Chemical sensing using nanostructured polythiophene transistors

The Recent Development in Chemoresistive-Based Heterostructure Gas Sensor Technology, Their Future Opportunities and Challenges: A Review

Atmospheric pollution has become a critical problem for modern society; therefore, the research in this area continually aims to develop a high-performance gas sensor for health care and environmental safety. Researchers have made a significant contribution in this field by developing highly sensitive sensor-based novel selective materials. The aim of this article is to review recent developments and progress in the selective and sensitive detection of environmentally toxic gases. Different classifications of gas sensor devices are discussed based on their structure, the materials used, and their properties. The mechanisms of the sensing devices, identified by measuring the change in physical property using adsorption/desorption processes as well as chemical reactions on the gas-sensitive material surface, are also discussed. Additionally, the article presents a comprehensive review of the different morphologies and dimensions of mixed heterostructure, multilayered heterostructure, composite, core-shell, hollow heterostructure, and decorated heterostructure, which tune the gas-sensing properties towards hazardous gases. The article investigates in detail the growth and interface properties, concentrating on the material configurations that could be employed to prepare nanomaterials for commercial gas-sensing devices.

Recent Advances in Perylene Diimide-Based Active Materials in Electrical Mode Gas Sensing

This review provides an update on advances in the area of electrical mode sensors using organic small molecule n-type semiconductors based on perylene. Among small organic molecules, perylene diimides (PDIs) are an important class of materials due to their outstanding thermal, chemical, electronic, and optical properties, all of which make them promising candidates for a wide range of organic electronic devices including sensors, organic solar cells, organic field-effect transistors, and organic light-emitting diodes. This is mainly due to their electron-withdrawing nature and significant charge transfer properties. Perylene-based sensors of this type show high sensing performance towards various analytes, particularly reducing gases like ammonia and hydrazine, but there are several issues that need to be addressed including the selectivity towards a specific gas, the effect of relative humidity, and operating temperature. In this review, we focus on the strategies and design principles applied to the gas-sensing performance of PDI-based devices, including resistive sensors, amperometric sensors, and operating at room temperature. The device properties and sensing mechanisms for different analytes, focusing on hydrazine and ammonia, are studied in detail, and some future research perspectives are discussed for this promising field. We hope the discussed results and examples inspire new forms of molecular engineering and begin to open opportunities for other rylene diimide classes to be applied as active materials.

Gate-tunable gas sensing behaviors in air-stable ambipolar organic thin-film transistors

Chemiresistive gas sensors, which exploit their electrical resistance in response to changes in nearby gas environments, usually achieve selective gas detection using multi-element sensor arrays. As large numbers of sensors are required, they often suffer from complex and high-cost fabrication. Here, we demonstrate an ambipolar organic thin-film transistor as a potential multi-gas sensing device utilizing gate-tunable gas sensing behaviors. Combining behaviors of both electron and hole carriers in a single device, the proposed device showed dynamic changes depending on gate biases and properties of target gases. As a result, the gas response as a function of gate biases exhibits a unique pattern towards a specific gas as well as its concentrations, which is very different from conventional unipolar organic thin-film transistors. In addition, our device showed an excellent air-stable characteristic compared to typical ambipolar transistors, providing great potential for practical use in the future.

Ambipolar organic field effect transistor shows a great potential to be used for multi-gas sensing device utilizing gate-tunable gas sensing behaviors.

Selective Ammonia-Sensing Platforms Based on a Solution-Processed Film of Poly(3-Hexylthiophene) and p-Doping Tris(Pentafluorophenyl)Borane

Here, we fabricate ammonia sensors based on organic transistors by using poly(3-hexylthiophene) (P3HT) blended with tris(pentafluorophenyl)borane (TPFB) as an active layer. As TPFB is an efficient p-type dopant for P3HT, the current level of the blend films can be easily modulated by controlling the blend ratio. The devices exhibit significantly increased on-state and off-state current levels owing to the ohmic current originated from the large number of charge carriers when the active polymer layer contains TPFB with concentrations up to 20 wt % (P3HT:TPFB = 8:2). The current is decreased at 40 wt % of TPFB (P3HT:TPFB = 6:4). The P3HT:TPFB blend with a weight ratio of 9:1 exhibits the highest sensing performances for various concentrations of ammonia. The device exhibits an increased percentage current response compared to that of a pristine P3HT device. The current response of the P3HT:TPFB (9:1) device at 100 ppm of ammonia is as high as 65.8%, 3.2 times that of the pristine P3HT (20.3%). Furthermore, the sensor based on the blend exhibits a remarkable selectivity to ammonia with respect to acetone, methanol, and dichloromethane, owing to the strong interaction between the Lewis acid (TPFB) and Lewis base (ammonia).

A Review of Chipless Remote Sensing Solutions Based on RFID Technology

Chipless Radio Frequency Identification (RFID) has been used in a variety of remote sensing applications and is currently a hot research topic. To date, there have been a large number of chipless RFID tags developed in both academia and in industry that boast a large variation in design characteristics. This review paper sets out to discuss the various design aspects needed in a chipless RFID sensor. Such aspects include: (1) Addressing strategies to allow for unique identification of the tag, (2) Sensing mechanisms used to allow for impedance-based response signal modulation and (3) Sensing materials to introduce the desired impedance change when under the influence of the target stimulus. From the tabular comparison of the various sensing and addressing techniques, it is concluded that although many sensors provide adequate performance characteristics, more work is needed to ensure that this technology is capable/robust enough to operate in many of the applications it has been earmarked for.

Selective Wet-Etching of Polymer/Fullerene Blend Films for Surface- and Nanoscale Morphology-Controlled Organic Transistors and Sensitivity-Enhanced Gas Sensors

Surface and nanoscale morphology of thin poly(3-hexylthiophene) (P3HT) films are effectively controlled by blending the polymer with a soluble derivative of fullerene, and then selectively dissolving out the fullerene from the blend films. A combination of the polymer blending with fullerene and a use of diiodooctane (DIO) as a processing additive enhances the molecular ordering of P3HT through nanoscale phase separation, compared to the pristine P3HT. In organic thin-film transistors, such morphological changes in the blend induce a positive effect on the field-effect mobility, as the mobility is ~5-7 times higher than in the pristine P3HT. Simple dipping of the blend films in butyl acetate (BA) causes a selective dissolution of the small molecular component, resulting in a rough surface with nanoscale features of P3HT films. Chemical sensors utilizing these morphological features show an enhanced sensitivity in detection of gas-phase ammonia, water, and ethanol.

Nozzle-jet printed flexible field-effect transistor biosensor for high performance glucose detection

Printable electronics is a subject of great interest for low-cost, facile and environmentally-friendly large scale device production. But, it still remains challenging for printable biosensor development. Herein, we present the fabrication of nozzle-jet printed flexible field-effect transistor (FET) glucose biosensor. The silver source-drain electrodes and ZnO seed layers were printed on flexible substrate by nozzle-jet printer followed by ZnO nanorods (ZnO NRs) synthesis and glucose oxidase (GOx) immobilization. Utilization of nozzle-jet printing methods resulted in highly reproducible electrodes with well-defined vertical grown ZnO NRs for high GOx loading and enhanced glucose sensing performance in a wide glucose detection range. The stability, anti-interference ability, reproducibility, reusability, and applicability in human serum samples were also assessed. Overall, biosensor fabrication using nozzle-jet printer will not only provide large scale production of highly reproducible electrodes but also reduce the fabrication cost. Additionally, printed electrodes can be modified accordingly for different analyte detection.

Electrical gas sensors based on structured organic ultra-thin films and nanocrystals on solid state substrates

Gas sensors, as useful tools to detect specific gas species such as toxic and explosive gases or volatile organic compounds, are the key components for environmental monitoring, fruit maturity and food safety monitoring, health care, and so on. The present commercial products based on porous metal oxide materials still face problems, such as high temperature operation and low level of selectivity. Thin films or nanostructures of organic materials with thickness or grain size down to nanometer scale represent promising candidates for gas sensing due to their potential for achieving high selectivity, portability and low cost. However, there are still challenges related to their stability, reproducibility and response/recovery speed despite the efforts in materials design, morphology control or device configuration, all of which have been expended during the last few decades. In this review, we summarize the progress of recent research on gas sensors based on organic ultra-thin films and nanostructures. We specifically discuss the effect of microstructure in the active layer on the sensing performance and mechanism.

Generation of highly ordered nanoporous Sb–SnO2 thin films with enhanced ethanol sensing performance at low temperature

Highly ordered nanoporous Sb–SnO₂ sensing films synthesized through psHT treatment present high sensitivity to 50 ppm ethanol at low temperature.

Highly sensitive and selective liquid-phase sensors based on a solvent-resistant organic-transistor platform

Liquid-phase sensing of various organic solvents is performed for the first time, using a solvent-resistant organic-transistor platform. Sensors composed of a cross-linked poly(3-hexylthiophene) (P3HT)-azide co-polymer and a calixarene derivative exhibit highly sensitive and selective sensing behavior, owing to the selective binding effects of the liquid analytes with the calixarene-functionalized P3HT-azide, extending the range of their use in practical applications.

Organic field-effect transistor-based gas sensors

This tutorial review reports the recent progress on OFET gas sensors, including their working principle, and protocols for high-performance sensing.

Electrochemical synthesis of poly(3-thiophene acetic acid) nanowires with water-soluble macromolecule templates

The interactions between template molecules and monomers play a key role in the formation of PTAA nanowires.

Field effect transistors based on polycyclic aromatic hydrocarbons for the detection and classification of volatile organic compounds

We show that polycyclic aromatic hydrocarbon (PAH) based field effect transistor (FET) arrays can serve as excellent chemical sensors for the detection of volatile organic compounds (VOCs) under confounding humidity conditions. Using these sensors, w/o complementary pattern recognition methods, we study the ability of PAH-FET(s) to: (i) discriminate between aromatic and non-aromatic VOCs; (ii) distinguish polar and non-polar non-aromatic compounds; and to (iii) identify specific VOCs within the subgroups (i.e., aromatic compounds, polar non-aromatic compounds, non-polar non-aromatic compounds). We further study the effect of water vapor on the sensor array's discriminative ability and derive patterns that are stable when exposed to different constant values of background humidity. Patterns based on different independent electronic features from an array of PAH-FETs may bring us one step closer to creating a unique fingerprint for individual VOCs in real-world applications in atmospheres with varying levels of humidity.

Prospective of Conducting Polymer Nanowire for Gas Sensing Application to its Physical Scaling

The effect of physical scaling on one dimensional (1-D) conducting polypyrrole (Ppy) nanowire device has been successfully studied. The synthesis, electrical characterization and ammonia gas sensing with 1-D Ppy nanowire device have been carried out in the present investigation. Ppy nanowires having ~80 to ~200 nm in diameter were synthesized by electrochemical polymerization in alumina template and 1.77 to 3 µm Ppy nanowire length were maintain by varying the distance between electrodes gap. We further demonstrated that gas sensors based on 1-D Ppy nanowire having high aspect ratio (length to diameter ratio, L:D) exhibits good sensitivity towards ammonia, and provided a reliable detection at concentration as low as approximately 1 ppm based on principal of physical scaling co-related to response resistance.

Polycyclic aromatic hydrocarbon for the detection of nonpolar analytes under counteracting humidity conditions

Real-world samples contain reducing and oxidizing chemical agents as well as large and small (bio)molecules, which are polar or nonpolar in nature. Sensing nonpolar analytes, which is of paramount importance for a wide variety of applications, is generally more difficult to achieve than sensing polar analytes. Here, we report on empirical observations of a unique polycyclic aromatic hydrocarbon derivative, referred as PAH-A, whose structure has a triangular-shaped aromatic core (with a carbon number of 60) and contains hydrophobic mesogens terminated with hydrophobic alkyl chains. We show that films made of PAH-A enable excellent sensitivity to nonpolar analytes, compared to polar analytes, in a setting of 5-40% counteracting relative humidity. This finding is based on monitoring the changes in the physical/optical properties of thin PAH-A films upon exposure to nonpolar and polar analytes, by means of quartz crystal microbalance and spectroscopic ellipsometry measurements. A comparison with other polycyclic aromatic hydrocarbon derivatives with different cores or organic functionalities is provided and discussed.

Heavy metallic oxide nanoparticles for enhanced sensitivity in semiconducting polymer x-ray detectors

Semiconducting polymers have previously been used as the transduction material in x-ray dosimeters, but these devices have a rather low detection sensitivity because of the low x-ray attenuation efficiency of the organic active layer. Here, we demonstrate a way to overcome this limitation through the introduction of high density nanoparticles having a high atomic number (Z) to increase the x-ray attenuation. Specifically, bismuth oxide (Bi(2)O(3)) nanoparticles (Z = 83 for Bi) are added to a poly(triarylamine) (PTAA) semiconducting polymer in the active layer of an x-ray detector. Scanning electron microscopy (SEM) reveals that the Bi(2)O(3) nanoparticles are reasonably distributed in the PTAA active layer. The reverse bias dc current-voltage characteristics for PTAA-Bi(2)O(3) diodes (with indium tin oxide (ITO) and Al contacts) have similar leakage currents to ITO/PTAA/Al diodes. Upon irradiation with 17.5 keV x-ray beams, a PTAA device containing 60 wt% Bi(2)O(3) nanoparticles demonstrates a sensitivity increase of approximately 2.5 times compared to the plain PTAA sensor. These results indicate that the addition of high-Z nanoparticles improves the performance of the dosimeters by increasing the x-ray stopping power of the active volume of the diode. Because the Bi(2)O(3) has a high density, it can be used very efficiently, achieving a high weight fraction with a low volume fraction of nanoparticles. The mechanical flexibility of the polymer is not sacrificed when the inorganic nanoparticles are incorporated.

Low temperature crystallization of transparent, highly ordered nanoporous SnO₂ thin films: application to room-temperature hydrogen sensing

High surface area highly ordered nanoporous thin films are the current gold standard for gas sensor use, however the nanostructure of such films is prone to collapse at annealing temperatures as low as 250 °C resulting in formation of a dense layer of limited utility. We report on a templating method used to deposit highly ordered nanoporous platinum (Pt)-doped tin dioxide (SnO(2)) thin films that are crystallized by a 100 °C water vapor hydrothermal treatment, with the low temperature process being compatible with a large variety of substrates including plastic. The resulting highly ordered nanoporous, transparent Pt-SnO(2) thin films are mechanically stable and can be annealed, as desired, at temperatures up to 800 °C for removal of the templating materials and tailoring of gas sensitivities without damage to the nanoporous structure. The synthesis method is general, offering a promising strategy for preparing high performance nanoporous metal oxide crystalline films for applications including gas sensing, photocatalysis, and 3(rd) generation photovoltaics. In our example application of the synthesized materials, we find that these Pt-SnO(2) films exhibit exceptional hydrogen gas sensing behavior, rapidly detecting low-level hydrogen concentrations at room temperature; for example, an eight order of magnitude change in electrical resistance is seen in response to 10 000 ppm H(2), with only minimal sensitivity to humidity.