Use of a graphite–polyurethane composite electrode for electroanalytical determination of indole-3-acetic acid in soil samples

A new electroanalytical methodology was developed for the quantification of the phytohormone indole-3-acetic acid (IAA), using a graphite–polyurethane composite electrode (GPU) and the square wave voltammetry (SWV), in 0.1 mol L^− 1 phosphoric acid solution (pH 1.6). Analytical curves were constructed under optimized conditions (f = 100 s^− 1, a = 50 mV, E_i = 5 mV) and the reached detection and quantification limits were 26 μg L^− 1 and 0.2 mg L^− 1, respectively. The developed methodology is simple and accurate for the routine determination of IAA. In order to verify the application of the electroanalytical methodology in fortified soil samples without previous treatment, an IAA assay was performed without serious interferences of the soil constituents.     

Voltammetric studies on the α-tocopherol anion and α-tocopheroxyl (Vitamin E) radical in acetonitrile

The α-tocopheroxyl radical was generated voltammetrically by one-electron oxidation of the α-tocopherol anion (E ^r_1/2=−0.73 V versus Ag|Ag⁺) that was prepared by reacting α-tocopherol with Et₄NOH in acetonitrile (with Bu₄NPF₆ as the supporting electrolyte). Cyclic voltammograms recorded at variable scan rates (0.05–10 V s⁻¹), temperatures (−20 to 20°C) and concentrations (0.5–10 mM) were modelled using digital simulation techniques to determine the rate of bimolecular self-reaction of α-tocopheroxyl radicals. The k values were calculated to be 3×10³ l mol⁻¹ s⁻¹ at 20°C, 2×10³ l mol⁻¹ s⁻¹ at 0°C and 1.2×10³ l mol⁻¹ s⁻¹ at −20°C. In situ electrochemical-EPR experiments performed at a channel electrode confirmed the existence of the α-tocopheroxyl radical.     

Assay of femtogram level nitrite in human urine using luminol–myoglobin chemiluminescence

In this paper, a sensitive flow injection chemiluminescence system luminol–myoglobin was described for determining femtogram nitrite. Nitrite bound myoglobin producing the ferric heme nitrite complexes, which catalyzed the electron transfer of luminol to myoglobin leading to fast chemiluminescence. The chemiluminescence intensity in the presence of nitrite was remarkably enhanced compared with that in the absence of it. Under the optimum reaction conditions the chemiluminescence increment produced was proportional to the concentration of nitrite in the range of 0.05 pg ml^− 1–1.0 ng ml^− 1 (R² = 0.9991), with a detection limit (3σ) of 20.0 fg ml^− 1. At the flow rate of 2.0 ml min^− 1, the whole process including sampling and washing could be completed in 0.5 min offering the sampling efficiency of 120 h^− 1 accordingly, and the relative standard deviation (RSD) was less than 2.60% (n = 5). It was satisfactory for the application to determine nitrite in human urine samples, and the possible mechanism was proposed.     

Oxidative degradation of non-ionic surfactant (TritonX-100) by chromium(VI)

Degradation of polyoxyethylene chain of non-ionic surfactant (TritonX-100) by chromium(VI) has been studied spectrophotometrically under different experimental conditions. The reaction rate bears a first-order dependence on the [Cr(VI)] under pseudo-first-order conditions, [TritonX-100]  [Cr(VI)] in presence of 1.16 mol dm⁻³ perchloric acid. The observed rate constant (k_obs) was 3.3 × 10⁻⁴ to 3.5 × 10⁻⁴ s⁻¹ and the half-life (t_1/2) was 33–35 min for chromium(VI). The effects of total [TritonX-100] and [H⁺] on the reaction rate were determined. Reducing nature of non-ionic TritonX-100 surfactant is found to be due to the presence of –OH group in the polyoxyethylene chain. It was observed that monomeric and non-ionic micelles of TritonX-100 were oxidized by chromium(VI). When [TritonX-100] was less than its critical micelle concentration (cmc) the k_obs values increased from 0.76 × 10⁻⁴ to 1.5 × 10⁻⁴ s⁻¹. As the [TritonX-100] was greater than the cmc, the k_obs values increases from 2.1 × 10⁻⁴ to 8.2 × 10⁻⁴ s⁻¹ in presence of constant [HClO₄] (1.16 mol dm⁻³) at 40 °C. A comparison was made of the oxidative degradation rates of TritonX-100 with different metal ion oxidants. The order of the effectiveness of different oxidants was as follows: permanganate > diperiodatoargentate(III) > chromium(VI) > cerium(IV).     

Real-time monitoring of metal deposition and segregation phenomena during preparation of PdCu membranes

The N₂ and H₂ evolution, respectively, were monitored during deposition of Pd and Cu from electroless plating baths to obtain in-process control of the composition during preparation of 3–7 μm thick PdCu membranes on tubular ceramic substrates. Compositions estimated by gas evolution compare favorably to those measured in post-mortem XRD and EDS analyses, mostly differing by not more than 1 at.%. This result suggests that use of gas evolution measurements to enable in-process control of composition to within 1 at.% is feasible. Annealing experiments in an H₂ atmosphere demonstrated that, at 893 K, only 48 h are needed to form a stoichiometrically homogeneous, 9.5 μm thick, face centered cubic (fcc) Pd₆₃Cu₃₇ membrane from sequentially deposited layers; at 723 K, the same transformation requires over 2 weeks. The appearance of transient body centered cubic (bcc) and fcc phases with lower Pd contents signaled compositional segregation in the initial stages of alloy formation at 723 and 773 K and could be a source of persistent stoichiometric heterogeneity particularly in bcc PdCu membranes. The H₂ fluxes of fcc Pd₅₈Cu₄₂ and Pd₇₀Cu₃₀ membranes were J_H2=(1.6±1.1) mol m⁻² s⁻¹ exp[(−24.8±0.4)kJ mol⁻¹/RT] and J_H2=(3.7±0.6) mol m⁻² s⁻¹ exp[(−21.3±1.0)kJ mol⁻¹/RT], respectively, at 100 kPa H₂ pressure difference.     

Electrocatalytic response of carbohydrates at copper-alloy electrodes

The voltammetric responses observed for carbohydrates and polyalcohols at 0.60 V in 0.10 M NaOH are significantly larger at preanodized CuMn (95:5) electrodes as compared to preanodized pure Cu electrodes. Apparent values for the number of electrons transferred (n_app) and the corresponding values of heterogeneous rate constants (k_app) are estimated for selected reactants from the slopes and intercepts, respectively, of Koutecký–Levich plots of background-corrected voltammetric currents obtained at CuMn and Cu rotated disk electrodes (RDEs). Values of n_app (and k_app) for sorbitol and glucose are 11.8 (9.2×10⁻³ cm s⁻¹) and 11.7 (8.0×10⁻³ cm s⁻¹), respectively, at a CuMn RDE. These are compared to the values 10.4 (1.8×10⁻³ cm s⁻¹) and 9.6 (2.0×10⁻³ cm s⁻¹) for sorbitol and glucose, respectively, at a Cu RDE. The larger sensitivities observed at the CuMn RDE in comparison to the Cu RDE are concluded to be the beneficial result of larger k_app values at the alloy electrode. Furthermore, the larger k_app values are speculated to result from enhanced preadsorption of the reactant species at Mn(IV) sites in the preanodized CuMn surface. In flow-injection measurements, the peak signals obtained for successive injections of glucose using a CuMn electrode (0.60 V vs. SCE) were quite stable with a standard deviation of 1.5%. However, large day-to-day variations (±15%) observed in the average peak signals are attributed to the temperature sensitivities of the k_app value and the diffusion coefficient for glucose.     

Studies of the hydrogen evolution reaction on smooth Co and electrodeposited Ni–Co ultramicroelectrodes

The hydrogen evolution reaction (HER) was studied on smooth Co and on electrodeposited Ni–Co ultramicroelectrodes (UMEs) in alkaline solutions at several temperatures by steady-state polarisation curves. The real electrochemical area was previously estimated by cyclic voltammetry to account for the large difference in roughness factor of the two surfaces. The values obtained for the Tafel slopes were very close to 2.303RT/βnF while the ‘apparent’ energies of activation were 59 and 41 kJ mol⁻¹ for Co and Ni–Co, respectively. A common Volmer–Heyrovsky mechanism with Heyrovsky as the rate-determining step (RDS) was initially proposed. This was confirmed when the experimental results were mathematically treated by a non-linear fitting procedure using the kinetic equations derived for that mechanism. The calculations revealed that Ni–Co is a more efficient catalyst for the HER then pure Co, with a rate constant value of 0.16×10⁻¹⁰ mol s⁻¹ cm⁻² at 25°C for the slow step. Although this value is more than one order of magnitude smaller than that already reported for deposited Ni, it is considerably larger than the one measured here (0.02×10⁻¹⁰ mol s⁻¹ cm⁻²) for pure Co at 25°C.     

Metal-mediated dearomatization leading to 2-azaspiro[4.5]decanes via tandem nucleophilic aromatic addition–Horner–Wadsworth–Emmons olefination–oxidative demetalation sequences

A ruthenium-mediated dearomatization sequence has been developed that delivers structurally intriguing azaspirolactam products in stereoselective fashion. Treatment of (η⁶-arene)Ru(cyclopentadienyl) complexes bearing N-benzyl-β-amido phosphonate side chains with excess NaH results in intramolecular nucleophilic aromatic addition to the ipso position of the coordinated arenes. Subsequent Horner–Wadsworth–Emmons (HWE) reaction with added aldehydes affords olefinated spirolactam cyclohexadienyl ruthenium complexes. Mild oxidation with CuCl₂ or CuBr₂ under CO effects removal and recovery of the CpRu(II) fragment. Substituents present on the cyclohexadienyl skeleton influence the outcome of demetalation and products obtained in this study include functionalized 2-azaspiro[4.5]decanes and tetrahydroisoquinolinones.     

Oxidation of ascorbic acid in the presence of phthalocyanine metal complexes. Chemical aspects of catalytic therapy of cancer 2. Catalysis by cobalt octacarboxyphthalocyanine. Reaction products

The products of ascorbic acid oxidation in the presence of cobalt octa-4,5-carboxy-phthalocyanine sodium salt (TPH) were identified. These include the ascorbate radical (A^·), hydroxyl radical (OH^·), and hydrogen peroxide (H₂O₂). The kinetics of accumulation and consumption of the reaction products was studied. For the concentration ranges of ascorbic acid = 0–2.5 ⋅ 10⁻³ mol L⁻¹ and the catalyst C _TPH = 0–3.5 ⋅ 10⁻⁵ mol L⁻¹, the the highest possible concentration of the ascorbate radical is ∼10⁻⁷ mol L⁻¹, the concentration of H₂O₂ is 7 ⋅ 10⁻⁴ (30% of the starting concentration of ascorbic acid) and the concentration of the hydroxyl radical is at most 10⁻⁶ mol L⁻¹.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2224–2228, October, 2004.     

Polyvinyl alcohol–ionic liquid composition for promoting the direct electron transfer and electrocatalysis of hemoglobin

The direct electron transfer and electrocatalysis of hemoglobin (Hb) entrapped in polyvinyl alcohol (PVA)–room temperature ionic liquid (i.e., 1-octyl-3-methylimidazolium hexafluorophosphate [OMIM]PF₆) composition has been investigated by using cyclic voltammetry and chronocoulometry. It is found that the composition can promote the direct electron transfer of Hb and the heterogeneous electron transfer rate constant (k_s) of immobilized Hb is enhanced to 19.9 s⁻¹. The immobilized Hb also shows high electro-catalytic activity towards the redox of oxygen, hydrogen peroxide and nitrite. The Michaelis constants (K_m) decrease to 1.2 × 10⁻⁴ M (for hydrogen peroxide) and 9.4 × 10⁻³ M (for nitrite). The surface concentration of electroactive Hb is estimated and it is ca. 1.4 × 10⁻¹⁰ mol cm⁻², meaning that several layers of immobilized Hb take part in the electrochemical reaction. When gold nanoparticles (GNP) is introduced into the composition, the resulting PVA–GNP–[OMIM]PF₆ composition presents better performance. The electrochemical characteristic of immobilized Hb is improved further. Thus PVA–GNP–[OMIM]PF₆ composition is more suitable for the immobilization of Hb. Therefore, it is a good strategy to prepare novel composition for protein immobilization by using several materials with different function.     

UV–visible spectrum of the phenyl radical and kinetics of its reaction with NO in the gas phase

Pulse radiolysis transient UV–visible absorption spectroscopy was used to study the UV–visible absorption spectrum (225–575 nm) of the phenyl radical, C₆H₅(), and kinetics of its reaction with NO. Phenyl radicals have a strong broad featureless absorption in the region of 225–340 nm. In the presence of NO phenyl radicals are converted into nitrosobenzene. The phenyl radical spectrum was measured relative to that of nitrosobenzene. Based upon σ(C₆H₅NO)_{270 nm}=3.82×10⁻¹⁷ cm² molecule⁻¹ we derive an absorption cross-section for phenyl radicals at 250 nm, σ(C₆H₅())_{250 nm}=(2.75±0.58)×10⁻¹⁷ cm² molecule⁻¹. At 295 K in 200–1000 mbar of Ar diluent k(C₆H₅()+NO)=(2.09±0.15)×10⁻¹¹ cm³ molecule⁻¹ s⁻¹.     

Reactions of CO2 with H, H2 and deuterated analogues

Combining a temperature variable 22-pole ion trap with a cold effusive beam of neutrals, rate coefficients k(T) have been measured for reactions of CO₂⁺ ions with H, H₂ and deuterated analogues. The neutral beam which is cooled in an accommodator to T_ACC, penetrates the trapped ion cloud with a well-characterized velocity distribution. The temperature of the ions, T_22PT, has been set to values between 15 and 300 K. Thermalization is accelerated by using helium buffer gas. For reference, some experiments have been performed with thermal target gas. For this purpose hydrogen is leaked directly into the box surrounding the trap. While collisions of CO₂⁺ with H₂ lead exclusively to the protonated product HCO₂⁺, collisions with H atoms form mainly HCO⁺. The electron transfer channel H⁺ + CO₂ could not be detected (<20%). Equivalent studies have been performed for deuterium. The rate coefficients for reactions with atoms are rather small. Within our relative errors of less than 15%, they do not depend on the temperature of the CO₂⁺ ions nor on the velocity of the atoms (k(T) lays between 4.5 and 4.7 × 10⁻¹⁰ cm³ s⁻¹ with H as target, and 2.2 × 10⁻¹⁰ cm³ s⁻¹ with D). For collisions with molecules, the reactivity increases significantly with falling temperature, reaching the Langevin values at 15 K. These results are reported as k = α (T/300 K)^β with α = 9.5 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.15 for H₂ and α = 4.9 × 10⁻¹⁰ cm³ s⁻¹ and β = −0.30 for D₂.     

The reduction mechanism of the C=O group. Part 2. The electrochemical reduction of p-diacetylbenzene in aqueous medium

A detailed study of the electrochemical reduction of diacetylbenzene A in aqueous medium between H_o = −5 and pH 14 is presented. The reactants are strongly adsorbed, so that the reactions are of a surface nature. From H_o = −5 to pH 6, a global 2e⁻ reduction yielding an enediol-type intermediate occurs. Analysis using the theory of the square schemes with protonations at equilibrium shows that, up to pH 4, the reaction is controlled by the first electron uptake, the paths being successively H⁺e⁻ and e⁻H⁺. The elementary electrochemical surface rate constants are 9.6 × 10⁷ s⁻ and 1.2 × 10⁶ s⁻ for AH⁺ and A respectively. From pH 6 to 14, a le⁻ adsorption wave, corresponding to the formation of (a) monoradical(s), appears and is followed by a le⁻ wave due to the reduction of the radical(s). A dimerization occurs, due to the coupling A⁻ + AH, as in the case of the monocarbonyl compounds. The rate of this surface process, k_d = 5 × 10¹³ cm² mol⁻¹ s⁻¹, is markedly smaller than the rate of the homogeneous reaction obtained in alkaline ethanol by Savéant et al. for the coupling of the radicals of benzaldehyde, benzophenone and acetophenone.     

Ionic liquid-based ultrasound-assisted dispersive liquid–liquid microextraction combined with electrothermal atomic absorption spectrometry for a sensitive determination of cadmium in water samples

A new method was developed for the determination of cadmium in water samples using ionic liquid-based ultrasound-assisted dispersive liquid–liquid microextraction (IL-based USA-DLLME) followed by electrothermal atomic absorption spectrometry (ETAAS). The IL-based USA-DLLME procedure is free of volatile organic solvents, and there is no need for a dispersive solvent, in contrast to conventional DLLME. The ionic liquid, 1-hexyl-3-methylimidazolium hexafluorophosphate (HMIMPF₆), was quickly disrupted by an ultrasonic probe for 1 min and dispersed in water samples like a cloud. At this stage, a hydrophobic cadmium–DDTC complex was formed and extracted into the fine droplets of HMIMPF₆. After centrifugation, the concentration of the enriched cadmium in the sedimented phase was determined by ETAAS. Some effective parameters of the complex formation and microextraction, such as the concentration of the chelating agent, the pH, the volume of the extraction solvent, the extraction time, and the salt effect, have been optimized. Under optimal conditions, a high extraction efficiency and selectivity were reached for the extraction of 1.0 ng of cadmium in 10.0 mL of water solution employing 73 µL of HMIMPF₆ as the extraction solvent. The enrichment factor of the method is 67. The detection limit was 7.4 ng L^− 1, and the characteristic mass (m₀, 0.0044 absorbance) of the proposed method was 0.02 pg for cadmium (Cd). The relative standard deviation (RSD) for 11 replicates of 50 ng L^− 1 Cd was 3.3%. The method was applied to the analysis of tap, well, river, and lake water samples and the Environmental Water Reference Material GSBZ 50009-88 (200921). The recoveries of spiked samples were in the range of 87.2–106%.     

Contribution of Na counterions to H-AOT&Na-AOT-based W/O microemulsion formation using aqueous NaOH solutions as estimated by pyranine absorbance

The objective of this study is to estimate the contribution of Na⁺ as a counterion in the formation of H-AOT&Na-AOT-based W/O microemulsions using aqueous NaOH solution by pyranine absorbance measurements. A mixture of an aqueous NaOH solution containing pyranine/H-AOT&Na-AOT/isooctane was emulsified by changing the mixing ratio of Na-AOT (X_Na-AOT = 0–1) and the mole fraction of NaOH (X_NaOH = [NaOH]/the AOT⁻ concentration in the water pool = 0–1). The phase behavior of the emulsified mixture was evaluated from the absorbance of pyranine at the isosbestic point and by visual observations. W/O microelumsions are formed at the mid-range of X_Na-AOT, whereas the emulsified mixture separates into two phases at lower X_Na-AOT and higher X_Na-AOT. The two phase boundaries shift toward lower X_Na-AOT as with increasing X_NaOH. The phase behavior depends on the degree of screening of electrostatic repulsions between the polar headgroups of AOT⁻ by the Na⁺ counterion. Interestingly, nano-sized W/O microemulsions are formed without phase separation using a highly concentrated NaOH aqueous solution when the Na-AOT mixing ratio is appropriately adjusted. The phase behavior was plotted as X_NaOH versus X_Na-AOT, and the correlation equations for the two phase boundaries were obtained by fitting the points. The contribution of the Na⁺ counterion from NaOH to W/O microemulsion formation was estimated by the correlation equations. The absorbance of pyranine and the size of W/O microemulsions, as measured by DLS, were plotted as a function of X_Na⁺=(x[Na⁺   from   NaOH]+[Na⁺   from   Na-AOT])/[AOT⁻], in which x is the ratio contributed by NaOH. The absorbance and size correlates well with X_Na⁺, indicating that X_Na⁺ is a meaningful parameter for quantitatively estimating phase behavior and size variation.     

Glass–ceramic Li2S–P2S5 electrolytes prepared by a single step ball billing process and their application for all-solid-state lithium–ion batteries

We report that glass–ceramic Li₂S–P₂S₅ electrolytes can be prepared by a single step ball milling (SSBM) process. Mechanical ball milling of the xLi₂S·(100 − x)P₂S₅ system at 55 °C produced crystalline glass–ceramic materials exhibiting high Li-ion conductivity over 10⁻³ S cm⁻¹ at room temperature with a wide electrochemical stability window of 5 V. Silicon nanoparticles were evaluated as anode material in a solid-state Li battery employing the glass–ceramic electrolyte produced by the SSBM process and showed outstanding cycling stability.     

Breaking the barrier to fast electron transfer

A study of the electron transfer for a non-glycosylated redox variant of GOx is reported, immobilised onto an electrode via a polyhistidine tag. The non-glycosylated variant allows the enzyme to be brought closer to the electrode, and within charge transfer distances predicted by Marcus' theory. The enzyme-electrode-hybrid shows direct very fast reversible electrochemical electron transfer, with a rate constant of ~ 350 s^− 1 under anaerobic conditions. This is 2 orders of magnitude faster than the enzyme-free flavin adenine dinucleotide (FAD). These results are discussed in the context of the conformation of FAD in the active site of GOx. Further data, presented in the presence of oxygen, show a reduced electron transfer rate (~ 160 s^− 1) that may be associated with the oxygen interaction with the histidines in the active site. These residues are implicated in the proton transfer mechanism and thus suggest that the presence of oxygen may have a profound effect in attenuating the direct electron transfer rate and thus moderating ‘short-circuit’ incidental electron transfer between proteins.     

A novel on-line preconcentration method for trace molybdenum determination by USN–ICP OES with biosorption on immobilized yeasts

A new system for on-line preconcentration of molybdenum by sorption on a minicolumn associated to inductively coupled plasma — optical emission spectrometry with ultrasonic nebulization was studied. It is based on the sorption of molybdenum on a column packed with immobilized baker's yeasts on controlled pore glass without further complexing reagent. The molybdenum preconcentrated by biosorption was subsequently eluted with hydrochloric acid. Considering a sample flow rate of 5.0 mL min^− 1, 10 mL of sample was preconcentrated in 2 min achieving a sensitive total enhancement factor of 480-fold, and the detection limit (3 s) obtained was 21 ng L^− 1. Additionally, the calculated precisions expressed as percent relative standard deviation (RSD%) was 1.9%.Satisfactory results were obtained for the determination of molybdenum in standard reference material NIST 1643e Trace Elements in Water and real water samples.     

On-line molecularly imprinted solid phase extraction for the selective spectrophotometric determination of catechol

A molecularly imprinted polymer has been synthesized for a selective on-line catechol extraction, followed by its spectrophotometric determination in guarana powder, mate tea and tap water samples. A clean-up column, containing a methacrylic polymer + C₁₈ solid phase, was also used in order to enhance selectivity. The imprinted polymer was prepared by bulk polymerization using catechol as template and 4-vinylpyridine as the functional monomer. Permanganate solution was used as spectrophotometric reagent, where Mn(VII) was reduced to Mn(II) by catechol in an acid medium, causing color loss, which was monitored at 528 nm. Physical (extraction flow rate, elution flow rate, coil length) and chemical (nature and concentration of the eluent, potassium permanganate concentration) variables were optimized, and the selectivity was appraised using three molecules (4-chloro-2-methylphenol, 2-cresol, 2-methoxyphenol) similar to catechol. These molecules did not present interference in 1:8, 1:10 and 1:10 (catechol/concomitant) proportions, respectively. The analytical calibration curve ranged from 3.0 up to 100 μmol L^− 1 (r > 0.999; seven concentrations levels, n = 3) and the limits of detection (LOD) and quantification (LOQ) were 0.8 and 2.7 μmol L^− 1, respectively. Precision, expressed as RSD, was of 3.0% (20 μmol L^− 1, n = 10), and the analytical frequency was 15 h^− 1. Accuracy was checked by the HPLC technique and the results did not present significant difference at 95% confidence levels according to the t test.     

Diffuse reflectance spectra of iron(III) vanadates

The electronic spectra of solid iron(III) vanadates FeVO₄ and Fe₂V₄O₁₃ were investigated by the diffuse reflectance technique in the spectral range 12 500–50 000 cm⁻¹. The spectra of investigated vanadates contain 2–3 intensive CT bands in the UV region and two lowest energy d–d bands in the 12 000–22 000 cm⁻¹ range. The presence of the weak bands for FeVO₄ and Fe₂V₄O₁₃ at 16 500 cm⁻¹ and 20 500 cm⁻¹ points to the lattice deffects (oxygen deficiency and the presence of the V⁴⁺ ions) in the structure of investigated vanadates.