Used in bioconjugation to attach polyethylene glycol (PEG) chains to primary amines on proteins, peptides, or amine-modified nucleic acids, improving solubility, stability, and biocompatibility while reducing immunogenicity. Commonly applied in drug delivery systems, PEGylation of therapeutic proteins, and surface modification of nanoparticles. The NHS ester reacts efficiently at pH 7–9, enabling conjugation under mild aqueous conditions. Also utilized in creating PEGylated hydrogels and functionalizing biomaterials for medical devices.