DNA-Templated Assembly of Au Nanoparticles via Step-by-Step Binding Reaction

Takuya Matsumoto, Hidekazu Tanaka, and Tomoji Kawai∗ The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan, and Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Kawaguchi Center Building , Hon-cho 4-1-8, Kawaguchi,Saitama 332-0012, Japan (Received 14 October 2004; Accepted 15 October 2004; Published 20 October 2004)

Molecular electronics, first suggested by Amirav and Ratner, has attracted much attention in the last quartercentury. This idea includes the concept that individual organic molecules can satisfy the requirements of electronic devices with their stable nanoscale structure and have energy levels that can be tailored by chemical synthesis. Notwithstanding the great anticipation of the potential of molecular-scale electronics, it is very difficult to fabricate molecular-devices, owing to the lack of effective technologies for assembling and wiring molecules.
Recently, we have found that gold nanoparticles (AuNP) can be combined with a DNA network [14] on the substrate by a convenient one-pot procedure [15]. Based on this finding, we propose a new strategy to assemble AuNPs by using a step-by-step Au-thiolate reaction. This method realizes formation of a continuous AuNP array with uniform height and small width. Figure 1 shows a schematic illustration of DNAtemplated, step-by-step growth of AuNP nanowires. Initially, an AuNP-combined DNA network is prepared on the substrate. Second, alkanedithiol molecules are attached to the network of AuNP combined with DNA. Third, AuNPs are attached to the opposite end of alkanedithiol molecules. After repeating the cycle of steps two and three above, the AuNPs, which were initially separated, are bridged along the DNA by the array of AuNPs and alkanedithiol. As a result, quasi-one-dimensional nanowires of AuNPs are formed along the DNA strands.
In this study, we chose butanedithiol for linker * Corresponding author: kawai@sanken.osaka-u.ac.jp molecule. This is because that the length of butanedithiol is approximately 0.5 nm, which is short enough to prevent formation of undesired structures like hairpin loops and to permit electron tunneling for hopping conduction. Furthermore, butanedithiol satisfy the well-known condition that the formation of self-assembled monolayer with standing molecules on Au surface requires molecular length to be longer than that of alkane chain with three or four carbons. All chemicals used to form AuNP nanowire were commercial products of reagent grade. The DNA used for network formation was a synthetic double-stranded complex of the polydeoxyriboadenylic acid and polydeoxyri-  bothymidylic acid (Poly(dA)-Poly(dT)) (Amersham Bioscience). The DNA was dissolved in distilled water to produce a concentration of 25 U (1 U = 50 ng/l). The AuNPs (BBInternational, EM.GC5) used had an average diameter of 5 nm and were supplied as an aqueous solution at a concentration of 5 × 10 13 particles/ml. The solution was centrifuged at 15,300 G at 4 • C for 1 hour. After centrifugation, we removed the clear supernatant and separated the useful portion, which contained an AuNP concentration of 3 × 10 14 particles/ml. In the case of cyclic reactions for growing the AuNP nanowire, ethanol was added to avoid the denaturing of AuNP-combined DNA networks. Butanedithiol (Sigma-Aldrich) used for linking molecules was diluted in ethanol to a concentration of 1 mM. Atomic force microscopy (AFM) observation was conducted by a scanning probe microscope (JEOL, JSPM4200) using tapping-mode operation in air at room temperature.
The AuNP-combined DNA network was formed by a simple procedure in which the mixed solution of AuNP and DNA, wherein the DNA concentration is adjusted to 10 U, is dropped on the substrate and the droplet was blown off a minute after the dropping it. Figure 2 ple was soaked in ethanol, which is the solvent of the Au-thiolate reaction, for 15 minutes, the DNA network is completely destroyed and no AuNPs remain on the surface as shown in Figure 2(b). To form an AuNP nanowire based on the AuNP-DNA network, it is necessary that the network does not degrade by the cyclical process represented in Fig. 1. To improve the adherability of the mica substrate surface to the DNA molecules, we examined the extent of the mica surfaces hydrophilicity and attempted to control it. The inset of Fig. 2(b) shows the contact angle for freshly cleaved mica, indicating its hydrophilic property. The mica surface can be changed to become hydrophobic by rinsing it with distilled water as shown in the inset of Fig. 2(c). After this treatment, AuNPcombined DNA network are not destroyed by ethanol ex-posure. The mechanism of this change is presumed to be a stoichiometry variation of surface cation of mica surface and the effect is often utilized for the fixation of biological materials in AFM imaging.
After the two cycle of the bonding reaction, the wirelike structures along DNA strands can be clearly observed, as shown in Fig. 3(a). The AuNP nanowire has an unbroken length of more than 1 mm. Figure 3(b) shows a magnified image of AuNPs nanowires in which individual AuNPs are resolved. The sectional profiles (Fig. 3(c)) show that the AuNP wire has a uniform height of 7 nm. Since the height is greater than the 5 nm diameter of the used AuNPs, we can conclude that the AuNPs are adsorbed onto the DNA molecules. On the other hand, height difference between the first and second AuNP layer is smaller than the diameter of the AuNPs. This is because the second layer of AuNPs are adsorbed on not on-top of, but hollow sites of first layer of AuNPs.
This height analysis is true of the whole surface, as shown in the histogram of height distribution (Fig. 3(d)). The histogram indicates four peaks: the first to fourth peaks correspond to the height of substrate surface, DNA network, first AuNP layer, and second AuNP layer, respectively. The peak intensity of the second layer is smaller than 2% of that of the first, which indicates that the binding reaction between the AuNPs prevent threedimensional growth, instead forcing growth to occur only along DNA strands. Actually, Fig. 3(e) shows that the extremities of AuNP wires are necessarily located on DNA strands and that AuNP wire has many kinks, reflecting the structure of the DNA network. Figure 4 shows a local area image and height profiles of AuNP-bounded DNA network with the resolution of individual AuNPs. The corrugation of AuNP array indicates that distances between AuNPs are in the range of 5∼8 nm, which are close to the sum of AuNP diameter (5 nm) and butanedithiol linker length (0.5 nm). This result suggests that a part of AuNPs are connected directly by butanedithiol molecules as expected in Figure 1.
Previous works have shown that it is difficult to produce fine, continuous AuNP wires of uniform height. For example, the electrolysis method provides continuous AuNP wires, but their shape is rugged and not well controlled. In contrast, the assembly of AuNP wires using the DNAtemplated bonding reaction can satisfy these factors concurrently. Moreover, we could not find any gaps between the AuNPs in the AFM images. Since the AuNPs are connected by buthanedithiol molecules, the gap width is expected to be less than 0.5 nm, so electrons might penetrate any gap by tunneling.
The binding reaction using the Au-thiolate reaction can be extended to various organic molecules possessing electronic, magnetic and photonic functions.
Step-by-step assembly using DNA templates shows promise for becoming a basis for realizing nanoscale molecular devices.
This work is supported by Grant-in-Aid for Center-of-Excellence (COE) Research from the Ministry of Education, Science, Sports and Culture of Japan, and Core Research for Evolutional Science and Technology (CREST) from Japan Science and Technology Corporation (JST).