| dc.description.abstract | Iron oxide nanoparticles (IONPs) with desirable surface chemistry display many unique properties that can be applied in modern biomedical applications, including cancer diagnostics, magnetic resonance imaging, hyperthermia, drug delivery system, and cell separation among others. However, the performance of IONPs in these biomedical applications depends on their particle size and particle size distribution (PSD). These biomedical applications demand a smaller size and narrow particle size distribution of IONPs to possess uniform physical and chemical properties. High surface to volume ratio and chemical activity causes aggregation of IONPs which increases their stability. To address these challenges, the surface of IONPs is covered with some non-toxic coating to enhance their dispersion and bio-compatibility.
Silica has proven to be a good coating material for IONPs since it increases their stability by screening the magnetic dipole interactions of the neighbouring particles. Several studies have been conducted for coating the surface of IONPs with silica using modified stöber's method. These studies report different reaction times and conditions to synthesize silica coated IONPs (SIONPs). In such a typical method, nature of solvent, ratio of silica precursor, and IONPs, concentration of ammonium hydroxide, surface chemistry of IONPs and reaction temperature are important parameters that affect the particle size and PSD of SIONPs. Different hypotheses regarding the effect of these parameters on the particle size and PSD of SIONPs have been discussed in literature.
Most of the research work discusses the effect of these parameters at similar conditions at reaction temperature higher than room temperature. Apart from this, there is a lack of research on the different types of morphologies that develop during the process. Further, there is little available literature about the changes in the kinetics of the process in response to reaction parameters.
This study discusses the effects of nature of solvent, surface chemistry of IONPs, mass of IONPs and concentration of ammonium hydroxide on the particle size, PSD and morphology of SIONPs, and it further presents interesting insights into kinetics of synthesis of SIONPs in response to these parameters. Apart from that, concentration of ammonium hydroxide used in this study is higher than usually reported in the literature. In addition to particle size and PSD, interesting morphologies of SIONPs have been observed using scanning electron microscope (SEM).
Two types of IONPs with different surface chemistry were synthesized via co-precipitation and were coated with silica. A series of reactions was conducted to determine the reaction time. Afterwards, a three-level full factorial experimental design was carried out to study the effect of nature of solvent, mass of IONPs, surface chemistry of IONPs and ammonium hydroxide concentration on the particle size, PSD and morphology of SIONPs by using SEM. In the next section, changes in the kinetics of the silica coating of IONPs are studied due to change in these parameters by measuring time-based changes in product's dry mass.
It was observed that iso-propanol gives larger particle size and broader PSD of SIONPs. Citrate coated IONPs also give large size and narrow PSD. Increase in the mass of IONPs increases the size of SIONPs and increase in the concentration of ammonium hydroxide decreases the size and increases PSD of SIONPs. Four different morphologies of SIONPs, i.e, spherical, irregular, agglomerates and incompletely fused, were observed in this study. Irregular shaped particles were formed when the concentration of ammonium hydroxide was less than 0.98 M. Incompletely fused particles were observed when the concentration of ammonium hydroxide was 1.53 M and ethanol was used as solvent. Spherical particles were formed when the ammonium hydroxide concentration was greater than or equal to 0.98 M. However, agglomerates were formed in all the experiments.
From the kinetics study of the process, the dry mass of product in iso-propanol was measured to be less than in ethanol, but the percentage of mass conversion was same in both solvents. Increase in the mass of IONPs increased the mass of magnetic particles of product, but the total dry mass of product remained same. Increase in the concentration of ammonium hydroxide increased the rate of reaction.
This study although performed within a stastical design, design parameters can be expanded to understand the effect of parameters such as temperature and ratio of silica precursor etc. Further it is suggested to continue to look at time-based evolution of particle size using SEM. This study showed that the size and PSD of SIONPs can be tuned by optimising these parameters depending on the end-use of the SIONPs in biomedical applications. | |