The NO2 gas sensing properties of calixarene/porphyrin mixed LB films

The NO2 gas-sensing properties of calix[8]arene/porphyrin LB films over the concentration range 0.13–4.6 ppm have been investigated. A fast (t50) response time of ∼10–15 s has been measured and the system has been shown to recover after gentle heating to around 70–90 °C. The kinetics of the adsorption process have revealed that 85–90% of the observed optical absorbance change occurs very quickly but that the final 10–15% change occurs with a much longer time constant. The former has been assigned to a fast, surface adsorption process and the latter can be thought of a slow, diffusion of analyte gas molecules through the bulk of the LB film. The concentration dependence of the sensing response follows simple Langmuir kinetics and indicates that these two-component materials may be attractive candidates for sensing NO2 in the sub 1 ppm concentration range. Pre-exposure of the mixed films to toluene vapour, prior to measuring the NO2 optical response, results in increasing the response time considerably. This is thought to occur as a result of blocking the calixarene cavities owing to their affinity for toluene.