Oxidation Of AA Research Articles

Nickel, copper and manganese hexacyanoferrate with poly(3,4-ethylenedioxythiophene) hybrid film modified electrode for selectively determination of ascorbic acid

The NiHCFPEDOT, CuHCFPEDOT and MnHCFPEDOT films were prepared on glassy car� bon electrode (GCE) by multiple scan cyclic voltammetry and characterized using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) techniques. The advantages of these films are demonstrated for selectivity detection of ascorbic acid using cyclic voltammetry and amperometric method. Interestingly, the NiHCFPEDOT and CuHCFPEDOT modified electrodes exhibited a wide linear response range (5 × 10 -6 -3 × 10 -4 M, R 2 = 0.9973 and 1.8 × 10 -3 -1.8 × 10 -2 M, R 2 = 0.9924). The electrochemical sensors facilitated the oxidation of AA but not responded to other electroac� tive biomolecules such as dopamine, uric acid, H2O2, glucose. The difference is MnHCFPEDOT/GCE that no response to AA. In addition, the NiHCFPEDO T and CuHCFPEDOT modified electrodes exhibited a distinct advantage of simple preparation, specificity, stability and reproducibility.

Determinação simultânea de ácido ascórbico e ácido acetilsalicílico usando análise por injeção em fluxo com detecção amperométrica pulsada

A simple flow system with multiple pulse amperometric detection using a single working electrode is proposed for simultaneous determination of ascorbic (AA) and acetylsalicylic (AAS) acids in pharmaceutical formulations. The procedure is based on application of two potential pulses: 0.90 V/50 ms: oxidation and determination of AA without the interference of AAS; 1.35 V/50 ms: oxidation of both compounds and quantification of AAS by current subtraction using a correction factor. Sampling rate was estimated as 125 injections per hour and the limits of detection were 0.17 and 0.16 µmol L-1 for AA and AAS, respectively. Results for commercial samples agreed with those obtained using HPLC.

Dietary proteins poorly contribute to the endogenous production of glucose in humans after egg ingestion

A high protein meal is considered to activate gluconeogenesis with a potential link with the satiating power of proteins. However, the contribution of dietary proteins to gluconeogenesis has never been directly determined. We used a multiple tracer methods, including the intrinsic and uniform labeling of egg proteins with 15N and 13C. After fasting overnight, eight healthy volunteers were intravenously infused with [6,6‐2H]‐glucose. Two hours later, they ingested 4 uniformly labeled eggs, cooked as an omelette (23g protein). The metabolic fate of 15N and 13C was followed through metabolic pools, and the endogenous production of glucose was determined for 8 h. After 4 h, oxidation of dietary AA was only the half of deamination while they were similar after 8 h, reaching 18 % of ingested proteins (i.e. 4.1 g). The endogenous production of glucose did not increase after meal ingestion, and significantly decreased after 6 h. The contribution of dietary amino acid to glucose was maximal between 3 and 5 h (representing 5% of produced glucose). On the 53 g of glucose that were produced trough 8 h, 1.8 g originated from dietary proteins (i.e. 3.4% of produced glucose and 44% of deaminated AA). We conclude that after fasting overnight and in the absence of glucose in the meal, a significant part of dietary AA‐derived carbon skeletons from deamination are converted to glucose but total dietary proteins poorly contributes to glucose production.

Facile synthesis of NiO nanoflowers and their electrocatalytic performance

In this paper, hierarchically structured NiO nanoflowers were facile synthesized by incorporating a convenient solution process with a subsequent thermal treating process. Their catalytic activity was then electrochemically investigated in detail. The NiO nanoflower modified biosensor exhibits excellent sensing performance for the determination of l-ascorbic acid with a response time less than 8 s, linear range between 0.005 and 3.5 mM, and sensitivity as 220.4 μA mM −1 cm −2. Besides, a high selectivity towards the oxidation of AA in the presence of dopamine (DA) and nitrite was also observed at their maximum physiological concentrations. The good analytical performance, long-term stability, low cost and straightforward fabrication process made the NiO nanomaterials promising for the development of effective electrochemical sensors for a wide range of potential applications in medicine, biotechnology and environmental chemistry.

Electrochemical deposition of 1-butyl-4-methyl-pyridinium tetrafluroborate ionic liquid on carbon paste electrode and its application for the simultaneous determination of dopamine, ascorbic acid and uric acid

Negatively charged 1-butyl-4-methyl-pyridiniumtetrafluroborate (BMPTB) ionic liquid was electrochemically deposited on the surface of carbon paste electrode (CPE) by cyclic voltammetric (CV) technique in 0.1 M nitric acid. It exhibited an electrocatalytic activity towards the oxidation of DA, AA, and UA in 0.1 M phosphate buffer solution (pH 6.6). The effect of concentration, scan rate, and pH was studied for dopamine. The BMPTB/CPE showed an excellent electrocatalytic activity for selective detection of DA in the presence of AA and UA by using CV and differential pulse voltammetry (DPV) respectively. Detection limit of dopamine was found to be 1 × 10 − 8 M. The present technique provides a novel method for the simultaneous determination of DA, AA and UA in their mixture sample.

Electrochemical determination of vitamin C in the presence of uric acid by a novel TiO2 nanoparticles modified carbon paste electrode

Abstract The electrochemical behavior of vitamin C (ascorbic acid or AA) is investigated on the surface of a carbon-paste electrode modified with TiO2 nanoparticles and 2,2′-(1,2 butanediylbis(nitriloethylidyne))-bis-hydroquinone (BBNBH). The prepared modified electrode showed an efficient catalytic role in the electrochemical oxidation of AA, leading to remarkable decrease in oxidation overpotential and enhancement of the kinetics of the electrode reaction. This modified electrode exhibits well-separated oxidation peaks for AA and uric acid (UA). The modified electrode is successfully applied for the accurate determination of AA in pharmaceutical preparations.

Ascorbic acid oxidation in SDS micellar systems

In this contribution the results of atmospheric and electrochemical oxidation of AA in the SDS micellar solutions and in the microemulsions pentanol/water stabilized by SDS are presented. The hydrophilic vitamin C readily dissolves in water and O/W microemulsions as well as undergoes the irreversible two-electron oxidation reaction. It was found that the atmospheric oxidation of AA is accelerated by the SDS up to the CMC and inhibited in the concentrated SDS solutions. We also found that the rate of atmospheric oxidation of ascorbic acid is higher in the water-in-oil (W/O) than in the oil-in-water (O/W) microemulsions and increases with the increasing oil content. The influence of the SDS on the electrochemical behavior of vitamin C was also studied. The general conclusion emerging from this investigation is that the increasing surfactant concentration shifts the ascorbic acid oxidation potential to higher values whereas the corresponding peak current values diminish. In the microemulsions the AA oxidation is slowed down by increasing the pentanol amount in the system.

Synthesis and Characterization of MWNTs/Au NPs/HS(CH2)6Fc Nanocomposite: Application to Electrochemical Determination of Ascorbic Acid

AbstractIn this article, a detailed electrochemical study of a novel 6‐ferrocenylhexanethiol (HS(CH2)6Fc) self‐assembled multiwalled carbon nanotubes‐Au nanoparticles (MWNTs/Au NPs) composite film was demonstrated. MWNTs/Au NPs were prepared by one‐step in situ synthesis using linear polyethyleneimine (PEI) as bifunctionalizing agent. HS(CH2)6Fc, which acted as the redox mediator, was self‐assembled to MWNTs/Au NPs via Au‐S bond. Transmission electron microscopy (TEM), energy‐dispersive X‐ray analysis (EDX), Fourier transformed infrared absorption spectroscopy (FT‐IR), UV‐visible absorption spectroscopy, and cyclic voltammetry were used to characterize the properties of the MWNTs/Au NPs/HS(CH2)6Fc nanocomposite. The preparation of the nanocomposite was very simple and effectively prevented the leakage of the HS(CH2)6Fc mediator during measurements. The electrooxidation of AA could be catalyzed by Fc/Fc+ couple as a mediator and had a higher electrochemical response due to the unique performance of MWNTs/Au NPs. The nanocomposite modified electrode exhibited excellent catalytic efficiency, high sensitivity, good stability, fast response (within 3 s) and low detection limit toward the oxidation of AA at a lower potential.

Analytical Application of Pyramidal, Rodlike, and Spherical Gold Nanostructures: Simultaneous Detection of Ascorbic Acid and Uric Acid

AbstractSimultaneous detection of ascorbic (AA) and uric acid (UA) is developed at pyramidal (NP), rodlike (NR), and spherical (NS) gold nanostructures, due to their high electrocatalytic activities toward the oxidation of AA and UA. Unlike at bare gold electrode, the fouling resulted from the oxidized product of AA is eliminated at the nanostructured gold electrode. The voltammetric signals of AA and UA are completely separated with a potential difference of 216 mV, 158 mV and 195 mV, respectively, at the pyramidal, rodlike, and spherical gold surfaces. The experimental results reveal that solution pH effects the peak separation of AA and UA, acidic solution is more favorable for the simultaneous determination of AA and UA than neutral one, than alkaline one. The coexistence of a large excess of AA does not interfere with the voltammetric sensing of UA, vice versa. All the three kinds of nanostructured gold electrodes show excellent sensitivity, stability, selectivity, low detection limit, quick response and wide linear range in the repeated detection of AA and UA. The practical utility of the present nanostructured gold electrodes is demonstrating by determining the concentration of AA in fruit juice and UA in urine sample.

L-Ascorbic acid as an alternative fuel for direct oxidation fuel cells

l-Ascorbic acid (AA) was directly supplied to polymer electrolyte fuel cells (PEFCs) as an alternative fuel. Only dehydroascorbic acid (DHAA) was detected as a product released by the electrochemical oxidation of AA via a two-electron transfer process regardless of the anode catalyst used. The ionomer in the anode may inhibit the mass transfer of AA to the reaction sites by electrostatic repulsion. In addition, polymer resins without an ionic group such as poly(vinylidene fluoride) and poly(vinyl butyral) were also useful for reducing the contact resistance between Nafion membrane and carbon black used as an anode, although an ionomer like Nafion is needed for typical PEFCs. A reaction mechanism at the two-phase boundaries between AA and carbon black was proposed for the anode structure of DAAFCs, since lack of the proton conductivity was compensated by AA. There was too little crossover of AA through a Nafion membrane to cause a serious technical problem. The best performance (maximum power density of 16 mW cm −2) was attained with a Vulcan XC72 anode that included 5 wt.% Nafion at room temperature, which was about one-third of that for a DMFC with a PtRu anode.

Spectrophotometric determination of ascorbic acid by the modified CUPRAC method with extractive separation of flavonoids–La(III) complexes

The proposed method for ascorbic acid: AA (Vitamin C) determination is based on the oxidation of AA to dehydroascorbic acid with the CUPRAC reagent of total antioxidant capacity assay, i.e., Cu(II)-neocuproine (Nc), in ammonium acetate-containing medium at pH 7, where the absorbance of the formed bis(Nc)-copper(I) chelate is measured at 450 nm. The flavonoids (essentially flavones and flavonols) normally interfering with the CUPRAC procedure were separated with preliminary extraction as their La(III) chelates into ethylacetate (EtAc). The Cu(I)-Nc chelate responsible for color development was formed immediately with AA oxidation. Beer's law was obeyed between 8.0 × 10 −6 and 8.0 × 10 −5 M concentration range, with the equation of the linear calibration curve: A 450 nm = 1.60 × 10 4 C (mol dm −3) − 0.0596. The relative standard deviation (R.S.D.) in the analysis of N = 45 synthetic mixtures containing 1.25 × 10 −2 mM AA with flavonoids was 5.3%. The Cu(II)-Nc reagent is a lower redox-potential and therefore more selective oxidant than the Fe(III)-1,10-phenanthroline reagent conventionally used for the same assay. This feature makes the proposed method superior for real samples such as fruit juices containing weak reductants such as citrate, oxalate and tartarate that may otherwise produce positive errors in the Fe(III)-phen method when equilibrium is achieved. The developed method was applied to some commercial fruit juices and pharmaceutical preparations containing Vitamin C + bioflavonoids. The findings of the developed method for fruit juices and pharmaceuticals were statistically alike with those of HPLC. The proposed spectrophotometric method was practical, low-cost, rapid, and could reliably assay AA in the presence of flavonoids without enzymatic procedures open to interferences by enzyme inhibitors.

Direct oxidation of l-ascorbic acid on a carbon black electrode in acidic media and polymer electrolyte fuel cells

Electrochemical oxidation of l-ascorbic acid (AA) on carbon electrodes in acidic media was investigated to use AA as a fuel for direct polymer electrolyte fuel cells (PEFCs). A higher current was obtained for the oxidation of AA by modifying glassy carbon electrodes with various carbon blacks. The anodic current density depended on the electrical double-layer capacitance formed by carbon black, Nafion ionomer, and electrolyte solution. Direct-type PEFCs, in which an aqueous solution of AA was used as a fuel, were fabricated with carbon black as anodes. A maximum power density of 15 mW cm −2 was attained at room temperature without any precious metal as an anode catalyst. This value is more than twice that obtained previously with palladium as an anode catalyst.

What is the True Supply of Amino Acids for a Dairy Cow?

Improving the prediction of milk protein yield relies on knowledge of both protein supply and requirement. Definition of protein/amino acid supply in ruminants is a challenging task, due to feedstuff variety and variability and to the remodeling of nutrient intake by the rumen microflora. The questions arise, therefore, how and where should we measure the real supply of AA in the dairy cow? This review will follow the downstream flow of AA from duodenum to peripheral tissue delivery, with a glance at the efficiency of transfer into milk protein. Duodenal AA flow comprises rumen undegrad-able feed, microbial protein, and endogenous secretions. Most attention has been directed toward definition of the first two contributions but the latter fraction can represent as much as 20% of duodenal flow. More information is needed on what factors affect its magnitude and overall impact. Once digested, AA are absorbed into the portal vein. The ratio of portal absorption to small intestinal apparent digestion varies among essential AA, from 0.43 (threonine) to 0.76 (phenylalanine), due to the contributions of preduodenal endogenous secretions to the digestive flow, non-reabsorption of endogenous secretions and gut oxidation of AA. Few data are available on these phenomena in dairy cows but the evidence indicates that they alter the profile of AA available for anabolic purposes. Recent comparisons of estimated duodenal flux and measured portal flux have prompted a revisit of the NRC (2001)approach to estimate AA flows at the duodenum. Changes to the model are proposed that yield predictions that better fit the current knowledge of AA metabolism across the gut. After absorption, AA flow first to the liver where substantial and differential net removal occurs, varying from zero for the branched-chain AA to 50% of portal absorption for phenylalanine. This process alters the pattern of net supply to the mammary gland. Overall, intermediary metabolism of AA between the duodenum and the mammary gland biologically explains the decreased efficiency of the transfer of absorbed AA into milk protein as maximal yield is approached. Therefore, variable, rather than fixed, factors for transfer efficiencies must be incorporated into future predictive models.

Electrooxidation of ascorbic acid on polyaniline and its implications to fuel cells

l-Ascorbic acid (AA) has been shown to undergo oxidation on polyaniline (PANI) without a platinum-group catalyst. A direct ascorbic acid fuel cell (DAAFC) has been assembled by employing an anode coated with PANI catalyst. From the experimental studies using cyclic voltammetry, amperometry and IR spectroscopy, it has been concluded that PANI facilitates the oxidation of AA. It has been possible to achieve a maximum power density of 4.3 mW cm −2 at a load current density of 15 mA cm −2 at 70 °C. As both AA and PANI are inexpensive and environmental-friendly, the present findings are expected to be useful for the development of cost-effective DAAFCs for several low power applications.

Exploration of synergism between a polymer matrix and gold nanoparticles for selective determination of dopamine

In this work, synergism between the conducting polymer matrix and gold nanoparticles is explored for dopamine sensing in the presence of excess ascorbic acid using poly (3,4-ethylenedioxythiophene) (PEDOT) in neutral phosphate buffer solutions (PBS 7.4). The electrodeposited thin films of PEDOT were characterized by atomic force microscopy (AFM) (and current sensing-AFM), and the incorporated Au nanoparticles by UV–Vis spectroscopy and electrochemical methods. Distinct “oxidized” and “reduced” regions of the polymer film were revealed in the CS-AFM. Electrochemical studies have shown that the catalytic oxidation of DA and AA on PEDOT modified electrodes can afford a peak potential separation of ∼230 mV. Further enhancement in the oxidation current for DA/AA oxidation was achieved by incorporation of Au nanoparticles in the PEDOT modified electrodes. The nanometer sized gold particles favour the nanomolar sensing of dopamine in the presence of excess of ascorbic acid (1.0 mM). The DA oxidation current increases linearly with DA concentration and the detection limit of DA is found to be ∼2 nM. Amperometric detection under stirred conditions is free from electrode fouling at lower concentrations of DA and is only slightly affected at concentrations beyond 20 μM. The combined effect of Au nanoparticles and the PEDOT matrix is based on the Au nano surrounded by a “hydrophobic sheath” tending to reside within these hydrophobic regions of PEDOT, increasing the DA oxidation current.

Electrochemical properties of a tetrabromo- p-benzoquinone modified carbon paste electrode. Application to the simultaneous determination of ascorbic acid, dopamine and uric acid

The electrochemical study of a tetrabromo- p-benzoquinone modified carbon paste electrode (TBQ-MCPE), as well as its efficiency for electrocatalytic oxidation of ascorbic acid, dopamine and uric acid, is described. Cyclic voltammetry was used to investigate the redox properties of this modified electrode at various solution pH values and at various scan rates. Three linear segments were found with slope values of −58.4 mV/pH, −28.1 mV/pH and 0.0 mV/pH in the pH range 2.0–7.1, pH 7.1–9.0 and pH 9.0–11.0, respectively. The apparent charge transfer rate constant, k s, and transfer coefficient, α, for electron transfer between TBQ and CPE were calculated as 3.79 ± 0.10 s −1 and 0.55, respectively. The electrode was also employed to study the electrocatalytic oxidation of AA, using cyclic voltammetry, chronoamperometry and differential pulse voltammetry as diagnostic techniques. It has been found that the oxidation of AA at the surface of TBQ-MCPE occurs at a potential of about 430 mV less positive than that of an unmodified CPE. The diffusion coefficient of AA was also estimated using chronoamperometry. The kinetic parameters such as the electron transfer coefficient, α, and heterogeneous rate constant, k h ′ , for oxidation of AA at the TBQ-MCPE surface was determined using cyclic voltammetry. Differential pulse voltammetry (DPV) exhibits two linear dynamic ranges and a detection limit of 0.62 μM for AA. In DPV, the TBQ-MCPE could separate the oxidation peak potentials of AA, DA and UA present in the same solution, though at the unmodified CPE the peak potentials were indistinguishable. This modified electrode was quite effective not only to detect AA, DA and UA, but also in simultaneous determination of each component concentration in the mixture.

Spectrophotometric determination of ascorbic acid using copper(II)–neocuproine reagent in beverages and pharmaceuticals

The proposed method for ascorbic acid (Vitamin C) (AA) determination is based on the oxidation of AA to dehydroascorbic acid with a Cu(II)-2,9-dimethyl-1,10-phenanthroline (neocuproine (Nc)) reagent in ammonium acetate-containing medium at pH 7, where the absorbance of the formed bis(Nc)-copper(I) chelate is measured at 450 nm. This chelate was formed immediately and the apparent molar absorptivity for AA was found to be 1.60 × 10 4 dm 3 mol −1 cm −1. Beer's law was obeyed between 8.0 × 10 −6 and 8.0 × 10 −5 M concentration range. The relative standard deviation for 90 μg AA was 3%. The Cu(II)-Nc reagent is a milder and therefore more selective oxidant than the conventional Fe(III)-1,10-phenanthroline (phen) reagent used for the same assay. This feature makes the proposed method superior for real samples such as fruit juices containing weak reductants such as citrate, oxalate and tartarate that otherwise produce positive errors in the Fe(III)-phen method when equilibrium is achieved. The developed method was applied to a number of commercial fruit juices, pharmaceutical preparations containing Vitamin C, and red wine. The meta-bisulfite content of wine was removed with an anion exchanger at pH 3 prior to analysis, and a difference extractive–spectrophotometric method of AA assay in wine was developed so as to suppress the interferences caused by wine anthocyanins and polyphenols. The findings of the developed method for fruit juices and pharmaceuticals were also statistically compared with those of HPLC so as to establish it as a reliable novel method.

Selective detection of l-glutamate using a microfluidic device integrated with an enzyme-modified pre-reactor and an electrochemical detector

A microfluidic device integrated with a nanoliter volume enzyme pre-reactor and an enzyme-modified electrode was developed for the highly selective continuous measurement of glutamate (Glu). The device consists mainly of two glass plates. One plate incorporates an electrochemical cell that consists of working electrode (WE), reference electrode (RE) and counter electrode (CE). The WE is modified with a bilayer film of Os-polyvinylpyrridine-based mediator containing horseradish peroxidase (Os-gel-HRP). The WE was operated at −50 mV versus Ag. The other plate has a thin layer flow channel integrated with a pre-reactor. The reactor has a number of micropillars (20 μm in diameter, 20 μm high and separated from each other by a 20 μm gap) modified with ascorbate oxidase (AAOx) to eliminate l-ascorbic acid (AA). The enzymatic oxidation of AA is superior to that obtained with our previously reported pre-electrolysis type micro-reactor since electrochemically reversible transmitters such as catecholamines do not provide a cathodic current at the WE. In addition, the high operation potential of the pre-reactor causes unknown electroactive species, which also cause interference at the detection electrode. As a result, we were able to detect 1 μM Glu continuously at a low flow rate even when AA concentration was 100 μM.

Dehydroascorbic Acid in Urine as a Possible Indicator of Surgical Stress

Background/Aim:L-Ascorbic acid (AA) is the predominant circulating form of vitamin C found in human blood. It has been hypothesized that surgical stress increases the vitamin C metabolite dehydroascorbic acid (DHAA). Vitamin C is mainly excreted through the kidneys. In this study, the ratio of AA to DHAA excreted in urine was determined in patients who had undergone total hip joint endoprosthesis surgery (n = 12), and the results were compared with data obtained from healthy controls (n = 12). Methods: All subjects received 1,000 mg sodium ascorbate intravenously three times a day (every 8 h) for 8 days, starting 2 days prior to surgery. Total urine was collected daily while subsequent determinations of AA and DHAA were performed photometrically. Results: Administration of vitamin C led to average daily excretions of the combined products AA + DHAA of 2,343 ± 438 mg/day (mean value ± confidence intervals). The initial average ratio DHAA/AA of all 24 probands was 0.064 (6% DHAA; 153 ± 76 mg/day). One day after surgery, an increase in the DHAA/AA ratio to 0.165 (15% DHAA; 332 ± 107 mg/day) was measured in the patients. The ratio decreased 2 days after surgery and returned to normal within 5 days. Conclusion: Our data indicate that surgery increases the oxidation of AA and urinary excretion of DHAA, as a result of the enhanced formation of free radicals.

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.