High-intensity first near-infrared emission through energy migration in multilayered upconversion nanoparticles

The development of Tm³⁺ 807 nm first near-infrared (NIR-I, 700-1000 nm) emission with second near-infrared (NIR-II, 1000-1700 nm) excitation is urgently needed, due to its potential application in biomedicine. In this work, a range of NaErF₄:Yb@NaYF₄:Yb@NaYF₄:Yb,Tm@NaYF₄ multilayer core-shell structure upconversion nanoparticles (UCNPs) were successfully prepared by a co-precipitation method. The strongest UC emissions can be obtained by changing the concentration of Yb³⁺ in the core and the first shell, and the proposed UC process was discussed in detail. The analysis shows that high-intensity NIR-I emission (807 nm) from Tm³⁺ and visible light from Er³⁺ were achieved through the energy migration among Yb³⁺ and the energy back transfer from Yb³⁺ to Er³⁺ under 1532 nm excitation. Besides, compared to bilayer UCNPs, multilayer core-shell UCNPs display superior optical performance. The high-intensity NIR-I emission at 807 nm (Tm³⁺:³H₄ → ³H₆) under 1532 nm NIR-II excitation demonstrates huge advantages in bioimaging.