Preparation method and application of allochroic dye based on bisazo structure

Abstract

The invention discloses a preparation method and application of an allochroic dye based on a bisazo structure, and belongs to the technical field of environment pH detection. The method comprises the following steps: adding 4-amino-5-hydroxy-2, 7-naphthalene disulfonic acid into a cyanuric chloride suspension, adjusting the pH value to 4-4.5, and carrying out a condensation reaction to obtain a first condensation solution; 4-amino-5-hydroxy-2, 7-naphthalene disulfonic acid is added into the first condensation solution for a condensation reaction, and a second condensation solution is obtained; and adding a diazonium salt into the secondary condensation liquid, adjusting the pH value to 1.0-2.0, carrying out a coupling reaction, and after the reaction is finished, purifying to obtain the bisazo color-changeable dye. The obtained color-changing dye is simple in synthesis process, easy to obtain, good in water solubility, high in sensitivity and good in selectivity, and the pH value of a solution can be rapidly detected through direct visual inspection.

Description

TECHNICAL FIELD

[0001] The present application relates to a preparation method of a color-changing dye based on a disazo structure and its application, and belongs to the technical field of detecting environmental pH changes. BACKGROUND

[0002] In recent years, azo dyes have been widely used in pH sensing due to their adjustable molecular structure, high molar extinction coefficient, and strong color development characteristics. Among them, disazo dyes significantly improve spectral response performance by expanding the molecular conjugated system. Compared with monoazo dyes, the disazo structure can cause a red shift of more than 100 nm in the maximum absorption wavelength (e.g., from 513 nm to 606 nm), significantly enhancing the naked-eye recognition. A study by Kursk State University in 2024 (Novel halochromic disazostilbene dye with intramolecular color mixing effect for textile sensor material) showed that disazo stilbene dyes exhibited a clear color gradient (green → orange → purple) in the pH range of 3-10, with a response time of only 3 seconds, making them suitable for textile sensor materials.

[0003] However, existing disazo dyes still have the following problems:

[0004] (1) Complex synthesis process: Most require multi-step diazo-coupling reactions, and the yield is low;

[0005] (2) Poor water solubility: Although the introduction of sulfonic acid groups can improve water solubility, it often sacrifices color fastness or color change sensitivity, such as the commonly used Runba HRR red disazo solvent dye for plastic rubber, Solvent Red 23, which has poor color fastness;

[0006] (3) Poor interval response: The color change is weak within a certain range, and the wavelength change range is small, making it difficult to meet the needs of food packaging, biological fluid detection, and other scenarios; for example, the color-changing reactive dye in patent CN 115368753 A only changes color at a pH of 6-7, and the color transition is not obvious, making it difficult to meet the needs of high-precision detection, and the study by Kursk State University also only exhibits a clear color change in the pH range of 3-10.

[0007] In addition, although patents CN 118290962 A, CN 115725189 A, and CN 117586642 A all mention a structure of disazo or polyazo dyes, none of them indicate that the dyes have the function of pH-responsive color change. SUMMARY

[0008] TECHNICAL PROBLEM

[0009] The double azo dyes in the prior art have problems of complex synthesis process, poor water solubility, poor interval response, etc., and therefore it is necessary to develop a double azo dye with simple synthesis, good water solubility, and high sensitivity in the weak acid interval.

[0010] Technical content

[0011] The present application solves the problems of the prior art by optimizing the molecular structure (introducing hydrophilic sulfonic acid groups and strong electron-withdrawing groups CN / NO2) and using a step-by-step condensation-diazo coupling process to successfully prepare a new nitro-containing double azo dye with obvious color gradient (red→purple→blue) and excellent water solubility in the pH 1-12 range.

[0012] In addition, the present application also uses polyethylene glycol (PVA) and synthetic dyes to prepare a pH-responsive color-changing film for acid-base environment detection by physical blending. The color-changing film can solve the current industrial pollution gas leakage, and the pH-responsive color-changing film is used for hydrochloric acid vapor and ammonia gas detection.

[0013] The present application provides a double azo color-changing dye for pH detection, and the structure of the dye is shown as formula I:

[0014]

[0015] In the formula, R is selected from

[0016] wherein M is H or Na; m=1, 2; n=1, 2 or 3; R' is selected from cyano (-CN) or nitro (-NO2).

[0017] Further, R is selected from

[0018] Specifically, R is selected from and R' is selected from cyano, or R is selected from and R' is selected from nitro.

[0019] Preferably, R is selected from R' is selected from cyano.

[0020] The present application also provides a preparation method of the above-mentioned double azo color-changing dye, comprising the following steps:

[0021] (1) 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid is added to a cyanuric chloride suspension, the pH is adjusted to 4-4.5 for condensation reaction to obtain a condensation liquid;

[0022] (2) 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid is added to the condensation liquid for condensation reaction to obtain a di-condensation liquid;

[0023] (3) adding diazonium salt into the di-condensed solution, adjusting pH value to 1.0-2.0 to carry out coupling reaction, and after the reaction is completed, purifying to obtain the bis-azo variable color dye.

[0024] Further, the structural formula of 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid is as follows:

[0025]

[0026] Further, the mass fraction of cyanuric chloride in the cyanuric chloride suspension in step (1) is 20%-25%.

[0027] Further, the cyanuric chloride suspension in step (1) is obtained by dispersing cyanuric chloride in water at 0-5 ℃ and beating up for 0.5-0.7 h.

[0028] Further, the molar ratio of cyanuric chloride to 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (1) is 1-1.02:1.

[0029] Further, the 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (1) needs to be prepared into an aqueous solution with a concentration of 20-25 wt% first, and then slowly added into the cyanuric chloride suspension within 40-80 min.

[0030] Further, the temperature of the condensation reaction in step (1) is 0-5 ℃, and the reaction time is 4-5 h.

[0031] Further, the pH value of the reaction in step (1) is adjusted to 4.0-4.5 by using sodium carbonate with a mass fraction of 20%.

[0032] Further, the structural formula of the product obtained in the mono-condensed solution in step (1) is as follows:

[0033]

[0034] Further, the addition amount of 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (2) is 80-120 wt% of the addition amount of 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (1); specifically, 100 wt%.

[0035] Further, the condensation reaction temperature in step (2) is 30-35 ℃, and the condensation reaction time is 2-3 h.

[0036] Further, the pH value of the condensation reaction in step (2) is 4-4.5.

[0037] Further, the structural formula of the product obtained in the di-condensed solution in step (2) is as follows:

[0038]

[0039] Further, the temperature of the coupling reaction in step (3) is 20-25℃, and the coupling time is 5-6h.

[0040] Further, the pH value of the coupling reaction in step (3) is 1.0-2.0.

[0041] Further, the purification in step (3) can adopt column chromatography, thin layer chromatography, high performance liquid chromatography and the like.

[0042] Specifically, the purification in step (3) is salting-out, filtration and drying; the salting-out is salting-out with 10% of the total mass of the reaction solution of potassium chloride; the filtration adopts suction filtration; and the drying temperature is 60-70℃.

[0043] Further, the structure of the diazonium salt in step (3) is as shown in the following formula:

[0044]

[0045] In the formula, R' is selected from cyano (-CN) or nitro (-NO2).

[0046] Further, the molar ratio of the diazonium salt in step (3) to 4-amino-5-hydroxy-2,7-naphthalenedisulfonic acid in step (1) is 1.8-2.2:1; and specifically, it is 2:1.

[0047] In an embodiment of the present application, the synthesis method of the diazonium salt is as follows:

[0048] 2-cyano-4-nitroaniline or 2,4-dinitroaniline is added into concentrated sulfuric acid, and then nitrous acid sulfuric acid is added dropwise; after the dropwise addition is completed, the reaction is carried out for a period of time to obtain the diazonium salt.

[0049] In an embodiment of the present application, the molar ratio of 2-cyano-4-nitroaniline or 2,4-dinitroaniline to nitrous acid sulfuric acid is 0.8-1.2:1.

[0050] In an embodiment of the present application, the mass fraction of the nitrous acid sulfuric acid is 30-50%.

[0051] In an embodiment of the present application, the reaction temperature is 20-25℃, and the time is 1-2h.

[0052] The double azo changeable dye provided by the present application is applied in the field of textile dyeing or the field of pH detection.

[0053] The application further provides a pH-responsive color-changing film, which is composed of a polymer, water and a bis-azo color-changing dye.

[0054] Further, the polymer comprises one or more of polyethylene glycol, gelatin and aqueous polyurethane.

[0055] The application further provides a preparation method of the pH-responsive color-changing film, comprising the following steps:

[0056] The polymer is mixed with water, and then the bis-azo color-changing dye is added and mixed to obtain a dispersion liquid; and then the dispersion liquid is uniformly poured into a mold for drying to obtain the pH-responsive color-changing film.

[0057] Further, the mass ratio of the polymer to water is 5-10:100.

[0058] Further, the mass ratio of the bis-azo color-changing dye to water is 0.05-0.1:100.

[0059] Further, the drying is performed at a temperature of 30-80 DEG C for 5-30 h.

[0060] In the application, the bis-azo color-changing dye for pH indicator is used to detect the pH in acidic and alkaline environments, and the mechanism is that the bis-azo color-changing dye for pH indicator has a significant shift in the absorption wavelength with the change of the solution pH, so that the pH value of the solution can be quickly judged by visually observing the color change of the solution. The application scenarios are not limited to environmental monitoring, ecological protection, disease monitoring, industrial production or pollution detection.

[0061] Advantages

[0062] (1) The bis-azo color-changing dye provided by the application has simple preparation process, low cost, good water solubility and excellent water solubility. Meanwhile, the bis-azo color-changing dye can be used for pH detection, has high sensitivity and reversibility.

[0063] (2) The bis-azo color-changing dye of the application can detect the pH of the solution without the aid of complex optical devices, and the rapid detection of pH can be realized by direct visual observation, which not only retains the advantages of simple operation, rapid reading and easy reading of traditional colorimetric detection, but also realizes the real-time monitoring effect that most pH indicators cannot achieve. In addition, when the pH-responsive film obtained by mixing the pH probe and PVA detects the pH of the solution, whether the acid and alkali gas leaks or not, the rapid monitoring of the acid and alkali gas can also be realized by direct visual observation. BRIEF DESCRIPTION OF DRAWINGS

[0064] Figure 1 is the nuclear magnetic hydrogen spectrum of the dye with a bis-azo structure obtained in Example 1;

[0065] Figure 2 is an infrared spectrum of the dye with a disazo structure obtained in Example 1;

[0066] Figure 3 is a UV absorption spectrum of the dye with a disazo structure obtained in Example 1 in aqueous solutions at different pH values;

[0067] Figure 4 is a color change photo of the pH indicator obtained in Example 1 in aqueous solutions at pH values of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, and 12.0, respectively;

[0068] Figure 5 is a color change photo of the pH-responsive color-changing film prepared from the pH indicator obtained in Example 1 in aqueous solutions at pH values of 4.0 and 11.0, respectively;

[0069] Figure 6 is a color change of the pH-responsive color-changing film prepared from the pH indicator obtained in Example 1 after exposure to an acid or base gas environment;

[0070] Figure 7 is a nuclear magnetic hydrogen spectrum of the dye with a disazo structure obtained in Example 2;

[0071] Figure 8 is an infrared spectrum of the dye with a disazo structure obtained in Example 2;

[0072] Figure 9 is a UV absorption spectrum of the dye with a disazo structure obtained in Example 2 in aqueous solutions at different pH values;

[0073] Figure 10 is a color change photo of the pH indicator obtained in Example 2 in aqueous solutions at pH values of 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, and 12.0, respectively;

[0074] Figure 11 is a color change photo of the pH-responsive color-changing film prepared from the pH indicator obtained in Example 2 in aqueous solutions at pH values of 4.0 and 11.0, respectively;

[0075] Figure 12 is a color change of the pH-responsive color-changing film prepared from the pH indicator obtained in Example 2 after exposure to an acid or base gas environment.

[0076] Figure 13 is a structural formula of the disazo variable color dye of the present application. DETAILED DESCRIPTION

[0077] The application will be further described below in connection with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not to limit the scope of the application. Furthermore, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content of the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0078] The H acid involved in the following examples of the application refers to 4-amino-5-hydroxy-2,7-naphthalenedisulfonic acid, and the corresponding structure is shown as follows:

[0079]

[0080] Example 1

[0081] Synthesis of a bisazo dye with the following structure:

[0082]

[0083] (1) Preparation of a mono-condensate: 1.96 g of cyanuric chloride is slurried with 5.88 g of deionized water at 0°C for 0.5-0.7 h. 3.99 g of H acid (0.01 mol) is accurately weighed and dissolved in 25 wt% aqueous solution, and the pH is adjusted to 4.5 with 10 wt% sodium hydroxide solution. The slurry of cyanuric chloride is added dropwise into the solution at 0°C within 1 h, and the temperature is maintained at 0°C and the pH is maintained at 4.5 for 4 h after the addition is completed. The reaction is terminated when no free amino group is detected by an amino reagent, and a mono-condensate system is obtained.

[0084] (2) Preparation of a di-condensate: 3.99 g of H acid (0.01 mol) is added to the mono-condensate system obtained in step (1), and the pH is adjusted to 4.5. The temperature is slowly increased to 30°C, and the pH and temperature are maintained for 3 h after the addition is completed. The reaction is terminated when no free amino group is detected by an amino reagent, and a di-condensate solution is obtained.

[0085] (3) Preparation of a diazonium salt: 3.36 g of 2-cyano-4-nitroaniline (0.02 mol) is accurately weighed and dissolved in 10 mL of concentrated sulfuric acid under stirring. After the solution is clear, 6.35 g of nitrous acid sulfuric acid (40 wt%) is slowly added dropwise, and the reaction is carried out at room temperature for 2 h to obtain a 2-cyano-4-nitroaniline diazonium salt.

[0086] (4) Coupling reaction: the 2-cyano-4-nitroaniline diazonium salt prepared in step (3) is added to the di-condensate solution prepared in step (2), and the pH is adjusted to 1.5 with 10% sodium hydroxide. The reaction is carried out at 20°C for 5 h to reach the coupling end point.

[0087] (5) Salting-out: KCl was weighed according to 10% of the total liquid volume at the end of the reaction, and the crude product of the disazo dye was obtained by filtration and drying, with a yield of 51.44% (calculated based on the amount of H acid).

[0088] (6) The crude probe powder obtained above was dissolved in 100 mL of ethanol, and an appropriate amount of water was added dropwise, and the pure product of the disazo dye was obtained by filtration and drying.

[0089] Characterization of the substance:

[0090] The proton nuclear magnetic resonance spectrum thereof is shown in Figure 1 , and the data are as follows: 1H NMR (400 MHz, DMSO-d6) δ 15.34-15.53 (m, 2H), 10.81 (s, 1H), 10.52 (s, 1H), 8.82 (d, J = 2.6 Hz, 2H), 8.78-8.75 (m, 2H), 8.42 (d, J = 8.9 Hz, 2H), 8.00 (d, J = 1.6 Hz, 2H), 7.60 (d, J = 15.4 Hz, 2H), 7.16 (s, 2H).

[0091] The infrared spectrum thereof is shown in Figure 2 , and the data are as follows: 3447; 3032; 2154; 1617; 1556; 1335; 1032 cm-1.

[0092] pH responsiveness of the indicator:

[0093] Absorption spectrum of the probe in different pH aqueous solutions: different pH (pH 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0) buffer solutions containing the reactive dye in Example 1 were prepared, and the concentration of the dye was 1.0 g / L. The ultraviolet absorption spectrum of each different pH solution was determined, and the results are shown in Figure 3 . With the increase of pH, the absorption intensity of the solution at 513 nm became weaker, a new absorption peak appeared at 606 nm and gradually increased, with a red shift of 93 nm. Moreover, the absorption spectrum of the dye had an isochromatic point at 540 nm.

[0094] Naked-eye color recognition of the indicator for different pH aqueous solutions:

[0095] Different pH (pH 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0) buffer solutions containing the probe in Example 1 were prepared, and the concentration of the probe was 1.0 g / L. The color of the solution at different pH values was observed by naked eye and photographed by camera, and the results are shown in Figure 4As shown, the color of the dye solution changed obviously from pink to blue as the pH value increased from 1 to 12. The UV-Vis spectral response of the dye in different pH aqueous solutions was consistent with the naked eye observation.

[0096] pH-responsive color-changing film:

[0097] First, polyethylene glycol (PVA) was physically blended with water, PVA accounted for 8%, and the mass ratio of pH probe and water was 0.0008:1. Specifically, 8 g of PVA was mixed with 100 mL of aqueous solution containing 0.08 g of pH probe at room temperature, stirred at a speed of 200 revolutions per minute for 45 minutes, and then degassed by reduced pressure for 15 min. The dispersion was obtained; then the dispersion was uniformly poured into a 10 cm diameter polytetrafluoroethylene culture dish, the film liquid mass was 25 g, and the film was dried at a temperature of 50°C for 24 h. After drying, the film was peeled off from the culture dish to obtain the pH-responsive color-changing film, and stored in a dark environment with a humidity of 40% for standby.

[0098] The naked-eye color recognition of the pH-responsive color-changing film to different pH aqueous solutions was tested: the pH-responsive color-changing film was immersed in buffer solutions with pH values of 4.0 and 11.0. The color change of the pH-responsive color-changing film in different pH aqueous solutions was observed by naked eye, and photographed by camera, and the results are shown in Figure 5 .

[0099] The naked-eye color recognition of the pH-responsive color-changing film to different pH aqueous solutions was tested: the pH-responsive color-changing film was immersed in buffer solutions with pH values of 4.0 and 11.0. The color change of the pH-responsive color-changing film in different pH aqueous solutions was observed by naked eye, and photographed by camera, and the results are shown in Figure 6 .

[0100] Example 2

[0101] Synthesis of a bis-azo dye with the following structure:

[0102]

[0103] (1) Preparation of the mono-condensation product: 1.96 g of cyanuric chloride was slurried with 5.88 g of deionized water at 0°C for 0.5-0.7 h. 3.99 g of H acid (0.01 mol) was accurately weighed and dissolved in 25 wt% aqueous solution. The pH was adjusted to 4.5 with 10 wt% sodium hydroxide solution, and the slurry of cyanuric chloride was added dropwise at 0°C over 1 h. After the addition was completed, the temperature was maintained at 0°C and the pH was maintained at 4.5 for 4 h. The reaction was terminated when no free amino group was detected by the amino reagent. The mono-condensation product was obtained.

[0104] (2) Preparation of the di-condensation product: 3.99 g of H acid (0.01 mol) was added to the mono-condensation product obtained in step (1), and the pH was adjusted to 4.5. The temperature was slowly increased to 30°C, and the pH and temperature were maintained for 3 h. The reaction was terminated when no free amino group was detected by the amino reagent. The di-condensation product was obtained.

[0105] (3) Preparation of the diazonium salt: 3.66 g of 2,4-dinitroaniline (0.02 mol) was accurately weighed and dissolved in 10 mL of concentrated sulfuric acid under stirring. After the solution was clear, 6.35 g of nitrous acid sulfuric acid (40% by mass) was slowly added dropwise. The reaction was carried out at room temperature for 2 h to obtain the 2,4-dinitroaniline diazonium salt.

[0106] (4) Coupling reaction: The 2,4-dinitroaniline diazonium salt prepared in step (3) was added to the di-condensation product prepared in step (2). The pH was adjusted to 1.5 with 10% sodium hydroxide, and the temperature was maintained at 20°C. The reaction was carried out at this pH and temperature for 5 h to reach the coupling end point.

[0107] (5) Salting-out: KCl was weighed at 10% of the total liquid volume at the end of the reaction, and the crude product was obtained by filtration and drying. The yield was 52.06% based on the amount of H acid used.

[0108] (6) The crude product was dissolved in 100 mL of ethanol, and an appropriate amount of water was added dropwise. The pure product was obtained by filtration and drying.

[0109] Characterization of the substance:

[0110] The proton nuclear magnetic resonance spectrum of the substance is shown in Figure 7, the data are as follows: 1H NMR (400 MHz, DMSO-d6) δ 15.79 (d, J = 25.7 Hz, 2H), 11.17 (s, 2H), 8.94 (d, J = 2.6 Hz, 2H), 8.61 (d, J = 2.8 Hz, 2H), 7.99 (d, J = 1.6 Hz, 2H), 7.60 (s, 2H), 7.27 (d, J = 11.1 Hz, 2H), 7.13 (d, J = 2.9 Hz, 2H).

[0111] The infrared spectrum thereof is shown in Figure 8 , the data are as follows: 3448; 2979; 1742; 1532; 1326; 1032 cm -1 .

[0112] pH response of the indicator:

[0113] Absorption spectra of the probe in aqueous solutions of different pH values: different pH value (pH 1.0, 2.0, 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0, 11.0, 12.0) buffer solutions containing the active dye in Example 1 were configured, and the concentration of the dye was 1.0 g / L. The UV absorption spectra of each different pH value solution were determined, and the results are shown in Figure 9 . With the increase of pH, the absorption intensity of the solution at 517 nm became weak, the maximum absorption wavelength red shifted to 540 nm, and at the same time a new peak was generated at 626 nm. Moreover, the absorption spectrum of the dye had an isochromatic point at 531 nm.

[0114] Naked-eye color recognition of the indicator to aqueous solutions of different pH values:

[0115] pH value buffer solutions containing the probe in Example 1 were configured, and the concentration of the probe was 1.0 g / L. The colors of the solutions of different pH values were observed by naked eye and photographed by a camera, and the results are shown in Figure 10 . When the pH value increased from 1.0 to 12.0, the color of the dye solution changed obviously from red to purple, and the UV-Vis spectral response of the dye in aqueous solutions of different pH values was consistent with the naked-eye observation.

[0116] pH-responsive color-changing film:

[0117] First, polyethylene glycol (PVA) was physically blended with water, PVA accounted for 8%, and the mass ratio of pH probe and water was 0.0008:1. Specifically, 8 g of PVA was mixed with 100 mL of an aqueous solution containing 0.08 g of pH probe at room temperature, stirred at a speed of 200 revolutions per minute for 45 minutes, and then degassed by the reduced pressure method for 15 minutes. A dispersion liquid was obtained; then the dispersion liquid was uniformly poured into a 10 cm diameter polytetrafluoroethylene culture dish, the film liquid mass was 25 g, and the film was dried at a temperature of 50°C for 24 h. After drying, the film was peeled off from the culture dish to obtain a pH-responsive color-changing film, and stored in a dark environment with a humidity of 40% for standby.

[0118] The naked-eye color recognition of the pH-responsive color-changing film to different pH aqueous solutions was tested: the pH-responsive color-changing film was immersed in buffer solutions with pH values of 4.0 and 11.0. The color change of the pH-responsive color-changing film in different pH aqueous solutions was observed by naked eye, and photographed by a camera, and the results are shown in Figure 11

[0119] The naked-eye color recognition of the pH-responsive color-changing film to different pH aqueous solutions was tested: the pH-responsive color-changing film was immersed in buffer solutions with pH values of 4.0 and 11.0. The color change of the pH-responsive color-changing film in different pH aqueous solutions was observed by naked eye, and photographed by a camera, and the results are shown in Figure 12

[0120] The above provided examples are not intended to limit the scope covered by the present application, and the described steps are not intended to limit the execution order. Those skilled in the art can make obvious improvements to the present application in combination with existing common knowledge, which also falls within the protection scope defined by the claims of the present application.​​

Claims

1. A bis-azo variable colour dye characterised in that, The structure of the dye is shown as formula I: wherein R is selected from Wherein, M is H or Na; R' is selected from cyano or nitro.

2. The use of the bis-azo reversible dye in claim 1 in the field of textile dyeing or pH detection.

3. The process for preparing the bis-azo variable dye as claimed in claim 1, characterized in that, The method comprises the following steps: (1) adding 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid to a chlorotriazine suspension, adjusting the pH to 4-4.5 for condensation reaction to obtain a condensation liquid; (2) adding 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid to the condensation liquid for condensation reaction to obtain a bis-condensation liquid; (3) adding diazonium salt to the bis-condensation liquid, adjusting the pH value to 1.0-2.0 for coupling reaction, and then purifying to obtain the bis-azo reversible dye; The structure of the diazonium salt is shown as follows: Wherein, R' is selected from cyano or nitro.

4. The preparation method according to claim 3, characterized in that, The molar ratio of chlorotriazine to 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (1) is 1-1.02:

1.

5. The method of claim 3, wherein the step of forming the first and second layers is performed by a method comprising: The condensation reaction temperature in step (1) is 0-5℃, and the reaction time is 4-5h.

6. The method of claim 3, wherein the step of forming the first and second layers is performed by a method comprising: The addition amount of 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (2) is 80-120wt% of the addition amount of 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (1).

7. The preparation method according to claim 3, characterized in that, The condensation reaction temperature in step (2) is 30-35℃, the condensation reaction time is 2-3h, and the pH value is 4-4.

5.

8. The preparation method according to claim 3, characterized in that, The coupling reaction temperature in step (3) is 20-25℃, the coupling time is 5-6h, and the pH value is 1.0-2.

0.

9. The preparation method according to claim 3, characterized in that, The molar ratio of diazonium salt to 4-amino-5-hydroxy-2,7-naphthalene disulfonic acid in step (1) is 1.8-2.2:

1.

10. A pH-responsive color-changing film, characterized by, The pH-responsive color-changing film is composed of a polymer, water, and the bis-azo reversible dye in claim 1; the polymer includes one or more of polyethylene glycol, gelatin, and aqueous polyurethane; The preparation method of the pH-responsive color-changing film comprises the following steps: Mixing the polymer and water, then adding the bis-azo reversible dye and continuing to mix to obtain a dispersion liquid; then uniformly pouring the dispersion liquid into a mold for drying to obtain the pH-responsive color-changing film; the mass ratio of the polymer to water is 5-10:100; the mass ratio of the bis-azo reversible dye to water is 0.05-0.1:100.

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