Hydrothermal synthesis of superparamagnetic and red luminescent bifunctional Fe3O4@Mn2+-doped NaYF4:Yb/Er core@shell monodisperse nanoparticles and their subsequent ligand exchange in water

We report the use of an efficient hydrothermal method to synthesize superparamagnetic and red luminescent bifunctional Fe 3 O 4 @Mn 2+ -doped NaYF 4 :Yb/Er nanoparticles (NPs) with core@shell structures via a seed-growth procedure. Oleic acid coated Fe 3 O 4 (OA-Fe 3 O 4 ) NPs were initially synthesized using a coprecipitation method. The as-synthesized OA-Fe 3 O 4 NPs were then used as seeds, on which the red upconversion luminescent shell (Mn 2+ -doped NaYF 4 :Yb/Er) was formed. Furthermore, hydrophobic to hydrophilic surface modification of the Fe 3 O 4 @Mn 2+ -doped NaYF 4 :Yb/Er NPs was achieved via a ligand exchange method where oleic acid was displaced by a PEG phosphate ligand [PEG   =   poly(ethylene glycol)]. These materials were characterized by means of transmission electron microscopy (TEM), X-ray diffraction (XRD), photoluminescence (PL) spectroscopy, and vibrating sample magnetometry (VSM). The Fe 3 O 4 cores were uniformly coated with a Mn 2+ -doped NaYF 4 :Yb/Er shell, and the bifunctional Fe 3 O 4 @Mn 2+ -doped NaYF 4 :Yb/Er NPs were monodispersed. Furthermore, the Fe 3 O 4 @Mn 2+ -doped NaYF 4 :Yb/Er NPs exhibited a saturated magnetization value of 6.2 emu/g and emitted red luminescence under a 980   nm laser. The obtained bifunctional Fe 3 O 4 @Mn 2+ -doped NaYF 4 :Yb/Er NPs may find potential applications in drug targeting, bioseparation, and diagnostic analysis. The synthetic method may be employed for the preparation of other bifunctional nanomaterials.