Nafion-coated Glassy Carbon Electrode Research Articles

Improved Voltammetric Determination of Kynurenine at the Nafion Covered Glassy Carbon Electrode - Application in Samples Delivered from Human Cancer Cells.

Nowadays, development of analytical methods responding to a need for rapid and accurate determination of human metabolites is highly desirable. Herein, an electrochemical method employing a Nafion-coated glassy carbon electrode (Nafion/GCE) has been developed for reliable determination of kynurenine (a key tryptophan metabolite) using a differential pulse adsorptive stripping voltammetry. To our knowledge, this is the first analytical method to allow for kynurenine determination at the Nafion-coated electrode. The methodology involves kynurenine pre-concentration in 0.1 M H2SO4 in the Nafion film at the potential of +0.5 V and subsequent stripping from the electrode by differential pulse voltammetry. Under optimal conditions, the sensor can detect 5 nM kynurenine (for the accumulation time of 60 seconds), but the limit of detection can be easily lowered to 0.6 nM by prolonging the accumulation time to 600 seconds. The sensor shows sensitivity of 36.25 μAμM−1cm−2 and 185.50 μAμM−1cm−2 for the accumulation time of 60 and 600 seconds, respectively. The great advantage of the proposed method is easy sensor preparation, employing drop coating method, high sensitivity, short total analysis time, and no need for sample preparation. The method was validated for linearity, precision, accuracy (using a high-performance liquid chromatography), selectivity (towards tryptophan metabolites and different amino acids), and recovery. The comprehensive microscopic and electrochemical characterization of the Nafion/GCE was also conducted with different methods including atomic force microscopy (AFM), optical profilometry, time-of-flight secondary ion mass spectrometry (TOF-SIMS), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV). The method has been applied with satisfactory results for determination of kynurenine concentration in a culture medium collected from the human ovarian carcinoma cells SK-OV-3 and to measure IDO enzyme activity in the cancer cell extracts.

A highly sensitive method for determination of β-blocker drugs using a Nafion-coated glassy carbon electrode

The electrochemical response of bisoprolol fumarate (BF) and propranolol hydrochloride (PR) is significantly enhanced by immobilization of a Nafion film on the surface of the glassy carbon electrode. The positively charged drug molecules were accumulated in 0.1 mol L −1 sulfuric acid solution at a potential of +0.6 V for 120 s, and then determined by square-wave anodic voltammetry. The enhancement mechanism and effect factors such as pH value, mass of Nafion, accumulation potential and pre-concentration time, were explored. The peak current is proportional to the concentration in the range from 5 × 10 −8 to 5 × 10 −7 mol L −1 for BF and from 8 × 10 −8 to 6 × 10 −7 mol L −1 for PR, with the sensitivities of 1.49 × 10 8 and 2.47 × 10 7 μA mol −1 L, respectively. The modified electrode was used to determine both β-blockers in commercial tablets. The quantification of BF in urine samples with no prior extraction was carried out after selective accumulation at the electrode surface under stirring. Together with low cost and ease preparation, the modified electrode is a promising sensor in electroanalysis, not only for the detection of BF and PR, but also for other β-blocker drugs.

A study on the electroanalytical performance of a bismuth film-coated and Nafion-coated glassy carbon electrode in alkaline solutions

Bismuth film electrodes are widely used for determination of heavy metal ions in acidic solutions, while alkaline solutions are rarely employed. We have compared the deposition of Bi(III) and Pb(II) on a Nafion-coated glassy carbon electrode in alkaline and acidic solutions. The results indicate that both Bi(III) and Pb(II) can be deposited in either alkaline or acidic solution, but the quantity of Pb(II) deposited in alkaline solution is less than that in acidic solution. The modified electrode was used to determine heavy metal ions in both alkaline and acidic solutions, and the results of the method agree well with those of atomic absorption spectroscopy.

Electrochemical preparation, characterization, and electrocatalytic studies of Nafion–ruthenium oxide modified glassy carbon electrode

Electrochemical synthesis of ruthenium oxide (RuOx) onto Nafion-coated glassy carbon (GC) electrode and naked GC electrode were carried out by using cyclic voltammetry. Electrochemical deposition of RuOx onto Nafion-coated electrode was monitored by in situ electro- chemical quartz crystal microbalance (EQCM). Surface characterizations were performed by scanning electron microscope (SEM) and atomic force microscope (AFM). SEM and AFM images revealed that ruthenium oxide particles incorporated onto the Nafion polymer film. In addition, a GC electrode modified with ruthenium oxide- Nafion film (RuOx-Nf-GC) was shown excellent electro- catalytic activity towards dopamine (DA) and ascorbic acid (AA). The anodic peak current increases linearly over the concentration range of 50 μM-1.1 mM for DA with the correlation coefficient of 0.999, and the detection limit was found to be (S/N=3) 5 μM. Owing to the catalytic effect of the modified film towards DA, the modified electrode resolved the overlapped voltammetric responses of AA and DA into two well-defined voltammetric peaks with peak-to- peak separation about 300 mV. Here, RuOx-Nf-GC electrode employed for determination of DA in the presence of AA. This modified electrode showed good stability and antifouling properties.

Ion-exchange voltammetry of dopamine at Nafion-coated glassy carbon electrodes: Quantitative features of ion-exchange partition and reassessment on the oxidation mechanism of dopamine in the presence of excess ascorbic acid

The incorporation/exclusion features of dopamine (DA), ascorbic acid (AA) and uric acid (UA) are evaluated for Nafion (NA)-coated glassy carbon electrodes (GCE) of different thicknesses. The ion-exchange partition of DA + between the NA film and the sodium phosphate electrolyte is evaluated by determining the partition coefficient ( k D) and the apparent diffusion coefficient ( D app) in thick NA films which were 401 and 1.5 × 10 − 9 cm 2 s − 1 , respectively. The solution diffusion coefficient was found to be 6.0 × 10 − 6 cm 2 s − 1 . Also, the effect of NA loading and of the voltammetric timescale on DA voltammetry in the presence of excess AA is assessed, at physiologic like conditions. It is demonstrated that, although AA is excluded at the NA coating, a catalytic regeneration of DA, induced by AA, occurs at the interface NA film/electrolyte resulting from the diffusion of the o-quinone product of DA oxidation from the electrode surface to that interface. The interference of AA in the voltammetric signal of DA is eliminated using 18 μg mm − 2 NA films and v ≥ 0.5 V s − 1 . Therefore, fast, selective and sensitive voltammetric analysis of DA at concentrations < 100 μM in the presence of excess AA, e.g., 1 mM is achieved.

Glucose Sensing Using p-Acetamidophenol as an Electron Mediator between Glucose Oxidase and Nafion-Coated Glassy Carbon Electrode

Glucose sensing with use of glucose oxidase, p-acetamidophenol, and Nafion-coated Glassy carbon electrode has been attempted aiming quantitative analysis of glucose without interference of electrochemically active species such as ascorbic acid, uric acid, and p-acetamidophenol. Electrochemical measurements of the three interfering species at a Nafion-coated glassy-carbon electrode have revealed that the electrode can oxidize only p-acetamidophenol. It has been also found that p-acetamidophenol works well as an electron mediator for glucose oxidase. Combination of those facts led to fabrication of a glucose sensor that avoids any interference by ascorbic acid and uric acid. When concentration higher than 5.0 mM is chosen for p-acetamidophenol as the electron mediator, further addition of p-acetamidophenol gives no significant changes in the oxidation currents. In addition, even oxygen dissolved in solution does not influence magnitude of sensing currents. Therefore, the glucose sensor prepared in this study is not disturbed by all electrochemically active substances which usually give the most serious interferences for oxidative glucose sensing.

A method for the quantification of low concentration sulfamethazine residues in milk based on molecularly imprinted clean-up and surface preconcentration at a Nafion-modified glassy carbon electrode

An electrochemical method for the determination of sulfamethazine at a low concentration level (25 μg l −1) in milk is reported. The method involves sample clean-up and selective preconcentration of sulfamethazine with a molecularly imprinted polymer (MIP), and a further electrode surface preconcentration of the analyte at a Nafion-coated glassy carbon electrode (GCE). Square wave (SW) oxidative voltammetry of accumulated sulfamethazine was employed for its quantification. Sulfamethazine electrode preconcentration was carried out in 0.1 mol l −1 Britton–Robinson buffer of pH 1.5, and by applying 5 min of accumulation at open circuit. A linear calibration graph was obtained for sulfamethazine at the Nafion-modified GCE over the 1.0 × 10 −8 to 1.0 × 10 −6 mol l −1 concentration range, with a detection limit of 6.8 × 10 −9 mol l −1 (1.9 μg l −1). This detection limit is remarkably better than those reported previously in the literature using electroanalytical techniques. Although the detection limit achieved was sufficient to allow the direct determination of sulfamethazine at the concentration level required in milk, a sample clean-up was shown to be necessary to obtain analytically useful SW voltammograms. This was accomplished by processing the deproteinized milk through a cartridge containing a molecularly imprinted polymer for sulfamethazine, also allowing a selective preconcentration of the analyte. Elution of the analyte from the MIP cartridges was carried out with 2 ml of a (9:1) MeOH:acetic acid mixture. Determination of sulfamethazine in milk samples was accomplished by interpolation into a calibration graph constructed with sulfamethazine standard solutions which were subjected to the same procedure than the deproteinized milk samples. Results obtained for five samples, spiked at the 25 μg l −1 level, showed a mean recovery of (100 ± 3)%.

Amperometric Detection of Dopamine in the Presence of Ascorbic Acid Using a Nafion Coated Glassy Carbon Electrode Modified with Catechin Hydrate as a Natural Antioxidant

Catechin hydrate (trans-3,3′,4′,5,7-penta-hydroxyflavone) is a derivative of catechol, and it is one of the natural antioxidant and anticarcinogenic compounds found in many fruits and agricultural foods. A new electrode was prepared by electrodeposition of a catechin-hydrate film on the surface of an activated glassy carbon electrode, followed by coating with nafion layer. Cyclic voltammetry was used for the deposition process, and the resulting modified electrode was found to retain the activity of the quinone/hydroquinone group anticipated for a surface-immobilized redox couple. The cyclic voltammograms of the modified electrode indicated that the surface-confined catechin-hydrate was strongly dependent on the solution pH. The electrochemical behaviour and stability of the modified electrode were studied by cyclic voltammetry. The apparent charge transfer rate constant (k s ) and electron transfer coefficient (α) between the electrode surface and adsorbed catechin hydrate were calculated as 0.41 and 0.31 sz−1 0.31, respectively. The modified electrode exhibits an electrocatalytic effect on dopamine oxidation in the pH range of 2 to 7. The modified electrode was used for the electrocatalytic oxidation of dopamine in the presence of ascorbic acid and the elimination of the interference by ascorbic acid (AA) when present in a 200-fold concentration excess. A very large decrease (400 mV) in the overpotential for the oxidation of dopamine and current increase were observed on the surface of the modified electrode. Dopamine was determined amperometrically at the modified electrode in the concentration range of 100 nM to 0.1 mM. The detection limit (signal to noise is 3) and sensitivity are 11 nM and 16 nA/µM, respectively. The modified electrode was employed to determine DA in biological mediums.

Trace Analysis of Lead at a Nafion-Modified Electrode Using Square-Wave Anodic Stripping Voltammetry

ASV using square-wave voltammetry was used with Nafion-coated glassy carbon electrodes for lead analysis and it was found that square-wave frequencies greater than 100 Hz could be employed satisfactorily for trace lead detection. Peak current increases with frequencies up to 1000 Hz at step potentials of up to 10 mV, though not linearly. However, the enhanced sensitivity allows the analysis of low levels of lead with concentrations down to 1 ppb detectable. This technique gave a value of 15.0±1.3 ppb lead in a diluted certified reference material with lead concentration of 14.7 ppb.

Trace Iron Determination by Cyclic and Multiple Square-Wave Voltammetry at Nafion Coated Electrodes. Applicationto Pore-Water Analysis

Nafion coated glassy carbon electrodes (NCE) are employed for preconcentrating and detecting Fe2+ and Fe3+ cations from aqueous solutions. The influence of the supporting electrolyte composition and of the redox state on the analyte partitioning within the Nafion coating are examined. By using cyclic voltammetry, the ion-exchange voltammetric determination of iron in the μM concentration range is achieved from voltammetric peak currents relevant to the reversible redox process: Fe3++e⇋Fe2+. Depending on the starting potential of the voltammetric scan, all the iron is initially converted to the Fe(III) or Fe(II) redox state, so that, from the voltammogram, the overall concentration of iron is always obtained. However, the simple measurement of the open circuit potential at the NCE before starting the scan allows one to get information on the ratio between Fe(III) and Fe(II) incorporated in the Nafion coating. From relevant ion-exchange distribution coefficients, open circuit potential values can be related to the Fe(III)/Fe(II) concentration ratio in the sample. The use at the NCE of a new electroanalytical technique named multiple square-wave voltammetry in the double differential mode allows the significative lowering of detection limits pushing the operative range of the method in the nM range. Application to analysis of Fe(II) in the pore-waters of sediments of the lagoon of Venice (Italy) is presented.

Nonequilibrium Thermodynamics Formalism for Charge Transport in Redox Polymer Electrodes

Onsager's formalism of nonequilibrium Thermodynamics is employed to analyze competitive diffusion processes pertaining to charge transport through redox polymer electrodes. The validity of reciprocity relations is demonstrated for electron hopping in the case of [Ru(NH3)6]3+/2+ couple on Nafion coated Glassy Carbon (GC) electrodes. The phenomenological coefficients pertaining to electron hopping and physical diffusion are estimated using the experimental data of Cp2FeTMA2+/1+ on Nafion coated GC electrodes. The redox conductivity and transport numbers are expressed in terms of Onsager's coefficients and the calculation of the phenomenological coefficients directly from the charge propagation behavior is demonstrated. The expression for entropy production is shown to yield new insights regarding the interpretation of diffusion coefficients in redox polymeric systems.

Stripping Voltammetry of Pb(II), Cu(II), and Hg(II) at a Nafion-Coated Glassy Carbon Electrode Modified by Neutral Ionophores

The analysis of Pb2+, Cu2+ and Hg2+ by stripping voltammetry using a Nafion-coated glassy carbon electrode modified with diethyldithiocarbamic acid (DDTC) or 1,4,7,10,13,16-hexaoxacyclooctadecane(18-crown-6) is described. Metal ions were accumulated at the electrode by complexing with the ionophore, reduction and detected by differential pulse anodic stripping voltammetry. A five-fold increase in the voltammetric response was achieved at the modified electrode compared to a bare glassy electrode. Parameters and conditions such as the modifier composition, the pH of supporting electrolyte, deposition potential, deposition time, and stripping mode were optimized. For five minutes preconcentration at DDTC in 100 mM KNO3, pH 2.5, linear calibration plots in the range 0.5 μM to 50 μM were obtained with a detection limit of 10 nM. For 18-crown-6, after accumulation at open circuit for 1 minute, a linear plot between 2 and 40 μM was achieved. The lifetimes of each electrode were more than three weeks.

Square-Wave Voltammetric Determination and ac Impedance Study of Dopamine on Preanodized Perfluorosulfonated Ionomer-Coated Glassy Carbon Electrodes

Preanodization of the Nafion-coated glassy carbon electrode (NCGCE) results in a large increase in the square-wave voltammetric peak current of dopamine (DA) with an excellent detection limit of 4.7 nM (S/N = 3) in pH 7.4 phosphate buffer solution. Chronocoulometry showed an increase in double-layer charge and the adsorption control of charge transfer. The surface coverage was calculated to be 3.1 × 10-10 mol/cm2, reminiscent of a monolayer coverage. The cause for such an increase in peak current on preanodization is unraveled using impedance measurements. The result reveals that the bulk resistance of Nafion is reduced progressively, and hence more solvent is found to permeate into the NCGCE, at more anodic preanodization potentials. The increase in the fraction of ion clusters in the bulk polymer exposing more SO3- and thus more cationic DA is exchanged to maintain the electroneutrality. The GCE/Nafion interfacial charge also increased with the preanodization potential due to the induction of >CO on the GCE surface. The interaction of the >CO with DA, possibly through hydrogen bonding, is envisaged to further increase the sensitivity in DA detection.

Selective voltammetric method for uric acid detection using pre-anodized Nafion-coated glassy carbon electrodes

Electrochemically pre-anodized Nafion-coated glassy carbon electrodes (NCGCEs) were used for the selective determination of uric acid (UA) by square-wave voltammetry. The major interference from ascorbic acid (AA) was overcome owing to the distinct ability of the pre-anodized NCGCE to yield its best performance at around pH 5. At pH 5, AA exists in the anionic form whereas UA is in the cationic form, since their pKa values are 4.1 and 5.4, respectively, at 25 °C. Consequently, the Nafion film repels the negatively charged AA and selective sensing of UA can be achieved. Moreover, the anodized surface has a high affinity towards UA through hydrogen bonding. Hence the combination of pre-anodization and the Nafion film modification works excellently in the determination of UA. Under optimized conditions, the detection limit for UA has been improved to 10 nM (S/N = 3). The practical analytical utility of the method is demonstrated by the measurement of UA in human urine without any preliminary treatment.

Stripping Potentiometry of Lead, Cadmium and Copper at a Nafion Coated Glassy Carbon Electrode with Encapsulated Mercury Acetate

The stripping potentiometric determination of lead, cadmium and copper with mercury film glassy-carbon electrodes coated with a Nafion membrane was investigated. The mercury film was plated using either mercury(II) acetate encapsulated within the Nafion membrane or a mercury(II) solution. Dissolved dioxygen was used as the stripping agent. The electrodes showed promising properties, particularly robustness and response repeatability. A linear dependence of the stripping time on concentration was found in the μg l−1 concentration range (s.d. of intercept ≤ 0.3 μg l−1, r.s.d. of slope ≤ 1%, for both lead and cadmium).

Spectroscopic and electrochemical response to nitrogen monoxide of a cationic iron porphyrin immobilized in nafion-coated electrodes or membranes

Nafion membranes containing immobilized FeIII tetrakis(N-methyl-4-pyridyl)porphyrin show spectroscopic changes in the UV–VIS range after exposure to gaseous NO, indicating the formation of a nitrosyl complex; electroreduction of NO is mediated by this metalloporphyrin when immobilized in Nafion-coated glassy-carbon electrodes and the use of these film electrodes for the amperometric determination of NO in aqueous solutions is demonstrated.

Incorporation of hydroxamic acid ligands into Nafion film electrodes

This paper describes studies on Nafion-coated glassy carbon electrodes incorporating hydroxamic acids. Desferrioxamine, a trihydroxamic acid, and glycine hydroxamate, a monohydroxamic acid, can be incorporated by ion exchange into the Nafion film. These ligands were detected in the film visually, after reaction with iron(III), and electrochemically, by voltammetric oxidation. The complex of iron(III) with desferrioxamine was electrochemically active at the iron(III) centre in the Nafion film. The electrochemistry of this complex in the film agrees well with the solution electrochemistry reported by other workers: irreversible at low pH and reversible at high pH. The ability of the modified electrode to act as a sensor for iron(III) was assessed. Complexation of iron(III) from dilute solution into the polymer film was possible, and 5 × 10–7 mol l–1 iron(III) could be detected after a 10 min preconcentration, with differential-pulse voltammetric detection in pH 10 ammoniacal buffer.

Electrochemical behaviour and preconcentration of uranyl(VI) at Nafion-coated glassy carbon electrodes

The electrochemical behaviour of UO 2 2+ solutions at Nafion ®-coated glassy carbon electrodes has been examined and compared with the behaviour observed at uncoated electrodes. At pH 2.4, two reduction processes are always observed at both electrodes; however, the mechanism involved in the first process is affected strongly by the presence of the polymeric coating. While in homogeneous solution the first reduction product UO 2 + is quite stable and reacts only with electrogenerated U III, in the presence of Nafion the local high concentration of H + inside the coating causes it to disproportionate. The relevance of the ion exchange competition between H + and Na + to this reaction is proved. The influence of the applied potential on the rate and extent of incorporation was also studied. The ion exchange characteristics of Nafion for the different oxidation states generated by reducing UO 2 2+ have been compared qualitatively and the selectivity coefficients for U IV and U VI determined. The values obtained indicate a selectivity of Nafion for U IV about 20 times higher than that for U VI, so that preconcentration from UO 2 2+ dilute solutions can be achieved by applying a suitable reducing potential at the modified electrode.

Determination of trace amounts of Eu 3+ and Yb 3+ ions at Nafion-coated thin mercury film electrodes

The use of Nafion-coated thin mercury film electrodes (NCTMFE) for determining trace amounts of Eu 3+ and Yb 3+ ions was examined. Ion-exchange preconcentration of submicromolar levels of the two rare earths was achieved efficiently at a rotating NCTMFE, while the use of “classical” Nafion-coated glassy carbon electrodes is restricted to the determination of Eu 3+ alone. Differential-pulse voltammetry was used to quantify the accumulated ions. The influence of variables such as rotation rate, preconcentration time and scan parameters was assessed. A preconcentration -voltammetry-regeneration scheme suitable for multiple analysis with the same modified electrode was developed. Calibration graphs with a linearity range extending up to 2 μM and detection limits of 0.03 and 0.08 μM for Eu 3+ and Yb 3+, respectively, were obtained. For Yb 3+, the detection limit can be lowered to 0.02 μM by electrocatalytic amplification of the signals achieved by operating in the presence of ammonium nitrate as supporting electrolyte. However, in this instance a more restricted linearity range is observed. The effect of competing incorporation caused by the presence in solution of an excess of La 3+ is also discussed, together with the competition between Eu 3+ and Yb 3+ when one of the two is present in large excess over the other.

Nafion-coated mercury-coated glassy carbon electrodes for metals analysis and speciation

The properties of mercury-coated glassy carbon electrodes coated with a thin membrane of Nation 117 were studied to evaluate the potential analytical applications in metal ion analysis. The Nation membrane was found to contain two types of binding sites — a strong site that bound metal ions in a nonelectroactive form and cation exchange sites that were useful for preconcentrations prior to differential pulse voltammetry. The exchange constants for the weak sites ranged from 0.413 for copper to 12.5 for methyl viologen. The differential pulse currents were increased by a factor of 75 to 100 and a linear calibration curve was observed from about 25 n M to 1000 n M for copper, lead, cadmium, and zinc. The ion exchange reaction is very sensitive to ionic strength and significant enrichments occur only in solutions of ionic strength below 0.10. The Nafion-coated glassy carbon electrode in a flow cell can be used to determine the free copper ion concentration in samples by a combination of ion exchange and differential pulse voltammetry. The flow-through system required about 15 mL of sample solution and an analysis time of about 10 min. The ion exchange measurement gave a linear calibration from 40 n M to 10 μ M for free copper.