Improved 800 nm emission of Tm³⁺ sensitized by Yb³⁺ and Ho³⁺ in β-NaYF₄ nanocrystals under 980 nm excitation.

In vitro and in vivo bioimaging, Yb/Tm doped fluoride nanocrystals (NCs) as nanoprobes have attracted much attention due to their near infrared (NIR) upconversion (UC) emission at 800 nm under NIR 980 nm excitation. Our paper presents a simple and general method which can further improve the intensity of NIR 800 nm emission of Tm³⁺ through adding Ho³⁺ as the second sensitizer of Tm³⁺ in Yb/Tm doped NaYF₄ nanorods. The intensity of the NIR 800 nm emission is demonstrated to increase by up to 3 times along with the adding of Ho³⁺. Experimental data illustrates that the sensitizations of Tm³⁺ by both Yb³⁺ and Ho³⁺ provide a more efficient energy transfer (ET) route for intense 800 nm emission than that by Yb³⁺ alone.