Used as an affinity labeling agent in biochemical research, particularly for identifying and modifying active site residues in enzymes. It selectively reacts with cysteine residues containing thiol groups, making it useful in studying enzyme mechanisms and protein structure. Commonly employed in the investigation of serine proteases and other hydrolases where it acts as an irreversible inhibitor. Also applied in the development of biosensors and labeled probes due to its strong electrophilic character and detectable nitrophenyl group.