t-Butoxycarbonyl-PEG1-NHS ester contains a t-boc protecting group and an NHS ester moiety. The t-boc group is removed under acidic conditions. The hydrophilic PEG spacer increases solubility in aqueous media. The NHS ester reacts specifically and efficiently with primary amines (e.g. the side chain of lysine residues or aminosilane-coated surfaces) at neutral or slightly basic condition to form a covalent bond.