Characterization of Langmuir-Blodgett Films of Porphyrins as Coating for QMB-Based Broad-Selective Sensors

In recent years Metalloporphyrins and their derivatives have been proposed as sensitive layer for mass variation based transducers for the detection of volatile compounds. Their sensitivity/selectivity properties have suggested their application in electronic nose systems. Despite the positive applications appeared so far a systematic study in the kind of interactions ruling the sensing mechanism has not yet been conducted. The selectivity toward a variety of compounds suggests the presence of weak Van der Walls or hydrogen bond interactions, but also certain evidences about coordination interactions with the central metal atom complexed in the metalloporphyrin compounds was reported. In this paper a study oriented towards a deeper comprehension of such mechanisms is introduced. From the experimental point of view, a set of quartz microbalances, coated with Langumuir-Blodget films of various metalloporphyrins, were exposed to a number of volatile compounds at various concentrations. Linear Sorption Energy Relationship theory provides, in many cases, a satisfactory modelization of of sorption sensors in which Van der Walls and hydrogen bonding are responsible of the interaction. Nervertheless the application of such a model results although not fully adequate in the case here discussed shown the role played by the central metal in determining the sensor selectivity, furthermore the results have allowed to hypotize the presence of a coordination mechanism, superimposed to the weak interactions, between volatile compounds and metalloporphyrin films.