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published_or_final_version / Chemistry / Doctoral / Doctor of Philosophy

Various metal phthalocyanines have been used as dyes, catalysts, indicators, electrophotographic receptors and more recently as active elements in chemical sensors and photoconductive materials for solar energy conversion applications. Of the MPc's, VOPc, GaPc-Cl and TiOPc have shown promise for solar energy conversion devices. GaPc-Cl has also shown promise as a chemical sensor. Up to this point in time, the focus of Pc research in this group has been in the direction of characterizing the photoelectrochemical properties of these materials. Recently, solid state studies of TiOPc have been done to determine the electronic properties of a tetravalent metal Pc, such as conductivity and photoconductivity. Such solid state measurements are facilitated by the use of interdigitated microelectrode arrays which are coated with thin films of various Pc's.

This work was undertaken to investigate the transpassive behaviour of the anodic film on two Fe-Cr alloys, namely Fe18Cr and Fe18Cr2Mo in acidic aqueous media in the pH range 0.5 to 3.8. Two electrochemical techniques were used, namely cyclic voltammetry and chronoamperometry. The two primary experimental variables in the cyclic voltammetric experiments were pH and sweep rate (2 - 800 mV/s). The main variables in the chronoamperometric experiments were the size of the potential step, the number of potential steps and the starting and ending potentials. Secondary experimental variables were temperature (25, 90°C), rotation rate (0, 150 rad/s), and the artificial addition of cations (Fe2+, Fe3+ and Cr3+) to some of the solutions. A voltammetric anodic peak, referred to as peak A, occurs in the transpassive region of the above Fe-Cr alloys, followed by a region of secondary passivity and then oxygen evolution. It was this peak that was investigated by cyclic voltammetric methods. The peak A current response was independent of rotation rate at pH 3.8 but was dependent on rotation rate at pH 0.5 with solutions of intermediate pH showing a gradual change in rotation rate dependence. This indicated a predominantly solid state process in less acidic solutions (pH 2.4 & 3.8) whereas in strongly acidic solutions (pH 0.5) the action of ions in solution must contribute significantly towards peak A processes. A method was developed to correct the peak A current response for the current due to oxygen evolution. The results of this method indirectly confirmed the hypothesis that more than one oxidation process contributes to the peak A current response. A diagnostic plot for diffusion control was applied to the peak height of peak A. The diagnostic involves plotting the peak height over the square root of the sweep rate versus the square root of the sweep rate. A process under diffusion control would give a horizontal line for this diagnostic plot. At pH 0.5 and at slow sweep rates (less than or equal to 60 mV/s) the diagnostic plot gave a positive deviation from the horizontal and this deviation was enhanced as the temperature was increased. As the pH was increased (towards pH 3.8), the deviation from the horizontal at slow sweep rates gradually became negative and this deviation was again enhanced when the temperature was increased. This phenomenon is explained in terms of the role of the hydronium ion. From the addition of Fe2+, Fe3+, and Cr3+ to pH 0.5 and pH 3.8 solutions it was noted that ferrous ions increased the peak A current response more than chromic ions of the same concentration. Ferric ions slightly decreased the peak A current response. Based on these results, reports in the literature, and the apparent role of the hydronium ion, a partial scheme was proposed in order to explain the role of Fe and Cr, from the alloy substrate, in the anodic film in the transpassive region. In chronoamperometric experiments, stepping to the transpassive region confirmed the phenomenon of the rising transient. A quantitative nucleation model - which was based on previous models from the literature - was generated. The model was successfully fitted to two rising transients, one from the pH 3.8, and the other from the pH 0.5 solution. The model also allows for the presence of a pre-existent laver at the starting potential of a chronoamperometric experiment after the electrochemical cleaning procedure. The model incorporates both diffusion controlled and charge transfer controlled steps. A key concept in the model is that of nucleation and "slow death" of corrosion pits growing into the electrode. "Death" of a pit occurs when it is covered by a nucleating and or growing passivating film. The rising transients were only obtained on Fe-Cr alloys (with one exception) when stepping to the transpassive region and also only in solutions where peak A was obtained in a cyclic voltammetric experiment. The exception to this was that in the pH 0.5 solution and at 90°C, rising transients were obtained when stepping to the passive region. This did not occur at 25°C. Rising transients were also obtained on pure iron when stepping to the passive region. In addition to the rising transient, a reverse rising transient was discovered. This reverse rising transient (which generated a cathodic current) was obtained when stepping the potential cathodically from the transpassive region. It was shown that the occurrence of the reverse rising transient was dependent on the presence of a stable, transpassive anodic film before the potential step. One indirect result from the discovery of the reverse rising transient was that it indicates that secondary passivity exists at least 200 mV into the oxygen evolution region. / Thesis (M.Sc.)-University of Natal, 1993.

The original purpose of this investigation was to attempt to apply the coulostatic method directly to a molten salt system. The inability to duplicate the reported capacity data for this system resulted in an investigation of the probable cause of this discrepancy between the data obtained by these different methods (14, 15).

Call number: LD2668 .T4 1986 T42 / Master of Science / Horticulture, Forestry, and Recreation Resources

Orientador: Marcos Fernando de Souza Teixeira / Coorientador: Carlos José Leopoldo Constantino / Banca: Orlando Fatibello Filho / Banca: Homero Marques Gomes / Resumo: O intuito desta pesquisa foi caracterizar eletrodos quimicamente modificados (EQM) pela formação de monocamadas auto-organizadas (SAM) com os tióis (11- mercaptoundecil)-N',N'',N'''-trimetilamônio e 6-(ferrocenil)hexanotiol sobre a superfície do eletrodo de ouro. E, ainda, verificar seu comportamento de transferência eletrônica com solução de hexacianoferrato(II) e (III) e sua aplicação como biossensor para glicose. As monocamadas auto-organizadas vêm sendo comumente usadas devido ao seu comportamento homogêneo, o que confere ao eletrodo maior sensibilidade e reprodutibilidade, tornando-se possível desenvolver eletrodos para vários fins e aplicações. Neste trabalho, foram estudadas as monocamadas formadas a partir dos tióis (11-mercaptoundecil)-N',N'',N'''- trimetilamônio e 6-(ferrocenil)hexanotiol. A caracterização eletroquímica e morfológica das monocamadas auto-organizadas foi realizada através da voltametria cíclica e microscopia eletrônica de varredura, respectivamente. Foi notado que as monocamadas mistas na proporção (1:2) apresentaram maior efeito catalítico, uma vez que se obtiveram maiores sinais analíticos, através do incremento das correntes anódicas e catódicas. A viabilidade da molécula de hexacianoferrato (III) foi verificada através da troca iônica destas moléculas eletroativas com as monocamadas auto-organizadas. Para a aplicação dos eletrodos modificados com monocamadas como biossensor para glicose, optou-se pela modificação na proporção (1:2) de (11-mercaptoundecil)-N',N'',N'''-trimetilamônio e 6- (ferrocenil)hexanotiol, respectivamente, assim como a utilização do mediador hexacianoferrato pré-concentrado por troca iônica durante 2 horas, em virtude que, neste tempo ocorreu uma maior concentração de espécies eletroativas sobre a superfície do ouro, tendo como favorecido o processo de transferência eletrônica. A construção do biossensor .. / Abstract: The purpose of this research was to characterize chemically modified electrodes (CME) by the formation of self-assembled monolayers (SAMs) with thiols (11- mercaptoundecyl)-N',N'',N'''trimethylammonium and 6-(ferrocenyl)hexanethiol on the surface of gold electrode. And also verify their behavior of electron transfer in solution with hexacyanoferrate (II) and (III) and its application as a biosensor for glucose. The self-assembled monolayers have been commonly used due to its homogenous, which gives the electrode higher sensitivity and reproducibility, becoming possible to develop electrodes for various purposes and applications. In this research, it was studied the monolayers formed from the thiols (11-mercaptoundecyl)- N',N'',N'''trimethylammonium and 6-(ferrocenyl)hexanethiol. Electrochemical and morphological characterization of self-assembled monolayers were performed using cyclic voltammetry and scanning electron microscopy, respectively. The electrochemical characterization of the different SAMs under study occurred in a solution of 0.10 mol L-1 hexacyanoferrate(II) and (III) potassium, verifying the electronic transfer mechanisms involved. It was noted that the self assembled monolayers in proportion (1:2) showed higher catalytic effect, since obtained higher analytical signals by increasing the anodic and cathodic currents. The viability of the hexacyanoferrate (III) molecule was verified by ion exchange these electroactive molecules with self-assembled monolayers. For the application of electrodes modified with monolayers as a biosensor for glucose, we opted for the change in the ratio (1:2) of (11-mercaptoundecyl)-N',N'',N'''trimethylammonium and 6- (ferrocenyl)hexanethiol, respectively, as well as the use of pre-concentrated by ion exchange mediator hexacyanoferrate for 2 hours, because that, this time there was a greater concentration of electroactive species on the surface of gold having favore / Mestre

Thesis (MTech (Chemistry))--Cape Peninsula University of Technology, 2015. / Immunosensors are analytical devices comprising antibody (Ab) molecules intimately integrated with electronic physicochemical transducers. Abs are responsible for specific recognition of an analyte so called antigen (Ag) while transducers are responsible for the conversion of chemical changes brought about by Ab-Ag interactions into measurable and processable signal. Amongst the many analytical tools, immunosensors have shown outstanding performance in applications in fields such as clinical diagnostics, agricultural purposes and environmental monitoring. They have come in place of the many conventional analytical methods which showed a number of disadvantages; high cost and longer time of operation, and requirement of highly knowledgeable personnel. On the other hand, immunosensors have shown potential to overcome these constraints. Their advantages include possibilities of portability, miniaturization, and simplified procedures. Of the possible fields of immunosensor applications, this study focussed on the environmental aspect. The safety of the environment is good for the well-being even though there are still some environmental threats that exist. Polychlorinated biphenyls (PCBs) have reportedly been found to be some of the potential substances to pose such threats due to their toxic and persistent behaviour. In this study, we have developed an electrochemical immunosensor as an analytical tool for the analysis and monitoring of PCBs. The development was based on the use of silver nanoparticles-doped polyaniline (PANI/Ag NPs) for modification of an electrode as a process for fabrication of the transducer. The PANI/Ag NPs composite was deposited on the glassy carbon (GC) and platinum (Pt) electrodes by oxidative electropolymerization of aniline in the presence of Ag NPs in 1 M HCl using cyclic voltammetry (CV) by ramping the potential from -0.1 to 1.4 V at 50 mV/s. The composite was then characterized and evaluated as a potential material for electrochemical transduction. Evaluation was on electroactivity, which is the main property of interest for materials used in the fabrication of electrochemical devices. The PANI composites were characterized using spectroscopic (FTIR), microscopic (TEM) and electrochemical CV techniques. Results confirmed the formation of PANI in its emeraldine form and the presence of Ag NPs. Characteristic functional groups and peaks of PANI were observed in FTIR and CV respectively. TEM micrograms showed one dimensional nanofibric tubes and crystalline-like structure of the composite. The incorporation of Ag NPs was indicated by the transition from the amorphous (PANI) to crystalline (PANI/Ag NPs) structure accompanied by increase in size as well as smoothness of the tubes. EDS-TEM counts increase of the chlorine (Cl) peaks is due to the closeness of these peaks to those of Ag, thus confirming incorporation of Ag NPs.

Thesis (B.S.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science; and, (B.S.)--Massachusetts Institute of Technology, Dept. of Chemistry, 1976. / Microfiche copy available in Archives and Engineering. / Bibliography: leaves 65-66. / B.S.

Thesis (M. Phil.)--University of Hong Kong, 1987.