Gas Sensor with Different Morphology of PANI Layer

This work presents the design of a polymer-film-based sensor for gas detection. Different types of polyaniline are used as active layers. The advantages of resistive sensors with PANI layers are easy preparation and low production cost. At room temperature, polymer films have a high sensitivity to gas concentrations. The developed sensor works on the idea of electrical resistance shifting with gas concentration. Three different polymerization solutions are employed to synthesize the polyaniline (PANI) active layers (aqueous solution, sulfuric acid solution, and acetic acid solution). Active layers are evaluated in a controlled environment for their ability to detect ammonia, carbon monoxide, nitrogen monoxide, acetone, toluene, and relative humidity in synthetic air. PANI layers polymerized in acetic acid solutions exhibit good sensitivity toward ammonia.


Introduction
The monitoring of gaseous substances, especially toxic substances, is very important in many areas such as automotive, aviation, agriculture, security, health care, defense and security, industry, and environmental monitoring [1,2]. Ammonia, nitrogen oxides, organic volatiles, etc., are among those gasses. Medical applications of gas sensors include respiratory monitoring by analyzing carbon dioxide in exhaled air [3,4]. Active layers with organic materials achieve good sensitivity. Sensor requirements aim to make them smaller and more efficient. Research has focused on increasing resistance to gasses and higher temperatures. Polymer active layers are used in sensor arrays [5,6].
The active layers using conducting polymers are attractive for room temperature operations and easy synthesis. Sensing layers can be synthesized by various methods such as the co-precipitation method, the hydrothermal method, the sol-gel method, and microwave-assisted techniques [7]. Chemical sensors based on conducting polymers such as polyaniline (PANI), polypyrrole (PPy), and polythiophene (PTh) are suitable for their low cost, flexibility, light weight, and simple production [8]. PANI exhibits good stability in a wide range of conductivity and can be easily doped [9]. These features make polyaniline promising for industrial applications and for sensors with high selectivity, fast response times, recovery times, and the option to tailor its particular characteristics [10].
Polyaniline can form various oxidation states (fully oxidized pernigraniline, protoemeraldine, emeraldine, nigraniline, and fully reduced leucoemeraldine). The fully oxidized and reduced polyaniline is not conductive. A conducting emeraldine salt can be obtained, if the oxidation states are slightly doped (especially the emeraldine form) [11].
Low-cost printed sensors for clinical diagnostics and environmental monitoring can take advantage of the polymer active layers, such as low curing temperature (60 • C to 120 • C), thin and smooth films, low-cost production, light weight, and large-area applications [12,13].
Detection of volatile organic compounds (VOCs) using PANI produced via in situ chemical polymerization was reported in [14]. This sensor is operated at room temperature. For the detection of ammonia gas, Safe et al. [15] fabricated a nanotube form of polyaniline with a response of 6% toward 20 ppm of ammonia.
Kumar et al. [16] prepared a PANI-based flexible sensor that detects ammonia in the range of 5-1000 ppm and operates at room temperature. In [17], the optimization of printed polyaniline for gas sensing applications was described. Hydrochloric acid-doped polyaniline was electrochemically synthesized and used as a gas sensor for ammonia in [18]. An ammonia gas sensor based on flexible PANI films was used for the rapid detection of spoilage in protein-rich foods [19]. In addition, polyaniline was doped with metals and metal oxides to increase sensing performance [20][21][22].
In this paper, we presented the design of a gas sensor with polyaniline layers prepared in different polymerization solutions: (i) aqueous solution, (ii) sulfuric acid solution, and (iii) acetic acid solution. Room temperature gas-sensing properties of the fabricated gas sensors to different gas environments (i.e., carbon oxide, carbon dioxide, ammonia, nitrogen dioxide, acetone, toluene, and relative humidity) were studied.

Sensor Platform
The sensor platform KBI2 Tesla Blatná a. s. was used for the study of polyaniline layers ( Figure 1). The sensor platform was fabricated by sputtering a thin layer of platinum on a 6.2 mm × 5.25 mm ceramic substrate (Al 2 O 3 ), followed by laser trimming to form the structures of the interdigital electrodes, temperature sensor (Pt1000), and heating element. The temperature sensor and the heating element were passivated by an insulating glass layer. Sensitive layers can be deposited on interdigital electrodes by various techniques, such as printing, dipping, screen printing, or immersion. In our study, we used the technique of dipping in a polymerization solution. The temperature element allows a constant temperature to be maintained or performs thermal cycling up to 450 • C. The power consumption as a function of temperature can be seen in Figure 2. At 450 • C, a sensor's power consumption is 3 W. Figure 1b shows the structure of the sensor platform. The width and spacing of platinum ID (interdigital) electrodes are 15 µm. There are 80 individual electrode fingers (40 pairs), and the length of each is 2 mm.

Material Synthesis and Sensor Fabrication
The sensor platforms were cleaned with acetone and isopropyl alcohol for 15 min prior to the deposition of the sensing layer. At room temperature, 0.2 M aniline hydrochloride was oxidized with 0.25 M ammonium persulfate to produce polyaniline in the form of protonated emeraldine salt ( Figure 3). This method was described in [9]. The polymerization was performed in different polymerization solutions: (i) aqueous solution, (ii) sulfuric acid solution, and (iii) acetic acid solution. Since polyaniline synthesis is an exothermic process, the temperature of the reaction mixture was monitored. The effect of temperature on PANI polymerization time is shown in Figure 4. The process of polymerization was finished at 37 • C within 15 min for the aqueous solution, at 36 • C within 10 min for the sulfuric acid solution, and at 35 • C within 25 min for the acetic acid solution. Before drying the sensors on a hotplate at 60 • C for 2 h and then by silica gel in a desiccator for 24 h, they were purified using 0.2 M hydrochloric acid and acetone.

Material Synthesis and Sensor Fabrication
The sensor platforms were cleaned with acetone and isopropyl alcohol for 15 min prior to the deposition of the sensing layer. At room temperature, 0.2 M aniline hydrochloride was oxidized with 0.25 M ammonium persulfate to produce polyaniline in the

Material Synthesis and Sensor Fabrication
The sensor platforms were cleaned with acetone and isopropyl alcohol for 15 prior to the deposition of the sensing layer. At room temperature, 0.2 M aniline hy chloride was oxidized with 0.25 M ammonium persulfate to produce polyaniline in form of protonated emeraldine salt ( Figure 3). This method was described in [9]. temperature on PANI polymerization time is shown in Figure 4. The process of polymerization was finished at 37 °C within 15 min for the aqueous solution, at 36 °C within 10 min for the sulfuric acid solution, and at 35 °C within 25 min for the acetic acid solution. Before drying the sensors on a hotplate at 60 °C for 2 h and then by silica gel in a desiccator for 24 h, they were purified using 0.2 M hydrochloric acid and acetone.

Preparation of the Gas Testing System
A measuring apparatus was designed for testing the gas sensors ( Figure 5). The apparatus allows for the precise adjustment of gas concentrations using Bronkhorst mass flow controllers MFC1 to MFC3 (F-201DV-AAD-22-K in the range 10 mL-500 mL, F-201EV-AAD-22-K in the range 40 mL-2000 mL). Synthetic air (SA: 21% O2 and the rest N2) is used as a carrier gas, which also serves as a purging gas. The exact concentration of the test gas can be obtained by mixing it with the carrier gas or by diluting the saturated vapor of the desired solvent from the bubbler with dry air. This second method is mainly used to generate different concentrations of humidity and volatile compounds (ethanol, temperature on PANI polymerization time is shown in Figure 4. The process of polymer ization was finished at 37 °C within 15 min for the aqueous solution, at 36 °C within 10 min for the sulfuric acid solution, and at 35 °C within 25 min for the acetic acid solution Before drying the sensors on a hotplate at 60 °C for 2 h and then by silica gel in a desiccato for 24 h, they were purified using 0.2 M hydrochloric acid and acetone.

Preparation of the Gas Testing System
A measuring apparatus was designed for testing the gas sensors ( Figure 5). The ap paratus allows for the precise adjustment of gas concentrations using Bronkhorst mass flow controllers MFC1 to MFC3 (F-201DV-AAD-22-K in the range 10 mL-500 mL, F 201EV-AAD-22-K in the range 40 mL-2000 mL). Synthetic air (SA: 21% O2 and the rest N2 is used as a carrier gas, which also serves as a purging gas. The exact concentration of the test gas can be obtained by mixing it with the carrier gas or by diluting the saturated vapo of the desired solvent from the bubbler with dry air. This second method is mainly used to generate different concentrations of humidity and volatile compounds (ethanol

Preparation of the Gas Testing System
A measuring apparatus was designed for testing the gas sensors ( Figure 5). The apparatus allows for the precise adjustment of gas concentrations using Bronkhorst mass flow controllers MFC1 to MFC3 (F-201DV-AAD-22-K in the range 10 mL-500 mL, F-201EV-AAD-22-K in the range 40 mL-2000 mL). Synthetic air (SA: 21% O 2 and the rest N 2 ) is used as a carrier gas, which also serves as a purging gas. The exact concentration of the test gas can be obtained by mixing it with the carrier gas or by diluting the saturated vapor of the desired solvent from the bubbler with dry air. This second method is mainly used to generate different concentrations of humidity and volatile compounds (ethanol, methanol, acetone, toluene, cyclohexane, etc.). The resulting mixture is then injected into the test chamber with the tested sensor. The volume of the test chamber is 50 mL. A Keithley 2400 sourcemeter Sensors 2023, 23, 1106 5 of 14 was used to measure the resistance. A relay multiplexer is used to switch individual sensors, which is controlled by a National Instruments USB-6351 DAQ (Data Acquisition) device (16 analog inputs, 24 digital inputs/outputs, 2 analog outputs, maximum sampling rate 125 MS/s). The resistance values of the sensors are measured every 250 ms. The connection of the sensors between the multiplexer and the sourcemeter is performed using a coaxial cable to reduce interference from external electromagnetic fields. The VICI EUTA-4VL4MWE2 four-port two-way valve is used to switch the flow of gasses (test gas-purge gas) to ensure a rapid increase to the target concentration of the test gas, and to avoid the influence of overflows during switching. The apparatus and measurement process are controlled by the LabView application.
puts, maximum sampling rate 125 MS/s). The resistance values of the sensors are m ured every 250 ms. The connection of the sensors between the multiplexer and sourcemeter is performed using a coaxial cable to reduce interference from external tromagnetic fields. The VICI EUTA-4VL4MWE2 four-port two-way valve is use switch the flow of gasses (test gas-purge gas) to ensure a rapid increase to the target centration of the test gas, and to avoid the influence of overflows during switching apparatus and measurement process are controlled by the LabView application.
The change in sensor resistance (∆R/R0) as a function of exposure time was inv gated. The response of the sensor is given by the change in relative resistance, where Rg represents the resistances upon exposure to a specific gas and R0 is the refer resistances in synthetic gas. The fabricated sensors were used for the detection of am nia (NH3), carbon dioxide (CO2), nitrogen dioxide (NO2), acetone (CH3COCH3), tol (C6H5CH3), and humid air (RH) under various concentrations at room temperature.

Scanning Electron Microscopy (SEM) and Raman Spectroscopy
Scanning electron microscopy (SEM, TESCAN FERA3 GM) was used to analyz surface morphology of the deposited active layers (Figure 6a-c). All active PANI l have nanostructure morphologies, especially when polymerized in sulfuric acid solu and acetic acid solution. Polyaniline synthesized in an aqueous solution forms a gran form, while that synthesized in an acidic environment forms nanotubes. Polyaniline pared in acetic acid exhibits the highest proportion of nanotubes.
The PANI layers were also examined by Raman spectroscopy to confirm the de ited sensitive layers and their purity. Raman spectroscopy was performed at room perature using a Renishaw inVia Qontor Raman spectrometer at a wavelength of 633 The change in sensor resistance (∆R/R 0 ) as a function of exposure time was investigated. The response of the sensor is given by the change in relative resistance, where R g represents the resistances upon exposure to a specific gas and R 0 is the reference resistances in synthetic gas. The fabricated sensors were used for the detection of ammonia (NH 3 ), carbon dioxide (CO 2 ), nitrogen dioxide (NO 2 ), acetone (CH 3 COCH 3 ), toluene (C 6 H 5 CH 3 ), and humid air (RH) under various concentrations at room temperature.

Scanning Electron Microscopy (SEM) and Raman Spectroscopy
Scanning electron microscopy (SEM, TESCAN FERA3 GM) was used to analyze the surface morphology of the deposited active layers (Figure 6a-c). All active PANI layers have nanostructure morphologies, especially when polymerized in sulfuric acid solution and acetic acid solution. Polyaniline synthesized in an aqueous solution forms a granular form, while that synthesized in an acidic environment forms nanotubes. Polyaniline prepared in acetic acid exhibits the highest proportion of nanotubes. The obtained spectra are shown in Figure 7. The spectrum of pristine PANI with main bands is described in Table 1 [23,24].   The PANI layers were also examined by Raman spectroscopy to confirm the deposited sensitive layers and their purity. Raman spectroscopy was performed at room temperature using a Renishaw inVia Qontor Raman spectrometer at a wavelength of 633 nm. The obtained spectra are shown in Figure 7. The spectrum of pristine PANI with main bands is described in Table 1 [23,24]. The obtained spectra are shown in Figure 7. The spectrum of pristine PANI with main bands is described in Table 1 [23,24].

Temperature Analysis and Current-Voltage Characteristics of Polyaniline Layers
The temperature dependencies of the prepared layers are shown in Figure 8a. The average values of the temperature coefficients of resistance (TCR) can be determined according to: where α is the temperature coefficient, ∆R is the difference in electrical resistance over a given temperature range, R 0 is the initial temperature, and ∆T is the temperature range.

Temperature Analysis and Current-Voltage Characteristics of Polyaniline Layers
The temperature dependencies of the prepared layers are shown in Figure 8a. The average values of the temperature coefficients of resistance (TCR) can be determined according to: where α is the temperature coefficient, ΔR is the difference in electrical resistance over a given temperature range, R0 is the initial temperature, and ΔT is the temperature range. Table 2 shows the average values of the temperature coefficients of the different sensitive PANI layers in a temperature range from 23 °C to 80 °C.  The temperature dependencies of the polyaniline layers exhibit a negative temperature coefficient. The polyanilines formed in acidic environments have a lower temperature  The temperature dependencies of the polyaniline layers exhibit a negative temperature coefficient. The polyanilines formed in acidic environments have a lower temperature dependence than the PANI formed in aqueous environments. The temperature dependencies show an exponential behavior, which is consistent with the temperature dependence of polyaniline described by the equation [25].
where σ is the specific conductance, d is the dimension of the sample, σ 0 and T 0 are the parameters. If the sample is three-dimensional, we obtain the so-called Mott relation of temperature dependence, where the exponent is equal to: The current-voltage characteristics shown in Figure 8b exhibit linear dependencies. The electrical resistance of the polyaniline layers at 25 • C and 50% relative humidity are shown in Table 3. It indicates that polyaniline formed in acidic media has lower electrical resistance than polyaniline formed in an aqueous solution.

Gas Sensing Analysis
The DC analysis was performed due to the expected use of the sensors in applications working with a DC power supply and the possibility of simple evaluation. The electrical resistance was measured as a function of the time changes in the gas concentrations. Figure 9a-d shows the responses of PANI layers toward ammonia (12.5 ppm NH 3 ), carbon monoxide (12.5 ppm CO), carbon dioxide (250 ppm CO 2 ), and nitrogen dioxide (12.5 ppm NO 2 ). The sensitive layers were also tested with volatile organic compounds (VOCs), 0.6% of acetone, 0.05% of toluene (Figure 10a,b), and toward humidity ( Figure 11). Three cycles with the specified concentrations (5 min of exposure to the test gas, 5 min of synthetic air (SA) purging, flow rate 100 mL·s −1 ) were performed during the testing. The measuring current was set to 10 µA to reduce the possibility of heating the layers. σ = σ0·exp(−(T0/T) ), where σ is the specific conductance, d is the dimension of the sample, σ0 and T0 are the parameters. If the sample is three-dimensional, we obtain the so-called Mott relation of temperature dependence, where the exponent is equal to: The current-voltage characteristics shown in Figure 8b exhibit linear dependencies. The electrical resistivity of the polyaniline layers at 25 °C and 50% relative humidity are shown in Table 3. It indicates that polyaniline formed in acidic media has lower electrical resistivity than polyaniline formed in an aqueous solution.

Gas Sensing Analysis
The DC analysis was performed due to the expected use of the sensors in applications working with a DC power supply and the possibility of simple evaluation. The electrical resistance was measured as a function of the time changes in the gas concentrations. Figure 9a-d shows the responses of PANI layers toward ammonia (12.5 ppm NH3), carbon monoxide (12.5 ppm CO), carbon dioxide (250 ppm CO2), and nitrogen dioxide (12.5 ppm NO2). The sensitive layers were also tested with volatile organic compounds (VOCs), 0.6% of acetone, 0,05% of toluene (Figure 10a,b), and toward humidity ( Figure 11). Three cycles with the specified concentrations (5 min of exposure to the test gas, 5 min of synthetic air (SA) purging, flow rate 100 mL•s −1 ) were performed during the testing. The measuring current was set to 10 µA to reduce the possibility of heating the layers.
The active layers exhibit the highest sensitivity to ammonia (Figure 9a). Polyaniline polymerized in acetic acid exhibits the greatest sensitivity. The reactions to carbon monoxide, carbon dioxide, and nitrogen dioxide show low sensitivity (Figure 9b-d). Significant sensitivity can be observed to acetone, toluene, and humidity (Figures 10a,b and 11). PANI layers exhibit a relatively fast response time to VOCs. Electrical resistance decreases with increasing relative humidity.  Ammonia is a gas generally detected by PANI layers because the nitrogen atoms of both compounds play a similar role in forming coordination bonds with protons. The deprotonation/protonation mechanism is used to explain the sensitivity and reversibility of the mineral acid-doped PANI layer to ammonia (Figure 12). The free nitrogen doublet of the ammonia molecule can form a coordination bond with the free atomic orbital of the donating proton. This reaction leads to the deprotonation of the polyaniline nitrogen atoms, involving the removal of charge carriers (polarons) and an increase in electrical resistance. Moreover, it may participate in the gas swelling reaction of the polymer [26].  Ammonia is a gas generally detected by PANI layers because the nitrogen atoms of both compounds play a similar role in forming coordination bonds with protons. The deprotonation/protonation mechanism is used to explain the sensitivity and reversibility of the mineral acid-doped PANI layer to ammonia (Figure 12). The free nitrogen doublet of the ammonia molecule can form a coordination bond with the free atomic orbital of the donating proton. This reaction leads to the deprotonation of the polyaniline nitrogen atoms, involving the removal of charge carriers (polarons) and an increase in electrical resistance. Moreover, it may participate in the gas swelling reaction of the polymer [26].  Ammonia is a gas generally detected by PANI layers because the nitrogen atoms of both compounds play a similar role in forming coordination bonds with protons. The deprotonation/protonation mechanism is used to explain the sensitivity and reversibility of the mineral acid-doped PANI layer to ammonia (Figure 12). The free nitrogen doublet of the ammonia molecule can form a coordination bond with the free atomic orbital of the donating proton. This reaction leads to the deprotonation of the polyaniline nitrogen atoms, involving the removal of charge carriers (polarons) and an increase in electrical resistance. Moreover, it may participate in the gas swelling reaction of the polymer [26]. The active layers exhibit the highest sensitivity to ammonia (Figure 9a). Polyaniline polymerized in acetic acid exhibits the greatest sensitivity. The reactions to carbon monoxide, carbon dioxide, and nitrogen dioxide show low sensitivity (Figure 9b-d). Significant sensitivity can be observed to acetone, toluene, and humidity (Figures 10a,b and 11). PANI layers exhibit a relatively fast response time to VOCs. Electrical resistance decreases with increasing relative humidity.
Ammonia is a gas generally detected by PANI layers because the nitrogen atoms of both compounds play a similar role in forming coordination bonds with protons. The deprotonation/protonation mechanism is used to explain the sensitivity and reversibility of the mineral acid-doped PANI layer to ammonia (Figure 12). The free nitrogen doublet of the ammonia molecule can form a coordination bond with the free atomic orbital of the donating proton. This reaction leads to the deprotonation of the polyaniline nitrogen atoms, involving the removal of charge carriers (polarons) and an increase in electrical resistance. Moreover, it may participate in the gas swelling reaction of the polymer [26].
Due to high sensitivity to ammonia, the sensing layers were further tested at both room temperature (Figure 13a) and at a higher temperature of 80 °C (Figure 13b) for various concentrations. When the layers are exposed to elevated temperatures up to 80 °C during testing with ammonia, a relatively good recovery of the sensor resistance to the initial value is observed. This can be explained by the better desorption of gas from the sensitive layer by supplying thermal energy. Figure 14 shows the response of the sensor layers to repeating the same ammonia concentration (12.5 ppm). The repeatability of the sensitive layer responses is observed from these waveforms.  Due to high sensitivity to ammonia, the sensing layers were further tested at both room temperature (Figure 13a) and at a higher temperature of 80 • C (Figure 13b) for various concentrations. When the layers are exposed to elevated temperatures up to 80 • C during testing with ammonia, a relatively good recovery of the sensor resistance to the initial value is observed. This can be explained by the better desorption of gas from the sensitive layer by supplying thermal energy. Figure 14 shows the response of the sensor layers to repeating the same ammonia concentration (12.5 ppm). The repeatability of the sensitive layer responses is observed from these waveforms. Due to high sensitivity to ammonia, the sensing layers were further tested at both room temperature (Figure 13a) and at a higher temperature of 80 °C (Figure 13b) for various concentrations. When the layers are exposed to elevated temperatures up to 80 °C during testing with ammonia, a relatively good recovery of the sensor resistance to the initial value is observed. This can be explained by the better desorption of gas from the sensitive layer by supplying thermal energy. Figure 14 shows the response of the sensor layers to repeating the same ammonia concentration (12.5 ppm). The repeatability of the sensitive layer responses is observed from these waveforms.   Figure 15 summarizes the responses of the PANI layers to the tested gasses. High sensitivity is evident for ammonia, especially for PANI polymerized in acetic acid. Furthermore, it can be observed that ammonia is a reducing gas, while the other tested gasses have an oxidizing character. Similar reduction behavior was observed for chloroform [27] and hydrogen sulfide [28]. Increased sensitivity to higher concentrations of acetone and toluene can also be observed.  Figure 15 summarizes the responses of the PANI layers to the teste sensitivity is evident for ammonia, especially for PANI polymerized in a thermore, it can be observed that ammonia is a reducing gas, while the oth have an oxidizing character. Similar reduction behavior was observed for and hydrogen sulfide [28]. Increased sensitivity to higher concentrations toluene can also be observed. Our results were compared with similar works, as shown in Table  doped with acrylic acid was synthesized and its production was described exposed to various concentrations of ammonia from 1 ppm to 600 ppm sponse was ΔR/R0 = 0.99 for a concentration of 58 ppm. Measurements we room temperature. In [30], a sensor on a flexible substrate with a PANI sen prepared. The authors used inkjet technology (modified Epson C46/C48   Figure 15 summarizes the responses of the PANI layers to the test sensitivity is evident for ammonia, especially for PANI polymerized in thermore, it can be observed that ammonia is a reducing gas, while the oth have an oxidizing character. Similar reduction behavior was observed for and hydrogen sulfide [28]. Increased sensitivity to higher concentrations toluene can also be observed. Our results were compared with similar works, as shown in Table  doped with acrylic acid was synthesized and its production was describe exposed to various concentrations of ammonia from 1 ppm to 600 ppm sponse was ΔR/R0 = 0.99 for a concentration of 58 ppm. Measurements we room temperature. In [30], a sensor on a flexible substrate with a PANI sen prepared. The authors used inkjet technology (modified Epson C46/C48 deposition of PANI on the substrate. A response to an ammonia concentr at 70 °C ΔR/R0 = 0.99 was achieved. Responses ΔR/R0 = 0.3 [31] and ΔR/R achieved using PANI doped with dodecylbenzenesulfonic acid. In [33], n with hydrochloric acid were created and the sensor response achieved Δ °C. Our results were compared with similar works, as shown in Table 4. In [29], PANI doped with acrylic acid was synthesized and its production was described. The layer was exposed to various concentrations of ammonia from 1 ppm to 600 ppm. The sensor response was ∆R/R 0 = 0.99 for a concentration of 58 ppm. Measurements were performed at room temperature. In [30], a sensor on a flexible substrate with a PANI sensitive layer was prepared. The authors used inkjet technology (modified Epson C46/C48 printer) for the deposition of PANI on the substrate. A response to an ammonia concentration of 50 ppm at 70 • C ∆R/R 0 = 0.99 was achieved. Responses ∆R/R 0 = 0.3 [31] and ∆R/R0 = 0.8 [32] were achieved using PANI doped with dodecylbenzenesulfonic acid. In [33], nanofibers doped with hydrochloric acid were created and the sensor response achieved ∆R/R 0 = 2.9 at 50 • C.

Conclusions
In this paper, we demonstrated the fabrication and characterization of gas sensors with polyaniline active layers. These layers were polymerized in different polymerization solutions: aqueous, sulfuric acid, and acetic acid. Commercial Tesla Blatná platforms were used for sensors with polyaniline layers that were inserted into the polymerization solution during polymerization. These were coated with sensitive PANI layers. The sensor could be prepared with such sensitive layers in a single step, which is a significant benefit.
The surface morphology of these layers was examined using a scanning electron microscope. Polyaniline synthesized in an aqueous solution formed a granular form and in an acidic environment formed nanotubes. PANI layers polymerized in an acidic solution exhibited lower temperature sensitivity.
The polyaniline sensor was tested for the detection of CO, CO 2 , NH 3 , NO 2 , acetone, and toluene. The highest response was obtained for polyaniline polymerized in acetic acid toward ammonia (∆R/R 0 = 0.95 for 50 ppm of NH 3 ). Better reversibility could be obtained at higher operating temperatures.

Conflicts of Interest:
The authors declare no conflict of interest.