Er self-sensitized nanoprobes with enhanced 1525 nm downshifting emission for NIR-IIb in vivo bio-imaging.

Traditional sensitizer (Yb or Nd) and activator (Er) co-doped lanthanide-based nanoprobes possessing emission of Er at 1525 nm have attracted much attention in NIR-IIb bio-imaging. However, the 1525 nm fluorescence efficiency was not high enough in such co-doped systems due to the serious back energy transfer from the activator to the sensitizer, resulting in a lot of excitation energy loss. Herein, we have designed an efficient NIR-IIb nanoprobe Er self-sensitized NaErF:0.5%Tm@NaLuF, where substantially all the excitation energy could contribute to Er ions and most energy transfer processes were confined among Er ions, avoiding the energy dissipation by heterogeneous sensitizer ions. The influence of the types of epitaxial heterogeneous shells, the doping effect and optimal doping concentration of Tm ions, as well as the critical shell thickness for obtaining the surface quenching-assisted downshifting emission are systematically investigated to acquire the most efficient 1525 nm luminescence under 800 nm excitation. The quantum yield in the 1500-1700 nm region reached 13.92%, enabling high-resolution through-skull cerebrovascular microscopy imaging and large-depth in vivo physiological dynamic imaging with an extremely low excitation powder density of 35 mW cm. The designed nanoprobe can be potentially used for brain science research and clinical diagnosis.