In situ neutron reflectometry investigation of gold-chemisorbed PEO layers of varying chain density: relationship of layer structure to protein resistance.

In work reported previously [L.D. Unsworth, H. Sheardown, J.L. Brash, Langmuir 21 (2005) 1036] we investigated protein interactions with polyethylene oxide (PEO) layers formed by chemisorption of thiol-PEO on gold. It was shown that, as a function of surface chain density, protein adsorption passed through a minimum. In follow-on work reported here, neutron reflectometry (NR) was used to investigate the formation and properties (volume fraction and chain density) of such PEO layers in situ, i.e., in contact with water. Chain density was varied by varying solubility conditions (far from and near the cloud point) and chemisorption time. Neutron experiments were carried out using neutrons of de Broglie wavelength 2.37 A. Contrast matching techniques were used to improve sensitivity. Layers formed under high solubility conditions were found to have PEO volume fraction, layer thickness and chain density of 0.33, 28 A, and 0.56 chains/nm2, respectively, after 0.5 h chemisorption; and 0.31, 28.5 A, and 0.59 chains/nm2, respectively, after 11 h, suggesting that the layer is fully formed within 0.5 h. Both chain density and PEO volume fraction in the chemisorbed layers were significantly greater when the layers were formed under low solubility conditions. The PEO layers shown in our previous work to have maximum protein resistance were found to have a PEO volume fraction of approximately 40%. Moreover the limiting volume fraction in the PEO films formed under low solubility conditions was approximately 57%, a value similar to the solubility limit of PEO in aqueous solution, suggesting that local regions in the layers may be phase separated under these conditions. This may result in increased hydrophobicity and may explain why protein adsorption was found to increase on the layers of higher chain density.