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

Abstract

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 (NO 2 ) 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 NO 2 . The PEI provides doping stability, enhanced electrical conductivity, and flexibility. As a result, the NO 2 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 NO 2 sensors. Our study contributes to the rational design of high-performance flexible gas sensors.

Side-Chain-Assisted Transition of Conjugated Polymers from a Semiconductor to Conductor and Comparison of Their NO2 Sensing Characteristics

“…From the signal calibration with the signal-to-noise ratio and RMS noise, the slope extracted from the linear curve fitting using the obtained I DS (t)/I DS (0) value of each NO 2 concentration marked a theoretical LOD of 1.59 ppb ( Figure 7 b). The extracted LOD was comparable to that of the most sensitive electrical NO 2 sensor that adopted crystalline CPs with good electrical properties [ 31 ].…”

“…CP-based electronic sensors have two different device configurations: chemiresistive- and FET-type sensors. Chemiresistive sensors utilize a change in resistance to detect the target gas; therefore, CP as a detection platform should have electrical conductor-like properties [ 31 , 32 , 33 ]. FET-based sensors, on the other hand, adopt semiconducting CPs to monitor current changes due to charge carrier mobility and/or concentration, which enable the delivery of multiparameter response characteristics, such as charge–carrier mobility, threshold voltage, on/off current, and conductivity [ 34 , 35 , 36 ].…”

State of the Art of Chemosensors in a Biomedical Context

“…Still, commonly used polymer materials are not strictly specific against a single analyte, but rather show variable sensitivity against various volatile compounds. Various conductive polymers are known for their affinity to a variety of these compounds, e.g., polythiopenes (e.g., P3HT) for ammonia and amines [136], polyanilines for hydrogen sulfide [137] (see Figure 7), xylenes [138], or NO 2 [139], polypyrrole for ammonia and indoles [140], poly(vinyl alcohol) for hazardous gases [141], poly(methyl methacrylate) for ethyl acetate and toluene [142]. Furthermore, a wide range of composites for conductive polymers (e.g., graphene composites) and nanowire application of CPs have been demonstrated as suitable approaches for gas sensing [143,144].…”

Solution-Processed Functionalized Graphene Film Prepared by Vacuum Filtration for Flexible NO2 Sensors

“…Real-time monitoring and analysis of NO 2 gas have proven difficult and constitute a challenging task. As per literature reports, chemiresistive gas sensors have clear attractive advantages over other families of gas sensors because they are easier to integrate with conventional microelectronics technology for implementation into portable devices [ 18 ]. Semiconducting metal oxides are the most investigated sensing materials for resistive sensors.…”

“…Moreover, the functionalization not only improves sensitivity but also may enhance selectivity toward specific gases even at room temperature. A wide variety of functional groups have been used, including polymers [ 18 , 19 , 20 ], metal oxides [ 21 , 22 ], metal nanoparticles [ 23 ], and small organic molecules [ 24 , 25 ]. Metalloporphyrins (MPor) and metallophthalocyanines (MPhc), as functional molecules, have attracted considerable interest due to their planar structure with metal atoms (Co, Fe, Zn, Ni, etc.)…”