Abstract:
A method and apparatus for detecting a particular chemical in a sample, includes placing the sample in contact with a semiconductive material provided on a flow cell. An electrical characteristic of the semiconductive material is detected by an interdigitated electrode, and a first signal indicative thereof of output. An optical characteristic of the semiconductive material is detected by a photodetector and a second signal indicative thereof is output. Based on the first and second signals, it is determined by a processor as to whether or not the particular chemical is present in the sample.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATIONS 
       [0001]    This application claims priority of U.S. Provisional Application No. 61/193,610, filed Dec. 10, 2008, which is hereby incorporated by reference. 
     
    
     FIELD 
       [0002]    The field is semiconductor sensors, including carbon nanotube sensors, intrinsic conducting polymer (ICP) sensors and the like. 
       BACKGROUND 
       [0003]    Sensor devices having sensor arrays are becoming very useful in today&#39;s society, with the threat of chemi and bio-terrorism being more and more prominent. In more detail, chemical and biological warfare pose both physical and psychological threats to military and civilian forces, as well as to civilian populations. 
         [0004]    An important feature of a sensor array unit is the ability to detect abnormalities in a sample, and to output an alarm when the abnormality is detected. Given that an abnormality may occur when only a very small concentration of a particular analyte exists in a sample, it is important that the sensor array unit is highly sensitive to such a very small concentration of the particular analyte. 
         [0005]    Semiconducting materials such as carbon nanotube sensors exhibit good properties for detecting trace amounts of certain chemicals. It is desirable to utilize carbon nanotube sensors for detecting many types of chemicals, and to develop metrics for assuring proper detection of those chemicals. 
       SUMMARY 
       [0006]    Accordingly, there is a need for a method and apparatus for detecting chemicals using semiconductor sensor materials. 
         [0007]    In accordance with one aspect, there is provided an apparatus for detecting a particular chemical. The apparatus includes a flow cell having an optically transparent window provided thereon. The apparatus also includes a light source disposed on the first side of the flow cell outside of the flow cell. The apparatus further includes a semiconductive material disposed within the flow cell where the optically transparent window is located. The apparatus still further includes at least one interdigitated electrode disposed within the flow cell where the optically transparent window is located, the electrode being in contact with the semiconductive material. The apparatus also includes a photodetector provided a second side of the flow cell opposite the first side of the flow cell, the photodetector being disposed outside of the flow cell. The apparatus further includes a processor that is electrically connected to the electrode and the photodetector and which receives first and second signals respectively output from the electrode and the photodetector with respect to a particular band. The processor determines whether or not the particular chemical is included in a sample incident on the apparatus. 
         [0008]    In accordance with another aspect, there is provided a method for detecting a particular chemical in a sample. The method includes placing the sample in contact with a semiconductive material provided on a flow cell. An electrical characteristic of the semiconductive material is detected by at least one interdigitated electrode, and a first signal indicative thereof of output. An optical characteristic of the semiconductive material film is detected by a photodetector, and outputting a second signal indicative thereof is output. Based on the first and second signals, it is determined by a processor as to whether or not the particular chemical is present in the sample. 
         [0009]    In accordance with yet another aspect, there is provided a computer readable medium embodying computer program product for detecting the presence or absence of a particular chemical in a sample. The computer program product, when executed by a computer or a microprocessor, causes the computer or the microprocessor to perform a step of placing the sample in contact with a semiconductive material provided on a flow cell. An electrical characteristic of the semiconductive material is detected by at least one interdigitated electrode, and a first signal indicative thereof of output. An optical characteristic of the semiconductive material is detected by a photodetector, and outputting a second signal indicative thereof is output. Based on the first and second signals, it is determined by a processor as to whether or not the particular chemical is present in the sample. 
         [0010]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as claimed. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments and, together with the description, serve to explain the principles of the invention. 
           [0012]      FIG. 1  is a plot showing changes in electrical characteristics of a poly aminobenzene sulfonic acid functionalized single walled carbon nanotubes (PABS-SWNT) film and changes in optical adsorption characteristic of the PABS-SWNT film over an S 11  band, when exposed to hydrogen cyanide (HCN), according to a first embodiment. 
           [0013]      FIG. 2  is a plot showing changes in electrical characteristics of a PABS-SWNT film and changes in optical adsorption characteristic of the PABS-SWNT film over an S 11  band, when exposed to hydrogen chloride (HCl), according to the first embodiment. 
           [0014]      FIG. 3  is a plot showing changes in electrical characteristics of a PABS-SWNT film and changes in optical adsorption characteristic of the PABS-SWNT film over an S 11  band, when exposed to chlorine (Cl 2 ), according to the first embodiment. 
           [0015]      FIG. 4  is a plot showing changes in electrical characteristics of a PABS-SWNT film and changes in optical adsorption characteristic of the PABS-SWNT film over an S 11  band, when exposed to ammonia (NH 3 ), according to the first embodiment. 
           [0016]      FIG. 5  is a plot showing the increased observed intensity of the S 11  band and the spectral features of the PABS-SWNT material as it is exposed to 30 ppm NH 3 , according to the first embodiment. 
           [0017]      FIG. 6  is a plot showing changes in electrical characteristics of an octadecylamine functionalized single wall carbon nanotubes (ODA-SWNT) film and changes in optical adsorption characteristic of the ODA-SWNT film over an S 11  band, when exposed to hydrogen cyanide (HCN), according to the first embodiment. 
           [0018]      FIG. 7  is a plot showing changes in electrical characteristics of an ODA-SWNT film and changes in optical adsorption characteristic of the ODA-SWNT film over an S 11  band, when exposed to hydrogen chloride (HCl), according to the first embodiment. 
           [0019]      FIG. 8  is a plot showing changes in electrical characteristics of an ODA-SWNT film and changes in optical adsorption characteristic of the ODA-SWNT film over an S 11  band, when exposed to chlorine (Cl 2 ), according to the first embodiment. 
           [0020]      FIG. 9  is a plot showing changes in electrical characteristics of an ODA-SWNT film and changes in optical adsorption characteristic of the ODA-SWNT film over an S 11  band, when exposed to ammonia (NH 3 ), according to the first embodiment. 
           [0021]      FIG. 10  is a block diagram of a sensor device according to a first embodiment. 
           [0022]      FIG. 11  is a view along an x-z axis of the sensor device according to the first embodiment. 
           [0023]      FIG. 12  is a view along an x-y axis of the sensor device according to the first embodiment. 
           [0024]      FIGS. 13   a - 13   c  respectively represent the density of states of semiconducting SWNTs, doped SWNTs, and metallic SWNTs, and  FIG. 13   d  is a schematic illustration of the S 11  and S 22  electronic spectrum of SWNTs. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. An effort has been made to use the same reference numbers throughout the drawings to refer to the same or like parts. 
         [0026]    Unless explicitly stated otherwise, “and” can mean “or,” and “or” can mean “and.” For example, if a feature is described as having A, B, or C, the feature can have A, B, and C, or any combination of A, B. and C. Similarly, if a feature is described as having A, B, and C, the feature can have only one or two of A, B, or C. 
         [0027]    Unless explicitly stated otherwise, “a” and “an” can mean “one or more than one.” For example, if a device is described as having a feature X, the device may have one or more of feature X. 
         [0028]    The inventors of this application have found that functionalized carbon nanotubes. In one embodiment, the nanotubes can be single-walled nanotubes. In another embodiment, the nanotubes can be poly aminobenzene sulfonic acid (PABS) functionalized. In a further embodiment, the nanotubes can be poly aminobenzene sulfonic acid functionalized single-walled nanotubes (PABS-SWNT). PABS-SWNTs display unique optical-electrical signatures when exposed to chemical vapors. Accordingly, it can be useful to measure the optical properties and electrical properties (conductance or resistance) of PABS-SWNT. In one embodiment, both the near infrared (NIR) absorption of the S 11  band and the electrical conductance (or resistance) of the PABS-SWNT material can be measured. In one embodiment, the measurement of NIR absorption of the S 11  band and the electrical conductance (or resistance) can be measured on the same sample, can be measured successively, and can be measured simultaneously. 
         [0029]    Any suitable chemical compound can be detected. In one embodiment, the chemical vapor can be hydrogen cyanide (HCN). Without being tied to any given theory, the inventors of this application have found that when PABS-SWNT material is exposed to HCN, the observed optical absorption of the S 11  band increases and the conductance of the material increases in direct proportion; i.e., the optical adsorption and the resistance of the materials change in opposite directions, as illustrated in  FIG. 1  (50 ppm HCN in-situ response). The optical adsorption data is shown by plot  110 , and the resistance data is shown by plot  120 . The S 11  band for a nanotube can depend on the diameter of the nanotube, and is typically a band within a range from approximately 100 nm to approximately 1500 nm. For example, a nanotube having a diameter of 1.01 nm can have an S 11  band at 1190 nm. The type of carbon nanotubes used in one embodiment is composed of nanotubes with a distribution of diameters which show typical S 11  band between approximately 1420 nm to approximately 2500 nm. 
         [0030]    The nature of electronic structure of SWNTs around the Fermi level can be associated with the interband transitions of interests.  FIGS. 13   a - 13   d  represent the density of states of semiconducting SWNTs and metallic SWNTs. For semiconducting SWNTs produced by chemical vapor deposition method, S 11  and S 22  refer to the first and second interband transitions which occur near approximately 4000 to approximately 7000 cm −1  (or 1428 nm-2500 nm) and approximately 7750 to approximately 11750 cm −1  (850 nm-1290 nm) respectively. The S 11  band can be more susceptible to doping effect. Without being tied to a particular theory, the wide band width likely is due to the mixing SWNTs of different diameter and bundle sizes. In more detail,  FIG. 13   a  shows a schematic representation of the density of states (DOS) of semiconducting SWNTs in which S 11  and S 22  correspond to the first and second interband transitions which occur in the near-IR spectral range,  FIG. 13   b  shows a schematic representation of the density of states (DOS) of hole doped semiconducting SWNTs in which the first interband transition (S 11  doped) is reduced in intensity due to depletion of the conduction band, and  FIG. 13   c  shows a schematic representation of the density of states (DOS) of metallic SWNTs.  FIG. 13   d  is a schematic illustration of the electronic spectrum (absorbance versus frequency) of SWNTs.  FIGS. 13   a - 13   d  are reproduced from M. E. Itkis, S. Niyogi, M. E. Meng, M. A. Hamon, H. Hu, R. C. Haddon, “Spectroscopic Study of the Fermi level Electronic Structure of single-Walled Carbon Nanotubes”,  Nano Lett.  2002, 2 pgs, 155-159, and M. E. Itkis, D. E. Perea, R. Jung, S. N iyogi, R. C. Haddon, “Comparison of Analytical Techniques for Purity Evaluation of Single-Walled Carbon Nanortubes”,  J. Am Chem. Soc.  2005, 127, pgs. 3439-3448. 
         [0031]    PABS-SWNT material can differentiate between HCN vapor and other chemicals, such as, for example, HCl, Cl 2 , and NH 3  (ammonia) as shown in  FIGS. 2 ,  3  and  4 . In more detail,  FIG. 2  shows an electrical resistance plot  210  and an optical absorption plot  220  with respect to the responses of a PABS-SWNT material to 50 ppm HCl,  FIG. 3  shows an electrical resistance plot  310  and an optical absorption plot  320  with respect to the responses of a PABS-SWNT material to 10 ppm Cl 2 , and  FIG. 4  shows an electrical resistance plot  410  and an optical absorption plot  420  with respect to the responses of a PABS-SWNT material to 300 ppm NH 3 .  FIG. 5  shows the increased observed intensity of the S 11  band and the spectral features of the PABS-SWNT material as it is exposed to 30 ppm NH 3 , whereby plot  520  shows the effects of exposure to NH 3  and plot  510  shows the “before exposure to NH 3 ” characteristics. The detection characteristics also can vary depending upon the functionalization of a carbon nanotube material. For example, HCN response can differ between PABS-SWNT and another carbon nanotube material, such as, for example, octadecylamine functionalized single wall carbon nanotubes (ODA-SWNT). 
         [0032]    For ODA-SWNT and other chemical vapor analytes, experiments performed by the inventors of this application have determined that the optical absorbance and electrical resistance change in direct relation with each other.  FIGS. 6 ,  7 ,  8  and  9  show the observed optical intensity versus resistance characteristics of the ODA-SWNT material as it is exposed to HCN, HCl, Cl 2 , and NH 3 , respectively. In more detail,  FIG. 6  shows an electrical resistance plot  610  and an optical absorption plot  620  with respect to the responses of an ODA-SWNT material to 50 ppm HCN,  FIG. 7  shows an electrical resistance plot  701  and an optical absorption plot  702  with respect to the responses of an ODA-SWNT material to 50 ppm HCl,  FIG. 8  shows an electrical resistance plot  810  and an optical absorption plot  820  with respect to the responses of an ODA-SWNT material to 10 ppm Cl 2 , and  FIG. 9  shows an electrical resistance plot  910  and an optical absorption plot  920  with respect to the responses of an ODA-SWNT material to 300 ppm NH 3 . 
         [0033]    Without being tied to a particular theory, the mechanism for the HCN “increase versus decrease” characteristics could be attributed to charge transfer competition between HCN, the functional group (PABS), and the modified SWNT band structure other than acid-base modulated SWNT band gap changes. See, for example, E. Bekyarova et al., “Mechanism of Ammonia Detection by Chemically Functionalized Single-Walled Carbon Nanotubes: in-situ Electrical and Optical Study of Gas Analyte Detection”, published in  J. Am. Chem. Soc.,  2007, vol. 129, pgs. 10700-10706. 
         [0034]    A sensor device can measure both the optical absorption and the electrical resistance changes, i.e., the optical-electrical signature as a metric. Any suitable analyte or combination of analytes can be examined using a coupled optical-resistance change in a functionalized carbon nanotube, such as, for example, a PABS-SWNT material. In addition to applications for chemical vapor detection, this phenomenon could be used as an actuator to trigger or control other devices or events, e.g., in chemical synthesis or chemical processing using gas. In one embodiment, the chemical synthesis or processing can be of HCN gas. 
         [0035]    A block diagram of a sensor device according to a first embodiment is shown in  FIG. 10 . The flow cell  700  can have optically transparent windows at appropriate wavelength for the nanotube S 11  absorption affixed with a mid-IR lasing LED light source  710  directly to a window on one side of the flowcell  700  and a photodetector  730  affixed directly to the window on the opposite side of the flowcell  700 . A sensing material, which corresponds to a nanotube film  720  (SWNT) in the first embodiment, can be deposited on the window on the opposite side of the flowcell  700 , with the photodetector  730  being disposed under an interdigitated electrode  740 , whereby the electrode  740  can measure the resistance of the nanotube film  720  and whereby the photodetector  730  can measure the optical adsorption characteristics of the nanotube film  720 . The photodetector  720  and the electrode  740  can be electrically connected to a microprocessor  750 , which respectively can receive a first and a second signals from these two elements, and which can interpret the first and second signals. Additional elements can be measured and, accordingly, additional signals can be received and interpreted. 
         [0036]      FIG. 11  is a view along an x-z axis of the sensor device according to the first embodiment, whereby the electrical leads to the microprocessor  750  are not shown for ease in explanation of that figure (but see  FIG. 10 ). An interdigitated electrode  740  is provided within an optically transparent window of the flow cell  700 , and the nanotube film (or layer)  720  is deposited on the electrode  740 . The electrode  740  and the nanotube film  720  can be sealed into the flowcell  700  within a top optically transparent glass plate  765   a  and a bottom optically transparent glass plate  765   b  that form the optically transparent window (with the electrode  740  and the nanotube film  720  sealed therebetween). The optically transparent window with the electrode  740  and nanotube film  720  provided therein is referred to as the bottom of the flowcell  700 , and the optically transparent window with no electrode  740  is referred to as the top of the flowcell  700 . On the outsides of the flowcell  720  are provided the LED  710  and the photodetector  730 . The LED  710  is affixed the top of the flowcell  720  and the photodetector  730  is affixed to the bottom of the electrode  740 . The electrode  740  can be chemically resistant, so that it will not break down over time as gases are input to and output from the flow cell  700  for detection of those gases. 
         [0037]    Both the top and the bottom of the flow cell  720  can be made with any optically transparent material, such as, for example, glass, plastic or crystal (the top optically transparent plate  765   a  and the bottom optically transparent plate  765   b ), so that the casing of the flow cell  700  will not interfere with light passing through the sensing material  720  (e.g., the nanotube film in the first embodiment). The top window of the flow cell  700  can be made of any optically transparent material, such as, for example, glass, plastic or crystal, for example. The bottom of the flow cell  700  can include an optically transparent plate (e.g., glass, plastic or crystal), the interdigitated electrode  740 , electrical leads (capable of connecting the electrode  740  to the processor  750 , and the sensing material  720  (e.g., the nanotube film). 
         [0038]    The optical window is the area of the flow cell  700  that light can pass through, unhindered by electrodes or electrical leads. This is where the optical sensing can take place, whereby this area of the flow cell  700  also can have the sensing material  720  deposited therein. In this embodiment, light can pass freely from a light source  710  (e.g., LED, incandescent bulb, fluorescent tube, etc.) affixed to the outside of the top of the flow cell  720  through the top plate  765   a  of the optically transparent window, then through free space, then through the sensing material  720 , and then through the bottom plate  765   b  of the optically transparent window of the flowcell  720 . The light then is incident on the photodetector  730  affixed to the outside of the bottom of the flowcell  720 .  FIG. 12  shows the electrode  740  provided only on the bottom of the flow cell  700 , whereby the x-z axis view of the flow cell  700  as shown in  FIG. 11  is of the bottom of the flow cell  700 , and whereby the x-z axis view of the top of the flow cell  700  is similar to  FIG. 11  except that there are no electrodes  740  present in that region of the flow cell  700 . The light source  710  is not shown in  FIG. 12 , whereby it is located on the other side of the flowcell  700  and is blocked from view by the photodetector  730  (but see  FIG. 10 ). 
         [0039]    By way of example and not by way of limitation, the microprocessor  750  can execute a program stored in a computer readable medium (e.g., a computer disk). The microprocessor  750  can access data stored in a memory (not shown), whereby the memory stores conductance and optical adsorption data corresponding to previous tests performed on known samples, whereby when there is a sufficient match between the stored memory data and the data corresponding to the first and second signals (e.g., their respective values are at least within at least approximately 85, at least approximately 90, or at least approximately 95% of each other over at least approximately 85, at least approximately 90, or at least approximately 95% of the S 11  band), then the microprocessor  750  can determine that there is a match, and that the particular chemical corresponding to the stored memory data is determined to exist in a sample incident on the flow cell  700  (and whereby the microprocessor  750  outputs an indication, such as an alarm, or visual display, to denote such a match to a user). In more detail, the microprocessor  750  processes and interprets the optical and conductance signals received from the photodetector  730  and the electrode  740 , and makes a decision as to whether or not to issue an alarm and whether or not to perform further agent classification/identification. 
         [0040]    The wide range of carbon nanotube bandgaps (from 0.4 to 6 eV) that are currently available makes carbon nanotubes very suitable for fabrication of sensors in the electromagnetic radiation band, e.g., from UV to IR. It also allows for building wide sensitive range radiation detectors. A wide variety of semiconductive materials could be used for a thin-film sensor according to the present invention, or placed adjacent to the sensor to make an array of sensors and provide additional discrimination of chemical vapors. 
         [0041]    One possible implementation of a mid-IR lasing LED light source  710  as shown in  FIG. 10  and  FIG. 11  would be a parabolic reflector, whereby such a parabolic reflector could minimize the number of optics required as the light source  710  would stay focused over a relatively short optical path length across the flowcell  700  and would be collected by the photodetector  730 . Any suitable parabolic reflector can be used, such as, for example, one manufactured by Dora Texas Corporation in Houston, Tex. The electrode  740  can be disposed such that it would not obstruct the light path. The electrical and optical signals can be collected from the same nanotube film  720  or can be collected from two separate nanotube films provided on the flow cell  700 . In one embodiment, the optical and electrical signals are collected by the same nanotube film  710 . The “two nanotube films” implementation can be used for, among other things, detecting particular chemicals in a sample at low concentration levels. 
         [0042]    Using a combination of using both optical and electrical signals to detect a particular chemical using a SWNT film in a flow cell can enable better selectivity for a range of chemicals in array based chemical sensors. 
         [0043]    An exemplary method of manufacturing a flow cell  700  in accordance with the first embodiment is described below. The flow cell  700  can be made starting from a glass slide, with gold deposited on the entire surface of the glass slide. Then, a gold design pattern  1210  can be made on the glass slide, as seen in  FIG. 12 , to thereby form a pattern that can be used to create an interdigitated electrode  740 . Next, a sensor material (e.g., SWNT) can be deposited on the interdigitated electrode  740  and an open area between two leads  1220   a  and  1220   b  (that connect to the processor  750 ) as a window for a light path through the flow cell  700 . The window for the light path is what is referred to above as the “optical window.” The bottom of the flow cell  700  can be covered with the interdigitated electrode  740  and an SWNT (sensor material)  720 , whereby the SWNT  720  can act as a resistor that changes its conductivity as the chemical nature or chemical environment changes (e.g., a chemiresistor). The flow cell  700  can then placed into a chamber, whereby spacers can be placed on all four sides of the electrode  740  in the z direction. Then, using adhesives, a clear, clean, glass ceiling can be sealed above the electrode  740 , leads, optical window, and SWNT  720 , whereby a space is left in the sides of the flow cell  700  for inlets and outlets for gas flow. 
         [0044]    The embodiments described above have been set forth herein for the purpose of illustration. This description, however, should not be deemed to be a limitation on the scope. Various modifications, adaptations, and alternatives may occur to one skilled in the art without departing from the claimed inventive concept. For example, while there has been demonstrated unique properties of ODA-SWNT and PABS-SWNT nanotube materials for detecting HCl, Cl 2 , HCN and NH 3 , other types of semiconductive materials could be used for a thin-film sensor, or placed adjacent to a thin-film sensor, to thereby make an array of sensors and provide additional discrimination of chemical vapors. By way of example, chemiresistive sensing materials such as Intrinsically Conductive Polymers (ICP) or metal decorated SWNT (MD-SWNT) can be utilized for the thin-film sensor provided on the flow cell. Also, other features within the full SWNT spectrum from IR to UV may hold relevant signatures that can be used to detect certain chemicals and gases using a nanotube material provided within a flow cell. The spirit and scope of the invention are indicated, but not limited, by the following claims.