Factors controlling the organ-specific T1 contrast effect of silica nanoparticles co-doped with both Mn2+ ions and oleate-coated iron oxides

Abstract

The present work introduces the synergistic effect of co-doping of both oleate-coated superparamagnetic iron oxide nanoparticles (SPIONs) and Mn(NO3)2 into silica nanoparticles (SNs) on the T1-relaxivity of Mn2+ ions. The observed synergism can be attributed to the limited oxidation of Mn2+ ions when they are doped into the outer layer of SNs doped with SPIONs, despite the alkaline synthesis conditions. The electrochemical behaviour of the manganese ions inside co-doped SNs corroborates the predominance of their oxidation state +2. Moreover, the T1 relaxivities of co-doped SNs have been determined to be 20.0 mM−1s−1 and 30.0 mM−1s−1 at 0.47 T. The T2 relaxivity of co-doped SNs can be tuned by incorporating 6 or 13 nm SPIONs with different saturation magnetizations, which allows the T2/T1 relaxation ratios to be limited to 0.8–5.9. The incorporation of amino groups on the surface of co-doped SNs by substituting silanol groups with propylamino groups reduces T1 relaxivity to 9.0 mM−1s−1, which is nevertheless sufficient to provide a brightening of the abdominal organs and mouse brain in magnetic resonance imaging at 11.7 T. The preferential localisation of co-doped SNs in the kidneys and intestines compared to the liver is a consequence of the specificity of amino-substituted SNs compared to bare SNs.