A starch-polyvinyl alcohol-based film and its preparation method and application

Abstract

The present invention provides a kind of starch polyvinyl alcohol based film and its preparation method and application.The film includes a first film layer and a second film layer arranged in a stacked manner, the first film layer is formed by a first film-forming liquid, the first film-forming liquid includes starch, polyvinyl alcohol and pH fluorescence response material, and the emission fluorescence intensity of the pH fluorescence response material changes with the change of pH value; the second film layer is formed by a second film-forming liquid, and the second film-forming liquid includes starch, polyvinyl alcohol and essential oil nanoemulsion, and the essential oil nanoemulsion has at least antibacterial and / or antioxidant properties. The packaging structure made of the starch polyvinyl alcohol based film provided by the present invention has a strong ultraviolet shielding ability, excellent antibacterial properties and antioxidant properties, can also achieve pH fluorescence response, and the essential oil nanoemulsion therein has good sustained-release characteristics, enriches the versatility of starch polyvinyl alcohol based film, and is expected to expand the application range of starch and polyvinyl alcohol in the field of food packaging.

Description

Technical Field

[0001] The invention belongs to the technical field of food packaging materials, and in particular relates to a starch-polyvinyl alcohol-based film and a preparation method and application thereof. Background Art

[0002] Food packaging plays an irreplaceable role in maintaining food freshness and reducing economic costs. In the "plastic age," non-degradable petroleum-based plastics used as food packaging have caused serious environmental problems, prompting researchers to study new types of food packaging.

[0003] Corn starch is a biodegradable natural polysaccharide that is economical, has excellent film-forming properties and regeneration, and is therefore widely used in the packaging field. However, corn starch-based films also have some disadvantages that are not conducive to industrial applications, such as insufficient mechanical properties and strong hydrophilicity. Currently, these problems of corn starch are generally improved by melt-mixing corn starch and polymers. Among them, polyvinyl alcohol, which is safe, degradable and has easy film-forming properties, is favored by researchers. A large number of studies have shown that films based on polyvinyl alcohol and corn starch have better performance in some aspects than films based only on corn starch. However, starch polyvinyl alcohol composite films have disadvantages such as poor UV shielding ability and lack of functional properties, which cannot meet the demand for multifunctional packaging materials in the food packaging field.

[0004] With the development of society, multifunctional food packaging is becoming a research trend in the field of food packaging. For example, the antibacterial and antioxidant properties of packaging materials have a significant impact on the safety and freshness of preserved food. At various links in the food supply chain, photocatalytic oxidation caused by ultraviolet radiation may affect the shelf life and quality stability of food, thereby generating a demand for packaging materials with ultraviolet light shielding effects. In addition, how to combine pH monitoring with packaging materials to achieve the purpose of conveniently detecting the freshness of high-protein foods. However, the functionality of current food packaging films is relatively simple, and the development of a multifunctional food packaging film is one of the problems that needs to be solved urgently. Summary of the Invention

[0005] In order to solve all or part of the above technical problems, the present invention provides the following technical solutions:

[0006] One of the objects of the present invention is to provide a starch-polyvinyl alcohol-based film, which includes a first film layer and a second film layer stacked together, wherein the first film layer is formed by a first film-forming liquid, which includes starch, polyvinyl alcohol and a pH fluorescence response material, and the emission fluorescence intensity of the pH fluorescence response material changes with the change of pH value; the second film layer is formed by a second film-forming liquid, which includes starch, polyvinyl alcohol and an essential oil nanoemulsion, and the essential oil nanoemulsion has at least antibacterial and / or antioxidant properties.

[0007] The present invention provides a multifunctional starch-polyvinyl alcohol-based film, in particular a starch-polyvinyl alcohol-based film for packaging. The pH fluorescence response material in the first film layer gives the film excellent pH-mediated fluorescence response ability, which can achieve fluorescence response to the pH environment inside the package, and then can detect the pH inside the package without damaging the packaging, and evaluate the preservation, freshness, food safety, etc. of the packaged goods based on the pH value; the essential oil nanoemulsion contained in the second film layer gives the film good antibacterial and antioxidant properties, which can inhibit the growth and reproduction of microorganisms in the packaged food, slow down possible lipid oxidation, and extend the shelf life of the food. The film containing the two-layer structure unexpectedly also has excellent ultraviolet light shielding ability, which is beneficial for the packaging structure based on the film to reduce oxidation caused by photocatalysis in the entire food supply chain. In addition, the film has at least a two-layer structure, and the double-layer structure film is more conducive to the slow release of the essential oil functional substances therein, so that it can exert its functional effects more lastingly.

[0008] In some embodiments, the pH fluorescence responsive material comprises carbon quantum dots, which contain amino and carboxyl groups that can undergo protonation / deprotonation reactions when the pH changes, thereby producing different emission fluorescence intensities. However, other materials with pH responsive functions can also be used as the pH fluorescence responsive material. However, since one of the main uses of the film of the present invention is to construct a stable pH responsive food packaging material, low-toxic carbon quantum dots are selected from the perspectives of stability and safety.

[0009] In some embodiments, the raw materials of the carbon quantum dots include a carbon source and a nitrogen source, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, and sodium citrate, and the nitrogen source includes urea.

[0010] In some preferred embodiments, the raw materials of the carbon quantum dots include urea and citric acid. The carbon quantum dots made from these two raw materials have low toxicity, can be used for food packaging, and have sensitive pH fluorescence response capabilities.

[0011] In some embodiments, the essential oil nanoemulsion includes one or more of, but is not limited to, a cinnamon essential oil nanoemulsion, a thymol nanoemulsion, an oregano essential oil nanoemulsion, a clove essential oil nanoemulsion, or a chamomile essential oil nanoemulsion. Essential oil nanoemulsions are obtained by emulsifying an oil phase containing essential oils and an aqueous phase. Compared to directly using essential oils, preparing an emulsion and then forming a film from it can address issues such as strong essential oil odor, rapid volatilization, and poor compatibility between the essential oil and the film matrix. Furthermore, the basic properties of the prepared film, such as mechanical properties, are superior to films directly incorporating essential oils.

[0012] In some embodiments, the raw materials of the essential oil nanoemulsion include essential oil, water, a nonionic surfactant, and a solubilizer.

[0013] In some embodiments, the nonionic surfactant comprises one or more of Tween 80, Tween 20, Tween 40, or Span 20, but is not limited thereto. The solubilizing agent comprises anhydrous ethanol, but is not limited thereto. The essential oil is specifically selected based on the essential oil nanoemulsion described above, for example, one or more of cinnamon essential oil, thymol essential oil, oregano essential oil, clove essential oil, and chamomile essential oil are used to form the corresponding essential oil nanoemulsion.

[0014] In some embodiments, the particle size of the essential oil nanoemulsion is 50 to 200 nm, preferably 56.26 to 123.56 nm. This smaller particle size is beneficial for the loading of the essential oil nanoemulsion on the film.

[0015] In some embodiments, the content of the pH-responsive fluorescent material in the first film-forming solution is 0.5-8% w / w, preferably 1-2% w / w, of the total weight of the starch and polyvinyl alcohol. If the content of the pH-responsive material is too low, the film may not respond sensitively to changes in pH. If the content is too high, the film may be dark in color, seriously interfering with the fluorescent response to pH.

[0016] In some embodiments, the volume proportion of the essential oil nanoemulsion in the second film-forming liquid is 0.99-16.67%, preferably 4.5-5.5%. If the content of the essential oil nanoemulsion is too low, it will not produce good antibacterial and antioxidant effects. If the content of the essential oil nanoemulsion is too high, it will cause serious deterioration of the performance of the film.

[0017] In some embodiments, the starch includes one or more of corn starch, tapioca starch, rice starch, potato starch or pea starch, but is not limited thereto.

[0018] In some embodiments, the thickness of the first film layer is 0.123-0.132 mm, and the thickness of the second film layer is 0.125-0.199 mm. In some typical embodiments, the thickness of the first film layer is 0.127-0.130 mm, and the thickness of the second film layer is 0.127-0.136 mm.

[0019] In some embodiments, the total thickness of the starch-polyvinyl alcohol-based film is 0.235-0.330 mm. In some typical embodiments, the total thickness of the starch-polyvinyl alcohol-based film is 0.255-0.265 mm.

[0020] A second object of the present invention is to provide a method for preparing a starch-polyvinyl alcohol-based film, comprising:

[0021] Providing a first film-forming solution containing starch, polyvinyl alcohol, and a pH fluorescence responsive material; providing a second film-forming solution containing starch, polyvinyl alcohol, and a cinnamon essential oil nanoemulsion; wherein the emission fluorescence intensity of the pH fluorescence responsive material changes with the change of pH value; and the essential oil nanoemulsion has at least antibacterial and / or antioxidant properties;

[0022] The first film-forming liquid and the second film-forming liquid are respectively made into a first film layer and a second film layer, and the first film layer and the second film layer are combined to obtain a starch-polyvinyl alcohol-based film.

[0023] In some embodiments, the pH fluorescence responsive material includes carbon quantum dots, which contain amino groups and carboxyl groups. The amino groups and carboxyl groups can undergo protonation / deprotonation reactions when the pH changes, thereby generating different emission fluorescence intensities.

[0024] In some embodiments, the method for preparing the carbon quantum dots comprises: subjecting a mixed solution containing 10-20% w / v carbon source and 5-15% w / v nitrogen source to a hydrothermal reaction at 160-200°C to obtain the carbon quantum dots. The carbon quantum dots thus prepared have good pH fluorescence response function, and the carbon quantum dots can be well combined with hydrophilic starch and polyvinyl alcohol. In some typical embodiments, the mixed solution contains 12.5-15% w / v nitrogen source and 7.5-10% w / v carbon source, and the hydrothermal reaction temperature can be 170-180°C.

[0025] In some embodiments, the method for preparing carbon quantum dots further comprises: ultrafiltration of the carbon quantum dot solution obtained by the hydrothermal reaction, followed by freeze-drying, and using the freeze-dried carbon quantum dots to prepare the first membrane-forming solution.

[0026] In some embodiments, the carbon source includes one or more of glucose, polyethylene glycol, citric acid, and sodium citrate, and the nitrogen source includes urea. Carbon quantum dots prepared using these carbon and nitrogen sources have good pH fluorescence response function.

[0027] In some embodiments, the hydrothermal reaction time is 5.5 to 6.0 hours.

[0028] In some embodiments, the content of the pH fluorescence responsive material in the first film-forming solution is 0.5-8% w / w of the total mass of the starch and polyvinyl alcohol, preferably 1-2% w / w.

[0029] In some embodiments, the essential oil nanoemulsion includes one or more of cinnamon essential oil nanoemulsion, thymol essential oil nanoemulsion, oregano essential oil nanoemulsion, clove essential oil nanoemulsion, or chamomile essential oil nanoemulsion, but is not limited thereto. When preparing the essential oil nanoemulsion, one or more of cinnamon essential oil, thymol essential oil, oregano essential oil, clove essential oil, or chamomile essential oil are selected for emulsification.

[0030] In some embodiments, the method for preparing the essential oil nanoemulsion comprises emulsifying a mixture containing 8-12% v / v essential oil, 78-82% v / v water, 4-8% v / v nonionic surfactant, and 1-5 v / v% solubilizer to obtain the essential oil nanoemulsion.

[0031] In some typical embodiments, a mixture containing 9-10% v / v essential oil, 80-81% v / v water, 5-6% v / v% nonionic surfactant and 2-3% v / v% solubilizer is emulsified to obtain the essential oil nanoemulsion.

[0032] In some embodiments, the nonionic surfactant includes one or more of Tween 20, Tween 40, Tween 80 or Span 20, but is not limited thereto; the solubilizer includes anhydrous ethanol, but is not limited thereto.

[0033] In some embodiments, the emulsification method includes homogenization and ultrasonication, but is not limited thereto. Other emulsification methods known in the art may also be used. For example, in a typical embodiment, the essential oil, water, a surfactant, and a solubilizing agent are first mixed and homogenized to obtain a coarse emulsion, and then ultrasonicated in an ice bath to obtain the essential oil nanoemulsion.

[0034] In some embodiments, the volume proportion of the essential oil nanoemulsion in the second film-forming liquid is 0.99% to 16.67%, preferably 4.5% to 5.5%.

[0035] In some embodiments, the starch includes one or more of corn starch, tapioca starch, rice starch, potato starch or pea starch, but is not limited thereto.

[0036] In some embodiments, the starch content in the first film-forming liquid and the second film-forming liquid is 2-8% w / v, preferably 4-5% w / v, and the polyvinyl alcohol content is 2-8% w / v, preferably 4-5% w / v.

[0037] In some typical embodiments, the preparation method includes: dissolving starch and polyvinyl alcohol in water and adding a plasticizer to form a base film-forming liquid; adding carbon quantum dots to the base film-forming liquid according to the aforementioned ratio to obtain a first film-forming liquid; and adding an essential oil nanoemulsion to the base film-forming liquid to obtain a second film-forming liquid. It should be noted that, in addition to starch and polyvinyl alcohol, the base film-forming liquid may also contain other conventional substances used in starch-polyethylene films in the prior art. This is not specifically limited in the present invention. However, for food packaging applications, the added substances may be safe. Examples of plasticizers include, but are not limited to, glycerin.

[0038] In some embodiments, the method of forming the first film-forming liquid and the second film-forming liquid into the first film layer and the second film layer respectively includes a casting method, but other film-forming methods may also be used, and the present invention does not specifically limit this.

[0039] In some embodiments, the preparation method specifically includes: first using the first film-forming liquid to prepare a first film layer, and then using the second film-forming liquid to in situ form a second film layer on the first film layer; or, first using the second film-forming liquid to prepare a second film layer, and then using the first film-forming liquid to in situ form a first film layer on the second film layer.

[0040] In some embodiments, the first film layer and the second film layer are manufactured in a constant temperature and humidity environment.

[0041] A third object of the present invention is to provide a packaging structure, wherein the part of the packaging structure used to accommodate the packaged object is made of the starch-polyvinyl alcohol-based film described in any technical solution, or the starch-polyvinyl alcohol-based film prepared by any method, and the second film layer of the starch-polyvinyl alcohol-based film is located on the side close to the packaged object.

[0042] The second layer of the starch-polyvinyl alcohol-based film is positioned on the side closest to the packaged product. This design ensures that the film layer containing the essential oil functional substance is located inside the packaging structure, in contact with the packaged product, while the film layer containing the pH-responsive fluorescent material is positioned on the outside. This design not only considers food safety, but also allows for the slow release of the essential oil, improving the utilization efficiency of the essential oil functional substance. Essential oil functional substances typically released on the non-food side are not fully utilized, resulting in significant losses.

[0043] It should be noted that, in addition to the first and second film layers described herein, the present invention does not exclude the possibility of introducing other film layers into the packaging structure. For example, a conventional film layer used in the food packaging field, such as a waterproof layer, may be applied to the side of the first film layer (the film layer containing the pH fluorescence responsive material) away from the second film layer. Alternatively, a conventional film layer used in the food packaging field may be applied to the side of the second film layer (the film layer containing the essential oil) away from the first film layer. Of course, the second film layer containing the essential oil described herein can be used directly in contact with the packaged food. The double-layer starch-polyvinyl alcohol-based film containing the first and second film layers described herein can meet the requirements of food packaging and also provide excellent antibacterial and antioxidant properties, sustained release of functional substances, and pH fluorescence responsiveness.

[0044] In some embodiments, the packaging structure is, for example, a packaging bag, and the bag body of the packaging bag for accommodating the packaged object is made of the starch-polyvinyl alcohol-based film.

[0045] A fourth object of the present invention is to provide the use of the starch-polyvinyl alcohol-based film described in any technical solution, the starch-polyvinyl alcohol-based film prepared according to any method, or the packaging structure described in any technical solution in food packaging.

[0046] In some embodiments, the food comprises a high-protein food.

[0047] Compared with the prior art, the present invention has at least the following beneficial effects:

[0048] (1) The film provided by the present invention has excellent pH-mediated fluorescence response ability, which can achieve fluorescence response to the pH environment inside the package, and realize the detection of the pH value of the packaged content without damaging the film, providing a new idea for conveniently detecting the freshness of high-protein foods; the film provided by the present invention has good antibacterial and antioxidant properties, which can inhibit the growth and reproduction of microorganisms in packaged foods, slow down the lipid oxidation that may occur in packaged foods, and extend the shelf life of foods;

[0049] (2) In particular, the film comprising the above-mentioned two-layer structure provided by the present invention unexpectedly also has excellent ultraviolet light shielding ability. The loading of carbon quantum dots and essential oil nanoemulsion, especially cinnamon essential oil nanoemulsion, and the preparation of the double-layer film improve the film's shielding ability against ultraviolet light, which is beneficial for the packaging to reduce photocatalytic oxidation throughout the food supply chain;

[0050] (3) The double-layer structure film provided by the present invention is conducive to the slow release of the functional substance essential oil therein, thereby facilitating its more lasting functional effect; and can reduce the loss of functional substances, improve the utilization rate of the essential oil nanoemulsion, and prolong the antibacterial and antioxidant effects of the packaging. BRIEF DESCRIPTION OF THE DRAWINGS

[0051] In order to more clearly illustrate the embodiments of the present application or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments recorded in this application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0052] Figure 1 1 and 2 are transmission electron microscope images of the carbon quantum dots prepared in Example 1 and Comparative Example 1 of the present invention.

[0053] Figure 2 This is the Fourier infrared spectroscopy analysis of the carbon quantum dots prepared in Example 1 of the present invention and Comparative Example 1.

[0054] Figure 3A 、 3B 3C, 3D, and 3E are excitation wavelength diagrams when the concentrations of the carbon quantum dots prepared in Example 1 of the present invention are 0.002 mg / mL, 0.004 mg / mL, 0.006 mg / mL, 0.008 mg / mL, and 0.010 mg / mL, respectively;

[0055] Figure 3F The fluorescence responses of the carbon quantum dots prepared in Example 1 at different pH values ​​when the concentrations are 0.002 mg / mL, 0.004 mg / mL, 0.006 mg / mL, 0.008 mg / mL, and 0.010 mg / mL;

[0056] Figure 4 is the particle size and potential of the cinnamon essential oil nanoemulsion prepared in Example 1 and Comparative Example 2 of the present invention;

[0057] Figure 5 1 is a Fourier transform infrared spectrum analysis diagram of the cinnamon essential oil nanoemulsion prepared in Example 1 of the present invention and Comparative Example 2;

[0058] Figure 6are the optical properties of the films of Examples 1-3 of the present invention and Comparative Examples 1-3;

[0059] Figure 7 is the fluorescence response of the film of Example 1 of the present invention to different pH values;

[0060] Figure 8 The fluorescence characteristics of the film of Example 2-3 of the present invention and the film of Example 2-3 are compared;

[0061] Figure 9 Schematic diagram of the release characteristics of the thin film cinnamon essential oil nanoemulsion of Example 1 of the present invention and Comparative Example 2;

[0062] Figure 10 Schematic diagram of the antibacterial properties of the films of Example 1 of the present invention and Comparative Examples 1-3;

[0063] Figure 11 Schematic diagram of the anti-oxidation properties of the films of Example 1 of the present invention and Comparative Examples 1-3;

[0064] Figure 12 Schematic diagram of the structure of the double-layer film in Example 1 of the present invention. DETAILED DESCRIPTION

[0065] The technical solutions of the present invention are described in detail below in conjunction with specific embodiments so that those skilled in the art can better understand and implement the technical solutions of the present invention. The specific functional details disclosed herein should not be interpreted as limiting, but rather as a basis for the claims and as a representative basis for teaching those skilled in the art to employ the present invention in various ways in virtually any appropriate detailed embodiment.

[0066] Unless otherwise specified, the raw materials and reagents used in the specific embodiments of the present invention are all commercially available.

[0067] As a typical embodiment of the technical solution of the present invention, a method for preparing a starch-polyvinyl alcohol-based film includes the following process steps:

[0068] (1) Urea and citric acid monohydrate aqueous solution are subjected to ultrasonic treatment and hydrothermal treatment in sequence, and the supernatant is collected after ultrafiltration, and after freezing, the supernatant is freeze-dried to obtain carbon quantum dots;

[0069] (2) Cinnamon essential oil nanoemulsion was prepared by homogenization and ultrasonication using cinnamon essential oil as the oil phase, deionized water as the aqueous phase, Tween 80 as the surfactant, and anhydrous ethanol as the solubilizer;

[0070] (3) corn starch and polyvinyl alcohol are heated and dissolved in deionized water, and after mixing, glycerol is used to plasticize to obtain a basic film-forming solution;

[0071] (4) Compounding the carbon quantum dots described in step (1) with the base film-forming solution described in step (3) by magnetic stirring to obtain a first film-forming solution, and casting the solution on a polytetrafluoroethylene plate and drying the solution in a constant humidity and constant temperature box to form a first film layer;

[0072] (5) Compounding the cinnamon essential oil nanoemulsion of step (2) with the base film-forming liquid of step (3) by magnetic stirring to obtain a second film-forming liquid, and casting the second film-forming liquid on a polytetrafluoroethylene plate containing the first film layer and drying the second film in a constant humidity and constant temperature box to form a double-layer film.

[0073] Example 1

[0074] This embodiment provides an antibacterial, antioxidant, slow-release intelligent corn starch polyvinyl alcohol-based double-layer film and a method for making the same, as follows:

[0075] Accurately weigh 6g of urea and 4g of citric acid monohydrate, add them to a beaker containing 40mL of deionized water, place the beaker in a steel basin containing ice and water, and perform ultrasonic treatment with an ultrasonic frequency of 500W and an ultrasonic time of 10min. After ultrasonication for 5s, stop for 5s, and then start ultrasonication for 5s, and repeat this cycle. After the ultrasonication is completed, all the obtained liquid is transferred to a high-pressure reactor lined with 50mL of polytetrafluoroethylene, the lid is tightened, and the reactor is placed in an oven at 180°C and heated for 6h for a hydrothermal reaction. After the reaction is completed, the oven door is opened and cooled overnight. The product obtained by the hydrothermal reaction is ultrafiltered using a 0.22μm polyethersulfone membrane, and the supernatant obtained by ultrafiltration is poured into a glass container. After freezing at -20°C, the carbon quantum dots are obtained by placing the product in a freeze dryer for 24h.

[0076] 10 g of corn starch was added to 200 ml of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a starch solution; 10 g of polyvinyl alcohol was added to 200 mL of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a polyvinyl alcohol solution; the prepared starch solution and polyvinyl alcohol solution were mixed, and 4 g of glycerol was added and stirred for 30 min to obtain a basic membrane-forming solution;

[0077] The carbon quantum dots prepared above were added to 50 ml of the base film-forming solution and uniformly mixed with a magnetic stirrer at 500 rpm to obtain a first film-forming solution, wherein the amount of carbon quantum dots added was 1 wt% of the total mass of corn starch and polyvinyl alcohol; the first film-forming solution was uniformly cast on a 150 mm × 150 mm polytetrafluoroethylene plate, which was then placed in a constant humidity and temperature chamber (50° C., 55% relative humidity) and dried for 12 h to obtain a first film layer;

[0078] 10 mL of cinnamon essential oil, 81 mL of deionized water, 6 mL of Tween 80, and 3 mL of anhydrous ethanol were mixed and homogenized (12000 rpm) for 2 min. The beaker was placed in a steel basin containing ice and water and subjected to ultrasonic treatment (500 W, 10 min, 5 s on, 5 s off) to obtain a cinnamon essential oil nanoemulsion; 2.5 mL of the cinnamon essential oil nanoemulsion was added to 50 mL of the base film-forming liquid and magnetically stirred at 500 rpm to uniformly mix to obtain a second film-forming liquid; the second film-forming liquid was cast on a polytetrafluoroethylene plate containing the first film layer and dried in a constant humidity and constant temperature box (50° C., 55% relative humidity) for 12 h to form a laminated second film layer on the first film layer, thereby constituting the corn starch polyvinyl alcohol-based double-layer film of this embodiment. Figure 12 Schematic diagram of the structure of the double-layer film.

[0079] Example 2

[0080] This embodiment provides an antibacterial, antioxidant, slow-release intelligent corn starch polyvinyl alcohol-based double-layer film and a method for making the same, as follows:

[0081] Accurately weigh 6g of urea and 4g of citric acid monohydrate, add them to a beaker containing 40mL of deionized water, place the beaker in a steel basin containing ice and water, and perform ultrasonic treatment with an ultrasonic frequency of 500W and an ultrasonic time of 10min. After ultrasonication for 5s, stop for 5s, and then start ultrasonication for 5s, and repeat this cycle. After the ultrasonication is completed, all the obtained liquid is transferred to a high-pressure reactor lined with 50mL of polytetrafluoroethylene, the lid is tightened, and the reactor is placed in an oven at 180°C and heated for 6h for a hydrothermal reaction. After the reaction is completed, the oven door is opened and cooled overnight. The product obtained by the hydrothermal reaction is ultrafiltered using a 0.22μm polyethersulfone membrane, and the supernatant obtained by ultrafiltration is poured into a glass container. After freezing at -20°C, the carbon quantum dots are obtained by placing the product in a freeze dryer for 24h.

[0082] 10 g of corn starch was added to 200 ml of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a starch solution; 10 g of polyvinyl alcohol was added to 200 mL of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a polyvinyl alcohol solution; the prepared starch solution and polyvinyl alcohol solution were mixed, and 4 g of glycerol was added and stirred for 30 min to obtain a basic membrane-forming solution;

[0083] The carbon quantum dots prepared above were added to 50 ml of the base film-forming solution and uniformly mixed with a magnetic stirrer at 500 rpm to obtain a first film-forming solution, wherein the amount of carbon quantum dots added was 0.5 wt% of the total mass of corn starch and polyvinyl alcohol; the first film-forming solution was uniformly cast on a 150 mm × 150 mm polytetrafluoroethylene plate, which was then placed in a constant humidity and temperature chamber (50° C., 55% relative humidity) and dried for 12 h to obtain a first film layer;

[0084] 10 mL of cinnamon essential oil, 81 mL of deionized water, 6 mL of Tween 80, and 3 mL of anhydrous ethanol were mixed and homogenized (12,000 rpm) for 2 min. The beaker was placed in a steel basin containing ice and water and ultrasonicated (500 W, 10 min, 5 s on, 5 s off) to obtain a cinnamon essential oil nanoemulsion. 1 mL of the cinnamon essential oil nanoemulsion was added to 50 mL of the base film-forming solution and uniformly mixed with magnetic stirring at 500 rpm to obtain a second film-forming solution. The second film-forming solution was cast on a polytetrafluoroethylene plate containing the first film layer and dried in a constant humidity and temperature chamber (50° C., 55% relative humidity) for 12 h to form a laminated second film layer on the first film layer, thereby constituting the corn starch polyvinyl alcohol-based double-layer film of this embodiment.

[0085] Example 3

[0086] This embodiment provides an antibacterial, antioxidant, slow-release intelligent corn starch polyvinyl alcohol-based double-layer film and a method for making the same, as follows:

[0087] Accurately weigh 6g of urea and 4g of citric acid monohydrate, add them to a beaker containing 40mL of deionized water, place the beaker in a steel basin containing ice and water, and perform ultrasonic treatment with an ultrasonic frequency of 500W and an ultrasonic time of 10min. After ultrasonication for 5s, stop for 5s, and then start ultrasonication for 5s, and repeat this cycle. After the ultrasonication is completed, all the obtained liquid is transferred to a high-pressure reactor lined with 50mL of polytetrafluoroethylene, the lid is tightened, and the reactor is placed in an oven at 180°C and heated for 6h for a hydrothermal reaction. After the reaction is completed, the oven door is opened and cooled overnight. The product obtained by the hydrothermal reaction is ultrafiltered using a 0.22μm polyethersulfone membrane, and the supernatant obtained by ultrafiltration is poured into a glass container. After freezing at -20°C, the carbon quantum dots are obtained by placing the product in a freeze dryer for 24h.

[0088] 10g corn starch was added to 200ml deionized water, and heated and stirred in a 90°C water bath for 30 minutes until completely dissolved to obtain a starch solution; 10g polyvinyl alcohol was added to 200ml deionized water, and heated and stirred in a 90°C water bath for 30 minutes until completely dissolved to obtain a polyvinyl alcohol solution; the prepared starch solution and polyvinyl alcohol solution were mixed, and 4g glycerol was added and stirred for 30 minutes to obtain a basic film-forming solution;

[0089] The carbon quantum dots prepared above were added to 50 ml of the base film-forming solution and uniformly mixed with a magnetic stirrer at 500 rpm to obtain a first film-forming solution, wherein the amount of carbon quantum dots added was 8 wt% of the total mass of corn starch and polyvinyl alcohol; the first film-forming solution was uniformly cast on a 150 mm × 150 mm polytetrafluoroethylene plate, which was then placed in a constant humidity and temperature chamber (50° C., 55% relative humidity) and dried for 12 h to obtain a first film layer;

[0090] 10 mL of cinnamon essential oil, 81 mL of deionized water, 6 mL of Tween 80, and 3 mL of anhydrous ethanol were mixed and homogenized (12,000 rpm) for 2 min. The beaker was placed in a steel basin containing ice and water and ultrasonicated (500 W, 10 min, 5 s on, 5 s off) to obtain a cinnamon essential oil nanoemulsion. 5 mL of the cinnamon essential oil nanoemulsion was added to 50 mL of the base film-forming solution and uniformly mixed with magnetic stirring at 500 rpm to obtain a second film-forming solution. The second film-forming solution was cast on a polytetrafluoroethylene plate containing the first film layer and dried in a constant humidity and temperature chamber (50° C., 55% relative humidity) for 12 h to form a laminated second film layer on the first film layer, thereby constituting the corn starch polyvinyl alcohol-based double-layer film of this embodiment.

[0091] Comparative Example 1

[0092] Comparative Example 1 provides a corn starch polyvinyl alcohol-based film with a carbon quantum dot content of 1 wt%, and the preparation method thereof is as follows:

[0093] Accurately weigh 6g of urea and 4g of citric acid monohydrate, add them to a beaker containing 40mL of deionized water, place the beaker in a steel basin containing ice and water, and perform ultrasonic treatment with an ultrasonic frequency of 500W and an ultrasonic time of 10min. After ultrasonication for 5s, stop for 5s, and then start ultrasonication for 5s, and repeat this cycle. After the ultrasonication is completed, all the obtained liquid is transferred to a high-pressure reactor lined with 50mL of polytetrafluoroethylene, the lid is tightened, and the reactor is placed in an oven at 180°C and heated for 6h for a hydrothermal reaction. After the reaction is completed, the oven door is opened and cooled overnight. The product obtained by the hydrothermal reaction is ultrafiltered using a 0.22μm polyethersulfone membrane, and the supernatant obtained by ultrafiltration is poured into a glass container. After freezing at -20°C, the carbon quantum dots are obtained by placing the product in a freeze dryer for 24h.

[0094] 10 g of corn starch was added to 200 ml of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a starch solution; 10 g of polyvinyl alcohol was added to 200 mL of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a polyvinyl alcohol solution; the prepared starch solution and polyvinyl alcohol solution were mixed, and 4 g of glycerol was added and stirred for 30 min to obtain a basic membrane-forming solution;

[0095] The carbon quantum dots prepared above were added to 50 ml of the basic film-forming liquid, and the mixture was evenly mixed with a magnetic stirrer at 500 rpm to obtain the final film-forming liquid. The amount of carbon quantum dots added was 1% of the total mass of corn starch and polyvinyl alcohol. The final film-forming liquid was evenly cast on a 150 mm × 150 mm polytetrafluoroethylene plate, and then placed in a constant humidity and constant temperature box (50°C, 55% relative humidity) and dried for 12 hours to obtain a carbon quantum dot-doped corn starch polyvinyl alcohol-based film, which was peeled off using tweezers.

[0096] Comparative Example 2

[0097] Comparative Example 2 provides a corn starch polyvinyl alcohol-based film with a cinnamon essential oil content of 2.5 mL, and the preparation method is as follows:

[0098] 10 g of corn starch was added to 200 ml of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a starch solution; 10 g of polyvinyl alcohol was added to 200 mL of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a polyvinyl alcohol solution; the prepared starch solution and polyvinyl alcohol solution were mixed, and 4 g of glycerol was added and stirred for 30 min to obtain a basic membrane-forming solution;

[0099] 10 mL of cinnamon essential oil, 81 mL of deionized water, 6 mL of Tween 80, and 3 mL of anhydrous ethanol were mixed and homogenized (12000 rpm) for 2 min. The beaker was placed in a steel basin containing ice and water and ultrasonicated (500 W, 10 min, 5 s on, 5 s off) to obtain a cinnamon essential oil nanoemulsion.

[0100] Add 2.5 ml of cinnamon essential oil nanoemulsion to 50 ml of base film-forming solution, stir magnetically at 500 rpm to mix uniformly, and obtain the final film-forming solution;

[0101] The final film-forming solution was cast on a 150 mm × 150 mm polytetrafluoroethylene plate and dried in a constant humidity and constant temperature box (50° C., 55% relative humidity) for 12 h to form a corn starch polyvinyl alcohol-based film containing cinnamon essential oil.

[0102] A second film layer is formed on the first film layer to form the corn starch polyvinyl alcohol-based double-layer film of this embodiment, and then the film is peeled off using tweezers.

[0103] Comparative Example 3

[0104] Comparative Example 3 provides a corn starch polyvinyl alcohol-based double-layer film, and the preparation method is as follows:

[0105] 10 g of corn starch was added to 200 ml of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a starch solution; 10 g of polyvinyl alcohol was added to 200 mL of deionized water, and heated and stirred in a 90°C water bath for 30 min until completely dissolved to obtain a polyvinyl alcohol solution; the prepared starch solution and polyvinyl alcohol solution were mixed, and 4 g of glycerol was added and stirred for 30 min to obtain a basic membrane-forming solution;

[0106] 50 mL of the base film-forming solution was taken and magnetically stirred at 500 rpm, then uniformly cast onto a 150 mm × 150 mm polytetrafluoroethylene plate and dried in a constant humidity and temperature chamber (50°C, 55% relative humidity) for 12 h to obtain the first film layer.

[0107] 50 mL of the basic membrane-forming solution was taken and magnetically stirred at 500 rpm, then uniformly cast on a polytetrafluoroethylene plate containing the first membrane layer and dried in a constant humidity and temperature box (50° C., 55% relative humidity) for 12 h to obtain a double-layer film.

[0108] In order to further illustrate the technical effects of the present invention, measurements were performed on relevant samples obtained from Examples 1-3 and Comparative Examples 1-3.

[0109] 1. Perform transmission electron microscopy and infrared spectroscopy analysis on the prepared carbon quantum dots.

[0110] (1) Transmission electron microscopy (TEM) of carbon quantum dots: Use a rubber-tipped dropper to draw up the supernatant of carbon quantum dots that has not been freeze-dried and drop it onto the ultrathin carbon film. After air-drying, observe and photograph under a transmission electron microscope.

[0111] Figure 1 The TEM image of the carbon quantum dots prepared in Example 1 shows that the prepared carbon quantum dots are spherical. Figure 1 The particle size analysis of the carbon quantum dots in the structure showed that the particle size ranged from 0.958 to 9.738 nm, with an average particle size of 5.496 nm.

[0112] (2) Infrared spectra of carbon quantum dots: The Fourier transform infrared spectra of all samples were collected using an attenuated total reflection Fourier transform infrared spectrometer with a scanning range of 4000 cm -1 -500cm -1 The carbon quantum dots were placed on the ATR crystal and pressure was applied to ensure good contact between the sample and the crystal. A background scan was performed before each scan to eliminate atmospheric interference.

[0113] Figure 2 This is the Fourier transform infrared spectrum of the carbon quantum dots prepared in Example 1. -1The broad vibration peak at 1552 cm is attributed to the stretching vibration of amide / NH and CH of carbon quantum dots; -1 Carboxylate COO was observed - The asymmetric stretching vibration of -1 -COO was observed - Symmetrical stretching vibration peak at 1347cm -1 The presence of the above functional groups indicates that the carbon quantum dots prepared in Example 1 contain nitrogen-containing groups and carboxyl groups, and the presence of these hydrophilic groups enables the carbon quantum dots to be loaded in the corn starch polyvinyl alcohol-based film.

[0114] (3) Measure the optimal excitation wavelength and optimal emission wavelength of carbon quantum dot solutions with different concentrations: Use a fluorescence spectrophotometer to measure the fluorescence emission spectra of carbon quantum dots with different concentrations at an excitation wavelength of 330-380 nm and an emission wavelength of 200-800 nm.

[0115] The fluorescence emission spectra of the carbon quantum dot solutions prepared in Example 1 with concentrations ranging from 2 to 10 μg / mL were quantitatively analyzed using a fluorescence spectrometer to determine the optimal excitation wavelength and the optimal emission wavelength of the carbon quantum dots. Figure 3A 、 3B , 3C, 3D, and 3E are the excitation spectra of the carbon quantum dots prepared in Example 1 with concentrations of 0.002 mg / mL, 0.004 mg / mL, 0.006 mg / mL, 0.008 mg / mL, and 0.010 mg / mL, respectively. Figure 3F The figure below shows the fluorescence response changes of carbon quantum dot solutions with different concentrations in different pH environments. The results show that within the excitation wavelength range of 330 to 380 nm, the fluorescence intensity of carbon quantum dot solutions with different concentrations is the highest at 440 nm. This observation indicates that the optimal emission wavelength of carbon quantum dots is 440 nm. Within the excitation wavelength range of 330-355 nm, the fluorescence intensity gradually increases, while within the excitation wavelength range of 355-380 nm, the fluorescence intensity gradually decreases. This indicates that the optimal excitation wavelength is 355 nm (pink mark). In addition, low wavelength excitation and high wavelength emission indicate that the substance is a fluorescent substance.

[0116] (4) Measure the response characteristics of carbon quantum dot solutions of different concentrations to pH (pH range 3.5-8.5). In order to determine the response characteristics of carbon quantum dots to pH value, phosphate buffer and standard hydrochloric acid solution were used as solvents to prepare carbon quantum dot solutions with different concentrations of pH values ​​of 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8 and 8.5. According to the optimal excitation wavelength and optimal emission wavelength obtained in (3), the relationship between the fluorescence intensity of the carbon quantum dot solution and pH value was analyzed using an F970Pro fluorescence spectrophotometer.

[0117] During storage and transportation, some proteins in high-protein foods are decomposed by microorganisms and converted into biogenic amines, thereby increasing the pH value of the packaging environment. The present invention uses phosphate buffer solutions with different pH values ​​(pH = 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 8.5) as solvents for carbon quantum dots and studies the response of the fluorescence intensity of carbon quantum dots to pH value. Figure 3F As shown in the figure, the fluorescence intensity of carbon quantum dots at various concentrations is positively correlated with the pH value. As the pH value increases, the fluorescence intensity also increases. This phenomenon can be attributed to the deprotonation of N atoms in carbon quantum dots as fluorophores. The research results also demonstrated the pH responsiveness of the prepared carbon quantum dots, indicating that they have the potential to respond to pH changes. Within a certain concentration range (2μg / mL-10μg / mL), the higher the concentration, the stronger the fluorescence intensity at the same pH value, which is consistent with the results of the previous study. Figure 3A -E gives the same result.

[0118] 2. Determination of the properties of the prepared cinnamon essential oil nanoemulsion

[0119] (1) The particle size and potential of cinnamon essential oil nanoemulsion were measured using a nanoparticle size analyzer.

[0120] Figure 4 The average particle size, polydispersity index, and zeta potential of the cinnamon essential oil nanoemulsion prepared in Example 1 are shown. The average particle size of the cinnamon essential oil nanoemulsion prepared in the present invention was 56.26 nm on the first day, but only 123.56 nm after 7 days. This small particle size facilitates the loading of the cinnamon essential oil nanoemulsion onto the film. It is generally believed that when the polydispersity index is less than 0.3, the emulsion has a better particle size distribution and is more conducive to its stability. The polydispersity index of the cinnamon essential oil nanoemulsion prepared in the present invention did not exceed 0.3 within 7 days, indicating that the cinnamon essential oil nanoemulsion has excellent stability. The small average particle size and polydispersity index are due to the strong emulsification ability of Tween 80. The absolute value of the zeta potential of a nanoemulsion stabilized with the nonionic surfactant Tween 80 is expected to be 0, but the prepared cinnamon essential oil nanoemulsion exhibits negative electronegativity, which may be due to the presence of ionizable groups on the surface of the cinnamon essential oil.

[0121] (2) Infrared spectra of cinnamon essential oil nanoemulsion: The Fourier transform infrared spectra of all samples were collected using an attenuated total reflection Fourier transform infrared spectrometer with a scanning range of 4000 cm -1 -500cm -1 The cinnamon essential oil nanoemulsion was dropped onto the ATR crystal, and pressure was applied to ensure good contact between the sample and the crystal. A background scan was performed before each scan to eliminate interference from atmospheric gases.

[0122] Figure 5 3352.50 cm in cinnamon essential oil. -1 The absorption peak corresponds to the stretching vibration of water OH. -1 and 1626.43cm -1 The stretching vibration of 745.89 cm corresponds to the stretching vibration of C=O of aldehyde carbonyl group. -1 and 687.05cm -1 They correspond to the =CH vibration absorption of the benzene ring and the vibration absorption of olefins, respectively.

[0123] III. Performance Measurement of Films Prepared in Example 1 and Comparative Examples 1-3

[0124] (1) Measurement of film optical properties: The transmittance of Examples 1-3 and Comparative Examples 1-3 (film size: 10×40 mm) within the ultraviolet-visible light wavelength range (200-800 mm) was measured using a UV-visible spectrophotometer. The films were attached vertically to the light-transmitting surface of a cuvette and wavelength scanning was performed.

[0125] Figure 6 It is the transmittance of the films of Examples 1-3 and Comparative Examples 1-3 of the present invention in the wavelength range of 200-800 nanometers.

[0126] The double-layer film composed of the carbon quantum dots and cinnamon essential oil nanoemulsion has excellent light transmittance. However, the high transmittance in the ultraviolet region means that the food is very susceptible to photocatalytic oxidation and deterioration (Comparative Example 3). Adding cinnamon essential oil or nanoemulsion to the single-layer film leads to a significant decrease in the ultraviolet transmittance, which is attributed to the aromatic groups of the cinnamon essential oil nanoemulsion and the aromatic sp of the carbon quantum dots. 2 π→π* electronic transition and n-→π* electronic transition of C=O group (Comparative Example 1 and Comparative Example 2). For the double-layer film containing cinnamon essential oil and carbon quantum dots, the transmittance is the worst and the ability to shield ultraviolet rays is the strongest, which is attributed to the stacking of the double-layer film. It also shows a dose correlation, the larger the dose, the worst transmittance (Examples 1-3). When 8% carbon quantum dots and 5mL of cinnamon essential oil nanoemulsion are incorporated (Example 3), the film is almost opaque, which is not conducive to gaining the favor of consumers, who are more inclined to choose transparent packaging films. Therefore, considering the aspects of light transmittance, antibacterial antioxidant properties and pH response functions, the amount of carbon quantum dots incorporated is preferably 1-2wt%, and the added volume proportion of cinnamon essential oil nanoemulsion is preferably 4.5-5.5%. The double-layer film constructed by the present invention has a very strong ability to shield ultraviolet rays and maintains a certain degree of transparency, which meets modern requirements for food packaging.

[0127] (2) Fluorescence response of the film of Example 1 to different pH values: Hydrochloric acid solution, ammonia solution and deionized water were used to prepare acetic acid solutions with pH values ​​of 3.5, 4, 4.5, 5, 5.5, 6, 6.5 and 7 and ammonia solution with pH values ​​of 7.5, 8 and 8.5, respectively. 200 μL of solutions with different pH values ​​were injected into a 24-well plate, a piece of film (30 mm × 30 mm) was placed on the well plate, the well plate cover was closed and allowed to stand for 30 minutes, and the fluorescence intensity corresponding to each well was measured. The fluorescence spectrophotometer setting program was as follows: Ex and Em slit widths were 5 nm, the angle between Ex and Em and the sample was 45°, the PMT voltage was 230 V, the excitation wavelength was 355 nm, the emission wavelength scanning range was 375-550 nm, the scanning speed was 1200 nm / min, and the scanning interval was 1 nm.

[0128] Figure 7 It is the fluorescence response curve of the film of Example 1 to different pH values. Volatile hydrochloric acid and ammonia water were used to simulate the storage environment of high-protein food to verify whether the film has pH-mediated fluorescence sensing function. Under irradiation with an excitation wavelength of 355nm, the film of Comparative Example 3 did not show an obvious fluorescence emission peak, while the fluorescence emission spectrum of the film of Example 1 showed a peak value near 440nm, indicating that fluorescence response can be achieved by incorporating carbon quantum dots into the film. After exposure to different pH solutions for 30 minutes, the fluorescence intensity of the film of Example 1 showed a pH response pattern, which was caused by the structural changes caused by the deprotonation of carbon quantum dots in an alkaline environment. The results show that the incorporation of carbon quantum dots into corn starch polyvinyl alcohol-based films can give the film pH-mediated fluorescence sensing properties (pH range of 3.5-8.5).

[0129] (3) Fluorescence properties of the films of Example 2-3 and Comparative Example 2-3: Films (30 mm × 30 mm) were placed in a fluorescence spectrophotometer for measurement. The fluorescence spectrophotometer settings were as follows: Ex and Em slit widths of 5 nm, Ex and Em angles with the sample of 45°, PMT voltage of 230 V, excitation wavelength of 355 nm, emission wavelength scan range of 375-550 nm, scan speed of 1200 nm / min, and scan interval of 1 nm.

[0130] Figure 8 These are the fluorescence characteristic curves of Example 2-3 and Comparative Example 2-3. For Comparative Example 2-3, no fluorescent substance carbon quantum dots were added, showing weak fluorescence characteristics. For Example 2-3, 0.5% and 8% carbon quantum dots were added, respectively, and no obvious fluorescence emission peak was shown at 440nm. This is attributed to the insensitivity of monitoring caused by the insufficient addition of 0.5%, and the excessive addition of 8%, which caused the carbon quantum dots to aggregate and quench. It can be explained that both insufficient and excessive addition are not conducive to the fluorescence emission behavior of the film.

[0131] (4) Release of cinnamon essential oil nanoemulsion in the films of Example 1 and Comparative Example 2: The sample was sealed at the mouth of a vial with a rubber band and placed in a constant humidity and constant temperature box at a relative humidity of 55% and a temperature of 25°C for 15 days. A sample was taken every 3 days, cut into pieces, placed in a sealed ethanol solution, stirred for 24 hours (500 rpm), and centrifuged at 5000 rpm for 20 minutes to obtain a supernatant. The absorbance at 285 nm was measured and the results were analyzed according to the pre-established standard curve (y = 98.515x + 0.0080, R 2 =0.9999), y is the absorbance value, and x is the concentration of cinnamon essential oil nanoemulsion (mg / mL). The remaining amount of cinnamon essential oil nanoemulsion is calculated based on the difference between the reduced cinnamon essential oil content in the film and the original cinnamon essential oil content in the sample.

[0132] The release behavior of the cinnamon essential oil nanoemulsion of the films of Example 1 and Comparative Example 2 was analyzed. Figure 9 It can be seen that the cinnamon essential oil nanoemulsion releases rapidly during the first three days of film application, likely due to the release of cinnamon essential oil from the exterior of the film. Over time, the difference in the amount of cinnamon essential oil remaining between Example 1 and Comparative Example 2 increases. The slope of the curve indicates that Example 1 releases cinnamon essential oil at a slower rate, while Comparative Example 2 releases it at a faster rate. This is likely due to the double-layer film in Example 1, where the outer layer acts as a barrier to the cinnamon essential oil nanoemulsion. On day 15, the cinnamon essential oil nanoemulsion in Comparative Example 2 has almost completely evaporated (2%), while 19% remains in Example 1. This demonstrates that the double-layer film design facilitates the slow release of functional substances, such as the cinnamon essential oil nanoemulsion, resulting in a more sustained functional effect.

[0133] (5) Antibacterial properties of Example 1 and Comparative Examples 1-3: The antibacterial properties of Example 1 and Comparative Examples 1-3 were evaluated using the plate count method. First, all samples (100 mg) were sterilized with ultraviolet light, taken out and placed in a 5 mL suspension of E. coli (10 5 CFU / mL) and incubate at 37°C in a shaker (180 rpm) for 6 h. Then, 100 μL of the E. coli suspension was evenly spread on an agar plate and incubated at 37°C in a shaker (180 rpm) for 24 h. The number of surviving colonies was recorded and the inhibition rate was calculated.

[0134] Using Escherichia coli as the target bacterial group, the antibacterial properties of Example 1 and Comparative Examples 1-3 were explored. Figure 10As shown. Comparative Example 3 is composed of a corn starch polyvinyl alcohol-based film and has no obvious antibacterial properties. Comparative Example 1 contains carbon quantum dots, and the unique and complex antibacterial activity mechanism of carbon quantum dots involves the production of ROS, degradation of cell structure, and cytoplasmic leakage due to DNA binding and gene expression regulation. Comparative Example 2 contains cinnamon essential oil nanoemulsion, and the main antibacterial effect is played by the aromatic substances therein, which interact with the bacterial cell wall, causing the cell wall to rupture, the cytoplasm to leak out, and ultimately leading to cell apoptosis. In Example 1, the combined effect of carbon quantum dots and cinnamon essential oil nanoemulsion improves the overall antibacterial ability of the film sample. By comparison, it can be seen that the combined use of carbon quantum dots and cinnamon essential oil nanoemulsion can synergistically improve the antibacterial effect.

[0135] (6) Antioxidant Properties of Example 1 and Comparative Examples 1-3: The antioxidant properties of the examples and comparative examples were tested using the DPPH test. DPPH was dissolved in methanol to prepare a 0.2 mM DPPH free radical solution. 20 mg of the sample was dissolved in 10 mL of the DPPH free radical solution and reacted in the dark for 1 h. The absorbance at 517 nm was measured using a UV-visible spectrophotometer. The DPPH free radical scavenging ability was calculated using the following formula:

[0136] DPPH free radical scavenging rate (%) = (A c -A s ) / A c ×100

[0137] Among them A s is the absorbance of the sample at 517 nm, A c The absorbance of the control at 517 nm.

[0138] In order to prevent food from being oxidized and deteriorated significantly during storage, transportation, and sales, the present invention evaluated the DPPH free radical scavenging ability of the films of Example 1 and Comparative Examples 1-3. The results are as follows: Figure 11 As shown. The film sample of Comparative Example 3 does not contain antioxidant substances, so it has almost no antioxidant capacity. Comparative Examples 2 and 1 contain a certain amount of cinnamon essential oil nanoemulsion and carbon quantum dots, respectively. Both substances can generate reactive oxygen species to eliminate free radicals, and have strong antioxidant properties. Moreover, when the two films are stacked layer by layer to form a double-layer film, their antioxidant capacity is further increased to 88%, indicating that the construction of a double-layer film is more conducive to the preparation of antioxidant films.

[0139] In addition, the present invention also replaces the corn starch in Example 1 with cassava starch, rice starch, potato starch, and pea starch; replaces the cinnamon essential oil nanoemulsion in Example 1 with thymol nanoemulsion, oregano essential oil nanoemulsion, clove essential oil nanoemulsion, and chamomile essential oil nanoemulsion; and replaces the citric acid used in preparing carbon quantum dots with glucose, polyethylene glycol, and sodium citrate, all of which achieve effects comparable to those of Example 1.

[0140] In summary, the present invention uses a layer-by-layer stacking method to construct a novel double-layer film packaging. A corn starch-based polyvinyl alcohol (PVA) film with pH-mediated fluorescence response is stacked layer by layer with a corn starch-based polyvinyl alcohol (PVA) film with antibacterial and antioxidant properties. This produces a corn starch-based polyvinyl alcohol (PVA) composite film with pH-responsive fluorescence, which exhibits sustained-release cinnamon essential oil nanoemulsion, excellent antibacterial and antioxidant properties, and enriches the functionality of corn starch-based polyvinyl alcohol films. This invention enables the preparation of multifunctional corn starch-based polyvinyl alcohol (PVA) films, potentially expanding the application areas of corn starch-based polyvinyl alcohol biodegradable films and providing new ideas for the development of pH-mediated fluorescence sensing films.

[0141] The various aspects, embodiments, features and examples of the present invention should be considered as illustrative in all respects and are not intended to limit the present invention, the scope of which is defined solely by the claims. Other embodiments, modifications and uses will be apparent to those skilled in the art without departing from the spirit and scope of the invention as claimed.

[0142] In addition, the inventors of this case also referred to the aforementioned embodiments and conducted experiments using other raw materials, process operations, and process conditions described in this specification, and obtained relatively ideal results.

[0143] Although the present invention has been described with reference to illustrative embodiments, it will be understood by those skilled in the art that various other changes, omissions, and / or additions may be made and that substantial equivalents may be substituted for the elements of the embodiments without departing from the spirit and scope of the present invention. Additionally, many modifications may be made to adapt specific circumstances or materials to the teachings of the present invention without departing from the scope of the present invention. Therefore, it is not intended herein to limit the present invention to the disclosed specific embodiments for carrying out the present invention, but rather to include all embodiments within the scope of the appended claims. Furthermore, unless specifically stated, any use of the terms first, second, etc. does not indicate any order or importance, but rather uses the terms first, second, etc. to distinguish one element from another.

Claims

1. A starch-polyvinyl alcohol-based film, characterized in that: The film includes a first film layer and a second film layer stacked together, the first film layer is formed by a first film-forming solution, the first film-forming solution includes starch, polyvinyl alcohol, and a pH fluorescence responsive material, the pH fluorescence responsive material includes carbon quantum dots, the carbon quantum dots contain amino groups and carboxyl groups, and the amino groups and carboxyl groups can undergo protonation / deprotonation reactions when the pH changes, thereby generating different emission fluorescence intensities, and the content of the pH fluorescence responsive material in the first film-forming solution is 0.5-8% w / w of the total mass of the starch and polyvinyl alcohol; The second film layer is formed by a second film-forming liquid, which includes starch, polyvinyl alcohol and essential oil nanoemulsion. The essential oil nanoemulsion has at least antibacterial and / or antioxidant properties, and the volume proportion of the essential oil nanoemulsion in the second film-forming liquid is 0.99~16.67%.

2. The starch-polyvinyl alcohol-based film according to claim 1, characterized in that: The starch comprises one or more of corn starch, tapioca starch, rice starch, potato starch or pea starch; And / or, the raw materials of the essential oil nanoemulsion include essential oil, water, a nonionic surfactant and a solubilizer, the nonionic surfactant includes one or more of Tween 20, Tween 40, Tween 80 or Span 20, and the solubilizer includes anhydrous ethanol; and / or, the particle size of the essential oil nanoemulsion is 50 to 200 nm; And / or, the essential oil nanoemulsion includes one or more of cinnamon essential oil nanoemulsion, thymol nanoemulsion, oregano essential oil nanoemulsion, clove essential oil nanoemulsion or chamomile essential oil nanoemulsion; And / or, the content of the pH fluorescence responsive material in the first film-forming solution is 1-2% w / w of the total mass of the starch and polyvinyl alcohol; And / or, the volume proportion of the essential oil nanoemulsion in the second film-forming liquid is 4.5-5.5%.

3. The starch-polyvinyl alcohol-based film according to claim 2, characterized in that: The raw materials of the carbon quantum dots include a carbon source and a nitrogen source. The carbon source includes one or a combination of glucose, polyethylene glycol, citric acid, and sodium citrate, and the nitrogen source includes urea.

4. The starch-polyvinyl alcohol-based film according to claim 1, wherein: The thickness of the first film layer is 0.123-0.132 mm, and the thickness of the second film layer is 0.125-0.199 mm; And / or, the total thickness of the starch-polyvinyl alcohol-based film is 0.235-0.330 mm.

5. A method for preparing a starch-polyvinyl alcohol-based film, characterized in that: include: Providing a first film-forming solution containing starch, polyvinyl alcohol and a pH fluorescent response material, and providing a second film-forming solution containing starch, polyvinyl alcohol and an essential oil nanoemulsion; The pH fluorescence responsive material comprises carbon quantum dots, which contain amino and carboxyl groups. The amino and carboxyl groups can undergo protonation / deprotonation reactions when the pH changes, thereby generating different emission fluorescence intensities. The essential oil nanoemulsion has at least antibacterial and / or antioxidant properties. The content of the pH fluorescence responsive material in the first film-forming liquid is 0.5-8% w / w of the total mass of the starch and polyvinyl alcohol; and the volume proportion of the essential oil nanoemulsion in the second film-forming liquid is 0.99-16.67%; The first film-forming liquid and the second film-forming liquid are respectively made into a first film layer and a second film layer, and the first film layer and the second film layer are combined to obtain a starch-polyvinyl alcohol-based film.

6. The preparation method according to claim 5, characterized in that: The essential oil nanoemulsion includes one or more of cinnamon essential oil nanoemulsion, thymol nanoemulsion, oregano essential oil nanoemulsion, clove essential oil nanoemulsion or chamomile essential oil nanoemulsion; and / or, the starch comprises one or more of corn starch, tapioca starch, rice starch, potato starch or pea starch; And / or, the content of the pH fluorescence responsive material in the first film-forming solution is 1-2% w / w of the total mass of the starch and polyvinyl alcohol; And / or, the volume proportion of the essential oil nanoemulsion in the second film-forming liquid is 4.5-5.5%; And / or, the starch content in the first film-forming liquid and the second film-forming liquid is 2-8% w / v, and the polyvinyl alcohol content is 2-8% w / v; And / or, the first film layer and the second film layer are made in a constant temperature and humidity environment; And / or, the preparation method specifically comprises: firstly preparing a first film layer using the first film-forming liquid, and then forming a second film layer in situ on the first film layer using the second film-forming liquid; Alternatively, the second film-forming liquid is first used to prepare the second film layer, and then the first film-forming liquid is used to in-situ form the first film layer on the second film layer.

7. The preparation method according to claim 6, characterized in that The preparation method of the carbon quantum dots comprises: subjecting a mixed solution containing 10-20% w / v carbon source and 5-15% w / v nitrogen source to a hydrothermal reaction at a temperature of 160-200° C. to obtain the carbon quantum dots.

8. The preparation method according to claim 7, characterized in that: The carbon source includes one or more of glucose, polyethylene glycol, citric acid, and sodium citrate, and the nitrogen source includes urea.

9. The preparation method according to claim 5, characterized in that The preparation method of the essential oil nanoemulsion comprises: emulsifying a mixture containing 8-12% v / v essential oil, 78-82% v / v water, 4-8% v / v nonionic surfactant and 1-5 v / v solubilizer to obtain the essential oil nanoemulsion.

10. The preparation method according to claim 9, characterized in that: The essential oil includes one or a combination of multiple of cinnamon essential oil, thymol essential oil, oregano essential oil, clove essential oil or chamomile essential oil.

11. The preparation method according to claim 9, characterized in that: The nonionic surfactant includes one or a combination of Tween 20, Tween 40, Tween 80 or Span 20.

12. The preparation method according to claim 9, characterized in that: The solubilizing agent includes anhydrous ethanol.

13. The preparation method according to claim 9, characterized in that: The emulsification method includes homogenization and ultrasonication.

14. A packaging structure, characterized in that: The portion of the packaging structure used to accommodate the packaged object is made of the starch-polyvinyl alcohol-based film described in any one of claims 1 to 4, or is made of the starch-polyvinyl alcohol-based film prepared by the method described in any one of claims 5 to 13, and the second film layer of the starch-polyvinyl alcohol-based film is located on the side close to the packaged object.

15. Use of the starch-polyvinyl alcohol-based film according to any one of claims 1 to 4, the starch-polyvinyl alcohol-based film prepared according to the method according to any one of claims 5 to 13, or the packaging structure according to claim 14 in food packaging.

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