Polymeric matrices for immobilising zinc tetraphenylporphyrin in absorbance based gas sensors

Abstract

Zinc tetraphenylporphyrin (ZnTPP) was incorporated in six silicone elastomers and three thermoplastics and its stability in these membranes on storage under ambient light and in the dark over thirty-five days period was measured using absorption spectroscopy. The characteristic Soret band absorbance of ZnTPP in all the polymeric membranes stored under ambient light decreased, with the absorbance of ZnTPP in silicone membranes was scarcely measurable after 14 days. However, when stored in the dark, the rate of decrease in the Soret band absorbance of ZnTPP within these membranes appears to be dependent on the concentration of ZnTPP incorporated in them. Possible explanations for this behaviour are discussed. Ethyl cellulose and poly(vinyl chloride) yielded active membranes with little or no change in their spectral characteristics on storage in the dark. Diethyl phthalate added to these thermoplastics as a plasticiser produced the desired effect of a reduction in the response and reversal times of the membranes on exposure to ammonia gas. The response characteristics of optimised sensing membranes of PVC exposed to ammonia gas were measured. These membranes responded to ammonia gas in less than 10 seconds and are capable of detecting ammonia gas at levels less than 0.5 mg m-3, however, these membranes were also found to be sensitive to relative humidity