Azido-PEG2-CH2CO2H is a water soluble click chemistry reagent containing an azide group with a terminal carboxylic acid. The azide group can react 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.