Edible intelligent packaging film based on carbon dot doping as well as preparation method and application of edible intelligent packaging film

Abstract

The invention provides an edible intelligent packaging film based on carbon dot doping and a preparation method and application thereof, and belongs to the technical field of food preservation. L-tryptophan and glucose are dissolved in water, an obtained carbon source solution reacts at the temperature of 120-180 DEG C, cooling, dialysis, filtration and freeze drying are conducted, and the carbon quantum dots are obtained; the preparation method comprises the following steps: dissolving chitosan in an aqueous solution of vitamin C, adding carbon quantum dots, uniformly stirring, and defoaming to obtain a composite solution; spraying the composite solution to form a film layer, and drying to obtain the edible intelligent food packaging film based on carbon dot doping.

Description

TECHNICAL FIELD

[0001] The application relates to an edible intelligent packaging film based on carbon dot doping and a preparation method and application thereof, and belongs to the field of food preservation. BACKGROUND

[0002] Traditional petroleum-based plastic packaging films are gradually difficult to meet the development needs of the food industry due to problems such as non-degradability, food contact safety hazards and single function. Among many alternative materials, chitosan, as a natural cationic polysaccharide, has good film-forming property, antibacterial activity and gas barrier property, and is natural, degradable and biocompatible, and has been widely studied in the field of food packaging.

[0003] However, the traditional chitosan edible film has obvious deficiencies: first, the antibacterial property is limited, and the action time is short, which is difficult to effectively prolong the shelf life of perishable food; second, it does not have intelligent monitoring function and cannot real-time feedback the freshness state of food, so consumers cannot judge whether the food is deteriorated. At present, there is no mature intelligent packaging film product on the market which integrates the functions of edibility, antibacterial property and visual indication. SUMMARY

[0004] Therefore, the application first provides a preparation method of an edible intelligent packaging film based on carbon dot doping.

[0005] Specifically, the application is realized by the following scheme: A preparation method of an edible intelligent packaging film based on carbon dot doping, the steps are as follows: Step one, L-tryptophan and glucose are dissolved in water, the obtained carbon source solution is reacted at 120-180 DEG C, and after cooling, it is subjected to dialysis, filtration and freeze-drying to obtain carbon quantum dot powder; Step two, chitosan is dissolved in a vitamin C (VC) aqueous solution, after stirring and dissolving, carbon quantum dots are added and stirred again to mix uniformly, and then defoaming is performed to obtain a composite solution; Step three, the composite solution is sprayed to form a film layer, and dried to obtain an edible intelligent food packaging film based on carbon dot doping.

[0006] The above method is based on natural materials as a matrix to prepare an edible intelligent packaging film with multiple functions such as antibacterial, pH response, fluorescent intelligent visualization, and has the advantages of long shelf life of fruits and vegetables, low loss, intelligent visualization of preservation degree, etc.

[0007] Further, as preferred: In step one, In the carbon source solution, the addition amount of L-tryptophan is 6-12 wt%, and the addition amount of glucose is 1-5 wt%.

[0008] The dialysis refers to treatment with a dialysis bag with a molecular weight cut-off of 3.5 kDa.

[0009] The filtration uses a 0.22 μm needle filter.

[0010] The freeze-drying time is 48-72 h.

[0011] The carbon quantum dots have carboxyl and amino functional groups on the surface and a particle size of less than 10 nm.

[0012] In step two, The temperature for stirring and dissolving is 20-40℃ and the time length is 6-12 h.

[0013] The time length for re-stirring is 30-60 min.

[0014] The defoaming uses ultrasonic defoaming and the defoaming time length is 10-30 min.

[0015] The concentration of vitamin C is 0.5-1.5 wt% and the pH is 3.8-4.5.

[0016] A plasticizer is also added, and the addition amount of the plasticizer in the composite solution is 0.5-1.5 wt%. Preferably, the plasticizer is glycerol or sodium citrate.

[0017] In the composite solution, the final concentration of tryptophan-derived carbon dots (Trp-CDs) corresponding to the carbon quantum dots is 0.01-0.1 wt%.

[0018] In the composite solution, the addition amount of chitosan is 1-3 wt%.

[0019] In step three, The drying is natural air drying or oven drying. More preferably, the natural air drying is at 20-30℃ for 12-24 h. The oven drying is at 25℃-30℃ for 18-24 h.

[0020] The thickness of the packaging film is 10-20 μm.

[0021] The applicant also provides an edible intelligent packaging film prepared by the above method, which is based on chitosan and doped with amino acid-derived carbon quantum dots (preferably with a doping amount of 0.01-0.1 wt%), has antibacterial properties and pH-responsive fluorescent visibility. The edible intelligent packaging film of the present application introduces carbon quantum dots synthesized by a hydrothermal method from a natural carbon source, endows the packaging film with excellent fluorescent properties, low toxicity, good biocompatibility and low cost, thereby preparing an edible packaging film with antibacterial properties and intelligent sensing functions.

[0022] The applicant also provides application of the edible intelligent packaging film prepared by the above method in food preservation, and the composite solution is sprayed on the surface of the food to be preserved. In particular, the application is applied to perishable fruits such as strawberries, blueberries and bananas, so as to achieve antibacterial preservation and monitor the fruit spoilage state through the change of fluorescence intensity.

[0023] The application provides a preparation method of a tryptophan carbon dot (Trp-CDs) doped edible intelligent packaging film, the prepared packaging film takes chitosan as a main matrix, the carbon quantum dots are synthesized from natural amino acids and glucose, the material source is natural, safe and edible, and the material is biodegradable in a natural environment, does not cause environmental pollution, meets the green development trend, and has excellent antibacterial performance. Chitosan has certain antibacterial activity, and cooperates with the carbon quantum dots, so that the growth and reproduction of common pathogenic bacteria such as escherichia coli and staphylococcus aureus can be effectively inhibited, and the preservation period of food is prolonged.

[0024] The above scheme provides that the doped carbon quantum dots in the packaging film have excellent fluorescence performance and are sensitive to pH changes. When food spoils, acidic or alkaline substances are generated to cause changes in the surrounding environment pH, and the fluorescence intensity of the packaging film changes, so that the freshness of the food can be visually monitored, and consumers can directly judge whether the food is deteriorated. Furthermore, by adding a plasticizer, the flexibility and anti-cracking property of the film body are improved, so that the packaging film has good mechanical properties; the packaging film is suitable for film forming processes such as dip coating, and can be directly sprayed on the surface of perishable fruits such as strawberries, blueberries and bananas, and the application mode is flexible and various. Experimental results show that the carbon quantum dot aqueous solution has good biological safety and does not cause secondary pollution to food, so that the safety of the food is ensured. BRIEF DESCRIPTION OF DRAWINGS

[0025] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiments or prior art description will be briefly introduced. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor.

[0026] Figure 1 TEM morphology of carbon quantum dots prepared in Example 1; Figure 2 SEM morphology of hydrogel film prepared in Example 1; Figure 3 Fluorescence spectrum of carbon quantum dots prepared in Example 1; Figure 4 Fluorescence pH response curve of carbon quantum dots prepared in Example 1; Figure 5A schematic diagram of the fluorescence acid-base response of the hydrogel prepared in Example 1; Figure 6 A comparison chart of the antibacterial properties of the hydrogel prepared in Example 1 and the carbon quantum dot doped hydrogel; Figure 7 A biological safety evaluation chart of the carbon quantum dot aqueous solution prepared in Example 1; Figure 8 A strawberry preservation evaluation chart of the hydrogel packaging film without spraying and the hydrogel packaging film with spraying; Figure 9 A banana preservation evaluation chart of the hydrogel packaging film with local spraying. DETAILED DESCRIPTION

[0027] In order to make the technical problems, technical solutions and beneficial effects to be solved in the present application more clear and explicit, the technical solutions in the embodiments of the present application will be further described in detail below in combination with the drawings in the embodiments of the present application. It should be understood that the specific embodiments described herein are only used to explain the present application and not used to limit the technical solutions of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor fall within the scope of protection of the present application.

[0028] Example 1

[0029] In this embodiment, the preparation of edible intelligent food packaging film based on carbon dot doping is carried out, and the specific steps are as follows: S1, 10 wt% of L-tryptophan and 3 wt% of glucose were mixed and dissolved in pure water, and the raw materials were uniformly dispersed by ultrasonic dispersion. The obtained mixed solution was poured into a reaction kettle and reacted at 160 ℃ for 4 h.

[0030] S2, the product obtained by S1 reaction was cooled to room temperature to obtain a light yellow liquid.

[0031] S3, after dialysis of the light yellow liquid Mn=3.5 kDa in a dialysis bag, 0.22 μm needle filter was used to filter out incomplete raw materials and impurities, and then freeze-dried for 72 h to obtain carbon quantum dot powder, which was marked as CDs and stored at 4 ℃ for standby.

[0032] S4: weigh ascorbic acid (VC) and add distilled water to stir until completely dissolved to obtain a VC aqueous solution with a pH of about 4.0 and a concentration of 1 wt%.

[0033] S5, according to the feeding ratio of 2 wt% of the total mass of the composite solution, chitosan powder was added to the VC aqueous solution, and stirring was carried out under the condition of 30 ℃. After the magnetic stirrer stirred for 12 h, the filter cloth was used to filter out the undissolved impurities to obtain a transparent or slightly emulsified chitosan-based liquid.

[0034] S6, the carbon quantum dots powder obtained in S3 was added into pure water to dissolve and obtain a carbon quantum dots aqueous solution, denoted as CDs aqueous solution.

[0035] S7, the carbon quantum dots aqueous solution of S6 was added into the chitosan-based solution of S5, and the final concentration of the carbon quantum dots was controlled to be 0.05 wt%, and the solution was stirred for 45 min. Glycerol was added in an amount of 1 wt% of the total mass of the composite solution, and the solution was stirred uniformly. After being placed in a water bath (25°C) for ultrasonic treatment for 10 min to remove bubbles, the solution was left to stand for 2 h to improve the stability of the solution, and a composite solution was obtained.

[0036] S8, 10 mL of the composite solution was sprayed on the surface of fruits to form a uniform film layer, and the film layer was naturally dried in a ventilated environment at 25°C for 24 h to obtain a packaging film with a thickness of about 15 μm.

[0037] The carbon quantum dots and the packaging film prepared in the above process were subjected to performance testing. 1) Identification: TEM morphology analysis was performed on the carbon quantum dots aqueous solution of S6, and the results are shown in FIG. 2. The particle size of the carbon quantum dots was less than 10 nm, and the dispersibility was good. SEM morphology analysis was performed on the packaging film obtained in S8, and the results are shown in FIG. 3. The microstructure of the film had a porous characteristic, indicating that the film had excellent gas permeability. Figure 1 Figure 2 2) Fluorescent performance: Fluorescent analysis was performed on the carbon quantum dots aqueous solution of S6, and pure water was used as a control group. The results are shown in FIG. 4. Pure water (i.e., the left sample in FIG. 4) had no fluorescence, while the carbon quantum dots aqueous solution (i.e., the right sample in FIG. 4) had good fluorescent performance.

[0038] Further tests were performed on the pH responsiveness of the carbon quantum dots aqueous solution of S6. The pH of the carbon quantum dots aqueous solution was controlled to be between 1 and 13 (the pH value can be adjusted by adding acid, base or buffer salt to the carbon quantum dots aqueous solution). The results are shown in FIG. 5. When the pH value was controlled to be 1, the corresponding fluorescent intensity was as high as 3100 a.u. When the pH value was controlled to be between 2 and 5, the corresponding fluorescent intensity was basically maintained at about 3000 a.u. When the pH value was adjusted to be 6, the fluorescent intensity decreased to about 1730 a.u. When the pH value was greater than 6, the fluorescent intensity of the carbon quantum dots decreased significantly to below 500 a.u. with the change of the pH value. Figure 3 Figure 3 The pH responsiveness test was performed on the packaging film obtained in S8, and the results were consistent with the pH responsiveness results of the carbon quantum dots. The results are shown in FIG. 6. Figure 3

[0039] Figure 4

[0040] Figure 5 ​​​​​​When acid is added dropwise to the packaging film, its fluorescence performance is enhanced; while alkali is added dropwise, the fluorescence is quenched.

[0041] 3) Antibacterial property: The antibacterial property test of the packaging film obtained in S8 was performed, and the results are shown in the following table: Figure 6 The antibacterial circle diameter of the packaging film of the present example to S. aureus is about 5.3 mm; the antibacterial circle diameter to E. coli reaches 3.4 mm, and the antibacterial effect is remarkable.

[0042] 4) Biological safety evaluation: The biological safety evaluation of the carbon quantum dot aqueous solution obtained in S6 was performed (the control group is tap water, and the judgment is based on the growth height of the plants), and the results are shown in the following table: Figure 7 The plants in the carbon quantum dot aqueous solution grow faster, which proves that the carbon quantum dot aqueous solution prepared in the present application has good biological safety.

[0043] 5) Fresh-keeping effect: (1) The obtained packaging film is used for strawberry packaging: four strawberries of the same batch are selected, two of which are sprayed with the composite solution of the present example, and the other two are not sprayed with the composite solution of the present example. The fresh-keeping effect control is performed under room temperature conditions, and the results are shown in the following table: Figure 8 The unsprayed strawberries show signs of deterioration on the third day, and the sprayed strawberries still maintain good freshness on the ninth day, and the fresh-keeping period is extended by about 5 days compared with the unsprayed packaging, and when the strawberries begin to rot, the fluorescence intensity of the packaging film decreases significantly, realizing the visual monitoring of freshness.

[0044] (2) Local spraying is performed on the same strawberry, and the results are shown in the following table: Figure 9 The unsprayed banana starts to appear black spots on the third day, and starts to rot on the fifth day, and by the ninth day, the banana is basically blackened; while the sprayed part (the area within the yellow circle) Figure 9 starts to appear black spots on the seventh day, but still maintains good condition until the eleventh day.

[0045] Comparative Example 1

[0046] The preparation of the packaging film is performed in the present comparative example, and the specific steps are as follows: S1: Ascorbic acid (VC) is weighed and added to distilled water and stirred until completely dissolved to obtain a VC aqueous solution with a pH of about 4.0 and a concentration of 1 wt%.

[0047] S2, according to the feeding amount of 2 wt% of the total mass of the composite solution, chitosan powder is added to the VC aqueous solution, and stirring is performed under the condition of 30°C. After the magnetic stirrer is stirred for 12 h, the filter cloth is used to filter the undissolved impurities, and a transparent or slightly emulsified chitosan-based liquid is obtained.

[0048] S3, glycerol was added into the chitosan base solution at a feeding amount of 1 wt% of the total mass of the composite solution, and after stirring uniformly, the solution was placed in a water bath at 25°C for ultrasonic treatment for 10 min to remove bubbles, and then was left to stand for 2 h to obtain a composite solution.

[0049] S4, 10 mL of the composite solution was sprayed on the surface of the fruit to form a uniform film layer, and the film layer was naturally dried in a ventilated environment at 25°C for 24 h to obtain a packaging film with a thickness of about 15 μm.

[0050] In the preparation process of Comparative Example 1, no carbon quantum dots were doped, and the obtained packaging film had certain antibacterial property and edibility, but did not have fluorescent response function and could not monitor the freshness of food, and the antibacterial performance was lower than that of Example 1, and the inhibition zone diameter of the packaging film against S. aureus was about 3.5 mm, and the inhibition zone diameter of the packaging film against E. coli was 2.1 mm.

[0051] Example 2

[0052] In this example, the settings were the same as those of Example 1, except that in S7, the final concentration of carbon quantum dots was controlled to be 0.01 wt% and 0.1 wt%, respectively.

[0053] The results showed that: The carbon quantum dots provided in this example had certain pH response and fluorescence. However, when the final concentration of carbon quantum dots was lower than 0.01 wt%, the fluorescence response sensitivity of the packaging film was low, the monitoring of the pH change of food spoilage was not obvious enough, and the influence on the antibacterial performance was poor. When the concentration was 0.1 wt%, the fluorescence response sensitivity was good, but the high concentration of carbon dots would damage the structure of the film, and the influence on the antibacterial performance was good.

[0054] Therefore, the final concentration of carbon quantum dots is preferably about 0.05 wt%.

[0055] Example 3

[0056] In this example, the settings were the same as those of Example 1, except that in S7, the amount of glycerol added was replaced by 0.5 wt% and 1.5 wt%, respectively.

[0057] The introduction of plasticizer can improve the flexibility and anti-cracking property of the film body, so that the packaging film has good mechanical properties.

[0058] When the amount of glycerol added was 0.5 wt%, it could be judged by hand feeling that the packaging film was brittle and easy to break; when the amount of glycerol added was 1.5 wt%, the film body was too soft and sticky, which affected the use; and when the amount of glycerol added was 1 wt%, the flexibility and anti-cracking property of the film body were good, so the amount of glycerol added was preferably 1 wt%.

[0059] Comparative Example 2

[0060] The preparation of the packaging film is carried out in the present comparative example, and the specific steps are as follows: S1: weigh ascorbic acid (VC) and add distilled water to stir until completely dissolved to obtain a VC aqueous solution with a pH of about 4.0 and a concentration of 1wt%.

[0061] S2, add 2wt% chitosan powder to the VC aqueous solution, and stir while adding at 30℃. After stirring for 12h by a magnetic stirrer, filter the undissolved impurities with filter cloth to obtain a transparent or slightly emulsified chitosan base solution.

[0062] S3, after ultrasonic defoaming for 10 min at 25℃ water bath, the chitosan base solution is placed for 2h. Take 10 mL of the obtained solution and spray it on the surface of the fruit to form a uniform film layer, and then dry it in a ventilated environment at 25℃ for 24h to obtain a packaging film with a thickness of about 15 μm.

[0063] Comparative Example 1 uses a traditional chitosan film, which is not doped with carbon quantum dots and does not add plasticizer during preparation. The obtained packaging film is easy to break, softens quickly in a humid environment, and does not have the function of fluorescence monitoring. The same batch of strawberries as Example 1 is preserved, and the preservation period is only extended by 2 days, and the preservation effect is obviously not as good as that of the packaging film of Example 1. The carbon quantum dots prepared in the present application are sensitive to pH changes. When the food spoils, acidic or alkaline substances will be produced, causing changes in the surrounding environment pH, and the fluorescence intensity of the packaging film will change accordingly, thereby realizing the visual monitoring of food freshness, and facilitating consumers to intuitively judge whether the food has deteriorated.

[0064] The above examples only express several feasible implementation manners of the present application, and the description is more specific and detailed, but it cannot be understood as a limitation on the scope of the present application patent, and the examples are not used to limit the protection scope in the claims of the present application. For ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, and any equivalent implementation or change made without departing from the present application shall be included in the present application technology.

Claims

1. A method for preparing an edible smart packaging film based on carbon doping, characterized in that, The steps are as follows: Step 1: Dissolve L-tryptophan and glucose in water, react the resulting carbon source solution at 120~180℃, cool, dialyze, filter, freeze dry to obtain carbon quantum dots; Step 2: Dissolve chitosan in an aqueous solution of vitamin C, then add carbon quantum dots, stir evenly, remove bubbles, and obtain a composite solution; Step 3: The composite solution is sprayed to form a film layer, which is then dried to obtain an edible smart food packaging film based on carbon doping.

2. The method for preparing an edible smart packaging film based on carbon doping according to claim 1, characterized in that: In step one, the amount of L-tryptophan added to the carbon source solution is 6-12 wt%, and the amount of glucose added is 1-5 wt%.

3. The method for preparing an edible smart packaging film based on carbon doping according to claim 1, characterized in that: The dialysis refers to treatment using a dialysis bag with a molecular weight cutoff of 3.5 kDa.

4. The method for preparing an edible smart packaging film based on carbon doping according to claim 1, characterized in that: The concentration of vitamin C is 0.5-1.5 wt%, and the pH is 3.8-4.

5.

5. The method for preparing an edible smart packaging film based on carbon doping according to claim 1, characterized in that: In step two, a plasticizer is also added, and the amount of plasticizer added to the composite solution is 0.5~1.5 wt%.

6. The method for preparing an edible smart packaging film based on carbon doping according to claim 5, characterized in that: The plasticizer is glycerol or sodium citrate.

7. The method for preparing an edible smart packaging film based on carbon doping according to claim 1, characterized in that: In the composite solution, the final concentration of tryptophan-derived carbon dots corresponding to carbon quantum dots is 0.01~0.1 wt%.

8. The method for preparing an edible smart packaging film based on carbon doping according to claim 1, characterized in that: The amount of chitosan added to the composite solution is 1-3 wt%.

9. An edible smart packaging film prepared by the method of claim 1, which is based on chitosan and doped with amino acid-derived carbon quantum dots, and has antibacterial properties and pH-responsive fluorescence.

10. The application of a carbon dot-doped edible smart packaging film prepared by the method of claim 1 in food preservation, wherein a composite solution is sprayed onto the surface of the food to be preserved.

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