DBCO (dibenzocyclooctyne) PEG acid or DBCO-PEG-carboxylic acid (DBCO-PEG-COOH) is a heterobifunctional reactive PEG derivative that can go Click Chemistry reaction without a need of any metal catalysts. The strain-promoted 1,3-dipolar cycloaddition of cyclooctynes and azides, also termed as the Cu-free click reaction, is a bioorthogonal reaction that enables the conjugation of two molecules in aqueous solution. DBCO PEG derivatives possess fast kinetics and stability in aqueous buffer. DBCO reagents can be used to label azide-modified biomolecules spontaneously without the need for toxic Cu catalysts. Carboxylic acid group, on the other hand, can be activated to react with amines, hydroxy and other functional groups.