Topographical analysis of As-induced folding of α-MT1a

Metallothionein binds multiple metals into two clustered domains. While the structure of the fully metalated protein is well known for the Cd- and Zn-containing protein, there is little known about the structures of the metal-free protein (apo-metallothionein) and even less about the partially metalated forms. However, the partially-metalated species are vitally important intermediates in the passage of the protein from translational synthesis to its homeostatic buffer or metal chaperone roles. Because multiple metals bind to metallothioneins, the partially-metalated species span a wide range depending on the metal bound. Up to 3 As³⁺ bind stepwise to the α-domain fragment in a manner that allows measurement of each of the 4 species simultaneously with the number of free cysteines diminishing by 3 for every As³⁺ bound: apo- (11 Cys), As₁- (8 Cys), As₂- (5 Cys) and As₃-α-MT (2 Cys). The cysteine modifier benzoquinone (Bq), was used to determine the relative accessibility of the free cysteines in the α-MT fragment as a function of the number of As³⁺ bound. The effect of each As³⁺ was to induce folding in the protein. The ESI-MS results show that the whole protein folds significantly even when just one of the three As³⁺ has bound. The profile of the Bq reacting with the unbound cysteines shows effects of steric hindrance in slowing down the reaction. By freezing the reaction midway to the endpoint, the mass spectral data show the ‘mid-flight’ concentrations of all the key species, 27 in all. Analysis of this mid-flight reaction profile gives insight into the topology of the partially metalated MT from the differential access to the unbound cysteinyl thiols by the Bq. Significantly, the metal-free, apo-α-MT also adopts a folded structure in the presence of the As³⁺ even though there is no As³⁺ bound. This can only happen if the apo-protein wraps around other metalated proteins in solution via protein–protein interactions.     

Noncooperative cadmium(II) binding to human metallothionein 1a

The two-domain (βα) mammalian metallothionein binds seven divalent metals, however, the binding mechanism is not well characterized and recent reports require the presence of the partially metallated protein. In this paper, step-wise metallation of the metal-free, two-domain βα-rhMT and the isolated β-rhMT using Cd(II) is shown to proceed in a noncooperative manner by analysis of electrospray ionization mass spectrometric data. Under limiting amounts of Cd(II), all intermediate metallation states up to the fully metallated Cd₃-β-rhMT and Cd₇-βα-rhMT were observed. Addition of excess Cd(II), resulted in formation of the supermetallated (metallation in excess of normal levels) Cd₄-β- and Cd₈-βα-metallothionein species. These data establish that noncooperative cadmium metallation is a property of each isolated domain and the complete two-domain protein. Our data now also establish that supermetallation is a property that may provide information about the mechanism of metal transfer to other proteins.     

A computational investigation of the interaction between As3+ and deoxynucleotides

A successful aptamer biosensor for As³⁺ in aqueous solution indicated that deoxynucleotides could be a potential ligand for arsenic absorption. However, the interaction mechanism between aptamer and As³⁺ is still not clear. In this paper, a density functional method was applied to investigate the interactions between As³⁺ and four kinds of deoxynucleotides in different charge states. According to the binding energy, dGMP was the best monomer with four different binding modes, and dGMP^3–As³⁺ with the O atom in the phosphate group was calculated to be the most stable one. Moreover, the interaction energy increased when the charge of deoxynucleotide was increased, suggesting that a negative charge was advantageous for deoxynucleotides to absorb As³⁺. The atoms in molecules (AIM), localised orbital locator (LOL) and Charge decomposition analyses (CDA) confirmed that the bond between N atoms and As³⁺ were covalent interactions and that between O atoms and As³⁺ were dominated by electrostatic interactions. As the charge grew, the bond strength of both interactions increased. At the same time, O atom in the vicinity of nucleobases began to participate in the interaction with As³⁺, which was originally bonded to N atoms.     

Arsenic-metalation of triple-domain human metallothioneins: Support for the evolutionary advantage and interdomain metalation of multiple-metal-binding domains

Metallothionein (MT) is a prominent metal-binding protein and in mammalian systems contains a two-domain βα motif, while in lower life forms MT often consists of only a single-domain structure. There are also unusual MTs from American oysters that consist of multiple domains (from one to four α domains). This report details the study of the As³⁺-metalation to two different concatenated triple β and α domain MTs using time-resolved electrospray ionization mass spectrometry (ESI MS). Analysis of kinetic ESI MS data show that ααα human MT and βββ human MT bind As³⁺ in a noncooperative manner and involves up to 11 sequential bimolecular reactions. We report the complete progress of the reactions for the As³⁺-metalation of both triple-domain MTs from zero and up to 9 (βββ) or 10 As³⁺ ions (ααα). The rate constants for the As³⁺-metalation are reported for both the βββ and ααα human MT. At room temperature (298 K) and pH 3.5, the sequential individual rate constants, k_n (n = 1-9) for the As³⁺-metalation of βββhMT starting at k_1βββ are 40, 36, 37, 26, 27, 17, 12, 6, and 1 M⁻¹ s⁻¹; while at room temperature (298 K) and pH 3.5, the sequential individual rate constants, k_n (n = 1-10) for the As³⁺-metalation of αααhMT starting at k_1ααα are 52, 45, 46, 42, 38, 36, 29, 25, 14, and 6 M⁻¹ s⁻¹. The trend in the rate constant values reported for these two triple-domain MT proteins supports our previous proposal that the rate constant values are proportionally related to the total number of equivalent binding sites. The rate of binding for the 1st As³⁺ is the fastest we have determined for any MT to date. Additionally, we propose that the data show for the first time for any MT species, that interdomain metalation occurs in the binding of the 10th and 11th As³⁺ to αααhMT.     

Tetraarsenictetrasulfide and Arsenic Trioxide Exert Synergistic Effects on Induction of Apoptosis and Differentiation in Acute Promyelocytic Leukemia Cells

Since arsenic trioxide (As³⁺) has been successfully used in the treatment of acute promyelocytic leukemia (APL), its adverse effects on patients have been problematic and required a solution. Considering the good therapeutic potency and low toxicity of tetraarsenictetrasulfide (As₄S₄) in the treatment of APL, we investigated the effects of combining As₄S₄ and As³⁺ on the apoptosis and differentiation of NB4 and primary APL cells. As₄S₄, acting similarly to As³⁺, arrested the G₁/S transition, induced the accumulation of cellular reactive oxygen species, and promoted apoptosis. Additionally, low concentrations of As₄S₄ (0.1–0.4 μM) induced differentiation of NB4 and primary APL cells. Compared with the As₄S₄- or As³⁺-treated groups, the combination of As₄S₄ and As³⁺ obviously promoted apoptosis and differentiation of NB4 and primary APL cells. Mechanistic studies suggested that As₄S₄ acted synergistically with As³⁺ to down-regulate Bcl-2 and nuclear factor-κB expression, up-regulate Bax and p53 expression, and induce activation of caspase-12 and caspase-3. Moreover, the combination of low concentrations of As₄S₄ and As³⁺ enhanced degradation of the promyelocytic leukemia-retinoic acid receptor α oncoprotein. In summary, As₄S₄ and As³⁺ synergistically induce the apoptosis and differentiation of NB4 and primary APL cells.     

Bismuth binding studies to the human metallothionein using electrospray mass spectrometry

Bismuth compounds are currently used to treat gastric ailments and to prevent the toxic side effects of cancer treatments. The affinity of bismuth for binding to sulfur compounds has been reported and one such target biomolecule is the cysteine-rich metalloprotein metallothionein. Renal mammalian metallothionein has been shown to be induced by Bi salts, with the Bi³⁺ binding to the renal MT. However, the exact metal-to-metallothionein stoichiometric ratios for the 2-domain βα mammalian protein and the individual β and α domain fragments remain unknown. We now report that the maximum metal-to-MT stoichiometries for the individual domain fragments and the entire 2-domain protein are Bi₃-S₉-βhMT, Bi₄-S₁₁-αhMT, and Bi₇-S₂₀-βαhMT, respectively. Electrospray mass spectrometry data also unambiguously show the existence of partially metalated Bi-containing MT species during the titration of apo-MT with Bi³⁺, which demonstrates that Bi-metalation to MT occurs in a noncooperative manner.     

Neoplastic transformation of BALB/3T3 cells by metals and the quest for induction of a metastatic phenotype

In the mouse embryo cell line BALB/3T3 Clone A31-1-1, dose-dependent morphologic neoplastic transformation was obtained with NaAsO₂, Na₂HAsO₄, CdCl₂, and K₂CrO₄. Cellular uptake was four fold higher for As³⁺ than for As⁵⁺, and As⁵⁺ was metabolized to As³⁺ in cytosol. Cytotoxicity and transformation rates were four fold higher for As³⁺ than As⁵⁺, but when correlated to cellular As burden they were equivalent. As³⁺ appears responsible for the transforming activity. The foci transformed by metals (or by other carcinogens) gave rise to tumorigenic cell lines (sc sarcomas in nude mice), none of which, however, induced metastases when tested by sc or by iv injection in nude mice. Thus carcinogens change this aneuploid cell line from a preneoplastic stage to the expression of malignant growth but not of metastatic activity. Metastatic and type IV collagenolytic activities can be induced by transfection of the c-Ha-ras oncogene and inhibited by the Ad2-E1a gene (so far shown in other cell types). It remains to be seen whether metal or other carcinogens can induce the nonmetastatic phenotype to become metastatic. The molecular mechanisms of metal carcinogenesis, studied in cell culture systems, in combination with other factors or oncogenes, may reveal the effect of individual metal carcinogens on discrete steps of the complex process of carcinogenesis.     

The structures and stabilities of small diarsenic-doped silicon clusters

The structures and stabilities of As₂-doped Si_n (n = 1-7) clusters have been investigated at the B3LYP level of theory, incorporating the 6-311+G^∗ basis set. An isosceles triangle is predicted to be the lowest-energy structure of the As₂Si cluster, whereas the global minimum of As₂Si₂ possesses an As-As-butterfly structure. The ground state structures for As₂Si₃, As₂Si₄ and As₂Si₅ are all bipyramids: trigonal, tetragonal and pentagonal, respectively, which could have important applications as building blocks to synthesize silicon nanowires. The most stable isomer of As₂Si₆ possesses a tricapped trigonal bipyramid structure. The lowest energy structure of As₂Si₇ can be viewed as a substitutional structure of the tricapped trigonal prism Si₉ isomer. In the majority of the lowest energy isomers, the two As atoms tend to be separated from each other, in order to maximize the number of Si-As bonds, and therefore locate at the axial vertex or face-capping atomic positions, especially for As₂Si₄-As₂Si₇. According to results of the incremental binding energies, the HOMO-LUMO gaps and the vertical ionization potentials, the As₂Si₃ and As₂Si₆ clusters are relatively stable compared to their neighbors. Natural bond orbital analyses suggest that delocalized electrons and multi-centered bonds play an important role in stabilizing the low-energy As₂Si_n structures.     

A novel vanadium transporter of the Nramp family expressed at the vacuole of vanadium-accumulating cells of the ascidian Ascidia sydneiensis samea

Background

Vanadium is an essential transition metal in biological systems. Several key proteins related to vanadium accumulation and its physiological function have been isolated, but no vanadium ion transporter has yet been identified.

Methods

We identified and cloned a member of the Nramp/DCT family of membrane metal transporters (AsNramp) from the ascidian Ascidia sydneiensis samea, which can accumulate extremely high levels of vanadium in the vacuoles of a type of blood cell called signet ring cells (also called vanadocytes). We performed immunological and biochemical experiments to examine its expression and transport function.

Results

Western blotting analysis showed that AsNramp was localized at the vacuolar membrane of vanadocytes. Using the Xenopus oocyte expression system, we showed that AsNramp transported VO²⁺ into the oocyte as pH-dependent manner above pH 6, while no significant activity was observed below pH 6. Kinetic parameters (K_m and V_max) of AsNramp-mediated VO²⁺ transport at pH 8.5 were 90 nM and 9.1 pmol/oocyte/h, respectively. A rat homolog, DCT1, did not transport VO²⁺ under the same conditions. Excess Fe²⁺, Cu²⁺, Mn²⁺, or Zn²⁺ inhibited the transport of VO²⁺. AsNramp was revealed to be a novel VO²⁺/H⁺ antiporter, and we propose that AsNramp mediates vanadium accumulation coupled with the electrochemical gradient generated by vacuolar H⁺-ATPase in vanadocytes.

General Significance

This is the first report of identification and functional analysis on a membrane transporter for vanadium ions.     

An insect model for assessing oxidative stress related to arsenic toxicity

The potential usefulness of an insect model to evaluate oxidative stress induced by environmental pollutants was examined with trivalent arsenic (As³⁺, NaAsO₂) and pentavalent arsenic (As⁵⁺, Na₂HAsO₄) in adult female house flies, Musca domestica, and fourth-instar cabbage loopers, Trichoplusia ni. M. domestica was highly susceptible to both forms of arsenic following 48 h exposure in the drinking water with LC₅₀s of 0.008 and 0.011% w/v for As³⁺ and As⁵⁺, respectively. T. ni larvae were susceptible to dietary As³⁺ with an LC₅₀ of 0.032% w/w but seem to tolerate As⁵⁺ well with an LC₅₀ of 0.794% concentration after 48 h exposure. The minimally acute LC₅ dose of both As³⁺ and As⁵⁺ varied considerably but averaged 0.005% for both insects. The potential of both valencies of arsenic for inducing oxidative stress in the insects exposed ad libitum to approximately LC₅ levels was assessed. The parameters examined were the alterations of the antioxidant enzyme activities of superoxide dismutase (SOD), catalase (CAT), glutathione transferase (GST), the peroxidase activity of glutathione transferase (GSTPX), and glutathione reductase (GR), and increases in lipid peroxidation and protein oxidation. SOD (1.3-fold), GST (1.6-fold), and GR (1.5-fold) were induced by As³⁺ in M. domestica but CAT and GSTPX were not affected. As⁵⁺ had no effect on M. domestica. In T. ni, the antioxidant enzyme activities were not affected by As³⁺ except for SOD which was suppressed by 29.4% and GST which was induced by 1.4-fold. As⁵⁺ had no effect except the suppression of SOD by 41.2%. Lipid peroxidation and protein oxidation, which represent stronger indices of oxidative stress, were elevated in both insects by up to 2.9-fold. However, based on the antioxidant enzyme response to the arsenic anions, the mode of action of arsenic induced oxidative stress may differ between the two insects. Until this aspect is further clarified, evidence at this time favors the prospect of As³⁺ as a pro-oxidant, especially for M. domestica. © 1995 Wiley-Liss, Inc.     

mCICR is required for As2O3-induced permeability transition pore opening and cytochrome c release from mitochondria

The permeability transition pore (PTP) is central for apoptosis by acting as a good candidate pathway for the release of Cyt. c and apoptosis induction factors (AIF). Arsenite induces apoptosis via a direct effect on PTP. To characterize the exact mechanism for arsenite induces PTP opening, the effect of Ca²⁺ on As₂O₃-induced PTP opening, the relationship between As₂O₃-induced PTP opening and Cyt. c release from mitochondria and calcium-induced calcium release from mitochondria (mCICR), and the effects of As₂O₃ on Ca²⁺-induced PTP opening were studied. The results showed As₂O₃ induces Cyt. c release by triggering PTP opening. Ca²⁺ is necessary for As₂O₃-induced PTP opening. As₂O₃-induced PTP opening and Cyt. c release depends on mCICR. As₂O₃ promotes PTP opening by lowering Ca²⁺-threshold. These results indicated As₂O₃ induce Cyt. c release from mitochondria by lowering Ca²⁺-threshold for PTP and triggering mCICR-dependent PTP opening. Suggesting that it is possible to control apoptosis by altering Ca²⁺ threshold and mCICR to modulate PTP opening and Cyt. c release.     

Phytofiltration of As3+, As5+, and Hg by the aquatic macrophyte Potamogeton pusillus L,and its potential use in the treatment of wastewater

The aim of this paper was to investigate the capacity of the aquatic macrophyte Potamogeton pusillus to remove As³⁺, As⁵⁺, and Hg from aqueous solutions. The plants were exposed to 0 mg.L^?1, 0.1 mg.L^?1, 0.5 mg.L^?1, 1 mg.L^?1, or 2 mg.L^?1 of As³⁺, As⁵⁺, and Hg for 20 days. The results obtained for the individual removal of As³⁺, As⁵⁺, and Hg from water solutions, together with their accumulation in P. pusillus, indicate that this plant can be effectively used for the removal of Hg and of moderate concentrations of As³⁺ or As⁵⁺ (0.1 mg.L^?1) from aquatic systems. Roots and leaves accumulated the highest amount of As when the plant was exposed to As⁵⁺, but when it was exposed to As³⁺, the root accumulated the highest amount of As, and the leaves, the highest amount of Hg. When compared to other aquatic plants species, the results showed that P. pusillus demonstrated a higher Hg accumulation (2465 ± 293 µg.g^?1) when the transfer coefficient was 40,580 ± 3762 L.kg ^?1, showing the great potential of this macrophyte for phytoremediation of water contaminated with Hg. To the extent of our knowledge, this is the first report on bioaccumulation of As³⁺, As⁵⁺, and Hg by P. pusillus.     

Characterization of α-Glucosyltransferase from Pseudomonas mesoacidophila MX-45

α-Glucosyltransferase was purified from Pseudomonas mesoacidophila MX-45. The molecular weight was estimated to be 63,000 by SDS–PAGE, and the isoelectric point was pi 5.4. For enzyme activity based on sucrose decomposition, the optimum pH and the optimum temperature were pH 5.8 and 40°C, respectively. The ranges of stable pH and temperature were pH 5.1–6.7 and below 40°C, respectively. The purified enzyme of MX-45 converted sucrose into trehalulose (1-O-α-d-glucopyranosyl-d-fructose) and isomaltulose (palatinose, 6–O-α-d-glucopyranosyl-d-fructose) simultaneously, and the ratio of trehalulose to isomaltulose increased at lower reaction temperatures. Therefore, optimum conditions for trehalulose production were pH 5.5–6.5 at 20°C. The yield of trehalulose from sucrose (20–40% solution) was 91%. The K_m for sucrose was 19.2 ± 3.3 mm estimated by the Hanes–Woolf plot. Product inhibition was observed, and the product inhibition constant was 0.17 m. Hg²⁺, Fe³⁺, Cu²⁺, Mg²⁺, Ag⁺, Pb²⁺, glucono-1,5-lactone, and Tris(hydroxymethyl)aminomethane inhibited the reaction.     

Dinuclear complexes with a μ-cyano ligand. Part X. Synthesis and characterization of several derivatives of cis-diaquaamminecobalt(III) complexes. Influence of pH

Six new dinuclear complexes, derived from cis-[Co(H₂O)₂(NH₃)₄]³⁺, cis-[Co(H₂O)₂(en)₂]³⁺ and [M(CN)₄^2? (M = Ni, Pd, Pt) were prepared and characterized by means of chemical analysis, electronic and IR measurements. The influence of the pH on the rate of the reaction was studied for the two derivatives of [Pd(CN)₄]^2?, showing that the best conditions to obtain the dinuclear compounds are at pH near 6, where the predominant species are cis-[Co(OH)(H₂O)(amine)₂]²⁺. The [Pt(CN)₄]^2? derivatives show PtPt interactions both in the solid state and in solution.     

Lewis acid catalysis of phosphoryl transfer from a copper(II)-NTP complex in a kinase ribozyme

The chemical strategies used by ribozymes to enhance reaction rates are revealed in part from their metal ion and pH requirements. We find that kinase ribozyme K28(1-77)C, in contrast with previously characterized kinase ribozymes, requires Cu²⁺ for optimal catalysis of thiophosphoryl transfer from GTPγS. Phosphoryl transfer from GTP is greatly reduced in the absence of Cu²⁺, indicating a specific catalytic role independent of any potential interactions with the GTPγS thiophosphoryl group. In-line probing and ATPγS competition both argue against direct Cu²⁺ binding by RNA; rather, these data establish that Cu²⁺ enters the active site within a Cu²⁺•GTPγS or Cu²⁺•GTP chelation complex, and that Cu²⁺•nucleobase interactions further enforce Cu²⁺ selectivity and position the metal ion for Lewis acid catalysis. Replacing Mg²⁺ with [Co(NH₃)₆]³⁺ significantly reduced product yield, but not k_obs, indicating that the role of inner-sphere Mg²⁺ coordination is structural rather than catalytic. Replacing Mg²⁺ with alkaline earths of increasing ionic radii (Ca²⁺, Sr²⁺ and Ba²⁺) gave lower yields and approximately linear rates of product accumulation. Finally, we observe that reaction rates increased with pH in log-linear fashion with an apparent pKa = 8.0 ± 0.1, indicating deprotonation in the rate-limiting step.     

Thermostable α‐Amylase and Glucoamylase from Thermomyces lanuginosus F1

An α‐amylase and a glucoamylase produced by Thermomyces lanuginosus F₁ were separated by ion‐exchange chromatography on Q‐Sepharose fast flow. The enzymes were further purified to electrophoretic homogeneity by chromatography on Sephadex G‐100 and Phenyl‐Sepharose CL‐4B.The molecular weights and isoelectric points of the enzymes were 55,000 Da and pHi 4.0 for α‐amylase and 70,000 Da and pH_i 4.0 for glucoamylase, respectively. The optimum pH and temperatures for the enzymes were found to be 5.0 and 60 °C for α‐amylase, and 6.0 and 70 °C for glucoamylase,respectively. Both enzymes were maximally stable at pH 4.0 and retained over 80% of their activity between pH 5.0 and 6.0 for 24 h. After incubation at 90 °C (1 h), the α‐amylase and glucoamylase retained only 6% and 16% of their activity, respectively. The enzymes readily hydrolyzed soluble starch, amylose, amylopectin and glycogen but hydrolyzed pullulan very slowly. Glucoamylase and α‐amylase had highest affinity for soluble starch with K_M values of 0.80 mg/ml and 0.67 mg/ml, respectively. The α‐amylase hydrolyzed raw starch granules with a predominant production of glucose and maltose. The activities of α‐amylase and glucoamylase increased in the presence of Mn²⁺, Co²⁺, Ca²⁺, Zn²⁺ and Fe²⁺, but were inhibited by guanidine‐HCl, urea and disodium EDTA. Both enzymes possess pH and thermal stability characteristics that may be of technological significance.     

Synthesis and characterization of new unsaturated macrobicyclic and bis(macrocyclic) copper(II) complexes containing N-CH2-N linkages

New copper(II) complexes [CuL²]²⁺ (L²=7,7,9-trimethyl-1,3,6,10,13-pentaazabicyclo[11,2,1^1.13]hexadec-9-ene) and [Cu₂(L³)(H₂O)₂]⁴⁺ have been prepared by the reaction of [CuL¹]²⁺ (L¹=5,5,7-trimethyl-1,4,8,11,14-pentaazatetradce-7-ene) and formaldehyde. The mononuclear complex [CuL²]²⁺ has a square-planar coordination geometry with a 5-6-5-6 chelate ring sequence and is relatively stable even in low pH at room temperature. The dinuclear complex [Cu₂(L³)(H₂O)₂]⁴⁺ consists of two unsaturated 15-membered pentaaza macrocyclic units (7,7,9-trimethyl-1,3,6,10,13-pentaazacyclopentadec-9-ene) that are linked together by a methylene group in a tilted face-to-face arrangement [Cu?Cu distance: 7.413(2) Å ]. Each macrocyclic unit of [Cu₂(L³)(H₂O)₂]⁴⁺ contains one four-membered chelate ring and has a severely distorted octahedral coordination polyhedron. The dinuclear complex is quite stable in aqueous solutions containing an excess of formaldehyde or in dry acetonitrile but is decomposed to [CuL¹]²⁺ and [CuL²]²⁺ in pure water.     

Characterization of arsenite-oxidizing bacteria to decipher their role in arsenic bioremediation

High arsenic groundwater contamination causes serious health risks in many developing countries, particularly in India and Bangladesh. The arsenic fluxes in aquifers are primarily controlled by bacterial populations through biogeochemical cycle. In this present study, two gram-positive rod-shaped bacteria were isolated from shallow aquifers of Bhojpur district in Bihar during the early winter season, able to withstand arsenite (As³⁺) concentration upto 70?mM and 1000?mM of arsenate (As⁵⁺) concentration. They showed high resistance to heavy metals up to 30?mM and utilized some complex sugars along with different carbon sources. Growth at wide range of temperature, pH and salinity were observed. Both these isolates showed high efficiency in converting As³⁺ into less toxic concentrations of As⁵⁺ respectively from arsenic enriched culture media. Along with superior arsenic transformation and arsenic resistance abilities, the isolates showed a wide variety of metabolic capacity in terms of utilizing a variety of carbon sources under aerobic conditions, respectively. This study reports the potential As³⁺-oxidizing bacteria that can play an important role in subsurface arsenic transformation that will aid in designing future bioremediation strategy for the arsenic affected areas.     

Structural,morphological and steady state photoluminescence spectroscopy studies of red Eu3+‐doped Y2O3 nanophosphors prepared by the sol‐gel method

Europium trivalent (Eu³⁺)‐doped Y₂O₃ nanopowders of different concentrations (0.5, 2.5, 5 or 7 at.%) were synthesized by the sol‐gel method, at different pH values (pH 2, 5 or 8) and annealing temperatures (600°C, 800°C or 1000°C). The nanopowders samples were characterized by X‐ray diffraction (XRD), field emission scanning electron microscopy (FE‐SEM), Fourier transform infrared spectroscopy (FT‐IR) and steady state photoluminescence spectroscopy. The effect of pH of solution and annealing temperatures on structural, morphological and photoluminescence properties of Eu³⁺‐doped Y₂O₃ were studied and are discussed. It was found that the average crystallite size of the nanopowders increased with increasing pH and annealing temperature values. The Y₂O₃:Eu³⁺ material presented different morphology and its evolution depended on the pH value and the annealing temperature. Activation energies at different pH values were determined and are discussed. Under ultraviolet (UV) light excitation, Y₂O₃:Eu³⁺ showed narrow emission peaks corresponding to the ⁵D_0–⁷F_J (J = 0, 1, 2 and 3) transitions of the Eu³⁺ ion, with the most intense red emission at 611 assigned to forced electric dipole ⁵D₀→⁷F₂. The emission intensity became more intense with increasing annealing temperature and pH values, related to the improvement of crystalline quality. For the 1000°C annealing temperature, the emission intensity presented a maximum at pH 5 related to the uniform cubic‐shaped particles. It was found that for lower annealing temperatures (small crystallite size) the CTB (charge transfer band) position presented a red shift. Copyright © 2015 John Wiley & Sons, Ltd.