Two-Step Synthesis of Narrow Size Distribution Nanoflowers Using

Chinese Academy of Sciences, Beijing 100190, People's Republic of China ..... As shown in Figure 4a, small flower-like nanoparticles with a size o...
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Two-Step Synthesis of Narrow Size Distribution Nanoflowers Using a Tree-Type Multi-Amine-Head Surfactant as a Template Wenfeng Jia, Jinru Li, Guanhua Lin, and Long Jiang* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Colloids and Surfaces, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

bS Supporting Information ABSTRACT: Unique, narrow polydispersity, hierarchical gold nanoflowers were synthesized by a two-step method using a tree-type multi-amine surfactant (C18N3) as the template and ascorbic acid (AA) as the reductant. Upon the addition of a stronger reductant AA, the C18N3 HAuCl4 composite changed to a black color, due to the gold nanoflower formation. Under a specific concentration ratio of C18N3, HAuCl4, and AA, narrow polydispersity flower-like gold structures were obtained. These nanoflowers were stable as an aqueous solution for months, with no deformation to particle size and no obvious change in the surface morphology. The experimental results demonstrated that the complexation between C18N3 and HAuCl4 is essential for the formation of gold nanoflowers with a narrow size distribution. The morphology and size of the composites can be easily adjusted by changing the concentration of C18N3, HAuCl4, and AA. The growth of nanoflowers is discussed in terms of two steps: the formation of complexes between C18N3 and HAuCl4, which determines the core size of the nanoflowers, and the surface nanocrystal growth, due to the reduction of HAuCl4 by AA, which determines the thickness and morphology of nanoflower surface layers. In the measurement of surface-enhanced Raman scattering (SERS) using rhodamine 6G, the gold nanoflowers exhibited a significant enhancement factor, indicating their potential in biosensing and nanodevice applications.

1. INTRODUCTION Gold nanostructures have attracted significant interest in recent years due to their unique structural features and fascinating optical, electronic, and chemical properties, as well as promising applications in nanoelectronics, optics, sensing, catalysis, biomedicine, and SERS.1 8 Because the properties and applications of gold nanostructures are largely dependent on their size, shape, and pattern, much effort has been devoted to developing a morphology-controlled synthesis of Au nanostructures,9 12 including shapes such as rod,13 wire,14 belt,15 plate and prism,16 18 polyhedra,19 21 cage,22 star and flower,23,24 branched,25 and dendritic26 particles, in the past decades. Among the various nanoarchitectures, a hierarchical morphology has great significance in terms of adsorption and surface reaction due to its high surface area and a greater concentration of active sites. For example, a high SERS response was reported for Au with hierarchical morphologies, such as “meatball” particles27 and nanoflowers.23 Although several methods have been developed to obtain such hierarchical nanostructures,28 36 a low cost and facile synthetic method for the preparation of controlled, and well-defined hierarchical gold nanostructures is still in great demand. It was recently reported that amphiphilic tree-type surfactants37 prepared by a facile coupling of tailored hydrophilic and hydrophobic branched segments produce a variety of morphologies in water and show outstanding uniformity in the size and morphology of the self-assemblies in comparison to typical surfactants. Recently, a novel single-chain surfactant r 2011 American Chemical Society

with multi-amine head groups, bis-(amidoethylcarbamoylethyl) octadecylamine (C18N3), which could self-assemble into various architectures in an aqueous solution, was synthesized.38 This kind of surfactant can function as a powerful structure-directing amphiphile, with greater versatility than commonly used surfactants or block copolymers, showing a significant advantage in the preparation of narrow polydispersity AgCl and Ag/AgCl hollow spheres,39 the fabrication of narrow polydispersity nano- and microplates,17 and the combination of gold nanoparticles (AuNPs) attached to the surface of polystyrene spheres to form a uniform nano- or microstructure.40 Herein, a facile, high-yielding, and reproducible synthesis of unprecedented hierarchical Au nanoflowers based on a twostep method is reported. First, C18N3 vesicles interacted with HAuCl4 and formed into templates, which were subsequently reduced by a stronger reductant, AA. In this manner, a variety of gold nanoflowers with narrow size distributions and unique morphologies can be prepared. The mechanism of nanoflower formation was also examined and is discussed in this work. Furthermore, these nanoflowers have a high SERS response for rhodamine 6G, indicating their potential application in biosensing and nanodevice fabrication.

Received: March 21, 2011 Revised: July 12, 2011 Published: July 18, 2011 3822

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to the formed gold films. SERS spectra were recorded using a 785 nm laser in a single scan. The laser power was 1 mW. Spectra were collected by focusing the laser beam onto the sample using a 50 objective, providing a spatial resolution of approximately 1 μm. The data acquisition time was 10 s for one accumulation. To test reproducibility, the measurements were performed at different positions on each sample.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization of Gold Nanostructures. The optimal conditions for the synthesis of AuNPs were

Figure 1. (a, b) SEM and (c, d) TEM images of gold nanoflowers synthesized under optimal conditions, in which 80 μL of an aqueous solution of HAuCl4 (48 mM) was added to 2.45 mL (0.5 mM) of an aqueous solution of C18N3, followed by the addition of 0.2 mL of an aqueous solution of ascorbic acid (20 mM).

2. EXPERIMENTAL SECTION 2.1. Materials. Chloroauric acid (HAuCl4 3 4H2O) and ascorbic acid were obtained from the Beijing chemical plant. Ultrapure deionized water (Milli-Q, MΩ cm 1) was used for all solution preparations. Bis-(amidoethylcarbamoylethyl)octadecylamine (C18N3) was synthesized by a two-step process as reported previously.38 2.2. Synthesis of Gold Nanostructures. To 2.45 mL of an aqueous solution of C18N3A (0.5 mM), 80 μL of an aqueous solution of HAuCl4 (48 mM) was added. The mixture was shaken gently. After 30 s, 0.2 mL of an aqueous solution of ascorbic acid (20 mM) was added one drop at a time. The mixture was shaken gently. The solution was maintained at room temperature for 1 h. The gold samples were purified by a centrifugation and resuspension procedure to remove excess reducing agents, salts, and amphiphiles. The sample solution was centrifuged at 3000 rpm for 5 min. The supernatant was removed, and ultrapure water was added to the precipitate. The residue was resuspended by sonication for 5 min. This rinsing procedure was repeated three times to obtain the final gold materials. The reactions were performed at room temperature (20 25 °C). 2.3. Characterization of Gold Nanostructures. The products were characterized by ultraviolet visible (UV vis) spectroscopy (Hitachi U-2800 spectrometer), scanning electron microscopy (SEM, Hitachi S4800, 10 kV), transmission electron microscopy (TEM JEOL2011, 200 kV), and X-ray diffraction (XRD, Rigaku Dmax-2000, Ni-filter Cu KR radiation). For the XRD measurements, the gold product was dispersed in water and several drops of the suspension were placed on a clean glass slide, and then allowed to dry in ambient air. For TEM and SEM measurements, the suspension was placed on a Formvar-covered copper grid and a silicon wafer, followed by air drying. 2.4. Raman Spectra Measurements. SERS measurements were performed using a confocal microprobe Raman spectrometer. R6G, from Aldrich, was used as a probe molecule. First, R6G was dissolved in ethanol, producing a 10 6 M solution. A total of 10 μL of gold nanoflowers was dropcast onto a silicon wafer and dried in ambient air. Once the film was dry, 10 μL of a 10 6 M ethanol solution was added

determined to be as follows: 80 μL of an aqueous solution of HAuCl4 (48 mM), added to 2.45 mL (0.5 mM) of an aqueous solution of C18N3, followed by the addition of 0.2 mL of an aqueous solution of ascorbic acid (20 mM). The molar ratio of [C18N3]/[HAuCl4]/[AA] was 1.00:3.12:3.25. The color of the solution changed from a light yellow to colorless, and finally to a dark green color, indicating the reduction of Au(III) to Au(0). Figure 1a d shows typical SEM and TEM images of the assynthesized gold nanoflowers after 1 h of storage. The flower-like colloidal particles appear quite monodisperse (Figure 1a). The size distribution reported in the histogram was obtained by dynamic light scattering (DLS), as shown in Supporting Information (Figure S1). The average diameter of approximately 650 nm is consistent with the electron microscopy measurements. The surface of the nanoflowers is composed of a large number of randomly arranged irregular ultrathin flakes with a zigzag fringe of approximately 20 50 nm in thickness. This kind of surface is expected to provide many active sites, which can significantly affect important properties such as the SERS of adsorbents41 and catalytic processes. The stability of the nanoflowers and of the functional molecules adsorbed on their surfaces over time is a crucial problem for their application. There was no obvious deformation of morphology observed under EM after adsorbing R6G for several months (Supporting Information, Figure S2), demonstrating that the nanoflowers possess very good mechanical stability. To determine the crystal structure of the as-synthesized nanostructures, a solution of gold nanoflowers was deposited on a glass substrate to form a thin film and examined by X-ray diffraction (XRD) measurements. The XRD pattern of the hierarchical nanoflowers is shown in Figure 2a, which exhibits sharp diffraction peaks exclusively attributed to Au crystals with a fcc structure (JCPDS No. 04-0784), indicating that the nanoflowers were pure, well-crystallized Au crystals. For further insight into the crystal structure, high-resolution transmission electron micrographs (HRTEM) were recorded. Figure 2b shows the lattice fringe spacing obtained from the rough fringe of the petals and the selected area electron diffraction pattern. The lattice fringe spacing determined at one of the rough fringes of petals was found to be d = 0.24 nm, corresponding to the (111) plane spacing of face-centered cubic (fcc) gold crystal. It is evident from the diffraction pattern that the rough fringe of the petals is a planar single crystal. In summary, a facile way to fabricate gold nanoflowers with a narrow size distribution was realized at an optimal molar ratio between C18N3, HAuCl4, and AA of [C18N3]/[HAuCl4]/[AA] = 1.00:3.12:3.25. It is interesting that the optimal molar ratio between [C18N3] and [AuCl4 ] is about 1:3, suggesting that one C18N3 molecule could chelated with three AuCl4 . 3.2. Effect of the HAuCl4 Concentration. To gain deeper insight into the growth mechanism of such nanostructures, the 3823

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Figure 2. (a) XRD pattern obtained from a thin film of nanostructures on a glass substrate; (b) high-magnification TEM view of typical fringe of petals; inset of b is a SAED pattern of the fringe of petals.

Figure 3. SEM images of Au nanostructures obtained with varying HAuCl4 concentrations at a constant concentration of C18N3 (0.5 mM) and AA (1.58 mM): (a) 0.54, (b) 1.27, (c) 1.45, (d) 1.63, (e) 1.81, and (f) 2.35 mM; The g j insets at the top right corners of panels b e represent the magnified images.

influence of the HAuCl4 concentration at a constant concentration of C18N3 (0.50 mM) and AA (0.50 mM) was investigated. The SEM images of the obtained gold nanostructures are shown in Figure 3. Because C18N3 vesicles can be formed when the concentration of this surfactant is higher than the CMC (0.05 mM) in a neutral medium,37,38 it is suggested that the gold nanoflowers were formed with C18N3 vesicles as a template. When a lower concentration of HAuCl4 (0.54 mM) was added to the C18N3 solution (0.50 mM), quasi-spherical gold nanostructures with a diameter of 200 nm and vertex protuberant polyhedrons with a diameter of 50 nm were obtained after the AA was added, as shown in Figure 3a. The rough surface of the nanostructures indicates the formation of C18N3 HAuCl4 complexes. Figure S3a,b, Supporting Information, shows the SEM and TEM images of the ball structure after centrifugation. Figure S3c, Supporting Information, shows the DLS of C18N3 and C18N3 HAuCl4 complexes having a similar size of about 160 nm, which is little smaller than the spherical nanoparticles. In addition, a cyro-TEM image of C12N3 having a similar molecular

structure to C18N3 except the length of the hydrophobic end is shown in Figure S3d, Supporting Information. Thus, it can be proposed that C18N3 aggregation in aqueous solution is spherical vesicle. Therefore, it is demonstrated that the formation of gold nanostructures was directed by the vesicles. When the concentration of HAuCl4 was increased to 1.27 mM, monodisperse bumpy nanoparticles with an average diameter 500 nm were obtained as shown in Figure 3b. As the concentration of HAuCl4 was increased to 1.45 mM, flower-like nanostructures emerged as shown in Figure 3c. The flower-like nanostructures with a diameter 650 nm formed many slices that resembled the petals of flowers with rough fringes. When the concentration of HAuCl4 was increased to 1.63 mM, the morphology of the flowers did not change significantly, but the petals became larger. However, when the concentration was increased to 1.81 mM, the roughness of the fringes of the petals and their size decreased. These experiments demonstrate that at different concentrations of HAuCl4 several kinds of nanostructures, such as vertex protuberant polyhedrons, branched nanostructures, and nanoflowers, could be formed. 3824

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Figure 4. SEM images of Au nanostructures obtained at different amounts of AA (2.0 mM): (a) 0.1, (b) 0.15, (c) 0.20, (d) 0.25, (e) 0.30, and (f) 0.35 mL in a solution containing HAuCl4 (1.6 mM) and C18N3 (0.5 mM).

Figure 5. SEM images of Au nanostructures obtained from an aqueous solution of HAuCl4 (1.6 mM) and AA (0.2 mL, 20 mM) with varying C18N3 concentrations: (a) 0.1, (b) 0.3, (c) 0.4, (d) 0.5, and (e) 0.6 mM.

It is important to point out that the size of the nanoflower depends on the concentration of HAuCl4. Figure 3 shows that the apparent size of the nanoflowers changes from 200 nm to approximately 1000 nm as the HAuCl4 concentration is increased from 0.54 to 2.35 mM, respectively. Because the complex ball was formed based on the C18N3 vesicle, which is very tough as shown in Figure S3, Supporting Information, the largest size formation at the optimal concentration of HAuCl4 is primarily attributed to the microcrystalline formation on the complex spherical surface. 3.3. Effect of the AA Concentration. To investigate the effect of AA on the morphology of the nanostructures, different concentrations of an aqueous AA solution were investigated at a constant concentration of C18N3 and HAuCl4. Figure 4 shows the SEM images of gold nanostructures obtained at different concentrations of AA. As shown in Figure 4a, small flower-like nanoparticles with a size of 300 nm were observed using 0.1 mL of AA (20 mM). The morphology of the petals is irregular. When 0.15 mL of AA was added, the size of the nanoflowers increased

to 800 1000 nm due to the growth of the gold petals. When the amount of AA increased from 0.2 to 0.25 mL, the density and thickness of the petals also increased. However, when the AA concentration exceeded 0.25 mL, instead of the large petals consisting of gold plates, many compact, bumpy nanospheres emerged. This phenomenon is consistent with the Weimarn Rule for the average size of crystals precipitated from a supersaturating solution:42 at a high concentration of AA, many more crystal seeds are formed due to the very fast reduction of Au3+, depressing the greater crystalline growth on the limited number of seeds on the vesicle surface. This phenomenon can control the surface morphology of the nanoflowers. 3.4. Complexation of Au Ions with C18N3 on the Surface of Vesicles. It is well-known that surface complexes play a very important role in the morphology of nanocrystals.43 Many reports have shown complex formation between HAuCl4 and dendrimers with multi-amine head groups.44 49 In order to demonstrate the interaction between the surfactant and HAuCl4, FT-IR was performed (Figure S4, Supporting Information), 3825

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Scheme 1. Schematic Illustration of the Proposed Mechanism for the Formation of Flower-like Nanostructures

Figure 6. SERS spectra of R6G form flower-like gold particles and the pure R6G solid powder on the quartz substrate.

indicating the existence of complexation between C18N3 and HAuCl4. However, the relationship between the morphology and the complex formation still remains unclear. To demonstrate the importance of C18N3 in the fabrication of nanoflowers, a control experiment was performed in which a HAuCl4 solution was reduced by AA directly. Figure S5c, Supporting Information, shows that without C18N3, the obtained nanoparticles have a spherical morphology and their dispersion has a strong absorbance peak at 605 nm (Figure S6b, Supporting Information), which indicates a spherical gold nanoparticle. However, when C18N3 is present, the obtained nanoparticle dispersion absorbs from the whole visible to near-infrared spectrum and becomes black, which indicates that a highly asymmetric morphology, such as microplates, exists on the surface of the particles. Because C18N3 has three active sites for dissociation in water, as reported in our previous work,38 it is reasonable to suggest that the chelation sites on the vesicle surface have different ability for reduction. The chelation will decrease the concentration of free ions around the chelation sites and, in turn, decrease the reduction rate, which is important for crystal growth. The first gold nuclei grow fast and depress the growth of other nuclei, such that a hierarchical gold nanoflower forms based on a hierarchical reduction of available growth sites on C18N3. The effect of the concentration of C18N3 was also investigated. Figure 5 shows the SEM images of particles obtained at different concentrations of C18N3, using a constant concentration of HAuCl4 and AA. As we reported elsewhere, vesicles can be formed if the concentration of the surfactant is greater than the critical micelle concentration (CMC) of C18N3 (0.05 mM) in a neutral medium.38 Because the lowest concentration of C18N3 utilized in this experiment was 0.1 mM, C18N3 aggregates existed as vesicles, and the gold nanoflowers always formed on the C18N3 vesicles. When the concentration of C18N3 was 0.1 mM, nanoflowers with a size of 500 nm formed along with a small nanoflower, as shown in Figure 5a. It can be attributed to the insufficiency of template. In order to demonstrate the importance of C18N3 vesicles on the nanoflower formation, the morphology of nanostructures were observed by SEM when the C18N3 concentrations were 0.05 (CMC) and 0.01 mM (Figure S5a,b, Supporting Information). It is interesting that, when the concentration of C18N3 changed from 0.3 to 0.6 mM, the size remained nearly the same, as shown in Figure 5, but the

density of flakes on the surface of nanoflower increased. Therefore, the surface morphology of nanoflowers can be tailored by changing the concentration of C18N3. pH measurements of these samples showed that they were almost the same as when the concentration of C18N3 changed from 0.1 to 0.6 mM (see Table S1, Supporting Information). Based on the fact that the pH value determines the sizes of C18N3 vesicles38 and that pH values are almost constant for a wide range of C18N3 concentrations, the pH value appeared to be a very important factor in the fabrication of gold nanoflowers. In summary, it is possible to modulate the size, polydispersity, thickness, and structure morphology of gold nanoflowers by changing the concentration of C18N3, HAuCl4, and AA. The formation process of flower-like gold nanostructures in the presence of a tree-type multi-amine surfactant (C18N3) is proposed in Scheme 1. 3.5. SERS Effect on Flower-like Gold Particles. Few studies have shown SERS of R6G from gold substrates because R6G does not strongly interact with the surface of gold. R6G on a quartz substrate gave a very weak response. Figure 6 shows the Raman spectra of R6G mixed with nanoflower particles, which clearly show the main vibrational features of R6G molecules.50 The enhanced factor (EF)51 for the R6G was approximately 106. This enhanced effect may be due to the petals with a zigzag fringe in the gold flowers, which have strong electromagnetic interactions with the dye molecules.41

4. CONCLUSIONS Hierarchical flower-like gold nanostructures were successfully generated by the reduction of an aqueous solution of HAuCl4 in the presence of a tree-type multiple-amine surfactant (C18N3) by ascorbic acid. The vesicles from the tree-type multiple-amine surfactants are formed instantly, serving as a template in the synthesis of various hierarchical flower-like gold structures. The morphology, size, and structure of the nanoflowers could be mediated by changing the amount of C18N3, HAuCl4, and AA in the reaction solution. The size of the gold nanoflowers could be changed from 200 to 1000 nm under these experimental conditions, and the as-prepared nanoflowers have a very high SERS effect for R6G. This work demonstrated that the tree-type surfactant C18N3 is essential for the fabrication of nano- and 3826

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Crystal Growth & Design mesostructures. This synthetic strategy may provide a new route for the facile fabrication of various metallic and nonmetallic nanostructures with novel morphologies and useful applications.

’ ASSOCIATED CONTENT

bS

Supporting Information. SEM image and histograms indicating the particles size distribution, SEM images involving the stability of nanoflowers, UV vis spectroscopy, IR, and pH measurement results. This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research is supported by the National Science Foundation of China (Grant Nos. 20933007, 90207026). ’ REFERENCES (1) Sperling, R. A.; Rivera Gil, P.; Zhang, F.; Zanella, M.; Parak, W. J. Chem. Soc. Rev. 2008, 37, 1896. (2) Wilson, R. Chem. Soc. Rev. 2008, 37, 2028. (3) Jain, P. K.; Huang, X.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578. (4) Sardar, R.; Funston, A. M.; Mulvaney, P.; Murray, R. W. Langmuir 2009, 25, 13840. (5) Huang, X.; Neretina, S.; El-Sayed, M. A. Adv. Mater. 2009, 21, 4880. (6) Hu, M.; Chen, J.; Li, Z.-Y.; Au, L.; Hartland, G. V.; Li, X.; Marquez, M.; Xia, Y. Chem. Soc. Rev. 2006, 35, 1084. (7) Imura, K.; Okamoto, H.; Hossain, M. K.; Kitajima, M. Nano Lett. 2006, 6, 2173. (8) Jiang, Y.; Horimoto, N. N.; Imura, K.; Okamoto, H.; Matsui, K.; Shigemoto, R. Adv. Mater. 2009, 21, 2309. (9) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Chem. Soc. Rev. 2008, 37, 1783. (10) Tao, A. R.; Habas, S.; Yang, P. Small 2008, 4, 310. (11) Xia, Y.; Xiong, Y.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (12) Nehl, C. L.; Hafner, J. H. J. Mater. Chem. 2008, 18, 2415. (13) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzan, L. M.; Mulvaney, P. Coord. Chem. Rev. 2005, 249, 1870. (14) Wang, C.; Hu, Y.; Lieber, C. M.; Sun, S. J. Am. Chem. Soc. 2008, 130, 8902. (15) Zhang, J.; Du, J.; Han, B.; Liu, Z.; Jiang, T.; Zhang, Z. Angew. Chem., Int. Ed. 2006, 45, 1116. (16) Sun, X.; Dong, S.; Wang, E. Angew. Chem. 2004, 116, 6520. (17) Lin, G. H.; Lu, W. S.; Cui, W. J.; Jiang, L. Cryst. Growth Des. 2010, 10, 1118. (18) Millstone, J. E.; Hurst, S. J.; Metraux, G. S.; Cutler, J. I.; Mirkin, C. A. Small 2009, 5, 646. (19) Li, C.; Shuford, K. L.; Park, Q. H.; Cai, W.; Li, Y.; Lee, E. J.; Cho, S. O. Angew. Chem. 2007, 119, 3328. (20) Ma, Y.; Zeng, J.; Li, W.; McKiernan, M.; Xie, Z.; Xia, Y. Adv. Mater. 2010, 22, 1930. (21) Sanchez-Iglesias, A.; Pastoriza-Santos, I.; Perez-Juste, J.; Rodríguez-Gonzalez, B.; GarcíadeAbajo, F. J.; Liz-Marzan, L. M. Adv. Mater. 2006, 18, 2529. (22) Skrabalak, S. E.; Chen, J.; Sun, Y.; Lu, X.; Au, L.; Cobley, C. M.; Xia, Y. Acc. Chem. Res. 2008, 41, 1587. (23) Zhong, L.; Zhai, X.; Zhu, X.; Yao, P.; Liu, M. Langmuir 2009, 26, 5876.

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