Abstract:
Disclosed herein is a unit for detecting an interaction between substances, including a reaction region for providing sites for the interaction between the substances, at least one pair of opposite electrodes disposed oppositely to each other so as to make it possible to impress an electric field on a medium contained in the reaction region, and an injection hole and an exhaust hole for feeding the medium containing the substances into the reaction region by capillarity.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to a technology for feeding a sample aqueous solution into a predetermined reaction region by utilizing capillarity, and, in addition, to a technology for performing a control of the high-order structures of substances, movements of the substances, fixation of the substances, removal of unnecessary substance, etc. by arranging opposite electrodes in the reaction region where the substances interact with each other and impressing a predetermined electric field.  
         [0002]     A principal background art relating to the present invention will be described. First, a first background art (related art) is a technology concerning a bioassay integrated substrate so-called DNA chip or DNA microarray (hereinafter referred to generically as “DNA chip”) in which predetermined DNAs are finely arranged by the microarray technique (see, for example, Japanese Patent Laid-open No. Hei 4-505763, and Japanese Translations of PCT for Patent No. Hei 10-503841). The DNA chip technology uses a structure in which a multiplicity of kinds of and a multiplicity of DNA oligo-chains, cDNAs (complementary DNAs) and the like are integrated on a glass substrate or a silicon substrate, and is characterized in that it is possible to perform collective analysis of intermolecular interactions such as hybridization. Therefore, DNA chips have been utilized for analysis of variations in genes, SNPs (single nucleotide polymorphism), gene expression frequency analysis, etc. and has come to be utilized widely in drug development, clinical diagnosis, pharmacological genomics, forensic medicine and other fields. Other than the DNA chips, there have also been developed protein chips comprising proteins on a substrate, biosensor chips for analyzing interactions between various substances, and the like.  
         [0003]     A second background art is a technology concerning actions of an electric field on substances present in an electrically charged state in a liquid phase. Specifically, it is known that a nucleotide chain (nucleic acid molecule) is stretched or moved under the action of an electric field in a liquid phase. The principle of this phenomenon is considered as follows. Phosphate ions (negative charges) constituting the skeleton of the nucleotide chain and hydrogen atoms (positive charges) formed by ionization of water present in the surroundings of the phosphate ions are considered to be forming ionic fogs, the polarization vectors (dipoles) generated by the negative charges and the positive charges are as a whole aligned in one direction upon application of a high-frequency high voltage, with the result of stretching of the nucleotide chain, and, in addition, when a nonuniform electric field with electric lines of force concentrated on a portion is impressed, the nucleotide chain is moved toward the portion on which the electric lines of force are concentrated (see Seiichi Suzuki, Takeshi Yamanashi, Shin-ichi tazawa, Osamu Kurosawa and Masao Washizu: “Quantitative analysis on electrostatic orientation of DNA in stationary AC electric field using fluorescence anisotropy”, IEEE Transaction on Industrial Applications, Vol. 34, No. 1, pp. 75-83 (1998)). Besides, it is known that when a DNA solution is placed in fine electrodes having a gap of several tens to several hundreds of micrometers and a high-frequency electric field of about 1 MV/m and 1 MHz is applied thereto, dielectric polarization occurs in the DNA present in a random coil form, resulting in that the DNA molecule is stretched in a straight line form in parallel to the electric field. Then, by this electrodynamic effect called “dielectrophoresis”, the polarized DNA is spontaneously drawn to the electrode end, and is fixed in the form of having one end in contact with the electrode edge (see Masao Washizu, “DNA handling conducted while viewing”, Visualized Information, Vol. 20, No. 76 (January, 2000)).  
         [0004]     The above-mentioned DNA chip technology is a technology in which a reaction region for providing sites for an interaction between substances in a liquid phase is preliminarily set on a substrate, and a detection nucleotide chain such as a probe DNA is preliminarily fixed in the reaction region, to thereby analyze the hybridization which is an interaction between the detection nucleotide chain and a complementary target nucleotide chain.  
         [0005]     However, there has been room for improvement in the technique for accurately putting a predetermined tiny volume of a sample aqueous solution to the reaction region being minute in volume. Particularly, for the purposes of suppressing the evaporation of the sample aqueous solution, preventing contamination, and the like, it is expected that a contrivance for forming the reaction region in a more closed space will be attempted now on. In this case, there will be the technical problem that it is difficult to feed the sample aqueous solution into the reaction region by directly using the widespread nozzle dripping system (spotting system).  
         [0006]     Next, the conventional DNA chips have had the problems that it takes a long time for performing a hybridization reaction since the efficiency of hybridization is poor and that detection accuracy is low since pseueo-positivity or pseudo-negativity is generated. The main causes of these problems include the steric hindrance arising from the high-order structure of the object substance showing the interaction, the interference (e.g., adhesion or contact) of the substances with the surrounding surfaces, and the presence of a substance which may cause a lowering in detection accuracy at the detection portion serving as a target for irradiation with detection excited rays.  
       SUMMARY OF THE INVENTION  
       [0007]     Accordingly, it is an object of the present invention to provide a novel technology for feeding a sample aqueous solution to a minute reaction region, and to provide a detecting unit with which it is possible to freely perform a control of high-order structures of substances, movements of the substances, fixation of the substances, removal of unnecessary substances, etc., and a method and a bioassay substrate using the detecting unit.  
         [0008]     First, according to the present invention, there are provided a unit for detecting an interaction between substances, comprising a reaction region for providing sites for the interaction between the substances, at least one pair of opposite electrodes disposed oppositely to each other so as to make it possible to impress an electric field to a medium, such as a buffer aqueous solution and a gel, contained in the reaction region, and an injection hole and an exhaust hole for feeding a medium, such as the aqueous solution and the gel, containing the substances into the reaction region by capillarity, and a bioassay substrate including a DNA chip provided with the detecting unit. In the present invention, a predetermined minute amount of the aqueous solution dropped to the injection hole can be securely fed toward the reaction region.  
         [0009]     Then, in order to prevent an electrochemical reaction due to an ionic solution which may be reserved in the reaction region, it is contrived that the surfaces on the reaction region side of the opposite electrodes are covered with an insulation film. The insulation film can be formed, for example, such a material as SiO 2 , SiN, SiOC, SiOF, SiC, TiO 2 , etc.  
         [0010]     In addition, a surface region surrounding an upper opening portion of the injection hole is made hydrophobic, whereby a hydrophilic medium present in a liquid droplet state (having been dropped) in the surrounding surface region can be smoothly fed into the reaction region which is hydrophilic.  
         [0011]     The surface of a first electrode on one side constituting the opposite electrodes can be used as a fixation surface for a detection substance. In this case, the surface of the first electrode is preliminarily subjected to a predetermined surface treatment suited to the desired fixation reaction such as bonding through an avidin-biotin bond or a disulfide bond (—S—S bond). In such a configuration, the electrode serves as an electrode transparent to detection excited rays, so that irradiation with the excited rays can be achieved also from the back side of the electrode.  
         [0012]     The area of the first electrode on one side constituting the opposite electrodes arranged in the detecting unit is made to be narrower than the area of a second electrode on the other side opposed to the first electrode, or the area of the portion, fronting on the reaction region, of the first electrode is made to be narrower than the area of the second electrode on the other side opposed to the first electrode, or the surface of the first electrode is formed as a rough surface or rugged surface, or conductive particulates are dispersed on the surface of the first electrode, whereby the surface of the first electrode can be provided with projected portions where the electric lines of force produced between the opposite electrodes are easily concentrated. As a result, a nonuniform electric field can be generated in the vicinity of the surface of the first electrode.  
         [0013]     Next, according to the present invention, there is provided a method of fixing a detection substance so contrived that the aqueous solution containing a predetermined detection substance is fed into the reaction region by use of the detecting unit, and the detection substance is fixed onto an electrode surface while impressing an electric field between the opposite electrodes.  
         [0014]     In this method, the detection substance such as a DNA probe can be moved while being stretched along the electric field, and can be aligned and fixed relative to a predetermined electrode surface.  
         [0015]     In addition, according to the present invention, there is provided a method of accelerating an interaction between substances, comprising a first procedure for feeding the aqueous solution containing a target substance into the reaction region, a second procedure for moving the target substance relative to an electrode surface where the detection substance is fixed while impressing an electric field between the opposite electrodes, and a third procedure for permitting an interaction between the detection substance and the target substance to proceed.  
         [0016]     According to this method, the target substance such as a single-chain DNA acting specifically on the fixed detection substance can be collected into an electrode surface region, where the detection substance is fixed, while being stretched under the action of the electric field, and, therefore, the efficiency of the interaction can be enhanced.  
         [0017]     The electric field to be impressed is suitably an AC electric field, since the AC electric field can move a nucleic acid such as DNA to a specified location where the electric field is high while stretching the nucleic acid. Besides, in the third procedure, the electric field is turned OFF, whereby the interaction such as hybridization depending on a natural Brownian motion of a natural substance can be permitted to proceed.  
         [0018]     Here, principal technical terms used in the present invention will be defined. First, the term “interaction” used in the present invention widely means chemical bondings inclusive of non-covalent bonding, covalent bonding, and hydrogen bonding and dissociation between substances, and includes hybridization which is a complementary bonding between nucleic acids (nucleotide chains), for example.  
         [0019]     Next, the term “opposite electrodes” means at least one pair of electrodes which are arranged oppositely to each other.  
         [0020]     The term “nucleotide chain” means a polymer of a phosphoric acid ester of a nucleotide in which a purine or pyrimidine base and a sugar are bonded by glycoside bonding, and widely includes oligonuleotides inclusive of probe DNAs, polynucleotides, DNAs (whole length or sections thereof) formed by polymerization of purine nucleotide with pyrimidine nucleotide, cDNAs (c probe DNAs) obtained by reverse transcription, RNAs, polyamide nucleotide derivatives (PNAs), etc.  
         [0021]     The term “hybridization” means a complementary chain (double chain) forming reaction between nucleotide chains having complementary base sequence structures. The term “mishybridization” means the complementary chain forming reaction which is not normal.  
         [0022]     The term “reaction region” means a region which can provide reaction sites for hybridization or other interactions, and examples thereof include a reaction site in the shape of a well capable of preserving or holding a medium such as a liquid phase and a gel. The interaction conducted in the reaction region is not narrowly limited, provided that the interaction conforms to the object or effects of the present invention. Examples of the interaction include not only an interaction between single-chain nucleic acids, i.e., hybridization but also an interaction between peptide (or protein) and a desired double-chain nucleic acid formed from a detection nucleic acid, an enzyme response reaction and other intermolecular interactions. Where the double-chain nucleic acid is used, for example, the bonding between a receptor molecule of a hormone receptor or the like which is a transcription factor and a response sequence DNA portion, and the like can be analyzed.  
         [0023]     The term “detection substance” is a substance which is preliminarily added into the reaction region and which is present in a free state in the region, or a substance which is present in the state of being fixed to a predetermined surface portion of the reaction region. The detection substance is a substance for capturing and detecting a substance showing a specific interaction with the substance, and includes detection nucleotide chains such as DNA probes.  
         [0024]     The term “target substance” means a substance which serves as a target of an interaction with the detection substance, and examples thereof include a nucleotide chain having a base sequence complementary to the DNA probe.  
         [0025]     The term “steric hindrance” means a phenomenon in which due to the presence of a bulky substituent group in the vicinity of a reaction center or the like in a molecule, the posture of a reaction molecule, or the steric structure (high-order structure), the access of molecules of the medium species becomes difficult and, as a result, it becomes difficult for the desired reaction (hybridization, in the present patent application) to take place.  
         [0026]     The term “dielectrophoresis” is a phenomenon in which molecules are driven toward the higher electric field side in a field where the electric field is anisotropic. Also where an AC voltage is applied, the polarity of polarization is reversed attendant on the reversion of the polarity of the applied voltage, so that the driving effect can be obtain in the same manner as in the case of DC (see “Micromachines and Material Technology (published by C. M. C.)” complied under the supervision of Teru Hayashi, pp. 3746, Chapter 5, Cell and DNA manipulation).  
         [0027]     The term “bioassay substrate” means an information integration substrate used for the purpose of biochemical or molecular biological analysis, and includes the so-called DNA chip.  
         [0028]     According to the present invention, the portion for detecting the interaction is simple in configuration or structure, so that the portion can be manufactured easily. A predetermined quantity of a sample aqueous solution can be securely fed into the reaction region of the detecting unit, so that there will be no loss of the sample aqueous solution. In addition, the sample aqueous solution is fed into the reaction region through a minute hole or holes by utilizing capillarity, so that the opening area of the reaction region can be set small, and evaporation of the aqueous solution can be restrained.  
         [0029]     The detection substance and the target substance can be collected onto an electrode surface under the action of an electric field, particularly an AC electric field, so that the concentrations of the substances are raised, and the efficiency of the interaction between the substances can be enhanced. The high-order structures of the detection nucleic acid such as DNA probe and the target nucleic acid can be controlled from a random coil state to a stretched state by the action of application of the electric field, so that the steric hindrance at the time of the interaction can be obviated. As a result, the efficiency and accuracy of the interaction can be enhanced, so that it is possible to shorten the operation time and to prevent pseudo-positivity and pseudo-negativity from being generated.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]     The above and other objects, features and advantages of the present invention will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings, in which:  
         [0031]      FIG. 1  shows a top plan view and a sectional view along arrows of line I-I of the top plan view, schematically showing the concept of the basic configuration of a unit ( 1   a ) for detecting an interaction between substances according to the present invention;  
         [0032]      FIG. 2  is a top plan view along line II-II of the sectional view in  FIG. 1 ;  
         [0033]      FIG. 3  is a sectional view along arrows of line I-I of  FIG. 1 , showing the configuration of a detecting unit ( 1   b ) in a modified form;  
         [0034]      FIG. 4  is a sectional view along arrows of line I-I of  FIG. 1 , schematically showing the condition where a DNA probe (D 1 ) is put into a reaction region ( 2 ) of the detecting unit ( 1   a );  
         [0035]      FIG. 5  is a sectional view along arrows of line III-III of  FIG. 1 , schematically showing the condition where the DNA probes (D 1 ) are aligned and fixed to a surface portion of an electrode (E 1 );  
         [0036]      FIG. 6  is a sectional view along arrows of line I-I of  FIG. 1 , showing the configuration of a detecting unit ( 1   c ) in a modified form;  
         [0037]      FIG. 7  is a sectional view along arrows of line I-I of  FIG. 1 , showing the configuration of a detecting unit ( 1   d ) in a modified form;  
         [0038]      FIG. 8  is a sectional view along arrows of line I-I of  FIG. 1 , showing the configuration of a detecting unit ( 1   e ) in a modified form;  
         [0039]      FIG. 9  is a partial sectional view showing a modified form of a fixation electrode (E 1 );  
         [0040]      FIG. 10  is a partial sectional view showing another modified form of the fixation electrode (E 1 );  
         [0041]      FIG. 11  is a partial sectional view showing a further modified form of the fixation electrode (E 1 );  
         [0042]      FIG. 12  is a sectional view along arrows of line I-I of  FIG. 1 , showing the configuration of a detecting unit ( 1   f ) in a modified form;  
         [0043]      FIG. 13  is an enlarged view for illustrating a preferable method of feeding an aqueous solution (L) which can be applied in common to the detecting units ( 1   a - 1   f ) according to the present invention;  
         [0044]      FIG. 14  is an enlarged view for illustrating another preferable method of feeding the aqueous solution (L) which can be applied in common to the detecting units ( 1   a - 1   f ) according to the present invention; and  
         [0045]      FIG. 15  is a view showing one example of a disk-like substrate ( 10 ) on which the detecting units ( 1   a - 1   f ) according to the present invention are arranged. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0046]     Now, a preferred embodiment of the present invention will be described below referring to the accompanying drawings. First,  FIG. 1  is a top plan view and a sectional view along arrows of line I-I of the top plan view, schematically showing the concept of the basic configuration of a unit for detecting an interaction between substances (hereinafter referred to simply as “detecting unit”) according to the present invention.  FIG. 2  is a top plan view along line II-II of the sectional view in  FIG. 1 .  
         [0047]     Symbol  1   a  in  FIG. 1  denotes the configuration of an essential part of one preferred embodiment of the detecting unit. The detecting unit  1   a  is formed on a substrate formed of a glass, a synthetic resin or the like, and is a portion contrived for detecting an interaction between substances. The size of the detecting unit  1   a  is, for example, about 100 μm in length by about 100 μm in width by about 5 μm in height (depth), and the height (depth) may be appropriately determined according to the molecular length of a detection substance such as probe DNA used (this applies also to other embodiments of the detecting unit).  
         [0048]     The detecting unit  1   a  and other detecting units  1   b  and the like described later are each provided with a reaction region  2  capable of reserving an aqueous solution to be used as sites for the interaction between the substances, holes  3  and  4  (described later) of about 10 μm×20 μm in size for communication between the reaction region  2  and the outside air, and a pair of opposite electrodes E 1 , E 2  disposed on the upper and lower sides of the reaction region  2  (see the sectional view in  FIG. 1 ).  
         [0049]     At least the electrode (e.g., electrode E 1 ) on one side, of the opposite electrodes E 1  and E 2 , or both the electrodes E 1  and E 2  may each be formed of a transparent conductor, for example, ITO (indium tin oxide). Where the electrode or electrodes are formed of the transparent conductor, the electrodes are transparent to detection excited rays, so that the system is suitable for use in the case of detecting the interaction in the reaction region based on measurement of light emission intensity by an optical means.  
         [0050]     The surfaces, fronting on the reaction region  2 , of the opposite electrodes E 1  and E 2  are covered respectively with insulation layers  5   a  and  5   b  (see the sectional view in  FIG. 1 ), which play the role of preventing an electrochemical reaction due to an ionic solution which may be reserved in the reaction region  2 . Incidentally, the insulation layers  5   a  and  5   b  can be formed of such a material as SiO 2 , SiN, SiOC, SiOF, SiC, TiO 2 , etc. In addition, the layer on the lower side of the opposite electrode E 1  shown is a substrate  6   a  formed of a synthetic resin or the like, and, similarly, the layer on the upper side of the opposite electrode E 2  shown is a substrate  6   b  formed of a synthetic resin or the like (see the sectional view in  FIG. 1 ). Therefore, the electrode E 1  is sandwiched between the insulation layer  5   a  and the substrate  6   a , whereas the electrode E 2  is sandwiched between the insulation layer  5   b  and the substrate  6   b.    
         [0051]     Symbol  7  shown in  FIG. 1  and the like denotes a spacer formed of an inorganic material such as SiO 2  or of, for example, a synthetic resin such as polyimide. Thus, the thickness of the spacer  7  determines the height (depth) of the reaction region  2  (see  FIG. 1 ), and the shape of the spacer  7  determines the shape and volume of the reaction region  2  (see  FIG. 2 ). Incidentally, as in the detecting unit  1   b  in a modified form shown in  FIG. 3 , a spacer  71  with such a shape and size as to be flush with wall surfaces  31 ,  41  of holes  3 ,  4  may be adopted, whereby a reaction region  21  in the form shown in  FIG. 3  can be formed.  
         [0052]     Next,  FIG. 4  is a sectional view along arrows of line I-I of  FIG. 1 , schematically showing the condition where a DNA probe D 1  which is a representative example of the detection substance is put into the reaction region  2  of the detecting unit  1   a . In this case, a switch S is turned OFF, so that application of an electric field between the electrodes E 1  and E 2  by a power source V is not being performed (see  FIG. 3 ).  
         [0053]     An aqueous solution L containing the DNA probe D 1  is dropped from a nozzle denoted by symbol N to the injection hole  3 , and then fed into the reaction region  2  by capillarity. In this instance, the hole denoted by symbol  4  functions as an exhaust hole for permitting the action of the capillarity. Incidentally, the DNA probe D 1  thus dropped has a random coil form high-order structure.  
         [0054]     Next,  FIG. 5  is a sectional view along arrows of line III-III of  FIG. 1 , schematically showing the condition where the DNA probes D 1  which are representative examples of the detection substance D are aligned and fixed to a surface portion of the electrode E 1 .  
         [0055]      FIG. 5  shows the condition in which immediately after the aqueous solution L containing the DNA probes D 1  is fed into the reaction region  2  (see  FIG. 4 ), the switch S is turned ON to impress a high-frequency AC electric field (symbol P in  FIG. 3 ) between the opposite electrodes E 1  and E 2  by the power source V, whereby the DNA probes D 1  are aligned and fixed to the surface of the electrode E 1  while being stretched along the electric lines of force. Incidentally, the electric field in this case is preferably about 1×10 6  V/m and about 1 MHz (see Masao Washizu and Osamu Kurosawa: “Electrostatic Manipulation of DNA in Microfabricated Structures”, IEEE Transaction on Industrial Application, Vol. 26, No. 26, pp. 1165-1172 (1990)). The DNA probe in the fixed state is denoted by symbol D 2  in  FIG. 5 , and symbol T denotes a target DNA having a base sequence complementary to the stretched and fixed DNA probe D 2 .  
         [0056]     Where the surface of the electrode E 1  is surface treated with streptoavidin, the system is suitable for fixation of the terminal end of biotinated DNA probe. Alternatively, where the surface of the electrode E 1  is surface treated with a thiol (SH) group, the system is suited to fixation of a DNA probe having a thiol group-modified terminal end by the disulfide bond (—S—S bond). Incidentally, the electrode E 2  on the upper side may be utilized as the fixation electrode.  
         [0057]     Here, as in the case of a detecting unit  1   c  shown in  FIG. 6 , it may be contrived that the area of the fixation electrode E 1  is designed to be narrower than the area of the opposite electrode E 2 , whereby the electric field P is concentrated more on the electrode E 11 , so that a nonuniform electric field is generated on the surface of the electrode E 11 . By this it is possible to enhance the effect of dielectrophoresis.  
         [0058]     In addition, in the case of a detecting unit  1   d  shown in  FIG. 7 , an end face  721  fronting on the reaction region  22  of the spacer  72  is formed as an inclined surface, whereby the area of the portion (see symbol E 12 ) fronting on the reaction region  22  of the electrode E 1  is narrowed, with the result that the effect of the dielectrophoresis can be enhanced similarly to the above.  
         [0059]     A detecting unit  1   e  shown in  FIG. 8  represents an embodiment in which, different from the detecting units  1   a  to  1   d  having electrodes opposed to each other and disposed on the upper and lower sides of the reaction region  2  and the like, a pair of electrodes E 1  and E 2  enabling application of an electric field are disposed at the bottom surface of the reaction region  2  so that their edge portions are opposed to each other. In this embodiment, the substrate  6   b  on the upper side is not provided with an electrode, so that the configuration is simplified. In this detecting unit  1   e , also, the electrode on one side may be formed to be narrower than the electrode on the other side, for the purpose of enhancing the effect of the dielectrophoresis in the same manner as in  FIG. 6 . Incidentally, the descriptions of other symbol portions relating to the detecting unit  1   e  are the same as in the above-described embodiments and, therefore, are omitted here.  
         [0060]     Here, as shown in FIGS.  9  to  11 , in the present invention, the surface of the fixation electrode E 1  may be processed into such a shape that the electric field (electric lines of force) will easily be concentrated thereon. Specifically, as shown in  FIG. 9 , when the electrode E 1  is provided with a rugged pattern, the electric field will be concentrated on the rugged portion. A method for forming such a rugged pattern may comprise the steps of forming an electrode pattern of, for example, ITO in an arbitrary shape on a lower substrate  6   a  by photolithography, then again forming a film of ITO thereon, and forming an insulation film  5   a  of SiO 2  or the like, whereby the electrode E 1  composed of the rugged pattern as shown in  FIG. 9  can be formed.  
         [0061]     Besides, as shown in  FIG. 10 , before an insulation layer  5   a  is formed on the surface of an electrode E 1  formed of ITO or the like, conductive particulates  8  may be dispersed, whereby a rugged pattern can be formed. Incidentally, as the particulates  8 , there can be adopted particulates of metals, conductive polymers, inorganic materials such as SiO 2 , and the like.  
         [0062]     Further, as shown in  FIG. 11 , in the case of forming an insulation layer  5   a  of SiO 2  or the like on the surface of an electrode E 1  formed of ITO or the like, the surface roughness of the insulation layer  5   a  may be controlled through film forming conditions to thereby form a film with a high surface roughness, whereby an electric field can be locally concentrated by utilizing the ruggedness. Incidentally, in controlling the surface roughness, a film of SiO 2  of not more than 10 nm in thickness, for example, may be again formed on the insulation layer  5   a  once formed, whereby the surface is provided with ruggedness.  
         [0063]     After the DNA probe is drawn by the coulomb force and fixed to the surface of the electrode E 1  or the like by concentrating the electric field as above-mentioned, a predetermined cleaning aqueous solution is fed into the reaction region  2  in the same manner as the above-mentioned aqueous solution L, or is forcibly injected into the reaction region  2 , or is put into the reaction region  2  by once opening the reaction region  2  through detaching the electrode substrate on the upper side, whereby surplus DNA probes and probe DNAs adsorbed non-specifically can be removed from the reaction region  2 , followed by drying.  
         [0064]     Into the reaction region  2  or the like of the detecting unit  1   a  or the like obtained as above-described, an aqueous solution containing a target DNA denoted by symbol T is fed via the injection hole  3  by utilizing capillarity. Here, by applying an electric field to the target DNA, the structure of the target DNA can be controlled from a random coil form high-order structure to a stretched structure, and the target DNA can be moved (dielectrophoresis) to the surface of the fixation electrode E 1  or the like, in the same manner as in the case of the DNA probe. Then, the electric field is once turned OFF, and the hybridization is made to proceed under natural Brownian motion.  
         [0065]     The hybridization can be detected by a method in which a fluorescent substance labeled on the DNA probe, a fluorescent intercalator inserted and bonded to a double-chain DNA or the like, for example, is irradiated with excited rays with a predetermined wavelength, and the hybridization is detected based on the intensity of the fluorescence. Incidentally, the fluorescent intercalator may be preliminarily mixed into the aqueous solution containing the target substance, or may be dropped into the reaction region  2  after the hybridization.  
         [0066]     Here, in a detecting unit  1   f  in a modified form as shown in  FIG. 12 , a double-chain DNA produced by hybridization is schematically drawn and denoted by symbol Dt. After the hybridization, however, surplus intercalator C may be present, and mishybridized DNA denoted by symbol M may also be present, leading to a lowering in detection accuracy.  
         [0067]     In view of this, a contrivance may be made in which opposite electrodes e 1 -e 2  having an opposition axis intersecting that of the opposite electrodes E 1 -E 2  are arranged, and a high-frequency AC electric field is impressed therebetween, thereby to draw the surplus intercalators C and mishybridized DNAs denoted by symbol M to the sides of the electrodes e 1  and e 2 , and to remove these substances C and M from the vicinity of the surface of the electrode E 1 , whereby detection accuracy can be enhanced. Incidentally, symbol S 2  in  FIG. 12  is a switch for impressing a voltage between the opposite electrodes e 1 -e 2  through a power source denoted by symbol V 2 .  
         [0068]      FIGS. 13 and 14  are views for illustrating a preferable method for feeding the aqueous solution L which can be applied in common to the above-described detecting units  1   a  to  1   f  according to the present invention. Incidentally, FIGS.  13  and  14  are enlarged views of part X showing the vicinity of the injection hole  3  shown in  FIG. 4 .  
         [0069]     The whole area of the upper substrate  6   b  or at least the surface of the vicinity of the injection hole  3  is preliminarily treated with an alkylsilane or the like to form a hydrophobic layer  9  (see  FIG. 13 ). Then, from above the injection hole  3 , a sample aqueous solution (containing a detection substance D and a target substance T) denoted by symbol L is dropped via a nozzle N by an ink jet system or a dispenser system, to form a liquid droplet as indicated by broken line in  FIG. 13 . Since the droplet Y is an aqueous solution and therefore hydrophilic as contrasted to the hydrophobic layer  9 , the droplet Y swiftly receives the action of capillarity and is gradually sucked into the reaction region  2 .  
         [0070]     Alternatively, as in the configuration shown in  FIG. 14 , the whole area of the upper substrate  6   a  or at least the surface of the vicinity of the injection hole  3  is preliminarily treated with an alkylsilane or the like to form a hydrophobic layer  9 , and only the vicinity of the injection hole  3  may be preliminarily irradiated with UV rays to form a hydrophilic layer  91 . In this case, the dropped liquid droplet Y can be once securely held in the vicinity of the injection hole  3  by the hydrophilic layer  91 , and thereafter the droplet  91  can be fed into the reaction region  2  under the action of capillarity.  
         [0071]     When such a method is used, only a required amount of the aqueous solution L can be fed into the reaction region  2 , so that loss of the aqueous solution can be suppressed. In addition, since the aqueous solution L fed in by utilizing the capillarity is enclosed in the reaction region  2  having few opening portions, the evaporation of the aqueous solution L can be restrained. From this point of view, it is desirable that the injection hole  3  and the exhaust hole  4  be as small as possible.  
         [0072]     However, the evaporation of the aqueous solution L cannot be prevented perfectly, since the reaction region  2  has opening portions. In view of this, it may be contrived that the detection substance D and the target substance T are dispersed in a gel, the gel is dropped toward the injection hole  3 , and the gel is fed into the reaction region  2  by capillarity. This makes it possible to confine the aqueous solution into the network structure of the gel, thereby restraining the evaporation of the aqueous solution.  
         [0073]     In addition, in order to prevent the evaporation of the aqueous solution L, there may be adopted a method in which a quick-drying polymer or the like is dropped to the injection hole  3  and the exhaust hole  4  to seal off the holes, after the aqueous solution L is fed into the reaction region  2 . In this case, if the intercalator is used, it is desirable to preliminarily mix the intercalator into the aqueous solution containing the target substance T.  
         [0074]     Where the detecting units denoted by symbols  1   a - 1   f  as above-described are arranged in a predetermined pattern on a substrate, it is possible to provide a bioassay substrate such as DNA chip with which interactions such as hybridization can be made to proceed speedily and collective analysis can be performed.  
         [0075]      FIG. 15  is a view showing one example of the bioassay substrate. As shown in  FIG. 15 , for example, a disk form substrate  10  can be provided with a multiplicity of detecting units  1   a  and the like which can be divided into groups.  
         [0076]     Incidentally, the detection of the interaction proceeding at any detecting unit  1   a  or the like provided on the substrate  10  can be carried out by use of a known optical detection means by which a fluorescent substance preliminarily marked onto the detection substance fixed to the surface of the electrode E 1  or a fluorescent intercalator inserted and bonded to a substance (double-chain nucleic acid) showing an interaction is irradiated with fluorescence exciting rays at a predetermined wavelength and the fluorescence is detected. Alternatively, a method may be adopted in which the light-emitting image of the detecting unit  1   a  and the like is picked up, and the quantity of light obtained from the image is quantitatively analyzed and detected.  
         [0077]     The present invention promises a high efficiency of the interaction such as hybridization at the detecting unit, so that it is possible to largely shorten the time required for the interaction. Besides, since it is possible to form an environment promising an easy progress of the interaction with high accuracy, it is possible to suppress the generation of pseudo-positivity or pseudo-negativity. Therefore, the present invention can be utilized for a bioassay substrate such as DNA chip which has such characteristics that the efficiency of the assay operation for interaction detection is excellent and that the detection accuracy is high.  
         [0078]     The present invention is not limited to the details of the above described preferred embodiments. The scope of the invention is defined by the appended claims and all changes and modifications as fall within the equivalence of the scope of the claims are therefore to be embraced by the invention.