Conjugated Polymer-Functionalized DNA Nanowires - American

Nagoya Municipal Industrial Research Institute, 3-4-41 Rokuban, Atsuta-ku, Nagoya, Aichi. 456-0058, Japan, and Faculty of Engineering, Shizuoka Univer...
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Langmuir 2005, 21, 7945-7950

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Fabricating and Aligning π-Conjugated Polymer-Functionalized DNA Nanowires: Atomic Force Microscopic and Scanning Near-Field Optical Microscopic Studies Hidenobu Nakao,*,† Hideki Hayashi,‡ Futoshi Iwata,§ Hidenori Karasawa,§ Koji Hirano,‡ Shigeru Sugiyama,† and Toshio Ohtani*,† National Food Research Institute, Kannondai 2-1-12, Tsukuba, Ibaraki 305-8642, Japan, Nagoya Municipal Industrial Research Institute, 3-4-41 Rokuban, Atsuta-ku, Nagoya, Aichi 456-0058, Japan, and Faculty of Engineering, Shizuoka University, 3-5-1 Johoku, Hamamatsu, Shizuoka 432-8561, Japan Received January 18, 2005. In Final Form: May 16, 2005 We report a simple method to functionalize DNA with π-conjugated polymer, forming highly aligned and integrated arrays of π-conjugated polymer nanowires of a few nanometers diameter. π-conjugated polymer, polyphenazasiline, having alkylammonium salts on the N atom (PPhenaz-TMA), synthesized in this study can be directly attached to DNA, which can be organized along stretched and aligned DNA molecules on surfaces as a template. Furthermore, PPhenaz-TMA/DNA nanowires were stretched and aligned on surfaces, even when PPhenaz-TMA/DNA complexes formed in solutions. The resulting PPhenazTMA/DNA nanowires could be easily converted to oxidized states or metallic nanowires by using adequate oxidant or metal salts. The direct visualization of PPhenaz-TMA/DNA nanowires and its structural changes have been studied by atomic force microscopy and scanning near-field optical microscopy.

Introduction π-conjugated polymers, such as polyaniline, polypyrrole, and polythiophene, have been widely studied and applied in various fields because of their unique electronic, optical, and catalytic properties. Like conventional silicon semiconductors, these polymers can change from an insulator to the metallic or semiconductor state and be reversibly modulated over many orders of magnitude by electrochemical or chemical doping.1-3 However, such polymers exceed conventional silicon semiconductors in their flexibility, ease of processing, and modifiable properties. Thus, controlled fabrication and patterning of such polymers on a nanometer scale could provide new platform technologies, such as nanodevices and nanosensors. By using photolithography,4 microcontact printing,5 membranetemplate synthesis6,7 and electrochemical dip-pen lithography,8 various microstructures of such polymers have been prepared. However, these techniques have limitations in terms of resolution of sub-100 nm dimensions, positioning, throughput speed, and cost. Developing a feasible technique for reproducibly aligning and integrating π-conjugated polymeric nanostructures is an important achievement in nanodevice and nanosensor development. * Corresponding authors. Telephone: +81-29-838-8054. Fax: +81-29-838-7181. E-mail: (H.N.) [email protected]; (T.O.) [email protected]. † National Food Research Institute. ‡ Nagoya Municipal Industrial Research Institute. § Shizuoka University. (1) Shirakawa, H. Synth. Met. 2002, 125, 3. (2) MacDiarmid, A. G. Synth. Met. 2002, 125, 11. (3) Heeger, A. J. Synth. Met. 2002, 125, 23. (4) Jager, E. W. H.; Smela, E.; Inganas, O. Science 2000, 290, 1540. (5) Yu, J. F.; Holdcroft, S. Chem. Commun. 2001, 1274. (6) Martin, C. R. Chem. Mater. 1996, 8, 1739. (7) Park, S.; Chung, S.-W.; Mirkin, C. A. J. Am. Chem. Soc. 2004, 126, 11772. (8) Maynor, B. W.; Filocamo, S. F.; Grinstaff, M. W.; Liu, J. J. Am. Chem. Soc. 2002, 124, 522.

However, to further miniaturize structures, templatedirected material assemblies allow directly physical patterning when individual biomolecules with well-defined architectures are used as the templates.9-11 Since DNA is a one-dimensional nanostructure having unique recognition, association, and binding properties, assemblies of nanoparticles and other nanomaterials onto highly aligned and patterned DNA allows the construction of circuit elements with various properties in nanoscale electronics.12-20 Thus, it is also possible to bind π-conjugated polymers along aligned and patterned DNA, which fabricates arrays of π-conjugated polymer nanowires.21,22 Polyaniline nanowires with 1 nm diameter have been fabricated on thermally oxidized Si surfaces by use of stretched and immobilized DNA strands as a template.22 In the formation of polyaniline nanowires, the aniline monomers along the DNA chains were polymerized (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) Braun, E.; Eichen, Y.; Silvan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (13) Yan, H.; Park, S. H.; Finkelstein, G.; Reif J. H.; LaBean, T. H. Science 2003, 301, 1882. (14) Keren, K.; Berman, R. S.; Buchstab, E.; Silvan, U.; Braun, E. Science 2003, 302, 1380. (15) Alivisatos, A. P.; Johnson, K. P.; Peng, X.; Wilson, T. E.; Loweth, C. J.; Bruchez, M. P.; Schultz, P. G. Nature 1996, 382, 609. (16) Harnack, O.; Ford, W. E.; Yasuda, A.; Wessels, J. M. Nano Lett. 2002, 2, 919. (17) Xin, H.; Woolley, A. T. J. Am. Chem. Soc. 2003, 125, 8710. (18) Mertig, M.; Ciacchi, L. C.; Seidel, R.; Pompe, W.; Vita, A. D. Nano Lett. 2002, 2, 841. (19) Kopaczynska, M.; Lauer, M.; Schulz, A.; Wang, T.; Schaefer, A.; Fuhrhop, J.-H. Langmuir 2004, 20, 9270. (20) Ganguli, M.; Babu, J. V.; Maiti, S. Langmuir 2004, 20, 5165. (21) Nagarajan, R.; Liu, W.; Kumar, J.; Tripathy, S. K.; Bruno, F. F.; Samuelson, L. A. Macromolecules 2001, 34, 3921. (22) Ma, Y.; Zhang, J.; Zhang, G.; He, H. J. Am. Chem. Soc. 2004, 126, 7097.

10.1021/la050145p CCC: $30.25 © 2005 American Chemical Society Published on Web 07/14/2005

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Figure 1. Chemical structure of PPhenaz-TMA.

enzymatically by adding horseradish peroxidase and H2O2, and its conductivity was sensitive to acid-base doping and undoping processes. In this approach, fabricated polyaniline nanowires are in the oxidation state having relatively high conductivity. However, mostly π-conjugated polymer must be in the neutral state for use as field effect transistors and various diodes. In this paper, we describe a simple method for fabricating highly aligned and integrated π-conjugated polymerfunctionalized DNA nanowires on surfaces. First, to directly attach π-conjugated polymer to DNA molecules, we attempted to synthesize a novel π-conjugated polymer. We recently reported a simple technique for DNAstretching and fixation using phenazasiline-containing π-conjugated polymer (PPhenaz)-coated substrates.23 In this method, many DNA molecules were sufficiently stretched and fixed on PPhenaz-coated surfaces by the π-π interaction between π-conjugated units in polymer and base pairs in DNA. However, PPhenaz and its derivatives displayed high efficiency as a hole transporting polymeric material24 and exhibited conductivity of 4-17 S/cm by electrochemical p-doping.25 Thus, PPhenaz having alkylammonium salts on the N atom (PPhenaz-TMA) were synthesized to enhance the solubility of PPhenaz in polar solvent such as water and interactions with DNA having negative charges, directly forming PPhenaz-TMA/DNA nanowires having excellent optical and electrical properties. By using atomic force microscopy (AFM) and scanning near-field microscopy (SNOM), we discuss structural changes of PPhenaz-TMA/DNA nanowires before and after treatments of adequate oxidant or metal salts. Experimental Section Materials. All chemicals used were of reagent grade. PPhenazTMA was prepared from quaternization of poly[10,10-dimethyl5-(3-dimethylamino)phenazasiline-2,8-diyl]26 by iodomethane. The chemical structure of PPhenaz-TMA is shown in Figure 1. Polycarbonate (Scientific Polymer Products, Inc.) were selected as a coating polymer in this study. λ-phage DNA was purchased from Wako Nippon Gene and used as obtained without purification. Fabrication of Aligned PPhenaz-TMA/DNA Nanowires on Surfaces. A 50 µL solution of polycarbonate (2 mg/mL in 1,2-dichloroethane) was spin-coated on a clean coverslip; a polycarbonate-coated glass substrate was used to provide a good formation of highly aligned DNA patterns on the substrate surface. Fabricating PPhenaz-TMA/DNA nanowires was accomplished by using two methods. In method I, a 5 µL droplet of λ DNA solution (4.5 ng/µL) solution in TE buffer (10 mM TrisHCl and 1 mM EDTA, pH 8.0) was deposited on the polycarbonate-

(23) Nakao, H.; Hayashi, H.; Yoshino, T.; Sugiyama, S.; Otobe, K.; Ohtani, T. Nano Lett. 2002, 2, 475. (24) Hayashi, H.; Nakao, H.; Adachi, A.; Kimura, H.; Okita, K.; Hayashi, T.; Tanaka, M. Chem. Lett. 2000, 688. (25) Hayashi, H.; Nakao, H.; Onozawa, S.-y.; Adachi, A.; Hayashi, T.; Okita. K. Polym. J. 2003, 35, 704. (26) Nakao, H.; Hayashi, H.; Okita, K. Polym. J. 2001, 33, 498.

Figure 2. AFM images of aligned bare DNA molecules: (a) image in a large scan range; (b) image in a smaller scan range; (c) scan profile along the white line of image b. The height scale is 5 nm in both images. coated surface and then sucked up with a pipet.23,27 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 20 µL of PPhenaz-TMA solution (0.05 mg/mL) for 5 min and then rinsed in water. In method II, we prepared a mixture of 1 µL of PPhenaz-TMA solution (1 mg/mL) and 5 µL of DNA solution (4.5 ng/µL in TE buffer, pH 8.0) and incubated it for 5 min at room temperature. Next, samples were stretched and fixed on the polycarbonatecoated surface according to the above method. AFM Measurements. AFM measurements were performed using an NVB 100 (Olympus Optical Co., Ltd.). The set-point voltage was adjusted to the lowest value so as not to damage any samples. We used a tapping mode and a standard silicon nitride probe with a 42 N/m spring constant (Model OMCL-AC160TS, Olympus Optical). The scanning rate was usually 0.4 Hz. SNOM Measurements. We observed near-field plasmon coupling on metal PPhenaz-TMA/DNA nanowires by using scattering-type apertureless SNOM (s-SNOM). The s-SNOM in this study employed light scattering (λ ) 532 nm) at the silvercoated sharp tip of a probing needle, such that the probing light field confined at the tip apex extends only about one tip radius (≈20 nm) , λ in any direction. Other detailed descriptions of the optical and AFM parts of our s-SNOM apparatus have been published elsewhere.29,30

Results and Discussion We attached PPhenaz-TMA to aligned DNA molecules on surfaces using method I. Figures 2 and 3 depict AFM images of DNA molecules before and after treatment with PPhenaz-TMA. The observed height (diameter) of λ DNA was 0.84 ( 0.13 nm before treatment with PPhenaz-TMA (Figure 2c) but increased to 1.31 ( 0.18 nm after treatment (Figure 3c). We believe that the height increase after treatment is caused by polymer deposition. Furthermore, the observed image shows that polymers are uniformly deposited on almost all DNA molecules (Figure 3b). Consequently, PPhenaz-TMA-functionalized DNA (PPhenaz-TMA/DNA) nanowires were created and highly aligned over a long distance on the surface (Figure 3a). The above result indicates that PPhenaz-TMA has a strong interaction with DNA molecules. This was confirmed by UV-vis and fluorescence measurements of (27) Nakao, H.; Gad, M.; Sugiyama, S.; Otobe, K.; Ohtani, T. J. Am. Chem. Soc. 2003, 125, 7162. (28) Nakao, H.; Shiigi, H.; Yamamoto, Y.; Tokonami, S.; Nagaoka, T.; Sugiyama, S.; Ohtani, T. Nano Lett. 2003, 3, 1391. (29) Iwata, F.; Mikage, K.; Sakaguchi, H.; Kitao, M.; Sasaki, A. Solid State Ionics 2003, 165, 7. (30) Iwata, F.; Mikage, K.; Sakaguchi, H.; Kitao, M.; Sasaki, A. J. Microsc. (Oxford) 2003, 210, 241.

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Figure 3. AFM images of PPhenaz-TMA attached to aligned DNA molecules: (a) image in a large scan range; (b) image in a smaller scan range; (c) scan profile along the white line of image b. The height scale is 5 nm in both images.

Figure 4. Interaction between PPhenaz-TMA and DNA: (a) absorption spectral changes of PPhenaz TMA (3.3 µg/mL) after addition of DNA (0.45 µg/ mL) in 3 mL of TE buffer (10 mM Tris-HCl and 1 mM EDTA, pH 8.0); (b) photoluminescence spectral changes of PPhenaz after addition of DNA in 3 mL of TE buffer.

adducts of DNA with PPhnenaz-TMA in aqueous buffer solution (Figure 4a,b). In Figure 4a, the UV-vis absorption spectrum of PPhenaz-TMA in aqueous buffer solution exhibits the π-π* absorption characteristic of the aromatic amine at 340 nm. As more DNA was added, the absorption peak at 340 nm decreased. This phenomenon known as hypochromism is generally observed for interactions

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Figure 5. AFM images of stretched and aligned DNA molecules after DNA-attaching of PPhenaz-TMA in solution: (a) image in a large scan range; (b) image in a smaller scan range; (c) scan profile along the white line indicated by I in image b; (d) scan profile along the white line indicated by II in image b. The height scale is 5 nm in both images.

between DNA and intercalator dye, indicating the intercalation of aromatic amine units of PPhenaz-TMA into the DNA base stack.31-34 From the molecular viewpoint, two benzene units of PPhemaz-TMA are fixed by the Si atom, and then aromatic amine units of them are almost planar.25 This fact favors the insertion of PPhenaz-TMA units into the hydrophobic interior of the DNA base stack. Furthermore, in the presence of DNA, strong quenching of the emission spectrum of PPhenaz-TMA was also observed, which was attributed to an electron transfer between DNA bases and the excited state of PPhenazTMA (Figure 4b).31 From the above results, one can state that PPhenaz-TMA strongly interacts with DNA through π-π interaction between aromatic amine units in polymers and base pairs in DNA. We also directly aligned PPhenaz-TMA/DNA nanowires on the surface using method II. AFM images showed that DNA molecules could be stretched and aligned, even when first combined with PPhenaz-TMA (Figure 5a,b). It is significant that directly created PPhenaz-TMA/DNA nanowires have gaps along DNA chains, as shown in Figure 5a,b. Our previous study indicated that the alignment after DNA-attaching of gold nanoparticles (AuNPs) in solution causes assemblies of AuNPs with large gaps along DNA chains,28 because DNA molecules (to which AuNPs were already attached) were stretched significantly by surface tension. In this case, PPhenazTNA/DNA wires were also strongly stretched on surfaces, resulting in partial attaching of PPhenaz-TMA along DNA chains. From a section profile (Figure 5c), observed grains along DNA chains are different in size. For example, the grain height indicated by I in the figure is 4.75 nm. However, the smaller grain height indicated by II is 1.44 nm. The reason for this is not clear; however, it might be due to the difference in the interaction of PPhenaz-TMA among DNA base sequences or structural changes of DNA induced by DNA-binding of PPhenaz-TMA. (31) Barker, K. D.; Benoit, B. R.; Bordelon, J. A.; Davis, R. J.; Delmas, A. A.; Mytykh, O. V.; Petty, J. T.; Wheeler, J. F.; Kane-Maguire, N. A. P. Inorg. Chim. Acta 2001, 322, 74. (32) Song, Y.-F.; Yang, P. Polyhedron 2001, 20, 501. (33) Yamamoto, T.; Shimizu, T.; Kurokawa, E. React. Funct. Polym. 2000, 43, 79. (34) Yang, D.; Strode, T.; Spielmann, H. P.; Wang, A. H. J.; Burke, T. G. J. Am. Chem. Soc. 1998, 120, 2979.

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Figure 6. Diagram showing the oxidation potential of PPhneazTMA, and the reduction potentials of Ag+, Au3+, and S2O8- vs SHE.

Figure 8. (a) AFM image of PPhneaz-TMA/DNA nanowires after immersion in a S2O8- solution for 5 min; (b) section profile along the white line of the AFM image. The height scale is 5 nm.

Figure 7. Absorption spectral changes of PPhenaz-TMA (10 µg/mL) after addition of 10 mM ammonium peroxodisulfate ((NH4)2S2O8) in 3 mL of TE buffer.

As described above, since PPhenaz derivatives exhibit a conductivity of 4-17 S/cm by electrochemical p-doping, fundamentally they have potentialities as the p-type semiconductors. PPhenaz-TMA also seems to be able to change its electroproperty by p-doping (oxidation of PPhenaz-TMA). To confirm this, we tried the oxidation of PPhenaz-TMA with ammonium peroxodisulfate ((NH4)2S2O8) as the oxidant. From electrochemical measurements of PPhenaz-TMA, the onset of oxidation was observed at +0.96 V (vs SHE). Conversely, the reduction potential of S2O82- is +2.01 V (vs SHE).35 Thus, the relative potential levels rationalize the spontaneous electron transfer from PPhenaz (oxidation) to S2O82- (reduction), as exhibited in Figure 6. Figure 7 depicts the absorption spectrum changes of PPhenaz-TMA with the addition of the oxidant. The absorption maxima of neutral PPhenaz-TMA around 340 nm decreased with an increase in the addition of the oxidant, and the new absorption bands around 475 nm and >1000 nm increased. These absorption bands are attributed to the absorption characteristics of oxidized radical cations of PPhenaz-TMA, which are widely delocalized in polymer chains.36 In such electric states, called polarons, many π-conjugated polymers possess relatively high electric conductivity.2 Thus, it seems that oxidized PPhenaz-TMA has higher electric conductivity than its neutral state. We then demonstrated the oxidation of PPhenaz-TMA/ DNA nanowires aligned on surfaces. After DNA-stretching and fixation on surfaces, aligned PPhenaz-TMA/DNA nanowires were formed by DNA-attaching of PPhenaz(35) Bard, A. J.; Parsons, R.; Jordan, J. Standard Potentials in Aqueous Solution; Dekker: New York, 1985. (36) Hayashi, H.; Nakao, H.; Imase, T. Polym. J. 2002, 34, 400.

Figure 9. (a) AFM image of PPhenaz-TMA/DNA nanowires after immersion in an Au3+ solution for 30 min; (b) section profile along the white line of the AFM image. The height scale is 5 nm.

TMA. Aligned PPhenaz-TMA/DNA nanowires were immersed in 20 µL of (NH4)2S2O8 solution (0.5 mM) for 5 min and then rinsed in water. Figure 8 presents AFM images of DNA/PPhenaz-TMA nanowires after treatment. After treatment, PPhenaz-TMA/DNA nanowires appeared significantly changed with increased height: observed heights were 1.98 ( 0.30 nm (Figure 8b). In this system, PPhenaz-TMA is oxidized by S2O82-, while S2O82- is reduced to SO42-; consequently, SO42- is incorporated into oxidized PPhenaz-TMA to compensate charges. In all cases, we observed a 0.6-0.8 nm increase of heights of resulting structures that roughly corresponded to the size of the SO42- anion or morphology changes of attached PPhenaz-TMA caused by oxidations. Thus, this result strongly suggests that PPhenaz-TMA/DNA nanowires are

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Figure 10. AFM and SNOM images of PPhenaz-TMA/DNA nanowires before and after immersion in an Au3+ solution: (a) AFM topographic image and (b) SNOM image of PPhenaz-TMA/DNA nanowires before treatment; (c) AFM topographic image and (d) SNOM image of PPhenaz-TMA/DNA nanowires after treatment.

oxidized with (NH4)2S2O8. We have already reported that the conductivity of PPhenaz increased with the growth of polaron bands by using spectroelectrochemical measurements.25 Oxidized PPhenaz-TMA/DNA nanowires might also have relatively high electric conductivity; however, we have not made direct electrical measurements for them. Therefore it is not possible to say with certainty that oxidation leads to conductivity changes in PPhenaz-TMA/ DNA nanowires. By using a metal salt solution capable of reducing PPhenaz-TMA, it is possible to deposit metals on PPhenazTMA/DNA nanowires. Choi et al. have reported spontaneous metal nanoparticle formation on single-walled carbon nanotube (SWCNT) sidewalls when nanotubes were immersed in some metal salt solutions.37 In this case, spontaneous metal deposition on SWCNTs reacts with metal salt solution such as AuCl4- and PtCl4- that have reduction potentials below the work function of SWCNTs. This process differs from typical electroless deposition that requires reducing agents or catalysts. In our case, it is expected that spontaneous electron transfer from PPhenaz-TMA to AuCl4- is possible, as illustrated in Figure 6. We then observed electroless deposition of Au on PPhenaz-TMA/DNA nanowires (Figure 9). After DNAstretching and fixation on surfaces, aligned PPhenaz-TMA/ DNA nanowires were formed by DNA-attaching of PPhenaz-TMA. Aligned PPhenaz-TMA/DNA nanowires were treated with 20 µL of HAuCl4 solution (0.5 mM) for 30 min in the dark and then rinsed in water. After treatment of PPhenaz-TMA/DNA nanowires in HAuCl4

solutions, we observed that heights of PPhenaz-TMA/DNA nanowires increased: observed heights were 2.14 ( 0.31 nm (Figure 9b). It is considered that the increased heights of PPhenaz-TMA/DNA nanowires after treatment were caused by Au metal deposition. Notice that Au deposition is on PPhenaz-TMA/DNA nanowires and not the background surface. However, treatment of PPhenaz-TMA/ DNA nanowires in Ag+ solutions having the higher reduction potential produces nonspecific particle deposition over the surface. Thus, metal depositions along PPhenaz-TMA/DNA nanowires are attributed to direct redox reaction between PPhenaz-TMA and metal ions. Metal depositions along PPhenaz-TMA/DNA nanowires can also be clarified by using SNOM. The strong interaction of nanometal particles with visible light originates from the excitation of collective oscillation of conduction electrons within these particles, called “surface plasmons”. The surface plasmons can be detected as resonance peaks in the light scattering spectra of these nanoparticles.38 Before treatment, the AFM topographic image of PPhenazTMA/DNA nanowires was observed, but the SNOM image of those was not observed (Figure 10a,b). However, the SNOM image of PPhenaz-TMA/DNA nanowires after treatment was clearly observed, synchronizing exactly with its AFM topographic image (Figure 10c,d). The observed SNOM image after treatment originates from near-field plasmon coupling between the probing tip and the nanometal particles on PPhenaz-TMA/DNA nanowires, which leads to enhanced light scattering. Since Au nanoparticles have a surface plasmon resonance peak at

(37) Choi, H. C.; Shim, M.; Bangsaruntip, S.; Dai, H. J. Am. Chem. Soc. 2002, 104, 9058.

(38) Taubner, T.; Keilmann, F.; Hillenbrand, R. Nano Lett. 2004, 4, 1669.

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about 520 nm, it is reasonable to consider that the strong light scattering arises from the interaction between Au nanometal and tip illuminated by laser at 532 nm. Thus, SNOM observation strongly revealed that PPhenaz-TMA/ DNA nanowires were converted to metallic nanowires in this redox system. In conclusion, π-conjugated polymer-functionalized DNA (PPhenaz-TMA/DNA) nanowires were fabricated and aligned on surfaces. PPhenaz-TMA strongly interacted with DNA molecules, which could organize PPhenazTMA along stretched and aligned DNA molecules on surfaces. Furthermore, PPhenaz-TMA/DNA nanowires were stretched and aligned on surfaces, even when PPhenaz-TMA/DNA complexes formed in solutions. AFM and SNOM observations revealed that PPhenaz-TMA/ DNA nanowires could be easily converted to oxidized states

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or metallic nanowires by using adequate oxidant or metal salts. This process occurs as a result of direct redox reaction between PPhenaz-TMA and redox species, indicating that its reaction is selective to PPhenaz-TMA attached to DNA. By choosing some redox systems, it is possible to tune electrical properties of PPhenaz-TMA/DNA nanowires from insulators to semiconductors or conductors for use as circuit elements. Thus, PPhenaz-TMA/DNA nanowires are attractive candidates in nanometer-scale science and an important step toward the construction of functional nanodevices using DNA and other biomolecules. 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). LA050145P