Protein adsorption onto polystyrene surfaces studied by XPS and AFM

Albumin has been adsorbed onto polystyrene substrates which have been prepared by controlled oxidation using an ultraviolet/ozone system. The composition and chemistry of PS and albumin adsorbed PS surfaces have been characterised using X-ray photoelectron spectroscopy and topography has been studied using atomic force microscopy. PS surface oxygen levels from 0 to 22 at.% were produced with minimal topographical change to the surface (AFM mean roughness within the range 0.2–0.5 nm across the whole oxidation range) which allowed isolation of the surface chemical effects on the albumin adsorption. Untreated (non-polar) PS surfaces with albumin adsorbed from aqueous solutions containing 0.1, 0.05 and 0.01 mg/cm3 albumin all gave effectively the same proteinaceous nitrogen level (4.5–5.0 at.%) indicating that the adsorbed levels are independent of the initial solution concentration. For PS surfaces containing <14 at.% oxygen the measured nitrogen levels are inversely related to oxygen level, these surfaces contain mainly C–OR groups. At substrate oxygen levels of 15–22 at.% surface nitrogen levels appear independent of substrate chemistry. Nitrogen levels of 0.8, 2.2 and 3.8 at.% were recorded for the three respective solution concentrations and AFM roughness increased with surface albumin concentrations. Quartz balance measurements indicate that more albumin is adsorbed on PS than on oxidised PS. Comparison of AFM images shows that the adsorbed albumin layer on PS is thicker than that on the oxidised material. AFM images of protein adsorbed onto non-polar surfaces from all three starting concentrations are characterised by ‘peak and ridge’ topography. This changes to a ‘grainy’ type structure for albumin adsorbed on oxidised substrates. Overall the observations are consistent with a substrate driven change in adsorption mechanism from non-polar to polar substrate which is accompanied by a change in albumin conformation.