NIR-II fluorescent nanophosphors for bio-imaging

The design of nanoprobes in the second window near-infrared region has grasped a substantial amount of attention due to the flexible emission in the second near-infrared region (NIR-II) region (1,000-1,700 nm). In addition, this region provides the advantage of reduced photon scattering with less autofluorescence for improvement during in vivo fluorescence imaging. NIR-II nanoprobes such as quantum dots, AIEgens, carbon nanotubes, and polymers are in a constant state of evolution for improved NIR-II emission. Among these probes, lanthanides are explored the most for NIR-II imaging applications. Moreover, nanophosphors, although in their nascent form, are interesting compounds due to their good luminescence properties with efficient energy transfer processes. Our review aims to give insight into nanophosphors, mainly for biological imaging applications. We will also provide a comparative study of lanthanides and nanophosphors for understanding the mechanism and importance of nanophosphors in the future bio-imaging field.