Nanocs dithio NHS functionalized polyethylene glycol (PEG-S-S-NHS) is a reducing reagent cleavable and amine (-NH2) reactive PEG derivative that can be used to modify protein, peptide or any other material surfaces with their available amino groups. Dithiol-NHS esters react with primary amine groups at pH 7~8.5 to form stable amide bonds, meanwhile, disulfide bond can be cleaved by DTT, TCEP and other disulfide bond reducing reagents.