Here, we report the synthesis of a bimetallic supramolecular polymer (SMP) for fabricating an electrochemical nitrite sensor and study the reaction mechanism of the selective oxidation of nitrite by cyclic voltammetry (CV) simulation through the kinetic parameters evaluation. Symmetrical ligand-bearing terpyridine moieties [4′,4⁗-(1,4-Phenylene)bis(2,2′:6′,2″-terpyridine)] were complexed with Ni(II) and Co(II) salts (Co:Ni:Ligand-0.5:0.5:1) (polyNiCo) to synthesize a heterometallo-SMP. The polyNiCo was characterized by using UV/vis spectrophotometric titration, SEM, EDS, FT-IR, EIS, and CV techniques. The molecular weight of the polymer was determined from the intrinsic viscosity measurement using the Mark–Houwink–Sakurada equation. While the spectroscopic data revealed the structural morphologies and properties of the polyNiCo, electroanalytical characterization studies confirmed the high electrochemical activity and suitability of the polyNiCo heterometallo-SMP as an electrochemical sensor. A glassy carbon electrode (GCE) was used as the base for fabricating ployNiCo_GCE and also for detecting the nitrite analyte through the oxidation process. The kinetics for the irreversible oxidation mechanism were studied using scan-rate and pH-variation methods. The electroactive surface area, electron transfer coefficient, heterogeneous electron transfer rate constant, etc. parameters were studied using the Butler–Volmer equations. We simulated the CV for the nitrite oxidation process at the polyNiCo_GCE based on the analysis of the kinetic parameters obtained from the electroanalytical experiments. An exceptional agreement between the experimental and the simulated CV was found, which confirmed the validity of the calculated kinetic parameters. Using CV and amperometry techniques, we studied the effectiveness of the polyNiCo_GCE for detecting the nitrite analyte at different concentrations. The amperometry technique showed a wide linear range of 2.5 μM–1.73 mM and a limit of detection (LOD) of 0.45 μM. The sensor was also tested for interference, stability, and reproducibility. Real sample analysis was performed using both CV and amperometry techniques, and the obtained results were compared with the results obtained by using standard solutions.
Here, we report a semiempirical quantum chemistry computational approach to understanding the electrocatalytic reaction mechanism (ERM) of a metallic supramolecular polymer (SMP) with nitrite through UV/vis spectral simulations of SMP with different metal oxidation states before and after interactions with nitrite. In one of our recent works, by analyzing the electrochemical experimental data, we showed that computational cyclic voltammetry simulation (CCVS) can be used to predict the possible ERM of heterometallo-SMP (HMSMP) during electrochemical oxidation of nitrite (IslamT. Islam, T. ACS Appl. Polym. Mater.20202273284). However, CCVS cannot predict how the ERM happens at the molecular level. Thus, in this work, we simulated the interactions between the repeating unit (RU) of the HMSMP polyNiCo and nitrite to understand how the oxidation process took place at the molecular level. The RU for studying the ERM was confirmed through comparing the simulated UV/vis and IR spectra with the experimental spectra. Then, the simulations between the RU of the polyNiCo and various species of nitrite were done for gaining insights into the ERM. The simulations revealed that the first electron transfer (ET) occurred through coordination of NO2 – with either of the metal centers during the two-electron-transfer oxidation of nitrite, while the second ET followed a ligand–ligand charge transfer (LLCT) and metal–ligand charge transfer (MLCT) pathway between the NO2 species and the RU. This ET pathway has been proposed by analyzing the transition states (TSs), simulated UV/vis spectra, energy of the optimized systems, and highest occupied molecular orbital–lowest occupied molecular orbital (HOMO–LUMO) interactions from the simulations between the RU and nitrite species.
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