Used in bioconjugation to link PEG chains to biomolecules like proteins, peptides, or antibodies that have primary amine groups. The NHS ester reacts efficiently with amines under mild conditions, forming a stable amide bond, while the carboxylic acid end allows further modification or attachment to surfaces. Commonly applied in drug delivery systems to improve solubility and stability of therapeutic agents, and in creating functionalized surfaces for biosensors or diagnostic devices. Also used in tissue engineering to modify scaffolds with bioactive molecules. Reactions typically occur in aqueous buffers at near-neutral pH, making it suitable for sensitive biological applications.