Development of Binary Pd@PPyC‐Bi₂O₃ Nanocomposite Sensor for Trace‐Level Detection of 2,4‐Dinitrophenol in Wastewater via Linear Sweep Voltammetry

ABSTRACT

Here, an electrochemical sensor has been developed for the efficient determination of 2,4‐dinitrophenol (2,4‐DNP) in aqueous medium at neutral pH (7.0) phase. The chemical sensing probe was developed by modifying a flat glassy carbon electrode (GCE) with palladium‐doped polypyrrole carbon–bismuth oxide nanocomposites (Pd@PPyC‐Bi₂O₃ NCs), synthesized via a hydrothermal route. The electrochemical performance of the NCs fabricated with PEDOT:PSS conducting polymers modified electrode (Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE) was evaluated by using linear sweep voltammetry (LSV), which revealed a wide linear detection range (LDR) of 10.0 – 90.0 µM for 2,4‐DNP. The calibration plot exhibited excellent linearity, with the sensor demonstrating a high sensitivity of 109.70 µA.µM^{− 1}.cm^{− 2}. The limit of detection (LOD) was obtained to be 0.92 ± 0.05 µM. In addition, the sensor exhibited good repeatability and stability under repeated measurements. Its practical applicability was further validated by successfully detecting 2,4‐DNP in environmental collected waste water samples from different sources, with satisfactory recovery results. Overall, this study introduces a sensor Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE probe as a good electrochemical platform for efficient monitoring of 2,4‐DNP by LSV, offering significant potential for environmental analysis and pollution control in a broad scales.

A novel Pd@PPyC‐Bi₂O₃ nanocomposite‐based electrochemical sensor was developed for highly sensitive and selective detection of toxic 2,4‐dinitrophenol. The sensor showed excellent electrocatalytic performance, low detection limit, and successful application in real water samples for environmental monitoring.

1. Introduction

Generally, 2,4‐Dinitrophenol (2,4‐DNP) is an organic pollutant that poses significant environmental and health risks [1, 2]. 2,4‐DNP has several industrial applications, which contribute to its widespread presence in the environment. Some of the key industrial applications of 2,4‐DNP include pesticide and herbicide production, dye and pigment manufacturing, explosives and propellant production, and pharmaceutical and chemical synthesis [3, 4]. In addition, 2,4‐DNP is used as a reagent in various chemical reactions and as an intermediate in the synthesis of certain pharmaceutical compounds. The improper disposal or accidental release of 2,4‐DNP during these processes can contribute to environmental contamination. Exposure to 2,4‐DNP can lead to adverse effects such as increased body temperature, nausea, vomiting, sweating, dizziness, and headaches [5, 6]. Exposure to 2,4‐DNP can lead to severe adverse health effects in humans and animals. it acts as an un‐coupler of oxidative phosphorylation, disrupting the normal cellular process of ATP production. In severe cases, 2,4‐DNP poisoning can lead to life‐threatening consequences, such as hyperthermia, metabolic acidosis, and even death [7, 8]. Due to its widespread use in various industries, including the production of pesticides, dyes, and explosives, the development of reliable and sensitive detection methods for 2,4‐DNP is of great importance.

Electrochemical sensing techniques have emerged as efficient approaches for the detection of 2,4‐DNP due to their inherent advantages, such as good sensitivity, good conductivity, fast response, and the potential for in situ monitoring [9, 10]. Considerable research efforts have been devoted to the design and fabrication of electro‐chemical sensors for 2,4‐DNP detection. These sensors have typically employed various electrode materials, including bare GCE, metal‐based nanostructures, and composite materials, to enhance the sensitivity and selectivity toward 2,4‐DNP [11, 12]. The use of nanocomposites (NCs) has demonstrated great potential for improving the performance of 2,4‐DNP electrochemical sensors [13, 14]. NCs can combine the unique properties of different nanomaterials, such as enhanced conductivity, catalytic activity, and surface area, to provide superior electrochemical performances [15, 16, 17, 18]. For instance, the integration of palladium (Pd) nanoparticles with conducting polymers, such as polypyrrole (PPy), and metal oxides, such as bismuth oxide (Bi₂O₃), can result in NCs with improved electron transfer kinetics, catalytic activity, and adsorption capabilities, which are useful for the determination of 2,4‐DNP.

It presents the improvement of a very sensitive chemical sensor for the accurate determination of 2,4‐DNP in environmental samples. The sensor utilizes a modified electrode of Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE which was manufactured by a hydrothermal process. The constructed sensor was characterized and assessed using LSV for the measurement of 2,4‐DNP in aqueous solutions at a pH of 7.00. The sensor demonstrated an extensive LDR, elevated sensitivity, and a minimal LOD, rendering it a potential instrument for the environmental monitoring of 2,4‐DNP.

2. Experimental Section

2.1. Materials and Methods

In this work, Bismuth (III) oxide (Bi₂O₃) nanopowder, Polypyrrole‐doped carbon black, PEDOT:PSS, palladium(II) chloride, isopropanol, methanol (CH₃OH), and ethanol (C₂H₅OH) were obtained with purity (99.99%) from Sigma–Aldrich and utilized as received without any further purification. The structural characteristics of the synthesized Pd@PPyC‐Bi₂O₃ NCs were analyzed by x‐ray diffraction (XRD) using a Bruker AXS D4 Endeavour diffractometer equipped with Cu Kα radiation (λα₁ = 154.060 pm; λα₂ = 154.439 pm). Surface morphology as well as microstructural details were investigated through field‐emission scanning electron microscopy (FESEM, JEOL‐6300F operated at 5.0 kV) and high‐resolution transmission electron microscopy (HRTEM, JEOL JEM‐2100F‐UHR at 200 kV) fitted with a 1k CCD detector and a Gatan GIF 2001 energy filter. Functional groups present in the samples were identified using Fourier‐transform infrared (FTIR) spectroscopy on a PerkinElmer Spectrum 100 instrument, recorded in the 400–4000 cm^{− 1} range using KBr pellets. Elemental composition and chemical states were examined by x‐ray photo‐electron spectroscopy (XPS) with a VG ESCALAB 200R system employing a monochromatic Mg Kα1 source (hν = 1253.6 eV) and a hemispherical analyzer. Specific surface area and pore characteristics were obtained from N₂ adsorption–desorption measurements at 77 K using a Quantachrome NOVA 4200 analyzer; all samples were degassed at 200°C for 12 h before the experiments. Optical absorption behavior was assessed by diffuse reflectance UV–vis (DRS) spectroscopy on a Shimadzu UV–vis 3600 instrument in the 200–800 nm wavelength range.

2.2. Preparation of PPyC‐Bi₂O₃ NCs

The 10% PPyC/Bi₂O₃ NCs were synthesized using the ultra‐sonication process. In summary, 1.0 g of Bi₂O₃ nanopowder was incorporated into 100.0 mL of deionized water containing 0.1 g of PPyC and mixed with continuous agitation for about 20 min. The resultant solution was then exposed to ultra‐sonication (20 kHz) for about 40 min. The resultant mixture was further washed with acetone, ethanol and then deionized water, followed by drying in an oven at 60.0°C for 12 h to produce PPyC/Bi₂O₃ nanostructures. Afterwards, the photo‐reduction approach, as previously documented [19, 20], was used for the synthesis of the necessary 1% Pd@10%PPyC/Bi₂O₃ photo‐catalytic framework. The target weight of palladium (II) chloride (solid), which contained 1.0 wt% of Pd, was precisely measured and thoroughly combined with 0.5 g of a 10% PPyC/Bi₂O₃ solution, then irradiated with mercury‐lamp (Osram) at an intensity of 2.0 mWcm⁻² for 14 h. Upon completion of the irradiation procedure, the mixture was further centrifuged to isolate the desired substance. Following that, the obtained final NCs material was washed with acetone, ethanol, and finally deionized water systematically, then dried at 65.0°C for 12 h to provide 1% Pd@10%PPyC/Bi₂O₃, referred to as Pd@PPyC/Bi₂O₃ throughout the text.

2.3. Modification of Working Electrode (WE) Using Pd@PPyC‐Bi₂O₃ NCs

Working electrode (WE) for the proposed sensor was prepared by modifying GCE electrode. First, the Pd@PPyC‐Bi₂O₃ NCs was dispersed in ethanol to form a uniform slurry. This mixture was carefully spread as a thin film onto the polished surface of the GCE and allowed to dry at room temperature. To improve adhesion and ensure the coating stayed firmly attached, several drops of a 5% PEDOT:PSS solution in ethanol were added on top of the dried layer. The coated electrode was then placed in an oven at 35°C for 1 h to complete the drying process. The electrochemical sensing setup was assembled using Metrohm Autolab equipment. The modified GCE served as the WE, while an Ag/AgCl (Saturated KCl) electrode and a platinum wire functioned as the reference and counter electrodes, respectively. A stock solution of 2,4‐DNP was prepared in a pH 7 buffer, and different concentrations ranging from 10.0 to 90.0 µM were used for testing. LSV was carried out to evaluate how good the sensor detected 2,4‐DNP. The resulting measurements were used to determine the sensor's sensitivity, LOD, LDR, stability, and how solution pH influenced the sensing performance.

2.4. Electrochemically Active Surface Area (EASA) of NCs Coated GCE

The electrochemically active surface area (EASA) of the electrodes was estimated from the electrochemical measurements using the Randles–Ševčík Equation (1):

$$I_{\mathrm{p}}=\left(2.69\times {10}^5\right)n^{3/2}A{D}^{1/2}C{\nu }^{1/2}$$ (1)

where I _p = peak current (A); n = number of electrons transferred; A = electroactive surface area (cm²); D = diffusion coefficient of [Fe(CN)₆]^{3 − /4 −} (7.6 × 10^{− 6} cm² s^{− 1}) , and C = concentration (5.0 mM); ν = scan rate (V s^{− 1}).

The calculated EASA values are found as 0.0475 and 0.0316 cm² for the Pd@PPyC‐Bi₂O₃ NCs coated and bare GCE, respectively. The electroactive surface area of NCs‐modified GCE electrode was found to be around 1.5 times bigger than that of the bare GCE. Thus, the insertion of the Pd@PPyC‐Bi₂O₃ NCs greatly increased the number of electro‐chemically accessible active sites. The expanded electroactive surface area offers additional adsorption sites for 2,4‐DNP molecules and promotes the charge transfer, hence contributing to the improved sensing performance of the manufactured sensor.

2.5. EIS Study for Pd@PPyC‐Bi₂O₃ NCs

The usual three‐electrode electrochemical cell was employed for the electrochemical impedance spectroscopy (EIS) experiments, where the bare GCE or Pd@PPyC‐Bi₂O₃/GCE was served as the WE, Ag/AgCl (3.0 M KCl) as the reference electrode and platinum wire as the counter electrode. The impedance spectra were measured at open circuit potential in 5.0 mM K₃[Fe(CN)₆]/K₄[Fe(CN)₆] with 0.1 M KCl using a sinusoidal AC perturbation of 5.0 mV over a frequency range of 100 kHz to 0.1 Hz. The resulting Nyquist plots were fitted with a modified Randles equivalent circuit to calculate the solution resistance (R _s), the charge transfer resistance (R _ct) and Warburg impedance (Zw: 0.0001).

3. Results and Discussion

3.1. FESEM Analysis

For the morphological analysis, Figure 1 displays the FESEM investigation of the Pd@PPyC‐Bi₂O₃ NCs. As illustrated in Figure 1a, the Bi₂O₃ component exhibits a spherical shape, appearing as an irregular aggregation of particular components. The polypyrrole‐carbon (PPyC) component forms a distinct network structure, as depicted in Figure 1b. When the Bi₂O₃ is added to the PPy, the Bi₂O₃ particles become adsorbed onto the PPyC network, as demonstrated in Figure 1c. Ultimately, with embedded of Pd nanoparticles to the PPyC/Bi₂O₃ system, the overall structure does not appear to change significantly, but the Pd nanoparticles are not observed as a distinct, identical shape, as shown in Figure 1d. Based on these observations, the prepared Pd@PPyC‐Bi₂O₃ material should be considered a NCs system, where the different components are closely combined without a significant change of their single morphologies.

FIGURE 1.

Morphological analysis by FESEM for Pd@PPyC‐Bi₂O₃ NCs. (a) Bi₂O₃, (b) PPyC, (c) PPyC‐Bi₂O₃, and (d) Pd@PPyC‐Bi₂O₃ NCs.

3.2. HRTEM Analysis of Pd@PPyC‐Bi₂O₃ NCs

The structural observations made in the HRTEM images (Figure 2) are similar to those observed in the FESEM investigation (Figure 1). As shown in Figure 2a, the Bi₂O₃ components appear as spherical molecules that are aggregated in an irregular shape, without forming any distinct morphological and structural shape. The polypyrrole‐carbon (PPyC) molecules, on the other hand, are interconnected with each other, as displayed in Figure 2b. When the Bi₂O₃ molecules are added to the PPyC, they become adsorbed onto the PPyC network, as perceived in Figure 2c. Finally, when Pd particles are added to the PPyC‐Bi₂O₃ system, the Pd particles are adsorbed onto the surface, as presented in Figure 2d. However, the Pd@PPyC‐Bi₂O₃ NCs does not form any particular shape or size in its overall structure. Consistent with the observations from the FESEM examination, the HRTEM images confirm that the organized Pd@PPyC‐Bi₂O₃ NCs should be reflected a NCs material, where the individual components are combined without forming a distinct, uniform structural arrangement. The lattice spacing has calculated and found as 0.23 nm, which is presented in Figure 2e. The selected area electron diffraction (SAED) is also measured and included in the Figure 2f.

FIGURE 2.

HR‐TEM evaluation of Pd@PPyC‐Bi₂O₃ NCs. (a) Bi₂O₃, (b) PPyC, (c) PPyC‐Bi₂O₃, (d) Pd@ PPyC‐Bi₂O₃ NCs, (e) Lattice spacing in magnified HRTEM image (0.23 nm), and (f) SAED.

3.3. EDS Analysis

The EDS analysis of the Pd@PPyC‐Bi₂O₃ NCs is presented in Figure 3. As observed in the previous FESEM (Figure 1) and HRTEM (Figure 2) analyses, the synthesized nanomaterials were determined to have a NCs structure. This observation is further confirmed with EDS data, as illustrated in Figure 3a. The elemental investigation of the Pd@PPyC‐Bi₂O₃ NCs, presented in Figure 3b, reveals the following composition: carbon (C) 60.15%, oxygen (O) 11.68%, bismuth (Bi) 26.31%, nitrogen (N) 1.25%, and palladium (Pd) 0.61%. Overall, the EDS results further support the NCs nature of the synthesized Pd@PPyC‐Bi₂O₃ material, with the presence of the expected elemental constituents.

FIGURE 3.

Elemental analysis of NCs. (a) SEM image and (b) elemental spectrum and elemental composition (inset).

3.4. XPS Analysis of Pd@PPyC‐Bi₂O₃ NCs

The oxidation states as well as binding energies of the elements present in the Pd@PPyC‐Bi₂O₃ NCs were analyzed using x‐ray photoelectron spectroscopy (XPS), as shown in Figure 4. The Bi4f spectrum in Figure 4a shows two characteristic peaks, Bi4f_7/2 and Bi4f_5/2, located at 160.5 and 166.0 eV. These values indicate that bismuth exists predominantly in the Bi^{3 +} state within the NCs [21, 22]. The XPS spectrum corresponding to palladium, presented in Figure 4b, displays a pair of doublet peaks. The Pd3d_5/2 signals appear at 336.5 and 339.0 eV, while the Pd3d_3/2 peaks are observed at 343.0 and 345.0 eV. These features confirm the expected electronic states of Pd in the material. Compared to the standard XPS spectra of metallic Pd⁰, the binding energies of the Pd peaks in the NCs have shifted slightly higher, from 336.5 to 339.0 eV and from 343.0 to 345.0 eV. These binding energy values are in good agreement with the expected values for Pd in the zero‐oxidation state (Pd⁰) [23, 24]. The O 1s XPS spectrum of the Pd@PPyC‐Bi₂O₃ NCs (Figure 4c) was found in single component. A magnified feature appears near 533.0 eV signifies oxygen present in O‐group in Metal‐oxygen bonds [25, 26]. The C 1s spectrum (Figure 4d) shows one clear peak at 286 eV. The peak at 286 eV is linked directly to C─H bonding in pyrrole, while the peak at 286 eV is associated with C─H groups. These signals confirm that the polypyrrole‐based matrix contains oxygen‐rich functionalities such as hydroxyl (C─O) and carbonyl (C═O) groups [27, 28]. Along with the carbon‐ and oxygen‐related features, the N 1s spectrum of the NCs also displays two peaks that reflect different nitrogen‐containing groups present within the composite material. A peak (at 398 eV) is associated with ═N─ functional groups, whereas a peak (at 401 eV) is assigned to N–H functional groups, as confirmed by reports earlier [29, 30]. The XPS study of the NCs yielded significant insights into its chemical composition as well as oxidation states. The study verified the presence of Bi³⁺, Pd⁰, O²⁺, along with C─O, C═O, ═N─, and N─H bonds in the prepared NCs.

FIGURE 4.

Binding energy as well as ionization states of constituting atoms of Pd@PPyC‐Bi₂O₃ NCs. (a) Bi4f, (b) Pd3d, (c) O1s, (d) C1s, (e) N1s orbitals, and (f) Full XPS survey spectrum.

3.5. Crystallinity, Optical, and Surface Area Analysis of Pd@PPyC‐Bi₂O₃ NCs

The crystalline structure and phase composition of the Pd@PPyC‐Bi₂O₃ NCs were evaluated using x‐ray diffraction (XRD), as presented in Figure 5a. The diffraction peaks corresponding to Bi₂O₃ show excellent agreement with the reference pattern for pure Bi₂O₃ (JCPDS No. 76–1730; 74–1375, and 76–0147). The major reflections appearing at the measured 2θ positions can be assigned to the (002), (012), (121), (200), (220), (212), (041), (123), (222), (241), (115), (622), (543), (127), (402), (800), (423), (028), and (356) lattice planes of the Bi₂O₃ phase [31, 32].

FIGURE 5.

(a) Crystallinity of prepared Pd@PPyC‐Bi₂O₃ NCs, (b) band‐gap energy assessment of NCs, (c) Surface area of NC using BET, and (d) pore size distribution.

This XRD analysis confirms the highly crystalline nature of the Bi₂O₃ phase in the Pd@PPyC‐Bi₂O₃ NCs, with no indication of impurities as secondary phases. The XRD analysis also reveals the crystalline phases of the palladium (Pd) component in the NCs. The detected diffraction peaks also correspond well to the (200) plane, as previously documented in the literature [33, 34]. In addition, the XRD pattern shows a distinct signal associated with the crystalline carbon phase, which aligns with the (002) plane reported in earlier studies [35, 36].

UV–vis spectroscopy was further used to study the optical properties of the Pd@PPyC‐Bi₂O₃ NCs, as presented in Figure 5b. From the absorption spectrum, the band‐gap energy was estimated to be 2.8 eV, suggesting that the NCs exhibit good electrical conductivity. The surface area of the material was also measured using the Brunauer–Emmett–Teller (BET) method. The Pd@PPyC‐Bi₂O₃ NCs showed a specific surface area of 78.19 m²/g, indicating a large number of active sites. This high surface area is beneficial for enhancing the electro‐chemical determination of 2,4‐DNP in buffer solution. Additionally, the pore size distribution of the Pd@PPyC‐Bi₂O₃ NCs was analyzed using the BJH method, and the results are presented in Figure 5d. The NCs exhibited an average pore diameter of 27.4 nm, indicating a mesoporous structure that is favorable for efficient mass transport and enhanced electrochemical activity. Furthermore, the cumulative pore volume determined from the BJH adsorption analysis was 1.3161 cm³g^{− 1}, confirming the presence of abundant mesopores that provide a high accessible surface area and numerous active sites for analyte adsorption, thereby contributing to the improved sensing performance of the fabricated electrode.

3.6. Electro‐Chemical Analysis of Pd@PPyC‐Bi₂O₃ NCs Fabricated GCE

Voltammetric measurements for the Pd@PPyC‐Bi₂O₃ NCs–modified PEDOT:PSS/GCE were performed in a 0.1 mM K₄[Fe(CN)₆], solution, as given in Figure 6a. The modified electrode displayed a peak‐to‐peak separation of +0.55 V, which is lower than the +0.58 V observed for the unmodified GCE. This reduced separation indicates enhanced electron‐transfer efficiency at the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE surface, making it more suitable for electrochemical sensing applications. Similar observations have been noted in earlier studies [37, 38, 39, 40]. To evaluate diffusion behavior at the electrode surface, the peak current was plotted against the square root of the scan rate over the range of 25–300 mV/s, using the same 0.1 mM K₄[Fe(CN)₆] system (Figure 6b). As illustrated in Figure 6c, the oxidation as well as reduction peak currents increased linearly with the square root of the scan rate. These linear trends confirm diffusion‐controlled electrochemical processes and can be defined according to the following relationships (Equations 2 and 3):

$$\begin{array}{ccc}& & {\mathrm{i}}_{\mathrm{p}}=59.745{\left(\mathrm{SR}\right)}^{0.5}+169.11;\hfill \\ & & R^2=0.9996\mathrm{at}\mathrm{oxidation}\mathrm{of}{\mathrm{K}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]\hfill \end{array}$$ (2)

$$\begin{array}{ccc}& & {\mathrm{i}}_{\mathrm{p}}=-45.791{\left(\mathrm{SR}\right)}^{0.5}-181.96;\hfill \\ & & {\mathrm{R}}^2=0.9949\mathrm{at}\mathrm{reduction}\mathrm{of}{\mathrm{K}}_4\left[\mathrm{Fe}{\left(\mathrm{CN}\right)}_6\right]\hfill \end{array}$$ (3)

FIGURE 6.

(a) Analysis of electrochemical performance with CV on NCs‐coated as well as bare‐GCE at 0.1 mM K₄[Fe(CN)₆], (b) scan rate effect on 25–300 mV/s, and (c) Square root of scan rates vs peak current.

3.7. LSV for Detection of 2,4‐DNP

LSV is widely regarded as a sensitive and dependable technique for detecting trace levels of organic pollutants. In this study, LSV was used to measure low concentrations of 2,4‐DNP in a conductive aqueous environment. For this purpose, a sensing platform was constructed by coating a GCE with Pd@PPyC‐Bi₂O₃ NCs (Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE). For the selectivity study, the fabricated Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE sensor was evaluated against several potentially interfering electroactive compounds, including 2‐nitrophenol (2‐NP), thiourea, hydrazine, 2,4‐DNP, urea, nitrite, xanthine, 4‐aminophenol (4‐AP), and 4‐chlorophenol, under identical experimental conditions using the same analyte concentration. As shown in Figures 7a, 2,4‐DNP produced the highest voltammetric peak current, demonstrating the excellent selectivity of the proposed sensing platform toward the target analyte. The superior response of 2,4‐DNP can be attributed to its two electron‐withdrawing nitro groups, which enhance its electrochemical activity and facilitate stronger adsorption onto the Pd@PPyC‐Bi₂O₃ NCs through π–π interactions, hydrogen bonding, and surface interactions with the catalytic Pd and Bi₂O₃ active sites. Furthermore, the synergistic effect of the highly conductive PPy‐derived carbon network, Pd nanoparticles, and Bi₂O₃ significantly accelerates interfacial electron transfer, resulting in an enhanced electrocatalytic response toward 2,4‐DNP compared with the other examined compounds. These results confirm the high selectivity and excellent recognition capability of the fabricated sensor for the sensitive determination of 2,4‐DNP. In addition, the pH of the phosphate buffer solution was found to be another important parameter affecting the electrochemical response of 2,4‐DNP, and therefore pH optimization was subsequently carried out to establish the optimum sensing conditions. The sensor's performance was tested using a range of pH values (5.8 to 8.5), as illustrated in Figure 7b. The results showed that the sensing performance of 2,4‐DNP was maximized at a pH of 7.1. Thus, the study has demonstrated that LSV using the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE sensor is an efficient and reliable analytical method for the detection of trace 2,4‐DNP in a conductive aqueous medium, with an optimum pH of 7.1. The electrochemical sensing of 2,4‐DNP was performed using LSV with a Pd@PPyC‐Bi₂O₃ NCs‐fabricated PEDOT:PSS/GCE as the WE in this analysis. In Figure 7c, the reduction peak current decreases proportionally as the 2,4‐DNP concentration increases from 10.0 to 90.0 µM at pH 7.1 phosphate buffer solution. To establish the calibration profile for the sensor, the LSV peak currents were plotted against the corresponding 2,4‐DNP concentrations, as displayed in Figure 7(d). The calibration curve showed strong linearity, with a regression coefficient (R²) of 0.9957, and the relationship between peak current and concentration is depicted by the linear Equation (4):

$${\mathrm{I}}_{\mathrm{p}}=-3.4664\mathrm{C}\left(\mu \mathrm{M}\right)+440.62$$ (4)

FIGURE 7.

LSV examination of 2,4‐DNP in phosphate buffer solution. Analytical parameter including (a) Selectivity, (b) pH optimization of 2,4‐DNP sensor with Pd@PPyC‐Bi₂O₃ NCs fabricated PEDOT:PSS/GCE, (c) LSV investigation of 2,4‐DNP in different analyte concentrations, and (d) calibration curve in 2,4‐DNP detection by LSV.

The observed linear relationship between the decreasing peak current as well as the concentration of 2,4‐DNP confirms that the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE functions effectively as an electrochemical sensor for quantifying 2,4‐DNP. The sensor exhibited a linear detection range (LDR) from 10.0 to 90.0 µM. Based on the slope of Equation (3) and the electroactive surface area of the GCE, the sensor's sensitivity was evaluated to be 109.6962 µA.µM^{− 1}.cm^{− 2}.Using a signal‐to‐noise ratio of 3, LOD for 2,4‐DNP was determined to be (0.92 ± 0.05) µM. This low LOD highlights the strong analytical capability of the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE for sensitive and reliable 2,4‐DNP measurement.

To evaluate the contribution of each component in the Pd@PPyC‐Bi₂O₃ NCs, separate electrodes were prepared using individual materials such as Bi₂O₃, along with the full Pd@PPyC‐Bi₂O₃ NCs. Their performance was compared through LSV measurements in a pH 7.1 buffer containing 50.0 µM 2,4‐DNP, as shown in Figure 8a. Among all tested electrodes‐including the bare GCE‐the Pd@PPyC‐Bi₂O₃ NCs–modified PEDOT:PSS/GCE shown the highest reduction peak current. This observation indicates that the collective prepared NCs presents significantly better catalytic activity as well as sensitivity toward the electrochemical reduction of 2,4‐DNP. The enhanced response can be explained by the cooperative interaction among the components: Bi₂O₃ and the PPyC matrix create a suitable structural platform that facilitates the uniform incorporation of Pd nanoparticles. This integrated architecture improves electron‐transfer properties and boosts catalytic efficiency, ultimately giving rise to the strongest reduction signal for 2,4‐DNP.

FIGURE 8.

(a) Electrochemical analysis for control experiment with 2,4‐DNP and (b) repeatability test of the fabricated 2,4‐DNP sensor based on Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE.

Reproducibility is a crucial metric for assessing the dependability of electrochemical sensors. Consequently, the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE sensor underwent a study of 2,4‐dinitrophenol (2,4‐DNP) under similar conditions, namely at pH 7.1 and a concentration (50.0 µM) 2,4‐DNP. Figure 8b illustrates that the LSV conducted over seven consecutive days yields equal and indistinguishable results. This experiment illustrates the exceptional dependability of the constructed 2,4‐DNP sensor using Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE. The reproducibility of the sensor response over multiple days indicates the stability (relative standard deviation, RSD 1.8%) and consistency of the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE electrode. This consistent performance suggests that the chemical sensor probe can give reliable and accurate measurements of 2,4‐DNP concentrations, even with repeated use over an extended period. The high reproducibility of the sensor is a crucial characteristic, as it ensures the reliability and trustworthiness of the analytical results obtained using the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE sensor. This finding further reinforces the potential of this newly NCs‐based electro‐chemical sensor for the sensitive and reliable detection of 2,4‐DNP in various potential applications. The reliability of this study was established by comparing the performance characteristics of 2,4‐DNP, including LDR, sensitivity, and LOD, against recently reported sources, as detailed in Table 1 [3, 41, 42, 43, 44, 45, 46]. Through this comparative analysis of various electrochemical sensing substrates, the Pd@PPyC‐Bi₂O₃ NCs modified GCE exhibited the best overall performance. The Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE exhibits a broad LDR, high sensitivity, low LOD, great repeatability and stability, and effective validation in real wastewater samples. The enhancements are mainly ascribed to the synergistic effect of the highly conductive Pd nanoparticles, the porous PPy‐derived carbon framework and the catalytically active Bi₂O₃ nanostructure, which simultaneously increase the electroactive surface area, accelerate the electron transfer kinetics and provide plentiful active sites for the electrocatalytic reduction of 2,4‐DNP. The comparison additionally emphasizes the practical advantages of the suggested sensor, including facile hydrothermal synthesis, low cost of fabrication, simple electrode preparation, rapid response and dependable performance under ecologically important settings. The presence of a comparison Table 1 with discussion definitely improves the manuscript as it places the present work in the current literature and highlights the scientific significance and practical advantages over other existing electro‐chemical sensing platforms.

TABLE 1.

Comparative evaluations of 2,4‐DNP sensors utilizing different fabricated sensing substrates by electrochemical methodologies.

Composite Materials	LOD	LDR	Sensitivity	Refs.
Au NPs@ZnO/CTSN NCs/GCE	0.45 µM	15~150 µM	20.99 µAµM−1cm−2	[3]
Pt NPs/PPy‐CB/ZnO NCs/GCE	1.25 µM	1.5~40.5 µM	7.89 µAµM−1cm−2	[41]
Ag‐Chitosan/SrSnO3 NC/GCE	0.18 µM	1.5∼13.5 µM	54.03 µAµM−1cm−2	[42]
PbO‐doped ZnO MSs/GCE	2.95 µM	3.23∼16.67 µM	32.19 µAµM−1cm−2	[43]
BaO NRs/GCE	0.50 µM	1.5~9.0 µM	17.60 µAµM−1cm−2	[44]
RV7/GCE	1.07 nM	5.0 nM–35.0 µM	—	[45]
NH2‐MIL‐125(Ti)	5 × 10−10M	0–70 µM	—	[46]
Molecularly imprinted polymer	16.3 µmol L−1	50–1000 µmol L−1	—	[47]
ZnS NPs‐CA‐rGO‐GCE	0.01 µM	1.0–23.0 µM	22.23 µAcm−2µM−1	[48]
MWCNT/GCE	1.44×10−4 mM	0.001 to 0.05mM		[49]
3D printed CF–G–PE	7.8 µM	50 µM up to 1.0 mM	—	[50]
Pd@PPyC‐Bi2O3 NCs/PEDOT:PSS/GCE	0.92 µM	10.0~90.0 µM	109.69 µAµM−1cm−2	This study

The sensing mechanism for detecting 2,4‐DNP using LSV with the Pd@PPyC‐Bi₂O₃ NCs‐modified PEDOT:PSS/GCE is illustrated in Scheme 1. In this process, 2,4‐DNP molecules first adsorb onto the active surface of the NCs‐modified GCE electrode. When an external potential is applied, the adsorbed 2,4‐DNP undergoes an electrochemical reduction, converting it into 2,4‐diaminophenol (2,4‐DAP). The hypothesized process is as follows: 2,4‐DNP molecules are adsorbed on the high surface area Pd@PPyC‐Bi₂O₃ NCs by hydrogen bonding, π–π interactions with the carbon framework, and surface contacts with the Pd and Bi₂O₃ active sites. The nitro groups of 2,4‐DNP are electro‐catalytically converted to the corresponding amino derivative (2,4‐DAP) by proton‐ and electron‐transfer reactions when the right voltage is applied. The high conductivity carbon network from PPy offers fast electron transport paths and the Pd nanoparticles increase the electron‐transfer kinetics due to their high electrical conductivity and catalytic activity. Meanwhile, Bi₂O₃ provides many active sites and increases adsorption of the analyte, thus promoting the redox process. The synergistic effect of Pd, PPyC and Bi₂O₃ greatly reduces the activation energy for electrochemical reduction of 2,4‐DNP, enhances the charge‐transfer efficiency and electroactive surface area, and results in a strong and reproducible voltammetric response. These combined effects are responsible for the greater sensitivities, low detection limits and outstanding selectivity of the fabricated sensor. This electrochemical reduction manner of 2,4‐DNP has been reported in comparable studies in the literature [37, 38, 39, 40]. The Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE platform facilitates this redox reaction, enabling the sensitive and selective detection of 2,4‐DNP through voltametric techniques.

SCHEME 1.

Schematic illustration of the electro‐chemical detection of 2,4‐DNP to 2,4‐DAP. (1) Electrochemical reduction of 2,4‐DNP to 2,4‐DAP, and (2) concentration‐dependent reactions of 2,4‐DNP.

3.8. Morphological Stability of the NCs/GCE After Electrochemical Sensing

To further study the structural stability of the constructed sensing platform, the surface morphology of the Pd@PPyC‐Bi₂O₃/PEDOT:PSS/GCE was investigated by FESEM following the repeated electrochemical sensing of 2,4‐DNP. The corresponding image is shown in Figure 9. Figure 9a, b showed that the nanostructured morphology of the modified electrode is still intact before and after the sensing procedure. The Pd@PPyC‐Bi₂O₃ NCs was evenly dispersed on the electrode surface without noticeable aggregation, cracking, delamination or collapse of the porous structure. The continuous conductive network and well attached NCs layer imply the effectiveness of the binder to maintain the mechanical integrity of the sensing film throughout electrochemical measurements. Moreover, no evident morphological deterioration or structural deformation was seen after several sensing cycles, which confirmed the great physicochemical stability and robust adhesion of the Pd@PPyC‐Bi₂O₃ NCs on the GCE surface. The morphology retained is in agreement with the remarkable repeatability, reproducibility and operational stability from the electrochemical experiments. Thus, the FESEM results demonstrate that the constructed sensor has adequate structural robustness to determine 2,4‐DNP electrochemically with reliability and repeatability in environmental water samples.

FIGURE 9.

FESEM image for Pd@PPyC‐Bi₂O₃ NCs. (a) Before Sensing analysis, (b) after sensing analysis. (c) EIS analysis (Bare, before, and after sensing).

The interfacial electron‐transfer properties of the prepared electrodes were further studied by EIS utilizing the ferri/ferrocyanide redox probe. Figure 9c shows the Nyquist plots of the bare GCE and Pd@PPyC‐Bi₂O₃/PEDOT:PSS/GCE. The impedance spectra are composed of a semi‐circular region at high frequencies and a linear diffusion region at low frequencies. The diameter of the semicircle is related to the charge‐transfer resistance (R _ct), which is a measure of the electron‐transfer kinetics at the electrode/electrolyte interface. The bare GCE showed a significantly broad semicircle with R _ct = 612.5 Ω indicating somewhat sluggish electron‐transfer kinetics. By comparison, the Pd@PPyC‐Bi₂O₃/PEDOT:PSS/GCE presented a much‐reduced semicircle with R _ct value of 93.8 Ω, indicating a notable drop in the interfacial resistance following the electrode alteration. The large lowering of R _ct is ascribed to the synergistic interaction between conductive PPy‐derived carbon, highly catalytic Pd nanoparticles and semiconducting Bi₂O₃. The conductive carbon network provides fast electron transport channels, Pd nanoparticles facilitate the heterogeneous electron‐transfer processes and Bi₂O₃ provides a large number of catalytic active sites. Thus, the NCs significantly improves the electrical conductivity and enables quick charge transfer during electrochemical oxidation/reduction processes [51].

Additionally, the EIS after‐sensing EIS (101.7 Ω) results show that the modified electrode still has a lower R _ct than the previous sensing GCE (97.8 Ω) but with a little increment in comparison to the coated GCE electrode (before‐sensing electrode). This tiny shift (before and after sensing) is due to the temporary adsorption of 2,4‐DNP and/or its electrochemical reduction products on the electrode surface (Figure 9c) that partially block a small fraction of the active sites. However, the low R _ct value after sensing reveals that the Pd@PPyC‐Bi₂O₃ NCs still possesses outstanding electrical conductivity, fast electron‐transfer kinetics, and structural integrity. The results are in good accord with the post sensing FESEM observations and stability, repeatability and reproducibility experiments, suggesting that the constructed sensor is robust enough for repeated electrochemical detection of 2,4‐DNP.

Additionally, the 2,4‐DNP electrochemical sensor using LSV with Pd@PPyC‐Bi₂O₃ NCs coated on PEDOT:PSS/GCE was employed to detect 2,4‐DNP in various environmental collected samples from different sources, including subterranean water, seawater, and tap water, as shown in Table 2. The electrochemical analysis used the standard addition method, measuring the concentration of 2,4‐DNP according to the calibration curve shown in Figure 7d, with the results displayed in Table 2. The data shown in the table demonstrates the great dependability of the 2,4‐DNP electrochemical sensor.

TABLE 2.

Real collected environmental samples analysis with Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE sensor probe by electrochemical technique.

Real samples	Added 2,4‐DNP conc. (µM)		Measured 2,4‐DNP conc. a by Pd@PPyC‐Bi2O3 NCs/PEDOT:PSS/GCE (µM)			Average recovery b (%)	RSD c (%) (n = 3)
			R1	R2	R3
Underground water		40.00	41.40	41.16	39.81	101.96	2.1
Sea water		40.00	40.34	41.14	40.16	101.37	1.3
Tap water		40.00	41.14	39.91	40.21	101.05	1.6

^a Mean of 3 repeated determinations (signal to noise ratio 3) with Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE.

^b Concentration of 2,4‐DNP determined/Concentration taken. (Unit: µM).

^c Relative standard deviation value indicates precision among 3 repeated measurements (R1, R2, & R3).

4. Conclusions

This work presents the development of highly sensitive as well as selective electro‐chemical sensor for detecting 2,4‐DNP via Pd@PPyC‐Bi₂O₃ NCs‐modified PEDOT:PSS/GCE. The analytical performance of our 2,4‐DNP sensor surpasses that of previously published electrochemical sensing devices, demonstrating a broad LDR, exceptional sensitivity, and a remarkable LOD. The chemical sensing process entails the adsorption of 2,4‐DNP onto the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE surface, succeeded by the electro‐chemical reduction of 2,4‐DNP to 2,4‐DAP under the applied voltage. This redox reaction forms the basis of the sensor's strong voltammetric response toward 2,4‐DNP. Combined with its simple preparation and low production cost, the Pd@PPyC‐Bi₂O₃ NCs/PEDOT:PSS/GCE shows great potential for fast, sensitive, and selective detection of 2,4‐DNP in both environmental and industrial approaches. Additional testing using real samples and long‐term stability studies will further confirm how well this sensor performs in practical applications.

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

The data that support the findings of this study are available on request from the corresponding author. The data are not publicly available due to privacy or ethical restrictions.