Protein-Enhanced NIR-IIb Emission of Indocyanine Green for Functional Bioimaging

Fluorescence imaging performed in the 1500-1700 nm spectral range (labeled near-infrared IIb, NIR-IIb) promises high imaging contrast and spatial resolution for its little photon scattering effect and minimum autofluorescence. Though inorganic and organic probes have been developed for NIR-IIb bioimaging, most are in the preclinical stage, hampering further clinical application. Herein, we showed that indocyanine green (ICG), a US Food and Drug Administration (FDA)-approved agent, exhibited a remarkable amount of NIR-IIb emission when dissolved into different protein solutions, including human serum albumin, rat bile, and fetal bovine serum. We performed fluorescence imaging in the NIR-IIb window to visualize structures of the lymph system, extrahepatic biliary tract, and cerebrovascular. The results demonstrated that proteins promoted NIR-IIb emission of ICG in vivo and that NIR-IIb imaging with ICG preserved a higher signal-to-background ratio and spatial resolution compared with the conventional NIR-II fluorescence imaging. Our findings confirm that NIR-IIb fluorescence imaging can be successfully performed using the clinically approved agent ICG. Further clinical application in the NIR-IIb region would hopefully be carried out with appropriate ICG-protein solutions.