NMR Relaxometric Properties of SPION-Loaded Solid Lipid Nanoparticles

Longitudinal and transverse relaxivities of solid lipid nanoparticles loaded with superparamagnetic iron oxide nanoparticles (SPION-SLNs) were thoroughly investigated with the aim of understanding the main parameters regulating the potential negative contrast properties of these systems. In particular, the longitudinal relaxivity (r1) of water protons in the 10 kHz to 35 MHz frequency range was determined by 1H fast field-cycling NMR, while transverse relaxivity (r2) was measured at 21 MHz. The reproducibility and stability of SPION-SLNs was also tested on samples arising from independent preparations and at different times after preparation. Water diffusion in proximity of superparamagnetic nanoparticles was found to be the mechanism of proton nuclear relaxation enhancement and characteristic parameters were quantitatively determined by fitting the experimental data acquired on different samples as a function of concentration and temperature. Although a variation ascribable to the formation of clusters with a different number of SPIONs inhomogeneously embedded in the lipid matrix was observed among different preparations, the relaxivity properties of the investigated SPION-SLNs were found to be better than those of SPION-based contrast agents commonly considered as standard in the literature, and remained stable for at least 2 months.