Date of Award

5-13-2011

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Plant, Soil & Insect Sciences

First Advisor

Baoshan Xing

Second Advisor

Stephen Simkins

Third Advisor

Richard W Vachet

Subject Categories

Plant Sciences

Abstract

Toxicity of nano-scaled Al2O3, SiO2, TiO2 and ZnO to bacteria (Bacillus subtilis, Escherichia coli and Pseudomonas fluorescens) was examined and compared to that of their respective bulk (micro-scaled) counterparts. All nanoparticles (NPs) but TiO2 showed higher toxicity than their bulk counterparts. Toxicity of released metal ions was differentiated from that of the oxide particles. ZnO was the most toxic among the three NPs, causing 100% mortality to the three tested bacteria. TEM images showed attachment of NPs to the bacteria, suggesting that the toxicity was affected by bacterial attachment.

The effects of oxide NPs on bacteria cells and bacterial surface biomolecules were studied by FTIR spectroscopy to provide a better understanding of their cytotoxicity. Lipopolysaccharide (LPS) and lipoteichoic acid could bind to oxide NPs through hydrogen bonding and ligand exchange, but the cytotoxicity of NPs seemed largely related to the function-involved or structural changes to proteins and phospholipids. The three NPs decreased the intensity ratio of β-sheets/α-helices, indicating protein structure change, which may affect cell physiological activities. The phosphodiester bond of L-α- Phosphatidyl-ethanolamine (PE) was broken by ZnO NPs, forming phosphate monoesters and resulting in the highly disordered alkyl chain. Such damage to phospholipid molecular structure may lead to membrane rupture and cell leaking, which is consistent with the fact that ZnO is the most toxic of the three NPs.

LPS and PE are amphiphilic biomolecules that are major constituents of the outer membrane of Gram-negative bacteria. Their micelles and vesicles were studied as model cell membranes to evaluate NP effects on membrane construction. The adsorption of polysaccharides on Al2O3 and TiO2 NPs dispersed LPS vesicles and micelles. LPS coated Al2O3 NPs, while it caused the aggregation of TiO2 NPs according to atom force microscopy images. Desorption from the two NPs was slow due to the firm hydrogen bonding. For PE, Al2O3 NPs induced large multilamillar vesicles, while ZnO NP converted vesicles to tiny aggregates due to molecular structure breakup. PE stability in solution was disturbed by adding NPs, but its stability was enhanced by increasing pH. The electrostatic force was the determining factor for the vesicle stability.

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