Characterisation and evaluation of porphyrins for the development of a proto-type vapour sensor

This thesis describes the influence of VOCs and NO2 (the analytes) on thin porphyrin films and examines the feasibility of utilising this behaviour in a sensing device. LB films of porphyrins exhibit intense optical absorption bands in the region from 400 to 800 nm. These molecules experience a significant change in their optical absorption properties as a result of exposure to appropriate analytes because the analytes bind to the porphyrins and distort their electronic energy levels. This principle has been adopted as the basis for a simple gas detection system. In-situ optical measurements have been performed to quantify the response of the films in a stream of gas of specified analyte concentration and their subsequent recovery. The sensing properties of a family of porphyrins exposed to analytes have been systematically evaluated.
Both metalled and free base porphyrins have shown response to a variety of analytes. A range of zinc porphyrins were exposed to amines, showing a large response to the analytes. Free base porphyrins showed a large response to carboxylic acids and nitrogen dioxide. It was expected that the porphyrins containing strong electron donating properties within their side groups would show a high response to analytes. Nevertheless, the interaction of porphyrin-analyte is not only limited to the electron donating nature of the side groups but also depends on the porphyrin-porphyrin interactions. Strong porphyrin-porphyrin interactions led to densely packed films; limiting the access of the analyte to the porphyrin binding sites. AFM analysis has shown that an open structure leads to a higher sensor response. Hence, a combination of the electron donating nature within the side groups and an open structure would be required to register a high sensor response.
The response of the sensing material has been improved by incorporating optically transparent host materials which disperse the porphyrin molecules and reduce detrimental aggregation on the monolayers. The expansion of various families of free base porphyrin monolayers upon the exposure to trifluoroacetic acid has been studied by developing a quartz crystal microbalance to determine how exposure to acid influences the molecular orientation on the Langmuir surface, and after deposition onto a substrate. Upon exposure, protonation of the porphine caused Coulombic repulsion between the molecules. This led to a new polar and charged system. The reorientation of the porphyrin molecules showed a lower density of the monolayers, reducing the initial pre-exposed aggregated state.
This thesis also presents a vapour chamber design which uses LB films of porphyrin as a sensitizer and a paired LED/photo detector as a transducer. This low cost sensor system is capable of the detecting different gaseous analytes. Regularly, absorption of porphyrins are modelled by the Langmuir absorption model. However, the response of these sensors has been shown to follow the Freundlich absorption model due to the non-linear response of the photo detectors. The sensor can detect concentrations of 2-methyl-2-butanol ranging from 4.36 to 41 ppt. The temperature dependence of the sensor was studied, leading to a calculated enthalpy of absorption with a value of 0.28 eV.