Ascorbate Anion Research Articles

Modification of carbon paste with congo red supported on multi-walled carbon nanotube for voltammetric determination of uric acid in the presence of ascorbic acid

A chemically modified carbon-paste electrode (CPE) is prepared by incorporating congo red (CR) immobilized on multi-walled carbon nanotube (MWCNT). The results show that CR is effectively immobilized on the surface of MWCNT under the ultrasonic agitation in aqueous solution and further incorporating the nafion. The prepared electrode, due to the electrostatic repulsions between the CR and ascorbate anion, is capable to mask the response of the ascorbic acid (AA) completely and provide an effective method for the detection of minor amounts of uric acid (UA) in the presence of high concentrations of AA. On the other hand, an increase in the microscopic area of the electrode by addition of MWCNT together with the electrocatalytic activity caused to a significant enhancement in the voltammetric response to UA. Optimization of the amounts of composite modifier in the matrix of CPE is performed by cyclic and differential pulse voltammetric measurements. The modified electrode shows a linear response to UA in the range of 1.0 × 10−7–1.0 × 10−4 M with a detection limit of 1.0 × 10−8 M. The electrode exhibits excellent accuracies for the determination of UA in the presence of high concentrations of AA (a recovery of 97.6%). The response of the electrode toward sulfhydryl compounds such as cysteine, penicillamine, and glutathione is not considerable. This reveals a good selectivity for the voltammetric response toward UA. The effective electrocatalytic property, ability for masking the voltammetric responses of the other biologically reducing agents, ease of preparation, and surface regeneration by simple polishing together with high reproducibility and stability of the responses make the modified electrode suitable for the selective and sensitive voltammetric detection of sub-micromolar amounts of UA in clinical and pharmaceutical preparations.

Electrochemiluminescence of Tris(2,2′‐bipyridine)ruthenium with Various Co‐reactants under Ultrasound Irradiation

AbstractElectrochemiluminescence (ECL) of tris(2,2′‐bipyridine)ruthenium, Ru(bpy)32+ in the presence of various co‐reactants, such as tripropylamine (TPA), oxalate ion (C2O42−), ascorbic acid (H2A) and dehydroascorbic acid (DHA), were investigated under ultrasound irradiation. In sono‐ECL experiments, an indium‐thin‐oxide (ITO) was used as working electrode, and a titanium tipped sonic horn probe (diameter 2 mm) which operated at a frequency of 20 kHz was set in the front of the ITO electrode. Under the ultrasound irradiation, ECL signals were found to be significantly enhanced when TPA and C2O42− were used as co‐reactants, only slightly enhanced in Ru(bpy)32+/DHA system, but total quenched in Ru(bpy)32+/H2A system. The difference of Ru(bpy)32+ ECL behaviors for various co‐reactant could to be due to the different kinetics of catalytic reactions associated in ECL schemes. ECL quenching effect observed in Ru(bpy)32+/H2A system was suggested to be due to electron transfer (ET) route between the excited state *Ru(bpy)32+ and ascorbate anion HA− diffused from the bulk solution, where the diffusional HA− species served as electron donor. The effect becomes more pronounced upon sonication because the effective collision frequency between *Ru(bpy)32+ and HA− would be significantly increased by the enhanced mass transport effect of ultrasound.

Self‐quenching in the electrochemiluminescence of Ru(bpy)32+ using ascorbic acid as co‐reactant

In this study, electrochemiluminescence (ECL) of Ru(bpy)(3)(2+) (bpy = 2,2'-bipyridyl) using ascorbic acid (H(2)A) as co-reactant was investigated in an aqueous solution. When H(2)A was co-existent in a Ru(bpy)(3)(2+)-containing buffer solution, ECL peaks were observed at a potential corresponding to the oxidation of Ru(bpy)(3)(2+), and the intensity was proportional to H(2)A concentration at lower concentration levels. The formation of the excited state *Ru(bpy)(3)(2+) was confirmed to result from the co-reaction between Ru(bpy)(3)(3+)and the intermediate of ascorbate anion radical (A(-)*), which showed the maximum ECL at pH = 8.8. It is our first finding that the ECL intensity would be quenched significantly when the concentration of H(2)A was relatively higher, or upon ultrasonic irradiation. In most instances, quenching is observed with four-fold excess of H(2)A over Ru(bpy)(3)(2+). The diffusional self-quenching scheme as well as the possible reaction pathways involved in the Ru(bpy)(3)(2+)-H(2)A ECL system are discussed in this study.

Electrical Conductance Studies in Aqueous Solutions with Aspartic Ions

Conductivity measurements in dilute aqueous solutions of L-ascorbic acid, sodium-L-ascorbate, magnesium-L-ascorbate, calcium-L-ascorbate and ferrous-L-ascorbate were performed in the (288.15 to 323.15) K temperature range. The limiting molar conductances of the ascorbic anion, λ∘(HAsc−, T), and the dissociation constants of ascorbic acid, K(T), were derived by the use of the Debye-Huckel equation for the activity coefficients and the Onsager and Quint and Viallard conductivity equations.

Nanocomposite of functionalized multiwall carbon nanotubes with nafion, nano platinum, and nano gold biosensing film for simultaneous determination of ascorbic acid, epinephrine, and uric acid

A unique bimetallic, nano platinum (Pt) with nano gold (Au) on nafion (NF) incorporated with functionalized multiwall carbon nanotubes ( f-MWCNTs) composite film ( f-MWCNTs–NF–PtAu) was developed by the potentiostatic method. The composite film exhibits promising efficient catalytic activity towards the oxidation of mixture of biochemical compounds and simultaneous measurement of ascorbate anion, epinephrine and urate anion in aqueous buffer solution (pH 6.75). Both, the cyclic voltammetry (CV) and differential pulse voltammetry (DPV) were used for the measurement of electroanalytical properties of neurotransmitters by means of composite film modified electrodes. Well-separated voltammetric peaks were obtained for ascorbate, epinephrine and urate anions with the peak separations of 0.222 and 0.131 V. The composite film can also be produced on gold and transparent semiconductor indium tin oxide electrodes for different kinds of studies such as electrochemical quartz crystal microbalance (EQCM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The incorporation of Pt and Au onto the f-MWCNTs–NF was revealed by the EQCM technique and the morphology of the film was studied using SEM, AFM and scanning electrochemical microscopy (SECM) techniques. Further, extensive studies were carried out using SECM for obtaining the surface current topographic images of composite film modified electrodes, and these indicated the presence of f-MWCNTs–NF–PtAu composite film on the electrode.

Critical Concentrations and Role of Ascorbic Acid (Vitamin C) in the Crystallization of Gold Nanorods within Hexadecyltrimethyl Ammonium Bromide (CTAB)/Tetraoctyl Ammonium Bromide (TOAB) Micelles

Growth of gold nanorods is investigated in the presence of ascorbic acid in an aqueous mixed micelle solution of hexadecyltrimethyl ammonium bromide (CTAB) and tetraoctyl ammonium bromide (TOAB) through photochemical reduction of HAuCl4·3H2O using 300 nm Hg lamps. The role of the ascorbic acid concentration, [H2A], in the formation of gold nanorods was studied using UV−vis absorption and transmission electron microscopy. Gold nanorods were produced only if [H2A]/[Au3+] ≥ 0.75 in the reaction cell. For 0.75 < [H2A]/[Au3+] < 6.2, the aspect ratio of nanorods increased as [H2A] increased. For [H2A]/[Au3+] > 6.2, the aspect ratio decreased, and shorter nanorods formed as [H2A] increased. These results are indicative of a lower and upper limit in the concentrations of ascorbic acid and its conjugate base (ascorbate anion) for the formation of gold nanorods. The roles of ascorbic acid and ascorbate ions are discussed, and these observations support the existence of soft templates formed by CTAB/TOAB micelles where Au nanorods grow. Evidence for parallel nucleation and growth processes is also presented.

Hyaluronan degradation by copper(II) chloride and ascorbate: rotational viscometric, EPR spin-trapping, and MALDI–TOF mass spectrometric investigations

The degradation of high-molar-mass hyaluronan (HA) by copper(II) chloride and ascorbate was studied by means of rotational viscometry. It was found that even small amounts of CuCl 2 present in the oxidative system led to the pronounced degradation of HA, reflected in a rapid decrease of the dynamic viscosity of the biopolymer solution. Such degradation was induced by free radicals generated in elevated amounts in the presence of copper ions. Electron paramagnetic resonance investigations performed on a model oxidative system containing Cu(II) and ascorbic acid proved the formation of relatively stable ascorbate anion radicals resulting from the reaction of ascorbic acid with hydroxyl radicals. In this way, by scavenging the hydroxyl radicals, ascorbic acid protected HA from their degradative action. Matrix-assisted laser desorption ionization–time-of-flight (MALDI–TOF) mass spectrometry was applied to analyze the degraded HA. The results showed that only regular fragmentation of hyaluronan occurred using the mentioned oxidative system that led to the formation of HA oligomers with unaffected primary chemical structure.

Hypotonic shock stimulates ascorbate release from coronary artery endothelial cells by a Ca 2+-independent pathway

In endothelial cells, anion channels open upon osmotic swelling during shear stress and hypotonic shock. Therefore, we examined the effects of hypotonic shock on release of the antioxidant anion ascorbate from pig coronary artery endothelial cells. Hypotonic shock potentiated ascorbate release from freshly isolated or cultured pig coronary artery endothelial cells; subsequently cultured endothelial cells were used. The hypotonic shock-induced increase in Asc release was rapid, depended on the degree of hypotonic shock, and not due to membrane leakiness. Stimulating P2Y 2 like receptors in endothelial cells with ATP causes ascorbate release via a Ca 2+-mediated pathway. Hypotonic shock-induced release differed from the Ca 2+-mediated Asc release because: (a) the increase in release with hypotonic shock was additive to that with ATP or A23187 (Ca 2+-ionophore), (b) apyrase, suramin or removing extracellular Ca 2+ did not affect the hypotonic shock-stimulated release, (c) anion channel blockers inhibited the release by the two pathways differently, and (d) hypotonic shock increased the ascorbate release from endothelial cells and cultured smooth muscle cells whereas the Ca 2+-mediated ascorbate release occurred only in endothelial cells. Accumulation of ascorbate by endothelial cells was examined at extracellular ascorbate concentrations of 10 (Na +-ascorbate symporter not saturated) and 5000 μM (Na +-ascorbate symporter saturated). Hypotonic shock and A23187 decreased ascorbate accumulation at 10 μM ascorbate but increased it at 5000 μM. The effects of the two treatments were additive and also differed from each other with substitution of gluconate for extracellular chloride. Thus, ascorbate release from endothelial cells can be potentiated by two distinct pathways — hypotonic shock mediated and ATP/Ca 2+ stimulated.

Electrocatalysis and simultaneous detection of dopamine and ascorbic acid using poly(3,4-ethylenedioxy)thiophene film modified electrodes

A novel voltammetric method using the PEDOT (poly(3,4-ethylenedioxy)thiophene) modified electrode was developed for simultaneous measurement of various combinations of dopamine (DA) and ascorbate anion (AA). Both the cyclic voltammetry (CV) and square wave voltammetry were used for the measurement of neurotransmitters by means of the PEDOT modified electrodes. Well-separated voltammetric peaks were observed for dopamine and ascorbate anion at the PEDOT modified electrodes with peak separation of 0.170 V. The electrostatic interaction between the heteroatom of PEDOT film and ascorbate anion was confirmed by adsorption of molybdate ions. A hydrophobic interaction was existed between the aromatic part of dopamine and the PEDOT film. The RRDE (rotating ring disk electrode) method was applied to study the redox reaction mechanism of dopamine and ascorbate anion. The Levich plot indicated that the electrocatalytic reaction between ascorbate anion and dopamine was mass transfer-controlled. The value of ‘diffusion co-efficient’ of dopamine and ascorbate anion were calculated to be 6.1 · 10 � 6 cm 2 s � 1 and 9.2 · 10 � 6 cm 2 s � 1 .

Simultaneous determination of dopamine and ascorbic acid on poly (3,4-ethylenedioxythiophene) modified glassy carbon electrode

Detection of dopamine (DA) in the presence of excess of ascorbic acid (AA) has been demonstrated using a conducting polymer matrix, poly (3,4-ethylenedioxythiophene) (PEDOT) film in neutral buffer (PBS 7.4) solution. The PEDOT film was deposited on a glassy carbon electrode by electropolymerization of EDOT from acetonitrile solution. Atomic force microscopy studies revealed that the electrodeposited film was found to be approximately 100 nm thick with a roughness factor of 2.6 nm. Voltammetric studies have shown catalytic oxidation of DA and AA on PEDOT modified electrode and can afford a peak potential separation of ∼0.2 V. It is speculated that the cationic PEDOT film interacts with the negatively charged ascorbate anion through favorable electrostatic interaction, which results in pre-concentration at a less anodic value. The positively charged DA tends to interact with the hydrophobic regions of PEDOT film through hydrophobic–hydrophobic interaction thus resulting in favorable adsorption on the polymer matrix. Further enhancement in sensitivity to micro molar level oxidation current for DA/AA oxidation was achieved by square wave voltammetry (SWV) which can detect DA at its low concentration of 1 μM in the presence of 1000 times higher concentration of AA (1 mM). Thus the PEDOT modified electrode exhibited a stable and sensitive response to DA in the presence of AA interference.

Immobilization of DNA on carbon fiber microelectrodes by using overoxidized polypyrrole template for selective detection of dopamine and epinephrine in the presence of high concentrations of ascorbic acid and uric acid

The overoxidized polypyrrole (PPyox) film as a template for DNA immobilization has been demonstrated in this paper. The DNA molecules inserted into the micropores of the ultrathin PPyox matrix under the driving forces of an electric field and were firmly immobilized on the carbon fiber electrode (CFE). Such a DNA-PPyox biocomposite layer exhibited more effective rejection of anionic ascorbate (AA) and uric acid (UA) and more preferential collection of the cationic dopamine (DA) and epinephrine (EP) than pure PPyox and DNA coatings. The DPV peak currents increased linearly with increasing DA and EP concentrations in the range of 3.0 x 10(-7) to 1.0 x 10(-5) M and 5.0 x 10(-7) to 2.0 x 10(-5) M with the lowest detected concentrations of 8.0 x 10(-8) M and 6.0 x 10(-8) M, respectively. The electrochemical signal of AA could be totally suppressed under a concentration of 20 mM and beyond this concentration, the overlapped responses of AA, DA/EP and UA could be resolved into three well-defined voltammetric peaks. The selectivity factors k(DA/AA) and k(EP/AA) were about 5000 and 2000 for an equal concentration in the presence of 0.5 mM UA. The properties of the biocomposite film have been characterized by atomic force microscopy and electrochemical investigations.

Calorimetric and spectrophotometric study on the interaction of hydroxypropyl-β-cyclodextrin with ascorbic acid

Hydroxypropyl-β-cyclodextrin forms a 1:1 molecular complex with ascorbic acid and exhibits slight affinity to the ascorbate anion in aqueous solutions, as found by calorimetry and UV spectrophotometry.

Electrochemistry of cytochrome c incorporated in Langmuir–Blodgett films of Nafion® and Eastman AQ 55®

Ultrathin films of Nafion® and Eastman-AQ 55® loaded with cytochrome c (cyt c) were obtained and transferred on indium tin oxide (ITO) electrodes via the Langmuir–Blodgett (LB) technique. The pressure-area isotherms for mixed ionomer-protein films indicate that the miscibility of cyt c in the interfacial layer is better for Nafion® than for AQ 55. Interestingly, these composite films maintain the electroactivity of cyt c without requiring the addition of promoters or mediators. Both for AQ 55-cyt c and Nafion-cyt c films, the half-wave potential for the reversible reduction of ferricytochrome c corresponds to the value expected for the weakly adsorbed protein. The modified electrodes show electrocatalytic reaction with ascorbate anion. Comparison with previous literature reports indicate that for Nafion the LB coating procedure is unique in keeping the electroactivity of cyt c.

Organic molecules and nanoparticles in inorganic crystals: Vitamin C in CaCO3 as an ultraviolet absorber

Organic molecules and nanoparticles embedded in inorganic crystalline lattices have been studied to add different properties and functions to composite materials. Calcium carbonate was precipitated by dropping an aqueous solution of CaCl2 into that of Na2CO3 containing dissolved vitamin C (ascorbic acid). The optical absorption ascribed to divalent ascorbate anions in the lattice was observed in the ultraviolet B (wavelength: 280–315 nm) region, while solid vitamin C exhibited absorption in the ultraviolet C (100–280 nm) region. The divalent ascorbate anion is only stable in CaCO3 due to the absence of oxygen molecules. Doping CaCO3 with nanoparticles of ZnO also enhanced the absorption in the ultraviolet A (315–380 nm) region. These composite materials are suggested for use as UV absorbers.

Selective determination of dopamine using unmodified, exfoliated graphite electrodes

Selective determination of a neurotransmitter, dopamine (DA), is achieved in the presence of ascorbic acid (AA) at neutral pH on an inert, unmodified exfoliated graphite electrode. The edge planes, exposed by roughening the surface of the electrode have been found to be very active for electrochemical sensing. A separation of 0.24 V in the oxidation potential and an increase of 100 times in the currents between DA and AA have been observed with the graphite exfoliated at 800 °C. The exfoliation temperature is found to play a major role in determining the activation of the surface and in turn the effective separation of the analytes. This is attributed to the trend in the O/C ratios on the graphite electrodes observed using X-ray photoelectron spectroscopy. The separation is mainly due to the repulsive interactions between the functional groups on the electrode surface and the ascorbate anion. This is confirmed by comparing the point of zero charge (pH PZC) of the electrode surface determined using pH titration and the p K a of AA. A very low detection limit of 50 nM of DA in the presence of 100 μM ascorbic acid has been observed using chronoamperometry. The exfoliated graphite is an unmodified electrode and hence just polishing the electrode could expose the active surface. The analytical utility of the exfoliated graphite electrodes for the selective determination of DA in the presence AA is presented.

The dual role of self-assembled monolayers of a tetraazamacrocyclic copper(II) complex in ascorbate oxidation catalysis

The copper(II) complex with 1,4,8-trimethyl-11-(2-thioethyl)-1,4,8,11-tetraazacyclotetradecane was adsorbed on the surface of glassy carbon (GCE) and gold electrodes using the self-assembly method. More stable coverage of the electrode was obtained using a gold substrate when the complex is anchored to the electrode through its thioethyl substituent. The amount of the complex on the electrode surface based on the charge of the Cu 2+/Cu + peak points to the nearly monolayer coverage of the electrode surface. The catalytic effect of the monolayer towards ascorbic acid oxidation was observed in neutral and alkaline solutions, and ascribed to the electrostatic interaction of ascorbate anion with the positively charged copper complex bound to the electrode surface. In dioxygenated solutions, the copper complex adsorbed on the electrode catalyses both the electrooxidation and auto-oxidation of ascorbate. The copper-catalyzed autoxidation of ascorbic acid leads to a decrease of the ascorbate electrooxidation peak with time. The progress of this reaction was monitored based on the changes of the electrooxidation current. Copper catalyzed autoxidation of ascorbate is known to require turnover between Cu 2+ and Cu + ions. This indicates that the molecules of the copper complex anchored through the thiol groups to the gold electrode, and tightly packed on the surface play a dual role in the ascorbate oxidation process. The role of the monolayer in enhancing the autoxidation of ascorbic acid resembles the function of copper in some biological complexes catalyzing in vivo ascorbate oxidation.

Simultaneous determination of the monoamine neurotransmitters and glucose in rat brain by microdialysis sampling coupled with liquid chromatography-dual electrochemical detector

A new type of chemically modified electrode based ring–disk electrode as the dual electrochemical detector (DECD) for high-performance liquid chromatography (HPLC) to simultaneously determine the monoamine neurotransmitters and glucose is described. The ring electrode was modified with an ion-exchange polymer-overoxidized polypyrrole (OPPy) and the disk electrode was modified with nano Au colloid and glucose oxidase (GOD). The electrochemical behaviors of dopamine (DA) and ascorbic acid (AA) at the OPPy chemically modified electrode (CME) were investigated by differential pulse voltammetry (DPV). It was found that the CME could permeate dopamine cations and repelled the ascorbate anions, which could be used to determine the monoamine neurotransmitters and avoid the interference of AA. The electrochemical behavior of glucose at the Nafion/GOD–Au colloid/GC CME was investigated by amperometry and flow injection analysis (FIA). It was found that the sensitivity of the CME increased apparently in determination of glucose. In order to obtain better separation and current responses of the analytes in HPLC-DECD, several operational parameters have been investigated. Under the optimum conditions, the method showed good stability and reproducibility. The application of this method coupled with microdialysis sampling for in vivo simultaneous determination of monoamine neurotransmitters and glucose in rat brain was satisfactory.

Electrochemical examination of the ascorbic acid radical anion in non-aqueous electrolytes

A quasi-reversible redox reaction involving ascorbic acid was observed in non-aqueous electrolytes at conductive diamond electrode. The chemical reversibility of these reactions is consistent with ascorbic acid being reduced to the ascorbic acid radical anion in a one-electron process, with subsequent reoxidation to ascorbic acid. This is the first report on the electrochemical production of the ascorbic acid radical anion in non-aqueous electrolytes. Ascorbyl 6-stearate and 4-hydroxy 2(5H)-furanone, which have somewhat similar structures as ascorbic acid, also showed one-electron transfer reduction reaction producing radicals with a single negative charge, suggesting that these compounds follow the same electrochemical behavior as ascorbic acid. The double bond and hydroxyl substituent on the five-membered ring are shown to be necessary for the stabilization of the radical anions. It was confirmed by the calculation of the total energy using molecular orbital methods that resonance structures involving the double-bond and hydroxyl group provide significant stabilization of the radical anions. Electrochemical preparation may be a useful method for the detailed study of radicals, their molecular structure and reactivity.

A Comparison of Atmospheric and Electrochemical Oxidation of Vitamin C in SDS Systems

ABSTRACT Ascorbic acid (AA, H2A) can be solubilized up to 60% by weight into a microemulsion formed by water, pentanol, and sodium dodecylsulfate (SDS). Addition of AA to the aqueous solution caused systematic reduction of the microemulsion region in the phase diagrams of the system. Ascorbic anion interacts with dodecylsulfate association structures changing W/O microemulsions towards O/W systems. The polar part of AA is exposed to the aqueous solution and subject to oxidation. The results of kinetic determination of AA decomposition are in accordance with a suggested model. They show enhanced oxidation of AA with increasing pentanol concentration in the system. The system aqueous AA solution/SDS, of concentration in excess of critical micellization concentration (cmc), was found to be more resistant to oxidation in air than aqueous AA solution. This was confirmed by electrochemistry studies of the anodic oxidation of AA on glassy carbon electrode. Sodium dodecylsulfate shifted the AA oxidation potential E pa to more positive values and decreased the peak current I pa . Antioxidation activity of AA was diminished in the presence of SDS micelles.

Kinetics and mechanism for reduction of halo- and haloam(m)ine platinum(IV) complexes by l-ascorbate

Reduction of the model platinum(IV) complexes cis-[PtCl 4(NH 3) 2] ( 1), trans-[PtCl 4(NH 3) 2] ( 2), trans-[PtCl 2(en) 2] 2+ ( 3), trans-[PtBr 2(NH 3) 4] 2+ ( 4), [PtCl 6] 2− ( 5), and [PtBr 6] 2− ( 6) with l-ascorbic acid (H 2Asc) in 1.0 M aqueous medium at 25 °C in the region 1.75≤pH≤7.20 has been investigated using stopped-flow spectrophotometry. The redox reactions follow the rate law: −d[Pt(IV]/d t= k[H 2Asc] tot[Pt(IV)] where k is a pH-dependent second-order rate constant and [H 2Asc] tot, the total concentration of ascorbic acid. The pH-dependence of k is attributed to parallel reduction of Pt(IV) by the protolytic species HAsc − and Asc 2−. Analysis of the kinetics data reveals that the ascorbate anion Asc 2− is up to seven orders of magnitude more reactive than HAsc − while H 2Asc is unreactive. Electron transfer from HAsc −/Asc 2− to the Pt(IV) compounds is suggested to take place by a mechanism involving a reductive attack on any one of the mutually trans-halide ligands by Asc 2− and/or HAsc − forming a halide-bridged activated complex. The rapid reduction of these complexes supports the assumption that ascorbate Asc 2− might be an important reductant at physiological conditions for anticancer active Pt(IV) pro-drugs capable of undergoing reductive trans elimination. The parameters Δ H ≠ and Δ S ≠ for reduction of Pt(IV) with Asc 2− have been determined from the study of the temperature dependence of k.