Gas phase preparation of magnetic nanoparticle hydrosols

The main purpose of this work was to produce nanoparticles with a pure iron core, a narrow size distribution and a high saturation magnetisation in order to improve their effectiveness in the hyperthermia treatment of tumours and in Magnetic Resonance Imaging (MRI) diagnosis. Gas phase Fe nanoparticles in a liquid suspension have been produced by co-deposition with water vapour in ultra-high vacuum (UHV) conditions. The water was injected from outside the vacuum as a molecular beam onto a substrate maintained at 77 K and formed an ice layer with a UHV compatible vapour pressure. Various coating ligands were added to the injected water in an attempt to stabilise the nanoparticle hydrosols. Transmission Electron Microscopy (TEM) images confirmed that the nanoparticles had a pure iron core with a thin oxide shell and a narrow size distribution with the most probable diameter of either 8.55 or 16 nm depending on the source conditions. It was found that short chain molecules are more effective in stabilising the gas phase nanoparticles. The size distribution of the nanoparticles in liquid suspensions analysed by a Nanosight LM10 particle sizer showed that, of the ligands tested, sorbitol and DMSA are the most suitable to prevent the agglomeration of the gas phase produced hydrophobic nanoparticles. UV-visible spectral measurements showed that DMSA coated nanoparticles transform into an oxide in a short time. In addition, a magnetometry study of sorbitol-coated iron nanoparticles showed that oxidation of the nanoparticles erodes the pure iron core to about 5 nm diameter in two months. MRI measurements of the sorbitol-coated iron nanoparticles show that their relaxivity is five times higher than commercial iron oxide nanoparticle suspensions (Resovist®). On the other hand, specific absorption rate (SAR) measurements of the nanoparticles by two different designs of heating coils were not accurate due to the low concentration of nanoparticle in solution. Hence, the heating performance of the nanoparticles was determined theoretically using a new model published by Vallejo-Fernandez [Vallejo-Fernandez 2013], which includes the heating mechanisms active over the whole size range of particles. The results show that the SAR of the pure iron core nanoparticles is significantly higher than iron oxide nanoparticles. Core-shell nanoparticle can also be produced in the gas phase by passing the core nanoparticles through hot crucible loaded with the shell material under UHV conditions. Composite Fe@FeO-Ag nanoparticles which can be used for multifunctional medical applications have been produced. It was also found that heating the nanoparticles in the gas phase using the empty crucible enabled the control of the nanoparticles’ shapes, which was found to change their MRI relaxivity.