Azido-PEG2-t-Boc-hydrazide is a PEG derivative containing an azide group and a Boc-protected hydrazide. The hydrophilic PEG spacer increases solubility in aqueous media. The azide group can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage. The Boc can be deprotected under mild acidic conditions to form a reactive hydrazide, which can then be coupled with various carbonyl groups.