JP3755247B2 - Odor sensor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an odor sensor which is a kind of gas sensor. The odor sensor of the present invention is used for quality inspection of foods and fragrances, quantitative detection of bad odor pollution, fire alarm by detection of burnt odor, quality inspection of foods and fragrances, and further, tracking, identification, authentication, drug inspection, etc. It can be used in a wide range of fields such as criminal investigations.
[0002]
[Prior art]
The odor sensor electrically (or optically) measures a physical change of an odor component contained in air (or supplied gas) caused by adhering to a sensitive surface of the sensor.
[0003]
As the odor sensor, a sensor using a metal oxide semiconductor as a sensitive film and utilizing the change in resistance value has been put into practical use. A so-called “electronic nose” using a plurality of sensors is commercialized by Prime Tech, France. In this type of odor sensor, the sensitive film is heated to a high temperature (350 ° C. or higher) to cause an oxidation-reduction reaction with the odor component adhering to the film surface. In this process, movement of electrons occurs, and the electric density changes as the electron density in the sensitive film and the thickness of the depletion layer change.
[0004]
Therefore, an odor sensor using a metal oxide semiconductor sensitive film can only detect a substance that causes a redox reaction, and cannot detect a substance that thermally decomposes at the above temperature. It had been. Further, it is necessary to wait for the sensor to rise to the operating temperature and stabilize at the time of analysis, and in particular, a long measurement time is required for repeated measurement. Furthermore, since the surface state of the sensitive film is relatively unstable, there is a problem that the change with time is large and the reliability is poor.
[0005]
In contrast, for example, Japanese Patent Application Laid-Open No. 61-147145 proposes a gas sensor using a conductive polymer. Further, an odor sensor using a conductive polymer mainly composed of polypyrrole as a sensitive film and utilizing a change in the resistance value thereof is commercialized by Aromascan and Neotronics, UK. With such a sensor, analysis can be performed while the sensitive film is maintained at room temperature.
[0006]
[Problems to be solved by the invention]
However, the conductive polymer such as polypyrrole used in the above conventional sensor must form a sensitive film mainly by an electrolytic polymerization method. Therefore, it is difficult to control the film thickness especially when the film is thinned. There were difficulties in uniformity and reproducibility. It was also difficult to create a sensitive film of any shape. Furthermore, productivity was also inferior.
[0007]
By the way, in this kind of odor sensor, it is known that the substance of the odor component that can be detected differs depending on the kind of the conductive polymer and the material to be doped even if it is the same conductive polymer. In research and development of conventional odor sensors, the main focus has been on expanding the range of substances of odor components that can be detected based on such a viewpoint. However, considering the future application field of odor sensors, it is also important to improve detection sensitivity in order to reliably detect a small amount of odor components. In this respect, the conventional odor sensor cannot be said to have sufficient detection sensitivity.
[0008]
The present invention has been made to solve the above-mentioned problems, and a first object thereof is to provide an odor sensor in which the uniformity and reproducibility of the sensitive film are good and the productivity is high. There is. A second object is to provide an odor sensor with high detection sensitivity.
[0009]
Means for Solving the Problem and Embodiment of the Invention
A first invention made to solve the above-mentioned problem is that a sensitive film is provided between two electrodes formed on an insulating substrate, and a target component in a gas adheres to the sensitive film between the electrodes. This is an odor sensor that measures odor by measuring the resistance change in which a sensitive film is formed by applying a solution obtained by dissolving a conductive polymer in a predetermined solvent on a substrate, and further using a dopant as a solvent. It is characterized in that the conductivity is increased by adding a dopant to the sensitive film by dissolving and bringing the solution into contact with the sensitive film.
[0010]
That is, the conductive polymer used for the sensitive film of the odor sensor of the first invention is soluble in a solvent such as chloroform. For this reason, a sensitive film is formed by dissolving a polymer polymerized by an oxidative polymerization method or the like in a solvent and applying the solution onto a substrate on which an electrode has been previously formed, and volatilizing the solvent. Can be formed. Therefore, the thickness of the sensitive film can be easily controlled, a highly uniform film can be formed, and the reproducibility is also good.
[0011]
As the predetermined solvent, an organic solvent having high volatility such as chloroform and toluene is suitable. Examples of the conductive polymer soluble in the solvent include poly (3-alkylthiophene) having a main chain of poly (3-alkylthiophene) in which an alkyl group having 4 or more carbon atoms is introduced at the 3-position of thiophene, specifically, , Poly (3-butylthiophene), poly (3-hexylthiophene), poly (3-octylthiophene), poly (3-decylthiophene), poly (3-dodecylthiophene), etc. Can be used. In addition, a conductive polymer having a main chain of poly (3-alkoxylthiophene) into which an alkoxyl group is introduced instead of the alkyl group can also be used. Furthermore, conductive polymers such as polyaniline can be used by using N-methylpyrrolidone (NMP) as a solvent.
[0012]
The conductive polymer is a conjugated polymer in which π electrons are incorporated into the polymer main chain, and it is known that the conductivity can be controlled by appropriate doping. The inventors of the present application have found that the response characteristics of the sensor depend on the conductivity of the conductive polymer of the sensitive film in the course of the experiment on the odor sensor using the conductive polymer as the sensitive film.
[0014]
That is, in the odor sensor having such a structure, when the conductivity of the conductive polymer of the sensitive film is in the range of 10 ⁻¹ to 10 ⁻⁵ [S / cm], the response characteristic to the odor component is good. Detection sensitivity is high. More preferably, a remarkable response characteristic is obtained when it is in the range of 10 ⁻² to 10 ⁻⁴ [S / cm].
[0015]
Usually, the conductive polymer constituting the sensitive film formed by applying a solution obtained by dissolving the conductive polymer in a predetermined solvent on the substrate as described above has a conductivity of 10 ⁻⁵ [ S / cm], the dopant can be added to the sensitive film to increase the conductivity by dissolving the dopant in a solvent and bringing the solution into contact with the sensitive film formed on the substrate.
[0016]
Examples of the dopant include proton acids (HNO ₃ , H ₂ SO ₄ , HClO ₄ , HF, HCl, FSO ₃ H, CF ₃ SO ₄ H, etc.), transition metal halides (FeCl ₃ , MoCl ₅ , WCl ₅ , SnCl ₄ , MoF ₅ , RuF ₅ , SnI _4, etc.), organic substances (TCNQ, TCNE, chloranil, etc.), porferins, polymers (polystyrene sulfonic acid, polyvinyl sulfonic acid, etc.) can be used.
[0017]
Such doping introduces a dopant that moves relatively freely into the conductive polymer film and carries electrons, thereby increasing the conductivity. In such a method, the amount of dopant to be added can be controlled by changing the concentration of the solution and the immersion time, so that the conductivity can be easily brought close to the target value.
[0018]
Further, instead of introducing a dopant, the conductive high polymer is added to a solvent in which a solvent in which the conductive polymer is easily soluble and a solvent having a higher polarity and dielectric constant than the solvent and in which the conductive polymer is difficult to dissolve. The conductivity of the sensitive film may be increased by dissolving the molecules and applying the solution onto the substrate to form a sensitive film. That is, the second invention made in order to solve the above-described problem is that a sensitive film is provided between two electrodes formed on an insulating substrate, and the target component in the gas adheres to the sensitive film. An odor sensor for measuring odor by measuring a change in resistance between electrodes, a first solvent in which a conductive polymer is easily soluble, a polarity and a dielectric constant higher than that of the first solvent, and the conductive polymer The sensitive film is formed by applying a solution in which the conductive polymer is dissolved in a solvent mixed with a second solvent that is difficult to dissolve, on the substrate. Examples of the solvent used here include an organic solvent in which the above-described conductive polymer is soluble (chloroform, toluene, etc.) and an organic solvent in which the conductive polymer is difficult to dissolve (tetrahydrofuran, dimethylformamide, dimethyl). A combination of sulfooxide, N-methylpyrrolidone, etc.) can be used.
[0019]
In the sensitive film formed in this way, a part of the polymer structure having regularity is deformed (for example, the distance between adjacent molecules is changed) and the conductivity is increased. In such an odor sensor, the structural change of the polymer itself contributes to electric conduction, so that there is no change with time and instability of the conductivity due to dropping off of the dopant and the like, and a more stable state can be maintained for a long time.
[0020]
【Example】
[Example 1]
A first embodiment (hereinafter referred to as “embodiment 1”) of an odor sensor according to the present invention will be described. This odor sensor is obtained by introducing ferric chloride as a dopant into poly (3-hexylthiophene) as a sensitive film. As described above, other conductive polymers and dopants can be used. FIG. 1 is a plan view (a) showing an example of the structure of the odor sensor 10 of the first embodiment, an enlarged view (b) and a sectional view (c) of a portion A of the plan view. Electrodes 12a and 12b made of gold and having a thickness of about 150 nm are formed in a comb shape at intervals of 5 μm on a glass substrate 11 that is an insulating material, and lead wires 13a and 13b are connected to the electrodes 12a and 12b. The materials of the substrate 11 and the electrodes 12a and 12b are not limited to those described above. On the substrate 11, a sensitive film 14 made of poly (3-hexylthiophene) is formed so as to cover the electrodes 12a and 12b. Ferric chloride is introduced into the sensitive film 14 as a dopant by the method described later.
[0021]
An example of the manufacturing method of the odor sensor 10 is as follows. First, the electrodes 12a and 12b are formed on the glass substrate 11 by a lift-off method. On the other hand, 3-hexylthiophene (manufactured by Tokyo Chemical Industry Co., Ltd.) in which 3-position of thiophene C ₄ H ₄ S is substituted with hexyl group C ₆ H ₁₃ is polymerized by oxidative polymerization to produce poly (3-hexylthiophene). This is dissolved in a chloroform solvent to prepare a solution having a concentration of 0.1 mol / L. This solution is applied onto the substrate 11 by spin coating to form a film body having a thickness of about 300 nm. In order to form such a film body, for example, a spinner may be operated at a rotation speed of 1500 rpm for 10 seconds. Thereafter, ferric chloride FeCl ₃ is dissolved in a nitromethane solvent to prepare a solution having a concentration of 0.1 mol / L, and the substrate on which the film body is formed is immersed in the solution for 10 minutes. Thereby, an appropriate amount of ferric chloride is introduced as a dopant into the poly (3-hexylthiophene) film, and the sensitive film 14 is completed. The lead wires 13a and 13b may be connected to the electrodes 12a and 12b by removing the film at a predetermined position after the formation of the sensitive film 14, or the film may not be formed at the predetermined position in advance. Good.
[0022]
In the state where the poly (3-hexylthiophene) film is formed on the substrate 11, the conductivity is a low value of 10 ⁻⁷ [S / cm] or less, but the dopant is introduced into the film as described above. By doing so, it is possible to obtain a preferable conductivity in the range of 10 ⁻² to 10 ⁻⁴ [S / cm].
[0023]
FIG. 2 is a configuration diagram of an evaluation apparatus for examining the gas response of the odor sensor 10. The clean air flow path 20 is provided with a valve 21, a flow cell 23, and a pump 24, and clean air flows through the flow path 20 by suction of the pump 24. A gas flow path connected to the odor substance container 22 is connected to the valve 21 so that an appropriate amount of odor component gas is mixed into clean air by the operation of the valve 21. The odor sensor 10 is disposed in the flow cell 23, and the resistance change of the electrode of the sensor 10 is measured by an ohmmeter 25.
[0024]
First, clean air that has passed through a desiccant (silica gel), activated carbon, and molecular sieves is allowed to flow at a flow rate of 200 mL / min for 10 seconds, thereby desorbing and removing impurities adhering to the sensitive film of the sensor 10. . Thereafter, a gas mixed with butyl acetate as an odor component in clean air is allowed to flow at the same flow rate for 30 seconds. Finally, only clean air is flowed again. During the above procedure, the resistance between the electrodes of the sensor 10 is continuously measured by the resistance meter 25.
[0025]
FIG. 3 is a diagram illustrating an actual measurement result of response characteristics of the odor sensor 10 according to the first embodiment. As shown in FIG. 3, when a gas containing an odor component starts to flow after 10 seconds, the resistance value increases immediately and steeply. That is, the detection response speed is extremely rapid. Moreover, since the difference in resistance value with respect to the presence or absence of an odor component is large, it can be seen that the detection sensitivity is high and it is effective for detecting a small amount of an odor component.
[0026]
Next, the relationship between the response rate to the odor component and the conductivity of the sensitive film 14 was examined. Here, the response rate means the rate of change in resistance between when the odor component is given and when it is not given. In order to create an odor sensor having a sensitive film having different conductivity, a plurality of solutions having different ferric chloride concentrations are prepared, and a substrate on which a poly (3-hexylthiophene) film is formed is formed on each solution. The introduction of the dopant was changed by immersion for the same time. FIG. 4 is a diagram showing actual measurement results of response rates of a plurality of odor sensors (having the same shape) created as described above. FIG. 4 shows that a response rate of about 10% or more is obtained when the conductivity is in the range of 10 ⁻¹ to 10 ⁻⁵ [S / cm]. Furthermore, it can be seen that a high response rate of 20% or more is obtained in the range of conductivity of 10 ⁻² to 10 ⁻⁴ [S / cm]. In the method for forming a sensitive film as described above, it is possible to easily control the conductivity to show such a high response rate.
[0027]
[Example 2]
Next, another example of the odor sensor according to the present invention (hereinafter referred to as “Example 2”) will be described. In Example 1 described above, doping was used to increase the electrical conductivity. However, in Example 2, a part of the polymer structure having regularity was deformed by changing the formation method of the film body itself, or between the polymers. The conductivity is increased by changing the distance.
[0028]
An example of the manufacturing method of the odor sensor 10 of the second embodiment is as follows. First, similarly to Example 1, electrodes 12a and 12b are formed on a glass substrate 11. Further, 3-hexylthiophene (manufactured by Tokyo Chemical Industry Co., Ltd.) is polymerized by an oxidative polymerization method to prepare poly (3-hexylthiophene), which is mixed into a mixed solution of chloroform and dimethyl sulfoxide (ratio is 9: 1) with a concentration of 0. Dissolve at 1 mol / L. Then, this solution is spin-coated on the substrate 11 in the same manner as in Example 1 to form a film body that covers the substrate 11 and the electrodes 12a and 12b. In Example 2, this film body becomes the sensitive film 14 as it is. Chloroform is a soluble solvent and dimethyl sulfoxide is an insoluble solvent for poly (3-hexylthiophene). The latter also has a higher polarity and dielectric constant than the former.
[0029]
Although the polymer structure of the sensitive film 14 formed by the above method is not completely elucidated, a part of the polymer structure arranged with regularity is missing or the distance between the polymers changes. It is presumed that the conductivity of the polymer increases because the electrons easily move inside the polymer.
[0030]
FIG. 5 is a diagram illustrating an actual measurement result of the response characteristic of the odor sensor according to the second embodiment. The measuring method is the same as the method of FIG. As can be seen from FIG. 5, when the gas containing the odor component starts flowing after 10 seconds, the resistance value increases immediately and steeply. Further, since the difference in resistance value with respect to the presence or absence of an odor component is also large as in the first embodiment, high detection sensitivity can be obtained.
[0031]
In the odor sensor of Example 1, the added dopant mainly contributes to electric conduction. For example, when the amount of the dopant drops off from the sensitive film with time and the amount thereof decreases, the conductivity tends to decrease. . However, in the odor sensor of the second embodiment, since the polymer structural change mainly contributes to electric conduction, the sensitive film 14 that is more stable over time is formed. FIG. 6 is a diagram illustrating a measurement result of resistance stability between the sensor of Example 1 and the sensor of Example 2. FIG. 6 shows that the sensor of Example 2 has a smaller resistance change with time and is more stable.
[0032]
In the above-described embodiment, the shape and size of the odor sensor can be changed as appropriate. Also, each numerical value such as the concentration at the time of manufacture can be appropriately changed.
[0033]
【The invention's effect】
As described above, in the odor sensor according to the first invention, a sensitive film made of a conductive polymer can be formed by applying a solution in which the conductive polymer is dissolved on a substrate and drying it. it can. For this reason, the control of the film thickness is easy, and the uniformity and reproducibility are very good. Moreover, since this odor sensor can detect an odor component at room temperature, it can also detect a component that is easily thermally decomposed.
[0034]
Further, in the odor sensor according to the second invention, since the conductivity of the conductive polymer used as the sensitive film is appropriately controlled, a high response characteristic can be obtained with respect to the odor component.
[0035]
Further, if the conductivity of the sensitive film is increased by dissolving the dopant in a solvent and bringing the solution into contact with the sensitive film formed on the substrate, a sensor having high response characteristics can be obtained.
[0036]
Furthermore, a solution in which the conductive polymer is dissolved in a solvent obtained by mixing a solvent in which the conductive polymer is easily soluble and a solvent having higher polarity and dielectric constant than the solvent and in which the conductive polymer is difficult to dissolve. If the conductivity is increased by forming a sensitive film by coating on a substrate, it is possible to obtain a stable sensor exhibiting high response characteristics and little change with time.
[Brief description of the drawings]
FIG. 1 is a configuration diagram of an odor sensor according to an embodiment of the present invention.
FIG. 2 is a configuration diagram of an odor sensor evaluation apparatus.
3 is a graph showing response characteristics of the odor sensor of Example 1. FIG.
FIG. 4 is a diagram showing a difference in response rate of an odor sensor due to a difference in conductivity.
FIG. 5 is a graph showing response characteristics of the odor sensor of Example 2.
6 is a graph showing the change over time in the resistance of the odor sensor of Example 2. FIG.
[Explanation of symbols]
11 ... Substrate 12a, 12b ... Electrode 13a, 13b ... Lead wire 14 ... Sensitive film

Claims (3)

This is an odor sensor for measuring odor by providing a sensitive film between two electrodes formed on an insulating substrate and measuring a resistance change between the electrodes when a target component in a gas adheres to the sensitive film. And
A sensitive film is formed by applying a solution in which a conductive polymer is dissolved in a predetermined solvent on a substrate, and further the dopant is dissolved in the solvent and the solution is brought into contact with the sensitive film to thereby add the dopant to the sensitive film. An odor sensor characterized by adding conductivity to increase conductivity. This is an odor sensor for measuring odor by providing a sensitive film between two electrodes formed on an insulating substrate and measuring a resistance change between the electrodes when a target component in a gas adheres to the sensitive film. And
The conductive polymer is dissolved in a solvent obtained by mixing a first solvent in which the conductive polymer is easily soluble and a second solvent having higher polarity and dielectric constant than the first solvent and in which the conductive polymer is difficult to dissolve. The odor sensor is characterized in that a sensitive film is formed by applying the solution on a substrate. The odor sensor according to claim 1, wherein the sensitive film is made of a conductive polymer having a conductivity of 10 ⁻¹ to 10 ⁻⁵ [S / cm].

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