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.     

Absolute standard redox potential of monolayer-protected gold nanoclusters

The redox properties of monolayer-protected gold nanoclusters (MPCs) are considered from both the theoretical and experimental viewpoints. The "absolute standard redox potential" ([E0((z/z)-1))]abs) of MPCs is first derived from electrostatic considerations. A linear dependence of the absolute standard redox potential on the valence state of MPCs is theoretically predicted and verified experimentally. By employing ferricinium/ferrocene (Fc+/Fc) as a reference redox couple, the average valence state of MPCs at a given potential can be estimated.     

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.     

Fluorescent Polypyrrole Nanospheres – Synthesis and Properties of “Wireless” Redox Probes

In this work a novel concept of monitoring of occurrence of redox reactions between conducting polymer nanospheres and redox species in a solution is proposed. The redox process is monitored in the emission mode (without wiring of the probe to an electrochemical measuring set‐up) as a change in emission spectrum of a dye (not participating in the redox process itself) but reporting the alteration of properties of highly sensitive conducting polymer nanoparticles. This approach is possible due to applied unique method of synthesis of conducting polymers nanospheres of highly active, unblocked surface. Thus the nanospheres redox state is affected by the solution redox potential, leading to change of their properties. If solvatochromic probe of sufficiently high brightness (pyrene) is present in nanospheres, a redox reaction between the conducting polymer and solution can be observed as change of emission spectrum of the probe. Thus a localized redox potential optical probe can be obtained. The emission properties of the dye incorporated were preserved in the nanospheres, moreover, the emission spectrum of the probe was affected by the change in redox potential of the solution, thus influencing the redox state and ultimately the properties of the conducting polymer. The emission changes observed were dependent on ion‐exchange properties of polypyrrole, i.e. depending on the dopant ions present in the polymer, the sensitivity of the optical probe can be tuned.     

A study of redox properties of hydralazine hydrochloride,an antihypertensive drug

Hydralazine hydrochloride itself is a reducing agent and its redox properties like other reducing agents vary as the oxidizing agent and applied conditions vary. The redox properties of hydralazine were studied by spectrophotometric method. Formal redox potential of hydralazine was calculated and effect of pH was observed on redox properties of hydralazine.     

Redox electrodeposition polymers: adaptation of the redox potential of polymer-bound Os complexes for bioanalytical applications

The design of polymers carrying suitable ligands for coordinating Os complexes in ligand exchange reactions against labile chloro ligands is a strategy for the synthesis of redox polymers with bound Os centers which exhibit a wide variation in their redox potential. This strategy is applied to polymers with an additional variation of the properties of the polymer backbone with respect to pH-dependent solubility, monomer composition, hydrophilicity etc. A library of Os-complex-modified electrodeposition polymers was synthesized and initially tested with respect to their electron-transfer ability in combination with enzymes such as glucose oxidase, cellobiose dehydrogenase, and PQQ-dependent glucose dehydrogenase entrapped during the pH-induced deposition process. The different polymer-bound Os complexes in a library containing 50 different redox polymers allowed the statistical evaluation of the impact of an individual ligand to the overall redox potential of an Os complex. Using a simple linear regression algorithm prediction of the redox potential of Os complexes becomes feasible. Thus, a redox polymer can now be designed to optimally interact in electron-transfer reactions with a selected enzyme.     

On-column electrochemical redox derivatization for enhancement of separation selectivity of liquid chromatography use of redox reaction as secondary chemical equilibrium

An on-column electrochemical redox derivatization for enhancement of high-performance liquid chromatography (HPLC) separation selectivity is presented using electrochemically modulated liquid chromatography (EMLC) and porous graphitic carbon (PGC) as the packing material. PGC therefore serves two purposes: it acts both as a chromatographic stationary phase and as a working electrode. The capability of on-column electrochemical redox derivatization was evaluated using hydroquinone and catechol as model compounds. By manipulation of the applied potential, hydroquinone and catechol will migrate as equilibrium mixtures, hydroquinone and p-benzoquinone and catechol and o-benzoquinone in the potential region of 25-125 mV and 150-200 mV (vs. Ag/AgCl), respectively. These redox reactions can be used as secondary chemical equilibria so that the corresponding equilibrium mixtures elute as single peaks and their retention times can be controlled by alterations in the potential applied to the PGC stationary phase. Homogeneity of the redox activity of the PGC stationary phase applied potential was also demonstrated.     

Coupled Redox and pH Potentiometric Responses of Electrodes Coated with Polypyrrole

ABSTRACT

The coupled influence of solution acidity and redox electrolytes on the open-circuit potentials of polypyrrole-modified electrodes was studied by recording the potential vs. time dependencies. It was observed that doping anions affect the relative rate of the responses resulting from pH influence (pH response) and from the presence of a redox couple in the solution, such as Fe(III)/Fe(II) or Fe(CN)6^3-/4-(redox response). Separation of the pH and redox responses was possible for polypyrrole doped with large organic anions of weak acids (Tiron, sulphosalicylic acid, Indigo Carmine). In such cases, a very fast pH response was recorded after which the polymer potential relaxed to the solution redox potential in each case. The final stable potential was determined entirely by the potential of the redox couple in solution. The final potential was pH-independent unless the redox potential was influenced by solution acidity. The solution pH determines only the shape of the potential vs. the time dependence, e.g. much faster responses were observed for solutions with a higher H* concentration.     

Ionization potential of clusters in liquids

Time-resolved observations of the fast electron transfer from an electron donor to metal ions adsorbed on metal clusters in solution have shown that a critical size of the cluster is required to make it capable of accepting electrons. The threshold is attributed to a size dependent redox potential of the cluster, increasing with the nuclearity (in contrast with the ionization potential in the gas phase which decreases when n increases): it corresponds to the nuclearity for which the cluster redox potential becomes more positive than the potential of the electron donor acting as a monitor.New data of redox potentials (or IP) of Ag_n clusters (hydroquinone as monitor) and Cu_n cluster (sulfonatopropylviologen anion as monitor) are derived. The influence of n and of the solvation or the ligand is discussed.     

Redox equilibria of iron oxides in aqueous-based magnetite dispersions: effect of pH and redox potential

The effect of pH and redox potential on the redox equilibria of iron oxides in aqueous-based magnetite dispersions was investigated. The ionic activities of each dissolved iron species in equilibrium with magnetite nanoparticles were determined and contoured within the Eh-pH framework of a composite stability diagram. Both standard redox potentials and equilibrium constants for all major iron oxide redox equilibria in magnetite dispersions were found to differ from values reported for noncolloidal systems. The "triple point" position of redox equilibrium among Fe(II) ions, magnetite, and hematite shifted to a higher standard redox potential and an equilibrium constant which was several orders of magnitude higher. The predominant area of magnetite stability was enlarged to cover a wider range of both pH and redox potentials as compared to that of a noncolloidal magnetite system.     

Synthetic seleno-glutaredoxin 3 analogues are highly reducing oxidoreductases with enhanced catalytic efficiency

Selenoenzymes have a central role in maintaining cellular redox potential. These enzymes have selenenylsulfide bonds in their active sites that catalyze the reduction of peroxides, sulfoxides, and disulfides. The selenol/disufide exchange reaction is common to all of these enzymes, and the active site redox potential reflects the ratio between the forward and reverse rates of this reaction. The preparation of enzymes containing selenocysteine (Sec) is experimentally challenging. As a result, little is known about the kinetic role of selenols in enzyme active sites, and the redox potential of a selenenylsulfide or diselenide bond in a protein has not been experimentally determined. To fully evaluate the effects of Sec on oxidoreductase redox potential and kinetics, glutaredoxin 3 (Grx3) and all three Sec variants of its conserved (11)CXX(14)C active site were chemically synthesized. Grx3, Grx3(C11U), and Grx3(C14U) exhibited redox potentials of -194, -260, and -275 mV, respectively. The position of redox equilibrium between Grx3(C11U-C14U) (-309 mV) and thioredoxin (Trx) (-270 mV) suggests a possible role for diselenide bonds in biological systems. Kinetic analysis is consistent with the hypothesis that the lower redox potentials of the Sec variants result primarily from the greater nucleophilicity of the active site selenium rather than its role as either a leaving group or a "central atom" in the exchange reaction. The 10(2)-10(4)-fold increase in the rate of Trx reduction by the seleno-Grx3 analogues demonstrates that oxidoreductases containing either selenenyl-sulfide or diselenide bonds can have physiologically compatible redox potentials and enhanced reduction kinetics in comparison with their sulfide counterparts.     

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.     

Potential shifts at electrodes coated with ion-exchange polymeric films

Voltammetry at electrodes modified with ion-exchange polymers, named "ion exchange voltammetry", has been recently developed for characterizing and determining quantitatively ionic electroactive analytes preconcentrated at the electrode surface. Like for other voltammetric techniques, characterization is based on the position of the response on the potential scale, but an appreciable difference is frequently observed between the formal half-wave potential for redox couples incorporated within ion-exchange polymeric films and those for the same redox couples in solution as measured at bare electrodes. Such a difference has been rationalized here by a generalized equation, inferred from a suitable elaboration of the Nernst equation, whose validity has been tested by a thorough investigation performed at glassy carbon electrodes modified with either cationic (Nafion) or anionic (Tosflex) polymeric films. With this purpose, the effect of both charge and concentration of the analyte and of the loading counterion, this last introduced as the cation or anion of the supporting electrolyte, of the ion-exchange selectivity coefficients of the redox partners and of their stoichiometric coefficients, as well as of the number of electrons involved in the charge transfer has been evaluated. The results obtained agree quite well with theoretical expectations and indicate that the potential shifts found are mainly conditioned by both charge and concentration of the counterion from the supporting electrolyte and by the ratio of the ion-exchange equilibrium constants for the two redox partners involved. Other parameters considered have no influence on the potential shift or lead to negligible effects, provided that the quantities of the redox partners incorporated within the ion-exchange coating represents less than 5% of the film capacity. Again in agreement with theoretical expectations, positive shifts are found for increasing supporting electrolyte concentrations when cationic redox species incorporated within cationic films are involved, while the opposite effect is found for anionic redox species incorporated within anionic films.     

The hydrogen peroxide–iodic acid–manganese (ii)–acetone oscillating system: Further observations

The redox potential and iodine concentration behavior of the title reaction and component reactions have been examined. The effect of hydrogen peroxide, potassium iodate, manganese (II) sulfate, sulfuric acid, and acetone concentration on the time period and redox potential behavior is reported. Iodine production and consumption rates for the component reactions are given, and some mechanistic suggestions, involving iodine dioxide as the one electron oxidant, are made.     

Electroanalytical measurements without electrolytes: Conducting polymers as probes for redox titration in non-conductive organic media

Electroanalytical methods have been applied only in conducting media. An application of conducting polymers allows to overcome this limitation. If such material is in electrochemical equilibrium with dissolved redox active species, its electrical conductivity depends on the redox potential of these species. Therefore, conductometric measurements with conducting polymers can provide about the same information as classical redox electrodes. The approach was applied for redox titration. Equivalent points obtained by this titration in aqueous and organic electrolytes were identical. Then the approach was applied for determination of bromine number by redox titration in non-conducting organic phase.     

Redox potential in acetone-butanol fermentations

In batch and continuous cultivations ofClostridium acetobutylicum ATCC 824 on lactose, a strong relationship was observed between redox potential of broth and the cellular metabolism. The specific productivity of butanol as well as of butyric acid was found to be maximum at a redox potential of -250 mV. The specific production rate of butyric acid decreased rapidly at redox potentials. For butanol, however, it achieved a lower but stable value. This was true for dynamic as well as steady states. The continuous fermentations involving lactose exhibited sustained oscillations at low dilution rates. These oscillations appear to be related to butanol toxicity to the growth of cells. At higher dilution rates where butanol concentrations were relatively low, no such oscillations were observed. It can be concluded that broth redox potential is an excellent indicator of the resulting fermentation product partitioning.     

A Stable Hexakis(guanidino)benzene: Realization of the Strongest Neutral Organic Four‐Electron Donor

The growing demand for efficient batteries has stimulated the search for redox‐active organic compounds with multistage redox behavior, as materials with large charge capacity. Herein we report the synthesis and properties of the first hexakis(guanidino)benzene derivative: a strong neutral organic electron donor with reversible multistage redox behavior and a record low redox potential for donation of four electrons. Detailed structural and spectroscopic characterization of three redox states (0, +2, and +4) reveal its unique electronic features. Despite its nitrogen richness, the compound is thermally robust and can be readily purified by sublimation.     

Design of an Os Complex‐Modified Hydrogel with Optimized Redox Potential for Biosensors and Biofuel Cells

Multistep synthesis and electrochemical characterization of an Os complex‐modified redox hydrogel exhibiting a redox potential ≈+30 mV (vs. Ag/AgCl 3 m KCl) is demonstrated. The careful selection of bipyridine‐based ligands bearing N,N‐dimethylamino moieties and an amino‐linker for the covalent attachment to the polymer backbone ensures the formation of a stable redox polymer with an envisaged redox potential close to 0 V. Most importantly, the formation of an octahedral N6‐coordination sphere around the Os central atoms provides improved stability concomitantly with the low formal potential, a low reorganization energy during the Os^3+/2+ redox conversion and a negligible impact on oxygen reduction. By wiring a variety of enzymes such as pyrroloquinoline quinone (PQQ)‐dependent glucose dehydrogenase, flavin adenine dinucleotide (FAD)‐dependent glucose dehydrogenase and the FAD‐dependent dehydrogenase domain of cellobiose dehydrogenase, low‐potential glucose biosensors could be obtained with negligible co‐oxidation of common interfering compounds such as uric acid or ascorbic acid. In combination with a bilirubin oxidase‐based biocathode, enzymatic biofuel cells with open‐circuit voltages of up to 0.54 V were obtained.     

Kinetics of redox polymer-mediated enzyme electrodes

Oxygen-reducing enzyme electrodes are prepared from laccase of Trametes versicolor and a series of osmium-based redox polymer mediators covering a range of redox potentials from 0.11 to 0.85 V. Experimentally obtained current density generated by the film electrodes is analyzed using a one-dimensional numerical model to obtain kinetic parameters. The bimolecular rate constant for mediation is found to vary with mediator redox potential from 250 s(-1) M(-1) when mediator and enzyme are close in redox potential to 9.4 x 10(4) s(-1) M(-1) when the redox potential difference is large. The value of the bimolecular rate constant for the simultaneously occurring laccase-oxygen reaction is found to be 2.4 x 10(5) s(-1) M(-1). The relationship between mediator-enzyme overpotential and bimolecular rate constant is used to determine the optimum mediator redox potential for maximum power output of a hypothetical biofuel cell with a planar cathode and a reversible hydrogen anode. For laccase of T. versicolor (E(e)(0) = 0.82), the optimum mediator potential is 0.66 V (SHE), and a molecular structure is presented to achieve this result.