Preparation of Surface Adsorbed and Impregnated Multi-walled Carbon Nanotube/Nylon-6 Nanofiber Composites and Investigation of their Gas Sensing Ability

We have prepared electrospun Nylon-6 nanofibers via electrospinning, and adsorbed multi-walled carbon nanotubes (MWCNTs) onto the surface of Nylon-6 fibers using Triton® X-100 to form a MWCNTs/Nylon-6 nanofiber composite. The dispersed MWCNTs have been found to be stable in hexafluoroisopropanol for several months without precipitation. A MWCNTs/Nylon-6 nanofiber composite based chemical sensor has demonstrated its responsiveness towards a wide range of solvent vapours at room temperature and only mg quantities of MWCNTs were expended. The large surface area and porous nature of the electrospun Nylon-6/MWCNT nanofibers facilitates greater analyte permeability. The experimental analysis has indicated that the dipole moment, functional group and vapour pressure of the analytes determine the magnitude of the responsiveness

Doctor of Philosophy

dissertationExplosives and drugs cause problems in society when used inappropriately. It is highly desired to detect these chemicals in a quick and reliable way with low cost. Vapor detection of explosives and drugs has been proven to be one of the most effective, practical, and noninvasive methods. Among all the methods developed so far, highly sensitive carbon nanotube-based (CNT-based) chemiresistive sensors remain promising. In this dissertation, we explored and developed three CNT-based sensors for the explosive and drug detection. In this dissertation, we proposed that the dominant mechanism of our oligomer-coated CNT-based sensors is due to the swelling of the oligomers. Based on this swelling mechanism, we have designed three oligomers or polymers functionalized CNT-based sensors for the detection of nitro-explosives, alkanes (related with ammonium nitrate/fuel oil), and amines (related with methamphetamine), respectively. Beyond the high sensitivity to the target analytes, the selectivity of the sensors was largely enhanced by the careful selection of oligomers and polymers. The three oligomers and polymers under investigation can enhance the interaction between the sensor and the analyte, and facilitate the dispersion of CNTs in a solution. For the detection of nitro-explosives, we chose an oligomer that had been successfully demonstrated as a fluorescence-based nitro-explosive sensing materials. For the detection of alkanes and amines, we introduced the alkane side chains and carboxylic acid functional groups in the polymer. This dissertation demonstrated three examples of oligomer or polymer functionalization CNT-based sensors for the detection of explosives and drugs. Meanwhile, the dominant mechanism of the sensors was proposed. This research paved ways for developing chemical vapor sensors with better sensitivity and selectivity in the future

Breakthroughs in the Design of Novel Carbon-Based Metal Oxides Nanocomposites for VOCs Gas Sensing

Nowadays, the detection of volatile organic compounds (VOCs) at trace levels (down to ppb) is feasible by exploiting ultra-sensitive and highly selective chemoresistors, especially in the field of medical diagnosis. By coupling metal oxide semiconductors (MOS e.g., SnO2, ZnO, WO3, CuO, TiO2 and Fe2O3) with innovative carbon-based materials (graphene, graphene oxide, reduced graphene oxide, single-wall and multi-wall carbon nanotubes), outstanding performances in terms of sensitivity, selectivity, limits of detection, response and recovery times towards specific gaseous targets (such as ethanol, acetone, formaldehyde and aromatic compounds) can be easily achieved. Notably, carbonaceous species, highly interconnected to MOS nanoparticles, enhance the sensor responses by (i) increasing the surface area and the pore content, (ii) favoring the electron migration, the transfer efficiency (spillover effect) and gas diffusion rate, (iii) promoting the active sites concomitantly limiting the nanopowders agglomeration; and (iv) forming nano-heterojunctions. Herein, the aim of the present review is to highlight the above-mentioned hybrid features in order to engineer novel flexible, miniaturized and low working temperature sensors, able to detect specific VOC biomarkers of a human's disease

Carbon-based materials for humidity sensing: a short review

Humidity sensors are widespread in many industrial applications, ranging from environmental and meteorological monitoring, soil water content determination in agriculture, air conditioning systems, food quality monitoring, and medical equipment to many other fields. Thus, an accurate and reliable measurement of water content in dierent environments and materials is of paramount importance. Due to their rich surface chemistry and structure designability, carbon materials have become interesting in humidity sensing. In addition, they can be easily miniaturized and applied in flexible electronics. Therefore, this short review aims at providing a survey of recent research dealing with carbonaceous materials used as capacitive and resistive humidity sensors. This work collects some successful examples of devices based on carbon nanotubes, graphene, carbon black, carbon fibers, carbon soot, and more recently, biochar produced from agricultural wastes. The pros and cons of the dierent sensors are also discussed in the present review

Hybrid Materials Based on Carbon Nanotubes and Graphene: Synthesis, Interfacial Processes, and Applications in Chemical Sensing

Development of hybrid nanostructures based on two or more building blocks can significantly expand the complexity and functionality of nanomaterials. For the specific objective of advanced sensing materials, single-walled carbon nanotubes and graphene have been recognized as ideal platforms, because of their unique physical and chemical properties. Other functional building blocks include polymers, metal and metal oxide nanostructures, and each of them has the potential to offer unique advances in the hybrid systems. In any case of constructing hybrid nanostructures, challenges exist in the controlling of composition, morphology and structure of different nanoscale building blocks, as well as the precise placement of these building blocks in the final assembly. Both objectives require systematical exploration of the synthetic conditions. Furthermore, there has been an increasing recognition of the fundamental importance of interface within the nanohybrid systems, which also requires detailed investigation. We have successfully developed several innovative synthetic strategies to regulate the assembly of nanoscale building blocks and to control the morphology of the hybrid systems based on graphitic carbon nanomaterials. We demonstrate the importance of surface chemistry of each building block in these approaches. Moreover, interfacial processes in the hybrid system have been carefully investigated to elucidate their impacts on the functions of the hybrid products. Specifically, we explored the synthesis and characterization of hybrid nanomaterials based on single-walled carbon nanotubes and graphene, with other building blocks including conducting polymers, metal, metal oxide and ceramic nanostructures. We demonstrated the development of core/shell morphology for polyaniline and titanium dioxide functionalized single-walled carbon nanotubes, and we showed a bottom-up synthesis of metal nanostructures that involves directed assembly and nanowelding of metal nanoparticles on the graphitic surfaces. Through electrical, electrochemical and spectroscopic characterizations, we further investigated their surface chemistry, interfacial interaction/processes, as well as their fundamental influence on the performance of the hybrid systems. We showed improved or even synergic properties for each hybrid system. Their chemical sensitivities, material stabilities, and charge separation efficiency were superior to individual components. These properties hold great promise in the real-world sensor applications, and can potentially benefit other research fields such as catalysis and green energy

Design, fabrication and characterisation of gas sensors based on nanohybrid materials

Hoy en día, la necesidad de monitorizar y controlar el medio ambiente es a cada vezmás importante debido al creciente nivel de gases tóxicos que provienen de la expansiónde las actividades industriales, amenazando así el medio ambiente y la salud humana. Eldesarrollo de la nano-tecnología ha permitido fabricar sensores de gases portables,altamente sensibles, selectivos, de bajo coste y de bajo consumo de potencia.Los nanotubos de carbono (NTC) están ganando un interés a cada vez más considerablepor parte de la comunidad científica debido a su geometría y morfología únicas y susexcelentes propiedades electrónicas, mecánicas, térmicas i ópticas. Esto hace de ellosunos candidatos prometedores para un amplio rango de aplicaciones como por ejemplonuevos sensores de gases con propiedades mejoradas. En este contexto, mediante lapresente tesis, se ha realizado un profundo estudio para explorar las propiedades dediferentes sensores basados en nano-materiales híbridos constituidos por nanotubos decarbono junto a otros materiales con el fin de detectar gases tóxicos de manera eficiente.El trabajo realizado consistió en el diseño, la fabricación, la caracterización, y laoptimización de nanosensores híbridos.Esta tesis fue financiada en el marco del proyecto Europeo "Nano2hybrids", cuyoobjetivo era de diseñar la interfaz de las nano-partículas del metal con los nanotubos decarbono a través del control de los defectos estructurales y químicos producidos por ladescarga de un plasma de radiofrecuencia y aplicarlo a la detección de gases. Elbenceno fue elegido como gas principal, debido a sus graves efectos tóxicos a niveles depocas ppb y también debido a la no existencia en el mercado de un detector de bajocoste para benceno. De hecho, no hay en el estado de la técnica, un sensor de gas quepuede detectar de forma selectiva este gas a nivel operativo de ppb y trabajando atemperatura ambiente. Así, el reto de esta tesis era obtener un sensor altamente sensible,selectivo y estable, portátil y de bajo coste para la detección de benceno.En este sentido, se estudiaron exhaustivamente diferentes materiales basados ennanotubos de carbono funcionalizados, decorados con nanopartículas de metal o biendecorados o mezclados con óxidos metálicos, en términos de su adecuación para ladetección de gases (por ejemplo, sus sensibilidad, selectividad, estabilidad, y elmecanismo de detección, etc.). En particular se estudió la detección de diferentes gasescomo (benceno (C6H6 ), monóxido de carbono (CO), dióxido de nitrógeno (NO2), eletileno (C2H4), el sulfuro de hidrógeno (H2S), amoníaco (NH3) y agua (H2O)). Nuestrastareas consistieron en investigar experimentalmente y teóricamente el efecto de lascondiciones de preparación de los materiales (p.e. el tratamiento con plasma, lanaturaleza del precursor y tamaño de las nanoparticulas de metales), fabricación delsensor (p.e., técnica de deposición, el efecto del tipo de metal del los electrodos delsensor), y de las condiciones de caracterización del sensor (p.e., temperatura deoperación, flujo de gas,) sobre las propiedades sensoras de los mismos. Todo ello hapermitido adquirir conocimientos, explicar los mecanismos de funcionamiento en elsensado de gases de los diferentes materiales investigados y con ello desarrollar unsensor de gases adecuado para la detección de benceno.Hemos encontrado que los materiales híbridos que consisten en nanotubos tratados conplasma de oxígeno y decorados con diferentes nanopartículas de metal, muestran unamayor capacitad de detección a temperatura ambiente respecto a los nanotubos decarbono en bruto o los funcionalizados sólo con plasma. Las propiedades interfacialesde los materiales híbridos resultantes pueden ser adaptadas, lo que ofrece una enormeflexibilidad para el ajuste de sus propiedades sensoras. Cuando se combinaron en unamatriz de micro-sensores que opera a temperatura ambiente, nanotubos decorados condiferentes metales, de forma que unos resulten sensibles al benceno y otros insensibles,esto permitió por primera vez la realización de un prototipo de bajo coste capaz dedetectar selectivamente y a temperatura ambiente el benceno presente a nivel de trazas(por debajo de 50 ppbs) en una mezcla de gases. El prototipo realizado presenta unostiempos de respuesta y de recuperación de 60 s y 10 minutos respectivamente además deuna buena estabilidad y reproducibilidad. Este prototipo se encuentra protegido por unapatente que ha sido licenciada a una compañía que se encargará de la comercializaciónindustrial del producto.In the last few years, there has been a growing demand for monitoring the environment,especially with the increasing concern by the release of toxic gases emitted by manmadeactivities. The development of nanotechnology has created a huge potential for buildinghighly sensitive, selective, low cost, and portable gas sensors with low powerconsumption.Nowadays, carbon nanotubes are receiving an intense interest from the scientificcommunity, due to their unique geometry, morphology, electronic, mechanical, thermaland optical properties, which make them a promising candidate for many industrialapplications including new gas sensors for the detection of toxic species. In this context,in this thesis a deep study is devoted to explore the sensing properties of differenthybrid nanomaterials based on carbon nanotubes for an efficient detection of toxicgases. The design, fabrication, characterization, and optimization of gas sensors usinghybrid materials have been carried out.This thesis was financially supported by the European project "Nano2hybrids", whichexploits the interface design of metal nanocluster-carbon nanotube hybrids via controlof structural and chemical defects in a plasma discharge, for designing gas sensors withsuperior performance. Benzene was chosen as the principal target gas due to its serioustoxic effects at low ppb levels and the fact that there are no reliable, low cost andselective benzene detectors in the market. In fact, no gas sensor able to selectivelydetect this gas at ppb levels and operating at ambient temperature has been reported upto now in the literature. So, the challenge of the project was to fabricate sensitive,highly selective, stable, portable, and low cost benzene gas sensor employing hybridnanomaterials.Herein, functionalized MWCNTs, metal decorated MWCNTs, and metal oxidedecorated MWCNTs or metal oxide and MWCNT mixtures were deeply investigated interms of their gas sensing performances (e.g, sensitivity, selectivity, stability, detectionmechanism,. etc) towards the detection of different gases (benzene (C6H6), carbonmonoxide (CO), nitrogen dioxide (NO2), ethylene (C2H4), hydrogen sulfide (H2S),ammonia (NH3), and water (H2O)). Our tasks were to investigate experimentally andtheoretically the effects of material preparation conditions (e.g., plasma treatment,nanocluster precursor and size), sensor fabrication (e.g., deposition technique,electrodes sensor metal), and sensor characterization conditions (e.g., operatingtemperature, gas flow) on the gas sensing properties of our devices, and to acquireknowledge in order to develop a selective benzene detector. Based on experimental andtheoretical results, different mechanisms for the interaction between gases and thehybrid materials tested have been proposed.We found that hybrid materials consisting of oxygen plasma treated multiwalled carbonnanotubes decorated with different metal nanoparticles showed room temperaturesensing capability. Responsiveness to gases of these hybrid materials was higher thanthat of pristine or plasma functionalized carbon nanotubes. Metal decoated CNTs can betailored for the recognition of different gases and vapors with different reactivities,which offers enormous flexibility for tuning the interfacial properties of the resultinghybrid materials and thus, of their sensing properties. When combined in a microsensorarray operating at room temperature, the use of benzene-sensitive and benzeneinsensitivemetal-decorated multiwalled carbon nanotubes, allowed for the first time theimplementation of a low cost detector prototype, which can selectively detect benzenewhen present at trace levels (below 50 ppb) in a gas mixture. Sensors present responseand recovery times of 60 s and 10 min respectively, good stability and reproducibility.This type of sensors are protected by a patent, and licensed to a company for industrialcommercialization

Highly-selective Chemiresistive Sensing and Analysis of Vapors Using Functionalized Nanotubes

Specifically, the project involves the development of a diversified array of nanostructured gas-sensors comprised of selectively, novel surface-functionalized carbon nanotubes (for analyte selectivity by virtue of functionality). Harnessing carbon nanotubes with various electron withdrawing and donating groups help in determining their affinity toward certain prognostic gaseous markers thus increasing specificity of such created sensors. We have devised synthetic routes that have led to the facile production of covalently polyfunctionalized nanotubes in high yield. Seven carbon nanotube analogues were systematically considered and then chemically synthesized, from pristine single-walled nanotubes (SWNT\u27s), for use as the main component of sensory units that was used for this study. The basic chemical structure of these functionalized nanotubes; namely: poly(p-phenol)-co-SWNT [1], poly(p-nitrobenzene)-co-SWNT [2], poly(p-fluorobenzene)-co-SWNT [3], poly(p-aniline)-co-SWNT [4], polybromide-SWNT [5] , poly(p-thiophenol-co-SWNT [6] and poly(p-benzonitrile-co-SWNT [7]. The ability to manufacture total organic sensors was demonstrated using carbon nanotube based architectures. These derivatized-nanotube-based materials are designed to serve as chemoreceptors that can facilitate the development of highly selective and sensitive chemical and biological sensor arrays through an electronic nose approach which mimics the mammalian olfactory system. Functionalized SWNTs (f-SWNTs) were dispersed in dimethyl formamide (DMF) and mCresol and spun-applied to the interdigitated regions of micro-lithographically fabricated, pre-cleaned interdigitated microsensor electrodes (IME 1025-M-Pt and Au). Measured changes in the electrical conductivities of an array of gas sensors upon exposure to selected vapors and inert explosive materials were monitored. These changes are transduced into electrical signals, which are preprocessed and conditioned before identification by a pattern recognition system. Preliminary chemisensory was conducted on four signature vapor components of RDX explosive. Sensor data from these individual detection methods was assessed by their own individual merits, after which they were amalgamate and reclassified to present each vapor as a unique data point on a 2-dimensional map and with a minimum loss of information. Extensive characterizations on the properties of these materials were carried out using various spectroscopic and electrical techniques to assess the usefulness of functionalized single-walled carbon nanotubes. It was found that the conductivity of two functionalized materials (poly(p-aniline)-co-SWNT [4] and polybromide-SWNT [5] ) were more conductive than the pristine SWNT. The development of consistent and successful functionalization techniques that allows for the construction of CNTs-based species of great usefulness, reversibility and selectivity for the use as sensing element, can be a challenge. We have demonstrated a proof-of-concept by exploring and using the functionalized carbon nanotubes for use as gas sensors, through the utilization of a stochastic fingerprinting methodology. However, further studies into electronic and electrochemical detection methods will provide more unique systems for R&D on the applicability of these materials to future technology

A Parametric study of gas sensing response of ZnO nanostructures and carbon nanotubes

Solid state chemical sensors are gaining popularity and finding extensive use in process control, environmental monitoring and residential safety. ZnO, a semiconducting metal oxide, and carbon nanotubes (CNTs) have attracted great interest over the years for their sensitivity to a variety of gases. Nanostructured sensing materials, such as nanowires, nanotubes and quantum dots offer an inherently high surface area, thus reducing operating temperatures and increasing sensitivity to low concentrations of analytes. In this work, ZnO nano-structures and CNTs have been tested as chemical sensors and a detailed study on the effect of different process parameters such as temperature, carrier gas flow, inter-electrode spacing, gas concentration and material properties on gas sensitivity is presented. Initial ZnO nanoparticles were prepared by a simple solution chemical process and characterized by Secondary Electron Microscopy (SEM) and Brunauer, Emmet and Teller (BET) Sorptometer to demonstrate the morphology and surface area respectively. The gas sensor platforms consisted of Pt inter-digitated fingers with a spacing of 10 μm. The sensor platform was dip-coated with ZnO nano-platelets suspended in terpineol to form a uniform film. Sensing was performed in a closed quartz chamber where, high purity N2 and dry industrial air were used as carrier and recovery gas respectively. Sensitivity of nano-platelets and porous films was measured for different concentrations of the analyte (H2). High response was observed at room temperature for H2 gas with sensitivities in excess 80% for 60ppm and about 55% for 80ppm of H2 gas at room temperature was observed for the nano-platelets and the porous films respectively. High sensitivity of the sensor at low temperatures is attributed to both the increased surface area of the porous ZnO nano-platelets and the presence of a Pt catalyst. Measurements at higher temperatures (150 °C) show even higher sensitivities, near 96% for a 20 ppm H2 concentration. Sensitivity with different gases and organic solvents was also measured at operating temperatures of 200oC. Values on the order of 60%, 42% and 29% for 315 PPM of CO, O2 and NH3 whereas sensitivity values of 77.76%, 70.26% and 38.43% for C2H5OH, CH3OH and H2O were recorded for concentration values approximating 500 PPM. The sensors depict incomplete recovery of resistance at room temperature. This effect is possibly due to the traces of elemental Zn in the material, which were not oxidized at the time of recovery. However, this effect was not observed at higher temperatures. Designed experiments conducted to understand effects of various device and process parameters show negative dependence of spacing on sensitivity with maximum effect of applied bias for lower concentration values. The sensitivity of the sensor was also recorded to increase with the increase in the number of electrodes. Higher sensitivity values nearing 70% were achieved with 30 IDEs for 60 PPM H2 when compared to 60% for 60 PPM of H2 with 20 IDEs. Interaction effects were observed and implemented to understand and model the behavior of the gas sensor. Sensitivity of arc produced CNTs was measured to various gases and organic solvents. Values on the order of 24% were observed at 80 PPM CO as compared to values of sensitivity about 15% for O2 and 3% for H2. Also, sensitivity value of 15% was measured for as low as 4 PPM of DMA which suggests the capability of PPB levels of DMA using CNTs. A brief comparison of sensitivity values achieved for ZnO nano-platelets and CNTs with similar analytes was also presented. Sensitivity to different analytes was measured using impedance spectroscopy for HiPCo produced SWCNT network. For experiments conducted with varying exposure time, sensitivity values nearing 6% for 0.01% (100 PPM) DMA for an exposure time of 25 minutes were recorded. Sensitivity values recorded for other solvents were 16.74%, 10.98%, 7.97%, 6.96% and 4.28% for concentration levels of 2.04%, 4.02%, 2.04%, 14% and 6.05% of NH3, IPA, CO, CH3OH and C2H5OH respectively. For experiments with varying concentration values of different analytes, higher response was observed for gaseous analytes. Results on the order of 15.27% and 3.82% were recorded for as low as 0.18% of both NH3 and CO. For the organic solvents, values approximating 2.64%, 2.36% and 0.10% for concentration levels of 0.29%, 0.92% and 0.42% of IPA, CH3OH and C2H5OH respectively. Results obtained with HiPCo produced SWCNT network at room temperature were comparable to the values of sensitivity shown by other researchers. Our future works entails correlating the sensitivity of the gas sensors to the material properties in addition to the device and the process parameters, with further development in methods for fabricating gas sensors and improvement in the selectivity of the sensor. For CNT based sensors, using as-grown multiwall carbon-nanotubes MWCNTs for gas sensor fabrication would be the next step in this research. In addition to developing standard fabrication techniques, further research is required for improving selectivity for different gases and organic solvents by decorating or filling CNTs with metal nano-particles or different groups of organic molecules. Also, future work will be focused to correlate sensitivity of HiPCo produced SWCNTs, Laser ablated SWCNTs and MWCNTs to their material properties

Advances in nanomaterials integration in CMOS-based electrochemical sensors: a review

The monolithic integration of electrochemical sensors with instrumentation electronics on semiconductor technology is a promising approach to achieve sensor scalability, miniaturization and increased signal to noise ratio. Such an integration requires post-process modification of microchips (or wafers) fabricated in standard semiconductor technology (e.g. CMOS) to develop sensitive and selective sensing electrodes. This review focuses on the post-process fabrication techniques for addition of nanomaterials to the electrode surface, a key component in the construction of electrochemical sensors that has been widely used to achieve surface reactivity and sensitivity. Several CMOS-compatible techniques are summarized and discussed in this review for the deposition of nanomaterials such as gold, platinum, carbon nanotubes, polymers and metal oxide/nitride nanoparticles. These techniques include electroless deposition, electro-chemical deposition, lift-off, micro-spotting, dip-pen lithography, physical adsorption, self-assembly and hydrothermal methods. Finally, the review is concluded and summarized by stating the advantages and disadvantages of these deposition methods