Exploring Electrochemical Sensing for Fungicide Detection: Utilization of Newly Synthesized Oligomers

The determination of thiabendazole is crucial for ensuring food safety, environmental protection, and compliance with regulatory standards. Accurate detection helps prevent harmful exposure, ensuring the safety of agricultural products and safeguarding public health. Therefore, this study investigates the electrochemical sensing capabilities of newly synthesized oligo 3-amino-5-mercapto-1,2,4-triazole (oligo AMTa) using hydrogen tetrachloroaurate (III) (HAuCl₄) as an oxidizing agent at room temperature for thiabendazole (TBZ) detection, employing a simple electrode fabrication process. The prepared oligo AMTa was thoroughly characterized using UV-visible spectroscopy, scanning electron microscopy (SEM), Energy Dispersive X-ray Analysis (EDAX), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), high-resolution mass spectroscopy (HR-MS), and Fourier-transform infrared spectroscopy (FT-IR) to confirm its oligomerization structure and properties. The IR spectrum of oligo AMTa reveals a new peak at 1449 cm^-1, indicating the conversion of -NH₂ groups to -N=N- groups during oligomerization, unlike AMTa. Additionally, the disappearance of the -SH group peak at 2615 cm^-1 in oligo AMTa suggests an S-S linkage involvement in the oligomerization process. In the oligo AMTa XPS spectrum, the presence of C=N is displayed by a small peak at 287.3 eV, and oligomerization via -NH and N=N is confirmed by the lack of a 284.0 eV peak for C-C or C=C. Gold nanoparticle formation is not demonstrated by the 84.8 eV peak, which implies that the gold atom is not in the Au⁰ state. The HR-MS spectrum of oligo AMTa shows a peak at <i>m</i>/<i>z</i> 564.08, indicating a chain of five monomers, and another peak at <i>m</i>/<i>z</i> 435.03, confirming the presence of a tetrameric form of AMTa. After that, the GC electrode was directly linked to the oligo AMTa by the potentiodynamic method. SEM, electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) were all employed to confirm the fabrication of oligo AMTa. The SEM image illustrates the formation of a particlelike structure with a uniform size of the oligomer after cycling in 0.1 M H₂SO₄. After electrocycling, the size of the oligomer was reduced from 2.6 μm to 30 nm. The oligo AMTa-modified electrode possesses the highest electroactive surface area and electrical conductivity due to several key factors. First, the presence of amino (-NH₂) and thiol (-SH) functional groups in AMTa enhances the surface coverage and density of electroactive sites, increasing the electroactive surface area. Additionally, the conjugated structure of AMTa facilitates efficient electron transfer, resulting in enhanced electrical conductivity compared to unmodified electrodes. Eventually, the electrochemical oxidation of TBZ occurred using the fabricated electrodes. The GC/oligo AMTa electrode exhibited a four-fold increase in oxidation current for TBZ compared to unmodified GC electrodes. This enhancement is due to the improved surface properties from the oligo AMTa modification, which significantly boosts TBZ adsorption through strong interactions like hydrogen bonding and π-π stacking. These interactions, along with the increased surface area and catalytic properties, facilitate effective electron transfer, resulting in a higher oxidation current. As an outcome, the film was employed to determine the sensitivity level of TBZ, and a LOD of 1.8 × 10^-11 M (S/N = 3) was found. The straightforward method's practical utility was proven by measuring TBZ in tap water, water spinach, and pear juice samples. The comprehensive characterization of oligo AMTa provided insights into its interaction mechanisms with thiabendazole, contributing to the development of a reliable, cost-effective, and efficient sensor.