JPH05188032A - Gas sensor - Google Patents

Abstract

PURPOSE:To obtain high responsiveness in a gas sensor formed in such a way as to detect gas included in gas to be detected by bringing an electrolytic solution into contact with the gas to be detected so as to generate potential difference between a pair of electrodes brought into simultaneous contact with the electrolytic solution and separated from each other. CONSTITUTION:Since an electrolytic solution 16 held in recessed grooves 22 is brought into contact with gas to be detected in the detecting face of a substrate 10, there is no holdup of the gas to be detected so as to obtain high responsiveness to the concentration change of carbon dioxide gas included in the gas to be detected. Also the interface between the gas to be detected and the electrolytic solution 16 in the recessed grooves 22 of an electrolytic solution holding means 18 is positioned on the surface of detecting electrodes 12, so that high responsiveness is obtained regarding the detection of existence and concentration of gas included in the gas to be detected.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas sensor for detecting a gas to be detected such as oxygen or carbon dioxide by utilizing an electrochemical reaction.

[0002]

2. Description of the Related Art A gas contained in a gas to be detected by bringing an electrolyte solution containing an enzyme into contact with the gas to be detected to generate a potential difference between a pair of electrodes that are in contact with the electrolyte solution and are separated from each other. A gas sensor for detecting is known. In such a type of gas sensor, for example, an equilibrium chemical reaction is generated between the gas to be detected and the water content in the electrolyte solution by a reaction promoting action of an enzyme, and is generated by a substance obtained by the chemical reaction. The gas to be detected is detected by detecting the ion concentration as the potential difference between the PH detection electrode and the reference electrode. Then, in order to improve the responsiveness of such a type of gas sensor, a substrate having a passage formed so as to communicate with the gas to be detected is provided with a gas permeable detection electrode across the passage, There has been proposed a gas sensor constituted by applying a gel electrolyte solution onto the gas permeable detection electrode. For example, JP-A-64
That is what is described in Japanese Patent Publication No. 88245.

[0003]

In the conventional gas sensor described above, the gel electrolyte solution is applied onto the gas-permeable electrode, so that the pores of the passage and the electrode are closed by the gel electrolyte solution. As a result, the gas to be detected unavoidably stays in the passage, and the electrode tends to be buried in the gel electrolyte solution, so that sufficient responsiveness has not been obtained yet.

The inventors of the present invention have conducted various studies in order to solve the above problems, and as a result, if a gas sensor is constructed so that the electrode is located at the interface between the electrolyte solution and the gas to be detected, a high response can be obtained. I found that I can be. The present invention was made based on such findings.

[0005]

That is, the gist of the present invention resides in that an electrolyte solution is brought into contact with a gas to be detected so that a potential difference is generated between a pair of electrodes which are both in contact with the electrolyte solution and separated from each other. A gas sensor for detecting a gas contained in the gas to be detected by generating (a) a substrate having a detection surface that is brought into contact with the gas to be detected,
(b) having a large number of recesses and protrusions formed on the detection surface of the substrate, an electrolyte solution holding means for holding the electrolyte solution in the recess, (c) in the recess of the electrolyte solution holding means While in contact with each of the held electrolyte solution, a detection electrode provided in the area where the electrolyte solution holding means of the detection surface of the substrate is provided, and (d) contact with the electrolyte solution on the detection surface of the substrate. A reference electrode, which is provided as possible.

[0006]

With this configuration, the electrolyte solution held in the large number of recesses of the electrolyte solution holding means is brought into contact with the gas to be detected on the detection surface of the substrate. Compared to the conventional gas sensor in which the gel electrolyte solution comes into contact with the gas to be detected, there is no retention of the gas to be detected in the passage and a high response to changes in the concentration of the gas contained in the gas to be detected. Sex is obtained. Further, since the detection electrode is provided in a region of the detection surface of the substrate where the electrolyte solution holding means is provided in a state of being in contact with the electrolyte solution held in the concave portion of the electrolyte solution holding means, the electrolyte solution The surface is located at the interface between the electrolyte solution and the gas to be detected in the recess of the holding means. Therefore, high responsiveness can be obtained with respect to the presence or absence of the gas contained in the gas to be detected and the detection of the concentration.

[0007]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a plan view showing a main part of a carbon dioxide gas sensor which is an embodiment of the present invention, and FIG. 2 is its II-II.
FIG.

In FIG. 1, a detection electrode 12 and a reference electrode 14 are provided on a detection surface, which is one surface of a flat substrate 10, with a predetermined gap D therebetween. Then, in the central portion of the detection surface of the substrate 10 and in the region overlapping the detection electrode 12 and the reference electrode 14, an electrolyte solution holding means 18 for holding the electrolyte solution 16 is provided as shown in FIG. There is. As a result, the detection electrode 12 and the reference electrode 14 are in a state of being electrically insulated from each other and in contact with the electrolyte solution 16 held by the electrolyte solution holding means 18.

The substrate 10 is made of an insulator such as ceramics, a semiconductor such as silicon, gallium arsenide, silicon carbide, or a conductor such as aluminum or copper.
However, when the substrate 10 is composed of a semiconductor or a conductor, the sensing electrode 12 and the reference electrode 14 are provided through a well-known insulating layer such as silicon oxide or aluminum oxide.
Are fixed to the substrate 10.

Iridium oxide (IrO ₂ ) is used for the sensing electrode 12 in this embodiment, but other materials known as PH sensing electrodes, such as platinum and platinum with platinum black, are used.
Gold, silver, iridium, palladium, silver palladium, etc. can be used. This sensing electrode 12 is preferably a vapor deposition,
It is formed by a thin film forming method that forms a metal layer with a film thickness of about several microns through a predetermined pattern formed on a photoresist film by sputtering, etc. It may be formed by a forming method. In addition, the reference electrode 14
In this embodiment, silver is used, but other metal selected depending on the combination with the material of the detection electrode 12, such as gold, may be used. Like the detection electrode 12, the reference electrode 14 is also formed by a thin film forming method, a thick film forming method, or the like.

The electrolyte solution 16 contains a predetermined enzyme, sodium chloride (NaCl), and sodium carbonate (Na).
HCO ₃ ) is an aqueous solution in which each is dissolved, and if necessary, one or more substances selected from methylcellulose, PVA, agar, gelatin, polyacrylamide, polyvinylpyrrolidone, polyacrylic acid, etc. are mixed. It is made into a gel by being made to. The sodium chloride reacts with the silver of the reference electrode 14 to determine the reference potential E ^O of the reference electrode 14 according to the concentration thereof. Further, the sodium carbonate is related to the detection sensitivity of carbon dioxide gas because it is ionized into Na ⁺ ions and HCO ₃ ⁻ ions in the electrolyte solution 16, and the concentration thereof increases the detection sensitivity of carbon dioxide gas. It is set to fall within the range of 10 ^{−3 to} 10 ⁻² . Further, the above-mentioned enzyme is a well-known one that is appropriately added as necessary, and for example, when detecting carbon dioxide, carbonic anhydrase or the like is used.

In the carbon dioxide sensor of this embodiment, the output voltage E _out, which is the potential difference generated between the detection electrode 12 and the reference electrode 14, is measured by the voltage difference meter 20 having a high input impedance of, for example, 10 ¹² ohms or more. Thus, the carbon dioxide concentration is detected according to the Nernst equation shown in Equation 1. In Expression 1, k is a constant.

[0013]

[Equation 1] E _out = E ^O −klog [CO _{2 in air} ]

That is, when the gas to be detected is humidified up to about 100% by a humidifier by a not-shown gas introducing device (not shown) and then passed in a direction parallel to the detection surface of the substrate 10, the detected gas is detected. The carbon dioxide gas contained in the gas reacts with the water in the electrolyte solution 16 and is ionized as shown in Formula 2. Since the reaction formula shown in Formula 2 is an equilibrium reaction, it reacts rightward or leftward depending on the carbon dioxide concentration. Hydrogen ion H ⁺ generated by this reaction
Is proportional to the concentration of carbon dioxide gas contained in the gas to be detected, and the potential difference E _out generated between the detection electrode 12 and the reference electrode 14 due to the PH in the electrolyte solution 16 which changes depending on the concentration of hydrogen ions H ^+. Changes, the relationship between the potential difference E _out and the carbon dioxide gas concentration in the electrolyte solution 16 can be expressed as shown in Equation 3. Then, the formula 1 is derived from the formula 3. In the above formula 3, n is the number of moles, R is a constant, T is an absolute temperature, and F is a Faraday constant.

[0015]

[Number 2] _{CO 2 + H 2 O = H} 2 CO 3 = H + + HCO 3 -

[0016]

[Equation 3] E _out = E ^O − (2.303 RT / nF) log [C
O _{2 dissolved} ]

As shown in detail in FIG. 2, a large number of concave grooves 22 having a rectangular cross section are formed in the central portion of the detection surface of the substrate 10, and by the concave grooves 22 and the ridges 24 between the concave grooves 22. The electrolyte solution holding means 18 is configured. The sensing electrode 12 and the reference electrode 14 are fixed in layers on the top surface of the ridge 24, and the concave groove 22 is filled with and holds the electrolyte solution 16. The boundary surface of the electrolyte solution 16 held in the groove 22 in this manner is located on the surfaces of the detection electrode 12 and the reference electrode 14. The width and depth of the groove 22 are set within a range of several microns to several tens of microns, for example.

Therefore, according to the carbon dioxide gas sensor of this embodiment, the electrolyte solution 16 held in the large number of grooves 22 of the electrolyte solution holding means 18 is brought into contact with the gas to be detected on the detection surface of the substrate 10. , A conventional gas sensor of a type in which a gel electrolyte solution comes into contact with a gas to be detected in the inner part of a passage of a substrate (Japanese Patent Laid-Open No. 64-882).
45), there is no retention of the gas to be detected in the passage, and a high responsiveness to changes in the concentration of carbon dioxide contained in the gas to be detected can be obtained.

Further, the detection electrode 12 is provided in the region where the electrolyte solution holding means 18 is provided on the detection surface of the substrate 10 while being in contact with the boundary surface of the electrolyte solution 16 held in the concave groove 22. Therefore, the boundary surface between the electrolyte solution 16 and the gas to be detected in the concave groove 22 of the electrolyte solution holding means 18 is located on the surface of the detection electrode 12. Therefore, high responsiveness can be obtained with respect to the presence or absence of the gas contained in the gas to be detected and the detection of the concentration thereof.

Next, another embodiment of the present invention will be described. In the following description, the same parts as those in the above-described embodiment are designated by the same reference numerals and the description thereof will be omitted.

In the embodiment shown in FIG. 3, the detection electrode 12 is also formed on the side wall surface in the concave groove 22 by, for example, sputtering in two directions shown by arrows. Similarly, the reference electrode 14 is also formed on the side wall surface inside the concave groove 22.
As a result, even if the amount of the electrolyte solution 16 held in the groove 22 varies, the boundary surface between the electrolyte solution 16 and the gas to be detected is reliably positioned on the surface of the detection electrode 12. Further, even if the liquid level of the electrolyte solution 16 held in the concave groove 22 changes during use, the electrolyte solution 1
Since the boundary surface between 6 and the gas to be detected is reliably positioned on the surface of the detection electrode 12, high durability performance can be obtained.

In the embodiment shown in FIG. 4, the concave groove 22 has a triangular sectional shape. Also in this embodiment, the same effect as that of the above-mentioned embodiment can be obtained. Further, in the case of this embodiment, by performing the sputtering once from the direction perpendicular to the detection surface of the substrate 10, the detection electrodes 12 are also formed all at once on the side wall surface inside the concave groove 22.

In the embodiment shown in FIG. 5, the electrolyte solution holding means 18 provided in the central portion on the detection surface of the substrate 10 overlapping the detection electrode 12 and the reference electrode 14 is formed by etching or sandblasting. A large number of fine irregularities on the order of several microns to several tens of microns R _max are formed on the ceramic substrate 10 by the surface roughening method. Also in this embodiment, since the electrolyte solution 16 is held in the recess and the boundary surface of the electrolyte solution 16 is surely positioned on the surface of the detection electrode 12, the same effect as the above-described embodiment can be obtained. ..

Although one embodiment of the present invention has been described in detail with reference to the drawings, the present invention can be implemented in other modes.

For example, in the above-described embodiment, the detection electrode 12 and the reference electrode 14 have a single shape that is in equilibrium with each other, but a large number of comb-tooth-shaped portions that are spaced apart from each other in parallel are provided. It may be provided. In this case, the comb-shaped portions are preferably formed every other protrusion 24.

Further, in the above-described embodiment, the two detection electrodes 12 and the reference electrode 14 are formed on the detection surface of the substrate 10, but other reference electrodes may be added if necessary. This added reference electrode is also provided in contact with the electrolyte solution 16.

The detection electrode 12 and the reference electrode 14 may be formed by a metallizing method in which molybdenum or manganese paste on a green sheet is printed, baked in a reducing atmosphere, and then plated with a predetermined metal. ..

Further, although the concave groove 22 of the embodiment shown in FIG. 1 is formed in the direction perpendicular to the longitudinal direction of the detection electrode 12 and the reference electrode 14, it may be formed in an oblique direction.
Further, they may be formed in two directions intersecting with each other.

Further, in the above-mentioned embodiment, the electrolytic solution holding means 18 is constituted by forming the concave groove 22 on the surface of the substrate 10, but a large number of thin film or thick film are laminated on the surface of the substrate 10. By forming the ridges of the book, the electrolyte solution may be retained in the recesses formed between the ridges.

Although the carbon dioxide gas sensor has been described in the above embodiments, the present invention can be applied to the detection of hydrocarbons, oxygen, nitrogen dioxide, hydrogen sulfide, hydrogen cyanide, nitrous oxide, nitric acid and the like.

Further, the reference electrode 14 of the above-mentioned embodiment is not required to be provided on the top surface of the ridge 24, as long as it is in a state of being in contact with the electrolyte solution 16, and the reference electrode 14 is provided in the concave groove 22. It may be provided only.

Although not illustrated one by one, the present invention can be implemented in various modified and improved modes based on the knowledge of those skilled in the art.

[Brief description of drawings]

FIG. 1 is a plan view showing a main part of a carbon dioxide sensor according to an embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II of FIG.

FIG. 3 is a view corresponding to FIG. 2 showing another embodiment of the present invention.

FIG. 4 is a view corresponding to FIG. 2 showing another embodiment of the present invention.

FIG. 5 is a view corresponding to FIG. 1 and showing another embodiment of the present invention.

[Explanation of symbols]

 10 Substrate 12 Detection Electrode 14 Reference Electrode 16 Electrolyte Solution 18 Electrolyte Solution Holding Means 22 Concave Groove (Concave) 24 Convex Strip (Convex)

Claims (1)

[Claims] 1. A gas contained in the gas to be detected is detected by bringing the electrolyte solution into contact with the gas to be detected to generate a potential difference between a pair of electrodes that are both in contact with the electrolyte solution and separated from each other. A gas sensor, which has a substrate having a detection surface that is brought into contact with the gas to be detected, and a large number of recesses and protrusions formed on the detection surface of the substrate, and holds the electrolyte solution in the recesses. An electrolyte solution holding means, and a detection electrode provided in a region of the detection surface of the substrate where the electrolyte solution holding means is provided in a state of being in contact with the electrolyte solution held in the recess of the electrolyte solution holding means, A gas sensor, comprising: a reference electrode provided so as to come into contact with the electrolyte solution on the detection surface of the substrate.