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Development and Characterisation of a Carbon Based pH Microsensor and Studies on Polypyrrole Nanowires
Herdman, Karen
In this thesis the electrochemical deposition of polypyrrole (PPy) into nanowire and bulk morphologies is reported. The electrochemical properties of the different conformations of PPy were examined in order to ascertain their conductivities and surface areas. This could determine the optimisation for further modification of the polymer with, e.g., copper structures forming an electrochemical sensor for the detection of the nitrate ion. PPy nanowires were electrodeposited onto gold electrodes using slightly acidic anions (Na2HPO4) and non-acidic anions (LiClO4) at a fixed potential of 0.80 V vs. SCE. The nanowires produced had an average diameter of ca. 89.2 nm, n = 50. Bulk PPy was electrodeposited using similar conditions, but the pH of the solution system was reduced using concentrated HClO4. This resulted in a bulk polymer with a higher surface area, so a second bulk polymer, of similar surface area to the nanowire films, was formed by reducing the electrodeposition time/charge consumed. Both of these bulk polymers were compared with the nanowire morphology of PPy using the electrochemical techniques of cyclic voltammetry, (CV), and electrochemical impedance spectroscopy, (EIS). Impedance data were fitted to equivalent circuits and the polymer resistance, the double-layer capacitance and the polymer capacitance were determined. The results indicated that the electrochemical properties of the polymers changed as the polymers went from an oxidised to a reduced state, i.e., their resistances and ability to store energy, confirming their optimum working potential range. The second section of this thesis seeks to develop a miniaturised analytical device that can deliver real-time information on changes in tissue pH. Many different pH probes exist, but they present many limitations including fragility, difficulty in miniaturising, potential drift, and difficulties in accurately measuring pH in solutions of varying ionic strength. There therefore remains a pressing need for more robust, pH sensors that can accurately sense pH changes in hostile surroundings, e.g., the highly resistive tissue found in the clinical environment. In this study, carbon paste electrodes (CPEs) and carbon fibre electrodes (CFEs) were modified with a quinone containing diazonium salt, FBRR, by electrochemical deposition. In buffered media, the quinone/hydroquinone redox system involves changes of the protonation state of the molecule, resulting in the observation that potentials vary with pH in a Nernstian manner. This behaviour was used as the basis of the electrochemical pH sensor. Various deposition conditions were employed to give the optimum and most efficient method, while organic and aqueous solvents were employed as the supporting electrolytes. Using either solvent the electrochemical techniques of CV, and linear sweep voltammetry, (LSV), were applied, optimising FBRR deposition by varying parameters such as potential window, scan rate and number of cycles/sweeps. Modified CPEs were calibrated for their pH response by CV, showing a response slope of −60.36 ± 0.89 mV/pH, n = 23. The surfaces of bare and derivatised CPEs were analysed by scanning electron microscopy, (SEM), coupled with energy dispersive X-ray, (EDX). After in-vitro development of a working pH sensor, a full characterisation was carried out, over the required pH range. This aimed to assess the sensor sensitivity, operational and storage stability, biocompatibility, and the effects that multiple interferences, found in the in-vivo environment had on the sensor performance. In-vivo voltammetry conditions were mimicked by changing the operational temperature and using a physiologically suitable reference electrode. An extensive study into the effect that the carbon: silicone oil content of CPEs had on the electrochemical properties of the modified electrodes was carried out using CV and corroborated by SEM and EDX surface analyses. These analyses concluded that although the electrodes appeared to perform better when exposed to proteins and lipids, the level of improvement did not justify the additional two days required in the manufacturing process of the sensor. The in-vivo application of the pH sensor was subsequently examined. The sensor was inserted into the hind limb muscle of anaesthetised rats. A pH change was induced locally to the limb by applying a tourniquet to restrict the blood flow and induce ischemia. This caused an increase in CO2 levels thus reducing the pH. After a short period of time, ca. 10 minutes, the tissue pH was allowed to recover. To induce an increase in the pH, injections of NaHCO3 were administered locally showing an immediate change in the observed potential, which recovered soon after. These changes in potential, of which pH was a contributing factor, were successfully recorded in real-time.
Keyword(s): Development; Characterisation; Carbon Based pH Microsensor; Polypyrrole Nanowires
Publication Date:
Type: Doctoral thesis
Peer-Reviewed: No
Institution: Maynooth University
Citation(s): Herdman, Karen (2016) Development and Characterisation of a Carbon Based pH Microsensor and Studies on Polypyrrole Nanowires. PhD thesis, National University of Ireland Maynooth.
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First Indexed: 2019-01-10 06:03:45 Last Updated: 2019-01-10 06:03:45