X-ray/red-light excited ZGGO:Cr,Nd nanoprobes for NIR-I/II afterglow imaging.

Zn2Ga3-x-yCrxNdyGe0.75O8 (ZGGO:Crx,Ndy) persistent luminescent nanoparticles (PLNPs) (x = 0, 0.02; and y = 0, 0.01, 0.02, 0.03, 0.04) were synthesized via a hydrothermal method in combination with a following heat treatment under vacuum. The effects of Nd3+ concentration on chemical composition, size distribution, luminescence property, the persistent energy transfer from Cr3+ to Nd3+ and afterglow imaging were studied in detail. When the Nd3+ concentration was increased from 0 to 0.04, the particle size increased from 45.4 to 86.2 nm. The afterglow emissions in the NIR-I (696 nm) and NIR-II (1067 nm) regions, which are ascribed to the 2E, 4T2→4A2 transitions of Cr3+ and the 4F3/2→4I11/2 transition of Nd3+, respectively, were simultaneously acquired after stopping the 635 nm excitation. Among the nanoparticles with different concentrations, ZGGO:Cr0.02,Nd0.02 exhibits the strongest NIR-II afterglow intensity and the corresponding energy transfer efficiency from Cr3+ to Nd3+ is found to be 25.9%. In addition, enhanced X-ray excited afterglow imaging can be observed in Nd3+/Cr3+ codoped nanoparticles dispersed in water, human serum albumin solution and simulated lysosomal environment compared to the Cr3+ singly doped nanoparticles. Renewable NIR afterglow imaging was realized through an X-ray reexcitation strategy. In particular, both the X-ray excitation strategy accompanied by NIR-I afterglow emission and the red light (635 nm) excitation strategy accompanied by NIR-II afterglow emission exhibit high tissue penetration capability. This study provides a further understanding of how to develop a suitable strategy for realizing deep tissue autofluorescence-free bioimaging.