6-Carboxyfluorescein N-hydroxysuccinimide ester is widely used as a fluorescent labeling reagent in biochemical and biomedical research. It is particularly effective for tagging proteins, peptides, and other biomolecules due to its reactive NHS ester group, which forms stable amide bonds with primary amines. This compound is commonly employed in fluorescence microscopy, flow cytometry, and immunoassays to visualize and track cellular processes or molecular interactions. Its high quantum yield and strong fluorescence make it a valuable tool for studying cell labeling, receptor binding, and intracellular trafficking. Additionally, it is utilized in the development of fluorescent probes and sensors for detecting specific analytes in biological samples.