The Effect of Silica Shell Thickness on Magnetic and Proton Relaxometric Properties: Fe₃O₄@mSiO₂ Nanoparticles

Here, we report the effect of silica shell thickness on proton relaxivities ( $r_{2}$ and $r_{1}$ ) and magnetic properties. Starting from hydrophobic 7 nm of Fe ₃ O ₄ , we have coated Fe ₃ O ₄ with cetyltrimethylammonium bromide (CTAB) for phase transformation from hydrophobic to hydrophilic. Core-shell structure Fe ₃ O ₄ @mSiO ₂ with different silica shell thickness have been obtained. The obtained Fe ₃ O ₄ @mSiO ₂ have been characterized by TEM, Fourier Transform Infrared (FTIR), Superconducting Quantum Interference Device (SQUID), and the proton relaxivities of the nanoparticles (NPs) have been determined. The presence of silica did alter the relaxivity with slightly increased the $r_{2}$ , and dramatically decreased the $r_{1}$ of Fe ₃ O ₄ @mSiO ₂ . We find that the $r_{2}$ and $r_{1}$ relaxivity decreased with the increase of silica shell thickness. Moreover, the high $r_{2}/r_{1}$ ratio shows that Fe ₃ O ₄ @mSiO ₂ may serve as $T_{2}$ contrast agents in magnetic resonance imaging (MRI).