A combination of surface analytical techniques, colloid probe Atomic Force Microscopy (AFM) and X-ray Photoelectron Spectroscopy (XPS) was used to optimise the grafting density of covalently attached 5, 20 and 40 kDa methoxy-terminated PEO layers (under marginal solvation (cloud point) conditions for the PEO molecules). The combination of these techniques allowed us to relate the PEO layer density and molecular conformations to the range, magnitude and types of forces generated by coatings of various grafting densities. The key optimisation parameter was the grafting time with the concentration of PEO in solution having a weaker effect. Oxidation of the substrate occurred, but did not significantly limit the surface density of the functional groups used to chemically attach the PEO molecules. Interactions between the substrate and silica were electrostatic in origin and did not contribute to the interaction between silica and the PEO surfaces due to salt screening effects Surfaces with dense, highly stretched PEO layers (brushes) generated purely repulsive forces at all separation distances, arising from compression by the silica spherical probe used. The force profiles for lower density surfaces comprised long-ranged attractive and short-ranged repulsive forces. The attractive forces were most likely due to attractive bridging interactions between the PEO chains and the SiO2 surface. For low grafting densities, i.e. inter-chain grafting distances, s > ??RF, the PEO layers were not strongly stretched and free to adsorb onto the opposing silica surface. XPS analysis demonstrated that HSA and Fibrinogen adsorbed onto low density 20 kDa PEO coatings (s > ??RF), most likely via diffusion through the PEO layer. No protein adsorption was found (detection limit > 10 ng/cm2) on high density, ???strongly stretched brush??? coatings (s < ?? RF). Analysis of data from the more sensitive Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) techniques indicated that low amounts of adsorbed HSA, lactoferrin, lysozyme, and IgG were present on high density 20 and 40 kDa surfaces; the most likely explanation being attractive interactions between the proteins and the PEO layers during the protein adsorption experiments. ToF-SIMS data obtained for the strongly stretched (s < ?? RF) 5 kDa PEO surfaces suggested that no protein was adsorbed, in line with the XPS data for the same surfaces.
| Identifer | oai:union.ndltd.org:ADTP/187126 |
| Date | January 2006 |
| Creators | Hamilton-Brown, Paul, Optometry & Vision Science, Faculty of Science, UNSW |
| Publisher | Awarded by:University of New South Wales. School of Optometry and Vision Science |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | Copyright Paul Hamilton-Brown, http://unsworks.unsw.edu.au/copyright |
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