Preparation method and application of photodynamic antibacterial film based on nmTiO2 (at) ZIF-L / natural polysaccharide

Abstract

The invention discloses a photodynamic antibacterial film based on nmTiO2 (at) ZIF-L / natural polysaccharide as well as a preparation method and application of the photodynamic antibacterial film. According to the film, zinc nitrate, dimethylimidazole and nano titanium dioxide are used as raw materials, a composite antibacterial material of ZIF-L in-situ coated titanium dioxide is successfully prepared through normal-temperature stirring with a one-pot method, and the composite antibacterial material is compounded with natural polysaccharide to prepare the preservative film. The film shows remarkable photodynamic antibacterial activity under irradiation of visible light, has certain performance on common pathogenic bacteria such as escherichia coli and staphylococcus aureus, has excellent DPPH free radical scavenging capacity and ideal gas selective permeability, and can maintain an optimal low-oxygen and carbon-suitable air conditioning environment in a package. Experiments prove that the film has a remarkable effect in banana preservation application, the preservation effect under an illumination condition is obviously superior to that under a dark condition, browning can be effectively delayed, softening is inhibited, and the shelf life is prolonged. The fresh-keeping agent has the advantages of easily available raw materials, simple process, greenness, safety and the like, and provides an innovative solution for the fresh-keeping of the picked fruits.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of photodynamic antibacterial and food preservation technology, and particularly relates to a visible light responsive nmTiO2@ZIF-L / natural polysaccharide photodynamic antibacterial film, a preparation method thereof and application thereof in banana preservation. BACKGROUND

[0002] The decay of fruits after harvest due to microbial infection and physiological metabolism is the main reason for huge economic losses. At present, chemical sterilizing agents (such as hypochlorous acid) are mainly used for preservation, but there are risks of chemical residues and induction of microbial drug resistance. In addition, traditional physical packaging films (such as polyethylene films) can only provide physical isolation and do not have active antibacterial function, so the preservation effect is limited.

[0003] Photodynamic antibacterial technology as a new type of non-thermal sterilization technology has attracted widespread attention because it is less likely to cause drug resistance. However, in the specific practice of applying it to fruit preservation films, some existing schemes still face some common technical bottlenecks: (1) many efficient photosensitizers need to be activated by ultraviolet light, which may cause damage to the nutritional ingredients of fruits, and its use has safety hazards, limiting its wide application in the food field; (2) the dispersibility of photosensitizers in the film matrix is poor and easy to agglomerate, which not only leads to a decrease in their optical activity and poor active oxygen yield, but also affects the transparency and uniformity of the film; (3) the mechanical strength and water resistance of the film itself are difficult to balance with the stability required for fruit preservation, such as swelling or peeling in high temperature and high humidity environments, resulting in ineffective antibacterial function and short-term preservation effect.

[0004] Therefore, it is urgent to develop a preservation film that is safe, efficient, stable and can be activated by visible light. SUMMARY

[0005] In view of the problem that the preservation effect of traditional fruit preservation films is not long-lasting and the single antibacterial effect of the material application is limited, the application aims to provide a nmTiO2@ZIF-L / natural polysaccharide photodynamic antibacterial film and a preparation method thereof. ZIF-L is synthesized by stirring at room temperature, and nmTiO2@ZIF-L / natural polysaccharide film is prepared by ultrasonic stirring, which solves the problem of poor antibacterial effect caused by passive physical barrier and is easy to recycle and reuse, and has good cycle stability.

[0006] To achieve the above purpose, the technical scheme adopted by the application is as follows:

[0007] In a first aspect, the application provides a nmTiO2@ZIF-L / natural polysaccharide photodynamic antibacterial film.

[0008] Based on the characteristics of nmTiO2 with strong stability and broad-spectrum high efficiency, the photodynamic antibacterial nmTiO2@ZIF-L / natural polysaccharide film has the functions of continuous sterilization and not easy to induce drug resistance of microorganisms, and solves the problem of poor antibacterial effect caused by the passive physical barrier of the traditional fruit preservative film.

[0009] In addition, the natural polysaccharide has good film-forming property, strong adhesion and good airtightness as a film-forming matrix. It has good compatibility with the MOF particles, can form a dense, uniform and high-mechanical-strength composite film, and effectively inhibits the respiration and water evaporation of fruits.

[0010] In a second aspect, the application provides a method for preparing the above-mentioned photodynamic antibacterial nmTiO2@ZIF-L / natural polysaccharide film, comprising the following steps:

[0011] S1: uniformly mix dimethyl imidazole with deionized water, then add nmTiO2, and then add prepared zinc nitrate solution to the reaction system, react at room temperature for 4h, and then obtain nmTiO2@ZIF-L after centrifugal washing;

[0012] S2: uniformly mix carboxymethyl cellulose with deionized water, then add appropriate amount of nmTiO2@ZIF-L, and then obtain nmTiO2@ZIF-L / natural polysaccharide film after stirring at room temperature for 5h.

[0013] Preferably, the concentration of nmTiO2 in the reaction system of step S1 is 2.5-5g / L.

[0014] When the concentration is lower than 2.5g / L, the functional units for photodynamic antibacterial effect in the system are too few. Although the nanoparticles can be highly dispersed, the active oxygen (ROS) generated in unit area or unit volume is insufficient, resulting in that the core antibacterial efficiency of the film cannot reach the expected threshold, and the preservation effect is limited. When the concentration is higher than 5g / L, the high nanoparticle content will increase the viscosity of the system, and aggravate the agglomeration between nanoparticles, resulting in resource waste and performance loss.

[0015] Preferably, the mass fraction of nmTiO2@ZIF-L in the nmTiO2@ZIF-L / natural polysaccharide film in step S2 is 20-40%.

[0016] The content of nmTiO2@ZIF-L in the reaction system plays a crucial role in enhancing the mechanical properties of the preservative film. At this preferred content, nmTiO2@ZIF-L composite particles act as highly efficient nano-enhancing fillers, uniformly dispersing in the polysaccharide film-forming matrix and forming dense interaction points with the polymer chains. This not only significantly improves the tensile strength, hardness, and wear resistance of the film, making it sufficient to withstand physical impact during post-harvest processing, packaging, and transportation of fruits, but more importantly, this content precisely balances the rigid enhancement and toughness retention. Too little filler will not have enough enhancement effect, while too much filler will easily lead to stress concentration and film brittleness; the content of the present application enables the film to have excellent strength and appropriate flexibility, which can elastically conform to the micro-deformation of the fruit surface, effectively preventing cracks and peeling caused by bending or shrinking, thereby providing a strong and durable "edible protective armor" for the fruit.

[0017] In terms of gas permeability, the selected content of nmTiO2@ZIF-L in the present application gives the preservative film a unique and intelligent gas regulation function. The uniformly dispersed ZIF-L component, with its regular microporous structure, creates a large number of gas adsorption sites in the film and exhibits differential transmission behavior for different gas molecules. At the same time, the rigid nmTiO2@ZIF-L particles create a highly tortuous diffusion path in the polysaccharide matrix, greatly hindering the rapid passage of oxygen molecules and effectively slowing down the respiration rate and oxidative deterioration of fruits. More importantly, the ZIF-L at this content can moderately adsorb and transport the carbon dioxide released by fruit respiration, cleverly avoiding the physiological damage caused by excessive accumulation of carbon dioxide in the film. Ultimately, the synergistic effect of this content actively creates and maintains an optimal micro-atmosphere environment with low oxygen and moderate carbon dioxide concentration around the fruit, significantly delaying aging from a physiological perspective and achieving dynamic and intelligent preservation effect.

[0018] In the third aspect, the functional material nmTiO2@ZIF-L exhibits certain antioxidant activity, which proves its dynamic and sustained free radical scavenging ability.

[0019] The functionalized material nmTiO2@ZIF-L prepared by the application is mixed with DPPH methanol solution, and then irradiated under light for 30 minutes, and then the absorbance is measured at a wavelength of 517 nm. The results show that the DPPH clearance rate of the material under light is significantly higher than that in the dark. The excellent clearance capacity is mainly due to the fact that the nano-titanium dioxide is excited under light, and the high-activity photo-generated holes and electrons can directly and efficiently reduce the DPPH free radical molecules. This property is crucial in actual preservation applications, meaning that when the film is in a light environment, not only can it kill microorganisms by producing reactive oxygen species (ROS), but it can also simultaneously exert a strong antioxidant effect, actively removing free radicals that accelerate fruit spoilage, thereby inhibiting browning, softening and nutrient loss, and providing a key basis for achieving efficient and dynamic photodynamic preservation.

[0020] In a fourth aspect, the application provides the use of the above-mentioned nmTiO2@ZIF-L / natural polysaccharide film in antibiosis. The nmTiO2@ZIF-L / natural polysaccharide film has inhibitory effects on Escherichia coli, Staphylococcus aureus and Aspergillus niger.

[0021] Experiments have found that the nmTiO2@ZIF-L / natural polysaccharide film prepared by the application has certain antibacterial properties for Escherichia coli, Staphylococcus aureus and Aspergillus niger, that is, its antibacterial properties are significantly enhanced under light conditions compared to dark conditions.

[0022] In a fifth aspect, the photodynamic antibacterial film provided by the application exhibits excellent and intelligent gas selective permeation performance.

[0023] The photodynamic antibacterial film can actively create an optimal preservation microenvironment for fruits. This is mainly due to the synergistic effect between the "labyrinthine" barrier structure constructed by the nmTiO2@ZIF-L composite particles and the polysaccharide matrix and the inherent gas adsorption selectivity of ZIF-L. On the one hand, the uniform dispersion of rigid composite particles greatly increases the diffusion path of oxygen molecules in the film, significantly reducing the oxygen permeation rate, thereby effectively inhibiting the respiration intensity of fruits and the reproduction of aerobic spoilage bacteria. On the other hand, the ZIF-L component has a certain preferential adsorption capacity for carbon dioxide molecules, which can accurately regulate the concentration of CO2 inside the package, avoiding the physiological toxicity of excessive accumulation of CO2 to fruits. Experimental data show that compared with fruits without the film, the fruits treated with the film can maintain a stable low-oxygen and moderate carbon dioxide atmosphere in the microenvironment inside the package during storage. This dynamic balancing ability not only provides physical barriers like traditional preservation films, but also actively and intelligently regulates the physiological metabolism of fruits, thereby delaying aging from the root cause and providing a key guarantee for long-term preservation.

[0024] In the sixth aspect, the photodynamic antibacterial film is applied to banana preservation, and remarkable technical effects are achieved.

[0025] Experiments prove that the preservation quality of bananas treated by the film and placed in visible light conditions is significantly better than that of the treatment group in dark conditions and all control groups. Specifically, in the equivalent storage period, the film treatment under light can effectively delay the browning process of banana peel, the color retention degree is improved compared with the dark treatment group, and the softening of the pulp is inhibited. More importantly, the film exhibits a light-dependent synergistic preservation mechanism - under light, the photosensitive component in the film is activated, not only enhancing the antibacterial activity against banana pathogenic bacteria, but also simultaneously removing the ethylene gas of ripening bananas, thereby double blocking the key path leading to banana corruption. Finally, the film can extend the shelf life of bananas to more than 7 days under light conditions.

[0026] Compared with the prior art, the application has the following beneficial effects:

[0027] The application has made a breakthrough: first, by constructing a composite structure of ZIF-L in situ wrapping nano-titanium dioxide, the technical bottleneck of easy aggregation and poor stability of traditional photosensitizers is fundamentally solved, and the excitation light source is successfully transferred from harmful ultraviolet light to safe visible light, realizing the functional leap from traditional preservation film "passive barrier" to "active photodynamic sterilization"; secondly, the application creates a synergistic mechanism of photodynamic sterilization, intrinsic antioxidant and intelligent gas regulation functions, which promote each other, and the inhibition rate of common pathogenic bacteria such as Escherichia coli is improved under light, while maintaining the best gas regulation environment of low oxygen / suitable carbon, and each function produces a synergistic effect of "1+1>2"; in addition, the one-pot synthesis process at room temperature adopted by the application significantly reduces the production cost while maintaining excellent performance. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 Figure 7 is a 7-day banana preservation experiment diagram of (i) CK group, (ii) carboxymethyl cellulose treatment group, (iii) ZIF-L / carboxymethyl cellulose treatment group, (iv) nmTiO2@ZIF-L / carboxymethyl cellulose light treatment group, and (v) nmTiO2@ZIF-L / carboxymethyl cellulose light avoidance treatment group;

[0029] Figure 2 Figure 6 is an electron microscope diagram of ZIF-L, nmTiO2, carboxymethyl cellulose, ZIF-L / carboxymethyl cellulose and nmTiO2@ZIF-L / carboxymethyl cellulose;

[0030] Figure 3Antibacterial circle diagram of nmTiO2, ZIF-L, carboxymethyl cellulose, ZIF-L / carboxymethyl cellulose and nmTiO2@ZIF-L / carboxymethyl cellulose under light and dark

[0031] Figure 4 Antibacterial circle diameter comparison diagram of nmTiO2@ZIF-L / carboxymethyl cellulose under light and dark DETAILED DESCRIPTION

[0032] For the purpose, technical scheme and advantages of the present application, the present application will be further described below in combination with specific examples. Those skilled in the art should understand that the specific examples described herein are only used to explain the present application and do not limit the present application. The test methods used in the examples are conventional methods unless otherwise specified. The materials, reagents, etc. used in the examples can be obtained from commercial channels unless otherwise specified.

[0033] The metal salt and organic ligand used in the embodiments of the present application are purchased from commercial reagent companies such as Shanghai Maikelin Biochemical Co., Ltd., National Reagent Corporation and Shanghai Bide Pharmaceutical Co., Ltd.

[0034] Example 1

[0035] This embodiment is to prepare functional material nmTiO2@ZIF-L, and the preparation method comprises the following steps:

[0036] 0.59 g of zinc nitrate hexahydrate was added to 40 mL of deionized water to prepare a 14.75 g / L aqueous solution to obtain solution A. 0.3 g of nanometer titanium dioxide was uniformly dispersed in solution A under magnetic stirring to obtain dispersion liquid B. 1.3 g of dimethyl imidazole was added to 40 mL of deionized water to prepare a 32.5 g / L aqueous solution to obtain solution C. Under constant temperature conditions at 25°C, solution C was first mixed with dispersion liquid B and stirred for 4 h. After the reaction was completed, centrifugation, washing and drying treatment were performed, and finally the nmTiO2@ZIF-L composite material was obtained.

[0037] Example 2

[0038] This embodiment is to prepare nmTiO2@ZIF-L / carboxymethyl cellulose film, and the preparation method comprises the following steps:

[0039] 2 g of carboxymethyl cellulose was prepared into a 20 g / L aqueous solution with 100 mL of deionized water, and the stirring speed was 800 rpm at room temperature for 1 h to obtain a uniform transparent film-forming matrix solution. Then 0.857 g of nmTiO2@ZIF-L composite material powder was added to the solution, and the nmTiO2@ZIF-L in the reaction system was 8.57 g / L. Under the same stirring conditions, the reaction was continued for 4 h to ensure that the composite material was fully dispersed, and finally the photodynamic antibacterial preservative film solution was prepared.

[0040] Example 3

[0041] This example is a banana preservation application experiment:

[0042] Select fresh bananas with consistent maturity and no mechanical damage, and randomly divide them into five groups:

[0043] Group I: No treatment, stored under visible light.

[0044] Group II: Treated with carboxymethyl cellulose, stored under visible light.

[0045] Group III: Treated with ZIF-L / carboxymethyl cellulose, stored under visible light.

[0046] Group IV: Treated with Example 2, stored under visible light.

[0047] Group V: Treated with Example 2, stored in the dark.

[0048] All experimental groups were stored in an environment with a temperature of 25°C and a relative humidity of 85%. The color, hardness, and weight changes of the bananas were observed and recorded regularly.

[0049] Performance test

[0050] 1. Morphology test

[0051] Scanning electron microscopy was used to characterize the morphology of the key materials involved in the invention, which clearly showed the microstructure of the materials and their evolution process:

[0052] The morphology of pure ZIF-L: as shown in Figure 2 a, the ZIF-L synthesized alone presents a typical two-dimensional leaf-like nanosheet structure, with smooth surface and clear edges, uniform size, confirming the successful synthesis of ZIF-L.

[0053] The morphology of pure nmTiO2: as shown in Figure 2 b, it is a regular spherical particle with a particle size distribution in the range of 20-30 nm and uniform dispersion.

[0054] The cross-sectional morphology of pure carboxymethyl cellulose film: as shown in Figure 2 c, the pure carboxymethyl cellulose film shows a continuous, dense and smooth amorphous film structure after film formation, which is a typical feature of polysaccharide film-forming matrix.

[0055] The cross-sectional morphology of ZIF-L / carboxymethyl cellulose film: as shown in Figure 2As shown in d, after simple physical blending of ZIF-L and CMC, ZIF-L nanosheets were observed to be randomly embedded in the carboxymethyl cellulose matrix. However, some regions showed ZIF-L aggregation, and there was a significant gap between the sheets and the matrix interface, indicating that the compatibility and binding force between the two need to be improved.

[0056] Morphology of nmTiO2@ZIF-L / carboxymethyl cellulose membrane: as follows Figure 2 As shown in Figure e, this is a visual representation of the core innovative structure of this invention. It can be observed that ZIF-L retains its complete two-dimensional sheet-like morphology, but smaller nano-titanium dioxide particles appear on the surface, confirming that nano-titanium dioxide has been successfully loaded onto the surface or interior of ZIF-L. The composite material and the polysaccharide matrix are tightly bonded, with no obvious phase separation, forming a dense network structure interwoven with "organic-inorganic" elements.

[0057] The morphology characterization results fully demonstrate that the one-pot in-situ synthesis strategy adopted in this invention successfully prepared a well-structured nmTiO2@ZIF-L composite material, and solved the key technical problems of poor dispersibility and weak interfacial bonding of nanomaterials in the film-forming matrix, laying a solid structural foundation for realizing the synergistic effect of multifunctional membranes.

[0058] 2. Antibacterial test

[0059] The antimicrobial properties of five materials against three common pathogens under light and dark conditions were systematically evaluated using the inhibition zone method.

[0060] Escherichia coli, Staphylococcus aureus, and Aspergillus niger were used as model organisms.

[0061] First, a 70 μL suspension of *Escherichia coli*, *Staphylococcus aureus*, and *Aspergillus niger* was spread on the surface of the culture medium. Then, the five samples, which had been sterilized under ultraviolet light, were cut into circular slices with a diameter of 7 mm and attached to the culture medium containing the bacterial suspension. Finally, the samples were incubated at 37°C for 1 day and photographed.

[0062] like Figure 3 As shown, the test results for (a) Escherichia coli, (b) Staphylococcus aureus, and (c) Aspergillus niger indicate that under the same concentration conditions, the three groups of samples (ii) ZIF-L, (iv) ZIF-L / carboxymethyl cellulose, and (v) nmTiO2@ZIF-L / carboxymethyl cellulose exhibited the Zn content of ZIF-L. 2+ The antibacterial properties of the samples containing nano-titanium dioxide (i) nmTiO2 and (v) nmTiO2@ZIF-L / carboxymethyl cellulose showed better antibacterial activity under light conditions than under dark conditions. However, (iii) carboxymethyl cellulose showed no antibacterial activity, a phenomenon verified in all three tested strains.

[0063] More importantly, Figure 4 By directly comparing the performance of nmTiO2@ZIF-L / carboxymethyl cellulose under light conditions (vi) and light-protected conditions (vii), a significant increase in the diameter of the inhibition zone under light conditions can be clearly observed. This result fully demonstrates the antibacterial enhancement effect of nano-titanium dioxide under light conditions through photocatalysis, providing direct evidence for the photodynamic antibacterial mechanism of the material.

[0064] The results of this study indicate that nmTiO2@ZIF-L / carboxymethyl cellulose membranes exhibit excellent antibacterial properties under light irradiation, and their application prospects in the field of food preservation are promising.

[0065] 3. Banana preservation experiment

[0066] A 7-day preservation experiment was conducted to systematically evaluate the effects of different material treatments on banana preservation. For example... Figure 1 As shown, the experiment was set up with the following five groups: (i) CK group, (ii) carboxymethyl cellulose treatment group, (iii) ZIF-L / carboxymethyl cellulose treatment group, (iv) nmTiO2@ZIF-L / carboxymethyl cellulose light treatment group, and (v) nmTiO2@ZIF-L / carboxymethyl cellulose light-protected treatment group. The changes in banana quality were observed and recorded on days 1, 3, 5, and 7, respectively.

[0067] The experimental results showed that the quality of bananas in each group differed significantly with prolonged storage time. By day 7, (i) bananas in group CK had completely browned, with blackened peels and significantly softened texture, losing their commercial value; (ii) the browning of bananas in the carboxymethyl cellulose treatment group was slightly delayed, but large areas of brown spots still appeared; (iv) the nmTiO2@ZIF-L / carboxymethyl cellulose light-treated group performed best, with the lightest browning, the peel still maintaining a bright yellow color, and the flesh maintaining good firmness; (v) the preservation effect of the nmTiO2@ZIF-L / carboxymethyl cellulose light-protected group was second only to group (iv).

[0068] In the preservation experiment on day 7, the (iv) nmTiO2@ZIF-L / carboxymethyl cellulose light-exposed group showed the best preservation effect, with the slowest decrease in firmness and the lowest weight loss rate, significantly better than the (i) CK group. This fully demonstrates the key role of light exposure in activating the photodynamic antibacterial film function of nmTiO2@ZIF-L / carboxymethyl cellulose, effectively delaying the softening process of bananas and reducing moisture loss.

[0069] The results of this study clearly show that nmTiO2@ZIF-L / carboxymethyl cellulose membrane can effectively delay the ripening process of bananas, inhibit browning, and maintain fruit quality under light conditions, demonstrating good application prospects in the field of postharvest preservation of fruits and vegetables.

Claims

1. A nmTiO2@ZIF-L / natural polysaccharide photodynamic antibacterial film, characterized in that, It is composed of the following components: nmTiO2, ZIF-L and natural polysaccharides.

2. The photodynamic antibacterial film according to claim 1, characterized in that, The selected natural polysaccharide is at least one of chitosan, sodium alginate, quaternary ammonium chitosan, pectin and carboxymethyl cellulose.

3. The photodynamic antibacterial film according to claim 1, characterized in that, The nano-titanium dioxide is anatase crystal type, hydrophilic spherical particles, and its particle size distribution is in the range of 20-30nm.

4. The photodynamic antibacterial film according to claim 1, characterized in that, The mass ratio of nmTiO2 to ZIF-L is 1:

2.

5. A method for preparing the photodynamic antibacterial film as described in any one of claims 1-4, characterized in that, Includes the following steps: S1: Add dimethylimidazole to deionized water, then add... nm After adding TiO2, the prepared zinc nitrate solution was added and the mixture was stirred at a certain temperature for 3-5 hours. After centrifugation and washing, nmTiO2@ZIF-L was obtained. S2: After mixing the natural polysaccharide with deionized water until homogeneous, nmTiO2@ZIF-L is added and stirred at a certain temperature to obtain nmTiO2@ZIF-L / natural polysaccharide membrane.

6. The method according to claim 5, characterized in that, The concentration of nmTiO2 in S1 is 2.5–5 g / L.

7. The method according to claim 5, characterized in that, The reaction temperature in SI is 20–40°C, and magnetic stirring is used at a speed of 400 rpm.

8. The method according to claim 5, characterized in that, The reaction temperature in S2 is 20–40°C, and the stirring is performed by electric stirring at a speed of 800 rpm.

9. The method according to claim 5, characterized in that, The volume ratio of the dimethylimidazole solution and the zinc nitrate solution in S1 is 1:1; the concentration of the dimethylimidazole is 25-40 g / L; the concentration of the zinc nitrate is 10-20 g / L; the concentration of the natural polysaccharide in S2 is 15-25 g / L; and the concentration of the nmTiO2@ZIF-L in S2 is 5-15 g / L.

10. The application of a photodynamic antibacterial film as described in any one of claims 1-4 in fruit preservation, characterized in that, The nmTiO2@ZIF-L / natural polysaccharide has a time-delaying effect on fruit preservation.

11. The application according to claim 10, characterized in that, The fruit in question is a banana, which has a shelf life of 7 days or more.

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