Surface Design of Eu-Doped Iron Oxide Nanoparticles for Tuning the Magnetic Relaxivity

Relaxivity tuning of nanomaterials with the intrinsic <i>T</i>₁–<i>T</i>₂ dual-contrast ability has great potential for MRI applications. Until now, the relaxivity tuning of T₁ and T₂ dual-modal MRI nanoprobes has been accomplished through the dopant, size, and morphology of the nanoprobes, leaving room for bioapplications. However, a surface engineering method for the relaxivity tuning was seldom reported. Here, we report the novel relaxivity tuning method based on the surface engineering of dual-mode <i>T</i>₁–<i>T</i>₂ MRI nanoprobes (DMNPs), along with protein interaction monitoring with the DMNPs as a potential biosensor application. Core nanoparticles (NPs) of europium-doped iron oxide (EuIO) are prepared by a thermal decomposition method. As surface materials, citrate (Cit), alendronate (Ale), and poly­(maleic anhydride-<i>alt</i>-1-octadecene)/poly­(ethylene glycol) (PP) are employed for the relaxivity tuning of the NPs based on surface engineering, resulting in EuIO-Cit, EuIO-Ale, and EuIO-PP, respectively. The key achievement of the current study is that the surface materials of the DMNP have significant impacts on the <i>r</i>₁ and <i>r</i>₂ relaxivities. The correlation between the hydrophobicity of the surface material and longitudinal relaxivity (<i>r</i>₁) of EuIO NPs presents an exponential decay feature. The <i>r</i>₁ relaxivity of EuIO-Cit is 13.2-fold higher than that of EuIO-PP. EuIO can act as <i>T</i>₁–<i>T</i>₂ dual-modal (EuIO-Cit) or <i>T</i>₂-dominated MRI contrast agents (EuIO-PP) depending on the surface engineering. The feasibility of using the resulting nanosystem as a sensor for environmental changes, such as albumin interaction, was also explored. The albumin interaction on the DMNP shows both <i>T</i>₁ and <i>T</i>₂ relaxation time changes as mutually confirmative information. The relaxivity tuning approach based on the surface engineering may provide an insightful strategy for bioapplications of DMNPs and give a fresh impetus for the development of novel stimuli-responsive MRI nanoplatforms with <i>T</i>₁ and <i>T</i>₂ dual-modality for various biomedical applications