Selection of gas sensing materials using the Hard Soft Acid Base theory; application to Surface Acoustic Wave CO2 detection

The Hard Soft Acid Base (HSAB) theory is introduced as a new tool to select or design sensitive materials for carbon dioxide detection with SAW-BAW (Surface Acoustic Waves - Bulk Acoustic Waves) devices. According to HSAB, CO2 is hard acid, thus small organic or inorganic molecules, or polymers which can act as hard bases could be suitable candidates as sensing layers for carbon dioxide detection. As a consequence of this theory, we propose the following polymers as potential candidates for CO2 sensing: simple polyallylamine, N-substituted polyallylamine, polydiallylamine and polyvinylamine, and mixtures of these polymers. The SAW device coated with one of the selected polymers, polyallyamine, shows good sensitivity for CO2 concentration (in the range 500–5000 ppm), long term stability and repeatability.