Nanomaterial-based Devices Research Articles

DNA Nanomaterial-Based Electrochemical Biosensors for Clinical Diagnosis.

Sensitive and quantitative detection of chemical and biological molecules for screening, diagnosis and monitoring diseases is essential to treatment planning and response monitoring. Electrochemical biosensors are fast, sensitive, and easy to miniaturize, which has led to rapid development in clinical diagnosis. Benefiting from their excellent molecular recognition ability and high programmability, DNA nanomaterials could overcome the Debye length of electrochemical biosensors by simple molecular design and are well suited as recognition elements for electrochemical biosensors. Therefore, to enhance the sensitivity and specificity of electrochemical biosensors, significant progress has been made in recent years by optimizing the DNA nanomaterials design. Here, the establishment of electrochemical sensing strategies based on DNA nanomaterials is reviewed in detail. First, the structural design of DNA nanomaterial is examined to enhance the sensitivity of electrochemical biosensors by improving recognition and overcoming Debye length. In addition, the strategies of electrical signal transduction and signal amplification based on DNA nanomaterials are reviewed, and the applications of DNA nanomaterial-based electrochemical biosensors and integrated devices in clinical diagnosis are further summarized. Finally, the main opportunities and challenges of DNA nanomaterial-based electrochemical biosensors in detecting disease biomarkers are presented in an aim to guide the design of DNA nanomaterial-based electrochemical devices with high sensitivity and specificity.

Water Disinfection Via Zinc Oxide (ZnO) Nanowires Chemically Fabricated on A Modified Polyurethane Substrate

Nowadays, water contamination is a big issue due to concerns about health and water scarcity. Unfortunately, most water for human consumption is contaminated with various pathogenic microorganisms that cause water-related diseases. Most traditional chemical and physical disinfectants are energy- and time-intensive and prone to generating harmful disinfection by-products. The recent controversy about waterborne diseases and the safety of commonly used disinfection methods has renewed interest in other forms of disinfection. Low-cost, high-efficiency, and low-energy devices should be developed for potential water disinfection, enabling safe drinking water access. Recently, many researchers have been working on improving the scalability and economics of nanomaterial-based devices to overcome many of the limitations of using traditional anti-microbial agents. Herein, we develop a safe and efficient new nanomaterial decontamination device targeting bacteria in drinking water. Zinc Oxide (ZnO) Nanowires and polyurethane sponges were utilized as affordable and available materials that would lower the cost of the filtration device. The device is based on an electroporation method that applies a low voltage of ~6 V to inactivated bacteria in water. The performance of our device was optimized using different values of voltages, flow rates, microorganism concentrations, and various operation times. By relying on nanotechnology-enabled electroporation principles, this method aims to address the limitations of traditional techniques and offer a feasible solution, especially in areas grappling with contamination issues that lack water treatment infrastructure.

Spray-lithography of hybrid graphene-perovskite paper-based photodetectors for sustainable electronics

Paper is an ideal substrate for the development of flexible and environmentally sustainable ubiquitous electronic systems. When combined with nanomaterial-based devices, it can be harnessed for various Internet-of-Things applications, ranging from wearable electronics to smart packaging. However, paper remains a challenging substrate for electronics due to its rough and porous nature. In addition, the absence of established fabrication methods is impeding its utilization in wearable applications. Unlike other paper-based electronics with added layers, in this study, we present a scalable spray-lithography on a commercial paper substrate. We present a non-vacuum spray-lithography of chemical vapor deposition (CVD) single-layer graphene (SLG), carbon nanotubes (CNTs) and perovskite quantum dots (QDs) on a paper substrate. This approach combines the advantages of two large-area techniques: CVD and spray-coating. The first technique allows for the growth of SLG, while the second enables the spray coating of a mask to pattern CVD SLG, electrodes (CNTs), and photoactive (QDs) layers. We harness the advantages of perovskite QDs in photodetection, leveraging their strong absorption coefficients. Integrating them with the graphene enhances the photoconductive gain mechanism, leading to high external responsivity. The presented device shows high external responsivity of ∼520 A W−1 at 405 nm at <1 V bias due to the photoconductive gain mechanism. The prepared paper-based photodetectors (PDs) achieve an external responsivity of 520 A W−1 under 405 nm illumination at <1 V operating voltage. To the best of our knowledge, our devices have the highest external responsivity among paper-based PDs. By fabricating arrays of PDs on a paper substrate in the air, this work highlights the potential of this scalable approach for enabling ubiquitous electronics on paper.

Nano(bio)Materials Do Not Affect Macrophage Phenotype—A Study Conducted by the REFINE Project

Macrophages are well known for their involvement in the biocompatibility, as well as biodistribution, of nano(bio)materials. Although there are a number of rodent cell lines, they may not fully recapitulate primary cell responses, particularly those of human cells. Isolation of tissue-resident macrophages from humans is difficult and may result in insufficient cells with which to determine the possible interaction with nano(bio)materials. Isolation of primary human monocytes and differentiation to monocyte-derived macrophages may provide a useful tool with which to further study these interactions. To that end, we developed a standard operating procedure for this differentiation, as part of the Regulatory Science Framework for Nano(bio)material-based Medical Products and Devices (REFINE) project, and used it to measure the secretion of bioactive molecules from M1 and M2 differentiated monocytes in response to model nano(bio)materials, following an initial assessment of pyrogenic contamination, which may confound potential observations. The SOP was deployed in two partner institutions with broadly similar results. The work presented here shows the utility of this assay but highlights the relevance of donor variability in responses to nano(bio)materials. Whilst donor variability can provide some logistical challenges to the application of such assays, this variability is much closer to the heterogeneous cells that are present in vivo, compared to homogeneous non-human cell lines.

Smart Fabric Textiles Using Nanomaterials: A Contemporary Overview

Recent years have witnessed the pivotal role of nanotechnology in shaping the landscape of smart fabric design. Nanomaterials have emerged as integral components, introducing sustainable attributes such as antimicrobial, ultraviolet resistant, electrically conductive, optical, hydrophobic, and flame-retardant properties into textiles and garments. The integration of nanomaterial-based smart devices into textiles has further expanded functionalities, encompassing energy harvesting and storage, sensing, drug release, and optics. Beyond the fashion industry, these advancements hold promise for applications in defense, healthcare, and on-body energy harnessing. This review aims to provide a comprehensive overview of the current trends in utilizing nanotechnology within modern textile industries, serving as both an informative resource and a catalyst for future research endeavors. Focusing on on-body electronics, the review delves into the potential of nanomaterials in achieving total energy reliance through innovative textile applications. Key scientific concepts explored include methods for nanomaterial functionalization and integration into textiles, with a keen emphasis on cost-effectiveness, comfort, wearability, energy conversion efficiency, and eco-sustainability. Recent developments are in nanogenerators, super capacitors, and photo electronic devices integrated into fabrics, with a specific focus on their efficiency and wearability. The discussion extends to possible remediation measures. Finally, the paper outlines the future outlook, envisioning further progress in the integration of smart nano-devices onto textile fabrics.

Nanotechnology and Prosthetic Devices: Integrating Biomedicine and Materials Science for Enhanced Performance and Adaptability

Nanomaterials are revolutionizing prosthetic device development. Nanotechnology has made prosthetic devices that replicate natural limb behavior and respond to users’ intentions possible. Nanomaterials improve prosthetic functionality, comfort, and lifespan. Nanocomposites, smart sensors, and medication delivery systems have addressed mechanical strength, control, and biocompatibility, resulting in enhanced prosthetic devices that improve user freedom, mobility, and quality of life. Biomedicine and materials science have helped nanomaterials reach their full potential, enabling their seamless integration into prosthetic devices and fostering interdisciplinary collaborations that advance prosthetics. The literature study shows substantial advances in nanomaterials for prosthetic devices; however, various gaps in present research and possible future research areas are indicated. First, long-term biocompatibility studies are needed to understand nanomaterials’ long-term effects on humans. Nanomaterial-based prosthetic devices must be tested and researched to assure safety and efficacy in real-world situations. Second, nanocomposites and nanoscale components must be standardized and quality-controlled to enable consistency and scalability in prosthetic devices. Third, nanoscale sensor and neural interface ethics must address privacy, security, and user consent issues. The nanomaterial-based prosthetic devices must be made more inexpensive and accessible to more disabled people. The study design was carried out to incorporate significant literature on the application of nanotechnology related to prosthetic devices. The literature was filtered from the Scopus database. The selected literature belongs to the original articles in which experimental work was carried out. Future research could combine nanotechnology with other developing technologies like artificial intelligence and robotics to produce more advanced and adaptable prosthetic devices.

Long and isolated graphene nanoribbons by on-surface polymerization on Au(111)

Low electronic gap graphene nanoribbons (GNRs) are used for the fabrication of nanomaterial-based devices and, when isolated, for mono-molecular electronics experiences, for which a well-controlled length is crucial. Here, an on-surface chemistry protocol is monitored for producing long and well-isolated GNR molecular wires on an Au(111) surface. The two-step Ullmann coupling reaction is sequenced in temperature from 100 °C to 350 °C by steps of 50 °C, returning at room temperature between each step and remaining in ultrahigh vacuum conditions. After the first annealing step at 100 °C, the monomers self-organize into 2-monolayered nano-islands. Next, the Ullmann coupling reaction takes place in both 1st and 2nd layers of those nano-islands. The nano-island lateral size and shape are controlling the final GNR lengths. Respecting the above on-surface chemistry protocol, an optimal initial monomer coverage of ~1.5 monolayer produces isolated GNRs with a final length distribution reaching up to 50 nm and a low surface coverage of ~0.4 monolayer suitable for single molecule experiments.

In Situ Sintering of CdSe/CdS Nanocrystals under Electron Beam Irradiation.

Colloidal semiconductor nanocrystals have attracted widespread attention due to their tremendous electrical and optical properties. Nanoparticles exhibit a strong tendency to aggregate and sinter in a short period of time during processing or use due to their large surface area-to-volume ratio, which may lead to significant changes in their required performance. Therefore, it is of great significance to conduct in-depth research on the sintering process and mechanism of nanoparticles to maintain their stability. Here, the sintering process of CdSe/CdS core/shell nanocrystals under continuous electron beam irradiation was studied using in situ transmission electron microscopy (TEM). In the early stages of sintering, CdSe/CdS nanocrystals approached each other at a distance of approximately 1-2 nm. As the exposure time to the electron beam increased, the movement of surface atoms on the nanocrystals led to contact between them. Subsequently, the atoms on the contact surfaces underwent rapid motion, resulting in the rapid formation of the neck between the particles. The neck formation between adjacent particles provides strong evidence of a sintering mechanism dominated by surface atom diffusion rather than Ostwald ripening. Further research in this area could lead to the development of improved methods to prevent sintering and enhance the stability of nanocrystals, ultimately contributing to the advancement of nanomaterial-based devices and materials with long-lasting performance.

Promising anisotropic mechanical, electronic, and charge transport properties of 2D InN alloys for photocatalytic water splitting

Two-dimensional (2D) materials with unique physical properties lead to new possibilities in future nanomaterial-based devices. Among them, 2D structures suitable to be the solar-driven catalyst for water-splitting reactions have become excessively important since the demand for clean energy sources has increased. Apart from the conventional crystals with well-known symmetries, recent studies showed that materials with exotic decorations could possess superior features in these kinds of applications. In this respect, we report novel 2D tetrahexagonal (th-) InN crystal and its ordered alloys In0.33X0.67N (X=Al, Ga) that can be utilized as effective catalysts for water splitting reactions. Proposed structures possess robust energetic, dynamical, thermal, and mechanical stability with a versatile mechanical response. After a critical tensile strain value, all monolayers exhibit strain-induced negative Poisson’s ratio in a particular crystal direction, making them half-auxetic materials. The examined materials are indirect semiconductors with desired band gaps and band edge positions for water-splitting applications. Due to their structural anisotropy, they have direction-dependent mobility that can keep the photogenerated charge carriers separated by reducing their recombination probability, which boosts the photocatalytic process. High absorption capacity in the wide spectral range underlines their potential performance. The versatile mechanical, electronic, and optical properties of 2D th-InN and its alloys, together with their remarkable structural stability, indicate that they can appropriately be exploited in the future for water splitting applications.

Constitutive Modeling of Mechanical Behaviors of Carbon-Based CNTs and GSs, and Their Sensing Applications as Nanomechanical Resonators: A Review.

Carbon-based nanomaterials, including carbon nanotubes (CNTs) and graphene sheets (GSs), have garnered considerable research attention owing to their unique mechanical, physical, and chemical properties compared with traditional materials. Nanosensors are sensing devices with sensing elements made of nanomaterials or nanostructures. CNT- and GS-based nanomaterials have been proved to be very sensitive nanosensing elements, being used to detect tiny mass and force. In this study, we review the developments in the analytical modeling of mechanical behavior of CNTs and GSs, and their potential applications as next-generation nanosensing elements. Subsequently, we discuss the contributions of various simulation studies on theoretical models, calculation methods, and mechanical performance analyses. In particular, this review intends to provide a theoretical framework for a comprehensive understanding of the mechanical properties and potential applications of CNTs/GSs nanomaterials as demonstrated by modeling and simulation methods. According to analytical modeling, nonlocal continuum mechanics pose small-scale structural effects in nanomaterials. Thus, we overviewed a few representative studies on the mechanical behavior of nanomaterials to inspire the future development of nanomaterial-based sensors or devices. In summary, nanomaterials, such as CNTs and GSs, can be effectively utilized for ultrahigh-sensitivity measurements at a nanolevel resolution compared to traditional materials.

The Significance and Implications of Nanotechnology in COVID-19

The coronavirus disease 2019 (COVID-19) caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) is a worldwide health hazard that has been classified as a pandemic by the World Health Organization (WHO). The task of developing efficient prevention and treatment measures for this pandemic is unparalleled. Due to nano-material's unique physicochemical features and controlled nano-bio interactions, nanotechnology has demonstrated significant potential in its capacity to combat a number of healthcare conditions. The application of nanotechnology for COVID-19 has been discussed in depth in this systematic review, which is divided into three sections: prevention, diagnostics, and treatment. To begin, we focused on nanotechnology-based protective equipment and disinfectants that can give much-needed protection against SARS-CoV-2. Again, nanoparticles can be used as an antigen carrier or adjuvant, paving the path for the development of a number of vaccines with preventive benefits. The capacity of nano-materials to magnify signal is then highlighted, which has been employed in the development of nano-biosensors and nano-imaging techniques that can be used for early-stage detection in conjunction with other diagnostic instruments. Finally, we discuss COVID-19 therapeutic approaches based on nano-materials. Nano-metals and their oxides affect cellular processes by interfering with the production of reactive oxygen species (ROS), which then give antiviral action. Various nano-products (polyethylenimine, squalene) can significantly lower the synthesis of inflammatory modulators (Cytokine storm), hence preventing Covid-19 infection. The review's limitations and nanoparticle's future directions for COVID-19 have been described briefly. This review is quite comprehensive and useful in terms of providing suggestions for developing nanomaterial-based devices to combat against COVID-19.

The Role of Nano-Sensors in Breath Analysis for Early and Non-Invasive Disease Diagnosis

Early-stage, precise disease diagnosis and treatment has been a crucial topic of scientific discussion since time immemorial. When these factors are combined with experience and scientific knowledge, they can benefit not only the patient, but also, by extension, the entire health system. The development of rapidly growing novel technologies allows for accurate diagnosis and treatment of disease. Nanomedicine can contribute to exhaled breath analysis (EBA) for disease diagnosis, providing nanomaterials and improving sensing performance and detection sensitivity. Through EBA, gas-based nano-sensors might be applied for the detection of various essential diseases, since some of their metabolic products are detectable and measurable in the exhaled breath. The design and development of innovative nanomaterial-based sensor devices for the detection of specific biomarkers in breath samples has emerged as a promising research field for the non-invasive accurate diagnosis of several diseases. EBA would be an inexpensive and widely available commercial tool that could also be used as a disease self-test kit. Thus, it could guide patients to the proper specialty, bypassing those expensive tests, resulting, hence, in earlier diagnosis, treatment, and thus a better quality of life. In this review, some of the most prevalent types of sensors used in breath-sample analysis are presented in parallel with the common diseases that might be diagnosed through EBA, highlighting the impact of incorporating new technological achievements in the clinical routine.

Self-Assembly of Cyclodextrin-Coated Nanoparticles:Fabrication of Functional Nanostructures for Sensing and Delivery.

In recent years, the bottom-up approach has emerged as a powerful tool in the fabrication of functional nanomaterials through the self-assembly of nanoscale building blocks. The cues embedded at the molecular level provide a handle to control and direct the assembly of nano-objects to construct higher-order structures. Molecular recognition among the building blocks can assist their precise positioning in a predetermined manner to yield nano- and microstructures that may be difficult to obtain otherwise. A well-orchestrated combination of top-down fabrication and directed self-assembly-based bottom-up approach enables the realization of functional nanomaterial-based devices. Among the various available molecular recognition-based "host-guest" combinations, cyclodextrin-mediated interactions possess an attractive attribute that the interaction is driven in aqueous environments, such as in biological systems. Over the past decade, cyclodextrin-based specific host-guest interactions have been exploited to design and construct structural and functional nanomaterials based on cyclodextrin-coated metal nanoparticles. The focus of this review is to highlight recent advances in the self-assembly of cyclodextrin-coated metal nanoparticles driven by the specific host-guest interaction.

Superior photodynamic effect of single-walled carbon nanotubes in aprotic media: a kinetic study

It has been confirmed that single-walled carbon nanotubes (SWCNTs) could generate reactive oxygen species in aprotic media by utilizing photon energy. However, the impact of photon irradiation on SWCNTs and the kinetics of the generation process in aprotic media are still unclear, which significantly limits the yield performance. In this work, the kinetics for photodynamic effects has been investigated by performing characterizations on ultraviolet-treated SWCNTs using Raman spectroscopy, conductive atomic force microscope (in-situ), kelvin probe force microscope, and X-ray photoelectron spectroscopy. It is found that ultraviolet-treated SWCNTs are observed to have more defects, lower conductivity, and less surface charge after energy conversion. Starting from the fundamental intrinsic properties of SWCNTs, the kinetics and formation of these changes are thoroughly discussed. It turns out that the dispersion, chirality, and structural integrity of SWCNTs are important for achieving high-performance photodynamic effects, which are validated using several different SWCNTs as well as other carbon nanomaterials. A type of (6,5) s-SWCNTs exhibited the highest energy efficiency among a variety of other carbon nanomaterials. The yield rate is 2.15 mM/h, and the energy consumption is determined to be 2.79 W·h/mM. This work is expected to help dramatically boost the energy efficiency of the photodynamic effect in aprotic media, and pave the way for designing high-performance carbon nanomaterial-based photoinduced devices.

Standardization of an in vitro assay matrix to assess cytotoxicity of organic nanocarriers: a pilot interlaboratory comparison

Nanotechnologies such as nanoparticles are established components of new medical devices and pharmaceuticals. The use and distribution of these materials increases the requirement for standardized evaluation of possible adverse effects, starting with a general cytotoxicity screening. The Horizon 2020 project “Regulatory Science Framework for Nano(bio)material-based Medical Products and Devices (REFINE)” identified in vitro cytotoxicity quantification as a central task and first step for risk assessment and development for medical nanocarriers. We have performed an interlaboratory comparison on a cell-assay matrix including a kinetic lactate dehydrogenase (LDH) release cell death and WST-8 cell viability assay adapted for testing organic nanocarriers in four well-characterized cell lines of different organ origins. Identical experiments were performed by three laboratories, namely the Biomedical Technology Center (BMTZ) of the University of Münster, SINTEF Materials and Chemistry (SINTEF), and the National Institute for Public Health and the Environment (RIVM) of the Netherlands according to new standard operating procedures (SOPs). The experiments confirmed that LipImage™ 815 lipidots® are non-cytotoxic up to a concentration of 128 µg/mL and poly(alkyl cyanoacrylate) (PACA) nanoparticles for drug delivery of cytostatic agents caused dose-dependent cytotoxic effects on the cell lines starting from 8 µg/mL. PACA nanoparticles loaded with the active pharmaceutical ingredient (API) cabazitaxel showed a less pronounced dose-dependent effect with the lowest concentration of 2 µg/mL causing cytotoxic effects. The mean within laboratory standard deviation was 4.9% for the WST-8 cell viability assay and 4.0% for the LDH release cell death assay, while the between laboratory standard deviation was 7.3% and 7.8% for the two assays, respectively. Here, we demonstrated the suitability and reproducibility of a cytotoxicity matrix consisting of two endpoints performed with four cell lines across three partner laboratories. The experimental procedures described here can facilitate a robust cytotoxicity screening for the development of organic nanomaterials used in medicine.Graphical abstract

Inferring the energy sensitivity and band gap of electronic transport in a network of carbon nanotubes

Since the industrialization of single-phase nanomaterial-based devices is still challenging, intensive research focus has been given to complex materials consisting of multiple nanoscale entities, including networks and matrices of nanowires, nanotubes, nanoribbons, or other large molecules; among these complex materials, networks of carbon nanotubes are a typical example. Detailed knowledge of the energy sensitivity and band gap of electronic transport in such a material system is difficult to detect, despite its importance in electronic, energetic and sensing applications. Here, we propose a new methodology to obtain these quantities using the measured Seebeck coefficient at a certain temperature but different Fermi levels. We discover that the network consisting of semiconducting (11,10)-carbon nanotubes actually exhibits metallic transport at room temperature. It is also interesting to verify that intrananotube ballistic transport is dominant over diffusive scattering by long-range disorder, as well as the quantum hopping resistance at the contact points. The transport asymmetry ratio between the holes and electrons (1.75) is similar to the value observed in pristine graphene samples (1.50).

Multifunctional Graphene Sensor Ensemble as a Smart Biomonitoring Fashion Accessory.

Biomonitoring wearable sensors based on two-dimensional nanomaterials have recently elicited keen research interest and potential for a new range of flexible nanoelectronic devices. Practical nanomaterial-based devices suited for real-world service, which exhibit first-rate performance while being an attractive accessory, are still distant. We report a multifunctional flexible wearable sensor fabricated using an ultrathin percolative layer of graphene nanosheets on laser-patterned gold circular interdigitated electrodes for monitoring vital human physiological parameters. This graphene on laser-patterned electrode (GLE) sensor displays an excellent strain resolution of 245 με (0.024%) and a record high gauge factor of 6.3 × 107, with exceptional stability and repeatability in its operating range. The sensor was tested for human physiological monitoring like measurement of heart rate, breathing rate, body temperature, and hydration level, which are vital health parameters, especially considering the current pandemic scenario. The sensor also served in applications such as a pedometer, limb movement tracker, and control switch for human interaction. The innovative laser-etch process used to pattern gold thin-film electrodes, with the multifunctional incognizable graphene layer, provides a technique for integrating multiple sensors in a wearable band. The reported work marks a giant leap from the conventional banal devices to a highly marketable multifunctional sensor array as a biomonitoring fashion accessory.

State-of-the-Art of Nanodiagnostics and Nanotherapeutics against SARS-CoV-2.

The pandemic outbreak of SARS-CoV-2, with millions of infected patients worldwide, has severely challenged all aspects of public health. In this regard, early and rapid detection of infected cases and providing effective therapeutics against the virus are in urgent demand. Along with conventional clinical protocols, nanomaterial-based diagnostics and therapeutics hold a great potential against coronavirus disease 2019 (COVID-19). Indeed, nanoparticles with their outstanding characteristics would render additional advantages to the current approaches for rapid and accurate diagnosis and also developing prophylactic vaccines or antiviral therapeutics. In this review, besides presenting an overview of the coronaviruses and SARS-CoV-2, we discuss the introduced nanomaterial-based detection assays and devices and also antiviral formulations and vaccines for coronaviruses.

Nanobiodevices for the Isolation of Circulating Nucleic Acid for Biomedical Applications

Methods that can rapidly and accurately extract or isolate nucleic acids would facilitate the capability for scientists to access key information regarding nucleic acid molecular signatures. Knowing these molecular signatures could contribute to development of strategies for detecting, treating, and diagnosing diseases based on nucleic acids. However, major impediments to accessing nucleic acids are their natural characteristics, including concentration and size. Here we review the development of nanomaterial-based devices to isolate circulating nucleic acids from biological samples, including blood, urine, cell, and virus; these devices enable enhancement of isolation and extraction efficiency compared to conventional methods.

Gas Sensors Based on Copper Oxide Nanomaterials: A Review

Metal oxide semiconductors have found widespread applications in chemical sensors based on electrical transduction principles, in particular for the detection of a large variety of gaseous analytes, including environmental pollutants and hazardous gases. This review recapitulates the progress in copper oxide nanomaterial-based devices, while discussing decisive factors influencing gas sensing properties and performance. Literature reports on the highly sensitive detection of several target molecules, including volatile organic compounds, hydrogen sulfide, carbon monoxide, carbon dioxide, hydrogen and nitrogen oxide from parts-per-million down to parts-per-billion concentrations are compared. Physico-chemical mechanisms for sensing and transduction are summarized and prospects for future developments are outlined.