Electrochemical study of pyrene using glassy carbon electrode modified with metal-oxide nanoparticles and a graphene oxide / multi-walled carbon nanotubes nanoplatform
This work describes and compares the electron transport and electrocatalytic properties of chemically synthesised cobalt oxide and nickel oxide using graphene oxide and acid treated multi-walled carbon nanotubes as supports grafted onto a glassy carbon electrode. The successful synthesis of the metal oxides, carbon nanomaterials and nanocomposites was confirmed using various spectroscopic techniques such as Ultra violet-Visible (UV-Vis), Fourier transform – infra red (FT-IR) , X-ray diffraction spectroscopy (XRD), Electron dispersive X-ray spectroscopy (EDX), Raman spectroscopy and microscopic techniques such as Scanning electron microscope (SEM) and Transmission electron microscope (TEM) . After electrode modification using the drop-casting method, comparative electrochemical studies were carried out in 0.1 M pH 7 phosphate buffer (PBS) and in 5mM ferricyanide/ ferrocyanide ([Fe(CN)$_6$$^3$$^-$]) /[Fe(CN)$_6$$^4$$^-$]) outer sphere model redox probe using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in order to establish the electron transfer properties of the modified electrodes. The electron transport between the various nanomaterials and / or their nanocomposites was detailed using EIS. This revealed that the modified electrodes have a pseudocapacitive nature as a result of the combined activity of the carbon nanomaterials (the double layer capacitor) and the electron conducting metal oxide nanoparticles. The performance of the modified electrodes relative to the unmodified GCE in pyrene was studied using cyclic voltammetry and impedance measurements. It was found that the metal oxides demonstrate better performance when the carbon nanotubes were used as a grafting support. The fMWCNT-MO modified electrodes demonstrated faster electron transport and a dramatically enhanced catalytic current when compared to the same metal oxides grafted onto the graphene oxide (GO). This inefficient performance of the GO based electrodes is associated with a larger proportion of unreactive basal planes exposed relative to the reactive edge planes of the GO. To better understand the mechanism of electrocatalytic oxidation of pyrene on the modified electrode surface, EIS was used to validate and compliment the results obtained using cyclic voltammetry. The charge transfer resistance, electron transfer rate constant (ks), Tafel value, limit of detection (LoD), sensitivity, adsorption equilibrium constant (β), Gibbs free energy change due to the adsorption (ΔG°$_a$$_d$$_s$) of pyrene onto the GCE-fMWCNT-Co$_3$O$_4$ were established and discussed. The LoD and ΔG°$_a$$_d$$_s$ for pyrene were 1.62 nM and -15.8 kJ/mol, respectively, over a linear dynamic range of 1.0 x 10$^-$$^9$– 100 x 10$^-$$^9$ M. The electro-oxidation of pyrene was a diffusion dominated process, but demonstrated adsorption thought to be as a result of a combination of the strong pi-pi electron interactions between pyrene and the MWCNT, thus the thin film formed on the surface of the electrode by the analyte and its reaction intermediates. In conclusion, this research work has demonstrated that cobalt oxide supported on acid functionalised multi-walled carbon nanotubes grafted onto glassy carbon electrode can be used as a sensitive and low cost electrochemical sensor for the detection of pyrene; one of a group of recalcitrant, ubiquitous, toxic and carcinogenic persistent organic pollutants.