Three-Dimensional pH Mapping within Model Hybrid Xerogel Thin Films.

When xerogel films derived from carboxyethylsilanetriol (COE) and tetraethoxysilane (TEOS) or 3-aminopropyltriethoxysilane (APTES), n-octyltriethoxysilane (C8), and TEOS are formed on Al2O3 they exhibit chemically segregated domains with unique chemistries and topographies. These characteristics are important for marine antifouling. By using the ratiometric fluorescent probe 5 (and 6)-carboxy SNARF-1 (C.SNARF-1) in concert with confocal fluorescence microscopy, we determine the pH in three dimensions within these hybrid films. For the COE/TEOS film, 4-5 μm diameter dendritically shaped features form, and they extend ∼100 nm above the film base. These dendritic features are acidic (pH < 7) in comparison to the film base. Their average diameter decreases as we progress from the solution-film interface toward the film-Al2O3 interface. Planes located at the solution-film interface, film center, and film-Al2O3 interface exhibit acidic surface areas that are 20% below, 50% above, and 70% below the average COE mole fraction used to create the film. In the APTES/C8/TEOS films, 1-3 μm diameter mesa-shaped features form, and they extend up to 450 nm above the film base. These mesa features are basic (pH > 7) in comparison to the film base and are columnar in shape, extending without change in diameter throughout the entire film. From the solution-film interface the planes located within the first 3/4 of the film exhibit basic surface areas that are equivalent to the average APTES mole fraction used to create the film. However, as one approaches the film-Al2O3 interface, many new 100-200 nm basic subsurface regions appear. The basic surface area in those film planes within 400-500 nm of the film-Al2O3 interface are enriched in APTES by up to 500% above the average APTES mole fraction used to create the film.