Limitations of Polymer Using in Gas Sensors

Strong water vapor influence (see Figs. 3.14 and 3.15c) is one of the main disadvantages of polymer-based gas sensors, because water vapor is the most common interferant for gas sensors operating at room temperature (Buchler and Ryan 1997 James et al. 2005 Pejcic et al. 2007). For example, Kaplan and Braham (1998) showed that an e-nose based on polymer gas sensors lost sensitivity in the presence of water vapor. Therefore, when humidity levels are not compensated for or controlled, it is possible that the humidity is responsible for some successful discriminations. For example, the 

14 The impedance response to humidity of undoped and doped anionic poly(phenylenevinylene) derivative. (Reprinted with permission from Liu et al. (2008). Copyright 2008 Elsevier) 

The required operating temperatures can add complexity to a gas sensor system. For example, sensors using Nafion 117 often do not reach a steady state at temperatures in excess of about 50 °C (Mika et al. 2007). 

This instability is caused by the rapid and continuous drying of the membrane in gas streams that affect the membrane s conductivity. The strong influence of the surrounding temperature on polymer conductivity (see Fig. 3.15b) is also a factor, which decreases detection limit of polymer-based chemiresistive sensors (Buchler and Ryan 1997). 

During sensor design, it is also necessary to take into account the fact that the conductivity of otherwise identical CP samples varies from preparation to preparation. Much of this variabihty is due to the method of preparation and the thickness of the flhn (Stussi et al. 1997), which has a considerable influence on the surface morphology. For example, Li et al. (2002) have shown that thin ( 300 run) films of PAni grown elecfiochemically are compact and smooth, but thicker films have filamental or dendritic surface sttucture. 

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