reportChemical Detection
9.2.3 Short description

Electronic Nose - Artificial noses also known as electronic noses are used to sense the
presence of toxic gases and bio agents, and convey its presence by means of an electrical signal. These artificial or electronic noses are also used in detecting explosives. A number of
physiochemical approaches are used in sensing, for example measurement of changes in conductivity for metals oxides and polymers, for piezoelectric materials changes in frequency are measured and fluorescent optical fibers changes in colour are measured. Nanosensors based on the human olfactory system have been studied in the United States for molecular recognition of species, pre-processing of the neural signal and transduction of the signal. Sensors based on the electrochemical are either conductance based or potentiometric (field effect transistor). Mass
change based piezoelectric sensors are quartz crystal microbalance and for surface acoustic
wave devices. Optical sensors are mainly either fluorescent based optical fibers or colorimetric
[4].

Conductance sensors are based on a metal oxide or a conducting polymer where binding of a compound causes a change in the resistance between two metal contacts. The main material used for metal oxide sensors are tin, zinc, titanium, tungsten, and iridium. These could be doped with palladium or platinum. A gas sensor array the size of a thumb nail size can sense industrial gases that maybe potentially toxic gases such as ammonia, formaldehyde and carbon monoxide based on oxides of tin and tungsten. Sensor layer thickness can range from 2-20nm [5]. Nanoscale tin oxide based sensors and its variations, with grain size of 8 nm, have effectively demonstrated the recognition of combustible gases such as propane, butane, LPG within their explosion limits [6].

Conductive Polymer - Conductive polymers used as sensors use a polymer to connect two electrodes. The polymer acts as the active sensing agent, the sensitivity of the sensor is higher than metal oxide sensors. The main shortcomings of these types of sensors are the complexity of fabrication and reproducibility of sensing function between batches [4,7]. The active sensor element is used to detect volatile organic compounds. Polymers such as polypyrole, polyaniline, polythiophene, and polyacetate are used due to their high sensitive to vapour and gases. Nanometre sized carbon black has also been used to make polystyrene conductive for sensing application [8]. Carbon black composite sensing arrays have been used to detect explosives and
chemical warfare agent such as sarin and soman [9].

Field effect transistors - Field effect transistors used as potentiometeric sensors have been
demonstrated in detecting gases by making the gates sensitive to gases. A volatile organic
compound produces a reaction in the sensing layer, which causes the physical property of the gate to change, thereby changing the threshold voltage and thus the channel conductivity. Noble metal catalysts such as platinum, palladium and iridium have been coated on metal oxide FET
[10]. Nose on a chip concept of sensors arrays of polymer gated FET's is used for sensing different odours. The sensing element is combined with a signal processing component, which is used to detect the presence of a gas, identified by a train of spikes in the frequency [11]. Methods such as statistical pattern recognition, neural networks, chemometrics, machine learning, and
biological cybernetics has been used to process electronic nose data [12].

Piezoelectric sensors - Piezoelectric devices working as an electronic nose work on the basis of measuring a change in mass. Piezoelectric crystals vibrate under the influence of an applied voltage, the mass of which determines the resonant frequency. Quartz crystal microbalance (QCM) and surface acoustic wave device are used as electronic noses [7]. QCM is used in
explosive detection, wherein the adsorption of a gas molecule on the surface of polymer changes the resonant frequency. Quartz crystal microbalance with immobilised nanoscaled ZSM-5 zeolite film has been developed as a sensor for nerve agent. A minimum concentration of 1 part per
million (ppm) was detected using the stimulant dimethylmethylphosphonate [13].

Surface Acoustic Wave sensors - Surface acoustic wave sensors are based on acoustic waves travelling on the surface of transducers. Adsorption of a gas molecule causes a change in the mass thereby causing a change in the frequency or phase shift. The advantage with these sensors is that they are easy to fabricate, while their drawback is that they are temperature sensitive, the noise in the signal increases with decreasing size [14]. IBM has demonstrated cantilever based sensors, in ambient air to detect ethenes, alcohols, natural flavours and water vapour using optical methods [15]. Microsensor systems Inc a leading producers of SAW sensors
have demonstrated the nerve gas agents and blister gas agents, with a sensitivity of 0.04 ppm in 20 seconds and 0.01 ppm in 120 seconds respectively [16]. Polymer coatings such as polysiloxane films that allow diffusion of chemical agents into the bulk of the film for optimal mass loading. These have been used both for SAW and QCM sensors [17,18]. Thin film piezoelectric
acoustic sensor works on the basis of change in thickness of the gas sorption layer on the substrate. These sensors can detect chemical and biological threats with a sensitivity of 100 ppm [19,20].

Flexural plate wave sensors - Flexural plate wave sensors are similar to SAW and QCM sensors detect an agent based on mass absorbed on a coating deposited on the sensor. It is known to have one of the highest levels of detection sensitivity being in the range of parts per trillion (ppt) [21]. Chemical vapour detection and biosensor array based on flexural plate wave sensor has been demonstrated. A siloxane polymer coating 50nm thick is applied on the surface
for the detection of specific chemical agents. A sensitivity of 10 ppm was demonstrated in the
experimental study [22].

Sensor Arrays - Sensor arrays have been integrated with support vector machines for detecting organophosphate based nerve agents. Support vector machines serve the purpose of data extraction, pre-processing and classification of chemical biological agent [23]. MEMS based sensor arrays have been used to detect nerve agents such as tabun and sarin, with a sensitivity of 4 and 26 parts per billion (ppb) respectively. Blister agents such as sulphur mustard were detected with a sensitivity of 16 ppb. Oxides of tin and titanium were deposited as nanostructured
thin films which act as the sensing element. These sensor arrays have demonstrated stability, high signal to noise ratio to the relevant chemical warfare agent [24].

Optical fibres - Optical fibres have been used in sensing application. The fibre is turned into a
sensor by coating the end with sensing materials or by removing the cladding and coating it with the sensing material. The sensing material used is primarily polymers containing chemically active fluorescent dyes. The presence of a target agent causes a change in the polarity of the dye
which further leads to a change in the wavelength [12]. These optical fibres have been used to detect explosives such as TNT at a sensitivity of 10-15 ppb, which is comparable to 1 ppb sensitivity of a dog's nose for the same agent. The sensitivity achieved was for a closed chamber however, field trials were not as successful in demonstrating the same result [25].

Cantilevers - Microcantilevers are similar in appearance to diving boards, and are machined from silicon or other materials. The length of these can vary from 100-200 microns and the thickness between 0.3 - 1 microns. Microcantilevers offer sensitivity at least an order of magnitude higher than QCM and SAW based sensors for chemical agents sensing [3]. Piezo resistive micro cantilever based sensors have been demonstrated to have excellent detection capabilities for chemical and explosive vapour detection. The cantilevers are coated with 4nm Ti film, 20nm gold layer and 4-mercaptobenzoic acid self assembled monolayer. Detection of dimethyl
methylphosphonate (DMMP), a stimulant for the nerve agents was demonstrated with parts per trillion detection capability within 10 seconds of exposure [26].

Chemiresistors - Chemiresistors are sensors that monitor a change in the resistance continuously with exposure to vapours. Carbon nanotubes have been used for organic vapour sensing. Single walled carbon nanotubes with diameter of 15-30 nm have been demonstrated as effective sensors for nerve gas agents Sarin and Soman. A network of films 1-2 microns thick on
a polyethylene terephthalate (PET) substrate can detect traces of chemical agent vapours with a sensitivity of 25 ppm. Strong sensors responses were obtained that were not affected by environmental conditions such as air quality and humidity does not interfere significantly [27]. Single strand DNA along with single walled carbon nanotubes field effect transistors have been used to detect chemical warfare agents. These sensors have show high sensitivity and stability up to 50 cycles of operation [28]. Detection of V type nerve agent has been experimentally
demonstrated using carbon nanotubes. The detection is based on enzyme catalyzed hydrolysis of nerve agents and amperometric detection of thiol containing hydrolysis product that is performed at the carbon nanotube modified screen printed electrode. The sensitivity demonstrated for such
sensors is 258 ppb [29].

Chemicapacitive sensor - A chemicapacitive sensor is a capacitor that has selectively
absorbing materials such as a polymer, as a dielectric. Volatile organic compounds are absorbed into dielectric, changing the permittivity leading to an increase or decrease in the capacitance. Polymer dielectrics are a type of chemicapacitive sensor that are used for detecting chemical warfare agents. These demonstrate a sensitivity of detection of 100 ppm for toxic industrial solvents and 1 ppm for chemical warfare agent as well as explosives [30]. Chemicapacitive sensors for toxic industrial chemicals have demonstrated a sensitivity of 0.0006 - 720 ppm for
analyte such as carbyl for the lower limit and carbon disulphide for the higher limit [31].

Spectroscopic Methods - Nerve gas agents such as sarin, soman, tabun, and VX, along with blister agents such as mustard gas, and lewisite were compared for their detection using techniques such as gas detection tube, flame photometric detector, ion mobility spectrometer, surface acoustic wave detector, photo ionisation detector, Fourier- transform infrared spectroscopy, gas chromatography - mass spectrometry for chemical warfare agents. Similar comparative study for biological warfare agents such as flow cytometry, bioluminescence detection, lateral flow immunoassay. Figure 1 below demonstrates the shortcomings of each of these techniques for onsite detection, according to the volatility and molecular weight. Some techniques proved to be suitable in detection of certain analytes while others resulted in false
positives or a slow response to the nerve agent [32].

Performance of onsite chemical and biological weapon detection

Figure CW.1 - Performance of onsite chemical and biological threat detection [32]

In another study chemical warfare agents were comparatively detected by different analytical
techniques such as gas chromatography-infrared detection-mass spectral detection (GC-IR-MS); liquid chromatography-mass spectrometry (LC-MS); nuclear magnetic resonance (NMR) using the nuclei H, C and P; and gas chromatography-atomic emission detection (GC-AED). It was observed in the study that each of the technique gave good identification of some of the components such as amines, phosphorous, or sulphur. Each of these techniques also missed out
on several major components [33]. Separation and detection of organophosphorus type nerve agents by gas chromatography with inductively coupled plasma mass spectrometry has been
demonstrated, less than 5 picogram sensitivity to river water and soil contamination [34].

Techniques such as laser photo-acoustic spectroscopy have demonstrated a detection of sarin with a sensitivity of 1.2 ppb, and with extremely low false positives of less than 1 in 1,000,000 in the presence of other trace gases [35]. Micro-X Ray fluorescence has unique capabilities suited to high throughput screening of combinatorial libraries of chemical warfare agents. High throughput screening of nerve agents such as VX has been demonstrated in nanogram
quantities. These could be coupled with other spectroscopic techniques such as Raman and IR to give additional information [36]. Contamination of portable water with nerve agent Sarin can have serious consequences such as bronchospasm and even death under conditions if enough quantity is consumed. Diffuse reflectance infrared spectrum investigation, using nanoparticulate magnesium oxide as a preconcentration medium produced a sensitivity of 98 ppb for Sarin
detection in water [37].

Laser induced breakdown spectroscopy has been used as a versatile sensory platform for
detecting chemical agents, biological agents such as anthrax and improvised explosive devices. This sensory platform can be couple with robotic platforms for toxic environments and with fiber optics. The LIBS technique has demonstrated effectively to distinguish chemical agents especially in the soil [38]. Phosphorus containing nerve agent stimulant detection with LIBS has
been demonstrated at a range of 20 meters [39].

Nanomaterials - Nanocomposites of tin oxide and indium oxide have been demonstrated as
excellent material for semiconductor gas sensors for toxic industrial gases. Addition of additives is shown to have enhanced sensitivity and selectivity performance significantly [40]. Toxic and explosive industrial gas such as methane, butane, propane, liquefied petroleum gas, and carbon monooxide have been detected by nanostructured tin oxide sensors. The sensor arrays demonstrated have a detection capability of less than 100 parts per million [6]. Tin, Niobium and
Vanadium Oxide thin films have been developed for the detection of nerve gas agents Sarin. For agglomerates of the size of 40 nm, a sensitivity of 70 ppb was demonstrated. The stability of such
thin films for gas sensing applications is being further researched [41].

Challenges of chemical sensing - Integration of large number of sensors in a limited area,
providing high sensitivity, and selectivity of the toxin. Another challenge is the environmental
conditions, whereby it is much easier to measure parameter in a laboratory condition as opposed to ambient air or in water [4]. Shortcomings of conductive polymers are that surface morphology is not predictable, therefore the surface conductivity and the sensing function are not reproducible
between batches, and more importantly it sensitivity to water vapour [4, 7]. There is no one device that meets the need of onsite detection of both chemical and warfare agents for onsite detection [32]. Shortcomings of chemical agents sensors are that no polymer coating for sensors can display complete selectivity to all possible interferents [22]. The costs of using mass spectrometry, gas chromatography, ion mobility spectrometry are expensive and not as easy to
use as electronic noses [26].

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Tags: electronic nose, conductive polymers, field effect transistors, piezoelectric sensors, SAW sensors, flexural plate wave, sensor array, optical fibres, cantilevers, chemiresistors, chemicapacitive, spectroscopy, nanomaterials, chemical sensing, chemical detection

reportChemical Detection 9.2.3 Short description

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9.2.3 Short description