Characterisation of magnetic nanoparticles and their evaluation for use in hyperthermia studies
Creators: name: Tonkin, David
Publisher: name: RMIT University
Year: 2024

description: There is a growing interest in the use of magnetic nanoparticles as agents for hyperthermia. Within the vast body of research, iron oxide nanoparticles are the most intensly studied due to their known metabolic pathways, low toxicity and suitable magnetic properties. In this thesis, however, novel alloys of FePt and CoFePt doped with silver have been chosen for hyperthermia studies, since their crystallinity, and subsequent magnetic properties can be easily fine tuned, allowing for versatile nanomaterials for RF heating applications. <br><br><br>The FePt and CoFePt magnetic nanoalloys utilised in this work were synthesised using a reverse micelle technique in which the reduction of Fe<sup>3+</sup>, Co<sup>2+</sup> and Pt<sup>2+</sup> cations, and the Ag<sup>+</sup> dopant was achieved with sodium borohydride. The resulting “as-­‐synthesised” nanoparticles were harvested by liquid-­‐liquid extraction, purified, and annealed at temperatures up to 600 °C in a tube furnace under a reducing atmosphere of 5% v/v H<sub>2</sub>. The effects of the thermal annealing were investigated by means of x-­ray diffraction, where it was found that the nanoparticles underwent several crystallographic phase changes, from chemically disordered, to pseudo cubic structures of A1 configuration, through to face centred tetragonal structures of L1<sub>0 </sub>conformation. The magnetic characteristics of these phases were determined by SQUID magnetometry, where samples of para-­ superpara-­ and ferro-­magnetic nature were able to be tailored by changing the inherent crystal structure. The effect of Ag dopant was examined, and, found to reside in the face-­‐centred cubic phases of FePt and CoPt, forming a strained cubic system with increased magnetic coercivity. <br><br>A novel differential adiabatic micro-­calorimeter was constructed that accounted for many shortcomings that have been reported in the literature, such as eddy current self heating of thermocouples and convective heat transfer to the sample from the electromagnet. A least-­squares lumped parameter model of the system was employed to analyse the data from a commercial magnetite sample, which overcame many of the uncertainties involved in calculating the specific absorption rate (SAR) of the nanoparticles in the AC magnetic field by the conventional <em>initial slope</em> method (ΔT/Δt).<br><br><br>It was found that the power output of the superparamagnetic iron oxide nanoparticles (SPION) in the viscous SPAN85 suspension medium increased linearly with nanoparticle mass, consistent with a Néelian heating mechanism as the predominant process. <br><br>The SAR values obtained from a commercial magnetite sample were consistent with others reported in the literature, but are believed to be more accurate after accounting for the aforementioned shortcomings. Studies of the literature revealed that SAR values are unique to each calorimeter, with myriad parameters effecting the SAR for a given material, such as particle size, viscosity of the suspension medium, the volume and geometry of the calorimeter vessel, the temperature recording method, the SAR calculation technique (initial slope or other) and the strength and frequency of the applied field, which make comparisons of SAR values from different research groups difficult. <br><br>Experimentally derived SAR values were determined, for the first time, for two selected alloys; a superparamagnetic alloy consisting of silver doped FePt annealed at 300 °C, and a weakly ferromagnetic alloy consisting of silver doped CoFePt annealed at 500 °C. The observed SAR values for these two alloys were lower than those of the commercial magnetite, which was attributed to the lower magnetic susceptibility of the superparamagnetic FePt alloy when compared to the commercial magnetite, and, for the ternary FeCoPt alloy, the limited amount of hysteretic heating from the very small amplitude of the magnetic field used in the calorimeter, which was restricted to maintain <em>H f</em> conditions within biologically relevant limits. <br>