Controlling Cr3+/Cr4+ concentration in single-phase host toward tailored super-broad near-infrared luminescence for multifunctional applications

Full control on the valence of the active ions in solids to improve properties is the central topic of chemistry and materials. Cr³⁺ and Cr⁴⁺ ions generally emit wavelength-different near-infrared (NIR) light. Here, we have developed a chromium valence-controllable single-phase phosphor, Mg₂GeO₄:Cr³⁺,Cr⁴⁺ to achieve super-broad NIR luminescence. High Li ⁺ content charge compensators can stabilize Cr³⁺, whereas high-temperature sintering tends to facilitate the formation of Cr⁴⁺. Through fine adjusting the synthesis conditions, pure Cr³⁺ or Cr⁴⁺ luminescence can be obtained with peak emission locating at 935 nm and 1190 nm, respectively. Super broad band dual emission spanning from 650 nm to 1600 nm is realized via fully controlling the concentration ratio of Cr³⁺ to Cr⁴⁺ in a single host. By measuring the transmission spectra of several foodstuff illuminated by our phosphors, non-destructive analysis in food safety areas can be realized. This study provides a new strategy for exploiting super broad band NIR luminescent materials.     

Photophysical properties of rhodamine isomers: A two-photon excited fluorescent sensor for trivalent chromium cation (Cr)

We introduce a new rhodamine-based fluorescent chemosensor, FD8 which exhibits a distinct two-photon excited fluorescence (TPEF) on/off characteristic upon binding Cr³⁺ ions. By coordination with metal cation, conformation of FD8 changes from spirocyclic to open-ring, resulting in remarkable enhancement of absorption and fluorescence both in one- and two-photon excitations. As a result, a 29-fold enhancement of two-photon excited fluorescent intensity was observed when 10 eq. Cr³⁺ was added to the FD8 solution. The detection limit of Cr³⁺ cation concentration down to 1 μM (0.01 eq. of FD8) was achieved under our experimental condition. Besides the excitation within ultraviolet regime by fluorescence resonance energy transfer (FRET) mechanism, the TPEF on/off behavior further extends the excitation to near infrared regime (the biological optimal window of 700-1200 nm), and shows more effective sensitivity. The broad excitation wavelength, on/off fluorescence and high selectivity to Cr³⁺ enable FD8 to be a powerful Cr³⁺ cation sensor with potential application, especially in biological detection. To the best of our knowledge, this is the first report about two-photon fluorescent sensor for Cr³⁺ ions.     

Heavy metal-induced inhibition of Aspergillus niger nitrate reductase: applications for rapid contaminant detection in aqueous samples

Enzyme inhibition assays have the potential to rapidly screen and identify heavy metals in environmental samples. Inhibition of nitrate reductase (NR) was examined as a method for detecting toxic metals. The activity of NR (EC 1.6.6.2) from Aspergillus niger was assayed as a function of metal concentration in the presence of Cd²⁺, Cr³⁺, Cr⁶⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺. NR exhibited sensitivity to these metals at concentrations below 10 μM. Various buffers were screened for their ability to protect NR activity from metal inhibition, and 3-(N-morpholino) propanesulfonic acid (MOPS) was selected as the buffering system for the NR assays as it exhibited the least interference with metal inhibition, thus providing increased assay sensitivity. The hypothesis that chelating agents could prevent the inhibition of NR activity by metal ions was also tested. Results indicated that 10 mM ethylenediaminetetraacetic acid (EDTA) could protect NR activity from inhibition by Cr³⁺, Cu²⁺, Cd²⁺, Ni²⁺, and Zn²⁺ at concentrations below 100 μM, but that the EDTA had no effect on NR inhibition by Cr⁶⁺. An amount of 10 mM nitrilotriacetic acid (NTA) prevented NR inhibition by Cd²⁺, Cu²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ at metal concentrations below 100 μM. However, 10 mM NTA was unable to protect the enzyme from inhibition by either Cr³⁺ or Cr⁶⁺. These results indicated that through specific metal chelation, a NR-based method for individually quantifying Cr³⁺ and Cr⁶⁺ species in aqueous solutions could be developed. The ability to restore activity to NR which been previously inhibited by exposure to 100 μM Pb²⁺, Cd²⁺, Zn²⁺, Cu²⁺, and Cr³⁺ was explored to determine whether NR activity could be recovered by EDTA additions for use in consecutive metal inhibition assays. The results showed NR activity could not be regained after exposure to Cr³⁺ or Cu²⁺, but did partially recover activity after Cd²⁺, Pb²⁺, and Zn²⁺ exposure.     

Simultaneous, sensitive and selective on-line chemiluminescence determination of Cr(III) and Cr(VI) by capillary electrophoresis

A novel in-capillary reduction and capillary electrophoretic (CE)-chemiluminescence (CL) method was developed for the sensitive and selective determination of chromium(III) and chromium(VI). The proposed method was based on the in-capillary reduction of Cr(VI) with acidic H₂O₂ to form Cr(III) using the zone-passing technique and chemiluminescence detection of Cr(III). The sample [Cr³⁺ and CrO₄²⁻], hydrochloric acid, and H₂O₂ (reductant) solution segments were injected for specified periods of time in this order from the anodic end of a capillary, followed by application of an appropriate running voltage between both ends. As both chromium species have opposite charges, Cr³⁺ migrates to the cathode while CrO₄²⁻ ion, moving oppositely to the anode, reacts with acidic H₂O_2, resulted in formation of Cr³⁺. Based on the migration time difference of both Cr³⁺ ions, they were separated by zone electrophoresis. Running buffer was composed of 0.02 mol l⁻¹ HAc-NaAc (pH 4.7) with 1×10⁻³ mol l⁻¹ EDTA. Parameters affecting CE-CL separation and detection, such as reductant concentration, mixing mode of the analytes with CL reagent, CL reaction reagent pH and concentration, stability of luminol-hydrogen peroxide mixed solution were optimized. The limits of detection for chromium(III) and chromium(VI) (3σ) were 6×10⁻¹³ mol l⁻¹ (mass concentration 12 zmol) and 8×10⁻¹² mol l⁻¹ (160 zmol), respectively. This method offered potential advantages of simplicity, sensitivity, selectivity and applicability to the determination of Cr(III) and Cr(VI) in environmental water.     

Investigation of Structural and Luminescent Properties of Sol-Gel-Derived Cr‑Substituted Mg3Al1−xCrx Layered Double Hydroxides

In the present work, Cr-substituted Mg₃Al_1−xCr_x layered double hydroxides (LDHs) were synthesised through the phase conversion of sol-gel-derived mixed-metal oxides in an aqueous medium. The chromium substitution level in the range of 1 to 25 mol% was investigated. It was demonstrated that all synthesised specimens were single-phase LDHs. The results of elemental analysis confirmed that the suggested synthetic sol-gel chemistry approach is suitable for the preparation of LDHs with a highly controllable chemical composition. The surface microstructure of sol-gel-derived Mg₃Al_1−xCr_x LDHs does not depend on the chromium substitution level. The formation of plate-like agglomerated particles, which consist of hexagonally shaped nanocrystallites varying in size from approximately 200 to 300 nm, was observed. Optical properties of the synthesised Mg₃Al_1−xCr_x LDHs were investigated by means of photoluminescence. All Cr-containing powders exhibited characteristic emission in the red region of the visible spectrum. The strongest emission was observed for the sample doped with 5 mol% Cr³⁺ ions. However, the emission intensity of samples doped with 1–10 mol% Cr³⁺ ions was relatively similar. A further increase in the Cr³⁺ ion concentration to 25 mol% resulted in severe concentration quenching.     

Prediction of Stability Constants for Cr(III) and Cr(II) Complexes

Regression analysis was used to derive equations for estimaing thermodynamic stability constants for complexes of Cr²⁺ (log^° ₁[Cr²⁺L] = 0.53log^° _n [H _n L]) and Cr³⁺ (log^° ₁[Cr³⁺L] = 0.88log^° _n [H _n L]) from the known protonation constants of H _n L ligands and for determining stability constants of Cr²⁺ and Cr³⁺ complexes from the available stability constants of Cu²⁺ complexes (log^° ₁[Cr²⁺L] = 0.76log^° ₁[Cu²⁺L] and log^° ₁[Cr²⁺L] = 0.60log^° ₁[Cr³⁺L], respectively). Parameters of the Panteleon–Ecka equation for calculating stability constants of Cr²⁺ complexes ( = 0.57) and Cr³⁺ complexes ( = 0.69) with two and three bidentate ligands were also determined. The ratio of logarithmic stability constants for complexes with the same metals but with different metal ionic charges was found to be approximately equal to the ratio of charges on the central ions. The stability constant of Cr(II) sulfate complex was calculated.     

Biosorption of metal ions with Penicillium chrysogenum

Biosorption of metal ions with Penicillium chrysogenum mycelium is described in this article. Alkaline pretreatment was used to remove proteins and nucleic acids from cells, and this treatment increased the adsorption capacities, for Cr³⁺ from 18.6 mg g⁻¹ to 27.2 mg g⁻¹, for Ni²⁺ from 13.2 mg g⁻¹ to 19.2 mg g⁻¹, for Zn²⁺ from 6.8 mg g⁻¹ to 24.5 mg g⁻¹. The adsorption of metal ions was strongly pH dependent. The mycelium could beused for large-scale removal of Cr³⁺ from tannery wastewater. The results show that this inexpensive mycelium adsorbent has potential in industry because of its high adsorption capacity. The main chelating sites are amino groups (−NH₂) of chitosan in the mycelium. A new model is established, which describes the relation of adsorption of metal ions on pH according to amino group chelating with metal ions and H⁺. The relative errors of simulation for Cu²⁺, Ni²⁺, Zn²⁺, and Cr³⁺ are 4.66%, 5.45%, 11.55%, and 1.69%, respectively.     

Chromium uptake from tricomponent solution in zeolite fixed bed

Removal of Cr³⁺,Ca²⁺,Mg²⁺ and K⁺ in equilibrium isotherms and in tricomponent solutions (Cr/Ca/K, Cr/Ca/Mg and Cr/Mg/K) were investigated in NaX and NaY packed beds at 30^∘C. The equilibrium selectivity was obtained as Cr⁺³ > Mg²⁺ > Ca²⁺≈K⁺ for zeolite NaY and Ca^{2 +}≫Cr^{3 +} > Mg^{2 +}≈K⁺ for zeolite NaX. The breakthrough curves showed sequential ion exchange where chromium ions are able to replace the competing cations. Some mass transfer parameters, such as length of unused bed and overall mass transfer coefficient, were investigated. Chromium retention was also investigated through a mass balance. Based on the breakthrough results, it was concluded that chromium-uptake mechanism was hardly influenced by the competition and interaction between the entering ions. NaY showed a higher affinity towards Cr³⁺ for both equilibrium and dynamic systems and its sites were more efficiently used in the ion exchange process. Chromium was less retained in NaX due to the high selectivity towards calcium ions.     

Magnetic entropy change in perovskite manganites La0.7A0.3MnO3 La0.7A0.3Mn0.9Cr0.1O3 (A = Sr,Ba, Pb) and Banerjee criteria on phase transition

The La_0.7A_0.3MnO₃, La_0.7A_0.3Mn_0.9Cr_0.1O₃ (A = Sr, Ba, Pb) polycrystalline perovskite manganites sample was prepared by the sol–gel technique. The replacement of partial Mn³⁺/Mn⁴⁺ by Cr³⁺ (Cr³⁺ with the same electronic configuration as Mn⁴⁺) cause the variety of magnetocaloric property and magnetic entropy change. The maximum magnetic entropy change ΔS_H = −1.16 J/kg K and the Relative Cooling Power (about 43.3 J/kg) were obtained from La_0.7Sr_0.3Mn_0.9Cr_0.1O₃ in the Cr³⁺ doped series under 1 T magnetic field variation. On this paper, Banerjee criteria had been remarked to distinguish the first-order or second-order phase transition on phase transition of the doped perovskite manganites.     

Trace metal-ion catalysis: kinetics and mechanism of oxidation of l-ascorbic acid by chromium(VI) in phosphate buffers

Summary The kinetics of oxidation of l-ascorbic acid (H₂A) by Cr^VI with and without added Cu^II conforms to the stoichiometry represented by the equation: 2Cr^VI + 3H₂A 2Cr^III + 3A + 6H⁺ where A is dehydroascorbic acid. The mode of the electron transfer from H₂A to Cr^VI is suggested to involve oxidative decomposition of an intermediate complex. Catalysis by Cu^II is indicated via complexation of the catalyst and substrate. The inhibitory effects of Cl^–, NO _f3 ^p– and SO _f4 ^p2– ions indirectly support complexation between Cr^VI and H₂A.     

Effect of electron correlation in Sr(Ca)Ru1−xCrxO3: Density functional calculation

We have investigated the electronic structure of Sr(Ca)Ru_1−xCr_xO₃ using the full potential linearized augmented plane wave method by different approximation such as LSDA and LSDA+U. The LSDA calculation suggest that Cr⁴⁺-Ru⁴⁺ hybridization is responsible for the high Curie temperature T_C in SrRu_1−xCr_xO₃, but it cannot completely describe its physical behavior. Our LSDA+U DOS results for SrRu_1−xCr_xO₃ clearly establishes renormalization of the intra-atomic exchange strength at the Ru sites, arising from the Cr-Ru hybridization. The antiferromagnetic coupling of Cr³⁺ with Ru^4+,5+ lattice increases the screening, which is consistent with the low magnetic moment of the Ru ions. The more distorted Ca-based compounds as compared to the Sr-based systems shows that the hybridization mechanism is not relevant for these compounds. The bigger exchange splitting of Ru 4d and Cr 3d at the Fermi level with Ru^4+,5+ and Cr^3+,4+ orbital occupancies of CaRu_0.75Cr_0.25O₃ in the LSDA+U calculation, compared with that of the LSDA calculation, shows that repulsion between electrons tend to keep the localized spins from overlapping. The low screening of the Ru t_2g electrons increases T_C in the Ca-based systems, which is consistent with the both high Ru exchange splitting and magnetic moment. The insulating behavior of the high Cr-doped systems can be explained by considering the Ru⁴⁺+Cr⁴⁺→Ru⁵⁺+Cr³⁺ charge transfer.     

Magnetic behaviour of the orthochromite La0.5Gd0.5CrO3

Magnetic behaviour of the perovskite La_0.5Gd_0.5CrO₃ has been studied. The orthochromite orders canted antiferromagnetically below Neel temperature T_N of ～225 K. Reversal of magnetization is observed in temperature dependence of magnetization measured in field cooled mode under external fields upto 500 Oe. In the field dependence of magnetization below T_N, a small hysteresis is observed with the magnetic anisotropy continuously increasing with lowering of temperature. Estimated values of Cr³⁺ moments, internal field due to sub-lattice of canted ordered Cr³⁺ and the paramagnetic Curie temperature of Gd³⁺ sub-lattice are found to be smaller than reported for GdCrO₃. Compared with Pr substituted analogue La_0.5Pr_0.5CrO₃, Cr³⁺ moment is about the same but the internal field at the Gd³⁺ sub-lattice is much smaller.     

Highly sensitive and selective reversible sensor for the detection of Cr

A new fluorescent sensor capable of sensing Cr³⁺ has been synthesized. Complexing with Cr³⁺ triggers the formation of a highly fluorescent ring-open form which is pink in color. The sensor shows extremely high fluorescence enhancement upon complexation with Cr³⁺ and it can be used as a ‘naked eye’ sensor. Binding of Cr³⁺ was found to be reversible as the pink color disappears with excess EDTA.     

Synthesis,structural characterization and electrochemical recognition of metal ions of two new ferrocenylhydrazone‐based receptors

Two new ferrocenylhydrazone‐based receptors FcL₁ and FcL₂ were prepared and the X‐ray crystal structure of FcL₁ was described. The electrochemical studies reveal that the receptor FcL₁ is responsive to Hg²⁺ and Cr³⁺, whereas, receptor FcL₂ only responsive to Hg²⁺. The maximum electrochemical shift of FcL₁ for Cr³⁺ is about 56 mV. Copyright © 2009 John Wiley & Sons, Ltd.     

Preparation and characterization of silicate gel glasses containing Cr2O3

The effect of thermal treatment on the structural incorporation of Cr₂O₃ in xCr₂O₃·(100 – x)SiO₂ and 5R _n O·xCr₂O₃·(95 – x)SiO₂ (where x=0.01–1 mol.% and R=Li, Na and Ca) gel glasses was studied by optical absorption spectrophotometry, DTA-TG, XRD and electron microscopy.Samples heat treated at 60°C have green color due to the presence of Cr³⁺ in octahedral coordination. The optical transmission, and color (yellow, orange or ambar), of the samples heat treated between 200 and 700°C prove that Cr³⁺ (octahedrally coordinated) and Cr⁶⁺ (tetrahedrally coordinated) are both present. Segregation of Cr₂O₃ take place at temperatures above 800°C.In reducing conditions the gel glasses were green due to the presence of a high content of Cr³⁺ ions. Samples containing Li or Na show crystalline phases at temperatures below 850°C.     

Analysis of dissolved Cr3+ and Cr6+ in water by APDC-MIBK extraction and atomic absorption spectrometry

Summary With the well-known APDC-MIBK extraction of dissolved trace metals from aqueous solutions for AAS analysis only Cr⁶⁺ is determined, whereas practically no Cr³⁺ is extracted. This technique was therefore modified (PHP-buffer; pH 4.7–5.5; 80° C; 20 min reaction time) so that hexavalent and trivalent chromium can be extracted simultaneously into the organic phase, thus allowing determination of the percentage of each of these ionic species in water (hexavalent chromium being determined separately). A procedure for the modified extraction is described.

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Analyse von gelöstem Cr³⁺ und Cr⁶⁺ in Wasser durch APDC-MIBK-Extraktion und Atomabsorptionsspektrometrie

Zusammenfassung Mit der bekannten APDC-MIBK-Extraktion gelöster Spurenmetalle aus wäßrigen Lösungen für die AAS-Analyse wird nur Cr⁶⁺ erfaßt, während Cr³⁺ praktisch nicht extrahiert wird. Die bisherige Methode wurde deshalb so modifiziert (PHP-Puffer; pH 4,7–5,5; 80° C; 20 min Reaktionszeit), daß mit dem sechswertigen auch das dreiwertige Chrom in die organische Phase extrahiert wird und so der Anteil beider lonenarten bestimmt werden kann (nach getrennter Bestimmung des Cr⁶⁺). Eine Arbeitsvorschrift wird angegeben.

     

Ratiometric near-infrared chemosensor for trivalent chromium ion based on tricarboyanine in living cells

A tricarboyanine derivative (IRPP) is applied as a ratiometric near-infrared chemosensor for detecting trivalent chromium ions (Cr³⁺) in living cells. Upon the addition of Cr³⁺ to a solution of IRPP, large-scale shifts in the emission spectrum (from 755 nm to 561 nm) are observed. In the newly developed sensing system, these well-resolved emission peaks yield a sensing system that covers a linear range from 1.0 × 10⁻⁷ to 1.0 × 10⁻⁵ M with a detection limit of 2.5 × 10⁻⁸ M. The experimental results show the response behavior of IRPP towards Cr³⁺ is pH independent under neutral conditions (6.0–7.5). Most importantly, the fast response time (less than 3 min) and selectivity for Cr³⁺ over other common metal ions provide a strong argument for the use of this sensor in real world applications. As a proof of concept, the proposed chemosensor has been used to detect and quantify Cr³⁺ in river water samples and to image Cr³⁺ in living cells with encouraging results.     

A highly selective colorimetric and fluorescent chemosensor for Cr2+ in aqueous solutions

A new rhodamine-based chemosensor was synthetized through a modified copper-catalyzed [3+2]-cycloaddition of an azidocoumarin with an alkynyl-rhodamine. Its sensing properties toward various metal cations in aqueous solutions were investigated by colorimetric changes, UV–vis and fluorescence spectroscopies. The sensor exhibited a high selectivity for Cr²⁺ over Cr³⁺ and other divalent cations such as Cu²⁺, Mg²⁺, Zn²⁺, Ca²⁺, Cd²⁺, Co²⁺, Hg²⁺ and Ni²⁺. The linear range of detection by fluorescence spectroscopy is 0.07–3.5 mM, with a detection limit of ca. 64 μM. The binding mode of Cr²⁺ with the sensor was rationalized through experimental evidences.     

In Vitro Selection of Chromium‐Dependent DNAzymes for Sensing Chromium(III) and Chromium(VI)

Chromium is a very important analyte for environmental monitoring, and developing biosensors for chromium is a long‐standing analytical challenge. In this work, in vitro selection of RNA‐cleaving DNAzymes was carried out in the presence of Cr³⁺. The most active DNAzyme turned out to be the previously reported lanthanide‐dependent Ce13d DNAzyme. Although the Ce13d activity was about 150‐fold lower with Cr³⁺ than that with lanthanides, the activity of lanthanides and other competing metals was masked by using a phosphate buffer; this left Cr³⁺ as the only metal that could activate Ce13d. With 100 μm Cr³⁺, the cleavage rate is 1.6 h^?1 at pH 6. By using a molecular beacon design, Cr³⁺ was measured with a detection limit of 70 nm , which was significantly lower than the United States Environmental Protection Agency (EPA) limit (11 μm ). Cr⁴⁺ was measured after reduction by NaBH₄ to Cr³⁺, and it could be sensed with a similar detection limit of 140 nm Cr⁴⁺; this value was lower than the EPA limit of 300 nm . This sensor was tested for chromium speciation analysis in a real sample, and the results supported its application for environmental monitoring. At the same time, it has enhanced our understanding of the interactions between chromium and DNA.     

Physicochemical and catalytic properties of sol gel-prepared copper-chromium oxides

Nanostructured copper-chromium oxides were prepared by the sol–gel process (SG) and were characterised by elemental analysis, thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and by their activity in methane combustion. A comparative study was made with copper chromites commercial catalysts. The as-synthesised copper chromites sample exhibited higher specific surface area (248 m² g⁻¹) with respect to commercial solids (42 m² g⁻¹). The surface quantitative analysis evidenced a Cr⁶⁺ enrichment for the SG catalyst (Cr⁶⁺/Cr³⁺=0.56) with respect to commercial sample (0.39), while the ratio of copper species Cu²⁺/(Cu° + Cu⁺) was the same in both solids. Catalytic activity of SG solids in methane combustion was found to be comparable to that of Pt/Al₂O₃ and superior to that of commercial copper chromites tested under the same conditions.