Ethylene Detection Based on Organic Field-Effect Transistors With Porogen and Palladium Particle Receptor Enhancements

A flexible and wearable paper-based chemiresistive sensor modified with SWCNTs-PdNPs-polystyrene microspheres composite for the sensitive detection of ethylene gas: A new method for the determination of fruit ripeness and corruption

This study developed a flexible and wearable paper-based chemoresistive sensor (FWPCS) by modifying a SWCNT-PdNP-polystyrene microsphere (SPPM) composite (SPPM/FWPCS) for the low-cost and online determination of fruit ripeness and corruption. A new method for the batch and low-cost fabrication of SPPM/FWPCSs based on laser direct writing was proposed. The sensing mechanism of FWPCS relies on the electron depletion layer in the sensing composite created by the Schottky barriers among SWCNTs, PdNPs, and the adsorbed oxygen, along with the construction of O₂^-. When the SPPM sensing film is exposed to ethylene, trapped electrons are released into the conduction band through oxidation and cleavage of ethylene, causing a decrease in resistance. The properties and morphology of the synthesized SPPM composite were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Additionally, the key parameters for the fabrication of SPPMs/FWPCS related to the sensing performance were optimized. The concentration of C₂H₄ can be detected down to 100 ppb using the SPPMs/FWPCS at 25 °C. Finally, the real-time determination of banana ripeness and corruption verified the feasibility of the sensor, indicating that the SPPMs/FWPCS has prospects in monitoring fruit ripeness and corruption during storage and transportation.

Recent Advances in Ethylene Gas Detection

The real-time detecting and monitoring of ethylene gas molecules could benefit the agricultural, horticultural and healthcare industries. In this regard, we comprehensively review the current state-of-the-art ethylene gas sensors and detecting technologies, covering from preconcentrator-equipped gas chromatographic systems, Fourier transform infrared technology, photonic crystal fiber-enhanced Raman spectroscopy, surface acoustic wave and photoacoustic sensors, printable optically colorimetric sensor arrays to a wide range of nanostructured chemiresistive gas sensors (including the potentiometric and amperometric-type FET-, CNT- and metal oxide-based sensors). The nanofabrication approaches, working conditions and sensing performance of these sensors/technologies are carefully discussed, and a possible roadmap for the development of ethylene detection in the near future is proposed.

Organic Thin-Film Transistors as Gas Sensors: A Review

Organic thin-film transistors (OTFTs) are miniaturized devices based upon the electronic responses of organic semiconductors. In comparison to their conventional inorganic counterparts, organic semiconductors are cheaper, can undergo reversible doping processes and may have electronic properties chiefly modulated by molecular engineering approaches. More recently, OTFTs have been designed as gas sensor devices, displaying remarkable performance for the detection of important target analytes, such as ammonia, nitrogen dioxide, hydrogen sulfide and volatile organic compounds (VOCs). The present manuscript provides a comprehensive review on the working principle of OTFTs for gas sensing, with concise descriptions of devices’ architectures and parameter extraction based upon a constant charge carrier mobility model. Then, it moves on with methods of device fabrication and physicochemical descriptions of the main organic semiconductors recently applied to gas sensors (i.e., since 2015 but emphasizing even more recent results). Finally, it describes the achievements of OTFTs in the detection of important gas pollutants alongside an outlook toward the future of this exciting technology.

Cobweb-like, Ultrathin Porous Polymer Films for Ultrasensitive NO2 Detection

Gas sensors based on polymer field-effect transistors (FETs) have drawn much attention owing to the inherent merits of specific selectivity, low cost, and room temperature operation. Ultrathin (<10 nm) and porous polymer semiconductor films offer a golden opportunity for achieving high-performance gas sensors. However, wafer-scale fabrication of such high-quality polymer films is of great challenge and has rarely been realized before. Herein, the first demonstration of 4 in. wafer-scale, cobweb-like, and ultrathin porous polymer films is reported via a one-step phase-inversion process. This approach is extremely simple and universal for constructing various ultrathin porous polymer semiconductor films. Thanks to the abundant pores, ultrathin size, and high charge-transfer efficiency of the prepared polymer films, our gas sensors exhibit many superior advantages, including ultrahigh response (2.46 × 10⁶%), low limit of detection (LOD) (<1 ppm), and excellent selectivity. Thus, the proposed fabrication strategy is exceptionally promising for mass manufacturing of low-cost high-performance polymer FET-based gas sensors.

Cascade Reaction-Based Chemiresistive Array for Ethylene Sensing

Chemiresistive sensors, which are based on semiconducting materials, offer real-time monitoring of environment. However, detection of nonpolar chemical substances is often challenging because of the weakness of the doping effect. Herein, we report a concept of combining a cascade reaction (CR) and a chemiresistive sensor array for sensitive and selective detection of a target analyte (herein, ethylene in air). Ethylene was converted to acetaldehyde through a Pd-catalyzed heterogeneous Wacker reaction at 40 °C, followed by condensation with hydroxylamine hydrochloride to emit HCl vapor. HCl works as a strong dopant for single-walled carbon nanotubes (SWCNTs), enabling the main sensor to detect ethylene with excellent sensitivity (10.9% ppm^-1) and limit of detection (0.2 ppm) in 5 min. False responses that occur in the main sensor are easily discriminated by reference sensors that partially employ CR. Moreover, though the sensor monitors the variation of normalized electric resistance (ΔR/R₀) in the SWCNT network, temporary deactivation of CR yields a sensor system that does not require analyte-free air for a baseline correction (i.e., estimation of R₀) and recovery of response. The concept presented here is generally applicable and offers a solution for several issues that are inherently present in chemiresistive sensing systems.

Realization of an Ultrasensitive and Highly Selective OFET NO2 Sensor: The Synergistic Combination of PDVT-10 Polymer and Porphyrin-MOF

Organic field-effect transistors (OFETs) are emerging as competitive candidates for gas sensing applications due to the ease of their fabrication process combined with the ability to readily fine-tune the properties of organic semiconductors. Nevertheless, some key challenges remain to be addressed, such as material degradation, low sensitivity, and poor selectivity toward toxic gases. Appropriately, a heterojunction combination of different sensing layers with multifunctional capabilities offers great potential to overcome these problems. Here, a novel and highly sensitive receptor layer is proposed encompassing a porous 3D metal-organic framework (MOF) based on isostructural-fluorinated MOFs acting as an NO₂ specific preconcentrator, on the surface of a stable and ultrathin PDVT-10 organic semiconductor on an OFET platform. Here, with this proposed combination we have unveiled an unprecedented 700% increase in sensitivity toward NO₂ analyte in contrast to the pristine PDVT-10. The resultant combination for this OFET device exhibits a remarkable lowest detection limit of 8.25 ppb, a sensitivity of 680 nA/ppb, and good stability over a period of 6 months under normal laboratory conditions. Further, a negligible response (4.232 nA/%RH) toward humidity in the range of 5%-90% relative humidity was demonstrated using this combination. Markedly, the obtained results support the use of the proposed novel strategy to achieve an excellent sensing performance with an OFET platform.

Gas Sensors Based on Nano/Microstructured Organic Field-Effect Transistors

Benefiting from the advantages of organic field-effect transistors (OFETs), including synthetic versatility of organic molecular design and environmental sensitivity, gas sensors based on OFETs have drawn much attention in recent years. Potential applications focus on the detection of specific gas species such as explosive, toxic gases, or volatile organic compounds (VOCs) that play vital roles in environmental monitoring, industrial manufacturing, smart health care, food security, and national defense. To achieve high sensitivity, selectivity, and ambient stability with rapid response and recovery speed, the regulation and adjustment of the nano/microstructure of the organic semiconductor (OSC) layer has proven to be an effective strategy. Here, the progress of OFET gas sensors with nano/microstructure is selectively presented. Devices based on OSC films one dimensional (1D) single crystal nanowires, nanorods, and nanofibers are introduced. Then, devices based on two dimensional (2D) and ultrathin OSC films, fabricated by methods such as thermal evaporation, dip-coating, spin-coating, and solution-shearing methods are presented, followed by an introduction of porous OFET sensors. Additionally, the applications of nanostructured receptors in OFET sensors are given. Finally, an outlook in view of the current research state is presented and eight further challenges for gas sensors based on OFETs are suggested.

Improved Room Temperature NO2 Sensing Performance of Organic Field-Effect Transistor by Directly Blending a Hole-Transporting/Electron-Blocking Polymer into the Active Layer

Over the past decades, organic field-effect transistor (OFET) gas sensors have maintained a rapid development. However, the majority of OFET gas sensors show insufficient detection capability towards oxidizing gases such as nitrogen oxide, compared with the inorganic counterpart. In this paper, a new strategy of OFET nitrogen dioxide (NO₂) gas sensor, consisting of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly(9-vinylcarbazole) (PVK) blend, is reported. Depending on the gate voltage, this sensor can operate in two modes at room temperature. Of the two modes exposed to NO₂ for 5 min, when the gate voltage is 0 V, the highest NO₂ responsivity of this OFET is >20 000% for 30 ppm (≈700% for 600 ppb) with the 1:1 P3HT/PVK blend, it is ≈40 times greater than that with the pure P3HT. The limit of detection of ≈300 ppb is achieved, and there is still room for improvement. While in the condition of -40 V, the response increases by 15 times than that with the pure P3HT. This is the first attempt to improve the OFET sensing performance using PVK, which usually functions as a hole-transport layer in the light- emitting device. The enhancement of sensing performance is attributed to the aggregation-controlling and hole-transporting/electron-blocking effect of PVK. This work demonstrates that the hole-transport material can be applied to improve the NO₂ sensor with simple solution process, which expands the material choice of OFET gas sensors.

Design of Novel Ceramic Preconcentrator and Integration in Gas Chromatographic System for Detection of Ethylene Gas from Ripening Bananas

In this paper, a novel ceramic preconcentrator is manufactured using aluminum nitride (ALN) ceramics. The preconcentrator consists of a heater, a preconcentrator body, a gas inlet and a gas outlet. The adsorption material, Carbosieve SII, is loaded into the preconcentrator. The preconcentrator is integrated with a previously developed micro gas chromatographic system filled with ethylene. When operated, adequate ethylene gas is adsorbed into the preconcentrator. The application of heat pulse also successfully desorbs the ethylene gas. Tests are conducted with ethylene gas at concentrations of 10 ppm, 5 ppm and 2.5 ppm and 400 ppb, respectively. The system is also tested with ethylene gas from ripening bananas over a period of three days. No interference signal is observed in the chromatogram because of other ripening gases (e.g., carbon dioxide, oxygen, alcohol) and humidity. A detection limit of 25 ppb is realized with this system. The developed preconcentrator has several applications, e.g., in food industry and environmental monitoring.

A Gas Chromatographic System for the Detection of Ethylene Gas Using Ambient Air as a Carrier Gas

Ethylene gas is a naturally occurring gas that has an influence on the shelf life of fruit during their transportation in cargo ships. An unintentional exposure of ethylene gas during transportation results in a loss of fruit. A gas chromatographic system is presented here for the detection of ethylene gas. The gas chromatographic system was assembled using a preconcentrator, a printed 3D printed gas chromatographic column, a humidity sensor, solenoid valves, and an electrochemical ethylene gas sensor. Ambient air was used as a carrier gas in the gas chromatographic system. The flow rate was fixed to 10 sccm. It was generated through a mini-pump connected in series with a mass flow controller. The metal oxide gas sensor is discussed with its limitation in ambient air. The results show the chromatogram obtained from metal oxide gas sensor has low stability, drifts, and has uncertain peaks, while the chromatogram from the electrochemical sensor is stable and precise. Furthermore, ethylene gas measurements at higher ppb concentration and at lower ppb concentration were demonstrated with the electrochemical ethylene gas sensor. The system separates ethylene gas and humidity. The chromatograms obtained from the system are stable, and the results are 1.2% repeatable in five similar measurements. The statistical calculation of the gas chromatographic system shows that a concentration of 2.3 ppb of ethylene gas can be detected through this system.

Sensitive and Selective NO2 Sensing Based on Alkyl- and Alkylthio-Thiophene Polymer Conductance and Conductance Ratio Changes from Differential Chemical Doping

NO₂-responsive polymer-based organic field-effect transistors (OFETs) are described, and room-temperature detection with high sensitivity entirely from the semiconductor was achieved. Two thiophene polymers, poly(bisdodecylquaterthiophene) and poly(bisdodecylthioquaterthiophene) (PQT12 and PQTS12, respectively), were used as active layers to detect a concentration at least as low as 1 ppm of NO₂. The proportional on-current change of OFETs using these polymers reached over 400% for PQTS12, which is among the highest sensitivities reported for a NO₂-responsive device based on an organic semiconducting film. From measurements of cyclic voltammetry and the electronic characteristics, we found that the introduction of sulfurs into the side chains induces traps in films of the PQTS12 and also decreases domain sizes, both of which could contribute to the higher sensitivity of PQTS12 to NO₂ gas compared with PQT12. The ratio of responses of PQTS12 and PQT12 is higher for exposures to lower concentrations, making this parameter a means of distinguishing responses to low concentrations for extended times from exposures to high concentrations from shorter times. The responses to nonoxidizing vapors were much lower, indicating good selectivity to NO₂ of two polymers. This work demonstrates the capability of increasing selectivity and calibration of OFET sensors by modulating redox and aggregation properties of polymer semiconductors.