Electroconductive Polyaniline–Ag-ZnO Green Nanocomposite Material

Abstract

Metal-conducting polyaniline (PANI)-based nanocomposite materials have attracted attention in various applications due to their synergism of electrical, mechanical, and optical properties of the initial components. Herein, metal-PANI nanocomposites, including silver nanoparticle-polyaniline (AgNP-PANI), zinc oxide nanoparticle-polyaniline (ZnONP-PANI), and silver-zinc oxide nanoparticle-polyaniline (Ag–ZnONP-PANI), were prepared using the two processes. Nanocomposite-based electrode platforms were prepared by depositing AgNPs, ZnONPs, or Ag–ZnONPs on a PANI modified glass carbon electrode (GCE) in the presence of 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide/N-Hydroxysuccinimide (EDC/NHS, 1:2) as coupling agents. The incorporation of AgNPs, ZnONPs, and Ag–ZnONPs onto PANI was confirmed by UV-Vis spectrophotometry, which showed five absorbance bands at 216 nm, 412 nm, 464 nm, 550 nm, and 831 nm (i.e., transition of π-π*, π-polaron band transition, polaron-π* electronic transition, and AgNPs). The FTIR characteristic signatures of the nanocomposite materials exhibited stretching arising from C–H aromatic, C–O, and C–N stretching mode for benzenoid rings, and =C–H plane bending vibration formed during protonation. The CV voltammograms of the nanocomposite materials showed a quasi-reversible behavior with increased redox current response. Notably, AgNP–PANI–GCE electrode showed the highest conductivity, which was attributed the high conductivity of silver. The increase in peak currents exhibited by the composites shows that AgNPs and ZnONPs improve the electrical properties of PANI, and they could be potential candidates for electrochemical applications.

1. Introduction

Nanomaterials have found more applications compared to their large-scale counterparts due to their distinctive physical, chemical, and biological properties [1,2]. The unique size-dependent properties of nanoparticles have considerably impacted all scopes of human life, making nanotechnology a promising field for various applications [3,4,5]. Their synthesis, design, and manipulation have recently found great attention due to their wide applications in various fields including medicine, sensors, renewable energies, cosmetology, and bio-therapeutic devices [6,7,8]. Nanomaterials, such as quantum dots, zinc oxide nanoparticles (ZnONPs), gold nanoparticles (AuNPs), and AgNPs, offer enzyme-free sensing strategies as electroactive labels for direct signal output [9,10] redox ability [11,12], excellent biocompatibility [13], and electro catalytic activity [14,15,16]. Additional advantages of these nanomaterials are simple preparation methods [16], synthesis over a wide range of sizes and shapes [17], and easy surface conjugation to various ligands [18,19].

Additional applications of ZnONPs and AgNPs include catalysts, optical materials, pigments, cosmetics, biocidal and functional devices [20]. This is attributed to their high-performance with high mechanical strength and thermal stability at room temperature [21]. Of note, ZnONPs and AgNPs have particularly attracted more attention in the field of sensors due to their good electrochemical performance with a relatively sharp peak [22,23,24], mild electrochemical detection conditions [25], low oxidation potential, and easy dissolution of metal ions without poisonous reagents like Br₂ [26,27,28,29,30]. For instance, AgNPs have demonstrated good sensing ability for hydrogen peroxide at concentrations as low as 0.9 µM [31], heavy metal ions such as Hg²⁺ with limit of detection of 25 nM [32,33] and microRNAs in serum samples up to 10⁻³ fM [34]. ZnONPs have also been used in the sensing and monitoring of nucleic acid hybridization [35] and toxic gases, such as CO [36]. AgNPs and ZnONPs have been widely used as fillers in polymer matrices to enhance their optical, catalytic, electrical, and conductivity properties [37]. For instance, Pavoski et al. (2019) [38] encapsulated AgNPs in silica and used them as antibacterial fillers in ethylene polymerization for potential application in active medicine packages. They exhibited good antibacterial activity towards Staphylococcus aureus with a 25 mm zone of inhibition. ZnONPs were incorporated in rubber, and they improved its mechanical properties including hardness, tensile strength, tear strength, and elongation at break [39]. Their incorporation as fillers in polymers has gained attention since they are environmentally friendly, can easily be synthesized by green methods, and they form strong bonds between the oxide groups on their surface with the polar groups in the polymer matrix [40,41].

Different metal and metal oxide nanoparticles have been incorporated into conducting polymers to form nanocomposites, a special class of materials with unique physical and chemical properties and wide application potential in diverse areas [42]. Conducting polymers such as polypyrroles (PPy), polythiophenes (PT), and polyanilines (PANI) have found widespread use in various applications since they are light weight, easy to process, and are resistant to corrosion [43,44]. They provide an effective immobilization platform with distinctive electron transfer capabilities, and possibilities for signal transduction [45]. Polyaniline (PANI) is an important conducting polymer employed in various applications due to its mechanical flexibility, low cost, good electrical conductivity, and thermal and chemical stability [46,47]. The polymer exhibits two redox couples that facilitate enzyme-polymer charge transfer, and its electrical properties can be controlled by protonation or changing the oxidation state of the polymer [48,49,50]. The functionalization of PANI with components such as metals, metalloids, and inorganic and organic compounds has attracted attention in order to improve and extend its functions [51]. Herein, AgNPs, ZnONPs, and Ag–ZnONPs were incorporated on the glassy carbon electrode that was previously modified by electrosynthesized PANI electrodes to form AgNP–PANI–GCE, ZnONP–PANI–GCE and Ag–ZnONP–PANI–GCE nanocomposites.

2. Materials and Methods

AgNPs, ZnONPs, and Ag–ZnONPs were synthesized as reported in our previous work [52]. Briefly, 20 mL of leaf extract of Bidens pilosa was added to 80 mL of 1 mM silver nitrate and/or zinc nitrate solution, the mixture allowed to stand for 2 h under nitrogen blanket, centrifuged at 14,500 rpm for 10 min, and dried at 60 °C for 8 h. Deionized water (resistivity, 18.2 MΩ.cm), aniline (C₆H₅NH₂, 99.5%), 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC, 99%), N-Hydroxysuccinimide (NHS, 98%), ethanol (C₂H₅OH, 99.98%), and hydrochloric acid (HCl, 37%) were obtained from (Sigma Aldrich, Johannesburg, South Africa).

3. Electrosynthesis of PANI

A single compartment cell with a three-electrode electrochemical system comprising platinum (Pt) wire, glassy carbon electrode (GCE), and Ag/AgCl (3 M NaCl) as counter, working, and reference electrodes was used for all electrochemical studies. Prior to electrosynthesis of PANI, GCE was thoroughly polished with 1.0, 0.3, and 0.05 μm slurries of alumina, followed by sonication of the cleaned electrode using ethanol and de-ionized water. The working solution for the electro-polymerization of PANI film contained 0.1 M aniline, and 1 M hydrochloric acid (HCl). The solution was degassed by bubbling argon gas through it for 7 min before the electro-polymerization was started, and a blanket of the argon gas was then maintained above the solution during the electrodeposition and characterization processes. The PANI film was electrodeposited on a GCE from 1 M HCl containing 0.1531 g of aniline by voltametric sweep between −0.2 V and 1.2 V/SCE, at 50 mV/s for 20 cycles. The PANI modified electrode (PANI|GCE) was washed thoroughly with distilled water to remove the excess of monomer, and characterized in 1 M HCl using CV from −1 V to 1.2 V at varying scan rates.

4. Preparation of AgNP–PANI, ZnONP–PANI and Ag–ZnONP–PANI Composites

10 µL solution containing 10 mM EDC and 20 mM NHS was dropped on the surface of the PANI–GCE modified electrode and retained for 30 min. Subsequently, 10 µL of AgNPs solution was incorporated on the surface of the PANI|GCE modified electrode to form the AgNP–PANI–GCE modified electrode. The AgNP–PANI–GCE modified electrode was allowed to stand at ambient temperatures for 12 h before electro-analysis using CV at scan rates of 5–100 mV/s in 0.1 M HCl solution. The same procedure was followed for ZnONPs and Ag–ZnONPs, and the modified electrodes were denoted as ZnONP–PANI–GCE and Ag–ZnONP–PANI–GCE electrodes, respectively.

5. Characterization of Nanoparticles and Nanocomposites

The Surface Plasmon Resonance (SPR) properties and spectral analysis of the nanoparticles, PANI, AgNP-PANI, ZnONP-PANI and Ag–ZnONP-PANI were analyzed using UV-Visible spectrometer (Shimadzu UV-1700 UV Pharmaspec from Kyoto, Japan) over a range of 200 to 900 nm and Fourier transform infrared (FTIR-JASCO 4100 form Tokyo, Japan). Fourier transform infrared spectrometry (FTIR) was employed to monitor functional groups present in the as-prepared nanomaterials and composite materials (PerkinElmer Spectrum 100, PerkinElmer Incorporated, Shelton, CT, USA). High resolution transmission electron microscopy and high-resolution scanning electron microscopy (SEM) experiments measured the internal structure of the materials as well as the morphological properties using a Tecnai G2 F20X-Twin MAT 200 KV high resolution (HRTEM) equipped with an energy-dispersive X-ray spectroscopy (EDS) detector (manufactured by Field Electron and Ion Company (FEI) Europe, Eindhoven, The Netherlands) and Auriga Scanning Electron Microscope (Carl Zeiss Microscopy GmbH, Jena, Germany).

Small angle X-ray Scattering (SAXS) was performed with a SAXSpace Spectrometer from Anton Paar, Graz, Austria, using 35 µL of non-diluted sample. The results from the SAXS analyses were Fourier-transformed, using the GIFT software, into a pair–distance distribution function (PDDF) by volume and size distribution. Electrochemistry characterization was performed using a CHI 760E electrochemical analyzer (CH Instruments Inc., Shanghai, China). The electrochemical behavior of AgNPs, ZnONPs, Ag–ZnONPs, PANI, AgNP-PANI, ZnONP-PANI, and Ag–ZnONP-PANI composites was investigated on a bare GCE using CV at a scan rate (ν) of 10 mV/s. All of the experiments were carried out in 1 M HCl within a potential window of −1.0 to 1.2 V starting with anodic sweep from −1.0 V.

6. Results and Discussion

6.1. Optical Properties of AgNPs, ZnONPs, and Ag–ZnONPs

The surface plasmon bands for AgNPs (red line) synthesized were at 428 nm, as shown in Figure 1A, which corresponds to the Surface Plasmon band of silver that is around 420–500 nm [53]. ZnONPs (Figure 1A, green line) exhibited a band at 324 nm, while Ag–ZnONPs (Figure 1A, blue line) obtained two bands at 300 nm and a broad band at 428 nm. In both bands for ZnONPs and Ag–ZnONPs, the band corresponding to ZnONPs showed a blue shift as compared to that of bulk ZnO (388 nm) [54]. The blue shift could be attributed to (i) big particle size of aggregation of particles which increase Raleigh scattering, thus causing a decrease in absorbance [55], or (ii) the surrounding medium due to adsorbed compounds from the leaf extract and interparticle interactions [53,56].

The optical energy band gaps for the nanoparticles were obtained using Tauc plots, as illustrated in Figure 1B1–B3. Graphs of (αhυ)² against energy (hυ) were plotted and a straight line to (αhυ)² = 0 axis, known as Tauc plots. The obtained values energy band gaps values were found to be 2.47 eV, 3.21 eV, and 2.33 eV for AgNPs, ZnONPs, Ag–ZnONPs, respectively. These values are in good agreement with the already reported studies: 2.47–2.9 eV [57,58] for AgNPs, and 3.20–3.30 eV [59,60] for ZnONPs.

6.2. Optical Properties of PANI, PANI-AgNPs, PANI-ZnONPs and PANI-Ag–ZnONPs

PANI film exhibited five absorbance bands at 216 nm, 412 nm, 464 nm, 550 nm, and 831 nm, as shown in Figure 2A. The band at 216 nm could correspond to π-π* transitions of the C=C bonds in the benzenoid and quinoid rings, i.e., the transition of an electron from the valance to the conduction band of benzenoid segments, due to the π-electronic configuration of PANI [61]. The band at 412 nm could be attributed to the π-polaron band transition at the same wavelength or to the oxidized dimer of aniline, diphenyl-p-phenylenediamine, which absorbs at 464 nm [61,62]. On the other hand, the band appearing at 550 nm is attributed to the polaron-π* electronic transition, related to the quinoid rings in the PANI as the transition from the emeraldine to the pernigraniline oxidation form occurs [62]. The minor peak at 831 nm, corresponds to n-π* polaronic transitions associated with delocalized proton charge carriers indicative, of the protonated emeraldine salt form of PANI [63]. The UV-Vis spectra for AgNP-PANI and Ag–ZnONP-PANI show similar bands to those of PANI and a minor band at 412 nm that corresponds to AgNPs [64]. From the results, it is clear that modification of PANI with the various nanoparticles increases the absorption of PANI.

Figure 2B1–B4 show the energy band gap energy values of 2.3 eV, 2.28 eV, 2.29 eV, and 2.27 eV associated with PANI (Figure 2B1), AgNP-PANI (Figure 2B2), ZnONP-PANI (Figure 2B3), and Ag–ZnONP-PANI (Figure 2B4), respectively. The obtained values are within the band gap energy range reported in previous studies for PANI (2.51–3.11 eV) [65,66,67], AgNP-PANI (2.07–2.37 eV) [37,65], and ZnONP-PANI (2.1–2.95 eV) [67,68,69]. The incorporation of nanoparticles in PANI increases absorption, which leads to a reduction in the energy band gap due to charge transfer transitions, and hence increase in optical conductivity [68]. This reduction in energy band gap can be attributed to the mobile electrons contained in the nanoparticles, and new excitation energy levels created below the regular band gap due to charge transfer from the nanoparticles to PANI [70]. This could also suggest that nanoparticles can increase the conductivity of PANI, and hence the possibility of using the nanocomposites in electrochemical devices and sensing platforms can be explored.

6.3. Characteristic Features of PANI, AgNP-PANI, ZnONP-PANI, and Ag–ZnONP-PANI

Figure 3 shows FTIR spectra for PANI, AgNP-PANI, ZnONP-PANI, and Ag–ZnONP-PANI. Herein, PANI (Figure 3, black line) exhibited absorption bands at 3537 and 3316 cm⁻¹ for the N–H stretching mode [71], 3052 cm⁻¹ for the C–H stretch aromatic [72], and 1648 cm⁻¹ for the carbonyl stretching vibration of the amino group [73]. The bands at 1323 and 1394 could be attributed to the C–N and C=N stretching [74], 1170 cm⁻¹ for the C–O stretch and C–N stretching mode for the benzenoid ring [75] or vibration of –NH⁺= structure [76], and 830 cm⁻¹ for the =C–H plane bending vibration which is formed during protonation [77,78]. As inspected carefully, the FTIR characteristic features of AgNP-PANI (Figure 3, red line), ZnONP-PANI (Figure 3, green line), and Ag–ZnONP-PANI (Figure 3, blue line) shows that vibrational modes of PANI are not altered, as they give similar bands at 3097 cm⁻¹ for the C–H stretch aromatic, 1648 cm-1 for the carbonyl stretch, and could also be attributed to existence of benzenoid rings, 1159 cm⁻¹ for the C–O stretch and C–N stretching mode for the benzenoid ring, and 836 cm⁻¹ for the =C–H plane bending vibration which is formed during protonation. The bands corresponding to the N–H stretch disappear in the nanocomposites’ IR bands, indicating formation of the amide bond between PANI and the compounds adsorbed onto the nanoparticles.

6.4. Size and Shape of AgNPs, ZnONPs, and Ag–ZnONPs

The size, shape, and d-spacing of nanoparticles were determined by HRTEM, HRSEM, and SAXS. HRTEM showed spherical nanoparticles that were crystalline in nature, with diameters ranging from 10 to 100 nm, as illustrated in Figure 4A–F. The HRSEM images of AgNPs, ZnONPs, and Ag–ZnONPs showed agglomerated spherical bubbles of different sizes, as shown in Figure 4G–I. Interestingly, AgNPs exhibited the largest particle sizes (with an average diameter of 70.4 nm), ZnONPs exhibited the smallest particle size (with two average diameters of 12.9 and 58.3 nm), and Ag–ZnONPs obtained a mixture of small and large particles (with average diameters of 12.9 and 73 nm) obtained using SAXS. It is worth noting that from the mixture of sizes observed for the Ag–ZnONPs from SAXS, a combination of AgNPs and ZnONPs could have been formed instead of the composite. The lattice fringes that were visible in HRTEM indicate good interplanar spacing (d-spacing), and the distance between the fringes represents the crystallinity of the nanoparticles. A first Fourier transform (FFT) was performed on the TEM images at 2 and 10 nm scales using ImageJ software to obtain the d-spacing from lattice fringes. The d-spacing obtained for AgNPs were 0.221 and 0.219 nm, for ZnONPs were 0.226 and 0.227 nm, and for Ag–ZnONPs were 0.217 and 0.225 nm. These values correspond to those reported in literature for AgNPs [79,80] and ZnONPs [81,82].

The band gap energy, d-spacing, and size of the nanoparticles obtained using SAXS, and calculated from TEM were summarized in

Table 1. A summary of the band gap energy, d-spacing and sizes of AgNPs, ZnONPs, and Ag–ZnONPs obtained using TEM and SAXS.

Nanoparticle	Band Gap Energy (eV)	d-Spacing (TEM)/nm	Calculated Size from d-Spacing (TEM)/nm	Size SAXS/nm
AgNPs	2.47	0.221	71.9	70.4
ZnONPs	3.21	0.226	55.8	12.9 58.3
Ag–ZnONPs	2.33	0.217 0.225	71.9 55.8	73 12.9

. It is evident that the sizes obtained from TEM were in correspondence to the sizes obtained from SAXS.

6.5. Internal Structure of AgNPs, ZnONPs, and Ag–ZnONPs

The internal structures of the nanoparticles were validated by SAXS, as demonstrated by the plots of logarithm of intensity vs. scattering vector, and the results obtained were Fourier transformed using GIFT software into a PDDF profiles and distribution of sizes by number and volume, as shown in Figure 5A–C. The sharp increase of intensity at low angles demonstrated by AgNPs could be due to attractive inter-particle interactions that lead to aggregated nanoparticles, as shown in size distribution by intensity and volume in Figure 5B,C [83]. Additionally, the plot of log of intensity vs. scattering vector shows no oscillations for all of the nanoparticles, which implies that the nanoparticles that were formed were of different sizes which conforms to the results reported using HRTEM [83]. The PDDF analysis of AgNPs (Figure 5D, red line) and ZnONPs (Figure 5D, green line) exhibited a spherically shaped characteristic features with maximum radiuses of 70.5 nm and 76.2 nm, respectively. When the AgNPs and ZnONPs were combined to form a composite material, the Ag–ZnONPs (Figure 5D, blue line) exhibited a dumbbell shape, indicating an inhomogeneous mixture of the small ZnONPs and the larger AgNPs which is confirmed by the size distribution by intensity and volume in Figure 5E,F [84,85]. The AgNPs obtained had average particle sizes of approximately 70.4 nm and agglomerated particles appearing at 116.8 nm, while ZnONPs exhibited particle sizes ranging from 12.9 nm to 58.3 nm, and were agglomerated at 106.4 nm. The Ag–ZnONPs demonstrated two distinct sizes at approximately 12.9 nm and 73 nm, which correspond to the established individual sizes of ZnONPs and AgNPs. This could suggest that the Ag–ZnONPs formed were actually a mixture of the two nanoparticles (AgNPs and ZnONPs) and not a composite. This is in line to what was evidenced from the TEM discussed previously. More so, the intensity at around 104 nm for ZnONPs and 116.8 nm for AgNPs disappear in the Ag–ZnONPs mixture, which could suggest that no agglomeration takes place when the two nanoparticles are synthesized simultaneously.

6.6. Electroanalysis of PANI and Composite Materials

Figure 6 shows the CVs recorded during the continuous scan of 40 cycles (scan rate of 50 mV/s) in a potential range of −1 to +1.2 V on the GCE containing 0.1 M aniline. For the first potential sweep, aniline was oxidized, resulting in an anodic current peak, and the product obtained from oxidation of aniline was deposited on the electrode surface to form PANI (a dark green film). As the number of scan cycles increased, the current peak also increased, indicating successive deposition of the PANI layers on the electrode surface.

The first pair of redox peaks, O¹R¹ (0.37 V, 0.09 V), was attributed to the interconversion of PANI reduced form, leucoemeraldine, state to the half oxidized emeraldine base state, and vice versa. The pair O³R³ (0.77 V, 0.56 V) was due to oxidation of emeraldine base state to the oxidized pernigraniline state, and vice versa. The O²R² (0.54 V, 0.45 V) peaks were attributed to either presence of ortho-coupled polymers [86] or the oxidation of segments of PANI chain to benzoquinone and hydroquinone species, whose potentials are very close to one another and tend to disappear after several scans [87,88]. The oxidation peak at O³ exhibited a broad oxidation wave compared to that of O¹, which could signify that charge transfer was more difficult for the second oxidation step than the first [85].

Figure 7 contains the CVs of PANI and its nanocomposites performed in 1 M HCl at scan rates of 10, 20, 30, and 50 mV/s. The anodic peak currents increased with an increase in scan rate, which implies that the protonation of PANI chains in HCl as the supporting electrolyte was controlled by the diffusion of H⁺ [89]. Since the first anodic oxidation peak was attributed to radical cations at the N-position which depends on H+ doping, the slope of the first anodic peak current vs. the square root of scan rate and the Randles–Ševčík equation were used to obtain the diffusion coefficient of H⁺ in the electrolyte across the PANI–GCE, AgNP–PANI–GCE, ZnONP–PANI–GCE, and Ag–ZnONP–PANI–GCE modified electrodes. The Randles–Ševčík equation is given by (Equation (1)):

$${\mathrm{i}}_{\mathrm{p}}=0.4463\mathrm{nFA}{[\frac{\mathrm{nF}}{\mathrm{RT}}]}^{1/2}{\mathrm{CD}}_{\mathrm{m}}{}^{1/2}{\mathsf{\nu }}^{1/2}$$

(1)

where i_p is the peak current in A, n = 1 is the number of electrons transferred, F = 96485 C mol⁻¹ is Faraday’s constant, A = 0.071 cm² is the surface area of the electrode, R = 8.314 J mol⁻¹, K⁻¹ is the gas constant, T = 289.15 K is the temperature, ν is the scan rate in V/s, C is the concentration of the electrolyte in mol cm⁻³, and D_m is the diffusion coefficient of the ions, i.e., m is H⁺ or counter ion (Cl⁻). Using the anodic peaks (a) from Figure 7 for PANI–GCE (Figure 7A), AgNP–PANI–GCE (Figure 7B), ZnONP–PANI–GCE (Figure 7C), and Ag–ZnONP–PANI–GCE (Figure 7D), the slopes for peak current vs. square root of scan rate (Figure 7E) were used to obtain the diffusion coefficients of H+ across the modified electrodes. The diffusion coefficients of H⁺ across the modified electrodes were obtained to be D_H+ = 1.20 × 10⁻⁷, 1.13 × 10⁻⁶, 1.33 × 10⁻⁷, and 6.57 × 10⁻⁸ cm²/s for PANI–GCE, AgNP–PANI–GCE, Ag–ZnONP–PANI–GCE, and ZnONP–PANI–GCE modified electrodes, respectively.

The increase of the second oxidation peak (i.e., peak b of Figure 7A), which corresponds to the doping process of counterions as they are inserted into the film to neutralize positive charge by H⁺ protonation during PANI oxidation, indicates that the doping behavior of these supporting electrolytes is controlled consistently by the surface electron transfer process [88]. Using the anodic peaks (b) from Figure 7 for PANI–GCE (Figure 7A), AgNP–PANI–GCE (Figure 7B), ZnONP–PANI–GCE (Figure 7C), and Ag–ZnONP–PANI–GCE (Figure 7D), the slopes for peak current vs. square root of scan rate (Figure 7F) were used to obtain the diffusion coefficients of the counter ion (Cl⁻) across the modified electrodes. The diffusion coefficient of the counter ions across the modified electrodes were obtained to be D_Cl⁻ = 7.854 × 10⁻⁸, 9.592 × 10⁻⁷, 1.930 × 10⁻⁷, and 8.764 × 10⁻⁸ cm²/s for PANI–GCE, AgNP–PANI–GCE, Ag–ZnONP–PANI–GCE and ZnONP–PANI–GCE modified electrodes, respectively.

The CVs for AgNPs (red line), ZnONPs (green line), and Ag–ZnONPs (blue line) show that the electron transfer is close to reversible as the anodic peak current are more defined in comparison to the cathodic peak currents (Figure 8A). From the CVs, it is clear that there is electrochemical dissolution (stripping) of nanoparticles on the electrode surface, and reduction in the generated metal ions in the oxidative part of the cycle. However, from the oxidative and reductive peaks, not all the metal ions are redeposited back, as demonstrated by the sharp anodic peaks corresponding to the oxidation of Ag⁰ to Ag⁺ at 0.23 V and 0.27 V for AgNP–GCE and Ag–ZnONP–GCE electrodes, respectively, whose subsequent reduction peaks are less intense and broad [90]. This could be attributed to the solubility of charged NPs and their dissolution away from the electrode, or that the rate of diffusion/adsorption of the NPs from solution to the electrode is lower than the scan rate [91,92]. The oxidation peak of AgNPs at 0.89 V could be due to the irreversible oxidation process of phenolic groups present in the stabilizing agents from the leaf extract of Bidens pilosa [93]. The reduction peaks observed for AgNPs, ZnONPs, and Ag–ZnONPs appeared at 0.54 V, −0.81 V, and −0.45 V, respectively.

CV voltammograms of the nanocomposites (Figure 8B) showed a quasi-reversible behavior similar to PANI, with increased oxidation and reduction peaks. Modification of PANI–GCE to AgNP–PANI–GCE led to an increase in anodic current (5.7 to 8.5 mA for AgNPs), while modification to ZnONP–PANI–GCE and Ag–ZnONP–PANI–GCE led to lower anodic currents (3.7 mA and 4.9 mA, respectively), as compared to PANI–GCE. This could be attributed to an increased distance due to the aggregation of dopants, yielding a thicker surface than the individual components, and hence resulting in slower electron transfer [94]. However, all the nanocomposites showed an increase in cathodic peak current (−4.6 mA, −4.5 mA, and −5.6 mA for AgNP–PANI–GCE, ZnONP–PANI–GCE, and Ag–ZnONP–PANI–GCE, respectively) as compared to −2.7 mA of PANI–GCE. The onset of oxidation of ZnONP–PANI–GCE and Ag–ZnONP–PANI–GCE occurs at more positive potentials, and their reduction peaks shift towards more negative potentials as compared to PANI–GCE and AgNP–PANI–GCE. This indicates that the oxides produced on the NPs in these nanocomposites are more stable, and are difficult to reduce [95]. This is in line with the CVs obtained for the nanoparticles, which showed that reduction occurred at more negative potentials of −0.61 V, −0.81 V, and −0.45 V for AgNPs, ZnONPs, and Ag–ZnONPs, respectively (Figure 8A). The increase in the peak currents depicted by the nanocomposites could be due to increased electron transfer rates resulting from an increased electroactive area, with the NPs acting as charge carriers along the PANI backbone [96], [97]. Modification of PANI with AgNPs showed the highest improvement in conductivity, which could be attributed to the high conducting properties of silver metal [37]. The peaks due to AgNP–PANI–GCE, ZnONP–PANI–GCE, and Ag–ZnONP–PANI–GCE show a slight anodic shift, which could be attributed to interaction between the nanoparticles and PANI chains and thiol groups adsorbed on the nanoparticle surfaces [98]. The preservation of the quasi-reversible electrode process and the increase in peak currents for the nanocomposites prove that AgNPs, ZnONPs, and Ag–ZnONPs improved the electrochemical properties of PANI–GCE [19]. The high surface area to volume ratio of the nanomaterials and the biocompatibility of PANI make the nanocomposites be good candidates for application in electrochemical devices [99].

To obtain the surface coverage, a slope of anodic peak vs. scan rate was used in the Brown–Anson equation (Equation (2)):

$${\mathrm{i}}_{\mathrm{p}}=\frac{{\mathrm{n}}^2{\mathrm{F}}^2\mathrm{A}\mathsf{\Gamma }}{4\mathrm{RT}}\mathsf{\nu }$$

(2)

where i_p = peak current in A, n = 1 is the number of electrons, F = 96485 C mol⁻¹ is Faraday’s constant, A = 0.071 cm², Γ is the surface area of the electroactive material in mol cm⁻², ν is scan rate in V/s, R = 8.314 J mol⁻¹ K⁻¹ is the gas constant, and T = 298.015 K is the temperature. The peak potential of the Brown–Anson peak remains relatively unchanged with the scan rate, as it represents the presence of surface-bound electroactive thin film on the electrode [100]. This is labelled as peak b in the CVs of Figure 7: for PANI–GCE (Figure 7A), AgNP–PANI–GCE (Figure 7B), ZnONP–PANI–GCE (Figure 7C), and Ag–ZnONP–PANI–GCE (Figure 7D). The Brown–Anson plots are presented in Figure 9, and the slopes were used to calculate Γ using Equation (2). The Γ values for PANI–GCE, AgNP–PANI–GCE, ZnONP–PANI–GCE, and Ag–ZnONP–PANI–GCE are 2.48 × 10⁻⁷, 8.86 × 10⁻⁷, 2.44 × 10⁻⁷, and 3.48 × 10⁻⁷ mol cm⁻², respectively.

7. Conclusions

AgNP-PANI, ZnONP-PANI, and Ag–ZnONP-PANI composites were successfully synthesized by two processes. The first process involves electrodeposition of PANI on a GCE, and the second process includes the adsorption of AgNPs, ZnONPs, and Ag–ZnONPs onto the PANI to for AgNP–PANI–GCE, ZnONP–PANI–GCE, and Ag–ZnONP–PANI–GCE via EDC/NHS as the coupling agents. SAXS confirmed the spherical nature of the nanoparticles, with sizes ranging from 0 to 130 nm. UV-Vis spectroscopy and FTIR showed the different electron transitions during electrodeposition of PANI and the different functional groups in the nanoparticle-PANI composites. CV analysis of PANI, AgNP–PANI–GCE, ZnONP–PANI–GCE, and Ag–ZnONP–PANI–GCE proved the conductivity of the composites to be higher than that of pure PANI and the nanoparticles alone. Since these nanoparticles improve the electrical properties of PANI, it suggests that the composites can be suitable for application in electrochemical devices and sensing platforms.