HOOC-PEG-NH2 (MW 1000) [HOOC-PEG-Amine (MW 1000)] is a heterobifunctional PEG crosslinker that enables the attachment of functional PEG to biomolecules, particles, and various material surfaces. The carboxyl group reacts with amino groups to form stable amide bonds, while it can also form less stable ester bonds with hydroxyl groups. Meanwhile, the amine group is reactive with numerous amine-reactive groups such as NHS esters and aldehydes.