Chemo-Electrical Gas Sensors Based on Conducting Polymer Hybrids

Sensitivity Enhancement of Acetone Gas Sensor using Polyethylene Glycol/Multi-Walled Carbon Nanotubes Composite Sensing Film with Thermal Treatment

There is a need to develop a chemiresistive gas sensor equipped with a thermostat over a wide area for the sensor, which can protect the sensor from the influence of ambient temperature due to the uniform temperature of the thermostat. In this paper, we demonstrated an acetone gas sensor based on a polyethylene glycol (PEG)/Multi-walled Carbon Nanotubes (MWCNTs) composite film, which was equipped with a thermostat. The sensor was operated at modest working temperatures for sensor sensitivity enhancement. The optimum design of the polyimide-based thermostat with widely uniform thermal distribution was investigated in detail. It was found that the temperature uniformity of the thermostat was achieved using double spiral geometry. The experimental results of the sensor response showed that the PEG/MWCNTs composite film with a moderate working temperature revealed a higher sensitivity than that without thermal treatment. Moreover, the sensing mechanisms of the PEG/MWCNTs composite gas sensor to acetone vapor were studied as well.

Combining SWNT and Graphene in Polymer Nanofibers: A Route to Unique Carbon Precursors for Electrochemical Capacitor Electrodes

It is important to fabricate nanostructured architectures comprised of functional components for a wide variety of applications because precise structural control in the nanometer regime can yield unprecedented, fascinating properties. Owing to their well-defined microstructural characteristics, it has been popular to use carbon nanospecies, such as nanotubes and graphene, in fabricating nanocomposites and nanohybrids. Nevertheless, it still remains hard to control and manipulate nanospecies for specific applications, thus preventing their commercialization. Herein, first, we report unique one-dimensional nanoarchitectures with meso-/macropores, consisting of single-walled nanotubes (SWNTs), graphene, and polyacrylonitrile, in which poly(vinyl alcohol) was employed as a dispersing agent and sacrificial porogen. One-dimensional SWNTs and two-dimensional graphene pieces were combined in the confined interior space of electrospun nanofibers, which led to unique microstructural characteristics such as enhanced ordering of SWNTs, graphene pieces, and polymer chains in the nanofiber interior. Next, the SWNT/graphene-in-polymer nanofiber (SGPNF) structures were converted into carbonized products (SGCNFs) with effective porosity and tunable electrochemical properties. Similar to SGPNFs, the microstructural and electrical properties of the SGCNFs depended on the incorporated amount of SWNT and graphene. At higher SWNT content, the mesopore volume proportion and specific discharge capacitance of the SGCNFs increased by max. 63 and 598%, respectively. The SGCNFs showed strong potential as a high-performance electrode material for electrochemical capacitors (max. capacitance: nonactivated ∼390 F g^-1 and activated ∼750 F g^-1). Flexible, all solid-state capacitor cells based on SGCNFs were also successfully demonstrated as a model application. The SGCNFs can be further functionalized by various methods, which will impart attractive properties for extended applications.

Conducting Polymers, Hydrogels and Their Composites: Preparation, Properties and Bioapplications

This review is focused on current state-of-the-art research on electroactive-based materials and their synthesis, as well as their physicochemical and biological properties. Special attention is paid to pristine intrinsically conducting polymers (ICPs) and their composites with other organic and inorganic components, well-defined micro- and nanostructures, and enhanced surface areas compared with those of conventionally prepared ICPs. Hydrogels, due to their defined porous structures and being filled with aqueous solution, offer the ability to increase the amount of immobilized chemical, biological or biochemical molecules. When other components are incorporated into ICPs, the materials form composites; in this particular case, they form conductive composites. The design and synthesis of conductive composites result in the inheritance of the advantages of each component and offer new features because of the synergistic effects between the components. The resulting structures of ICPs, conducting polymer hydrogels and their composites, as well as the unusual physicochemical properties, biocompatibility and multi-functionality of these materials, facilitate their bioapplications. The synergistic effects between constituents have made these materials particularly attractive as sensing elements for biological agents, and they also enable the immobilization of bioreceptors such as enzymes, antigen-antibodies, and nucleic acids onto their surfaces for the detection of an array of biological agents. Currently, these materials have unlimited applicability in biomedicine. In this review, we have limited discussion to three areas in which it seems that the use of ICPs and materials, including their different forms, are particularly interesting, namely, biosensors, delivery of drugs and tissue engineering.

Comparative Studies on Two-Electrode Symmetric Supercapacitors Based on Polypyrrole:Poly(4-styrenesulfonate) with Different Molecular Weights of Poly(4-styrenesulfonate)

Poly(4-styrenesulfonate)-conducting polymer (PSS-CP) is advantageous for thin-film electrode manufacturing due to its high conductivity, high charge storage, structural stability, and excellent ink dispersion. In this work, comparative studies of two-electrode symmetric supercapacitors using Polypyrrole:Poly(4-styrenesulfonate) (PPy:PSS), with different molecular weights (Mw's) of Poly(4-styrenesulfonate) (PSS) as the electrodes, were performed. PPy:PSS can be easily prepared using a simple solution process that enables the mass production of thin-film electrodes with improved electrical and electrochemical properties. As-prepared PPy:PSS, with different PSS molecular weights, were assembled into two-electrode supercapacitors based on coin cell structures. It was confirmed that the electrical and electrochemical properties of PPy:PSS were improved with increasing PSS molecular weight. The coin cell, using PPy:PSS with a PSS molecular weight of 1.0 × 10⁶ g/mol, exhibited higher areal capacitance (175.3 mF/cm²), higher volumetric capacitance (584.2 F/cm³), and longer cycling stability (86.3% after 5000 cycles) compared to those of PPy:PSS with PSS molecular weights of 2.0 × 10⁵ and 7.0 × 10⁴ g/mol. This work provides an efficient approach for producing cost-effective and miniaturized supercapacitors with high conductivity and high specific capacitance for practical applications in a variety of electronic devices.

Influence of Moisture on the Energy-Level Alignment at the MoO3/Organic Interfaces

MoO₃ is widely used in polymer-based organic solar cells as an anode buffer layer because of its high workfunction and formation of a strong dipole at the MoO₃/polymer interface facilitating charge transfer across the MoO₃/polymer interface. In the present work, we show that exposure of the MoO₃/polymer interface to moisture attracts water molecules to the interface via diffusion. Because of their own strong dipole, water molecules counter the dipole at the MoO₃/polymer interface. As a consequence, the charge transfer across the MoO₃/polymer will reduce and affect the charge transport across the interface. The outcome of this work thus suggests that it is critical to keep the MoO₃/polymer interface moisture-free, which requires special precautions in device fabrications. The composition of the MoO₃/P3HT:PC₆₁BM interface is analyzed with X-ray photoelectron spectroscopy and the depth profiling technique, neutral impact collision ion scattering spectroscopy. The results show that the concentration of oxygen increases upon exposure but leaves the oxidation state of Mo unchanged. The valence electron spectroscopy technique shows that the dipole across the MoO₃/P3HT:PC₆₁BM interface decreases even for short-time exposure to atmosphere because of the diffusion of water molecules to the interface. The far-ranging consequences for organic electronic devices are discussed.

Biodegradable Contrast Agents for Photoacoustic Imaging

Over the past twenty years, photoacoustics—also called optoacoustics—have been widely investigated and, in particular, extensively applied in biomedical imaging as an emerging modality. Photoacoustic imaging (PAI) detects an ultrasound wave that is generated via photoexcitation and thermoelastic expansion by a short nanosecond laser pulse, which significantly reduces light and acoustic scattering, more than in other typical optical imaging and renders high-resolution tomographic images with preserving high absorption contrast with deep penetration depth. In addition, PAI provides anatomical and physiological parameters in non-invasive manner. Over the past two decades, this technique has been remarkably developed in the sense of instrumentation and contrast agent materials. In this review, we briefly introduce state-of-the-art multiscale imaging systems and summarize recent progress on exogenous bio-compatible and -degradable agents that address biomedical application and clinical practice.

Nanosensors based on polymer vesicles and planar membranes: a short review

This review aims to summarize the advance in the field of nanosensors based on two particular materials: polymer vesicles (polymersomes) and polymer planar membranes. These two types of polymer-based structural arrangements have been shown to be efficient in the production of sensors as their features allow to adapt to different environment but also to increase the sensitivity and the selectivity of the sensing device. Polymersomes and planar polymer membranes offer a platform of choice for a wide range of chemical functionalization and characteristic structural organization which allows a convenient usage in numerous sensing applications. These materials appear as great candidates for such nanosensors considering the broad variety of polymers. They also enable the confection of robust nanosized architectures providing interesting properties for numerous applications in many domains ranging from pollution to drug monitoring. This report gives an overview of these different sensing strategies whether the nanosensors aim to detect chemicals, biological or physical signals.

Fluorinated Metal Phthalocyanines: Interplay between Fluorination Degree, Films Orientation, and Ammonia Sensing Properties

In this work, the sensor response of MPcFx (M = Cu, Co, Zn; x = 0, 4, 16) films toward gaseous NH₃ (10⁻50 ppm) was studied by a chemiresistive method and compared to that of unsubstituted MPc films to reveal the effects of central metals and F-substituents on the sensing properties. A combination of atomic force microscopy and X-ray diffraction techniques have been used to elucidate the structural features of thin MPcFx films deposited by organic molecular beam deposition. It has been shown that the sensor response of MPcF₄ films to ammonia is noticeably higher than that of MPc films, which is in good correlation with the values of binding energy between the metal phthalocyanine and NH₃ molecules, as calculated by the density functional theory (DFT) method. At the same time, in contrast to the DFT calculations, MPcF16 demonstrated the lesser sensor response compared with MPcF₄, which appeared to be connected with the different structure and morphology of their films. The ZnPcF₄ films were shown to exhibit a sensitivity to ammonia up to concentrations as low as 0.1 ppm, and can be used for the selective detection of ammonia in the presence of some reducing gases and volatile organic compounds. Moreover, the ZnPcF₄ films can be used for the detection of NH₃ in the gas mixture simulating exhaled air (N₂ 76%, O₂ 16%, H₂O 5%, and CO₂ 3%).

Comprehensive Study of Poly(2,3,5,6-tetrafluoroaniline): From Electrosynthesis to Heterojunctions and Ammonia Sensing

In this work, we report for the first time on a comprehensive study of poly(2,3,5,6-tetrafluoroaniline) (PTFANI). Contrary to the nonfluorinated polyaniline (PANI) or its analogues bearing one fluorine atom, PTFANI is a poorly conductive material. We present a comprehensive study of the electrosynthesized PTFANI from its monomer in an acidic aqueous medium. PTFANI was fully characterized by a potential-pH diagram, spectroelectrochemistry, and electrochemical quartz crystal microbalance (EQCM) measurements, as well as by a morphological study. Combined with the X-ray photoelectron spectroscopy (XPS) analysis, it allowed us to understand the redox properties of this polymer compared to those of the unsubstituted PANI. At pH < 1.85, no proton transfer occurred during the electrochemical process, but the insertion of anions at the site of the protonated imines was demonstrated through the EQCM and XPS experiments. PTFANI showed a lower ratio of 1 ClO₄^- per 3 2,3,5,6-tetrafluoroaniline units compared to that of PANI. The behavior at pH > 1.85 was different; no anion upload was observed during the electron transfer, but 1 H⁺ per electron was involved during the transition between the leucoemeraldine and emeraldine base forms. It should also be noted that the oxidation of the emeraldine into the pernigraniline form was not accessible in PTFANI because of the electron-withdrawing effects of the fluorine atoms. However, we took advantage of the unique behavior of PTFANI to build heterojunctions, by combining with a highly conductive molecular material, namely lutetium bisphthalocyanine, LuPc₂. The obtained double-lateral heterojunction exhibited a particularly interesting sensitivity to ammonia, even under humid atmospheres, with a limit of detection of 450 ppb. This work paves the way for the use of PTFANI in other electronic devices and as a sensor not only in the field of air quality monitoring but also in the field of health diagnosis in measuring the human breath.

Chemical Design of Functional Polymer Structures for Biosensors: From Nanoscale to Macroscale

Over the past decades, biosensors, a class of physicochemical detectors sensitive to biological analytes, have drawn increasing interest, particularly in light of growing concerns about human health. Functional polymeric materials have been widely researched for sensing applications because of their structural versatility and significant progress that has been made concerning their chemistry, as well as in the field of nanotechnology. Polymeric nanoparticles are conventionally used in sensing applications due to large surface area, which allows rapid and sensitive detection. On the macroscale, hydrogels are crucial materials for biosensing applications, being used in many wearable or implantable devices as a biocompatible platform. The performance of both hydrogels and nanoparticles, including sensitivity, response time, or reversibility, can be significantly altered and optimized by changing their chemical structures; this has encouraged us to overview and classify chemical design strategies. Here, we have organized this review into two main sections concerning the use of nanoparticles and hydrogels (as polymeric structures) for biosensors and described chemical approaches in relevant subcategories, which act as a guide for general synthetic strategies.

A Study on the Impact of Poly(3-hexylthiophene) Chain Length and Other Applied Side-Chains on the NO₂ Sensing Properties of Conducting Graft Copolymers

The detection and concentration measurements of low concentrations of nitrogen dioxide (NO₂) are important because of its negative effects on human health and its application in many fields of industry and safety systems. In our approach, conducting graft copolymers based on the poly(3-hexylthiophene) (P3HT) conducting polymer and other side-chains, polyethylene glycol (PEG) and dodec-1-en, grafted on a poly(methylhydrosiloxane) backbone, were investigated. The grafts containing PEG (PEGSil) and dodec-1-en (DodecSil) in two variants, namely, fractions with shorter (hexane fraction -H) and longer (chloroform fraction -CH) side-chains of P3HT, were tested as receptor structures in NO₂ gas sensors. Their responses to NO₂, within the concentration range of 1–20 ppm, were investigated in an nitrogen atmosphere at different operating temperatures—room temperature (RT) = 25 °C, 50 °C, and 100 °C. The results indicated that both of the copolymers with PEG side-chains had higher responses to NO₂ than the materials with dodec-1-en side-chains. Furthermore, the results indicated that, in both cases, H fractions were more sensitive than CH fractions. The highest response to 1 ppm of NO_2, from the investigated graft copolymers, had PEGSil H, which indicated a response of 1330% at RT and 1980% at 100 °C. The calculated lower-limit of the detection of this material is lower than 300 ppb of NO₂ at 100 °C. This research indicated that graft copolymers of P3HT had great potential for low temperature NO₂ sensing, and that the proper choice of other side-chains in graft copolymers can improve their gas sensing properties.

Optical Fiber Sensors Based on Polymeric Sensitive Coatings

Polymer technology is one of the fastest growing fields of contemporary research due to the possibility of using a wide variety of synthetic chemical routes for obtaining a polymeric network with a well-defined structure, resulting in materials with outstanding macroscopic properties. Surface engineering techniques based on the implementation of polymeric structures can be used as an interesting tool for the design of materials with functional properties. In this sense, the use of fabrication techniques for the design of nanostructured polymeric coatings is showing an important growth due to the intrinsic advantages of controlling the structure at a nanoscale level because physical, chemical, or optical properties can be considerably improved in comparison with the bulk materials. In addition, the presence of these sensitive polymeric coatings on optical fiber is a hot topic in the scientific community for its implementation in different market niches because a wide variety of parameters can be perfectly measured with a high selectivity, sensitivity, and fast response time. In this work, the two main roles that a polymeric sensitive matrix can play on an optical fiber for sensing applications are evaluated. In a first section, the polymers are used as a solid support for the immobilization of specific sensitive element, whereas in the second section the polymeric matrix is used as the chemical transducer itself. Additionally, potential applications of the optical fiber sensors in fields as diverse as biology, chemistry, engineering, environmental, industry or medicine will be presented in concordance with these two main roles of the polymeric sensitive matrices.

Facile Synthesis of Polyaniline Nanotubes Using Self-Assembly Method Based on the Hydrogen Bonding: Mechanism and Application in Gas Sensing

Based on hydrogen bonding, the highly uniform polyaniline (PANI) nanotubes were synthesized by self-assembly method using citric acid (CA) as the dopant and the structure-directing agent by optimizing the molar ratio of CA to aniline monomer (Ani). Synthesis conditions like reaction temperature and mechanical stirring were considered to explore the effects of hydrogen bonding on the morphologies. The effects of CA on the final morphology of the products were also investigated. The as-synthesized CA doped polyaniline (PANI) nanomaterials were further deposited on the plate electrodes for the test of gas sensing performance to ammonia (NH₃). The sensitivity to various concentrations of NH₃, the repeatability, and the stability of the sensors were also tested and analyzed. As a result, it was found that the PANI nanomaterial synthesized at the CA/Ani molar ratio of 0.5 has highly uniform tubular morphology and shows the best sensing performance to NH₃. It makes the PANI nanotubes a promising material for high performance gas sensing to NH₃.

Impact of Temperature and UV Irradiation on Dynamics of NO₂ Sensors Based on ZnO Nanostructures

The main object of this study is the improvement of the dynamics of NO₂ sensors based on ZnO nanostructures. Investigations presented in this paper showed that the combination of temperature and ultraviolet (UV) activation of the sensors can significantly decrease the sensor response and regeneration times. In comparison with the single activation method (elevated temperature or UV), these times for 1 ppm of NO₂ decreased from about 10 min (or more) to less than 40 s. In addition, at the optimal conditions (200 °C and UV), sensors were very stable, were fully scalable (in the range on NO₂ concentration of 1-20 ppm) and baseline drift was significantly reduced. Furthermore, in this paper, extensive studies of the influence of temperature and carrier gas (nitrogen and air) on NO₂ sensing properties of the ZnO nanostructures were conducted. The NO₂ sensing mechanisms of the sensors operating at elevated temperatures and under UV irradiation were also discussed. Our study showed that sensor responses to NO₂ and response/regeneration times are comparable from sensor to sensor in air and nitrogen conditions, which suggests that the proposed simple technology connected with well-chosen operation conditions is repeatable. The estimated limit of detection of the sensors is within the level of ≈800 ppb in nitrogen and ≈700 ppb in air.