Indocyanine green (ICG) is a near-infrared fluorescence imaging dye. ICG has a maximum absorbance at a wavelength around 780 nm and its fluorescence emission peaks at about 805 nm, which is ideal for in vivo imaging applications.

ICG-NHS binds efficiently to nucleophiles, particularly to epsilon-amino functions of solvent accessible lysines on various substrates (antibodies, proteins, peptides, enzymes) and small molecules. ICG has been approved by the FDA and has been found in numerous applications in therapeutics and diagnostics including detection of solid tumors, localization of lymphnodes, visualization of retinal and choroidal vasculature, angiography during reconstructive surgery.