Polypyrrole nanotubes conjugated with human olfactory receptors: high-performance transducers for FET-type bioelectronic noses

Advances in Bioelectrode Design for Developing Electrochemical Biosensors

The critical performance factors such as selectivity, sensitivity, operational and storage stability, and response time of electrochemical biosensors are governed mainly by the function of their key component, the bioelectrode. Suitable design and fabrication strategies of the bioelectrode interface are essential for realizing the requisite performance of the biosensors for their practical utility. A multifaceted attempt to achieve this goal is visible from the vast literature exploring effective strategies for preparing, immobilizing, and stabilizing biorecognition elements on the electrode surface and efficient transduction of biochemical signals into electrical ones (i.e., current, voltage, and impedance) through the bioelectrode interface with the aid of advanced materials and techniques. The commercial success of biosensors in modern society is also increasingly influenced by their size (and hence portability), multiplexing capability, and coupling in the interface of the wireless communication technology, which facilitates quick data transfer and linked decision-making processes in real-time in different areas such as healthcare, agriculture, food, and environmental applications. Therefore, fabrication of the bioelectrode involves careful selection and control of several parameters, including biorecognition elements, electrode materials, shape and size of the electrode, detection principles, and various fabrication strategies, including microscale and printing technologies. This review discusses recent trends in bioelectrode designs and fabrications for developing electrochemical biosensors. The discussions have been delineated into the types of biorecognition elements and their immobilization strategies, signal transduction approaches, commonly used advanced materials for electrode fabrication and techniques for fabricating the bioelectrodes, and device integration with modern electronic communication technology for developing electrochemical biosensors of commercial interest.

Artificial Olfactory Biohybrid System: An Evolving Sense of Smell

The olfactory system can detect and recognize tens of thousands of volatile organic compounds (VOCs) at low concentrations in complex environments. Bioelectronic nose (B-EN), which mimics olfactory systems, is becoming an emerging sensing technology for identifying VOCs with sensitivity and specificity. B-ENs integrate electronic sensors with bioreceptors and pattern recognition technologies to enable medical diagnosis, public security, environmental monitoring, and food safety. However, there is currently no commercially available B-EN on the market. Apart from the high selectivity and sensitivity necessary for volatile organic compound analysis, commercial B-ENs must overcome issues impacting sensor operation and other problems associated with odor localization. The emergence of nanotechnology has provided a novel research concept for addressing these problems. In this work, the structure and operational mechanisms of biomimetic olfactory systems are discussed, with an emphasis on the development and immobilization of materials. Various biosensor applications and current developments are reviewed. Challenges and opportunities for fulfilling the potential of artificial olfactory biohybrid systems in fundamental and practical research are investigated in greater depth.

Nanomaterial Gas Sensors for Biosensing Applications: A Review

BACKGROUND

Nanomaterial is one of the most used materials for various gas sensing applications to detect toxic gases, human breath, and other specific gas sensing. One of the most important applications of nanomaterial based gas sensors is biosensing applications. In this review article, the gas sensors for biosensing are discussed on the basis of crystalline structure and different categories of nanomaterial.

METHODS

In this paper, firstly, rigorous efforts have been made to find out research questions by going through a structured and systematic survey of available peer reviewed high quality articles in this field. The papers related to nanomaterial based biosensors are then reviewed qualitatively to provide substantive findings from the recent developments in this field.

RESULTS

In this mini-review article, firstly, classifications of nanomaterial gas sensors have been presented on the basis of the crystalline structure of nanomaterial and different types of nanomaterial available for biosensing applications. Further, the gas sensors based on nanomaterial for biosensing applications are collected and reviewed in terms of their performance parameters such as sensing material used, target gas component, detection ranges (ppm-ppb), response time, operating temperature and method of detection, etc. The different nanomaterials possess slightly different sensing and morphological properties due to their structure; therefore, it can be said that a nanomaterial must be selected carefully for a particular application. The 1D nanomaterials show the best selectivity and sensitivity for gases available in low concentration ranges due to their miniaturised structure compared to 2D and 3D nanomaterials. However, these 2D and 3D nanomaterials also so good sensing properties compared to bulk semiconductor materials. The polymer and nanocomposites which are also discussed in this patent article have opened the door for future research and have great potential for new generation gas sensors for detecting biomolecules.

CONCLUSION

These nanomaterials extend great properties towards sensing the application of different gases for a lower concentration of particular gas particles. Nano polymer and nanocomposites have great potential to be used as gas sensors for the detection of biomolecules.

Field-Effect Sensors Using Biomaterials for Chemical Sensing

After millions of years of evolution, biological chemical sensing systems (i.e., olfactory and taste systems) have become very powerful natural systems which show extreme high performances in detecting and discriminating various chemical substances. Creating field-effect sensors using biomaterials that are able to detect specific target chemical substances with high sensitivity would have broad applications in many areas, ranging from biomedicine and environments to the food industry, but this has proved extremely challenging. Over decades of intense research, field-effect sensors using biomaterials for chemical sensing have achieved significant progress and have shown promising prospects and potential applications. This review will summarize the most recent advances in the development of field-effect sensors using biomaterials for chemical sensing with an emphasis on those using functional biomaterials as sensing elements such as olfactory and taste cells and receptors. Firstly, unique principles and approaches for the development of these field-effect sensors using biomaterials will be introduced. Then, the major types of field-effect sensors using biomaterials will be presented, which includes field-effect transistor (FET), light-addressable potentiometric sensor (LAPS), and capacitive electrolyte–insulator–semiconductor (EIS) sensors. Finally, the current limitations, main challenges and future trends of field-effect sensors using biomaterials for chemical sensing will be proposed and discussed.

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.

Improvement of capacitive performance of polyaniline based hybrid supercapacitor

This study focuses on synthesis of polyaniline composites and application as electrode material in the fabrication of hybrid supercapacitors. Hybrid supercapacitors were fabricated using aluminum foil as current collector, and conductive Polyaniline (PANI) composites as electrode materials. Cobalt oxide (Co2O3), ammonium peroxydisulfate (APS) were used along with PANI in the preparation of electrodes of the supercapacitor. Polyaniline and its various composites were used in the cathode and activated carbon in the anode (positive electrodes) of the asymmetric hybrid supercapacitor. Electrochemical performance of the supercapacitors has been evaluated on the basis of capacitive properties (i.e. capacitance, energy density and power density). Area of the supercapacitor, applied voltage for charging and composition of cathode materials have been optimized in this study. The optimum area of the supercapacitor was found 30 cm2 at optimum voltage 5 V. The result showed that supercapacitor fabricated using polyaniline (PANI) as cathode material with Al foil as current collector provided highest capacitance of 249 F/g, Energy density of 31 Wh/kg and Power density of 18 W/kg. Among various PANI composites (Ni-PANI, Cu-PANI, CNF-PANI) synthesized in this study, Ni-PANI composite showed highest capacitance of 336 F/g, energy density of 42 Wh/kg, and power density of 31 W/kg. The result indicates that Ni-PANI composite has high potential as cathode materials for the hybrid supercapacitor.

Advancements in Microfabricated Gas Sensors and Microanalytical Tools for the Sensitive and Selective Detection of Odors

In recent years, advancements in micromachining techniques and nanomaterials have enabled the fabrication of highly sensitive devices for the detection of odorous species. Recent efforts done in the miniaturization of gas sensors have contributed to obtain increasingly compact and portable devices. Besides, the implementation of new nanomaterials in the active layer of these devices is helping to optimize their performance and increase their sensitivity close to humans' olfactory system. Nonetheless, a common concern of general-purpose gas sensors is their lack of selectivity towards multiple analytes. In recent years, advancements in microfabrication techniques and microfluidics have contributed to create new microanalytical tools, which represent a very good alternative to conventional analytical devices and sensor-array systems for the selective detection of odors. Hence, this paper presents a general overview of the recent advancements in microfabricated gas sensors and microanalytical devices for the sensitive and selective detection of volatile organic compounds (VOCs). The working principle of these devices, design requirements, implementation techniques, and the key parameters to optimize their performance are evaluated in this paper. The authors of this work intend to show the potential of combining both solutions in the creation of highly compact, low-cost, and easy-to-deploy platforms for odor monitoring.

Ultrasensitive Stress Biomarker Detection Using Polypyrrole Nanotube Coupled to a Field-Effect Transistor

Stress biomarkers such as hormones and neurotransmitters in bodily fluids can indicate an individual’s physical and mental state, as well as influence their quality of life and health. Thus, sensitive and rapid detection of stress biomarkers (e.g., cortisol) is important for management of various diseases with harmful symptoms, including post-traumatic stress disorder and depression. Here, we describe rapid and sensitive cortisol detection based on a conducting polymer (CP) nanotube (NT) field-effect transistor (FET) platform. The synthesized polypyrrole (PPy) NT was functionalized with the cortisol antibody immunoglobulin G (IgG) for the sensitive and specific detection of cortisol hormone. The anti-cortisol IgG was covalently attached to a basal plane of PPy NT through an amide bond between the carboxyl group of PPy NT and the amino group of anti-cortisol IgG. The resulting field-effect transistor-type biosensor was utilized to evaluate various cortisol concentrations. Cortisol was sensitively measured to a detection limit of 2.7 × 10⁻¹⁰ M (100 pg/mL), with a dynamic range of 2.7 × 10⁻¹⁰ to 10⁻⁷ M; it exhibited rapid responses (<5 s). We believe that our approach can serve as an alternative to time-consuming and labor-intensive health questionnaires; it can also be used for diagnosis of underlying stress-related disorders.

The Emergence of Insect Odorant Receptor-Based Biosensors

The olfactory receptor neurons of insects and vertebrates are gated by odorant receptor (OR) proteins of which several members have been shown to exhibit remarkable sensitivity and selectivity towards volatile organic compounds of significant importance in the fields of medicine, agriculture and public health. Insect ORs offer intrinsic amplification where a single binding event is transduced into a measurable ionic current. Consequently, insect ORs have great potential as biorecognition elements in many sensor configurations. However, integrating these sensing components onto electronic transducers for the development of biosensors has been marginal due to several drawbacks, including their lipophilic nature, signal transduction mechanism and the limited number of known cognate receptor-ligand pairs. We review the current state of research in this emerging field and highlight the use of a group of indole-sensitive ORs (indolORs) from unexpected sources for the development of biosensors.

A 3D-Printed Hybrid Nasal Cartilage with Functional Electronic Olfaction

Advances in biomanufacturing techniques have opened the doors to recapitulate human sensory organs such as the nose and ear in vitro with adequate levels of functionality. Such advancements have enabled simultaneous targeting of two challenges in engineered sensory organs, especially the nose: i) mechanically robust reconstruction of the nasal cartilage with high precision and ii) replication of the nose functionality: odor perception. Hybrid nasal organs can be equipped with remarkable capabilities such as augmented olfactory perception. Herein, a proof-of-concept for an odor-perceptive nose-like hybrid, which is composed of a mechanically robust cartilage-like construct and a biocompatible biosensing platform, is proposed. Specifically, 3D cartilage-like tissue constructs are created by multi-material 3D bioprinting using mechanically tunable chondrocyte-laden bioinks. In addition, by optimizing the composition of stiff and soft bioinks in macro-scale printed constructs, the competence of this system in providing improved viability and recapitulation of chondrocyte cell behavior in mechanically robust 3D constructs is demonstrated. Furthermore, the engineered cartilage-like tissue construct is integrated with an electrochemical biosensing system to bring functional olfactory sensations toward multiple specific airway disease biomarkers, explosives, and toxins under biocompatible conditions. Proposed hybrid constructs can lay the groundwork for functional bionic interfaces and humanoid cyborgs.

Bead-String-Shaped DNA Nanowires with Intrinsic Structural Advantages and Their Potential for Biomedical Applications

Due to high programmability and good biocompatibility, DNA has been recognized as a powerful building block for engineering of sophisticated nanostructures for different purposes. Herein, we present the first example of a bead-string-shaped DNA nanowire (BS-nanow) with long-range structural order for in vivo bioimaging and targeted drug delivery. BS-nanow is assembled from DNA tetrahedron units with precise nanometer-scale spatial control, capable of accommodating chemotherapeutic agents with high payload capacity (1204 binding sites) as well as possessing a 60-fold enhanced binding affinity for target cells. Confocal fluorescence imaging and in vivo experiments on CEM subcutaneous tumor-bearing mice show that specific bioimaging of living cells and even systemic delivery of drugs into internal tumor organs and tissues were accomplished, thereby achieving an efficient inhibition of tumor growth in the xenograft model without systemic toxicity. BS-nanow's show potential in vivo applications in accurate diagnosis and targeted therapy for human cancer.

Cranial Nerve Stimulation for Olfaction (Cranial Nerve 1)

Like sensory maps in other systems, the sense of smell has an organizational structure based on converging projections of olfactory receptor neurons containing unique odorant receptors onto the olfactory bulb in synaptic aggregations termed glomeruli. This organizational structure provides the potential for electrical stimulation and restoration of smell. Prior animal and human studies support the feasibility of an olfactory stimulation device, encouraging ongoing work in development of olfactory implants.

Activation of Haa1 and War1 transcription factors by differential binding of weak acid anions in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Haa1 and War1 transcription factors are involved in cellular adaptation against hydrophilic weak acids and lipophilic weak acids, respectively. However, it is unclear how these transcription factors are differentially activated depending on the identity of the weak acid. Using a field-effect transistor (FET)-type biosensor based on carbon nanofibers, in the present study we demonstrate that Haa1 and War1 directly bind to various weak acid anions with different affinities. Haa1 is most sensitive to acetate, followed by lactate, whereas War1 is most sensitive to benzoate, followed by sorbate, reflecting their differential activation during weak acid stresses. We show that DNA binding by Haa1 is induced in the presence of acetic acid and that the N-terminal Zn-binding domain is essential for this activity. Acetate binds to the N-terminal 150-residue region, and the transcriptional activation domain is located between amino acid residues 230 and 483. Our data suggest that acetate binding converts an inactive Haa1 to the active form, which is capable of DNA binding and transcriptional activation.

Polypyrrole-coated phase-change liquid perfluorocarbon nanoparticles for the visualized photothermal-chemotherapy of breast cancer

A theranostic nanoplatform (DTX/PFH@PPy-FA) for multi-modal imaging-guided photothermal-chemotherapy has been constructed. Lipid-perfluorohexane (PFH) nanodroplet loaded with docetaxel (DTX) was coated with a polypyrrole (PPy) shell. Then the folic acid (FA) molecule with active tumor-targeting function was modified on the surface of PPy shell. Due to the good photothermal conversion performance, PPy shell can raise the temperature under the near infrared laser irradiation, which not only produces photothermal effect to kill tumor cells, but also promotes liquid-gas phase change of PFH, and produces ultrasound imaging effect. The results of photothermal experiment and imaging experiment confirmed that the obtained DTX/PFH@PPy-FA possessed good photothermal, photoacoustic imaging and ultrasound imaging effects in vitro and in vivo. The results of in vitro cell experiments showed that DTX/PFH@PPy-FA had a active targeting ability to tumor cells, and its photothermal-chemotherapy synergistically inhibited the proliferation of tumor cells. In vivo study on 4T1-bearing BALB/c mice indicated that the photothermal-chemotherapy of DTX/PFH@PPy-FA not only effectively inhibited the growth of 4T1 breast cancer, but also inhibited lung metastasis. This multifunctional nanoparticle is expected to become a new nanoplatform for the visualized photothermal-chemotherapy of cancer. STATEMENT OF SIGNIFICANCE: In this work, we presented a multi-modal imaging-guided photothermal-chemotherapy theranostic nanoplatform (DTX/PFH@PPy-FA) for visualized treatment of breast cancer. The docetaxel (DTX) loaded perfluorohexane (PFH) nanodroplets (DTX/PFH@SPC) were firstly prepared and then coated with polypyrrole shell (PPy). Then, PEGylated folic acid was covalently modified to obtain the folate-targeted multifunctional nanoparticle (DTX/PFH@PPy-FA). Due to the good photothermal conversion performance, PPy shell can raise the temperature under the near infrared laser irradiation, which not only produces photothermal effect to kill tumor cells, but also promotes liquid-gas phase change of PFH, and produces good ultrasound imaging effect. The PPy shell also imparts photoacoustic imaging characteristics to the nanoparticles. Experimental results show that our prepared DTX/PFH@PPy-FA possesses folic acid-mediated tumor targeting ability, ultrasound and photoacoustic imaging, and photothermal-chemotherapy synergistic effect. This multi-functional nanoparticle is expected to become a new platform for the visualized photothermal-chemotherapy of breast cancer.

Visual detection of odorant geraniol enabled by integration of a human olfactory receptor into polydiacetylene/lipid nano-assembly

A new polydiacetylene lipid/human olfactory receptor nano-assembly was fabricated for the visual detection of an odorant for the first time. The assembly consisted of phospholipid-mixed polydiacetylenes (PDAs) and human olfactory receptors (hORs) in detergent micelles. To overcome the limitations of bioelectronic noses, hOR-embedded chromatic complexes (PDA/hORs) were developed, introducing PDAs that showed color and fluorescence transitions against various stimuli. The chromatic nanocomplexes reacted with target molecules, showing a fluorescence intensity increase in a dose-dependent manner and target selectivity among various odorants. As a result, a color transition of the assembly from blue to purple occurred, allowing the visual detection of the odorant geraniol. Through circular dichroism (CD) spectroscopy and a tryptophan fluorescence quenching method, the structural and functional properties of the hORs embedded in the complexes were confirmed. Based on this first work, future array devices, integrating multiple nano-assemblies, can be substantiated and utilized in environmental assessment and analysis of food quality.

Combining SWNT and Graphene in Polymer Nanofibers: A Route to Unique Carbon Precursors for Electrochemical Capacitor Electrodes

It is important to fabricate nanostructured architectures comprised of functional components for a wide variety of applications because precise structural control in the nanometer regime can yield unprecedented, fascinating properties. Owing to their well-defined microstructural characteristics, it has been popular to use carbon nanospecies, such as nanotubes and graphene, in fabricating nanocomposites and nanohybrids. Nevertheless, it still remains hard to control and manipulate nanospecies for specific applications, thus preventing their commercialization. Herein, first, we report unique one-dimensional nanoarchitectures with meso-/macropores, consisting of single-walled nanotubes (SWNTs), graphene, and polyacrylonitrile, in which poly(vinyl alcohol) was employed as a dispersing agent and sacrificial porogen. One-dimensional SWNTs and two-dimensional graphene pieces were combined in the confined interior space of electrospun nanofibers, which led to unique microstructural characteristics such as enhanced ordering of SWNTs, graphene pieces, and polymer chains in the nanofiber interior. Next, the SWNT/graphene-in-polymer nanofiber (SGPNF) structures were converted into carbonized products (SGCNFs) with effective porosity and tunable electrochemical properties. Similar to SGPNFs, the microstructural and electrical properties of the SGCNFs depended on the incorporated amount of SWNT and graphene. At higher SWNT content, the mesopore volume proportion and specific discharge capacitance of the SGCNFs increased by max. 63 and 598%, respectively. The SGCNFs showed strong potential as a high-performance electrode material for electrochemical capacitors (max. capacitance: nonactivated ∼390 F g^-1 and activated ∼750 F g^-1). Flexible, all solid-state capacitor cells based on SGCNFs were also successfully demonstrated as a model application. The SGCNFs can be further functionalized by various methods, which will impart attractive properties for extended 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.

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.

Functional expression of olfactory receptors using cell-free expression system for biomimetic sensors towards odorant detection

How to obtain sufficient functional olfactory receptors and coupled to transducers with high efficiency are crucial to biomimetic olfactory-based biosensors. In this study, a cell-free expression system was employed to prepare functional olfactory receptors that were utilized as sensitive elements for biomimetic olfactory receptor-based biosensors. A nematode olfactory receptor, ODR-10, was used as a model of olfactory receptors and expressed in Escherichia coli (E. coli) cell-free expression system. To improve the coupling efficiency and obtain the on-chip purification, a His₆-tag was expressed as a fusion to ODR-10, which makes the expressed ODR-10 capable of selectively binding to the sensor surface modified with anti-His₆-tag aptamers. Electrolyte-insulator-semiconductor (EIS) sensors were utilized as the transducer to measure the capacitance changes induced by the ODR-10 responding to its natural ligand, diacetyl. The results show that ODR-10 was successfully expressed using E. coli cell-free expression system and the expression could be promoted by adding 0.5% Brij58 as the detergent. Capacitance measurement results indicate that this olfactory receptor-based biosensor can detect diacetyl with high specificity and sensitivity. Concentration-dependent linear response to diacetyl ranging from 0.01 nM to 1 nM was obtained. The detection limit was as low as 0.01 nM. All the results demonstrated that the cell-free expression system provides a new approach for the preparation of functional olfactory receptors, which have great potential to be applied in biomimetic sensors towards chemical sensing.

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.

Chemical Design of Functional Polymer Structures for Biosensors: From Nanoscale to Macroscale

Over the past decades, biosensors, a class of physicochemical detectors sensitive to biological analytes, have drawn increasing interest, particularly in light of growing concerns about human health. Functional polymeric materials have been widely researched for sensing applications because of their structural versatility and significant progress that has been made concerning their chemistry, as well as in the field of nanotechnology. Polymeric nanoparticles are conventionally used in sensing applications due to large surface area, which allows rapid and sensitive detection. On the macroscale, hydrogels are crucial materials for biosensing applications, being used in many wearable or implantable devices as a biocompatible platform. The performance of both hydrogels and nanoparticles, including sensitivity, response time, or reversibility, can be significantly altered and optimized by changing their chemical structures; this has encouraged us to overview and classify chemical design strategies. Here, we have organized this review into two main sections concerning the use of nanoparticles and hydrogels (as polymeric structures) for biosensors and described chemical approaches in relevant subcategories, which act as a guide for general synthetic strategies.

Shining Light on Molecular Mechanism for Odor-selectivity of CNT-immobilized Olfactory Receptor

Olfactory receptor (OR)-based bioelectronic nose is a new type of bio-affinity sensor applied for detecting numerous odorant molecules. In order to elucidate the effect of the adsorption of nanomaterial carriers on the receptor structure and its selectivity to odors, we used a systematic computation-scheme to study two OR models immobilized onto carbon nanotube. Our result indicates that there is a multistep OR-adsorption process driven by hydrophobic interaction. Many allosteric communication pathways exist between the absorbed residues and the pocket ones, leading to a significant shrinkage of the pocket. Consequently, the size-selectivity of the receptor to the odors is changed to some extent. But, the odor size and its hydrophobicity, rather than specific functional groups of the odor, still play a determinant role in binding OR, at least for the 132 odors under study. Regardless of the limitation for the odor size in initial recognition, the different-size odors could induce significant changes in the pocket conformation so that it could better match the pocket space, indicating the importance of the ligand-fit binding. Due to the CNT-induced shrinkage of the pocket, the CNT immobilization could increase the binding affinity through enhancing van der Waals interaction, in particular for the large odors.

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.

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.

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.

Platinum nanoparticles immobilized on polypyrrole nanofibers for non-enzyme oxalic acid sensor

Oxalic acid (OA), naturally available in many fruits and vegetables, reacts easily with Ca²⁺ and Mg²⁺ ions to produce an insoluble salt.

Plasma surface modification of polymers for sensor applications

Polymeric sensors play an increasingly important role in monitoring the environment we live in, providing relevant information for a host of applications.

Application of Electronic Nose Systems on Animal-Source Food

Electronic nose, which is designed to perceive artificially the odour-active molecules in a sample headspace, has seen an increased use in the food industry as a rapid and reliable tool for quality assessment, classification, and authentication of several food items. The use of chemometrics and pattern recognition methods, together with gas sensors, emerged to be a very powerful analytical approach. In this chapter, an overview of the recent achievements in the field of electronic nose applications on animal-source food is given. Moreover, the authors deal with the recent research trends to overcome the actual sensor shortcomings, including sensor fusion techniques and their applications to evaluate animal-source foods and novel electronic nose systems.

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.

Dopamine Receptor D1 Agonism and Antagonism Using a Field-Effect Transistor Assay

The field-effect transistor (FET) has been used in the development of diagnostic tools for several decades, leading to high-performance biosensors. Therefore, the FET platform can provide the foundation for the next generation of analytical methods. A major role of G-protein-coupled receptors (GPCRs) is in the transfer of external signals into the cell and promoting human body functions; thus, their principle application is in the screening of new drugs. The research community uses efficient systems to screen potential GPCR drugs; nevertheless, the need to develop GPCR-conjugated analytical devices remains for next-generation new drug screening. In this study, we proposed an approach for studying receptor agonism and antagonism by combining the roles of FETs and GPCRs in a dopamine receptor D1 (DRD1)-conjugated FET system, which is a suitable substitute for conventional cell-based receptor assays. DRD1 was reconstituted and purified to mimic native binding pockets that have highly discriminative interactions with DRD1 agonists/antagonists. The real-time responses from the DRD1-nanohybrid FET were highly sensitive and selective for dopamine agonists/antagonists, and their maximal response levels were clearly different depending on their DRD1 affinities. Moreover, the equilibrium constants (K) were estimated by fitting the response levels. Each K value indicates the variation in the affinity between DRD1 and the agonists/antagonists; a greater K value corresponds to a stronger DRD1 affinity in agonism, whereas a lower K value in antagonism indicates a stronger dopamine-blocking effect.

A highly sensitive FET-type aptasensor using flower-like MoS2 nanospheres for real-time detection of arsenic(iii)

Arsenic (As) contamination in drinking water is problematic due to its threat to human health. Highly stable, sensitive and selective sensors are needed for As(iii), which is the most toxic of the various forms of arsenic. Herein, we describe the fabrication of field-effect transistor (FET)-type aptasensors for As(iii) detection that is based on carboxylic polypyrrole (CPPy)-coated flower-like MoS₂ nanospheres (CFMNSs). Arsenic-binding aptamer-conjugated CFMNSs were integrated into a liquid-ion gated FET system, leading to extraordinary performance with a rapid response (on a time scale of less than 1 s). Field-induced current changes occurred through the interaction between the aptamer and As(iii), resulting in sensitive discrimination of As(iii) at unprecedentedly low concentrations (ca. 1 pM). Moreover, the CFMNS-based aptasensor selectively recognized As(iii) among numerous other metal ions and accurately detected As(iii) in a mixed solution. The FET aptasensor could also discriminate target analytes in a real sample derived from river water. This MoS₂-based aptasensor is a promising tool for the detection of As(iii) and could be used in a wide range of practical applications.

Electrical and Electrochemical Properties of Conducting Polymers

Conducting polymers (CPs) have received much attention in both fundamental and practical studies because they have electrical and electrochemical properties similar to those of both traditional semiconductors and metals. CPs possess excellent characteristics such as mild synthesis and processing conditions, chemical and structural diversity, tunable conductivity, and structural flexibility. Advances in nanotechnology have allowed the fabrication of versatile CP nanomaterials with improved performance for various applications including electronics, optoelectronics, sensors, and energy devices. The aim of this review is to explore the conductivity mechanisms and electrical and electrochemical properties of CPs and to discuss the factors that significantly affect these properties. The size and morphology of the materials are also discussed as key parameters that affect their major properties. Finally, the latest trends in research on electrochemical capacitors and sensors are introduced through an in-depth discussion of the most remarkable studies reported since 2003.

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.

Bioelectronic Nose: An Emerging Tool for Odor Standardization

Odors are perceived differently as a function of individual human experience, and communicating about odors between individuals is therefore very difficult. There is a need to classify and standardize odors, but appropriate tools have not yet been developed. A bioelectronic nose mimics human olfaction and detects target molecules with high sensitivity and selectivity. This new tool has great potential in many applications and is expected to accelerate odor classification and standardization. In particular, a multiplexed bioelectronic nose can provide complex odor information using pattern recognition techniques, and could even reproduce odors via an integrated olfactory display system. We expect that a bioelectronic nose will be a useful tool for odor standardization by providing codes for odors that enable us to communicate odor information.

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 Using Odorant Binding Protein-Derived Peptide and Carbon Nanotube Field-Effect Transistor for the Assessment of Salmonella Contamination in Food

Salmonella infection is the one of the major causes of food borne illnesses including fever, abdominal pain, diarrhea, and nausea. Thus, early detection of Salmonella contamination is important for our healthy life. Conventional detection methods for the food contamination have limitations in sensitivity and rapidity; thus, the early detection has been difficult. Herein, we developed a bioelectronic nose using a carbon nanotube (CNT) field-effect transistor (FET) functionalized with Drosophila odorant binding protein (OBP)-derived peptide for easy and rapid detection of Salmonella contamination in ham. 3-Methyl-1-butanol is known as a specific volatile organic compound, generated from the ham contaminated with Salmonella. We designed and synthesized the peptide based on the sequence of the Drosophila OBP, LUSH, which specifically binds to alcohols. The C-terminus of the synthetic peptide was modified with three phenylalanine residues and directly immobilized onto CNT channels using the π-π interaction. The p-type properties of FET were clearly maintained after the functionalization using the peptide. The biosensor detected 1 fM of 3-methyl-1-butanol with high selectivity and successfully assessed Salmonella contamination in ham. These results indicate that the bioelectronic nose can be used for the rapid detection of Salmonella contamination in food.

Human Dopamine Receptor-Conjugated Multidimensional Conducting Polymer Nanofiber Membrane for Dopamine Detection

In the brain and central nervous system, dopamine plays a crucial role as a neurotransmitter or a local chemical messenger for interneuronal communication. Dopamine is associated with renal, hormonal, and cardiovascular systems. Additionally, dopamine dysfunction is known to cause serious illnesses, such as Parkinson's disease and Alzheimer's disease. Therefore, dopamine detection is essential for medical diagnosis and disease prevention and requires a novel strategy with high sensitivity and selectivity and a rapid response. Herein, we present a novel human dopamine receptor (hDRD1)-conjugated multidimensional conducting polymer nanofiber (NF) membrane for the selective and sensitive detection of dopamine. The membrane, which consists of multidimensional carboxylated poly(3,4-ethylenedioxythiophene) (MCPEDOT) NFs with nanorods, is used as a transistor in a liquid-ion gated field-effect transistor (FET)-based biosensor. Interestingly, hDRD1 is first expressed in Escherichia coli before it is immobilized onto the MCPEDOT NF. The hDRD1-MCPEDOT NF-based FET exhibits a rapid real-time response (<2 s) with high dopamine selectivity and sensitivity performance (approximately 100 fM). Furthermore, this FET device can be integrated into a poly(dimethylsiloxane)-based microfluidic system and also can retain its high performance in the integrated system, which results in the generation of large-scale dopamine biosensors with a novel geometry.

Carboxylic Acid-Functionalized Conducting-Polymer Nanotubes as Highly Sensitive Nerve-Agent Chemiresistors

Organophosphates are powerful inhibitors of acetylcholinesterase, which is critical to nerve function. Despite continuous research for detecting the highly toxic organophosphates, a new and improved methodology is still needed. Herein we demonstrate simple-to-fabricate chemiresistive gas sensors using conducting-polymer polypyrrole (PPy) nanotube transducers, which are chemically specific and capable of recognizing sub-ppb concentrations (ca. 0.5 ppb) of dimethyl methylphosphonate (DMMP), a simulant of nerve agent sarin. Interestingly, the introduction of carboxylic groups on the surface of PPy nanotube transistors resulted in enhanced sensitivity to DMMP via intermolecular hydrogen bonding. Furthermore, it was found that the sensitivity of the nanotube transducer depended on the degree of the carboxylic group introduced. Finally, a sensor array composed of 5 different transducers including the carboxylated nanotubes exhibited excellent selectivity to DMMP in 16 vapor species.

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.

Conducting Polymer Based Nanobiosensors

In recent years, conducting polymer (CP) nanomaterials have been used in a variety of fields, such as in energy, environmental, and biomedical applications, owing to their outstanding chemical and physical properties compared to conventional metal materials. In particular, nanobiosensors based on CP nanomaterials exhibit excellent performance sensing target molecules. The performance of CP nanobiosensors varies based on their size, shape, conductivity, and morphology, among other characteristics. Therefore, in this review, we provide an overview of the techniques commonly used to fabricate novel CP nanomaterials and their biosensor applications, including aptasensors, field-effect transistor (FET) biosensors, human sense mimicking biosensors, and immunoassays. We also discuss prospects for state-of-the-art nanobiosensors using CP nanomaterials by focusing on strategies to overcome the current limitations.