This Ph. D. thesis presents new methods for the determination of selected clinically relevant electrochemically active compounds. The first part deals with development of determination of tumour biomarkers homovanillic acid (HVA) and vanillylmandelic acid (VMA) in human urine with the use of hollow-fibre based liquid-phase microextraction (HF-LPME) and differential pulse voltammetry (DPV) at cathodically pre-treated boron doped diamond electrode (BDDE). Optimum conditions for HF-LPME-DPV of HVA and VMA were as follows: butyl benzoate as supported liquid membrane formed on porous polypropylene hollow-fibre, 0.1 mol L−1 HCl as donor phase and 30 min extraction time. Optimum acceptor phases were 0.1 mol L−1 phosphate buffer of pH 6 with ionic strength set to 0.55 mol L−1 for HVAand 0.1 mol L−1 NaOH for VMA, respectively. HF-LPME-DPV concentration dependencies for HVA and VMAwere linear in the range from 0.4 to 100 µmol L−1 and 0.5 to 100 µmol L−1 . Limits of quantification (LOQ)/detection (LOD) were 1.2/0.4 µmol L−1 for HVA and 1.7/0.5 µmol L−1 for VMA, respectively. The applicability of the developed methods was verified by analysis of human urine. In the second part, voltammetric behaviour of heavy metal poisoning antidote 2,3- dimercapto-1-propane-sulfonic acid (DMPS) was investigated with the use...

>Magister Scientiae - MSc / This project focuses on the development of simple, highly sensitive, accurate, and low cost electrochemical sensors based on the modification of pencil graphite electrodes by the electrochemical reduction of graphene oxide-metal salts as nanocomposites (ERGO-MC-PGE; MC = Sb or Au nanocomposite). The electrochemical sensors ERGO-Sb-PGE and ERGO-Au-PGE were used in the determination of paracetamol (PC) in pharmaceutical formulations using adsorptive stripping differential pulse voltammetry. The GO was prepared from graphite via a modified Hummers’ method and characterized by FTIR and Raman spectroscopy to confirm the presence of oxygen functional groups in the conjugated carbon-based structure whilst, changes in crystalline structure was observed after XRD analysis of graphite and GO. / 2023-10-07

Indiana University-Purdue University Indianapolis (IUPUI) / The reaction between elemental sulfur and sulfide is a lynchpin in the biotic and abiotic cycling of sulfur. This dissertation is focused on the reactivity of elemental sulfur nanoparticles (S8weimarn, S8raffo) among other forms of elemental sulfur (S8aq, S8aq-surfactant, α-S8), and how the variation of their surface area, character and coatings reflect on the analytical, physical-chemical and geochemical processes involving sulfur cycling. A comprehensive electrochemical investigation utilizing mercury-surface electrodes showed that elemental sulfur compounds are represented by three main voltammetric signals, corresponding to potentials at -1.2V, -0.8V, and -0.6V in the absence of organics at circumneutral pH. Dissolved S8aq-surfactant signals can be found from -0.3V up to -1.0V, depending on the surfactant in the system. Variations in current response resulted from differences in electron transfer efficiency among the forms of S8, due to their molecular structural variability. Based on this observation a new reaction pathway between S8 and Hg-surface electrodes is proposed, involving an amalgam-forming intermediate step. The kinetics of the nucleophilic dissolution of S8nano by sulfide, forming polysulfides, were investigated under varying surface area, surface character and presence or absence of surfactant coatings on S8nano. Hydrophobic S8weimarn and hydrophilic S8raffo show kinetic rate laws of 𝑟𝑆8𝑤𝑒𝑖𝑚𝑎𝑟𝑛 = 10−11.33 (𝑒 −700.65 𝑅𝑇 ) (Molar(S8)/second/dm-1) and𝑟𝑆8𝑟𝑎𝑓𝑓𝑜 = 10−4.11 𝑖−0.35 (𝑒 −615.77 𝑅𝑇 ) (Molar(S8)/second), respectively. The presence of surfactant molecules can influence the reaction pathways by dissolving S8nano and releasing S8aqsurfactant, evolving the rate-limiting step as a function of the degree of the solubilization of S8nano. The reaction rate of S8biological can be compared with those of S8raffo and S8weimarn in circumneutral pH values and T=50oC, making the forms of S8nano successful abiotic analogue models of microbially produced S8biological. Field observations and geochemical kinetic modeling in the geothermal features of Yellowstone indicate that the nucleophilic dissolution reaction appears to be a key abiotic pathway for the cycling of sulfur species and the enhancement of elemental sulfur bioavailability. Furthermore, in situ and ex situ voltammetry in the same geothermal waters disclosed chaotic variability in chemical gradients of sulfide (observed over small temporal and spatial scales) which can be considered as an ecological stressor capable of influencing single cell physiology and microbial community adaptation.

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Electrochemical behavior of mono- and bimetallic chromiumtricarbonyl and cationic manganesetricarbonyl of fluxional dibenzo[a,e]cyclooctatetraene (DBCOT) complexes were studied via cyclic voltammetry over a range of scan rates (20 – 2000 mV/s) and temperatures (0 °C and 25 °C). The presented work displays electrochemical reduction mechanisms associated with eight-membered ring coordinated M(CO)3 systems that undergo rapid ring inversion in solution. The electrochemical studies of these complex systems exhibit comparitively similar behaviors, which suggest relatively undifferentiated mechanisms. Slight differences between the chromium and isoelectronic cationic manganese are seen in their chemical reactions in solution and the potential at which they reduce. The significance of the electrochemical studies of these complexes are justified by their potential contribution to nanotechnology considering the possibility of generating a cylindrical nanostructure containing the tub-shaped eight-membered ring ð-coordinated M(CO)3 system. The high probability of a haptotropic shift to the eight-membered ring upon reduction could prove to be beneficial to electrocatalysis.

Deriving electrical energy from glucose and other carbohydrates under mild conditions is an important research objective because these biomolecules are abundant, renewable, and can provide 12 to 24 electrons per molecule, yielding substantial electrical power. It was previously observed that disubstituted viologens, salts of N,N'-disubstituted 4,4'-bipyridine, are able to oxidize glucose under alkaline conditions. Building on that initial result, the objective of this work was to understand and quantify the effectiveness and utility of viologens as catalysts for use in direct carbohydrate fuel cells.The extent that viologens oxidize carbohydrates, the conditions under which that oxidation occurs, and the mechanism for the oxidation were examined using oxygen-uptake and other methods. Viologens were found to catalytically oxidize carbohydrates extensively in alkaline solution. Viologens were also found to react with the enediol form of the carbohydrate, initiating carbohydrate oxidation with subsequent reduction of the viologen. If the viologen/carbohydrate ratio is low, electron transfer from the carbohydrate to the viologen becomes limiting and the carbohydrates undergoing oxidation rearrange into unreactive intermediates such as carboxylic acids and alcohols. At high catalyst ratios, excess viologen more rapidly oxidizes the carbohydrate and minimizes formation of unreactive intermediates. We also found that viologen polymers were more efficient than an equivalent concentration of monomers, suggesting that the higher localized concentration in polymeric viologen acts to efficiently oxidize carbohydrates and simulates high viologen/carbohydrate ratios.Monoalkyl viologens, aminoviologens, indigo carmine, and methylene blue were investigated by the method of cyclic voltammetry to inform their use as catalysts in the oxidation of carbohydrates. Redox potentials, diffusion coefficients, and heterogeneous electron-transfer rate constants were determined. Stability in alkaline solution and aqueous solubility were also examined in a semi-quantitative fashion. A comparison between the catalysts was made and viologens were found to be superior based on the examined parameters.The catalytic oxidation of carbohydrates by viologen was also examined using a fuel cell-like device. For the conditions in which a test cell was operated, oxidation efficiencies of up to 33% were observed, compared to previously reported values from about 2.5% to 80%. Anode polarization curves were obtained and used to determine the behavior of the viologen-controlled anode as a function of pH, viologen and carbohydrate concentration, and carbohydrate identity. pH was found to have a stronger effect on the performance at the anode for carbohydrates with a higher number of carbons than those with a lower number.

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