Biotin-PEG3-azide is PEG derivative containing a biotin group and an azide group. The azide group can react with either alkyne moiety in Cu(I)-catalyzed Click Chemistry reaction or DBCO moiety in copper-free Click Chemistry reaction to form a stable triazole linkage. The hydrophilic PEG spacer increases solubility in aqueous media of the molecules conjugated to the biotin compound. It also helps to minimize steric hindrance involved with the binding to avidin molecules.