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Frontiers in Sensors ·

Volume 7 - 2026 | https://doi.org/10.3389/fsens.2026.1889042

Processable polyaniline modified biodegradable substrates as the electrode material for gas sensor application

  • P. C. Himadri Reddy1
  • M. S. Sunitha2
  • Saravanan Chandrasekaran1*corresponding author
  1. 1Department of Chemistry, School of Engineering, Presidency University, Bengaluru, India
  2. 2Department of Chemistry, Nitte Meenakshi Institute of Technology, Nitte Deemed to Be University, Bengaluru, Karnataka, India
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Abstract

Gas sensors are extremely important because of their broad application in industrial production and everyday life. However, many conventional gas sensors require high operating temperatures and are fabricated from non-biodegradable materials, leading to increased energy consumption and disposal concerns. To address these limitations, processable conducting materials with sensing properties coated on natural and biodegradable substrates are highly desirable. In this study, organically processable polyaniline was synthesized, coated on natural substrates such as banana bracts (BB), fish scales (FS), and paper and was used as the electrode for sensing toxic gases such as ammonia (NH 3), carbon monoxide (CO), and sulfur dioxide (SO 2). Organically processable polyaniline (PANI)-maleic acid (MA)-dioctyl sodium sulfosuccinate (PAMD) was synthesized by the chemical oxidative polymerization of aniline in the presence of MA dopant, dioctyl sodium sulfosuccinate (DSS) surfactant, and ammonium peroxydisulfate initiator. The integration of DSS along with the PANI chains was confirmed by Fourier-transform infrared (FT-IR) spectroscopy, X-ray diffraction (XRD) studies, and UV–visible (UV–VIS) spectroscopy. The PAMD dispersion coated on BB, FS, and paper substrates was exposed to different toxic gases, and the corresponding resistivity changes were monitored. Among the tested substrates, PAMD coated on FS substrate exhibited the highest gas-sensing response, showing good linearity toward NH 3, CO, and SO 2 detection. The sensor also demonstrated low detection limits, excellent repeatability, and retained a high percentage of its initial response after multiple sensing cycles, confirming its good reusability and long-term stability.

Keywords

1 Introduction

Sensors for environmental monitoring are in high demand, particularly for detecting harmful gases and vapors in homes, industries, and workplaces. Among these, ammonia (NH3), carbon monoxide (CO), and sulfur dioxide (SO2) are of major concern due to their adverse effects on human health and the environment (Behera et al., 2013; Chen et al., 2007; Zhu et al., 2026; Dominguez et al., 2026; Madake et al., 2026). NH3 is widely used in fertilizers, refrigeration, and chemical industries, and excessive exposure can cause irritation to the eyes, skin, and respiratory tract. CO is a colourless and odorless toxic gas produced from the incomplete combustion of fuels, and even low concentrations can lead to serious poisoning or death. SO2, commonly released from industrial emissions and fossil fuel combustion, contributes to respiratory problems, acid rain, and atmospheric pollution. Therefore, the development of sensitive, selective, and rapid gas sensors capable of operating at room temperature is highly important for real-time monitoring and safety in domestic and industrial environments (Sunny et al., 2025; Hu et al., 2025; Hassan et al., 2025; George et al., 2025).

The disposal of electronic waste (e-waste) generated from used sensor devices is one of the fastest growing global issues. It is also important to recover valuable metals (e.g., from the electrode materials of sensor devices) and safely dispose of substrate materials from used sensor products. Existing gas sensor materials are commonly coated or fabricated on polymeric substrates such as polydimethylsiloxane, polybutadiene, polyurethane, polystyrene, polyethylene terephthalate, polycarbonate, nylon, and polyimide along with electrode materials (Seekaew et al., 2014; Andrysiewicz et al., 2020; Constantinoiu and Viespe, 2019; Park et al., 2017; Pasahan and Abadie, 2012). These petroleum-based polymeric substrate materials are non-biodegradable, and hence the recovery of metals from used electronic devices is a challenging process (Sarath et al., 2015). These environmental and economic challenges can be efficiently addressed by adopting biodegradable materials as substrate materials, which can disintegrate at the end of the sensor lifetime and facilitate the recovery of harmful and valuable materials from used sensors. Therefore, the development of biodegradable gas sensors has emerged as a promising frontier with far-reaching environmental implications (Molina et al., 2023; Ko et al., 2020; Singh et al., 2023a; Liu et al., 2016; Pena-Pereira et al., 2023).

Conducting polymer (CP) materials are organic polymer materials used as an active material for various gas sensors (Torad et al., 2020; Verma et al., 2023; Wang et al., 2020; Reddy P C et al., 2024; Bai and Shi, 2007). Among the various types of CPs, processable polyaniline (PANI) is one of the promising materials for gas sensor applications (Virji et al., 2004; Kroutil et al., 2023; Fratoddi et al., 2015; Singh et al., 2023b; Haynes and Gouma, 2009; P et al., 2010; Jain et al., 2026; Jian et al., 2019). Due to the less soluble nature of PANI in aqueous and non-aqueous media, its utilization for various applications is limited. Therefore, the synthesis of processable PANI has been an attractive research area (Kinlen et al., 1998; Kinlen et al., 1999; Li et al., 1987; Cho et al., 2019; Amarnath et al., 2007; John et al., 2008; Palaniappan and Amarnath, 2005). Soluble PANI can be achieved by substituting alkyl or alkoxy groups on the aromatic ring by copolymerization or by surfactant integration with the polymeric chain. Swaruparani et al. (2009) have synthesized soluble PANI copolymerized with o-anisidine. Soluble and processable PANI salt (Shah et al., 2019) has been synthesized by inverse emulsion polymerization pathway using diesel dispersion medium. Pa et al. (2002) described an emulsion polymerization pathway for the preparation of organically soluble PANI sulfate. Miras et al. (2008) introduced additional functional groups on PANI to alter properties such as its solubility. Zeghioud et al. (2015) prepared conducting and soluble PANI salt by chemical polymerization in the presence of itaconic acid. Soluble PANI was synthesized by Dan and Sengupta (2007) using benzene diazonium chloride salt in aqueous HCl medium. Bilal et al. (2012) synthesized completely soluble PANI-DBSA salts via inverse polymerization. Bhadra et al. (2010) reported water-soluble sulfonated PANI by stirring it with fuming sulfuric acid. Kang et al. (2017) synthesized water-soluble PANI/polystyrene sulfonate by using dialysis membrane. Water-soluble PANI using different types of cellulose derivatives has been synthesized by Hussin et al. (2017). Although PANI-based composites have demonstrated excellent gas sensing performance, their integration with biodegradable substrates remains largely unexplored (Jain et al., 2026).

In this study, a processable PANI doped with maleic acid and integrated with anionic surfactant dioctyl sodium sulfosuccinate (PAMD) was synthesized by chemical oxidative polymerization using ammonium persulfate (APS) oxidant, maleic acid (MA) as a dopant, and dioctyl sodium sulfosuccinate as a surfactant. Synthesized processable PAMD was characterised by UV–VIS, Fourier-transform infrared (FT-IR), and X-ray diffraction (XRD) analysis. One of the distinguishing features of PAMD is its exceptional solubility in organic solvents, and we have taken this advantage of this as an active material for gas sensor applications. PAMD was coated on naturally abundant biodegradable substrate materials—banana bracts (BB), fish scales (FS), and paper—to fabricate room temperature chemiresistive gas sensors. All PAMD coated substrates were subjected to a gas sensor study to assess the sensor response of PAMD-coated biodegradable materials to gases such as NH3, CO, and SO2. This research will provide a low-cost, sustainable, and environmentally friendly sensing platform while maintaining the desirable electrical properties of PAMD.

2 Experimental section

2.1 Materials

Aniline, maleic acid (MA), acetone, and methanol were obtained from SD Fine-chem Limited (India). The aniline was distilled under reduced pressure and stored in dark bottles at 5 °C until use. Dioctyl sodium sulphosuccinate (DSS), N,N-dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO) purchased from SRL Pvt. Ltd. (India) and ammonium persulfate purchased from Thermo Fisher Scientific India Pvt. Ltd. were used without further purification. Biodegradable materials such as FS recovered from fish waste, BB collected from the local vegetable market, and A4-size printing paper were used as received. FS and BB were washed thoroughly with deionized water and dried at room temperature before coating with processable PANI.

2.2 Synthesis of PAMD

In a typical synthesis of PAMD, 0.978 g (0.0022 mol) of DSS was dissolved in 50 ml of distilled water, stirred for 10 min, and divided into two parts. To the first part, 2.51 g (0.011 mol) of APS was added. To the second part, 1 ml (1.0297 g = 0.011 mol) of aniline and 1.276 g (0.011 mol) of MA were added (Himadri Reddy et al., 2024). This was added to the first part in portions with constant stirring over 20 min. Stirring continued for 4 h at room temperature. The dark green product formed was filtered and washed with distilled water, acetone, and methanol until the filtrate became colourless.

2.3 Coating procedure of PAMD on biodegradable substrates

We stirred 0.1 g of the PAMD with 1 ml of DMF to form a thick green paste. Selected FS was cleaned, dried, and cut into 1 × 1 cm dimensions. The paste was applied on one side of FS using a brush and allowed to dry. After drying, one more coating was applied and dried further. To obtain good contact of the probes with the material, a silver coating was applied at two diametric edge points. After drying the silver paint, the substrate was kept in the gas sensing chamber, the probes were set on the silver painted points, and electric resistance was recorded. A similar coating procedure was followed for BB and paper. As a representative, the material coated on FS is shown in Figure 1. The weight difference between the substrate materials before and after coating was approximately 26 mg.

PAMD salt as **(a)** a pellet; PAMD coated on **(b)** FS, **(c)** BB, and **(d)** paper substrate materials.
Figure 1. PAMD salt as (a) a pellet; PAMD coated on (b) FS, (c) BB, and (d) paper substrate materials.

2.4 Characterization

The conducting polymer PAMD was characterized by UV–visible, FT-IR spectroscopy, and XRD analysis. UV–VIS spectra were obtained using a Systronics PC-based double-beam UV–VIS spectrophotometer 2202. FT-IR spectroscopy was carried out using a Bruker, ALPHA II Compact FT-IR Spectrometer. XRD patterns of the sample were recorded using a Philips Diffractometer PW 1710 from 2θ values 10°–90° at a step of 2°/min using monochromatic Cu Kα radiation at 40 kV excitation voltage and 30 mA tube current.

As all the substrate materials used for gas sensor studies are biodegradable in nature, we did not perform biodegradable studies on the above materials. The SEM image was reported for the same sample in Himadri Reddy et al. (2024).

2.4.1 Conductivity measurement

To determine electrical conductivity, a portion of the prepared material was pressed into pellets. We used 0.14 g of the sample to prepare each pellet with a 13 mm diameter (radius 6.5 mm and thickness 0.85 mm) by applying 5 tons of pressure with a pressure holding time of 5 min. The silver paste was applied at the diagonal edge points of the pellet and allowed to dry. A two-probe digital multimeter (Model No: V8222H) was used to measure the resistance of the material. The two probes of the multimeter were placed on the silver coated points. Resistance at 25 °C was recorded after attaining stability. The electrical conductivity of the sample was calculated from the resistance. The average values of at least nine pairs of consistent readings at different points on the pressed pellet were taken. The conductivity was calculated using Equation 1:where σ is the electrical conductivity, l is the thickness of the pellet of PAMD, R is the bulk resistance of the PAMD, and A is the area of the pellet.

σ = l R X A (1)

2.5 Gas sensing study

To carry out the gas sensing study, a portion of the prepared soluble PAMD material was pressed into pellets. We used 0.15 g of the sample to prepare each pellet, with a diameter of 13 mm and a thickness of 1.2 mm, by applying 5 tons of pressure with a pressure holding time 5 min. To get good contact of the probes with the pellet, a silver coating was applied at two diametric edge points. After drying the silver paint, the pellet was kept in the gas chamber, and the probes were set on the silver painted points to record the electric resistance. The concentration of the gases used was the cylinder’s maximum concentration (parts per million-ppm) available for each gas in the laboratory. The cylinder concentration was different for different gases. First, synthetic air (80% nitrogen and 20% oxygen) was passed for approximately 30 min to attain stability in resistance (in place of N2 gas synthetic air was used). Then the specified gas and synthetic air was passed into the gas chamber alternatively at 2-min intervals, which were automatized and computer controlled. The changes in resistance were recorded every minute by a multimeter. The gas sensing study on pellet-form of PAMD and PAMD coated on biodegradable material was carried out for 1) NH3 (5 ppm (parts per million)), 2) CO (7.1 ppm), and 3) SO2 (3 ppm) gases at room temperature. As we used all the toxic gases in lowest concentrations from 3 ppm to 7.1 ppm, no limit of detection (LOD) studies were performed on the reported materials.

A Keithley digital source meter (Model No: 2,450) was used to record the change in electric resistance when the gas interacts with PAMD. The experimental set-up for gas sensing study is shown in Figure 2.

Set-up of gas sensing chamber and Keithley digital source meter.
Figure 2. Set-up of gas sensing chamber and Keithley digital source meter.

3 Results and discussion

3.1 Synthesis of organic soluble PAMD

To achieve the highly soluble PANI, several molar ratios of the aniline were varied: APS: MS: DSS (Scheme 1). The conductivity of the PAMD synthesized with the 1: 1: 1: 0.2 M ratio was 10−4 S cm-1, and good solubility in organic solvents such as DMF and DMSO solvents was obtained. The primary goal of employing DSS in this study is to increase the solubility of PANI by introducing hydrophilic and hydrophobic functionality to the PANI chains, allowing the PANI to be processable in both aqueous and non-aqueous conditions. Therefore, the concentration of DSS was varied while keeping the other components constant. We observed that it was extremely difficult to prepare the PAMD materials at low concentrations of DSS and that they are highly dispersible at higher concentration of DSS in the reaction medium. As this research is mainly focused on sensor studies, those results were not included in the manuscript. Thus, further characterisation and gas sensor studies were performed on PAMD materials, which was prepared from 1: 1: 1: 0.2 M ratio of aniline: APS: MS: DSS.

Synthesis of PAMD by chemical oxidation method.
Figure 3. Synthesis of PAMD by chemical oxidation method.

3.1.1 UV–visible spectroscopy

UV–visible spectroscopy of PAMD was carried out by dispersing the powder sample in deionized water. The sample displayed four bands: 300, 430, 590, and 880 nm. The UV–visible spectrum of PAMD is shown in Figure 3. Generally, the undoped emeraldine base form of PANI shows two absorption bands at 320 and 630 nm due to the π→π* transition of the benzenoid and quinoid ring chain, respectively (Ebrahim et al., 2014; Baruah and Mahanta, 2016). However, for the doped form of PANI, the peak at 630 nm disappeared, and new peaks appeared at around 420 and 850 nm, signifying the protonation of PANI (Izumi et al., 2007). Similarly, in the present case, the bands at 300, 430, 590, and 880 nm are associated with π–π, polaron-π, oxidized polymer-pernigraniline (de Albuquerque et al., 2004; Albuquerque et al., 2000), and π-polaron transition, respectively, signifying the protonated state of PAMD (Chellachamy Anbalagan and Sawant, 2016). The band present at 590 nm indicates that the prepared PAMD was not completely doped, which led to the low conductivity of the material (10−4 S cm-1).

UV–VIS spectra of PAMD salt.
Figure 4. UV–VIS spectra of PAMD salt.

3.1.2 FT-IR spectra

Figure 4 shows the FT-IR spectra of PAMD. The band located at 3,226 cm-1 was due to -NH stretching, and the bands at 2,925 and 2,855 were due to the symmetrical and asymmetrical stretching of aliphatic C-H of alkyl substituents of dioctyl sodium sulfosuccinate. Thus the integration of surfactant molecules with PANI chains is evident. The bands in the region of 1,554, 1,448, and 1,302 cm−1 which were also present in the conventional PANI respectively correspond to the ring-stretching vibrations of the quinoid, benzenoid rings, and C-N stretching vibration of PANI (Mohamed et al., 2015; Olad et al., 2012). An intense band at 1725 cm-1, which is attributed to the carbonyl group, was observed, confirming the presence of dopant maleic acid in the polymer chain (Palaniappan and Amarnath, 2006). The band at 791 cm-1 is attributed to CH out-of-plane bending. This band has been used as the signature peak to study the type of benzene substitution and is attributed to the 1,4-disubstituted system (Massoumi et al., 2016).

FT-IR spectra of PAMD salt.
Figure 5. FT-IR spectra of PAMD salt.

3.1.3 X-ray diffraction

PAMD was analysed by XRD studies (Figure 5). It has a highly rigid backbone due to its aromatic nature and linear structure, and it displays a partial crystalline nature. However, in the presence of some sulfonic acid dopants, PANI chains tend to be in highly ordered arrays which increases their crystallinity (Zhang et al., 2005). Significant transformation in crystallinity in PANI after doping with various dopants has also been reported (Chellachamy Anbalagan and Sawant, 2016). In the present study, the sample displayed two peaks at around 2θ = ∼20° and ∼25°. The former was attributed to the periodicity parallel to the polymer chain, while the latter was due to the periodicity perpendicular to the polymer chain (Chellachamy Anbalagan and Sawant, 2016). These findings collectively indicate that the structural features of PAMD were essentially such as conventional PANI, and dopant and surfactant did not affect the structural property of PANI.

XRD of PAMD salt.
Figure 6. XRD of PAMD salt.

3.2 Gas sensing property of PAMD pellet form and coated on biodegradable substrates

We measured gas sensing using a standard chemiresistive sensing approach, where changes in electrical resistance upon exposure to target gases were monitored using a Keithley source meter. The experimental procedure followed commonly reported methodologies for conducting polymer-based gas sensors operating at room temperature. Table 1 shows that the resistance value of PAMD pellets was in the kilo-ohms (kΩ) range, while coated PAMD with substrates was in the mega-ohms (MΩ) range. This is owing to the low concentration of PAMD in the dilute PAMD/DMF solution, as well as the non-conducting nature of the substrate materials, which increases the overall resistance of the material. Notably, the initial resistance values of PAMD materials are in the following order: pellet < paper substrate < BB substrate < FS substrate. This could because the compact packing of PAMD particles in a pressed pellet increases the contact between the particles, resulting in less resistance (higher conductivity) than PAMD material coated on various substrates. Among the various substrate materials, PAMD coated on paper substrate has lower resistance, while the FS substrate is higher. Differences in resistance values may be attributed to the porous nature of the substrate materials. The porosity of the paper enables the integration of a greater number of PAMD particles than other substrate materials, resulting in less material resistance.

TABLE 1. Initial resistance of PAMD pellet and PAMD coated materials.

MaterialInitial resistance (NH3)Initial resistance (CO)Initial resistance (SO2)
PAMD pellet0.657 kΩ0.714 kΩ0.715 kΩ
PAMD FS-substrate6.024 MΩ7.416 MΩ5.202 MΩ
PAMD BB-substrate2.981 MΩ3.178 MΩ3.706 MΩ
PAMD paper-substrate2.143 MΩ2.379 MΩ2.361 MΩ

3.2.1 Ammonia gas sensing property of PAMD

3.2.1.1 PAMD pellet ammonia sensing

The sensing capabilities of the PAMD pellet with 5 ppm of NH3 gas were investigated. After exposing the pellet to NH3 for 2 mins, a significant increase in resistance was observed, from 0.657 kΩ to 0.663 kΩ. After 2 min, synthetic air was introduced into the chamber, and the resistance changed to 0.665 kΩ (Figure 6a).

Repetitive response behaviour of **(a)** PAMD pellet, **(b)** PAMD coated on FS, **(c)** PAMD coated on BB, and **(d)** PAMD coated on paper with 5 ppm ammonia gas.
Figure 7. Repetitive response behaviour of (a) PAMD pellet, (b) PAMD coated on FS, (c) PAMD coated on BB, and (d) PAMD coated on paper with 5 ppm ammonia gas.

3.2.1.2 PAMD coated on FS ammonia sensing

The sensing capabilities of the PAMD coated on FS with NH3 were investigated (Figure 6b). After exposing the material coated on FS to NH3 for 2 min, a significant rise in resistance from 6.024 MΩ to 7.001 MΩ was observed. After 2 min, synthetic air was introduced into the chamber, and the resistance dropped to 6.345 MΩ.

3.2.1.3 PAMD coated on BB ammonia sensing

The sensing capabilities of the PAMD coated on BB with NH3 were investigated (Figure 6c). After exposing the material coated on BB to NH3 for 2 min, we observed a significant rise in resistance from 2.981 MΩ to 3.384 MΩ. After 2 min, synthetic air was introduced into the chamber, and the resistance dropped to 3.0638 MΩ.

3.2.1.4 PAMD coated on paper ammonia sensing

The sensing capabilities of the PAMD coated on paper with NH3 were investigated (Figure 6d). After exposing PAMD to NH3 for 2 min, a significant rise in resistance from 2.143 MΩ to 2.680 MΩ was observed. After 2 min, synthetic air was introduced into the chamber, and the resistance decreased to 2.225 MΩ.

It is notable that the resistance of all the above materials was marginally greater than the baseline resistance before exposure to synthetic air, suggesting that the initial resistance was not reached. As evidenced by response-recovery, it takes more time to regain the initial resistance. The variation in resistance for ten cycles is shown in the graph in Figures 6a–d.

3.2.2 Sensing of carbon monoxide

3.2.2.1 PAMD pellet carbon monoxide sensing

Investigation was carried out on the ability of PAMD pellet to sense CO gas (Figure 7a). During the exposure of the substrate to CO for 2 minutes, we recorded a decrease in resistance from an initial value of 0.714 kΩ to 0.713 kΩ. Subsequently, when CO was stopped and synthetic air was passed into the chamber for 2 min, the resistance was restored to 0.714 kΩ.

Repetitive response behaviour of **(a)** PAMD pellet, **(b)** PAMD coated on FS, **(c)** PAMD coated on BB, and **(d)** PAMD coated on paper with 7.1 ppm carbon monoxide gas.
Figure 8. Repetitive response behaviour of (a) PAMD pellet, (b) PAMD coated on FS, (c) PAMD coated on BB, and (d) PAMD coated on paper with 7.1 ppm carbon monoxide gas.

3.2.2.2 PAMD coated on FS CO sensing

Investigation was carried out on the ability of PAMD coated on FS to sense CO. During exposure of the substrate to CO for 2 minutes, we recorded a decrease in resistance from an initial value of 7.416 MΩ to 6.808 MΩ. Subsequently, when CO was stopped and synthetic air was passed into the chamber for 2 min, the resistance increased to 7.371 MΩ.

3.2.2.3 PAMD coated on BB CO sensing

Investigation was carried out on the ability of PAMD coated on BB to sense CO. During the exposure of the substrate to CO for 2 minutes, we recorded a decrease in resistance from an initial value of 3.178 MΩ to 3.131 MΩ. Subsequently, when CO was stopped and synthetic air was passed into the chamber for 2 min, the resistance increased to 3.137 MΩ.

3.2.2.4 PAMD coated on paper CO sensing

Investigation was done on the ability of PAMD coated on thick paper to sense CO. During the exposure of the substrate to CO for 2 min, we recorded a decrease in resistance from an initial value of 2.379 MΩ to 2.371 MΩ. Subsequently, when CO was stopped and synthetic air was passed into the chamber for 2 min, the resistance increased to 2.378 MΩ.

It is significant that the resistance decreased on exposure of the material to CO and increased when exposed to synthetic air. Recovery of the resistance (baseline resistance of the materials) occurred in less than 2 min. This suggests that the material has an excellent recovery time which is less than the response time. The experiment was extended through ten cycles of CO and synthetic air exposure to assess the sensor’s recovery and consistency. One notable finding from the above investigation was that, for all the cycles, all the materials exhibited a continuous decrease in resistance from the base line resistance.

3.2.3 Sensing of sulfur dioxide

3.2.3.1 PAMD pellet SO2 sensing

The gas sensing performance of the PAMD pellet toward SO2 gas was investigated. For 2 min, the chamber was filled with a regulated concentration of three parts per million (ppm) of SO2. During the exposure of the substrate to SO2 for 2 minutes, we recorded a decrease in resistance from an initial value of 0.715 kΩ to 0.714 kΩ. Subsequently, when SO2 was stopped and synthetic air was passed into the chamber for 2 minutes, the resistance increased to 0.715 kΩ.

3.2.3.2 PAMD coated on FS SO2 sensing

Investigation was carried out on the ability of PAMD coated on FS to sense SO2 gas. During exposure of the substrate to SO2 for 2 min, we recorded a decrease in resistance from an initial value of 5.202 MΩ to 4.955 MΩ. Subsequently, when SO2 was stopped and synthetic air was passed into the chamber for 2 min, the resistance increased to 5.205 MΩ.

3.2.3.3 PAMD coated on BB SO2 sensing

A study was conducted on the SO2 sensing capability of the PAMD coated on BB. During exposure of the substrate to SO2 for 2 min, we recorded a decrease in resistance from an initial value of 3.706 MΩ to 3.559 MΩ. Subsequently, when SO2 was stopped and synthetic air was passed into the chamber for 2 min, the resistance increased to 3.584 MΩ.

3.2.3.4 PAMD coated on paper SO2 sensing

The ability of the PAMD coated on thick paper to sense SO2 was investigated. During exposure of the substrate to SO2 for 2 min, we recorded a decrease in resistance from an initial value of 2.361 MΩ to 2.348 MΩ. Subsequently, when SO2 was stopped and synthetic air was passed into the chamber for 2 min, the resistance increased to 2.359 MΩ.

It is notable that the resistance decreased on exposure of the material to SO2 and increased when exposed to synthetic air. Recovery of resistance (baseline resistance value) occurred in less than 2 min. This suggests that the material has an excellent recovery time which is less than the response time. The experiment was extended through ten cycles of SO2 and synthetic air exposure to assess the sensor’s recovery and consistency. Throughout these cycles, the resistance changes were recorded systematically and plotted as a graph (Figures 8a–d).

Repetitive response behaviour of **(a)** PAMD pellet, **(b)** PAMD coated on FS, **(c)** PAMD coated on BB, and **(d)** PAMD coated on paper with 3 ppm of SO<sub>2</sub> gas.
Figure 9. Repetitive response behaviour of (a) PAMD pellet, (b) PAMD coated on FS, (c) PAMD coated on BB, and (d) PAMD coated on paper with 3 ppm of SO2 gas.

3.3 Discussion on the gas sensing property of PAMD in pellet form and coated on biodegradable substrates

From the above gas sensing studies, the type of gas and the substrate materials used in this study determine the gas sensing behaviour of the PAMD materials. Smaller resistance variations to all gases are observed when pellets are used as the sensor material (Table 2). As mentioned earlier, PAMD materials are tightly packed in pressed pellet form where particles do not have adequate interactions with analytes (gases), resulting in minor resistance changes. On the other hand, PAMD coated on FS substrate showed higher gas sensing responses than BB and paper substrate materials. Higher resistance changes were observed with the FS substrate due to the availability of loosely packed PAMD materials for interaction with analytes. PAMD materials occupy the pores of BB and paper substrates where analytes have limited access, resulting in significantly lower resistance changes than FS substrate material. Moreover, when the materials were exposed to NH3 gas, resistance increased continuously from the initial resistance; when exposed to CO and SO2 gases, resistance decreased from the initial values. The reason is well-known: PANI is a p-type conducting polymer, and resistance increases in the presence of ammonia (reducing gas) as electrons occupy holes (Savin et al., 2022). As a result, increasing resistance is more effective when the material is exposed to NH3 than decreasing resistance after this gas is removed. In the case of CO, the positive charge of resonance structure of CO (+C≡O-) removes electrons from the unprotonated amine nitrogen and introduces holes in PANI, decreasing the resistance of the PAMD material (Nasresfahani et al., 2020; Densakulprasert et al., 2005). The SO2-sensing process in PANI can be explained in terms of charge balance between PANI and the electron acceptor SO2. When PANI interacts with SO2, electrons may be transferred from PANI to the electron-accepting SO2, introducing additional holes in PANI and decreasing the resistance of PAMD materials (Chaudhary and Kaur, 2015a; Kiattibutr et al., 2002).

TABLE 2. Gas sensor response behaviour of PAMD pellet and PAMD-coated materials to various gases.

GasesResistance changes (Rg-Ra)-pellet (kΩ)Resistance changes (Rg-Ra)-FS-substrate (MΩ)Resistance changes (Rg-Ra)-BB-substrate (MΩ)Resistance changes (Rg-Ra)-paper-substrate (MΩ)
NH3+0.006+0.977+0.367+0.537
CO−0.001−0.608−0.047−0.008
SO2−0.001−0.247−0.148−0.013

Rg, resistance in the presence of gas (on); Ra, resistance in the absence of gas (off).

(+), resistance increased; (−), resistance decreased.

The resistance-reversible behaviour of the PAMD sensor materials was studied and is summarized in Table 3. Regardless of the gases, the resistance of the PAMD pellet was almost the reverse of its initial value. All the substrate materials showed reversible behaviour according to the type of gas. With NH3, FS-substrate material showed 0.4 MΩ difference from the initial value, while other substrates exhibited a 0.08 MΩ difference. This is because NH3 adsorbed onto the surface of the materials adheres more firmly, requiring more time for desorption. As explained earlier, the greater the amount of PAMD particles on FS-substrate materials, the more the interaction with NH3, resulting in large variations in the initial and final resistance values under an air atmosphere. The reversibility of the substrate materials with CO-paper substrate material is restored back to the original value compared with FS and BB substrate materials. This demonstrates less interaction of paper-coated PAMD material with CO and that. the porous nature of the paper discharges CO at lab air atmosphere. Compared to FS and paper substrate materials, BB substrate is less reversible with SO2. This is due to the strong interaction of BB-substrate-coated PAMD material with SO2, which reduces the final resistance of the material.

TABLE 3. Reversible behaviour of PAMD pellet and PAMD-coated materials under different gases.

GasesResistance changes (Rf-Ri)-pellet (kΩ)Resistance changes (Rf-Ri)-FS-substrate (MΩ)Resistance changes (Rf-Ri)-BB-substrate (MΩ)Resistance changes (Rf-Ri)-paper-substrate (MΩ)
NH3+0.008+0.411+0.083+0.081
CONo difference−0.045−0.041−0.001
SO2No difference−0.003−0.122−0.002

Rf, final resistance after exposing to air; Ri, initial resistance of the PAMD material.

(+), resistance increased; (−), resistance decreased.

As shown in Table 4, the sensing performance of the PAMD-coated biodegradable substrate materials is practically comparable to that of other previously reported PANI-based gas sensor materials. Notably, the PAMD materials exhibited measurable sensor responses towards even low concentrations of NH3 (5 ppm), CO (7.1 ppm), and SO2 (3 ppm) gases without any additional reinforcing or functional additives, indicating that the reported materials showing inherent gas-sensing capability.

TABLE 4. Comparison of the sensing properties of the PANI-MA-DSS (PAMD) material with previously reported PANI-based gas sensor materials.

Sl. NoPANI-based gas sensor materialTarget gasDetection rangeOperating temperatureReference
1PANI/HCl/polyamide 6 (PANI/HCl/PA6) nanofiberNH350–250 ppmRoom temperaturePang et al. (2021)
2DBSA-doped soluble PANINH31–1,200 ppmRoom temperatureYadav et al. (2019)
3PANI/HCl/paper substrateNH312–1,000 ppmRoom temperatureGupta et al. (2023)
4PANI-grafted/polydopamine (PD)/polyethylene terephthalate (PET) filmNH312–50 ppm50 °CMatsuguchi et al. (2024)
5PANI/HCl/graphene oxide (GO) compositeCO500 ppmRoom temperatureAigerim et al. (2026)
6PANI/HCl/SnO2CO25–200 ppm30 °C–50 °CJian et al. (2019)
7PANI nanofiber/Cu(II)–L-aspartic acid nanofibersCO200–8,000 ppmRoom temperatureNami-Ana et al. (2021)
8Ag/PANI/SnO2SO250 ppmUV-light at 20 °CXu et al. (2021)
9PANI/cetyltrimethylammonium bromide (CTAB)SO210 ppmRoom temperatureChaudhary and Kaur (2015b)
10PANI/maleic acid (MA)/dioctyl sodium sulfosuccinate (DSS)/various substratesNH3 COSO25 ppm7.1 ppm3 ppmRoom temperatureCurrent study

4 Conclusion

The synthesis and characterization of highly organic soluble PAMD salts coated on biodegradable substrates for gas sensor applications was successfully accomplished in this study. The chemical oxidation polymerization method, utilizing dioctyl sodium sulphosuccinate (DSS) as an anionic surfactant and ammonium persulfate (APS) as the oxidizing agent, formed PAMD materials. PAMD exhibited excellent solubility in organic solvents such as DMF and DMSO, enhancing its versatility and potential applications. Importantly, PAMD was effectively coated on various biodegradable substrates, including dry banana bracts, fish scales, and paper. This achievement highlights the potential utilization of sustainable and eco-friendly materials for gas sensor applications. Furthermore, PAMD coated biodegradable substrates displayed sensitive gas-sensing responses to a range of analytes, including ammonia, carbon monoxide, and sulfur dioxide. This indicates their potential for practical gas sensor applications in diverse environmental monitoring and safety scenarios. Overall, this research contributes to the development of environmentally conscious gas sensor technology by combining biodegradable substrates with organic soluble PAMD. Such an approach aligns the growing demand for sustainable materials with green technologies, particularly in environmental monitoring and safety applications. Future studies may focus on optimizing coating thickness, investigating selectivity toward a wider range of toxic gases, evaluating long-term environmental stability, assessing the effects of humidity on sensor performance, developing flexible sensor fabrication, and exploring real-time wearable applications for environmental monitoring.

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Footnotes

Edited by: Kavya S. Keremane, The Pennsylvania State University (PSU), United States

Reviewed by: Kalappa Prashantha, Adichunchanagiri University (ACU), India

Murugan Veerapandian, Central Electrochemical Research Institute (CSIR), India

Statements

Conflict of interest

The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The handling editor KSK declared a past co-authorship with author MSS.

Funding

The author(s) declared that financial support was received for this work and/or its publication. Authors SC and PCH are thankful to the management of Presidency University, Bengaluru, for providing financial support through the Presidency University seed grant (File No: RI&C/Funded Projects/Seed grant/2021/8). MSS is thankful to the management of NMIT, Nitte Deemed to be University for providing support through a seed grant.

Data availability

The original contributions presented in the study are included in the article/supplementary material; further inquiries can be directed to the corresponding author.

Author contributions

PCH: Formal analysis, Investigation, Methodology, Writing – original draft, Writing – review and editing. MSS: Writing – original draft, Writing – review and editing, Methodology. SC: Conceptualization, Funding acquisition, Investigation, Methodology, Project administration, Writing – original draft, Writing – review and editing.

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Citation

Reddy, P. C. H., et al. (2026). Processable polyaniline modified biodegradable substrates as the electrode material for gas sensor application. Frontiers in Sensors, Volume 7. https://doi.org/
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Copyright © 2026 Himadri Reddy, Sunitha and Chandrasekaran.

License: https://creativecommons.org/licenses/by/4.0/

*Correspondence: Saravanan Chandrasekaran

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