Electrode In Aqueous Solution Research Articles

Simultaneous detection of dopamine and acetaminophen by modified gold electrode with polypyrrole/aszophloxine film

The differential pulse voltammetric method was developed for simultaneous determination of dopamine (DA) and acetaminophen (AC) using the Ppyox/AZ/Au (poly overoxidized pyrrole–aszophloxine–gold) modified electrode. Polypyrrole film was prepared by incorporation of aszophloxine (AZ) as doping anion, during the electropolymerization of pyrrole onto a gold (Au) electrode in aqueous solution using the cyclic voltammetric (CV) technique. Then the polypyrrole film was overoxidized at constant potential. Well-separated voltammetric peaks were observed for dopamine (DA) and acetaminophen (AC) with peaks separation of about 200mV. The current of the oxidation peaks showed a linear dependent on the concentrations of DA and AC in the range of 0.1–30μM and 0.2–100μM respectively. The detection limit of 0.05 and 0.08μM (S/N=3) were obtained for DA and AC respectively. The modified electrode was used for determination of DA and AC in some real samples such as human serum and tablet samples.

Nickel(ii) macrocycles: highly efficient electrocatalysts for the selective reduction of CO2 to CO

A series of molecular materials that are structurally similar to the NiII macrocycle [Ni(cyclam)]2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) have been used as electrocatalysts for the reduction of CO2 at a mercury pool working electrode in aqueous solution. At pH 5, with an applied potential of −0.96 V vs. NHE (overpotential of −0.55 V), the complexes are highly efficient, having both high rate constants and Faradaic efficiencies (F.E.s) for the selective reduction of CO2 to CO. When the pH is below the pKa (pH < 2) of the Ni(H) species (pKas: 0.5–2), the F.E.s are still high but product selectivity changes to yield predominantly H2 from the reduction of water. At least two of the complexes investigated are better electrocatalysts than [Ni(cyclam)]2+, probably due to: (i) surface geometries that are suitable for adsorption onto the mercury electrode surface, and (ii) electronic effects of methyl groups or cyclohexane rings on the cyclam backbone. Mechanistic studies by pulse radiolysis show evidence of Ni(CO2) adducts for two of the catalysts, with KCO2 ∼ 10 M−1 for the reaction of NiI with CO2 in aqueous solution.

Integrated photosystem II-based photo-bioelectrochemical cells

Photosynthesis is a sustainable process that converts light energy into chemical energy. Substantial research efforts are directed towards the application of the photosynthetic reaction centres, photosystems I and II, as active components for the light-induced generation of electrical power or fuel products. Nonetheless, no integrated photo-bioelectrochemical device that produces electrical power, upon irradiation of an aqueous solution that includes two inter-connected electrodes is known. Here we report the assembly of photobiofuel cells that generate electricity upon irradiation of biomaterial-functionalized electrodes in aqueous solutions. The cells are composed of electrically contacted photosystem II-functionalized photoanodes and an electrically wired bilirubin oxidase/carbon nanotubes-modified cathode. Illumination of the photoanodes yields the oxidation of water to O(2) and the transfer of electrons through the external circuit to the cathode, where O(2) is re-reduced to water.

Dechlorination of chloroacetic acids by electrocatalytic reduction using activated silver electrodes in aqueous solutions of different pH

Chloroacetic acids (CAAs, including trichloroacetic acid, dichloroacetic acid and monochloroacetic acid) are byproducts formed upon the addition of chlorine to water for disinfection purpose and they pose a potential human health risk. To catalyze the dechlorination of CAAs, an activated silver electrode was prepared by an oxidation–reduction cycle process. Surface structure and roughness factor of the activated silver electrode and its catalytic activity (including stability) towards the dechlorination of CAAs in aqueous solutions of different pH were studied by SEM, underpotential deposition (UPD) method, cyclic voltammetry (CV) and controlled-current electrolysis, respectively. Experimental evidence is presented that the activated silver electrode, having a porous surface with a roughness factor of approx. 63.9, is an excellent electrocatalytic material for direct dechlorination of CAAs and the complete dechlorination of trichloroacetic acid to safe acetic acid with excellent current efficiency (near 95%) can be successfully achieved by using this peculiar electrode in alkaline aqueous solutions. The reduction of trichloroacetic acid in a stepwise dechlorination fashion was suggested and trichloroacetic acid (TCAA) → dichloroacetic acid (DCAA) → monochloroacetic acid (MCAA) → acetic acid (AA) probably is the main reductive reaction route independent of pH.

A combination of in situ ESR and in situ NMR spectroelectrochemistry for mechanistic studies of electrode reactions: the case of p‐benzoquinone

A combined in situ NMR and in situ ESR spectroelectrochemical study of a reaction mechanism is presented detecting and describing the whole number of paramagnetic and diamagnetic intermediates and final products in an electrode reaction. While in situ NMR spectroelectrochemistry provides a powerful method for the study of structural or electronic changes of diamagnetic molecules in any electrochemical reaction mechanism, in situ ESR spectroelectrochemistry is the method of choice to detect paramagnetic structures and to characterise their electronic state via the g-value and hyperfine splitting in redox reactions. To demonstrate the power of this combination of magnetic spectroscopies in electrochemistry, the reduction of p-benzoquinone to hydroquinone is followed by both these spectroelectrochemical methods at selected pH values, thus considering the influence of the proton on the reaction mechanism. The results of both in situ spectroelectrochemical methods at the same redox system are used to get the complete reaction mechanism of p-benzoquinone at electrodes in aqueous solutions.

Optimization of Nd(III) pre-concentration on a rotating disk mercury film electrode in aqueous solution

Abstract Current work indicates that trivalent lanthanides will consistently sorb to a rotating disk mercury film electrode in bulk aqueous solution thus allowing the user to pre-concentrate trivalent lanthanides for further separation. However, the mechanism of sorption to the mercury film is largely unknown, making it difficult if not impossible to predict a priori the conditions under which pre-concentration is optimized. In the present study, we explore some of the electrochemical factors affecting this pre-concentration process to optimize the technique for use in a wide variety of aqueous media. Trivalent neodymium (Nd(III)) is used as a model for the trivalent f-element cations. The factors investigated include pH of solution, rotation rate of the electrode, concentration of mercury, potential application waveforms, order of deposition, and time dependency.

Preparation, characterization and the electrogenerated chemiluminescence behavior of WO 3 nanocrystals

The (WO 3) NCs/Nafion film electrode was prepared by immobilizing the synthesized WO 3 NCs on the surface of a glassy carbon electrode (GCE) with the help of Nafion. The ECL emission of the electrode in aqueous solution was affected by the buffer solution with respect to the pH and composition. And the strongest ECL was observed in an NH 3–NH 4Cl buffer with pH being 8.2. Only a weak ECL peak at 1.14 V which was originated from the annihilation process between oxidized and reduced species of WO 3 NCs was found. By adding coreactant triproplamine (TPrA) in buffer solution, an additional ECL peak at 1.13 V which was attributed to the electron-transfer reaction between the oxidized WO 3 NCs and reduced intermediate of TPrA was observed. The (WO 3) NCs/Nafion film electrode exhibits excellent ECL property and good stability, which would promote the potential application of WO 3 NCs as a luminescence material for solid-state ECL detection.

Voltammetric study on interaction of cysteine with type I-collagen in the aqueous medium

Interaction of cysteine with type I-collagen from bovine achilles tendon in the aqueous solutions has been examined using square wave voltammetry (SWV) and cyclic voltammetry (CV) techniques. In the absence of cysteine, type I-collagen gives a reversible peak at −0.114 V in Britton-Robinson (B-R) buffer (pH 4.0). The electrochemical parameters (Ip/f, Ep/f, Ep/pH, Ip/pH, Ip/v, Ip/v1/2) of type I-collagen have been also studied. In addition, it has been determined that there is a linear relationship between current and concentration of type I-collagen. On the other hand, cysteine exhibits a reversible peak at −0.383 V due to the reduction of mercurous cysteine thiolate. By using a hanging mercury drop electrode in aqueous solutions, SWV and CV voltammograms obtained for type I-collagen in the presence of cysteine indicate that there is an interaction between type I-collagen and cysteine. In the presence of cysteine, peak current of type I-collagen decreases and a new peak is observed at −0.121 V for cysteine which is bonded to type I-collagen.

Front end with offset-free symmetrical current source optimized for time domain impedance spectroscopy

Fast impedance measurements are often performed in time domain utilizing broad bandwidth excitation signals. Other than in frequency domain measurements harmonic distortion cannot be compensated which requires careful design of the analog front end. In order to minimize the influence of electrode polarization and noise, especially in low-frequency measurements, current injection shows several advantages compared to voltage application. Here, we show an active front end based on a voltage-controlled current source for a wide range of impedances. Using proper feedback, the majority of the parasitic capacitances are compensated. The bandwidth ranges from dc to 20 MHz for impedance magnitude below 5 kΩ. The output is a symmetric signal without dc-offset which is accomplished by combination of a current conveyor and a voltage inverter. An independent feedback loop compensates the offset arising from asymmetries within the circuitry. We focused especially on the stability of the current source for usage with small metal electrodes in aqueous solutions. At the monitor side two identical, high input impedance difference amplifiers convert the net current through the object and the voltage dropping across into a 50 Ω symmetric output. The entire circuitry is optimized for step response making it suitable for fast time domain measurements.

Bubble dynamic templated deposition of three-dimensional palladium nanostructure catalysts: Approach to oxygen reduction using macro-, micro-, and nano-architectures on electrode surfaces

Three-dimensional (3D) palladium (Pd) nanostructures (that is, nano-buds or nano-dendrites) are fabricated by bubble dynamic templated deposition of Pd onto a glassy carbon electrode (GCE). The morphology can be tailored by changing the precursor concentration and reaction time. Scanning electron microscopy images reveal that nano-buds or nano-dendrites consist of nanoparticles of 40–70 nm in diameter. The electrochemical reduction of oxygen is reported at such kinds of 3D nanostructure electrodes in aqueous solution. Data were collected using cyclic voltammetry. We demonstrate the Pd macroelectrode behavior of Pd nanostructure modified electrode by exploiting the diffusion model of macro-, micro-, and nano-architectures. In contrast to bare GCE, a significant positive shift and splitting of the oxygen reduction peak (vs Ag/AgCl/saturated KCl) at Pd nanostructure modified GCE was observed.

Theoretical Treatment of a Cathodic Stripping Mechanism of an Insoluble Salt Coupled with a Chemical Reaction in Conditions of Square Wave Voltammetry. Application to 6‐Mercaptopurine‐9‐D‐Riboside in the Presence of Ni(II)

AbstractCathodic stripping mechanism of an insoluble salt coupled with a homogenous chemical reaction is considered both theoretically and experimentally under conditions of square‐wave voltammetry. For the mercury electrode in aqueous solution, the electrode reaction is described as L(aq)+Hg(l)=HgL(s)+2e−, where L(aq) is the reactive ligand that forms a sparingly soluble compound HgL(s). The electrode reaction is coupled with a homogenous, first‐order chemical reaction, A(aq)=L(aq). Theoretical predictions are confirmed by experiments with 6‐mercaptopurine‐9‐D‐riboside in the presence of nickel(II) ions.

Direct oxidation of methionine at screen printed graphite macroelectrodes: Towards rapid sensing platforms

The direct electrochemical oxidation of methionine has been achieved at bare carbon based electrodes and for the first time at screen printed graphite electrodes in aqueous solutions. Due to scales of economy and intended use as a potential point-of-care sensor, the quantification of methionine was explored at screen printed electrodes, allowing linear ranges over the range 0.05–5.0 mM with a detection limit of 95 × 10 −6 mol L −1 possible in model solutions. Application of this sensor was used for the determination of methionine in a pharmaceutical product containing a complex mixture of vitamins, amino acids, chelated minerals and additional factors with the results agreeing with manufacturers’ specification suggesting that this sensing platform holds promise as a rapid, sensitive and disposable sensor for methionine determination.

Cathodic electrochemiluminescence from self-designed near-infrared-emitting CdTe/CdS/ZnS quantum dots on bare Au electrode

Cathodic electrochemiluminescence (ECL) from self-designed near-infrared (NIR)-emitting CdTe/CdS/ZnS quantum dots (QDs) on bare Au electrode in aqueous solution was studied. Strong and stable ECL signals at − 1.25 V with an onset potential of − 0.98 V produced by the highly effective electron-transfer reactions between electron-injected CdTe/CdS/ZnS QDs and reduced S 2O 8 2− are observed. Passivation of the QD surfaces with ZnS shell increased ECL intensity by 9 times when compared to CdTe/CdS QDs. The good correspondence of the ECL emission peak with the photoluminescence results suggested the surface states of the QDs had been largely passivated by ZnS shell. Furthermore, the effects of pH, buffer solutions, electrode materials and concentrations of K 2S 2O 8 on ECL intensity were investigated, and a possible ECL mechanism was also proposed.

Electrochemical Oxidation of an Immunosuppressant, Mycophenolate Mofetil, and Its Assay in Pharmaceutical Formulations

Electrochemical oxidation of mycophenolate mofetil (MMF) has been studied at a glassy carbon electrode in aqueous solution over a wide pH range. MMF was oxidized on glassy carbon electrode (GCE) by an irreversible process that was controlled mainly by diffusion. The irreversibility of the electrode process was verified by different criteria. A probable mechanism for electrochemical oxidation of MMF was proposed. Differential-pulse voltammogram of the drug showed two oxidation peaks at 0.631 V and at 0.921 V (verses SCE) in phosphate buffer of pH 6.0. This process could be used to determine MMF in the concentration range of5.0×10−7to7.5×10−4 M with a limit of detection of1.48×10−7 M. The method was successfully applied for the analysis of MMF in pure and dosage forms and in biological fluids.

Simultaneous Detection of Ascorbic Acid and Hydrogen Peroxide by Flow‐Injection Analysis with a Thin Layer Dual‐Electrode Detector

AbstractBased on preliminary voltammetric investigations at unmodified and modified electrodes in aqueous solutions buffered at different pHs, a profitable thin layer dual‐electrode detector was developed for the selective and simultaneous determination of ascorbic acid (AA) and hydrogen peroxide present in the same samples. It consists of a small thickness (0.1 mm) cell, used as a detector for a flow injection system, in which a glassy carbon (GC) and a polyeugenol coated Pt electrode are encased. The GC electrode is selective for AA, thanks to the potential applied (0.3 V vs. Ag/AgCl,Cl−sat), while H2O2 alone can be oxidized at the Pt electrode (0.75 V), thanks to the size selectivity and electrostatic screen properties displayed by the polyeugenol coating layer which prevents oxidation of ascorbate anions. Repeatable sharp peaks (±4.5 %) were detected for both analytes over wide linear ranges (0.005–1.000 mM) and detection limits of about 10−6 M (176 ppb) and 5×10−7 M (16 ppb) were inferred for AA and H2O2 respectively. This flow injection approach was applied to the analysis of some orange‐taste soft drinks, used as prototype real samples, spiked with controlled amounts of both analytes, thus inferring good recoveries which pointed out that deviations never exceeding 5 % affected the relevant accuracy.

Deposition of new thia-containing Schiff-base iron (III) complexes onto carbon nanotube-modified glassy carbon electrodes as a biosensor for electrooxidation and determination of amino acids

Multiwall carbon nanotubes (MWCNTs) were used as an immobilization matrix to incorporate an Fe (III)–Schiff base complex as an electron-transfer mediator onto a glassy carbon electrode surface. First, the preheated glassy carbon was subjected to abrasive immobilization of MWCNTs by gently rubbing the electrode surface on filter paper supporting the carbon nanotubes. Second, the electrode surface was modified by casting 100 μL of an Fe (III)-complex solution (0.01 M in ACN). The cyclic voltammograms of the modified electrode in an aqueous solution displayed a pair of well-defined, stable and nearly reversible reductive oxidation redox systems with surface confined characteristics. Combinations of unique electronic and electrocatalytic properties of MWCNTs and Fe (III)–Schiff base complexes resulted in a remarkable synergistic augmentation of the response. The electrochemical behavior and stability of the modified electrode in aqueous solutions at pH 1–9 were characterized by cyclic voltammetry. The apparent electron transfer rate constant ( K s) and transfer coefficient ( a) were determined by cyclic voltammetry and were approximately 7 s −1 and 0.55, respectively. The modified electrodes showed excellent catalytic activity towards the oxidation of amino acids at an unusually positive potential in acidic solution. They also displayed inherent stability at a wide pH range, fast response time, high sensitivity, low detection limit and had a remarkably positive potential oxidation of amino acids that decreased the effect of interferences in analysis. The linear concentration range, limits of detection (LOD), limits of quantization (LOQ) and relative standard deviation of the proposed sensor for the amino acid detection were 1–55,000, 1.10–13.70, 2.79–27.14 and 1.30–5.11, respectively.

Synthesis, characterization and electroanalytical application of a new SiO 2/SnO 2 carbon ceramic electrode

A new SiO 2/SnO 2 carbon ceramic composite was prepared by the sol–gel method, and its potential application in electrochemistry as a novel electrode material has been studied. The prepared xerogel was structurally and electrochemically characterized by scanning electron microscopy coupled to energy dispersive spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction and cyclic voltammetry. The composite was pressed in a rigid disk-shape and used as a conductive substrate to immobilize a water-soluble organic–inorganic hybrid polymer, 3-n-propyl-4-picolinium chloride silsesquioxane. The oxidation of nitrite was studied on this polymer film coated electrode in aqueous solution using cyclic voltammetry and differential pulse voltammetry. This modified electrode exhibited a better defined voltammetric peak shifted negatively about 60 mV. The linear detection limit found for nitrite was from 1.3 × 10 −5 to 1.3 × 10 −3 mol l −1 and the detection limit was 3.3 × 10 −6 mol l −1.

CO 2 attraction by specifically adsorbed anions and subsequent accelerated electrochemical reduction

The electrochemical reduction of CO 2 was studied on a copper mesh electrode in aqueous solutions containing 3 M solutions of KCl, KBr and KI as the electrolytes in a two and three phase configurations. Electrochemical experiments were carried out in a laboratory-made, divided H-type cell. The working electrode was a copper mesh, while the counter and reference electrodes were Pt wire and Ag/AgCl electrode, respectively. Results of our work suggest a reaction mechanism for the electrochemical reduction of CO 2 in the two phase configuration where the presence of Cu-X as the catalytic layer facilitates the electron transfer from the electrode to CO 2. Electron-transfer to CO 2 may occur via the X ad −(Br −, Cl −, I −)–C bond, which is formed by the electron flow from the specifically adsorbed halide anion to the vacant orbital of CO 2. The stronger the adsorption of the halide anion to the electrode, the more strongly CO 2 is restrained, resulting in higher CO 2 reduction current. Furthermore, it is suggested that specifically adsorbed halide anions could suppress the adsorption of protons, leading to a higher hydrogen overvoltage. These effects may synergistically mitigate the overpotential necessary for CO 2 reduction, and thus increase the rate of electrochemical CO 2 reduction.

Voltammetric detection of riboflavin based on ordered mesoporous carbon modified electrode

The potential application of ordered mesoporous carbon (OMC)-modified glassy carbon electrode (OMC/GCE) in electrochemistry as a novel electrode material was investigated. X-ray diffraction, transmission electron micrographs, and cyclic voltammetry were used to characterize the structure and electrochemical behaviors of this material. Compared to GC electrode, the peak currents of potassium ferricyanide (K3[Fe(CN)6]) increase and the peak potential separation (ΔEp) decreases at the OMC/GC electrode. These phenomena suggest that OMC-modified GC electrode possesses larger electrode area and faster electron transfer rate, as compared with bare GC electrode. Furthermore, riboflavin was detected using OMC/GC electrode in aqueous solutions. The results showed that, under an optimum condition (pH 7.0), the OMC/GC electrode exhibited excellent response performance to riboflavin in the concentration range of 4.0 × 10−7 to 1.0 × 10−6 M with a high sensitivity of 769 µA mM−1. The detection limit was down to around 2 × 10−8 M. With good stability and reproducibility, the present OMC/GC electrode was applied in the determination of vitamin B2 content in vitamin tablets, and satisfactory results were obtained.

Investigations on current transients in porous alumina films during re-anodizing using the electrochemical quartz crystal microbalance

Current transients and mass variations in as-prepared and heat-treated anodic alumina films were measured during re-anodizing by means of voltammetry and electrochemical quartz crystal microbalance (EQCM), respectively. Aluminum electrodes (100 nm) on quartz crystals were prepared by thermal evaporation. Anodic alumina films were formed on the surface of Al electrodes in aqueous solutions of oxalic (0.3 M) and phosphoric (0.6 M) acid in the potentiostatic regime. The EQCM experiments did not detect an overshoot in the mass variation of the Al electrode during re-anodizing of heat-treated anodic alumina films. The observed current overshoot in transients proved the presence of electrons and electron holes injected from the contacts in the bulk of the oxide. This can be explained by the emergence of excess electrons in the barrier layer of the alumina films due to a change in the mobility of the electrons.