Nutrient control is especially important to improve the yield of crops. It is necessary for continuous nutrient control to continuously measure the concentration of the nutrient components. Especially, the concentrations of K⁺, NO₃⁻, NH₄⁺, H₂PO₄⁻, and HPO₄²⁻ are important factors for plant growth. Electrochemical measurements using ion-selective electrodes (ISEs) are very attractive because the culture solution can be used for measurements of the nutrient concentrations without any pretreatment, and the concentrations can be rapidly evaluated on the spot. The authors’ research group has improved stable ISEs of NO₃⁻, H₂PO⁴⁻, and HPO₄²⁻ suitable for long-term measurements. As for the NO₃⁻-ISE, the sensing function was improved by eliminating the effect of coexisting Cl⁻. As for the H₂PO⁴⁻-ISE; the potential response can be stabilized by controlling the dissolved oxygen concentration in the solution. In this critical review, the preparation methods of these ISEs and their physicochemical properties are explained. We made a simple automatic management system for the nutrient concentration in a hydroponic culture solution using K⁺-, NO₃⁻-, and H₂PO⁴⁻-ISEs. In addition, we made a new pH sensor using Pd, which is one of hydrogen storage metals, since a glass electrode is easily broken, and it is difficult to miniaturize. The physicochemical properties of the pH sensor using a Pd electrode is also described.

Nanocarbon films fabricated by using unbalanced magnetron (UBM) or electron cyclotron resonance (ECR) sputtering show a wide potential window and high stability. We fabricated nitrogen-doped nanocarbon film electrodes by using sputtering in the presence of N₂ gas, a heat treatment in the presence of ammonia (NH₃), and a plasma treatment using NH₃ vapor. The nitrogen concentration, sp²/sp³ ratio and the structure of nitrogen-containing surface functional groups of the carbon films were dependent on the fabrication methods and plasma treatment conditions, such as the nitrogen or ammonia gas concentrations, which caused a positive shift of the oxygen reduction reaction (ORR) potential. For detecting biomolecules, the oxidation peaks of nicotinamide adenine dinucleotide (NADH) and L-ascorbic acid shifted to more negative potentials. Furthermore, the UBM carbon film showed improved biocompatibility after a NH₃ plasma treatment since the ΔE of ferrocyanide ([Fe(CN)₆]³⁻) showed excellent redox properties with very small ΔE increase, even in a solution containing a large concentration of bovine serum albumin (BSA). The magnitude of the current for serotonin oxidation (neurotransmitter) was almost unchanged by continuous CV measurements, although the oxidation product of serotonin is adsorptive to the electrode surface. The UBM sputtered nanocarbon film introduced nitrogen-containing functional groups onto the surface, which suggests an anti-fouling property by the adsorption of serotonin oxidized products and proteins.

The adsorption properties of dodecylsulfate and decylsulfate ions have been evaluated using electrocapillary (phase boundary potential dependence of interfacial tension) measurements at the electrochemical liquid/liquid interfaces between water (W) and three highly hydrophobic ionic liquids (ILs). The three ILs investigated are composed of a common anion, bis(nonafluorobutanesulfonyl)amide (C₄C₄N⁻), and three different cations: N-octadecylisoquinolinium (C₁₈Iq⁺), trioctylmethylammonium (TOMA⁺), and trihexyltetradecylphosphonium (THTDP⁺). For all the three ILs, the adsorption activity of the alkylsulfate ions is enhanced when the IL side of the IL/W interface is positively charged, compared with the conventional oil/water interface. This IL peculiarity is likely to originate from the stabilization of surface-active anions at the IL/W interface via the electrostatic interaction with IL cations. The enhancement is the highest at the [C₁₈Iq⁺][C₄C₄N⁻]/W interface among the three ILs, suggesting that the positively charged moiety of C₁₈Iq⁺, which is relatively exposed to neighboring ions, stabilizes more the surface-active anions adsorbed at the IL/W interface. In contrast, TOMA⁺ and THTDP⁺ have positively charged moieties buried in their alkyl chains, markedly for the latter IL, which weakens the interaction with the surface-active anions, although the adsorption activity is still higher than the case with the oil/water interface.

Azulene sulfonates with different alkyl groups were examined by ion transfer voltammetry at a nitrobenzene|water interface. All of the 8 azulene sulfonates showed reversible voltammograms of the monoanion. The parameters of hydrophobicity (standard potentials and standard Gibbs energies of transfer) of the azulene sulfonates were estimated from the mid-point potentials of the voltammograms. Those standard Gibbs energies of transfer were compared with the percentages of ulcer inhibition. As the hydrophobicity increased, the percentage of ulcer inhibition became higher. But the most hydrophobic ion gave the lowest anti-ulcer activity, probably because of the very low solubility in water and the accumulation in membrane.

We have estimated the effect of a chemical reagent based on the oxygen consumption and the motion of zebrafish embryos by using an LSI-based amperometric sensor array device (Bio-LSI). Bio-LSI with 400 gold microelectrodes in a 6 mm × 6 mm region enabled us to visualize the distribution of the concentration of redox species as images. We applied –0.2 V vs. Ag/AgCl to all electrodes of a Bio-LSI chip to obtain the distribution of the oxygen concentrations in the presence of zebrafish embryos. The motions of the embryos were observed and traced by a CMOS camera. The reduction currents, obtained at the electrodes immediately under the embryos, are smaller than that obtained at the electrodes without the embryos. When the embryos moved to the position above other electrodes, the positions of electrodes with the low reduction current synchronized to the position of the embryos. This was due to the consumption of oxygen by the respiration of embryos and the inhibition of the diffusion of oxygen to the electrodes. When a solution containing 2,4-dinitrophenol (DNP), that is an uncoupler of oxidative phosphorylation, was added, the reduction current drastically decreased and then gradually returned to the original level. The active motions of the embryos coupled with the decrease of the reduction current were observed immediately after adding DNP, while no motion was observed after turning toward original level. Embryos would transiently enhance the consumption of oxygen by respiration due to the compensation of its depression of ATP synthesis, and then be led to death. The present system based on the reduction currents of oxygen and the motions of embryos would provide simple and high-throughput toxicity tests for various chemical agents.

The electrochemiluminescence (ECL) spectra of lucigenin were measured at an indium tin oxide electrode in an O/W emulsion. Lucigenin generated two luminescence species in the emulsion. One was an illuminant with an emission of around 420 nm, which was produced in an organic droplet; the other was with an emission at around 500 nm that was produced at an oil/water interface. The dispersion state in the emulsion was widely changed by controlling the concentration of surfactant and organic solvent. The particle size distribution measurements by dynamic light scattering (DLS) were carried out in parallel with ECL measurements. The relationship between the two kinds of light emission and the population of the particle size was examined, and it was found that the intensity of the emission at around 500 nm at the interface depended on the population of relatively small particles in the range of 100–10000 nm, while that around 420 nm depended on the population of relatively large particles in the range of 10000–100000 nm. It has been demonstrated based on the above relationship that the particle size distribution in the fast transition of the dispersed state was predicted from the ECL time-resolved spectra. This result suggested the superiority of in situ ECL measurement compared with DLS measurements to require the integration during several minutes.

Bubble-attaching electrodes have been constructed to determine the kinetics of the oxygen dissolution to an aqueous solution. Since the four-electron reduction of oxygen is accelerated by a multi-copper oxidase modified porous gold electrode, the mass-transfer process of oxygen in the solution is the rate-determining step. The bubble attached electrode provides a steady-state oxygen reduction current. The current when the bubble has been detached from the electrode surface is highly reproducible. The dissolution-rate constants of the buffer solutions are estimated from the current by a comparison between the experimental results and the numerical simulation results. The increase in the concentration of the buffer solution increases the dissolution rate constants of oxygen while decreasing the saturated concentration and diffusion coefficient of oxygen.

By using the post-synthesis method, nitrogen-doped carbon (N-carbon) can be synthesized on the surface of carbon nanotubes (CNTs) by low-temperature heating. Furthermore, by using various nitrogen-containing small organic molecules as a N-carbon source, it would be possible to control of the skeleton structure, nitrogen introduction amount, and introduction form of N-carbon. These properties affect the oxygen-catalyzed reduction reaction (ORR) activity. In the present study, melamine, L-aspartic acid, and poly-L-lysine were used as N-carbon sources, and N-carbon was prepared on single-walled carbon nanotubes (SWCNTs) at a low-temperature (200 °C) by using the post-synthesis method. As a result, it was found that it has an ORR activity equivalent to that of N-carbon synthesized at high temperature. It was also found that the nitrogen/carbon ratio of organic molecules and the defective state of SWCNT have great influence on the ORR activity.