Electrochemical characterization of screen-printed and conventional carbon paste electrodes

This work compares the electroactivity of a conventional carbon paste electrode and non-pretreated commercially available screen-printed carbon electrodes (from Alderon Biosciences, University of Florence and DropSens) towards some benchmark redox couples like hexaammineruthenium (III), ferricyanide, p-aminophenol and hydroquinone. While cyclic voltammograms of Ru³⁺ did not show significative electron transfer reactivity differences between the electrodes tested, the other redox systems exhibited higher reversible behaviours on DropSens electrodes. Scanning electron microscopy and roughness analysis with a profilometer were applied to detect the surface morphology of the working electrodes. The roughness evaluated of the screen-printed carbon working electrodes increased in this order Alderon < University of Florence < DropSens. Finally, the most electrochemically active and rough unpretreated electrode (DropSens commercial screen-printed electrode) was used to study the electrochemical-chemical reaction mechanism of indigo carmine oxidation in 0.1 M sulphuric acid. This study showed that the adsorption of the oxidation product of indigo carmine is stabilized when it is adsorbed on the surface of the electrode.     

The effect of electrolytic oxidation on the electrochemical properties of multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWNTs) were electrochemically oxidized by a constant-potential electrolysis method and then investigated in detail using scanning electron microscope, transmission electron microscope, FT-IR, electrical impedance spectroscopy, and cyclic voltammetry. The FT-IR spectra showed that the amount of hydroxyl generated on the surface of MWNTs increased with increasing the electrochemical oxidation time of MWNTs. The CV results, being conducted in nitrobenzene solution, showed that the nitrobenzene reduction current increased with the increase in oxidation time of the MWNTs within the first 60 min of electrolysis. An electrical equivalent circuit model for electrical impedance spectroscopy was further established to analyze the surface capacitance and resistance of the MWNTs, and the model results showed that the capacitance of the oxidized MWNTs increased greatly while the charge transfer resistance decreased, suggesting electrochemical oxidized MWNTs modified pyrolytic carbon electrode being an effective electrochemical sensor for nitrobenzene determination.     

Low potential detection of glucose at carbon nanotube modified glassy carbon electrode with electropolymerized poly(toluidine blue O) film

A mediator glucose biosensor has been constructed by immobilizing glucose oxidase at electropolymerized poly(toluidine blue O) film on carbon nanotube modified glass carbon electrode. The toluidine blue O moieties served as redox mediators for enzymatic glucose oxidation and as polymeric network to maintain the biosensor activity. Great enhancement in current response was observed for the glucose biosensor. The detection potential could be decreased to −0.1 V (versus Ag|AgCl), where common interferences such as ascorbic acid, uric acid and acetamidophenol were not oxidized to cause interferences. The amperometric glucose biosensor offered a sensitivity of 14.5 mA M⁻¹ cm⁻² for the linear range of 1-7 mM.     

Tantalum oxide-based compounds as new non-noble cathodes for polymer electrolyte fuel cell

Tantalum oxide-based compounds were examined as new non-noble cathodes for polymer electrolyte fuel cell. Tantalum carbonitride powder was partially oxidized under a trace amount of oxygen gas at 900 °C for 4 or 8 h. Onset potential for oxygen reduction reaction (ORR) of the specimen heat-treated for 8 h was 0.94 V vs. reversible hydrogen electrode in 0.1 mol dm⁻³ sulfuric acid at 30 °C. The partial oxidation of tantalum carboniride was effective to enhance the catalytic activity for the ORR. The partially oxidized specimen with highest catalytic activity had ca. 5.25 eV of ionization potential, indicating that there was most suitable strength of the interaction of oxygen and tantalum on the catalyst surface.     

Electrocatalytic oxidation of some amino acids on a nickel-curcumin complex modified glassy carbon electrode

This study investigated the electrocatalytic oxidation of alanine, l-arginine, l-phenylalanine, l-lysine and glycine on poly-Ni(II)-curcumin film (curcumin: 1,7-bis [4-hydroxy-3-methoxy phenyl]-1,6-heptadiene-3,5-dione) electrodeposited on a glassy carbon electrode in alkaline solution. The process of oxidation and its kinetics were established by using cyclic voltammetry, chronoamperometry and electrochemical impedance spectroscopy techniques. Voltammetric studies indicated that in the presence of amino acids the anodic peak current of low valence nickel species increased, followed by a decrease in the corresponding cathodic current. This indicates that amino acids were oxidized on the redox mediator which was immobilized on the electrode surface via an electrocatalytic mechanism. Using Laviron's equation, the values of α and k_s for the immobilized redox species were determined as 0.43 ± 0.03 and 2.47 ± 0.02 × 10⁶ s⁻¹, respectively. The rate constant, the electron transfer coefficient and the diffusion coefficients involved in the electrocatalytic oxidation of amino acids were determined.     

Poly(indole-6-carboxylic acid) and tetracyanoquinodimethane-modified electrode for selective oxidation of dopamine

Poly(indole-6-carboxylic acid) (PICA) was synthesized electrochemically over glassy carbon electrode (GCE) through potentiodynamic mode of polymerization. The resulting polymer was soluble in Tris-HCl buffer (pH 7.0). The processable polymer was cast over desired electrode surface along with organic redox mediator tetracyanoquinodimethane (TCNQ) as an electron transfer relay using Nafion^®. Nafion^® was used to solubilize TCNQ as well as to introduce permselectivity to the blend of polymer and TCNQ. The above blend was cast over GCE and characterized by cyclic voltammetry followed by its application in electrochemical sensing of dopamine (DA) and ascorbic acid (AA). The modified electrode was found to be selective for DA analysis. The lowest detection limit of DA sensing was found to be 4 μМ with a sensitivity of 18 μA ± 6 nA/mM of DA.     

Poly(taurine)/MWNT-modified glassy carbon electrodes for the detection of acetaminophen

A novel dye-polymer/CNT, poly(taurine)/MWNT-modified glassy carbon electrode was fabricated. This electrode is based on an electrochemically polymerized taurine layer coated on a MWNT film. The application of this electrode for voltammetric detection of acetaminophen is described. The electroactive surface area of the modified electrode was calculated to be 0.37 cm². Acetaminophen is oxidized at 0.38 V and then reduced at 0.27 V on the modified electrode. The irreversible oxidation process is due to the conversion of acetaminophen into imidogenquinone; the reduction process is ascribed to the reverse electrode reaction. The adsorption-controlled anodic peak current is proportional to the acetaminophen concentration (from 1.0 μM to 0.1 mM) with a detection limit of 0.5 μM. The detection of acetaminophen in drugs was conducted.     

Poly-phenothiazine derivative-modified glassy carbon electrode for NADH electrocatalytic oxidation

Electropolymerization of a new phenothiazine derivative (bis-phenothiazin-3-yl methane; BPhM) on glassy carbon (GC) electrode generates a conducting film of poly-BPhM, in stable contact with the electrode surface. The heterogeneous electron-transfer process corresponding to the modified electrode is characterized by a high rate constant (50.4 s⁻¹, pH 7). The GC/poly-BPhM electrode shows excellent electrocatalytic activity toward NADH oxidation. The rate constant for catalytic NADH oxidation, estimated from rotating disk electrode (RDE) measurements and extrapolated to zero concentration of NADH, was found to be 9.4 × 10⁴ M⁻¹ s⁻¹ (pH 7). The amperometric detection of NADH, at +200 mV vs. SCE, is described by the following electroanalytical parameters: a sensitivity of 1.82 mA M⁻¹, a detection limit of 2 μM and a linear domain up to 0.1 mM NADH.     

Chemically immobilised carbon nanotubes on silicon: Stable surfaces for aqueous electrochemistry

Diazonium ion chemistry has been used to electrochemically graft aminophenyl layers onto p-type silicon (1 0 0) substrates. A condensation reaction was used to immobilise single-walled carbon nanotubes with high carboxylic acid functionality directly to this layer. Electrochemical monitoring of the aminophenyl groups confirmed the formation of an amide linkage between the single-walled carbon nanotubes and the aminophenyl layer. The carbon nanotube electrode showed high stability and good electrochemical performance in aqueous solution. At moderate scan rates the Ru(NH₃)₆^+3/+2 couple exhibited quasi-reversible electron transfer kinetics with a standard heterogenous rate constant of 1.2 × 10⁻³ cm s⁻¹ at the covalently-linked carbon nanotube surface. The electrode thus combines the advantages of a silicon substrate for easy integration into sophisticated electrical and electronic devices, carbon nanotubes for desirable electrochemical properties, and stability in aqueous medium for future applications in environmental sensing.     

Gold supported catalytic layer: An intermediate step between fundamental and applied fuel cell studies

We report the preparation and electrochemical characterization of gold supported catalytic layer electrodes which have been prepared using a similar methodology to that employed in the preparation of conventional catalyst coated membranes. Consequently, the so-prepared catalytic layers have comparable properties (morphology and thickness) than those employed in direct liquid fuel cells. Using this working electrode configuration, and the so-called hydrogen adsorption-desorption region, fundamental electrochemical parameters such as electrochemically accessible Pt surface area, roughness factor and Pt-catalyst utilization of the catalytic layer have been evaluated. The electrochemically accessible Pt surface area, roughness factor and Pt-catalyst utilization have been found independent of the metal loading in the range of 0.1-0.5 mg Pt cm⁻². However for 1 mg Pt cm⁻², an important decrease on electrochemically accessible Pt surface area and Pt-catalyst utilization has been observed. On the other hand, when formic acid electrooxidation is used, a sudden decrease on the electrocatalytic activity has been observed as metal loading increases. These results clearly indicate that the formic acid electrooxidation process is strongly dependant of the accessibility of the reactant into the inner part of the electrodes, highlighting its mass-transport controlled reaction properties. These findings point out that, from an applied point of view, it is recommendable using catalytic layers as thinner as possible as well as high formic acid concentrations. These experimental conditions will maximize the Pt-catalyst utilization by minimizing the accessibility problems of the reactant into the inner part of the electrodes.     

Electrodeposition of platinum on polytyramine-modified electrodes for electrocatalytic applications

Cyclic voltammetry was used in order to deposit conducting polytyramine (PTy) on graphite substrates. In acidic media, ca. 40 deposition cycles resulted in the formation of stable polymer films with reasonably good resistivity. Electrochemical deposition of platinum on the PTy-covered graphite substrate allowed us to obtain a composite material that exhibits, for a platinum loading of 0.34 mg cm⁻², a specific electrochemically active surface area of the Pt particles of ca. 57 m² g⁻¹. Good electrocatalytic activity of this electrode material for phenol oxidation in acidic media was found, and the results suggested that, when deposited as small particles in a PTy matrix, platinum is less sensitive to deactivation by phenol oxidation polymeric products.     

Electrocatalytic hydrazine oxidation on quinizarine modified glassy carbon electrode

The electrocatalytic oxidation of hydrazine has been studied on glassy carbon modified by electrodeposition of quinizarine, using cyclic voltammetry and chronoamperometry techniques. It has been shown that the oxidation of hydrazine to nitrogen occurs at a potential where oxidation is not observed at the bare glassy carbon electrode. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized quinizarine were calculated as 4.44 s⁻¹ and 0.66, respectively. The heterogenous rate constant for oxidation of hydrazine at the quinizarine modified electrode surface was also determined and found to be about 4.83 × 10³ M⁻¹ s⁻¹. The diffusion coefficient of hydrazine was also estimated as 1.1 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometry.     

Synergistic effect of a catechin-immobilized poly(3,4-ethylenedioxythiophene)-modified electrode on electrocatalysis of NADH in the presence of ascorbic acid and uric acid

Catechin is a polyphenolic flavonoid that can be isolated from a variety of natural sources, including tea leaves, grape seeds, and the wood and bark of trees such as acacia and mahogany. In our experiments, catechin was immobilized on PEDOT/GC (poly(3,4-ethylenedioxythiophene)/glassy carbon)-modified electrodes and used as a mediator for NADH (nicotinamide adenine dinucleotide) oxidation. The effect of the PEDOT thickness on the surface coverage of the catechin molecules was studied using cyclic voltammetry. The electrochemical properties and the effect of pH on the redox properties of the immobilized catechin molecules were studied by cyclic voltammetry in phosphate solution. The electrocatalytic oxidation of NADH at different electrode surfaces such as the bare GC-, the PEDOT/GC-, the catechin/GC- and the catechin/PEDOT/GC-modified electrodes was explored in phosphate solution at pH 7. In the catechin/PEDOT/GC-modified electrode, the PEDOT film plays an important role in resolving the oxidation potentials of ascorbic acid and NADH into two peaks that occur at the same potential for the catechin/GC-modified electrode surface. The heterogeneous electron transfer rate constant for NADH oxidation at the catechin/PEDOT/GC-modified electrode was determined using the rotating disk electrode technique and found to be 9.88 × 10³ M⁻¹ s⁻¹. The amperometric determination of NADH at the catechin/PEDOT/GC electrode was tested. The sensitivity of the electrode was 19 nA/μM.     

Electrocatalytic oxidation of ascorbic acid by [Fe(CN)6] redox couple electrostatically trapped in cationic N,N-dimethylaniline polymer film electropolymerized on diamond electrode

Multinegatively charged metal complex, hexacyanoferrate ([Fe(CN)₆]⁴⁻), was electrostatically trapped in the cationic polymer film of N,N-dimethylaniline (PDMA) which was electrochemically deposited on the boron-doped diamond (BDD) electrode by controlled-potential electro-oxidation of the monomer. This ferrocyanide-trapped PDMA film was used to catalyze the oxidation of ascorbic acid (AA). Increase in the oxidation current response with a negative shift of the anodic peak potential was observed at the cationic PDMA film-coated BDD (PDMA|BDD) electrode, compared with that at the bare BDD electrode. A more drastic enhancement in the oxidation peak current as well as more negative shift of oxidation potential was found at the ferrocyanide-trapped PDMA film-coated BDD ([Fe(CN)₆]^3−/4−|PDMA|BDD) electrode. This [Fe(CN)₆]^3−/4−|PDMA|BDD electrode can be used as an amperometric sensor of AA. Ferrocyanide, electrostatically trapped in the polymer film shows more electrocatalytic activity than that coordinatively attached to the polymer film or dissolved in the solution phase. The electrocatalytic current depends on the surface coverage of ferricyanide, Γ_Fe, within the polymer film. Diffusion coefficient (D) of AA in the solution was estimated by rotating disk electrode voltammetry: D = (5.8 ± 0.3) × 10⁻⁶ cm² s⁻¹. The second-order rate constant for the catalytic oxidation of AA by ferricyanide was also estimated to be 9.0 × 10⁴ M⁻¹ s⁻¹. In the hydrodynamic amperometry using the [Fe(CN)₆]^3−/4−|PDMA|BDD electrode, a successive addition of 1 μM AA caused the successive increase in current response with equal amplitude and the sensitivity was calculated as 0.233 μA cm⁻² μM⁻¹.     

Voltammetric determination of bisoprolol fumarate in pharmaceutical formulations and urine using single-wall carbon nanotubes modified glassy carbon electrode

The electrochemistry of bisoprolol fumarate (BF) has been investigated by differential pulse voltammetry at a single-wall carbon nanotubes (SWNTs) modified glassy carbon electrode (GCE). The prepared electrode showed an excellent electrocatalytic activity towards the oxidation of BF leading to a marked improvement in sensitivity as compared to bare glassy carbon electrode where electrochemical activity for the analyte cannot be observed. The SWNTs-modified GCE exhibited a sharp anodic peak at a potential of ∼950 mV for the oxidation of BF. Under optimum conditions linear calibration curve was obtained over the BF concentration range 0.01-0.1 mM in 0.5 M phosphate buffer solution (pH 7.2) with a correlation coefficient of 0.9789 and detection limit of 8.27 × 10⁻⁷ M. The modified electrode has been applied for the drug determination in human urine with no prior extraction and in commercial tablets. The proposed method has also been validated.     

Electrocatalytic oxidation and simultaneous determination of uric acid and ascorbic acid on the gold nanoparticles-modified glassy carbon electrode

A new gold nanoparticles-modified electrode (GNP/LC/GCE) was fabricated by self-assembling gold nanoparticles to the surface of the l-cysteine-modified glassy carbon electrode. The modified electrode showed an excellent character for electrocatalytic oxidization of uric acid (UA) and ascorbic acid (AA) with a 0.306 V separation of both peaks, while the bare GC electrode only gave an overlapped and broad oxidation peak. The anodic currents of UA and AA on the modified electrode were 6- and 2.5-fold to that of the bare GCE, respectively. Using differential pulse voltammetry (DPV), a highly selective and simultaneous determination of UA and AA has been explored at the modified electrode. DPV peak currents of UA and AA increased linearly with their concentration at the range of 6.0 × 10⁻⁷ to 8.5 × 10⁻⁴ mol L⁻¹ and 8.0 × 10⁻⁶ to 5.5 × 10⁻³ mol L⁻¹, respectively. The proposed method was applied for the detection of UA and AA in human urine with satisfactory result.     

Microwave activation of electrochemical processes: High temperature phenol and triclosan electro-oxidation at carbon and diamond electrodes

The electrochemical oxidation of phenolic compounds in aqueous media is known to be affected by the formation of electro-polymerized organic layers which lead to partial or complete electrode blocking. In this study the effect of high intensity microwave radiation applied locally at the electrode surface is investigated for the oxidation of phenol and triclosan in alkaline solution at a 500 μm diameter glassy carbon or at a 500 μm × 500 μm boron-doped diamond electrode. The temperature at the electrode surface and mass transport enhancement are determined by calibration with the Fe(CN)₆^3−/4− redox system in aqueous 0.3 M NaOH and 0.2 NaCl (pH 12) solution. The calibration shows that strong thermal and mass transport effects occur at both glassy carbon and boron-doped diamond electrodes. The average electrode temperature reaches up to 390 K and mass transport enhancements of more than 20-fold are possible. For the phenol electro-oxidation at glassy carbon electrodes and at a concentration below 2 mM a multi-electron oxidation (ca. 4 electrons) occurs in the presence of microwave radiation. For the electro-oxidation of the more hydrophobic triclosan only the one-electron oxidation occurs. Although currents are enhanced in presence of microwave radiation, rapid blocking of the electrode surface in particular at high phenol concentrations still occurs.     

Preparation of polypyrrole/ferrocyanide films modified carbon paste electrode and its application on the electrocatalytic determination of ascorbic acid

Functionalized polypyrrole film were prepared by incorporation of (Fe(CN)₆)⁴⁻ as doping anion, during the electropolymerization of pyrrole onto a carbon paste electrode (CPE) in aqueous solution by using potentiostatic method. The electrochemical behavior of the (Fe(CN)₆)³⁻/(Fe(CN)₆)⁴⁻ redox couple in polypyrrole was studied by cyclic voltammetry and double step potential chronoamperometry methods. In this study, an obvious surface redox reaction was observed and dependence of this reaction on the solution pH was illustrated. The electrocatalytic ability of polypyrrole/ferrocyanide films modified carbon paste electrode (Ppy/FCNMCPEs) was demonstrated by oxidation of ascorbic acid. It has been found that under optimum condition (pH 7.00), the oxidation of ascorbic acid at the surface of such electrode occurs at a potential about 540 mV less positive than unmodified carbon paste electrode. The kinetic parameters such as electron transfer coefficient, α and catalytic reaction rate constant, k_h′, were also determined by using various electrochemical approaches.The catalytic oxidation peak current showed a linear dependent on the ascorbic acid concentration and a linear calibration curve was obtained in the range of 4.5×10⁻⁴ to 9.62×10⁻³ M of ascorbic acid with a correlation coefficient of 0.9999. The detection limit (2σ) was determined as 5.82×10⁻⁵ M.     

Diesel fuel desulfurization with hydrogen peroxide promoted by formic acid and catalyzed by activated carbon

Desulfurization of diesel fuels with hydrogen peroxide was studied using activated carbons as the catalysts. Adsorption and catalytic properties of activated carbons for dibenzothiophene (DBT) were investigated. The higher the adsorption capacity of the carbons is, the better the catalytic performance in the oxidation of DBT is. The effect of aqueous pH on the catalytic activities of the activated carbons was also investigated. Oxidation of DBT is enhanced when the aqueous pH is less than 2, and addition of formic acid can promote the oxidation. The effect of carbon surface chemistry on DBT adsorption and catalytic activity was also investigated. Adsorption of DBT shows a strong dependence on carboxylic group content. The oxidative removal of DBT increases as the surface carbonyl group content increases. Oxidative desulfurization of a commercial diesel fuel (sulfur content, 800 wt. ppm) with hydrogen peroxide was investigated in the presence of activated carbon and formic acid. Much lower residual sulfur content (142 wt. ppm) was found in the oxidized oil after the oxidation by using the hydrogen peroxide-activated carbon-formic acid system, compared with a hydrogen peroxide-formic acid system. The resulting oil contained 16 wt. ppm of sulfur after activated carbon adsorption without any negative effects in the fuel quality, and 98% of sulfur could be removed from the diesel oil with 96.5% of oil recovery. Activated carbon has high catalytic activity and can be repeatedly used following simple water washing, with little change in catalytic performance after three regeneration cycles.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.