Enhanced near-infrared II luminescence in NaErF4 based core/shell nanocrystals via incorporating high Ce3+ ions concentration

Near-infrared II (NIR-II, 1000-1700 nm) fluorescent probes have received extensive attention in the field of biomedicine due to their deep tissue penetration, high spatial resolution and superior signal-to-noise ratio. In this work, the Ce3+ ions with high concentration were doped into both the core and shell layer of the NaErF4@NaYF4 NCs. Compared to the NaErF4@NaYF4 NCs, the NIR-II emission intensities of the NaErF4:20Ce@NaYF4:20Ce core@shell NCs increases about 5.1 times under 808 nm excitation. We verify that doping part of Ce3+ ions in the shell could not only preserve the high concentration of Er3+ ions in the core, but also facilitate the nonradiative relaxations of 4I(11/2)-> I-4(13/2) followed by the enhanced NIR-II emission intensity. The emission light could be clearly observed even at the depth of 6 mm, which show promising application in deep tissue imaging.