Using PEI solution, preparation and stabilization of silver nanoparticles were carried out to make chelate of transition metal ions (silver metal ions) with amino groups in PEI, which were confirmed by UV-vis spectroscopy. As shown in Supplementary Figure S1, the color change from colorless to brown solution confirmed the preparation of PEI-Ag NPs. UV-vis spectrum of PEI- Ag NPs exhibited the absorption (λ_max) peak at 430 nm which is in good agreement with data described in the literature (Kim et al., 2010; Imar et al., 2015; Wen et al., 2017).

FTIR spectra exhibited characteristics vibration bands of Dawson Polyoxometalate compounds in 4000-500 cm⁻¹. The peak values revealed terminal W=O and v (P-O) vibrations, whereas bridge’s vibrations of inter and intra tungstate-oxygen-tungstate (W-Ob-W) bands in the range of 700 and 900 cm⁻¹ were presented in Supplementary Table S1 (43). Supplementary Figure S2 showed the physical appearance of Dawson-type POM. In Supplementary Figure S3, the characterization bands of heteropolyanions are between 700 and 1,100 cm⁻¹, which proved the Dawson structure.

The spectra Supplementary Figure S4 showed that the designed materials were in amorphous form and the main diffraction angles were recorded at 2 thetas (12°, 15°, 24°, 26°, 31°, 32°, 36°, 55°, and 53°) and two thetas (26.7°, 28.55°) for the Dawson POM and fabricated PDDA/P₂W₁₈ POM/AgNPs/SCPE respectively. There was good agreement with the data described in the literature (Modvig et al., 2019). P₂W₁₈ POM and PEI-AgNPs were used for multilayer formations on the screen-printed electrode (SCPE), and XRD characterization showed the amorphous structure of fabrication of PDDA/P₂W₁₈ POM/PEI-AgNPs on the active surface of screen printed electrodes (SCPEs) with multilayers (Zhao et al., 2021).

Changes in films morphology of fabricated samples with P₂W₁₈ POM and PEI-AgNPs deposited on PDDA modified electrodes were examined via scanning electron microscope. SEM image of a modified electrode with POM and PEI- AgNPs (outer layer) deposit film exhibits the aggregates with an average size of 500 nm, as shown in Figure 1A. SEM image of modified electrode PEI stabilized AgNPs and POM (outer layer) deposit film exhibit the construction of identical agglomerates with size is 500 nm, as shown in Figure 1B.

Surface images of Dawson POM and PEI-stabilized AgNPs deposited on screen-printed electrodes were performed by Atomic force microscope (AFM) and are shown in Figure 2. Atomic force microscope images were conducted at intervals throughout the multilayers via deposition method to investigate the variations in the topography during the process. Images of samples such as blank/SCPE (A), PDDA/SCPE (B), PDDA/cobalt substituted Dawson POM/PEI-AgNPs/SCPE with outer layer PEI-AgNPs (C), and PDDA/PEI-AgNPs/cobalt substituted Dawson POM/SCPE with outer layer cobalt substituted Dawson POM (D) were used to find the topography of the multilayers. An AFM image obtained with PDDA/SCPE has a more spherical structure indicative of polymeric film. These structures of all LBL films have a hemispherical shape (Anwar et al., 2014).

The electrochemical redox properties of cobalt substituted Dawson POM were recorded in 3 ml acetate buffer (0.1M Na₂SO₄ and 20 mM CH₃COOH) via cyclic voltammetry (CV) technique. One reversible redox pair I-I’ exhibited and redox potential E_1/2 = (Epa + Epc) is—0.650 mV correlated with redox behavior of tungsten-oxo framework of (P₂W₁₈) Dawson type Polyoxometalate in cyclic voltammograms (Imar et al., 2015), which are shown in Figure 3. [ P 2 W 18 O 62 ] 6 − + 2 e − + 2 H + ⇌ [ H 2 P 2 W 18 O 62 ] 6 − [ H 2 P 2 W 18 O 62 ] 6 − + 2 e − + 2 H + ⇌ [ H 4 P 2 W 18 O 62 ] 6 −

The polydimethyldiallyammonium chloride (8% PDDA) was introduced to prepare glassy carbon electrodes (GCE). A glassy carbon electrode was polished with alumina powder and dipped in 0.5 ml 8% PDDA solution (polycations). CV was recorded in 10 ml acetate buffer pH 3.5 in Supplementary Figure S5A. Multilayer formation depends on cationic and anionic moieties. Multilayers were built up with P₂W₁₈ POM and AgNPs stabilized with polyethyleneimine on PDDA/GCE modified electrode with layer-by -layer (LBL) method and degassing with nitrogen gas before each layer formation. CV was recorded in 0.1 M acetate buffer pH 3.5 at a scan rate of 100 mV/S after formation. In multilayer formation, the outermost layer was P₂W₁₈ Dawson Polyoxometalate, anionic moieties. In Supplementary Figure S5B, the peak of current (uA) increases gradually in the redox couple I, increasing the number of layers and decreasing peak to peak separation in recorded CV. The incorporative AgNPs exhibited a small increase in the current peak during the construction of multilayers. A slight anodic shift was examined for the W-O redox process with layer-by-layer assembly immobilization associated with the POM solution response, which demonstrated the interactions between POM and PEI-stabilized AgNPs within the construction of multilayers (Anwar et al., 2014).

For scan rate study, a glassy carbon electrode was modified with P2W18 POM and PEI-AgNPs. After making multilayers, a scan rate study of fabricated P₂W₁₈ POM/AgNPs/GCE modified electrode was carried out via cyclic voltammetry. 0.1 M acetate buffer, pH 3.5 was used in the 3-electrode system. Figure 4 shows cyclic voltammograms of multilayers of PDDA/POM/AgNPs at 10 to 400 scan rate mVS⁻¹ on GCE. In cyclic voltammograms, redox couples exhibited a gradual change in oxidation and reduction peaks. Scan rate study depends on the anionic moieties (POM as the outermost layer). The current was increased with the increase of scan rate mVS⁻¹.

The pH study of P₂W₁₈ POM depends on the pH of the buffer. In electrochemical measurements, reduction of P₂W₁₈ POM went together with protonation. Oxidation and reduction peaks of P₂W₁₈ POM vary with the pH of the buffer. Therefore, the electrochemical study was pH-dependent. Three different types of acetate buffers were prepared for pH study. The first acetate buffer was 0.1 M Na₂SO₄ at (pH = 2, 2.5, and 3). The second acetate buffer was 0.1M Na₂SO₄ and 20 mM CH₃COOH at (pH = 3.5, 4, 4.5, and 5). The third was 0.1M Na₂SO₄ and Na₂H₂PO₄ at (pH = 5.5 and 6). All pH studies were carried out via the cyclic voltammetry technique. Figure 5A presents fabricated P2W18 POM/PEI-AgNPs/GCE, which was used for pH studies at a scan rate of 100 mVS⁻¹. It was discovered that film showed the stable redox activity linked with Tungsten-oxo centers within Dawson POM between pH ranges 2 to 6 based on acetate buffers solution. Dawson POM exhibited a redox process and shifted cathodically with the pH increase and the number of protons linked with each redox process. In Figure 5B, the current decreases when the pH value of the electrolyte increases. The resulting slope value of Dawson POM was examined at 0.017 mV/pH due to the addition of proton per monoelectronic redox process in solution (Anwar et al., 2014; Zhou et al., 2021). Calculated E_1/2 values at different pH are presented in Table 1 (Sukirtha and Usharani, 2013).

The permeability of layer-by-layer (LBL) films was investigated through cyclic voltammetry (CV) towards cationic and anionic redox probes, such as potassium ferricyanide K₃[Fe(CN)₆], and characterized by monoelectronic redox process. Redox transformations occurred on the electrode surface or within LBL film after diffusion by the redox probe. Figure 6A exhibited the cyclic voltammogram of bare GCE and fabricated GCE with PDDA, P₂W₁₈ POM, and PEI-AgNPs layer-by-layer film depending on the number of layers and outermost layer (Fay et al., 2005). Fe (CN)₆ can penetrate the multilayers and react at the bare GCE. Repulsion produced between the outer anionic layer and Fe (CN)6 significantly increases the peak-to-peak separation. Fabricated GCE with LBL films led to suppressing the redox probes’ peaks, which created difficulty for the probe for diffusion in the layer-by-layer (LBL) films at the GCE. Surface permeability of fabricated GCE (eight multilayers) with cationic PEI-AgNPs (outermost layer) exhibited redox activity of the anionic probe at the GCE due to electrostatic interactions between the anionic probe and cationic layer (outermost layer).

In Figure 6B, fabricated GCE (16 multilayers) with anionic POM (outermost layer) showed no redox activity due to unfavorable electrostatic repulsion between the anionic probe and outermost layer. The cathodic current increased the redox processes by increasing film thickness determined in the presence of anionic probe, resulting in electrocatalytic reduction due to electron transfer from cationic species (Naseer et al., 2015). The experiment was carried out in 0.1 M acetate buffer at pH 2, 0.1M Na₂SO_4, and 1 mM Potassium ferricyanide K₃[Fe(CN)₆] solution was prepared for permeability at scan rate 100 mV/S (Anwar et al., 2014). A small reduction in current peaks associated with the redox process of the electrode can be investigated with the increase in peak-to-peak separation on fabricated GCE while decreases in peak-to-peak separation can be seen on bare GCE (Imar et al., 2015).

The electrochemical performance of P₂W₁₈ Polyoxometalate towards methyl parathion was investigated in 0.04 M Britton Robison Buffer at pH 4.5 via cyclic voltammetry. Chemical mechanism of methyl parathion reduction is given in Figure 7. Figure 8A shows the CV curve of P₂W₁₈ Polyoxometalate in the presence and absence of methyl parathion along with fabricated PDDA/P₂W₁₈ POM/PEI-AgNPs/GCE. With the addition of 20 µL methyl parathion, the reduction peak current increases gradually. The penetration of transition centers with heteropolyanions enhanced the catalytic behavior and generated highly reduced products by the electrons from the reduced W-O framework. These results exhibited a reduced form of HPA that catalyzes the methyl parathion reduction. Catalytic current peak (−0.7V) increased with the increase of MP concentration. Sensitivities were assessed with a linear range of 0.2–2.0 mM. The detection limit was achieved at 29 µM methyl parathion detection at LBL composite PDDA, P₂W₁₈ and PEI-AgNPs and showed the limit of detection was found at 1 ppm (Hou et al., 2019). In Figure 8B, the reduction current peak increases with the addition of the concentration of the analyte. The reduction current peak is directly proportional to the concentration of methyl parathion.