In Vivo High-Contrast Biomedical Imaging in the Second Near-Infrared Window Using Ultrabright Rare-Earth Nanoparticles

Intravital luminescence imaging in the second near-infrared window (NIR-II) enables noninvasive deep-tissue imaging with high spatiotemporal resolution of live mammals because of the properties of suppressed light scattering and diminished autofluorescence in the long-wavelength region. Herein, we present the synthesis of a downconversion luminescence rare-earth nanocrystal with a core-shell-shell structure (NaYF₄@NaYbF₄:Er,Ce@NaYF₄:Ca). The structure efficiently maximized the doping concentration of the sensitizers and increased Er³⁺ luminescence while preventing cross relaxation. Furthermore, Ce³⁺ doping in the middle layer efficiently limited the upconversion pathway and increased downconversion by 24-fold to produce bright 1550 nm luminescence under 975 nm excitation. Finally, optimizing the inert shell coating of NaYF₄:Ca and liposome encapsulation reduced the luminescence quenching impact by water and improved biological metabolism. Thus, our synthesized biocompatible, ultrabright NIR-II probes provide high contrast and resolution for through-scalp and through-skull luminescence imaging of mice cerebral vasculature without craniotomy as well as imaging of mouse hindlimb microvessels.