The application of metalloporphyrins as coating material for quartz microbalance-based chemical sensors

The results of both optimization and tests to prove the suitability of an array of quartz microbalance sensors (QMBs) modified with various metalloporphyrins for the determination of food freshness are presented and discussed. As far as optimization is concerned, it was found that a minimum amount of 50 mu g of metalloporphyrin must be used for the modification of the quartz microbalance sensors in order to obtain the maximum sensitivity. The sensory behavior of five different porphyrins was subsequently studied. QMBs were modified using four different meso-tetraphenylporphyrins: phenyl, p-nitrophenyl, p-bromophenyl, p-methoxyphenyl and an octa-alkylporphyrin (etio-porphyrin I), all loaded with a Co2+ metal ion. A clear decrease in the sensitivity for the etio-porphyrin I was observed whereas for the meso-tetraphenylpolphyrins the best response was obtained for the p-nitrophenyl derivative. These results can be attributed to the different electron densities which are present at the metal center of the macrocycle. The determination of the response behavior with respect to methanol, thiophene, diethylamine and triethylamine of a sensor array consisting of rhodium, ruthenium, cobalt, and manganese meso-tetraphenylporphyrin revealed that there is a clear difference in terms of the sensitivity and hence, the selectivity for the various QMBs. The rhodium and the cobalt-based QMBs were alike and demonstrated a preference for the gases with soft donating sites, i.e. thiophene and the amines. The QMBs based on ruthenium and manganese demonstrated distinctly different behavior. The ruthenium-based QMB demonstrated no clear preference for gases with either hard or soft donating sites, whereas the manganese-based QMB preferred gases with hard donating sites, i.e. methanol. These results led to the overall conclusion that this sensor array could be used for the analysis of complex gas mixtures, where the most prevalent gases fall under the categories of the amines, the alcohols and the sulphides