Chalcogen Bonding in the Molecular Dimers of WCh2 (Ch = S, Se, Te): On the Basic Understanding of the Local Interfacial and Interlayer Bonding Environment in 2D Layered Tungsten Dichalcogenides.

Layered two-dimensional transition metal dichalcogenides and their heterostructures are of current interest, owing to the diversity of their applications in many areas of materials nanoscience and technologies. With this in mind, we have examined the three molecular dimers of the tungsten dichalcogenide series, (WCh2)2 (Ch = S, Se, Te), using density functional theory to provide insight into which interactions, and their specific characteristics, are responsible for the interfacial/interlayer region in the room temperature 2H phase of WCh2 crystals. Our calculations at various levels of theory suggested that the Te···Te chalcogen bonding in (WTe2)2 is weak, whereas the Se···Se and S···S bonding interactions in (WSe2)2 and (WS2)2, respectively, are of the van der Waals type. The presence and character of Ch···Ch chalcogen bonding interactions in the dimers of (WCh2)2 are examined with a number of theoretical approaches and discussed, including charge-density-based approaches, such as the quantum theory of atoms in molecules, interaction region indicator, independent gradient model, and reduced density gradient non-covalent index approaches. The charge-density-based topological features are shown to be concordant with the results that originate from the extrema of potential on the electrostatic surfaces of WCh2 monomers. A natural bond orbital analysis has enabled us to suggest a number of weak hyperconjugative charge transfer interactions between the interacting monomers that are responsible for the geometry of the (WCh2)2 dimers at equilibrium. In addition to other features, we demonstrate that there is no so-called van der Waals gap between the monolayers in two-dimensional layered transition metal tungsten dichalcogenides, which are gapless, and that the (WCh2)2 dimers may be prototypes for a basic understanding of the physical chemistry of the chemical bonding environments associated with the local interfacial/interlayer regions in layered 2H-WCh2 nanoscale systems.

Oligomers of Cyclic Oligochalcogenides for Enhanced Cellular Uptake.

Monomeric cyclic oligochalcogenides (COCs) are emerging as attractive transporters to deliver substrates of interest into the cytosol through thiol-mediated uptake. The objective of this study was to explore COC oligomers. We report a systematic evaluation of monomers, dimers, and trimers of asparagusic, lipoic, and diselenolipoic acid as well as their supramolecular monomers, dimers, trimers, and tetramers. COC dimers were more than twice as active as the monomers on both the covalent and noncovalent levels, whereas COC trimers were not much better than dimers. These trends might suggest that thiol-mediated uptake of COCs is synergistic over both short and long distances, that is, it involves more than two COCs and more than one membrane protein, although other interpretations cannot be excluded at this level of complexity. These results thus provide attractive perspectives for structural evolution as well as imminent use in practice. Moreover, they validate automated HC-CAPA as an invaluable method to collect comprehensive data on cytosolic delivery within a reasonable time at a level of confidence that is otherwise inconceivable.

Probing for Thiol-Mediated Uptake into Bacteria.

Cellular uptake mediated by cyclic oligochalcogenides (COCs) is emerging as a conceptually innovative method to penetrate mammalian cells. Their mode of action is based on dynamic covalent oligochalcogenide exchange with cellular thiols. To test thiol-mediated uptake in bacteria, five antibiotics have been equipped with up to three different COCs: One diselenolane and two dithiolanes. We found that the COCs do not activate antibiotics in Gram-negative bacteria. In Gram-positive bacteria, the COCs inactivate antibiotics that act in the cytoplasm and reduce the activity of antibiotics that act on the cell surface. These results indicate that thiol-mediated uptake operates in neither of the membranes of bacteria. COCs are likely to exchange with thiols on the inner, maybe also on the outer membrane, but do not move on. Concerning mammalian cells, the absence of a COC-mediated uptake into bacteria observed in this study disfavors trivial mechanisms, such as passive diffusion, and supports the existence of more sophisticated, so far poorly understood uptake pathways.

Biosynthesis of bismuth selenide nanoparticles using chalcogen-metabolizing bacteria.

Cost and energy reductions in the production process of bismuth chalcogenide (BC) semiconductor materials are essential to make thermoelectric generators comprised of BCs profitable and CO2 neutral over their life cycle. In this study, as an eco-friendly production method, bismuth selenide (Bi2Se3) nanoparticles were synthesized using the following five strains of chalcogen-metabolizing bacteria: Pseudomonas stutzeri NT-I, Pseudomonas sp. RB, Stenotrophomonas maltophilia TI-1, Ochrobactrum anthropi TI-2, and O. anthropi TI-3 under aerobic conditions. All strains actively volatilized selenium (Se) by reducing selenite, possibly to organoselenides. In the growth media containing bismuth (Bi) and Se, all strains removed Bi and Se concomitantly and synthesized nanoparticles containing Bi and Se as their main components. Particles synthesized by strain NT-I had a theoretical elemental composition of Bi2Se3, whereas those synthesized by other strains contained a small amount of sulfur in addition to Bi and Se, making strain NT-I the best Bi2Se3 synthesizer among the strains used in this study. The particle sizes were 50-100 nm in diameter, which is sufficiently small for nanostructured semiconductor materials that exhibit quantum size effect. Successful synthesis of Bi2Se3 nanoparticles could be attributed to the high Se-volatilizing activities of the bacterial strains. Selenol-containing compounds as intermediates of Se-volatilizing metabolic pathways, such as methane selenol and selenocysteine, may play an important role in biosynthesis of Bi2Se3.

Role of the Chalcogen (S, Se, Te) in the Oxidation Mechanism of the Glutathione Peroxidase Active Site.

The oxidation by H2 O2 of the human phospholipid hydroperoxide glutathione peroxidase (GPx4), used as a model peroxidase selenoenzyme, as well as that of its cysteine (Cys) and tellurocysteine (Tec) mutants, was investigated in silico through a combined classic and quantum mechanics approach to assess the role of the different chalcogens. To perform this analysis, new parameters for selenocysteine (Sec) and tellurocysteine (Tec) were accurately derived for the AMBER ff14SB force field. The oxidation represents the initial step of the antioxidant activity of GPx, which catalyzes the reduction of H2 O2 and organic hydroperoxides by glutathione (GSH). A mechanism involving a charge-separation intermediate is feasible for the Cys and Sec enzymes, leading from the initial thiol/selenol form to sulfenic/selenenic acid, whereas for the Tec mutant a direct oxidation pathway is proposed. Activation strain analyses, performed for Cys-GPx and Sec-GPx, provided insight into the rate-accelerating effect of selenium as compared to sulfur and the role of specific amino acids other than Cys/Sec that are typically conserved in the catalytic pocket.

Rhizopus stolonifer mediated biosynthesis of biocompatible cadmium chalcogenide quantum dots.

We report an efficient method to biosynthesize biocompatible cadmium telluride and cadmium sulphide quantum dots from the fungus Rhizopus stolonifer. The suspension of the quantum dots exhibited purple and greenish-blue luminescence respectively upon UV light illumination. Photoluminescence spectroscopy, X-ray diffraction, and transmission electron microscopy confirms the formation of the quantum dots. From the photoluminescence spectrum the emission maxima is found to be 424 and 476nm respectively. The X-ray diffraction of the quantum dots matches with results reported in literature. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay for cell viability evaluation carried out on 3-days transfer, inoculum 3×105 cells, embryonic fibroblast cells lines shows that more than 80% of the cells are viable even after 48h, indicating the biocompatible nature of the quantum dots. A good contrast in imaging has been obtained upon incorporating the quantum dots in human breast adenocarcinoma Michigan Cancer Foundation-7 cell lines.

Anion recognition by a bidentate chalcogen bond donor.

Perfluoroaryl-substituted tellurophenes act as anion receptors through noncovalent chalcogen bonding interactions. Linking two tellurophenes through an ethynylene group results in a significant level of chelate cooperativity, thus demonstrating that chalcogen bonding can be used to achieve multidentate anion recognition.

Stochastic phase-change neurons.

Artificial neuromorphic systems based on populations of spiking neurons are an indispensable tool in understanding the human brain and in constructing neuromimetic computational systems. To reach areal and power efficiencies comparable to those seen in biological systems, electroionics-based and phase-change-based memristive devices have been explored as nanoscale counterparts of synapses. However, progress on scalable realizations of neurons has so far been limited. Here, we show that chalcogenide-based phase-change materials can be used to create an artificial neuron in which the membrane potential is represented by the phase configuration of the nanoscale phase-change device. By exploiting the physics of reversible amorphous-to-crystal phase transitions, we show that the temporal integration of postsynaptic potentials can be achieved on a nanosecond timescale. Moreover, we show that this is inherently stochastic because of the melt-quench-induced reconfiguration of the atomic structure occurring when the neuron is reset. We demonstrate the use of these phase-change neurons, and their populations, in the detection of temporal correlations in parallel data streams and in sub-Nyquist representation of high-bandwidth signals.

Microorganism mediated biosynthesis of metal chalcogenides; a powerful tool to transform toxic effluents into functional nanomaterials.

Cadmium contained in soil and water can be taken up by certain crops and aquatic organisms and accumulate in the food-chain, thus removal of Cd from mining or industrial effluents - i.e. Ni-Cd batteries, electroplating, pigments, fertilizers - becomes mandatory for human health. In parallel, there is an increased interest in the production of luminescent Q-dots for applications in bioimaging, sensors and electronic devices, even the present synthesis methods are economic and environmentally costly. An alternative green pathway for producing Metal chalcogenides (MC: CdS, CdSe, CdTe) nanocrystals is based on the metabolic activity of living organisms. Intracellular and extracellular biosynthesis of can be achieved within a biomimetic approach feeding living organisms with Cd precursors providing new routes for combining bioremediation with green routes for producing MC nanoparticles. In this mini-review we present the state-of-the-art of biosynthesis of MC nanoparticles with a critical discussion of parameters involved and protocols. Few existing examples of scaling-up are also discussed. A modular reactor based on microorganisms entrapped in biocompatible mineral matrices - already proven for bioremediation of dissolved dyes - is proposed for combining both Cd-depletion and MC nanoparticle's production.

Ultra-low Doping on Two-Dimensional Transition Metal Dichalcogenides using DNA Nanostructure Doped by a Combination of Lanthanide and Metal Ions.

Here, we propose a novel DNA-based doping method on MoS2 and WSe2 films, which enables ultra-low n- and p-doping control and allows for proper adjustments in device performance. This is achieved by selecting and/or combining different types of divalent metal and trivalent lanthanide (Ln) ions on DNA nanostructures, using the newly proposed concept of Co-DNA (DNA functionalized by both divalent metal and trivalent Ln ions). The available n-doping range on the MoS2 by Ln-DNA is between 6 × 10(9) and 2.6 × 10(10 ) cm(-2). The p-doping change on WSe2 by Ln-DNA is adjusted between -1.0 × 10(10) and -2.4 × 10(10 ) cm(-2). In Eu(3+) or Gd(3+)-Co-DNA doping, a light p-doping is observed on MoS2 and WSe2 (~10(10 ) cm(-2)). However, in the devices doped by Tb(3+) or Er(3+)-Co-DNA, a light n-doping (~10(10 ) cm(-2)) occurs. A significant increase in on-current is also observed on the MoS2 and WSe2 devices, which are, respectively, doped by Tb(3+)- and Gd(3+)-Co-DNA, due to the reduction of effective barrier heights by the doping. In terms of optoelectronic device performance, the Tb(3+) or Er(3+)-Co-DNA (n-doping) and the Eu(3+) or Gd(3+)-Co-DNA (p-doping) improve the MoS2 and WSe2 photodetectors, respectively. We also show an excellent absorbing property by Tb(3+) ions on the TMD photodetectors.

Microbial synthesis of chalcogenide semiconductor nanoparticles: a review.

Chalcogenide semiconductor quantum dots are emerging as promising nanomaterials due to their size tunable optoelectronic properties. The commercial synthesis and their subsequent integration for practical uses have, however, been contorted largely due to the toxicity and cost issues associated with the present chemical synthesis protocols. Accordingly, there is an immediate need to develop alternative environment-friendly synthesis procedures. Microbial factories hold immense potential to achieve this objective. Over the past few years, bacteria, fungi and yeasts have been experimented with as eco-friendly and cost-effective tools for the biosynthesis of semiconductor quantum dots. This review provides a detailed overview about the production of chalcogen-based semiconductor quantum particles using the inherent microbial machinery.

Rational design of a chalcogenopyrylium-based surface-enhanced resonance Raman scattering nanoprobe with attomolar sensitivity.

High sensitivity and specificity are two desirable features in biomedical imaging. Raman imaging has surfaced as a promising optical modality that offers both. Here we report the design and synthesis of a group of near-infrared absorbing 2-thienyl-substituted chalcogenopyrylium dyes tailored to have high affinity for gold. When adsorbed onto gold nanoparticles, these dyes produce biocompatible SERRS nanoprobes with attomolar limits of detection amenable to ultrasensitive in vivo multiplexed tumour and disease marker detection.

Reduction of chalcogen oxyanions and generation of nanoprecipitates by the photosynthetic bacterium Rhodobacter capsulatus.

The facultative photosynthetic bacterium Rhodobacter capsulatus is characterized in its interaction with the toxic oxyanions tellurite (Te(IV)) and selenite (Se(IV)) by a highly variable level of resistance that is dependent on the growth mode making this bacterium an ideal organism for the study of the microbial interaction with chalcogens. As we have reported in the past, while the oxyanion tellurite is taken up by R. capsulatus cells via acetate permease and it is reduced to Te(0) in the cytoplasm in the form of splinter-like black intracellular deposits no clear mechanism was described for Se(0) precipitation. Here, we present the first report on the biotransformation of tellurium and selenium oxyanions into extracellular Te(0) and Se(0)nanoprecipitates (NPs) by anaerobic photosynthetically growing cultures of R. capsulatus as a function of exogenously added redox-mediator lawsone, i.e. 2-hydroxy-1,4-naphthoquinone. The NPs formation was dependent on the carbon source used for the bacterial growth and the rate of chalcogen reduction was constant at different lawsone concentrations, in line with a catalytic role for the redox mediator. X-ray diffraction (XRD) analysis demonstrated the Te(0) and Se(0) nature of the nanoparticles.

Metal vs. chalcogen competition in the catalytic mechanism of cysteine dioxygenase.

Why the cysteine dioxygenase (CDO) cannot catalyze the oxidation of selenocysteine (Sec) but that of cysteine (Cys) is still an open question. In order to solve this question, the CDO model complex, the active site of CDO, and their selenium-substituted complexes have been selected as the computational models in this work. The stepwise donation of electron density during the first two reaction steps has been explored. In the first step, the electron density-donor ability of Se to donate to Fe is stronger than that of S; in the second step, S has the better electron density-donor ability to donate to O(2) than Se. Under the influence, in the Cys-bound complexes, the change of the oxidation state for the Fe center is II→III→II, while the Fe center in the Sec-bound complexes remains in the II oxidation state throughout. Considering that the ferric-superoxo species is an active oxidant and exhibits high reactivity in such reaction, it is speculated that the valence change of the Fe center makes the Cys-bound complexes effectively catalyze the oxidation of Cys, while the Sec-bound complexes cannot catalyze the oxidation of Sec. The competition for donation of electron density determines the valence change and the reaction ability.

Regioselective deiodination of thyroxine by iodothyronine deiodinase mimics: an unusual mechanistic pathway involving cooperative chalcogen and halogen bonding.

Iodothyronine deiodinases (IDs) are mammalian selenoenzymes that catalyze the conversion of thyroxine (T4) to 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) by the outer- and inner-ring deiodination pathways, respectively. These enzymes also catalyze further deiodination of T3 and rT3 to produce a variety of di- and monoiodo derivatives. In this paper, the deiodinase activity of a series of peri-substituted naphthalenes having different amino groups is described. These compounds remove iodine selectively from the inner-ring of T4 and T3 to produce rT3 and 3,3'-diiodothyronine (3,3'-T2), respectively. The naphthyl-based compounds having two selenols in the peri-positions exhibit much higher deiodinase activity than those having two thiols or a thiol-selenol pair. Mechanistic investigations reveal that the formation of a halogen bond between the iodine and chalcogen (S or Se) and the peri-interaction between two chalcogen atoms (chalcogen bond) are important for the deiodination reactions. Although the formation of a halogen bond leads to elongation of the C-I bond, the chalcogen bond facilitates the transfer of more electron density to the C-I σ* orbitals, leading to a complete cleavage of the C-I bond. The higher activity of amino-substituted selenium compounds can be ascribed to the deprotonation of thiol/selenol moiety by the amino group, which not only increases the strength of halogen bond but also facilitates the chalcogen-chalcogen interactions.

Structure and mechanism of the chalcogen-detoxifying protein TehB from Escherichia coli.

The oxyanion derivatives of the chalcogens tellurium and selenium are toxic to living organisms even at very low levels. Bacteria have developed mechanisms to overcome their toxicity by methylating them. The structure of TehB from Escherichia coli has been determined in the presence of the cofactor analogues SAH (S-adenosylhomocysteine) and sinefungin (a non-hydrolysable form of S-adenosyl-L-methionine) at 1.48 Å (1 Å=0.1 nm) and 1.9 Å respectively. Interestingly, our kinetic data show that TehB does not discriminate between selenium or tellurite oxyanions, making it a very powerful detoxifying protein. Analysis of the active site has identified three conserved residues that are capable of binding and orientating the metals for nucleophilic attack: His176, Arg177 and Arg184. Mutagenesis studies revealed that the H176A and R184A mutants retained most of their activity, whereas the R177A mutant had 65% of its activity abolished. Based on the structure and kinetic data we propose an SN2 nucleophilic attack reaction mechanism. These data provide the first molecular understanding of the detoxification of chalcogens by bacteria.

Hepatic element concentrations of lesser scaup (Aythya affinis) during spring migration in the upper Midwest.

High concentrations of some hepatic elements might be contributing to the decline of the continental lesser scaup (Aythya affinis) population. We evaluated hepatic element concentrations of male and female lesser scaup collected from the upper Midwest (Iowa, Minnesota, North Dakota, and South Dakota) during the 2003 and 2004 spring migrations. We measured concentrations of 24 elements in livers of 117 lesser scaup. We found that only selenium concentrations were at levels (>3.0 µg/g wet weight [ww)]) proposed to adversely affect reproduction. Approximately 49% of females (n = 61) had individual hepatic concentrations >3.0 µg/g ww selenium (Se). Our observed hepatic concentration of Se was similar to that reported in lesser scaup collected from the mid-continental United States but less than Se concentrations reported from the Great Lakes region. We found that the liver cadmium (Cd) concentration for males was significantly higher than that for females. Gender differences in hepatic Cd concentrations have not been previously reported for lesser scaup, but Cd is known to have negative impacts on male reproduction. Our results indicate that lesser scaup migrating through the upper Midwest in spring have elevated Se levels and that males carry a significantly greater Cd burden than females. Moreover, elemental concentrations might be high enough to affect reproduction in both male and female lesser scaup, but controlled laboratory studies are needed to adequately assess the effects of Se and Cd on lesser scaup reproduction.

Distribution patterns of chalcogens (S, Se, Te, and 210Po) in various tissues of a squid, Todarodes pacificus.

In order to elucidate the relationship between the chemically similar chalcogen elements S, Se, Te, and 210Po in marine invertebrates, we conducted a comparative study of the distribution patterns of these elements in a squid, Todarodes pacificus. Elemental concentrations of the four chalcogens were determined in (mantle) muscle, gill, stomach, and hepatopancreas tissues. No relationship between chalcogen concentrations and morphological parameters (mantle length, body weight, and sex) was evident. Gills showed slightly elevated levels of Se and 210Po, which may indicate absorption and uptake of these elements over the gill surface. All four chalcogens have their highest concentrations in the hepatopancreas and the lowest concentrations in the muscle tissue. However, concentration differences between tissues, revealed by (1) bioaccumulation values based on reference seawater values and (2) internal relative enrichment factors (IREF) based on enrichment of hepatopancreas compared to muscle tissue, were least pronounced for S, most distinct for 210Po, and moderate for Se and Te. Furthermore, no significant correlation for Se, Te, and 210Po with S within tissue concentrations, and only a slightly negative correlation between S and 210Po in the squid muscle and hepatopancreas tissues were found, which indicates either an antagonistic effect between, or a disconnection of the two elements through metabolic processing. Overall, the distribution patterns of Se and Te resemble those of essential trace elements, such as Zn and Cu, whereas 210Po is partitioned in a manner similar to toxic heavy metals, such as Cd and Ag.