Insight on the silver catalyst distribution during silicon nanowire array formation: an X-ray reflectivity study.

Although metal-catalysts are commonly used to create nanoscale materials at surfaces, little is quantitatively known or understood about the depth distribution profile of the catalyst during the growth process. Using X-ray reflectivity, we report the first quantitative investigation, with nanoscale resolution, of the Ag metal-catalyst depth distribution profile during metal-assisted chemical etch (MACE) growth of Si nanowire (SiNW) arrays on Si(100). Given the very low optical reflectivity of these nanowire arrays, specular reflection from these materials in the X-ray region is extremely challenging to measure because it probes interfaces on the nanoscale. Nevertheless, we demonstrate that with suitable investigation, X-ray specular reflection can be measured and utilized to obtain unique structural information about the composition profile of both Ag and Si. The measurements, which also include X-ray diffraction and complementary electron microscopy, reveal that the Ag nanoparticles distribute along the length of the nanowires upon etching with a Ag density that increases towards the etch front. The Ag nanoparticles coarsen with etch time, indicating a high mobility of Ag ions even though we also find that the Ag does not migrate from the SiNW region into the etch bath during etching. The Ag density gradient and the Ag mobility suggest the existence of a strong chemical force that attracts Ag towards the etch front. These results provide unique and important new insight into the growth process for creating SiNWs from wet chemical etching using metal-catalysts.

Ultra-sensitive and selective NH3 room temperature gas sensing induced by manganese-doped titanium dioxide nanoparticles.

The study of the fabrication of ultra-high sensitive and selective room temperature ammonia (NH3) and nitrogen dioxide (NO2) gas sensors remains an important scientific challenge in the gas sensing field. This is motivated by their harmful impact on the human health and environment. Therefore, herein, we report for the first time on the gas sensing properties of TiO2 nanoparticles doped with various concentrations of manganese (Mn) (1.0, 1.5, 2.0, 2.5 and 3.0mol.% presented as S1, S2, S3, S4 and S5, respectively), synthesized using hydrothermal method. Structural analyses showed that both undoped and Mn-doped TiO2 crystallized in tetragonal phases. Optical studies revealed that the Mn doped TiO2 nanoparticles have enhanced UV→Vis emission with a broad shoulder at 540nm, signifying induced defects by substituting Ti4+ ions with Mn2+. The X-ray photoelectron spectroscopy and the electron paramagnetic resonance studies revealed the presence of Ti3+ and singly ionized oxygen vacancies in both pure and Mn doped TiO2 nanoparticles. Additionally, a hyperfine split due to Mn2+ ferromagnetic ordering was observed, confirming incorporation of Mn ions into the lattice sites. The sensitivity, selectivity, operating temperature, and response-recovery times were thoroughly evaluated according to the alteration in the materials electrical resistance in the presence of the target gases. Gas sensing studies showed that Mn2+ doped on the TiO2 surface improved the NH3 sensing performance in terms of response, sensitivity and selectivity. The S1 sensing material revealed higher sensitivity of 127.39 at 20 ppm NH3 gas. The sensing mechanism towards NH3 gas is also proposed.

An in ovo investigation into the hepatotoxicity of cadmium and chromium evaluated with light- and transmission electron microscopy and electron energy-loss spectroscopy.

Excessive agriculture, transport and mining often lead to the contamination of valuable water resources. Communities using this water for drinking, washing, bathing and the irrigation of crops are continuously being exposed to these heavy metals. The most vulnerable is the developing fetus. Cadmium (Cd) and chrome (Cr) were identified as two of the most prevalent heavy metal water contaminants in South Africa. In this study, chicken embryos at the stage of early organogenesis were exposed to a single dosage of 0.430 µM physiological dosage (PD) and 430 µM (×1000 PD) CdCl2, as well as 0.476 µM (PD) and 746 µM (×1000 PD) K2Cr2O7. At day 14, when all organ systems were completely developed, the embryos were terminated and the effect of these metals on liver tissue and cellular morphology was determined with light- and transmission electron microscopy (TEM). The intracellular localization of these metals was determined using electron energy-loss spectroscopy (EELS). With light microscopy, the PD of both Cd and Cr had no effect on liver tissue or cellular morphology. At ×1000 PD both Cd and Cr caused sinusoid dilation and tissue necrosis. With TEM analysis, Cd exposed hepatocytes presented with irregular chromatin condensation, ruptured cellular membranes and damaged or absent organelles. In contrast Cr caused only slight mitochondrial damage. EELS revealed the bio-accumulation of Cd and Cr along the cristae of the mitochondria and chromatin of the nuclei.

Synthetic Hemozoin (ß-Hematin) Crystals Nucleate at the Surface of Neutral Lipid Droplets that Control Their Sizes.

Emulsions of monopalmitoylglycerol (MPG) and of a neutral lipid blend (NLB), consisting of MPG, monostearoylglycerol, dipalmitoylglycerol, dioleoylglycerol and dilineoylglycerol (4:2:1:1:1), the composition associated with hemozoin from the malaria parasite Plasmodium falciparum, have been used to mediate the formation of ß-hematin microcrystals. Transmission electron microscopy (TEM), electron diffraction and electron spectroscopic imaging/electron energy loss spectroscopy (ESI/EELS) have been used to characterize both the lipid emulsion and ß-hematin crystals. The latter have been compared with ß-hematin formed at a pentanol/aqueous interface and with hemozoin both within P. falciparum parasites and extracted from the parasites. When lipid and ferriprotoporphyrin IX solutions in 1:9 v/v acetone/methanol were thoroughly pre-mixed either using an extruder or ultrasound, ß-hematin crystals were found formed in intimate association with the lipid droplets. These crystals resembled hemozoin crystals, with prominent {100} faces. Lattice fringes in TEM indicated that these faces made contact with the lipid surface. The average length of these crystals was 0.62 times the average diameter of NLB droplets and their size distributions were statistically equivalent after 10 min incubation, suggesting that the lipid droplets also controlled the sizes of the crystals. This most closely resembles hemozoin formation in the helminth worm Schistosoma mansoni, while in P. falciparum, crystal formation appears to be associated with the much more gently curved digestive vacuole membrane which apparently leads to formation of much larger hemozoin crystals, similar to those formed at the flat pentanol-water interface.