Abstract:
A sensor for sensing a property of a plurality of analytes includes a substrate having a resonant frequency that varies based on contact with a predetermined property of an analyte. The substrate has an analyte contact surface and a non-analyte contact surface located opposed to the analyte contact surface. The analyte contact surface is configured to receive a plurality of analytes. A plurality of pairs of electrodes are operatively connected with the nonanalyte contact surface, each of the electrodes being spaced apart one from another.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 10/863,831, entitled “Corrosive or Conductive Liquid/Gas Sensor Using Lateral-Field-Excited Resonator”. 
    
    
     GOVERNMENT INTEREST 
     The invention described herein may be manufactured, used, imported, sold, and licensed by or for the Government of the United States of America without the payment of any royalty thereon or therefor. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to acoustic sensors and, more particularly, to sensors comprising quartz crystal microbalance resonators. 
     2. Related Art 
     Sensors comprising quartz crystal microbalance (QCM) resonators are generally known. QCM resonators may function as acoustic wave resonators to provide highly sensitive detection mechanism for fluid analytes. As illustrated in  FIG. 1 , a typical QCM resonator is shown generally at  100  and comprises a piezoelectric crystal substrate  102  located between a pair of electrodes  104  having leads  106 . In this configuration, an electric field may be generated by the electrodes  104  and extend therebetween along a transverse axis, or through the thickness, of the piezoelectric crystal substrate  102 . Hence in this configuration the QCM resonator may be termed a thickness field excitation (TFE) resonator. The electrodes  104  and the crystal  102  are dimensioned to achieve an optimal resonance condition. 
     One particular example of a TFE resonator is described in U.S. Pat. No. 6,544,478 to Oyama et al wherein the resonator is arranged in a multi-channel structure. The resonator includes a crystal substrate that has four mutually opposed electrodes disposed on opposite sides of the substrate. In operation, the TFE resonator may be used to detect and quantitatively analyze components of a sample from a variation in fundamental resonant frequency and impedance when a surface of one of the pair of electrodes is immersed into either a sample gas or solution. 
     While the above TFE resonators have been suitable for use with non-caustic analytes, it has been found that when these resonators are immersed into a caustic substance the electrodes tend to deteriorate. Also, use of these resonators is restricted to non-conductive analytes because of the possibility that the electric field may become shorted. Accordingly, to date, no suitable QCM resonator is available for analyzing a caustic or conductive analyte. 
     SUMMARY OF THE INVENTION 
     In accordance with an embodiment of the present invention, a sensor for sensing a property of a plurality of analytes comprises a substrate having a resonant frequency that varies based on contact with a predetermined property of an analyte. The substrate has an analyte contact surface and a non-analyte contact surface located opposed to the analyte contact surface. The analyte contact surface is configured to receive a plurality of analytes. A plurality of pairs of electrodes are operatively connected with the nonanalyte contact surface, each of the electrodes being spaced apart one from another. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The following detailed description is made with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective view of a QCM resonator in accordance with the prior art; 
         FIG. 2  is a perspective view of a QCM resonator in accordance with an embodiment of the present invention; 
         FIG. 3  is a cross section of a sensor device including a QCM resonator in accordance with another embodiment of the present invention; 
         FIG. 4  is an exploded view of the sensor device of  FIG. 3 ; 
         FIG. 5  is a perspective view showing an analyte contact surface of a sensor device in accordance with another embodiment of the present invention; 
         FIG. 6  is another perspective view of the sensor device of  FIG. 5  showing an electrode contact surface; 
         FIG. 7  is a sectional view taken along line VII of  FIG. 5 ; and 
         FIG. 8  is a diagram showing the sensor device of  FIG. 5  in circuit with switches and readout electronics in accordance with a further embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One embodiment of the present invention concerns a quartz crystal microbalance (QCM) resonator that is suitable for use with a caustic or conductive analyte. In another embodiment of the present invention, a sensor device employing a QCM resonator suitable for use with a caustic or conductive analyte is presented. 
     Referring now to  FIG. 2 , a QCM resonator in accordance with one embodiment of the present invention is illustrated generally at  10 . In this embodiment, the QCM resonator  10  comprises a substrate  12 , electrodes  14  and electrode leads  16 . 
     The substrate  12  may comprise a piezoelectric crystal material such as quartz that functions such that when contacted with a property of an analyte to be measured, varies in resonant frequency and impedance in a known manner. Examples of properties of an analyte to be measured include viscosity and density. The substrate  12  may comprise any suitable outer geometrical configuration such as square or circular and comprises an electrode depositing surface  18  and an analyte contact surface  20 . Although not illustrated as such, the analyte contact surface  20  may be coated with a material such as an antibody and/or a polymer that may enhance sensitivity or selectivity of the QCM resonator  10  in a known manner. 
     In accordance with a feature of this embodiment of the present invention, the electrodes  14  are located away from any contact with an analyte that may be limited to the analyte contact surface  20 . As illustrated, both of the electrodes are located on the electrode depositing surface  18 , although, other locations on the substrate may be possible. The electrodes  14  may comprise any suitable, highly conductive, metallic substance, although gold is preferred, and may be applied to the substrate  12  via photolithography or deposited via, e.g., evaporation, sputtering, or electroplating. Electrode leads  16  may be connected at one end to the electrodes  14  and at the other to a suitable AC source at the resonant frequency of the resonator  52  and measuring device (not shown). 
     In this configuration, an electric field may be generated by the electrodes  16  along a lateral axis of the piezoelectric crystal substrate  12 . Hence in this configuration the QCM resonator may be termed a lateral field excitation (LFE) resonator. As in the TFE case, the electrodes  16  and the crystal  12  may be dimensioned to achieve an optimal resonance condition. 
     In another embodiment of the present invention, illustrated in  FIGS. 3 and 4  a sensor device  50  comprises a QCM resonator  52  and a housing  54  for an analyte  56 . The QCM resonator  52  may be similar to the QCM resonator  10  described above and similarly comprises a substrate  58  including an electrode depositing surface  60 , electrodes  62  deposited to the electrode depositing surface and electrode leads  64 . 
     The housing  54  may comprise an analyte support container  66  and a base  68 . The analyte support container  66  and the base  68  may each comprise a moldable polymeric material such as a polyethylene or a polyamide and may also each comprise generally cylindrical outer configurations, as illustrated. The analyte support container  66  is illustrated as having a generally closed configuration including a chamber  69  for the analyte  56  and an aperture  70 , although, it will be understood that the analyte support container may comprise a lid or cover (not shown) or be connected to a pipe or conduit (also not shown) for communication of the analyte to the chamber in a continuous flow-like process. 
     The base  68  comprises an open end (not numbered) that is preferably dimensioned to receive the analyte support container  66  (best seen in  FIG. 3 ). A seal, such as an O-ring  72 , is provided to seal the analyte  56  adjacent an analyte support surface  74  from a cavity  76  of the base  68 . A spring  78  may be interposed between the base  68  and the QCM resonator  52  for biasing the resonator adjacent the O-ring  72 , which is in turn biased adjacent the analyte support container  66 . Another O-ring  80  may be employed to insulate the spring  78  and prevent shorting the electrodes  62 . This embodiment allows an easy replacement of the QCM resonator  52  when the need for replacement of the QCM resonator  52  arises. This may be when the resonator  52  is damaged or a different coating for sensing a different analyte may be necessary. 
     In another embodiment of a sensing device in accordance with the present invention, illustrated generally at  200  in  FIGS. 5 through 7 , multiple QCM resonators  202 , each comprising a plurality of electrode pairs  203 , are located on a substrate  204 . The substrate  204  may be composed of a similar material as that of the substrate  12 , described above in connection with  FIG. 2 , although, it will be understood that a layered structure comprising a glass slide (not shown) and a piezoelectric film (also not shown) may be substituted for the substrate. The substrate  204  may comprise wells  206  that may be formed by etching and may each comprise a generally rectangular configuration, as illustrated, although other configurations, such as circular or plate-shaped, may be used. The wells  206  may function to receive a sample, or differing samples of, fluid analyte (not shown). A coating  208  may be applied to the substrate  204  within the wells  206  as shown and may comprise an antibody and/or a polymer as described above for enhancing sensitivity or selectivity. It will be appreciated that each well  206  may comprise a coating that comprises a different material in order to, e.g., vary the analysis for one particular sample fluid analyte. For example, different coatings comprising differing antibodies dispersed in a polymer carrier may be applied to various wells  206  for testing one particular analyte for different reactions in a known manner. 
     Referring now to  FIG. 8 , each electrode pair  203  may be connected in a parallel circuit via lines  210  to readout electronics  212 . Switches  214  may be interposed between the electrode pairs  203  and the readout electronics  212  for operation of each QCM resonator  202 . In operation, the readout electronics  212  may be operated in a known manner and switches  214  may be sequentially closed to energize a particular electrode pair  203 . 
     While the present invention has been described in connection with what are presently considered to be the most practical and preferred embodiments, it is to be understood that the present invention is not limited to these herein disclosed embodiments. Rather, the present invention is intended to cover all of the various modifications and equivalent arrangements included within the spirit and scope of the appended claims.