Cyanine3.5 carboxylic acid chloride is an activated near-infrared fluorescent dye. Its carboxylic acid group is converted into an acyl chloride (-COCl) form, which has higher reactivity. It can react quickly with the amine group (-NH₂) in biological molecules to generate a stable amide bond (-CO-NH-). It is suitable for labeling biological molecules such as proteins and peptides, and has a wide range of applications in biological imaging and analysis.