A 980 nm laser-activated upconverted persistent probe for NIR-to-NIR rechargeable in vivo bioimaging

Long-lasting persistent luminescent nanoparticles (PLNPs) with efficient near-infrared (NIR) emission have emerged as a new generation of probes for in vivo optical bioimaging owing to their advantages of zero-autofluorescence benefited from the self-sustained emission after excitation, deep penetration depth, and a high signal-to-noise ratio. However, most of the PLNPs are charged by ultraviolet (UV) or visible light, remarkably limiting their applications for in vivo long-term bioimaging. Here we demonstrate 980 nm laser activated upconversion-PLNPs (UC-PLNPs) with efficient NIR emission. The NIR-emitting UC-PLNPs (Zn₃Ga₂GeO₈:Yb/Er/Cr) were synthesized by a sol-gel method with subsequent calcination. Owing to the efficient energy-transfer between Er and Cr ions, these UC-PLNPs present long-lasting up to 15 h NIR emission at 700 nm after the excitation of a 980 nm laser; in which both excitation and emission bands fall within the biological transparent window. The results of in vitro/in vivo toxicity assessments indicate that UC-PLNPs after surface modification present low biotoxicity and side effects in living animals. More importantly, the synthesized UC-PLNPs can be effectively recharged by 980 nm laser to restore in vivo persistent bioimaging signals and can also be employed as nanoprobes for in vivo UC optical bioimaging. This is the first demonstration of rechargeable UC-PLNPs for NIR-to-NIR in vivo bioimaging. We believe that the synthesized UC-PLNPs by combining UC and persistent luminescence properties into a single host may have potential applications in the bioimaging area and pave the way for widely using PLNPs for in vivo renewable long-lasting bioimaging.