In Situ Polypeptide Directed Silica Biomineralization
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In Situ Polypeptide Directed Silica Biomineralization Diana D. Glawe,1 Francisco Rodríguez, 2 Morley O. Stone,2 and Rajesh R. Naik 2 1 Department of Engineering Science, Trinity University, San Antonio, TX, USA. 2 Materials Directorate, Air Force Research Laboratories, Wright-Patterson AFB, OH, USA. ABSTRACT Silica-precipitating polypeptides were deposited onto an indium tin oxide (ITO) surface from bulk solution with and without the assistance of an externally applied electrostatic field. Exposure of the peptide-coated surface to an alkoxide precursor produced biosilica structures that were securely attached to the electrode surface. The silica morphologies resulting from the test cases using an externally applied electrostatic field during peptide deposition were distinct from the morphologies resulting from cases without an applied field. The silica morphologies observed on the ITO surface were different from the usual silica morphology resulting from static conditions. Peptide size was also shown to influence resulting biosilica morphology. The experimental results presented herein demonstrate the feasibility of creating biosilica nanostructures with controlled morphologies using polypeptides in vitro. INTRODUCTION The impetus of peptide- mediated silica research stems from the desire to mimic and leverage nature's ability to create repeated silica structures with high fidelity to enable bioengineering of materials and devices for opto- mechanical and biomedical applications, among others. Numerous examples of nanopatterning and nanostructure are commonly found in nature—mostly apparent in marine diatoms and sponges [1,2]. The diatom cell walls are considered a paradigm for the controlled production of nanostructure silica. The formation of the nanostructure silica in vivo is a complex process involving silica-templating proteins within the silica deposition vesicles (SDV) that result in the precipitation of silica. In vitro, peptides such as silcateins, silaffins and poly-L- lysine have been shown to precipitate silica from a silane precursor [3,4,5]. In addition, previous studies have demonstrated the impact of chemical and physical influences on the morphological outcome of in vitro peptide- mediated biosilica [6,7]. The experimental results presented herein demonstrate the deposition of polypeptides onto ITO surfaces in the presence and absence of an externally applied electrostatic field, followed by in situ peptidemediated biosilica formation. EXPERIMENTAL DETAILS An electrostatic field applied externally between two flat electrodes was used to induce electrophoresis [8] of polypeptide molecules suspended in a sodium phosphate buffer solution, yielding an electrophoretic- mediated deposition of polypeptides on a cathode electrode surface. (Figure 1A & B). A highly polished flat copper electrode with a diameter of 2.5cm was positioned at a height (h) approximately 1mm above the flat ITO-coated slide. Predetermined voltages (V) in the range of 1-12 volts were applied between the electrodes, driving
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