Abstract:
An array of piezoelectric resonators used in a sensor device in order to identify chemical and biological agents. The resonators can operate as bulk acoustic wave (BAW), surface acoustic wave (SAW), or Love mode devices. The sensor device integrates gravimetric, calorimetric, thermal gravimetric, voltage gravimetric and optical detection methods into one sensor system, improving the accuracy of identifying hazardous agents. For gravimetric detection, dual-mode resonators provide simultaneous calorimetric and gravimetric data, one type from each mode. Resonators with heaters on the surfaces will provide thermal gravimetric data. An optical detector can be used to analyze the optical signal from the surface of a coated resonator. Additionally, voltage gravimetric measurements can be made with an electric field set up between the resonator and an external electrode. Thermal voltage gravimetric measurements can be made by adding an integrated heater on the resonator with an external electrode. An alarm can be activated upon the identification of a hazardous agent. The sensor device can utilize other valuable information, including traceable time, GPS location, and variables related to temperature, humidity, air speed, and air direction.

Description:
FIELD OF THE INVENTION 
     The present invention relates to biological and chemical sensors integrating several physical measurements of target agents. 
     BACKGROUND OF THE INVENTION 
     The fast and accurate identification of biological and chemical agents is not only of great interest within the sensor community, but performs a public service by saving lives. The wide dissemination of inexpensive and accurate sensor systems with very low or zero false alarm rates is critical in order to respond to terrorist threats or accidental exposures. False alarms are very costly and could lead to dilatory responses to subsequent real terrorist threats and accidental exposures. 
     The effectiveness of the first responders depends upon their knowing what hazardous substance has been detected, the concentration of the hazardous substance and the time of the initial exposure. A sensor system which is fast, inexpensive and accurate, and with a low false alarm rate, is critical in both military and civilian applications. 
     The false alarm rate can be reduced significantly through the use of multiple orthogonal detection methods. Orthogonal methods detect different physical characteristics of a target agent or substance. For example, optical and gravimetric effects are orthogonal. Gravimetric effects result from mass changes on the resonator, while optical techniques look at the interaction of electromagnetic radiation. 
     For example, U.S. Pat. No. 5,744,902 to Vig describes detectors using a dual-mode sensor using both a gravimetric and a calorimetric analysis of chemical/biological agents. 
     However, other than gravimetric and calorimetric, none of the prior art detection systems integrates two or more orthogonal measurements (selected from the following methods: gravimetric, calorimetric, thermal gravimetric, voltage gravimetric, and optical detection methods) into one sensor system, thereby substantially improving the identification of hazardous agents and reducing the false alarm rate. 
     SUMMARY OF THE INVENTION 
     The present invention provides an array of piezoelectric resonators, which are used as a “laboratory” for measuring mechanical, physical and chemical effects. The array can be manufactured from a single resonator, or individual resonators can be formed into an array, depending on the application. 
     The resonators in the array can be arranged into configurations for each test. For gravimetric detection, dual-mode resonators will provide simultaneous calorimetric and gravimetric data, one type from each mode. Resonators with heaters on the surfaces will provide thermal gravimetric data. Further, the heaters can make the resonators “self-cleaning.” An optical detector can be used to analyze the optical signal from the surface of a coated resonator; incorporating gold-nano particles into the coating and the electrode of the resonator can enhance the optical signal. Additionally, voltage gravimetric measurements can be made with an electric field set up between the resonator and an external electrode. Thermal voltage gravimetric measurements can be made by adding an integrated heater on the resonator with an external electrode. 
     The array of piezoelectric resonators can operate as bulk acoustic wave (BAW), surface acoustic wave (SAW), or Love mode devices. 
     The integration of gravimetric, calorimetric, thermal gravimetric and optical analytical methods into one sensor system greatly reduces the false alarm rate for detecting chemical and biological agents. For example, after the optical sensor provides data to identify a target agent, the resonators can be used to determine its concentration. 
     The sensors can be used in buildings and open spaces to monitor terrorist threats for Homeland Security and to identify hazardous waste in environmental applications. They can be used to monitor chemical and biological agents in military and commercial settings. Further, they can be used to detect toxic mold in buildings. 
     It is an object of the present invention to provide a sensor system capable of detecting a wide variety of chemical and biological agents and concentrations, with an extremely low false alarm rate. 
     Another object of the present invention is to combine two or more orthogonal detection methods based on gravimetric, calorimetric, thermal gravimetric, voltage gravimetric, and optical measurements. 
     Still another object of the present invention is to provide a sensor system which is fast and accurate, yet inexpensive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a top view of an array of bulk acoustic wave (BAW) devices. 
         FIG. 2  is a cross-sectional side view of the array of the BAW devices illustrated in  FIG. 1  taken along line  2 — 2 . 
         FIG. 3  is a cross-sectional side view of a BAW device, with an external electrode plate. 
         FIG. 4  is a cross-sectional side view of a BAW device integrated with a fluorescent optical detector system. 
         FIG. 5  is a representational top view of an array of SAW devices. 
         FIG. 6  is a representational side view of the array of SAW devices. 
         FIG. 7  is a top detail view of a single surface acoustic wave (SAW) device. 
         FIG. 8  is a sectional view of the single SAW device illustrated in  FIG. 7  taken along line  8 — 8 . 
         FIG. 9  is a schematic diagram illustrating the sensor device of the present invention. 
         FIG. 10  is a representational top view of the sensor device of the present invention, embodied in a hand-held unit. 
         FIG. 11  is a representational cross-sectional side view of the sensor device illustrated in  FIG. 10  taken along line  11 — 11 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The integrated sensors of the present invention use piezoelectrically-based resonators designed from a piezoelectric crystal such as quartz, lithium niobate, lithium tantalate, langasite, or Gallium Orthophosphate. The resonators can operate as bulk acoustic wave (BAW), surface acoustic wave (SAW), or Love mode devices. In all cases, the frequency is influenced by material deposited onto the surfaces. When mass is deposited onto the crystal, a change in frequency of the resonator occurs. 
     The sensors are miniature laboratories capable of measuring mechanical, physical and chemical effects. The sensors described herein can be used for detecting the presence and concentration of chemical and biological agents in a medium of air or liquids. A sensor array is formed from a number of resonators, each a multiple (2, 3 or more) mode piezoelectric resonator, which is energy trapped, having a highly smooth surface relative to the wavelength of the mode. Electrodes formed on each resonator excite the resonator. A coating of nano particles (gold, carbon or another material) can be used to enhance the absorption sites for the target agent (and the resonator&#39;s gravimetric response), as well as the optical reflectivity for optical detection. A sensor coating on the resonators will bond, chemically or physically, to certain target agents. Each resonator can have a different sensor coating; some can have no coating at all. The different sensor coatings applied to the resonators in an array are selected so that orthogonal physical properties can be measured, thereby allowing the user to look at the target agent from different directions. A heating element on the resonator controls its temperature and is used to generate data for use in thermal-gravimetric analysis (mass change with heat). 
     Using conventional means, the medium to be tested is concentrated and then delivered to the surface of the crystal resonators. An excitation circuit causes the multiple modes of the resonators to be excited at the same time so that the mass change and temperature change can be measured independently, allowing the mass loading to be calculated accurately. A circuit with variable drive levels can be used to detect when the particles on the surface of the resonator become detached. An optical sensor focused on the surface of the resonator can be used to identify the atomic absorption wavelengths of the target agent. The optical sensor can be transmitted, reflected or fluorescent light. A circuit measures the power dissipated in the crystal via the heating element and can be used to determine the heat of reaction between the target agent and the coating on the surface of the resonator (the additional heat generated in the resonator causes a decrease in the heat required to maintain the crystal at a predetermined temperature). A measurement circuit is used to collect the gravimetric, calorimetric, thermal-gravimetric and optical data. An analysis algorithm is used to determine the identify of the target agent. A communications system is used to relay information about the detected agent and its concentration. Finally, an alarm system can be utilized when the target agent and/or its concentration are identified as hazardous. 
     As shown in Table 1 below, typically the following physical characteristics can be measured: 
     
       
         
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 PHYSICAL 
                   
                 VARIABLE 
               
               
                 CHARACTERISTICS 
                   
                 CONTROL 
               
               
                 MEASURED 
                 MEASUREMENT 
                 PARAMETER 
               
               
                   
               
             
             
               
                 Gravimetric 
                 Frequency of 
                 None 
               
               
                 (mass change) 
                 first mode 
               
               
                 Calorimetric 
                 Frequency of first 
                 None 
               
               
                 (heat generated) 
                 and second mode 
               
               
                 Elastic Properties 
                 Impedance of 
                 None 
               
               
                 of film 
                 first mode 
               
               
                 Drive Power Effects on 
                 Impedance of 
                 Drive Power 
               
               
                 Impedance of first mode 
                 first mode 
               
               
                 Drive Power Effects on 
                 Frequency of 
                 Drive Power 
               
               
                 Frequency of first mode 
                 first mode 
               
               
                 Thermal-Gravimetric 
                 Frequency of 
                 None 
               
               
                 (mass change 
                 first mode vs. 
               
               
                 with temperature) 
                 temperature of resonator 
               
               
                 Voltage-Gravimetric 
                 Frequency of 
                 None 
               
               
                 (mass change 
                 first mode vs. 
               
               
                 with electric field 
                 temperature of resonator 
               
               
                 Thermal-Voltage 
                 Frequency of 
                 Temperature 
               
               
                 Gravimetric (mass 
                 first mode, temperature 
                 or Voltage 
               
               
                 change with electric field 
                 or resonator, and 
               
               
                 and temperature) 
                 electric field 
               
               
                   
               
             
          
         
       
     
     When enough physical characteristics of a target agent are measured, the accuracy of the identification is greatly increased, resulting in zero or near-zero false alarms. In addition, thresholds can be set to permit certain concentrations of target agents to be tolerated, with an alarm sounding only when the concentration reaches an unacceptable level. 
     The present invention can be embodied in a bulk acoustic wave (BAW) array  10 , such as the one shown in  FIG. 1  and  FIG. 2 . Formed on a single crystal quartz wafer  11  is an arrangement of BAW resonator plates  12 . The BAW resonator plates  12  can be round, as shown, or can have another shape, such as square or hexagonal. The BAW resonator plates  12  can be arrayed in a 3×4 array, as shown, or can be arrayed 2×2, 2×n, 3×3, 3×n, 4×4, or 4×n. The BAW resonator plates  12  have contoured surfaces to improve the short term stability of the resonators by reducing the noise flow. 
     Depending on its position in the BAW array  10 , each BAW resonator has either a top edge electrode  13  or a top center electrode  14  (on the top side of the crystal quartz wafer  11 ), as well as with a bottom edge electrode  15  or a bottom center electrode  16  (on the bottom side of the crystal quartz wafer  11 ). The electrodes  13 ,  14 ,  15 ,  16  can be coated with nano particles (e.g., gold) to enhance the absorption of the target agent as well as the optical reflectivity. 
     Some of the BAW resonator plates  12  will have been coated with a sensor coating  17 , the sensor coatings  17  having been collectively designed to differentially absorb to a specific chemical or biological agent, mass loading the resonator and giving off heat in the reaction. The heat of reaction can be detected by operating the resonator on two modes, using one mode which is temperature compensated (changes very little with temperature) and another which has a large temperature coefficient. For example, a BAW resonator operating on the third overtone C mode, designed with minimum frequency shift over the temperature range can be used with a third overtone B mode over the same temperature range designed to have a large frequency shift with temperature. A third overtone C mode, designed with minimum frequency shift over a temperature range, could be used with a fundamental C mode, designed to have a higher frequency shift over the same temperature range. 
     The material used for each sensor coating  17  can be a metal, metallic alloy, polymer, ceramic, carbon, nano-structure, or gold nano-particle. A different coating  17  can be used on each BAW resonator plate  12  in order to detect different target agents. 
     The BAW sensor array  10  shown in  FIG. 1  and  FIG. 2  also shows the integrated heater element  18  on two of the BAW resonator plates  17 . The heater element  18  can be used to control the temperature of the BAW resonators. In addition, thermal gravimetric data detectors  19  can be used to monitor the current or voltage through the heater element  18  in order to determine the heat of reaction between the thin film and the target agent; the heat generated in the reaction will decrease the amount of heat required to maintain the resonators at a predetermined temperature. The heater elements  18  can also be used to “self-clean” the resonators and regenerate sensor coatings  17  which have become saturated. 
     Data collected from the BAW resonator plates  12  includes gravimetric/calorimetric data, thermal-gravimetric data. In addition, information related to the elastic properties of the monolayer can be determined from the loss in the BAW resonators. Further, a circuit with variable drive levels can detect when the particles on the surface become detached. 
       FIG. 3  shows another embodiment of the present invention. A portion of the crystal quartz wafer  11  has a BAW resonator plate  12 , with a top electrode  20  and a bottom electrode  21 , which can be coated with nano particles to enhance the absorption of the target agent. The BAW resonator plate  12  has been coated with a sensor coating  17  to bind with a specific chemical or biological agent. A heater element  18  can be used to control the temperature of the BAW resonator plate  12 . An external electrode plate  22  has been arranged to set up an electrical field between the top electrode  20  and the external electrode plate  22 , which provides for the voltage gravimetric measurement of mass loss with applied electric field. 
       FIG. 4  shows the present invention integrated with an optical detector. A portion of the crystal quartz wafer  11  has a BAW resonator plate  12  with a top electrode  20  and a bottom electrode  21 , which can be coated with nano particles to enhance optical reflectivity. The top electrode  20  on the BAW resonator plate  12  has been coated with a sensor coating  23  which is capable of fluorescing. The optical detector system consists of an optical source  24 , such as an organic light emitting diode (OLED), a separation barrier, or shield  25 , and an optical detector  26 , arranged to detect the fluorescence of biological or chemical agents adsorbed onto the top electrode  20  on the BAW resonator plate  12 . The optical detector system is used to identify the atomic absorption wavelengths of the target agent. 
     The present invention can also be embodied in a surface acoustic wave (SAW) array  27 , such as the one shown in  FIGS. 5 through 8 . A surface acoustic wave device, such as the one shown, is formed from a quartz crystal designed to support high-frequency acoustics oscillators, which are sensitive to surface effects. The SAW array  27  shown in  FIG. 5  and  FIG. 6  has a surface acoustic wave (SAW) substrate  28 . Each SAW resonator has an input electrode  29  and an output electrode  30  coupled to the substrate  28 . A sensor coating  32  can cover a portion of the substrate  28  (as shown) or can cover the electrodes  29 ,  30  and the entire upper planar surface of the substrate  28 , so long as the sensor coating  32  material would not corrode the electrodes  29 ,  30 . 
     The SAW resonator can be arrayed in a 3×4 SAW array  27 , as shown, or can be arrayed 2×2, 2×n, 3×3, 3×n, 4×4, or 4×n. The electrodes  29 ,  30  can be coated with nano particles (e.g., gold) to enhance the absorption of the target agent as well as the optical reflectivity. 
     Each of the SAW resonators in the SAW array  27  can have a different sensor coating  32  designed to chemically attach to a specific chemical or biological agent, mass loading the resonator and giving off heat in the reaction. 
     The SAW array  27  shown in  FIGS. 5 and 6  has integrated heater elements  33  encircling the electrodes  29 ,  30  of several of the SAW resonators. The heater elements  33  can be used to control the temperature of the SAW resonators. In addition, the current or voltage through the heater elements  33  can be monitored to determine the heat of reaction, which will decrease the amount of heat required to maintain the resonators at a predetermined temperature. Heat from the heater elements  33  can also be used to “self-clean” the resonators and regenerate sensor coatings  32  which have become saturated. 
     As shown in  FIG. 6 , the SAW substrate  28  is thermally insulated by stand-offs  35 . 
     A single SAW resonator  36  is shown in detail in  FIG. 7  and  FIG. 8 . The input electrode  29  and output electrode  30 , with a sensor coating  32  in between, are disposed on substrate  28 . Electrode wires  31  connect the electrodes  29 ,  30  to a power source (not shown). Similarly, heater element wires  34  connect the heater element  33  to a power source (not shown). In  FIG. 8  the electrode contacts  37  and heater contacts  38  can be seen. 
       FIG. 9  is a schematic diagram illustrating the sensor device of the present invention. The quartz resonator  40  is a resonator formed of a piezoelectric material. As noted supra, the resonator can operate as a bulk acoustic wave (BAW), surface acoustic wave (SAW), or Love mode device. The quartz resonator  40  is excited by electrical signals of varying frequency from the C-mode oscillator  41  and the B-mode or other temperature mode oscillator  42 . A resistance measurement  43  is delivered to the measurement system  44 , as well as data collected from the C-mode oscillator  41  and the temperature mode oscillator  42 . The resonator drive control can cause effects on the frequency and impedance of the C-mode oscillator  41 , which are transmitted to the measurement system  44 . A sensor coating  46  is generally applied to the surface of the quartz resonator  40 . A heater  47  can be attached to or embedded in the surface of the quartz resonator  40 . A heater control circuit  48  controlled by heater microcontroller  49  affects the temperature of the heater  47 , controlling the temperature of the quartz resonator  40 ; the temperature measurements are transmitted to measurement system  44 . A gas concentrator  50 , controlled by microcontroller  51 , concentrates the target agent and forces it across the surface of the quartz resonator  40 . 
     The data from measurement system  44  is delivered to the memory and CPU  52  for analysis and correlation. The results of the analysis are sent to the display  53  for reading by the operator. An alarm sounds if the target agent and/or its concentration are identified as hazardous. The antenna  55  can be used to transmit the information to a remote location. 
     The traceable time  56  provides the time (G.M.T.) at which a target agent is being tested. Traceable time, with an accuracy suitable for the application, is critical. In an application where the wind speed could be sixty miles per hour (60 mph), the air is moving at 88 feet per second. Time synchronization within a sensor network must be accurate enough to be usable for predicting the position of a hazardous cloud. Time inaccuracies of 10 seconds in 60 mph wind will lead to errors of 880 feet. Time synchronization using traceable time to 1 millisecond will reduce this error to less than one foot. 
     The GPS receiver  57  gives the exact location at which the target agent is being tested, detailing the latitude, longitude, and altitude of the test. 
     Environmental variables  58  provides valuable information relating to such factors as temperature, humidity, wind or air speed, and wind or air direction. 
     Table 2, below, shows the characteristics measured by the present invention, the measurement means, and the resulting measurements. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 CHARACTERISTIC 
                 MEASUREMENT 
                   
               
               
                 MEASURED 
                 MEANS 
                 MEASUREMENT 
               
               
                   
               
             
             
               
                 Gravimetric 
                 From C-mode 
                 Frequency or voltage 
               
               
                   
                 oscillator 
               
               
                 Temperature 
                 From B-Mode 
                 Frequency or 
               
               
                   
                 or other tem- 
                 voltage → temperature 
               
               
                   
                 perature mode 
               
               
                   
                 oscillator 
               
               
                 Resistance (loss) 
                 Resonator peak 
                 Slope of peak at 
               
               
                   
                 width 
                 frequency 
               
               
                 Drive 
                 Current of crystal 
                 Current or voltage 
               
               
                 Heater current 
                 Current 
                 Current 
               
               
                 Time 
                 Clock 
                 Date and time 
               
               
                 Location 
                 GPS Receiver 
                 Latitude, longitude, 
               
               
                   
                   
                 altitude 
               
               
                 Temperature 
                 Thermometer 
                 Degrees 
               
               
                 Humidity 
                 Barometer 
                 Barometric pressure 
               
               
                 Air speed and 
                 Anemometer 
                 Velocity and direction 
               
               
                 direction 
               
               
                   
               
             
          
         
       
     
       FIGS. 10 and 11  show a typical sensor device  60  of the present invention embodied in a hand-held configuration. Disposed within a conventional rectangular housing  61  is the sensor array  62 , which can be comprised of piezoelectric-based resonators designed from quartz, lithium, niobate, lithium tantalate, langasite, Gallium Orthophosphate, or any piezoelectric crystal. The resonators (described supra in more detail) in the sensor array  62  can operate as bulk acoustic wave (BAW), surface acoustic wave (SAW), or Love mode devices. The sensor array  62  is connected electronically to the sensor array  63 , which generally consists of several circuits, including an excitation circuit for each of the multiple modes; a circuit with variable drive levels; a circuit to provide heat; a circuit used to measure the power dissipated in the crystal via the heater and further used to determine the heat of reaction between the target agent and the coating on the resonator surface; a measurement circuit used to collect data, incurring resonant frequencies and magnitudes of impedance over a frequency range; and an optical sensor. The sensor array electronics  62  are connected to microcontrollers  64 ,  65 ,  66 ,  67  and to the memory  68 ,  69 , which together correlate and characterize the data, comparing, for instance, the sensed frequencies with reference frequencies. A battery  70  provides power for operation of the hand-held embodiment  60 . An antenna  71  can be used to transmit data to a remote location. 
     As shown in  FIG. 11 , an air-borne target agent is pulled through filter  72  by air pump  73 , and is then concentrated by concentrator  74 . The target agent has been forced across the sensor array  62 , using a piezoelectric fan or MEMS-based fan. Then, a pumping system  75  removes it from the hand-held embodiment  60 , forcing it out through exit filter  76 . The results of the analysis of the data related to the target agent are shown on the display screen  77 . 
     Although the description contains much specificity, these details should not be construed as limiting the scope of the invention, but merely providing illustrations of some of the presently preferred embodiments of this invention. Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given.