Abstract:
A detection system for an enclosed container for an enclosed cargo container includes a sensor device for sensing a material harmful to human beings within an enclosed cargo container and a detection device coupled to the sensor device for transmitting a corresponding signal to a monitoring device outside the cargo container. Containers which have harmful materials within them can be inspected or stopped before entering the country.

Description:
This application claims the priority, as a continuation-in-part, of application Ser. No. 12/707,062, filed on Feb. 17, 2010 (now U.S. Pat. No. 7,911,336, issued on Mar. 22, 2011), and claims the priority, as a continuation-in-part, of application Ser. No. 11/705,142, filed on Feb. 9, 2007 (now U.S. Pat. No. 7,667,593, issued on Feb. 23, 2010), from which the 12/707,062 application claims priority, and claims the priority, as a continuation-in-part, of application Ser. No. 10/998,324, filed on Nov. 29, 2004 (now U.S. Pat. No. 7,176,793), from which the 11/705,142 application claims priority. This application also claims the benefit of Application Ser. No. 61/321,257, filed on Apr. 6, 2010, and of Application Ser. No. 61/385,340, filed on Sep. 22, 2010. The disclosures of all of these applications are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The present exemplary embodiment relates to the detection arts. It finds particular application in conjunction with cargo containers which are used to ship products, foodstuffs, and other materials from one country to another, and will be described with particular reference thereto. However, it is to be appreciated that the present exemplary embodiment is also amenable to other like applications. 
     Cargo containers are widely used for shipping materials by land or by water from one country to another. Knowing the contents of such containers has become of increasing importance in detecting potential threats. It has thus become extremely important to monitor the contents of such containers for harmful materials, such as explosives, harmful biological and chemical materials, and radiation materials. 
     U.S. Pat. No. 7,176,793 discloses a detection device in the form of a strip for use in an enclosed container. The detection strip includes sensors of macro, meso or nanosize, all of which are referred to as nanosensors, for detecting materials that are harmful to human beings within an enclosed container and for transmitting a corresponding resonance frequency. One or more detection strips are initially placed within a container, depending on the size of the container. The detection devices are designed to send off specific resonant frequency signals which can be detected by voltage changes and/or current changes which are correlated to any harmful material detected within the container. A serial number computer chip is provided for specifically identifying the detection device and transmitting a corresponding resonance frequency, which allows the container to be identified. A power source is provided for operating the detection strip. A hand-held or stationary monitor is provided for monitoring the container for any signals given off from the detection strips within the container. The detection devices are designed to give off a predetermined amount of background signal. In consequence, if no such signals are received, the container is highly suspect as being tampered with, allowing such a container to be quickly removed and its contents examined. 
     For some applications, hazardous materials may be at relatively low concentrations, for example hazardous nuclear materials may be distributed in amongst other materials or chemical or biological warfare agents may be in small concentrations within the container. 
     The exemplary embodiment provides a solution to this problem by mounting a sensor to one or more interior walls of the container which provides a unique fingerprint for a hazardous material. The signals output by the sensor can be received by one or more detection devices which communicate the signals to an exterior monitor. 
     BRIEF DESCRIPTION 
     In accordance with one aspect of the exemplary embodiment, a detection system for an enclosed container includes a sensor device comprising carbon nanoparticles for detecting materials harmful to human beings, such as explosives, e.g., nitro-containing explosives such as trinitrotoluene (TNT) and/or peroxide-based explosives, within an enclosed container and transmitting a corresponding detection signal and at least one detection device which detects the detection signal and outputs a signal responsive thereto. 
     In another aspect, in combination, a cargo container in which any suitable cargo is placed for transport from one place to another place, a detection system disposed in the container for detecting tampering with the container, the detection system including a sensor device, means of storing and transmitting information acquired by the sensor, and a power source for operating the detection system. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a shipping container and a container monitoring system in accordance with one aspect of the exemplary embodiment; 
         FIG. 2  is a top plan view of an array of sensor devices in a row manufactured by a screen-printing process; 
         FIG. 3  is a functional block diagram which illustrates a sensing system comprising an individual sensor device of the type illustrated in  FIG. 2 ; 
         FIG. 4  is a side sectional view of the sensor device of  FIG. 2 ; 
         FIG. 5  illustrates sensor device performance for three individual dinitrotoluenes (DNTs) in a 3.5 wt % NaCl solution; 
         FIGS. 6-9  illustrate sensor device performances in mixed DNT species solutions (3.5 wt % NaCl) as compared to those for individual DNT species; 
         FIG. 10  illustrates sensor device performance for three individual DNTs using a gelled ionic liquid electrolyte sensor device; 
         FIGS. 11-14  show performance of the gelled ionic liquid electrolyte sensor device in the mixed DNT species solutions (3.5 wt % NaCl) as compared to those for individual DNT species; 
         FIG. 15  illustrates a testing chamber for gas phase explosive detection; 
         FIG. 16  is a plot illustrating the performance of the gelled ionic liquid electrolyte sensor device for gas phase DNT detection of 2-4 DNT; and 
         FIG. 17  is a plot illustrating the performance of the gelled ionic liquid electrolyte sensor device for gas phase DNT detection of 2-6 DNT. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the exemplary embodiment relate to a sensing system suitable for detection of trinitrotoluene and other harmful species at low concentrations in a shipping container. 
     With reference to  FIG. 1 , there is shown a container  10  which can be of any size, including large cargo containers. Cargo containers  10  are generally made of metal and include eight walls, namely, a bottom  12  with a pair of opposing, upstanding similar sides  14 , a pair of similar opposing ends  16 , and a top  18 , for covering and closing the cargo container  10 . The walls  12 ,  14 ,  16 ,  18  may be made of metal, such as steel or alumina, or from non-metallic material, such as carbon fiber, or a combination thereof. The walls  12 ,  14 ,  16 ,  18  define an interior space  20  for receiving a cargo  22 , such as a liquid, solid, or other material. Each wall  12 ,  14 ,  16 ,  18  has an interior surface  24 , some or all of which may be in contact with the cargo  22 . 
     A container monitoring system  26  includes one or more detection systems  28 , for monitoring conditions within the container  10 . Each detection system  28  includes one or more sensor devices  30  carried within the container, e.g., on the interior surfaces  24  of the walls. The sensor device(s)  30  may detect harmful materials, such as explosives, radioactive materials, harmful chemicals, such as chemical warfare agents, nerve gases, biological materials, such as such as gases, anthrax and other germ warfare agents, narcotics and other illegal drugs, or combinations thereof. At least one of the sensor devices  30  is configured for generating a signal which is indicative of the presence of a nitrogen-based explosive, such as trinitrotoluene (TNT) and/or a peroxide based explosive, such as triacetone triperoxide (TATP) or hexamethylenetriperoxidediamine (HMTD), or a combination thereof. 
     The detection system  28  also includes a detection device  40  in communication with the sensor device(s)  30 . The exemplary detection device  40  is positioned within the container  10  and receives signals from the sensor device  30  and may also apply a voltage to the sensor device  30 , such an alternating square wave voltage. In the exemplary embodiment, the detection device(s) is/are fixed to an interior surface  24  of a container wall. The exemplary detection devices  40  are capable of withstanding extremes of temperatures, humidity, vibrations, and salt air. Signals, such as current voltage changes, are carried, e.g., by appropriate wiring, from the sensor device  30  to the detection device  40 . The detection device  40  may be configured, for example, as illustrated in U.S. Pat. No. 7,292,828 and/or as described in copending application Ser. No. 61/321,257, filed Apr. 6, 2010. For example, the detection device includes a power source  42 , a current monitor  44 , and a local data adapter/collector (LDA)  46  capable of multiplexing data which collects signals from the current monitor  44 . A transmitter  48 , incorporated in or separate from the LDA is capable of data transmission by satellite uplink and/or by direct line of sight up to 15-30 miles. U.S. Pat. No. 7,292,828, the disclosure of which is incorporated herein by reference, discloses one multichannel transmitter which employs wireless telemetry to send signals indicative of harmful materials to a remote receiver that may be used herein. In other embodiments, encrypted RF data signals are sent from the transmitter  48  to a transponder  50  ( FIG. 1 ) which transmits the signals to a remote or local data transmission device. In a container with multiple detection devices  40 , each one may have its own LDA/transmitter which communicates with transponder  50 . 
     Transponder  50  may be exterior or interior of the container  10 . In one embodiment, the transponder  50  (e.g., conforming to an RF protocol like Dash-7, ISO 18,000-7) may be mounted to a door  52  of the container  10 , as illustrated in  FIG. 1 , to transmit signals from the detection devices  40  to a location outside the container. In one embodiment, transponder  50  may be mounted to an inside of the door  52 , i.e., within the container interior  20 , with an antenna  54  for the transponder being located on the outside of the door. The outside antenna  54  may include an indicator system to provide a visual or aural indication of the status of the container  10 . For example, an orange light may be illuminated if the detection system  28  is not working properly and a red light may be illuminated if the detection system  28  has a sensor hit. The antenna  54  may communicate using IS018,000-7 protocol (Dash-7) or other RF protocol to a ship controller. A commercially available tamper-proof container door latch including a transponder  50  with an antenna  54  that uses Dash-7 sensing of temperature, humidity, shock and entry may be used as the transponder  50 , with suitable modifications to enable transmission of data, which may be encrypted, from the detection system  28 . Exemplary transponder devices of this type are sold under the tradename SAVI® (e.g., SAVI®ST-675) by Lockheed Martin Co. 
     In one embodiment, transmitter  48  or transponder  50  transmits the signals to a portable receiver  60  and/or to a local or remote data transmission, logging, and/or data analysis device  62 ,  64  (e.g., via a satellite link  66 ). In a container with multiple detection devices, each one may have its own detection device which communicates with transponder  50  ( FIG. 1 ). The detection system may, for example, send a warning that the container  10  has been tampered with to the monitoring device  60 ,  62 ,  64  when a threshold level of the harmful substance is detected. The threshold may be, in some cases, the lowest level detectable by the sensor device, or one which is a predetermined amount above a normal background level. 
     The detection system  28  may include a global positioning system (GPS) computer chip  70  for providing a signal representative of the location of the detection system  28  and its associated container  10 . For containers  10  which are below deck and/or covered by many other containers, the GPS chip  70  may receive a signal from a corresponding GPS chip in a local container if the satellite signal is too weak to be picked up. 
     The detection system  28  may include an encrypted serial numbered (ESN) computer chip  72  may also be embedded in or otherwise supported by the strip  54 . The ESN chip  72  generates a signal corresponding to the device&#39;s unique serial number which may also be transmitted via the LDA/transmitter  46 ,  48 . The components  42 ,  44 ,  46   48 ,  70 ,  72 , of the detection device  40  may all be mounted on a common strip  74  formed form plastic or the like and components  44 ,  46   48 ,  70 ,  72 , may all be powered by a single power source  42  or by separate power sources, such as a battery. For example, a low voltage motion activated power source  40  is carried by the strip  74 . The power source  42  may be disconnected from the components by a magnetic switch  76  which completes the circuit with the components  44 ,  46   48 ,  70 ,  72  only intermittently. The container  10 , when moved, may activate the power source  42  to maintain operation of the detection device  40 . In this way, the power source is not drained two quickly. A battery thus may last for about two years before it needs to be replaced. 
     The power source  42  may be configured for applying a square wave voltage to the sensor device  30 , e.g., alternating between positive and negative potentials (vs. reference) with a wavelength of about 1-10 minutes. The exemplary sensor device  30  is a three-electrode system including a counter electrode  82 , a working electrode  84 , and a reference electrode  86 , which are exposed to the gaseous environment in the container in a detection region  88  of the sensor device  30 . The square wave source  42  is connected to the working and reference electrodes  84 ,  86 . The current monitor  44  is connected to the working and counter electrodes  84 ,  82 . 
     As illustrated in  FIG. 4 , the exemplary working and counter electrodes  84 ,  82  include a layer  90  of particles, e.g., micro-, meso- or nano-sized particles of active carbon (porous carbon), which may be in the form of nanotubes. The active carbon nanoparticles may be combined with iridium particles, although in one embodiment, iridium particles are not used. The reference electrode may be a silver/silver chloride (Ag/AgCl) electrode. A respective silver contact pad  94  is connected with each of the electrodes. An insulation layer  96  may cover part of the electrodes, leaving tips of the electrodes exposed to the detection environment in region  88 . 
     The sensor devices  30  shown in  FIGS. 3 and 4  can fabricated on a substrate  100  formed from polyester or other electrically non-conductive material, such as other polymeric materials, alumina (Al 2 O 3 ), ceramic based materials, glass or a semi-conductive substrate, such as silicon, silicon oxide and other covered substrates. Multiple sensor devices  30  can thus be formed on a common substrate ( FIG. 2 ). As will be appreciated, variations in the geometry and size of the electrodes are contemplated. The sensor device can operate in various electrochemical modes, including cyclic voltammetry, time-dependent current measurement, and square-wave voltammetry, or the like. Of these electrochemical measurement techniques, square wave voltammetry (SWV) allows a shorter detection time. 
     The sensor device  30  may be fabricated by cost-effective thick film screen and/or ink-jet printing processes. For example, in the case of the carbon electrodes, active carbon is mixed with a binder, deposited like an ink on the substrate, and allowed to dry. 
     The sensor device  30  can be used in an environment in which a conductive electrolyte exists, such as in sea water. The sensor device  30  also can be integrated with a conductive electrolyte layer, such as an ionic liquid or polymeric electrolyte  102 . In this embodiment, the sensor device  30  may include electrodes  82 ,  84 ,  86  and an electrolyte layer  102 , all laid down on a suitable substrate  100 , as shown in the cross sectional view in  FIG. 4 . This provides a complete electrochemical sensing system which can be used and deployed in an environment where a conductive electrolyte is not available. The electrolyte  102  may be in the form of a gel which includes ionic liquid in a binder, optionally with a solvent. A suitable ionic liquid is a salt which is a liquid at ambient temperatures. Exemplary ionic liquids have a melting point below 100° C., generally below 50° C. Exemplary cations in the salt include alkyl imidazolium groups and alkyl and aryl pyridinium groups. Examples of ionic liquids include 1-butyl-3-methylimidazolium hexafluorophosphate, 1-hexyl-3-methylimidazolium hexafluorophosphate, 1-octyl-3-methylimidazolium hexafluorophosphate, 1-decyl-3-methylimidazolium hexafluorophosphate, 1-dodecyl-3-methylimidazolium hexafluorophosphate, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulphonyl)amide, 1-hexyl-3-methylimidazolium bis(trifluoromethylsulphonyl)amide, 1-hexylpyridinium tetrafluoroborate, 1-octylpyridinium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate, 1-methyl-3-ethyl imidazolium chloride, 1-ethyl-3-butyl imidazolium chloride, 1-methyl-3-butyl imidazolium chloride, 1-methyl-3-butyl imidazolium bromide, ethyl pyridinium bromide, ethyl pyridinium chloride, ethylene pyridinium dibromide, ethylene pyridinium dichloride, butyl pyridinium chloride, and benzyl pyridinium bromide. As an example, the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate can be used as an electrolyte for detection of TNT. 
     The ionic liquid in layer  102  has desirable properties for use as an electrolyte including good stability at high temperature, relatively insensitive to moisture or humidity effects, and good ionic conductivity at ambient or low temperature. The exemplary ionic liquid is in a gel form covering the surface areas of working electrode  84 . The ionic liquid may be hydrophilic such that it does not dry out when placed in a shipping container for an extended period. It should also be stable over a wide range of temperatures, such as up to 50° C. or higher. 
     A mixture of the ionic liquid, one or more of a gelling agent and a binder may be used to form the layer  102 . A binder of polyvinylpyrrolidone (PVP), a resin, such as polyvinyl butyral (PVB) and a thermoplastic fluoropolymer, such as polyvinyldifluoride (PVDF) is used in the exemplary embodiment. A solvent may also be used which allows the mixture to be applied as an “ink” after which the solvent is evaporated. The solvent used to prepare this binder ink can be, for example, N-methylpyrrolidone (NMP). The solvent can be present at, for example, from 10-99 weight % of the ink, e.g., at about 90 wt. %. A ratio of binder to ionic liquid in the mixture can be, for example, from 1:0 to 1:1, e.g., about 3:10. The mixture is applied over the exposed working electrode  84 . The deposition of the ionic liquid film can also be accomplished by spin-coating, ink-jet printing, and other dispensing techniques. 
     Harmful species, such as TNT, are transported through the ionic liquid to the sensor surface and cause a change in the signal transmitted by the sensor device  30 . 
     As will be appreciated, a range of the binder to ionic liquid ratio can be used depending on the environmental conditions where the sensor device will be applied. Also, different ionic liquids and binder materials can be selected and used depending on the suitability of the applications of the sensor device. 
     Based on studies with related nitrotoluene compounds, the exemplary sensor device should readily detects TNT and other hazardous nitro-compounds in air or water at concentrations of as low as 1 ppm, and, with more sensitive current detector  44 , could detect nitro compounds in the ppb range. The detection system  28  provides a unique fingerprint for aromatic nitro compounds, allowing non-harmful gases such as nitrogen oxides and ammonia to be easily eliminated from the detection scheme. The detection device can thus compare a current profile received from the sensor in response to an applied voltage with that of a known harmful substance to determine if there is a match and outputs a signal based thereon. 
     For detection of radiation generating materials, in addition to or as an alternative to the nanosensors, a radiation detection system  104  ( FIG. 4 ) comprising one or more conventional scintillation counters may be employed to detect neutrons and high energy gamma rays or their reaction bi-products. The radiation detection system  104  may be linked to the detection device  40  to provide counts corresponding to detected radiation. In the embodiment shown in  FIG. 4 , the radiation detection system  104  is mounted to the same adhesive-backed strip  74 , which carries the detection device  40  and optionally the sensor device  30 . For example, radiation detection system  104  may include plastic scintillators which can very easily be shaped and machined to the forms desired in detectors (cylinders, rods, flat sheets, fibers, microspheres and thin films). Scintillators coupled to a photomultiplier tube detect ionizing radiation, such as the photons produced when high energy gamma rays emitted by the radiation generating materials interact with solid materials, or neutrons. Solid materials, such as those with high stopping power, may be placed intermediate the container interior and the scintillators of the radiation detection system  104  to absorb gamma rays and generate the detectable photons. As the solid material, LaBr 3 (Ce), for example, offers a higher stopping power for gamma rays (density of 5.08 g/cm 3  versus 3.67 g/cm 3  for NaI(TI)). LYSO (Lu 1.8 Y 0.2 SiO 5 (Ce)) has an even higher density (7.1 g/cm 3 ), is non-hygroscopic, and has a high light output (32 photons/keV γ), in addition to being rather fast (41 ns decay time). 
     Gamma radiation may also be detected through its ability to dissociate atmospheric nitrogen and oxygen, resulting in the formation of nitrogen dioxide, which in turn serves as an ozone catalyst and thus can be detected through reductions in ozone levels. Other detection methods include the use of photodissociative bacteria or algae which respond to the photons generated, as well as topaz/silica, which turns from clear to blue in the presence of some forms of radiation. 
     Without intending to limit the scope of the exemplary embodiment, the following examples demonstrate the sensitivity of the exemplary detection system  28 . 
     EXAMPLES 
     Example 1 
     Evaluation in Liquid Electrolyte 
     A screen-printed active carbon sensor device  30  with an active carbon working electrode  84 , an active carbon counter electrode  82 , and an Ag/AgCl reference electrode  86  was prepared. In the following examples, the carbon was not in extremely finely divided form, e.g., in the form of nanotubes, which is expected to yield improved results and lower (more sensitive) detection levels. 
     The detection of dinitrotoluene (DNT) isomers indicates the feasibility of detection of TNT, and other related explosive substances. In the following examples, three DNT species are used for the demonstration of the validity of the developed detection system  28 . The three DNT species are 3,4-dinitrotoluene, 2,6-dinitrotoluene, and 2,4-dinitrotoluene. The sensor device  30  is suited to the detection of nitro-based explosives using electrochemical techniques. 
     In this study, the detection of the three selected DNT species is first carried out in a 3.5% NaCl solution in deionized water (a common level of salt content in sea water). Solutions with each DNT species at a concentration of 100 ppm were prepared. Square wave voltammetry is employed in the detection using a wavelength of 5 minutes and a potential in the range of −1 to about +0.2V, relative to the reference electrode.  FIG. 5  shows the sensor device response (current profile) to each of the respective DNT solutions. The results shown are an average of several cycles. The current is the difference, in amps, between the applied current and the output current. 
     As is apparent from  FIG. 5 , each individual DNT species shows a distinct current profile which is a characteristic fingerprint of the DNT species in the respective solution. It demonstrates that the sensor device  30  can be used to detect individual DNT species as shown, indicating the sensitivity of the device for such small differences in chemical structure. 
       FIGS. 6-9  show the testing results of the sensor device in the presence of different mixtures of the DNT species. The SWV scans of the system containing multiple components compare favorably to the individual species scans. For example, in the mixture solution containing both 2,6 and 2,4 DNTs (x in  FIG. 6 ) two distinct current peaks are observed at the potential of approximately −0.7 and −0.9 V versus the Ag/AgCl reference electrode. A similar trend in the peak locations can be observed from the individual DNTs. While there are changes in the shape and size of the peak currents, the peak current in the mixture solution can be readily matched to those individual DNT species. Similarly, the results from the tests of other combined mixtures of DNTs can be matched by the locations of the individual DNT also, as shown  FIGS. 7-9 . 
     Example 2 
     Evaluation with Working Electrode Having Liquid Electrolyte Layer 
     The sensor device  30  coated with the ionic liquid electrolyte layer  102  ( FIG. 4 ) was tested for the detection of the DNTs. Specifically, a liquid electrolyte layer  102  composed of 1-butyl-3-methylimidazolium hexafluorophosphate and binding components of PVB/PVDF/PVP was prepared. First, a binder mixture of polyvinylpyrrolidone (PVP), polyvinyl butyral (PVB) polyvinyldifluoride (PVDF), and 90 wt % of N-methylpyrrolidone (NMP) was prepared. A 3:10 ratio of binder to the ionic liquid is used, based on various experimental evaluations of the properties of the gelled ionic liquid prepared. For demonstration purposes, the deposition of the gelled ionic liquid film on the surface of the sensor device was performed using a pipette. The film is dried at 37° C. for one hour in room air evaporating the solvent in the binder ink. After the solvent is evaporated, the sensor devices are cooled in room temperature overnight. The curing conditions for the film can be changed without directly affecting the performance of the sensor device. Square wave voltammetry was used for detection, as in Example 1. 
       FIG. 10  shows the testing results obtained in the presence of individual DNT species using the ionic liquid coated sensor device prototype using square wave voltammetry as for Example 1. The concentration of each DNT is 100 ppm, and the DNT is dissolved in DI water as the medium for the test.  FIG. 10  demonstrates the performance of the gelled ionic liquid electrolyte sensor device prototype for the detection of individual DNT species in 3.5 wt. % NaCl solution.  FIG. 10 , similar to the results shown in  FIG. 5 , shows a reasonable characteristic fingerprint for each individual DNT. 
       FIGS. 11-14  show a family of test results in combined mixtures of DNTs. The test results in the combined mixtures of DNTs compare well with the individual DNT&#39;s in terms of its peak current and potential locations using the ionic liquid electrolyte film as the ionic conductive layer. This observation is similar to that obtained in NaCl solution shown in  FIGS. 6-9 . The experimental results shown in  FIGS. 6-9  and  11 - 14  illustrate that there is a difference in the test results using the sensor device with and without the ionic liquid layer integrated with the sensor device. However, the detection of DNTs using the ionic liquid layer sensor device is readily achieved. For the ionic liquid layer electrode, there is a shift in some of the locations of the peak currents of the mixtures as compared to the individual DNTs. This shift may be due to the preparation method and could be eliminated in an automated process. 
     Example 3 
     Evaluation of Liquid Electrolyte Layer with Gaseous DNTS 
     In another evaluation, the feasibility of using the sensor device  30  with the ionic liquid electrolyte layer  102  to detect gaseous DNT species was undertaken. A simulated container  10 ′ in the form of an enclosure or test chamber was designed and constructed, as illustrated in  FIG. 15 . 
     The temperature and the quantity of the DNT in the gas phase can be controlled using a flexible polyimide (Kapton) coated heater  140  with a current source. A digital thermometer allows for the continuous monitoring of the temperature inside the test chamber. The test chamber is completely sealed during the testing with electrical leads integrated into the side wall of the chamber providing electrical connections for the electrode elements of the sensor device  30  and the current of the heater. 
     The performance of the gas phase testing of DNTs is carried out using this test chamber. A sensor device  30 , with a gelled ionic liquid layer  102  was prepared in the manner described for Example 2. 2,4 and 2,6 DNTs are used in this gas phase testing. Square wave voltammetry scans are used at a time interval of 5 minutes over a total time period of 20 minutes in the potential range of −0.9 V to +0.3 V vs. the Ag/AgCl reference electrode. Other experimental parameters for this test are the same as those employed in the previous Examples.  FIGS. 16 and 17  show the comparison of the results in the presence and absence of the DNTs in the gas phase inside the test chamber. 
     There is a detectable difference between the sensor device outputs in the presence of the DNT and the background air (the absence of the DNT) in this gas phase testing. The test results demonstrate that the peak current at a specific potential of this gelled ionic liquid layer coated sensor device  30  can be used to identify the presence of the DNT in the air versus that of the background air. It can therefore be inferred that such a gelled ionic liquid covered sensor device operated in the SWV mode is thus capable of detecting DNT (and hence TNT) vapors in a closed environment, such as a cargo container  10 . 
     In summary, these examples demonstrate that a screen-printed active carbon sensor device with a carbon working, a carbon counter and an Ag/AgCl reference electrode is capable of detecting either individual or a combination of DNT species. Due to the similar structure of DNT to TNT, the capability of the sensor device to detect nitro containing explosives can be assumed. It is found that this sensor device can detect DNTs in a 3.5 wt % NaCl solution (salinity of typical sea water) using the square wave voltammetry measuring technique. 
     Further, in Examples 2 and 3, the integration of an ionic liquid layer  102  serving as electrolyte composed of 1-butyl-3-methylimidazolium hexafluorophosphate and binding components of PVB/PVDF/PVP has been tested. The testing results show that this sensor device with the gelled ionic liquid electrolyte film is effective in measuring DNTs in solution. 
     Testing of the sensor device performance in the presence of gas phase DNTs has also been performed in Example 3. The experimental results show that the sensor device  30  has the ability to distinguish the presence of the DNT species and the absence of the DNT (air background). This detection is achieved in 20 minutes of testing time in a closed environment, such as a sealed container. The sensor device fabricated is capable of detecting DNT, or nitro containing explosives in air, at low concentration. This provides for detection of explosives in various environments. The sensor device can be manufactured cost-effectively. 
     The exemplary embodiment has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the exemplary embodiment be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.