Emerging NIR-II luminescent bioprobes based on lanthanide-doped nanoparticles: From design towards diverse bioapplications

The second near-infrared (NIR-II) fluorescence has shown great application prospects in the biological field because of its deep tissue penetration, weak tissue scattering, low autofluorescence, and good temporal and spatial resolution. Among many optical bioprobes, lanthanide-doped nanoparticles (LDNPs) are one of the most promising materials due to their special optical properties, such as long luminescence lifetimes, large Stokes or anti-Stokes shifts, narrow-band emission and high resistance to photobleaching and blinking. More excitingly, the abundant loadable and modifiable sites on the surface of nanoparticles provide good conditions for the construction of functional bioprobes based on LDNPs. Herein, we summarize the common synthesis method of LDNPs, the strategies to boost NIR-II luminescence and the means of surface functionalization modifications. This is followed by a review of recent research advances of NIR-II luminescent LDNPs in optical sensing, bioimaging, and image-guided therapy. Finally, the opportunities and challenges of NIR-II luminescent bioprobes based on LDNPs in bioapplications are discussed.