No description available.

No description available.

Zinc oxide is a II-VI semiconductor with considerable potential for optoelectronic and power-electronic applications in the UV spectrum, due to its wide direct band gap (3.35 eV at 300 K), high exciton binding energy (60 meV), high melting point, and excellent radiation hardness. A key requirement for many device applications is the consistent production of high performance Schottky contacts. Schottky contact formation to n-type ZnO was investigated via systematic studies into the relative performance of different metal and metal oxide Schottky contacts to hydrothermal and melt grown, bulk ZnO. The results of these studies can be explained by the dominating influence of two key mechanisms in the formation of high quality contacts: the removal of the natural hydroxide termination of ZnO and the associated surface accumulation layer, and the minimisation of process induced oxygen vacancies which tend to pin the barrier height of ZnO Schottky contacts in the 0.6 - 0.8 eV range. These investigations also led to the discovery of a new technique for the consistent production of high quality ZnO Schottky contacts, using the deposition of metal oxide films in reactive oxygen ambients. Specifically, silver oxide, iridium oxide, and platinum oxide films were used to consistently produce highly rectifying, very low ideality factor Schottky contacts to bulk ZnO, with figures of merit significantly better than those published in the literature. In addition, a number of previously unreported, surface polarity related effects were discovered in the electrical and optical properties of ZnO, which increase in magnitude with decreasing carrier concentration of the ZnO material. For example, metal oxide Schottky contacts fabricated on the Zn-polar surface of hydrothermal ZnO have significantly higher barrier heights than those on the O-polar surface, and low temperature (4 K) photoluminescence emission, from free excitons and excitons bound to ionised donors, is also significantly stronger from the Zn-polar face of the same material. These effects are thought to be related to the large spontaneous polarisation (-0.057 Cm-2) of ZnO, and indicate that surface polarity is an important variable when comparing experiment results with theoretical models, and in the future design of ZnO based devices.

A study of the reactions of various S-nitrosothiols, particularly S-nitroso-N- acetylpenicillamine (SNAP), was undertaken. These compounds were known to produce nitric oxide (NO) when decomposing, which has important and diverse biological roles. An example of their use in physiological research was demonstrated. The Griess method was used to determine the stoichiometry of nitrite production from S-nitrosothiol decomposition in various buffer solutions. In all cases the production was found to be almost quantitative. The kinetic measurement of SNAP decomposition in a variety of buffers and pH was undertaken. The results were complex and often erratic, conforming to first order but also half order kinetics in many cases. There was some indication that decomposition products and light could affect the reaction. The presence of disulphide (dimer) as a major reaction product was confirmed in the case of SNAP. Free-radical traps were used to probe the decomposition mechanism, as were hemin and haemoglobin as NO detectors to determine decomposition kinetics. The true agent of S-nitrosothiol decomposition was found to be intrinsic copper in the water supply and buffer salts. S-nitrosothiols were found to be stoichiometrically decomposed by Hg(^2+) ions, but catalytically decomposed by Cu(^2+) ions. Kinetic measurement confirmed the complex nature of the catalysis. The importance of SNO and NH(_2), and SNO and COO- as binding sites was demonstrated. Some explanation was found for the differing structure/reactivity relationships observed. It was shown that transnitrosation of a thiol could occur, involving thiolate anion attack upon the S-nitrosothiol. However, the reaction appeared to be very slow at physiological pH. The nitrosation of N-methylaniline by S-nitrosothiols was found to occur only in the presence of oxygen - direct transfer of NO did not occur, nitrosation being mediated after SNAP decomposition.

No description available.

A detailed study concerning the aqueous decomposition characteristics of S-nitrosothiols in both the presence and absence of cupric ions was undertaken. Spectrophotometric measurements established that the true catalytic species generating nitric oxide from S-nitrosothiols is Cu(^+), formed by the reduction of copper(II) ions by thiolate, which is present as an impurity in solution. Introduction of the specific cuprous ion chelator neocuproine inhibited reaction, with the concentration of thiol in situ having a significant influence on the absorbance/time traces obtained. Under certain conditions thiolate ions clearly promoted S-nitrosothiol decomposition, whereas at times an opposite effect was noted. These results have been correlated with the reductive ability and chelation properties towards Cu(^2+) of each thiol in question. Structure/reactivity studies were extended further to include a range of S-nitrosated aromatic and heterocyclic thiols which generated the corresponding disulfides in distilled water yet reformed the appropriate thione at pH 7.4, along with nitric oxide in both media. A mechanism has been proposed which accounts for these observations. The reaction of S-nitrosothiols with cupric ions bound to biologically significant molecules such as amino acids, peptides and proteins was followed. Despite Cu(^2+) being chelated in this manner, S-nitrosothiol decomposition was apparent, albeit at a slower rate than that seen when copper(II) sulfate pentahydrate was utilised. Thiolate ions were capable of reducing Cu(^2+) Cu(^+) which was bound to such molecules suggesting a possible mechanism for nitric oxide formation from S-nitrosothiols in vivo. The blue copper protein ceruloplasmin also promoted NO generation under physiological conditions. A brief investigation into the direct reaction of thiolate ion with its corresponding S-nitrosothiol was also carried out. It was discovered that the major reaction product in this instance is ammonia and not nitric oxide, suggesting that a different copper-ion independent process is occurring involving direct interaction between the two species.

No description available.

No description available.

Graphene is a single layer of graphite with many unique mechanical, electrical, and optical properties. In addition, graphene is also known to adsorb wide range of biomolecules including single-stranded DNA. On the other hand, the adsorption of double-stranded DNA was much weaker. To properly disperse in water, graphene oxide (GO) is often used due to its oxygen-containing groups on the surface. Recently, it was discovered that it could efficiently quench the fluorescence of fluorophores that were adsorbed. With these properties, it is possible to prepare DNA-based optical sensors using GO. Majority of the DNA/GO-based fluorescent sensors reported so far were relied on the complete desorption of DNA probes. Even though all these reports demonstrated the sensitivity and selectivity of the system, the fundamentals of binding between DNA and GO were hardly addressed. Understanding and controlling binding between biomolecules and inorganic materials is very important in biosensor development. In this thesis, adsorption and desorption of DNA on the GO surface under different buffer conditions including ionic strength, pH, and temperature were systematically evaluated. For instance, adsorption is favored in a lower pH and a higher ionic strength buffer. It was found that once a DNA was adsorbed on the surface, little desorption occurred even in low salt buffers. Even with high pH or temperature, only small percentage of adsorbed DNA can be desorbed. To completely desorb the DNA, complementary DNA is required. The energies and activation energies associated with DNA adsorption/desorption were measured and molecular pictures of these processes were obtained. With the fundamental understanding of the DNA/GO interaction, we demonstrated that it is possible to achieve sensor regeneration without covalent immobilization. In addition, we also achieved the separation of double-stranded DNAs from single-stranded ones without using gel electrophoresis. We also studied the fluorescence property of DNA near the GO surface using covalently attached DNA probes. It was found that the fluorophore quantum yield and lifetime changed as a function of DNA length. This study is important for rational design of covalently linked DNA sensors. This study confirmed that fluorescence quenching by GO occurs in a distance-dependent manner. Energy transfer occurred between the fluorophore and GO to result in reduced quantum yield, shorter lifetime, and lower fluorescence intensity. Although fluorescent sensors based on covalently attached DNA probes on GO have not yet been reported, the study presented here clearly supported its feasibility.

Riparian soils are thought to be potential hotspots for nitrous oxide (N2O) fluxes from incomplete denitrification, with soil moisture cited as a primary controller, however, because there are multiple potential pathways for N2O production in soils, each with their own environmental regulators, the timing and magnitude of N2O fluxes in difficult to predict. Often empirical observations have failed to yield consistent relationships between environmental factors in lab and field scenarios. This thesis characterizes the hydrological controls (soil moisture, water table depth, and precipitation) on N2O fluxes from different positions on the riparian landscape (dry, loamy upland, and wet, organic lowland) in the field during the growing season. Nitrous oxide and carbon dioxide (CO2) fluxes in the field, as well as environmental and climatic variables, were measured in the field. Over the three year study period N2O fluxes were consistently correlated with soil temperature during the growing season, but not with any hydrological factors. However, direct relationship between soil hydrology and N2O fluxes was more evident on an “episodic” time scales. Lab experiments were used to assess the influence of AHC on N2O production under controlled conditions. Experiment 1 employed intact soil cores collected from the upland and lowland positions of the riparian landscape and the cores were subjected to one of two contrasting moisture regimes (wet-dry-wet or dry-wet-dry). Experiment 2 used homogenized soils from the upland and lowland positions on the landscape to create a multi-factorial experiment that simultaneously altered soil moisture and soil substrate concentrations (nitrate, ammonium, organic carbon). The lab results showed that different AHC resulted in differences to the timing and magnitude of N2O fluxes, and that these patterns differed with soil type. Nitrous oxide production was often correlated with soil moisture in the lowland soils regardless of AHC. The results from Experiment 2 suggested that the upland soils were C limited, which resulted in an unpredictable relationship between soil moisture and N2O production during different AHC. The lowland soils were less affected by AHC as they were not N or C limited like the upland soils. It can be concluded from this research that the relationship between soil moisture and N2O fluxes is influenced by AHC through the influence of AHC on soil N and C dynamics. Given the differences in C and N dynamics between soils types, and the influence of AHC on soil C and N, it can be concluded that a derived relationship between soil moisture and N2O fluxes may not be directly transferable between soil types unless C and N are considered.