Vesicle-Directed Generation of Gold Nanoflowers by Gemini

Spectrophotometric evidence to the formation of AuCl4–CTA complex and synthesis of gold nano-flowers with tailored surface textures. Mohammad Naved ...
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Vesicle-Directed Generation of Gold Nanoflowers by Gemini Amphiphiles and the Spacer-Controlled Morphology and Optical Property Ling Zhong, Xiaodong Zhai, Xuefeng Zhu, Pingping Yao, and Minghua Liu* Beijing National Laboratory for Molecular Sciences (BNLMS), Key Laboratory of Colloid and Interface Science, Institute of Chemistry, The Chinese Academy of Sciences, Beijing 100190, PR China Received October 8, 2009. Revised Manuscript Received November 22, 2009 In this article, we developed an effective approach to generate gold nanoflowers (AuNFs) by vesicles made from a series of gemini amphiphiles (G2-G10) with different spacer lengths. The gemini amphiphiles were found to form vesicles in aqueous solution. Upon mixing with vesicles in the presence of AgNO3, HAuCl4 could be reduced into gold nanoflowers by ascorbic acid. The vesicles directed the growth of the AuNFs, and the spacer length of the gemini amphiphiles showed obvious control of the morphology and optical properties of the formed AuNFs. At a lower HAuCl4 concentration, the minimum-sized AuNFs were formed when vesicles from the amphiphile with a spacer length of 4 were applied. Upon increasing the spacer length, branched nanoflowers are predominantly produced. A seed-growth mechanism together with the conformational change of the spacer of the gemini amphiphiles was proposed according to the studies on reaction processes. In addition, the formed gold nanoflowers showed obvious surface-enhanced Raman scattering activity for R6G. The present method provided an efficient, controllable way to synthesize branched gold nanostructures.

Introduction Gold nanostructures have been attracting considerable interest because of their unique structural features and important applications in many fields such as optical devices, catalysis, biological assay, surface-enhanced Raman scattering (SERS), and so on.1-3 The properties of the gold nanostructures are largely dependent on their sizes and shapes;4 therefore, the synthesis of gold nanostructures with controlled size and shape is of vital importance. Besides various simplex Au nanostructures such as particles,5 rods,6 and prisms,7 the assembly of gold nanostructures is also significant. Upon assembly, the properties of the gold nanostructures can be tuned to a large extent. Very recently, complicated nanostructures such as multibranched gold nanostructures have been reported to have a better SERS effect and catalyst properties.8-10 Although several methods have been developed to obtain such multi-

branched gold nanostructures,11-25 controllable and well-defined multibranched gold structures are still difficult to prepare. In this article, we report a simple one-pot method for the fabrication of gold nanoflowers (AuNFs) in high yield. Our approach is to use a unique kind of vesicle composed of gemini amphiphiles to direct the synthesis of the gold nanostructures. By mixing the vesicles with a certain amount of AgNO3 and HAuCl4, AuNFs were formed in a controlled manner. Gemini amphiphiles can be regarded as dimeric amphiphiles that are covalently connected by a spacer between the two headgroups and could exhibit superior assembly performance that is orders of magnitude more surface-active than for comparable conventional surfactants.26-28 Many parameters of the gemini amphiphiles such as the length of the alkyl chain, headgroup, counterion, and spacer can be used to tune the properties of the formed organized structures.29,30 Liposomes have been successfully used to synthesize platinum nanocages and dendritic nanosheets effectively.31

*Corresponding author. E-mail: [email protected]. (1) Daniel, M. C.; Astruc, D. Chem. Rev. 2004, 104, 293. (2) Hu, M.; Chen, J. Y.; Li, Z. Y.; Au, L.; Hartland, G. V.; Li, X. D.; Marquez, M.; Xia, Y. N. Chem. Soc. Rev. 2006, 35, 1084. (3) (a) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano Lett. 2006, 6, 2173. (b) Jiang, Y.; Horimoto, N. N.; Imura, K.; Okamoto, H.; Matsui, K.; Shigemoto, R. Adv. Mater. 2009, 21, 2309. (4) Nehl, C. L.; Hafner, J. H. J. Mater. Chem. 2008, 18, 2415. (5) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (6) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. Rev. 2005, 249, 1870. (7) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L. D.; Schatz, G. C.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 5312. (8) Xie, J. P.; Zhang, Q. B.; Lee, J. Y.; Wang, D. I. C. ACS Nano 2008, 2, 2473. (9) Jena, B. K.; Raj, C. R. Langmuir 2007, 23, 4064. (10) Liao, H. G.; Jiang, Y. X.; Zhou, Z. Y.; Chen, S. P.; Sun, S. G. Angew. Chem., Int. Ed. 2008, 47, 9100. (11) Jena, B. K.; Raj, C. R. Chem. Mater. 2008, 20, 3546. (12) Yamamoto, M.; Kashiwagi, Y.; Sakata, T.; Mori, H.; Nakamoto, M. Chem. Mater. 2005, 17, 5391. (13) Bakr, O. M.; Wunsch, B. H.; Stellacci, F. Chem. Mater. 2006, 18, 3297. (14) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Y. Nano Lett. 2004, 4, 327. (15) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (16) Kuo, C. H.; Huang, M. H. Langmuir 2005, 21, 2012. (17) Wu, H. L.; Chen, C. H.; Huang, M. H. Chem. Mater. 2009, 21, 110. (18) Yuan, H.; Ma, W. H.; Chen, C. C.; Zhao, J. C.; Liu, J. W.; Zhu, H. Y.; Cao, X. P. Chem. Mater. 2007, 19, 1592.

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(19) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (20) Lu, L. H.; Ai, K.; Ozaki, Y. Langmuir 2008, 24, 1058. (21) Li, Z. Q.; Li, W. Y.; Camargo, P. H. C.; Xia, Y. Angew. Chem., Int. Ed. 2008, 47, 9653. (22) Su, Y. H.; Lai, W. H.; Chen, W. Y.; Hon, M. H.; Chang, S. H. Appl. Phys. Lett. 2007, 90, 181905. (23) Guo, S.; Wang, L.; Wang, E. Chem. Commun. 2007, 3163. (24) Pastoriza-Santos, I.; Liz-Marzan, L. M. Adv. Funct. Mater. 2009, 19, 679. (25) Qin, Y.; Song, Y.; Sun, N.; Zhao, N.; Li, M.; Qi, L. Chem. Mater. 2008, 20, 3965. (26) (a) Oda, R.; Huc, I.; Schmutz, M.; Candau, S. J.; MacKintosh, F. C. Nature 1999, 399, 566. (b) Oda, R.; Huc, I.; Candau, S. J. Chem. Commun. 1997, 2105. (27) Menger, F. M.; Keiper, J. S. Angew. Chem., Int. Ed. 2000, 39, 1906. (28) Zana, R. Adv. Colloid Interface Sci. 2002, 97, 205. (29) (a) Chen, X. D.; Wang, J. B.; Shen, N.; Luo, Y. H.; Li, L.; Liu, M. H.; Thomas, R. K. Langmuir 2002, 18, 6222. (b) Jiang, M.; Zhai, X. D.; Liu, M. H. Langmuir 2005, 21, 11128. (c) Zhai, X. D.; Zhang, L.; Liu, M. H. J. Phys. Chem. B 2004, 108, 7180. (d) Zhong, L.; Jiao, T. F.; Liu, M. H. Langmuir 2008, 24, 11677. (30) Zhang, G. C.; Zhai, X. D.; Liu, M. H. Langmuir 2009, 25, 1366. (31) (a) Song, Y. J.; Yang, Y.; Medforth, C. J.; Pereira, E.; Singh, A. K.; Xu, H. F.; Jiang, Y. B.; Brinker, C. J.; Swol, F. V.; Shelnutt, J. A. J. Am. Chem. Soc. 2004, 126, 635. (b) Song, Y. J.; Steen, W. A.; Pena, D.; Jiang, Y. B.; Medforth, C. J.; Huo, Q.; Pincus, J. L.; Qiu, Y.; Sasaki, D. Y.; Miller, J. E.; Shelnutt, J. A. Chem. Mater. 2006, 18, 2335. (c) Song, Y. J.; Garcia, R. M.; Dorin, R. M.; Wang, H. R.; Qiu, Y.; Shelnutt, J. A. Angew. Chem., Int. Ed. 2006, 45, 8126.

Published on Web 12/10/2009

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Chart 1. Chemical Structural Formula of Gn

However, there is no report using gemini vesicles in the synthesis of AuNFs. According to our previous work, our designed gemini amphiphiles could form stable vesicles at a lower concentration in aqueous dispersions and showed an excellent propensity to induce the aggregation of a cyanine dye.30 In this article, we have also found that such vesicles can direct the growth of AuNFs. With these vesicles, not only can AuNFs be synthesized in an efficient way, but also the morphologies and their optical properties can be regulated by the different spacer lengths of the gemini amphiphiles.

Experimental Section Materials. Chloroauric acid (HAuCl4 3 4H2O), AgNO3, and ascorbic acid were from the Beijing Chemical Plant. Ultrapure deionized water (Milli-Q, 18 MΩ cm) was used for all solution preparations. Vesicle Preparation. The series of geminis (abbreviated as Gn, n = 2, 4, 6, 8, and 10) was synthesized according to a previous paper,29c and the structures are shown in Chart 1. The series of geminis have the same headgroups and different lengths of the alkyl chain spacer. All experiments were performed at room temperature. The geminis were first dissolved in chloroform to a concentration of 10-3 M. Then a 25 μL gemini solution was spread on the bottom of a glass sample container. After complete solvent evaporation, 5 mL of pure water was added to the container. With intense sonication (250 W) for 2 h, transparent vesicle stock solutions were obtained. Before the synthesis of gold nanostructures, the stock solutions of the vesicles were aged for 1 h to exclude the influence of temperature change from sonication. Synthesis of Gold Nanostructures. A 50 μL 10-3 M AgNO3 aqueous solution was injected into 5 mL of a 5 μM vesicle solution. Then a 10-2 M HAuCl4 aqueous solution was added to achieve Au concentrations of 10-4, 2  10-4, 5  10-4, and 10-3 M. After that, 1 mL of solution was taken from the above solution, and an ascorbic acid aqueous solution with a concentration of 10-2 M was added dropwise under gentle shaking in volumes of 20, 40, 100, and 200 μL. The gold samples were purified by a centrifugation and resuspension procedure in order to remove excess reducing agents, salts, and amphiphile. The sample solution was centrifuged at 3000 rpm for 4 min using an Anke TGL-16c centrifuge. Then the supernatant was removed and ultrapure water was added to the precipitate. The residue was resuspended by sonication for 5 min. The rinsing procedure was repeated three times to obtain the final gold materials. Characterization of Gold Nanostructures. Sonication experiments were carried out on a KQ-250B ultrasonic cleaner. UV-vis spectra were recorded with a Jasco UV-550 spectrophotometer. XPS data were obtained with an ESCALab 220i-XL electron spectrometer from VG Scientific using 300W Al KR X-ray radiation. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. For TEM and SEM characterization, the samples were prepared by adding drops of gold solutions onto carbon-coated copper grids or a silicon substrate and drying in air. TEM measurements were made on a JEOL TEM-2010 electron microscope. SEM images were taken from a Hitachi S4300 scanning electron microscope. To observe vesicles in the gold sample better, high-resolution SEM images were taken from a Hitachi S4800 scanning electron microscope. XRD data were obtained from a Hitachi Rigaku D/max 2500 X-ray diffractometer (Japan) with Cu KR radiation (λ = 0.154 nm). Langmuir 2010, 26(8), 5876–5881

Figure 1. (a, b) SEM and (c, d) TEM images of gold nanoflowers synthesized by the reduction of HAuCl4 (10-3 M) in a G4 (5 μM) vesicle aqueous solution with ascorbic acid in the presence of 0.01 mM AgNO3. (e) SAED pattern of gold nanoflowers.

Raman Spectra Measurements. R6G was used as a probe molecule from Aldrich. First, R6G was dissolved in ethanol, and a 2  10-5 M solution was obtained. Two-hundred microliters of gold nanoflowers (HAuCl4 10-3 M, AgNO3 10-5 M, AA 200 μL, G4 5 μM) and bumped particles (HAuCl4 10-4 M, AgNO3 10-5 M, AA 20 μL, G4 5 μM) was cast onto a glass slide and dried under nitrogen. Then 10 μL of a 2  10-5 M R6G ethanol solution was spread onto the formed gold films. SERS spectra were recorded on a Renishaw inVia Plus Raman microscope with excitation from a 785 nm laser in one scan. The laser power was 1 mW.

Results and Discussion Synthesis of Gold Nanoflowers via Vesicles. Vesicles were prepared via solvent evaporation and a subsequent sonication procedure in aqueous solution.30 In a typical experiment to synthesize AuNFs, AgNO3 and HAuCl4 aqueous solutions were consecutively added to a 5 μM vesicle dispersion to achieve concentrations of 10-5 and 10-3 M, respectively. Then an ascorbic acid (AA) aqueous solution was added to the mixed solution dropwise. The color of the solution changed from light yellow to colorless and then to dark green, indicating the reduction of Au(III) to Au(I) and then to metal Au. Because the series of gemini amphiphiles showed a similar tendency in the synthesis of gold nanostructures, we take G4 as an example to illustrate the generation of AuNFs. Figure 1 shows typical SEM and TEM images of synthesized gold nanostructures from G4 vesicles. Flowerlike structures with an average diameter of 406 ( 89 nm (the maximum size) were observed. The branch structures can be seen clearly in SEM and TEM images. Selected-area electron diffraction (SAED) indicates that the flowerlike nanostructures are face-centered-cubic (fcc) crystalline and randomly orientated. From the XRD data (Figure S1 in Supporting Information), peaks ascribed to {111}, {200}, {220}, and {311} can be clearly seen. The peak corresponding to {111} was more intense than those corresponding to the other planes. The conventional ratio of {111} to {200} in gold nanoparticles is 1.9. In our nanoflowers, the ratio was 3.5, which is much larger than the conventional value, demonstrating that the {111} plane was the predominant orientation. Effect of HAuCl4 Concentration. To gain deep insight into the growth mechanism of such AuNFs, the influence of the HAuCl4 concentration was investigated. The SEM pictures of DOI: 10.1021/la903809k

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Figure 2. SEM images and UV-vis spectra of gold nanostructures obtained in the 5 μM G4 vesicle aqueous solution with a varying HAuCl4

concentration: (a) 1  10-4, (b) 2  10-4, (c) 5  10-4, and (d) 1  10-3 M. The concentration of AgNO3 is 10-5 M; the volume of AA is 20, 40, 100, and 200 μL, respectively.

Figure 3. (a-c, f ) TEM and (d, e) SEM images of gold nanostructures with HAuCl4 (10-3 M) and Ag ions (10-5 M) in a G4 vesicle solution

(5 μM) after adding various volumes of AA (10-2 M): (a) 60, (b) 80, (c-e) 150, and (f ) 200 μL.

the formed gold nanostructures and the corresponding UV-vis spectra are shown in Figure 2. At a lower concentration of HAuCl4 (e.g., 10-4 M), quasispherical bumped nanoparticles with an average diameter of 65 ( 15 nm are obtained, as shown in Figure 2a. The SPR absorption appears at 592 nm in Figure 2, which is obviously red shifted compared to that of typical spherical particle, which is around 520 nm. The SPR absorption of gold nanostructures is very dependent on the size, shape, and particulate interactions. As the size of gold nanoparticle increases, the SPR band will shift to a longer wavelength. When anisotropic structures are formed, multiple bands may appear or SPR red shifts to long wavelengths. Therein, the influence of shape on the SPR is more obvious than that of size. Thus, in our experiment the large red shift of SPR in gold particles should be due to the bumped structure. As the concentration of HAuCl4 is increased to 2  10-4 M, although the bumped AuNPs are still the main components, some multibranched nanostructures emerge (Figure 2b). In this case, the SPR band shifts to a longer wavelength of 604 nm. When the HAuCl4 concentration increases up to 5  10-4 M, flowerlike structures are clearly observed and a broad SPR band appears at 691 nm. Upon 5878 DOI: 10.1021/la903809k

increasing the concentration further, large flowerlike structures can always be observed. In the case of 10-3 M HAuCl4, the SPR band is broadened and appears at longer wavelength, which is beyond the visible region. From the above experiments, it can be suggested that nanoparticle seeds may be initially formed. When the amount of HAuCl4 is not enough, bumped particles are mainly formed. When a sufficient HAuCl4 source is present in the system, the original seeds could further grow into gold nanoflowers. To verify this, we added HAuCl4 to the solution, in which bumped spherical nanoparticles were mainly formed, to a total concentration of 10-3 M; after reduction by AA, gold nanoflowers were also obtained (Figure S2), indicating the seed-growth mechanism. Effect of the Amount of Reduction Agent. However, because the reaction was completed within a few minutes, it is difficult to monitor the reaction process as a function of time. Instead, we monitored the growth process by taking TEM images after adding a different volume of AA aqueous solution. Figure 3 shows the SEM and TEM images of the gold nanostructures formed when various amounts of AA were added. As shown in Figure 3a, small nanoparticles with a size of 20-60 nm are seen after 60 μL of AA was added. These star-shaped, faceted particles Langmuir 2010, 26(8), 5876–5881

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Figure 4. SEM images of gold nanostructures fabricated from a 10-4 M HAuCl4 solution in 5 μM (a) G2, (b) G4, (c) G6, (d) G8, and (e) G10 vesicle solutions. (f-j) SEM images of the gold nanostructures formed from a 2  10-4 M HAuCl4 solution with (f ) G2, (g) G4, (h) G6, (i) G8, and ( j) G10 vesicles. The AgNO3 concentration is 10-5 M. The bar scale is 500 nm.

were mainly covered by {111} and {200} surface orientations (as shown in the high-resolution TEM image in Figure S3). Similar results have been reported in other research on gold and silver rods where surfactants stabilized the {110} and {100} surfaces, leading to growth along the {111} facets.32-34 After a total of 80 μL of AA was added, large starlike structures can be observed, as shown in Figure 3b. As the volume of AA increased to more than 150 μL, flowerlike nanostructures have mainly been formed (Figure 3c-f ). These morphological changes clearly indicated that the flowers were grown from seeds. Furthermore, the highresolution SEM images were also taken as shown in Figure 3d,e. It is interesting to find that some circular nanostructures, with sizes of several tens of nanometers to 200 nm, attached to the gold nanoflowers. Because these structures have shapes and sizes that are similar to those of our vesicles, it seems that Au structures were from the vesicles. This clearly indicated that our vesicles worked as a template to direct the growth of these gold nanoflowers. Spacer Effect. Previously, we found that the spacer length of the gemini amphiphile could affect the packing of the molecules. Interestingly, our vesicles from the gemini amphiphiles show an obvious spacer effect. First, the spacer length influences the shapes of obtained gold nanostructures. When the HAuCl4 concentration is 10-4 M, bumped spherical particles are mainly obtained in all geminis except G10, in which branched Au is predominantly generated, as shown in Figure 4a-e. When the gold concentration is increased to 2  10-4 M, branched Au can be found in G6, G8, and G10, but bumped particles are still the main product in G2 and G4 solutions though branches can be found, as shown in Figure 4f-j. As the gold concentration was increased further, the influence of the spacer decreased. From the results above, it is clear that at a determined concentration a long spacer favors the formation of branched gold nanostructures. However, the spacer effect on morphology can also be observed in the changes in optical properties, as shown in Figure 5. The gold nanostructures produced from 10-4 M HAuCl4/G2 and 10-4 M HAuCl4/G4 vesicles have maximum absorption wavelengths at 604 and 592 nm, respectively, consistent with the small particles generated. As the spacer increases, SPR peaks appear at longer wavelengths of 604 and 605 in the G6 and G8 systems. From G2 to G8, the SPR absorptions all have similar spectral features and the maximum wavelengths all appear at around 600 nm, indicating that the spectral difference is mainly induced by size. The SPR of gold in G4 appears at the shortest wavelength, indicating the minimum size, which can be proven later. However, the gold in G10 has two SPR bands at 636 and 750 nm. (32) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771. (33) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (34) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765.

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Figure 5. UV-vis spectra of gold nanostructures synthesized by the reduction of HAuCl4 at (a) 10-4, (b) 2  10-4, (c) 5  10-4, and (d) 10-3 M in 5 μM Gn vesicle solution (10-5 M AgNO3).

As discussed above, when anisotropic structures were formed, SPR would obviously red shift or appear in the form of multiple peaks. Thus, the large spectral difference between G10 and other geminis clearly indicates different gold nanostructure, which is proven by the SEM images in Figure 4e. As the gold concentration is increased to 2  10-4 M, the gold absorptions in G2 and G4 also appear at shorter wavelengths of 617 and 604 nm. From G6 to G10, SPR bands appear at much longer wavelengths, 743, 761, and 773 nm. Compared with the SEM images, the large red shift of G6, G8, and G10 is ascribed to branched gold nanostructures. At the same time, the SPR of gold in G4 also appears at the shortest wavelength, implying the minimum size. As the concentration is further increased to 5  10-4 M, only G4 has SPR at a shorter wavelength position, 691 nm, and SPR bands in other geminis are all beyond the visible region. It is interestingly noted that SPR in G4 solution always appeared at the shortest wavelength, implying the smallest size of gold nanostructures in the G4 system. The spacer effect can be further observed from the size of the synthesized gold nanostructures. The average size of the gold nanostructures in each case was obtained by counting at least DOI: 10.1021/la903809k

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Figure 6. (a) Size of gold nanostructures and (b) maximum absorption wavelength as a function of the number of methylenes in the gemini spacer with HAuCl4 concentrations of (2) 10-4, (() 2  10-4, (b) 5  10-4, and (9) 10-3 M.

300 gold structures. The average size was plotted against the spacer length, as shown in Figure 6. Simultaneously, the absorption maximum of these nanostructures was also plotted against the spacer length. When the HAuCl4 concentration is low (e.g., 10-4 M and 5  10-4 M), the size of the obtained Au decreases from G2 to G4 and then increases from G4 to G6 and approaches the same value when the spacer length increases to G6, G8, and G10. The results are consistent with the UV-vis spectra indicating that gold in the G4 solution always had an absorption at the shortest wavelength. However, when the HAuCl4 concentration is increased to 10-3 M, the spacer effect becomes less obvious. SERS Effect on Gold Nanoflowers. It is well known that gold and silver have good SERS effects and have potential applications in identification and biological arrays.35 There have been few reports on the SERS of R6G from gold substrates because the interaction between R6G and gold is weak.36 Usually, a Ag substrate is used to obtain the SERS of R6G.37 However, owing to the stability of gold, good biocompatibility, low cyctotoxicity, and wide SPR absorption, it is indeed necessary to develop a proper gold substrate for Raman spectra in biological applications. Figure 7 shows the Raman spectrum of solid R6G (Figure 7a) and the SERS spectra of R6G from our AuNFs (10-3 M HAuCl4, 10-5 M AgNO3 in G4) and bumped particles (10-4 M HAuCl4, 10-5 M AgNO3 in G4). The pure R6G solid powder sample has a strong fluorescent background, and the Raman signal has overlapped with fluorescence in the region of 500-2000 cm-1. By comparison, for R6G adsorbed on AuFNs, the fluorescent background is weakened to a large extent and intensive Raman signals of R6G are detected. This enhanced effect may be due to the branches on gold flowers that have strong electromagnetic interactions with the dye molecules. At the same time, we also collected the SERS spectrum of R6G on our bumped nanoparticles. A good enhanced effect is also obtained, and this may be due to the rough surface of nanoparticles. The flowerlike structure has shown a strong enhancement of the electromagnetic field near the surface of these complex Au nanostructures, thus they showed high SERS activity. However, the optical properties of gold nanoflowers are sensitive to size and branch structure, thus they are of interest in optical applications. Moreover, gold nanoflowers also exhibited good electrocatalytic activity (e.g., oxidation of methanol and reduction of oxygen).9,25 (35) Kneipp, N.; Kneipp, H.; Itzkan, I.; Dasari, R. R.; Feld, M. S. Chem. Rev. 1999, 99, 2957. (36) Gupta, R.; Weimer, W. A. Chem. Phys. Lett. 2003, 374, 302. (37) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. Nano Lett. 2003, 3, 1229.

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Figure 7. (a) Raman spectrum of an R6G solid powder and SERS spectrum of R6G adsorbed on (b) gold nanoflowers and (c) bumped nanoparticles. Scheme 1. Chemical Structures of Geminis and Cartoon Illustration of the Formation of AuNFs

Discussion On the basis of the above results, it is suggested that the presence of vesicles in the dispersion is very important. We found that if the concentration of Gn is lowered to 2 μM then only a few gold flowers can be found (Figure S4). A vesicle is usually adopted as an organized structure to control the growth of the nanostructures. Our vesicle has positive charged sites that could preadsorb HAuCl4. Upon mixing the gemini vesicles with HAuCl4, the AuCl4- ions would accumulate on both the inner and outer spheres of the vesicles. Upon reduction with ascorbic acid, which can also penetrate into the vesicles, seed nanoparticles were produced both inside and between the vesicles (Scheme 1). These seeds were suggested to be predominantly localized within the two neighboring covalently linked headgroups. In addition, as discussed above, the geminis could stabilize {110} or {100} faces of the gold seed and induce the preferential deposition of Au on {111} faces. Because the vesicles have a global structure, the nanostructures grow into a 3D nanoflower. The spacer length as well as its conformational change can subtly influence the growth of the gold nanostructure. Because the two headgroups of the gemini amphiphiles are covalently linked, the alkyl spacer will change their conformation to a curved shape when forming the circular vesicles. Such a conformational change is related to the spacer length. When the spacer length is 2, it could not be curved because it contains only one C-C bond. As the spacer length increases, the alkyl spacer can change its conformation to facilitate the circular vesicle structure. Our previous work showed that the spacer of G6 starts to bend at the air/water interfacial assembly.29c We have used the series of geminis to control dye aggregation, where a dye molecule acts as a probe of Langmuir 2010, 26(8), 5876–5881

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the conformational change.30 In aqueous solution, the bending of the spacer may be started from G4 because of the strong ionic interaction of the headgroup with water. Thus, we observed the minimum in the case of the G4 vesicle. Because of the bending of the alkyl spacer, the gold nanoseeds tend to grow into branched structures; such an effect is particularly obvious when the AuCl4concentration is lower. With the increment of the alkyl spacer, the tendency of the alkyl chain to change to a curved conformation is increased. Therefore, we observed branched structure even when the concentration of gold ions is lower with vesicles from a longer spacer. At a higher concentration of AuCl4-, the whole structure of the gemini rather than the subtle conformation of the alkyl spacer influences the growth of the gold nanostructures more; therefore, we obtained the AuNFs when any of the gemini vesicles were used. It should be noted that the presence of AgNO3 in the system is very important in forming the AuNFs. We have found that without Ag ions no gold flowers can be obtained, though some anisotropic structures have also been generated (Figure S5). Multibranched nanostructures definitely emerged when AgNO3 was added. Moreover, the relative ratio of Ag(I) to Gn is important. When the concentration of Ag(I) ions is larger than 2Gn, quasi-spherical structures with some short branches were found (Figure S6). Thus, both the existence of Ag(I) ions and the proper concentration determine the anisotropic structures of AuNFs. Although the role of silver ions is not yet clear, it has been reported that in the synthesis of gold nanorods, silver ions could combine with Br- ions and enhance the formation of rods.38-40 Our XPS data on gold (38) (a) Jana, N. R. Small 2005, 1, 875. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. Adv. Mater. 2001, 13, 1389. (39) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (40) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (41) Liu, X. H.; Luo, X. H.; Lu, S. X.; Zhang, J. C.; Cao, W. L. J. Colloid Interface Sci. 2007, 307, 94.

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flowers also showed the existence of AgBr, with Ag 3d5/2 appearing at 367.8, in good agreement with the literature value.41 Thus it may be the combination of gemini and the deposition of AgBr that leads to the formation of AuNFs. In addition, because of the limited spaces in vesicles, the AuNFs grow mainly in the outer sphere, as observed in the SEM pictures.

Conclusions Gold nanoflowers were successfully generated by the reduction of HAuCl4 in a gemini vesicle by ascorbic acid with the assistance of AgBr deposition. The vesicle from the gemini amphiphiles worked as a direct precursor in the synthesis of AuNFs, and the spacer length of the gemini amphiphiles had an obvious control effect on the morphologies and the optical properties of the formed AuNFs. It is suggested that the alkyl spacer changed from a linear zigzag conformation to a bended one during vesicle formation. The longer spacer tended to form a bending conformation that favors the formation of gold nanoflowers. On designing proper molecules, we successfully realized control over the nanoscale growth of gold by tuning the chemical structure. It is believed that our method provides a simple approach to regulating gold nanostructures as well as their optical properties. Such a method may also be applied to other metals, and this work is underway. Acknowledgment. This work was supported by the National Natural Science Foundation of China (no. 20533050) and the Fund of the Chinese Academy of Sciences. Supporting Information Available: XRD data, SEM, a high-resolution TEM image. This material is available free of charge via the Internet at http://pubs.acs.org.

DOI: 10.1021/la903809k

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