PANI-Based Sensors: Synthesis and Application

Abstract

In this chapter, we will present different methods of synthesis of PANI-based nanocomposites and their applications as bionanosensors, pH, and gas nanosensors. In this chapter, a comparison of various methods of synthesis of PANI-based nanocomposites with carbon nanotubes and graphene, as well as the production of nanosensors based on Screen Printed Electrodes will be given. Parallel, complete electrochemical and physical characterization of SPE-based nanosensor electrodes will be presented. For biosensing applications, various pharmaceutical active components will be reported. For pH testing, results of seawater testing in various parts of Europe (Sardinia, Barcelona, Napoli) will be reported. Gas-sensing analysis was done for SO₄, CO₂, and NH₃ gases.

1. Introduction

Electroconductive polymers are a class of modern materials particularly interesting to researchers because of their proven sensing ability [1]. These low-cost materials have demonstrated desired properties for developing room temperature operable gas sensors. Conductive polymers receive attention for their vast use in electrochemical applications, mainly for the improvement of electronics such as sensors, optoelectronic, and photonic devices [2].

Last decade, particular interest has been directed toward the design and development of chemical sensors based on electro-conductive polymers such as polyaniline (PANI). Especially, great interest was given to PANI-based composites due to the fact that composite materials made of conductive polymer or semi-conductors have been extensively used to improve the properties and sensing performance of gas sensing at room temperature [3]. In addition to electronic conductive polymers, nanomaterials are a crucial sensing component for the further development of gas sensors. Their ability to interact with the surroundings at a nanoscale level, large specific surface area, and high reactivity show the incredible performance of nanomaterials employed as excellent sensors with superior properties. Polymer-based electrodes can be implemented as an exceptional system for the detection of gaseous chemicals [4]. Polymer-based chemical sensors with specific functionalities can successfully recognize a particular compound within a mixture, providing rapid identification and qualitative analysis. Introducing a second component such as nanomaterials into PANI enhances its performance because of the synergetic effect between the two structures and thereby expands its application scope in the field of electronic devices.

Due to its p-type semiconductivity, polyaniline is characterized as a versatile electro polymer because of its excellent optical and electrical properties, conductivity, and ability to work at room temperature [3, 5]. Owing to the unique conduction mechanism, ease of synthesis, low cost, and high environmental stability, PANI is serving as a potential candidate for the fabrication of sensitive layers of novel gas sensors [6]. PANI’s outstanding conductivity arises from its simple and reversible acid/base doping-dedoping chemistry. The doping process includes removal of electrons from the polymer’s backbone, causing the cation radical to act as a charge carrier [7].

Not only PANI but also nano-sized PANI with high surface-to-volume ratio and unique electrical properties that can enhance the gas adsorption/desorption and thus promote the response and recovery processes is one of the ideal candidates for the development of PANI composites-based gas. The PANI/PVA composites have been reported for carbon dioxide sensing by Doan et al. [8]. PANI/TiO2 nanocomposites and PANI/sodium superoxide composite exhibited high sensitivity toward the CO2 sensor, and the fabricated sensor shows good response and recovery time [9]. The sensor performance of pure and PANI/Cu–ZnS composite were studied by Parangusan et al. [10]. Their results indicated that the PANI/Cu–ZnS composite exhibits a higher sensor response upon exposure to CO2 gas at room temperature. They discussed the enhanced sensing related to the p/n hetero-junction and porous microstructure of the core-shell structure. He et al. successfully produced a PANI/WO3@cotton thread-based flexible sensor that is capable of detecting NH3 at room temperature [11]. The WO3 nano-blocks were synthesized using a hydrothermal method, while the PANI/WO3@cotton thread was prepared by in-situ polymerization. The molar ratio of WO3 was varied and for 10% of WO3, the obtained sensor demonstrated the highest gas response upon exposure to 100 ppm NH3.

Shen et al. designed a wireless passive gas sensor based on alumina ceramic in which the sensor components were based on acidified CNT and PANI composites [12]. The gas sensing layer worked by physical adsorption of NH3 molecules. The wireless passive measurement was realized through the changes in resonant frequency. The relevant tests have shown that the sensor was able to detect a wide range of concentrations with a sensitivity of 0.04 MHz/ppm in the concentration of 300 ppm NH3. Parmar and his team worked on PANI and PANI/Graphene film-based sensors for the detection of the toluene [13]. The graphene–PANI ratio in the nanocomposite polymer film was optimized at 1:2. For the film’s preparation they used N-methyl-2-pyrrolidone (NMP) solvent. The sensing behaviors of the films were analyzed at different temperatures (30, 50, and 100°C) for 100 ppm toluene in air. They found out that the nanocomposite Graphene-PANI films have exhibited better overall toluene sensing behavior in terms of sensor response and recovery time as well as repeatability [13].

Gaikwad et al. [14] reported the synthesis of PANI nanofibers and two nanocomposite systems: Polyaniline/Graphene Oxide (PANI/GO) and Polyaniline/Graphene Oxide/Zinc Oxide (PANI/GO/ZnO), used for sensing of NH3, LPG, CO₂ and H2S gases at room temperature. The authors observed better selectivity and sensitivity from all systems toward NH₃ at room temperature. PANI/GO/ZnO nanocomposite showed the best performance with a response of 5.706 for 1000 ppm NH₃ at around 80°C and a rapid recovery time of only 90 seconds. Polyaniline/zinc oxide (PANI/ZnO) hybrid film-based sensors have been developed for ammonia detection at room temperature by Zhu et al. [15]. The obtained sensor exhibited a p-type semiconductor behavior and better response than pristine PANI. ZnO nanorod arrays incorporated into the PANI create a nanoscale gap for gas diffusion and provide abundant adsorption sites, thus enhancing the sensors’ response. The length of the nanorods is proportional to the sensitivity i.e., longer nanorods provide an efficient gap for gas diffusion, leading to better sensitivity.

Liu et al. [16] proposed an NH3 gas detector based on pristine and nanostructuralized PANI thin film, prepared by chemical oxidation polymerization and spin coating approach with further etching via reactive ion etching (RIE). The nanostructuralized PANI thin film sensor showed an increased response from 1.16 to 3.19 while increasing the NH3 concentration from 3 ppm to 990 ppm. The response, reproducibility, and selectivity toward NH3 showed superior properties of the nanostructuralized PANI film compared to the pristine PANI film sensor.

Macangnano et al. [17] investigated systems of PANI nanofibrous layers with different electrospinnable hosting polymers (polystyrene (PS), polyvinylpyrrolidone (PVP), and polyethylene oxide (PEO)), tested against traces of nitrogen oxide and ammonia. The authors reported good selectivity and rapid sensor response due to the components’ both high porosity and high interaction surface. The hosting polymers are reported as modulators of the sensors’ properties such as: PANI-PEO, a sensor with good electrical performance and high sensitivity to NH₃, but unstable while subjected to environmental stress; PANI-PVP, with good selectivity toward NO₂ but unstable while exposed to humidity; PANI-PS, stable and promising sensor, with good performance even when exposed to high humidity (up to 50% RH) and high temperature. Electrospun nanofibers have been confirmed as promising candidates for the development of gas sensors with high sensitivity, offering an improvement of the surface area-to-volume ratio. Korent et al. [18] described a novel and controllable method for the synthesis of PANI sensors using electrochemical deposition via cyclic voltammetry (CV) on golden screen-printed electrodes (SPEs), used for the detection of NH3. The proposed sensor showed good performances in terms of repeatability, reproducibility, and sensitivity in the range of 32–1100 ppb of NH3. Zhu et al. [19] developed a high-performance ammonia gas sensor based on self-assembly polyaniline films, prepared with the assistance of sodium dodecyl benzene sulfonate (SDBS). The SDS-functionalized PANI film-based sensor showed a good response toward NH₃ in the concentration range of 5.4–40 ppm NH₃, a low detection limit of 0.1 ppm, and a recovery time of 12 s. The surfactant functionalized PANI film enhances the gas sensing performance as a result of its structure ordered to accelerate the electron transport rate and the protonation/deprotonation properties.

Moreover, PANI is investigated in depth as an active component for other electrical devices such as pH sensors. Since solution pH has a crucial role in chemical reactions, exact determination and monitoring of pH are important in various fields [20].

In this chapter, we will discuss the synthesis and application of PANI, as one of the most promising conductive polymer substrates for the development and improvement of electrical devices such as sensors.

2. Synthesis and application of PANI-based nanocomposites

Polyaniline can be produced by chemical or electrochemical polymerization. Both include redox processes. Chemical polymerization is carried out in strongly acidic aqueous solutions in the presence of various oxidizing agents such as K₂Cr₂O₇, (NH)₄S₂O₈, or KIO3 [21, 22, 23]. Its advantage is obtaining PANI to a large volume scale [23] compared to electropolymerization which is limited to dimensions of the electrode. Electrochemical polymerization is carried out in an aqueous solution of aniline in strong oxidizing protonic acid (mostly H₂SO₄) [24], using different electrochemical techniques such as: potentiostatic, galvanostatic, and cyclic potentiodynamic methods [25]. The product of electrolysis—polyaniline, is obtained on the anode (oxidative process). Advantages of the electrochemical route of polyaniline synthesis, especially for PANI-based composites and sensors are [26]: (1) good adhesion on the transducer surface (ex. screen printed electrodes), (2) control of layer thickness (controlled electron transfer with the electrode), (3) control of the morphology (using an appropriate electrolyte and its hydrodynamic regime), (4) no need for polymerization initiator or heating and (5) higher conductivity of the produced polymer/composite [27]. Depending on the applied potential, the mechanism of the PANI electropolymerization involves the exchange of electrons of the aniline with electrode forming different electronic states of the polyaniline such as leucoemeraldine (fully reduced), emeraldine (half-oxidized), and pernigraniline (fully oxidized) [28, 29, 30], where emeraldine is the highly conductive one. Electropolymerization of PANI was proved as an auto-catalyzed process [31].

The following text will explained our own procedure for the electrochemical synthesis of polyaniline and nanocomposite based on PANI, reinforced with carbon nanostructures (CNSs)—graphene(G) and multi-walled carbon nanotubes (MWCNTs) [32, 33]. The used graphene was obtained by molten salt electrolysis in the lab of the Faculty of Technology and Metallurgy in Skopje. Before the usage, the graphene was treated in a 10 wt% solution of H₂O₂ for 2 h and later, in a 40 wt% solution of HF for 1 h. MWCNTs were received from JRC (No.231, ISPRA, d = 10÷40 nm, purity ∼94%) and used without additional treatment.

The first step in the determination of the electropolymerization conditions was cyclic voltammetry scanning of both systems—pure PANI and PANI + CNSs. The measurements were performed in three-electrode cells, where the working and counter electrodes were platinum tiles with a working surface of 10 cm², while as a reference one, a saturated calomel electrode (SCE) was used. Two types of electrolytes were prepared: 0.1 M aniline +0.5 M H2SO4 and 0.1 M aniline +0.5 M H₂SO₄ + CNSs. Before being dispersed into the electrolyte, CNSs were sonicated in an ultrasonic bath for 30 minutes. During the measurements, the electrolyte was stirred by a magnetic stirrer (200 rpm) at ambient temperature. The electrochemical set-up was consisted of potentiostat/galvanostat METROHM Autolab PGSTAT 128 N, three-electrode cells, and corresponding software for data acquisition (Figure 1). The potential range of scanning was from −0.2 to 1 V.

Figure 1.

Electrochemical set-up for cyclic voltammetry measurements.

The electrochemical spectra of both studied systems are shown in Figure 2. Both spectra show similar redox peaks, but the spectrum of the G/PANI shows a much higher current response as a result of the presence of graphene with high electrical conductivity. So, there is more electron exchange and electrochemical processes occur faster in this system. Because redox peaks of the pure PANI system are not so pronounced, this spectrum is shown separately in the inset of Figure 2. Corresponding potential values of peaks appearance are summarized in Table 1.

Figure 2.

Cyclic electrochemical spectra of electropolymerization of pure PANI (0.1 M aniline +0.5 M H2SO4) and G/PANI nanocomposite (0.1 M aniline +0.5 M H2SO4 + 3%wt. graphene related to aniline weight). In the inset, only the spectrum of electropolymerization of pure PANI is shown.

System	O1	O2	O3	O4	R1	R2	R3	R4
PANI	0.19	0.48	0.56	0.82	0.037	0.45	0.52	0.79
G/PANI	0.26	0.50	0.58	0.79	0.076	0.43	0.50	0.70

Table 1.

Potential position of the characteristic redox peaks for the studied system, from cyclic voltammograms in Figure 2.

The peak O1 in the anodic potential region corresponds to oxidation of the reduced form of PANI—leucoemeraldine to half-oxidized form—emeraldine [34, 35, 36]. The opposite peak in cathodic region R1, corresponds to the reverse reaction. According to the literature data [34, 37], this process occurs with the removal of electrons from the nitrogen atoms of the amine between the benzene rings, where the nitrogen atom acquires a cationic character. The middle redox pairs O2/R2 and O3/R3 can be ascribed to the oxidation/reduction of intermediate products [37, 38]. The oxidation peak O2 corresponds to the formation of benzoquinone, while the opposite cathodic peak R2 to its reduction to hydroquinone [35]. These redox reactions are the result of over-oxidation or degradation of PANI film [39]. O3/R3 is related to the formation of p-aminophenol/benzoquinoneimine [39, 40]. The last peak O4 denotes the oxidation of half-oxidized emeraldine to the fully oxidized pernigraniline form of PANI [34, 35, 36]. The opposite reaction is denoted with peak R4 in the cathodic region. Form the cyclic voltammogram, the potential region of formation of the partially oxidized emeraldine (the electroconductive form of PANI) can be estimated. According to the voltammogram it takes place in the potential region of 0.64–0.8 V. But, in this potential region is possible to obtain pernigraniline —a non-conductive form of PANI. In order to avoid this and to determine precisely the working potential of PANI electropolymerization, the steady-state polarization measurement was performed.

The steady-state change of the current in the potential region from 0.6 to 1.1 V is shown in Figure 3. As can be seen, the oxidation of emeraldine begins at 0.7 V vs. SCE and completed to pernigraniline at 0.9 V. It was expected that the formation of electroconductive PANI should occur in the middle of the oxidation region of emeraldine, i.e., at 0.8 V. But, the electropolymerization at this potential lead to the formation of a dark blue film of non-conductive pernigraniline. Therefore, the electropolymerization should be performed at a lower potential. The next attempt of electropolymerization at 0.75 V, lead to the formation of desirable conductive, green-colored emeraldine.

Figure 3.

Steady-state polarization curve in the system 0.1 M aniline +0.5 M H2SO4.

After the determination of the optimal electropolymerization potential, PANI and CNSs/PANI films were deposited on screen-printed electrodes (SPE) aimed for different sensing applications. Electropolymerization was performed at potentiostatic conditions, at 0.75 V, using an electrochemical set-up (potentiostat/galvanostat WENKING HC 500) as is shown in Figure 4.

Figure 4.

Electrochemical set-up for electropolymerization of PANI on screen printed electrodes (SPE).

During the potentiostatic electropolymerization (E = const.), it was observed that current density continuously increased, highlighting the autocatalytic character of the electropolymerization process. This is illustrated in Figure 5, where the change of current density during the time for electropolymerization of pure PANI and nanocomposites G/PANI and MWCNTs/PANI are shown. Increasing of the current density can be explained by the initial formation of the polymer/composite film, which possesses increased surface roughness related to the initial pure electrode surface [35, 41]. As the thickness of the polymer/composite film increases, surface roughness increases and consequently, current density increases. Initially, there is not so pronounced current increase. This can be ascribed to the induction period of the polymer/composite film formation. The induction period consists of a few consequent processes: (1) oxidation of aniline to radical cations, (2) their polymerization to an oligomer of aniline, and (3) formation of PANI by autocatalytic reaction [42, 43, 44]. Another observation that we can highlight from Figure 5 is that the electropolymerization of composites is considerably more intensive than that of pure PANI. This is the result of the much higher electrical conductivity of CNSs included in the polymer film, enabling faster electrons exchange of electrons and consequently, higher current density. Nanocomposite reinforced with MWCNTs has shown the highest current density of the electropolymerization process.

Figure 5.

Change of the current density with time during the potentiostatic electropolymerization of PANI, G/PANI, and MWCNTs/PANI, at 0.75 V vs. SCE.

Cyclic voltammetry was also used for the determination of the double layer capacity of the obtained PANI and CNSs/PANI nanocomposite, using an electrochemical set-up as in Figure 1. The voltammograms were scanned at different scan rates: 10, 20, 50, and 100 mV·s⁻¹ and shown in Figure 6 for pure PANI and in Figure 7 for G/PANI nanocomposite. Double layer capacity C_dl can be calculated by the following equation [45]:

Figure 6.

Cyclic electrochemical spectra of pure PANI (0.1 M aniline +0.5 M H2SO4) at different scan rates: 10, 20, 50, and 100 mV·s⁻¹. In inset is shown the change of capacitance current density by the change of scan rate.

Figure 7.

Cyclic electrochemical spectra of pure PANI (0.1 M aniline +0.5 M H2SO4 + 3%wt. graphene related to aniline weight) at different scan rates: 10, 20, 50, and 100 mV·s⁻¹. In inset is shown the change of capacitance current density by the change of scan rate.

$$C_{\mathit{dl}}=\frac{d{i}_{\mathit{cap}.}}{{\mathit{dv}}_i},$$E1

where v_i is scan rate and i_cap. is the capacitance current density at intersected potential in the region of double-layer charging and discharging as shown in Figures 6 and 7. Capacitance current density is the arithmetic mean of the absolute values of anodic and cathodic current density:

$$i_{\mathit{cap}.}=\frac{\left|i_{\mathit{\text{anodic}}}\right|+\left|i_{\mathit{\text{cathodic}}}\right|}{2},$$E2

The change of the capacitance current density by the change of the scan rate is shown in the inset of Figures 6 and 7. The double layer capacity of G/PANI nanocomposite was determined to be 121.07 mF·cm⁻². This is about 70 times higher related to the double layer capacity of the pure PANI. Incorporation of only 3% graphene in the polymer matrix of PANI, the electrochemical characteristics of the nanocomposite can be remarkably improved. Except the sensing application, these nanocomposites have good potential in electrochemical and energy storage devices [46, 47, 48].

3. pH sensing

The PANI-based pH sensors, obtained with electropolymerization have been tested to follow the pH changes of the water, on a laboratory scale, displaying quite good potentiality (Figure 8) [32, 33]. Besides the pure PANI, also several nanocomposites MWCNTs/PANI and G/PANI containing 1, 3, 5, and 10 wt% of nanostructures were created. The devices require immersion in the water sample for 30 sec. For 1 minute a voltammogram was recorded. The sensing response was expressed by the anodic peak at positive current values. The applied potential range during the measurement was −0.8 to +0.8 V, performed with a scan rate of 0.05 V·s⁻¹.

Figure 8.

PANI-based pH sensors, obtained with electro polymerization.

They are suitable for the analysis of samples with pH values in the range required by the target application (Figure 9) and they displayed the ability to detect pH in both laboratory (buffers and seawater simulating solutions) and real samples. Experimentally obtained PANI-based SPE-sensors, created in the FP7-COMMONSENS project, were compared with commercial PANI-based SPE [49].

Figure 9.

Validation curve for pH range = 7.9 ÷8.5.

Actually, it has been verified that the presence of many possible components in real seawater does not affect the efficiency of the device working and not disturbing the measurements. Even in the case of good agreement of the electrode response to the pH variation and the results of the laboratory pH meters, commonly used as a reference, it is important to improve the sensitivity of the systems in order to allow more precise results. This can be achieved by: (i) more controlled nanocomposite deposition on the electrode, (ii) obtaining a layer with fixed thickness, and (iii) constant and uniform covering degree of the sensing electrode surface.

The precision value obtained during field testing activities (Figure 10) showed that the SPE-based pH sensor needs to be optimized.

Figure 10.

Field testing at CNR in Oristano, September 2016, pH and electrical resistivity (MΩ] on abscissas and time [minutes] in ordinates.

The comparison of the measurements performed in the sample of the lab tank with the measurements performed directly in the lagoon has shown that both curves followed the same trend in the pH range of around 8 (Figure 11). Namely, the presented resistivity curve for 16 minutes at pH = 8.1 in the seawater was compared with the resistivity curve for 16 minutes in the lab tank when pH was changed in the range from 7.9 to 8.7 by adding NaOH drops in the water glass. It is evident that the resistivity values are in different ranges. For constant pH = 8.1 while for increasing pH from 7.9 to 8.7, resistivity in the lab tank was measured from 400 (pH = 7.9) to 500 kΩ (pH = 8.7).

Figure 11.

Comparison of lagoon and lab tank testing by SPE of 3% G/PANI nanocomposite sensing element, with resistivity [kΩ] on abscissas and time [minutes] in ordinates.

Because by this technique a current is applied during the measurement, a more stable signal can be obtained. Once the applied current is optimized, the sensor needs to be tested using artificial samples. In the case of achieving good linear response, reproducibility, and precision values, the sensor can be tested using real seawater samples. The time needed to perform this action has been estimated to be of about 6–8 months. Also, with better resolution of the measurements, it can be possible to have better precision in the measured potential.

The stability of the pH sensor based on SPE of MWCNTs/PANI nanocomposites was tested in seawater at pH = 8.4. The obtained data are presented in Table 2.

Sample 3% wt MWCNT/PANI	R [MΩ]
	1 min	2 min	3 min	5 min	10 min	12 min stabilized
First day	1.26	1.40	1.50	1.81	2.31	2.48
5 days	1.60	1.67	1.76	1.80	2.01	2.06
10 days	1.70	1.80	1.83	1.91	1.99	2.04
21 days	1.31	1.36	1.37	1.33	1.39	1.40

Table 2.

Stability of SPE-MWCNT nanosensors in seawater at pH = 8.4.

3.1 Gas sensing

Industrialization, urbanization, and rapid technological modernization are three crucial factors contributing to over-exploitation of natural resources. With domination of the nonrenewable fossil fuels in the global energy supply, environmental pollution is worsening everywhere on the globe. In the past decade, atmospheric pollution has become a global phenomenon reaching life-threatening levels. Ammonia (NH₃) and nitrogen dioxide (NO2) are one of the most abundant inorganic toxic pollutants known for their worsening effect on the air quality, causing serious eye, skin, and mucous membrane respiratory irritation even when found in traces [50, 51]. Serving as a serious threat to human health, it is important to find an efficient and rapid solution for maintaining air quality in both indoor and outdoor environments. Researchers are constantly developing strategies and solutions for clean technology environmental applications to lower pollution damage. However, the first prerequisite procedure for environmental pollution treatment is monitoring. Therefore, it is imperative to develop novel sensors with outstanding characteristics such as higher sensitivity, selectivity, and accuracy, for effectively enriching pollutants in traces.

Even though many analytical instruments based on colorimetry, luminescence, or IR absorption are used for the concentration measurement of toxic gases, screen-printed electrochemical gas sensors are receiving constant attention because of their low-cost, simple design, and miniature size [52]. The screen-printing technique is one of the easiest and cheapest methods for chemiresistive sensor development which serves as an alternative to the traditional electrodes. The electrochemical principle requires a three-electrode system consisted of working, counter, and reference electrodes (Figure 12), where the sensing component or composite is deposited or printed onto the working electrode on various types of plastic or ceramic substrates. The principle is widely recognized and serves as one of the most promising approaches for sensor development because of its superior features—rapid in-situ analysis, high sensitivity and selectivity, portable miniaturized size, and low-cost [53]. Referred as economical electrochemical substrates, screen-printed electrodes are continually improving with respect to both their format and printing materials [52].

Figure 12.

Screen-printed electrode.

By now, there has been a large amount of research related to conducting polymer-based gas sensors, including our work [54] and other numerous reviews [2, 55, 56, 57]. Conductive polymers are explored as active substrates in gas sensors because of their broad properties in terms of conductivity, sensitivity, selectivity, and redox characteristics [17]. Among all of them, PANI, an intrinsically conducting polymer with excellent properties, has experience as a frequently investigated gas sensing component in chemiresitive sensors. PANI’s nanodispersions have shown large potential for sensing applications, regarding the fact that they are inkjet and screen-printable, facilitating the conductive polymer pattern directly to the substrate [58]. Due to PANI’s classification as a semiconductor, it is often used for accurate and rapid detection of reducing ammonia gas, a highly desirable task not only in environmental applications but also in the automotive and chemical industry and medical applications.

Chemoresistive sensors based on PANI have routinely been applied for ammonia detection, such as electrospun PANI fibers-based highly sensitive chemiresistive sensor described by Zhang et al. [59], MWCNT/PANI nanocomposite based SPE gas sensor proposed by Chepishevski et al. [60], inkjet-printed polyaniline nanoparticles based ammonia sensor described by Crowley et al. [61], modified gas sensor based on PANI nanofibers employing the techniques of electrical impedance spectroscopy with frequency response analysis and amperometry reported by Basak et al. [62] and many more. A highly sensitive and flexible ammonia gas sensor based on PANI film as an active sensing layer has been reported by Kumar et al. [63]. Their room temperature functioning sensor operated in the range of 5–1000 ppm, offered good reproducibility, long-term stability, and mechanical robustness, indicating the promising application of PANI films for portable on-site detection. Nanostructured PANI-based composites are suggested by Wojkiewicz and al. [64] for ppb range ammonia sensing. The author and his team studied three types of PANI composites with different morphologies: two core–shell systems with a poly(butyl acrylate) (PBuA) or poly(vinylidene fluoride) (PVDF) core and a PANI shell, and a composite based on PANI nanofibers embedded in a polyurethane (PU) matrix, and all of them showed high performances in terms of response time, reversibility and detection limit. The detection limit has been reported as below 100 ppb for films formed of the core–shell nanoparticles and below 20 ppb for the nanofiber-based composites. The team concluded that better sensitivity is achieved for the films made of the contacting core–shell nanoparticles. Talwar et al. [65] proposed a novel synthesis of ZnO-assisted PANI nanofibers and investigated their sensing response regarding ammonia gas. The fabricated sensor showed excellent selectivity and its sensing response has been proportional to the concentration of ammonia gas. Sutar et al. [66] prepared nanofibrous PANI films using an amino-silane self-assembled monolayer (SAM) employed as artificial seeds for the self-organization of PANI during polymerization. Chemiresistor sensors that have been developed using the fabricated nanofibrous PANI films as a sensitive layer, showed high sensitivity to very low concentrations (0.5 ppm).

One of the aims of this chapter is to show the results obtained from the electrochemical characterization of pristine PANI electrodes, as proposed sensors tested against ammonia (NH₃) vapors with different concentrations. The electrochemical characterization included resistance change monitoring of commercial PANI electrodes exposed to variable ammonia vapor concentrations of 3, 6.2, 12.5, and 25% (wt.). Commercial screen-printed PANI electrodes with 4 mm in diameter were ordered from Dropsens, Spain. In order to obtain better results, electrodes were exposed to external thermal excitation, measuring the sensing activity by heating and evaporating the ammonia solution around 50°C. The experimental setup scheme is shown in Figure 13.

Figure 13.

Experimental setup for gas-sensing characterization.

The working mechanism is quite simple. The ammonia solution is evaporated in a closed circuit and ammonia gas is released. Because of the heating and humidity, the atmosphere gets more conductive, so it can absorb and more evenly distribute excess charges. Gas molecules travel and interact with the surface of the PANI electrode. When PANI is exposed to ammonia, its conductivity starts to change due to the deprotonation mechanism of amine groups in emeraldine salt converting it to emeraldine base following equation:

$$\text{PANI}-{\mathrm{H}}^{+}+\text{NH}3\leftrightharpoons \text{PANI}+\text{NH}4{}^{+},$$E3

proving excellent selectivity toward NH3 [67]. PANI’s conductivity can be adjusted by changing its oxidation and protonation state and the material can be found in three key oxidative states: leucoemeraldine—fully reduced state, emeraldine—half-oxidized state, and pernigraniline—fully oxidized state.

The electrochemical characterization results obtained from the commercial PANI electrodes testing confirm electrochemical resistance decrease and conductivity increase, over time and at all concentrations. This effect is expected, as exposure to the ammonia vapors should indeed cause a resistance change which manifests as a decrease and conductivity increase.

Because of the conductive nature, PANI electrodes show high conductivity, as shown in Figure 14. Moreover, the tested electrodes show a non-linear response. The measured data suggests that it takes approximately 6 minutes to reach equilibrium, at all concentrations. This phenomenon happens because of the required time for the solution to create a humid environment. The sensor shows a stable and good response for all concentration levels. However, the highest conductivity is measured while testing the electrodes in the most concentrated ammonia solution i.e., 25%. The lowest response from the obtained measurement is shown for the ammonia solution with a concentration of 6.2%. After around 10 minutes, saturation is achieved at all concentrations and the curves show steadiness and linearity for the rest of the measurement.

Figure 14.

Resistivity changes of commercial screen-printed PANI electrodes in various concentrations of NH3 ions.

We tested the irradiation treatment of PANI electrodes by using 50 and 100 kGy e-beam irradiation. Comparison is given in Figure 15. Evidently, a higher effect was achieved with 50 kGy.

Figure 15.

Resistivity changes of irradiated SPE- PANI electrodes 3% of NH3 ions.

Looking through the literature data, this behavior agrees with the experimental results for pure PANI-based gas sensors [7, 68, 69]. PANI is found to exhibit detectable sensitivity expressed as increasing resistivity change for ammonia gas [18]. Improved response and sensitivity can be achieved by the addition of other conductive materials to the PANI matrix, which can act synergistically, such as carbon materials (CNT, graphene), metals (Ag, Au, Pt, Cu), inorganic nanoparticles (CeO₂, TiO₂, ZrO₂, Fe₂O₃, Fe₃O₄), chalcogenides (CdS, ZnS, CdSe), polymers (polyvinyl alcohol (PVA), polyvinyl acetate (PVAc) and polymethyl methacrylate (PMMA)), etc. [2, 56].

4. Conclusions

PANI and PANI-based nanocomposites with CNT and G were employed in the design of highly sensitive electrochemical sensors for the monitoring of the pH of sea and ocean waters and gases compounds. PANI and its nanocomposites possess excellent electrocatalytic properties for the modified sensors, such as enhanced detection sensitivity, electrocatalytic effects, high conductivity, and reduced fouling. These superior attributes endow CNTs, graphene, and PANI nanocomposites with great advantages for enhanced monitoring of pH and gas sensing applications. Carbon nanostructures (CNT, G) have shown that they may play a critical role in the future improvement of PANI-based sensor development of advanced points of other sensing applications.

Acknowledgments

The research reported in this chapter was part and financed by several projects: the FP7 COMMON SENSE (Fp7-614155) project, NANO IRA NET-MAK1003 from the IAEA project, and the bilateral scientific project between the Faculty of Technology and Metallurgy—University Ss Cyril and Methodius in Skopje from the Republic of North Macedonia and Institute of Solid State Physics—University of Technology Graz from Austria.

Conflict of interest

The authors declare no conflict of interest.