Keywords

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14.1 Applications

The human nose can discriminate among hundreds of thousands of odorant molecules [1, 2]. The olfactory sense in all animals, including human, is used to evaluate food, drink and environmental toxic materials. Recently, many studies on sensing devices mimicking the human olfactory system have been reported, and the possibility for numerous applications has been suggested [3–6]. The bioelectronic nose, described in this book, has a similar function to human olfaction, and a huge variety of possible applications (Table 14.1)

Table 14.1 Examples of applications of bioelectronic noses
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14.1.1 Diagnosis of Disease

Generally, numerous volatile and non-volatile organic compounds , including odorous compounds, are emitted from various parts of human body, such as scalp, feet, oral cavity, axillae, genital, and skin [7]. The emission of volatile compounds is influenced by diet, stress, metabolic diseases, and immune status of the individual. Thus, the change of body odor potentially represents various diseases and even mental health (Table 14.2).

Table 14.2 Key volatile compounds from patients’ breath with diseases
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Ancient physicians considered that exhaled breath from human was associated with a certain disease, and might reflect the disorder in physiological and pathophysiological processes [36]. For example, diabetes gives a fruit-like smell of acetone in a patient’s breath [16]. Diabetic patients cannot metabolize carbohydrates, including glucose, but catabolize fats into ketone bodies, such as acetone. Urine, sweat, and skin, as well as the exhaled breath are the major paths to emit odorous compounds, and they provide a lot of information about the condition of the human body. Many current researches focus on the analysis of the breath or urine odor to achieve a non-invasive and easy diagnosis .

Many studies using conventional sensing techniques have tried to analyze the breath in order to diagnose various diseases, such as a cancer, diabetes, liver failure, bacterial infection, etc. The correlation between the exhaled breath and diseases has been continuously suggested, since the first breath test as a medical assessment was tried in the eighteenth century [37]. Gas chromatography-mass spectroscopy (GC-MS) was developed for the separation and identification of volatile odor compounds in the 1960s, and breath test detecting volatile organic compounds (VOCs) from exhaled breath was used in the 1970s [38]. From the exhaled breath, many compounds have been identified, from small inorganic molecules, such as nitric oxide and carbon monoxide, to organic compounds, such as acetone, methanol and isoprene [39–42]. However, GC-MS is a labor-intensive and time-consuming technique. In addition, the collection and pre-treatment processes of the breath sample is difficult. Thus, the extensive applications of conventional methods to the disease diagnosis are not practically possible .

Electronic noses for diseases diagnosis were developed to overcome the limitation of GC-MS. The electronic nose consists of several sensor arrays and recognizes specific odors through analyzing the response patterns generated by odor stimulation [43]. Many researches demonstrating the capability of the electronic nose on medical applications have been presented [44]. Electronic noses usually target odors from human exhaled breath or urine, and can be used for the diagnosis of a wide range of diseases, such as lung cancer [45–48], chronic obstructive pulmonary disease [47, 49], and asthma [49, 50]. Recently, a gold nanoparticle-based electronic nose, which can diagnose lung cancer using exhaled breath, has been reported [51]. Peng et al. demonstrated that the sensor can discriminate patients’ breath with high accuracy. However, the electronic nose still has limitations in terms of selectivity and sensitivity. Most patients do not have a single disease. For instance, a person who has high blood pressure is also likely to have diabetes or lung cancer. In other words, the combinations of patients’ diseases are countless, and the resultant chemical/odor changes are necessarily complex. Thus, high selectivity capable of discriminating specific diseases is required for precise diagnosis. Moreover, the most important point on medical applications is an early diagnosis. Therefore, sufficient sensitivity has to be guaranteed for practical diagnosis.

Bioelectronic noses mimicking human olfaction can achieve an extremely high sensitivity and selectivity. Various biological components, such as living cells, proteins, and even peptides, can be used for the development of bioelectronic noses. Lin et al. constructed a quartz crystal coated with a synthesized peptide which is derived from an olfactory receptor [52]. The peptide-based sensor was able to selectively detect trimethylamine, a biomarker of uremia . This study first showed that the bioelectronic nose was able to be used for the detection of the odorous biomarker.

Recently, Lim et al. developed the bioelectronic nose for the diagnosis of lung cancer [53] . The specific olfactory receptor that binds to heptanal was selected . In the blood from lung cancer patients , the concentration of heptanal is significantly higher than that in the blood from non-patients [54]. By functionalizing the single-walled carbon nanotube field-effect transistors (SWNT-FETs) with nanovesicles containing the selected olfactory receptors, a highly sensitive and selective bioelectronic nose was fabricated. The sensor can detect 100 fM of hetpanal from human blood plasma, even though the plasma sample was not pre-treated (Fig. 14.1).

Fig. 14.1
figure 1

Detection of heptanal from human blood plasma using a nanovesicle-based bioelectronic nose. a A real-time measurement for the influence of plasma on the conductance change in the bioelectronics nose. The bioelectronics nose was fabricated using nanovesicles with olfactory receptor, Gαolf, and receptor-transporting protein 1S (RTP1S). Human blood plasma samples were diluted to different ratios. The 1/10−7-diluted plasma did not show any effect on the conductance, which means that no pretreatment process is required when 1/10−7-diluted plasma is used in the experiment. b The real-time detection of heptanal from 1/10−7-diluted plasma using a bioelectronic nose. A bioelectronic nose based on nanovesicles with olfactory receptor (OR), Gαolf, and RTP1S was able to detect 1 × 10−13 M heptanal (square), whereas a bioelectronic nose without ORs was not (triangle). (Reprinted from Ref. [53] with permission from John Wiley & Sons Inc.)

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Most hormone receptors as well as olfactory receptors belong to a GPCR . Therefore, other types of GPCR-based biosensors for the diagnosis of diseases can be developed through a similar fabrication process to that of an OR-based bioelectronic nose. A biosensor which sensitively and selectively detects a human parathyroid hormone (hPTH) has been reported [55]. The sensor was fabricated with human parathyroid hormone receptors (hPTHRs) over-expressed in E. coli and conducting polymer nanoparticles. The hPTHRs and conducting polymer nanoparticles play roles in the selective discrimination of hPTHs and in the conversion of bio-signals into the sensitive electrical signals, respectively. The hPTHR-based sensor was able to detect hPTHs from real serum samples with a high sensitivity and selectivity . Human acetylcholine receptor was also used for the development of diagnostic devices [56]. The receptor was over-expressed in E. coli and immobilized on the SWNT-FETs. The sensor was able to easily and rapidly detect acetylcholine with high sensitivity and selectivity. These techniques, when applied to the development of biosensors, are expected to offer numerous applications in biomedical fields.

14.1.2 Assessment of Food Quality

Quality and spoilage control of foods are very important to many industries and consumers because a good quality is directly connected to a consumption of products, consumers’ choice of brands and even consumers’ health. A consumers’ repeat purchase behavior depends on whether the product can consistently satisfy the consumers’ expectation of quality. With growing consumer expectation of quality and safety for foods, interest in the effective sensing system for measuring the quality of foods has increased. Various devices have been developed during a couple of decades to satisfy this interest. However, most methods require the destruction of samples and random sampling processes. Therefore, a novel concept of device to assess the quality of foods based on the sense of smell has been suggested.

Off-odor from foods is generated by the oxidation of foods or the contamination by bacteria or fungi [57–59]. Intake of spoiled or contaminated foods can cause severe health problems to human. Thus, it is important to determine the quality of foods before consumption. This is possible by detecting odorous compounds emitted from low-quality foods. In this respect, effective methods to analyze odor from foods have been intensively developed long ago.

In order to detect VOCs generated from foods, GC-MS has been commonly utilized. Especially, through a complementing solid-phase microextraction (SPME) technique, the sensitivity of GC-MS has been remarkably improved [60]. SPME and GC-MS techniques are still used to analyze VOCs from various spoiled or contaminated foods [61–63]. High-performance liquid chromatography (HPLC) and ion-exchange chromatography are also suitable for this purpose, and have been broadly used [64–66]. These techniques can precisely analyze all VOCs generated from foods; however, all kinds of standard molecules have to be procured. In the case of real foods, numerous chemical substrates are decomposed by many factors, such as oxidation, bacterial growth, and thermal degradation. Thus, it is impossible to analyze all VOCs generated in an actual condition rather than a controlled condition. Moreover, large-sized instruments and complicated pretreatment processes make the on-site and real-time analysis difficult.

Many electronic noses that measure the quality of foods have been developed [67–70]. The electronic noses classify food odors through recognizing the response patterns generated from a sensor array. Such devices have many advantages as practical sensors. First of all, their operation can be quite simple. The electronic nose recognizes the odor from foods in a similar manner to the human nose, and subsequent processes can be automated. In addition, the devices can be miniaturized; hence, several portable types of electronic nose have already been commercialized. However, the electronic nose still has limitations. In most cases, various kinds of foods are mixed and even cooked. The quality and combination of ingredients differ at all times. This means that the database of incalculable odor patterns has to be built so as to determine the quality of foods in actual conditions. The electronic nose can differentiate between spoiled and non-spoiled foods in ideal conditions, but cannot selectively recognize the odor from certain spoiled foods when various foods are mixed. Therefore, more advanced concepts of artificial smelling devices are required.

The bioelectronic nose is expected to play this role. Bioelectronic noses have excellent selectivity due to their primary sensing elements, an olfactory receptor [4]. They can recognize only their counterpart ligands, irrespective of mixed degrees or physical conditions of foods. This is a very important characteristic of the sensing device when the sensor is applied within the food industry. It has been reported that the amount of linear aldehydes, especially hexanal, increases by the oxidation of unsaturated fatty acids [71]. Thus, hexanal is generally considered as an indicator of the degree of decomposition of foods which contain fatty acids. Park et al. developed a bioelectronic nose that can sensitively and selectively detect hexanal [72]. One of the canine olfactory receptors was utilized as a primary sensing element in the form of a nanovesicle. The nanovesicle containing the canine olfactory receptor on its membrane was combined with SWNT-FETs. This bioelectronic nose was able to detect hexanal at a concentration as low as 1 fM with excellent selectivity. Its sensitivity and selectivity facilitated the detection of hexanal from spoiled milk without any pretreatment processes. These results showed that the bioelectronic nose will be an excellent sensing device for the assessment of foods quality .

The remarkable selectivity and sensitivity of the bioelectronic nose resulted in another interesting application. Although many foods are mixed, the degree of decomposition of specific foods can be easily measured. Recently, Lim et al. developed a peptide receptor-based bioelectronics nose (PRBN) by using single walled-carbon nanotube field-effect transistors (SWNT-FETs) functionalized with olfactory receptor-derived peptides (ORPs) [73]. The peptide, which is derived from one of the canine olfactory receptors, specifically binds to trimethylamine, an indicator of seafood decomposition . Even though a small peptide rather than a whole protein was utilized, the selectivity of the olfactory receptor still remained. Thus, the sensor was able to detect trimethylamine from spoiled seafood samples without any pretreatment processes (Fig. 14.2a), and determine the degree of spoilage (Fig. 14.2b, c) . Moreover, spoiled seafood was specifically discriminated among other kinds of spoiled foods (milk, tomato, broccoli, and beef) and fresh seafood (Fig. 14.2d). By virtue of its excellent sensitivity and selectivity of the bioelectronic nose, no pretreatment processes of food samples were required. In the near future, the freshness of each food ingredient in a household refrigerator may be automatically measured by the bioelectronic nose.

Fig. 14.2
figure 2

Detection of trimethylamine (TMA) from spoiled seafood using peptide receptor-based bioelectronic noses. a A real-time measurement data exhibiting the conductance change generated by repeated treatments with a spoiled oyster sample, which was produced by storing the oyster sample at 25 °C for two days. Consistent and prompt responses were generated by the four treatments. The gray dotted line represents the initial base line. b Real-time measurements of conductance changes generated by treatments with oyster (red line), shrimp (blue line), and lobster (green line) samples spoiled for different periods of time. Significant decreases in the conductance were observed for the 2 day-spoiled oyster, 1 day-spoiled shrimp, and 2 day-spoiled lobster and the responses increased with an increase in the spoilage time. c Response patterns versus the degree of spoilage of the three spoiled seafood samples (oyster, shrimp, and lobster). The generated responses tended to increase with the degree of spoilage. d Real-time recognition and distinction of spoiled oyster from other types of spoiled foods (milk, tomato, broccoli, and beef) and fresh oyster. The sample solutions of milk, tomato, broccoli, and beef spoiled for four days and the fresh oyster had no significant effect on the conductance. However, the injection of the oyster sample that had been spoiled for two days caused a sharp decrease in conductance. For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article. (Reprinted from Ref. [73] with permission from Elsevier)

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14.1.3 Determination of Fragrance and Flavor

The sense of smell in human is not considered to be a crucial function for survival, although odor perception plays an important role in the survival of all animals. Nevertheless, the olfactory sense in human is an important motivation factor inducing the development of a fragrance and flavor industry. Many industries are interested in scent, which attracts customers to their products. Depending on the product in question, a customers’ propensity to consume products can easily rely on preference to a specific smell. Therefore, the importance of a good scent is emphasized in most products in our daily lives, such as wines, cuisine, cosmetics, perfume, and coffee. This indicates that the control of fragrance and flavor is very important to various industrial fields.

The determination of a good smell largely depends on personal disposition and experience, and can be very subjective. Therefore, to objectively judge the fragrance and flavor, well-trained experts play a role in the development of many products. For instance, a sommelier is a wine connoisseur and a perfumer develops scents for perfumes and cosmetics. However, the method that a person directly smells has many limitations. Many parameters, such as health, fatigue and the specific conditions of a person can affect the smelling ability; therefore, reproducibility and repeatability cannot be guaranteed through this method. In the case of the fragrance- and flavor-related industries, a vast number of odorous components are used. Also, the actual smell may be extremely changeable depending on the mixing ratio of the used odorants. Thus, it is impossible to determine scent through analytical techniques such as GC-MS . Such techniques can only identify components comprising the smell [74, 75]. This is the most serious drawback of conventional techniques on the applications for the fragrance and flavor industry.

Electronic noses can provide a more objective value of odor. Several groups have reported results showing the capability of electronic noses on applications for the measurement of scent [76, 77]. Electronic noses have successfully discriminated perfumery compounds through a pattern recognition process. However, the patterns generated by the odor stimulation cannot fundamentally represent the real odor that a person perceives, because electronic noses do not use human olfactory receptors. The electronic nose detects volatile compounds including odorants. Thus, various odorless compounds, such as CO2, CO, and even water vapor, can induce significant responses in the electronic nose. Hence, in the fragrance and flavor industry where the human perception has to be objectively analyzed, the electronic nose has limitations.

Bioelectronic noses utilize olfactory receptors which originally act as a primary sensing element for the recognition of odors. The biological processes for odor perception can be reconstructed in vitro using bioelectronic noses. Olfactory receptors of the bioelectronic nose recognize odorants with excellent selectivity. This recognition is identical to that of the human nose [78]. If the sensor is functionalized with all kinds of human olfactory receptors, it can perfectly represent the same response patterns as those which the human nose produces for the perception of odors. In order to apply the sensor to the fragrance and flavor industry, the sensor should objectively analyze the type and intensity of the smell based on the human olfactory sense. This is only possible when human olfactory receptors are incorporated to the sensor system. Eventually, the bioelectronic nose will provide a simple, fast and reliable analysis method to measure scent without the help of the human nose .

14.1.4 Monitoring of Environmental Pollutants

Recently, there has been a growing need to more consistently and specifically monitor environmental pollutants. In a narrow sense, environmental pollution means the emission of greenhouse gases or toxic compounds generated from industrial processes into the atmosphere, soil, or water. This is strictly regulated by law, because pollutants can be seriously harmful to human health. In the broader sense, there are a lot of pollutions that are not officially regulated, but can give rise to unpleasant feelings by malodor. For instance, garbage thrown in the street, spoiled fish in the wastebasket, or smoking in the public area can cause malodor. Thus, it is also a kind of environmental pollutions although such odor is not restricted. All environmental pollutants, regardless of their regulation, need to be monitored to improve quality of life. For this purpose, the development of further effective methods to monitor various pollutants is required.

GC-MS has been generally used for the detection of environmental pollutants in the atmosphere [79–81]. HPLC is suitable to analyze the composition of water [82–84]. Such techniques can precisely quantify the amount of chemical compounds. Thus, they are effective to monitor restricted pollutants. However, they have disadvantages in terms of portability and immediacy. Because of the large-sized instrument, an on-site measurement is difficult, and sampling processes should be conducted. In addition, real-time measurements are difficult due to the complexity of analysis processes. Furthermore, equipment such as GC and HPLC is too expensive to be used for the broad and continuous detection of less severe pollutants, such as the odor from rotten foods or smoking.

Electronic noses can be effectively and extensively applied to the monitoring of such environmental pollutants. Many electronic noses have been developed for various purposes, such as the monitoring of urban pollution, water pollution, and the quality of indoor air [85–89]. Severe environmental pollutants are mainly toxic gases which are major targets of the electronic noses; thus, electronic noses are appropriate for the monitoring of pollutants. In addition, the sensing processes can be simplified. This is very important to detect hazardous chemicals such as CO, HF, and NO2. However, the sensitivity of electronic noses is insufficient. The sensor should detect toxic gases before the amount of toxins reaches a dangerous level, and can also selectively recognize the existence of pollutants under any circumstances. However, electronic noses are easily affected by a variety of factors, such as humidity, electromagnetic fields, and temperature.

Bioelectronic noses are being regarded as a better sensor system to monitor a number of environmental pollutants in terms of high sensitivity and selectivity. There is, however, a potential complication as to whether a given biological element is active under dry conditions, because the sensors should be able to detect gas compounds. Wu and Lo addressed this problem by using a synthetic peptide which mimics the binding site of olfactory receptors [90, 91]. The sensor functionalized with peptide receptors was able to selectively detect trimethylamine and ammonia, which are well known as air pollutants due to their pungent odor. Lee et al. demonstrated that whole olfactory receptor proteins were active in the dry condition [78]. They functionalized a conducting polymer nanotube-based sensor with a human olfactory receptor which had been expressed in E. coli. This sensor not only detected specific odorants with high sensitivity and selectivity, but also showed human nose-like behaviors such as antagonism. In order to make a better tertiary structure of olfactory receptors, the whole proteins were trapped in a ‘nanodisc’, a self-assembling nano-scale membrane assembly [6]. The nanodiscs containing olfactory receptors were utilized for the functionalization of carbon nanotubes, and the sensor successfully detected gaseous odorants. All things taken together, the bioelectronic nose can fully supplement the weakness of the electronic nose, and will be widely used for the monitoring of a range of environmental pollutants .

14.1.5 Other Applications

There are various other fields where bioelectronic noses can be effectively utilized besides the examples that have been previously discussed. First of all, the bioelectronic nose can be a perfect alternative to direct smelling. To this day, trained dogs play major roles in the detection of narcotics and explosives, such as their use in customs and airports, as well as military institutions. Although dogs have an excellent smelling ability, they have critical limitations such as high costs for training and maintenance. Also, the natural olfactory system is easily and rapidly adapted to the repetitive exposure to odors; thus, the dogs cannot continuously search drugs and explosives [92]. Hence, bioelectronic noses may replace the role of dogs within such industries.

Bioelectronic noses can be utilized for the process monitoring using their excellent sensitivity and selectivity. For instance, they can selectively detect impurities that are contained in food and beverage products during mass production processes. Thus, the quality control of products can be improved. Also, a sensor which detects the smell generated from coffee roasting can accurately determine the degree of roasting. The volatile metabolites generated from bacterial fermentation processes can also be analyzed. Consequently, the progress of such processes can be easily monitored in real-time using the bioelectronic noses. Bioelectronic noses can be effectively applied to not only the examples described above, but also any cases where the process has a resultant smell or chemical release.

14.2 Perspectives

14.2.1 Standardization of Smell

In contrast to the senses of vision and hearing, the sense of smell does not have a method to precisely express the information of smell or flavors, even though it plays an important role in our daily life. It is very difficult to create a database and to standardize the information obtained from the olfactory sense, because it usually responds to complex components consisting of a vast variety of chemical elements. The classification and description of smell depends on quite subjective and abstractive expressions and smell cannot be precisely described or quantified using these kinds of expressions.

In the early classification of smells, they were grouped based on the expertise of scholars, such as a chemist and botanist, and the classification of smells through experiments was begun in the 1900s [93]. The odor prism was proposed by Henning (1916) suggesting verbal odor descriptions using six odor qualities, which are spice, fragrant, resinous, ethereal, foul, and burnt [94]. From the middle of the 1930s, there have been many trials to classify odor quality by connecting the odor perception to its chemical structure [95–99], but it is still not possible to explain odor sensation and perception. Prior to finding the number of human olfactory receptors, several scientists tried to quantify and classify odors based on the function and type of olfactory receptors [100–103], but this approach has been largely suspended, as it has been identified that the number of human olfactory receptors are at least 320 [104].

Standardization and classification of smell is important not only for the general odor, but also for the malodor. Since standardization of method for measuring malodor was begun by many countries in the 1970s, studies for standardization of malodor have mainly depended on a sensory test using human olfaction. Up to now, malodor is recognized by several detection methods like scentometer and olfactometer, which have been widely used, but these methods do not provide the exact information together with lots of variables, mainly due to dependence on human olfaction. The human-like smell sensing system is useful for these reasons.

Various studies are being currently carried out also for the display of olfaction, for example, the development of a device exuding flavor through the internet, and the development of a method and device for smell transmission. However, these kinds of trials have a limitation, in that the device does not deliver the exact information of a smell because there is no criterion for a variety of smells. Standardization of smell will be an important criterion for digitalization of emotional expression of smell, as well as for various applications of smell and flavor, and therefore, needs to be achieved by mimicking human olfaction as closely as possible.

The bioelectronic nose may be the best device mimicking the human olfactory system, because it utilizes the human olfactory receptors as a sensing element and employs the olfactory signaling processes. It is expected that a method for the standardization of smell can be developed through the qualitative and quantitative measurement by bioelectronic nose, and eventually, that a relationship between data from bioelectronics nose and sensory evaluation data from human olfaction would be established. The established standard of smell can be widely utilized in various industries such as food, beverage, agriculture, flavor and perfume, as well as biomedicine and environmental monitoring.

14.2.2 Multi-channel Bioelectronic Nose

The human olfactory system has an array type recognition system, which consists of about 390 different olfactory neuron cells . Each type of olfactory neuron cell has one kind of its own olfactory receptor on the cell surface. The olfactory system efficiently recognizes even a wide range of odor mixtures, as well as single kind of odor molecule, by employing the array type recognition system. This suggests that multiple odor compounds can be precisely identified using array type sensing system, as in the case of the human olfactory system.

Therefore it is necessary to make an array type of sensor for identifying the profile of a mixture, as well as for enhancing the discriminative ability of the bioelectronic nose. The array type sensor is integrated with several different types of sensors, the number of which depends on the type of samples to be analyzed. The greater the number of sensor integrated in one platform, the better sensor system is able to identify analytes. In contrast to the conventional electronic nose, using limited number of chemical sensor array, bioelectronic noses, especially those using olfactory receptors as a sensing material, are the best example for developing a multiplexed sensing platform. The bioelectronic nose can utilize lots of biomaterials as a sensing material, for example, 390 different types of human olfactory receptors, which are about 40 % out of approximately 1,000 olfactory receptor genes, are functional, and can be used as sensing materials.

Recently, researchers have tried to integrate a large number of olfactory receptors on a single platform to detect a variety of odor molecules simultaneously, and are developing data processing techniques for the analysis of electrical information produced from a bioelectronic nose consisting of multi-channel sensors (Fig. 14.3). This kind of multi-channel bioelectronic nose can be usefully applied to the analysis of a whole profile of an odorant mixture. For example, various severe diseases, including cancer and diabetes, which produce specific odor molecules as a biomarker, emit not one specific biomarker but many disease-specific biomarkers through the exhaled breath. It is very important to identify multiple biomarkers for the exact diagnosis of these complex diseases . Thus, most target samples to be identified are complex mixtures containing a variety of different odor components. So, many analytical methods have been used to recognize patterns of specific signals obtained from the response of electronic noses to complex mixtures, as well as in order to detect a specific biomarker. Nevertheless, there is a limit to the number of sensor elements which can be integrated in a single platform, and a vast array of potential data to be analyzed. A multi-channel bioelectronic nose would provide a powerful tool for a great advance in various fields of application of these analytical methods.

Fig. 14.3
figure 3

Multi-channel platform of bioelectronic nose. In each channel, each type of olfactory receptor is combined with a carbon nanotube FET sensor. Signals are generated from the interaction of various olfactory receptors with odor molecules

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14.2.3 Visualization of Smell

In order to discriminate among thousands of odors, we do not have any reliable artificial sensing device, but still depend on our human sensory system. However, the information obtained through our natural sensory system does not provide us with the objective information about the odors. Up to now, complex experimental steps, large-scale equipment, well-trained experts, or electronic noses have partially fulfilled the need for the detection or analysis of odors. In addition, it is difficult to collect the data on the odorant response pattern to be used as an objective index. However, if the odor is visualized, the visualized pattern can be used as a code for the odor like a QR code.

Several approaches have been taken to visualize the olfactory signal transduction in the cell-based system. Calcium imaging , cAMP response element (CRE) reporter assay , and bioluminescence resonance energy transfer (BRET) assay are representative methods used for the visualization of odorant response. When the cells expressing ORs are stimulated with specific odorants, the cellular signaling cascade is generated and the ion-channels open. The calcium imaging detects the calcium ion which enters into the cells through the ion-channel, using calcium binding fluorescent dye, and this system is the most widely used odorant detection method [105]. However, its use is limited due to the fact that calcium imaging is time-consuming and labor-intensive, and thus, CRE reporter assay has been developed as an alternative method [106, 107]. In the CRE reporter system, the secondary signal messenger cAMP generated through the odorant stimulation binds to, and activates the signaling molecules, resulting in activation of the CRE promoter to express the luminescence or fluorescence protein as a reporter . For the protein-based visualization of odorant binding response, the BRET assay was carried out by the insertion of sequences encoding a bioluminescent donor and a fluorescent acceptor protein at the C-terminus of olfactory receptor ODR-10 and the third intracellular loop, respectively [108]. The conformational change of the olfactory receptor upon ligand stimulation induced the change of BRET signal. These methods were applied as useful visualization techniques, but have limited use in various olfactory receptors because of the labor-intensive and time-consuming processes involved.

Recently, a simple and low-cost miniaturized odorant screening platform was fabricated by a Micro-Electro-Mechanical system (MEMs) and a reverse transfection technique was used to visualize the odorant response in a high-throughput format [109]. Each different olfactory receptor DNA with transfection reagent was inserted into each PEG microwell, then HEK293 cells containing the CRE report system were spread over the PEG microwell array plate. Using this method, each different olfactory receptor was expressed in each well and the odorant response of each OR was detected using fluorescent CRE reporter protein simultaneously. Since this OR DNA printed-based microwell can be stored stably, a large quantity of the microwell can be fabricated and the odorant response can be visualized easily through seeding the cells containing the CRE reporter system.

Cell-based visualization systems can generate intuitive images of smell. However, a labor-intensive and skilled handling of cells is required to obtain reproducible and reliable results. Furthermore, the detection of odors in a gas-phase is difficult in the case of the cell-based systems, because a liquid environment is essential for the survival of cells. Bioelectronic noses can offer a perfect solution to these problems if several technologies are combined. First of all, the technique that integrates a lot of independent electronic modules into a single chip with a high density should be applied because all kinds of olfactory receptors have to be used in a multi-channel sensor array. Also, signal processing methods to analyze the parallel signals generated from the multi-channel sensor and to express the results in visual images must be established. In addition, conventional microfluidic systems that constantly supply a small amount of gas samples to the sensor platform can be effectively integrated. A bioelectronic nose that detects gaseous odorants has already been successfully demonstrated [78]. These integrations will facilitate the development of an effective sensor that easily converts smells into a visual representation (Fig. 14.4).

Fig. 14.4
figure 4

Visualization of odor compounds originating from various sources using a bioelectronic nose. (Reprinted from Ref. [48] with permission from Nature Publishing Group)

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14.3 Conclusion

The bioelectronic nose consists of two major parts, which are primary and secondary transducers. The primary transducer is a biological recognition element, and the secondary transducer is a non-biological signal transducing element. The representative primary transducers are cells, nanovesicles, proteins and peptides. The cell acts as a live sensing material, and may be the best system for mimicking the human nose because it has all the components necessary for signaling induced by analytes. However, it is difficult for cells to be maintained on the sensor platform for a long period. Nanovesicles also can be a good candidate for biosensing material. Although they are not alive, they still have all the components necessary for the olfactory signal transduction like cells . They can be stored in a freezer for several months. Olfactory receptor proteins and peptides are most appropriate for the commercial biosensor, because they are quite stable even at room temperature. The representative secondary transducers are QCM, SPR, EIS , microelectrode, carbon nanotube, conducting polymer nanotube, conducting polymer nanoparticles, and graphene . The integration of the olfactory biological recognition elements with these nano-devices enables the bioelectronics nose to have extremely high selectivity and sensitivity.

The unique characteristic of the bioelectronic nose is to mimic the human olfactory system in order to detect smell as accurately as a human nose. Eventually, the most ideal bioelectronics nose mimicking the human olfactory system will be developed through the integration of 390 types of human olfactory receptors on a single sensor platform because human olfactory system has 390 different functional olfactory receptors. It will be very meaningful to develop the bioelectronic nose consisting of 390 receptor elements in terms of the artificial realization of the human olfactory sense.

The bioelectronic nose can provide a powerful tool to detect and discriminate among smell compounds, which are emitted from almost everything, and exist everywhere. The change of smell in regard to type and concentration provides us with information on the objects and environments surrounding us. The bioelectronic nose can be applied in various fields, such as in the standardization of smell, development of code for each smell, medical diagnosis, quality assessment of food and beverage, as well as scent-related industries, including perfume, cosmetics, wine and coffee, agricultural application, environmental monitoring, process monitoring , and the detection of explosives, toxicants and drugs. Consequently, the development of a bioelectronic nose system is expected to play an important role in improving the quality of human life.