Used as a bifunctional crosslinking agent in bioconjugation, enabling the attachment of molecules via click chemistry. The azide group reacts selectively with alkynes under copper-catalyzed or strain-promoted conditions, while the carboxylic acid allows for amide bond formation with amines. Ideal for modifying peptides, proteins, and nanoparticles with polyethylene glycol spacers to improve solubility, reduce aggregation, and extend circulation time in biological systems. Commonly applied in drug delivery, diagnostic probes, and surface functionalization.