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 Nd(3+) between 720 and 760 nm plus efficient spatial energy transfer and migration through Nd(3+) →Yb(3+) →Yb(3+) →Tm(3+) . 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.