Used in bioconjugation for labeling biomolecules such as proteins, peptides, and antibodies due to its reactive NHS ester group, which readily forms stable amide bonds with primary amines. The fluorescein moiety provides a fluorescent tag, enabling detection and visualization in applications like fluorescence microscopy, flow cytometry, and immunoassays. Its long spacer arm improves solubility and reduces steric hindrance, enhancing labeling efficiency. Commonly used in developing diagnostic probes and studying molecular interactions in biological systems.