Engineering diatom peptides for the synthesis of silica nanomaterials

Abstract

The ability to fabricate silica materials with highly organized nanostructures is of increasing demand across the medical, optical, energy, and mechanical fields. Diatoms, a class of eukaryotic algae, produce intricately-patterned silica structures under ambient conditions through a process initiated by post-translationally modified silaffin peptides that nucleate silicic acid. Designing these peptides would enable the production of silica nanostructures with desired properties; however, the functional effects of the modifications are poorly understood. In this thesis, I use Escherichia coli to express and modify recombinant silaffin R5 peptide from the diatom Cylindrotheca fusiformis. A library of 38 enzymes is tested for R5 modifications in vitro, from which active methyltransferases, kinases, acetyltransferases, oxidases, and myristoyltransferases are identified from diatoms, humans, yeast, and bacteria. Modified R5 peptides are used for silica precipitation and the impacts on particle size, shape, porosity, and surface area are quantified. I then used these individually characterized modifications to build synthetic enzyme pathways in vitro and in vivo, and demonstrate that introducing multiple modifications to R5 has additive effects on silica morphology. In the second part of this thesis, I apply the R5 peptide to synthesize silica coated core-shell nanoparticles for a range of core materials (Fe₃O₄ TiO₂, ZnO, HfO₂, and Ta₂O₅), and show that silica shell thickness can be tuned (2.3 - 120 nm) by altering the concentration of R5 used in the reaction. Together, these projects illustrate a design-driven approach for rapidly engineering and synthesizing silica nanostructures and multifunctional composite nanomaterials under ambient conditions, with potential applications in biomedicine, electronics, and photonics.