Sulfo-Cyanine5.5 azide tripotassium, a near-infrared (NIR) fluorescent dye, is an azide-functionalized Sulfo-Cyanine5.5 (Ex=678 nm, Em=694 nm). Sulfo-Cyanine5.5 azide tripotassium can react with alkynes for Click Chemistry labeling[1]. It contains an azide group and can undergo copper-catalyzed azide-alkyne cycloaddition (CuAAc) with molecules containing alkyne groups. It can also undergo ring strain-promoted alkyne-azide cycloaddition (SPAAC) with molecules containing DBCO or BCN groups.