Lanthanide-doped near-infrared II luminescent nanoprobes for bioapplications

Luminescent biosensing in the second nearinfrared (NIR-II) region is featured with superior spatial resolution and high penetration depth by virtue of the suppressed scattering of long-wavelength photons. Hitherto, the reported NIR-II nanoprobes are mostly based on carbon nanotubes, organic fluorophores or semiconducting quantum dots. As an alternative, trivalent lanthanide ions (Ln3+) doped nanoparticles have been emerging as a novel class of promising nanoprobes. In this review, we highlight the recent progress in the design of highly efficient Ln3+-doped NIR-II nanoparticles towards their emerging bioapplications, with an emphasis on autofluorescence-free bioimaging, sensitive bioassay, and accurate temperature sensing. Moreover, some efforts and challenges towards this rapidly expanding field are envisioned.