Highly Ordered Assemblies of Au Nanoparticles Organized on DNA

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Highly Ordered Assemblies of Au Nanoparticles Organized on DNA

2003 Vol. 3, No. 10 1391-1394

Hidenobu Nakao,† Hiroshi Shiigi,‡ Yojiro Yamamoto,‡ Shiho Tokonami,‡ Tsutomu Nagaoka,§ Shigeru Sugiyama,† and Toshio Ohtani*,† National Food Research Institute, 2-1-12 Kannondai, Tsukuba, Ibaraki 305-8642, Japan, Department of Applied Chemistry, Faculty of Engineering, Yamaguchi UniVersity, 2-16-1 Tokiwadai, Ube 755-8611, Japan, and Research Institute of AdVanced Science & Technology, Osaka Prefecture UniVersity, 1-2 Gakuen, Sakai 599-8570, Japan Received August 7, 2003; Revised Manuscript Received August 29, 2003

ABSTRACT We developed a simple method for highly ordered assemblies of gold nanoparticles (AuNPs) along DNA molecules on substrates, and achieved assemblies with well-aligned and long-range order by using well-stretched DNA templates. In addition, oxidized aniline-capped AuNPs (AN− AuNPs) prepared in this study were strongly attached to DNA. Two different assembly methods were carried out, and consequently continuous depositions and necklace-like depositions of AN−AuNPs along DNA molecules were achieved.

Although the realization of nanoscale electronics is a subject of great priority for industrial technology, modern techniques based on optical lithography have grown increasingly complicated with attempts to achieve further miniaturization. Successful routes toward nanoscale electronics include the use of macromolecules and small molecules as building blocks for electronic devices. Nanoparticles (NPs), such as metal and semiconductor colloids with diameters of one to a few nanometers, are particularly attractive candidates.1-7 Metal and semiconductor NPs have already been used as the functional components of nanoscale electronic devices.4-7 Thus, the ability to organize, interconnect and address NPs on surfaces is an important hurdle to overcome in order to fully realize nanoscale electronics. Many natural materials that have specific binding properties in molecular recognition and self-assembly can be used as a template to organize and interconnect assemblies of NPs on surfaces.8-11 DNA in particular has been widely investigated as a template because of its structurally controlled nanowire with ∼2 nm, welldefined polymeric sequence and many functionalities, that can make possible linear assemblies of NPs or other nanomaterials.12-19 Although some methods for assembling NPs onto DNA have been reported, to date those with wellaligned and long-range order are almost nonexistent. * Corresponding author: phone:+81-29-838-8054, fax: +81-29-8387181, e-mail: [email protected]. † National Food Research Institute. ‡ Yamaguchi University. § Osaka Prefecture University. 10.1021/nl034620k CCC: $25.00 Published on Web 09/20/2003

© 2003 American Chemical Society

In this study, we demonstrated a simple method for highly ordered assemblies of gold NPs (AuNPs) along DNA molecules on substrates. To securely attach AuNPs to DNA molecules, preparations of various surface-functionalized AuNPs have been reported. In numerous other studies, such AuNPs were prepared by thiolate ligand (cationic thiols16,17,20 or intercalators21) exchange. However, we attempted a onestep preparation of surface-functionalized AuNPs without ligand exchange. Novel surface-functionalized AuNPs or “AN-AuNPs” were prepared based on the conventional reduction22 of HAuCl4 using aniline as a reducer, so that AN-AuNPs would have a positive charge and aromatic ring on the surface (due to the formation of oxidized aniline during preparation). The simple preparative procedure was as follows. A 0.1 M aniline (0.5 mL, Wako Pure Chemical Industry) reducer was added to a 0.03% chloroauric aqueous solution acid (25 mL, Wako Pure Chemical Industry) then stirred at 80 °C for 20 min. The resulting solution was stored in polypropylene containers. One milliliter of the stocked solution was centrifuged at 15 000 rpm for 30 min at 5 °C, then the supernatant was removed. The precipitate was redispersed in 1 mL of distilled water. The result was ANAuNPs (3.0 × 10-4 g/L). The UV spectrum of AN-AuNPs possessed a plasmon resonance peak at 554 nm (Figure 1a). The maximum of the peak position was shifted 34 nm toward the bathochromic side, compared with that (about 520 nm) of generally reported AuNPs coated by surfactants23 such as tetra-n-octylammmonium bromide or methyl-tri-n-octylammonium chloride. The bathochromic shift indicated that

Figure 1. (a) UV spectrum of AN-AuNPs. (b) Histogram showing the size distribution of AN-AuNPs measured by AFM observations.

Figure 2. Procedure of assemblies of gold nanoparticles onto DNA.

AuNPs were coated with a monolayer having a larger dielectric constant.23,24 In addition, characterization of prepared particles by electrophoresis analysis and zeta-potential measurements revealed the presence of positive charges on surfaces of those. Based on these results, we suggest that AN-AuNPs have a monolayer of oxidized aniline on surfaces. We then experimented with assemblies of AN-AuNPs organized on DNA molecules. As shown in Figure 2, two different procedures were used. In Method I, first, DNAstretching and fixation were carried out on the surface according to previously reported methods,25,26 which resulted in highly aligned DNA patterns formed on surfaces. Briefly, a 5 µL droplet of a λ-DNA solution (4.5 ng/µL, Wako Nippon Gene) was deposited on a polycarbonate-coated coverslip, then sucked up with a pipet. Air-water interface motion was induced by sucking, and DNA molecules were stretched and aligned along the central direction of the droplet. Next, DNA molecules were treated with 10 µL ANAuNPs solution (3.0 × 10-4 g/L) for 5 min, then rinsed in water. Before treatment with AN-AuNPs, the height (diameter) of DNA molecules imaged by atomic force microscopy (AFM) on the surface was ∼1.0 nm, which well agreed 1392

Figure 3. AFM images of highly ordered assemblies of ANAuNPs onto DNA molecules using method I. (a) Image in a large scan range. (b) Image in a smaller scan range. (c) Scan profile along the white line of the image b. The height scale is 5 nm in both images.

with that of a single double-stranded DNA in previous study.25 Figure 3 shows AFM images of DNA molecules after treatment of AN-AuNPs. AFM observation revealed that many DNA molecules on surfaces had contiguous particles with raised height (Figure 3b), indicating that observed heights of DNA molecules were 2.14 ( 0.35 nm (Figure 3c). The majority of the particles that can be distinguished from the background are 1.56 ( 0.21 nm (Figure 1b). Consequently, it is considered reasonable and proper that the increased DNA molecule heights after treatment were caused by particle deposition. Additionally, the above result indicates that AN-AuNPs have strong interaction with DNA molecules. The strong interaction in this system is considered to be due to the electrostatic interaction between the negatively charged phosphate backbone of DNA and AN-AuNPs with an oxidized aniline shell. Conceivably, other interactions such as hydrogen-bonding interactions also might happen between AN-AuNPs and DNA molecules.13 It is reasonable to expect that AN-AuNPs having amino groups interact with nitrogen or oxygen atoms of the bases that may be exposed in the major or minor grooves of the double helix. To see whether AN-AuNPs have specific interaction with DNA, more precise determination of the chemical nature and structure of the aniline coating on the AN-AuNPs surfaces is in progress. Figure 3b suggests that AN-AuNPs are uniformly deposited on almost all DNA molecules. Since the tip radius limits the resolution of the AFM instrument, the actual deposition might be less homogeneous. All of the linear structures observed by AFM also suggested that assemblies of AN-AuNPs on surfaces were highly oriented over long distances (Figure 3a). Although further optimizations of treatment time and concentration of AN-AuNPs are necessary to minimize nonspecific depositions on surfaces, our method makes it possible to easily achieve highly oriented assemblies of ANAuNPs on aligned DNA molecules as templates. Nano Lett., Vol. 3, No. 10, 2003

Figure 4. AFM images of highly ordered assemblies of ANAuNPs onto DNA molecules using method II. (a) Image in a large scan range. (b) Image in a smaller scan range. (c) Scan profile along particles line from I to II in the image b. Red marks is showing the particle height indicated by the white arrow in the image b. The height scale is 5 nm in both images.

As shown in Figure 2, we also demonstrated direct assemblies of AN-AuNPs on surfaces using Method II. First, we prepared a mixture of 1 µL of AN-AuNPs solution (3.0 × 10-4 g/L) and 5 µL λ-DNA solution (4.5 ng/µL) and incubated for 30 min. Next, samples were stretched and fixed on surfaces according to the above method. AFM images showed that DNA molecules were well stretched and aligned, even when combined with AN-AuNPs (Figure 4). It is interesting to note that AN-AuNPs with larger interparticle spacing are assembled along DNA molecules like a necklace. Since DNA molecules (to which AN-AuNPs were already attached) were stretched significantly by surface tension, larger interparticle spacing occurred. Interparticle spacing is obviously larger (Figure 4b) than in Method I (Figure 3b). Although the actual interparticle spacing is unknown because resolution is limited by the tip radius of AFM, it seems that interparticle spacing is nearly uniform along the DNA molecules. This indicates that DNA stretching induced by the surface tension is almost constant over the entire length of DNA. By controlling the surface tension (the speed of the air-water interface movement or surface hydrophobicity), it might be possible to control the interparticle spacing of NPs on DNA molecules. Though the bare DNA on interparticle is invisible after preparation by Method II, it might be due to the increase in background roughness caused by sample deposition. From a section profile along the aligned particles line (Figure 4c), many particle heights (2.97 ( 0.35 nm) assembled by using Method II exceeded those (about 2.0 nm) in the case of Method I. In addition, we could not observe the presence of particles exceeding about 3 nm in the background as shown in Figure 4b. From these results, the presence of the grain in the case of Method II cannot be regarded as a single particle. Thus, it seems that particles exceeding about 3.0 nm in height contain more than one AN-AuNPs. In Method I, since DNA molecules were Nano Lett., Vol. 3, No. 10, 2003

already stretched and fixed on surfaces, DNA attaching of AN-AuNPs is limited to almost one side. On the other hand, DNA attaching of AN-AuNPs in the solution phase (Method II) can occur from any direction. Consequently, we consider that the larger height measured in Method II is caused by binding of more than one particle surrounding DNA at close positions. In summary, we developed simple methods for highly ordered assemblies of AuNPs, which enabled us to easily organize AuNPs with well-aligned and long-range order on DNA molecules. AN-AuNPs prepared in this study strongly interacted with DNA molecules. Two approaches used in this study enabled different formations of linear arrays of AN-AuNPs. Specifically, linear arrays of AN-AuNPs with interparticle spacing could be organized onto DNA molecules like a necklace. By depositing different metals or NPs on interparticle spacing, it will be possible to tune electrical or optical properties of linear arrays. This method will also be helpful for investigating and constructing metal nanoparticle plasmon “waveguides”,27 indicating the energy transport along a nanoparticle chain with a short distance. Assembling NPs with various properties onto higher-ordered patterns of DNA molecules could help the further realization of nanoscale electronics. Acknowledgment. One of the authors, H.N., is thankful for Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists (JSPS). References (1) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (2) Shiigi, H.; Yamamoto, Y.; Yakabe, H.; Tokonami, S.; Nagaoka, T. Chem. Commun. 2003, 1038. (3) Torimoto, T.; Yamashita, M.; Kuwabata, S.; Sakata, T.; Mori, H.; Yoneyama, H. J. Phys. Chem. B 1999, 103, 7799. (4) Kim, S.-H.; Markovich, G.; Rezvani, S.; Choi, S. H.; Wang, K. L.; Heath, J. R. Appl. Phys. Lett. 1999, 74, 317. (5) Berven, C. A.; Clarke, L.; Mooster, J. L.; Wybourne, M. N.; Hutchison, J. E. AdV. Mater. 2001, 13, 109. (6) Sato, T.; Ahmed, H.; Brown, D.; Johnson, B. F. J. Appl. Phys. 1997, 82, 696. (7) Thelander, C.; Magnusson, M. H.; Deppert, K.; Samuelson, L.; Poulsen, P. R.; Nygård, J.; Borggreen J. Appl. Phys. Lett. 2001, 79, 2106. (8) Yamashita, I. Thin Solid Films 2001, 393, 12. (9) Macmillan, R. A.; Paavola, C. D.; Howard, J.; Chan, S. L.; Zaluzec, N. J.; Trent, J. D. Nature Mater. 2002, 1, 247. (10) Lee, S. W.; Mao, C.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892. (11) Scheibel, T.; Parthasarathy, R.; Sawicki, G.; Lin, X.-M.; Jaeger, H.; Lindquist, S. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4527. (12) Coffer, J. L.; Bigham, S. R.; Li, X.; Pinizzotto, R. F.; Rho, Y. G.; Pirtle, R. M.; Pirtle, I. L. Appl. Phys. Lett. 1996, 69, 3851. (13) Harnack, O.; Ford, W. E.; Yasuda, A.; Wessels, J. M. Nano Lett. 2002, 2, 919. (14) Keren, K.; Krueger, M.; Gilad, R.; Ben-Yoseph, G.; Silvan, U.; Braun, E. Science 2002, 72, 297. (15) Mertig, M.; Ciacchi, L. C.; Seidel, R.; Pompe, W.; Vita, A. D. Nano Lett. 2002, 2, 841. (16) Warnerd, M. G.; Hutchison, J. E. Nature Mater. 2003, 2, 272. (17) Yonezawa, T.; Onoue, S.; Kimizuka, S. Chem. Lett. 2002, 1172. (18) Monson, C. F.; Woolley, A. T. Nano Lett. 2003, 3, 359. (19) Xin, H.; Wooly, A. T. J. Am. Chem. Soc. 2003, 125, 8710. 1393

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Nano Lett., Vol. 3, No. 10, 2003