Organically modified silicates: promising sensing materials with high porosity for SAW sensors

Sol-gel materials are widely used for the preparation of selective materials in analytical chemistry. They promise chemical stability and their properties can be varied in a wide range. A striking feature is the opportunity to prepare sol-gel materials with tailored porosity. High porosity is attained by use of organically modified materials, e.g., silicates, and subsequent heat treatment at temperatures ≥400°C. We have tested the suitability of organically modified silicates as the sensing materials of surface acoustic wave (SAW) sensors for the detection of gaseous analytes. The materials are prepared with tetraethoxysilane (TEOS) as precursor and alkyl or aryl derivatives of TEOS as organic modifiers. For this reason, SAW sensors and corresponding model sensors are coated with thin layers of the sensing materials by an electrospray procedure, followed by thermal treatment (up to 600°C). The sorption of the organic analytes is tested with model sensors by means of thermogravimetric analysis (TGA). Heat treatment causes a decrease of the mass of the sensing layer and a simultaneous increase of its sorption capacity and sorption rate. The sorption capacity depends on the organic functionality and the concentration of the modifier. In particular, after treatment at temperatures above 400°C, a considerable increase of the sorption capacity is observed. Furthermore, attempts are made with surface modification of silicate layers by organic polymers, e.g., polyisobutylene.