Arsenic Clusters Research Articles

Synthesis and Characterization of Novel Cobalt Carbonyl Phosphorus and Arsenic Clusters.

Phosphorus- and arsenic-containing cobalt clusters are an interesting class of compounds that continue to provide new structures with captivating bonding patterns. Although the first members of this family were reported 45 years ago, the number of such species is still limited within the broad family of transition metal complexes bearing pnictogen atoms. Herein, we present the reaction of Co2(CO)8 as a cobalt source with a number of phosphorus- and arsenic-containing compounds under variable reaction conditions. These reactions result in various known and novel cobalt phosphorus and cobalt arsenic clusters in which different nuclearity ratios between P/As and Co exist. All those clusters were characterized by X-ray structural analysis and partly by IR, 31P{1H} NMR, EI-MS and elemental analysis. This comprehensive study is the first detailed study in this field that reveals the richness of compounds that could be obtained only by modifying the ratio of used reactants and the involved reaction conditions.

GIS based hotspot analysis and health risk assessment of groundwater arsenic from an unconfined deep aquifer of Lahore, Pakistan.

Use of groundwater for drinking purpose poses serious hazards of arsenic contamination particularly in plains of western Himalayan region. Therefore, current study was designed to investigate the level of Arsenic (As) in the water obtained from tubewells in a metropolitan city of Lahore, Pakistan and assess the human health risk. So, a total of 73 tubewells were sampled randomly in the manner that the whole study region was covered without any clustering. The water samples were analyzed for As using atomic absorption spectrophotometer. These samples were also tested for total dissolved solids, chlorides, pH, alkalinity, turbidity, hardness and calcium. GIS based hotspots analysis technique was used to investigate the spatial distribution patterns. Our results revealed that only one sample out of total 73 had arsenic level below the WHO guideline of 10μg/L. The spatial distribution map of arsenic revealed that the higher concentrations of arsenic are present in the north-western region of Lahore. The cluster and outlier analysis map using Anselin Local Moran's I statistic indicated the presence of an arsenic cluster in the west of River Ravi. Furthermore, the optimized hotspot analysis based on Getis-Ord Gi* statistics confirmed the statistical significance (P < 0.05) and (P < 0.01) of these samples from the vicinity of River Ravi. Regression analysis showed that variables such as turbidity, alkalinity, hardness, chlorides, calcium and total dissolved solids were significantly (all P < 0.05) associated with level of Arsenic in tubewells. Whereas, PH and electrical conductivity and other variables like town, year of installation, depth and diameter of the wells were not significantly associated with Arsenic concentrations in tubewells. Principal component analysis (PCA) exhibited that the random distribution of tubewell samples showed no distinct clustering with towns studied. Health risk assessment based on hazard and Cancer risk index revealed serious risk of developing carcinogenic and non-carcinogenic diseases particularly in children. The health risk due to prevalence of high As concentration in tubewells' water need to be mitigated immediately to avoid worst consequences in future.

A systematic DFT study of arsenic doped iron cluster AsFe n (n = 1–4)

Abstract The research on metallic clusters in relevance to its far-reaching involvement in the high technology sector, solid-state physics and catalysis is an interesting and significant field of study. In this report, the investigation of arsenic doped iron cluster, AsFe n (n = 1–4) aided by conceptual density functional theory (CDFT) method has been performed. CDFT based global descriptors-mainly HOMO–LUMO energy gap and other parameters of these clusters are worked out. Obtained data shows that band energy gap varies in the magnitude of 1.451–3.138 eV. Uppermost magnitude of HOMO–LUMO energy gap i.e. 3.138 eV is observed for AsFe while AsFe4 show the smallest energy gap. It is noted that band gap of these systems decreases with increase in the cluster size, ‘n’. Direct association concerning both parameters HOMO–LUMO energy gap and molecular hardness of AsFe n clusters have been found. It indicates that among the studied compound AsFe is the most stable system whereas AsFe4 is the least stable. Dipole moment of the clusters is observed in the variation of 2.303 Debye to 3.853 Debye, signifying that the bond within the clusters is ionic in nature. The computed bond length between Fe–Fe in AsFe n is in agreement with the experimental data.

Fundamentals and application in phytoremediation of an efficient arsenate reducing bacterium Pseudomonas putida ARS1

Arsenic is a ubiquitous environmental pollutant. Microbe-mediated arsenic bio-transformations significantly influence arsenic mobility and toxicity. Arsenic transformations by soil and aquatic organisms have been well documented, while little is known regarding effects due to endophytic bacteria. An endophyte Pseudomonas putida ARS1 was isolated from rice grown in arsenic contaminated soil. P. putida ARS1 shows high tolerance to arsenite (As(III)) and arsenate (As(V)), and exhibits efficient As(V) reduction and As(III) efflux activities. When exposed to 0.6 mg/L As(V), As(V) in the medium was completely converted to As(III) by P. putida ARS1 within 4 hr. Genome sequencing showed that P. putida ARS1 has two chromosomal arsenic resistance gene clusters (arsRCBH) that contribute to efficient As(V) reduction and As(III) efflux, and result in high resistance to arsenicals. Wolffia globosa is a strong arsenic accumulator with high potential for arsenic phytoremediation, which takes up As(III) more efficiently than As(V). Co-culture of P. putida ARS1 and W. globosa enhanced arsenic accumulation in W. globosa by 69%, and resulted in 91% removal of arsenic (at initial concentration of 0.6 mg/L As(V)) from water within 3 days. This study provides a promising strategy for in situ arsenic phytoremediation through the cooperation of plant and endophytic bacterium.

Arsenic clustering in arsenian pyrite: A combined photoemission and theoretical modeling study

Abstract Arsenian pyrite is known to have a strong association with gold in most auriferous refractory deposits, and thus understanding the chemical speciation of arsenic in localized environments in arsenian pyrite provides an important basis for determining its reactivity and mobility. However, arsenic is fast-oxidizing among elements in the Fe-As-S system and hence it may exist in various chemical states, which renders it difficult to establish arsenic nature under pristine conditions, particularly in arsenian pyrite. Herein, arsenian pyrite samples were analyzed on a synchrotron soft X-ray spectroscopy beamline under ultrahigh vacuum conditions, and As-3d as well as S-2p spectra were collected. A comparison between the spectrum of bulk As-3d in the samples with its bulk counterpart in arsenopyrite revealed a 0.6 eV shift toward lower binding energies. This observation was similar to loellingite (FeAs2), where the binding energy shift was attributed to high electron density on As of the As-As dimer. Formation of As clusters resulting in comparable binding energy shifts was also proposed from the spectroscopic studies. The experiments were complemented by a series of first-principles calculations simulating four experimentally observed pyrite surfaces where surficial S atoms were randomly substituted by As. As such, six arsenian pyrite crystal surfaces were modeled, two of which constituted surficial As clusters replacing both S and Fe atoms. The surfaces were geometrically optimized, and surface energies were calculated along with the corresponding electronic structure providing a detailed distribution of partial charges for surficial atoms obtained from Löwdin population analysis. The calculated partial charges of atoms located at the surface arsenian pyrite indicated that while the electron density on the As atom of As-S dimers in arsenian pyrite is less negative than the As in bulk arsenopyrite, it is more negative for the As atom of As-As dimers, which were only seen in the surficial As clusters. This validated the description of As presence in arsenian pyrite as local clusters inducing localized lattice strain due to increased bond distances. Our findings offer a good background for future studies into the reactive sites in arsenian pyrite and how that compares with associated minerals, arsenopyrite, and pyrite.

Synthesis and Redox Chemistry of a Homoleptic Iron Arsenic Prismane Cluster

AbstractThe redox chemistry of the homoleptic iron prismane cluster [{Cp*Fe}3(μ3,η4:4:4‐As6)] (A, Cp*=C5Me5) is investigated both electrochemically and synthetically. While its first oxidation leads to the diamagnetic species [{Cp*Fe}3(μ3,η4:4:4‐As6)][X] ([1][TEF], [1][FAl], [TEF]−=[Al{OC(CF3)3}4]−, [FAl]−=[FAl{O(1‐C6F5)C6F10}3]−), the second oxidation yields the paramagnetic [{Cp*Fe}3(μ3,η4:4:4‐As6)][TEF]2 ([2][TEF]2). The reduction of A leads to the monoanionic compound [K@[2.2.2]‐cryptand][{Cp*Fe}3(μ3,η4:4:4‐As6)] ([K@crypt][3]), while a second reduction could only be traced spectroscopically. All compounds were comprehensively characterized, revealing the structural changes accompanying the described redox processes. All findings are supported by spectroscopic as well as computational studies.

Synthesis and Structure of Binary Copper/Silver–Arsenic Clusters Derived from Zintl Ion As73–

Comprehensive SummaryPolyarsenides containing coinage metals, [As7Cu(PPh3)]2– (1) and [M2As14]4– (M = Cu, 2; Ag, 3), were synthesized by reactions of the nominal composition K3As7 with Group 11 metal complexes. The possible intermediate, cluster 1, was isolated from the solution phase through subtle changes in reaction conditions in the formation process of complex 2. Hence, we establish the pathway of the bimetallic bridged clusters [Cu2As14]4– by the oxidation of the [As7Cu(PPh3)]2–. Quantum chemical calculations reveal the presence of metallophilic interaction in clusters 2 and 3.

Correlative STEM-HAADF and STEM-EDX tomography for the 3D morphological and chemical analysis of semiconductor devices

3D analysis of an arsenic-doped silicon fin sample is performed in a transmission electron microscope (TEM). High angle annular dark-field scanning TEM (STEM-HAADF) and energy-dispersive x-ray spectroscopy (STEM-EDX) modes are used simultaneously to extract 3D complementary multi-resolution information about the sample. The small pixel size and angular step chosen for the STEM-HAADF acquisition yield reliable information about the sidewall roughness and the arsenic clusters’ average volume. The chemical sensitivity of STEM-EDX tomography gives insights into the 3D conformality of the arsenic implantation and its depth distribution. Non-negative matrix factorization method is employed to identify the chemical phases present in the sample automatically. A total variation minimization algorithm, implemented in 3D, produces high-quality volumes from heavily undersampled datasets. The extension of this correlative approach to electron energy-loss spectroscopy STEM tomography and atom probe tomography is also discussed.

Formation of arsenic clusters in InAs nanowires with an Al2O3 shell.

An in-depth understanding of thermal behavior and phase evolution is required to apply heterostructured nanowires (NWs) in real devices. The intermediate status during the vaporization process of InAs NWs in an Al2O3 shell was studied by conducting quenching during in situ heating experiments, using a transmission electron microscope. The formation of As clusters in the amorphous Al2O3 shell was confirmed by analyzing the high-angle annular dark field images and energy-dispersive X-ray spectra. The As clusters existed independently in the shell and were also observed at the end of the InAs pieces obtained after quenching. The formation process of the As clusters was demonstrated from a theoretical perspective. Moreover, an ab initio molecular dynamics simulation (AIMD) was conducted to study the atomic and molecular behaviors.

Origin of two special Raman modes in dilute nitrides

Two special Raman modes at 197 cm−1 and 255 cm−1 have been observed and discussed for a long time without persuasive conclusion in dilute nitride materials. In this work, using Raman spectroscopy and transmission electron microscopy (TEM), we studied Molecular Beam Epitaxy grown GaAsN/GaAs multiple quantum well samples with nitrogen concentration ranging from 0.6% to 6.1% and undergoing post-growth rapid thermal annealing (RTA) processes in the temperature range of 650 °C–950 °C. We assigned these two modes to the double degeneracy Eg and Ag modes from arsenic crystalline clusters on sample surface according to the Raman measurements and TEM observations. The physical mechanisms involved in the RTA processes at different temperatures were proposed.

Infrared Photodissociation Spectroscopy of Heteronuclear Arsenic-Iron Carbonyl Cluster Anions.

Heteronuclear arsenic-iron carbonyl cluster anions AsmFe(CO)n- (m, n = 2, 3) have been generated in the gas phase and investigated by mass-selected infrared photodissociation spectroscopy and density functional theory calculations at the B3LYP/BP86/TPSS levels. All the AsmFe(CO)n- (m, n = 2, 3) cluster anions are determined to contain Fe(CO)n- fragments, which can be regarded as being formed by replacing one arsenic atom of the arsenic clusters Asm+1 with the Fe(CO)n- group. Bonding analyses indicated that each As2Fe(CO)n- (n = 2, 3) cluster anion involves two Fe-As single bonds and one As-As double bond. Each As3Fe(CO)n- (n = 2, 3) cluster anion has three Fe-As single bonds and three As-As single bonds. The Fe(CO)3- group with a 15-electron configuration is valence isoelectronic to the As atom and can serve as a building block for forming heteronuclear arsenic-iron carbonyl clusters.

Characteristics of Amorphous As2S3 Semiconductor Films Obtained via Spin Coating

Centrifugation is used in fabricating, e.g., films with large areas and/or thicknesses of several micrometers. However, it has yet to be widely employed for chalcogenide compounds, due to their relatively weak solubility in most solvents. Determining the optimum conditions for preparing solutions of chalcogenide compounds and obtaining films via centrifugation is therefore of great interest. Specific features of amorphous arsenic sulfide (As2S3) films prepared via the centrifugation of solutions in n-butylamine have been studied. These films were characterized by means of X-ray diffraction analysis, IR spectroscopy, atomic-force microscopy and Raman spectroscopy. It was shown that amorphous As2S3 films have a greater elasticity modulus than those of analogous composition produced via thermal evaporation in vacuum, or As2S3 glass. A structural model based on arsenic sulfide clusters whose surfaces are bound by negatively and positively charged ions is used to explain the experimental results obtained in this work. DC measurements show that the amorphous films exhibit semiconductor-type conductivity. Their room temperature conductivity is ~10−15 S/cm, which indicates good dielectric properties. The films are optically transparent starting from the yellow spectral range, making them promising functional materials for engineering applications in optics and photonics.

First-Principles Studies on the Structural and Electronic Properties of As Clusters.

Based on the genetic algorithm (GA) incorporated with density functional theory (DFT) calculations, the structural and electronic properties of neutral and charged arsenic clusters Asn (n = 2–24) are investigated. The size-dependent physical properties of neutral clusters, such as the binding energy, HOMO-LUMO gap, and second difference of cluster energies, are discussed. The supercluster structures based on the As8 unit and As2 bridge are found to be dominant for the larger cluster Asn (n ≥ 8). Furthermore, the possible geometric structures of As28, As38, and As180 are predicted based on the growth pattern.

Understanding arsenic incorporation in CdTe with atom probe tomography

Overcoming the open circuit voltage deficiency in Cadmium Telluride (CdTe) photovoltaics may be achieved by increasing p-type doping while maintaining or increasing minority carrier lifetimes. Here, routes to higher doping efficiency using arsenic are explored through an atomic scale understanding of dopant incorporation limits and activation in molecular beam epitaxy grown CdTe layers. Atom probe tomography reveals spatial segregation into nanometer scale clusters containing > 60 at% As for samples with arsenic incorporation levels greater than 7–8 × 1017 cm−3. The presence of arsenic clusters was accompanied by crystal quality degradation, particularly the introduction of arsenic-enriched extended defects. Post-growth annealing treatments are shown to increase the size of the As precipitates and the amount of As within the precipitates.

An {As4 Cu4 [Cu(H2 O)]12 } Cluster Incorporated within Four [Nb7 O22 ]9- Units.

A gigantic 16-copper-containing 28-niobate cluster, Na14 K7 H5 {As4 Cu4 [Cu(H2 O)]12 Nb28 O109 }⋅37.5 H2 O (1), has been prepared by a one-pot reaction, which is the first example of arsenic copper clusters containing polyoxoniobate, and the first tetrahedral arrangement of both the Cu cluster and the [Nb7 O22 ]9- subunits.

The Arsenic Resistance-Associated Listeria Genomic Island LGI2 Exhibits Sequence and Integration Site Diversity and a Propensity for Three Listeria monocytogenes Clones with Enhanced Virulence.

In the foodborne pathogen Listeria monocytogenes, arsenic resistance is encountered primarily in serotype 4b clones considered to have enhanced virulence and is associated with an arsenic resistance gene cluster within a 35-kb chromosomal region, Listeria genomic island 2 (LGI2). LGI2 was first identified in strain Scott A and includes genes putatively involved in arsenic and cadmium resistance, DNA integration, conjugation, and pathogenicity. However, the genomic localization and sequence content of LGI2 remain poorly characterized. Here we investigated 85 arsenic-resistant L. monocytogenes strains, mostly of serotype 4b. All but one of the 70 serotype 4b strains belonged to clonal complex 1 (CC1), CC2, and CC4, three major clones associated with enhanced virulence. PCR analysis suggested that 53 strains (62.4%) harbored an island highly similar to LGI2 of Scott A, frequently (42/53) in the same location as Scott A (LMOf2365_2257 homolog). Random-primed PCR and whole-genome sequencing revealed seven novel insertion sites, mostly internal to chromosomal coding sequences, among strains harboring LGI2 outside the LMOf2365_2257 homolog. Interestingly, many CC1 strains harbored a noticeably diversified LGI2 (LGI2-1) in a unique location (LMOf2365_0902 homolog) and with a novel additional gene. With few exceptions, the tested LGI2 genes were not detected in arsenic-resistant strains of serogroup 1/2, which instead often harbored a Tn554-associated arsenic resistance determinant not encountered in serotype 4b. These findings indicate that in L. monocytogenes, LGI2 has a propensity for certain serotype 4b clones, exhibits content diversity, and is highly promiscuous, suggesting an ability to mobilize various accessory genes into diverse chromosomal loci.IMPORTANCEListeria monocytogenes is widely distributed in the environment and causes listeriosis, a foodborne disease with high mortality and morbidity. Arsenic and other heavy metals can powerfully shape the populations of human pathogens with pronounced environmental lifestyles such as L. monocytogenes Arsenic resistance is encountered primarily in certain serotype 4b clones considered to have enhanced virulence and is associated with a large chromosomal island, Listeria genomic island 2 (LGI2). LGI2 also harbors a cadmium resistance cassette and genes putatively involved in DNA integration, conjugation, and pathogenicity. Our findings indicate that LGI2 exhibits pronounced content plasticity and is capable of transferring various accessory genes into diverse chromosomal locations. LGI2 may serve as a paradigm on how exposure to a potent environmental toxicant such as arsenic may have dynamically selected for arsenic-resistant subpopulations in certain clones of L. monocytogenes which also contribute significantly to disease.

Impact of LT-GaAs layers on crystalline properties of the epitaxial GaAs films grown by MBE on Si substrates

GaAs films with low-temperature GaAs (LT-GaAs) layers were grown by molecular beam epitaxy (MBE) method on vicinal (001) Si substrates oriented 6° off towards [110]. The grown structures were different with the thickness of LT-GaAs layers and its arrangement in the film. The processes of epitaxial layers nucleation and growth were controlled by reflection high energy electron diffraction (RHEED) method. Investigations of crystalline properties of the grown structures were carried out by the methods of X-ray diffraction (XRD) and transmission electron microscopy (TEM). The crystalline perfection of the GaAs films with LT-GaAs layers and the GaAs films without ones was comparable. It was found that in the LT- GaAs/Si layers the arsenic clusters are formed, as it occurs in the LT-GaAs/GaAs system without dislocation. It is shown that large clusters are formed mainly on the dislocations. However, the clusters have practically no effect on the density and the propagation path of threading dislocations. With increasing thickness of LT-GaAs layer the dislocations are partly bent along the LT-GaAs/GaAs interface due to the presence of stresses.

Properties of neutral and charged cobalt-doped arsenic CoAsn(0±1) (n = 1–15) clusters by density functional theory

The structural, electronic and magnetic properties of neutral and charged CoAsn (n=1–15) have been systematically investigated using spin polarized density functional theory to highlight the effects of a cobalt atom on properties of arsenic clusters. Compared with the neutral clusters, the ground state structures of CoAsn+ and CoAsn− were obviously rearranged due to the extraction or the addition of an electron. The optimized clusters reveal that the most stable structures and their corresponding isomers have three dimensional configurations. The relative stabilities have been studied in terms of the binding energies, fragmentation energies and second-order difference of energies for all CoAsn(0±1) structures. The values of HOMO–LUMO gaps have a decreasing tendency along with the increasing number of As atoms in the cluster. The HOMO–LUMO gaps decrease considerably in doped clusters which suggest an increase of metallic property. Total magnetic moment depends on the geometry, the position of Co atom in the cluster, the charge transfer and orbital hybridization. The vertical ionization potential (VIP), vertical electronic affinity (VEA), chemical hardness (η), adiabatic electronic affinity (AEA) and adiabatic ionization potential (AIP) have also been investigated and discussed.

Functional analysis of ars gene cluster of Pannonibacter indicus strain HT23(T) (DSM 23407(T)) and identification of a proline residue essential for arsenate reductase activity.

Arsenic is a naturally occurring ubiquitous highly toxic metalloid. In this study, we have identified ars gene cluster in Pannonibacter indicus strain HT23(T) (DSM 23407(T)), responsible for reduction of toxic pentavalent arsenate. The ars gene cluster is comprised of four non-overlapping open reading frames (ORFs) encoding a transcriptional regulator (ArsR), a low molecular weight protein tyrosine phosphatases (LMW-PTPase) with hypothetical function, an arsenite efflux pump (Acr3), and an arsenate reductase (ArsC). Heterologous expression of arsenic inducible ars gene cluster conferred arsenic resistance to Escherichia coli ∆ars mutant strain AW3110. The recombinant ArsC was purified and assayed. Site-directed mutagenesis was employed to ascertain the role of specific amino acids in ArsC catalysis. Pro94X (X = Ala, Arg, Cys, and His) amino acid substitutions led to enzyme inactivation. Circular dichroism spectra analysis suggested Pro94 as an essential amino acid for enzyme catalytic activity as it is indispensable for optimum protein folding in P. indicus Grx-coupled ArsC.

Draft Genome Sequence of Anaeromyxobacter sp. Strain PSR-1, an Arsenate-Respiring Bacterium Isolated from Arsenic-Contaminated Soil.

Here, we report a draft genome sequence of Anaeromyxobacter sp. strain PSR-1, an arsenate-respiring bacterium isolated from arsenic-contaminated soil. It contained three distinct arsenic resistance gene clusters (ars operons), while no respiratory arsenate reductase gene (arr) was identified.