Effect of silica shell thickness of Fe₃O₄-SiO _x core-shell nanostructures on MRI contrast

Core-shell magnetic nanostructures (MNS) such as Fe₃O ₄-SiO_x, are being explored for their potential applications in biomedicine, such as a T₂ (dark) contrast enhancement agent in magnetic resonance imaging (MRI). Herein, we present the effect of silica shell thickness on its r ₂ relaxivity in MRI as it relates to other physical parameters. In this effort initially, monodispersed Fe ₃O₄ MNS (nominally 9 nm size) were synthesized in organic phase via a simple chemical decomposition method. To study effect of shell thickness of silica of Fe₃O₄-SiO_x core shell on r ₂ relaxivity, the reverse micro-emulsion process was used to form silica coating of 5, 10 and 13 nm of silica shell around the MNS, while polyhedral oligomeric silsesquioxane was used to form very thin layer on the surface of MNS; synthesized nanostructures were characterized by transmission electron microscopy (TEM) and high resolution TEM (HRTEM), superconducting quantum interference device magnetometry and MRI. Our observation suggests that, with increase in thickness of silica shell in Fe₃O ₄-SiO_x core-shell nanostructure, r ₂ relaxivity decreases. The decrease in relaxivity could be attributed to increased distance between water molecules and magnetic core followed by change in the difference in Larmor frequencies (Δω) of water molecules. These results provide a rational basis for optimization of SiO_x-coated MNS for biomedical applications.