Interdisciplinary collaboration among various fields is essential for advancements in science. In medicine, innovative approaches created through the collaboration are required for the analysis of mechanisms underlying complicated biological events as well as the development of therapies for various diseases in a super-aging society. ‘Physiology’ is a fundamental field in medicine—physiologists intend to ask what a ‘living’ organism is and seek to understand the functions and roles of its constituents, organs and cells. The observations obtained in physiological studies contribute to the elucidation of pathological processes and drug discovery. Historically, a variety of biosensors fabricated on the basis of the principles of electrochemistry have been used in physiological experiments. To develop innovative technologies and explore new scientific fields, The Physiological Society of Japan is set to collaborate with The Electrochemical Society of Japan. In this article, I summarize the research direction of physiology and discuss the challenges in and prospects for the interdisciplinary collaboration between the two scientific communities.

Functional analyses of the membrane proteins on live cells using ion-sensitive field effect transistors (ISFETs) are described in this review. Expressions of human epidermal growth factor receptor (HER2) and epidermal growth factor receptor (EGFR) on live cancer cells have been detected using cell-based field effect transistors (FETs) in combination with enzymatic signal amplification. A good correlation could be obtained between the pH values measured with the cell-based FETs and the fluorescence intensities measured using the fluorescence-activated cell sorting (FACS), with a correlation coefficient of 0.976. The interactions between membrane proteins/transporters and ligands at cell membranes using a cell-based FET with an oocyte were monitored non-invasively. Xenopus laevis oocytes were injected with the capped human organic anion transporting peptide C (hOATP-C) cRNA. Estrone-3-sulfate (E3S) was used as a substrate for hOATP-C during the uptake measurements. The transporting kinetics of the substrate when mediated by the wild-type and the mutant-type transporters could be distinguished using the cell-based FETs. It was found that the signal generation mechanism of the cell-based transistor could be explained by direct or indirect proton transport via the transporters. Measurements of expression levels of membrane proteins is important to analyze their signaling pathways and cellular outcomes. Moreover, membrane proteins and transporters constitute one of the most extensively studied classes of drug targets. Therefore, a system based on cell-based FETs would be suitable for rapid and cost-effective identification of biomarkers and high throughput analysis of drug candidates.

Carbon-based electrode materials have been widely applied for the electrochemical analysis of biomolecules. In addition to traditional carbon electrodes such as glassy carbon and carbon paste, a wide variety of carbon materials such as nanocarbons and boron doped diamond (BDD) electrodes have been employed for electrochemical analysis and biosensors in the last 25 years. Of the carbon electrode materials, carbon films are practically advantageous because they can be fabricated reproducibly with a wide range of shapes and sizes. In this paper, we report the application of sputter deposited nanocarbon film electrodes for the electrochemical analysis of biomolecules. The pure nanocarbon film electrodes have been employed for detecting DNA methylation, and lipopolysaccharides (LPS). Nitrogen-containing carbon films show improved electrochemical activity for biomolecules and excellent biocompatibility with interferents such as proteins. Metal nanoparticle embedded or modified carbon film electrodes show excellent electrocatalytic performance with sugars.

Although intensive research in the bioelectroanalysis domain in the last twenty years led to great improvements in protein-based electrode and device performances, their large application remains limited by their stability overtime under operational and resting conditions. One under-studied issue is the role played by the spatial distribution of enzymes on electrocatalysis. Actually, high fluxes in metabolic pathways involve compartmentalization and spatial organization of active biomolecules. In a mimicking way, it can be expected that controlled localization of proteins on electrode surfaces may play a role in the overall electron transfer processes and bioelectrocatalysis performances. In this short review, we will discuss recent developments in surface patterning allowing to tune in a controlled manner the localization and density of enzymes on the electrode surface. We will investigate how mixed functional layers, electrode and biological materials can serve as protein platforms to provide such electrode patterning.

A novel artificial insulin receptor was developed, that can self-assemble on a gold surface and alter its structure in response to insulin recognition via partial domains of the intrinsic insulin receptor. The candidates for the artificial insulin receptor were designed by fusing the αCT segment and L1CR domain of the insulin receptor with a gold-binding peptide to have self-assembling abilities on a gold surface. The proteins were termed 3GαL and αL3G, based on the order of these domains, and expressed in mammalian cells. A quartz crystal microbalance technique confirmed the ability of both proteins to self-assemble on the gold surface. Electrochemical impedance spectroscopy measurements using gold electrodes modified with these proteins revealed that 3GαL altered its structure in response to insulin recognition, even on a gold surface, confirming that it works as an artificial insulin receptor that self-assembles on a gold surface. We expect that 3GαL will contribute to the development of various biosensors that utilize gold surfaces as insulin-recognition elements.

Bioconversion of methane to methanol by methanotrophs under mild conditions is a promising approach for efficiently utilizing methane. Here, we present an electrochemical technique based on open-circuit potential (OCP) measurements to monitor the metabolic activity of Methylococcus capsulatus (Bath), a representative methanotrophic model. This technique is based on the extracellular electron transfer (EET) mechanism, in which intracellular electrons in living cells are exchanged across the cell membrane with an extracellular electrode. Without using artificial electron mediators in our study, we observed that OCP shifted to negative when methane metabolism was activated. By manipulating the culture conditions with the absence or presence of copper supplement to regulate the expression of outer membrane cytochromes (OMCs), the cells with a high OMC expression level, known to serve as conduits for EET, responded with increased sensitivity to stimulation with excess NADH compared to the cells with a low OMC expression level. We, therefore, used the instinctive EET capacity of M. capsulatus (Bath) for real-time OCP measurement to monitor the bioconversion of methane to methanol. Our measurements showed that the OCP levels change with intracellular redox variations and reflect methanol production rates. Our findings may facilitate the development of a methanotrophic bioprocess that allows more effective and efficient control of intracellular redox status using OCP monitoring based on EET.

Fluorescent glucose analogs, 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-D-glucose (2-NBDG) and 2-[N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino]-2-deoxy-L-glucose (2-NBDLG) were applied to the detection of metastasis dependent glucose uptake and intracellular movement. The uptakes of both analogs by highly metastatic mouse melanoma cell line, Nanog⁺BL6 were significantly greater than those by the control cell line, BL6. Nucleus shape in Nanog⁺BL6, however, was not influenced by high concentration of 2-NBDLG, suggesting less active state of 2NBDLG in cells than in other cells reported elsewhere. We focused the involvement of Slc37a4 that was the top rank gene among those up-regulated by Nanog overexpression. When Slc37a4 in Nanog⁺BL6 was knocked down, the uptake of 2-NBDG decreased to 62 % but 2-NBDLG uptake was unchanged. This suggests that combined use of 2-NBDG and 2-NBDLG can detect intracellular glucose movement relevant to cancer cells with different metastatic potentials.

Cell adhesion to culture substrates plays a crucial role in cellular activities, such as proliferation, differentiation, and apoptosis. Recently, electrochemiluminescence (ECL) imaging has been utilized for analyzing cell adhesion. In addition to ECL imaging, electrochemical impedance spectroscopy (EIS) is widely used as a conventional method for evaluating cell adhesion. However, the relationship between the results obtained from ECL imaging and EIS has not yet been investigated. In this study, the relationship between the results obtained using two methods is explored, and their advantages and disadvantages have been discussed. Endothelial cells were cultured on an extracellular matrix-coated indium tin oxide electrode for 24 h, which was further used for ECL imaging and EIS. To the best of our knowledge, this is the first report of ECL imaging and EIS of the same samples for cell adhesion analysis. This comprehensive analysis of cell adhesion using EIS and ECL imaging could be further used to evaluate drugs that target cell adhesion.

Controlling redox enzyme adsorption on an electrode surface is a key feature for designing efficient and stable bioelectrodes for biofuel cell applications. Here, we report an analysis of the adsorption mechanism of laccase, which catalyzes the 4-electron reduction reaction of oxygen, on multi-walled carbon nanotubes (MWCNTs). The adsorption of laccase on the surface of MWCNTs can be explained by the Langmuir model with a pseudo-first-order kinetics. We found that laccase adsorption is an exothermic process, mainly driven by hydrophobic interactions between laccase and the MWCNTs. Our investigation also revealed that electrostatic interactions are not the main driving forces and play a small role in laccase adsorption. A clear understanding and devising an efficient method of enzyme adsorption will provide important guidelines for optimizing, not only the surface material design but also the adsorption method for high-performance electrodes.

Direct electron transfer (DET)-enabled oxidoreductase has been utilized to develop mediator-free bioelectronic devices. Fungal pyrroloquinoline quinone-dependent dehydrogenase from Coprinopsis cinerea is an attractive quinohemoprotein in which the catalytic PQQ domain and cytochrome domain perform DET. Here, we examined the difference in the distance from the active center to the protein surface between PQQ and heme. Then, we studied the direct bioelectrocatalysis of the full-length enzyme by regulating the distance between the enzyme and electrode with various alkyl chains of self-assembled monolayers (SAMs). The catalytic currents by the full-length enzyme were obtained on SAM of chain alkyl lengths of C6 or more, while no catalytic currents have been obtained by the isolated PQQ domain. After approximately 15 Å from PQQ in the active site to the electrode surface, the onset potential of the current shifted from the redox potential of PQQ_red/PQQ_semi to Heme_red/Heme_ox. The results indicated that the chain length-dependent electron transfer pathway from the PQQ domain directly to the electrode changed via the cytochrome domain. The amount of electroactive cytochrome domain that had immobilized decreased with increasing pH (pH 6.0 to pH 9.5). In direct bioelectrocatalysis through interdomain electron transfer of the cytochrome domain, k_cat was found to be pH dependent with an optimal pH of 8.5; therefore, the rate-limiting step that governs pH dependence is likely the IET process.

Given that trace level H₂S quantitation in biological samples currently requires expensive and time-consuming instrumental procedures, we herein developed an electrochemical Ag/C sensor for the detection of H₂S in protein-containing neutral solutions. The detection principle was based on the increase in galvanic current that was elicited by the sulfidation of the Ag electrode and was linearly dependent on the concentration of H₂S. This linear relationship enabled the quantitation of H₂S in the concentration range of 0.037–6.7 µmol L⁻¹ in mock biological samples without any pretreatment, highlighting the potential of the Ag/C sensor for the analysis of water quality and food, blood, and other biological samples.

Carbon paste electrodes (CPEs) are widely used because of their malleability and ease of modification of functional molecules. This study introduces the application of Au paste electrodes (AuPEs) as a method for amplifying electrochemiluminescence (ECL), with a focus on bipolar electrochemical analyses. First, the ECL intensities generated by the cathodic and anodic reactions of [Ru(bpy)₃]²⁺ at the glassy carbon disc electrode, Au disc electrode, CPE, and AuPE was compared using a three-electrode system. We confirmed that the utilization of the AuPEs resulted in ECL intensities that were 1.4–1.7 times higher than those achieved through the use of other electrodes. A similar ECL enhancement effect was observed with the AuPE mixed with N,N′-dimethyl-3,4,9,10-perylenetetracarboxylicdiimide (PDI-CH₃) as a cathodic luminophore. This PDI-CH₃ mixed AuPE was used as the cathode in a closed bipolar electrode system. The system successfully detected dopamine concentrations of 1.0 mmol dm⁻³ in a sample cell. We hypothesize that the observed enhancements in the ECL were attributable to the surface plasmon field-enhancement effect of the Au particles. These results can be applied to highly sensitive bipolar electrochemical microscopy for imaging the dynamics of intracellular transmitter molecules.

The split G-quadruplex-based DNA-nanotweezwers-based electrochemical DNA sensor was characterized by focusing on the blocking ability of the mixed self-assembled monolayer (SAM) against the nonspecific adsorption of hemin and the direct reduction of oxygen on a gold electrode. We found that the mixed SAM composed of MCH and MHA (concentration of each alkanethiol in SAM formation solution was 1.0 mM : 0.1 mM) effectively suppressed the nonspecific adsorption of hemin and the direct reduction of oxygen on the gold electrode.

To perform on-site bacterial testing at food and pharmaceutical manufacturing sites, it is desired to develop a new method that can quickly measure the number of viable bacterial cells. We have succeeded in measuring the number of viable bacteria using small and inexpensive disposable electrode chips focusing on electrochemical methods that realize quick detection and device miniaturization. The oxidized form of the tetrazolium salt, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), is soluble and highly permeable through cell membranes. MTT is a useful indicator for evaluating cell activity that not only turns color as a result of structural changes related to intracellular metabolism but also causes a clear current response. The carbon-screen-printed electrode chip provides a distinct current response related to the MTT redox reaction in a small volume of bacterial suspension (50 µL). Based on the fact that the current reaction of MTT was strongly dependent on intracellular metabolism, the number of viable cells in a bacterial suspension could be measured electrochemically. Current changes for live cells occurred within 10 min and increased with the incubation time. After only 60 min of incubation, we successfully estimated the number of viable cells in a bacterial suspension of 10³ CFU mL⁻¹. This technology eliminates the need for complicated testing, expensive equipment, and lengthy culture testing times, thereby enabling the confirmation of food safety before shipping to prevent food poisoning.

Microbial fuel cells (MFC) are technologies that use microorganisms that transfer electrons to the anode, which flows to the cathode to find a final electron acceptor. Oxygen (O₂) is the most widely used electron acceptor as it can diffuse through air-cathodes in single-chamber MFCs. However, microorganisms need neutral to slightly acid pH to survive, which is detrimental to the oxygen reduction reaction (ORR). Therefore, catalysts are needed at the air-cathodes to sustain a stable operation of single-chamber MFCs. Here, we report that the use of small amount (0.15 mg cm⁻²) of a Fe-N-C catalyst with carbon black in air-cathodes promote the ORR in neutral media and can sustain a stable MFC operation, keeping cell voltages of 0.3 V for 8 days.

The electric organ of a typical electric fish was artificially constructed by use of a model-cell system combining liquid-membrane cells mimicking the function of K⁺ and voltage-gated Na⁺ channels. The relation between the power generation of the electric organ and the rapid synchronization was investigated using the external electric stimulus. As for a model electrocyte, only a K⁺-channel-mimicking cell was set on the head side and both K⁺-channel and voltage-gated Na⁺-channel mimicking cells were placed on the caudal side. The potential difference between one and another side through the electrocyte changed from 0 V to about 0.15 V after the external electric stimulus was applied. In the firing state, the electric current due to the transfer of K⁺ and Na⁺ flowed through the model electrocyte. When multiple model electrocytes were in series, the simultaneous ignitions by opening of voltage-gated Na⁺ channels generate a large voltage through the electrocyte aggregates. In this case, the total voltage was the sum of the potential differences of the respective electrocytes. When several model electrocytes were in parallel, the total current was the sum of the currents of all electrocytes. The rapid synchronization of the electric organ of the electric eel (0.5 ms level) seems to be caused by circulating leak currents among neighboring cells through neural and vascular networks.

Molecular recognition elements like enzymes, antibodies, and nucleic acids, which are involved in specific binding, are important components in biosensing technologies. These biomolecular recognition elements are based on molecular interactions such as hydrogen bonding, van der Waals forces, and hydrophobic interactions. However, these interactions are often affected by the solution environment such as pH, temperature, and salt concentration, which are the rate-limiting factors for biosensing applications. In this study, we focused on molecular recognition using photocrosslinkable artificial nucleic acids. Photocrosslinkable artificial nucleic acids can form covalent bonds with target nucleic acids upon photoirradiation after hybridization. The covalent bonds formed are stronger than those in conventional molecular recognition and are not affected by the solution environment. Herein, we propose a biosensing system that combines molecular recognition by photocrosslinkable artificial nucleic acids, isothermal amplification by hybridization chain reaction, and electrochemical detection of miR-21 as the target molecule, which has recently attracted attention as a cancer biomarker. This technology eliminates non-specific binding and enables biosensing measurements with a suppressed background.

In the present study, a progesterone (P4) biosensor was developed to detect cattle estrus. Thionin, which has been reported to oxidize steroid hormones, was immobilized on an electrode via 10-carboxy-1-decanethiol with the aim of continuous measurement of P4 in the cattle body. On the screen-printed electrode, Au nanoparticles were electrodeposited on the surface of the carbon electrode to increase surface area of electrode. Finally, the thionin-modified electrode surface was covered with Nafion™. As a result, the influence of contaminants (BSA, l-ascorbic acid) was avoided. The detection range of the prepared sensor for P4 was 1 nM (= n mol/l)–20 nM. When bovine plasma was used as a biological sample, it was confirmed that the current response in i-t measurement increased due to the addition of P4. The fabricated biosensor was able to detect P4 for 4 days. It is expected that the P4 biosensor used in the present study will enable accurate understanding of cattle estrus.

Power characteristics become one of the important performance measures of lithium-ion batteries as high-power applications such as electric vehicles are emerging. Among several electrochemical steps that limit the power characteristics, phase transition kinetics is known as the limiting step for two-phase coexisting (biphasic) materials. In this study, we used LiFePO₄ as a model biphasic material and investigated the intrinsic factor that limits the phase transition behavior. When the same LiFePO₄ electrodes were tested in non-aqueous and aqueous electrolytes, the activation energy for the aqueous system was lower. In addition, impedance measurements using 4-electrode cells show that the charge-transfer resistance at the electrode/electrolyte interface in the aqueous media is also lower than that in the non-aqueous media, suggesting more facile solvation/de-solvation process in the aqueous media. This indicates that the rearrangement of the phase transition boundary (LiFePO₄/FePO₄) is sufficiently fast and other factors such as charge-transfer at the electrode/electrolyte interface affects the whole reaction rate.

Metal-free hydrogen-bonding conductive polymer catalysts (HCPCs) are emerging alternatives to platinum-group-metals (PGMs) for electrocatalytic hydrogen evolution reaction (HER). In this study, electrodes coated with poly-neutral red (PNR) films were fabricated by oxidative chemical vapor deposition (oCVD) and electropolymerization deposition (EPD) to study their activities as HER-electrocatalysts in acidic media. PNR (EPD) exhibited a superior performance, i.e., an overpotential of 194 mV (at 10 mA cm⁻²) owing to a high exchange current density of 0.711 mA cm⁻². The activity is substantiated by spectroelectrochemical measurements and time-dependent density functional theory (TDDFT) calculation unveiling a doubly reduced and doubly protonated PNRH₂ as the reaction intermediate for the evolution of H₂. It resulted in a first-order kinetic constant of k = 9.98 × 10⁻⁴ s⁻¹ to indicate a slow dissociation of PNRH₂ to release H₂. Large Tafel slope of 176 mV dec⁻¹ also limits the electrocatalytic activity of PNR.

The transition between MgM₂O₄ (M = Mn, Fe, Co) spinels (SPs) and MgMO₂ rock salts (RS) has attracted considerable interest for cathode reactions in future magnesium battery applications. To improve the cycling performance, one should suppress the consumption of solvent molecules. In this study, we investigated ether solvent decomposition on MgM₂O₄ SP and MgMO₂ RS surfaces using first-principles calculations. We found that the C–H bond dissociation of ether molecules on the SP surface was exothermic, while the C–H bond dissociation on RS and C–O bond dissociation on both SP and RS surfaces were endothermic, irrespective of the transition metal element. The products of C–H dissociation reactions at the SP surfaces have occupied states originating from SP surfaces inside the bandgap. As the SP surface is destabilized by C–H dissociation, the electrons at this level can be extracted as an oxidative current.

We demonstrated a bright yellow phosphorescent electrogenerated chemiluminescence (ECL) cell with a maximum luminance of over 100 cd m⁻² and a maximum current efficiency of 2.84 cd A⁻¹ by adding a redox mediator to the iridium complex solution, which are both among the highest values ever reported for the iridium complex-based ECL cells. Here, bis(2-phenylbenzothiazolato)(acetylacetonate)iridium(III) (Ir(BT)₂(acac)) was used as a luminescent material, while 4,5-bis(carbazol-9-yl)-1,2-dicyanobenzene (2CzPN) was used as the mediator. The working mechanism for the developed cell was discussed based on the electrochemical and photophysical properties of the employed materials.

This study introduces a facile mechanochemical synthesis of K_xMn[Fe(CN)₆] (KMnHCF) and carbon nanotube (CNT) composite (KMnHCF@CNT) as a positive electrode material for potassium-ion batteries. The KMnHCF@CNT, synthesized by a simultaneous process of mechanochemical synthesis and carbon compositing, shows a homogeneous composite and achieves a much higher electron conductivity of 7.16 × 10⁻¹ S cm⁻¹ than the KMnHCF and CNT mixture (2.35 × 10⁻² S cm⁻¹) synthesized by the two-step process. The improved electron conductivity demonstrates reduced carbon content in the electrode and excellent rate performance of maintaining 80 mAh g⁻¹ at 20 C in potassium cells.

Prostate specific antigen (PSA), as a biomarker, plays important roles in early diagnosis of male prostate diseases, especially prostate cancer. In this study, a novel electrochemical probe polymer of poly(ferriporphyrin-co-acrylamide) was synthesized, by further combined with reduced graphene oxide and gold nanoparticles, and oriented immobilization of PSA antibody, a label-free electrochemical immunosensor was obtained, and then successfully applied in the determination of PSA in 10 % non-antigen serum system. The prepared biosensor expressed high selectivity toward PSA, with a LOD of 0.001 ng mL⁻¹, a linear range of 0.01–110 ng mL⁻¹, and a sensitivity of 15.78 µA mL ng⁻¹. What’s more, it showed good stability, well repeatability and high selectivity in the real PSA sample detection. This work provided certain references for the design of the new electrochemical probes and the detection of tumor markers.

The voltage difference of battery pack is a very important index for the state evaluation of energy storage battery. When the voltage difference is too large inside the battery pack, it may cause a series of safety problems. By predicting the voltage difference of battery pack, potential dangerous situations can be detected as early as possible, and necessary measures can be taken to ensure the safety of the energy storage battery, so as to realize the reliability improvement, efficiency improvement, and safety guarantee of the energy storage system. Through the multi-step prediction for the voltage difference of the energy storage battery pack, the variation trend of the voltage difference can be predicted in advance, so as to warn the possible voltage difference over-limit fault. At present, there are many methods for multi-step prediction of time series data, but which one is most suitable for predicting the voltage difference of the energy storage battery pack is still lack of research. In this paper, the stationarity and correlation of energy storage battery pack’s voltage difference data are analyzed and processed, and different multi-step prediction algorithms are used to predict the voltage difference of energy storage battery pack. The prediction results generated by different models are compared and analyzed, and the most suitable model selection for predicting the voltage difference of energy storage battery pack is discussed.

Li-metal batteries employing Li-metal anodes with the highest capacity density have attracted considerable attention. However, the Li-metal deposition is often nonuniform, which reduces the cycle performance of Li-metal batteries. The three-dimensionally ordered macroporous polyimide (3DOM PI) separator has provided much better Li deposition/dissolution cycle performance owing to its unique and ordered pore structure, but its low mechanical strength is an obstacle to practical use. In this study, we added polybenzimidazole (PBI) to increase the mechanical strength of the 3DOM PI separator. Both the 3DOM PI and 3DOM PI+PBI separators with pore sizes of 300 and 100 nm were prepared to investigate the effect of pore size on the mechanical strength of the separator. The 3DOM PI+PBI separators exhibited higher tensile strength, higher thermal stability, better Li deposition/dissolution cycle performance, and higher charge–discharge performance of the Li-metal battery than the 3DOM PI separators. Here, it was suggested that the increase in the mechanical strength of the separator prevented the compression of the pores in the separator by external pressure and provided a more uniform Li⁺ ion flux inside the separator.

All-solid-state sodium secondary batteries have attracted attention as next-generation batteries owing to their balanced performances in terms of energy density, battery life, abundant availability of sodium resources, and resulting cost reduction. For the positive electrode materials, NaFe_0.5Mn_0.5O₂ is promising because it consists of abundant elements. However, its application in all-solid-state batteries with sulfide solid electrolytes are hindered by side reactions with the solid electrolytes, which lower the operating voltage. In this study, the electrode performances of all-solid-state sodium batteries were enhanced by mixing Na₃PO₄ with NaFe_0.5Mn_0.5O₂ particles. Subsequent heat treatment further improved the electrode performance, resulting in an increased discharge voltage and a reversible capacity of 140 mAh g⁻¹.