Epitaxial growth of ultrathin layers on the surface of sub-10 nm nanoparticles: the case of β-NaGdF:Yb/Er@NaDyF nanoparticles.

Upconversion core-shell nanoparticles have attracted a large amount of attention due to their multifunctionality and specific applications. In this work, based on a NaGdF sub-10 nm ultrasmall nanocore, a series of core-shell upconversion nanoparticles with uniform size doped with Yb, Er and NaDyF shells with different thicknesses were synthesized by a facile sequential growth process. NaDyF coated upconversion luminescent nanoparticles showed an obvious fluorescence quenching under excitation at 980 nm as a result of energy resonance transfer between Yb, Er and Dy. NaGdF:Yb,Er@NaDyF core-shell nanoparticles with ultrathin layer shells exhibited a better -weighted MR contrast.