Engineering of Spiky Hedgehog Particles

Abstract

Hedgehog particles (HPs) represent a platform for engineering dispersion stability by marked reduction of van der Waals forces from nanoscale spikes. This dissertation utilizes the dispersion stability of HPs in a broad array of applications including sensing in biofluids, catalysis in high ionic strengths and nonpolar environments, and for the creation of stimuli-responsive materials. New chemistries of HPs including development of core materials, spike materials, and nanoparticle coatings have been utilized to achieve functional properties. In chapter II, we introduced functional sensing nanoparticles and polyelectrolytes through the deposition of layer-by-layer nanoparticle films on HPs. Gold-modified HPs were used to overcome the problem of colloidal aggregation in surface enhanced Raman spectroscopy probes resulting in an order of magnitude enhancement in sensing capability directly in biofluids. In this study, we also explored the limits of engineering dispersible structures, demonstrating that nanostars, did not exhibit strong enough surface corrugation and very thick branching nanoparticle and polymer layers could disrupt the dispersion stability on HPs. HPs originally, consisted of commercial carboxylated polystyrene cores. For catalysis and other applications, the production of both more chemically resistant and lab made materials is required. In chapter III, we developed a method utilizing layer-by-layer assembly to engineer microscale lab-made SiO2 cores with nanoscale ZnO spikes that can be dispersed in both high ionic strength and nonpolar environments. Electrolyte environments enable control of product selectivity and yield but see little use with dispersible heterogeneous catalysts due to poor catalyst stability. SiO2 HPs in concentrated electrolytes allowed for control of product selectivity and greatly enhanced yield in the photooxidation of 2-phenoxy-1-phenylethanol. SiO2 HPs can engineer different reaction pathways in a high ionic strength environment. Chapter IV involved the development of active core materials with complementary properties and multifunctional core and spike materials. Omnidispersible HPs carrying stiff ZnO nanospikes were prepared with Fe2O3 hematite microcubes, Fe3O4 magnetite microcubes, hollow Au microspheres, and hollow TiO2 microcubes. Inorganic core HPs maintained dispersion in a wide array of nonpolar environments and exhibited enhanced thermal and chemical stability. In a water and cyclohexane emulsion system, hematite HP proceeded in a novel chemical pathway to produce cyclohexene oxide, a valuable intermediate. In Chapter V, a tunable polymer-based shell was developed for control of stability based on environmental conditions. In order to create a flexible polymer-based HP or Tendril particle (TP), polyallylamine films were crosslinked with glutaraldehyde. Hollow, flexible polymer spikes with controlled zinc oxide content were formed which can be loaded with small molecules and nanoparticles. TPs consisted of a flexible shell but retained remarkable dispersion properties including in heptane and high ionic strength media. Through addition of Poly(N-isopropylacrylamide-co-acrylic acid) (PAA-NIPAM) and dopamine subunits, controlled aggregation was observed in response to high temperature and pH respectively. In conclusion, this thesis centers on the development of omnidispersible colloids that can function in high ionic strength and nonpolar environments. We utilize these colloids to develop new chemical pathways in these complex fluid environments and demonstrate their ability to function in sensing in biofluids and as an environmentally responsive material.