Chlorine gas sensing using thin films of meso-tetra(p-stearamidophenyl)porphyrin

The generation, emission and use of toxic, flammable and asphyxiant gases is prevalent throughout all sections of industry today. Safety at work, control of hazardous substances, and monitoring the effect of pollutants on the environment are high on the governments list of priorities. The gas sensing industry is eager to identify cheap and inherently simple sensing methodologies with more sensitive sensing agents in order to meet new legislation for the detection of toxic gases at ever lower concentrations with greater selectivity, reversibility and reliability. One strategy is to utilise simple colour changes in thin films of chromophores which occur as a result of exposure to low levels of such toxic gases. This paper describes the modification, when exposed to chlorine gas, of the optical spectrum of a deposited thin porphyrin Langmuir–Blodgett (LB) film coated onto a plastic substrate. Films of this porphyrin (see structure of the symmetrically substituted meso-tetra(p-stearamidophenyl)porphyrin molecule) indeed exhibit a typical response time of <30 s. Our spectroscopic measurements have been carried out, in situ, enabling dynamic gas response and recovery data to be presented. The data has been compared to that expected from simple adsorption kinetics. The dependence of the gas response, on film thickness and deposition surface pressure, have also been investigated.