Highly Enhanced Cooperative Upconversion Luminescence through Energy Transfer Optimization and Quenching Protection.

Upconversion luminescence nanomaterials have shown great potential in biological and physical applications because of their unique properties. However, limited research exists on the cooperative sensitization upconversion emission in Tb(3+) ions over Er(3+) ions and Tm(3+) ions because of its low efficiency. Herein, by optimizing the doping ratio of sensitizer and activator to maximize the utilization of the photon energy and introducing the CaF2 inert shell to shield sensitizer from quenchers, we synthesize ultrasmall NaYbF4:Tb@CaF2 nanoparticles with a significant enhancement (690-fold) in cooperative sensitization upconversion emission intensity, compared with the parent NaYbF4:Tb. The lifetime of Tb(3+) emission in NaYbF4:Tb@CaF2 nanoparticles is prolonged extensively to ∼3.5 ms. Furthermore, NaYbF4:Tb@CaF2 was applied in in vitro and in vivo bioimaging. The presented luminescence enhancement strategy provides cooperative sensitization upconversion with new opportunities for bioapplication.