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PubMed2005

Effects of rottlerin on silica-exacerbated systemic autoimmune disease in New Zealand mixed mice.

Environmental crystalline silica exposure has been associated with formation of autoantibodies and development of systemic autoimmune disease, but the mechanisms leading to these events are unknown. Silica exposure in autoimmune-prone New Zealand mixed (NZM) mice results in a significant exacerbation of systemic autoimmunity as measured by increases in autoantibodies and glomerulonephritis. Previous studies have suggested that silica-induced apoptosis of alveolar macrophages (AM) contributes to the generation of the autoantibodies and disease. Rottlerin has been reported to inhibit apoptosis in many cell types, possibly through direct or indirect effects on PKCdelta. In this study, rottlerin reduced silica-induced apoptosis in bone marrow-derived macrophages as measured by DNA fragmentation. In NZM mice, RNA and protein levels of PKCdelta were significantly elevated in AM 14 wk after silica exposure. Therefore, rottlerin was used to reduce apoptosis of AM and evaluate the progress of silica-exacerbated systemic autoimmune disease. Fourteen weeks after silica exposure, NZM mice had increased levels of anti-histone autoantibodies, high proteinuria, and glomerulonephritis. However, silica-instilled mice that also received weekly instillations of rottlerin had significantly lower levels of proteinuria, anti-histone autoantibodies, complement C3, and IgG deposition within the kidney. Weekly instillations of rottlerin in silica-instilled NZM mice also inhibited the upregulation of PKCdelta in AM. Together, these data demonstrate that in vivo treatment with rottlerin significantly decreased the exacerbation of autoimmunity by silica exposure.

Acetophenones↗
PubMed1997

Involvement of the ICE family of proteases in silica-induced apoptosis in human alveolar macrophages.

Exposure to silica dust can result in lung inflammation that may progress to fibrosis for which there is no effective clinical treatment. The mechanisms involved in the development of pulmonary silicosis have not been well defined; however, most current evidence implicates a central role for alveolar macrophages in this process. We have previously demonstrated that fibrotic agents, such as asbestos and silica, induce apoptosis in human alveolar macrophages. The goal of this study was to identify molecular events in the silica-induced apoptotic process to better understand the mechanism by which fibrotic agents may be inducing apoptosis in human alveolar macrophages. To elucidate the possible mechanism by which silica causes apoptosis, we investigated the involvement of the interleukin-converting enzyme (ICE) family of proteases. Human alveolar macrophages were treated with silica in vitro and were examined for the involvement of ICE, Ich-1L, and cpp32beta in silica-induced apoptosis. Pretreatment of cells with 10 microM of the ICE inhibitor z-Val-Ala-Asp-fluoromethyl ketone and the cpp32beta inhibitor Asp-Glu-Val-Asp-fluoromethyl ketone completely blocked silica-induced apoptosis. Additionally, an increased formation of the active p20 fragments of ICE and Ich-1L as well as degradation of the inactive zymogen form of cpp32beta protein were observed in silica-treated human alveolar macrophages, indicating activation of these proteases. Furthermore, degradation of the nuclear protein poly(ADP-ribose) polymerase was observed within 2 h of silica treatment. These results suggest that silica-induced apoptosis involves activation of the ICE family of proteases and is the first step in elucidating the intracellular mechanism of particulate-induced apoptosis in human alveolar macrophages.

Adult↗
PubMed1998

Upregulation of urokinase in alveolar macrophages and lung tissue in response to silica particles.

Impaired fibrinolytic activity and persistent fibrin deposits in lung tissue have been associated with lung fibrotic disorders. The present study examined the sources of plaminogen activator (PA) changes induced by a single intratracheal administration of silica particles (5 mg) in the mouse lung. We found in both control and silica-treated animals that amiloride almost totally abolished PA activity in bronchoalveolar lavage (BAL) fluid (BALF), indicating that initial upregulation (from day 1) as well as sustained PA activity (up to day 30) observed in response to silica is related to changes in urokinase-type PA (uPA). The upregulation of BALF uPA activity was associated with a marked and persistent increase in uPA mRNA levels in lung tissue. Changes in uPA expression were also reflected in the BAL cell fraction. A maximal and constant increase in cell uPA activity was associated with the early response to silica, whereas significant but lower upregulation was still noted at the fibrotic stage. From days 3 to 30, a progressive increase in uPA mRNA levels was noted in BAL inflammatory cells elicited by silica. Because the number of BAL neutrophils was strongly correlated with BALF and BAL cell-associated uPA activity, their involvement in uPA upregulation was addressed by inducing neutropenia with cyclophosphamide (200 mg/kg ip) before administration of the silica. Neutrophilic depletion did not, however, reduce, and even increased, the BAL cell-associated uPA activity. At the BALF level, neutropenia did not change PA activity in silica-treated mice, pointing to alveolar macrophages as the principal source of uPA in response to silica. Immunohistochemical stainings identified alveolar macrophages and pneumocytes as uPA-expressing cells in silica-treated animals (day 30). Intense and heterogenous staining was observed in silicotic nodules. These findings indicate that urokinase produced by alveolar macrophages is operative not only at the alveolitis stage but also later in the fibrotic process, produced by silica particles, supporting the role of uPA in fibrogenesis.

Amiloride↗
PubMed2001

Reactive oxygen species and caspase activation mediate silica-induced apoptosis in alveolar macrophages.

Alveolar macrophages (AMs) are the principal target cells of silica and occupy a key position in the pathogenesis of silica-related diseases. Silica has been found to induce apoptosis in AMs, whereas its underlying mechanisms involving the initiation and execution of apoptosis are largely unknown. The main objective of the present study was to examine the form of cell death caused by silica and the mechanisms involved. Silica-induced apoptosis in AMs was evaluated by terminal deoxynucleotidyltransferase-mediated dUTP nick end-labeling assay and cell cycle/DNA content analysis. The elevated level of reactive oxygen species (ROS), caspase-9 and caspase-3 activation, and poly(ADP-ribose) polymerase (PARP) cleavage in silica-treated AMs were also determined. The results showed that there was a temporal pattern of apoptotic events in silica-treated AMs, starting with ROS formation and followed by caspase-9 and caspase-3 activation, PARP cleavage, and DNA fragmentation. Silica-induced apoptosis was significantly attenuated by a caspase-3 inhibitor, N-acetyl-Asp-Glu-Val-Asp aldehyde, and ebselen, a potent antioxidant. These findings suggest that apoptosis is an important form of cell death caused by silica exposure in which the elevated ROS level that results from silica exposure may act as an initiator, leading to caspase activation and PARP cleavage to execute the apoptotic process.

Animals↗
PubMed1991

Alveolar type II cells isolated after silica-induced lung injury in rats have increased surfactant protein A (SP-A) receptor activity.

We examined surfactant secretion and its regulation by surfactant protein A (SP-A) in alveolar type II cells isolated from silica-treated rats to determine the role of SP-A-mediated regulatory control of phospholipid secretion in the pathogenesis of silica-induced alveolar proteinosis. Type II cells were isolated at weekly intervals for 28 d after silica or saline instillation. The maximum total binding of [125I]SP-A (internalized and surface-bound SP-A) to type II cells increased with time after silica instillation and, at 21 d, was 4-fold greater than that of type II cells isolated from saline-treated rats (272.8 +/- 42.5 and 65.4 +/- 9.8 ng/10(5) cells, respectively; P less than 0.05). Type II cells isolated from silica-treated rats showed a 2-fold increased surface binding and a 3-fold increased internalization compared to control cells. The receptor affinity for SP-A was the same for type II cells isolated from silica- and saline-treated animals. Type II cells isolated 14 d after silica instillation were separated into normotrophic and hypertrophic populations by centrifugal elutriation. Hypertrophic cells showed significantly elevated maximum total binding compared to normotrophic cells. The secretion of [3H]phosphatidylcholine [( 3H]PC) by type II cells from silica- and saline-treated animals was also compared. Type II cells from silica-treated animals showed lower basal and tetradecanoyl phorbol acetate (TPA)-stimulated [3H]PC secretion than cells from saline-treated animals at each time point after instillation. SP-A inhibited TPA-stimulated [3H]PC secretion similarly in type II cells isolated after either silica or saline instillation.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed2006

Silica-directed mast cell activation is enhanced by scavenger receptors.

Inhalation of crystalline silica results in pulmonary fibrosis and silicosis. It has been suggested that mast cells play a role in these conditions. How mast cells would influence pathology is unknown. We thus explored mast cell interactions with silica in vitro and in B6.Cg-kit(W-sh) mast cell-deficient mice. B6.Cg-kit(W-sh) mice did not develop inflammation or significant collagen deposition after instillation of silica, while C57Bl/6 wild-type mice did have these findings. Given this supporting evidence of a role for mast cells in the development of silicosis, we examined the ability of silica to activate mouse bone marrow-derived mast cells (BMMC), including degranulation (beta-hexosaminidase release); production of reactive oxygen species (ROS) and inflammatory mediators; and the effects of silica on Fc epsilon RI-dependent activation. Silica did not induce mast cell degranulation. However, TNF-alpha, IL-13, monocyte chemotactic protein-1, protease activity, and production of ROS were dose-dependently increased after silica exposure, and production was enhanced after Fc epsilon RI stimulation. This mast cell activation was inhibited by anti-inflammatory compounds. As silica mediates some effects in macrophages through scavenger receptors (SRs), we first determined that mast cells express scavenger receptors; then explored the involvement of SR-A and macrophage receptor with colleagenous structure (MARCO). Silica-induced ROS formation, apoptosis, and TNF-alpha production were reduced in BMMC obtained from SR-A, MARCO, and SR-A/MARCO knockout mice. These findings demonstrate that silica directs mast cell production of inflammatory mediators, in part through SRs, providing insight into critical events in the pathogenesis and potential therapeutic targets in silicosis.

Animals↗
PubMed2006

Effects of nitric oxide synthase inhibitor omega-nitro-L-arginine methyl ester, on silica-induced inflammatory reaction and apoptosis.

BACKGROUND: Although nitric oxide is overproduced by macrophages and neutrophils after exposure to silica, its role in silica-induced inflammatory reaction and apoptosis needs further clarification. In this study, rats were intratracheally instilled with either silica suspension or saline to examine inflammatory reactions and intraperitoneally injected with omega-nitro-L-arginine methyl ester (L-NAME), an inhibitor of nitric oxide synthases, or saline to examine the possible role of nitric oxide production in the reaction. RESULTS: Results showed that silica instillation induced a strong inflammatory reaction indicated by increased total cell number, number of neutrophils, protein concentration and lactate dehydrogenase (LDH) activity in bronchoalveolar lavage fluid (BALF). There were no significant differences in these indices between silica-instilled groups with and without L-NAME injection (p > 0.05) except LDH level. The results also showed that apoptotic leucocytes were identified in BALF cells of silica-instilled groups whereas no significant difference was found between silica-instilled groups with and without L-NAME injection in the apoptotic reaction (p > 0.05). Silica instillation significantly increased the level of BALF nitrite/nitrate and L-NAME injection reduced this increase. CONCLUSION: Intratracheal instillation of silica caused an obvious inflammatory reaction and leucocyte apoptosis, but these reactions were not influenced by intraperitoneal injection of L-NAME and reduced production of NO. This supports the possibility that silica-induced lung inflammation and BALF cell apoptosis are via NO-independent mechanisms.

Journal Article↗
PubMed1994

Mechanisms of carcinogenesis by crystalline silica in relation to oxygen radicals.

The carcinogenic effects of crystalline silica in rat lungs were extensively demonstrated by many experimental long-term studies, showing a marked predominance for adenocarcinomas originating from alveolar type II cells and associated with areas of pulmonary fibrosis (silicosis). In contrast with its effects in rats, silica did not induce alveolar type II hyperplasia and lung tumors in mice and hamsters, pointing to a critical role for host factors. Using these animal models, we are investigating the role of cytokines and other cellular mediators on the proliferation of alveolar type II cells. Immunohistochemical localization of TGF-beta 1 precursor in alveolar type II cells adjacent to silicotic granulomas was shown to occur in rats, but not in mice, and hamsters, suggesting a pathogenetic role for this regulatory growth factor. Recent investigations in our laboratory on the biologic mechanisms of crystalline silica included determination of anionic sites on crystalline silica surfaces by binding of the cationic dye Janus Green B; binding of crystalline silica to DNA, demonstrated by infrared spectrometry; production of oxygen radicals by crystalline silica in aqueous media; induction of DNA strand breakage and base oxidation in vitro and its potentiation by superoxide dismutase and by hydrogen peroxide; and induction by crystalline silica of neoplastic transformation and chromosomal damage in cells in culture. On the basis of these in vitro studies, we propose that DNA binding to crystalline silica surfaces may be important in silica carcinogenesis by anchoring DNA close to sites of oxygen radical production on the silica surface, so that the oxygen radicals are produced within a few A from their target DNA nucleotides.

Animals↗
PubMed1997

Silica-induced oxygen radical generation in alveolar macrophage.

Silica is a well-known occupational fibrogenic agent and its primary target cell is alveolar macrophage. Particle-stimulated macrophages are believed to release various mediator which can regulate the inflammation as well as pulmonary fibrosis. Even though oxygen radicals play the major role among these mediators, the mechanisms concerning the stimulation of alveolar macrophages are not clear yet. The present study was carried out to investigate the signal transduction pathway on oxygen radical generation in silica-stimulated alveolar macrophages. Silica induced oxygen radical generation in a dose-response pattern. Extracellular calcium depletion, calcium channel blockers, and calcium release blocker decreased the effect of silica on oxygen radical generation. Silica increased intracellular calcium through the influx of calcium through the calcium channel and the calcium release from the intracellular calcium store. To know the role of protein kinase C (PKC), phospholipase C (PLC), and protein tyrosine kinase (PTK) in silica-induced oxygen radical generation, we pretreated alveolar macrophages with inhibitors of these enzymes. Inhibitors of PKC (sphingosine and staurosporine), PLC (neomycin and U-73122), and PTK (genistein and erbstatin) suppressed the silica-induced oxygen radical generation. Silica increased the PLC activity at the concentration of 5 mg/ml. The inhibitors of PTK and PLC suppressed the action of silica on the PLC activity. From these results, we suggest that silica induces oxygen radical generation through PTK, PLC, and PKC in alveolar macrophages.

Animals↗
PubMed2004

Silicosis and smoking strongly increase lung cancer risk in silica-exposed workers.

It remains controversial whether silica is a human lung carcinogen. In this study, we estimated the relative risks of lung cancer due to silica and silicosis by meta-analysis. We collected papers published from 1966-2001 which epidemiologically reported on the relationship between silica/silicosis and lung cancer. We removed papers which did not exclude the effects of asbestos and radioactive materials including radon. We selected the most recent one if some papers were based on the same cohort. Based on the selected papers, we summarized the lung cancer risks from silica, silicosis and non-silicosis with exposure to silica, by meta-analysis using a random effects model. The pooled relative risks were 1.32 (95% confidence interval (CI), 1.23-1.41) for silica, 2.37 (95% CI, 1.98-2.84) for silicosis and 0.96 (95% CI, 0.81-1.15) for non-silicosis with exposure to silica. Since some papers on silica did not exclude silicosis, the risk due to silica itself may be smaller than 1.32. It was less possible that silica exposure directly increases lung cancer risk. On the other hand, the relative risk, 2.37 for silicosis suggested that silicosis increases lung cancer risk. Meta-analysis also revealed that cigarette smoking strongly increased the lung cancer risk in silicotic patients (relative risk, 4.47; 95% CI, 3.17-6.30). Thus, the present study suggested the great importance of preventing silicosis and smoking cessation in reducing lung cancer incidence in silica-exposed workers.

Carcinogens↗
PubMed2001

Multifactorial aetiology of lung cancer among silica-exposed workers.

In 1997, the International Agency for Research on Cancer (IARC) upgraded its evaluation of crystalline silica to a Group 1 human carcinogen. Criticism against such decision is based on the lack of consistency in experimental results across animal species, violation of an important principle for causality, such as the replication of findings under different circumstances of exposure, and the lack of a clear dose-response curve. The most recent epidemiological literature on the silica-silicosis-lung cancer link replicates the inconsistent findings that have been characterising 50 years of scientific debate in the occupational arena. Exposure circumstances capable of modifying the silica-lung cancer association include chronic bronchitis, composition of the dust mixing, particularly concerning co-occurrence of other known or probable lung carcinogens, total respirable dust, concentration of silica in respirable dust, type of crystalline silica and particle surface characteristics. The hypothesis of a silicosis-mediated pathway points toward an unspecific mechanism shared with other fibrotic conditions, for which silica might be just one of the triggers. In envisaging a multivariate multistep model of lung cancer among silica-exposed workers, silica might be considered as a "passive components of the sufficient cause", i.e., one of the associated risk factors, concurrent or subsequent to the "active component(s) of the sufficient cause" (including, for instance, smoking, asbestos, radon-daughters, arsenic, hexavalent chromium, nickel, polycyclic aromatic hydrocarbons (PAH), and diesel exhausts among the external risk factors; and DNA repair enzymes polymorphism and spontaneous inactivation of tumour suppressor genes among the internal risk factors), which adds up in modulating the tumoral development in not easily predictable directions. If silica acts as a human lung carcinogen depending on certain occupational exposure circumstances, perhaps those circumstances and not silica itself should be classified for their human carcinogenicity potential.

Carcinogens, Environmental↗
PubMed2001

[Evaluation of historical exposure to silica dust for the workers employed in geologic exploration industry].

OBJECTIVE: To evaluate their level of exposure to silica dust in the workers employed in geologic exploration industry using a quantitative method. METHODS: Monitoring data of exposure to silica dust since 1950s were collected for the workers in various jobs from the factories and mines of the bureaux of geological exploration in the nine provinces, with 30,000 pieces of figures in total. Job title and history of employment were determined for 1,627 employees. Levels of exposure to silica dust in various factories and mines were estimated based on the data mentioned-above, including amount of respirable dust, total dust in the lungs, and proportion of free silica dust to the total silica dust. RESULTS: Concentration of total silica dust was 14 mg/m3 in average in the factories and mines, with a range of 29 mg/m3 during the earlier years to 3 mg/m3 in recent years, and that of respirable silica dust was 3 mg/m3, in average, with (28.0 +/- 8.2)% of free silica dust. Different indices of exposure to silica dust were calculated for individuals, based on their history of employment and level of exposure. CONCLUSION: Assessment of exposure to silica dust during the past years, based on the monitoring data, could provide basis for evaluation of dose-response relationship.

Air Pollutants, Occupational↗
PubMed2004

[Effect of niacin on nitric oxide synthase expression in rat lung exposed to silica].

OBJECTIVE: To evaluate the effects of niacin supplemented in diet on temporal expression of nitric oxide synthase in rat lung exposed to silica by tissue array technology. METHODS: Wistar rats were randomly divided into three experimental groups: saline-treated group, silica-treated group, niacin-treated group. There are 48 animals in each group. Animal models were established by direct tracheal instillation of silica into the rat lungs. Plasma level of niacin was measured by high performance liquid chromatography (HPLC). The expression of iNOS protein in the paraffin-embedded lung sections was measured with streptavidin/peroxidase (SP) immunohistochemistry on tissue microarray and quantified by Image-Pro Plus. RESULTS: Plasma level of niacin in niacin-treated group were significantly elevated by 5.946 4, 17.422 0, 21.398 0, 16.091 0, 4.414 3 and 7.130 5 mg/L at 1, 3, 7, 14, 21 and 28 days after instillation of silica, as compared to control and silica-treated groups. Seven days after instillation of silica, iNOS integrated optical density (IOD) of the lung, total NOS and iNOS activities in bronchial alveolar lavage fluid (BALF) supernatant in silica-treated group significantly elevated by 273 421, 2.61 kU/L and 1.89 kU/L, respectively, in the saline-treated group, with statistical significance. Niacin treatment could significantly decrease silica-elevated iNOS integrated optical density (IOD) of the lung, total NOS and iNOS activities in BALF supernatant by 248.292, 1.50 kU/L and 0.91 kU/L in the silica-treated group, respectively, with statistical significance. CONCLUSIONS: It is suggested that treatment with niacin could effectively attenuate the over expression of nitric oxide synthase in the rat lung induced by silica particles in our study.

Animals↗
PubMed1989

Enhanced macrophage-fibroblast interactions in the pulmonary interstitium increases fibrosis after silica injection to monocyte-depleted mice.

The role of interstitial vs. alveolar macrophages in the generation of pulmonary fibrosis after silica was examined. Using whole body irradiation to delay the inflammatory response and so retard particulate clearance, many more instilled silica particles reached the interstitial macrophages in the first 2 weeks than after silica alone. This was followed by greatly increased fibroblast proliferation and deposition of collagen in the irradiation plus silica group, which developed large interstitial granulomas at the sites of silica retention. Although alveolar macrophages containing silica were seen in both silica groups, more interstitial particles were observed after combined irradiation and silica, significantly more silica was recovered in a residue from the lungs at 16 weeks, and pulmonary fibrosis at 8-16 weeks was greater than in all other groups. The results indicate that increased fibroblast growth and collagen synthesis in vivo are associated with phagocytosis of silica by interstitial macrophages rather than by free alveolar macrophages. It is suggested that transfer of a macrophages-derived growth factor to fibroblasts is more efficient when it occurs within the pulmonary interstitium.

Animals↗
PubMed1982

Deposition and translocation of inhaled silica in rats. Quantification of particle distribution, macrophage participation, and function.

Chronic exposure to silica dust causes fibrotic lung disease. Using brief exposures, we have attempted to define the initial patterns of dust deposition, anatomical compartments through which silica is translocated, and the participation of pulmonary macrophages in clearing the inhaled dust. To accomplish this, particle distribution and translocation at the alveolar level were studied in rats exposed to aerosolized alpha-quartz. Animals were exposed to 109 mg. per cu. m. of crystalline silica in inhalation chambers for 3 hours and sacrificed at varying times after exposure. The lungs were fixed by vascular perfusion through the right ventricle and tissue blocks were prepared for transmission and scanning electron microscopy. Lungs of additional animals were lavaged to recover populations of pulmonary macrophages for in vitro studies. Scanning electron microscopy in concert with back-scattered electron imaging showed that 24 hours postexposure there was a significant decrease in the number of silica particles per unit area of alveolar duct surface when compared with lung tissue from animals sacrificed immediately after exposure. Transmission electron microscopy revealed that silica crystals had been translocated to alveolar type I cells, interstitium, and macrophages. The percentage of silica-containing macrophages on alveolar surfaces increased from 36 +/- 2 per cent (mean +/- S.E.) immediately after exposure to 66 +/- 2 per cent during the 24 hours following exposure. This high percentage of macrophage participation was maintained through a 24-day postexposure period and then returned to 25 +/- 2 per cent 42 days after exposure. The percentage of silica-containing macrophages recovered by lavage were remarkably similar to those studied in situ: 24 +/- 4 per cent immediately postexposure, 62 +/- 3 per cent from 12 hours through 24 days, and 28 +/- 4 per cent 42 days postexposure. Although the percentage of macrophages with silica remained steady, the amount of silica per cell decreased during this 12-hour to 24-day period. Metabolic and viability studies of lavaged macrophages in vitro showed no differences between sham and silica-exposed animals. We propose that the events reported here represent normal, steady state clearance of a subpathogenic dose of potentially toxic particulates.

Animals↗
PubMed2001

Effect of silicon carbide whisker-silica heat treatment on the reinforcement of dental resin composites.

The strength and fracture resistance of dental resin composites need to be improved to extend their use to large stress-bearing crown and multiple-unit applications. Recent studies showed that the addition of ceramic whiskers significantly reinforced resin composites. The aim of the present study was to use a silicon carbide whisker-silica particle mixture to reinforce resin composites, and to investigate the effect of whisker-silica mixture heat-treatment on composite properties. The whiskers were blended with silica particles and were thermally fused in an attempt to roughen the whiskers for improved retention in the matrix. The mixtures were heat-treated at temperatures of 500, 650, 800, 950, and 1100 degrees C for 10 min, 30 min, and 3 h. The mixtures were then silanized, incorporated into a dental resin, and the paste was placed into 2 x 2 x 25 mm molds and heat-cured at 140 degrees C for 30 min. A 3-point flexural test was used to measure flexural strength and work-of-fracture. Two commercial indirect composites were tested as controls. Two-way ANOVA showed that there was no significant effect from temperature or time. Therefore, all the whisker-silica mixture samples were combined into one group (n = 96), and compared to composites filled with silica only or whisker only, and the two indirect control composites. The whisker-silica mixture group had a flexural strength (mean +/- SD) of (186 +/- 24) MPa, significantly higher than (99 +/- 29) MPa for silica only, (131 +/- 22) MPa for whisker only, and (109 +/- 23) MPa and (114 +/- 18) MPa for the two indirect composites (Tukey's multiple comparison test; family confidence coefficient = 0.95). Similar results were obtained on work-of-fracture. Scanning electron microscopy revealed rough fracture surfaces for the whisker-silica composites, indicating crack deflection and bridging by whiskers as toughening mechanisms. Whisker-silica mixture minimized whisker entanglement and enhanced whisker-resin bonding, resulting in substantially stronger and tougher dental resin composites.

Carbon Compounds, Inorganic↗
PubMed1986

Silica in higher plants.

Opaline silica deposits are formed by many vascular (higher) plants. The capacity of these plants for silica absorption varies considerably according to genotype and environment. Plant communities exchange silica between soil and vegetation, especially in warmer climates. Silica deposition in epidermal cell walls offers mechanical and protective advantages. Biogenic silica particles from plants are also implicated in the causation of cancer. Recent techniques are reviewed which may aid in the identification of plant pathways for soluble silica movement to deposition sites and in the determination of ionic environments. Botanical investigations have focused on silicification of cell walls in relation to plant development, using scanning and transmission electron microscopy combined with X-ray microanalysis. Silica deposition in macrohair walls of the lemma of canary grass (Phalaris) begins at inflorescence emergence and closely follows wall thickening. The structure of the deposited silica may be determined by specific organic polymers present at successive stages of wall development. Lowering of transpiration by enclosure of Phalaris inflorescences in plastic bags reduced silica deposition in macrohairs. Preliminary freeze-substitution studies have located silicon, as well as potassium and chloride, in the cell vacuole and wall deposition sites during initial silicification.

Climate↗
PubMed2005

Silica-gel-confined ionic liquids: a new attempt for the development of supported nanoliquid catalysis.

A new concept of designing and synthesizing highly dispersed ionic-liquid catalysts was developed through physical confinement or encapsulation of ionic liquids (with or without metal complex) in a silica-gel matrix through a sol-gel process. We studied ionic liquids such as EMImBF4, BuMImBF4, DMImBF4, CMImBF4, BuMImPF6, either with or without [Pd(PPh3)2Cl2] and [Rh(PPh3)3Cl], in a silica-gel matrix (E = ethyl, Bu = butyl M = methyl, D = decyl, C = cetyl and Im = imidazolium). The contents of ionic liquids and loadings of Pd or Rh were 8-53 wt % and 0.1 approximately 0.15 wt %, respectively. Analyses of FT-Raman spectra showed that abnormal Raman spectra of the confined ionic liquids were observed in comparison with the bulk and pure ionic liquids. EMImBF4 and BuMImBF4 ionic liquids could be completely washed out from the silica-gel matrix under vigorous reflux conditions, but ionic liquids with larger molecular size, for example, DMImBF4 or CMImBF4, could be confined into the silica-gel nanopores relatively firmly. These results suggested that the ionic liquids were physically confined or encapsulated into the silica gel. The N2 adsorption measurements indicated that the silica-gel skeleton was mesoporous with 50-110 A pore size after the BuMImBF4 ionic liquid was removed completely. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis showed that the silica-gel matrix was amorphous and non-uniformly mesoporous. Carbonylation of aniline and nitrobenzene for synthesis of diphenyl urea, carbonylation of aniline for synthesis of carbamates, and oxime transformation between cyclohexanone oxime and acetone were used as test reactions for these catalysts. Catalytic activities were remarkably enhanced with much lower amounts of ionic liquids needed with respect to bulk ionic-liquid catalysts or silica-supported ionic-liquid catalysts prepared with simple impregnation, in which the ionic liquid may be deposited as a thin layer on the support. Such unusual enhancement in catalytic activities may be attributed to the formation of nanoscale and high-concentration ionic liquids due to the confinement of the ionic liquid in silica gel; this results in unusual changes in the symmetry and coordination geometry of the ionic liquids.

Journal Article↗