Sulfo-Cyanine5.5 amine potassium is a water-soluble cyanine dye suitable for far-red/near-infrared applications such as in vivo imaging. The dye has four sulfonate groups, making it highly hydrophilic and water-soluble. Like other cyanines, sulfo-Cyanine5.5 has an excellent extinction coefficient, making it a bright fluorescent marker in the far-red region. This is an amine-containing fluorescent dye. The amine group is separated from the fluorophore by a relatively long linker that facilitates conjugation. Aliphatic primary amine groups can be coupled with various electrophiles (activated esters, epoxides, etc.) and can also be used for enzymatic transamination labeling.