Controlled Assembly of Monolayer-Protected Gold Clusters by

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NANO LETTERS

Controlled Assembly of Monolayer-Protected Gold Clusters by Dissolved DNA

2004 Vol. 4, No. 1 95-101

Gangli Wang and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 Received October 22, 2003; Revised Manuscript Received November 20, 2003

ABSTRACT One-dimensional chains of gold nanoparticles coated with cationic trimethyl(mercaptoundecyl)ammonium (TMA) monolayers have been electrostatically assembled along DNA molecules in solution by careful control of the relative molar quantities of nanoparticles and DNA base pairs. The nanoparticles are monolayer-protected Au clusters (MPCs) with average diameter 4.4 nm. Transmission electron microscopy (TEM) images of dried solution droplets in which the DNA base pair/MPC mole ratio was about 20:1 display chains of nanoparticles a few microns long in which the Au cores are lined up with nearly contacting cationic monolayer edges along the anionic DNA template. The gold core edge-edge distance in the TEM image is 2.9±0.9 nm, which is roughly twice the TMA monolayer thickness. The linear structures formed at a 10:1 DNA base pair/MPC mole ratio appear to persist stably in solutions. The Au surface plasmon resonance band in UV−vis spectra intensifies and red-shifts as the DNA/MPC mole ratio is increased; precipitation occurs at higher ratios. Other and more complex patterns of DNA/MPC binding are observed in TEM images prepared from solutions with different DNA/MPC mole ratios. We believe entropic effects are significant in the assembly process and lead to strong binding constants.

Introduction. This paper describes the assembly of linear chains of cationic nanoparticles along DNA molecules dissolved in solution. A capacity to prepare soluble supramolecular structures provides building blocks for further manipulation of DNA-templated structures; the present material might, for example, be used as soluble conductive nanowires in which conductivity occurs by electron hopping1 between metal-based nanoparticles. Other applications can be imagined in which incorporating specific nanoparticle linkage chemistry2 yields directed connections or junctions between nanoparticle chains, and three-dimensionally selfassembled nanostructures that offer new and different properties. This reasoning exemplifies the idea of bottomup assembly of nanostructures, which becomes attractive3,4 at the dimension limit of traditional microfabrication. Preparing nanostructures in solutions additionally has the attraction of scale of preparation and potential for further processing. The gold nanoparticles used in the present work are examples of the stable, monolayer-protected clusters (MPCs) that became accessible through synthetic innovations such as the Brust reaction,5 in which MPCs with hydrophobic alkanethiolate monolayers were prepared. There was subsequently described6 a related synthesis of luminescent7 water-soluble MPCs with tiopronin thiolate monolayers (tiopronin is N-(2-mercaptopropionyl) glycine) and of watersoluble MPCs8 with N,N,N-trimethyl(mercaptoundecyl)ammonium9 (TMA) thiolate ligands. The monolayers of these 10.1021/nl034922m CCC: $27.50 Published on Web 12/13/2003

© 2004 American Chemical Society

and other MPCs can be manipulated using ligand or placeexchange6,10-12 reactions, which are convenient routes to further functionalize MPCs. There have to date been reported a number of ways to assemble nanoparticles onto surfaces, using various linking materials. For example, metal ions2 and polymers13 have been used to bring MPCs with functionalized ligands together in films, and dithiols have been used to assemble MPCs both in solutions and on surfaces.1,15 DNA has been recognized as an attractive scaffolding material because of its very long linear structure and its mechanical rigidity over short distances.16 Gold nanoparticles have been absorbed along DNA chains pre-immobilized on solid substrates,17 and cationic CdS nanoparticles adsorbed to DNA layers that in turn had been adsorbed18 at an amphiphile monolayer. Conductive nanowires have been prepared on surface adsorbed DNA templates by binding silver19 and palladium20 ions followed by reduction to the metal state. Nanoparticle assemblies in solution have also been described. Mirkin, et al.21 used sequence specific molecular recognition properties to connect DNA oligonucleotides to metal colloids (sizes > 10 nm) bearing thiolated DNA complements, producing analytically useful optical changes. Alivisatos et al.22 used an analogous approach to fashion two or three-dimensional assemblies of nanoparticles. Rotello et al.23 exploited electrostatic binding of the cationic TMA MPC in a study of inhibition of DNA transcription, and protein-

modified gold nanoparticle assembly by DNA has been investigated.24 We have employed the DNA-intercalative properties of the ethidium species for DNA-binding of ethidium-labeled nanoparticles.25 These have been important steps but have not so far provided routes to geometrical control of the nanoparticle assembly in a solution. Bulky 3-D complexes lacking an organizational pattern are typically found in these experiments, which were directed at other objectives. As noted above, the objective here was a controllable assembly of MPCs onto DNA molecules in solutions, using electrostatic binding, and calculating on entropic changes as principle assembly driving forces. Warner and Hutchison26 were the first to describe a systematic approach to obtaining a controlled DNA-template assembly of nanoparticles in solutions and described formation of linear nanoparticle structure. In this paper, we show that crucial variables in controlled assembly are molar ratios of nanoparticles and DNA base pairs and the length of the DNA chains employed. By systematically varying the former, we have been able to isolate conditions for reproducibly preparing, in solution, with subsequent isolation for transmission electron microscopy (TEM) analysis, reasonably ordered chains of cationic gold MPCs lined up along DNA chains. The separations between the nanoparticles appear, from the similarity of their edgeedge separation (2.9 nm) to the thickness of two TMA monolayers, to be sterically determined by the MPC dimension, with implications of future control of this variable through the MPC monolayer. That is, the MPCs are essentially close-packed on the DNA surface. This result is found only at a specific mole ratio of DNA base pairs to nanoparticles; different structures (by TEM analysis) are found at other mole ratios. Results and Discussion. The electrostatic binding of nanoparticles to the negatively charged phosphate backbone of DNA was achieved by using a water-soluble, polycationic MPC with a trimethyl(mercaptoundecyl)ammonium (TMA) protecting monolayer. The TMA MPCs were prepared according to Cliffel et al.8 in a reaction using a 3:1 mole ratio of the thiolate to AuCl4-. TEM results agreed with the previously reported average Au core size of 4.4 nm; the overall nanoparticle diameter is ca. 7.6 nm, assuming that the 1.6 nm long TMA chains are extended. The average nanoparticle composition is about Au3000(TMA)910, so that the formal charge per MPC is ca. +910. Its counterions are chloride. DNA sodium salt from herring testes (abbrev. long DNA, from Sigma) and prehybe DNA (abbrev. short DNA, 300-1300 bp from Lofstrand Labs) were used as received. UV-Vis Spectra Show the MPC-DNA Interaction. Since the MPCs are polycationic (ca. 910 cationic ligands) and the MPC diameter (ca. 7.6 nm) is far larger than the spacing between DNA base pairs (ca. 0.34 nm), multiple base pair MPC charge interactions can occur between each single MPC nanoparticle and a segment of DNA chain. The DNA chains can further bind to multiple MPCs along their overall lengths. The strong electrostatic interaction from multiple interaction sites can readily lead to precipitation, as described by others.23,26 96

Figure 1. UV absorbance spectra taken during titration of 10.1 pmol of TMA MPC (2 mL 5.1 × 10-8 M TMA MPC) with 4.4 × 10-4 M long chain DNA. The first 5 spectra (left to right spectra see arrow at 600 nm) correspond to additions of 5, 10, 16, 22, and 24 µL DNA solution (22:1, 43:1, 69:1, 94:1, 104:1 mol DNA base pairs to MPCs). The last and most right-hand spectrum was taken after 5 min from 24 µL addition. Inset: Absorbance (left axis) and wavelength maxima (right) changes of surface plasma band as a function of the relative molar concentrations of DNA base pairs and MPC.

With the above multiple charge interactions in mind, we chose experimental conditions such that the concentration of MPC charges in the solution was greater than the total concentration of charges due to the base pairs of DNA. MPC solutions were gradually titrated with DNA solution; the MPCs were initially more abundant, and each added DNA chain became saturated by MPCs. The molar concentration of DNA base pairs grew to exceed the molar concentration of MPCs, but the concentration of base pair charges never exceeded the total concentration of charges on the (+910) MPCs. The initial MPC concentrations in the solutions were calculated taking the molar absorptivity of the SPR band at 530 nm as  ) 3000 per gold atom;27 the DNA concentration in the titrant solution was measured using UV absorbance at 260 nm following standard protocol. The TMA MPCs have a core size (4.4 nm diameter), sufficient to exhibit a prominent surface plasmon resonance (SPR) absorbance band in aqueous solutions at 530 nm. Binding of MPCs to DNA brings them close together, modifying their local environment and changing the SPR absorbance. The SPR absorbance is known27 to be sensitive to local environment. Figure 1 shows a typical (long) DNA titration of an MPC solution. For both long and short DNA, as DNA was gradually added, the SPR band intensity gradually increased and the peak maximum shifted to lower energy (see Figure inset). Both changes were small effects but are qualitatively those expected if the MPCs were being brought into a closer average proximity to one another than is the case in free solutions. At a certain point in the titration, however, the energy of the SPR band abruptly decreased by ca. 8 nm, to 544 nm, and the absorbance, instead of increasing gradually as before, decreased slightly (see insert). Within minutes after this last addition of DNA, precipitation Nano Lett., Vol. 4, No. 1, 2004

was noticed in the solution. The SPR absorbance continued to further decrease, and at the same time, the absorbance due to DNA at 260 nm began to decrease with time. After a few hours, the MPCs had completely precipitated and would not redissolve in water. It was evident that an insoluble complex between the nanoparticles and DNA is formed when the MPC/DNA ratio reaches a certain value. Prior to that value, the solutions of MPC/DNA complex were stable for days. The onset of precipitation in the above experiment did not change with DNA chain length; precipitation occurred for long and short DNA when their base pair concentrations exceeded 94 and 90, respectively, times larger than the MPC concentration. The ratio that leads to precipitation is both reproducible and specific; the spectrum in a solution with an 80-fold concentration ratio was quite stable. We presume that precipitation is associated with the base pair sites on the added DNA chains becoming insufficiently screened by interaction with MPC nanoparticles, leading to cross-linking between chains of interaction partners that is extensive enough to seriously diminish the solubility of the MPC/DNA complex. That long and short DNA chains behave similarly means that the ratio of DNA to nanoparticles is more important, in assembling a nanoparticle/DNA complex, than the dimension of the DNA. This observation has attractive implications for assembling physically quite large nanostructures based on nanoparticle/DNA interactions. The ratio of concentration of DNA base pair charges to those of that of MPC charges is 0.2 at the onset of precipitation. The charge ratio, and specifically the mutual compensation of charges, may be a more significant measure of the solubility limit of DNA/MPC assembly. The spacing between base pairs along a B type DNA chain is known to be 0.34 nm. In comparison, the terminal quaternary ammonium cations on the MPC monolayer surface lie on average, about 0.5 nm apart (given the TMA MPC diameter, 7.6 nm, and the +910 charges). This lack of commensurate registry of charges between DNA and MPC surfaces should be of little consequence, since the cation sites on the MPC monolayer have considerable flexibility of motion, allowing cations on the monolayer surface to laterally approach the DNA chain. In this way, over a length scale equivalent to the TMA MPC diameter, as many as 40 phosphate groups (20 base pairs) could charge compensate 40 quaternary ammonium sites of the TMA MPC. Interaction with all 20 base pairs would require introducing some curvature into the DNA duplex structure (see cartoon A), and that all of the +910 charges on a given TMA MPC could be charge

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compensated by a single DNA chain is highly implausible. It is more plausible that MPCs could experience charge compensating interactions simultaneously with multiple DNA chains over different parts of the MPC monolayer surface (see cartoon B). This picture implies that the high charge of the MPC could act as the dominant organizing force, of multiple DNA strands along an MPC “backbone”. We should note that the preceding DNA/MPC titrations were done without adding any electrolyte to the solutions, so that the Cl- counterions in the original MPC solution provide the charge compensation to the nanoparticle to the extent that the DNA did not. Adding further indifferent electrolyte to the solution would undoubtedly lessen the strength of the DNA/MPC interaction, so that precipitation would occur at a higher ratio of base pair to nanoparticle concentrations. This comment emphasizes that, as in any form of molecular electrostatic interaction, indifferent electrolytes can exert a charge screening effect and modify the degree of interaction. Transmission Electron Microscopy Imaging of MPC Assemblies Formed With Long DNA.28 The region of the DNA titration in Figure 1 involving small mole ratios of DNA base pairs to MPC nanoparticles was thought to present the simplest situation, and one most likely to avoid crosslinking or interactions of MPCs with multiple DNA chains. Figure 2 shows images (at different magnifications) obtained from a solution in which the mole ratio of base pairs to MPCs was 10:1. (The ratio of base pair charges to MPC charges is 1:45). Only the Au core of the nanoparticles appears in the image; the surrounding TMA monolayer appears as “blank” space between cores. A substantial fraction of the MPC cores observed have become aligned into more or less linear chains. This alignment result was typical at a 10:1 mole ratio of base pairs to MPCs; additional examples are found in Supporting Information Figure S-2. It was interesting to see that the linear MPC-DNA chains were formed on length scales of several 100s of nanometers to even a few microns (Figure 2B). Further, the chains display relatively little curvature over many MPC and base pair units; electrostatic repulsion between adjacent cationic MPCs may play a role in this. In addition to the aligned MPCs, there are numerous randomly scattered MPCs, as is typical in TEM images of TMA MPCs in the absence of DNA (Figure S-1). TMA MPCs are neVer observed to form isolated chains in TEM images taken in the absence of DNA. Scattered aggregates appear in the left side of Figure 2B. Figure S-2 shows what appear to be overlapping chains, but there are also instances (such as Figure 2A, lower left) of short segments of aligned MPCs branching off from the DNA, whose significance is not understood. Details of the effect of concentration and the blank spacing between the linear aligned nanoparticles are discussed later, using data from a mixture of short DNA with MPCs. Chains of aligned MPCs were common in TEM images prepared using solutions containing mole ratios between 10:1 and 20:1 base pairs per MPC. As the mole ratio increased, there was a greater incidence (Figure S-3) of looping of the 97

Figure 2. TEM images from solution mixtures of long chain DNA and TMA MPC in which the MPC and DNA concentrations were 5 × 10-8 M and 5 × 10-7 M (base pair/MPC mole ratio 10:1; base pair/MPC charge ratio 0.022). See Figure S-5 (Supporting Information) for enlargements of these figures.

chains and multiples of aligned chains, similar to those reported by Warner and Hutchison.26 The most central result, however, was that linear chains of MPCs aligned by DNA were obtained at low mole ratios of DNA base pairs to MPCs, and that these linear structures were persistent in solutions for many days according to sampling by TEM images. Transmission Electron Microscopy Imaging of MPC Assemblies Formed With Short DNA (chain length ca. 100400 nm). The chain length of the long (natural) DNA was unknown. Further experiments were conducted using shortened, prehybe DNA (“short chain DNA”, Lofstrand Labs, 300∼1300 base pairs). Following the linear MPC chain result of Figure 2, 5×10-8 M TMA MPC was mixed with short chain DNA solutions to obtain a 15:1 mole ratio of DNA base pairs to MPC. Figure 3 shows several results of TEM examination of the complexes formed. As in Figure 2, there is again a preponderance of aligned chains of MPCs. Figures 3A and 3B show several short chains of MPCs aligned into a long 98

Figure 3. TEM images from solution mixtures of short chain DNA and TMA MPC in which the MPC and DNA concentrations were, respectively, 5 × 10-8 M and 7.5 × 10-7 M (base pair/MPC mole ratio 15:1; base pair/MPC charge ratio 0.033). In Panel C the TEM specimen has been negatively stained with uranyl acetate so that the DNA chain shows up as a gray shadow along the aligned chain of MPCs. See Figure S-6 (Supporting Information) for enlargements of these figures.

chain, as if there were occasional pieces of a DNA molecule that somehow were not labeled with the nanoparticles. That the MPCs were in fact aligned by DNA chains was confirmed by Figure 3C, in which the DNA was stained, producing a gray shadow that follows the course of the chain of MPCs. Figure 3C again shows a definite branching of the chain, which could be overlap of two different DNA chain Nano Lett., Vol. 4, No. 1, 2004

Figure 4. TEM images from solution mixtures of short chain DNA and TMA MPC in which the MPC concentration was 5 × 10-8 M and the DNA concentrations were, Panels A and B: 1 × 10-6 M (base pair/MPC mole ratio 20:1; base pair/MPC charge ratio 0.044); Panels C and D: 2 × 10-6 M (base pair/MPC mole ratio 40:1; base pair/MPC charge ratio 0.088); and Panel E: 3 × 10-6 M (base pair/MPC mole ratio 60:1; base pair/MPC charge ratio 0.12). See Figure S-7 (Supporting Information) for enlargements of these figures.

segments, or dehybridization of a section of the DNA duplex. It is interesting that the lengths of the aligned assemblies varies between 10s and 100s of nm, which is consistent with the overall DNA chain lengths (100-400 nm). Several lone and a few clumps of MPCs are common in the images, but the main population of MPCs appears in the chain-aligned form. As in the titration of Figure 1, we went on to inspect solutions in which the mole ratio of DNA base pairs to MPCs was increased, from 20:1 to 40:1 to 60:1. As was the case with long chain DNA in Figure 2, the incidence of linear, Nano Lett., Vol. 4, No. 1, 2004

aligned chains of MPCs fell and more complex and disordered assemblies were found. Figure 4A shows that at a DNA base pair/MPC mole ratio of 20:1, some aligned structures are still present. However, in different areas of the same sample (Figure 4B), we see images that suggest that the aligned chains are becoming bundled together. Chainlike threads of MPCs can easily be picked out in Figure 4B. At a higher, 40:1 DNA base pair/MPC mole ratio, the bundling is more extensive and threads of MPCs aligned 3 and 4 abreast can be seen (Figure 4C), with practically no chains of singly aligned MPCs. Crowding together of TMA 99

MPCs can be seen in images prepared for MPC solutions lacking DNA (Figure S-4), but they do not display the aligned threads of nanoparticles that are evident in Figure 4B and 4C. A curious feature seen in Figure 4C (and less distinctly in Figure 4B) are “nanochannels” of roughly similar widths (10-20 nm) that lie between the bundles of nanoparticles. The nanochannels themselves are devoid of nanoparticles. The origin of this peculiar effect is uncertain. A possible source is a “rafting” of the DNA/MPC assembly at the airwater interface during drop-drying on the TEM grid. The TMA MPC is somewhat nonpolar (dissolves in ethanol), and the contact angle of a droplet of an aqueous MPC solution on a Parafilm surface was 79°, while that of pure water on this surface was 89°. The latter observation showed that the MPC has some surfactant characteristics, which would be accentuated during the final stages of drying of the DNA/ MPC droplet on the TEM grid. Whether the nanochannels represent excess DNA that has microphase segregated from the DNA/MPC complexes or are just empty space is not known. Different regions in TEM samples of the 40:1 mixture showed DNA/MPC assemblies (Figure 4D) that were much less well-organized, and were similar to what was seen at the higher DNA/MPC mole ratio (60:1) in Figure 4E. It appears that many parts of the DNA/MPC assembly overlapped so that the image is no longer one of a single layer of complex on the grid surface. This indicated that stronger interactions between DNA and MPCs have taken hold, although no precipitation has commenced. Further addition of DNA does cause precipitation as reported in Figure 1. Further Comments on the Assembly Mechanism. We conclude with a discussion of several further aspects of the DNA-nanoparticle assembly. First, it is evident in the TEM images of Figures 2, 3, and 4A that the MPC gold cores in single, aligned MPC chains are spaced apart by rather uniform distances (2.9(0.9 nm, hundreds of examples were read with Scion image software). Adjacent threads of MPCs in the more complex assemblies of Figures 4B and 4C are similarly spaced. This spacing between gold core edges is the same as can be seen in TEM images prepared from more concentrated TMA MPC-only solutions,8 which is reported to be 3 nm. (Such an image is given here as Figure S-4.) The spacing is approximately twice the length of the TMA ligand chain, which if extended is 1.6 nm. The TMA ligands do not interdigitate as is common29 for alkanethiolate-coated MPCs, owing to charge repulsion between the terminal quaternary ammonium groups.8 We conclude from the edge-edge distance analysis that the TMA MPCs are crowded together, with monolayers of adjacent nanoparticles in near-contact, along the DNA duplex. The spacing between nanoparticles is determined by its monolayer and not by the DNA, although DNA induces the nanoparticles cast from a very dilute solution to be brought together (relative to a TMA MPC-only solution, Figure S-1) and to be aligned in chains or threads. This is a reasonable result since each MPC carries +910 charges and 100

interaction of an MPC with any given DNA chain can charge compensate only 20 to 40 of them. The edge-edge distance analysis also means, second, that the number of MPCs interacting with a given DNA chain is sterically determined, and seems to lie near the maximum that is sterically possible. The DNA/MPC binding constant must be quite large, which is expected owing to the large number (20 to 40) of individual cation-anion electrostatic interactions, which releases the same number of Na+ and Cl- counterions. The electrostatic interaction should entail a large positive entropy change, akin to the well-known chelate effect in metal complex formation, and to the behavior of polyelectrolytes interacting with one another. For reference, the equilibrium constant KN for binding an N-valent counterion to the DNA could be expressed as ∂ log KN ) -N ∂ log c where c refers to electrolyte concentration (which is very small). Detailed discussions of counterion interactions with DNA can be found in the literature.30 Third, we note that the TEM images of “bundled” DNA/ MPC assemblies (Figure 4B for example) show chains of MPCs that are 3-5 abreast, and that lie flat on the TEM grid surface. Are these assemblies also planar in solution? We believe this is somewhat unlikely and that there is probably some three-dimensionality to the nanostructures formed in solution. That is, the “flattened bundles” in the TEM image may be “ropes” that have formed when in solution and that are thin enough to become flattened during the droplet casting and evaporation process. Whether this entails a “rafting” effect as discussed above in connection with Figure 4C, or a strong interaction with the grid surface itself, or some other factor, is not known. Finally, we mentioned the formation of soluble conductive nanowires in the Introduction as one potentially useful aspect of DNA-induced chains of metal nanoparticles. If one considers the hypothetical electron-hopping conductivity along linear aligned MPC chains as in Figures 2 and 3A, the relevant ingredients are the gold edge-edge separation (sterically determined as noted above) and the electronic coupling term β for electron tunneling though the monolayer chains. If one assumes a β of 0.8 Å-1 and considering the 3 nm edge separation, the molecular resistance between adjacent MPCs could be roughly in the range of 108 Ω/MPC, based on a previous discussion.1 Acknowledgment. This research was supported in part by grants from the Office of Naval Research and the National Science Foundation. We thank Professor Sergei Sheiko (UNC-CH) and Dr. Hui Xu (UNC-CH) for helpful discussions. Supporting Information Available: Additional TEM images of TMA MPCs and TMA MPC complexes with DNA. This material is available free of charge via the Internet at http://pubs.acs.org. Nano Lett., Vol. 4, No. 1, 2004

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