Influence of the nature of the surface of polypyrrole films upon their interaction with volatile organic compounds

Although conducting polymers have been increasingly used in sensing devices to assess the quality of food and beverages, at present the details of the interaction mechanism between volatile organic compounds (VOCs) and the polymeric films used are not completely understood. In this work we examine how the sensitivity of polypyrrole films doped with different counter-ions (metanesulfonic acid sodium salt, octanesulfonic acid sodium salt, lithium perchlorate, p-toluenesulfonic acid monohydrate and anthraquinone-2-sulfonic acid sodium salt) towards polar (methanol, ethanol, 2-propanol and ethyl acetate) and apolar (hexane, benzene and carbon tetrachloride) VOCs can be correlated both to the chemical nature of the compounds and to the properties of the thin films determined by contact-angle measurements. The intensity of the observed response is higher for the case of the polar compounds, but in fact all sensors have shown sensitivity towards the examined VOCs. By adopting a principal component analysis it was also possible to discriminate between the different apolar compounds tested and between the members of the homologous series of alcohols. We have found that a positive correlation exists between the intensity of the response and the values of the dielectric constant, acidity, basicity and dipole moment of the VOCs, and that the sensitiveness to the polar [apolar] compounds is well correlated to the value of the polar [dispersive] component of the free surface energy of the thin films. Our results confirm that in the design of specific chemical sensors it is important to take into account the surface characteristics of the polymeric films.