Direct High-Yield Synthesis of High Aspect Ratio Gold Nanorods

Feb 28, 2007 - Direct high-yield synthesis of high aspect ratio gold nanorods can be achieved by seed-mediated growth approach using cetyltrimethylamm...
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Direct High-Yield Synthesis of High Aspect Ratio Gold Nanorods

CRYSTAL GROWTH & DESIGN 2007 VOL. 7, NO. 4 831-835

Hsiang-Yang Wu, Wan-Ling Huang, and Michael H. Huang* Department of Chemistry, National Tsing Hua UniVersity, Hsinchu 30013, Taiwan ReceiVed NoVember 6, 2006; ReVised Manuscript ReceiVed December 24, 2006

ABSTRACT: We report a new development for the direct high-yield synthesis of high aspect ratio gold nanorods. By using a modified seed-mediated synthesis approach for the preparation of high aspect ratio gold nanorods with the addition of an appropriate amount of nitric acid during nanorod growth, uniform and monodispersed gold nanorods were synthesized in large quantity. The formation of triangular nanoplate byproducts was substantially reduced by replacing trisodium citrate with cetyltrimethylammonium bromide (CTAB) surfactant as the capping agent in the preparation of gold seeds. The percent yield of nanorods produced can be as high as over 90% after a simple purification step. The nanorods have an average length of 355.3 ( 31.3 nm and an average diameter of 18.7 nm, giving them an average aspect ratio of 19. These nanorods can spontaneously self-assemble into highly ordered side-by-side packing structures over a large area. The X-ray diffraction pattern revealed a very strong (111) diffraction peak. The ultra-small gold seeds used for nanorod growth were ∼1-2 nm in diameter and showed a weak surface plasmon resonance (SPR) absorption band centered at ∼480 nm. The nanorods showed a transverse SPR absorption band at 497 nm and a longitudinal SPR band at ∼2135 nm. Because of the large reduction in the amount of triangular and truncated triangular nanoplates formed, the absorbance of the nanoplates at 830 nm has been lowered substantially. Introduction In the past few years, there has been a growing interest in using gold nanorods to form interesting superstructures. Endto-end linkages of gold nanorods into chainlike structures via covalent bonding and hydrogen bonding have been demonstrated.1 This nanorod linkage strategy has been used for the selective detection of cysteine and glutathione.2 Side-by-side assembly of gold nanorods via interlinking molecules have also been studied.3 In addition to applications in chemical analysis, gold nanorods may also be considered as components of nanoscale electronic devices. For nanoscale circuit design and fabrication of functional architecture such as wire interconnections, long nanorods are more preferable to short nanorods. Although several different approaches such as electrochemical,4 photochemical,5 and seed-mediated methods6-8 have been used to grow gold nanorods, only nanorods with relatively low aspect ratios (that is, 1.5-10) can be produced in high yield to over 90%. High aspect ratio gold nanorods (that is, ∼15-25) can be prepared via the seed-mediated synthesis approach,9 but the percent yield is relatively low (∼20-40% for aspect ratio of ∼25).7 The side products in the formation of high aspect ratio gold nanorods are mostly spherical nanoparticles, triangular and truncated triangular nanoplates, and some short nanorods. To obtain a relatively pure long nanorod sample, a separation scheme using centrifugation, followed by repeated precipitation and redispersion in hot cetyltrimethylammonium bromide (CTAB) solution, has been reported.10 This separation procedure is time-consuming, and fine-tuning of the separation conditions is required. A method that can directly synthesize long nanorods in high yield with minimum purification steps is certainly more desirable. We have recently reported a seed-mediated approach for the synthesis of long gold nanorods with average aspect ratios of 21-23 and a monodispersed size distribution by the addition of nitric acid during nanorod growth.11 The production of high aspect ratio gold nanorods was greatly enhanced to 60-70%, as determined by counting the gold nanostructures in the * To whom correspondence [email protected].

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scanning electron microscopy (SEM) images, such that the nanorods can spontaneously self-assemble into a high-density three-dimensional packing structure. However, there was still a significant amount of triangular, truncated triangular, and hexagonal nanoplates about 120-200 nm in width present in the sample. To improve the percent yield of nanorods, the formation of nanorods should increase, and the amount of nanoplates produced must be reduced. In a separate work on the thermal aqueous solution approach for the synthesis of triangular and hexagonal gold nanoplates, we noted that trisodium citrate may play a key role in assisting the formation of nanoplates.12 With this insight, we have used CTAB instead of trisodium citrate as the capping agent for the preparation of gold seeds. The resulting gold seeds are smaller than those synthesized with trisodium citrate. This seemingly minor change in the synthetic procedure led to a remarkably high yield of gold nanorods. Here we present a simple and effective procedure for the direct high-yield synthesis of high aspect ratio gold nanorods in large quantity with relatively few nanoplate side products. The key step is the preparation of gold seeds by adding a small volume of ice-cold aqueous NaBH4 solution, a strong reducing agent, to an aqueous solution of HAuCl4 and CTAB surfactant. Transfer of a small volume of this gold seed solution into a growth solution containing HAuCl4, CTAB, and ascorbic acid starts the nanorod growth. The nanorod growth procedure is the same as before, with the addition of 300 µL of 0.1 M nitric acid to the last growth solution to enhance the synthesis of nanorods with the highest aspect ratio possible.11 After the formation of nanorods, a simple removal of the top solution containing mostly spherical nanoparticles, followed by redispersion and centrifugation of the precipitate in water, yields highly pure gold nanorods in large quantity. Experimental Section Preparation of Gold Seeds. A volume of 0.25 mL of 0.01 M HAuCl4 (Aldrich, g99.9%) and 0.3645 g of CTAB surfactant (Aldrich, > 99.0%) were added to a flask. Deionized water was added to bring the total solution volume to 10 mL. The concentration of CTAB in this solution is 0.1 M. In a separate flask, 10 mL of ice-cold 0.01 M NaBH4 (Aldrich, 98%) was

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prepared. Then 350 µL of the NaBH4 solution was transferred to the CTAB-HAuCl4 solution, and the reaction was stirred for 3 min. The initial yellowish solution turned colorless and then faint brown upon the addition of NaBH4, indicating the formation of ultra-small gold particles. Preparation of Growth Solution. A volume of 100 mL of 2.5 × 10-4 M HAuCl4 aqueous solution was prepared. Then 1 × 10-2 mol of CTAB was added to the solution with stirring until the CTAB powder was completely dissolved. Synthesis of High Aspect Ratio Gold Nanorods. Two 25mL and one 100-mL conical flasks were labeled A, B, and C. In flasks A and B, 25 µL of 0.1 M L(+)-ascorbic acid (Riedelde-Hae¨n, 99.7%) was added to 4.5 mL of growth solution. In flask C, 250 µL of 0.1 M ascorbic acid and 300 µL of 0.1 M nitric acid were added to 45 mL of growth solution. Next, 400 µL of the gold seed solution was added to the solution in flask A, and the sample was stirred for 3 s. Then 400 µL of the solution in flask A was immediately added to flask B, and the sample was stirred for 5 s. Finally, 4 mL of the solution in flask B was transferred to flask C, and the sample stirred for 5 s. Flask C was left undisturbed in a water bath set at 26-27 °C for 12 h for the reaction to go to completion. The top solution contained mostly spherical nanoparticles and was removed, leaving only high aspect ratio nanorods that settled to the bottom of the flask as a precipitate. Then 10 mL of deionized water was added to redisperse the precipitate, and the solution color was brown. The nanorods were concentrated by centrifugation at 2000 rpm for 20 min (Hermle Z323 centrifuge). Again the top solution containing mainly spherical nanoparticles and CTAB surfactant was carefully removed with a pipet. The nanorod precipitate was then withdrawn with a pipet and diluted with deionized water for spectral analysis. Instrumentation. SEM images were obtained using a JEOL JSM-6330F scanning electron microscope. A drop of the nanorod sample was added to a clean silicon substrate and dried in the air for the SEM images. Transmission electron microscopy (TEM) images were acquired using a JEOL 2000FX electron microscope operating at 200 kV. High-resolution TEM images were obtained using a JEOL JEM-4000EX electron microscope operating at 400 kV. UV-vis absorption spectra were taken on a JASCO V-570 spectrophotometer. A background subtraction was employed for each spectrum to remove any absorption of water in the near-infrared (NIR) region. To obtain absorption spectra of the nanorods in the NIR region, the concentrated nanorod precipitate after centrifugation was transferred to a microscope cover glass and dried in a vacuum oven. With this treatment, the nanorods were found to show random orientation, and the possible effect of interparticle coupling on their plasmon resonances is minimized. The cover glass was mounted on a sample holder for measurements with a background subtraction. Results and Discussion In this study, the gold seeds were prepared by replacing trisodium citrate with CTAB as the capping agent to significantly reduce the formation of nanoplate byproducts. El-Sayed et al. have used a similar approach to improve the yield of gold nanorods with aspect ratios of 1.5-10.6b,6c However, AgNO3 and a cosurfactant were added to assist the nanorod growth, and silver ions were considered to play a key role in the nanorod formation. The growth mechanism of gold nanorods with and without the presence of silver ions has been shown to be different.13 In this regard, our reasoning for using CTAB as the capping agent and the resulting production of high aspect ratio gold nanorods without the presence of silver ions are quite different from their work.

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Figure 1. (a) SEM image of the high-density arrays of high aspect ratio gold nanorods over a large area. Scale bar ) 1 µm. (b) SEM image of the upper right corner in panel a. These long nanorods tend to pack into bundles. Few triangular and truncated triangular nanoplates are observed in this image. Scale bar ) 500 nm.

Figure 1 shows a typical SEM image of gold nanorods obtained by the present synthetic method. A large quantity of high aspect ratio gold nanorods was observed. These nanorods also self-assemble into high-density side-by-side packing structures. Notice that there are very few triangular and truncated triangular nanoplates present in this sample, demonstrating the success of the current synthetic strategy in producing high aspect ratio gold nanorods with high purity. It is estimated that the percent yield of nanorods produced can be as high as over 90%, as determined by examining and counting the gold nanostructures shown in many SEM images, and represents a dramatic improvement over other known methods. These nanorods are straight and very uniform in dimensions with an average length of 355.3 ( 31.3 nm (100 nanorods were counted) and an average diameter of 18.7 nm (80 nanorods were counted), giving them an average aspect ratio of 19. Thus, despite only a slight modification in the nanorod synthesis procedure, the improvement in the nanorod yield is quite significant, considering that no simple and effective method is yet available in the literature to produce high aspect ratio gold nanorods with such a high yield. The results also show that this present work is not an incremental advance in the research on the synthesis of high aspect ratio gold nanorods, as the formation of a large quantity of nanoplate byproducts is a typical problem in the growth of long nanorods. A schematic diagram illustrating the key differences between this study and our previous study on the

Synthesis of High Aspect Ratio Gold Nanorods

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Figure 2. (a) Length distribution histogram of the gold nanorods. (b) UV-vis absorption spectrum of the gold seeds. (c and d) UV-vis absorption spectra of the gold nanorods. Absorbance value in panel d cannot be directly compared with that in panel c. This is because in panel d nanorods were added to a microscope cover glass and dried for spectral measurements.

Figure 3. SEM image of the high aspect ratio gold nanorods selfassembled into high-density and highly ordered packing arrays over an extremely large area. Scale bar ) 1 µm.

synthesis of high aspect ratio gold nanorods is provided in Supporting Information. Figure 2a gives a length distribution histogram of the gold nanorods prepared in this study. These nanorods show a relatively narrow size distribution. Because an extremely large number of long nanorods were synthesized, they can spontaneously form an ultrahigh-density and highly ordered packing superstructure over a very large area upon the evaporation of a water droplet on a substrate. Interestingly, these nanorods can also form a curved or wavelike pattern over a very large area despite their long lengths (Figure 3). Such ultrahigh-density packing of gold nanorods has not been fabricated before. These features suggest the possibility of using these nanorods to prepare highly oriented nanorod arrays on substrates through

an applied directional flow; Langmuir-Blodgett technique has been used to form monolayers of aligned silver nanowires.14 Formation of more complicated and functional architecture from the assembled nanorods may be conceived. The crystal structure of these high aspect ratio gold nanorods has been characterized before by transmission electron microscopy.11 Figure 4 shows some TEM images of the high aspect ratio gold nanorods synthesized in this study. The dimensions of these nanorods are relatively uniform. The dark bands or fringes on these nanorods should result from the slight variations in the diameters of these nanorods and suggest that the nanorod surfaces are not atomically flat. The X-ray diffraction pattern of these nanorods further reveals a very strong (111) diffraction peak. The intensity of the (222) diffraction peak is also strong (see Figure 5). This result is consistent with that obtained from the TEM analysis that the gold (111) lattice planes are aligned parallel to the direction of the long axis of the gold nanorod. The optical properties of the gold seeds and the high aspect ratio nanorods were characterized by UV-vis absorption spectroscopy. Figure 2b shows a UV-vis absorption spectrum of the gold seeds. A weak band centered at ∼480 nm was observed. The particle size of the gold seeds, as determined by the high-resolution TEM images, is about 1-2 nm (see Supporting Information). Previously, the gold seeds prepared using trisodium citrate as the capping agent have diameters of 2-4 nm and showed a single surface plasmon resonance (SPR) band at 508 nm.11,15 In general, spherical gold nanoparticles with a uniform and smaller diameter exhibit an absorption band at shorter wavelengths. The UV-vis absorption spectra of the high aspect ratio gold nanorods are shown in Figure 2c,d. The spectra show an absorption band at 497 nm, which should correspond to the transverse SPR absorption band of the nanorods. The longitudinal SPR band appears at ∼2135 nm. The position of the longitudinal SPR band is in fair agreement with that predicted using the discrete dipole approximation

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Figure 4. TEM image of the high aspect ratio gold nanorods. Again these long nanorods have the tendency to form parallel alignment. Scale bar ) 100 nm. Inset shows a higher magnification TEM image of some closely spaced gold nanorods. The uniform spacing between adjacent nanorods may be attributed to the presence of capping CTAB surfactant. Scale bar of the inset ) 50 nm.

Figure 5. XRD pattern of the high aspect ratio gold nanorods. The intensity of the (111) diffraction peak is very strong. The intensity of the (222) diffraction peak is also strong relative to the intensities of the other diffraction peaks.

method (that is, λmax ) 96AR + 418, where AR refers to aspect ratio), considering the length distribution of the nanorods and the relatively broad width of the band.16 Two more bands centered at ∼830 and 1030 nm are attributed to light absorption by the triangular and truncated triangular nanoplates (∼90150 nm in width). Since much fewer nanoplates were formed, the absorbance of the nanoplate bands was substantially lowered such that the nanoplate absorbance is similar to that of the transverse SPR band of the nanorods. Previously, the absorbance of the nanoplates was much stronger than that of the nanorods.11 This is strong evidence of the significant reduction in the formation of triangular nanoplates. Please note that a direct comparison of the relative absorbance values to determine the

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absolute percentages of different gold nanostructures is not accurate, as they may have different extinction coefficients. The growth mechanism of these high aspect ratio gold nanorods has been proposed before.9b,17 The nanorods grow by adding more gold ions in the growth solution to the seed particles and having CTAB in the solution form elongated bilayer structure over the growing nanorod surface.9b Alternatively, AuCl2--adsorbed CTAB micelles (through reduction of AuCl4- to AuCl2- by ascorbate and the displacement of bromide ions) are transported to the growing CTAB-capped seed particles.17 The deposition of AuCl2--adsorbed CTAB micelles to the tips of the seeds leads to rod formation. Our gold nanorods should also be formed following the proposed growth mechanism as they are also seed-mediated. However, in terms of the growth mechanism, our study is focused more on which aspects significantly improved yield of high aspect ratio nanorods. Two factors are believed to contribute to the ability of the present method to directly prepare such a large quantity of high-purity long gold nanorods. First, the addition of nitric acid during nanorod growth was found to greatly enhance the formation of nanorods.11 It has been shown that micelles formed by the cetyltrimethylammonium nitrate (CTAN) surfactant have a larger aggregation number and micellar size than those formed by the CTAB surfactant.18 CTAN micelles formed at high surfactant concentrations tend toward prolate ellipsoidal or rodshaped. Ropelike CTAB-templated mesoporous silica MCM41 can be made by the addition of nitric acid to induce the elongation of the surfactant micelles.19 On the other hand, replacement of bromide ions with chloride ions leads to shorter nanorods, as the micellar size is smaller and more spherically shaped.17,18 Thus, a suitable amount of nitrate ions can assist the formation of elongated CTAB micelles and significantly increase the percent yield of long gold nanorods. Furthermore, nitrate ions have recently been shown to stabilize the {100} planes of cubic Cu2O crystals.20 Since the side faces of the gold nanorods are bounded by {100} faces, adsorption of nitrate ions on the side faces of the gold nanorods may also stabilize the nanorod structure and facilitate the formation of high aspect ratio nanorods.11 Second, the modified synthesis of gold seeds without the use of trisodium citrate, but with CTAB surfactant, appears to be effective in reducing the formation of nanoplates. Citrate ions could modify the crystal growth direction and facilitates the formation of gold nanoplates.12 Inhibition of small gold nanoplate formation at the very early stage of the nanorod growth process ensures a significant reduction of the nanoplate byproducts in the final nanorod sample. The use of smaller gold seeds may also be helpful in raising the nanorod yield. The combined effect of these key steps leads to a very high percent yield of long nanorods. Conclusion In summary, by using a modified seed-mediated synthesis procedure with the use of CTAB surfactant as the capping agent in the preparation of gold seeds and the addition of an appropriate amount of nitric acid in the growth solution, gold nanorods with high aspect ratios can be produced in a very large quantity with a minimum amount of triangular nanoplate formation. With a simple purification procedure, the nanorod yield can reach over 90%. These long nanorods also readily form ultrahigh-density packing on substrates. With this direct and simple synthesis of highly pure high aspect ratio gold nanorods, molecular sensing and nanostructure integration using long gold nanorods may become more attractive. It is expected that the demonstration of research ideas using gold nanorods can now be more readily extended to these long nanorods.

Synthesis of High Aspect Ratio Gold Nanorods

Acknowledgment. The authors thank the National Science Council of Taiwan for financial support (Grant NSC 94-2113M-007-012). Supporting Information Available: A scheme showing the key differences between our previous study on high aspect ratio gold nanorods and this current study, and TEM images of the gold seed particles. This materials is available free of charge via the Internet at http://pubs.acs.org.

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