Metal Oxide Nanoparticles in Complex Environments: Characterization, Implications and Biomolecule-Nanoparticle Interactions
Nanoscience and nanotechnology are research areas that have shown great promise towards addressing clean and sustainable energy, environmental protection, and human health. Metal oxide nanoparticles are widely used in various applications, including removing pollutants from contaminated water, tracking cancer cell growth, targeted drug delivery. These nanoparticles are highly reactive, and their abundance in the environment brings potential concerns to their exposure, leading to increased interactions with biomolecules that have impactful environmental and health effects. Ecological systems have multi-components, including natural organic matter, oxyanions, and biological macromolecules; biological systems also contain complexity as proteins and nutrients can all be found at the nanomaterial-water interface upon nanoparticle exposure. Although significant research has been pursued on the surface transformations of metal oxide nanoparticles, multi-component adsorption kinetics, changes in adsorbate structure, and the impacts on the nanomaterial properties in complex environments remain unclear.
Once nanoparticles are introduced in complex aqueous biological and environmental systems, proteins adsorb onto their surfaces and form a dynamic layer termed "corona." Newly occurred corona may change the nanoparticle interfacial state and its biological and ecological identity. If altered, the new identity influences the nanoparticle fate within the surrounding complex media. Details of protein and amino acid (building block of proteins) interactions with nanoparticles and substantial structural change on nanoparticle surfaces remain unclear. These processes can be affected by various factors due to the complexities of nano-bio surface interactions. Therefore, it is necessary to study multiple parameters individually, and a systematic study on the impacts of influential factors on the adsorption at the nano-bio interface is strongly desired.
The research presented in this dissertation pursues a greater understanding of metal oxide nanoparticle characterizations, implications, and biomolecule-nanoparticle interactions from studies of amino acid and protein adsorption. Nanoparticle- and environmental-related factors, including effects of pH, nanoparticle-type, biomolecule concentration, pre-adsorbed phosphate and lipopolysaccharides, and nanoparticle production in a workplace environment (occupational health study), were investigated. We studied the influencing factors of the complex environment individually to examine each aspect in detail. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR), as well as various microscopic and spectroscopic tools, were employed to help better understand the impact of these factors.
In this dissertation, adsorption of α-amino acids, lysine, glutamic acid, glycine, and serine, onto TiO2 nanoparticles in buffered solutions was determined. The predominant molecular surface species and the adsorption affinity were highly pH-dependent. Adsorption of lysine and glycine were increased proportionally with changes in pH, whereas glutamic acid adsorption decreased with increasing pH. We attributed these differences to the functional groups of different species and the TiO2 surface charge at each pH. Furthermore, the effects of nanoparticle type and amino acid concentration on the mechanisms of amino acids, lysine, glutamic acid, aspartic acid, and arginine, adsorption on α-Fe2O3 nanoparticles were investigated. The detailed chemistry in the adsorption processes implied the formation of outer-sphere and inner-sphere complex differences between different nanomaterials.
Combined in-situ ATR-FTIR and curve-fitting provides insights and a greater understanding of changes in secondary structures of bovine serum albumin (BSA) and βeta-lactoglobulin (β-LG) upon adsorbed onto α-Fe2O3 nanoparticles in the presence and absence of co-adsorbed phosphate. The results indicated that structural changes were time-dependent, and the existence of pre-adsorbed phosphate influenced adsorption and desorption kinetics. An additional part of this work showed that pre-adsorbed lipopolysaccharide additionally played a role in the interaction of Immunoglobulin G (IgG) adsorbed onto α-Fe2O3 nanoparticles. In agreement with the β-LG adsorption, a significant change in Amide I/II ratio was observed for adsorbed IgG, indicating changes in the protein secondary structure compared to the solution phase. Deconvolution analyses revealed that the α-helix content of the adsorbed IgG was higher than the unbound conformation in the presence of lipopolysaccharides.
Moreover, to complement our understanding of protein and amino acid adsorption, we also investigated airborne nanoparticle presence in different production sites in an occupational health study. Identified nanoparticles in these settings were characterized by two forms: sub-micrometer fractal-like agglomerates from activities such as welding; and super-micrometer particles (nanoparticle collectors) with nanoparticles coagulated on their surfaces. These agglomerates were proposed to affect deposition and transport inside the respiratory system. The respirable incidental nanoparticles would have corresponding health implications regarding their primary and/or secondary sites of uptake.
Overall, the research in this dissertation provides essential insights into understanding the behavior of metal oxide nanoparticles in complex environments. Studies on amino acid and protein adsorption, along with the detailed characterization of the nano-bio interface with spectroscopic and microscopic methods, allowed us to understand the effects of a multitude of influences on biomolecule-nanoparticle surface interactions.