Microwave Synthesis of Electrically Conductive Gold Nanowires on

Synthesis of Electrically Conductive Gold Nanowires on DNA Scaffolds ... The DNA is a polydispersed mixture of DNA chains of broad size distributi...
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Langmuir 2008, 24, 9668-9674

Microwave Synthesis of Electrically Conductive Gold Nanowires on DNA Scaffolds Subrata Kundu* and Hong Liang Materials Science and Mechanical Engineering, Texas A&M UniVersity, College Station, Texas 77843-3123 ReceiVed January 14, 2008. ReVised Manuscript ReceiVed June 13, 2008 Biological molecules, in particular DNA, have shown great potential to be used as interconnects of nanodevices and computational elements. In this research, we synthesized electrically conductive gold nanowires for the first time exploiting an electroless and microwave heating method for 120-180 s. Our results indicate that DNA serves as a reducing and nonspecific capping agent for the growth of nanowires. The current voltage (I-V) characteristics of the Au nanowires are continuous, exhibiting Ohmic behavior having low contact resistance with the gold electrodes. The nanowires have a diameter of 10-15 nm in solution and of 20-30 nm in immobilized DNA with resistivity comparable to pure metals. The method is highly selective with deposition confined to the DNA itself. The nanowires we fabricated can be used as building blocks for functional nanodevices, sensors, and optoelectronics.

Introduction Biomimetic “bottom up” strategies are currently being explored for making one-dimensional nanostructures for application in functional nanodevices using molecular and nanoparticulate building blocks.1,2 Nanowires are applied for connecting and fabrication of microscopic electrodes or quantum devices.3 Biological molecules, such as nucleic acids, peptides, and amino acids have shown great potentials in fabrication and construction of nanostructures and nanodevices.4,5 DNA molecules have been reported as the basic building blocks for assembly of devices and computational elements for interconnections.6 There are numerous advantages to apply DNA molecules in such applications. First, the intermolecular interactions in DNA are most readily programmed and reliably predicted. Among the four base molecules in DNA, adenine (A) pairs with thymine (T) and guanine (G) with cytosine (C). This structure makes DNA an effective genetic material for programmed self-assembly. Second, the versatile chemical structure allows DNA to self-assemble into complex structures7 like squares,8 cubes,9 T-junctions,10 etc., to make complex circuit elements. Third, the negative charge on DNA due to phosphate ions results in electrostatic repulsion that enables the DNA to easily bind with metal cations11,12 and metal nanoparticles13,14 easily resulting its functionalized structure with thiol (S-H) or disulfide (S-S) groups. Finally, the double * Corresponding author: e-mail, [email protected]; phone, 979-862-2578; fax, 979-845-3081. (1) Kolvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (2) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (3) Gudiksen, M. S. ; Lauhon, L. J. ; Wang, J. ; Smith, D. C. ; Lieber, C. M. Nature 2002, 415, 617. (4) Lee, S. W.; Mao, C. B.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892. (5) Mcmillan, R. A.; Paavola, C. D.; Howard, J.; Zaluzec, N. J.; Trent, J. D. Nat. Mater. 2002, 1, 247. (6) Braun, E.; Keren, K. AdV. Phys. 2004, 53, 441. (7) Le, J. D.; Pinto, Y.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. Nano Lett. 2004, 4, 2343. (8) Seeman, N. C. Curr. Opin. Struct. Biol. 1996, 6, 519. (9) Zhang, J.; Liu, Y.; Ke, Y.; Yan, H. Nano Lett. 2006, 6, 248. (10) SanMartin, M. C.; Gruss, C.; Carazo, J. M. J. Mol. Biol. 1997, 268, 15. (11) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (12) Richter, J.; Seidel, R.; Kirsch, R.; Mertig, M.; Pompe, W.; Plaschke, J.; Schackert, H. K. AdV. Mater. 2000, 12, 507. (13) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature 1996, 382, 607. (14) Warner, M. G.; Hutchison, J. E. Nat. Mater. 2003, 2, 272.

helix chain structure provides DNA with high mechanical strength.15 Mirkin and co-workers13 described a method of assembling colloidal gold nanoparticles into macroscopic aggregates using DNA as the linking element. Nanowires of noble metals like gold,13 silver,11 palladium,12 platinum,16 and copper17 have been metallized on DNA. Synthesis methods, however, required long processing times and high temperatures with multiple steps. Recently, the microwave (MW) as a heat source has been used to synthesize metallic nanostructures, such as Ag,18 Au/Pd,19 and semiconductor rods and wires20 at significantly higher speeds compared to the conventional thermal convection. To the best of our knowledge, using the MW technique to make electrically conductive continuous Au nanowires with diameters of 10-30 nm within 180 s of radiation time has not been achieved. In the present study, we put forward a one-step in situ microwave approach for the synthesis of Au nanowires on DNA chains in solution using MW heating for 180 s. The synthesis was done by exposing the DNA/Au salt solution to a MW oven causing highly selective Au deposition on DNA chains in solution or immobilization on a solid substrate resulting in conductive nanowires. The key advantage was the in situ synthesis of conductive Au nanowires on the DNA without involving any other additives like reducing agents or conventional low molecular weight amphiphiles, e.g., surfactants that do not yield nanowires.

Experimental Section Reagents. High molecular weight double-stranded sodium salt of deoxyribonucleic acid (DNA, salmon testes) with average size of ∼50 kbp (actual size 48700 bp) was purchased from Sigma. The DNA is a polydispersed mixture of DNA chains of broad size distribution. A Tris-EDTA buffer (pH 7.4) was purchased directly from Sigma. Hydrogen tetrachloroaurate trihydrate (HAuCl4 · 3H2O, 99.9%), purchased from Aldrich was used without further purification. Adenosine 5′-triphosphate disodium salt (ATP) was purchased from Sigma and used as received. Ultrapure distilled water (UPD water) (15) Wirtz, D. Phys. ReV. Lett. 1995, 75, 2436. (16) Seidel, R.; Ciacchi, L. C.; Weigel, M.; Pompe, W.; Mertig, M. J. Phys. Chem. B 2004, 108, 10801. (17) Monson, C. F.; Woolley, A. T. Nano Lett. 2003, 3, 359. (18) Pastoriza-Santos, I.; Liz-Marza´n, L. Langmuir 2002, 18, 2888. (19) Harpeness, R.; Gedanken, A. Langmuir 2004, 20, 3431. (20) Panda, A. B.; Glaspell, G. P.; El-Shall, M. S. M. J. Am. Chem. Soc. 2006, 128, 2790.

10.1021/la801633r CCC: $40.75  2008 American Chemical Society Published on Web 08/01/2008

Gold Nanowires on DNA Scaffolds DNAse, RNAse free was used in all synthesis procedures and purchased from Invitrogen Corp. Instruments. The ultraviolet-visible (UV-vis) absorption spectra were recorded in an Ocean Optics absorbance spectrophotometer, and the Hitachi (model U-4100) UV-vis-NIR spectrophotometer was equipped with 1 cm quartz cuvette holder for liquid samples. A high-resolution transmission electron microscopy (HR-TEM, ZEOL ZEM-2010) was used at an accelerating voltage of 200 kV. The X-ray diffraction (XRD) analysis was done with a scanning rate 0.020 s-1 in the 2θ range 20-80° using a Rigeku Dmax γA X-ray diffractometer with Cu KR radiation (λ ) 0.154178). Energy dispersive X-ray spectra (EDS) were recorded with the instrument connected with HR-TEM during TEM experiments. Field emission scanning electron microscopy (FE-SEM) analysis was done using the Hitachi S-4700. A domestic microwave (MW) oven (Gold star company, EM-Z200S, 1000 W, 60 Hz) was used for MW irradiation for all synthesis experiments. Synthesis of Gold Nanowires on DNA by MW Irradiation. A stock DNA solution (60 µg/mL) was prepared by mixing appropriate amounts of DNA with Tris-EDTA buffer (pH 7.4) using DNAse, RNAse free water and was stirred overnight. The buffer solution helps to prepare a homogeneous DNA solution without any pop off of A and G bases in DNA and was stored in a refrigerator. A stock solution of 1.23 × 10-4 (M) aqueous gold chloride (HAuCl4) solution was made. The Au(III) solution was mixed with the stock DNA solution at R ) 0.5, 1, 2, 3, and 5 (R is the molar concentration of AuCl4- ion relative to DNA base-pair), and the mixture was stirred for 5 min using a magnetic stirrer. The UV-visible spectra were taken immediately after thorough mixing. The resulting solution was heated in MW for 120-180 s with intermittent pauses after every 10 s to cool the reaction vessel. The gold particle formation started just after 120 s of MW heating as observed by the UV-visible spectrometry. The formation of gold nanoparticles was evident by appearance of a light pinkish coloration of the solution. Preparation of Samples for HR-TEM, FE-SEM, and I-V Studies. Samples for HR-TEM analysis were prepared by placing a drop of fresh gold-DNA solution on carbon film coated copper (Cu) grids, followed by slow evaporation of the solvent at ambient conditions. FE-SEM and I-V studies were performed on Si chips with 100 nm SiO2 layer and a pair of gold electrodes spaced 2 µm apart between the alternate electrodes. The chip was thoroughly cleaned with ethanol and piranha (30% H2O2 and 70% H2SO4) followed by a treatment with HF and a final cleaning with ethanol. DNA chains were then stretched across the electrode gap using the moving meniscus of a drop of solution, caused by evaporation. Subsequently, the chip was washed by DNAse, RNAse free water and allowed to dry in the air. This chip with DNA chains bridging between the Au electrodes was placed in a Au(III) solution and exposed with MW for 3 min followed by a gentle wash. The chip was subsequently dried in a vacuum at 250 °C for 2 h. I-V measurements using a pair of Au electrodes were performed on a home-built system. The same samples were then used for the FESEM characterization to obtain the gold nanowires.

Results and Discussion UV-Visible Spectroscopy Study. Self-assembly of nanostructures using DNA has been extended for the synthesis of conductive nanowires.21,22 The basic idea is constructing a metal nanowire attached to two gold electrodes separated by a definite distance (2-50 µm). In our process, we exposed the DNA chains to a gold salt solution in a 1000 W MW oven for 180 s (see details in Experimental Section). In solution the nanowires are 1-2 µm long because the gold deposition on DNA is restricted due to limited Au available per DNA chain, R (the molar concentration of AuCl4- ion relative to DNA base-pair). This is also evident from the surface plasmon peak at ∼532 nm which (21) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (22) Martin, C. R.; Dermody, D. J.; Reiss, B. D.; Fang, M.; Lyon, L. A.; Natan, M. J.; Mallouk, T. E. AdV. Mater. 1999, 11, 1021.

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Figure 1. UV-visible spectrum at various stages of DNA-gold nanowires synthesis: (A) absorption spectra of bare DNA strands in water; (B) spectrum of the mixture of DNA, gold(III) solution before MW irradiation; (C) surface plasmon resonance (SPR) band for gold nanowires with absorption maxima at ∼532 nm. Inset shows the pink color gold solution synthesized after 180 s of MW irradiation.

Figure 2. UV-visible absorption spectrum of DNA-Au nanowire synthesis with different R values, (A) R ) 1, (B) R ) 2, and (C) R ) 3, for the DNA-Au(III) complex after 180 s of MW exposure.

is characteristic of nanometer size gold particles. Electron microscopy images also showed that the nanowires are highly dense on the DNA chain. The selective deposition of Au on DNA chains makes an electrically conductive nanowire. It should be noted that these wires have been assembled on top of the previously deposited gold electrodes. The repeated current-voltage (I-V) measurement revealed a linear Ohmic relationship. This indicated that there was good contact between nanowires and the electrodes indicated no isolated or separated particles on the wire. It has been reported in literature that a nonlinear I-V behavior is an indication of poor conductivity.11 We synthesized the nanowires that were micronmeters long and the average wire diameter was 10-15 nm in solution (see Figure 3) and 20-30 nm in immobilized DNA on solid substrate (see Figure 7). Figure 1A compares the UV-visible spectrum of the solution at various stages of the process. The aqueous gold chloride solution gets precipitated to microparticles in the absence of any stabilizer when exposed with MW or visible light as shown in TEM (see Supporting Information). The rate of this reduction is very slow

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Figure 3. TEM images and diffraction patterns of the DNA-Au nanowires at R ) 2 for different MW exposure times: (a) after 120 s of MW exposure, the particles grow in wirelike morphology with average diameter ∼10 nm, inset shows the selected area electron diffraction pattern of the Au nanowires indicates crystalline nature of the particles; (b) after 150 s of MW exposure, the Au particles on DNA with wirelike structure having average diameter ∼10-12 nm and length 1-2 µm; (c) after 180 s of MW exposure, the formation of a gold nanowire, the diameter of the wires is ∼10-15 nm; (d) linear structure of DNA-Au nanowire having diameter 10-15 nm after 180 s of MW exposure.

but increases significantly in the presence of organic ligands.23 It was reported that the DNA forms a stable complex with AuCl4at pH ∼5.24 The aqueous DNA solution has an absorption band at ∼260 nm (curve A, Figure 1). The mixing of HAuCl4 with DNA (curve B, Figure 1) resulted a slight shift and ∼10% increase of absorbance value of DNA at 260 nm. But this small shift does not confirm any complex formation as there is no considerable shift of the absorption maximum for DNA at 260 nm. All the absorbance measurements were done after subtracting the blank from the test samples. After MW exposure for 150 s, the solution turned pink with appearance of an additional absorption band at 532 nm due to the surface plasmon resonance (SPR) mode of gold nanoparticles. The sharp SPR peak observed in curve C, Figure 1, indicates the formation of gold nanoparticles of 10-40 nm range.25–29 The decrease and red shift of absorption peaks for DNA (at 260 nm) on MW exposure is attributed to the aggregation of DNA strands. This is similar to UV influence supercoiling/cross-linking of DNA strands due to trimeric (23) Gachard, E. ; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 22, 1257. (24) Wilkins, R. J. Nucleic Acids Res. 1978, 5, 3731. (25) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (26) Kundu, S.; Panigrahi, S.; Praharaj, S.; Basu, S.; Ghosh, S. K.; Pal, A.; Pal, T. Nanotechnology 2007, 18, 75712. (27) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782. (28) Kundu, S.; Pal, A.; Ghosh, S. K.; Nath, S.; Panigrahi, S.; Praharaj, S.; Pal, T. Inorg. Chem. 2004, 43, 5489. (29) Kundu, S.; Maheshwari, V.; Saraf, R. F. Langmuir 2008, 24, 551.

complex formation.30 The SPR band for gold nanoparticles for R ) 2 ceased to increase beyond 180 s of MW exposure, indicating completion of the reaction. For subsequent studies, we fixed the exposure time to 180 s and varied R from 0.5 to 5 by changing the Au content. The inset of Figure 1 compares the image of the well-known pink color Au nanoparticles synthesized at R ) 2. The SPR peak increases as R increases from 0.5 to 2 but begins to broaden and red shifts for R > 2 as shown in Figure 2. The inset of Figure 2 shows the image of three different gold nanoparticle solutions synthesized with different R values indicated by A (R ) 1), B (R ) 2), and C (R ) 3) respectively. The broadening and red shifting indicate the formation of larger size nanoparticles or aggregation of smaller particles with a broad size distribution. Therefore, R ) 2 and 180 s of MW heating are the optimum conditions in our synthesis process. High-Resolution Transmission Electron Microscopy (HRTEM) Study. Figure 3 compares the structure of the particles observed using transmission electron microscopy (TEM) at R ) 2 with MW exposure for 180 s. Figure 3a shows the TEM image of the particles obtained from a solution after 120 s of MW exposure. Here the particles that started to grow on the DNA chain have an average diameter of ∼10 nm. Figure 3b shows the growth of the Au particles on DNA with wirelike structures after 150 s of MW exposure. The average diameter of the wire at this (30) Yamada, M.; Kato, K.; Nomizu, M.; Sakairi, N.; Ohkawa, K.; Yamamoto, H.; Nishi, N. Chem.-A Eur. J. 2002, 8, 1407.

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Figure 4. TEM images of the DNA-Au nanowires at different R values for 180 s of MW exposure time: (a) R ) 1, average diameter of the discrete particles ∼8-10 nm; (b) R ) 3 and (c) R ) 5, respectively. Compared to R ) 2, the particles for R ) 3 and 5 are larger in diameter, average diameter ∼25-30 nm.

stage is ∼10-12 nm and length 1-2 µm. Figure 3c shows a perfect wire of Au formed after 180 s of MW exposure. The diameter of the wires measured under TEM was ∼10-15 nm larger than the diameter of the DNA itself (∼2 nm). This indicates the highly selective deposition of Au on DNA. The diameter of the wires remaining constant with further increase in exposure time confirms the completion of the reaction. Figure 3d shows the linear structure of the DNA-Au nanowire has a diameter of 10-15 nm that is formed after 180 s of MW exposure at R ) 2. This is consistent with the SPR band shown in Figure 1C. The length of the nanowire also increases with increasing irradiation time and remains fixed after 180 s. The inset of Figure 3a shows the selected area electron diffraction pattern of the Au nanoparticles signifies the single crystalline nature of the particles. We studied our reactions with different R. When R ) 0.5, no particles were formed in our experimental condition as confirmed from the color and UV-visible spectra. The formation of particles started at R ) 1. The TEM images with different R values (R ) 1, 3, and 5) are shown in Figure 4. With an increase in the amount of HAuCl4 for R > 2 (pH at R ) 2 is 3.5), the aggregation increases due to reduced pH (at R ) 3, pH ∼3.1 and R ) 5, pH ∼2.3) and particles become larger. As shown in Figure 2C, the Au nanoparticle solution also turns blue in color due to the increase of gold. The TEM images show that with R > 2, the average particle size is in the 25-30 nm range consistent with UV-visible spectra. The larger particle formation at higher R may be due to

Figure 5. EDS of the DNA-Au nanowires. The spectrum consists of different peaks for gold, copper, carbon, chromium, nitrogen, and phosphorus.

growth of Au nuclei on DNA and particle-particle aggregation to minimize surface energy of the Au particles. Energy Dispersive X-ray Spectroscopy (EDS) Analysis. Figure 5 presents the results obtained by energy dispersive X-ray spectroscopy (EDS) analysis, which is used to examine the chemical composition of nanomaterial. The spectrum consists of different peaks for gold, copper, carbon, chromium, nitrogen, and phosphorus. The gold peak came from the gold nanowire and the nitrogen and phosphorus peaks from the DNA. The Cu and C peaks came from the carbon-coated copper grid. The Cr peak came from the sample holder used for TEM.

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Figure 6. Powder X-ray diffraction pattern of the DNA-Au nanowires. The peaks were assigned to diffraction from the (111), (200), (220), and (311) planes of fcc gold with lattice constant 0.405 nm.

X-ray Diffraction (XRD) Analysis. The XRD patterns of the gold nanowires recorded from the DNA-Au nanowire samples are displayed in Figure 6. The peaks were assigned to diffraction from the (111), (200), (220), and (311) planes of face-centered cubic (fcc) gold, respectively. The lattice constant 0.405 nm is within the error of reported value with a ) 0.4078 nm given by JCPDS file no. 4-0784. The gold nanoparticles were sized by X-ray diffraction peak line width broadening using the Debye formula for small crystalline spheres.31 The mean diameters of the particles are consistent with the results of TEM measurements. The gold nanowires were found have good crystallinity which is similar to that previously observed by Chen’s group.32 It is to be noted that the results of the ratio (R) between the intensities of (200) and (111) diffraction peaks is slightly higher than the conventional value, indicating that the nanowires are abundant in {100} planes and tend to be preferentially oriented parallel to the surface of the supporting substrate. The ratio of intensities between peaks of (220) and (111) is slightly higher than the conventional value. This is due to a relative abundance of {110} facets on the surface of the gold nanowire. According to Wang and co-workers,33 the shape of a fcc nanocrystal was mainly determined by the ratio between growth rates along 〈100〉 and 〈111〉 directions. As DNA was used as a stabilizer, it is believed that the selective interaction between DNA and various crystal facets of the fcc gold could greatly reduce the growth rate along the 〈100〉 direction and enhance the growth rate along the 〈111〉 direction. Effects of Reaction Variables. In our experiments, we varied the concentration of Au(III) ions, concentration of DNA solution, study with free DNA bases, and the MW exposure time. Controlled experiments show that we have obtained the nanowire only on the concentration given in the Experimental Section. When the R value was larger than 2, the large sized particles were formed with blue-colored solution and became precipitated (Figure 4). Similar behavior was observed with long MW exposure time (>10-30 min) (see Supporting Information). We studied our reaction with free DNA bases but formed only spherical gold particles (confirmed by TEM images, see Supporting Information). (31) Guinier, A., X-Rar Diffraction; W. H.,. Freeman: San Franscisco, CA, 1963. (32) Hu, M. S.; Chen, H. L.; Shen, C. H.; Hong, L. S.; Huang, B. R.; Chen, K. H.; Chen, L. C. Nat. Mater. 2006, 5, 102. (33) Wang, Z. L. J. Phys. B: At., Mol. Opt. Phys. 2000, 104, 1153.

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Field Emission Scanning Electron Microscopy (FE-SEM) and Conductivity (Current, I vs Voltage, V) Studies. For the synthesis of Au nanowire on immobilized DNA on a solid substrate, the effective R is large and DNA does not have to be stabilized (i.e., soluble) in the solution. As a result, continuous Au wire can be formed and the yield is high (∼85%). Although in certain places on the substrate some discontinuous wires were also found, these were negligible compared to the continuous wire. Au nanowires are deposited on immobilized DNA strands and stretched across gold electrodes spaced 2 µm apart on a SiO2 (0.1 µm)/Si (substrate) chip. The electrodes were first bridged by stretching bare DNA strands, using the receding meniscus of a drop (10 µg/mL DNA) placed on them.34 After gentle washing, the chip was immersed in a 1.23 × 10-4 M solution of gold salt and exposed to MW for 180 s. The Au deposits selectively from the solution onto the DNA chains and forms continuous DNA gold nanowires, as seen by the field emission scanning electron microscopy (FESEM) (see Figure 7a). Consistent with the observation on the Au deposition on solution suspended DNA, in Figure 7a, the process is highly selective and only limited to the DNA strands. The inset of Figure 7a shows the corresponding highly magnified image with selective deposition in DNA chains. In a few areas “flakelike” particles around DNA were observed; this is due to the stretching of DNA on solid substrate having a higher R than that in the solution. Figure 7b shows the corresponding I-V characteristics of a single bridge spanning between the electrodes. The behavior is Ohmic with low resistance and no hysterisis indicating good contacts, continuous structure, and no capacitance. The low resistance of 83.57 Ω, which is comparable to bulk gold, and the Ohmic behavior of the wire are important considerations in using such an approach for interconnection and circuitry. Figure 7c shows I-V characteristics of three different Au nanowires spanning between the electrodes. The behavior is Ohmic with no hysteresis; this also indicates good contacts and continuous structure. The Ohmic resistances for the linear fit are 83.57, 89.55, and 109.00 Ω for the three bridges and indicate good reproducibility between the different nanowires. The slight difference in conductivity of the nanowires is due to a difference in contact resistance of the nanowire with the gold electrodes. The continuous structure and Ohmic behavior of the wire are important considerations in using such an approach for interconnection and circuitry. Reaction Mechanism for the Nanowire Synthesis. During the MW irradiation, all the processes were carried out at room temperature and under ambient pressure. The mechanism of the Au nanowire synthesis on DNA is found to be a stepwise process. Initially, the Au(III) salt mixed with the DNA with an slight increase of the absorption value as discussed above in Figure 1B. Second, on exposure to MW for 180 s the Au(III) reduces to Au(0) in the presence of DNA, forming a stabilized Au seed on the DNA. The process catalyzed by the DNA is indirectly evident with no Au nanowires formed for similar (or greater) exposure of the Au(III) solution without DNA. We tested our reactions with another biomolecule, adenosine 5′-triphosphate (ATP), which has a similar charge to DNA. The process produced small spherical gold seeds (∼5-8 nm) after 180 s of MW irradiation (see Supporting Information) although, the particles became precipitated after a day due to the absence of any specific stabilizer. Similarly, in our reaction small gold seeds formed initially during nucleation and then growth of the seed particles took place over the DNA chains in multisteps to form wire. Once the initial deposition (∼5-7 nm) of gold over DNA took place (after >120 (34) Bensimon, A.; Simon, A.; Chiffaudel, A.; Croquette, V.; Heslot, F.; Bensimon, D. Science 1994, 265, 2096.

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Figure 7. FE-SEM images and current (I)-voltage (V) characteristics of DNA-Au nanowires. (a) FE-SEM image of a single DNA-Au nanowire stretched and synthesized across a 2 µm gap. The inset shows the corresponding high magnified image. (b) Current-voltage characteristics of a single bridge spanning between the electrodes corresponding to Figure 7a. The Ohmic resistance for the linear fit is 83.57 Ω for the single bridges. (c) I-V characteristics of three different nanowires spanning between the electrodes indicates good reproducibility between the different nanowires. The Ohmic resistances for the linear fit are 83.57, 89.55, and 109.00 Ω, respectively. Scheme 1. Schematic Presentation of the Au Nanowires Synthesis on DNA

s of MW irradiation), the excess DNA in the solution has a reducing function that was enhanced by the preformed gold seeds on DNA. The preformed gold seeds on DNA not only allowed very fast reduction of the remaining gold ions in the solution but also protected the DNA from MW degradation. After 180 s of MW irradiation, all the gold ions were reduced by DNA to form gold nanowires shown in Scheme 1. The completion of the reaction (i.e., reduction of all gold ions) was also confirmed by the SPR band for gold nanoparticles that ceased to increase beyond 180 s of MW exposure (Figure 1). The mechanism of the nanowire formation is slightly different on a solid substrate (surface) than in solution. As we discussed in the section above, on a solid substrate, the effective R is large and DNA does not have to be stabilized in the solution. Here we first stretched the DNA strands on solid substrate and then immersed the substrate on Au(III) solution. We believe that few DNA strands (∼10-12%) washed

out from the substrate during MW heating. Then these DNA strands dissolve in the Au(III) solution and help the reduction process occurred in multiple steps to form a continuous wire as shown in Figure 7a. We believe that DNA acts both as a reducing agent and as a stabilizer35 in our processes, which produce reactive intermediates like free radicals during microwave heating. The hydroxyl groups of DNA (present in the deoxyribose sugar) initiate the reduction of Au(III) in the presence of MW heating, nucleating the Au on DNA and also capping the Au nanoparticles with DNA. It was assumed that the radical or solvated electrons formed during the MW irradiation of DNA solution containing HAuCl4 was responsible for the reduction of Au(III) to form the nucleation centers. Once the nucleation of Au(0) started, growth took place in multiple steps, and finally nanoclusters were formed and stabilized on the DNA chain as nanowire. This was indirectly proved by the above experiment of gold salt with ATP in the absence of DNA. As mentioned earlier, both nucleation and growth were carried out through microwave heating in the presence of DNA. The DNA serves both as a reducer and as a nonspecific capping agent for the synthesis of Au nanowire which is comparable with other literature reports where poly(vinyl alcohol),36 TX-100,37 ascorbic acid,38 dendrimers39 (containing hydroxyl group), etc., were used for similar purposes. The poly(sulfonated styrene), PSS, has a similar charge like DNA but does not have the specific chemistry like DNA and no Au deposition was found after 120 s of MW irradiation. (35) Sinha, R. P.; Hader, D. P. Photochem. Photobiol. Sci. 2002, 1, 225. (36) Henglein, A. Langmuir 1999, 15, 6738. (37) Pal, A. Talanta 1998, 46, 583. (38) Pal, A.; Pal, T. J. Raman Spectrosc. 1999, 30, 199. (39) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir 1998, 14, 3157.

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As an application, we anticipate Au wiring by the present avenue will most likely be performed either through solution directly (Figure 3) or by immobilizing DNA on the solid substrate followed by Au deposition (Figure 7a). In such a scenario, the DNA will most likely be protected from the MW radiation after the first 3-4 nm of Au deposition on the DNA. We believe that the degradation of DNA due to MW heating will not affect the quality of Au wiring, consistent with the low contact resistance and high currents noted in parts b and c of Figure 7. Logically, formation of continuous wires on any macromolecules will result in the loss of functionality of the molecule for subsequent use, due to complete coverage by the metal species. The functionality of DNA is critical in the process of immobilization forming patterned templates. Control experiments indicate that the role of the DNA is essential. Our results have showed that replacing the DNA with a monomer of DNA, surfactants, or other biomolecules like ATP leads to spherical, agglomerated particles from micrometers to the nanometer scale. It has been discussed that the DNA serves as nonspecific capping agents for the growth of the nanowires. The MW exposure in this process is subsequent to the formation of DNA-Au nanowire and does not affect the quality of nanowires or the DNA’s functionality. This process will be valuable to fabricate other monometallic (e.g., Ag, Pd, and Pt) and hybrid (e.g., Au@Ag, Au@Pd, and Au@Pt) nanowires that will be discussed in the future.

Conclusion In summary, we have described a simple one-step method to synthesize electrically conductive Au nanowires on DNA in

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solution and immobilized on a solid substrate using MW irradiation for 120-180 s. The nanowires are micrometers long with a diameter of 10-15 nm in solution and of 20-30 nm in immobilized DNA. The nanowires exhibit Ohmic behavior, with low resistance and no hysterisis indicative of continuous metallic structure. The one-step in situ process that does not disturb the overall conformation of the DNA chain can be extended to fabricate intricate circuitry using the property of DNA to form complex shapes by hybridization. For future applications, the approach can be used to fabricate building blocks for functional nanodevices, sensors, and optoelectronics. In those the DNA is used to form “circuit lines” followed by deposition of gold described herewith. Acknowledgment. This research was in part sponsored by the NSF-0506082, the Department of Mechanical Engineering, Texas A&M University, and the Texas Engineering Experiments Station. We wish to acknowledge Mr. Damon Bennett from Texas A&M University for proofreading the paper. Support for TEM and EDS by Dr. Zhiping Luo at the Microscopy Imaging Center (MIC), Texas A&M University, and for HR-SEM by Dr. Dwight Romanovicz at the Biological Science Department, University of Texas, Austin, was greatly appreciated. Supporting Information Available: Transmission electron micrograph (TEM) images for different controlled experiments. This material is available free of charge via the Internet at http://pubs.acs.org. LA801633R