Azide-PEG-Amine(N3-PEG-NH2) is a linear bifunctional PEG linker with reactive azide (N3) that can be used to modify proteins, peptides, and other materials. The PEG moiety in the heterofunctional PEG derivatives provides water solubility, biocompatibility and flexibility. Applications are particularly suitable for the development of antibody drug conjugates (ADCs).