GdF₃ hollow spheres: Self-assembly and multiple emission spanning the UV to NIR regions under 980 nm excitation

Materials with a hollow structure have drawn a lot of attentions due to their potential applications in controlled drug delivery and catalysis. When these hollow materials are integrated with upconversion nanocrystals, it is possible to modulate the release of loaded drugs with ultraviolet emission and to further trace the particles by near-infrared emission. However, reports of upconversion particles with intrinsic pores are scarce. In this work, monodispersed submicrometer GdF₃:Er,Yb hollow spheres have been synthesized via an effective one-pot hydrothermal route. These hollow spheres show an average diameter of ∼260 nm, and a shell thickness of about 90 nm. The surface of the hollow spheres is composed of small nanoparticles with a size of 16 nm. By modulating the amount of chelator, a formation mechanism for the hollow spheres has been proposed. Under excitation at 980 nm, these hollow spheres exhibit unique strong upconversion emissions spanning from the UV to the NIR, indicating that these hollow spheres hold promise for encapsulating drugs with controlled release and bioimaging in the NIR tissue transparent window.