Rhodamine-based fluorescent sensor for mercury in buffer solution and living cells

A novel fluorescent sensor based on thiooxorhodamine B has been prepared to detect Hg²⁺ in aqueous buffer solution. It demonstrates high selectivity for sensing Hg²⁺ with about 383-fold enhancement in fluorescence emission intensity and micromolar sensitivity (K_d = 7.5 × 10⁻⁶ mol L⁻¹) in comparison with alkali and alkaline earth metal ions (K⁺, Na⁺, Mg²⁺, Ca²⁺) and other transition metal ions (Mn²⁺, Ni²⁺, Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺, Ag⁺, Pb²⁺, Cr³⁺, Fe³⁺). Meanwhile the distinct color changes and rapid switch-on fluorescence also provide ‘naked eyes’ detection for Hg²⁺ over a broad pH range. Moreover, such sensor is cell-permeable and can visualize the changes of intracellular mercury ions in living cells using fluorescence microscopy.     

Linear sweep anodic stripping voltammetry of heavy metals from nitrogen doped tetrahedral amorphous carbon thin films

Nitrogen doped tetrahedral amorphous carbon (ta-C:N) thin films were deposited on p-Si (1 1 1) substrates (1 × 10⁻³ to 6 × 10⁻³ Ω cm) by a filtered cathodic vacuum arc technique with different nitrogen flow rates (3 and 20 sccm). The ta-C:N film coated samples were used as working electrodes to detect trace heavy metals such as zinc (Zn), lead (Pb), copper (Cu) and mercury (Hg) by using linear sweep anodic stripping voltammetry in 0.1 M KCl solutions (pH 1). The influence of nitrogen flow rate on the sensitivity of the films to the metal ions was investigated. The results showed that the current response of the ta-C:N film electrodes was significant to differentiate all the tested trace metal ions (Zn²⁺, Pb²⁺, Cu²⁺, and Hg²⁺) and the three ions (Pb²⁺ + Cu²⁺ + Hg²⁺) could be simultaneously identified with good stripping peak potential separations.     

Electroanalytical studies on cobalt(II) selective potentiometric sensor based on bridge modified calixarene in poly(vinyl chloride)

A bridge modified 4-tert-butylthiacalix[4]arene (I) has been employed as electroactive material in the preparation of cobalt selective sensor. Polyvinyl chloride (PVC)-based membranes of (I) using sodium tetraphenylborate (NaTPB) as anion discriminator and bis(2-ethylhexyl) sebacate (DOS), chloronaphthalene (CN), tri-n-butylphosphate (TBP), o-nitrophenyl octyl ether (NPOE), tris(2-ethylhexyl)phosphate (TEHP) as plasticizers were prepared and investigated as cobalt selective sensors. A number of membranes of different compositions were prepared and investigated. The best performance was observed with the membrane having composition of 2:66:1.5:127 (mg) = I:NaTPB:PVC:NPOE. The potential response of this membrane is linear to Co²⁺ ions in the concentration range 5.3 × 10⁻⁶ to 1.0 × 10⁻¹ M with near-Nernstian slope of 30.0 mV/decade of activity and a detection limit of ∼0.3 ppm. This membrane also showed lowest response time of 10 s and works satisfactorily in partially non-aqueous medium. The selectivity studies of this sensor, evaluated with fixed interference method and matched potential method, show that the sensor under consideration possesses excellent selectivity for Co²⁺ over a large number of mono-, bi- and trivalent cations such as Na⁺, K⁺, Ag⁺, Ca²⁺, Mg²⁺, Cu²⁺, Hg²⁺, Pb²⁺, Li⁺, Ba²⁺, Zn²⁺, Sr²⁺, Cr³⁺, Ni²⁺, etc. The sensor could be used as an indicator electrode in the quantification of Co²⁺ by potentiometric titration against EDTA as well as in determination of cobalt content in wastewater and beer samples.     

Preparation and characterisation of a poly(acrylamidoglycolic acid-co-acrylamide) hydrogel for selective binding of Cu and application to diffusive gradients in thin films measurements

A poly(acrylamidoglycolic acid-co-acrylamide) [poly(AAGA-co-AAm)] hydrogel was prepared by copolymerising 2-acrylamidoglycolic acid (AAGA) with acrylamide (AAm). The copolymer hydrogel composition and structure was characterised by FTIR spectroscopy and elemental microanalysis and found to contain 3.5 AAGA monomer units for each AAm monomer unit. This was similar to the monomer ratios used in the synthesis. The metal ion binding properties of the hydrogel were characterised for a range of metal ions (Cu²⁺, Cd²⁺, K⁺, Na⁺, Mg²⁺ and Ca²⁺) under varying conditions of pH, ionic strength, metal concentration and time. The hydrogel was shown to bind Cu²⁺ and Cd²⁺ strongly under non-competitive binding conditions, with binding capacities of 5.3 and 5.1 μmol cm⁻², respectively. The binding capacity of each metal decreased, under competitive binding conditions (with a range of metal ions present at 17.8 μN), to 1.3 and 0.17 μmol cm⁻², respectively, indicating stronger selectivity for Cu²⁺. The metal ions were readily recovered (>94%) by eluting with 2 M nitric acid solution for 24 h. The binding capacities for Cu²⁺ and Cd²⁺ were also found to decrease with increasing ionic strength and at pH values <5. The copolymer was found to have an equilibrium swelling ratio (q_w) of over 500 at a maxima of pH 5.4 and at low ionic strengths. Finally, the copolymer hydrogel was tested as a binding phase with the diffusive gradients in thin films technique. A linear mass vs. time relationship was observed for Cu²⁺ in synthetic Windermere water with a recovery of approximately 100%.     

Novel chemosensory materials based on polyfluorenes with 2-(2′-pyridyl)-benzimidazole and 5-methyl-3-(pyridin-2-yl)-1,2,4-triazole groups in the side chain

Polyfluorenes with 2-(2′-pyridyl)-benzimidazole (P1, P2 and P4) and 5-methyl-3-(pyridin-2-yl)-1,2,4-triazole (P3) groups in the side chain were synthesized by Suzuki polycondensation. The responsive properties of polymers on metal ions and H⁺ were investigated by absorption and emission spectra. The fluorescences of polymers (P1-P4) were completely quenched upon the transition metal ions such as Co²⁺, Ni²⁺, Fe³⁺ and Ag⁺ due to the enhanced electronic communication properties of conjugated polymers. The obvious differences to Ni²⁺ ion responsive sensitivity were observed between P1 and P4 polymers. The fluorescences of P1 and P4 were quenched to 50 (I₀/I) and to 22 (I₀/I) upon the addition of a Ni²⁺ solution of 3.2 × 10⁻⁶ M, as well as 5.0 × 10⁻⁶ M, respectively, owing to the different conjugated backbone. The fluorescences of P2 and P3 were completely and hardly quenched upon the addition of a Al³⁺ solution of 1.0 × 10⁻⁴ M, respectively, owing to the different receptors in the side chain. P2 showed good selectivity to Ni²⁺ ion in the range of quencher concentration as low as 5 ppm, owing to the different chelating abilities of receptor with ions. Cu²⁺ and Mn²⁺ ions hardly quenched the fluorescences of polymers (P1-P4), which were different from the oligopyridyl-functionalized conjugated polymers. The results further opened the opportunities to develop the tailored sensory materials through the appropriate alteration of receptors in the side chain and the conjugated backbone.     

Cationic conjugated polyelectrolyte-based fluorometric detection of copper(II) ions in aqueous solution

A series of water-soluble cationic polyfluorene copolymer containing 2,2′-bipyridine moieties (PFP-P_1-3) in the backbone were designed and synthesized as the fluorescent probes for Cu²⁺ ions. In the absence of the Cu²⁺ ion, the PFP-P₂ exhibits strong fluorescence emission in aqueous solution. Upon adding the Cu²⁺ ion, the PFP-P₂ coordinates to Cu²⁺ ions through weak N?Cu interactions, and its fluorescence is efficiently quenched by the Cu²⁺ ion with a Stern-Volmer constant (K_sv) of 1.44 × 10⁷ M⁻¹. The new method has high sensitivity with a detection limit of 20 nM. The minor interference from other heavy metal ions clearly shows that the PFP-P₂ can be used as the Cu²⁺ ion probe with good selectivity.     

Study on the photocatalytic degradation of trichlorfon in suspension of titanium dioxide

In this paper, the photocatalytic degradation of trichlorfon, an organophosphorous pesticide, was studied by using TiO₂ as a photocatalyst. The effects of various parameters, such as the amount of the photocatalyst, illumination time, reaction temperature, electron acceptors, metal ions, anions, and initial pH value on the photocatalytic degradation of trichlorfon were investigated. The best conditions for the photocatalytic degradation of trichlorfon were obtained. The results show that the optimum amount of the photocatalyst used is 8.0 g L^− 1. The photodegradation efficiency of trichlorfon increases with the increase of the illumination time or reaction temperature. The photodegradation efficiency of trichlorfon is increased rapidly by adding a small amount of H₂O₂, K₂S₂O₈, KBrO₃, Fe³⁺ and Cu²⁺, however, with the addition of Na⁺, K⁺, Mg²⁺, Ca²⁺, Zn²⁺, Co²⁺ and Ni²⁺, or with the addition of trace amount of SO₄²⁻, Cl⁻, Br⁻, there are no obvious effects on the photocatalytic degradation reactions. Alkaline mediums are favorable for the photocatalytic degradation of trichlorfon. The possible roles of the additives on the reactions and the possible mechanisms of effect were also discussed.     

Factors influencing the structure of electrochemically prepared α-MnO2 and γ-MnO2 phases

The α- and γ-phases of MnO₂ prepared by electrolysis of MnSO₄ and M_xSO₄ (where M = Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺ or Mg²⁺) in aqueous solutions at various pH and voltage E_v values under ambient conditions have been systematically studied. The structures of powdery MnO₂ produced are found to depend on the radius of the M^z+ counter cation in addition to the pH and E_v conditions. In order to achieve the α-phase for MnO₂ formation under neutral pH condition, the radius of counter cation must be equal to or greater than 1.41 Å, the size of the K⁺ cation. The relative concentration ratio of [MnO₄⁻]_transient/[Mn²⁺], which is related to the pH-E_v conditions, also affects the structure of MnO₂ produced with counter ions smaller than K⁺. For samples prepared in acidified solution with the counter ions of Li⁺, Na⁺ or Mg²⁺ at 2.2 V, the electrolysis products display the γ-MnO₂ phase while those prepared at 2.8 V electrolysis produce a mixture of γ-MnO₂ and α-MnO₂ phases. Single phase of α-MnO₂ is identified in the 5 V electrolysis products. Furthermore, the valence state of manganese was found to decrease as the applied voltage was reduced from 5.0 to 2.2 V. This implies that the lower [MnO₄⁻]_transient/[Mn²⁺] ratio or the less oxidative condition is responsible for the non-stoichiometric MnO₂ structure with oxygen deficiency.     

A potentiometric and voltammetric sensor based on polypyrrole film with electrochemically induced recognition sites for detection of silver ion

Conducting polypyrrole membranes were deposited on glassy carbon electrodes by electropolymerizing pyrrole in the presence of Eriochrome Blue-Black B (EBB) as the counter anion. The electrodes were then subjected to several oxidation/reduction potential steps in pure silver nitrate solution for successive accumulation/stripping of silver species. This electrochemically mediated doping/templating generated selective recognition elements in the EBB/PPy film for silver ions. The resulting sensor exhibited a considerable enhancement in the potentiometric and voltammetric response characteristics: extending the linear dynamic range and lowering the detection limit. In the potentiometric mode, the sensor showed highly reproducible response with a Nernstian slope of 58.5 ± 0.3 mV per decade of Ag⁺ activity over a linear range spanning seven orders of magnitude (1 × 10⁻⁸ to 1 × 10⁻¹ M Ag⁺), with a detection limit of ∼6 × 10⁻⁹ M. The electrodes demonstrated high selectivity over a large number of cations including alkali, alkaline earth and several transition and heavy metal ions, and could be used over a wide pH range of 1-8.5. The EBB/PPy modified electrode was also used for preconcentration and differential pulse anodic stripping voltammetric (DPASV) measurements. The DPASV peak current was dependent on the concentration of Ag⁺ over the range 3 × 10⁻¹⁰ to 1 × 10⁻⁴ M. The presence of 1000-fold excess of Cd²⁺, Cu²⁺, Cr³⁺, Co²⁺, Mn²⁺, Fe²⁺, Fe³⁺, Ni²⁺ and Pb²⁺ can be tolerated in the determination of silver ion.     

Neodymium(III) cathodic processes in molten fluorides

The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF-CaF₂. A preliminary study based both on thermodynamic and experimental data showed that it is not possible to observe the Nd(III)/Nd(0) system in LiF-KF and LiF-NaF melts; because the K⁺ and Na⁺ cation reduction waves hide the Nd³⁺ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 °C using solutions of NdF₃ in fluoride solvents without K⁺ and Na⁺ ions, such as LiF-CaF₂, by cyclic voltammetry, chronopotentiometry and square wave voltammetry. Experimental results show that neodymium trifluoride is reduced in Nd(0) in a one-step process exchanging three electrons (Nd(III) + 3e⁻ → Nd(0)). The electrode process is shown to be diffusion controlled. Nd(III) diffusion coefficient is in the range of 1.1-1.3 × 10⁻⁵ cm² s⁻¹ at 810 °C.     

Polymer-based fluorescence sensor incorporating triazole moieties for Hg detection via click reaction

The polymer could be obtained by the polymerization of 1,4-dibutoxy-2,5-diethynylbenzene (M-1) with 1,4-diazidobenzene (M-2)via click reaction. The polymer show blue fluorescence. The responsive optical properties of the polymer on various transition metal ions were investigated by fluorescence spectra. Compared with other cations, such as Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺ and Zn²⁺, Hg²⁺ can exhibit the most pronounced fluorescence response of the polymer Hg²⁺ can exhibit the most pronounced fluorescence response of the polymer due to triazole moiety in the polymer main chain as the metal binding ligand. The results indicate this kind of conjugated polymer with triazole moiety synthesized by click reaction can be used as a selective fluorescence sensor for Hg²⁺ detection.     

Design and synthesis of novel fluorescence sensing perylene diimides based on photoinduced electron transfer

Two novel tetraester- and PAMAM-branched perylene diimides were synthesized and configured as “fluorophore-spacer-receptor” systems based on photoinduced electron transfer. Due to their long alkylester and alkylamine terminal groups the examined compounds were well soluble in organic solvents. Photophysical characteristics of the dyes were investigated in DMF and water/DMF (1:1, v/v) solution. The ability of the synthesized perylene diimides to detect cations was evaluated by the changes in their fluorescence intensity in the presence of metal ions (Zn²⁺, Co²⁺, Cu²⁺, Fe³⁺, Pb²⁺, Hg²⁺, Ag⁺ and Ni²⁺) and protons. The dyes under study displayed “off-on” switching in its fluorescence as a function of pH, which is attributed to disallowing photoinduced electron transfer from the receptor moiety to the fluorophore. PAMAM-branched dye displayed a good pH sensor activity (FE = 6.4), however the pH sensing ability of tetraester was substantially higher (FE = 184). In the presence of Cu²⁺ and Pb²⁺ ions tetraester quenched its fluorescence intensity (FQ = 22 and 12 respectively), while PAMAM-branched dye enhanced its fluorescence intensity with pronounced selectivity to Cu²⁺ and Fe³⁺ (FE = 3.2 and 4.9, respectively). The results obtained indicate the potential of the novel compounds as fluorescent detectors for metal ions with pronounced selectivity towards Cu²⁺, Pb²⁺ and Fe³⁺ ions and highly efficient “off-on” pH switches, especially a tetraester-branched perylene diimide.     

Study of the potentiometric response of the doped spinel Li1.05Al0.02Mn1.98O4 for the optimization of a selective lithium ion sensor

In this paper, we studied the development of a selective lithium ion sensor constituted of a carbon paste electrode modified (CPEM) with an aluminum-doped spinel-type manganese oxide (Li_1.05Al_0.02Mn_1.98O₄) for investigating the influence of a doping ion in the sensor response. Experimental parameters, such as influence of the lithium concentration in the activation of the sensor by cyclic voltammetry, pH of the carrier solution and selectivity for Li⁺ against other alkali and alkaline-earth ions were investigated. The sensor response to lithium ions was linear in the concentration range 5.62 × 10⁻⁵ to 1.62 × 10⁻³ mol L⁻¹ with a slope 100.1 mV/decade over a wide pH 10 (Tris buffer) and detection limit of 2.75 × 10⁻⁵ mol L⁻¹, without interference of other alkali and alkaline-earth metals, demonstrating that the Al³⁺ doping increases the structure stability and improves the potentiometric response and sensitivity of the sensor. The super-Nernstian response of the sensor in pH 10 can be explained by mixed potential arising from two equilibria (redox and ion-exchange) in the spinel-type manganese oxide.     

A highly selective fluorescent sensor for Hg based on the water-soluble poly(p-phenyleneethynylene)

The conjugated polymer P-1 could be synthesized by the polymerization of 4,7-diethynyl-benzo[2,1,3]thiadiazole (M-1) and 1,4-bis[3′-(N,N-diethylamino)-1′-oxapropyl]-2,5-diiodobenzene (M-2) via Pd-catalyzed Sonogashira reaction. The water-soluble conjugated polyelectrolyte P-2 could be obtained by the reaction of P-1 with ethyl bromide. Both P-1 and P-2 can emit orange fluorescence. The responsive optical properties of P-1 and P-2 on Hg²⁺ were investigated by fluorescence spectra. Hg²⁺ can lead to nearly complete fluorescence quenching of P-1. On the contrary, Hg²⁺ can show the most pronounced fluorescence enhancement response of P-2 in aqueous solution without interference from those coexistent ions, such as K⁺, Mg²⁺, Pb²⁺, Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺, Fe³⁺ and Zn²⁺. The results also exhibit that this kind of water-soluble conjugated polyelectrolyte can be used as a highly sensitive and selective fluorescence sensor for Hg²⁺ detection in water.     

Theoretical and practical investigations of copper ion selective electrode with polymeric membrane based on N,N′-(2,2-dimethylpropane-1,3-diyl)-bis(dihydroxyacetophenone)

A novel ion-selective poly(vinyl chloride) (PVC) membrane sensor for Cu²⁺ ions based on N,N′-(2,2-dimethylpropane-1,3-diyl)-bis(dihydroxyacetophenone) (NDHA) as a new ionophore was prepared and studied. The best performance was observed for the membrane composition, including 30:65:1:4 (wt%) = PVC:DBP:KTpClPB:NDHA. The electrode showed a good Nernstian slope of 30.0 ± 0.5 mV/decade in a wide linear range activity of 3.0 × 10⁻⁷ to 1.0 × 10⁻² mol dm⁻³ Cu(NO₃)₂ with limit of detection 2.5 × 10⁻⁷. Sensor exhibited a fast response time (t_95% < 10 s) and could be used for about 4 months in the pH range of 3.0–7.4. The proposed potentiometric sensor was found to work satisfactorily in partially non-aqueous media up to 30 (vol%) content of methanol, ethanol and acetone. Applications of this electrode for the determination of copper in real samples, and as an indicator electrode for potentiometric titration of Cu²⁺ ion using EDTA, were reported. In order to predict the extraction ability of NDHA for different metallic ions, the complexes [M(NDHA)] and [M(H₂O)₆] (where M = Cu²⁺, Co²⁺, Hg²⁺, Pb²⁺, Ag⁺, Mg²⁺, Ca²⁺, Mn²⁺, Zn²⁺, Cd²⁺, K⁺ and Al³⁺) were investigated using ab initio theoretical calculations. The metal binding capability was evaluated using the binding energy. Results of our study could be useful for prediction of the extraction power of this Schiff base and could play a guiding role in planning experiments.     

A new rhodamine-based single molecule multianalyte (Cu, Hg) sensor and its application in the biological system

A new single molecule multianalyte sensor, vanillic aldehyde rhodamine 6G hydrazone has been designed for the selective detection of Cu²⁺ and Hg²⁺ ions. UV/Vis spectroscopy indicates that the sensor is a good chromogenic chemosensor for Cu²⁺ in 1:99 (v/v) ethanol-water media. Whereas, other ions, such as Li⁺, Na⁺, Mg²⁺, K⁺, Ca²⁺, Cr³⁺, Mn²⁺, Fe³⁺, Co²⁺, Ni²⁺, Zn²⁺, Ag⁺, Cd²⁺, Ba²⁺, Hg²⁺ and Pb²⁺ failed to generate a distinct response. Fluorescence spectral data reveals that the sensor is an excellent fluorescent chemosensor for Hg²⁺ in aqueous ethanol solution and with no fluorescent response toward other ions. The spectroscopic behavior of the sensor in living cells indicated that it can be used for the detection of Cu²⁺ and Hg²⁺ in environmental and biological systems. Mechanisms for the high selectivity of the sensor to Cu²⁺ and Hg²⁺ are discussed.     

Faradaic rectification studies of some reactions at the mercury/solution interface

The kinetic parameters of Hg₂²⁺, 1·0 M HClO₄/Hg and Fe(Ox)₃³⁻, Fe(Ox)₃⁴⁻, 0·5 M K₂ (Ox), 0·7 N H₂SO₄/Hg reactions have been obtained by faradaic rectification at radio frequencies. The value of I_a°, the exchange current density for discharge of 1·0 mM and 0·5 mM of mercurous ions on mercury in 1·0 M perchloric acid is 3·9 A/cm² and 1·1 A/cm² respectively. The rate constant k_a° for the reaction is of the order of 0·9 × 10⁻² cm/s at 35°C and the transfer coefficient is 0·56. The values of kinetic parameters obtained for the discharge of mercurous ions have been compared with data obtained by others using different methods. The value of α for Fe(Ox)₃³⁻/Fe(Ox)₂⁴⁻, 0·5 M K₂(Ox)/Hg reaction is 1·0.     

A highly selective and sensitive fluorescence chemosensor based on optically active polybinaphthyls for Hg

The chiral polymer was synthesized by the polymerization of 4,7-diethynylbenzo[2,1,3]-thiadiazole (M-1) with (R)-6,6′-dibutyl-3,3′-diiodo-2,2′-bis(diethylaminoethoxy)-1,1′-binaphthyl (R-M-1) via Pd-catalyzed Sonogashira reaction. The chiral polymer has orange fluorescence due to the extended π-electronic structure between binaphthyl unit and benzo[2,1,3]thiadiazole (BT) group via ethynyl bridge. The responsive optical properties of the polymer on various metal ions were investigated by fluorescence spectra. The fluorescence of the chiral polymer can produce the pronounced enhancement as high as 1.8-fold upon addition of 1:2 molar ratio of Hg²⁺. Compared with other cations, such as K⁺, Mg²⁺, Pb²⁺, Co²⁺, Ni²⁺, Ag⁺, Cd²⁺, Cu²⁺, Zn²⁺, Mn²⁺ and Fe³⁺, Hg²⁺ can produce the pronounced fluorescence response of the polymer. The result indicates this kind of chiral polybinaphthyls incorporating diethylamino and benzo[2,1,3]thiadiazole (BT) moieties as receptors exhibits highly sensitive and selective behavior for Hg²⁺ detection.     

Removal, preconcentration and determination of trace heavy metal ions in water samples by AAS via chemically modified silica gel N-(1-carboxy-6-hydroxy) benzylidenepropylamine ion exchanger

The use of chemically modified silica gel N-(1-carboxy-6-hydroxy) benzylidenepropylamine (SiG-CHBPA), ion exchanger for removal and preconcentration of Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd and Pb in natural water samples collected from River Nile, Mediterranean Sea and other locations followed by their consecutive AAS determination was described. The effects on the percentage of recovered metal ions including mass change of ion exchanger, stirring time, pH of sample solutions and eluent concentration were studied. The distribution coefficient K_d, ml g^− 1 and the percentage concentration of the studied metal ions on the ion exchanger at equilibrium, C_M,_eqm ,% (Recovery, %) were studied as a function of experimental parameters. The logarithmic values of the distribution coefficients, log K_d are 3-6.3. The interfering effects of some foreign ions on the removal, preconcentration and determination of the investigated metal ions were described. The metal-chelates formed between the ion exchanger and the studied metal ions were characterized by IR (absorption and reflectance), UV spectrometry, potentiometric titration and thermal analysis (TG and DTG). The reliability of the present method was confirmed by the comparison with a standard solvent extraction method. The present method is simple and rapidly applicable for the determination of the studied metal ions, ng ml^− 1 in different natural water samples.     

Synthesis and transport properties in La2−xAxMo2O9−δ (A = Ca, Sr, Ba, K) series

The La_2−xA_xMo₂O_9−δ (A = Ca²⁺, Sr²⁺, Ba²⁺ and K⁺) series has been synthesised as nanocrystalline materials via a modification of the freeze-drying method. The resulting materials have been characterised by X-ray diffraction (XRD), thermal analysis (TG/DTA, DSC), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The high-temperature β-polymorph is stabilised for dopant content x > 0.01. The nanocrystalline powders were used to obtain dense ceramic materials with optimised microstructure and relative density >95%. The overall conductivity determined by impedance spectroscopy depends on both the ionic radius and dopant content. The conductivity decreases slightly as the dopant content increases in addition a maximum conductivity value was found for Sr²⁺ substitution, which show an ionic radii slightly higher than La³⁺ (e.g. 0.08 S cm⁻¹ for La₂Mo₂O₉ and 0.06 S cm⁻¹ for La_1.9Sr_0.1Mo₂O_9−δ at 973 K). The creation of extrinsic vacancies upon substitution results in a wider stability range under reducing conditions and prevents amorphisation, although the stability is not enhanced significantly when compared to samples with higher tungsten content. These materials present high thermal expansion coefficients in the range of (13-16) × 10⁻⁶ K⁻¹ between room temperature and 753 K and (18-20) × 10⁻⁶ K⁻¹ above 823 K. The ionic transport numbers determined by a modified emf method remain above 0.98 under an oxygen partial pressure gradient of O₂/air and decreases substantially under wet 5% H₂-Ar/air when approaching to the degradation temperature above 973 K due to an increase of the electronic contribution to the overall conductivity.