Method for preparing dimensionally stable smart indicating labels using wood preserving lignin

Abstract

This invention relates to a method for preparing dimensionally stable smart indicator labels using wood with retained lignin. It addresses the technical problems of existing blending methods for preparing smart indicator films, such as high cost and difficulty in identifying color changes due to humidity and transparency variations. The method involves: mixing epichlorohydrin, triethylamine, and ethanol to obtain a reaction solution; soaking poplar veneer in an alkaline solution to swell it before adding it to the reaction solution to obtain quaternized poplar veneer; then immersing the quaternized poplar veneer in a dye solution to adsorb bromothymol blue, followed by soaking in deionized water until neutral, and freeze-drying to obtain the smart indicator label. This label is highly sensitive to acetic acid gas, exhibiting a clear color change. When indicating milk freshness, the label changes from blue to green when °T is near the critical value specified by the national standard, providing a quick and accurate indication of milk freshness. This invention's smart indicator label can be used in the field of food testing.

Description

Technical Field

[0001] This invention relates to a method for preparing intelligent indicator labels, belonging to the field of food safety testing technology. Background Technology

[0002] Food quality is a key factor affecting consumer health. Every year, foodborne illnesses caused by spoiled food result in significant productivity and economic losses. With the development of new materials and innovative methods, the functional capabilities of packaging materials are rapidly expanding to enable real-time detection and reporting of food quality. Smart indicator labels convey information about food quality and safety to consumers by responding to various food quality indicators, thereby enabling real-time monitoring of packaged foods or their surrounding environment. Developing simple, accurate, low-cost, and scalable detection technologies is crucial for quantifying food quality, reducing the risk of foodborne illnesses, and promoting the intelligent development of the food industry.

[0003] Currently, smart food labels are mostly prepared by blending pH-sensitive natural pigments (anthocyanins, carotenoids, curcumin, betaine) or chemical indicators (bromocresol green, bromocresol violet, bromothymol blue, methyl red, methyl orange, etc.) with film-forming matrices such as plant polysaccharides, proteins, cellulose, and chitosan. On the one hand, the color and chemical properties of natural pigments are unstable and easily affected by oxygen, light, heat, and humidity; while chemically synthesized dyes may pose a health hazard if they migrate into food. On the other hand, this method has high raw material acquisition costs, the mechanical properties of the prepared film are affected by humidity, and color recognition is affected by transparency. Therefore, it is necessary to find a low-cost, stable, and opaque renewable material as a substrate to firmly anchor chemically synthesized dyes to the substrate surface. Summary of the Invention

[0004] The present invention aims to solve the technical problems of existing methods for preparing smart indicator films by blending, such as high cost and difficulty in identifying color changes due to humidity and transparency. Instead, it provides a method for preparing size-stable smart indicator labels by utilizing wood to retain lignin.

[0005] The method of preparing dimensionally stable smart indicator tags using wood with retained lignin according to the present invention is carried out according to the following steps:

[0006] I. Preparation of Quaternized Poplar Veneer (QW):

[0007] a. Mix epichlorohydrin and triethylamine in a molar ratio of 1:(1~1.1), then add an ethanol aqueous solution with a mass percentage concentration of 45%~55% equal to the volume of triethylamine, and place it in a constant temperature and constant speed magnetic stirring water bath at a temperature of 45~48℃ and a stirring speed of 0~400r for 3~3.5h to obtain the reaction solution;

[0008] b. Soak the poplar veneer in an alkaline solution with a NaOH concentration of 0.1% to 1.5% for 2 to 2.5 hours to allow it to swell.

[0009] c. Pour the swollen poplar veneer together with the alkaline solution into the reaction solution, place it in a constant temperature and speed magnetic stirring water bath at 65℃ and a stirring speed of 0-400r for 3-3.5h, remove it, wash it with anhydrous ethanol and deionized water in sequence, freeze dry it to obtain quaternized poplar veneer (QW).

[0010] II. Preparation of Smart Indicator Tags

[0011] a. Add bromothymol blue (BTB) and NaOH to deionized water at a mass concentration of 0.05–0.06 g / L and a NaOH concentration of 0.1–0.15 mol / L, mix well, and obtain a dye solution.

[0012] b. Immerse quaternized poplar veneer in a dye solution and adsorb it in a constant temperature water bath at 50–55℃ for 10–12 hours. Remove the veneer, wash away any excess dye with 0.1–0.15 mol / L NaOH, then soak it in deionized water until neutral. Freeze-dry to obtain a dimensionally stable smart indicator tag. This smart indicator tag is poplar veneer anchored to bromothymol blue (ABW).

[0013] Furthermore, in step b of step one, the ratio of the mass of the oven-dried poplar veneer to the volume of the alkali solution is 1 g : (40-150) mL.

[0014] Furthermore, in step c of step one, the ratio of the mass of the oven-dried poplar veneer to the volume of epichlorohydrin in the reaction solution is 1 g : (20-30) mL; the ratio of the mass of the oven-dried poplar veneer to the volume of the alkaline solution is 1 g : (40-80) mL.

[0015] Furthermore, during the adsorption process in step 2b, the surface is turned over to ensure uniform adsorption.

[0016] Wood is a ubiquitous renewable resource, primarily composed of cellulose, hemicellulose, and lignin. It possesses a unique, naturally occurring, anisotropic hierarchical porous structure ranging from the nanoscale to the macroscale, along with a large specific surface area and abundant active functional groups, making it suitable for developing functional materials based on renewable resources. This invention modifies fast-growing poplar wood through a mild alkaline treatment that preserves lignin, retaining the wood's porous structure, locking in color, preventing fading, eliminating pollution, providing a large indicator capacity, and exhibiting rapid liquid absorption through capillary action. While preserving lignin, a large number of quaternary ammonium salt groups are grafted onto the lignin on the wood surface, firmly anchoring anionic indicators to the wood surface. Simultaneously, BTB is used as a pH indicator; its stable and vivid color changes accurately and quickly indicate the freshness of food.

[0017] The smart indicator tag of this invention is a pH-responsive smart wood film formed by the adsorption of BTB by quaternized poplar veneer. This pH-responsive material has the following advantages:

[0018] This invention swells wood through mild alkali treatment and grafts quaternary ammonium salt groups onto lignin and cellulose. This method protects the dimensional stability of the wood while quaternizing the lignin. BTB is then adsorbed onto the wood surface and into the pores. The electrostatic interaction between the cationic groups of the modified lignin and cellulose and the anionic groups of BTB firmly anchors BTB to the wood surface and pores. The pH-responsive smart tag prepared by this invention exhibits excellent pH response performance, dimensional stability, and resistance to BTB release. Immersing the tag in buffer solutions of different pH values ​​immediately results in a significant color change; the dimensional stability of the tag is minimally affected by quaternization; no BTB is released after soaking the tag in deionized water for 24 hours, and no BTB is released after shaking the tag in 50% and 95% ethanol aqueous solutions for 24 hours. The mild alkaline treatment method of this invention preserves the porous structure of the wood, allowing it to quickly absorb liquid through capillary action during application, thereby increasing the speed of the indication reaction. The pH-responsive smart tag prepared by this invention has high sensitivity to acetic acid gas, and when indicating milk freshness, the tag changes from blue to green when °T is near the critical value specified by the national standard, which can quickly and accurately indicate the freshness of milk.

[0019] This invention provides a method for modifying wood to retain lignin as much as possible, resulting in a new material that is green, dimensionally stable, and exhibits easily identifiable color changes, thus indicating food freshness. It offers a new approach to expanding the application of low-quality wood in other fields, fully utilizing the structural and chemical characteristics of wood, with a simple preparation process and low raw material costs. The intelligent indicator label of this invention can be used in the field of food testing. Attached Figure Description

[0020] Figure 1The graph shows the single-plate dimensions and volume change rate of QW-1 prepared in step one of Example 1 and QW-2 prepared in step one of Example 2.

[0021] Figure 2 The graph shows the change rate of single-layer plate mass and porosity of QW-1 prepared in step one of Example 1 and QW-2 prepared in step one of Example 2.

[0022] Figure 3 The FTIR spectra are those of the original poplar veneer (OW), QW-1 prepared in step one of Example 1, and QW-2 prepared in step one of Example 2;

[0023] Figure 4 These are the FTIR spectra of poplar veneer (OW), quaternized poplar veneer QW-2, and smart indicator tag ABW-2 from Example 2;

[0024] Figure 5 The XRD patterns are of the original poplar veneer (OW), QW-1 prepared in step one of Example 1, and QW-2 prepared in step one of Example 2;

[0025] Figure 6 These are the XRD patterns of poplar veneer (OW), quaternized poplar veneer QW-2, and smart indicator tag ABW-2 from Example 2;

[0026] Figure 7 The XPS full spectrum and N1s fine spectrum of the original poplar veneer (OW), QW-1 prepared in step one of Example 1, and QW-2 prepared in step one of Example 2 are shown.

[0027] Figure 8 These are scanning electron microscope images of the original poplar veneer (OW), QW-2 prepared in step one of Example 2, and ABW-2 prepared in step two;

[0028] Figure 9 These are color photographs of OBW prepared in Comparative Example 1, ABW-1 prepared in Example 1, and ABW-2 prepared in Example 2;

[0029] Figure 10 These are photographs of OBW-2 prepared in Comparative Example 1 and ABW-2 prepared in Example 2 releasing BTB in water;

[0030] Figure 11 These are color photographs of the smart indicator tags prepared in Examples 1 and 2 in buffer solutions with different pH values;

[0031] Figure 12 This is a schematic diagram of the smart indicator tags prepared in Examples 1 and 2 responding to acetic acid gas;

[0032] Figure 13These are graphs showing the color changes of the smart indicator tags prepared in Examples 1 and 2 in response to acetic acid gas under different humidity levels.

[0033] Figure 14 These are migration diagrams of the smart indicator tags prepared in Examples 1 and 2 in an ethanol-water solution;

[0034] Figure 15 It shows the pH and °T changes during the storage of fresh milk and the corresponding colorimetric reaction diagram of the smart indicator label. Detailed Implementation

[0035] The beneficial results of the present invention will be verified using the following examples.

[0036] Example 1: The method for preparing dimensionally stable smart indicator tags using wood with retained lignin in this example is carried out according to the following steps:

[0037] I. Preparation of Quaternized Poplar Veneer:

[0038] a. Mix 60 mL of epichlorohydrin and 117 mL of triethylamine, then add 117 mL of 50% ethanol aqueous solution, and place in a constant temperature and speed magnetic stirring water bath at 45 °C and a stirring speed of 0 rpm for 3 h to obtain the reaction solution.

[0039] b. Cut poplar veneer with a thickness of 1 mm into poplar veneers with a length and width of 50.00 mm (OW). Soak 3 grams of oven-dried poplar veneer in 450 mL of NaOH with a mass percentage concentration of 0.1% for 2 hours to swell.

[0040] c. Pour the swollen poplar veneer together with 225 mL of alkaline solution into 294 mL of reaction solution, place it in a constant temperature and speed magnetic stirring water bath at 65℃ and 0 r for 3 h, take it out, wash it with anhydrous ethanol and deionized water in sequence, freeze dry it to obtain quaternized poplar veneer, denoted as QW-1.

[0041] II. Preparation of Smart Indicator Tags:

[0042] a. Add BTB and NaOH to deionized water at a mass concentration of 0.05 g / L and a NaOH concentration of 0.1 mol / L, mix well, and obtain a dye solution;

[0043] b. Immerse the quaternized poplar veneer in the dye solution and adsorb it in a constant temperature water bath at 50℃ for 12 hours. During adsorption, turn the veneer over to ensure uniform adsorption. Remove the veneer, wash away any excess dye with 0.1mol / L NaOH, then immerse it in deionized water until neutral, and freeze-dry to obtain the smart indicator tag. This smart indicator tag is used to anchor BTB poplar veneer and is designated ABW-1.

[0044] Example 2: The method for preparing dimensionally stable smart indicator tags using wood with retained lignin in this example is carried out according to the following steps:

[0045] I. Preparation of Quaternized Poplar Veneer:

[0046] a. Mix 90 mL of epichlorohydrin and 176 mL of triethylamine, then add 176 mL of 50% ethanol aqueous solution, and place in a constant temperature and constant speed magnetic stirring water bath at 45℃ and a stirring speed of 400 rpm for 3 hours to obtain the reaction solution.

[0047] b. Cut poplar veneer with a thickness of 1 mm into poplar veneers with a length and width of 50.00 mm (OW). Soak 3 grams of oven-dried poplar veneer in 450 mL of NaOH with a mass percentage concentration of 1.5% for 2 hours to swell.

[0048] c. Pour the swollen poplar veneer together with 225 mL of alkaline solution into 442 mL of reaction solution, place it in a constant temperature and speed magnetic stirring water bath at 65℃ and 400 r for 3 h, take it out, wash it with anhydrous ethanol and deionized water in sequence, freeze dry it to obtain quaternized poplar veneer, denoted as QW-2.

[0049] II. Preparation of Smart Indicator Tags:

[0050] a. Add BTB and NaOH to deionized water at a mass concentration of 0.05 g / L and a NaOH concentration of 0.1 mol / L, mix well, and obtain a dye solution;

[0051] b. Immerse the quaternized poplar veneer in the dye solution and adsorb it in a constant temperature water bath at 50℃ for 12 hours. During adsorption, turn the veneer over to ensure uniform adsorption. Remove the veneer, wash away any excess dye with 0.1mol / L NaOH, then immerse it in deionized water until neutral, and freeze-dry to obtain the smart indicator tag. This smart indicator tag is used to anchor BTB poplar veneer and is designated ABW-2.

[0052] Comparative Example 1: The preparation method of the poplar veneer with BTB adsorption in this comparative example is carried out according to the following steps:

[0053] a. Add BTB and NaOH to deionized water at a mass concentration of 0.05 g / L and a NaOH concentration of 0.1 mol / L, mix well, and obtain a dye solution;

[0054] b. Immerse poplar veneer in dye solution, adsorb in a constant temperature water bath at 50℃ for 12 hours, remove, wash off the floating color with 0.1mol / L NaOH, then soak in deionized water until neutral, freeze dry to obtain poplar veneer adsorbed with BTB, denoted as OBW.

[0055] The dimensional and volume change rates, as well as the mass and porosity change rates, of QW-1 prepared in step one of Example 1 and QW-2 prepared in step one of Example 2 were tested. The dimensional and volume change rates are as follows: Figure 1 As shown, the rate of change of mass and porosity is as follows: Figure 2 As shown, from Figure 1 It can be seen that quaternization has no significant effect on the dimensional and volumetric change rates of poplar veneer, with the order being tangential > radial > axial. As the quaternization conditions change, the dimensional and volumetric change rates of the veneer increase slightly, but not exceeding 2%, indicating that quaternization has a small impact on the dimensional stability of poplar veneer. Figure 2 It can be seen that the quaternized poplar veneer QW-1 prepared under the quaternization conditions of a material-to-liquid ratio of 1:20, 0.1% NaOH, and a rotation speed of 0 r showed a 2.30% decrease in mass and a 0.74% increase in porosity; while the quaternized poplar veneer QW-2 prepared under the conditions of a material-to-liquid ratio of 1:30, 1.5% NaOH, and a rotation speed of 400 r showed a 7.03% increase in mass and a 1.75% decrease in porosity. This is because a small amount of lignin and hemicellulose is removed during the alkali swelling process, leading to a decrease in veneer mass and an increase in porosity. With the change in quaternization conditions, the grafting rate of quaternary ammonium salts significantly increased, compensating for the weight loss caused by alkali swelling, resulting in a significant increase in veneer mass and a decrease in porosity.

[0056] FTIR spectra were performed on the original poplar veneer (OW), QW-1 prepared in step one of Example 1, and QW-2 prepared in step one of Example 2. The obtained FTIR spectra are shown below. Figure 3 As shown. The FTIR spectra of poplar veneer (OW), quaternized poplar veneer QW-2, and smart indicator tag ABW-2 in Example 2 are shown below. Figure 4 As shown. By Figure 3 It can be seen that the main FTIR characteristic absorption peaks of the original poplar veneer are: 3343 cm⁻¹. -1 (OH stretching vibration), 2917cm -1 and 2850cm -1 (CH stretching vibration), 1732cm -1 (acetyl stretching vibrations of lignin and hemicellulose), 1592cm -1 (lignin aromatic ring stretching vibration), 1235cm -1 (Phenolic ether bond stretching vibration), 1031 cm⁻¹ -1(CO stretching of cellulose and lignin). With changes in quaternization conditions, 1732 cm⁻¹ -1 The peak weakens or even disappears, 1235cm -1 The disappearance of the peak indicates that the cleavage of acetyl and phenolic ether bonds during quaternization leads to the degradation of hemicellulose and lignin. (1592 cm⁻¹) -1 The peak remained unchanged, indicating that the aromatic ring structure of lignin was not disrupted; 1546 cm⁻¹ -1 Peak disappearance 792cm -1 Peak enhancement, 1395cm -1 1263cm -1 New peaks appear at [locations], corresponding to -N. + The stretching vibrations of CN and COC in (C2H5)3 demonstrate the successful grafting of quaternary ammonium salts onto wood. From Figure 4 It can be seen that after BTB adsorption, the 1546 cm⁻¹ -1 Peak and 776cm -1 The peak disappears at 1333cm. -1 A stretching vibration peak of the sulfonic acid group (S=O) was detected at the site.

[0057] The XRD patterns of the original poplar veneer (OW), QW-1 prepared in step one of Example 1, and QW-2 prepared in step one of Example 2 are shown below. Figure 5 As shown, the XRD patterns of poplar veneer (OW), quaternized poplar veneer QW-2, and smart indicator tag ABW-2 in Example 2 are as follows. Figure 6 As shown. Figure 5 As shown, the XRD characteristic peaks of the original poplar veneer are at 16.02°, 21.87°, and 34.59°, corresponding to the (101), (002), and (040) crystal planes of cellulose, respectively. The original poplar veneer, quaternized poplar veneer, and anchored BTB poplar veneer are all cellulose type I. The crystallinity of cellulose was calculated using the Segal method: the crystallinity of the original poplar veneer was 61.10%, the crystallinity of the quaternized poplar veneer ranged from 61.98% to 62.40%, and the crystallinity of the anchored BTB poplar veneer ranged from 63.61% to 66.37%. The increased crystallinity is due to the OH groups generated during quaternization and adsorption. - The effect of this process leads to the removal of some lignin and hemicellulose, increasing the relative content of cellulose and thus increasing crystallinity. Figure 6 In the middle, the diffraction peak at 21.87° shifts to the right. According to Bragg's equation, as 2θ increases, the interplanar spacing decreases, indicating that OH... - This may have caused defects in the cellulose.

[0058] XPS full spectrum and N1s fine spectrum of the original poplar veneer (OW), QW-1 prepared in step one of Example 1, and QW-2 prepared in step one of Example 2 are as follows: Figure 7 As shown in the full spectrum (a) and N1s fine spectrum (bd), it can be seen that the original poplar veneer ( Figure 7 The binding energies of C, O, and N are 285.5 eV, 532.4 eV, and 400.0 eV, respectively. The N 1s spectrum ( Figure 7 The fitting peak for b) is at 399.6 eV. Quaternized poplar veneer ( Figure 7 The fitted peaks of the N1s spectrum of the cdN were at 399.15 eV and 401.64 eV, corresponding to the ternary and quaternary valence states of N, respectively, confirming the occurrence of quaternization modification. The former originates from the wood itself or triethylamine mixed in during the quaternization modification process. The results further confirmed the FTIR analysis. Table 1 shows the effect of different quaternization conditions on the elemental content of poplar veneer. With the change of quaternization conditions, the N content in poplar veneer increased. + The proportion increased.

[0059] Table 1. Effects of different quaternization conditions on elemental content in poplar veneer.

[0060]

[0061]

[0062] The microstructure of OW, QW-2, and ABW-2 in Example 1 was tested, and the resulting scanning electron microscope images are shown below. Figure 8 As shown, from Figure 8 It can be seen that the original veneer OW exhibits a hierarchical porous structure with a dense intercellular layer and tightly connected wood rays. Quaternized QW-2 shows slight damage to the intercellular layer, resulting in fine cracks and wood ray separation. Quaternization increases the specific surface area and porosity of the veneer, which is beneficial for subsequent adsorption. Tiny granular structures appear on the inner walls of the wood fibers, which are aggregates of quaternary ammonium salts. After BTB adsorption, the fibers swell, the granular structure disappears, and the wood surface becomes smooth, indicating that BTB molecules are uniformly adsorbed onto the pores and inner walls of the wood.

[0063] Table 2. Effects of different quaternization conditions on the BTB adsorption capacity and color of poplar veneer.

[0064]

[0065] The adsorption capacity of quaternized poplar veneers QW-1 and QW-2 for bromothymol blue (BTB) was calculated based on the initial BTB solution concentration, residual solution concentration, and the concentration of washed-off floating color. As shown in Table 2, under the same adsorption conditions, the adsorption capacity of the original poplar veneer OW was only 0.222 mg / g. After quaternization, the adsorption capacity of the veneer for BTB increased significantly. Changing the quaternization conditions can increase the grafting rate of the quaternary ammonium salt, thereby further increasing the adsorption capacity of the veneer. When the quaternization conditions were a material-to-liquid ratio of 1:30, 1.5% NaOH, and a rotation speed of 400 rpm, 1 g of quaternized poplar veneer could adsorb 8.504 mg of BTB, and the surface color was more uniform. With the increase of adsorption capacity, the L* and b* values ​​gradually decreased, indicating that the color of the veneer gradually deepened and the blue hue intensified. Specific colors are shown in Table 2. Figure 9 As shown.

[0066] The smart indicator tag ABW-2 prepared in Example 2 and the OBW prepared in Comparative Example 1 were soaked in deionized water. After 24 hours, the OBW prepared from the original poplar veneer almost faded to its original wood color, while the soaking solution turned a deep blue. Figure 10 As shown in (a); while the soaking solution of the smart indicator label ABW-2 prepared from quaternized poplar veneer is colorless, as shown in (a). Figure 10 As shown in (b), the absorbance measured with a UV-Vis spectrophotometer is 0. After blotting the surface of the veneer with a tissue to remove moisture, no color transfer occurs. Figure 10 As shown in (c), this demonstrates that after quaternization, bromothymol blue is firmly anchored to the single plate due to ionic bonding, resulting in a water-stable label.

[0067] Table 3 presents the color changes of the smart indicator tags prepared in Examples 1 and 2 in buffer solutions with different pH values. Specific colors are as follows: Figure 11 As shown, the chromaticity parameters are listed in Table 3. From Figure 11 As shown in Table 3, the label color changes significantly with increasing pH. ABW-1 appears orange at pH=2, yellow at pH=4, light green at pH=6, green at pH=7, lake blue at pH=8, dark blue at pH=10, and light blue at pH=12. ABW-2 appears orange at pH=2 and pH=4, dark green at pH=6, peacock blue at pH=7, and dark blue at pH>7. The L* value first decreases and then increases, indicating that the brightness first decreases and then increases. The change of a* from positive to negative indicates that the label changes from a reddish tone to a greenish tone. The decrease of b* from a positive value to a minimum negative value followed by an increase indicates that the label changes from a yellowish tone to a blue tone, with the strongest blue tone at pH=10. The color changes of ABW-1 at all pH levels are visually distinguishable (ΔE>5), while the color changes of ABW-2 are only visually distinguishable at pH≤8. The results indicate that the label has good pH sensitivity.

[0068] Table 3. Colorimetric parameters of the smart indicator tags prepared in Examples 1 and 2 at different pH values.

[0069]

[0070] The smart indicator tags prepared in Examples 1 and 2 were tested for their response to acetic acid gas. The test method is as follows: Figure 12 As shown: The smart indicator tag was suspended in a sealed transparent container. The humidity inside the container was adjusted to 11% RH, 33% RH, 53% RH, and 75% RH, respectively. 0.5 ml of glacial acetic acid was injected, and the response performance of the pH smart indicator tag to acetic acid gas was tested. The results show that with the increase of humidity and response time, the color change of the tag becomes more obvious. The color changes of ABW in response to acetic acid gas under different humidity conditions are shown below. Figure 13 As shown in Table 4, the chromaticity parameters are because in a high humidity environment, the label has a higher moisture content, which is conducive to the ionization of CH3COOH into more H+. + As time progresses, more and more acetic acid molecules are adsorbed, promoting structural changes in BTB molecules in an acidic environment. This demonstrates the label's good responsiveness to acetic acid gas.

[0071] Table 4. Colorimetric parameters of the smart indicator tags prepared in Examples 1 and 2 in response to acetic acid gas under different humidity levels.

[0072]

[0073]

[0074]

[0075] The migration of the smart indicator tags prepared in Examples 1 and 2 was tested in food simulants. A 50% ethanol aqueous solution was used as the simulant for oily foods, and a 95% ethanol aqueous solution was used as the simulant for fatty foods. 1 g of chopped ABW was placed in 50 mL of ethanol aqueous solution and shaken for 24 h in a water bath at 200 rpm. The absorbance of the solution in the 380–800 nm range was then measured using a UV-Vis spectrophotometer. The migration of ABW in the 50% and 95% ethanol aqueous solutions is shown below. Figure 14 As shown, ABW-1 exhibited slight migration in both 50% and 95% ethanol aqueous solutions, with greater migration in the 50% ethanol aqueous solution, while ABW-2 showed no migration in either solution. These results indicate that quaternized lignocellulose in ABW-2 is more effective at anchoring BTB.

[0076] Based on the migration of ABW in food simulants, ABW-2 was selected for indicating milk freshness. Fresh milk was stored in a 40°C incubator. Over time, lactic acid produced by microbial growth and metabolism gradually accumulated, leading to a decrease in pH. Figure 15 As shown in (a), the titratable acidity increases, as Figure 15 As shown in (b), milk with a temperature of °T ≤ 18°C ​​is considered good quality, and milk with a temperature of °T ≤ 20°C is considered acceptable. The freshness of the milk is tested by immersing a 6.0mm × 6.0mm smart indicator label ABW-2 in the milk for 1 minute. As time progresses, the label's b* value changes from negative to positive, as shown in (b). Figure 15 As shown in (a), the label changes from a blue hue to a yellow hue. At °T = 18.76, b* = 0.92; at °T = 20.63, b* = 6.92. This is because acidic substances such as lactic acid in milk provide an acidic environment when they come into contact with the label, causing the label to change from blue to green near the threshold specified in the national standard (GB 19301-2010) as °T increases. Figure 15 As shown in (c), this effectively indicates the spoilage of the milk.

Claims

1. A method for preparing dimensionally stable smart indicator tags using wood with retained lignin, characterized in that, This method is performed in the following steps: I. Preparation of Quaternized Poplar Veneer: a. Mix epichlorohydrin and triethylamine in a molar ratio of 1:(1~1.1), then add an ethanol aqueous solution with a mass percentage concentration of 45%~55% equal to that of triethylamine, and place it in a constant temperature and constant speed magnetic stirring water bath at a temperature of 45~48℃ and a stirring speed of 0~400 rpm for 3~3.5h to obtain the reaction solution; b. Soak the poplar veneer in an alkaline solution with a NaOH concentration of 0.1%~1.5% for 2~2.5 hours to allow it to swell; c. Pour the swollen poplar veneer together with the alkaline solution into the reaction solution, place it in a constant temperature and speed magnetic stirring water bath at 65℃ and a stirring speed of 0~400 rpm for 3~3.5h, take it out, wash it with anhydrous ethanol and deionized water in sequence, freeze dry it to obtain quaternized poplar veneer. II. Preparation of Smart Indicator Tags a. Add bromothymol blue and NaOH to deionized water at a mass concentration of 0.05~0.06 g / L and a NaOH concentration of 0.1~0.15 mol / L, mix well to obtain a dye solution; b. Immerse the quaternized poplar veneer in the dye solution and adsorb it in a constant temperature water bath at 50~55℃ for 10~12h. Then take it out, wash off the floating color with NaOH at a concentration of 0.1~0.15mol / L, and then soak it in deionized water until neutral. Finally, freeze dry to obtain a dimensionally stable smart indicator label.

2. The method for preparing dimensionally stable smart indicator tags using wood with retained lignin according to claim 1, characterized in that, In step b of step one, the ratio of the mass of the oven-dried poplar veneer to the volume of the alkali solution is 1 g : (40~150) mL.

3. The method for preparing dimensionally stable smart indicator tags using wood with retained lignin according to claim 1 or 2, characterized in that, In step 1c, the ratio of the mass of the oven-dried poplar veneer to the volume of epichlorohydrin in the reaction solution is 1g:(20~30)mL; the ratio of the mass of the oven-dried poplar veneer to the volume of the alkaline solution is 1g:(40~80)mL.

4. The method for preparing dimensionally stable smart indicator tags using wood with retained lignin according to claim 1 or 2, characterized in that, During the adsorption process in step b of step two, the surface is turned over to ensure uniform adsorption.

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