published_or_final_version / Paediatric Dentistry / Master / Master of Dental Surgery

Atomically thin Molybdenum disulfide, MoS2, a star member of the group VI transition metal dichalcogenide(TMDC) family has been attracting rising interests for its potential applications in emerging electronics and optoelectronics. Bulk MoS2is a semiconductor with an indirect gap located between the top of valence band at Γ points and the bottom of conduction band in mid of K and Γ points in its Brillouin zone. Atomically thin MoS2 films including monolayers and multilayers, being chemically inert, present a class of intrinsic 2D semiconductors which are widely regarded as a platform for ultimate electronics. As yet tremendous efforts focus on the optical properties and electric transport study. In this thesis, we report the experimental study of photocurrent measurements on MoS2thin films. The sample preparation, device fabrication, optical and electric characterizations are introduced. The experiments have been carried out on a field effect transistor (FET) structured MoS2 device. The photocurrent spectroscopy reveals the interband excitonic transitions at spin-split bands around K valleys. The results demonstrate that MoS2has potential applications in optoelectronics. / published_or_final_version / Physics / Master / Master of Philosophy

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Bioavailability of selenium (Se) in processed tuna and wheat products was studied in humans and rats. The protein source of the rat diets was torula yeast with Se supplied by either raw, precooked or canned tuna, or whole wheat flour, bread or bran. Sodium selenite was used as a control. Each Se source was fed at three levels; 0.05, 0.10 and 0.15 ppm. Using increase in glutathione peroxidase (GSH-Px) activity in various tissues of rats as an indicator of bioavailabiiity, no difference was seen among the three tuna products or among the three wheat products tested. However, significantly lower GSH-Px activity was found in the combined tuna groups as co»pared to the combined wheat groups, suggesting that the Se an wheat was more available than that in tuna. Se concentration m four rat tissues (Liver, Kidney, Whole blood and muscle) was also measured. A significant increase in the liver Se content of rats fed canned tuna over those fed raw or precooked tuna was observed. Since this did not correspond with an increase in GSH-Px activity it was concluded that it did not represent increased bioavaiiability of canned tuna. In the human experiment, eight young men ate controlled diets where the Se was supplied by either whole wheat bread or canned tuna for two week periods. The Se content of the tuna diet was 331.5 ug/day and the bread diet was 354 ug/ day. No difference was observed in whole blood GSH-Px or Se due to the tuna or bread diets but this may be due to the short time period. No significant difference in excretion of Se was observed in the balance study. On the tuna diet, the subjects excreted 72.7% of the Se consumed and on the bread diet they excreted 70.4%. / Graduation date: 1982

In this five-week feeding study male New Zealand rabbits were fed diets containing cyclopropene fatty acids (CPFA), diets containing cholesterol, and diets with both CPFA and cholesterol added to test the effect of CPFA on cholesterol metabolism and atherosclerosis induction. When CPFA-fed animals were compared with control animals, they tended to have higher plasma cholesterol levels, higher liver cholesterol levels and a high incidence of aortic atherosclerosis. Control animals had no atherosclerosis. A similar pattern was seen when animals fed cholesterol were compared with those fed both cholesterol and CPFA. Mean triglyceride levels were higher in CPFA-fed rabbits than in controls. A wide range of sensitivity to the CPFA was observed. Some animals had plasma cholesterol levels eight to ten times normal after three weeks of feeding; others showed only slight changes after five weeks. Histological examination of liver cells from CPFA-fed animals showed evidence of alteration in cellular morphology and, in some cases, extensive damage. In some liver cells of CPFA-fed rabbits, subcellular organelles appear to be aligned in fiber-like structures. The most extreme signs of toxicity were noted in livers of rabbits fed CPFA and cholesterol. After five weeks on experimental diets, adipose tissue of the rabbits contained approximately 2% CPFA. Ratios of 16:0 to 16:1 and 18:0 to 18:1 fatty acids increased in liver lipid and erythrocyte ghost lipid after CPFA feeding. Over the five-week study there were no differences in rate of weight gain or in feed conversion ratios in CPFA-fed animals. / Graduation date: 1974

Normal human skin fibroblast cells were used to study the effect of zinc supplementation of the media on cell growth and the competitive binding activity of low density lipoprotein (LDL). Cells were grown in the media containing Dulbecco's Modified-Eagle Medium (DMEM), 5% (v/v) fetal calf serum (FCS), and various levels of zinc. Cell counts and protein determination revealed that there was no stimulatory effect of zinc on the growth of cells, showing a flat growth curve with up to 6 μg/ml zinc supplementation. However, zinc supplementation of greater than 6 μg/ml to the medium appeared to be toxic to the cell and thereby prevented growth. When zinc was removed from the medium using Epoxy-activated Sepharose 6B coupled with iminodiacetate, zinc concentration in the medium was markedly reduced to 0.045 μg/ml from 0.210 μg/ml. The cell growth study using this zinc depleted medium exhibited a growth curve similar to that obtained from the earlier study, suggesting that 0.045 μg/ml of zinc in the control medium was still sufficient to support normal cell growth. For the LDL binding study, cells were grown in the media with various levels of zinc supplementation for 7 days and the competitive binding activity of LDL was determined. When cells were grown in the zinc removed medium with 1.5 μg/ml zinc supplementation, the maximum amounts of ¹²⁵I-LDL bound and internalized in the cells were observed; however, higher levels of zinc supplementation to the growth medium caused decreased ¹²⁵I-LDL binding to the cell receptors. These results suggest that zinc may be involved in the binding of LDL to the receptors. / Graduation date: 1983

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In response to a variety of hyperthermic treatments, rat embryonic fibroblasts synthesize heat shock proteins (hsps), including those with molecular weights of 68,000 (hsp 68), 70,000 (hsp 70) and 89,000 (hsp 89). Hyperthermic stresses, which produce the hsps, also cause expression of thermotolerance. The dependence of thermotolerance expression on hsp synthesis was investigated in this mammalian cell line under different heating conditions. Temperature shift experiments showed that hsp synthesis and thermotolerance expression were dependent not only on the absolute hyperthermic temperature, but also on the difference between the initial incubation temperature and the hyperthermic temperature. Small temperature differences which produced no cell killing did not cause detectable synthesis of hsp 68. Increasing the difference of the initial and hyperthermic temperatures reduced cell survival and increased the synthesis of hsp 68. Thermotolerance could be expressed by surviving cells following an initial heat stress even when both heat shock and general protein synthesis were inhibited. Cells exposed to cycloheximide were heated, incubated at their initial temperature for six hours and reheated in the presence of the drug. The inhibitor was then removed and the cells plated for colony formation. The hsps were expressed during this latter incubation period. The regulation of hsp 70 in rat fibroblasts was investigated next. Hsp 70 synthesis rates correlated with the amount of hsp 70 encoding mRNA. The time course of heat shock synthesis and general protein synthesis recovery were each dependent on the duration of the heat stress. Inhibiting protein synthesis with cycloheximide resulted initially in the accumulation of the RNA encoding hsp 70 but did not effect the normal turnover of this RNA species. The conclusions based on these findings are that thermal survival adaptation can be expressed in the absence of hsp 68 synthesis. Hsp 68 is expressed by cells that will ultimately die (see Chapter 2). The hsps do not appear to protect cells against subsequent heat stress. They may function in a repair capacity (see Chapter 3). Hsp 70 expression is primarily regulated by transcription in Rat-1 cells. Hsp 70 does not act to regulate its own turnover (see Chapter 4).

Results of two studies are reported here, a greenhouse study and a field study. In the greenhouse study, dry matter yield and nitrogen (total and 15N) uptake of leaves, stems and roots of tomato plants (Lycopersicum esculentum Mill., cv. Columbia) subjected to saline stress by NaCl were studied. The integrated effects of responses of these tissues to salinity on the whole plant basis and levels of Na⁺, Cl⁻ and K⁺ accumulation in these tissues were also studied. The treatments consisted of low (control, 0.3 bar), medium (4.3 bars), and high (8.3 bars) salinity. The saline treatments were prepared by adding NaCl to nutrient solution in sand culture. The plants were 80 days old at the start of the treatments and each was in a pot containing 1.8 kg of quartz sand. The ¹⁵N was provided to plants by adding K¹⁵NO₃ to the pots and the 15N treatment continued with the saline treatments up to 30 days. The plants were harvested at each 5-day interval during the treatment period. Dry matter production and nitrogen (total and ¹⁵N) uptake were significantly lower for saline treatments as compared with the control. Differences in dry matter production and ¹⁵N uptake on whole plant basis appeared in the latter part of the treatment period between the two saline treatments. For most of the parameters studied, the leaves were found to be affected most by salinity, the roots were intermediate in their response and the stems were the least affected by salinity. The effect of salinity on the studied parameters were attributed to osmotic effects and specific ion effects of Na⁺ and/or Cl⁻. A field study with two cultivars--Columbia and Pearson was conducted at the Safford Agricultural Center. Three N treatments were used: 0 kg N/ha, 84 kg N/ha and 168 kg N/ha and two treatments consisting of two water sources--river water with an EC of 1.15 dS/m and more saline well water of EC of 2.21 dS/m. Columbia had a significantly higher yield of tomatoes than Pearson for both water types. The N treatments had no effect on tomato yield apparently due to high residual N remaining in the field from the previous crop. Commercially acceptable fresh market yields were approached with both varieties and waters in spite of moderate salinity and sodium under heavy textural soil conditions, high temperatures and the presence of certain diseases in the area.

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