Showing papers on "Cyclic voltammetry published in 1993"

Cyclic Voltammetry: Simulation And Analysis Of Reaction Mechanisms

Abstract: Useful Equations 1. The Reduction Potential and Electrode Kinetics 1.1 The Reduction Potential 1.2 Electrode Kinetics References 2. The Cyclic Voltammetric Experiment 2.1 An Overview 2.2 The Electrochemical Cell 2.3 Electrochemical Mechanisms: E&C Notation 2.4 Distortions of the Faradaic Response 2.5 Microelectrodes and Fast Scan Voltammetry 2.6 Potential Step Methods and Cyclic Voltammetry 2.7 Construction of a Fast Potentiostat 2.8 Determination of the Number of Electrons References 3. A Survey of Electrochemical Mechanisms 3.1 The CE Mechanism 3.2 Multielectron Transfer 3.3 Protonations at Equilibrium 3.4 Catalytic Mechanisms 3.5 The Reduction of Nitrobenzoic Acid 3.6 Reduction of the Nit rosonium Cation and Its Complexes 3.7 Reactivity of 17--, 18--, and 19--Electron Tungsten Complexes 3.8 Mechanisms Involving Adsorption References 4. The Simulation of Electrochemical Experiments 4.1 The Discretized Diffusion Equation 4.2 Evaluation of the Boundary Conditions 4.3 Dimensionless Units 4.4 Solution Chemical Kinetics 4.5 A Sample Simulation Program References 5. CVSIM: A General Program for the Simulation of Cyclic Voltammetry Experiments 5.1 An Overview of CVSIM 5.2 Extensions of the Simulation Method 5.3 Accuracy of Simulations 5.4 Installation and Use of CVSIM and CVGRAF 5.5 Examples References 6. CVFIT: Simplex Data Analysis with CVSIM 6.1 CVFIT: Simplex Data Analysis with CVSIM 6.2 Instructions for the use of CVFIT 6.3 A Prototype Analysis: The EC Mechanism 6.4 Some Final Comments on Simulation Analysis References Appendix: Summary of Instructions for CVSIM, CVGRAF, CVFIT, and DSTEP Index

Sequence-selective biosensor for DNA based on electroactive hybridization indicators.

Abstract: Deoxyribonucleic acid was covalently immobilized onto oxidized glassy carbon electrode surfaces that had been activated using 1-[3-(dimethylamino)-propyl]-3-ethylcarbodimide hydrochloride and N-hydroxysulfosuccinimide. This reaction is selective for immobilization through deoxyguanosine (dG) residues. Immobilized DNA was detected voltammetrically, using tris (2,2'-bipyridyl)cobalt(III) perchlorate and tris (1,10-phenanthroline)cobalt(III) perchlorate (Co(bpy)3(3+) and Co(phen)3(3+). These complexes are reversibly electroactive (1e-) and preconcentrate at the electrode surface through association with double-stranded DNA. Voltammetric peak currents obtained with a poly(dG)poly(dC)-modified electrode depend on [Co(bpy)3(3+)] and [Co(phen)3(3+)] in a nonlinear fashion and indicate saturation binding with immobilized DNA. Voltammetric peak currents for Co(phen)3(3+) reduction were used to estimate the (constant) local DNA concentration at the modified electrode surface; a binding site size of 5 base pairs and an association constant of 1.74 x 10(3) M(-1) yield 8.6 +/- 0.2 mM base pairs. Cyclic voltammetric peak separations indicate that heterogeneous electron transfer is slower at DNA-modified electrodes than at unmodified glassy carbon electrodes. A prototype sequence-selective DNA sensor was constructed by immobilizing a 20-mer oligo (deoxythymidylic acid) (oligo(dT)20), following its enzymatic elongation with dG residues, which yielded the species oligo(dT)20(dG)98. Cyclic voltammograms of 0.12 mM Co(bpy)3(3+) obtained before and after hybridization with poly-(dA) and oligo(dA)20 show increased cathodic peaks after hybridization. The single-stranded form is regenerated on the electrode surface by rinsing with hot deionized water. These results demonstrate the use of electroactive hybridization indicators in a reusable sequence-selective biosensor for DNA.

Thioaromatic monolayers on gold: a new family of self-assembling monolayers

Abstract: Self-assembling (SA) monolayers of the aromatic compounds thiophenol (TP), p-biphenyl mercaptan (BPM), and p-terphenyl mercaptan (TPM) were prepared on gold electrodes. Contact angle (CA) measurements and ellipsometry indicate that TP forms poorly defined layers, while BPM and TPM form monolayers with reproducible CAs and ellipsometric thicknesses. BPM and TPM films are substantially more stable than TP layers toward various electrochemical conditions and replacement by alkanethiols as shown by the combined CAs, ellipsometric thicknesses, and the electrochemical measurements of the corresponding monolayer-covered electrodes. The surface coverages of the three monolayer systems, determined from the electrochemical responses of the respective electrodes, show that the blocking efficiency increases with the number of phenyl rings in the molecule. Three electrochemical techniques, namely gold oxide formation/removal, cyclic voltammetry (CV), and ac impedance were used to determine the monolayer surface coverage. The ac impedance technique proves to be superior for surface coverage determination, mainly due to the milder conditions applied. Molecular mechanics calculations show that BPM and TPM might form (√3 x √3)R30° overlayers on Au(llI) with the molecules nearly perpendicular to the gold surface. Such overlayers would have favorable intermolecular interactions and retain the usual structure for sulfur on Au(llI).

Electrode Kinetics for Chemists, Chemical Engineers and Materials Scientists

Abstract: General considerations polarizable and nonpolarizable interphases the potentials of phases the driving force two cases of special interest components of the measured potential the meaning of the normal hydrogen scale fundamental measurements in electrochemistry measurement of current and potential cell geometry and the choice of reference electrodes electrode kinetics some basic concepts relating electrode kinetics to chemical kinetics methods of measurement single-step electrode reactions the overpotential fundamental equations of electrode kinetics the symmetry factor in electrode kinetics multistep electrode processes mechanistic criteria some specific examples the ionic-double-layer capacitance theories of double-layer structure electrocapillarity thermodynamics methods of measurement and results intermediates in electrode reactions adsorption isotherms for intermediates formed by charge transfer the adsorption pseudocapacitance electrosorption phenomenology isotherms experimental techniques fast transients large-amplitude transients linear potential sweep and cyclic voltammetry electrochemical impendance spectroscopy microelectrodes applications batteries and fuel cells corrosion electroplating.

The electrochemical activity of boron-doped polycrystalline diamond thin film electrodes

Abstract: The electrochemical activity of [open quotes]as grown[close quotes] boron-doped polycrystalline diamond thin film electrodes has been studied using cyclic voltammetry, chronoamperometry, and ac impedance without external illumination. The resistivity of these materials after doping is ca. 10 ohm-cm. The diamond electrodes possess a low double layer capacitance and a relatively high polarization resistance toward surface oxidation. Slow electrode kinetics are observed for Fe(CN)[sub 6][sup 3[minus]/4[minus]] at the [open quotes]as grown[close quotes] surface; however, the electrode response was observed to be extremely stable over a 2-month period as compared with freshly polished glassy carbon. The results suggest that diamond electrodes may have suitable properties as an electrode material for use in electroanalysis. 30 refs., 8 figs., 4 tabs.

Self-assembly of molecular superstructures studied by in situ scanning tunneling microscopy: DNA bases on gold (111)

Abstract: Wc have studied the DNA bases adenine, thymine, guanine, and cytosine adsorbed onto Au(111) using in situ scanning tunneling microscopy (STM), atomic force microscopy (AFM), and cyclic voltammetry. Adenine, guanine, and cytosine adsorbed spontaneously onto the electrode with the counter electrode disconnected and yieldcd stable images over a range of electrode potentials. Thymine was not adsorbed until an electron-transfer reaction occurred at +0.4 V (SCE). Adenine and guanine formed polymeric aggregates in which the bases stacked with repeat distances of 3.4±0.2 A (adenine) and 3.3±0.3 A (guanine)

A Study of the Oxidation of Phenol at Platinum and Preoxidized Platinum Surfaces

Abstract: The oxidation of phenol at the platinum electrode was studied in aqueous acidic solutions. The effects of electrode surface oxide on the oxidation reactions of phenol and on electrode passivation by reaction products were investigated using cyclic voltammetry and chronoamperometry. X‐ray photoelectron spectrometry was used to detect changes in the nature of the passive film. Phenol reacted at both the inner and outer Helmholtz layers at platinum metal electrodes. Phenol in the inner Helmholtz layer is adsorbed irreversibly and is conductive. Its oxidation involves ring cleavage with a greater than 18 eq/mol. The outer Helmholtz layer reactions are characterized by rapid simple oxidations involving minimal rearrangement of the reactant molecule. This implies that once stable oxidized products such as benzoquinone and polymers with quinone or ether structures are formed they must move from the outer to the inner Helmholtz layer to be oxidized further by ring‐cleavage reactions. We postulate that the bulk of the initial current flow during phenol oxidation is due to the simple fast outer Helmholtz reactions. This initial current continues until the buildup of unreactive products blocks further outer Helmholtz reactions and the slower inner layer reactions predominate. This electrode behavior changed if the electrode was preoxidized producing a platinum oxide coating. The inner layer reactions were greatly reduced at a platinum oxide coated electrode resulting in lower passivated electrode current flow. The onset of passivation however was delayed at the oxide coated electrode. This is attributed to a weaker adsorption of reaction products at the electrode surface requiring additional reaction to produce a stable passive film. The final resulting passive films at platinum and platinum oxide electrodes were chemically similar based on x‐ray photoelectron spectrometric analysis but differed in thickness indicating that the electrode passivation is not due simply to the thickness of the passive film.

Interactions between organized, surface-confined monolayers and vapor-phase probe molecules. 7. Comparison of self-assembling n-alkanethiol monolayers deposited on gold from liquid and vapor phases

Abstract: : We report a comparative study of the structure and chemistry of methyl-terminated n-alkanethiol self-assembling monolayers (SAMs) formed from liquid and vapor phases. Three different SAMs are considered: Au/HS(CH2)nCH3, n=5, 11, and 15. Liquid-phase-deposited films were prepared by exposure of Au substrates to dilute ethanol solutions of the n-alkanethiols followed by ethanol rinsing, and vapor-phase-deposited SAMs were prepared by exposure of the Au surface to 10%-of-saturation n-alkanethiol vapors followed by N2 purging, which removes loosely bound n-alkanethiol molecules from the surface. The matrix of 6 organic surfaces were studied by FTIR external reflectance spectroscopy (FTIR- ERS), ellipsometry, and cyclic voltammetry, which provide information about the average structure of the SAMs, and a newly developed scanned tunneling microscope (STM)-based method, which provides information about individual SAM defect structures. FTIR-ERS and ellipsometry indicate no detectable differences between liquid- and vapor-phase-deposited SAMs. Data obtained using cyclic voltammetry and STM show that the barrier properties of SAMs depend on the ambient phase from which the SAM assemblies, the length of the n-alkanethiol, and the chemical nature of the molecular probe used to evaluate the monolayer structure.

Electrochemical applications of the bending beam method. 2. Electroshrinking and slow relaxation in polypyrrole

Abstract: Cyclic voltammetry of chloride-doped polypyrrole (PPy(Cl)) in an aqueous solution of tetraethylammonium chloride and dodecyl benzenesulfonate-doped polypyrrole (PPy(DBS)) in an aqueous solution of sodium dodecylbenzenesulfonate is studied. Emphasis is put on the effect of holding the polymers at the negative or positive potential limit on the subsequent cyclic voltammograms. The bending beam method is used to detect the volume changes in PPy(Cl) and PPy(DBS) during doping and undoping under constant potentials and while the polymers are held at the undoped or doped state.

Electrochemistry at .omega.-hydroxy thiol coated electrodes. 4. Comparison of the double layer at .omega.-hydroxy thiol and alkanethiol monolayer coated Au electrodes

Abstract: Tbe diffuse-layer potential and potential of zero charge at alkanethiol and ω-hydroxy thiol monolayers adsorbed at Au electrodes are extracted from differential capacitance data and compared with the predictions of a two-capacitor model for the electrode double layer. Modeling the double layer as an electrolyte-independent monolayer capacitor in series with a diffuse-layer capacitor calculated from the Gouy-Chapman theory, close agreement is found between the calculated and measured diffuse-layer potential versus electrode potential curves

A fully active monolayer enzyme electrode derivatized by antigen-antibody attachment

Abstract: The immobilization of monolayers and submonolayers of glucose oxidase on carbon electrodes by adsorption of rabbit IgG (antigen) and reaction with a glucose oxidase conjugated antibody, the antirabbit IgG produced in goat, is described. As revealed by radioactive 125 I labeling and by cyclic voltammetry, using ferrocene methanol as mediator, the enzyme monolayers thus immobilized are fully active and persistent. The fact that the mediator couple remains reversible in the presence of the enzyme film allows a particularly simple and quick derivation of primary and secondary plots characterizing the enzyme kinetics from the cyclic voltammetric responses

Optimization of waveforms for pulsed amperometric detection of carbohydrates based on pulsed voltammetry

Abstract: Liquid chromatography (LC) with pulsed amperometric detection (PAD) at Au electrodes has gained prominence for analysis of complex mixtures of polyalcohols and carbohydrates. The most commonly used PAD waveform consists of three potentials and four time parameters, all of which can be varied independently but the effects of which are not necessarily mutually exclusive. Choice of potential values in PAD waveforms has traditionally been based on cyclic voltammetry (CV)

Reversible Electrochemistry of Fumarate Reductase Immobilized on an Electrode Surface. Direct Voltammetric Observations of Redox Centers and Their Participation in Rapid Catalytic Electron Transport

Abstract: Fumarate reductase (Escherichia coli) can be immobilized in an extremely electroactive state at an electrode, with retention of native catalytic properties. The membrane-extrinsic FrdAB component adsorbs to monolayer coverage at edge-oriented pyrolytic graphite and catalyzes reduction of fumarate or oxidation of succinate, depending upon the electrode potential. In the absence of substrates, reversible redox transformations of centers in the enzyme are observed by cyclic voltammetry. The major component of the voltammograms is a pair of narrow reduction and oxidation signals corresponding to a pH-sensitive couple with formal reduction potential E degree' = -48 mV vs SHE at pH 7.0 (25 degrees C). This is assigned to two-electron reduction and oxidation of the active-site FAD. A redox couple with E degree' = -311 mV at pH 7 is assigned to center 2 ([4Fe-4S]2+/1+). Voltammograms for fumarate reduction at 25 degrees C, measured with a rotating-disk electrode, show high catalytic activity without the low-potential switch-off that is observed for the related enzyme succinate dehydrogenase. The catalytic electrochemistry is interpreted in terms of a basic model incorporating mass transport of substrate, interfacial electron transfer, and intrinsic kinetic properties of the enzyme, each of these becoming a rate-determining factor under certain conditions. Electrochemical reversibility is approached under conditions of low turnover rate, for example, as the supply of substrate to the active site is limited. In this situation, electrocatalytic half-wave potentials, E1/2, are similar for oxidation of bulk succinate and reduction of bulk fumarate and coincide closely with the E degree' value assigned to the FAD. At 25 degrees C and pH 7, the apparent KM for fumarate reduction is 0.16 mM, and kcat is 840 s-1. Accordingly the second-order rate constant, kcat/KM, is 5.3 x 10(6) M-1 s-1. Under the same conditions, oxidation of succinate is much slower. As the supply of fumarate to the enzyme is raised to increase turnover, the electrochemical reaction eventually becomes limited by the rate of electron transfer from the electrode. Under these conditions a second catalytic wave becomes evident, the E1/2 value of which corresponds to the reduction potential of the redox couple suggested to be center 2. This small boost to the catalytic current indicates that the low-potential [4Fe-4S] cluster can function as a second center for relaying electrons to the FAD.

Hydrogen evolution on platinum single crystal surfaces: effects of irreversibly adsorbed bismuth and antimony on hydrogen adsorption and evolution on platinum (100)

Abstract: A study of the hydrogen evolution reaction (HER) has been carried out on platinum (111), (100), and (110) oriented electrodes in acidic medium (H 2 SO 4 0.5 M). No influence of the surface structure of the electrode has been observed. It has been confirmed that H upd cannot be the intermediate involved in the hydrogen evolution reaction. A detailed investigation follows concerning the HER on Pt(100) electrodes poisoned by irreversibly adsorbed adatoms. The Bi-Pt(100) and Sb-Pt(100) systems have been studied and in both cases the same general behavior has been observed. The basic HER mechanism does not change with adatom adsorption, while its rate is lowered as a consequence of poisoning

Temperature dependence of the Tafel slope for oxygen reduction on platinum in concentrated phosphoric acid

Abstract: Oxygen reduction on bright platinum in concentrated H3PO4 has been investigated with the rotating disc electrochemical technique at temperatures from 25 to 250° C and oxygen pressures up to 1.77 MPa. Cyclic voltammetry has been employed to study the anodic film formed on platinum in concentrated H3PO4 and the possible electroreduction of H3PO4 on platinum. The apparent transfer coefficient for the oxygen reduction has been found to be approximately proportional to temperature rather than independent of temperature. Such behaviour is difficult to reconcile with accepted theories for the effect of electrode potential on the energy barriers for electrode processes. It is of importance to establish an understanding of this phenomenon. Possible factors which can contribute to the temperature dependence of the transfer coefficient but which would not necessarily result in a direct proportionality to temperature include potential dependent adsorption of solution phase species, restructuring of the solution in the compact layer, proton and electron tunnelling, a shift in rate-determining step, changes in the symmetry of the potential energy barrier, penetration of the electric field into the electrode phase, insufficient correction for ohmic losses, and impurity effects.

Examination of Conditions under Which the Reduction of the Cobaltocenium Cation Can Be Used as a Standard Voltammetric Reference Process in Organic and Aqueous Solvents

Abstract: The two one-electron proccesses for reduction of the cobaltocenlum cation as the hexafluorophosphate salt have been studied extensively in both aprotic and protic solvents at a hanging mercury drop electrode and at conventionally and microsized platinum, goid, and glassy carbon disk electrodes of various radii. Under carefully chosen conditions, both the Cc + /Cc 0 and Cc 0 /Cc - couples (Cc=cobaltocene) can be used as reversible voltammetric reference systems

Morphology of electrochemically prepared polyaniline. Influence of polymerization parameters

Abstract: The kinetics of electrochemical aniline polymerization in aqueous acidic solutions have been investigated by chronocoulometry as a function of the electrolyte anion (i.e. HSO4–, Cl–, NO3–, ClO4–, CF3CO2–), concentration of aniline and para-phenylenediamine (PPDA) as an additive and also in terms of potential or current parameters during film preparation. The morphology of the resulting films was examined by scanning electron microscopy and by porosity measurements. Porosity values were determined based on film thicknesses (40–2000 µm) of polyaniline (PANI) layers saturated with electrolyte solution. While the standard recipes for electrochemical aniline polymerization yield films of porosities P>90%, much denser films (P≈60%) were prepared from aqueous solutions of 0.5 mol dm–3 aniline in 1.0 mol dm–3 HClO4 in the presence of 25 mmol dm–3 PPDA. The results are discussed in terms of a qualitative model for film growth and in view of potential applications in batteries.

Tetrathiafulvalene- and dithiafulvene-substituted mesitylenes, new .pi.-donor molecules with 3-fold symmetry and the formation of an unprecedented new class of electroactive polymers

Abstract: Novel π-donor molecules with 3-fold symmetry are built on the 1,3,5-benzene core, substituted with three identical moieties, i.e. -CH=H(TTF) (8) (TTF=tetrathiafulvalene), -CH 2 S((TTF)Me 3 ) (9), 1,4-dithiafulven-6-yl (10a,b), and 6-methyl-1,4-dithiafulven-6-yl (11a,b). Cyclic voltammetry experiments demonstrate that oxidation of 10a,b gives rise to the precipitation of an electroactive polymer on the electrode, while 9 can be reversibly oxidized to 9 3+ and 9 6+ . Electrocrystallization of 9 in the presence of TBA 2 (Mo 6 Cl 14 ) affords crystals of (9 2+ ).Mo 6 Cl 14 2- , whose magnetic properties are investigated by EPR

Nature of surface bonding on voltammetrically oxidized noble metals in aqueous media as probed by real-time surface-enhanced Raman spectroscopy

Abstract: The nature of the metal-oxygen bonding formed during the initial phase of voltammetric electrooxidation of platinum, rhodium, ruthenium, and gold surfaces in aqueous media has been examined by means of surface-enhanced Raman spectroscopy (SERS). The first three surfaces were formed by electrodeposition as ultrathin (ca. three monolayer) films on a SERS-active gold substrate, enabling intense Raman spectra to be obtained for oxidation of the transition-metal overlayers. Sequences of SER spectra were typically obtained in real time during cyclic potential excursions in acidic (0.1 M HClO 4 ) and basic (0.1 M KOH) media, enabling the evolution of the surface vibrational properties to be correlated with the simultaneous voltammetric (current-potential) response