Lanthanide-based nanoparticles (Ln3+-based NPs) are promising candidates as magnetic resonance imaging (MRI) contrast agents. The present thesis aims to investigate the effect of the crystalline phase of Ln3+-based NPs on their MRI contrast performance. Understanding the phase-dependent MRI contrast behaviour of Ln3+-based NPs will provide insights into the development of brighter MRI contrast agents for future in vivo biomedical applications. A set of NaGdF4 NPs (6-8 nm) in cubic and hexagonal phases in the same size range was synthesized by employing a microwave-assisted approach, allowing the influence of host crystallinity on MRI T1 relaxivity to be investigated (chapter 4). The results showed that cubic NaGdF4 NPs exhibited superior performance as MRI T1 contrast agents than their hexagonal analogues, irrespective of the chosen surface modification, e.g. small citrate groups or longer chain poly(acrylic acid). NaDyF4 NPs (3 nm) were synthesized in both phases to assess whether phase-dependent MRI contrast behaviour consistently exists in other Ln3+-base NPs of the NaLnF4 family (chapter 5). Again, it was demonstrated that cubic NaDyF4 NPs had a better contrast performance as T2 contrast agents than the hexagonal NPs. Alternatively, cubic NaEuF4 NPs, exhibiting additional optical properties (e.g. red emission under UV excitation), were prepared as potential candidates for the preparation of chemical exchange saturation transfer (CEST) contrast agents (chapter 5). Chapter 6 introduces preliminary dispersion stability studies of cubic NaGdF4 NPs dispersed in different buffer solutions, the obtained hydrodynamic diameters indicated that NaGdF4 NPs possessed better dispersity in saline than that in PBS solution.

Lanthanide-based nanoparticles are of interest for optical displays, catalysis, telecommunication, bio-imaging, magnetic resonance imaging, multimodal imaging, etc. These applications are possible partly because the preparation of lanthanide-based nanoparticles has made tremendous progress. Now, nanoparticles are routinely being made with a good control over size, crystal phase and even shape. Despite the achievements, little attention is given to the fundamental physical chemistry aspects, such as crystal structure, architecture, cation exchange, etc. The results of the study on the crystal structures of LnF3 nanoparticles show that the middle GdF3 and EuF3 nanoparticles have two crystal phases, which has then been tuned by doping with La3+ ions. However, the required doping level is very different from the bulk. While the results for the bulk are well explained by thermodynamic calculations, kinetics is actually responsible for the results of the undoped and doped GdF3 and EuF3 nanoparticles. The attempt to make LnF3 core-shell nanoparticles led to the finding of cation exchange, a phenomenon that upon exposure of LnF3 nanoparticles to an aqueous solution containing Ln3+ ions, the Ln3+ ions in the nanoparticles are replaced by the Ln3+ ions in the solution. The consequence of the cation exchange is that LnF3 core-shell nanoparticles are unlikely to form in aqueous media using a core-shell synthesis procedure. It has also been verified that nanoparticles synthesized using an alloy procedure do not always have an alloy structure. This means that the core-shell and alloy structure of nanoparticles in the literature may not be true. The investigation of the architecture of nanoparticles synthesized in aqueous media is extended to those synthesized in organic media. The dopant ion distribution in NaGdF4 nanoparticles has been examined. It has been found that they don’t have the generally assumed statistical dopant distribution. Instead, they have a gradient structure with one type of Ln3+ ions more concentrated towards the center and the other type more concentrated towards the surface of the nanoparticles. With the understanding of these physical insights, lanthanide-based core-shell nanoparticles are prepared using the cation exchange. These core-shell nanoparticles containing a photoluminscent core and a paramagnetic shell are promising candidates for multimodal imaging. / Graduate