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

The development of Tm 3+ 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 NaErF 4 :Yb@NaYF 4 :Yb@NaYF 4 :Yb,Tm@NaYF 4 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 Yb 3+ 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 Tm 3+ and visible light from Er 3+ were achieved through the energy migration among Yb 3+ and the energy back transfer from Yb 3+ to Er 3+ 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 (Tm 3+ : 3 H 4 → 3 H 6 ) under 1532 nm NIR-II excitation demonstrates huge advantages in bioimaging.