Bio-inspired oil-core silica-shell nanocapsules for controlled-release applications

Abstract

Silica nanocapsules having core–shell architecture are of great interest in many current and emerging areas of technology. The core provides high-capacity loading of various actives, while the silica shell serves as protective envelope and diffusion barrier enabling controlled release of the active. Currently, production of silica nanocapsules is mainly based on templating methods using chemical surfactants to stabilize the template and to induce hydrolysis and condensation of silica precursors forming solid silica shell surrounding the template. Major drawbacks with the current methods are extreme pH conditions and/or elevated temperatures involving toxic chemicals that might pose undesirable environmental side effects and require tedious procedures for removing the chemicals. Inspired by Nature, biomimetic approaches have recently been developed to synthesize silica-based nanomaterials under mild physiological conditions through the use of biomolecules (i.e., peptides and proteins). This contrast in processing conditions and the growing demand for benign synthesis methods having minimum environmental risks have spurred much interest in biomimetic approaches. However, their utility is limited to biosilicification in bulk aqueous solution or at solid–liquid interfaces. There is no current biosilicification at oil–water interfaces, and thus fabrication of oil-core silica-shell nanocapsules using biomolecules has not been realized yet. This thesis addresses the question on benign synthesis method for forming biocompatible oil-core silica-shell nanocapsules, by designing and utilizing biomolecules, in lieu of chemical surfactants, to facilitate formation and stabilization of oil droplets and to direct nucleation and growth of silica shells at the oil–water interfaces. The major outcomes of the work presented in this thesis are: (1) development of a designed bifunctional modular peptide, SurSi, comprising a surface-active module and a biosilicification-active module that led to novel emulsion and biomimetic dual-templating approach for making oil-core silica-shell nanocapsules with tunable silica shell thickness; (2) facile encapsulation and sustained release of model insecticide, fipronil, from the silica nanocapsules in vitro in water and in vivo against a model insect, termite; (3) development of a designed bifunctional modular protein, D4S2, for making silica nanocapsules, thereby extending conceptual design of SurSi peptide, so that the protein could be produced renewably through biological expression in industrially-relevant, microbial cell-factory Escherichia coli (E. coli); and (4) large-scale production of D4S2 protein for synthesis of fipronil-encapsulated silica nanocapsules which could then be used for termite field trial. This work introduces, to the best of our knowledge, the first design and use of bifunctional modular biomolecules for making oil-core silica-shell nanocapsules. The dual-templating method developed in this thesis represents a new strategy for forming silica nanocapsules using components and processes expected to have minimal environment footprint. Active molecule can be easily encapsulated in silica nanocapsules with high encapsulation efficiency, by directly dissolving the active in the oil core prior to silica shell formation, and then released controllably from the nanocapsules with the release kinetics depending on the silica shell thicknesses. Furthermore, the use of bifunctional modular protein that can be produced through E. coli expression system has opened opportunities for sustainable and scalable bioprocess routes to produce oil-filled silica nanocapsules, thus makes the platform developed in this thesis suitable for large-scale applications especially in the fields of biomedical and agricultural applications that demand biocompatibility, high encapsulation efficiency and controlled-release properties.