HOOC-PEG-NH2 (MW 20000) [HOOC-PEG-Amine (MW 20000)] is a heterobifunctional PEG crosslinker that facilitates the attachment of functional PEG to biomolecules, particles, and various material surfaces. The carboxyl group reacts with amines to form stable amide bonds and can also form less stable ester bonds with hydroxyl groups. Additionally, the amine group is reactive with numerous amine-targeting groups such as NHS esters and aldehydes.