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PubMed2004

Multi-step synthesis of nanoparticles performed on millisecond time scale in a microfluidic droplet-based system.

This paper reports a plug-based, microfluidic method for performing multi-step chemical reactions with millisecond time-control. It builds upon a previously reported method where aqueous reagents were injected into a flow of immiscible fluid (fluorocarbons)(H. Song et al., Angew. Chem. Int. Ed., 2003, 42, 768). The aqueous reagents formed plugs--droplets surrounded and transported by the immiscible fluid. Winding channels rapidly mixed the reagents in droplets. This paper shows that further stages of the reaction could be initiated by flowing additional reagent streams directly into the droplets of initial reaction mixture. The conditions necessary for an aqueous stream to merge with aqueous droplets were characterized. The Capillary number could be used to predict the behavior of the two-phase flow at the merging junction. By transporting solid reaction products in droplets, the products were kept from aggregating on the walls of the microchannels. To demonstrate the utility of this microfluidic method it was used to synthesize colloidal CdS and CdS/CdSe core-shell nanoparticles.

Cadmium Compounds↗
PubMed2004

Single-molecule measurements of gold-quenched quantum dots.

We report the study of the quenching of quantum dots (CdSe) by gold nanoparticles at the single-molecule level. Double-stranded DNA is used as a rigid spacer to tune the distance between the two nanoparticles. The width of the fluorescent intensity distribution, monitored at different interparticle distances, reflects both the nanoparticle heterogeneity and the fluorescence intermittency of the quantum dot. The fluorescence distribution emitted by single CdSe nanocrystals can easily be distinguished from the fluorescence of partially quenched CdSe. Our results show that the distance-dependence quenching is compatible with a Förster-type process.

Cadmium Compounds↗
PubMed1963

REDUCTION OF SELENITE BY INTACT YEAST CELLS AND CELL-FREE PREPARATIONS.

Falcone, Giuseppe (Rutgers, The State University, New Brunswick, N.J.), and Walter J. Nickerson. Reduction of selenite by intact yeast cells and cell-free preparations. J. Bacteriol. 85:754-762. 1963.-Nonproliferating cell suspensions of Candida albicans rapidly reduced selenite to red, metallic selenium in the absence of added substrate. Cell suspensions reduced selenite optimally at pH 4.2. No added metabolite was found to be stimulatory; reduction was inhibited by methionine and formate as well as by fluoride, dinitrophenol, and certain sulfhydryl poisons. Cell-free preparations capable of reducing selenite were obtained from C. albicans and from baker's yeast disintegrated in a Hughes press. The enzymatic system had optimal activity at pH 7 with 10(-2)m selenite. Activity of the system was lost on dialysis but was restored upon the addition of dialyzable substances or of boiled, undialyzed extract.

Candida↗
PubMed2004

Cadmium sulfide and cadmium selenide/cadmium sulfide nanoparticles stabilized in water with poly(cysteine acrylamide).

Cysteine acrylamide (N-acryloyl L-cysteine) stabilizes CdS nanoparticles as the particles form in aqueous dispersions. Cysteine acrylamide also exchanges for citrate on the surfaces of CdSe and core/shell CdSe/CdS nanoparticles to provide greater stability. Heating of the nanoparticle dispersions polymerizes the cysteine acrylamide on the surface to form a more efficient polydentate stabilizer. The polymer-coated nanoparticle dispersions are colloidally stable even after removal of low molecular weight solutes by dialysis. Emission quantum yields of the polymer-coated CdSe and CdSe/CdS samples were 0.9% and 2.6%, respectively, after aging of the samples in light. CdSe/CdS coated with poly(cysteine acrylamide) is colloidally stable for at least two years in the dark at 5 degrees C.

Acrylic Resins↗
PubMed2005

Single-crystal CdSe nanowires prepared via vapor-phase growth assisted with silicon.

Hexagonal cadmium selenide (CdSe) nanowires, with diameter around 20 nm, were synthesized using a simple vapor-phase growth. Silicon (Si) powder acts as a source material assisting the synthesis, which is very important to the formation of the CdSe nanowires. We also suggest that self-catalysis at the Cd-terminated (0001) surface, together with the assistance action of Si, leads to the formation of wire-like structures to be formed. Meanwhile, the assistance of Si is responsible for the fineness and uniformity of the CdSe nanowires. The possible growth mechanism of the CdSe nanowires is proposed, and the optical property of the as-grown CdSe nanowires is characterized.

Cadmium Compounds↗
PubMed2004

Studies on tellurium-containing heterocycles. Part 20. Reactions of 2-benzoselenopyrylium salts and 2-benzotelluropyrylium salts with nucleophiles: formation of 1-functionalized 1H-isoselenochromenes and 1H-isotellurochromenes.

The reactions of the 2-benzoselenopyrylium (1A) and 2-benzotelluropyrylium cations (1B) with a variety of nucleophiles have been investigated. LiAlH(4), sodium alkoxide (NaOMe, NaOi-Pr and NaOt-Bu), diethylamine, n-butylamine and acetone reacted with 1 to give the 1H-isochromenes (2) and the corresponding 1-substituted products (4-9) under mild conditions in almost good to high yields. The 1-alkyl(phenyl)isoselenochromenes (10-13) and 1-benzylisochromenes (18A, 18B), which were produced by the reaction of the salts 1 with Grignard reagents, were converted to the corresponding 1,3-disubstituted 2-benzopyrylium salts (14-17, 19) by treatment with triphenylcarbenium tetrafluoroborate (Ph(3)C(+) BF(4)(-)), respectively. The 1-benzylselenopyrylium salts (19A) and 1-benzyltelluropyrylium salts (19B) exist in the solvent as an equilibrium mixture of the salts (19) and the corresponding (Z)-benzylidene compounds (20).

Acids, Heterocyclic↗
PubMed1987

Toxic and carcinogenic agents in dry and moist snuff.

The oral use of snuff is causatively associated with cancer of the oral cavity. Since most epidemiologic studies to date relate to the long-term use of dry snuff, which has dominated the U.S. smokeless tobacco market in the past, the concentrations of several toxic and carcinogenic agents in the three most popular dry snuff brands have been compared with those in the five most popular moist snuff brands sold in the United States. All eight samples were analyzed for nitrate, alkaloids, polyphenols, volatile carbonyl compounds, lead, cadmium, selenium, and the carcinogenic compounds benzo[a]pyrene (CAS: 50-32-8), polonium-210 (CAS: 13981-52-7), volatile N-nitrosamines (VNAs), N-nitrosodiethanolamine (CAS: 1116-54-7), and the tobacco-specific N-nitrosamines (TSNAs). Most of the snuff brands were rich in nitrate (greater than or equal to 1.5%), total polyphenols (greater than 2%), and in nicotine (greater than or equal to 1.5%), which is the habituating factor in tobacco use. Concentrations of the VNAs were significantly above the permissible limits set for some food products; the concentrations of the TSNAs in both snuff types exceeded the levels of nitrosamines in other consumer products by at least two to three orders of magnitude. The extremely high levels of the TSNAs in snuff have remained unchanged during the last decade and present the major carcinogenic risk factor for the oral use of snuff. Polonium-210 contributes further to the carcinogenic risk associated with snuff. The chemical-analytical data presented in this study do not indicate marked differences in the carcinogenic potential of moist snuff compared to dry snuff.

Acetone↗
PubMedDate not supplied

Chemoprevention with triphenylselenonium chloride in selenium-deficient rats.

Cancer chemoprevention by high levels of selenium, including compounds like sodium selenite or selenomethionine, is generally not accompanied by increases in known selenoenzymes. There has been no information on whether selenoenzymes are obligatory mediators of the anticarcinogenic effect of selenium. Our previous experience with triphenylselenonium chloride suggests that it might be an ideal agent for studying selenium chemoprevention while simultaneously precluding the synthesis of selenoenzymes. Triphenylselenonium chloride has excellent tumor inhibitory activity but does not support the repletion of selenoenzymes in animals that have been deprived of a bioavailable form of selenium. In the present experiments, we evaluated the efficacy of mammary cancer protection by this compound in rats fed either a selenite-deficient (< 0.01 ppm Se) or selenite-adequate (0.1 ppm Se) diet. We also measured the activities of liver glutathione peroxidase and thioredoxin reductase as markers of selenium bioavailability in these different treatment conditions. In carcinogen-treated control animals not receiving triphenylselenonium chloride, mammary tumor incidence and the total number of tumors were similar between the selenite-deficient and selenite-adequate groups. Thus the correction of selenium deficiency by the addition of 0.1 ppm Se as selenite did not have detectable anticarcinogenic effects. Supplementation of triphenylselenonium chloride at a level of 30 ppm Se suppressed mammary tumorigenesis by approximately 50% regardless of dietary selenium nutritional status. However, this supplement had little effect on tissue selenium levels and did not increase liver glutathione peroxidase or thioredoxin reductase activities. In contrast, a level of 0.1 ppm Se as selenite did not affect mammary tumorigenesis but markedly increased tissue selenium concentrations and selenoenzyme activities. It is concluded that triphenylselenonium chloride does not release inorganic selenium for selenoprotein synthesis and that its anticancer activity involves mechanisms that are probably intrinsic to the compound. This study also shows for the first time that selenium chemoprevention is possible in an environment of severely depressed selenoenzyme expression. Thus selenium chemoprevention efficacy can be separated experimentally from selenoprotein synthesis using this model system.

Animals↗
PubMed2006

Selenium speciation analysis using inductively coupled plasma-mass spectrometry.

Selenium exists in several oxidation states and a variety of inorganic and organic compounds, and the chemistry of selenium is complex in both the environment and living systems. Selenium is an essential element at trace levels and toxic at greater levels. Interest in speciation analysis for selenium has grown rapidly in this last decade, especially in the use of chromatographic separation coupled with inductively coupled plasma-mass spectrometry (ICP-MS). Complete characterization of selenium compounds is necessary to understand selenium's significance in metabolic processes, clinical chemistry, biology, toxicology, nutrition and the environment. This review describes some of the essential background of selenium, and more importantly, some of the currently used separation methodologies, both chromatographic and electrophoretic, with emphasis on applications of selenium speciation analysis using ICP-MS detection.

Chromatography, Gas↗
PubMed2006

Imaging Escherichia coli using functionalized core/shell CdSe/CdS quantum dots.

The internalization of a series of water-soluble CdSe/CdS quantum dots (QDs) stabilized by citrate, isocitrate, succinate, and malate by Escherichia coli is established by epifluorescence and confocal fluorescence scanning microscopy, fluorimetry, and UV-vis spectroscopy on whole and lysed bacterial cells. The organic-acid-stabilized QDs span a range in size from 3.8+/-1.1 to 6.0+/-2.4 nm with emission wavelengths from 540 to 630 nm. QDs of different sizes (i.e., 3.8-6 nm) can enter the bacterium and be detected on different fluorescence channels with little interference from other QDs as a result of the distinct emission profiles (i.e., 540-630 nm, respectively). Costaining QD-labeled E. coli with 4',6-diamidino-2-phenylindole dihydrochloride (DAPI) demonstrates that the QDs and DAPI are colocalized within E. coli, whereas costaining QD-labeled E. coli with membrane dye FM4-64 shows that the FM4-64 is localized in the outer bacterial membrane and that the QDs are inside.

Cadmium Compounds↗
PubMed1989

Selenotrisulfide inhibits initiation by RNA polymerase II but not elongation.

We previously reported that RNA polymerase II (purified from wheat germ) is inhibited by selenotrisulfides, the products of the reaction of selenite with sulfhydryl compounds [Frenkel, Walcott, and Middleton, Molecular Pharmacology 31, 112 (1987)]. We have now found that the initiation stage of the reaction is inhibited by selenotrisulfide but the elongation stage of the reaction is not. The actual start of the RNA chain is not inhibited by the selenotrisulfide, but rather the formation of the enzyme-DNA binary complex. Selenotrisulfide has a similar differential effect on initiation and elongation by RNA polymerase II from HeLa cells; in contrast, with E. coli RNA polymerase, it inhibits elongation as well.

Cysteamine↗
PubMed2006

A highly sensitive system for urea detection by using CdSe/ZnS core-shell quantum dots.

An original and novel assay system with urease as a catalyst and CdSe/ZnS quantum dots (QDs) as an indicator has been developed for quantitative analysis of urea. By mixing urease and QDs, the determination of urea can be performed in a quantitative manner. The detection is based on the enhancement of QD photoluminescence (PL) intensity, which is correlated to the enzymatic degradation of urea. By controlling the buffer concentration and pH, PL enhancement due to the degradation of urea is linear in the urea concentration ranging from 0.01 to 100mM. This property makes the urease/QDs system to be a promising urea-biosensing system. The newly developed system is a superior design and possesses many advantages, including its simple preparation, low cost, no enzyme immobilization required, high flexibility, and good sensitivity.

Biosensing Techniques↗
PubMed2004

The interaction between some diamines and CdSe quantum dots.

The interaction of some diamines (ethylenediamine (EDA), 1,6-hexanediamine (HDA), o-phenylenediamine (OPD)) with CdSe quantum dots (QDs) is reported. With increasing concentration of EDA from 0 to 2.0 x 10(-6) mol l(-1), slight fluorescence enhancement is observed. However, the CdSe QDs fluorescence quenching is seen at relatively higher concentration of EDA. There is a red-shift of 0-7 nm in fluorescence emission spectra of CdSe QDs when the concentration of EDA is changed from 2.0 x 10(-6) to 8.0 x 10(-6) mol l(-1). The resonance light scattering (RLS) spectra of CdSe QDs have little change when the concentration of EDA is less than 5.0 x 10(-6) mol l(-1). It indicates there are little large particles formed in the solution. However, a significant increase of the RLS is observed in the 300-500 nm wavelength range after adding higher concentration than 5.0 x 10(-6) mol l(-1) EDA, which could be attributed to the large particles formed. The interaction between HDA and CdSe QDs is similar to that of EDA. However, with the OPD, it is found that the interaction is much different from those of EDA, HDA, and that the quenching, even at low concentration, is effective for CdSe QDs emission. The quenching phenomenon could be explained by a surface bound complexation equilibrium model.

Cadmium Compounds↗