Rare-earth-doped nanoparticles are one of the emerging probes for bioimaging due to their visible-to-near-infrared (NIR) upconversion emission via sequential single-photon absorption at NIR wavelengths. The NIR-excited upconversion property and high photostability make this probe appealing for deep tissue imaging. So far, upconversion nanoparticles include ytterbium ions (Yb3 + ) codoped with other rare earth ions, such as erbium (Er3 + ) and thulium (Tm3 + ). In these types of upconversion nanoparticles, through energy transfer from Yb3 + excited with continuous wave light at a wavelength of 980 nm, upconversion emission of the other rare earth dopants is induced. We have found that the use of the excitation of Er3 + in the 1550-nm wavelength region allows us to perform deep tissue imaging with reduced degradation of spatial resolution. In this excitation–emission process, three and four photons of 1550-nm light are sequentially absorbed, and Er3 + emits photons in the 550- and 660-nm wavelength regions. We demonstrate that, compared with the case using 980-nm wavelength excitation, the use of 1550-nm light enables us to moderate degradation of spatial resolution in deep tissue imaging due to the lower light scattering coefficient compared with 980-nm light. We also demonstrate that live cell imaging is feasible with this 1550 nm excitation.
Keywords: 1550 nm wavelength; erbium; high-resolution; rare-earth-doped nanoparticle; upconversion emission imaging.