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The aim of our study was to enhance the reusability of a taste sensor by increasing the efficiency of the cleaning process. In this paper, we proposed a surfactant cleaning method for two types of bitterness sensor using lipid polymer membranes with high hydrophobicity. To select suitable surfactants for cleaning, the usability of four types of surfactant with different polarities and ionicities was evaluated. Among these surfactants, the anionic surfactant linear alkylbenzene sulfonate (LAS) showed no impact on the initial membrane potential and completely removed residue substances of high-concentration quinine hydrochloride adsorbed on the bitterness sensor used for hydrochloride salts. On the other hand, the amphoteric surfactant lauryl dimethylaminoacetic acid (LDA) showed no impact on the initial membrane potential and almost completely removed residue substances of high-concentration iso- $\alpha $ acid adsorbed on the bitterness sensor used for acidic bitter materials. Moreover, we found that the combined use of the conventional cleaning solution and LDA acid solution in turn greatly improved the reusability of an astringency sensor in multiple measurements of black tea.

An odor sensing system with chemosensitive resistors was used to identify the gases generated from overheated cables to prevent fire. Three different electric cables for a distribution cabinet were used. The cables had an insulation layer made of polyvinyl chloride (PVC) or cross-linked polyethylene (XLPE). The heat resistance of the cables was tested by differential thermal and thermogravimetric analyses. The thermal decomposition products of the cables were investigated by gas chromatography-mass spectrometry (GC-MS). For the odor sensing system, two types of 16-channel array were used to detect the generated gases. One contains high-polarity GC stationary phase materials and the other contains GC stationary phase materials of high to low polarity. The system could distinguish among three cable samples at 270 °C with an accuracy of about 75% through both arrays trained with machine learning. Furthermore, the system could achieve a recall rate of 90% and a precision rate of 70% when the abnormal temperature was set above the cables’ allowable conductor temperature at 130 °C. The odor sensing system could effectively detect the abnormal heating of the cables before the occurrence of fire. Therefore, it is helpful for fire prediction and detection systems in factories and substations.

Technologies for quantifying bitterness are essential for classifying medicines. As previously reported, taste sensors with lipid polymer membranes can respond to bitter hydrochloride substances in pharmaceuticals. However, the acid hydrolysis reaction between the lipid phosphoric acid di-n-decyl ester (PADE) and the plasticizer tributyl o-acetylcitrate (TDAB) led to a deterioration in sensor responses during storage. Given the cost of transportation and preservation for commercialization, membrane components that maintain physical and chemical stability during long-term storage are needed. Here we present a membrane electrode based on hydrophobic tetrakis [3,5-bis (trifluoromethyl) phenyl] borate (TFPB) and a plasticizer 2-nitrophenyl octyl ether (NPOE) for the quantification of pharmaceutical bitterness, they maintain a stable response before and after accelerated deterioration, as well as high selectivity and sensitivity. It is a first attempt to use a completely dissociative substance to replace non-completely dissociative lipids. Our work offsets the long-term stability issue of a bitterness sensor with a negatively charged hydrophobic membrane. Meanwhile, we provide the opportunity to select surface charge modifiers for a membrane surface using ester plasticizers containing oppositely charged impurities.

A taste sensor with lipid/polymer membranes is one of the devices that can evaluate taste objectively. However, the conventional taste sensor cannot measure non-charged bitter substances, such as caffeine contained in coffee, because the taste sensor uses the potentiometric measurement based mainly on change in surface electric charge density of the membrane. In this study, we aimed at the detection of typical non-charged bitter substances such as caffeine, theophylline and theobromine included in beverages and pharmaceutical products. The developed sensor is designed to detect the change in the membrane potential by using a kind of allosteric mechanism of breaking an intramolecular hydrogen bond between the carboxy group and hydroxy group of aromatic carboxylic acid (i.e., hydroxy-, dihydroxy-, and trihydroxybenzoic acids) when non-charged bitter substances are bound to the hydroxy group. As a result of surface modification by immersing the sensor electrode in a modification solution in which 2,6-dihydroxybenzoic acid was dissolved, it was confirmed that the sensor response increased with the concentration of caffeine as well as allied substances. The threshold and increase tendency were consistent with those of human senses. The detection mechanism is discussed by taking into account intramolecular and intermolecular hydrogen bonds, which cause allostery. These findings suggest that it is possible to evaluate bitterness caused by non-charged bitter substances objectively by using the taste sensor with allosteric mechanism.

In this study, we were challenging to identify characteristic compounds in breast cancer cell lines. GC analysis of extracts from the culture media of breast cancer cell lines (MCF-7, SK-BR-3, and YMB-1) using a solid-phase Porapak Q extraction revealed that two compounds of moderate volatility, 1-hexadecanol and 5-(Z)-dodecenoic acid, were detected with markedly higher amount than those in the medium of fibroblast cell line (KMST-6). Furthermore, LC-TOF/MS analysis of the extracts clarified that in addition to the above two fatty acids, the amounts of five unsaturated fatty acids [decenoic acid (C10:1), decadienoic acid (C10:2), 5-(Z)-dodecenoic acid (C12:1), 5-(Z)-tetradecenoic acid (C14:1), and tetradecadienoic acid (C14:2)] in MCF-7 medium were higher than those in medium of KMST-6. Interestingly, H2O2-oxidation of 5-(Z)-dodecenoic acid and 5-(Z)-tetradecenoic acid produced volatile aldehydes that were reported as specific volatiles in breath from various cancer patients, such as heptanal, octanal, nonanal, decanal, 2-(E)-nonenal, and 2-(E)-octenal. Thus, we concluded that these identified compounds over-produced in breast cancer cells in this study could serve as potential precursors producing reported cancer-specific volatiles.

We have developed a method to quantify the sweetness of negatively charged high-potency sweeteners coexisting with other taste substances. This kind of sweetness sensor uses lipid polymer membranes as the taste-sensing part. Two types of outputs have been defined in the measurement of the taste sensor: one is the relative value and the other is the CPA (the change in membrane potential caused by adsorption) value. The CPA value shows a good selectivity for high-potency sweeteners. On the other hand, the relative value is several times higher than the CPA value, but the relative value is influenced by salty substances. In order to obtain both high sensitivity and selectivity, we established a model for predicting the concentration of sweeteners with a nonlinear regression analysis method using the relative values of both the sweetness sensor and the saltiness sensor. The analysis results showed good correlations with the estimated concentration of acesulfame potassium coexisting with salty substances, as represented by R2 = 0.99. This model can correspond well to the prediction of acesulfame K in a concentration of 0.2&ndash, 0.7 mM, which is commonly used in food and beverages. The results obtained in this paper suggest that this method is useful for the evaluation of acesulfame K using the taste sensors.

A taste sensor based on lipid/polymer membranes has been reported being possible to detect the masking of bitter substances or masking on bitterness receptors (physical masking or biochemical masking). However, it was difficult to express the bitterness suppression by sweeteners, which is decided by the balance of substances produced in human’s brain (functional masking). High-potency sweeteners are one of the sweeteners used for bitterness-masking in food and pharmaceutical industry. The objective of this study is to evaluate the bitterness-masking effect of high-potency sweeteners using the taste sensor. A bitterness sensor was used to evaluate the bitterness of quinine hydrochloride, and sweetness sensors for high-potency sweeteners were used to evaluate the sweetness of aspartame and saccharine sodium. The sensory evaluation was also carried out to examine the bitterness suppression effect of high-potency sweeteners. The bitterness-prediction formulas were proposed with the aid of a model regression analysis using two outputs from the bitterness sensor and the sweetness sensor for high-potency sweeteners. As a result, the predicted bitterness showed a good correlation with the human taste when aspartame or saccharine sodium was added to quinine hydrochloride. Thus, this study provided an effective method to evaluate the bitterness suppressed by high-potency sweeteners.

We investigated the bitter compounds in coffee brews using multivariate analysis of the data obtained from analytical instrument and electronic taste sensor experiments. Coffee brews were prepared from coffee beans roasted to four different degrees. Each brew was fractionated into four fractions by liquid-liquid extraction. The relative amounts of 30 compounds in each fraction were analyzed by analytical instruments, and the bitterness response value of each fraction was analyzed by a taste sensor. Candidate bitter compounds in the coffee brews were identified with reference to their variable importance in projection and by coefficient of projection to latent structure regression (PLS-R) analysis. PLS-R analysis suggested that nicotinic acid, l-lactic acid, and nicotinamide contributed to the bitterness of the coffee brews. In fact, the coffee brews with added nicotinic acid, l-lactic acid, and nicotinamide had an increased bitterness response value compared to those without.

Electronic tongues have been identified as useful tools for quantifying the taste of food and beverages. However, the bitterness evaluation of coffee in the presence of high-potency sweeteners has not been performed yet. In this study, we investigated an approach to quantify the bitterness of coffee with different roasting degrees and the sweetness of acesulfame potassium (Ace K) added to the coffee using taste sensors with lipid polymer membranes. A formula for predicting coffee bitterness with the addition of Ace K was proposed using the responses of a bitterness sensor and a sweetness sensor. We found that the predicted coffee bitterness showed a good correlation with human taste.

The saltiness enhancement effect is the effect whereby saltiness is enhanced by adding specific substances to salt (sodium chloride). Since this effect can be used in the development of salt-reduced foods, a method to objectively evaluate the saltiness with this effect is required. A taste sensor with lipid/polymer membranes has been used to quantify the taste of food and beverages in recent years. The sensor electrodes of this taste sensor have the feature of selectively responding to each of the five basic tastes, which is realized by the lipid/polymer membranes. In this study, we developed a new saltiness sensor based on the lipid/polymer membrane with the aim of quantifying the saltiness enhancement effect. In addition to the conventional components of a lipid, plasticizer, and polymer supporting reagent, the membrane we developed comprises ionophores, which selectively capture sodium ions. As a result, the response of the sensor increased logarithmically with the activity of NaCl in measured samples, similarly to the taste response of humans. In addition, all of the sensor responses increased upon adding saltiness-enhancing substances, such as citric acid, tartaric acid and branched-chain amino acids (BCAAs), to NaCl samples. These findings suggest that it is possible to quantify the saltiness enhancement effect using a taste sensor with lipid/polymer membranes.

This paper reports the improvement of a bitterness sensor based on a lipid polymer membrane consisting of phosphoric acid di-n-decyl ester (PADE) as a lipid and bis(1-butylpentyl) adipate (BBPA) and tributyl o-acetylcitrate (TBAC) as plasticizers. Although the commercialized bitterness sensor (BT0) has high sensitivity and selectivity to the bitterness of medicines, the sensor response gradually decreases to almost zero after two years at room temperature and humidity in a laboratory. To reveal the reason for the deterioration of the response, we investigated sensor membranes by measuring the membrane potential, contact angle, and adsorption amount, as well as by performing gas chromatography-mass spectrometry (GC-MS), liquid chromatography-tandem mass spectrometry (LC-MS/MS). We found that the change in the surface charge density caused by the hydrolysis of TBAC led to the deterioration of the response. The acidic environment generated by PADE promoted TBAC hydrolysis. Finally, we succeeded in fabricating a new membrane for sensing the bitterness of medicines with higher durability and sensitivity by adjusting the proportions of the lipid and plasticizers.

High-potency sweeteners are applied to low-calorie diets and bitterness-masking ingredients in pharmaceutical products. We have studied taste sensors with lipid polymer membranes based on potentiometric measurement system for high-potency sweeteners. However, the sensor also responds to astringency substances because of hydrophobic characteristics of the lipid polymer membrane. In this study, we developed a new taste sensor using a lipid polymer membrane for saccharin sodium and acesulfame potassium as negatively charged high-potency sweeteners. We optimized the quantities and types of lipids and plasticizers for the fabrication of the sensor with high selectivity and sensitivity. We succeeded in the fabrication of the new sensor, the output of which could be suppressed under −5 mV for astringency substances. Moreover, the sensor has a good sensitivity and selectivity for saccharin sodium and acesulfame potassium.

The detection and recognition of metabolically derived aldehydes, which have been identified as important products of oxidative stress and biomarkers of cancers; are considered as an effective approach for early cancer detection as well as health status monitoring. Quartz crystal microbalance (QCM) sensor arrays based on molecularly imprinted sol-gel (MISG) materials were developed in this work for highly sensitive detection and highly selective recognition of typical aldehyde vapors including hexanal (HAL); nonanal (NAL) and bezaldehyde (BAL). The MISGs were prepared by a sol-gel procedure using two matrix precursors: tetraethyl orthosilicate (TEOS) and tetrabutoxytitanium (TBOT). Aminopropyltriethoxysilane (APT); diethylaminopropyltrimethoxysilane (EAP) and trimethoxy-phenylsilane (TMP) were added as functional monomers to adjust the imprinting effect of the matrix. Hexanoic acid (HA); nonanoic acid (NA) and benzoic acid (BA) were used as psuedotemplates in view of their analogous structure to the target molecules as well as the strong hydrogen-bonding interaction with the matrix. Totally 13 types of MISGs with different components were prepared and coated on QCM electrodes by spin coating. Their sensing characters towards the three aldehyde vapors with different concentrations were investigated qualitatively. The results demonstrated that the response of individual sensors to each target strongly depended on the matrix precursors; functional monomers and template molecules. An optimization of the 13 MISG materials was carried out based on statistical analysis such as principle component analysis (PCA); multivariate analysis of covariance (MANCOVA) and hierarchical cluster analysis (HCA). The optimized sensor array consisting of five channels showed a high discrimination ability on the aldehyde vapors; which was confirmed by quantitative comparison with a randomly selected array. It was suggested that both the molecularly imprinting (MIP) effect and the matrix effect contributed to the sensitivity and selectivity of the optimized sensor array. The developed MISGs were expected to be promising materials for the detection and recognition of volatile aldehydes contained in exhaled breath or human body odor.

Objective taste evaluation has been much in demand in the food, beverage and pharmaceutical industries. A taste-sensing system, which is an electronic tongue with “global selectivity,” is one of the methods used for objective taste evaluation. A taste sensor electrode responds to only one of the basic tastes (saltiness, sourness, sweetness, bitterness and umami) as a change in membrane potential caused by interactions with tastants. Sweet substances are compounds with diverse chemical structures and sizes. Since the taste-sensing system is a potentiometric measurement system using a change in membrane potential, three types of sweetness sensors are required, one for sweeteners with each type of electric charge (uncharged, positively charged and negatively charged). A sweetness sensor for uncharged sweeteners has been developed. Therefore, negatively charged sweeteners, such as saccharine sodium and acesulfame potassium, were chosen as the target substances in this study. We investigated the responses of various sensor membranes using a lipid and nine kinds of plasticizers to each basic taste sample. Furthermore, not only the selectivity of the membranes but also the concentration dependence of their response to sweeteners was investigated. As a result, one of the developed sensors showed responses of over 20 mV to 5 mM saccharine sodium and 10 mM acesulfame potassium in CPA value measurement (CPA: change in membrane potential caused by adsorption). On the other hand, the sensor also showed nearly zero responses to other basic taste samples. In addition, saltiness was the only interfering taste, and the responses to target substances in relative value measurement were over 140 mV. The developed sweetness sensor had high selectivity and concentration-dependent responses. Hence, we concluded that the sensor is suitable for use as a sweetness sensor for high-potency sweeteners with a negative electric charge.

This review describes an “electronic dog nose” based on a surface plasmon resonance (SPR) sensor and an antigen–antibody interaction for security and safety. We have concentrated on developing appropriate sensor surfaces for the SPR sensor for practical use. The review covers different surface fabrications, which all include variations of a self-assembled monolayer containing oligo(ethylene glycol), dendrimer, and hydrophilic polymer. We have carried out detection of explosives using the sensor surfaces. For the SPR sensor to detect explosives, the vapor or particles of the target substances have to be dissolved in a liquid. Therefore, we also review the development of sampling processes for explosives, and a protocol for the measurement of explosives on the SPR sensor in the field. Additionally, sensing elements, which have the potential to be applied for the electronic dog nose, are described.

Taste evaluation technology has been developed by several methods, such as sensory tests, electronic tongues and a taste sensor based on lipid/polymer membranes. In particular, the taste sensor can individually quantify five basic tastes without multivariate analysis. However, it has proven difficult to develop a sweetness sensor, because sweeteners are classified into three types according to the electric charges in an aqueous solution; that is, no charge, negative charge and positive charge. Using membrane potential measurements, the taste-sensing system needs three types of sensor membrane for each electric charge type of sweetener. Since the commercially available sweetness sensor was only intended for uncharged sweeteners, a sweetness sensor for positively charged high-potency sweeteners such as aspartame was developed in this study. Using a lipid and plasticizers, we fabricated various lipid/polymer membranes for the sweetness sensor to identify the suitable components of the sensor membranes. As a result, one of the developed sensors showed responses of more than 20 mV to 10 mM aspartame and less than 5 mV to any other taste. The responses of the sensor depended on the concentration of aspartame. These results suggested that the developed sweetness sensor had high sensitivity to and high selectivity for aspartame.

A taste sensor using a lipid/polymer membrane, i.e., an electronic tongue with global selectivity, has been developed for objective evaluation of the taste of foods and beverages. Moreover, the taste sensor has been also contributing to safety of foods, e.g., the sensor membrane with strong hydrophobicity was used to detect sodium dodecyl sulfate (SDS), a negatively charged surfactant, which was generally added into the pesticide because of its strongly melting effect. An immersion process in monosodium glutamate (MSG) solution, called “MSG preconditioning” was needed to obtain the change in membrane electric potential caused by adsorption (CPA) for sensor membrane before measurement. However, what happened to sensor membrane during MSG preconditioning is unclear. In this paper, we examined the relationship between the CPA value and the period of MSG preconditioning. The amount of adsorbed SDS and MSG was measured to figure out whether the CPA value is related to the amount of adsorption. As a result, with the precondition process progressed, the CPA values showed concentration dependence on SDS concentration, and increased to a peak by preconditioning for one day then decreased to a stable state after that. The amount of adsorbed SDS depended on the SDS concentration but did not change with the increasing of preconditioning time. In conclusion, we revealed that the most suitable time of MSG preconditioning for the membrane for SDS was one day. The CPA value was affected by both the surface charge density and the amount of absorption.

Recently, highly functional biosensors have been developed in preparation for possible large-scale terrorist attacks using chemical warfare agents. Practically applicable sensors are required to have various abilities, such as high portability and operability, the capability of performing rapid and continuous measurement, as well as high sensitivity and selectivity. We developed the detection method of capsaicinoids, the main component of some lachrymators, using a surface plasmon resonance (SPR) immunosensor as an on-site detection sensor. Homovanillic acid, which has a vanillyl group similar to capsaicinoids such as capsaicin and dihydrocapsaicin, was bound to Concholepas concholepas hemocyanin (CCH) for use as an immunogen to generate polyclonal antibodies. An indirect competitive assay was carried out to detect capsaicinoids using SPR sensor chips on which different capsaicin analogues were immobilized. For the sensor chip on which 4-hydroxy-3-methoxybenzylamine hydrochloride was immobilized, a detection limit of 150 ppb was achieved. We found that the incubation time was not required and the detection can be completed in five minutes.

In this study, we modified a surface plasmon resonance immunosensor chip with a polymer using surface-initiated atom transfer polymerization (SI-ATRP) for the highly sensitive detection of 2,4,6-trinitrotoluene (TNT). To immobilize a TNT analogue on the polymer, mono-2-(methacryloyloxy)ethylsuccinate (MES), which has a carboxyl group, was used in this study. However, the anti-TNT antibody may adsorb non-specifically on the polymer surface by an electrostatic interaction because MES is negatively charged. Therefore, a mixed monomer with MES and diethylaminoethylmethacrylate (DEAEM), which has a tertiary amino group and is positively charged, was prepared to obtain electroneutrality for suppressing the nonspecific adsorption. The detection of TNT was performed by inhibition assay using the polymer surface. To ensure high sensitivity to TNT, the affinity between the surface and the antibody was optimized by controlling the density of the initiator for ATRP by mixing two types of self-assembled monolayer reagents. As a result, a limit of detection of 5.7 pg/mL (ppt) for TNT was achieved using the optimized surface.

To detect 2,4,6-trinitrotoluene (TNT) using a displacement method over a surface plasmon resonance immunosensor, we developed sensor surfaces modified with a poly-(N-vinylformamideco-vinylamine), which was synthesized from a monomer, N-vinylformamide, and was hydrolyzed to produce amine groups, and TNT analogs. Poly-(N-vinylformamide-co-vinylamine) that reacted with succinic acid was used as a matrix of a sensor surface for increasing the number of antibody binding sites. 2,4-Dinitrophenyl glycine, 3-nitrophenyl acetic acid, and 4-nitrophenyl acetic acid were immobilized on the polymer-modified sensor surfaces. We evaluated the suppression ability of the sensor surface for nonspecific adsorption and determined the limit of detection (LOD) for TNT. As a result, a sensor chip suppressed nonspecific adsorption for 1 mg/mL bovine serum albumin and lysozyme. TNT was measured by the displacement method, and the LOD achieved over a 3-nitrophenyl acetic acid-modified sensor chip was 0.9 ng/mL (ppb).

A chemiresistive sensor based on polyaniline (PANI) nanofibers decorated with highly dispersed gold nanoparticles (AuNPs) is developed to detect the volatile sulfur compounds (VSCs) of human expired breath. The PANI nanofibers with a characteristic of horizontal orientation on the insulating gap area of an interdigitated electrode are prepared by a temple-free electrochemical polymerization. The formation of the fine AuNPs on the PANI nanofibers is realized by the redox reaction between HAuCl 4 and PANI in the form of emeraldine. The PANI/AuNPs sensors exhibit good sensing responses to H 2 S and CH 3 SH gases. The sensing ability of the fabricated electrodes on VSCs contained in human breath is confirmed by their response upon exposure to the expired breath of a healthy volunteer after ingesting raw garlic. The developed nanosensors, together with a multichannel sensing system, are expected to be used in breath analysis and disease diagnose related to malodor biomarker gases.

By investigating the electrochemical nucleation and growth of polyaniline (PANI) on the insulating gap area of an interdigitated electrode, a template-free, in-situ approach is developed to obtain PANI nanowires and nanofibers with horizontal orientation on solid substrates. Experimental results show that the deposition process of PANI on the gap area is significantly influenced by polymerization conditions (such as polymerization current and time) and surface characteristics of the substrates (such as hydrophilicity/hydrophobicity and roughness). The concentration of solution-formed oligomers and the nucleation amount of the oligomers on the solid surface determine the morphology and orientation of the nanostructures. Controlling the deposition with high oligomer concentrations and limited nucleation amounts on the substrate is the key to the horizontal orientation. Gas sensing experiments confirm that the horizontal orientation of the nanostructures helps to improve the sensitivity and response time of ...

Quartz crystal microbalance (QCM) gas sensors based on the alternate adsorption of TiO(2) and polyacrilic acid (PAA) were developed for the sensitive detection of amine odors. Individual TiO(2) gel layers could be regularly assembled with a thickness of approximately 0.3 nm by the gas-phase surface sol-gel process (GSSG). The thickness of the poly(acrylic acid) (PAA) layer is dependent on its molecular weight, showing different thicknesses of approximately 0.4 nm for PAA(25) (Mw 250,000) and 0.6-0.8 nm for PAA(400) (Mw 4,000,000). The QCM sensors showed a linear response to ammonia in the concentration range 0.3-15 ppm, depending on the deposition cycle of the alternate TiO(2)/PAA layer. The ammonia binding is based on the acid-base interaction to the free carboxylic acid groups of PAA and the limit of detection (LOD) of the 20-cycle TiO(2)/PAA(400) film was estimated to be 0.1 ppm when exposed to ammonia. The sensor response was very fast and stable in a wide relative humidity (rH) range of 30-70%, showing almost the same frequency changes at a given concentration of ammonia. Sensitivity to n-butylamine and ammonia was higher than to pyridine, which is owing to the difference of molecular weight and basicity of the amine analytes. The alternate TiO(2)/PAA(400) films have a highly effective ability to capture amine odors, and the ambient ammonia concentration of 15 ppm could be condensed up to approximately 20,000 ppm inside the films.

In order to obtain practicable nano-conducting polymer resistive sensors, we investigated the electrochemical deposition of polyaniline (PANI) on the insulating gap area of an interdigitated electrode with a gap width as great as 100 μm. We revealed that the nucleation and growth of PANI on the insulating substrate were influenced by the surface character of the substrate such as hydrophilicity and roughness. By controlling the polymerization conditions, homogeneous PANI films with various nanostructures could be obtained across the insulating gap to form resistive junctions. Among them, a loose 2D nanowire network structure showed the best sensing performance upon exposure to ammonia gas with a low concentration. Keywords: Polyaniline, Nanowire network, Resistive junction, Gas sensor, Electropolymerization

This paper reports the development of membrane sensors based on an artificial lipid and plasticizers with high selectivity and sensitivity to drug bitterness by using bis(1-butylpentyl) adipate (BBPA), bis(2-ethylhexyl) sebacate (BEHS), phosphoric acid tris(2-ethylhexyl) ester (PTEH), and tributyl o-acetylcitrate (TBAC) as a plasticizer and phosphoric acid di-n-decyl ester (PADE) as an artificial lipid to optimize surface hydrophobicity of the sensors. In addition, a sensor with highly correlated bitterness sensory score was developed by blending BBPA and TBAC to detect the bitterness suppression effect of sucrose, and other bitter-masking materials. Therefore, this sensor can be used to evaluate the bitterness of various drug formulations with high accuracy. Copyright © 2009 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.

A surface plasmon resonance (SPR) immunosensor for detection of 2,4-dinitrotoluene (2,4-DNT), which is a signature compound of 2,4,6-trinitrotoluene-related explosives, was developed by using a novel oligo (ethylene glycol) (OEG)-based sensor surface. A rabbit polyclonal antibody against 2,4-DNT (anti-DNPh-KLH-400 antibody) was prepared, and the avidity for 2,4-DNT and recognition capability were investigated by indirect competitive ELISA. The sensor surface was fabricated by immobilizing a 2,4-DNT analog onto an OEG-based self-assembled monolayer formed on a gold surface via an OEG linker. The fabricated surface was characterized by Fourier-transform infrared-refractive absorption spectrometry (FTIR-RAS). The immunosensing of 2,4-DNT is based on the indirect competitive principle, in which the immunoreaction between the anti-DNPh-KLH-400 antibody and 2,4-DNT on the sensor surface was inhibited in the presence of free 2,4-DNT in solution. The limit of detection for the immunosensor, calculated as three times the standard deviation of a blank value, was 20 pg mL(-1), and the linear dynamic range was found to be between 1 and 100 ng mL(-1). Additionally, the fabricated OEG-based surface effectively prevented non-specific adsorption of proteins, and the specific response to anti-DNPh-KLH-400 antibody was maintained for more than 30 measurement cycles.

Taste sensor responds to different taste qualities by unique patterns of output signals. The sensor uses lipid membranes as recognition element of tastes. Food constituents affect the membrane potential of the sensor mainly with shielding effect and/or adsorption. However suitable concentration of lipid in membrane has not been studied up to now. In this paper, we changed the concentration of lipid dioctyl phosphate (C) and trioctylmethyl ammonium chloride (T) in membranes, and investigated the response of the sensor to taste substances. Dilution of lipid improved the detection limit of the sensor to adsorptive taste substances, whereas the detection limit to non-adsorptive taste substances did not remarkably changed.

By a chemical oxidative polymerization of aniline on solid substrates treated with Au nanoparticles (Au-NPs) deposition and 4-aminothiophenol (ATP) modification, polyaniline (PANI) fibers were obtained in micro-/nanosize with two-dimensional network structures. The observation by laser scanning microscopy (LSM) indicated that the formation of PANI fibrous network occurred during the drying process rather than the polymerization. It was suggested the ATP modification on Au-NPs deposited substrate resulted in an inhomogeneous grafting of solution-formed PANI on the solid substrates, which self-aggregated to the fibrous network structure with the evaporation of water due to interchain interactions. The influence of polymerization conditions on the formation of fibrous network and the conducting property of the prepared film were discussed. Results from the gas sensing of the as-prepared PANI sensor upon exposure to NH 3 confirmed that the sensitivity and responding rapidity was improved greatly by the PANI 2D fibrous network structure. Especially, a rapid response in resistance change on NH 3 with low concentration indicated that the developed method in this work can be used to fabricate highly sensitive PANI gas sensors.

A taste sensor is composed of several kinds of lipid/polymer membranes as transducers which convert taste information to electric signal. Thus, the role of membranes is very important to detect various taste components. In this paper, we developed novel membranes which specifically respond to quinine that is typical bitter substances. These membranes were composed of hydrophobic ionic liquid such as N, N, N-trimethyl-N-propylammonium bis(trifluoromethansulfonyl)imide, 1-butyl-3-methylimidazolium hexafluorophosphate and 1-butylpyridinium hexafluorophosphate, a plasticizer, 2-nitrophenyl octyl ether and a polymer, polyvinyl chloride. In addition to quinine, they also showed response to both several kinds of alkaloids such as caffeine and strychnine, and non-alkaloid such as phenylthiocarbamide. The order of these responses was equal to that of the tongue glossopharyngeal nerve of flog. Furthermore, there were the other alkaloids which response to these membranes. Especially in these alkaloids, they showed high response to denatonium benzoate and berberin chloride which have a strong bitter taste.

Simple, automated and cost-effective sensor systems intended for detection of low-molecular-weight (LMW) fragrant compounds are indispensable at automated, bottle-filling production lines of food and beverage industries. Here, we report our investigations on the development of a highly regenerable and storageable surface plasmon resonance (SPR)-based biosensor for detection of trace amounts of benzaldehyde (BZ), a characteristic fragrant compound of peach products. The sensing surface was fabricated by self-assembling of long-chain polyethyleneglycol(PEG)-dialkanethiols on thin Au-films and by followed covalent-binding of a BZ analog on the self-assembly of PEG-dialkanethiols. The all-chemical based BZ-bound PEG-monolayer chip shows specific binding affinity toward anti-BZ antibody (BZ-Ab). The principle of indirect competitive immunoassay, promising highly sensitive and consistent detection of small molecular analytes, has been applied, wherein immunospecific binding of BZ-Ab onto the BZ-bound sensor surface is inhibited by analyte BZ present in test sample. The immunosensor exhibited excellent sensitivity for BZ detection over a wide concentration range of 0.1–80 ng/ml (ppb). The BZ-bound monolayer surface was highly regenerable and storageable for repeated use of a same sensor chip. The assay reproducibility was proved through sequential multiple analysis of the same sensor chip for more than 50 regeneration cycles. Sensitivity was enhanced further by an add-on strategy involving anti-(rabbit IgG)-functionalized Au nanoparticles (nAu-Ab), with which the sensor-signal is enhanced by about 15–18 times and the amount of BZ-Ab antibody required for an analysis could be reduced to one-tenth. With the enhancement technique, detection of as low as 7 pg/ml (ppt) BZ directly from beverage samples has been demonstrated.

A biological olfaction system is one of the highest performance chemical sensing systems for low-molecular-weight compounds. An ability of recognizing a various molecular properties based on their structures is essential for a biological odor recognition process. For the development of a sensor mimicking the olfactory system, we have formed benzene-patterned self-assembled monolayer (SAM) on the sensor surface using SAMs with nanostructure that serves as adsorption sites for benzene ring, which is representative substructure of aromatics. The structure of the benzene-patterned SAM surface was analyzed by contact-angle measurement, ellipsometry, cyclic voltammetry (CV) and atomic force microscopy (AFM). Moreover, the electrodes evaluated as sensor surfaces with cyclic surface-polarization impedance (cSPI) sensor that measures changes in impedance of the electrode surface by adsorption of substances. The results of cSPI indicated the benzene-patterned SAM surface could distinguish benzene by recognizing of the hydrophobicity and the molecular size.

Recent concern on international terrorism and weapons of mass destruction demands the development of novel analytical methods for identification and quantification of explosive molecules. In this article, we describe the development of high-performance immunosensors for detection of 2,4,6-trinitrotoluene (TNT), a prime component of the landmines and bombs used by terrorist and military forces. The immunosensors were constructed by physical adsorption and self-assembly methods, and their binding interactions with a monoclonal anti-TNT antibody were evaluated for TNT detection using the surface plasmon resonance technique. A home-made 2,4,6-trinitrophenyl-keyhole limpet hemocyanine conjugate was used for physical adsorption. A poly(ethylene glycol) hydrazine hydrochloride thiolate was used in the construction of self-assembled monolayer surface and was immobilized with trinitrophenyl-β-alanine by the amide coupling method. The immunosensors were highly selective, regenerable, rapid, and exhibited remarkable...

A taste sensor using lipid-polymer membranes has been developed to evaluate the taste of foods, beverages and medicines. The response of the taste sensor, measured as a change in the membrane potential caused by adsorption (CPA), corresponds to the aftertaste felt by humans. The relationships between the CPA value and the amount of adsorbed taste substances, quinine and iso-α acid (bitterness), and tannic acid (astringency), have been studied so far. However, that of epigallocatechin gallate (EGCg) has not been clarified, although EGCg is abundantly present in green tea as one of its astringent substances. This study aimed at clarifying the response of the taste sensor to EGCg and its relationship with the amount of EGCg adsorbed onto lipid-polymer membranes. The lipid concentration dependence of the CPA value was similar to that of the amount of adsorbed EGCg, indicating a high correlation between the CPA value and the amount of adsorbed EGCg. The CPA value increased with increasing amount of adsorbed EGCg, however, the CPA value showed a tendency of leveling off when the amount of adsorbed EGCg further increased.

The global demands for landmine abolition become stronger, whereas the development of landmine detection technology progresses slowly. At present, the detection activities are mainly performed by means of a metal detector or a well-trained dog that needs much time and cost. Therefore, a more efficient landmine detection technique is desired. Most landmines contain aromatic nitro compounds, such as 2,4-dinitrotoluene (DNT) and 2,4,6-trinitrotoluene (TNT) as explosive charges; therefore, these substances are clues for landmines. In this study, we tried to detect amount of aromatic nitro compounds high-sensitively using a surface-polarization controlling method that measures electrochemical impedance of an electrode surface where explosive compounds adsorbed. As a result, we could detect the aromatic nitro compounds at detection limits of sub-μM level and distinguish the aromatic nitro compounds from other aromatic compounds at 100 μM at least. Furthermore, in order to improve specificity and sensitivity to aromatic nitro compounds, the electrode surface was modified with arenes that makes charge-transfer complex with aromatic nitro compounds. We adopted anthracene as the arene to functionalize the electrode surface. The modified electrode makes it possible to detect the aromatic nitro compounds selectively. This method enables us to develop a sensor for landmine detection.

This paper reports a new measurement method to detect ppb levels of noncharged organic substances using lipid/polymer membrane sensors. Noncharged organic substances have large influences on the adsorption of positively-charged lipids to negatively-charged membranes. Organic solvents (trichloroethylene) and endocrine disrupting chemicals (di-2-ethylhexylphthalate) were detected by utilizing the sensor output, which is the change of membrane potential caused by interactions among the lipid membrane, noncharged organic substances, and lipids in solution. This new potentiometric method has a possibility of detection of a trace amount of noncharged toxic substances.

Monosodium glutamate (MSG) induces a favorable taste, termed umami. The action of MSG on lipid membranes of a taste sensor was investigated. In general, food constituents monotonously increase the electric potential of the negatively charged membranes. Contrary to this, MSG decreases the potential at lower concentration and increases it at higher concentration, thus the effect of MSG on lipid membrane is biphasic. On the decrease of the membrane potential, a model was proposed in which the dissociated carboxyl group of MSG accelerates the dissociation of phosphate group of the membrane lipid.

Mimicking the biological olfaction, large odor-sensor arrays can be used to acquire a broad range of chemical information, with a potentially high degree of redundancy, to allow for enhanced control over the sensitivity and selectivity of artificial olfaction systems. The arrays should consist of the largest possible number of individual sensing elements while being miniaturized. Chemosensitive resistors are one of the sensing platforms that have a potential to satisfy these two conditions. In this work we test viability of fabricating a 16-element chemosensitive resistor array for detection and recognition of volatile organic compounds (VOCs). The sensors were fabricated using blends of carbon black and gas chromatography (GC) stationary-phase materials preselected based on their sorption properties. Blends of the selected GC materials with carbon black particles were subsequently coated over chemosensitive resistor devices and the resulting sensors/arrays evaluated in exposure experiments against vapors of pyrrole, benzenal, nonanal, and 2-phenethylamine at 150, 300, 450, and 900 ppb. Responses of the fabricated 16-element array were stable and differed for each individual odorant sample, proving the blends of GC materials with carbon black particles can be effectively used for fabrication of large odor-sensing arrays based on chemosensitive resistors. The obtained results suggest that the proposed sensing devices could be effective in discriminating odor/vapor samples at the sub-ppm level.

Phospholipids, such as phosphatidic acid, suppress bitter taste without affecting other taste qualities. In the present study, we detected and quantified this suppression effect with an electronic tongue whose transducer is composed of several kinds of lipid/polymer membranes with different characteristics. We measured a phospholipid cocktail and various kinds of taste substances with five basic taste qualities. The responses to quinine hydrochloride and L-tryptophan, which have a bitter taste, were reduced as the phospholipid concentration was increased, and the responses to the other taste substances were not affected by the phospholipids, as with the human sensation test. Furthermore, the change of bitter intensity caused by phospholipid was quantified by principal component analysis and the tau scale, which expresses the relationship between taste intensity and taste substance concentration. The results are compared with those of the human sensory test and discussed.

It has been noticed recently that there is a possibility of elution of chemical substances from packages of foods. The chemicals act as harmful substances such as environmental hormones for biological bodies at very low concentration. It is therefore necessary to develop a sensing device, which can detect such substances easily and quickly. The purpose of the present study is high sensitive measurement of environmental hormones using electrode polarity controlling method. The responding ability of the polarity-controlled sensor covers various chemicals from electrolytes to nonelectrolytes and from hydrophilic to hydrophobic substances. As a result, we could sensitively detect bisphenol-A at about 50ppb and dibutyl phthalate at about 3ppb, and the sensor could distinguish such chemicals from ordinary taste substances.

A chloride ion-selective solvent polymeric membrane electrode (Cl-ISE) using quaternary ammonium salts for determination of the ethanol concentration during sake fermentation is described. The ethanol response by a Cl-ISE is based on the change in the transfer activity coefficient of Cl- from water to ethanol. The logarithm value of the transfer activity coefficient increases linearly with increasing ethanol concentration. The Cl-ISE using a mixture of o-nitro-phenyloctyl ether and n-tetradecylalcohol as plasticizers and tetradodecylammonium salts as sensing materials showed good linearities for ethanol (−2.39 mV/%) and for Cl- (−53.6 mV/decade). Furthermore, the interference from co-existent components in sake mash with the response of the Cl-ISE was improved in comparison with that using other plasticizers and sensing materials. The Cl-ISE was found to show good accuracy in the measurement of ethanol in sake mash.

The present study investigates adsorption of taste substances to lipid membranes for the multichannel taste sensor. To clarify the mechanism of adsorption of taste substances to a lipid membrane, we measured adsorption quantities of taste substances, such as amino acids and quinine-HCl, with fluorometry, and the surface of the lipid membrane was analyzed with ESCA (Electron spectroscopy for chemical analysis). Furthermore, we measured potential responses of a lipid membrane and pH of monosodium glutamate and lipid solutions. The results showed that amino acids adsorb to the lipid membrane, but no specific adsorption site exists on the membrane. The adsorption occurred by weak interaction between a phosphoric acid group of lipid molecules and an amino group of amino acids. Quinine-HCl adsorbed to membrane with strong electrostatic coupling and weak hydrophobic effects to lipid molecules in the membrane.

An ethanol sensor using two chloride ion-selective polymeric membrane electrodes (Cl-ISEs) was developed. Two different polymeric membranes were used, one with o-nitro-phenyloctyl ether (NPOE) and the other with n-decylalcohol (DA) as the plasticizer. The electrical potential of the Cl-ISE fell linearly with increasing ethanol concentration at a given concentration of Cl-. The electrical potential equation is viewed as a function of the concentration of ethanol and Cl-. The responses to ethanol and Cl- differed between plasticizers. The two Cl-ISEs showed nearly Nernstean slopes for Cl- in ethanol solution (0-20%). In addition, each electrode had good linearity for ethanol with a slope of −2.51 mV/ethanol % (NPOE) and −1.01 mV/ethanol% (DA), respectively. The concentration of ethanol and Cl- can be estimated by solving the simultaneous equations derived from the electrical potentials of the two different Cl-ISEs. To decrease the interference from co-existing components, KCl solution was added to the sample until the total concentration was 30 mM. The ethanol sensor using the two Cl-ISEs showed good accuracy in the measurement of ethanol in sake and shochu.

This paper describes an application of taste sensor for detection of cyanide. The taste sensor consists of multichannel sensors with lipid membranes which are modeled on living organisms. One of characteristics of this sensor is a global selectivity. In this study, we have developed a new effective method which is able to control the charge of membranes for high sensitivity and selectivity of lipid membrane sensors. Using this method, sensors for free cyanide and complex cyanide were developed. We could detect total cyanide under the guideline limit. It was possible to detect free cyanide below the concentration 0.5ppm in river water. Measurement time is within five minutes. In conclusion, the taste sensor has a very powerful performance and advantage for real time monitoring of water quality.

Response mechanism of a membrane made of trioctylmethylammonium chloride (TOMA) to ethanol was studied. The response of the TOMA membrane to ethanol increased linearly with increasing ethanol concentration (0-20%) at constant KCl concentration. The slope of response to ethanol was-1.73 (mV/ethanol%) using the multiple regression analysis (0-20% ethanol, 1-100 mMKCI). The value of this slope calculated using the transfer activity coefficient of chloride ion from water to ethanol showed a good agreement with the experimental value. On the other hand, the results obtained in AgCl and anion exchange membrane disagreed with the theoretical values.

Impedance changes in phospholipid LB (Langmuir-Blodgett) and protein-mixed phospholipid LB films by several taste substances were measured. The resistance of the phospholipid LB film increased by bitter and sweet substances, whereas the resistance decreased for salty, sour and umami substances. For sweetness the larger change was observed in the protein-mixed phospholipid LB film, and hence discrimination between sweetness and bitterness was possible.

Interactions between a lipid-protein membrane and taste substances were investigated using SPR (surface plasmon resonance). The used membrane was the LB (Langmuir-Blodgett) multi-layered film composed of DHP (dihexadecyl phosphate) and the sugar-binding protein ConA. The LB membrane including ConA showed larger responses to almost all the taste substances, such as quinine, caffeine, sucrose and NaCl than the LB membrane without ConA. Suggestion was also made on the interactions of astringent and pungent substances with the charged lipid LB membrane.

We have developed a new portable taste sensor with a lipid/polymer membrane and conducted experiments to evaluate the sensor’s performance. The fabricated sensor consists of a taste sensor chip (40 mm × 26 mm × 2.2 mm) with working and reference electrodes and a portable sensor device (80 mm × 25 mm × 20 mm). The working electrode consists of a taste-sensing site comprising a poly(hydroxyethyl)methacrylate (pHEMA) hydrogel layer with KCl as the electrolyte layer and a lipid/polymer membrane as the taste sensing element. The reference electrode comprises a polyvinyl chloride (PVC) membrane layer with a small hole and a pHEMA layer with KCl. The whole device is the size of a USB memory stick, making it suitable for portable use. The sensor’s response to tannic acid as the standard astringency substance showed good accuracy and reproducibility, and was comparable with the performance of a commercially available taste sensing system. Thus, it is possible for this sensor to be used for in-field evaluations and it can make a significant contribution to the food industry, as well as in various fields of research.