A theaflavonol carbon quantum dot preservative film with monitoring and antibacterial effects, and a preparation method and application thereof

Abstract

The application discloses a theaflavins carbon quantum dots preservative film with monitoring and antibacterial effects, and a preparation method and application thereof, relates to the field of novel functional nanomaterials, and comprises a polyvinyl alcohol nanofiber matrix, theaflavins-based carbon quantum dots, citric acid and graphite phase carbon nitride dispersed in the matrix; the theaflavins-based carbon quantum dots are in-situ embedded in the polyvinyl alcohol nanofiber through electrospinning and are cross-linked and fixed in the polyvinyl alcohol nanofiber through thermal esterification to form a three-dimensional cross-linked network structure. The prepared carbon quantum dots form a heterojunction with graphite phase carbon nitride through an electrostatic field domain, and the antibacterial efficiency is synergistically enhanced, the photo-thermal performance is significantly better than that of tea polyphenol carbon quantum dots, and active oxygen can be generated. The application of the preservative film in fruit preservation has the dual effects of fruit freshness indication based on fluorescence quenching and antibacterial preservation, and the fruit storage period can be prolonged by 100% at room temperature.

Description

Technical Field

[0001] This invention relates to the field of novel functional nanomaterials, and in particular to a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects, its preparation method, and its application. Background Technology

[0002] Bananas and strawberries hold significant economic value in the global fruit industry; however, they are highly susceptible to spoilage after harvesting due to microbial contamination and their own metabolic processes, resulting in substantial economic losses. While traditional low-temperature refrigeration and gas packaging technologies can preserve fruit to some extent, their high energy consumption and costs remain significant problems. Furthermore, the use of chemical preservatives often carries safety risks such as residues and drug resistance. Therefore, there is an urgent need to develop new preservation materials that can effectively inhibit microbial growth while ensuring food quality and safety.

[0003] Polyvinyl alcohol (PVA) electrospun films have shown great potential in the food packaging field due to their high porosity, biodegradability, non-toxicity, and good film-forming properties. However, the antibacterial properties of pure PVA electrospun films are limited, failing to meet the preservation requirements of foods with high microbial loads. Furthermore, pure PVA is highly hydrophilic, easily absorbing water and swelling, losing its barrier properties when exposed to high-moisture fruits such as strawberries or in high-humidity environments. In addition, chemical cross-linking agents commonly used to address the water resistance of PVA, such as glutaraldehyde, are highly toxic and strictly limited to food contact materials. Therefore, researchers often load inorganic nano-antibacterial agents such as nano-silver and zinc oxide, or natural antibacterial agents such as tea polyphenols and chitosan, into electrospun fibers to impart antibacterial activity. However, inorganic nanoparticles may pose biotoxicity and environmental risks, while natural antibacterial agents still suffer from problems such as easy decomposition, narrow antibacterial spectrum, and slow action.

[0004] Carbon quantum dots (CQDs) have attracted widespread attention in the antibacterial field due to their excellent biocompatibility, low toxicity, and superior optical properties. In particular, their unique photothermal and photodynamic effects offer the possibility of developing multi-effect synergistic antibacterial strategies that combine near-infrared light triggering with physical and chemical approaches. However, applying CQDs to food preservation, especially as antibacterial active ingredients integrated into packaging materials, still faces three major challenges: First, the controllability and synergy of carbon quantum dot (CQD) performance are insufficient. Most current research focuses on a single antibacterial mode of CQDs, such as chemical, photothermal, or photodynamic antibacterial action. Performance regulation often relies on complex post-synthetic modifications or heteroelement doping, which are cumbersome and lack a clear mechanism for regulating the intrinsic electronic structure of the material. Furthermore, conventional inorganic heavy metal semiconductors introduced to improve photogenerated carrier separation efficiency pose serious food safety risks. The key to improving the antibacterial efficiency of CQDs lies in finding a simple and fundamental method to achieve synergistic regulation and simultaneous enhancement of their photothermal, photodynamic, and chemical antibacterial properties.

[0005] Secondly, there are issues regarding the functional integration and long-term stability of carbon quantum dots (CQDs). Pure CQDs present challenges in dispersibility, stability, and recyclability when used for food preservation. Composites with polymeric substrates such as polyvinyl alcohol (PVA) offer an ideal solution. However, simple physical blending can lead to CQD aggregation or leakage in high-humidity environments, affecting performance durability and posing safety hazards. Therefore, developing a composite fiber membrane structure that achieves stable and uniform dispersion of CQDs, constructs a robust network, and fully leverages their multifunctional synergistic effects is crucial for their practical application.

[0006] Third, carbon quantum dots (CQDs) have limited functional applications. When used for preservation, CQDs only provide simple antibacterial effects and cannot simultaneously monitor food freshness. Therefore, developing a multifunctional material that integrates antibacterial and freshness monitoring has significant application potential. Summary of the Invention

[0007] In view of the defects and shortcomings of the existing technology, the purpose of this invention is to provide a theaflavin carbon quantum dot preservation film that has both monitoring and antibacterial functions, so as to solve the problem that the carbon quantum dot preservation film in the existing technology cannot simultaneously have antibacterial and detection preservation effects while preventing leakage in high humidity environments.

[0008] The present invention also aims to provide a preparation method for preparing a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0009] The present invention also aims to provide the application of theaflavin carbon quantum dot preservation film that has both monitoring and antibacterial effects.

[0010] In a first aspect, the present invention discloses a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial functions, comprising a polyvinyl alcohol nanofiber matrix, and theaflavin-based carbon quantum dots, citric acid and graphitic carbon nitride dispersed in the matrix. Theaflavin-based carbon quantum dots were embedded in situ through electrospinning and then fixed in polyvinyl alcohol nanofibers by thermal esterification, forming a three-dimensional cross-linked network structure.

[0011] Preferably, theaflavin-based carbon quantum dots are prepared by the following method: Theaflavins were dissolved in deionized water and then placed in a sealed reaction vessel. The reaction was carried out under programmed temperature rise, followed by natural cooling and centrifugation. The supernatant was filtered through a microporous membrane, and the filtrate was dialyzed and then freeze-dried to obtain theaflavin-based carbon quantum dots.

[0012] Preferably, the volume-to-mass ratio of deionized water to theaflavins is 30-60 mL: 1 g.

[0013] Preferably, the programmed temperature rise process involves raising the temperature to 180–250°C at a rate of 5–10°C / min and maintaining it at 170–190°C for 8–12 hours.

[0014] Preferably, the centrifugation speed is 8000-10000 r / min and the centrifugation time is 15-30 min.

[0015] Preferably, the aqueous membrane used for filtration has a pore size of 0.18–0.24 μm; the dialysis bag has a molecular weight cutoff of 400–600 Da; the dialysis time is 20–28 h; and the freeze-drying temperature is -30–-50 °C, with a drying time of 18–36 h.

[0016] Preferably, theaflavins are prepared by the following method: Tea leaves were pulverized into a fine powder, mixed with distilled water, and laccase was added to react. The reaction was stopped by heating, cooled to room temperature, and extracted with ethyl acetate. The ethyl acetate phase was dried under reduced pressure to obtain theaflavins.

[0017] Preferably, the fine powder is 100-200 mesh.

[0018] Preferably, the distilled water is 15 to 30 times the weight of the tea leaves.

[0019] Preferably, the ratio of laccase added to tea leaves is (500-800U):1g.

[0020] Preferably, the laccase reaction temperature is 40-60℃, the reaction time is 2-3 hours, and the reaction is stopped when the temperature is raised to 90-100℃.

[0021] Preferably, the temperature is cooled to room temperature of 25–35°C.

[0022] Preferably, the amount of ethyl acetate added is 1 / 3 to 1 times the volume of distilled water, the extraction time is 30 to 60 min, the vacuum drying is carried out at 50 to 60 °C, 0.01 to 0.2 Pa, and the drying time is 8 to 16 h.

[0023] Secondly, the present invention also provides a preparation method for preparing a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects, comprising the following steps: S1. Polyvinyl alcohol is added to deionized water to prepare a solution, citric acid, graphitic carbon nitride, and theaflavin-based carbon quantum dots are added and mixed to form a carbon quantum dot electrostatic spray precursor liquid. S2. The carbon quantum dot electrospinning precursor liquid is electrospun, dried, and then thermally crosslinked to prepare the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0024] Preferably, the polyvinyl alcohol mass fraction is 5% to 15%.

[0025] Preferably, the amount of citric acid added is 5% to 12% of the mass of polyvinyl alcohol.

[0026] Preferably, the amount of graphitic nitrogen oxide added is 10% to 30% of the mass of theaflavin-based carbon quantum dots.

[0027] Preferably, the mass-to-volume ratio of theaflavin-based carbon quantum dots to deionized water is 1–5 mg / mL.

[0028] The preferred electrospinning method specifically involves using an aluminum plate as a receiver and collecting samples after electrostatic spraying.

[0029] Preferably, the applied voltage for electrospinning is 16–20 kV, the feed speed is 0.4–0.8 mL / h, and the receiving distance is 10–20 cm.

[0030] Preferably, the drying temperature is 30–50°C and the drying time is 20–30 hours.

[0031] Preferably, the temperature for thermal esterification crosslinking is 100–140°C, and the treatment time is 1–3 h.

[0032] Thirdly, the present invention also provides an application of a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial functions in fruit preservation, namely, wrapping fresh fruit with 0.5% to 1.5% of its mass of the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial functions, irradiating it under 808nm near-infrared light for 5 to 15 minutes, and then storing it at room temperature. The preservation film of the present invention has fluorescence response characteristics to changes in the microenvironment. When the fruit is fresh, the preservation film exhibits bright blue fluorescence under ultraviolet light. As the fruit rots and releases volatile gases, the fluorescence quenches and dims.

[0033] The beneficial effects of this invention are: 1. The carbon quantum dots prepared in this invention inhibit bacteria through a chemical-photodynamic-photothermal synergistic mechanism, and significantly improve the ROS yield with the support of Z-shaped spatial confinement heterojunction, achieving highly efficient bactericidal effect against Escherichia coli and Staphylococcus aureus with an inhibition rate of up to 99%.

[0034] 2. The electrospun antibacterial nanofiber membrane prepared by this invention has excellent density, film-forming properties, and extensibility; moreover, due to the construction of a food-grade cross-linked network, it possesses excellent high-humidity hydrolysis resistance and material leakage prevention capabilities. As a packaging film for fruits such as strawberries and bananas, it can effectively delay the decay and spoilage of the fruits.

[0035] 3. The preservation film prepared by this invention also has a microenvironment self-responsive intelligent indication function, which can intuitively and non-destructively monitor the freshness of fruit by observing the quenching and darkening of fluorescence under ultraviolet light.

[0036] 4. The preparation method of the present invention has mild reaction conditions and uses physical electrostatic field and natural food-grade crosslinking agent to replace complex chemical synthesis and highly toxic reagents, making it suitable for industrial production. Attached Figure Description

[0037] Figure 1 These are transmission electron microscopy (TEM) images, where 1A-1B are carbon dots of tea polyphenols (TP-CQDs) and 1C-1D are carbon dots of theaflavins (TFs-CQDs). Figure 2 The images are XPS spectra, where 2A-2B are the total XPS measured spectra of TP-CQDs and 2C-2D are the high-resolution C1s XPS spectra of TFs-CQDs. Figure 3 The UV-Vis absorption spectra of (A) TP-CQDs and (C) TFs-CQDs; the PL emission spectra of (B) TP-CQDs and (D) TFs-CQDs; Figure 4 CV curves for (A) TP-CQDs and (B) TFs-CQDs; Figure 5 Figures showing the irradiation degradation of (A) TP-CQDs and (B) TFs-CQDs under an 808 nm light source in DCFH-DA. Figure 6 For near-infrared laser (808nm, 2W / cm) 2 Temperature variation curves of different concentrations of (A) TP-CQDs and (B) TFs-CQDs under irradiation over time; Figure 7 The bactericidal effects of (A) TP-CQDs and (B) TFs-CQDs at 500 μg / mL on Staphylococcus aureus and Escherichia coli under NIR irradiation and without NIR irradiation; Figure 8 Electron micrograph of polyvinyl alcohol electrospun nanofiber membrane; Figure 9 Electron micrograph of TFs-CQDs-polyvinyl alcohol electrospun nanofiber membrane; Figure 10 To compare the effects of different fiber membranes on fruit preservation, (A) weight loss rate of strawberries and (B) bananas; Figure 11 Photofluorescence spectra and corresponding fluorescence images of theaflavin carbon quantum dot preservation film, which has both monitoring and antibacterial effects, on fruits at different freshness levels. Detailed Implementation

[0038] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0039] A theaflavin carbon quantum dot preservation film with both monitoring and antibacterial functions includes a polyvinyl alcohol nanofiber matrix, and theaflavin-based carbon quantum dots, citric acid and graphitic carbon nitride dispersed in the matrix. Theaflavin-based carbon quantum dots were embedded in situ through electrospinning and then fixed in polyvinyl alcohol nanofibers by thermal esterification, forming a three-dimensional cross-linked network structure.

[0040] The inventors discovered that the degree of conjugation of the precursor can be directly transferred to the synthesized CQDs and determine the band structure, thereby achieving precise and linear control of the photothermal properties of CQDs and simultaneously optimizing their photodynamic and chemical antibacterial properties, achieving efficient synergy among the three. Based on this discovery, the inventors used theaflavins as a precursor to prepare theaflavin-based carbon quantum dots (TFs-CQDs) via a one-step hydrothermal method. Further, using electrospinning technology, TFs-CQDs, graphitic nitrogen oxides, food-grade citric acid, and polyvinyl alcohol were composited. Utilizing the spatial confinement effect of a high-voltage electrostatic field, the two-dimensional graphitic nitrogen oxides and the zero-dimensional TFs-CQDs were tightly bonded to form a Z-shaped metal-free photocatalytic heterojunction, further accelerating the separation of photogenerated electron-hole pairs. Simultaneously, subsequent high-temperature treatment caused the polycarboxyl groups of citric acid to undergo in-situ esterification and polycondensation reactions with the hydroxyl / amino groups on the surface of polyvinyl alcohol and TFs-CQDs, constructing a stable three-dimensional cross-linked network. This film not only solves the problems of high-humidity leakage and stabilization of CQDs, but also enhances their initial fluorescence by restricting the non-radiative transitions of carbon dots through a rigid network. Furthermore, it induces fluorescence quenching upon contact with volatile amine gases produced by fruit and vegetable spoilage, providing an early warning of spoilage. It also introduces a synergistic antibacterial mechanism of chemical-photodynamic-photothermal interaction into the food packaging field, allowing its powerful antibacterial function to be activated on demand through convenient near-infrared light irradiation. This preservation film also features fruit freshness monitoring, exhibiting bright blue fluorescence under ultraviolet light excitation. As fruit spoils and releases volatile amine gases such as ammonia, putrescine, and cadaverine, the amino groups in the gas structure can specifically bind to the carboxyl groups on the carbon dot surface, causing fluorescence quenching and darkening. Therefore, it possesses both fruit freshness indication and antibacterial preservation functions.

[0041] This invention uses theaflavins as a precursor. Theaflavins with a higher degree of oxidative polymerization form a denser conjugated aromatic structure and a higher degree of graphitization during carbonization, resulting in higher sp. values ​​for its TF-CQDs. 2The hybrid carbon network is more complete, and the band gap is narrower. This structural characteristic enhances near-infrared light absorption, promotes efficient separation of photogenerated carriers, generates reactive oxygen species (ROS) through electronic transitions, and improves the photodynamic effect. Simultaneously, excited-state energy is rapidly converted into heat energy through lattice vibrations, significantly improving photothermal conversion efficiency. This determines the band structure of the synthesized CQDs, enabling precise and linear control of their photothermal properties, and simultaneously optimizing their photodynamic and chemical antibacterial properties, achieving a highly efficient synergy among the three.

[0042] In some embodiments, theaflavin-based carbon quantum dots are prepared by the following method: Theaflavins were dissolved in deionized water and then placed in a sealed reaction vessel. The reaction was carried out under programmed temperature rise, followed by natural cooling and centrifugation. The supernatant was filtered through a microporous membrane, and the filtrate was dialyzed and then freeze-dried to obtain theaflavin-based carbon quantum dots.

[0043] The volume-to-mass ratio of ionized water to theaflavins is 30–60 mL: 1 g.

[0044] The programmed temperature rise process involves raising the temperature to 180–250°C at a rate of 5–10°C / min, and then maintaining it at 170–190°C for 8–12 hours.

[0045] Centrifuge at 8000–10000 r / min for 15–30 min.

[0046] The aqueous membrane used for filtration has a pore size of 0.18–0.24 μm; the dialysis bag has a molecular weight cutoff of 400–600 Da, and the dialysis time is 20–28 h; the freeze-drying temperature is -30 to -50 °C, and the drying time is 18–36 h.

[0047] By employing the above technical solutions, this invention uses theaflavins as a precursor to prepare TFs-CQDs via a one-step hydrothermal method. A deionized water to theaflavins ratio of 30–60 mL:1 g ensures an appropriate reaction concentration; programmed temperature control achieves complete carbonization of theaflavins and quantum dot formation; centrifugation removes large particulate impurities; filtration through a 0.18–0.24 μm microporous membrane removes unreacted substances; and dialysis with a 400–600 Da dialysis bag for 20–2 hours removes small molecule impurities. The prepared TFs-CQDs exhibit significantly superior photothermal properties and reactive oxygen species generation capacity compared to tea polyphenol carbon quantum dots (TP-CQDs).

[0048] In some embodiments, theaflavins are prepared by the following method: Tea leaves were pulverized into a fine powder, mixed with distilled water, and laccase was added to react. The reaction was stopped by heating, cooled to room temperature, and extracted with ethyl acetate. The ethyl acetate phase was dried under reduced pressure to obtain theaflavins.

[0049] The fine powder is 100-200 mesh.

[0050] The amount of distilled water should be 15 to 30 times the weight of the tea leaves.

[0051] The ratio of laccase added to tea leaves by mass is (500-800U):1g.

[0052] The reaction temperature for laccase is 40–60℃, the reaction time is 2–3 hours, and the reaction is stopped when the temperature is raised to 90–100℃.

[0053] Cool to room temperature of 25-35℃.

[0054] The amount of ethyl acetate added is 1 / 3 to 1 times the volume of distilled water. The extraction time is 30 to 60 min. The vacuum drying is carried out at 50 to 60 °C, 0.01 to 0.2 Pa, and the drying time is 8 to 16 h.

[0055] The theaflavins of this invention differ from commercially available theaflavins. This invention is based on the molecular genetic effects of theaflavins with specific structures, relying on laccase-catalyzed oxidation to prepare narrow-bandgap carbon quantum dots. By employing the above technical solutions, the efficient preparation of the unique theaflavins of this invention is ensured. Pulverizing tea leaves to 100-200 mesh increases the specific surface area and improves the catalytic efficiency of laccase; using 15-30 times the mass of distilled water ensures a sufficient reaction environment; using 500-800 U / g tea leaves, a reaction temperature of 40-60℃, and a reaction time of 2-3 hours effectively converts tea polyphenols to theaflavins; raising the temperature to 90-100℃ rapidly inactivates laccase and terminates the reaction; ethyl acetate extraction effectively separates the theaflavins.

[0056] A preparation method for preparing a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects includes the following steps: S1. Polyvinyl alcohol is added to deionized water to prepare a solution, citric acid, graphitic carbon nitride, and theaflavin-based carbon quantum dots are added and mixed to form a carbon quantum dot electrostatic spray precursor liquid. S2. The carbon quantum dot electrospinning precursor liquid is electrospun, dried, and then thermally crosslinked to prepare the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0057] The mass fraction of polyvinyl alcohol is 5% to 15%.

[0058] The amount of citric acid added is 5% to 12% of the mass of polyvinyl alcohol.

[0059] The amount of graphitic nitrogen oxide added is 10% to 30% of the mass of theaflavin-based carbon quantum dots.

[0060] The mass-to-volume ratio of theaflavin-based carbon quantum dots to deionized water is 1–5 mg / mL.

[0061] By adopting the above technical solutions, a polyvinyl alcohol mass fraction of 5% to 15% provides a suitable spinning viscosity; a citric acid dosage of 5% to 12% serves as both a crosslinking agent and an esterification reaction catalyst; a graphitic carbon nitride dosage of 10% to 30% can form heterojunctions with CQDs; and a TFs-CQDs concentration of 1% to 5 mg / mL ensures effective functional loading.

[0062] In some embodiments, electrospinning specifically includes using an aluminum plate as a receiver and collecting samples after electrostatic spraying.

[0063] The applied voltage for electrospinning is 16–20 kV, the feed speed is 0.4–0.8 mL / h, and the receiving distance is 10–20 cm.

[0064] By employing the above technical solutions and electrospinning parameters, uniform and fine nanofibers can be obtained. During the electrospinning process, the spatial confinement effect of the high-voltage electrostatic field promotes the close bonding of two-dimensional graphitic carbon nitride and zero-dimensional TFs-CQDs, forming a Z-shaped metal-free photocatalytic heterojunction. This heterojunction structure further accelerates the separation efficiency of photogenerated electron-hole pairs, significantly improves the reactive oxygen species yield, and thus enhances the photodynamic antibacterial effect.

[0065] In some embodiments, the drying temperature is 30–50°C and the drying time is 20–30 h.

[0066] By adopting the above technical solutions, the temperature for thermal esterification crosslinking is 100-140℃, and the treatment time is 1-3 hours.

[0067] By employing the above technical solutions, during the thermo-induced esterification crosslinking process, the polycarboxyl groups of citric acid undergo in-situ esterification and polycondensation reactions with the hydroxyl and / or amino groups on the surface of polyvinyl alcohol and TFs-CQDs, constructing a stable three-dimensional crosslinked network. This network not only solves the problems of high-humidity leakage and stabilization of CQDs, but also restricts the non-radiative transitions of carbon dots through a rigid network, enhancing their initial fluorescence intensity. Furthermore, it induces fluorescence quenching upon contact with volatile amine gases produced by fruit and vegetable spoilage, thus providing an early warning of spoilage.

[0068] Thirdly, the present invention also provides an application of a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial functions in fruit preservation, namely, wrapping fresh fruit with 0.5% to 1.5% of its mass of the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial functions, irradiating it under 808nm near-infrared light for 5 to 15 minutes, and then storing it at room temperature. The preservation film of the present invention has fluorescence response characteristics to changes in the microenvironment. When the fruit is fresh, the preservation film exhibits bright blue fluorescence under ultraviolet light. As the fruit rots and releases volatile gases, the fluorescence quenches and dims.

[0069] Example Example 1: A theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects was prepared by the following method: S1. Grind 1 kg of tea leaves into a fine powder of 100 mesh, add 15 L of distilled water and mix, add 500,000 U of laccase, react at 40℃ for 3 h, then raise the temperature to 90℃ to stop the reaction, cool to 25℃, add 15 L of ethyl acetate and extract for 60 min, dry the ethyl acetate phase at 50℃ and 0.01 Pa for 16 h to obtain 210 g of theaflavins.

[0070] S2. Weigh 100g of theaflavins and dissolve them in 6L of deionized water, then place the solution into a high-temperature reactor. Increase the temperature to 180℃ at a rate of 5℃ / min, maintain at 170℃ for 12 hours, and allow to cool naturally to room temperature. Centrifuge at 8000r / min for 30 minutes. Filter the supernatant using a 0.18μm pore size aqueous phase membrane. Dialyze the filtrate using a dialysis bag with a molecular weight cutoff of 400Da for 28 hours. Freeze-dry the dialyzed solution at -30℃ for 36 hours. 12g of brownish-red carbon quantum dot powder is obtained, which is theaflavin-based (TFs-CQDs) carbon quantum dots.

[0071] S3. Prepare a 5% polyvinyl alcohol aqueous solution by dissolving 5g of polyvinyl alcohol in 100mL of deionized water. Then add 0.75g of citric acid, 30mg of graphitic nitrogen oxides, and 100mg of TFs-CQDs carbon quantum dots to prepare a carbon quantum dot concentration of 1mg / mL for electrostatic spraying. Electrospinning is performed under an applied voltage of 16kV, a propulsion speed of 0.4mL / h, and a receiving distance of 10cm. The spun fibers are collected and vacuum dried at 30℃ for 30h. The spun film is then thermally esterified and cured at 100℃ for 3h to obtain 5g of theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0072] The above-mentioned plastic wrap was used for fruit preservation: 100 g of fresh bananas and 100 g of strawberries were first washed with deionized water, and then the fruit was gently wrapped in a 1 g electrospun film loaded with carbon quantum dots. The wrapped fruit was then irradiated under 808 nm near-infrared light for 15 minutes and stored at room temperature. During storage, the plastic wrap exhibited bright blue fluorescence under UV excitation. After 15 days, as the fruit spoiled and released volatile gases, the fluorescence quenched and dimmed.

[0073] Example 2: A theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects was prepared by the following method: S1. Grind 1 kg of tea leaves into a fine powder of 200 mesh, add 30 L of distilled water and mix, add 800,000 U of laccase, react at 60 °C for 2 h, then raise the temperature to 100 °C to stop the reaction, cool to 35 °C, add 10 L of ethyl acetate and extract for 60 min, dry the ethyl acetate phase at 60 °C and 0.2 Pa for 8 h to obtain 250 g of theaflavins.

[0074] S2. Weigh 100g of theaflavins and dissolve them in 3L of deionized water, then place the solution into a high-temperature reactor. Increase the temperature to 250℃ at a rate of 10℃ / min, maintain at 190℃ for 8 hours, and allow to cool naturally to room temperature. Centrifuge at 10000r / min for 15min, and filter the supernatant using a 0.24μm pore size aqueous phase membrane. Dialyze the filtrate using a dialysis bag with a molecular weight cutoff of 600Da for 20 hours. Freeze-dry the dialyzed solution at -50℃ for 18 hours. 15g of brownish-red carbon quantum dot powder is obtained, which is theaflavin-based (TFs-CQDs) carbon quantum dots.

[0075] S3. Prepare a 15% polyvinyl alcohol aqueous solution by dissolving 15g of polyvinyl alcohol in 100mL of deionized water. Then add 0.75g of citric acid, 30mg of graphitic nitrogen oxides, and 300mg of TFs-CQDs carbon quantum dots to prepare a carbon quantum dot concentration of 3mg / mL for electrostatic spraying. Electrospinning is performed under an applied voltage of 20kV, a propulsion speed of 0.8mL / h, and a receiving distance of 20cm. The spun fibers are collected and vacuum dried at 50℃ for 20h. The spun film is then thermally esterified and cured at 120℃ for 2h to obtain 15g of theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0076] The above-mentioned plastic wrap was used for fruit preservation: 100g of fresh bananas and 100g of strawberries were first washed with deionized water, and then the fruit was gently wrapped in an electrospun film loaded with 3g of theaflavins and carbon quantum dots. The wrapped fruit was then irradiated under 808nm near-infrared light for 5 minutes and stored at room temperature. During storage, the plastic wrap exhibited bright blue fluorescence under UV excitation. After 15 days, as the fruit spoiled and released volatile gases, the fluorescence quenched and dimmed.

[0077] Example 3: A theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects was prepared by the following method: S1. Grind 1 kg of tea leaves into a fine powder of 150 mesh, add 20 L of distilled water and mix, add 600,000 U of laccase, react at 50 °C for 2.5 h, then raise the temperature to 95 °C to stop the reaction, cool to 30 °C, add 10 L of ethyl acetate and extract for 50 min, dry the ethyl acetate phase at 55 °C and 0.1 Pa for 10 h to obtain 220 g of theaflavins.

[0078] S2. Weigh 100g of theaflavins and dissolve them in 4L of deionized water, then place the solution into a high-temperature reactor. Heat the solution to 200℃ at a rate of 6℃ / min, maintain the temperature at 180℃ for 10 hours, and allow it to cool naturally to room temperature. Centrifuge at 9000r / min for 20 minutes. Filter the supernatant through a 0.20μm pore size aqueous phase membrane. Dialyze the filtrate through a dialysis bag with a molecular weight cutoff of 500Da for 24 hours. Freeze-dry the dialyzed solution at -40℃ for 30 hours. 17g of brownish-red carbon quantum dot powder is obtained, which is theaflavin-based (TFs-CQDs) carbon quantum dots.

[0079] S3. Prepare a 10% polyvinyl alcohol aqueous solution by dissolving 10g of polyvinyl alcohol in 100mL of deionized water. Then add 1.0g of citric acid, 40mg of graphitic nitrogen oxides, and 200mg of TFs-CQDs carbon quantum dots to prepare a carbon quantum dot concentration of 2mg / mL for electrostatic spraying. Electrospinning is performed under an applied voltage of 18kV, a propulsion speed of 0.6mL / h, and a receiving distance of 15cm. The spun fibers are collected and vacuum dried at 40℃ for 25h. The spun film is then thermally esterified and cured at 140℃ for 1h to obtain 10g of theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0080] The above-mentioned plastic wrap was used for fruit preservation: 100g of fresh bananas and 100g of strawberries were first washed with deionized water. The fruit was gently wrapped in an electrospun membrane loaded with 2g of theaflavins and carbon quantum dots. Then, the wrapped fruit was irradiated under 808nm near-infrared light for 10 minutes and stored at room temperature. During storage, the plastic wrap exhibited bright blue fluorescence under UV excitation. After 15 days, as the fruit spoiled and released volatile gases, the fluorescence quenched and dimmed.

[0081] Comparative Example Comparative Example 1, a theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects, differs from Example 1 only in that it does not contain theaflavin carbon quantum dots, yielding 4.8g of pure polyvinyl alcohol electrospun film. Fruit preservation results are as follows... Figure 10 As shown, compared to the naturally placed blank control group, it has virtually no preservation effect.

[0082] Comparative Example 2, a theaflavins carbon quantum dot preservation film with both monitoring and antibacterial effects, differs from Example 1 only in that laccase is not added in step S1, yielding 185g of tea polyphenols. Subsequent steps use the tea polyphenol preparation method to produce carbon quantum dots (TP-CQDs), resulting in 4.9g of a tea polyphenol carbon quantum dot electrospun film (TP-CQDs-PVA). Fruit preservation results are as follows... Figure 10As shown, the electrospun film also has a preservation effect, but the theaflavin carbon quantum dot preservation film (TFs-CQDs-PVA), which has both monitoring and antibacterial effects, has the most significant preservation effect. Compared with pure polyvinyl alcohol electrospun film, its preservation time can be extended by 100% under the same mass loss rate.

[0083] Performance testing: 1. The carbon quantum dots of Example 1 and Comparative Example 2 were characterized by transmission electron microscopy using a JEM-2100 transmission electron microscope from JOEL Corporation, Japan. 1A and 1B are the tea polyphenol carbon dots (TP-CQDs) of Comparative Example 2; 1C and 1D are theaflavin carbon dots (TFs-CQDs) of Comparative Example 2.

[0084] Figure 1 Figure (AD) shows the morphology and state of CQDs under transmission electron microscopy (TEM). The CQDs in the figure all exhibit a similar near-spherical morphology with a particle size of less than 10 nm, consistent with the basic appearance of CQDs. Figures (1B) and (1D) show that the monodisperse carbon quantum dot particles all possess lattice fringes, indicating that carbon quantum dots have a certain degree of crystallinity. The lattice spacings are 0.30 nm and 0.63 nm, respectively, indicating that the lattice structures of TP-CQDs and TFs-CQDs are not the same.

[0085] 2. The carbon quantum dots of Example 1 and Comparative Example 2 were characterized by XPS spectra. 2A and 2B are the total XPS measured spectra of TP-CQDs of Comparative Example 2, and 2C and 2D are the high-resolution C1s XPS spectra of TFs-CQDs of Example 1.

[0086] Figure 2 (AD) indicates that the total XPS measured spectra of the two CQDs consist of C1s and O1s peaks, suggesting that the CQDs are composed of both C and O elements. The elemental valence states of the two carbon points were investigated using high-resolution XPS spectroscopy. The spectra showed three bands at approximately 284.8 eV, 286.3 eV, and 288.9 eV, indicating the presence of C=C / CC, CO, and C=O, respectively. This demonstrates that both TP-CQDs and TFs-CQDs are composed of –O–H, C=C, C–C, and C–O, C=O chemical bonds, inheriting to some extent the functional groups of the precursors.

[0087] 3. The carbon quantum dots of Example 1 and Comparative Example 2 were characterized by PL emission spectra. 3A is the UV-Vis absorption spectrum of TP-CQDs and 3C is the TFs-CQDs; 3B is the PL emission spectrum of TP-CQDs and 3D is the TFs-CQDs.

[0088] The UV-Vis absorption spectra of the two CQDs exhibit the same characteristics, such as Figure 3As shown in (A and C), the absorption peak at around 210 nm corresponds to the sp of the carbon nucleus. 2 π-π hybrid structures (C=C bonds) The absorption peak observed at approximately 280 nm is attributed to the n-π transition of the oxygen-containing group (C=O / OH). Transition. Additionally, due to the π-π transition of the carbon nucleus conjugated structure... Electronic transitions cause the CQDs solution to exhibit an absorption band at 500 nm in the UV-Vis absorption spectrum, displaying a continuous, broad absorption range from near-infrared to visible light. However, the excitation-dependent fluorescence spectral characteristics of the two types of carbon quantum dots (CQDs) are significantly different, such as... Figure 3 Images (B and D) show maximum excitation wavelengths of 360 nm and 410 nm, and maximum emission wavelengths of 400 nm and 490 nm, respectively. This indicates that the π-conjugated system of tea polyphenols significantly increases after polymerization into theaflavins, leading to a red shift in fluorescence. Furthermore, calculations revealed relative fluorescence quantum yields of TP-CQDs and TFs-CQDs of 0.99% and 0.62%, respectively. The low fluorescence quantum yields suggest low light energy conversion efficiency, indicating that absorbed photon energy is primarily dissipated through non-radiative transitions rather than released in fluorescent form.

[0089] 4. The carbon quantum dots of Example 1 and Comparative Example 2 were characterized by CV curves. 4A is the CV curve of TP-CQDs and 4B is the CV curve of TFs-CQDs.

[0090] Figure 4 (A and B) are the CV curves of TP-CQDs and TFs-CQDs. It can be seen that the band gaps of TP-CQDs and TFs-CQDs are 1.796 eV and 0.987 eV, respectively, with the band gap gradually decreasing. This means that the higher the degree of conjugation of carbon quantum dots, the lower the band gap. This is because highly graphitized carbon quantum dots have higher crystallinity, and their internal sp... 2 Hybridized carbon atoms form a more complete conjugated system. This structure allows electrons to move more freely within the conjugated system, thus reducing the band gap. Furthermore, a narrower band gap means a lower photon energy threshold, making carbon quantum dots more easily excited in the near-infrared and more effectively converting absorbed light energy into heat energy.

[0091] 5. The carbon quantum dots of Example 1 and Comparative Example 2 were characterized by DCFH-DA degradation under irradiation. 5A is the DCFH-DA diagram of TP-CQDs and 5B is the DCFH-DA diagram of TFs-CQDs under an 808 nm light source.

[0092] The DCFH-DA probe itself is non-fluorescent, and its detection principle is based on the specific oxidation reaction of reactive oxygen species (ROS). After being oxidized by ROS, the probe generates a strongly fluorescent product DCF. The fluorescence intensity is positively correlated with the ROS level. The higher the fluorescence increment rate, the easier it is for the material to generate ROS after photoexcitation.

[0093] from Figure 5 The test results (A and B) show that both CQDs have the ability to generate reactive oxygen species (ROS) under 808nm near-infrared laser irradiation. Under laser irradiation for 0-300s, the fluorescence intensity of TFs-CQDs increased from 40 to 210, an increase of 170, while the fluorescence intensity of TP-CQDs only increased from 60 to 100, an increase of 40. This indicates that the ROS generation capacity of TFs-CQDs is significantly stronger than that of TP-CQDs. The narrower the band gap of carbon quantum dots, the higher the electron-hole pair separation efficiency, thus generating ROS more effectively. When there is a large amount of ROS, it attacks lipid molecules on the bacterial cell membrane, leading to membrane structure damage; it also damages bacterial DNA and proteins, interfering with their normal metabolic functions, ultimately causing bacterial death. Therefore, a stronger ROS generation capacity shows greater application potential in photo-assisted sterilization.

[0094] 6. Carbon quantum dots from Example 1 and Comparative Example 2 were subjected to near-infrared laser (808 nm, 2 W / cm²) testing. 2 Temperature variation curves over time for 6A (TP-CQDs) and 6B (TFs-CQDs) of different concentrations under irradiation.

[0095] Using water as a control, CQDs were irradiated with near-infrared (NIR) light at a wavelength of 808 nm to investigate their NIR-responsive photothermal properties. Under constant laser power irradiation (2 W / cm²), the NIR response photothermal properties were investigated. 2 Under different concentrations (0.5 to 2.0 mg / mL), the temperature change over time for each CQD was recorded.

[0096] Figure 6 (A and B) show the effect of near-infrared laser (808 nm, 2 W / cm²). 2 The temperature rise curves of different concentrations of CQDs under irradiation change over time. The CQD concentration is positively correlated with the system temperature; an increase in the concentration gradient significantly increases the temperature rise. This is because higher concentrations of CQDs solution result in higher near-infrared absorption at 808 nm, leading to higher converted heat energy. Compared to TP-CQDs, TFs-CQDs show a more significant temperature rise, reaching 68℃ after 10 minutes of irradiation at 2 mg / mL, while TP-CQDs only reach 57.5℃. This indicates that the thermal effect of TFs-CQDs is more significant.

[0097] 7. The bactericidal effects of carbon quantum dots in Example 1 and Comparative Example 2 were characterized by using 500 μg / mL TP-CQDs and TFs-CQDs against Staphylococcus aureus and Escherichia coli under NIR irradiation and without NIR irradiation.

[0098] The synergistic antibacterial effect of CQDs on Gram-positive bacteria (Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli) in the near-infrared light (808 nm, 2 W / cm²) irradiation group and the dark treatment group was quantitatively evaluated by plate counting method, revealing its triple action mechanism of chemical, photodynamic and photothermal antibacterial action.

[0099] The results are as follows Figure 7 As shown, without the addition of CQDs, the near-infrared laser irradiation group (808nm, 2W / cm²) 2 There was no significant difference in colony survival rate between the group exposed to 808nm near-infrared laser (15 min) and the group exposed to darkness, indicating that simple 808nm near-infrared laser irradiation did not produce an effective antibacterial effect. However, under laser-free conditions, the antibacterial activity of CQDs against Staphylococcus aureus corresponded to the results of previous inhibition zone experiments, confirming its inherent chemical antibacterial effect. Under 808nm laser (2W / cm²) conditions, the antibacterial activity of CQDs against Staphylococcus aureus was [not specified in the original text]. 2 Under activation (15 min), the synergistic antibacterial efficacy of CQDs against Staphylococcus aureus was significantly enhanced. In particular, with the increase of conjugation degree, the photothermal conversion efficiency and ROS yield of CQDs were higher, thus exhibiting a better synergistic antibacterial effect.

[0100] 8. The polyvinyl alcohol electrospun fiber film prepared in Comparative Example 1 and the theaflavin carbon quantum dot preservation film prepared in Example 1 were characterized by electron microscopy.

[0101] Figure 8-9 The figures show the polyvinyl alcohol electrospun fiber membrane prepared in Comparative Example 1 and the theaflavin carbon quantum dot preservation film (TFs-CQDs-PVA membrane) prepared in Example 1, which combines monitoring and antibacterial functions. As can be seen from the figures, the polyvinyl alcohol nanofiber membrane appears shriveled and thin. In contrast, the theaflavin carbon quantum dot preservation film, which combines monitoring and antibacterial functions, has thicker and fuller fiber strips. This is because the composite of CQDs, graphitic carbon nitride, citric acid, and polyvinyl alcohol utilizes the spatial confinement effect of a high-voltage electrostatic field to promote the tight adhesion of two-dimensional graphitic carbon nitride and zero-dimensional TFs-CQDs. Subsequent high-temperature treatment causes in-situ esterification and polycondensation reactions between the polycarboxyl groups of citric acid and the hydroxyl / amino groups on the surface of polyvinyl alcohol and CQDs, constructing a stable three-dimensional cross-linked network. This membrane achieves molecular-level uniform fixation of carbon quantum dots in nanofibers. While preventing leakage and aggregation of carbon quantum dots, it ensures functional durability and safety. Simultaneously, the carbon dots also endow the electrospun fibers with better toughness and extensibility.

[0102] 9. Characterization of preservation effect: The preservation films prepared in Example 1 and Comparative Examples 1-2 were applied to the photodynamic antibacterial fruit preservation effect and the fluorescence spectra of fruits (bananas) with different freshness levels.

[0103] The rotting of bananas and strawberries during storage causes a decline in their quality, and changes in quality effectively reflect their freshness. Figure 10 It can be seen that the polyvinyl alcohol electrospun preservation film prepared in Comparative Example 1, without the addition of any carbon quantum dots, has almost no preservation effect. The mass decay rate of bananas and strawberries in this group is close to that of the untreated control group. Under the premise that other conditions remain unchanged, the TP-CQDs-PVA electrospun film prepared by adding tea polyphenol carbon quantum dots in Comparative Example 2 has a certain preservation effect on bananas and strawberries, and the mass decay rate is effectively reduced. The TP-CQDs-PVA electrospun film prepared with added tea polyphenol carbon quantum dots has a mass decay rate of 6 days for strawberries, which is 42.8% longer than the 4.2 days of the control group, and the TP-CQDs-PVA film prepared with added tea polyphenol carbon quantum dots has a mass decay rate of 5.8 days for bananas, which is 41.5% longer than the 4.1 days of the control group. In contrast, the theaflavins carbon quantum dot preservation film (TFs-CQDs-PVA film) with both monitoring and antibacterial effects has the most significant preservation effect. The TP-CQDs-PVA film prepared with added tea polyphenol carbon quantum dots has a mass decay rate of 9 days for strawberries, which is 114.3% longer than the 4.2 days of the control group, and the TP-CQDs-PVA film prepared with added tea polyphenol carbon quantum dots has a mass decay rate of 9 days for bananas, which is 119.5% longer than the 4.1 days of the control group. This is because the conjugation degree of theaflavins' carbon quantum dots is higher than that of tea polyphenols, resulting in a better photothermal synergistic bactericidal effect and a more significant effect on fruit preservation.

[0104] Strawberries and bananas coated with TFs-CQDs-PVA membrane were observed after 5, 10, and 15 days at room temperature. No obvious changes were observed in their appearance. However, under UV light, the TFs-CQDs-PVA membrane gradually changed from its original bright blue fluorescence to a dark gray. Figure 11 As can be seen, as bananas rot, amine gases in the fruit specifically bind to the functional groups such as carboxyl groups on the carbon dots, resulting in a significant decrease in the fluorescence intensity of the carbon dots. Simultaneously, as shown in the illustration, with the decrease in fluorescence intensity, the electrospun fiber membrane loaded with carbon quantum dots changes from its original bright blue fluorescence to dark gray, achieving effective monitoring of fruit freshness visible to the naked eye.

[0105] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects, characterized in that, It includes a polyvinyl alcohol nanofiber matrix, and theaflavin-based carbon quantum dots, citric acid and graphitic carbon nitride dispersed in the matrix; The theaflavin-based carbon quantum dots are embedded in situ through electrospinning and then fixed in polyvinyl alcohol nanofibers by thermal esterification crosslinking to form a three-dimensional crosslinked network structure.

2. The theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, The theaflavin-based carbon quantum dots were prepared by the following method: Theaflavins were dissolved in deionized water and then placed in a sealed reaction vessel. The reaction was carried out under programmed temperature rise, followed by natural cooling and centrifugation. The supernatant was filtered through a microporous membrane, and the filtrate was dialyzed and then freeze-dried to obtain theaflavin-based carbon quantum dots.

3. The theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 2, characterized in that, The theaflavins were prepared by the following method: Tea leaves were pulverized into a fine powder, mixed with distilled water, and laccase was added to react. The reaction was stopped by heating, cooled to room temperature, and extracted with ethyl acetate. The ethyl acetate phase was dried under reduced pressure to obtain theaflavins.

4. The theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 3, characterized in that, The distilled water is 15 to 30 times the weight of the tea leaves; The ratio of the amount of laccase added to the mass of tea leaves is (500-800U):1g; The laccase reaction temperature is 40-60℃, the reaction time is 2-3h, and the reaction is terminated when the temperature is raised to 90-100℃. The programmed heating process involves raising the temperature to 180–250°C at a rate of 5–10°C / min, and then maintaining it at 170–190°C for 8–12 hours.

5. The theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, The polyvinyl alcohol has a mass fraction of 5% to 15%; The amount of citric acid added is 5% to 12% of the mass of polyvinyl alcohol; The amount of graphitic nitrogen oxide added is 10% to 30% of the mass of theaflavin-based carbon quantum dots.

6. A preparation method for preparing the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects as described in any one of claims 1 to 5, characterized in that, Includes the following steps: S1. Polyvinyl alcohol is added to deionized water to prepare a solution, citric acid, graphitic carbon nitride, and theaflavin-based carbon quantum dots are added and mixed to form a carbon quantum dot electrostatic spray precursor liquid. S2. The carbon quantum dot electrospinning precursor liquid is electrospun, dried, and then thermally crosslinked to prepare the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects.

7. The preparation method according to claim 6, characterized in that, The mass-to-volume ratio of the theaflavin-based carbon quantum dots to deionized water is 1–5 mg / mL.

8. The preparation method according to claim 6, characterized in that, The electrospinning specifically involves using an aluminum plate as a receiver and collecting samples after electrostatic spraying. The electrospinning process involves applying a voltage of 16–20 kV, a feed rate of 0.4–0.8 mL / h, and a receiving distance of 10–20 cm.

9. The preparation method according to claim 6, characterized in that, The drying temperature is 30–50°C, and the drying time is 20–30 hours. The temperature for the thermally induced esterification crosslinking is 100–140°C, and the treatment time is 1–3 h.

10. The application of the theaflavin carbon quantum dot preservation film with both monitoring and antibacterial effects as described in any one of claims 1 to 5 in fruit preservation, characterized in that, Wrap fresh fruit in a theaflavins carbon quantum dot preservation film containing 0.5% to 1.5% of its weight, which has both monitoring and antibacterial effects, and irradiate it under 808nm near-infrared light for 5 to 15 minutes, then store it at room temperature. The plastic wrap has fluorescence response characteristics to changes in the microenvironment. When the fruit is fresh, the plastic wrap exhibits bright blue fluorescence under ultraviolet light. As the fruit rots and releases volatile gases, the fluorescence quenches and dims.

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