Nanomaterials for IoT Sensing Platforms and Point-of-Care Applications in South Korea.

Herein, state-of-the-art research advances in South Korea regarding the development of chemical sensing materials and fully integrated Internet of Things (IoT) sensing platforms were comprehensively reviewed for verifying the applicability of such sensing systems in point-of-care testing (POCT). Various organic/inorganic nanomaterials were synthesized and characterized to understand their fundamental chemical sensing mechanisms upon exposure to target analytes. Moreover, the applicability of nanomaterials integrated with IoT-based signal transducers for the real-time and on-site analysis of chemical species was verified. In this review, we focused on the development of noble nanostructures and signal transduction techniques for use in IoT sensing platforms, and based on their applications, such systems were classified into gas sensors, ion sensors, and biosensors. A future perspective for the development of chemical sensors was discussed for application to next-generation POCT systems that facilitate rapid and multiplexed screening of various analytes.

Sub-Parts-per-Million Hydrogen Sulfide Colorimetric Sensor: Lead Acetate Anchored Nanofibers toward Halitosis Diagnosis.

Lead(II) acetate [Pb(Ac)2] reacts with hydrogen sulfide to form colored brownish precipitates of lead sulfide. Thus far, in order to detect leakage of H2S gas in industrial sectors, Pb(Ac)2 has been used as an indicator in the form of test papers with a detection limit only as low as 5 ppm. Diagnosis of halitosis by exhaled breath needs sensors able to detect down to 1 ppm of H2S gas. In this work, high surface area and porous Pb(Ac)2 anchored nanofibers (NFs) that overcome limitations of the conventional Pb(Ac)2-based H2S sensor are successfully achieved. First, lead(II) acetate, which melts at 75 °C, and polyacrylonitrile (PAN) polymer are mixed and stirred in dimethylformamide (DMF) solvent at 85 °C, enabling uniform dispersion of fine liquid droplets in the electrospinning solution. During the subsequent electrospinning, Pb(Ac)2 anchored NFs are obtained, providing an ideal nanostructure with high thermal stability against particle aggregation, numerous reactions sites, and enhanced diffusion of H2S into the three-dimensional (3D)-networked NF web. This newly obtained sensing material can detect down to 400 ppb of H2S at a relative humidity of 90%, exhibiting high potential feasibility as a high-performance colorimetric sensor platform for diagnosis of halitosis.

Nitrogen-Doped Single Graphene Fiber with Platinum Water Dissociation Catalyst for Wearable Humidity Sensor.

Humidity sensors are essential components in wearable electronics for monitoring of environmental condition and physical state. In this work, a unique humidity sensing layer composed of nitrogen-doped reduced graphene oxide (nRGO) fiber on colorless polyimide film is proposed. Ultralong graphene oxide (GO) fibers are synthesized by solution assembly of large GO sheets assisted by lyotropic liquid crystal behavior. Chemical modification by nitrogen-doping is carried out under thermal annealing in H2 (4%)/N2 (96%) ambient to obtain highly conductive nRGO fiber. Very small (≈2 nm) Pt nanoparticles are tightly anchored on the surface of the nRGO fiber as water dissociation catalysts by an optical sintering process. As a result, nRGO fiber can effectively detect wide humidity levels in the range of 6.1-66.4% relative humidity (RH). Furthermore, a 1.36-fold higher sensitivity (4.51%) at 66.4% RH is achieved using a Pt functionalized nRGO fiber (i.e., Pt-nRGO fiber) compared with the sensitivity (3.53% at 66.4% RH) of pure nRGO fiber. Real-time and portable humidity sensing characteristics are successfully demonstrated toward exhaled breath using Pt-nRGO fiber integrated on a portable sensing module. The Pt-nRGO fiber with high sensitivity and wide range of humidity detection levels offers a new sensing platform for wearable humidity sensors.

Innovative Nanosensor for Disease Diagnosis.

As a futuristic diagnosis platform, breath analysis is gaining much attention because it is a noninvasive, simple, and low cost diagnostic method. Very promising clinical applications have been demonstrated for diagnostic purposes by correlation analysis between exhaled breath components and specific diseases. In addition, diverse breath molecules, which serve as biomarkers for specific diseases, are precisely identified by statistical pattern recognition studies. To further improve the accuracy of breath analysis as a diagnostic tool, breath sampling, biomarker sensing, and data analysis should be optimized. In particular, development of high performance breath sensors, which can detect biomarkers at the ppb-level in exhaled breath, is one of the most critical challenges. Due to the presence of numerous interfering gas species in exhaled breath, selective detection of specific biomarkers is also important. This Account focuses on chemiresistive type breath sensors with exceptionally high sensitivity and selectivity that were developed by combining hollow protein templated nanocatalysts with electrospun metal oxide nanostructures. Nanostructures with high surface areas are advantageous in achieving high sensitivity because the sensing signal is dominated by the surface reaction between the sensing layers and the target biomarkers. Furthermore, macroscale pores between one-dimensional (1D) nanostructures can facilitate fast gas diffusion into the sensing layers. To further enhance the selectivity, catalytic functionalization of the 1D metal oxide nanostructure is essential. However, the majority of conventional techniques for catalytic functionalization have failed to achieve a high degree of dispersion of nanoscale catalysts due to aggregation on the surface of the metal oxide, which severely deteriorates the sensing properties by lowering catalytic activity. This issue has led to extensive studies on monolithically dispersed nanoscale particles on metal oxides to maximize the catalytic performances. As a pioneering technique, a bioinspired templating route using apoferritin, that is, a hollow protein cage, has been proposed to obtain nanoscale (∼2 nm) catalyst particles with high dispersity. Nanocatalysts encapsulated by a protein shell were first used in chemiresistive type breath sensors for catalyst functionalization on 1D metal oxide structures. We discuss the robustness and versatility of the apoferrtin templating route for creating highly dispersive catalytic NPs including single components (Au, Pt, Pd, Rh, Ag, Ru, Cu, and La) and bimetallic catalysts (PtY and PtCo), as well as the core-shell structure of Au-Pd (Au-core@Pd-shell). The use of these catalysts is essential to establish high performance sensors arrays for the pattern recognition of biomarkers. In addition, novel multicomponent catalysts provide unprecedented sensitivity and selectivity. With this in mind, we discuss diverse synthetic routes for nanocatalysts using apoferritin and the formation of various catalyst-1D metal oxide composite nanostructures. Furthermore, we discuss detection capability of a simulated biomarker gas using the breath sensor arrays and principal component analysis. Finally, future prospects with the portable breath analysis platform are presented by demonstrating the potential feasibility of real-time and on-site breath analysis using chemiresistive sensors.

Applications and Advances in Bioelectronic Noses for Odour Sensing.

A bioelectronic nose, an intelligent chemical sensor array system coupled with bio-receptors to identify gases and vapours, resembles mammalian olfaction by which many vertebrates can sniff out volatile organic compounds (VOCs) sensitively and specifically even at very low concentrations. Olfaction is undertaken by the olfactory system, which detects odorants that are inhaled through the nose where they come into contact with the olfactory epithelium containing olfactory receptors (ORs). Because of its ability to mimic biological olfaction, a bio-inspired electronic nose has been used to detect a variety of important compounds in complex environments. Recently, biosensor systems have been introduced that combine nanoelectronic technology and olfactory receptors themselves as a source of capturing elements for biosensing. In this article, we will present the latest advances in bioelectronic nose technology mimicking the olfactory system, including biological recognition elements, emerging detection systems, production and immobilization of sensing elements on sensor surface, and applications of bioelectronic noses. Furthermore, current research trends and future challenges in this field will be discussed.

Metal Organic Framework-Templated Chemiresistor: Sensing Type Transition from P-to-N Using Hollow Metal Oxide Polyhedron via Galvanic Replacement.

Facile synthesis of porous nanobuilding blocks with high surface area and uniform catalyst functionalization has always been regarded as an essential requirement for the development of highly sensitive and selective chemical sensors. Metal-organic frameworks (MOFs) are considered as one of the most ideal templates due to their ability to encapsulate ultrasmall catalytic nanoparticles (NPs) in microporous MOF structures in addition to easy removal of the sacrificial MOF scaffold by calcination. Here, we introduce a MOFs derived n-type SnO2 (n-SnO2) sensing layer with hollow polyhedron structures, obtained from p-n transition of MOF-templated p-type Co3O4 (p-Co3O4) hollow cubes during galvanic replacement reaction (GRR). In addition, the Pd NPs encapsulated in MOF and residual Co3O4 clusters partially remained after GRR led to uniform functionalization of efficient cocatalysts (PdO NPs and p-Co3O4 islands) on the porous and hollow polyhedron SnO2 structures. Due to high gas accessibility through the meso- and macrosized pores in MOF-templated oxides and effective modulation of electron depletion layer assisted by the creation of numerous p-n junctions, the GRR-treated SnO2 structures exhibited 21.9-fold higher acetone response (Rair/Rgas = 22.8 @ 5 ppm acetone, 90%RH) compared to MOF-templated p-Co3O4 hollow structures. To the best of our knowledge, the selectivity and response amplitudes reported here for the detection of acetone are superior to those MOF derived metal oxide sensing layers reported so far. Our results demonstrate that highly active MOF-derived sensing layers can be achieved via p-n semiconducting phase transition, driven by a simple and versatile GRR process combined with MOF templating route.

Heterogeneous Sensitization of Metal-Organic Framework Driven Metal@Metal Oxide Complex Catalysts on an Oxide Nanofiber Scaffold Toward Superior Gas Sensors.

We report on the heterogeneous sensitization of metal-organic framework (MOF)-driven metal-embedded metal oxide (M@MO) complex catalysts onto semiconductor metal oxide (SMO) nanofibers (NFs) via electrospinning for markedly enhanced chemical gas sensing. ZIF-8-derived Pd-loaded ZnO nanocubes (Pd@ZnO) were sensitized on both the interior and the exterior of WO3 NFs, resulting in the formation of multiheterojunction Pd-ZnO and ZnO-WO3 interfaces. The Pd@ZnO loaded WO3 NFs were found to exhibit unparalleled toluene sensitivity (Rair/Rgas = 4.37 to 100 ppb), fast gas response speed (∼20 s) and superior cross-selectivity against other interfering gases. These results demonstrate that MOF-derived M@MO complex catalysts can be functionalized within an electrospun nanofiber scaffold, thereby creating multiheterojunctions, essential for improving catalytic sensor sensitization.

Silver Nanowire Embedded Colorless Polyimide Heater for Wearable Chemical Sensors: Improved Reversible Reaction Kinetics of Optically Reduced Graphene Oxide.

Optically reduced graphene oxide (ORGO) sheets are successfully integrated on silver nanowire (Ag NW)-embedded transparent and flexible substrate. As a heating element, Ag NWs are embedded in a colorless polyimide (CPI) film by covering Ag NW networks using polyamic acid and subsequent imidization. Graphene oxide dispersed aqueous solution is drop-coated on the Ag NW-embedded CPI (Ag NW-CPI) film and directly irradiated by intense pulsed light to obtain ORGO sheets. The heat generation property of Ag NW-CPI film is investigated by applying DC voltage, which demonstrates unprecedentedly reliable and stable characteristics even in dynamic bending condition. To demonstrate the potential application in wearable chemical sensors, NO2 sensing characteristic of ORGO is investigated with respect to the different heating temperature (22.7-71.7 °C) of Ag NW-CPI film. The result reveals that the ORGO sheets exhibit high sensitivity of 2.69% with reversible response/recovery sensing properties and minimal deviation of baseline resistance of around 1% toward NO2 molecules when the temperature of Ag NW-CPI film is 71.7 °C. This work first demonstrates the improved reversible NO2 sensing properties of ORGO sheets on flexible and transparent Ag NW-CPI film assisted by Ag NW heating networks.

Metal Chelation Assisted In Situ Migration and Functionalization of Catalysts on Peapod-Like Hollow SnO2 toward a Superior Chemical Sensor.

Rational design of nanostructures and efficient catalyst functionalization methods are critical to the realization of highly sensitive gas sensors. In order to solve these issues, two types of strategies are reported, i.e., (i) synthesis of peapod-like hollow SnO2 nanostructures (hollow 0D-1D SnO2 ) by using fluid dynamics of liquid Sn metal and (ii) metal-protein chelate driven uniform catalyst functionalization. The hollow 0D-1D SnO2 nanostructures have advantages in enhanced gas accessibility and higher surface areas. In addition to structural benefits, protein encapsulated catalytic nanoparticles result in the uniform catalyst functionalization on both hollow SnO2 spheres and SnO2 nanotubes due to their dynamic migration properties. The migration of catalysts with liquid Sn metal is induced by selective location of catalysts around Sn. On the basis of these structural and uniform functionalization of catalyst benefits, biomarker chemical sensors are developed, which deliver highly selective detection capability toward acetone and toluene, respectively. Pt or Pd loaded multidimensional SnO2 nanostructures exhibit outstanding acetone (R air /R gas = 93.55 @ 350 °C, 5 ppm) and toluene (R air /R gas = 9.25 @ 350 °C, 5 ppm) sensing properties, respectively. These results demonstrate that unique nanostructuring and novel catalyst loading method enable sensors to selectively detect biomarkers for exhaled breath sensors.

WO3 Nanofiber-Based Biomarker Detectors Enabled by Protein-Encapsulated Catalyst Self-Assembled on Polystyrene Colloid Templates.

A novel catalyst functionalization method, based on protein-encapsulated metallic nanoparticles (NPs) and their self-assembly on polystyrene (PS) colloid templates, is used to form catalyst-loaded porous WO3 nanofibers (NFs). The metallic NPs, composed of Au, Pd, or Pt, are encapsulated within a protein cage, i.e., apoferritin, to form unagglomerated monodispersed particles with diameters of less than 5 nm. The catalytic NPs maintain their nanoscale size, even following high-temperature heat-treatment during synthesis, which is attributed to the discrete self-assembly of NPs on PS colloid templates. In addition, the PS templates generate open pores on the electrospun WO3 NFs, facilitating gas molecule transport into the sensing layers and promoting active surface reactions. As a result, the Au and Pd NP-loaded porous WO3 NFs show superior sensitivity toward hydrogen sulfide, as evidenced by responses (R(air)/R(gas)) of 11.1 and 43.5 at 350 °C, respectively. These responses represent 1.8- and 7.1-fold improvements compared to that of dense WO3 NFs (R(air)/R(gas) = 6.1). Moreover, Pt NP-loaded porous WO3 NFs exhibit high acetone sensitivity with response of 28.9. These results demonstrate a novel catalyst loading method, in which small NPs are well-dispersed within the pores of WO3 NFs, that is applicable to high sensitivity breath sensors.

High throughput ultralong (20 cm) nanowire fabrication using a wafer-scale nanograting template.

Nanowires are being actively explored as promising nanostructured materials for high performance flexible electronics, biochemical sensors, photonic applications, solar cells, and secondary batteries. In particular, ultralong (centimeter-long) nanowires are highly attractive from the perspective of electronic performance, device throughput (or productivity), and the possibility of novel applications. However, most previous works on ultralong nanowires have issues related to limited length, productivity, difficult alignment, and deploying onto the planar substrate complying with well-matured device fabrication technologies. Here, we demonstrate a highly ordered ultralong (up to 20 cm) nanowire array, with a diameter of 50 nm (aspect ratio of up to 4,000,000:1), in an unprecedented large (8 in.) scale (2,000,000 strands on a wafer). We first devised a perfectly connected ultralong nanograting master template on the whole area of an 8 in. substrate using a top-down approach, with a density equivalent to that achieved with e-beam lithography (100 nm). Using this large-area, ultralong, high-density nanograting template, we developed a fast and effective method for fabricating up to 20 cm long nanowire arrays on a plastic substrate, composed of metal, dielectric, oxide, and ferroelectric materials. As a suggestion of practical application, a prototype of a large-area aluminum wire grid polarizer was demonstrated.

Selective diagnosis of diabetes using Pt-functionalized WO3 hemitube networks as a sensing layer of acetone in exhaled breath.

Thin-walled WO(3) hemitubes and catalytic Pt-functionalized WO(3) hemitubes were synthesized via a polymeric fiber-templating route and used as exhaled breath sensing layers for potential diagnosis of halitosis and diabetes through the detection of H(2)S and CH(3)COCH(3), respectively. Pt-functionalized WO(3) hemitubes with wall thickness of 60 nm exhibited superior acetone sensitivity (R(air)/R(gas) = 4.11 at 2 ppm) with negligible H(2)S response, and pristine WO(3) hemitubes showed a 4.90-fold sensitivity toward H(2)S with minimal acetone-sensing characteristics. The detection limit (R(air)/R(gas)) of the fabricated sensors with Pt-functionalized WO(3) hemitubes was 1.31 for acetone of 120 ppb, and pristine WO(3) hemitubes showed a gas response of 1.23 at 120 ppb of H(2)S. Long-term stability tests revealed that the remarkable selectivity has been maintained after aging for 7 months in air. The superior cross-sensitivity and response to H(2)S and acetone gas offer a potential platform for application in diabetes and halitosis diagnosis.

Functionalized metal-organic frameworks for heavy metal ion removal from water.

Water purification is becoming increasingly important due to the scarcity and industrial contamination of water. Although traditional adsorbents such as activated carbon and zeolites can remove heavy metal ions from water, they have slow kinetics and low uptake. To address these problems, metal-organic framework (MOF) adsorbents have been developed, which are characterized by facile synthesis, high porosity, designability, and stability. Water-stable MOFs, such as MIL-101, UiO-66, NU-1000, and MOF-808, have attracted considerable research interest. Thus, in this review, we summarize the developments of these MOFs and highlight their adsorption performance characteristics. Moreover, we discuss functionalization methods that are typically used to improve these MOFs' adsorption performance. This minireview is timely and will help readers understand the design principles and working phenomena of next-generation MOF-based adsorbents.

Temperature-Dependent n-to-p-Type Transition of 2D Mn Oxide Nanosheets toward NO2 for Flexible Gas Sensor Application.

Rapid and on-site detection of nitrogen dioxide (NO2) is important for environmental monitoring as NO2 is a highly toxic chemical emitted from automobiles and power plants. In this study, we proposed atomically thin two-dimensional (2D) Mn oxide nanosheets (NSs) assembled on a flexible heating substrate for application in flexible and wearable NO2 sensors. A liquid-phase exfoliation technique was used to obtain individual Mn oxide layers that formed a homogeneous suspension. A flexible heater was fabricated by partially embedding Ag nanotubes (NTs) on the surface of a colorless polyimide (CPI) film for use as a sensor substrate. Temperature-dependent NO2 sensing properties were investigated via control of the operating temperature using a Ag NT-embedded CPI heating film. As a result, the n-type sensing behavior of the Mn oxide NSs exhibited a response [(Rgas - Rair)/Rair × 100 (%)] of 1.20 ± 0.21% for 20 ppm NO2 at room temperature (25 °C). Meanwhile, n-p transition occurred with p-type sensing property as the operating temperature increased to 150 °C with an improved response [(Rair - Rgas)/Rair × 100 (%)] of 4.10 ± 0.42% for 20 ppm NO2. The characteristic n-p transition of Mn oxide NSs at different operating temperatures was evidenced by the surface-adsorbed oxygen ions (i.e., O2- and O-) and nitrate species (NO3- and NO32-).

Towards the fastest kinetics and highest uptake of post-functionalized UiO-66 for Hg2+ removal from water.

Recent advances in adsorbents have improved the removal of mercury ions from wastewater. Metal-organic frameworks (MOFs) have been increasingly used as adsorbents due to their high adsorption capacity and ability to adsorb various heavy metal ions. UiO-66 (Zr) MOFs are mainly used because they are highly stable in aqueous solutions. However, most functionalized UiO-66 materials are unable to achieve a high adsorption capacity because of the undesired reactions that occur during post-functionalization. Herein, we report a facile post-functionalization method to synthesize a MOF adsorbent with fully active amide- and thiol-functionalized chelating groups, termed UiO-66-A.T. UiO-66-A.T. was synthesized via a two-step reaction by crosslinking with a monomer containing a disulfide moiety, followed by disulfide cleavage to activate the thiol groups. UiO-66-A.T. removed Hg2+ from water with a maximum adsorption capacity of 691 mg g-1 and a rate constant of 0.28 g mg-1 min-1 at pH 1. In a mixed solution containing 10 different heavy metal ions, UiO-66-A.T. has a Hg2+ selectivity of 99.4%, which is the highest reported to date. These results demonstrate the effectiveness of our design strategy for synthesizing purely defined MOFs to achieve the best Hg2+ removal performance to date among post-functionalized UiO-66-type MOF adsorbents.

Computational Prediction of Stacking Mode in Conductive Two-Dimensional Metal-Organic Frameworks: An Exploration of Chemical and Electrical Property Changes.

Conductive two-dimensional metal-organic frameworks (2D MOFs) have attracted interest as they induce strong charge delocalization and improve charge carrier mobility and concentration. However, characterizing their stacking mode depends on expensive and time-consuming experimental measurements. Here, we construct a potential energy surface (PES) map database for 36 2D MOFs using density functional theory (DFT) for the experimentally synthesized and non-synthesized 2D MOFs to predict their stacking mode. The DFT PES results successfully predict the experimentally synthesized stacking mode with an accuracy of 92.9% and explain the coexistence mechanism of dual stacking modes in a single compound. Furthermore, we analyze the chemical (i.e., host-guest interaction) and electrical (i.e., electronic structure) property changes affected by stacking mode. The DFT results show that the host-guest interaction can be enhanced by the transition from AA to AB stacking, taking H2S gas as a case study. The electronic band structure calculation confirms that as AB stacking displacement increases, the in-plane charge transport pathway is reduced while the out-of-plane charge transport pathway is maintained or even increased. These results indicate that there is a trade-off between chemical and electrical properties in accordance with the stacking mode.

Technology Roadmap for Flexible Sensors.

Humans rely increasingly on sensors to address grand challenges and to improve quality of life in the era of digitalization and big data. For ubiquitous sensing, flexible sensors are developed to overcome the limitations of conventional rigid counterparts. Despite rapid advancement in bench-side research over the last decade, the market adoption of flexible sensors remains limited. To ease and to expedite their deployment, here, we identify bottlenecks hindering the maturation of flexible sensors and propose promising solutions. We first analyze challenges in achieving satisfactory sensing performance for real-world applications and then summarize issues in compatible sensor-biology interfaces, followed by brief discussions on powering and connecting sensor networks. Issues en route to commercialization and for sustainable growth of the sector are also analyzed, highlighting environmental concerns and emphasizing nontechnical issues such as business, regulatory, and ethical considerations. Additionally, we look at future intelligent flexible sensors. In proposing a comprehensive roadmap, we hope to steer research efforts towards common goals and to guide coordinated development strategies from disparate communities. Through such collaborative efforts, scientific breakthroughs can be made sooner and capitalized for the betterment of humanity.

Flexible Chemiresistive Cyclohexanone Sensors Based on Single-Walled Carbon Nanotube-Polymer Composites.

We report a chemiresistive cyclohexanone sensor on a flexible substrate based on single-walled carbon nanotubes (SWCNTs) functionalized with thiourea (TU) derivatives. A wrapper polymer containing both 4-vinylpyridine (4VP) groups and azide groups (P(4VP-VBAz)) was employed to obtain a homogeneous SWCNT dispersion via noncovalent functionalization of SWCNTs. The P(4VP-VBAz)-SWCNT composite dispersion was then spray-coated onto an organosilanized flexible poly(ethylene terephthalate) (PET) film to achieve immobilizing quaternization between the pyridyl groups from the polymer and the functional PET substrate, thereby surface anchoring SWCNTs. Subsequent surface functionalization was performed to incorporate a TU selector into the composites, resulting in P(Q4VP-VBTU)-SWCNT, for the detection of cyclohexanone via hydrogen bonding interactions. An increase in conductance was observed as a result of the hydrogen-bonded complex with cyclohexanone resulting in a higher hole density and/or mobility in SWCNTs. As a result, a sensor device fabricated with P(Q4VP-VBTU)-SWCNT composites exhibited chemiresistive responses (ΔG/G0) of 7.9 ± 0.6% in N2 (RH 0.1%) and 4.7 ± 0.4% in air (RH 5%), respectively, upon exposure to 200 ppm cyclohexanone. Selective cyclohexanone detection was achieved with minor responses (ΔG/G0 < 1.4% at 500 ppm) toward interfering volatile organic compounds (VOC). analytes. We demonstrate a robust sensing platform using the polymer-SWCNT composites on a flexible PET substrate for potential application in wearable sensors.

Functional Single-Walled Carbon Nanotubes for Anion Sensing.

We report an anion-sensing platform wherein conductance changes are triggered by chemical interactions between selectors and anions. The selector design incorporates both a cationic moiety (i.e., pyridinium) and a thiourea-based dual-hydrogen-bond donor. Anion binding by a model selector (2) was studied using 1H NMR and UV-vis titrations, which reveal a binding strength toward acetate ions (AcO-) followed by Cl- > Br- > NO3-. These studies reveal that selector 2 is deprotonated upon addition of AcO-, whereas it undergoes hydrogen bonding associated with Cl-, Br-, and NO3-. The cationic pyridinium moiety improves anion binding affinity by lowering the pKa value of selector 2 and enhancing the hydrogen-bond donor capability as confirmed by spectroscopic titrations and DFT calculations. The selector is covalently attached to poly(4-vinylpyridine) (P4VP), which wraps single-walled carbon nanotubes (SWCNTs) (i.e., P4VP-2-SWCNT) to transduce an electrical signal. As a result, continuous anion sensing was achieved with high sensitivity represented by a normalized resistance change of 101.9 ± 10.3% toward 16.7 mM AcO-, whereas negligible sensitivity was observed toward Cl-, Br-, and NO3-. The sensitivity transition was attributed to the internal charge transfer of 2 by deprotonation of the thiourea proton upon addition of AcO-.

Interpenetrating Polymer Semiconductor Nanonetwork Channel for Ultrasensitive, Selective, and Fast Recovered Chemodetection.

Organic semiconductor (OSC)-based gas detection has attracted considerable attention due to the facile manufacturing process and effective contact with target chemicals at room temperature. However, OSCs intrinsically suffer from inferior sensing and recovery capability due to lack of functional sites and deep gas penetration into the film. Here, we describe an interpenetrating polymer semiconductor nanonetwork (IPSN) channel possessing unreacted silanol (Si-OH) groups on its surface to overcome bottlenecks that come from OSC-based chemodetection. On the top of the IPSN, moreover, we introduced electron-donating amine (NH2) groups as a chemical receptor because they strongly interact with the electron-withdrawing nature of NO2 gas. The NH2-IPSN-based field-effect transistor exhibited high-performance chemodetection such as ultrasensitivity (990% ppm-1 at 5 ppm) and excellent NO2 selectivity against other toxic gases. Impressively, the gas recovery was significantly improved because the NH2 chemical receptors anchored on the surface of the IPSN suppress deep gas penetration into the film. This work demonstrates that our NO2 chemodetection is expected to provide inspiration and guideline for realization of practical gas sensors in various industries and daily life.