Nanoassembled Thin Film Gas Sensors. III. Sensitive Detection of Amine Odors Using TiO<sub>2</sub>/Poly(acrylic acid) Ultrathin Film Quartz Crystal Microbalance Sensors

Quartz crystal microbalance (QCM) gas sensors based on the alternate adsorption of TiO<sub>2</sub> and polyacrilic acid (PAA) were developed for the sensitive detection of amine odors. Individual TiO<sub>2</sub> gel layers could be regularly assembled with a thickness of ∼0.3 nm by the gas-phase surface sol−gel process (GSSG). The thickness of the poly(acrylic acid) (PAA) layer is dependent on its molecular weight, showing different thicknesses of ∼0.4 nm for PAA<sub>25</sub> (Mw 250 000) and 0.6−0.8 nm for PAA<sub>400</sub> (Mw 4 000 000). The QCM sensors showed a linear response to ammonia in the concentration range 0.3−15 ppm, depending on the deposition cycle of the alternate TiO<sub>2</sub>/PAA layer. The ammonia binding is based on the acid−base interaction to the free carboxylic acid groups of PAA and the limit of detection (LOD) of the 20-cycle TiO<sub>2</sub>/PAA<sub>400</sub> film was estimated to be 0.1 ppm when exposed to ammonia. The sensor response was very fast and stable in a wide relative humidity (rH) range of 30−70%, showing almost the same frequency changes at a given concentration of ammonia. Sensitivity to <i>n</i>-butylamine and ammonia was higher than to pyridine, which is owing to the difference of molecular weight and basicity of the amine analytes. The alternate TiO<sub>2</sub>/PAA<sub>400</sub> films have a highly effective ability to capture amine odors, and the ambient ammonia concentration of 15 ppm could be condensed up to ∼20 000 ppm inside the films.