Effects of Alpha-Tocopherol Antioxidant on the Bonding Properties of Resin Adhesive to Dentin Bleached with Sodium Perborate

Title from PDF of title page, viewed August 29, 2017Thesis advisor: Mary P. Walker and Yong WangVitaIncludes bibliographical references (pages 28-30)Thesis (M.S.)--School of Dentistry. University of Missouri--Kansas City, 2017Previous reports have shown diminished capacity for bonding after bleaching teeth. Reversal of the bond strengths back to pre-bleached levels can be obtained with application of 10% alpha-tocopherol in a 2-hour time frame, or by delaying bonding for two weeks. This study evaluated the effectiveness of a 5-minute application of 20% alpha-tocopherol to reverse the deleterious effect of nonvital bleaching on dentin bonding. Thirty third molars were prepared to obtain a flat dentin surface and assigned to three groups: unbleached, bleached, and bleached followed by 5-minute treatment with 20% alpha-tocopherol. The dentin surfaces of the bleached groups were exposed to sodium perborate (2 g/mL) for seven days. The post-bleach treatment group was subsequently treated with 20% alpha tocopherol for 5 minutes, then all groups were restored with restorative composite. After 24 hour storage at 37oC and 100% humidity, restored tooth specimens were sectioned into 1 mm2 dentin-composite beams. Four to six beams from each tooth were subjected to microtensile bond strength testing. Following microtensile testing, 2 beams from representative specimens were further evaluated with Raman microspectroscopy for depth of penetration and degree of conversion of adhesive resin. Mean bond strength values (MPa) for each group: unbleached control=26.2, bleached control=20.3, post-bleach treatment group=18.5. A 1-factor ANOVA and Tukey post hoc test (α=0.05) indicated that bleaching had a detrimental effect on bond strength and that short-term alpha-tocopherol treatments did not improve post-bleach bond strength. While Raman microspectroscopy revealed depth of penetration and degree of conversion for the post-bleach alpha-tocopherol group were similar to the bleached control, both values were markedly lower than the unbleached control group. Collectively, the results suggest that the application of 20% alpha-tocopherol as a post-bleach treatment in a clinically relevant time frame was not effective in counteracting the deleterious effect of bleaching on bond strength, and composite resin bonding procedures should be delayed following tooth bleaching.Introduction -- Materials and methods -- Results -- Discussion -- Conclusion

Effects of Bulk and Surface Conductivity on the Performance of CdZnTe Pixel Detectors

We studied the effects of bulk and surface conductivity on the performance of high-resistivity CdZnTe (CZT) pixel detectors with Pt contacts. We emphasize the difference in mechanisms of the bulk and surface conductivity as indicated by their different temperature behaviors. In addition, the existence of a thin (10-100 A) oxide layer on the surface of CZT, formed during the fabrication process, affects both bulk and surface leakage currents. We demonstrate that the measured I-V dependencies of bulk current can be explained by considering the CZT detector as a metal-semiconductor-metal system with two back-to-back Schottky-barrier contacts. The high surface leakage current is apparently due to the presence of a low-resistivity surface layer that has characteristics which differ considerably from those of the bulk material. This surface layer has a profound effect on the charge collection efficiency in detectors with multi-contact geometry; some fraction of the electric field lines originated on the cathode intersects the surface areas between the pixel contacts where the charge produced by an ionizing particle gets trapped. To overcome this effect we place a grid of thin electrodes between the pixel contacts; when the grid is negatively biased, the strong electric field in the gaps between the pixels forces the electrons landing on the surface to move toward the contacts, preventing the charge loss. We have investigated these effects by using CZT pixel detectors indium bump bonded to a custom-built VLSI readout chip

Greenhouse gas balance over thaw-freeze cycles in discontinuous zone permafrost

Peat in the discontinuous permafrost zone contains a globally significant reservoir of carbon that has undergone multiple permafrost-thaw cycles since the end of the mid-Holocene (~3700 years before present). Periods of thaw increase C decomposition rates which leads to the release of CO2 and CH4 to the atmosphere creating potential climate feedback. To determine the magnitude and direction of such feedback, we measured CO2 and CH4 emissions and modeled C accumulation rates and radiative fluxes from measurements of two radioactive tracers with differing lifetimes to describe the C balance of the peatland over multiple permafrost-thaw cycles since the initiation of permafrost at the site. At thaw features, the balance between increased primary production and higher CH4 emission stimulated by warmer temperatures and wetter conditions favors C sequestration and enhanced peat accumulation. Flux measurements suggest that frozen plateaus may intermittently (order of years to decades) act as CO2 sources depending on temperature and net ecosystem respiration rates, but modeling results suggest that—despite brief periods of net C loss to the atmosphere at the initiation of thaw—integrated over millennia, these sites have acted as net C sinks via peat accumulation. In greenhouse gas terms, the transition from frozen permafrost to thawed wetland is accompanied by increasing CO2 uptake that is partially offset by increasing CH4 emissions. In the short-term (decadal time scale) the net effect of this transition is likely enhanced warming via increased radiative C emissions, while in the long-term (centuries) net C deposition provides a negative feedback to climate warming

Method for Measurement of Viral Fusion Kinetics at the Single Particle Level

Membrane fusion is an essential step during entry of enveloped viruses into cells. Conventional fusion assays typically report on a large number of fusion events, making it difficult to quantitatively analyze the sequence of the molecular steps involved. We have developed an in vitro, two-color fluorescence assay to monitor kinetics of single virus particles fusing with a target bilayer on an essentially fluid support

Novel liquid crystalline organic semiconducting oligomers incorporating N-heterocyclic carbazole moieties for fluorescent OLEDs

A novel class of nematic liquid crystalline organic semiconducting oligomers incorporating N-heterocyclic carbazole moieties has been synthesised using simple and highly efficient reaction pathways. The electroluminescent colour of these novel oligomers can be varied in a controlled manner by molecular design. The values of the ionization potential and the electron affinity of these electroluminescent oligomers can also be matched by structural design to the HOMO energy level of the electron-blocking layer and the LUMO energy level of electron-transporting layer in the OLEDs to create low charge-injection barriers for electrons and holes, respectively leading to electroluminescence with an efficacy up to 4.1 cd A-1

Properties of Pt Schottky Type Contacts On High-Resistivity CdZnTe Detectors

In this paper we present studies of the I-V characteristics of CdZnTe detectors with Pt contacts fabricated from high-resistivity single crystals grown by the high-pressure Brigman process. We have analyzed the experimental I-V curves using a model that approximates the CZT detector as a system consisting of a reversed Schottky contact in series with the bulk resistance. Least square fits to the experimental data yield 0.78-0.79 eV for the Pt-CZT Schottky barrier height, and <20 V for the voltage required to deplete a 2 mm thick CZT detector. We demonstrate that at high bias the thermionic current over the Schottky barrier, the height of which is reduced due to an interfacial layer between the contact and CZT material, controls the leakage current of the detectors. In many cases the dark current is not determined by the resistivity of the bulk material, but rather the properties of the contacts; namely by the interfacial layer between the contact and CZT material.Comment: 12 pages, 11 figure