Clusters of Magnetic Nanoparticles as Contrast Agents for MRI: Effect of Aggregation on Transverse Relaxivity

The effect of aggregation of magnetic nanoparticles on the transverse relaxivity (r
₂
) is analyzed with respect to the size of clusters. The nanoparticles employed are based on La
_0.75
Sr
_0.25
MnO
₃
ferromagnetic phase with the mean size of crystallites dXRD = 26 nm synthetized via sol-gel route followed by thermal treatment and mechanical processing. The subsequent silica coating provides colloidally stable particles whose magnetic cores are mostly composed of compact clusters of manganite crystallites. The product has been subjected to repeated differential centrifugation and several size fractions, possessing the same dXRD but differing in the effective size of magnetic cores, are isolated. Thorough analyses of their size distributions by transmission electron microscopy and dynamic light scattering measurements are carried out together with SQUID magnetometry. The concentration of particles in aqueous suspensions is accurately determined by atomic absorption spectroscopy and a detailed study of transverse relaxation at the magnetic field B
₀
= 0.5 T is performed. The highest r
₂
values are clearly observed for midsized clusters and the temperature dependence of r
₂
resembles the evolution of magnetization with temperature. Supplemental samples with different thicknesses of the silica shell are also synthetized and thoroughly analyzed.