Microtubule Templated Synthesis of Inorganic Nanomaterials
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Microtubule Templated Synthesis of Inorganic Nanomaterials Andrew K. Boal, Thomas J. Headley, Ralph G. Tissot, and Bruce C. Bunker* Biomolecular Materials and Interfaces, MS1413 Sandia National Laboratory PO Box 5800 Albuquerque, NM 87123 USA ABSTRACT Protein microtubules (MTs) have been used as templates for the biomimetic synthesis of metal oxide, metal sulfide, and metallic nanomaterials. These materials were coated onto MTs via three distinct synthetic pathways: metal ion hydrolysis which yielded iron oxide or zinc oxide-coated microtubules, metal ion/sulfide co-precipitation which yielded zinc sulfide coated MTs, and metal ion reduction which yielded gold-coated MTs. The growth process of metal oxide coating involves heterogeneous nucleation on the MT surface and produces even, microcrystalline films. Metal sulfide and metal coating initially involves the formation of nanoparticle arrays that decorate the MT surface and can eventually lead to either semi- or fully continuous coatings. INTRODUCTION Biomimetic materials synthesis employing biomolecular templates upon which the growth of an inorganic phase is nucleated provides a useful pathway for the synthesis of nanomaterials.[1] Microtubules (MTs) made from tubulin polymerization are an under explored but potentially useful biomimetic synthesis template. The length of the MT can be varied between 1 to 1000 mm,[2] enabling the production of nanowires with aspect ratios as high as 40,000. Another attractive aspect of MTs is their wide range of dynamic behaviors. MTs themselves can be alternatively grown or shortened which, in conjunction with their conjugation to an electronic interface, could be utilized to grow MTs in place. Additionally, MTs can be transported to a desired location in a two-dimensional synthetic system by the motor protein kinesin. These properties combined with the ability to use the MT as a template for the formation of inorganic nanowires will allow active fabrication of electronic interconnects in a microelectronic device. Biomimetic synthetic processes utilizing MTs as templates require several steps to produce a coated MT. MTs are first formed by the guanosine triphosphate (GTP) induced polymerization of tubulin (Figure 1a). While the dynamic nature of MTs is one of their attractive features, these properties can also be detrimental to many potential mineralization processes. For example, several metal ions, including Cd2+ which is of interest in the fabrication of semiconducting materials, are well known to induce the depolymerization of MTs.[3] To overcome this difficulty, gluteraldehyde is used to produce a crosslinked microtubule (CLMT) which is more resistant to metal ion-induced depolymerization (Figure 1b).[4] With a stable template in hand, an inorganic coating reaction can then be used to produce the coated MT (Figure 1c). We have previously demonstrated this process in the preparation of lepidocrocite coated MTs (FeOOH-MTs).[5] FeOOH-MTs were prepared by exposing an anaerobic solution of MTs and Fe2+ ions to air. Oxida
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