808 nm NIR light excited single-band red upconversion emission in lanthanide-doped KMnF₃ nanocrystals

A new type of upconversion nanoparticles (UCNPs) whose excitation and emission peaks are both located in the biological window has been developed based on the combination of Nd³⁺ sensitization and efficient energy transfer between Mn²⁺ and Er³⁺ ions. In our design, Nd³⁺ ions act as sensitizers for efficient absorption of 808 nm NIR photons, while the usage of Yb³⁺ ions as energy migrators facilitates the efficient energy transfer from Nd³⁺ to Er³⁺ ions. More importantly, the introduction of Mn²⁺ ions plays a key role in the realization of single-band red UC emission of Er³⁺ ions via the efficient energy transfer process between Mn²⁺ and Er³⁺. Consequently, upon 808 nm laser excitation, single-band red UC emission can be obtained in the as-synthesized Nd³⁺/Yb³⁺/Er³⁺ tri-doped KMnF₃ nanocrystals, which could be considered as attractive alternative to conventionally used UCNPs for deep-tissue imaging with mitigation of the attenuation effect associated with the emission peaks outside biological window and the overheating constraints imposed by the conventional 980 nm laser excitation.