Micropyramidal Flexible Ion Gel Sensor for Multianalyte Discrimination and Strain Compensation.

Highly flexible, mechanically stable, and sensitive NO2 gas sensors based on reduced graphene oxide nanofibrous mesh fabric for flexible electronics

Owing to the unique properties such as pristine interfaces, ultra‐thin and uniform thickness, 2D nanomaterials have attracted the attention from many researchers. Particularly, rGO and MXene 2D nanomaterials have received intense research interest in the application of gas sensors due to their high electrical conductivity. The coating of these 2D nanomaterials onto plastic, paper or textile substrate formed flexible, stretchable and portable gas sensors as an alternative to the conventional rigid gas sensors. To date, numerous research works on flexible gas sensors using either graphene or MXene have been published. However, the reported data is scattered and varied by multiple factors. In this short review, we revisit the advances in the current application of 2D conductive nanomaterials (mainly rGO and MXene) and their nanocomposites with metal oxides on flexible gas sensors. We focus on recent developments regarding the working mechanism, advantages, disadvantages, and performances (including gas response, selectivity, response time, and recovery time) of each type of gas sensor. Finally, this short review sheds light on future directions and prospects in the field of flexible gas sensors using 2D nanomaterials.

With the rapid development of nanotechnology, silicon nanowire (SiNW) array gas sensors have shown great potential for gas detection due to their high sensitivity, fast response, and excellent selectivity. In this study, we fabricated a flexible SiNW array gas sensor and measured its performance in acetone gas detection under various conditions, including different acetone gas concentrations (ranging from 0.1 to 200 ppm), humidity levels, and bending conditions. To further investigate the sensor’s performance, we developed a multiphysics finite element model using COMSOL software. The model simulation results were in good agreement with the experimental data. Experimental and modeling results showed that the main factors influencing sensor performance were the changes in nanowire morphology, diffusion dynamics, and adsorption, while the underlying mechanisms were discussed. This study provides helpful theoretical support for the design and optimization of flexible SiNW array gas sensors, offering insights for the development of highly sensitive and durable flexible gas sensors.

This study investigates the synthesis of zinc oxide nanoparticles (ZnO NPs) at room temperature (RT) and their integration into flexible gas sensors for diverse environmental and industrial applications. ZnO, a group II-VI semiconductor with a wide bandgap (3.0–3.37 eV), exhibits high sensitivity, stability, and cost-effectiveness, making it an excellent candidate for gas sensing applications. However, traditional ZnO synthesis methods often demand high operating temperatures and involve lengthy, energy-intensive processes, limiting their use in flexible and wearable sensor technologies. Addressing these challenges, we propose an efficient RT synthesis method leveraging variations in solution pH to optimize the physiochemical properties of ZnO NPs, thereby advancing their practical applications.ZnO NPs were synthesized by adjusting the pH of aqueous solutions (neutral to high basicity) and employing bath sonication. The RT synthesis process eliminates the need for high-temperature hydrothermal methods, resulting in energy savings and reduced environmental impact. The synthesized NPs underwent extensive characterization to determine their size, structure, surface area, and thermal stability. A subset of NPs was calcinated at 500 °C to investigate the impact of thermal treatment on gas-sensing properties.The synthesized ZnO NPs demonstrated a strong dependence on solution pH. At higher basicity levels (pH ≈ 13), the NPs exhibited minimal organic residue (4.2 wt%), high crystallinity, and a narrow size distribution (30–80 nm). These features were corroborated by thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and X-ray diffraction (XRD), which revealed structural transformations and superior thermal stability. In contrast, NPs synthesized at neutral pH exhibited significant organic impurities and less uniformity, indicating the critical role of solution basicity in determining the quality of ZnO NPs.The ZnO NPs were incorporated into flexible gas sensors fabricated using the doctor blade technique, depositing nanocomposite pastes onto pre-screened carbon electrodes on polyethylene terephthalate (PET) substrates. These sensors demonstrated remarkable mechanical flexibility, maintaining stability under repetitive bending and stress. The integration of pre- and post-calcinated ZnO NPs enabled a comparative analysis of their sensing performance. Gas sensing tests were conducted under ambient conditions (RT and moderate relative humidity), exposing the sensors to various volatile organic compounds and analyzing their response and recovery times.Sensors containing pre-calcinated ZnO NPs (RT.pH13, Figure 1A) exhibited heightened sensitivity to gases such as styrene and acetic acid (Figure 1(C,D)), while post-calcinated sensors (RT.pH13.C, Figure 1B) showed improved selectivity towards benzene, acetone, and ethanol (Figure 1(C,D)). These variations in gas sensitivity were attributed to differences in surface properties, such as particle size and organic impurity levels, influenced by the calcination process. For instance, the removal of organic residues through calcination enhanced the specificity of the RT.pH13.C sensors, reducing noise and improving stability. Additionally, the RT.pH13 sensors demonstrated partial recovery after exposure to styrene, suggesting possible chemical interactions, whereas the RT.pH13.C sensors exhibited more reversible responses indicative of physical sorption mechanisms.The sensors' temperature and humidity performance were also evaluated, revealing the calcination process's role in improving thermal and humidity stability. The RT.pH13.C sensors maintained consistent behavior under varying environmental conditions, demonstrating their suitability for applications in challenging industrial settings. In contrast, the RT.pH13 sensors exhibited greater sensitivity to humidity and temperature fluctuations, likely due to the presence of residual organic materials.This study underscores the potential of RT-synthesized ZnO NPs as a viable alternative to traditional hydrothermal methods. By optimizing solution pH and minimizing post-synthesis processing steps, the proposed method offers a cost-effective, scalable, and environmentally friendly approach to ZnO NP production. The integration of these NPs into flexible sensors highlights their practical utility for Internet of Things (IoT) applications, including environmental monitoring, healthcare diagnostics, and industrial safety.Future research will focus on expanding the range of target gases and exploring additional modifications to further enhance sensor performance. Collaborations with interdisciplinary teams are anticipated to validate these findings and accelerate the deployment of ZnO-based sensors in real-world applications. This work bridges the gap between nanomaterial synthesis and application, paving the way for innovative advancements in flexible gas sensor technology. Figure 1

Strip-type flexible capacitive humidity sensor based on composite of polyimide and poly(glycidyl methacrylate): Fabrication, humidity sensitive performance and potential for detecting water content in liquids

Lightweight and flexible gas sensors are essentially required for the fast detection of toxic gases to pass on the early warning to deter accident situations caused by gas leakage. In view of this, we have fabricated a thin paper-like free-standing, flexible, and sensitive carbon nanotube (CNT) aerogel gas sensor. The CNT aerogel film synthesized by the floating catalyst chemical vapor deposition method consists of a tiny network of long CNTs and ∼20% amorphous carbon. The pores and defect density of the CNT aerogel film were tuned by heating at 700 °C to obtain a sensor film, which showed excellent sensitivity for toxic NO2 and methanol gas in the concentration range of 1-100 ppm with a remarkable limit of detection ∼90 ppb. This sensor has consistently responded to toxic gas even after bending and crumpling the film. Moreover, the film heat-treated at 900 °C showed a lower response with opposite sensing characteristics due to switching of the semiconductor nature of the CNT aerogel film to n-type from p-type. The annealing temperature-based adsorption switching can be related to a type of carbon defect in the CNT aerogel film. Therefore, the developed free-standing, highly sensitive, and flexible CNT aerogel sensor paves the way for a reliable, robust, and switchable toxic gas sensor.

Wearable biosensors have received tremendous attention over the past decade owing to their great potential in predictive analytics and treatment toward personalized medicine. Flexible electronics could serve as an ideal platform for personalized wearable devices because of their unique properties such as light weight, low cost, high flexibility and great conformability. Unlike most reported flexible sensors that mainly track physical activities and vital signs, the new generation of wearable and flexible chemical sensors enables real-time, continuous and fast detection of accessible biomarkers from the human body, and allows for the collection of large-scale information about the individual's dynamic health status at the molecular level. In this article, we review and highlight recent advances in wearable and flexible sensors toward continuous and non-invasive molecular analysis in sweat, tears, saliva, interstitial fluid, blood, wound exudate as well as exhaled breath. The flexible platforms, sensing mechanisms, and device and system configurations employed for continuous monitoring are summarized. We also discuss the key challenges and opportunities of the wearable and flexible chemical sensors that lie ahead.

Advances in stretchable and flexible sensors are meeting the demands of an expanding range of applications, such as wearable healthcare, in vivo monitoring, and soft interactive robots. Unlike conventional rigid sensors, soft and elastic material-based flexible sensors are more adaptable to applied objects which involve dynamic and irregular surfaces. Flexible optical fiber sensors offer superior advantages over conventional flexible electronic sensors, including high sensitivity, rapid response time, high biocompatibility, immunity to electromagnetic interference, and enhanced safety. These desirable features have propelled them to become a prominent category, witnessing substantial advancements in recent years. This technology has successfully facilitated a diverse range of applications, such as human health monitoring, biological measurements, and human-machine interfaces. In this review, we provide a concise overview of the latest developments in flexible optical fiber sensing, focusing on key aspects, including preparation materials, methodologies, and applications. Furthermore, we provide valuable insights into the potential challenges and opportunities that lie ahead in this field of sensing technology.

Patternable production of SrTiO3 nanoparticles using 1-W laser directly on flexible humidity sensor platform based on ITO/SrTiO3/CNT

Due to the increased world‐wide threat to health from air pollution, as well as the need for personalized diagnostic and physiological monitoring of exhaled breath, flexible gas sensor nodes have become an emerging area of critical importance for wearable electronics applicable. In this study, WO3−δ films are prepared on flexible substrates at room temperature using a single‐step, solid‐state deposition, granule spray in vacuum (GSV) process. The GSV‐WO3−δ films are successfully utilized to fabricate an ultrasensitive flexible gas sensor. The performance of the sensor can be enhanced by its increased surface area and large number of defect sites on the surface utilizing high‐impulse hollow sphere granules. The gas sensor is extremely responsive and selective to low concentration of NO2. The gas‐sensing responses are linear for the concentrations ranging from sub‐ppm to tens of ppm with detection limit of 1.88 (±0.128) for NO2. The response time is on the order of few seconds for gas detection (17 s) and recovery (25 s). Beyond this sensing performance, the GSV‐WO3−δ film is highly advantageous in terms of production cost and time for the realization of mass production of flexible sensing devices, compared to conventional gas sensors that require high‐cost and time‐consuming processes.

The sensitive and selective detection of ammonia (NH 3 ) gas at sub-ppm level and room temperature has attracted significant attention due to its promising applications in environment monitoring and medical diagnosis. Furthermore, these applications require flexible and wearable functionalities for both potential and practical use. Here, we report an NH 3 gas sensor based on a porous copper bromide (CuBr) film, converted from the two-dimensional (2D) copper nanosheets (Cu NSs). The porous CuBr film as a sensing material is prepared by an all-solution and vacuum-free process at low temperature, enabling device fabrication on a plastic substrate. The optimized gas sensor, achieved by controlling the morphology and surface coverage of the porous CuBr film, exhibits a sensitive and selective NH 3 gas response at sub-ppm levels and room temperature. The mechanical bending test over 1000 bending cycles at a bending radius (r) of 2 mm reveals the feasibility of flexible and wearable applications. • Development of flexible NH 3 gas sensors based on CuBr film using a low-temperature and vacuum-free solution process. • A porous CuBr film converted from 2D copper nanosheets at low temperature. • Sub-ppm level NH 3 detection at room temperature (30 °C) and selective detection, • Gas response exceeding 5 even after 1000 bending cycles (bending radius: ~ 2 mm). • Potential as large-area fabrication and flexible/wearable electronics.

Flexible chemiresistive sensor of polyaniline coated filter paper prepared by spraying for fast and non-contact detection of nitroaromatic explosives

Flexible H2S sensors were fabricated based on ultrathin dinaphtho [3, 4-d:3’, 4’-d’] benzo [1, 2-b:4, 5b’] dithiophene (Ph5T2) single-crystal organic field-effect transistors, and response performances at different deformation states were investigated. The flexible devices exhibited strikingly different sensing behaviors under different deformation states toward H2S at room temperature. In a tensile state with ${r} = \textsf {7}$ mm, the flexible sensor presented an unprecedented response as high as 400% at 1 ppm H2S, which is almost one order of magnitude higher than the response in a flat state and surpasses all reported flexible H2S sensors. Such a surprising improvement provides an effective way to enhance the response of gas sensors and shows the potential advantage of using flexible sensors in tensile states.

Flexible multidimensional sensors are essential for smart wearables in motion monitoring, recognition and health tracking. However, conventional strain sensors have limitations for application in complex three‐dimensional (3D) motion analysis due to their tradeoff between the detection direction and strain sensitivity, and the process compatibility for system integration in flexible sensors. This work presents a novel flexible multidimensional sensor capable of simultaneous detection of in‐plane (along X ‐ and Y ‐axes) and out‐of‐plane pressure ( Z ‐axis) strain direction using an integrated thin film sensor. The 3D thin film sensor comprises a tri‐layer structure of an upper pressure sensor component and a stack of orthogonally aligned nanofibrous strain layers, forming a cross‐structured sensor with strong orthogonally directional selectivity. The integrated 3D thin film sensor is fabricated by electrospinning and infiltration, achieving a high gauge factor (GF) of 925 and a wide strain range of 630%. The sensor exhibits an in‐plane GF of 293.2 along the X ‐axis and that of 1.3 along the Y ‐axis, yielding a high selectivity ratio of 6.15, revealing a mechanical stability over 5500 cycles with minimal crosstalk. It accurately distinguishes strain direction (0°–90°) and magnitude. Z ‐axis pressure is detected via a capacitive mechanism, thus enabling full 3D force perception. Integrated with a long short‐term memory network, the 3D sensor achieves 95.83% accuracy in recognizing complex motions, surpassing single‐axis recognition by 21.7%. This work demonstrates a compact, high‐performance approach to multidimensional sensing for intelligent wearable motion monitoring and recognition systems.

Sensors are core elements to directly obtain information from surrounding objects for further detecting, judging and controlling purposes. With the rapid development of soft electronics, flexible sensors have made considerable progress, and can better fit the objects to detect and, thus respond to changes more sensitively. Recently, as a newly emerging electronic ink, liquid metal is being increasingly investigated to realize various electronic elements, especially soft ones. Compared to conventional soft sensors, the introduction of liquid metal shows rather unique advantages. Due to excellent flexibility and conductivity, liquid-metal soft sensors present high enhancement in sensitivity and precision, thus producing many profound applications. So far, a series of flexible and wearable sensors based on liquid metal have been designed and tested. Their applications have also witnessed a growing exploration in biomedical areas, including health-monitoring, electronic skin, wearable devices and intelligent robots etc. This article presents a systematic review of the typical progress of liquid metal-enabled soft sensors, including material innovations, fabrication strategies, fundamental principles, representative application examples, and so on. The perspectives of liquid-metal soft sensors is finally interpreted to conclude the future challenges and opportunities.