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Graphical abstract:

Graphical abstract:

Graphical abstract:

Graphical abstract:

A liquid-membrane type nitrate-selective electrode was improved to lower the influence of contaminants by modifying its inner electrode system from Ag | AgCl | Cl−to Ag | Ag+. The NO3−-selective electrode displayed a linear response to the concentration of NO3−with a Nernstian slope of −53 ± 1 mV decade−1, in the concentration region between 10−5and 2 mol dm−3(M). The NO3−detection limit was about 10−5M. The electrochemical response of this electrode was stable for more than 30 days. The deterioration in responding characteristics due to the coexistence of Cl-was suppressed by use of the Ag | Ag+redox couple in the absence of Cl−inside the NO3−-selective electrode.

In order to improve plant factories, an appropriate control system on fertilization is urgently required. An automatic management system to control nutrient concentration was constructed using a programmable logic controller (PLC) and ion selective electrodes (ISEs) of nitrate, phosphate, and potassium ion. The concentration of nutrient components in a culture solution was monitored using these ISEs. When the concentration of the nutrient components diminished to the threshold set as an optimum condition (0.1 - 2.0 mM), an appropriate amount of a concentrated solution of each nutrient component was added to the culture solution using solenoid valves connected with the PLC. The present cultivation system was simply constructed without any computers and pumps. Three kinds of automatic control systems simultaneously worked and did not influence each other.

A phosphate ion-selective electrode using molybdenum metal was constructed. The modified molybdenum electrode responded to HPO42-in the presence of molybdenum dioxide and molybdophosphate (PMo12O403-) on the surface. The electrode exhibited a linear response to HPO42-in the concentration range between 1.0 × 10 5 and 1.0 × 10 1M (mol dm−3) in the pH range from 8.0 to 9.5 with a detection limit of 1.0 × 10−6M. The sensor showed near Nernstian characteristics (27.8 ± 0.5 mV dec−1) at pH 9.0. Since the responding potential was attributed to the activity of HPO42-, the potential at a given concentration of phosphate depended on the pH. The electrode indicated a good selectivity with respect to other common anions such as NO3-, SO42-, Cl-, HCO3-and CH3COO. The modified molybdenum electrode can be continuously used for over a 1 month with good reproducibility. The feasibility of the electrochemical sensor was proved by successful for the detection of phosphate in real samples.

We constructed a bio-solar cell based on photosynthesis. The bioanode comprises thylakoid membranes from spinach on carbon nanotubes immobilized on a gold-spattered indium tin oxide electrode. [Ru(NH3)6]Cl3was used as a soluble mediator. The bioanode exhibited a photocurrent density of 180 µA cm-2at 0 V vs. Ag|AgCl|sat. KCl. The biocathode comprises bilirubin oxidase for direct-electron-transfer-type bioelectrocatalytic O2reduction. The cell was constructed without a separator and exhibited an open-circuit voltage of 0.61 V and a maximum power density of 50 µW cm-2.A bio-solar cell consisting of a bioanode with thylakoid membranes as biocatalysts and [Ru(NH3)6]Cl3as a mediator and a gas-diffusion-type bioanode with bilirubin oxidase as a direct electron transfer-type biocatalyst was constructed without a separator and exhibited an open-circuit voltage of 0.61 V and a maximum power density of 50 µW cm-2.

An automatic management system for nutrient solutions was constructed using a programmable logic controller (PLC) and a K+-ion selective electrode (K+-ISE). The concentration of K+was monitored by the K+-ISE. When the concentration of K+fell to the threshold limit, an appropriate amount of a concentrated K+solution was added to the hydroponic solution. The volume was also maintained at a constant level by addition of water. This system can be constructed simply and inexpensively without any computers and pumps.

Using two types of liquid membranes having Na+- and Cl--permselectivities, novel power generation devices were composed. Electric power was generated based on salinity gradient energy conversion. Since the electric resistance of the dilute solution of NaCl was reduced by the addition of NaCl, the power generation capability of the electrical generator was improved compared with that in the absence of NaCl in analogy with reverse electrodialysis using ion-exchange membranes.A novel reverse electrodialysis (RED) system was constructed with two kinds of liquid membranes: one is a Na+-permselective liquid membrane, which is composed of sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaTFPB) and 2-nitrophenyl octyl ether (NPOE), and the other is a Cl--permselective membrane, which is made of tetraoctylammonium chloride (TOcACl) and NPOE. The study indicates that the liquid membranes can be alternatives to ion-exchange membranes used in RED systems.

In this study, nanostructured porous gold electrodes were prepared by the anodization of gold in the presence of oxalic acid or glucose as a reductant, and applied as scaffolds for direct electron transfer (DET)-type bioelectrocatalysis. Gold cations generated in the anodization seem to be reduced by the reductant to construct a porous gold structure. The DETtype performance of the electrode was examined using two DET-type model enzymes, bilirubin oxidase (BOD) and peroxidase (POD), for the four-electron reduction of dioxygen and the two-electron reduction of peroxide, respectively. BOD and POD on the anodized porous gold electrodes exhibited well-defined sigmoidal steady-state waves corresponding to DET-type bioelectrocatalysis. Scanning electron microscopy images revealed sponge-like pores on the electrodes. The anodized porous gold electrodes demonstrate promise as scaffolds for DET-type bioelectrocatalysis.

Shewanellacan transfer electrons to various extracellular electron acceptors. We electrochemically investigated the pathway of extracellular electron transfer from Shewanellastrain Hac319 to electrodes. A resting cell suspension of Shewanellastrain Hac319 containing lactate produced a steady-state sigmoidal wave in the presence of flavin mononucleotide (FMN) in cyclic voltammetry, but not in the absence of FMN. A harvested cell suspension without cell-washing also produced a similar catalytic wave without any external addition of free FMN. The midpoint potentials of the two sigmoidal waves were identical to the redox potential of free FMN. The data indicate that FMN secreted from the Shewanellastrain Hac319 works as an electron-transfer mediator from the cell to electrodes. An addition of cyanide to a resting cell suspension of Shewanellastrain Hac319 increased the rate of the FMN reduction in the presence of lactate, while it decreased the respiration rate. By considering the fact that cyanide is coordinated to the heme moiety of hemoproteins and shifts the redox potential to the negative potential side, the data indicate that the electron derived from lactate is predominantly transferred in a down-hill mode from an electron donor with a redox potential more negative than that of FMN without going through outer membrane cytochromes cmolecules.

Amperometric biosensors were constructed for the simultaneous detection of lactate enantiomers. The enantioselectivity of the sensor is based on NAD-dependent L- and D-lactate dehydrogenases that, respectively, oxidize L- and D-lactates into pyruvate. The NADH formed during the enzymatic reduction was catalytically oxidized at Meldola's blue-adsorbed mesoporous electrodes. Stable amperometric measurements were performed in a two-electrode system using Ag|AgCl|sat. KCl as a counter electrode via a salt bridge. The response of the sensor reached a pseudo-steady state within 60 s. The agreement of the sensitivities for L- and D-lactates and the pseudo-steady-state characteristics of the sensors demonstrate that the current is strongly influenced by the diffusion of lactates at the edge of the electrode, enabling reproducible measurements. The pseudo steady-state characteristics are also realized at the chip-type electrode. The sensor was successfully applied for the detection of D- and L-lactates in horse serum.

We performed numerical simulations on an extremely fast, mediated, electron transfer-type bioelectrocatalytic reaction using a microband electrode. The simulations under fast-enzyme-kinetics conditions predicted that the decrement of the current density by increaseing the microband thickness would effectively improve the upper limit of detection. These predictions were accurate for an ultrathin-ring with thickness of 100 nm and gold leaf with thickness of 10 μm electrodes, acting as novel amperometric glucose sensors with FAD-dependent glucose dehydrogenase. The gold leaf electrode provided pseudo-steady-state currents which were proportional to the glucose concentration up to a concentration of 20 times higher than the mediator concentration.

Non-catalytic direct electron transfer (DET) signal of Compound I of horseradish peroxidase (POD) was first detected at 0.7 V on POD/carbon nanotube mixture-modified electrodes. Excellent performance of DET-type bioelectrocatalysis was achieved with POD immobilized with glutaraldehyde on Ketjen Black (KB)-modified electrodes for H2O2reduction with an onset potential of 0.65 V (vs.Ag | AgCl | sat. KCl) without any electrode surface modification. The POD-immobilized KB electrode was found to be suitable for detecting H2O2with a low detection limit (0.1 μM at S/N= 3) at –0.1 V. By co-immobilizing glucose oxidase (GOD) and POD on the KB-modified electrode, a bienzyme electrode was constructed to couple the oxidase reaction of GOD with the DET-type bioelectrocatalytic reduction of H2O2by POD. The amperometric detection of glucose was performed with a high sensitivity (0.33 ± 0.01 μA cm−2μM−1) and a low detection limit (2 μM at S/N= 3).

To investigate the effect of mesoporous media as electrodes for the observation of a noncatalytic Faradaic current of redox proteins adsorbed on an electrode (i.e., redox peaks in adsorption voltammetry), we propose a simulation model with randomly oriented redox proteins to obtain adsorption voltammograms. Furthermore, we compare the simulated currents of redox proteins adsorbed on planar and mesoporous electrodes. In this model, a redox protein is postulated as a sphere with the active site located at a certain distance from the center of the protein. For a mesoporous electrode, a spherical pore with a radius of Rpis assumed. When Rpis close to the radius of the protein, a redox peak is obtained even when the active site is located at a great distance from the protein surface; however, the peak is not observed for a planar electrode. This is ascribed to an increase in the number of proteins with orientations suitable for electron transfer because of the curvature effect of the pore. This work indicates that mesoporous media with appropriate pore sizes will provide suitable platforms for several redox proteins to produce observable Faradaic currents.

Ion transport through a single channel of gramicidin A (GA) within the bilayer lipid membrane (BLM) between two aqueous phases (W1 and W2) has been analyzed based on the electroneutrality principle. The single-channel current increases in proportion to the magnitude of the applied membrane potential and is also dependent on the permeability coefficients of electrolyte ions (K+and Cl−). By varying the ratio of the concentration of KCl in W1 to that in W2 the ratio of the diffusion coefficient of K+in the BLM to that of Cl−in the BLM can be evaluated.

We have focused on the catalytic properties of tungsten-containing formate dehydrogenase (FoDH1) from Methylobacterium extorquensAM1 to construct a bioelectrochemical interconversion system between formate (HCOO−) and hydrogen carbonate (HCO3−). FoDH1 catalyzes both of the HCOO oxidation and the HCO3−reduction with several artificial dyes. The bi-molecular reaction rate constants between FoDH1 and the artificial electron acceptors and NAD+(as the natural electron acceptor) show the property called a linear free energy relationship (LFER), indicating that FoDH1 would have no specificity to NAD+. Similar LFER is also observed for the catalytic reduction of HCO3−. The reversible reaction between HCOO−and HCO3−through FoDH1 has been realized on cyclic voltammetry by using methyl viologen (MV) as a mediator and by adjusting pH from the thermodynamic viewpoint. Potentiometric measurements have revealed that the three redox couples, MV2+/MV·+, HCOO−/HCO3−, FoDH1 (ox/red), reach an equilibrium in the bulk solution when the two-way bioelectrocatalysis proceeds in the presence of FoDH1 and MV. The steady-state voltammograms with two-way bioelectrocatalytic properties are interpreted on a simple model by considering the solution equilibrium.

The fabrication of ultrathin-ring electrodes with a diameter of 2 mm and a thickness of 100 nm is established. The ultrathin-ring electrodes provide a large density of pseudo-steady-state currents, and realize pseudo-steady-state amperometry under quiescent conditions without a Faraday cage. Under the limiting current conditions, the current response at the ultrathin-ring electrode can be well explained by the theory of the microband electrode response. Cyclic voltammograms at the ultrathin-ring electrode show sigmoidal characteristics with some hysteresis. Numerical simulation reveals that the hysteresis can be ascribed to the time-dependence of pseudo-steady-state current. The performance of amperometry with the ultrathin-ring electrode has been verified in its application to redox enzyme kinetic measurements.

The propagation of the change in potential differences across liquid membranes from the potential-sending cell to the potential-receiving cell was investigated by use of a system combined with three liquid membrane cells, which were composed of two aqueous phases and a 1,2-dichloroethane solution phase. The ionic composition of one potential-sending cell (S) was identical to that of the receiving cell (Rec), and that of another potential-sending cell (Ap) was different from that of the Rec. When the connection of cell Rec was switched from cell S to cell Ap, the change in the membrane potential was caused by the circulating current. The greater the ratio of the interfacial area of the membrane of cell Ap to that of cell Rec, the faster the change in the membrane potential propagated from cell Ap to cell Rec.

Membrane-bound [NiFe]-hydrogenase (MBH) from Hydrogenovibrio marinusis an O2-tolerant enzyme and allows direct-electron-transfer (DET) bioelectrocatalysis for H2-oxidation. MBH is a promising bioelectrocatalyst for bioanode of enzymatic H2–O2biofuel cells. From the practical viewpoint of electricity production, the H2-depletion near the electrode surface and the oxidative and reversible inactivation called “anaerobic inactivation” of [NiFe]-hydrogenases limit the H2-oxidation at high potentials. We have already proposed a gas-diffusion system to avoid inactivation in our previous study. In this research, we have analyzed the kinetics of the electrochemically induced anaerobic inactivation and the DET bioelectrocatalytic reaction of MBH on electrodes. When the inactivation is considered as a competitive inhibition-like reaction, the maximum value of the apparent Michaelis constant reaches 6.5 mM (at Ketjen Black-modified electrode) as analyzed in our kinetic model. Since the value is larger than the saturated H2-concentration in solution (0.74 mM), we conclude that high-speed H2-supply realized by a gas-diffusion electrode is essential to compete with the inactivation. Furthermore, a gas-diffusion bioanode with MBH can eliminate the H2-depletion near the electrode surface and has reached about 10 mA cm−2at 0 V (vs. Ag|AgCl|sat. KCl electrode) under quiescent (passive) and H2-atmospheric conditions.

On the basis of the dependency of adsorption and partitioning of ionic surfactant on the phase-boundary potential (? OWf ), it is possible to understand in a unified manner various instability phenomena at the liquid|liquid interface in the presence of ionic surfactants, such as spontaneous emulsification, oscillation of ? OWf and interfacial tension, and irregularly increased currents on voltammograms of the transfer of surface-active ions across the interface. These instabilities are associated with the electrochemical instability, which defines a thermodynamically unstable condition at the liquid|liquid interface in terms of ? OWf and is a fundamental concept to understand the instabilities in the presence of ionic surfactants. The characteristic manifestations of the electrochemical instability, that is, the irregularly increased current on voltammograms in the transfer of surface-active ions, the spontaneous emulsification, and the spontaneous oscillation of ? OWf can be designed based on the criterion of the electrochemical instability.

A membrane-bound [NiFe]-hydrogenase (MBH) from Hydrogenovibrio marinusallows the direct-electron-transfer (DET)-type bioelectrocatalysis for H2oxidation and is an O2-tolerant promising enzyme for the construction of enzymatic H2–O2biofuel cells. However, the oxidative and reversible inactivation called “anaerobic inactivation” limits the H2oxidation at high potentials. In order to avoid this inhibition, we have constructed an MBH-adsorbed gas-diffusion-type electrode, in which H2is spontaneously supplied to the electrode from the gas phase.

The propagation of the change in membrane potential between two aqueous phases across two parallel membranes from the sending site to the receiving site was elucidated. The potential differences at every interface are controlled by the ion transfers and the circulating current flowed to keep the electroneutrality of every phase.

This paper describes a diffusion-controlled electrolysis of glucose with mediated bioelectrocatalysis at microdisk-electrodes. Under conditions of an extremely fast enzyme reaction, compared with the diffusion of glucose, the current in potential-step chronoamperometry reaches an almost steady state within 10 s, and is proportional to the glucose concentration. The current can be detected at +0.1 V (vs. Ag|AgCl) with 9,10-phenanthrenequinone as a mediator. The addition of carboxymethylcellulose increased the linear range up to 10 mM.