Showing papers on "Buffer solution published in 2011"

Dopamine—Melanin Film Deposition Depends on the Used Oxidant and Buffer Solution

Abstract: The deposition of “polydopamine” films, from an aqueous solution containing dopamine or other catecholamines, constitutes a new and versatile way to functionalize solid−liquid interfaces. Indeed such films can be deposited on almost all kinds of materials. Their deposition kinetics does not depend markedly on the surface chemistry of the substrate, and the films can reach thickness of a few tens of nanometers in a single reaction step. Up to now, even if a lot is known about the oxidation mechanism of dopamine in solution, only little information is available to describe the deposition mechanism on surfaces either by oxidation in solution or by electrodeposition. The deposition kinetics of melanin was only investigated from dopamine solutions using oxygen or ammonium persulfate as an oxidant and from a tris(hydroxymethyl) aminomethane (Tris) containing buffer solutions at pH 8.5. Many other oxidants could be used, and the buffer agent containing a primary amine group may influence the deposition process. ...

Adsorption of chiral aromatic amino acids onto carboxymethyl-β-cyclodextrin bonded Fe3O4/SiO2 core–shell nanoparticles

Abstract: Surface of magnetic silica nanoparticles is modified by grafting with carboxymethyl-β-cyclodextrin (CM-β-CD) via carbodiimide activation. The functionalized magnetic core–shell nanoparticles (MNPs) are characterized by Transmission Electron Microscopy (TEM), Fourier Transform Infra Red (FTIR) spectroscopy, X-ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Vibrating Sample Magnetometer (VSM). These nano-sized particles are scrutinized for adsorption of certain chiral aromatic amino acid enantiomers namely, d - and l -tryptophan (Trp), d - and l -phenylalanine (Phe) and d - and l -tyrosine (Tyr) from phosphate buffer solutions. Adsorption capacities of the coated magnetic nanoparticles toward amino acid enantiomers are in the order: l -Trp > l -Phe > l -Tyr and under the same condition, adsorption capacities are higher for l -enantiomers than the corresponding d -enantiomers. All the equilibrium adsorption isotherms are fitted well to Freundlich model. FTIR studies depict significant changes after adsorption of amino acids onto nanoparticles. The stretching vibration frequencies of N H bonds of the amino acid molecules are changed with complex formation through host–guest interaction. The structure and hydrophobicity of amino acid molecules emphasize the interactions between amino acid molecules and the nano-adsorbents bearing cyclodextrin, thus play important roles in the difference of their adsorption behaviors.

Sensitive determination of dopamine in the presence of uric acid and ascorbic acid using TiO2 nanotubes modified with Pd, Pt and Au nanoparticles

Abstract: A simple modified TiO2 nanotubes electrode was fabricated by electrodeposition of Pd, Pt and Au nanoparticles. The TiO2 nanotubes electrode was prepared using the anodizing method, followed by modifying Pd nanoparticles onto the tubes surface, offering a uniform conductive surface for electrodeposition of Pt and Au. The performance of the modified electrode was characterized by cyclic voltammetry and differential pulse voltammetry methods. The Au/Pt/Pd/TiO2 NTs modified electrode represented a high sensitivity towards individual detection of dopamine as well as simultaneous detection of dopamine and uric acid using 0.1 M phosphate buffer solution (pH 7.00) as the base solution. In both case, electro-oxidation peak currents of dopamine were linearly related to accumulated concentration over a wide concentration range of 5.0 × 10−8 to 3.0 × 10−5 M. However in the same range of dopamine concentration, the sensitivity had a significant loss at Pt/Pd/TiO2 NTs electrode, suggesting the necessity for Au nanoparticles in modified electrode. The limit of the detection was determined as 3 × 10−8 M for dopamine at signal-to-noise ratio equal to 3. Furthermore, the Au/Pt/Pd/TiO2 NTs modified electrode was able to distinguish the oxidation response of dopamine, uric acid and ascorbic acid in mixture solution of different acidity. It was shown that the modified electrode possessed a very good reproducibility and long-term stability. The method was also successfully applied for determination of DA in human urine samples with satisfactory results.

Fluoride-containing bioactive glasses: Fluoride loss during melting and ion release in tris buffer solution

Abstract: Melt-derived bioactive glasses (SiO2–P2O5–CaO–Na2O–CaF2; CaF2 0 to 17.76 mol%) lost fluoride during melting, but nominal and analysed CaF2 contents in the glass correlated linearly. Analysed CaO contents were increased, showing that fluoride was lost as hydrofluoric acid after reaction with atmospheric water during melting. Weight loss on ignition reduced linearly with increasing CaF2, suggesting that CaF2 impedes absorption of atmospheric water. pH changes in tris buffer solution showed that pH is controlled by the silicate matrix (via ion exchange processes), and fluoride release contributes less to the overall pH. Glasses formed apatite in tris buffer; phosphate concentration of the glass was the limiting factor, resulting in fluorite formation for increasing fluoride content in the glass and calcite formation for the fluoride-free composition. These results allow for tailoring of novel fluoride-containing bioactive glasses to address specific needs, particularly in dentistry and for remineralising toothpastes.

Novel pyrazoline-based selective fluorescent sensor for Zn2+ in aqueous media

Abstract: This work describes the preparation of a novel pyrazoline compound and the properties of its UV–vis absorption and fluorescence emission. Moreover, this compound can be used to determine Zn 2+ ion with high selectivity and a low detection limit in the HEPES (20 mM HEPES, pH = 7.2, 50% (v/v) CH 3 CN) buffer solution. This sensor forms a 1:1 complex with Zn 2+ and shows a fluorescent enhancement by chelation enhanced fluorescence effect with good tolerance of other metal ions. In addition, this sensor is very sensitive with fluorometric detection limit of 0.12 μM.

Dual molecular light switches for pH and DNA based on a novel Ru(II) complex. A non-intercalating Ru(II) complex for DNA molecular light switch.

Abstract: A new Ru(II) complex of [Ru(phen)2(Hcdpq)](ClO4)2 {phen = 1,10-phenanthroline, Hcdpq = 2-carboxyldipyrido[3,2-f:2′,3′-h]quinoxaline} was synthesized and characterized. The spectrophotometric pH and calf thymus DNA (ct-DNA) titrations showed that the complex acted as a dual molecular light switch for pH and ct-DNA with emission enhancement factors of 17 and 26, respectively. It was shown to be capable of distinguishing ct-DNA from yeast RNA with this binding selectivity being superior to two well-known DNA molecular light switches of [Ru(bpy)2(dppz)]2+ {bpy =2,2′-bipyridine, and dppz = dipyrido-[3,2-a:2′,3′-c]phenazine}and ethidium bromide. The complex bond to ct-DNA probably in groove mode with a binding constant of (4.67 ± 0.06) × 103 M–1 in 5 mM Tris-HCl, 50 mM NaCl (pH = 7.10) buffer solution, as evidenced by UV–visible absorption and luminescence titrations, the dependence of DNA binding constants on NaCl concentrations, DNA competitive binding with ethidium bromide, and emission lifetime and viscosit...

Salt Effect on the Aqueous Two-Phase System PEG 8000−Sodium Sulfate

Abstract: The effect of added salt (NaCl or KCl) on the polyethylene glycol 8000 (PEG)−sodium sulfate (Na2SO4) aqueous two-phase system (ATPS), containing 0.01 mol·L−1 of sodium phosphate buffer (NaPB), pH 7.4, has been investigated at 296.15 K. Phase diagrams determined by the cloud point method, including tie-lines assigned from mass phase ratios according to the lever arm rule, are presented for the different PEG 8000−Na2SO4 ATPSs, all containing 0.01 mol·L−1 NaPB, pH 7.4, and increasing the concentration of a neutral salt, such as NaCl or KCl, up to 1.0 mol·L−1. Experimental binodal results were satisfactorily correlated with an empirical mathematical model, the Merchuk equation. The results indicate that the addition of both salts causes a depression of the binodal relatively to that for the salt-free system. The salting-out ability of the cations follows the Hofmeister series (Na+ > K+) and can be related to the ions Gibbs free energy of hydration (ΔGhyd).

Determination of morphine at gold nanoparticles/Nafion® carbon paste modified sensor electrode

Abstract: A novel and effective electrochemical sensor for the determination of morphine (MO) in 0.04 mol L−1 universal buffer solution (pH 7.4) is introduced using gold nanoparticles electrodeposited on a Nafion modified carbon paste electrode. The effect of various experimental parameters including pH, scan rate and accumulation time on the voltammetric response of MO was investigated. At the optimum conditions, the concentration of MO was determined using differential pulse voltammetry (DPV) in a linear range of 2.0 × 10−7 to 2.6 × 10−4 mol L−1 with a correlation coefficient of 0.999, and a detection limit of 13.3 × 10−10 mol L−1, respectively. The effect of common interferences on the current response of morphine namely ascorbic acid (AA) and uric acid (UA) is studied. The modified electrode can be used for the determination of MO spiked into urine samples, and excellent recovery results were obtained.

The Stability of bFGF Against Thermal Denaturation

Abstract: The influence of sulphated ligand and pH on thermal denaturation of basic fibroblast growth factor (bFGF) was investigated by differential scanning calorimetry (DSC), and verified by fluorescence spectrophotometry. Purity of bFGF before and after heat denaturation was assessed by SDS-PAGE analysis. In DSC studies the samples were heated to 95 degrees C. The midpoint of the temperature change in the thermogram was designated as Tm. Sulphated ligand experiments were undertaken in potassium phosphate (pH 6.5) and sodium acetate buffers. Control thermograms (with no ligand) showed a Tm at 59 degrees C in potassium phosphate buffer. Higher Tm values were noted as sulphated ligand concentration was increased. Similarly when heparin was added, the Tm moved to a higher temperature. A ratio as low as 0.3:1 of heparin to bFGF, increased the Tm to 90 degrees C, which is a 31 degrees C shift in Tm. The effect of pH on thermal denaturation of bFGF was studied in a citrate-phosphate-borate buffer system. A shift in Tm from 46 to 65 degrees C was observed as the pH is changed from 4 to 8. Changes in protein conformation as a function of pH were monitored by fluorescence spectroscopy. It was found that a pH range from 5 to 9 is optimal for the stability of bFGF formulations. In a stability study it was noted that heparin protected bFGF from thermal denaturation only at high temperature.

Effect of pH on the interaction of baicalein with lysozyme by spectroscopic approaches.

Abstract: The interaction mechanism of baicalein and lysozyme (Lys) has been characterized by fluorescence, synchronous fluorescence, ultraviolet-vis absorbance, and three-dimensional (3D) fluorescence. The structural characteristics of baicalein and Lys were probed, and their binding affinities were determined under different pH conditions (pH 7.4, 4.5, and 2.5). The results showed that the binding abilities of the drug to Lys increased under lower pH conditions (pH 4.5 and 2.5) due to the alterations of the protein secondary and tertiary structures or the structural change of baicalein. The effect of baicalein on the conformation of Lys was analyzed using UV, synchronous fluorescence and three-dimensional (3D) fluorescence under different pH conditions. These results indicate that the binding of baicalein to Lys causes apparent change in the secondary and tertiary structure of Lys. In the presence of Cu(2+), the decrease of the binding constant in buffer solution of pH 2.5 may result from the competition of the metal ion and baicalein binding to Lys. In addition, the presence of Cu(2+) increased the binding constants of baicalein-Lys complex under higher pH conditions (pH 7.4 and 4.5). The possible site of binding of baicalein to Lys has been proposed to explain these observations.

Diffusion coefficient in native mucus gel of rat small intestine.

Abstract: The diffusion coefficients of [3H] water, urea, benzoic acid, antipyrine, aminopyrine, alpha-methyl-glucoside, L-phenylalanine and of hydrogen ions were measured at 38 degrees C in native mucus gel from rat small intestine. The diffusion in the gel was reduced to 37-53% (for hydrogen ions to 7%) compared with buffer solution. In addition, the buffering capacity of the gel retarded the permeation of hydrogen ions before a steady state flux was attained. A model calculation revealed that in the preparation a gel layer of 80 microns thickness represents 23% of the total permeation resistance for substances with high epithelial permeability. The aqueous part of the pre-epithelial diffusion resistance amounts to 77% of the total resistance.

Shift of pH-Value During Thermal Treatments in Buffer Solutions and Selected Foods

Abstract: The pH value is one of the most important process parameters during thermal treatments, regardless if the medium is a simple buffer solution or a complex food matrix. When the temperature increases after an initial measurement of the pH at ambient temperature (25°C), a significant pH shift could occur, which could produce incomparable results in different buffer solutions or lead to side reactions during food preservation. Consequently, a measurement cell was constructed to record online the pH-value and temperature up to 130°C. By applying the Nernst equation, it was possible to exclude the temperature-dependent influence of the pH glass electrode on the total pH value. The pH shift was measured over a wide temperature range (ΔT 20–130°C) in the most commonly used buffer solutions and some selected food matrices. The ΔpH of certain buffer solutions, namely TRIS and ACES, showed a significant pH decrease of −2.01 ± 0.08 (ΔT 20–130°C) and −1.27 ± 0.1 (ΔT 20–130°C), respectively, whereas the pH of PBS buffe...

Surface-enhanced Raman spectroscopy investigation of the potential-dependent acid-base chemistry of silver-immobilized 2-mercaptobenzoic acid.

Abstract: The acid−base chemistry of 2-mercaptobenzoic acid (2-MBA) immobilized on a polycrystalline silver surface was investigated by surface-enhanced Raman spectroscopy under potential control. The COO− bending mode of the benzoate form and the C−COOH stretching mode of the benzoic acid form of 2-MBA were used to determine the relative deprotonated and protonated populations of the bound ligand, respectively. In addition, shifts in the symmetric carboxylate stretching mode of 2-MBA reveal interactions between the benzoate group and the silver surface, interactions which could be displaced by acetate and other buffer anions from solution. It was found that the applied potential has a significant effect on the proton dissociation equilibrium of immobilized 2-MBA. This effect arises from the surface potential governing the activity of protons at the interface, which changes the interfacial pH relative to bulk solution. The results are fit to a Poisson−Boltzmann model, corrected for potential distribution across the...

Spectrofluorimetric determination of folic acid in tablets and urine samples using 1,10-phenanthroline-terbium probe.

Abstract: A new spectrofluorimetric method was reported for the determination of folic acid (FA), based on its quenching effect on the fluorescence intensity of Tb3+–1,10-phenanthroline complex as a fluorescent probe. The quenched fluorescence intensity at an emission wavelength of 545 nm was proportional to the concentration of FA in Tris–HCl buffer solution of pH 6.2. The effects of pH, time, order of addition of reagents, temperature and concentrations of Tb3+, buffer and 1,10-phenanthroline were investigated and optimized. The linear range for the determination of FA was 0.01–1.1 mg/L. The detection limit was 0.003 mg/L and the relative standard deviation for replicated determination of 1 mg/L of folic acid was 1.2%. This method was simple, practical and relatively free from interference effects. It was successfully applied to assess FA in pharmaceutical tablets and urine samples. Copyright © 2010 John Wiley & Sons, Ltd.

Voltammetric studies of the interaction of rutin with DNA and its analytical applications on the MWNTs–COOH/Fe3O4 modified electrode

Abstract: Multi-walled carbon nanotubes functionalized with a carboxylic acid group (MWNTs–COOH)/iron oxide (Fe 3 O 4 ) modified glassy carbon electrode (MWNTs–COOH/Fe 3 O 4 /GCE) and DNA/MWNTs–COOH/Fe 3 O 4 /GCE were prepared. The electrochemical behaviors of rutin (RU) were investigated on MWNTs–COOH/Fe 3 O 4 /GCE by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) in britton–robinson buffer solution (B–R). The interaction of RU with DNA was also explored. Dramatic decrease of peak current without obvious peak potential shift were observed in both cases of DNA in the solution and immobilized on the electrode surface. In addition, the electron transfer coefficient ( α ) and the rate constant ( k s ) kept unchanged in the absence and presence of DNA. So interaction of DNA with RU formed a non-electroactive complex. The binding constant and binding ratio was obtained in the process. The interaction was also confirmed by UV–visible spectroscopy. The reduction peak current was linear with the concentration of RU in the range of 2.50 × 10 −8 to 1.37 × 10 −6 M, with a detection limit of 7.5 nM. The MWNTs–COOH/Fe 3 O 4 /GCE showed comparatively low detection limit, rapid response, simplicity for the determination of RU.

Sensitive voltammetric determination of rutin in pharmaceuticals, human serum, and traditional Chinese medicines using a glassy carbon electrode coated with graphene nanosheets, chitosan, and a poly(amido amine) dendrimer

Abstract: The electrochemical behavior of rutin was investigated in pH 6.0 buffer solution using a glassy carbon electrode coated with graphene nanosheets, chitosan and a poly (amidoamine) dendrimer in pH 6.0 buffer solution. The results indicate that the modified electrode displays electrochemical redox activity towards rutin, and that the oxidation peak current of rutin increases significantly compared to that at other electrodes. The amount of immobilized graphene and dendrimer, pH value, scan rate, accumulation time and accumulation potential were optimized. The kinetic parameters, charge transfer coefficient, transfer electron number, proton transfer number, standard rate constant, were calculated. Under the optimized conditions, the oxidation peak current is proportional to the concentration of rutin in the range between 0.001 and 2.0 μmol L−1 (R = 0.9991). The detection limit is 0.6 nmol L−1 (at S/N = 3). The electrode exhibits satisfactory selectivity and reproducibility and was applied to the determination of rutin in pharmaceutical preparations, spiked human serum, and traditional Chinese medicine, with recoveries between 97.2 and 104.67%.

Spectroscopic Studies on the Oxidation of Catechin in Aqueous Solution

Abstract: The spectroscopic behavior of catechin (5,7,3',4'-tetrahydroxyflavan-3-ol), has been studied in the presence and the absence of air using UV-vis absorption spectrophotometry and fluorescence spectroscopy. The UV-vis absorption spectrum of catechin shows a very sharp and strong absorption maximum peak at 275 nm in deaerated water. New absorption maximum peaks appeared in aerated water, as well as in basic aqueous solution, caused by the oxidation of catechin. The absorbances in the UV-vis absorption spectrum of catechin decreased when the solution was left in the dark for a long time. The fluorescence emission spectrum of catechin after a long time period differs markedly from that in freshly prepared solution; the fluorescence maxia shifted as time passes after adding catechin to the solutions. When the deaerated basic catechin solutions were left in the dark for a long time, their fluorescence quantum yields were found to be nearly zero. This suggests that the oxidized catechin molecules were seen to have slowly undergone successive chemical reactions in basic buffer solution.