Covalent Attachment and Derivatization of Poly(L-lysine) Monolayers on Gold Surfaces As Characterized by Polarization-Modulation FT-IR Spectroscopy

A monolayer of poly(L-lysine) (PL) is attached covalently via amide bonds to an alkanethiol self-assembled monolayer (SAM) on a gold surface. The amide bond is formed in two steps: the terminal carboxylic acid groups of an 11-mercaptoundecanoic acid (MUA) SAM are first activated to the N-hydroxysulfosuccinimide (NHSS) ester, followed by reaction of this MUA-NHSS ester monolayer with the amino groups of PL to create multiple amide bond linkages to the surface. The reactivity and packing density of the MUA-NHSS esters are investigated in detail by reacting these intermediates with ammonia (NH3). In the NH3 experiments, approximately 50% of the carboxylic acids in the MUA monolayer are converted to amides during the first cycle of this two-step surface reaction. This reaction yield of 50% is limited by the steric packing of the NHSS ester intermediate. However, after three cycles of MUA activation to the NHSS ester and reaction with NH3, nearly all of the MUA molecules (∼80%) are converted to amides. Polarization-modulation Fourier transform infrared reflection-absorption spectroscopy (PMFT-IRRAS) is employed to study both the NH3 and PL reactions on the gold surface. The PM-FT-IRRAS spectrum of a covalently attached PL monolayer indicates that amide bonds are formed with the underlying MUA molecules. This conclusion is confirmed by the fact that the covalent PL monolayer resists desorption despite immersion into solutions of pH < 2 or pH > 12. Finally, the PL is derivatized with a bifunctional NHSS ester-maleimide molecule either by reaction in solution prior to covalent attachment or by reaction with PL already adsorbed to the surface. Up to 50% of the total number of lysine amino groups are converted to maleimide groups, which can be used for the subsequent attachment of sulfhydryl-containing biomolecules.