Azido-PEG10-acid is a water soluble PEG compound containing an azide group with a terminal carboxylic acid. The azide (N3) group is reactive with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. The terminal carboxylic acid can react with primary amine groups in the presence of activators (e.g. EDC, or HATU) to form a stable amide bond.