Spin dynamics of Superparamagnetic Iron Oxide Nanoparticles investigated by Solid State NMR

Abstract

The long term aim of the research presented on this thesis is to bridge the gap in the knowledge of the mesoscopic magnetic phenomena between the smallest of the nanoparticles and the largest molecular iron clusters. Starting from this basic idea, the main objective of this work is the investigation of the spin dynamics of fine superparamagnetic (SPM) nanoparticle ensembles using monodispersed powder samples of organic-coated ferrite particles by resorting to Nuclear Magnetic Resonance. Probing of the local dynamics is made possible by the magnetic dipolar coupling between the superparamagnetic moment of the ferrite core and hydrogen nuclear spins (proton spins) in the organic material of the encapsulating shell. Thus, even though the chosen NMR probes do not sense each single Fe spin in the inorganic core on a true local level, the dynamic magnetic behavior of the nanoparticle as a whole can still be observed. The importance of the proposed investigation resides in its novelty, since a complete study of magnetic nanoparticles encompassing AC susceptibility, DC magnetometry, NMR solid-state spectroscopy, NMR relaxometry and Mössbauer spectroscopy has never been attempted. This multi-technique approach allows access to key parameters such as the saturation magnetization, the average magnetic anisotropy energy and its distribution, the characteristic time for the electronic magnetization relaxation, and to information about interparticle interactions. Additionally, NMR relaxometry has been exploited to obtain the frequency dependence of the longitudinal and transverse relaxivities, and an estimation of the diffusional correlation time, three quantities which are tightly related to the MRI efficiencies of the investigated materials. The interplay of these and other parameters have been evaluated and correlated with structural parameters, such as the magnetic ion species, the particle size and the particle topology. The work carried out in Pavia represents the very first attempt at studying the spin dynamics of magnetic nanocrystals as a function of temperature with NMR: an important goal was showing that an original NMR approach to the study of the dynamic properties of fine superparamagnetic particle is possible and that the NMR approach deserves the same consideration as other experimental techniques usually selected for this kind of investigation, e.g. Mössbauer spectroscopy, Ferromagnetic spectroscopy, and AC/DC magnetometry. The fundamental physics involved in the phenomenology of magnetic nanoparticles is of great interest for the application of iron-oxide based nanoparticles as functionalizable MRI negative contrast agents. Indeed, the magnetic properties and the spin dynamics have to be investigated in order to reach a better understanding of the underlying physical mechanisms that cause the decrease in the relaxation time of water proton spins. In particular, the role of the magnetic anisotropy in determining the contrasting efficiency as a function of the static magnetic field needs to be defined. In order to study these aspect, relaxivity curves on a wide variety of samples were analyzed: the longitudinal relaxivity has been measured to validate a descriptive physical model upon which formulate practical considerations about the selection of a certain material and particle size to be used for synthesizing a superparamagnetic nanoparticle-based contrast agent; complementary transverse relaxivity measurements have been carried out to actually quantify the nanoparticle systems as negative contrast agents. Relying on the good results obtained in the fundamental research, and considering how the topology and the magnetic properties come together to confer good contrasting efficiency to a magnetic nanoparticle system, a candidate material has been envisioned, created and ultimately selected for preliminary in vivo tests in mice.