A carbon dot / silk protein antibacterial fresh-keeping liquid and a preparation method and application of a composite film thereof

Abstract

The present application relates to a kind of carbon dots / silk protein antibacterial fresh-keeping fluid and the preparation method and application of its composite film, the volume ratio of glycerol and carbon dots / silk protein mixed solution in the antibacterial fresh-keeping composite film is 1:20-1:500, the content of silk protein in carbon dots / silk protein mixed solution is 0.9-2.7wt%, and the concentration of carbon dots is 0.5-5mg / ml;The present application selects natural silk and horseradish, which are both biomass materials, to prepare carbon dots / silk protein mixed solution with fresh-keeping effect, glycerol is added to carbon dots / silk mixed solution, and carbon dots / silk protein antibacterial fresh-keeping composite film can be prepared;The fresh-keeping film has good water vapor and oxygen permeability, good fresh-keeping effect, and is degradable, with wide application prospect.

Description

Technical Field

[0001] This invention belongs to the field of food processing, specifically relating to a carbon dot / silk protein antibacterial preservative liquid and its composite film preparation method and application. Background Technology

[0002] Approximately one-third of the world's food is lost or wasted, resulting in losses of up to $1 trillion. Premature spoilage of perishable foods, particularly fresh fruits and vegetables, is a major cause of food waste in the supply chain, primarily due to their short post-harvest lifespan and susceptibility to spoilage. This situation is exacerbated by the extensive transportation, heavy transit, and multiple layers of intermediaries involved in the food supply chain. To extend the shelf life of perishable foods, various physical and chemical packaging technologies have been developed. These methods include cryopreservation, controlled atmospheres, and plastic packaging, which work by slowing the respiration rate of fruits and reducing microbial growth. Currently, petroleum-based plastic wrap is a common means of preservation in retail; however, these petroleum-based plastic products have limited functionality and are difficult to degrade. Some specific chemical preservation technologies, such as ozone, hydrogen peroxide, and irradiation, are also used to inhibit spoilage by removing preservatives. However, some of these technologies have drawbacks, including being time-consuming, expensive, environmentally unfriendly, and potentially causing irreversible damage to fruits and vegetables. Furthermore, the excessive use of preservatives has also raised further food safety concerns. Currently, widely used food wraps are mainly petroleum-based materials, with limited functionality and poor biodegradability. Developing new biodegradable food wraps contributes to sustainable development, and some biodegradable materials and multifunctional additives are emerging as alternatives to existing food wraps.

[0003] Silk fibroin, also known as silk protein, is a product of natural silk degumming. It possesses excellent processing properties, allowing it to be processed into various shapes, and is also degradable by proteases and soil microorganisms. Carbon dots, a novel zero-dimensional nanomaterial, exhibit excellent optical properties and have been extensively studied in recent years. Carbon dots have also been proven to possess excellent antibacterial and antioxidant capabilities, showing potential application value in mitigating microbial proliferation in fruits and vegetables. This invention addresses the shortcomings of existing technologies by developing a carbon dot / silk fibroin mixed solution. On one hand, this solution can be used as a post-harvest preservation solution for fruits, forming a thin preservative film directly on the fruit surface through an impregnation method. On the other hand, by introducing an appropriate amount of glycerol into the carbon dot / silk fibroin mixed solution, transparent, stretchable, large-size films with biodegradability can be manufactured, demonstrating potential application value. Summary of the Invention

[0004] In view of the shortcomings of the prior art, the present invention aims to provide a method for preparing a preservative liquid and a preservative film based on carbon dots and silk protein, and its application in fruit preservation.

[0005] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0006] An antibacterial preservative solution of carbon dots / silk protein, wherein the raw materials of the antibacterial preservative solution contain an aqueous solution of silk protein and an aqueous solution of carbon dots; the silk protein content in the carbon dots / silk protein mixed solution is 0.9-2.7 wt%, and the concentration of carbon dots is 0.5-5 mg / ml.

[0007] An antibacterial and preservative composite film of carbon dots / silk protein, wherein the volume ratio of glycerol to carbon dots / silk protein mixed solution is 1:20-1:500.

[0008] A method for preparing a carbon dot / silk fibroin antibacterial preservative solution includes the following steps:

[0009] (1) Preparation of silk protein aqueous solution: Wash and dry silkworm cocoons or silk waste and then crush them. Boil the crushed silkworm cocoons or silk waste in an alkaline solution at 100℃ for 30 minutes and then replace with a new alkaline solution. Heat to 100℃ and boil for another 30 minutes. Wash the alkaline-boiled silkworm cocoons or silk with clean water and dry them for later use. Dissolve the dried silk in a dispersion at 60℃ for 4 hours. Dialyze the silk solution with a dialysis bag with a molecular weight of 3500KDa for 24 hours. Remove the dispersion to obtain a pure silk protein aqueous solution. Adjust the concentration of the silk protein aqueous solution to 1-3wt% for later use.

[0010] (2) Preparation of antibacterial and antioxidant carbon dot solution: Peel and grind the seeds of Moringa. Weigh 2-10g of the crushed seeds into a 50ml polytetrafluoroethylene reaction vessel, add 30ml of water, place in a drying oven at 140 degrees Celsius for 2h, and then cool naturally to room temperature. Centrifuge the cooled reaction solution at 7000r / min for 10 minutes. Discard the precipitate and keep the supernatant. Dialyze the solution using a dialysis bag with a molecular weight of 1000KDa for 24h to obtain the purified antibacterial carbon dot aqueous solution. Concentrate the solution by heating to control the concentration of the carbon dot stock solution within the range of 5-50mg / ml.

[0011] (3) Preparation of carbon dot / silk protein mixed solution: The carbon dot aqueous solution and the silk protein solution are mixed in a ratio of 1:9. Specifically, the antibacterial carbon dot solution is slowly added dropwise to the silk protein solution, and the mixture is stirred gently and mixed evenly. The bubbles are removed by ultrasonication to obtain the antibacterial mixed solution of carbon dot / silk protein, which is the antibacterial preservation solution of carbon dot / silk protein. The silk protein content in the carbon dot / silk protein mixed solution is 0.9-2.7wt%, and the carbon dot concentration is 0.5-5mg / ml.

[0012] Furthermore, the silkworm cocoons are broken into pieces ranging from 0.5cm*0.5cm to 1cm*1cm, or the silk waste is cut into pieces of 1cm*1cm*1cm with scissors.

[0013] Furthermore, the alkaline solution is a sodium carbonate solution with a ratio of 2.2g:1L water, and the dispersion is a lithium bromide solution with a concentration ratio of 16g:20mL water.

[0014] Furthermore, the application of the aforementioned carbon dot / silk protein antibacterial preservative solution in fruit and vegetable preservation.

[0015] A method for preparing a carbon dot / silk fibroin antibacterial and preservative composite film, comprising the above-obtained carbon dot / silk fibroin antibacterial mixed solution, plus the following steps.

[0016] (4) Preparation of carbon dot and silk protein preservation film: Add glycerin to the carbon dot / silk protein mixture to obtain a composite solution. The volume ratio of glycerin to carbon dot / silk protein mixture is 1:20-1:500. Pour the composite solution onto a plastic substrate and use a scraper to adjust the coating thickness to 2 mm. Spread the solution evenly on the plastic substrate and then transfer it to a 30°C oven for 6-10 hours to dry into a film. The film can then be peeled off from the plastic substrate to obtain the carbon dot / silk protein antibacterial preservation composite film.

[0017] Furthermore, the plastic substrate is made of polymethyl methacrylate (PMMA) or polyethylene plastic, and the PMMA substrate is a circular substrate with a diameter of 9cm or a square substrate of 20cm×30cm.

[0018] Furthermore, the application of the aforementioned carbon dot / silk protein antibacterial preservation composite film in fruit and vegetable preservation.

[0019] Beneficial effects of this invention:

[0020] This invention uses natural silk and natural moringa as raw materials, both of which are environmentally friendly biomass raw materials. The carbon dot / silk protein mixed solution has antibacterial properties and can be directly used as a soaking solution for fruits, forming a protective film on the surface of the fruits, thereby extending the shelf life and making it convenient to use. Furthermore, the mixed solution can also be cast into a film for use as a preservation film. It can be prepared in large sizes, has good water vapor and oxygen permeability, and is biodegradable, with broad application prospects.

[0021] (1) The natural silk and moringa used in this invention are both biomass materials, with no toxic side effects, thus avoiding the use of non-degradable materials such as petroleum-based materials. Secondly, silk and moringa are very easy to obtain, abundant in source, and inexpensive. Furthermore, the source of silk can be silkworm cocoons or silkworm waste. The highest temperature in the entire processing is a 100-degree Celsius water bath, and no higher temperature is required, making the operation simple.

[0022] (2) In this invention, by processing and breaking the structure of silk fibers, the hydrogen bonds of the silk fibers combine with carbon dots and glycerol, which can transform the porous silk fibers into a denser film. Moringa is processed to obtain antibacterial and antioxidant carbon dots, which can effectively kill some foodborne pathogens, such as Escherichia coli and Salmonella. The carbon dots have good water solubility and are easy to process.

[0023] (3) The antibacterial carbon dots / silk protein mixture can be used directly as a soaking solution for fresh fruit. This mixture can be used directly as a preservative coating, forming a thin protective film on the fruit surface in situ. Silk protein has high viscosity, making the carbon dots and silk protein mixture more likely to adhere to and wet the fruit surface, resulting in a more even coating. Testing the hydrophobic angles of the carbon dot aqueous solution and the carbon dot / silk protein mixture on different fruit surfaces demonstrates that the mixture has higher wettability. After soaking and drying the fruit, a dense, thin carbon dot / silk protein film forms on the fruit surface, exerting antibacterial and preservative effects. This film is washable and can be washed off before the fruit is consumed, leaving no residue.

[0024] (4) By adding a small amount of glycerol to the mixed solution of antibacterial carbon dots / silk protein, it can also be processed into a large-size antibacterial preservation composite film that is stretchable, breathable, and antibacterial at room temperature. The oxygen permeability and water vapor permeability of the film are tested and found to be comparable to existing fruit preservation films. Furthermore, the film is biodegradable in soil and can serve as a potential alternative to commercial petroleum-based preservation films. It will not cause environmental pollution after use. Attached Figure Description

[0025] Figure 1 This is a diagram showing the inhibitory effect of carbon dots on Escherichia coli.

[0026] Figure 2 The diagram shows the inhibitory effect of carbon dots on Salmonella.

[0027] Figure 3 This is a diagram illustrating the antioxidant properties of carbon dots.

[0028] Figure 4 Fourier transform infrared spectrum of carbon dot / silk protein mixed solution;

[0029] Figure 5 To demonstrate the hydrophobic angle of the carbon dot / silk protein mixed solution on the fruit surface;

[0030] Figure 6 Tensile properties test diagram of carbon dot / silk protein antibacterial and food preservation composite film;

[0031] Figure 7-11 Optical images of the preservation films prepared in Examples 2 to 6;

[0032] Figure 12-16Scanning electron micrographs of the food preservation films prepared in Examples 2 to 6;

[0033] Figure 17 The image shows the preservation effect of a carbon dot / silk protein mixed solution on grapes.

[0034] Figure 18 The image shows the preservation effect of a carbon dot / silk protein mixed solution on strawberries.

[0035] Figure 19 The image shows the preservation effect of a carbon dot / silk protein mixed solution on bananas.

[0036] Figure 20 The effect of carbon dot / silk protein mixed solution on the weight of fruit after storage is shown in the figure.

[0037] Figure 21 Image showing the washing effect of a carbon dot / silk protein mixed solution;

[0038] Figure 22 The image shows mold growth on strawberries that were not covered with the carbon dot / silk protein antibacterial preservation composite film.

[0039] Figure 23 This image shows the preservation effect of strawberries coated with a carbon dot / silk protein antibacterial preservation composite film.

[0040] Figure 24 Figure showing the effect of carbon dot / silk protein antibacterial preservation composite film on strawberry firmness;

[0041] Figure 25 Figure showing the effect of carbon dot / silk protein antibacterial preservation composite film on the weight loss rate of strawberries;

[0042] Figure 26 The graph shows the biodegradability test results of the carbon dot / silk protein antibacterial and preservation composite film. Detailed Implementation

[0043] Example 1: A method for preparing a carbon dot / silk protein antibacterial preservative solution, comprising the following steps:

[0044] (1) Preparation of silk protein aqueous solution: Wash the silkworm cocoons to remove impurities, dry them and cut them into pieces of 0.5cm*0.5cm to 1cm*1cm. Place the broken silkworm cocoons in a sodium carbonate aqueous solution and boil at 100℃ for 30 minutes. Then change the alkaline solution, wash them, boil them at 100℃ for another 30 minutes and rinse them with water. Then place them in a 60-degree drying oven to dry them and obtain degummed silk. Place the degummed silk in 0.2g of 10ml of 0.02M lithium bromide (LiBr) solution and dissolve it completely. The dissolution conditions are 60℃ for 4 hours to fully destroy the various high molecular chains in the silk. Dialyze the silk solution with a dialysis membrane with a molecular weight of 3500KDa for 24 hours. Change the water every hour for the first 4 hours and every 5 hours for the next 20 hours to remove LiBr. The silk protein aqueous solution can be obtained. Adjust the concentration of the silk protein aqueous solution to 1-3wt% for later use.

[0045] (2) Preparation of antibacterial and antioxidant carbon dot solution: Moringa seeds were selected as the precursor. After peeling, they were ground and weighed into 3g in a 50ml polytetrafluoroethylene reactor. 30ml of water was added and the mixture was placed in a drying oven at 140℃ for 2h. After natural cooling to room temperature, the cooled reaction solution was centrifuged at 7000r / min for 10 minutes to remove the precipitate and keep the supernatant. The supernatant was dialyzed with a dialysis membrane with a molecular weight of 1000KDa for 24h to obtain the antibacterial carbon dot solution. The concentration of the carbon dot stock solution was controlled within the range of 5-50mg / ml by heating and concentration for later use. Antibacterial carbon dots exhibit excellent antibacterial effects against *Escherichia coli* (ATCC 11775) and *Salmonella* (ATCC 43971). Their antibacterial performance against foodborne pathogens was tested using the dilution plate method. At a carbon dot concentration of 0.6 mg / ml, after incubation with bacteria for 3 hours, the inhibition rate against *Escherichia coli* reached 99.96%. Figure 1 The inhibition rate against Salmonella reached 99.99%. Figure 2 Meanwhile, carbon dots also possess antioxidant properties. The antioxidant capacity of the carbon dots was tested using the Absolute Antioxidant Test Kit (ABTS) method. At a concentration of 85 μg / ml, the antioxidant rate reached over 85%. Figure 3 Fresh fruits undergo browning during storage, resulting in oxidation of biomolecules and a darkening of color. Carbon dots have strong antioxidant capabilities, indicating that they can effectively resist the browning reaction of fruits during storage.

[0046] (3) Preparation of carbon dot / silk protein mixed solution: Dissolve 1 ml of 10 mg / ml carbon dot solution in 9 ml of 2 wt% silk protein solution as a dispersant. After gentle stirring and mixing, sonicate for 10 minutes to remove air bubbles, resulting in a carbon dot / silk protein mixed solution with an antibacterial carbon dot concentration of 1 mg / ml and a silk protein content of 1.8 wt%, which is the carbon dot / silk protein antibacterial preservative solution. Fourier transform infrared spectroscopy showed that after mixing carbon dots and silk protein aqueous solution, hydrogen bonds formed between the carbon dots and silk protein (…). Figure 4 Hydrophobicity tests showed that the carbon dot / silk protein mixed solution exhibited higher wettability and better diffusion on fruit surfaces compared to pure carbon dot aqueous solutions. The mixed solution adhered more easily and evenly to the fruit surface. Figure 5 ).

[0047] Example 2: A method for preparing a carbon dot / silk protein antibacterial and preservative composite film, using the carbon dot / silk protein antibacterial and preservative liquid obtained in Example 1, with the following additional steps.

[0048] (4) Preparation of carbon dot and silk protein antibacterial and food preservation composite film: A small amount of glycerol (the volume ratio of glycerol to carbon dot / silk protein mixture is 1:300) was added to the above mixed solution. The composite solution was poured onto a polymethyl methacrylate (PMMA) plate. The coating height was adjusted to 2 mm using a scraper to spread the solution evenly on the PMMA substrate. After being placed in a room temperature environment for 20 h, the film and PMMA substrate were gently peeled off to obtain the carbon dot / silk protein antibacterial and food preservation composite film. Infrared spectroscopy proved that hydrogen bonds were formed between carbon dots and silk protein in the mixed solution, which was beneficial to the formation of the film. The addition of glycerol further enhanced this hydrogen bonding effect. The prepared food preservation film had good stretchability, with a maximum deformation of up to 140%. Figure 6 The prepared food preservation film, such as... Figure 7 As shown, the diameter is 9 cm, and the scanning electron microscope image is as follows. Figure 12 As shown.

[0049] The plastic wrap has a certain degree of air permeability, with a water vapor permeability of 2300g / (㎡-day) and an oxygen permeability of 11200cc / (㎡-day) (Table 1);

[0050] In step (1) of the above embodiments one and two, the silkworm cocoons can be replaced with waste silk materials, which are cut into pieces of 1cm*1cm*1cm for use.

[0051] Example 3: Following the same steps as in Example 2, a 3wt% aqueous solution of silk fibroin and a carbon dot solution concentration of 50 mg / ml were prepared. In step (3) of the mixed solution preparation, the concentration of the mixed carbon dot solution was controlled to 5 mg / ml. Following the same steps as in Example 1, a carbon dot / silk fibroin mixed solution with a silk fibroin content of 2.7wt% was obtained. Then, glycerol was added to the mixed solution to obtain a composite solution (the volume ratio of glycerol to carbon dot / silk fibroin was 1:100). The composite solution was then poured onto a polypropylene plastic plate, dried, and peeled off to obtain a carbon dot / silk fibroin antibacterial and preservative composite film.

[0052] Example 4:

[0053] In step (3) of Example 2, the concentration of the 9 ml silk fibroin solution was changed to 1.5 wt%, and the concentration of the 1 ml carbon dot solution was changed to 10 mg / ml. Other steps were the same as in Example 2, resulting in a carbon dot / silk fibroin mixed solution with an antibacterial carbon dot concentration of 1 mg / ml and a silk fibroin content of 1.35 wt%. Glycerin was then added to the mixed solution to obtain a composite solution (glycerin to dot / silk fibroin volume ratio of 1:500). The composite solution was then poured onto a polypropylene plastic plate, dried, and peeled off to obtain the carbon dot / silk fibroin antibacterial preservation composite film.

[0054] Example 5:

[0055] In step (3) of Example 2, the concentration of the 9 ml silk protein solution was changed to 1 wt%, and the concentration of the 1 ml carbon dot solution was changed to 5 mg / ml. The other steps were the same as in Example 2. After mixing, the carbon dot / silk protein mixed solution had a carbon dot concentration of 0.5 mg / ml and a silk protein content of 0.9 wt%. Glycerin was then added to the mixed solution to obtain a composite solution (the volume ratio of glycerin to carbon dot / silk protein was 1:20). The composite solution was then poured onto a polypropylene plastic plate, dried, and peeled off to obtain the carbon dot / silk protein antibacterial and food-preserving composite film.

[0056] Example 6

[0057] In step (3) of Example 2, the volume of silk fibroin was adjusted to 18 ml (2 wt%), and 2 ml of carbon dot solution with a concentration of 20 mg / ml was added to make the carbon dot concentration in the mixed solution 2 mg / ml and the silk fibroin content 1.8 wt%. Then, glycerol was added to the mixed solution to obtain a composite solution (the volume ratio of glycerol to carbon dot / silk fibroin was 1:33). The solution was then poured onto a 20 cm × 30 cm PMMA substrate, dried, and peeled off to obtain a large-size film.

[0058] The water vapor and oxygen permeability tests of the plastic wraps prepared in Examples 2 to 6 are shown in Table 1.

[0059] Table 1. Test of water vapor and oxygen transmission rate of plastic wrap

[0060]

[0061] To verify that the carbon dot / silk protein mixture prepared in Example 1 can be directly used as a fruit preservative, grapes, strawberries, and bananas were selected as test models, using the carbon dot / silk protein mixture from Example 1 as an example. The preservation effect of the mixture on the selected fruits was tested. Fruits with a carbon dot / silk protein mixture coating on their surface showed significantly less spoilage, while uncoated fruits showed higher levels of spoilage and severe mold growth. Figure 17-19 Before and after storage, the weight changes of the fruit were tested. Fruits with a carbon dot / silk protein mixed solution coating on their surface showed significantly smaller weight loss rates. Figure 20 To confirm that the film formed on the fruit surface by the mixed solution is washable, the film was placed in water. Within 15 seconds, the film rapidly dissolved in the water, indicating that the mixture has good water-washing properties (e.g., Figure 21 This indicates that the carbon dot / silk protein mixture solution can be used directly as a preservative coating.

[0062] Introducing appropriate amounts of glycerol into a mixed solution can produce large-size films. To verify the preservation performance of this film, the carbon dot / silk protein antibacterial preservation composite film prepared in Example 2 was used. Taking strawberries as an example, the preservation performance of this film on strawberries was tested. After 3 days of storage, uncovered strawberries showed mold growth, decreased firmness, and the most severe weight loss, exceeding 15%. Strawberries covered with the prepared film showed no bacterial growth, and exhibited the highest firmness, with a weight loss of less than 10%. Figure 22-25 );

[0063] To verify the biodegradability of the prepared film, the carbon dot / silk protein antibacterial and preservative composite film prepared in Example 2 was used. The film was placed in soil, and its weight change was tested after 30 days. Before burying in the soil, the film was weighed using a balance. After 30 days, all buried films were removed, the surface soil residue was washed away with clean water, and the films were dried in a 37°C drying oven for 6 hours before being weighed again. The film showed a weight loss exceeding 35% after 30 days in the soil, indicating that it is biodegradable in soil. Figure 26 ).

Claims

1. A carbon dot / silk protein antibacterial preservative solution, characterized in that: The antibacterial preservative liquid raw material contains a silk protein aqueous solution and a carbon dot aqueous solution; the silk protein content in the carbon dot / silk protein antibacterial preservative liquid is 0.9-2.7 wt%, and the carbon dot concentration is 0.5-5 mg / ml; The method for preparing the antibacterial preservative solution includes the following steps: (1) Preparation of silk protein aqueous solution: Wash and dry silkworm cocoons or silk waste and then crush them. Boil the crushed silkworm cocoons or silk waste in an alkaline solution at 100℃ for 30 minutes and then replace with a new alkaline solution. Heat to 100℃ and boil for another 30 minutes. Wash the alkaline-boiled silkworm cocoons or silk with clean water and dry them for later use. Dissolve the dried silk in a dispersion at 60℃ for 4 hours. Dialyze the silk solution with a dialysis bag with a molecular weight of 3500KDa for 24 hours. Remove the dispersion to obtain a pure silk protein aqueous solution. Adjust the concentration of the silk protein aqueous solution to 1-3wt% for later use. (2) Preparation of antibacterial and antioxidant carbon dot solution: Peel and grind the seeds of Moringa. Weigh 2-10g of the crushed seeds into a 50ml polytetrafluoroethylene reaction vessel, add 30ml of water, place in a drying oven at 140 degrees Celsius for 2h, and then cool naturally to room temperature. Centrifuge the cooled reaction solution at 7000r / min for 10 minutes. Discard the precipitate and keep the supernatant. Dialyze the solution using a dialysis bag with a molecular weight of 1000KDa for 24h to obtain the purified antibacterial carbon dot aqueous solution. Concentrate the solution by heating to control the concentration of the carbon dot stock solution within the range of 5-50mg / ml. (3) Preparation of carbon dot / silk protein antibacterial preservative solution: The carbon dot aqueous solution and the silk protein solution are mixed in a ratio of 1:

9. Specifically, the antibacterial carbon dot solution is slowly added dropwise to the silk protein solution, and the mixture is stirred gently and mixed evenly. The air bubbles are removed by ultrasonication to obtain the carbon dot / silk protein antibacterial preservative solution. The silk protein content in the carbon dot / silk protein antibacterial preservative solution is 0.9-2.7wt%, and the carbon dot concentration is 0.5-5mg / ml.

2. The antibacterial preservative solution of carbon dots / silk protein according to claim 1, characterized in that: In step (1) of the method for preparing the antibacterial preservative liquid, the silkworm cocoons are broken into pieces ranging from 0.5cm×0.5cm to 1cm×1cm, or the silkworm waste is cut into pieces of 1cm×1cm×1cm by scissors.

3. The antibacterial preservative solution of carbon dots / silk protein according to claim 1, characterized in that: In step (1) of the method for preparing the antibacterial preservative solution, the alkaline solution is a sodium carbonate solution with a ratio of 2.2g:1L water, and the dispersion is a lithium bromide solution with a concentration ratio of 16g:20mL water.

4. The application of the carbon dot / silk protein antibacterial preservative solution according to claim 1 in the preservation of fruits and vegetables.

5. A method for preparing a carbon dot / silk protein antibacterial and preservative composite film, characterized in that: Using the antibacterial preservative solution of carbon dots / silk protein obtained according to claim 2 or 3, plus the following steps... (4) Preparation of carbon dot and silk protein preservation film: Add glycerin to the carbon dot / silk antibacterial preservation liquid to obtain a composite solution; pour the composite solution onto a plastic substrate, adjust the coating thickness to 2 mm with a scraper, spread the solution evenly on the plastic substrate, and then transfer it to a 30-degree oven for 6-10 hours to dry into a film. The film can then be peeled off from the plastic substrate to obtain the carbon dot / silk protein antibacterial preservation composite film.

6. The method for preparing the carbon dot / silk fibroin antibacterial and preservative composite film according to claim 5, characterized in that: The volume ratio of glycerol to carbon dot / silk protein antibacterial preservative solution is 1:20-1:

500.

7. A method for preparing a carbon dot / silk fibroin antibacterial and preservative composite film according to claim 5 or 6, characterized in that: The plastic substrate is made of polymethyl methacrylate (PMMA) or polyethylene plastic. The PMMA substrate is a circular substrate with a diameter of 9cm or a square substrate with a diameter of 20cm×30cm.

8. The application of the antibacterial preservation composite film prepared by the method of carbon dot / silk protein according to claim 5 or 6 in the preservation of fruits and vegetables.

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