Designing Upconversion Nanocrystals Capable of 745 nm Sensitization and 803 nm Emission for Deep-Tissue Imaging

A crystal design strategy is described that generates hexagonal‐phased NaYF4:Nd/Yb@NaYF4:Yb/Tm luminescent nanocrystals with the ability to emit light at 803 nm when illuminated at 745 nm. This is accomplished by taking advantage of the large absorption cross‐section of Nd3+ between 720 and 760 nm plus efficient spatial energy transfer and migration through Nd3+→Yb3+→Yb3+→Tm3+. Mechanistic investigations suggest that a cascaded two‐photon energy transfer upconversion process underlies the emission mechanism. This protocol enables deep‐tissue imaging to be achieved while mitigating the attenuation effect associated with the visible emission and the overheating constraint imposed by conventional 980 nm excitation. Intense NIR emission from a Tm3+ emitter has been realized in Nd3+‐sensitized NaYF4:Nd/Yb@NaYF4:Yb/Tm core‐shell nanocrystals upon biocompatible 745 nm excitation. The low attenuation coefficient of biotissue for light in the region of 700 to 900 nm makes these nanocrystals attractive for deep‐tissue imaging applications with minimized overheating effects (see figure).