NANO LETTERS
Fabrication of Coaxial Metal Nanocables Using a Self-Assembled Peptide Nanotube Scaffold
2006 Vol. 6, No. 8 1594-1597
Ohad Carny,† Deborah E. Shalev,‡ and Ehud Gazit*,† Department of Molecular Microbiology and Biotechnology, George S. Wise Faculty of Life Sciences, Tel AViV UniVersity, Tel AViV 69978, Israel, and Wolfson Centre for Applied Structural Biology, Hebrew UniVersity of Jerusalem, Edmond Safra Campus, GiVat Ram, Jerusalem 91904, Israel Received February 28, 2006; Revised Manuscript Received June 7, 2006
ABSTRACT The design and fabrication of complex nanostructures with specific geometry and composition is one of the main challenges of nanotechnology. Here we demonstrate the devise of metal−insulator−metal, trilayered, coaxial nanocables. Such coaxial geometry may give rise to useful and unique electromagnetic properties. We have fabricated these nanostructures using a scaffold of self-assembled peptide nanotubes. Gold nanoparticles were bound to the surface of peptide nanotubes via a common molecular recognition element that was included in various linker peptides. This enabled us to promote site-specific metal reduction and to create the coaxial nanostructure. Using electron microscopy, 1H NMR spectra, and energy-dispersive X-ray analysis, we monitored the different steps within the process, gaining further understanding of its mechanism.
A major challenge in the field of nanotechnology is to design and fabricate complex nanostructures with a specific geometry and composition by bottom-up assembly.1 Multishell coaxial geometry represents such an intricately designed architecture, whose merits in electromagnetic interference shielding have been well-known since Nikola Tesla first patented his basic trilayered (metal-insulator-metal) coaxial cable in 1894.2 Here, we demonstrate the fabrication of metal-insulator-metal, trilayered, “Teslaian” coaxial nanocables by a designed bottom-up self-assembly process. We have characterized the self-assembly process of diphenylalanine peptides (phe-phe), which forms well-ordered and durable peptide nanotubes.3-5 On the basis of our understanding of the self-assembly process, we have used specific peptide recognition motifs for the manipulation of the selfassembled peptide nanotubes into coaxial nanocables. Although other researchers presented inorganic approaches for the fabrication of multishell coaxial nanocables, either with multiple semiconductor layers6,7 or with multiple metal layers,8 we suggest a different strategy. Peptide chemistry holds biological specificity on one hand and chemical diversity on the other hand. The potential usage of peptides and other biological polymers as building blocks for mo* To whom correspondence should be addressed at the Department of Molecular Microbiology and Biotechnology, Tel Aviv University, Tel Aviv 69978, Israel. E-mail:
[email protected]. Tel: + 972-3-640-9030. Fax: + 972-3-640-5448. † Tel Aviv University. ‡ Hebrew University of Jerusalem. 10.1021/nl060468l CCC: $33.50 Published on Web 07/07/2006
© 2006 American Chemical Society
lecular recognition is well established, as evidenced by the diversity of biological polymer-based nanometric materials that have been produced.9-27 NMR spectroscopy was used to follow the dynamics of nanotube assembly. The 1H NMR spectra of diphenylalanine, taken throughout the peptide self-assembly process, exhibited a sigmoidal decrease in the integral of the diphenylalanine peaks with time, together with an inversely correlated increase of the water signal. The decrease in the integral of the diphenylalanine signal is consistent with its migration from the liquid phase into an aggregative phase. This occurs in parallel to an increase in the integral of the water signal, probably because the diphenylalanine molecules shed their water of hydration as they assemble, which demonstrates the existence of buried monomers inside the nanostructures that are not accessible to the solvent. Both processes had a lag time of 20 min beyond the initial 10 min, during which the temperature was reduced from 60 to 32 °C and the spectrometer was set up (Figure 1). This behavior is typical of a nucleation-dependent self-assembly process, which occurs in other highly ordered self-assembly processes such as protein crystal growth, microtubule formation, amyloid fibril formation, and others.28 This implies that peptide nanotubes have an ordered and hierarchical assembly mechanism. On the basis of our understanding of the peptide nanotube assembly process, we designed a method (Figure 2) for surface decoration of the peptide nanotube. We used linker peptides, which are able to both associate by weak bonds to
Figure 1. NMR analysis of peptide nanotube assembly. 1H NMR data of 1D spectra of diphenylalanine at the beginning (a) and end (b) of the peptide self-assembly dynamics experiment; and (c) the percentage of peak integrals against time, relative to the measurements at time zero: the curve fits (R2 ) 0.99), a sigmoid curve [y ) 75 + 32/(1 + exp((x - 53)/31))].
Figure 2. Model for fabricating a coaxial nanowire. A model diagram describing (from left to right) the self-assembly of the peptide nanotube; reduction of silver in the hollow pore of the peptide nanotube; binding of linker peptides to peptide nanotube surface; attachment of gold nanoparticles; and electroless deposition of a gold cover over the peptide nanotube using the bonded gold nanoparticles as nucleation sites.
the peptide nanotube surface and covalently bind chemical inserts that are subsequently introduced. For this purpose, three candidate linker peptides were designed: cys-phe-phe (CFF), phe-phe-cys (FFC), and cys-glyser-gly-phe-phe (CGSGFF). These peptides contain the diphenylalanine motif, which can interact with the peptide nanotube surface, and a cysteine residue, which provides the reactive thiol group for the ensuing chemical modifications. Examinations by electron microscopy revealed that none of Nano Lett., Vol. 6, No. 8, 2006
these peptides were able to form peptide nanotubes by themelves, although CFF forms other kinds of assemblies.26 To visualize the binding sites of the linker peptides, we used gold nanoparticles, 20 nm in diameter, which bind rapidly to cysteine thiol groups. These gold nanoparticles can be viewed easily using a transmission electron microscope (TEM). The binding of the linker peptides to the diphenylalanine peptide nanotubes was done using two strategies: (1) The 1595
Figure 3. Electron microscopy analysis of the gold-nanoparticles’ binding process. TEM images of (a) gold nanoparticles; (b) a peptide nanotube; (c) a control of peptide nanotubes that were incubated with gold nanoparticles without the linker, in which only minor binding of gold nanoparticles occurred; and (d) peptide nanotubes that were incubated with linker peptides and then with gold nanoparticles, in which a massive binding of gold nanoparticles occurred.
linker peptides were added to the diphenylalanine solution after 8 h of incubation (after the peptide nanotubes were already self-assembled). A high density of bound gold nanoparticles was observed, even after short incubation periods (5 min), signifying that a large number of linker peptides, which adhered to the nanotubes’ surface, were capable of gold nanoparticle binding. Quantification of bounded linker peptides under these experimental conditions was performed using Ellman’s reagent,29 displaying 50-55% of binding by the various linker peptides. (2) The linker peptides were added to the diphenylalanine solution at time zero of the self-assembly process. Only low densities of bound gold nanoparticles were noted, indicating that a significant decrease in the gold nanoparticles’ binding ability occurred. This may be due to buried linker peptides, which were trapped within the internal layers of the peptide nanotube and therefore could not form the thiol-gold contact. Quantification of bounded linker peptides by the reduction reaction of Ellman’s reagent showed ∼30% less reactivity of the reagent, compared with the first strategy. This phenomenon provided us with a glimpse of the peptide nanotubes’ self-assembly process, suggesting the existence of layers buried deep inside the nanotubes, which are inaccessible to the 20-nm-diameter gold nanoparticles. Using TEM analysis, we confirmed that gold nanoparticles were clearly bound to the surface of the peptide nanotubes. There were no visible differences in the amount of binding or binding patterns among the different linkers. The binding was spread evenly on the surface of the nanotubes. A control, in which no linker peptides were added, showed a very low capability of binding of gold nanoparticles to the peptide nanotubes; this sporadic binding may be associated with the weak chemical activity of the amine termini of the diphenylalanine peptides (Figure 3). Another visible characteristic of the gold nanoparticles binding over the peptide nanotubes’ surface was the formation of linear binding strips. A significant fraction of the gold nanoparticles (∼20%) were bound in linear arrays. This indicates that the packing of the peptide monomers over the surface of the nanotubes is very ordered (Figure 4). The binding and even distribution of the gold nanoparticles on the surface of the peptide nanotube was ideal for site1596
Figure 4. Ordered organization of the nanoparticles on the nanotubes. The red arrows in the TEM image denote “lines” of bounded particles. This indicates that the peptide nanotubes’ assemblies have some level of ordered organization.
Figure 5. Metallic coating and filling of the peptide nanotubes. (a) TEM micrograph of a peptide nanotube covered with a thin and continuous gold coating. (b) Energy-dispersive X-ray analysis of a gold-coated peptide nanotube. (c) TEM micrograph of a silverfilled peptide nanotube covered with only a partial gold coating after an early halt of the gold reduction process. (d) Energydispersive X-ray analysis of coaxial nanowires with a silver inner filling and a gold outer coating.
specific initiation of electroless deposition of metal ions over the peptide nanotubes’ surface. The bound gold nanoparticles gave rise to site-specific nucleation sites for controlled and selective reduction of gold ions over the surface of the peptide nanotubes. To form a thin plating, we used 1.4-nmdiameter gold particles for nucleation in order to achieve a moderate reduction of gold. Examination of the products under TEM verified the formation of metal coatings over the surface of the peptide nanotubes (Figure 5a). The metal coatings were ∼20 nm thick and were present continuously on all of the nanotube parts. Energy-dispersive X-ray analysis confirmed the presence of gold on the peptide nanotube structures (Figure 5b). Nano Lett., Vol. 6, No. 8, 2006
To form trilayered (silver-peptide-gold) coaxial nanowires, we applied a method that was used previously by our group for reducing silver nitrate in the hollow pore of the peptide nanotubes,3 followed by a gold metal coating, as described in this work. To directly confirm the formation of the three layers, we sampled the reaction mixture shortly after initiating the gold reduction process, as a means of enabling only partial metal coating. This partial gold coating enabled us to check for the presence of an inner silver nanowire filling directly by TEM (Figure 5c). Use of energydispersive X-ray analysis confirmed the existence of the silver filling in fully gold-coated peptide nanotubes as well (Figure 5d). In this work, we suggest a different approach for molecular molding and for designing nanoscale heterostructures, based on short peptide recognition elements. We present a general protocol for embellishing the surface of diphenylalaninebased peptide nanotubes, which might be applied similarly in other self-assembled systems. The NMR dynamics data and binding patterns of the linker peptides over the surface of the tubes support the previously proposed model for the tubes’ self-assembly and structure3,26 and provide some insight into the self-assembly process. Acknowledgment. We thank members of the Gazit laboratory for their helpful discussions, and especially Meital Reches for her help in the acquisition of energy-dispersive X-ray analysis data. We thank the Israel Science Foundation (F.I.R.S.T program) for financial support. Supporting Information Available: Materials and methods. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312-1319. (2) Tesla, N. U.S. Patent 514167, 1894. (3) Reches, M.; Gazit, E. Science 2003, 300, 625-627.
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