A photothermal and thermoresponsive antibacterial film, its preparation method and application

Abstract

The present invention discloses a photothermal and thermoresponsive antibacterial film, its preparation method and application, belonging to the technical field of functional packaging. The present invention prepares a photothermal and thermoresponsive antibacterial film by compounding a substance with photothermal conversion performance and a natural plant essential oil inclusion complex into a film-forming substrate, realizing the combination of contact sterilization and release sterilization; at the initial stage of food storage, the antibacterial film can be irradiated with light of a specific wavelength to rapidly heat up the film, killing most of the microorganisms on the food at an appropriate temperature, and at the same time accelerating the diffusion rate of the essential oil from the inclusion complex under the drive of thermal power, greatly improving the release rate. Thus, through the synergistic sterilization of the release type and the contact type, the effect of double sterilization is achieved; the advantages of the two sterilization methods are fully utilized, and the problems existing in the two sterilization methods are overcome, greatly improving the antibacterial efficiency of the film.

Description

Technical Field

[0001] The present invention relates to the technical field of functional packaging, and particularly to a photothermal and thermoresponsive antibacterial film and its preparation method and application. Background Art

[0002] With the development of society and the progress of life, people have higher and higher requirements for food. In particular, food preservation has become a key issue of current concern. The primary problem of food preservation is food spoilage. During the storage of food, the microorganisms remaining on the food surface will accelerate food spoilage. To address the problem of microorganisms accelerating food spoilage, the commonly used simple and effective method in the prior art is to endow food packaging with antibacterial properties, thereby extending the shelf life of food.

[0003] Currently, incorporating antibacterial agents into packaging through physical or chemical methods is an effective way to endow packaging with antibacterial properties; conventional antibacterial methods are generally divided into two types, namely, release type and contact type. The release type mainly adds some active substances as antibacterial agents to the packaging. Then, during the storage of food, the active substances will slowly be released from the packaging to inhibit the growth of the colonies remaining on the food, thereby effectively extending the shelf life of food; natural plant essential oils are often added to the packaging as active substances to prepare antibacterial fresh-keeping packaging due to their natural and good antibacterial effect; although natural plant essential oils have the advantages of being natural and having a good antibacterial effect, they are extremely volatile and often diffuse in large quantities at the initial stage of storage, unable to achieve a long-term continuous antibacterial effect. If the essential oils are embedded, although the problem of a large amount of essential oil release is solved, to a certain extent, the release of the essential oils is also restricted, and the release efficiency of the essential oils cannot be accurately controlled, resulting in the release amount of the essential oils being insufficient to achieve an ideal antibacterial effect. In addition, the antibacterial effects of most natural plant essential oil release-type antibacterial agents on different types of bacteria also vary greatly, and a single type of plant essential oil cannot achieve a broad-spectrum bactericidal effect.

[0004] In addition to natural plant essential oils, release-type antibacterial agents also use metal ions such as silver, manganese, copper, and iron as antibacterial agents. These metal ions react with components such as proteins and phospholipids on the cell membrane to destroy the integrity of the cell membrane, causing the cell to lose selective permeability, thereby resulting in the death of bacteria; although metal ions have a good bactericidal effect on bacteria, there are also significant potential food safety problems. Among them, the control of the addition amount of metal ions is a key issue. If the addition amount of metal ions is small, the bactericidal effect is not significant. If the addition amount of metal ions is large, the excess metal ions will migrate to the food in large quantities, thereby triggering major food safety problems.

[0005] The most representative of contact sterilization is photothermal sterilization. Photothermal sterilization involves incorporating a material with photothermal conversion properties into the packaging, which releases heat energy under the irradiation of light with a specific wavelength. By destroying the cell membrane at high temperature and denaturing proteins, bacterial cells are killed. It has the advantages of convenient use, high efficiency, and broad-spectrum antibacterial activity, and is a powerful antibacterial strategy. However, photothermal contact sterilization also has the following disadvantages: First, contact sterilization can only be achieved in the part of the packaging that comes into contact with the food. Since most foods on the market are irregular in shape, although there are methods in the prior art to make the packaging fit tightly with the food, it is still difficult to ensure that the packaging completely covers all parts of the food without dead angles, making it difficult to sterilize the food comprehensively. Second, most of the currently used photothermal materials are nanomaterials with photothermal conversion properties. By doping the nanomaterials into the packaging, the packaging is given the property of photothermal conversion. However, due to their small size and electrostatic interactions between them, nanomaterials tend to agglomerate. Once agglomeration occurs, it will affect the dispersion of the nanomaterials in the packaging. If the dispersion is uneven, it will lead to uneven photothermal heating, seriously affecting the effect of photothermal sterilization.

[0006] In summary, due to the obvious disadvantages of both release-type and contact-type antibacterial methods, which greatly limit the application of antibacterial packaging in actual production, it is of great significance for the development of antibacterial food packaging to develop a packaging that can make up for the disadvantages of release-type and contact-type antibacterial methods and has high antibacterial effects. Summary of the Invention

[0007] The object of the present invention is to provide a photothermal and thermoresponsive release antibacterial film, its preparation method and application to solve the problems existing in the above-mentioned prior art. The antibacterial film provided by the present invention combines contact-type photothermal sterilization and release-type slow release of active substances for antibacterial. At the initial stage of preservation, all bacteria on the food are killed by high temperature and thermoresponsive rapid release of active substances, and during the subsequent storage period of the food, the active substances will continue to be slowly released, achieving the effect of long-term antibacterial.

[0008] To achieve the above object, the present invention provides the following solutions:

[0009] One of the technical solutions of the present invention: A photothermal and thermoresponsive release antibacterial film, the raw materials include a solvent, and the following components in the mass percentage of the solvent: 1-10% of a film-forming substrate, 0.5-3% of a natural plant essential oil inclusion, 0.001-4% of a photothermal conversion substance, and 0-0.6% of glycerol. That is, based on the mass of the solvent, the dosages of the film-forming substrate, natural plant essential oil inclusion, photothermal conversion substance, and glycerol are 1-10%, 0.5-3%, 0.001-4%, and 0-0.6% of the solute mass, respectively.

[0010] Preferably, for the photothermal and thermoresponsive antibacterial film, the raw materials include a solvent, and the following components in the mass percentage of the solvent: 10% of a film-forming substrate, 1% of a natural plant essential oil inclusion complex, and 0.0027% of a photothermal conversion substance.

[0011] Furthermore, the film-forming substrate is one or more of polyvinyl alcohol (PVA), chitosan, carboxymethyl cellulose, polylactic acid, and pectin.

[0012] Furthermore, when the film-forming substrate is pectin and / or chitosan, the dosage of glycerol is not zero.

[0013] Furthermore, the natural plant essential oil inclusion complex is composed of a natural plant essential oil and cyclodextrin, and can be called a natural plant essential oil cyclodextrin inclusion complex. The natural plant essential oil cyclodextrin inclusion complex is substantially a microcapsule structure, and the natural plant essential oil is embedded through the microcapsule to improve the stability and release efficiency of the natural plant essential oil in the film.

[0014] Furthermore, the natural plant essential oil is one or more of cinnamon essential oil, thymol essential oil, oregano essential oil, eugenol essential oil, neroli essential oil, peppermint essential oil, and tea tree essential oil.

[0015] Furthermore, the natural plant essential oil is preferably cinnamon essential oil.

[0016] Furthermore, the cyclodextrin is preferably β-cyclodextrin.

[0017] Cyclodextrin also has good stability at high temperatures and is not easily changed by temperature changes. Selecting cyclodextrin to prepare the natural plant essential oil inclusion complex can maintain the stability of the inclusion complex in a high-temperature environment. If substances such as starch or proteins are used as microcapsules to embed essential oils, they are easily denatured by temperature changes, resulting in the complete release of the essential oil and the loss of the sustained-release effect.

[0018] Furthermore, the photothermal conversion substance is nanoparticles.

[0019] Furthermore, the photothermal conversion substance is one or more of gold nanoparticles, silver nanoparticles, manganese oxide nanoparticles, and lignin nanoparticles.

[0020] Furthermore, the photothermal conversion substance is preferably gold nanoparticles.

[0021] Furthermore, the surface of the photothermal conversion substance is coated with a silica shell layer, that is, the gold nanoparticles, silver nanoparticles, manganese oxide nanoparticles and lignin nanoparticles are specifically gold-core silica-shell core-shell nanoparticles, silver-core silica-shell core-shell nanoparticles, manganese oxide-core silica-shell core-shell nanoparticles and lignin-core silica-shell core-shell nanoparticles with a silica shell layer on the surface. The above-mentioned core-shell nanoparticles are obtained by coating a layer of silica shell on the nanoparticles through surface modification. Since the nanoparticles have poor stability in the reaction solution and are prone to aggregation, a layer of silica shell is coated through surface modification to improve the stability of the nanoparticles in the reaction system, and to avoid aggregation and precipitation during the mixing reaction with the film-forming substrate, so that the nanoparticles are unevenly distributed when the final film is formed, resulting in uneven photothermal temperature at each position on the film, which greatly affects the photothermal conversion efficiency of the film.

[0022] Furthermore, the thickness of the silica shell layer is 15 nm.

[0023] Furthermore, the operation of coating the silica shell layer on the surface is as follows: add 3-aminopropyltrimethoxysilane solution to the aqueous solution of nanoparticles, stir at room temperature for 10 min, then add sodium metasilicate solution and continue stirring for more than 3 min, and finally carry out a condensation reflux reaction at 90 °C for 2 h.

[0024] Furthermore, the concentration of 3-aminopropyltrimethoxysilane is 1 mM; the sodium metasilicate solution is calculated as SiO2 and has a concentration of 0.54 wt%, and the pH of the sodium metasilicate solution is adjusted to 9.5 - 11 with HCl.

[0025] Furthermore, the volume ratio of the aqueous solution of nanoparticles, 3-aminopropyltrimethoxysilane solution and sodium metasilicate solution is 30:0.4:5.

[0026] Furthermore, when the aqueous solution of nanoparticles is an aqueous solution of manganese oxide nanoparticles or an aqueous solution of lignin nanoparticles, the concentration of the aqueous solution of nanoparticles is 4 mg / 30 mL;

[0027] When the aqueous solution of nanoparticles is an aqueous solution of gold nanoparticles (AuNPs), its preparation method is as follows: first add 1 wt% HAuCl4 solution to water at a volume ratio of 1:99, heat to boiling at 125 °C, then add 0.7 vol% sodium citrate solution, and carry out a condensation reflux reaction for 10 min to obtain an aqueous solution of AuNPs; the concentration of the sodium citrate solution is 25.4 mM;

[0028] When the aqueous solution of nanoparticles is an aqueous solution of silver nanoparticles, its preparation method is as follows: First, dissolve silver nitrate in water, heat it to boiling under stirring, and then dropwise add sodium citrate solution, and continue to react for 1.5 hours to obtain an aqueous solution of silver nanoparticles; the concentration of the sodium citrate solution is 38.8 mmol / L, and the dosage ratio of silver nitrate, water and sodium citrate solution is 9 mg: 49 mL: 1 mL.

[0029] Further, the solvent is one or more of water, acetic acid and dichloromethane.

[0030] The second technical solution of the present invention: The preparation method of the above-mentioned photothermal and thermoresponsive release antibacterial film includes the following steps:

[0031] Weigh each raw material according to the mass percentage composition of the raw materials, mix the other raw materials except the natural plant essential oil inclusion compound to obtain a composite solution; add the natural plant essential oil inclusion compound to the composite solution to obtain a film-forming solution; pour the film-forming solution into a vessel for film formation, and dry it to obtain the photothermal and thermoresponsive release antibacterial film.

[0032] Further, according to the different raw materials selected, the step of mixing the other raw materials except the natural plant essential oil inclusion compound to obtain a composite solution; adding the natural plant essential oil inclusion compound to the composite solution to obtain a film-forming solution specifically selects one of the following methods:

[0033] Method 1: Take a part of the solvent and mix it with the photothermal conversion substance to obtain a photothermal conversion substance solution; mix the film-forming substrate, the photothermal conversion substance solution and the remaining solvent, and heat and stir at 75-90 °C for 2-3 h to obtain a composite solution; cool the composite solution to 50-55 °C and then add the natural plant essential oil inclusion compound, and continue to heat and react at 50-55 °C for 0.5 h to obtain a film-forming solution (when the film-forming substrate is only PVA and no additional plasticizer glycerol is required, select Method 1);

[0034] Method 2: Take a part of the solvent and mix it with the photothermal conversion substance to obtain a photothermal conversion substance solution; mix the film-forming substrate, the photothermal conversion substance solution and the remaining solvent, and heat and stir at 75-90 °C for 2-3 h, then add glycerol, and continue to heat and stir for 2-3 h to obtain a composite solution; cool the composite solution to 50-55 °C and then add the natural plant essential oil inclusion compound, and continue to heat and react at 50-55 °C for 0.5 h to obtain a film-forming solution (when the film-forming substrate is only pectin and the plasticizer glycerol needs to be added, select Method 2);

[0035] Method 3: Take a part of the solvent and mix it with the photothermal conversion substance to obtain a photothermal conversion substance solution; mix the film-forming substrate, the photothermal conversion substance solution, glycerol, and the remaining solvent, and stir at room temperature for 12 h to obtain a composite solution; add the natural plant essential oil inclusion complex to the composite solution, and continue stirring and reacting for 0.5 h to obtain a film-forming solution (when the film-forming substrate is only chitosan and the plasticizer glycerol needs to be added, select Method 3);

[0036] Method 4: Mix the film-forming substrate, the photothermal conversion substance, and the solvent, and stir and react at room temperature for 2 - 3 h to obtain a composite solution; add the natural plant essential oil inclusion complex to the composite solution, and continue stirring and reacting for 0.5 h to obtain a film-forming solution (when the film-forming substrate is only polylactic acid and no additional plasticizer glycerol is required, select Method 4);

[0037] Method 5: Take a part of the solvent and mix it with the photothermal conversion substance to obtain a photothermal conversion substance solution; mix the film-forming substrate, the photothermal conversion substance solution, and the remaining solvent, and stir at room temperature for 0.5 h to obtain a composite solution; add the natural plant essential oil inclusion complex to the composite solution, and continue stirring for 0.5 h to obtain a film-forming solution (when the film-forming substrate is only carboxymethyl cellulose and no additional plasticizer glycerol is required, select Method 5);

[0038] Method 6: Take a part of the solvent and mix it with the photothermal conversion substance to obtain a photothermal conversion substance solution; take a part of the solvent and mix it with one of the film-forming substrates to obtain a film-forming substrate solution 1; mix the other of the film-forming substrates with the photothermal conversion substance solution and the remaining solvent to obtain a film-forming substrate solution 2; mix the film-forming solution 1 and the film-forming solution 2 to obtain a composite solution; add the natural plant essential oil inclusion complex to the composite solution, and continue stirring for 0.5 h to obtain a film-forming solution (when the film-forming substrate is a composite of two materials, select Method 6).

[0039] Further, the rotation speed of the stirring in Methods 1 and 2 is 700 rpm.

[0040] The third technical solution of the present invention: The above-mentioned photothermal and thermoresponsive release antibacterial film is applied to food packaging.

[0041] Further, the specific operation of the application is: Use the photothermal and thermoresponsive release antibacterial film to seal-pack the food, and after the seal-packaging is completed, irradiate the photothermal and thermoresponsive release antibacterial film with 808 nm near-infrared light for 5 min.

[0042] The present invention discloses the following technical effects:

[0043] (1) The present invention prepares a photothermal and thermoresponsive release antibacterial film by compounding a photothermal conversion substance with a photothermal conversion effect and a natural plant essential oil inclusion complex into a film-forming substrate, realizing the combination of contact sterilization and release sterilization. At the initial stage of food storage, the antibacterial film can be irradiated with light of a specific wavelength, causing the film to rapidly heat up. By an appropriate temperature, most of the bacteria on the food can be killed. At the same time, as the temperature rises, the plant essential oil accelerates its diffusion rate from the inclusion complex under the drive of thermal dynamics, and the release rate is greatly increased. Thus, through the synergistic sterilization of the release type and the contact type, the effect of double sterilization is achieved, solving the problem that contact sterilization cannot cover food comprehensively, resulting in poor sterilization efficiency. At the same time, it also solves the problem that after the embedding of volatile active substances, it is difficult to control the release rate, the release efficiency is low, and the sterilization effect is not ideal. It makes full use of the advantages of the two sterilization methods and overcomes the problems existing in the two sterilization methods, greatly improving the antibacterial efficiency of the film.

[0044] (2) In addition to being able to sterilize through the synergistic effect of the release type and the contact type at the initial stage of food storage, during the food storage period, due to the barrier of the packaging, bacteria in the external environment are difficult to enter the interior of the packaging, and the remaining plant essential oil can continue to be slowly released at room temperature, achieving a continuous antibacterial effect, further prolonging the shelf life of the food.

[0045] (3) The antibacterial film of the present invention has good sterilization effect, simple preparation process, small amount of raw materials used, low overall cost, and through the synergistic sterilization of the release type and the contact type, effectively reduces the addition amount of antibacterial agents, making the composite film as a whole have good biocompatibility, meeting the requirements of food fresh-keeping packaging. In summary, the antibacterial film of the present invention has great application prospects in the field of food packaging.

[0046] (4) The present invention modifies and coats a silica shell layer on the surface of the nanomaterial to improve the stability of the nanomaterial in the reaction system, enabling the nanomaterial to be evenly distributed in the film, thus solving the problem that the nanoparticles are unevenly distributed during the final film formation, resulting in uneven photothermal temperature, and effectively improving the photothermal conversion efficiency of the packaging. In addition, silica also has good biocompatibility, so coating the silica shell layer can further improve the biocompatibility of the photothermal conversion substance, and further improve the safety of the film. Description of the Drawings

[0047] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0048] Figure 1 Photothermal conversion temperature diagram of the films prepared in Examples 1-6;

[0049] Figure 2 Photothermal conversion temperature diagram of the films prepared in Examples 7-8 and Comparative Examples 1-3;

[0050] Figure 3 Photothermal conversion temperature diagram of the films prepared in Examples 14-16;

[0051] Figure 4 Colony diagram of Staphylococcus aureus in the film antibacterial experiment;

[0052] Figure 5 Colony diagram of Escherichia coli in the film antibacterial experiment;

[0053] Figure 6 Inhibitory rate diagram of Staphylococcus aureus in the film antibacterial experiment;

[0054] Figure 7 Inhibitory rate diagram of Escherichia coli in the film antibacterial experiment;

[0055] Figure 8 Line graph of essential oil release amount of the film;

[0056] Figure 9 Line graph of essential oil release rate of the films prepared in Example 3 and Comparative Example 5;

[0057] Figure 10 Colony diagrams of the blank group, Comparative Example 1 and Comparative Example 2 on the second and fifth days in the beef preservation experiment;

[0058] Figure 11 Colony diagrams of Comparative Example 3, Comparative Example 4 and Example 3 on the second and fifth days in the beef preservation experiment;

[0059] Figure 12 Colony quantity comparison diagram of different groups on the second and fifth days in the beef preservation experiment;

[0060] Figure 13 TVB-N comparison diagram of different groups on the second and fifth days in the beef preservation experiment. Detailed implementation manners

[0061] The various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation on the present invention, but rather as a more detailed description of certain aspects, features, and implementation schemes of the present invention.

[0062] It should be understood that the terms described in the present invention are only for describing specific embodiments and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0063] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0064] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific embodiments of the present invention specification, which are obvious to those skilled in the art. Other embodiments obtained from the specification of the present invention are obvious to those skilled in the art. The specification and examples of the present invention are merely exemplary.

[0065] Regarding the use of "comprising", "including", "having", "containing", etc. in this article, they are all open-ended terms, meaning including but not limited to.

[0066] The preparation method of the natural plant essential oil β-cyclodextrin clathrate used in the following examples and comparative examples is as follows: Dissolve 9 g of β-cyclodextrin in a mixed solution of 90 mL of ethanol and deionized water (1:2, v / v), and continuously stir with a magnetic stirrer at a rotation speed of 700 rpm for 3 h under the condition of 55 ± 1 °C to obtain a β-cyclodextrin solution. Dissolve 1 g of cinnamon essential oil in 10 mL of ethanol, and slowly add it to the β-cyclodextrin solution under continuous stirring, so that the mass ratio of cinnamon essential oil to β-cyclodextrin is 10:90. The resulting mixed solution is stirred at 50 ± 1 °C for 2.5 h. After the mixed solution is cooled to room temperature, it is placed in a 4 °C environment and frozen overnight. The precipitated precipitate is purified by vacuum filtration and rinsed twice with 30 vol% ethanol, and then freeze-dried for 12 h to obtain the dry powder of the natural plant essential oil β-cyclodextrin clathrate, which is placed in a sealed container and stored at 4 °C for subsequent use.

[0067] The preparation method of the gold-core silica-shell core-shell nanoparticles used in the following examples and comparative examples is as follows: First, add 1 mL of 1 wt% HAuCl4 solution to 99 mL of deionized water, heat to boiling at 125 °C, and then add 0.7 mL (25.4 mM) of sodium citrate solution under magnetic stirring at a rotation speed of 700 rpm, and carry out a condensation reflux reaction for 10 min to obtain an aqueous solution of AuNPs. After the aqueous solution of AuNPs is cooled to room temperature, add 0.4 mL (1 mM) of 3-aminopropyltrimethoxysilane solution to 30 mL of the aqueous solution of AuNPs, stir at a rotation speed of 500 rpm at room temperature for 10 min, then add 5 mL of sodium metasilicate solution (0.54 wt% SiO2, adjusted to pH 10 with HCl) and continue stirring for 10 min. Finally, carry out a condensation reflux reaction at 90 °C for 2 h to obtain an aqueous solution of gold-core silica-shell core-shell nanoparticles, centrifuge, and dry to obtain a solid product.

[0068] The preparation method of the silver-core silica-shell core-shell nanoparticles used in the following examples is as follows: First, dissolve 9 mg of silver nitrate in 49 mL of water, and heat to boiling under magnetic stirring at a rotation speed of 700 rpm. Then, dropwise add 1 mL of 38.8 mmol / L sodium citrate solution and continue the reaction for one and a half hours to obtain an aqueous solution of silver nanoparticles. After the aqueous solution of silver nanoparticles is cooled to room temperature, add 0.4 mL (1 mM) of 3-aminopropyltrimethoxysilane solution to 30 mL of the aqueous solution of silver nanoparticles, stir at a rotation speed of 500 rpm at room temperature for 10 min, then add 5 mL of sodium metasilicate solution (0.54 wt% SiO2, adjusted to pH 10 with HCl) and continue stirring for 10 min. Finally, carry out a condensation reflux reaction at 90 °C for 2 h to obtain an aqueous solution of silver-core silica-shell core-shell nanoparticles, centrifuge, and dry to obtain a solid product.

[0069] The preparation method of the manganese oxide-core silica-shell core-shell nanoparticles used in the following examples is as follows: Add 4 mg of manganese oxide nanoparticles to 30 mL of deionized water, stir evenly, then add 0.4 mL (1 mM) of 3-aminopropyltrimethoxysilane solution, stir at a rotation speed of 500 rpm at room temperature for 10 min, then add 5 mL of sodium metasilicate solution (0.54 wt% SiO2, adjusted to pH 10 with HCl) and continue stirring for 10 min. Finally, carry out a condensation reflux reaction at 90 °C for 2 h to obtain an aqueous solution of manganese oxide-core silica-shell core-shell nanoparticles, centrifuge, and dry to obtain a solid product.

[0070] The preparation method of the lignin core - silica shell core - shell nanoparticles used in the following examples is as follows: Add 4 mg of lignin nanoparticles to 30 mL of deionized water. After stirring evenly, add 0.4 mL (1 mM) of 3 - aminopropyltrimethoxysilane solution, and stir at a speed of 500 rpm for 10 min at room temperature. Then add 5 mL of sodium metasilicate solution (0.54 wt% SiO2, pH adjusted to 10 with HCl) and continue stirring for 10 min. Finally, carry out a condensation reflux reaction at 90 °C for 2 h to obtain an aqueous solution of lignin core - silica shell core - shell nanoparticles, centrifuge, and dry to obtain a solid product.

[0071] The zeta potential values and particle sizes of the above - mentioned photothermal conversion substances before and after coating with a silica shell were measured, and the results are shown in Table 1:

[0072] Table 1 Zeta potential values and particle sizes of various photothermal conversion substances before and after coating with a silica shell

[0073] Table 1

[0074]

[0075] Table 1 shows the changes in zeta potential values and particle sizes of various photothermal conversion substances before and after coating with a silica shell. It can be seen that after coating with a silica shell, the potential values of the nanoparticles have all changed. This is because after coating the nanoparticles with a silica shell, due to the negative charges on the silica shell, the overall core - shell nanoparticles show a higher negative potential value than the nanoparticles. The higher the electric potential on the particle surface, the greater the electrostatic repulsion force between particles with the same kind of charge, making it not easy for particles to aggregate with each other. Therefore, the dispersion of particles in the polymer substrate will be more uniform. In addition, the change in particle size also well illustrates the formation of the silica shell. With the addition of the same amount of sodium metasilicate, a silica layer of about 15 nm is formed on the nanoparticles. The presence of the silica layer can protect the internal nanoparticles from the external environment and improve the stability of the nanoparticles.

[0076] The preparation method of the zein-pectin stabilized cinnamon essential oil Pickering emulsion used in the following comparative examples is as follows: Dissolve 4 g of zein in 100 mL of ethanol (80 vol%) at room temperature, and then homogenize it at 10000 rpm for 1 min to obtain a zein nano-dispersion. Dissolve 2 g of pectin in 100 mL of deionized water, heat it to 70 °C until completely dissolved, and centrifuge it at 11000 rpm for 20 min to obtain a pectin solution. Subsequently, under the condition of magnetic stirring (800 rmp) at 70 °C, add the pectin solution drop by drop to the zein nano-dispersion at a mass ratio of 10:4 (zein nano-dispersion / pectin solution), and evaporate the ethanol for 20 min. After cooling, adjust the pH to 3 with hydrochloric acid to obtain a zein / pectin composite colloid particle dispersion. Then homogenize it at 8000 rpm for 5 min. Finally, add cinnamon essential oil (1:1, v / v) to the zein / pectin composite colloid particle dispersion and homogenize it at 14000 rpm for 4 min to obtain a zein-pectin stabilized cinnamon essential oil Pickering emulsion.

[0077] Example 1

[0078] Add 3 g of PVA, 22.5 mL of deionized water and 7.5 mL of an aqueous solution of gold-core silica-shell core-shell nanoparticles (0.04 mg / mL) to a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution; after the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamon essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene Petri dish with a diameter of 90 mm, and place it on a 35 °C hot stage to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0079] Example 2

[0080] Add 3 g of PVA, 15 mL of deionized water and 15 mL of an aqueous solution of gold-core silica-shell core-shell nanoparticles (0.04 mg / mL) to a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution; after the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamon essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene Petri dish with a diameter of 90 mm, and place it on a 35 °C hot stage to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0081] Example 3

[0082] Add 3 g of PVA, 10 mL of deionized water and 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution; after the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot table at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0083] Example 4

[0084] Add 3 g of PVA, 5 mL of deionized water and 25 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution; after the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot table at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0085] Example 5

[0086] Add 3 g of PVA and 30 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution; after the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot table at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0087] Example 6

[0088] Add 3 g of PVA and 30 mL (0.1 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution. After the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot table at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0089] Example 7

[0090] Add 3 g of PVA and 30 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution. After the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.15 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue to react at 50 °C for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot table at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0091] Example 8

[0092] Add 3 g of PVA and 30 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution. After the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.9 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot table at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive release antibacterial film.

[0093] Example 9

[0094] Add 0.6 g of pectin, 10 mL of deionized water, and 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 75 °C, and carry out a condensation reflux reaction for 2 h under the condition that the stirring bar rotates at 700 rpm. Then add 0.18 g of glycerol and continue to stir and react at 75 °C for 2 h to obtain a composite solution. After the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamon essential oil β-cyclodextrin inclusion complex, and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a 35 °C hot stage to dry for 24 h to obtain a photothermal and thermoresponsive antibacterial film.

[0095] Example 10

[0096] Add 0.3 g of chitosan, 0.075 g of glycerol, 10 mL of 3 wt% acetic acid aqueous solution, and 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Stir at a speed of 900 rpm for 12 h using mechanical stirring at room temperature to obtain a composite solution. After the reaction is completed, add 0.3 g of cinnamon essential oil β-cyclodextrin inclusion complex and continue to react for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a 35 °C hot stage to dry for 24 h to obtain a photothermal and thermoresponsive antibacterial film.

[0097] Example 11

[0098] Add 1.5 g of polylactic acid, 0.8 mg of gold-core silica-shell core-shell nanoparticles, and 30 mL of dichloromethane into a 100 mL round-bottom flask. Stir and react at a speed of 700 rpm for 3 h by magnetic stirring at room temperature to obtain a composite solution. Subsequently, add 0.3 g of cinnamon essential oil β-cyclodextrin inclusion complex and continue to react for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a 35 °C hot stage to dry for 24 h to obtain a photothermal and thermoresponsive antibacterial film.

[0099] Example 12

[0100] Add 0.6 g of carboxymethyl cellulose, 10 mL of deionized water and 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, stir mechanically at a speed of 400 rpm for 0.5 h at room temperature to obtain a composite solution. Then add 0.3 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue stirring and reacting for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot plate at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive antibacterial film.

[0101] Example 13

[0102] Dissolve 0.3 g of chitosan in 30 mL of 1 wt% acetic acid aqueous solution, and stir (250 rpm) at 50 °C for 2 h to obtain a chitosan solution. Mix 3 g of polyvinyl alcohol, 20 mL of deionized water and 40 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles, and stir (700 rpm) under reflux condensation reaction at 90 °C for 3 h to obtain a polyvinyl alcohol solution. Then mix the polyvinyl alcohol solution and the chitosan solution evenly by mechanical stirring (600 rpm) to obtain a composite solution. Subsequently, add 0.45 g of cinnamomum essential oil β-cyclodextrin inclusion complex and continue stirring and reacting for 0.5 h to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot plate at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive antibacterial film.

[0103] Example 14

[0104] Same as Example 3, the only difference is that 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles is replaced with 20 mL (0.2 mg / mL) of an aqueous solution of silver-core silica-shell core-shell nanoparticles.

[0105] Example 15

[0106] Same as Example 3, the only difference is that 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles is replaced with 20 mL (0.6 mg / mL) of an aqueous solution of manganese oxide-core silica-shell core-shell nanoparticles.

[0107] Example 16

[0108] Same as Example 3, the difference is that 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles is replaced with 20 mL (60 mg / mL) of an aqueous solution of lignin-core silica-shell core-shell nanoparticles.

[0109] Control Example 1

[0110] Add 3 g of PVA, 10 mL of deionized water, and 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene Petri dish with a diameter of 90 mm, and place it on a hot plate at 35 °C to dry for 24 h to obtain an antibacterial film.

[0111] Control Example 2

[0112] Add 3 g of PVA and 30 mL of deionized water into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution; after the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of cinnamomum essential oil β-cyclodextrin inclusion complex, and continue to stir and react at 50 °C for 0.5 h to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene Petri dish with a diameter of 90 mm, and place it on a hot plate at 35 °C to dry for 24 h to obtain an antibacterial film.

[0113] Control Example 3

[0114] Add 3 g of PVA and 30 mL of deionized water into a 100 mL round-bottom flask. Subsequently, place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a film-forming solution; pour the film-forming solution into a disposable polypropylene Petri dish with a diameter of 90 mm. Place it on a hot plate at 35 °C to dry for 24 h to obtain a film.

[0115] Control Example 4

[0116] Commercial ordinary PE plastic wrap with a thickness of 0.2 mm.

[0117] Control Example 5

[0118] Add 3 g of PVA, 10 mL of deionized water and 20 mL (0.04 mg / mL) of an aqueous solution of gold-core silica-shell core-shell nanoparticles into a 100 mL round-bottom flask. Then place the round-bottom flask in an oil bath, control the temperature of the oil bath to be maintained at 90 °C, and carry out a condensation reflux reaction for 3 h under the condition that the stirring speed of the magnetic stirrer is 700 rpm to obtain a composite solution. After the reaction is completed, lower the temperature of the composite solution to 50 °C, add 0.3 g of Pickering emulsion of cinnamon essential oil stabilized by zein-pectin nanoparticles, and continue to stir and react at 50 °C for 10 min to obtain a film-forming solution. Pour the film-forming solution into a disposable polypropylene petri dish with a diameter of 90 mm, and place it on a hot stage at 35 °C to dry for 24 h to obtain a photothermal and thermoresponsive antibacterial film.

[0119] Test Example 1

[0120] Testing of the mechanical properties of the film

[0121] Testing method: Use a universal tensile testing machine (SUST, China) for testing. The size of the test sample is a dumbbell-shaped sample of 2×35×0.2 mm. 3 During testing, the distance between the sample clamps is 15 mm, the testing speed is 10 mm / min, and each sample is tested in parallel 6 times. The final result is the average value.

[0122] Table 2 shows the results of the mechanical tensile experiments of the films prepared in Examples 1-16 and Comparative Examples 1-3 of the present invention.

[0123] Table 2

[0124]

[0125]

[0126] As can be seen from Table 2, in Examples 1-6, as the addition amount of the gold-core silica-shell core-shell nanoparticles increases from 0.001-0.01%, the elongation at break of the film shows a gradually increasing trend, and the strength of the film also gradually increases but the change trend is not significant; in Examples 5, 7 and 8, when the addition amount of the gold-core silica-shell core-shell nanoparticles is fixed and the addition amount of the natural plant essential oil cyclodextrin inclusion complex is changed from 0.5-3%, the elongation at break of the film shows a gradually increasing trend, and the strength of the film also shows an increasing trend.

[0127] Comparing Comparative Examples 1-3, it can be seen that the addition of gold-core silica-shell core-shell nanoparticles and natural plant essential oil cyclodextrin inclusion complexes will slightly reduce the mechanical properties of the polyvinyl alcohol film. However, due to the excellent mechanical properties of the polyvinyl alcohol film itself, the addition of gold-core silica-shell core-shell nanoparticles and natural plant essential oil cyclodextrin inclusion complexes does not have a significant impact on the mechanical properties of the film, and the reduction range is within an acceptable range. The film as a whole still has excellent mechanical properties.

[0128] In addition, in the examples of the present invention, in addition to exploring the mechanical properties of the film prepared with polyvinyl alcohol as the film-forming substrate, the mechanical properties of the films prepared with other different film-forming substrates and different photothermal conversion materials were also explored. It can be seen from Examples 9-16 that after adding gold-core silica-shell core-shell nanoparticles and natural plant essential oil cyclodextrin inclusion complexes to each film-forming substrate, the mechanical properties of the prepared films are within the reasonable range of the mechanical properties of the films prepared with a single film-forming substrate; in the case of the same film-forming substrate, different photothermal conversion materials also have little impact on the mechanical properties of the film.

[0129] Test Example 2

[0130] Testing of the photothermal conversion performance of the film

[0131] Testing method: The film was irradiated with an 808 nm wavelength infrared laser, and at the same time, the change of the film's temperature with time was monitored and recorded by an infrared thermal imager.

[0132] Figure 1 The following are the photothermal conversion experimental results of the films prepared in Examples 1-6 of the present invention. As shown in the figure, in Examples 1-6, as the addition amount of gold-core silica-shell core-shell nanoparticles increased from 0.001% to 0.01%, the photothermal conversion performance of the film also continuously improved. The highest temperature that the film's photothermal conversion could reach rose from 40±2°C to 107±1°C, and the films in Examples 1-6 could all reach the highest temperature within 1 minute, which proves that the film has good photothermal conversion performance;

[0133] Figure 2 The following are the photothermal conversion experimental results of the films prepared in Examples 7-8 and Comparative Examples 1-3. It can be seen from Comparative Examples 2 and 3 that without adding gold-core silica-shell core-shell nanoparticles, the film does not have the performance of photothermal conversion, while in Examples 7 and 8, the film has the same photothermal conversion performance when adding the same content of gold-core silica-shell core-shell nanoparticles; in addition, by comparing the results of Example 2 and Comparative Example 1, and Example 5 and Examples 7-8, it can be seen that the addition of natural plant essential oil cyclodextrin inclusion complexes does not affect the photothermal performance of the film.

[0134] Figure 3Photothermal conversion experimental results of the films prepared in Examples 14 - 16. In these three examples, the photothermal conversion materials were changed based on Example 3. In this test example, with the same photothermal conversion temperature as the benchmark, the photothermal conversion performance of different photothermal materials was explored. Among them, the addition amount of gold-core silica-shell core-shell nanoparticles was the least, followed by silver-core silica-shell core-shell nanoparticles and manganese oxide-core silica-shell core-shell nanoparticles. The lignin-core silica-shell core-shell nanoparticles had the largest addition amount. Thus, it can be seen that the gold-core silica-shell core-shell nanoparticles had the best photothermal conversion performance. When the same photothermal conversion temperature could be achieved, their addition amount was much less than that of other materials. And the less the addition amount of the material, the less the amount migrating out of the film, and the higher the safety of the film.

[0135] Test Example 3

[0136] Antibacterial performance test of the film

[0137] Test method: The inhibition rate of the film against Escherichia coli and Staphylococcus aureus was tested by the spread plate method. Specifically, first cut the film into circular pieces with a diameter of 5 mm, take 10 μL of the bacterial suspension with a specific concentration and drop it on the circular piece. The experimental group was irradiated with 808 nm near-infrared light for 5 min, and the control group was directly placed for 5 min and then the circular piece was rinsed with 400 μL of PBS buffer. Finally, take 100 μL of the dilution and add it to the culture medium and place it in a biochemical incubator for 12 h.

[0138] After demonstrating that the film had excellent photothermal conversion performance, considering the influence of high temperature on food, the higher the temperature, the more likely it was to affect the quality of food. In the present invention, Example 3 with a photothermal temperature in the range of 68 - 70 °C was selected as the optimal group for the next antibacterial experiment. Example 3 could maximize the effects of photothermal sterilization and promoting essential oil release without affecting the food quality. Further, Staphylococcus aureus was used as Gram-positive bacteria and Escherichia coli was used as Gram-negative bacteria to explore the antibacterial effect of the film; Figure 4 、 5 Figures 6 and 7 are the antibacterial experimental results of the films prepared in Example 3 of the present invention and Comparative Examples 1 - 3. From Figure 4 and Figure 5 the number of colonies on the culture dishes, it can be seen that the pure PVA film of Comparative Example 3 did not have antibacterial effect, while the PVA film prepared in Comparative Example 2 with only the inclusion complex of essential oil had a certain antibacterial effect on Staphylococcus aureus, but did not have antibacterial effect on Escherichia coli. This shows that the cinnamomum essential oil used in the present invention has antibacterial effect on Staphylococcus aureus, but is not applicable to Escherichia coli. This also verifies the problem that a single antibacterial agent cannot achieve broad-spectrum sterilization in diffusion sterilization.

[0139] In contrast, the films containing gold-core silica-shell core-shell nanoparticles prepared in Example 3 and Comparative Example 1 showed significant bactericidal effects on both bacteria under 808 nm near-infrared light illumination. However, the film in Comparative Example 1 did not completely kill Staphylococcus aureus, while the film in Example 3 achieved complete killing of both bacteria. This is mainly because Escherichia coli is more sensitive to temperature and can be completely killed at an environmental temperature of 70 °C, while Staphylococcus aureus is relatively more heat-resistant and usually requires a temperature above 80 °C to be completely killed. In addition, in the absence of light, Example 3 showed a certain bactericidal effect on Staphylococcus aureus due to the simultaneous incorporation of natural essential oil inclusion compounds, while Comparative Example 1 did not have a bactericidal effect in the absence of light.

[0140] Based on the results of the above antibacterial experiments, it can be concluded that the films in Comparative Example 2 containing only natural plant essential oil inclusion compounds and Comparative Example 1 containing only gold-core silica-shell core-shell nanoparticles could not achieve complete sterilization. However, by incorporating natural plant essential oil inclusion compounds and gold-core silica-shell core-shell nanoparticles into the PVA film together, the present invention achieved a 100% antibacterial rate against Staphylococcus aureus and Escherichia coli under infrared light illumination.

[0141] Test Example 4

[0142] Essential oil thermoresponsive release test

[0143] Test method: Take two films of specific sizes from the same film and immerse them in a glass reagent bottle containing 95 vol% ethanol. Then, place the glass reagent bottles in a water bath at room temperature and 70 °C respectively. Take out 2 mL of the solution from the bottle at specific intervals and measure the ultraviolet spectrum of the taken-out solution. Calculate the content of cinnamomum essential oil in the solution based on the absorbance of the characteristic peak of cinnamomum essential oil at 285 nm and the concentration standard curve of cinnamomum essential oil in ethanol, and infer the release amount of cinnamomum essential oil from the film. After the test, pour the solution back into the glass bottle.

[0144] Specifically, to prove that increasing the temperature can promote the accelerated release of cinnamomum essential oil from cyclodextrin, in the present invention, two films of the same size were taken from the film prepared in Example 3 and placed into glass bottles containing 95 vol% ethanol respectively. The control group was kept at room temperature all the time, while the experimental group was first placed in hot water at 70 °C for 5 min and then at room temperature. Subsequently, it was placed in hot water at 70 °C for 5 min again every two hours and repeated until the test was completed. The test results are as follows Figure 8As shown, after heating for 5 minutes, the content of cinnamon essential oil in the experimental group solution was about 2.5 times that of the control group placed at room temperature. And every time it was heated for 5 minutes subsequently, the content of cinnamon essential oil in the solution increased significantly, proving that the method of promoting the accelerated release of cinnamon essential oil from cyclodextrin by heating in the present invention is effective and the effect is very remarkable.

[0145] The present invention also explored the influence of different embedding materials on the heat-responsive release. A piece of film with the same size was taken from the films prepared in Example 3 and Comparative Example 5 respectively, and then put into a glass bottle containing 95 vol% ethanol. Both groups of films were first placed in hot water at 70 °C for 5 minutes, and then placed at room temperature. Subsequently, the ultraviolet spectrum was measured every two hours and repeated in cycles until the test ended. The test results are as Figure 9 shown. After soaking in hot water at 70 °C for 5 minutes, the release rate of cinnamon essential oil in the film of Comparative Example 5 rapidly increased to 60%, while on the contrary, the release rate of cinnamon essential oil in the film of Example 3 steadily increased to 30%. Subsequently, the release rate of cinnamon essential oil in the film of Comparative Example 5 reached more than 90% at the 6th hour, while the release rate of cinnamon essential oil in the film of Example 3 continued to increase steadily within 12 hours. It can be analyzed from the above experimental results that for the cinnamon essential oil stabilized by zein-pectin nanoparticles, after heating, since substances such as protein pectin are easily denatured by high temperature, a large amount of cinnamon essential oil is released in a short time, greatly weakening the continuous antibacterial effect, while cyclodextrin is relatively more heat-resistant and can still maintain its activity after high temperature and continuously and stably release cinnamon essential oil.

[0146] Application Example 1

[0147] Beef freshness preservation application experiment

[0148] Experimental method: Select fresh beef as the object of freshness preservation, wrap the beef with a film, seal it with a sealant, and irradiate the sample with 808 nm near-infrared light for 5 minutes after sealing. At the same time, un-packaged beef and beef packaged with commercial PE fresh-keeping film were used as controls. All samples were uniformly placed in an indoor environment at 25 ± 1 °C for 5 days, and three groups of samples were taken on the 2nd day and the 5th day respectively for relevant index tests.

[0149] In addition to providing the above method for preparing the antibacterial film, the present invention also provides an example of applying the antibacterial film of the present invention to food packaging. Specifically, in this application example, it is manifested as the fresh-keeping packaging application of beef. Specifically, the films prepared in Comparative Examples 1-3, the commercial PE fresh-keeping film in Comparative Example 4, and the film prepared in Example 3 are used to encapsulate beef, and a blank control group is set at the same time. The samples in Comparative Example 1 and Example 3 are irradiated with 808 nm near-infrared light for 5 minutes, and the other groups are not irradiated. Finally, the samples of each group are uniformly placed in an indoor environment at 25±1°C for 5 days, and samples are taken and homogenized on the 2nd and 5th days. After that, the homogenate is diluted and spread on a culture dish for cultivation; the colony maps of the samples of each group are as Figure 10 、 11 shown. Combining with the total colony count map in Figure 12 , it can be concluded that after the beef is placed for 2 days, the total colony counts of Comparative Examples 1-3 and Example 3 are nearly half less than those of the blank group and Comparative Example 4, and the total colony count of Comparative Example 4 is more than that of the blank group; comparing the total colony counts of Comparative Examples 1-3 and Example 3, the beef samples with synergistic antibacterial effects of photothermal and essential oil release have fewer total colony counts compared with the pure PVA film, single photothermal and single essential oil antibacterial.

[0150] By the 5th day, the total colony counts of the blank group and Comparative Example 4 increased sharply. The total number of bacteria in the blank group increased by about 1 time, and the total number of bacteria in the PE fresh-keeping film group increased by about 3 times. In contrast, the total colony counts of Comparative Examples 1-3 and Example 3 on the 5th day increased less compared with the total colony counts on the 2nd day. Among them, the total colony count of Comparative Example 3 is the largest, followed by Comparative Example 2, Comparative Example 1, and Example 3 respectively.

[0151] Based on the above total colony count data, first comparing the total colony counts on the 2nd day, it can be seen that the total colony counts of Comparative Examples 1-3 and Example 3 are less than those of the blank control group. This shows that the wrapping of the film blocks the invasion of external bacteria to a certain extent. The reason why the total colony count of the commercial PE fresh-keeping film is more than that of the blank control group may be that the commercial PE fresh-keeping film has poor water permeability, and the water exuded by the beef during storage cannot penetrate the fresh-keeping film, resulting in continuous accumulation of water in the environment sealed by the fresh-keeping film, which exacerbates the speed of bacterial growth; after adding antibacterial agents in Comparative Examples 1-2 and Example 3, the total colony counts are less than those in Comparative Example 3, and after the samples in Example 3 and Comparative Example 1 are irradiated, the total colony counts are less than those in Comparative Example 2 with only the inclusion of essential oil. From this, it can be inferred that the effect of photothermal sterilization is better than that of essential oil slow-release sterilization. Further, the total colony count of Example 3 is significantly less than that of Comparative Example 1, which proves that the synergistic bactericidal effect of photothermal and essential oil release is better than that of single photothermal sterilization;

[0152] Secondly, compare the total number of colonies on the 5th day. The growth rates of the total number of colonies in Control Example 2 and Example 3 are lower than that in Control Example 1. From this, it can be inferred that during storage, the film can continue to slowly release essential oils, effectively inhibiting the growth of bacteria in the samples. Therefore, the increase in the total number of colonies on the 5th day is less compared to the 2nd day, proving that the slowly released essential oils have a continuous antibacterial effect.

[0153] In addition to the total number of colonies, total volatile basic nitrogen (TVB-N) is also commonly used to reflect the spoilage of food, as Figure 13 shown. Compared with the 0th day, after each group of samples was placed for 2 days, TVB-N increased. Among them, the increase in TVB-N in Control Example 4 and the blank group was relatively large, and the TVB-N values of the PE plastic wrap group and the blank group had exceeded the maximum limit of 15 mg / 100 g specified in GB 2707-2016 "National Food Safety Standard Fresh (Frozen) Livestock and Poultry Products"; the average TVB-N values of Control Examples 1-3 were equal (14.78 mg / 100 g) and very close to the maximum limit of 15 mg / 100 g, while the increase in Example 3 was the smallest and the average TVB-N was lower than 15 mg / 100 g;

[0154] By the 5th day, the TVB-N of the blank group and the samples in Control Example 4 increased significantly, and the increase in the PE plastic wrap group was even more rapid, indicating that the degree of spoilage of the samples in this group was very serious. Compared with the blank group and the PE plastic wrap group, the increase in TVB-N in the other sample groups was not large, but except that the average TVB-N value of Example 3 was still below 15 mg / 100 g, the average TVB-N values of the other groups of samples had exceeded 15 mg / 100 g, exceeding the maximum limit specified in GB 2707-2016, indicating that the beef samples had undergone unacceptable spoilage.

[0155] The described embodiments are only descriptions of the preferred embodiments of the present invention and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. Application of a photothermal and thermoresponsive antibacterial film in food packaging, characterized in that, The specific operation of the application is as follows: using the photothermal and thermoresponsive antibacterial film to seal and package food, and after the sealing and packaging is completed, irradiating the photothermal and thermoresponsive antibacterial film with 808 nm near-infrared light for 5 minutes; The raw materials of the photothermal and thermoresponsive antibacterial film include a solvent, and the following components in the mass percentage of the solvent: 1-10% of a film-forming substrate, 0.5-3% of a natural plant essential oil inclusion complex, 0.001-4% of a photothermal conversion substance, and 0-0.6% of glycerol; The natural plant essential oil inclusion complex is composed of natural plant essential oil and cyclodextrin; The photothermal conversion substance is one or more of gold nanoparticles, silver nanoparticles, manganese oxide nanoparticles, and lignin nanoparticles; The surface of the photothermal conversion substance is coated with a silica shell layer.

2. The application according to claim 1, characterized in that The film-forming substrate is one or more of polyvinyl alcohol, chitosan, carboxymethyl cellulose, polylactic acid, and pectin.

3. The application according to claim 1, wherein The natural plant essential oil is one or more of cinnamon essential oil, thymol essential oil, oregano essential oil, eugenol essential oil, neroli essential oil, peppermint essential oil, and tea tree essential oil.

4. The application according to claim 1, wherein The solvent is one or more of water, acetic acid, and dichloromethane.

5. The application according to claim 1, wherein The preparation method of the photothermal and thermoresponsive antibacterial film includes the following steps: Weigh each raw material according to the mass percentage composition, mix the other raw materials except the natural plant essential oil inclusion complex to obtain a composite solution; add the natural plant essential oil inclusion complex to the composite solution to obtain a film-forming solution; pour the film-forming solution into a vessel for film formation and dry it to obtain the photothermal and thermoresponsive antibacterial film.

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