Downshifting Luminescence Rare-Earth Nanoparticles for High-Resolution In Vivo Imaging at NIR IIb/c Window.

Rare-earth nanoparticles (RENPs) operating in the NIR-IIb/c (1600-2000 nm) window hold great promise for bioimaging due to superior spatial resolution and deep tissue penetration. However, achieving high-brightness RENPs with precise emission control remains a significant challenge. Here, we report the rational design of highly Tm3+-doped α-phase core@shell@shell@shell RENPs with a cascade Nd3+-sensitized energy transfer architecture. Optimized RENPs exhibited a quantum yield (QY) of ∼23.5% in the NIR-IIb/c region under 980 nm excitation and achieved a QY of ∼11.3% under 808 nm excitation. Notably, the emission efficiency of highly Tm3+-doped systems (≥8%) is strongly dependent on the thickness of the intermediate Yb3+ shell, while low Tm3+ doping levels (≤4%) show minimal sensitivity to shell thickness. Furthermore, the engineered RENPs exhibit excellent biocompatibility and enable high-contrast deep-tissue vascular imaging in vivo. Our work provides a promising strategy for developing next-generation RENPs tailored for high-performance, noninvasive NIR-IIb/c bioimaging applications.