Flexible Gas Sensor Research Articles

A Novel Gas Sensor for Detecting Pork Freshness Based on PANI/AgNWs/Silk.

A novel, operational, reliable, flexible gas sensor based on silk fibroin fibers (SFFs) as a substrate was proposed for detecting the freshness of pork. Silk is one of the earliest animal fibers utilized by humans, and SFFs exposed many biological micromolecules on the surface. Thus, the gas sensor was fabricated through polyaniline (PANI) and silver nanowires (AgNWs) and deposited on SFFs by in-suit polymerization. With trimethylamine (TMA) as a model gas, the sensing properties of the PANI/AgNWs/silk composites were examined at room temperature, and the linear correlativity was very prominent between these sensing measures and the TMA measures in the range of 3.33 μg/L–1200 μg/L. When the pork sample is detected by the sensor, it can be classified into fresh or stale pork with the total volatile basic nitrogen (TVB-N) as an index. The result indicated that the gas sensor was effective and showed great potential for applications to detect the freshness of pork.

Microstructural Control of Soluble Acene Crystals for Field-Effect Transistor Gas Sensors.

Microstructural control during the solution processing of small-molecule semiconductors (namely, soluble acene) is important for enhancing the performance of field-effect transistors (FET) and sensors. This focused review introduces strategies to enhance the gas-sensing properties (sensitivity, recovery, selectivity, and stability) of soluble acene FET sensors by considering their sensing mechanism. Defects, such as grain boundaries and crystal edges, provide diffusion pathways for target gas molecules to reach the semiconductor-dielectric interface, thereby enhancing sensitivity and recovery. Representative studies on grain boundary engineering, patterning, and pore generation in the formation of soluble acene crystals are reviewed. The phase separation and microstructure of soluble acene/polymer blends for enhancing gas-sensing performance are also reviewed. Finally, flexible gas sensors using soluble acenes and soluble acene/polymer blends are introduced, and future research perspectives in this field are suggested.

Femtosecond Laser-Assisted Device Engineering: Toward Organic Field-Effect Transistor-Based High-Performance Gas Sensors.

Organic electronic-based gas sensors hold great potential for portable healthcare- and environment-monitoring applications. It has recently been shown that introducing a porous structure into an organic semiconductor (OSC) film is an efficient way to improve the gas-sensing performance because it facilitates the interaction between the gaseous analyte and the active layer. Although several methods have been used to generate porous structures, the development of a robust approach that can facilely engineer the porous OSC film with a uniform pore pattern remains a challenge. Here, we demonstrate a robust approach to fabricate porous OSC films by using a femtosecond laser-processed porous dielectric layer template. With this laser-assisted strategy, various polymeric OSC layers with controllable pore size and well-defined pore patterns were achieved. The consequent porous p-type polymer-based device exhibits enhanced sensitivity to the ammonia analyte in the range from 100 ppb to 10 ppm with remarkable reproducibility and selectivity. The micropattern of the active layer was precisely controlled by generating various pore densities in the predecorated templates, which results in modulated ammonia sensitivities ranging from 30 to 65% ppm-1. Furthermore, we show that this approach can be used to fabricate flexible gas sensors with enhanced sensing performance and mechanical durability, which indicate that this femtosecond laser-assisted approach is very promising for the fabrication of next-generation wearable electronics.

One-pot hydrothermal growth of indium oxide-CNT heterostructure via single walled carbon nanotube scaffolds and their application toward flexible NO2 gas sensors

Single walled carbon nanotubes (SWNTs) are recognized as a promising sensing material for flexible NO2 gas sensors, whereas poor recovery at room temperature is a significant barrier to practical application. This study reports a one-pot hydrothermal synthesis of an indium oxide/single-walled carbon nanotube heterostructure (h-In2O3/SWNT) for efficient recovery in gas sensors. This results in the stable formation of 2-dimensional electron gases and their favorable electron compensation in the channel layers. The h-In2O3/SWNT FET gas sensors (h-GS) exhibit a sensing resolution of 1 ppm, negligible degradation over 9 consecutive cycled exposures with a 2-day-interval, and a significant improvement in recovery within 10 min at room temperature. Surprisingly, even when the flexible h-GS is evaluated under repeated tensile strain of 0.88 %, electrical reliability and reproducibility of the h-GS is fully validated. Thus, with its demonstrated robust sensing properties and compensation scheme, the h-In2O3/SWNT is expected to be a promising candidate for use as a novel sensing layer in a wearable gas sensor, resolving one of the long-standing issues associated with gas sensor sluggish recovery and reliability for impact.

Tough and Antifreezing MXene@Au Hydrogel for Low-Temperature Trimethylamine Gas Sensing.

Trimethylamine (TMA) is one of the important chemical indexes to judge the freshness of marine fish. It is necessary to develop a low temperature TMA sensor to help the monitoring and prediction of the quality of marine fish in cold chain. Herein, a flexible low temperature TMA gas sensor featuring antifreezing and superior mechanical properties was developed based on the Au nanoparticle-modified MXene (MXene@Au) composite. MXene@Au was synthesized and then polymerized with a hydrogel composed of acrylamide (AM), N,N'-methylenebisacrylamide (BIS), sodium carboxymethyl cellulose (CMC), and EG, and the resultant MXene@Au hydrogel was found to exhibit excellent antifreezing performance even at extremely low temperature as well as high tensile strength, ultrastretchability, and toughness, which enabled an efficient gas sensing platform for TMA detection at low temperature. The TMA sensing properties of the flexible MXene@Au DN hydrogel sensor at 25 °C and a low temperature of 0 °C were studied, and a linear relationship between TMA sensitivity and concentration was built. The excellent sensing properties were maintained even under deformation. The application of the MXene@Au DN hydrogel sensor in detection of fish freshness at 0 °C was investigated. The result indicated the potential application of the flexible MXene@Au DN hydrogel gas sensor in dynamic quality monitoring and prediction of marine fish products during its transportation and storage in the cold chain.

Organic/Inorganic-Based Flexible Membrane for a Room-Temperature Electronic Gas Sensor.

A room temperature (RT) H2S gas sensor based on organic–inorganic nanocomposites has been developed by incorporating zinc oxide (ZnO) nanoparticles (NPs) into a conductivity-controlled organic polymer matrix. A homogeneous solution containing poly (vinyl alcohol) (PVA) and ionic liquid (IL) and further doped with ZnO NPs was used for the fabrication of a flexible membrane (approx. 200 μm in thickness). The sensor was assessed for its performance against hazardous gases at RT (23 °C). The obtained sensor exhibited good sensitivity, with a detection limit of 15 ppm, and a fast time response (24 ± 3 s) toward H2S gas. The sensor also showed excellent repeatability, long-term stability and selectivity toward H2S gas among other test gases. Furthermore, the sensor depicted a high flexibility, low cost, easy fabrication and low power consumption, thus holding great promise for flexible electronic gas sensors.

Plasma Surface Modification of Flexible Substrates to Improve Grafting for Various Gas Sensing Applications: A Review

Surrounding air pollution is one of the important warnings to human health. It is necessary to detect hazardous pollutant gases in the air. The gas sensor is the candidate for such detection. Progress of smart and flexible sensors has been receiving growing curiosity in recent years because of their enormous sensing applications. Such smart and flexible sensors can be manufactured by depositing conducting materials, such as conducting polymer, carbon nanotube (CNT), graphene oxide, and metallic nanoparticles, onto nonconducting polymeric flexible substrates, such as paper, plastic films, and textiles. Most of the flexible polymeric substrates, such as polyethylene, polyester, nylon, and polypropylene, have low surface energy and poor wettability, which weakens the adhesion between conducting material and flexible substrate. Plasma surface modification is a novel technique to enhance surface properties to produce stable composites. This review focuses on various types of plasmas for surface modification and their effects on the surface after plasma treatment to improve the grafting and adhesion of the sensing layer with the substrate. The aim of this article is to systematically overview the development of plasma-enhanced smart and flexible gas sensors to date and to envisage their future progress.

Effect of the deposition method and substrate on an ultra-stable CoV2O6 hydrogen gas sensor, operating at room temperature

The constantly increasing demand for flexible, reliable and stable gas sensors, operating at room temperature, which would be integrated with present electronic circuits for real life applications is a true fact. In the present work, the gas sensing properties of bimetallic cobalt‑vanadium-oxide system (CoV2O6) under the effect of deposition method (spin coating, direct ink writing and drop-casting) as well as the type of substrate (glass, flexible PET), at room temperature were examined. The surface morphology as well as the film thickness of the sensing material were investigated by Scanning Electron Microscopy. The spin-coated sensor on glass substrate exhibited the higher response (65.2%) towards 1000 ppm H2 at room temperature, compared to those that were deposited with the other methods, showing a response and recovery time of 94 s and 74 s, respectively. The CoV2O6 flexible PET sensor exhibited a response of 36.6% against 1000 ppm H2, at room temperature, with a response and recovery time of 120 s and 80 s, respectively. In addition, both operation under bending conditions at 180o and the reliability, in terms of repeatability of signal and stability after 1 year, were studied for the CoV2O6 flexible PET sensor. Finally, a sensing mechanism is proposed taking into account the thickness and architecture of the films as well as film variations and imperfections of the active sensing surface, such as micro-cracks, edges and pores. These parameters play a dominant role on the gas sensing performance far greater than that of the substrate or the metal electrodes. The realization of a CoV2O6 flexible sensor demonstrates its potential for real-life applications.

Flexible all-biomass gas sensor based on doped carbon quantum dots/nonwoven cotton with discriminative function

Carbon quantum dots (CQDs) co-doped with N, P and S derived from expired milk was prepared by a simple hydrothermal method. By dipping pure cotton face towel (PCFT) into CQDs ink, a flexible all-biomass CQDs/PCFT sensor was prepared for the first time. Due to the heteroatom doping, extremely small particle size of CQDs and excellent permeability of CQDs/PCFT film, the flexible CQDs/PCFT sensor showed the high sensitivity and bending stability. In the range of 0–60° bending states, the responses of CQDs/PCFT sensor to four target analytes changed by less 5.0%. After 3000 bending of 60°, the maximum change of the response to the target analytes was only 6.4%. Interestingly, due to the abundant functional groups and defects of CQDs, the flexible CQDs/PCFT sensor displayed sensing curves of different shapes for different target analytes. In this way, by establishing a database of sensing curves of target analytes, multiple analytes can be detected discriminatively by relying only on single sensor with the help of image recognition. This work provided a reference for the development of cotton fiber based all biomass flexible gas sensor.Graphical abstract

Ultrasensitive flexible NO2 gas sensors via multilayer porous polymer film

Micro-nano structure engineering of active materials in sensors have attracted broad attention for hazardous component detection, which enables creating electronic noses that could detect gas analytes efficiently without introducing abundant functional groups and defects. However, challenges still remain on realizing flexible gas sensors with both decent sensing performance and good mechanical robustness, which are extremely important for wearable electronics. Here we present a scalable and facile manufacturing method to construct the three-dimensional (3D) multilayer porous organic semiconductors (OSCs) structure for transistor based gas sensor, showing largely enhanced gas sensing properties as well as bendability down to 1 mm radius with stable operation. The multilayer porous film of OSCs is assembled via breath figure method, followed by layer-by-layer transferring process. The formed 3D porous structure provides efficient hazardous gas diffusion pathways, abundant gas molecules adsorption active sites and immunity towards mechanical deformation simultaneously. Consequently, an obvious increase of responsivity (from 122% to 1053% at 30 ppm) was achieved for NO2 detection, along with an ultralow limit of detection (∼2.3 ppb), and good gas selectivity. This work demonstrates a scalable and low-cost fabrication strategy for high-performance flexible sensors, which delivers an alternative way for next-generation wearable electronics.

A flexible and wearable NO2 gas detection and early warning device based on a spraying process and an interdigital electrode at room temperature

Flexible sensors used to detect NO2 gas generally have problems such as poor repeatability, high operating temperature, poor selectivity, and small detection range. In this work, a new spraying platform with a simple structure, low cost, and good film-forming consistency was designed and built to make a sensitive film (rGO/SnO2) for NO2 gas sensors. The relationship between the solid content of rGO and SnO2 nanoparticles, annealing temperature, and sensor performance was studied. The results show that the interdigital electrode-sensitive film formed by spraying 0.25 ml of a 0.4 wt% rGO/SnO2 mixture and annealing at 250 °C exhibited the best comprehensive performance for NO2 detection. The sensor’s response value for 100 ppm NO2 gas was 0.2640 at room temperature (25 °C), and the response time and recovery time were 412.4 s and 587.3 s, respectively. In the range of 20–100 ppm, the relationship between the response and NO2 concentration was linear, and the correlation coefficient was 0.9851. In addition, a soft-monitoring node module with an overlimit warning function for NO2 gas was designed and manufactured based on flexible electronics. Finally, the flexible sensor and node module were embedded into woven fabric that could be used to make a mask or a watch that could detect NO2 gas, realizing the practical application of flexible NO2 gas sensors in the field of wearable electronics.

Intrinsically Breathable and Flexible NO2 Gas Sensors Produced by Laser Direct Writing of Self-Assembled Block Copolymers.

The surge in air pollution and respiratory diseases across the globe has spurred significant interest in the development of flexible gas sensors prepared by low-cost and scalable fabrication methods. However, the limited breathability in the commonly used substrate materials reduces the exchange of air and moisture to result in irritation and a low level of comfort. This study presents the design and demonstration of a breathable, flexible, and highly sensitive NO2 gas sensor based on the silver (Ag)-decorated laser-induced graphene (LIG) foam. The scalable laser direct writing transforms the self-assembled block copolymer and resin mixture with different mass ratios into highly porous LIG with varying pore sizes. Decoration of Ag nanoparticles on the porous LIG further increases the specific surface area and conductivity to result in a highly sensitive and selective composite to detect nitrogen oxides. The as-fabricated Ag/LIG gas sensor on a flexible polyethylene substrate exhibits a large response of -12‰, a fast response/recovery of 40/291 s, and a low detection limit of a few parts per billion at room temperature. Integrating the Ag/LIG composite on diverse fabric substrates further results in breathable gas sensors and intelligent clothing, which allows permeation of air and moisture to provide long-term practical use with an improved level of comfort.

Synthesis, characterization and mechanism of electrospun carbon nanofibers decorated with ZnO nanoparticles for flexible ammonia gas sensors at room temperature

A flexible gas sensor based on ZnO nanoparticles decorated carbon nanofibers (ZnO@CNF) was fabricated by electrospinning, preoxidation, oxidation and carbonization to detect ammonia at room temperature. SEM, TEM, EDS, XPS, XRD and Raman spectra were used to analyze the morphology, structure and composition of the composite. By using electrospinning technology, preoxidation at 220 °C in air, oxidation at 410 °C and carbonization at 600 °C in argon, ZnO@CNF with a diameter of about 200 nm were synthesized. The preoxidation stage includes cyclization, dehydrogenation and oxidation of PAN, while the carbonization was contributed to the decreasing of carbon nanofiber diameter. The gas sensing test results show that ZnO@CNF has higher response, better selectivity than CNF to ammonia at room temperature, while greatly reducing the optimum working temperature of pure ZnO. The flexibility test of ZnO@CNF shows that the composite of metal oxides and carbon materials has a certain degree of flexibility. With the advantages of flexibility, room temperature operation and good response to ammonia, gas sensors based on ZnO@CNF are expected to become a potential candidate for the detection of ammonia in exhaled gas of patients with gastrointestinal diseases or kidney diseases in the future.

High-Performance, Flexible NO2 Chemiresistors Achieved by Design of Imine-Incorporated n-Type Conjugated Polymers.

Flexible and mechanically robust gas sensors are the key technologies for wearable and implantable electronics. Herein, the authors demonstrate the high‐performance, flexible nitrogen dioxide (NO2) chemiresistors using a series of n‐type conjugated polymers (CPs: PNDIT2/IM‐x) and a polymer dopant (poly(ethyleneimine), PEI). Imine double bonds (C = N) are incorporated into the backbones of the CPs with different imine contents (x) to facilitate strong and selective interactions with NO2. The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO2 sensors with PNDIT2/IM‐0.1 and PEI (1:1 by weight ratio) exhibit outstanding sensing performances, such as excellent sensitivity (ΔR/R b = 240% @ 1 ppm), ultralow detection limit (0.1 ppm), high selectivity (ΔR/R b < 8% @ 1 ppm of interfering analytes), and high stability, thereby outperforming other state‐of‐the‐art CP‐based chemiresistors. Furthermore, the thin film of PNDIT2/IM‐0.1 and PEI blend is stretchable and mechanically robust, providing excellent flexibility to the NO2 sensors. Our study contributes to the rational design of high‐performance flexible gas sensors.

Study on the preparation and performance of flexible sulfur dioxide gas sensors based on metal-organic framework

Study on the preparation and performance of flexible sulfur dioxide gas sensors based on metal-organic framework

Laser-localized hydrothermal synthesis of flexible ZnO gas sensor for room-temperature detection of nitrogen dioxide

Laser-localized hydrothermal synthesis of flexible ZnO gas sensor for room-temperature detection of nitrogen dioxide

Flexible and disposable paper-based gas sensor using reduced graphene oxide/chitosan composite

Nitrogen dioxide (NO2) is a representative toxicant in air pollution that mostly arises from the exhaust gas released by automobiles. It is related to various respiratory diseases such as pneumonia and sudden infant death syndrome. Additionally, because the toxicity of nitrogen dioxide is high in overpopulated areas (i.e., a capital or metropolis), the development of simple, practical, and facile sensors is highly needed. This work presents a flexible and disposable paper-based NO2 sensor based on a reduced graphene oxide/chitosan (rGO/CS) composite. The synthesized rGO/CS composite can be easily flexed and deformed into various shapes, which are attributed to chitosan molecules that function as a dispersion and reduction agent and support material. In addition, this composite can be attached to paper owing to its adhesive property; hence it can be utilized in versatile applications in a disposable manner. By analyzing the conductive change of the rGO/CS composite when it reacts with NO2, we can detect nitrogen dioxide in a concentration range of 0–100 ppm with a detection limit of 1 ppm. Moreover, we performed NO2 detection in the exhaust gas released by automobiles using the rGO/CS composite for practical application. The results indicated that the rGO/CS composite has the potential to be used in feasible gas sensing as a facile and disposable sensor under various conditions.

Self-Powered and Flexible Gas Sensor Using Defect-Engineered Ws2/G Heterostructure

Self-Powered and Flexible Gas Sensor Using Defect-Engineered Ws2/G Heterostructure

Flexible Acetone Gas Sensor based on ZIF-8/Polyacrylonitrile (PAN) Composite Film

Flexible Acetone Gas Sensor based on ZIF-8/Polyacrylonitrile (PAN) Composite Film

Ligand-assisted deposition of ultra-small Au nanodots on Fe2O3/reduced graphene oxide for flexible gas sensors.

The development of flexible room-temperature gas sensors is important in environmental monitoring and protection. In this contribution, by using 1-octadecanethiol (ODT) as a surface ligand, Au nanodots (NDs) with ultra-small size of ∼1.7 nm were deposited on the surface of α-Fe2O3/reduced graphene oxide (rGO). The Au ND–ODT/α-Fe2O3/rGO composite was fabricated into flexible gas sensors, which could detect NO2 gas down to 200 ppb at room temperature. Compared with α-Fe2O3/rGO, Au ND–ODT/α-Fe2O3/rGO showed enhanced sensing performance because of the beneficial effects of Au NDs, including facilitating the adsorption of NO2 molecules and forming ohmic-like contact with rGO and α-Fe2O3. In addition, the sensing performance of the composite was also influenced by the surface ligands of the Au NDs. Ligands with less polar terminal groups were found to be beneficial to charge transfer in the sensing film. Moreover, Au ND–ODT/α-Fe2O3/rGO-based flexible sensors showed negligible performance deterioration under moderately bent conditions, suggesting their potential to be used in portable and wearable devices.