Biotin-PEG4-alkyne is PEG derivative containing a biotin group and an alkyne group. The alkyne group can react with azide moiety in Cu(I)-catalyzed 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.