Relaxometric Studies of γ-Fe2O3@SiO2 core shell nanoparticles: when the coating matters

Iron oxide nanoparticles with a constant superparamagnetic core coated with a silica shell with a thickness ranging from 0.6 to 71 nm were prepared by a fast and facile soft chemistry approach. The increase of the coating thickness of the γ-Fe2O3@SiO2 nanoparticles causes a significant decrease of the r1 and r2 relaxivities of their aqueous suspensions. The sizes of the nanoparticles obtained by relaxometry are significantly lower than those measured by electron microscopy. Their magnetizations measured by relaxometry also decrease relative to the values obtained by magnetometry, which correspond to the core. However, this "magnetic dilution" is smaller than expected if the entire silica shell was water impermeable. Both results indicate that a significant part of the silica coating is permeable to water. The adequate silica shell thickness may, thus, be tuned to allow for both a sufficiently high response as contrast agent and an adequate grafting of targeted biomolecules