t-Boc-aminooxy-PEG4-propargyl is a PEG derivative containing a propargyl group and t-Boc-aminooxy group. The propargyl group can be reacted with azide-bearing compounds or biomolecules via copper catalyzed azide-alkyne Click Chemistry to yield a stable triazole linkage. The protected aminooxy can be deprotected under mild acidic conditions and then can be reacted with an aldehyde or ketone group to form a stable oxime linkage. The hydrophilic PEG spacer increases solubility in aqueous media.