Cy5.5 diacid is widely used as a fluorescent dye in biomedical imaging due to its near-infrared emission properties, which allow deep tissue penetration and minimal background interference. It is commonly employed in in vivo imaging for tracking biological processes, such as tumor targeting, vascular imaging, and monitoring drug delivery systems. Its high stability and water solubility make it suitable for conjugation with peptides, antibodies, and nanoparticles, enhancing its application in molecular diagnostics and therapeutic research. Additionally, it is utilized in fluorescence microscopy and flow cytometry for cellular and subcellular analysis, providing high-resolution imaging and quantification of biological samples.