Separation and indirect detection by capillary zone electrophoresis of ppb (w/w) levels of aluminum ions in solutions of multiple cations

Solutions of multiple cations in aqueous solutions at concentrations as low as 200 ppb were analyzed by capillary zone electrophoresis. Aluminum ions were cleanly separated from Li⁺, K⁺, Ca²⁺, Cr³⁺, Zn²⁺, CU²⁺, and other ions less than 6 min after injection of the solution on a 50 cm × 50 μm I.D. uncoated fused-silica capillary column at 15 kV. Indirect detection at 204 nm was carried out using a pH 2.8 background electrolyte containing 5.2 mM ephedrine as a UV-absorbing co-ion and 4.7 mM -hydroxyisobutyric acid as a completing counter ion. Mobilities for Al³⁺ and 14 other complexed cations were determined for this electrolyte.     

钙离子和镁离子浓度变化对磷脂酰乙醇胺-磷脂酰甘油双分子层膜的影响

Ca²⁺ and Mg²⁺ ions are the main divalent cations in living cells and play vital roles in the structure and function of biological membranes. To date, the differences in the effects of these two ions on the Escherichia coli (E. coli) inner membrane at various concentrations remain unknown. Here, the effects of Ca²⁺ and Mg²⁺ ions on a mixed lipid bilayer composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) in a 3 : 1 ratio (mol/mol), which mimics the E. coli inner membrane, were quantitatively differentiated at different concentrations by dynamic light scattering (DLS), zeta potential measurements and all-atom molecular dynamics (AA-MD) simulations. The DLS results demonstrated that the POPE/POPG liposomes were homogeneous and monodisperse in solutions with Ca²⁺ or Mg²⁺ ion concentrations of 0 and 1 mmol∙L^-1. As the Ca²⁺ or Mg²⁺ ion concentration was increased to 5-100 mmol∙L^-1, lipid aggregation or the fusion of unilamellar liposomes occurred in the ion solutions. The zeta potential measurements showed that both the Ca²⁺ and Mg²⁺ ions had overcharging effects on the negatively charged POPE/POPG liposomes. The AA-MD simulation results indicated that the Ca²⁺ ions irreversibly adsorbed on the membranes when the simulation time was longer than 100 ns, while the Mg²⁺ ions were observed to dynamically adsorb on and desorb from the membranes at various concentrations. These results are consistent with the DLS and zeta potential experiments. The average numbers of Ca²⁺ and Mg²⁺ ions in the first coordination shell of the oxygen atoms of the phosphate, carbonyl and hydroxyl groups of POPE and POPG (i.e., the first coordination numbers) in the pure membrane and membranes containing 5 and 100 mmol∙L^-1 ions were calculated from the radial distribution functions. The results indicated that the primary binding site of these two ions on POPE and POPG at the concentrations studied was the negatively charged phosphate group. Thus, these results might explain the overcharging effects of both the Ca²⁺ and Mg²⁺ ions on the POPE/POPG liposomes. Moreover, as the Ca²⁺ concentration increased, the area per lipid of the lipid bilayers decreased, and the membrane thickness increased, while the Mg²⁺ ions had negligible effects on these membrane parameters. In addition, these ions had different effects on the orientation of the lipid head groups. These simulation results may be used to provide the possible explanations for the differences between Ca²⁺ and Mg²⁺ ions in DLS and zeta potential measurements at the atomic level. The experimental results and MD simulations provide insight into various biological processes regulated by divalent cations, such as membrane fusion.     

Influence of K+, Na+ or Ca2+ Ions on the Interaction Between AmB and Saturated Phospholipids by Langmuir Technique

Interaction between amphotericin B(AmB) and cell membrane is influenced by different metal cations. In the presence of K⁺, Na⁺ or Ca²⁺ ions, the surface pressure-area isotherms and the elastic modulus of an amphotericindipalmitoylphosphatidylcholine(AmB-DPPC) mixed monolayer were discussed. And the excess free energy and entropies of mixing were calculated according to the surface pressure-area isotherms. The phase transition of the mixed monolayer needed a higher concentration of AmB in the sequence Na⁺ > pure buffer > K⁺ > Ca²⁺. When the molar fraction of AmB(x_AmB) was 0.5, the molecular interaction changed from attraction to repulsion and the mixed monolayer turned to ordered state from disorder state under the induction of K⁺ or Ca²⁺ ions at all surface pressure in our experiment. At high surface pressure, the disorder of monolayer enhanced in the presence of Na⁺ ions at x_AmB > 0.1. At different molar ratios of AmB, the influences of these metal cations were discrepant. These cations may influence AmB molecules to form pores on the monolayer. It is helpful to understand the reduction of AmB's toxicity as theoretical reference.     

Gas phase hydrogen deuterium exchange reactions of a model peptide: FT-ICR and computational analyses of metal induced conformational mutations

We utilized gas phase hydrogen/deuterium (H/D) exchange reactions and ab initio calculations to investigate the complexation between a model peptide (Arg-Gly-AspRGD) with various alkali metal ions. The peptide conformation is drastically altered upon alkali metal ion complexation. The associated conformational changes depend on both the number and type of complexing alkali metal ions. Sodium has a smaller ionic diameter and prefers a multidentate interaction that involves all three amino acids of the peptide. Conversely, potassium and cesium form different types of complexes with the RGD. The [RGD + 2Cs − H]⁺ species exhibit the slowest H/D exchange reactivity (reaction rate constant of 6 × 10⁻¹³ cm³molecule⁻¹s⁻¹ for the fastest exchanging labile hydrogen with ND₃). The reaction rate constant of the protonated RGD is two orders of magnitude faster than that of the [RGD + 2Cs − H]⁺. Addition of the first cesium to the RGD reduces the H/D exchange reaction rate constant (i.e., D₀) by a factor of seven whereas sodium reduces this value by a factor of thirty. Conversely, addition of the second alkali metal ions has the opposite effect; the rate of D₀ disappearance for all [RGD + 2Met − H]⁺ species (MetNa, K, and Cs) decreases with the alkali metal ion size.     

离子选择性微电极用于原位测量离子扩散系数

扩散系数是描述物质扩散过程的重要参数,而用膜池法、放射性或荧光示踪法、分子动力学模拟等现有方法无法原位进行生物体系中离子扩散系数的实时测量。 本文利用离子选择性微电极响应迅速、高选择性、高灵敏度、高空间分辨率、对样品无污染等优势,通过分析单个植物细胞原生质体在培养液中破裂时所形成的离子浓度脉冲信号,建立了相应的点源扩散模型,推导出了描述离子浓度随时间变化的理论公式,并通过该公式对实验测得的脉冲信号进行拟合,得到了离子的扩散系数,从而建立了一种用离子选择性微电极原位测定离子扩散系数的新方法,并将其应用于芦荟细胞原生质体破裂时离子扩散系数的测定,得到了Ca²⁺、Na⁺和K⁺的扩散系数分别为(6.51±0.12)×10^-6、(2.93±0.15)×10^-5和(3.03±0.35)×10^-5 cm²/s。 对比发现,拟合得到的Ca²⁺、Na⁺和K⁺扩散系数均略高于已报道的数值(纯水中),这一现象的产生可能是因为原生质体是在低渗液中吸水膨胀,细胞膜内压力升高产生内外压力差,该压力差会加速细胞破裂时离子的扩散。 这一方法对生物体系无干扰,较好地解决了生物体系中离子扩散系数原位实时测量的难题。     

Simultaneous separation of common mono- and divalent cations on silica gel modified with aluminium by non-suppressed ion chromatography with conductimetric detection and nitric acid–15-crown-5 as eluent

The modification of silica gel with aluminium by a coating method was very effective for the preparation of silica-based stationary phases which acted as a cation exchanger under strongly acidic conditions. However, the separation of common mono- and divalent cations (Li⁺, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) on an aluminium-adsorbing silica (Al-Silica) column was moderate by a conductimetric detection ion chromatography (IC) with strongly acidic eluents. Then, the addition of various crown ethers (12-crown-4, 15-crown-5 and 18-crown-6) in acidic eluent was carried out. As a result, it was found that 15-crown-5 was most effective for the improvement of peak resolution. Excellent separation of these cations was achieved in 20 min by elution with 2 mM nitric acid–2 mM 15-crown-5. The proposed IC was successfully applied to the determination of major cations in various natural waters.     

Separation and detection of common mono- and divalent cations by ion chromatography with an ODS column and conductivity/UV detection

The retention and detection behavior of common mono- and divalent cations (M⁺, alkali metal (Li⁺, Na⁺, K⁺, Rb⁺, Cs⁺) and ammonium ions (NH₄⁺); M²⁺, alkaline earth metal ions (Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺) was examined using an ODS column (150×4.6 mm I.D.) and conductivity (CD)/UV detection. The results obtained were as follows: (1) for M⁺, the mobile phase, 0.1 mM sodium dodecyl sulphate (SDS)+10 mM HNO₃ and indirect CD detection were effective. (2) Addition of Ce(III) in the mobile phase accelerated the elution of both M⁺ and M²⁺. The separation of above 10 cations on an ODS column was achieved for the first time without any coelution of cations and disturbance by system peak. Addition of higher SDS resulted in good separation of M⁺ and M²⁺ with longer retention times. CD detection was possible for M⁺ and M²⁺ and UV detection for M²⁺. (3) For M²⁺, the mobile phase, 0.8 mM Ce(III)+0.1 mM SDS+1 mM HNO₃ and indirect UV detection were effective. The IC methods were applied to real samples.     

Ion chromatography of organic-rich natural waters from peatlands II. Na, NH4, K, Mg and Ca

Organic rich natural waters from peat bogs in continental (Switzerland) and maritime (Shetland Islands, Scotland) environments were analysed for Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺ using ion chromatography. These cations were determined simultaneously in surface and pore water samples from the continental bogs using a 250-μl injection loop in an isocratic separation. Using this loop, detection limits of the order of 1 ng/g were achieved. An organics-removal cartridge (Dionex OnGuard P) was used to remove humic materials. Analyses of deionized water filtered through these cartridges showed acceptably low blank values (e.g., ca. 5 ng/g) and appeared to have no significant effect on the measured cation concentration. For the maritime bog waters, the low concentrations of NH₄⁺ (ca. 1 μg/g) compared with Na⁺ (ca. 100 μg/g) required improved peak separation. This was accomplished using a gradient separation beginning with 40 mM HCl—1 mM , -2,3-diaminopropionic acid monochloride (DAP) and switching to 40 mM HCl-12 mM DAP after 2 min. Using a 25 μl injection loop, Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺ were determined simultaneously in less than 25 min. In this instance, even with Na⁺/NH₄⁺ > 100, there was no interference from Na⁺ in the determination of NH₄⁺ (baseline separated).     

FT-IR and Raman spectroscopic study of di-urea cross-linked poly(oxyethylene)/siloxane ormolytes doped with Zn ions

The interactions occurring in di-urea (NHC(O)NH) cross-linked poly(oxyethylene) (POE)/siloxane hybrids (di-ureasils) doped with zinc triflate (Zn(CF₃SO₃)₂) were investigated by Fourier Transform infrared (FT-IR) and Raman (FT-Raman) spectroscopies. Bonding of the Zn²⁺ ions to the urea carbonyl oxygen atoms occurs in the entire range of compositions studied (∞ > n ≥ 1, where n, salt content, is the molar ratio of oxyethylene moieties per Zn²⁺ ion). At n > 20 the incorporation of the guest cations progressively reduces the number of free CO groups. At n = 20 the saturation of the urea cross-links is attained and the Zn²⁺ ions start to coordinate to the POE chains giving rise to the formation of a crystalline POE/Zn(CF₃SO₃)₂ complex. The latter process occurs at the expense of the destruction of the hydrogen-bonded POE/urea structures of the host di-ureasil structure. New hydrogen-bonded associations, more ordered than the urea–urea aggregates present in the non-doped matrix and including Zn²⁺OC coordination, emerge in parallel. “Free” and weakly coordinated CF₃SO₃⁻ ions, present in all the xerogels studied, appear to be the main charge carriers of the conductivity maximum of this family of ormolytes located at n = 60 at 30 °C. In materials with n ≤ 20 contact ion pairs, “cross-link separated” ions pairs and higher ionic aggregates appear. The data reported demonstrate that the behaviour of the di-ureasils doped with triflate salts depends on the type of cation.     

Synthesis and fluorescence behaviour of crown and azacrown ethers carrying the dansyl fluorophore as a pendant in acetonitrile solution

The influence of Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, Ba²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Pb²⁺ and Al³⁺ ions on the spectroscopic properties of the dansyl group covalently linked to crown ether or diazacrown ethers was investigated by means of absorption and emission spectrophotometry. Interaction of the alkali metal ions with all fluoroionophores studied is weak, while alkaline earth metal ions interact strongly causing about 50% quenching of dansyl fluorescence of A21C5-Dns and A₂18C6-Dns. The Cu²⁺, Pb²⁺ and Al³⁺ cations interact very strongly with dansyl chromophore regardless the crown ether type, causing a major change in absorption spectrum of the chromophore and forming non-fluorescent complexes. The Co²⁺, Ni²⁺, Zn², Mg²⁺ and Ag⁺ cations interact moderately with all fluoroionophores studied causing about 20% of fluorescence quenching of dansyl, except for a strong dansyl fluorescence quenching of 15C5-Dns by Co²⁺ ion. The quenching efficiency of didansylated fluoroionophores by the alkali metal ions and alkaline earth metal ions is weaker than monodansylated ones.     

Calcium as a factor influencing the response of Serratia marcescens strain SMG40 to growth surfaces

Serratia marcescens, like several other bacterial species, is able to adhere to and swarm on growth surfaces, and this property is correlated with changes in the structures and functions of cells which pass from broth to solid media. In the case of the pigmented form of S. marcescens strain SMG40, the response to the contact with agar was influenced by the presence of Ca²⁺ ions as follows. (1) In the swimming phase, i.e. in soft 0.35% agar media, expansion was not modified upon the addition of 5–35 mM Ca²⁺ ions. (2) On growth surfaces inducing swarming, i.e. on media containing 0.75% agar (low agar), Ca²⁺ ions used at low concentrations (e.g. 0.5mM) enhanced both growth and expansion. (3) At higher concentrations (from 3 mM). the Ca²⁺ ions inhibited expansion but not growth. Correlatively, the cell density per unit of the surface occupied by bacteria inhibited from swarming was increased: expansion rates of swarms at successive periods were lower and showed less fluctuation than normal. In a non-pigmented but still good swarmer mutant, found to be more sensitive to calcium, expansion was reduced to 50% with at most 0.5 mM Ca²⁺ ions. One-step revenants recovered pigmentation and the initial response to Ca²⁺ ions described in points (2) and (3) above. The inhibitory activity of CaCl₂ was partly antagonized by NaCl. Other properties implied in the interactions of Serratia cells with agar, i.e. pigmentation, adherence to solid surfaces and the production of an extracellular surfactant were not inhibited by Ca²⁺ ions.     

Ion-exchange properties of hypercrosslinked polystyrene impregnated with methyl orange

A novel bipolar stationary phase (HCPS–MO) was prepared by impregnation of hypercrosslinked polystyrene (HCPS) with methyl orange (MO; 4-dimethylamino-4′-sulfoazobenzene) and its ion-exchange properties were studied. Simultaneous separation of cations and anions on HCPS–MO is possible, although it behaves preferentially as a cation-exchanger. Unusual selectivity of HCPS-MO for alkali and alkaline-earth metal cations: Na⁺+K⁺+4⁺+ and Mg²⁺2+2+2+ was observed. The effect of temperature on retention of alkali and alkaline-earth metal cations was studied. Separation of Na⁺, K⁺, Rb⁺, NH₄⁺, Cs⁺, Mg²⁺ and Ca²⁺ on HCPS–MO with diluted cerium(III) nitrate solution as an eluent in single run is presented.     

Molecular dynamics study of the influence of calcium ions on the conformation of gelsolin S2 domain

Gelsolin is an actin-severing protein whose action is promoted by Ca²⁺ ions and inhibited by binding to lipid phosphoinositides incorporated in the inner leaflet of the plasma membrane inner lipid bilayer. In this study, we carried out molecular dynamics (MD) simulations to investigate the influence of calcium cations on the conformation of gelsolin S2 domain. First, gelsolin S2 domain taken from the crystal structure of apo-gelsolin (PDB code: 1D0N) was subjected to three 1100 ps MD simulations in a periodic water box with the 5.0 force field at T=298 K. In the first simulation (S2_Ca²⁺) excess concentration of Ca²⁺ was applied, in the second one (S2_phys) the concentration of Ca²⁺ ions was physiological and in the third one (S2_w) no Ca²⁺ ions were added. The results of MD simulations showed high conformational flexibility of the N-terminal part of the S2 domain. S2_w deviated from the starting structure considerably more that S2_phys and S2_Ca²⁺ suggesting that Ca²⁺ ions stabilize the conformation of the S2 domain of gelsolin.     

The dependence of photoinduced energy transfer on the orientation of the acceptor with respect to the π-plane of the donor in naphthalene-linked crown ether–Tb complexes

The photoinduced energy transfer (ET) from naphthalene (N) to Tb³⁺ has been studied in the complexes of Tb³⁺ ion with 2,3-naphtho-17-crown-5 ether(I), 2,3-naphtho-20-crown-6 ether(II), 1,8-naphtho-21-crown-6 ether(III) and 1,5-naphtho-22-crown-6 ether(IV), respectively, using nitrate (NO₃⁻) ion as the counter anion in EtOH glass at 77 K. The ligands are so designed that the Tb³⁺ ion can be complexed with a predetermined orientation with respect to the naphthalene molecular plane. In systems I and II, the Tb³⁺ ion is along the Z-axis; in system III, it is along the Y-axis and in IV, it is along the X-axis, where Z- and Y- are the molecular in-plane long and short axes of the naphthalene molecular plane respectively and X- is the out-of plane axis perpendicular to the naphthalene molecular plane. Present studies indicate that the efficiency of energy transfer (ET) and the quenching of naphthalene phosphorescence show a strong dependence on the orientation of the acceptor metal ion (Tb³⁺) with respect to the π-plane of the donor naphthalene moiety. The ET studies suggest that an exchange mechanism involving the lowest (ππ^*) triplet state of N and the ⁵D₄ state of Tb³⁺ ion is predominantly operating. Our observation further indicates that for a given orientation in a complex the emission intensity of the various transitions (⁵D₄ → ⁷F_J, J=2–6) for Tb³⁺, vis-a-vis ET efficiency varies considerably with ΔJ values (=0, +1 and +2).     

Tripodal lipophilic ionophores: synthesis, cation binding and transport through liquid membranes

New C₃-symmetric lipophilic tripodal ionophores, Et(CH₂OCH₂COR)₃; R=NMePh (1), R=NEtPh (2), R=piperidyl (3), have been prepared and their binding abilities for alkali and alkaline earth metal cations evaluated by extraction equilibrium and cation transport through bulk liquid membranes. Experiments show that these ionophores have considerable potential for transporting lithium, sodium and calcium ions relative to potassium and magnesium ions. The cation transport rates by ionophores 1 and 2 decrease in the order Li⁺>Na⁺>Ca²⁺>Ba²⁺>K⁺>Mg²⁺, and the selectivities of Li⁺/K⁺, Na⁺/K⁺ and Ca²⁺/Mg²⁺ are 6.47–7.24, 6.05–6.19 and 9.39–16.13, respectively. The extraction selectivity sequences of the ionophores 1 and 2 are in agreement with the descending order of the cation transport rate, and the complexation constants in chloroform phase were estimated.     

Density functional theory assessment of the thermal degradation of diclofenac and its calcium and iron complexes

Thermogravimetric analyses of diclofenac sodium, its Ca²⁺ and Fe³⁺ complexes manifested a decreasing trend of the onset decomposition temperatures at which these compounds dissociated. The drop in the temperature was metal ion dependent; the sodium salt showed thermal stability up to 245 °C, whereas the complexes started their degradation processes at temperatures starting from 90 °C. While G* for the cleavage of the acetate moiety in the sodium salt was 63.76 kJmol⁻¹, it was 82.06 and 140.57 kJmol⁻¹ in the cases of Ca²⁺ and Fe³⁺, respectively. However, their complete fusion took place at 187.65, 150.34 and 98.77 °C, respectively, displaying a reversed trend which is probably indicative of some catalytic part on the binding metals.

Using the Gaussian 98 W package of programs, ab initio molecular orbital treatments were applied to diclofenac and its Ca²⁺ and Fe³⁺ metal complexes to study their electronic structure at the atomic level. The thermochemistry of diclofenac sodium was followed through the TG fragmentation peak temperatures using the density functional theory calculations at the 6-31G(d) basis set level. The FT-IR data were in good agreement with the theoretically calculated values.

Single point calculations at the B3LYP/ 6-311G(d) level of theory, were used to compare the geometric features, energies and dipole moments of these compounds to detect the effect of the binding metal ions on the thermal dissociation of their diclofenac complexes.     

Acid-rain monitoring in East Asia with a portable-type ion-exclusion–cation-exchange chromatographic analyzer

A monitoring system consisting of a portable-type conductimetric ion-exclusion–cation-exchange chromatographic (CEC) analyzer and a meteorological satellite data analyzer has been investigated for the evaluation of the effects of acid precipitation on natural and urban environments in East Asia. The portable ion-exclusion–CEC analyzer uses a polymethacrylate-based weakly acidic cation-exchange resin column in the H⁺-form and a weak-acid eluent (tartaric acid–methanol–water) and is applied for the simultaneous determination of anions (SO₄²⁻, NO₃⁻, and Cl⁻) and cations (Na⁺, NH₄⁺, K⁺, Mg²⁺, and Ca²⁺) in precipitation transported from mainland China to central Japan, as mapped by the meteorological satellite data analyzer. Linear calibration graphs of peak area versus concentration for anions and cations were observed in the concentration range 0–1.0 mM for the anions and 0–0.5 mM for the cations. Detection limits at a signal-to-noise ratio of 3 were in the range 5.18–12.1 ppb for the anions and 6.58–16.5 ppb for the cations. The practical utility of this monitoring system is presented.     

Simultaneous determination of anions and cations by ion-exclusion chromatography–cation-exchange chromatography with tartaric acid/18-crown-6 as eluent

Ion-exclusion chromatography–cation-exchange chromatography was developed for the simultaneous separation of common inorganic anions and cations (Cl⁻, NO₃⁻ and SO₄²⁻; Na⁺, NH₄⁺, K⁺, Mg²⁺ and Ca²⁺) on a weakly acidic cation-exchange column by elution with weak acid. Generally, the resolution among these monovalent cations was only moderate, thereby hindering the determination of these analytes in natural-water samples. Therefore, 18-crown-6 was added to the eluent to improve the resolution. A good separation of these anions and cations on a weakly acidic cation-exchange column was achieved in 30 min by elution with 5 mM tartaric acid/6 mM 18-crown-6/methanol–water (7.5:92.5). The ion-exclusion chromatography–cation-exchange chromatography method developed here was successfully applied to the separation of major anions and cations in an environmental water sample.     

A calorimetrical study of the polynuclear Zn(II) and Pb(II) and the polymeric Ni(II) and Cd(II) complexes of 2-mercaptoethanol and 3-mercapto-1,2-propanediol

The ΔG, ΔH and ΔS values for the dissociation of 2-mercaptoethanol (MEL) and 3-mercapto-1,2-propanediol and for the formation of complexes between these ligands and the metal ions Ni²⁺, Zn²⁺, Cd²⁺ and Pb²⁺ have been determined calorimetrically in 0.5 M KNO₃ and at 25°C.     

PVC membrane based potentiometric sensors for uranium determination

Two novel uranyl PVC matrix membrane sensors responsive to uranyl ion are described. The first sensor incorporates tris(2-ethylhexyl)phosphate (TEHP) as both electroactive material and plasticizer and sodium tetraphenylborate (NaTPB) as an ion discriminator. The sensor displays a rapid and linear response for UO₂²⁺ ions over the concentration range 1×10⁻¹–2×10⁻⁵ mol l⁻¹ UO₂²⁺ with a cationic slope of 25.0±0.2 mV decade⁻¹. The working pH range is 2.8–3.6 and the life span is 4 weeks. The second sensor contains O-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N′,N′-bis(tetra-methylene)uronium hexafluorophosphate (TPTU) as a sensing material, sodium tetraphenylborate as an ion discriminator and dioctyl phenylphosphonate (DOPP) as a plasticizer. Linear and stable response for 1×10⁻¹–5×10⁻⁵ mol l⁻¹ UO₂²⁺ with near-Nernstian slope of 27.5±0.2 mV decade⁻¹ are obtained. The working pH range is 2.5–3.5 and the life span of the sensor is 6 weeks. Interference from many inorganic cations is negligible for both sensors. However, interference caused by some ions (e.g. Th⁴⁺, Cu²⁺, Fe³⁺) is eliminated by a prior ion exchange or solvent extraction step. Direct potentiometric determination of as little as 5 μg ml⁻¹ uranium in aqueous solutions shows an average recovery of 97.2±1.3%. Application for the determination of uranium at levels of 0.01–1 wt.% in naturally occurring and certified ores gives results with good correlation with data obtained by X-ray fluorescence.