In this Letter, I present a versatile strategy to enhance the near-infrared to near-infrared (NIR-to-NIR) upconversion luminescence from sub-10-nm ultra-small colloidal nanoparticles through lanthanide doping under 980 nm laser excitation. It is interesting that the NIR-to-NIR upconversion emission at 801 nm of nanoparticles can be improved by increasing the doping concentration or by introducing another lanthanide activator ( or ) as a sensitizer. The luminescence enhancement effect showed a strong dependence on the doping concentrations of activator ions (, , or ). Particularly, adding 1 mol. % ions into nanoparticles induced a 2.85-fold enhancement in NIR 801 nm emission of ions. The related upconversion emission mechanisms were investigated and discussed.
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