JPH0129259B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to electrodes used in electrochemical analysis and electrolysis of solutions, particularly reference electrodes used as counter electrodes to working electrodes, which are highly accurate and stable against changes in ionic species and their activities in solutions. A reference electrode having a high electrode potential is provided. In this specification, "reference electrode"
is a general term for electrodes that are used as a standard for the potential used to measure the relative value of an electrode's potential, and that are used in combination with an arbitrary electrode to form a type of battery in order to measure the potential of that electrode. Ion sensors For example, in electrodes whose characteristics are detected as current, such as transistor-type electrodes that utilize field effects, this is also the electrode configuration that serves as the reference for the current used to measure the relative value of the electrode characteristics. Defined as inclusive of. Among electrochemical analysis methods, in the analysis method classified under the name potentiometry, a working electrode (indicator electrode) is inserted into the sample solution, and the potential generated at the interface between the electrode and the solution is the same as that of the sample. The concentration of sample components is analyzed by measuring this potential, utilizing a phenomenon that is related to the components and concentration of the solution. Furthermore, when implementing this method, since it is impossible to directly measure the interfacial potential, a reference electrode is usually inserted into the same sample solution as a counter electrode to the working electrode, and the potential of the reference electrode is always constant. The working electrode potential is estimated by measuring the potential difference between both electrodes on the assumption that . Calomel electrodes, silver-silver chloride electrodes, etc. are generally used as reference electrodes, but calomel and silver chloride, which are sparingly soluble metal salts, also react with chlorine in the solution with which they come into contact. Since the potential changes depending on the ion activity, specifically, these electrodes are immersed in an aqueous solution with a constant chloride ion activity, and a liquid junction is partially placed on the boundary wall between the solution and the sample solution. is provided to enable electrical continuity between the internal solution and the sample solution. In another example, an aqueous solution with a constant chloride ion activity is poured into the reference electrode in small quantities, so that the solution containing the sample solution does not come into contact with the reference electrode and change the internal electrode potential. However, these methods do not require precise ion-selective electrode potential due to fluctuations in potential generated at the liquid junction (generally called junction potential) and changes in chloride ion activity due to alternating current between the internal and external liquids. This results in errors in the measurement of , and in turn makes precise chemical analysis of trace samples such as biochemical analysis difficult. FIG. 1 shows an outline of a conventional analytical instrument for potentiometry. In the figure, 1 is a working electrode (ion selective electrode), 2 is a reference electrode, and a container 2B has a pinhole type liquid junction 2A.
inserted into the internal fluid of the 3 is the sample solution, 4
The amplifier amplifies the potential difference between the electrodes 1 and 2, and the output thereof is displayed by the meter 5. Multiplier 4
In the input circuit, a shield 6 is applied to the lead wire connecting the electrode 1 and the amplifier 4 for the purpose of noise reduction. In such a configuration, as described above, the liquid junction 2
Changes in the juncture potential at A and fluctuations in internal ion activity due to the exchange of internal and external fluids through the liquid junction 2A are unavoidable, making it difficult to maintain the potential of the reference electrode 21 stably with high precision. . In order to solve the above problems, the inventor of the present application proposed using a polymer membrane with an inert molecular structure at the interface where the ion electrode contacts the sample solution.
Furthermore, we proposed the use of polymer membranes produced by plasma polymerization for this purpose. (Patent application No. 55-162225
No. (Japanese Unexamined Patent Publication No. 57-86037), No. 55-168982,
(Unexamined Japanese Patent Publication No. 57-93247), No. 56-133087 (Unexamined Japanese Patent Publication No. 58-34352)) As a result of investigating the characteristics of the ion-inactive membrane of the above-mentioned polymer membrane, the present inventor found that In other words, it was clarified that a membrane with a heterogeneous cross-sectional structure, in which the electrode conductor side has an extremely dense structure and the outer side, that is, near the surface in contact with the sample solution, has a rough structure is extremely effective as an ion-insensitive membrane for the reference electrode. did. FIG. 2 is a schematic diagram of a reference electrode according to an embodiment of the present invention, in which the surface of a silver wire 21 has a silver vapor deposited film 22 on one side and an inert polymer film 24 on the other side.
The film 23 is coated with a cellulose film 23 having
is fixed to the silver wire 21 with a stopper 25. The inert polymer membrane 24 has an inert molecular structure that does not generate an interfacial potential depending on the ion species and their activities in the sample fluid with which it comes in contact, or does not change its size even if an interfacial potential is generated. (hereinafter referred to as "ionically inert") polymer membrane. Its thickness is usually about 1000 Å to 1000 μm, and its resistance value is, for example, about 10 MΩ to 5000 MΩ. Experimental example Ag was sputtered to a thickness of 5 μm on one side of a 25 μm thick cellulose, and a multilayer film was formed on the other side by polymerizing 7000 Å thick polystyrene from styrene monomer by glow discharge for 90 seconds. The conditions for glow discharge are as follows. Vacuum degree: 5 × 10 ^-2 Torr, temperature: room temperature, discharge frequency: 10KHz discharge output: 1Watt After fixing this multilayer film on a 6 mm diameter silver wire 21 with a stopper 25, it was immersed in a 0.1M KCl aqueous solution. ⁺ and Cl ^- were impregnated and fixed in the rough surface structure to form a reference electrode having the configuration shown in FIG. 2. This electrode was placed in sample solution 3 and the following measurements were performed. [1] The above electrode and a sodium ion selective electrode are combined and inserted into an aqueous NaCl solution, and the generated potential difference E is plotted against each value of Na activity,
The broken line A in FIG. 3 was obtained. Similarly, a solid line B was obtained by plotting the potential difference generated when the same Na-selective electrode was combined with a conventional silver-silver chloride reference electrode. As can be seen from this graph, the performance of the two is in good agreement. [] When the reference electrode of the present invention was combined with a conventional silver-silver chloride electrode (internal solution: 0.1M-KCl), the potential differences for various ions were measured.
The results shown in Figure 4 were obtained. _NaHCO3 , NaCl, KCl
For each electrolyte solution, no potential difference occurred over a wide concentration range. Hydrogen ions were measured over a wide range from PH=9 to PH=2, but no change in potential difference was observed. Therefore, the results of experiments [1] and [] prove that the electrode shown in FIG. 2 above can be used satisfactorily as a reference electrode in the same way as the silver-silver chloride electrode. [] Figure 5 is a comparison diagram of the performance of the conventional silver-silver chloride reference electrode and the reference electrode shown in Figure 2 above with respect to Na ⁺ ion concentration in human serum, and each point indicates the same Na ⁺ ion concentration. The horizontal axis X shows the measured value for the concentration, and the horizontal axis X shows the measured value using the combination of the conventional silver-silver chloride type electrode and the Na ⁺ selective electrode, and the vertical axis Y shows the measured value using the same Na ⁺ selective electrode in the book [ ] mentioned above. It shows the measured values when combined with the reference electrode of the invention. Both results show satisfactory agreement. [] Figure 6 shows Na ⁺ when the reference electrode shown in Figure 2 above is combined with a Na ⁺ ion selective electrode.
The relationship between ion activity and generated potential difference is shown.
In the case of an aqueous solution of NaCl (marked with ○) and in human serum from which ions have been removed (Ion Free Serum),
The measured values are completely consistent with those obtained when NaCl is dissolved (△・mark), indicating that the reference electrode of the present invention can also be applied to measuring the concentration of Na ⁺ ions in human serum. [] Table 1 shows the influence of various organic interfering substances on the measured values when inorganic electrolyte components contained in blood are measured using the reference electrode of the present invention. The leftmost column shows the names of coexisting substances that are expected to interfere with the measurement, and the middle column shows the normal concentration range of these interfering substances in blood and the concentration of interfering substances in the sample used for the experiment. Furthermore, the rightmost column shows the degree of interference (the value when there is no influence is 100). As can be seen from the table, the reference electrode of the present invention

【table】

[Table] This allows accurate measurements to be made without being affected by large amounts of interfering substances that far exceed the normal concentration range in blood. As described above, the reference electrode of the present invention made of a plasma polymerized membrane is insensitive to ions in any electrolyte solution. In order to further elucidate the properties of this film, the surface of the plasma-polymerized polystyrene film was analyzed using a scanning electron microscope. This surface structure is shown in Figures 7 to 10 in comparison with that of a commercially available polystyrene membrane. Looking at the structure of the film when the frequency of plasma discharge is varied, it is found that at 10KHz to 100KHz, the film structure has a non-uniform structure and a structure of coarse particles of around 1 μm near the surface of the film is formed. It was observed that the inside of the specimen maintained a dense structure. On the other hand, by forming a film of this plasma-polymerized polystyrene on metals such as silver and aluminum, various concentrations of NaCl and
Since it does not respond at all even when placed in a KCl solution, it is clear that it does not have through holes (pinholes) through which these ions can pass. From these facts, in a plasma polymerized film, the polymer that adheres at the beginning of the discharge progresses cross-linking during the subsequent discharge and becomes a dense structure, but the polymer that adheres on top of it gradually becomes coarser. ,
As seen in FIGS. 9 and 10, a rough particle surface of about 1 μm is formed on the surface. As a result, as shown in FIG. 11, it was found that the inside was dense, there were no pores through which ions could pass, and the film had a membrane structure in which coarse particles were gradually formed near the surface. (The reference numerals in FIG. 11 are the same as in FIG. 2.) Therefore, a membrane structure is formed in which the ionized solution can enter in the layer near the surface, but not in the deep layer. A characteristic of plasma polymerized films is that they have a film structure with a heterogeneous cross-section, but it takes a relatively long time (for example, 1 to
It is desirable to perform the discharge over a period of 5 minutes). Therefore, as shown in Figure 2, the electrode was constructed with a surface plasma polymerized film in such a way that its outer surface, which is in contact with the liquid, had a rough texture, and this was left overnight in a KCl aqueous solution slightly warmed above room temperature. As a result, a membrane electrode containing an aqueous solution containing K ⁺ and Cl ^- ions is formed to approximately half the thickness of the membrane. This part containing K ⁺ and Cl ^- acts as a salt bridge, so to speak. These ions are not easily released because the membrane has a small pore diameter and a large number of pores are formed in a complicated manner.
This point is extremely effective for constructing a reference electrode with stable performance. Note that depending on the manufacturing method, it is also possible to make the particles angular in shape or to have a net-like structure. (In this specification, these will be collectively referred to as "tissues.") Above, from Figure 2 onwards, another polymer material (cellulose) is inserted between the electron conductive material and the ion-inactive polymer membrane. Although the interposed electrode configuration and its performance have been described, other types of polymers can be used instead of cellulose, and polymers such as xylene and epoxy can also be used instead of styrene as inert polymers.
In addition, examples of electronically conductive materials include Ag and Al.
In addition, arbitrary metals such as W, Co, and Cu, and carbon can also be used. In addition, the structure of the reference electrode of the present invention, which uses an inert polymer membrane with a cross-sectional structure consisting of particles that are dense inside and coarse near the surface, in the part that comes into contact with the sample liquid, is as shown in Figure 2. The method is not limited to one in which another polymer substance is interposed between the ion-inactive polymer membrane. That is, (a) By interposing an ionic conductor, for example, a membrane (AgCl + Ag ₂ S membrane) in which silver chloride and silver sulfide are mixed and fixed by dissolving them, between the electron conductor and the ion-inactive polymer membrane, Electrodes with uniform performance and uniform characteristics can be obtained. (b) An ion-inactive polymer film may be coated on the gate portion of an ion-selective field effect transistor (abbreviated as “ISFET”). ISFET is a field effect transistor with a source, drain, and gate on a silicon substrate, and an ion-selective ion-sensitive film is covered on the insulating layer of the channel between the source and drain. The potential generated between the ion-sensitive membrane and the sample solution when brought into contact with the sample solution is used as the gate input to change the current between the source and drain, and this is used as the output to detect the concentration of the target ion in the sample solution. can. In the inactive ISFET to which the present invention is applied, the outermost surface of the electrically insulating layer constituting the gate portion is covered with the above-mentioned inert polymer film, and the current flowing between the drain and source is controlled by the sample solution in contact with the gate. It remains constant regardless of concentration and acts as a reference electrode. By utilizing this electrode, the entire electrode system including the working electrode and reference electrode can be significantly miniaturized, and both can be formed on a common semiconductor substrate to the extent that they can be attached to the tip of a syringe and inserted into a blood vessel. It also becomes possible to miniaturize it. (c) A stable reference electrode can be constructed by coating a sensitive part made of an ion-selective material such as glass or ceramic with an ion-inactive heterogeneous membrane. (d) A stable reference electrode with a simple structure can be constructed by directly coating an ion-inactive heterogeneous polymer membrane on an electron conductor. As described above, according to the present invention, the part in contact with the sample liquid is an ion inert membrane having a heterogeneous membrane cross-sectional structure that is dense in the deep part and relatively coarse in the vicinity of the surface in contact with the sample liquid. By having a structure coated with , it is possible to provide an extremely stable reference electrode, which is extremely useful for measuring ion activity in various fields such as analytical chemistry and biometry.

[Brief explanation of drawings]

FIG. 1 is an example of a conventional reference electrode, FIG. 2 is a schematic configuration diagram of a reference electrode according to an embodiment of the present invention, FIGS. 3 to 6 are characteristic diagrams showing the performance of the device in FIG. 2, and FIG. Figures 10 to 10 are comparison diagrams (crystal structure photographs) of membrane surfaces between a normal polymer membrane and the ion-inactive polymer membrane of the present invention. Figure 11 is an explanatory diagram of the cross-sectional structure of the inert polymer membrane of the present invention. . 3...Sample liquid, 21...Silver wire, 22...Silver vapor deposited film, 23...Cellulose film, 24...Heterogeneous cross-section ion inert film.

Claims (1)

[Scope of Claims] 1. A cross section of a polymer membrane produced by a plasma polymerization method in which the frequency of plasma discharge is 10 KHz to 100 KHz, and which has a dense structure inside and a rough structure near the surface in contact with the sample liquid. A reference electrode characterized in that its wetted part is covered with an ion-inactive membrane having a heterogeneous structure. 2. The reference electrode according to claim 1, wherein the polymer is polystyrene. 3. A patent claim, characterized in that the interior of the ion-inactive membrane has no pinholes, and the surface portion where the structure is rough is impregnated with a solution containing a specific ionic species, and this membrane forms a salt bridge. The reference electrode according to item 1. 4. The reference electrode according to claim 1, wherein the ion inert film is formed directly on an electron conductor. 5. The reference electrode according to claim 1, wherein the ion inert film is formed on a surface of a gate portion of a field effect irradiator. 6. The reference electrode according to claim 1, characterized in that an ion-conductive substance is interposed between the ion-inactive film and an electron conductor constituting the electrode conductor; 7. 2. The reference electrode according to claim 1, wherein a polymer substance is interposed between the active film and the electron conductor constituting the electrode conductor.