Abstract:
This invention describes a quantitative, inexpensive, disposable immunosensor that requires no wash steps and thus generates no liquid waste. Moreover, in preferred embodiments of the sensor no timing steps are required of the user, and the sensor can be readily adapted to antigen-antibody interactions over a wide kinetic range.

Description:
This application is a continuation of application Ser. No. 09/616,433, filed Jul. 14, 2000 now abandoned, which is incorporated herein by reference in its entirety and to which we claim priority. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a device and method for performing immunoassays. The device comprises a disposable immunosensor. 
     BACKGROUND OF THE INVENTION 
     Biomedical sensors are used to report the presence and/or concentration of a wide variety of analytes. When the analyte is a protein, then the sensing element used is usually an antibody since the interaction of the antibody with the protein (antigen) is very specific. Such immunoassays usually fall into two categories: a “yes/no answer” obtained, e.g., by simple visual detection, or a concentration of the antigen determined by a quantitative method. Most of the quantitative methods involve expensive pieces of equipment such as scintillation counters (for monitoring radioactivity), spectrophotometers, spectrofluorimeters (see, e.g., U.S. Pat. No. 5,156,972), surface plasmon resonance instruments (see, e.g., U.S. Pat. No. 5,965,456), and the like. It would therefore be advantageous to develop a quantitative immunoassay that is both inexpensive and simple enough to use to be suitable for home or field use. 
     Conventional immunoassays are classified into two categories: competition assay and sandwich assay. In a competition assay, the antigen in the test sample is mixed with an antigen-probe complex and the mixture then competes for binding to the antibody. The probe may be an enzyme, a fluorophore or a chromophore. Secondly, in a sandwich immunoassay, the antigen in the test sample binds to the antibody and then a second antibody-probe complex binds to the antigen. In these prior art assay methods, one or more washing steps are usually required. The washing steps introduce complexity into the assay procedure and can generate biohazardous liquid waste. It would therefore be advantageous to develop a device for performing an immunoassay that does not require any washing steps. Of necessity, such a device would be designed to be a single use disposable device. 
     SUMMARY OF THE INVENTION 
     This invention describes a quantitative, inexpensive, disposable immunosensor that requires no wash steps and thus generates no liquid waste. Moreover, in preferred embodiments of the sensor, no timing steps are required of the user, and the sensor can be readily adapted to antigen-antibody interactions over a wide kinetic range. 
     In one embodiment of the present invention, a disposable device is provided for use in detecting a target antigen in a fluid sample having a pH, the device including a reaction chamber having an internal surface, a proximal end, and a distal end; an immobilized antibody fixed within the reaction chamber, the antibody being capable of binding to the target antigen; a reporter complex present within the reaction chamber, the complex including a probe, the reporter complex being capable of mixing with the sample; a detection chamber having a wall, an internal surface, a distal end and a proximal end; a sample ingress at the distal end of the reaction chamber, and a sample passageway between the distal end of the reaction chamber and the proximal end of the detection chamber. 
     In one aspect of this embodiment, an agent contained within the reaction chamber and capable of preventing non-specific binding of proteins to the reaction chamber internal surface is included. The agent may be selected from the group consisting of a surfactant and a blocking protein, for example, bovine serum albumin. 
     In another aspect of this embodiment, the reporter complex further includes a second antigen capable of competing with the target antigen for binding to the immobilized antibody, or a second antibody capable of binding to the target antigen. 
     In another aspect of this embodiment, the probe is selected from the group consisting of chromophores and fluorophores. The probe may include an enzyme, such as glucose oxidase or glucose dehydrogenase. An enzyme substrate may also be included, for example, an oxidizable substrate such as galactose, acetic acid, or glucose. 
     In another aspect of this embodiment, the detection chamber further includes a mediator. The mediator may include dichlorophenolindophenol, complexes between transition metals and nitrogen-containing heteroatomic species, or ferricyanide. 
     In another aspect of this embodiment, the device further includes a buffer capable of adjusting the pH of the sample, such as one including phosphate or citrate. 
     In another aspect of this embodiment, the immobilized antibody and/or the reporter complex is supported on a reaction chamber interior surface. The reporter complex may be separated from the immobilized antibody by less than about 1 millimeter. 
     In another aspect of this embodiment, the device further includes a stabilizer that stabilizes one or more of the antigen, the enzyme, and the antibody. 
     In another aspect of this embodiment, the enzyme substrate is supported on a detection chamber interior surface. 
     In another aspect of this embodiment, the device further includes a support material. The support material may be contained within the detection chamber, and one or more substances such as an enzyme substrate, a mediator, and a buffer may be supported on or contained with the support material. The support material may also be contained within the reaction chamber, and one or more substances such as the immobilized antibody, the reporter complex, and an agent capable of preventing non-specific binding of proteins to the reaction chamber internal surface may be supported on or contained within the support material. The support material may include a mesh or fibrous filling material including a polymer selected from the group consisting of polyolefin, polyester, nylon, cellulose, polystyrene, polycarbonate, polysulfone, and mixtures thereof; a porous material such as a macroporous membrane including a polymeric material selected from the group consisting of polysulfone, polyvinylidene difluoride, nylon, cellulose acetate, polymethacrylate, polyacrylate, and mixtures thereof; or a sintered powder. 
     In another aspect of this embodiment, the detection chamber includes at least two electrodes. The electrodes may include a material selected from the group consisting of palladium, platinum, gold, iridium, carbon, carbon mixed with binder, indium oxide, tin oxide, and mixtures thereof. 
     In another aspect of this embodiment, the detection chamber wall is transparent to a radiation emitted or absorbed by the probe, the radiation being indicative of the presence or absence of the reporter complex in the detection chamber. 
     In another aspect of this embodiment, a detector capable of detecting a condition wherein the reaction chamber is substantially filled is included. A piercing means capable of forming a detection chamber vent in the distal end of the detection chamber may also be included. A reaction chamber vent at the distal end of the reaction chamber may be included as well. 
     In a second embodiment of the present invention, a method of manufacture of a disposable device for use in detecting a target antigen in a fluid sample having a pH is provided, the method including the steps of forming a first aperture extending through a first sheet of material having a proximal end and a distal end, the first aperture defining a reaction chamber side wall, a detection chamber side wall and a first sample passageway between the reaction chamber distal end and the detection chamber proximal end; mounting a first layer toga first side of the first sheet and extending over the aperture to define a first reaction chamber end wall and a first detection chamber end wall; mounting a second layer to a second side of the first sheet and extending over the aperture to define a second reaction chamber end wall and a second detection chamber end wall in substantial overlying registration with the first layer, whereby the sheet and layers form a strip having a plurality of exterior surfaces; forming a second passageway extending through an exterior surface of the strip and into the reaction chamber at the reaction chamber distal end, the second passageway defining a reaction chamber vent; forming a third passageway extending through the an exterior surface of the strip and into the reaction chamber at the reaction chamber proximal end, the third passageway defining a sample ingress; immobilizing an antibody within the reaction chamber; and placing a reporter complex in the reaction chamber, the complex including a probe. 
     In one aspect of this embodiment, the aperture extends through the proximal end of the first sheet to form the third passageway. 
     In another aspect of this embodiment, the first sheet, the first layer and the second layer include an electrically resistive material, the first layer includes a first electrode wherein the first electrode faces the first side of the first sheet, and the second layer includes a second electrode wherein the second electrode faces the second side of the sheet. At least one of the electrodes may include a material selected from the group consisting of palladium, platinum, gold, iridium, carbon, carbon mixed with binder, indium oxide, tin oxide, and mixtures thereof. The first electrode may substantially cover the first detection chamber end wall and the second electrode substantially covers the second detection chamber end wall. At least one of the electrodes may be a sputter coated metal deposit. The second electrode may be mounted in opposing relationship a distance of less than about 500 microns from the first electrode; less than about 150 microns from the first electrode; or less than about 150 microns and greater than about 50 microns from the first electrode. 
     In another aspect of this embodiment, the layers are adhered to the sheet, for example, by an adhesive such as a heat activated adhesive, pressure sensitive adhesive, heat cured adhesive, chemically cured adhesive, hot melt adhesive, and hot flow adhesive. At least the sheet, or one of the layers may include a polymeric material such as polyester, polystyrene, polycarbonate, polyolefin, and mixtures thereof, or polyethylene terephthalate. At least one of the layers may be transparent to a wavelength of radiation including infrared radiation, visible light, and ultraviolet radiation. 
     In another aspect of this embodiment, the method further includes providing an enzyme substrate and a mediator, wherein the enzyme substrate and the mediator are contained within the detection chamber, wherein the probe is an enzyme, and wherein the mediator is capable of mediating a reaction between the enzyme and the electrode, to indicate the occurrence of an electrochemical reaction. 
     In another aspect of this embodiment, the method further includes the step of providing a buffer, wherein the buffer is capable of adjusting the pH of the sample. 
     In a third embodiment of the present invention, a method of manufacture of a disposable device for use in detecting a target antigen in a fluid sample having a pH is provided, the method including forming a first aperture extending through a first sheet of electrically resistive material having a proximal end and a distal end, the first aperture having a first aperture reaction chamber part and a first aperture detection chamber part and defining a first portion of a reaction chamber side wall, a detection chamber side wall and a sample passageway between the reaction chamber distal end and the detection chamber proximal end; forming a second aperture extending through a second sheet of electrically resistive material having a proximal end and a distal end, the second aperture defining a second portion of the reaction chamber side wall; forming a third aperture extending through a third sheet of electrically resistive material having a proximal end and a distal end, the third aperture defining a third portion of the reaction chamber side wall; mounting a first side of the second sheet to a first side of the first sheet, the second sheet extending over the first aperture detection chamber part whereby to define a first detection chamber end wall, the second portion of the reaction chamber side wall in substantial registration with the first portion of the reaction chamber side wall; mounting a first side of the third sheet to a second side of the first sheet, the third sheet extending over the first aperture detection chamber part whereby to define a second detection chamber end wall, the third portion of the reaction chamber side wall in substantial registration with the first portion of the reaction chamber side wall; mounting a first layer to a second side of the second sheet and extending over the second aperture to define a first reaction chamber end wall; mounting a second layer to a second side of the third sheet and extending over the third aperture to define a second reaction chamber end wall in substantial overlying registration with the first thin layer, whereby the sheets and layers form a strip having a plurality of exterior surfaces; forming a second passageway extending through the outside of the strip and into the reaction chamber at the reaction chamber distal end, the second passageway defining a reaction chamber vent; forming a third passageway extending through the outside of the strip and into the reaction chamber at the reaction chamber proximal end, the third passageway defining a sample ingress; immobilizing an antibody within the reaction chamber; and placing a reporter complex in the reaction chamber, the reporter complex including a probe. 
     In a fourth embodiment of the present invention, a method for determining a presence or an absence of a target antigen in a fluid sample is provided, the method including providing a disposable device including a reaction chamber having an internal surface, a proximal end, and a distal end, an immobilized antibody fixed within the reaction chamber, the antibody being capable of binding to the target antigen, a reporter complex present within the reaction chamber, the complex including a probe, the reporter complex being capable of mixing with the sample, a detection chamber having a wall, an internal surface, a distal end and a proximal end, a sample ingress at the distal end of the reaction chamber, and a sample passageway between the distal end of the reaction chamber and the proximal end of the detection chamber, wherein the reporter complex further includes a second antigen capable of competing with the target antigen for binding to the immobilized antibody; contacting a fluid sample with the sample ingress; substantially filling the reaction chamber with the fluid sample by allowing the sample to flow from the sample ingress toward the reaction chamber; allowing a predetermined time to lapse, the time being sufficient for substantially all reporter complex to bind to the immobilized antibody in the absence of antigen in the sample; substantially filling the detection chamber with the fluid sample by allowing the sample to flow from the reaction chamber through the sample passageway toward the detection chamber; detecting a presence or an absence of the antigen-probe complex within the detection chamber, the presence or absence of the antigen-probe complex being indicative of a presence or an absence of the antigen in the sample. 
     In one aspect of this embodiment, then method further includes piercing the wall of the detection chamber so as to form a detection chamber vent at the distal end of the detection chamber, the piercing step immediately following the lapse of the predetermined time. 
     In a fifth embodiment of the present invention, a method of manufacture of a disposable device for use in detecting a target antigen in a fluid sample having a pH is provided, the device having a plurality of exterior surfaces, the method including forming a first aperture extending through a first sheet of electrically resistive material, the first aperture having a detection chamber part and defining a detection chamber side wall, the detection chamber having a proximal end and a distal end; mounting a first layer to a first side of the first sheet and extending over the aperture to define a first detection chamber end wall; mounting a second layer to a second side of the first sheet and extending over the aperture to define a second detection chamber end wall in substantial overlying registration with the first layer, whereby the sheet and layers form a strip; forming a second aperture extending through the strip, the strip having a proximal end and a distal end, the second aperture having a reaction chamber part, the reaction chamber having a distal end and a proximal end, and the second aperture defining a reaction chamber side wall and a sample passageway between the reaction chamber distal end and the detection chamber proximal end; mounting a first side of a third layer to a first side of the strip, the third extending over the second aperture reaction chamber part to define a first reaction chamber end wall; mounting a first side of a fourth layer to a second side of the strip, the fourth layer extending over the second aperture reaction chamber part whereby to define a second reaction chamber end wall in substantial registration with the first reaction chamber end wall; forming a third aperture extending through a surface of the device and into the reaction chamber at the reaction chamber distal end, the third aperture defining a reaction chamber vent; forming a fourth aperture extending through a surface of the device and into the reaction chamber at the reaction chamber proximal end, the fourth aperture defining a sample ingress; immobilizing an antibody within the reaction chamber, and placing a reporter complex in the reaction chamber, the reporter complex including a probe. 
     In one aspect of this embodiment, the first sheet, the first layer and the second layer include an electrically resistive material, the first layer includes a first electrode wherein the first electrode faces the first side of the first sheet, and the second layer includes a second electrode wherein the second electrode faces the second side of the sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a top view (not to scale) of an immunosensor incorporating an electrochemical cell. 
         FIG. 2  shows a cross-sectional view (not to scale) along line A-A′ of an embodiment of the immunosensor of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description and examples illustrate a preferred embodiment of the present invention in detail. Those of skill in the art will recognize that there are numerous variations and modifications of this invention that are encompassed by its scope. Accordingly, the description of a preferred embodiment should not be deemed to limit the scope of the present invention. 
     Disclosed is a single step, no-wash immunosensor. The sensor is a single use, disposable device that utilizes two adjacent chambers, a reaction chamber and a detection chamber. In the reaction chamber, the antigen-antibody reactions take place and in the detection chamber the results of those reactions are detected and the presence or absence of antigen in the sample is inferred. 
     Any suitable detection method can be utilized. Suitable detection methods include, e.g., visual detection wherein the development of a color is observed, or spectroscopic detection wherein reflected or transmitted light is used to measure changes in light absorbance. In a preferred embodiment, the detection method is electrochemical wherein the electrical current or potential generated indirectly by the products of antigen/antibody reactions is measured. 
     Methods and devices for obtaining electrochemical measurements of fluid samples are discussed further in U.S. patent application Ser. No. 09/615,691, now U.S. Pat. No. 6,638,415, filed on Jul. 14, 2000, entitled “ANTIOXIDANT SENSOR,” U.S. patent application Ser. No. 09/616,512, now U.S. Pat. No. 6,632,349 filed on Jul. 14, 2000, entitled “HEMOGLOBIN SENSOR,” and U.S. patent application Ser. No. 09/616,556, filed on Jul. 14, 2000, now U.S. Pat. No. 6,445,115, entitled “ELECTROCHEMICAL METHOD FOR MEASURING CHEMICAL REACTION RATES,” each of which is incorporated herein by reference in its entirety. 
     The timing of the various test stages, i.e., the reaction stage and the detection stage may be done manually. Alternatively, timing may be done automatically in response to a trigger signal generated when the reaction chamber is filled. 
     An embodiment of the sensor suitable for use with electrochemical detection is illustrated in  FIGS. 1 and 2 .  FIG. 1  is a top view of the sensor strip and  FIG. 2  is a cross-sectional view, showing details of the reaction chamber and the detection chamber. 
     The Sensor 
     The immunosensors of the present invention may be prepared using well-known thin layer device fabrication techniques as are used in preparing electrochemical glucose sensing devices (see, e.g., U.S. Pat. No. 5,942,102, incorporated herein by reference in its entirety). Such techniques, with certain modifications, are also used to prepare immunosensors utilizing non-electrochemical detection methods. 
     In a preferred embodiment of the immunosensor, illustrated in  FIGS. 1 and 2 , the detection chamber  28  comprises an electrochemical cell. The reaction chamber  22  and detection chamber  28  are prepared by first forming an aperture extending through a sheet of electrically resistive material  36 . The aperture is shaped such that it defines a sidewall of both the reaction chamber  22  and the detection chamber  28 , as well as the sample passageway  38  between the two chambers  22  and  28 . By extending the aperture from the proximal end  24  of the reaction chamber  22  through to the edge  37  of the sheet  36 , the sample ingress  24  is also formed. In one embodiment, the thickness of the sheet  36  defines the entire height of the reaction chamber  22  and detection chamber  28 , which are the same. In another embodiment, the height of the reaction chamber  22  is greater than that of the detection chamber  28 . A reaction chamber  22  of greater height than the detection chamber  28  is prepared by layering multiple sheets  32 ,  34 , and  36  together. Sheet  36  in the middle of the layer has an aperture defining the sidewalls  74  and  76  of both the reaction chamber  22  and detection chamber  28  as described above. The middle sheet  36  is then sandwiched between two or more additional sheets  32  and  34 , the additional sheets  32  and  34  having an aperture defining the side wall  74  of the reaction chamber  22  only, the sheets  32  and  34  thereby defining end walls  60  and  62  of the detection chamber  28 . In this embodiment, the end walls  60  and  62  of the detection chamber must also comprise thin electrodes  52  and  54 , which may be prepared as described below. 
     After the sidewalls  74  and  76  of the reaction chamber  22  and detection chamber  28  are formed, a first thin electrode  52  is then mounted on one side  70  of the sheet of electrically resistive material  36 , extending over the aperture forming the detection chamber  28  and forming an end wall  60 . The first thin electrode  52  may be adhered to the sheet  36 , e.g., by means of an adhesive. Suitable adhesives include, for example, heat activated adhesives, pressure sensitive adhesives, heat cured adhesives, chemically cured adhesives, hot melt adhesives, hot flow adhesives, and the like. The first thin electrode  52  is prepared by coating (e.g., by sputter coating) a sheet of electrically resistive material  32  with a suitable metal, for example, platinum, palladium, carbon, indium oxide; tin oxide, mixed indium/tin oxides, gold, silver, iridium, mixtures thereof, and the like. Materials suitable for use as thin electrodes  52  and  54  must be compatible with the reagents present in the sensor  20 , i.e., they will not react chemically with reagents. 
     A second thin electrode  54  is then mounted on the opposite side of the sheet of electrically resistive material  36 , also extending over the aperture forming the detection chamber  28 , so as to form a second end wall  62 . In a preferred embodiment, the thin electrodes  52  and  54  are mounted in opposing relationship at a distance of less than about 500 microns, more preferably less than 150 microns, and most preferably between 50 and 150 microns. If a sample ingress  24  has not already been formed, then one must be provided, e.g., by forming a notch in the edge  37  of the device  20  that intersects the proximal end of the reaction chamber  22 . 
     The electrodes  52  and  54  are provided with connection means allowing the sensor  20  to be placed in a measuring circuit. At least one of the electrodes  52  or  54  in the cell is a sensing electrode, i.e., an electrode sensitive to the amount of reduced redox agent in the antioxidant case or oxidized redox agent in the oxidant case. In the case of a potentiometric sensor  20  wherein the potential of the sensing electrode  52  or  54  is indicative of the level of analyte present, a second electrode  54  or  52 , acting as reference electrode is present which acts to provide a reference potential. In the case of an amperometric sensor  20  wherein the sensing electrode current is indicative of the level of analyte in the sample, at least one other electrode  54  or  52  is present which functions as a counter electrode to complete the electrical circuit. This second electrode  54  or  52  may also function as a reference electrode. Alternatively, a separate electrode (not shown) may perform the function of a reference electrode. 
     If the immunosensor  20  is operated as an electrochemical cell, then the sheets  32 ,  34 , and  36  containing the apertures defining the reaction chamber  22  and/or detection chamber  28  should comprise electrically resistive materials. Suitable electrically resistive materials include, for example, polyesters, polystyrenes, polycarbonates, polyolefins, mixtures thereof, and the like. A preferred polyester is polyethylene terephthalate. If the immunosensor  20  is operated using a detection method other than an electrochemical detection method, then the materials need not be electrically resistive. However, the polymeric materials described above are preferred for use in constructing the immunosensors of a preferred embodiment because of their ease of processing, low cost, and lack of reactivity to reagents and samples. In the case of a detection method involving absorbance, transmission, or emission of light of a particular frequency, then the end walls  60  and/or  62 , sheets  32 ,  34  and layers  46 ,  42  above the end walls of the detection chamber  28  should be transparent to that light frequency. 
     Reagents for use in the cell, e.g., immobilized antibody, enzyme-linked antigen, buffer, mediator, and the like, may be supported on the internal surfaces  40 ,  48 , and/or sidewall  74  of the reaction chamber  22  or on the end walls  60 ,  62 , and/or sidewall  76  of the detection chamber  28 , on an independent support contained within chambers, within a matrix, or may be self supporting. If the reagents are to be supported on the chamber walls or electrodes  52  and  54 , the chemicals may be applied by use of printing techniques well known in the art, e.g., ink jet printing, screen printing, lithography, and the like. In a preferred embodiment, a solution containing the reagent is applied to a surface within a chamber and allowed to dry. 
     Rather than immobilize or dry the antibodies  44 , the enzyme-linked antigen  50 , or other chemicals onto the internal surfaces  40 ,  48 , onto the end walls  60 ,  62 , or onto the sidewalls  74 , and/or  76  of the reaction chamber  22  or detection chamber  28 , it may be advantageous to support them on or contain them within one or more independent supports which are then placed into a chamber. Suitable independent supports include, but are not limited to, mesh materials, nonwoven sheet materials, fibrous filling materials, macroporous membranes, or sintered powders. The advantages of independent supports include an increased surface area, thus allowing more antibody and enzyme-linked antigen to be included in the reaction chamber, if desired. In such an embodiment, the antibody is immobilized on one piece of porous material and placed in the first reaction chamber and the enzyme-linked antigen is dried onto a second piece of porous material, which is then placed into the reaction chamber. Alternatively, either the antibody or the enzyme-linked antigen are incorporated onto the porous material and the other component supported on the reaction chamber wall as described above. In yet another embodiment, the walls of the reaction chamber themselves are porous, with the antibody and/or probe enzyme-linked antigen incorporated in them. In this embodiment, the liquid is able to wick into the porous wall, but not leak out of the defined area. This is accomplished by using a macroporous membrane to form the reaction chamber wall and compressing the membrane around the reaction chamber to prevent leakage of sample out of the desired area. 
     Suitable independent supports such as mesh materials, nonwoven sheet materials, and fibrous fill materials include, polyolefins, polyesters, nylons, cellulose, polystyrenes, polycarbonates, polysulfones, mixtures thereof, and the like. Suitable macroporous membranes may be prepared from polymeric materials including polysulfones, polyvinylidene difluorides, nylons, cellulose acetates, polymethacrylates, polyacrylates, mixtures thereof, and the like. 
     The protein or antibody may be contained within a matrix, e.g., polyvinyl acetate. By varying the solubility characteristics of the matrix in the sample, controlled release of the protein or antibody into the sample may be achieved. 
     In all cases, the materials used within the sensor are in a form amenable to mass production, and the cells themselves are designed to be able to be used for a single experiment then disposed of. 
     A preferred embodiment of an immunosensor that is fabricated as described above is illustrated in  FIGS. 1 and 2 . In this preferred embodiment, the sheets of electrically resistive material  32  and  34  are coated with electrically conductive material that forms the thin electrodes  52  and  54 . The electrically conductive material is coated on the surface of end walls  60  or  62  facing the detection chamber  28  and an adhesive layer (not shown) is coated on the surface  33  or  35  facing layer  42  or  46 , respectively. 
     Using the Sensor to Determine the Presence or Absence of an Antigen 
     In a preferred embodiment, the sensor  20  is an electrochemical cell utilizing an enzyme, e.g., glucose oxidase or glucose dehydrogenase, as the probe, as illustrated in  FIG. 1 , a top view of such a sensor  20 , and  FIG. 2 , a cross section of the sensor through line A-A′. The presence or absence of an analyte is inferred in this embodiment as follows. 
     The user first introduces sample into the reaction chamber  22  of the sensor  20  through the sample ingress  24 . The sample is drawn into the reaction chamber  22  under the influence of capillary or wicking action. During filling the reaction chamber vent  26  is open to the atmosphere, thus allowing air displaced by the sample to escape. Sample will be drawn into the reaction chamber  22  until it is filled up to the reaction chamber vent  26 , whereupon filling will stop. The volume of the reaction chamber  22  is chosen so as to be at least equal to and preferably larger than the volume of the detection chamber  28 . 
     The dashed circle in  FIG. 1  denotes an aperture  30  piercing sheets  32 ,  34 , and  36  but not layers  42  and  46 , the aperture in sheet  34  opening into the detection chamber  28 . Since layers  42  and  46  are not pierced initially, the only opening to the atmosphere of the detection chamber  28  is the sample passageway  38  opening into the reaction chamber  22 . Thus, when the reaction chamber  22  fills with sample, it blocks the sample passageway  38  to the detection chamber  28 . This traps air in the detection chamber  28  and substantially prevents it from filling with sample. A small amount of sample will enter the detection chamber  28  during the time between when the sample first contacts the opening of the sample passageway  38  to the detection chamber  28  and when it contacts the far side of the opening of the sample passageway  38 . However, once the sample has wet totally across the opening to the detection chamber  28 , no more filling of the detection chamber  28  will take place. 
     The internal surface  40  of the layer  42 , which forms the base of the reaction chamber  22 , is coated with antibodies  44  to the antigen to be detected. The antibodies  44  are adsorbed or otherwise immobilized on the surface  40  of the layer  42  such that they are not removed from the layer  42  during a test. Optionally, after application of the antibodies  44  to the internal surface  40  of the layer  42 , an agent designed to prevent non-specific binding of proteins to this surface can be applied (not shown). An example of such an agent well known in the art is bovine serum albumin (BSA). A nonionic surfactant may also be used as such an agent, e.g., Triton X100™ manufactured by Rohm &amp; Haas of Philadelphia, Pa., or Tween™ manufactured by ICI Americas of Wilmington, Del. The nonionic surfactant selected must not denature proteins. The antibodies  44  coating the internal surface  40  of the layer  42  are in the dry state when ready to be used in a test. 
     Another layer  46  defines the internal surface  48  of the reaction chamber  22 . On the internal surface  48  of the layer  46  are coated the enzyme-linked antigens  50  to be detected. Examples of suitable enzymes include, but are not limited to, glucose oxidase and glucose dehydrogenase. The enzyme-linked antigen  50  is dried onto the internal surface  48  of the layer  46  in such a way that it can be liberated into the sample when the internal surface  48  is wet by the sample. The internal surface  48  of the layer  46  and the method for coating on the enzyme-linked antigen  50  are therefore chosen such that only a weak bond between the enzyme-linked antigen  50  and the internal surface  48  of the layer  46  exists. The rate of dissolution of the enzyme-linked antigen  50  from the internal surface  48  is chosen such that little dissolution has occurred during the time taken for the sample to fill the reaction chamber  22 . In this manner, the enzyme-linked antigen  50  will be evenly distributed throughout the area of the reaction chamber  22  after filling. 
     The relative amounts of enzyme-linked antigen  50  and antibody  44  are chosen such that there is a slight excess of antibody  44  over enzyme-linked antigen  50 . In this context, a slight excess is defined to be such that the excess is small when compared to the number of antigen molecules to be detected in the sample. 
     Thus, when sample fills the reaction chamber  22  the enzyme-linked antigen  50  enters and mixes with the sample. Sufficient time is then allowed for the enzyme-linked antigen  50  to come into contact with the antibodies  44 . Since there is excess of antibodies  44 , if no antigen is present in the sample then substantially all of the enzyme-linked antigen  50  will bind to the antibodies  44  and so be effectively immobilized. If antigen is present in the sample, the antigen, being smaller than the enzyme-linked antigen  50  and already present throughout the volume of the sample, will contact and bind to the antibodies  44  before the enzyme-linked antigen  50  contacts the antibodies  44 . The antibodies  44  will therefore be blocked and prevented from binding to the enzyme-linked antigen  50 . So if antigen is initially present in the sample then, at the end of the reaction step, enzyme-linked antigen  50  will remain mobile in the sample. If no antigen is initially present in the sample, the enzyme-linked antigen  50  will be immobilized to the antibodies  44  on the internal surface  48  of layer  46  at the end of the reaction step. 
     The end of the reaction step is a predetermined time after the sample is introduced into the reaction chamber  22 . The predetermined time is set such that there is sufficient time for substantially all of the enzyme-linked antigen  50  to bind to the antibodies  44  under the test conditions when no antigen is initially present in the sample. 
     The time that the sample is introduced into the reaction chamber  22  can be indicated by the user, for example, by depressing a button on a meter connected to the sensor  20 . This action is used to trigger a timing device. In the case of visual detection, no meter device is necessary. In such an embodiment, the user manually times the reaction period. 
     In the case where electrochemical detection is used to detect the result of the antibody/antigen reactions, the indication that sample has been introduced into the reaction chamber  22  can be automated. As described above, when sample fills the reaction chamber  22 , a small portion of the detection chamber  28  at its opening into the reaction chamber  22  will be wet by sample. If electrochemical detection is employed then at least two thin electrodes  52  and  54  will be present in the detection chamber  28 . If the thin electrodes  52  and  54  are placed in the detection chamber  28 , such that at least a portion of each thin electrode  52  and  54  is contacted by the sample during the filling of the reaction chamber  22 , the presence of the sample will bridge the thin electrodes  52  and  54  and create an electrical signal which can be used to trigger the timing device. 
     A predetermined time after the timing device has been triggered, either by the user or automatically, the antibody/antigen reaction phase of the test is deemed to be completed. When the antibody/antigen phase of the test is completed, the vent  56  to the atmosphere is opened. For example, a solenoid activated needle in the meter may be used to pierce layer  42  or layer  46  or both layers  42  and  46 , thus opening the distal end  58  of the detection chamber  28  to the atmosphere. The piercing can be automatically performed by the meter, as in the example above, or manually by the user in the case of visual detection wherein no meter may be used, e.g., the user inserts a needle through the layers  42  and  46 , thereby forming the vent  56 . 
     The opening of the vent  56  to the atmosphere allows the air trapped in the detection chamber  28  escape, thereby allowing the detection chamber  28  to be filled with reacted sample from the reaction chamber  22 . The reacted sample will be drawn into the detection chamber  28  due to increased capillary force in the detection chamber  28  compared to that present in the reaction chamber  22 . In a preferred embodiment, the increased capillary force is provided by suitably coating the surfaces of end walls  60  and  62  of the detection chamber  28  or, more preferably, by choosing the capillary distance for the detection chamber  28  to be smaller than that of the reaction chamber  22 . In this embodiment, the capillary distance is defined to be the smallest dimension of the chamber. 
     Optionally disposed in the detection chamber  28  are dried reagents  64  comprising an enzyme substrate and a mediator, capable of reacting with the enzyme part of the enzyme-linked antigen  50  to produce a detectable signal. The enzyme substrate and mediator, if present, are to be of sufficient amount such that the rate of reaction of any enzyme present with the enzyme substrate is determined by the amount of enzyme present. For instance, if the enzyme were glucose oxidase or glucose dehydrogenase, a suitable enzyme mediator and glucose (if not already present in the sample) would be disposed into the detection chamber  28 . Buffer may also be included to help adjust the pH of the sample in the detection chamber  28  if necessary. In an embodiment wherein an electrochemical detection system is used, ferricyanide is a suitable mediator. Other suitable mediators include dichlorophenolindophenol and complexes between transition metals and nitrogen-containing heteroatomic species. The glucose, mediator and buffer reagents are present in sufficient quantities such that the rate of reaction of the enzyme with the enzyme substrate is limited by the concentration of the enzyme present. 
     When the detection chamber  28  is filled, the dried reagents  64  dissolve into the sample. The enzyme component of the enzyme-linked antigen  50  reacts with the glucose and the mediator to produce reduced mediator. This reduced mediator is electrochemically oxidized at an electrode  52  or  54  acting as an anode in the detection chamber  28  to produce an electrical current. In one embodiment, the rate of change of this current with time is used as an indicator of the presence and amount of enzyme that is present in the reacted sample. If the rate of change of current is less than a predetermined threshold value, then it is indicative of no significant amount of enzyme-linked antigen  50  present in the reacted sample, indicating the lack of antigen present in the original sample. If the rate of change of current is higher than the threshold rate, it indicates that enzyme-linked antigen  50  is present in the reacted sample, and thus antigen is also present in the sample initially. In one embodiment, the rate of change of the current is used to give a measure of the relative amount of antigen initially present in the sample. 
     In a preferred embodiment of the electrochemical detection system, the thin electrodes  52  and  54  in the detection chamber  28  are formed as electrically conductive layers coated onto the surfaces of end walls  60  and  62 , e.g., by sputtering as disclosed in WO97/18464. These conductive layers are of materials that do not react chemically with reagent present and are useful as electrodes  52  and  54  at the potential of choice. Examples of suitable materials include, but are not limited to, palladium, platinum, gold, iridium, carbon, carbon mixed with a binder, indium oxide, tin oxide, and mixed oxides of indium and tin. 
     In this embodiment, an inert, electrically insulating sheet  36  separates the electrode-bearing sheets  32  and  34 . Preferably, insulating sheet  36  functions to keep sheets  32  and  34  at a predetermined separation. Provided this separation is small enough, e.g., less than 500 micron and more preferably from 50 to 150 microns, the current flowing between the electrodes  52  and  54  will be directly proportional to the concentration of reduced mediator after a suitably short time relative to the detection time employed. In this embodiment, the rate of current rise is directly related to the rate of the enzyme reaction and therefore the amount of enzyme present. 
     In  FIG. 1 , a connection end  66  is shown. The thin electrodes  52  and  54  (not shown) in the detection chamber  28  can be placed in electrical connection with a meter (not shown) through the connection end  66 . The connection means (not shown) connects the meter (not shown) to the thin electrodes  52  and  54  in the detection chamber  28  via conducting tracks (not shown). In the preferred embodiment illustrated in  FIG. 2 , the conducting tracks are extensions of the electrically conductive material that form the thin electrodes  52  and  54  and are coated onto the internal surfaces of sheets  32  and  34 . The meter in connection with the connection end  66  (not shown) is capable of applying a potential between the thin electrodes  52  and  54  in the detection chamber  28 , analyzing the electrical signals generated, displaying a response and optionally storing the response in memory. 
     In other embodiments utilizing electrochemical detection, stripes of conducting material on one or both internal faces of the detection chamber are used, with the provision that at least two electrodes are present, i.e., a sensing electrode and a counter/reference electrode. Optionally, a third electrode, serving as a separate reference electrode, is included. 
     In the case of an embodiment wherein visual detection or reflectance spectroscopy is the detection method used, at least the sheet  32  and layer  46  or sheet  34  and layer  42  are transparent to the wavelength of radiation that is to be observed. In the case of visual detection, a simple color change in the detection chamber  28  is observed. In the case of reflectance spectroscopy, detection radiation is shone through sheet  32  and layer  46  or sheet  34  and layer  42 , and radiation reflected from the solution in the detection chamber  28  is analyzed. In the case of transmission spectroscopy used as the detection method, at least sheet  32  and layer  46 , or sheet  34  and layer  42 , are transparent to radiation at the wavelength of choice. Radiation is shone through the sample in the detection chamber  28  and the attenuation of the beam is measured. 
     In a preferred embodiment of a method of constructing the sensor, sheet  36  comprises a substrate with a layer of adhesive (not shown) coated on its upper surface  70  and lower surface  72 . Examples of materials suitable for the substrate of sheet  36  include polyester, polystyrene, polycarbonate, polyolefins, and, preferably, polyethylene terephthalate. Examples of suitable adhesives are pressure sensitive adhesives, heat and chemically curing adhesives and hot melt and hot flow adhesives. 
     Use of Melitten as a Probe 
     Conventional ELISAs link an antigen to an enzyme. However, it is also possible to link the antigen to melittin, a polypeptide found in bee venom. In this embodiment, a probe-linked antigen comprising an antigen-melittin complex can bee dried on a wall of the reaction chamber, as described above. The detection chamber can contain a mediator comprising ferrocyanide in liposomes or lipid vesicles. If the antigen-melittin complex reaches the liposomes, they will burst and release the ferrocyanide. This leads to a rapid amplification of the signal, i.e., a small amount of free antigen competes with the antigen-melittin complex for binding sites on the antibodies and results in a large concentration of ferrocyanide. 
     Use of Horse Radish Peroxidase and Alkaline Phosphatase in Electrochemical Assays 
     Conventional ELISAs use horse radish peroxidase (HRP) or alkaline phosphatase (AP) as the enzymes in a calorimetric assay. However, substrates have been developed which allow both these enzymes to be used in an electrochemical assay. In this embodiment, AP can be used with p-aminophenyl phosphate and HRP can be used with tetrathiafulvalene. 
     The above description discloses several methods and materials of the present invention. This invention is susceptible to modifications in the methods and materials, as well as alterations in the fabrication methods and equipment. Such modifications will become apparent to those skilled in the art from a consideration of this disclosure or practice of the invention disclosed herein. Consequently, it is not intended that this invention be limited to the specific embodiments disclosed herein, but that it cover all modifications and alternatives coming within the true scope and spirit of the invention as embodied in the attached claims.