Composites Based On Conductive Polymer With Carbon Nanotubes In DMMP Gas Sensors – An Overview

A number of recent terrorist attacks make it clear that rapid response, high sensitivity and stability are essential in the development of chemical sensors for the detection of chemical warfare agents. Nerve agent sarin [2-(fluoro-methyl-phosphoryl) oxypropane] is an organophosphate (OP) compound that is recognized as one of the most toxic chemical warfare agents. Considering sarin’s high toxicity, being odorless and colorless, dimethyl methylphosphonate (DMMP) is widely used as its simulant in the laboratory because of its similar chemical structure and much lower toxicity. Thus, this review serves to introduce the development of a variety of fabricated chemical sensors as potential sensing materials for the detection of DMMP in recent years. Furthermore, the research and application of carbon nanotubes in DMMP polymer sensors, their sensitivity and limitation are highlighted. For sorption-based sensors, active materials play crucial roles in improving the integral performances of sensors. The novel active materials providing hydrogen-bonds between the polymers and carbon nanotubes are the main focus in this review

CHARACTERIZATION OF CARBON NANOTUBES BASED RESISTIVE AND CAPACITIVE GAS SENSORS

A preliminary gas detection study was conducted on as-grown multi-walled carbon nanotubes and anodized aluminum oxide (MWNTs/AAO) template. The material demonstrated room temperature gas sensitivity and p-type semiconductor characteristics. Plasma-etched MWNTs/AAO templates were employed to construct capacitive gas sensors. The capacitances of the sensors were sensitive to both reducing and oxidizing gases at room temperature. Single-walled carbon nanotubes (SWNTs) dispersed in binder andamp;aacute;-terpineol were applied on sensor platforms to form resistive gas sensors. The sensors demonstrated excellent sensitivity to low concentrations of reducing and oxidizing gases at room temperature, which suggests the p-type semiconducting behavior of SWNTs. The sensor recovery was found to be incomplete at room temperature in flow of nitrogen and air, thus possible solutions were investigated to enhance sensor performance. The sensor operating principles and suggestions for possible future work are discussed. The room temperature and air background functionality of the sensor suggest that SWNT is a promising gas sensing material for application in ambient conditions

Single-walled carbon nanotube networks and related composite materials for gas sensing applications

In this thesis, the gas sensing properties of single-walled carbon nanotube (SWCNT) networks and SWCNT-Zeolite composite materials were investigated in a variety of environmental conditions. The aim of the project was to establish the effect that adsorbed water vapour had on the electrical properties of SWCNT networks, along with any subsequent impact on the NO2 sensing responses of SWCNT-based chemiresistors. Motivated by these investigations, the sensitivity of the SWCNT networks to water vapour was exploited to develop the water-assisted regeneration (WAR) method, enabling the improved recovery of the baseline sensing signal. Zeolites, known as molecular sieves due to their selective adsorption properties, were utilised in SWCNT-Zeolite composite sensing layers to reduce the cross-sensitivity of functionalised SWCNTs to water vapour. Functionalisation of the SWCNTs with a range of anionic, cationic and nonionic surfactants to aid solution processing was found to enhance the conductancehumidity effect, in some cases by a factor of 10. An interesting bi-directional switch in conductance change was observed when anionic (conductance decrease) vs cationic (conductance increase) were used. Under experimental conditions, fluctuations in atmospheric humidity levels were shown to alter the gas sensing characteristics of the SWCNT networks. Formed from interconnected metallic and semiconducting SWCNTs, the chemiresistive sensors demonstrated increased response magnitudes, adsorption rates and recovery rates at higher levels (A 50% RH) of relative humidity. Raman spectroscopy, UV-Vis-NIR spectroscopy, electron microscopy and electrical characterisation techniques were used in conjunction with gas sensing experiments to study changes in the properties of the sensing elements, helping to elucidate potential mechanisms. Extraction of key sensing parameters was facilitated by the application of a model for completely irreversible adsorption of NO2, whilst a model based on partially reversible desorption was found to best describe the sensing data

Carbon Nanostructure-Based Field-Effect Transistors for Label-Free Chemical/Biological Sensors

Over the past decade, electrical detection of chemical and biological species using novel nanostructure-based devices has attracted significant attention for chemical, genomics, biomedical diagnostics, and drug discovery applications. The use of nanostructured devices in chemical/biological sensors in place of conventional sensing technologies has advantages of high sensitivity, low decreased energy consumption and potentially highly miniaturized integration. Owing to their particular structure, excellent electrical properties and high chemical stability, carbon nanotube and graphene based electrical devices have been widely developed for high performance label-free chemical/biological sensors. Here, we review the latest developments of carbon nanostructure-based transistor sensors in ultrasensitive detection of chemical/biological entities, such as poisonous gases, nucleic acids, proteins and cells

Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges

Integration of Biomolecular Recognition Elements with Solid-State Devices

Continued advances in stand-alone chemical sensors requires the introduction of new materials and transducers, and the seamless integration of the two. Electronic sensors represent one of the most efficient and versatile sensing transducers that offer advantages of high sensitivity, compatibility with multiple types of materials, network connectivity, and capability of miniaturization. With respect to materials to be used on this platform, many classes and subclasses of materials, including polymers, oxides, semiconductors, and composites have been investigated for various sensing environments. Despite numerous commercial products, major challenges remain. These include enhancing materials for selectivity/specificity, and low cost integration/ miniaturization of devices. Breakthroughs in either area would signify a transformative innovation. In this thesis, a combined materials and devices approach has been explored to address the above challenges. Biomolecular recognition elements, exemplified by aptamers, are the most recent addition to the library of tunable materials for specific detection of analytes. At the same time, nanoscale electrical devices based on tunnel junctions offer the potential for simple design, large scale integration, field deployment, network connectivity, and importantly, miniaturization to the molecular scale. To first establish a framework for studying sorption properties of solid oligonucleotides, custom designed aptamers sequences were studied to determine equilibrium partition coefficients. Linear-solvation-energy-relationship (LSER) analysis provides quantifications of non-covalent bonding properties and reveals the dominance of hydrogen bonding basicity in oligonucleotides. We find that DNA-analyte interactions have selective sorption properties similar to synthetic polymers. LSER analysis provides a chemical basis for material-analyte interactions. Oligonucleotide sequences were integrated with gold nanoparticle chemiresistors to transfer the selective sorption properties to microfabricated electrical devices. Responses generated by oligonucleotides under dry conditions were similar to standard organic mediums used as capping agents and suggests that DNA-based chemiresistor sensors operate with a similar mechanism based on sorption induced swelling. The equilibrium mass-sorption behavior of bulk DNA films could be translated to the chemiresistor sensitivity profiles. Our work establishes oligonucleotides, including aptamers, as a class of sorptive materials that can be systematically studied, engineered, and integrated with nanoscale electronic sensor devices. Experiments to investigate secondary structure effects were inconclusive and we conclude that further work should investigate DNA aptamers in buffered, aqueous environments to unequivocally establish the ability of chemiresitors to signal molecular recognition. Concurrent with the above studies, device integration and miniaturization was investigated to combine many sensing materials into a single, compact design. Arrays of nanoscale chemiresistors with critical features on the order of 10 – 100 nm were developed, using dielectrophoretic assembly of gold nanoparticles to control placement of the sensing material with nanometer accuracy. The nanoscale chemiresistors achieved the smallest known gold nanoparticle chemiresistors relying on just 2 – 3 layers of nanoparticles within 50 nm gaps, and were found to be more robust and less dependent on film thickness than previously published designs. Due to shorter diffusion paths, the sensors are also faster in response and recovery. A proof-of-concept, integrated single-chip sensor array was created and it showed similar response patterns as non-integrated sensor arrays. Dielectrophoresis is established as a key enabler for nanoscale, integrated devices. Based on the major findings of the thesis work, additional investigations were initiated to investigate the potential for nanoscale chemiresitor sensors to operate in buffered, aqueous (liquid) flow cells. Preliminary experiments show that chemiresistor sensing is transferable to liquid environments where analyte molecules are observed to partition from the bulk liquid to the sensing materials, leading to a detectable change of the device electrical properties. Comparing micron- and nano-scale devices fabricated using aqueous oligonucleotide-functionalized gold nanoparticles, it was found that nanoscale chemiresistors are more resistant to solvent damage than 5 µm chemiresistors. We conclude that future experiments to investigate aptamer sensing in aqueous solutions is a promising direction. Overall, this thesis is a significant contribution to materials development and device design to attain improved sensor selectivity and higher levels of device integration. First, it offers a scheme for design, selection, and validation of materials that confer analyte-specific interactions. Second, it paves the way for large scale sensor integration and parallel operation on a single chip. Lastly, it offers an approach to combine biomolecular recognition elements with electronic devices into robust, nanoscale detection systems. Based on the major findings of the thesis work, additional investigations were initiated to investigate the potential for nanoscale chemiresitor sensors to operate in buffered, aqueous (liquid) flow cells. Preliminary experiments show that chemiresistor sensing is transferable to liquid environments where analyte molecules are observed to partition from the bulk liquid to the sensing materials, leading to a detectable change of the device electrical properties. Comparing micron- and nano-scale devices fabricated using aqueous oligonucleotide-functionalized gold nanoparticles, it was found that nanoscale chemiresistors are more resistant to solvent damage than 5 µm chemiresistors. We conclude that future experiments to investigate aptamer sensing in aqueous solutions is a promising direction. Overall, this thesis is a significant contribution to materials development and device design to attain improved sensor selectivity and higher levels of device integration. First, it offers a scheme for design, selection, and validation of materials that confer analyte-specific interactions. Second, it paves the way for large scale sensor integration and parallel operation on a single chip. Lastly, it offers an approach to combine biomolecular recognition elements with electronic devices into robust, nanoscale detection systems

Rational Nanocarbon Composites for Novel Sensing and Catalysis

Carbon nanomaterials have extraordinary electrical properties due to their covalent structure which enable a wide range of sensing and catalytic applications. While these structures are impressive on their own, combinations with other materials enable broader extensions of properties and applications. Most functionalization schemes to form composite materials to this date have resulted in degradation of the excellent physical properties. Careful introduction of defects or composite formation is a potential route to avoid loss of outstanding electrical properties and enable a wide range of nanomaterial composites. We first investigated how covalent organic frameworks can be used as a templating strategy to control oxidation while retaining sp2 conjugation to make holey graphene from single layer graphene. Holey graphene can then be used as a substrate for formation of size-controlled gold and palladium nanoparticles without reducing agent. Composite holey graphene nanoparticle materials exhibit strong electronic coupling between the individual nanomaterials and were leveraged to perform sensing of hydrogen sulfide and hydrogen gases for gold and palladium-based composites, respectively. This covalent organic framework template strategy can be further extended into formation of bulk holey graphene through patterning many graphene sheets at once on highly ordered pyrolytic graphite (HOPG). Once exfoliated, many solution phase reactions with metal salts were explored but silver, gold, copper, and nickel were all shown to form size-controlled nanoparticles in aqueous solution in a concentration dependent manner. So-called graphene nanoparticle compounds showed strong electronic coupling between constituent nanoparticles and nickel-based compounds showed promising activity for oxygen evolution reaction reaching mass current activities of 10,000 mA/mg of Ni at 1.7 V versus reversible hydrogen electrode. The final project leveraged growth mechanics of porous frameworks with nanocarbon materials to combine a copper catecholate based metal organic framework (MOF) grown on single-walled carbon nanotubes (SWCNTs). These composites were used to facilitate a liquid-based field-effect transistor (FET) sensing modality where ionic strength and migration of ions into pores in the presence of homologous series of carbohydrates altered observed current leading to a size selective sensor. The results of this report illustrate how careful control of nanomaterial interfaces can be leveraged for novel sensing and catalysis

Low electrical resistivity carbon nanotube and polyethylene nanocomposites for aerospace and energy exploration applications

An investigation was conducted towards the development and optimization of low electrical resistivity carbon nanotube (CNT) and thermoplastic composites as potential materials for future wire and cable applications in aerospace and energy exploration. Fundamental properties of the polymer, medium density polyethylene (MDPE), such as crystallinity were studied and improved for composite use. A parallel effort was undertaken on a broad selection of CNT, including single wall, double wall and multi wall carbon nanotubes, and included research of material aspects relevant to composite application and low resistivity such as purity, diameter and chirality. With an emphasis on scalability, manufacturing and purification methods were developed, and a solvent-based composite fabrication method was optimized. CNT MDPE composites were characterized via thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), Raman spectroscopy, and multiple routes of electron microscopy. Techniques including annealing and pressure treatments were used to further improve the composites' resulting electrical performance. Enhancement of conductivity was explored via exposure to a focused microwave beam. A novel doping method was developed using antimony pentafluoride (SbF 5 ) to reduce the resistivity of the bulk CNT. Flexible composites, malleable under heat and pressure, were produced with exceptional electrical resistivities reaching as low as 2*10 -6 Ω·m (5*10 5 S/m). A unique gas sensor application utilizing the unique electrical resistivities of the produced CNT-MDPE composites was developed. The materials proved suitable as a low weight and low energy sensing material for dimethyl methylphosphonate (DMMP), a nerve gas simulant

Carbon Nanomaterials based on Graphene in (Electro-)chemical Sensors: Characterization, Modification and Application

The variability of graphene with its exceptional properties gives rise to improve material chemistry in various fields of applications. The development of graphene is still in the beginning and up to now only a few niche products have reached the market and are highlighted as well as the aim of this work in chapter 1. The goal of this thesis was the investigation of graphene in electrochemical sensor applications. It holds great promise in terms of miniaturization, improving sensitivity and developing new sensor concepts. Many approaches are already described for biosensor applications, often utilizing graphene in an amperometric detection scheme. Therefore, the impact of preparation technique on the detection of the model analyte H2O2 was investigated, applying different graphene materials as electrode material. Further, graphene was studied as tunable sensor material in gas sensing applications and how it can be customized for the room temperature detection of CO2. Chapter 2 summarizes the author’s publications and patents developed in the frame of this work. The perspectives of graphene in electrochemical sensors were investigated by the means of research performed in this field and are described in chapter 3. The most prominent preparation techniques and the application in biosensor and gas sensor technologies are discussed. Every method provides graphene materials of different characteristics, scalability and further usability. It was shown that defects in the ideal sp2 carbon lattice decide on the sensor performance, but also on device fabrication and appropriate functionalization. A higher quality of graphene can lead to more sensitivity and reliable device production in electrochemical biosensor technologies. In contrast, a defective structure can enhance the sensitivity and applicability in gas sensing applications, providing additional adsorption sites. In chapter 4, the experimental work, performed during this work, is described in detail. Chapter 5 comprises the results on graphene and graphene composite materials applied in the electrochemical detection of H2O2 and as tunable recognition element in gas sensors. In a first part, graphene materials derived by different preparation techniques were studied as electrode material. The electrochemical behavior of the different materials has been investigated as well as the feasibility in device fabrication was compared. It was shown, that the quality of the graphene has an enormous impact on the reductive amperometric detection of H2O2. A defective structure like in reduced graphene oxide leads to almost no significant improvement in signal enhancement compared to a standard carbon disc electrode. In contrast, fewer defects like in graphene prepared by chemical vapor deposition, resulted in a higher sensitivity, which is 50 times better compared to reduced graphene oxide. This technique was found to be most suitable for the production of highly sensitive electrodes to be further used in amperometric detection and development of biosensors. Whereas, a material of high quality is desired in electrochemical biosensor applications, defects are beneficial using graphene as transducer in a chemiresistive setup for the detection of gaseous analytes. In the next part of the work, reduced graphene oxide was demonstrated to be an applicable candidate for gas detection at moderate (85 °C) or even room temperature. Analyte gases like NO2, CH4 and H2 were detected due to fast changes in the electrical resistance at of 85 °C. To overcome the poor selectivity, the material was further altered with octadecylamine, metal nanoparticles such as Pd and Pt, and metal oxides such as MnO2, and TiO2. This changed the sensor response towards the studied gases and the different response patterns for six different materials allowed a clear discrimination of all test gases by pattern recognition based on principal component analysis. Based on the feasibility of this concept, a graphene-based sensor for the room temperature detection of CO2 was developed. Decoration of reduced graphene oxide with CuO nanoparticles led to an improved sensing performance for the target analyte. Different levels of metal oxide doping were applied by wet chemical and electrochemical preparation methods and the resulting composite materials were characterized. It was shown that a complete coverage obtained by wet chemical functionalization leads to highest sensitivity, comparable to a commercial CO2 sensor, which was also tested in the frame of this work. An array consisting of reduced graphene oxide and the composite with CuO nanoparticles was capable to differentiate CO2 from NO2, CO, H2 and CH4. This sensor material can lead to the development of miniaturized chemical sensors comprising high and adjustable sensitivity, which can be applied for monitoring air quality and ventilation management. The presented sensor concept based on customized graphene materials can be tailored for the versatile use in appropriate applications. One of the main challenges remains the reproducible large-scale production of graphene and functionalized graphene combined with reliable transfer techniques in terms of an industrial application. This problem has not been solved completely up to now. But the extensive research going on in this field will lead to a solution in near future and will help graphene to find its way to be integrated into many electrochemical sensor devices. Further studies should preferably aim for large scale production of the material and devices. The use of high quality graphene with a distinct introduction of defects and a better controlled way of functionalization may be a route to tune the material properties in favorable directions

Group 14 and 15 elements as building blocks for low dimensional functional nanostructures

Carbon nanotubes (CNTs) are an interesting allotrope of carbon which can have a wide range of applications as they have extraordinary mechanical, electrical and thermal properties. All this makes CNTs an interesting nanomaterial for different applications ranging from mechanical sensors to electrical microelectrodes and even biological applications due to their biological compatibility. Beside the excellent material properties, the use of special structuring of these materials is of great importance. The random orientation of CNTs cannot be controlled which usually leads to irreproducible material which is not suitable for real world applications. Therefore, the controlled growth of vertically aligned carbon nanotubes (VACNTs) is considered in this work. VACNTs have been grown on Si/SiO2 substrates using a water-assisted chemical vapor deposition technique. By this technique highly crystalline, pure, low-layer multiwalled CNTs with a vertical orientation to the substrate are obtained. The parameters for the growth are optimized and even structuring of the VACNTs is possible obtaining VACNTs with different heights in one synthesis step. This structuring is the used to construct a nano-microstructured artificial-hair-cell-type sensor as an example for a mechanical sensor which can measure three-dimensional forces by the changing contact resistance between neighboring CNT bundles of different heights. Because to the excellent electrical properties together with the highly-increased surface area due to the vertical alignment of VACNTs, they are compared to randomly oriented CNTs for microelectrode applications. In this, the advantage of vertical alignment becomes clear in the dramatic decrease in impedance and enormous increase in capacity. These microelectrodes are then tested for biological applications for which the compatibility and growth pattern of cortical neurons on VACNTs is studied. While CNTs represent one-dimensional systems, also two-dimensional materials related to carbon such as graphene oxide and reduced graphene oxide are studied and used for gas-adsorption as well as liquids absorbents in combination with bacterial cellulose in the form of aerogels. Finally, phosphorene as an example of a two-dimensional material closely related to graphene is synthesized. Phosphorene, although structurally similar to graphene, it has a band gap in contrast to graphene which makes it an interesting material field-effect transistor devices as shown in this work