Enhancing hydrofluoric acid sensitivity in silicon polymer composites through TiO2 blending

The detection of liquid or vapor hydrofluoric acid (HF), a toxic industrial chemical, is an important area for sensor research. The approach presented here uses a conjugated silicon polymer specifically designed to react with the fluoride anion of HF, resulting in a change of the polymer conductivity. The conductivity response of polymer based coatings prepared with and without titanium dioxide (TiO₂) to aqueous HF exposure are compared, as is the response of the silicon polymer composite to HF vapor. Coatings were directly compared to the same concentration of hydrochloric, sulfuric, nitric acid, and deionized water. While many chemiresistor-based sensors depend on the physical swelling of the polymer coating in the presence of an analyte, and are therefore reversible, these sensors were designed with a polymeric coating that reacts with the analyte, thereby effecting a permanent change in properties. A permanent change in impedance of 30% in less than 600 sec. with a selectivity of 20:1 towards fluorine was observed. The lower limit of detection has not been established yet, but an impedance change of 6% occurs when the sensor is exposed to 1 ppm HF. While an impedance change greater than 100% in vapor phase hydrofluoric acid has been shown, the mechanism to explain the enhanced selectivity toward fluorine when TiO₂ is added to the sensor coating formulation is currently unknown.