Electrochemical response of ascorbic acid at conducting and electrogenerated polymer modified electrodes for electroanalytical applications: a review

The present short review deals with electroanalytical aspects of electrochemical response of ascorbic acid (Vitamin C) at conducting and electrogenerated polymer modified electrodes. Two main topics are considered: (i) electrocatalytic oxidation of ascorbate at conducting polymer modified electrodes, leading to electroanalytical techniques for ascorbate assay, and (ii) retardation of ascorbate penetration through a layer of electrogenerated polymers, leading to permselective coatings and their diverse uses, especially for biosensing devices.     

Free-radical-mediated oxidation of ascorbic acid by peroxomonosulphate

The kinetics of the oxidation of ascorbic acid (AH₂) by peroxomonosulphate (PMS) were determined in aqueous medium in acidic (pH 4.4), neutral (pH 7.0) and alkaline (pH 9.0) conditions over the temperature range 13-28°C. The reactions were found to obey total second-order kinetics, first-order each with respect to peroxomonosulphate and ascorbic acid concentration, obeying-d[AH₂]/dt = k ₂[PMS][AH₂]. Dehydroascorbic acid was detected as the product of the reaction. The stoichiometry of the reaction, [peroxomonosulphate]/[ascorbicacid] = 1 : 1, indicated the absence of self-decomposition of peroxomonosulphate. The influence of neutral salt (NaClO₄) was found to increase the reaction rate. Evidence for the formation of radical intermediates was obtained. A mechanism involving the formation of hydroxyl, sulphate and ascorbate free radicals as intermediates is proposed. The rate and activation parameters were evaluated to substantiate the mechanism proposed. A comparison is made with the corresponding reactions of the similar peroxides, S₂O₈ ^2- and P₂O₈ ^4-.     

Simultaneous determination of ascorbic acid and dehydroascorbic acid by HPLC with postcolumn derivatisation and fluorometric detection

Summary A reproducible method is described for the separation and simultaneous and specific quantitation of ascorbic acid and dehydroascorbic acid by ion-pairing reversed-phase HPLC with fluorometric detection. Copper sulphate and copper acetate were compared as oxidizing reagents for ascorbic acid and 1,2-diaminobenzene dihydrochloride and 1,2-diamino-3,4-dimethylbenzene dihydrochloride as derivatising reagents. The HPLC-method was applied to human plasma. The detection limit reaches 16 ng for ascorbic acid and 3 ng for dehydroascorbic acid. Sample preparation is carried out by solid phase extraction with a recovery of 98%; it is compared with conventional precipitation of plasma proteins by metaphosphoric acid.     

Isotachophoretic analysis of the dihydrofolate reductase reaction in the presence of methotrexate and ascorbic acid

The antifolate methotrexate (MTX) is widely used in cancer chemotherapy. In this study, we show that MTX (MTX-Glu1) and MTX-polyglutamates (MTX-Glu2-5) strongly inhibited the growth of the leukemic cell line MOLT-4. This effect, however, was mitigated by ascorbic acid. We investigated whether ascorbic acid is able to reduce dihydrofolic acid (DHF) to tetrahydrofolic acid (THF) directly or by circumventing the MTX inhibition of dihydrofolate reductase (DHFR). The inhibition of this NADPH-dependent reduction of DHF by MTX-Glun in the absence or presence of ascorbate, was determined by analytical isotachophoresis. Using 0.01 M HCl/histidine, pH 6.0, as a leading electrolyte (L) and 0.005 M 2-(N-morpholino)ethanesulfonic acid (MES)/histidine, pH 6.0, as a terminating electrolyte (T), MTX-Glun derivatives including MTX-Glu1 could be easily separated, whereas the quantitative estimation of THF was not possible. A quantitative characterization of the DHFR reaction by measuring NADPH, NADP+ and ascorbate was achieved with another system (L: 0.01 M HCI/beta-alanine, pH 3.73; T: 0.01 M caproic acid, pH 3.27). Nanomolar concentrations of MTX-Glu1-5 inhibited consumption of NADPH and production of NADP+. Ascorbic acid was not able to reduce DHF, neither directly nor after inhibition of DHFR by MTX. However, ascorbic acid seemed to diminish the oxidation of THF and this may account for its capacity to reduce the inhibitory effect of MTX on MOLT-4 cells.     

All-solid-state potentiometric sensors for ascorbic acid by using a screen-printed compatible solid contact

The development of all-solid-state potentiometric ion selective electrodes for monitoring of ascorbic acid, by using a screen-printed compatible solid contact is described. The applied methodology is based on the use of PVC membrane modified with some firstly-tested ionophores (triphenyltin(IV)chloride, triphenyltin(IV)hydroxide and palmitoyl-l-ascorbic acid) and a novel one synthesized in our laboratory (dibutyltin(IV) diascorbate). Synthesis protocol and some preliminary identification studies are given. A conductive graphite-based polymer thick film ink was used as an internal solid contact between the graphite electrode and the PVC membrane. The presence and the nature of the solid contact (plain or doped with lanthanum 2,6-dichlorophenolindophenol (DCPI)) seem to enhance the analytical performance of the electrodes in terms of sensitivity, dynamic range, and response time. The analytical performance of the constructed electrodes was evaluated with potentiometry, constant-current chronopotentiometry and electrochemical impedance spectroscopy (EIS). The interference effect of various compounds was also tested. The potential response of the optimized Ph₃SnCl-based electrode was linear against ascorbic acid concentration range 0.005-5.0 mM. The applicability of the proposed sensors in real samples was also tested. The detection limit was 0.002 mM ascorbic acid (50 mM phosphate, pH 5 in 50 mM KCl). The slope of the electrodes was super-Nernstian and pH dependent, indicating a mechanism involving a combination of charge transfer and ion exchange processes. Fabrication of screen-printed ascorbate ISEs has also been demonstrated.     

Reductive dissolution of ferrihydrite by ascorbic acid and the inhibiting effect of phospholipid

The interaction of ascorbic acid with ferrihydrite nanoparticles with and without adsorbed phospholipid has been investigated with atomic force microscopy (AFM), attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR), density functional theory (DFT) cluster calculations, and batch geochemical methods. Both batch geochemical rate measurements and in situ AFM showed that ferrihydrite particles dissolved in the presence of ascorbic acid over a period of hours. The area-normalized dissolution rate derived from AFM measurements of isolated ferrihydrite particles was relatively constant over the period of dissolution and was faster than the dissolution rate derived from batch reaction methods. Results from ATR-FTIR interpreted in view of theoretical calculations suggested that exposure of ferrihydrite to ascorbic acid led to an adsorbed monodentate ascorbate surface complex. Ferrihydrite dissolution was suppressed if particles were exposed to an organic lipid prior to or during exposure to ascorbic acid. AFM analysis of the lipid layer showed that its thickness was close to 7 nm, the expected value for lipid assembled into a bilayer structure.     

Tear analysis of ascorbic acid,uric acid and malondialdehyde with capillary electrophoresis

Tears have a significant role in antioxidant defense in ocular tissues and since their collection is quick and noninvasive, their analysis would facilitate monitoring of pathophysiological changes. However, their low volume and low content of antioxidants makes analysis difficult; methods of high sensitivity are needed. In this paper, we present a method for tear analysis of two antioxidant molecules (ascorbic and uric acid) and of a lipid peroxidation indicator (malondialdehyde) with capillary electrophoresis. Tears were collected with Schirmer strips, extracted with a low‐pH phosphate buffer, centrifuged through membrane filters and an antioxidant was added. They were stable at ?70°C for 15 days. After pilot experiments, optimum electrophoretic separation was achieved in a 25 mm borate buffer, pH 10.0, containing 100 mm sodium dodecyl sulfate at 25°C and 20 kV. The developed method has good repeatability (<5% RSD), precision (<15% relative error values) and high sensitivity (LLOQ values of 20, 2.3 and 2.5 μM for ascorbate, urate and malondialdehyde, respectively). It was applied to the analysis of tears from healthy individuals and the antioxidant levels are in agreement with those obtained with other techniques. This method might serve as a tool to clarify the role of endogenous antioxidants in the pathophysiology of ocular diseases. Copyright © 2009 John Wiley & Sons, Ltd.     

Voltammetric and amperometric determination of ascorbic acid at a chemically modified carbon fibre microelectrode

A carbon fibre microelectrode was modified by electrodeposition of a thin copper-heptacyanonitrosylferrate (CuHNF) film on its surface. The film showed the ability to catalyse electrochemical oxidation of ascorbic acid. The catalytic reaction was limited either by diffusion, or by the electrochemical reaction of the catalyst. A linear, cyclic voltammetric response for ascorbic acid was obtained between 5.0 x 10(-5) and 6.0 x 10(-3)M. By amperometric measurements the linear range could be extended to 8.0 x 10(-6)-2.0 x 10(-3)M. The modified electrode showed good stability and reproducibility.     

Improved extraction method for avoiding the interference of ascorbic acid in the spectrophotometric determination of nitrite in meat products

In minced meat products, ascorbic acid interferes in the usual spectrophotometric determination of nitrite by the diazotisation-coupling technique with sulphanilamide and N-(1-naphthyl)ethylenediamine. In a modified extraction method, the enzyme ascorbate oxidase was shown to diminish the interference of the acid. Within 10 min the enzyme was able to reduce the level of ascorbic acid in the minced meat specimens from 1000 p.p.m. down to the tolerance limit for interference. This appeared to be superior to the extraction method involving the use of water and was comparable to a previously described method based on the use of 0.01 M sodium hydroxide. As isoascorbic acid is not approved as a food additive in Norway no attempt was made to investigate the influence of this acid.     

Nafion-离子液体-修饰碳糊电极在抗坏血酸和尿酸存在下选择性测定多巴胺

用Nafion和亲水性离子液体溴化1-辛基-3-甲基咪唑([OMIM]Br)作修饰剂制作了Nafion-离子液体-修饰碳糊电极;在0.1 mol/L磷酸盐缓冲溶液(pH 7.40)中,用循环伏安法(CV)和方波伏安法(SWV)研究了多巴胺在该修饰电极上的电化学行为,建立了抗坏血酸和尿酸存在下选择性测定多巴胺的新方法.研究表明,该修饰电极降低了多巴胺氧化、还原反应的过电位,增大了其氧化、还原反应的峰电流,而抗坏血酸和尿酸在该修饰电极上无响应;在方波伏安曲线上,多巴胺的氧化电流与其浓度在3.0×10-8～2.0×10-6 mol/L范围内呈线性关系,检出限为1.0×10-8 mol/L.该法可用于注射液和模拟生物样品中多巴胺的测定.     

Ru(III)-EDTA catalyzed oxidation of ascorbic acid by hydrogen peroxide in aqueous solution

The kinetics of oxidation of ascorbic acid to dehydroascorbic acid by hydrogen peroxide catalyzed by ethylenediaminetetraacetatoruthenate(III) has been studied over the pH range 1.50 – 2.50, at 30°C and μ = 0.1 M KNO₃. The reaction has a first-order dependence on ascorbic acid and Ru(III)-EDTA concentrations, an inverse first-order dependence on hydrogen ion concentration, and is independent of hydrogen peroxide concentration in the pH range studied. A mechanism has been proposed in which ascorbate anion forms a kinetic intermediate with the catalyst in a pre-equilibrium step. Ruthenium(III) is reduced to ruthenium(II) in a rate-determining step and is reoxidized with hydrogen peroxide back to the Ru(III) complex in a fast step.     

The application of conducting polymer nanoparticle electrodes to the sensing of ascorbic acid

An ascorbic acid sensor was fabricated via the drop-casting of dodecylbenzene sulphonic acid (DBSA)-doped polyaniline nanoparticles onto a screen-printed carbon-paste electrode. The modified electrode was characterised with respect to the numbers of drop cast layers, optimum potential and operating pH. The sensor was found to be optimal at neutral pH and at 0 V vs. Ag/AgCl. Under these conditions, the sensor showed good selectivity and sensitivity in that it did not respond to a range of common interferents such as dopamine, acetaminophen, uric acid and citric acid, but was capable of the detection of ascorbic acid at a sensitivity of 0.76 μA mM⁻¹ or 10.75 μA mM⁻¹ cm⁻² across a range from 0.5 to 8 mM (r² = 0.996, n = 6), and a limit of detection of 8.3 μM (S/N = 3). The sensor was compared to a range of other conducting polymer-based ascorbate sensors and found to be comparable or superior in terms of analytical performance.     

Highly sensitive chemiluminescence flow sensor for ascorbic acid

A highly sensitive chemiluminescence(CL) flow sensor is proposed for the determination of ascorbic acid. The analytical reagents luminol and iron(II) are immobilized on anion-exchange and cation-exchange resins, respectively, and can be eluted by sodium sulphate. The calibration graphs are linear in the range 1 × 10^–9 to 1 × 10^–6 g mL^–1 and the detection limit is 4.0 × 10^–10 g mL^–1. The sensor has been applied successfully to the determination of ascorbic acid in vegetables.     

Fabrication of multiwalled carbon nanotubes/polyaniline modified Au electrode for ascorbic acid determination

An ascorbate oxidase (AsOx) (E.C.1.10.3.3) purified from Lagenaria siceraria fruit was immobilized covalently onto a carboxylated multiwalled carbon nanotubes and polyaniline (c-MWCNT/PANI) layer electrochemically deposited on the surface of an Au electrode. The diffusion coefficient of ascorbic acid was determined as 3.05 × 10(-4) cm(2) s(-1). The behavior of different electrolytes on electro-deposition was also studied. An ascorbate biosensor was fabricated using a AsOx/c-MWCNT/PANI/Au electrode as a working electrode, Ag/AgCl (3 M/saturated KCl) as standard and Pt wire as an auxiliary electrode connected through a potentiostat. Linear range, response time and detection limit were 2-206 μM, 2 s and 0.9 μM respectively. The biosensor showed optimum response at pH 5.8 and in a broader temperature range (30-45 °C), when polarized at +0.6 V. The biosensor was employed for determination of ascorbic acid level in sera, fruit juices and vitamin C tablets. The sensor was evaluated with 91% recovery of added ascorbic acid in sera and 6.5% and 11.4% within and between batch coefficients of variation respectively for five serum samples. There was a good correlation (r = 0.98) between fruit juice ascorbic acid values by the standard 2,6-dichlorophenolindophenol (DCPIP) method and the present method. The enzyme electrode was used 200 times over a period of two months, when stored at 4 °C. The biosensor has advantages over earlier enzyme sensors in that it has no leakage of enzyme, due to the covalent coupling of enzyme with the support, lower response time, wider working range, higher storage stability and no interference by serum substances.     

Oxidimetric determination of ascorbic acid with potassium hexacyanoferrate(III) in acid medium

Conditions have been developed for the titrimetric determination of ascorbic acid with hexacyanoferrate(III), with potentiometric and visual end-points, in sulphuric, hydrochloric or phosphoric acid media. Several organic substances likely to be present in plant tissues do not interfere.     

Benzoquinone modified electrode for sensing NADH and ascorbic acid

A benzoquinone modified basal plane pyrolytic graphite electrode shows electrocatalytic activity for the oxidation of NADH and ascorbic acid in phosphate buffer (pH 7.3). The modified electrode shows a linear variation of catalytic current with concentration in the range 1-10 mM for both NADH and ascorbic acid. The rate constants have been estimated from the surface coverage data.     

Electrochemical polymerization of aniline over tetracyanoquinodimethane encapsulated ormosil matrix: application in the electrocatalytic oxidation of ascorbic acid and acetylthiocholine

Organically modified sol-gel glass (ormosil) matrix is utilized as a template for the electrochemical polymerization of aniline. The ormosil matrix is further modulated by encapsulation of: (a) tetracyanoquinodimethane (TCNQ), and (b) tetracyanoquinodimethane together with palladium (Pd). The presence of tetracyanoquinodimethane within ormosil matrix considerably influences the polymerization process of aniline and the same is significantly enhanced as compared to that of control. The presence of palladium within ormosil network further influence the polymerization process as compared to that of TCNQ only. The polyaniline obtained as PAni-TCNQ and PAni-TCNQ-Pd composites has been utilized for fabricating the modified electrodes. These modified electrodes are used to study the electrochemical sensing of ascorbic acid and acetylthiocholine. The results based on cyclic voltammetry, differential pulse voltammetry and amperometry justify that the electrode material exhibits excellent electrocatalytic activity for the oxidation of ascorbic acid and acetylthiocholine with major findings as compared to the control: (1) a negative shift to the order of ～340 mV vs. Ag/AgCl in the anodic overpotential for the electro-oxidation of ascorbic acid, (2) a significant increase in the anodic peak current for the oxidation of ascorbic acid, (3) an increase in the sensitivity of ascorbic acid analysis to the order of 7-fold for the modified electrodes, (4) acetylthiocholine undergoes direct oxidation with considerable increase in both anodic and cathodic peak currents and (5) an increase in the sensitivity of acetylthiocholine analysis to the order of 5-fold for the modified electrodes.     

Electrocatalytic determination of ascorbic acid on a glassy carbon electrode chemically modified with cobalt pentacyanonitrosylferrate.

An electrochemically prepared thin film of cobalt pentacyanonitrosylferrate (GC/CoPCNF) was used as a surface modifier for glassy carbon electrodes. The oxidation of ascorbic acid on a glassy carbon electrode modified with GC/CoPCNF as a working electrode was studied using cyclic voltammetry, rotating disk electrode (RDE) voltammetry and chronoamperometry in a 0.25 M KNO3 + 0.25 M phosphate buffer (pH 7) solution. The glassy carbon modified with CoPCNF showed good electrocatalytic activity toward ascorbic acid oxidation. The kinetics of the catalytic reaction was investigated, and the average value of the rate constant (k) for the catalytic reaction and the diffusion coefficient (D) were evaluated by different approaches for ascorbic acid, and were found to be 3.3 +/- 0.3 x 10(2) M(-1) s(-1) and 3.2 +/- 0.3 x 10(-6) cm2 s(-1), respectively.     

Improved voltammetric peak separation and sensitivity of uric acid and ascorbic acid at nanoplatelets of graphitic oxide

In this study, much improved voltammetric peak separation and sensitivity of uric acid and ascorbic acid was observed at a screen-printed carbon electrode modified with nanoplatelets of graphitic oxide (GO). Electrochemical sensing of uric acid and ascorbic acid was further used to explore the role of oxygen functionalities and edge plane sites on electrocatalysis at GO. We successfully apply a microwave-assisted hydrothermal elimination method to remove the oxygen-containing functional groups from the GO surface. The edge plane on GO can be retained and the density of oxygen-containing functional groups can be easily controlled by varying the microwave treatment temperature. Using this platform, such discrimination to peak separation is attributed to the very distinct behavior of uric acid and ascorbic acid to form hydrogen bonds with oxo-surface groups (especially COOH group) at GO.     

Flow injection fluorometric determination of ascorbic acid using perylenebisimide-linked nitroxide

A simple and sensitive flow injection fluorometric method for the determination of ascorbic acid is described. Perylenebisimide-linked nitroxide (PBILN) is used as a fluorescent reagent, which permits the selective determination of ascorbic acid. The fluorescence of the perylenebisimide moiety in PBILN is quenched by the nitroxide moiety, which is linked to the perylenebisimide. When a stream of a solution of ascorbic acid is merged with a stream of PBILN, the ascorbic acid reacts with the nitroxide moiety of PBILN to form hydroxylamine, and the fluorescence properties of the perylenebisimide moiety are recovered. As a result, a peak-shaped fluorescence signal is produced, which can be observed by a fluorescence detector located downstream. Under optimized conditions, a good linear relationship between the concentration of ascorbic acid and peak height in the concentration range from 0.5 to 10 μmol L⁻¹ was found and the detection limit (S/N = 3) was 0.28 μmol L⁻¹. The relative standard deviation for the determination of 4.0 μmol L⁻¹ ascorbic acid samples was 1.0% (n = 5). The proposed method was applied to the determination of ascorbic acid in several soft drink beverages and the analytical results were in good agreement with those obtained using a conventional method.