ABSTRACT: Selective solid-state sensors for chromium(VI) and arsenic(V) based on redox quenching of the luminescence from nanostructured porous silicon and polysiloles will be reported. Tetraphenyl(polysiloles), copolymers, and corresponding germanium derivatives have been shown to sense electron acceptor molecules such as nitroaromatics either in the vapor phase or in aqueous solution. Detection at ppt levels is achieved by quenching of the intense green luminescence of the polymers by excited state electron transfer to analyte. Colloidal nanoparticulate (~120 nm by atomic force microscopy) suspensions of these polymers materials display increased sensitivity toward chromate detection; however, nitrate and perchlorate show only a weak quenching ability.
Colloidal suspensions of luminescent tetraphenylsiloles and functionalized tetraphenylsilole monomers have been prepared. The detection sensitivity can be enhanced to determine sub ppm levels of analyte by addition of surface ionizable groups, such as amines. Arsenate, another drinking water contaminant of concern, can also be detected using this approach. Work has also focused on surface functionalization as a means to enhance binding of the chromate and arsenate anions to nanostructured porous silicon photonic crystals, and to stabilize the material towards oxidation in air and water environments. In the present funding period we showed that replacing residual Si-H species on the surface with methyl groups could increase the chemical stability of porous silicon. We found that functionalized porous silicon surfaces can be reductively coupled with iodomethane (CH3I), resulting in replacement of most of the remaining Si-H bonds. Several experiments designed to quantify the stability of the modified porous silicon samples were performed, involving the use of chemical oxidants and solutions that mimic those used in bioassay applications or that might be encountered in environmental sensor applications. The use of the modified nanostructures as concentrators for environmental pollutants will be discussed.