A surface forces and protein adsorption study of grafted PEO layers

摘要

A combination of surface analytical techniques, colloid probe Atomic Force Microscopy(AFM) and X-ray Photoelectron Spectroscopy (XPS) was used to optimise the graftingdensity of covalently attached 5, 20 and 40 kDa methoxy-terminated PEO layers (undermarginal solvation (cloud point) conditions for the PEO molecules). The combination ofthese techniques allowed us to relate the PEO layer density and molecular conformationsto the range, magnitude and types of forces generated by coatings of various graftingdensities. The key optimisation parameter was the grafting time with the concentration ofPEO in solution having a weaker effect. Oxidation of the substrate occurred, but did notsignificantly limit the surface density of the functional groups used to chemically attach thePEO molecules. Interactions between the substrate and silica were electrostatic in originand did not contribute to the interaction between silica and the PEO surfaces due to saltscreening effectsSurfaces with dense, highly stretched PEO layers (brushes) generated purely repulsiveforces at all separation distances, arising from compression by the silica spherical probeused. The force profiles for lower density surfaces comprised long-ranged attractive andshort-ranged repulsive forces. The attractive forces were most likely due to attractivebridging interactions between the PEO chains and the SiO2 surface. For low graftingdensities, i.e. inter-chain grafting distances, s ½RF, the PEO layers were not stronglystretched and free to adsorb onto the opposing silica surface.XPS analysis demonstrated that HSA and Fibrinogen adsorbed onto low density 20 kDaPEO coatings (s ½RF), most likely via diffusion through the PEO layer. No proteinadsorption was found (detection limit 10 ng/cm2) on high density, ‘strongly stretchedbrush’ coatings (s ½ RF). Analysis of data from the more sensitive Time-of-FlightSecondary Ion Mass Spectrometry (ToF-SIMS) techniques indicated that low amounts ofadsorbed HSA, lactoferrin, lysozyme, and IgG were present on high density 20 and 40 kDasurfaces; the most likely explanation being attractive interactions between the proteins andthe PEO layers during the protein adsorption experiments. ToF-SIMS data obtained forthe strongly stretched (s ½ RF) 5 kDa PEO surfaces suggested that no protein wasadsorbed, in line with the XPS data for the same surfaces.