Novel Schiff base Cu( <scp>ii</scp> ) and Au( <scp>iii</scp> ) complexes: spectroscopic, computational, and electrochemical insights for H ₂ O ₂ sensor applications

A Schiff base ligand, called (<i>E</i>)-2-((1<i>H</i>-pyrrol-2-yl)methyleneamino) benzenethiol (H₂L), was synthesized from 2-aminothiophenol and pyrrole-2-carbaldehyde; the copper(ii) and gold(iii) coordination complex were then successfully synthesized and characterized by elemental analysis, FT-IR, UV-vis, NMR, XRD, SEM, TGA/DTA and conductivity measurements. Based on spectroscopic and structural data, ligand H₂L behaves as a tridentate ligand to Cu(ii) <i>via</i> its thiophenolic sulfur, azomethine nitrogen and pyrrolic nitrogen atoms, and as a bidentate ligand to Au(iii) <i>via</i> sulfur and azomethine nitrogen. Density Functional Theory (DFT/B3LYP/LANL2DZ) studies provided additional information on molecular orbital, electronic reactivity, charge distributions, and thermodynamic stability and showed that the gold(iii) half-sandwich complex was more exothermic, favoring thermodynamic stability, because of the stronger Au-ligand based on covalency and relativistic orbital interactions. Electrochemical analysis using screen-printed electrodes (SPEs) modified with H₂L/Cu and H₂L/Au nanomaterials showed significantly improved electron-transfer kinetics and capacitive behavior, with the H₂L/Au system exhibiting the lowest charge-transfer resistance (19 Ω) and highest specific capacitance (774 F g^-1). The H₂L/Au-modified electrode also exhibited excellent electrocatalytic activity for non-enzymatic hydrogen peroxide sensing, with a wide linear range (0.05-1725 μM), ultra-low detection limit (0.025 μM), high sensitivity, and outstanding stability. The practical applicability of the H₂L/Au-modified electrode was demonstrated with real-sample analysis of water, milk, cheese salami, and juice, showing excellent recovery of hydrogen peroxide in each sample. In summary, the combination of Schiff base coordination chemistry and transition-metal centers resulted in multifunctional nanocomposites with high thermal and electrochemical stability, making them suitable nanomaterials for possible energy-storage devices and electrochemical biosensors.