Real-time monitoring of odorant-induced cellular reactions using surface plasmon resonance

Biosensors for Odor Detection: A Review

Animals can easily detect hundreds of thousands of odors in the environment with high sensitivity and selectivity. With the progress of biological olfactory research, scientists have extracted multiple biomaterials and integrated them with different transducers thus generating numerous biosensors. Those biosensors inherit the sensing ability of living organisms and present excellent detection performance. In this paper, we mainly introduce odor biosensors based on substances from animal olfactory systems. Several instances of organ/tissue-based, cell-based, and protein-based biosensors are described and compared. Furthermore, we list some other biological materials such as peptide, nanovesicle, enzyme, and aptamer that are also utilized in odor biosensors. In addition, we illustrate the further developments of odor biosensors.

The convergence of traditional and digital biomarkers through AI-assisted biosensing: A new era in translational diagnostics?

Advances in consumer electronics, alongside the fields of microfluidics and nanotechnology have brought to the fore low-cost wearable/portable smart devices. Although numerous smart devices that track digital biomarkers have been successfully translated from bench-to-bedside, only a few follow the same fate when it comes to track traditional biomarkers. Current practices still involve laboratory-based tests, followed by blood collection, conducted in a clinical setting as they require trained personnel and specialized equipment. In fact, real-time, passive/active and robust sensing of physiological and behavioural data from patients that can feed artificial intelligence (AI)-based models can significantly improve decision-making, diagnosis and treatment at the point-of-procedure, by circumventing conventional methods of sampling, and in person investigation by expert pathologists, who are scarce in developing countries. This review brings together conventional and digital biomarker sensing through portable and autonomous miniaturized devices. We first summarise the technological advances in each field vs the current clinical practices and we conclude by merging the two worlds of traditional and digital biomarkers through AI/ML technologies to improve patient diagnosis and treatment. The fundamental role, limitations and prospects of AI in realizing this potential and enhancing the existing technologies to facilitate the development and clinical translation of "point-of-care" (POC) diagnostics is finally showcased.

Advances in the Production of Olfactory Receptors for Industrial Use

In biological olfactory systems, olfactory receptors (ORs) can recognize and discriminate between thousands of volatile organic compounds with very high sensitivity and specificity. The superior properties of ORs have led to the development of OR-based biosensors that have shown promising potential in many applications over the past two decades. In particular, newly designed technologies in gene synthesis, protein expression, solubilization, purification, and membrane mimetics for membrane proteins have greatly opened up the previously inaccessible industrial potential of ORs. In this review, gene design, expression and solubilization strategies, and purification and reconstitution methods available for modern industrial applications are examined, with a focus on ORs. The limitations of current OR production technology are also estimated, and future directions for further progress are suggested.

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.

An Overview of Artificial Olfaction Systems with a Focus on Surface Plasmon Resonance for the Analysis of Volatile Organic Compounds

The last three decades have witnessed an increasing demand for novel analytical tools for the analysis of gases including odorants and volatile organic compounds (VOCs) in various domains. Traditional techniques such as gas chromatography coupled with mass spectrometry, although very efficient, present several drawbacks. Such a context has incited the research and industrial communities to work on the development of alternative technologies such as artificial olfaction systems, including gas sensors, olfactory biosensors and electronic noses (eNs). A wide variety of these systems have been designed using chemiresistive, electrochemical, acoustic or optical transducers. Among optical transduction systems, surface plasmon resonance (SPR) has been extensively studied thanks to its attractive features (high sensitivity, label free, real-time measurements). In this paper, we present an overview of the advances in the development of artificial olfaction systems with a focus on their development based on propagating SPR with different coupling configurations, including prism coupler, wave guide, and grating.

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.

Discovery of Orphan Olfactory Receptor 6M1 as a New Anticancer Target in MCF-7 Cells by a Combination of Surface Plasmon Resonance-Based and Cell-Based Systems

Olfactory receptors (ORs) account for 49% of all G protein-coupled receptors (GPCRs), which are important targets for drug discovery, and hence ORs may also be potential drug targets. Various ORs are expressed in breast cancer cells; however, most of them are orphan receptors, and thus, their functions are unknown. Herein, we present an experimental strategy using a surface plasmon resonance (SPR) system and a cell-based assay that allowed the identification of orphan OR6M1 as a new anticancer target in the MCF-7 breast cancer cell line. After the construction of stable OR6M1-expressing cells, the SPR-based screening of 108 chemicals for ligand activity was performed against OR6M1-expressing whole cells (primary screening) or membrane fragments (secondary screening). As a result, anthraquinone (AQ) and rutin were discovered to be new OR6M1 ligands. Based on calcium imaging in OR6M1-expressing Hana3A cells, AQ and rutin were classified as an OR6M1 agonist and antagonist, respectively. Cell viability and live/dead assays showed that AQ induced the death of MCF-7 cells, which was inhibited by rutin. Therefore, OR6M1 may be considered an anticancer target, and AQ may be considered a chemotherapeutic agent. This combined method can be widely used to discover the ligands and functions of other orphan GPCRs.

Emergent Biosensing Technologies Based on Fluorescence Spectroscopy and Surface Plasmon Resonance

The purpose of this work is to provide an exhaustive overview of the emerging biosensor technologies for the detection of analytes of interest for food, environment, security, and health. Over the years, biosensors have acquired increasing importance in a wide range of applications due to synergistic studies of various scientific disciplines, determining their great commercial potential and revealing how nanotechnology and biotechnology can be strictly connected. In the present scenario, biosensors have increased their detection limit and sensitivity unthinkable until a few years ago. The most widely used biosensors are optical-based devices such as surface plasmon resonance (SPR)-based biosensors and fluorescence-based biosensors. Here, we will review them by highlighting how the progress in their design and development could impact our daily life.

2D Nanomaterial-Based Surface Plasmon Resonance Sensors for Biosensing Applications

The absorption and binding energy of material plays an important role with a large surface area and conductivity for the development of any sensing device. The newly grown 2D nanomaterials like black phosphorus transition metal dichalcogenides (TMDCs) or graphene have excellent properties for sensing devices' fabrication. This paper summarizes the progress in the area of the 2D nanomaterial-based surface plasmon resonance (SPR) sensor during last decade. The paper also focuses on the structure of Kretschmann configuration, the sensing principle of SPR, its characteristic parameters, application in various fields, and some important recent works related to SPR sensors have also been discussed, based on the present and future scope of this field. The present paper provides a platform for researchers to work in the field of 2D nanomaterial-based SPR sensors.

The Convergence of Cell-Based Surface Plasmon Resonance and Biomaterials: The Future of Quantifying Bio-molecular Interactions-A Review

Cell biology is driven by complex networks of biomolecular interactions. Characterizing the kinetic and thermodynamic properties of these interactions is crucial to understanding their role in different physiological processes. Surface plasmon resonance (SPR)-based approaches have become a key tool in quantifying biomolecular interactions, however conventional approaches require isolating the interacting components from the cellular system. Cell-based SPR approaches have recently emerged, promising to enable precise measurements of biomolecular interactions within their normal biological context. Two major approaches have been developed, offering their own advantages and limitations. These approaches currently lack a systematic exploration of 'best practices' like those existing for traditional SPR experiments. Toward this end, we describe the two major approaches, and identify the experimental parameters that require exploration, and discuss the experimental considerations constraining the optimization of each. In particular, we discuss the requirements of future biomaterial development needed to advance the cell-based SPR technique.

Peptide hormone sensors using human hormone receptor-carrying nanovesicles and graphene FETs

Hormones within very low levels regulate and control the activity of specific cells and organs of the human body. Hormone imbalance can cause many diseases. Therefore, hormone detection tools have been developed, particularly over the last decade. Peptide hormones have a short half-life, so it is important to detect them within a short time. In this study, we report two types of peptide hormone sensors using human hormone receptor-carrying nanovesicles and graphene field-effect transistors (FETs). Parathyroid hormone (PTH) and glucagon (GCG) are peptide hormones present in human blood that act as ligands to G protein-coupled receptors (GPCRs). In this paper, the parathyroid hormone receptor (PTHR) and the glucagon receptor (GCGR) were expressed in human embryonic kidney-293 (HEK-293) cells, and were constructed as nanovesicles carrying the respective receptors. They were then immobilized onto graphene-based FETs. The two hormone sensors developed were able to detect each target hormone with high sensitivity (ca. 100 fM of PTH and 1 pM of GCG). Also, the sensors accurately recognized target hormones among different types of peptide hormones. In the development of hormone detection tools, this approach, using human hormone receptor-carrying nanovesicles and graphene FETs, offers the possibility of detecting very low concentrations of hormones in real-time.

Membrane protein-based biosensors

This review highlights recent development of biosensors that use the functions of membrane proteins. Membrane proteins are essential components of biological membranes and have a central role in detection of various environmental stimuli such as olfaction and gustation. A number of studies have attempted for development of biosensors using the sensing property of these membrane proteins. Their specificity to target molecules is particularly attractive as it is significantly superior to that of traditional human-made sensors. In this review, we classified the membrane protein-based biosensors into two platforms: the lipid bilayer-based platform and the cell-based platform. On lipid bilayer platforms, the membrane proteins are embedded in a lipid bilayer that bridges between the protein and a sensor device. On cell-based platforms, the membrane proteins are expressed in a cultured cell, which is then integrated in a sensor device. For both platforms we introduce the fundamental information and the recent progress in the development of the biosensors, and remark on the outlook for practical biosensing applications.

Silver Nanoparticles: Synthesis and Application for Nanomedicine

Over the past few decades, metal nanoparticles less than 100 nm in diameter have made a substantial impact across diverse biomedical applications, such as diagnostic and medical devices, for personalized healthcare practice. In particular, silver nanoparticles (AgNPs) have great potential in a broad range of applications as antimicrobial agents, biomedical device coatings, drug-delivery carriers, imaging probes, and diagnostic and optoelectronic platforms, since they have discrete physical and optical properties and biochemical functionality tailored by diverse size- and shape-controlled AgNPs. In this review, we aimed to present major routes of synthesis of AgNPs, including physical, chemical, and biological synthesis processes, along with discrete physiochemical characteristics of AgNPs. We also discuss the underlying intricate molecular mechanisms behind their plasmonic properties on mono/bimetallic structures, potential cellular/microbial cytotoxicity, and optoelectronic property. Lastly, we conclude this review with a summary of current applications of AgNPs in nanoscience and nanomedicine and discuss their future perspectives in these areas.

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.

An overview of an artificial nose system

The present review describes recent advances in the development of an artificial nose system based on olfactory receptors and various sensing platforms. The kind of artificial nose, the production of olfactory receptors, the sensor platform for signal conversion and the application of the artificial nose system based on olfactory receptors and various sensing platforms are presented. The associated transduction modes are also discussed. The paper presents a review of the latest achievements and a critical evaluation of the state of the art in the field of artificial nose systems.

Applications and Advances in Bioelectronic Noses for Odour Sensing

A bioelectronic nose, an intelligent chemical sensor array system coupled with bio-receptors to identify gases and vapours, resembles mammalian olfaction by which many vertebrates can sniff out volatile organic compounds (VOCs) sensitively and specifically even at very low concentrations. Olfaction is undertaken by the olfactory system, which detects odorants that are inhaled through the nose where they come into contact with the olfactory epithelium containing olfactory receptors (ORs). Because of its ability to mimic biological olfaction, a bio-inspired electronic nose has been used to detect a variety of important compounds in complex environments. Recently, biosensor systems have been introduced that combine nanoelectronic technology and olfactory receptors themselves as a source of capturing elements for biosensing. In this article, we will present the latest advances in bioelectronic nose technology mimicking the olfactory system, including biological recognition elements, emerging detection systems, production and immobilization of sensing elements on sensor surface, and applications of bioelectronic noses. Furthermore, current research trends and future challenges in this field will be discussed.

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.

Quantification of interacting cognate odorants with olfactory receptors in nanovesicles

This study aims to improve our understanding of the interaction between olfactory receptors and odorants to develop highly selective biosensing devices. Natural nanovesicles (NVs) from Saccharomyces cerevisiae, ~100 nm in diameter, carrying either the human OR17-40 or the chimpanzee OR7D4 olfactory receptor (OR) tagged with the c-myc epitope at their N-terminus, are presented as model systems to quantify the interaction between odorant and olfactory receptors. The level of expression of olfactory receptors was determined at individual NVs using a novel competitive ELISA immunoassay comparing the values obtained against those from techniques involving the solubilization of cell membrane proteins and the identification of c-myc-carrying receptors. Surface Plasmon Resonance (SPR) measurements on L1 Biacore chips indicate that cognate odorants bind to their Ors, thereby quantifying the approximate number of odorants that interact with a given olfactory receptor. The selectivity of OR17-40-carrying NVs towards helional and OR7D4-carrying NVs towards androstenone has been proven in cross-check experiments with non-specific odorant molecules (heptanal and pentadecalactone, respectively) and in control receptors.

Field-Effect Transistor Biosensor Platform Fused with Drosophila Odorant-Binding Proteins for Instant Ethanol Detection

Odorant-binding proteins (OBPs) have attracted considerable attention as sensing substrates for the development of olfactory biosensors. The Drosophila LUSH protein is an OBP and is known to bind to various alcohols. Technology that uses the LUSH protein has great potential to provide crucial information through odorant detection. In this work, the LUSH protein was used as a sensing substrate to detect the ethanol concentration. Furthermore, we fused the LUSH protein with a silicon-on-insulator (SOI)-based ion-sensitive field-effect transistor (ISFET) to measure the electrical signals that arise from molecular interactions between the LUSH and ethanol. A dual-gate sensing system for self-amplification of the signal resulting from the molecular interaction between the LUSH and ethanol was then used to achieve a much higher sensitivity than a conventional ISFET. In the end, we successfully detected ethanol at concentrations ranging between 0.001 and 1% using the LUSH OBP-fused ISFET olfactory sensor. The OBP-fused SOI-based olfactory ISFET sensor can lead to the development of handheld sensors for various purposes such as detecting toxic chemicals, narcotics control, testing for food freshness, and noninvasive diagnoses.

Diverse Applications of Nanomedicine

The design and use of materials in the nanoscale size range for addressing medical and health-related issues continues to receive increasing interest. Research in nanomedicine spans a multitude of areas, including drug delivery, vaccine development, antibacterial, diagnosis and imaging tools, wearable devices, implants, high-throughput screening platforms, etc. using biological, nonbiological, biomimetic, or hybrid materials. Many of these developments are starting to be translated into viable clinical products. Here, we provide an overview of recent developments in nanomedicine and highlight the current challenges and upcoming opportunities for the field and translation to the clinic.

Molecular cloning, expression profile, odorant affinity, and stability of two odorant-binding proteins in Macrocentrus cingulum Brischke (Hymenoptera: Braconidae)

The polyembryonic endoparasitoid wasp Macrocentrus cingulum Brischke (Hymenoptera: Braconidae) is deployed successfully as a biocontrol agent for corn pest insects from the Lepidopteran genus Ostrinia in Europe and throughout Asia, including Japan, Korea, and China. The odorants are recognized, bound, and solubilized by odorant-binding protein (OBP) in the initial biochemical recognition steps in olfaction that transport them across the sensillum lymph to initiate behavioral response. In the present study, we examine the odorant-binding effects on thermal stability of McinOBP2, McinOBP3, and their mutant form that lacks the third disulfide bonds. Real-time PCR experiments indicate that these two are expressed mainly in adult antennae, with expression levels differing by sex. Odorant-binding affinities of aldehydes, terpenoids, and aliphatic alcohols were measured with circular dichroism spectroscopy based on changes in the thermal stability of the proteins upon their affinities to odorants. The obtained results reveal higher affinity of trans-caryophelle, farnesene, and cis-3-Hexen-1-ol exhibits to both wild and mutant McinOBP2 and McinOBP3. Although conformational flexibility of the mutants and shape of binding cavity make differences in odorant affinity between the wild-type and mutant, it suggested that lacking the third disulfide bond in mutant proteins may have chance to incorrect folded structures that reduced the affinity to these odorants. In addition, CD spectra clearly indicate proteins enriched with α-helical content.

Bioelectronic nose and its application to smell visualization

There have been many trials to visualize smell using various techniques in order to objectively express the smell because information obtained from the sense of smell in human is very subjective. So far, well-trained experts such as a perfumer, complex and large-scale equipment such as GC-MS, and an electronic nose have played major roles in objectively detecting and recognizing odors. Recently, an optoelectronic nose was developed to achieve this purpose, but some limitations regarding the sensitivity and the number of smells that can be visualized still persist. Since the elucidation of the olfactory mechanism, numerous researches have been accomplished for the development of a sensing device by mimicking human olfactory system. Engineered olfactory cells were constructed to mimic the human olfactory system, and the use of engineered olfactory cells for smell visualization has been attempted with the use of various methods such as calcium imaging, CRE reporter assay, BRET, and membrane potential assay; however, it is not easy to consistently control the condition of cells and it is impossible to detect low odorant concentration. Recently, the bioelectronic nose was developed, and much improved along with the improvement of nano-biotechnology. The bioelectronic nose consists of the following two parts: primary transducer and secondary transducer. Biological materials as a primary transducer improved the selectivity of the sensor, and nanomaterials as a secondary transducer increased the sensitivity. Especially, the bioelectronic noses using various nanomaterials combined with human olfactory receptors or nanovesicles derived from engineered olfactory cells have a potential which can detect almost all of the smells recognized by human because an engineered olfactory cell might be able to express any human olfactory receptor as well as can mimic human olfactory system. Therefore, bioelectronic nose will be a potent tool for smell visualization, but only if two technologies are completed. First, a multi-channel array-sensing system has to be applied for the integration of all of the olfactory receptors into a single chip for mimicking the performance of human nose. Second, the processing technique of the multi-channel system signals should be simultaneously established with the conversion of the signals to visual images. With the use of this latest sensing technology, the realization of a proper smell-visualization technology is expected in the near future.

Bioelectronic nose: Current status and perspectives

A characteristic feature of human and animal organs of smell is the ability to identify hundreds of thousands of odours. It is accompanied by particular smell sensations, which are a basic source of information about odour mixture. The main structural elements of biological smell systems are the olfactory receptors. Small differences in a structure of odorous molecules (odorants) can lead to significant change of odour, which is due to the fact that each of the olfactory receptors is coded with different gene and usually corresponds to different type of odour. Discovery and characterisation of the gene family coding the olfactory receptors contributed to the elaboration and development of the electronic smell systems, the so-called bioelectronic noses. The olfactory receptors are employed as a biological element in this type of instruments. An electronic system includes a converter part, which allows measurement and processing of generated signals. A suitable data analysis system is also required to visualise the results. Application potentialities of the bioelectronic noses are focused on the fields of economy and science where highly selective and sensitive analysis of odorous substances is required. The paper presents a review of the latest achievements and critical evaluation of the state of art in the field of bioelectronic noses.

Bioanalytical and chemical sensors using living taste, olfactory, and neural cells and tissues: a short review

Biosensors utilizing living tissues and cells have recently gained significant attention as functional devices for chemical sensing and biochemical analysis. These devices integrate biological components (i.e. single cells, cell networks, tissues) with micro-electro-mechanical systems (MEMS)-based sensors and transducers. Various types of cells and tissues derived from natural and bioengineered sources have been used as recognition and sensing elements, which are generally characterized by high sensitivity and specificity. This review summarizes the state of the art in tissue- and cell-based biosensing platforms with an emphasis on those using taste, olfactory, and neural cells and tissues. Many of these devices employ unique integration strategies and sensing schemes based on sensitive transducers including microelectrode arrays (MEAs), field effect transistors (FETs), and light-addressable potentiometric sensors (LAPSs). Several groups have coupled these hybrid biosensors with microfluidics which offers added benefits of small sample volumes and enhanced automation. While this technology is currently limited to lab settings due to the limited stability of living biological components, further research to enhance their robustness will enable these devices to be employed in field and clinical settings.

Olfactory receptor screening assay using nanovesicle-immobilized carbon nanotube transistor

Olfactory receptor (OR) genes are considered to be the largest superfamily of the mammalian genome, and in the case of humans, approximately 390 kinds of functional ORs play a role in perceiving odors. In spite of their significance in olfaction, the function of all ORs has not yet been fully revealed. In order to efficiently identify specific ligands of orphan ORs, methods that can generate olfactory signals in a reliable manner and that can convert the cellular signals into measurable responses are required. Here, we describe an OR screening assay method using olfactory sensors that are based on cell-derived nanovesicles combined with single-walled carbon nanotube field-effect transistors (SWNT-FETs). The nanovesicles contain ORs on their surface membrane and induce influx of calcium ions similar to olfactory signal transduction. This ion influx causes an electrical current change along the carbon nanotube, and then this change is measured by the SWNT-FET sensor. This technique facilitates the simple and rapid screening of OR functions.

Coupling of olfactory receptor and ion channel for rapid and sensitive visualization of odorant response

In the human smell sensing system, there are about 390 kinds of olfactory receptors (ORs) which bind to various odorants with different affinities and specificities. Characterization and odorant binding pattern analysis of the ORs are essential for understanding of human olfaction and to mimic the olfactory system in various applications. Although various cell-based odorant screening systems have been developed for this purpose, many human ORs (hORs) still remain orphan because of the time-consuming and labor-intensive experimental procedures of the available screening methods. In this study, we constructed an ion channel-coupled hOR for simple odorant detection by rapidly visualizing the odorant response to overcome the limitations of conventional screening systems. The hORs were coupled to the Kir6.2 potassium channel and the fusion proteins were expressed in HEK293 cells. In this system, when an odorant binds to the hORs coupled to the ion channel, a conformational change in the OR occurs, which consequently opens the ion channel to result in ion influx into the cell. This ion influx was then visualized using a membrane potential dye. Cells expressing ion channel-coupled hORs showed high sensitivity and selectivity to their specific odorants, and the odorant-hOR binding pattern was visualized to identify the response of individual hORs to various odorants, as well as the response of various hORs to various odorants. These results indicate that the ion channel-coupled hOR system can be effectively used not only for simple and fast high-throughput odorant screening, but also to visualize the odorant-hOR response pattern.

Bioelectronic nose combined with a microfluidic system for the detection of gaseous trimethylamine

A bioelectronic nose based on a novel microfluidic system (μBN) was fabricated to detect gaseous trimethylamine (TMA) in real-time. Single-walled carbon nanotube-field effect transistors (SWNT-FETs) were functionalized with olfactory receptor-derived peptides (ORPs) that can recognize the TMA molecules. The ORP-coated SWNT-FETs were assembled with a microfluidic channel and were sealed with top and bottom frames. This simple process was used to complete the μBNs, and a well-defined condition was achieved to detect the gaseous molecules. The μBNs allowed us to detect gaseous TMA molecules down to 10 parts per trillion (ppt) in real-time and showed high selectivity when distinguishing gaseous TMA from other gaseous odorants. The sensor was used to determine the quality of seafood (oysters), and spoiled seafood and other types of spoiled foods were also successfully discriminated without any pretreatment processes. These results indicate that portable-scale platforms can be manufactured by using μBNs and can be applicable for real-time on-site gas analysis.

Ion-channel-coupled receptor-based platform for a real-time measurement of G-protein-coupled receptor activities

A simple but efficient measurement platform based on ion-channel-coupled receptors and nanovesicles was developed for monitoring the real-time activity of G-protein-coupled receptors (GPCRs). In this work, an olfactory receptor (OR), the most common class A GPCR, was covalently fused with a Kir6.2 channel so that the GPCR action directly induced the opening of the ion channels and changes in the electrical membrane potential without complex cellular signaling processes. This strategy reduced the measurement errors caused by instability of various cellular components. In addition, rather than using whole cells, a cell-surface-derived nanovesicle was used to preserve the membrane-integrated structure of GPCRs and to exclude case-dependent cellular conditions. Another merit of using the nanovesicle is that nanovesicles can be easily combined with nanomaterial-based field-effect transistors (FETs) to build a sensitive and stable measurement platform to monitor GPCR activities with high sensitivity in real-time. Using a platform based on carbon nanotube FETs and nanovesicles carrying Kir6.2-channel-coupled ORs, we monitored the real-time response of ORs to their ligand molecules. Significantly, since this platform does not rely on rather unstable cell signaling pathways, our platform could be utilized for a rather long time period without losing its functionality. This system can be utilized extensively for simple and sensitive analysis of the activities of various GPCRs and should enable various academic and practical applications.

Ligands binding and molecular simulation: the potential investigation of a biosensor based on an insect odorant binding protein

Based on mimicking biological olfaction, biosensors have been applied for the detection of various ligands in complex environment, which could represent one of the most promising research fields. In this study, the basic characters of one insect odorant binding protein (OBP) as a biosensor were explored. To explore the molecular recognition process, the tertiary structure of the protein was modeled and the protein-ligand interactions with 1,536,550 chemicals were investigated by the molecular docking. The availability of large amount of recombinant SlitOBP1 overcame the difficulty to obtain biological sensing material. After obtained the purified recombinant protein, the result of fluorescence binding assays proved the candidate protein has good affinities with the majority of the tested chemicals. With the aid of simulation docking, the key conserved amino acids within the binding site were identified and then mutated to alanine. After mutation, the protein-ligand binding characteristics were recorded, and the competitive binding assays were carried out to provide experimental verification. The detailed information on its structure and affinities investigated in this study could allow the design of specific mutants with desired characteristics, which provides a solid base for tailoring OBP for biosensor and provides a role model for screening the other elements in olfactory system for different applications.

Taking your breath away: metabolomics breathes life in to personalized medicine

Breath-based metabolomics (breathomics) is an exciting developing area of biotechnology that centers on the capture, identification, and quantification of volatile organic compound (VOC) patterns in human breath and their utilization as tools in the diagnosis of a broad spectrum of medical problems. With the age of personalized medicines demanding rapid bespoke diagnosis and treatment, this area of molecular diagnostics is beginning to see an upsurge in biotechnological advancement. Here, we discuss recent improvements and directions in the development of breath VOC analysis and diagnosis platforms that offer the potential for disease biomarker discovery and disease prognosis.

Surface plasmon resonance for cell-based clinical diagnosis

Non-invasive real-time observations and the evaluation of living cell conditions and functions are increasingly demanded in life sciences. Surface plasmon resonance (SPR) sensors detect the refractive index (RI) changes on the surface of sensor chips in label-free and on a real-time basis. Using SPR sensors, we and other groups have developed techniques to evaluate living cells' reactions in response to stimuli without any labeling in a real-time manner. The SPR imaging (SPRI) system for living cells may visualize single cell reactions and has the potential to expand application of SPR cell sensing for clinical diagnosis, such as multi-array cell diagnostic systems and detection of malignant cells among normal cells in combination with rapid cell isolation techniques.

Nanovesicle-based bioelectronic nose for the diagnosis of lung cancer from human blood

A human nose-mimetic diagnosis system that can distinguish the odor of a lung cancer biomarker, heptanal, from human blood is presented. Selective recognition of the biomarker is mimicked in the human olfactory system. A specific olfactory receptor recognizing the chemical biomarker is first selected through screening a library of human olfactory receptors (hORs). The selected hOR is expressed on the membrane of human embryonic kidney (HEK)-293 cells. Nanovesicles containing the hOR on the membrane are produced from these cells, and are then used for the functionalization of single-walled carbon nanotubes. This strategy allows the development of a sensitive and selective nanovesicle-based bioelectronic nose (NvBN). The NvBN is able to selectively detect heptanal at a concentration as low as 1 × 10(-14) m, a sufficient level to distinguish the blood of a lung cancer patient from the blood of a healthy person. In actual experiments, NvBN could detect an extremely small increase in the amount of heptanal from human blood plasma without any pretreatment processes. This result offers a rapid and easy method to analyze chemical biomarkers from human blood in real-time and to diagnose lung cancer.

Highly selective and sensitive detection of neurotransmitters using receptor-modified single-walled carbon nanotube sensors

We present receptor-modified carbon nanotube sensors for the highly selective and sensitive detection of acetylcholine (ACh), one kind of neurotransmitter. Here, we successfully expressed the M1 muscarinic acetylcholine receptor (M1 mAChR), a family of G protein-coupled receptors (GPCRs), in E. coli and coated single-walled carbon nanotube (swCNT)-field effect transistors (FETs) with lipid membrane including the receptor, enabling highly selective and sensitive ACh detection. Using this sensor, we could detect ACh at 100 pM concentration. Moreover, we showed that this sensor could selectively detect ACh among other neurotransmitters. This is the first demonstration of the real-time detection of ACh using specific binding between ACh and M1 mAChR, and it may lead to breakthroughs for various applications such as disease diagnosis and drug screening.

Application of SPR imaging sensor for detection of individual living cell reactions and clinical diagnosis of type I allergy

A technique to visualize living cell activation in a real time manner without any labeling is required in the fields of life sciences and medicine. We have reported that surface plasmon resonance (SPR) sensors detect large changes of refractive index (RI) with living cells, such as mast cells, human basophils and lymphocytes. However conventional SPR sensors detect only an average change of RI with thousands of cells at detectable area on a sensor chip. Therefore, we developed an SPR imaging (SPRI) sensor with a CMOS camera and an objective lens in order to visualize RI of individual living cells and their changes upon stimuli. The SPRI sensor we developed could detect reactions of individual rat basophilic leukemia (RBL-2H3) cells and mouse keratinocyte cells in response to specific or nonspecific stimuli. Moreover, the sensor could detect the reactions of individual human basophils isolated from patients in response to antigens (allergens). Thus the technique can visualize the effect of various stimuli, inhibitors and/or conditions on cell reactions as change of intracellular RI distribution at single cell levels. Establishment of the technique to rapidly isolate cells from patient blood should enable us to utilize SPRI system as a high throughput screening system in clinical diagnosis, such as type I hypersensitivity and drug hypersensitivity, and as a tool to reveal novel phenomena in evanescent fields around plasma membranes.

Recent advances in olfactory receptor-based biosensors

The biological olfactory system can recognize and discriminate thousands of volatile organic compounds (VOCs) with extremely high sensitivity and specificity. The most fundamental elements are olfactory receptors (ORs) in the cilia of olfactory sensory neurons (OSNs), which contribute greatly to the high-performance olfactory system. The excellent properties of ORs are generally recognized in the development of biomimetic OR-based biosensors. Over the past two decades, much work has been done in developing OR-based biosensors due to their promising potential in many applications. In this article, we will outline the latest advances of OR-based biosensors. Two current crucial issues in this field will be discussed, namely, the production methods and immobilization techniques of ORs. We will also elaborate on various OR-based biosensors and their latest developments. Finally, current research trends and future challenges in this field will be discussed.

Mimicking the human smell sensing mechanism with an artificial nose platform

Sensing smell is a highly complex biological process, and characterizing and mimicking the interaction between the olfactory receptor (OR) protein and its ligands is extremely challenging. Herein, we report a highly sensitive and selective human nose-like nanobioelectronic nose (nbe-nose), which responds to gaseous odorants sensitively and selectively, has a signal specificity pattern similar to that in the cellular signal transduction pathway, and maintains an antagonistic behavior similar to the human nose. The human olfaction mechanism was mimicked by using carboxylated polypyrrole nanotubes (CPNTs) functionalized with human OR protein. The nbe-nose was able to detect gaseous odorants at a concentration as low as 0.02 parts-per-trillion (ppt), which was comparable to a highly trained, human expert's nose. The nbe-nose can be used scientifically for smell mechanism studies. It can be also applied to various fields that rely on smell monitoring for industrial and public purposes.

Bioelectronic nose with high sensitivity and selectivity using chemically functionalized carbon nanotube combined with human olfactory receptor

Single-walled carbon nanotubes (swCNTs) hold great promise for use as molecular wires because they exhibit high electrical conductivity and chemical stability. However, constructing swCNT-based transducer devices requires controlled strategies for assembling biomolecules on swCNTs. In this study, we proposed a chemically modified swCNT. The swCNT was functionalized with 1,5-diaminonaphthalene via π-stacking, for reliable attachment of the human olfactory receptor 2AG1 (hOR2AG1). The human olfactory receptor was then anchored. We investigated the use of this functionalized CNT in the fabrication of a highly sensitive and selective bioelectronic nose. For the bioelectronic nose, the swCNT-field effect transistor (FET) platform was composed of polyethylene glycol (PEG)-coated regions to prevent non-specific absorption and chemically modified swCNTs regions containing hOR2AG1, which can bind to the specific odorant. This approach allowed us to create well-defined micron-scale patterns of hOR2AG1 on the swCNTs. Our bioelectronic nose displayed ultrahigh sensitivity down to concentrations as low as 1fM due to the enhanced hOR2AG1-odorant interaction through the tight binding of hOR2AG1 on the chemically modified swCNTs. In addition, the approach described here may provide an alternative route for multiplexed detection of diverse odorants and to improve the sensitivity of sensor devices.

Recent advances in electronic and bioelectronic noses and their biomedical applications

Significant effort has been made in the development of an artificial nose system for various applications. Advances in sensor technology have facilitated the development of high-performance electronic and bioelectronic noses. Numerous articles describe the advantages of artificial nose systems for biomedical applications. Recent advances in the development of electronic and bioelectronic noses and their biomedical applications are reviewed in this article.

Mimicking nature's noses: from receptor deorphaning to olfactory biosensing

The way in which organisms detect specific volatile compounds within their environment, and the associated neural processing which produces perception and subsequent behavioural responses, have been of interest to scientists for decades. Initially, most olfaction research was conducted using electrophysiological techniques on whole animals. However, the discovery of genes encoding the family of human olfactory receptors (ORs) paved the way for the development of a range of cellular assays, primarily used to deorphan ORs from mammals and insects. These assays have greatly advanced our knowledge of the molecular basis of olfaction, however, while there is currently good agreement on vertebrate and nematode olfactory signalling cascades, debate still surrounds the signalling mechanisms in insects. The inherent specificity and sensitivity of ORs makes them prime candidates as biological detectors of volatile ligands within biosensor devices, which have many potential applications. In the previous decade, researchers have investigated various technologies for transducing OR:ligand interactions into a readable format and thereby produce an olfactory biosensor (or bioelectronic nose) that maintains the discriminating power of the ORs in vivo. Here we review and compare the molecular mechanisms of olfaction in vertebrates and invertebrates, and also summarise the assay technologies utilising sub-tissue level sensing elements (cells and cell extracts), which have been applied to OR deorphanization and biosensor research. Although there are currently no commercial, "field-ready" olfactory biosensors of the kind discussed here, there have been several technological proof-of-concept studies suggesting that we will see their emergence within the next decade.