Size and Surface Effects on the MRI Relaxivity of Manganese Ferrite Nanoparticle Contrast Agents

TLDR

Superparamagnetic MnFe₂O₄ nanocrystals of varying sizes were synthesized in high‑boiling ether and rendered water‑soluble via ligand exchange, amphiphilic polymer encapsulation, or lipid micelle embedding, and their size‑dependent relaxivity was qualitatively compared to theoretical models. Micellar‑embedded particles exhibited markedly higher transverse relaxivity (r₂*) than polymer‑coated counterparts, and for polymer‑coated nanocrystals transverse relaxivity increased with particle size.

Abstract

Superparamagnetic MnFe2O4 nanocrystals of different sizes were synthesized in high-boiling ether solvent and transferred into water using three different approaches. First, we applied a ligand exchange in order to form a water soluble polymer shell. Second, the particles were embedded into an amphiphilic polymer shell. Third, the nanoparticles were embedded into large micelles formed by lipids. Although all approaches lead to effective negative contrast enhancement, we observed significant differences concerning the magnitude of this effect. The transverse relaxivity, in particular r2*, is greatly higher for the micellar system compared to the polymer-coated particles using same-sized nanoparticles. We also observed an increase in transverse relaxivities with increasing particle size for the polymer-coated nanocrystals. The results are qualitatively compared with theoretical models describing the dependence of relaxivity on the size of magnetic spheres.