Screen-printed single-walled carbon nanotube networks and their use for dimethyl methylphosphonate detection

Single-walled carbon nanotube (SWNT) films were prepared on silicon/silica substrates by screen-printed technique at a wafer scale, and their sensing properties to dimethyl methylphosphonate (DMMP) were studied. The SWNT networks were characterized by field-emission scanning electron microscope. The resistance responses to different concentrations of DMMP vapors were investigated at room temperature. The results showed that the resistance changes of the screen-printed SWNT films increased rapidly in varying concentrations ranging from 20 to 200?ppm. The sensor exhibited high resistance responses, good reproducibility and excellent long-term stability for DMMP vapor detection. The screen-printed SWNT networks would be potentially extended to large-scale, low cost and simple manufacturing sensor applications.     

Hole doping and surface functionalization of single-walled carbon nanotube chemiresistive sensors for ultrasensitive and highly selective organophosphor vapor detection

We developed a chemiresistive sensor based on doped and functionalized semiconducting single-walled carbon nanotube (SWNT) networks for ultrasensitive and rapid detection of dimethyl methylphosphonate (DMMP) (simulant of nerve agent sarin) vapor. The semiconducting SWNT network was deposited between interdigitated electrodes and modified by solid organic acid tetrafluorohydroquinone (TFQ). The TFQ molecules could not only selectively bind DMMP onto the sidewalls of SWNTs via the strong hydrogen bonding interaction, but also tailor the electronic properties of SWNTs via heavy hole doping. This synergetic effect significantly improved the sensitivity of the devices, and enabled the sensors to easily detect DMMP at 20 parts-per-trillion (ppt) concentration with a response time of less than 2 min, without the need for pre-concentration of the analytes. This sensitivity is about five orders of magnitude higher than that of the unmodified SWNT chemiresistor, and also significantly higher than that of the functionalized SWNT chemiresistors previously reported. Moreover, the SWNT-TFQ sensors could be recovered when DMMP is replaced with referencing gas. The SWNT-TFQ sensors also show excellent selectivity toward DMMP over some interfering organic vapors. The response mechanism, i.e. charge transfer and dedoping was investigated.     

Flexible carbon nanotube sensors for nerve agent simulants

Chemiresistor-based vapour sensors made from network films of single-walled carbon nanotube (SWNT) bundles on flexible plastic substrates (polyethylene terephthalate, PET) can be used to detect chemical warfare agent simulants for the nerve agents Sarin (diisopropyl methylphosphonate, DIMP) and Soman (dimethyl methylphosphonate, DMMP). Large, reproducible resistance changes (75-150%), are observed upon exposure to DIMP or DMMP vapours, and concentrations as low as 25?ppm can be detected. Robust sensor response to simulant vapours is observed even in the presence of large equilibrium concentrations of interferent vapours commonly found in battle-space environments, such as hexane, xylene and water (10?000?ppm each), suggesting that both DIMP and DMMP vapours are capable of selectively displacing other vapours from the walls of the SWNTs. Response to these interferent vapours can be effectively filtered out by using a 2?μm thick barrier film of the chemoselective polymer polyisobutylene (PIB) on the SWNT surface. These network films are composed of a 1-2?μm thick non-woven mesh of SWNT bundles (15-30?nm diameter), whose sensor response is qualitatively and quantitatively different from previous studies on individual SWNTs, or a network of individual SWNTs, suggesting that vapour sorption at interbundle sites could be playing an important role. This study also shows that the line patterning method used in device fabrication to obtain any desired pattern of films of SWNTs on flexible substrates can be used to rapidly screen simulants at high concentrations before developing more complicated sensor systems.     

A new polysiloxane coating on QCM sensor for DMMP vapor detection

A new material-poly{methyl [3-(2-hydroxyl, 4,6-bistrifluoromethyl)phenyl]propylsiloxane} (PMTFMPS), which was sensitive to toxic organophosphate vapor, was synthesized via O-alkylation, claisen rearrange reaction and hydrosilylation reaction. The polymer was coated on quartz crystal microbalance (QCM) to investigate its gas sensitive properties to nerve agents’ simulant dimethyl methylphosphonate (DMMP) vapor, as well as other interfering vapors. It was found that QCM sensor responded linearly to DMMP vapor with a slope of 27 Hz/ppm in the 10–50 ppm range. The material was much more sensitive to DMMP than to other interfering vapors, thus high selectivity to DMMP was demonstrated. The influence of humidity on the sensor response was also examined. The results showed that the frequency shifts were about 60% when tested in 77% RH wet air than in dry N₂. When compared with our previously studied unfluorinated phenol-modified siloxane PMPS, PMTFMPS exhibited sensitivity enhancement of 2.3 times and an increased resistance to humidity variations.     

Undoped, nitrogen-doped and boron-doped multiwalled carbon nanotube/poly(vinyl alcohol) composite as active layer in simple hydrostatic pressure sensors

We report on hydrostatic pressure sensors prepared using 10 % w/w multiwalled carbon nanotube (MWCNT)/poly(vinyl alcohol) composites. Three types of carbon nanotubes were used in the composites: undoped MWCNTs, nitrogen-doped MWCNTs and boron-doped MWCNTs (B-MWCNTs). The sensor response was tested using an alternating current input, measuring the capacitance and conductance outputs. The sensors based on the three composites show a linear capacitance and conductance sensitivity pressure dependence in the range 50–120 kPa, but a higher sensitivity to pressure dependence above this interval. The highest angular coefficient of the sensitivity, which reached 0.092 kPa^?1, was observed for the capacitance sensitivity of the B-MWCNT based composite.     

Intrinsic optical-fiber sensor for nerve agent sensing

A novel chemical-sensing technique to detect the nerve agent sarin stimulant dimethylmethylphosphonate (DMMP) is presented. This technique uses a combination of doped polypyrrole as an active chemical material coated on an optical fiber to form an intrinsic fiber-optic sensor. Sensitivity of up to 26 ppm of DMMP with response time of a few seconds is demonstrated. Influence of three different dopants, i.e., 1,5 naphthalene disulphonic acid, anthraquinone 2 sulphonic acid, and hydrochloric acid is investigated for sensor response and sensitivity. Two polymer processing techniques, i.e., in situ deposition and monomer vapor phase deposition is investigated for optimal polypyrrole morphology for DMMP sensitivity. The influence of substrate nature, i.e., hydrophilic and hydrophobic, on sensor sensitivity is studied. Organophosphate specific binding sites have been created in polypyrrole structure using Cu/sup 2+/ ions to enhance DMMP response. The selectivity issue is addressed by testing the sensor in the presence of other gases like ammonia, water vapor, and acetone which influence the electronic properties of polypyrrole.     

Rational design of a Nile Red/polymer composite film for fluorescence sensing of organophosphonate vapors using hydrogen bond acidic polymers

The solvatochromic dye Nile Red dispersed in selected hydrogen bond acidic polymer matrixes demonstrated strong fluorescence enhancement at the presence of dimethyl methylphosphonate (DMMP) vapors. Two hydrogen bond acidic polymers were examined as dye matrixes, one with fluorinated alcohol groups on a polystyrene backbone (PSFA) and the other with fluorinated bisphenol groups alternating with oligo(dimethylsiloxane) segments (BSP3). The combination of hydrogen bond acidic polymer (a strong sorbent for DMMP) with the solvatochromic dye led to initial depression of the dye fluorescence and a significant red shift in the absorbance and fluorescence spectra. DMMP sorption changed the dye environment and dramatically altered the fluorescence spectrum and intensity, resulting in a strong fluorescence enhancement. It is proposed that this fluorescence enhancement is due to the competition set up between the dye and the sorbed vapor for polymeric hydrogen-bonding sites. The highest responses were obtained with BSP3. DMMP detection has been demonstrated at sub-ppm DMMP concentrations, indicating very low detection limits compared to previous Nile Red/polymer matrix fluorescence vapor sensors. Nile Red/poly(methyl methacrylate) films prepared for comparisons exhibited substantially lower response to DMMP. Rational selection of polymers providing high sorption for DMMP and competition for hydrogen-bonding interactions with Nile Red yielded flourescent films with high sensitivity.     

Electrochemical co-deposition and characterization of MnO2/SWNT composite for supercapacitor application

Manganese oxide/single-wall carbon nanotubes (MnO₂/SWNT) composite was co-deposited by the potentiostatic method on a graphite slice. Morphological and structural performances for MnO₂/SWNT composite were characterized by means of scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The wall surface of SWNT was wrapped by ramsdellite MnO₂ nanoparticles to fabricate MnO₂/SWNT coaxial nanotubes, which further interconnected other MnO₂ particles to form the porous MnO₂/SWNT composite. The electrochemical properties were examined by cyclic voltammograms, galvanostatic charge and discharge and electrochemical impedance spectrum. A high specific capacitance of 421 F g^?1 was obtained for overall MnO₂/SWNT composite electrode at the constant current density of 1 A g^?1 in 3 mol L^?1 KCl solution.     

Detection of a nerve agent simulant using single-walled carbon nanotube networks: dimethyl-methyl-phosphonate

Single-walled carbon nanotube (SWNT) networks were used to detect hazardous dimethyl-methyl-phosphonate (DMMP) gas in real time, employing two different metals as electrodes. Random networks of SWNTs were simply obtained by drop-casting a SWNT-containing solution onto a surface-oxidized Si substrate. Although the electrical responses to DMMP at room temperature were reversible for both metals, the Pd-contacting SWNT network sensors exhibited a higher response and a shorter response time than those of the Au-contacting SWNT network sensors at the same DMMP concentration, due to the stronger interactions between the SWNTs and Pd surface atoms. In Pd-contacting SWNT network sensors, the response increased linearly with increasing DMMP concentration and reproducible response curves were obtained for DMMP levels as low as 1 ppm. These results indicate that SWNT networks in contact with Pd electrodes can function as good DMMP sensors at room temperature with scalable and fast response and excellent recovery.     

Influence of pyrolytic temperature on the properties of ZnO films optimized for H2 sensing application

In this work, we report the influence of pyrolytic temperature on the properties of ZnO films deposited by a novel spray pyrolysis deposition route. XRD results revealed an improvement in crystal quality of the films with increase in growth temperature. The optical measurements of the films show a maximum transmittance of ~85 % and the band gap of ~3.5 eV. Photoluminescence spectra revealed that the UV emission peaks at 385 nm is improved with increase in growth temperature upto 300 °C, which corresponds to the increase of optical quality and decrease of Zn interstitial defect in the films. Gold ohmic contacts were evaporated on the optimized ZnO film prepared at the substrate temperature of 300 °C, and response of the film to different concentrations of hydrogen (150–500 ppm) at room temperature was investigated. The ZnO sensor showed significant sensitivity to hydrogen for concentration as low as 150 ppm at room temperature, and the sensor response was observed to increase with increase in hydrogen concentration. The increased sensitivity of the film was attributed to the large roughness of the film revealed from AFM analysis. The results ensure the application of our novel sensor, to detect H₂ at low concentration and at room temperature.     

Role of defects in single-walled carbon nanotube chemical sensors

We explore the electronic response of single-walled carbon nanotubes (SWNT) to trace levels of chemical vapors. We find adsorption at defect sites produces a large electronic response that dominates the SWNT capacitance and conductance sensitivity. This large response results from increased adsorbate binding energy and charge transfer at defect sites. Finally, we demonstrate controlled introduction of oxidation defects can be used to enhance sensitivity of a SWNT network sensor to a variety of chemical vapors.     

Enhanced acetone detection using Au doped ZnO thin film sensor

Zinc oxide (ZnO) thin films are prepared using sol–gel method for acetone vapor sensing. Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O) was taken as starting material and a stable and homogeneous solution was prepared in ethanol by deliquescing the zinc acetate and distinct amount of monoethanolamine as a stabilizing agent. The prepared solution was then coated on silicon substrates by spin coating method and then annealed at 650 °C for preparing ZnO thin films. The thickness of the film was maintained at 410 nm. The structural, morphological and optical studies were done for the synthesized ZnO thin films. The operating temperature and sensor response is considered to be an important parameter for the gas sensing behavior of any material. Therefore, the present study examined the effect of sensing behavior of 3% v/v gold (Au) doped ZnO thin films as a sensor. The response characteristics of 410 nm ZnO thin film for temperature ranging from 180 to 360 °C were determined for the acetone vapors. The reported study provides a significant development towards acetone sensors, where a very high sensitivity with rapid response and recovery times are reported with lowered optimal operating temperature as compared to bare ZnO nano-chains like structured thin films. In comparison to the bare ZnO thin films giving a response of 63 at an operating temperature of 320 °C, a much better response of 132.3 was observed for the Au doped ZnO thin films at an optimised operating temperature of 280 °C for a concentration of 500 ppm of acetone vapors.     

Nanoporous carbon materials with enhanced supercapacitance performance and non-aromatic chemical sensing with C1/C2 alcohol discrimination

We have investigated the textural properties, electrochemical supercapacitances and vapor sensing performances of bamboo-derived nanoporous carbon materials (NCM). Bamboo, an abundant natural biomaterial, was chemically activated with phosphoric acid at 400 °C and the effect of impregnation ratio of phosphoric acid on the textural properties and electrochemical performances was systematically investigated. Fourier transform-infrared (FTIR) spectroscopy confirmed the presence of various oxygen-containing surface functional groups (i.e. carboxyl, carboxylate, carbonyl and phenolic groups) in NCM. The prepared NCM are amorphous in nature and contain hierarchical micropores and mesopores. Surface areas and pore volumes were found in the range 218–1431 m² g^?1 and 0.26–1.26 cm³ g^?1, respectively, and could be controlled by adjusting the impregnation ratio of phosphoric acid and bamboo cane powder. NCM exhibited electrical double-layer supercapacitor behavior giving a high specific capacitance of c.256 F g^?1 at a scan rate of 5 mV s^?1 together with high cyclic stability with capacitance retention of about 92.6% after 1000 cycles. Furthermore, NCM exhibited excellent vapor sensing performance with high sensitivity for non-aromatic chemicals such as acetic acid. The system would be useful to discriminate C₁ and C₂ alcohol (methanol and ethanol).     

Fast response time alcohol gas sensor using nanocrystalline F-doped SnO2 films derived via sol–gel method

Pure and fluorine-modified tin oxide (SnO₂) thin films (250–300 nm) were uniformly deposited on corning glass substrate using sol–gel technique to fabricate SnO₂-based resistive sensors for ethanol detection. The characteristic properties of the multicoatings have been investigated, including their electrical conductivity and optical transparency in visible IR range. Pure SnO₂ films exhibited a visible transmission of 90% compared with F-doped films (80% for low doping and 60% for high doping). F-doped SnO₂ films exhibited lower resistivity (0· 12 × 10^???4 Ω  cm) compared with the pure (14·16 × 10^???4 Ω  cm) one. X-ray diffraction and scanning electron microscopy techniques were used to analyse the structure and surface morphology of the prepared films. Resistance change was studied at different temperatures (523–623 K) with metallic contacts of silver in air and in presence of different ethanol vapour concentrations. Comparative gas-sensing results revealed that the prepared F-doped SnO₂ sensor exhibited the lowest response and recovery times of 10 and 13 s, respectively whereas that of pure SnO₂ gas sensor, 32 and 65 s, respectively. The maximum sensitivities of both gas sensors were obtained at 623 K.     

Vapor sensing performances of PVDF nanocomposites containing titanium dioxide nanotubes decorated multi-walled carbon nanotubes

Inorganic nanocarbon hybrid materials are good alternatives for superior electrochemical performance and specific capacitance to their traditional counterparts. Nanocarbons act as a good template for the growth of metal nanoparticles on it and their hybrid combinations enhance the charge transport and rate capability of electrochemical materials without sacrificing the specific capacity. In this study, titanium dioxide nanotubes (TNT) are synthesized hydrothermally in the presence of multi-walled carbon nanotubes (MWCNT) where the latter acts as base template material for the metal oxide nanotube growth. The MWCNT–TNT hybrid material possesses very high dielectric strength and this is used to enhance the dielectric property of the polymer polyvinyledene fluoride (PVDF). Solution mixing was used to prepare the PVDF/MWCNT–TNT nanocomposites by varying the filler concentrations from 0.5 to 2.5 wt%. Excellent vapor sensing was noticed for the PVDF nanocomposites with different rate of response towards commonly used laboratory solvents. The composites and the fillers were characterized for its morphology and structural properties using scanning and transmission electron microscopy, X-ray diffraction studies and infrared spectroscopy. Vapor sensing was measured as relative resistance variations against the solvent vapors, and the dielectric properties of the composites were measured at room temperature during the frequency 10²–10⁷ Hz. Experimental results revealed the influence of filler synergy on the properties of PVDF and the enhancement in the solvent vapor detectability and dielectric properties reflects the ability of these composite films in flexible vapor sensors and in energy storage.     

Crosslinking with ammonium zirconium carbonate improves the formation and properties of spruce galactoglucomannan films

Spruce (Picea abies) O-acetyl-galactoglucomannans (GGMs), low-value by-products from the forestry industry were upgraded to sustainable film-forming materials by crosslinking with ammonium zirconium carbonate (AZC). The purpose of crosslinking was to enhance the film formation, reduce the need of polyol plasticizers, and decrease the sensitivity of the film properties to moisture. Tensile testing showed that AZC-crosslinked GGM can be used to prepare strong and stiff films, with tensile strength up to 52 MPa and Young’s modulus of 4.7 GPa. Dynamic mechanical analysis, performed as a function of relative humidity (RH), showed that AZC-crosslinked GGM films retained their stiffness at higher RH than the reference films without AZC. Water vapor sorption and permeability analyses were done to further study the effect of moisture on the film properties, and those showed that the effect of sorbitol as a plasticizer depended greatly on RH. The oxygen permeability of the AZC-crosslinked GGM films was in the range of 4–11 [cm³ μm/(m² day kPa)]. GGM films could offer a bio-based and biodegradable alternative to existing synthetic oxygen barrier materials, on the condition that they are protected from the effects of moisture, e.g., by hydrophobic laminated layers.     

A low-cost method for producing high-performance nanocomposite thin-films made from silica and CNTs on cellulose substrates

We show that thin films of silica loaded with 22 wt% of carbon nanotubes (CNTs) can be deposited on cellulose substrate via the sol–gel route by a well-controlled process. The high loadings are obtained by airbrush spraying of a diluted sol solution (which contained a much smaller concentration of CNTs) followed by drying at 200 °C. The films are nearly continuous despite the fibrous structure of the substrate. The high degree of connectivity of the stranded structure of the CNTs yields a specific electrical conductivity of 3 × 10³ Ω⁻¹ m⁻¹. In contrast, films made with high loadings of carbon black have poor electrical conductivity. Results from mechanical tensile tests of samples are also reported. This economical method of producing CNT dispersed thin films could find application in catalysis, as electrodes in fuel cells and batteries, and in sensor technologies.     

Effect of rf power on the structural properties of indium tin oxide thin film prepared for application in hydrogen gas sensor

Indium tin oxide films were grown on glass substrate by rf magnetron sputtering at 648 K. Influence of rf power on structural properties of the ITO films was studied. XRD measurements showed (222) preferred orientation under the optimized deposition conditions. The surface morphology of ITO films analyzed by scanning electron microscope appears to be uniform over the entire surface area, the film exhibited dense layers with fine grains. Finally, ITO sensor device was fabricated and the sensing properties of the device towards hydrogen gas were investigated. The variation in sensitivity of the ITO sensor with operating temperature and with concentration of hydrogen gas was studied. The maximum response was found to be 1.6 at 400 K, for 1,000 ppm of hydrogen gas, and the response of the sensor was found to decrease with increase in concentration of H₂ gas.     

Effects of active chitosan-flaxseed mucilage-based films on the preservation of minced trout fillets: A comparison among aerobic,vacuum, and modified atmosphere packaging

The objectives of the current study were to examine physical–mechanical, structural, and morphological characteristics of chitosan-flaxseed mucilage films enriched with Ziziphora clinopodioides essential oil (ZEO; 0%, 0.25%, and 0.5%) and sesame oil (SO; 0% and 0.75%) and to provide useful information for the preservation of minced trout fillet's using antimicrobial films under aerobic, vacuum, and modified atmosphere conditions for 16 days. The films showed thickness, tensile strength, puncture force, puncture deformation, water vapor transmission rate, water vapor permeability, swelling index, and oxygen permeability values ranging 0.082–0.86 mm, 33.34–46.83 MPa, 25.69–53.08 N, 11.45–28.45 mm, 17.48–26.73 g/m² h, 8.57–12.49 × 10⁻⁴ g mm/m² h Pa, 12.45–38.43%, and 3.02–13.32 × 10⁻¹² cm³/m² s Pa, respectively. The following order of effect on the microbial spoilage population of treated samples was found in the applied packaging methods: modified atmosphere packaging > vacuum packaging > aerobic packaging. The final microbial population of treated samples was 0.35–4.91 log CFU/g lower than the controls after 16 days of refrigerated storage. At the end of the storage, the total volatile base nitrogen, peroxide value, and thiobarbituric acid reactive substances of untreated samples were 34.02–48.6 mg of N/100 g, 1.43–2.32 meq of peroxide/1000 g, and 3.33–4.24 mg of malondialdehyde/kg, respectively. The lowest corresponding values were recorded for the treated samples with ZEO 0.5% + SO 0.75% films by 14.26–17.73 mg of N/100 g, 0.48–0.86 meq of peroxide/1000 g, and 1.08–1.48 mg of malondialdehyde/kg, respectively.     

MWCNT-polymer composites as highly sensitive and selective room temperature gas sensors

Multi-walled carbon nanotubes (MWCNTs)-polymer composite-based hybrid sensors were fabricated and integrated into a resistive sensor design for gas sensing applications. Thin films of MWCNTs were grown onto Si/SiO(2) substrates via xylene pyrolysis using the chemical vapor deposition technique. Polymers like PEDOT:PSS and polyaniline (PANI) mixed with various solvents like DMSO, DMF, 2-propanol and ethylene glycol were used to synthesize the composite films. These sensors exhibited excellent response and selectivity at room temperature when exposed to low concentrations (100 ppm) of analyte gases like NH(3) and NO(2). The effect of various solvents on the sensor response imparting selectivity to CNT-polymer nanocomposites was investigated extensively. Sensitivities as high as 28% were observed for an MWCNT-PEDOT:PSS composite sensor when exposed to 100 ppm of NH(3) and - 29.8% sensitivity for an MWCNT-PANI composite sensor to 100 ppm of NO(2) when DMSO was used as a solvent. Additionally, the sensors exhibited good reversibility.