Gold particles supported on self-organized nanotubular TiO2 matrix as highly active catalysts for electrochemical oxidation of glucose

Au/TiO₂/Ti electrode was prepared by a two-step process of anodic oxidation of titanium followed by cathodic electrodeposition of gold on resulted TiO₂. The morphology and surface analysis of Au/TiO₂/Ti electrodes was investigated using scanning electron microscopy and EDAX, respectively. The results indicated that gold particles were homogeneously deposited on the surface of TiO₂ nanotubes. The nanotubular TiO₂ layers consist of individual tubes of about 60–90 nm in diameter, and the electrode surface was covered by gold particles with a diameter of about 100–200 nm which are distributed evenly on the titanium dioxide nanotubes. This nanotubular TiO₂ support provides a high surface area and therefore enhances the electrocatalytic activity of Au/TiO₂/Ti electrode. The electrocatalytic behavior of Au/TiO₂/Ti electrodes in the glucose electro-oxidation was studied by cyclic voltammetry. The results showed that Au/TiO₂/Ti electrodes exhibit a considerably higher electrocatalytic activity toward the glucose oxidation than that of gold electrode.     

Determination of Trace Amount of Lead(II) in Sweet Fruit‐Flavored Powder Drinks by Differential Pulse Adsorptive Stripping Voltammetry at Carbon Paste Electrode

A new method is described for the determination of lead based on the cathodic adsorptive stripping of the lead–nuclear fast red (NFR) at a carbon paste electrode (CPE). The differential pulse voltammograms of the adsorbed complex of lead–NFR are recorded from ?0.10 to ?0.60 V (versus Ag/AgCl electrode). Optimal conditions were found to be an electrode containing 25% paraffin oil and 75% high purity graphite powder, 4.0×10^?5 mol L^?1 NFR; buffer solution (pH of 3.0), accumulation potential and time, ?0.20 V, 60 and 120 s (for high and low concentration of lead), respectively. The results show that the complex can be adsorbed on the surface of the CPE, yielding one peak at ?0.34 V, corresponding to reduction of NFR in the complex at the electrode. The detection limit was found to be 0.2 ng mL^?1 with a 120s accumulation time. The linear ranges are from 0.5 to 50 (t_acc=120 s) and 50 to 200 ng mL^?1 (t_acc=60 s). Application of the procedure to the determination of lead in lake water, bottled mineral water, synthetic samples and sweet fruit‐flavored powder drinks samples gave good results.     

Preparation and electrocatalytic property of lead dioxide prepared by pulse electrodeposition with different pulse current density

Lead dioxide electrodes were prepared by pulse electrodeposition in the lead nitrate solution with different pulse current density. The effects of pulse current density on the morphology and structure of lead dioxide electrodes were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD), respectively. The SEM and XRD results show that the increase of pulse current density can make the morphology more fine, and the crystal size of lead dioxide decreases with the increase of pulse current density. The anodic polarization curves demonstrate that the oxygen evolution overpotentials of lead dioxide electrodes also enhance with the increase of pulse current density. The stability of lead dioxide electrodes enhances with the increase of pulse current density until 15 mA cm^–2, then the stability decreases. The electrocatalytic property of lead dioxide electrodes was examined for the electrochemical oxidation of rhodamine B (RhB). The results show that the RhB removal efficiency on the lead dioxide electrodes increases with the increase of pulse current density, which can be attributed to the increase of oxygen evolution overpotential.     

Relaxation of the Adsorption with Charge Transfer: The Iodide Ion Adsorption on a Platinum Electrode

The relaxation of the iodide ion adsorption on platinum electrodes with smooth and developed surfaces is studied in the time range 10^–4to 10³s and frequency range 10^–3to 10⁵Hz. Mathematical models matching experimental data to within the experimental accuracy are designed. Most adsorption centers on platinum have relaxation times > 0.1 s. The fraction of adsorption centers with > 100 s increases on the electrode with the developed surface, and in the ranges t< 0.1 s and F> 2 Hz, characteristics of this electrode are almost completely defined by the surface porosity. It is discovered that the electrode surface becomes smoother, leading to a slow drift of characteristics with time.     

The electrocatalytic reduction of carbon dioxide using cobalt tetrakis (4-trimethylammoniophenyl) porphyrin under high pressure

The electrocatalytic reduction of carbon dioxide in aqueous solution using Co [p-N(CH₃)₃) TPPI under pressure (4～22 kg · cm^?2) was carried out on solid-state metal electrodes such as In, Sn, Pb and Pb-Hg. The reduction products were mainly carbon monoxide and a small amounts of formic acid. The effects of the pressure of carbon dioxide, electrolytes, catalyst concentration, electrode materials and potentials on the reduction of carbon dioxide were studied. The optimum voltage for maximum current efficiency of reduction products was at ?1.2 Vvs. SCE for all four kinds of metal electrodes. High current efficiencies (>90%) were obtained with lead and amalgamated lead electrodes in an aqueous solution of NaHCO₃ containing 7.4 × 10×⁵ mol/L of catalyst.     

Study of a Catalytic Mechanism in Additive Differential Pulse Techniques

The new electrochemical double pulse technique, known as additive differential normal pulse voltammetry (ADNPV) when there is no restriction on the duration of both pulses, and additive differential pulse voltammetry (ADPV) when t₂?t₁, has been applied to a pseudo‐first‐order catalytic mechanism. The expressions obtained here are applicable to planar and spherical electrodes, of any radius. This is of great interest since the size of the electrode plays an important role in the preponderating of diffusive and kinetics processes. The signal obtained with this technique presents the same morphological characteristics as the triple pulse technique, double differential pulse voltammetry (DDPV) and is more advantageous than DDPV and than the double pulse one, differential pulse voltammetry (DPV).     

Enhanced Photoelectrochemical Property of Zn Loaded TiO2 Nanotube Arrays Electrode

TiO₂ nanotube arrays (TNTs) electrode loaded with Zn nanoparticles was prepared by anodization and the size of Zn nanoparticle loaded on TNTs electrode was controlled bychronoamperometry deposition time. Results of SEM and XRD analysis show that Zn nanoparticles had a diameter of about 15-25 nm when the deposition time was 3-5 s. The UV-Vis diffuse reflectance spectra show the Zn loaded harvest light with 480-780 nm more effectively than the unloaded sample. The photocurrent response of Zn loaded TNTs electrodes were studied, the results showed that TNTs electrodes loaded with Zn nanoparti-cles has 50% increased photocurrent response under high-pressure mercury lamp irradiation compared with unloaded TNTs electrode.     

Determination of chromium(VI) in dialysis fluids by alternating current and differential pulse voltammetry

Alternating current (ACV) and differential pulse voltammetry (DPV) are employed for the determination of Cr(VI) in dialysis fluids, using 0.1 mol/1 dibasic ammonium citrate as supporting electrolyte (pH 5.9). A three-electrode cell was used: The working electrode was a long-lasting sessile-drop mercury electrode (LLSDME) with a drop time of 240 to 300 s. Precision, expressed as relative standard deviation (s _r%), and accuracy, expressed as relative recovery (R%) are also reported.     

Influence of lead dioxide electrodes morphology on kinetics and current efficiency of oxygen-ozone evolution reactions

Influence of electrode morphology on electrochemical properties of lead dioxide electrodes (β-PbO₂) for oxygen-ozone evolution reactions in acid medium was investigated using scanning electronic microscopy (SEM), cyclic voltammetry (CV), polarization curves (PC), and determination of the current efficiency (Φ). Experimental findings revealed that application of high electrodeposition current densities furnishes more rough β-PbO₂ films. Surface characteristics were verified by SEM images and the analysis of interfacial pseudo-capacitances and morphology factor (φ). Kinetic study of the overall electrode process (O₂ + O₃) based on the analysis of the Tafel slope revealed that the electrode morphology and electrolyte composition considerably affect the electrode kinetics. In most cases, the existence of two Tafel slopes distributed in the low and high overpotential domains was observed. Abnormal Tafel slopes (b ≠ 120 mV) obtained for the primary water discharge step during water electrolysis were interpreted considering the apparent charge transfer coefficient (α _apa). Optimum conditions for the ozone production were obtained for the less rough β-PbO₂ electrode immersed in a sulfuric acid solution (1.0 mol dm⁻³) containing KPF₆ (30 × 10⁻³ mol dm⁻³), where the current efficiency of 15 mass % for the ozone production was obtained.     

Barium Sulfate Effects on the Electrochemical Behaviors of Nanostructured Lead Dioxide and Commercial Positive Plates of Lead-Acid Batteries

Positive electrode with uniform lead dioxide nanostructures directly synthesized by cyclic voltammetry (CV) method on the lead substrate in 1 M sulfuric acid solution including different concentration of barium sulfate. The effect of potential scan rate, sulfuric acid and barium sulfate concentration were studied on the morphology and particle size of lead dioxide using scanning electron microscopy (SEM) and X-ray diffraction techniques (XRD). The effect of barium sulfate was studied on the CV parameters including anodic peak current (I _p^a), cathodic peak current (I _p^c), anodic peak potential (E _p^a) and cathodic peak potential (E _p^c) during synthesis process. Finally, the effect of barium sulfate on the discharge capacity and cycle life of nanostructured positive electrodes and commercial positive plates was investigated. Both CV and battery test results showed that barium sulfate with concentration of 1 × 10⁻⁵ M can be used as suitable additive for positive paste of lead-acid batteries.     

Removal of Methylene Blue from Aging Water Solutions Treated by a Submerged Arc

Low voltage, low energy submerged pulsed arcs with a pulse repetition rate of 100 Hz, energies of 2.6–192 mJ and durations of 10–40 μs, followed by aging in the dark, were used to decompose 10 mg/l methylene blue (MB) dissolved in 40 ml of water, with the addition of 0.5 % H₂O₂. Electrode pairs composed of Fe/Fe, Ti/Ti, Cu/Cu, Cu/Fe, Fe/Cu, Ti/Fe, Fe/Ti, Cu/Ti and Ti/Cu were used. MB degraded during arc treatment, and during post arc treatment aging. The aging degraded MB faster (by a factor of ~2–3) when the MB solution was subjected to arcing with dissimilar electrodes when one of them was Cu, than for arcing with other used electrode pairs. The impact of the arc treatment time and the electrode materials on the MB removal ratio (C₀–C_ta)/C₀ was determined as a function of aging time t_a, where C₀ and C_ta are the MB concentrations initially and after t_a. For a pulse duration of 10 μs and pulse energies of 2–20 mJ, the MB removal rate increased linearly with treatment time and its growth rate increased with pulse energy. The linear dependence of the MB removal rate on treatment time was violated with pulse duration of 40 μs and pulse energies of 30–200 mJ. Kinetics of the MB degradation during aging of the arc treated solution was well described by the 1st order linear rate equation.     

Hierarchically organized titanium dioxide nanostructured electrodes: Quantum-sized nanowires grown on nanotubes

Titanium dioxide nanotube electrodes were fabricated by anodization of titanium and decorated with quantum-sized rutile nanowires (2 nm in diameter) by chemical bath deposition. The length of the nanotubes (120 nm in external diameter) was varied between 4 and 10 μm by changing the anodization time. The hierarchically organized electrodes present good mechanical properties and an enhanced capacity for reversible charge accumulation. The photoelectrocatalytic properties of such electrodes have been tested by photo-oxidizing both water and oxalic acid, turning out to be superior to those of bare nanotubes, which are ascribed to an enhanced interfacial area while keeping the favorable transport properties (for both electrons and chemicals) typical of nanotube electrodes.     

Electrochemical Properties and Application of Mixed Silver‐Bismuth Electrodes

Electrochemical properties of silver electrodes with 2, 4, 6, 10 and 15% bismuth have systematically been investigated with cyclic voltammetry (CV), differential pulse voltammetry (DPV), and electrochemical impedance spectroscopy (EIS). Increased overpotential towards hydrogen evolution reaction (HER) was found as a result of increasing amount of added bismuth. This was also demonstrated in acid solution where zinc was successfully detected on mixed electrodes, but failed on pure silver electrodes. Formation and decomposition of oxide products formed on the different electrode surfaces were studied by cyclic voltammetry, and in addition to known species found on sliver, also peaks attributed to bismuth were achieved and examined. Zinc, cadmium, and lead were measured in the low μg/L range on the mixed electrodes, and good linearity (r²=0.998) was found for 2 to 10 μg/L. Lead was measured down to 0.1 μg/L. Further it was found in DPASV that the zinc peak significantly shifted towards a more negative value with increasing amount of bismuth in the silver electrodes. The same was also observed for cadmium and lead, but in a less extent. Finally a silver electrode containing 15% bismuth was used for continuous analyses in a polluted river for 6 weeks.     

Amyloid–β peptides time-dependent structural modifications: AFM and voltammetric characterization

The human amyloid beta (Aβ) peptides, Aβ_1-40 and Aβ_1-42, structural modifications, from soluble monomers to fully formed fibrils through intermediate structures, were investigated, and the results were compared with those obtained for the inverse Aβ_40-1 and Aβ_42-1, mutant Aβ_1-40Phe¹⁰ and Aβ_1-40Nle³⁵, and rat Aβ_1-40Rat peptide sequences. The aggregation was followed at a slow rate, in chloride free media and room temperature, and revealed to be a sequence-structure process, dependent on the physicochemical properties of each Aβ peptide isoforms, and occurring at different rates and by different pathways. The fibrilization process was investigated by atomic force microscopy (AFM), via changes in the adsorption morphology from: (i) initially random coiled structures of ∼0.6 nm height, corresponding to the Aβ peptide monomers in random coil or in α-helix conformations, to (ii) aggregates and protofibrils of 1.5–6.0 nm height and (iii) two types of fibrils, corresponding to the Aβ peptide in a β-sheet configuration. The reactivity of the carbon electrode surface was considered. The hydrophobic surface induced rapid changes of the Aβ peptide conformations, and differences between the adsorbed fibrils, formed at the carbon surface (beaded, thin, <2.0 nm height) or in solution (long, smooth, thick, >2.0 nm height), were detected. Differential pulse voltammetry showed that, according to their primary structure, the Aβ peptides undergo oxidation in one or two steps, the first step corresponding to the tyrosine amino acids oxidation, and the second one to the histidine and methionine amino acids oxidation. The fibrilization process was electrochemically detected via the decrease of the Aβ peptide oxidation peak currents that occurred in a time dependent manner.     

An innovative electrochemical approach for voltammetric determination of levodopa using gold nanoparticles doped on titanium dioxide nanotubes

Catalytic nanotubes made from titanium dioxide (TiO₂-NTs) and covered with gold nanoparticles (Au-NPs) were prepared via galvanic deposition of the Au-NPs on the TiO₂-NTs. The morphology and surface characteristics of the resulting electrodes were investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results indicated that the Au-NPs were homogeneously deposited on the surface of TiO₂-NTs which consist of individual tubes of about 40?C80?nm in diameter. The AuNPs with a size of 80?C100?nm are well-dispersed on the surface of the TiO₂-NTs. The electro-catalytic activity of the electrodes towards the electro-oxidation of levodopa was studied by cyclic voltammetry, differential puls voltammetry. The results showed that the electrodes exhibit a considerably higher activity toward the oxidation of levodopa. The oxidation peak current linearly depends on the concentration of levodopa in the 10 to 70???M concentration range. Levodopa was determined by the method in pharmaceutical preparations, and results were found to be satisfactory.

Ti/TiO2 indicator electrodes formed by plasma electrolytic oxidation for potentiometric analysis

Ti/TiO₂ indicator electrodes were prepared by plasma electrolytic oxidation (PEO) method in the tetraborate electrolyte and were used for potentiometric indication of chemical reactions of different types and for analysis of surface and industrial wastewaters on the example of potentiometric determination of alkalinity and chloride. The electrodes formed at current densities of 0.05, 0.1, 0.15 and 0.2 A/cm² are different in composition, surface morphology and electroanalytical properties. The electrodes formed at a current density of 0.05 A/cm² exhibit the highest pH-sensitivity and generate the highest analytical signal at the equivalence point in the acid–base and precipitation titrations. The maximum analytical signal at the equivalence point, exceeding in magnitude the analytical signal, obtained by classical Pt electrode in oxidation–reduction and complexometric titrations generates PEO layers formed at a current density of 0.05 A/cm² and a platinum-modified nanoparticles. The results of the potentiometric titration of the surface and technogenic waters using as indicator Ti/TiO₂ electrodes are comparable with the conventionally used glass electrode (to determine alkalinity) and Ag electrode (to the determine chloride) and the results of visual titration. The advantage of the obtained metal oxide systems is the ability to determine two hydrochemical parameters due to their multifunctionality and opportunity to work with a single electrode. In addition, these sensors offer some analytical characteristics such as sensitivity, good reproducibility, high mechanical stability and a simple preparation procedure.     

Normal Pulse Voltammetric Determination of Subnanomolar Concentrations of Chromium(VI) with Continuous Wavelet Transformation

The renewable mercury film‐modified silver solid amalgam annular band electrode (MF‐AgSAE) applied for quantitative determination of sub‐nanomolar concentrations of Cr(VI) using differential pulse (DP) and normal pulse (NP) catalytic adsorptive striping voltammetry (CAdSV) is presented. In this context a signal processing algorithm is described and applied for the transformation of sigmoidal shaped NP curves to peak shaped curves. The method utilizes continuous wavelet transform (CWT) and a specially constructed mother wavelet defined using the ideal wave‐shaped curve. It simplifies the interpretation of sigmoidal curves. In the effect the new strategy of Cr(VI) determination is 10 times more sensitive than differential pulse and square‐wave techniques. The reproducibility is below 3–5 % (n=3) for the 0.2–2.2 nM concentration range of Cr(VI). The detection limit for 30 s preconcentration is equal to 0.05 nM with sensitivity of 0.809±0.012 µA nM^?1 and is limited by the purity of the used reagents. The correlation coefficient is equal to 0.9993. For 2 nM of Cr(VI), in the tested range, 0≤t_acc≤60 s, the relation wave height? accumulation time (I_w–t_acc) is linear. The operation and effectiveness of the proposed procedures was confirmed by the quantitative determination of Cr(VI) in supporting electrolyte and CRM (surface water samples and urine) with known amounts of the analyte. The obtained results show substantial improvement of the performance of NP CAdSV technique.     

An Effective Non-Enzymatic Glucose Biosensor Based on Nanostructured CuxO/Cu Electrodes Synthesized in Situ by Copper Anodization

Potentiostatic anodization was developed to synthesize copper oxide/copper (Cu_xO/Cu, x=1,2) electrode with nano structure for sensitive non-enzymatic glucose detection. At a catalytic potential of 0.55 V, the CuO/Cu electrode presented a high sensitivity of 2954.38 μA mM⁻¹ cm⁻² to glucose and a linear range of 0.1 mM to 1.3 mM. The response time is less than 3 s with addition of 0.1 mM glucose. The CuO/Cu electrode above was anodized in 1M KOH solution at −100 mV and the morphology was compact nanoparticles and sparsely dispersed nanosheets, which enlarged the surface area and provided abundant electrocatalytic active sites. Compared the sensing property of electrodes with different morphologies, it indicated that nanostructure was significant to the efficient glucose catalytic oxidation process and it could be regulated by changing the potential and electrolyte concentration during anodization.     

Application of Polymer‐Modified Hanging Mercury Drop Electrode in the Indirect Determination of Certain β‐Lactam Antibiotics by Differential Pulse,Ion‐Exchange Voltammetry

A selective and sensitive polymer‐modified electrode was developed for β‐lactam antibiotics (cefaclor, amoxycillin and ampicillin) present in formulated and blood plasma samples for the quantitative analysis in aqueous environment. The detection was made using an ion‐exchange voltammetric technique, in differential pulse mode, on poly(N‐chloranil N,N,N′,N′‐tetramethylethylene diammonium dichloride)‐modified hanging mercury drop electrode of a three‐electrode system (PAR Model 303A) attached with a Polarographic Analyzer/Stripping Voltammeter (PAR Model 264A). Antibiotics, which are electroinactive compounds, were essentially converted to their electroactive oxazolone analogues through acid treatment under drastic conditions (0.1 mol L^?1 HCl, ～85 °C, 2 h). These analytes in the form of their respective oxazolones were indirectly analyzed by oxazolone entrapment in the polymeric film through ion‐exchange process at modified electrode surface (accumulation potential ?0.20 V (vs. Ag/AgCl), accumulation time 120 s, pH 7.4, KH₂PO₄‐NaOH buffer (ionic strength 0.1 mol L^?1), scan rate 10 mV s^?1, pulse amplitude 25 mV). The limit of detection of cefaclor‐derived oxazolone was found to be 2.12 nmol L^?1 (0.82 ppb, S/N 3, RSD 3.21%) in terms of cefaclor (a representative β‐lactam) concentration.     

Characterization and Optimization of Interdigitated Ultramicroelectrode Arrays as Electrochemical Biosensor Transducers

Interdigitated ultramicroelectrode arrays (IDUAs) were fabricated on glass wafers and investigated to obtain optimal oxidation and reduction reactions of potassium ferro/ferrihexacyanide, Fe^2+/3+(CN)₆, when using a 2‐electrode set up. These electrodes will be used as transducers in portable microfluidic‐based biosensors in the future for the detection in an aqueous, biocompatible matrix. IDUAs were designed to maximize the signal‐to‐noise ratio (S/N) investigating electrode height, gap size, finger width, and material. Interesting differences in the electrode materials gold and platinum were found, which were due to the oxidization of platinum and gold during the IDUA fabrication process. It resulted in gold IDUAs being by far superior in respect to signal‐to‐noise ratio and overall signal magnitude to those made of platinum. The effects of gap size, electrode width and number of electrode fingers were as expected. Optimal electrode heights were in the range of 70 nm–140 nm, much larger and smaller electrodes had lower signal‐to‐noise ratios due to overall reduced signal or increased background. The optimized IDUA was made out of gold, had 400 fingers with a finger width of 2.7 μm, a finger height between 70 nm and 140 nm and a gap size of 0.9–1 μm. A detection limit of as low as 0.1 μM ferro/ferrihexacyanide measured in a simple 2‐electrode set up was obtained with a signal‐to‐noise ratio of 9.7.