Functionalisation of Si/SiO₂ and glass surfaces with ultrathin dextran films and deposition of lipid bilayers

Dextran with molecular weight of 500 kDalton was covalently coupled to glass and Si/SiO₂-surfaces by epoxy functionalisation or by photo reactive functionalisation of the solid surface. With the described methods we can control the deposited mass density between 0.3 and 4.8 ng/mm² corresponding to mean film thicknesses of a dry dextran film between 2 Å and 30 Å. We studied the structural properties (thickness, density) of ultrathin layers coupled to silicon wafers or glass surfaces in humid atmosphere and under water by ellipsometry and reflection interference contrast microscopy (RICM) and developed a new method to measure the interfacial forces in ultrathin films. We demonstrate, that these hydrophilic polymer films form soft cushions which can be reversibly swollen both under water and in humidified air. In water the films swell up to thicknesses of about 600 to 800 Å as measured by reflection interference contrast microscopy (RICM) and ellipsometry. The interaction of the polymer films with Si/SiO₂-surfaces in contact with humidified air was studied by ellipsometry. We measured the thickness of the polymer layers as a function of the relative humidity of the atmosphere surrounding the sample in a hydration chamber. Depending on the humidity of the surrounding air and induced by the hydration the film thickness changes by about a factor of 10. Coupling of N-hydroxysuccinimide to dextran enables the functionalisation of the dextran cushions with a broad range of different specific binding molecules for various detection tasks. The possibility of soft hydrated dextran films as a cushion for the deposition of self healing lipid bilayers is shown.