This paper reports the results for the pH of six buffer solutions free of chloride ion with compositions: (a) BES (0.03 mol·kg(-1)) + NaBES (0.09 mol·kg(-1)); (b) BES (0.02 mol·kg(-1)) + NaBES (0.04 mol·kg(-1)); (c) BES (0.04 mol·kg(-1)) + NaBES (0.08 mol·kg(-1)); (d) BES (0.04 mol·kg(-1)) + NaBES (0.04 mol·kg(-1)) (e) BES (0.05 mol·kg(-1)) + NaBES (0.05 mol·kg(-1)); and (f) (0.06 mol·kg(-1)) + NaBES (0.06 mol·kg(-1)). The remaining eight buffer solutions (g) to (n) have saline media of the ionic strength I = 0.16 mol·kg(-1), matching closely to that of the physiological sample. Conventional pa(H) values, designated as pH(s), for all six buffer solutions (a) - (f) without the chloride ion and eight buffer solutions with the chloride ion (g) - (n) at I = 0.16 mol·kg(-1) from (278.15 K to 328.15) K have been calculated. The operational pH values for five buffer solutions at T = 298.15 K and T = 310.15 K have been determined based on the difference in the values of the liquid junction potentials between the blood phosphate standard and the experimental buffer solutions. Five of these buffers are recommended as secondary standards for the physiological pH range 7.5 to 8.5.
We validate, by comparison with experimental data, a theoretical description of the amperometric response of microbiosensors formed via enzyme entrapment. The utility of the theory is further illustrated with two relevant examples supported by experiments: (1) quantitative detection of glucose and (2) quantitative detection of adenosine triphosphate (ATP).
Binding of a few ligand molecules with its receptors on cell surface can initiate cellular signaling transduction pathways, and trigger viral infection of host cells. HIV-1 infects host T-cells by binding its viral envelope protein (gp120) with its receptor (a glycoprotein, CD4) on T cells. Primary strategies to prevent and treat HIV infection is to develop therapies (e.g., neutralizing antibodies) that can block specific binding of CD4 with gp120. The infection often leads to the lower counts of CD4 cells, which makes it an effective biomarker to monitor the AIDS progression and treatment. Despite research over decades, quantitative assays for effective measurements of binding affinities of protein-protein (ligand-receptor, antigen-antibody) interactions remains highly sought. Solid-phase electrochemiluminescence (ECL) immunoassay has been commonly used to capture analytes from the solution for analysis, which involves immobilization of antibody on solid surfaces (micron-sized beads), but it cannot quantitatively measure binding affinities of molecular interactions. In this study, we have developed solution-phase ECL assay with a wide dynamic range (0-2 nM) and high sensitivity and specificity for quantitative analysis of CD4 at femtomolar level and their binding affinity with gp120 and monoclonal antibodies (MABs). We found that binding affinities of CD4 with gp120 and MAB (Q4120) are 9.5×10(8) and 1.2×10(9) M(-1), respectively. The results also show that MAB (Q4120) of CD4 can completely block the binding of gp120 with CD4, while MAB (17b) of gp120 can only partially block their interaction. This study demonstrates that the solution-phase ECL assay can be used for ultrasensitive and quantitative analysis of binding affinities of protein-protein interactions in solution for better understating of protein functions and identification of effective therapies to block their interactions.
An implantable, micromachined microprobe with a microsensor array for combined monitoring of the neurotransmitters, glutamate (Glut) and dopamine (DA), by constant potential amperometry has been created and characterized. Microprobe studies in vitro revealed Glut and DA microsensor sensitivities of 126±5 nA·μM(-1)·cm(-2) and 3250±50 nA·μM(-1)·cm(-2), respectively, with corresponding detection limits of 2.1±0.2 μM and 62±8 nM, both at comparable ~1 sec response times. No diffusional interaction of H(2)O(2) among arrayed microelectrodes was observed. Also, no responses from the electroactive interferents, ascorbic acid (AA), uric acid (UA), DOPA (a DA catabolite) or DOPAC (a DA precursor), over their respective physiological concentration ranges, were detected. The dual sensing microbe attributes of size, detection limit, sensitivity, response time and selectivity make it attractive for combined sensing of Glut and DA in vivo.
Electrochemistry and ion transport in a planar array of mechanically-driven, droplet-based ion sources are investigated using an approximate time scale analysis and in-depth computational simulations. The ion source is modeled as a controlled-current electrolytic cell, in which the piezoelectric transducer electrode, which mechanically drives the charged droplet generation using ultrasonic atomization, also acts as the oxidizing/corroding anode (positive mode). The interplay between advective and diffusive ion transport of electrochemically generated ions is analyzed as a function of the transducer duty cycle and electrode location. A time scale analysis of the relative importance of advective vs. diffusive ion transport provides valuable insight into optimality, from the ionization prospective, of alternative design and operation modes of the ion source operation. A computational model based on the solution of time-averaged, quasi-steady advection-diffusion equations for electroactive species transport is used to substantiate the conclusions of the time scale analysis. The results show that electrochemical ion generation at the piezoelectric transducer electrodes located at the back-side of the ion source reservoir results in poor ionization efficiency due to insufficient time for the charged analyte to diffuse away from the electrode surface to the ejection location, especially at near 100% duty cycle operation. Reducing the duty cycle of droplet/analyte ejection increases the analyte residence time and, in turn, improves ionization efficiency, but at an expense of the reduced device throughput. For applications where this is undesirable, i.e., multiplexed and disposable device configurations, an alternative electrode location is incorporated. By moving the charging electrode to the nozzle surface, the diffusion length scale is greatly reduced, drastically improving ionization efficiency. The ionization efficiency of all operating conditions considered is expressed as a function of the dimensionless Peclet number, which defines the relative effect of advection as compared to diffusion. This analysis is general enough to elucidate an important role of electrochemistry in ionization efficiency of any arrayed ion sources, be they mechanically-driven or electrosprays, and is vital for determining optimal design and operation conditions.
A new, sensitive platform for the simultaneous electrochemical assay of Zn(II), Cd(II) and Pb(II) in aqueous solution has been developed. The platform is based on a new bimetallic Hg-Bi/single-walled carbon nanotubes (SWNTs) composite modified glassy carbon electrode (GCE), demonstrating remarkably improved performance for the anodic stripping assay of Zn(II), Cd(II) and Pb(II). The synergistic effect of Hg and Bi as well as the enlarged, activated surface and good electrical conductivity of SWNTs on GCE contribute to the enhanced activity of the proposed electrode. The analytical curves for Zn(II), Cd(II) an Pb(II) cover two linear ranges varying from 0.5 to 11 μg L(-1) and 10 to 130 μg L(-1) with correlation coefficients higher than 0.992. The limits of detection for Zn(II), Cd(II) are lower than 2 μg L(-1) (S/N = 3). For Pb(II), moreover, there is another lower, linear range from 5 to 1100 ng L(-1) with a coefficient of 0.987 and a detection limit of 0.12 ng L(-1). By using the standard addition method, Zn(II), Cd(II) and Pb(II) ions in river samples were successfully determined. These results suggest that the proposed method can be applied as a simple, efficient alternative for the simultaneous monitoring of heavy metals in water samples. In addition, this method demonstrates the powerful application of carbon nanotubes in electrochemical analysis of heavy metals.
A generalized description of the response behavior of potentiometric polymer membrane ion-selective electrodes is presented on the basis of ion-exchange equilibrium considerations at the sample-membrane interface. This paper includes and extends on previously reported theoretical advances in a more compact yet more comprehensive form. Specifically, the phase boundary potential model is used to derive the origin of the Nernstian response behavior in a single expression, which is valid for a membrane containing any charge type and complex stoichiometry of ionophore and ion-exchanger. This forms the basis for a generalized expression of the selectivity coefficient, which may be used for the selectivity optimization of ion-selective membranes containing electrically charged and neutral ionophores of any desired stoichiometry. It is shown to reduce to expressions published previously for specialized cases, and may be effectively applied to problems relevant in modern potentiometry. The treatment is extended to mixed ion solutions, offering a comprehensive yet formally compact derivation of the response behavior of ion-selective electrodes to a mixture of ions of any desired charge. It is compared to predictions by the less accurate Nicolsky-Eisenman equation. The influence of ion fluxes or any form of electrochemical excitation is not considered here, but may be readily incorporated if an ion-exchange equilibrium at the interface may be assumed in these cases.
Carbon fiber/epoxy composite materials, which are manufactured using the pultrusion process, are commercially available in various shapes and sizes at very low cost. Here we demonstrate the application of such a material as an electrochemical detector in a flow system. Cyclic voltammetry shows that the material's electrochemical behavior resembles that of glassy carbon. Using tube and rod composites, we successfully fabricated a ring-disk electrode with a 20 μm gap between the ring and the disk. The narrow gap is favorable for mass transfer in the generator-collector experiment. This composite ring-disk electrode is assembled in a thin-layer radial-flow cell and used as an electrochemical detector. The disk electrode, placed directly opposite to the flow inlet, is operated as a generator electrode with the ring electrode being a collector. The high collection efficiency on the ring electrode (0.8 for a chemically reversible species) enhances the detection selectivity.
Electrochemical detection is becoming increasingly important for the detection of biological species. Most current biological research with electrochemical detection is done with carbon fiber electrodes due to their many beneficial properties. The ability to build electrochemical sensor from noble metals instead of carbon fibers may be beneficial in developing inexpensive multiplexed electrochemical detection schemes. To advance understanding and to test the feasibility of using noble metal electrochemical sensors the detection of dopamine, a biologically important small molecule was studied here. Specifically, dopamine detection on gold microelectrodes was characterized and compared to P-55 carbon fiber microelectrodes of the same geometry, using background subtracted fast scan cyclic voltammetry. While not as sensitive to dopamine as carbon fibers, it was observed that gold microelectrodes have six times the saturation coverage per area and 40 times the linear working range. Selectivity to dopamine, in comparison to several other neurotransmitters and their derivatives, is also quantitatively described.
Graphene has remarkable electrochemical properties that make it an ideal material for constructing biosensors,however it has not been explored for DNA biosensing. Herein, we report on a chitosan-modified graphene platform for the electrochemical detection of changes in DNA sequences. For this purpose, graphene synthesized chemically and characterized by Raman spectroscopy and Transmission electron microscopy, was covalently modified with positively charged chitosan to facilitate the immobilization of a single-stranded DNA `capture' oligonucleotide. The covalent attachment of chitosan to graphene was confirmed by FT-IR spectroscopy and then the capture DNA was immobilized on to the chitosan modified graphene electrode. Then, the target DNA (complementary or mismatched `mutant' DNA) was applied to the electrode and cyclic voltammetry was performed. The results of the voltammetric experiments indicate that the chitosan modified graphene electrodes immobilized with ssDNA+complementary DNA exhibit a significantly higher magnitude of redox peak current than the chitosan modified graphene electrodes immobilized with the non-complementary mutant DNAs. Together, these results demonstrate that the chitosan-graphene platform provides a rapid, stable and sensitive detection of mismatched DNA and has the potential to be used for point-of-care diagnostic tests for specific DNA mutations associated with disease conditions.
Passing currents through ion-selective membranes has contributed to the development of a variety of novel methods. In this work, chronopotentiometric (CP) transients with two transition times (breakpoints) are presented for the first time, with the theoretical interpretation of such voltage transients. The validity of our theory has been confirmed in experiments utilizing ETH 5294 chromoionophore-based pH sensitive membranes with and without lipophilic background electrolyte and ETH 5234 ionophore-based calcium selective membranes in which the ionophore forms 3:1 complexes with Ca(2+) ions. The conditions under which two breakpoints can be identified in the chronopotentiometric voltage transients are discussed.Spectroelectrochemical microscopy (SpECM) is used to show that the two breakpoints in the CP curves emerge approximately when the free ionophore and ion-ionophore complex concentrations approach zero at the opposite membrane-solution interfaces. The two breakpoint times can be utilized to follow simultaneously the concentration changes of the free ionophore, the ion-ionophore complex, and the mobile anionic sites in cation-selective membranes. In membranes with known composition, the time instances where breakpoints occur can be used to estimate the free ionophore and the ion-ionophore complex diffusion coefficients.
A theoretical treatment of the time-dependent potential response of ion-selective electrodes to sample solutions containing primary and interfering ions is presented. The theory accounts for the influence of ion fluxes in the electrode membrane and the contacting aqueous sample layer and describes the variations in the apparent selectivity behavior as a function of the measuring time. The applicability of the theory is demonstrated by comparing predicted response curves with results of virtual experiments based on computer simulation. A close and convincing agreement was achieved for a large series of different examples, which confirms that the new theory can be successfully applied for general cases.
A simple but powerful numerical simulation for analyzing the electrochemical behavior of ion-selective membranes and liquid junctions is presented. The computer modeling makes use of a finite-element procedure in the space and time domains, which can be easily processed (e. g., with MS Excel software) without the need for complex mathematical evaluations. It leads to convincing results on the dynamic evolution of concentration profiles, potentials, and fluxes in the studied systems. The treatment accounts for influences of convection, flow, or stirring in the sample solution that act on the boundary diffusion layer and it is even capable of including the effects of an electrolyte flow through the whole system. To minimize the number of arbitrary parameters, interfacial reactions are assumed to be near local equilibrium, and space-charge influences are considered via phase-boundary potential differences. The applicability of the computer simulation is demonstrated for different ion-selective membranes as well as for liquid junctions. The numerical results are in excellent agreement with experimental data.
A straightforward theoretical description of the time-dependent response of ion-selective membrane electrodes to multiple sample changes is presented. The derivation makes use of an approximation for the ion fluxes in the membrane, and of the superposition of partial fluxes induced by the step-changes. The general theory allows for any number of samples and ions. It is applied for the analysis of memory effects that reflect the influence of preceding samples on subsequent measurements. Various phenomena are discussed, including super-, near-, or sub-nernstian responses, shifts of apparent reference potentials, and potential dips with domains of reversed slopes. The theoretical results agree well with virtual experiments based on computer simulation.
Thyrotropin-releasing Hormone (TRH) forms an electroactive Cu(II) complex in aqueous solution. Rotating ring-disk electrochemistry reveals oxidation at the disk electrode and reduction at the ring electrode. The plot of limiting current vs. square root of rotation frequency deviates from the Levich equation, indicating both preceding and following chemical reactions. The reaction following the oxidation is a multiple-electron ECE-type of process that has been seen before in Cu(II)-peptide electrochemistry. The preceding reaction is unusual. The deviation from diffusion-controlled behavior is more pronounced at higher initial concentration of Cu(II) and peptide. We propose that a non-electroactive dimer, Cu(II)(2)-TRH(2), is in a slow equilibrium with the electroactive Cu(II)-TRH. Simulation of the RRDE behavior of the postulated Cu(II)-TRH system has succeeded in matching experimental data. Capillary electrophoresis indicates that there is a negative charge on the dimer. It is suggested that a hydroxo-bridge may link the two Cu(II) centers. Calculations verify that bi-nuclear Cu(II)(2)-TRH(2) complexes are possible.
This study focuses on the cyclic voltammetry behavior at shallow recessed microdisc electrode, particularly on the transition from cottrellian behavior to steady state behavior. Diffusion to the inlaid and recessed microdisc electrode is simulated. From the shape of the CVs, for a given radius and potential scan rate, the transition time from planar diffusion to hemispherical diffusion presents a minimum as the recess increases. Theoretical prediction was confirmed by fitting the simulated CVs with experimental results. Dimensionless transition scan rate has been defined and determined by simulation for inlaid and recessed microdisc electrodes.
Nitric oxide (NO) has many important physiological roles in the body. Since NO electrodes can directly measure NO concentration in the nM range and in real time, NO electrode methods have been generally used in laboratories for measuring NO concentration in vivo and in vitro. This review focuses on the application of electrode methods in studies of NO diffusion and metabolic kinetics. We have described the physical and chemical properties that need to be considered in the preparation of NO stock solution, discussed the effect of several interfering factors on the measured curves of NO concentration that need to be eliminated in the experimental setup for NO measurements, and provided an overview of the application of NO electrode methods in measuring NO diffusion and metabolic kinetics in solution and in biological systems. This overview covers NO metabolism by oxygen (O2), superoxide, heme proteins, cells and tissues. Important conclusions and physiological implication of these studies are discussed.
We study propagation of a particle that jumps between two states, in which it moves with different velocities and diffusion coefficients. To simplify analysis, in the main part of the paper we derive formulas assuming that in one of the states the particle is immobile. A generalization to the case when the particle is mobile in both states is given at the end of the paper. The formulas show how the effective drift velocity and effective diffusion coefficient depend on jump rates between the two states as well as on the particle velocities and diffusion coefficients in these states. Specifically, we find that the effective diffusion coefficient can exhibit a non-monotonic behavior as a function of the ratio of the jump rates.
Scanning electrochemical microscopy (SECM) is developed as a powerful approach to electrochemical characterization of individual one-dimensional (1D) nanostructures under unbiased conditions. 1D nanostructures comprise high-aspect-ratio materials with both nanoscale and macroscale dimensions such as nanowires, nanotubes, nanobelts, and nanobands. Finite element simulations demonstrate that the feedback current at a disk-shaped ultramicroelectrode tip positioned above an unbiased nanoband, as prepared on an insulating substrate, is sensitive to finite dimensions of the band, i.e., micrometer length, nanometer width, and nanometer height from the insulating surface. The electron-transfer rate of a redox mediator at the nanoband surface depends not only on the intrinsic rate but also on the open-circuit potential of the nanoband, which is determined by the dimensions of the nanoband as well as the tip inner and outer radii, and tip-substrate distance. The theoretical predictions are confirmed experimentally by employing Au nanobands as fabricated on a SiO(2) surface by electron-beam lithography, thereby yielding well defined dimensions of 100 or 500 nm in width, 47 nm in height, and 50 μm in length. A 100 nm-wide nanoband can be detected by SECM imaging with ∼2 μm-diameter tips although the tip feedback current is compromised by finite electron-transfer kinetics for Ru(NH(3))(6) (3+) at the nanoband surface.
The ability to quickly and inexpensively fabricate planar solid state nanogaps has enabled research to be effectively performed on devices down to just a few nanometers. Here, nanofabricated electrode pairs with electrode-to-electrode spacings of <4, 6 and 20 nm are utilized for monitoring an electroactive molecules, dopamine, in ionic solution. The results show a several order of magnitude enhancement of the electrochemical signal, collected current, for the solid state nanogaps with 6 nm electrode-electrode spacings as compared to traditional microelectrodes. The data from the <4 nm and 20 nm solid state nanogaps verify that this enhancement is due to cycling of the redox molecules in the confined geometry of the nanogap. In addition the data collected for the <4 nm nanogap emphasizes and reinforces that scaling does have limits and that as device sizes move to the few nanometer scale, the influence of a molecule's size and other physical properties becomes increasingly important and can eventually dominate the generated signals.
We investigate the nonlinear dynamics of transpassive electrodissolution of nickel in sulfuric acid in an epoxy-based microchip flow cell. We observed bistability, smooth, relaxation, and period-2 waveform current oscillations with external resistance attached to the electrode in the microfabricated electrochemical cell with 0.05 mm diameter Ni wire under potentiostatic control. Experiments with 1mm × 0.1 mm Ni electrode show spontaneous oscillations without attached external resistance; similar surface area electrode in macrocell does not exhibit spontaneous oscillations. Combined experimental and numerical studies show that spontaneous oscillation with the on-chip fabricated electrochemical cell occurs because of the unusually large ohmic potential drop due to the constrained current in the narrow flow channel. This large IR potential drop is expected to have an important role in destabilizing negative differential resistance electrochemical (e.g., metal dissolution and electrocatalytic) systems in on-chip integrated microfludic flow cells. The proposed experimental setup can be extendend to multi-electrode configurations; the epoxy-based substrate procedure thus holds promise in electroanalytical applications that require collector-generator multi-electrodes wires with various electrode sizes, compositions, and spacings as well as controlled flow conditions.
This work demonstrates the first cyclic voltammetry in a perfluorocarbon solvent without use of a cosolvent. The novel electrolyte tetrabutylammonium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate (NBu(4)BArF(104); 80 mM) allows for voltammetry of ferrocene in perfluoro(methylcyclohexane) by lowering the specific resistance to Ω268 k cm at 20.8 °C. Despite significant solution resistance, the resulting voltammograms can be fitted quantitatively without difficulty. The thus determined standard electron transfer rate constant, k°, for the oxidation of ferrocene in perfluoro(methylcyclohexane) is somewhat smaller than for many solvents commonly used in electrochemistry, but can be explained readily as the result of the viscosity and size of the solvent using Marcus theory. Dielectric dispersion spectroscopy verifies that addition of NBu(4)BArF(104) does not significantly raise the overall polarity of the solution over that of neat perfluoro(methylcyclohexane).
The applicability of extremely thin non-electroneutral membranes for ion-selective electrodes (ISEs) is investigated. A theoretical treatment of potential and concentration profiles in space-charge membranes of < 1 μm thickness is presented. The theory is based on the Nernst-Planck equation for ion fluxes, which reduces to Boltzmann's formula at equilibrium, and on the Poisson relationship between space-charge density and electric field gradient. A general solution in integral form is obtained for the potential function and the corresponding ion profiles at equilibrium. A series of explicit sub-solutions is derived for particular cases. Membrane systems with up to three different ion species are discussed, including trapped ionic sites and co-extracted ions. Solid-contacted thin membranes (without formation of aqueous films at the inner interface) are shown to exhibit a sub-Nernstian response. The theoretical results are confirmed by numerical simulations using a simplified finite-difference procedure based on the Nernst-Planck-Poisson model, which are shown to be in excellent agreement.
We have developed a theoretical description of the amperometric response of ultramicroelectrode (UME) biosensors formed via enzyme entrapment. Our model allows for multiple enzymes and co-substrates, and results in a closed-form analytical expression for the steady-state current response of the disk ultramicroelectrode. It captures the effects of enzyme-entrapment domain size, species transport properties (which can be different in the polymer and surrounding electrolyte), enzyme kinetics, and axisymmetric diffusion. Assumptions inherent in the derivation are carefully explained, as are the resulting limits on the applicability of the results. The ability to theoretically predict the response of enzyme entrapped UMEs should enable improved design, operation, and data interpretation for this important class of biosensors.
In situ electrochemical scanning tunneling microscopy measurements of the anodic oxidation of Cu(0 0 1) in 0.1 M NaOH are reported. Adsorption-induced surface reconstruction is observed in the underpotential range of oxidation with the formation of dimers of superimposed Cu atoms ejected from the substrate and stabilized by adsorbed OH groups, presumably in bridging positions. The reconstruction causes the reorientation of the substrate step edges and the formation of holes and ad-islands of monoatomic height. The dimers of superimposed Cu atoms are alternatively aligned along the 〈1 0 0〉 directions to form zig-zag arrangements. Long range ordering is observed in areas of limited lateral extension with c(2×6) and c(6×2) domains. In the potential range of Cu(I) oxide formation, a facetted Cu2O layer grows with a Cu2O(0 0 1) ∣∣ Cu(0 0 1)[1 0 0] epitaxial relationship. The 45° rotation between the close-packed directions of the oxide lattice and metal lattice results from the orientation of the dimers of superimposed Cu atoms in the precursor adsorbed OH layer. The surface of the oxide layer is facetted due to a tilt of ∼3% between oxide and metal lattices. Its (0 0 1) terraces have an identical chemical termination and are presumably hydroxylated.
The electrochemical reduction of oxygen on phenanthrenequinone-modified glassy carbon electrodes (GC) has been studied using a rotating ring–disk electrode (RRDE). Phenanthrenequinone (PQ) was covalently attached to the surface of GC by the electrochemical reduction of the corresponding diazonium salt. The redox potential of surface-bound PQ in 0.1 M KOH was 300 mV more positive than that of anthraquinone (AQ). The PQ modified electrode showed a much higher electrocatalytic activity for oxygen reduction. The results have been analysed using a surface redox catalytic cycle in which the key reactive intermediate is the semiquinone radical anion. The more positive redox potential of PQ compared with AQ and the larger value of the rate constant of reaction of the radical with O2 result in an enhanced electrocatalytic activity of the functionalised electrode, producing H2O2 with a 100% yield.
The duplex copper oxide layer formation process in 0.1 M borax solution has been investigated using different potential/time programs, open circuit potential decay, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The importance of time in the stability attained by the different oxide species was underscored. The diffusional processes involved in the duplex layer formation and oxide thickness values dependent on experimental conditions were characterized and estimated respectively through EIS data.
5,10,15,20-tetraphenylporphyrin (TPP) and 5,10,15,20-tetra-(4-chlorophenyl)porphyrin (TClPP) were synthesized and formed adlayers on iron surface. The surface properties of the porphyrin adlayers on iron electrode were characterized by Fourier transform infrared reflection spectroscopy (FT-IR), fluorescence spectrum (FS), scanning electron microscopy (SEM) and electrochemical methods including electrochemical impedance spectroscopy (EIS) and polarization curves. FT-IR and FS results indicated that TPP and TClPP were able to form adlayers on the iron surface. The electrochemistry results showed that both adlayers of TPP and TClPP were able to protect iron from corrosion effectively and the protection efficiency of TPP was higher than that of TClPP. The surface state of iron was characterized by SEM after the iron electrode was corroded in H2SO4 solutions (0.5 M). Besides, quantum chemical calculation was applied to optimize the structure of the two molecules and was able to explain the experimental results to some extent. Results indicated that TPP and TClPP were good inhibitors for iron corrosion in H2SO4 solutions.
The hydrogen oxidation reaction kinetics have been measured as a function of temperature on Ru(0001) and Ru(10−10) surfaces in H2SO4 and HClO4 solutions by using a rotating disk electrode. The reaction is essentially under kinetic control at temperatures between 25 and 60 °C. It has a pronounced structural sensitivity, having higher rates on a Ru(10−10) than on a Ru(0001) surface. The reaction is strongly inhibited by Ru oxide formation at low overpotentials for hydrogen oxidation, which causes a current peak in the polarization curve and a negligible oxidation current at large overpotentials. The structural dependence appears to be predominantly determined by the properties of the oxidized surfaces of Ru(0001) and Ru(10−10). The kinetics are faster in H2SO4 than in HClO4 solution due to a slower surface oxidation in the former acid. The apparent electrochemical activation energy for the Ru(0001) surface is about 120 kJ mol−1, while 80 kJ mol−1 is observed for the Ru(10−10) surface. The exchange current densities of 0.13 and 0.16 mA cm−2 at 40 °C for Ru(0001) and Ru(10−10), respectively have been determined from the linear part of the polarization curves. The origin of the structural effects on the hydrogen oxidation kinetics on Ru surfaces has been discussed. In sharp contrast to hydrogen oxidation, the hydrogen evolution kinetics shows a very small structural dependence.
Spontaneous deposition of Pt on a Ru(0001) single crystal surface has been demonstrated by in situ scanning tunneling microscopy and linear sweep voltammetry techniques. The immersion of a ultra-high vacuum (UHV) prepared Ru single crystal in a platinum-ion-containing solution results in the formation of monolayer-to-multilayer Pt deposits without application of an external potential. The coverage and morphology of the Pt deposit depend on the concentration of platinum ions and the time of immersion. This simple method can be applied to Pt deposition on Ru nanoparticles, which can considerably reduce Pt loading in Pt/Ru electrocatalysts. The electrochemical behavior of such Pt/Ru(0001) bimetallic surfaces depends on the coverage of the Ru electrode and amount and morphology of the Pt deposit, which indicates a pronounced electronic modification of the Pt adlayer due to a strong Pt/Ru interaction.
Cyclic voltammetry (CV) was used to investigate the electrocatalytic oxidation of formic acid on smooth and rough Ru(0001) electrodes in HClO4 solution. Ex-situ electron diffraction (LEED and RHEED) and Auger electron spectroscopy (AES) were applied to characterize the Ru electrode surfaces and the adsorbate structure. The CV for a smooth Ru(0001) surface in a 0.1 M HCOOH+0.1 M HClO4 solution no longer exhibits the H- and OH-adsorption peaks in the double layer region due to complete blocking of H/OH-adsorption by COad species, as has been confirmed by the observation of an ordered (2×2)-CO phase in good agreement with in-situ IR measurements. We find that the major pathway for HCOOH oxidation is via dehydration at electrode potentials below 0 V involving no faradaic current formation in agreement with the literature, since no anodic current peak occurs in this potential region. An anodic current peak appears at 0.6 V, which is ascribed to COad electrooxidation with concomitant O/OH adsorption as confirmed by a (1×1)-O phase and an increase in the Auger O-signal, which causes inhibition of HCOOH adsorption and decomposition leading to deceleration of formic acid oxidation at higher potentials, E>0.7 V.
Potent catalysis by Ru electrodes has been reported by various workers. Reported here are studies of chemisorption, surface vibrational spectroscopy and electrochemical reactivity at Ru(001) single-crystal electrode surfaces. Electrochemical oxidation of methane on these Ru electrode surfaces in aqueous electrolytes was investigated. The influence of surface oxide and electrodeposited silver on methane oxidation were explored. Immersion of Ru(001) into pure water at open circuit forms a layer of adsorbed hydrous oxides with an ordered (2 x 2) structure as measured by Auger spectroscopy and low energy electron diffraction (LEED). Anodization of Ru(001) in 1 M HClO4 produces a disordered Ru O/OH film consisting of several atomic layers. The high resolution energy electron loss spectrum of this O/OH layer exhibits Ru-O and O-H stretching bands, and the layer is not removed by subsequent electrolysis at negative potentials. Various submonolayer and multiple-layer amounts of silver were electrodeposited on Ru(001). A continuous film is formed, based upon attenuation of the substrate Auger signal. The silver layer lacks long-range order, as judged by LEED. Under the present conditions, namely Ru(001) single-crystal surfaces with or without the O/OH and/or silver layers in aqueous electrolytes, the faradaic current due to oxidation of methane is generally less than 1 muA cm-2.
Cyclic voltammetry, controlled-potential electrolysis, GC, GC–MS, HPLC, and HPLC–ESI–MS have been employed to investigate the catalytic reduction of 1,1,2-trichloro-1,2,2-trifluoroethane (chlorofluorocarbon 113, CFC-113, or FreonTM 113) by cobalt(I) salen electrogenerated at a carbon cathode in dimethylformamide (DMF) containing tetra-n-butylammonium tetrafluoroborate (TBABF4) as a supporting electrolyte. A cyclic voltammogram for the reduction of cobalt(II) salen in the presence of excess CFC-113 exhibits a large prewave, attributed to both the formation of a 1,1-dichloro-1,2,2-trifluoroethylcobalt(III) salen complex and its reduction to chlorotrifluoroethene and cobalt(II) salen. This prewave is followed by a smaller wave which involves the reduction of cobalt(II) salen to cobalt(I) salen; a slower catalytic reaction of chlorotrifluoroethene to form trifluoroethene takes place at this same potential, but only when CFC-113 has been completely consumed. Controlled-potential electrolyses carried out at a potential slightly more negative than this second wave also lead to other products, one confirmed to be 1,1,1,2-tetrafluoroethane and at least two other species suspected to be difluoro compounds. From results obtained by means of cyclic voltammetry and controlled-potential electrolysis, along with already published knowledge about the electrochemistry of cobalt-containing complexes, a mechanism is proposed to explain our findings. Catalyst death is also addressed in the light of data from controlled-potential electrolysis and from experiments done with the aid of HPLC and HPLC–ESI–MS.
The coordination chemistry of 2,8-dithia,(2,9)-1,10-phenanthrolinophane (L) towards transition and post-transition metal ions such as Cu(II), Cu(I), Cd(II), Co(II), Ni(II), Pb(II) and Zn(II) was studied at the polarized water/1,2-dichloroethane interface by cyclic voltammetry. The dependence of the half-wave transfer potential on the ligand and metal concentrations was used to formulate the stoichiometry, and to evaluate the association constants of the complexes formed between L and Pb(II). (c) 2005 Elsevier B.V. All rights reserved.
The ion transfer of 1,10-phenanthroline (phen) across the water/nitrobenzene interface has been studied in the pH range 1–7 by electrochemical methods, adding phen to the aqueous phase or to the organic (nitrobenzene) phase. Current-scan polarography at an ascending water electrode and potential-scan cyclic voltammetry at a stationary plane electrode were used. With each addition, an anodic wave caused by the transfer of a protonated phen (Hphen+) from the aqueous to the organic phase was observed. Here a detailed mechanism of the ion transfer is proposed by considering branch diffusions of phen species from the interface to either bulk phase by which the Hphen+ transfer producing the anodic wave is depressed. With the addition of phen to the aqueous phase, the wave became deformed, noisy and steep, and finally separated into two waves as the phen concentration was increased and the pH was decreased. This is probably caused by the interfacial adsorption of Hphen+. The adsorption phenomenon is also discussed with the aid of electrocapillary curves.
The present paper investigates voltammetrically the properties of the layers formed, whereas Part I of this series studied the electrochemical formation of l-α-lecithin (dipalmitoyl, DPPC) adsorbed layers at the water/1,2-dichloroethane interfaces. These are primarily determined by ion pairing between the positively charged layer and the anions present on the aqueous side of the interface. Cerium(IV) sulfate has been used as the source of the model anion and a five-step mechanism proposed for the formation and behaviour of DPPC layers. For the sake of comparison, the same experiments have been carried out with trimethyloctadecylammonium cation (TODA+) which have helped in elucidation of certain parts of the mechanism. In the absence of anions of higher charges, both DPPC and TODA+ form monolayers at the interface and the process involves simple free diffusion transport of the layer components through the bulk of the solution. In their presence, both DPPC and TODA+ form multilayers, the transport of the components is slower and is given by lateral diffusion along the layer surface and diffusion through the layer. The passage of ions across the interface is virtually not hindered by the monolayers, unless the ions are very large. However, DPPC and TODA+ multilayers strongly slow down or even prevent passage of larger ions (trimetylammonium (TMA+), and K+ ions assisted by a crown compound were used in the study).
The facilitated transfer of silver, Ag+, has been studied at the Interface between Two Immiscible Electrolyte Solutions (ITIES). The transfer was achieved with the assistance of a calixarene-based silver ionophore. An investigation of the mechanistic details of the transfer was conducted using cyclic voltammetry at both micro and macro liquid|liquid interfaces. The mechanism was found to follow a Transfer by Interfacial Complexation (TIC)/Transfer by Organic phase Complexation (TOC) mechanism. The complex stoichiometry was found to shift from 1:1 to 1:2, metal:ligand, with increasing ionophore concentration. The logarithms of the complex association constants, , were estimated at 12.4 and 14.5, respectively. The charge transfer current was also found to be limited by diffusion of the transferring species and was unchanged by the presence of a range of interferents. The system thus shows promise for selective analytical applications.
Cyclic voltammetry applied to liquid/liquid interfaces and nuclear magnetic resonance spectroscopy were used to analyze the stability of tylosin A in aqueous solutions in acid conditions. Cyclic voltammetry was employed to characterize the charge transfer processes for tylosin A and tylosin B across the H2O/1,2-dichloroethane interface. Electrochemical study includes the transfer potential dependence of pH value, the elucidation of the mechanism of transfer using mechanical control of convective flux, and the determination of partition coefficient values and diffusion coefficient values employing the change of voltammetric current with external parameters. Degradation process of tylosin A was monitored and tylosin B or desmycosin was isolated and then identified and characterized by nuclear magnetic resonance spectroscopy and electrochemical techniques.
Rotating ring-disk electrode (RRDE) voltammetry has been used for studying the electrochemical oxidation processes of N-substituted 2,6-dimethyl-3,5-di(ethoxycarbonyl)-1,4- and 1,2-dihydropyridines in acetonitrile.1,2-Dihydropyridines oxidise more easily by 200-300 mV than the corresponding 1,4-dihydropyridines; introduction of electron donating substituents into positions 1 and 4 of the ring facilitates, whilst that of electron accepting ones impedes electrooxidation. Changing the electrode material (platinum, carbon, gold) does not affect the potential of the first stage of electrooxidation substantially.RRDE methods were applied within a wide temperature range (from +40 to −40°C) to determine the kinetic characteristics of decay of the primary products of one-electron oxidation of 1,2-dihydropyridine cation radicals. Cation radicals have also been identified by ESR methods and hyperfine structure constants of their ESR spectra have been determined. A possible mechanism and intermediate stages of the electrochemical oxidation reaction are proposed and compared with the electrochemical reduction mechanism of pyridinium salts.
The synthesis, spectroscopic and electrochemical characterizations of two neutral bisdithiolene nickel complexes are described. Electrochemical reversibility and bandgap have been measured by different techniques. The influence of the nature and length of the alkyl chain, the type of electrode (Pt microdisk for solutions, ITO-coated glass, Pt wire, and CME (cavity microelectrode) for condensed state) and the solvent have been investigated. Optical transmission measurements in the visible–near-IR (VIS–NIR) region allowed the determination of the optical bandgap.
The assisted transfer of proton by interfacial complexation with 2-acetylpyridine-4-phenyl-3-thiosemicarbazone (APPT) has been investigated at the micro interface of two immiscible electrolyte solutions (ITIES) hosted by a 25 μm diameter pipette. Thermodynamics of proton transfer has been investigated by using cyclic voltammetry. The dependence of half wave potential on the ligand concentration suggests that the equilibrium is effectively displaced towards a 1:1 (proton:ligand) stoichiometry, with a formation constant of 6.3 × 109. The formal Gibbs energy of proton transfer, the diffusion coefficient of proton in aqueous solution and diffusion coefficient of APPT in organic phase have been evaluated. The kinetic parameters of ion transfer process have also been studied by micropipette voltammetry.
Thionine (Thio+) is a dye used in photogalvanic and dye-sensitized solar cells to convert light into chemical energy. The photophysical properties of this cell are strongly affected by aggregate formation and polymerization of thionine because the monomer is the photoactive species. In this paper, we report the electrochemical interfacial transfer of thionine from water to 1,2 DCE and the further photochemical reaction upon illumination of this organic phase. Cyclic voltammetry coupled to interfacial absorbance measurements allowed us to perform a kinetic analysis. Values of εThio = (2.5 ± 0.1)104 M−1 cm−1, εproduct = (1.4 ± 0.1)10 4 M−1 cm−1 and a kinetic constant k = 0.28 ± 0.05 s−1 for the chemical reaction, were calculated by simulation of the interfacial absorbance profiles. NMR and UV–vis spectra of the phases at different pH values contributed to identify the nature of the reacting species of the chemical reaction and showed the important influence of pH on the kinetic of the process. The effect of the organic supporting electrolyte on the kinetics of the reaction was also analyzed.
The transfer of ions at the water ⋎ 1,2-dichloroethane interface mediated either by erythromycin or by its decomposition products in the presence and absence of a phospholipid monolayer was analysed by dc cyclic voltammetry and electrochemical impedance measurements in a four-electrode system. The composition of the liquid phases has an important effect on the transfer processes. Significant differences in their parameters were found depending on the phase in which the antibiotic was initially present, i.e. erythromycin as a base in the organic phase or protonated erythromycin in the aqueous phase. A dependence on pH was also noticed. These results suggest that the ion transfer mechanisms can be modified by the experimental conditions. The charge transfer resistance (Rct) for erythromycin and for its hydrolysis products was measured with and without the phospholipid monolayer. Except when erythromycin was in the organic phase, a general increase in Rct, more important for the hydrolysed antibiotic, was observed, indicating that the loss of the sugar units or the opening of the lactone ring hinders the transport process at the phospholipid modified interface.
The transfer of phenothiazine derivatives (promazine, chlorpromazine, triflupromazine, methotrimeprazine, perphenazine and fluphenazine) across the water∣1,2-dichloroethane interface was studied using cyclic voltammetry. The partition coefficients of ionic species of phenothiazines, logPXH+, were calculated from the transfer potentials measured at pH < pKa. These values were related with the Hammet parameter of substituents in order to find a dependence with the presence of electron acceptor substituents in the molecule. These electron acceptor groups affect the biological activity of these drugs. The results indicated that logPXH+ can be used in structure–activity correlations as it takes into account the effects of substituents on the main positions within the phenothiazine molecule.
The ion transfer and adsorption behavior of the free base of water-soluble porphyrins were studied at the polarized water/1,2-dichloroethane interface by potential modulated fluorescence (PMF) spectroscopy. The PMF response indicated the presence of adsorption process for all systems depending on the Galvani potential difference. The adsorption from the organic side of the interface was found for cationic meso-tetrakis(N-methylpyridyl)porphyrin (H2TMPyP4+) at potentials more negative than its formal ion transfer potential. The emission spectrum for the interfacial species could be obtained successfully by analyzing the dependence of PMF intensity on the wavelength, and the emission maximum wavelength of the interfacial species was significantly different from the bulk species measured in the aqueous and organic phases. It suggests that the solvation structure of interfacial species is modified from both the aqueous and organic bulk species. The presence of adsorption process for anionic porphyrin systems, meso-tetrakis(4-sulfonatophenyl)porphyrin and protoporphyrin IX, was also found by analyzing the PMF responses.
The facilitated potassium transfer by monoaza-crown-6 (A18C6) across the water—1,2-dichloroethane interface has been investigated by cyclic voltammetry. The pH of the aqueous phase determines the nature of and the type of assisted ion transfer, i.e. proton transfer, assisted metal ion transfer or both. The stability constants of protonated A18C6 and potassium-A18C6 in 1,2-dichloroethane, and potassium-A18C6 in water, have been evaluated from the voltammetric data.
The electrochemical oxidation of 6-hydroxy-1,2,3,4-tetrahydro-β-carboline (1), an alkaloid which occurs naturally in the mammalian brain, has been studied in aqueous solution particularly at physiological pH. The first voltammetric oxidation peak of 1 observed at the pyrolytic graphite electrode generates a radical intermediate which dimerizes to give 5,5′-bi-(6-hydroxy-1,2,3,4-tetrahydro-β-carboline) (3). However, the putative radical intermediate can also be further oxidized (1e) to give a C(5)-centered carbocation which can either dimerize in an ion-substrate reaction to give 3 or be attacked by water to give 5,6-dihydroxy-1,2,3,4-tetrahydro-β-carboline (8) which is rapidly oxidized further to 1,2,3,4-tetrahydro-β-carboline-5,6-dione (9). In the presence of glutathione dione 9 forms the 8-S-glutathionyl conjugate of 8 which is easily oxidized to the 8-S-glutathionyl conjugate of 9. It is suggested that 1 might be an alkaloid which is elevated in the brain as a result of chronic alcoholism, and the roles of the oxidative transformations of this compound in some of the addictive and neurophathological consequences of ethanol consumption are discussed.
An investigation into the oxidative electrochemistry of 1,3,5-Tris[4-[(3-methylphenyl)phenylamino]phenyl]benzene (TMPB) is described. In particular, the heterogeneous electron-transfer kinetics in N,N-dimethylformamide (DMF) and dichloromethane (DCM) are studied using the high-speed microband channel electrode. Experiments to investigate the standard electrochemical rate constants, k0, transfer coefficients, α, and formal potentials, Ef0, of the observable oxidations of TMPB in both DMF and DCM solutions containing 0.10 M tetrabutylammonium perchlorate (TBAP) are reported for 12.5 μm platinum microband electrodes using a range of linear flow velocities from 12 to 25 m s−1. The measured values of k0 for the two measurable oxidations in DMF are 1.03 ± 0.41 and 1.02 ± 0.20 cm s−1, and for the three oxidations in DCM are 0.44 ± 0.04, 0.17 ± 0.03 and 0.08 ± 0.02 cm s−1, respectively. The values of α for these oxidations in DMF are 0.48 ± 0.06 and 0.52 ± 0.02, and in DCM are 0.52 ± 0.02, 0.62 ± 0.03 and 0.53 ± 0.07, respectively. The respective formal oxidation potentials (all measured vs Ag) are 0.533 ± 0.002 and 0.766 ± 0.003 V in DMF, and 0.161 ± 0.002, 0.495 ± 0.002 and 1.128 ± 0.004 V in DCM. The presence of the monocation radical in DCM is confirmed by ESR measurement.Experiments are also presented to explore the voltammetry of TMPB in microdroplets of toluene and also in the solid-phase, when in contact with aqueous solutions of sodium fluoride, perchlorate, nitrate and sulphate. It was found that, when TMPB is dissolved in a toluene microdroplet, anion insertion accompanies the first oxidation for the case of perchlorate and nitrate, with anion-facilitated dissolution occurring for sulphate and fluoride. More complex reactions occur at more positive potentials. In the solid-phase, however, slow anion-facilitated dissolution still occurs for fluoride and sulphate, and rapid direct dissolution takes place in the case of perchlorate.