Core–shell NaYF4:Yb3+,Tm3+@FexOy nanocrystals for dual-modality T2-enhanced magnetic resonance and NIR-to-NIR upconversion luminescent imaging of small-animal lymphatic node

New core–shell structured NaYF4:Yb3+,Tm3+@FexOy nanocrystals, with 20 nm Yb3+,Tm3+-co-doped NaYF4 nanocrystals as a core and 5 nm FexOy nanocrystals as a shell, have been synthesized and characterized by TEM and XRD analysis. These core–shell nanocrystals exhibit excellent near-infrared upconversion luminescence (UCL) emission at 800 nm under excitation by a continuous-wave 980 nm laser and superparamagnetic properties with a saturation magnetization (Ms) of ∼12 emu/g. Water-soluble nanocrystals were obtained by surface ligand exchange of oleic acid-coated precursor nanocrystals, and their internalization within living cells has been investigated by laser scanning UCL microscopy. Furthermore, the obtained core–shell nanocrystals have been applied in dual-modality T2-enhanced magnetic resonance (MR) and UCL imaging in vivo of the lymphatic system. Moreover, the toxicity of NaYF4:Yb3+,Tm3+@FexOy nanocrystals has also been evaluated by MTT assay, IC50 values, and histological analysis of lymphatic node sections.