Selective adsorption of protein on polymer surfaces studied by soft X-ray photoemission electron microscopy

X-ray photoemission electron microscopy (X-PEEM) using synchrotron radiation illumination in the C 1s, N 1s and O 1s regions has been used to characterize a phase segregated polystyrene/polymethylmethacrylate (PS/PMMA) polymer thin film, and to map the adsorption of fibrinogen (a blood plasma protein) on this surface from both isotonic, buffered, and low ionic strength, unbuffered aqueous solutions at varying fibrinogen concentrations. The concentration dependence of the coverage correlates with independent, non-spatially resolved measurements using 125I-radiolabeled protein. At low concentrations (<0.1mg/ml) of the buffered solution, adsorption of fibrinogen occurs with strong preference for PS domains. In contrast, adsorption from similar concentrations of unbuffered solution strongly prefers the interface of the PS and PMMA domains. Increasing the solution concentration up to 1mg/ml of both buffered and unbuffered solutions leads progressively to full surface coverage (close-packed monolayer). These results demonstrate for the first time that X-PEEM with tunable soft X-rays has the sensitivity to locate and detect adsorbed proteins at the submonolayer level, while simultaneously detecting the spatial distribution of phases, and protein distribution relative to the phases, at the surface of an underlying microphase separated polymer substrate.