Rare-earth doped hexagonal NaYbF4 nanoprobes with size-controlled and NIR-II emission for multifunctional applications

Abstract

Hexagonal NaYbF₄-based fluorescent nanoparticles are widely used in near-infrared bioimaging, especially in the second near-infrared (NIR-II) window region, which has obvious advantages in imaging depth and contrast. In this region, the autofluorescence of biological tissues and the absorption and scattering of photons by tissues are significantly weakened. However, there are still many difficulties in controlling the size, morphology, crystalline phase, and monodispersity of such nanomaterials. In this study, we selected Tm³⁺ with characteristic emission at 1470 nm as the activation center, hexagonal NaYbF₄ as the main body, and produced controllable NaYbF₄:Tm³⁺ nanoparticles by the thermal decomposition method. In addition, we found that the cubic NaYbF₄:Tm³⁺ nanoparticles with irregular shapes can be transformed into highly monodisperse hexagonal NaYbF₄:Tm³⁺,Gd³⁺ nanoparticles by doping appropriate concentrations of Gd³⁺. At the same time, Yb³⁺ in the nucleus can increase the absorption cross-section at 980 nm and thus enhance the fluorescence emission of Tm³⁺. Ce³⁺ can inhibit the upconversion pathway of Tm³⁺ and thus enhance the downshifting emission at 1470 nm. Then, we epitaxially grew a layer of NaYF₄ inert shell with certain biocompatibility on the hexagonal nanoparticle core to suppress the surface-related quenching mechanism, which can significantly improve the downshifting efficiency of NaYbF₄:Tm³⁺,Gd³⁺,Ce³⁺. Finally, we used distearoyl phosphatidylethanolamine-polyethylene glycol (DSPE-mPEG₂₀₀₀) to modify the surface of NaYbF₄:Tm³⁺,Gd³⁺,Ce³⁺@NaYF₄ core–shell nanocrystals to make them highly biocompatible for bioimaging and magnetic resonance imaging (MRI).