Chemical sensors based on surface-modified sol-gel-coated infrared waveguides

Attenuated total reflectance sampling, or waveguide sampling, is commonly used to obtain infrared spectra of aqueous solutions. The detection limits achieved for the determination of organic analytes in aqueous solutions using waveguide sampling are limited by the strong water absorption features and relatively small effective sample thickness due to the short penetration depth of the evanescent wave into the sample. Sol-gel processing has been used to produce a porous silica film on the surface of an infrared waveguide. The surface of the sol-gel film was modified with a silanizing reagent to produce a hydrophobic surface. The resulting waveguide was used to obtain infrared spectra from samples of benzonitrile in water. The benzonitrile spectral features obtained with the modified waveguide are enhanced by more than a factor of 10(3) when compared with those obtained with an unmodified waveguide.