Adsorption of macromolecules onto functionalized surfaces
Three experiments are discussed. Each experiment uses surface plasmon resonance (SPR) spectroscopy to examine the adsorption of organic molecules from solution onto a solid substrate. A description of experimental techniques and materials is included. An extensive background discussion precedes the description of each experiment.
For the first experiment, SPR and x-ray photoelectron spectroscopies are used to determine the amount of a polyelectrolyte adsorbed to a surface with a pre-selected charge density (1). The observed behavior indicates that the Coulomb attraction between the surface charges and the polymer charges is the chief interaction controlling the adsorption for high surface charge densities, i.e., charge densities which correspond to average charge spacings less than the charge separation between the polyelectrolyte segments. When the average distance between adjacent surface charges exceeds the length of a fully extended polymer segment, the polymer layer appears to maintain a constant density. A clear shift in the adsorption occurs when the average charge separation distance on the substrate equals the intercharge separation for the polyelectrolyte.
The second experiment uses SPR spectroscopy to monitor the in situ adsorption kinetics of an antibody protein from aqueous solution onto an antigen-coated surface (2). We attribute the shape of the adsorption isotherm as indicative of an evolution in the orientation of the adsorbed antibodies. The kinetics data are interpreted in terms of random sequential adsorption.
The third experiment describes efforts to obtain an image of domain growth during a phase transition in a monolayer of amphiphilic molecules using surface plasmon resonance microscopy. The feasibility of using SPR microscopy is demonstrated. It is shown that the polarizability of the molecules in the monolayer may be higher than the polarizability of those in solution, providing evidence that the transition is occurring. The results suggest that the phase segregation, if it occurs, possesses length scales shorter than the resolution afforded by SPR microscopy.