Ultrasmall monodisperse NaYF₄:Yb³⁺/Tm³⁺ nanocrystals with enhanced near-infrared to near-infrared upconversion photoluminescence

Photoluminescent NaYF₄:Yb³⁺/Tm³⁺ nanocrystals are ideally suited for in vitro and in vivo photoluminescence (PL) bioimaging due to their virtue of near-infrared to near-infrared (NIR-to-NIR) upconversion (UC); they display PL with a peak at ∼800 nm if excited at-980 nm. Here, we report the synthesis of monodisperse NaYF₄:Yb ³⁺/Tm³⁺ nanocrystals of ultrasmall size (7-10 nm) with high UC efficiency. The intensity of their NIR UC emission was demonstrated to increase by up to 43 times along with an increase in the relative content of Yb³⁺ ions from 20 to 100%, with a corresponding decrease in the Y³⁺ content from 80 to 0%. The achieved ultrasmall NaYbF ₄:2% Tm³⁺ nanocrystals manifest NIR PL emission, which is 3.6 times more intense than that from 25-30 nm sized NaYF₄:20% Yb³⁺/2% Tm³⁺ nanocrystals, previously synthesized and used for in vitro and in vivo bioimaging. An optimization of both size and UC PL efficiency of NIR-to-NIR nanocrystals provides us with highly efficient optical imaging probes for bioapplications.