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PubMed2005

Restructuring and redispersion of silver on SiO2 under oxidizing/reducing atmospheres and its activity toward CO oxidation.

The effects of oxygen-hydrogen pretreatments of nanosilver catalysts in cycle mode on the structure and particle size of silver particles, and subsequently the activity of the catalyst toward CO oxidation (or CO selective oxidation in the presence of H2), are reported in this paper. Ag/SiO2 catalyst with silver particle sizes of ca. 6 approximately 8 nm shows relatively high activity in the present reaction system. The adopting of a cycle of oxidation/reduction pretreatment has a marked influence on the activity of the catalyst. Oxygen pretreatment at 500 degrees C results in the formation of subsurface oxygen and activates the catalyst. As evidenced by in-situ XRD and TEM, the following H2 treatment at low temperatures (100 approximately 300 degrees C) causes surface faceting and redispersing of the silver particles without destroying the subsurface oxygen species. The subsequent in-situ FTIR and catalytic reaction results show that CO oxidation occurs at -75 degrees C and complete CO conversion can be obtained at 40 degrees C over such a nanosilver catalyst pretreated with oxygen at 500 degrees C followed by H2 at 100 degrees C. However, prolonged hydrogen treatment at high temperatures (>300 degrees C) after oxygen pretreatment at 500 degrees C induces the aggregation of silver particles and also depletes so much subsurface oxygen species that the pathway of CO oxidation by the subsurface oxygen species is inhibited. Meanwhile, the ability of the catalyst to adsorb reactants is greatly depressed, resulting in a 20 approximately 30% decrease in the activity toward CO oxidation. However, the activity of the catalyst pretreated with oxygen at 500 degrees C followed by hydrogen treatment at high temperatures (>300 degrees C) is still higher than that directly pretreated with H2. This kind of catalytic behavior of silver catalyst is associated with physical changes in the silver crystallites because of surface restructuring and crystallite redispersion during the course of oxygen-hydrogen pretreatment steps.

Journal Article↗
PubMed2005

Characterization of polypyrrole-silver nanocomposites prepared in the presence of different dopants.

Conducting polypyrrole (PPy) powder synthesized by using FeCl3 x 6 H2O and/or Fe(NO3)3 oxidants was impregnated in silver salt solutions. The stability and decomposition of the material was followed by thermogravimetric measurements. The total silver content was determined by atom absorption spectroscopy (ICP-AAS). The heat and electric conductivities of the composites were measured and correlated with the silver content. The incorporated silver was speciated and measured by X-ray diffraction (XRD). The spectra proved that the chemical state of the silver incorporated into the composite depends on the anion used in the polymerization process. In the case of the polymerization in a nitrate ion containing solution, the impregnation leads exclusively to the formation of metallic silver. The size distribution of the AgCl and Ag nanoparticles, determined from transmission electron microscopy (TEM) pictures in the different composites, proves the formation of a rather uniform species below 10 and 7 nm, respectively. The observations can be correlated with the different interactions in the PPy-chloride/nitrate-silver systems. The redox type interaction based conclusions can be considered as a guide during the preparation of other metal-conducting polymer composites.

Journal Article↗
PubMed2005

Molecular numbers in core and shell: structural dependence of reactivity of alkylcarboxylate-stabilized silver nanoparticles.

Spectroscopic, chemical, thermal, and voltammetric analyses on six kinds of alkylcarboxylate-stabilized silver nanoparticles 4.7 nm in diameter were carried out with an aim to reveal the effect of alkylcarboxylates on the optical, thermal, geometric, and electrochemical properties of the nanoparticles. These nanoparticles are composed of silver atoms and silver alkylcarboxylates having even numbers, m, of carbon atoms from 8 to 18. As a measure of the structure of the nanoparticles, the ratio of the number of silver atoms (nAg) to that of alkylcarboxylates (ns) per nanoparticle was evaluated by means of titration through chemical oxidation, voltammetric currents, and thermal gravimetric analysis. It increased with an increase in m and ranged from 1.3 to 9.8. Properties of the nanoparticle have been exhibited in absorbance of the UV-vis spectra at the point of the proportionality to n(Ag), voltammetric currents of which values were close to the theoretical values at the diffusion of particle itself, and the m-independent kinetic energy of the thermal decomposition and the overpotential of the reduction. They are not observed for the composed species, that is, silver atoms and silver alkylcarboxylate molecules.

Journal Article↗
PubMed2005

Preparation and characterization of Cr(CO)(4)dpp (chromium tetracarbonyl 2,3-bis(2'-pyridyl)pyrazine) adsorbed on silver nanoparticles.

A transition metal carbonyl species, Cr(CO)(4)dpp, has been successfully attached to bare silver nanoparticles prepared by laser ablation of a metal foil in ethanol. Transmission electron microscopy (TEM) images have shown that at least a portion of the silver nanoparticles have been capped by the chromium species, and ligand shells corresponding to Cr(CO)(4)dpp multilayer adsorption onto the silver nanoparticles of 30-50 nm diameter have been observed. The detection of the strongest Raman-active nu(CO) band of Cr(CO)(4)dpp at 2004 cm(-1) revealed that the species has been adsorbed without decomposition. The time-of-flight secondary ion mass spectrometry (TOF-SIMS) signals recorded of the chromium-capped silver nanoparticles were also consistent with the nondecomposition adsorption process. Density functional calculations have been used to reproduce the Raman spectrum using Ag(7)(+) as a model surface. A large binding energy of about 122 kJ/mol has also been computed between silver and nitrogen atoms thus lending support to Cr(CO)(4)dpp being chemisorbed onto the silver surface.

Journal Article↗
PubMed2005

Solvent-adaptable silver nanoparticles.

A simple and efficient way of obtaining silver nanoparticles that are dispersible both in organic and in aqueous solvents using a single capping agent is described. The silver nanoparticles are initially prepared in water in the presence of aerosol OT [sodium bis(2-ethylhexyl)-sulfosuccinate, AOT]. Thereafter, transfer of the AOT-capped silver nanoparticles to an organic phase is induced by the addition of a small amount of orthophosphoric acid during shaking of the biphasic mixture. The AOT-stabilized silver nanoparticles could be separated out from the organic phase in the form of a powder. The hydrophobic nanoparticles thus prepared are stable and are readily resuspended in a variety of other polar (including water) and nonpolar solvents without further surface treatment. The amphiphatic nature of the silver surface is brought about by a small orientational change in the AOT monolayer on the silver surface in response to the polarity of the solvent.

Journal Article↗
PubMed2005

ATR-SEIRAS study of the adsorption of acetate anions at chemically deposited silver thin film electrodes.

The adsorption of acetate anions at silver thin film electrodes has been studied by in-situ infrared spectroscopy experiments with a Kretschmann internal reflection configuration. Stable silver thin films were chemically deposited on germanium substrates. Ex-situ STM images show mean grain sizes ranging from ca. 20 to 90 nm for deposition times between 2 and 20 min, respectively. The thickness of the silver film, measured by AFM, is typically around 10 nm for a deposition time of 10 min and increases up to 50 nm for a deposition time of 20 min. Roughness factors around 2.3 have been obtained for the silver films from the charge involved in lead underpotential deposition (UPD). A noticeable enhancement of the infrared absorption of adsorbed species (SEIRA effect) is observed when the silver films are used as electrodes under internal total reflection conditions. Maximum intensities of the adsorbate bands were observed for a deposition time of 10 min and an angle of incidence around 65 degrees . The potential-dependent infrared spectra of acetate and interfacial water are consistent with previously proposed models involving the existence of weakly hydrogen-bonded water molecules at potentials below the potential of zero charge and the reorientation of water molecules at potentials above the potential of zero charge. Results reported in this work suggest a weak interaction between acetate and water molecules adsorbed at the silver thin film electrodes.

Journal Article↗
PubMed2001

Silver(I) ion-selective membrane based on Schiff base-p-tert-butylcalix[4]arene.

A PVC membrane electrode for silver(I) ion based on Schiff base-p-tert-butylcalix[4]arene is reported. The electrode works well over a wide range of concentration (1.0 x 10(-5)-1.0 x 10(-1) mol dm-3) with a Nernstian slope of 59.7 mV per decade. The electrode shows a fast response time of 20 s and operates in the pH range 1.0-5.6. The sensor can be used for more than 6 months without any divergence in the potential. The selectivity of the electrode was studied and it was found that the electrode exhibits good selectivity for silver ion over some alkali, alkaline earth and transition metal ions. The silver ion-selective electrode was used as an indicator electrode for the potentiometric titration of silver ion in solution using a standard solution of sodium chloride; a sharp potential change occurs at the end-point. The applicability of the sensor to silver(I) ion measurement in water samples spiked with silver nitrate is illustrated.

Journal Article↗
PubMed2002

An electrochemical DNA hybridization detection assay based on a silver nanoparticle label.

A novel, sensitive electrochemical DNA hybridization detection assay, using silver nanoparticles as the oligonucleotide labeling tag, is described. The assay relies on the hybridization of the target DNA with the silver nanoparticle-oligonucleotide DNA probe, followed by the release of the silver metal atoms anchored on the hybrids by oxidative metal dissolution and the indirect determination of the solubilized Ag(I) ions by anodic stripping voltammetry (ASV) at a carbon fiber ultramicroelectrode. The influence of the relevant experimental variables, including the surface coverage of the target oligonucleotide, the duration of the silver dissolution steps and the parameters of the electrochemical stripping measurement of the silver(I) ions, is examined and optimized. The combination of the remarkable sensitivity of the stripping metal analysis at the microelectrode with the large number of silver(I) ions released from each DNA hybrid allows detection at levels as low as 0.5 pmol L(-1) of the target oligonucleotides.

DNA Mutational Analysis↗
PubMedDate not supplied

Early healing events in a porcine model of contaminated wounds: effects of nanocrystalline silver on matrix metalloproteinases, cell apoptosis, and healing.

A porcine model of wound healing was employed to examine the impact of nanocrystalline silver-coated dressings on specific wound healing events. Full-thickness wounds were created on the backs of pigs, contaminated with an experimental inoculum containing Pseudomonas aeruginosa, Fusobacterium sp., and coagulase-negative staphylococci, and covered with dressing products either containing silver or not. Nanocrystalline silver-coated dressings promoted rapid wound healing, particularly during the first several days post-injury. Healing was characterized by rapid development of well vascularized granulation tissue that supported tissue grafting 4 days post-injury, unlike control dressed wounds. The proteolytic environment of wounds treated with nanocrystalline silver was characterized by reduced levels of matrix metalloproteinases. Matrix metalloproteinases have been shown to be present in chronic ulcers at abnormally high levels, as compared with acute wounds, and may contribute to the nonhealing nature of these wounds. Cellular apoptosis occurred at a higher frequency in the nanocrystalline silver-treated wounds than in wounds dressed with other products. The results suggest that nanocrystalline silver may play a role in altering or compressing the inflammatory events in wounds and facilitating the early phases of wound healing. These benefits are associated with reduced local matrix metalloproteinase levels and enhanced cellular apoptosis.

Animals↗
PubMed1991

Polyamines in plants infected by citrus exocortis viroid or treated with silver ions and ethephon.

The levels of polyamines in leaves of Gynura aurantiaca DC and tomato, Lycopersicon esculentum Mill. cv Rutgers, infected with citrus exocortis viroid (CEVd) or treated with silver nitrate or ethephon (2-chloroethylphosphonic acid) were measured by HPLC in relation to development of symptoms. Previously it had been demonstrated that treatment of G. aurantiaca plants with silver nitrate or ethephon closely mimicked the effects of viroid infection in the plants. In the studies reported here, a marked decrease in putrescine level was observed in plants infected by CEVd or treated with silver ions or ethephon. There was no significant change in either spermidine or spermine levels. Treatment of G. aurantiaca plants with specific inhibitors of ethylene biosynthesis (aminoethoxyvinylglycine, Co(2+)) or ethylene action (norbornadiene) prevented the decrease of putrescine associated with silver nitrate treatment and had no effect on spermidine or spermine levels. The development of viroid-like symptoms, the production of associated pathogenesis-related proteins, and the rise in protease activity induced by silver nitrate, were all suppressed by exogenous application of putrescine. The decreased level of putrescine as an ethylene-mediated step in the transduction of the viroid and silver or ethephon signaling is discussed.

Journal Article↗
PubMed2004

Ultraviolet-visible and surface-enhanced Raman scattering spectroscopy studies on self-assembled metalloporphyrin films on organic monolayer-modified ultra-thin silver substrates.

A self-assembled monolayer (SAM) film of 5,10,15,20-tetra-(para-chlorophenyl)-porphyrin terbium (or lutetium) hydroxy compound (TbOH/LuOH) fabricated on a silver substrate premodified with a SAM of 4-mercaptopyridine (PySH) was studied by ultraviolet-visible (UV-Vis) spectroscopy and surface-enhanced Raman scattering (SERS) spectroscopy. PySH can modify the substrate and deliver its pyridyl group pointing out from the silver surface. Thus, we can investigate the effects of the central metals of the metalloporphyrins in the formation of the composite films. For the TbOH-PySH composite film, a new absorption band arising from TbOH appears at 425 nm, and a band at 512 nm due to the PySH-modified silver substrate shifts to a longer wavelength (538 nm). The results suggest that TbOH is successfully assembled on the top of PySH-modified silver film and that there is an interaction between TbOH and PySH in the film. The frequency shifts and relative intensity changes of bands due to PySH in the SERS spectra imply the coordination of the pyridyl group on Tb in the SAM. As for the LuOH-PySH composite film, its SERS spectrum shows bands arising from both the LuOH and PySH moieties, indicating that LuOH is assembled on the PySH-modified silver film. Furthermore, a band at 1221 cm(-1) due to the in-plane C-H bending mode of PySH disappears, implying that the pyridyl moiety of PySH becomes more parallel to the silver surface upon the formation of the LuOH-PySH composite film. Additionally, an absorption band at 515 nm shifts to a longer wavelength (541 nm) and becomes broad upon the formation of the composite film, suggesting an interaction between LuOH and PySH in the film. By comparing the spectral changes between the two self-assembled composite films, we find that the central metal is crucial in the formation of the composite films. The structure and orientation of the composite films depend on the central metal of the metalloporphyrin compounds.

Journal Article↗
PubMed2004

Cloning of the cDNA for thyroid stimulating hormone beta subunit and changes in activity of the pituitary-thyroid axis during silvering of the Japanese eel, Anguilla japonica.

The purposes of this study were: (1) to clone the cDNA encoding pituitary thyroid-stimulating hormone beta subunit (TSH beta) of the Japanese eel, Anguilla japonica, together with its genomic DNA sequence, for phylogenetic analysis, and to study the regulation of the TSH beta gene expression in cultured pituitaries; and (2) to investigate the transcript levels of pituitary TSH beta mRNA and the serum thyroxine profiles at different stages of ovarian development before and during silvering in the wild female eels. The maturity of female eels was divided into four stages, juvenile, sub-adult, pre-silver, and silver, based on skin color and oocyte diameter. The genomic DNA of the TSH beta subunit contains two introns and three exons, and the TSH beta protein possesses a putative signal peptide of 20 amino acids and a mature peptide of 127 amino acids. The amino acid sequence identities of TSH beta mature peptide of Japanese eel compared with those of teleosts and other vertebrates are: European eel (98.4%), salmonids (60.6-61.3%), carps (52.0-56.7%), sturgeon (48.4%), and tetrapods (42.9-45.2%). In in vitro studies of the regulation of TSH beta mRNA it was found that thyrotropin-releasing hormone increased while thyroxine decreased its expression. RT-PCR and real-time quantitative PCR analysis showed that the transcript levels of TSH beta subunit increased during eel silvering. The serum thyroxine levels also increased in parallel with TSH beta mRNA expression during silvering, supporting the hypothesis that the hypothalamus-pituitary-thyroid axis is correlated to silvering in the wild female Japanese eels.

Amino Acid Sequence↗
PubMedDate not supplied

Microleakage of Class 2 glass-ionomer-silver restorations in primary molars.

The aims of this study were to assess microleakage at the cervical margins of class 2 "sandwich" restorations placed with two glass-ionomer-silver cements in primary molars, to compare the quality of the occlusal margins of these restorations to those prepared with Miracle Mix and Ketac Silver, and to assess by scanning electron microscopy (SEM) the marginal micromorphology of those restorations. Fifty-two class 2 restorations were prepared in extracted primary molars and were restored as follows: 1) Ketac Silver + Estilux Posterior (sandwich), 2) Miracle Mix + Estilux Posterior (sandwich), 3) Ketac Silver only, and 4) Miracle Mix only. No or minimal leakage was evident in most of the occlusal margins, whereas severe leakage was observed in almost 70% of the cervical margins of the Ketac Silver groups. Scanning electron microscopy evaluation demonstrated good adaptation at the buccal and lingual margins of all the restorations. Sixty-seven percent of the Miracle Mix restorations had no defects at the cervical margins, as opposed to only 17% of those with Ketac Silver.

Cermet Cements↗
PubMed2002

[Genetic analysis on diversity of reproductive modes of silver crucian carp inferred by transferrin and isozyme markers].

Silver crucian carp (Carassius auratus gibelio) has been known for its gynogenetic reproduction. In this paper, eggs from clone F of gynogenetic silver crucian carp were inseminated respectively by sperms of clone D, clone A and common carp, and polymorphic patterns of transferrin and isozymes of the produced three offspring FD, FA and FL were studied comparatively to explore the diversity in reproductive modes of silver crucian carp. As control, zymograms of FL progenies exhibited a maternally clonal inheritance, and gynogenesis was reconfirmed. However, differentiation of morphs and electrophoretic patterns was both observed among the FD progenies, and detection of bands specific for the clone F or clone D in some FD individuals urged the occurrence of recombination. Furthermore, extreme linkage disequilibrium for different protein loci suggested that linkage groups composed of different genes might function as the fundamental unit in the recombination. With respect to FA group, phenotypes of parental clones were both detected in the F2 generation (FA x FA progenies) while only maternal phenotypes were detected in the F1 generation. It appeared that both the parental chromosome sets could be transmitted to the offspring but expression of paternal genes were fundamentally upset in the F1 generation because of incompatibility of the parental genes. Generally, genetic analysis of FD and FA offspring primarily exhibited a particular syngamy for silver crucian carp. Besides the clonal reproduction of gynogenesis, syngamy could provide opportunities of recombination for silver crucian carp, which might remove some genetic loads from the genome and introduce new genotypes. Diversity in reproductive modes might play an important role in ecological adaptation of silver crucian carp.

Animals↗
PubMed1988

Retinal fine structure in the European eel Anguilla anguilla. VIII. Photoreceptors of the sexually mature silver eel stage.

The morphology of the photoreceptors of the sexually mature silver eel has been investigated by electron microscopy and these findings compared with observations made on the photoreceptors of the glass, yellow and sexually immature silver eel stages of the lengthy life cycle of the European eel (Anguilla anguilla). As in previous stages the photoreceptors of the sexually mature silver eel are adequately described as either rods or single cones. The rod: cone ratio at this stage is about 200: 1 which is the highest ratio of any of the stages investigated. Rods and perhaps also cones are capable of photomechanical movements. In the light-adapted stage studied, rod inner segments are the same width in the ellipsoid region as the outer segments but the rod myoid region is much thinner. Rod outer segments in the mature silver eel are slightly longer than that observed in the immature silver eel but remain much the same diameter as in previous stages. Cone outer segments taper distally and are not as wide as the inner segments. Both rods and cones display organelles in the inner segment region indicative of active protein production as it is well established that the inner segment is the synthetic center of these cells. In most cones degenerative changes were noted in the mitochondria of the ellipsoid and in the presence empty vacuoles within their cytoplasm. Cone nuclei are large and vesicular and in the light-adapted state are always located sclerad to the external limiting membrane. Rod nuclei are smaller and more electron dense and located within the outer nuclear layer. Both rods and cones still show both invaginated (ribbon) and superficial synaptic sites. In the sexually mature silver eel, therefore, the rod: cone ratio becomes higher still; the rod outer segments become somewhat longer in length and cone cell degenerative changes are even more widespread.

Anguilla↗
PubMed1977

Solubility studies of silver sulfadiazine.

The solubility of silver sulfadiazine as a function of pH was determined in nitric acid-potassium nitrate buffer for pH 2-3 and in 2-(N-morpholino)ethanesulfonic acid buffer for pH 6-7. As the salt of a weak organic acid, silver sulfadiazine exhibits the anticipated increase in solubility with an increasing hydrogen-ion concentration. Measurement of the silver-ion concentration was carried out using a silver-ion selective electrode. The methods of known subtraction and known addition were utilized to measure the total concentration of the silver ion in solution. Evidence was obtained to indicate that the salt is completely ionized in aqueous solution.

Chemical Phenomena↗
PubMed1995

Adherence of organisms to silver-coated surfaces.

Pure silver-, silver oxide- and silver chloride-treated surfaces in comparison to polypropylene inhibited both growth and adherence from saline of Serratia marcescens, Staphylococcus epidermidis, Pseudomonas aeruginosa and Candida albicans. These same organisms demonstrated enhanced adherence to an Ion-Beam-Assisted-Deposited silver surface followed by loss of viability. This type of surface in contrast to the other silver surfaces did not produce zones of inhibition in agar diffusion tests.

Anti-Bacterial Agents↗
PubMed1983

Chemical reduction of silver chromate: a procedure for electron microscopical analysis of Golgi-impregnated neurons.

A procedure is described by which the solid silver chromate precipitate present in Golgi-impregnated neurons is reduced by a photographic developer into metallic silver grains or particles. The remaining silver chromate is subsequently removed, and the fine structure and synaptic relationships of the impregnated neurons can be studied with the electron microscope. Three developers, Hydrochinone, Kodalith and D 19 were tested on cortical neurons in Vibratome sections of rapid Golgi-impregnated rat brains. Considerable differences were observed as regards penetration of the developer into the sections, uniformity of development, size and location of the silver particles produced during the development, and the quality of preservation of the ultrastructural details both in the impregnated neurons and the surrounding neuropil. Following the chemical reduction procedure the metallic silver deposit can be converted into metallic gold by a gold toning technique. Of the 3 tested developers, Kodalith and D 19 met the demands: good ultrastructural preservation, high contrast at both light microscopic (LM) and electron microscopic (EM) levels, reliable and easy to perform, minimum of test runs. Because of the high contrast provided by Kodalith and D 19, these substances are well suited for neuroanatomical tracing studies which make use of a transport or degeneration technique combined with Golgi-EM. In contrast to Kodalith, D 19 has little effect on the ultrastructure of the impregnated cells. Therefore, this developer can also be used for ultrastructural studies of Golgi-impregnated, light microscopically identified neurons.

Animals↗