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

Abstract

The effect of aggregation of magnetic nanoparticles on the transverse relaxivity (r2) is analyzed with respect to the size of clusters. The nanoparticles employed are based on La0.75Sr0.25MnO3 ferromagnetic phase with the mean size of crystallites dXRD = 26 nm synthetized via sol-gel method 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. The highest r2 values are clearly observed for midsized clusters and the temperature dependence of r2 resembles the evolution of magnetization with temperature. Supplemental samples with different thicknesses of the silica shell are also synthetized and thoroughly analyzed.