(α-NaYbF ₄:Tm ³⁺)/CaF ₂ core/shell nanoparticles with efficient near-infrared to near-infrared upconversion for high-contrast deep tissue bioimaging

We describe the development of novel and biocompatible core/shell (α-NaYbF ₄:Tm ³⁺)/CaF ₂ nanoparticles that exhibit highly efficient NIR _in-NIR _out upconversion (UC) for high contrast and deep bioimaging. When excited at ∼980 nm, these nanoparticles emit photoluminescence (PL) peaked at ∼800 nm. The quantum yield of this UC PL under low power density excitation (∼0.3 W/cm ²) is 0.6 ± 0.1%. This high UC PL efficiency is realized by suppressing surface quenching effects via heteroepitaxial growth of a biocompatible CaF ₂ shell, which results in a 35-fold increase in the intensity of UC PL from the core. Small-animal whole-body UC PL imaging with exceptional contrast (signal-to-background ratio of 310) is shown using BALB/c mice intravenously injected with aqueously dispersed nanoparticles (700 pmol/kg). High-contrast UC PL imaging of deep tissues is also demonstrated, using a nanoparticle-loaded synthetic fibrous mesh wrapped around rat femoral bone and a cuvette with nanoparticle aqueous dispersion covered with a 3.2 cm thick animal tissue (pork).