Biological approaches to synthesis and assembly of semiconductor and metallic nanomaterials

Sweeney, Rozamond Yvonne
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The goal of this work is to use proteins, viruses, and whole organisms to direct the growth and assembly of semiconductor and metallic materials. The motivation for this work was to find a new way to build inorganic materials and devices with greater ease, more precise control, and smaller features than is possible with current synthetic methods. A biological method to efficiently synthesize large quantities of cadmium sulfide nanocrystals in the bacteria E. coli was discovered. The physical properties of the nanocrystals were characterized by electron microscopy and photoluminescence spectroscopy. Next, the genetic and physiological parameters that play a role in the synthesis of cellular nanocrystals were explored. In particular, a strain and growth phase dependence for E. coli nanocrystal formation was determined, indicating that the capacity for nanocrystal synthesis in E. coli is intrinsic and can be genetically controlled. This result is a first step towards understanding this mechanism of biologically-encoded nanomaterial synthesis, and it suggests the possibility of genetically engineering E. coli to produce nanocrystals with precise control over composition, size, and crystal type. Recently, it was discovered that filamentous viruses can be genetically engineered to direct the formation of semiconductor and magnetic nanowires. To follow-up on this project, a method for precisely directing the assembly of the viruses was developed. In order to begin ordering the viruses, the viral coat proteins were engineered to display a type of protein domain, called a leucine zipper, which can form non-covalent dimeric, trimeric, or tetrameric interactions with other leucine zippers. The leucine zipper, attached at the ends of the virus, caused individual viruses to adhere to each other end-to- end, producing one-and two-dimensional arrays. This method was also shown to be an effective way to alternate assembly of different types of viruses. By controlling the placement of the virus-templated nanowires from the bottom-up, the nanowires might become technologically useful for applications that require precise ordering, such as electronic and photonic circuits, sensors, or liquid crystal displays.