Controlled Electron Transfer Reaction Research Articles

Interfacial Synthesis of Nylon‐6,6 and Its Modification with Silver‐Based Nanoparticles at the Electrified Liquid‐Liquid Interface

AbstractIn this work, the synthesis of Nylon‐6.6 together with silver nanoparticles (presumably Ag2O, Ag NPs) was carried out at a polarized liquid‐liquid interface (LLI). The reduction of silver ions to Ag‐NPs was driven by an electrochemically controlled electron transfer reaction between silver cations initially dissolved in the aqueous phase and ferrocene species present in the organic phase. The formation of Ag2O is assumed to originate from Ag+ precipitation to AgOH further converted into Ag2O. The polyamide formation was derived from the interfacial polycondensation reaction happening between 1,6‐diaminohexane and adipoyl chloride. We studied the possibility to form given material simultaneously (interfacial polycondensation and Ag+ reduction occurring in one step) and sequentially (electrochemical Ag+ reduction by ferrocene from the organic phase was performed after polyamide deposition). The Ag‐based NPs‐polyamide material was synthesized at the macroscopic and microscopic LLI. As the support for the latter, fiberglass membranes was used. The resulting material was characterized by infra‐red (IR) spectroscopy and scanning electron microscopy (SEM).

ZnO@MnO2 nanocomposite modified carbon paste electrode for electrochemical detection of dopamine

In this study, we have compared the performance of a bare carbon paste electrode (BCPE) against the ZnO@MnO2 nanocomposite modified carbon paste electrode (MCPE) in the electrochemical determination of dopamine (DA). Chemical and structural features of the ZnO@MnO2 nanocomposites were characterized using scanning electron microscopy (SEM), energy dispersive X-Ray analysis (EDX), transmission electron microscopy (TEM), X-Ray diffraction (XRD), thermo-gravimetric analysis (TGA) with differential thermal Analysis (DTA) and Brunauer-Emmet-Teller (BET) surface area analysis. The cyclic voltammetry (CV) measurement of the ZnO@MnO2 nanocomposite MCPE showed better oxidative catalytic activity (3.6 μA) towards the DA than BCPE with higher efficiency (44.50%) in sensing the dopamine, making MCPE suitable for developing biosensors. The electrochemical studies were carried out in 5 × 10−5 M DA and obtained results were measured in the potential range of −0.2 to 0.6 V vs. saturated calomel electrode (SCE). The electrochemical performance of electrochemical impedance spectroscopy (EIS) and Tafel technique were measured for the BCPE and MCPE using 0.2 M phosphate buffer to maintain pH 7.2 with the scan rate of 50 mV/s. The redox peak current for MCPE Vs scan rate had good linearity with correlation coefficients of R2 = 0.9996, which represents the diffusion controlled process was under controlled electron transfer reaction. It is expected that the good electrocatalytic behavior of MCPE can be utilized for the fabrication of biosensors.

Redox properties of plant biomass-derived black carbon (biochar).

Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars). Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and envirotechnical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700 °C) show the highest capacities to accept and donate electrons. Combined electrochemical, elemental, and spectroscopic analyses of the thermosequence chars provide evidence that the pool of redox-active moieties is dominated by electron-donating, phenolic moieties in the low-HTT chars, by newly formed electron accepting quinone moieties in intermediate-HTT chars, and by electron accepting quinones and possibly condensed aromatics in the high-HTT chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".

Electrochemical Behavior of Hydrazine Borane in Methanol Solution

Electrochemical behavior of hydrazine borane (HB) was investigated on gold electrode in 0.5 M LiClO4 solution in methanol using cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry. Two irreversible peaks at 164 and 530 mV are attributed to direct electro-oxidation of HB on gold electrode in methanol. Both EIS results and CV data at different scan rates indicate a diffusion controlled electron transfer reaction. Furthermore, the Tafel slope (b = 0.191 V) and charge transfer coefficient (α = 0.69) were obtained for the HB oxidation at the gold/solution interface. A possible ECE (electrochemical-chemical-electrochemical) mechanism is proposed for the irreversible two electron transfer reaction. This mechanism was also confirmed by the results of Nicholson-Shain test. Additionally, diffusion coefficient of HB was found to be 1.6 × 10−5 cm2.s−1 using chronoamperometry. First oxidation peak of HB appears at a lower potential than that of ammonia borane (AB), clearly indicating that HB is a stronger reducing agent as compared to AB in methanol solution.

Voltammetrically Controlled Electron Transfer Reactions from Alkanethiol Modified Gold Electrode Surfaces to Low Molecular Weight Molecules Deposited within Lipid (Lecithin) Bilayers

A procedure was developed for initiating electron transfer from a gold electrode to a low molecular weight electron acceptor present inside supported lipid (lecithin) bilayers, followed by further electron transfer to an electron acceptor present in an aqueous solution. The electron acceptors present in the lecithin bilayers and aqueous phase were 7,7,8,8-tetracyanoquinodimethane (TCNQ) and [Fe(III)(CN)(6)](3-), respectively. A polished planar gold disk electrode was first coated via self-assembly procedures with an alkanethiol monolayer. A phospholipid layer consisting of multiple bilayers of lecithin containing TCNQ was subsequently deposited onto the alkanethiol monolayer. The Au/alkanethiol/lecithin-TCNQ electrode was placed in an aqueous solution containing various amounts of [Fe(III)(CN)(6)](3-) and [Fe(II)(CN)(6)](4-), with 0.5 M KCl as the supporting electrolyte. In the absence of TCNQ inside the alkanethiol/lecithin layers, only a small background current was observed. When TCNQ was included in the alkanethiol/lecithin layers, the voltammetry showed features typical of a catalytic process, due to the TCNQ being reduced to TCNQ(-*) within the lecithin bilayers and then undergoing oxidation back to TCNQ via interaction with [Fe(III)(CN)(6)](3-) at the lecithin-aqueous solution interface. The procedures for preparing the alkanethiol/lecithin-TCNQ coatings were optimized in order to obtain the most reproducible voltammetric response. Experiments were also performed using tetrathiafulvalene (TTF) as an electron donor in the lipid bilayer phase.

Statistics and kinetics of single-molecule electron transfer dynamics in complex environments: A simulation model study

Dynamics of the environments of complex systems such as biomolecules, polar solvents, and glass plays an important role in controlling electron transfer reactions. The kinetics is determined by the nature of a complex multidimensional landscape. By quantifying the mean and high-order statistics of the first-passage time and the associated ratios, the dynamics in electron transfer reactions controlled by the environments can be revealed. We consider real experimental conditions with finite observation time windows. At high temperatures, exponential kinetics is observed and there are multiple kinetic paths leading to the product state. At and below an intermediate temperature, nonexponential kinetics starts to appear, revealing the nature of the distribution of local traps on the landscape. Discrete kinetic paths emerge. At very low temperatures, nonexponential kinetics continues to be observed. We point out that the size of the observational time window is crucial in revealing the intrinsic nature of the real kinetics. The mean first-passage time is defined as a characteristic time. Only when the observational time window is significantly larger than this characteristic time does one have the opportunity to collect enough statistics to capture rare statistical fluctuations and characterize the kinetics accurately.

Coupling between electroosmotically driven flow and bipolar faradaic depolarization processes in electron-conducting microchannels

A quantitative theory is proposed for the analysis of steady electroosmotically driven flows within conducting cylindrical microchannels. Beyond a threshold value of the electric field applied in the electrolyte solution and parallel to the conducting surface, electrochemical oxidation and reduction reactions take place at the two extremities of the substrate. The spatial distribution of the corresponding local faradaic currents along the bipolar electrode is intrinsically coupled to that of the electric field in solution. The nonuniform distribution of the electric field alters the double layer composition, and in particular the zeta-potential value, along the conducting surface via the occurrence of concomitant electronic and ionic double layer charging processes. The combined spatial dependencies of the lateral electric field and electrokinetic potential considerably affects the distribution of the electroosmotic velocity field in the directions parallel and perpendicular to the surface depolarized by faradaic processes. In this paper, the coupling between bipolar electrodic behavior and electroosmosis is explicitly investigated for the case of irreversible—that is, kinetically controlled—electron transfer reactions. Typical simulation results are presented and illustrate the possibility of controlling and optimizing electroosmotic flows in conducting channels by electrochemical means.

Redox processes in a tin doped melt with the basic composition 20Na 2O · 80SiO 2 studied by square-wave voltammetry and impedance spectroscopy

Redox processes in a tin doped melt with the basic composition 20Na 2O · 80SiO 2 were studied by square-wave voltammetry and impedance spectroscopy. At temperatures of 1350 and 1450 °C, the recorded square-wave voltammograms exhibited a distinct peak attributed to the reduction of Sn 4+ to Sn 2+. Impedance spectra recorded at the same temperatures could be simulated with a simple equivalent circuit assuming a diffusion controlled electrode reaction. At temperatures T ⩽ 1250 °C, and small step times, the voltammograms exhibited a second peak which was much stronger affected by the frequency (step time) applied than expected from a diffusion controlled electron transfer reaction. In order to fit the impedance spectra at these temperatures, a more complex equivalent circuit was necessary, taking into account adsorption phenomena. In all impedance spectra, the Warburg parameter obtained from simulation, which stands for the diffusion from and to the electrode, exhibited a minimum which was equal to the voltammetric peak at more negative potentials. The voltammetric peak at less negative potentials is caused by the reduction of Sn 4+ to an adsorbed state. Within this reduction step, a monolayer of Sn 2+ species is formed.

Langevin‐schroedinger formulation of electronic tunneling through a molecular bridge with a dissipative acceptor

AbstractModeling electronic tunneling through molecular bridges is desired in order to understand the mechanism of long‐range electron transfer reactions in nature, as well as for the design of novel molecular electronics devices. Particularly interesting is the effect of the nuclear motion at the molecular bridge on the electron transfer mechanism and rate. In this work we study the effect of electronic nuclear coupling at the molecular bridge on a unidirectional electronic tunneling process from an electron donor into a dissipative acceptor, as may appear in controlled electron transfer reactions at biological membranes, or in heterogeneous electron transfer reactions. The model includes a collection of harmonic bath modes coupled to the dissipative acceptor site and a single mode at the molecular bridge. The parameters of the dissipative bath are tuned such that the electronic population decays from the donor to the acceptor. This process is simulated using a time‐dependent nonlinear Langevin‐Schroedinger equation, based on a mean‐field approximation for the electronic‐nuclear coupling at the acceptor site and a numerically exact treatment of the electronic‐nuclear coupling at the molecular bridge. The simulations at zero temperature and weak electronic‐nuclear coupling demonstrate that electronic tunneling is promoted by coupling to the nuclear mode at the bridge. This result is consistent with our previous studies of electronic tunneling oscillations in a symmetric donor‐bridge‐acceptor complex, and it emphasizes the importance of electronic nuclear coupling in analyzing long‐range electron transfer processes through molecular bridges or wires.

Experimental Study of the Interplay between Long-Range Electron Transfer and Redox Probe Permeation at Self-Assembled Monolayers: Evidence for Potential-Induced Ion Gating

Evidence for the competition between long-range electron transfer across self-assembled monolayers (SAMs) and incorporation of the redox probe into the film is reported for the electroreduction of Ru(NH(3)) at hydroxyl- and carboxylic-acid-terminated SAMs on a mercury electrode, by using electrochemical techniques that operate at distinct time scales. Two limiting voltammetric behaviors are observed, consistent with a diffusion control of the redox process at mercaptophenol-coated electrodes and a kinetically controlled electron transfer reaction in the presence of neutral HS-(CH(2))(10)-COOH and HS-(CH(2))(n)()-CH(2)OH (n = 3, 5, and 10) SAMs. The monolayer thickness dependence of the standard heterogeneous electron transfer rate constant shows that the electron transfer plane for the reduction of Ru(NH(3)) at hydroxyl-terminated SAMs is located outside the film | solution interface at short times. However, long time scale experiments provide evidence for the occurrence of potential-induced gating of the adsorbed structure in some of the monolayers studied, which takes the form of a chronoamperometric spike. Redox probe permeation is shown to be a kinetically slow process, whose activation strongly depends on redox probe concentration, applied potential, and chemical composition of the intervening medium. The obtained results reveal that self-assembled monolayers made of mercaptobutanol and mercaptophenol preserve their electronic barrier properties up to the reductive desorption potential of a fully grown SAM, whereas those of mercaptohexanol, mercaptoundecanol, and mercaptoundecanoic acid undergo an order/disorder transition below a critical potential, which facilitates the approach of the redox probe toward the electrode surface.

Smooth landscape solvent dynamics in electron transfer reactions

Solvent effects play a major role in controlling electron–transfer reactions. The solvent dynamics happens on a very high-dimensional surface, and this complex landscape is populated by a large number of minima. A critical problem is to understand the conditions under which the solvent dynamics can be represented by a single collective reaction coordinate. When this unidimensional representation is valid, one recovers the successful Marcus theory. In this study the approach used in a previous work [V. B. P. Leite and J. N. Onuchic; J. Phys. Chem. 100, 7680 (1996)] is extended to treat a more realistic solvent model, which includes energy correlation. The dynamics takes place in a smooth and well behaved landscape. The single shell of solvent molecules around a cavity is described by a two-dimensional system with periodic boundary conditions with nearest neighbor interaction. It is shown how the polarization-dependent effects can be inferred. The existence of phase transitions depends on a factor γ proportional to the contribution from the two parameters of the model. For the present model, γ suggests the existence of “weak kinetic phase transitions,” which are used in the analysis of solvent effects in charge–transfer reactions.

Factors controlling electron transfer reactions and stabilisation of uncommon oxidation states of chromium

Some experimental approaches to seek semi-quantitative understanding of factors controlling outer sphere electron transfer reactions of some transition metal complexes have been made. The relative importance of nuclear and electronic factors to outer sphere processes has been examined. By the manipulation of Franck-Condon or nuclear factors, it has now been possible to gain access into the chemistry of chromium in unusual oxidation states. An example of a reorganisation controlled electron transfer reaction involving Cr(IV)-Cr(III) system has been demonstrated. The bimolecular rate of reduction of diperoxoaquaethylenediamine chromium(IV) and diperoxodiethylenetriamine chromium(IV) is independent of the nature of th reductant employed viz. Fe2+ or VO2+ indicating that the generation of6 coordinate Cr(IV) species from7 coordinate of diperoxochromium(IV) reactant may be rate limiting. Similarly by increasing the barrier for the6 coordinate to4 coordinate structures through equatorial coordination of macrocyclic ligands, it has now been possible to detect through cyclic voltommogram the formation of relatively stable Cr(IV) species in the electrochemical oxidation of Cr(Me4[14] tetraene)(H2O) 2 3+ in aqueous sulphuric acid media. The kinetics and mechanism of the cerium(IV) and iodosyl benzene oxidation of Cr(salen)(H2O) 2 + and Cr(salprn)(H2O) 2 + have been investigated and kinetic and spectroscopic evidence for the formation of Cr(IV) transients and stable Cr(V) products has been presented. The relative importance of Franck-Condon factors in the oxidation of Cr(III) to Cr(IV) and Cr(V) states in different macrocyclic and multidentate ligand environments has been discussed.

Extremely Sensitive Optical Detection System for Stopped‐flow Measurements of Reactions with Very High Rate Constants or Small Absorbance Changes

AbstractStopped‐flow experiments are usually performed with optical detection. In case of low absorptivities or low concentrations of the measured substances a very high sensitivity is necessary. A special optical detection system with this feature is described. By a differential configuration of the detectors and the use of high light intensities fluctuations of the light source and detector shot noise were greatly reduced in their effects. The detection system is capable of performing extremely sensitive kinetic measurements from 290 to 620 nm within the time‐scale of stopped‐flow experiments. Kinetic traces with a full scale absorbance down to 5 · 10−4 were recorded. In this extreme case, the signal to noise ratio was still as good as 25:1. With typical values for absorptivities, optical path length, and dead time, even diffusion controlled electron transfer reactions could thus be observed in a stopped‐flow apparatus.