STUDIES ON SILOXANE POLYMERS FOR NIR-EVANESCENT WAVE ABSORBENCY SENSORS

The properties of siloxane membranes were optimized for the applicatio n in optical sensors for monitoring nonpolar organic compounds in aque ous media. The sensing principle is based on the enrichment of these s ubstances in a hydrophobic polysiloxane membrane and the measurement o f their evanescent wave near-infrared (NIR) absorption spectra by mean s 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 comp ound for the different water/polymer systems, was increased by a facto r 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 i s the refractive index Oil), which can be adjusted by the degree of ph enylation. By approaching it to the value of the waveguide, the evanes cent wave interaction increases and thus the sensitivity. The RI of th e synthesized siloxanes varied between 1.41 and 1.55. The siloxanes as well as the enriched compounds show absorption bands in the same regi on of the NIR range. This leads to interferences in the spectral detec tion of the analyte. To avoid these interferences deuterated polysilox anes were synthesized by reaction of (CD3)(2)SiCl2 with methanol/aceti c acid and curing of the diols by a condensation reaction. The present ed results show that the partition coefficient and the RI of the silox ane polymers can be optimized with regard to the application in EFA se nsors. (C) 1997 Elsevier Science S.A.