Depth profiling of functionalized silane films on quartz and silicon substrates and of urease immobilized on such films by angle-resolved X-ray photoelectron spectroscopy

The thickness of nonfunctional C 18 , carboxypentadecyl, and ω-aminododecanoylaminopropyl silane films covalently anchored on quartz and silicon substrates were evaluated by angle-resolved X-ray photoelectron spectroscopy using an algorithm developed by Hill and co-workers, making use of two sets of calculations. In one set, the ratio between the Si(2p) binding energy peak intensities of the silane overlayer and the substrate was utilized to compute the silane film thickness, and calculations used an experimentally determined normalization parameter. In the second set, the ratio between the intensities of the C(1s) binding energy peak of the silane layer and the Si-(2p) signal of the substrate was used along with an appropriate experimentally obtained normalization parameter to calculate the silane layer thickness. A similar pair of computations were applied to measure the thickness of urease layers covalently attached to the ω-carboxy and ω-amino functionalized silane surfaces using N(1s) and Si(2p) binding energy peak intensities and normalization parameters. Good correlation between the theoretically estimated and experimentally obtained (by XPS and corroborated by ellipsometry) thickness values was achieved. Both polymerizable and nonpolymerizable silanes gave monolayer level surface films under the experimental conditions employed. A difference in the urease layer thickness based on the length and structure of the alkyl chain of the silane was observed.