Spin Dynamics of Core-Shell Magnetic Nanoparticles as a function of diverse Size, Coating and Metal Ion Doping: Unravelling the Fundamental Physical Mechanisms to Envision State of the Art Theranostic Applications

This PhD thesis work, Spin dynamics of core-shell magnetic nanoparticles as a function of diverse size, coating and metal Ion doping: unravelling the fundamental physical mechanisms to envision state of the art theranostic applications, aimed to understand the how different morpho-structural properties of magnetic nanoparticles (MNPs) can influence their fundamental physical properties, to tailor them to specific fields of application. In this case, the focus was directed towards contrast agents (CAs) for Magnetic Resonance Imaging (MRI) and therapy agents for Magnetic Fluid Hyperthermia (MFH), highlighting their dual diagnostic and therapeutic functionality as theranostic agents. This is achieved by the synthesis of iron oxide-based magnetic nanoparticles having different size, coating, and elemental composition, exploring different synthesis techniques, including eco-friendly sustainable methods designed to reduce the use of hazardous chemicals and minimize environmental impact. Special attention was given to the development of a nanoparticle coating protocol for the MNPs, which is crucial for enabling the experiments underlying this study as well as for future in vivo applications. The investigation over these nanosystems was conducted through 1H Nuclear Magnetic Resonance relaxometry, calorimetry, magnetometry and morpho-dimensional characterization techniques. The synthesis of most of the MNPs and part of their characterization were made possible by a strong collaboration with the Instituto de Ciencia de Materiales-Consejo Superior de Investigaciones Cientificas (ICMM-CSIC) of Madrid. The results highlight a strong size dependence of both relaxometric and hyperthermic properties: larger MNPs exhibit higher anisotropy and magnetic moments, translating into significantly enhanced r₂ relaxivities and SAR values. Surface coating was also shown to modulate the relaxometric performance. Variations in coating type (e.g., DMSA, PAA, CM-D) led to differences in spin dynamics at certain sizes, particularly evident in the frequency-dependent dispersion of r1 of 4.4 nm particles coated with DMSA or PAA, and in the r₂ of 11 nm particles coated with DMSA, PAA, or CM-D, indicating the surface disorder and the kind of coating, significant for tuning relaxometric behavior. Coating had negligible influence on SAR values, suggesting that heating efficiency is dominated by core magnetic volume and dynamics, at least for the studied size and coating combinations. Elemental doping with Zn and Mn further impacted magnetic and relaxometric properties. For 4 nm particles, Zn content of x = 0.1 yielded higher r₁ and r₂ values compared to undoped samples. For 15 nm particles, Zn doping up to x = 0.4 maximized anisotropy and transverse relaxivity. Across all tested conditions, doping consistently improved relaxometric performance. In conclusion, particle size, surface coating, and elemental composition emerged as key parameters for tailoring MNPs toward efficient and tunable theranostic systems. This study also confirms the viability of green microwave-assisted synthesis in producing high-quality, biocompatible MNPs with properties suitable for both MRI enhancement and localized hyperthermia.