Synthesis of Plasmonic Nanoparticle Superlattices Using DNA-Mediated Assembly and Directional Entropic Forces

The assembly of inorganic nanoparticles into ordered arrays for applications in functional devices has attracted significant interest in the field of nanoscience in recent years. This is especially true for optical metamaterials, which have the potential to exhibit properties that do not exist in nature. However, bottom-up methods utilizing colloidal metal nanoparticles have not been fully explored due to the difficulty of creating well-defined hierarchical structures in which the particle position and crystallographic symmetry can be precisely controlled. To this end, the work of this dissertation seeks to enhance the understanding of the rational design and synthesis of nanoparticle superlattices for the purpose of creating `designer materials' that exhibit predetermined properties. Chapter 1 describes some of the emergent properties and potential applications of noble metal nanoparticle superlattices as well as the state of the art in nanoparticle assembly. Chapter 2 explores the emergent optical properties of silver nanoparticle superlattices synthesized using DNA-mediated assembly and predicts with electrodynamics simulations that the assemblies should exhibit optical metamaterial properties including Epsilon-Near-Zero (ENZ) behavior. Heterogeneous binary superlattices consisting of silver and gold nanoparticles with various crystallographic symmetries are also investigated. In Chapter 3, a novel class of gold-free spherical nucleic acids based upon biocompatible silica shells is presented in order to avoid concerns about the long-term toxicity of gold nanoparticles used for biological applications such as gene regulation. Chapters 4 and 5 explore the assembly of anisotropic gold nanoparticles, which have the potential to display rich assembly behavior due to their reduced symmetry, using a novel directional entropic force approach (DEFA). In this method, charged surfactant micelles induce attractive depletion forces that cause the anisotropic nanoparticles to assemble into non-close-packed superlattices in solution with their facets aligned. Furthermore, the assemblies are reversible and tunable through modification of several experimental parameters. As a whole, the work reported in this dissertation serves to gain a deeper understanding of the relationship between nanoparticle building blocks, the colloidal crystals they form, and the emergent properties they exhibit, thus allowing for a more rational approach to the synthesis of designer materials.