Ammonia gas sensors: A comprehensive review.

The strong and controllable chemical sensitivity of organic semiconductors (OSCs) and the amplification capability of transistors in circuits make use of OSC-based field-effect transistors compelling for chemical sensors. Analytes detected and assayed range from few-atom gas-phase molecules that may have adverse health and security implications to biomacromolecules (proteins, nucleic acids) that may be markers for physiological processes and medical conditions. This review highlights recent progress in organic field-effect transistor (OFET) chemical sensors, emphasizing advances from the past 5 years and including aspects of OSC morphology and the role of adjacent dielectrics. Design elements of the OSCs and various formats for the devices are illustrated and evaluated. Challenges associated with the present state of the art and future opportunities are also discussed.

Chemical and Biomolecule Sensing with Organic Field-Effect Transistors

Flexible sensors have the potential to be seamlessly applied to soft and irregularly shaped surfaces such as the human skin or textile fabrics. This benefits conformability dependant applications including smart tattoos, artificial skins and soft robotics. Consequently, materials and structures for innovative flexible sensors, as well as their integration into systems, continue to be in the spotlight of research. This review outlines the current state of flexible sensor technologies and the impact of material developments on this field. Special attention is given to strain, temperature, chemical, light and electropotential sensors, as well as their respective applications.

Flexible Sensors—From Materials to Applications

In this work, we successfully demonstrate a fast method to determine the fish freshness by using a sensing system containing an ultrasensitive amine gas sensor to detect the volatile amine gas from the raw fish meat. When traditional titration method takes 4 h and complicated steps to test the total volatile basic nitrogen (TVB-N) as a worldwide standard for fish freshness, our sensor takes 1 min to deliver an electrical sensing response that is highly correlated with the TVB-N value. When detecting a fresh fish with a TVB-N as 18 mg/100 g, the sensor delivers an effective ammonia concentration as 100 ppb. For TVB-N as 28–35 mg/100 g, a well-accepted freshness limit, the effective ammonia concentration is as 200–300 ppb. The ppb-regime sensitivity of the sensor and the humidity control in the sensing system are the keys to realizing fast and accurate detection. It is expected that the results in this report enable the development of on-site freshness detection and real-time monitoring in a fish factory.

One-Minute Fish Freshness Evaluation by Testing the Volatile Amine Gas with an Ultrasensitive Porous-Electrode-Capped Organic Gas Sensor System

The thin-film structures of chemical sensors based on conventional organic field-effect transistors (OFETs) can limit the sensitivity of the devices toward chemical vapors, because charge carriers in OFETs are usually concentrated within a few molecular layers at the bottom of the organic semiconductor (OSC) film near the dielectric/semiconductor interface. Chemical vapor molecules have to diffuse through the OSC films before they can interact with charge carriers in the OFET conduction channel. It has been demonstrated that OFET ammonia sensors with porous OSC films can be fabricated by a simple vacuum freeze-drying template method. The resulted devices can have ammonia sensitivity not only much higher than the pristine OFETs with thin-film structure but also better than any previously reported OFET sensors, to the best of our knowledge. The porous OFETs show a relative sensitivity as high as 340% ppm−1 upon exposure to 10 parts per billion (ppb) NH3. In addition, the devices also exhibit decent selectivity and stability. This general and simple strategy can be applied to a wide range of OFET chemical sensors to improve the device sensitivity.

Porous Organic Field-Effect Transistors for Enhanced Chemical Sensing Performances

Ammonia (NH 3 ) gas sensors based on organic field-effect transistor (OFET) using poly(3-hexylthiophene) (P3HT) blended with polystyrene (PS) as a semiconductor layer were fabricated. An optimized composition of 1.6 wt% P3HT in PS matrix exhibited the best performance to various concentrations of NH 3 , which is comparable to that of pure P3HT (8 wt%). The results showed the percentage responses of saturation current were 52% and 16% under 50 ppm and 5 ppm NH 3 , respectively. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH 3 gas. By analyzing the morphologies of blend films and the electrical characteristics of OFET sensors, it was found that the film of P3HT blended with PS has more interface to interact with NH 3 , resulting in more efficient detection to NH 3 even in the range of low concentration. Besides, the PS matrix would prevent the gas from diffusing into the semiconductor/dielectric interface directly, which was beneficial to the selectivity of P3HT/PS blend OFET sensor. Moreover, the sensing property was related to the solvents and molecular weight of PS. In addition, the environmental stability of OFET sensors was measured after storing the sensors under ambient atmosphere for 40 days, and the device with the blend semiconducting layer exhibited the superior stability.

Poly(3-hexylthiophene)/polystyrene (P3HT/PS) blends based organic field-effect transistor ammonia gas sensor

Ammonia (NH3) gas sensors based on organic field-effect transistor (OFET) using poly(vinyl alcohol) (PVA) as a gas accumulation layer were fabricated. The results showed the percentage responses of saturation current (Ion) was 50.3% under 0.5 ppm NH3, which is over one order of magnitude higher than that of pure PMMA dielectric. Also, it showed that there was a remarkable shift in the field-effect mobility after exposed to NH3 gas. Moreover, the percentage responses of Ion would increase to 66.9% and 30.2% under 0.5 ppm and 0.2 ppm NH3 after applied a gate voltage pulse, respectively. The sensing properties of these OFET gas sensors can realize absorption and desorption with opposite gate bias, exhibiting non-volatile states of Ion and VT under the programming-erasing bias as used for memory elements. By analyzing the morphologies of dielectric and organic semiconductor, and the electrical characteristics of OFET sensors, the interaction between NH3 and PVA will be strengthened under the influence of the applied electrical field on the hydroxyl dipoles. The bias-induced re-orientation of the hydroxyl dipoles can modulate the influence of NH3 on the trapping (absorption) and detrapping (desorption) processes of charge carriers at the pentacene/PVA interface.

Poly(vinyl alcohol) as a gas accumulation layer for an organic field-effect transistor ammonia sensor

Biocompatible silk fibroin (SF):poly(vinyl alcohol) (PVA) blends were prepared as the dielectric layers of organic field-effect transistors (OFETs). Compared with those with pure SF dielectric layer, an optimal threshold voltage of ~0 V, high on/off ratio of ~104, and enhanced field-effect mobility of 0.22 cm2/Vs of OFETs were obtained by carefully controlling the weight ratio of SF:PVA blends to 7:5. Through the morphology characterization of dielectrics and organic semiconductors by utilizing atom force microscopy and electrical characterization of the devices, the performance improvement of OFETs with SF:PVA hybrid gate dielectric layers were attributed to the smooth and homogeneous morphology of blend dielectrics. Furthermore, due to lower charge carrier trap density, the OFETs based on SF:PVA-blended dielectric exhibited a higher bias stability than those based on pure SF dielectric.

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Biocompatible/Degradable Silk Fibroin:Poly(Vinyl Alcohol)-Blended Dielectric Layer Towards High-Performance Organic Field-Effect Transistor.

To investigate the origins of hydroxyl groups in a polymeric dielectric and its applications in organic field-effect transistors (OFETs), a polar polymer layer was inserted between two polymethyl methacrylate (PMMA) dielectric layers, and its effect on the performance as an organic field-effect transistor (OFET) was studied. The OFETs with a sandwiched dielectric layer of poly(vinyl alcohol) (PVA) or poly(4-vinylphenol) (PVP) containing hydroxyl groups had shown enhanced characteristics compared to those with only PMMA layers. The field-effect mobility had been raised more than 10 times in n-type devices (three times in the p-type one), and the threshold voltage had been lowered almost eight times in p-type devices (two times in the n-type). The on-off ratio of two kinds of devices had been enhanced by almost two orders of magnitude. This was attributed to the orientation of hydroxyl groups from disordered to perpendicular to the substrate under gate-applied voltage bias, and additional charges would be induced by this polarization at the interface between the semiconductor and dielectrics, contributing to the accumulation of charge transfer.

https://www.mdpi.com/1996-1944/9/7/545/pdf

Xin Yang

Papers

Poly(3-hexylthiophene)/polystyrene (P3HT/PS) blends based organic field-effect transistor ammonia gas sensor

Poly(vinyl alcohol) as a gas accumulation layer for an organic field-effect transistor ammonia sensor

Biocompatible/Degradable Silk Fibroin:Poly(Vinyl Alcohol)-Blended Dielectric Layer Towards High-Performance Organic Field-Effect Transistor.

Tailoring the Dielectric Layer Structure for Enhanced Performance of Organic Field-Effect Transistors: The Use of a Sandwiched Polar Dielectric Layer