Design of Silicon Dioxide Nano Materials and Understanding their Colloidal Behaviors for Sensing and Biological Applications

Abstract

Due to their well-known synthetic strategies and design flexibilities, amorphous silica nanomaterials have been studied in many colloidal application fields with various physical and chemical modifications optimized for specific purposes. Amorphous silica nanomaterials can be prepared with simple hydrolysis, condensation, and polymerization reactions among silica precursor molecules in different aqueous synthetic conditions for the production of an intended design and property. The forming and breaking of the siloxane bonds, which are the infrastructure of the designed silica nanomaterials, are the key factors to define materials’ function and stability in colloidal state. These siloxane bonds are likely to undergo hydrolysis in aqueous dispersion, and the understanding of the physical and chemical changes of the silica nanomaterials in colloidal state can be a proper way to estimate their environmental impact and to evaluate their potential capacities in the related applications. Throughout the thesis, the silica nanomaterials’ changes during colloidal dispersions as well as the effect of the physical/chemical properties on the designed silica materials’ performances on sensing and biological applications were investigated for the purpose of the production of more sustainable and prospective nanotechnology. The treatment of the agricultural plants with silicon has been known to enhance their resistance ability against fungal disease, and the silica nanoparticles have been extensively studied as a strategic tool for more efficient silicon delivery. Chapter One discusses the design of a set of silica nanoparticles and their applications to the plants to obtain the biochemical information behind the particle-plant interactions. Specifically, the particles were chemically tuned to possess different dissolution rates in aqueous media to elaborate the effect of silicon supplying rates on the growth and disease suppression capacity of Citrullus lanatus with Fusarium fungal infection. This work demonstrates that the silicon releasing rates of the silica nanoparticles can be critical for the plants growth under the infected condition, and the silica nanoparticles can be tuned to dissolve at different rates with proposed synthetic designs, which can be utilized for more sustainable and plant-beneficial nanomaterials. Chapter Two assesses the relationship between the silica nanoparticles’ morphology and the maintenance of the surface electric potentials in colloidal state. The ligand- functionalized silica nanoparticles, which possessed the positive zeta potentials in aqueous colloidal dispersion, showed different surface electric potential sustentation behaviors depending on their morphologies. This work demonstrates that the colloidal v stability of this surface-modified silica nanoparticles can be affected by not just the ligand itself but also the degradation of the silica at inner regions. The drug delivery application is one of the most active fields where silica nanoparticles have been applied and studied. Their high colloidal stability, however, can be a problematic for in vivo applications due to the potential bioaccumulation issue. In Chapter Three, a pH-responsive swelling polymer is coated with silica shell to enhance the structural rigidity of the nanomaterials as well as to facilitate their degradation. This silica- coated polymer design proposes the drug delivery nanoparticle platform in which the polymer and the silica can induce synergetic effect to enhance the colloidal stability, drug loading efficiency, and biocompatibility of the nanomaterials. Silica is often used to protect the core materials from aggregation or degradation in many fields of nanotechnology. Chapter Four of this work reviews how the silica shell and the other protecting agents preserve and improve the plasmonic properties of the silver and gold nanomaterials in colloidal state and other environmental conditions. Chapter Five assesses the interaction between a model hydrophobic analyte molecule and a mesoporous silica-coated gold nanorods substrate in a series of the designed colloidal surface-enhanced Raman spectroscopic measurements. Environmental factors, such as temperature and ionic strength, and silica shell characteristics such as pore size and surface hydrophobicity were varied to evaluate their effects on Raman signal intensities of trans-1,2-bis(4-pyridyl)-ethylene molecules and to maximize the signal intensities in this colloidal measurement platform.