A self-healing detection hydrogel and its preparation method and uses

Abstract

The present invention belongs to the technical field of food safety, and particularly relates to a detection-type hydrogel with self-healing function, a preparation method and uses thereof. In the preparation method of the hydrogel, a dynamic reversible borate ester bond is formed between the boric acid group in 4-carboxylphenylboronic acid and the polyhydroxy groups in polyvinyl alcohol, and a hydrogel matrix with a three-dimensional network structure is formed under the cross-linking action of the borate ester bond. Then, carboxymethyl chitosan is used to compound with the borate ester bond to form a modified hydrogel matrix. Bromothymol blue sodium salt and litmus essence are loaded into the three-dimensional network structure of the modified hydrogel matrix as a composite indicator. Through the synergistic effect of the composite indicator and the modified hydrogel matrix, a hydrogel with dual functions is prepared, which can realize the detection of the freshness of grains while having the self-healing function, providing the possibility for the repeated use of the hydrogel.

Description

Technical Field

[0001] The present invention belongs to the field of food safety, and particularly relates to a self-healing detection hydrogel and a preparation method and use thereof. Background Art

[0002] Grains are an important part of the global human food supply chain, and ensuring their quality and freshness is crucial for public health and food safety. As grains may be affected by various factors such as temperature, humidity, oxygen, and microbial activity during planting, storage, transportation, and sales, freshness detection has become an important means to ensure grain quality, reduce food waste, and extend the shelf life.

[0003] Currently, the freshness detection technologies for grains mainly include physical methods, chemical methods, and biological methods. Physical detection methods mainly include sensory evaluation and foreign object detection, etc., and judge the freshness of grains through methods such as human eye observation, touch, smell, and appearance. However, these methods have problems such as strong subjectivity, large errors, and low efficiency, and it is difficult to meet the requirements of the modern grain logistics system. Chemical detection methods involve quantitative analysis of specific chemical components or metabolites in grains to judge their freshness. Common methods include: gas chromatography and high performance liquid chromatography, which are used to detect volatile organic compounds and other metabolites in grains, such as ammonia, hydrogen sulfide, etc. Titration methods and spectroscopy methods, which are used to determine the humidity, pH value, and certain specific chemical components in grains. Although these methods have high accuracy, they require complex pretreatment and professional operators, have a long detection cycle, and are costly. Biological detection methods use biosensors and cell technologies to evaluate the freshness of grains by detecting the activity or metabolites of microorganisms in grains. For example: enzyme sensors, which judge the freshness of grains by detecting the activity of specific enzymes or their reaction products. Bacterial detection methods, which detect pathogenic bacteria and spoilage bacteria in grains through culture or molecular biology techniques. Although biological methods have high sensitivity and high selectivity, they usually require professional equipment and technical support, are complex to operate, and have a long detection time. Therefore, it is of great significance to design a simple and highly safe freshness detection method.

[0004] Food freshness indicating materials generally identify and sense the acidity and alkalinity of the surrounding environment through internal indicators, and accompany color changes. Then, through the process of color diffusion and transmission, the color of the food freshness indicating materials changes, so as to achieve the purpose of detecting and warning the freshness of food. Such materials can be applied to rapid detection of food safety.

[0005] Hydrogel is a polymer material with high water content that forms a three-dimensional network structure through chemical or physical interactions. It has excellent flexibility, good biocompatibility, environmental responsiveness, and advantages such as simple preparation method and portability, and can be used as a carrier for cereal freshness indicator materials. In the prior art, the preparation method of hydrogel mainly involves directly mixing the preparation raw materials and forming hydrogel through physical cross-linking. For example, sodium alginate and calcium salt form hydrogel in an aqueous solution through the action of calcium ions and -COOH. Although this method is simple to operate, each preparation raw material can only be cross-linked through intermolecular interactions such as ionic interaction, hydrogen bond, and crystallization during the preparation process, making it difficult for the preparation raw materials to effectively combine, resulting in poor mechanical strength and stability of the prepared hydrogel material. Therefore, when using hydrogel to detect the freshness of cereals, since it is necessary to fold and wrap the cereals with hydrogel, the poor mechanical properties will cause the hydrogel to be easily damaged and unable to be reused when used as a detection material, resulting in a decrease in the sensitivity and failure of the hydrogel as an indicator material, thus limiting the application of hydrogel materials in detecting and indicating the freshness of cereals. Summary of the Invention

[0006] In order to solve the technical problems of poor mechanical strength and stability of the hydrogel matrix during the detection of cereal freshness, and easy damage and inability to be reused during use, the present invention provides a self-healing detection hydrogel, a preparation method and uses thereof. The present invention realizes the preparation of a dual-functional hydrogel by loading a composite indicator in the self-healing hydrogel. That is, while realizing the detection of cereal freshness, it has the self-healing function, providing the possibility for the repeated use of the hydrogel.

[0007] The present invention uses raw materials with high safety, low price and easy availability to prepare hydrogel. The preparation method is simple and the functionality is clear. Dynamic reversible borate ester bonds are formed between 4-carboxyphenylboronic acid and the polyhydroxy groups in polyvinyl alcohol, and a hydrogel matrix with a three-dimensional network structure is formed under the cross-linking action of the borate ester bonds. Then, carboxymethyl chitosan and the borate ester bonds are compounded to form a modified hydrogel matrix. Using sodium bromothymol blue and litmus essence as composite indicators, they are loaded into the three-dimensional network structure of the modified hydrogel matrix to obtain a self-healing detection hydrogel.

[0008] GB / T 29405-2012 of the national standard points out that the free fatty acid value on the surface of grains will continuously increase with the prolongation of storage time. Therefore, the free fatty acids on the surface of grains are characteristic substances for detecting the freshness of grains, and also the reason for the identification of the achievements of the present invention and the response to the change of environmental pH value. The indication range of sodium bromothymol blue is 6.0 - 7.0, and the indication range of litmus extract is 5.0 - 8.0. After the two are compounded, the indication range of the indicator can be accurately up to 5.0 - 7.0, which is more targeted and accurate for the detection of free fatty acids on the surface of grains. Then, after the achievement of the present invention completely coats the grains with hydrogel for 4 minutes, due to the existence of multiple cross-linking points inside the hydrogel matrix, according to the principle of similar compatibility, the composite indicator is loaded into the cross-linked network in the hydrogel matrix and dispersed throughout the hydrogel in the form of points. The accumulation of free fatty acids on the surface of grains presents an acidic environment. After the composite indicator in the hydrogel captures the acidic substance, the points loaded with the indicator change color first. Since the composite indicator is water-soluble and there are a large number of water molecules in the hydrogel, when the indicator in the hydrogel matrix changes color by capturing free fatty acids, according to the principle of similar solubility, the color diffuses with the water molecules. By observing the color change on the surface of grains, the storage time of rice can be judged, and the freshness detection of grains can be realized. The whole process is simple to operate and the result is accurate. Finally, after cutting and separating the unreacted part of the hydrogel, its self-healing function without external force can be utilized to quickly recover into a detection-type hydrogel of standard size and can be reused repeatedly.

[0009] The first object of the present invention is to provide a preparation method of a detection-type hydrogel with self-healing function, using 4-carboxyphenylboronic acid, polyvinyl alcohol and carboxymethyl chitosan as the matrix of the hydrogel, and then compounding two composite reagents of sodium bromothymol blue and litmus extract, including the following steps:

[0010] Step 1: Add the polyvinyl alcohol solution to the 4-carboxyphenylboronic acid solution. Under the action of ultrasonic waves, the boric acid group in 4-carboxyphenylboronic acid and the polyhydroxy group in polyvinyl alcohol generate dynamic reversible borate ester bonds. Under the cross-linking action of the borate ester bonds, a hydrogel matrix with a three-dimensional network structure is formed to obtain a first mixed solution.

[0011] It should be noted that in the three-dimensional network structure of the hydrogel, the hydrophilic polymer segments formed by the reaction of 4-carboxyphenylboronic acid and polyvinyl alcohol serve as the diffusion path of color molecules, and within one minute, the color of the indicator diffuses to other parts of the hydrogel matrix, presenting a color change macroscopically.

[0012] As one of the matrix materials for forming borate ester bonds, 4-carboxyphenylboronic acid is insoluble in water. Using other organic solvents will affect the generated products, and only alkaline reagents can be used. Therefore, 4-carboxyphenylboronic acid is dissolved in sodium hydroxide solution. However, the formed 4-carboxyphenylboronic acid solution shows an alkaline environment. Neither an acidic environment nor an alkaline environment is conducive to the self-healing performance of the hydrogel and will affect the pH responsiveness of the composite indicator. Therefore, in the present invention, the pH value is adjusted to 7-7.5 by dropping hydrochloric acid to ensure a neutral environment for the 4-carboxyphenylboronic acid solution and no impurities are generated.

[0013] The specific preparation steps of the 4-carboxyphenylboronic acid solution described in the present invention are as follows: Add 4-carboxyphenylboronic acid to sodium hydroxide solution and stir at room temperature for 1 h to 2 h until completely dissolved. The stirring speed is 200 r / min to 350 r / min. Drop hydrochloric acid into the obtained solution to adjust the pH value to 7-7.5, and then continue to stir at room temperature for 1 h to 2 h. Secondly, add polyvinyl alcohol powder to deionized water to obtain a polyvinyl alcohol solution. In a preferred embodiment of the present invention, the dissolution temperature of the polyvinyl alcohol solution is 90 °C to 95 °C, and the dissolution time is 2 h to 3 h. Finally, add the polyvinyl alcohol solution to the 4-carboxyphenylboronic acid solution and mix evenly by ultrasonic. In order to fully mix the polyvinyl alcohol solution into the 4-carboxyphenylboronic acid solution to form borate ester bonds. In a preferred embodiment of the present invention, the first mixed solution is ultrasonically mixed at room temperature, the ultrasonic power is 20 W to 45 W, and the ultrasonic time is 6 min to 10 min. Ultrasonic is used to accelerate the dissolution of the polyvinyl alcohol solution in the 4-carboxyphenylboronic acid solution.

[0014] In some preferred embodiments, the volume ratio of the 4-carboxyphenylboronic acid solution to the polyvinyl alcohol solution is 28.6-36.6:63.4-71.4. In the present invention, the volume ratio of the 4-carboxyphenylboronic acid solution to the polyvinyl alcohol solution can be any ratio between 28.6-36.6:63.4-71.4. The boronic acid group of 4-carboxyphenylboronic acid generates dynamic reversible borate ester bonds with the polyhydroxy groups in polyvinyl alcohol. The ratio of 4-carboxyphenylboronic acid to polyvinyl alcohol directly determines the number of borate ester bonds and thus directly affects the self-healing function of the hydrogel. In some preferred embodiments, the concentration of the 4-carboxyphenylboronic acid solution is 2 wt% to 6 wt%, the concentration of the sodium hydroxide solution is 5 wt% to 9 wt%, and the concentration of the polyvinyl alcohol solution is 4 wt% to 8 wt%.

[0015] Step 2: Add an aqueous carboxymethyl chitosan solution to the first mixed solution, and under ultrasonic conditions, carboxymethyl chitosan is combined with borate ester bonds to form a modified hydrogel matrix to obtain a second mixed solution.

[0016] The specific preparation steps of the carboxymethyl chitosan aqueous solution of the present invention are as follows: Add carboxymethyl chitosan to water, and after mixing evenly, obtain the carboxymethyl chitosan aqueous solution. In some preferred embodiments, the concentration of the carboxymethyl chitosan aqueous solution is 2wt% - 6wt%. Carboxymethyl chitosan is a water-soluble polysaccharide with good water solubility, safety and non-toxicity. Compared with other saccharides, carboxymethyl chitosan has stronger biological activity, pH sensitivity, biocompatibility and antibacterial activity. In order to fully dissolve carboxymethyl chitosan in water, in the preferred embodiments of the present invention, the dissolution temperature of the carboxymethyl chitosan aqueous solution by ultrasonic mixing is 40°C - 45°C, the ultrasonic power is 20W - 45W, and the ultrasonic time is 10min - 15min. By compounding carboxymethyl chitosan with borate bonds, the viscosity of the hydrogel is effectively reduced, and at the same time, the hardness of the hydrogel is increased, so that the hydrogel has a self-healing function while maintaining a good shape. In some preferred embodiments of the present invention, the second mixed solution is ultrasonically mixed at room temperature, the ultrasonic power is 20W - 45W, and the ultrasonic time is 5min - 9min.

[0017] Step 3: Mix the aqueous solution of sodium bromothymol blue and the aqueous solution of litmus essence to prepare a composite indicator.

[0018] The indication range of a single indicator is relatively small and lacks specificity. By compounding indicators, the pH detection range of the indicator can be adjusted more precisely, making it more suitable for the detection of rice freshness. In order to enable the aqueous solution of sodium bromothymol blue and the aqueous solution of litmus essence to be fully compounded, in some preferred embodiments of the present invention, the composite indicator is ultrasonically mixed at room temperature, the ultrasonic power is 20W - 45W, and the ultrasonic time is 4min - 8min.

[0019] Step 4: Add the composite indicator to the second mixed solution, mix well and let it stand, so that the composite indicator is loaded into the three-dimensional network structure of the modified hydrogel matrix in the second mixed solution, and obtain a detection-type hydrogel with self-healing function.

[0020] In order to fully mix the composite indicator with the second mixed solution, in some preferred embodiments of the present invention, the hydrogel is ultrasonically mixed at room temperature, the ultrasonic power is 20W - 45W, and the time is 4min - 8min. In order to make the second mixed solution form a standard size for facilitating the detection of rice freshness, pour the well-mixed second mixed solution into an organic mold and let it stand at room temperature. In some preferred embodiments of the present invention, the standing temperature of the hydrogel is room temperature and the time is 12h - 36h.

[0021] The second object of the present invention is to provide a hydrogel prepared by the above preparation method.

[0022] The third objective of the present invention is to provide a hydrogel prepared by the above preparation method, which has dual functions of pH-responsive detection and self-healing.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] In the present invention, dynamic reversible borate ester bonds are formed between the boric acid groups in 4-carboxyphenylboronic acid and the polyhydroxy groups in polyvinyl alcohol. Under the cross-linking action of the borate ester bonds, a hydrogel matrix with a three-dimensional network structure is formed. Then, carboxymethyl chitosan is compounded with the borate ester bonds to form a modified hydrogel matrix. Sodium bromothymol blue and litmus essence are used as a composite indicator and loaded into the three-dimensional network structure of the modified hydrogel matrix. Through the synergistic effect of the composite indicator and the modified hydrogel matrix, a hydrogel with dual functions is prepared, which can detect the freshness of grains while having self-healing function, providing the possibility for the repeated use of the hydrogel.

[0025] The matrix raw materials of the present invention are 4-carboxyphenylboronic acid, polyvinyl alcohol and carboxymethyl chitosan, and each raw material has the advantages of high safety, environmental friendliness, low price and easy availability. 4-carboxyphenylboronic acid is safe and stable at normal temperature and pressure. Polyvinyl alcohol is a water-soluble environmental protection material widely used in industries such as food, medicine and polymer chemical industry. Carboxymethyl chitosan is a water-soluble polysaccharide, which is safe, non-toxic and has stronger biological activity, biocompatibility and antibacterial properties.

[0026] In the process of preparing the hydrogel of the present invention, a polyvinyl alcohol solution is added to the 4-carboxyphenylboronic acid solution. Under the action of ultrasonic waves, dynamic reversible borate ester bonds are formed between the boric acid groups in 4-carboxyphenylboronic acid and the polyhydroxy groups in polyvinyl alcohol. Under the cross-linking action of the borate ester bonds, a hydrogel matrix with a three-dimensional network structure is formed. Since the borate ester bond belongs to a dynamic covalent bond and can spontaneously break and form dynamically, the self-healing function of the hydrogel is endued, and it also provides the possibility for its repeated use. In addition, the unique borate ester bond is a hydrophilic polymer chain segment, which serves as a diffusion path for color molecules. Within one minute, the color of the indicator diffuses to other parts of the hydrogel matrix, presenting a color change macroscopically. These high-concentration color molecules will surround the grains until the color on the surface of the grains changes, realizing the function of indicating the freshness of grains.

[0027] On the basis of preparing the hydrogel matrix, an aqueous solution of carboxymethyl chitosan is added to the first mixed solution, and ultrasonic treatment is continued to enable carboxymethyl chitosan to be compounded with the borate ester bonds under the action of ultrasonic waves to form a modified hydrogel matrix. By compounding carboxymethyl chitosan with the borate ester bonds, the viscosity of the hydrogel is effectively reduced, and at the same time, the elasticity, hardness, ductility and portability of the hydrogel are improved.

[0028] Based on the preparation of the modified hydrogel matrix, the sodium bromothymol blue aqueous solution and the litmus extract aqueous solution are mixed to prepare a composite indicator. By regulating the pH indication range of the composite indicator, the free fatty acid value formed on the surface of grains can be detected more accurately, and then the storage time of grains can be detected more accurately.

[0029] Based on the preparation of the modified hydrogel matrix, the composite indicator is added to the second mixed solution, mixed evenly and then left standing, so that the composite indicator is loaded into the pores of the three-dimensional network structure of the modified hydrogel matrix in the second mixed solution, and a detection hydrogel with self-healing function is obtained. The use method of the hydrogel of the present invention is simple, has high safety, and the detection results are easy to observe. Moreover, for the hydrogel after one use, the unreacted part can be separated. After multiple small-volume hydrogels self-heal without external force, the hydrogel can be reused, which is convenient for consumers to use daily. Description of the Drawings

[0030] Figure 1 It shows the color development effects after wrapping new rice with Examples 1-5 of the present invention.

[0031] Figure 2 It shows the color development effects after wrapping old rice with Examples 1-5 of the present invention.

[0032] Figure 3 It shows the color development effects after wrapping aged rice with Examples 1-5 of the present invention.

[0033] Figure 4 It shows the healing situation of the hydrogel material prepared in Example 3 of the present invention. Figure (a) shows the used hydrogel after excision, Figure (b) shows the combined unused hydrogel, and Figure (c) shows the self-healed hydrogel.

[0034] Figure 5 It shows the tensile and rebound photos of the hydrogel material prepared in Example 3 of the present invention. Figure (a) shows before stretching, and Figure (b) shows after stretching. Detailed Embodiments

[0035] In order to enable those skilled in the art to better understand and implement the technical solutions of the present invention, the present invention will be further described below in conjunction with specific embodiments and drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0036] In the description of the present invention, unless otherwise specified, the reagents used are commercially available, and the methods used are conventional techniques in the art.

[0037] The following is further illustrated through specific examples.

[0038] Example 1

[0039] This example provides a method for preparing a detection-type hydrogel with self-healing function, including the following steps:

[0040] Step 1: Add a polyvinyl alcohol solution to a 4-carboxyphenylboronic acid solution. Under ultrasonic action, the boronic acid group in 4-carboxyphenylboronic acid and the polyhydroxy group in polyvinyl alcohol form dynamic reversible borate ester bonds. Under the cross-linking action of the borate ester bonds, a hydrogel matrix with a three-dimensional network structure is formed to obtain a first mixed solution.

[0041] ① Add 2 g of 4-carboxyphenylboronic acid to 98 mL of a sodium hydroxide solution with a mass fraction of 5 wt%, stir at room temperature for 1 h until completely dissolved, the stirring speed is 200 r / min, add hydrochloric acid dropwise to the obtained solution to adjust the pH value to 7, and then continue to stir at room temperature for 1 h to obtain a 4-carboxyphenylboronic acid solution.

[0042] ② Add 4 g of polyvinyl alcohol powder with a molecular weight of 1750 ± 50 to 96 mL of deionized water, and stir at 90 °C for 2 h to obtain a polyvinyl alcohol solution.

[0043] ③ According to the volume percentage, add 28.6% of the 4-carboxyphenylboronic acid solution to 71.4% of the polyvinyl alcohol solution for ultrasonic mixing to obtain a first mixed solution. Perform ultrasonic treatment at room temperature, the ultrasonic power is 25 W, and the time is 6 min.

[0044] Step 2: Add an aqueous carboxymethyl chitosan solution to the first mixed solution. Under ultrasonic conditions, carboxymethyl chitosan is combined with the borate ester bond to form a modified hydrogel matrix to obtain a second mixed solution.

[0045] ① Weigh 2 g of carboxymethyl chitosan, and perform ultrasonic mixing with 98 mL of deionized water to obtain an aqueous carboxymethyl chitosan solution. The ultrasonic temperature is 40 °C, the ultrasonic power is 45 W, and the ultrasonic time is 10 min.

[0046] ② Mix the aqueous carboxymethyl chitosan solution and the first mixed solution according to a volume ratio of 2:7. The mixing method is to pour the aqueous carboxymethyl chitosan solution into the first mixed solution, and perform ultrasonic treatment at room temperature for 5 min, the ultrasonic power is 20 W, to obtain a second mixed solution.

[0047] Step 3: Mix an aqueous solution of sodium bromothymol blue and an aqueous solution of litmus essence to prepare a composite indicator.

[0048] ① Weigh 1 g of sodium bromothymol blue and add it to 99 mL of deionized water to obtain an aqueous solution of sodium bromothymol blue.

[0049] ② Weigh 1 g of litmus essence and add it to 99 mL of deionized water to obtain a litmus essence aqueous solution.

[0050] ③ According to the volume percentage, add 66.7% of the bromothymol blue sodium salt aqueous solution to 33.4% of the litmus essence aqueous solution and perform ultrasonic mixing to obtain a composite indicator. Perform ultrasonic treatment at room temperature with an ultrasonic power of 20 W and an ultrasonic time of 4 min.

[0051] Step 4: Add the composite indicator to the second mixed solution, and the volume ratio of the composite indicator to the second mixed solution is 1:9, so that the composite indicator is loaded into the three-dimensional network structure of the modified hydrogel matrix in the second mixed solution. Perform ultrasonic treatment at room temperature with an ultrasonic power of 20 W and an ultrasonic time of 4 min. After mixing evenly, pour it into an organic mold and let it stand at room temperature for 24 h to obtain a detection-type hydrogel with self-healing function.

[0052] Example 2

[0053] This example provides a preparation method of a detection-type hydrogel with self-healing function, including the following steps:

[0054] Step 1: Add a polyvinyl alcohol solution to a 4-carboxyphenylboronic acid solution. Under ultrasonic action, the boronic acid group in 4-carboxyphenylboronic acid and the polyhydroxy group in polyvinyl alcohol form a dynamic reversible borate ester bond, and a hydrogel matrix with a three-dimensional network structure is formed under the cross-linking action of the borate ester bond to obtain a first mixed solution.

[0055] ① Add 3 g of 4-carboxyphenylboronic acid to 97 mL of a 5 wt% sodium hydroxide solution, stir at room temperature for 1 h until completely dissolved, with a stirring speed of 200 r / min. Gradually add hydrochloric acid dropwise to the obtained solution to adjust the pH value to 7, and then continue to stir at room temperature for 1 h to obtain a 4-carboxyphenylboronic acid solution.

[0056] ② Add 5 g of polyvinyl alcohol powder with a molecular weight of 1750 ± 50 to 95 mL of deionized water, and stir at 90 °C for 2 h to obtain a polyvinyl alcohol solution.

[0057] ③ According to the volume percentage, add 30.6% of the 4-carboxyphenylboronic acid solution to 69.4% of the polyvinyl alcohol solution and perform ultrasonic mixing to obtain a first mixed solution. Perform ultrasonic treatment at room temperature with an ultrasonic power of 25 W and an ultrasonic time of 6 min.

[0058] Step 2: Add a carboxymethyl chitosan aqueous solution to the first mixed solution. Under ultrasonic conditions, carboxymethyl chitosan is combined with the borate ester bond to form a modified hydrogel matrix to obtain a second mixed solution.

[0059] ① Weigh 2 g of carboxymethyl chitosan and mix it with 98 mL of deionized water by ultrasonic treatment to obtain an aqueous carboxymethyl chitosan solution. The ultrasonic temperature is 40 °C, the ultrasonic power is 45 W, and the ultrasonic time is 10 min.

[0060] ② Mix the aqueous carboxymethyl chitosan solution and the first mixed solution at a volume ratio of 2:7. The mixing method is to pour the aqueous carboxymethyl chitosan solution into the first mixed solution, and ultrasonically treat it at room temperature for 5 min with an ultrasonic power of 20 W to obtain a second mixed solution.

[0061] Step 3: Mix the aqueous solution of sodium bromothymol blue and the aqueous solution of litmus extract to prepare a composite indicator.

[0062] ① Weigh 2 g of sodium bromothymol blue and add it to 98 mL of deionized water to obtain an aqueous solution of sodium bromothymol blue.

[0063] ② Weigh 2 g of litmus extract and add it to 98 mL of deionized water to obtain an aqueous solution of litmus extract.

[0064] ③ According to the volume percentage, add 68.7% of the aqueous solution of sodium bromothymol blue to 31.3% of the aqueous solution of litmus extract and ultrasonically mix them to obtain a composite indicator. The ultrasonic temperature is room temperature, the ultrasonic power is 25 W, and the ultrasonic time is 4 min.

[0065] Step 4: Add the composite indicator to the second mixed solution. The volume ratio of the composite indicator to the second mixed solution is 1:9, so that the composite indicator is loaded into the three-dimensional network structure of the modified hydrogel matrix in the second mixed solution. Perform ultrasonic treatment at room temperature with an ultrasonic power of 20 W and an ultrasonic time of 4 min. After mixing evenly, pour it into an organic mold and let it stand at room temperature for 24 h to obtain a self-healing detection hydrogel.

[0066] Example 3

[0067] This example provides a preparation method of a self-healing detection hydrogel, including the following steps:

[0068] Step 1: Add a polyvinyl alcohol solution to a 4-carboxyphenylboronic acid solution. Under ultrasonic action, the boronic acid group in 4-carboxyphenylboronic acid and the polyhydroxy group in polyvinyl alcohol form a dynamic reversible borate ester bond. Under the cross-linking action of the borate ester bond, a hydrogel matrix with a three-dimensional network structure is formed to obtain a first mixed solution.

[0069] ① Add 4 g of 4-carboxyphenylboronic acid to 96 mL of a 5 wt% sodium hydroxide solution, stir at room temperature for 1 h until completely dissolved, the stirring speed is 200 r / min, dropwise add hydrochloric acid to the obtained solution to adjust the pH value to 7, and then continue to stir at room temperature for 1 h to obtain a 4-carboxyphenylboronic acid solution.

[0070] ② Add 6 g of polyvinyl alcohol powder with a molecular weight of 1750 ± 50 to 94 mL of deionized water, and stir at 90 °C for 2 h to obtain a polyvinyl alcohol solution.

[0071] ③ According to the volume percentage, add 32.6% of 4-carboxyphenylboronic acid solution to 67.4% of the polyvinyl alcohol solution and mix ultrasonically to obtain a first mixed solution. The ultrasonic temperature is room temperature, the ultrasonic power is 25 W, and the ultrasonic time is 6 min.

[0072] Step 2: Add an aqueous carboxymethyl chitosan solution to the first mixed solution, and the carboxymethyl chitosan and borate bonds are compounded under ultrasonic conditions to form a modified hydrogel matrix, obtaining a second mixed solution.

[0073] ① Weigh 4 g of carboxymethyl chitosan and 96 mL of deionized water, mix them ultrasonically to obtain a carboxymethyl chitosan solution. The ultrasonic temperature is 40 °C, the ultrasonic power is 45 W, and the ultrasonic time is 10 min.

[0074] ② Mix the aqueous carboxymethyl chitosan solution and the first mixed solution in a volume ratio of 2:7. The mixing method is to pour the aqueous carboxymethyl chitosan solution into the first mixed solution, and ultrasonically mix at room temperature for 5 min with an ultrasonic power of 20 W to obtain a second mixed solution.

[0075] Step 3: Mix an aqueous solution of sodium bromothymol blue and an aqueous solution of litmus essence to prepare a composite indicator.

[0076] ① Weigh 3 g of sodium bromothymol blue and add it to 97 mL of deionized water to obtain an aqueous solution of sodium bromothymol blue.

[0077] ② Weigh 3 g of litmus essence and add it to 97 mL of deionized water to obtain an aqueous solution of litmus essence.

[0078] ③ According to the volume percentage, add 70.7% of the aqueous sodium bromothymol blue solution to 29.3% of the aqueous litmus essence solution and mix ultrasonically to obtain a composite indicator. Ultrasonically mix at room temperature for 4 min with an ultrasonic power of 25 W.

[0079] Step 4: Add the composite indicator to the second mixed solution. The volume ratio of the composite indicator to the second mixed solution is 1:9, so that the composite indicator is loaded into the three-dimensional network structure of the hydrogel matrix in the second mixed solution. Perform ultrasonic treatment at room temperature with an ultrasonic power of 20 W and an ultrasonic time of 4 min. After mixing evenly, pour it into an organic mold and let it stand at room temperature for 24 h to obtain a detection-type hydrogel with self-healing function.

[0080] Example 4

[0081] This embodiment provides a method for preparing a detection-type hydrogel with self-healing function, including the following steps:

[0082] Step 1: Add a polyvinyl alcohol solution to a 4-carboxyphenylboronic acid solution. Under ultrasonic action, the boronic acid groups in 4-carboxyphenylboronic acid and the polyhydroxy groups in polyvinyl alcohol form dynamic reversible borate ester bonds. Under the cross-linking action of the borate ester bonds, a hydrogel matrix with a three-dimensional network structure is formed to obtain a first mixed solution.

[0083] ① Add 5 g of 4-carboxyphenylboronic acid to 95 mL of a sodium hydroxide solution with a mass fraction of 5 wt%. Stir at room temperature for 1 h until completely dissolved, with a stirring speed of 200 r / min. Dropwise add hydrochloric acid to the obtained solution to adjust the pH value to 7, and then continue to stir at room temperature for 1 h to obtain a 4-carboxyphenylboronic acid solution.

[0084] ② Add 7 g of polyvinyl alcohol powder with a molecular weight of 1750 ± 50 to 93 mL of deionized water, and stir at 90 °C for 2 h to obtain a polyvinyl alcohol solution.

[0085] ③ According to the volume percentage, add 34.6% of the 4-carboxyphenylboronic acid solution to 65.4% of the polyvinyl alcohol solution for ultrasonic mixing to obtain a first mixed solution. Ultrasonic at room temperature for 6 min, with an ultrasonic power of 25 W.

[0086] Step 2: Add an aqueous carboxymethyl chitosan solution to the first mixed solution. Under ultrasonic conditions, carboxymethyl chitosan and the borate ester bonds are compounded to form a modified hydrogel matrix to obtain a second mixed solution.

[0087] ① Weigh 5 g of carboxymethyl chitosan, and mix it with 95 mL of deionized water by ultrasonic to obtain a carboxymethyl chitosan solution. The ultrasonic temperature is 40 °C, the ultrasonic power is 45 W, and the ultrasonic time is 10 min.

[0088] ② Mix the aqueous carboxymethyl chitosan solution and the first mixed solution according to a volume ratio of 2:7. The mixing method is to pour the aqueous carboxymethyl chitosan solution into the first mixed solution, and ultrasonic at room temperature for 5 min, with an ultrasonic power of 20 W, to obtain a second mixed solution.

[0089] Step 3: Mix an aqueous solution of bromothymol blue sodium salt and an aqueous solution of litmus spirit to prepare a composite indicator.

[0090] ① Weigh 4 g of bromothymol blue sodium salt and add it to 95 mL of deionized water to obtain an aqueous solution of bromothymol blue sodium salt.

[0091] ② Weigh 4 g of litmus spirit and add it to 95 mL of deionized water to obtain an aqueous solution of litmus spirit.

[0092] ③ Add 72.7% of the aqueous solution of sodium bromothymol blue to 27.3% of the aqueous solution of litmus essence by volume percentage, and perform ultrasonic mixing to obtain a composite indicator. The ultrasonic temperature is at room temperature, the ultrasonic power is 25 W, and the ultrasonic time is 4 min.

[0093] Step 4: Add the composite indicator to the second mixed solution. The volume ratio of the composite indicator to the second mixed solution is 1:9, so that the composite indicator is loaded into the three-dimensional network structure of the hydrogel matrix in the second mixed solution. Perform ultrasonic treatment at room temperature with an ultrasonic power of 20 W and an ultrasonic time of 4 min. After mixing evenly, pour it into an organic mold and let it stand at room temperature for 24 h to obtain a self-healing detection hydrogel.

[0094] Example 5

[0095] This example provides a preparation method of a self-healing detection hydrogel, including the following steps:

[0096] Step 1: Add the polyvinyl alcohol solution to the 4-carboxyphenylboronic acid solution. Under ultrasonic action, the boronic acid group in 4-carboxyphenylboronic acid and the polyhydroxy group in polyvinyl alcohol form dynamic reversible borate ester bonds, and a hydrogel matrix with a three-dimensional network structure is formed under the cross-linking action of the borate ester bonds to obtain a first mixed solution.

[0097] ① Add 6 g of 4-carboxyphenylboronic acid to 94 mL of a 5 wt% sodium hydroxide solution, stir at room temperature for 1 h until completely dissolved, with a stirring speed of 200 r / min. Dropwise add hydrochloric acid to the obtained solution to adjust the pH value to 7, and then continue to stir at room temperature for 1 h to obtain a 4-carboxyphenylboronic acid solution.

[0098] ② Add 8 g of polyvinyl alcohol powder with a molecular weight of 1750 ± 50 to 92 mL of deionized water, and stir at 90 °C for 2 h to obtain a polyvinyl alcohol solution.

[0099] ③ Add 36.6% of the 4-carboxyphenylboronic acid solution to 63.4% of the polyvinyl alcohol solution by volume percentage and perform ultrasonic mixing to obtain a first mixed solution. The ultrasonic temperature is at room temperature, the ultrasonic power is 25 W, and the ultrasonic time is 6 min.

[0100] Step 2: Add the aqueous solution of carboxymethyl chitosan to the first mixed solution. Under ultrasonic conditions, carboxymethyl chitosan and the borate ester bond are compounded to form a modified hydrogel matrix to obtain a second mixed solution.

[0101] ① Weigh 6 g of carboxymethyl chitosan and 94 mL of deionized water, and perform ultrasonic mixing to obtain a carboxymethyl chitosan solution. The ultrasonic temperature is 40 °C, the ultrasonic power is 20 W, and the ultrasonic time is 10 min.

[0102] ② The carboxymethyl chitosan aqueous solution and the first mixed solution were mixed at a volume ratio of 2:7. The mixing method was to pour the carboxymethyl chitosan aqueous solution into the first mixed solution, and ultrasonicate for 5 minutes at room temperature with an ultrasonic power of 20 W to obtain a second mixed solution.

[0103] Step 3: Mix the aqueous solution of bromothymol blue sodium salt and the aqueous solution of litmus to prepare a composite indicator.

[0104] ① Weigh 5 g of bromothymol blue sodium salt and add it to 95 mL of deionized water to obtain a bromothymol blue sodium salt aqueous solution.

[0105] ② Weigh 5 g of litmus essence and add it into 95 mL of deionized water to obtain a litmus essence aqueous solution.

[0106] ③ According to volume percentage, 74.7% of bromothymol blue sodium salt aqueous solution was added to 25.3% of litmus aqueous solution for ultrasonic mixing to obtain a composite indicator, and ultrasonicated for 4 minutes at room temperature with an ultrasonic power of 25W.

[0107] Step 4, adding the composite indicator to the second mixed solution, the volume ratio of the composite indicator to the second mixed solution is 1:9, so that the composite indicator is loaded in the three-dimensional network structure of the modified hydrogel matrix in the second mixed solution, and ultrasonic treatment is performed at room temperature, the ultrasonic power is 20 W, the ultrasonic time is 4 minutes, and after mixing evenly, pouring into an organic mold, standing at room temperature for 24 hours, a detection hydrogel with self-healing function is obtained.

[0108] Experimental Section

[0109] 1. Rice freshness indicator

[0110] The present invention respectively tests the rice freshness of the hydrogels prepared in Examples 1 to 5, and the results are shown in Tables 1 to 3 and Figures 1 to 3 As shown, the specific operations are as follows:

[0111] The hydrogel materials prepared in Examples 1 to 5 were cut into small pieces of 5 cm×5 cm in size. Several grains of new rice, old rice and aged rice were taken respectively, where the new rice was the newly harvested rice of this year, the old rice was the rice stored for more than 1 year but less than 3 years, and the aged rice was the rice stored for 3 years or more, and were placed on the surface of the hydrogel prepared in Examples 1 to 5 respectively, and the gel around the rice was folded and wrapped to be sealed, and placed at room temperature for 4 minutes, and the gel wrapped around the rice was opened and the rice was taken out to observe the color change on the surface of the rice.

[0112] Table 1 Color development time after coating of new rice in Examples 1 to 5

[0113] New rice Example 1 Example 2 Example 3 Example 4 Example 5 Color development time / min 6 5 4 4 4

[0114] Table 1 shows the color development times of the newly harvested rice after being coated in Examples 1 to 5. Figure 1 Fig. shows the color development effects of the newly harvested rice after being coated in Examples 1 to 5. In the figure, from left to right, they are the color development effects of the newly harvested rice after being coated in Example 1, Example 2, Example 3, Example 4, and Example 5 respectively. As Figure 1 shown, at room temperature, the hydrogels prepared in Examples 1 to 5 contact and wrap the rice, and color changes begin to appear on the surface of the rice. The color development of the rice is blue-violet or light purple, indicating that when the color development is blue-violet or light purple and the color of the rice is the same as that of the hydrogel, the tested rice is newly harvested rice. It can be seen from Table 1 that the color development time of the rice in Example 3 is 4 min and basically does not change anymore.

[0115] Table 2 shows the color development times of the aged rice after being coated in Examples 1 to 5

[0116] Old rice Example 1 Example 2 Example 3 Example 4 Example 5 Color development time / min 6 5 4 4 4

[0117] Table 2 shows the color development effects of the aged rice after being coated in Examples 1 to 5. Figure 2 Fig. shows the color development effects of the aged rice after being coated in Examples 1 to 5. In the figure, from left to right, they are the color development effects of the aged rice after being coated in Example 1, Example 2, Example 3, Example 4, and Example 5 respectively. It can be Figure 2 seen that at room temperature, the hydrogels prepared in Examples 1 to 5 contact and wrap the rice, and the color development of the rice is violet or purple-pink, which is inconsistent with the color of the hydrogel that has not contacted the rice. This is because the acidic environment generated by the free fatty acids on the surface of the rice reacts with the indicator in the hydrogel in the presence of water molecules and the color changes. It indicates that when the color development is violet or purple-pink and the color of the rice is inconsistent with the color of the hydrogel, the tested rice is aged rice. It can be seen from Table 2 that the color development time of the rice in Example 3 is 4 min and basically does not change anymore.

[0118] Table 3 shows the color development effects of the deteriorated rice after being coated in Examples 1 to 5

[0119] Aged rice Example 1 Example 2 Example 3 Example 4 Example 5 Color development time / min 6 5 4 4 4

[0120] Table 3 shows the color development times of the deteriorated rice after being coated in Examples 1 to 5. Figure 3 Fig. shows the color development effects of the deteriorated rice after being coated in Examples 1 to 5. In the figure, from left to right, they are the color development effects of the deteriorated rice after being coated in Example 1, Example 2, Example 3, Example 4, and Example 5 respectively. As Figure 3As shown, at room temperature, the hydrogel prepared by Examples 1 to 5 contacts and wraps the rice, and the color of the rice shows full powder or pink, which is inconsistent with the color of the hydrogel that has not contacted the rice. This is because the acidic environment generated by the free fatty acids on the surface of the rice reacts with the indicator in the hydrogel in the presence of water molecules and causes a color change. It shows that when the color shows full powder or pink and the color of the rice is inconsistent with the color of the hydrogel, the tested rice is aged rice. As can be seen from Table 3, the color development time of the rice in Example 3 is 4 minutes and basically no longer changes.

[0121] The freshness indication of rice is detected using the free fatty acids on the surface of the rice as characteristic substances. When the hydrogel contacts and wraps the rice, after the composite indicator in the hydrogel captures the acidic substances, the sites loaded with the indicator change color first. Since the composite indicator is water-soluble and there are a large number of water molecules in the hydrogel, after the indicator in the hydrogel matrix changes color by capturing free fatty acids, according to the principle of "like dissolves like", the color diffuses with the water molecules, and the hydrophilic polymer segments formed by the reaction of 4-carboxylphenylboronic acid and polyvinyl alcohol serve as the diffusion path for the color molecules. Within one minute, the color of the indicator diffuses to other parts of the hydrogel matrix, presenting a color change macroscopically. These high-concentration color molecules will surround the rice until the color of the rice surface changes, achieving the function of indicating the freshness of the rice. Through the color change on the surface of the rice, the freshness of the rice can be qualitatively judged:

[0122] When the color shows light purple or blue-violet and the color of the rice is consistent with the color of the untested hydrogel, the tested rice is new rice;

[0123] When the color shows violet or purple-pink and the color of the rice is inconsistent with the color of the untested hydrogel, the tested rice is old rice;

[0124] When the color shows pink or pinkish-red and the color of the rice is inconsistent with the color of the untested hydrogel, the tested rice is aged rice.

[0125] As Figures 1 to 3 shown, the color development of the rice ranges from blue-violet to pink. Since the indication range of sodium bromothymol blue is 6.0 - 7.0, under acidic conditions, bromothymol blue presents yellow; the indication range of litmus extract is 5.0 - 8.0, and under acidic conditions, litmus extract presents red. The present invention can expand and precisely control the indication range of the composite indicator to 5.0 - 7.0, and can more accurately detect the color change on the surface of the rice for the free fatty acids on the surface of the rice. Moreover, the detection-type hydrogel prepared by the present invention has an obvious detection effect on rice with different storage times.

[0126] 2. Self-healing performance test

[0127] The present invention conducts self-healing performance tests on the hydrogels prepared in Examples 1 to 5, and the results are shown in Table 4 and Figure 4 as follows:

[0128] After taking out the rice with freshness indication completed from the hydrogels prepared in Examples 1 to 5, cut off the used gel part, take a new gel of appropriate size and fill it into the notch. Observe the healing situation at the notch after 5 minutes as Figure 4 and the healing time of the hydrogel is shown in Table 4.

[0129] Table 4 Self-healing time of the hydrogels prepared in Examples 1 to 5

[0130]

[0131] Table 4 shows the self-healing time of the hydrogels prepared in Examples 1 to 5 after being indicated by fresh rice, old rice, and aged rice. It can be seen from Table 4 that after Example 3, the self-healing time of the hydrogel is 5 minutes, and the healing time of the hydrogel basically no longer changes. This shows that after 5 minutes, the hydrogel can achieve complete healing.

[0132] Figure 4 Figure (a) shows the hydrogel after the used part is cut off, Figure (b) shows the unused hydrogel filled into the cut part, and Figure (c) shows the hydrogel after self-healing. As shown in Figure (a), the hydrogel after freshness indication is cut off. As shown in Figure (b), the unused hydrogel is filled into the cut part. As shown in Figure (c), as time goes by, the hydrogel heals from the beginning to complete healing, indicating that the hydrogel material prepared in Example 3 can completely heal, and it shows that the detection-type hydrogel prepared by the present invention has a self-healing function.

[0133] 3. Tensile test

[0134] At room temperature, gently hold both ends of the hydrogel prepared in Example 3 with your hands, and it can be slowly stretched with a pulling force of 2 N to 3 N. At the same time, use a ruler to measure the stretching length. Note to keep the stretching speed uniform and observe the elastic recovery of the hydrogel to confirm that it can return to its original shape.

[0135] Figure 5 Figure (a) shows the hydrogel before stretching, and Figure (b) shows the hydrogel after stretching. As can be seen from Figure (a), the original length of the hydrogel is 2.3 cm. After being slowly stretched, as shown in Figure (b), the hydrogel is 7.4 cm, stretched to 3 to 4 times its original length. The hydrogel still maintains a transparent elastic state without breakage or fracture, and after 3 minutes, the hydrogel can return to its original shape. This shows that the prepared hydrogel has excellent tensile resilience performance and is not easily damaged in practical applications.

[0136] Although the embodiments of the present invention have been disclosed as above, they are not limited to the applications listed in the specification and embodiments. It can be fully applied to various fields suitable for the present invention. For those familiar with the field, additional modifications can be easily achieved. Therefore, without departing from the general concept defined by the claims and the equivalent scope, the present invention is not limited to the specific details and the illustrated examples described herein.

Claims

1. A method for preparing a detection-type hydrogel with self-healing function, characterized in that: The following steps are involved: Adding a polyvinyl alcohol solution to a 4-carboxyphenylboronic acid solution, under the action of ultrasound, the boronic acid groups in the 4-carboxyphenylboronic acid and the polyhydroxy groups in the polyvinyl alcohol generate dynamic reversible borate ester bonds, and under the cross-linking action of the borate ester bonds, a hydrogel matrix with a three-dimensional network structure is formed to obtain a first mixed solution; adding a carboxymethyl chitosan aqueous solution to the first mixed solution, and continuing ultrasonication to allow the carboxymethyl chitosan to be complexed with the borate ester bond under the action of ultrasound to form a modified hydrogel matrix, thereby obtaining a second mixed solution; The bromothymol blue sodium salt aqueous solution and the litmus aqueous solution are mixed to prepare a composite indicator; Adding the composite indicator to the second mixed solution and mixing well, and then standing still, so that the composite indicator is loaded in the three-dimensional network structure of the modified hydrogel matrix in the second mixed solution, to obtain a detection-type hydrogel with self-healing function; The concentration of the 4-carboxyphenylboronic acid solution is 2wt% to 6wt%; The concentration of the polyvinyl alcohol solution is 4wt% to 8wt%; The volume ratio of the 4-carboxyphenylboric acid solution to the polyvinyl alcohol solution is 28.6-36.6:63.4-71.

4.

2. The method for preparing the detection-type hydrogel with self-healing function according to claim 1, characterized in that: The concentration of the carboxymethyl chitosan aqueous solution is 2wt% to 6wt%; The volume ratio of the first mixed solution, the carboxymethyl chitosan aqueous solution and the composite indicator is 7-9:2-4:1-2.

3. The method for preparing the detection-type hydrogel with self-healing function according to claim 1, characterized in that: The volume ratio of the bromothymol blue sodium salt aqueous solution to the litmus aqueous solution is 66.7-74.7:25.3-33.4; The concentrations of the solutions of bromothymol blue sodium salt and litmussine are both 1 wt% to 5 wt%.

4. The method for preparing the detection-type hydrogel with self-healing function according to claim 1, characterized in that: The dissolution temperature of the polyvinyl alcohol solution is 90° C. to 95° C., and the dissolution time is 2 h to 3 h.

5. The method for preparing the detection-type hydrogel with self-healing function according to claim 1, characterized in that: The first mixed solution is ultrasonically mixed at room temperature, the ultrasonic power is 20W to 45W, and the ultrasonic time is 6min to 10min.

6. The method for preparing the detection-type hydrogel with self-healing function according to claim 1, characterized in that: The second mixed solution is ultrasonically mixed at room temperature, the ultrasonic power is 20W to 45W, and the ultrasonic time is 5min to 9min.

7. The method for preparing a detection-type hydrogel with self-healing function according to claim 1, characterized in that: The hydrogel is placed at room temperature for 12 to 36 hours.

8. A detection-type hydrogel with self-healing function prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the detection hydrogel with self-healing function according to claim 8 in detecting the freshness of grains.

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