Control of the Specific Adsorption of Proteins onto Gold Surfaces with Poly(L-lysine) Monolayers

Abstract

Monolayers of the polypeptide poly(L-lysine) (PL) are used to control the specific adsorption of proteins onto gold surfaces. A PL monolayer modified with biotin is electrostatically adsorbed onto a vapor-deposited gold film that has been coated with a self-assembled monolayer of the alkanethiol 11-mercaptoundecanoic acid (MUA). The immobilized biotin moieties act as specific adsorption sites for the protein avidin. Adsorption of the biopolymers onto the gold surface is monitored with a combination of surface plasmon resonance (SPR) and fluorescence measurements. By varying the percent biotinylation of the lysine residues on the PL prior to deposition, the surface coverage of avidin can be controlled to create either full or partial monolayers. The thickness of a full monolayer of avidin is 41 A, as determined by the SPR measurements. At high surface coverages of avidin, an excess of biotin sites is required to overcome steric hindrance. The PL monolayer and any adsorbed avidin can be easily rinsed from the surface with a low or high pH solution. This removal allows for quantitation of the adsorbed molecules by fluorescence measurements in solution rather than on the gold surface. In this manner, fluorescein-labeled PL and avidin are used to determine absolute surface coverages of 4 x 10 14 lysine residues cm -2 for the PL monolayer and 3 x 10 12 avidin molecules cm -2 for the full avidin monolayer. SPR imaging experiments are employed to verify that UV photopatterning of the MUA/PL bilayers can be used to spatially direct the adsorption of avidin onto the gold surface. The polylysine attachment methodology will be beneficial in the fabrication of adsorption biosensors.