Patent Publication Number: US-2011064615-A1

Title: Piezoelectric sensor and sensing instrument

Description:
TECHNICAL FIELD 
     The present invention relates to a piezoelectric sensor having an adsorption layer that is composed of an antibody provided on a front surface of an electrode formed on one surface side of a piezoelectric piece and for detecting an antigen adsorbed to the antibody by an antigen-antibody reaction in accordance with a change in an oscillation frequency of the piezoelectric piece, and to a sensing instrument using the above piezoelectric sensor. 
     BACKGROUND ART 
     As a method for sensing presence/absence of a trace substance, an environmental pollutant such as, for example, a mouse IGg, or a disease marker such as a hepatitis C virus and a C-reactive protein (CPR), in a sample solution, or for measuring these substances, there has been widely known a measurement method using: a quartz-crystal sensor that includes a quartz-crystal resonator; and a measuring device that is electrically connected to the above quartz-crystal sensor and includes an oscillator circuit or the like for oscillating the quartz-crystal resonator (for example, Patent Document 1). 
     To explain concretely, in the measurement method, the quartz-crystal sensor including the quartz-crystal resonator called a Langevin-type resonator that is provided with, for example, a plate-shaped quartz-crystal piece, and a pair of foil-shaped electrodes for excitation provided on one surface side and the other surface side of the quartz-crystal piece to sandwich the quartz-crystal piece respectively is structured so that the electrode on the one surface side comes into contact with a measurement atmosphere (sample solution) and the electrode on the other surface side faces an airtight space, an antibody capturing an antigen by an antigen-antibody reaction is formed as an adsorption layer on a front surface of the electrode on the one surface side, and the method utilizes a property that when the antigen is captured to the above adsorption layer, a natural frequency of the quartz-crystal resonator changes in accordance with an adsorption amount of the antigen. Then, a difference between the natural frequency of the quartz-crystal resonator before the antigen is absorbed to the adsorption layer and the natural frequency of the quartz-crystal resonator after the antigen is absorbed to the adsorption layer, namely a change amount, is obtained, and presence/absence or a concentration of a substance to be measured is detected in accordance with the above change amount. 
       FIG. 10  shows one example of a structure of the periphery of the quartz-crystal resonator provided in the quartz-crystal sensor. In  FIG. 10 ,  11  denotes a wiring substrate, and the quartz-crystal resonator  10  is placed on the above wiring substrate  11 . The above quartz-crystal resonator  10  has electrodes  13  for excitation provided on one surface side and the other surface side of a plate-shaped quartz-crystal piece  12 , and the electrodes  13  are electrically connected to electrodes  11   a  provided on a wiring substrate  11  side via conductive adhesives  14  each made of a conductive filler and a binder. 
     In  FIG. 10 ,  15  denotes a through hole bored in the wiring substrate  11  in a thickness direction, and in  FIG. 10 ,  15   a  denotes a sealing member covering the through hole  15  from a rear surface side of the substrate  11 . A region surrounded by the sealing member  15   a , the through hole  15 , and the quartz-crystal resonator  10  forms an airtight space, and the electrode  13  on the rear surface side of the quartz-crystal resonator  10  faces the above airtight space. In  FIG. 10 ,  16  denotes a plate-shaped quartz-crystal pressing member made of, for example, rubber or the like, and it presses the quartz-crystal resonator  10  toward the substrate  11  to fix a position of the quartz-crystal resonator  10 . 
     In  FIG. 10 ,  17  denotes an opening portion provided to penetrate the quartz-crystal pressing member  16  in the thickness direction and it faces the electrode  13  on the front surface side of the quartz-crystal resonator  10 . In  FIG. 10 ,  18  denotes an annular projection of the quartz-crystal pressing member  16 . Then, a predetermined amount of a sample solution is stored in a solution storage space  19  surrounded by the opening portion  16  and the annular projection  18 , and thereby the electrode  13  comes into contact with a measurement atmosphere. 
     Further, the electrodes  13  provided on the one surface side and the other surface side of the quartz-crystal piece  12  of the quartz-crystal resonator  10  are each composed of, as shown in  FIG. 11 , two layers, which are a gold (Au) layer  100  and a base layer  101  made of metal such as, for example, chromium (Cr) or nickel (Ni), in general. The above two layers are formed by, for example, sputtering. The reason why gold is used for the upper layer is to oscillate quartz-crystal effectively, and the reason why metal such as chromium or nickel is used for the lower layer is to increase adhesion force between the gold layer  100  and the quartz-crystal piece  12 . Then, a film thickness of the gold layer  100  is set to 2000 Å in order to oscillate the quartz-crystal piece  12  stably, and a film thickness of the base layer  101  is set to 100 Å in order to obtain adhesion between the quartz-crystal piece  12  and the gold layer  100  sufficiently. 
     Further, conventionally, in order to join the electrodes  11   a  on the wiring substrate  11  and the electrodes  13  for excitation on the quartz-crystal resonator  10 , the conductive adhesive  14  in which a conductive filler made of, for example, silver (Ag) is dispersed in a silicone resin being a binder is used. However, with the above conductive adhesive  14 , after the resin in the periphery of Ag first cures, the resin in the periphery of a front surface portion of the gold layer  100  cures, and thus Ag that has been joined to a front surface of the gold layer  100  moves in a direction of going away from the front surface of the gold layer  100  due to curing shrinkage, and consequently a resin film is formed on the front surface of the gold layer  100  to hamper a current-carrying characteristic. Thus, by precipitating the metal of the base layer  101 , which is, for example, chromium, to the front surface of the gold layer  100  by means of thermal diffusion and utilizing the fact that the resin in the periphery of Ag and the resin in the periphery of a Cr front surface portion cure at the same speed, the movement of Ag due to curing shrinkage has been suppressed (Patent Document 2). 
     However, in the quartz-crystal sensor, as shown in  FIG. 11 , antibodies  201  each capturing an antigen  200  by an antigen-antibody reaction are attached to the front surface of the electrode  13  to thereby form an adsorption layer  202 , and thus the following problem occurs when chromium is precipitated to the front surface of the gold layer  100 . That is, the antibody  201 , which is, for example, a protein or the like, easily attaches to gold but does not easily attach to chromium, so that if chromium is precipitated to the front surface of the gold layer  100 , an attachment amount of the antibody  201  on the front surface of the electrode  13  is reduced to reduce detecting ability of the quartz-crystal sensor. Thus, it has been considered that such thermal diffusion processing is not performed, and as the conductive adhesive  14 , one in which a binder cures in a state where a conductive filler is joined to the front surface of the gold layer  100  is used. Concretely, the conductive adhesive  14  with a conductive filler made of, for example, silver and a binder made of an epoxy resin has been used. Incidentally, in recent years, the quartz-crystal sensor has been required to detect a trace substance of, for example, dioxin or the like with high precision, and it has been necessary to respond to such a requirement. 
     On the other hand, Patent Document 3 has described that in annealing processing, a mold process or the like to be performed after joining a connection electrode (lead) to an electrode film formed on a front surface of a quartz-crystal piece by a solder, a solder component diffused on a front surface of the electrode film diffuses into the electrode film in a joining process, and thus in order to prevent the above, chromium is formed on an upper surface of the electrode film and such a chromium component is thermally diffused in the electrode film in a film thickness direction. Further, it has been described that, in this invention, a film thickness of the electrode film is set to not less than 1000 Å nor more than 5000 Å, but there has been no description with regard to the above-described problem. 
     PRIOR ART DOCUMENT 
     Patent Document 1 Japanese Patent Application Laid-open No. 2001-194866 
     Patent Document 2 Japanese Patent Application Laid-open No. 2000-151345 (paragraph 0006 to paragraph 0008, paragraph 0014 and paragraph 0015)
 
Patent Document 3 Japanese Patent Application Laid-open No. 2002-50937 (paragraph 0012 and paragraph 0067)
 
     SUMMARY OF THE INVENTION 
     The present invention has been made under such circumstances, and an object thereof is, in a piezoelectric sensor having an adsorption layer that is composed of an antibody provided on a front surface of an electrode that is formed on one surface side of a piezoelectric piece and for detecting an antigen adsorbed to the antibody by an antigen-antibody reaction in accordance with a change in an oscillation frequency of the piezoelectric piece, to improve detecting ability of the piezoelectric sensor. 
     The present invention is characterized in that a piezoelectric sensor for sensing an antigen in a sample solution based on a natural frequency of a piezoelectric resonator, the piezoelectric sensor includes: 
     a holder having a hole portion formed therein; 
     a piezoelectric resonator having electrodes that are each made of a gold layer formed on one surface side and the other surface side of a piezoelectric piece via adhesive layers respectively and provided to cover the hole portion and to make the electrode on the other surface side face the hole portion; 
     an antibody provided on a front surface of the electrode on the one surface side and capturing an antigen by an antigen-antibody reaction; and 
     conductive paths for connecting the electrodes to an oscillator circuit, and in which 
     the gold layer on the one surface side is one formed to be a film having a thickness that is equal to or more than 3000 Å by sputtering. 
     Concrete examples of the above-described piezoelectric sensor are cited. The holder is a wiring substrate provided with the conductive paths, 
     in order to connect the electrodes to the conductive paths, a conductive adhesive is provided over the electrodes and the conductive paths, and 
     the adhesive contains a conductive filler and a binder made of an epoxy resin. The adhesive layer is preferably at least one type selected from, for example, chromium, titanium, nickel, aluminum, and copper. 
     Further, a sensing instrument of the present invention includes: the piezoelectric sensor of the present invention; and a measuring device main body for detecting the natural frequency of the piezoelectric resonator. 
     According to the present invention, the thickness of the gold layer in the electrode formed on the front surface of the piezoelectric piece is set to 3000 Å or more, and thereby an adsorption amount of an antigen to the adsorption layer formed on the front surface of the gold layer is increased as shown in later-described examples. It is inferred that this is because, by sputtering, gold atoms are deposited to increase the thickness of the gold layer, and thereby the front surface of the gold layer is coarsened to increase a contact area with the antibody on the front surface of the gold layer, and when the adsorption layer is formed on the front surface of the gold layer, an amount of the antibody to attach to the front surface of the gold layer is increased. That is, it is considered that by increasing the thickness of the gold layer, an attachment amount of the antibody on the front surface of the gold layer is increased, and thereby it becomes possible to capture a larger number of the antigens by the antibodies. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a vertical cross-sectional view of a quartz-crystal sensor according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view showing a structure of a quartz-crystal piece according to the embodiment of the present invention; 
         FIG. 3  is a conceptual diagram explaining an adsorption layer to be formed on a front surface of an electrode of the quartz-crystal piece; 
         FIG. 4(   a ) and  FIG. 4(   b ) are conceptual diagrams each explaining formation of an adsorption layer on a front surface of a gold layer; 
         FIG. 5  is a perspective view of the quartz-crystal sensor; 
         FIG. 6  is an exploded perspective view of the quartz-crystal sensor; 
         FIG. 7  is a perspective view of a rear surface side of a quartz-crystal pressing member constituting the quartz-crystal sensor; 
         FIG. 8  is a block diagram of a sensing instrument including the quartz-crystal sensor; 
         FIG. 9  is an explanatory graph showing a result of experiments performed to confirm an effect of the present invention; 
         FIG. 10  is a schematic vertical side view showing a substantial part of a conventional quartz-crystal sensor; and 
         FIG. 11  is a conceptual diagram explaining an adsorption layer to be formed on a front surface of an electrode of a quartz-crystal piece shown in  FIG. 10 . 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     An embodiment of a quartz-crystal sensor being one example of a piezoelectric sensor according to the present invention will be explained with reference to  FIG. 1  to  FIG. 7 .  FIG. 1  is a vertical cross-sectional view showing a quartz-crystal sensor  20  being one example of the piezoelectric sensor according to the present invention, and  FIG. 2  is a plan view showing a structure of a quartz-crystal resonator  2  being a piezoelectric resonator provided in the piezoelectric sensor. Further,  FIG. 5  is a perspective view of the quartz-crystal sensor  20 , and  FIG. 6  is an exploded perspective view showing upper surface sides of respective components of the quartz-crystal sensor  20 . As shown in  FIG. 1 ,  FIG. 5 , and  FIG. 6 , the quartz-crystal sensor  20  is structured in a manner that respective components of a sealing member  3 A, a wiring substrate  3  being a holder, the quartz-crystal resonator  2 , a quartz-crystal pressing member  4 , and a solution injection cover  5  are stacked in this order from the bottom. 
     As shown in  FIG. 1 , the quartz-crystal resonator  2  has electrodes  22 ,  23  for excitation formed on one surface side and the other surface side of a quartz-crystal piece  21  being a piezoelectric piece. The electrode  22  formed on the one surface side of the quartz-crystal piece  21  is continuously formed on a peripheral edge portion of the other surface side, and the electrode  23  formed on the other surface side of the quartz-crystal piece  21  is continuously formed on a peripheral edge portion of the one surface side. As shown in  FIG. 2 , the electrodes  22 ,  23  are each formed in a manner that a gold (Au) layer  70  for efficiently oscillating quartz-crystal and a base layer  71  being an adhesive layer made of metal selected from, for example, chromium (Cr), titanium (Ti), nickel (Ni), aluminum (Al), and copper (Cu) for increasing adhesion force between the gold layer  70  and the quartz-crystal piece  21  are stacked in order from the base layer  71 . 
     Further, a thickness of the gold layer  70  is set to 3000 Å or more, and is set to 3000 Å in this example, and the thickness of the gold layer  70  is set to such a size, thereby increasing an adsorption amount of an antigen  74  to an adsorption layer  7  formed on a front surface of the gold layer  70  as shown in later-described examples. As a reason why the adsorption amount of the antigen  74  to the adsorption layer  7  is increased, it is inferred that this is because, as will be described later, by sputtering, gold atoms are deposited on a front surface of the base layer  71  to increase the thickness of the gold layer  70 , namely, gold atoms are newly deposited on irregularly deposited gold atoms and the above deposition is performed repeatedly to form the gold layer  70  having the thickness of 3000 Å, so that consequently the front surface of the gold layer  70  is coarsened to thereby increase a contact area with an antibody  72  on the front surface of the gold layer  70 , resulting that, when the adsorption layer  7  is formed on the front surface of the gold layer  70  as will be described later, an amount of the antibody  72  to attach to the front surface of the gold layer  70  is increased. 
     An upper limit value of the thickness of the gold layer  70  is set to 10000 Å, and in the case when the thickness is increased more than this, an oscillation frequency jump easily occurs in the quartz-crystal resonator  2 . Further, a thickness of the base layer  71  is set to 10 to 500 Å in order to sufficiently obtain adhesion between the quartz-crystal piece  21  and the gold layer  70 , and is set to 100 Å in this example. As for the electrodes  22 ,  23 , for example, the base layer  71  and the gold layer  70  are stacked on the entire both surfaces of the quartz-crystal piece  21  in this order by sputtering, and next a mask with a predetermined pattern is formed on the both surfaces of the quartz-crystal piece  21  to perform etching, and thereby electrode patterns with a two-layer structure are obtained. 
     Further, as will be described later, the electrode  22  is provided to face a solution storage space  45  to which a sample solution is supplied, and thus as shown in  FIG. 3 , the antibodies  72  to capture the antigens  74  by an antigen-antibody reaction are formed as the adsorption layer  7  on the electrode  22 , and further substances for blocking (blockers)  73  are adsorbed to spaces between the antibodies  72  facing one another so that the antigen  74  being a substance to be measured is not adsorbed to the front surface of the electrode  22 . 
     Here, the formation of the adsorption layer  7  on the front surface of the gold layer  100  will be described in detail. In this embodiment, as described previously, by sputtering, gold is applied to the front surface of the base layer  71  to form a film and the gold layer  70  having the thickness of 3000 Å is formed, thereby coarsening the front surface of the gold layer  70  to increase a contact area with the antibody  72  on the front surface of the gold layer  70 , resulting that, as shown in  FIG. 4(   a ), a large number of the antibodies  74  come to attach to the front surface of the gold layer  70 . On the other hand, a gold layer  100  constituting electrodes  13  on a quartz-crystal resonator  12  to be used for a conventional quartz-crystal sensor has a thickness smaller than that of the gold layer  70  in this embodiment, so that a front surface of the gold layer  100  is hardly coarsened, and as shown in  FIG. 4(   b ), an attachment amount of an antibody  201  on the front surface of the gold layer  100  is reduced as compared with that of the antibody  74  on the front surface of the gold layer  70  in this embodiment. That is, by increasing the thickness of the gold layer  70 , an attachment amount of the antibody  74  on the front surface of the gold layer  70  is increased. 
     Next, the wiring substrate  3  being a holder holding the quartz-crystal resonator will be explained. The above wiring substrate  3  is formed by, for example, a printed circuit board, and an electrode  31  and an electrode  32  are provided to be apart from each other in a direction from a front end side toward a rear end side on a front surface of the wiring substrate  3 . Further, as shown in  FIG. 1 , conductive adhesives  8  each made of a conductive filler and a binder are bonded to regions where the electrodes  31 ,  32  of the wiring substrate  3  are provided, and as will be described later, the electrodes  22 ,  23  formed on the peripheral edge portion of the other surface side of the quartz-crystal piece  21  are set to overlap the electrodes  31 ,  32  on a wiring substrate  3  side via the conductive adhesives  8 . As the conductive adhesive  8 , one in which a binder cures in a state where a conductive filler is joined to the front surface of the gold layer  100  is used, and concretely the conductive adhesive  8  with a conductive filler made of, for example silver or gold, which is made of silver (Ag) in this example, and a binder made of an epoxy resin is used. 
     Here, the conductive adhesive  8  to be used in this embodiment will be explained in detail. The above conductive adhesive  8  uses an epoxy resin with a fast curing speed as a binder, so that a timing that the resin in the periphery of Ag cures and a timing that the resin in the periphery of a front surface portion of the gold layer  70  cures are substantially the same, and as a result, in a state where Ag is joined to the front surface of the gold layer  70 , the resin rapidly cures. Thus, in the conductive adhesive  8  to be used in this embodiment, as has also been described in “Conventional Art”, a phenomenon that since the curing speed of the resin in the periphery of the front surface portion of the gold layer  100  is slower than that of the resin in the periphery of Ag, Ag joined to the front surface of the gold layer  70  moves in a direction of going away from the front surface of the gold  100  due to curing shrinkage does not occur. 
     The wiring substrate  3  will be explained again, and between the electrodes  31  and  32  on the wiring substrate  3 , a through hole  33  being a hole portion bored in the wiring substrate  3  in the thickness direction is formed to be apart from the electrodes  31 ,  32 . As will be described later, the above through hole  33  forms a recessed portion to be an airtight space faced by the electrode  23  on the rear surface side of the quartz-crystal resonator  2 . Note that the hole portion may also be formed not to be penetrated to be a recessed portion having a bottom portion, but it is preferably a through hole. Further, at positions closer to the rear end side than a place where the electrode  32  is formed, two parallel line-shaped conductive path patterns are formed as connection terminal portions  34 ,  35  respectively. The connection terminal portion  34  is electrically connected to the electrode  31  via a pattern  34   a , and the other connection terminal portion  35  is electrically connected to the electrode  32  via a pattern  35   a.    
     In  FIG. 6 ,  36  denotes a weir and the weir  36  serves to fix the position of the quartz-crystal resonator  2 , and the quartz-crystal resonator  2  is placed on a region surrounded by the weir  36 . In  FIGS. 6 ,  37   a ,  37   b , and  37   c  denote engagement holes, and they are bored in the wiring substrate  3  in the thickness direction. These engagement holes  37   a ,  37   b , and  37   c  are engaged with engagement projections  51   a ,  51   b , and  51   c  provided on a lower surface of the cover  5  respectively. Further, in  FIGS. 6 ,  38   a ,  38   b , and  38   c  denote cutout portions formed in a peripheral edge portion of the wiring substrate  3 , and they are engaged with claw portions  52   a ,  52   b , and  52   c  bending inward and provided on a peripheral edge portion of the lower surface of the cover  5  respectively. The sealing member  3 A is a film member and together with the through hole  33 , it forms the recessed portion to be the airtight space. 
     In  FIG. 6  and  FIG. 7 ,  4  denotes the quartz-crystal pressing member, and the quartz-crystal pressing member  4  is formed in a plate shape provided with rectangular-shaped cutout portions  41   a ,  41   b , and  41   c  corresponding to the cutout portions  38   a ,  38   b , and  38   c  respectively. Further, as shown in  FIG. 1  and  FIG. 7 , in a lower surface of the quartz-crystal pressing member  4 , a recessed portion  42  housing the quartz-crystal resonator  2  is formed. At a center of a ceiling surface portion (a bottom surface portion if the description is based on the direction in  FIG. 7 ) of the above recessed portion  42 , an annular projection  43  slightly larger than the through hole  33  in the upper surface of the wiring substrate  3  is provided. In a front surface side of the quartz-crystal pressing member  4 , an opening portion  44  is formed, and the above opening portion  44  communicates with a space surrounded by the annular projection  43 . 
     A peripheral side surface  44   a  of the opening portion  44  and an inner peripheral side surface  43   a  of the annular projection  43  are inclined inward/downward, and a tip portion  47  of the annular projection  43  presses the peripheral edge portion of the quartz-crystal piece  20 . A region surrounded by the peripheral side surfaces  43   a ,  44   a  and the quartz-crystal resonator  2  forms the solution storage space  45  storing the sample solution. 
     Further, in  FIG. 6 ,  46   a ,  46   b  denote engagement holes bored to penetrate the pressing member  4  in the thickness direction, and they are formed to correspond to the engagement holes  37   a ,  37   b  of the wiring substrate  3  and the engagement projections  51   a ,  51   b  of the solution injection cover  5 . In  FIG. 6 ,  46   c  denotes an arc-shaped cutout portion formed at a center of a rear-side edge, and it corresponds to the engagement hole  37   c  of the wiring substrate  3  and the engagement projection  51   c  of the solution injection cover  5 . 
     At a front side and a rear side on an upper surface of the cover  5 , an injection port  53  and a check port  54  for the sample solution are formed respectively. In the lower surface of the cover  5 , an injection channel  55  that is a groove is formed along a longitudinal direction of the cover  5 , and one end and the other end of the above injection channel  55  are connected to the injection port  53  and the check port  54  respectively. Further, the injection channel  55  is provided to face the opening portion  44 , and the sample solution injected into the injection port  53  is supplied to the solution storage space  45  through the injection channel  55 . Further, on the lower surface of is the cover  5 , an annular weir  56  surrounding the injection channel  55  is provided to prevent the sample solution from leaking. 
     The above-described quartz-crystal sensor  20  is assembled in the following manner. First, the through hole  33  in the wiring substrate  3  is covered by the sealing member  3 A to form the recessed portion in the substrate  3 . Subsequently, a predetermined amount of the conductive adhesive  8  is applied to the front surfaces of the electrodes  31 ,  32  on the wiring substrate  3 . Thereafter, the quartz-crystal resonator  2  is placed on the wiring substrate  3  so that the electrodes  22 ,  23  formed on the peripheral edge portion of the other surface side of the quartz-crystal piece  21  overlap the electrodes  31 ,  32  on the wiring substrate  3  side and the electrode  23  formed on a center portion of the other surface side of the quartz-crystal piece overlaps the recessed portion. 
     Next, after the solution injection cover  5  and the pressing member  4  are stacked on each other by engaging the engagement projections  51   a  to  51   c  of the solution injection cover  5  with the engagement holes  46   a ,  46   b  and the cutout portion  46   c  of the quartz-crystal pressing member  4 , they are stacked on the wiring substrate  3  so that the claw portions  52   a ,  52   b , and  52   c  of the solution injection cover  5  and the cutout portions  38   a ,  38   b , and  38   c  of the wiring substrate  3  are fit to each other, and are pressed toward the wiring substrate  5 . Thereby, the claw portions  52   a  to  52   c  of the solution injection cover  5  each bend toward an outer side of the wiring substrate  3 , and as soon as the claw portions  52   a  to  52   c  further reach the lower surface of the peripheral edge portion of the wiring substrate  3  via the cutout portions  38   a  to  38   c  respectively, the claw portions  52   a  to  52   c  return to the original shape due to its inward restoring force respectively, and as soon as the wiring substrate  3  is sandwiched by the respective claw portions  52   a  to  52   c  to be caught thereby, the pressing member  4  sandwiched between the wiring substrate  3  and the cover  5  is pressed by them. 
     Due to elasticity of the pressed pressing member  4 , the annular projection  43  presses a portion, of the front surface of the quartz-crystal resonator  2 , outside the recessed portion toward the wiring substrate  3  side, so that the position of the quartz-crystal resonator  2  is fixed, the peripheral edge portion thereof comes into close contact with the wiring substrate  3  to turn the recessed portion formed by the through hole  33  and the sealing member  3 A into an airtight space, the electrode  23  formed on the center portion of the other surface side of the quartz-crystal piece  21  faces the above airtight space, the conductive adhesives  8  formed on the front surfaces of the electrodes  31 ,  32  of the wiring substrate  3  and the electrodes  22 ,  23  formed on the peripheral edge portion of the other surface side of the quartz-crystal piece  21  are bonded, and thereby the electrodes  22 ,  23  and the electrodes  31 ,  32  on the wiring substrate  3  side are electrically connected respectively. 
     Next, an operation of the above-described quartz-crystal sensor  20  will be explained. First, an operator injects the sample solution into the injection port  53  of the solution injection cover  5  by using, for example, an injector. The sample solution injected into the injection port  53  is supplied to the solution storage space  45  for the sample solution formed by the opening portion  44  and the annular projection  43 , and the electrode  22  on the front surface side of the quartz-crystal resonator  2  comes into contact with the sample solution, and the antigen  74  in the sample solution is adsorbed to the adsorption layer  7  composed of the antibodies  72  formed on the front surface of the electrode  22  by an antigen-antibody reaction. Then, when the antigen  74  is adsorbed to the adsorption layer  7 , a natural frequency of the quartz-crystal resonator  2  reduces in accordance with an adsorption amount of the antigen  74 . Thereby, a difference between the natural frequency of the quartz-crystal resonator  2  before the antigen  74  is adsorbed to the adsorption layer  7  and the natural frequency of the quartz-crystal resonator  2  after the antigen  74  is adsorbed to the adsorption layer  7 , namely a change amount, is obtained. 
     According to the above-described embodiment, in the electrodes  22 ,  23  formed on the front surface of the quartz-crystal piece  21 , the thickness of the gold layer  70  is set to 3000 Å, and is set to 3000 Å in this example, and thereby an adsorption amount of the antigen  74  to the adsorption layer  7  formed on the front surface of the gold layer  70  is increased as shown in the later-described examples. It is inferred that this is because, since, as described above, by sputtering, gold atoms are deposited on the front surface of the base layer  71  to increase the thickness of the gold layer  70 , namely gold atoms are newly deposited on gold atoms deposited irregularly and the above deposition is performed repeatedly to form the gold layer  70  having the thickness of 3000 Å, consequently the front surface of the gold layer  70  is coarsened to increase a contact area with the antibody  72  on the front surface of the gold layer  70 , and when the adsorption layer  7  is formed on the front surface of the gold layer  70  as will be described later, an amount of the antibody  72  to attach to the front surface of the gold layer  70  is increased. That is, it is considered that by increasing the thickness of the gold layer  70 , an attachment amount of the antibody  72  on the front surface of the gold layer  70  is increased, and thereby it becomes possible to capture a larger number of the antigens  74  by the antibodies  72 . 
     Further, in the above-described embodiment, the thickness of the gold layer  70  is set to 3000 Å or more, and is set to 3000 Å in this example, thereby enabling the following effect to be obtained. For example, chromium being the metal of the base layer  71  to be used for increasing the adhesion force between the gold layer  70  and the quartz-crystal piece  21  gradually diffuses into the gold layer  70  as time passes. When a thickness of the gold layer  100  is 2000 Å as is a conventional quartz-crystal sensor shown in  FIG. 11  and  FIG. 12 , chromium precipitates to the front surface of the gold layer  100  for half a year to one year, and by the above precipitated chromium, antibodies  201  attaching to the front surface of the gold layer  100  are desorbed to shorten a usable life of the quartz-crystal sensor, but by setting the thickness of the gold layer  70  to 3000 Å, it takes one year or longer for chromium to precipitate to the front surface of the gold layer  100 , and thus the effect of extending a usable life of the quartz-crystal sensor also exists. 
     Further, the above-described quartz-crystal sensor  20  is used as a sensing unit of a sensing instrument when connected to a measuring device main body  7  having a configuration as shown in  FIG. 8  that is a block diagram, for example. In  FIG. 8 ,  62  denotes an oscillator circuit oscillating the quartz-crystal piece  21  of the quartz-crystal sensor  20 ,  63  denotes a reference clock generating unit generating a reference frequency signal, and  64  denotes a frequency difference detector formed by, for example, a heterodyne detector, which, based on a frequency signal from the oscillator circuit  62  and a clock signal from the reference clock generating unit  63 , extracts a frequency signal corresponding to a frequency difference therebetween.  65  denotes an amplifying unit,  66  denotes a counter counting a frequency of an output signal from the amplifying unit  65 , and  67  denotes a data processing unit. 
     The frequency of the quartz-crystal sensor  20  is 9.2 MHz, and thus as the frequency of the reference clock generating unit  63 , for example, 10 MHz is selected. When the antigen  74  being a substance to be measured, which is, for example, dioxin, is not adsorbed to the above-described adsorption layer  7  provided on the quartz-crystal resonator  2  of the quartz-crystal sensor  20 , the frequency difference detector  64  outputs a frequency signal (frequency difference signal) corresponding to 1 MHz that is a difference between the frequency from a quartz-crystal sensor side and the frequency of the reference clock, but when the antigen  74  contained in the sample solution is adsorbed to the adsorption layer  7  on the quartz-crystal resonator  2 , the natural frequency of the quartz-crystal resonator  2  changes and thereby the frequency difference signal also changes, so that a counter value in the counter  66  changes, thereby enabling the concentration of the substance to be measured or the presence/absence of the substance to be detected. 
     EXAMPLES 
     Experiments that have been performed to confirm the effect of the present invention will be explained. 
     Example 1 
     In the quartz-crystal sensor  20  shown in  FIG. 1 , the adsorption layer  7  was formed on the front surface of the electrode  22  composed of the gold layer  70  having the thickness of 3000 Å and the base layer  71  having the thickness of 100 Å with the antibodies  72 . The above adsorption layer  7  was formed in the following manner. First, 0.2 ml of a buffer solution was supplied into the solution storage space  45 , and next 0.2 ml of a sample solution in which 100 μg/ml of a protein called BSA (Bovine Serum Albumin) being the antibody  72  is contained was supplied into the solution storage space  45 . Thereby, the antibody  72  attached to the front surface of the electrode  22  and the adsorption layer  7  was formed. 
     After the adsorption layer  7  was formed, 1 ml of a sample solution in which 10 μg/ml of, for example, a mouse IGa being the antigen  74  is contained was injected into the injection port  53  of the quartz-crystal sensor. Then, an amount of the antigen  74  adsorbed to the adsorption layer  7  on the front surface of the electrode  22  was obtained by taking a difference between the natural frequency of the quartz-crystal resonator  2  before the antigen  74  is adsorbed to the adsorption layer  7  and the natural frequency of the quartz-crystal resonator  2  after the antigen  74  is adsorbed to the adsorption layer  7 . 
     Example 2 
     In the same manner as that of Example 1 except that the thickness of the gold layer  70  was set to 4000 Å, the adsorption layer  7  was formed, and thereafter a sample solution in which a mouse IGg is contained was injected to obtain an amount of the antigen  74  adsorbed to the adsorption layer  7  on the front surface of the electrode  22 . 
     Example 3 
     The same experiment as that of Example 2 except that the thickness of the gold layer was set to 5000 Å was performed. 
     Example 4 
     The same experiment as that of Example 2 except that the thickness of the gold layer was set to 6000 Å was performed. 
     Example 5 
     The same experiment as that of Example 2 except that the thickness of the gold layer was set to 7000 Å was performed. 
     Comparative Example 1 
     In the same manner as that of Example 1 except that the thickness of the gold layer  70  was set to 1000 Å, the adsorption layer  7  was formed, and thereafter a sample solution in which a mouse IGg is contained was injected to obtain an amount of the antigen  74  adsorbed to the adsorption layer  7  on the front surface of the electrode  22 . 
     Comparative Example 2 
     In the same manner as that of Example 1 except that the thickness of the gold layer  70  was set to 2000 Å, the adsorption layer  7  was formed, and thereafter a sample solution in which a mouse IGg is contained was injected to obtain an amount of the antigen  74  adsorbed to the adsorption layer  7  on the front surface of the electrode  22 . 
     (Results and Discussion) 
     As shown in  FIG. 9 , the adsorbed amounts of the antigen  74  in Examples 1 to 5 were 9.8 ng/cm 2 , 11.0 ng/cm 2 , 11.7 ng/cm 2 , 12.0 ng/cm 2 , and 12.2 ng/cm 2  respectively. Further, the adsorbed amounts of the antigen  74  in Comparative Example 1 and Comparative Example 2 were 6.5 ng/cm 2  and 8.0 ng/cm 2 . That is, it is found that the thickness of the gold layer  100  is increased, thereby increasing the adsorbed amount of the antigen  74  to the adsorption layer  7  formed on the front surface of the gold layer  100 . It is inferred that this is because, when, as described above, by sputtering, gold atoms are deposited on the front surface of the base layer  71  to increase the thickness of the gold layer  70 , with that, the front surface of the gold layer  70  is coarsened to increase a contact area with the antibody  201  on the front surface of the gold layer  70 , and thereby an attachment amount of the antibody  72  on the front surface of the gold layer  100  is increased. Thus, it is found that, when the thickness of the gold layer  100  is set to 3000 Å or more, an attachment amount of the antibody  72  is increased to thereby enable high sensitivity in the piezoelectric sensor to be obtained. Note that, as an expression to obtain a reaction amount based on a measurement frequency, an expression created by Sauerbrey was used.