Probing the electronic structure of [2Fe-2S] clusters with three coordinate iron sites by use of photoelectron spectroscopy

Five series of [2Fe-2S] complexes, [Fe(2)S(2)Cl(2)(-)(x)(CN)(x)](-), [Fe(2)S(2)(SEt)(2)(-)(x)Cl(x)](-), [Fe(2)S(2)(SEt)(2)(-)(x)(CN)(x)](-), [Fe(2)S(2)Cl(2)(-)(x)(OAc)(x)](-) (OAc = acetate), and [Fe(2)S(2)(SEt)(2)(-)(x)(OPr)(x)](-) (OPr = propionate) (x = 0-2), were produced by collision-induced dissociation of the corresponding [4Fe-4S] complexes, and their electronic structures were studied by photoelectron spectroscopy. All the [2Fe-2S] complexes contain a [Fe(2)S(2)](+) core similar to that in reduced [2Fe] ferredoxins but with different coordination geometries. For the first three series, which only involve tricoordinated Fe sites, a linear relationship between the measured binding energies and the substitution number (x) was observed, revealing the independent ligand contributions to the total electron binding energies. The effect of the ligand increases in the order SEt --> Cl --> CN, conforming to their electron-withdrawing ability in the same order. The carboxylate ligands in the [Fe(2)S(2)Cl(2)(-)(x)(OAc)(x)](-) and [Fe(2)S(2)(SEt)(2)(-)(x)(OPr)(x)](-) complexes were observed to act as bidentate ligands, giving rise to tetracoordinated iron sites. This is different from their monodentate coordination behavior in the [4Fe-4S] cubane complexes, reflecting the high reactivity of the unsatisfied three-coordinate iron site in the [2Fe-2S] complexes. The [2Fe-2S] complexes with tetracoordinated iron sites exhibit lower electron binding energies, that is, higher reductive activity than the all tricoordinate planar clusters. The electronic structures of all the [2Fe-2S] complexes were shown to conform to the "inverted energy level scheme".     

Voltammetric determination of traces of Cr(VI) in the flow system in the presence of bipyridyne

The voltammetric method of Cr(VI) determinations in the flow system, based on the combination of selective accumulation of the product of Cr(VI) reduction on HMDE and a very sensitive method of chromium determination in the presence of bipyridine [Z. Gao K.S. Siow, Electroanalysis 8 (1996) 602] is proposed. The calibration graphs were linear from 3x10(-9) to 3x10(-8) and from 5x10(-10) to 5x10(-9) mol l(-1) for accumulation times 120 and 600 s, respectively. The relative standard deviation (R.S.D.) was 6.5% (n=5) for Cr(VI) concentration 1x10(-8) mol l(-1) and the accumulation time 120 s. The influence of foreign ions commonly present in water samples is presented. Validation of the method was made by comparison of the results of analyses of tap water by another electrochemical method and by recovery test for river and mineral water.     

Determination of sulfur dioxide in vitriol plant wastewater by using a polyNiMe4TAA electrochemically modified Pt microelectrode

A Pt microelectrode modified with nickel(II) polytetramethyldibenzo[b,i]tetraaza[14]annulene was prepared by electropolymerization of nickel(II) tetramethyldibenzo[b,i]tetraaza[14]annulene monomers and applied to determine sulfur dioxide in vitriol plant wastewater. For determination of SO2 with this electrochemically modified Pt microelectrode, the linear range was from 9.6 x 10(-6) to 2.4 x 10(-4) mol L-1, the sensitivity was 1.8 x 10(-4) A L mol-1, the detection limit was calculated to be 4.8 x 10(-6) mol L-1 (S/N = 3), the response time was less than 20 s and the relative standard deviation was found to be 2.1% on analyzing 4.8 x 10(-5) mol L-1 SO2 solution repeatedly (n = 7). These results demonstrated good accuracy compared with those obtained by the conventional iodimetric method.     

An optical fiber sensor for berberine based on immobilized 1,4-bis(naphth[2,1-d]oxazole-2-yl)benzene in a new copolymer

A new copolymer made of 8-quinolyl methacrylate, methyl methacrylate and n-butyl acrylate(PQMB) has been used as a solid substrate for the preparation of an optical fiber berberine sensor. The sensing is based on the fluorescence of 1,4-bis(naphth[2,1-d]oxazole-2-yl)benzene(BNOB) immobilized in the copolymer quenched by berberine with the formation of 1:1 complex between them. The membrane composition was optimized using the orthogonal experimental design. Using the sensor described above, berberine in a sample solution from 4.02x10(-7) mol l(-1) to 2.82x10(-4) mol l(-1) can be detected. The sensor has satisfactory reversibility and a short response time of less than 30 s. The relative standard deviations for repeated measurements (n=15) of 8.05x10(-6) mol l(-1) and 4.02x10(-5) mol l(-1) beberine are 0.43% and 0.33%, respectively. The sensor shows good selectivity over alkali and alkali-earth metal salts and some common pharmaceutical species and can be used for the determination of berberine in pharmaceutical preparations.     

Flow-injection chemiluminometric determination of some thioxanthene derivatives in pharmaceutical formulations and biological fluids using the [Ru(dipy)3(2+)]-Ce(IV) system.

A flow-injection (FI) methodology using tris(2,2'-dipyridyl)ruthenium(II), [Ru(dipy)3(2+)], chemiluminescence (CL) was developed for the rapid and sensitive determination of three thioxanthene derivatives, namely zuclopenthixol hydrochloride, flupentixol hydrochloride and thiothixene. The method is based on the CL reaction of the studied thioxanthenes with [Ru(dipy)3(2+)] and Ce(IV) in a sulfuric acid medium. Under the optimum conditions, calibration graphs were obtained over the concentration ranges 0.002-6 migrograms/ml for zuclopenthixol hydrochloride, 0.5-15 micrograms/ml for flupentixol hydrochloride and 0.05-7.5 micrograms/ml for thiothixene. The limits of detection (s/n = 3) were 4.2 x 10(-9) mol/l zuclopenthixol hydrochloride, 2 x 10(-8) mol/l flupentixol hydrochloride and 4.5 x 10(-8) mol/l thiothixene. The method was successfully applied to the determination of these compounds in dosage forms and biological fluids.     

Determination of the herbicide desmetryne in organised media by adsorptive stripping voltammetry

An electroanalytical method for the determination of the herbicide desmetryne at nanomolar levels in dispersed media, based on adsorptive stripping voltammetry, is reported. The adsorption of desmetryne at the hanging mercury drop electrode was checked both in micellar solutions, where the anionic surfactant sodium pentanesulphonate was chosen as the most suitable surfactant agent, and in oil-in-water emulsions prepared with ethyl acetate as the organic solvent. In a micellar medium formed with 0.02% sodium pentanesulphonate and with 0.1 mol l(-1) Britton-Robinson buffer (pH 1.5), the herbicide could be determined over the 1.0 x 10(-8)-4.0 x 10(-7) mol l(-1) concentration range, when an accumulation potential of -0.70 V was applied for 50 s. On the other hand, in an oil-in-water emulsion formed with 2% ethyl acetate and 0.04% sodium pentanesulphonate as emulsifying agent in 0.1 mol l(-1) HClO(4), desmetryne could be determined over the 2.0 x 10(-9)-1.0 x 10(-7) mol l(-1) concentration range. The limits of detection were 2.4 x 10(-9) and 4.2 x 10(-10) mol l(-1) in micellar and emulsified media, respectively, with R.S.D.s (n=10) 3.6 and 3.7%. The degree of interference from some other s-triazines on the desmetryne differential pulse response was also evaluated. Finally, the method developed in emulsified medium was applied to the determination of desmetryne in spiked apple juice.     

Redispergation of TiO2 particles in aqueous solutions

The colloidal stability of TiO2 dispersions in aqueous solutions was studied. Aqueous solutions of ATLAS G-3300 (1.57 x 10(-3) mol/l), TRITON X-100 (5 x 10(-5) mol/l), and PMAA (4 x 10(-6) and 5.81 x 10(-3) mol/l) have been used as medium for redispergation of TiO2 particles. Stability of dispersions was investigated at different pH values by two different methods. By using analytical centrifuge the sedimentation velocity of TiO2 particles was directly measured and by means of light scattering the particle size of dispersed particles has been monitored. Combination of these two methods allowed determination of the aggregation degree of TiO2 particles as well as structure of the aggregates formed in aqueous phase. It has been found that redispergation process does not provide complete separation of virgin TiO2 particles. Even in the case of stable dispersions some aggregates were found, which consisted of 2-4 virgin TiO2 particles. With increasing colloidal stability of dispersions aggregates appear to be spherically shaped. In the system where TRITON X-100 was used, formation of secondary aggregates by fusion of primary ones was observed.     

Adsorption voltammetry of the scandium-alizarin red S complex onto a carbon paste electrode

For the first time, a new method is described for the determination of scandium based on the cathodic adsorptive stripping of the scandium-alizarin red S complex onto a carbon paste electrode. The second-order derivative linear scan voltammograms of the complex are recorded by use of model JP-303 polarographic analyzer from 0.0 to -1.0 V (versus SCE). Optimum conditions are found to be: an acetic acid (0.36 mol l(-1))-potassium biphthalate (0.064 mol l(-1)) buffer solution (pH 4.0) containing 2.0x10(-5) mol l(-1) alizarin red S, a preconcentration potential of 0.0 V, a preconcentration time of 60 s, a rest time of 10 s and a scan rate of 100 mV s(-1). The results show that the complex can be adsorbed on the surface of a carbon paste electrode, yielding one peak at -0.58 V, corresponding to the reduction of alizarin red S in the complex at the electrode. The detection limit is found to be 6.0x10(-10) mol l(-1) for 3 min of preconcentration time. The linear range is 1.0x10(-9) to 4.0x10(-7) mol l(-1). Application of the procedure to the determination of scandium in the ore samples gave good results.     

Electrochemiluminescent determination of L-cysteine with a flow-injection analysis system.

A flow-injection electrochemiluminescent method for L-cysteine determination has been developed based on its enhancement of the electrochemiluminecence of luminol at a glassy carbon electrode. This method is simple and sensitive for cysteine determination. Under the selected experimental parameters, the linear range for cysteine concentration was 1.0 x 10(-6) - 5.0 x 10(-5) mol/l, and the detection limit was 0.67 micromol/l (S/N = 3). The relative standard deviation for 11 measurements of 1.0 x 10(-5) mol/l cysteine was 4.5%. The proposed method has been applied to the detection of cysteine in pharmaceutical injections with satisfactory results.     

Determination of diclofenac sodium by capillary zone electrophoresis with electrochemical detection

Capillary zone electrophoresis was employed for the determination of diclofenac sodium using an end-column amperometric detection with a carbon fiber microelectrode, at a constant potential of 0.83 V vs. saturated calomel electrode. The optimum conditions of separation and detection are 4.90 x 10(-3) mol/l Na2HPO4-3.10 x 10(-3) mol/l NaH2PO4 (pH 7.0) for the buffer solution, 10 kV for the separation voltage, 5 kV and 10 s for the injection voltage and the injection time, respectively. The limit of detection is 2.5 x 10(-6) mol/l or 5.2 fmol (S/N=2). The relative standard deviation is 0.8% for the migration time and 4.7% for the electrophoretic peak current. The method was applied to the determination of diclofenac sodium in human urine.     

Determination of dysprosium by resonance light scattering technique in the presence of BPMPHD

Dysprosium has been determined by resonance light scattering (RLS) method in the presence of 1,6-bi(1'-phenyl-3'-methyl-5'-pyrazolone-4'-)hexanedione (BPMPHD) at pH 5.5. The Dy-BPMPHD system has three characteristic peaks at 358, 399 and 450 nm, especially the peak at 358 nm, which is proportional to the concentration of Dy(3+) in the range of 1.0 x 10(-10)-1.0 x 10(-5) mol l(-1). The detection limit (S/N=2) is 5.6 x 10(-12) mol l(-1). Synthetic samples are determined satisfactorily. A new sensitive method for detection of dysprosium has been proposed.     

Evaluation of luminol-H(2)O(2)-KIO(4) chemiluminescence system and its application to hydrogen peroxide, glucose and ascorbic acid assays

A novel chemiluminescence (CL) system was evaluated for the determination of hydrogen peroxide, glucose and ascorbic acid based on hydrogen peroxide, which has a catalytic-cooxidative effect on the oxidation of luminol by KIO(4). Hydrogen peroxide can be directly determined by luminol-KIO(4)-H(2)O(2) CL system. The detection limit was 3.0x10(-8) mol l(-1) and the calibration graph was linear over the range of 2.0x10(-7)-6.0x10(-4) mol l(-1). The relative standard deviation of H(2)O(2) was 1.1% for 2.0x10(-6) mol l(-1) (N=11). Glucose was indirectly determined through measuring the H(2)O(2) generated by the oxidation of glucose in the presence of glucose oxidase at pH 7.6. The present method provides a source for H(2)O(2), which, in turn, coupled with the luminol-KIO(4)-H(2)O(2) CL reaction system. The CL was linearly correlated with glucose concentration of 0.6-110 mug ml(-1). The relative standard deviation was 2.1% for 10 mug ml(-1) (N=11). Detection limit of glucose was 0.08 mug ml(-1). Ascorbic acid was also indirectly determined by the suppression of luminol-KIO(4)-H(2)O(2) CL system. The calibration curve was linear over the range of 1.0x10(-7)-1.0x10(-5) mol l(-1) of ascorbic acid. The relative standard deviation was 1.0% for 8.0x10(-7) mol l(-1) (N=11). Detection limit of ascorbic acid was 6.0x10(-8) mol l(-1). These proposed methods have been applied to determine glucose, ascorbic acid in tablets and injection.     

UV-detection capillary electrophoresis for benzo[a]pyrene and pyrene following a two-step concentration system using homogeneous liquid-liquid extraction and a sweeping method

10(-9) mol l-1 levels of polycyclic aromatic hydrocarbons (PAHs) suspected for certain noxious materials can be determined and separated by UV-detection capillary electrophoresis following a two-step concentration system. When the conditions of the homogeneous liquid-liquid extraction were [THF]T = 5 vol%, [HCl]T = 0.66 mol l-1, and [PFOA]T = 2 x 10(-3) mol l-1 (i.e., the volume of sample solution; 50 ml-->sedimented phase; 30 microliters), the extraction percentages of benzo[a]pyrene (Bap) and pyrene (Py) at 10(-6) mol l-1 were 102 and 97.5%, respectively. Also, when the total concentration factor(volume ratio), which includes that of the sweeping method, was 8335-fold, the determination range of Bap was 6.4 x 10(-9)-8.0 x 10(-7) mol l-1 and for Py, 8.0 x 10(-9)-7.0 x 10(-7) mol l-1. The detection limits (3 sigma) of Bap and Py were 1.6 x 10(-9) and 4.8 x 10(-9) mol l-1, respectively.     

Electrochemical determination of hydrogen peroxide using o -dianisidine as substrate and hemoglobin as catalyst

A new electrochemical method for the determination of microamounts of hydrogen peroxide utilizing o-dianisidine (ODA) as substrate and hemoglobin (Hb) as catalyst is described in this paper. Hb can be used as mimetic peroxidase and it can catalyse the reduction of hydrogen peroxide with the subsequent oxidation of ODA. The oxidative reaction product is an azo compound, which is an electroactive substance and has a sensitive second-order derivative polarographic reductive peak at the potential of -0.58 V (vs. SCE) in pH 80 Britton-Robinson (B-R) buffer solution. The conditions of Hb-catalytic reaction and polarographic detection of the reaction product were carefully studied. By using this polarographic peak and under optimal conditions, the calibration curve for the H₂O₂ was constructed in the linear range of 2.0 x 10^-7 ∼ 10 x 10^-4 mol/l with the detection limit of 5.0 x 10^-8 mol/l. This method can also be used to the determination of Hb content in the range of 20 x 10^-9 ∼ 30 x 10^-7 mol/l with a detection limit of 10 x 10^-9 mol/l. The proposed method was further applied to the determination of the content of H₂O₂ in fresh rainwater with satisfactory results. The catalytic reaction mechanism and the electrode reductive process of the reaction product were carefully studied.     

Polarographic determination of total iron content using a Fe(2+/3+)-Methylthymol Blue-NO2- system.

In the buffer solution of NH3-NH4Cl (pH = 8.5, 0.04 mol l(-1)), iron-Methylthymol Blue (MTB) can produce a sensitive polarographic wave at -1.10 V (vs. SCE) in the NaNO2. The peak current is linear with the concentration of the iron in the range of 3 x 10(-8)-5 x 10(-6) mol l(-1), and the detection limit is 1 x 10(-8) mol l(-1). By studying the characteristics of the wave and the electrode reaction mechanism, we can prove that the catalytic wave is an adsorption wave and that the peak current comes from the reduction of Fe(II). The molar ratio of iron to ligand was found to be 1:1. Adsorption particles are neutral molecules, the saturated adsorption quantity of the complex on the mercury electrode is 1.92 x 10(-9) mol cm(-2), according with the Frumkin isothermal formula. In the experiments, the adsorption coefficient (beta) is 4.05 x 10(5); the adsorption factor (gamma) is 0.70: the electron transfer number (n) is 2; the free energy (deltaG(o)) is 31.99 kJ mol(-1); the transfer coefficient of the irreversible adsorption is 0.42-0.45; and the reaction velocity constant (Ks) is 1.35 s(-1). This method, whose result is satisfying, can be applied to the detection of trace total iron contents in medicinal products.     

Characterisation and optimisation of an immunoprobe for triazines

The characterisation and optimisation of an optical immunoassay with label free detection based on Reflectometric Interference Spectroscopy (RIfS) is presented. The immunoprobe is operated in a sequential scheme, where Fab-fragments react with analyte molecules in a first step. In a second step the optical transducer is used to quantify the amount of unoccupied Fab- fragments in the reaction mixture binding to the hapten-modified transducer surface. For optimisation of the test, the Fab-fragment concentration was varied between 2x10(-8) mol/l and 2.5x 10(-9) mol/l. Down to a concentration of 5x10(-9) mol/l a reduction in the limit of detection has been observed. At the lowest concentration investigated no further improvement has been found due to a reduced binding of the analyte and a strong decrease of antibody binding at the transducer surface. This finding could be explained by the thermodynamics of the antigen-antibody reaction and the performance of the optical transducer used. The limit of detection obtained is discussed with respect to thermodynamics, transducer characteristics and immunoprobe test format.     

Carbon paste electrode for trace zirconium(IV) determination by adsorption voltammetry.

A very sensitive and selective procedure was developed for trace measurement of zirconium based on the cathodic adsorptive stripping voltammetry of the zirconium-alizarin red S(ARS) complex at a carbon paste electrode (CPE). The 2nd-order derivative linear scan voltammograms of the zirconium-ARS complex were recorded by a model JP-303 polarographic analyzer from 0.0 to -1.0 V (vs. SCE). Optimal analytical conditions were found to be: an acetic acid (0.1 mol l(-1))-potassium biphthalate (0.08 mol l(-1)) buffer solution (pH 4.8) containing 4.0 x 10-6 mol l(-1) ARS; accumulation potential, 0.0 V; accumulation time, 180 or 90 s; rest time, 10 s; scan rate, 250 mV s(-1). The results showed that the complex can be adsorbed on the surface of the CPE, yielding one peak at -0.51 V, corresponding to the reduction of ARS in the complex at the electrode. The detection limit was found to be 1.0 x 10(-10) mol l(-1) (S/N = 3) for 240 s accumulation. The linear range was 2.0 x 10(-10)-4.0 x 10(-7) mol l(-1). The developed method was applied to the determination of trace zirconium in the ore samples with satisfactory results.     

Terbium-sensitized fluorescence method for the determination of pazufloxacin mesilate and its application.

A simple, rapid and highly sensitive fluorometric method for the determination of pazufloxacin mesilate (PZFX) is described. It is based on the formation of the complex [Tb(PZFX)2](3+), which shows the intensive characteristic bands of Tb3+. Optimum conditions for the determination were investigated. Under the optimum experimental condition, the fluorescence intensity responds linearly to the PZFX concentration in the range of 2.0 x 10(-8) - 5.0 x 10(-6) mol/l with a detection limit of 6.2 x 10(-9) mol/l. The method has been successfully applied to the determination of PZFX in urine and serum samples.     

Binding nucleophiles to [Fe4Y4Cl4](2-) (Y = S or Se) can increase or suppress the rate of proton transfer to the cluster

In the proton transfer reactions between [Fe 4Y 4Cl 4] (2-) (Y = S or Se) and [pyrH] (+) (pyr = pyrrolidine) in the presence of a variety of nucleophiles (L = I (-), Br (-), PhS (-), EtS (-) or ButNC), initial binding of the nucleophile can occur to generate [Fe 4Y 4Cl 4(L)] ( n- ). The subsequent rate of proton transfer depends markedly on the nature of L. Stopped-flow kinetic studies show that proton transfer from [pyrH] (+) to [Fe 4Y 4Cl 4] (2-) { (S) k 4 = (2.1 +/- 0.5) x 10 (4) dm (3) mol (-1) s (-1); (Se) k 4 = (8.0 +/- 0.5) x 10 (3) dm (3) mol (-1) s (-1)} is increased by prior binding of L = PhS (-) or Bu ( t )NC to form [Fe 4Y 4Cl 4(L)] (n-) ( (S) k 7 (L) approximately 1 x 10 (6) dm (3) mol (-1) s (-1)), but prior binding of L = I (-), Br (-), or EtS (-) to the clusters inhibits the rate of proton transfer {e.g. (S) k 7 (I) = (6.0 +/- 0.8) x 10 (2) dm (3) mol (-1) s (-1); (Se) k 7 (I) = (4.5 +/- 0.5) x 10 (2) dm (3) mol (-1) s (-1)}. This behavior is correlated with the bonding characteristics of L and the effect this has on bond length reorganization within the cluster upon proton transfer.     

Stopped-flow spectrophotometric determination of hydrogen peroxide with hemoglobin as catalyst

A rapid and sensitive method was proposed for the determination of hydrogen peroxide based on the catalytic effect of hemoglobin using o-phenylenediamine as the substrate. Stopped-flow spectrophotometric method was used to study the kinetic behavior of the oxidation reaction. The catalytic effectiveness of hemoglobin was compared with other four kinds of catalysts. The initial rate of the formation of the reaction product 2,3-diaminophenazine at the wavelength of 425 nm was monitored, permitting a detection limit of 9.2x10(-9) mol/l H(2)O(2). A linear calibration graph was obtained over the H(2)O(2) concentration range 5.0x10(-8)-3.5x10(-6) mol/l, and the relative standard deviation at a H(2)O(2) concentration of 5.0x10(-7) mol/l was 2.08%. Satisfied results were obtained in the determination of H(2)O(2) in real samples by this method.