TCO-PEG3-NH2 is a click chemistry reagent featuring a TCO group, allowing it to participate in an inverse electron demand Diels-Alder reaction (iEDDA) with molecules containing tetrazine groups. The amino group of TCO-PEG3-NH2 can react with compounds containing carboxyl, ester, or other active groups to form stable amide bonds, enabling site-specific conjugation or modification of biomolecules.