Lanthanide dopant-induced formation of uniform sub-10 nm active-core/active-shell nanocrystals with near-infrared to near-infrared dual-modal luminescence

Abstract

We present a novel strategy involving the lanthanide dopant-induced formation of ultrasmall (sub-10 nm) uniform Ln3+:BaF2/Ln3+:SrF2 active-core/active-shell architectures. The lanthanide ions doped in the shell are demonstrated to play a key role to retard the growth of the core/shell nanocrystals. Particularly, adopting ∼3 nm Tm3+,Yb3+:BaF2 nanocrystals as cores prepared by a solvothermal reaction, growth of Gd3+,Nd3+:SrF2 shells is successfully induced on the surfaces of these cores through a thermal decomposition process, forming ∼7 nm highly uniform and monodisperse Tm3+,Yb3+:BaF2/Gd3+,Nd3+:SrF2 active-core/active-shell nanocubes. In this architecture, the Gd3+,Nd3+:SrF2 shell not only benefits the enhancement of the near-infrared to near-infrared upconversion luminescence of the Tm3+,Yb3+:BaF2 core, but also acts as the host to realize the near-infrared to near-infrared downconversion luminescence of Nd3+ dopants and the paramagnetism of Gd3+ ones. Importantly, the doping of Tm3+/Yb3+ and Nd3+ into the core and shell respectively effectively suppresses the adverse energy transfer from Tm3+ to Nd3+ as well as from Nd3+ to Yb3+, resulting in lessening of the quenching for both Tm3+ upconversion and Nd3+,Yb3+ downconversion emissions. These hydrophobic core/shell nanocrystals are further converted into hydrophilic ones using thioglycolic acid as the surface ligand. The sub-10 nm water-soluble active-core/active-shell architectures with near-infrared to near-infrared dual-modal luminescence and proper paramagnetism may find potential applications in biomedical imaging and detection.