Titre de la dissertation: Superparamagnetic iron oxide nanoparticles: magnetometry and Monte Carlo simulations of their magnetic and nuclear magnetic resonance relaxation properties

Promoteurs de thèse: Monsieur Yves Gossuin et Monsieur Quoc Lam Vuong

Résumé de la dissertation: Due to their high saturation magnetization and absence of remnant magnetization, superparamagnetic iron oxide nanoparticles (SPIONs) can be used for many applications, especially in the field of biophysics. In particular, they can serve as T₂ contrast agents in nuclear magnetic resonance imaging (MRI). In practice, for stability and functionalization, SPIONs are usually coated with a non-magnetic layer, typically a sugar or a polymer in which water diffusion is slowed down. Understanding the magnetic properties of SPIONs and the mechanisms that govern the T₂ contrast they induce is important to optimize their efficiency.

In the first part of this work, the magnetic properties of SPIONs submitted to an external magnetic field were studied using a Metropolis algorithm. The influence on the magnetization curves of the size distribution of the nanoparticles, of uniaxial anisotropy, and of dipolar interaction between the cores was examined, as well as the influence of drying the samples under a zero or non-zero magnetic field. The obtained results showed that fitting of the magnetization curves of particles with magnetic anisotropy to a Langevin model (including a size distribution of the particles), as is often done in literature, can lead in certain cases to erroneous values of the size distribution parameters. The simulation results were also compared to experimental results obtained for commercial iron oxide nanoparticles with a vibrating sample magnetometer.

In the second part, Monte Carlo simulations of water protons transverse relaxation induced by SPIONs were performed to study the influence of the coating on SPIONs efficiency. As diffusion is the underlying mechanism that explains high field T₂ transverse relaxation, the coating is expected to affect relaxation and therefore contrast efficiency. Indeed, experimental evidence in literature shows that coating material and thickness impact transverse relaxation rates. The simulation results obtained in this thesis provide an explanatory framework to that experimental evidence, through an exchange model. Moreover, experimental optimums were identified to take advantage of that effect, in terms of SPION size, coating material, and coating layer thickness.