Propargyl-PEG4-O-C1-NHS ester is widely used in bioconjugation and chemical biology for labeling and modifying biomolecules. Its NHS ester group reacts efficiently with primary amines, enabling covalent attachment to proteins, peptides, and other amine-containing compounds. The propargyl group allows for further functionalization via click chemistry, particularly copper-catalyzed azide-alkyne cycloaddition (CuAAC), making it valuable for creating complex molecular architectures. This compound is instrumental in drug delivery systems, where it facilitates the targeted modification of therapeutic agents. It is also employed in the development of diagnostic tools, such as fluorescent probes and imaging agents, by enabling precise attachment of detection moieties. Additionally, it plays a role in surface modification and material science, where it aids in the functionalization of polymers and nanoparticles for enhanced biocompatibility or specific interactions.