We report the fine-tuning of the relaxometry of gamma-Fe2O3@SiO2 core-shell nanoparticles by adjusting the thickness of the coated silica layer. It is clear that the coating thickness of Fe2O3@SiO2 nanoparticles has a significant impact on the r(1) (at low B0 fields), r(2), and r(2)* relaxivities of their aqueous suspensions. These studies clearly indicate that the silica layer is heterogeneous and has regions that are porous to water and others-that are not. It is also shown, that the viability and the mitochondrial dehydrogenase expression of the microglial cells do not appear to be sensitive to the vesicular load with these core-shell nanoparticles. The adequate silica-shell thickness can therefore be tuned to allow for both a sufficiently high response as contrast agent, and-adequate grafting of targeted biomolecules.