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 with amine groups at pH 7–9, forming a stable amide bond, and the reaction is typically carried out in aqueous buffers. Enables controlled functionalization in bioengineering and pharmaceutical formulations.