N-(Propargyl-PEG4)-biocytin is a biotin PEG derivative containing a carboxylic group and an alkyne group. Reaction of carboxylic with primary amino (-NH2) forms stable, irreversible amide bonds. 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.