Determination of p-nitroaniline by the tartrate-acetone-Mn2+-KBrO3-H2SO4 double organic substrate oscillating system using non-equilibrium stationary state

This paper described the determination of p-nitroaniline in a double organic substrate oscillating system of tartrate-acetone-Mn²⁺-KBrO₃-H₂SO₄. Under the optimum conditions, temperature was chosen as a control parameter to design the bifurcation point and proposed a convenient method for determination of p-nitroaniline. Results showed that the system consisting of 3.5 mL 0.06 mol L⁻¹ tartrate, 4.0 mL 0.7 mol L⁻¹ H₂SO₄, 1.5 mL 1.5×10⁻⁴ mol L⁻¹ MnSO₄, 4.0 mL 0.4 mol L⁻¹ acetone and 7.0 mL 0.05 mol L⁻¹ KBrO₃ was very sensitive to the surrounding at 33.5°C. A good linear relationship between the potential difference and the negative logarithm concentration of p-nitroaniline was obtained to be in the range of 2.50×10⁻⁷∼3.75×10⁻⁵ mol L⁻¹ with a lower detection limit of 2.50×10⁻⁸ mol L⁻¹.      

Determination of furfural in an oscillating chemical reaction using an analyte pulse perturbation technique

A rapid and convenient method for the determination of furfural is presented that is based upon sequential perturbation of the Mn(II)-catalyzed B-Z oscillating system with different amounts of furfural using a continuous-flow stirred tank reactor (CSTR). When the sample was injected, the change in the amplitude and/or period was linearly proportional to the logarithm of the concentration of furfural over the range 3×10⁻⁸∼1×10⁻⁵ mol L⁻¹. This method gave a detection limit of 3×10⁻⁹ mol L⁻¹ under optimum conditions. Finally, the possible mechanism of furfural perturbation in the oscillating reaction is discussed. When the furfural was injected into the Mn(II)-catalyzed B-Z oscillating system, the change in the amplitude and/or period was linearly proportional to the logarithm of the concentration of furfural over the range 3×10⁻⁸~1×10⁻⁵ mol L⁻¹, with a detection limit of 3×10⁻⁹ mol L⁻¹ under optimum conditions.      

Determination of europium by using the chemical oscillating system of Ce(IV)-KBrO<Subscript>3</Subscript>-acetone-oxalic acid-H<Subscript>2</Subscript>SO<Subscript>4</Subscript>

A sensitive and convenient method for the determination of trace europium ions using an oscillating chemical reaction involving Ce(IV) - KBrO₃ - acetone - oxalic acid - H₂SO₄ was proposed. The results indicated that the changes in oscillating period (T) was linearly proportional to the negative logarithmic concentration of Eu³⁺ (-log C) in the range of 1.41 × 10⁻⁸ ˜ 1.41 × 10⁻⁴ mol L⁻¹ (r = 0.9982) with a detection limit of 1.04 × 10⁻⁹ mol L⁻¹. The recoveries were limited to the range of 99.5% to 100.8%. Under the same conditions, other rare earth ions did not interfere with the determination of Eu³⁺. In addition, a perturbation mechanism was also discussed briefly.      

A kinetic method for <Emphasis Type="Italic">para</Emphasis>-nitrophenol determination based on its inhibitory effect on the catalatic reaction of catalase

The inhibitory effect of para-nitrophenol on the catalytic reaction of catalase was investigated. Michaelis-Menten kinetic parameters were determined from Lineweaver-Burk plots obtained in the absence or in the presence of the inhibitor. The inhibitor pattern, revealed by the Lineweaver-Burk plots, suggested a fully mixed inhibition mechanism. Spectrophotometric monitoring of the indicator reaction: in conjunction with initial rate measurements was employed for the kinetic determination of the inhibitor. Calibration plots of initial rate vs. para-nitrophenol concentration were linear in the concentration range 0.9·10⁻⁵–2.5·10⁻⁵ mol/L and the detection limit was 3·10⁻⁶ mol/L (417 μg/L) para-nitrophenol. Interferences from other phenolic compounds like orto-cresole, meta-and orto-nitrophenol were observed.     

Voltammetric sensor for nitrite determination based on its electrocatalytic reduction at the surface of p-duroquinone modified carbon paste electrode

A p-duroquinone (tetramethyl-p-benzoquinone) modified carbon paste electrode (DMCPE) was employed to study the electrocatalytic reduction of nitrite in aqueous solutions using cyclic voltammetry (CV), double potential-step chronoamperometry, and differential pulse voltammetry (DPV). It has found that under an optimum condition (pH 1.00), the reduction of nitrite at the surface of DMCPE occurs at a potential of about 660 mV less negative than that of an unmodified carbon paste electrode (CPE). The catalytic rate constant, k′_h, based on Andrieux and Saveant theoretical model was calculated as for scan rate 10 mV s^-1. Also, the apparent diffusion coefficient, D _app, was found as 2.5 × 10^–10 and 3.61 × 10^–5 cm² s^-1 for p-duroquinone in carbon paste matrix and nitrite in aqueous buffered solution, respectively. The values for αn_α were estimated to be −0.65 and −0.19 for the reduction of nitrite at the surface of DMCPE and CPE, respectively. The electrocatalytic reduction peak currents showed a linear dependence on the nitrite concentration, and a linear analytical curve was obtained in the ranges of 5.0 × 10^–5 M to 8.0 × 10^–3 M and 6.0 × 10^–6 M to 8.0 × 10^–4 M of nitrite concentration with CV and DPV methods, respectively. The detection limits (2σ) were determined as 2.5 × 10^–5 M and 4.3 × 10^–6 M by CV and DPV methods. This method was also applied as a simple, selective and precise method for determination of nitrite in real samples (the weak liquor from the wood and paper factory of Mazandaran province in Iran) by using a standard addition method.     

Electrochemical behavior and voltammetric determination of metol using a multiwall carbon nanotubes modified electrode

Electrochemical behavior of metol, which coexists with p-benzeneiol (HQ) at a glassy-carbon electrode modified with multiwall carbon nanotubes (MWNT/GCE), is studied in the thesis. The results indicate that metol yields a well-defined peak of which two concomitant reductive peaks separate and the potential separation reaches to 178 mV, and that concomitant HQ has almost no interference with the reduction signal of metol. The values of the reductive peak current (I _pc) are found to be linearly related to metol concentration over the range of 8.0 × 10⁻² −1.0 × 10⁻⁵ M, with a detection limit of 5.0 × 10⁻⁶ M. Some common matter has no interference with the determination of metol. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 1, pp. 31–35. The text was submitted by the authors in English.     

A facile and fast electrochemical method for the simultaneous determination of <Emphasis Type="Italic">o</Emphasis>-dihydroxybenzene and <Emphasis Type="Italic">p</Emphasis>-dihydroxybenzene using a surfactant

A fast and convenient analytical technique has been suggested to detect dihydroxybenzene (DHB) isomers directly and simultaneously. In 0.1 M HAc-NaAc buffer solution (pH 5.6), the reduction peaks of p -DHB and o -DHB were partly separated by differential pulse voltammetry at the glassy carbon electrode. By adding a surfactant (cetyl pyridinium chloride) to enhance the detection sensitivity and using a semi-derivative technique to improve the separation, p-DHB and o-DHB were detected simultaneously. The linear calibration ranges were 1.2 × 10⁻⁶ to 1.0 × 10⁻⁴ M for p -DHB and 8.2 × 10⁻⁷ to 1.0 × 10⁻⁴ M for o -DHB, with detection limits of 3.6 × 10⁻⁷ and 2.4 × 10⁻⁷ M, respectively. The proposed method has been applied to the direct determination of DHB isomers in water sample, and the results were excellent. The text was submitted by the authors in English.     

The electrochemical polymerization of <Emphasis Type="Italic">o</Emphasis>-phenylenediamine on <Emphasis Type="SmallCaps">l</Emphasis>-tyrosine functionalized glassy carbon electrode and its application

The polymerization of o-phenylenediamine (OPD) on l-tyrosine (Tyr) functionalized glassy carbon electrode (GCE) and its electro-catalytic oxidation towards ascorbic acid (AA) had been studied in this report. l-Tyrosine was first covalently grafted on GCE surface via electrochemical oxidation, which was followed by the electrochemical polymerization of OPD on the l-tyrosine functionalized GCE. Then, the poly(o-phenylenediamine)/l-tyrosine composite film modified GCE (POPD-Tyr/GCE) was obtained. X-ray photo-electron spectroscopy (XPS), field emission scanning electron microscope (SEM), and electrochemical techniques have been used to characterize the grafting of l-tyrosine and the polymerization and morphology of OPD film on GCE surface. Due to the doping of the carboxylic functionalities in l-tyrosine molecules, the POPD film showed good redox activity in neutral medium, and thus, the POPD-Tyr/GCE exhibited excellent electrocatalytic response to AA in 0.1 mol l⁻¹ phosphate buffer solution (PBS, pH 6.8). The anode peak potential of AA shifted from 0.58 V at GCE to 0.35 V at POPD-Tyr/GCE with a greatly enhanced current response. A linear calibration graph was obtained over the AA concentration range of 2.5 × 10⁻⁴–1.5 × 10^–3 mol l⁻¹ with a correlation coefficient of 0.9998. The detection limit (3δ) for AA was 9.2 × 10⁻⁵ mol l⁻¹. The modified electrode showed good stability and reproducibility and had been used for the determination of AA content in vitamin C tablet with satisfactory results.     

A sensitive nicotine sensor based on molecularly imprinted electropolymer of o-aminophenol

A sensitive nicotine sensor based on a molecularly imprinted electropolymer of o-aminophenol is proposed and its configuration and performance are studied in detail. On the condition of weak acidity, the sensitive layer was prepared by electropolymerization of o-aminophenol on a gold electrode in the presence of the template (nicotine). The sensor exhibits good selectivity and sensitivity to nicotine. The determination limit is 2.0×10⁻⁷ mol/L and a linear relationship between the current and concentration is found in the range of 4.0×10⁻⁷ ∼ 3.3 ×10⁻⁵ mol/L. The sensor has also been applied to the analysis of nicotine in tobacco samples with recovery rates ranging from 99.0% to 102%. __________ Translated from Journal of Hunan university (Nature Science), 2005, 32(3) (in Chinese)     

The electrochemical behavior of p-benzenediol on a self-assembled monolayers Pt electrode modified with N-(2-mercapto-1,3,4-thiadiazol-5-yl)-N′-(4-substituted-arylacetyl) urea

Two novel N-(2-mercapto-1,3,4-thiadiazol-5-yl)-N′-(4-substituted-arylacetyl) urea compounds have been synthesized, characterized by NMR and MS, and used as self-assembly reagents to form self-assembled monolayers (SAMs) on Pt electrodes. The modified electrodes were characterized by electrochemical methods. The electrochemical behavior of p-benzenediol at the SAMs electrodes was investigated. It was found that the electrochemical response to p-benzenediol is controlled by diffusion and can be electrocatalyzed to obtain more symmetrical redox peaks and higher voltammetric current response at the SAMs electrodes, with a peak separation of 80 mV. For p-benzenediol the process at the SAMs electrodes is quasi-reversible with a rate constant of 0.6742 s⁻¹. The SAMs electrodes have been used to determine p-benzenediol by differential pulse voltammetry. The peak current was linear for concentrations of p-benzenediol in the range 1×10⁻⁷−5×10⁻⁴ mol L⁻¹ and the detection limit was 4.0×10⁻⁸ mol L⁻¹. The SAMs electrodes were used to determine p-benzenediol in real photographic developer and in a synthetic waste water sample; the standard addition recovery was in the range 96.6–100.4%.      

Microencapsulated Engineered <Emphasis Type="Italic">Lactococcus lactis Cells</Emphasis> for Heterologous Protein Delivery: Preparation and In Vitro Analysis

This article demonstrates the potential of encapsulated, engineered Lactococcus lactis as a vehicle for the oral delivery of therapeutic proteins. Using alginate-poly-l-lysine-alginate membrane-encapsulated L. lactis engineered to secrete the reporter protein Staphylococcal aureus nuclease, we show comparable viability and protein secretion between free and immobilized cells. After 12 h, microcapsules with a cell density of 4.8 × 10⁵ colony forming unit (CFU) ml⁻¹ grew to 2.2 × 10⁸ CFU ml⁻¹ and released 0.24 arbitrary unit (AU) ml⁻¹ of nuclease, producing similar results as free cells, which grew from 3.4 × 10⁵ to 1.9 × 10⁸ CFU ml⁻¹ and secreted 0.21 AU ml⁻¹ of nuclease. Moreover, encapsulated cells at a density of 4.4 × 10⁷ CFU ml⁻¹ grew to 2.2 × 10¹⁰ CFU ml⁻¹ in 12 h and secreted 15.3 AU ml⁻¹ of nuclease although 3.1 × 10⁷ CFU ml⁻¹ of free cells reached only 2.3 × 10⁹ CFU ml⁻¹ and released 5.6 AU ml⁻¹ of nuclease. We also show the sustained stability of the microcapsules during storage at 4°C over 8 weeks.     

Electrochemical behaviors of metol on ionic-liquid-modified carbon paste electrode and its analytical application

The electrochemical behaviors of metol on an ionic liquid N-butylpyridinium hexafluorophosphate modified carbon paste electrode (IL-CPE) were studied in this paper. The results indicated that a pair of well-defined quasi-reversible redox peaks of metol appeared with the decrease of overpotential and the increase of redox peak current, which was the characteristics of electrocatalytic oxidation. The electrocatalytic mechanism was discussed and the electrochemical parameters were calculated with results of the charge-transfer coefficient (α) as 0.45, the electrode reaction rate constant (k _s) as 4.02 × 10⁻³ s⁻¹, and the diffusion coefficient (D) as 6.35 × 10⁻⁵ cm²/s. Under the optimal conditions, the anodic peak current was linear with the metol concentration in the range of 5.0 × 10⁻⁶ ∼ 1.0 × 10⁻³ mol/L (n = 11, γ = 0.994) and the detection limit was estimated as 2.33 × 10⁻⁶ mol/L (3σ). The proposed method was successfully applied to determination of metol content in synthetic samples and photographic solutions.     

Kinetic and equilibrium study of the deprotonation of 4-nitrophenyl[bis(ethylsulphonyl)]methane by organic bases in acetonitrile in the presence of common cation BH<Superscript>+</Superscript> and tetrabutylammonium perchlorate

The results of kinetic and equilibrium experiments with the set of reaction of proton abstraction from 4-nitrophenyl[bis(ethylsulphonyl)]methane in acetonitrile are reported. Two strong organic bases are used: 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 7-methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD). The rates of proton transfer reaction have been measured by T-jump method in the presence of perchlorate of the appropriate base as a common cation BH⁺ and supporting electrolyte-tetrabutylammonium perchlorate (TBAP) in the temperature range between 20–40°C are: k _H =1.32×10⁷−2.00×10⁷ and 2.82×10⁷−4.84×10⁷ dm ³mol⁻¹s⁻¹ for MTBD and TBD respectively. The enthalpies of activation ΔH _MTBD ^≠ =13.5 and ΔH _TBD ^≠ =18.1 kJmol⁻¹. The entropies of activation are negative: ΔS _MTBD ^≠ =−62.3 and ΔS _TBD ^≠ =−40.3 Jmol⁻¹K⁻¹. The change of the absorbance of the anion of 4-nitrophenyl[bis9ethylsulphonyl)]methane at the temperature 25°C in the presence of common cation BH⁺ gives the equilibrium constants K=705 and 906 M⁻¹ for MTBD and TBD respectively. Kinetic and equilibrium results are discussed. The possible mechanism of proton transfer reaction between 4-nitrophenyl[bis(ethylsulphonyl)]methane and cyclic organic bases: MTBD and TBD in acetonitrile is proposed.     

Analytical applications of electrode sensitive to labetalol in pharmaceuticals

The analytical properties of an ion-selective electrode sensitive to labetalol with a liquid membrane, based on ion-pair complexes with sodium tetraphenylborate (TPB-Na⁺) are described. The studied electrode can be used for the determination of labetalol hydrochloride as a protonated form of labetalol in pharmaceuticals. The calibration curve, e.g. EMF=f(pC _LabHCl ) is linear in the range from 10⁻⁵ to 10⁻² mol L⁻¹ with a correlation coefficient of 0.9992 and slope of 61.13 mV/decade, which is close to the Nernstian slope. The detection limit of the examined electrode is 7.20×10⁻⁶ mol L⁻¹. The influence of pH of the tested solutions on the formulation of the electrode is not as considerable since the electrode works correctly in the pH range 3.0–8.0. The main attributes of the developed electrode are: stability, good reproducibility of EMF and short response time, close to 30 seconds depending on labetalol concentration in the solution. The electrode shows good selectivity for many inorganic ions. The selectivity for drug cations is weaker due to the structural similarity of the interfering cations to labetalol. The results of labetalol determination using direct potentiometry in drugs such as Pressocard (Polpharma) and Trandate (GlaxoWellcome) were compatible with the quantity of labetalol declared by the manufacturer, and with parallel UV spectrophotometric and HPLC determinations.     

Perchlorate selective membrane electrodes based on a platinum complex

Three platinum(II) complexes were synthesized and studied to characterize their ability as an anion carrier in a PVC membrane electrode. The polymeric membrane electrodes (PME) and also coated glassy carbon electrodes (CGCE) prepared with one of these complexes showed excellent response characteristics to perchlorate ions. The electrodes exhibited Nernstian responses to ClO₄ ⁻ ions over a wide concentration range from 1.5 × 10⁻⁶ to 2.7 × 10⁻¹ M for PME and 5.0 × 10⁻⁷ to 1.9 × 10⁻¹ M for CGCE with low detection limits (9.0 × 10⁻⁷ M for PME and 4.0 × 10⁻⁷ M for CGCE). The electrodes possess fast response time, satisfactory reproducibility, appropriate lifetime and, most importantly, good selectivity toward ClO₄ ⁻ relative to a variety of other common anions. The potentiometric response of the electrodes is independent of the pH of the test solution in the pH range 2.0–9.0. The proposed sensors were used in potentiometric determination of perchlorate ions in mineral water and urine samples. Correspondence: Ahmad Soleymanpour, Department of Chemistry, Damghan Basic Science University, Damghan, Iran.     

Flow screen-printed amperometric detection of p-nitrophenol in alkaline phosphatase-based assays

p-Nitrophenyl phosphate is one of the most widely used substrates for alkaline phosphatase in ELISAs because its yellow, water-soluble product, p-nitrophenol, absorbs strongly at 405 nm. p-Nitrophenol is also electroactive; an oxidative peak at 0.97 V (vs. an Ag pseudoreference electrode) is obtained when a bare screen-printed carbon electrode is used. When an amperometric detector was coupled to a flow-injection analysis system the detection limit achieved for p-nitrophenol was 2×10⁻⁸ mol L⁻¹, almost two orders of magnitude lower than that obtained by measuring the absorbance of the compound. By use of this electrochemical detection method, measurement of 7×10⁻¹⁴ mol L⁻¹ alkaline phosphatase was achieved after incubation for 20 min. The feasibility of coupling immunoassay to screen-printed carbon electrode amperometric detection has been demonstrated by performing an ELISA for detection of pneumolysin, a toxin produced by Streptococcus pneumoniae, which causes respiratory infections. The method is simple, reproducible, and much more sensitive than traditional spectrophotometry.     

Fluorometric enzyme immunosensing system based on natural product resveratrol for horseradish peroxidase and Ag/SiO<Subscript>2</Subscript> nanoparticles

The properties of resveratrol (3′, 4′, 5-trihydroxystlbene, RST) were for the first time evaluated as a potential substrate for horseradish peroxidase (HRP)-catalyzed fluorogenic reaction. The properties of RST for use as fluorogenic substrates for HRP and its application in immunoassays were compared with commercially available substrates such as p-hydroxyphenylpropionic acid (pHPPA), chavicol and Amplex red by a fluoroimmunosensing method in the use of Schistosomia japonicum antibody (SjAb) as a model analyte. The fluoroimmunosensing device was constructed by dispersing Schistosomia japonicum antigen (SjAg), nano-Ag/SiO₂ particles and sol-gel at low temperature. In pH 5.8 Britton-Robinson buffer (B-R), HRP-SjAb conjugates can catalyze the polymerization reaction of RST by H₂O₂ forming fluorescent dimmers. The increase of the fluorescence intensity of the dimmers product at emission of 462 nm (excitation: 315 nm) is proportional to the concentration of HRP-SjAb binding to the SjAg entrapped in the nano-Ag/SiO₂ particles-sol-gel matrix. A competitive binding assay has been used to determine SjAb in rabbit serum with the aid of SjAb labeled with HRP. Substrate RST showed comparable ability for HRP detection and its enzyme-linked immunosensing reaction system, in a linear detection ranging of 1.5×10⁻⁶–7.3×10⁻⁴ g/L and with a detection limit of 1.5×10⁻⁶ g/L. The immobilized biocomposites surface could be regenerated by simply polishing with an alumina paper, with an excellent reproducibility (RSD = 4.7%). The proposed method has been successfully used for analysis of the rabbit serum sample with satisfactory results. Supported by the Projects of Scientific Research Fund of Hunan Provincial Education Department of China (Grant Nos. 05B020 and 06C098)     

Kinetic determination of Ag+ using a novel Belousov-Zhabotinskii oscillating system catalyzed by a macrocyclic complex

A new methodological approach for the determination of Ag⁺ by the perturbations caused by different amounts of Ag⁺ on a novel Belousov-Zhabotinskii oscillating system is proposed. Such a novel oscillating system involves the participation of an unsaturated macrocyclic Cu(II) complex, [CuL](ClO₄)₂, as catalyst and malic acid as substrate. The ligand L in the complex is 5,7,7,12,14,14-hexemethyl-1,4,8,11-tetraazacy-clotetradeca-4,11-diene. The linear relationship between the change in the oscillating period of the chemical system and the logarithm of [Ag⁺] is found in the range 6.25 × 10⁻⁵ to 1.00 × 10⁻⁷ M. The obtained RSD is 0.8%. The probable mechanism involving the perturbation of Ag⁺ is also discussed. The text was submitted by the authors in English.     

Simultaneous Determination of Triptolide and Tripdiolide in Extract of Tripterygium wilfordii Hook. f. by LC–APCI-MS

A novel method using liquid chromatography coupled with atmospheric pressure chemical ionization mass spectrometry in the negative selected ion monitoring mode has been developed and validated for rapid simultaneous determination of triptolide and tripdiolide in the extract of Tripterygium wilfordii Hook. f. The molecular ions m/z [M–H]⁻ 359 and 375 were selected for the quantification in selected ion monitoring mode for triptolide and tripdiolide. Standard calibration curve was linear over the concentration range of 0.12–24 and 0.15–30 μg mL⁻¹ for triptolide and tripdiolide. The relative standard deviations of intra- and inter-day were in the range of 4.7–9.9 and 8.9–12.6%. The average recoveries were between 96.4 and 104.6%. The limits of quantitation were 2.0 × 10⁻³ and 2.5 × 10⁻³ μg mL⁻¹ for triptolide and tripdiolide.     

Voltammetric Behavior of Strychnine, and its Determination in Strychno Nux-Vomica Seeds Extract

The voltammetric behavior of strychnine has been studied with a pyrolytic graphite (PG) electrode. The redox process taking place at the PG electrode is discussed. The cyclic voltammetric response has also been evaluated with respect to various experimental conditions, such as scan rate, pH of the supporting electrolyte, strychnine concentrations and accumulation time. A highly sensitive voltammetric method for the determination of strychnine is consequently developed. The linear calibration is in the range of 1×10⁻⁶ M – 1.1×10⁻⁴ M, with the limit of detection (LOD) being 1×10⁻⁸ M. The precision is excellent with a relative standard deviation (RSD) of 2.3%. The proposed cyclic voltammetric methodology has been applied to the determination of strychnine in the extract of Strychno nux-vomica seeds using the standard addition method. Consistent results have been obtained from both the electrochemical approach described here and the previously reported HPLC method.