Self-assembled macroscopic structures of gold nanoparticles

Self-assembled Macroscopic Structures of Gold Nanoparticles Nadja Bigall, Stephen G. Hickey, Nikolai Gaponik, Alexander Eychmüller Physical Chemistry, TU Dresden, Bergstraße 66b, 01062 Dresden, Germany ABSTRACT Macroscopic materials built from assembled gold nanoparticles are presented. The structure is characterized via SEM. It turns out to consist of a long, tube-like network with several bifurcations. The tubes have diameters in the range of one micrometer. Electron microscopy also indicates that the nanoparticles of the system are still separated from each other. The procedure is likely to be a template mediated assembly with a carbon compound as template, even though the complete growth mechanism still has to be investigated. Keywords: gold, nanoparticles, self-assembly, template-assisted, microtubes

1. INTRODUCTION The investigation of new techniques, such as template free [1-6] and template induced [7-14] assemblies of nanoparticles, is in the focus of nanoscience since these structures have widespread applications and thus promise much scientific and industrial progress [15,16]. Metal nanoparticles, for example, have interesting optical properties due to the size and shape dependent position of their plasmon resonance. Assembled structures of these colloids can have additional (even completely different) interesting optical or physical properties, depending on the regularity of their superstructures and the resulting distances between the nanoparticles. Gold nanoparticles of different sizes are synthesized in aqueous solution using a slight variation of the reduction of tetrachloroaureate by sodium borohydrate and sodium citrate [17]. The as-prepared particles can subsequently be used for template induced assembly yielding macroscopic superstructures in the range of centimeters. There are different possibilities to build the template for the assembly, depending on the proposed purpose. For example, for the preparation of regular two- or three-dimensional superstructures, it is possible to build opal-like assemblies of microspheres, whereby the lattice parameter is tuneable via the size of the spheres [18-20]. Chemical modification of the surface influences the assembly of the gold nanoparticles onto the template. In principe, the same strategy of template induced assembly can lead to one-dimensional tube-like microstructures which result in a cloud-like network in the range of cubic centimeters, due to several bifurcations in the template structure [21]. Here the outer diameter of the tubes is approximately 1.3 microns, the inner diameter approximately one micron, which is very large compared to the particle diameter which is in the range of 15 nanometers. The characteristic shape of this structure of high inner and outer surface has the possibility to be of great interest for different future applications, e.g. for plasmonic studies, whispering gallery mode investigations and as substrates for surface enhanced Raman spectroscopy.

2. METHODOLOGY The scanning electron micrographs (SEM) have been recorded on a Zeiss DSM 982 Gemini instrument. 90 mL of a solution of 0,011 % tetrachloroaureate (Acros) in bidestilled water were stirred in a conical flask. A solution of 2 mL of a solution containing 1 % sodium citrate (Sigma) and 0,05 % of citric acid (Grüssing, 99 %) in bidestilled water was prepared, as well as a solution containing the same ingredients and 0,072 % of sodium borohydrate. After 25 minutes, first 2 mL of the citrate solution were added to the tetrachloroaureate solution, followed about one minute later by 1 mL of the sodium borohydrate solution. The reaction took place at room temperature. The solution became violet

ICONO 2007: Novel Photonics Materials; Optics and Optical Diagnostics of Nanostructures edited by Oleg A. Aktsipetrov, Vladimir M. Shalaev, Sergey V. Gaponenko, Nikolay I. Zheludev Proc. of SPIE Vol. 6728, 67281N, (2007) · 0277-786X/07/$18 · doi: 10.1117/12.752365 Proc. of SPIE Vol. 6728 67281N-1

but changed over night to the typical red color of small gold nanoparticles. After leaving the solution under ambient conditions for about two months, the typical cloudlike macroscopic structure had developed.

3. DATA Figure 1 shows a typical absorption spectrum of the gold colloidal solution before generation of the superstructures. The absorption maximum corresponding to the plasmon resonance is at a wavelength of 517 nm. Using data from the literature it is possible to determine the mean size of the gold nanoparticles to be 15 nm [22]. The macroscopic structures had a volume ranging from a few cubic millimeters to cubic centimeters. For example, a washed sample with about 0.07 cm3 had a total mass of 0.50 mg. So these tube-like structures have an average density of 100 mg per cubic centimeter. SEM data indicate that the inner diameter is 1.0 µm (figure 2) whereas the outer diameter is 1.3 µm with a broad standard deviation of 28 % (obtained from figure 3).

4. RESULTS After about two months a cloud-like macroscopic structure had developed, floating in the solution of the gold nanoparticles (figure 4). Via scanning electron microscopy the superstructure is identified to be a multibifurcational network (figure 5). A look at one end of these structures (figure 2) shows the assembled structure to be hollow in character. The thickness of the tube wall is about 0.2 µm. The diameter of the branches is from 0.8 µm to 1.6 µm, whereby the main diameter is 1.3 µm (figure 3). The bifurcations appear on the length scale of 100 µm (figure 5) which is two orders of magnitude larger than the diameter of the tubes. At higher resolutions the gold nanoparticles of this superstructure are clearly visible and turn out to still remain separated from each other (figure 6). From differences in the SEM contrasts (figure 2) and from closely related previous work in the literature [3] we presume the existence of a template inducing the assembly of the gold nanoparticles.

5. CONCLUSIONS We have synthesized a microscopic superstructure assembled from gold nanoparticles, most probably on a template. The superstructure consists of tube-like branches with many y-shaped bifurcations leading to a net-like structure. The mean outer diameter of the tube-like structure is about 1.3 µm and the thickness of the wall is about 0.2 µm. The outer volume of these structures is of the dimension of cubic centimeters. The average density of the macrostructure turns out to be very low with 100 mg per cubic centimeter. Scanning electron micrographs confirm that the gold nanoparticles remain well separated. Although the growth mechanism still has to be clarified, the synthesis of these gold microtubes is another approach for obtaining superstructures and may in future be projected onto different colloidal systems.

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Fig. 1. Typical absorption spectrum one month after the injection of the reducing agents

2 µm Fig. 2. The Scanning electron micrograph of one end of the structure indicates its hollow nature.

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Fig. 3. The SEM shows that the outer diameters are in the range between 0.8 and 1.6 µm, resulting in a mean diameter of 1.3µm.

Fig.4. Photograph of the cloudlike branched macroscopic structure inside the impregnation solution of the gold nanoparticles. The volume of the macrostructure can exceed some cubic centimeters.

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50 µm Fig.5. In the SEM overview the multibranched structure is clearly visible.

500 nm Fig.6. At higher resolutions the SEM clearly shows that the nanoparticles remain separate even after the assembly.

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REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.

Z. Tang, Z. Zhang, Y. Wang, S. C. Glotzer,N. A. Kotov Science 314, 274 (2006) Y. Gao, Y. Bao, M. Beerman, A. Yasuhara, D. Shindo, K. M. Krishnan Appl. Phys. Lett. 84, 17, 3361 (2004) Y. X. Zhang, H. C. Zeng J. Phys. Chem. B 110, 16812 (2006) M. Li, H. Schnablegger, S. Mann nature 402, 393 (1999) J. J. Diao, J. Sun, J. B. Hutchison, M. E. Reeves Appl. Phys. Lett. 87, 103113 (2005) K. Naka, H. Itoh, Y. Chujo Nano Lett., 2, 11 (2002) J. Sharma, R. Chhabra, Y. Liu, Y. Ke, H. Yan Angew. Chem. Int. Ed. 45, 730 (2006) W. Shenton, D. Pum, U. B. Sleytr, S. Mann nature 389, 585 (1997) Y. Lu, H. Fan, A. Stump, T. L. Ward, T. Rieker, C. J. Brinker nature 398, 223 (1999) C. Minelli, C. Hinderling, H. Heinzelmann, R. Pugin, M. Liley Langmuir 21, 7080 (2005) A. N. Shipway, M. Lahav, I. Willner Adv. Mater. 12, 13, 993 (2000) T. O. Hutchinson, Y.-P. L., C. Kiely, C. J. Kiely, M. Brust Adv. Mater. 13, 23, 1800 (2001) H. Nakao, H. Shiigi, Y. Yamamoto, S. Tokonami, T. Nagaoka, S. Sugiyama, T. Ohtani Nano Lett. 3, 10, 1391 (2003) Y. Wang, Z. Tang, S. Tan, N. A. Kotov Nano Lett. 5, 2, 243 (2005) R. J. Tseng, C. Tsai, L. Ma, J. Ouyang, C. S. Ozkan, Y. Yang nature nanotechnology 1, 72 (2006) I. W. Hamley Angew. Chem. Int. Ed. 42, 1692 (2003) K. R. Brown, D. G. Walter, M. J. Natan, Chem. Mater. 12, 306 (2000) L. Lu, I. Randjelovic, R. Capek, N. Gaponik, J. Yang, H. Zhang, A. Eychmüller, Chem. Mater. 17, 5731 (2005) L. Lu, A. Eychmüller, A. Kobayashi, Y. Hirano, K. Yoshida, Y. Kikkawa, K. Tawa, Y. Ozaki, Langmuir 22, 2605 (2006) L. Lu, R. Capek, A. Kornowski, N. Gaponik, A. Eychmüller Angew. Chem. Int. Ed. 44, 5997 (2005) T. Wang, R. Zheng, X. Hu, L. Zhang, S. Dong, J. Phys. Chem. B 110, 14179 (2006) W. Haiss, N. T. K. Thanh, J. Aveyard, D. G. Fernig Anal. Chem.79, 4215 (2007)

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