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

Graphical abstract: An efficient hydrothermal method was used to fabricate the superparamagnetic and red luminescent bifunctional Fe₃O₄@Mn^2(*)+-doped NaYF₄:Yb/Er nanoparticles (NPs) with core@shell structures through a seed-growth procedure. Then using PEG phosphate ligand to displace oleate from the as-synthesized NPs, hydrophilic Fe₃O₄@Mn²⁺-doped NaYF₄:Yb/Er NPs with good water solubility are obtained. - Highlights: • Homogeneous size distribution of magnetic-upconversion core@shell structured nanoparticles (NPs) were synthesized. • The core@shell nanostructures were obtained by seed-growth method. • The oleic acid coated Fe₃O₄ NPs were used as seeds and cores. • The magnetic-upconversion NPs emitted red luminescence under a 980 nm laser. • Synthesized magnetic-upconversion NPs were phase transferred using ligand exchange process. - Abstract: We report the use of an efficient hydrothermal method to synthesize superparamagnetic and red luminescent bifunctional Fe₃O₄@Mn²⁺-doped NaYF₄:Yb/Er nanoparticles (NPs) with core@shell structures via a seed-growth procedure. Oleic acid coated Fe₃O₄ (OA-Fe₃O₄) NPs were initially synthesized using a coprecipitation method. The as-synthesized OA-Fe₃O₄ NPs were then used as seeds, on which the red upconversion luminescent shell (Mn²⁺-doped NaYF₄:Yb/Er) was formed. Furthermore, hydrophobic to hydrophilic surface modification of the Fe₃O₄@Mn²⁺-doped NaYF₄: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 Fe₃O₄ cores were uniformly coated with a Mn²⁺-doped NaYF₄:Yb/Er shell, and the bifunctional Fe₃O₄@Mn²⁺-doped NaYF₄:Yb/Er NPs were monodispersed. Furthermore, the Fe₃O₄@Mn²⁺-doped NaYF₄: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 Fe₃O₄@Mn²⁺-doped NaYF₄: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.