Abstract
The grafting of polymers and oligomers of ethylene oxide onto surfaces is widely used to prevent nonspecific adsorption of biological material on sensors and membrane surfaces. In this report, we show for the first time the robust covalent attachment of short oligoethylene oxide-terminated alkenes (CH3O(CH2CH2O)(3)(CH2)(11)-(CH=CH2) [EO3] and CH3O(CH2CH2O)(6)(CH2)(11)-(CH=CH2) [EO6]) from the reaction of alkenes onto silicon-rich silicon nitride surfaces at room temperature using UV light. Reflectometry is used to monitor in situ the nonspecific adsorption of bovine serum albumin (BSA) and fibrinogen (FIB) onto oligoethylene oxide coated silicon-rich silicon nitride surfaces (EOn-SixN4, x > 3) in comparison with plasma-oxidized silicon-rich silicon nitride surfaces (SiOy-SixN4) and hexadecane-coated SixN4 surfaces (C-16-SixN4). A significant reduction in protein adsorption on EOn-SixN4 surfaces was achieved, adsorption onto EO3 SixN4 and EO6-SixN4 were 0.22 mg m(-2) and 0.08 mg m(-2), respectively. The performance of the obtained EO3 and EO6 layers is comparable to those of similar, highly protein-repellent monolayers formed on gold and silver surfaces. EO6-SixN4 surfaces prevented significantly the adsorption of BSA (0.08 mg m-2). Atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), X-ray reflectivity and static water contact angle measurements were employed to characterize the modified surfaces. In addition, the stability of EO6-SixN4 surfaces in phosphate-buffered saline solution (PBS) and alkaline condition (pH 10) was studied. Prolonged exposure of the surfaces to PBS solution for 1 week or alkaline condition for 2 h resulted in only minor degradation of the ethylene oxide moieties and no oxidation of the SixN4 substrates was observed. Highly stable antifouling coatings on SixN4 surfaces significantly broaden the application potential of silicon nitride-coated microdevices, and in particular of microfabricated filtration membranes.