Discrimination of Umami Tastants Using Floating Electrode-Based Bioelectronic Tongue Mimicking Insect Taste Systems

Bioelectronic Tongues Mimicking Insect Taste Systems for Real-Time Discrimination between Natural and Artificial Sweeteners

A bioelectronic tongue (B-ET) mimicking insect taste systems is developed for the real-time detection and discrimination of natural and artificial sweeteners. Here, a carbon nanotube field-effect transistor (CNT-FET) was hybridized with nanovesicles including the honeybee sugar taste receptor, gustatory receptor 1 of Apis mellifera (AmGr1). This strategy allowed us to detect glucose, a major component of nectar, down to 100 fM in real time and identify sweet tastants from other tastants. It could also be utilized for the detection of glucose in dextrose tablet solutions. Importantly, we demonstrated the discrimination between natural and artificial sweeteners down to 10 pM even in real beverages such as decaffeinated coffee using our hybrid platform. In this respect, our B-ET mimicking insect taste systems can be a powerful tool for various applications such as food screening and basic studies on insect taste systems.

Bioelectrical Nose Platform Using Odorant-Binding Protein as a Molecular Transporter Mimicking Human Mucosa for Direct Gas Sensing

Recently, various bioelectronic nose devices based on human receptors were developed for mimicking a human olfactory system. However, such bioelectronic nose devices could operate in an aqueous solution, and it was often very difficult to detect insoluble gas odorants. Here, we report a portable bioelectronic nose platform utilizing a receptor protein-based bioelectronic nose device as a sensor and odorant-binding protein (OBP) as a transporter for insoluble gas molecules in a solution, mimicking the functionality of human mucosa. Our bioelectronic nose platform based on I7 receptor exhibited dose-dependent responses to octanal gas in real time. Furthermore, the bioelectronic platforms with OBP exhibited the sensor sensitivity improved by ∼100% compared with those without OBP. We also demonstrated the detection of odorant gas from real orange juice and found that the electrical responses of the devices with OBP were much larger than those without OBP. Since our bioelectronic nose platform allows us to directly detect gas-phase odorant molecules including a rather insoluble species, it could be a powerful tool for versatile applications and basic research based on a bioelectronic nose.

Progress in the Development of Detection Strategies Based on Olfactory and Gustatory Biomimetic Biosensors

The biomimetic olfactory and gustatory biosensing devices have broad applications in many fields, such as industry, security, and biomedicine. The development of these biosensors was inspired by the organization of biological olfactory and gustatory systems. In this review, we summarized the most recent advances in the development of detection strategies for chemical sensing based on olfactory and gustatory biomimetic biosensors. First, sensing mechanisms and principles of olfaction and gustation are briefly introduced. Then, different biomimetic sensing detection strategies are outlined based on different sensing devices functionalized with various molecular and cellular components originating from natural olfactory and gustatory systems. Thereafter, various biomimetic olfactory and gustatory biosensors are introduced in detail by classifying and summarizing the detection strategies based on different sensing devices. Finally, the future directions and challenges of biomimetic biosensing development are proposed and discussed.

Evaluation of site-selective drug effects on GABA receptors using nanovesicle-carbon nanotube hybrid devices

Site-selective drug effects on the ion-channel activities of γ-aminobutyric acid type A (GABA_A) receptors are evaluated by using a nanovesicle-carbon nanotube hybrid device. Here, nanovesicles containing GABA_A receptors are immobilized on the channel region of a carbon nanotube field-effect transistor. The receptor responses of this hybrid device to GABA are detected with a high sensitivity down to ∼1 aM even in the presence of other neurotransmitters. Further, sensitivity differences between two GABA_A-receptor-subunit compositions of α5β2γ2 and α1β2γ2 are assessed by normalizing the dose-dependent responses obtained from these hybrid devices. Specifically, the GABA concentration that produces 50% of maximal response (EC₅₀) is obtained as ∼10 pM for α5β2γ2 subunits and ∼1 nM for α1β2γ2 subunits of GABA_A receptor. Significantly, the potency profiles of both antagonist and agonist of GABA_A receptor can be evaluated by analyzing EC₅₀ values in the presence and absence of those drugs. A competitive antagonist increases the EC₅₀ value of GABA by binding to the same site as GABA, while an allosteric agonist reduces it by binding to a different site. These results indicate that this hybrid device can be a powerful tool for the evaluation of candidate drug substances modulating GABA-mediated neurotransmission.

Self-healable and anti-freezing ion conducting hydrogel-based artificial bioelectronic tongue sensing toward astringent and bitter tastes

Numerous efforts have been attempted to mimic human tongue since years. However, they still have limitations because of damages, temperature effects, detection ranges etc. Herein, a self-healable hydrogel-based artificial bioelectronic tongue (E-tongue) containing mucin as a secreted protein, sodium chloride as an ion transporting electrolyte, and chitosan/poly(acrylamide-co-acrylic acid) as the main 3D structure holding hydrogel network is synthesized. This E-tongue is introduced to mimic astringent and bitter mouth feel based on cyclic voltammetry (CV) measurements subjected to target substances, which permits astringent tannic acid (TA) and bitter quinine sulfate (QS) to be detected over wide corresponding ranges of 29.3 mM-0.59 μM and 63.8 mM-6.38 μM with remarkable respective sensitivities of 0.2 and 0.12 wt%^-1. Besides, the taste selectivity of this E-tongue is performed in the presence of various mixed-taste chemicals to show its high selective behavior toward bitter and astringent chemicals. The electrical self-healability is shown via CV responses to illustrate electrical recovery within a short time span. In addition, cytotoxicity tests using HeLa cells are performed, where a clear viability of ≥95% verified its biocompatibility. The anti-freezing sensing of E-tongue tastes at -5 °C also makes this work to be useful at sub-zero environments. Real time degrees of tastes are detected using beverages and fruits to confirm future potential applications in food taste detections and humanoid robots.

Human-like performance umami electrochemical biosensor by utilizing co-electrodeposition of ligand binding domain T1R1-VFT and Prussian blue

Over the past decades, due to the desire for artificial umami flavors, apparatuses for detecting the umami taste have constantly been developed. Nevertheless, most information on umami is still acquired through human sensory assessment, which makes it difficult to establish an umami standard or quantify the umami flavor. In this study, the ligand binding domain called venus flytrap (VFT) domain of the umami taste receptor protein T1R1 was used as a recognition element, and an electrochemical biosensor based on a double-signal amplification strategy was constructed using single-walled carbon nanotubes (SWCNTs) and Prussian blue (PB). Moreover, the umami taste of four representative umami substances, inosine-5'-monophosphate (IMP), monosodium L-glutamate (MSG), beefy meaty peptide (BMP), and sodium succinate (WSA), were successfully quantitatively measured using differential pulse voltammetry (DPV) at an electrochemical workstation. Based on an equation (S/N = 3), the low detection limits (LODs) of IMP, MSG, BMP, and WSA were 0.1, 0.1, 0.1, and 0.01 pM, respectively. Meanwhile, a normalized signal intensity of more than 90% was kept for 4 days. The results showed that the biosensor could be used to detect umami substances with high sensitivity and selectivity, and was shown to have human-like performance. To develop the T1R1-VFT biosensor using the above-mentioned method, we utilized the ligand binding domain of the human umami receptor, rather than the entire umami receptor protein, which had a complex structure, having the following advantages: volume reduction, simplicity, and stability. This method has great potential for the detection of umami tastes, instead of using sensory evaluation, and for the development of new artificial flavorings.

Tropism of Extracellular Vesicles and Cell-Derived Nanovesicles to Normal and Cancer Cells: New Perspectives in Tumor-Targeted Nucleic Acid Delivery

The main advantage of extracellular vesicles (EVs) as a drug carrier system is their low immunogenicity and internalization by mammalian cells. EVs are often considered a cell-specific delivery system, but the production of preparative amounts of EVs for therapeutic applications is challenging due to their laborious isolation and purification procedures. Alternatively, mimetic vesicles prepared from the cellular plasma membrane can be used in the same way as natural EVs. For example, a cytoskeleton-destabilizing agent, such as cytochalasin B, allows the preparation of membrane vesicles by a series of centrifugations. Here, we prepared cytochalasin-B-inducible nanovesicles (CINVs) of various cellular origins and studied their tropism in different mammalian cells. We observed that CINVs derived from human endometrial mesenchymal stem cells exhibited an enhanced affinity to epithelial cancer cells compared to myeloid, lymphoid or neuroblastoma cancer cells. The dendritic cell-derived CINVs were taken up by all studied cell lines with a similar efficiency that differed from the behavior of DC-derived EVs. The ability of cancer cells to internalize CINVs was mainly determined by the properties of recipient cells, and the cellular origin of CINVs was less important. In addition, receptor-mediated interactions were shown to be necessary for the efficient uptake of CINVs. We found that CINVs, derived from late apoptotic/necrotic cells (aCINVs) are internalized by in myelogenous (K562) 10-fold more efficiently than CINVs, and interact much less efficiently with melanocytic (B16) or epithelial (KB-3-1) cancer cells. Finally, we found that CINVs caused a temporal and reversible drop of the rate of cell division, which restored to the level of control cells with a 24 h delay.

Biohybrid sensor for odor detection

Biohybrid odorant sensors that directly integrate a biological olfactory system have been increasingly studied and are suggested to be the next generation of ultrasensitive sensors by taking advantage of the sensitivity and selectivity of living organisms. In this review, we provide a detailed description of the recent developments of biohybrid odorant sensors, especially considering the requisites for their perspective of on-site applications. We introduce the methodologies to effectively capture the biological signals from olfactory systems by readout devices, and describe the essential properties regarding the gaseous detection, stability, quality control, and portability. Moreover, we address the recent progress on multiple odorant recognition using multiple sensors as well as the current screening approaches for pairs of orphan receptors and ligands necessary for the extension of the currently available range of biohybrid sensors. Finally, we discuss our perspectives for the future for the development of practical odorant sensors.

Bioelectronic Skin Based on Nociceptive Ion Channel for Human-Like Perception of Cold Pains

A bioelectronic skin device based on nociceptive ion channels in nanovesicles is developed for the detection of chemical cold-pain stimuli and cold environments just like human somesthetic sensory systems. The human transient receptor potential ankyrin 1 (hTRPA1) is involved in transmission and modulation of cold-pain sensations. In the bioelectronic skin, the nanovesicles containing the hTRPA1 nociceptive ion channel protein reacts to cold-pain stimuli, and it is electrically monitored through carbon nanotube transistor devices based on floating electrodes. The bioelectronic skin devices sensitively detect chemical cold-pain stimuli like cinnamaldehyde at 10 fm, and selectively discriminate cinnamaldehyde among other chemical stimuli. Further, the bioelectronic skin is used to evaluate the effect of cold environments on the response of the hTRPA1, finding that the nociceptive ion channel responds more sensitively to cinnamaldehyde at lower temperatures than at higher temperatures. The bioelectronic skin device could be useful for a basic study on somesthetic systems such as cold-pain sensation, and should be used for versatile applications such as screening of foods and drugs.

Micelle-stabilized Olfactory Receptors for a Bioelectronic Nose Detecting Butter Flavors in Real Fermented Alcoholic Beverages

A bioelectronic nose device based on micelle-stabilized olfactory receptors is developed for the selective discrimination of a butter flavor substance in commercial fermented alcoholic beverages. In this work, we have successfully overexpressed ODR-10, a type of olfactory receptor, from Caenorhabditis elegans using a bacterial expression system at a low cost and high productivity. The highly-purified ODR-10 was stabilized in micelle structures, and it was immobilized on a carbon nanotube field-effect transistor to build a bioelectronic nose for the detection of diacetyl, a butter flavor substance, via the specific interaction between diacetyl and ODR-10. The bioelectronic nose device can sensitively detect diacetyl down to 10 fM, and selectively discriminate it from other substances. In addition, this sensor could directly evaluate diacetyl levels in a variety of real fermented alcoholic beverages such as beer, wine, and makgeolli (fermented Korean wine), while the sensor did not respond to soju (Korean style liquor without diacetyl). In this respect, our sensor should be a powerful tool for versatile food industrial applications such as the quality control of alcoholic beverages and foods.

Bioelectronic sensor mimicking the human neuroendocrine system for the detection of hypothalamic-pituitary-adrenal axis hormones in human blood

In the neuroendocrine system, corticotropin-releasing hormone (CRH) and adrenocorticotropic hormone (ACTH) play important roles in the regulation of the hypothalamic-pituitary-adrenal (HPA) system. Disorders of the HPA system lead to physiological problems, such as Addison's disease and Cushing's syndrome. Therefore, detection of CRH and ACTH is essential for diagnosing disorders related to the HPA system. Herein, receptors of the HPA axis were used to construct a bioelectronic sensor system for the detection of CRH and ACTH. The CRH receptor, corticotropin-releasing hormone receptor 1 (CRHR1), and the ACTH receptor, melanocortin 2 receptor (MC2R), were produced using an Escherichia coli expression system, and were reconstituted using nanodisc (ND) technology. The receptor-embedded NDs were immobilized on a floating electrode of a carbon nanotube field-effect transistor (CNT-FET). The constructed sensors sensitively detected CRH and ACTH to a concentration of 1 fM with high selectivity in real time. Furthermore, the reliable detection of CRH and ACTH in human plasma by the developed sensors demonstrated their potential in clinical and practical applications. These results indicate that CRHR1 and MC2R-based bioelectronic sensors can be applied for rapid and efficient detection of CRH and ACTH.

Nafion-Radical Hybrid Films on Carbon Nanotube Transistors for Monitoring Antipsychotic Drug Effects on Stimulated Dopamine Release

We developed floating electrode-based carbon nanotube biosensors for the monitoring of antipsychotic drug effects on the dopamine release from PC12 cells under potassium stimulation. Here, carbon nanotube field-effect transistors with floating electrodes were functionalized with 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS^•) radicals by Nafion films. This method allows us to build selective biosensors for dopamine detection with a detection limit down to 10 nM even in the presence of other neurotransmitters such as glutamate and acetylcholine, resulting from the selective interaction between ABTS^• radicals and dopamine. The sensors were also utilized to monitor the real-time release of dopamine from PC12 cells upon the stimulation of high-concentrated potassium solutions. Significantly, the antipsychotic effects of pimozide on the dopamine release from potassium-stimulated PC12 cells could also be evaluated in a concentration-dependent manner by using the sensors. The quantitative and real-time evaluation capability of our strategy should provide a versatile tool for many biomedical studies and applications.

Carbon Nanotube Chemical Sensors

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Preliminary research on the receptor–ligand recognition mechanism of umami by an hT1R1 biosensor

The biosensor reflecting the linkage of the umami signal during conduction and amplification, and the study on the receptor–ligand recognition mechanism.

Human-like smelling of a rose scent using an olfactory receptor nanodisc-based bioelectronic nose

We report a strategy for the human-like smelling of a rose scent utilizing olfactory receptor nanodisc (ND)-based bioelectronic nose devices. In this strategy, a floating electrode (FE)-based carbon nanotube (CNT) field effect transistor (FET) was functionalized with human olfactory receptor 1A2 (hOR1A2)-embedded NDs (hOR1A2NDs). The hOR1A2NDs responded to rose scent molecules specifically, which were monitored electrically using the underlying CNT-FET. This strategy allowed us to quantitatively assess the contents of geraniol and citronellol, the main components of a rose scent, as low as 1 fM and 10 fM, respectively. In addition, it enabled us to selectively discriminate a specific rose odorant from other odorants. Significantly, we also demonstrated that the responses of hOR1A2NDs to a rose scent could be strongly enhanced by enhancer materials like a human nose. Furthermore, the method provided a means to quantitatively evaluate rose scent components in real samples such as rose oil. Since our method allows one to quantitatively evaluate general rose scent ingredients just like a human nose, it could be a powerful strategy for versatile basic research and various applications such as fragrance development.

Progress in the development of olfactory-based bioelectronic chemosensors

Artificial chemosensory devices have a wide range of applications in industry, security, and medicine. The development of these devices has been inspired by the speed, sensitivity, and selectivity by which the olfactory system in animals can probe the chemical nature of the environment. In this review, we examine how molecular and cellular components of natural olfactory systems have been incorporated into artificial chemosensors, or bioelectronic sensors. We focus on the biological material that has been combined with signal transduction systems to develop artificial chemosensory devices. The strengths and limitations of different biological chemosensory material at the heart of these devices, as well as the reported overall effectiveness of the different bioelectronic sensor designs, is examined. This review also discusses future directions and challenges for continuing to advance development of bioelectronic sensors.

Magnetically-focusing biochip structures for high-speed active biosensing with improved selectivity

We report a magnetically-focusing biochip structure enabling a single layered magnetic trap-and-release cycle for biosensors with an improved detection speed and selectivity. Here, magnetic beads functionalized with specific receptor molecules were utilized to trap target molecules in a solution and transport actively to and away from the sensor surfaces to enhance the detection speed and reduce the non-specific bindings, respectively. Using our method, we demonstrated the high speed detection of IL-13 antigens with the improved detection speed by more than an order of magnitude. Furthermore, the release step in our method was found to reduce the non-specific bindings and improve the selectivity and sensitivity of biosensors. This method is a simple but powerful strategy and should open up various applications such as ultra-fast biosensors for point-of-care services.

High-Speed Lateral Flow Strategy for a Fast Biosensing with an Improved Selectivity and Binding Affinity

We report a high-speed lateral flow strategy for a fast biosensing with an improved selectivity and binding affinity even under harsh conditions. In this strategy, biosensors were fixed at a location away from the center of a round shape disk, and the disk was rotated to create the lateral flow of a target solution on the biosensors during the sensing measurements. Experimental results using the strategy showed high reaction speeds, high binding affinity, and low nonspecific adsorptions of target molecules to biosensors. Furthermore, binding affinity between target molecules and sensing molecules was enhanced even in harsh conditions such as low pH and low ionic strength conditions. These results show that the strategy can improve the performance of conventional biosensors by generating high-speed lateral flows on a biosensor surface. Therefore, our strategy can be utilized as a simple but powerful tool for versatile bio and medical applications.

Bioelectronic Nose Using Olfactory Receptor-Embedded Nanodiscs

Olfactory receptors (ORs) are the largest family of the G protein-coupled receptors (GPCRs), which are significantly involved in many human diseases and 40% of all drug targets. A platform containing stable and high-quality OR would be a powerful tool for the development of a practical biosensor that can be applied to various applications, such as the early diagnosis of diseases, assessment of food quality, and drug and fragrance development. Significant efforts have been made to develop the biosensor using GPCRs; nevertheless, they remain a challenge. This chapter describes an attractive methodology for the development of a stable bioelectronic nose using OR-embedded nanodiscs. The ORs were produced in Escherichia coli (E. coli), purified with column chromatography, reconstituted into nanodiscs and applied to a carbon nanotube-field effect transistor (CNT-FET) with floating electrodes. The nanodisc-based bioelectronic nose exhibits high-performance in terms of sensitivity, selectivity and stability. This strategy can be used as a practical method for the receptor-based sensing approach, which represents significant progress in nano-bio technology toward a practical biosensor.

The bioelectronic nose and tongue using olfactory and taste receptors: Analytical tools for food quality and safety assessment

Food intake is the primary method for obtaining energy and component materials in the human being. Humans evaluate the quality of food by combining various facets of information, such as an item of food's appearance, smell, taste, and texture in the mouth. Recently, bioelectronic noses and tongues have been reported that use human olfactory and taste receptors as primary recognition elements, and nanoelectronics as secondary signal transducers. Bioelectronic sensors that mimic human olfaction and gustation have sensitively and selectively detected odor and taste molecules from various food samples, and have been applied to food quality assessment. The portable and multiplexed bioelectronic nose and tongue are expected to be used as next-generation analytical tools for rapid on-site monitoring of food quality. In this review, we summarize recent progress in the bioelectronic nose and tongue using olfactory and taste receptors, and discuss the potential applications and future perspectives in the food industry.

Nanodisc-Based Bioelectronic Nose Using Olfactory Receptor Produced in Escherichia coli for the Assessment of the Death-Associated Odor Cadaverine

Cadaverine (CV), a death-associated odor, is an important target molecule for various sensor applications, including the evaluation of food spoilage. In this study, we developed an oriented nanodisc (ND)-functionalized bioelectronic nose (ONBN), based on carbon nanotube transistors and nanodiscs embedded with an olfactory receptor produced in Escherichia coli (E. coli) for detection of CV. To fabricate ONBN devices, a trace-amine-associated receptor 13c (TAAR13c) binding to CV was produced in E. coli, purified, reconstituted into NDs, and assembled, in the desired orientation, onto a carbon- nanotube-based field-effect transistor with floating electrodes. The ONBN showed high performance in terms of sensitivity and selectivity. Moreover, the ONBN was used to measure CV in diverse real-food samples for the determination of food freshness. These results indicate ONBN devices can be utilized to evaluate the quality of food samples quantitatively, which should enable versatile practical applications such as food safety and preservative development. Moreover, the ONBN could provide a useful tool for detection of corpses, which could be practically used in disaster responses.

Nanoscale hybrid systems based on carbon nanotubes for biological sensing and control

This paper provides a concise review on the recent development of nanoscale hybrid systems based on carbon nanotubes (CNTs) for biological sensing and control. CNT-based hybrid systems have been intensively studied for versatile applications of biological interfaces such as sensing, cell therapy and tissue regeneration. Recent advances in nanobiotechnology not only enable the fabrication of highly sensitive biosensors at nanoscale but also allow the applications in the controls of cell growth and differentiation. This review describes the fabrication methods of such CNT-based hybrid systems and their applications in biosensing and cell controls.