Nonexponential relaxations in sensor arrays: forecasting strategy for electronic nose performance

The ability to select definite and unique selectivity profile from a variety of molecular organic materials predetermines their using as sensitive layers for cross-selective sensor massifs. However, to gain greater insight into how the instrumental performance depends on various parameters of such sensitive layers there is a need for a flexible, yet cost effective methodology for the estimation of the natural recourses of organic materials to monitor complex gas mixtures. In the present work the use of the cluster analysis method in the ‘fuzzy logic’ concept for the optimization of the cross-selective sensor arrays (‘electronic nose’, EN) is considered. This approach enables to purposefully form the sensor elements arrays with definite chemical functionality optimized for the solution of the specific applied problems. The criteria of the optimization of the sensor response, number and type of the sensor elements are considered with the goal of the improvement of the classification of the pharmaceutical products of the wide use. base of analysis of the response kinetic peculiarities the physical mechanisms were considered, which determines the peculiarities of the adsorption–desorption processes at the interface. It was shown that adsorption of molecules from complex mixtures decays nonexponentially in general: in a “weak” adsorbers films it is exponential and approaches a stretched exponential for “strong” adsorbers film. Moreover the stretched exponentiality in adsorption–desorption processes can be described in terms of probabilistic model of relaxation based on the self-similarity of distributed random transition rates. We have demonstrated the first application of the stretched exponential low for nonexponential relaxations of QCM sensors with organic sensitive layers. There is a good agreement between molecular structure of materials, interfacial dynamics of sensors and discrimination ability of massifs thereon.