Ag nanoparticles exhibit various colors depending on their localized surface plasmon resonance (LSPR). Based on this phenomenon, Ag deposition-based electrochromic devices can represent various optical states in a single device such as the three primary colors (cyan, magenta, and yellow), silver mirror, black and transparent. A control of the morphology of Ag nanoparticles can lead to dramatic changes in color, as their size and shape influence the LSPR band. In this research, we focused on the diffusion rate of Ag⁺ ions when Ag nanoparticles are electrochemically deposited. Consequently, well-isolated Ag nanoparticles were obtained due to the slow growth rate by using an electrolyte with a low concentration of Ag⁺ ions, resulting in an improvement in the color quality of cyan and magenta. Additionally, spherical Ag nanoparticles were deposited in the same device by optimizing their voltage application conditions, which represented yellow and green colors. In particular, green coloration is a unique phenomenon because it can appear by the combination of two absorption peaks of LSPR. As a result of investigating the finite-difference time-domain method, it was observed that the LSPR band in the long wavelength region was originated from the effects of the connection between Ag particles.

Silver deposition-based electrochromic (EC) device enabled various optical states by using either the electrodeposition of silver thin films or silver nanoparticles on an indium tin oxide electrode. These optical states, including chromatic colors such as cyan, magenta, yellow, and green, are based on absorption by the localized surface plasmon resonance of silver nanoparticles. However, an ideal counter-reaction material that does not affect the color representation by silver nanoparticles and does not dissolve the deposited silver has not been reported. To achieve both color retention and chromatic color representation, MnO2 was utilized as the counter electrode material. The EC properties of the novel device with the MnO2 counter electrode were investigated in terms of charge density and its ability to change color. The novel EC device achieved both representations of chromatic color and retained its color without power supply.

The use of taste sensors with lipid/polymer membranes is one of the methods to evaluate taste. As previously reported, taste sensors can detect non-charged substances such as caffeine by modifying the lipid/polymer membranes with hydroxybenzoic acids (HBAs). The mechanism of caffeine detection by taste sensors was identified to be an allosteric one. Generally, the allosteric mechanism, defined as “regulation at distant sites”, is used to describe the regulation process for proteins. In this study, to improve the sensitivity of taste sensors to caffeine and its analogs using the allosteric mechanism, we used various modifiers of lipid/polymer membranes, and we detected caffeine using taste sensors with the modified membranes. The detection of the caffeine analogs theophylline and theobromine was also analyzed. The results of caffeine detection clarified that the molecular structure underlying the allosteric mechanism capable of effective caffeine detection involves both the carboxyl and hydroxyl groups, where the hydroxyl group can form intermolecular H bonds with caffeine. Furthermore, the taste sensors with a modifier, which has the molecular structure underlying the allosteric mechanism, showed high sensitivity to caffeine and caffeine analogs. The use of an allosteric mechanism may help improve the sensitivity of taste sensors to other non-charged pharmaceutical substances, such as dexamethasone and prednisolone, in the future.

A taste sensor employs various lipid/polymer membranes with specific physicochemical properties for taste classification and evaluation. However, phosphoric acid di(2-ethylhexyl) ester (PAEE), employed as one of the lipids for the taste sensors, exhibits insufficient selectivity for umami substances. The pH of sample solutions impacts the dissociation of lipids to influence the membrane potential, and the response to astringent substances makes accurate measurement of umami taste difficult. This study aims to develop a novel taste sensor for detecting umami substances like monosodium L-glutamate (MSG) through surface modification, i.e., a methodology previously applied to taste sensors for non-charged bitter substance measurement. Four kinds of modifiers were tested as membrane-modifying materials. By comparing the results obtained from these modifiers, the modifier structure suitable for measuring umami substances was identified. The findings revealed that the presence of carboxyl groups at para-position of the benzene ring, as well as intramolecular H-bonds between the carboxyl group and hydroxyl group, significantly affect the effectiveness of a modifier in the umami substance measurement. The taste sensor treated with this type of modifier showed excellent selectivity for umami substances.

The saltiness enhancement effect can be produced by adding specific substances to dietary salt (sodium chloride). This effect has been used in salt-reduced food to help people forge healthy eating habits. Therefore, it is necessary to objectively evaluate the saltiness of food based on this effect. In a previous study, sensor electrodes based on lipid/polymer membrane with Na⁺ ionophore have been proposed to quantify the saltiness enhanced by branched-chain amino acids (BCAAs), citric acid, and tartaric acid. In this study, we developed a new saltiness sensor with the lipid/polymer membrane to quantify the saltiness enhancement effect of quinine by replacing a lipid that caused an unexpected initial drop in the previous study with another new lipid. As a result, the concentrations of lipid and ionophore were optimized to produce an expected response. Logarithmic responses have been found on both NaCl samples and quinine-added NaCl samples. The findings indicate the usage of lipid/polymer membranes on novel taste sensors to evaluate the saltiness enhancement effect accurately.