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.     

Chemical agent identification by field-based attenuated total reflectance infrared detection and solid-phase microextraction

Attenuated total reflectance Fourier transform infrared (ATR-FT-IR) spectroscopy is used to identify liquid and solid-phase chemicals. This research examines the feasibility of identifying vapor-phase chemicals using a field-portable ATR-FT-IR spectrometer (TravelIR) combined with solid-phase microextraction (SPME). Two nerve agent simulants, diisopropyl methylphosphonate (DIMP) and di-methyl methylphosphonate (DMMP), and three sorbent polymers were evaluated. Each polymer was deposited as a thin film on the instrument's sampling interface to partition and concentrate the simulants from air samples prepared in Tedlar bags. The lowest vapor concentrations identified were 50 ppb (v/v) (DIMP) and 250 ppb (v/v) (DMMP). The ATR-FT-IR instrument demonstrated a linear response at concentrations of 1 ppm (v/v) and below. Increasing the sample exposure time, the sample air velocity, and the film thickness was demonstrated to increase the amount of analyte extracted from the air sample. This research demonstrates that it is feasible to use a portable ATR-FT-IR spectrometer with SPME sampling to detect and identify vapor-phase chemicals.     

Dimethyl methylphosphonate detection with a single-walled carbon nanotube capacitive sensor fabricated by airbrush technique

Single-walled carbon nanotube (SWNT) films were prepared on interdigitated electrodes by airbrush technique, and their sensing properties to dimethyl methylphosphonate (DMMP) were studied. The SWNT films were characterized by field-emission scanning electron microscope. The response to different concentrations of DMMP vapors were investigated at room temperature. The results showed that the capacitance of airbrush SWNT sensor decreased rapidly in varying concentrations ranging from 12 to 60 mg/m³ (2.4–12 ppm). The capacitance sensitivity was about 12.5 % when exposed to 12 mg/m³ DMMP vapor. The capacitance sensitivity was higher when the initial capacitance and loss tangent were higher and the SWNT film was denser. It was found that the capacitance sensitivity was nearly 10 times to the resistance sensitivity. The airbrush SWNT sensor exhibited highly and fast capacitance response, good repeatability and selectivity for DMMP vapor.     

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.     

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.     

Structural elucidation of direct analysis in real time ionized nerve agent simulants with infrared multiple photon dissociation spectroscopy

Infrared multiple photon dissociation (IRMPD) was used to generate vibrational spectra of ions produced with a direct analysis in real time (DART) ionization source coupled to a 4.7 T Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer. The location of protonation on the nerve agent simulants diisopropyl methylphosphonate (DIMP) and dimethyl methylphosphonate (DMMP) was studied while solutions of the compounds were introduced for extended periods of time with a syringe pump. Theoretical vibrational spectra were generated with density functional theory calculations. Visual comparison of experimental mid-IR IRMPD spectra and theoretical spectra could not establish definitively if a single structure or a mixture of conformations was present for the protonated parent of each compound. However, theoretical calculations, near-ir IRMPD spectra, and frequency-to-frequency and statistical comparisons indicated that the protonation site for both DIMP and DMMP was predominantly, if not exclusively, the phosphonyl oxygen instead of one of the oxygen atoms with only single bonds.     

Biocompatibility of native and functionalized single-walled carbon nanotubes for neuronal interface

Single-walled carbon nanotubes (SWNTs) have unique mechanical, electrical, and optical properties and can be easily chemically modified; features that make them excellent candidate materials for applications as sensors and stimulators in neuronal tissue engineering. The purpose of this study was to demonstrate that SWNTs can support neuronal attachment and growth, that simple chemical modifications can be employed to control cell growth, that SWNTs do not interfere with ongoing neuronal function, and that neurons can be electrically coupled to SWNTs. Growth and attachment of the neuroblastoma*glioma NG108, a model neuronal cell, was assessed on unmodified SWNT substrates or substrates from SWNTs modified with 4-benzoic acid or 4-tert-butylphenyl functional groups using a simple functionalization method. SWNT films support cell growth, but at a reduced level compared to tissue culture-treated polystyrene. The order of viability and cell attachment was tissue culture treated polystyrene > SWNTs > 4-tert-butylphenyl-functionalized SWNTs > 4-benzoic acid-functionalized SWNTs. Decreased cell growth after culture on untreated (non adherent) polystyrene suggested that cell attachment was a critical determinant of proliferation and cell growth on SWNTs. Fluorescence and scanning electron microscopy revealed decreased neurite outgrowth in NG108 grown on SWNT substrates. We are also among the first groups to demonstrate electrical coupling of SWNTs and neurons by demonstrating that NG108 and rat primary peripheral neurons showed robust voltage-activated currents when electrically stimulated through transparent, conductive SWNT films. Our data suggest that SWNTs are flexible resource materials for tissue engineering application involving electrically excitable tissues such as muscles and nerves.     

Detection and classification characteristics of arrays of carbon black/organic polymer composite chemiresistive vapor detectors for the nerve agent simulants dimethylmethylphosphonate and diisopropylmethylphosponate

Arrays of conducting polymer composite vapor detectors have been evaluated for performance in the presence of the nerve agent simulants dimethylmethylphosphonate (DMMP) and diisopropylmethylphosponate (DIMP). Limits of detection for DMMP on unoptimized carbon black/ organic polymer composite vapor detectors in laboratory air were estimated to be 0.047-0.24 mg m(-3). These values are lower than the EC50 value (where EC50 is the airborne concentration sufficient to induce severe effects in 50% of those exposed for 30 min) for the nerve agents sarin (methylphosphonofluoridic acid, 1-methylethyl ester) and soman (methylphosphonofluoridic acid, 1,2,2-trimethylpropyl ester), which has been established as approximately 0.8 mg m(-3). Arrays of these vapor detectors were easily able to resolve signatures due to exposures to DMMP from those due to DIMP or due to a variety of other test analytes (including water, methanol, benzene, toluene, diesel fuel, lighter fluid, vinegar, and tetrahydrofuran) in a laboratory air background. In addition, DMMP at 27 mg m(-3) could be detected and differentiated from the signatures of the other test analytes in the presence of backgrounds of potential interferences, including water, methanol, benzene, toluene, diesel fuel, lighter fluid, vinegar, and tetrahydrofuran, even when these interferents were present in much higher concentrations than that of the DMMP or DIMP being detected.     

Hierarchical Three-Dimensional Layer-by-Layer Assembly of Carbon Nanotube Wafers for Integrated Nanoelectronic Devices

We report a general approach to overcome the enormous obstacle of the integration of CNTs into devices by bonding single-walled carbon nanotubes (SWNTs) films to arbitrary substrates and transferring them into densified and lithographically processable "CNT wafers". Our approach allows hierarchical layer-by-layer assembly of SWNTs into organized three-dimensional structures, for example, bidirectional islands, crossbar arrays with and without contacts on Si, and flexible substrates. These organized SWNT structures can be integrated with low-power resistive random-access memory.     

Integration of carbon nanotubes as hole transport electrode in polymer/fullerene bulk heterojunction solar cells

Transparent and conductive single-walled carbon nanotube (SWNT) thin films were fabricated onto glass substrates and their optical and electrical properties were evaluated. Particular attention was given to the dependence of the conductivity and optical transparency on the thickness of the films. Furthermore, the SWNT thin films were integrated in organic photovoltaic devices as the hole transport electrode. The best photovoltaic performance was observed for the devices utilizing 80 nm SWNT films with a sheet resistance of 362 Ω/sq, and a transmittance of 64% at 520 nm. The experiments reveal that SWNTs films can be used as transparent electrodes for efficient, flexible organic photovoltaic devices.     

Assessment of the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes

Scanning electrochemical microscopy (SECM) has been employed in the feedback mode to assess the electrochemical behavior of two-dimensional networks of single-walled carbon nanotubes (SWNTs). It is shown that, even though the network comprises both metallic and semiconducting SWNTs, at high density (well above the percolation threshold for metallic SWNTs) and with approximately millimolar concentrations of redox species the network behaves as a thin metallic film, irrespective of the formal potential of the redox couple. This result is particularly striking since the fractional surface coverage of SWNTs is only approximately 1% and SECM delivers high mass transport rates to the network. Finite element simulations demonstrate that under these conditions diffusional overlap between neighboring SWNTs is significant so that planar diffusion prevails in the gap between the SECM tip and the underlying SWNT substrate. The SECM feedback response diminishes at higher concentrations of the redox species. However, wet gate measurements show that at the solution potentials of interest the conductivity is sufficiently high that lateral conductivity is not expected to be limiting. This suggests that reaction kinetics may be a limiting factor, especially since the low surface coverage of the SWNT network results in large fluxes to the SWNTs, which are characterized by a low density of electronic states. For electroanalytical purposes, significantly, two-dimensional SWNT networks can be considered as metallic films for typical millimolar concentrations employed in amperometry and voltammetry. Moreover, SWNT networks can be inexpensively and easily formed over large scales, opening up the possibility of further electroanalytical applications.     

A sensor of alcohol vapours based on thin polyaniline base film and quartz crystal microbalance

Thin films of polyaniline base, emeraldine base (EB), coating on the quartz crystal microbalance (QCM) electrode were used as a sensitive layer for the detection of a number of primary aliphatic alcohols such as ethanol, methanol, 2-propanol and 1-propanol vapours. The frequency shifts (Δf) of the QCM were increased due to the vapour adsorption into the EB film. Δf were found to be linearly correlated with the concentrations of alcohols vapour in part per million (ppm). The sensitivity of the sensor was found to be governed by the chemical structure of the alcohol. The sensor shows a good reproducibility and reversibility. The diffusions of different alcohols vapour were studied and the diffusion coefficients (D) were calculated. It is concluded that the diffusion of the vapours into the EB film follows Fickian kinetics.     

Structural study of single-walled carbon nanotube films doped by a solution method

Herein, we have studied a structure of SWNT films doped by organic molecules. In this study, we reacted organic molecules with SWNT films by a vapor phase and a liquid phase, respectively. The structure of doped SWNT films was investigated using synchrotron X-ray powder diffraction measurements, and we found the clear difference between the vapor phase reaction (v-doped) and the liquid phase reaction (I-doped). In v-doped SWNT films, organic molecules are predominantly encapsulated inside SWNTs, although molecules adsorbed on the surface of SWNT bundles in l-doped SWNT films.     

Transparent conductive thin film synthesis based on single-walled carbon nanotubes dispersion containing polymethylmethacrylate binder

Transparent conductive hybrid thin films of single-walled carbon nanotubes (SWNTs) and polymethyl methacrylate (PMMA) are fabricated using dispersions containing SWNTs and water-borne PMMA binder. The polymer binder was used as adhesion promoter between the SWNTs and the substrate. The polymer binder content in the SWNTs dispersion is varied to obtain the optimum optical transmittance, electrical conductivity, and mechanical adhesion. The PMMA and SWNT network formed the composite over substrate. The fabricated SWNTs/PMMA hybrid films are immersed in nitric acid (HNO3) and thionyl chloride (SOCl2) to improve electrical conductivity. SWNTs films with 0.2-0.6 mg/ml polymer binder have sheet resistance of 80-140 ohms/sq at a transmittance of about 80% and a strong adhesion on glass substrate. Furthermore, the electrical stability of the films is improved via the PMMA addition. This results indicates that the SWNTs/PMMA hybrid films fabricated by this method can be used as an alternative of indium tin oxide (ITO) film on flexible substrate.     

Chemical vapor detection using a capacitive micromachined ultrasonic transducer

Distributed sensing of gas-phase chemicals using highly sensitive and inexpensive sensors is of great interest for many defense and consumer applications. In this paper we present ppb-level detection of dimethyl methylphosphonate (DMMP), a common simulant for sarin gas, with a ppt-level resolution using an improved capacitive micromachined ultrasonic transducer (CMUT) as a resonant chemical sensor. The improved CMUT operates at a higher resonant frequency of 47.7 MHz and offers an improved mass sensitivity of 48.8 zg/Hz/μm(2) by a factor of 2.7 compared to the previous CMUT sensors developed. A low-noise oscillator using the CMUT resonant sensor as the frequency-selective device was developed for real-time sensing, which exhibits an Allan deviation of 1.65 Hz (3σ) in the presence of a gas flow; this translates into a mass resolution of 80.5 zg/μm(2). The CMUT resonant sensor is functionalized with a 50-nm thick DKAP polymer developed at Sandia National Laboratory for dimethyl methylphosphonate (DMMP) detection. To demonstrate ppb-level detection of the improved chemical sensor system, the sensor performance was tested at a certified lab (MIT Lincoln Laboratory), which is equipped with an experimental chemical setup that reliably and accurately delivers a wide range of low concentrations down to 10 ppb. We report a high volume sensitivity of 34.5 ± 0.79 pptv/Hz to DMMP and a good selectivity of the polymer to DMMP with respect to dodecane and 1-octanol.     

In situ infrared aerosol spectroscopy for a variety of nerve agent simulants using flow-through photoacoustics

We present newly measured results of an ongoing experimental program established to measure optical cross sections in the mid- and long-wave infrared for a variety of chemically and biologically based aerosols. For this study we consider only chemically derived aerosols, and in particular, a group of chemical compounds often used as simulants for the detection of extremely toxic organophosphorus nerve agents. These materials include: diethyl methylphosphonate (DEMP), dimethyl methylphosphonate (DMMP), diisopropyl methylphosphonate (DIMP), and diethyl phthalate (DEP). As reported in a prior study [Appl. Opt. 44, 4001 (2005)], we combine two optical techniques well suited for aerosol spectroscopy [i.e., flow-through photoacoustics and Fourier transform infrared (FTIR) emission spectroscopy], to measure in situ the absolute extinction and absorption cross sections over a variety of wavelengths spanning the IR spectral region from 3 to 13 mum. Aerosol size distribution(s), particle number density, and dosimetric measurements are recorded simultaneously in order to present optical cross sections that are aerosol mass normalized, i.e., m(2)/gram. Photoacoustic results, conducted at a series of CO(2) laser lines, compare well with measured broadband FTIR spectral extinction. Both FTIR and photoacoustic data also compare well with Mie theory calculations based on measured size distributions and previously published complex indices of refraction.     

Carbon nanotube based transparent conductive thin films

Carbon nanotube (CNT) based optically transparent and electrically conductive thin films are fabricated on plastic substrates in this study. Single-walled carbon nanotubes (SWNTs) are chemically treated with a mixture of concentrated sulfuric acid and nitric acid before being dispersed in aqueous surfactant-contained solutions. SWNT thin films are prepared from the stable SWNT solutions using wet coating techniques. The 100 nm thick SWNT thin film exhibits a surface resistivity of 6 kohms/square nanometer with an average transmittance of 88% on the visible light range, which is three times better than the films prepared from the high purity as-received SWNTs.     

Charge Transfer from Carbon Nanotubes to Silicon in Flexible Carbon Nanotube/Silicon Solar Cells

Mechanical fragility and insufficient light absorption are two major challenges for thin flexible crystalline Si‐based solar cells. Flexible hybrid single‐walled carbon nanotube (SWNT)/Si solar cells are demonstrated by applying scalable room‐temperature processes for the fabrication of solar‐cell components (e.g., preparation of SWNT thin films and SWNT/Si p–n junctions). The flexible SWNT/Si solar cells present an intrinsic efficiency ≈7.5% without any additional light‐trapping structures. By using these solar cells as model systems, the charge transport mechanisms at the SWNT/Si interface are investigated using femtosecond transient absorption. Although primary photon absorption occurs in Si, transient absorption measurements show that SWNTs also generate and inject excited charge carriers to Si. Such effects can be tuned by controlling the thickness of the SWNTs. Findings from this study could open a new pathway for designing and improving the efficiency of photocarrier generation and absorption for high‐performance ultrathin hybrid SWNT/Si solar cells.     

Raman Spectroscopy and SEM Study of SWNTs in Aqueous Solution and Films with Surfactant or Polymer Surroundings

Raman spectra of single-walled carbon nanotubes (SWNTs) in aqueous solutions with sodium dodecylsulfate (SDS) or fragmented single-stranded DNA (ss-DNA) and films obtained from these solutions have been studied. Scanning electron microscope (SEM) film study shows that micelles formed by SDS molecules around SWNT in solution do not keep individual nanotubes from sticking together in bundles during drying out the film. DNA wrapped around SWNT precludes the full nanotubes sticking in the film that facilitates the following splitting of these bundles.     

Chemical vapour deposition of single walled carbon nanotubes freely suspended over nanotube supports

Single walled carbon nanotubes (SWNTs) suspended above the substrate can be fabricated simply and rapidly by chemical vapour deposition growth over pre-grown multi-walled carbon nanotubes (MWNTs). SWNTs are suspended either on a randomly organized carbon nanotube network on an unpatterned substrate, or between organized pillars made from vertically aligned nanotube forests on a patterned substrate. All nanotubes are produced during a single growth run using a two step growth technique. This approach enables the fabrication of laterally suspended SWNT networks which are well suited for optical applications.