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Photolytic Metallization of Au Nanoclusters and Electrically Conducting Micrometer Long Nanostructures on a DNA Scaffold Subrata Kundu, Vivek Maheshwari, and Ravi F. Saraf* Department of Chemical and Biomolecular Engineering, UniVersity of Nebraska, Lincoln, Nebraska 68588-0643 ReceiVed August 7, 2007. In Final Form: October 25, 2007 An electroless, photolytic method is described to synthesize Au nanoclusters and electrically conductive, micronmeter long nanostructures on DNA. Electrical characterization indicates that the Au nanostructures are continuous, exhibiting Ohmic behavior with very low contact resistance with the electrodes. The nanoclusters have a size of 10-40 nm, and the nanostructure have a diameter of 40-70 nm with resistivity comparable to that of pure metal. The method is highly selective with deposition confined to the DNA template.
Introduction Bottom-up approaches to make one-dimensional nanoscale structures for nanoelectronics is of great interest to translate from the device level to the circuit level.1,2 Nanostructures, in particular, will play an important role as both interconnects and active components in fabricating nanoscale electronic and photonic devices.3 Biomacromolecules such as proteins, amino acids, peptides, and nucleic acids with the ability to self-assemble have been shown as templates to metallize metal nanoclusters and nanostructures.4-6 Deoxyribonucleic acid (DNA), in particular, has been widely investigated as a template for metallization, because (i) its well-defined polymeric sequence and versatile chemical structure allows DNA to self-assemble into a hierarchy of complex structures7 such as cubes,8 squares,9 and “T” junctions10 that may be leveraged to make complex circuit elements, (ii) the negative charge on DNA allows binding to metal cations11,12 and metal nanoparticles,13,14 functionalizing the DNA structures, and (iii) the double-helix rigid chain structure gives DNA high mechanical strength.15 Silver,11 palladium,12 platinum,16 and copper17 have been metallized on DNA, leading to formation of nanoclusters and nanostructures. However, the * To whom correspondence should be addressed. E-mail: rsaraf@ unlnotes.unl.edu. (1) Lopes, W. A.; Jaeger, H. M. Nature 2001, 414, 735. (2) Liz-Marzan, L. M.; Norris, D. J.; Bawendi, M. G.; Betley, T.; Doyle, H.; Guyot-Sionnest, P.; Klimov, V. I.; Kotov, N. A.; Mulvaney, P.; Murray, C. B.; Schiffrin, D. J.; Shim, M.; Sun, S.; Wang, C. MRS Bull. 2001, 26, 981. (3) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (4) Scheibel, T.; Parthasarathy, R.; Sawicki, G.; Lin, X. M.; Jaeger, H.; Lindquist, S. L. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 4527. (5) Lee, S. W.; Mao, C. B.; Flynn, C. E.; Belcher, A. M. Science 2002, 296, 892. (6) Mcmillan, R. A.; Paavola, C. D.; Howard, J.; Zaluzec, N. J.; Trent, J. D. Nat. Mater. 2002, 1, 247. (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. (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.
list does not include the metallization of gold (Au) nanoclusters and conducting nanostructures on DNA. Here we report a one-step in situ method for metallization of Au nanoclusters on DNA chains in solution and further the technique by selectively metallizing electrically conducting microns long Au nanostructures on DNA immobilized on a solid substrate. The metallization occurs by exposing the DNA/Au salt solution to UV radiation, causing highly selective Au deposition on the DNA chain, in solution or immobilized on a solid substrate. Combined with the ability to make preformed scaffolds,18,19 the selective metallization on immobilized DNA will be an important addition to the rapidly growing nanofabrication tool kit. Experimental Section Reagents and Instruments. Double-stranded salmon testes DNA with an average size of 500 bp was purchased from Sigma. The DNA is a polydispersed mixture of DNA chains of broad size distribution. Hydrogen tetrachloroaurate trihydrate (HAuCl4‚3H2O; 99.9%), purchased from Aldrich, is used without further purification. Ultrapure distilled (UPD) water, DNAse- and RNAse-free, is used in all synthesis procedures and purchased from Invitrogen Corp. All ultraviolet-visible (UV-vis) absorption spectra are recorded in an ocean-optics absorbance spectrophotometer equipped with a 1 cm quartz cuvette holder for the liquid sample. A xenon lamp source (Newport Corp.) providing an intensity of 2 µW (at 260-270 nm) on the sample is used for photoirradiation. Transmission electron microscopy (TEM) analysis is performed on a Hitachi-H-9000 NAR, and the samples are prepared by placing a drop of fresh gold-DNA solution on carbon-film-coated copper (Cu) grids, followed by slow evaporation of solvent at ambient conditions. General Route for the Synthesis of Au Nanoclusters and Nanostructures on DNA by Photoreduction of a DNA-Au(III) Solution. A DNA solution (10µg/mL) is prepared with DNAse- and RNAse-free water and stirred overnight. A stock solution of 2.05 × 10-5 M aqueous gold chloride (HAuCl4; Au(III)) solution is made. The Au(III) solution is mixed with the DNA solution at R ) 0.5, 2, 3, and 4 (R is the molar concentration of AuCl4- ion relative to DNA base pairs). An increase is observed in the UV-vis absorption spectrum of the mixed solution (DNA-Au(III)) compared to the pure DNA solution. The UV-vis spectrum is taken immediately after thorough mixing. The photoreduction is performed by directly (18) Claridge, S. A.; Goh, S. L.; Frechet, J. M. J.; Williams, S. C.; Micheel, C. M.; Alivisatos, A. P. Chem. Mater. 2005, 17, 1628. (19) Pinto, Y. Y.; Le, J. D.; Seeman, N. C.; Musier-Forsyth, K.; Taton, T. A.; Kiehl, R. A. Nano Lett. 2005, 5, 2399.
10.1021/la702416z CCC: $40.75 © 2008 American Chemical Society Published on Web 12/21/2007
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placing the cuvette under 260 nm of UV light irradiation with a varying time of exposure. After photoreduction the UV-vis spectrum is again recorded. Sample Preparation for I-V and Field Emission Scanning Electron Microscopy (FESEM) Study. The samples are prepared on a silicon (Si) chip with a 100 nm SiO2 layer and a pair of gold electrodes spaced 20 µm apart. The chip is thoroughly cleaned with ethanol and piranha solution (30% H2O2 and 70% H2SO4) followed by treatment with HF and a final cleaning with ethanol. DNA chains are then stretched across the electrode gap using the moving meniscus of a drop of solution, caused by evaporation. Subsequently, the chip is washed by DNAse- and RNAse-free water and allowed to dry in air. The chip, with DNA chains bridging the 20 µm gap between the Au electrodes, is placed in a Au(III) solution (2.05 µM) and irradiated with UV light (260 nm) for 18 h followed by a gentle wash. The chip is subsequently dried in a vacuum at 25 °C for 2 h. I-V measurements using a pair of Au electrodes are performed on a home-built system. Field emission scanning electron microscopy (FESEM) characterization is performed on the samples after I-V measurements to visualize the nanostructures.
Results and Discussion The DNA chains are exposed to a gold salt solution under UV irradiation (see the Experimental Section), resulting in nucleation of Au seeds on DNA. The nuclei autocatalytically grow to form nanocluster/nanostructure-decorated DNA chains. In solution the Au metallization on DNA is restricted to discrete nanoclusters due to the limited Au available per DNA chain, R. This is evident by an absorbance peak at ∼532 nm which corresponds to the plasmon resonance characteristic of nanometer size Au clusters. TEM shows that the nanoclusters are highly dense on the DNA chains. In contrast, metallization of Au on the immobilized DNA, not restricted by R, makes an electrically conducting nanostructure. The current through the structure, I, as a function of the applied bias, V, follows Ohm’s law, implying good contacts with the electrodes and no isolated clusters. Poor contact and nanoscale defects would lead to a nonlinear I-V behavior.11 FESEM images show the nanostructures to be micrometers in length with a diameter of 40-70 nm. It is known that, on UV irradiation of an aqueous HAuCl4 solution, gold microparticles are produced that immediately precipitate due to the absence of any stabilizers, and the rate of photoreduction is very slow but increases significantly in the presence of organic ligands.20 Following this approach, a DNAAu(III) solution is made by mixing 2.05 × 10-5 M HAuCl4 in water with DNA. R is varied from 0.5 to 4. As shown in Figure 1A for R ) 2, on mixing with HAuCl4, a ∼11% increase in the optical density of DNA at 260 nm is observed, which we believe is due to interaction between DNA and Au(III).22 On photoirradiation of the solution at 260 nm (intensity 2 µW) for 18 h, the solution turns pink with the appearance of an additional absorption band at 532 nm due to the surface plasmon mode in Au nanoclusters. The sharp surface plasmon peak observed in Figure 1A clearly signifies the formation of Au nanoparticles in the 10-40 nm size range.23-25 The decrease in and red shift of the absorption intensity peak for DNA (260 nm) on irradiation is attributed to UV-influenced supercoiling and aggregation/ (20) Gachard, E.; Remita, H.; Khatouri, J.; Keita, B.; Nadjo, L.; Belloni, J. New J. Chem. 1998, 22, 1257. (21) Wilkins, R. J. Nucleic Acids Res. 1978, 5, 3731. (22) Berti, L.; Alessandrini, A.; Facci, P. J. Am. Chem. Soc. 2005, 127, 11216. (23) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Mater. 2001, 13, 2313. (24) Logunov, S. L.; Ahmadi, T. S.; ElSayed, M. A.; Khoury, J. T.; Whetten, R. L. J. Phys. Chem. B 1997, 101, 3713. (25) Kundu, S.; Panigrahi, S.; Praharaj, S.; Basu, S.; Ghosh, S. K.; Pal, A.; Pal, T. Nanotechnology 2007, 18, 75712.
Figure 1. Absorption spectra of DNA-gold nanoclusters at different stages of synthesis. (A) Absorption spectra of (a) bare DNA strands, (b) a DNA-Au(III) mixture (R ) 2), and (c) the mixture after UV photoreduction. The peak at 532 nm corresponds to plasmon resonance in Au. (B) Absorption spectra of DNA-Au(III) complex after UV exposure for 18 h for R values of (a) 0.5, (b) 2, and (c) 4.
cross-linking of DNA strands due to trimeric complex formation26 (as is evident in the TEM images). Monitoring the surface plasmon peak at 532 nm as a function of the irradiation time indicates that no significant Au formation occurs before 12 h of exposure. Furthermore, the plasmon absorption peak ceases to increase beyond 18 h of exposure, indicating completion of the reaction. For subsequent studies we fix the exposure time to 18 h and vary R from 0.5 to 4 by changing the Au content. Figure 1B compares the Au nanocluster formation on DNA, indicating that the plasmon peak increases as R increases from 0.5 to 2 and begins to broaden and red shift for R > 2. This broadening and red shift are indicative of larger size nanoclusters (or agglomeration of small clusters) with a broad size distribution. Thus, R ) 2 is the optimum condition for Au nanocluster synthesis. Figure 2 shows TEM images of the gold nanoclusters metallized at R ) 2 for UV exposure of 18 h. The TEM samples are prepared by depositing the DNA-Au nanoclusters on a carbon-coated copper grid by solution casting followed by air drying. Figure 2A shows a nominally linear structure with few entanglements, and Figure 2B shows a highly branched structure. Both kinds of structures are present in the sample and span micrometers in length with densely decorated discrete Au nanoclusters. A more clear visual of their discreteness and branching is seen in the corresponding insets. The highly aggregated structure is attributed (26) Yamada, M.; Kato, K.; Nomizu, M.; Sakairi, N.; Ohkawa, K.; Yamamoto, H.; Nishi, N. Chem.sEur. J. 2002, 8, 1407.
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Figure 3. (A) and (B) are the TEM image of the gold-DNA nanoclusters synthesized at R ) 3 and 4, respectively. Compared to those at R ) 2, the particles are larger in diameter.
Figure 2. TEM images of DNA-gold nanoclusters, synthesized at R ) 2, deposited on a carbon-film-coated copper grid. The contrast in the low- and high-magnification images is primarily from Au due to its high atomic number (Z) compared to DNA that is composed of low-Z atoms. (A) Gold nanoclusters on a nominally linear chain of DNA with few entanglements. (B) Au on a highly entangled web of chains of DNA. (C) High-magnification TEM image of goldDNA nanoclusters, synthesized under identical conditions. The insets clearly show that the nanoclusters are discrete and assemble to form a branched chainlike structure.
to chemical cross-linking of DNA on UV exposure, which can lead to a network spanning over 2 µm and aggregation due to charge neutralization of the DNA from HAuCl4.26 Thus, despite the low molar ratio of Au to DNA, the aggregation of DNA
significantly reduces the effective sites on DNA available for Au deposition. From the estimated Au clusters per chain length (in Figure 2) and R, the effective number of chains available for deposition is reduced by an order of magnitude due to aggregation. Nevertheless, the chain structure of the cross-linked aggregated DNA (seen in Figure 2) after Au metallization deposition is clearly visible, indicating that the deposition is highly selective on DNA. The ∼20 nm size of the discrete nanoclusters for R ) 2, estimated from TEM images in Figure 2, is consistent with the UV-vis observation of Figure 1B. The nanoclusters on highly branched sections of DNA (i.e., inset of Figure 2B) are 30-50% smaller than the more linear superstructure shown in Figure 2A,C. On a further increase of HAuCl4, i.e., R > 2 (the pH at R ) 2 is 3.52), the aggregation will increase due to a lower pH (at R ) 3, pH ≈ 3.1) and the particles will become larger due to the increased amount of Au. The metallization at R ) 3 and 4 (Au content 1.5 and 2 times higher than that at R ) 2) results in a blue color solution (see the absorption spectra in Figure 1B), signifying the formation of larger Au clusters with polydispersity in size. The TEM images in Figure 3 (for R ) 3 and 4) show a particle size in the range 20-120 nm, which is consistent with the spectra in Figure 1B that indicate a broader and red-shifted plasmon peak compared to that at R ) 2. We attribute the large particle formation at high R to both the growth of the Au nuclei on the chain and particle-particle aggregation to minimize the surface energy of the Au particles. Similar to those at R ) 2, we note that the clusters are chainlike in Figure 3, indicating metallization on DNA. The length distribution is due to the polydispersed molecular weight of DNA. For metallization of Au on immobilized DNA, the effective R is very large and DNA does not have to be stabilized (i.e., soluble) in the solution. As a result, continuous Au nanostructures,
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Figure 4. (A) FESEM image of a DNA-gold wire stretched and synthesized across a 20 µm gap. The inset shows the contact between the DNA-Au wire and the Au electrode pad. (B) Current-voltage characteristics of the same stretched DNA-gold wire. The ohmic resistance from a linear fit is 76 Ω for the single bridge. (C) An irreversible electrical breakdown occurs on biasing the same device at a bias greater than 0.5 V. (D) FESEM image of the same wire after the electrical breakdown. The two insets show the location of the single break in the nanowire bridge which occurred close to the left electrode.
micrometers in length, instead of a discrete necklace can be formed. Au nanostructures are metallized on immobilized DNA strands, stretched across gold electrodes spaced 20 µm apart on a SiO2 (1 µm)/Si (substrate) chip. The electrodes are first bridged by stretching bare DNA strands, using the receding meniscus of a drop (10 µg/mL DNA) placed on them.27 After gentle washing, the chip is immersed in a 2.05 µM solution of gold salt and exposed to UV light irradiation for 18 h. The Au deposits selectively from the solution onto the entangled DNA chains, forming continuous DNA-templated gold nanostructures, as seen from FESEM images (see Figure 4A,D). Consistent with observation on the Au metallized on solution-suspended DNA, in Figure 4A, the metallization process is highly selective and limited to the DNA strands. Figure 4B shows 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 76 Ω and ohmic behavior of the wire are important considerations in using such an approach for interconnection and circuitry. Assuming the total length as 4 times the gap and nanowire diameter of ∼50 nm, the resistivity is comparable to that of bulk Au. On application of the larger voltage (see Figure 4C), an electrical breakdown is observed at ∼0.7 V. Figure 4D shows the FESEM image of the same sample indicating that the electrical breakdown was at a single point in the bridge. Phenomenologically, the mechanism of Au metallization on DNA is a three-step process: (i) The Au(III) salt interacts with the DNA, as evidenced by the absorption spectra discussed above in Figure 1A. (ii) On exposure to UV radiation at 260 nm that coincides with the DNA absorption peak (see Figure 1A), Au(27) (a) Bensimon, A.; Simon, A.; Chiffaudel, A.; Croquette, V.; Heslot, F.; Bensimon, D. Science 1994, 265, 2096. (b) Klein, D. C. G.; Gurevich, L.; Janssen, J. W.; Kouwenhoven, L. P.; Carbeck, J. D.; Sohn, L. L. Appl. Phys. Lett. 2001, 78, 2396.
(III) reduces to Au, forming a stabilized Au seed on DNA. The photolytic process catalyzed by DNA is indirectly evident: No Au is formed at similar (or greater) exposure of the Au(III) solution either at 260 nm without DNA or at 300 nm (away from the DNA absorption peak) with DNA. (iii) On subsequent exposure to UV light, the Au(III) from the solution spontaneously reduces to form Au, growing the seed that is stabilized on DNA. The total quantum yield of the process, for 3 mL of the Au(III) and DNA mixture (R ) 2), is 0.009, comparable to reported values of Au(III) photoreduction in the presence of a liposome.28 The complete process results from two important aspects of Au(III) reduction. One is the well-known phenomenon of photoreduction of Au(III) to Au, which is a two-photon process.29 It is also known that in the presence of organic agents such as ethylene glycol30 and poly(vinyl alcohol)31 the photoreduction process is accelerated and mediated by their hydroxyl groups. We believe similarly that the hydroxyl groups of DNA (present in the deoxyribose sugar) initiate the reduction of Au(III) in the presence of UV light,32 nucleating the Au on DNA and also capping the Au nanoparticles with DNA. The growth proceeds by photoreduction, leading to formation of nanoparticles in solution and continuous Au nanostructures on surface-immobilized DNA. For the selective deposition on DNA leading to a Au wire both the 260 nm excitation and Au(III)-DNA complex formation are critical. For example, poly(sulfonated styrene) that absorbs around 260 nm but does not have a specific chemistry to complex with Au(III) exhibits no evidence of any photocatalytic Au deposition under similar conditions. (28) Sato, T.; Ito, T.; Iwabuchi, H.; Yonezawa, Y. J. Mater. Chem. 1997, 7, 1837. (29) Kurihara, K.; Kizling, J.; Stenius, P.; Fendler, J. H. J. Am. Chem. Soc. 1983, 105, 2574. (30) Eustis, S.; Hsu, H. Y.; El-Sayed, M. A. J. Phys. Chem. B 2005, 109, 4811. (31) Henglein, A. Langmuir 1999, 15, 6738. (32) Sinha, R. P.; Hader, D. P. Photochem. Photobiol. Sci. 2002, 1, 225.
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In an application, we anticipate that Au wiring by the reported avenue will most likely be performed by immobilizing the DNA followed by Au deposition (Figure 4). In such a scenario, the DNA will most likely be protected from the UV radiation after the first 3-4 nm of Au deposition on DNA. Thus, the photodegradation of DNA will not affect the quality of Au wiring, consistent with the low contact resistance and high currents noted in Figure 4. Logically, formation of continuous wires on any macromolecules will result in loss of functionality of the molecule for subsequent use, due to the complete coverage by the metal species. The functionality of the DNA is critical in the process of immobilization, forming patterned templates. As the UV exposure in this process is subsequent to formation of the DNA template, photodegradation of DNA is not a factor affecting the quality of the nanowires or the versatility of DNA functionality. Furthermore, due to the over 3-4 orders larger electromigration in Ag than Au, the latter is more suitable for nanoscale electronics compared to the Ag wire on DNA.22 Although the Ag mineralization demonstrated in ref 22 is also highly selective on DNA, the reported AFM images show that the particles are not percolating as shown for Au in Figure 4.
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Conclusions In summary, we have described a simple method to synthesize Au nanoclusters on DNA in solution and electrically conductive, micrometer long Au nanostructures on immobilized DNA. The nanostructures display ohmic behavior, with low resistance and no hysterisis, indicative of a continuous metallic structure. The one-step process that does not perturb 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. Although futuristic, the approach can be used to wire electronic nanodevices where the DNA, for example, may be manipulated by flow/surface tension27 and hybridization7 to form “circuit lines” followed by the gold deposition process described above to achieve wiring. Acknowledgment. Financial support from the NSF (Grant CMS-0330227) is appreciated. We acknowledge Dr. X. Z. Li at the electron microscopy center of the University of Nebraska, Lincoln, for help with HR-TEM. LA702416Z
