Biologically Inspired Synthesis Route to Three-Dimensionally Structured Inorganic Thin Films

Hindawi Publishing Corporation Journal of Nanomaterials Volume 2008, Article ID 352871, 6 pages doi:10.1155/2008/352871 Review Article Biologically Inspired Synthesis Route to Three-Dimensionally Structured Inorganic Thin Films Birgit Schwenzer1, 2 and Daniel E. Morse1, 2, 3 1 Institute for Collaborative Biotechnologies, University of California, Santa Barbara, CA 93106-5100, USA 2 California NanoSystems Institute, University of California, Santa Barbara, CA 93106-5100, USA 3 Department of Molecular Cellular and Developmental Biology, University of California, Santa Barbara, CA 93106-9610, USA Correspondence should be addressed to Prof. Daniel E. Morse, d Received 2 October 2007; Accepted 26 December 2007 Recommended by Ping Xiao Inorganic thin films (hydroxide, oxide, and phosphate materials) that are textured on a submicron scale have been prepared from aqueous metal salt solutions at room temperature using vapor-diffusion catalysis. This generic synthesis approach mimics the essential advantages of the catalytic and structure-directing mechanisms observed for the formation of silica skeletons of marine sponges. Chemical composition, crystallinity, and the three-dimensional morphology of films prepared by this method are extremely sensitive to changes in the synthesis conditions, such as concentrations, reaction times, and the presence and nature of substrate materials. Focusing on different materials systems, the reaction mechanism for the formation of these thin films and the influence of different reaction parameters on the product are explained. Copyright © 2008 B. Schwenzer and D. E. Morse. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Nanostructured thin films as well as thin films with no surface texture are currently used for many applications such as electro-optical devices [1], batteries [2], solar cell technology [3], and gas sensors [4]. These applications require high purity, defect-free materials. The use of metal organic chemical vapor deposition (MOCVD), molecular-beam epitaxy (MBE), or liquid-phase epitaxy (LPE) techniques represents the state-of-the-art fabrication processes for such highquality semiconductor thin films. However, in recent years, lower-cost approaches to fabricate these films have emerged, among them hydrothermal [5] and electrochemical [6] synthesis methods. In addition, synthesis techniques that mimic biomineralization processes have received much attention because of the inherently benign and malleable conditions under which biominerals are produced. An increasing number of research groups have been studying biomimetic [7] or biologically inspired [8] pathways towards preparation of semiconductor materials in the past few years using biological principles of materials formation. Biomineralization, including the formation of bones, teeth, shells, or the silica skeletons of marine sponges, takes place in vivo under ambient conditions. These biomineralization processes often produce highly ordered structures on the nanoscopic as well as the macroscopic scale, and generally nano- or micrometer-sized defects within these structures do not propagate, but are corrected on a short-range scale. The lessons learned from these biomimetic systems follow a general model [9, 10]: the reaction environment is anisotropically preorganized; this provides a framework from which the material can grow in an anisotropic manner. Kinetic control is imposed at the stage of nucleation, typically at an interface in the reaction environment, by closely controlling the supply of molecular precursor chemicals. In addition, crystal growth is vectorially regulated by a template, as seen both in natural systems [11–13] and artificial systems such as those based on virus protein coats [14], ferritin-like cage proteins [14, 15], and others [16]. Based on previous research in our group on silicatein (for silica protein) [13, 17, 18], an enzymatic biocatalyst discovered in the biologically fabricated needles of silica made by a sponge (Tethya Aurantia), we used these biomimetic 2 concepts for the development of a bioinspired synthesis route to nanostructured inorganic materials. We observed that silicification in Tethya Aurantia is controlled by occluded protein filaments (silicateins) that serve as both templates and catalysts for the deposition of opal-like SiO2 [17–19]. In vitro experiments with isolated silicatein filaments yielded SiO2 nanoparticles similar to those formed in vivo, when the proteins were exposed to silicon tetraethoxide (TEOS) [18]. Recognizing silicatein to be a specialized member of the superfamily of hydrolytic enzymes [17, 18], Zhou et al. used site-directed mutagenesis to confirm the proposition that the sidechains of two specific amino acids, serine and histidine, play an essential role as partners in the catalytically active center of silicatein [19]. Based on these findings, the following mechanism for the formation of SiO2 was proposed [18]: resulting from the close proximity of the hydroxyl group of the serine, the hydrogen atom of this functional group becomes a bridging hydrogen atom linking the oxygen atom of the hydroxyl group and the nitrogen atom in the 3 positions of the imidazole ring on the histidine. This partial withdrawal of the proton leads to an increased nucleophilicity of the oxygen atom on the serine, facilitating a nucleophilic addition onto the electron-deficient silicon center of TEOS. An EtO− group is cleaved from the TEOS precursor, reacting with the electron-deficient bridging hydrogen to form EtOH, and the addition of a water molecule then initiates hydrolysis to yield silanol, with restoration of the enzyme’s catalytic center with its –CH2 –O· · · H· · · N– bond. Subsequently either further hydrolysis takes place, or several silicon alkoxide molecules react via condensation to form SiO2 as the final product [18]. Mimicking this hydrolysis/condensation mechanism with its –CH2 –O· · · H· · · N– sequence as the reactive center for catalysis, silica, silsesquioxanes, and metal oxide materials has been prepared using block copolypeptides of the essential catalytic amino acids [20] or small functionalized molecules [21] in an effort to remove the biomolecule from the process. In a further translation step, self-assembled monolayers (SAMs) of suitable hydroxyl- and imidazole-terminated molecules have been patterned on gold-coated silicon wafers [22]. Similarly, the same catalytic activity has been achieved by functionalizing gold nanoparticles with the respective surface coating to mimic the catalytic center of silicatein [23]. While these approaches were successful for the preparation of a variety of metal oxide materials, such as TiO2 [24] and Ga2 O3 [25], the use of an organic template to direct synthesis is not feasible for d (...truncated)