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Preparation of Poly(3,4-ethylenedioxythiophene) Films on Piezoelectric Quartz Crystal and their Gas Sensitivities

Conducting polymers has been a subject of growing interest in recent years for sensing application, since they have the capability of offering amplified sensitivity and real time response as a result of enhanced interaction between the sensitive layer and the analyte [1-5]. However, QCM NO2 sensors did not yet have a large diffusion: the principal aim of this work is to realize a new gas sensor applying the quartz crystal microbalance (QCM) technology. The sensitive layer is obtained by electropolymerization of EDOT on metal electrodes of a 10 MHz AT-cut commercial quartz crystal. The sensor performance in terms of sensitivity and selectivity were assessed exploiting an ad hoc chemical sampling unit, and a low noise conditioning and processing electronic system. This latter consists of an oscillator hosting the piezoelectric resonator, and of a frequency measurement system granting a (short time) relative accuracy of approximately 10, which ensures a mass resolution in the order of the ng. The electropolymerization process used for the deposition of the sensing layer allows to control and standardize the deposition process and to obtain sensing layers with a high reproducibility degree. In this paper, the authors want to show the different behavior as gas sensors between three different films of poly(3,4-ethilenedioxythiophene) (Aldrich): the first film was only with LiClO4 (Fluka ) as support electrolyte. The second was in LiClO4 and NaPSSa (Fluka ) and the third was in TEAPF6 (Fluka ) and anhydrous acetonitrile (Aldrich, 99,8% pure, packaged under nitrogen) as solvent. The sensitive layers topography were measured using a Molecular Imaging AFM instrument . A set of sensors were tested for NO2 sensitivity (in a range of 2-10 ppm). The frequency shift of each sensor was measured during chemical transient alternating the exposure to a reference gas and a gas under test in different measurement protocols. In this abstract we report a measurement obtained with the following protocol: phase 0→ exposure to reference gas (N2, duration 120s); phase 1→ exposure to the gas under test (180s), and phase 2→ recovery in N2 (600s). All the measurement were conducted with a constant airflow of 200ml/min, at 35°C constant room temperature and 0% relative humidity. As shown in fig.1, this kind of sensors shows a NO2 sensitivity of the order of 2.6 Hz /ppm, and a good linearity. Preliminary tests for the sensitivity to interfering gasses such as CO and O2 also showed a good sensor selectivity toward NO2.