The Formation of an Electrochemical Sensor for the Selective Detection of Dopamine

In this thesis results are presented and discussed on the formation, characterisation, optimisation and performance of a polypyrrole-sulfonated β-cyclodextrin (PPy-SCD) film that can be used as novel dopamine (DA) sensor. This material exhibited a higher sensitivity to DA in comparison to the several other polypyrrole modified electrodes that were investigated in this study and it was also highly selective towards DA in the presence of a wide variety of major interfering compounds. The sulfonated β-cyclodextrins were immobilised in a polypyrrole film via a potentiostatic mode of growth from an aqueous solution of pyrrole and sulfonated β-cyclodextrin. No supporting electrolyte was used to ensure that the polymer was only doped by the anionic sulfonated β-cyclodextrin. The DA was detected at this material in a sulfate supporting electrolyte, giving peak oxidation and reduction currents at approximately 0.450 V vs. SCE and 0.320 V vs. SCE, respectively, for a 1.0 x 10-3 mol dm-3 DA solution. However, similar DA oxidation currents and potentials were obtained in a chloride supporting electrolyte solution. This increases the potential of the PPy-SCD film as a possible in-vivo DA sensor seeing as most biological samples have a high saline content. The oxidation of DA at the PPy-SCD modified electrode was optimised by varying a range of factors and conditions. Using these optimised parameters a limit of detection of 3.36 x 10-6 mol dm-3 was evaluated using constant potential amperometry. An attempt to increase this sensitivity further by incorporating ultrathin fibers into the polymer matrix proved to be unsuccessful. It was determined that a 1:1 inclusion complex was formed between the DA and the sulfonated β-cyclodextrin, whereby the hydrophobic moiety of the DA was included in the cavity. Complex formation constants of 199.54 ± 4.88 mol-1 dm3 and 240.18 ± 5.87 mol-1 dm3 were obtained, respectively, from cyclic voltammetry and rotation disk voltammetry in solution. In fact, this complexation in conjunction with oxidation at the polypyrrole film is responsible for the significant DA signal observed at the PPy-SCD modified electrode. A wide variety of interfering compounds were also examined to establish their effect, if any, on the determination of DA at the PPy-SCD modified electrode. The interference from several major interfering compounds, such as ascorbic acid and uric acid, could be effectively blocked. No interference was exhibited from 5-hydroxytryptamine and 5- hydroxyindole acetic acid at low concentrations of DA and the oxidation peaks of DA and acetaminophen were effectively separated with the acetaminophen having no influence on the oxidation peak current or potential of the DA signal. However, the interference from epinephrine, homovanillic acid and 3,4-dihydroxyphenylacetic acid could not be eliminated