Heavy and precious metal toxicity evaluation using a horseradish peroxidase immobilised biosensor
Environmental pollution is always the hottest topic in public conversation and one of the most concerned aspects of human health. The thin film sputtered microelectrode devices have been developed to improve the quality of human health, by offering better monitoring capabilities. This thesis is divided into three parts and the studies were performed on chemical sensor technology currently available and under development using modified methods. In the first part of this thesis: (i) the studies are related to synthesis, characterization and polymerisation of polyaniline (PANI) and polyaniline-co-poly(2,2´-dithiodianline) (PANI-co-PDTDA). Polyaniline (PANI) and the copolymer of aniline with dithiodianiline, an aniline derivative containing S-S-links were of interest in polymer synthesis. Electrochemical synthesis was carried out in 1 M HCl and different concentrations of H2SO4 (1, 2.5, and 5 M) solutions for PANI and PANI-co-PDTDA respectively. The PANI and PANI-co-PDTDA were grown electrochemically on the surface of a glassy carbon electrode (GCE) by repetitive cyclic voltammetric scanning. Cyclic voltammetry (CV) was used to evaluate the differences between the electrochemical characteristics associated with growth of the copolymer and homopolymer, polyaniline (PANI). The surface concentration of PANI was estimated to be 2.64 × 10-1 mol.cm-2 while the film thickness was estimated to be 7.09 × 10-10cm and 1.49 × 10-9cm for scan rate and aquare root scan rate. In contrast, PANI-co-PDTDA concentrations (1, 2, 5 and 5 M H2SO4 solutions) gained a surface concentration (G) falling in the range 6.1 x 10-2 - 7.9 x 102 mol.cm-2 and a film thickness in the range 8.16 x 10 -9- 2.05x10-8cm. The second section of this thesis focused on the development of two sensors, Pt/PANI/HRP and Pt/PANI-co-PDTDA/HRP biosensors. The biosensor described in this chapter focus on the use of horseradish peroxidise (HRP) with hydrogen peroxide as substrate, was constructed with the aim of further investigation of inhibition by heavy metals (Cd2+, Pb2+ and Hg2+). To achieve this, the enzyme HRP as the catalytic bio-element, was immobilised on the surface of a platinum electrode with PANI as a mediator. Immobilisation of HRP in conducting polymer matrices of PANI and PANI-co-PDTDA were achieved by electrochemical polymerisation. The use of amperometric detection allowed for the coupling of the biosensor with a portable potentiostat system (PalmSens). Differential pulse voltammetry (DPV) as technique was used as a detection method for inhibition determination. Selection of suitable pH values for biosensor performance was evaluated and the system showed optimal performance at pH 6.8 and 7.2 for Pt/PANI/HRP and Pt/PANIco- PDTDA/HRP biosensors, respectively. The biosensors developed in this work showed detection limits (LODs) of 0.32 mM and 0.0483 mM for PANI/HRP and PANI-co- PDTDA/HRP, respectively. For the Pt/PANI/HRP biosensor, the apparent Michaelis-Menten constant (Km app) value and maximum current (Imax) were evaluated from Lineweaver-Burk plots at various H2O2 concentrations. The values were found to be 0.6 mM and 1.7 μA for the Pt/PANI/HRP biosensor, while for the Pt/PANI-co-PDTDA/HRP biosensor the results were 0.7 mM and 0.27 μA, respectively. The third section investigated the adsorptive cathodic differential pulse stripping voltammetric (AdDPSV) determination of platinum group metals (PGMs), using an ex situ bismuth coated screen printed carbon electrode (SPCE/Bi) as the working electrode and ammonium buffer solution (pH = 9.2) as the supporting electrolyte. The cathodic stripping differential pulse method was used for investigating the electrochemical behaviour and the quantitative analysis of platinum group metals (Pt, Pd and Rh) at the SPCE/Bi surface in the presence of dimethylgloxime (DMG) as a complexing agent. In order to determine the metals at improved detection limits ensuring repeatability and sensitivity, a complete optimization study of voltammetric parameters was performed. The proposed method was successfully applied to the determination of the real samples (sediments & water) collected in the platinum mining area in the North-West and Limpopo Provinces, South Africa. The results were compared with those obtained by the glassy carbon bismuth film (GC/BiF) voltammetric and ICP-AES spectrometry techniques. Well-shaped voltammograms with clear peak potentials were obtained in the analysis of the real samples, offering excellent perspectives on the use of the constructed modified electrodes. The calibration curves for all PGMs investigated were linear with the limit of detection (LOD) at approximately 0.008, 0.006, and 0.005 μg.L-1 for Pd, Pt and Rh, respectively.