Synthesis, Characterisation and Application of Iron Nanoparticles from Organometallic Precursors

Abstract

Magnetic nanomaterials are actively researched due to their size and shape dependent magnetisation. Of particular interest are iron-based nanomaterials which are currently widely investigated as potential magnetic resonance imaging contrast agents, drug delivery vehicles, and as theranostic agents.

This thesis is concerned with the solution phase synthesis of iron/iron oxide core-shell nanoparticles via the hot-injection of iron containing organometallic precursors at various reaction temperatures. Solution synthesis allows for precise control over nanoparticle synthesis utilising changes in reaction temperature, capping agents and reaction time to favour a discrete nanoparticle size or shape. It was observed that the nanoparticle size could be controlled by careful choice of the iron precursor and reaction temperature. The nanoparticle morphology, crystallinity, and chemical composition were studied by transmission electron microscopy (TEM), selected area electron diffraction (SAED), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The magnetic properties of the nanoparticles where characterised using the superconducting quantum interference device (SQUID) and magnetic resonance imaging (MRI).

Chapters 1 and 2 provide an introduction to the background for this thesis, including an extensive literature review on the current state of iron nanoparticle synthesis, and the experimental methods used in this research. The two main synthetic strategies employed were bench-top method and Fischer-Porter bottle method.

In Chapter 3, iron/iron oxide core-shell nanoparticles were synthesised from the precursor (η5-cyclopentadienyl)(η5-cyclohexadienyl)iron(II), [Fe(η5-C5H5)(η5-C6H7)], at reaction temperatures ≤ 200°C. [Fe(η5-C5H5)(η5-C6H7)] has successfully been used to synthesise iron nanoparticles in the Fischer-Porter bottle, however remained to be examined via the bench-top method. The lack of size and shape control of the nanoparticles synthesised, regardless of altering the reaction temperature, capping agent or reaction time was proposed to be due to having insufficient thermal energy at reaction temperatures of ≤ 200°C to induce a rapid nucleation burst.

In Chapter 4, the reaction temperature was increased to 300°C, and [Fe(η5-C5H5)(η5-C6H7)] along with other organometallic precursors were investigated. It was found that at high reaction temperatures, the asymmetry of [Fe(η5-C5H5)(η5-C6H7)] was resulting in a complex decomposition pathway, and the insufficient separation of nucleation and growth stages as the reaction proceeded. Subsequent decomposition of precursors reported in literature yielded monodisperse nanoparticles, and the decomposition of ferrocene and bis(indenyl)iron(II) resulted in both large and small monodisperse iron/iron oxide core-shell nanoparticles respectively. It was concluded that organometallic iron sandwich compounds that had identical ligands resulted in monodisperse nanoparticles due to a more uniform decomposition, and the final size was linked to the stability of the precursor.

In Chapter 5, the synthesis of various pseudoferrocene compounds were investigated as potential iron precursors. The pseudoferrocene compound bis(η5-1,3,5-exo-6-tetramethylcyclohexadienyl) iron(II), [Fe(η5-C6H3Me4)2], was found to be the most suitable precursor for the hot-injection synthesis of 14-16 nm iron nanoparticles due to its relative stability, symmetry and ease of synthesis. The successful synthesis of monodisperse 14 nm iron/iron oxide core-shell nanoparticles in the Fischer-Porter bottle was also achieved.

In Chapter 6, the formation of nanoparticles from [Fe(η5-C6H3Me4)2] was investigated. It was found that the growth temperature had to be finely controlled to ensure no secondary burst nucleation events occurred, resulting in nanoparticle polydispersity. The nanoparticle formation over time was monitored, with the formation of the coreshell structure observed, with fast growth the α-Fe core occurring early on in the reaction. The final nanoparticles had a high magnetic saturation of 148 emu.g-1(Fe), and was proven to be an optimal T2 contrast enhancers for MRI imaging with a relaxivity of 332 mM-1.s-1, three times that for iron oxide nanoparticles.

Finally, Chapter 7 provides the overall conclusions of this research, and the future plans for this work. In particular the chapter discusses the key variables required for the formation of monodisperse iron/iron oxide core-shell nanoparticles from organometallic sandwich compounds with a focus on the precursor structure and growth temperature.