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Experimental and Theoretical Studies for Optimization of Polythiophene Gas Sensor Arrays

Organic transistor-based gas sensor arrays have received attention as candidates for low-cost electronic noses for environmental and product quality monitoring. Polythiophene-based organic transistors are excellent candidates for this sensor array since they have rich chemistry that can be exploited and can easily be integrated into an array (1). While polythiphenes as gas sensors have been studied to some extent (2), there has not yet been a thorough examination of the relevant factors that contribute to the sensor response. We present a systematic experimental and theoretical investigation of some of the important factors that affect the sensing response of functionalized polythiophenes. Through a combination of detailed experiments coupled to DFTbased theoretical investigations, the major dependencies were elucidated, thus establishing optimization pathways for realization of low-cost electronic noses. Functionalized polythiophenes were used as the active material of bottom-gated transistors by spin-coating onto a thermally grown silicon dioxide substrate with evaporated gold source and drain pads. The analytes were introduced in a dark, isolated chamber while the electrical characteristics of the polythiophene transistors were monitored in real time before, during, and after the exposure. Sensor responses are reported as percent changes in current from their baseline values. We find that discrimination of the analyte is affected by the side chain’s length and its functional group. Increasing the length of the side chain increases the discrimination between analytes of varying lengths (Fig. 1). The change is different depending on what functional group is present in the side chain. The presence of an ester increases the sensor response at longer chain lengths while the opposite is true in the absence of a functional group. These factors also affect the sensitivity of the polythiophenes to the analyte. Figure 2 shows the sensitivity to butylamine for an ester-functionalized thiophene. Examination of the tabulated values for different thiophenes with amine analytes (Table I) shows that sensitivity is determined most strongly by the analyte. To help explain the interaction between functional groups, density functional theory (DFT) calculations were employed. DFT does not account for morphological factors but is useful to examine the interactions between functional groups. The calculations show that the position of the analyte and the separation varies depending on the analyte. Butylamine has a favorable interaction with 3-butylthiophene exclusively at the sulfur atom of the thiophene ring. Figure 3 shows the potential plot of this system. This results in equilibrium distance of 3.55 angstroms for butylamine. By contrast, nitrobutane interacts with the face of the thiophene ring and has an equilibrium distance of 6.5 angstroms. Further exploration of the butylamine-thiophene interaction reveals that no charge transfer takes place, which correlates with the reversibility of the sensor response. The calculations also show that the presence of the amine shifts the HOMO level of the thiophene system. These results therefore suggest ways to tailor polythiophene in order to engineer sensor arrays for electronic nose applications. Figure 1: The polythiophene (PT) response as a function of side chain length for non functionalized and ester functionalized polythiophenes.