Used in bioconjugation processes to link molecules selectively, particularly in drug delivery and protein labeling. The azide group reacts efficiently via click chemistry with alkynes, enabling modular assembly of complex biomolecular structures. PEG spacer enhances solubility and reduces aggregation, while the carboxylic acid allows for easy activation and coupling to amines. Commonly applied in the development of antibody-drug conjugates, diagnostic probes, and functionalized nanoparticles. Its short PEG chain provides minimal steric interference, making it suitable for precise molecular attachments.