Charge transport through Geobacter sulfurreducens biofilms grown on graphite rods

Biofilms of the electroactive bacterium Geobacter sulfurreducens were induced to grow on graphite-rod electrodes under a potential of 0 V (vs Ag/AgCl) in the presence of acetate as an electron donor. Increased anodic currents for bioelectrocatalytic oxidation of acetate were obtained when the electrodes were incubated for longer periods with periodic electron-donor feeding. The maximum current density for acetate oxidation increased 2.8-fold, and the biofilm thickness increased by 4.25-fold, over a time period of 83-147 h. Cyclic voltammetry in the presence of acetate supports a model of heterogeneous electron transfer, one electron at time, from biofilm to electrode through a dominant redox species centered at -0.41 V vs Ag/AgCl. Voltammetry performed under nonturnover conditions provided an estimate of the surface coverage of the redox species of 25 nmol/cm(2). This value was used to estimate a redox species concentration of 7.3 mM within the 34-μm-thick biofilm and a charge-transport diffusion coefficient of 3.6 × 10(-7) cm(2)/s. This value of diffusion coefficient is greater than that observed in traditional thin-film voltammetric studies with redox polymer films containing much higher surface concentrations of redox species and might be associated with proton transport to ensure electroneutrality within the biofilm upon electrolysis.     

Comparative study of biocatalytic reactions of high and low redox potential fungal and plant laccases in homogeneous and heterogeneous reactions

The homogeneous catalytic and heterogeneous bioelectrocatalytic properties of high redox potential fungal laccases and low redox potential plant laccase have been compared. The fungal and plant laccases exhibit radically different catalytic activities as a function of pH with respect to substrates donating only electrons and substrates donating both hydrogen atoms and electrons, as well as in the bioelectrocatalytic reaction of dioxygen reduction. It is suggested that the difference between the biocatalytic properties of these enzymes correlates with their role in lignin metabolism.     

Long tethers binding redox centers to polymer backbones enhance electron transport in enzyme "Wiring" hydrogels

A redox hydrogel with an apparent electron diffusion coefficient (D(app)) of (5.8 +/- 0.5) x 10(-)(6) cm(2) s(-)(1) is described. The order of magnitude increase in D(app) relative to previously studied redox hydrogels results from the tethering of redox centers to the backbone of the cross-linked redox polymer backbone through 13 atom spacer arms. The long and flexible tethers allow the redox centers to sweep electrons from large-volume elements and to collect electrons of glucose oxidase efficiently. The spacer arms make the collection of electrons from glucose oxidase so efficient that glucose is electrooxidized already at -0.36 V versus Ag/AgCl, the reversible potential of the redox potential of the FAD/FADH(2) centers of the enzyme at pH 7.2. The limiting current density of 1.15 mA cm(-)(2) is reached at a potential as low as -0.1 V versus Ag/AgCl. The novel redox center of the polymer is a tris-dialkylated N,N'-biimidazole Os(2+/3+) complex. Its redox potential, -0.195 V versus Ag/AgCl, is 0.8 V reducing relative to that of Os(bpy)(2+/3+), its 2,2'-bipyridine analogue.     

Theory of ballistic electron emission microscopy

The present status of theories for interpreting experimental ballistic electron emission microscopy (BEEM) data is reviewed. Current formalisms may be divided into two broad classes: one-electron theories, where carriers do not exchange energy with other excitations in the solid, and scattering approaches, where such losses are considered. While the former theories have been formulated with the help of Green's functions (GFs), the latter have relied more on simulation by Monte-Carlo techniques. For the one-electron approach, we discuss why the originally suggested free propagation of carriers (e.g., ballistic electrons) does not offer a consistent interpretation of the experimental database and should be replaced instead by considering the coherent propagation of electrons interacting with the periodic potential in the metal base. Bridging towards the scattering formalisms, it is shown how GFs incorporating a complex self-energy are still a feasible approach, when only a single inelastic source of scattering (e.g., electron–electron (e–e) interaction) is operative. Within this one-electron scheme, the importance of an accurately computed transmission coefficient at the metal-semiconductor interface is stressed, when aiming to obtain absolute BEEM currents. Analyzing results from scattering techniques, it is argued that this coefficient should be modified to take into account the back-injection of electrons from the semiconductor into the metal. A general expression for BEEM currents is given that can be used to simulate results in real-space, reciprocal-space or energy-space (spectroscopy with BEEM). Some experimental results are discussed in relation to the theories presented.     

Mechanistic study of the oxidation of -ascorbic acid by chloranil at the nitrobenzene water interface

The redox reaction between -ascorbic acid in water and chloranil in nitrobenzene has been studied by means of polarography with an ascending water electrode as well as cyclic voltammetry with a stationary interface. Through accurate measurement of the limiting currents, it has been suggested that the redox reaction should be a two-electron reaction rather than a one-electron reaction described previously. A spectrophotometric technique has also been used to observe that the redox reaction proceeds spontaneously under certain conditions even without electrochemical control. Based on these findings, it has been concluded that the present heterogeneous charge transfer reaction is the ion transfer of chloranil semiquinone radical, which is driven by the homogeneous electron transfer between ascorbic acid and chloranil in the aqueous phase.     

Heterogeneous ECE Processes at Channel Electrodes: Analytical Solution for all Reaction Rate Constants and Different Diffusion Coefficients

An analytical solution is developed for heterogeneous ECE processes occurring at channel electrode surface for both laminar and turbulent flow. The solution explicitly links the behavior of ECE processes and the parameters. A simple expression of the effective number of electrons transferred, covering all the reaction rate constants and different diffusion coefficients of the reactant A and the intermediate product B, is obtained. Excellent agreement with previous numerical and analytical results is shown . Parametric studies illustrate the effects of diffusion coefficients, hydrodynamic factors and reaction rate constants on the effective number of electrons transferred and the currents.     

On the Initial Stages of Electrooxidation of Aqueous Maleic Acid on Bi‐Doped PbO2

Oxidation of maleic acid in aqueous solution on bismuth‐doped lead oxide has been studied. The effects of hydrodynamic conditions on the oxidation rate have been identified. The number of electrons transferred during the initial stages of oxidation as well as the apparent heterogeneous rate constant was obtained from the combined analysis of rotating ring‐disk currents and the decrease of concentration at constant potential, determined from UV‐vis measurements. The number of electrons involved during electrochemical oxidation was found to be 12, indicating full mineralization to CO₂, and the heterogeneous rate constant for oxidation at 1.6 V was 9.8×10⁵ cm s^?1.     

Electrochemical characterization of anodic biofilms enriched with glucose and acetate in single-chamber microbial fuel cells

This study used a simple and efficient electrochemical technique, cyclic voltammogram (CV), to quantitatively measure the electron transfer capability of anodic biofilms enriched with acetate and glucose in single-chamber microbial fuel cells (MFCs). Two pairs of distinct redox peaks were observed by CV measurements in both biofilms, identical to the CV features of a pure Geobacter strain. The CVs also revealed a higher density of electroactive species in the acetate-enriched biofilm than that in the glucose-enriched biofilm. Based on the scan rate analysis, the apparent electron transfer rate constants (k(app)) in the acetate-enriched biofilm and glucose-enriched biofilm were determined to be 0.82 and 0.15s(-1), respectively, which supported the higher power output of the MFC fed with acetate. Meanwhile, the pH dependence of the biofilms was studied by monitoring the changes of the biofilm redox peak currents and potentials. It is concluded that redox reaction of the electrochemical active species in biofilms is pH dependent, and both electrons and protons are involved in the redox reactions.     

Improvement of spectral resolution in the presence of periodic noise and microphonics for hyper-pure germanium detector gamma-ray spectrometry using a new digital filter

The radiation dose of workers and patients resulting from inhaling radon and through the consumption of spring waters was examined in the hospital near the Héviz lake in Hungary. The radiation dose originating from radon was 2.15–3.95 mSv·y⁻¹ concerning workers at the spa. The radiation dose originating from radon in the case of those regularly taking a bath was an average of 0.75 mSv·y⁻¹. Due to the limited duration of treatments a bound effective dose of maximum 100 μSv·y⁻¹ may originate from radon and inhaling radon, while a maximum of 1.4 μSv·y⁻¹ may originate from ingestion of ²²²Rn, ²²⁶Ra, ²³⁴U and ²³⁸U radionuclides.     

Electron transfer of horse spleen ferritin at gold electrodes modified by self-assembled monolayers

Electron transfer is known to be an important step in the sequestering of iron by cellular ferritin. In this work, direct electron transfer between ferritin and a gold electrode was performed in order to probe its electron transfer kinetics. Gold electrodes were modified by the formation of self-assembled monolayers of 3-mercapto-propionic acid on the gold surface. Cyclic voltammetry using these electrodes shows that ferritin exhibits slow electron transfer kinetics at low potentials, yet fairly well-defined current—potential curves. In addition, the voltammetry indicates that adsorption of ferritin precedes the electron transfer step. Controlled potential electrolysis measurements yielded an n-value of 1910 electrons transferred per mole of ferritin. Cyclic voltammetry of a solution containing ferritin as well as nitrilotriacetate yields no electrolytic currents at potentials where the iron—nitrilotriacetate complex undergoes redox reactions, indicating that the currents observed in the voltammetry of ferritin were not due to free iron in the ferritin sample. In addition, the voltammetry of iron-free ferritin (apoferritin) did not yield appreciable currents, providing additional support to the suggestion that the observed voltammetric currents were due to the redox reactions of ferritin iron. Self-assembled monolayers containing carboxylate end groups effectively promoted the direct electron transfer of ferritin at a gold electrode, thus demonstrating that the electron transfer mechanisms of ferritin can now be probed electrochemically.     

Some studies of electrodes made from single crystals of TTF /sd TCNQ

Electrodes made from single crystals of tetrathiafulvalenium tetracyanoquinodimethanide (TTF. TCNQ) have been used to study the electrochemistry of the conducting organic salt and to investigate the mechanism of the electrochemical oxidation of glucose oxidase at conducting salt electrodes.The single crystal electrodes exhibit much lower non-Faradaic currents than the corresponding polycrystalline electrodes prepared as sublimed films or as pressed pellets. This leads to much lower background current levels and hence more clearly defined electrochemistry for solution species. Studies of the ac impedance behaviour and the electrochemistry of outer sphere redox species indicate that TTF·TCNQ electrodes behave as conventional metallic electrodes within their stable potential range.Results for the electrochemistry of glucose oxidase at the single crystal electrodes are inconsistent with a simple homogeneous mediation mechanism or with simple heterogeneous redox catalysis. Similarities with results obtained for TTF modified glucose oxidase suggest that the enzyme may undergo direct electrochemistry after modification by hydrophobic interaction with TTF molecules derived from the conducting salt electrode.     

Tertiary amine mediated aerobic oxidation of sulfides into sulfoxides by visible-light photoredox catalysis on TiO2

The selective oxidation of sulfides into sulfoxides receives much attention due to industrial and biological applications. However, the realization of this reaction with molecular oxygen at room temperature, which is of importance towards green and sustainable chemistry, remains challenging. Herein, we develop a strategy to achieve the aerobic oxidation of sulfides into sulfoxides by exploring the synergy between a tertiary amine and titanium dioxide via visible-light photoredox catalysis. Specifically, titanium dioxide can interact with triethylamine (TEA) to form a visible-light harvesting surface complex, preluding the ensuing selective redox reaction. Moreover, TEA, whose stability was demonstrated by a turnover number of 32, plays a critical role as a redox mediator by shuttling electrons during the oxidation of sulfide. This work suggests that the addition of a redox mediator is highly functional in establishing visible-light-induced reactions via heterogeneous photoredox catalysis.     

有序中孔炭/四氧化三铁复合材料的制备及其用于血红蛋白的电化学研究

采用微湿含浸法制备了有序中孔炭/四氧化三铁磁性材料.采用透射电镜和X射线衍射对复合材料进行了表征.将血红蛋白(Hb)固定于材料表面,对其直接电化学行为进行了研究,结果表明Hb在该材料内仍保持了其生物活性,在pH=7.0的PBS缓冲液中,血红蛋白表现出一对峰形良好的准可逆氧化还原峰,为Hb的Fe(Ⅲ)/Fe(Ⅱ)电对的特征峰,求出式电位E0’为-0.306 V,电子转移数为n=1.226,电荷传递系数为α=0.51,表观异相电子转移速率常数为KS=0.0144s-1.在3.00×10-6到1.50×10-4mol/L浓度范围内,血红蛋白的浓度与其响应电流呈良好的线性关系,线性相关系数为R=0.9924,最低检测限为0.270×10-6mol/L.     

Measurement of the concentration of Mn2+ and Mn3+ in the manganese-catalyzed 1,4-cyclohexanedione-acid-bromate reaction using redox-triggered magnetic resonance spectroscopy

A new nuclear magnetic resonance (NMR) experiment is reported, where the spectrometer is triggered using the output from a combination redox electrode. This technique was used to probe redox oscillations in the 1,4-cyclohexanedione-acid-bromate reaction. Manganese(III) acetate or manganese(II) sulfate was used as the catalyst, and the periodic change in concentration of Mn2+/Mn3+ ions was determined as a function of redox potential. The concentration of Mn3+ ions was at a maximum at high redox potential and at a minimum at low redox potential. Also, redox potentials were found to not be dominated by the Mn2+/Mn3+ couple.     

A Multiple Evaluation Approach of Commercially Available Screen‐Printed Nanostructured Carbon Electrodes

Commercially available carbon‐based screen‐printed electrodes were studied by cyclic voltammetry and electrochemical impedance spectroscopy in their behavior towards electron transfer to the soluble fast redox probes hexacyanoferrate(III), hexaammineruthenium(III) and methyl‐viologen. Semi‐infinite linear diffusion was observed for hexacyanoferrate(III) probe, with heterogeneous electron transfer rate constants significantly favored on nanotubes‐modified surfaces. Finite diffusion was observed for methyl‐viologen on graphene electrodes, which was reflected in the enhancement of the faradic currents by 4‐folds. Hexaammineruthenium(III) showed mixed diffusion behavior. These characteristics are reflected in the voltammetric behavior of lead(II) and cadmium(II) stripping from in‐situ deposited bismuth layer.     

Spectroelectrochemical Investigation on Neutral Red

Neutral Red can be used as an indicator, a stain reagent or a mediator compound in the studies of biological redox systems. No reports dealing with the electrode process of Neutral Red, especially, about its kinetics have been published. In this paper we report the determinations of formal reduction potentials, the number of electrons transferred, diffusion coefficient as well as the rate constant of heterogeneous electron transfer     

Electrochemical studies of the oxidation mechanism of polyamide on glassy carbon electrode

Polyamides containing N-methyl pyrroles and N-methyl imidazoles are a type of small molecule that can bind and recognize the bases of DNA with high affinity and specificity. Five polyamides were studied at glassy carbon electrode in acetate buffer by cyclic and differential pulse voltammetry to clarify their redox pathways. The polyamide electrochemical responses are compared by peak currents and peak potentials. The slopes of the three anodic E_p vs. pH plots of a typical polyamide are linear and show 0.059, 0.057, 0.056 V per pH in acid media, respectively, which correspond to a mechanism involving the equal number of electrons and protons. A possible mechanism for the redox pathway of various polyamides is proposed: the oxidation product of imidazole ring is acylamide and the results of in situ UV–Vis spectroscopy at Pt web electrode support the proposed mechanism. electrospray ionization mass spectroscopy (ESI-MS) indicates that one or two oxygen atoms are added into polyamide molecule after electrochemical oxidation.     

Simultaneous determination of concentration,diffusion coefficient and number of electrons for electroactive species by combining suitable electroanalytical measurements

The electroanalytical method described for the simultaneous determination of concentration, the number of electrons involved in the redox process and diffusion coefficient is based on evaluation of the ratios between the currents recorded for the analyte and for an easily standardized reference species dissolved in the same medium. Three different electroanalytical techniques are used in which the currents exhibit, for two techniques at least, different dependences on both the diffusion coefficient and electron number. The approach is applicable to diffusion-controlled processes, regardless of the degree of reversibility involved. Reliability tests with electroactive organic compounds dissolved in dimethyl sulphoxide show that both accuracy and precision are within 10% depending on the chosen combination of techniques.     

(Photo)electrochemical behavior of selected organic compounds on TiO2 electrodes. Overall relevance to heterogeneous photocatalysis

Oxidation potentials of resorcinol, 4-chlororesorcinol, 4,6-dichlororesorcinol, catechol, 3,4-dihydroxybenzoic acid and 1,2,4-trihydroxybenzene were measured on particulate TiO₂ (Degussa P-25) thin films, immobilized on optically transparent SnO₂ conducting glass electrodes, by cyclic voltammetry in 0.5M KCl aqueous electrolyte solutions. The effect of adsorption on oxidation potentials was examined with the compounds adsorbed on the TiO₂ particle surface. Scan rate dependencies of oxidation peak currents indicate that adsorbed species are consistently characterized by less positive oxidation potentials compared to those attributed to solution free species; the difference ranges from about 0.2 to 0.8 V. Results show that depending on the nature of the working electrode, associating a single oxidation potential to such compounds does not adequately describe their electrochemical behavior. Such observations have relevance in heterogeneous photocatalysis in that predictions of whether a substance will be photooxidized or photoreduced cannot be based on Fermi levels (redox potentials) of the redox couples in homogeneous solutions.     

A multilayer model for the study of space distributed redox modified electrodes: Part I. Description and discussion of the model

A multilayer model for the study of space distributed redox modified electrodes (redox polymer electrodes, or adsorption of an electroactive substance in several layers) is described and discussed in detail. It is shown in particular that it is equivalent to a system in which the electrons diffuse in the coating.