A method for preparing alginate-based hydrogel colorimetric labels

Abstract

This disclosure discloses a method for preparing an alginate-based hydrogel colorimetric label. The method includes: adding alginate and a regulating material in a predetermined ratio to an aqueous solution containing an indicator solution and glycerol, stirring and heating to obtain a mixed solution; preparing an alginate hydrogel film from the mixed solution using a casting or casting method; uniformly spraying or coating a metal ion crosslinking agent onto the surface of the alginate hydrogel film for crosslinking treatment, and after standing, obtaining a crosslinked alginate hydrogel film; immersing the crosslinked alginate hydrogel film in an ethanol solution, and then inducing solvent exchange through ultrasound to obtain the alginate-based hydrogel colorimetric label. This application can improve the elastic modulus and colorimetric resolution of the colorimetric label.

Description

Technical Field

[0001] This disclosure belongs to the field of food packaging technology, specifically relating to a method for preparing an alginate-based hydrogel colorimetric label. Background Technology

[0002] With the rapid development of technology, people's demands for food safety and quality are constantly increasing. Due to various physical, chemical, and biological factors, fresh food loses its nutritional value and may even spoil during transportation, storage, and distribution, posing significant risks to human health and the environment. Smart packaging that can trace and monitor food freshness in real time can effectively provide information on real-time changes in food quality, improving product safety and quality by providing early warnings of potential problems.

[0003] Colorimetric indicator labels are a type of simple, efficient, and low-cost gas sensor that changes color in response to external stimuli, indicating changes in food quality. Typically, colorimetric labels consist of two parts: a solid support layer and an indicator bonded to it. Alginate hydrogels are a type of natural biomaterial with advantages such as low cost, biosafety, and environmental friendliness, making them a suitable solid support layer material.

[0004] Alginate hydrogels have poor mechanical properties and are easily soluble in water, which limits their practical production and application. Current research focuses on alginate aerogel colorimetric labels; however, aerogels, by removing water, lose their advantage of readily reacting with gases. Alginate hydrogels, containing a certain amount of free and bound water, are themselves pH-sensitive materials and possess abundant ion-rich channels and pore structures, providing more reaction sites for gas detection and indicators attached to them, thus exhibiting higher sensitivity to gases volatile from various foods.

[0005] Currently, there are two key issues in the research of hydrogel-based colorimetric labels: firstly, the elastic modulus of hydrogels is relatively low; secondly, the colorimetric labels suffer from uneven color development, i.e., low resolution. These two issues affect the manufacturability and accuracy of hydrogel-based colorimetric labels, thus limiting their market application in packaging. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the purpose of this disclosure is to provide a method for preparing alginate-based hydrogel colorimetric labels. This disclosure can improve the manufacturability and measurement accuracy of alginate-based colorimetric indicator labels, thereby promoting the market application of alginate-based hydrogel colorimetric indicator labels.

[0007] To achieve the above objectives, this disclosure provides the following technical solutions:

[0008] A method for preparing an alginate-based hydrogel colorimetric label, the method comprising: adding alginate and a regulating material in a predetermined ratio to an aqueous solution containing an indicator solution and glycerol, stirring and heating to obtain a mixed solution; preparing an alginate hydrogel film from the mixed solution using a casting method or a casting method; uniformly spraying or coating a metal ion crosslinking agent onto the surface of the alginate hydrogel film for crosslinking treatment, and after standing, obtaining a crosslinked alginate hydrogel film; immersing the crosslinked alginate hydrogel film in an ethanol solution, and then inducing solvent exchange with ultrasonication to obtain an alginate-based hydrogel colorimetric label.

[0009] Optionally, the alginate includes sodium alginate or sodium oxidized alginate.

[0010] Optionally, the control material includes gelatin or agar.

[0011] Optionally, the indicator solution is prepared by dissolving the indicator in anhydrous ethanol or an aqueous solution.

[0012] Optionally, the indicator includes any one or two of the following: methyl red, bromothymol blue, methylene blue, bromocresol purple, and anthocyanins.

[0013] Optionally, the metal ion crosslinking agent includes any one of the following: calcium ion solution, aluminum ion solution, and zinc ion solution.

[0014] Optionally, the concentration of the metal ion crosslinking agent is 0.25 mol / L to 0.5 mol / L.

[0015] Optionally, the standing time after the metal ion crosslinking agent is sprayed or coated onto the surface of the sodium alginate hydrogel film for crosslinking treatment is 10 min to 60 min.

[0016] Optionally, the concentration of the ethanol solution is 10% v / v to 30% v / v.

[0017] Optionally, the duration of the assisted ultrasound is 0.5 h to 3 h.

[0018] Compared with the prior art, the beneficial effects of this disclosure are as follows: This disclosure can construct an interconnected porous dual-network structure of alginate hydrogel through a stepwise cross-linking synergistic solvent-induced process, while also being able to regulate the distribution of free water inside the hydrogel and improve the elastic modulus and colorimetric resolution of the colorimetric label. Attached Figure Description

[0019] Figure 1 This is a schematic flowchart of a method for preparing an alginate-based hydrogel colorimetric label according to an embodiment of this disclosure;

[0020] Figure 2 The images show the microstructure of the surface and cross-section of sodium alginate and its composite hydrogel film before induction.

[0021] Figure 3 These are the microstructure results of the surface and cross-section of sodium alginate and its composite hydrogel film after induction;

[0022] Figure 4 This is a laser confocal observation result of sodium alginate and its composite hydrogel film before induction;

[0023] Figure 5 This is a laser confocal microscopy observation result of sodium alginate and its composite hydrogel film after induction;

[0024] Figure 6 The graph shows the elastic modulus results of sodium alginate and its composite hydrogel before induction.

[0025] Figure 7 The graph shows the elastic modulus results of sodium alginate and its composite hydrogel after induction.

[0026] Figure 8 This is a comparison chart of the elastic modulus of sodium alginate-based hydrogel samples prepared by the method described in this application with those of sodium alginate-based hydrogel samples from other sources;

[0027] Figure 9 The image shows the colorimetric response results of sodium alginate and its composite hydrogel before induction.

[0028] Figure 10 The image shows the colorimetric response results of sodium alginate and its composite hydrogel after induction.

[0029] Figure 11 The graph shows the sensitivity results of sodium alginate and its composite hydrogel before induction.

[0030] Figure 12 This is a graph showing the sensitivity results of sodium alginate and its composite hydrogel after induction.

[0031] Figure 13 This is a comparison chart of the quantitative results of color uniformity of sodium alginate and its composite hydrogel before and after the induction process. Detailed Implementation

[0032] The following will refer to the appendix. Figures 1 to 13 Specific embodiments of this disclosure are described in detail. While specific embodiments of this disclosure are shown in the accompanying drawings, it should be understood that this disclosure may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of this disclosure to those skilled in the art.

[0033] It should be noted that certain terms are used in the specification and claims to refer to specific components. Those skilled in the art will understand that different terms may be used to refer to the same component. This specification and claims do not distinguish components based on differences in terminology, but rather on differences in function. The terms "comprising" or "including" used throughout the specification and claims are open-ended and should be interpreted as "comprising but not limited to." The following descriptions are preferred embodiments for carrying out this disclosure; however, these descriptions are for the purpose of understanding the general principles of the specification and are not intended to limit the scope of this disclosure. The scope of protection of this disclosure is determined by the appended claims.

[0034] To facilitate understanding of the embodiments of this disclosure, further explanations and descriptions will be provided below with reference to the accompanying drawings and specific embodiments. The accompanying drawings do not constitute a limitation on the embodiments of this disclosure.

[0035] Figure 1 This is a schematic flowchart illustrating a method for preparing an alginate-based hydrogel colorimetric label according to an exemplary embodiment of this disclosure, as shown below. Figure 1 As shown, the preparation method includes the following steps:

[0036] S1: Mix 1% v / v methyl red indicator aqueous solution and 1.5% v / v glycerol aqueous solution to obtain a mixed solvent. Then add sodium alginate and gelatin in a mass ratio of 1:1 to the mixed solvent, heat to 60°C, and stir magnetically for 2 hours to obtain a 4wt% mixed solution.

[0037] In this step, the methyl red indicator is prepared by dissolving 100 mg of methyl red powder in 100 mL of anhydrous ethanol.

[0038] S2: The mixed solution is prepared into a sodium alginate hydrogel film (SA / Gel, Sodium Alginate / Gelatin) by casting or casting method.

[0039] In this step, a sodium alginate hydrogel film is prepared using the casting method, specifically including:

[0040] Prepare the mold: Select a mold of appropriate size and flatness, usually in the shape of a flat plate, and ensure that its surface is clean and smooth.

[0041] Pouring solution: Slowly and evenly pour the well-stirred alginate-based mixed solution into the mold, taking care to avoid introducing air bubbles.

[0042] Standing and evaporation: Let the solution stand naturally at room temperature or under controlled temperature for a period of time. During this period, the solvent will gradually evaporate and form a wet film of a certain thickness.

[0043] Drying and curing: If necessary, the film can be further dried in the air or placed in an oven at a low temperature to fully cure it.

[0044] The preparation of sodium alginate hydrogel films by the coating or doctor blade method specifically includes:

[0045] Equipment setup: Adjust the blade gap on the casting machine to the desired film thickness and prepare the conveyor belt or support plate.

[0046] Coating solution: Use a doctor blade or similar tool to evenly coat the mixed solution onto the moving support surface, and control the final thickness of the film by adjusting the blade height.

[0047] Drying process: As the support plate moves, the solvent in the solution evaporates rapidly, leaving a uniform wet film; then the drying process can be accelerated by heating or other means to promote rapid film setting.

[0048] Film collection: Once the film is fully dried and has sufficient mechanical strength, it is peeled off from the support plate, completing the preparation.

[0049] S3: A calcium ion solution (e.g., calcium chloride solution) with a concentration of 0.25 mol / L is uniformly sprayed or coated onto the surface of the sodium alginate hydrogel film, and cross-linking treatment is performed at room temperature. After standing for 10 minutes, the cross-linked sodium alginate hydrogel film is obtained.

[0050] In this step, alginate molecules contain a large number of carboxyl groups (~COOH). When a calcium ion (Ca²⁺) solution is uniformly sprayed or coated onto the surface of the sodium alginate hydrogel film, the calcium ions form complex bonds with the carboxyl groups. Each calcium ion can form cross-linking points with multiple carboxyl groups, thereby connecting multiple alginate molecules together to form a three-dimensional network structure. This three-dimensional network structure significantly enhances the mechanical properties of the hydrogel, improving its elastic modulus and mechanical strength, while reducing the swelling and flowability of the hydrogel.

[0051] S4: The cross-linked sodium alginate hydrogel film is placed in a 10% v / v ethanol solution at room temperature, ensuring that the sodium alginate hydrogel film is completely submerged in the ethanol solution. The container containing the sodium alginate hydrogel film and the ethanol solution is placed in an ultrasonic cleaner for ultrasonic treatment. The ultrasonic power is set to 600W and the ultrasonic time is set to 0.5h. After the ultrasonic treatment is completed, the sodium alginate hydrogel film is removed from the ethanol solution and dried to obtain a sodium alginate-based hydrogel colorimetric label.

[0052] In this step, the propagation of ultrasound in the solution generates a cavitation effect, where tiny bubbles rapidly form, grow, and collapse. The collapse of these bubbles creates strong local pressure and flow, accelerating the diffusion and exchange of solvent molecules. Through ultrasound treatment, ethanol molecules can penetrate the sodium alginate hydrogel film more quickly, simultaneously displacing free water molecules within the film. This not only shortens the solvent exchange time but also improves its efficiency. Furthermore, the strong local flow generated by ultrasound helps to uniformly disperse solvent molecules, reducing uneven solvent distribution within the sodium alginate hydrogel film. Ultrasonic-treated sodium alginate hydrogel films exhibit more uniform colorimetric response, significantly improving the resolution and accuracy of colorimetric labels. Moreover, the mechanical vibration of ultrasound helps to tightly align the polymer chains within the sodium alginate hydrogel film, enhancing its structural stability. This results in ultrasonic-treated sodium alginate hydrogel films with better mechanical strength and durability, making them less prone to breakage or deformation.

[0053] It should be noted that without ultrasonic treatment of the sodium alginate hydrogel film, the solvent exchange process mainly relies on natural diffusion. This means that the free water inside the sodium alginate hydrogel film cannot be completely replaced by ethanol, resulting in the physical properties of the sodium alginate hydrogel film (such as elastic modulus and mechanical strength) failing to meet requirements. Furthermore, the uneven distribution of solvent within the sodium alginate hydrogel film leads to uneven distribution of the chromogenic agent within the film, causing uneven color changes in response to the target gas on the colorimetric label, thus affecting the resolution and accuracy of the colorimetric label.

[0054] In another exemplary embodiment, this disclosure also provides a method for preparing an alginate-based hydrogel colorimetric label, the method comprising the following steps:

[0055] S10: Mix 1% v / v bromothymol blue indicator aqueous solution and 2% v / v glycerol aqueous solution to obtain a mixed solvent. Then add sodium alginate and gelatin in a mass ratio of 1:5 to the mixed solvent, heat to 70°C, and stir magnetically for 2 hours to obtain an 8wt% mixed solution.

[0056] In this step, the bromothymol blue indicator is prepared by dissolving 100 mg of bromothymol blue powder in 100 mL of anhydrous ethanol.

[0057] S20: The mixed solution is prepared into a sodium alginate hydrogel film using a casting method or a casting method;

[0058] S30: A 0.3 mol / L aluminum ion solution (e.g., aluminum chloride solution) is uniformly sprayed or coated onto the surface of the sodium alginate hydrogel film, and crosslinking is performed at room temperature. After standing for 30 minutes, the crosslinked sodium alginate hydrogel film is obtained.

[0059] S40: The cross-linked sodium alginate hydrogel film is placed in a 20% v / v ethanol solution at room temperature, ensuring that the sodium alginate hydrogel film is completely submerged in the ethanol solution. The container containing the sodium alginate hydrogel film and the ethanol solution is placed in an ultrasonic cleaner for ultrasonic treatment. The ultrasonic power is set to 600W and the ultrasonic time is set to 2h. After the ultrasonic treatment is completed, the sodium alginate hydrogel film is removed from the ethanol solution and dried to obtain the sodium alginate hydrogel colorimetric label.

[0060] In another exemplary embodiment, this disclosure also provides a method for preparing an alginate-based hydrogel colorimetric label, the method comprising the following steps:

[0061] S100: Mix 10% v / v anthocyanin indicator aqueous solution and 10% v / v glycerol aqueous solution to obtain a mixed solvent. Then add sodium alginate (SA) and agar in a mass ratio of 1:9 to the mixed solvent, heat to 80°C, and stir magnetically for 2 hours to obtain a 9wt% mixed solution.

[0062] In this step, the anthocyanin indicator is prepared by dissolving 100 mg of anthocyanin powder in 100 mL of deionized water.

[0063] In addition, methylene blue and bromocresol purple can also be used as indicators.

[0064] S200: The mixed solution is prepared into a sodium alginate hydrogel film using a casting method or a casting method;

[0065] S300: A zinc ion solution (e.g., zinc chloride solution) with a concentration of 0.5 mol / L is uniformly sprayed or coated onto the surface of the sodium alginate hydrogel film, and crosslinking treatment is performed at room temperature. After standing for 60 min, the crosslinked sodium alginate hydrogel film is obtained.

[0066] S400: The cross-linked sodium alginate hydrogel film is placed in a 30% v / v ethanol solution at room temperature, ensuring that the sodium alginate hydrogel film is completely submerged in the ethanol solution. The container containing the sodium alginate hydrogel film and the ethanol solution is placed in an ultrasonic cleaner for ultrasonic treatment. The ultrasonic power is set to 600W and the ultrasonic time is set to 3h. After the ultrasonic treatment is completed, the sodium alginate hydrogel film is removed from the ethanol solution and dried to obtain a sodium alginate-based hydrogel colorimetric label.

[0067] In the above embodiments, the concentration of the metal ion crosslinking agent (i.e., calcium ion solution, aluminum ion solution, and zinc ion solution) is 0.25 mol / L to 0.5 mol / L. 0.25 mol / L is the minimum effective concentration, which allows for sufficient crosslinking between the metal ions and the carboxyl groups in alginate. At this concentration, the number of metal ions is sufficient to form enough crosslinking points, enhancing the structural stability and mechanical properties of the hydrogel. 0.5 mol / L is the maximum effective concentration, providing a stronger crosslinking effect and further improving the mechanical properties of the hydrogel, while avoiding the negative effects of excessively high concentrations. If the concentration is too low (less than 0.25 mol / L), the crosslinking effect will be insufficient, resulting in inadequate mechanical properties of the hydrogel, making it prone to cracking or deformation during use. If the concentration is too high (greater than 0.5 mol / L), although the crosslinking effect will be better, excessively high concentrations may lead to the following adverse effects: firstly, excessive metal ions can cause over-crosslinking of the hydrogel, making it too rigid and losing necessary flexibility and elasticity; secondly, high concentrations of metal ions can damage the structure of the hydrogel, making the film brittle and prone to cracking. Furthermore, this application uses 0.3 mol / L as the optimal concentration of the metal crosslinking agent because: a concentration of 0.3 mol / L provides sufficient metal ions for an effective crosslinking reaction, enhancing the structural stability and mechanical properties of the hydrogel while maintaining appropriate flexibility and elasticity. This concentration avoids insufficient crosslinking due to being too low, and excessive stiffness due to being too high. Testing has shown that this concentration, while ensuring the stability of the hydrogel network structure, optimizes the response speed and accuracy of the indicator to the target gas (e.g., ammonia).

[0068] In the above embodiments, the standing time after spraying or coating the metal ion crosslinking agent onto the surface of the sodium alginate hydrogel film for crosslinking treatment is set to 10–60 min. 10 min is the shortest effective time, within which metal ions can penetrate to the film surface and form preliminary crosslinking points with carboxyl groups. 60 min is the longest effective time, within which metal ions have sufficient time to penetrate into the film interior and form more crosslinking points with carboxyl groups, enhancing the structural stability and mechanical properties of the film. If the time is too short (<10 min), firstly, the crosslinking reaction will be insufficient, resulting in inadequate mechanical properties of the hydrogel, making it prone to cracking or deformation during use; secondly, due to the presence of free water, the response time of the hydrogel to the target gas will be prolonged, reducing its sensitivity. If the time is too long (>60 min), the hydrogel will be over-crosslinked, making it too rigid and losing necessary flexibility and elasticity. Furthermore, prolonged crosslinking reactions can damage the structure of the hydrogel, making the sodium alginate hydrogel film brittle and prone to cracking. Furthermore, this application uses 30 minutes as the optimal settling time because within 30 minutes, metal ions have sufficient time to penetrate into the sodium alginate hydrogel film and form more crosslinking points with carboxyl groups. This helps enhance the structural stability and mechanical properties of the film, ensuring that the crosslinking effect reaches an ideal state. If the settling time is less than 30 minutes, metal ions may not have enough time to penetrate into the sodium alginate hydrogel film and form sufficient crosslinking points with carboxyl groups. This will result in an insufficiently dense crosslinking network, thus relatively weakening the structural stability and mechanical properties of the hydrogel. On the other hand, if the settling time is greater than 30 minutes, it will relatively increase the possibility of over-crosslinking of the hydrogel, making it too rigid and losing the necessary flexibility and elasticity. Such over-crosslinking will not only affect the feel and user experience of the material, but may also cause damage to the hydrogel structure, making the film brittle and fragile.

[0069] In the above embodiments, the concentration of the ethanol solution is set to 10% v / v to 30% v / v. Experiments have verified that 10% v / v is the minimum effective concentration; at this concentration, the number of ethanol molecules is sufficient to initiate the solvent exchange process without significantly affecting the physical properties of the film. 30% v / v is the maximum concentration; at this concentration, the number of ethanol molecules is sufficient to completely displace free water, improving the physical properties of the film. Furthermore, if the concentration is too low (<10% v / v), the solvent exchange effect will be insufficient, and the free water inside the sodium alginate hydrogel film cannot be completely replaced. This will lead to uneven distribution of the indicator inside the film, resulting in inconsistent color changes and affecting the resolution and accuracy of the colorimetric label. Conversely, if the concentration is too high (>30% v / v), the sodium alginate hydrogel film will be over-dehydrated, becoming dry, brittle, and losing its necessary flexibility. In addition, excessive ethanol will affect the activity of the indicator, reducing its responsiveness to the target gas.

[0070] In summary, setting the concentration of the ethanol solution to 10% v / v to 30% v / v ensures that the solvent exchange process is both sufficient and not excessive, while avoiding adverse effects on the physical properties and structure of the film. This range of ethanol concentration balances the solvent exchange effect and the physical properties of the film, ensuring that the prepared alginate-based colorimetric indicator label has good physical properties and colorimetric response.

[0071] In the above embodiments, the ultrasonic treatment time of 0.5h to 3h was experimentally verified. 0.5h is the minimum effective treatment time; within this time, ultrasound can significantly accelerate the penetration of ethanol molecules into the sodium alginate hydrogel film, initially displacing free water. 3h is the maximum treatment time; within this time, ethanol molecules can completely penetrate into the film, maximizing the dissolution of free water. Testing showed that if the ultrasonic time is too short (less than 0.5h), solvent exchange will be insufficient, meaning that a significant amount of free water remains inside the sodium alginate hydrogel film. This will affect the physical properties of the sodium alginate hydrogel film, including its elastic modulus and mechanical strength. Furthermore, the presence of free water will prolong the response time of the sodium alginate hydrogel film to the target gas, thus reducing the sensitivity of the indicator tag. Conversely, if the ultrasound time is too long (greater than 3 hours), firstly, it will cause the polymer chains inside the sodium alginate hydrogel film to break, making the sodium alginate hydrogel film brittle and prone to cracking; secondly, excessive ethanol penetration will cause the sodium alginate hydrogel film to be over-dehydrated, making it dry, hard and brittle, losing the necessary flexibility, thereby affecting the activity of the indicator and reducing its responsiveness to the target gas.

[0072] In summary, the ultrasonic treatment time is set to 0.5 to 3 hours to ensure sufficient solvent exchange while avoiding damage to the sodium alginate hydrogel film structure, thereby ensuring that the prepared alginate-based colorimetric indicator label has good physical properties and colorimetric response. Furthermore, this application uses 2 hours as the optimal ultrasonic treatment time because, within the 0.5 to 3 hour range, 2 hours ensures that ethanol molecules fully penetrate into the sodium alginate hydrogel film and maximally displace free water, which helps improve the film's physical properties, such as elastic modulus and mechanical strength, while ensuring uniform distribution of the indicator within the film. Ultrasonic treatment exceeding 2 hours may cause polymer chain breakage within the sodium alginate hydrogel film, making the film brittle and prone to cracking. In addition, prolonged ultrasonic treatment may lead to excessive dehydration of the film, making it dry, brittle, and losing necessary flexibility, affecting its practical application performance. In summary, 2 hours is considered the optimal ultrasonic treatment time because it ensures sufficient solvent exchange while avoiding damage to the sodium alginate hydrogel film structure, thus ensuring that the prepared alginate-based colorimetric indicator label has good physical properties and colorimetric response.

[0073] This application employed a confocal laser microscope (Leica TCS SP8 STED) and an Olympus large metallurgical microscope (OLYMPUS GX71) to compare and analyze the surface and cross-section of sodium alginate hydrogel films before and after induction. Sample SA is a sodium alginate hydrogel film, and samples SG1, SG2, and SG3 represent sodium alginate to gelatin ratios of 3:1, 1:1, and 1:3, respectively. Figure 2 The images show the microstructure of the surface and cross-section of sodium alginate and its composite hydrogel film before induction. Figure 3 These are microstructure results of the surface and cross-section of sodium alginate and its composite hydrogel film after induction. The results show that the distribution of pore structure shapes on the surface and cross-section of sodium alginate and its composite hydrogel film before induction is significantly different, with the surface showing larger pores compared to the cross-section. After induction, the surface pores tend to be denser and more uniform, while the cross-section shows larger pores.

[0074] Figure 4 This is a laser confocal observation result of sodium alginate and its composite hydrogel film before induction; Figure 5 This is a laser confocal microscopy observation result of sodium alginate and its composite hydrogel film after induction. Figure 4 The distribution of the colorimetric agent on the surface of the sodium alginate composite hydrogel film before induction is uneven. Figure 5The results show that the distribution of the chromogenic agent in the cross-section of the induced sodium alginate hydrogel film (SA) is uneven, while the chromogenic agent in the surface and cross-section of the induced sodium alginate composite hydrogel film (SA / Gel) is more uniformly distributed throughout the hydrogel. These results indicate that the structure and chromogenic agent distribution of the sodium alginate composite hydrogel film are more uniform after the induction process.

[0075] This application also compared and analyzed the mechanical properties of sodium alginate and its composite hydrogel film before and after induction. The mechanical properties of the hydrogel were characterized using a universal tensile testing machine (UTM2103, Shenzhen Sansi) at room temperature (25℃) and 75% RH. For the tensile test, the hydrogel was cut into 50 mm × 10 mm × 2 mm strips, and the tensile speed was 50 mm / min. The elastic modulus was calculated using the slope of the initial linear region (strain 5–10%) of the stress-strain curve. Figure 6 The graph shows the elastic modulus results of sodium alginate and its composite hydrogel before induction. Figure 7 The graph shows the elastic modulus results of sodium alginate and its composite hydrogel after induction. Figure 8 This is a comparison chart of the elastic modulus of sodium alginate-based hydrogel samples prepared by the method described in this application with those of sodium alginate-based hydrogel samples from other sources. Figure 6 In the study, the stress-strain curves of sodium alginate and its composite hydrogel before induction showed a small slope in the initial linear region (strain 5%–10%), indicating that the stiffness of sodium alginate and its composite hydrogel before induction was low. Figure 7 In the study, the slope of the stress-strain curves of sodium alginate and its composite hydrogels after induction significantly increased in the initial linear region (strain 5%–10%), indicating that the elastic modulus of sodium alginate and its composite hydrogels was greatly improved after induction, making them harder and thus achieving better mechanical strength. Figure 8 It is evident that the elastic modulus of the sodium alginate-based hydrogel sample prepared in this application is significantly higher than that of sodium alginate-based hydrogel samples from other sources. Specifically, the elastic modulus of the sodium alginate-based hydrogel sample after induction treatment increased by approximately 35 times compared to that before induction. This indicates that the method of this application not only enhances the mechanical strength of the material itself but also exhibits significant performance advantages compared to similar materials in the prior art. This enhanced mechanical property is crucial for practical applications, such as in the food packaging field, where it is necessary to ensure that labels can withstand certain physical stresses without damage.

[0076] This application also tested the colorimetric response and sensitivity of sodium alginate-based hydrogels to ammonia. The samples were placed in glass petri dishes, and a 1.4% ammonia solution (800 mM) was added. The color changes of the samples were recorded, and the color parameters (L*, a*, b*) and (R, G, B) of the label color development were obtained using Adobe Photoshop CC 2019. Figure 9 The graph shows the colorimetric response of sodium alginate and its composite hydrogel to ammonia before induction. Figure 10 The image shows the colorimetric response of sodium alginate and its composite hydrogel to ammonia after induction. Comparison revealed that this method produces a more uniform colorimetric label based on the sodium alginate-based composite hydrogel.

[0077] Figure 11 The graph shows the sensitivity results of sodium alginate and its composite hydrogel before induction. Figure 12 This is a graph showing the sensitivity results of sodium alginate and its composite hydrogel after induction. Figure 12 In the study, the sensitivity of induced sodium alginate and its composite hydrogel was significantly improved. The results showed that the sensitivity of the induced sodium alginate composite hydrogel was increased by about 16% compared with that of the sodium alginate hydrogel.

[0078] To further quantify the resolution of colorimetric labels, point calculations are performed on the colors within the distribution matrix of the color display area to obtain an RGB three-coordinate scatter plot. The clustering trend of points in the scatter plot can quantify the color uniformity of the colorimetric label, i.e., the colorimetric resolution. Figure 13 This paper presents a comparison of the quantitative results of color uniformity of sodium alginate and its composite hydrogel before and after induction processing. The figure uses an RGB three-axis scatter plot to quantify the colorimetric resolution of the colorimetric label. Figure 13 As shown, the scatter plot of the colorimetric response of the untreated sample exhibits a large dispersion, meaning the points are widely spaced and show little aggregation. This wide distribution indicates significant differences in color parameters (e.g., RGB values) across different regions, suggesting an uneven distribution of the chromogenic agent throughout the hydrogel, resulting in noticeable regional variations in the colorimetric response. In the treated sample, the points in the scatter plot are more tightly clustered, forming more defined clusters and showing less dispersion. This tight clustering indicates reduced differences in color parameters between regions, demonstrating a more uniform distribution of the chromogenic agent within the hydrogel, leading to a more consistent response of the entire label to environmental changes. Furthermore, with improved color uniformity, the sample's sensitivity to environmental changes increases, enabling more accurate reflection of subtle environmental variations. Higher resolution means that even under low concentration conditions, rapid and accurate color changes can be achieved.

[0079] In summary, this method significantly improves the colorimetric response sensitivity and color change uniformity of sodium alginate composite hydrogels, resulting in higher resolution of colorimetric changes. This demonstrates the importance of the induction process for improving label sensitivity and resolution.

[0080] The above are merely preferred embodiments of this application and do not limit the patent scope of this application. Any equivalent structural or procedural transformations made using the content of this application's specification and drawings, or direct or indirect applications in other related technical fields, are similarly included within the patent protection scope of this application.

Claims

1. A method for preparing an alginate-based hydrogel colorimetric label, characterized in that, The preparation method comprises: adding alginate and gelatin or agar as a regulating material in a set ratio into an aqueous solution containing an indicator solution and glycerol to stir and heat to obtain a mixed solution; preparing the mixed solution into an alginate hydrogel film based on a casting method or a flow casting method; uniformly spraying or coating a metal ion crosslinking agent on the surface of the alginate hydrogel film to perform crosslinking treatment, and obtaining a crosslinking-treated alginate hydrogel film after standing; immersing the crosslinking-treated alginate hydrogel film into an ethanol solution, and obtaining an alginate-based hydrogel colorimetric label after assisting ultrasonic induction through solvent exchange; wherein the concentration of the metal ion crosslinking agent is 0.25 mol / L-0.5 mol / L; the standing time of the alginate hydrogel film after the metal ion crosslinking agent is sprayed or coated on the surface of the sodium alginate hydrogel film to perform crosslinking treatment is 10 min-60 min; the concentration of the ethanol solution is 10% v / v-30% v / v; the assisting ultrasonic time is 0.5 h-3 h.

2. The production method according to claim 1, characterized by, The alginate comprises sodium alginate or oxidized sodium alginate.

3. The method of claim 1, wherein, The indicator solution is prepared by dissolving an indicator in anhydrous ethanol or an aqueous solution.

4. The production method according to claim 3, characterized by, The indicator comprises any one or two of the following: methyl red, bromothymol blue, methylene blue, bromocresol purple and cyanidin.

5. The preparation method according to claim 1, characterized in that, The metal ion crosslinking agent comprises any one of the following: a calcium ion solution, an aluminum ion solution and a zinc ion solution.

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