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 (Fe ₃O ₄) nanoparticles coated in silica for biocompatibility. The T ₁ and T ₂ 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 T ₂ relaxivity (R ₂) is more than 50 times larger than the T ₁ relaxivity (R ₁) for the nanoparticle contrast agent, which reflects the fact that the T ₂ relaxation is mainly influenced by outer sphere processes. The high R ₂/R ₁ ratio demonstrates that silica-coated iron oxide nanoparticles may serve as a T ₂ contrast agents in magnetic resonance imaging with high efficacy.