Abstract:
The purpose of the invention is to provide a method for quantifying a chemical substance with high accuracy using substitutional stripping voltammetry and a sensor chip used therefor. 
     A sensor chip comprising a stripping electrode which is covered with stripping gel and a method utilizing the sensor chip. A reaction represented by the following formula (III) occurs at the stripping electrode. 
       [Chem. 3] 
       Ag+X ⊖ →AgX↓+ e   ⊖   (III)

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to a method for quantifying a chemical substance with substitutional stripping voltammetry and a sensor chip used therefor. 
         [0003]    2. Description of the Background Art 
         [0004]    Patent Document 1 discloses substitutional stripping voltammetry. The substitutional stripping voltammetry allows a chemical substance contained in a solution to be electrochemically quantified with high sensitivity. 
         [0005]      FIG. 1  shows the system for the substitutional stripping voltammetry disclosed in the Patent Document 1. 
         [0006]    The system comprises a pair of comb-shaped working electrodes  1 , a stripping electrode  2 , a reference electrode  3 , a counter electrode  4 , a solution  5 , a stripping liquid  6 , a salt bridge  7 , an ion conductor  8 , a potentiostat  9 , a recorder  10 , and a switch box  11 . 
         [0007]    The solution  5  contains a chemical substance to be quantified and an oxidation-reduction substance. The stripping liquid  6  contains a standard electrolyte and a support electrolyte. 
         [0008]      FIG. 2  shows a sensor chip  101   a  employed for the substitutional stripping voltammetry disclosed in the Patent Document 1. 
         [0009]    The sensor chip  101   a  comprises a plurality of electrodes  2  to  4  on the surface thereof. Furthermore, the container  64  covers the surface of the sensor chip  101   a . The container  64  comprises a first penetrated opening  64   a  and a second penetrated opening  64   b . The solution  5  and the stripping liquid  6  are supplied to the first penetrated opening  64   a  and the second penetrated opening  64   b , respectively. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         [PTL 1] Japanese Patent Publication No. 3289059B. 
       
     
       SUMMARY OF THE INVENTION 
     Technical Problem 
       [0011]    The evaporation of the stripping liquid  6  changes the concentration of the standard electrolyte. This causes the quantification accuracy of the chemical substance to be lowered. 
         [0012]    The purpose of the invention is to provide a method for quantifying a chemical substance with high accuracy using substitutional stripping voltammetry and a sensor chip used therefor. 
       Solution to Problem 
       [0013]    The following items [1] to [11] solve the above problem(s). 
         [0000]    [1]: A method for quantifying a chemical substance contained in a sample solution, comprising the following steps (a) to (e): 
         [0014]    a step (a) of preparing a sensor chip ( 300 ), wherein,
       the sensor chip comprises a substrate ( 30 ), a pair of working electrodes ( 31   a / 31   b ), a counter electrode ( 33 ), a stripping electrode ( 34 ), and a stripping gel ( 35 ),   the pair of working electrodes ( 31   a / 31   b ) is composed of a first working electrode ( 31   a ) and a second working electrode ( 31   b ),   the surface of the stripping electrode ( 34 ) comprises silver,   the stripping gel ( 35 ) covers the stripping electrode ( 34 ),   the stripping gel ( 35 ) does not cover the pair of the working electrodes ( 31   a / 31   b ) or the counter electrode ( 33 ),   the stripping gel ( 35 ) contains a standard electrolyte and an ionic liquid,   the stripping gel ( 35 ) contains no water,   the ionic liquid is hydrophobic and nonvolatile,   the ionic liquid is consisted of a cation and an anion,   the standard electrolyte is consisted of the cation and a halide ion,       
 
         [0025]    a step (b) of supplying the sample solution to the surface of the sensor chip to cover the surface with the sample solution, wherein,
       the sample solution contains the chemical substance and an oxidation-reduction substance or contains the chemical substance modified with an oxidation-reduction substance,       
 
         [0027]    a step (c) of applying a potential to the first working electrode ( 31   a ) with potentiostat, and connecting the second working electrode ( 31   b ) to the stripping electrode ( 34 ) to generate reactions represented by the following formulas (I) to (III) in the respective surface of the first working electrode ( 31   a ), the second working electrode ( 31   b ), and the stripping electrode ( 34 ), respectively, 
         [0028]    the first working electrode ( 31   a ): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, n represents an integer, and m represents a positive integer.) 
         [0029]    the second working electrode ( 31   b ): 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, n represents an integer, and m represents a positive integer.) 
         [0030]    the stripping electrode ( 34 ): 
         [0000]      [Chem. 3] 
         [0000]      Ag+X ⊖ →AgX↓+ e   ⊖   (III)
 
         [0000]    (wherein, X represents iodine atom, bromine atom, or chlorine atom.)
       wherein, the silver halide is deposited on the surface of the stripping electrode ( 34 ),       
 
         [0032]    a step (d) of applying a potential to the stripping electrode ( 34 ) in a condition where no potential is applied to either the first working electrode ( 31   a ) or the second working electrode ( 31   b ) to measure a current flowing through the stripping electrode ( 34 ), 
         [0033]    a step (e) of calculating the concentration of the oxidation-reduction substance (reductant) to quantify the chemical substance on the basis of the calculated concentration. 
         [0000]    [2]: A method according to above item 1, wherein, 
         [0034]    the sensor chip ( 300 ) further comprises a cover ( 37 ) with an inlet ( 36 ), 
         [0035]    a space is formed between the cover ( 37 ) and the sensor chip ( 300 ), 
         [0036]    in the step (b), the sample solution is supplied through an inlet ( 36 ) to the surface of the sensor chip ( 300 ). 
         [0000]    [3]: A method according to above item 2, wherein, 
         [0037]    the sensor chip ( 300 ) further comprises an air vent ( 38 ), 
         [0038]    in the step (b), the air which the has filled the space is drained through the air vent ( 38 ). 
         [0000]    [4]: A method according to above item 2, wherein, 
         [0039]    after the step (b), the space is filled with the sample solution. 
         [0000]    [5]: A method according to above item 1, wherein, 
         [0040]    the cation and the anion are selected from the following groups (I) and (II), respectively: 
         [0041]    Group (I): a cation represented by the following formula IV-(1) to IV-(6). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  are the same as or different from each other, and represent hydrogen atom, a straight or branched alkyl group which may contain heteroatom, an aralkyl group, or an aryl group, and R 9 , R 10 , R 11 , and R 12  are the same as or different from each other, and represent a straight or branched alkyl group which may contain heteroatom, an aralkyl group, or an aryl group.) 
         [0042]    Group (II): an anion represented by the following formula V-(1) or V-(2). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, Rf 1  and Rf 2  are the same as or different from each other, and represent a perfluoroalkyl group having carbon number of 1 to 4.)
 
[6]: A method according to above item 1, wherein,
       the ionic liquid is selected from the following:       1,3-Dimethylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Ethyl-3-methylimidazolium triflate,   1-Ethyl-3-methylimidazolium bis(pentafluoroethanesulfonyl)imide,   1,3-Diethylimidazolium bis(trifluoromethanesulfonyl)imide,   1,3-Diethylimidazolium triflate,   1-Butyl-3-ethylimidazolium triflate,   1,2-Dimethyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Butyl-3-methylimidazolium triflate,   1-isoPropyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,   1,2-Dimethyl-3-propyllimidazolium bis(trifluoromethanesulfonyl)imide,   N,N-propylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide,   Propyltrimethyammonium bis(trifluoromethanesulfonyl)imide,   N,N-Methylpropylpiperidinium bis(trifluoromethanesulfonyl)imide, or   N-Butylpyridinium bis(trifluoromethanesulfonyl)imide.
 
[7]: A sensor chip for a substitutional stripping voltammetry, comprising:
   
 
         [0060]    a substrate ( 30 ), 
         [0061]    a pair of working electrodes ( 31   a / 31   b ), 
         [0062]    a counter electrode ( 33 ), 
         [0063]    a stripping electrode ( 34 ), and 
         [0064]    a stripping gel ( 35 ), wherein 
         [0065]    the pair of working electrodes ( 31   a / 31   b ) is composed of a first working electrode ( 31   a ) and a second working electrode ( 31   b ), 
         [0066]    the surface of the stripping electrode ( 34 ) comprises silver, 
         [0067]    the stripping gel ( 35 ) covers the stripping electrode ( 34 ), 
         [0068]    the stripping gel ( 35 ) does not cover the pair of the working electrodes ( 31   a / 31   b ) or the counter electrode ( 33 ), 
         [0069]    the stripping gel ( 35 ) contains a standard electrolyte and an ionic liquid, 
         [0070]    the ionic liquid is hydrophobic, 
         [0071]    the ionic liquid is consisted of a cation and an anion, 
         [0072]    the standard electrolyte is consisted of the cation and a halide ion. 
         [0000]    [8]: A sensor chip according to above item 7, wherein, 
         [0073]    the sensor chip ( 300 ) further comprises a cover ( 37 ) with an inlet ( 36 ), 
         [0074]    a space is formed between the cover ( 37 ) and the sensor chip ( 300 ). 
         [0000]    [9]: A sensor chip according to above item 8, wherein, the sensor chip ( 300 ) further comprises an air vent ( 38 ).
 
[10]: A sensor chip according to above item 7, wherein,
 
         [0075]    the cation and the anion are selected from the following groups (I) and (II), respectively: 
         [0076]    Group (I): a cation represented by the following formula IV-(1) to IV-(6). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0077]    (wherein, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  are the same as or different from each other, and represent hydrogen atom, a straight or branched alkyl group which may contain heteroatom, an aralkyl group, or an aryl group, and R 9 , R 10 , R 11 , and R 12  are the same as or different from each other, and represent a straight or branched alkyl group which may contain heteroatom, an aralkyl group, or an aryl group.) 
         [0078]    Group (II): an anion represented by the following formula V-(1) or V-(2). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, Rf 1  and Rf 2  are the same as or different from each other, and represent a perfluoroalkyl group having carbon number of 1 to 4.)
 
[11]: A sensor chip according to above item 7, wherein,
 
         [0079]    the ionic liquid is selected from the following:
   1,3-Dimethylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Ethyl-3-methylimidazolium triflate,   1-Ethyl-3-methylimidazolium bis(pentafluoroethanesulfonyl)imide,   1,3-Diethylimidazolium bis(trifluoromethanesulfonyl)imide,   1,3-Diethylimidazolium triflate,   1-Butyl-3-ethylimidazolium triflate,   1,2-Dimethyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,   1-Butyl-3-methylimidazolium triflate,   1-isoPropyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide,   1,2-Dimethyl-3-propyllimidazolium bis(trifluoromethanesulfonyl)imide,   N,N-Propylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide,   Propyltrimethyammonium bis(trifluoromethanesulfonyl)imide,   N,N-Methylpropylpiperidinium bis(trifluoromethanesulfonyl)imide, or   N-Butylpyridinium bis(trifluoromethanesulfonyl)imide.   
 
       Advantageous Effects of Invention 
       [0096]    The present invention provides a method for quantifying a chemical substance with high accuracy using substitutional stripping voltammetry and a sensor chip used therefor. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0097]      FIG. 1  shows the system for the substitutional stripping voltammetry disclosed in Patent Document 1. 
           [0098]      FIG. 2  shows a sensor chip  101   a  employed for the substitutional stripping voltammetry disclosed in Patent Document 1. 
           [0099]      FIG. 3  shows the sensor chip  300  according to the embodiment 1. 
           [0100]      FIG. 4  schematically shows the sensor chip  300  which is connected to the potentiostat  42 . 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0101]    An embodiment of the present invention is described below with reference to  FIG. 3  and  FIG. 4 . 
         [0102]    (Step (a)) 
         [0103]    First, a sensor chip  300  is prepared. 
         [0104]      FIG. 3  shows the sensor chip  300  according to the embodiment 1.  FIG. 4  schematically shows the sensor chip  300  which is connected to the potentiostat  42 . The sensor chip  300  comprises a substrate  30 , a pair of comb-shaped working electrodes  31   a / 31   b , a reference electrode  32 , a counter electrode  33 , a stripping electrode  34 , a striping gel  35 , and a cover  37 . The cover  37  is comprised optionally, and comprises an inlet  36  for injecting a sample solution and an air vent  38 . 
         [0105]    The pair of the comb-shaped working electrodes  31   a / 31   b  is composed of a first working electrode  31   a  and a second working electrode  31   b.    
         [0106]    The reference electrode  32  is comprised optionally. In light of high accuracy of the quantification, it is preferred that the sensor chip  300  comprises the reference electrode  32 . 
         [0107]    The substrate  30  comprises an electrode region  30   a  and a connection region  30   b . The cover  37  covers the electrode region  30   a  on the quantification of the chemical substance, whereas the cover  37  does not cover the connection region  30   b.    
         [0108]    The pair of comb-shaped electrodes  31   a / 31   b , the reference electrode  32 , the counter electrode  33 , and the stripping electrode  34  are formed in the electrode region  30   a . Each of these electrodes comprises a lead wire(s)  39 . All the lead wires  39  are not electrically connect to each other. In the electrode region  30   a , they are covered with an insulator film (not shown), which prevents them from coming into contact with the sample solution. All the terminals of the lead wires  39  are extended into the connection region  30   b , which is inserted into the connector (not shown) of the potentiostat  42  shown in  FIG. 4 . 
         [0109]    An example of the shape of the substrate  30  is a rectangle, a square, and an ellipse. The surface of the substrate  30  comprises an insulator layer (not shown). Preferably, the surface of the substrate  30  is flat in light of formation of the electrodes. 
         [0110]    The pair of comb-shaped electrodes  31   a / 31   b  are disposed anywhere in the electrode region  30   a  as long as they are not connected with the stripping gel  35  and other electrodes electrically. Preferably, they are disposed on the periphery of the center of the electrode region  30   a . An example of the material of the pair of comb-shaped electrodes  31   a / 31   b  is gold, platinum, or glassy carbon in light of stability against electrochemical reactions. The pair of comb-shaped electrodes  31   a / 31   b  face each other and are engaged. 
         [0111]    The reference electrode  32  is also disposed anywhere in the electrode region  30   a . Preferably, it is disposed on the periphery of the pair of comb-shaped electrodes  31   a / 31   b . On the electrochemical measuring, the reference electrode  32  has constant potential. An example of the reference electrode  32  is a silver/silver chloride electrode. 
         [0112]    The counter electrode  33  may be disposed anywhere in the electrode region  30   a . The shape of the counter electrode  33  is not also limited. It is preferred that the area of the counter electrode  33  is approximately twenty to thirty times as large as the area of the pair of comb-shaped electrode  31   a / 31   b  and the area of the stripping electrode  34 . An example of the material of the counter electrode  33  is gold, platinum, and glassy carbon in light of stability against electrochemical reactions similarly to the pair of comb-shaped electrodes  31   a / 31   b.    
         [0113]    The stripping electrode  34  comprises silver on the surface thereof. 
         [0114]    The stripping gel  35  covers the stripping electrode  34 , which prevents the stripping electrode  34  from coming into contact with the sample solution. It is preferred that the stripping gel  35  is located on the periphery of the pair of comb-shaped electrodes  31   a / 31   b  to lower the resistance therebetween. The stripping gel  35  is preferably a thin film. 
         [0115]    The stripping gel  35  does not cover other electrodes  31  to  33  other than the stripping electrode  34 . In the case that the stripping gel  35  contacts with at least one of the other electrode  31  to  33 , the stripping gel  35  would connect electrically to at least one electrode. This causes the quantification of the chemical substance to be impossible. 
         [0116]    The inlet  36  and the air vent  38  may be disposed on the top or side plate of the cover  37 . The shapes of the inlet  36  and the air vent  38  are not limited. The cover  37  covers the entire of the electrode region  30   a . The cover  37  prevents the sample solution supplied therein from flowing to the connection region  30   b  and out of the sensor chip  300 . The cover  37  is provided optionally. The sample solution may be applied to the surface of the sensor chip  300  without the cover  37 . 
         [0117]    Next, the stripping gel  35  is described below in more detail. 
         [0118]    The stripping gel  35  contains the standard electrolyte and the ionic liquid. The ionic liquid serves as a support electrolyte. The stripping gel  35  is configured so as the standard electrolyte and the ionic liquid are not mixed with the sample solution. A method for configuring the stripping gel  35  is not limited. An example is supporting and/or including the standard electrolyte and the ionic liquid in a hydrophobic polymer. An example of the hydrophobic polymer is poly(vinylidene fluoride-hexafluoropropylene), polymethyl methacrylate, polyacrylonitrile, and polybutylacrylate. 
         [0119]    The ionic liquid is hydrophobic. The hydrophobic ionic liquid is composed of the following cation and anion. 
         [0120]    Cation: a cation represented by the following formula IV-(1) to IV-(6). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  are the same as or different from each other, and represent hydrogen atom, a straight or branched alkyl group which may contain heteroatom, an aralkyl group, or an aryl group, and R 9 , R 10 , R 11 , and R 12  are the same as or different from each other, and represent a straight or branched alkyl group which may contain heteroatom, an aralkyl group, or an aryl group.) 
         [0121]    Preferably, in the imidazolium ion represented by the formula IV-(1), R 1  is selected from the group consisting of methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, and t-butyl group, R 2  is hydrogen atom or methyl group, R 3  is an alkyl group having carbon number of 1 to 6 which may contain hetero atom, and R 4  and R 5  are hydrogen atom. 
         [0122]    Preferably, in the isoquinolium ion represented by the formula IV-(2), R 2  is an alkyl group having carbon number of 1 to 6 which may contain hetero atom, and R 1 , R 3 , R 4 , R 5 , R 6 , R 7 , and R 8  are hydrogen atom. 
         [0123]    Preferably, in the pyridinium ion represented by the formula IV-(3), R 1  is an alkyl group having carbon number of 1 to 6 which may contain hetero atom, and R 2 , R 3 , R 4 , R 5 , and R 6  are hydrogen atom. 
         [0124]    Preferably, in the pyrrolidinium ion represented by the formula IV-(4), R 1  is selected from the group consisting of methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, and t-butyl group, R 2  is an alkyl group having carbon number of 1 to 6 which may contain hetero atom, R 3 , R 4 , R 5 , and R 6  are hydrogen atom. 
         [0125]    Preferably, in the piperidinium ion represented by the formula IV-(5), R 1  is selected from the group consisting of methyl group, ethyl group, n-propyl group, isopropyl group, n-butyl group, isobutyl group, and t-butyl group, R 2  is an alkyl group having carbon number of 1 to 6 which may contain hetero atom, and R 3 , R 4 , R 5 , R 6 , and R 7  are hydrogen atom. 
         [0126]    Preferably, in the ammonium ion represented by the formula IV-(6), R 9 , R 10 , R 11 , and R 12  are the same as or different from each other, and represent an alkyl group having carbon number of 1 to 6 which may contain halogen atom, a phenyl group, or a benzyl group. 
         [0127]    Anion: an anion represented by the following formula V-(1) or V-(2). 
         [0000]    
       
                 
         
             
             
         
       
     
         [0000]    (wherein, Rf 1  and Rf 2  are the same as or different from each other, and represents a perfluoroalkyl group having carbon number of 1 to 4.) 
         [0128]    Preferably, in the anion represented by the formula V-(1), Rf 1  and Rf 2  are identical perfluoromethyl group or perfluoroethyl group. 
         [0129]    Preferably, in the anion represented by the formula V-(2), Rf 1  is trifluoromethyl group. 
         [0130]    More specifically, the ionic liquid is exemplified below.
   1,3-Dimethylimidazolium bis(trifluoromethanesulfonyl)imide   1-Ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide   1-Ethyl-3-methylimidazolium triflate   1-Ethyl-3-methylimidazolium bis(pentafluoroethanesulfonyl)imide   1,3-Diethylimidazolium bis(trifluoromethanesulfonyl)imide   1,3-Diethylimidazolium triflate   1-Butyl-3-ethylimidazolium triflate   1,2-Dimethyl-3-ethylimidazolium bis(trifluoromethanesulfonyl)imide   1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide   1-Butyl-3-methylimidazolium triflate   1-isoPropyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide   1,2-Dimethyl-3-propyllimidazolium bis(trifluoromethanesulfonyl)imide   N,N-Propylmethylpyrrolidinium bis(trifluoromethanesulfonyl)imide   Propyltrimethyammonium bis(trifluoromethanesulfonyl)imide   N,N-Methylpropylpiperidinium bis(trifluoromethanesulfonyl)imide   N-Butylpyridinium bis(trifluoromethanesulfonyl)imide   
 
         [0147]    The standard electrolyte is consisted of the above-mentioned cation and a halide ion. The halide ion denotes chloride ion, bromide ion, or iodide ion. 
         [0148]    It is preferred that the standard electrolyte comprises identical or similar cation to that of the ionic liquid in light of solubility. The “similar cation” means any cation represented by the formula IV-(1), when the cation of the ionic liquid is a cation represented by the formula IV-(1). Specifically, when the ionic liquid is 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, the standard electrolyte may preferably be 1-butyl-3-methylimidazolium halide. 
         [0149]    (Preparation Method of the Sensor Chip) 
         [0150]    A procedure to prepare the sensor chip of the present embodiment is described below. 
         [0151]    (Formation of Electrodes) 
         [0152]    An insulated substrate coated by a photoresist is exposed to an ultraviolet through a mask for patterning, followed by alkaline developing. A metal is sputtered on the patterned substrate  30 . The residue photoresist is dissolved into an organic solvent to remove the unnecessary metal. The whole surface of the substrate  30  is coated by an insulating film. The insulating film over the electrodes is removed by dry etching to form the pair of comb-shaped electrodes  31   a / 31   b , the reference electrode  32 , the counter electrode  33  and the stripping electrode  34 . The Ag/AgCl electrode is formed by applying an Ag/AgCl paste on the reference electrode  32 . 
         [0153]    (Formation of Stripping Gel  35 ) 
         [0154]    The stripping gel  35  may be formed as below. 
         [0155]    First, poly(vinylidene fluoride-hexafluoropropylene) is dissolved in acetone by ultrasonic wave on ice cooling to prepare an acetone solution. 1-Butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide containing 1-butyl-3-methylimidazolium iodide is added to the acetone solution. Subsequently, the acetone solution is stirred and dropped on the stripping electrode  34 . Finally, the acetone is evaporated to form the stripping gel  35 . 
         [0156]    (Step (b)) 
         [0157]    A sample solution is supplied to the surface of the above-mentioned sensor chip  300  to cover the surface with the sample solution. 
         [0158]    The sample solution contains a chemical substance to be quantified according to the present invention. An example of the chemical substance is an antigen, an antibody, a nucleic acid, a cell, bacteria, virus, a hapten, and a sugar. 
         [0159]    Preferably, the cover  37  is provided with the sensor chip  300 . The sample solution supplied through the inlet  36  covers the surface of the electrode region  30   a  of the sensor chip  300 . The air in the space between the cover  37  and the sensor chip  300  is drained through the air vent  38 . 
         [0160]    More preferably, the space between the cover  37  and the sensor chip  300  is filled with the sample solution. This allows the volume of the liquid solution to be constant. 
         [0161]    The sample solution contains a chemical substance to be quantified and an oxidation-reduction substance. In the present invention, the sample solution contains the oxidation-reduction substance in the reduction condition. The chemical substance to be quantified and an oxidation-reduction substance may be distinct. For example, the chemical substance to be quantified is an enzyme, and the oxidation-reduction substance is an electric mediator such as potassium ferrocyanide. Or, the chemical substance to be quantified may be modified with the oxidation-reduction substance. The protein modified with a ferrocenecarboxylic acid (hereinafter, “FcCOOH”) is exemplified. 
         [0162]    The substitutional stripping voltammetry comprises a step (c) and a step (d). 
         [0163]    (Step (c)) 
         [0164]    In the step (c), the switch  41   a  and the switch  41   b  are connected to the respective terminals A, and a constant potential is applied to the first working electrode  31   a . Furthermore, the second working electrode  31   b  is electrically connected to the stripping electrode  34  to form a redox cycle between the pair of comb-shaped electrodes  31   a / 31   b.    
         [0165]    When the oxidation-reduction substance is ferrocenecarboxylic acid, the following reactions represented by the following chemical formulas (VI) to (VIII) are caused on the comb-shaped electrodes  31   a , the comb-shaped electrodes  31   b , and the stripping electrode  34 . 
         [0166]    Silver halide is deposited on the surface of the stripping electrode  34 , which is composed of silver. 
         [0167]    Comb-Shaped Working Electrode  31   a : 
         [0000]    
       
                 
         
             
             
         
       
     
         [0168]    Comb-Shaped Working Electrode  31   b : 
         [0000]    
       
                 
         
             
             
         
       
     
         [0169]    Stripping Electrode  34 : 
         [0000]      [Chem. 8] 
         [0000]      Ag+X ⊖ →AgX↓+ e   ⊖   (VIII)
 
         [0000]    (wherein, X represents iodine atom, bromine atom, or chlorine atom.) 
         [0170]    (Step (d)) 
         [0171]    In the step (d), the switch  41   a  and the switch  41   b  are connected to the respective terminals B. No potential is applied to either the first working electrode  31   a  or the second working electrode  31   b . The stripping electrode  34  is swept with the potentiostat  42 . This cause electrolysis of the silver halide which has been deposited at the step (c), thus resultant halide ion is dissolved in the stripping gel as shown in the following chemical formula (IX). 
         [0172]    Stripping Electrode  34 : 
         [0000]      [Chem. 9] 
         [0000]      AgX+ e   ⊖ →Ag+X ⊖   (VIII)
 
         [0000]    (wherein, X represents iodine atom, bromine atom, or chlorine atom.) 
         [0173]    If the stripping gel  35  contains water, the water inhibits the reaction represented by the formula (VIII). Because water has an affinity with the halide ion. Therefore, the stripping gel  35  contains no water. However, the stripping gel may contain water as long as the content of the water is within a range of a determination precision is not adversely affected. The ionic liquid is nonvolatile. Accordingly, unlike prior arts, the eveporation of the stripping gel  35  is suppressed in the step (c) and in the step (d). This allows the concentration of the standard electrolyte to be maintained. As a result, the quantification of the chemical substance is allowed to be more accurate. This characterizes the present invention. 
         [0174]    In the longer period the constant potential is applied in the step (c), the higher sensitivity is achieved, since the deposition amount of silver halide is increased. 
         [0175]    The amount of the current flowing on the dissolution in the step (d) is proportional to the deposited amount of the silver halide. The deposited amount of the silver halide is proportional to the product of the concentration of the oxidation-reduction substance (reductant) by the period when the potential is applied in the step (c). Namely, the following equation is satisfied. 
         [0000]    
       
         
           
             
               
                 
                   
                     ( 
                     
                       the 
                        
                       
                           
                       
                        
                       deposition 
                        
                       
                           
                       
                        
                       amount 
                        
                       
                           
                       
                        
                       of 
                        
                       
                           
                       
                        
                       the 
                        
                       
                           
                       
                        
                       silver 
                        
                       
                           
                       
                        
                       halide 
                     
                     ) 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             
                               the 
                                
                               
                                   
                               
                                
                               concentration 
                                
                               
                                   
                               
                                
                               of 
                                
                               
                                   
                               
                                
                               the 
                                
                               
                                   
                               
                                
                               oxidation 
                                
                               
                                 - 
                               
                                
                               reduction 
                                
                               
                                   
                               
                                
                               substance 
                             
                           
                         
                         
                           
                             
                               ( 
                               reductant 
                               ) 
                             
                           
                         
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         
                           
                             
                               the 
                                
                               
                                   
                               
                                
                               period 
                                
                               
                                   
                               
                                
                               when 
                                
                               
                                   
                               
                                
                               the 
                             
                           
                         
                         
                           
                             
                               poyenial 
                                
                               
                                   
                               
                                
                               is 
                                
                               
                                   
                               
                                
                               applied 
                                
                               
                                   
                               
                                
                               in 
                                
                               
                                   
                               
                                
                               the 
                                
                               
                                   
                               
                                
                               step 
                                
                               
                                   
                               
                                
                               
                                 ( 
                                 c 
                                 ) 
                               
                             
                           
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   [ 
                   
                     Math 
                     . 
                     
                         
                     
                      
                     1 
                   
                   ] 
                 
               
             
           
         
       
     
         [0176]    Accordingly, the concentration of the oxidation-reduction substance (reductant) is calculated from the amount of the current flowing in the step (d). The chemical substance is quantified on the basis of the concentration of the oxidation-reduction substance (reductant). Needless to say, similarly to a typical procedure, when the chemical substance is quantified from the current, a standard curve which has been prepared is used. 
       INDUSTRIAL APPLICABILITY 
       [0177]    The present invention provides a method for quantifying a chemical substance with high accuracy using substitutional stripping voltammetry and a sensor chip used therefor. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               300  sensor chip 
               30  substrate 
               30   a  electrode region 
               30   b  connection region 
               31   a  first working electrode 
               31   b  second electrode 
               32  reference working electrode 
               33  counter electrode 
               34  stripping electrode 
               35  stripping gel 
               36  inlet 
               37  cover 
               38  air vent 
               39  lead wire 
               41  switch box 
               41   a  switch 
               41   b  switch 
               42  potentiostat 
             W working electrode of potentiostat 
             R reference electrode of potentiostat 
             C counter electrode of potentiostat