t-Butoxycarbonyl-PEG4-NHS ester has a t-Boc protecting group and an NHS ester moiety. The t-butyl group can be deprotected under acidic conditions. NHS ester can react specifically and efficiently with primary amines such as the side chain of lysine residues or aminosilane-coated surfaces at neutral or slightly basic condition to form a covalent bond. The hydrophilic PEG linker increases the water solubility of the compound in aqueous media.