TITANIUM-CONTAINING XEROGEL COATINGS AS ANTI-FOULING/FOULING-RELEASE SURFACES USING NATURALLY OCCURRING REAGENTS IN THE SEA  

An anti-fouling (AF) strategy employable on submerged surfaces uses naturally occurring reagents in seawater to create biocides in situ. Halide salts are slowly oxidized by H₂O₂ to give hypohalous acids, which are known to have biocidal properties at greater than μM concentrations. In the open ocean, concentrations of H₂O₂ approach 0.2 μM and can be much higher (up to 50 μM) in coastal areas where concentrations in rain water and runoff are in the range of 16-526 μM.  Hydrogen peroxide is also formed on submerged surfaces by organisms in the biofilm.  At these concentrations of H₂O₂ and in the presence of the 0.5 M chloride, 1 mM bromide and 1 μM iodide found in seawater, the production of positive halogen species cannot compete with the decomposition or consumption rate of positive halogen species. A catalyst to accelerate the formation of hypohalous acids is necessary for anti-fouling characteristics.

In prior work, we demonstrated that compounds derived from organoselenides and tellurides are efficient catalysts for the activation of H₂O₂, that these materials are better catalysts sequestered in xerogels than in solution, and that the presence of H₂O₂ and surface-sequestered catalyst give reduced settlement of fouling organisms on surfaces. We now describe a second generation of coatings with improved characteristics for the activation of H₂O₂. We have found that silicon-based xerogel coatings [(tetra)alkoxysilane and organo trialkoxy silanes] incorporating metal alkoxides of titanium, vanadium, and other transition metals (1-20 mole-%) in the inorganic matrix perform as catalysts for the activation of hydrogen peroxide to produce hypohalous acids on the coating surface.  As part of the inorganic matrix, the titanium and other transition metal modifications minimally impact the organization of the organic modifications on the xerogel surface. The organic modifications impart the characteristics of surface energy, charge, and topography - all critical features in the settlement of fouling organisms.  

Initial evaluations of catalytic activity indicate that the transition metal-containing xerogels catalyze the oxidations of bromide and chloride with H₂O₂ and allow the bromination and chlorination, respectively, of organic substrates. Initial biological screening has shown reduced settlement of algal zoospores on titanium-containing xerogels in the presence of H₂O₂ relative to catalyst-free controls and H₂O₂-free conditions.  The titanium-containing xerogels show reduced adhesion of algal sporelings in the presence of H₂O₂.