The effects of secondary doping on ink-jet printed PEDOT:PSS gas sensors for VOCs and NO2 detection

In the study of conductive conjugated polymers, electrical doping has long played an important role. A new polymeric gas sensor has been successfully fabricated by means of an ink-jet printer using a conductive aqueous formulation of poly(3,4-ethylenedioxythiophene) poly(styrene-sulfonate) (PEDOT:PSS). A simple yet robust treatment method for the irreversible secondary doping was performed (by H2SO4 and MeOH post-treatments) to enhance conductivity and improve gas sensing performance. Real time gas sensing measurements were carried out by exposing the devices with eight different analytes in a low concentrations range of VOCs vapors up to 5 % of the saturated vapor pressure, 10 ppm of NO2 and up to 10 % of relative humidity (RH) at 21 degrees C, exploiting dry air as carrier and diluting gas. The gas response, obtained as the ratio between the steady-state resistance variation and the baseline resistance of the device, was evaluated for different PEDOT:PSS post-treated sensors. An unexpected behavior of PEDOT:PSS post-treated with concentrated H2SO4 was observed, while MeOH and diluted H2SO4 post-treated sensors exhibited improved response towards all investigated analytes. The best performances were obtained towards 5 % of ammonia and NO2 with a gas response of 6 % and 28 % respectively with the device post-treated with pure methanol and 16 % with the sensor post-treated with diluted sulfuric acid. Furthermore, long-term stability and the influence of temperature were evaluated on the fabricated sensors. Altogether, these promising results allow a better understanding of the secondary doping effects on the electrical and sensing properties, paving the way for electronic nose development.