Used extensively in bioconjugation for labeling biomolecules such as proteins, peptides, and antibodies. Its NHS ester group reacts efficiently with primary amines, forming stable amide bonds, making it ideal for fluorescent tagging in biological imaging. Sulfo-Cyanine5.5 is water-soluble and exhibits strong absorption and fluorescence in the near-infrared range, enabling deep tissue imaging with low background autofluorescence. Commonly applied in fluorescence microscopy, flow cytometry, and in vivo imaging studies. The sulfo group enhances solubility and reduces nonspecific binding, improving signal-to-noise ratio in complex biological environments.