Biomedical subjects

Q Li

Publications and source records attributed to Q Li.

At least 37 records · Page 2Linked to original sources

PubMed2006

The C-terminal peptide of connective tissue growth factor blocks the full molecule binding to tubular epithelial cell.

OBJECTIVES: Interstitial fibrosis is a critical pathologic change in chronic allograft nephropathy. The cytokine connective tissue growth factor (CTGF, also CCN2) plays an important role in epithelial-mesenchymal transformation (EMT) of tubular epithelial cells to renal interstitial fibrosis. The hexadeca-peptide within the C-terminal of CTGF (named P2) contains the unique binding domain of CTGF to its potential receptor, integrin alphavbeta3. This study examined whether P2 bound preferentially to the receptor and served as an inhibitor of CTGF. METHODS: All studies used an established rat kidney tubular epithelial cell line NRK-52E. Chemically synthesized P2 was purified, and some of it labeled with FITC. The affinity of CTGF or P2 to NRK-52E cells was examined by a solid-phase cell adhesion assay. Competitive binding between P2 and CTGF to NRK-52E cells was examined with flow cytometric analysis. RESULTS: Both P2 and CTGF bound to the NRK-52E cells, mediating cell adhesion. When the cells were incubated in the mixture of P2 and CTGF, P2 bound to the cells preferentially. Furthermore, when cells were preincubated with excessive CTGF, it became difficult for subsequent P2 binding to occur. CONCLUSIONS: P2 and CTGF seemed to bind to cell membranes at the same binding domain. P2 competitively blocked CTGF binding, acting as a CTGF inhibitor.

Animals↗

PubMed2006

Simvastatin down regulates mRNA expression of RANTES and CCR5 in posttransplant renal recipients with hyperlipidemia.

Chemokines and hyperlipidemia are involved in the mechanism of chronic allograft nephropathy (CAN). In this study, the mRNA expression of RANTES and its receptor CCR5 on peripheral blood mononuclear cells were measured in renal transplant recipients with hyperlipidemia, and the effect of simvastatin treatment observed to investigate the mechanism and prevention of CAN. Sixty recipients selected from 167 renal transplant recipients were divided into two groups: group A without hyperlipidemia (n = 30) and group B with hyperlipidemia (n = 30). The control group consisted of 30 healthy volunteers. The recipients in group B were treated with simvastatin for 3 months. We estimated serum lipid levels and mRNA expressions of RANTES and CCR5. The mRNA expressions of RANTES and CCR5 were significantly higher in renal transplant recipients compared with controls. The expressions were much higher in group B than in group A patients. In group B patients, serum lipid levels decreased dramatically after simvastatin treatment. Meanwhile, the mRNA expressions of RANTES and CCR5 were reduced significantly after 1.5 months of simvastatin treatment to a level significantly lower than that in group A after 3 months of treatment. The increased expressions of RANTES and CCR5 mRNAs in renal transplant recipients with hyperlipidemia might be involved in CAN due to hyperlipidemia. Simvastatin seemed to reduce the chemokine transcripts in renal recipients with hyperlipidemia.

Adult↗

PubMed2006

Unrelated donor bone marrow transplantation for beta-thalassemia major: an experience from China.

Although related bone marrow transplantation (BMT) is effective for thalassemia, less than 30% of patients have sibling donors. Here, we report unrelated BMT in nine thalassemic children using a high-resolution HLA typing technique to identify donors. HLA mismatches between donors and recipients were 0, 1 and 2 in 2, 5 and 2 cases, respectively. The results showed that white blood cells, platelets and hemoglobin all returned to normal at various time points, and blood transfusion was eliminated from 13 to 62 days after transplantation. Full engraftment was achieved in eight patients while ABO blood types were replaced with that of donors in five of the six ABO mismatched recipients. Acute skin GVHD was found in seven patients and acute liver GVHD in one. One patient with acute intestinal GVHD eventually developed chronic GVHD. One patient died of pulmonary hemorrhage in spite of having a fully functional graft. We conclude that this is the first successful application of unrelated BMT for thalassemia major in Chinese people and that the results will certainly expand donor resources and greatly enhance the survival and quality of life of thalassemic patients.

ABO Blood-Group System↗

PubMed2006

Changes and relations of circulating visfatin, apelin, and resistin levels in normal, impaired glucose tolerance, and type 2 diabetic subjects.

Visfatin and apelin are two novel adipocyte- secreted hormone proposed to link obesity with insulin resistance. In this study we investigated whether plasma visfatin and apelin levels were altered in normal, impaired glucose tolerance, and type 2 diabetic subjects. We also assessed the association between plasma visfatin, or apelin and body composition, metabolic parameters, and resistin concentrations in these subjects. The visfatin levels of fasting and 2-h post-glucose load were found to be significantly decreased in diabetics compared with the controls ( P<0.05). In contrast, basal apelin levels were significantly increased in the IGT and diabetic subjects compared with the controls ( P<0.05 and P<0.01). The apelin levels of 2-h post-glucose load were significantly higher than the basal levels in every group (all P<0.05). Fasting plasma visfatin was found to correlate positively and significantly with BMI, WHR, and fasting plasma resistin, but negatively with HbA1c and 2 h OGTT glucose. Multiple regression analysis showed that WHR, HbA1c, 2 h OGTT glucose were independent related factors influencing plasma visfatin levels. Fasting plasma apelin levels correlated positively with HOMA-IR, BMI, TC, LDL-C, FBG and Fasting plasma insulin. Multiple regression analysis also showed that HOMA-IR, BMI, and TC were independent related factors influencing plasma apelin levels. The present work indicates the potential link of visfatin and apelin with the pathogenesis of insulin resistance and T2DM.

Adult↗

PubMed2006

Individual-level predictors of cessation behaviours among participants in the International Tobacco Control (ITC) Four Country Survey.

BACKGROUND: The International Tobacco Control (ITC) Four Country Survey (ITC-4) is a prospective cohort study designed to evaluate the psychosocial and behavioural impact of national-level tobacco control policies enacted in the Australia, Canada, the UK, and the USA. Wave 1 of ITC-4 survey was conducted between October 2002 and December 2002. Wave 2 survey was conducted between May 2003 and August 2003. OBJECTIVE: To test for individual-level predictors of smoking cessation behaviours (that is, quit attempts and smoking cessation) among cigarette smokers in the ITC Four Country Study measured between Wave 1 and Wave 2. This set of predictors will serve as the base for evaluating the added effect of tobacco control policies and other factors. METHODS: Respondents included in this study are 6682 adult current smokers in the Wave 1 main survey who completed the Wave 2 follow-up (1665 were in Canada, 1329 were in the USA, 1837 were in the UK and 1851 were in Australia). RESULTS: Factors predictive of making a quit attempt included intention to quit, making a quit attempt in the previous year, longer duration of past quit attempts, less nicotine dependence, more negative attitudes about smoking, and younger age. Lower levels of nicotine dependence were the main factor that predicted future cessation among those that made a quit attempt. CONCLUSION: Intention to quit and other cognitive variables were associated with quit attempts, but not cessation. Behavioural variables related to task difficulty, including measures of dependence, predicted both making attempts and their success. Predictors of making quit attempts and cessation were similar for each of the four countries, but there were some differences in predictors of success.

Adolescent↗

PubMed2006

Postoperative transhepatic arterial chemoembolization and portal vein chemotherapy for patients with hepatocellular carcinoma: a randomized study with 131 cases.

OBJECTIVE: To explore the value of postoperative transarterial chemoembolization (TACE) and portal vein chemotherapy (PVC) in patients with hepatocellular carcinoma (HCC) in the prevention of recurrence, and to evaluate prognostic factors in a randomized study. METHOD: 131 patients with HCC were randomly divided into 3 groups: operation only (group A, n = 45); operation plus TACE (group B, n = 39), and operation plus TACE and PVC (group C, n = 47). Disease-free survival rates as well as prognostic factors were analyzed. RESULTS: Most of the side effects and complications related to the operation, catheters and local chemotherapy were liver decompensation (16.1%), catheter obstruction (12.9%), and nausea and loss of appetite (25.8%), respectively. The disease-free survival curves were significantly different between the 3 groups as estimated by the Kaplan-Meier method (p < 0.05). Group C had a significantly higher disease-free survival rate compared to group A (p < 0.05). But no statistical differences were found between groups A and B and groups B and C (both p > 0.05). Tumor number and treatment modalities were independent prognostic factors for HCC patients (p < 0.05). CONCLUSION: Postoperative TACE combined with PVC may benefit the survival of patients with HCC. In specialized medical centers, aggressive methods such as TACE and PVC should be attempted on HCC patients without contraindications.

Antineoplastic Combined Chemotherapy Protocols↗

PubMed2006

Correlation of four vascular specific growth factors with carcinogenesis and portal vein tumor thrombus formation in human hepatocellular carcinoma.

The aim of the present study was to detect the correlation between the expression of vascular endothelial growth factor (VEGF), angiopoietin 2 (Ang2), ephrinB2 and endocrine gland-derived vascular endothelial growth factor (EG-VEGF) and carcinogenesis or portal vein tumor thrombus (PVTT) formation in human hepatocellular carcinoma (HCC). The expression of VEGF, Ang2, ephrinB2 and EG-VEGF was detected by RT-PCR in 54 cases HCC without PVTT (group A), 9 cases HCC with PVTT (group B), 10 normal liver tissues (group D) and 10 cirrhosis tissues (group C). The samples were also stained with CD34 by immunohistochemistry. Quantitation of microvessel density (MVD) and semi-quantitation of VEGF, Ang2, ephrinB2 and EG-VEGF expression were analyzed to find the relations. The MVD was 146.69 +/- 77.79, 214.07 +/- 54.41, 32.85 +/- 8.49 and 34.83 +/- 8.29 in group A-D respectively with significant difference (F = 19.77, P = 0.000). The MVD in group A was higher than that in group C P = 0.006, but lower than that in group B P < or = 0.05 or 0.01. The expression levels of VEGF165, VEGF189, Ang2 and EG-VEGF mRNA were significantly different among the groups. The expression levels of VEGF165, Ang2 and EG-VEGF mRNA in group A were all higher than those in group C, but lower than those in group B P < 0.05 or 0.01. The MVD was significantly correlated with VEGF165, VEGF189, Ang2 and EG-VEGF mRNA with Spearman's related coefficient being 0.764, 0.510, 0.640 and 0.366 in HCC (P = 0.000, 0.000 0.000 and 0.003). In conclusion VEGF, Ang2 and EG-VEGF mRNA may play a role in angiogenesis and carcinogenesis of HCC. They can promote PVTT formation in HCC by modulating angiogenesis.

Angiopoietin-2↗

PubMed2005

Molecular simulation of RMM: ordered mesoporous SBA-15 type material having microporous ZSM-5 walls.

SBA-15 is a novel porous material with uniform size mesopores arranged in a regular pattern. The adjacent mesopores are connected to each other by microporous walls. The major disadvantages of these materials are that the walls are amorphous and have low thermal, hydrothermal, and mechanical stability. Recently, there have been a few attempts to either coat the walls of SBA-15 by microporous crystalline zeolites or to fabricate SBA-15 using CMK-3 in such a way that the walls are made up of ZSM-5. The present work provides a first-ever study of RMM (replicated mesoporous materials, which are SBA-15 like ordered mesoporous materials with walls made up of ZSM-5) using molecular modeling. A random orientation of the unit cells and the distribution of sizes of the supercells located at nucleation sites would be ideal to model the RMM. However, such a study would introduce more uncertainties with regard to voids between the individual supercells, noncrystalline silica, and the location of active sites where the nucleation occurs. In a simpler model studied in the present work, the walls of SBA-15 were made up of regularly arranged ZSM-5 having the same orientation. The structure was characterized by estimating the nitrogen accessible area/volume by Connolly surfaces, small-angle and wide-angle X-ray diffraction patterns, methane adsorption, and ice as a probe to study the pore structure. It was found that RMMs have significantly higher methane adsorption capacity compared to SBA-15 and the majority of methane is adsorbed in the microporous walls of RMM. Further research in the field of RMM is needed to obtain the details of zeolitic wall structure.

Journal Article↗

PubMed2005

Pre-protachykinin A mRNA is colocalized with tyrosine hydroxylase-immunoreactivity in bulbospinal neurons.

Previous studies have generated controversy about the extent of co-localization between substance P- and catecholamine-containing neurons that project to the spinal cord. In earlier studies, estimates using immunofluorescence after colchicine have ranged from almost all, to almost none. We sought to resolve this issue by combining in situ hybridization and immunofluorescence. Catecholamine (A1 to A7, C1 to C3; tyrosine hydroxylase immunoreactive) neurons in the rat brainstem were examined to determine their content of mRNA for the preprotachykinin-A gene. In the A1 to A7 and the C1 to C3 cell groups, preprotachykinin-A mRNA was found only in substantial amounts in the C1-C3 cell groups. On average 20.9+/-0.9% (234/1120, n=3) of rostral C1 neurons contained preprotachykinin-A mRNA. Co-localization was also observed in C2 and C3 neurons to a similar extent. Retrograde tract-tracing with cholera toxin B subunit was used to identify bulbospinal neurons and 17.9+/-2.7% (96/529 cells) of the bulbospinal tyrosine hydroxylase-containing neurons of the rostral C1 cell group were found to contain preprotachykinin-A mRNA. In addition a new population of non-catecholaminergic bulbospinal preprotachykinin-A neurons is described in an area corresponding to the recently described caudal pressor area. To confirm that the preprotachykinin-A mRNA observed in cells in the medulla was converted to protein, dual immunofluorescence for fiber labeling at the confocal level was carried out. This confirmed colocalization of substance P and tyrosine hydroxylase in the intermediolateral cell column, but nowhere else, in a small number of cases. The results provide evidence for a much larger population of substance P/neurokinin A containing neurons in the brainstem than was previously suspected. Furthermore, many of these neurons are catecholaminergic and spinally projecting. The specific sympathetic outflow that these neurons influence remains to be determined.

Animals↗

PubMed2005

Is downregulation of MHC class I antigen expression in human non-small cell lung cancer associated with prolonged survival?

PURPOSE: To characterize HLA class I antigen expression in non-small cell lung cancer (NSCLC) lesions, and to assess the clinical significance of these molecules' downregulation. METHODS: One hundred and ninety primary formalin fixed, paraffin embedded NSCLC lesions were stained with HLA class I heavy chain-specific mAb HC-10. Results were scored as percentage of stained tumor cells and categorized into three groups: 0-24% (negative), 25-75% (heterogeneous) and >75% (positive). HLA class I antigen expression was correlated with clinical and pathologic predictors of time to progression and survival and analyzed using the chi-square test. Association between HLA class I antigen expression and survival was assessed using Cox regression models, while controlling for confounders. RESULTS: HLA class I antigen expression was negative, heterogeneous and positive in 153, 25 and 12 primary NSCLC lesions, respectively. Independent variables significantly associated with survival included tumor stage, PS and weight loss. The median survival times were 40.6, 44.0 and 17.9 months for patients with a HLA class I antigen expression scored as negative, heterogeneous and positive, respectively. CONCLUSION: HLA class I antigen defects were found with high frequency (93.6%) in NSCLC lesions. HLA class I antigen downregulation was associated with improved survival, although this association was not statistically significant. These results, which parallel similar findings in uveal melanoma and in breast carcinoma, raise the possibility that NK cells may play a role in the control of NSCLC tumors.

Adult↗

PubMed2005

Polyunsaturated eicosapentaenoic acid changes lipid composition in lipid rafts.

BACKGROUND: Polyunsaturated fatty acids (PUFAs) modulate immune responses particularly by affecting T cell function and are applied clinically as adjuvant immunosuppressants in the treatment of various inflammatory diseases. However, the molecular mechanisms of PUFA-induced immunosuppressive effects are not yet elucidated. Membrane lipid rafts are functional plasma membrane microdomains characterized by a unique lipid environment. Since lipid interactions are crucial for the formation of lipid rafts, the immunomodulatory effects of PUFAs may be due to changes of fatty acid composition in lipid rafts. AIM OF THE STUDY: We investigated the effects of eicosapentaenoic acid (EPA, 20:5 n - 3) supplementation on modulating lipid composition and fatty acyl substitution in their cytoplasmic and exoplasmic lipid leaflet in lipid rafts. METHODS: The human Jurkat E6-1 T cells were cultured in EPA-supplemented medium and the cells treated with stearic acid served as a control. Lipid rafts were isolated by discontinuous sucrose density gradient ultracentrifugation. The lipids in raft and soluble fractions from EPA-treated and control T cells were extracted and separated by gas chromatography. Raft phospholipids were analyzed by mass spectrometry. RESULTS: Our results showed that EPA treatment could alter lipid composition resulting in a considerable increase of unsaturated fatty acyl chains in lipid rafts from EPA-treated T cells compared with control cells. Effective incorporation of EPA to rafts was not only in the exoplasmic but also in the cytoplasmic membrane lipid leaflet. EPA treatment altered the lipid environment in lipid rafts. EPA presented an inhibiting effect on Jurkat T cells proliferation and inhibited IL-2Ralpha expression on the surface of T cells. CONCLUSIONS: Our data provided evidence for an important modification in lipid composition of membrane lipid rafts and T cell function by EPA supplementation.

Cell Division↗

PubMed2005

Effects of carnosine on amygdaloid-kindled seizures in Sprague-Dawley rats.

The effects of carnosine (beta-alanyl-L-histidine) on amygdaloid-kindled seizures were investigated in rats. I.p. injection of carnosine (500, 1000, 1500 mg/kg, i.p.) significantly decreased seizure stage, afterdischarge duration and generalized seizure duration, and significantly prolonged generalized seizure latency of amygdaloid-kindled seizures, in a dose-dependent, and time-related manner. The protective effect of carnosine (1500 mg/kg) was completely antagonized by histamine H1-antagonists pyrilamine (2, 5 mg/kg, i.p.) and diphenhydramine (5, 10 mg/kg, i.p.), but not by histamine H2-antagonist zolantidine even at a high dose of 10 mg/kg. Carnosine (1500 mg/kg, i.p.) caused a significant increase of carnosine and histidine levels in the hypothalamus, thalamus, hippocampus, amygdala and cortex, as well as histamine levels in the hippocampus and amygdala. I.c.v. injection of alpha-fluoromethylhistidine (50 microg, i.c.v.), a selective and irreversible histidine decarboxylase inhibitor, only partially reversed the inhibition of amygdaloid-kindled seizures induced by carnosine. In addition, carnosine significantly decreased glutamate contents in the amygdala and hippocampus. These results indicate that carnosine could protect against amygdaloid-kindled seizures in rats, and its action may be due to the activation of histamine postsynaptic H1-receptors via two different mechanisms, one being carnosine's direct action, and the other being indirectly mediated by histaminergic pathway. The study suggests that carnosine may be an endogenous anticonvulsant factor in the brain and could be used as a new antiepileptic drug in the future.

Amygdala↗

PubMed2005

Guest-host encapsulation of microporous zeolites in ordered mesoporous materials by molecular simulations.

The present work provides the first study of ordered mesoporous materials SBA-15 coated with microporous zeolites ZSM-5 using molecular simulations. Several model structures with characteristics such as periodic arrangement of mesopores, randomly arranged micropores, surface hydroxyls and bulk deformations of SBA-15 were used. Cartesian coordinates of ZSM-5 unit lattice were obtained from the literature and the 100 face of H-ZSM-5 unit cell was then placed on the surface of SBA-15 and the entire structure was equilibrated to obtain final configuration. The resulting structure was characterized using simulated small angle and wide angle X-ray diffraction, Connolly surface area (to compare BET area), accessible pore volume for nitrogen molecules (to compare with t-plot volume of micro and mesopores) and methane adsorption at 303 K. The orientation of ZSM-5 on the SBA-15 had no effect on the surface area, pore volume or adsorption capacity. In order to find out if the addition of microporous ZSM-5 should increase the total methane adsorption capacity due to addition of micropores, we studied adsorption on bare and coated SBA-15. However, total adsorption capacity was found to decrease, while the number of methane molecules adsorbed per unit cell of the SBA-15 structure increased. An existing experimental method (J. Am. Chem. Soc., 2004, 126, 14324) of the synthesizing hybrid ZSM-5/SBA-15 structure was studied using accessible micropore volume (by t-plot). It was found that the procedure made all the micropores inaccessible. A modification of the method or use of other host materials is suggested to use the benefits of narrow micropore distribution in ZSM-5.

Adsorption↗

PubMed2005

Reversible resistance modulation in mesoscopic silver wires induced by exposure to amine vapor.

Ensembles of silver nanowires (AgNEs) with diameters ranging from 200 nm to 1.0 microm have been prepared by electrochemical step edge decoration. These AgNEs showed a rapid (< 5 s), reversible increase in resistance upon exposure to the vapor of ammonia, trimethylamine, and pyridine. The amplitude of the resistance change was up to +3000% (DeltaR/Ro)-more than 2 orders of magnitude larger than can be explained based on boundary layer scattering effects. We experimentally probe the mechanism for this resistance modulation in the case of ammonia, and we propose a model to describe it. Conductive tip atomic force microscopy was used to probe individual sections of nanowires in AgNEs; these data revealed that the resistance change caused by NH(3) exposure was concentrated within a minority (approximately 10%) of the 5-microm wire segments that were probed--not uniformly distributed along each nanowire. All AgNEs showed a temperature dependence of their resistance, alpha, that was smaller than expected for silver metal. Highly sensitive AgNEs sometimes showed a negative alpha, characteristic of semiconductors, but negative alpha values were never observed for AgNEs with a low sensitivity to NH3. AgNEs did not respond to hydrocarbons, O2, H2O, N2, CO, or Ar, but a large (DeltaR/Ro > |-50%|) irreversible decrease in resistance was seen upon exposures to acids including HCl, HNO3, and H2SO4. Based on these and other data, we propose a model in which oxidized constrictions in silver nanowires limit the conductivity of the wire and provide a means for "gating" conduction based on the protonation state of the oxide surface.

Journal Article↗

PubMed2005

Room-temperature chemisorption and thermal evolution of 1,1-dichloroethylene and monochloroethylene on Si(111)7 x 7: formation of vinylidene and vinylene adspecies.

The room-temperature (RT) adsorption and thermal evolution of 1,1-dichloroethylene (1,1-C2H2Cl2 or iso-DCE) and monochloroethylene (C2H3Cl or MCE) on Si(111)7 x 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm(-1) suggests that upon adsorption iso-DCE dissociates via C-Cl bond breakage on the 7x7 surface to form mono-sigma-bonded 1-chlorovinyl (ClC=CH2) and/or di-sigma-bonded vinylidene (: C=CH(2)) adspecies. Upon annealing to 450 K, the 1-chlorovinyl adspecies undergoes further dechlorination to vinylidene adspecies, which may be converted to di-sigma-bonded vinylene (HC=CH) before dehydrogenating to hydrocarbon fragments above 580 K. TDS studies reveal both molecular desorption of iso-DCE near 350 K and C2H2 fragments near 700 K, and the presence of the latter confirms the existence of the di-sigma-bonded vinylene adspecies. Like the other chlorinated ethylene homologues, iso-DCE also exhibits TDS features of an etching product SiCl2 at 800-950 K and a dehydrochlorination product HCl at 700-900 K. Unlike iso-DCE, MCE is found to adsorb on the 7 x 7 surface predominantly through a [2 + 2] cycloaddition mechanism at RT, with similar di-sigma bonding structure as ethylene. The thermal evolution of MCE however follows that of iso-DCE, with the formation of vinylene above 580 K. Despite the lack of TDS feature attributable to HCl, weaker SiCl2 TDS feature could be observed at 800-950 K. For both iso-DCE and MCE, strong recombinative desorption of H2 is observed near 780 K. The differences in the Cl content among iso-DCE, MCE, and ethylene therefore play a key role in the RT chemisorption and thermally driven chemical processes on Si(111)7 x 7.

Journal Article↗

PubMed2005

Structure and transport properties of nanostructured materials.

In the present manuscript, we have presented the simulation of nanoporous aluminum oxide using a molecular-dynamics approach with recently developed dynamic charge transfer potential using serial/parallel programming techniques (Streitz and Mintmire Phys. Rev. B 1994, 50, 11996). The structures resembling recently invented ordered nanoporous crystalline material, MCM-41/SBA-15 (Kresge et al. Nature 1992, 359, 710), and inverted porous solids (hollow nanospheres) with up to 10 000 atoms were fabricated and studied in the present work. These materials have been used for separation of gases and catalysis. On several occasions including the design of the reactor, the knowledge of surface diffusion is necessary. In the present work, a new method for estimating surface transport of gases based on a hybrid Monte Carlo method with unbiased random walk of tracer atom on the pore surface has been introduced. The nonoverlapping packings used in the present work were fabricated using an algorithm of very slowly settling rigid spheres from a dilute suspension into a randomly packed bed. The algorithm was modified to obtain unimodal, homogeneous Gaussian and segregated bimodal porous solids. The porosity of these solids was varied by densification using an arbitrary function or by coarsening from a highly densified pellet. The surface tortuosity for the densified solids indicated an inverted bell shape curve consistent with the fact that at very high porosities there is a reduction in the connectivity while at low porosities the pores become inaccessible or dead-end. The first passage time distribution approach was found to be more efficient in terms of computation time (fewer tracer atoms needed for the linearity of Einstein's plot). Results by hybrid discrete-continuum simulations were close to the discrete simulations for a boundary layer thickness of 5lambda.

Journal Article↗

PubMed2005

Molybdenum disulfide nanowires and nanoribbons by electrochemical/chemical synthesis.

Molybdenum disulfide nanowires and nanoribbons have been synthesized by a two-step, electrochemical/chemical synthetic method. In the first step, MoO(x) wires (a mixture of MoO(2) and MoO(3)) were electrodeposited size-selectively by electrochemical step-edge decoration on a highly oriented pyrolytic graphite (HOPG) surface. Then, MoO(x) precursor wires were converted to MoS(2) by exposure to H(2)S either at 500-700 degrees C, producing "low-temperature" or LT MoS(2) nanowires that were predominantly 2H phase, or above 800 degrees C producing "high-temperature" or HT MoS(2) ribbons that were predominantly 3R phase. The majority of these MoS(2) wires and ribbons were more than 50 microm in length and were organized into parallel arrays containing hundreds of wires or ribbons. MoS(2) nanostructures were characterized by X-ray photoelectron spectroscopy, scanning and transmission electron microscopy, selected area electron diffraction, X-ray diffraction, UV-visible absorption spectrometry, and Raman spectroscopy. HT and LT MoS(2) nanowires were structurally distinct: LT MoS(2) wires were hemicylindrical in shape and nearly identical in diameter to the MoO(x) precursor wires from which they were derived. LT MoS(2) wires were polycrystalline, and the internal structure consisted of many interwoven, multilayer strands of MoS(2); HT MoS(2) ribbons were 50-800 nm in width and 3-100 nm thick, composed of planar crystallites of 3R-MoS(2). These layers grew in van der Waals contact with the HOPG surface so that the c-axis of the 3R-MoS(2) unit cell was oriented perpendicular to the plane of the graphite surface. Arrays of MoS(2) wires and ribbons could be cleanly separated from the HOPG surface and transferred to glass for electrical and optical characterization. Optical absorption measurements of HT MoS(2) nanoribbons reveal a direct gap near 1.95 eV and two exciton peaks, A1 and B1, characteristic of 3R-MoS(2). These exciton peaks shifted to higher energy by up to 80 meV as the wire thickness was decreased to 7 nm (eleven MoS(2) layers). The energy shifts were proportional to 1/ L( parallel)(2), and the effective masses were calculated. Current versus voltage curves for both LT and HT MoS(2) nanostructures were probed as a function of temperature from -33 degrees C to 47 degrees C. Conduction was ohmic and mainly governed by the grain boundaries residing along the wires. The thermal activation barrier was found to be related to the degree of order of the crystallites and can be tuned from 126 meV for LT nanowires to 26 meV for HT nanoribbons.

Journal Article↗

PubMed2005

Thermal chemistry of styrene on Si(100)2x1 and modified surfaces: electron-mediated condensation oligomerization and posthydrogenation reactions.

The room-temperature (RT) adsorption and surface reactions of styrene on Si(100)2x1 have been investigated by thermal desorption spectrometry, low-energy electron diffraction, and Auger electron spectroscopy. Styrene is found to adsorb on Si(100)2x1 at a saturation coverage of 0.5 monolayer, which appears to have little effect on the 2x1 reconstructed surface. The chemisorption of styrene on the 2x1 surface primarily involves bonding through the vinyl group, with less than 15% of the surface moiety involved in bonding through the phenyl group. Except for the 2x1 surface where molecular desorption is also observed, the adsorbed styrene is found to undergo, upon annealing on the 2x1, sputtered and oxidized Si(100) surfaces, different thermally induced processes, including hydrogen abstraction, fragmentation, and/or condensation oligomerization. Condensation oligomerization of styrene has also been observed on Si(100)2x1 upon irradiation by low-energy electrons. In addition, large postexposure of atomic hydrogen to the chemisorbed styrene leads to Si-C bond cleavage and the formation of phenylethyl adspecies. Hydrogen therefore plays a decisive role in stabilizing and manipulating the processes of different surface reactions by facilitating different surface structures of Si.

Adsorption↗