Electrochemical properties of nanoporous based materials doped with metal oxide nanoparticles for potential application as sensors

PhD (Chemistry), North-West University, Mafikeng Campus, 2015This work describes successful synthesis of three metal oxides nanoparticles (MO = NiO, ZnO and Fe₃O₄) obtained by chemical synthesis, and modified with multiwalled carbon nanotubes (MWCNT), polyaniline (PANI) and MWCNT functionalized nylon 6,6 nanofibers on glassy carbon electrode (GCE). Successful characterization of the electrodes with the MO (NiO, ZnO and Fe₃O₄), MWCNT/MO, PANI/MO and Nylon 6,6/MWCNT/MO nanocomposites were confirmed by field emission scanning electron microscopy (FESEM), high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscopy (HRTEM), x-ray diffraction spectroscopy (XRD), electron dispersive x-ray spectroscopy (EDX), Fourier transformed infra-red spectroscopy (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. Electron transport (ET) properties of the modified electrodes was explored using cyclic voltammetry (CV) and electrochemical impedance spectroscopic techniques (EIS) with ferricyanide/ferrocyanide ([Fe(CN)6]³⁻/⁴⁻) as the redox probe. The mechanism of electron transport between the MO nanomaterial, their nanocomposites`and the electrocatalytic oxidation or reduction of the analyte at the modified electrode was extensively studied and discussed using electrochemical impedance spectroscopy (EIS). The electron transfer constant differs in terms of materials and electrical equivalent circuits used in the fitting or modelling process. The presence of MWCNT, PANI and Nylon 6, 6 also enhances the MO modified electrodes ET compared with electrodes without CNTs, PANI and Nylon 6,6. The electrocatalytic properties of the modified electrodes were explored using the following analytical probes: dopamine, serotonin, phenanthrene and lindane. Electrochemical oxidation of dopamine and serotonin on a glassy carbon electrode (GCE) modified with multi-walled carbon nanotubes doped with metal oxides (GCE/MWCNT/NiO, GCE/MWCNT/ZnO, GCE/MWCNT/Fe₃O₄) was examined by cyclic voltammetry, EIS and square wave voltammetry in 0.1 M phosphate buffer solution PBS at pH 7 and results discussed. The results were compared with those obtained on bare GCE, GCE/MWCNT and MO GCE modified electrodes (GCE/NiO, GCE/ ZnO, GCE/ Fe₃O₄). The nanocomposite modified electrodes exhibit excellent electrocatalytic activity towards the electrochemical oxidation of serotonin at large peak current and lower oxidation potentials compared to other electrodes investigated. It was found that the multi-walled carbon nanotubes improve remarkably the reactivity of NiO, ZnO and Fe₃O₄ for dopamine and serotonin oxidation. The GCE/MWCNT/NiO, GCE/MWCNT/ZnO, GCE/MWCNT/Fe₃O₄ electrodes exhibited good linear properties in the concentration range studied with a limit of detection of 0.00799 nM, 374.2 nM and 1386 nM respectively for dopamine; and 50.8 nM, 60.6 nM and 1090 nM respectively for serotonin. GCE-MWCNT-NiO was the best electrode in terms of dopamine and serotonin current response, electrode stability, resistance to fouling and limit of detection towards the analytes. The interference study also revealed no AA interference signal to DA at AA concentration 1000 times that of DA. Similar results were obtained for GCE/Nylon6,6/MWCNT/NiO, GCE/Nylon6,6/MWCNT/ZnO, GCE/Nylon6,6/MWCNT/Fe3O4 for electrochemical detection of dopamine with a limit of detection of 0.0176 nM, 0.0149 nM and 0.0428 nM respectively towards DA; and 339 nM, 0.117 nM, and 1160 nM respectively towards serotonin. GCE/Nylon6,6/MWCNT/ZnO modified electrode gave the best limit of detection towards both DA and serotonin. In the same vein, GCE-PANI-NiO, GCE-PANI-ZnO, GCE-PANI-Fe₃O₄ electrodes exhibited good electrocatalytic properties towards DA with a limit of detection of 11.5 nM, 63.3 nM and 1110 nM respectively. The electrocatalytic oxidation of phenanthrene on PANI-NiO on modified glassy carbon electrode (GCE-PANI-NiO) was studied using cyclic voltammetry, square wave voltammetry and impedance spectroscopy and discussed. Results showed that detection of phenanthrene was enhanced by the nanostructure of PANI-NiO film on GCE. The square wave voltammetry analysis shows a very low detection limit of 0.732 x 10⁻³ nM for phenanthrene with the linear range of 7.6 x 10⁻³ nM – 1.4 x 10⁻² nM. The Tafel value of 227 mVdec⁻¹ suggests adsorption of phenanthrene oxidation intermediates on the GCE-PANI-NiO electrode. The GCE-PANI-NiO modified electrodes gave better performance towards phenanthrene in terms current response, oxidation potential, current recovery, stability and resistance to electrode fouling effects PANI-MO and Nylon6,6/MWCNT/MO modified glassy carbon electrodes were prepared and used for the electrochemical reduction of lindane. The modified electrodes offer a high sensing current for lindane, and highly stable with respect to time, so that the single electrode can be used for the multiple analysis of the lindane sample. Cyclic voltammetry and square wave voltammetry were used as the sensing techniques. The dynamic range for the lindane determination was between 9.9 × 10⁻¹² mol/L to 5 × 10⁻⁶ mol/L with detection limits 51 and 32 nM for Nylon6,6/MWCNT/ZnO and Nylon6,6/MWCNT/Fe₃O₄ sensors respectively, and 239 nM, 44.7 nM for PANI/ZnO and PANI/Fe₃O₄ sensors respectively. The LoD value reveals that the best electrode is Nylon6,6/MWCNT/Fe₃O₄. The analytical utility of the proposed method was checked with drinking water samples.Doctora