Flexible and transparent gas molecule sensor integrated with sensing and heating graphene layers

2D nanomaterials for realization of flexible and wearable gas sensors: A review

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Soft Human-Machine Interface Sensing Displays: Materials and Devices

The development of human-interactive sensing displays (HISDs) that simultaneously detect and visualize stimuli is important for numerous cutting-edge human-machine interface technologies. Therefore, innovative device platforms with optimized architectures of HISDs combined with novel high-performance sensing and display materials are demonstrated. This study comprehensively reviews the recent advances in HISDs, particularly the device architectures that enable scaling-down and simplifying the HISD, as well as material designs capable of directly visualizing input information received by various sensors. Various HISD platforms for integrating sensors and displays are described. HISDs consist of a sensor and display connected through a microprocessor, and attempts to assemble the two devices by eliminating the microprocessor are detailed. Single-device HISD technologies are highlighted in which input stimuli acquired by sensory components are directly visualized with various optical components, such as electroluminescence, mechanoluminescence and structural color. The review forecasts future HISD technologies that demand the development of materials with molecular-level synthetic precision that enables simultaneous sensing and visualization. Furthermore, emerging HISDs combined with artificial intelligence technologies and those enabling simultaneous detection and visualization of extrasensory information are discussed.

Direct Induction of Porous Graphene from Mechanically Strong and Waterproof Biopaper for On-Chip Multifunctional Flexible Electronics

Graphene with a 3D porous structure is directly laser-induced on lignocellulosic biopaper under ambient conditions and is further explored for multifunctional biomass-based flexible electronics. The mechanically strong, flexible, and waterproof biopaper is fabricated by surface-functionalizing cellulose with lignin-based epoxy acrylate (LBEA). This composite biopaper shows as high as a threefold increase in tensile strength and excellent waterproofing compared with pure cellulose one. Direct laser writing (DLW) rapidly induces porous graphene from the biopaper in a single step. The porous graphene shows an interconnected carbon network, well-defined graphene domains, and high electrical conductivity (e.g., ≈3 Ω per square), which can be tuned by lignin precursors and loadings as well as lasing conditions. The biopaper in situ embedded with porous graphene is facilely fabricated into flexible electronics for on-chip and paper-based applications. The biopaper-based electronic devices, including the all-solid-state planer supercapacitor, electrochemical and strain biosensors, and Joule heater, show great performances. This study demonstrates the facile, versatile, and low-cost fabrication of multifunctional graphene-based electronics from lignocellulose-based biopaper.

Enabling Quick Response to Nitrogen Dioxide at Room Temperature and Limit of Detection to Ppb Level by Heavily n-Doped Graphene Hybrid Transistor

Sensitive detection of nitrogen dioxide (NO₂) is of significance in many areas for health and environmental protections. In this work, we developed an efficient NO₂ sensor that can respond within seconds at room temperature, and the limit of detection (LOD) is as low as 100 ppb. Coating cyano-substituted poly(p-phenylene vinylene) (CN-PPV) films on graphene (G) layers can dope G sheets effectively to a heavy n state. The influences of solution concentrations and annealing temperatures on the n-doping effect were investigated in detail. The CN-PPV-G transistors fabricated with the optimized parameters demonstrate active sensing abilities toward NO₂. The n-doping state of CN-PPV-G is reduced dramatically by NO₂, which is a strong p-doping compound. Upon exposure to 25 ppm of NO₂, our CN-PPV-G sensors react in 10 s, indicating it is almost an immediate response. LOD is determined as low as 100 ppb. The ultrahigh responding speed and low LOD are not affected in dry air. Furthermore, cycling use of our sensors can be realized through simple annealing. The superior features shown by our CN-PPV-G sensors are highly desired in the applications of monitoring the level of NO₂ in situ and setting immediate alarms. Our results also suggest that transfer curves of transistors can react very promptly to the stimulus of target gas and, thus, are very promising in the development of fast-response sensing devices although the response values may not reach maximum as a tradeoff.

Unique Photoactivated Time-Resolved Response in 2D GeS for Selective Detection of Volatile Organic Compounds

Volatile organic compounds (VOCs) sensors have a broad range of applications including healthcare, process control, and air quality analysis. There are a variety of techniques for detecting VOCs such as optical, acoustic, electrochemical, and chemiresistive sensors. However, existing commercial VOC detectors have drawbacks such as high cost, large size, or lack of selectivity. Herein, a new sensing mechanism is demonstrated based on surface interactions between VOC and UV-excited 2D germanium sulfide (GeS), which provides an effective solution to distinguish VOCs. The GeS sensor shows a unique time-resolved electrical response to different VOC species, facilitating identification and qualitative measurement of VOCs. Moreover, machine learning is utilized to distinguish VOC species from their dynamic response via visualization with high accuracy. The proposed approach demonstrates the potential of 2D GeS as a promising candidate for selective miniature VOCs sensors in critical applications such as non-invasive diagnosis of diseases and health monitoring.

Temperature-Dependent n-to-p-Type Transition of 2D Mn Oxide Nanosheets toward NO2 for Flexible Gas Sensor Application

Rapid and on-site detection of nitrogen dioxide (NO₂) is important for environmental monitoring as NO₂ is a highly toxic chemical emitted from automobiles and power plants. In this study, we proposed atomically thin two-dimensional (2D) Mn oxide nanosheets (NSs) assembled on a flexible heating substrate for application in flexible and wearable NO₂ sensors. A liquid-phase exfoliation technique was used to obtain individual Mn oxide layers that formed a homogeneous suspension. A flexible heater was fabricated by partially embedding Ag nanotubes (NTs) on the surface of a colorless polyimide (CPI) film for use as a sensor substrate. Temperature-dependent NO₂ sensing properties were investigated via control of the operating temperature using a Ag NT-embedded CPI heating film. As a result, the n-type sensing behavior of the Mn oxide NSs exhibited a response [(R_gas - R_air)/R_air × 100 (%)] of 1.20 ± 0.21% for 20 ppm NO₂ at room temperature (25 °C). Meanwhile, n-p transition occurred with p-type sensing property as the operating temperature increased to 150 °C with an improved response [(R_air - R_gas)/R_air × 100 (%)] of 4.10 ± 0.42% for 20 ppm NO₂. The characteristic n-p transition of Mn oxide NSs at different operating temperatures was evidenced by the surface-adsorbed oxygen ions (i.e., O₂^- and O^-) and nitrate species (NO₃^- and NO₃^2-).

MoS2 Defect Healing for High-Performance Chemical Sensing of Polycyclic Aromatic Hydrocarbons

The increasing population and industrial development are responsible for environmental pollution. Among toxic chemicals, polycyclic aromatic hydrocarbons (PAHs) are highly carcinogenic contaminants resulting from the incomplete combustion of organic materials. Two-dimensional materials, such as transition metal dichalcogenides (TMDCs), are ideal sensory scaffolds, combining high surface-to-volume ratio with physical and chemical properties that are strongly susceptible to environmental changes. TMDCs can be integrated in field-effect transistors (FETs), which can operate as high-performance chemical detectors of (non)covalent interaction with small molecules. Here, we have developed MoS₂-based FETs as platforms for PAHs sensing, relying on the affinity of the planar polyaromatic molecules for the basal plane of MoS₂ and the structural defects in its lattice. X-ray photoelectron spectroscopy analysis, photoluminescence measurements, and transfer characteristics showed a notable reduction in the defectiveness of MoS₂ and a p-type doping upon exposure to PAHs solutions, with a magnitude determined by the correlation between the ionization energies (E_I) of the PAH and that of MoS₂. Naphthalene, endowed with the higher E_I among the studied PAHs, exhibited the highest output. We observed a log-log correlation between MoS₂ doping and naphthalene concentration in water in a wide range (10^-9-10^-6 M), as well as a reversible response to the analyte. Naphthalene concentrations as low as 0.128 ppb were detected, being below the limits imposed by health regulations for drinking water. Furthermore, our MoS₂ devices can reversibly detect vapors of naphthalene with both an electrical and optical readout, confirming that our architecture could operate as a dual sensing platform.

High-Performance and Rapid-Response Electrical Heaters Derived from Cellulose Nanofiber/Silver Nanowire Nanopapers for Portable Thermal Management

High-performance electrical heaters with outstanding flexibility, superior portability, and mechanical properties are highly desirable for portable thermal management. However, it is still a huge challenge to simultaneously achieve competent electrical heating performances and excellent mechanical properties. Herein, inspired by the Janus structure, versatile electrical heaters are developed via a sequential assembly followed by a hot-pressing strategy. The elaborately designed Janus structure is composed of a nanofibrillated cellulose (NFC) layer and a partially wrapped silver nanowire (AgNW) skeleton in the NFC substrate. Owing to the perfect introduction of nano-soldered points induced by thermal welding decoration, the resultant NFC/AgNW papers (NAPs) possess great flexibility, excellent mechanical strength (176.75 MPa), extremely low sheet resistance (0.60 Ω/sq), and superior electrical stabilities against mechanical deformations. Moreover, benefitting from these fascinating attributes, the NAP-based electrical heaters exhibit a remarkable heating temperature (∼220 °C), ultrafast electro-thermal response (<10 s), and groundbreaking long-term stability (∼105 °C for >186 h) and repeatability (>20,000 cycles) with low AgNW contents and driving voltages (0.5-5.0 V), which far surpass those of the previously reported and conventional indium tin oxide-based Joule heaters. Impressively, large-area production feasibilities of NAPs are demonstrated and assembled into multifunctional applications, including personal thermal management, healthcare thermotherapy, multifunctional cups, and smart homes, indicating their promising potential for wearable devices, artificial intelligence, and specific heating systems in the fields of aerospace, military, and intelligent life.

Gate-Tuned Gas Molecule Sensitivity of a Two-Dimensional Semiconductor

In this work, we develop a gate-tunable gas sensor based on a MoS2/hBN heterostructure field effect transistor. Through experimental measurements and numerical simulations, we systematically reveal a principle that relates the concentration of the target gas and sensing signals (ΔI/I0) as a function of gate bias. Because a linear relationship between ΔI/I0 and the gas concentration guarantees reliable sensor operation, the optimal gate bias condition for linearity was investigated. Taking NO2 and NH3 as target molecules, it is clarified that the bias condition greatly depends on the electron accepting/donating nature of the gas. The effects of the bandgap and polarity of the transition metal dichalcogenides (TMDC) channel are also discussed. In order to achieve linearly increasing signals that are stable with respect to the gas concentration, a sufficiently large VBG within VBG > 0 is required. We expect this work will shed light on a way to precisely design reliable semiconducting gas sensors based on the characteristics of TMDC and target gas molecules.

Gas sensing devices based on two-dimensional materials: a review

Gas sensors have been widely utilized penetrating every aspect of our daily lives, such as medical industry, environmental safety testing, and the food industry. In recent years, two-dimensional (2D) materials have shown promising potential and prominent advantages in gas sensing technology, due to their unique physical and chemical properties. In addition, the ultra-high surface-to-volume ratio and surface activity of the 2D materials with atomic-level thickness enables enhanced absorption and sensitivity. Till now, different gas sensing techniques have been developed to further boost the performance of 2D materials-based gas sensors, such as various surface functionalization and Van der Waals heterojunction formation. In this article, a comprehensive review of advanced gas sensing devices is provided based on 2D materials, focusing on two sensing principles of charge-exchange and surface oxygen ion adsorption. Six types of typical gas sensor devices based on 2D materials are introduced with discussion of latest research progress and future perspectives.

The Combination of Two-Dimensional Nanomaterials with Metal Oxide Nanoparticles for Gas Sensors: A Review

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects.

Revolution in Flexible Wearable Electronics for Temperature and Pressure Monitoring—A Review

In the last few decades, technology innovation has had a huge influence on our lives and well-being. Various factors of observing our physiological characteristics are taken into account. Wearable sensing tools are one of the most imperative sectors that are now trending and are expected to grow significantly in the coming days. Externally utilized tools connected to any human to assess physiological characteristics of interest are known as wearable sensors. Wearable sensors range in size from tiny to large tools that are physically affixed to the user and operate on wired or wireless terms. With increasing technological capabilities and a greater grasp of current research procedures, the usage of wearable sensors has a brighter future. In this review paper, the recent developments of two important types of wearable electronics apparatuses have been discussed for temperature and pressure sensing (Psensing) applications. Temperature sensing (Tsensing) is one of the most important physiological factors for determining human body temperature, with a focus on patients with long-term chronic conditions, normally healthy, unconscious, and injured patients receiving surgical treatment, as well as the health of medical personnel. Flexile Psensing devices are classified into three categories established on their transduction mechanisms: piezoresistive, capacitive, and piezoelectric. Many efforts have been made to enhance the characteristics of the flexible Psensing devices established on these mechanisms.

Moisture-resistant, stretchable NOx gas sensors based on laser-induced graphene for environmental monitoring and breath analysis

The accurate, continuous analysis of healthcare-relevant gases such as nitrogen oxides (NO_x) in a humid environment remains elusive for low-cost, stretchable gas sensing devices. This study presents the design and demonstration of a moisture-resistant, stretchable NO_x gas sensor based on laser-induced graphene (LIG). Sandwiched between a soft elastomeric substrate and a moisture-resistant semipermeable encapsulant, the LIG sensing and electrode layer is first optimized by tuning laser processing parameters such as power, image density, and defocus distance. The gas sensor, using a needlelike LIG prepared with optimal laser processing parameters, exhibits a large response of 4.18‰ ppm^-1 to NO and 6.66‰ ppm^-1 to NO₂, an ultralow detection limit of 8.3 ppb to NO and 4.0 ppb to NO₂, fast response/recovery, and excellent selectivity. The design of a stretchable serpentine structure in the LIG electrode and strain isolation from the stiff island allows the gas sensor to be stretched by 30%. Combined with a moisture-resistant property against a relative humidity of 90%, the reported gas sensor has further been demonstrated to monitor the personal local environment during different times of the day and analyze human breath samples to classify patients with respiratory diseases from healthy volunteers. Moisture-resistant, stretchable NO_x gas sensors can expand the capability of wearable devices to detect biomarkers from humans and exposed environments for early disease diagnostics.

Physical and Chemical Sensors on the Basis of Laser-Induced Graphene: Mechanisms, Applications, and Perspectives

Laser-induced graphene (LIG) is produced rapidly by directly irradiating carbonaceous precursors, and it naturally exhibits as a three-dimensional porous structure. Due to advantages such as simple preparation, time-saving, environmental friendliness, low cost, and expanding categories of raw materials, LIG and its derivatives have achieved broad applications in sensors. This has been witnessed in various fields such as wearable devices, disease diagnosis, intelligent robots, and pollution detection. However, despite LIG sensors having demonstrated an excellent capability to monitor physical and chemical parameters, the systematic review of synthesis, sensing mechanisms, and applications of them combined with comparison against other preparation approaches of graphene is still lacking. Here, graphene-based sensors for physical, biological, and chemical detection are reviewed first, followed by the introduction of general preparation methods for the laser-induced method to yield graphene. The preparation and advantages of LIG, sensing mechanisms, and the properties of different types of emerging LIG-based sensors are comprehensively reviewed. Finally, possible solutions to the problems and challenges of preparing LIG and LIG-based sensors are proposed. This review may serve as a detailed reference to guide the development of LIG-based sensors that possess properties for future smart sensors in health care, environmental protection, and industrial production.

Charge transfer driven by redox dye molecules on graphene nanosheets for room-temperature gas sensing

Special functional groups to modify the surface of graphene have received much attention since they enable the charge transfer enhancement, thus realizing gas-sensing at room temperature. In this work, three typical redox dye molecules, methylene blue (MB), indigo carmine (IC) and anthraquinone-2-sulfonate (AQS), were selected to be supramolecularly assembled with reduced graphene oxide (rGO), respectively. Remarkably, three graphene-based materials AQS-rGO (response = 3.2, response time = 400 s), IC-rGO (response = 4.3, response time = 300 s) and MB-rGO (response = 7.1, response time = 100 s) exhibited excellent sensitivity and short response time toward 10 ppm NO₂ at room temperature. The corresponding NO₂ sensing mechanism of the obtained materials was further investigated by cyclic voltammetry (CV) measurements. CV was conducted to measure the anodic peak potential (E_pa) of three redox dyes. Interestingly, it is obvious that the E_pa values were positively correlated with the gas sensitivity and response time of the three materials. To explore the mechanism, UV-vis spectroscopy was adopted to analyze the lowest unoccupied molecular orbitals (LUMOs) of three redox dye molecules. The results show that the oxidation abilities of three redox dyes were also positively correlated with the gas sensitivity and response time of three corresponding graphene-based materials.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide (NO2), a hazardous gas with acidic nature, is continuously being liberated in the atmosphere due to human activity. The NO2 sensors based on traditional materials have limitations of high-temperature requirements, slow recovery, and performance degradation under harsh environmental conditions. These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials. Molybdenum disulfide (MoS2) has emerged as a potential candidate for developing next-generation NO2 gas sensors. MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies, facile integration with other materials and compatibility with internet of things (IoT) devices. The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices (resistor and transistor), layer thickness, morphology control, defect tailoring, heterostructure, metal nanoparticle doping, and through light illumination. Moreover, the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively. Finally, the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2. Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

Fabrication of fine-pored polydimethylsiloxane using an isopropyl alcohol and water mixture for adjustable mechanical, optical, and thermal properties

A fabrication method for obtaining fine-pored PDMS is presented. Low-cost, volatile, and easily accessible IPA is used as a co-solvent in water and PDMS emulsions, allowing porous PDMS with adjustable mechanical, optical and thermal properties.

Crystallization of ZnO thin films without polymer substrate deformation via thermal dissipation annealing method for next generation wearable devices

The synthesis method of transparent and flexible ZnO thin films is currently considered the most important factor for the fabrication of next generation wearable devices. To fabricate transparent and flexible devices by using sol–gel spin-coated ZnO thin films, an annealing step is necessary; however, annealing processes at high temperatures decompose polymer substrates due to their low melting temperature. It was found that sol–gel spin-coated ZnO thin films can be crystallized through the mobility difference of ZnO molecules placed at the surface of ZnO thin films. Especially, ZnO thin films can be annealed at high temperature (above 500 °C) by using a thermal dissipation annealing (TDA) method without the deformation of the polymer substrate. A transparent and flexible ultraviolet photodetector based on ZnO thin films annealed with the TDA method exhibited fast rise and decay time constants, a high on/off current ratio, and reproducible photocurrent characteristics. Thus, these results indicated that the TDA method is a feasible alternative route for the fabrication of next generation wearable devices.

Thermal dissipation annealing method is an effective way of fabricating transparent and flexible optoelectronics for next generation wearable devices.

Homecare Robotic Systems for Healthcare 4.0: Visions and Enabling Technologies

Powered by the technologies that have originated from manufacturing, the fourth revolution of healthcare technologies is happening (Healthcare 4.0). As an example of such revolution, new generation homecare robotic systems (HRS) based on the cyber-physical systems (CPS) with higher speed and more intelligent execution are emerging. In this article, the new visions and features of the CPS-based HRS are proposed. The latest progress in related enabling technologies is reviewed, including artificial intelligence, sensing fundamentals, materials and machines, cloud computing and communication, as well as motion capture and mapping. Finally, the future perspectives of the CPS-based HRS and the technical challenges faced in each technical area are discussed.

Ultrasensitive Field-Effect Biosensors Enabled by the Unique Electronic Properties of Graphene

This review provides a critical overview of current developments on nanoelectronic biochemical sensors based on graphene. Composed of a single layer of conjugated carbon atoms, graphene has outstanding high carrier mobility and low intrinsic electrical noise, but a chemically inert surface. Surface functionalization is therefore crucial to unravel graphene sensitivity and selectivity for the detection of targeted analytes. To achieve optimal performance of graphene transistors for biochemical sensing, the tuning of the graphene surface properties via surface functionalization and passivation is highlighted, as well as the tuning of its electrical operation by utilizing multifrequency ambipolar configuration and a high frequency measurement scheme to overcome the Debye screening to achieve low noise and highly sensitive detection. Potential applications and prospectives of ultrasensitive graphene electronic biochemical sensors ranging from environmental monitoring and food safety, healthcare and medical diagnosis, to life science research, are presented as well.

Wearable Electronics Based on 2D Materials for Human Physiological Information Detection

Recently, advancement in materials production, device fabrication, and flexible circuit has led to the huge prosperity of wearable electronics for human healthcare monitoring and medical diagnosis. Particularly, with the emergence of 2D materials many merits including light weight, high stretchability, excellent biocompatibility, and high performance are used for those potential applications. Thus, it is urgent to review the wearable electronics based on 2D materials for the detection of various human signals. In this work, the typical graphene-based materials, transition-metal dichalcogenides, and transition metal carbides or carbonitrides used for the wearable electronics are discussed. To well understand the human physiological information, it is divided into two dominated categories, namely, the human physical and the human chemical signals. The monitoring of body temperature, electrograms, subtle signals, and limb motions is described for the physical signals while the detection of body fluid including sweat, breathing gas, and saliva is reviewed for the chemical signals. Recent progress and development toward those specific utilizations are highlighted in the Review with the representative examples. The future outlook of wearable healthcare techniques is briefly discussed for their commercialization.

Two-Dimensional, Few-Layer MnPS3 for Selective NO2 Gas Sensing under Ambient Conditions

In this study, two-dimensional few-layer MnPS₃ is introduced as a selective and reversible NO₂ gas sensor in dry nitrogen (N₂) under ambient conditions. The solvent exfoliation technique is utilized to exfoliate bulk MnPS₃ into a few layers, which are further assembled as thin films by the vacuum filtration method. The films are subsequently transferred onto a sensing device and used for NO₂ sensing. Exfoliated MnPS₃ shows excellent sensitivity toward NO₂ gas with a low detection limit of a few tens of ppb at 25 °C. A sensitivity of 9530% is obtained at 35 ppm concentration of NO₂ with the theoretical limit of detection calculated to be ∼9.5 ppb. The sensor is highly selective toward NO₂ gas (with respect to interferents NO, NH₃, H₂, CO, CO₂, C₂H₂, and O₂) and is fully reversible under ambient conditions. The time constant is determined to be in the range of 30-160 s for adsorption and desorption processes. Raman spectroscopy reveals that the mechanism of sensing is based on charge transfer interactions between the sensor and analyte. This study opens up ways to fabricate gas sensors using few-layer metal phosphochalcogenides (MPX₃).

Piezoresistive and chemiresistive gas sensing by metal-free graphene layers

Graphene is an ideal candidate to use in various applications as a component in semiconductor devices with excellent properties, such as its atomic thickness, optical transparency, chemical stability, and high electrical and thermal conductivities. The high gas sensitivities of graphene functionalized with metal, metal oxides, and other groups have been improved through intensive research. However, the development of a metal-free graphene gas sensor and clarification of its mechanism still remain a challenge. In this study, H₂, CO₂, NH₃, and He gas sensing performances are demonstrated using two- to multilayered graphene, directly fabricated on a quartz substrate. The sheet resistances of more than 100 graphene layers were considerably changed from 3% to 6% by He gas injection, caused by its piezoresistive property. The anomalous resistance changes by piezoresistivity is a result of electron transfer path changes associated with graphene assemble structure changes by insertion of He gas between graphene crystal units and pressing graphene units. The sheet resistances of the synthesized graphene were found to dramatically change through physical adsorption and chemisorption. The chemisorption of NH₃ gas on functional oxygen groups at graphene edges was responsible for the chemiresistive behavior of the material. The gas sensing and piezoresistive mechanisms of graphene determined in this work sheds light on the development of a graphene gas sensor.

Graphene-based wearable sensors

The human body is a "delicate machine" full of sensors such as the fingers, nose, and mouth. In addition, numerous physiological signals are being created every moment, which can reflect the condition of the body. The quality and the quantity of the physiological signals are important for diagnoses and the execution of therapies. Due to the incompact interface between the sensors and the skin, the signals obtained by commercial rigid sensors do not bond well with the body; this decreases the quality of the signal. To increase the quantity of the data, it is important to detect physiological signals in real time during daily life. In recent years, there has been an obvious trend of applying graphene devices with excellent performance (flexibility, biocompatibility, and electronic characters) in wearable systems. In this review, we will first provide an introduction about the different methods of synthesis of graphene, and then techniques for graphene patterning will be outlined. Moreover, wearable graphene sensors to detect mechanical, electrophysiological, fluid, and gas signals will be introduced. Finally, the challenges and prospects of wearable graphene devices will be discussed. Wearable graphene sensors can improve the quality and quantity of the physiological signals and have great potential for health-care and telemedicine in the future.

An Ultraflexible and Stretchable Aptameric Graphene Nanosensor for Biomarker Detection and Monitoring

An ultraflexible and stretchable field-effect transistor nanosensor is presented that uses aptamer-functionalized monolayer graphene as the conducting channel. Specific binding of the aptamer with the target biomarker induces a change in the carrier concentration of the graphene, which is measured to determine the biomarker concentration. Based on a Mylar substrate that is only 2.5-μm thick, the nanosensor is capable of conforming to underlying surfaces (e.g., those of human tissue or skin) that undergo large bending, twisting, and stretching deformations. In experimental testing, the device is rolled on cylindrical surfaces with radii down to 40 μm, twisted by angles ranging from -180° to 180°, or stretched by extensions up to 125%. With these large deformations applied either cyclically or non-recurrently, the device is shown to incur no visible mechanical damage, maintain consistent electrical properties, and allow detection of TNF-α, an inflammatory cytokine biomarker, with consistently high selectivity and low limit of detection (down to 5 × 10^-12M). The nanosensor can thus potentially enable consistent and reliable detection of liquid-borne biomarkers on human skin or tissue surfaces that undergo large mechanical deformations.

Reduced Graphene Oxide/Mesoporous ZnO NSs Hybrid Fibers for Flexible, Stretchable, Twisted, and Wearable NO2 E-Textile Gas Sensor

E-textiles are gaining growing popularity recently due to low cost, light weight, and conformable compatibility with clothes in wearable and portable smart electronics. Here, an easy-handing, low cost, and scalable fabricating strategy is reported to fabricate conductive, highly flexible, and mechanically stretchable/twisted fiber gas sensor with great wearability and knittability. The proposed gas sensor is built using commercially available cotton/elastic threads as flexible/stretchable templates and reduced graphene oxide/mesoporous zinc oxide nanosheets as sensing layers to form conducting fibers. The as-prepared fiber demonstrates sensitive sensing response, excellent long-term stability (84 days), low theoretical detection limit (43.5 ppb NO₂), great mechanical deformation tolerance (3000 bending cycles, 1000 twisting cycles and 65% strain strength), and washing durability in room-temperature gas detection. More significantly, scalable wearable characteristics including repairability, reliability, stability, and practicability have been efficiently improved, which are achieved by knotting the fractured fibers, incorporating multiple sensors in series/parallel and weaving multisensor array networks integrated into clothes. The good sensing properties, superior flexibility, and scalable applications of wearable fibers may provide a broad window for widespread monitoring of numerous human activities in personal mobile electronics and human-machine interactions.

High Selectivity Gas Sensing and Charge Transfer of SnSe2

SnSe₂ is an anisotropic binary-layered material with rich physics, which could see it used for a variety of potential applications. Here, we investigate the gas-sensing properties of SnSe₂ using first-principles calculations and verify predictions using a gas sensor made of few-layer SnSe₂ grown by chemical vapor deposition. Theoretical simulations indicate that electrons transfer from SnSe₂ to NO₂, whereas the direction of charge transfer is the opposite for NH₃. Notably, a flat molecular band appears around the Fermi energy after NO₂ adsorption and the induced molecular band is close to the conduction band minimum. Moreover, compared with NH₃, NO₂ molecules adsorbed on SnSe₂ have a lower adsorption energy and a higher charge transfer value. The dynamic-sensing responses of SnSe₂ sensors confirm the theoretical predictions. The good match between the theoretical prediction and experimental demonstration suggests that the underlying sensing mechanism is related to the charge transfer and induced flat band. Our results provide a guideline for designing high-performance gas sensors based on SnSe₂.

Inexpensive Graphene Oxide Heaters Lithographed by Laser

In this paper, we present a simple and inexpensive method for the fabrication of high-performance graphene-based heaters on different large-scale substrates through the laser photothermal reduction of graphene oxide (laser-reduced graphene-oxide, LrGO). This method allows an efficient and localized high level of reduction and therefore a good electrical conductivity of the treated films. The performance of the heaters is studied in terms of steady-state temperature, power consumption, and time response for different substrates and sizes. The results show that the LrGO heaters can achieve stable steady-state temperatures higher than 200 °C when a voltage of 15 V is applied, featuring a time constant of around 4 s and a heat transfer coefficient of ~200 °C cm²/W. These characteristics are compared with other technologies in this field, demonstrating that the fabrication approach described in this work is competitive and promising to fabricate large-scale flexible heaters with a very fast response and high steady-state temperatures in a cost-effective way. This technology can be easily combined with other fabrication methods, such as screen printing or spray-deposition, for the manufacturing of complete sensing systems where the temperature control is required to adjust functionalities or to tune sensitivity or selectivity.

Gas Sensor by Direct Growth and Functionalization of Metal Oxide/Metal Sulfide Core-Shell Nanowires on Flexible Substrates

We have developed a novel fabrication method for flexible gas sensors for toxic gases based on sequential wet chemical reaction. In specific, zinc oxide (ZnO) nanowires were locally synthesized and directly integrated on a flexible polymer substrate using localized hydrothermal synthesis methods and their surfaces were selectively functionalized with palladium (Pd) nanoparticles using a liquid phase deposition process. Because the entire process is conducted at a low temperature in a mild precursor solution, it can be applied for flexible substrates. Furthermore, the surface of ZnO nanowires was sulfurized by hydrogen sulfide (H₂S) gas to form zinc oxide/zinc sulfide (ZnO/ZnS) core-shell nanowires for stable sensing of H₂S gas. The locally synthesized ZnO/ZnS core-shell nanowires enable an ultracompact-sized device, and Pd nanoparticles improve the sensing performance and reduce the operating temperature (200 °C). The device shows a high sensitivity [(G_gas - G_air)/G_air × 100% = 4491% to 10 ppm], fast response (response/recovery time <100 s) to hydrogen sulfide, and outstanding selectivity (>100 times) to other toxic gases (e.g., carbon monoxide, acetone, ethanol, and toluene). Moreover, vertically synthesized nanowires provide a long bending path, which reduces the mechanical stresses on the structure. The devices showed stable gas sensing performance under 9 mm positive radius of curvature and 5 mm negative radius of curvature. The mechanical robustness of the device was also verified by numerical simulations which showed dramatic decrease of maximum stress and strain to 4.2 and 5.0%, respectively.

Shape-Adaptable 2D Titanium Carbide (MXene) Heater

Prior to the advent of the next-generation heater for wearable/on-body electronic devices, various properties are required, including conductivity, transparency, mechanical reliability, and conformability. Expansion to two-dimensional (2D) structure of metallic nanowires based on network- and mesh-type geometries has been widely exploited for realizing these heaters. However, the routes led to many drawbacks such as the low-density cross-bar linking, self-aggregation of wire, and high junction resistance. Although 2D carbon nanomaterials such as graphene and reduced graphene oxide (rGO) have shown their potentials for the purpose, CVD-grown graphene with sufficiently high conductivity was limited due to its poor processability for large-area applications, while rGO fabricated with a complex reduction process involving the use of toxic chemicals suffered from a low electrical conductivity. In this study, we demonstrate a simple and robust process, utilizing electrostatic assembling of negatively charged MXene flakes on a positively treated surface of substrate, for fabricating a metal-like 2D MXene thin film heater (TFH). Our TFH showed a high optical property (>65%), low sheet resistance (215 Ω/sq), fast electrothermal response (within dozens of seconds) with an intrinsically high electrical conductivity, and mechanical flexibility (up to 180° bending). Its capability for forming a firm and stable ionic-type interface with a counterpart surface allows us to develop a shape-adaptable and patchable thread heater (TH) that can be shaped on diverse substrates even under harsh conditions of conventional sewing or weaving processes. This work suggests that our shape-adaptable MXene heaters are potentially suitable not only for wearable devices for local heating and defrosting but also for a variety of emerging applications of soft actuators and wearable/flexible healthcare monitoring and thermotherapy.

Octahedral SnO2/Graphene Composites with Enhanced Gas-Sensing Performance at Room Temperature

Although high-energy facets on metal oxides are usually active and preferred for gas sensing, it is difficult to expose them according to thermodynamics. In this work, nanocomposites of SnO₂ and graphene are prepared by a hydrothermal method. The SnO₂ nanoparticles change from a lance shape to an octahedral shape as the concentration of HCl in the solution is increased gradually from 6.5 to 10 vol %. However, the SnO₂ nanoparticles have an elongated octahedral shape if the concentration of HCl is increased further. The octahedral SnO₂ nanoparticles are mainly surrounded by high-surface-energy {221} facets, thus facilitating gas sensing. First-principles calculation shows that the surface energy and adsorption energy of the {221} facets are larger than those of the stable {110} facets, and so, the gas adsorption capacity on the {221} facets is better. Furthermore, because the Fermi level of the SnO₂{221} facet is higher than that of graphene, the electrons are transferred from SnO₂ nanoparticles to graphene sheets, enabling effective electron exchange between the composite and external NO₂ gas. The excellent gas-sensing properties of the octahedral SnO₂/graphene composites are ascribed to the high-surface-energy {221} facets exposed.

Synergy between nanomaterials and volatile organic compounds for non-invasive medical evaluation

This article is an overview of the present and ongoing developments in the field of nanomaterial-based sensors for enabling fast, relatively inexpensive and minimally (or non-) invasive diagnostics of health conditions with follow-up by detecting volatile organic compounds (VOCs) excreted from one or combination of human body fluids and tissues (e.g., blood, urine, breath, skin). Part of the review provides a didactic examination of the concepts and approaches related to emerging sensing materials and transduction techniques linked with the VOC-based non-invasive medical evaluations. We also present and discuss diverse characteristics of these innovative sensors, such as their mode of operation, sensitivity, selectivity and response time, as well as the major approaches proposed for enhancing their ability as hybrid sensors to afford multidimensional sensing and information-based sensing. The other parts of the review give an updated compilation of the past and currently available VOC-based sensors for disease diagnostics. This compilation summarizes all VOCs identified in relation to sickness and sampling origin that links these data with advanced nanomaterial-based sensing technologies. Both strength and pitfalls are discussed and criticized, particularly from the perspective of the information and communication era. Further ideas regarding improvement of sensors, sensor arrays, sensing devices and the proposed workflow are also included.

Photoactivated Mixed In-Plane and Edge-Enriched p-Type MoS2 Flake-Based NO2 Sensor Working at Room Temperature

Toxic gases are produced during the burning of fossil fuels. Room temperature (RT) fast detection of toxic gases is still challenging. Recently, MoS₂ transition metal dichalcogenides have sparked great attention in the research community due to their performance in gas sensing applications. However, MoS₂ based gas sensors still suffer from long response and recovery times, especially at RT. Considering this challenge, here, we report photoactivated highly reversible and fast detection of NO₂ sensors at room temperature (RT) by using mixed in-plane and edge-enriched p-MoS₂ flakes (mixed MoS₂). The sensor showed fast response with good sensitivity of ∼10.36% for 10 ppm of NO₂ at RT without complete recovery. However, complete recovery was obtained with better sensor performance under UV light illumination at RT. The UV assisted NO₂ sensing showed improved performance in terms of fast response and recovery kinetics with enhanced sensitivity to 10 ppm NO₂ concentration. The sensor performance is also investigated under thermal energy, and a better sensor performance with reduced sensitivity and high selectivity toward NO₂ was observed. A detailed gas sensing mechanism based on the density functional theory (DFT) calculations for favorable NO₂ adsorption sites on in-plane and edge-enriched MoS₂ flakes is proposed. This study revealed the role of favorable adsorption sites in MoS₂ flakes for the enhanced interaction of target gases and developed a highly sensitive, reversible, and fast gas sensor for next-generation toxic gases at room temperature.

Fully Stretchable and Humidity-Resistant Quantum Dot Gas Sensors

Stretchable gas sensors that accommodate the shape and motion characteristics of human body are indispensable to a wearable or attachable smart sensing system. However, these gas sensors usually have poor response and recovery kinetics when operated at room temperature, and especially suffer from humidity interference and mechanical robustness issues. Here, we demonstrate the first fully stretchable gas sensors which are operated at room temperature with enhanced stability against humidity. We created a crumpled quantum dot (QD) sensing layer on elastomeric substrate with flexible graphene as electrodes. Through the control over the prestrain of the flexible substrate, we achieved a 5.8 times improvement in NO₂ response at room temperature with desirable stretchability even under 1000 stretch/relax cycles mechanism deformation. The uniformly wavy structural configuration of the crumpled QD gas-sensing layer enabled an improvement in the antihumidity interference. The sensor response shows a minor vibration of 15.9% at room temperature from relative humidity of 0 to 86.7% compared to that of the flat-film sensors with vibration of 84.2%. The successful assembly of QD solids into a crumpled gas-sensing layer enabled a body-attachable, mechanically robust, and humidity-resistant gas sensor, opening up a new pathway to room-temperature operable gas sensors which may be implemented in future smart sensing systems such as stretchable electronic nose and multipurpose electronic skin.

High-Quality AZO/Au/AZO Sandwich Film with Ultralow Optical Loss and Resistivity for Transparent Flexible Electrodes

Transparent flexible electrodes are in ever-growing demand for modern stretchable optoelectronic devices, such as display technologies, solar cells, and smart windows. Such sandwich-film-electrodes deposited on polymer substrates are unattainable because of the low quality of the films, inducing a relatively large optical loss and resistivity as well as a difficulty in elucidating the interference behavior of light. In this article, we report a high-quality AZO/Au/AZO sandwich film with excellent optoelectronic performance, e.g., an average transmittance of about 81.7% (including the substrate contribution) over the visible range, a sheet resistance of 5 Ω/sq, and a figure-of-merit (FoM) factor of ∼55.1. These values are well ahead of those previously reported for sandwich-film-electrodes. Additionally, the interference behaviors of light modulated by the coat and metal layers have been explored with the employment of transmittance spectra and numerical simulations. In particular, a heater device based on an AZO/Au/AZO sandwich film exhibits high performance such as short response time (∼5 s) and uniform temperature field. This work provides a deep insight into the improvement of the film quality of the sandwich electrodes and the design of high-performance transparent flexible devices by the application of a flexible substrate with an atomically smooth surface.

Mimicking a Dog's Nose: Scrolling Graphene Nanosheets

Inspired by the densely covered capillary structure inside a dog's nose, we report an artificial nanostructure, i. e., poly(sodium p-styrenesulfonate)-functionalized reduced graphene oxide nanoscrolls (PGNS), with high structural perfection and efficient gas sensing applications. A facile supramolecular assembly is introduced to functionalize graphene with the functional polymer, combined with the lyophilization technique to massively transform the planar graphene-based nanosheets to nanoscrolls. Detailed characterizations reveal that the bioinspired nanoscrolls exhibit a wide-open tubular morphology with uniform dimensions that is structurally distinct from the previously reported ones. The detailed morphologies of the graphene-based nanosheets in each scrolling stage during lyophilization are monitored by cryo-SEM. This unravels an asymmetric polymer-induced graphene scrolling mechanism including the corresponding scrolling process, which is directly presented by molecular dynamics simulations. The fabricated PGNS sensors exhibit superior gas sensing performance with reliable repeatability, excellent linear sensibility, and, especially, an ultrahigh response ( R_a/ R_g = 5.39, 10 ppm) toward NO₂. The supramolecular assembly combined with the lyophilization technique to fabricate PGNS provides a strategy to design biomimetic materials for gas sensors and chemical trace detectors.

Improved NO2 Gas Sensing Properties of Graphene Oxide Reduced by Two-beam-laser Interference

We report on the fabrication of a NO₂ gas sensor from room-temperature reduction of graphene oxide(GO) via two-beam-laser interference (TBLI). The method of TBLI gives the distribution of periodic dissociation energies for oxygen functional groups, which are capable to reduce the graphene oxide to hierarchical graphene nanostructures, which holds great promise for gaseous molecular adsorption. The fabricated reduced graphene oxide(RGO) sensor enhanced sensing response in NO₂ and accelerated response/recovery rates. It is seen that, for 20 ppm NO₂, the response (R_a/R_g) of the sensor based on RGO hierarchical nanostructures is 1.27, which is higher than that of GO (1.06) and thermal reduced RGO (1.04). The response time and recovery time of the sensor based on laser reduced RGO are 10 s and 7 s, which are much shorter than those of GO (34 s and 45 s), indicating that the sensing performances for NO₂ sensor at room temperature have been enhanced by introduction of nanostructures. This mask-free and large-area approach to the production of hierarchical graphene micro-nanostructures, could lead to the implementation of future graphene-based sensors.

Raman spectroscopy of graphene-based materials and its applications in related devices

Graphene-based materials exhibit remarkable electronic, optical, and mechanical properties, which has resulted in both high scientific interest and huge potential for a variety of applications. Furthermore, the family of graphene-based materials is growing because of developments in preparation methods. Raman spectroscopy is a versatile tool to identify and characterize the chemical and physical properties of these materials, both at the laboratory and mass-production scale. This technique is so important that most of the papers published concerning these materials contain at least one Raman spectrum. Thus, here, we systematically review the developments in Raman spectroscopy of graphene-based materials from both fundamental research and practical (i.e., device applications) perspectives. We describe the essential Raman scattering processes of the entire first- and second-order modes in intrinsic graphene. Furthermore, the shear, layer-breathing, G and 2D modes of multilayer graphene with different stacking orders are discussed. Techniques to determine the number of graphene layers, to probe resonance Raman spectra of monolayer and multilayer graphenes and to obtain Raman images of graphene-based materials are also presented. The extensive capabilities of Raman spectroscopy for the investigation of the fundamental properties of graphene under external perturbations are described, which have also been extended to other graphene-based materials, such as graphene quantum dots, carbon dots, graphene oxide, nanoribbons, chemical vapor deposition-grown and SiC epitaxially grown graphene flakes, composites, and graphene-based van der Waals heterostructures. These fundamental properties have been used to probe the states, effects, and mechanisms of graphene materials present in the related heterostructures and devices. We hope that this review will be beneficial in all the aspects of graphene investigations, from basic research to material synthesis and device applications.

Direct growth of graphene on rigid and flexible substrates: progress, applications, and challenges

Graphene has recently been attracting considerable interest because of its exceptional conductivity, mechanical strength, thermal stability, etc. Graphene-based devices exhibit high potential for applications in electronics, optoelectronics, and energy harvesting. In this paper, we review various growth strategies including metal-catalyzed transfer-free growth and direct-growth of graphene on flexible and rigid insulating substrates which are "major issues" for avoiding the complicated transfer processes that cause graphene defects, residues, tears and performance degradation in graphene-based functional devices. Recent advances in practical applications based on "direct-grown graphene" are discussed. Finally, several important directions, challenges and perspectives in the commercialization of 'direct growth of graphene' are also discussed and addressed.

Nitrogen-Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen-doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen-doping is carried out under thermal annealing in H₂ (4%)/N₂ (96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1-66.4% relative humidity (RH). Furthermore, a 1.36-fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt-nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real-time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt-nRGO fiber integrated on a portable sensing module. The Pt-nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.

Chemical sensing with 2D materials

During the last decade, two-dimensional materials (2DMs) have attracted great attention due to their unique chemical and physical properties, which make them appealing platforms for diverse applications in sensing of gas, metal ions as well as relevant chemical entities.

Gas molecule sensing of van der Waals tunnel field effect transistors

van der Waals (vdW) heterostructures with two-dimensional (2D) crystals such as graphene, hexagonal boron nitride (hBN) and transition metal dichalcogenides (TMDCs) allow us to demonstrate atomically thin field-effect transistors (FETs), photodetectors (PDs) and photovoltaic devices capable of higher performance and greater stability levels than conventional devices. Although there have been studies of gas molecule sensing with 2D crystal channels, vdW heterostructures based on 2D crystals have not been employed thus far. Here, utilizing graphene/WS₂/graphene (G/WS₂/G) vdW heterostructure tunnel FETs, we demonstrate the rectification behavior of the sensitivity signal by tuning the WS₂ potential barriers as a function of the gas molecule concentration and devise a fingerprint map of the sensitivity variation corresponding to an individual ratio of two different molecules in a gas mixture. Because the separation of different gas molecule concentrations from gas mixtures is in high demand in the gas-sensing research field, this result will greatly assist in the progress on selective gas sensing.

Metal-Organic Framework-Templated PdO-Co3O4 Nanocubes Functionalized by SWCNTs: Improved NO2 Reaction Kinetics on Flexible Heating Film

Detection and control of air quality are major concerns in recent years for environmental monitoring and healthcare. In this work, we developed an integrated sensor architecture comprised of nanostructured composite sensing layers and a flexible heating substrate for portable and real-time detection of nitrogen dioxide (NO₂). As sensing layers, PdO-infiltrated Co₃O₄ hollow nanocubes (PdO-Co₃O₄ HNCs) were prepared by calcination of Pd-embedded Co-based metal-organic framework polyhedron particles. Single-walled carbon nanotubes (SWCNTs) were functionalized with PdO-Co₃O₄ HNCs to control conductivity of sensing layers. As a flexible heating substrate, the Ni mesh electrode covered with a 40 nm thick Au layer (i.e., Ni(core)/Au(shell) mesh) was embedded in a colorless polyimide (cPI) film. As a result, SWCNT-functionalized PdO-Co₃O₄ HNCs sensor exhibited improved NO₂ detection property at 100 °C, with high sensitivity (S) of 44.11% at 20 ppm and a low detection limit of 1 ppm. The accelerated reaction and recovery kinetics toward NO₂ of SWCNT-functionalized PdO-Co₃O₄ HNCs were achieved by generating heat on the Ni(core)/Au(shell) mesh-embedded cPI substrate. The SWCNT-functionalized porous metal oxide sensing layers integrated on the mechanically stable Ni(core)/Au(shell) mesh heating substrate can be envisioned as an essential sensing platform for realization of low-temperature operation wearable chemical sensor.

UV-Activated MoS2 Based Fast and Reversible NO2 Sensor at Room Temperature

Two-dimensional materials have gained considerable attention in chemical sensing owing to their naturally high surface-to-volume ratio. However, the poor response time and incomplete recovery at room temperature restrict their application in high-performance practical gas sensors. Herein, we demonstrate ultrafast detection and reversible MoS₂ gas sensor at room temperature. The sensor's performance is investigated to NO₂ at room temperature, under thermal and photo energy. Incomplete recovery and high response time of ∼249 s of sensor are observed at room temperature. Thermal energy is enough to complete recovery, but it is at the expense of sensitivity. Further, under photo excitation, MoS₂ exhibits an enhancement in sensitivity with ultrafast response time of ∼29 s and excellent recovery to NO₂ (100 ppm) at room temperature. This significant improvement in sensitivity (∼30%) and response time (∼88%) is attributed to the charge perturbation on the surface of the sensing layer in the context of NO₂/MoS₂ interaction under optical illumination. Moreover, the sensor shows reliable selectivity toward NO₂ against various other gases. These unprecedented results reveal the potential of 2D MoS₂ to develop a low power portable gas sensor.

Defect-engineered graphene chemical sensors with ultrahigh sensitivity

We report defect-engineered graphene chemical sensors with ultrahigh sensitivity (e.g., 33% improvement in NO2 sensing and 614% improvement in NH3 sensing). A conventional reactive ion etching system was used to introduce the defects in a controlled manner. The sensitivity of graphene-based chemical sensors increased with increasing defect density until the vacancy-dominant region was reached. In addition, the mechanism of gas sensing was systematically investigated via experiments and density functional theory calculations, which indicated that the vacancy defect is a major contributing factor to the enhanced sensitivity. This study revealed that defect engineering in graphene has significant potential for fabricating ultra-sensitive graphene chemical sensors.