General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems

The equation of the linear potential sweep voltammogram is derived for any degree of reversibility of the electrochemical reaction for the following methods: surface voltammetry when both the oxidized and the reduced forms are strongly adsorbed, and a Langmuir isotherm is obeyed, thin layer voltammetry, and linear potential sweep coulometry. The results are expressed in one mathematical form valid for the three cases. The transfer coefficient and the rate constant of the electrochemical reaction can be deduced from an experimental study of the variations of the peak potentials as a function of the sweep rate.     

A multilayer model for the study of space distributed redox modified electrodes: Part II. Theory and application of linear potential sweep voltammetry for a simple reaction

The theory of linear potential sweep voltammetry for a simple redox system in the case of a space distributed redox modified electrode (redox polymer electrode, or adsorption of an electroactive substance in several layers) is established on the basis of the model discussed in Part I of this series. When the rate of the reaction of electron exchange inside the coating is rapid, the theory for a monolayer adsorption is directly applicable. When it is slow, the peaks have a particular shape, with a “tailing” of the current; their characteristics are discussed. The theory is applied to the study of the multilayer adsorption of benzo(c)-cinnoline.     

Computer simulation of the kinetic behaviour of surface reactions driven by a linear potential sweep: Part I. Model 1-electron reaction with a single adsorbed species

The kinetic behaviour of a simple 1 e surface reaction is treated theoretically, especially with regard to (a) characteristic aspects of its behaviour when significant attractive or repulsive interactions arise in the electrodeposited film and (b) formulation of characteristic features of its kinetic and equilibrium behaviour which can provide a reference case for distinguishing, by comparison, the behaviour of other more complex sequential reactions treated in Part II.     

Hydrogen bonding interactions between adsorbed polymer molecules and crystal surface of acetaminophen

The objective of this work was to investigate whether or not the hydrogen bonding interaction between polymer and crystal surface can be detected by the etching pattern changes in the presence of polymers. The (010) face of acetaminophen single crystal was used as a model solid surface. The etching patterns on the (010) face of acetaminophen crystal by water are in the directions of a- and c-axes, which are the same as the directions of the dominant attachment energies on the (010) face. In the presence of polymer, the hydrogen bonding interactions between adsorbed polymer and crystal surface can affect surface diffusion of acetaminophen molecules and change the etching patterns in the direction of a-axis, i.e., the direction of one hydrogen bond chain. Studies with 2-hydroxyethyl cellulose (HEC), hydroxypropyl cellulose (HPC), hydroxypropyl methylcellulose (HPMC) and poly(vinyl alcohol) (PVA) showed that polymers, which can form hydrogen bonds with acetaminophen crystal surface, can change etching patterns in the direction of a-axis. Study with Dextran suggested that if a polymer cannot form hydrogen bonds with crystal surface due to steric repulsion, it will not change the etching pattern in the direction of a-axis. Studies with poly(ethylene glycol) (PEG) and poly(propylene glycol) (PPG) further confirmed that only if a polymer can form hydrogen bonds with acetaminophen on crystal surface, the etching patterns in the direction of a-axis will be affected. The study results suggest that in the presence of polymers, the etching pattern change in the direction of hydrogen bond chain, the a-axis of acetaminophen crystals, can be used to indicate the existence of the hydrogen bonding interactions between adsorbed polymers and acetaminophen crystal surface.     

Steric protection of a modified glass surface with adsorbed polyacrylamide: interactions with blood proteins

Glass beads chemically modified with aluminol (A1OH) groups were coated with a monolayer of high-molecular-mass polyacrylamide at different temperatures and exposed to 125I-labelled albumin and fibrinogen solutions for 1 h. No adsorption of albumin was observed, whereas fibrinogen adsorption increased as a function of temperature. These observations were interpreted in terms of the density and stability of the adsorbed polymer layer. At 25 degrees C, where fibrinogen adsorption was already important, it was shown, using 3H-labelled polyacrylamide, that fibrinogen partly displaced the polymer from its adsorption sites.     

On the ratio of processes of adsorbed oxygen layer formation and palladium surface layer dissolution at linear anodic potential sweep

The surface layer dissolution on certain palladium deposits (Pd/Pt) and smooth polycrystalline Pd in sulfuric acid solutions is quantitatively studied at anodic linear potential sweep using atomic absorption analysis. The strong effect of the conditions of Pd/Pt electrolytic generation on the processes of palladium dissolution and oxygen adsorption is shown. The scanning electron microscopy method is used to trace the relationship between the synthesis conditions and the palladium deposit morphology. It is shown that palladium dissolution in the course of the anodic potential sweep is the most active for deposits with the highest specific surfaces. At the oxygen adsorption potentials, the charge value consumed in Pd dissolution decreases as the potential sweep rate increases.     

Evidence for a surface reaction between H and CO adsorbed on polycrystalline Pt

Temperature programmed desorption (TPD) experiments have been performed after dosing an ion-sputtered and annealed polycrystalline Pt surface with CO and H₂. The CO-dosing was done both sequentially and simultaneously. With increasing residence time on the surface, the H₂-TPD spectra exhibit conversion of the original H₂-like features to features similar to the CO-TPD peaks. This provides evidence for the formation of a CO/H surface species through a chemical reaction between adsorbed H and adsorbed CO. The reaction occurs more rapidly when the sample is dosed or held at elevated temperatures, indicating that the reaction is activated.     

Effect of mono-CDNP substitution of lysine residues on the redox reaction of cytochrome c electrostatically adsorbed on a mercaptoheptanoic acid modified Au(111) surface

The effect of charge-inverting modification of single surface lysine residue on the electron transfer (ET) reaction of horse heart cytochrome c (cyt c) is examined for 12 different types of mono-4-chloro-2,5-dinitrobenzoic acid substituted cyt c (mCDNPc) adsorbed on a Au(111) electrode modified with a self-assembled monolayer (SAM) of 7-mercapto-heptanoic acid (MHA). A negative shift in the redox potential by 10-35 mV as compared to that of native cyt c and a monolayer coverage in the range of 13-17 pmol cm(-2) are observed for electroactive mCDNPc's. The magnitude of the decrease in the ET rate constant (k(et)) of mCDNPc's compared with that of native cyt c depends on the position of the CDNP substitution. For mCDNPc's in which the modified lysine residue is outside of the interaction domain of cyt c with the SAM, the ratio of the k(et) of mCDNPc to that of native cyt c is correlated to the change in the dipole moment vector of cyt c due to the CDNP modification. This correlation suggests that the dipole moment of cyt c determines its orientation of adsorption on the SAM of MHA and significantly affects the rate of the ET. The CDNP modification of lysine residues at the interaction domain significantly decreases the rate, demonstrating the importance of the local charge environment in determining the rate of ET.     

Theoretical study of a two-step reversible electrochemical reaction associated with irreversible chemical reactions in thin layer linear potential sweep voltammetry

A theoretical study of a two-step reversible electrochemical reaction OPR is made in thin layer linear potential sweep voltammetry when O, P and R can undergo one or several of the following irreversible chemical reactions: P→Z, R→Z, 2P→Z, 2R→Z, O+P→Z, P+R→Z and O+R→Z. The equations of the reduction and oxidation peaks are calculated. Criteria permitting the determination of the reaction involved are given. Values of the disproportionation constant K larger than 0.05 can be calculated from the experimental curves, and values of the rate constant of the chemical reaction k when 10^?3<k (s^?1)<1 for a first order reaction and 1<k (mol l^?1 s^?1)<10⁴ for a second order reaction.     

A comparison between the use of a redox mediator in solution and of surface modified electrodes in the electrocatalytic oxidation of nicotinamide adenine dinucleotide

Cyclic voltammetry was successfully applied to study the oxidation of nicotinamide adenine dinucleotide (NADH) both in homogeneous and heterogeneous phase. The first case was realized with a solution containing p-methylamino-phenolsulphate (MAP) as redox mediator and the diaphorase (DI) from Clostridium kluveri as enzyme while the second one by using both a glassy carbon (GC) and a carbon nanotube paste (CNTP) electrode modified with electrodeposited films derived from 3,4-dihydroxybenzaldehyde (3,4-DHB). Such systems were successively coupled with glucose dehydrogenase (GDH) reaction to realize the redox chain present in glucose biosensors. A critical comparison of the two systems was also reported.     

Effect of ionic composition of solutions on the methanol reaction with adsorbed oxygen on smooth polycrystalline platinum electrodes: Transients of the open-circuit potential

Transients of the open-circuit potential observed in the reaction of methanol with oxygen (O_ads) preliminarily adsorbed on smooth polycrystalline platinum (pcPt) are measured in 0.05 M HClO₄, 0.5 M HClO₄, 0.05 M H₂SO₄, 0.05 M H₂SO₄ + 0.45 M Na₂SO₄, and 0.05 M H₂SO₄ + 0.45 M Cs₂SO₄. It is shown that the solution pH has a weak effect on the transient characteristics (when the reversible hydrogen electrode potential scale is used). This confirms the chemical nature of rate-controlling stages in the reaction mechanism. The changes in the reaction rate, observed upon going from one electrolyte to another, are preferentially associated with the involvement of solution ions in the formation of activated surface complexes that include CH₃OH, O_ads, and supporting-electrolyte components.     

Evaluation of redox mediators for amperometric biosensors: Ru-complex modified carbon-paste/enzyme electrodes

The properties of reagentless amperometric biosensors are mainly governed by the interaction of the used redox enzyme and the redox mediators used to facilitate the electron-transfer reaction. Both the used redox mediators and the redox enzymes differ concerning their hydrophilicity and their properties within the matrix of a carbon-paste electrode. Since there is no general procedure which is applicable for any enzyme in combination with any redox mediator, optimisation is necessary for each possible combination. Three approaches for the development of biosensors were investigated using carbon-paste electrodes enriched with redox mediator as a base in all sensor architectures. A class of redox mediators with the common formula Ru(LL)(2)(X)(2) (where LL are 1,10-phenantroline or 2,2'-bipyridine type ligands, and X is an acido ligand) was investigated. In the first approach, enzymes were integrated into the carbon paste; in the second, the enzymes were adsorbed on the surface of the mediator-containing carbon-paste electrode and held in place by a Nafion film; and in the third approach, enzymes were entrapped in polymer films, which were electrochemically deposited onto the electrode's surface. The properties of the obtained biosensors strongly depend on the sensor architecture and the specific features of the used enzyme. Thus, our investigation using three different sensor architectures can provide valuable information about the possible interaction between a specific enzyme and a redox mediators with specific properties.     

On the measurement of the equilibrium potential of a hindered redox reaction

Ohne Zusammenfassung     

An empirical potential for interactions of large molecules: Application to binding of dipeptides to DNA

An empirical potential is introduced that is suitable for calculation of the interaction enegies of biomolecules with thousands of atoms. The potential consists of electrostatic, repulsion, and dispersion energy terms. The approach used for parametrization of the potential is entirely different from that used with other existing potentials. Namely, all the terms were parametrized independently to retain their physical significance. The sum of the electrostatic and repulsion terms mimic the SCF interaction energy calculated using Huzinaga's minimal basis set MINI -1. The dispersion energy is very important and is usually predominant for the interactions of large (polar) molecules in the gas phase as well as in the liquid phase.     

A.C. Polarography and faradaic impedance of strongly adsorbed electroactive species: Part III. Theoretical complex plane analysis for a surface redox reaction

The complex plane analysis of a cell impedance in the case of a surface redox system (strong adsorption of both O and R) is treated theoretically. The adsorption rate does not control the kinetics of the system. These systems present the general property that the equation of the curves obtained does not depend on the isotherm when the angular frequency is varied. When the concentration or the potential is varied, on the contrary, the curves differ according to the isotherm; either a Langmuir or a Frumkin isotherm is considered. The use of certain representations for the characterization of adsorption is discussed.