A highly selective amperometric sensor for ascorbic acid based on mesopore-rich active carbon-modified pyrolytic graphite electrode

A kind of new mesopore-rich active carbon (MRAC) was firstly prepared by carbonizing bamboo micro-particles, which was utilized for constructing MRAC-modified pyrolytic graphite electrode (MRAC/PGE). The electrochemical behavior of ascorbic acid (AA) was in detail investigated at the MRAC/PGE. The proposed MRAC/PGE showed an excellent electrocatalytic response towards AA oxidation in neutral buffer solution. The oxidation potential of AA significantly decreased at MRAC/PGE. The oxidation current of AA obtained at the MRAC/PGE is 4.6-fold higher than that of the bare PGE. Using amperometric method, the anodic current is linear with AA concentration in the range of 0.5-2000 μM, with a detection limit about 0.3 μM. Furthermore, the proposed electrode can also avoid major interferences such as dopamine, uric acid, urea, and so on. This new method has been successfully applied for AA assay in urine and pharmaceutical preparations.     

Electrodeposition of high Mo content amorphous/nanocrystalline Ni–Mo alloys using 1-ethyl-3-methyl-imidazolium chloride ionic liquid as an additive

In the following research, adherent, compact and bright Ni–Mo alloy has been electrodeposited on mild steel in the presence of ionic liquid additive 1-ethyl-3-methyl-imidazolium chloride in ammonia citrate solution using pulse plating technique. The textural components of the coatings were evaluated employing X-ray diffraction (XRD) method. Microstructure, morphology and chemical composition of the coatings were studied using scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) analysis. The results revealed that in the presence of ionic liquid at pH of 8.5, Ni–Mo films containing more than 49 wt.% Mo have been carried out whilst no proper deposits have been conducted in additive free solution at the same condition. SEM micrographs showed different morphology of Ni–Mo deposits in the presence of ionic liquid additive. Also XRD patterns represented that present conditions lead to amorphous/nanocrystalline Ni–Mo coatings. Temperature and pH were varied for studying their effects on Ni–Mo characteristics. Moreover the effect of substrate preparation on morphology of coatings was also investigated.     

A comparative study of Pt and Pt–Pd core–shell nanocatalysts

This comparative study characterizes two types of metallic and core–shell bimetallic nanoparticles prepared with our modified polyol method. These nanoparticles consist of Pt and Pt–Pd core–shell nanocatalysts exhibiting polyhedral morphologies. The controlled syntheses of Pt metallic nanoparticles in the 10-nm regime (4–8 nm) and Pt–Pd bimetallic core–shell nanoparticles in the 30-nm regime (15–25 nm) are presented. To realize our ultimate research goals for proton exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs), we thoroughly investigate the dependence of the electrocatalytic properties of the nanoparticles on the structure, size and morphology. Significant differences in the electrocatalysis are also explained in experimental evidences of both Pt and Pt–Pd nanocatalysts. We suggested that the core–shell controlled morphologies and nanostructures of the Pd nanoshell as the Pd atomic monolayers will not only play an important role in producing inexpensive, novel Pt- and Pd-based nanocatalysts but also in designing more efficient Pt- and Pd-based nanocatalysts for practical use in DMFC technology. Our comparative results show that Pt–Pd nanocatalysts with Pd nanoshells exhibited much better electrocatalytic activity and stabilization compared to Pt nanocatalysts. Interestingly, we found that the size effect is not as strong as the nanostructuring effect on the catalytic properties of the researched nanoparticles. A nanostructure effect of the core–shell bimetallic nanoparticles was identified.     

Kinetic study of nitrate reduction on Pt(1 1 0) electrode in perchloric acid solution

The kinetics of electrocatalytic reduction of nitrate on Pt(1 1 0) in perchloric acid was studied with cyclic voltammetry at a very low sweep rate of 1 mV s⁻¹, where pseudo-steady state condition was assumed to be achieved at each electrode potential. Stationary current-potential curves in perchloric acid in the absence of nitrate showed two peaks at 0.13 V and 0.23 V (RHE) in the so-called adsorbed hydrogen region. The nitrate reduction proceeded in the potential region of the latter peak in the pH range studied. The reaction orders with respect to NO₃⁻ and H⁺ were observed to be close to 0 and 1, respectively. The former value means that the adsorbed NO₃⁻ at a saturated coverage is one of the reactants in the rate-determining step (rds). The latter value means that hydrogen species is also a reactant above or on the rds. The Tafel slope of nitrate reduction was −66 mV per decade, which is taken to be approximately −59 mV per decade, indicating that the rds is a pure chemical reaction following electron transfer. We discuss two possible reaction schemes including bimolecular and monomolecular reactions in the rds to explain the kinetics and suggest that the reactants in the rds are adsorbed hydrogen and adsorbed NO₃⁻ with the assistance of the results in our recent report for nitrate reduction on Pt(S)[n(1 1 1) × (1 1 1)] electrodes: the nitrate reduction mechanism can be classified within the framework of the Langmuir-Hinshelwood mechanism.     

Electropolymerization of chlorinated phenols on a Pt electrode in alkaline solution Part I: A cyclic voltammetry study

Cyclic voltammetry studies were carried out on a platinum electrode in alkaline 1 M NaOH solutions containing 0.1 M of phenol, monochlorophenols, dichlorophenols, trichlorophenols and pentachlorophenol in order to compare their electropolymerization ability and the degree of deactivation of the electrode. These show that fouling of the Pt electrode occurs during the electrooxidation of all the phenols studied. Almost full deactivation of the electrode occurs in the case of phenol and 3-chlorophenol after five cycles, while 4-chlorophenol, 3,4-dichlorophenol, 2,4-dichlorophenol and 2,4,5-trichlorophenol deactivate the electrode completely after the first cycle. Partial deactivation of the electrode occurs in the case of 2-chlorophenol, 2,3-dichlorophenol, 2,6-dichlorophenol, 2,5-dichlorophenol and 2,4,6-trichlorophenol. A weak fouling of the electrode occurs in the case of 3,5-dichlorophenol, 2,3,6-trichlorophenol and pentachlorophenol. Such differences in the degree of electrode deactivation may be explained by the different structure (permeability) of the polymeric tars formed. The structure of the tars depends on the monomer structure. Presumably, more regular and dense polymer structures deactivate the electrode more rapidly. More branched, irregular high molecular weight substances deactivate the electrode more slowly.     

A novel amperometric sensor for peracetic acid based on a polybenzimidazole–modified gold electrode

We have developed a peracetic acid (PAA) sensor based on a polybenzimidazole–modified gold (PBI/Au) electrode. Fourier transform infrared and X-ray photoelectron spectroscopy indicated that PAA oxidized 69.4% of the imine in PBI to form PBI N-oxide, increasing the electrochemical reduction current during cyclic voltammetry. The chemical oxidation of the PBI/Au electrode by PAA, followed by its electrochemical reduction, allowed PAA to be detected directly and consecutively by assessing its reduction current. The PAA sensor had a broad linear detection range (3.1 μM–1.5 mM) and a rapid response time (3.9 s) at an applied potential of −0.3 V. Potentially interfering substances, such as hydrogen peroxide, acetic acid, and oxygen, had no effect on the ability of the probe to detect PAA, indicating high selectivity of the probe. Furthermore, the detection range, response time, and sensitivity of the sensor could all be improved by modification of the smooth planar electrode surface to a porous three-dimensional configuration. When compared to the analytical characteristics of other PAA sensors operating under optimal conditions, the three-dimensional PBI/Au electrode offers a rapid detection time, a usable linear range, and a relatively low detection limit.     

Thermodynamic analysis of (bi)sulphate adsorption on a Pt(1 1 1) electrode as a function of pH

A complete thermodynamic study of (bi)sulphate adsorption on Pt(1 1 1) electrodes from solutions at four different pHs (pH 0.43, 2.1, 3.1 and 4.1) is reported. The effect of pH on the sum of the Gibbs excesses of sulphate and bisulphate species, standard Gibbs energies of adsorption and formal partial charge numbers is analyzed. The results provide relevant information on the nature of species involved in the different voltammetric features. The experiments at pH 0.43 were performed in a higher base electrolyte concentration (0.5 M), that allows the study of (bi)sulphate adsorption in a broader range of concentrations. Under these conditions, two adsorption steps are clearly defined, associated to two different voltammetric features, between 0.30 and 0.60 V and between 0.65 and 0.90 V (standard hydrogen scale, SHE). Once the pH is increased, a marked decrease in absolute value of the (bi)sulphate adsorption Gibbs energy is observed, concomitant with an increasing amount of OH co-adsorption.     

Electropolymerization of chlorinated phenols on a Pt electrode in alkaline solution Part III: A Fourier transformed infrared spectroscopy study

FTIR spectroscopy was employed to investigate high molecular weight substances formed on a platinum electrode surface during the electrochemical oxidation of phenol and its chlorinated derivatives. Potentiodynamic (potential range from –0.80 V to 0.85 V vs SHE; scan rate 200 mV s^–1) and potentiostatic (at 0.78 V vs SHE) electropolymerization was used in alkaline solutions (1 M NaOH) containing 0.1 M of phenol, monochlorophenols, dichlorophenols, trichlorophenols and pentachlorophenol. The IR spectra of the corresponding monomers were recorded for the comparison. The FTIR spectroscopy studies revealed that the polymers formed under potentiodynamic and potentiostatic conditions are of aromatic nature (–C=C– stretching vibrations at 1450–1600 cm^–1), they have ether-linkages (=C—O—C= stretching vibrations at 1100–1300 cm^–1) and quinone groups (–C=O stretching vibrations at 1630–1800 cm^–1 and –C—H out-of-plane bending at 760 cm^–1). The intensities of the hydroxyl group bands in most of the polymers are rather weak compared to those in the corresponding monomers. Vibrations at 2850–2960 cm^–1, which are present in most of the IR spectra of polymers formed under cyclic voltammetry conditions, correspond to the stretching vibrations of the sp³ hybridized C—H bond and suggest that the cleavage of the benzene ring occurs to some extent during electrooxidation–electropolymerization of phenol and its chlorinated derivatives when reaching the potential of oxygen evolution (0.85 V vs SHE).     

Nafion/lead oxide–manganese oxide combined catalyst for use as a highly efficient alkaline air electrode in zinc–air battery

In this study, we report the application of an inexpensive and easily prepared lead oxide–manganese oxide catalyst combined with Nafion (designated as Nf/PbMnO_x) as a highly efficient air-cathode for a zinc–air battery. We verify the mechanistic study of the reduction of O₂ for Nf/PbMnO_x in alkaline aqueous solution using rotating ring/disk electrode voltammetry, and also an electrochemical approach using a wall-jet screen-printed ring disk electrode. The presence of Nf/PbMnO_x shows great catalytic activity for the disproportionation reaction of HO₂⁻ to O₂ and OH⁻ with an overall 4e⁻ reduction of O₂ in the first reduction reaction. The 4e⁻ reduction of O₂ was eventually achieved at the Nf/PbMnO_x through evidence from the slope of Koutecky–Levich plots. With these inherent features, we then fabricated the zinc–air battery with the Nf/PbMnO_x catalyst and examine the performance for a practical application with air cathodes.     

Thermodynamic approach to the double layer capacity of a Pt(1 1 1) electrode in perchloric acid solutions

A thermodynamic method based on the work done by Frumkin and Petrii [A.N. Frumkin, O.A. Petrii, Electrochim. Acta 20 (1975) 347], to calculate the so-called double layer capacity for a Pt(1 1 1) electrode is proposed. The analysis requires careful measurement of the total charge density versus potential curves for a series of solutions with composition (0.1 − x) M KClO₄ + x M HClO₄. A method in which the total charge densities are determined by integration of cyclic voltammograms recorded in solutions with or without chloride is described. Following this procedure the double layer capacity curves were calculated. The double layer capacity curves displayed three peaks that were tentatively assigned to the solvent reorientation, onset of OH adsorption and completion of the OH adlayer. In the hydrogen adsorption region, the double layer capacity values were 14 ± 5 μF/cm², in good agreement with previous estimates reported in the literature by using other approaches.     

Electropolymerization of chlorinated phenols on a Pt electrode in alkaline solution. Part IV: A gas chromatography mass spectrometry study

The electropolymerization of phenol and chlorinated phenols (monochlorophenols, dichlorophenols, 2,3,6-, 2,4,6-, 2,4,5-trichlorophenols and pentachlorophenol) was studied on a platinum electrode at 0.78 V vs SHE in alkaline 1 M NaOH aqueous solutions containing 0.1 M of the phenols. The low molecular weight reaction products were investigated by means of gas chromatography mass spectrometry (GCMS). Product analyses show that oligomers (dimers, trimers and tetramers) are present in the polymer mixtures formed. The MS spectra reveal the ether-linked nature of the oligomers formed during the electrooxidation-electropolymerization of the phenolic compounds. The mass spectra of the low molecular weight substances formed suggest that the oxidation–polymerization of phenols proceeds following two different mechanisms: (i) through the quinol-ether route (without chlorine elimination) and (ii) via the nucleophilic-radical substitution (S_RN1) route (with some elimination of chlorine from ortho and/or para positions).     

A novel hydrazine electrochemical sensor based on a zirconium hexacyanoferrate film-bimetallic Au–Pt inorganic–organic hybrid nanocomposite onto glassy carbon-modified electrode

This work describes the electrochemical behavior of zirconium hexacyanoferrate (ZrHCF) film immobilized on the surface of bimetallic Au–Pt inorganic–organic hybrid nanocomposite glassy carbon electrode and its electrocatalytic activity toward the oxidation of hydrazine. The electrode possesses a three-dimensional (3D) porous network nano architecture (NFs). The surface structure and composition of the sensor was characterized by scanning electron microscopy (SEM). Electrocatalytic oxidation of hydrazine on the surface of modified electrode was investigated with cyclic voltammetry and chronoamperometry methods and the results showed that the ZrHCF film displays excellent electrochemical catalytic activities toward hydrazine oxidation. The modified electrode indicated reproducible behavior and high level of stability during the electrochemical experiments.     

Thermodynamic studies of phosphate adsorption on Pt(1 1 1) electrode surfaces in perchloric acid solutions

The thermodynamics of the so-called perfectly polarizable electrode was employed to analyze the total charge densities for a nearly defect-free Pt(1 1 1) electrode in a series of NaH₂PO₄ solutions with an excess of inert electrolyte (0.1 M HClO₄) at constant ionic strength and pH. Thermodynamic analysis using both electrode potential and charge density as independent electrical variables is described. The Gibbs excess, Gibbs energy of adsorption and charge numbers both at constant electrode potential and constant chemical potential for anion adsorption at the Pt(1 1 1) surface have been determined. The calculated electrosorption valencies and charge numbers at constant chemical potential are close to two electrons per adsorbed anion, suggesting that in the absence of co-adsorbed species, HPO₄²⁻ is the predominant adsorbed species. The maximum Gibbs excess of adsorbed hydrogenphosphate attains a value of ≈3.2 × 10¹⁴ ions cm⁻² which corresponds to a coverage of ≈0.22 ML.     

XPS study of irreversibly adsorbed arsenic on a Pt(1 1 1) electrode

The chemical composition of an irreversibly adsorbed layer of arsenic on Pt(1 1 1) in sulfuric acid solution has been studied by X-ray photoelectron spectroscopy (XPS). From the chemical shift of the As 3d level, a change in the valence state from As(0) to As(III) with positive-going electrode potential is deduced, the total amount of As on the surface remaining constant. The As coverage derived from XPS is around 0.33 ML, which is in agreement with the charge under the current peak in the cyclic voltammogram. From the coadsorption of anions, accompanying the valence transition of As, As(III) is assumed to exist as As(OH)₃ on the surface.     

Hydrogenation of Citral using Monometallic Pt and Bimetallic Pt‐Ru Catalysts on a Mesoporous Support in Supercritical Carbon Dioxide Medium

Supercritical carbon dioxide was shown to be a suitable reaction medium for the highly efficient hydrogenation of citral using monometallic Pt and bimetallic Pt‐Ru supported on a mesoporous material, MCM‐48, as catalyst. A remarkable change in the product distribution was observed after the addition of Ru to the monometallic Pt catalyst in supercritical carbon dioxide. The monometallic Pt catalyst was found to be highly selective to the unsaturated alcohol (geraniol+nerol) at a temperature of 323 K within 2 h whereas the bimetallic catalyst becomes selective to the partially saturated aldehyde (citronellal) under the same reaction conditions. Phase behavior plays an important role in the product distribution. Highest conversion and high selectivity to citronellal were achieved in the homogeneous phase for the Pt‐Ru catalyst while on the other hand the unsaturated alcohol (geraniol+nerol) was produced in the heterogeneous phase for the monometallic Pt catalyst. An XPS study offers strong evidence of the electronic modification of Pt after the addition of Ru in the bimetallic catalyst. The change in product distribution on the Pt‐Ru bimetallic catalyst may be explained by the appreciable interaction between the medium and the metal particles promoted by the presence of metallic Ru.     

A selective determination of levodopa in the presence of ascorbic acid and uric acid using a glassy carbon electrode modified with reduced graphene oxide

A selective determination of levodopa (LD) in the presence of ascorbic acid (AA) and uric acid (UA) has been investigated at a glassy carbon electrode modified with reduced graphene oxide (rGO). The graphene oxide was synthesized chemically by Hummers method and characterized by energy-filtered transmission electron microscopy (EF-TEM). The reduced graphene oxide modified glassy carbon electrode (rGO/GCE) showed excellent electrochemical performance in the simultaneous electrochemical detection of LD, AA, and UA due to the unique properties of graphene, such as large surface area, facile electronic transport and high electrocatalytic activity. The redox characteristics of rGO/GCE were investigated with cyclic voltammetry (CV) and differential pulse voltammetry (DPV). Well-resolved oxidation peak potentials, corresponding to the oxidation of AA, LD, and UA, were observed from their mixture solution at 0.098, 0.285, and 0.423 V, respectively. The rGO/GCE showed that LD can be detected without the interference of AA and UA. Under the optimized conditions, the oxidation peak current of LD is linear with the concentration of LD from 2.0 to 100 μM with the detection limit of 1.13 μM (S/N = 3). The present electrode system was also successfully applied to direct determination of LD in commercially available tablets and urine samples.     

Non–Pt Catalyst Layer Operation in a PEM Fuel Cell: A Variable‐thickness Regime

A recently pubilshed experimental polarization curve of a PEM fuel cell with the non–Pt cathode catalyst layer (CCL) exhibits unusual feature: in the region of small current densities, the curve is close to linear. We report a model for the CCL performance which explains this effect. The model includes finite rate of the oxygen adsorption on the catalyst surface. Qualitatively, due to a very high exchange current density of the non–Pt catalyst, the ORR rate close to the membrane is determined by the potential–independent oxygen adsorption rate. This leads to a specific regime of the CCL operation, when only part of the CCL thickness contributes to current production, while the rest part is completely inactive. With the growth of the cell current, the active part increases in width, while the inactive part shrinks. The resulting polarization curve appears to be close to linear.     

Electrocatalytic oxidation of ascorbic acid using a single layer of gold nanoparticles immobilized on 1,6-hexanedithiol modified gold electrode

This paper describes the electrocatalytic oxidation of ascorbic acid (AA) in phosphate buffer solution by the immobilized citrate capped gold nanoparticles (AuNPs) on 1,6-hexanedithiol (HDT) modified Au electrode. X-ray photoelectron spectrum (XPS) of HDT suggests that it forms a monolayer on Au surface through one of the two SH groups and the other SH group is pointing away from the electrode surface. The free SH groups of HDT were used to covalently attach colloidal AuNPs. The covalent attachment of AuNPs on HDT monolayer was confirmed from the observed characteristic carboxylate ion stretching modes of citrate attached with AuNPs in the infra-red reflection absorption spectrum (IRRAS) in addition to a higher reductive desorption charges obtained for AuNPs immobilized on HDT modified Au (Au/HDT/AuNPs) electrode in 0.1 M KOH when compared to HDT modified Au (Au/HDT) electrode. The electron transfer reaction of [Fe(CN)₆]^4−/3− was markedly hindered at the HDT modified Au (Au/HDT) electrode while it was restored with a peak separation of 74 mV after the immobilization of AuNPs on Au/HDT (Au/HDT/AuNPs) electrode indicating a good electronic communication between the immobilized AuNPs and the underlying bulk Au electrode through a HDT monolayer. The Cottrell slope obtained from the potential-step chronoamperometric measurements for the reduction of ferricyanide at Au/HDT/AuNPs was higher than that of bare Au electrode indicating the increased effective surface area of AuNPs modified electrode. The Au/HDT/AuNPs electrode exhibits excellent electrocatalytic activity towards the oxidation of ascorbic acid (AA) by enhancing the oxidation peak current to more than two times with a 210 mV negative shift in the oxidation potential when compared to a bare Au electrode. The standard heterogeneous electron transfer rate constant (k_s) calculated for AA oxidation at Au/HDT/AuNPs electrode was 5.4 × 10⁻³ cm s⁻¹. The oxidation peak of AA at Au/HDT/AuNPs electrode was highly stable upon repeated potential cycling. Linear calibration plot was obtained for AA over the concentration range of 1–110 μM with a correlation coefficient of 0.9950. The detection limit of AA was found to be 1 μM. The common physiological interferents such as glucose, oxalate ions and urea do not show any interference within the detection limit of AA. The selectivity of the AuNPs modified electrode was illustrated by the determination of AA in the presence of uric acid.     

A carboxyalkanethiol monolayer modified electrode for blocking of ascorbic acid oxidation and its application to a biosensor

Upon the application of amperometric biosensor to the biological fluid, ascorbic acid interferes the amperometric determination of analytes, because the oxidative potential of ascorbic acid is lower than that of electro active substances such as H₂O₂ produced by the enzymatic reaction. In this study we propose a method to block ascorbic acid based on the electrostatic interaction with self-assembled monolayer (SAM) and its application of the surface modified electrode to biosensor. In order to form SAM on the gold electrode with carboxyl group, 7-carboxy-heptanethiol (7-CHT) was used. The 7-CHT modified electrode did not show anodic response to ascorbic acid, but oxidized phenanthroline cobalt complex [Co(phen)₃²⁺], which can be used as a mediator of biosensor. Thus, the 7CHT-modified electrode was applied to biosensor mediated with Co(phen)₃²⁺. Fructose dehydrogenase (FDH) was immobilized to the 7-CHT modified electrode. Fructose was determined selectively with the FDH/7-CHT modified electrode at the range of 0.2-2 mM.     

One‐step oxidation of aniline by peroxotitanium acid to polyaniline–titanium dioxide: A highly stable electrode for a supercapacitor

In this study, to make a stable electrode material for a supercapacitor, we selected a polyaniline and titanium dioxide (TiO₂) hybrid material. Peroxotitanium acid was used to oxidize aniline in the presence of sulfuric acid to a poly(aniline sulfate) salt–titanium oxide composite in one step. IR, X‐ray diffraction, and energy dispersive X‐ray analysis (EDAX) analyses supported the formation of the composite. The poly(aniline sulfate) salt–titanium oxide composites (50 wt % each) showed an amorphous, flakelike morphology having a conductivity value of 8 × 10^?3 S/cm with an excellent yield and stability (300°C).This composite material in the cell configuration showed a specific capacitance of 320 F/g at a 0.33 A/g discharge current density. Thirty thousand charge–discharge (CD) cycles at a heavy CD current density of 3.3 A/g were carried out on the supercapacitor cell. The values of equivalent series resistance (ESR) (8–9 Ω) and efficiency (100–98%) were found to be independent of the cycle number with an excellent retention capacity of 83%. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41711.