Vapor Sensor Research Articles

Low-density polyethylene-multi-walled carbon nanotube nanocomposite membranes with enhanced conductivity for highly sensitive vapor sensing

A facile technique was reported for fabricating high conductivity and improved strength of linear low-density polyethylene/multi-walled carbon nanotubes (LLDPE/MWNTs) composite films by the ultrasonication anchoring technique and compression molding treatment. Thermal property, mechanical property, electrical conductivity, microstructures, optical property, and organic vapor sensing behaviors of the MWNTs/LLDPE composite films were studied. The MWNTs are uniformly anchored onto the surface of LLDPE matrix, and the conductive networks are easily formed by the ultrasonication anchoring technique. After compression molding treatment, the incorporation of MWNTs causes an easier formation of LLDPE extended-chain, which is wrapped around of MWNTs shish. The MWNTs/LLDPE composite films exhibit an excellent conductivity of 2.79 × 105 Ω∙cm with 0.15 wt % MWNTs. Meanwhile, the tensile strength of the composite films reaches 18.9 MPa. Interestingly, the transparency is not significantly reduced. The sensitivity and reproducibility of vapor sensing behaviors have been demonstrated during immersion-drying runs toward two representative solvents, i.e., acetone and xylene. This work opens up a new direction for the conductivity optimization of MWNTs/LLDPE composite films with a broad prospect in the field of vapor sensor.

Highly sensitive and selective chlorobenzene vapor sensor based on ZIF-8/GO one-dimensional photonic crystal

In this study, one-dimensional photonic crystal (1DPC) chlorobenzene sensors assembled by ZIF-8, GO, and TiO2 were proposed. The optical properties of 1DPC based on ZIF-8, GO, and TiO2 were studied by varying the number of periods, and the optimized structure of 1DPC based on ZIF-8, TiO2, and GO was to be 2-period. In comparison to the ZIF-8/TiO2 1DPC sensor, the fabricated ZIF-8/GO 1DPC sensor showed better optical sensing properties of chlorobenzene vapor, which showed stronger selectivity, higher sensitivity, and lower limit of detection (0.55 ppm). The sensitivity of the ZIF-8/GO 1D PC sensor (0.18 nm/ppm) is almost 3 times higher than that of ZIF-8/TiO2 1D PC sensor (0.05 nm/ppm) at 0–100 ppm concentration of chlorobenzene vapor. The relationship between the reflected spectra shifts and chlorobenzene of low concentrations was presented to be high linearity. Moreover, the ZIF-8/GO 1DPC sensor showed ultrafast response time (500 ms), outstanding stability, and recyclability. The mechanism of 1DPC sensor based on ZIF-8/GO for enhancing the selectivity and sensitivity to chlorobenzene was also discussed. Given its excellent properties, ZIF-8/GO 1DPC sensor can be an attractive option of high-performance chlorobenzene sensor for environmental quality monitoring.

Vapor Sensing of Colorectal Cancer Biomarkers in Isolation by Bare and Functionalized Nanoelectromechanical Sensors

Vapor Sensing of Colorectal Cancer Biomarkers in Isolation by Bare and Functionalized Nanoelectromechanical Sensors

Molecularly Imprinted Polymer Based Antenna Sensor for Methanol Vapor Sensing

Molecularly Imprinted Polymer Based Antenna Sensor for Methanol Vapor Sensing

Organic vapor sensing behaviors of 4D printed thermoplastic polyamide elastomer by selective laser sintering

Thermoplastic polyamide elastomer (TPAE) films were prepared by selective laser sintering technology and their organic vapor sensing behaviors were investigated. The stable responsive electrical signal was attributed to the conductive pathways evolution formed by polyetheramine soft segments. And the response behaviors of the TPAE films were evaluated using organic vapors of different polarities. Unlike conventional materials that only respond to some specific organic vapors, TPAE showed a high negative vapor coefficient (NVC) effect for all tested vapors. Rapid response, high responsivity and good repeatability were observed in continuous immersion drying run (IDR) experiments at 20 °C. The sensing behaviors during the breathing tests showed a good discrimination for different ethanol vapor concentrations. This study not only provides a guidance for the preparation of organic vapor sensors with fast response, high responsivity and stable repeatability, but also provides a strategy to broaden the application of TPAE into 4D printing field.

Field Deployable Sensor Arrays with Molecular Selectivity for Soil Health

Monitoring soil health is very important in agriculture. Soil health can be monitored by the sensing the metabolomic activities of microbiota as increased concentrations in CO2, CH4, N2O, vapors in the soil. A vapor sensor based on microfabricated cantilever array is an ideal sensor platform for field deployable miniature sensors. Many such platforms, such as cantilever arrays, have been demonstrated as sensitive sensors for chemical and biological detection. In fact, the array-based sensor platform satisfies almost all of the requirements of an ideal high-performance sensor, such as its very high sensitivity, miniature size, array based multi-analyte detection, and low power consumption. However, poor molecular selectivity, especially for small molecules at trace level, as well as the lack of reproducibility, pose formidable challenges in translating this highly sensitive sensor platform into a practical reality. Traditionally, selectivity in chemical and biological sensing is achieved by using immobilized receptors or chemical interfaces on sensor surfaces. While poor selectivity in small molecule detection using reversible receptors is due to the lack of uniqueness of receptor-analyte interaction, nonuniformity in the immobilized receptor graft density is the reason for unacceptable rates of reproducibility in these sensors. We have used a multi-modal, multi-physics approach in overcoming these challenges. Multi-modal data when analyzed using deep learning techniques, show enhanced selectivity, sensitivity, and reliability. In order to operate these sensors in a field-scale study, a long range through-the-soil (TTS) wireless power transfer method is developed. Energy from an adjacent solar panel array is distributed wirelessly via conduction currents that propagate outward along and through the soil. Power transfer within a 0.25-ha (0.6acre) area is demonstrated using an average input power of 6W.This research is supported by NSF-SitS Award Nos. 2226614 and 2226612

Multipurpose fluorescent carbon dots from papaya seed waste as sensing materials for Cu2+ detection and diethyl ether vapor sensor via electronic nose system

Herein, carbon dots (CDs) as biocompatible, fluorescent carbon-based nanomaterials were synthesized from papaya seed waste as renewable carbon sources for the first time via a facile acid pyrolysis method. The papaya seed-derived CDs showed blue fluorescence emission under UV light (365 nm) with a quantum yield of 2.74%, and contained oxygen-, and nitrogen- containing functional groups. Due to their surface functionality, the CDs have a great potential for using as fluorescence sensing probe in metal ion sensing application. The CD solution exhibited the most selective detection to Cu2+ as presented the highest fluorescence quenching with the limit of detection (LOD) of 5.16 μM. The CD-paper-based fluorescent sensor was also developed for practical application, and the RGB value was used to compare the sensitivity of CDs toward metal ions. The CD sensing film was also prepared for diethyl ether vapor sensing via optical electronic nose system. The principal component analysis (PCA) score plots revealed the total variance of 99.3%, indicating that the CDs can be used to discriminate different concentrations of diethyl ether/ethanol vapor mixtures. This work demonstrated that the papaya seed-derived CDs have a great attention to be alternative materials for developing sensing materials in both solution and film forms.

A Halogen-Bonded Organic Framework (XOF) Emissive Cocrystal for Acid Vapor and Explosive Sensing, and Iodine Capture.

There is a strong and urgent need for efficient materials that can capture radioactive iodine atoms from nuclear waste. This work presents a novel strategy to develop porous materials for iodine capture by employing halogen bonding, mechanochemistry and crystal engineering. 3D halogen-bonded organic frameworks (XOFs) with guest-accessible permanent pores are exciting targets in crystal engineering for developing functional materials, and this work reports the first example of such a structure. The new-found XOF, namely TIEPE-DABCO, exhibits enhanced emission in the solid state and turn-off emission sensing of acid vapors and explosives like picric acid in nanomolar quantity. TIEPE-DABCO captures iodine from the gas phase (3.23 g g-1 at 75 °C and 1.40g g-1 at rt), organic solvents (2.1g g-1 ), and aqueous solutions (1.8g g-1 in the pH range of 3-8); the latter with fast kinetics. The captured iodine can be retained for more than 7 days without any leaching, but readily released using methanol, when required. TIEPE-DABCO can be recycled for iodine capture several times without any loss of storage capacity. The results presented in this work demonstrate the potential of mechanochemical cocrystal engineering with halogen bonding as an approach to develop porous materials for iodine capture and sensing.

An Effective Resistive-Type Alcohol Vapor Sensor Using One-Step Facile Nanoporous Anodic Alumina.

With the increases in work environment regulations restricting alcohol to 1000 ppm, and in drink-driving laws, testing for alcohol with a simple method is a crucial issue. Conventional alcohol sensors based on sulfide, metal oxide, boron nitride or graphene oxide have a detection limit in the range of 50-1000 ppm but have disadvantages of complicated manufacture and longer processing times. A recent portable alcohol meter based on semiconductor material using conductivity or chemistry measurements still has the problem of a complex and lengthy manufacturing process. In this paper, a simple and effective resistive-type alcohol vapor sensor using one-step anodic aluminum oxide (AAO) is proposed. The nanoporous AAO was produced in one-step by anodizing low-purity AA1050 at room temperature of 25 °C, which overcame the traditional high-cost and lengthy process at low temperature of anodization and etching from high-purity aluminum. The highly specific surface area of AAO has benefits for good sensing performance, especially as a humidity or alcohol vapor sensor. With the resistance measurement method, alcohol vapor concentration of 0, 100, 300, 500, 700 and 1000 ppm correspond to mean resistances of 8524 Ω, 8672 Ω, 9121 Ω, 9568 Ω, 10,243 Ω, and 11,045 Ω, respectively, in a linear relationship. Compared with other materials for detecting alcohol vapor, the AAO resistive sensor has advantages of fast and simple manufacturing with good detection limits for practical applications. The resistive-type alcohol vapor-sensing mechanism is described with respect to the resistivity of the test substance and the pore morphology of AAO. In a human breath test, the AAO sensor can quickly distinguish whether the subject is drinking, with normal breath response of -30% to -40% and -20% to -30% response after drinking 50 mL of wine of 25% alcohol.

Capacitive polymer sensors: Factors influencing performance and design principles

Capacitive polymer sensors present a promising approach for machine olfaction arrays capable of detecting and classifying various analytes. In this study, a comprehensive analysis of factors influencing capacitive vapor sensors is performed. An in-depth analysis of performance of sensing polymers with a wide range of solubility interactions and dielectric constants is performed providing a foundation for sensor design principles. Unique frequency-dependent signal-to-noise behavior was observed for each polymer-analyte pair due to variations in polarization mechanisms, which was in turn used to tune the array for a better sensitivity. Film thickness and background humidity on array sensitivity was also explored. Design principles based on this study are used to optimize sensor arrays for the classification and quantification of analytes exhibiting distinct solubility interactions (alcohol, ketone, ester, and two hydrocarbons). Electrochemical impedance spectroscopy analysis provides valuable insight into the different physical processes and polarization mechanisms for each sensing polymer. The design principles explored have broad implications for other dielectric-based sensing systems, including integrated circuit-based arrays such as field-effect transistors.

Phosphoric acid-catalyzed synthesis of fused pyridocoumarins and their acidochromic applications

Pyridocoumarins are well known to exhibit various potent applications in optoelectronics, fluorescent probes, and sensors due to their unique photophysical properties. A concise and effective approach has been demonstrated to synthesize pyridocoumarins via a relatively inexpensive phosphoric acid-catalyzed Povarov reaction and their photophysical properties has also been investigated. The structures of all the synthesized compounds were well characterized by using 1H NMR, 13C NMR, IR, HRMS, and single crystal X-ray structure analysis. Detailed spectroscopic studies revealed that all the synthesized pyridocoumarins displayed acidochromic properties in solutions, which could be utilized as acid-base vapor sensors, and anti-counterfeiting agents. The detection limit was also calculated for all the synthesized acid sensors, which were found to be in the range of 41–103.8 ppm (parts per million). DFT and TDDFT studies were also performed to understand the protonation sites and the electronic transitions involved in these compounds. HOMO–LUMO energy gap of the synthesized pyridocoumarins ranges from 3.50 to 3.90 eV.

Exploring a Redox-Active Ionic Porous Organic Polymer in Environmental Remediation and Electrochromic Application.

Here, we report the design and synthesis of a redox-active multifunctional ionic porous organic polymer iPOP-Bpy with exchangeable Br- ions, incorporating viologen as a redox-active building block. The material shows not only excellent iodine uptake capacity in the vapor phase (540 wt %) but also in the organic (1009.77 mg g-1) and aqueous phases (3921.47 mg g-1) with very fast adsorption kinetics in all cases. The material also shows its utility in being used as a solid-state NH3 vapor sensor as it shows very fast color switching in the presence of NH3 vapor. Furthermore, the material found application as a p-type complementary electrochromic electrode and was fabricated into a bilayer device. Excellent coloration efficiency, high switching speed, and good color contrast were obtained as investigated using bias-dependent optical and spectroelectrochemical studies, paving the way for fabricating power-efficient solid-state electrochromic devices.

Enhanced ethanol vapor sensing by 2D CuO-ZnO composite at ambient conditions

Gas sensors find application in the fields of food packaging, medical, agriculture, industries, transport, storage, and warehousing, etc. An efficient gas sensor should be sensitive and selective towards analyte, have fast response and recovery time and operate under moderate conditions. The commercial sensors have an in-built heating element. Thus, there is scope for finding materials that can function as room temperature ethanol sensors. Here, a composite of two-dimensional copper oxide (CuO) with zinc oxide (ZnO) nanoparticles is reported as a potential ethanol (EtOH) vapor sensor under ambient conditions. CuO is synthesized with the help of liquid–liquid phase separation technique yielding architecturally aligned 2D structures. ZnO nanoparticles are synthesized via microwave assisted wet chemical route separately. X-ray diffraction studies show the phase formation of pure CuO and ZnO individually. The CuO and ZnO samples are ground mechanically in 1:1 wt ratio to form a composite referred to as CZC. The optical absorption studies show bandgap of CuO structures as 1.41 eV, ZnO as 3.13 eV, and of CZC as 1.24 eV. A slurry of the composite is then coated on a glass substrate to study the resistance changes after ethanol vapor exposure. It is observed that while pure CuO is almost unresponsive to EtOH vapors, ZnO has sluggish response, the CZC samples responded fast. The best average response time is observed to be ∼ 4 sec while recovery time is ∼ 12 sec. The sensors show good reversibility as they return to their initial state without applying any heat treatment. The change in EtOH sensing in composites is attributed to the formation of heterojunction between CuO-ZnO which facilitates faster electron transfer across the heterojunction and the presence of large surface area renders an increased number of adsorption sites.

Directional Activated Exciton Highway via Fractal Electric Field Modulation for Ultrasensitive Carbon Nanotube-Based Sensors.

The electrical vapor sensor based on carbon nanotubes (CNTs) has attracted wide attention due to its excellent conductivity, stable interfacial structure, and low dimensional quantum effects. However, the conductivity and contact interface activity were still limited by the random distribution of coated CNTs, which led to limited performance. We developed a new strategy to unify the CNT directions with image fractal designing of the electrode system. In such a system, directional aligned CNTs were gained under a well-modulated electric field, leading to microscale CNT exciton highways and molecule-scale host-guest site activation. The carrier mobility of the aligned CNT device is 20-fold higher than that of the random network CNT device. With excellent electrical properties, such modulated CNT devices based on fractal electrodes behave as an ultrasensitive vapor sensor for methylphenethylamine, a mimic of illicit drug methamphetamine. The detection limit reached as low as 0.998 ppq, 6 orders of magnitude sensitive than the reported 5 ppb record based on interdigital electrodes with random distributed CNTs. Since the device is easily fabricated in wafer-level and compatible with the CMOS process, such a fractal design strategy for aligned CNT preparation will be widely applied in a variety of wafer-level electrical functional devices.

Thin film preparation of polyphenol oxidase enzyme (PPO) and investigation of its organic vapor interaction mechanism

This study investigated spin coated thin films of polyphenol oxidase (PPO) enzyme as vapor sensor to detect chloroform, acetone, ethyl acetate, isopropyl alcohol and toluene. Thin film of enzyme was produced onto a bio-composite (gelatine chitosan) first layer via 5000 rpm spin speed. The density and the viscosity of the enzyme were 1.2 g ml−1 and 68 mPa.s respectively. UV–visible spectroscopy and quartz crystal microbalance (QCM) measurements were carried out to analyze the reproducibility of PPO spun film. It was found that the PPO enzyme can be transferred onto a solid substrate as a solid state thin film form. The sensor films of PPO enzyme were exposed to various volatile organic compounds (VOCs) (chloroform, acetone, ethyl acetate, isopropyl alcohol and toluene) with different fixed concentrations. The sensing responses of PPO thin films versus five vapors were investigated using QCM as the time dependence frequency recording method. The PPO sensor films exhibited high sensitivity and fast responses against all VOCs. But the response rate and magnitude were changed depending on the chemical structure and the molecular size of the analyte vapor. Recorded frequency changes as monitored by QCM technique have been integrated with the Fick’s second law of diffusion to determine the diffusion coefficients of analyte vapors. The results showed that the interaction characteristics between PPO and the analytes can be considered in terms of two main processes which are surface interaction and diffusion. And it was concluded that the formation of these two processes during the interaction depend on the molecular size and functional group of the analytes. These results showed that enzymes can be integrated into vapor sensor system as active layer and are promising for further sensor studies.

High sensitivity ethanol vapor sensor based on a nematic liquid crystal film embedded optical fiber Sagnac interferometer

Interferometric optical fiber sensors have become the preferred choice for ethanol vapor detection because of their high sensitivity and figure of merit. However, the response time of interferometric fiber optic ethanol vapor sensors is very long. To address this issue, we experimentally investigated an ethanol vapor sensor based on a nematic liquid crystal (NLC) film embedded optical fiber Sagnac interferometer. The high birefringent NLC film, which worked as the sensing media of ethanol vapor for its absorption of ethanol vapor, was penetrated into the Sagnac ring to generate the spectral interference. The results showed that the measurement sensitivity of ethanol gas concentration reached 2.22 pm ppm−1. The detection range was about 1210 ppm–10 000 ppm. Most importantly, the response time of the proposed sensor is only 15 s. The designed sensor, which showed the advantages of fast response, high sensitivity, and stability, could be a competitive candidate for ethanol vapor sensing.

Leveraging Coupled Solvatofluorochromism and Fluorescence Quenching in Nitrophenyl-Containing Thiazolothiazoles for Efficient Organic Vapor Sensing.

Solvatofluorochromic molecules provide strikingly high fluorescent outputs to monitor a wide range of biological, environmental, or materials-related sensing processes. Here, thiazolo[5,4-d]thiazole (TTz) fluorophores equipped with simple alkylamino and nitrophenyl substituents for solid-state, high-performance chemo-responsive sensing applications are reported. Nitroaromatic substituents are known to strongly quench dye fluorescence, however, the TTz core subtly modulates intramolecular charge transfer (ICT) enabling strong, locally excited-state fluorescence in non-polar conditions. In polar media, a planar ICT excited-state shows near complete quenching, enabling a twisted excited-state emission to be observed. These unique fluorescent properties (spectral shifts of 0.13 - 0.87eV and large transition dipole moments Δµ = 20.4 - 21.3 D) are leveraged to develop highly sought-after chemo-responsive, organic vapor optical sensors. The sensors are developed by embedding the TTz fluorophores within a poly(styrene-isoprene-styrene) block copolymer to form fluorescent dye/polymer composites (ΦF = 70 - 97%). The composites respond reversibly to a comprehensive list of organic solvents and show low vapor concentration sensing (e.g., 0.04% solvent saturation vapor pressure of THF - 66ppm). The composite films can distinguish between solvent vapors with near complete fluorescent quenching observed when exposed to their saturated solvent vapor pressures, making this an extremely promising material for optical chemo-responsive sensing.

Facile controlled synthesis of bifunctional ZnO nanoparticles for application as a high-performance self-powered UV photosensor and highly selective vapor sensor

ZnO has unique multifunctional and morphological properties, and has received significant attention in the field of next-generation sensors and photonic devices. Herein, we report bifunctional ZnO nanoparticles (NPs) used for the selective detection of hazardous gases and self-powered photo response. ZnO nanoparticles with a wurtzite structure and particle size in the range of 10–20 nm were synthesized and fully characterized. ZnO NPs were explored as selective self-powered UV light (370 nm) photosensors and gas sensors. The photoresponse upon illumination with 370-nm light exhibited self-power behavior with an on/off ratio of 1.5x 104, responsivity (R(λ)) of 649 mA/W, external quantum efficiency (EQE) of 217%, and detectivity (D) of 2.4 × 1013 Jones at a bias voltage of 0 V. Maximum sensitivities of 211% and 96% were observed for propanol and chloroform, respectively, among the different gas environments, with quick response, short recovery times, and good repeatability. A possible mechanism has been proposed using the morphology, structural, and electrical characterization results. Thus, our study opens a path for synthesis of multifunctional nanomaterials and their exploration for multiple applications.

Zn-doped SnO2 nanoparticles for ethanol vapor sensor: a combined experimental and first-principles study

Zn-doped SnO2 nanoparticles for ethanol vapor sensor: a combined experimental and first-principles study

ZnGa2O4 and ZnGa2O4:N thin films applied as sensors for detection of acetaldehyde in ethanol

Acetaldehyde, a byproduct of fermentation, is a highly toxic contaminant. Therefore, it is important to develop cost-effective, fast-response and portable acetaldehyde sensors to monitor the formation of this contaminant in the process. Herein, we have evaluated the potential use of ZnGa2O4 and ZnGa2O4:N (referred as ZnGaON) films as vapor sensors of ethanol containing acetaldehyde. Sensors based on ZnGa2O4 films were successfully prepared by PLD on Al2O3 substrates containing interdigitated Pt electrodes. The nitridation was carried out under NH3 gas flow at 700 °C, to obtain ZnGaON films. The prepared sensors presented high sensitivity to acetaldehyde, allowing identification of acetaldehyde in ethanol (300 ppm). This acetaldehyde concentration corresponds to the limit established by Brazialian legislation to sugarcane alcoholic beverages (cachaça). The ZnGaON sensor should be operated at moderate temperature (450 °C), and response time of 43 s and recovery time of 26 s are quite fast. The results strongly suggest that ZnGaON may be considered a potential candidate for the development of an efficient screening test for a highly sensitive and selective identification of acetaldehyde in beverages, foods, or fuel.