Used in bioconjugation to link PEG chains to biomolecules like proteins, peptides, or antibodies through primary amine groups. The NHS ester reacts efficiently with amines under mild conditions, forming a stable amide bond while the COOH end allows further modification. Commonly applied in drug delivery systems to improve solubility and stability of therapeutic agents, reduce immunogenicity, and extend circulation time in vivo. Also utilized in surface modification of nanoparticles, biosensors, and medical devices to enhance biocompatibility and prevent nonspecific binding. Enables controlled functionalization in hydrogel formation and tissue engineering scaffolds.