Confinement of Silver Triangles in Silver Nanoplates Templated by Duplex DNA K. Swaminathan Iyer,† Charles S. Bond,*,† Martin Saunders,‡ and Colin L. Raston*,† Centre for Strategic Nano-fabrication, School of Biomedical, Biomolecular and Chemical Sciences, The UniVersity of Western Australia, Crawley, WA-6009, Australia, and Centre for Microscopy, Characterisation and Analysis, The UniVersity of Western Australia, Crawley, WA-6009, Australia
CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 5 1451–1453
ReceiVed NoVember 12, 2007; ReVised Manuscript ReceiVed February 22, 2008
ABSTRACT: The use of DNA as a molecular tool in developing novel nanostructures is based on its ability to form self-organizing three-dimensional structures. In the current report the palindromic DNA dodecamer d(CGTAGATCTACG) was employed as a scaffold to fabricate silver nanostructures. The utility of DNA dodecamer to self-assembly into closed pack monolayers and thereby control the formation of silver nanoplates and the consequent confinement is reported. Fabricating nanostructures with well -defined shape and size has attracted the imagination of the scientific community because of their unique chemical and physical properties. In the case of noble metal nanoparticles, for example, this relates to their applications in the fields of catalysis, electronics, photonics, biological labeling, and information storage.1 It is well-known that simultaneous fine control of the size and shape of nanoparticles in building up novel structures is rather challenging. A gamut of literature reporting methods exists for the synthesis of nanoparticles with specific shapes including spheroids, nanowires, nanocubes, nanoplates, and nanoprisms.2–4 Most of the techniques used to gain access to these different shapes are based on directed growth of particles in the presence of readily available commercial surfactants. These include thiols, carboxylic acids, and amines, and they usually lead to shapes of simple topologies. Developing techniques and synthetic protocols to gain access to new nanostructures of higher complexity is important in defining and fine-tuning shape- and size-dependent properties of nanomaterials for end use applications. The use of DNA as templates, scaffolds, and interconnects has been successfully employed in the fabrication of metal nanowires.5 The power of DNA as a molecular tool in developing nanostructures is based on a number of its properties: it forms self-organizing threedimensional structures; its structure is determined by its sequence of polymer subunits; effectively any DNA sequence can be synthesized; and any sequence of sufficient length can be amplified to macroscopic quantities by the polymerase chain reaction (PCR). We previously reported the use of spinning disk processing (SDP) as a continuous flow technology for fabricating silver nanoparticles with the ability to efficiently manipulate their size, shape, agglomeration, phase, and defects.6 Herein we report the remarkable ability of a DNA scaffold to template the formation of nanoplates of silver under process intensification using SDP, with selfassembled DNA strands sandwiched between the nanoplates effectively confining a smaller triangle of silver, [AgNano @ DNA @ AgNano]. We note that SDP is a relatively new technique for fabricating nanomaterials, which also includes nanoparticles of carotenoids, and fullerene silver nanocomposites. Plug flow conditions, and turbulent mixing and shearing across a rotating disk coupled with the ability to vary the disk speed, concentration of reactants, and other parameters enables fine-tuning the conditions for building up nanomaterials of higher complexity.7,8 The palindromic DNA dodecamer d(CGTAGATCTACG) was employed in the current investigation as the scaffold. Short * To whom correspondence should be addressed. Phone: +61 86488 3045. Fax: +61 86488 8683. E-mail: (C.L.R.)
[email protected]; (C.S.B.)
[email protected]. † Centre for Strategic Nano-fabrication, School of Biomedical, Biomolecular and Chemical Sciences. ‡ Centre for Microscopy, Characterisation and Analysis.
Figure 1. Self-assembled monolayers of DNA d(CGTAGATCTACG) as derived from the crystal structure. (A) Side view; (B) top view.
palindromic sequences spontaneously form stable DNA duplexes with mechanical rigidity and relatively precise geometry. They hold promise in the bottom-up self-assembly of nanodevices.9 d(CGTAGATCTACG) has been previously crystallized and its 3D crystal structure solved.10,11 Interestingly, we note that the crystal is formed from layers of closely packed vertically aligned duplexes of thickness 3.2 nm (Figure 1).10,11 We have previously reported that the reduction of silver ions in water involving process intensification using SDP in the presence of starch stabilized fullerene nanowhiskers results in complete coating of the fullerene array by the metal.7 Translating such reduction under the same conditions in the presence of the DNA strand in place of the fullerene nanowhiskers is a potential route to metal coated DNA. DNA can form stable complexes with Ag+ principally due to interactions between silver and the nucleotide bases, rather than the anionic phosphate residues.9 Metal ions commonly occupy interaction sites formed in the major and/or minor grooves of duplex DNA.12 TEM analysis of the products obtained by reacting a solution of Ag+ and DNA [AgNO3 (10 mM) and DNA (0.003 wt %)] with
10.1021/cg7011167 CCC: $40.75 2008 American Chemical Society Published on Web 04/19/2008
1452 Crystal Growth & Design, Vol. 8, No. 5, 2008
Communications
Figure 2. (A) TEM image of hexagonal silver nanoplate surrounded by a sheath of DNA monolayer (red arrows), and the corresponding EDS spectra obtained at (B) the center of the nanoplate and (C) the edge of the nanoplate.
Figure 3. TEM images of silver-DNA hybrids. (A) DNA nanosandwiches (red arrows), (B) truncated triangular nanoplate surrounded by hexagonal nanoplate with sandwich layer of DNA in between (as shown by the red arrows), and (C) dendritic nanoplates (inset: single crystal diffraction pattern).
ascorbic acid solution (10 mM) in water showed that most silver nanoplates have hexagonal and truncated triangular shapes. A typical image of the hexagonal nanoplate is shown in Figure 2A. Interestingly, it was also observed that the nanoplates were indeed surrounded by a polymeric sheath of around 4 nm in thickness. The sheath material was confirmed as DNA using energy dispersive spectroscopy, which revealed a dominant phosphorus signal around the edges of the nanoplates. The spectra were obtained by focusing the beam around the edges of the nanoplates, and the center of the silver nanoplates.13 Representative spectra are as shown in Figure 2B,C. The end-to-end distance of the double helix of the DNA dodecamer, as established from the crystal structure, is close to 3.2 nm.11 It is therefore reasonable to assume that the sheath around the nanoplates in the present study corresponds to a monolayer of DNA self-assembled around the edges, functioning as a surfactant to stabilize the outer periphery of the nanostructures. Complementary fluorescence analysis of suspensions of the nanoplates was undertaken by treating the solution with ethidium bromide (EtBr), which is a commonly used dye for detecting DNA, with intercalation between adjacent base pairs resulting in a strong orange-red fluorescence.14 Indeed, samples which had been washed to remove free nucleic acid from solution showed strong fluorescence confirm-
ing that the sheath layer surrounding the particles were indeed composed of DNA (Supporting Information). We conclude that, rather than coating the DNA dodecamer with silver, nanoplates of silver form which are templated by the DNA with the metal presumably following the contour of self-assembled monolayers of the DNA. An appealing feature of this current investigation is the presence of the novel DNA-sandwich structures, [AgNano @ DNA @ AgNano] (Figure 3A,B), which is without precedent. The thickness of the sandwich layer as measured using transmission electron microscopy (TEM) was again found to be around: 4-5 nm. Here the DNA acts as a divergent ditopic surfactant, and while the detailed mechanism of the formation of these nanostructures is yet to be established, it is believed that the seed mediated ripening of the nanoplates results in the formation of these novel structures. Silver nanoplates stabilized by DNA monolayers may act as seeds in serving as active centers for Ostwald ripening resulting in an onion shell assembly. These sandwich structures can be used for encapsulation of strands of DNA to protect them from modification of solution environment (pH, enzymes, or biochemical environment) or to block the DNA activity. Moreover, the drive for Ostwald ripening also results in the formation of occasional (
