Optimization of the composition of bimetallic core/shell Fe₂O₃/Au nanoparticles for MRI/CT dual-mode imaging

Bimetallic core/shell Fe₂O₃/Au nanoparticles are promising candidate dual-mode contrast agents for magnetic resonance imaging (MRI) and computed tomography (CT) imaging. However, the gold coating on the hybrid nanoparticles (hybrids) affects the MRI and CT imaging quality. A thick gold nanoshell increases the X-ray attenuation effect but decreases the magnetic saturation of the hybrids. Therefore, we studied the effect of the Fe₂O₃ and Au composition on these properties to find a suitable hybrid for MRI and CT imaging. Water-soluble, Au-coated magnetic nanoparticles were synthesized by iteratively reducing Au³⁺ onto the Fe₂O₃ surface via hydroxylamine seeding. The properties of the hybrids obtained after different numbers of Au seeding cycles were studied using transmission electron microscopy, UV–Vis spectrophotometry, a vibrating swatch gaussmeter, MRI, CT, and an MTT assay. The hybrids obtained after three Au seeding cycles had an Fe₂O₃:Au molar ratio of 7.2:26.8, a mean diameter of 48.3 nm, a UV–Vis absorbance peak of 550 nm, a saturation magnetization of 49.0 emu/g, and no cytotoxicity at a concentration of 500 μg/mL after incubation with RAW 264.7 cells for up to 72 h. The hybrids obtained after three Au seeding cycles are the preferred candidates for MRI and CT applications because of their relatively high R2 relaxivity (95 mM⁻¹ s⁻¹) and X-ray attenuation (1.87 times that of iodine) compared to those of the other hybrids investigated in this study.