Tris(hydroxymethyl)phosphine-Capped Gold Particles Templated by

NANO LETTERS

Tris(hydroxymethyl)phosphine-Capped Gold Particles Templated by DNA as Nanowire Precursors

2002 Vol. 2, No. 9 919-923

Oliver Harnack, William E. Ford, Akio Yasuda, and Jurina M. Wessels* Materials Science Laboratories, Sony International (Europe) GmbH, AdVanced Technology Center Stuttgart, Heinrich-Hertz-Strasse 1, D-70327 Stuttgart, Germany Received May 17, 2002; Revised Manuscript Received July 6, 2002

ABSTRACT Tris(hydroxymethyl)phosphine-capped gold nanoparticles (ca. 1−2 nm diameter) bind densely to DNA, although both species are negatively charged. The particles are also active catalysts for electroless plating of gold. Electroless plating after adsorption to calf thymus DNA immobilized on silicon provides nanowires as narrow as ca. 30−40 nm and electrical conductivities ca. one-thousandth that of bulk gold. Here we summarize our physical characterization of the particles and their DNA conjugates and initial electrical measurements after plating.

The steady trend in the electronics industry toward components having ever-smaller dimensions has stimulated the development of alternative “bottom-up” fabrication technologies to compete with conventional micro- and nanolithography, which are expected to become extremely expensive as the feature sizes of future electronic circuits approach the limits of optical lithography.1 In contrast, bottom-up approaches rely on self-assembly (or self-organization) and often utilize biological molecules as templates.2 Low fabrication costs and feature sizes below the current limit of optical lithography are some of the major advantages of bottom-up approaches. These features also characterize the approach that we report here for fabricating DNA-templated nanowires, which could possibly be used as interconnects for future electronic circuits. Several methods have already been described in the literature for metallizing DNA to produce nanowires.3-8 Metallization is required due to the relatively poor intrinsic electric conductivity of DNA.9 Conceptually, the metallization processes described previously involve two steps: (1) formation of conjugates between DNA and metal nanoparticles (NPs) and (2) enlargement of the NPs by electroless plating until they coalesce. Processes used for step (1) can be further distinguished according to whether the metal NPs in the DNA-NP conjugate are generated in-situ3-5,8 or exsitu.6,7 In the reported ex-situ approach,6,7 lysine-capped gold colloidal particles were attached to DNA via electrostatic interaction between the positively charged particles and the negatively charged phosphate groups of the DNA. We have discovered that negatively charged tris(hydroxymethyl)phosphine-capped gold nanoparticles (THP-AuNPs) also * Corresponding author. E-mail: [email protected]. 10.1021/nl020259a CCC: $22.00 Published on Web 08/06/2002

© 2002 American Chemical Society

bind densely to DNA. Electroless gold plating of the resulting DNA-AuNP conjugates provides nanowires as narrow as ca. 30-40 nm in width and longer than 2 µm showing ohmic behavior and resistivity of ca. 10-5 Ωm. Duff et al.10-13 first reported the synthesis and characterization of THP-AuNPs. Their standard preparation, which involves adding a solution of HAuCl4 to a solution of hydrolyzed tetrakis(hydroxymethyl)phosphonium chloride (THPC), gives an aqueous colloidal sol of THP-AuNPs with a mean particle diameter of ca. 1.5 nm.10,11 Several applications for these particles have since been found, including supported catalysts for CO oxidation14 and the formation of insulator-metal core-shell particles as active components of photonic devices.15-19 Vogel et al.13 observed that THP-AuNPs are negatively charged and only adsorb significantly to alumina and titania powders in aqueous suspensions at pH values below the isoelectric points (IEP) of the oxides, when the oxide surface is positively charged. Adsorption to silica (IEP ) pH 2) is negligible over the pH range 2-10.13 Westcott et al.20 also observed very little, if any, adsorption of THP-AuNPs to larger silica NPs unless the latter are functionalized with amino or thiol terminal groups. In contrast, we have observed that THP-AuNPs can bind densely to calf thymus DNA, which is negatively charged due to the double-stranded sugar-phosphate backbone. As described below, this behavior allows for DNA-templating of the particles and subsequent particle growth, since the THP-AuNPs are active catalysts toward electroless gold plating.12,15-19 The THP-AuNPs were synthesized according to a procedure similar to that developed by Duff et al. (experimental procedures and other details are available in Supporting

Information). We verified that the particles were negatively charged and estimated that each particle had an average net charge of approximately 15 e based on lithium ion exchange and ICP-AES analysis. Further confirmation of the negative charge was provided by gel electrophoresis, which showed that the particles migrated toward the positive electrode (anode). To metallize the DNA, it was first immobilized on a silicon substrate using O2-plasma treatment to enhance immobilization21 and spin-coating to elongate the molecules.22,23 The THP-AuNP sol was then applied to the substrate for 2-15 min, rinsed, and dried. Although the particles in the assynthesized solution bind to DNA, better templating was achieved if they were precipitated with ethanol and then redissolved in ethanol-water mixtures just before being applied to the DNA-substrate. The samples were subsequently treated with a commercially available electroless gold plating solution to provide conductive nanowires suitable for SEM and electrical characterization. A series of metallization experiments were performed using different ethanol-water compositions (0%, 50%, 90%, and 95% ethanol). The best results with respect to density of particle templating, avoidance of excess particles on the substrate, and lack of defects in the resultant wires were so far obtained as the ethanol concentration was increased until an optimal value of about 95% ethanol. Templating was still observed at even higher ethanol concentrations; however, the particles tended to precipitate spontaneously from solution. Figure 1A shows a tapping mode AFM image of DNA molecules on a silicon substrate after treatment with a solution of THP-AuNPs in 95% ethanol. The DNA molecules are decorated with particles, but some excess particles are also visible on the substrate. The particle density is greatest in places where two or more DNA molecules are intertwined. Figure 1B is a height profile of a section of the AFM image that includes part of a bare DNA molecule (∼0.6 nm), a section of DNA upon which particles are deposited, and two isolated particles on the substrate, one of which is average size (∼1.3 nm) and the other about twice as large (∼2.9 nm). Figure 1C is a histogram of the heights of the particles present on the substrate, i.e., without underlying DNA. The majority of the particles that can be distinguished from the background are between 0.7 and 2 nm in size. The slightly granular nature of the background prevents discrimination of particles smaller than 0.7 nm. The larger particles may be clusters of smaller ones, since THP-AuNPs are known to cluster, especially in ethanol.20 Gel filtration experiments provide strong evidence of cluster formation in aqueous solution as well. The particles elute completely within the void volume of a Sephadex G-50 column (see Supporting Information), indicating an effective hydrodynamic radius greater than 2.6 nm.24 This result is consistent with the observation of Duff et al. that Visking dialysis tubing retains the color in a standard sol despite a nominal pore size of 6 nm.11 The particle clustering does not result directly in fusion, however, as is evident from the lack of a 920

Figure 1. (A) Topographic AFM image of calf thymus DNA immobilized on a silicon (native oxide) substrate and treated with a solution of THP-AuNPs in 95% ethanol-water for 7 min. The height scale range is 2.2 nm. (B) Line profile showing the variation in height along a 350-nm section indicated by the dotted white line in the upper left quadrant of the AFM image. (C) Histogram of the heights ((0.1 nm) of particles not associated with DNA in the AFM image. The distribution has a mean value of 1.44 nm (standard deviation ) 0.82 nm).

pronounced surface plasmon absorption band in the UVvisible absorption spectrum (data not shown). Nano Lett., Vol. 2, No. 9, 2002

Figure 2. SEM image after electroless gold plating showing conductive nanowires. DNA immobilized on a silicon (200-nm oxide) substrate was treated with a solution of THP-AuNPs in 95% ethanol-water for 7 min and then with an electroless gold plating solution for 7 min. The bar scale is 300 nm.

Figure 2 is an SEM image obtained after electroless plating. It shows a homogeneous coverage of the DNA molecules by enlarged gold particles ca. 30-40 nm in size without obvious gaps between the particles. The good contrast against the silicon surface indicates that the structures are highly conductive. It is clear from this image that the metallization was restricted almost entirely to the DNA. At its narrowest points, the wire structure consists of strings of enlarged gold particles. Previously we showed that such structures do not arise from reaction between components of the plating solution and DNA.8 Since the conditions of the present experiments were somewhat different, we confirmed that result in control experiments in which the plating solution was applied to DNA without pretreatment with THP-AuNPs (see Supporting Information). To demonstrate the electrical transport capabilities of our DNA nanowires, electrically contacted networks of wires were fabricated by spin-coating DNA onto 15 nm thick interdigitated gold electrodes on silicon substrates having a 400 nm thermal silicon oxide layer on top. The metallization was performed as described above, leading to randomly aligned wires across the 1-4 µm wide electrode gaps. Figure 3 is an SEM image showing a metallized DNA network between two gold electrodes. The inset represents a typical current-voltage plot for this network, showing linear ohmic behavior with an overall resistance of 86 Ω. This value is significantly lower than the typical resistance of nonmetallized DNA networks aligned across similar electrodes.9 In a further experiment, the resistive change of a network was monitored while using an AFM tip to cut a selected single metallized DNA wire within the network. Figure 4 shows SEM images of a selected DNA wire between a 1 µm gap before (A) and after (B) cutting, together with the recorded change of the network resistance while the wire was cut (C). The single wire resistance Rsw was determined by the simple equation 1/Rsw ) 1/R1 - 1/R2, where R1 is the network resistance before cutting the wire and R2 afterwards. From the resistance change from R1 ) 93.4 Ω to R2 ) 97.1 Ω, as Nano Lett., Vol. 2, No. 9, 2002

Figure 3. SEM image of a network of metallized DNA molecules between gold electrodes on a silicon (400-nm oxide) substrate. The bar scale is 1 µm. Inset: current-voltage characteristics of this network at room temperature.

Figure 4. (A) SEM image of a single metallized DNA wire selected for cutting with an AFM tip (contact mode). (B) Change of the network resistance during cutting. (C) SEM image of the same spot as in (A) after the cut was performed.

shown in Figure 4C, we derived a single wire resistance Rsw of 2.4 kΩ. By taking the wire dimensions into account, a resistivity of about 3 × 10-5 Ω m was estimated, which is about 1000 times higher than the bulk resistivity of gold (2 × 10-8 Ω). In general, surface scattering and transport through grain boundaries is responsible for significantly higher resistivity of granular nanowires compared to the corresponding bulk materials. Details on further studies of the resistivity of our metallized DNA wires will be published elsewhere. Good reproducibility of the results could be observed with freshly prepared nanoparticle solutions. However, as was described by Duff et al.,11 the sols underwent a color change from orange-brown to red upon storage, accompanied by a growth of the plasmon absorption band. This change was also accompanied by a decrease in the capability of the 921

particles to bind to DNA. Although the origin of this change is not understood, the growth of the nanoparticles may be related to oxidation of the THP ligand.25 Our current understanding of the binding mechanism of THP-AuNPs to DNA and the factors controlling the efficiency of binding is still limited. Preliminary experiments show that the THP-AuNPs preferably adsorb to hydrophilic areas on substrate surfaces (e.g., oxygen plasma treated silicon oxide) rather than to hydrophobic areas (e.g., amorphous carbon layer). Thus the THP-AuNPs may tend to move selectively toward hydrophilic DNA molecules on a silicon oxide surface. This process would be even more effective when the solvent is made less polar, which occurs when the ethanol content of ethanol-water mixtures is increased. As noted above, the tendency of THP-AuNPs to self-associate into clusters increases with the ethanol content in ethanol-water mixtures.20 This effect may reflect a decrease in the Debye screening length as the dielectric constant decreases, but the solvent may also affect the gold nanoparticles via solvation, hydrogen bonding, or other localscale effects.20 Such effects may also favor particle-DNA interactions. For a potential binding mechanism between THP-AuNPs and DNA, it is reasonable to expect that hydrogen-bonding interactions may be important. The THP ligands that cap the particles each provide three hydroxyl groups capable of serving as either O-H donors or O acceptors in hydrogen bonds of moderate strength (4-15 kcal mol-1).26 Likewise, double-stranded DNA has a number of groups that can serve as acceptors or donors in hydrogen bonds of moderate strength. These include the phosphate (PdO, P-O-) O acceptors along the polymeric backbone and nitrogen and oxygen atoms of the bases that may be exposed in the major or minor grooves of the double helix. Cooperative, e.g. multivalent “bifurcated” and “trifurcated”,26 hydrogen bond bridging within the conjugates between THP-AuNPs and DNA could further strengthen these interactions. For example, Hayashida et al.27 found that calix[4]resorcarene-based saccharide bundles were firmly bound to calf thymus DNA in water via multivalent saccharide-phosphate hydrogen bonding. Other possible binding mechanisms include ligand replacement by groups on DNA that can coordinate to the AuNP surface, since phosphine ligands are known to be labile on gold nanoparticles,28 and covalent bonding between THP and amino groups of the DNA bases, since THP is known to undergo Mannich-type condensation reactions with N-H group containing compounds.29-31 In summary, we have found a new process for metallizing DNA via the templating of gold nanoparticles capped with the tris(hydroxymethyl)phosphine ligand. The efficiency of templating can be controlled by the solvent, which influences particle-particle and particle-DNA interactions. Issues for further investigation include determination of the binding mechanism of the THP-AuNPs to DNA and how the solvent and substrate influence the process. The nanowires produced via subsequent electroless gold plating have electrical conductivities ca. 1/1000 that of bulk gold. This metallization process is faster than in-situ approaches that employ metal 922

salts or complexes as precursors. Attachment of THPAuNPs to DNA and subsequent growth by electroless plating can be accomplished within ca. 10 min, while in-situ approaches for the formation of nanowires typically require more than 1 h.4-5,8 Acknowledgment. This project was partially funded by the European Commission through the IST-FET “Nanotechnology Information Devices” initiative (www.cordis.lu/ist/ fetnid.htm), BIOAND project IST-1999-11974. We gratefully acknowledge the invaluable technical assistance of Zoi Karipidou and Frank Scholz during the course of this work, and Andrea Schroedter (Universita¨t Hamburg) for performing the gel electrophoresis measurements. Supporting Information Available: Experimental details regarding (a) materials, (b) instrumentation, (c) particle synthesis, (d) particle precipitation and redispersion in ethanol-water mixtures, (e) DNA immobilization, (f) metallization of immobilized DNA, (g) control experiments, (h) estimation of the net negative charge per particle, and (i) gel electrophoresis of the particles. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Saxl, O. Opportunities for Industry in the Application of Nanotechnology; The Institute of Nanotechnology: Stirling, Scotland, 2001 (http://www.nano.org.uk/contents.htm). (2) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4129. (3) Braun, E.; Eichen, Y.; Silvan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (4) Richter, J.; Seidel, R.; Kirsch, R.; Mertig, M.; Pompe, W.; Plaschke, J.; Schackert, H. K. AdV. Mater. 2000, 12, 507. (5) Richter, J.; Mertig, M.; Pompe, W.; Mo¨nch, I.; Schackert, H. K. Appl. Phys. Lett. 2001, 78, 536. (6) Kumar, A.; Pattarkine, M.; Bhadbhade, M.; Mandale, A. B.; Ganesh, K. N.; Datar, S. S.; Dharmadhikari, C. V.; Sastry, M. AdV. Mater. 2001, 13, 341. (7) Sastry, M.; Kumar, A.; Datar, S.; Dharmadhikari, C. V.; Ganesh, K. N. Appl. Phys. Lett. 2001, 78, 2943. (8) Ford, W. E.; Harnack, O.; Yasuda, A.; Wessels J. M. AdV. Mater. 2001, 13, 1793. (9) Storm, A. J.; van Noort, J.; de Vries, S.; Dekker, C. Appl. Phys. Lett. 2001, 79, 3881. (10) Duff, D. G.; Baiker, A.; Edwards, P. P. J. Chem. Soc., Chem. Commun. 1993, 96. (11) Duff, D. G.; Baiker, A.; Edwards, P. P. Langmuir 1993, 9, 2301. (12) Duff, D. G.; Baiker, A.; Gameson, I.; Edwards, P. P. Langmuir 1993, 9, 2310. (13) Vogel, W.; Duff, D. G.; Baiker, A. Langmuir 1995, 11, 401. (14) Grunwaldt, J.-D.; Kiener, C.; Wo¨gerbauer, C.; Baiker, A. J. Catal. 1999, 181, 223. (15) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (16) Oldenburg, S. J.; Hale, G. D.; Jackson, J. B.; Halas, N. J. Appl. Phys. Lett. 1999, 75, 1063. (17) Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J. Appl. Phys. Lett. 1999, 75, 2897. (18) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729. (19) Graf, C.; van Blaaderen, A. Langmuir 2002, 18, 524. (20) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (21) Harnack, O.; Ford, W. E.; Wessels, J. European Patent Application No. 00125433.3 (Nov. 20, 2000): Process for immobilization of nucleic acid molecules on a substrate. (22) Yakota, H.; Sunwoo, J.; Sarikaya, M.; van den Engh, G.; Aebersold, R. Anal. Chem. 1999, 71, 4418. (23) Ye, J. Y.; Umemura, K.; Ishikawa, M.; Kuroda, R. Anal. Biochem. 2000, 281, 21. Nano Lett., Vol. 2, No. 9, 2002

(24) The fractionation range of Sephadex G-50 for globular proteins is 1.5-30 kDalton according to the manufacturer’s specifications (Amersham Biosciences AB). The 30 kDalton exclusion limit corresponds to a hydrodynamic radius of 2.6 nm, assuming the empirical globular shape MW/radius relationship (Protein Solutions, Inc., http://www.protein-solutions.com/calc.htm). (25) Ellis, J. W.; Harrison, K. N.; Hoye, P. A. T.; Orpen, A. G.; Pringle, P. G.; Smith, M. B. Inorg. Chem. 1992, 31, 3026. (26) Steiner, T. Angew. Chem., Int. Ed. Engl. 2002, 41, 49.

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