Abstract:
A device is provided for determining the degree of the presence of an unwanted environmental agent. The apparatus comprises a device ( 30, 40 ) having first ( 31, 41 ) and second ( 33, 46 ) conducting layers with alternatively interdigitized fingers ( 34, 36, 42, 43, 44, 47, 48, 49 ) coupled to a nano-structure ( 32, 45 ) having a high aspect ratio, wherein sections ( 35, 37, 50, 51, 52, 53, 54 ) of the nano-structure between each of the fingers are substantially equal in length. Circuitry ( 62 ) coupled to the first and second conducting layers determines the occurrence of a change in a material characteristic of the sections of the nano-structure.

Description:
FIELD OF THE INVENTION  
       [0001]     The present invention generally relates to a device for determining the presence of an unwanted environmental agent, and more particularly to a nanotube device for determining the degree of the presence of the unwanted environmental agent.  
       BACKGROUND OF THE INVENTION  
       [0002]     Emergency responders, such as fire fighters, police, or HAZMAT personnel, many times arrive at the site of an emergency situation without the ability to detect environmental hazards such as toxic industrial chemicals, chemical warfare agents, or radiation. Furthermore, if it is known that an environmental hazard is present, they cannot determine the severity, or concentration, of the hazard. Such inability may result in physical harm to the emergency responders. Large quantities of toxic industrial chemicals may be present in populated areas: industrial sites, storage depots, transportation and distribution facilities, resulting in the potential for accidents such as the accidental release of methylisocyanate in Bhopal, India in 1984. Other toxic industrial chemicals, for example, include ammonia, chlorine, hydrogen chloride, and sulfuric acid. Chemical warfare agents are usually more lethal than toxic industrial chemicals. Nerve agents are the most common chemical warfare agents, such as the nerve agent Sarin that was used in the 1995 Tokyo subway gas attack. Other chemical warfare agents, for example, include Tabun, sulfur mustard, and hydrogen cyanide.  
         [0003]     Chemical warfare agents typically are medium to high volatility and therefore may be detected in the gas phase. Electronic monitors for chemical warfare agents are based on electronic detection using ion-mobility-spectrometry, photo-ionization and flame-ionization. These tools offer a broadband response with high levels of sensitivity, but most suffer from interference effects caused by what is often a highly complex chemical background mix at the scene, and most commercial tools exhibit high false-positive responses to contaminants. Furthermore, these devices are not designed to be wearable, and most tools, although handheld, are relatively bulky and fully engage the user, thereby detracting from other important duties.  
         [0004]     Known colorimetric methods for detecting such chemical and biological hazards include simple color-change badges generally having a life span of approximately 8 hours, to tubes providing quantitative data with high specificity, but both require the user to assess the color change to determine the hazard level. Furthermore, gas tubes are sensitive to physical abuse and are limited in some cases to only one or in other cases a few hazards requiring the user to know what type or types of hazards are suspected.  
         [0005]     Radiological threats have become more relevant with the so-called ‘dirty bomb’, which combines explosive blast with surreptitious ingredients of radionuclides such as Cs-137, a beta and gamma emitter. Radiological monitors (dosimeters) have been available for many years, mostly for occupational safety monitoring. Pager style, wearable units, having audio/visual alerts built-in are available for such monitoring. Also, a variety of miniature radiation detectors exist, such as small Geiger-Muller tubes, selective scintillation layers with photo-sensors, and silicon diodes. Probes can be attached to other types of monitors, covering any of the radiation species, but these monitors are at best hand-held, and must be maintained regularly. Recently, colorimetric badges that detect radiation have been developed; however, these require the user to constantly monitor its status.  
         [0006]     Carbon is one of the most important known elements and can be combined with oxygen, hydrogen, nitrogen and the like. Carbon has four known unique crystalline structures including diamond, graphite, fullerene and carbon nanotubes. In particular, carbon nanotubes refer to a helical tubular structure grown with a single wall or multi-wall, and commonly referred to as single-walled nanotubes (SWNTs), or multi-walled nanotubes (MWNTs), respectively. These types of structures are obtained by rolling a sheet formed of a plurality of hexagons. The sheet is formed by combining each carbon atom thereof with three neighboring carbon atoms to form a helical tube. Carbon nanotubes typically have a diameter in the order of a fraction of a nanometer to a few hundred nanometers.  
         [0007]     Carbon nanotubes can function as either a conductor, like metal, or a semiconductor, according to the rolled shape and the diameter of the helical tubes. With metallic-like nanotubes, it has been found that a one-dimensional carbon-based structure can conduct a current at room temperature with essentially no resistance. Further, electrons can be considered as moving freely through the structure, so that metallic-like nanotubes can be used as ideal interconnects. When semiconductor nanotubes are connected to two metal electrodes, the structure can function as a field effect transistor wherein the nanotubes can be switched from a conducting to an insulating state by applying a voltage to a gate electrode. Therefore, carbon nanotubes are potential building blocks for nanoelectronic devices because of their unique structural, physical, and chemical properties.  
         [0008]     Existing methods for the production of nanotubes, include arc-discharge and laser ablation techniques. Unfortunately, these methods typically yield bulk materials with tangled nanotubes. Recently, reported by J. Kong, A. M. Cassell, and H Dai, in Chem. Phys. Lett. 292, 567 (1988) and J. Hafner, M. Bronikowski, B. Azamian, P. Nikoleav, D. Colbert, K. Smith, and R. Smalley, in Chem. Phys Lett. 296, 195 (1998) was the formation of high quality individual single-walled carbon nanotubes (SWNTs) demonstrated via thermal chemical vapor deposition (CVD) approach, using Fe/Mo or Fe nanoparticles as a catalyst. The CVD process has allowed selective growth of individual SWNTs, and simplified the process for making SWNT based devices. However, the choice of catalyst materials that can be used to promote SWNT growth in a CVD process has been limited to only Fe/Mo nanoparticles. Furthermore, the catalytic nanoparticles were usually derived by wet chemical routes, which are time consuming and difficult to use for patterning small features.  
         [0009]     Another approach for fabricating nanotubes is to deposit metal films using ion beam sputtering to form catalytic nanoparticles. In an article by L. Delzeit, B. Chen, A. Cassell, R. Stevens, C. Nguyen and M. Meyyappan in Chem. Phys. Lett. 348, 368 (2002), CVD growth of SWNTs at temperatures of 900° C. and above was described using Fe or an Fe/Mo bi-layer thin film supported with a thin aluminum under layer. However, the required high growth temperature prevents integration of CNTs growth with other device fabrication processes.  
         [0010]     Single walled carbon nanotubes have been shown to be a highly sensitive chemical and biological sensor. The utility of detecting the presence or absence of a specific agent is one type of known detection scheme. As the agent attaches itself to a nanotube, the measurable resistance of the nanotube changes. As the resistance changes, a quantitative result, e.g., concentration, may be determined. Known nanotube systems use a single nanotube (only one path for determining resistance), a parallel array of nanotubes, or a network array of nanotubes to determine the presence of an unwanted agent.  
         [0011]     However, known nanotube systems do not provide a dynamic range that spans several orders of magnitude. An accurate method is needed that gives a greater degree of accuracy and reliability.  
         [0012]     Accordingly, it is desirable to provide a device for determining the degree of the presence of an unwanted environmental agent. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.  
       BRIEF SUMMARY OF THE INVENTION  
       [0013]     A device is provided for determining the degree of the presence of an unwanted environmental agent. The apparatus comprises a device having first and second conducting layers with alternatively interdigitized fingers coupled to a nano-structure having a high aspect ratio, wherein sections of the nano-structure between each of the fingers are substantially equal in length. Circuitry coupled to the first and second conducting layers determines the occurrence of a change in a measurable characteristic of the sections of the nano-structure. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]     The present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and  
         [0015]      FIG. 1  is a schematic of a known device including electrodes across two nano-structure;  
         [0016]      FIG. 2  is a schematic of a known device including electrodes across five nano-structure;  
         [0017]      FIG. 3  is a schematic of a first embodiment of the present invention;  
         [0018]      FIG. 4  is a schematic of a second embodiment of the present invention; and  
         [0019]      FIG. 5  is a block diagram of another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0020]     The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention.  
         [0021]     When a molecule attaches itself to a nano-structure, such as a carbon nanotube, a characteristic of the material changes, such as the change in a current flowing in the nanotube that is measurable in a manner known to those skilled in the art. While a carbon nanotube is the preferred embodiment of the nano-structure, other embodiments would include all other nano-structures with a high aspect ratio (length versus width), for example, carbon fibers, metal nanowires, semiconductor nano-wires, nano-ribbons, and tubes formed with other materials such as boron nitride. Additionally, the nano-structure may be coated with a substance for determining specific environmental agents. And while a change in current is the preferred embodiment for the measurable material characteristic, other embodiments would include, for example, magnetic, optical, frequency, and mechanical.  
         [0022]     By measuring this change in the current, it is known that a determination may be made as to the number of molecules that have attached to the carbon nanotube, and therefore, a correlation to the concentration of the molecules in the environment around the carbon nanotube. Known systems place an electrode across a carbon nanotube to measure this change in the material characteristic.  
         [0023]     However, since a single section of a nanotube may not provide enough area for a sufficient reading, several sections of a plurality of nanotubes are positioned across the pair of electrodes. For example,  FIG. 1  illustrates electrodes  10  and  12  coupled across two nanotubes  14 , and  FIG. 2  illustrates electrodes  10  and  12  coupled across five nanotubes  14 . However, when several nanotubes are placed in parallel, the length (since they are rarely parallel) and diameter of each nanotube will vary. Additionally, the number of nanotubes coupled between electrodes may not be known. This results in an inaccurate and non-standard reading.  
         [0024]     Referring to  FIG. 3 , a cross section of a first embodiment of the present invention comprises a device  30  including a first electrode  31  electrically coupled to a nanotube  32 . A second electrode  33  includes a first arm  34  electrically coupled to the nanotube  32 , thereby defining a first portion  35  of nanotube  32  between the electrode  31  and the first arm  34 . A second arm  36  of the second electrode  33  is electrically coupled to the nanotube  32 , thereby defining a second portion  37  of nanotube  32  between the electrode  31  and the second arm  36 . By using two sections of the same nanotube  32 , the diameter will be the same and since the nanotube will be substantially straight and the first and second arms  34  and  36  will be equally spaced on either side of electrode  31 , the sections will be the same length.  
         [0025]     While only two sections  35  and  37  of the nanotube  32  are shown in  FIG. 3 , it should be understood that any number of sections could be used. See for example  FIG. 4 , where device  40  comprises a first arm  41  having three arms  42 ,  43 , and  44  equally spaced apart and making electrical contact with nanotube  45 . A second arm  46  comprises arms  47 ,  48 , and  49  equally spaced and making electrical contact with nanotube  45 . Each of the arms  42 ,  43 ,  44 ,  47 ,  48 , and  49  cooperate to define nanotube sections  50 ,  51 ,  52 ,  53 , and  54 .  
         [0026]     Referring to  FIG. 5 , an exemplary system  60  includes the device  30  or  40 , for example, having their electrodes  31  and  33 , or  41  and  46  coupled to a power source  61 , e.g., a battery. A circuit  62  determines the current between the electrodes and supplies the information to a processor  63 . The information may be transferred from the processor  63  to a display  64 , an alert device  65 , or an RF transmitter  66 .  
         [0027]     The nanotubes  32  and  45  may be grown in any manner known to those skilled in the art, and are typically 100 nm to 1 cm in length and less than 1 nm to 100 nm in diameter. The conductive layers, or electrodes  31 ,  33 ,  41 , and  46 , may comprise any conductive material, but preferably would comprise layers of chromium and gold, titanium and gold, palladium, or gold. Contact between the nanofubes  32  and  45  and electrodes  31 ,  33 ,  41 , and  46  is made during fabrication, for example, by any type of lithography, e-beam, optical, soft lithography, or imprint technology.  
         [0028]     Referring to  FIG. 6 , the use of an increasing number of interdigited finger devices gives the ability to determine the concentration range of the environmental agent. Placing the device  30  and device  40  on the same nanostructure allows for a determination of the concentration level of the environmental agent. Having many more devices with 7, 9, 11, . . . N fingers, where N corresponds to the highest concentration value to be measured, allows for a more accurate determination of the concentration value. For a given concentration, the devices with the smaller number of fingers will saturate (maximum possible reading) before the devices with larger number of fingers. For example, sequential reading of the devices in an ascending order until determining that a device has not saturated will indicate the concentration of the environmental agent.  
         [0029]     Referring to  FIG. 7 , multiples of one device having the same number of fingers, e.g., device  30 , on the same nano-structure provide better stringency (precision). Therefore, the preferred embodiment would have devices with, for example, 3, 3, . . . x; 5, 5, . . . y; 7, 7, z . . . N, where x, y, and z are determined based on the desired stringency. Having multiple devices with the same number of fingers allows for averaging the reading from each device, resulting in a more precise reading of the presence of the environmental agent.  
         [0030]     The sensor described herein provides a larger dynamic sensitivity range while not degrading any of the performance due to variations in the sensing element by sensing the current through sections of the same long nanotube, thereby eliminating the need to make shorter nanotubes identical in diameter and chirality. A single long nanotube has the same diameter and chirality along its entire length. Dynamic range, or the ability to accurately detect the number of agents in the environment, is thereby increased.  
         [0031]     While at least one exemplary embodiment has been presented in the foregoing detailed description of the invention, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended claims.