Core@shell Fe3O4@Mn2+-doped NaYF4:Yb/Tm nanoparticles for triple-modality T1/T2-weighted MRI and NIR-to-NIR upconversion luminescence imaging agents

Core@shell structures of Fe3O4@Mn2+-doped NaYF4:Yb/Tm nanoparticles (NPs) with multifunctional properties were prepared using a hydrothermal route with a seed-growth procedure. The effect of Mn2+ ions on the phase, shape, and upconversion luminescence (UCL) of Fe3O4@Mn2+-doped NaYF4:Yb/Tm NPs was explored using X-ray diffraction, transmission electron microscopy, dynamic light scattering, and photoluminescence spectroscopy. Furthermore, hydrophobic to hydrophilic surface modification of the Fe3O4@Mn2+-doped NaYF4:Yb/Tm NPs was achieved by coating the NPs with an amphiphilic polymer (poly(maleic anhydride-alt-1-octadecene) (C18PMH)) modified with amine-functionalized methyl ether poly(ethylene glycol). Then, the NIR-to-NIR UCL ascribed to the Tm3+ ions, T1-weighted MRI ascribed to the Mn2+ ions, and T2-weighted MRI ascribed to the Fe3O4 core of the hydrophilic Fe3O4@Mn2+-doped NaYF4:Yb/Tm NPs were evaluated. The results indicate that Mn2+ doping is an effective method to control the size/shape and improve the UCL intensity of Fe3O4@Mn2+-doped NaYF4:Yb/Tm NPs, which are promising as imaging agents for NIR-to-NIR UCL and T1/T2-weighted MRI in biomedical research.