Particle size dependence of relaxivity for silica-coated iron oxide nanoparticles

We investigate the particle size dependence of the relaxivity of hydrogen protons in an aqueous solution of iron oxide (Fe3O4) nanoparticles coated in silica for biocompatibility. The T1 and T2 relaxation times for various concentrations of silica-coated nanoparticles were determined by a magnetic resonance scanner. We find that the relaxivity increased linearly with increasing particle size. The T2 relaxivity (R2) is more than 50 times larger than the T1 relaxivity (R1) for the nanoparticle contrast agent, which reflects the fact that the T2 relaxation is mainly influenced by outer sphere processes. The high R2/R1 ratio demonstrates that silica-coated iron oxide nanoparticles may serve as a T2 contrast agents in magnetic resonance imaging with high efficacy.