphological Stability. Morphological stability of ultrathin Au NWs was evaluated on a JEOL TEM (JEM1011). Electron resistance of Au NWs was evaluated by exposure to 100 kV of a TEM electron beam. Thermal stability was evaluated by heating Au NWs on a TEM copper grid at 150 °C for 3 h.

Figure 2. TEM images of ultrathin Au NWs (a, b) before and (c, d) after transfer from toluene phase to water phase.

Figure 3. TEM images of Au nanocrystals at pH values of (a) 11.7, (b) 7.4, (c) 6.7, and (d) 3.0.

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RESULTS AND DISCUSSION pH Resistance of Water-Soluble Ultrathin Au NWs. Ultrathin Au NWs with diameters of 2 nm and lengths above a few hundred nanometers were synthesized by reducing HAuCl4 in a C18AA-toluene gel according to the literature (Figure 2a).30,31 A periodic fringe of 0.23 nm corresponding to the (111) lattice spacing along the long axis of the Au NWs in a high-resolution TEM image indicated that ultrathin Au NWs were grown in the (111) direction (Figure 2b). Since silica coating of Au NPs or Au nanorods is usually conducted in polar solvents,1,28,36 the ultrathin Au NWs dispersed in toluene were then transferred into the water phase. The length of ultrathin Au NWs was changed to short-length one during the phase transfer because of a fragile property of the NWs (Figure 2c,d). To coat Au NWs with silica by hydrolysis of TEOS, basic or acidic conditions are required to promote the production of silica, and thus we examined the pH resistance of the ultrathin Au NWs dispersed in water. The original pH of the aqueous dispersion was 8.3. Figure 3 shows TEM images of ultrathin Au NWs at various pH values. The morphology of ultrathin Au NWs was maintained under basic conditions of pH 11.7 (Figure 3a) and neutral conditions of pH 7.4 and 6.7 (Figure

3b,c) but changed to NPs under acidic conditions of pH 3.0 (Figure 3d). This is likely caused by a change in the packing structure of C18AA adsorbed on the Au surface due to protonation of the tertiary amine moiety to form a quaternary amine at pH ∼6.37 It was found that the use of basic catalysts such as ammonia was most favorable for SiO2 coating of Au NWs. Preparation of Silica-Coated Ultrathin Au NWs. The conventional preparation method for SiO2 coating of Au NPs or nanorods was performed in ethanol solution of TEOS under basic conditions of ammonia.1,23,36 In order to cover Au NWs with a SiO2 shell, 1.1 mM ethanol solution of TEOS (5 mL) and 1.0 wt % ammonia solution (10.3 mL) were added to water-soluble Au NWs (2 mL). However, the Au NWs rapidly precipitated and the color changed from dark red to transparent. The precipitation was caused by the addition of ethanol (or a high concentration of ethanol) because similar precipitation was observed by adding pure ethanol into the water-soluble Au NWs. This phenomenon implied that C18AA-capped ultrathin Au NWs have a low dispersibility in ethanol. Since the dispersion of Au NCs can be easily improved 1889

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respectively. Figure 6 indicated that silica-coated Au NWs were not aggregated in the solution.

by changing the capping molecules, we applied ligand exchange methods to make ethanol-dispersible Au NWs. According to a previous report on silica coating of Au NPs,28 we used 3mercaptopropanoic acid (3-MPA) as the exchange capping molecule. We previously showed that ultrathin Au NWs are fragile, and their morphology is easily changed when the capping molecules are exchanged completely from C18AA to thiol compounds because C18AA covering on Au NWs is strictly necessary to retain their original shape.30 Thus, we needed to find a partial exchange condition for dispersibility in water/ethanol while retaining the NWs morphology. 1.5 mL of aqueous solutions of varying concentrations of 3-MPA (0.2, 0.7, 2.0, and 20 mM) was added into a 2 mL aqueous dispersion of Au NWs, and the mixtures were left for 24 h at room temperature to examine the morphological changes. The morphology of the Au NWs was retained at [3-MPA] < 0.7 mM, whereas they lost their original shape at high concentrations of [3-MPA] > 2.0 mM (Figure 4). Since full

Figure 5. TEM images of silica-coated ultrathin Au NWs synthesized at TEOS concentrations of (a, b) 1.1 mM and (c, d) 0.5 mM. The original pH value was ∼10.

Figure 6. Low-magnification TEM images of silica-coated ultrathin Au NWs synthesized at TEOS concentrations of (a) 1.1 mM and (b) 0.5 mM. Magnification is (a) 15 000 and (b) 30 000.

Figure 4. TEM images of Au nanocrystals after adding 3-MPA aqueous solutions of (a) 20, (b) 2, (c) 0.7, and (d) 0.2 mM.

Evaluating the Stability of Silica-Coated Ultrathin Au NWs. After drying the silica-coated Au NWs, they could be completely dispersed in water, whereas a dried sample of the original bare Au NWs did not disperse in any solvents. Furthermore, a dried sample of silica-coated Au NWs preserved for a few months also had a high dispersibility in water. From this dispersibility improvement, it is naturally expected that silica coating would also improve the morphological stability of Au NWs and provide other environmental resistances such as toward heat and pH. It is well-known that ultrathin Au NWs are easily broken upon exposure to a high-energy electron beam during TEM observations.6,18,20 Figure 7 shows TEM images of 6−10 nm silica-coated Au NWs under exposure to a TEM electron beam with accelerating voltage of 100 kV compared to those of the original Au NWs. Although the bare Au NWs were cut within 1 min (Figure 7b), the silica-coated ultrathin Au NWs kept their original morphology after an exposure of 1 min. Even a longer exposure time of ∼10 min did not affect the morphology (Figure 7d), proving that silica-coating improves the resistance of Au NWs to a high-energy electron beam. Furthermore, the silica-coated Au NWs had thermal and pH resistances. When Au NWs on a TEM-copper grid were heated at 150 °C for 3 h, the bare Au NWs turned into NPs and completely disappeared (Figure 8a), whereas the silica-coated Au NWs retained their

exchange of the capping molecule from C18AA to thiol compound brings about the shape change of Au NWs as abovementioned, at high concentration of [3-MPA] > 2.0 mM the exchange ratio of 3-MPA to C18AA is thought to be a quite high, and consequently it is difficult to keep the shape of Au NWs due to a lower covering density of C18AA. Furthermore, when 15 mL ethanol was added into the former dispersion (2.0 mL) containing Au NWs with no morphological change, the Au NWs of [3-MPA] = 0.7 mM maintained their dispersibility, but those of [3-MPA] = 0.2 mM precipitated and consequently the solution became transparent. Therefore, we performed the coating of Au NWs with SiO2 at a concentration of [3-MPA] = 0.7 mM. An ethanol solution of 0.5 or 1.1 mM TEOS (5.0 mL) and a 1 wt % NH3 solution (10.3 mL) were added to the dispersion of Au NWs capped with 3-MPA (2.0 mL), and the mixture was left for 24 h without stirring at room temperature. The original pH value was ∼10. As shown in Figure 5, we successfully prepared Au NWs covered with an ultrathin SiO2 shell. The thicknesses depended on the concentration of TEOS and were 6−10 and 14−18 nm at [TEOS] = 0.5 and 1.1 mM, respectively. Low-magnification TEM images of as-prepared silica-coated Au NWs and silica-coated Au NWs collected by centrifugation are shown in Figure 6 and Figure S1, 1890

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capping molecule, and TEOS concentration. Further, we showed that the silica coating produced the improvement of morphological stability of C18AA-capped ultrathin Au NWs against external stimuli, such as pH change and heat. The thickness of the silica-coating layer was controllable from 6 to 18 nm by changing the TEOS concentration. The silica-coated Au NWs had high dispersibility in water and could be redispersed in water even after drying. It was also demonstrated that the silica-coated Au NWs have high morphological stability against external stimuli such as a TEM electron beam, heat, and pH changes compared with the bare Au NWs.

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ASSOCIATED CONTENT

S Supporting Information *

TEM image of silica-coated Au NWs. This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 7. TEM images of ultrathin Au NWs after irradiation of TEM electron beam for (a) 0 and (b) 1 min. TEM images of silica-coated ultrathin Au NWs after irradiation of TEM electron beam for (c) 0 and (d) 10 min.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for JSPS Fellows from the Ministry of Education, Culture, Sports, Science, and Technology of Japan.

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REFERENCES

(1) Pastoriza-Santos, I.; Pérez-Juste, J.; Liz-Marzán, L. M. SilicaCoating and Hydrophobation of CTAB-Stabilized Gold Nanorods. Chem. Matter. 2006, 18, 2465−2467. (2) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment. J. Phys. Chem. B 2003, 107, 668−677. (3) Liz-Marzán, L. M. Tailoring Surface Plasmons through the Morphology and Assembly of Metal Nanoparticles. Langmuir 2006, 22, 32−41. (4) Burda, C.; Chen, X.; Narayanan, R.; El-Sayed, M. A. Chemistry and Properties of Nanocrystals of Different Shapes. Chem. Rev. 2005, 105, 1025−1102. (5) Haes, A. J.; Van Duyne, R. P. A Nanoscale Optical Biosensor: Sensitivity and Selectivity of an Approach Based on the Localized Surface Plasmon Resonance Spectroscopy of Triangular Silver Nanoparticles. J. Am. Chem. Soc. 2002, 124, 10596−10604. (6) Sakai, T.; Alexandridis, P. Ag and Au Monometallic and Bimetallic Colloids: Morphogenesis in Amphiphilic Block Copolymer Solutions. Chem. Mater. 2006, 18, 2577−2583. (7) Brust, M.; Walker, M.; Bethell, D.; Sciffrin, D. J.; Whyman, R. Synthesis of Thiol-Derivatised Gold Nanoparticles in a Two-Phase Liquid−Liquid System. J. Chem. Soc., Chem. Commun. 1994, 801−802. (8) Sakai, T.; Alexandridis, P. Mechanism of Gold Metal Ion Reduction, Nanoparticle Growth and Size Control in Aqueous Amphiphilic Block Copolymer Solutions at Ambient Conditions. J. Phys. Chem. B 2005, 109, 7766−7777. (9) Sakai, T.; Alexandridis, P. Size- and Shape-Controlled Synthesis of Colloidal Gold through Autoreduction of the Auric Cation by Poly(ethyleneoxide)−Poly(propylene oxide) Block Copolymers in Aqueous Solutions at Ambient Conditions. Nanotechnology 2005, 16, S344−S353. (10) Murphy, C. J.; Jana, N. R. Controlling the Aspect Ratio of Inorganic Nanorods and Nanowires. Adv. Mater. 2002, 14, 80−82. (11) Jana, N. R.; Gearheart, L.; Murphy, C. J. Wet Chemical Synthesis of Silver Nanorods and Nanowires of Controllable Aspect Ratio. Chem. Commun. 2001, 617−618.

Figure 8. TEM images of nanocrystals after heating (a) ultrathin Au NWs and (b) silica-coated ultrathin Au NWs for 3 h.

Figure 9. TEM images of silica-coated ultrathin Au NWs at (a) pH 8.0 and (b) pH 3.0.

original shape (Figure 8b and Figure S2). In addition, as mentioned in the previous section (Figure 3), the bare Au NWs were easily transformed under acidic conditions of pH 3, whereas the silica-coated Au NWs retained their morphology between pH 3 and pH 10, showing high pH resistance (Figures 5 and 9). Therefore, these results prove that silica coating is a very valuable method for imparting environmental resistance to ultrathin Au NWs. Providing high thermal stability would be effective for new nanocatalysts since they can then function under high-performance conditions at higher temperature.

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CONCLUSION This paper demonstrated a preparation method for silica-coated straight ultrathin Au nanowires (NWs) and described detailed preparation conditions such as pH, concentration of 3-MPA as 1891

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(12) Smith, D. K.; Miller, N. R.; Korgel, B. A. Iodide in CTAB Prevents Gold Nanorod Formation. Langmuir 2009, 25, 9518−9524. (13) Lu, X.; Yavuz, M. S.; Tuan, H.-Y.; Korgel, B. A.; Xia, Y. Ultrathin Gold Nanowires Can Be Obtained by Reducing Polymeric Strands of Oleylamine−AuCl Complexes Formed via Aurophilic Interaction. J. Am. Chem. Soc. 2008, 130, 8900−8901. (14) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Shape-Controlled Synthesis of Metal Nanocrystals: Simple Chemistry Meets Complex Physics. Angew. Chem., Int. Ed. 2009, 48, 60−103. (15) Huo, Z.; Thung, C.; Huang, W.; Zhang, X.; Yang, P. Sub-Two Nanometer Single Crystal Au Nanowires. Nano Lett. 2008, 8, 2041− 2044. (16) Halder, A.; Ravishankar, N. Ultrafine Single-Crystalline Gold Nanowire Arrays by Oriented Attachment. Adv. Mater. 2007, 19, 1854−1858. (17) Ravishankar, N. Seeing Is Believing: Electron Microscopy for Investigating Nanostructures. J. Phys. Chem. Lett. 2010, 1, 1212−1220. (18) Pazos-Pérez, N.; Baranov, D.; Irsen, S.; Hilgendorff, M.; LizMarzán, L. M.; Giersig, M. Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media. Langmuir 2008, 24, 9855−9860. (19) Feng, H.; Yang, Y.; You, Y.; Li, G.; Guo, J.; Yu, T.; Shen, Z.; Wu, T.; Xing, B. Simple and Rapid Synthesis of Ultrathin Gold Nanowires, Their Self-Assembly and Application in Surface-Enhanced Raman Scattering. Chem. Commun. 2009, 1984−1986. (20) Wang, C.; Sun, S. Facile Synthesis of Ultrathin and SingleCrystalline Au Nanowires. Chem.Asian J. 2009, 4, 1028−1034. (21) Xu, J.; Wang, H.; Liu, C.; Yang, Y.; Chen, T.; Wang, Y.; Wang, F.; Liu, X.; Xing, B.; Chen, H. Mechanical Nanosprings: Induced Coiling and Uncoiling of Ultrathin Au Nanowires. J. Am. Chem. Soc. 2010, 132, 11920−11922. (22) Liz-Marzán, L. M.; Giersig, M.; Mulvaney, P. Synthesis of Nanosized Gold−Silica Core−Shell Particles. Langmuir 1996, 12, 4329−4335. (23) Graf, C.; Vossen, D. L. J.; Imhof, A.; van Blaaderen, A. A General Method To Coat Colloidal Particles with Silica. Langmuir 2003, 19, 6693−6700. (24) Liu, S.; Hang, M. Synthesis, Functionalization, and Bioconjugation of Monodisperse, Silica-Coated Gold Nanoparticles: Robust Bioprobes. Adv. Funct. Mater. 2005, 15, 961−967. (25) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Magnetic Silica Dispersions: Preparation and Stability of SurfaceModified Silica Particles with a Magnetic Core. Langmuir 1994, 10, 92−99. (26) Bechger, L.; Koenderink, A. F.; Vos, W. L. Emission Spectra and Lifetimes of R6G Dye on Silica-Coated Titania Powder. Langmuir 2002, 18, 2444−2447. (27) Ryan, J. N.; Elimelech, M.; Baseman, J. L.; Magelky, R. D. SilicaCoated Titania and Zirconia Colloids for Subsurface Transport Field Experiments. Environ. Sci. Technol. 2000, 34, 2000−2005. (28) Cho, E. C.; Choi, S.-W.; Camargo, P. H. C.; Xia, Y. ThiolInduced Assembly of Au Nanoparticles into Chainlike Structures and Their Fixing by Encapsulation in Silica Shells or Gelatin Microspheres. Langmuir 2010, 26, 10005−10012. (29) Joo, S. H.; Park, J. Y.; Tsung, C.-K.; Yamada, Y.; Yang, P.; Somorjai, G. A. Thermally stable Pt/Mesoporous Silica Core−Shell Nanocatalysts for High-Temperature Reactions. Nat. Mater. 2009, 8, 126−131. (30) Imura, Y.; Tanuma, H.; Sugimoto, H.; Ito, R.; Hojo, S.; Endo, H.; Morita, C.; Kawai, T. Water-Dispersible Ultrathin Au Nanowires Prepared Using a Lamellar Template of a Long-Chain Amidoamine Derivative. Chem. Commun. 2011, 47, 6380−6382. (31) Morita, C.; Tanuma, H.; Kawai, C.; Ito, Y.; Imura, Y.; Kawai, T. Room-Temperature Synthesis of Two-Dimensional Ultrathin Gold Nanowire Parallel Array with Tunable Spacing. Langmuir 2013, 29, 1669−1675. (32) Imura, Y.; Maezawa, A.; Morita, C.; Kawai, T. Neuron-Shaped Gold Nanocrystals and Two-Dimensional Dendritic Gold Nanowires Fabricated by Use of a Long-Chain Amidoamine Derivative. Langmuir 2012, 28, 14998−15004.

(33) Imura, Y.; Morita, C.; Endo, H.; Kondo, T.; Kawai, T. Reversible Phase Transfer and Fractionation of Au Nanoparticles by pH Change. Chem. Commun. 2010, 46, 9206−9208. (34) Morita, C.; Sugimoto, H.; Matsue, K.; Kondo, T.; Imura, Y.; Kawai, T. Changes in Viscosity Behavior from a Normal Organogelator to a Heat-Induced Gelator for a Long-Chain Amidoamine Derivative. Chem. Commun. 2010, 46, 7969−7971. (35) Morita, C.; Aoyama, T.; Imura, Y.; Kawai, T. Novel Thermoresponsive Coloring Phenomena in Water/Surfactant/Oil Emulsions. Chem. Commun. 2011, 47, 11760−11762. (36) Han, Y.; Jiang, J.; Lee, S. S.; Ying, J. Y. Reverse MicroemulsionMediated Synthesis of Silica-Coated Gold and Silver Nanoparticles. Langmuir 2008, 5842−5848. (37) Wang, W.; Lu, W.; Jiang, L. Influence of pH on the Aggregation Morphology of a Novel Surfactant with Single Hydrocarbon Chain and Multi-Amine Headgroups. J. Phys. Chem. B 2008, 112, 1409− 1413.

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