Date of Award

8-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Materials Science and Engineering

Advisor

Mefford, Olin T

Committee Member

Ballato , John

Committee Member

Kitchens , Christopher L

Committee Member

Peng , Fei

Abstract

ABSTRACT
Iron oxide nanoparticles (IONPs) have been widely studied in the theranostics application due to their promising magnetic properties, low cytotoxicity and attractive biocompatibility. Despite the numerous studies on the kinetic mechanisms of IONPs synthesis and thus the resulting size, shape and crystallinity; there are still considerable unsolved issues in quantitatively depicting the dependence between particle morphology and the reaction conditions.
To begin to explain some of these phenomena, the kinetic mechanism for the morphology and crystalline changes of IONPs with the ligand/precursor ratio in nanoparticle synthesis was investigated. During the synthesis of nanoparticles via thermal decomposition of iron precursors, the capping ligand-precursor ratio influences the resulting size of the iron oxide nanoparticles. As the molar ratio of aliphatic amines to iron precursor is increased, the average diameter of the synthesized iron oxide nanoparticles decreases. This trend is opposite to previously reported results. We investigated this phenomenon by independently varying the ligand chain length, the ligand-precursor molar ratio, and the degree of saturation of the aliphatic chain. Nuclear magnetic resonance spectra of precursor illustrated the presence of a primary amine peak before heating and the peak absence after heating, potentially indicating that the primary amine acts as reducing agent to promote the decomposition of the iron precursor. We hypothesize that the amine groups play a dominant role in the nucleation of the particles, while the chain length and degree of aliphatic saturation have only a minor effect on particle size. The nanoparticles' size and crystallinity were characterized with high resolution transmission electron microscopy, dynamic light scattering, and X-ray diffraction, and the magnetic properties were characterized by magnetometry.
Known ligand/precursor ratio effects on the IONPs size distribution, here in, we report that the resulting size of iron oxide nanoparticles synthesized by thermodecomposition could be modified by changing the ligand molecule length in the reaction solution. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) measurements show that IONPs diameter increased from 16 nm to 25 nm with ligand length at a low molar ratio of ligand to iron precursor (1:2). However, there was no observable dependence of particle size on the ligand chain length at higher molar ratios (30:1). In addition, particle size evolution differences with the reaction time between different ligand lengths in the solution were verified by dynamic light scattering (DLS) and AC susceptibility. To understand the mechanism of these phenomena and the factors contributing to the reaction, the kinetic process of the particle formation was simulated by the Monte Carlo algorithm. The goal was to investigate the effect of the length of the ligand molecule on the nucleation stage and growth of the particles at different ligand/precursor ratios and reaction time regimes. The subsequent results agreed well with the experimental findings suggesting our hypothesized mechanism of particle growth is correct.
To transfer the synthesized IONPs from a hydrophobic environment into an aqueous system for biomedical applications, we transferred particles from organic solvent to aqueous solution by a one-step approach and investigated the heating efficiency changes of IONPs between in toluene and water in a bio-friendly alternating field (147 kHz and 41 kA/m) using AC calorimetery. Using specific absorption rate (SAR) to evaluating heating efficiency, a maximum SAR value was obtained with particles diameter of 22nm in both toluene and aqueous solution. Those particles with sizes greater or lower than the 22nm particles exhibited lower SAR values which suggests that the 22nm particles are at the optimal size at which the total contribution of the Brownian and the Néel relaxation mechanisms were maximized. It was observed that the SAR value is significantly affected by the concentration of iron in toluene, which is opposite to the published report. This could be due to the interparticle colloidal interactions in the AC field and form the localized ordering structure which could restrain the relaxation of IONPs. A pH dependency of SAR was observed in aqueous solution, which confirms that the pH will tune the surface charge of the nanoparticles and further affect the colloidal stability and SAR value. The results above have the implications for IONPs size control and prediction in synthesis and optimization of IONPs colloidal performance in biomedical applications.

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