Faculty of Applied Sciences

The two case studies in this work involve the development and fabrication of an electrochemical biosensor using various enzymes for the evaluation of the electrochemical responses, relating to the total phenolic (TP) content and the antioxidant activities in wine and tea samples respectively. The modification of the glassy carbon electrode (GCE) was carried out using green apple as an enzymatic source of polyphenol oxidase and laccase enzyme. The experimental variables were optimized using the Box-Behnken experimental design as a predictive model, for a better understanding of the parameters and their interaction responses with each other during an electrochemical analysis. This multivariate optimization method is based on a factorial design, where the three most influential factors include the electrolyte pH, the deposition time (td) and the scan rate (sr). The design was run in a single block fashion while random order of experiment was selected to provide greater protection against the effects of outlying variables. The optimized results obtained yielded the most suitable conditions for the determination of the TP content in wine samples. They were selected as follows: phosphate buffer of pH 7.65 as supporting electrolyte, td 29.8 s and sr 25.0 mV/s respectively. The method was optimized for the current signal at a deposition potential of 0.2 V and within an oxidation potential of -0.2 V to 0.6 V. Good analytical responses were obtained with apple sensors for the detection of TP content in wine samples, with a higher concentration in red wines than in white wines. Differential pulse voltammetry (DPV) and cyclic voltammetry (CV) were used to establish and interpret the redox mechanisms of flavonoids present in alcoholic and non-alcoholic beverages. The sensor responses were evaluated by first, investigating the changes in the total phenolic (TP) content in wine samples using catechin as a standard. Thereafter, the electrochemical behaviour of rutin and ascorbic acid, antioxidant capacities (trolox reagents) were established in tea samples, yielding a positive linear correlation between the trolox equivalent antioxidant capacities (TEAC) and TP content (R2 = 0.9812 ± 0.012). DPV was applied to the laccase modified GCE, and the experimental results indicate that this sensor shows good reducing properties. The scavenging ability of 2,2’-Azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS) a diammonium salt, was assessed in the sample extracts, which yielded half-maximal effective concentration (EC50) values of 10.80 μg/ml and 11.62 μg/ml for ascorbic acid and rutin respectively. These findings indicated that the experimental design was a convenient method to evaluate the statistical significance of the optimised parameters, and the positive linearity for the TEAC and the TP content confirms the robustness of this methodology.

This study involves the enantioresolution of (±) catechin with the highly sulphated beta cyclodextrin (HS-β-CD) as a chiral selector using capillary electrophoresis (CE). The purpose of this study was to be tter understand enantioresolution amongst host-guest interactions. Furthermore, molecular docking was carried out to elucidate the mechanism of the enantioselective separations of (±) catechin enantiomers obtained in Electrokinetic chroma tography (EKC). A large difference in the interaction energies observed between the two enantiomers represents significant enantiodifferentiation. Our results also suggest that the host-guest interactions between the phenyl ring of the ligand and the open cavity of the HS-β-CD are due mainly to hydrophobic interactions. Interestingly, the stronger interactions observed with (+)-catechin is consistent with the elution order observed in the CE experiments.

Faculty of Applied Sciences

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