How to prevent the loss of surface functionality derived from aminosilanes

Aminosilanes are common coupling agents used to functionalize silica surfaces. A major problem in applications of 3-aminopropylsilane-functionalized silica surfaces in aqueous media was encountered: the loss of covalently attached silane layers upon exposure to water at 40 degrees C. This is attributed to siloxane bond hydrolysis catalyzed by the amine functionality. To address the issue of loss of surface functionality and to find conditions where hydrolytically stable amine-functionalized surfaces can be prepared, silanization with different types of aminosilanes was carried out. Hydrolytic stability of the resulting silane-derived layers was examined as a function of reaction conditions and the structural features of the aminosilanes. Silane layers prepared in anhydrous toluene at elevated temperature are denser and exhibit greater hydrolytic stability than those prepared in the vapor phase at elevated temperature or in toluene at room temperature. Extensive loss of surface functionality was observed in all 3-aminopropylalkoxysilane-derived layers, independent of the number and the nature of the alkoxy groups. The hydrolytic stability of aminosilane monolayers derived from N-(6-aminohexyl)aminomethyltriethoxysilane (AHAMTES) indicates that the amine-catalyzed detachment can be minimized by controlling the length of the alkyl linker in aminosilanes.