Preparation and electrochemical characterisation of anion exchange polymer coated electrodes towards sensing applications

This thesis concerns with the investigation of electrochemical processes at the electrode interface, using different electrode materials coated with anion exchange polymers. There is an ongoing interest in analytical chemistry, with focus on developing sensors with high sensitivity and selectivity to detect specific analytes, sensors can be fabricated to enhance these characteristics. Anion conducting polymers usually used in Anion exchange membrane fuel cells (AEMFC), which gives the potential to use non-platinum based electrocatalysts in its operation, are useful in this respect in electroanalytical applications, as their selectivity towards negative species in solution allow their preconcentration at the electrode surface. Two polymers were used in this project, Hexamethyl-p-terphenyl poly(benzimidazolium) - HMT-PMBI and poly(ethylene-co-tetrafluoroethylene)-g-poly(benzyl-trimethylammonium chloride) – BTMA. The former was utilized by dropcasting solution onto glassy carbon electrode (GCE) and layering indium tin oxide substrate (ITO) with Langmuir Schaefer (LS) films and BTMA was used to fabricate a modified carbon paste electrode. Amperometric sensors are very attractive compared to other analytical methods such as, atomic absorption spectrometry (AAS) and molecular absorption spectrometry (MAS) because of their potential low cost and the possibility to perform measurements in situ using portable devices. Applications for these devices can be utilised in many industries, including medical, environmental and food. A common example of a commercial sensor is the blood glucose sensor, which uses the enzyme glucose oxidase to break blood glucose down. To investigate the basic electrochemical properties of the different electrodes, redox mediators were used. These oxidation and reduction reactions agree with the theory i.e. Randles – Sevcik equation and the calculation of the apparent diffusion coefficient. This was calculated to be 10-9 – 10-10 for HMT-PMBI coated electrode. The significance of this is, the smaller the diffusion coefficient the slower the rate of diffusion through a substance.In order to ascertain the suitability of the as-prepared ionomer films and paste for electrocatalytic sensing studies, investigations into the electrochemical behavior of HMT-PMBI and BTMA towards the detection of mercury (Hg), uric acid (UA) and ascorbic acid (AA). These have been selected to study because they are negative species, and hence the polymer is positive which will have selectivity towards these. All modified electrodes show attractive limit of detections for UA and AA i.e. in the range of harmful levels in blood and urine – over 530 µM in females and 619 µM in males for UA, and sensitives compared to selected peer reviewed journals which have also used sensors for the detection of analytes in this project. Where the detection limits range from 0.05 – 15 µM81-95 for UA and 0.002 – 300 µM for AA80-87,114-120. Compared to this work where the detection limits for BTMA-CPE range between 0.84 – 9.5 μM for UA, 1.43 – 50 μM for AA, the HMT-PMBI GCE range between 6 – 18 μM for UA, 17 – 81 μM for AA and the 20L HMT-PMBI ITO range between 2.2 – 26 μM for UA, 16.9 – 19.1 μM for AA.