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<sup>3+</sup> ions over Er<sup>3+</sup> ions and Tm<sup>3+</sup> 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 CaF<sub>2</sub> inert shell
to shield sensitizer from quenchers, we synthesize ultrasmall NaYbF<sub>4</sub>:Tb@CaF<sub>2</sub> nanoparticles with a significant enhancement
(690-fold) in cooperative sensitization upconversion emission intensity,
compared with the parent NaYbF<sub>4</sub>:Tb. The lifetime of Tb<sup>3+</sup> emission in NaYbF<sub>4</sub>:Tb@CaF<sub>2</sub> nanoparticles
is prolonged extensively to ∼3.5 ms. Furthermore, NaYbF<sub>4</sub>:Tb@CaF<sub>2</sub> was applied in <i>in vitro</i> and <i>in vivo</i> bioimaging. The presented luminescence
enhancement strategy provides cooperative sensitization upconversion
with new opportunities for bioapplication
