CN116023371B - Preparation method and application of a water-soluble weak acid color-changing pH probe with D-π-A structure - Google Patents

Abstract

The present invention discloses a method for preparing a water-soluble weak acid color-changing pH probe having a D-π-A structure and its application, and belongs to the technical field of detectable environmental pH. The present invention prepares a pH probe having a D-π-A structure shown in general formula (I) by the following method: (1) preparation of a monocondensate, (2) preparation of a dicondensate, (3) preparation of a diazonium salt, (4) coupling reaction, and then salting out and purifying. The pH probe obtained by the present invention has a simple synthesis process, is easy to obtain, has good water solubility, and can be directly observed by the naked eye to quickly detect the pH value of the solution, with high sensitivity and good selectivity.

Description

A preparation method of a water-soluble weak acid color-changing pH probe with a D-π-A structure and its application

Technical Field

The invention relates to a preparation method of a water-soluble weak acid color-changing pH probe with a D-π-A structure and application thereof, belonging to the technical field of environmental pH detection.

Background Art

As a measure of acidity, the rapid and accurate measurement of pH is particularly important in environmental protection, human health, biophysiology, chemistry, physics, mining and industrial production. In fact, spoiled foods, such as fish, meat and milk, usually have a significant change in pH compared with fresh foods due to the release of a large number of metabolites, such as organic acids, biogenic amines, esters and volatile fatty acids. Therefore, monitoring the change in pH value will be a fast, convenient, interference-free and reliable method for food safety assessment. At present, the main methods for determining pH are pH test paper measurement, electrode method and sensor. Among them, the pH test paper measurement method, although simple and easy to use, has low accuracy and is greatly affected by subjective factors, which is not conducive to the accurate measurement of pH value. The electrode method has greatly improved the accuracy of the pH test paper measurement method and has long been commercialized, but it has the disadvantages of being interfered by electrochemical interference, metal ion interference, easy machine damage, cell damage, large errors in the determination of strong acids and strong bases, and is not suitable for in vivo pH and food safety testing. Currently, fluorescent probe method, UV-visible absorption method, gas chromatography-mass spectrometry (GC-MS) and liquid chromatography have been used for food safety monitoring, but they are usually time-consuming, use expensive instruments, require qualified personnel or cannot be used for on-site testing. Such methods are difficult to implement outside the laboratory. Therefore, developing a class of simple, feasible pH probes that can achieve real-time detection remains an important research direction.

Summary of the invention

The present invention provides a preparation and application of a water-soluble color-changing pH probe with a D-π-A structure by improving the materials used. First, a new benzothiazole azo dye with good water solubility is synthesized by condensation and diazo coupling reaction. Then, a pH-responsive color-changing film for smart packaging is prepared by blending water-based polyurethane (WPU) and the synthesized dye. In order to solve the current food safety problem, the pH-responsive color-changing film is used for monitoring the freshness of yogurt.

The present invention provides a water-soluble weak acid color-changing pH probe having a D-π-A structure through condensation, diazotization and coupling reaction. The structure of the probe is shown in general formula (I):

Wherein, R is selected from:

wherein M is H or Na; n=1, 2 or 3.

The present invention also provides a method for preparing the above-mentioned water-soluble weak acid color-changing pH probe having a D-π-A structure, comprising:

(1) adding substrate A shown in formula 1 to a cyanuric chloride suspension, adjusting the pH to 4-4.5 for condensation reaction, and obtaining a condensate containing a compound of formula 2;

(2) adding the substrate B shown in formula 3 to the obtained first liquid to carry out a condensation reaction to obtain a second liquid containing the compound of formula 4;

(3) adding 3-amino-5-nitrobenzisothiazole to concentrated sulfuric acid, then dropping a nitrous acid solution, reacting for a period of time after the dropwise addition is completed, and then using aminosulfonic acid to eliminate excess nitrous acid after the reaction is completed; obtaining a diazonium salt shown in Formula 5;

(4) adding the diazonium salt obtained in step (3) to the dicondensate obtained in step (2), adjusting the pH value to 4.5-5 to carry out coupling reaction; after the reaction is completed, salting out, filtering, and drying to obtain a water-soluble weak acid color-changing pH probe having a D-π-A structure represented by general formula (I);

In one embodiment of the present invention, in step (1), substrate A is first prepared into an aqueous solution of substrate A, and then added into the cyanuric chloride suspension.

In one embodiment of the present invention, the substrate mass concentration in the aqueous solution of substrate A is 20% to 25%.

In one embodiment of the present invention, in step (1), the molar ratio of cyanuric chloride to substrate A is 1 to 1.02:1.

In one embodiment of the present invention, in step (1), the mass fraction of cyanuric chloride in the cyanuric chloride suspension is 25% to 35%.

In one embodiment of the present invention, in step (1), the condensation reaction temperature is 0-5°C and the time is 3-6h.

In one embodiment of the present invention, in step (1), the pH value of the aqueous solution of substrate A is 5.5-6.0, which is adjusted by using a sodium hydroxide solution with a mass fraction of 10%.

In one embodiment of the present invention, in step (1), 15% by mass of sodium carbonate is used to adjust the pH value of the reaction to 4.0-4.5.

In one embodiment of the present invention, in step (1), the cyanuric chloride suspension is obtained by dispersing cyanuric chloride in water at 0-5°C and beating for 0.5-0.7h.

In one embodiment of the present invention, the molar ratio of substrate B in step (2) to substrate A in step (1) is (0.8-1.2):1; specifically, 1:1 can be selected.

In one embodiment of the present invention, the condensation reaction temperature in step (2) is 28 to 30° C., and the condensation reaction time is 2 to 3 hours.

In one embodiment of the present invention, the pH value of the condensation reaction in step (2) is 4 to 4.5. The above pH value can be adjusted by using 15% by mass of sodium carbonate.

In one embodiment of the present invention, the molar ratio of 3-amino-5-nitrobenzisothiazole in step (3) to substrate A in step (1) is (0.8-1.2):1; specifically, 1:1 can be selected.

In one embodiment of the present invention, in step (3), the molar ratio of 3-amino-5-nitrobenzisothiazole to nitrous acid 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 reaction temperature in step (3) is 25-30° C. and the reaction time is 1-2 h.

In one embodiment of the present invention, the pH value of the coupling reaction in step (4) is 4.5-5.0. The above pH value can be adjusted by using 10% by mass sodium hydroxide.

In one embodiment of the present invention, the coupling reaction in step (4) is carried out at a temperature of 15 to 25° C. and for 5 to 6 hours.

In one embodiment of the present invention, the salting-out process in step (4) is as follows: weighing potassium chloride according to 10% of the total mass of the reaction solution at the end for salting-out; filtering is performed by suction filtration; and the drying temperature is 55-60°C.

In one embodiment of the present invention, the structure of the pH probe is specifically:

In one embodiment of the present invention, the preparation method of the above-mentioned water-soluble weak acid color-changing pH probe having a D-π-A structure comprises:

(1) adding K acid aqueous solution to a slurried cyanuric chloride suspension, adjusting the pH to 4-4.5 to carry out a condensation reaction to obtain a condensate;

(2) adding acid J to the first condensate of R obtained in step (1) to undergo condensation reaction to obtain a second condensate;

(3) adding 3-amino-5-nitrobenzisothiazole to concentrated sulfuric acid under stirring, and then adding nitrous acid and sulfuric acid dropwise. After the addition is complete, stirring and reacting for 1-2 hours, and using aminosulfonic acid to eliminate excess nitrous acid; performing a diazotization reaction to obtain a diazotized salt;

(4) adding the diazonium salt obtained in step (3) to the dicondensate obtained in step (2), adjusting the pH value to 4.5-5 for coupling reaction, salting out, filtering, and drying to obtain a water-soluble weak acid color-changing heterocyclic pH probe.

A water-soluble weak acid color-changing pH probe with a D-π-A structure of the present invention, during the synthesis process, the electron-donating and electron-withdrawing groups in the coupling component are different from those of the existing probes. Although the electron-withdrawing group in the present invention has only one water-soluble group at the 7th position, the positions of the electron-donating amino group (2nd position) and the hydroxyl group (5th position) on the naphthalene ring are farther apart than those in the prior art, and the electron cloud density of the azo nitrogen atom is relatively weakened, so that the prepared pH probe has good light-resistant oxidation stability. In the structure of the existing probe, the electron-withdrawing group contains two water-soluble groups (3rd and 6th positions), and the electron-donating group includes an amino group (1st position) and a hydroxyl group (8th position), and is located in the ortho position, which increases the electron cloud density of the azo nitrogen atom, resulting in the problem of poor light-resistant stability of the obtained probe. Combined with the comparison results, it can be inferred that the weak acid color-changing performance of the pH probe is closely related to the D-π-A structure formed by the azo group, and the destruction of the azo group will cause the probe to lose its color-changing ability. The probe of the structure of the present invention is more suitable for outer packaging materials.

The water-soluble weak acid color-changing pH probe with a D-π-A structure of the present invention is used for detecting the pH in a solution and detecting whether the yogurt is spoiled. The pH value can be quickly detected by directly observing the color change of the solution or the pH-responsive color-changing membrane with the naked eye without the aid of any optical device.

The invention discloses an application of a water-soluble weak acid color-changing pH probe having a D-π-A structure. The pH probe is applied to environmental monitoring, ecological protection, disease monitoring, industrial production, and sewage discharge detection.

In the present invention, the benzisothiazole water-soluble compound used as a pH probe is used to detect pH in acidic and neutral environments. The mechanism is that the absorption wavelength of the benzisothiazole water-soluble compound as the pH probe changes significantly with the change of the solution pH, so the pH value of the solution can be quickly determined by visually observing the change in the solution color.

Beneficial effects:

(1) The preparation process of the water-soluble benzisothiazole compound used as a pH probe of the present invention is simple and low in cost;

(2) The water-soluble benzisothiazole compound used as a pH probe of the present invention is reversible when used for solution pH detection;

(3) The pH probe of benzisothiazole water-soluble compounds can detect the pH of the solution without the aid of complex optical devices. It can be quickly detected by naked eye observation. It not only retains the advantages of traditional colorimetric detection, which is simple to operate and quick to read, but also achieves the real-time monitoring effect that most fluorescent probes cannot achieve. The pH probe and WPU mixed to obtain a pH-responsive membrane to detect the pH of the solution and whether the yogurt is spoiled, which can also be used to quickly detect food by naked eye observation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG1 is a hydrogen nuclear magnetic spectrum of the pH probe having a D-π-A structure obtained in Example 1;

FIG2 is an infrared spectrum of the pH probe having a D-π-A structure obtained in Example 1;

FIG3 is a graph showing ultraviolet absorption spectra of the pH probe obtained in Example 1 in aqueous solutions of different pH values;

FIG4 is a photograph of the color change of the pH probe obtained in Example 1 in aqueous solutions with pH values of 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0, respectively;

FIG5 is a photograph of the color change of the pH-responsive color-changing film made from the pH probe obtained in Example 1 in 3.0 and 10.0 aqueous solutions, respectively;

FIG6 is a photograph of the color change of the pH-responsive color-changing membrane made by the probe in Example 1 after being placed in yogurt at 5° C. and 30° C. for the same period of time.

FIG7 is a graph showing ultraviolet absorption spectra of the probe in Comparative Example 1 in aqueous solutions of different pH values;

Figure 8 is a photograph of the color change of the probe in Comparative Example 1 in aqueous solutions with pH values of 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0 and 10.0, respectively;

FIG9 is a photograph showing the color change of the pH-responsive color-changing membrane made from the probe in Comparative Example 1 after being placed in yogurt at 5° C. and 30° C. for the same period of time.

DETAILED DESCRIPTION

The present invention will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms fall within the scope limited by the appended claims of the application equally.

Example 1

Synthesis of a pH probe with the following structure:

(1) Preparation of a condensate: 1.92 g of cyanuric chloride was slurried in 4.4 g of ice-water mixture at 0°C for 0.5 to 0.7 h. 3.91 g of K acid (0.01 mol, structural formula: ) is prepared into a 24.2wt% aqueous solution, the pH is adjusted to 5.5 with a 10wt% sodium hydroxide solution, and it is added dropwise to the slurried cyanuric chloride at 0°C within 1 hour. After the addition is completed, the temperature is maintained at 0°C and the pH is 4.5 for 4 hours. The end point of the contraction reaction is the absence of free amino groups detected by an amino reagent to obtain a contraction system.

(2) Preparation of dicondensate: 2.61 g of J acid (0.01 mol, structural formula: ) is added to the monocondensate system obtained in step (1), the pH is adjusted to 4.5, the temperature is slowly raised to 28°C, and the reaction is carried out under such pH and temperature conditions. The reaction endpoint is detected by TLC to obtain a dicondensate.

(3) Preparation of diazonium salt: Accurately weigh 1.95 g of 3-amino-5-nitrobenzisothiazole (0.01 mol), dissolve it in 5 ml of concentrated sulfuric acid under stirring, and after the red powder is dissolved, slowly drop 3.175 g of nitrous acid solution (mass fraction is 40%), and react at room temperature for 2 h. After the reaction is completed, use aminosulfonic acid to eliminate excess nitrous acid to obtain 3-amino-5-nitrobenzisothiazole diazonium salt.

(4) Coupling reaction: Add the 3-amino-5-nitrobenzisothiazole diazonium salt prepared in step (3) to the dicondensate prepared in step (2), adjust the pH to 5 with 10% sodium hydroxide, and maintain the temperature at 20° C. React at this pH and temperature for 5 hours to reach the coupling endpoint.

(5) Salting out: KCl was weighed according to 10% of the total liquid volume at the end of the reaction for salting out, and the crude pH probe was obtained by filtration and drying. The yield was 83.5% (calculated based on the amount of K acid added).

(6) The crude probe powder obtained above was dissolved in 100 mL of ethanol, and then an appropriate amount of water was added dropwise, and the mixture was filtered and dried to obtain a pure pH probe.

Material Characterization:

Its hydrogen nuclear magnetic resonance spectrum is shown in Figure 1, and the data are as follows: ¹ H NMR, (DMSO-d ₆ ,δ _H ,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,1 H,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). Its infrared spectrum is shown in Figure 2, and the data are as follows: 3431,3060,1741,1607,1541,1488,1425,1309,1106,1029,833,794cm ^-1 .

pH responsiveness of the probe:

Absorption spectra of probe in aqueous solutions with different pH values: buffer solutions with different pH values (pH 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) containing the active dye in Example 1 were prepared respectively, and the concentration of the probe was 1.0 g/L. The ultraviolet absorption spectra of the solutions with different pH values were measured, and the results are shown in Figure 3. As the pH value increases, the absorption intensity of the solution at 534 nm becomes weaker, a new absorption peak appears at 663 nm and gradually increases, and the wavelength is red-shifted by 129 nm. In addition, the absorption spectrum of the dye has an isochromatic point at 590 nm.

Naked eye color recognition of probes for aqueous solutions of different pH values: buffer solutions containing the probes in Example 1 were prepared with pH values of 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 1.0 g/L. The colors of solutions of different pH values were observed with the naked eye and photographed with a camera. The results are shown in FIG4 . When the pH value increases from 3 to 10, the color of the dye solution obviously changes from slightly reddish to green, and the UV-Vis spectral response of the dye in aqueous solutions of different pH values is consistent with that observed with the naked eye.

pH responsive color-changing membrane: First, the volume ratio of waterborne polyurethane (WPU) and water is set to 20:1, and the mass ratio of pH probe and water is 0.0056:1. Specifically, 10g of WPU containing sulfonic acid groups is mixed with 0.5mL of aqueous solution containing 2.8mg of pH probe at room temperature, stirred at 200 rpm for 45 minutes, and then degassed by decompression for 15 minutes. A dispersion is obtained; the dispersion is then evenly coated on a 6cm diameter culture dish, dried at 60°C for 12h, and the membrane is peeled off from the culture dish after drying to obtain a pH responsive color-changing membrane, which is stored in a dark environment with a humidity of 30% for standby use.

Test the naked eye color recognition of pH responsive color-changing membrane for different pH aqueous solutions: immerse the pH responsive color-changing membrane in buffer solutions with pH values of 3.0 and 10.0. Observe the color changes of the pH responsive color-changing membrane in aqueous solutions with different pH values with the naked eye, and take pictures with a camera. The results are shown in Figure 5.

Naked eye color recognition of pH-responsive color-changing membrane for detecting milk spoilage: The pH-responsive color-changing membrane was immersed in two cups of the same yogurt, and stored at 5°C and 30°C for 36 hours, respectively. The L*a*b* values of the pH-responsive color-changing membrane are shown in Table 1. The red light (a*>0) of the deteriorated sample of the pH-responsive color-changing membrane after storage at 30°C for 36 hours increased from 4.79 to 22.27, and the color difference value reached 17.65 compared with the fresh yogurt (blank sample). These results are consistent with the obvious red color on the pH-responsive color-changing membrane. Since the color of the membrane changes significantly with the change of pH, it shows that the prepared pH-responsive color-changing membrane can be successfully used as part of the intelligent packaging system. The color of the pH-responsive color-changing membrane before and after the yogurt spoilage was observed by naked eye and photographed with a camera. The results are shown in Figure 6 and Table 1.

Table 1

Comparative Example 1

Probe:

Absorption spectra of probe in aqueous solutions with different pH values: buffer solutions with different pH values (pH 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, 10.0) containing the active dye in Example 1 were prepared respectively, and the concentration of the probe was 1.0 g/L. The ultraviolet absorption spectra of the solutions with different pH values were measured, and the results are shown in FIG7 . As the pH value increases, the absorption intensity of the solution at 590 nm becomes weaker, a new absorption peak appears at 665 nm and gradually increases, and the absorption spectrum of the dye has an isosbestic point at 625 nm.

The probe is recognized by naked eye for different pH aqueous solutions: The pH values of the probes configured with the above structure are 3.0, 4.0, 5.0, 6.0, 7.0, 8.0, 9.0, and 10.0 buffer solutions, and the concentration of the probe is 1.0 g/L. The color of the solutions with different pH values is observed by naked eye, and it can be seen that when the pH value increases from 3 to 10, the color of the dye solution changes from blue to green. As shown in Figure 8, the UV-Vis spectrum response of the dye in different pH aqueous solutions is consistent with the naked eye observation. However, in the existing invention, the maximum absorption wavelength of the probe changes with the pH value by only 75 nm, while the maximum absorption wavelength of the present invention is red-shifted by 129 nm, indicating that the color change of the probe of the present invention is more obvious and easier to identify and detect with the naked eye. When the pH value is 4.0, 5.0, and 6.0, the color of the probe solution of the present invention changes from red → brown → green, and the color of the solution changes significantly, while the color of the existing probes is blue, which further confirms that the probe of the present invention has better responsiveness under weakly acidic conditions.

The corresponding pH-responsive color-changing membrane was obtained using the probe of the above structure in the same way. The L*a*b* values of the pH-responsive color-changing membrane in the spoiled samples stored at 5°C and 30°C for 36 hours, as well as in the fresh yogurt (blank sample), were measured, and the results are shown in Table 2. The green light (a*<0) of the pH-responsive color-changing membrane in the spoiled sample stored at 30°C for 36 hours increased from 1.27 to 1.59. Compared with the fresh yogurt (blank sample), the color difference value was only 3.25, which also shows that the color on the pH-responsive color-changing membrane did not change significantly with the change of pH, and all showed blue. The color of the pH-responsive color-changing membrane before and after the yogurt deterioration was observed by naked eye and photographed with a camera. The results are shown in Figure 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.