A study on the estimating DPV surface coverages for chemically modified electrodes

This thesis is about estimating the surface coverages (Г) of immobilised redox molecules at modified electrode surfaces using differential pulse voltammetry (DPV). The cyclic voltammetry (CV) technique less sensitive than DPV technique because of the background current that can obscure the Faradaic current of the attached redox molecules. As a result, CV technique less sensitive for measurements of low surface concentrations of immobilised redox species. This is not the case for DPV. Currently, there is no specific agreed procedure to determine the surface coverages by DPV. Thus to establish the method to determine the surface coverages of immobilised redox molecules at the surface of the electrode by DPV measurement was a prime interest in this research. Secondary to that point, exploring a new strategy in preparing chemically modified electrodes in order to obtain a better surface coverages of covalently immobilised redox molecules at electrode surfaces was also an aim of this thesis. Enhanced DPV currents were obtained when an external resistance (Rext) of suitable magnitude was added in the DPV cell circuit for electrode modified with either for organic, DNA labelled or inorganic redox systems. This motivated us to develop an experimental model for estimating the DPV surface coverages (ГDPV, exp) of attached redox molecules at electrode surfaces at suitable Rext. Moreover, a simple equation for estimating the ΓDPV, exp based on DPV parameters is presented in this study and shown to work well, given several underlying assumptions. Our approach makes use of an additional Rext in the DPV cell circuit and several non-adjustable DPV parameters. Subsequently, the ΓDPV, exp of immobilised redox probes can be estimated by integrating the area under the oxidation peak of the differential pulse voltammograms (DPVs) at a suitable Rext. The experimental approach was verified by numerical modelling using a specially developed DPV simulation in MATLAB. The electrochemical parameters used for the DPV simulation, such as uncompensated solution resistance in an electrochemical cell (Ru), double layer capacitance (Cdl) and electrode kinetics (ks and α) were obtained from cyclic voltammetry (CV), chronoamperometry and electrochemical impedance spectroscopy (EIS) measurements vii made on the modified electrodes. Good agreement was obtained between the experimental DPV and the simulations. The second part of this thesis was focused on extracting thermodynamic and kinetic parameters such as the standard potential (E°), the diffusion coefficient (Do), surface coverages, the rate constant for adsorption (kA), the rate constant for homogenous reaction (kH), the rate of electron transfer (ks), and the transfer coefficient (α) for the electrochemical grafting of primary amine linkers onto GC electrode surfaces by numerical simulation. These parameters are impossible to access by experimental approaches. The grafting process was performed in two different solvents; by electrochemical grafting from neat ACN and by electrochemical grafting in a mixed solvent of ACN and sodium hydrogen carbonate (NaHCO3). In the case of the mixed-solution method, the ratio of ACN to NaHCO3 was 4:1. The primary amine of the EDA-Boc molecule was studied as an experimental model in this work. Using a specially developed CV simulation for inhibiting species at the surface of the electrode, the experimental CVs for the electrochemical grafting of EDA-Boc in the two solutions were simulated. Good agreement between experimental CV and the simulated CV was obtained. Moreover, this simulation allows us to extract the aforementioned parameters, which are not electrochemically accessible. To investigate the effect of the presence of NaHCO3 on immobilised EDA-Boc at the surface of GC electrodes, the anthraquinone (AQ) was covalently coupled to the grafted EDA (after removal of the Boc group). The CV and DPV surface coverages of covalently attached AQ on the EDA linker based on the two methods of electrografting were compared. The developed DPV simulation was also employed in this work in order to show the utility and flexibility. A good agreement between experimental and simulated DPV was achieved.