CN116023371B - Preparation method and application of pH probe with D-pi-A structure and changeable color of water-soluble weak acid - Google Patents
Preparation method and application of pH probe with D-pi-A structure and changeable color of water-soluble weak acid Download PDFInfo
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- 238000001514 detection method Methods 0.000 claims abstract description 11
- 239000012954 diazonium Substances 0.000 claims abstract description 8
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- 238000012544 monitoring process Methods 0.000 claims description 8
- LDTCWISGJYTXDC-UHFFFAOYSA-N 5-nitro-1,2-benzothiazol-3-amine Chemical compound C1=C([N+]([O-])=O)C=C2C(N)=NSC2=C1 LDTCWISGJYTXDC-UHFFFAOYSA-N 0.000 claims description 7
- 238000001035 drying Methods 0.000 claims description 7
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- LNOPIUAQISRISI-UHFFFAOYSA-N n'-hydroxy-2-propan-2-ylsulfonylethanimidamide Chemical compound CC(C)S(=O)(=O)CC(N)=NO LNOPIUAQISRISI-UHFFFAOYSA-N 0.000 claims description 4
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- WCUXLLCKKVVCTQ-UHFFFAOYSA-M Potassium chloride Chemical compound [Cl-].[K+] WCUXLLCKKVVCTQ-UHFFFAOYSA-M 0.000 description 4
- CDBYLPFSWZWCQE-UHFFFAOYSA-L Sodium Carbonate Chemical compound [Na+].[Na+].[O-]C([O-])=O CDBYLPFSWZWCQE-UHFFFAOYSA-L 0.000 description 4
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- IAZDPXIOMUYVGZ-WFGJKAKNSA-N Dimethyl sulfoxide Chemical compound [2H]C([2H])([2H])S(=O)C([2H])([2H])[2H] IAZDPXIOMUYVGZ-WFGJKAKNSA-N 0.000 description 2
- LFQSCWFLJHTTHZ-UHFFFAOYSA-N Ethanol Chemical compound CCO LFQSCWFLJHTTHZ-UHFFFAOYSA-N 0.000 description 2
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- Organic Low-Molecular-Weight Compounds And Preparation Thereof (AREA)
- Investigating Or Analyzing Non-Biological Materials By The Use Of Chemical Means (AREA)
- Investigating Or Analysing Materials By The Use Of Chemical Reactions (AREA)
Abstract
The invention discloses a preparation method and application of a pH probe with a D-pi-A structure and changeable color by water-soluble weak acid, and belongs to the technical field of environmental pH detection. The invention prepares the pH probe with the D-pi-A structure shown in the general formula (I) by the following method: (1) preparation of a polycondensate, (2) preparation of a di-polycondensate, (3) preparation of diazonium salt, and (4) coupling reaction, and then salting out and purification are carried out. The pH probe obtained by the invention has simple and easily obtained synthesis process, good water solubility, high sensitivity and good selectivity, and can be used for directly and quickly detecting the pH value of the solution by naked eye observation.
Description
Technical Field
The invention relates to a preparation method and application of a pH probe with a D-pi-A structure and a variable color of water-soluble weak acid, belonging to the technical field of environmental pH detection.
Background
The pH value is used as a measurement index of acidity, and the rapid and accurate measurement of the pH value is particularly important in the processes of environmental protection, human health, biological physiology, chemistry, physics, mining, industrial production and the like. In fact, spoiled foods, such as fish, meat and milk, often have a significantly altered pH compared to fresh foods due to the release of large amounts of metabolites, such as organic acids, biogenic amines, esters and volatile fatty acids. Therefore, monitoring changes in pH would be a quick, convenient, undisturbed and reliable food safety assessment method. The current method for measuring pH mainly comprises a pH test paper measuring method, an electrode method, a sensor and the like. The pH test paper measuring method is simple and easy to operate, but has low accuracy, is greatly influenced by subjective factors and is unfavorable for accurate measurement of the pH value. The electrode method has greatly improved accuracy compared with the pH test paper measurement method, and has been applied commercially for a long time, but has the defects of electrochemical interference, metal ion interference, easy machine damage, cell damage and the like, and has the defects of large error in measurement of strong acid and strong alkali, inapplicability to detection of living pH and food safety and the like. Currently, fluorescent probe methods, ultraviolet-visible absorption methods, gas chromatography-mass spectrometers (GC-MS) and liquid chromatography have been used for food safety monitoring, but they are generally long-term, expensive instruments are used, qualified personnel are required or cannot be used for on-site detection, and such methods are difficult to implement outside a laboratory, so developing a pH probe which is convenient and feasible and can realize real-time detection is still an important research direction.
Disclosure of Invention
The invention provides a preparation method and application of a water-soluble color-changeable pH probe with a D-pi-A structure by improving the used materials. Firstly, a novel benzothiazole azo dye with good water solubility is synthesized through condensation and diazo coupling reaction. A pH-responsive color-changing film for smart packaging was then prepared with aqueous polyurethane (WPU) and a synthetic dye blend. In order to solve the existing food safety problem, the pH response type color-changing film is used for monitoring the freshness of the yoghourt.
The invention provides a pH probe with a D-pi-A structure, which is prepared by condensation, diazotization and coupling reaction and is provided with a water-soluble weak acid, wherein the structure of the probe is shown as a general formula (I):
Wherein R is selected from:
wherein M is H or Na; n=1, 2 or 3.
The invention also provides a preparation method of the pH probe with the D-pi-A structure and changeable color by using the water-soluble weak acid, which comprises the following steps:
(1) Adding a substrate A shown in a formula 1 into cyanuric chloride suspension, and adjusting pH=4-4.5 to perform condensation reaction to obtain a condensed liquid containing a compound shown in a formula 2;
(2) Adding a substrate B shown in a formula 3 into the obtained first condensed liquid to perform condensation reaction to obtain a second condensed liquid containing a compound shown in a formula 4;
(3) Adding 3-amino-5-nitrobenzoisothiazole into concentrated sulfuric acid, then dropwise adding a nitrous acid solution, reacting for a period of time after the dropwise adding is finished, and eliminating excessive nitrous acid by sulfamic acid after the reaction is finished; obtaining diazonium salt shown in formula 5;
(4) Adding the diazonium salt obtained in the step (3) into the binary liquid obtained in the step (2), and adjusting the pH value to 4.5-5 for coupling reaction; after the reaction is finished, salting out, filtering and drying to obtain a pH probe with a D-pi-A structure and a variable color of water-soluble weak acid shown in the general formula (I);
In one embodiment of the invention, in step (1), substrate a is first formulated as an aqueous solution of substrate a and then added to the cyanuric chloride suspension.
In one embodiment of the invention, the substrate A is present in an aqueous solution at a substrate mass concentration of 20% to 25%.
In one embodiment of the invention, in step (1), the molar ratio of cyanuric chloride to substrate A is from 1 to 1.02:1.
In one embodiment of the invention, in step (1), the mass fraction of cyanuric chloride in the cyanuric chloride suspension is 25% -35%.
In one embodiment of the present invention, in step (1), the temperature of the condensation reaction is 0 to 5 ℃; the time is 3-6h.
In one embodiment of the invention, in step (1), the aqueous solution of substrate A has a pH of 5.5 to 6.0 and is obtained by adjusting it with 10% by mass of sodium hydroxide solution.
In one embodiment of the invention, in the step (1), sodium carbonate with the mass fraction of 15% is adopted to adjust the pH value of the obtained reaction to be 4.0-4.5.
In one embodiment of the present invention, in the step (1), the cyanuric chloride suspension is obtained by dispersing cyanuric chloride in water at 0 to 5 ℃ and beating for 0.5 to 0.7 hours.
In one embodiment of the invention, the molar ratio of substrate B in step (2) to substrate A in step (1) is (0.8-1.2): 1; specifically, the ratio of the two components is 1:1.
In one embodiment of the present invention, the condensation reaction temperature in step (2) is 28 to 30℃and the condensation reaction time is 2 to 3 hours.
In one embodiment of the present invention, the pH of the condensation reaction in step (2) is from 4 to 4.5. The pH can be adjusted by using 15% by mass of sodium carbonate.
In one embodiment of the invention, the molar ratio of 3-amino-5-nitrobenzoisothiazole in step (3) to substrate A in step (1) is (0.8-1.2): 1; specifically, the ratio of the two components is 1:1.
In one embodiment of the present invention, the molar ratio of 3-amino-5-nitrobenzoisothiazole to nitrous acid in step (3) is (0.8-1.2): 1.
In one embodiment of the present invention, the mass fraction of the nitrous acid solution in step (3) is 40%.
In one embodiment of the present invention, the temperature of the reaction in step (3) is 25 to 30℃and the time is 1 to 2 hours.
In one embodiment of the invention, the pH of the coupling reaction in step (4) is from 4.5 to 5.0. The pH can be adjusted by 10% sodium hydroxide.
In one embodiment of the present invention, the temperature of the coupling reaction in step (4) is 15 to 25 ℃; the time is 5-6 h.
In one embodiment of the present invention, the salting-out process in step (4) is: weighing potassium chloride accounting for 10% of the total mass of the reaction liquid at the end of the reaction, and salting out; the filtering adopts a suction filtration mode; the drying temperature is 55-60 ℃.
In one embodiment of the present invention, the pH probe has a structure specifically including:
In one embodiment of the present invention, the method for preparing the pH probe having a D-pi-A structure, which is changeable in color by the above-mentioned water-soluble weak acid, comprises:
(1) Adding an aqueous solution of K acid into cyanuric chloride suspension which is uniformly pulped, and adjusting pH to be 4-4.5 for condensation reaction to obtain a condensed liquid;
(2) Adding J acid into the first condensed liquid of R obtained in the step (1), and performing condensation reaction to obtain second condensed liquid;
(3) Adding 3-amino-5-nitrobenzoisothiazole into concentrated sulfuric acid under stirring, dropwise adding nitrous acid sulfuric acid, stirring for reacting for 1-2h after the dropwise adding is finished, and eliminating excessive nitrous acid by sulfamic acid; diazotizing to obtain diazonium salt;
(4) Adding the diazonium salt obtained in the step (3) into the binary liquid obtained in the step (2), regulating the pH value to 4.5-5 for coupling reaction, salting out, filtering and drying to obtain the water-soluble weak acid-changeable heterocyclic pH probe.
The pH probe with the D-pi-A structure and the changeable color of the water-soluble weak acid is different from the existing probe in electron donating and withdrawing groups in coupling components in the synthesis process. In the invention, although only one water-soluble group is arranged at the 7-position of the electron-withdrawing group, the positions of the electron-donating amino group (2-position) and the hydroxyl group (5-position) on the naphthalene ring are far away from each other compared with the prior art, and the electron cloud density of azo nitrogen atoms is relatively weakened, so that the prepared pH probe has better photooxidation resistance. In the structure of the existing probe, the electron withdrawing group contains two water-soluble groups (3 and 6 positions), the electron donating group contains an amino group (1 position) and a hydroxyl group (8 position), and the electron donating group is positioned at the ortho position, so that the electron cloud density of azo nitrogen atoms is increased, and the problem of poor photostability of the obtained probe is caused. From the comparison, it can be assumed that: the weak acid color-changing performance of the pH probe is closely related to the D-pi-A structure formed by azo groups, and the damage of the azo groups can lead to the probe losing the color-changing capability, so that the probe with the structure is more suitable for being used as an outer packaging material.
The application of the pH probe with the D-pi-A structure and the changeable color of the water-soluble weak acid is used for detecting the pH value in the solution and detecting whether the yoghurt is deteriorated or not, and the pH value can be rapidly detected by directly observing the color change of the solution or the pH response type color-changing film through naked eyes without any optical device.
The invention relates to an application of a pH probe with a D-pi-A structure and a changeable color of water-soluble weak acid, which is applied to environmental monitoring, ecological protection, disease monitoring, industrial production and pollution discharge detection.
In the invention, the application of the benzisothiazole water-soluble compound for the pH probe in detecting the pH in an acidic and neutral environment is characterized in that the absorption wavelength of the benzisothiazole water-soluble compound for the pH probe is obviously shifted along with the change of the pH of the solution, so that the pH value of the solution can be rapidly judged by visual inspection of the change of the color of the solution.
The beneficial effects are that:
(1) The benzisothiazole water-soluble compound serving as the pH probe has the advantages of simple preparation process and low cost;
(2) The benzisothiazole water-soluble compound serving as the pH probe has reversibility when being used for detecting the pH of a solution;
(3) The pH of the benzisothiazole water-soluble compound detection solution of the pH probe can be detected rapidly by visual observation directly without using a complex optical device, so that the advantages of simplicity and convenience in operation, rapidness and readability of the traditional colorimetric detection are maintained, and the effect of real-time monitoring which cannot be achieved by most fluorescent probes is realized; when the pH probe and the WPU are mixed to obtain the pH responsive film to detect whether the pH of the solution and the yoghurt are deteriorated, the quick detection of food can be directly realized through naked eye observation.
Drawings
FIG. 1 is a nuclear magnetic resonance hydrogen spectrum of a pH probe having a D-pi-A structure obtained in example 1;
FIG. 2 is an infrared spectrum of a pH probe having a D-pi-A structure obtained in example 1;
FIG. 3 is a graph showing the ultraviolet absorption spectrum of the pH probe obtained in example 1 in aqueous solutions of different pH;
FIG. 4 is a photograph showing the color change of the pH probe obtained in example 1 in aqueous solutions having pH values of 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0, respectively;
FIG. 5 is a photograph showing the color change of the pH-responsive color-changing film obtained by the pH probe obtained in example 1 in aqueous solutions of 3.0 and 10.0, respectively;
FIG. 6 is a photograph showing the color change of the pH responsive color-changing film obtained by the probe of example 1 in yogurt at 5℃and 30℃respectively over the same time.
FIG. 7 is a graph showing ultraviolet absorption spectra of the probe of comparative example 1 in aqueous solutions of different pH;
FIG. 8 is a photograph showing the color change of the probe of comparative example 1 in aqueous solutions having pH values of 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0, respectively;
FIG. 9 is a photograph showing the color change of the pH responsive color-changing film obtained by the probe of comparative example 1 in yogurt at 5℃and 30℃respectively over the same time.
Detailed Description
The application is further described below in conjunction with the detailed description. It is to be understood that these examples are illustrative of the present application and are not intended to limit the scope of the present application. Furthermore, it should be understood that various changes and modifications can be made by one skilled in the art after reading the teachings of the present application, and such equivalents are intended to fall within the scope of the application as defined in the appended claims.
Example 1
The synthesis of the pH probe has the structure:
(1) Preparation of a condensate: 1.92g of cyanuric chloride is beaten in 4.4g of ice-water mixture for 0.5 to 0.7h at 0 ℃. Accurately weighing 3.91g K acid (0.01 mol, structural formula is shown in the specification) ) Preparing 24.2wt% aqueous solution, regulating the pH to 5.5 by using 10wt% sodium hydroxide solution, dripping the aqueous solution into cyanuric chloride which is uniformly pulped within 1 hour at the temperature of 0 ℃, keeping the temperature of 0 ℃ after dripping, reacting for 4 hours at the pH=4.5, and detecting no free amino as a condensation reaction end point by using an amino reagent to obtain a condensation system.
(2) Preparation of the polycondensate: 2.61g J acids (0.01 mol, formula)) Adding the mixture into the polycondensate system obtained in the step (1), regulating the pH value to 4.5, slowly heating to 28 ℃, keeping the pH value and the temperature for reaction, and detecting the end point of the reaction by TLC (thin layer chromatography) detection to obtain a second shrinking liquid.
(3) Preparation of diazonium salt: 1.95g of 3-amino-5-nitrobenzoisothiazole (0.01 mol) is accurately weighed, dissolved in 5ml of concentrated sulfuric acid under stirring, slowly dripped into 3.175g of nitrous acid solution (the mass fraction is 40%) after red powder is dissolved, reacted at room temperature for 2 hours after dripping, and excessive nitrous acid is eliminated by sulfamic acid after reaction, so as to obtain the 3-amino-5-nitrobenzoisothiazole diazonium salt.
(4) Coupling reaction: adding the 3-amino-5-nitrobenzoisothiazole diazonium salt prepared in the step (3) into the two-shrinking liquid prepared in the step (2), adjusting the pH to 5 by using 10% sodium hydroxide, and keeping the temperature at 20 ℃. At this pH, the reaction was carried out at this temperature for 5h, reaching the coupling end point.
(5) Salting out: KCl is weighed according to 10% of the total liquid amount at the end of the reaction, salting out is carried out, the pH probe crude product is obtained by suction filtration and drying, and the yield is 83.5% (calculated by K acid feeding).
(6) Dissolving the crude probe powder in 100mL of ethanol, dropwise adding a proper amount of water, filtering and drying to obtain a pH probe pure product.
Characterization of the substance:
The nuclear magnetic resonance hydrogen spectrum is shown in figure 1, the data is shown as follows ：1H NMR,(DMSO-d6,δH,ppm):15.41(s,1H,–OH),11.20(s,1H,–NH),10.22(s,1H,–NH),9.11(s,1H,Ar–H),8.81(s,1H,Ar–H),8.69(s,1H,Ar–H),8.30(d,J＝8.3Hz,2H),8.13–8.08(m,1H),8.03(s,1H,Ar–H),7.65–7.63(m,1H),7.39(s,1H,Ar–H),7.26(s,1H,Ar–H),7.13–7.09(m,1H)., the infrared spectrum is shown in figure 2, and the data is shown as follows: 3431,3060,1741,1607,1541,1488,1425,1309,1106,1029,833,794cm -1.
PH responsiveness of the probe:
Absorption spectra of probes in aqueous solutions of different pH: buffer solutions containing the reactive dyes of example 1 at different pH values (pH 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) were prepared, respectively, and the probe concentration was 1.0g/L. The UV absorption spectra of the solutions at various pH values were measured and the results are shown in FIG. 3, in which as the pH increases, the absorption intensity of the solution at 534nm becomes weaker, a new absorption peak at 663nm appears and increases gradually, and the wavelength is red shifted by 129nm. And the absorption spectrum of the dye has an isochromatic point at 590 nm.
Naked eye chromogenic recognition of probes for aqueous solutions of different pH values: buffer solutions containing the probes of example 1 at pH values of 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0 were prepared, and the concentration of the probe was 1.0g/L. The color of the solution at different pH values was visually observed and photographed by a camera, and as a result, see FIG. 4, the color of the dye solution was clearly changed from reddish to green when the pH value was increased from 3 to 10, and the UV-Vis spectral response of the dye in the aqueous solution at different pH values was consistent with visual observation.
PH-responsive color-changing film: first, the volume ratio of aqueous polyurethane (WPU) to water was set to 20:1, and the mass ratio of ph probe to water was 0.0056:1. Specifically, 10g of WPU containing sulfonic acid groups was mixed with 0.5mL of an aqueous solution containing 2.8mg of a pH probe at room temperature, stirred at 200 rpm for 45 minutes, and then deaerated by a reduced pressure method for 15 minutes. Obtaining a dispersion liquid; the dispersion was then spread evenly on a 6cm diameter petri dish, dried at 60 ℃ for 12 hours, and the film was peeled off from the petri dish after drying to obtain a pH-responsive color-changing film, which was stored in a dark environment with 30% humidity for use.
And testing the naked eye color development recognition of the pH responsive color-changing film on water solutions with different pH values: the pH-responsive color-changing film was immersed in a buffer solution having a pH of 3.0 and 10.0. The color change of the pH responsive color-changing film in the aqueous solution of different pH values was visually observed and photographed by a camera, and the result is shown in FIG. 5.
Naked eye color development identification for detecting milk deterioration by using pH responsive color-changing film: the pH-responsive color-changing film was immersed in two cups of the same yogurt, and after storage at 5 ℃ and 30 ℃ for 36 hours, the pH-responsive color-changing film had a value of L x a x b as shown in table 1, and the color difference value of the modified sample after storage at 30 ℃ for 36 hours was increased from 4.79 to 22.27 when the red light (a x 0) was increased to 17.65 as compared with the fresh yogurt (blank). These results are consistent with the apparent red color on the pH-responsive color-changing film. As the color of the film changes significantly with changes in pH, it is shown that the prepared pH-responsive color-changing film can be successfully used as part of a smart packaging system. The color of the pH responsive color-changing film before and after the yogurt was spoiled was visually observed and photographed by a camera, and the results are shown in FIG. 6 and Table 1.
TABLE 1
Comparative example 1
And (3) probe:
Absorption spectra of probes in aqueous solutions of different pH: buffer solutions containing the reactive dyes of example 1 at different pH values (pH 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) were prepared, respectively, and the probe concentration was 1.0g/L. The UV absorption spectra of the solutions at the various pH values were measured and as shown in FIG. 7, the absorption intensity of the solution at 590nm was decreased with increasing pH, a new absorption peak was developed at 665nm and gradually increased, and the absorption spectrum of the dye had an isochromatic point at 625 nm.
Naked eye chromogenic recognition of probes for aqueous solutions of different pH values: the pH values of the probes with the above-described structure were 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0, respectively, and the concentration of the probes was 1.0g/L. The color of the solution with different pH values is observed by naked eyes, and can be seen: as the pH increases from 3 to 10, the color of the dye solution changes from blue to green, as shown in fig. 8, and the UV-Vis spectral response of the dye in aqueous solutions at different pH is consistent with visual observations. However, the maximum absorption wavelength of the probe in the prior invention is only red-shifted by 75nm along with the change of the pH value, and the maximum absorption wavelength of the probe is red-shifted by 129nm, which proves that the color change of the probe is more obvious and the detection is easier to be recognized by naked eyes. When the pH value is 4.0, 5.0 and 6.0, the color of the probe solution of the invention is obviously changed from red to brown to green, and the color of the solution is blue, thus further proving that the probe of the invention has better responsiveness under the weak acid condition.
The corresponding pH-responsive color-changing film was obtained by the same method using the probe of the above-described structure. The values of L a b in the modified samples and fresh yogurt (blank) after the pH-responsive color-changing films were each stored at 5 ℃ and 30 ℃ for 36 hours were measured, and the results are shown in table 2. The modified samples of the pH-responsive color-changing film after 36h storage at 30 ℃ increased from 1.27 to 1.59 for green light (a < 0) with a color difference value of only 3.25 compared to fresh yogurt (blank), which also indicated that the color on the pH-responsive color-changing film did not change significantly with pH, and all showed blue color. The color of the pH responsive color-changing film before and after deterioration of the yogurt was visually observed and photographed by a camera, and the results are shown in FIG. 9 and Table 2.
TABLE 2
Claims (10)
1. A water-soluble weak acid-variable pH probe having a D-pi-a structure, characterized in that the probe has the structure shown below:
。
2. the method for preparing a pH probe having a D-pi-A structure, which is changeable in color by a water-soluble weak acid as claimed in claim 1, comprising the steps of:
(1) Adding a substrate A shown in a formula 1 into cyanuric chloride suspension, and adjusting pH=4-4.5 to perform condensation reaction to obtain a condensed liquid containing a compound shown in a formula 2;
(2) Adding a substrate B shown in a formula 3 into the obtained first condensed liquid to perform condensation reaction to obtain a second condensed liquid containing a compound shown in a formula 4;
(3) Adding 3-amino-5-nitrobenzoisothiazole into concentrated sulfuric acid, then dropwise adding a nitrous acid solution, reacting for a period of time after the dropwise adding is finished, and eliminating excessive nitrous acid by sulfamic acid after the reaction is finished; obtaining diazonium salt shown in formula 5;
(4) Adding the diazonium salt obtained in the step (3) into the binary liquid obtained in the step (2), and adjusting the pH value to 4.5-5 for coupling reaction; after the reaction is finished, salting out, filtering and drying to obtain a pH probe with a D-pi-A structure and changeable color of water-soluble weak acid;
、、、、，
R is 。
3. The preparation method according to claim 2, wherein in the step (1), the molar ratio of cyanuric chloride to the substrate A is 1-1.02:1.
4. The preparation method according to claim 2, wherein the temperature of the condensation reaction is 0-5 ℃; the time is 3-6h.
5. The process according to claim 2, wherein the molar ratio of substrate B in step (2) to substrate A in step (1) is from (0.8 to 1.2): 1.
6. The preparation method according to claim 2, wherein the condensation reaction temperature in the step (2) is 28-30 ℃ and the condensation reaction time is 2-3 hours.
7. The process according to claim 2, wherein the molar ratio of 3-amino-5-nitrobenzoisothiazole to nitrous acid in step (3) is (0.8-1.2): 1.
8. The preparation method according to claim 2, wherein the reaction temperature in the step (3) is 25-30 ℃ and the reaction time is 1-2 hours.
9. The preparation method according to any one of claims 2 to 8, wherein the temperature of the coupling reaction in step (4) is 15 to 25 ℃; the time is 5-6 h.
10. The use of a water-soluble weak acid-variable pH probe with a D-pi-A structure in the preparation of environmental monitoring, ecological protection, disease monitoring, industrial production and pollution discharge detection reagents according to claim 1.
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