Patent Publication Number: US-2007109541-A1

Title: Differential surface plasmon resonance measuring device and its measuring method

Description:
TECHNICAL FIELD  
      The present invention relates to a differential surface plasmon resonance measuring apparatus and a method for differentially measuring surface plasmon resonance.  
     BACKGROUND ART  
      While the industry highly developed in the latter half of the 20 century in Japan brought material wealth to our life, it has left negative legacies that have a serious impact on human society, such as air, water, and soil pollution and juvenile drug abuse. Among these problems, air and water pollution caused by inorganic materials have fairly overcome. However, endocrine disrupters as represented by for, example, dioxin, whose effects on living bodies were found in the early 1990s, are of concern. Specifically, solutions for environmental pollution with some artificial low-molecular-weight organic compounds, physical and mental decay by drug abuse, and soil pollution are left to the 21 century, and they are considered to be public concerns that should be overcome immediately. In view of metrological chemistry, analysis of those organic compounds, which are minor constituents and need measuring with reliability, is performed by gas chromatography and mass spectroscopy, which are expensive analysis and require a lot of skill for operation. Accordingly, information sufficient to know the actual conditions of such pollution cannot be obtained. This is one of the causes of difficulty in solving the problems.  
      In addition to the above-mentioned gas chromatography and mass spectroscopy, general approaches for analyzing organic compounds include liquid chromatography, optical measurements based on chemical reactions using fluorescence reagents or illuminant reagents, enzyme immunoassay, and a surface plasmon resonance measurement. Among these, simple is the surface plasmon resonance measurement. The reasons are as follows.  
      In the surface plasmon resonance measurement, optical resonance (surface plasmon resonance, SPR) is measured which is generated in the region of 100 nm by an interaction between materials induced by irradiating a metal surface in a plasma state. This measurement has the following advantages: 
      (1) Chemical reactions at the surface of a sensor can be tracked in real time;     (2) Since the interaction between materials occurs in the region of 100 nm, samples to be analyzed can be in small amount;     (3) Even a small amount of sample can be concentrated with a high sensitivity because of the above (2);     (4) The detecting system uses a glass prism, and accordingly the detector can be extremely small; and     (5) A gold membrane is used to generate plasmon resonance, and consequently, it becomes easy to fix inductors, such as antibodies, and a detecting system selectively detecting a measuring object can be designed.    

      Accordingly, the surface plasmon resonance measurement is thought of as an optimal approach for developing a ubiquitous palm-size-oriented field apparatus for measuring a low-molecular-weight environmental organic pollutant.  
      Surface plasmon resonance is a phenomenon in which when light enters a prism that is coated with a metal thin film by vapor deposition, evanescent waves always generated at the surface of the prism resonate with surface plasmon waves excited at a gold surface, thereby reducing reflection. The incident angle inducing the surface plasmon resonance depends on the permittivity of the sample solution. By fixing a material interactive with the measuring object to the surface of a metal thin film to form a functional film, a chemical sensor measuring a variety of organic compounds can be achieved.  
      This phenomenon has been known in the field of optics in applied physics since a long time ago. More specifically, Wood found the phenomenon in 1902 and Nylander developed a sensor using the phenomenon in 1982. Scientific applications of the phenomenon have not been made until recently, and real-time measurement of interaction between a biomembrane and a material was made possible by fixing an antibody or the like to a gold surface. In general, in order to measure the interaction between the biomembrane and the material, an equilibrium method is performed in which their equilibrium state is measured over a period of several days. The surface plasmon resonance measurement allows real-time measurement of the equilibrium state, and accordingly, its various applications, such as immunosensors measuring immune response and protein interaction analysis, have been made widely in the fields of science and industry, such as analytical chemistry, biochemistry, drug chemistry, and medical measurement.  
      The principle of surface plasmon resonance will now be described.  
      If light is emitted to a glass substrate whose one surface is coated with a metal thin layer deposited at a thickness of several tens nanometers, such as of gold or silver, from the other surface side, wave propagation called surface plasmon occurs. The surface plasmon results from quantization of fluctuations of free electrons less constrained in the metal. Free electrons can propagate with a crude density equal to that of sound waves in the direction of the tangent at the metal surface. If free electrons are vibrated with electromagnetic waves having the same propagation speed, the electrons resonate and thus surface plasmon occurs.  
      Since in a metal, electrons move freely around the cations, the metal can be considered to be solid-state plasma. The solid-state plasma has surface plasma oscillations (their quantum refers to surface plasmon), which result from collective electron excitation, in the vicinity of its surface. The surface plasmon is surface waves present only at a metal surface, and the relationship between its wave number K sp  and frequency ω is given as follows, depending not only on the permittivity ε m  of the metal, but also on the refractive index n s  of the medium (sample) in contact with the metal:  
               Ksp   =       c   ω     ⁢           ɛ   m     ⁢     n   s   2           ɛ   m     +     n   s   2               ,           (   1   )             
 
      where c represents the velocity of light in a vacuum.  
      If the wave number K sp  of the surface plasmon with a frequency ω at the surface of the metal (whose permittivity ε m  has been known) is obtained, the refractive index n s  of the sample can be determined from equation (1).  
       FIG. 1  is a schematic representation of the principle of surface plasmon resonance.  
      In this figure, reference numeral  1  represents a prism (refractive index n D ),  2  represents a metal thin film (permittivity ε),  3  represents a sample solution,  4  represents an incident light (wave number K p ),  5  represents evanescent waves (wave number K ev ),  6  represents reflected light,  7  represents a CCD detector, and  8  represents surface plasmon (wave number K sp ).  
      As shown in  FIG. 1 , the metal thin film  2  is deposited on the surface of the prism  1  and brought into contact with the sample (in this case, sample solution)  3 . When incident light  4  comes to the bottom (sensor surface) of the prism  1  at an angle of the critical angle or more from the prism  1  side, the evanescent waves  5  penetrate the sample  3 . If plane waves (wave number K p ) acting as incident light  4  enter at an incident angle θ, the wave number K ev  of the evanescent waves  5  becomes a component of the spatial frequency of the incident light  4  along the bottom of the prism:
 
 K   ev   =K   p  sin θ  (2)
 
 When the incident angle is the critical angle or more, the relationship K p  sin θ&gt;K s  holds (K s  represents the wave number of light propagating through the sample  3 ). Hence,
 
 K   ev   =K   p  sin θ&gt; K   s   (3)
 
 The wave number K ev  of the evanescent waves  5  is larger than the wave number K s  of the light propagating through the sample  3 . Therefore an incident angle θ sp  satisfying the relationship K ev =K sp  exists. Light  4  entering at this angle θ sp  resonates with the evanescent waves  5  to excite surface plasmon  8 . Once the surface plasmon  8  is excited by the evanescent waves  5 , part of the energy of the light transfers to the surface plasmon  8  and, thus, the intensity of the reflected light  6  returning into the prism  1  is reduced. By measuring the dependency of the reflectance at the prism  1  side on the wave number K ev  of the evanescent waves or on the incident angle of the incoming plane waves, an absorption peak is observed which indicates the excitation of the surface plasmon  8 . The wave number K sp  of the surface plasmon  8  is derived from the absorption peak position (wave number K ev  or incident angle θ sp ), and the refractive index n s  of the sample can be obtained from equations (1) and (2). The refractive index n s  of the sample solution  3  depends on the concentration of the sample. Thus, the concentration can be determined by measuring the refractive index. 
 
      If a material interactive with the measuring object is fixed to the surface of the metal thin film  2  to form a functional film  9 , as shown in  FIG. 2 , the permittivity and the thickness of the functional film  9  are varied (by various types of reaction and binding) to change the resonance angle. By measuring the changes of this angle in real time, the state, speed, and quantity of various types of reaction and binding, and sample concentration can be known. Incidentally,  FIG. 2  shows an immunological measurement using surface plasmon resonance.  
      A known plasmon resonance measuring apparatus will now be described.  
       FIG. 3  is a schematic diagram of a differential surface plasmon resonance measuring apparatus.  
      In this figure, reference numeral  11  represents a light source,  12  represents a beam splitter,  13  represents an SPR detector,  14  represents a sample photodetector,  15  and  18  represent preamplifiers,  16  and  19  represent A/D converters,  17  represents a reference photodetector,  20  represents an interface (I/F), and  21  represents a computer.  
      As shown in  FIG. 3 , in a known optical system, light from the light source  11  is split into two paths of light beams by the beam splitter  12 , and thus the beams are irradiated to predetermined two points of the SPR detector  13  including a prism. The two independent photodetectors  14  and  17  detect the reductions of the beams resulting from surface plasmon resonance and the preamplifiers  15  and  18  amplify the signals.  
       FIG. 4  shows a detecting system of the known surface plasmon resonance measuring apparatus.  
      In this figure, reference numeral  22  represents a prism,  23  represents an optical interface oil layer,  24  represents a sensor,  25  represents a sample,  26  represents a liquid pump,  27  represents a flow cell,  28  represents a flow cell holder, and  29  represents light.  
      As shown in this figure, the known detector includes the liquid pump  26 , the flow cell  27 , the cell holder  28 , the sensor  24 , the prism  22 , and the optical interface oil layer  23  for ensuring optical matching with the prism  22 .  
      [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2000-039401  
      [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2001-183292  
      [Patent Document 3] Japanese Unexamined Patent Application Publication 2001-255267  
      [Patent Document 4] Japanese Patent No. 3356212  
      [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2003-185572  
     DISCLOSURE OF INVENTION  
      Unfortunately, the multi-path system as shown in  FIG. 3  is limited in downsizing the prism because light is split into two paths of light beams by the beam splitter  12 . Also, the multi-path system needs two detecting systems, and accordingly requires some space in the structure. That is why the downsizing of the apparatus to a palm sized model is limited.  
      In general, surface plasmon resonance measuring apparatuses, which are commercially available from, for example, Biacore K. K., Nippon Laser Electronics, have large dimensions of 760 (W) by 350 (D) by 610 cm (H) and a weight of 50 kg (BIAcore 1000 of Biacore K. K.), and they are limited to laboratory use.  
      Accordingly, for measuring the surface plasmon resonance of a sample in practice, the sample has to be brought to a laboratory. It has been impossible to obtain living measurement results on site.  
      The measuring system using the detecting system as shown in  FIG. 4  is inevitably large and cannot satisfy the requirements for on-site-oriented ubiquitous, palm-sized differential surface plasmon resonance measuring apparatus that can make measurement anytime anywhere.  
      In order to overcome the disadvantages in the surface plasmon resonance measurement, the present invention provides a palm-sized inexpensive differential surface plasmon resonance measuring apparatus including an optical system and a detecting system to which new ideas have been applied. The apparatus is intended for use in measurement of an environmental organic pollutant and its measurement results are reliable. Also anyone can easily operate the apparatus without special experience anytime and anywhere including outdoors, in much the same way as sensors, such as pH glass electrodes.  
      In view of the above circumstances, the object of the present invention is to provide a small, inexpensive differential surface plasmon resonance measuring apparatus that is intended for ubiquitous measurements performed without special experience, and to a method for differentially measuring surface plasmon resonance.  
      In order to accomplish the object:  
      [1] A differential surface plasmon resonance measuring apparatus is provided which includes: an incident light optical system in which light enters at an incident angle in a range including the resonance angle; a sample setting device including a sample solution-fixing portion and a reference solution-fixing portion on a thin film deposited on a prism, the sample solution-fixing portion and the reference solution-fixing portion lying in the region irradiated with a beam of the incident light; a projection optical system for splitting light reflected from the sample solution-fixing portion and the reference solution-fixing portion into their respective beams and turning the directions of the beams to project the beams on a single line; and a liner CCD sensor including a CCD on the single line, the CCD receiving the beams.  
      [2] In the differential surface plasmon resonance measuring apparatus of the above [1], the projection optical system includes a plurality of mirrors for splitting the light reflected from the sample solution-fixing portion and the reference solution-fixing portion into their respective beams and turning the directions of the beams to project the beams on the single line.  
      [3] In the differential surface plasmon resonance measuring apparatus of the above [2], the plurality of mirrors includes a first mirror for reflecting the reflected light from the sample solution-fixing portion at a first angle and a second mirror for reflecting the reflected light from the reference solution-fixing portion at a second angle.  
      [4] The differential surface plasmon resonance measuring apparatus of the above [1] further includes an adhesive optical interface film disposed on the prism and the optical interface film has a refractive index matched with the refractive index of the prism.  
      [5] A method for differentially measuring surface plasmon resonance is provided which includes: emitting light from a light source having a specific wavelength so as to form a line focus on a sensor including a prism and a glass substrate; generating surface plasmon resonances at sensing portions of a sample cell and a reference cell that are provided on the line focus at a predetermined distance to reduce the intensity of the light reflected from the sensing portions; allowing the beams of the reflected light to reflect from light-splitting mirrors having different angles with the beams maintaining a distance equal to the predetermined distance between centers of the sensing portions and thus splitting the reflected light into two optical paths; and pressing an electrode-type combination sensor cell including sensing films corresponding to the sample portion and the reference portion on an adhesive optical interface film disposed on the prism, having a refractive index matched with that of the prism, whereby an optical system performing detection in two regions of a single CCD line sensor measures the surface plasmon resonances generated in the sample cell and the reference cell, with optical matching maintained between the sensor, the optical interface film, and the prism.  
      [6] In the method for differentially measuring surface plasmon resonance of the above [5], the optical interface film is a polymeric adhesive optical interface film.  
      [7] In the method for differentially measuring surface plasmon resonance of the above [6], the polymeric film is made of polyvinyl chloride.  
      [8] In the method for differentially measuring surface plasmon resonance of the above [6] or [7], the sample cell is disposed on the adhesive optical interface film without using a matching oil having the same refractive index as the prism and the glass substrate.  
      [9] In the method for differentially measuring surface plasmon resonance of the above [8], a substance interactive with a functional material and having a refractive index that is varied by the interaction is measured in a chemical sensor-like system.  
      [10] In the method for differentially measuring surface plasmon resonance of the above [9], an antibody is fixed to the sample cell so that an antigen-antibody reaction is measured in an immunosensor-like system.  
      [11] In the method for differentially measuring surface plasmon resonance of the above [5], the electrode-type combination sensor cell is pressed at a force of about 20 N. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic representation of the principle of surface plasmon resonance;  
       FIG. 2  is a schematic representation of an immunological measurement using surface plasmon resonance;  
       FIG. 3  is a schematic diagram of a known differential surface plasmon resonance measuring apparatus;  
       FIG. 4  is a schematic diagram of a detecting system of a known differential surface plasmon resonance measuring apparatus;  
       FIG. 5  is a schematic diagram of a differential surface plasmon resonance measuring apparatus of the present invention;  
       FIG. 6  is a schematic diagram of an optical system of a palm-sized differential surface plasmon resonance measuring apparatus of the present invention;  
       FIG. 7  is a representation of measuring points of the palm-sized differential surface plasmon resonance measuring apparatus of the present invention, viewed from above a combination dual sensor cell;  
       FIG. 8  is a schematic diagram of splitting mirrors (step  1 ) of the palm-sized differential surface plasmon resonance measuring apparatus of the present invention;  
       FIG. 9  is a schematic diagram of the splitting mirrors (step  2 ) of the palm-sized differential surface plasmon resonance measuring apparatus of the present invention;  
       FIG. 10  is a schematic representation of two detections with a single photodetector using the splitting mirrors, in the palm-size differential surface plasmon resonance measuring apparatus of the present invention;  
       FIG. 11  shows schematic diagrams of a detecting system of the palm-sized differential surface plasmon resonance measuring apparatus of the present invention;  
       FIG. 12  shows a surface plasmon resonance measurement in a chemical sensor-like system;  
       FIG. 13  is a schematic diagram of a surface exposed to light of an electrode-type SPR combination sensor cell.  
       FIG. 14  is a schematic diagram of a positioning guide of an SPR sensor cell;  
       FIG. 15  is a schematic diagram of a detecting system using an adhesive optical interface film;  
       FIG. 16  is a flow chart of a process for forming a polymeric adhesive optical interface film;  
       FIG. 17  shows plots of relationships between the intensity of differential SPR and the changes in resonance angle;  
       FIG. 18  shows plots of the stability of the resonance angle in use of a PBS buffer solution (pH  7 . 4 ) according to the present invention;  
       FIG. 19  is a plot showing the stability of the resonance angle signals of a single-type apparatus;  
       FIG. 20  is a schematic diagram of a prototype of a combination sensor cell according to the present invention; and  
       FIG. 21  is a plot showing the 2,4-dichlorophenol concentration-following capability of a differential surface plasmon resonance measuring apparatus of the present invention.  
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION  
      The apparatus of the present invention uses a combination sensor cell to embody the advantage that surface plasmon resonance can be measured in real time, in the fullest sense. The apparatus is small, resistant to disturbances, and very easy to operate.  
      Specifically, an apparatus achieving ubiquitous measurement of environmental pollutants has to be: (1) highly sensitive; (2) easy to operate; (3) with no moving parts; (4) small; (5) light; (6) capable of on-site measurement; (7) portable; (8) inexpensive; (9) battery-operated; ( 10 ) reliable; and ( 11 ) like a chemical sensor. The apparatus of the present invention satisfies these requirements for ubiquitous measurement.  
      The present invention aims for providing a sensor that anyone can easily use for ubiquitous surface plasmon resonance measurement anywhere. The surface plasmon resonance measuring sensor includes: (1) an optical system emitting light to a sample sensing portion and a reference sensing portion on a thin film, allowing the reflected light to reflect from splitting mirrors to split the light into beams, and focusing the beams on a single linear CCD sensor side by side; (2) a detecting system including a sensor cell with which the sample sensing portion and the reference sensing portion are easily disposed and held on a thin film deposited on a prism; and (3) an adhesive optical interface film exhibiting high adhesion, having a refractive index equivalent to that of a matching oil, eliminating the mechanical pressing of a sensor base and the prism on each other that is necessary during the use of the matching oil, and being an alternative to the known matching oil and easy to operate on site. The surface plasmon resonance measuring sensor of the present invention can simultaneously measure both a sample and a reference, and thus can achieve real-time measurement. In addition, the sensor can be so small, sensitive, and easy to operate as to achieve ubiquitous measurement.  
     EMBODIMENTS  
      Embodiments of the present invention will now be described.  
       FIG. 5  is a schematic diagram of a differential surface plasmon resonance measuring apparatus of the present invention.  
      In this figure, reference numeral  31  represents a light source,  32  represents an SPR detector including a sample cell  32 A and a reference cell  32 B,  33  represents light-splitting mirrors,  34  represents a sample light beam,  35  represents a reference light beam,  36  represents a photodetector,  37  represents a preamplifier,  38  represents an A/D converter,  39  represents an interface, and  40  represents a computer.  
      The differential surface plasmon resonance measuring apparatus of the present invention, which aims for overcoming the structural limitation in downsizing the known SPR measuring apparatuses, includes a novel SPR optical system featuring a single light source and a single photodetector by use of the light-splitting mirrors  33 , as shown in  FIG. 5 , thereby being downsized to a palm size (10 cm (H) by 170 cm (W) by 50 cm (D)) and a light weight (770 g). The palm-sized differential surface plasmon resonance measuring apparatus of the present invention includes the optical system, a detecting system, an electrical system, a notebook computer (Windows XP-compliant), and computer software (DUAL SPRWIN) for taking in SPR signals and converting the signals into concentrations.  
       FIG. 6  is a schematic diagram of the optical system of the palm-sized differential surface plasmon resonance measuring apparatus, and  FIG. 7  shows measuring points of the plasmon resonance measuring apparatus, viewed from above a combination dual sensor cell.  
      In these figures, reference numeral  41  represents incident light from a light source LED (wavelength: 770 nm),  42  represents a prism,  43  is an polymeric adhesive optical interface film,  44  represents a glass substrate,  45  represents sensing films,  45 A represents a sample sensing film,  45 B represents a reference sensing film, and d represents a distance between the centers of the sample sensing film  45 A and the reference sensing film  45 B (distance between measuring points  45   a  and  45   b  of both sensing films). The distance in the present embodiment is set at 5 mm in view of the overall size. Reference numeral  46  represents a sensor support, and  47  represents an electrode-type combination dual sensor cell. The sample sensing film  45 A and the reference sensing film  45 B, where SPR&#39;s are measured, are disposed at lower portions of the cell. These components constitute a sensor that measures SPR&#39;s of a sample and a reference while the polymeric adhesive optical interface film  43  is pressed on the glass substrate  44  underlying the lower surfaces of the sensing films  45 . Reference numeral  48  represents a cylindrical lens,  49  represents a planoconvex lens,  50  represents a SPR reflected light,  51  represents a reflector,  52  represents a slit,  53  represents a splitting mirror unit including a splitting mirror  53 A and the other splitting mirror  53 B,  54  represents a reflected light beam from one splitting mirror  53 A,  55  represents a reflected light beam from the other splitting mirror  53 B,  56  represents a linear CCD sensor projecting the reflected light beams  54  and  55  on a line.  
      As described above, the light-splitting mirror unit  53  including the two mirrors  53 A and  53 B splits light into two light beams: a reflected light beam from the measuring point  45   a  (see  FIG. 7 ) of the sample sensing film  45 A and a reflected light beam from the measuring point  45   b  (see  FIG. 7 ) of the reference sensing film  45 B. More specifically, reflected light  50  generated at the sample sensing film  45 A and the reference sensing film  45 B underlying the dual sensor cell  47  by SPR&#39;s according to the permittivities of the sample and the reference is split into the reflected light beam  54  from the sample and the reflected light beam  55  from the reference by the two light-splitting mirrors  53 A and  53 B. These reflected light beams  54  and  55  are projected on a line by the linear CCD sensor  56 .  
      In the present invention, light  41  emitted from the single light source is irradiated to the sample sensing film  45 A and the reference sensing film  45 B so as to be line-focused on them, so that the surface plasmon phenomenon occurs at the surfaces of the sample sensing film  45 A and the reference sensing film  45 B. The reflected light  50  from the sensing films is split into the sample light beam and the reference light beam by the splitting mirror unit  53  and projected on a line of the single linear CCD sensor  56  without losing its energy. The present invention features the electrode-type combination dual sensor cell  47  for a differential application, the line-focus image forming technique, and the reflected light-splitting mirrors.  
      The positions of splitting mirrors of the plasmon resonance measuring apparatus will be further described below.  
       FIG. 8  is a schematic diagram showing positions of the splitting mirrors (step  1 ) of the plasmon resonance measuring apparatus,  FIG. 9  is a schematic diagram of another splitting mirror (step  2 ) of the plasmon resonance measuring apparatus, and  FIG. 10  is a schematic representation of two optical detections with a single photodetector using the splitting mirrors.  
      In order to project the reflected light  50  from the sensing films  45 A and  45 B on equally divided sensor regions of the linear CCD sensor  56  via the two splitting mirrors  53 A and  53 B, the angles of the two mirrors  53 A and  53 B are adjusted in two steps.  
      The following describes in detail how the splitting mirrors  53 A and  53 B are arranged and how the angles of the mirrors are adjusted.  
      The splitting mirrors  53 A and  53 B for splitting the reflected light  50  into a sample light beam and a reference light beam are arranged such that light-splitting points  58 A and  58 B on the respective splitting mirrors  53 A and  53 B lie on a Z axis  57 , including the light beams from the measuring point  45   a  on the sample sensing film and the measuring point  45   b  on the reference sensing film at the distance d therebetween, as shown in  FIG. 10 .  
      As shown in  FIG. 8 , the splitting mirrors  53 A and  53 B are arranged respectively at angles of α and β with respect to the Z axis  57  extending as the center line of these angles through the light-splitting points  58 A and  58 B so as to split the SPR reflected light  50  from the measuring points  45   a  and  45   b  on the sample sensing film and the reference sensing film into two beams in the direction toward the linear CCD sensor  56  (rightward direction).  
      Turning then to  FIG. 9 , the angles of the splitting mirrors  53 A and  53 B are adjusted to θ and γ so that the SPR reflected light  50  at the measuring points  45   a  and  45   b  on the sample sensing film and the reference sensing film is collected from the Z axis  57  on the XY plane including a linear optical element of the linear CCD sensor  56  and on a line of the linear CCD sensor  56 .  
      As described above, the two reflected light beams  54  and  55  are formed with no difference in optical path from the detecting point to the light-receptive point with the distance d maintained between the reflected light beams from the measuring points  45   a  and  45   b  of the sample sensing film and the reference sensing film, by adjusting the angles of the two splitting mirrors  53 A and  53 B in two steps. Thus, the reflected light  50  from the measuring points  45   a  and  45   b  of the sample sensing film and the reference sensing film can be equally split to light beams with no distortion to form an SPR signal image on the linear optical element of the linear CCD sensor  56 . Thus, a palm-sized apparatus can be achieved.  
      The detecting system of the palm-sized differential surface plasmon resonance measuring apparatus will now be described.  
      In order to achieve the palm-sized differential surface plasmon resonance measuring apparatus, the detecting system as well as the optical system should be downsized to a palm size. Accordingly, in order to accomplish the principal object of the present invention, it is desired to develop a chemical sensor-like detecting system not using a liquid pump or the like.  
       FIG. 11  schematically shows a detecting system of the palm-sized differential surface plasmon resonance measuring apparatus of the present invention.  FIG. 11 ( a ) is a schematic diagram of the detecting system of the palm-sized differential surface plasmon resonance measuring apparatus, and  FIG. 11 ( b ) is a sectional view taken along line A-A in  FIG. 11 ( a ).  
      In these figures, reference numeral  61  represents a prism,  62  represents a polymeric adhesive optical interface film,  63  represents a sensor,  64  represents a dual sensor cell including a sample cell  64 A and a reference cell  64 B,  65  represents a sensor cell guide,  66  represents a sensor cell support tube, and  67  represents a sensor cell cap.  
      As clearly shown in these figures, the detecting system of the differential surface plasmon resonance measuring apparatus of the present invention does not require any pump or any sensor holder. For example, a pH measuring glass electrode, which is a well-known chemical sensor, includes a pH-sensing glass membrane at the end of a glass or plastic sensor cell support tube. The pH measuring glass electrode is inserted into a sample in combination with a silver-silver chloride reference electrode, thereby generating a potential difference according to the pH of the sample. To make clear the concept of the SPR measurement in a chemical sensor-like system according to the present invention, a technique for surface plasmon resonance (SPR) measurement ( FIG. 12 ( b )) is compared to a known technique for pH measurement using a pH measuring glass electrode ( FIG. 12 ( a )).  
      In  FIG. 12 ( a ), reference numeral  71  represents a sample,  72  represents a cylindrical combination pH electrode including a pH measuring glass electrode  73  and a silver-silver chloride reference electrode  74 ,  75  represents a potentiometer for measuring potentials generated between the pH measuring glass electrode  73  and the silver silver-chloride reference electrode  74 . The combination pH electrode  72  has a bar shape with a diameter of about 12 mm and a length of about 150 mm. If a detecting portion is designed in such a chemical sensor form, the detecting portion becomes separable and the structure of the apparatus proper can be simplified and easily downsized. Such apparatuses also include dissolved oxygen meters and ion concentration meters.  
       FIG. 12 ( b ) shows the overview of an SPR measurement in a chemical sensor-like system according to the present invention in comparison with the simplest chemical measurement system, pH measuring glass electrode ( FIG. 12  ( a )).  
      In  FIG. 12 ( b ), reference numeral  81  represents an SPR detector for detecting the changes of SPR signals, and  82  represents a polymeric adhesive optical interface film for transmitting light with certain energy to the sensor through a prism. The polymeric adhesive optical interface film adheres to the prism. Reference numeral  83  represents an SPR-measuring electrode-type combination dual sensor cell, namely, a combination SPR electrode, corresponding to the combination pH electrode  72  shown in  FIG. 12  ( a ). The combination SPR electrode is in a cylindrical form measuring about 14 mm in diameter by about 25 mm in length. This sensor cell for measuring SPR can be called “SPRODE”, namely, SPR electrode, in the sense of a bar-shaped sensor, in comparison to the bar-shaped sensors for measuring potential or current that are called “ELECTRODES”. Reference numeral  84  represents a sensor cell guide for securing the sensor cell during measurement,  85  represents a sample cell,  86  represents a reference cell,  85 A represents a sample solution, and  86 B represents a reference solution. Reference numeral  87  represents a glass substrate (film) with a thickness of about 0.1 mm, serving as a base of the sensor cell,  88  represents a gold thin-film deposited at a thickness of 45 nm on the glass substrate  87 , and  89  represents a sensing film formed of, for example, an antibody chemically fixed to the gold thin film  88 . Reference numeral  90  represents a plastic sensor cell support,  91  represents a silicon sheet of about 1 mm in thickness, and  92  represents a sensor cell cap.  
       FIG. 13  shows the surface exposed to light of the electrode-type combination SPR sensor cell. In  FIG. 13 , reference numeral  101  represents light forming an image in a line focus at the interface between the prism and the sensor cell,  104 A represents the center of the sample sensing film disposed at the bottom of the sample cell, and  104 B represents the center of the reference sensing film, or the deposited gold thin film, disposed at the bottom of the reference cell. These points  104 A and  104 B have a constant distance of 5 mm. Reference numeral  112  represents the glass substrate (film),  113  represents the reference sensing film disposed on the glass substrate  112 ,  114  represents the sample sensing film to which an antibody or the like is fixed, which is disposed on the glass substrate  112 , and  115  represents the sensor cell support tube.  
       FIG. 14  shows a positioning guide of the SPR sensor cell. In  FIG. 14 , reference numeral  109  represents the sensor cell guide, and  117  represents the sensor cell cap. Reference numerals  118 A and  118 B represent screw holes used for fixing the sensor cell guide  109  to the SPR detector being the main body of the apparatus. Reference numeral  119 A represents a measuring position guideline of the sensor cell guide, and  119 B represents a measuring position guideline of the sensor cell.  
      In the present invention, SPR is induced by irradiating the prism to form a line focus having a width of about 100 μm and a length of about 10 mm. Accordingly, the reaction points on the sample sensing film and the reference sensing film where SPR occurs are positioned on the line focus with the distance d of 5 mm maintained between the centers of the sample and reference sensing films. Then, after the measuring position guideline  119 A of the sensor cell guide is aligned with the line focus, the sensor cell guide  109  is fixed to the body using the screw holes  118 A and  118 B. The screw holes  118 A and  118 B serve for fixing the sensor cell guide  109  to the body and for positioning for SPR detection, and their diameter can be arbitrary set.  
      The polymeric adhesive optical interface film will now be described.  
      Basically, the deposited gold film acting as the base of the sensor should be directly formed on the prism. Unfortunately, this process increases running cost for measurement. As an alternative to such a gold film, microscope cover glasses on which gold has been deposited are often used as the base of the sensor. In this approach, a matching oil having the same refractive index as the prism and the glass substrate acting as the sensor base has to be used to ensure the optical matching between the prism and the glass substrate. In addition, the flow cell, the glass substrate, and the prism are mechanically, uniformly pressed on each other with the oil therebetween to maintain smoothness because surface plasmon resonance occurs at a depth of 100 nm or less from the gold surface. However, such an approach is unsuitable for on-site measurement. Accordingly, in the present invention, a newly developed polymeric adhesive optical interface film is used which ensures optical matching and is easily fixed to the sensor cell.  
      An oil-free adhesive optical interface film has already been reported by the present inventors. However, this film has problems in repeatability, transparency, and adhesion. The polymeric adhesive optical interface film of the present invention is an improved type of that oil-free film.  
      In order to use a polymeric film as an alternative to the matching oil, the polymeric film must: (1) be transparent and colorless, (2) be highly adhesive, (3) have a refractive index same as or similar to that of the matching oil, and (4) in an analytical chemistry sense, produce SPR signals that are absolutely the same as the matching oil or that relatively correspond to the matching oil. The inventors first conducted research for a method for producing a polymeric film satisfying these requirements. The novel adhesive optical interface film is formed of an easily available polyvinyl chloride (PVC, polymerization degree: 700) in the similar manner to the general PVC wrapping film formation.  
       FIG. 15  is a schematic diagram of a detecting system using the polymeric adhesive optical interface film.  
      In this figure, reference numeral  121  represents a prism,  122  represents the polymeric adhesive optical interface film,  123  represents a glass film (substrate),  124  represents a deposited gold film,  125  represents a sample sensing film,  126  represents a sample solution. As shown in the figure, the optical interface is defined by a solid film.  
       FIG. 16  is a flow chart of a process for forming the polymeric adhesive optical interface film. The basic procedure of the process will now be described with reference to this figure.  
      It has been found that a transparent colorless film can be formed with a good reproducibility by the following procedure: PVC powder is dissolved in tetrahydrofuran (THF); subsequently a plasticizer, 2-ethylhexyl phthalate (DOP), and tritolyl phosphate (TCP) are added to the solution; and then, the solution is cast in a petri dish, followed by heat drying at 120° C. for 2 hours in a Corning plate drier capable of temperature control. Effects of drying temperature were investigated. As a result, it has been found that lower drying temperature is suitable for forming films used for SPR. According to experimental results, films formed at 80° C. were most superior in adhesion and separable in use for SPR.  
      Probably, the plasticizer, which has a high affinity for PVC and accordingly weakens the interaction with it to lower the melting point, enhances sliding of the PVC molecules at a controlled drying temperature of 80° C., consequently, producing a rubber elasticity and an adhesiveness. Also, PVC molecules are released from their intermolecular force by the plasticizer and the effect of temperature, and enhance their sliding so that the resulting film becomes flexible. It is believed that the resulting film is thus turned into an amorphous state from a crystalline state.  
      Then, PVC films having various compositions with different proportions of plasticizer and PVC were formed, and the refractive indices of the PVC films were measured with an Abbe refractometer produced by ATAGO. As a result, a film having a composition containing 0.5 g each of DOP and TCP relative to 0.2 g of PVC had a refractive index of 1.5211, exhibiting the closest value to the refractive index of the matching oil 1.5150. The film of this composition is thus employed as the adhesive optical interface film. This film is brought into close contact with the end of the sensor cell or the prism in advance, and loaded in the cylindrical support of the sensor cell along the guideline of the sensor cell. The sensor cell is pressed on the prism at about 20 newtons (N) with an index finger. Thus, optimal SPR signals can be obtained.  
      Up to this point, the essential elements of the present invention, namely, the optical system, the detecting system, and the polymeric adhesive optical interface film, have been described in detail. The palm-sized differential surface plasmon resonance measuring apparatus including these elements has the following specifications:  
      (1) Apparatus  
     
         
          Principle: surface plasmon resonance (SPR)  
          Differential system: splitting mirror, single light-receptive element  
          SPR measuring configuration: Kretchmann configuration  
          Measuring range: 65° to 75°  
          Power source: AC/DC (100 V or 9 V battery)  
          Maximum continuous operation time: 10 hours  
          Dimensions: 170 by 100 by 50 mm  
          Body weight: 770 g 
 
 (2) Optical System 
 
          Light source: point source LED (wavelength: 770 nm, half-width: 50 nm)  
          Prism material: BK7  
          Polarizing filter: extinction ratio, 0.00071  
          Light-receptive element: 2048-pixel CCD line sensor  
          Gold base size: within 14 mm square  
          Optical interface: adhesive PVC film 
 
 (3) Detecting System 
 
          Sensor cell: Combination SProde  
          Flow cell: flow rate, 1 to 100 μL/min  
          Sample volume: 1 μL or more 
 
 (4) Performance 
 
 Resonance angle stability 
 
          Single line: 0.0002°  
          Differential line: 0.0004°  
       
    
      An example of the differential surface plasmon resonance apparatus, prototyped according to the present invention will now be described.  
      An optical system, a combination sensor cell, and an adhesive optical interface PVC film for differentially detecting surface plasmon resonance were prototyped and assembled into a differential surface plasmon resonance measuring apparatus. The apparatus was subjected to performance tests.  
      1. Relationship between SPR Intensity and Changes in Resonance Angle  
       FIG. 17  shows plots of relationships between the intensity of differential SPR and the changes in resonance angle.  FIG. 17 ( a ) shows SPR curves of a blank cell prepared by filling the sample cell A and the reference cell B of the combination sensor cell with pH 7.4 buffer solution. Since the SPR intensities of the sample cell A and the reference cell B in the blank test are the same, their SPR&#39;s coincide with each other.  FIG. 17 ( b ) shows SPR curves when the reference cell B contains the same buffer solution and the sample cell A contains 0.1 mol/l glucose adjusted with the pH 7.4 buffer solution. It has been found that the resonance angle is varied according to the changes in glucose concentration, and that a differential surface plasmon resonance measuring apparatus can be achieved by calculating the difference between the sample resonance angle and the reference resonance angle. The difference of the SPR curves suggests that the SPR&#39;s at the two points of the combination sensor cell were correctly separated by the splitting mirror.  
      2. Resonance Angle Stability in the Present Invention  
       FIG. 18  shows plots of the stability of the resonance angle in use of a PBS buffer solution (pH 7.4) according to the present invention.  FIG. 18 ( b ) shows the changes in single line resonance angle [B] of the reference cell and  FIG. 18 ( a ) shows the changes in differential line resonance angle [A-B] being the difference between the sample cell and the reference cell. These results show that the stability of the single line resonance angle according to the present invention was 0.0002° and the stability of the differential line resonance angle was 0.0004°. On the other hand, the stability of a single mode surface plasmon resonance measuring apparatus based on the same principle was 0.001° as shown in  FIG. 19 . It has been shown that the angular resolution of the differential apparatus according to the present invention is 5 times or more increased than that of the single mode apparatus. The differential line angle stability was 0.0004° and larger than the single line angle stability. This is probably because of negative variations resulting from the subtraction between the resonance angles of the sample cell A and the reference cell B. The optical system of the present invention uses the same 2048-pixel CCD line sensor as in the single mode apparatus. Since, in the differential apparatus, reflected light from the sensor cell is split into two beams by the splitting mirrors, the practical pixel number is reduced by about half to 500 from 900. Thus, the present inventors thought that the single line resolution was thus 5 times increased. Then, the practical pixel number was reduced to 250 by adjusting the splitting mirrors. The results are shown in Table 1.  
                       TABLE 1                                      ITEMS                                                         Practical   Single Line   Differential Line       MEASURING   Light   CCD Line   CCD Signal   Pixel   Resonance Angle   Resonance Angle       SYSTEM   Source   Sensor   Processing   Number   Stability   Stability               Single   770 nm   2048   Moving   900   0.001            Differential-1   LED   pixel   average -   500   0.0002    0.00004                   of 7 cycle       Differential-2           measurements   250   0.00004   0.00008                   with 3-sec.                   sampling                    
      As clearly shown in Table 1, the stability in single line resonance angle was 0.00004° and thus the resolution was further 5 times increased as expected. For the same reason, the stability in differential line resonance angle was 0.00008°. As described above, it has been found that resolution of the resonance angle of the differential surface plasmon resonance measuring apparatus using the optical system of the present invention is inversely proportional to the practical pixel number.  
      3. Application to Immunological Measurement  
      To show the possibility of applying the differential surface plasmon resonance measuring apparatus of the present invention to immunological measurement, a combination sensor cell was prototyped and the SPR of 2,4-dichlorophenol, which has been known as a dioxin analog, was measured.  
       FIG. 20  shows the structure of the prototyped combination sensor cell. As shown in this figure, the combination sensor cell  130  having a body of 14 mm in diameter and 20 mm in height includes a glass substrate  135  having a 45 nm thick deposited gold film  136 , an epoxy resin support tube  134 , and a sensor cell cap  131  of 16 mm in diameter. The sample cell  132  and the reference cell  133  each have an internal diameter of 3.5 mm and the distance between these two cells is set at 5 mm. To the sample cell  132  of the combination sensor cell  130 , 2,4-dichlorophenol antibody was fixed in a conventional manner to prepare 2,4-dichlorophenol combination immunosensors. Four 2,4-dichlorophenol combination immunosensors were prepared. Reference cells  133  were each filled with PBS buffer solution (pH 7.4) as a reference solution. Sample cells  132  were respectively filled with 10, 25, 50, 100 ppm 2,4-dichlorophenol solution whose concentrations were adjusted with the PBS buffer solution, and thus, measuring sensor cells were prepared. For determination, the combination sensor cell was gently dropped onto the adhesive optical interface PVC film previously fixed to the body of the measuring apparatus along the sensor guide, and a pressure of about 20 N was applied with an index finger. Reflected light beams of the sample and the reference reduced by SPR generated at each point on the SPR sensing surfaces of the sample cell  132  and the reference cell  133  on the 10 mm line focus on the prism were measured with a CCD light-receptive element.  
       FIG. 21  shows a thus obtained calibration curve. As clearly shown in this figure, although the sensor cells have variations from each other, a satisfactory calibration curve was obtained with a multiple correlation coefficient of 0.973 in the 2,4-dichlorophenol concentration region of 10 to 100 ppm. Thus, it has been shown that the differential surface plasmon resonance measuring apparatus of the present invention, in which an antibody is fixed to the sample cell of the combination sensor cell, can readily measure antigen-antibody reactions in real time without labeling the antibody. Although this section has described an SPR immunosensor, it goes without saying that the present invention can use any chemical sensor-like system capable of being generally used for SPR measurement, and that any material can be sensed in a chemical sensor-like system as long as the material can produce an interaction with a functional material and consequently varies the refractive index.  
      Examples of the present invention have been described. The present invention allows an SPR measuring apparatus whose application has been limited to research use in laboratories to achieve the following: 
      (1) By newly designing an optical system, the SPR measuring apparatus can be downsized, be differentially operated, and have a high resolution.     (2) In order to simplify the determination procedure, an inexpensive portable downsized apparatus is provided using a newly designed polymeric adhesive optical interface film and combination sensor cell that anyone can easily operate anywhere, thereby achieving ubiquitous measurement.    

      It has been concerned that chemical pollutants, especially, low-molecular-weight organic compounds including endocrine disrupters, such as dioxin, stimulant drugs, and narcotic drugs may affect society. However, the amount of scientific information about those harmful organic chemical compounds is small because apparatuses for measuring those compounds are expensive and difficult to operate. Accordingly, portable apparatuses are desired which anyone can easily operate anywhere on site and which can provide various types of information. Unfortunately, only inorganic compounds, such as pH, DO (dissolved oxygen), and specific ions, can be measured by palm-sized immersion type sensor-like apparatuses and there is no simplified apparatus for measuring organic compounds so far. The present invention provides a combination sensor cell including a sample sensing film to which a material to be sensed is fixed and a reference sensing film. Thus, it is believed that the present invention can lead the way to a novel sensor-pressing SPR measurement (SProde method) for organic compounds. However, the concentrations in the environment of low-molecular-weight organic compounds, such as endocrine disrupters, are as extremely low as the order of, normally, ppt (pg/mL) to ppb (ng/mL).  
      The detection sensitivity of the immunological SPR measurement for low-molecular-weight organic compounds having molecular weights of about 200 is about 100 ng/L. In order to enhance the detection sensitivity of immunological measurement for low-molecular-weight organic compounds, such as 2,4-dichlorophenol (molecular weight: 175), a competitive method is generally employed in which an antibody and an antigen are added to be brought into competition with a antibody-fixed sensor. The detection sensitivity of this method is about 5 ppd. However, the method increases the number of steps in the procedure by one, and accordingly impairs the advantage that SPR can be measured in real time. However, the lack of absolute detection sensitivity in the SPR measurement can be compensated by solid state extraction that can be directly measured. The solid state extraction is suitable for compensating the lack of detection sensitivity in SPR measurement because it is versatile and readily allows 1000-times concentration. Table 2 shows the results of a study of the possibility that solid phase extraction achieves 125-times concentration of 2,4-dichlorophenol.  
                           TABLE 2                       2,4-dichlorophenol   Concentrated to   Concentration   Concentration       in raw water (ppb)   (ppm)   rate   error (%)                                                10   11.67   117   −6       30   31.04   103   −18       50   61.25   123   −2                  
 
      A divinylbenzene column ENV +  (solid weight: 200 mg; reservoir volume: 6 mL) produced by IST was used as the extraction column. Concentration was performed under the following conditions: sample volume of 1 L (pH 2); flow rate of 60 ml/min; and eluate of 8 mL of 0.1% formic acid/50% methanol. Table 2 clearly shows that 2,4-dichlorophenol on the order of ppb can be concentrated certainly to the order of ppm with an average error of −8.7%, in spite of extraction loss. Even at this time a combination of the above results and solid phase extraction can achieve determination of 2,4-dichlorophenol on the order of ppb with the measuring apparatus of the present invention. Since SPR occurs at a depth of 100 nm from the interface with the sensor, a sample volume of about 1 μL can suffice for determination. By microsizing the extraction column for extracting 1 μL of sample for analysis, taking this advantage of SPR, the concentration efficiency can further be enhanced. In this instance, 1 ppt of 2,4-dichlorophenol can be determined from 1 L of raw water.  
      Up to this point examples according to the present invention have been described, and it has been shown that the differential surface plasmon resonance measuring apparatus of the present invention, which is small like a palm, exhibit basic performances superior or equal to the known SPR measuring apparatus whose application is limited to laboratory use. In addition, the apparatus of the present invention is so portable and easy that anyone can operate anywhere. If the apparatus of the present invention is spread to society, ubiquitous approaches using the differential plasmon resonance measuring apparatus can be proposed to various fields associated with organic compounds, such as those of environment, analytical chemistry, medical drugs, safety, chemical industry, and research, and thus a large amount of important information can be produced. The information contributes to appropriate decisions in various fields. Thus, the present invention certainly helps to improve and develop human society.  
      The present invention provides the following advantages:  
      1. Optical System  
      The optical system according to the present invention has the following features. While the known optical system measures a single point of a sample chip on the prism, the present invention makes it possible to measure two points of a sensor on the prism by splitting reflected light from two points including SPR signals on the prism into two beams and projecting the beams on two positions of a photodetector by two splitting mirrors. If the same linear CCD element as in the known apparatus is used, the theoretical resolution of SPR signals is reduced to half that of the known apparatus. However, there is no problem in angular resolution because a sufficient number of data points are ensured for high-resolution peak detection with computer calculation.  
      If two linear CCD sensors are used for the known optical system, electronics must include two preamplifiers and two A/D converters, consequently increasing costs and size. The advantages of the optical system according to the present invention are clearly shown in the following:  
      (1) Design for Small Differential System  
      Taking advantages of the structure using a single linear CCD sensor, a novel optical system can be provided which can lead to a downsized differential surface plasmon resonance measuring apparatus. Actually, a prototype measured 170 cm (W) by 100 cm (H) by 50 cm (D) and weighed 770 g. Also, the optical system according to the present invention can be used as the known single-mode optical system by replacing the splitting mirrors with a reflector.  
      (2) Temperature Compensation  
      In the one-point measurement by the known apparatus, SPR signals are derived from the measurement of the refractive index (or permittivity) of the sample in principle. Consequently, the signals drift depending on temperature. In order to compensate the drift, it has been necessary that a semiconductor temperature sensor or the like having a high resolution be additionally installed to measure temperature, and that SPR signal data be calibrated according to the obtained temperature. If the SPR signals are used for a biosensor in an antigen-antibody reaction, their variations are extremely small. Then, a sample solution and another sample for temperature compensation having a high temperature coefficient are placed at the two measuring points on a sample chip in differential SPR, as an alternative to use of the temperature sensor. In addition to this, for temperature compensation of SPR signals, a temperature sensor matching with the temperature characteristics of the signals is used. In the present invention, by placing a sample solution and a reference solution for temperature compensation that has the same composition as the sample solution but not containing the measuring object at the two measuring points on a single sensor, the refractive indices and temperature changes of the solutions having the same composition at the two measuring points on the same sensor are each compensated in real time. Thus, SPR signals according to an antigen-antibody reaction can be measured.  
      (3) Real-time SPR Measurement  
      A sample and a reference solution having the same composition as the sample but not containing the measuring object are placed in the sample cell and the reference cell on the same sensor, respectively. These two points are simultaneously measured such that SPR signals of the sample are measured while SPR signals of the zero point before reaction are continuously being measured. Thus SPR signals after reaction with the measuring object can be obtained from the difference between the two points in real time. Alternatively, a sample solution and a reference solution having the same composition as the sample but containing a known concentration of the measuring object may be placed in the sample cell and the reference cell on the same sensor, respectively. These two points are simultaneously measured such that the difference between the SPR signals of the reference solution and the SPR signals of the sample solution is simultaneously observed on a single detector while the SPR signals of the reference solution acting as the reference of reaction quantity are continuously being measured. In this measurement, the comparison of concentrations of the measuring object can be observed in real time. Thus, the measurement can be applied to screening and on-off alarms.  
      (4) Multipoint Measurement  
      The present invention has been described in detail using a two-point differential system in which SPR signals at two points at a distance of 5 mm of the SPR combination sensor cell, through the prism and the adhesive optical interface PVC film are divided into signals of the sample and the reference by splitting mirrors and are measured on a photodetector. By reducing the distance between the measuring points or by downsizing the sensor to a microchip, or reversely by increasing the dimensions of the sensor or the prism to some extent, multi-point SPR at 3 to n measuring points can be measured with a single photodetector with basically the same optical system and  3  to n splitting mirrors.  
      2. Detecting System  
      (1) Combination Sensor Cell (SProde)  
      The combination sensor cell is brought by expanding the ranges of the possibility of the known surface plasmon measurement, and it results from the development of a chemical sensor-like system for SPR measurement. In general, pH measuring glass electrodes are called pH electrodes in the sense that they are bar-shaped sensors for measuring electrochemical phenomena that occur at the interface of the glass membrane selectively responding to hydrogen ion concentration. The SPR combination sensor cell of the present invention measures surface plasmon resonance, which is the change of light occurring at the interface of a sensing film selectively responding to a measuring object, in a form of a bar-shaped sensor similar to the pH electrode. The combination sensor cell of the present invention can be defined as a novel chemical sensor based on the detection of light according to SPR, and thus can be called SProde. This produces a new area in the field of chemical sensors.  
      As described above, the SPR-measuring chemical sensor-like system is expected to expand the ranges of its application so that surface plasmon measuring apparatuses, which have been intended only for research use, are simplified into on-site field apparatuses that anyone can easily operate anywhere.  
      In the known method for detecting surface plasmon resonance, a flow cell using a cell holder is mechanically brought into close contact with the sensor with an optical matching oil therebetween. Then, a reference solution is delivered to the flow cell with a pump and the light intensity of the SPR generated at this moment is stored. Subsequently, a sample is subjected to the same operation and the difference between the SPR signals is compared to the concentration of the measuring object. However, this method limits the downsizing and simplification of the apparatus for on-site field use because this method uses an expensive pump for delivering the sample and the cell holder for mechanically pressing the flow cell for SPR measurement. Furthermore, the detecting system of this method still has problems to be solved. Specifically, this method involves a time lag between the measurements of the reference solution and the sample solution, and does not embody real-time measurement in the fullest sense. Also, two flow paths are required for a differential system, and the structure thus has a limitation in terms of space.  
      The present invention solves these problems the known method has by providing the combination sensor cell and the polymeric adhesive optical interface film. In the present invention, visible light acting as energy for inducing SPR is emitted from an LED light source (wavelength: 770 nm) and line-focused on the sample sensing film and the reference sensing film on the combination sensor cell so as to measure SPR signals simultaneously generated at sample sensing film and the reference sensing film. Thus, relative values according to the SPR signals of the sample and the reference are obtained in real time in the fullest sense. For a large number of samples to be measured, the present invention can be easily used in on-off/screening applications.  
      (2) Real-Time Sensing  
      For a simplified immunological method, an enzyme-immunological approach that has been known as the ELISA method is generally employed. This method does not realize real-time measurement because antigen-antibody reaction to be measured is introduced to an enzyme system and B/F separation is required to eliminate the influences of physical nonspecific adsorption. These steps increase the time of determination and thus unsuitable for on-site field measurement. On the other hand, SPR measurement can realize real-time measurement in principle. Measurements using a flow cell are however not performed in real time in a strict sense because samples are measured after measuring a blank and then the difference between their signals is taken. In the system using combination sensor cell of the present invention, in which no pump is used for introducing samples or a blank to the sensor, SPR occurs at points at a predetermined distance on the line focus on the sample cell and the reference cell. Consequently, reflected light detected by the CCD line sensor can be measured in real time with no time lag.  
      (3) Reliability Of SPR Immunological Measurement  
      As described in the above (2), the known ELISA method requires B/F separation to obtain information according to immune response. The combination sensor cell system of the present invention can simultaneously measure the SPR occurring in the sample cell and the reference cell. By thus measuring signals according to the immune response, nonspecific signals, and signals according to bulk components in the sample cell and measuring nonspecific signals and signals according to the bulk components in the reference cell, the signals according to the immune response can be selectively measured without the step of B/F separation.  
      3. Polymeric Adhesive Optical Interface Film  
      In the known method, in order to ensure optical matching between the prism and the sensor, a matching oil having the same refractive index as the prism and the glass substrate being a sensor base is applied to the prism before each measurement, and the sensor including the glass substrate is disposed on the prism. In addition, in order to bring the sensor into close contact with the prism, the sensor is mechanically pressed with a sensor holder. Furthermore, the matching oil is toxic, and it must be meticulously used. Use of such oil is unsuitable for apparatuses intended for use in the field. In the present invention, it suffices that the combination sensor cell is pressed on an adhesive interface film with an index finger, as long as the adhesive interface PVC film is fixed to the prism in advance. Since the film is adhesive, no sensor cell holder is required. Also, since the solvent in the film is solidified with PVC, it is much safer than the oil. The polymeric adhesive optical interface film makes the differential surface plasmon resonance measuring apparatus of the present invention downsized and makes its operation safe and easy.  
      The major advantages of the present invention has been described. The differential surface plasmon resonance measuring apparatus embodies the advantages that SPR can be measured in real time in the fullest sense by use of the combination sensor cell, and the apparatus is small and resistant to disturbances, and very easy to operate.  
      The present invention is not limited to the foregoing embodiments and examples, and various modifications can be made according to the spirit of the invention without being rejected from the scope of the invention.  
     INDUSTRIAL APPLICABILITY  
      The differential surface plasmon resonance measuring apparatus and the method for differentially measuring surface plasmon resonance according to the present invention are suitable for ubiquitous on-site measurement using a palm-size-oriented apparatus for measuring a low-molecular-weight environmental organic pollutant.