Used in bioconjugation, endo-BCN-PEG3-NHS ester facilitates the attachment of biomolecules to surfaces or other molecules, enhancing drug delivery systems. The endo-BCN (bicyclo[6.1.0]nonyne) group enables copper-free click chemistry via strain-promoted azide-alkyne cycloaddition (SPAAC) with azide-containing molecules, allowing precise and rapid modifications of proteins, peptides, antibodies, and nucleic acids. The NHS ester group ensures efficient coupling with primary amines, such as lysines on proteins, while the PEG3 spacer increases solubility and reduces steric hindrance. Its applications span targeted cancer therapies, diagnostic imaging, and the development of advanced bioconjugates for research and therapeutic purposes.