(A) Schematic figure showing the synthesis route of PEGylated Fe3O4@β-NaGdF4: Yb/Er or Yb/Tm nanophosphors. TEM image of (B) Fe3O4 seeds after surface modification, (C) Fe3O4@Gd2O(CO3)2•H2O:Yb/Er@AL/PEI, (D) PEGylated Fe3O4@NaGdF4:Yb/Er, (E) PEGylated Fe3O4@NaGdF4:Yb/Tm. Upconversion luminescent spectra and images for (F) PEGlayted Fe3O4@β-NaGdF4:Yb/Er, (G) PEGlayted Fe3O4@β-NaGdF4:Yb/Tm nanophosphor, (H) Extinction spectrum of a 0.18 mg/mL colloidal solution of oleic acid-coated magnetite cores in a 1 cm path length cuvette.