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All-solid-state NO 3 - , K + , NH 4 + , Na + , and Ca 2+ ion-selective sensors (ISEs) were prepared using polyvinyl butyral (S-LEC®K KX-5). In the present case, polyvinyl butyral was used as a porous material to keep the internal solution of the respective ISE. All sensors exhibited near-Nernst responses in the concentration region between approximately 10 -5 and 0.1 mol dm -3 . To avoid the influence of KCl as interfering ions, MgSO 4 was used as an electrolyte within the salt bridge. Although the liquid junction potential was generated, the potential difference was stabilized within about few minutes. The NO 3 - -ISE showed high stability with no potential drift during 12 h of continuous measurements and maintained high sensitivity even after 3 weeks of storage in ultrapure water. Solidification of the internal solution is expected to make the sensor smaller and increase its mechanical strength. As an actual measurement, the sodium concentration in plasma samples using the Na + -ISE was measured to confirm agreement with literature values., (© 2024. The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.)

The mechanism of directional propagation of action potential throughout a single cell was examined using a liquid-membrane model cell system. In the experiments on the liquid-membrane model cell system, liquid-membrane cells were constructed to mimic the function of K + and voltage-gated Na + channels, which play important roles in action potential propagation. These channel-mimicking cells were connected electrically, and a model cell system was composed of four parts within the one cell. When one voltage-gated Na + channel-mimicking cell was connected to form the action potential and generated the inflow current at the one part, action potential occurred in the surrounding area due to the local circulating current and propagated to the other parts. The action potential propagation throughout the cell by a brief electrical stimulus (10 ms) was easier than that by a long electrical stimulus (2 s). The long electric stimulus thus caused hyperpolarized region within the cell. Moreover, the increase in resistance corresponding to the extracellular fluid weakened the action potential propagation. In the simulation experiments using the software LTspice, the characteristics of K + and Na + channel-mimicking cells were reproduced in the electrical circuit also. A model cell aggregate consisting of closely packed three model cells and the extracellular fluid was constructed in the electric circuit. When one cell fired, the electrical signal propagated to the neighboring cells through the intercellular and extracellular fluids. This result suggests that electrical propagation can occur between independent cells in closely packed tissues without chemical transmission or direct propagation across the gap junctions., (© 2023. The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.)

In enzyme-based biosensors, Ag + eluted from the reference electrode inhibits the enzyme activity. Herein, to suppress the inhibition of bilirubin oxidase (BOD) by Ag + , kinetic analysis was used to examine the effect of Ag + on the activity of BOD. It was confirmed that the addition of Ag + decreased the bioelectrocatalytic activity of BOD. Atomic absorption spectroscopy (AAS) suggested that Ag + was attached to BOD. Moreover, the changes in the visible absorption spectra after Ag + addition showed that Ag + was bound to the type I Cu sites in BOD. During oxygen reduction by BOD, the direct-electron-transfer-type bioelectrocatalytic current decreased after Ag + was added. The decay of the catalytic current was evaluated using kinetic analysis (assuming a pseudo-first-order reaction). Based on the analysis, the inhibition of BOD was suppressed when the Ag + concentration was below 0.1 µM. Referring to the solubility product of AgCl, Cl - at a concentration of 1 mM suppressed the inhibition of the enzymatic activity by 95%., (© 2022. The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.)

A pair of symmetrical cathodic and anodic peaks is observed in cyclic voltammograms for the ion transport across a bilayer lipid membrane (BLM) between two aqueous phases in the presence of tetraphenylborate (TPhB - ). Although TPhB - serves as a carrier of a hydrophilic counter ion (Na + ) under the steady-state condition, the reason for the appearance of symmetrical peaks has not been clearly explained until now. From the chronoamperometric analysis, it is turned out that the symmetrical peaks are attributed to the translocation of TPhB - between two adsorbed layers on the surface of the BLM., (© 2022. The Author(s), under exclusive licence to The Japan Society for Analytical Chemistry.)

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 NO 3 - -selective electrode displayed a linear response to the concentration of NO 3 - with a Nernstian slope of -53 ± 1 mV decade -1 , in the concentration region between 10 -5 and 2 mol dm -3 (M). The NO 3 - detection limit was about 10 -5 M. 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 NO 3 - -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 HPO 4 2- in the presence of molybdenum dioxide and molybdophosphate (PMo 12 O 40 3- ) on the surface. The electrode exhibited a linear response to HPO 4 2- in the concentration range between 1.0 × 10 -5 and 1.0 × 10 -1 M (mol dm -3 ) in the pH range from 8.0 to 9.5 with a detection limit of 1.0 × 10 -6 M. 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 HPO 4 2- , 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 NO 3 - , SO 4 2- , Cl - , HCO 3 - and CH 3 COO - . 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.

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.

A liquid-membrane type nitrate-selective electrode was constructed, in which the responding membrane contained polyvinylchloride, o-nitrophenyloctylether and tetraheptylammonium nitrate. The NO 3 - -selective electrode displayed a linear response to the concentration of NO 3 - with a Nernstian slope of -53.3 ± 1.0 mV decade -1 , in the 10 -5 - 10 -1 mol dm -3 (M) NO 3 - concentration range. The NO 3 - detection limit was about 10 -6 M. The electrochemical response of this electrode was stable for more than 30 days. Measurements performed using the NO 3 - -sensor indicated that in the presence of green plants, the concentration of NO 3 - in a hydroponic solution decreased from 0.20 to 0.05 mM over a three-day period.

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 DET-type 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.

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.

Shewanella can transfer electrons to various extracellular electron acceptors. We electrochemically investigated the pathway of extracellular electron transfer from Shewanella strain Hac319 to electrodes. A resting cell suspension of Shewanella strain 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 Shewanella strain Hac319 works as an electron-transfer mediator from the cell to electrodes. An addition of cyanide to a resting cell suspension of Shewanella strain 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 c molecules.

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 H 2 O 2 reduction 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 H 2 O 2 with 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 H 2 O 2 by POD. The amperometric detection of glucose was performed with a high sensitivity (0.33 ± 0.01 μA cm -2 μM -1 ) and a low detection limit (2 μM at S/N = 3).

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.

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.

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.