Copper-based mof supported nanotitania / polycaprolactone composite film

Abstract

The application discloses a copper-based MOF loaded nanometer titanium dioxide / polycaprolactone composite film and relates to the technical field of food preservation packaging materials. The preparation method of the composite film adopts a one-pot hydrothermal method to prepare copper-based metal organic framework loaded nanometer titanium dioxide; polycaprolactone is dissolved in a volatile organic solvent, the copper-based metal organic framework loaded nanometer titanium dioxide is added, and stirring is uniformly carried out to obtain a film forming solution; flow casting is carried out to form a film, and the solvent is volatilized, so that the composite film is obtained. The composite film prepared by the application can effectively inhibit nanometer titanium dioxide from agglomerating, can maintain good mechanical properties of polycaprolactone, and has excellent fruit preservation effect.

Description

Technical Field

[0001] This invention relates to the field of food preservation packaging materials. More specifically, this invention relates to a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film. Background Technology

[0002] Fruits remain living tissues after harvest, continuously undergoing respiration, transpiration, and related physiological and biochemical metabolism during storage, transportation, and sales. This makes them susceptible to problems such as water loss, softening, browning, flavor degradation, nutrient loss, and spoilage. For climacteric fruits and vegetables such as bananas, apples, pears, mangoes, and tomatoes, the ripening process is typically accompanied by a rapid increase in respiration intensity and ethylene release, accelerating aging and spoilage, resulting in shorter shelf life and greater distribution losses. Therefore, developing multifunctional preservation packaging materials that can effectively delay ripening and inhibit microbial infection is of great significance.

[0003] Current methods for preserving fruits and vegetables mainly include low-temperature storage, controlled atmosphere storage, chemical preservative treatment, edible coatings, and functional packaging. Among these, packaging materials can achieve preservation by regulating the microenvironment surrounding the fruit, reducing moisture loss, blocking external contamination, and inhibiting microbial growth. For climacteric fruits, ideal packaging materials should not only possess good mechanical and barrier properties but also the ability to regulate the composition of the microenvironment's gases and provide long-lasting antibacterial effects to meet their comprehensive preservation needs during storage and transportation.

[0004] Polycaprolactone (PCL) is a biodegradable polyester material with good flexibility, processability, and biocompatibility, showing promising application prospects in the field of biodegradable packaging. Using PCL as a substrate for food preservation films can help reduce the environmental impact of traditional plastic packaging. However, PCL films themselves still have problems such as low mechanical strength and insufficient gas barrier properties. When used alone for fruit preservation, their overall performance is still insufficient to meet practical requirements. Therefore, it is usually necessary to improve their performance through inorganic functional particle composite modification.

[0005] Nano-titanium dioxide (TiO2) possesses high chemical stability, safety, and photocatalytic activity. Under light irradiation, it can generate reactive oxygen species, thereby achieving a certain antibacterial effect. It also exhibits some degradation ability for small molecule gases such as ethylene, showing great application potential in food packaging and preservation materials. However, nano-TiO2 is prone to agglomeration in polymer matrices, exhibiting poor dispersibility and easily affecting the structural uniformity and performance of the film. Furthermore, its effectiveness is highly dependent on ambient light; its activity drops sharply under weak light conditions, and the duration and stability of its antibacterial efficacy still need improvement.

[0006] Metal-organic frameworks (MOFs) are a class of porous crystalline materials formed by the coordination of metal ions or metal clusters with organic ligands. They possess characteristics such as large specific surface area, tunable pore structure, and strong loading capacity. Copper-based metal-organic frameworks (Cu-MOFs), in addition to their antibacterial potential, can also serve as dispersion carriers for functional particles, improving the loading efficiency and dispersion effect of active components, and achieving sustained antibacterial effects through the slow release of copper ions. Therefore, combining TiO2 with Cu-MOFs holds promise for improving the dispersion state of TiO2 through the porous structure and interfacial interactions of Cu-MOFs, thereby enhancing the antibacterial and gas regulation capabilities of the composite system.

[0007] However, existing technologies still have the following shortcomings: First, most preservation film systems have relatively simple functions and it is difficult to simultaneously take into account mechanical properties; second, the aggregation problem of nano-TiO2 in polymer matrices has not been effectively solved; and third, research and technical solutions for in-situ composite and synergistic introduction of TiO2 and Cu-MOFs into preservation film systems using biodegradable PCL as the matrix and applied to the preservation of respiration-climacteric fruits are still relatively insufficient.

[0008] Therefore, the technical problem to be solved by the present invention is to provide a biodegradable composite film and its preparation method that can simultaneously solve the aggregation of nano-TiO2, improve the antibacterial efficacy under low light conditions, and have good mechanical properties and barrier properties, so as to meet the comprehensive preservation needs of respiratory climacteric fruits during storage and transportation. Summary of the Invention

[0009] The first aspect of this invention provides a method for preparing copper-based MOF-supported nano-titanium dioxide, which adopts a one-pot hydrothermal method to simultaneously complete Cu-MOF synthesis and TiO2 loading, simplifying the process, avoiding TiO2 agglomeration, and improving the stability of the filler.

[0010] The second aspect of this invention provides a method for preparing a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film. The process is simple, the conditions are controllable, no complex equipment is required, and it can be prepared on a large scale. The filler has good compatibility with the PCL matrix, and the film is uniform.

[0011] The third aspect of the present invention provides a composite film prepared by the above preparation method, which uses biodegradable PCL as a matrix, is environmentally friendly, and has multiple functions such as mechanical reinforcement, barrier improvement, antibacterial and preservation.

[0012] The fourth aspect of this invention provides the application of the above-mentioned composite film in the preservation of climacteric fruits. Through the synergistic effects of antibacterial activity, delayed ripening and senescence, and water retention and barrier, the film delays fruit senescence and spoilage, extends shelf life, and reduces storage and transportation losses.

[0013] To achieve these objectives and other advantages according to the present invention, a method for preparing a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film is provided, comprising the following steps: A one-pot hydrothermal method was used to prepare copper-based metal-organic framework-supported nano-titanium dioxide. Polycaprolactone was dissolved in a volatile organic solvent at a mass-to-volume ratio of 1:5 to 1:10. A copper-based metal-organic framework supported on nano-titanium dioxide was added at a mass ratio of 0.5 wt% to 5 wt% of the polycaprolactone. The mixture was stirred until homogeneous to obtain a film-forming solution. The film-forming solution was cast into a film, and the volatile organic solvent was evaporated to remove it, resulting in a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film.

[0014] Preferably, the one-pot hydrothermal method includes: mixing copper chloride dihydrate and gallic acid in a molar ratio of 1:1 to 3:1, adding nano-titanium dioxide, wherein the amount of nano-titanium dioxide added is 0.1wt% to 7.0wt% of the total mass of copper chloride dihydrate and gallic acid, to obtain a precursor mixture, and hydrothermally reacting the precursor mixture at 120°C to 180°C for 12h to 36h, followed by cooling, to obtain a copper-based metal-organic framework supported on nano-titanium dioxide.

[0015] Preferably, the amount of nano-titanium dioxide added is 3.6 wt% of the total mass of copper chloride dihydrate and gallic acid.

[0016] Preferably, the weight-average molecular weight of polycaprolactone is between 50,000 g / mol and 100,000 g / mol.

[0017] Preferably, the volatile organic solvent is at least one of dichloromethane, chloroform, or tetrahydrofuran.

[0018] A copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film was prepared according to the preparation method described above.

[0019] The application of the copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film in the preservation of respiratory climacteric fruits.

[0020] Preferably, the climacteric fruit is at least one of banana, apple, pear, mango, or tomato.

[0021] The present invention has at least the following beneficial effects: First, this invention simultaneously synthesizes copper-based metal-organic frameworks and loads nano-titanium dioxide using a one-pot hydrothermal method. When the amount of nano-titanium dioxide added is 3.6 wt% of the total mass of copper chloride dihydrate and gallic acid, the actual loading rate is the highest and the synergistic antibacterial effect is optimal.

[0022] Secondly, by loading nano-titanium dioxide onto a copper-based metal-organic framework, the composite particles obtained in this invention exhibit an antibacterial rate of 99.74% against Staphylococcus aureus and 99.81% against Escherichia coli, which are significantly higher than the 74.19% and 92.68% of pure copper-based metal-organic frameworks, respectively, thus demonstrating a synergistic antibacterial effect.

[0023] Third, this invention uses a copper-based metal-organic framework loaded with nano-titanium dioxide and polycaprolactone to cast a composite film. The amount of composite particles added in the composite film is adjustable in the range of 0.5wt% to 5wt%, resulting in a uniform film structure and good filler dispersion.

[0024] Fourth, this invention utilizes the antibacterial and preservative functions of nano-titanium dioxide loaded in a copper-based metal-organic framework in a composite membrane, along with the barrier and water-retaining effects of a polycaprolactone matrix, to synergistically delay the aging and spoilage of climacteric fruits such as bananas, apples, pears, mangoes, and tomatoes, thereby reducing post-harvest storage and transportation losses.

[0025] Other advantages, objectives and features of the present invention will become apparent in part from the following description, and in part from those skilled in the art through study and practice of the invention. Attached Figure Description

[0026] Figure 1 Fourier transform infrared spectra of GA and Cu-MOFs; Figure 2 Fourier transform infrared spectra of TiO2@Cu-MOFs with different TiO2 addition amounts; Figure 3 XRD patterns of TiO2, Cu-MOFs and TiO2@Cu-MOFs with different TiO2 addition amounts; Figure 4 Thermogravimetric curves (a) and differential thermogravimetric curves (b) of Cu-MOFs and TiO2@Cu-MOFs with different TiO2 addition amounts; Figure 5 EDS elemental analysis results of TiO2@Cu-MOFs (TCM-x) with different amounts of nano-TiO2 added; Figure 6 SEM images of TiO2@Cu-MOFs (TCM-x) with different amounts of nano-TiO2 added; Figure 7 XPS spectra of Cu-MOFs and TiO2@Cu-MOFs (TCM-3.6) at the optimal addition level; Figure 8 Antibacterial plate images of TiO2, Cu-MOFs, and TiO2@Cu-MOFs with different TiO2 additions against Staphylococcus aureus and Escherichia coli; Figure 9 Statistical results of the antibacterial rates of Cu-MOFs and TiO2@Cu-MOFs with different TiO2 additions against Staphylococcus aureus and Escherichia coli; Figure 10 DSC melting curves (a) and crystallization curves (b) of different composite films are shown. Figure 11 XRD patterns of different composite films; Figure 12 Results for water contact angles of different composite membranes; Figure 13 The tensile properties of different composite films are shown. Figure 14 The images show the actual product (a) and the mass change curve (b) of the mango preserved using TiO2@Cu-MOFs / PCL. Detailed Implementation

[0027] The present invention will now be described in further detail with reference to the accompanying drawings, so that those skilled in the art can implement it based on the description.

[0028] It should be understood that terms such as "having," "comprising," and "including" as used herein do not exclude the presence or addition of one or more other elements or combinations thereof. It should be noted that the experimental methods described in the following embodiments, unless otherwise specified, are conventional methods, and the reagents and materials, unless otherwise specified, are commercially available and therefore should not be construed as limiting the invention.

[0029] A method for preparing copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite films includes the following steps: Step 1: A copper-based metal-organic framework supported on nano-titanium dioxide is prepared using a one-pot hydrothermal method. The copper-based metal-organic framework is a porous crystalline material formed by the coordination of copper ions and organic ligands. In this application, the copper-based metal-organic framework is used as a carrier for the nanofiller. The nano-titanium dioxide is an inorganic nanomaterial with photocatalytic activity. In this application, the nano-titanium dioxide is used as a nanofiller. The framework synthesis and nanoparticle loading are completed simultaneously in the same reaction system using a one-pot hydrothermal method. Preferably, the one-pot hydrothermal method includes: mixing copper chloride dihydrate and gallic acid in a molar ratio of 1:1 to 3:1, where copper chloride dihydrate provides copper ions as the metal source for the copper-based metal-organic framework, and gallic acid is a natural organic ligand that can coordinate with copper ions to form a framework structure; adding nano-titanium dioxide, the amount of nano-titanium dioxide added being 0.1wt% to 7.0wt% of the total mass of copper chloride dihydrate and gallic acid, preferably 3.6wt%, to achieve the highest loading rate of the filler, to obtain a precursor mixture; hydrothermally reacting the precursor mixture at 120℃ to 180℃ for 12h to 36h, followed by cooling, to obtain a copper-based metal-organic framework supported by nano-titanium dioxide; achieving uniform dispersion of nano-titanium dioxide on the copper-based metal-organic framework through one-pot hydrothermal loading, avoiding nanoparticle agglomeration, and preparing a copper-based metal-organic framework supported by nano-titanium dioxide with a complete structure and uniform loading, ensuring the porous structure of the composite filler; Step 2: Dissolve polycaprolactone in a volatile organic solvent at a mass-to-volume ratio of 1:5 to 1:10. Add a nano-titanium dioxide-supported copper-based metal-organic framework (MOF), with the amount of MOF added being 0.5 wt% to 5 wt% of the mass of PVC. Stir until no agglomeration or precipitation occurs to obtain the film-forming solution. PVC is a biodegradable aliphatic polyester with a weight-average molecular weight of 50,000 g / mol to 100,000 g / mol. Within this molecular weight range, PVC is readily soluble in volatile solvents such as dichloromethane and, after film formation, exhibits moderate crystallinity, tensile strength, and flexibility. Preferably, the weight-average molecular weight of PVC is 80,000, ensuring that PVC possesses solubility, film-forming properties, and mechanical properties. The film-forming process... To prevent issues such as stringing and cracking, the volatile organic solvent is at least one of dichloromethane, chloroform, or tetrahydrofuran, ensuring rapid and complete dissolution of polycaprolactone without any residue after evaporation, thus not affecting film performance. To ensure film quality, a dense and defect-free structure, dichloromethane is preferred as the volatile organic solvent. Preferably, the amount of TiO2@Cu-MOFs filler added is 0.65 wt% of the mass of polycaprolactone. At this addition amount, the composite film achieves a good balance between mechanical property retention and antibacterial functionality. Those skilled in the art will understand that, depending on the specific application scenario and the emphasis on mechanical strength and antibacterial rate, the amount of filler added can be appropriately increased or decreased from 0.65 wt%, for example, adjusted within the range of 5 wt% to 10 wt%, and a composite film that balances mechanical integrity and preservation functionality can usually still be obtained. Step 3: Slowly pour the film-forming solution into a flat mold, remove surface air bubbles, place it in a constant temperature environment, cast the film, evaporate and remove the volatile organic solvent. After the solvent is completely removed, peel the film off the mold to obtain a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film.

[0030] The prepared composite film uses biodegradable polycaprolactone as a matrix, combining the antibacterial properties of a copper-based metal-organic framework with the photocatalytic antibacterial properties of nano-titanium dioxide, and is expected to meet the safety and functional requirements of food packaging. Applying the composite film to the preservation of climacteric fruits is anticipated to have a positive effect on delaying fruit senescence and spoilage and reducing postharvest losses through the synergistic effects of antibacterial activity, delayed fruit ripening and senescence, and water barrier properties, and is compatible with the physiological characteristics of bananas, apples, pears, mangoes, and tomatoes.

[0031] <Example 1> A method for preparing copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite films includes: Copper chloride dihydrate and gallic acid were weighed and mixed evenly at a molar ratio of 2:1. Nano-titanium dioxide was added to the mixture at an amount of 3.6 wt% of the total mass of copper chloride dihydrate and gallic acid. The mixture was stirred and dispersed to obtain a precursor mixture. The precursor mixture was transferred to a hydrothermal reactor and hydrothermally reacted at 180°C for 24 h. After the reaction was completed, the temperature was lowered to room temperature to prepare copper-based metal-organic framework supported nano-titanium dioxide (hereinafter referred to as TCM-3.6). Polycaprolactone (weight-average molecular weight 80,000 g / mol) was dissolved in dichloromethane at a mass-to-volume ratio of 1:5 (w / v). The prepared TCM-3.6 was added to the solution at an amount of 0.65 wt% of the mass of polycaprolactone. The mixture was stirred until homogeneous, and the bubbles were removed to obtain the film-forming solution. The film-forming solution was cast uniformly into a flat mold, and the solvent was evaporated by ventilation at room temperature. After the solvent was completely removed, the film was peeled off to obtain a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film (hereinafter referred to as TiO2@Cu-MOFs / PCL).

[0032] <Comparative Example 1> Without adding any functional fillers, the rest of the preparation process is exactly the same as in Example 1, and a pure polycaprolactone membrane, i.e., PCL, is obtained.

[0033] <Comparative Example 2> Without adding nano-titanium dioxide, pure copper-based metal-organic frameworks (referred to as Cu-MOFs) were prepared by a one-pot hydrothermal method. Cu-MOFs were added to a polycaprolactone / dichloromethane solution, and the rest of the preparation process was exactly the same as in Example 1, thus obtaining a copper-based metal-organic framework / polycaprolactone membrane, namely Cu-MOFs / PCL.

[0034] <Comparative Example 3> Without loading a copper-based metal-organic framework, only pure nano-titanium dioxide was added to a polycaprolactone / dichloromethane solution. The rest of the process was exactly the same as in Example 1, and a nano-titanium dioxide / polycaprolactone film, namely TiO2 / PCL, was prepared.

[0035] <Comparative Example 4> Following the one-pot hydrothermal method of Example 1, with a fixed molar ratio of copper chloride dihydrate to gallic acid of 2:1, and the amount of nano-TiO2 added was set to 1.8wt% and 5.4wt%, respectively, copper-based metal-organic framework supported nano-titanium dioxide with different amounts of TiO2 added was prepared, namely TCM-1.8 and TCM-5.4.

[0036] Experiment 1: Infrared Spectroscopic Characterization Experiment 1.1: To verify the successful synthesis of copper-based metal-organic frameworks (Cu-MOFs), Fourier transform infrared spectroscopy was performed. The infrared absorption spectra of pure gallic acid (GA) monomer and Cu-MOFs prepared according to Comparative Example 2 were measured using the ATR method. The experimental results are as follows: Figure 1 As shown.

[0037] like Figure 1 As shown, compared with GA, Cu-MOFs exhibit better performance in the 3500-3200 cm⁻¹ range. -1 The stretching vibration peak of the phenolic hydroxyl group within the range was significantly weakened, indicating that the phenolic hydroxyl group participated in the interaction with Cu. 2+ Coordination function; 1700 cm -1 The stretching vibration peak of the nearby free carboxyl group C=O was significantly weakened, indicating that the carboxyl group also participated in the coordination process; in addition, the 1300-1000 cm⁻¹ peak... -1 The shift in the phenol-oxygen-CO vibration peak within the range further indicates that a stable coordination structure has been formed between the organic ligand and the metal ion. Combined with the Cu-OC related characteristic absorption peaks appearing in the fingerprint region, this study demonstrates the successful preparation of Cu-MOFs.

[0038] Experiment 1.2: To characterize the functional group structure, chemical bonding state, and loading of TiO2@Cu-MOFs (TCM-x) with different TiO2 addition amounts, and to verify the successful loading and optimal loading of TiO2, Fourier transform infrared spectroscopy (ATR) was performed. The tested samples included Cu-MOFs prepared according to the method of Comparative Example 2, TCM-1.8 and TCM-5.4 prepared according to the method of Comparative Example 4, and TCM-3.6 prepared according to the method of Example 1. The experimental results are as follows: Figure 2 As shown.

[0039] like Figure 2As shown, compared with Cu-MOFs, the positions of the main characteristic peaks of TCM-x did not change significantly, indicating that the introduction of TiO2 did not disrupt the original coordination framework structure of Cu-MOFs. With increasing TiO2 content, the characteristic peaks of Cu-MOFs gradually weakened, indicating that TiO2 gradually covered the surface of Cu-MOFs; when the content continued to increase, the peak intensity further decreased, indicating that some TiO2 may have locally aggregated, thus affecting the exposure degree of active sites on the Cu-MOF surface. Overall, the FTIR results show that TiO2 can achieve effective loading while maintaining the integrity of the MOF host structure.

[0040] Experiment 2: X-ray diffraction characterization To verify the crystal phase integrity and TiO2 loading state of copper-based metal-organic framework-loaded nano-titanium dioxide under different nano-titanium dioxide addition amounts, powder X-ray diffraction (PXRD) tests were performed. A PXRD instrument was used with CuKα radiation source, wavelength 0.15418 nm, and scanning range 2θ of 5°–80°. Test samples included TiO2, TCM-3.6 prepared according to the method in Example 1, Cu-MOFs prepared according to the method in Comparative Example 2, and TCM-1.8 and TCM-5.4 prepared according to the method in Comparative Example 4. The experimental results are as follows: Figure 3 As shown.

[0041] like Figure 3 As shown, Cu-MOFs exhibit distinct diffraction peaks at 2θ values ​​of 10.16°, 13.54°, 20.56°, 28.42°, 31.56°, and 42.98°, which are largely consistent with the characteristic peaks of related Cu-based MOFs reported in the literature, indicating that the target MOF structure has been successfully formed. Meanwhile, weak diffraction peaks appear near 2θ values ​​of 36.56° and 42.39°, which can be attributed to a small amount of Cu₂O impurity phase.

[0042] After TiO2 composite, the characteristic diffraction peaks of Cu-MOFs remained clearly visible, indicating that the TiO2 loading process did not disrupt the crystal structure of the MOFs. Simultaneously, characteristic diffraction peaks corresponding to nano-TiO2 appeared in the composite sample, indicating that TiO2 was successfully loaded onto the Cu-MOFs surface and existed in a crystalline form. With increasing TiO2 addition, the intensity of the diffraction peaks gradually increased; however, when the addition amount increased to 0.15 g, the intensity of the characteristic peaks of Cu-MOFs decreased and the peak shape broadened, indicating that excess TiO2 was more likely to agglomerate on the surface, thereby reducing the crystal order of the system. In summary, TCM-3.6 achieved a good balance between TiO2 loading and MOF crystallinity, and therefore can be considered a superior formulation.

[0043] Experiment 3: Thermogravimetric and Differential Thermogravimetric Characterization To verify the effect of nano-titanium dioxide loading on the thermal stability of copper-based metal-organic frameworks, thermogravimetric (TG) and differential thermogravimetric (DTG) tests were performed. Under a nitrogen atmosphere, the samples were heated from room temperature to 600°C at a heating rate of 10°C / min. The test samples included TCM-3.6 prepared according to the method in Example 1, Cu-MOFs prepared according to the method in Comparative Example 2, and TCM-1.8 and TCM-5.4 prepared according to the method in Comparative Example 4. TG and DTG curves were recorded, and the experimental results are shown below. Figure 4 As shown.

[0044] like Figure 4 As shown in b, the DTG curve of pure Cu-MOFs shows a strong water adsorption and removal peak near 100℃. Combined with existing test results, it can be seen that the intensity of this peak decreases significantly with increasing TiO2 loading, confirming that TiO2 nanoparticles have successfully covered the Cu-MOFs surface. The particles occupy the pore inlets, directly disrupting the material's adsorption capacity for environmental water molecules. The peak position and shape near 200℃ do not show significant shifts, indicating that TiO2 loading does not change the internal coordination structure of Cu-MOFs, and the combination is mainly through physical adsorption. It should be noted that the area near 300℃ represents the collapse stage of the material's main framework. The DTG peak temperature of the TiO2@Cu-MOFs composite material shifts slightly towards lower temperatures compared to pure Cu-MOFs, and the starting point of the weight loss step in the corresponding TG curve also shifts to the left. This indicates that the addition of TiO2 slightly accelerates the thermal decomposition process of the Cu-MOFs framework. The cause of this phenomenon can be preliminarily inferred to be that TiO2 nanoparticles and Cu-MOFs are only physically adsorbed together, and there are a large number of lattice mismatches and structural defects at the interface between the two phases. These high-energy sites will become preferential nucleation sites for thermal decomposition when heated, inducing the MOF framework to collapse at a lower temperature.

[0045] like Figure 4 As shown in Figure a, the final char yield test results of the TG curve show that the residual mass of the TCM-1.8 sample at 600℃ is slightly lower than that of the composite sample of pure Cu-MOFs and high TiO2 addition. This difference can be explained by the fact that a lower TiO2 addition introduces a large number of interfacial defects, promoting more thorough decomposition of organic components and leading to a decrease in char yield. When the TiO2 addition is increased to 3.6% or higher, although the catalytic decomposition effect still exists, the physical contribution of TiO2 itself as an inorganic residue to the total residual weight gradually increases, offsetting the mass loss caused by the premature decomposition of organic components, thus bringing the char yield back to the level of pure Cu-MOFs.

[0046] In summary, thermogravimetric analysis confirmed that TiO2 loading reduced the hygroscopicity of Cu-MOFs, and the positions of each pyrolysis peak did not change significantly, indicating that TiO2 exists on the surface of Cu-MOFs by physical adsorption.

[0047] Experiment 4: Scanning Electron Microscopy-Energy Dispersive Spectroscopy (SEM-EDS) Characterization To visually observe the microstructure, dispersion state, and elemental distribution of composite particles with different TiO2 addition amounts, and to verify the dispersion suppression effect of copper-based metal-organic frameworks on nano-titanium dioxide, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) elemental distribution tests were performed. Samples were dispersed on conductive adhesives, sputtered with gold, and their morphology was observed using field emission scanning electron microscopy (FETS). Simultaneously, EDS surface scanning was performed to detect the uniformity of Ti and Cu elemental distribution. Test samples included TCM-3.6 prepared according to the method in Example 1, Cu-MOFs prepared according to the method in Comparative Example 2, and TCM-1.8 and TCM-5.4 prepared according to the method in Comparative Example 4. The EDS surface distribution and SEM morphology results are shown below. Figure 5 , Figure 6 As shown.

[0048] like Figure 6 As shown in the SEM images, Cu-MOFs exhibit a relatively complete crystal morphology and a relatively smooth surface, indicating that the material has been successfully formed. After the introduction of TiO2, the surface of the composite material becomes significantly rougher, and a large number of fine particles can be observed adhering to the Cu-MOF surface, indicating that TiO2 has been effectively loaded onto the MOF framework surface. With increasing TiO2 addition, the particle density on the sample surface gradually increases, but under high addition conditions, significant particle accumulation occurs in localized areas.

[0049] like Figure 5 As shown, the EDS surface distribution results also indicate that Ti elements aggregate in some areas, consistent with the SEM observations. In particular, the Ti element mass fraction in TCM-5.4 is lower than that in TCM-3.6, indicating that when excessive TiO2 is added, local agglomeration and uneven distribution occur in the system, leading to a decrease in the effective dispersion loading. Therefore, an appropriate amount of TiO2 is beneficial for forming a more uniform loading layer on the Cu-MOF surface, while excessive addition is detrimental to the stable construction of the composite structure.

[0050] Experiment 5: X-ray photoelectron spectroscopy (XPS) characterization To characterize the surface elemental composition and chemical valence state of TCM-3.6 under optimal loading, XPS analysis was performed to analyze the surface elemental composition and chemical state of the samples. X-ray photoelectron spectroscopy was used to measure the full spectrum and high-resolution Ti2p and Cu2p spectra. The binding energy was calibrated with C1s. The tested samples included pure Cu-MOFs prepared according to Comparative Example 2 and TCM-3.6 prepared according to the method in Example 1. The experimental results are as follows: Figure 7 As shown.

[0051] like Figure 7As shown, in the high-resolution Cu 2p spectra, both samples exhibit distinct satellite peaks near 940-945 eV and 960-965 eV, indicating that copper in the samples is mainly in the form of Cu. 2+ The form exists; simultaneously, a small amount of low-valent copper-related signals can be observed near 932 eV and 952 eV, indicating that a small amount of reduction may occur during the synthesis process. The characteristic peaks of CC / C=C, CO, and C=O appearing in the C 1s spectrum correspond to different carbon chemical environments in the organic ligands, respectively; the peak near 531.8 eV in the O 1s spectrum can be attributed to the Cu-O coordination structure, indicating that Cu... 2+ A stable skeletal structure is formed with the organic ligand.

[0052] Compared to Cu-MOFs, the Cu 2p, C 1s, and O 1s spectra in TCM-3.6 remained largely consistent, indicating that the MOF bulk structure remained intact after the introduction of TiO2. In the Ti 2p spectrum of the composite material, characteristic peaks of approximately 458.5 eV and 464.3 eV were observed, corresponding to the Ti 2p peaks in TiO2, respectively. 3 / 2 and Ti 2p 1 / 2 The signal was observed; simultaneously, the peak intensity related to lattice oxygen in the O 1s spectrum increased, further indicating that TiO2 had been successfully loaded onto the Cu-MOFs surface. Combined with XRD and SEM results, it can be seen that TiO2 mainly composites with Cu-MOFs in the form of surface loading, without causing significant damage to the MOF framework.

[0053] Test 6: Antibacterial Test Experiment 6.1: To evaluate the antibacterial effect of copper-based metal-organic framework-loaded nano-titanium dioxide with different amounts of nano-titanium dioxide, an antibacterial plate count test was conducted. The plate viable count method was used, and the test strain was Staphylococcus aureus (S. aureus). Staphylococcus aureus Gram-positive representatives) and Escherichia coli ( Escherichia coli (Gram-negative representative), the sample was co-cultured with bacterial suspension and then spread onto agar medium. It was incubated at 37℃ for 24 h, and colony growth was observed and recorded. Test samples included a control group (blank), TiO2, TCM-3.6 prepared according to Example 1, Cu-MOFs prepared according to Comparative Example 2, and TCM-1.8 and TCM-5.4 prepared according to Comparative Example 4. The experimental results are as follows: Figure 8 As shown.

[0054] Experiment 6.2: To evaluate the antibacterial properties of composite fillers with different amounts of nano-titanium dioxide, a quantitative test of the antibacterial rate was conducted. Following the GB / T 31402-2015 standard, the test samples were mixed with bacterial suspensions (test strains were Staphylococcus aureus and Escherichia coli, initial concentration approximately 10).5 After 24 hours of contact with CFU / mL, the colony count was counted. Test samples included TCM-3.6 prepared according to Example 1, Cu-MOFs prepared according to Comparative Example 2, and TCM-1.8 and TCM-5.4 prepared according to Comparative Example 4. The antibacterial rate was calculated using the formula: Antibacterial rate (%) = (Number of colonies in the blank group - Number of colonies in the sample group) / Number of colonies in the blank group × 100%. The test results are as follows: Figure 9 As shown.

[0055] like Figure 9 As shown, pure TiO2 did not exhibit significant antibacterial activity against either strain, which is consistent with its photocatalytic activity being dependent on ultraviolet light excitation. In the absence of sufficient ultraviolet light, TiO2 struggles to efficiently generate reactive oxygen species with bactericidal effects; therefore, its antibacterial effect is limited when used alone.

[0056] like Figure 8 As shown, in comparison, Cu-MOFs alone exhibited better antibacterial activity, with antibacterial rates of 74.19% and 92.68% against Staphylococcus aureus and Escherichia coli, respectively. This is mainly related to the continuous release of copper ions from Cu-MOFs: the released Cu... 2+ It can disrupt bacterial cell membrane structure, interfere with intracellular enzyme activity, and induce oxidative stress, thereby achieving antibacterial effects. Furthermore, TiO2@Cu-MOFs exhibited the best antibacterial performance among all samples, with the composite material prepared by adding 0.1 g TiO2 achieving antibacterial rates of 99.74% and 99.81% against the two bacterial strains, respectively. Plate colony plots also showed that the number of colonies in this group of samples was extremely low, almost completely inhibiting bacterial proliferation.

[0057] This result demonstrates a significant synergistic effect between TiO2 and Cu-MOFs. Cu-MOFs provide a stable antibacterial copper source, while the introduction of TiO2 helps improve the interfacial reaction environment, thereby further enhancing the antibacterial activity of the composite system. Therefore, TiO2@Cu-MOFs not only overcomes the problem of insufficient antibacterial ability of pure TiO2 under dark conditions but also outperforms Cu-MOFs alone, showing promising application prospects as an active packaging functional filler.

[0058] This is likely because the increased loading of TiO2 nanoparticles on the Cu-MOF surface enhances the effective specific surface area and surface roughness of the filler, thereby increasing the physical contact and probability of damage between the filler and the bacterial cell wall. Furthermore, the oxygen vacancy defects on the TiO2 surface may promote the growth of Cu in Cu-MOFs. 2+ Local adsorption and enrichment of copper ions increase the effective antibacterial copper ion concentration in the microenvironment surrounding bacteria.

[0059] Experiment 7: Differential Scanning Calorimetry Characterization To evaluate the effects of different functional fillers on the crystallization behavior and thermal properties of polycaprolactone (PCL) matrix, DSC tests were performed. Test samples included PCL prepared according to Comparative Example 1, Cu-MOFs / PCL prepared according to Comparative Example 2, TiO2 / PCL prepared according to Comparative Example 3, and TiO2@Cu-MOFs / PCL prepared according to Example 1. Under a nitrogen atmosphere, the temperature was increased from -50°C to 100°C at a rate of 10°C / min, held at that temperature for 3 min to eliminate thermal history, and then decreased to -50°C at a rate of 10°C / min. The heating / cooling curves and the cooling curve were recorded. The experimental results are as follows: Figure 10 As shown 。

[0060] like Figure 10 As shown, all sample groups exhibited significant endothermic peaks within the 55-65℃ range, corresponding to the melting process of the PCL matrix. Compared to pure PCL, the melting peak of Cu-MOFs / PCL shifted slightly towards lower temperatures, indicating that the introduction of Cu-MOFs interfered with the orderly stacking of PCL molecular chains to some extent, resulting in a slight decrease in crystal integrity. Conversely, the melting peak of TiO2 / PCL shifted slightly towards higher temperatures, indicating that TiO2, as a heterogeneous nucleating agent, facilitated the formation of a relatively more complete crystal structure in PCL. For the TiO2@Cu-MOFs / PCL composite film, its melting peak shifted further towards higher temperatures, indicating that a stronger interfacial interaction was formed between the composite filler and PCL, restricting the thermal motion of chain segments and making the crystal structure more stable under heat. The cooling crystallization curves showed that the crystallization temperature of all filler-modified PCL composite films was higher than that of pure PCL, indicating that TiO2, Cu-MOFs, and their composite fillers all played a certain nucleation role. Overall, the introduction of TiO2@Cu-MOFs has a certain regulatory effect on the thermal behavior of PCL, but does not destroy its basic melting and crystallization characteristics.

[0061] Experiment 8: X-ray diffraction characterization To analyze the influence of functional fillers on the crystal phase structure of polycaprolactone composite films, XRD tests were performed. Powder X-ray diffractometer was used with CuKα radiation source and a scanning range of 2θ = 5°–50°. Test samples included PCL prepared according to Comparative Example 1, Cu-MOFs / PCL prepared according to Comparative Example 2, TiO2 / PCL prepared according to Comparative Example 3, and TiO2@Cu-MOFs / PCL prepared according to Example 1. The experimental results are as follows: Figure 11 As shown.

[0062] like Figure 11As shown, pure PCL exhibits characteristic diffraction peaks near 18.5°, 21.5°, and 24.5°, with 21.5° and 24.5° being typical diffraction peaks for the orthorhombic PCL crystal system, indicating a relatively well-defined semi-crystalline structure. The addition of TiO2 did not significantly alter the positions of the main characteristic peaks of PCL, suggesting that the introduction of TiO2 did not change the basic crystal form of PCL, but rather primarily affected its crystallization behavior through heterogeneous nucleation.

[0063] When Cu-MOFs were introduced, the peak intensity near 18.5° increased and exceeded that at 21.5°, indicating that Cu-MOFs had a certain regulatory effect on the crystal orientation of PCL. This may be related to the abundant hydroxyl and carboxyl sites on the surface of Cu-MOFs, and the interfacial interaction between Cu-MOFs and PCL molecular chains restricts the free movement of chain segments, thus affecting the crystal growth mode. For the TiO2@Cu-MOFs / PCL composite film, the difference in peak intensity between 18.5° and 21.5° further increased, indicating that the composite filler had a more significant effect on the arrangement of PCL chain segments. Combined with the DSC results, it can be seen that TiO2@Cu-MOFs can not only maintain the crystal structure characteristics of PCL, but also regulate its crystal orientation through interfacial interactions.

[0064] Experiment 9: Contact Angle Test To evaluate the surface hydrophilicity and hydrophobicity of different composite membranes, predict their water vapor barrier capacity and antifouling performance, and adapt them to fruit preservation requirements, water contact angle tests were conducted. The sitting drop method was used, with 2 μL of deionized water dropped onto the membrane surface at room temperature. The static contact angle was recorded using a contact angle meter. Five different locations were tested for each sample, and the average value was taken. The tested samples included PCL prepared according to Comparative Example 1, Cu-MOFs / PCL prepared according to Comparative Example 2, TiO2 / PCL prepared according to Comparative Example 3, and TiO2@Cu-MOFs / PCL prepared according to Example 1. The experimental results are as follows: Figure 12 As shown.

[0065] like Figure 12 As shown, the contact angle of the pure PCL membrane is about 83°, exhibiting certain hydrophobicity; after adding Cu-MOFs, the contact angle increases to about 86°; after adding TiO2, the contact angle is about 81°; while the contact angle of the TiO2@Cu-MOFs / PCL composite membrane is about 87°, which is the highest among all samples.

[0066] This is likely because the introduction of TiO2@Cu-MOFs filler, the micro-rough structure formed by the filler on the matrix surface, and the filler's own low surface energy cause the composite film surface to change from moderately hydrophobic to strongly hydrophobic. For food packaging materials, a moderate increase in the hydrophobicity of the outer surface of the film helps reduce the adhesion of environmental moisture and dust, and reduces the probability of initial adhesion of microorganisms to the film surface, thereby physically improving the hygiene level of the packaging.

[0067] Experiment 10: Tensile Property Test To evaluate the influence of functional fillers on the mechanical properties of polycaprolactone composite films and to verify the basic mechanical properties of the films, tensile property tests were conducted. Following GB / T 1040.3-2006, the films were cut and tested using a universal testing machine at a tensile rate of 50 mm / min to measure the tensile strength and elongation at break. Test samples included PCL prepared according to Comparative Example 1, Cu-MOFs / PCL prepared according to Comparative Example 2, TiO2 / PCL prepared according to Comparative Example 3, and TiO2@Cu-MOFs / PCL prepared according to Example 1. The test results are as follows: Figure 13 As shown.

[0068] like Figure 13 As shown, pure PCL exhibits the best overall mechanical properties, with an elongation at break of 345% and a tensile strength of 16.5 MPa, indicating that the PCL-based film possesses good flexibility and a continuous, dense structure. After adding Cu-MOFs, the elongation at break of the composite film decreased to 302%, and the tensile strength decreased to 14.5 MPa, indicating that the addition of fillers disrupted the continuous phase structure of PCL to some extent, but the overall decrease was relatively small. The TiO2@Cu-MOFs / PCL composite film had an elongation at break of 318% and a tensile strength of 13.0 MPa, with its mechanical properties falling between those of Cu-MOFs / PCL and TiO2 / PCL. This demonstrates that TiO2@Cu-MOFs can impart functionality to the film while still maintaining the ductility and load-bearing capacity of the PCL matrix. In contrast, the tensile strength of TiO2 / PCL decreased more significantly, reaching only 11.0 MPa, while its elongation at break (348%) was similar to that of pure PCL. This indicates that TiO2 alone is more likely to cause stress concentration, leading to earlier membrane fracture under stress. Overall, the introduction of functional fillers weakens the mechanical strength of PCL to varying degrees, but TiO2@Cu-MOFs / PCL maintains good mechanical suitability while improving antibacterial, antioxidant, and surface properties.

[0069] Experiment 11: Fruit Preservation Test To evaluate the practical preservation effect of the TiO2@Cu-MOFs / PCL composite film and verify its effect on extending the shelf life of fresh fruits, a mango preservation simulation test was conducted. Mangoes were placed in a blank control group and a TiO2@Cu-MOFs / PCL composite film packaging group, respectively, and stored continuously at room temperature for 7 days. Changes in the appearance of the mangoes and the loss of quality were observed and recorded. The test samples included the TiO2@Cu-MOFs / PCL composite film prepared according to Example 1 and the blank control group. The experimental results are as follows: Figure 14 As shown.

[0070] like Figure 14 As shown, mangoes in the blank control group developed obvious black spots on the second day of storage, and began to rot and turn black over a large area on the third day. Simultaneously, their quality declined rapidly with prolonged storage, resulting in significant quality loss within 7 days. In contrast, mangoes packaged with the TiO2@Cu-MOFs / PCL composite film showed no obvious rot or mold growth during the 7-day storage period, maintaining a good appearance, and exhibiting a significantly slower rate of quality decline. Their quality loss within 7 days was far lower than that of the blank control group. This indicates that the TiO2@Cu-MOFs / PCL composite film can simultaneously achieve the dual effects of inhibiting spoilage and reducing moisture loss, effectively extending the shelf life of fruits such as mangoes.

[0071] Although this embodiment uses mango as a representative climacteric fruit to verify the preservation effect, those skilled in the art will understand that bananas, apples, pears, tomatoes, etc., are also typical climacteric fruits. Their common physiological characteristics of postharvest ripening and senescence are: respiration intensity and ethylene release rapidly increase during ripening, thereby accelerating fruit softening, water loss, browning, and spoilage. The preservation mechanism of the copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film described in this invention mainly relies on the continuous release of Cu from the copper-based metal-organic framework. 2+ The invention utilizes a broad-spectrum antibacterial effect, the ability of nano-titanium dioxide to degrade gases such as ethylene under weak light conditions, and the excellent water vapor barrier and water retention properties of the polycaprolactone matrix. This preservation mechanism does not depend on a specific fruit variety but rather targets the common aging and spoilage pathways of climacteric fruits. Based on the actual effect of mangoes maintaining good appearance and significantly reducing quality loss during a 7-day storage period at room temperature, and combined with the aforementioned common mechanism of action, it is reasonable to expect that the composite film of this invention can also effectively delay the ripening and aging process of other climacteric fruits such as bananas, apples, pears, and tomatoes, inhibit microbial spoilage, and reduce post-harvest storage and transportation losses. Therefore, all fruit varieties defined in claim 8 are within the reasonable protection scope of this invention.

[0072] In summary, this invention employs a one-pot hydrothermal method, fixing the molar ratio of copper chloride dihydrate to gallic acid at 1:1 to 3:1, and investigates the effect of the amount of nano-titanium dioxide added (0.1wt% to 7.0wt%) on the loading rate. It was found that the actual loading rate was highest when the addition amount was 3.6wt% (TCM-3.6). Antibacterial tests showed that TCM-3.6 achieved antibacterial rates of 99.74% and 99.81% against Staphylococcus aureus and Escherichia coli, respectively, significantly higher than the 74.19% and 92.68% of pure copper-based metal-organic frameworks. When the addition amount was low (1.8wt%), the antibacterial rates were 91.89% and 99.77%, respectively; when the addition amount was high (5.4wt%), the rates decreased to 53.25% and 50.94% due to aggregation, confirming the optimal loading amount and synergistic antibacterial effect. By combining TCM-3.6 with polycaprolactone to form a film, a composite packaging film with both biodegradability and stable antibacterial properties can be directly obtained without complex processes. When the addition amount is 0.5wt% to 5wt% of the mass of polycaprolactone, the composite film exhibits good overall performance. The addition of TiO2@Cu-MOFs ensures that the composite film maintains good packaging suitability. Relying on synergistic antibacterial effects, its antibacterial and preservation effects are significantly better than those of pure PCL and single-filler modified films. It can effectively delay the spoilage of climacteric fruits and reduce post-harvest losses, providing a practical technical solution for biodegradable antibacterial preservation packaging.

[0073] This invention is the first to construct a copper-based metal-organic framework (MOF)-supported nano-titanium dioxide and its polycaprolactone (PCO2) composite antibacterial film system. Using Cu-MOFs as a carrier to support TiO2, it solves the problems of PCL's single function, weak antibacterial properties, poor barrier properties, and the tendency of nano-TiO2 to aggregate and its insufficient stability under low light. This achieves a multi-functional synergistic effect of antibacterial and preservation. Compared with the blank control group, this composite film can effectively extend the shelf life of climacteric fruits such as mangoes and bananas, demonstrating excellent preservation effects.

[0074] The number of devices and processing scale described herein are for the purpose of simplifying the description of the invention. Applications, modifications, and variations of the invention will be readily apparent to those skilled in the art.

[0075] Although embodiments of the present invention have been disclosed above, they are not limited to the applications listed in the specification and embodiments. They can be applied to various fields suitable for the present invention. For those skilled in the art, other modifications can be easily made. Therefore, without departing from the general concept defined by the claims and their equivalents, the present invention is not limited to the specific details and illustrations shown and described herein.

Claims

1. A method for preparing copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite films, characterized in that, Includes the following steps: A one-pot hydrothermal method was used to prepare copper-based metal-organic framework-supported nano-titanium dioxide. Polycaprolactone was dissolved in a volatile organic solvent at a mass-to-volume ratio of 1:5 to 1:

10. A copper-based metal-organic framework supported on nano-titanium dioxide was added at a mass ratio of 0.5 wt% to 5 wt% of the polycaprolactone. The mixture was stirred until homogeneous to obtain a film-forming solution. The film-forming solution was cast into a film, and the volatile organic solvent was evaporated to remove it, thus obtaining a copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film.

2. The preparation method according to claim 1, characterized in that, The one-pot hydrothermal method includes: mixing copper chloride dihydrate and gallic acid in a molar ratio of 1:1 to 3:1, adding nano-titanium dioxide, the amount of nano-titanium dioxide added being 0.1wt% to 7.0wt% of the total mass of copper chloride dihydrate and gallic acid, to obtain a precursor mixture, and hydrothermally reacting the precursor mixture at 120℃ to 180℃ for 12h to 36h, followed by cooling to obtain a copper-based metal-organic framework supported on nano-titanium dioxide.

3. The preparation method according to claim 2, characterized in that, The amount of nano-titanium dioxide added is 3.6 wt% of the total mass of copper chloride dihydrate and gallic acid.

4. The preparation method according to claim 1, characterized in that, Polycaprolactone has a weight-average molecular weight of 50,000 g / mol to 100,000 g / mol.

5. The preparation method according to claim 1, characterized in that, The volatile organic solvent is at least one of dichloromethane, chloroform, or tetrahydrofuran.

6. The copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film prepared by any one of claims 1 to 5.

7. The application of the copper-based MOF-supported nano-titanium dioxide / polycaprolactone composite film according to claim 6 in the preservation of respiratory climacteric fruits.

8. The application according to claim 7, characterized in that, The climacteric fruit is at least one of banana, apple, pear, mango, or tomato.

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