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• A model based on the electrochemical potential of all ions at the liquid–liquid interface. • Evaluation of the voltammograms affected by the IR drops. • Estimation of capacitive currents under the voltammetric condition. • Reproduction of electrocapillary curves that satisfy thermodynamic requirements. The liquid–liquid interface between two immiscible electrolyte solutions is an important subject in electrochemistry because ion-transfer reactions can be controlled externally. The electrochemical potentials of ions determine their behavior in a liquid–liquid two-phase system. The electrochemical potential is a continuous function of the system assuming a linear transition region at the interface. A simulation model that considers the electrochemical potential of all ions in the system was constructed for the liquid–liquid interface under electrochemical-measurement conditions. This model facilitates evaluating ion-transfer voltammograms affected by IR drops and separating charging currents from Faraday currents. Moreover, the model enables electrocapillary curves to be determined. [ABSTRACT FROM AUTHOR]

The expression mechanism of permselectivity through a gramicidin A (gA) channel between two aqueous phases (W1 and W2) was investigated. When the concentration of CsCl or CsBr in W1 was equivalent to that in W2, the single‐channel current was proportional to the absolute value of the applied membrane potential. Although the single‐channel current linearly increased with increasing electrolyte concentration in W1 and W2 until about 0.1 M (mol dm−3), it began to level off around 0.1 M, indicating that ion permeation across the channel pore is the rate‐determining step and that the saturation of the transporting ion within the channel pore provokes the leveling off. In the case of asymmetric composition of the electrolyte in W1 and W2, the monovalent cation and the counter anion were transported in the opposite direction through the gA channel pore or the bilayer lipid membrane around the gA channel. Finally, the experimental data was fitted using the Goldman‐Hodgkin‐Katz equation based on the relationship between the membrane potential and the single‐channel current to define the ratio of the diffusion coefficients of Cs+, Cl−, and Br− as 5.7 : 1.0 : 0.26. [ABSTRACT FROM AUTHOR]

In nerve cells, changes in local membrane potentials are generated and propagated along a nerve axon mainly by the function of K+ and Na+ channels. Generally, concurrent monitoring of multi-points on an axon is performed based on the voltage-clamp method. As the respective membrane potentials have been evaluated by considering the relations between the applied potential, the local current, and conductance, experimental values are not directly evaluated. We directly measured the actual membrane potentials and local currents of the respective cells using a nerve-model system comprising liquid-membrane cells. It was then proven that the action potential spreads along the axon toward the axon terminal due to the function of both the channel-type receptors in the synapse and voltage-gated Na+ channels on the axon, and that hyperpolarization cannot be caused by only the operation of the delayed-K+ and the voltage-gated Na+ channels. • Membrane potentials and currents were directly observed at multipoints in a model. • Action potentials spread by synaptic ligand and voltage-gated Na+ channels. • Hyperpolarization was not caused by delayed-K+ or voltage-gated Na+ channels. [ABSTRACT FROM AUTHOR]

Mediatorless bienzyme-type amperometric biosensors that couple a hydrogen peroxidase (H 2 O 2)-generating oxidase (OxD) reaction on the electrode surface with a direct electron transfer-type bioelectrocatalytic reduction of H 2 O 2 by horseradish peroxidase (HRP) allow the detection of the substrate of the OxD without any mediator. Constructed was an analytical model involving four steps: the diffusion of the substrate of an OxD, the diffusion of oxygen, the enzymatic reaction of the OxD, and the enzymatic reaction of HRP to understand the response of the mediatorless bienzyme-type biosensors. Pyruvate oxidase was used as a model OxD to detect pyruvate. The steady-state amperometric response of the pyruvate sensor in a rotating disk mode as functions of the pyruvate concentration and the rotating speed was well explained on the model to yield the kinetic parameters for the enzymatic reactions and the mass transfer in the biosensor. Based on the kinetic parameters, we constructed a mediatorless bienzyme-type microelectrode to detect pyruvate with a steady-state diffusion-controlled performance under quiescent conditions. Unlabelled Image • Bienzyme and mediatorless amperometric pyruvate sensors are constructed. • Kinetics of the oxidase and peroxidase type sensor is modeled. • The model explains the responses of the constructed pyruvate sensor for the pyruvate and H 2 O 2. • The rate determining step in the constructed sensor is predicted by the model. • The mass-transfer controlled microdisk-type pyruvate sensor is realized. [ABSTRACT FROM AUTHOR]

A pH sensor was fabricated using a modified hydrogen-storage palladium electrode. The Pd electrode stores H 2 generated electrochemically at suitable potentials. After the Pd electrode had become coated with palladium hydride (PdH x), the resting potential of the modified Pd electrode responded to changes in the pH with a near-Nernstian response of −56.2 ± 0.2 mV pH −1 in the range from pH 1 to pH 13. Although such Pd electrodes are usually unstable due to the release of H 2 , their stability could be dramatically increased by covering with a Teflon tube. The Pd electrode can be easily regenerated by electrolysis. As H 2 gas is spontaneously released from the modified Pd electrode, the partial pressure of H 2 gas on the surface of the Pd electrode is almost constant (H 2 gas is almost saturated in the vicinity of the electrode). Accordingly, the potential is assumed to be determined by the pH value of the aqueous solution based on the redox potential of the H 2 |H+ couple. When the pH value of the solution is almost constant, the modified Pd electrode can act as a reference electrode without any leakage of the inner electrolyte that occurs with Ag|AgCl|sat. KCl electrodes. Unlabelled Image • A pH-responsive electrode was fabricated using hydrogen-storing palladium. • The modified electrode showed a near-Nernstian response (−56 mV pH–1) to the pH value. • The electrode was stabilized by covering with a Teflon tube and regenerated by the electrolysis. • When the pH value of the test solution is almost constant, the modified Pd electrode can serve as a reference electrode. [ABSTRACT FROM AUTHOR]

Abstract Ketjen Black (KB), a mesoporous carbon material, was modified with gold nanoparticles (AuNPs) and 4-mercaptobenzoic acid (MBA). The MBA/AuNP-modified KB was utilized as an effective scaffold for the direct electron transfer (DET)-type bioelectrocatalytic reduction of dioxygen by bilirubin oxidase from Myrothecium verrucaria (BOD). Owing to the MBA/AuNP modification, the limiting current density of the steady-state catalytic wave was doubled and the half-wave potential of the catalytic wave was shifted to the positive direction by 140 mV. The kinetic analysis of the steady-state catalytic wave indicated that the modification narrowed the orientation distribution of the adsorbed BOD and shortened the mean length between the redox copper site of BOD and the electrode surface. The electrostatic attraction and π–π interactions between BOD and MBA are likely responsible for the favorable orientation of the enzyme. Graphical abstract Unlabelled Image Highlights • Ketjen Black was modified with 4-mercaptobenzoic acid and AuNPs. • Modified electrode was an effective scaffold for DET-type bioelectrocatalysis of BOD. • Half-wave potential of the catalytic wave was close to the formal potential of BOD. • Kinetic analysis of the wave provided information about the orientation of BOD. • BOD seemingly adsorbed in homogeneous and productive orientations on the electrode. [ABSTRACT FROM AUTHOR]

The ionization constant of water, K W , has been determined potentiometrically by use of an ionic liquid salt bridge (ILSB) consisting of a moderately hydrophobic N -ethyl- N -heptylpyrrolidinium bis(pentafluoroethanesulfonyl)amide at 25 °C. The value obtained for p K W = -log 10 (K W ) on the assumption that the liquid junction potential across the ILSB inserted between aqueous ca. 0.001 mol dm−3 solutions of HCl and NaOH is negligible was 13.81 ± 0.01(STDEV), which is smaller than 13.9965, reported about 90 years ago [1,2] , based on potentiometry of the cells claimed to be without liquid junction. The present results seem to challenge the unproven assumption of no liquid junction in the so-called Harned cells in that if our E cell value were greater by 10 mV, p K W would have been in perfect agreement with their 13.99, although it is very difficult to expect such a substantial magnitude of potential difference across the ILSB; logically, one may expect the opposite case, that is, that their value bears ca. 10 mV surplus possibly due to the hidden liquid junctions in the Harned cell. Relevant physicochemical properties of N -ethyl- N -alkylpyrrolidinium+ (alkyl = pentyl, hexyl and heptyl) and its ionic liquids formed in combination with bis(pentafluoroethanesulfonyl)amide− at 25 °C are also reported. • The ionization constant of water has been determined potentiometrically at 25 °C. • The cell with an ionic liquid salt bridge gave p K W = 13.81 ± 0.01(STDEV). • The value is considerably smaller than p K W = 13.99 by R. S. Harned et al. in 1930s. • The so-called Harned cells may contain hidden liquid junction potentials. • The above discrepancy corresponds to ca. 10 mV at the HCl|NaOH interface. [ABSTRACT FROM AUTHOR]

Abstract We constructed a bioelectrochemical formate generating system without external power supply. CO 2 was reduced with tungsten-containing formate dehydrogenase adsorbed on a biocathode on which benzyl viologen was adsorbed as a mediator, while the electron for the CO 2 reduction was supplied by the oxidation of H 2 with [NiFe]‑hydrogenase adsorbed on a bioanode functionalized with p -phenylenediamine (PDA2+) to adsorb H 2 ase in orientations suitable for direct communication with the electrode. Gas-diffusion-system was employed for high-speed supply of both of the gaseous substrates. The two electrodes were short-circuited through an ammeter and a switch. In this system, the formate generation rate of the biocathode was 290 ± 20 pmol cm−2 s−1 at pH 6.5 and 25 °C. On the other hand, the formate oxidation by H+ as the reversed reaction was also confirmed in the cell. Graphical abstract Unlabelled Image Highlights • A formate generating system from CO 2 and H 2 without external power supply was constructed. • Formate dehydrogenase and hydrogenase were used for CO 2 reduction and H 2 oxidation. • The rate of the formate generation reached 290 ± 20 pmol cm−2 s−1. • The reversed reaction (H 2 production from formate and H+) was also realized in this system. • The system was discussed in view of thermodynamics of bioelectrochemistry. [ABSTRACT FROM AUTHOR]

Phosphorous is one of three major nutritional elements for plants and usually exists as phosphate ions in nature. For hydroponic culturing and wastewater treatment, the development of a high-performance phosphate sensor would be very helpful. A novel phosphate ion-selective electrode was constructed using a cobalt phosphate surface coated cobalt electrode. The potential response seems to be caused by the formation of Co(H 2 PO 4 ) 2 in the coexistence of CoO and Co(OH) 2 . The sensor exhibited a linear response to H 2 PO 4 − in the concentration range from 1.0 × 10 −5 to 1.0 × 10 −1  mol L −1  at a pH range from 4.0 to 6.5 with a slope of −39 mV dec −1 . The sensor was unaffected by common anions, such as chloride, carbonate, and sulfate. The electrode maintained stability for at least 4 weeks in a live hydroponics system when sufficient Co 3 (PO 4 ) 2 ·8H 2 O was deposited on the Co electrode. [ABSTRACT FROM AUTHOR]

A model of a non-polarizable interface is developed and considered in terms of the specific adsorption of charged redox species and the nonspecific adsorption of the supporting electrolyte. The adsorption coefficients of charged redox species depend on their charges and the potential at the inner Helmholtz plane. The adsorbed amounts of the redox species and supporting electrolyte are estimated using the Frumkin isotherm and Gouy–Chapman theory, respectively. The potential dependence of the adsorbed amount of either one or both of the oxidized and reduced forms of the redox species is maximized near its standard potential. The electrocapillary equation at a non-polarizable liquid metal–solution interface under an externally controlled potential is revisited. The derived electrocapillary equation facilitates calculating the electrocapillary curve of the non-polarizable interface based on the quantity of adsorbed ions. The electrocapillary curve calculated based on the model may have a concave part, which would indicate that the interface is thermodynamically forbidden, near the standard redox potential of the redox couple. [ABSTRACT FROM AUTHOR]

The effects of three types of water-soluble carbon nanotubes (CNTs) of different lengths on the direct electron transfer (DET)-type bioelectrocatalysis of redox enzymes were investigated. Bilirubin oxidase (BOD), copper efflux oxidase (CueO), and a membrane-bound NiFe hydrogenase (H 2 ase) were used as model redox enzymes for four-electron dioxygen (O 2 ) reduction (in the case of BOD and CueO) and two-electron dihydrogen (H 2 ) oxidation (in the case of H 2 ase). As a result, diffusion-controlled O 2 reduction in an O 2 -saturated neutral buffer was realized by BOD on CNTs of a length of 1 μm, but the catalytic current densities decreased as the length of CNTs increased. However, almost opposite trends were obtained when CueO and H 2 ase were utilized as the biocatalysts. Factors of the CNTs and the enzymes affecting the characteristics of the DET-type bioelectrocatalysis of the three enzymes were discussed. Finally, the electrostatic interaction between an enzyme (especially the portion near the redox active center) and CNTs is proposed as one of the most important factors governing the performance of DET-type bioelectrocatalysis. [ABSTRACT FROM AUTHOR]

Tungsten-containing formate dehydrogenase from Methylobacterium extorquens AM1 (FoDH1) catalyzes formate oxidation with NAD + . FoDH1 shows little direct communication with carbon electrodes, including mesoporous Ketjen Black-modified glassy carbon electrode (KB/GCE); however, it shows well-defined direct electron transfer (DET)-type bioelectrocatalysis of carbon dioxide reduction, formate oxidation, NAD + reduction, and NADH oxidation on gold nanoparticle (AuNP)-embedded KB/GCE treated with 4-mercaptopyridine. Microscopic measurements reveal that the AuNPs ( d = 5 nm) embedded on the KB surface are uniformly dispersed. Electrochemical data indicate that the pyridine moiety on the AuNPs plays important roles in facilitating the interfacial electron transfer kinetics and increasing the probability of productive orientation of FoDH1. The formal potential of the electrochemical communication site, which is most probably an ion‑sulfur cluster, is evaluated as − 0.591 ± 0.005 V vs. Ag | AgCl | sat. KCl from Nernst analysis of the steady-state catalytic waves. [ABSTRACT FROM AUTHOR]

A mediator-less amperometric biosensor for putrescine detection was proposed to obtain a (pseudo) steady-state catalytic current. Putrescine oxidase (PuOD) and peroxidase (POD) were co-immobilized with glutaraldehyde on a Ketjen Black (KB)-based mesoporous electrode. A POD-catalyzed direct electron transfer-type reduction wave of H 2 O 2 generated by the reaction of PuOD was observed at a PuOD/POD-immobilized and KB-modified rotating disk glassy carbon electrode with an onset potential of 0.60 V vs. Ag | AgCl. A PuOD/POD-immobilized and KB-modified microdisk electrode produced a spherical diffusion-controlled (pseudo) steady-state catalytic current under quiescent conditions in the presence of putrescine. The bienzyme mesoporous microelectrode exhibited a linear range from 17 μM to 500 μM with a sensitivity of 0.33 ± 0.01 mA mM − 1 cm − 2 and a lower detection limit of 5 μM (S/N > 3). [ABSTRACT FROM AUTHOR]

Electrostatic interactions between proteins are key factors that govern the association and reaction rate. We spectroscopically determine the second-order reaction rate constant ( k ) of electron transfer from [NiFe] hydrogenase (H 2 ase) to cytochrome (cyt) c 3 at various ionic strengths ( I ). The k value decreases with I . To analyze the results, we develop a semi-analytical formula for I dependence of k based on the assumptions that molecules are spherical and the reaction proceeds via a transition state. Fitting of the formula to the experimental data reveals that the interaction occurs in limited regions with opposite charges and with radii much smaller than those estimated from crystal structures. This suggests that local charges in H 2 ase and cyt c 3 play important roles in the reaction. Although the crystallographic data indicate a positive electrostatic potential over almost the entire surface of the proteins, there exists a small region with negative potential on H 2 ase at which the electron transfer from H 2 ase to cyt c 3 may occur. This local negative potential region is identical to the hypothetical interaction sphere predicted by the analysis. Furthermore, I dependence of k is predicted by the Adaptive Poisson–Boltzmann Solver considering all charges of the amino acids in the proteins and the configuration of H 2 ase/cyt c 3 complex. The calculation reproduces the experimental results except at extremely low I . These results indicate that the stabilization derived from the local electrostatic interaction in the H 2 ase/cyt c 3 complex overcomes the destabilization derived from the electrostatic repulsion of the overall positive charge of both proteins. [ABSTRACT FROM AUTHOR]

Direct electron transfer (DET)-type bioelectrocatalysis is an essential technique for constructing simple bioelectrochemical devices such as biosensors, bioreactors, and biofuel cells. Bilirubin oxidase (BOD), a biocatalyst for the four-electron reduction of dioxygen (O 2 ) into water, is a promising enzyme for powerful DET-type biocathodes. Mesoporous carbon materials are often used in BOD-modified biocathodes. However, the supply of O 2 becomes a key factor governing BOD-catalyzed current densities owing to its low solubility. In this study, we analyzed steady-state rotating disk voltammograms of a BOD-catalyzed reaction in a rigid manner by taking into account the mass-transport as well as the enzymatic and the interfacial electron transfer kinetics. Non-catalytic redox signals of adsorbed BOD were also analyzed to obtain the surface redox properties of BOD. The analysis revealed that modification of electrodes with bilirubin and/or negatively charged carbon nanotubes to improve the DET-type catalytic performance increased the amount of BOD molecules with the proper orientation for bioelectrocatalysis. The interfacial electron transfer kinetic characteristics remained almost unchanged. [ABSTRACT FROM AUTHOR]

A new gas-diffusion-type biocathode was constructed for carbon dioxide (CO 2 ) reduction. In this work, tungsten-containing formate dehydrogenase (FoDH1), which is a promising enzyme for interconversion of formate and CO 2 , was used as a catalyst and was absorbed on a Ketjen Black (KB)-modified electrode. We used 1,1′-trimethylene-2,2′-bipyridinium dibromide as a mediator, and the hydrophobicity of the FoDH1-absorbed electrode was optimized according to the weight ratio of the polytetrafluoroethylene binder to KB. We achieved cathodic current densities of about 20 mA cm − 2 under mild and quiescent conditions (neutral pH, atmospheric pressure, and room temperature). [ABSTRACT FROM AUTHOR]

A reversible surface-confined system for potentiometric coulometry studies was fabricated by the deposition of a liquid film on an electrode surface, and the system was used to detect charge accumulation due to the peroxidase (POD)-catalyzed reduction of H 2 O 2 . The components of the liquid film were 1-ethyl-3-methylimidazolium bis(nonafluorobutanesulfonyl)imide as the electrolyte, dibutyl phthalate as the medium, and ferrocene as the redox-active mediator. When the thickness of the liquid film was less than that of the diffusion layer in the minimum potential step of the apparatus, a symmetric voltammogram, according to the Nernst equation, was recorded. The reduction of H 2 O 2 by POD was carried out on the liquid-film-modified electrode and studied by potentiometric coulometry. The addition of H 2 O 2 shifts the electrode potential positively, due to the oxidation of ferrocene. The change in the potential is in agreement with the Nernst equation. Thus, potentiometric coulometry, carried out on the liquid-film-modified electrode, enables the determination of the substrate without the requirement of a calibration curve. [ABSTRACT FROM AUTHOR]

d -Fructose dehydrogenase (FDH) is a heterotrimeric-membrane-bound enzyme capable of direct electron transfer (DET)-type bioelectrocatalysis. Subunit II contains three heme C moieties and is presumed to play a key role in the DET reaction because the subunit I/III subcomplex (without subunit II) lacks DET-type activity. We constructed an expression system for subunit II. A non-turnover signal from subunit II was not observed on voltammograms, and a subunit II-adsorbed electrode did not exhibit DET-type activity in the presence of the subunit I/III subcomplex and fructose. Gel filtration column chromatography indicated that subunit II formed multimeric complexes with four or more subunits. The aggregation of subunit II seemed to interfere with its direct communication with the electrodes. In contrast, when subunit II was mixed with the subunit I/III complex in a solution, they formed a 1:1 full complex, and the complex recovered DET-type bioelectrocatalytic activity. These results strongly suggest that the heme C in subunit II is the electron transfer site for the DET-type bioelectrocatalytic activity of FDH. [ABSTRACT FROM AUTHOR]

We show a great possibility of mediated enzymatic bioelectrocatalysis in the formate oxidation and the carbon dioxide (CO 2 ) reduction at high current densities and low overpotentials. Tungsten-containing formate dehydrogenase (FoDH1) from Methylobacterium extorquens AM1 was used as a catalyst and immobilized on a Ketjen Black-modified electrode. For the formate oxidation, a high limiting current density ( j lim ) of ca. 24 mA cm − 2 was realized with a half wave potential ( E 1/2 ) of only 0.12 V more positive than the formal potential of the formate/CO 2 couple ( E °′ CO2 ) at 30 °C in the presence of methyl viologen (MV 2 + ) as a mediator, and j lim reached ca. 145 mA cm − 2 at 60 °C. Even when a viologen-functionalized polymer was co-immobilized with FoDH1 on the porous electrode, j lim of ca. 30 mA cm − 2 was attained at 60 °C with E 1/2 = E °′ CO2 + 0.13 V. On the other hand, the CO 2 reduction was also realized with j lim ≈ 15 mA cm − 2 and E 1/2 = E °′ CO2 − 0.04 V at pH 6.6 and 60 °C in the presence of MV 2 + . [ABSTRACT FROM AUTHOR]

Effects of chemical modification of mesoporous Ketjen Black (KB) electrodes on direct electron transfer (DET)-type bioelectrocatalytic reduction of dioxygen by bilirubin oxidase (BOD) were investigated under air-saturated neutral conditions. Several amines were electrochemically oxidized at KB-modified electrode to generate nitrogen–carbon bond. The modification with negatively charged aromatic amines such as 4-aminobenzoic acid (4-ABA) drastically increased the catalytic current density compared with that by positively charged and non-charged aromatic compounds and negatively charged non-aromatic compound. Considering the basic amino acid residues around the type I site of BOD, it can be concluded that weakly negative charge on electrode surface induces a favorable orientation of BOD for the DET-type catalysis via the electrostatic interaction, while the π–π interaction is also essential for effective orientation of BOD on the electrode surface. The 4-ABA-modification leads to an increase in the heterogeneous electron transfer rate constant and a decrease in the randomness of the orientation as well as a slight increase in the surface concentration of BOD. [ABSTRACT FROM AUTHOR]

Effects of the electrode potential on a direct electron transfer (DET)-type bioelectrocatalysis of fructose dehydrogenase (FDH) at Au electrodes were investigated. Adsorbed FDH showed the highest DET activity at an adsorption potential ( E ad ) around the point of zero charge ( E pzc ). Since FDH stock solution contains 2-mercaptoethanol (ME) for stabilization, ME is partially bound to the Au electrode. However, the DET activity drastically decreased at E ad >> E pzc . Au oxide layer is formed at the positive potentials to hinder the interfacial electron transfer. In contrast, only slight decrease in the DET activity was observed at sufficiently negative E ad (<< E pzc ), where ME is reductively desorbed from the Au electrode, but co-exists in the solution. In contrast, when FDH and ME were adsorbed on Au electrodes at an open circuit potential and the FDH- and ME-adsorbed Au electrode was held at such a negative hold potential ( E ho ) in the buffer without ME, the DET activity drastically decreased. An addition of ME in the test solution prevented the decrease in the DET activity at the negative E ho . These results indicate that ME close to adsorbed FDH plays a significant role in the stabilization of FDH adsorbed on Au electrodes. [ABSTRACT FROM AUTHOR]

Effects of the electrode poential on the activity of an adsorbed enzyme has been examined by using copper efflux oxidase (CueO) as a model enzyme and by monitoring direct electron transfer (DET)-type bioelectrocatalysis of oxygen reduction. CueO adsorbed on bare Au electrodes at around the point of zero charge ( E pzc ) shows the highest DET activity, and the activity decreases as the adsorption potential ( E ad ; at which the enzyme adsorbs) is far from E pzc . We propose a model to explain the phenomena in which the electrostatic interaction between the enzyme and electrodes in the electric double layer affects the orientation and the stability of the adsorbed enzyme. The self-assembled monolayer of butanethiol on Au electrodes decreases the electric field in the outside of the inner Helmholtz plane and drastically diminishes the E ad dependence of the DET activity of CueO. When CueO is adsorbed on bare Au electrodes under open circuit potential and then is held at hold potentials ( E ho ) more positive than E pzc , the DET activity of the CueO rapidly decreases with the hold time. The strong electric field with positive surface charge density on the metallic electrode ( σ M ) leads to fatal denaturation of the adsorbed CueO. Such denaturation effect is not so serious at E ho << E pzc , but the electric field with negative σ M induces an orientation inconvenient for the DET reaction during the adsorption process. A positively charged neomycin shows a promoter ability to CueO adsorbed at E ad << E pzc . The phenomenon is also explained on the proposed model. [ABSTRACT FROM AUTHOR]

Highlights: [•] Diffuse double layers overlap with each other in the micropore. [•] The overlapping of the diffuse double layer affects the double layer capacitance. [•] The electric field becomes weak in the micropore. [•] The electroneutrality is unsatisfactory in the micropore. [ABSTRACT FROM AUTHOR]

Abstract: A sensitive d-amino acid oxidase (DAAO)/peroxidase (POD) bienzyme biosensor is constructed, in which pentacyanoferrate-bound poly(1-vinylimidazole) polymer (PVI[Fe(CN)5]) is selected as a mediator. Reductive current of PVI[Fe(CN)5] related to the H2O2 concentration generated in the DAAO reaction was measured at −0.1V vs. Ag|AgCl with DAAO/POD/PVI[Fe(CN)5]-modified electrode. The result revealed that PVI[Fe(CN)5] is suitable as a mediator for this bienzyme system due to its appropriate formal potential and its extremely low reactivity against DAAO. The stability of DAAO was improved by adding free flavin adenine dinucleotide and the electrode composition was optimized for the detection of d-alanine. Nafion and ascorbate oxidase-immobilized films worked successfully to prevent severe interference from uric acid and ascorbic acid. The low detection limits of d-alanine (2μM) and d-serine (2μM) imply its possibility for the determination of extremely low concentration of d-amino acids in physiological fluids. The proposed bienzyme biosensor is proved to be capable of detecting d-amino acids in urine. [Copyright &y& Elsevier]

Abstract: The slow relaxation of the electrical double layer on the ionic liquid (IL) side of the electrified interface, which was recently found at the IL|water interface, is detected voltammetrically also at a polycrystalline Pt electrode in two ionic liquids, trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide ([TOMA+][C4C4N−]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide ([C2mim+][C1C1N−]). The potential-step transients at the Pt electrode show exponential decays, in which the slower relaxation time is on the order of 0.1–0.4s in both [TOMA+][C4C4N−] and [C2mim+][C1C1N−]. The slow relaxation commonly seen at the Pt electrode in highly viscous [TOMA+][C4C4N−] (2.0Pas) and much less viscous [C2mim+][C1C1N−] (0.0325Pas), which was also seen at the IL|water interface, suggests the presence of a common origin of the slow relaxation on the ionic liquid side of the electrified interface, possibly the slow rearrangement of ions in the electrical double layer that consists of an ionic liquid. An even slower mode of the relaxation was detected in cyclic voltammograms as the time-dependent double layer capacitance. [Copyright &y& Elsevier]

Abstract: A moderately hydrophobic ionic liquid, tributyl(2-methoxyethyl)phosphonium bis(pentafluoroethanesulfonyl)amide ([TBMOEP+][C2C2N−]), shows a very stable liquid junction potential upon contact with an aqueous solution whose ionic strength is as low as 1μmoldm−3. The stability with the maximum excursion of the potential within ±0.5mV for 30min is very promising for accurate determination of pH and other single ion activities potentiometrically. [ABSTRACT FROM AUTHOR]

Abstract: The ultraslow relaxation, on the order of a few seconds or longer, of the structure of the electrical double layer in response to the change in the phase-boundary potential across the ionic liquid (IL)|water(W) interface, which was recently reported for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide, has been confirmed in two new hydrophobic ionic liquids, trihexyltetradecylphosphonium bis(nonafluorobutanesulfonyl)amide and trihexyltetradecylphosphonium tetrakis(pentafluorophenyl)borate. A comparison of the degree of the hysteresis in electrocapillary curves for these ILs with those for trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide demonstrates that the degree of the hysteresis is not correlated with the viscosity of these ILs. The ultraslow relaxation of the electrical double layer seems to be a general feature of ILs at electrified interfaces. [ABSTRACT FROM AUTHOR]

Electrocapillary curves obtained under the thermodynamic equilibrium at the interface between trioctylmethylammonium bis(nonafluorobutanesulfonyl)amide and aqueous lithium chloride have been analyzed to study the adsorption of Li+and Cl−ions at the interface. The specific adsorption of Li+and Cl−is negligible in almost the entire range of the polarized potential window. At the positive extreme where the excess surface charge density is greater than 3 μC·cm−2, the deviation of the experimental relative surface excesses of Li+and Cl−from the Gouy’s theory was detected. This deviation suggests the coordination of adsorbed bis(nonafluorobutanesulfonyl)amide to Li+in the aqueous side of the interface. The double layer capacitance on the ionic liquid side of the interface is deduced to be 125 μF·cm−2at the potential of zero charge. [ABSTRACT FROM AUTHOR]

• Electrochemical property of FDH lacking a cytochrome subunit (ΔC FDH) was studied. • DET of ΔC FDH was observed at porous Au electrodes modified with thiols. • Electrode-active site of ΔC FDH was estimated by kinetic analysis of DET signals. • Uncharged aromatic thiols on porous Au electrodes enhance DET of ΔC FDH. • An enhancement mechanism for DET by aromatic thiols was proposed. A heterotrimeric membrane-bound d -fructose dehydrogenase (FDH) from Gluconobacter japonicus , composed of subunits L, C, and S, shows strong direct electron transfer (DET)-type bioelectrocatalytic performance in the two-electron oxidation of d -fructose. The electrode-active site of the enzyme is the heme c moiety in the subunit C. A previous study reported the construction of the subunit L/S subcomplex (ΔC FDH) lacking the subunit C. In this work, we attempted to realize the DET-type reaction of ΔC FDH using porous gold electrodes functionalized with several thiols, and investigated the influence of the thiols on DET. Kinetic analysis of the steady-state catalytic waves revealed that the electrode-active site of ΔC FDH is the [3Fe-4S] iron-sulfur cluster in the L subunit. The experimental results demonstrated that uncharged aromatic thiols on the electrode enhance the DET-type reaction of ΔC FDH. Based on the pH-dependent profile of the DET-type activity and the surface conditions of the modified thiols evaluated by electrochemical reductive desorption, we suggest an enhancement mechanism for DET by aromatic thiols. [ABSTRACT FROM AUTHOR]

• Membrane-bound aldehyde dehydrogenase (AlDH) was examined as a bioelectrocatalyst. • AlDH enabled the direct electron transfer (DET)-type acetaldehyde oxidation. • The bioelectrocatalysis of AlDH was kinetically and thermodynamically analyzed. • Cyanide (CN−) inhibition proved the involvement of hemes c in DET by AlDH. • CN−-coordinated AlDH catalyzed the DET-type acetate reduction under acidic conditions. The bioelectrocatalytic properties of membrane-bound aldehyde dehydrogenase (AlDH) from Gluconobacter oxydans NBRC12528 were evaluated. AlDH exhibited direct electron transfer (DET)-type bioelectrocatalytic activity for acetaldehyde oxidation at several kinds of electrodes. The kinetic and thermodynamic parameters for bioelectrocatalytic acetaldehyde oxidation were estimated based on the partially random orientation model. Moreover, at the multi-walled carbon nanotube-modified electrode, the coordination of CN− to AlDH switched the direction of the DET-type bioelectrocatalysis to acetate reduction under acidic conditions. These phenomena were discussed from a thermodynamic viewpoint. [ABSTRACT FROM AUTHOR]

In nerve cells, the concurrent monitoring of multipoint on the axon has been conducted based on the voltage‐clamp method using two long parallel electrodes inside and outside the axon. As the respective membrane potentials have been evaluated by considering the clamping potential, the local current, and the conductance, the membrane potentials were not actually evaluated. We directly measured the actual membrane potentials and local currents of the respective cells using a nerve‐model‐system comprising some liquid‐membrane cells. It is proved that the action potential propagation is artificially facilitated or prevented by both the external electric generator and two long electrodes. [ABSTRACT FROM AUTHOR]

Abstract: The charge accumulation due to peroxidase (POD)-catalyzed reduction of H2O2 in a test solution (4μL) by Os(II) in a POD/PVI[Os(dmebpy)2Cl]-immobilized layer on an electrode (PVI=poly(1-vinylimidazole), dmebpy=4,4′-dimethyl-2,2′-bipyridine) was monitored potentiometrically for the detection of H2O2. Before potentiometry, the Os(II)/Os(III) ratio of the modified electrode was controlled by pre-electrolysis at a given potential in a separated electrolysis cell. The redox potential of the Os polymer film in the test solution shifted to the positive side on the addition of H2O2 and reached a constant value due to the accumulation of Os(III) in the film. The total amount of the accumulated charge was determined from the area of the portion corresponding to the redox potential shift on a reversible cyclic voltammogram recorded separately. The low detection limit (5pmol H2O2) was realized with 82–90% of the recovery percentage. [Copyright &y& Elsevier]

Bioelectrocatalysis has become one of the most important research fields in electrochemistry and provided a firm base for the application of important technology in various bioelectrochemical devices, such as biosensors, biofuel cells, and biosupercapacitors. The understanding and technology of bioelectrocatalysis have greatly improved with the introduction of nanostructured electrode materials and protein-engineering methods over the last few decades. Recently, the electroenzymatic production of renewable energy resources and useful organic compounds (bioelectrosynthesis) has attracted worldwide attention. In this review, we summarize recent progress in the applications of enzymatic bioelectrocatalysis. [ABSTRACT FROM AUTHOR]

Bioelectrocatalysis provides the intrinsic catalytic functions of redox enzymes to nonspecific electrode reactions and is the most important and basic concept for electrochemical biosensors. This review starts by describing fundamental characteristics of bioelectrocatalytic reactions in mediated and direct electron transfer types from a theoretical viewpoint and summarizes amperometric biosensors based on multi-enzymatic cascades and for multianalyte detection. The review also introduces prospective aspects of two new concepts of biosensors: mass-transfer-controlled (pseudo)steady-state amperometry at microelectrodes with enhanced enzymatic activity without calibration curves and potentiometric coulometry at enzyme/mediator-immobilized biosensors for absolute determination. [ABSTRACT FROM AUTHOR]

Diffusion-controlled amperometric biosensors for dissolved oxygen (O 2) were constructed by immobilization of multi-copper oxidases (copper efflux oxidase and bilirubin oxidase) on porous gold microdisk electrodes fabricated by anodization in a glucose solution. The immobilized enzymes rapidly consumed O 2 near the electrode at potentials more negative than 0.2 V vs. Ag|AgCl|sat. KCl via direct electron transfer-type bioelectrocatalysis and the reduction current reached the steady state limiting value under static conditions. The fabricated biosensor exhibited a linear response to dissolved O 2 concentration and was almost identical to the theoretical sensor, based on nonlinear diffusion of O 2 around the microdisk electrode. The biosensor response was fast enough to monitor the catalytic consumption of dissolved O 2 by glucose oxidase and exhibited storage stability for more than six days. • Porous structures on a gold surface enhance the bioelectrocatalytic activity of multi-copper oxidases. • Diffusion-controlled biocatalytic oxygen reduction occurs at microelectrodes. • Oxygen biosensors operate without calibration. • Biosensors function under neutral and weakly acidic conditions. • Response of the biosensor is fast enough to monitor oxidase reactions. [ABSTRACT FROM AUTHOR]

Direct electron transfer (DET)-type bioelectrocatalysis, which couples the electrode reactions and catalytic functions of redox enzymes without any redox mediator, is one of the most intriguing subjects that has been studied over the past few decades in the field of bioelectrochemistry. In order to realize the DET-type bioelectrocatalysis and improve the performance, nanostructures of the electrode surface have to be carefully tuned for each enzyme. In addition, enzymes can also be tuned by the protein engineering approach for the DET-type reaction. This review summarizes the recent progresses in this field of the research while considering the importance of nanostructure of electrodes as well as redox enzymes. This review also describes the basic concepts and theoretical aspects of DET-type bioelectrocatalysis, the significance of nanostructures as scaffolds for DET-type reactions, protein engineering approaches for DET-type reactions, and concepts and facts of bidirectional DET-type reactions from a cross-disciplinary viewpoint. [ABSTRACT FROM AUTHOR]

Bilayer lipid membranes (BLM) are widely employed to study the electrochemical behavior across cell membranes, particularly the ion transport in the presence of hydrophobic ions or ionophores. However, owing to the physical fragility of BLMs, they are readily broken down by external shocks, such as the applications of a potential difference, current, and vibration. Thus, in this study, a facile method for enhancing the physical stability of BLMs was developed using a track-etched membrane (TM). Physically stable BLMs were rapidly fabricated within the apertures of TM by dropping an n -decane solution containing lecithin and cholesterol on TM. Therefore, BLM formation was confirmed by capacitance measurement, and the estimated membrane thickness and mean capacitance were 3.5 ± 0.4 nm and 0.51 ± 0.05 μF cm−2, respectively. The applied membrane potential could be extended to ≥1.5 V, which is approximately 10 times higher than those of conventional BLMs, and the lifetime exceeded 40 min. The applicability of the fabricated BLM was verified by electrochemical measurements during ion transport. [Display omitted] [ABSTRACT FROM AUTHOR]

This review summarizes the bioelectrocatalytic properties of d -fructose dehydrogenase (FDH), while taking into consideration its enzymatic characteristics. FDH is a membrane-bound flavohemo-protein with a molecular mass of 138 kDa, and it catalyzes the oxidation of d -fructose to 5-keto- d -fructose. The characteristic feature of FDH is its strong direct-electron-transfer (DET)-type bioelectrocatalytic activity. The pathway of the DET-type reaction is discussed. An overview of the application of FDH-based bioelectrocatalysis to biosensors and biofuel cells is also presented, and the benefits and problems associated with it are extensively discussed. Unlabelled Image • Bioelectrocatalytic properties of d -fructose dehydrogenase (FDH) are reviewed. • Recent findings about FDH are summarized especially DET mechanism. • The application of FDH to biosensors and biofuel cells are simmarized. • The benefits and problems of FDH-based reaction are detailed for future works. [ABSTRACT FROM AUTHOR]

Porous gold (Au) electrodes prepared by anodization of Au electrodes in buffer solutions containing glucose were utilized as scaffolds for direct electron transfer (DET)-type bioelectrocatalysis of bilirubin oxidase (BOD). The three-dimensional porous structure appeared to promote DET-type bioelectrocatalysis of BOD. The current density of DET-type bioelectrocatalysis of BOD increased with the formation of a three-dimensional porous structure at the beginning of anodization; however, it showed saturation characteristics with a further increase in the non-Faradaic current. Moreover, the glucose concentration during anodization affected the activity of DET-type bioelectrocatalysis of BOD. Scanning electron microscopy of the prepared porous Au electrodes showed that the structure of the electrode surface depended on the glucose concentration. Achieving a pore size distribution suitable for substrate transport and enzyme orientation seemed to be dependent on the glucose concentration. Unlabelled Image • Anodization of gold electrodes in glucose solutions provides the porous structure. • The surface structure is affected by the glucose concentration during anodization. • Porous gold electrodes are effective scaffolds for bioelectrocatalysis. • Bioelectrocatalysis of bilirubin oxidase depends on the porous structure. [ABSTRACT FROM AUTHOR]

Abstract d -Fructose dehydrogenase (FDH), a membrane-bound heterotrimeric enzyme, shows strong activity in direct electron transfer (DET)-type bioelectrocatalysis. An FDH variant (Δ1 c 2 c FDH) which lacks 199 amino acid residues including two heme c moieties from N-terminus was constructed, and its DET-type bioelectrocatalytic performance was evaluated with cyclic voltammetry at Au planar electrodes. A DET-type catalytic current of d -fructose oxidation was clearly observed on Δ1 c 2 c FDH-adsorbed Au electrodes. Detailed analysis of the steady-state catalytic current indicated that Δ1 c 2 c FDH transports the electrons to the electrode via heme 3 c at a more negative potential and at more improved kinetics than the recombinant (native) FDH. Graphical abstract Unlabelled Image Highlights • An FDH variant which lacks heme 1 c and heme 2 c moieties (Δ1 c 2 c FDH) was constructed. • Δ1 c 2 c FDH showed high activity of the direct electron transfer-type bioelectrocatalysis. • Δ1 c 2 c FDH transferred the electrons at potentials more negative than the native FDH. • The energy loss in the DET-type bioelectrocatalysis of FDH has decreased. • The interfacial electron transfer kinetics has been improved. [ABSTRACT FROM AUTHOR]

Review: 43 refs. [ABSTRACT FROM AUTHOR]

A method to stably immobilize microbes on electrodes was developed. Resting cells of Methylobacterium extorquens AM1( Me AM1) were caged within multiwalled carbon nanotubes (MWNTs)by adding the cells to a water dispersion of MWNTs then allowing the resulting mixture to dry on electrodes. The Me AM1-MWCNTs electrode thus obtained displayed excellent activities in the bidirectional bioelectrocatalysis due to formate dehydrogenase(s) in the resting cells; formate oxidation and carbon dioxide reduction proceeded at steady-state catalytic current densities of 0.6 ± 0.1 and −0.8 ± 0.1 mA cm −2 , respectively, using methyl viologen as mediator under very mild conditions (pH 7.0, atmospheric pressure, and 37 °C). In addition, the catalytic signal was stable for more than one week under continuous operation. [ABSTRACT FROM AUTHOR]

A change in the membrane potential in nerve cells is thought to be generated and propagated mainly by a function of K + and Na + channels. The concurrent monitoring of multipoints on the axon has been generally conducted on the basis of the voltage-clamp or current-clamp method. Given that the respective membrane potentials have been evaluated by considering the applied potential, local current, and conductance, the propagation of the change in the membrane potential was measured. By using a nerve model system composed of some liquid membrane cells, we directly measured the actual membrane potentials and the local currents of the respective cells. We demonstrated that the local membrane current caused by an external voltage induced a change in the membrane potential and that the change was propagated by connecting the liquid membrane cells and mimicking voltage-gated Na + channels. It has been proved that hyperpolarization hardly occurs on the occasion of existence of the flux of K + and Na + only in the present model system and that the change in the membrane potential corresponding to the action potential is directionally propagated. [ABSTRACT FROM AUTHOR]

Abstract: The blocking effects of the cationic procaine, a typical local anesthetic (LA), on ion transport through gramicidin A (gA) channels between two aqueous phases (W1 and W2) were electrochemically elucidated. Although the gA channels promoted the permeation of monovalent cations, especially Cs+, the addition of procaine to W1 decreased the permeation of Cs+ through these channels from W1 to W2. This can be explained based on the following mechanism. Hydrophobic cationic procaine tends to approach the pore of a gA channel. Since it is too large to enter the pore, it cannot pass through the channel. Thus, cationic procaine inhibits the permeation of Cs+ from W1 to W2 by competing with Cs+ for access to the entrances of the gA channels. It is postulated that the decrease in the apparent activity of Cs+ caused by this competition prevents ion transport through the gA channels. [ABSTRACT FROM AUTHOR]

d -Fructose dehydrogenase (FDH), a heterotrimeric membrane-bound enzyme, exhibits strong activity in direct electron transfer- (DET-) type bioelectrocatalysis. We constructed a variant (Δ1 c FDH) that lacks 143 amino acid residues involving one heme c moiety (called heme 1 c ) on the N-terminus of subunit II, and characterized the bioelectrocatalytic properties of Δ1 c FDH using cyclic voltammetry. A clear DET-type catalytic oxidation wave of d -fructose was observed at the Δ1 c FDH-adsorbed Au electrodes. The result clearly indicates that the electrons accepted at the flavin adenine dinucleotide catalytic center in subunit I are transferred to electrodes via two of the three heme c moieties in subunit II without going through heme 1 c . In addition, the limiting current density of Δ1 c FDH was one and a half times larger than that of the native FDH in DET-type bioelectrocatalysis. The downsizing protein engineering causes an increase in the surface concentration of the electrochemically effective enzymes and an improvement in the heterogeneous electron transfer kinetics. [ABSTRACT FROM AUTHOR]

Tungsten-containing formate dehydrogenase (FoDH1) with a molecular mass of 170 kDa from Methylobacterium extorquens AM1 catalyzes the oxidation of formate (HCOO − ) to carbon dioxide (CO 2 ) with NAD + as a natural electron acceptor in solution. FoDH1 does not produce any direct electron transfer (DET)-type bioelectrocatalytic wave at planar electrodes, but can adsorb on and communicate with mesoporous carbon electrodes. The curvature effect of mesoporous structures seems to increase the number of enzymes with orientations suitable for electrochemical communication. However, adsorption proceeds slowly on Ketjen Black-modified electrode and the catalytic current density remains low. Most probably, the size of the mesopores is too small to effectively trap FoDH1. The adsorbed FoDH1 catalyzes DET-type bioelectrocatalytic interconversion of the CO 2 /HCOO − and NAD + /NADH redox couples. Most probably, one of the iron–sulfur clusters located near the enzyme surface communicates with mesoporous electrodes. When the communication proceeds effectively, FoDH1 behaves as a novel bidirectional catalyst for the substrates, since FoDH1 can realize fast uphill intramolecular electron transfer. The non-covalently bound flavin mononucleotide (FMN) cofactor in FoDH1 is dissociated from some FoDH1 molecules and adsorbs on the mesoporous electrode to give a symmetrical surface-confined redox wave. Although adsorbed FMN cannot participate in mediated electron transfer (MET)-type bioelectrocatalysis, dissociated FMN in solution works as a mediator for MET-type bioelectrocatalysis of the HCOO − oxidation at planar electrodes. [ABSTRACT FROM AUTHOR]

[Display omitted] • A new electrochemical consequtive detection system of NO 2 – and NO 3 – was constructed. • The concentration of NO 3 – in the solution was detected using an ion selective electrode. • NO 2 – was oxidized to NO 3 – using a carbon felt electrode and the concentration was determined by the Coulomb number. • The concentration of NO 3 – was detected using an ion selective electrode again. Since NO 2 − often coexists with NO 3 − in environment and waste fluids, it is important to evaluate precisely the concentrations of NO 2 − and NO 3 − simultaneously. A new electrochemical consequtive detection system of NO 2 − and NO 3 − was constructed. After the concentration of NO 3 − in the sample solution was detected using an ion selective electrode (NO 3 −-ISE), persisting NO 2 − in the sample solution was oxidized to NO 3 − using carbon felt as a working electrode. Then, the concentration of NO 3 − in the sample solution was measured using the NO 3 −-ISE again, whereas the concentration of NO 2 − was determined by comparing the results before and after electrolytic oxidation. On the basis of the Coulomb number and via electrolysis, our investigations detected the sample solution's concentrations of NO 2 − and NO 3 − successively, thereby easing the evaluation of the concentration of NO 2 − for cases above 10−5 mol dm−3. [ABSTRACT FROM AUTHOR]

A membraneless direct electron transfer (DET)-type dihydrogen (H 2 )/air-breathing biofuel cell without any mediator was constructed wherein bilirubin oxidase from Myrothecium verrucaria (BOD) and membrane-bound [NiFe] hydrogenase from Desulfovibrio vulgaris Miyazaki F (MBH) were used as biocatalysts for the cathode and the anode, respectively, and Ketjen black-modified water proof carbon paper (KB/WPCC) was used as an electrode material. The KB/WPCC surface was modified with 2-aminobenzoic acid and p -phenylenediamine, respectively, to face the positively charged electron-accepting site of BOD and the negatively charged electron-donating site of MBH to the electrode surface. A gas-diffusion system was employed for the electrodes to realize high-speed substrate supply. As result, great improvement in the current density of O2 reduction with BOD and H 2 reduction with MBH were realized at negatively and postively charged surfaces, respectively. Gas diffusion system also suppressed the oxidative inactivation of MBH at high electrode potentials. Finally, based on the improved bioanode and biocathode, a dual gas-diffusion membrane- and mediatorless H 2 /air-breathing biofuel cell was constructed. The maximum power density reached 6.1 mW cm −2 (at 0.72 V), and the open circuit voltage was 1.12 V using 1 atm of H 2 gas as a fuel at room temperature and under passive and quiescent conditions. [ABSTRACT FROM AUTHOR]

Transport of hydrophobic tetraalkylammonium chlorides (tetrabutylammonium, tetrapentylammonium, tetrahexylammonium and tetraheptylammonium) across a porous polytetrafluoroethylene resin film impregnated by 1-octanol between two aqueous phases is interpreted by considering the steady-state flows of both the tetraalkylammonium ion and the chloride ion within both the aqueous and the membrane phases. The parabolic relation between the hydrophobicity and the permeability in the quantitative structure-activity relationship is attributable to whether the transport of both ions within the aqueous phases or that within the membrane phase is the rate-determining step in the cell system. [ABSTRACT FROM AUTHOR]