Controlled Synthesis of Dendritic Gold Nanostructures Assisted by

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Controlled Synthesis of Dendritic Gold Nanostructures Assisted by Supramolecular Complexes of Surfactant with Cyclodextrin Teng Huang, Fei Meng, and Limin Qi* Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry, Peking University, Beijing 100871, People’s Republic of China Received November 19, 2009. Revised Manuscript Received December 16, 2009 Controlled synthesis of well-defined planar Au nanodendrites with a symmetric single-crystalline structure consisting of trunks and side branches grown along the Æ211æ directions was realized by reducing chloroauric acid in aqueous mixed solutions of dodecyltrimethylammonium bromide (DTAB) and β-cyclodextrin (β-CD). It has been revealed that the formation of the supramolecular complexes of DTAB with β-CD due to host-guest interaction is indispensable for the fabrication of these unique planar Au nanodendrites, and a proper CD-to-DTAB molar ratio is essential to their exclusive formation. A variety of Au nanostructures, such as branched particles consisting of rodlike branches and flowerlike particles consisting of platelike petals, could be readily obtained by simply changing the CD-to-DTAB molar ratio. Moreover, the obtained Au nanodendrites exhibited both a good electrocatalytic activity toward the oxidation of methanol and a good surface-enhanced Raman scattering (SERS) sensitivity for detecting p-aminothiophenol (PATP) molecules, indicating their potential applications including catalysis, biosensing, and nanodevices.

Introduction In recent years, gold nanostructures have stimulated great interest because of their fascinating optical, electronic, and chemical properties and promising applications in nanoelectronics, biomedicine, sensing, and catalysis.1 Since the intrinsic properties and relevant applications of Au nanostructures are largely determined by their size and shape, great efforts have been devoted to the morphology-controlled synthesis of Au nanostructures in the past decade.2 In particular, a variety of wet chemical methods have been developed to fabricate Au nanostructures with various shapes such as rods,3 wires,4 belts,5 plates,6 polyhedra,7 and branched particles.8 However, it remains a challenge to synthesize Au nanostructures with hierarchical architectures, which is a crucial step toward integrating nanoscale Au building blocks into complex functional systems. In this *To whom correspondence should be addressed. E-mail: [email protected]. Fax þ86-10-62751708.

(1) (a) Sperling, R. A.; Gil, P. R.; Zhang, F.; Zanella, M.; Parak, W. J. Chem. Soc. Rev. 2008, 37, 1896. (b) Wilson, R. Chem. Soc. Rev. 2008, 37, 2028. (c) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578. (2) (a) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. Rev. 2008, 37, 1783. (b) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (3) (a) Gou, L.; Murphy, C. J. Chem. Mater. 2005, 17, 3668. (b) Xu, X.; Cortie, M. B. Adv. Funct. Mater. 2006, 16, 2170. (4) (a) Lu, X.; Yavuz, M. S.; Tuan, H.-Y.; Korgel, B. A.; Xia, Y. J. Am. Chem. Soc. 2008, 130, 8900. (b) Wang, C.; Hu, Y.; Lieber, C. M.; Sun, S. J. Am. Chem. Soc. 2008, 130, 8902. (5) (a) Zhang, J.; Du, J.; Han, B.; Liu, Z.; Jiang, T.; Zhang, Z. Angew. Chem., Int. Ed. 2006, 45, 1116. (b) Zhao, N.; Wei, Y.; Sun, N.; Chen, Q.; Bai, J.; Zhou, L.; Qin, Y.; Li, M.; Qi, L. Langmuir 2008, 24, 991. (6) (a) Sun, X.; Dong, S.; Wang, E. Angew. Chem., Int. Ed. 2004, 43, 6360. (b) Li, C.; Cai, W.; Cao, B.; Sun, F.; Li, Y.; Kan, C.; Zhang, L. Adv. Funct. Mater. 2006, 16, 83. (7) (a) Ma, Y.; Kuang, Q.; Jiang, Z.; Xie, Z.; Huang, R.; Zheng, L. Angew. Chem., Int. Ed. 2008, 47, 8901. (b) Niu, W.; Zheng, S.; Wang, D.; Liu, X.; Li, H.; Han, S.; Chen, J.; Tang, Z.; Xu, G. J. Am. Chem. Soc. 2009, 131, 697. (c) Jeong, G. H.; Kim, M.; Lee, Y. W.; Choi, W.; Oh, W. T.; Park, Q.-H.; Han, S. W. J. Am. Chem. Soc. 2009, 131, 1672. (8) (a) Yuan, H.; Ma, W.; Chen, C.; Zhao, J.; Liu, J.; Zhu, H.; Gao, X. Chem. Mater. 2007, 19, 1592. (b) Li, Z.; Li, W.; Camargo, P. H. C.; Xia, Y. Angew. Chem., Int. Ed. 2008, 47, 9653.

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regard, single-crystalline, comblike gold nanostructures were successfully obtained in aqueous solutions of mixed cationic/ anionic surfactants.5b It is worth noting that metallic nanodendrites with hyperbranched architectures have attracted much attention due to their importance in understanding the fascinating fractal growth phenomena and their potential applications in functional devices, plasmonics, biosensing, and catalysis; recent examples include the solution-phase synthesis of dendritic nanostructures of Ag,9 Cu,10 Pd,11 and Pt.12 However, there are rare reports on the dendritic gold nanostructures of the hyperbranched type, and the obtained gold nanodendrites are usually polycrystalline aggregates.13 Recently, we reported the preparation of hierarchical, 3-fold symmetrical, single-crystalline gold dendrites on zinc substrate in the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate.14 It would be more desirable to develop facile methods for the controlled synthesis of singlecrystalline dendritic gold nanostructures with a well-defined crystal orientation in aqueous solutions. As a class of water-soluble and nontoxic cyclic oligosaccharides with a hydrophilic exterior and a hydrophobic interior, cyclodextrins (CDs) have been extensively investigated in host-guest chemistry for construction of versatile supramolecular complexes owing to their special hydrophobic cavities.15 In recent years, (9) (a) Zhou, Y.; Yu, S. H.; Wang, C. Y.; Li, X. G.; Zhu, Y. R.; Chen, Z. Y. Adv. Mater. 1999, 11, 850. (b) Wen, X.; Xie, Y.-T.; Mak, M. W. C.; Cheung, K. Y.; Li, X.-Y.; Renneberg, R.; Yang, S. Langmuir 2006, 22, 4836. (c) Han, Y.; Liu, S.; Han, M.; Bao, J.; Dai, Z. Cryst. Growth Des. 2009, 9, 3941. (10) Qiu, R.; Cha, H. G.; Noh, H. B.; Shim, Y. B.; Zhang, X. L.; Qiao, R.; Zhang, D.; Kim, Y.; Pal, U.; Kang, Y. S. J. Phys. Chem. C 2009, 113, 15891. (11) Song, Y.-J.; Kim, J.-Y.; Park, K.-W. Cryst. Growth Des. 2009, 9, 505. (12) Song, Y.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H.; Jiang, Y.; Brinker, C. J.; van Swol, F.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (13) (a) Pang, S.; Kondo, T.; Kawai, T. Chem. Mater. 2005, 17, 3636. (b) Lu, G.; Li, C.; Shi, G. Chem. Mater. 2007, 19, 3433. (14) Qin, Y.; Song, Y.; Sun, N.; Zhao, N.; Li, M.; Qi, L. Chem. Mater. 2008, 20, 3965. (15) Douhal, A. Chem. Rev. 2004, 104, 1955.

Published on Web 12/31/2009

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CDs have been employed in the facile synthesis,16 surface functionalization,17 and controlled assembly of Au nanoparticles.18 For example, the facile synthesis and one-dimensional assembly of R-CD-capped Au nanoparticles were realized by using R-CD as both the reducing and stabilizing agents recently.19 However, there have been only few reports on the use of natural CDs to synthesize Au nanostructures with multiple shapes because of their relatively weak ability in controlling the shape of Au nanoparticles. It is noted that surfactants, which are very effective in morphological control of Au nanocrystals, can be included by CDs through forming host-guest complexes.20 Nowadays, the formation of supramolecular complexes via the host-guest interaction between surfactants and β-CD is a widely investigated topic,21 which inspires us to explore the use of mixed surfactant/ β-CD solutions for the shape-controlled synthesis of Au nanostructures. The formed surfactant-CD inclusion complexes may exert a subtle control on the growth of Au nanocrystals in solution due to the special adsorption behaviors of the supramolecular complexes, thereby leading to a more delicate control of morphology. Herein, we report the controlled synthesis of novel, planar, single-crystalline, dendritic Au nanostructures with their trunks and side branches grown along the Æ211æ directions by reducing chloroauric acid in aqueous solutions of dodecyltrimethylammonium bromide (DTAB) and β-CD. The formation of inclusion complexes of DTAB and β-CD due to host-guest interaction is indispensable for the formation of such Au nanodendrites. To the best of our knowledge, this is the first report on the shapecontrolled synthesis of Au nanostructures with the assistance of surfactant-CD supramolecular complexes. Moreover, both the electrocatalytic activity toward the oxidation of methanol and the surface-enhanced Raman scattering (SERS) sensitivity for detecting p-aminothiophenol (PATP) molecules have been investigated to demonstrate the potential applications of the obtained dendritic gold nanostructures.

Experimental Section Materials. Hydrochloroauric acid trihydrate (HAuCl4 3 3H2O, 99.9%) and ascorbic acid (AA, 99.7%) were obtained from Beijing Chemical Reagents Co. Dodecyltrimethylammonium bromide (DTAB), β-cyclodextrin (β-CD), and p-aminothiophenol (PATP) were purchased from Alfa Aesar. All the other chemicals were of analytical grade. The water used was deionized. Synthesis. The synthesis of dendritic Au nanostructures was simply achieved by the reduction of HAuCl4 with AA in aqueous mixed DTAB/β-CD solutions. In a typical synthesis, 3.85 mL of water, 0.50 mL of 0.05 M DTAB, and 0.25 mL of 0.05 M β-CD were first mixed at room temperature to give a clear solution, which was held at 27 C for ∼1 h. Then, 0.10 mL of 0.01 M HAuCl4 and 0.30 mL of 0.10 M AA were added and mixed with the solution successively. The final concentrations of DTAB, β-CD, HAuCl4, and AA were 5.0, 2.5, 0.2, and 6.0 mM, respectively, (16) Pande, S.; Ghosh, S. K.; Praharaj, S.; Panigrahi, S.; Basu, S.; Jana, S.; Pal, A.; Tsukuda, T.; Pal, T. J. Phys. Chem. C 2007, 111, 10806. (17) Park, C.; Youn, H.; Kim, H.; Noh, T.; Kook, Y. H.; Oh, E. T.; Park, H. J.; Kim, C. J. Mater. Chem. 2009, 19, 2310. (18) (a) Liu, J.; Alvarez, J.; Ong, W.; Kaifer, A. E. Nano Lett. 2001, 1, 57. (b) Liu, Y.; Yang, Y.-W.; Chen, Y. Chem. Commun. 2005, 4208. (c) Liu, Z.; Jiang, M. J. Mater. Chem. 2007, 17, 4249. (19) Huang, T.; Meng, F.; Qi, L. J. Phys. Chem. C 2009, 113, 13636. (20) (a) Dharmawardana, U. R.; Christian, S. D.; Tucker, E. E.; Taylor, R. W.; Scamehorn, J. F. Langmuir 1993, 9, 2258. (b) Mwakibete, H.; Cristantino, R.; Bloor, D. M.; Wyn-Jones, E.; Holzwarth, J. F. Langmuir 1995, 11, 57. (c) Dorrego, A. B.; Garcia-Rio, L.; Herves, P.; Leis, J. R.; Mejuto, J. C.; Perez-Juste, J. Angew. Chem., Int. Ed. 2000, 39, 2945. (21) (a) Xing, H.; Lin, S.-S.; Yan, P.; Xiao, J.-X. Langmuir 2008, 24, 10654. (b) Jiang, L.; Deng, M.; Wang, Y.; Liang, D.; Yan, Y.; Huang, J. J. Phys. Chem. B 2009, 113, 7498.

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giving a CD-to-DTAB molar ratio of 0.5. Finally, the reaction mixture was kept at 27 C under static conditions for ∼12 h, leading to the formation of Au nanodendrites. The CD-to-DTAB molar ratios or the concentrations of DTAB were varied to examine their effects on the synthesis of Au nanostructures. The samples were collected by centrifugation and washed three times with deionized water. Characterization. The products were characterized by scanning electron microscopy (SEM, Hitachi S4800, 15 kV), transmission electron microscopy (TEM, JEOL JEM-200CX, 160 kV), and high-resolution TEM (HRTEM, FEI Tecnai F30, 300 kV) together with associated energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD, Rigaku Dmax-2000, Ni-filtered Cu KR radiation), and UV-vis spectrophotometry (PerkinElmer Lambda 35). Electrospray ionization mass spectrometry (ESI-MS) measurements were performed on a Fourier transform ion cyclotron resonance mass spectrometer (FTMS, Bruker APEX IV) with deionized water as the solvent. Electrochemical Measurements. A CHI 660C electrochemical workstation (Shanghai CH Instruments, China) with a conventional three-electrode cell was used to perform electrochemical measurements. The working electrode was a glassy carbon electrode with a diameter of 4 mm. A KCl-saturated calomel electrode (SCE) was used as the reference electrode and a platinum electrode as the auxiliary electrode. All electrochemical experiments were conducted at ambient temperature (25 ( 2 C) under N2 protection. For the preparation of Au dendritemodified electrodes, the prepared dendrite Au nanostructures were dispersed in deionized water to obtain a uniform suspension by sonication. Glassy carbon (GC) electrodes were first polished with 0.05 μm alumina slurry and then washed ultrasonically in distilled water and ethanol for a few minutes. The GC electrodes were coated by casting the gold nanodendrite suspension and dried under an infrared lamp. Finally, 1 wt % Nafion solution in alcohol was cast on the surface of the sample and dried naturally in the air. For comparison purposes, a commercial polycrystalline Au (poly-Au) electrode was mechanically cleaned, electrochemically activated with vitriol, and then used for electrochemical measurements without the casting of a Nafion thin film. Raman Measurements. Raman measurements were conducted with a Renishaw System 1000 Raman imaging microscope (Renishaw plc, U.K.) equipped with a 25 mW (632.8 nm) He-Ne laser (model 127-25RP, Spectra-Physics) and a Peltier-cooled CCD detector (576 pixels 384 pixels). A 50 objective (NA = 0.80) mounted on an Olympus BH-2 microscope was used to focus the laser onto a spot ∼1 μm diameter and collect the backscattered light from the sample. For the preparation of SERS samples, the p-aminothiophenol (PATP) molecules were assembled on the surface of gold nanodendrites by immersing in 5 mM PATP solution for 12 h to ensure a saturated coverage of PATP, i.e., formation of a complete self-assembled film of PATP. Then, the solids were collected and rinsed thoroughly with ethanol, followed by redispersing in ethanol by sonication. Finally, one drop of the suspension was dropped onto the surface of a silicon wafer and dried in an atmosphere of nitrogen. Typically, the incident light was focused onto single planar Au nanodendrites lying on the Si substrate. For comparison purposes, Au nanosheets were prepared as the reference sample using the reported method,6b and single Au nanosheets lying on the Si substrate were used for the SERS measurement by using the method identical to the measurement for Au nanodendrites.

Results and Discussion Figure 1 presents typical SEM images of the Au product obtained by reduction of HAuCl4 in aqueous solution of 5.0 mM DTAB and 2.5 mM β-CD. As shown in Figure 1a, the product consists almost entirely of hyperbranched dendritic DOI: 10.1021/la904393n

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Figure 1. SEM images of Au nanodendrites grown in mixed DTAB/β-CD solution. [DTAB] = 5.0 mM; [β-CD]/[DTAB] = 0.5.

Figure 2. XRD pattern of Au nanodendrites.

structures up to ∼2 μm in length. The aggregation of the dendrites is flowerlike, implying that these dendrites might grow radially from central particles. The enlarged images shown in Figure 1b,c suggest that each dendrite has a planar, highly symmetric structure, which consists of a pronounced trunk with two groups of symmetrical side branches grown on the trunk in parallel. It can be seen that the diameters of both the trunks and side branches are in the range of 50-200 nm, and the angles between the trunks and the side branches are all ∼60. Figure 1d shows that some parallel rodlike leaves or sub-branches (∼50 nm in diameter) can further grow on the side branches with a growth angle ∼60, resulting in the formation of a self-similar dendrite with a three-order structure. The XRD pattern of the hierarchical dendrites is shown in Figure 2, which exhibits sharp diffraction peaks exclusively attributed to Au crystals with the fcc structure (JCPDS No. 04-0784), indicating that the dendrites are pure, well-crystallized Au crystals. TEM observations were carried out to determine the crystal orientation of the obtained dendritic Au nanostructures (Figure 3). Figure 3a presents a typical TEM image of an individual Au dendrite lying on the copper grid. The dendrite exhibits a symmetrical planar structure with a pronounced trunk and two groups of parallel branches touching the grid, which is consistent with the SEM observations. The related electron diffraction (ED) pattern of the whole dendrite (Figure 3b) shows 7584 DOI: 10.1021/la904393n

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Figure 3. TEM image (a), ED pattern (b), HRTEM image (c), and EDS spectrum (d) of Au nanodendrites. The ED pattern shown in panel b corresponds to the whole region of panel a.

the forbidden (1/3){422} reflections, which can be indexed to the [111] zone axis of the cubic Au, indicating that the whole Au dendrite is a planar single crystal with the (111) plane as the top surface. Both the trunk and the side branches are grown along the Æ211æ direction, and the angle between them is ∼60, identical to the theoretical angle between two equivalent Æ211æ directions of the cubic Au in the (111) projection plane. The HRTEM image shown in Figure 3c exhibits clear lattice fringes with a d spacing of 0.24 nm, which is in good agreement with the 3  {422} superlattice spacing of cubic Au crystals, confirming that the nanodendrite is a single crystal with the trunk grown along the Æ211æ direction. Therefore, it may be concluded that the entire dendrite is a planar, symmetrical, single-crystalline gold structure with Æ211æ-oriented trunk and branches, indicating an interesting fractal growth. To our knowledge, such well-defined singlecrystalline dendrites with Æ211æ-oriented trunks and branches in the same plane have not been reported for gold so far. The associated energy-dispersive X-ray spectroscopy (EDS) spectrum presented in Figure 3d shows only the Au signals except the Cu and C signals arising from the carbon-covered copper grid used during the HRTEM analysis, confirming the formation of pure Au crystals. It is known that anisotropic gold nanoparticles normally exhibit two principal surface plasmon resonance (SPR) absorption peaks characteristic of the short (transverse band) and long (longitudinal band) axes.3 Figure 4 shows the UV-vis absorption spectrum of the as-prepared Au nanodendrites suspended in water. The spectrum displays a gradual increase in absorption from ∼500 nm to the near-IR region without indication of leveling off, which could be attributed to the longitudinal plasmon band, indicating a remarkable overlapping between the transverse band and the longitudinal band. It is well-known that the position and intensity of the longitudinal band depend largely on the size, aspect ratio, and mutual coupling of Au nanocrystals.1c,3 The observed overlapping between the transverse band and the longitudinal band for the obtained Au nanodendrites could be attributed to the polydispersity in the length and diameter of the trunks and branches, which may lead to a variety of sizes and aspect ratios. The multiple coupling between neighboring trunks and side branches could also result in a longitudinal plasmon band with the position lying in a wide range of wavelengths. So the observed absorption spectrum may represent a contour combinLangmuir 2010, 26(10), 7582–7589

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Figure 4. UV-vis absorption spectrum of Au nanodendrites.

Figure 5. TEM images of Au products obtained at early stages of the formation of Au nanodendrites. Reaction time: (a) 15 min and (b) 1 h.

ing the size, aspect ratio, and coupling factors of the hierarchical dendritic structures. The representative TEM images of the gold products obtained at the early stages of the dendrite formation are presented in Figure 5. After 15 min of reaction, thorny nanoparticles of about 100-200 nm were produced with short branches emanating from these nuclei (Figure 5a). When the reaction time was prolonged to 1 h, flowerlike aggregates with dendritic petals appeared (Figure 5b), indicating further growth from the original short branches. After 12 h of reaction, each petal would finally evolve into a planar symmetrical dendrite with a two- or three-order hierarchy as shown in Figure 1. It was found that the CD-to-DTAB molar ratio showed a remarkable effect on the formation of the Au nanostructures with a variety of morphologies (Figure 6). In the typical synthesis of Au nanodendrites described above, the concentrations of DTAB and β-CD were 5.0 and 2.5 mM, respectively, corresponding to a CD-to-DTAB molar ratio of 0.5. If no β-CD was added to the reaction system, the product consisted of irregular particles exhibiting faceted surfaces with sizes in the range of 300500 nm (Figure 6a). When 1.5 mM β-CD was added to the solution giving a CD-to-DTAB molar ratio of 0.3, branched particles (∼1 μm) consisting of rodlike branches were the predominant product (Figure 6b). The length and diameter of the branches were ∼300 and ∼100 nm, respectively. When the CDto-DTAB molar ratio was increased to 0.5, flowerlike aggregates consisting of well-defined, symmetric, planar Au dendrites were obtained, as shown in Figure 1. If the CD-to-DTAB molar ratio was increased to 1, very similar Au dendrites were obtained (Figure 6c). On increasing the CD-to-DTAB molar ratio to 2 or higher, flowerlike particles (∼800 nm) consisting of platelike petals became the predominant product (Figure 6d,e), which could be partially ascribed to the more effective capping of β-CD molecules to prevent further growth of Au crystals due to Langmuir 2010, 26(10), 7582–7589

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the relatively high CD concentration (10 mM). Figure 6f shows a typical SEM image of the product formed in the presence of 2.5 mM β-CD but in the absence of DTAB, which suggests that only spherical Au nanoparticles mainly in the range of 60100 nm in diameter with coarse surfaces were obtained without DTAB. Similar Au nanoparticles could also be generated when neither DTAB nor β-CD was added, indicating the relatively weak shape-controlling ability of CDs for Au nanoparticles. Therefore, the morphology of the Au products is significantly influenced by the CD-to-DTAB molar ratio, and a proper molar ratio of CD-to-DTAB (i.e., 0.5-1) is essential to the exclusive formation of symmetric planar Au nanodendrites. In short, when 5 mM DTAB was present in the solution, the morphology of the Au products could be adjusted from irregular particles, through branched particles consisting of rodlike branches, flowerlike aggregates consisting of symmetric planar dendrites, to flowerlike particles consisting of platelike petals with increasing CDto-DTAB molar ratio from 0 to 2. In the synthesis of dendritic Au nanostructures, the formation of DTAB/β-CD inclusion complexes played an important role. It is well-known that CDs are able to form host-guest complexes with most surfactants with high binding constants by including surfactants into CD cavities.20,21 In particular, the 1:1 inclusion complex can be easily formed between DTAB and β-CD with the binding constant of the supramolecular complex running up to 105 M-1.22 In the current reaction system, the formation of the 1:1 inclusion complex in the growth solution of the Au nanodendrites was supported by ESI-MS data, which gave a peak with m/z 1362.63, corresponding to the 1:1 inclusion complex of DTAB (minus the bromine anion) and β-CD (Supporting Information, Figure S1). In order to reveal the effect of the supramolecular DTAB/β-CD inclusion complexes on the formation of Au nanodendrites, two control experiments were carried out (Supporting Information, Figure S2). When 2.5 mM 1-adamantanol was added to the mixed DTAB/β-CD solutions, Au nanodendrites could not be obtained anymore and irregular particles exhibiting rough surfaces were the predominant product (Figure S2a), which are quite similar to that obtained only in the presence of 5 mM DTAB, as shown in Figure 6a. Adamantanol is reported to have a very high binding ability with the β-CD cavity.18b Therefore, DTAB molecules included in the CD cavity would be driven out when 1-adamantanol was added to the system, leading to the disassembly of the DTAB/β-CD inclusion complexes. On the other hand, when DTAB, β-CD, HAuCl4, and ascorbic acid (AA) were directly mixed without incubation of the mixed DTAB/ β-CD solution for 1 h in advance as in the typical synthesis of Au nanodendrites, many irregular particles coexisted with fragments of branched particles (Figure S2b). In this situation, there was not enough time for DTAB and β-CD molecules to form supramolecular complexes via host-guest interactions, resulting in the absence of the DTAB/β-CD inclusion complexes in the formation of Au crystals. These results indicate that symmetric planar Au nanodendrites cannot be obtained if the DTAB/β-CD inclusion complexes do not exist during the growth of Au crystals. In other words, the formation of supramolecular complexes of DTAB with β-CD due to host-guest interaction is indispensable for the fabrication of such Au nanodendrites. Moreover, the effect of the DTAB concentration on the formation of Au nanostructures has also been investigated (Supporting Information, Figure S3). As shown in Figure 6a, (22) (a) Wan Yunus, W. M. Z.; Taylor, J.; Bloor, D. M.; Hall, D. G.; Wyn-Jones, E. J. Phys. Chem. 1992, 96, 8979. (b) Funasaki, N.; Ohigashi, M.; Hada, S.; Neya, S. Langmuir 2000, 16, 383. (c) Tutaj, B.; Kasprzyk, A.; Czapkiewicz, J. J. Inclusion Phenom. Macrocyclic Chem. 2003, 47, 133.

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Figure 6. SEM images of Au nanostructures obtained in DTAB (a), mixed DTAB/β-CD (b-e), and β-CD (f) solutions. [β-CD]/[DTAB]: (b) 0.3, (c) 1, (d, e) 2. (a-e) [DTAB] = 5.0 mM; (f) [β-CD] = 2.5 mM.

irregular particles exhibiting faceted surfaces were the predominant product in the DTAB solution with a DTAB concentration of 5 mM. If the DTAB concentration was decreased to 2.5 mM, irregular branched structures consisting of rodlike branches were obtained (Figure S3a), which could be attributed to the adsorption of less DTAB molecules on the surfaces of the growing Au crystals. On the contrary, if the DTAB concentration was increased from the standard concentration (5 mM) to 10 mM, irregular polyhedral particles together with a few wirelike particles were obtained (Figure S3b). It is noted that all the DTAB concentrations employed in this study were less than the critical micellar concentration (cmc) of DTAB in aqueous solution, which is ∼14.4 mM,23 suggesting that the DTAB molecules existed in solutions as free molecules rather than micelles. Therefore, a less DTAB concentration could lead to the adsorption of less DTAB molecules on the Au particle surfaces. These results indicate that less adsorption of DTAB molecules would be favorable for the branching or dendritic growth of gold crystals, possibly due to the weaker stabilizing effect of the surfactant molecules. However, regular planar Au nanodendrites cannot be obtained by simply changing the DTAB concentration in the absence of β-CD. When β-CD was added to the DTAB solutions with a CD-to-DTAB molar ratio of 0.5, well-defined planar dendrites were obtained at a DTAB concentration of 5 mM, as shown in Figure 1; however, irregular branched structures and irregular particles were obtained at DTAB concentration of 2.5 and 10 mM, respectively (Figure S3c,d). These results suggest that the decrease in the concentration of the DTAB/β-CD mixture generally favored the branching or dendritic growth of gold crystals and the presence of an appropriate concentration of DTAB/β-CD inclusion complexes is essential to the formation of well-defined planar Au nanodendrites. It has been documented that hierarchical branching morphologies are generally formed through a self-organization process under nonequilibrium conditions and that the crystallization pattern is generally affected by the distance between the growth condition and the equilibrium state, i.e., the driving force for crystallization.24 A delicate balance between the diffusion rate and the reaction rate could be (23) Junquera, E.; Pena, L.; Aicart, E. Langmuir 1995, 11, 4685. (24) Imai, H. Top. Curr. Chem. 2007, 270, 43. (25) (a) Ma, Y.; Qi, L.; Ma, J.; Cheng, H. Cryst. Growth Des. 2004, 4, 351. (b) Liu, B.; Yu, S.-H.; Li, L.; Zhang, Q.; Zhang, F.; Jiang, K. Angew. Chem., Int. Ed. 2004, 43, 4745.

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responsible for the formation of highly symmetrical dendrites with a single-crystalline structure. In the present situation, the decrease in the concentration of DTAB or the DTAB/β-CD mixture would lead to a weaker protection of the Au particles from growing and thus a faster reaction rate, thereby favoring the branching or dendritic growth. Meanwhile, the specific adsorption of the DTAB molecules and the DTAB/β-CD inclusion complexes on the Au particle surfaces would considerably influence the growth process. In the mixed DTAB/β-CD solutions with suitable compositions, the DTAB/β-CD inclusion complexes and the DTAB molecules may interact with growing Au crystals in a synergistic way, which could result in a delicate balance between the diffusion rate and the reaction rate, favoring the formation of the unique planar Au dendrites with a symmetric single-crystalline structure. Considering that gold nanobelts5b and nanoplates6 with the top (111) surface have been frequently observed, it may be reasonably assumed that Au crystals with the cubic structure have a tendency to form nanostructures with a dominant (111) surface in the presence of certain additives. Therefore, the inherent crystalline structure of Au could largely contribute to the formation of the planar Au nanodendrites with the top (111) surface. It may be noted that a variety of hierarchical dendrites, such as dendritic PbS25a and PbWO425b structures, have been reported in the literature, which have been explained in terms of diffusion-limited growth or oriented attachment process. In the current situation, a diffusion-limited growth sounds more plausible since no evidence for the presence of nanoscale building blocks was observed during the formation of the hierarchical, single-crystalline Au structures. On the basis of the above experimental results, a tentative growth mechanism for the Au nanostructures obtained at various molar ratios of CD-to-DTAB is illustrated in Figure 7. When the DTAB concentration was fixed at 5 mM, irregular particles exhibiting faceted surfaces were the predominant product in the absence of β-CD. Upon the addition of β-CD, the 1:1 inclusion complex would easily form between DTAB and β-CD via the host-guest interaction and the formed DTAB/β-CD complexes would replace some of the DTAB molecules to adsorb on the Au particle surfaces. At a CD-to-DTAB molar ratio of 0.3, branched particles were obtained, which could be ascribed to the weaker protection of the mixed adsorption film of the DTAB molecules and the DTAB/β-CD complexes on specific surfaces of Au crystals. If the CD-to-DTAB molar ratio was increased to 0.5, Langmuir 2010, 26(10), 7582–7589

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Figure 7. Schematic illustration of the possible formation mechanism of Au nanostructures obtained at various molar ratios of CD-to-DTAB. [β-CD]/[DTAB]: (A) 0; (B) 0.3; (C) 0.5; (D) 2. All scale bars are 200 nm in panels a-d.

more DTAB molecules on the Au crystal surfaces were replaced by the DTAB/β-CD inclusion complexes, which may lead to a delicate balance between the diffusion rate and the reaction rate under nonequilibrium conditions, thus favoring the formation of the unique planar Au dendrites with a single-crystalline structure. In this case, the specific adsorption of the DTAB/ β-CD inclusion complexes on Au crystal surfaces could also considerably contribute to the formation of the planar Au nanodendrites with the top (111) surface. On further increasing the CD-to-DTAB molar ratio to 2, flowerlike particles consisting of platelike petals became the predominant product, which could be partially ascribed to the more effective capping of β-CD molecules to prevent further growth of Au crystals due to the high concentration of free β-CD in solution. Nevertheless, a detailed investigation on the structure of DTAB/β-CD inclusion complexes and the interaction between the DTAB/β-CD inclusion complexes and gold crystals is needed to fully elucidate the growth mechanism of the unique Au nanodendrites, which is currently underway in our lab. It has been reported that gold nanocrystals can exhibit a good electrocatalytic activity for methanol oxidation and that the catalytic property largely depends on their size and shape.5b,14 Langmuir 2010, 26(10), 7582–7589

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Accordingly, the as-prepared well-defined planar gold dendrites were deposited onto the glassy carbon (GC) electrode to prepare gold dendrite-modified GC electrode (denoted as gold dendrite electrode), and its electrocatalytic activity toward the oxidation of methanol was investigated by measuring its cyclic voltammograms (CVs) in KOH solutions. For comparison, the electrocatalytic activity of a commercial polycrystalline Au (abbreviated as poly-Au) electrode, which was first mechanically cleaned and then electrochemically activated with vitriol, was also measured in KOH solution. The surface area of the gold dendrite electrode was calculated to be 0.031 cm2 from the charge consumed during the reduction of surface oxides using the reported value of 400 μC/cm2 for a clean Au electrode.14 Since this method is a standard electrochemical method frequently used for determining the real surface area of gold, the surface areas of both gold electrodes were calculated by this method, and the measured current was normalized with the surface area to obtain the current density. Figure 8 displays the typical CVs recorded for both the gold dendrite electrode and the poly-Au electrode in 0.1 M KOH solution with and without 2.0 M CH3OH. In the absence of methanol, a broad oxidation wave and a reduction peak are observed for both modified electrodes. The oxidation peak can be ascribed to formation of gold surface oxides, and the reduction peak can be ascribed to subsequent removal of the oxides.5b,14 As compared with the voltammetric behavior of the poly-Au electrode, the oxidation peak of the gold dendrite electrode occurs at a much less positive potential. Moreover, the current density of the oxidation peak for gold surface oxides at the gold dendrite electrode is significantly higher than that at poly-Au electrode, indicating that the gold nanodendrite is oxidized more readily than poly-Au under otherwise identical conditions. When methanol was added, the CVs of both two gold electrodes clearly show that methanol oxidation occurs in a potential region from -0.2 to 0.4 V versus KCl-saturated calomel electrode (SCE), which may be ascribed to the oxidation of methanol to formates via a four-electron-transfer reaction.5b Apparently, the poly-Au electrode shows a CV curve very similar to that for the Au dendrite electrode. However, the current density calculated for the oxidation of methanol at the Au dendrite electrode (∼56 μA/cm2) is considerably higher than that at poly-Au electrode (∼34 μA/cm2), indicating that the prepared Au dendrite electrode has higher electrocatalytic activity toward methanol oxidation than the electrochemically activated poly-Au electrode. The higher electrocatalytic activity could be ascribed to the special morphology of the gold dendrites containing numerous nanobranches, which is reminiscent of the high electrocatalytic activity toward methanol oxidation observed from the nested Au nanobelt electrode5b and the 3-fold symmetrical Au dendrite electrode.14 The existence of more defects or hot spots in the Au dendrites as well as the exposed surfaces with specific crystal planes for the single-crystalline Au dendrites could considerably contribute to their higher electrocatalytic activity as compared to the poly-Au.14 Nevertheless, it would be worthwhile to perform a more detailed investigation to elucidate the exact mechanism for the high electrocatalytic activity of the obtained Au nanodendrites. On the other hand, it has been reported that metal nanostructures with sharp corners or edges are specially active SERS substrates,1c,2b which implies that the obtained unique Au nanodendrites could be further used as an effective SERS substrate. The SERS sensitivity of the as-prepared Au nanodendrites was investigated using p-aminothiophenol (PATP) as a model molecule. For comparison purposes, smooth Au nanosheets with the top (111) surface were prepared and used as reference sample for DOI: 10.1021/la904393n

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Figure 8. Cyclic voltammograms of gold dendrite electrode (a) and polycrystalline Au electrode (b) in 0.1 M KOH solutions without CH3OH or with 2.0 M CH3OH. The potential scan rate was 10 mV s-1.

conditions. It has been proposed that the dendritic Ag nanostructures exhibited excellent SERS properties,9b,9c which could be ascribed to the huge local electromagnetic field created between neighboring branched building blocks9c,27 and the numerous tips of the dendrites.9c,28 In the present case, the observed large enhancement in Raman scattering of the PATP molecules on the dendritic Au nanostructures could be also ascribed to the giant local electromagnetic field due to the formation of many hot spots at tips of the dendrites as well as the existence of many junctions between trunks and side branches in the Au dendrites. Therefore, the obtained dendritic Au nanostructures could be used as an effective SERS substrate that may find many potential applications including biosensing.

Conclusions Figure 9. Raman spectrum of solid PATP (a) and SERS spectra of PATP molecules adsorbed on different substrates: (b) Au nanosheets; (c) Au nanodendrites.

the SERS measurement (Supporting Information, Figure S4). The Raman spectra of PATP solid and the PATP molecules adsorbed on the two SERS substrates are presented in Figure 9. Compared with the normal Raman spectrum of solid PATP shown in Figure 9a, noticeable changes in the frequency shift of the bands can be observed from the SERS spectra on the two Au substrates, indicating that the thiol group in PATP directly contacted with the gold surface.5b,19 The SERS spectra obtained from the two Au substrates are both dominated with the a1 vibrational modes (in-plane, in-phase modes), such as ν(CC) and ν(CS) at 1577 and 1078 cm-1. The predominance of a1 modes in the SERS spectrum may imply that the enhancement via an electromagnetic (EM) mechanism is significant.26 Moreover, for the SERS spectrum obtained from the Au nanodendrites, the b2 modes (in-plane, out-of-phase modes) located at 1433, 1389, 1176, 1141, and 1006 cm-1 are also apparent. Generally, the apparent enhancement of b2 modes may be ascribed to the charge transfer (CT) of the metal to the adsorbed molecules, which demonstrates that the PATP molecules contact with the gold surface by forming a strong Au-S bond.5b,19 Obviously, the SERS intensity from Au nanodendrites gains considerably larger enhancement than that from the smooth Au nanosheets. The SERS intensity of the peak at 1577 cm-1 (a1 vibrational modes) from a planar Au nanodendrite with the incident light focused onto a circular area ∼1 μm in diameter is about 5 times stronger than that from a smooth Au nanosheet under otherwise identical (26) Fleischmann, M.; Hendra, P.; McQuilan, A. Chem. Phys. Lett. 1974, 26, 163.

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Controlled synthesis of well-defined planar Au nanodendrites with a symmetric single-crystalline structure consisting of trunks and side branches grown along the Æ211æ directions was successfully realized by the reduction of chloroauric acid in aqueous DTAB/β-CD solutions. It has been revealed that the formation of supramolecular complexes of DTAB with β-CD due to hostguest interaction is indispensable for the fabrication of these unique planar Au nanodendrites, and a proper CD-to-DTAB molar ratio is essential to their exclusive formation. In the mixed DTAB/β-CD solutions with suitable compositions, the DTAB/ β-CD inclusion complexes and the DTAB molecules may interact with growing Au crystals in a synergistic way, which could result in a delicate balance between the diffusion rate and the reaction rate under nonequilibrium conditions, favoring the formation of the unique planar Au dendrites with a symmetric single-crystalline structure. Moreover, a variety of Au nanostructures, such as branched particles consisting of rodlike branches and flowerlike particles consisting of platelike petals, could be readily obtained in the mixed DTAB/β-CD solutions by simply changing the CD-to-DTAB molar ratio. To the best of our knowledge, this is the first report on the shape-controlled synthesis of Au nanostructures with the assistance of supramolecular complexes of surfactants with CDs. This synthetic strategy may open a new route for the facile fabrication and hierarchical assembly of metal nanostructures in solution. The obtained Au nanodendrites exhibited both a good electrocatalytic activity toward the oxidation of methanol and a good SERS sensitivity for detecting PATP molecules. These well-defined planar gold nanodendrites with a single-crystalline nature would be ideal candidates for (27) Garcia-Vidal, F. J.; Pendry, J. B. Phys. Rev. Lett. 1996, 77, 1163. (28) Kottmann, J. P.; Martin, O. J.; Smith, D. R.; Schultz, S. Phys. Rev. B 2001, 64, 235402.

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investigating the electrical and photonic properties of hierarchical architectures of one-dimensional metal nanostructures and could find potential applications including catalysis, biosensing, and nanodevices. Acknowledgment. Financial support from NSFC (Grants 20873002, 20673007, 20633010, and 50821061), MOST (Grant

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2007CB936201), and SRFDP (Grant 20070001018) is gratefully acknowledged. Supporting Information Available: ESI-MS spectrum of the growth solution and SEM images of the Au products obtained under different conditions. This material is available free of charge via the Internet at http://pubs.acs.org.

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