Development of an optical membrane for humidity

4-(N,N-dioctylamino)-4-trifluoroacetyl-azobenzene (ETH^T 4001), together with the catalyst tridodecylmethylammonium chloride, is dissolved in the hydrophilic polymer polyurethane Tecoflex. The resulting membrane layers show high sensitivity toward water vapour and allow the application of the membranes for humidity measurements. Upon exposure to humid air, the membrane exhibits a decrease in absorbance at a wavelength around 490 nm and an increase at around 430 nm. This signal change is caused by the conversion of the trifluoroacetyl group of the reactand into a diol, thus changing the electron delocalisation of the reactand. The sensor layer exhibits a dynamic range from 1% to 100% RH with highest sensitivity in the 5%–40% RH range. The limit of detection is 0.5% RH. The amount of added catalyst enables the sensitive range to be tailored. The selectivity over ethanol and carbonate is sufficient for the membrane to be used for long-term measurements of air. The change in colour of the humidity-sensitive membrane from red to yellow also means it can be used as an optical test strip.