Used in bioconjugation to label thiol-containing biomolecules such as cysteine residues in proteins or peptides. The maleimide group reacts selectively with sulfhydryl groups under mild conditions, forming a stable thioether bond. This allows site-specific attachment of the ICG fluorophore, which is highly fluorescent in the near-infrared (NIR) region. Due to its NIR emission, ICG Maleimide is widely used in fluorescence imaging for in vivo diagnostics, including tumor detection, lymph node mapping, and real-time surgical guidance. It is also employed in immunoassays and receptor targeting when conjugated to antibodies or ligands. Its high sensitivity and low background interference make it valuable in research and clinical settings.