Mechanismus der Elektroreduktion von V(+V)

In papers containing data about electrode reactions of oxo-ions there are great differences concerning the mechanisms of the electrode reactions. In this work the electrode reaction of VO₃ ^– is described and interpreted. All effects observed at the reduction of VO₃ ^– are in accord with the proposed theory.

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Functionalization of platinum nanoparticles for electrochemical detection of nitrite

In this work, a novel electrochemical method for nitrite detection by using functionalized platinum nanoparticles (PtNPs) is proposed. Firstly, a gold electrode is immobilized with 4-(2-aminoethyl)benzenamine. Then, PtNPs are modified with 5-[1, 2]dithiolan-3-yl-pentanoic acid [2-(naphthalene-1-ylamino)-ethyl]amide (DPAN). Consequently, in the presence of nitrite ions, Griess reaction occurs between 4-(2-aminoethyl)benzenamine on the electrode and DPAN on PtNPs, thus PtNPs are localized onto the electrode surface. So, PtNPs-electrocatalyzed reduction of H₂O₂ can be achieved to correlate the electrochemical signal with the concentration of nitrite ions. The linear concentration range can be as wide as 10–1,000 μM, while the detection limit is as low as 5 μM. The proposed method has been also successfully applied to the detection of nitrite with the local lake water, and the result is well consistent with that obtained by UV-visible spectrophotometric method. So, this method has potential use for monitoring nitrite in drinking water supplies in the future.     

Glassy carbon electrodes modified with gold nanoparticles for the simultaneous determination of three food antioxidants

Electrochemical behavior of three antioxidants: butylated hydroxyanisole (BHA), butylated hydroxytoluene (BHT) and butylated hydroquinone (TBHQ), was investigated at a glassy carbon electrode modified with gold nanoparticles (AuNPs/GCE). This electrode was characterized by scanning electron microscopy (SEM). The experimental results indicated that the modified electrode was strongly electroactive during the redox reactions of BHA, BHT and TBHQ, and this was confirmed by the observed increased redox peak currents and shifted potentials; in addition, the oxidation products of BHA and TBHQ were found to be the same. The experimental conditions were optimized and the oxidation peaks of BHA and BHT were clearly separated. Based on this, an electrochemical method was researched and developed for the simultaneous determination of BHA, BHT and TBHQ in mixtures with the use of first derivative voltammetry; the linear concentration ranges were 0.10–1.50 μg mL⁻¹, 0.20–2.20 μg mL⁻¹ and 0.20–2.80 μg mL⁻¹, and detection limits were 0.039, 0.080 and 0.079 μg mL⁻¹, for BHA, BHT and TBHQ, respectively. The proposed method was successfully applied for the analysis of the three analytes in edible oil samples.     

Graphene oxide modified glassy carbon electrode for determination of linagliptin in dosage form,biological fluids,and rats' feces using square wave voltammetry

An applicable square wave anodic adsorptive stripping voltammetric (SWAdASV) technique was utilized for linagliptin determination. A glassy carbon electrode was modified with graphene oxide to increase the electrode reactivity. The method is cheap, accurate, precise, and selective, with a good linearity range and a low detection limit. The proposed method was the first one to determine linagliptin in the feces, which is the main route for excreting the drug from the body. The electrode was characterized using various techniques, including Scanning Electron Microscope (SEM), Fourier-transform infrared (FTIR), and X-ray powder diffraction (XRD), and the oxidation mechanism of the drug was examined. The proposed method has a linear range of 9.45–103.96 ng mL^?1. The detection limit was 4.0 ng mL^?1. The modified electrode was employed efficiently to determine the drug in tablet formulations, spiked human urine, plasma, and rats' feces with high recoveries. The proposed method's results were statistically compared with those of another previously published method.     

Voltammetric study on the complex of thiram-copper(II) and its application

Based on the formation of tetramethylthiuram disulfide (thiram)-copper(II) complex by square-wave voltammetry on a glassy carbon electrode, a new method is proposed for determination of thiram. There is a good linear relationship between peak current and concentration of thiram in the range of  mol l⁻¹ (r=0.999). The method is applied to the determination of thiram residue in plant with satisfactory results, compared with results achieved by using HPLC. Two oxidation peaks and two reduction peaks are obtained by cyclic voltammetry on a glassy carbon electrode in 0.2 mol l⁻¹ HAc-NaAc (pH 4.0) solution containing thiram-copper(II) complex. The electrode reaction process is quasi-reversible with adsorptive characteristics. The mechanism of electrode reaction is discussed.     

Voltammetric determination of traces of cobalt(II) with a chemically modified carbon paste electrode

Summary A new preconcentration and voltammetric determination method for cobalt(II) in aqueous solution with a chemically modified electrode is proposed. The accumulation behaviour and voltammetry of cobalt(II) has been investigated with a carbon paste electrode modified with cationexchanger and 1,10-phenanthroline. The electrochemical response is characterized with respect to carbon paste composition, pH, preconcentration time, cobalt(II) concentration and other variables. For a 3-min preconcentration time, the electrode gives good linearity for 1×10^–7 to 4×10^–6 mol/l Co(II), a detection limit of 8×10^–8 mol/l. The response can be reproduced with a 4.0% relative standard deviation. The method is fairly free from many coexisting ions interferences. A rapid and convenient renewal procedure allows the use of a single electrode in multiple analytical determinations over several days. Satisfactory results are obtained for the determination of cobalt in a variety of certified standard reference materials.     

Enzymatic detection of glucose using fluoride-selective electrodes with polymeric membranes

The feasibility of using polymeric membrane fluoride-selective electrodes based on zirconium(IV) 5,10,15,20-tetraphenylporphyrin as a detector in a flow-injection analysis (FIA) system for glucose determination was examined. The optimization of enzymatic reactions, FIA system configuration and enzyme-immobilization process was performed. It was shown that the resulting flow-injection system exhibits good working parameters, such as reproducibility, linear range of glucose concentration (3 × 10⁻³–10⁻¹ M), sampling rate (60 samples per minute) and lifetime (over 1 month). The performance of the polymeric membrane electrode was similar to that of a crystalline LaF₃ electrode. The results of glucose determination in synthetic samples with the proposed system show good agreement with real glucose concentrations.     

Electrode reactions of palladium(II) in chloride solution at carbon paste electrodes modified with derivatives of N-benzoylthiourea

The study of the electrode reactions of palladium(II) at non-modified carbon paste electrodes (CPEs) in chloride solution has revealed the existence of a chloropalladate(II) complex at the electrode surface. The complex is formed during the application of anodic potentials after preceding palladium deposition. In the present paper the electrode reactions of Pd^II at CPEs modified with some N′,N′-disubstituted derivatives of N-benzoylthiourea [as selective ligands for palladium(II)] are studied in chloride solution by cyclic voltammetry. Two reduction peaks are observed in the cathodic scans recorded after deposition of palladium and anodization of the electrode. From the results it is concluded that [in addition to the chloropalladate(II) complex, observed at the non-modified electrode] a second palladium complex is formed at positive potentials. The formation of the palladium(II) complex of the N-benzoylthiourea derivatives by ligand exchange at the electrode surface is assumed. The ligand exchange itself occurs without charge transfer across the electrode|solution interface; therefore, it cannot be detected electrochemically. After palladium deposition and anodic treatment a pronounced "inverse" peak (i.e., an anodic peak in the cathodic scan) with peak currents up to 100 μA is observed at about +0.8 V. Its peak current increases with the amount of deposited palladium and the number of cycles. The reactions at the electrode surface are discussed. The results of the study reveal the existence of two different surface complexes of palladium(II) at ligand-modified CPEs, but the surface reactions could not be elucidated in detail. Electronic Publication     

Formation of a novel electroactive film by electropolymerization of 5-amino-1-naphthol

The electrooxidation of 5-amino-1-naphthol in acetonitrile solutions led to the formation of a purplish gray, air-stable, polymeric film (poly(5-amino-1-naphthol), poly(5,1-ANT)), which presents a nonfibrillar morphology, on basal-plane pyrolytic graphite and In-Sn oxide conducting glass (ITO) electrodes. The film showed a reversible, well-defined oxidation-reduction response in both aqueous solutions of pH 1–13 and protic non-aqueous solutions, and was semiconducting. The formal redox potential (E^o') was 0.075 V vs. a sodium chloride saturated calomel electrode in 0.2 M NaClO₄ aqueous solution (pH 1.0). The pH dependence of (E^o') suggested that the electrode processes at pH 1-5 and pH 8–13 are 1e⁻ − 1H⁺and 1e^t- − 2 H⁺ reactions, respectively. The poly(5,1-ANT) film-coated ITO electrode displayed an electrochromic character: the oxidized form is purplish gray and the reduced one is pale brownish yellow. In addition, based on the electrochemical and IR absorption spectroscopic characterization of the film, some possible candidates for the structures of electroactive moieties of poly(5,1-ANT) are proposed.     

Electrochemical DNA biosensors based on platinum nanoparticles combined carbon nanotubes

Platinum nanoparticles were used in combination with multi-walled carbon nanotubes (MWCNTs) for fabricating sensitivity-enhanced electrochemical DNA biosensor. Multi-walled carbon nanotubes and platinum nanoparticles were dispersed in Nafion, which were used to fabricate the modification of the glassy carbon electrode (GCE) surface. Oligonucleotides with amino groups at the 5′ end were covalently linked onto carboxylic groups of MWCNTs on the electrode. The hybridization events were monitored by differential pulse voltammetry (DPV) measurement of the intercalated daunomycin. Due to the ability of carbon nanotubes to promote electron-transfer reactions, the high catalytic activities of platinum nanoparticles for chemical reactions, the sensitivity of presented electrochemical DNA biosensors was remarkably improved. The detection limit of the method for target DNA was 1.0 × 10⁻¹¹ mol l⁻¹.     

Electroreduction of oxygen on octadecylmercaptan self-assembled monolayers

The reduction of oxygen has been studied on octadecylmercaptan self-assembled monolayers adsorbed on gold substrates in borate buffer solutions with a rotating disc electrode. A great inhibition of the oxygen reduction and other electrochemical reactions by these monolayers has been found. However, after polarisation at –0.80 V_SHE the protecting properties of the film against electron transfer reactions are lost, and a behaviour similar to bare gold is observed. Ex situ XPS indicates that the thiol monolayer has not been desorbed to a large extent during oxygen reduction. Disorders of the monolayer structure and desorption of thiol molecules are proposed as the main reasons for the accessibility of electrochemical reactions to the surface.     

Direct electrochemical redox of tyrosinase at silver electrodes

The direct electron transfer reactions between tyrosinase and silver electrode were investigated by using cyclic voltammetry and potential-step chronoamperometry as well as current-step chronopotentiometry techniques. The kinetics of these reactions is quasi-reversible with two electron transfer reactions and 0.030 s(-1) apparent electrode reaction rate constant. The results demonstrate that neither electrode surface modification nor the inclusion of mediators is necessary to study the electron transfer reactions of tyrosinase at silver electrodes. Moreover, both the anodic and the cathodic currents are linear relationship with the tyrosinase concentration in the range of 1 x 10(-9) approximately 5 x 10(-8)moll(-1). It is possible to be used as a method of analyzing tyrosinase concentration.     

Apoferritin protein nanoparticles dually labeled with aptamer and horseradish peroxidase as a sensing probe for thrombin detection

A novel and ultrasensitive sandwich-type electrochemical aptasensor has been developed for the detection of thrombin, based on dual signal-amplification using HRP and apoferritin. Core/shell Fe₃O₄/Au magnetic nanoparticles (AuMNPs) loading aptamer1 (Apt1) was used as recognition elements, and apoferritin dually labeled with Aptamer2 (Apt2) and HRP was used as a detection probe. Sandwich-type complex, Apt1/thrombin/Apt2–apoferritin NPs–HRP was formed by the affinity reactions between AuMNPs–Apt1, thrombin, and Apt2–apoferritin–HRP. The complex was anchored on a screen-printed carbon electrode (SPCE). Differential pulse voltammetry (DPV) was used to monitor the electrode response. The proposed aptasensor yielded a linear current response to thrombin concentrations over a broad range of 0.5–100 pM with a detection limit of 0.07 pM (S/N = 3). The detection signal was amplified by using apoferritin and HRP. This nanoparticle-based aptasensor offers a new method for rapid, sensitive, selective, and inexpensive quantification of thrombin, and offers a promising potential in protein detection and disease diagnosis.     

Electrocatalytic oxidation of glucose on nanoporous gold membranes

With characteristic of structural integrity and high surface area, nanoporous gold (NPG) prepared by dealloying method is proposed to be a highly sensitive catalyst for glucose electrooxidation. It can be found that a-NPG which obtained by electrochemical corrosion method has the highest sensitivity for glucose electrooxidation among the three studied samples. Under alkaline conditions, the catalytic current density of a-NPG is over 1.5 times and 17 times higher than that of f-NPG (prepared by free corrosion) and poly-Au electrode, respectively. Using a-NPG sample for glucose detection, the obtained minimum sensible concentration are 413 nM in alkaline media and 1 μM in neutral solutions. The a-NPG electrode also shows stable recovery and reproducibility characteristics. These results indicate that NPG may work as an efficient electrode material for electrochemical sensors and a promising catalyst for alkaline glucose fuel cells.     

A solid paraffin-based carbon paste electrode modified with 2-aminothiazole organofunctionalized silica for differential pulse adsorptive stripping analysis of nickel in ethanol fuel

A solid paraffin-based carbon paste electrode modified with 2-aminothiazole organofunctionalized silica (SiAt-SPCPE) was applied to Ni²⁺ determination in commercial ethanol fuel samples. The proposed method comprised four steps: (1) Ni²⁺ preconcentration at open circuit potential directly in the ethanol fuel sample, (2) transference of the electrode to an electrochemical cell containing DMG, (3) differential pulse voltammogram registering and (4) surface regeneration by polishing the electrode. The proposed method combines the high Ni²⁺ adsorption capacity presented by 2-aminothiazole organofunctionalized silica with the electrochemical properties of the Ni(DMG)₂ complex, whose electrochemical reduction provides the analytical signal.All experimental parameters involved in the proposed method were optimized. Using a preconcentration time of 20 min, it was obtained a linear range from 7.5 × 10⁻⁹ to 1.0 × 10⁻⁶ mol L⁻¹ with detection limit of 2.0 × 10⁻⁹ mol L⁻¹. Recovery values between 96.5 and 102.4% were obtained for commercial samples spiked with 1.0 μmol L⁻¹ Ni²⁺ and the developed electrode was totally stable in ethanolic solutions. The contents of Ni²⁺ found in the commercial samples using the proposed method were compared to those obtained by graphite furnace atomic absorption spectroscopy by using the F- and t-test. Neither the F- nor t-values exceeded the critical values at 95% confidence level, confirming that there are not statistical differences between the results obtained by both methods. These results indicate that the developed electrode can be successfully employed to reliable Ni²⁺ determination in commercial ethanol fuel samples without any sample pretreatment or dilution step.     

Direct amperometric determination of tert-butylhydroquinone in biodiesel

The direct amperometric determination of tert-butylhydroquinone (TBHQ) in biodiesel at an unmodified glassy carbon electrode is reported. A biodiesel aliquot was added into an electrochemical cell containing a 75% (v/v) ethanol-water solution under stirring (with final concentration of 50 mmol L⁻¹ HClO₄). The amperometric method involved the continuous application of three sequential potential pulses to the working electrode (700 mV for 300 ms, 0 mV for 100 ms and −50 mV for 1 s). TBHQ was continuously monitored at the first (direct oxidation) and optionally at the second (reduction) potential pulse while the third potential pulse was applied for cleaning of the electrode surface. For comparison, the samples were also analyzed by high-performance liquid-chromatography and a good agreement between the results was verified. Recovery values for spiked samples were between 90% and 95% and the reproducibility of the proposed method was around 5% (n = 5). The proposed method can be easily adapted for on-site analysis.     

Square-wave voltammetric (SWV) determination of Captopril in reconstituted serum and pharmaceutical formulations

A simple, fast and sensitive square-wave voltammetric (SWV) method for the determination of trace amounts of Captopril in pharmaceutical formulation and reconstituted serum is reported. A three-electrode system containing the static mercury drop electrode (SMDE) working electrode, Pt auxiliary electrode and Ag/AgCl reference electrode was used throughout. Sodium sulfite was used as both supporting electrolyte and oxygen removing agent. No nitrogen purging is needed for oxygen removal from sample solution. Calibration graph showed good linearity in the concentration range of 0.5-50.0 μg mL⁻¹ of Captopril and regression coefficient of 0.9957 is obtained. R.S.D. for eight replicate measurements and LOD of the proposed method are 1.2% and 6.28 × 10⁻³ μg mL⁻¹, respectively. The effect of various parameters (equilibration time, scan increment, pulse height, drop size, frequency and sodium sulfite concentration) on the determination were investigated. The procedure was successfully applied to the determination of Captopril in pharmaceutical formulation and reconstituted serum.     

Voltammetric determination of trace mercury at a sonogel-carbon electrode modified with poly-3-methylthiophene

The sonogel-carbon electrode is a new class of sol-gel electrode that exhibit favourable mechanics and electrics properties to be used as electrochemical sensor. In this paper, a modified sonogel-carbon electrode is proposed to determine mercury at trace levels. The modified electrode is obtained by electropolymerization of 3-methylthiophene on the surface of a bare sonogel-carbon electrode. This electrode shows high selectivity and sensitivity and linear response towards Hg(II), with a detection limit of 1.4 × 10⁻³ mg l⁻¹. The electrode is reusable by a simple chemical cleaning procedure. No deterioration was observed in the electrode response during at least 1 week of successive measurements.     

Mesoporous platinum hosts for electrodeliquidliquid – Triple phase boundary redox systems

Mesoporous metal hosts are attractive electrode materials for complex electrode reactions, for example those involving a system of two immiscible liquids. Here we show that a solution of tetraphenylporphyrianto manganese (MnTPP) in 4-(3-phenylpropyl)-pyridine (PPP) organic liquid can be immobilized into mesoporous platinum thin films electrodeposited from a liquid crystalline template. When immersed in an aqueous solution, the organic liquid remains immobilized inside the mesoporous platinum framework. Well-defined, stable, and reproducible ion transfer voltammograms are recorded. The effects of mesoporous platinum membrane thickness (volume), scan rate, and the type of aqueous electrolyte anion (for Cl⁻, , , CN⁻, SCN⁻ and ) are investigated. Mesoporous platinum is proposed as a very effective electrode material for liquidliquid anion sensing and for other applications of electrochemically driven liquidliquid redox systems.     

Photocurrent kinetics at the electron emission from metal into electrolyte solution X. Discharge of short-lived intermediate species

Electrode reactions of intermediate species (IS), generated by a short pulse of laser photoemission (LPE), result in the time-dependent change of emitted charge Q(t). Analytical expressions for the kinetic curves Q(t) are derived by solving non-stationary diffusion equations for e⁻_aq and IS. For the IS adsorption Gibbs energy less than −25 kJ mol ⁻¹, kinetic curves are exponential over the very wide range of electrode reaction rate constant W, from 1 up to 10⁷ s⁻¹. The dependence Q(t) ∝ t⁻¹² is typical for the case of activated adsorption of IS or their discharge from the non-adsorbed state. Voltammograms of IS generated by pulse radiolysis, modulated photolysis and pulsed or alternating photoemission current are demonstrated to be described by similar expressions. The difference between half-wave and equilibrium potentials depends on the reactant and product lifetimes and rates of desorption. A characteristic trapezoid of Tafel lines is introduced as a new way to characterize completely the kinetics of two-electron electrode reactions. The relations obtained were applied to the analysis of hydrogen evolution reactions and carbon dioxide and formaldehyde reduction, where hydrogen atoms and organic radicals HCO₂ and CH₂OH adsorbed on a mercury electrode participate as IS.