Studies on siloxane polymers for NIR-evanescent wave absorbance sensors

Abstract

The properties of siloxane membranes were optimized for the application in optical sensors for monitoring nonpolar organic compounds in aqueous media. The sensing principle is based on the enrichment of these substances in a hydrophobic polysiloxane membrane and the measurement of their evanescent wave near-infrared (NIR) absorption spectra by means of a light conducting element, which is in contact with the membrane. An improvement of the membrane extraction properties was achieved by insertion of phenyl groups in the polydimethylsiloxane backbone. The partition coefficient of trichloroethene, which was used as model compound for the different water/polymer systems, was increased by a factor of 1.7 by using a phenylmethylsiloxane (molar phenyl/methyl ratio: 0.67) instead of poly(dimethylsiloxane). Another important property of siloxanes as sensing membranes in evanescent wave absorbance sensors is the refractive index (RI), which can be adjusted by the degree of phenylation. By approaching it to the value of the waveguide, the evanescent wave interaction increases and thus the sensitivity. The RI of the synthesized siloxanes varied between 1.41 and 1.55. The siloxanes as well as the enriched compounds show absorption bands in the same region of the NIR range. This leads to interferences in the spectral detection of the analyte. To avoid these interferences deuterated polysiloxanes were synthesized by reaction of (CD 3) 2SiCl 2 with methanol/acetic acid and curing of the diols by a condensation reaction. The presented results show that the partition coefficient and the RI of the siloxane polymers can be optimized with regard to the application in EFA sensors.