Site-Sensitive Gas Sensing and Analyte Discrimination in Langmuir-Blodgett Porphyrin Films

We have studied the controlled adsorption of molecules of volatile compounds on thin Langmuir-Blodgett films of porphyrins deposited onto the surface of a quartz microbalance. The modification of the film optical anisotropy has been measured by reflectance anisotropy spectroscopy (RAS), and simultaneously, the amount of adsorbed molecules has been monitored. Optical spectra measured after adsorption of single analytes are interpreted straightforwardly in terms of anisotropies related to the H-like and J-like aggregation modes of porphyrins. By an appropriate selection of analytes, we have then investigated the effect of the exposure to controlled vapor mixtures. The experimental RAS line shape variation with respect to the signal measured before exposure is the sum of the variations produced by single analytes. This shows that the identification of types and even quantities of gases and vapors in gas sensing experiments is possible, and the responses of the sensing layer to single analytes could be listed in a library.