Enhanced Electrocatalytic Behaviour of Poy(aniline-co-2-hydroxyaniline) Coated Electrodes for Hydrogen Peroxide Electrooxidation

Abstract

Polymer-coated electrodes are widely used for the detection and oxidation of hydrogen peroxide (H₂O₂). Conducting polyaniline (PANI), poly (2-hydroxyanilne) (PHA), and their copolymer poly(aniline-co-2-hydroxyaniline) (PACHA) were electrochemically synthesized on a gold substrate for H₂O₂ detection and analysis. Cyclic voltammetry (CV), square wave voltammetry (SWV), and differential pulse voltammetry (DPV) techniques were used for electroanalysis. Both PACHA and PANI greatly reduced the gold overpotential for H₂O₂ oxidation with enhanced current densities. The PACHA- and PANI-coated electrodes showed oxidative peaks at 0.30 and 0.50 V, respectively, in the presence of 1.4 × 10⁻⁶ M H₂O₂, while PHA-coated electrodes exhibited no response. The fabricated electrodes displayed a linear response towards H₂O₂ in range of 2 × 10⁻⁷ to 1.4 × 10⁻⁶ M, with very low detection limits (LODs) of 1 × 10⁻⁷ M (for PACHA) and 1.15 × 10⁻⁷ M (for PANI) evaluated from CV data. In case of SWV and DPV, the LODs were found to be 1.78 × 10⁻⁷ M (for PACHA) and 1 × 10⁻⁷ M (for PANI), respectively. The materials exhibit high sensitivity of 650 A/Mcm² and show good stability. The PACHA-coated electrode shows better capacitance (1.84 × 10⁻³ F) than PHA- (2.52 × 10⁻⁴ F) and PANI-coated (1.17 × 10⁻³ F) electrodes.

1. Introduction

Hydrogen peroxide (H₂O₂) has been of great value in biological systems and commercial applications, i.e., for bleaching, sterilizing, oxidizing, and for natural organic matter removal from water [1]. Though H₂O₂ is of great help, its excessive production and use can be hazardous, as it causes environmental pollution [2]. H₂O₂ produced in mitochondria by oxidases, and is the main cause of various disorders like myocardial infarction, neurodegeneration, and Alzheimer’s disease [3,4]. A high level of H₂O₂ is also toxic to cells, because it can disrupt its morphology and function [5,6]. Thus, detection and determination of H₂O₂ plays a vital role in many disciplines, like biochemistry, clinical, environmental chemistry, and pharmaceutical analysis.

In the recent few decades, many techniques, such as chromatography, fluorometry, spectrophotometry, and titrimetry, have been applied for the detection of H₂O_2. However_, electrochemical techniques have gained more attention because of their high sensitivity, low cost, robust response, and easy operation. Many trials have shown that the reduction and oxidation of H₂O₂ occurring at solid electrodes displays the large overpotential and slow electrode dynamics, which has an adverse effect on detection performance, and also results in noisiness from other electroactive species in the practical determination of H₂O₂. Consequently, enzyme-involved biosensors have been used for the analysis of H₂O₂. Although the enzyme-modified electrodes show low detection limits, they have certain drawbacks like poor stability, a high cost of enzymes, and immobilization complexity. Fortunately, the emergence of coating electrodes with polymers opens a new doorway for H₂O₂ sensing, due to their greater stability and other properties, such as reducing overpotential, preventing electrode/electrode fouling, and interfering when the reagent solution is at a high concentration [7,8]. Polymers latch onto electrode surfaces, either by fixing the membranes (desired polymer membrane) that are available commercially or by electro-polymerization [9]. The latter method can be more useful than other methods because of its simplicity and the reproducibility of the construction process in a single step.

An electrode of the desired properties can be made by choosing an appropriate monomer and adequate experimental conditions. Thus, the thickness of the polymer membrane and other physical properties can be regulated by varying experimental conditions during the process of electro-polymerization. Electrodes modified with conducting polymers and nitrogen-doped graphene have been used for the oxidation of H₂O₂ [10], catechol [11], and ascorbic acid [12].

Conducting polymers are widely used for electrode surface coating, in order to enhance the bare electrode sensing properties. They have a large scope of applications in the realm of fuel cells [13], super capacitors [14], sensors [15], and electrocatalysis [16]. The most-used polymers are polyaniline (PANI), polypyrrole, polythiophene, and their derivatives. Due to its susceptibility and being easy to synthesize, PANI has attracted more attention. Some features of PANI can be upgraded via doping [17], interfacial polymerization, copolymerization, and the formation of bi-layer structures [18]. In addition, 2-hydroxyaniline, a derivative of aniline, contains hydroxyl and amino groups in its structural unit, and can be polymerized. The hydroxyl and amino groups in poly (2-hydroxyaniline) (PHA) can be oxidized and reduced electrochemically, and have been utilized in the fabrication of sensors [19].

A copolymer of aniline and 2-hydroxyaniline, poly (aniline-co-2-hydroxyaniline) (PACHA), with improved electrochemical properties both in neutral and basic media compared to its respective homopolymers, has been reported by our and other groups in the last decade [20,21]. Previous studies have shown that the anodic current response of PACHA is 1.3 times greater than PHA [22]. Furthermore, the electrochemical impedance spectroscopy revealed that the capacitance and conductivity of PACHA-coated electrodes increase by increasing the upper potential. The PACHA-modified electrode exhibits superior performance to PANI in the methanol oxidation reaction in direct methanol fuel cells (DMFCs). It is considered a good auxiliary material for the catalytic layer in DMFCs [22]. The copolymer is also used for the sensing of arsenate, uricase, and ascorbic acid [23]. It has been tested for probing radical scavengers of catechin and pyrogallol [24,25]. The fibers of PACHA were effectively used for the electro-oxidation of catechol in Na₂SO₄ solution [26].

In this work, we have tried to apply PACHA-coated electrode for the detection of H₂O₂ at a very minute level, by taking advantage of the redox behaviour of PACHA and comparing its electrocatalytic and capacitance properties with its homopolymers, as well as with previously reported data. The PACHA coating greatly reduces gold overpotential for H₂O₂ oxidation, even 200 mV lower than a PANI coating, with enhanced anodic current density. The PACHA-modified electrode displayed a linear response towards H₂O₂ in the range of 2 × 10⁻⁷ to 1.4 × 10⁻⁶ M, with a very low detection limit (LOD) of 1 × 10⁻⁷ M, and exhibited high sensitivity and good stability.

2. Results and Discussion

2.1. Cyclic Voltammetry Analysis

Figure 1 shows the cyclic voltammograms of bare gold electrodes, as well as PACHA-, PANI-, and PHA-coated electrodes in 0.1 M sodium citrate solution, with and without H₂O₂.

It can be perceived from Figure 1a that bare gold electrodes show a very small current response towards H₂O₂ at large oxidation potential. In Figure 1b, the copolymer displays a couple of electrochemical responses because of the redox nature of the copolymer. The anodic and cathodic peaks are observed at 0.32 V and 0.03 V, respectively, while after the addition of H₂O₂ the anodic and cathodic peaks shift to 0.30 V and −0.10 V. The curve also shows great increase in current peaks by the addition of H₂O₂. As shown in Figure 1c, the case of PANI is similar. PANI exhibits good electrochemical response towards H₂O₂. The anodic and cathodic peaks are observed at 0.75 V and −0.18 V, but after the addition of H₂O₂ the peaks shift to 0.50 V and −0.11 V, respectively. An increase in current peaks is also observed when H₂O₂ is added. From Figure 1, it is evident that both PANI and PACHA possess effectual electrochemical responses towards H₂O₂, offering a way to be used as H₂O₂ sensors. Based on the quantity of anodic peak currents, PANI is more electro-active towards H₂O₂ than PACHA. A possible reason might be the large separation of the polymer chain by side groups in the copolymer [21]. Interestingly, the H₂O₂ oxidation peak was observed at 0.30 V on the PACHA-coated electrodes. This potential was lower by 0.21 V compared to the H₂O₂ oxidation peak observed for the PANI-coated electrodes. This finding shows that the overpotential for H₂O₂ oxidation is greatly increased when the PACHA-coated electrode was used, indicating that the coating is favorable for H₂O₂ detection purposes. The PACHA detects H₂O₂ at a lower potential, not only with respect to PANI [26], but also poly(aniline-co-p-aminophenol) [27] and poly(thiophene-2-aminophenol-3-thiopheneacetic acid) [28]. The current response of PHA towards H₂O₂ is almost negligible compared to the response of PACHA and PANI; this might be due to the compact structural morphology and lower conductivity of PHA, as PHA is a redox polymer, having a ladder-like structure in which the electron transfer takes place by a hopping process, as there is no continuous path for the electron flow; thus, the conduction of the polymer is reduced. The inconsequential response of PHA towards H₂O₂ analysis may also be attributed to the pH of the solution in which the experiment is carried out, because the electrochemical activity of PHA quietly decreases in buffer solution of neutral pH.

2.1.1. Scan Rate and Rate Constant Analysis

The electrochemical response of PANI- and PACHA-coated electrodes towards H₂O₂ was checked both at lower and higher scan rates. A total of 1.4 × 10⁻⁶ M of H₂O₂ was taken for the purpose. It can be seen from Figure 2 that the current peak increases linearly with the square root of the scan rate (ν1/2) for H₂O₂, thus indicating that the redox reaction of H₂O₂ on the surface of PANI- and PACHA-coated electrodes is a diffusion-controlled process.

For a redox couple having an intermediate electron transfer rate constant, the anodic peak current (I_pa) and scan rate (v) can be related by Equation (1):

$$I_{pa}=0.496nFAC{D}^{\frac{1}{2}}{\left(\frac{\left(\alpha n_{\alpha }\right)Fv}{RT}\right)}^{\frac{1}{2}}$$

(1)

where F is the Faraday constant, A is the area of electrode, C is concentration, D is the diffusion coefficient (in cm² s⁻¹), α is the charge transfer coefficient, R is universal gas constant, T is the absolute temperature, and n_α is the number of electrons transferred in a reaction. The αn_α can be calculated from Equation (2):

$$|E_{pa}-E_{pa/2}|=1.857RT/F\alpha n_{\alpha }$$

(2)

where $E_{pa}$ is the anodic peak potential and $E_{pa/2}$ is the half-peak potential. By substituting the $\alpha n_{\alpha }$ value in Equation (1), the diffusion coefficient ca be estimated. The k_s (rate constant for a charge transfer reaction) value for a quasi-reversible redox reaction can also be resolute from Equation (3):

$$k_s=2.18{\left[D\left(\alpha n_{\alpha }\right)vF/RT\right]}^{\frac{1}{2}}exp\left[{\alpha }^{2}nF\left(E_{pc}-E_{pa}\right)/RT\right]$$

(3)

The resulting αn_α values for H₂O₂ at the scan rate of 0.05 Vs⁻¹ for PANI- and PACHA-coated electrodes are 0.20 and 0.25, respectively. The diffusion coefficient of PANI was found to be 6.9 cm² s⁻¹, which is more than PACHA (3.47 cm² s⁻¹), meaning that H₂O₂ diffuses faster into PANI than PACHA. The k_s value for PANI is 3.42 × 10⁻¹ cm s⁻¹, while for PACHA it decreases to 2.83 × 10⁻¹ cm s⁻¹; the reason for this may be that the charge transfer between the PANI and H₂O₂ takes place at a faster rate.

2.1.2. Electrochemical Response with an Increase in Concentration

Figure 3 shows cyclic voltammograms of PACHA- and PANI-coated electrodes in different concentrations of H₂O₂. It is observed that the oxidation peak constantly increases with the increase in H₂O₂ concentration, which means that the current peak has a direct and linear relation with H₂O₂ concentration [29].

The calibration curves obtained are shown in Figure 3b,d. The coefficient (R²) values attained from the linear calibration curve of H₂O₂ concentration ranging from 2 × 10⁻⁷ to 14 × 10⁻⁷ M at PACHA and PANI coated electrodes were 0.996 and 0.980, respectively. The limit of detection (LOD) was calculated with the help of following equation [30]:

$$\mathrm{LOD}=3\sigma /m$$

(4)

where σ stands for the standard deviation of the blank/regression line, and m is the slope of the calibration line. The limit of detection and limit of quantification calculated were found to be 1 × 10⁻⁷ M, and 3.3 × 10⁻⁷ M for PACHA, while for PANI they increase to 1.15 × 10⁻⁷ M and 3.8 × 10⁻⁷ M, respectively.

2.2. Square Wave Voltammetry Analysis

Square wave voltammetry (SWV) and Differential Pulse Voltammetry (DPV) were also employed to scrutinize the oxidation of H₂O₂ on PACHA-, PANI-, and PHA-coated gold electrodes, because of their high current sensitivity and low contribution to the background current.

Figure 4 shows the square wave voltammograms of bare as well as PHA-, PANI-, and PACHA-coated gold electrodes in 0.1 M acetate buffer containing 14 × 10⁻⁷ M H₂O₂. Oxidation peaks were obtained, and the peak potential and the current at different electrodes are quite indistinguishable. The anodic peaks at bare and PHA-coated electrodes were at −0.60 V and −0.07 V, respectively, with a very small peak current.

The oxidation of H₂O₂ by a PACHA-coated electrode gives rise to a single peak at 0.08 V, while PANI results at 0.20 V. The peak current in the case of PANI is greater than PACHA; however, the voltammetry peak for a PANI-coated electrode is broader, which makes the determination of individual concentration complicated [31]. PACHA resolved the merged anodic peaks in distinct voltammetry peaks. From the peak current analysis, it is clear that PACHA- and PANI-coated electrodes enhance the electrochemical response towards H₂O₂. The SWV results are in good agreement with the CV results.

Electrochemical Response with an Increase in Concentration

Figure 5 shows SWV voltammograms of PACHA and PANI in different concentration of H₂O₂. It can be observed that the current peak increases with the increase in H₂O₂ concentration. The calibration curve obtained is shown in the inset. The coefficient (R²) value obtained from the linear calibration curve of the H₂O₂ concentration at PACHA was 0.944. The limit of detection and limit of quantification calculated were found to be 1.78 × 10 ⁻⁷ M and 5.6 × 10 ⁻⁷ M for PACHA, respectively, while for PANI they cannot be evaluated due to the broader current peaks.

2.3. Differential Pulse Voltammetry Analysis

Differential pulse voltammograms (DPVs) of H₂O₂ for PACHA-, PANI-, and PHA-coated as well as bare electrodes are shown in Figure 6.

The oxidation peak of H₂O₂ for the PANI- and PACHA-coated electrodes are observed at 0.20 V and 0.10 V, respectively. It can be seen that the oxidation peak currents of H₂O₂ for the PACHA- and PANI-coated electrodes are much higher than the bare and PHA-coated electrodes. Similar to CV and SWV, the DPV results show that the oxidation of H₂O₂ takes place at a higher potential with PANI-coated electrodes compared to PACHA-coated electrodes. The DPV data are in agreement with the CV and SWV data. However, the current response in the case of SWV is greater than that of CV and DPV. This may be because of the efficient elimination of capacitance current [32].

Electrochemical Response with Increase in Concentration

DPVs of PACHA and PANI in different concentrations of H₂O₂ are given in Figure 7. Figure 7b,d shows the calibration curve with coefficient R² values of 0.963 for PANI and 0.971 for PACHA. A direct relationship between current peak and H₂O₂ concentration is attained. The curves also show a peak shift with the increase of H₂O₂ concentration; the reason for this may be that a high voltage is required for the electrons to surpass the interface [33]. The limit of detection (LOD) and limit of quantification (LOQ) obtained were 1 × 10⁻⁷ M and 3.3 × 10⁻⁷ M, respectively, for both PANI- and PACHA-coated electrodes. The DPV results validate the CV and SWV data.

The LOD of H₂O₂ is given in

Table 1. Comparison of various characteristics of PANI and PACHA along with the reported data.

Electrode Technique	Detection Potential	Linearity (M)	LOD (M)	LOQ (M)	Sensitivity A/Mcm2
PACHA–Au, CV	0.3 V	2 × 10−7–1.4 × 10−6	1 × 10−7	3.3 × 10−7
PANI–Au, CV	0.5 V	2 × 10−7–1.4 × 10−6	1.15 × 10−7	3.8 × 10−7
PACHA–Au, SWV	0.08 V	2 × 10−7–1.4 × 10−6	1.78 × 10−7	5.6 × 10−7
PACHA–Au, DPV	0.1 V	2 × 10−7–1.4 × 10−6	1 × 10−7	3.3 × 10−7	650
PANI–Au, DPV	0.20 V	2 × 10−7–1.4 × 10−6	1 × 10−7	3.3 × 10−7	434
rGO-Nf@Ag6			5.35 × 10−7 [34]
Silicon/silver nanocomposite		1.65 × 10−6 –0.5 × 10−3	0.45 ×10−6 [35]
Au/graphene/HRP/CS			1.7 × 10−6 [36]
MWCNT–graphene		20 × 10−6–2.1 × 10−3	9.4 × 10−6 [36]
BGNs		1 × 10−3–20 × 10−3	3.8 × 10−6 [36]		266.7 × 10−6 [36]
Ag NS/ITO		0.2 × 10−3–4 × 10−3	1 × 10−6 [37]
Graphene–hAuPd		0.1 × 10−6–20 × 10−6			5.059 [36]
RGO–MWCNT–Pt/Mb					1.99 × 10−6 [36]

. It can be seen from Table 1 that the LODs obtained from different techniques are quite comparable with each other, and are much lower than the already reported data [34,35,36,37]. It can be concluded from these results that electrodeposited PACHA and PANI on the surface of an Au electrode enables the detection of H₂O₂ at a low concentration level. The LOD and LOQ determined with a DPV have the same values both for PANI- and PACHA-coated electrodes, despite the higher peak current of PANI compared to PACHA. The DPV curves of PANI do not show sharp peaks, but rather wide current plateaus spreading over the region from 50 to 200 mV. The DPV curves of PACHA show sharp peaks around 50 mV; however, the current peaks shift towards 100 mV with the increase in concentration of ascorbic acid (AA). The LOD and LOQ values evaluated from DPV curves depend not only on the peak current, but also the linearity and slope of the calibration curve. The R² value of PACHA (Figure 7b) is higher than for PANI (Figure 7d); therefore, the LOD and LOQ values determined with a PACHA-coated electrode resemble those determined with a PANI-coated electrode.

2.4. Stability, Sensitivity, and Reproducibility

The stability of the electrodes was determined by keeping the electrodes in 1 M solution of citrate buffer (pH 5) for 10 days and recording CV before and after immersion in 1.4 × 10⁻⁶ M H₂O₂ solution. The resulted CV graph is provided in Figure S1 of the supporting file. The results show that the current peak decreased very slightly for the PACHA-coated electrode, indicating its good stability. DPV measurements were carried out in order to analyze the sensitivity of the sensors. The resulted data was plotted as a calibration curve of current versus concentration in Figure S2. The sensitivity of electrodes was evaluated using Equation (5) [38]:

$$Sensitivity=\raisebox{1ex}{$m$}\!\left/ \!\raisebox{-1ex}{$A$}\right.$$

(5)

In the above equation, m is the slope of calibration curve and A is the area of active sites of electrode, where A = 0.5 cm². The resulted sensitivity value for PACHA was 650 A/Mcm² in terms of current, while for PANI it was 434 A/Mcm². The sensitivity was found to be much greater than previously reported H₂O₂ sensors [36,39]. The current values assure the high sensitivity of both PACHA- and PANI-coated electrodes. The reproducibility of the sensor was also scrutinized with the help of cyclic voltammetry. The electrochemical behavior of PACHA was noted in solutions of different concentrations. The derived data was reported in Figure S3 in the form of a calibration curve. The LOD calculated from the reproducibility measurement was 1 × 10⁻⁷, which is same as the LOD reported earlier, thus assuring the repeatability of the PACHA-coated electrode.

2.5. Electrochemical Impedance Measurements

Electrochemical impedance spectroscopy (EIS) is considered to be a promising tool for interfacial properties studies of surface modified electrodes. The data is usually presented in the form of Nyquist and Bode plots. The diameter of the semicircle in a Nyquist plot represents the charge transfer resistance (Rct), and the straight line shows the diffusion-related process [40]. Figure 8 shows Nyquist plots of bare gold and polymer-coated electrodes in citrate buffer (pH 5) containing 1.4 × 10⁻⁶ M H₂O₂ solution. As can be easily observed, PACHA exhibits both a semicircle and a straight line. However, the semicircle diameter is very small, suggesting low Rct to the H₂O₂ dissolved in the citrate buffer solution. The Nyquist plots of bare Au and PANI-coated electrodes indicate that the impedance is due to the interfacial charge transfer within the applied frequency range [41]. However, the PHA shows only a diffusional, limited electrochemical process [42].

The Bode plot of capacitance versus frequency, obtained with PANI-, PACHA-, PHA-coated electrodes, is given in Figure 9a. It is clear from the figure that the capacitance decreases with the increase in frequency. However, the lowering of the capacitance value is only observed in the low frequency range, whereas the ion permeability of the films remains the same at the higher frequency range. The resulted capacitance values of PACHA–Au, PANI–Au, PHA–Au, and Au were 1.84 × 10⁻³ F, 1.17 × 10⁻³ F, 2.52 × 10⁻⁴ F, and 1.21 × 10⁻² F, respectively. The capacitance of PACHA is greater than its homopolymers, which means that PACHA is more permeable and has a higher tendency for charge storage than PANI and PHA.

The phase angle decreases in order of PHA > PANI > Au > PACHA, as shown in Figure 9b. The decrease in phase angle specifies the reduction in inductive parameters like charge transfer resistance, membrane resistance, and solution resistance [43]. The decrease in phase angle also enhances the inductive behavior of the electrode. A capacitor with promising inductive behavior requires a low phase angle. It can be seen in Figure 9b that Au and PACHA exhibit low phase angles; hence, it is assumed that their inductive properties are better than both the homo-polymers. The phase angle analysis is in agreement with capacitance.

3. Materials, Instruments, and Methods

3.1. Materials and Instrument

Reagent grade aniline (Riedel–De Haën, Seelze, Germany) was distilled and then stored in nitrogen atmosphere. The reagent-grade 2-hydroxyaniline (ACROS), reagent-grade sulfuric acid (Riedel–De Haën, Seelze, Germany), H₂O₂ (analytical-grade), citric acid, and sodium citrate were used as received. Gamry Reference 3000 workstation (Warminster, PA, United States) was used for carrying out electrochemical measurements in a three-electrode assembly. Gold sheets of 0.5 cm² area, gold wire, and a saturated calomel electrode were used as working, counter, and reference electrodes, respectively. LSV DPV EIS measurements were carried out in the frequency range of 0.1 Hz to 10⁵ Hz at 0.5 V, with 5 mV AC perturbation. Square wave voltammograms were recorded in the range of −0.2 to 0.5 V at 15 Hz with 4 mV step size, 50 mV pulse size and 0.06 s pulse time. Differential pulse voltammograms were recorded with 2 mV step size, 50 mV pulse size and 0.2 s pulse time in the potential range of -0.2 to 0.56 V.

3.2. Electrode Preparation

The PANI, PHA, and PACHA were prepared by the electrooxidation of aniline, 2-hydroxyaniline, and its copolymer in 0.5 M sulfuric acid solution at 50 mV/s, while keeping the electrode potential range from −0.2–0.85 V (for aniline and 2-hydroxyaniline) and −0.2–1.1 V (for the copolymer). The resulted CVs are shown in Figure 10.

The cyclic voltammograms (CVs) in Figure 10a display the growth of polyaniline film during the electrolysis of 20 mM aniline in 0.5 M sulfuric acid solution. There were three anodic peaks at 0.2 V, 0.48 V, and 0.79 V, while three cathodic peaks at 0.6 V, 0.39 V, and 0.0 V were observed. The peak currents increase constantly with the increase in the number of potential cycles. This indicates that as the electrolysis proceeds, the film of polyaniline grows more rapidly. After electrolysis, a green film of polyaniline was formed on the working electrode.

Figure 10b shows the cyclic voltammograms for electrolysis of 2 mM 2-hydroxyaniline in 0.5 M H₂SO₄ solution. Two anodic peaks can be observed, one at 0.64 V, which is attributed to the oxidation of a hydroxyl group in the phenyl ring, and the other at 0.77 V, which is due to the oxidation of an amino group in the phenyl ring of 2-hydroxyaniline. Oxidation currents at 0.64 V and 0.77 V decrease quickly with increase in the number of potential cycles, while a peak at 0.13 V developed, which is attributable to the oxidation of poly (2-hydroxyaniline). After 25 potential cycles, a thin golden film of poly (2-hydroxyaniline) was obtained on the working electrode. The slenderness of the film is due to the inhibited growth during the electrolysis, owing to the low conductance of poly (2-hydroxyaniline) [21].

Cyclic voltammograms for electrolysis of the solution containing a mixture of 2 mM 2-hydroxyaniline and 20 mM aniline in 0.5 M H₂SO₄ is given in Figure 10c. There are three anodic peaks and four cathodic peaks on the curves—i.e., changed from the curves in Figure 10a,b. The anodic peak at 0.66 V in Figure 10c is due to the oxidation of 2-hydroxyaniline, compared with that in Figure 10b. The formation of the three cathodic peaks indicates the formation of PACHA [21]. The oxidation peak at 0.24 V and reduction peak at 0.28 V and 0.5 V occurs on the cyclic voltammogram. This extra pair of redox peaks in Figure 10c is neither from the redox of 2-hydroxyaniline nor the polyaniline film, but rather is caused by the copolymer itself. Finally, a deep yellow film was formed on the working electrode. The fabricated electrodes were then used to check out their electrochemical behavior towards H₂O₂ oxidation.

4. Conclusions

In conclusion, PACHA- and PANI-coated electrodes were successfully applied for the detection and determination of H₂O₂. The PACHA and PANI films can effectively support electron transfer, which makes them show a high electrochemical response towards H₂O₂. The fabricated electrodes show good sensitivity of 650 A/Mcm² (in the case of PACHA) and 464 A/Mcm² (in the case of PANI). Moreover, the LOD calculated from the calibration curve (CV current response to H₂O₂ concentration) was as low 1 × 10⁻⁷ M, in the case of PACHA-coated electrodes, and 1.15 × 10⁻⁷ M in the case of PANI-coated electrodes. Considering the advantages, like good electrochemical behaviour, high sensitivity, good stability, and reproducibility, the materials show pronounced potential for the detection and determination of H₂O₂. Furthermore, this opens a new doorway for PACHA and PANI to be used in analytical devices. It also offers a way to detect H₂O₂ in many enzymes-catalyzed reactions, environmental samples, food, and clinical industries.