Amino-PEG1-azide is a PEG derivative containing an amino group with an azide group. The hydrophilic PEG spacer increases solubility in aqueous media. The amino group is reactive with carboxylic acids, activated NHS esters, carbonyls (ketone, aldehyde) etc. The azide group can react with alkyne, BCN, DBCO via Click Chemistry to yield a stable triazole linkage.