Showing papers on "Buffer solution published in 2005"

Poly(p-aminobenzene sulfonic acid)-modified glassy carbon electrode for simultaneous detection of dopamine and ascorbic acid

Abstract: A glassy carbon electrode (GCE) is modified with electropolymerized film of p-aminobenzene sulfonic acid (p-ABSA) in pH 70 acetate buffer solution (ABS) Cyclic voltammetry (CV), different pulse voltammetry (DPV) and amperometric curve were used to study the electrochemical properties of the polymer film The polymer film-modified electrode is used to electrochemically detect dopamine (DA) in the presence of ascorbic acid (AA) Polymer film showed excellent electrocatalytic activity for the oxidation of DA and AA The DA and AA anodic peak potential values at the modified electrode are 196 and −8 mV, respectively, which can be obtained from DPV recordings In pH 70 ABS, the anodic peak current increases linearly over three concentration intervals of DA, viz, 10 × 10−7–10 × 10−6, 10 × 10−6–10 × 10−5 and 10 × 10−5–10 × 10−4 mol l−1, with the correlation coefficient, 09984, 09973 and 09921, respectively, and the detection limit (S/N = 3) is 20 × 10−8 mol l−1 AA has no interference with the determination of DA because of the very distinct attracting interaction between DA cations and the negatively poly(p-ABSA) film The proposed method exhibits good recovery and reproducibility

Particle Size Control of Cryptomelane Nanomaterials by Use of H2O2 in Acidic Conditions

Abstract: A new soft-step chemistry method has been developed to prepare pure cryptomelane-type manganese oxide materials (OMS-2) with the smallest particle sizes ever reported. The synthetic procedure is based on the reduction of KMnO4 by H2O2 under acidic conditions followed by reflux. An acetate-containing buffer solution and HNO3 are used to control the pH of the reaction mixture. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials have been investigated by X-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy (HRTEM), potentiometric titration, thermogravimetric analysis, and N2 sorption analyses. Both the concentration of H2O2 and the nature of the acid used affect the crystalline phase formation, microstructure, thermal stability, and the composition of the final product. HRTEM images reveal that the OMS-2 nanofibers are not oriented preferentially and present significant twinning, along with discontinuity...

Potentiometric enzyme electrode for urea determination using immobilized urease in poly(vinylferrocenium) film

Abstract: A new enzyme electrode for the determination of urea was developed by immobilizing urease in poly(vinylferrocenium) (PVF+) matrix. A PVF+ClO4− film was coated on Pt electrode at +0.7 V by electrooxidation of poly(vinylferrocene) in metylene chloride containing 0.1 M tetrabutylammonium perchlorate (TBAP). The enzyme modified electrode PVF+E− was prepared by anion-exchange in an enzyme solution in 1.0 mM phosphate buffer at pH 7.0. FTIR spectroscopy was used to identify PVF+ClO4−, and PVF+E−. UV spectroscopy was also used to prove the enzyme immobilization. The effects of polymeric film thickness, concentration of enzyme solution, immobilization time of the enzyme, pH and temperature of the medium, concentration of the buffer solution and possible interferents on the measured potential values were investigated. The potentiometric enzyme electrode developed in this study provided linearity to urea in the 5 × 10−5 to 1 × 10−1 M urea concentration range. The detection limit under the optimum working conditions was determined as 5 × 10−6 M for urea. The enzyme electrode was found to be stable for 24 days. The apparent Michaelis-Menten consant (KM app) value and the activation energy, Ea, of this immobilized enzyme system were found to be 4.48 × 10−5 M and 4.97 kcal mol−1 for urea, respectively.

Phosphate coatings on magnesium alloy AM60: Part 2: Electrochemical behaviour in borate buffer solution

Abstract: A zinc phosphate coating was formed on the surface of magnesium alloy AM60 by immersing the specimen in a phosphating bath. Corrosion potential measurements, anodic polarisation curves and electrochemical impedance spectroscopy (EIS) were used to assess the corrosion protection of the coating. The electrochemical measurements were performed in borate solution at pH=9.2. This slightly alkaline solution was chosen because it can be used to differentiate the electrochemical behaviour of phosphated and non-phosphated magnesium alloys. The phosphate coating considerably slowed down the metal dissolution process. Measurements performed on phosphated and non-phosphated specimens in borate solution showed that the phosphate coating afforded considerable protection against corrosion. From the results obtained, it was deduced that the optimum phosphating time was 10 min. The results were fitted to an equivalent electrical circuit.

Voltammetric Detection of Heparin at Polarized Blood Plasma/1,2-Dichloroethane Interfaces

Abstract: Heparin, a highly negatively charged polysaccharide, which has been used widely as an anticoagulant and antithrombotic, was detected by ion-transfer voltammetry at the interface between 1,2-dichloroethane and an aqueous buffer solution or undiluted blood plasma. Quaternary ammoniums with different numbers of methyl and long alkyl groups were examined as positively charged heparin ionophores using pipet electrodes filled with the organic electrolyte solutions of their tetrakis(4-chlorophenyl)borate salts. It was shown that octadecyltrimethylammonium most selectively facilitates interfacial heparin adsorption without interference from potential-dependent ionophore transfer into the aqueous phase. Water-filled pipet electrodes were also used to study the stoichiometry of the interfacial complex between a heparin molecule and multiple ionophore molecules, which is discussed as a counterion condensation effect. Stripping voltammetry based on facilitated heparin adsorption and desorption gives a detection limit...

Amperometric nitrite sensor based on hemoglobin/colloidal gold nanoparticles immobilized on a glassy carbon electrode by a titania sol-gel film

Abstract: A novel amperometric nitrite sensor was developed based on the immobilization of hemoglobin/colloidal gold nanoparticles on a glassy carbon electrode by a titania sol-gel film. The sensor shows a pair of well-defined and nearly reversible cyclic voltammogram peaks for Hb Fe(III)/Fe(II) with a formal potential (E°′) of −0.370 V, and the peak-to-peak separation at 100 mV s−1 was 66 mV vs. Ag/AgCl (3.0 M KCl) in a pH 6.9 phosphate buffer solution. The formal potential of the Hb Fe(III)/Fe(II) couple shifted linearly with pH with a slope of −50.0 mV/pH, indicating that electron transfer accompanies single-proton transportation. The sensor exhibited an excellent electrocatalytic response to the reduction of nitrite. The reduction overpotential was 0.45 V below that obtained at a colloidal gold nanoparticles/TiO2 sol-gel film-modified GCE. The linear range for nitrite determination for the sensor was 4.0×10−6 to 3.5×10−4 M, with a detection limit of 1.2×10−6 M. The stability, repeatability and selectivity of the sensor were also evaluated.

A third-generation hydrogen peroxide biosensor fabricated with hemoglobin and Triton X-100

Abstract: A new film of Triton X-100 to incorporate hemoglobin for biosensor fabrication is presented in this paper. In this film, hemoglobin displays a pair of redox peaks in pH 6.0 NaAc–HAc buffer solution with a formal potential of −0.257 V (versus SCE) and shows a thin-layer behavior. Without electron mediator or promoter, the modified electrode has an electrocatalytic activity to the reduction of hydrogen peroxide (H2O2), which shows a linear dependence on the H2O2 concentration ranging from 1.0 × 10−6 to 1.0 × 10−4 mol/L. The detection limit of 3.0 × 10−7 mol/L is estimated when the signal-to-noise ratio is 3. The K M app value of hemoglobin in Triton X-100 film has been determined to be 4.27 mmol/L. Ascorbate, uric acid, dopamine, catechol, cystine and epinephrine will not interfere with the sensitive determination of H2O2.

A facile and sensitive chemiluminescence detection of amino acids in biological samples after capillary electrophoretic separation

Abstract: It was found that native amino acids enhanced the chemiluminescence (CL) reaction between luminol and BrO - in an alkaline aqueous solution. This has led to the development of a facile and highly sensitive CL detection scheme for the determination of amino acids in biological samples after capillary electrophoretic (CE) separation. The CE-CL conditions were optimized. An electrophoretic buffer of 2.5 x 10 - 2 M sodium borate (pH 9.4) containing 1 x 10 - 4 M luminol was used. The oxidizer solution of 8 x 10 - 4 M NaBrO in 0.1 M sodium carbonate buffer solution (pH 12.5) was introduced post-column. Under the optimal conditions, the detection limits were 1.0 x 10 - 7 M for glutamic acid (Glu) and 1.3 x 10 - 7 M (S/N = 3) for aspartic acid (Asp). The relative standard deviations (RSDs) of peak area and migration time were in the ranges of 3.8-4.3% and 1.4-1.6%, respectively. The present method was applied to the determination of excitatory amino acids (i.e., Asp and Glu) in rat brain tissue and monkey plasma. The levels of these major excitatory amino acids in monkey plasma were quantified for the first time and found to be 1.17 ′0.17 x 10 - 5 M (mean ′ SD, n = 6) for Glu and 1.64 ′ 0.19 x 10 - 6 M for Asp, which were comparable with the levels in human plasma.

Synthesis of Poly(ε‐lysine)‐Grafted Dextrans and Their pH‐ and Thermosensitive Hydrogelation with Cyclodextrins

Abstract: To give pH sensitivity to a thermoreversible supramolecular-structured hydrogel system, poly(epsilon-lysine) (PL), as a cationic polymer, was grafted to dextran and used for inclusion complexation with alpha-cyclodextrins (alpha-CDs). The synthesized graft copolymer was characterized by 1H NMR spectroscopy, and the hydrogel formation was confirmed by X-ray diffraction and solid-state 13C NMR analysis. The hydrogelation was induced from a phase-separated structure of hydrated dextrans and hydrophobically aggregated inclusion complexes in buffer solution at pH 10.0. The prepared hydrogels showed thermoreversible gel-sol transitions as well as pH-sensitive phase transitions, which were recorded by the changes in UV/Vis transmittance. A rapid phase transition from gel to sol was observed upon decreasing the pH value to 4.0, which resulted from the dissociation process between the protonated guest polymer and alpha-CDs. The stimuli-responsive physical properties of the hydrogels were improved by modulating the degree of substitution of the grafted PL and the combination with alpha-CDs.

A potentiometric acetylcholinesterase biosensor based on plasma-polymerized film

Abstract: A potentiometric acetylcholinesterase biosensor based on pH-sensitive PVC membrane with plasma-polymerized ethylenediamine film has been proposed. Glow discharge plasma technique was utilized to deposit a film containing amino groups on the surface of pH-sensitive PVC membrane. Such an approach makes it possible to fabricate a very simple self-mounted acetylcholinesterase membrane directly attached to PVC pH-sensing electrode surface. The deposition conditions were optimized and the optimum plasma polymerization parameters were as follows: applied power, 75 W; pressure, 130 Pa; plasma exposure time, 60 min and flow rates, 10 ml min−1. Two neutral carriers, DOODA and DAMAB were compared as the pH-carrier, and the biosensor based on DAMAB showed better response characteristics. The effect of pH of the medium was also examined. The potential responses of biosensor increase with the acetylcholine chloride concentration over the concentration range of 10−6 to 10−1 mol dm−3 and the detection limit is 0.002 mmol dm−3 in pH 8.0 PB buffer solution. The biosensor was employed more than 200 times in 20 days while the maximum response span of biosensor retained 90%.

On Calibration of pH Meters

Abstract: The calibration of pH meters including the pH glass electrode, ISE electrodes, buffers, and the general background for calibration are reviewed. Understanding of basic concepts of pH, pOH, and electrode mechanism is emphasized. New concepts of pH, pOH, as well as critical examination of activity, and activity coefficients are given. The emergence of new solid state pH electrodes and replacement of the salt bridge with a conducting wire have opened up a new horizon for pH measurements. A pH buffer solution with a conducting wire may be used as a stable reference electrode. The misleading unlimited linear Nernstian slope should be discarded. Calibration curves with 3 nonlinear portions for the entire 0—14 pH range due to the isoelectric point change effect are explained. The potential measurement with stirring or unstirring and effects by double layer (DL) and triple layer (TL) will be discussed.

Adsorptive voltammetry of Hg(II) ions at a glassy carbon electrode coated with electropolymerized methyl-red film

Abstract: The glassy carbon electrode coated with electropolymerized methyl-red film, 1.2 × 10−6 m in thickness, (PMRE) showed high sensitivity towards Hg(II) ions. PMREs were adopted to accumulate and detect Hg(II) ions in a pH 2.56 Britton–Robinson buffer solution. Cyclic voltammogram of the accumulated Hg species on PMREs exhibited an anodic wave at 0.64 V and a cathodic wave at 0.13 V, due to the oxidation of accumulated Hg species on PMREs and the reduction of Hg(II) ions in the solution, respectively. For this heterogeneous adsorption of Hg(II) ions onto PMREs, the maximum surface concentration, adsorption equilibrium, and Gibbs energy change were evaluated to be 5.12 × 10−6 mol m−2, 3.7 × 105 l mol−1, and −30.1 kJ mol−1, respectively. The anodic peak current at 0.64 V was linear with the concentration of Hg(II) ions in the range of 1.1 × 10−10 to 1.1 × 10−7 M with a detection limit of 4.4 × 10−11 M. The proposed method was utilized successfully for the detection of Hg(II) ions in the lake water.

Surface-induced unfolding of human lactoferrin

Abstract: We have determined the structural conformations of human lactoferrin adsorbed at the air/water interface by neutron reflectivity (NR) and its solution structure by small angle neutron scattering (SANS). The neutron reflectivity measurements revealed a strong structural unfolding of the molecule when adsorbed at the interface from a pH 7 phosphate buffer solution (PBS with a total ionic strength at 4.5 mM) over a wide concentration range. Two distinct regions, a top dense layer of 15−20 A on the air side and a bottom diffuse layer of some 50 A into the aqueous subphase, characterized the unfolded interfacial layer. At a concentration around 1 g dm-3, close to the physiological concentration of lactoferrin in biological fluids, the adsorbed amount was 5.5 × 10-8 mol m-2 in the absence of NaCl, but the addition of 0.3 M NaCl reduced protein adsorption to 3.5 × 10-8 mol m-2. Although the polypeptide distributions at the interface remained similar, quantitative analysis showed that the addition of NaCl reduced...