Carbon Paste Macrocycle Doped Composite Electrodes for the Selective Electrochemical Detection of Dopamine

The neurotransmitter dopamine (DA) has shown to play a very important role in the functioning of the central nervous system. Thus, the determination of DA is of great importance in the fields of neurochemistry and biomedical chemistry. In this thesis, a number of carbon paste electrodes modified with macrocycles for the electrochemical detection of DA is reported. The different macrocycles employed were based on cyclodextrin derivatives and consist of sulfated β-CD (S-β-CD), carboxymethyl β-CD (CM-β-CD), Ferrocene complex β-CD (Fc-β-CD), Heptakis 6-deoxy-6-(1-(4,5-dicarboxyl)-1,2,3-triazolyl)-β-CD (CD6.6) and Heptakis (6-(4-hydroxymethyl-1H-[1, 2, 3] triazol-1-yl)-6-deoxy)-β-cyclodextrin (CD6.7). The fabricated electrodes were characterized by using surface techniques and electrochemical methods such as energy dispersive X-ray (XRD), scanning electron microscopy (SEM), electrochemical impedance spectroscopy (ESI) and cyclic voltammetry (CV). The detection of DA at all modified electrodes (except for CD6.7) resulted in an enhancement of the oxidation signal response over that of the bare electrode. High performance of the electrochemical detection of DA was obtained at S-β-CD modified CPE such as a wide concentration range (from 5 × 10-7 M to 5 × 10-4 M) and low detection limit (1.33 × 10-7 M). It was shown that the sensitivity of the developed sensor towards the detection of DA depends on the amount of S-β-CD incorporated within the paste. The optimum sensor architecture was made by impregnating 0.545 g S-β-CD in a carbon paste containing 0.71 g graphite and 200 μL silicone oil. In addition, graphite was replaced by graphene and the electrochemical behaviour of DA at the S-β-CD modified graphene electrode was investigated by DPV. The results showed that the S-β-CD modified graphene electrode exhibited excellent electrochemical oxidation of DA in phosphate buffer solution (pH 6.8) compared to the bare graphite paste electrode. CD6.6 and CD6.7 were synthesised in order to exhibit further increase in the signal responses of DA. The formation of inclusion complexes of CD6.6 and CD6.7 with DA was studied by 1H-NMR spectroscopy. A 2:1 and 1:1 stoichiometry was obtained in the case of CD6.6:DA and CD6.7:DA, respectively. The bare GPE was modified with CD6.6 and the electrode was miniaturised from macro to micro-size level. This configuration also was shown to improve the detection of DA (3 × 10-7 M). This thesis also aimed to utilise these modified electrodes to enhance the selectivity for DA over two interferents, ascorbic acid (AA) and serotonin (5-HT). Improvements in the selectivity of DA were obtained at the S-β-CD modified GPE as AA was excluded at the electrode surface. Moreover, the results showed that DA, AA and 5-HT in coexisting solutions can be simultaneously oxidised at significantly different potentials in the presence of S-β-CD modified CPE. The voltammetric response of the three compounds could be completely separated at the modified electrode using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques at optimal conditions. DPV provided larger peak potential separations and higher response sensitivities of DA, AA, and 5-HT compared to CV