Ytterbium as the Energy Regulator to Enhance the NIR‐IIb Luminescence of Er³⁺‐Rich Sub‐20 nm Nanocrystals for Ratiometric Fluorescence Imaging

Fluorescence/luminescence bioimaging, utilizing rare‐earth doped nanocrystals (RENCs) within the NIR‐IIb (1500–1700 nm) sub‐window, has drawn interest for its deep penetration depth and superior resolution. Herein, Er 3+ /Yb 3+ couple is rationally designed in topologically arranged RENCs that facilitate a boosted 1530 nm emission of Er 3+ via premeditated Yb 3+ ‐mediated energy transfer processes under 808 nm excitation. To maximize this modulation effect for boosting 1530 nm emission, a regional active ions concentration and spatial regulation strategy is explored in the optimized concentration and core–shell–shell structure (NaErF 4 :20%Yb@NaYbF 4 :20%Er@NaYF 4 with a quantum yield of 16.46% in NIR‐IIb region). Leveraging the superior emission and multi‐wavelength excitation characteristics of the as‐obtained nanocrystals, a NIR‐IIb‐based ratiometric fluorescence imaging measurement is achieved under 808 and 980 nm excitation, which cannot only show the 2D distribution of probes but also provide spatial depth information (range from 0 to 2.8 mm with a tissue depth resolution of 66 µm). The findings hold significant potential for advancing both vascular science and clinical medicine.