Simultaneous determination of paracetamol and ascorbic acid using tetraoctylammonium bromide capped gold nanoparticles immobilized on 1,6-hexanedithiol modified Au electrode

Tetraoctylammonium bromide stabilized gold nanoparticles (TOAB-AuNPs) attached to 1,6-hexanedithiol (HDT) modified Au electrode was used for the simultaneous determination of paracetamol (PA) and ascorbic acid (AA) at physiological pH. The attachment of TOAB-AuNPs on HDT modified Au surface was confirmed by attenuated total reflectance (ATR)-FT-IR spectroscopy and atomic force microscope (AFM). The ATR-FT-IR spectrum of TOAB-AuNPs attached to the HDT monolayer showed a characteristic stretching modes corresponding to -CH₂ and -CH₃ of TOAB, confirming the immobilization of AuNPs with surface-protecting TOAB ions on the surface of the AuNPs after being attached to HDT modified Au electrode. AFM image showed that the immobilized AuNPs were spherical in shape and densely packed to a film of ca. 7 nm thickness. Interestingly, TOAB-AuNPs modified electrode shifted the oxidation potential of PA towards less positive potential by 70 mV and enhanced its oxidation current twice when compared to bare Au electrode. In addition, the AuNPs modified electrode separated the oxidation potentials of AA and PA by 210 mV, whereas bare Au electrode failed to resolve them. The amperometry current of PA was increased linearly from 1.50 × 10⁻⁷ to 1.34 × 10⁻⁵ M with a correlation coefficient of 0.9981 and the lowest detection limit was found to be 2.6 nM (S/N = 3). The present method was successfully used to determine the concentration of PA in human blood plasma and commercial drugs.     

Electrochemical evaluation of the surface of chalcopyrite during dissolution in sulfuric acid solution

The dissolution of a massive chalcopyrite electrode (98.1% chalcopyrite, 1.9% siderite) was studied in 0.5 M sulfuric acid solution. Different anodic potentials were applied and the behavior of the electrode was observed by means of EIS, potentiodynamic, and Mott-Schottky techniques. Electrochemical impedance spectroscopy studies at open circuit potential (around −235 mV vs. MSE) proved the existence of a thin surface layer on the electrode. This layer was stable up to 100 mV vs. MSE and was assumed to be Cu_1−xFe_1−yS₂ (y?x) based on reports from previous studies. By increasing the potential to the range of 100-300 mV vs. MSE, the previously formed layer partially dissolved and a second layer (Cu_1−x−zS₂) formed on the surface. Both of the layers showed the characteristics of passive layers at low potentiodynamic scan rate (0.05 mV s⁻¹) while at high scan rates they acted like pseudo-passive layers. However, in the potential range of 300-420 mV vs. MSE, both of these surface layers dissolved and active dissolution of the electrode started. Further increase in potential caused the formation of a CuS layer which hindered the dissolution rate of the electrode. The formation of CuS is concomitant with Fe₂(SO₄)₃ formation and the latter may act as a nucleation precursor for jarosite at higher potentials (around 750 mV vs. MSE). Jarosite precipitation on the electrode surface hindered the dissolution of chalcopyrite at higher potentials. Different equivalent electrochemical circuits were modeled for each potential range and the model regression results compared with the experimental results of EIS to determine the proposed sequence of chalcopyrite dissolution.     

Characterization of permselective coatings electrosynthesized on Pt-Ir from the three phenylenediamine isomers for biosensor applications

The permeability of polymers, electrosynthesized at neutral pH on Pt-Ir cylinders from each of the three isomers of phenylenediamine (oPD, mPD and pPD), to H₂O₂ (signal transduction molecule in many oxidase-based biosensors) and ascorbic acid (AA, archetypal interference species in biological applications of biosensors) was measured, and used to determine the permselectivity of the three polymers. PmPD was the coating with the greatest permselectivity for H₂O₂ over AA, for low concentrations of AA. For AA levels greater than 200 μM, however, poly-ortho-phenylenediamine (PoPD) was superior. Furthermore, stability studies indicated that the permselectivity of PmPD degraded rapidly, even after 1 day, supporting the choice of PoPD as the permselective membrane for biosensor implantation where AA levels are high, such as in brain monitoring. A variety of techniques were used to gain further insight into the PPD layers, specifically electrochemical quartz crystal microbalance, mass spectrometry and scanning electron microscopy. Together these studies indicate that PmPD forms the thickest layer (∼15 nm) and is the least soluble of the polymers, that the PoPD layer is ∼4 nm thick and may consist mostly of tetramers, while PpPD is the thinnest (∼3 nm) and appears to consist of trimers.     

Prussian Blue-modified microelectrodes for selective transduction in enzyme-based amperometric microbiosensors for in vivo neurochemical monitoring

Prussian Blue-modified carbon fiber microelectrodes (CFE/PBs) are proposed as an alternative to the more conventional metal transducers used for H₂O₂-detecting biosensors in brain extracellular fluid (ECF). The main advantages of this approach are the very small dimensions (∼10 μm diameter) and the low applied potentials needed (0.0 V versus SCE). Electrocatalytic and physiochemical properties of PB deposits were studied using cyclic voltammetric (CV), amperometric and spectroscopic methods (FTIR and VIS). Optimized CFE/PB microsensors displayed a H₂O₂ current density of 1.00 ± 0.04 A M⁻¹ cm⁻² with a detection limit of 10⁻⁸ M. Furthermore, to improve stability and selectivity properties, several polymeric films were investigated: Nafion, poly(o-phenylenediamine) (PoPD), and a hybrid configuration of these two polymers. Finally, the PoPD film was selected due to its excellent anti-interference properties. The use of this permselective film also enhanced the stability of PB against solubilization at high pH, albeit at the expense of slightly lower H₂O₂ sensitivity (0.48 ± 0.02 A M⁻¹ cm⁻²) and higher detection limit (∼10⁻⁷ M). However, the use of the PoPD film significantly enhanced the selectivity against the main endogenous brain interference species (ascorbic acid, uric acid, dopamine and 3,4-dihydroxyphenylacetic acid), expressed as the ratio of the sensitivity slopes (SH₂O₂/Sinterference), which was close to 600 for all interference molecules studied. A prototype of a CFE/PB-based glucose microbiosensor design is presented, together with preliminary studies of its characteristics in vitro and its functionality in brain ECF in vivo.     

Electropolymerization kinetic study of 3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine and its optical optimization for electrochromic window applications

Poly (3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine), PProDOT-Me₂, is one of the most promising conducting polymers in the alkylenedioxythiophene based family for electrochromic window applications. In the electropolymerization kinetic study of 3,3-dimethyl-3,4-dihydro-2H-thieno[3,4-b][1,4]dioxepine (ProDOT-Me₂), microgravimetry and chronoamperometry were used to determine the reaction orders with respect to the electrolyte and monomer, and the corresponding general kinetic equation of electropolymerization. This study presents that monomer concentration has a strong impact on electropolymerization mechanism. The relationship between film thickness and polymerization time was analyzed indicating that saturation of polymerization reduced the increase rate of film thickness with polymerization time. Also, the electropolymerization conditions were optimized to reach high contrast (Δ%T > 70%) with the minimum of transmittance (%T_min < 1) for electrochromic window applications.     

Electrochemical preparation and kinetic study of poly(o-tolidine) in aqueous medium

Poly(o-tolidine), PoT, film was prepared by electrochemical oxidation of the monomer, oT, in 0.1 M HCl + 0.1 M KClO₄. The presence of KClO₄ in the formation medium was found to be essential for the electropolymerization process to proceed. Increasing the upper potential limit up to +1.5 V, instead of +1.0 V, leads to appearance of a new anodic peak at +1.36 V and enhancement of the polymer formation of PoT without changing the film structure. The electrochemical behavior of the formed polymer films was investigated in 1.0 M HClO₄. The kinetic parameters were calculated from the values of the charge consumed during the electropolymerization process. The rate of the polymerization reaction was found to depend on the concentration of the monomer rather than the electrolyte. The polymerization rate is first order with respect to the monomer concentration and zero order with respect to the electrolyte. The electrolyte plays no active role in the kinetics of the electropolymerization process and its role is most likely limited to polymer doping.     

The influence of potential on the anodic dissolution of SIMFUEL (UO2)

The influence of potential on the anodic dissolution of SIMFUEL (doped uranium dioxide) has been characterized over the range 0-500 mV (versus SCE). Cathodic stripping voltammetry was used to determine the changes in surface reactivity of UO₂ in neutral solutions after different anodic oxidation timescales. Scanning electron microscopy (SEM) was used to view the damage to the SIMFUEL electrode surface which was minimal at E < 200 mV but present as local pits and eroded grains after oxidation at higher potentials. Long-term anodic oxidation at potentials below 200 mV suggests that local acidification can develop within surface asperities in the fuel and pores in corrosion product deposits accumulated on the electrode surface.     

Efficient electrochemical reduction of nitrate to nitrogen on tin cathode at very high cathodic potentials

The electrochemical reduction of nitrate on tin cathode at very high cathodic potentials was studied in 0.1 M K₂SO₄, 0.05 M KNO₃ electrolyte. A high rate of nitrate reduction (0.206 mmol min⁻¹ cm⁻²) and a high selectivity (%S) of nitrogen (92%) was obtained at −2.9 V versus Ag/AgCl. The main by-products were ammonia (8%) and nitrite (<0.02%). Small amounts of N₂O and traces of NO were also detected.As the cathodic potential increases, the %S of nitrogen increases, while that of ammonia displays a maximum at −2.2 V. The %S of nitrite decreases from 65% at −1.8 V to <0.02% at −2.4 V. The kinetic analysis indicated that the formation of nitrogen and ammonia proceeds through the intermediate nitrite.The reduction follows first order kinetics for both nitrate and nitrite at more cathodic potentials than −2.4 V, while at less negative potentials the kinetics is more complicated.The %Faradaic efficiency (%FE) of the reduction at −2.9 V was about 60% initially and decreased to 22% at 40 min.A cathodic corrosion of tin was observed, which was more intensive in the absence of nitrate. At potentials more negative than −2.4 V, small amounts of tin hydride were detected.     

Electropolymerization of phenol, o-nitrophenol and o-methoxyphenol on gold and carbon steel materials and their corrosion protection effects

This paper reports the results of a comparative study of the electropolymerization of phenol, o-methoxyphenol and o-nitrophenol by cyclic voltammetry on gold and carbon steel electrodes. The aim of this work is to synthesize homogeneous and adherent polyphenols film to protect carbon steel material against corrosion. Gold electrodes were used to optimize the experimental parameters such as the initial concentration of the monomer, the pH, the potential scan rate and the anodic potential limit value. Results showed that poly-o-metoxyphenol synthesized on carbon steel using optimized parameters obtained from gold electrode leads to more effective protection. This is probably due to the electron-donating mesomeric effet (+M) of the methoxy group which stabilized the phenoxy radicals obtained during the monoelectronic discharge of o-methoxyphenol. The best electropolymerization conditions involved an aqueous solution of 0.04 M o-methoxyphenol at pH 10.7, 5 mV s⁻¹ potential scan rate and an anodic potential limit (1.64 V/SCE) avoiding the degradation of the polymer film. The application of these conditions on steel electrodes leads to the formation of a stable, adherent and inhomogeneous film of polymer. A polymerization mechanism was proposed with consideration of results from literature.     

Experimental evidence of an upper limit for hydrogen storage at 77 K on activated carbons

An upper limit for hydrogen storage at 77 K on activated carbons was clearly observed in the present experimental work. Such a limit is around 6.4 wt.%, i.e., close to the theoretical limit of 6.8 wt.%. Results of hydrogen storage were obtained in three independent laboratories using volumetric and gravimetric devices. Lab-made activated carbons (ACs) were found to have higher capacities than those of the commercial material AX-21. A maximum excess hydrogen storage capacity of 6.0 wt.% at 77 K and 4 MPa was obtained. This maximum was reduced to 0.6 wt.% at 298 K and 5 MPa. ACs with surface areas (S_BET) as high as 3220 m² g⁻¹ were prepared from chemical activation of anthracites with alkali (Na and K) hydroxides. At 77 K and 4 MPa, excess hydrogen storage capacity was directly correlated with S_BET for ACs having S_BET values lower than 2630 m²/g. Hydrogen uptake at 77 K also correlated with micropore volume and strongly depended on average pore diameter.     

Electrochemical study on cobalt film modified glassy carbon electrode and its application

A novel electroactive material for ascorbic acid (AA) determination was successfully prepared by plating/potential cycling method. The cobalt film was first deposited on the surface of glassy carbon electrode (GCE) in CoSO₄ solution by potential cycling, and then a cobalt film on the surface of GCE was activated by potential cycling in 0.1 mol L⁻¹ NaOH. The electrochemical performance of the resulted film (Co/GCE) and factors affecting its electrochemical activity were investigated by cyclic voltammetry and amperometry. This film electrode exhibited good electrocatalytic activity to the oxidation of AA. This biosensor had a fast response of AA less than 3 s and excellent linear relationships were obtained in the concentration range of 3 × 10⁻⁷ to 1 × 10⁻⁴ mol L⁻¹ with a detection limit of 2 × 10⁻⁷ mol L⁻¹ (S/N = 3) under the optimum conditions. Moreover, the selectivity, stability and reproducibility of this biosensor were evaluated with satisfactory results.     

Electroreduction of oxygen on Co-based catalysts: determination of the parameters affecting the two-electron transfer reaction in an acid medium

Co-based catalysts for the oxygen reduction reaction (ORR) in an acid medium have been prepared from cobalt acetate (CoAc) adsorbed on nine different carbons (previously enriched in surface nitrogen or not). The catalysts were obtained by heat-treating these materials at 900 °C in a reducing environment rich in NH₃. In this work, the emphasis was mainly placed on the electrochemical production of H₂O₂ as measured by the rotating ring-disk electrode (RRDE) technique. It is shown that all Co-based catalysts are active for ORR. The activity and specificity of the catalysts for peroxide production depend essentially on three factors: (i) the potential applied to the disk, (ii) the type of carbon support; and (iii) the concentration of the cobalt precursor. At identical Co loadings (2000 ppm), the percentage of peroxide produced at the disk (%H₂O₂) reaches a maximum in the 0.3-0.1 V versus SCE potential range and decreases for more negative potentials. When the potential is set at a constant value (100 mV versus SCE for instance), a strong effect of the carbon support on %H₂O₂ and on the ring current I_R is noticed, with lower values of %H₂O₂ and I_R corresponding to higher nitrogen content at the surface of the catalysts, while higher values of disk current I_D are obtained under the same conditions. A figure of merit for the electroreduction of oxygen to hydrogen peroxide was obtained for each catalyst by multiplying I_D (representing their activity for ORR) by %H₂O₂ (representing their specificity for H₂O₂ production). According to this figure of merit, the best catalysts for peroxide production are made with Ketjenblack, Black Pearls, Vulcan, and Norit carbon supports. For Co loadings higher than 2000 ppm, it is shown that increasing the loading by more than one order of magnitude (from 2000 to 50,000 ppm) has practically no effect on %H₂O₂ and I_R, while I_D decreases.     

Effect of chloride ions on the corrosion behaviour of steel in 0.1 M citrate

The corrosion behaviour of steel was studied in aerated near neutral citrate solutions without and with various concentrations of NaCl. The potentiodynamic anodic polarization curve in 0.1 M citrate solution exhibits four anodic peaks A1, A2, A3 and A4 prior to the oxygen evolution reaction. Addition of Cl⁻ ions to the solution enhances the four peaks currents, specially A3, which is followed by pitting corrosion. The negative going scans of the cyclic voltammograms show two anodic reactivation peaks A5 and A6 and one cathodic plateau P1. A diffusion controlled process in the potential range of A1, A2 and P1 was detected by RDE experiments. The potentiostatic current time transients, at different concentrations of NaCl and applied potentials E_a > A3, were studied. The pit nucleation rate (t_i⁻¹) is found to increase with increasing the concentration of NaCl and the applied anodic potential. The impedance spectra exhibit four different behaviours depending on the potential range used. They were fitted with a single time constant circuit at E_a < −700 mV. However, at −700 mV < E_a < −480 mV, they were fitted with a circuit with two time constants. At E_a > −480 mV, the second semicircle is replaced by negative polarization resistance which is disappeared at E_a > −300 mV. The electrode impedance was found to decrease with the applied potential.     

Permeability and permselectivity of polyphenylenediamine films synthesized at a palladium disk electrode

Three phenylenediamine isomers (including ortho-, meta- and para-derivatives) were electrochemically polymerized on palladium disk electrodes. The permeability and permselectivity of polyphenylenediamine films for hydrogen peroxide, ascorbic acid, uric acid, acetaminophen, and cysteine were compared. The resulting polyphenylenediamine electrodes showed rapid electrochemical responses to hydrogen peroxide and good anti-fouling ability to possible interferences. Experimental results provide useful information for the fabrication of membrane-based enzyme electrodes.     

Protection of type 430 stainless steel against pitting corrosion by ladder conductive polymer

Poly(o-phenylenediamine) (PoPD) was electropolymerized by cyclic voltammetry (CV) on 430 stainless steel from sulfuric acid solution containing o-phenylenediamine monomer. The formation of the polymer film is slower than that of polyaniline (PANI) film. Transparent and compact layers (∼1.0 μm) of PoPD deposited after 100 cycles, while thicker (∼3 μm), grainy and porous layers of PANI formed after 50 cycles. The PoPD layers protect the steel substrate from pitting in 3% NaCl but the layers of PANI fail, and pitting and crevice corrosion were observed on the steel surface. Both polymers keep the steel substrate in a passive state in sulfuric acid. After aging in acid solution the underlying oxides were investigated after peeling off the polymer layers; this showed an excellent passive film formed under PoPD. The passive steel was completely free from pitting after immersion in the chloride solution for 1 week.     

Electrochemical enhancement of adsorption capacity of activated carbon fibers and their surface physicochemical characterizations

The adsorption of activated carbon fibers (ACFs) and their surface characteristics were investigated before and after electrochemical polarization. The adsorption kinetics of m-cresol showed the dependence on polarized potential, and the adsorption rate constant increased by 77.1%, from 6.38 × 10⁻³ min⁻¹ at open-circuit (OC) to 1.13 × 10⁻² min⁻¹ at polarization of 600 mV. The adsorption isotherms at different potentials were in good agreement with Langmuir isotherm model, and the maximum adsorption capacity increased from 2.28 mmol g⁻¹ at OC to 3.67 mmol g⁻¹ at polarized potential of 600 mV. These indicated that electrochemical polarization could effectively improve the adsorption rate and capacity of ACFs. The surface characteristics of ACFs before and after electrochemical polarization were evaluated by N₂ adsorption-desorption isotherms, scanning electron microscope (SEM), zeta potential and Fourier transform infrared spectroscopy (FTIR). The results showed that the BET specific surface area and pore size increased as the potential rose. However, the surface chemical properties of ACFs hardly changed under electrochemical polarization of less than 600 mV. This study was beneficial to understand the mechanism of electrochemically enhanced adsorption.     

Development of functional mesoporous microparticles for controlled drug delivery

Mesoporous microbeads can be easily obtained by radical polymerization of biocompatible glycerol dimethacrylate (GDMA) in supercritical carbon dioxide. Small mass density microparticles (ρ = 0.19-0.37 g cm⁻³) with controlled size (1-3 μm) and homogeneous morphology are obtained by the addition of different stabilizers to the polymerization media. The microbeads were obtained in quantitative yield as white, dry powders directly from the reaction vessel possessing a pollen-like morphology. The (S)-ibuprofen loading (up to 120 mg g⁻¹) and release profile from the PGDMA microbeads is highly promising which makes them potential drug delivery vehicles.     

Effect of polymerization potential on the actuation of free standing poly-3,4-ethylenedioxythiophene films in a propylene carbonate electrolyte

The linear actuation of poly-3,4-ethylenedioxythiophene (PEDOT) films polymerized at different potentials (0.8-1.3 V) at −27 °C in propylene carbonate (PC) solutions of TBACF₃SO₃ (tetrabutylammonium trifluoromethanesulfonate) was examined under isotonic (constant force) and isometric (constant length) conditions. The actuation properties were evaluated by electrochemomechanical deformation measurements (ECDM) during cyclic voltammetry, square wave potential steps and long term cycling. The ECDM response revealed mixed ion actuation behaviour for PEDOT films polymerized at the potential extremes of 0.8 and 1.3 V. At intermediate polymerization potentials from 0.9 to 1.2 V, cation-driven actuation was observed involving immobilized triflate anions (CF₃SO₃⁻). Long term experiments (50 cycles) showed that films prepared at polymerization potentials of 0.8 V exhibited mainly anion-driven actuation, during potential steps to and from 1.0 V; conversely PEDOT prepared at a polymerization potential of 1.1 V showed exclusively cation-driven actuation. PEDOT films prepared at a polymerization potential of 1.1 V showed the maximum cation-driven actuation during cyclic voltammetry experiments including long term cycling. SEM images showed an open porous structure in all of the PEDOT films.     

Improvement of performances of complex non-platinum electrode materials for hydrogen evolution

Structural and electrochemical characteristics of hypo-hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied and modified in order to improve their performances. All studied materials were of general composition 10% Ni + 18% TiO₂ + C.All materials were prepared of amorphous or crystalline TiO₂, crystalline Ni or NiCo (10-20 nm) and Vulcan XC-72, by sol-gel procedure.Both, material's intrinsic catalytic activity and surface area were affected by applied modifications. As a result, the electrocatalytic activity was improved, e.g. transformation of TiO₂ into anatase form lowers the HER overpotential for 60 mV. Introduction of MWCNTs was even more effective, lowering η for 120 mV. Co addition to metallic phase lowers η for utmost 195 mV.Combined modification of TiO₂ and carbon substrate lowers η for 145 mV, while the complete modification of all three catalyst's components was the most effective with 230 mV decrease of overpotential.     

Study of structural and electrochemical characteristics of Co-based hypo-hyper d-electrocatalysts for hydrogen evolution

Structural and electrochemical characteristics of hypo-hyper d-electrocatalytic materials aimed for preparation of electrodes for hydrogen evolution were studied. The basic catalytic material was prepared of 10% amorphous Co (grain size <2 nm), 18% amorphous TiO₂ and Vulcan XC-72, by sol-gel procedure. A number of modifications were applied aimed at improving the materials performances: (i) TiO₂ was transformed into anatase by heating at 480 °C for 1 h, (ii) multiwalled carbon nanotubes (MWCNT) were used as a catalyst support instead of Vulcan XC-72 and (iii) Mo was added to Co phase in a quantity of 25 at.% (Mo:Co = 1:3).Both, material's intrinsic catalytic activity and surface area were affected by these modifications. As a result, the electrocatalytic activity for hydrogen evolution was improved, e.g. transformation of TiO₂ into anatase form lowers the HER overpotential (η) for 15 mV at 60 mA cm⁻². Introduction of MWCNTs lowered η for 30 mV, while addition of Mo to metallic phase for 40 mV.The complete modification of all three catalyst's components (10% MoCo₃ + 18% anatase + MWCNTs) was the most effective with 60 mV decrease of overpotential.Characterization was made by XRD, SEM, IR and XPS methods. Surface area was measured by means of cyclic voltammetry.