Structural Engineering of Carbon and Metal Nanostructures For Antibacterial Applications
Abstract
ABSTRACT
STRUCTURAL ENGINEERING OF CARBON AND METAL NANOSTRUCTURES FOR ANTIBACTERIAL APPLICATIONS
by
Mauricio D. Rojas-Andrade
Antibiotic resistance is a particularly alarming issue in world health today, as the rise and prevalence of antibiotic-resistant microorganisms significantly increases death rates and costs of treatment in even the most developed nations. According to the World Health Organization, many countries around the world have observed last-resort antibiotics to be ineffective in over half of patients afflicted by common pathogenic bacteria such as Escherichia coli and Staphylococcus Aureus, necessitating the search for novel antibacterial agents. Recently, nanostructured materials have been utilized for this application, with promising results observed for a wide variety of different compositions and morphologies. This has prompted significant research efforts toward the understanding of the antimicrobial activities of nanostructured materials in order to determine the nature of their unique cytotoxic mechanisms and consequently, the root of their antibacterial efficacy. This dissertation presents the antibacterial activities of novel carbon and metal nanostructures, focusing on the connection between their structural characteristics and their mechanisms of cytotoxicity.
In the first chapter, the antibacterial activity of silver nanostructures synthesized by a green, photochemical method is reported. By utilizing high-resolution transmission microscopy (HRTEM) and x-ray diffraction (XRD), a correlation between the surface morphology and crystal structure of silver nanostructures to their antibacterial activity is established. Silver nanostructures structures composed of (111) faceted surfaces are proposed to be more cytotoxic towards bacterial cells due to slow oxidation and fast dissolution kinetics outside and inside bacterial cells respectively. This chapter develops the foundation for silver nanostructure toxicity, with the fundamental mechanisms being applicable to all metal nanostructures.
In chapter 2, the antibacterial activities of Ag, Cu, and bimetallic, AgCu alloy nanoparticles is presented. A comprehensive characterization of Ag, Cu, and AgCu alloy nanoparticle structures is first presented, followed by a thorough analysis of their antibacterial activities. AgCu alloy nanoparticles with an average size of ~5 nm and an equal composition of Ag and Cu were found to be the most effective at inhibiting bacterial growth. The mechanisms of Ag, Cu, and AgCu alloy nanoparticles cytotoxicity is then further investigated using fluorescence microscopy and electron paramagnetic resonance (EPR) experiments. AgCu alloy nanoparticles are concluded to exhibit their marked activity due to enhanced reactive oxygen species (ROS) generation resulting from increased Fenton reactions catalyzed by copper species stabilized in the homogenous bimetallic alloy structure.
Finally, in chapter 3, the antibacterial activity of graphene oxide quantum dots (GOQD) is reported. The as-prepared structures were synthesized through an established top-down approach, and a sodium borohydride-reduced derivative (rGOQD) was synthesized using these as the precursor. Using a variety of spectroscopic techniques, the structural properties are characterized and differences between as-prepared and reduced GOQD established. Their cytotoxicity toward bacterial cells with and without light irradiation is presented, with GOQDs demonstrating apparent activity under dark conditions, and rGOQD only under light irradiation. A mechanism of cytotoxicity and phototoxicty is proposed, which can be used to establish a foundation by which the cytotoxicity of all carbon nanostructures can be understood.