Multi-functional NaErF4:Yb nanorods: enhanced red upconversion emission, in vitro cell, in vivo X-ray, and T2-weighted magnetic resonance imaging

In this paper, multi-functional hexagonal phase NaErF4:Yb nanorods were synthesized by a facile hydrothermal method. The upconversion luminescence (UCL) intensity and red to green ratio of the multi-functional NaErF4 nanorods can be improved by Yb3+ doping. More importantly, owing to the decreased distance of Er and Yb, the significant enhancement of red UCL can be obtained, which is different to the usual green UCL of Yb/Er doped NaYF4 host. In addition, the intensity of UCL is strongest when the Yb3+-doped concentration reached 30%. The in vitro cell imaging and localized UCL spectra taken from HeLa cells revealed that these NaErF4: 30% Yb3+ nanorods are ideal nanoprobes with absence of autofluorescence for optical bioimaging. Moreover, these nanorods possess large X-ray absorption ions (Er3+ and doped Yb3+), and were successfully used as contrast agents for in vivo X-ray bioimaging for the first time. In addition to the excellent UCL and X-ray absorption properties, these nanorods present significant paramagnetic properties and can be used as T-2-weighted magnetic resonance imaging (MRI) agents. Therefore, these enhanced red UCL NaErF4 nanocrystals with excellent paramagnetic properties and X-ray absorption properties can be used as promising multi-modal nanoprobes for optical bioimaging, MRI, computed X-ray tomography (CT), and may have potential applications in bioseparation.