Epitaxial growth of ultrathin layers on the surface of sub-10 nm nanoparticles: the case of β-NaGdF(4):Yb/Er@NaDyF(4) 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(4) sub-10 nm ultrasmall nanocore, a series of core–shell upconversion nanoparticles with uniform size doped with Yb(3+), Er(3+) and NaDyF(4) shells with different thicknesses were synthesized by a facile sequential growth process. NaDyF(4) coated upconversion luminescent nanoparticles showed an obvious fluorescence quenching under excitation at 980 nm as a result of energy resonance transfer between Yb(3+), Er(3+) and Dy(3+). NaGdF(4):Yb,Er@NaDyF(4) core–shell nanoparticles with ultrathin layer shells exhibited a better T(1)-weighted MR contrast.