The commercial poly(3,4-ethylene dioxythiophene) doped with polystyrene sulfonated acid (PEDOT-PSS) polymers were obtained in aqueous solution from Bayer Corporation; 2-(4-Dimethylaminophenylazo)benzoic acid, 4-Dimethylaminoazobenzene-2′-carboxylic acid, Acid Red (Methyl Red dye) and Ethanol Alcohol (99.8%) were purchased from Sigma-Aldrich and Vetec, respectively. For gas sensing, nitrogen was used (inert carrier) and ambient air under relative humidity (RH) at 46 % to simulate environment conditions.

The electrical measurements (current-voltage and current-time) of Au | Polymers-dye blend | Au were measured using KEITHLEY 6487 in DC voltage supply. The I(V) characteristics were obtained under variable temperature in a controlled chamber with 20 mV scanning, but the responses as a function of time was performed in real ambient conditions with nitrogen or/and ethanol flow on the sensor. Moreover, the electric resistance of the sensor element was calculated by Ohm’s Law, resulting in a very low power consumption of 20 μW when operating in air [12].

In general, gas sensing experiments were performed in nitrogen the as gas carrier without or with ethanol vapor i.e., without ethanol to measure the influence of the RH on the sensitivity and with ethanol to obtain responses at analyte. The limit detection of 26.25 ppm of the nitrogen/ethanol mixture can be determined from measured gas-phase concentrations using Henry’s Law [13]. In this case, 5.0 mL of ethanol was used with concentration a in aqueous phase at 5.0 mM and Henry’s Law constant for standard conditions of 1.9 × 10² atm/M, so their evaporation is carried by nitrogen until the surface of the sensor element.

The morphology was investigated by atomic force microscopy (AFM) of films deposited on glass substrates by spin-coating at 1,000 or 2,000 rpm. After evaporation of the solvent, a thin film shows a granular morphology, with a typical grain size of about 10–80.49 nm at 1,000 rpm. Figure 3a,b shows pictures of topographies - for which the surface area can be observed to decrease with the increasing rotation. The AFM images of the as-deposited films reveal a change on the surface; Figure 3(a) shows topography for Polymers-dye blend deposited at 2,000 rpm with a grain size of 26.50 nm and a surface area of ∼100.55 μm², in comparison with Figure 3(b) with a ∼112.57 μm² surface area when deposited at 1,000 rpm, though both samples were prepared using equal conditions. The higher surface area for chemiresistors is very important to improve the adsorption of analyte enhancing gas sensitivity [14]. In this case, the better sensors manufactured were deposited at 1,000 rpm and obtained ∼200 nm of thickness. Moreover, thin films prepared above this rotation show increased surface defects and small, non-reproducible electric responses.

In addition to nanostructured surfaces i.e., relative to modification of nano-sized grains on thin films, the increase of the surface area by inclusion of nanostructures is a good method. Recently, Vander Wal et al. [15] have shown good responses with a nanostructured surface sensor: in this work, nano-sized grains were obtained on the surface by rotation controlled.

With respect to the real-time responses of the sensor under nitrogen or analyte vapor, both measurements were obtained quickly and were reversible responses. In Figure 4, the nitrogen supply was turned ON after 13 s on the thin film surface of the sensor under 46% RH to simulate local ambient conditions, and an immediate increase in electric current was observed. In the next periodic process, the current levels increased slowly. This is due to the fact that water molecules trap electrons and change the ion transfer in the thin film, so that after removal of the water molecules on the film surface, the sensitivity increases while decreasing the resistance [16].

In general, the adsorption of the analyte and water vapors are cooperative [17,18]. Figure 5 shows the response to ethanol vapor/nitrogen mixture at ambient room temperature with 46% RH and a sensitivity of 1.15 ± 0.08. The sensitivity was obtained from the ratio between ambient resistance and resistance under analyte vapor (R_a/R_v), when ethanol vapor at 26.25 ppm is carried by nitrogen gas to the chemiresistor increasing the current through the thin film in as little as ∼0.6 s, so the resistance decrease quickly by 0.15% i.e., applying Ohm’s Law (R = V/I) and normalized resistance change (ΔR/R₀%), where R₀ is the initial resistance in air and R the resistance measured in real-time. Furthermore, the sensing efficiency decrease only 1.26 % after 10 cycles (∼200 s).

In this chemiresistor, the MR dye molecule can be controlled by acid/base reactions. Consequently, the ions of the ethanol are transferred by MR to the PEDOT-PSS by adsorption in the diimide site, increasing the doped levels of the polymer blends and reducing the resistance. Similar process are related by Sadek or Krivan et al. [19,20]. In comparison with ZnO:Al thin film gas sensor [21], relative sensitivity for ethanol vapor/air mixture was ∼1.25 at 120 °C for 400 ppm concentration, but PEDOT-PSS doped with MR dye obtained responses at room temperature for detection limit of 26.25 ppm. The measures were realized applying DC bias of 2.50 V.