الفهرس 
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Abstract
Abstract br Thesis Title: Novel Electrochemical Study of Antioxidants: Nano-Structured br Sensors and Bioelectrochemical Applications. br Eelectrochemical oxidation mechanism of gallic acid, GA, in aqueous br phosphate buffer solutions, PBS, of different pHs was studied at glassy-carbon br electrode, GCE. The study was performed using cyclic, convolution–deconvolution br sweep voltammetry, double potential step sweep -#99;-#104;-#114;ono- amperometry and - br coulometry. It gives two irreversible diffusion-controlled cyclic voltammetric waves br at the entire range of pH. The electrochemical oxidation mechanism was proposed to br be an ECEC, first -#111;-#114;-#100;-#101;-#114; mechanism in which the two electron transfer steps and the br two chemical follow-up deprotonation reactions are irreversible. The proposed br mechanism was confirmed by comparing the simulated response with the br experimental ones. Then, a direct determination of GA was achieved at a GCE br modified working electrode with polyepinephrine (PEP/GCE) in PBS of pH 1.88 by br cyclic voltammetry (cv) and adsorptive stripping square wave voltammetric method br (AdSWV). Using the optimal experimental conditions, the response of gallic acid br varies linearly with concentration in the range -#102;-#114;-#111;-#109; 10.00 x 10-7 to 2.00 x 10-5 mol L-1 br with a detection limit of 6.63 x 10-7 mol L-1. The adsorptive stripping square wave br voltammetric method was used to estimate the GA contents in tea sample.. Thus, br PEP/GCE was successfully applied for the determination of GA contents without br interference of ascorbic acid and caffeic acid. br Mechanistic electrochemical oxidation of quercetin was investigated at GCE br modified with multi-walled carbon nanotubes, MWCNTs, in aqueous 0.2 M PBS with br different pH values. The investigation was carried out using cyclic voltammetry, br double potential step sweep -#99;-#104;-#114;ono-amperometry and coulometry. It was found that br the oxidation proceeds in sequential steps, related with the five-hydroxyl groups in the br three aromatic rings. The mechanism was proposed to be an ECEC, first-order br kinetics. The proposed mechanism was confirmed on comparing the digital simulated br cyclic voltammetric responses with the experimental ones. 1 br The electrode homogeneous and heterogeneous kinetic parameters of electrode br reaction are estimated -#102;-#114;-#111;-#109; the simulated data. Moreover, quercetin molecules adsorb br on the electrode surface. A new electrochemical method for the determination of br quercetin based on carbon paste electrode, CPE, modified with a quercetin selfassembled br monolayer, QSAM/CPE, was established by linear sweep voltammetry. br Using the optimal experimental conditions, the calibration curve for quercetin was br linear in the concentration range -#102;-#114;-#111;-#109; 1.0 to 18.0 µM with a detection limit of 8.40 x br 10-7 mol L-1. br Electrochemical investigation of oxidation kinetics and mechanism of caffeic br acid, CAF, was carried out in aqueous PBS with different pH values. Bare glassy br carbon electrode, GCE and GCE-modified with multi-walled carbon nanotubes, br MWCNTs/GC electrodes were used. On using MWCNTs/GC electrode, markedly br improve the cyclic voltammetric responses, giving rise to chemically reversible, and br concerted two-electron oxidation process. Importantly, the oxidation behaviour is a br function of scan rate, concentration, and pH. The mechanism was proposed to be br consistent with transfer of two electrons coupled with two protons. It was concluded br that CAF oxidizes chemically following ECEC, radical-radical mechanism. The br proposed mechanism was confirmed on comparing the experimental data with the br simulated ones. br Interaction of DNA with CAF was studied in PBS at pH 2.21. On addition of br DNA, CAF interacts with DNA giving CAF-DNA complex by intercalative binding br mode. The apparent binding constant of the CAF-DNA complex is determined using br amperometric titrations. Biosensor, DNA/GCE, was used to detect the oxidative br damage of DNA and its protection by CAF. Damage is caused by the reactive oxygen br species, •OH, generated -#102;-#114;-#111;-#109; Fenton system. It was found that CAF has a capability of br scavenging the hydroxide radical and protect the DNA immobilized on the GCE br surface. Finally, the construction of NADH/n-Au/GCE biosensor was used to detect br the oxidative damage of NADH and its protection by CAF. The results showed that br the scavenging potency of CAF to •OH was 66.92 % and relatively stable. 2 br Keywords: Gallic acid, Quercetin, Caffeic acid, Oxidation, Electrochemistry, Digital br Simulation, Biosensor, DNA, NADH, Gold nanoparticles and MWCNTs. br 3