Method for producing ionic liquid dye, method for producing gas sensor material using the same, gas sensor, compound detection method, and method for examining basicity or acidity of chemical species contained in measurement object gas

Abstract

To provide a method for producing an ionic liquid dye which can clearly grasp a color tone change when brought into contact with a detection object compound even in a state of being impregnated in a base material.SOLUTION: A method for producing an ionic liquid dye includes: an ion exchange step of converting a salt of a cation having one or two or more substituents whose total number of carbon atoms contained in the substituents is 10 to 40 into a hydroxide of the cation by ion exchange; and a reaction step of reacting a dye compound having an acidic group with the hydroxide of the cation to prepare an ionic liquid dye.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing an ionic liquid dye, a method for producing a gas sensor material using the dye, a gas sensor, a method for detecting a compound, and a method for determining the basicity or acidity of a chemical species contained in a gas to be measured. [Background technology]

[0002] It is well known that basic gases such as ammonia and acidic gases such as hydrogen chloride are useful compounds used in the synthesis of various chemical products, and are produced and consumed in large quantities in the modern chemical industry. However, despite their usefulness, it is widely known that these gases are harmful to the human body at high concentrations, and that even at low concentrations, such as those not regulated in the atmosphere by the Offensive Odor Control Law, they can have an impact on the environment.

[0003] For these reasons, various gas sensors are used. For example, Patent Document 1 proposes a nitrogen oxide gas sensor, and Patent Document 2 proposes a semiconductor-type ammonia gas sensor that uses a metal oxide as a detection site. These sensors use electrochemical techniques and are highly sensitive, but they tend to have problems with power sources and installation locations. Furthermore, many of these gas sensors indicate the presence of a detection target in the form of numerical data, making it difficult to intuitively determine its presence.

[0004] Under these circumstances, the present inventors have proposed the use of conjugated compounds that are liquid at room temperature as sensors for acidic and basic gases (see Patent Documents 3 to 5). These conjugated compounds exhibit fluorescence and change the color and intensity of their fluorescence when exposed to acidic gases or ammonia, making them useful for intuitively detecting the presence of these gases by visual inspection. The conjugated compound described in Patent Document 3 is capable of detecting both acidic and basic substances, while the conjugated compound described in Patent Document 4 detects ammonia, a basic gas, and the conjugated compound having a pyridine ring described in Patent Document 5 changes its color and fluorescence emission in the presence of an acid or a base.

[0005] Furthermore, Patent Document 6 describes that a dye is converted into an ionic liquid, and that the ionic liquid dye exhibits a reversible color change when it comes into contact with an acidic compound or a basic compound, and that this ionic liquid dye can be used as a gas sensor. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Special Publication No. 2012-504237 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-071658 [Patent Document 3] Japanese Patent Application Publication No. 2018-076251 [Patent Document 4] Japanese Patent Publication No. 2021-143140 [Patent Document 5] Japanese Patent Application Publication No. 2023-110935 [Patent Document 6] International Publication No. WO2018 / 151311 Summary of the Invention [Problem to be solved by the invention]

[0007] The ionic liquid dye described in Patent Document 6 certainly exhibits a color change when it is brought into contact with an acidic or basic compound in a liquid state, but the inventors' investigations have revealed that when it is impregnated into a substrate such as filter paper, it exhibits a blackish color, and the color change upon contact with an acidic or basic compound cannot be clearly detected, or the sensitivity to the color change is poor. When actually used as a gas sensor material, it is desirable for it to be impregnated into or coated on a substrate for ease of portability and use, and it is problematic that the color change upon contact with the target compound in such a state cannot be clearly detected.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a method for producing an ionic liquid dye that allows a clear understanding of the color change that occurs when the dye comes into contact with a compound to be detected, even when the dye is impregnated into a substrate. [Means for solving the problem]

[0009] The ionic liquid dye described in Patent Document 6 is prepared, for example, by contacting an organic phase containing trihexyltetradecylphosphonium bromide dissolved in a non-aqueous organic solvent as the cation source for the ionic liquid with an aqueous phase consisting of an aqueous sodium hydroxide solution containing a dye compound with an acidic substituent, followed by stirring at room temperature. The dye compound is initially neutralized with sodium hydroxide and present in the aqueous phase as a sodium salt. With stirring, the dye compound undergoes cation exchange with trihexyltetradecylphosphonium bromide present in the organic phase and migrates to the organic phase. Because the trihexyltetradecylphosphonium cation is a bulky cation, the salt formed between this cation and the anion of the dye compound can become an ionic liquid. Therefore, by finally distilling off the organic solvent from the organic phase, the ionic liquid dye is obtained as a salt formed between the trihexyltetradecylphosphonium cation and the anion of the dye compound. The ionic liquid dye thus obtained exhibits a blackish color when impregnated into a substrate, and as already mentioned above, the change in color upon contact with an acidic compound or a basic compound is not clearly noticeable or the sensitivity to the color change is poor.

[0010] The present inventors have discovered that, in preparing an ionic liquid dye, which is a salt of a bulky cation and an anion of a dye compound, rather than cation-exchanging a salt of the dye compound with a salt of the bulky cation in the presence of a strong base compound as in the invention of Patent Document 6, the salt of the bulky cation can be anion-exchanged in advance with an ion-exchange resin to obtain a hydroxide of the cation, and this hydroxide (which exhibits the basicity of the bulky cation) can then be directly neutralized with a dye compound that has acidic properties (i.e., a dye compound that forms a salt with a basic compound) to prepare an ionic liquid dye, or by cation-exchanging a dye compound that is already in the form of a salt, such as a sodium salt, with the cation. The resulting ionic liquid dye does not take on a blackish hue when impregnated into a substrate, and the color change upon contact with an acidic or basic compound is clearly visible. The present invention is based on these findings and provides the following.

[0011] (1) The present invention provides a method for producing an ionic liquid dye, comprising: an ion exchange step of converting a salt of a cation having one or more substituents, the substituents having a total of 10 to 40 carbon atoms, into a hydroxide of the cation by an ion exchange method; and a reaction step of preparing an ionic liquid dye by reacting a dye compound that forms a salt with a basic compound or is already a salt with the hydroxide of the cation.

[0012] (2) The present invention also provides a method for producing an ionic liquid dye according to (1), wherein the salt of the cation is a tetraalkylphosphonium salt in which the total number of carbon atoms contained in the four alkyl groups is 10 to 40.

[0013] (3) The present invention also provides a method for producing an ionic liquid dye according to (2), wherein the tetraalkylphosphonium salt is a trihexyltetradecylphosphonium salt.

[0014] (4) The present invention also provides a method for producing an ionic liquid dye according to any one of (1) to (3), wherein the ion exchange step is carried out using an anion exchange resin.

[0015] (5) The present invention also provides a method for producing an ionic liquid dye according to any one of (1) to (4), wherein the dye compound has a sulfo group, a carboxy group, or a phenolic hydroxyl group.

[0016] (6) The present invention also provides a method for producing an ionic liquid dye according to any one of (1) to (5), wherein the dye compound is bromothymol blue, thymol blue, bromocresol purple, methyl orange, or rose bengal.

[0017] (7) The present invention also relates to a method for producing a gas sensor material, characterized in that the ionic liquid dye obtained by the method for producing an ionic liquid dye according to any one of items (1) to (6) is used as a gas sensor material, or a substrate impregnated with or coated with the ionic liquid dye.

[0018] (8) The present invention also provides a method for producing a gas sensor material according to (7), wherein the object to be detected is an acidic or basic compound.

[0019] (9) The present invention also provides a gas sensor comprising, as a constituent member, the gas sensor material obtained by the manufacturing method described in (7).

[0020] (10) The present invention also provides a compound detection method, which comprises exposing a gas sensor material obtained by the manufacturing method described in (7) to a gas to be measured, and identifying the chemical species actually contained in the gas to be measured from among candidate chemical species that may be contained in the gas to be measured based on the change in color of the gas sensor material that accompanies the exposure.

[0021] (11) The present invention also provides the detection method according to (10), wherein when there are a plurality of chemical species candidates, the chemical species included in the candidates differ from one another in basicity or acidity.

[0022] (12) The present invention also provides the detection method according to (10) or (11), wherein the chemical species included in the candidate chemical species is a volatile amine compound.

[0023] (13) The present invention also provides a method for determining the basicity or acidity of chemical species contained in a gas to be measured, by exposing a gas sensor material obtained by the manufacturing method described in (7) to the gas sensor material and observing the color change of the gas sensor material upon exposure. [Effects of the Invention]

[0024] According to the present invention, there is provided a method for producing an ionic liquid dye that allows a clear understanding of the change in color tone upon contact with a target compound even when the dye is impregnated in a substrate. [Brief explanation of the drawings]

[0025] [Figure 1] FIG. 1 is a chart showing the change in absorption spectrum of the BTB ionic liquid (Example 1) obtained by the production method of the present invention when exposed to hydrochloric acid gas or ammonia gas. [Figure 2] FIG. 2 is a chart showing the change in absorption spectrum of the TB ionic liquid (Example 2) obtained by the production method of the present invention when exposed to hydrochloric acid gas or ammonia gas. [Figure 3] FIG. 3 is a chart showing the change in absorption spectrum of the MO ionic liquid (Example 3) obtained by the production method of the present invention when exposed to hydrochloric acid gas. [Figure 4] FIG. 4 is a chart showing the change in absorption spectrum of the BCP ionic liquid (Example 4) obtained by the production method of the present invention when exposed to hydrochloric acid gas or ammonia gas. [Figure 5] FIG. 5 is a chart showing the change in absorption spectrum of the RB ionic liquid (Example 5) obtained by the production method of the present invention when exposed to hydrochloric acid gas. DETAILED DESCRIPTION OF THE INVENTION

[0026] Hereinafter, one embodiment of the method for producing an ionic liquid dye and the gas sensor material of the present invention, one embodiment of the gas sensor material of the present invention, one embodiment of the method for detecting a compound of the present invention, and one embodiment of the method for determining the basicity or acidity of a chemical species contained in a gas to be measured will be described. Note that the present invention is not limited to the following embodiments and can be practiced with appropriate modifications within the scope of the present invention.

[0027] <Method of manufacturing ionic liquid dye> First, one embodiment of the method for producing an ionic liquid dye of the present invention will be described. Here, the ionic liquid dye is a salt of a dye compound that has become an anion and a cation equipped with a bulky substituent, and is liquid at room temperature. Ionic liquids composed of salts that are liquid at room temperature are widely known, but the ionic liquid dye referred to in the present invention is an ionic liquid in which the anion portion of the ionic liquid is composed of the anion of a dye compound. In the ionic liquid dye, the bulky cations act as steric hindrance, preventing contact between anions and preventing interaction, resulting in a liquid state without crystallization. Therefore, as described above, the ionic liquid dye remains liquid even at room temperature without crystallization. In the present invention, room temperature refers to a temperature of approximately 10°C to 40°C.

[0028] The method for producing an ionic liquid dye of the present invention comprises an ion exchange step of converting a salt of a cation having one or more substituents, each of which has a total of 10 to 40 carbon atoms, into a hydroxide of the cation by ion exchange, and a reaction step of preparing an ionic liquid dye by reacting a dye compound that forms a salt with a basic compound or is already a salt with the hydroxide of the cation. Each step is described below.

[0029] [Ion exchange process] The ion exchange step is a step of converting a salt of a cation having one or more substituents, the total number of carbon atoms of which is 10 to 40, into a hydroxide of the cation by an ion exchange method. The total number of carbon atoms is preferably about 15 to 40, and more preferably about 20 to 40.

[0030] As described above, the salt of the cation used in this step has one or more substituents, each of which has a total of 10 to 40 carbon atoms. These substituents make the cation bulky and contribute to converting the dye compound into an ionic liquid. Examples of the substituent include an alkyl group having about 6 to 20 carbon atoms and which may have a branch, and an alkylaryl group or aralkyl group having such an alkyl group. One or more of these substituents are contained in the cation. For example, when the cation is a quaternary ammonium or quaternary phosphonium, preferred embodiments include one in which four of these substituents are bonded to the nitrogen atom or phosphorus atom, which is the central element of the cation.

[0031] Examples of the cation include phosphonium cations, ammonium cations, pyrazolium cations, pyrrolidinium cations, imidazolium cations, pyridinium cations, sulfonium cations, and piperidinium cations. Among these, phosphonium cations or ammonium cations are preferred, and phosphonium cations are more preferred. For example, examples of the phosphonium cation include tetraalkylphosphonium cations, and examples of such tetraalkylphosphonium cations include, but are not limited to, trihexyltetradecylphosphonium cations represented by the following chemical formula:

[0032] [ka]

[0033] The anions that make up the salts of cations include Cl - , Br - , I - , NCO3 - , CH3COO - , BF4 - , PF6 - , AlCl4 - , Al2Cl7 - , ClO4 - , NO3- , CF3COO - , CH3SO3 - , CF3SO3 - , (CF3SO2)2N - , (CF3SO2)3C - , AsF6 - , SbF6 - , NbF6 - , C4F9SO3 - , (C2F5SO2)2N - , (CF3SO2)(CF3CO)N - Among these, Cl - or Br - For example, if the cation is the trihexyltetradecylphosphonium cation, its chloride (i.e., Cl - salts of Br - and salts) are commercially available.

[0034] The salt of the cation is subjected to anion exchange by an ion exchange method, and converted into the hydroxide of the cation. The ion exchange method can be any known method without particular limitation, but among these, an ion exchange method using an anion exchange resin is preferred.

[0035] Anion exchange resins are resins made of polymers that have positively charged fixed ions in their molecules and negatively charged ions that electrically neutralize them. In the present invention, the negative charges are OH - An anion exchange resin with the following structure is used. Such anion exchange resin can absorb Cl contained in water. - and Br - and other anions, and instead - The anion exchange resin is a styrene or acrylic base material containing quaternary ammonium (-N(CH3)3 + As such an anion exchange resin, various types are commercially available, and such commercially available products may be obtained and used.

[0036] When ion exchange is performed using an anion exchange resin, a sufficient amount of anion exchange resin is added to an aqueous solution of a cation salt and stirred under heating. A preferred temperature is about 50°C, and a preferred stirring time is about 2 to 3 hours. After stirring is completed, the anion exchange resin is removed by filtration, and the water solvent is distilled off from the resulting filtrate. This yields a hydroxide of the cation, in which the cation salt has been exchanged for hydroxide ions.

[0037] The hydroxide of the cation obtained in this step is subjected to a reaction step.

[0038] [Reaction process] The reaction step is a step of preparing an ionic liquid dye by reacting a dye compound that forms a salt with a basic compound or that is already a salt with the hydroxide of the cation.

[0039] The dye compound that forms a salt with a basic compound or is already a salt is not particularly limited, but preferred examples include those that have a sulfo group, a carboxy group, a phenolic hydroxyl group, or a group that is a salt thereof in their structure, i.e., those that have an acidic group or a group that is a salt thereof. Furthermore, the dye compound may contain one or more of these groups. Hereinafter, sulfo groups, carboxy groups, phenolic hydroxyl groups, or groups that are a salt thereof will be collectively referred to as "acidic groups."

[0040] Examples of such dye compounds include azo dyes, porphyrin dyes, phthalocyanine dyes, triphenylmethane dyes, naphthoquinone dyes, anthraquinone dyes, naphthalenetetracarboxylic acid diimide dyes, perylenetetracarboxylic acid diimide dyes, oligophenylene dyes, oxazole dyes, coumarin dyes, phthalein dyes, dicyanomethylene dyes, quinacridone dyes, perylene dyes, quinophthalone dyes, oxazine dyes, polymethine dyes, organometallic complex dyes, photochromic dyes, anthracene, rubrene, diphenyltetracene, diphenylanthracene, and xanthene dyes, which have a parent skeleton and form salts with basic compounds or are already salts. Among these, preferred examples include bromothymol blue (BTB), thymol blue (TB), bromocresol purple (BCP), methyl orange (MO), and rose bengal (RB), which are represented by the following chemical formulas: The following BTB, TB, and BCP have a phenolic hydroxyl group, which is an acidic group, and the following MO and RB have a salt formed by the phenolic hydroxyl group or sulfo group.

[0041] [ka]

[0042] Of the above dye compounds, BTB, TB, BCP, and RB each have two acidic groups, and MO has one acidic group. That is, when BTB, TB, BCP, and RB become one anion molecule, they form an ionic liquid dye with two cation molecules. Furthermore, one MO molecule forms an ionic liquid dye with one cation molecule. When the above dye compounds become ionic liquid dyes, BTB, TB, and BCP change color when they come into contact with an acidic or basic gas, and MO and RB change color when they come into contact with an acidic gas.

[0043] To react a dye compound with a cation hydroxide, the dye compound and the cation hydroxide are dissolved in an organic solvent and stirred. The amount of cation hydroxide used should be equal to the number of equivalents of acidic groups contained in the dye compound. For example, if the dye compound is BTB, which has two acidic groups per molecule, two moles of cation hydroxide are used per mole of BTB. A preferred organic solvent used in the reaction is a mixed solvent of dichloromethane and ethanol. The stirring time can be approximately 12 hours, and the stirring temperature can be approximately 40°C.

[0044] By stirring the dye compound and the hydroxide of the cation in the solvent as described above, a neutralization reaction or a cation exchange reaction occurs between them to form an ionic liquid dye. After the reaction is completed, the solvent is removed, for example, by distillation under reduced pressure, to obtain the ionic liquid dye.

[0045] <Method of manufacturing gas sensor material> The present invention also provides a method for producing a gas sensor material, characterized in that the gas sensor material is the above-mentioned ionic liquid dye itself or a substrate impregnated with or coated with the dye ions. Next, one embodiment of this method will be described.

[0046] As already explained, the ionic liquid dye prepared by the method for producing an ionic liquid dye of the present invention, even when impregnated into a substrate, undergoes a clear, visually observable color change upon contact with a target gas, and the detection sensitivity is also high. Therefore, by using the gas sensor material prepared by the method for producing a gas sensor material of the present invention, it is possible to quickly visually confirm the presence or absence of a target gas.

[0047] The substrate for impregnating the ionic liquid dye is not particularly limited as long as it is liquid-absorbent, and examples thereof include paper, wood, nonwoven fabric, etc. Among these, paper is preferred as the substrate, and among papers, filter paper is particularly preferred.

[0048] Examples of substrates for applying the ionic liquid dye include resin sheets, resin panels, and glass.

[0049] The ionic liquid dye of the present invention exhibits a significant color change when it comes into contact with an acidic or basic compound, and therefore, the gas sensor material of the present invention can preferably detect acidic or basic compounds. Examples of such gases to be detected include aliphatic carboxylic acids such as acetic acid, alkylamines such as methylamine, hydrogen chloride, and SO. x , NO x , ammonia, etc.

[0050] <Gas sensor> A gas sensor including the gas sensor material obtained by the above manufacturing method as a constituent member also constitutes one aspect of the present invention.

[0051] The gas sensor material of the present invention indicates the presence of a target substance as a reversible change in color tone, and can therefore be preferably used as a component of a gas sensor equipped with a mechanism for detecting this color tone change by electrical, optical, or other means. As already explained, the ionic liquid dye obtained by the method for producing an ionic liquid dye of the present invention, unlike the ionic liquid dye described in Patent Document 6, exhibits a clear, visible color change upon contact with an acidic or basic compound. This difference between the ionic liquid dye obtained by the method for producing an ionic liquid dye of the present invention and the ionic liquid dye described in Patent Document 6 is presumed to be due to the presence of trace amounts of impurities contained in the ionic liquid dye. However, to determine the chemical species of these trace impurities, multiple procedures for separating and analyzing each of the various impurities believed to be present must be repeated many times, which requires a significant amount of trial and error and is therefore not practical.

[0052] <Method for detecting compounds> The present invention also provides a compound detection method, which comprises exposing a gas sensor material obtained by the above manufacturing method to a target gas, and identifying the actual chemical species contained in the target gas from among candidate chemical species that may be contained in the target gas based on the color change of the gas sensor material that accompanies the exposure. The present invention also includes an embodiment in which a gas sensor including the gas sensor material obtained by the above manufacturing method as a component is exposed to the target gas to perform similar detection. This is because, when detecting a compound using this gas sensor, the target gas is necessarily exposed to the gas sensor material, which causes a color change of the gas sensor material.

[0053] For example, when the ionic liquid dye of BCP (bromocresol purple) is used as a gas sensor material, the gas sensor material exhibits different color changes when exposed to ammonia gas and when exposed to triethylamine gas. Therefore, when the target gas contains either ammonia gas or triethylamine gas (i.e., the candidate chemical species are ammonia and triethylamine), the color change of the gas sensor material can be observed to determine which gas is contained in the target gas. This detection is possible because the ionic liquid dye of the present invention can signal the presence of the target substance in the form of a clear color change.

[0054] Since this color change occurs significantly depending on the pH, when multiple candidate chemical species are included, it is preferable that the chemical species included in the candidates have different basicities or acidities. In the above example of ammonia gas and triethylamine gas, ammonia gas is more basic than triethylamine gas, and if an ionic liquid dye of BCP is used as a gas sensor material, it will show different color changes when exposed to ammonia gas (changing to blue) and when exposed to triethylamine gas (changing to green).

[0055] Furthermore, preferred examples of chemical species included in the candidate chemical species include volatile amine compounds, such as alkylamines such as trimethylamine, triethylamine, dimethylamine, diethylamine, methylamine, ethylamine, propylamine, and cyclopropylamine, and ammonia.

[0056] <Method for determining the basicity or acidity of chemical species contained in the gas to be measured> The present invention also includes a method for determining the basicity or acidity of chemical species contained in a gas to be measured, based on the color change of the gas sensor material resulting from the exposure, by exposing the gas to the gas sensor material obtained by the above-described manufacturing method. The present invention also includes an embodiment in which the gas sensor includes the gas sensor material obtained by the above-described manufacturing method as a component, and determines the basicity or acidity of chemical species contained in the gas to be measured. This is because, when a measurement is performed using this gas sensor, the gas sensor material is necessarily exposed to the gas to be measured, which causes a change in the color of the gas sensor material.

[0057] As described in the compound detection method above, the ionic liquid dye of the present invention exhibits different color changes depending on the pH of the gas to which it is exposed, i.e., the basicity or acidity. This property is utilized in the present invention. The details of this have already been described, so a detailed explanation will be omitted here. [Example]

[0058] The present invention will be explained in more detail below by showing examples, but the present invention is not limited to the following examples in any way.

[0059] Preparation of cation hydroxides [ka]

[0060] Commercially available trihexyltetradecylphosphonium bromide (2.50 g, 4.43 mmol) was added to deionized water (30 mL) and stirred. 10 g of a strongly basic anion exchange resin (Fujifilm Wako Pure Chemical Industries, Ltd., No. 8) was added and stirred at 50°C for 3 hours. The mixture was then cooled to room temperature, and the anion exchange resin was removed by suction filtration. The filtrate was concentrated under reduced pressure and washed twice with ethanol to obtain the hydroxide of the cation as a pale yellow liquid. Hereinafter, the trihexyltetradecylphosphonium cation will be referred to as [P 66614 ] + The hydroxide of the cation obtained above is also called [P 66614 ] + OH - Also called.

[0061] Preparation of BTB ionic liquid (Example 1) [ka]

[0062] Bromothymol blue (BTB, 0.70 g, 1.10 mmol) and [P 66614 ] + OH - The ionic liquid (1.10 g, 2.20 mmol) was dissolved in a mixture of dichloromethane (DCM, 10 mL) and ethanol (EtOH, 10 mL) and stirred at 40 °C for 12 h. The reaction mixture was then extracted with chloroform and washed three times with saturated brine (50 mL). The solvent was concentrated under reduced pressure to give a deep red BTB ionic liquid (1.50 g, 84% yield).

[0063] Preparation of TB ionic liquid (Example 2) [ka]

[0064] Thymol blue (TB, 1.04 g, 2.34 mmol) and [P 66614 ] + OH -The TB ionic liquid (2.35 g, 4.69 mmol) was dissolved in a mixture of DCM (10 mL) and EtOH (10 mL) and stirred at 40°C for 12 hours. The reaction mixture was then extracted with chloroform and washed three times with saturated brine (50 mL). The solvent was concentrated under reduced pressure to give a deep red TB ionic liquid (yield: 2.48 g, 72%).

[0065] Preparation of MO ionic liquid (Example 3) [ka]

[0066] Methyl orange (MO, 1.0 g, 3.06 mmol) and [P 66614 ] + OH - The product (1.53 g, 3.06 mmol) was dissolved in a mixture of DCM (10 mL) and EtOH (10 mL) and stirred at 40°C for 12 hours. The reaction mixture was then extracted with chloroform and washed three times with saturated brine (50 mL). The solvent was concentrated under reduced pressure to give a deep red MO ionic liquid (yield: 2.10 g, 83%).

[0067] Preparation of BCP ionic liquid 1 (Example 4) [ka]

[0068] Bromocresol purple (BCP, 1.08 g, 2.00 mmol) and [P 66614 ] + OH - The product (2.00 g, 4.00 mmol) was dissolved in a mixture of DCM (10 mL) and EtOH (10 mL) and stirred at 40°C for 12 hours. The reaction mixture was then extracted with chloroform and washed three times with saturated brine (50 mL). The solvent was concentrated under reduced pressure to give an orange BCP ionic liquid (yield 1.53 g, 50%).

[0069] Preparation of RB ionic liquid (Example 5) [ka]

[0070] Rose Bengal (RB, 1.38 g, 1.42 mmol) and [P 66614 ] + OH - The product (1.42 g, 1.77 mmol) was dissolved in a mixture of DCM (10 mL) and EtOH (10 mL) and stirred at 40°C for 12 hours. The reaction mixture was then extracted with chloroform and washed three times with saturated brine (50 mL). The solvent was concentrated under reduced pressure to give a pink BCP ionic liquid (yield 1.53 g, 75%).

[0071] Preparation of BCP ionic liquid 2 (comparison example) [ka]

[0072] Commercially available trihexyltetradecylphosphonium bromide (1.92 g, 3.73 mmol) was dissolved in DCM (10 mL), and BCP (1.0 g, 1.85 mmol) was dissolved in 1 M aqueous NaOH (10 mL). These two solutions were mixed and stirred at 40 °C for 12 hours. The aqueous phase was then discarded, and fresh deionized water (50 mL) was added and stirred three times to wash the organic phase. The organic phase was dried over anhydrous magnesium sulfate, filtered off, and the organic solvent was evaporated to obtain a dark brown BCP ionic liquid (yield: 1.80 g, 65%). In this comparative example, the same dye as in Example 4 was used for ionic liquid conversion; however, the color tone of the resulting ionic liquid dye differed between Example 4 and the comparative example.

[0073] [Acid / basic gas detection test 1] 10.0 mg of each of the ionic liquid dyes obtained in Examples 1 to 5 was weighed out and dissolved in 1.0 mL of dichloromethane. This was dropped onto a glass substrate and spin-coated at 3,000 rpm for 30 seconds to form a thin film. The thin film was exposed to hydrochloric acid gas or ammonia gas, and then the absorption spectrum was measured to examine the ability to recognize acidic and basic gases. The spectral changes obtained by this measurement upon exposure to BTB, TB, MO, BCP, and RB gases are shown in Figures 1 to 5, respectively.

[0074] [Acid / basic gas detection test 2] 10.0 mg of each of the ionic liquid dyes obtained in Examples 1 to 5 was weighed out and dissolved in 2.0 mL of dichloromethane. This was impregnated into filter paper, and the methylene chloride was evaporated. The filter paper was exposed to hydrochloric acid gas or ammonia gas, and the color change was observed. The results are shown in Table 1. As shown in Table 1, it was found that the BTB ionic liquid has the ability to detect hydrochloric acid gas and ammonia gas, the TB ionic liquid has the ability to detect hydrochloric acid gas and ammonia gas, the MO ionic liquid has the ability to detect hydrochloric acid gas, the BCP ionic liquid has the ability to detect hydrochloric acid gas and ammonia gas, and the RB ionic liquid has the ability to detect hydrochloric acid gas.

[0075] [Acid / basic gas detection test 3] 10.0 mg of the BCP ionic liquid dye obtained in the comparative example was weighed out and dissolved in 2.0 mL of dichloromethane. This was impregnated into filter paper, and the methylene chloride was evaporated. The filter paper was exposed to hydrochloric acid gas or ammonia gas, and the color change was observed. The results are shown in Table 1. As also shown in Table 1, the filter paper impregnated with the BCP ionic liquid obtained in the comparative example, unlike Example 4, was gray before exposure, and although a color change was observed when exposed to hydrochloric acid gas, the sensitivity was poor, and no color change was visually observed when exposed to ammonia gas.

[0076] [Table 1]

[0077] As described above, the ionic liquid dye obtained by the production method of the present invention has better sensitivity to color change upon exposure to acidic or basic gases during substrate impregnation than the ionic liquid dye obtained by the method described in Patent Document 6, and it was found that the color change can be easily recognized with the naked eye.

[0078] [Color change test depending on the chemical species exposed] A thin film was formed using the ionic liquid dye of Example 4, and the thin film was exposed to ammonia gas (NH3) or triethylamine gas (NET3). The color tone at that time was expressed in the Lab color system (L * , a * , b * ) and the difference in each parameter before and after the color change (ΔL * , Δa * , Δb * ) and the color difference (ΔE * ab) are shown in Table 2. The thin film of the ionic liquid dye of Example 4 was yellow, but when it was exposed to ammonia gas it turned blue, and when it was exposed to triethylamine gas it turned green. As shown in Table 2, the difference in color tone between when it was exposed to ammonia gas and when it was exposed to triethylamine was measured by the color difference (ΔE * These results indicate that the ionic liquid dye of the present invention has gas selectivity, changing color depending on the gas to which it is exposed, and that this difference can be mechanically distinguished, for example, as a difference in color difference in the Lab color system.

[0079] [Table 2]

Claims

1. an ion exchange step of converting a salt of a cation having one or more substituents, the total number of carbon atoms contained in the substituents being 10 to 40, into a hydroxide of the cation by an ion exchange method; a reaction step of reacting a dye compound that forms a salt with a basic compound or that is already a salt with a hydroxide of the cation to prepare an ionic liquid dye.

2. 2. The method for producing an ionic liquid dye according to claim 1, wherein the salt of the cation is a tetraalkylphosphonium salt having four alkyl groups each having 10 to 40 carbon atoms in total.

3. 3. The method for producing an ionic liquid dye according to claim 2, wherein the tetraalkylphosphonium salt is a trihexyltetradecylphosphonium salt.

4. The method for producing an ionic liquid dye according to claim 1 , wherein the ion exchange step is carried out using an anion exchange resin.

5. The method for producing an ionic liquid dye according to claim 1 , wherein the dye compound has a sulfo group, a carboxy group, or a phenolic hydroxyl group.

6. 2. The method for producing an ionic liquid dye according to claim 1, wherein the dye compound is bromothymol blue, thymol blue, bromocresol purple, methyl orange, or rose bengal.

7. A method for producing a gas sensor material, characterized in that the ionic liquid dye itself obtained by the method for producing an ionic liquid dye according to any one of claims 1 to 6, or a substrate impregnated with or coated with the dye ions, is used as a gas sensor material.

8. 8. The method for producing a gas sensor material according to claim 7, wherein the detection target is an acidic or basic compound.

9. A gas sensor comprising, as a constituent member, the gas sensor material obtained by the manufacturing method according to claim 7.

10. 10. A compound detection method comprising exposing a gas sensor material obtained by the manufacturing method according to claim 7 to a gas to be measured, and identifying a chemical species actually contained in the gas to be measured from among candidate chemical species that may be contained in the gas to be measured based on a change in color of the gas sensor material resulting from the exposure.

11. 11. The detection method according to claim 10, wherein when there are a plurality of chemical species candidates, the chemical species included in the candidates differ from one another in basicity or acidity.

12. The detection method according to claim 10, wherein the chemical species included in the candidate chemical species is a volatile amine compound.

13. A method for determining the basicity or acidity of chemical species contained in a gas to be measured, comprising exposing a gas sensor material obtained by the manufacturing method according to claim 7 to the gas sensor material, and determining the basicity or acidity of chemical species contained in the gas to be measured based on the change in color of the gas sensor material resulting from the exposure.

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