Determination of trace transition elements in ultra high purity sodium and calcium carbonates by neutron activation analysis

Résumé On mesure le cuivre 64 et le manganèse 56 dans du carbonate de sodium ultra pur par spectroscopie du rayonnement gamma après élimination du²⁴Na par passage sur colonnes de pentoxide d'antimoine hydraté (PAH); On a aussi déterminé le cobalt, le chrome et le fer par détection spectrométrique non-destructive du rayonnement gamma du⁶⁰Co,⁵⁹Fe et⁵¹Cr. Dans le cas de CaCo₃, après irradiation et dissolution, on a déterminé simultanément le⁶⁴Cu et le⁵⁶Mn, par une séparation radiochimique sélective à pH 3 à 4 avec un mélange de dithizone (H₂D) et d'acide pynolidinedithiocarbamique (HPDC) dans CHCL₃. L'interférence radioactive de⁴⁷Sc et⁴⁷Ca produits pendant 100 heures d'irradiation de CaCO₃, étaient supprimées par extraction sélective de ce dernier en milieu HCl 5,0 M par l'oxyde de Tri-n-octylphosphine (TOPO) 0,1 M dans du cyclohexane. On a utilisé en mélange de 0,1 M Thenoyltrifluoroacetone (HTTA) 0,1 M et de TOPO 0,1 M dans du cyclohexane pour enlever le⁴⁷Ca à pH≥8.0. Après avoir éliminé l'activité des isotopes de la matrice, on mesure les pics photoélectriques de⁵⁹Fe.⁶⁰Co et⁵¹Cr pour doser ces impuretés.      

The analytical concentration of humic substances from natural waters

The thermodynamics of the adsorption of humic and fulvic acids on the macroreticular polystyrene resin Amberlite XAD-2 have been investigated with a view to optimizing the conditions for its application to the analytical concentration of these compounds. Under the optimal conditions, recoveries of humic and fulvic acids of above 92 and 75%, respectively, were achieved. It has been shown that these compounds can be fractionated on a molecular weight basis during the desorption stage by serial elution at selected pH values.     

The synthesis of bioactive indolocarbazoles related to K-252a

A range of functionalised indolocarbazoles, related to the natural product K-252a, have been prepared, starting from a readily available bridged cyclopentene. Sequences of transformations, involving initial hydroboration-oxidation to give a ketone, or by dihydroxylation and cyclic sulfate formation, enable the preparation of diverse indolocarbazole products. Issues of imide nitrogen protection for the indolocarbazole, and opportunities for asymmetric desymmetrisation of key intermediates were also explored. A novel chiral lithium amide base mediated transformation of a cyclic sulfate intermediate gave the anticipated ketone product in up to 87% ee. A number of compounds, in the form of unprotected imide substituted indolocarbazoles, were screened for biological activity and were found to be potent inhibitors of a number of kinase enzymes.     

Multicolor polymeric carbon dots: synthesis,separation and polyamide-supported molecular fluorescence

Multicolor carbon dots (CDs) have been developed recently and demonstrate great potential in bio-imaging, sensing, and LEDs. However, the fluorescence mechanism of their tunable colors is still under debate, and efficient separation methods are still challenging. Herein, we synthesized multicolor polymeric CDs through solvothermal treatment of citric acid and urea in formamide. Automated reversed-phase column separation was used to achieve fractions with distinct colors, including blue, cyan, green, yellow, orange and red. This work explores the physicochemical properties and fluorescence origins of the red, green, and blue fractions in depth with combined experimental and computational methods. Three dominant fluorescence mechanism hypotheses were evaluated by comparing time-dependent density functional theory and molecular dynamics calculation results to measured characteristics. We find that blue fluorescence likely comes from embedded small molecules trapped in carbonaceous cages, while pyrene analogs are the most likely origin for emission at other wavelengths, especially in the red. Also important, upon interaction with live cells, different CD color fractions are trafficked to different sub-cellular locations. Super-resolution imaging shows that the blue CDs were found in a variety of organelles, such as mitochondria and lysosomes, while the red CDs were primarily localized in lysosomes. These findings significantly advance our understanding of the photoluminescence mechanism of multicolor CDs and help to guide future design and applications of these promising nanomaterials.

Understanding the origin and sensitivity of carbon dot emission will improve their utility in various applications.

Since the accidental discovery of luminescent carbon fragments in 2004,¹ carbon dots (CDs) have attracted great research interest due to the diverse synthetic methods, tunable luminescence, and applicability in a broad range of fields, including bio-imaging,^2–4 sensing,^5,6 and light emitting diodes (LEDs).^7,8 Typically, CDs are fluorescent carbon nanostructures of sizes less than 10 nm, composed of carbon, oxygen, and nitrogen.^9–12 CDs can be produced through bottom-up methods, which involve small molecular precursors like citric acid, malic acid, urea, ethylenediamine, and so on.^13–15 In a high temperature reaction, polymerization and dehydration occur among various functional groups, and the resulting products are usually a mixture of small molecule residues, oligomers, and long chain polymers.¹⁶ The unclear fluorescence mechanisms and poorly understood internal structure of CDs limit the ability to understand, tune, and fully exploit their fluorescence properties.Fortunately, in recent years, breakthrough syntheses of multicolor CDs have been achieved.^17–19 Several different multicolor CDs have been synthesized with aromatic compounds such as phenylenediamine.^4,20–22 However, it should be noted that precursors such as aniline and phenol may have toxic effects on human health and the environment,^23,24 and thus should be avoided where possible. Syntheses of colorful CDs from non-aromatic compounds such as citric acid and urea often employ solvothermal methods. Utilizing different solvents such as formamide and dimethylformamide have been shown to play a significant role in tuning CD emission.^25,26 In addition, chromatographic post-treatment of as-made CDs plays a critical role in obtaining different colored fractions, using techniques such as anion-exchange column chromatography,²⁶ normal phase silica chromatography,²⁷ and reversed phase silica chromatography.¹⁵ Compared with high performance liquid chromatography (HPLC), the aforementioned column chromatography techniques help to separate CDs on a larger scale. These separations are based on charge²⁶ or polarity,²¹ and are efficient in isolating the desired fractions with distinct colors so that detailed structural characterization can be performed.To gain insight into the fluorescence mechanism of these multicolor CDs, researchers have considered three hypotheses: quantum size effects,²⁸ the inclusion of molecular fluorophores,²⁹ and surface state-induced emission.³⁰ For example, Rogach and coworkers developed solid-state CDs with tunable fluorescence via the seeded growth method. They attributed the tunable emission to the size of π-conjugated domains.³¹ Yang and coworkers synthesized CDs by hydrothermal treatment of citric acid and ethylenediamine. They identified a small molecule fluorophore, IPCA (1,2,3,5-tetrahydro-5-oxo-imidazo[1,2-α]pyridiine-7-carboxylic acid) from CD column separation fractions, which contributed to the blue fluorescence.¹³ Xiong and coworkers synthesized CDs from urea and p-phenylenediamine that emitted a range of colors and separated them with silica column chromatography. They found the degree of carbon oxidation increased as the emission redshifted and thus, they endorsed the surface state hypothesis.²¹ In addition to the above mechanisms, computational methods such as density functional theory (DFT) have also been applied to analyze the fluorescence origins of CDs. The charge transfer between functional groups on the polymeric unit of CDs made from citric acid and ethylenediamine was found to facilitate blue emission.¹⁶The goal of present work is to understand the fluorescence origin of multicolor CDs. The model multicolor CDs were obtained by reacting citric acid and urea in formamide via a microwave-assisted hydrothermal treatment. An automated chromatographic apparatus was employed to separate as-made CD mixtures into distinct color fractions. The individual separation process took around 20 minutes, and the obtained CD fractions exhibit discrete illumination-induced emission throughout the visible region of the spectrum. Interestingly, the sizes of separated CD fractions are not statistically different from one another, suggesting that the quantum size effects are not the source of differential emission. Solvatochromism experiments showed that the blue and green fractions have similar fluorescence behavior as a function of solvent polarity, but the red fraction behaved differently. Using computational simulations, three models of the fluorescence origin were constructed and evaluated, showing that the formation of small blue fluorescent molecules is likely and pyrene analogs could be the origins for various emission colors. Moreover, two representative CD fractions, the blue- and red-emitting fractions, were chosen for subsequent cell imaging experiments. The localization pattern for the CD fractions differed: blue-emitting CDs were observed in a wide range of organelles, while red-emitting CDs were primarily enclosed in lysosomes. Understanding the origin and the sensitivity of CD emission will improve their utility in bioimaging applications.     

The determination of selenium in sea water,silicates and marine organisms

Spectrophotometric procedures are described for the determination of selenium in sea water, silicates (especially marine sediments) and marine organisms. Coprecipitation with iron(III) hydroxide at pH 4–6 is used to concentrate selenium and to separate it from many of the commoner elements. Separation from iron and other cations is achieved by ion exchange. Selenium is determined photometrically with diaminobenzidine. Isotope dilution with selenium-75 is used to correct results for the small losses occurring during the analysis. Silicates can be decomposed without loss of selenium by means of a mixture of hydrofluoric and nitric acids. The method of Cummins et al., with sulphuric and perchloric acids in presence of molybdate ion, is highly satisfactory for the decomposition of bio-materials. For sea water, which contains ca. 0.4–0.5 <mg Se/l, a standard deviation of 0.03 μg/l was obtained. A silicate sediment and a sea weed containing ca. 1.5 μg Se/g and 0.8 μg Se/g respectively gave coefficients of variation of 8.0% and 4.7%. The U.S. Geological Survey standard granite G1 was found to contain 2.5 ± 0.1 μg Se/g.     

Chelating resins for the concentration of trace elements from sea water and their analytical use in conjunction with atomic absorption spectrophotometry

An investigation has been made of the uptake of trace elements from both distilled water and sea water by the chelating ion-exchange resins Chelex-100 and Permutit S1005. The resins retained the following elements with an efficiency of ca. 100%: Ag, Bi, Cd, Cu, In, Pb, Mo, Ni, rare earths, Re (90% only), Sc, Th, W, V, Y and Zn. Manganese was retained quantitatively only by the Chelex resin. The following elements are removed with 100% efficiency by means of₂N mineral acids: Bi, Cd, Co, Cu, In, Ni, Pb, rare earths, Sc, Th, Y and Zn. Ammonia (4 N) completely removes molybdenum, tungsten, vanadium and rhenium. The resins have been used in conjunction with atomic absorption spectrophotometry for the simultaneous determination of zinc, cadmium, copper, nickel and cobalt in sea waters.     

Speciation of chromium in Australian fly ash

The concentrations of chromium (III) and (VI) in fly ash from nine Australian coal fired power stations were determined. Cr(VI) was completely leached by extraction with 0.01 M NaOH solution and the concentration was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES). This was confirmed by determining Cr(III) and Cr(VI) in the extracts of fly ash that had been spiked with chromium salts. These analytical measurements were done using a combination of ion-exchange chromatography and ICP-AES. The elutant was 0.05 M HNO₃ containing 0.5%-CH₃OH. When the column was operated at a flow rate of 1.2 ml min⁻¹ and samples were injected by use of a sample loop with a volume of 100 μl, Cr(III) and Cr(VI) in sample solution was exclusively separated within approximately 10 min. The detection limits (3σ) were 5 ng for Cr(III) (0.050 mg l⁻¹) and 9 ng for Cr(VI) (0.090 mg l⁻¹), respectively. A relative standard deviation of 1.9% (n = 6) was obtained for the determination by IC-ICP-AES of 0.25 mg l⁻¹ Cr(III) and Cr(VI).     

Generalized RNA-directed recombination of RNA

RNA strand exchange through phosphor-nucleotidyl transfer reactions is an intrinsic chemistry promoted by group I intron ribozymes. We show here that Tetrahymena and Azoarcus ribozymes can promote RNA oligonucleotide recombination in either two-pot or one-pot schemes. These ribozymes bind one oligonucleotide, cleave following a guide sequence, transfer the 3' portion of the oligo to their own 3' end, bind a second oligo, and catalyze another transfer reaction to generate recombinant oligos. Recombination is most effective with the Azoarcus ribozyme in a single reaction vessel in which over 75% of the second oligo can be rapidly converted to recombinant product. The Azoarcus ribozyme can also create a new functional RNA, a hammerhead ribozyme, which can be constructed via recombination and then immediately promote its own catalysis in a homogeneous milieu, mimicking events in a prebiotic soup.