Diffusionless System Research Articles

Approximate calculation of voltammetric peak of diffusionless system

AbstractVoltammetric peak of the adsorbed electroactive system may be expressed by a parametric equation; no exact explicit relation is achievable. Present work describes an approximate algorithm (Lorentzian‐Gaussian algorithm [LGA]) for the computation of voltammetric peak of diffusionless system with high precision. This algorithm may be used for calculation of voltammetric peaks of adsorbed species; redox behavior of mixed ionic‐electronic conductors (Prussian blue, electroactive polymers, etc) can also be covered by LGA if diffusion and charge transfer does not impede the process. This algorithm can be useful in computer simulation of mass transfer in thin films of Prussian blue and other mixed ionic‐electronic conductors. Despite being an approximate algorithm, LGA has a remarkable accuracy. Unfortunately, this accuracy has been achieved at the expense of simplicity: LGA makes use of nine adjustable coefficients.

A study of ion exchange at the poly(butyl viologen)-electrolyte interface by SECM

In this work, the ion exchange characteristics of poly(butyl viologen) (PBV) thin films on a platinum electrode has been investigated by cyclic voltammetric (CV) scans. Since ferrocyanide anions (Fe(CN) 6 4−) were added during the polymerization of the PBV thin-film for its stability, Fe(CN) 6 4− could form charge transfer complex with monomer and co-deposited with polymer. Scanning electrochemical microscopy (SECM) was used to probe the released Fe(CN) 6 4− ions from PBV film with Os(bpy) 3Cl 2 as a mediator for the approaching process in 0.5 M KCl medium. Mass changes during the redox process of the film were also monitored in-situ by electrochemical quartz crystal microbalance (EQCM). The ion exchange and transport behavior was observed during CV cycling of the film of the SECM and EQCM. The insertion and extraction of anions were found to be potential-dependence. Moreover, the decrease in tip current of released Fe(CN) 6 4− with increasing cycle number accounted for the ion exchange between Fe(CN) 6 4− and Cl − in the KCl electrolyte. However, the Fe(CN) 6 4−/Fe(CN) 6 3− redox couple was found to be highly stable between 0.0 and 0.5 V (vs. Ag/AgCl/saturated KCl) in the phosphate buffer solution. Therefore, the electrochemical property of Fe(CN) 6 4−/Fe(CN) 6 3− redox couple was studied at different scan rates using CV technique. The peak currents were directly proportional to the scan rate as predicted for a surface confined diffusionless system. The surface coverage ( Γ) and the concentration of Fe(CN) 6 4− were determined to be 1.88 × 10 −8 mol/cm 2 and 0.641 mol/dm 3, respectively. By neglecting cations incorporation during redox reaction of the PBV film and also based on the results obtained from energy-dispersive X-ray spectroscopy for the films of as-deposited, reduced and oxidized states, an ion exchange mechanism was proposed.

General Behavior of the I–E and ΔI–E Curves Obtained when a Multistep Potential is Applied to an Electroactive Monolayer

AbstractSimple, analytical and fully explicit expressions for the I–E curves of a charge transfer process of a diffusionless system corresponding to staircase voltammetry (SCV) are presented. We have analyzed the reversibility influence and our results indicate that SCV curves obtained at different sampled times tend to coincide as the rate constant decreases. Moreover, the current–potential SCV response converts to that corresponding to linear sweep voltammetry (LSV) when the pulse amplitude ΔE→0. We have also obtained the theoretical expression for the response of differential staircase voltammetry (DSCV) which presents two peaks and a cross potential at which ΔI=0.

Diffusion accelerates and enhances chirality selection

The diffusion effect on chirality selection in a two-dimensional reaction-diffusion model is studied by Monte Carlo simulations. The model consists of achiral reactants A which turn into either of the chiral products R or S in a solvent of chemically inactive vacancies V . The reaction contains a nonlinear autocatalysis as well as a recycling process, and the chiral symmetry breaking is monitored by an enantiomeric excess (ee) phi. Without dilution a strong nonlinear autocatalysis ensures chiral symmetry breaking. By diluting a diffusionless system, the ee phi decreases and a racemic state is recovered below a critical concentration c(c) . When the diffusion is allowed, the steady value of phi increases and c(c) decreases. As for the relation between the ee phi and the concentration c , a formula interpolating between the diffusionless (D=0) and the homogeneous (D=infinity) limits is proposed by incorporating the diffusional enhancement of the concentrations of chiral species. Diffusion also accelerates the development of the chiral order, and the time required to establish the order in a system of a size L(2) is inversely proportional to the diffusion constant D as L(2)/D .

Dynamics of Electron Transport by Elastic Bending of Short DNA Duplexes. Experimental Study and Quantitative Modeling of the Cyclic Voltammetric Behavior of 3‘-Ferrocenyl DNA End-Grafted on Gold

The dynamics of electron transport within a molecular monolayer of 3'-ferrocenylated-(dT)(20) strands, 5'-thiol end-grafted onto gold electrode surfaces via a short C2-alkyl linker, is analyzed using cyclic voltammetry as the excitation/measurement technique. It is shown that the single-stranded DNA layer behaves as a diffusionless system, due to the high flexibility of the ss-DNA chain. Upon hybridization by the fully complementary (dA)(20) target, the DNA-modified gold electrode displays a highly unusual voltammetric behavior, the faradaic signal even ultimately switching off at a high enough potential scan rate. This remarkable extinction phenomenon is qualitatively and quantitatively justified by the model of elastic bending diffusion developed in the present work which describes the motion of the DNA-borne ferrocene moiety as resulting from the elastic bending of the duplex DNA toward and away from the electrode surface. Its use allows us to demonstrate that the dynamics of electron transport within the hybridized DNA layer is solely controlled by the intrinsic bending elasticity of ds-DNA. Fast scan rate cyclic voltammetry of end-grafted, redox-labeled DNA layers is shown to be an extremely efficient method to probe the bending dynamics of short-DNA fragments in the submillisecond time range. The persistence length of the end-anchored ds-DNA, a parameter quantifying the flexibility of the nanometer-long duplex, can then be straightforwardly and accurately determined from the voltammetry data.

Integration analysis of the cyclic voltammograms of the electrode reaction in a diffusionless system

Abstract A method of integration analysis of the cyclic voltammogram of the electrode reaction in a diffusionless system is presented. The direct results of the integration analysis are the concentrations of the oxidized and reduced form of the electroactive substances, the forward and backward reaction rates. From these two rates, electrochemical parameters can be estimated according to Butler–Volmer kinetics. The method presented can be applied to the cases in which the cyclic voltammetric oxidation–reduction peak potential separation is greater than only 20 mV. The application of the method to the experiment was satisfactory.

Threshold Phenomena and Solitary Traveling Waves in a Class of Reaction-Diffusion Systems

In this paper we consider a class of two-species reaction-diffusion systems in which one species may vary on a faster time scale than the other. The system is assumed to have a unique stable steady state, and conditions on the reaction terms ensure that one of the nullclines of the diffusionless system has a kink in it. We exploit the existence of the two time scales in order to analyze the system asymptotically, and show that it exhibits threshold behavior in the spatially uniform case and a solitary traveling wave in the spatially nonuniform case. We consider examples of physical systems which exhibit these phenomena and show that each may be modeled by a reaction-diffusion system satisfying the conditions of the paper.