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

Abstract

The application discloses a theabrownin carbon quantum dot preservative film with monitoring and antibacterial effects, and a preparation method and application thereof. The preparation method comprises the following steps: (1) theabrownin preparation, (2) carbon quantum dot preparation, (3) theabrownin carbon quantum dot, citric acid and graphite phase carbon nitride composite electrospinning nanofiber film preparation and heat-induced esterification curing. The prepared carbon quantum dots are closely combined with graphite phase carbon nitride through an electrostatic field, and the antibacterial efficiency is synergistically improved, the photo-thermal performance is significantly better than that of theabrownin carbon quantum dots, and active oxygen ability is 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 preservation materials, and in particular to a tea brown carbon quantum dot preservation film with both monitoring and antibacterial functions, its preparation method, and its application. Background Technology

[0002] Fruits, rich in nutrients and with high water content, are highly susceptible to spoilage, making preservation a crucial aspect of fruit storage. While traditional low-temperature refrigeration and gas packaging technologies can achieve fruit preservation 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, safe and efficient preservation materials have been a hot research topic.

[0003] Natural antibacterial agents such as tea polyphenols suffer from problems such as easy decomposition, narrow antibacterial spectrum, and poor efficacy, severely limiting their application in fruit preservation. Recent studies have found that 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 solutions. However, applying CQDs to food preservation, especially as antibacterial active ingredients integrated into packaging materials, still faces challenges such as a lack of synergistic effects, low antibacterial efficiency, poor dispersibility and stability, and difficulty in recycling. Furthermore, they cannot achieve food freshness monitoring functions beyond antibacterial properties. Summary of the Invention

[0004] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the purpose of this invention is to provide a theabrownin carbon quantum dot preservation film with both monitoring and antibacterial functions, as well as its preparation method and application. The theabrownin carbon quantum dot preservation film of this invention not only achieves efficient sterilization against Escherichia coli and Staphylococcus aureus, but also can intuitively and non-destructively monitor the freshness of fruits through the quenching and darkening phenomenon of fluorescence under ultraviolet light.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] This invention provides a method for preparing a theabrownin-carbon quantum dot preservation film with both monitoring and antibacterial effects, comprising the following steps:

[0007] (1) Preparation of theabrownin: Tea leaves are crushed into fine powder, mixed with distilled water, laccase is added to react, the reaction is stopped by heating, cooled to room temperature, ethanol is added to precipitate, centrifuged, and the supernatant is dried under reduced pressure to obtain theabrownin;

[0008] (2) Add theabrownin to the deionized water solution, disperse it by ultrasonication, place it in a sealed reaction vessel, heat it to react, cool it naturally to room temperature, centrifuge it, filter the supernatant through a microporous membrane, dialyze the filtrate and freeze dry it to obtain theabrownin-based carbon quantum dots, namely TBs-CQDs carbon quantum dots;

[0009] (3) Preparation of electrospun nanofiber membrane: Polyvinyl alcohol was prepared into a polyvinyl alcohol solution with deionized water, and citric acid, graphitic carbon nitride and TBs-CQDs carbon quantum dots were added and mixed to form a carbon quantum dot electrostatic spraying precursor liquid. After electrostatic spraying, the sample was collected, vacuum dried and thermally esterified to obtain a theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0010] Preferably, in step (1):

[0011] Tea leaves are pulverized into a fine powder of 100-200 mesh. Distilled water is added at a ratio of 15-30 times the mass of tea leaves. The mass ratio of laccase to tea leaves is 1000-2000 U: 1 g. The reaction temperature is 40-60℃, and the reaction time is 4-8 h. The reaction is stopped by heating to 90-100℃ and then cooled to 25-35℃. The amount of ethanol added is 3-4 times the volume of distilled water. The centrifugation speed is 4000-6000 rpm, and the centrifugation time is 15-30 min. The vacuum drying is carried out at 50-60℃, 0.01-0.2 Pa, and the drying time is 8-16 h.

[0012] Preferably, in step (2):

[0013] The volume-to-mass ratio of deionized water to theaflavins was 30-60 mL: 1 g; the ultrasonic power was 1500-2500 W, and the ultrasonic time was 10-30 min; the temperature was heated to 170-190℃ and held for 8-12 h; the centrifugation speed was 8000-12000 r / min, and the centrifugation time was 15-30 min; the pore size of the aqueous phase membrane used for filtration was 0.18-0.24 μm; the molecular weight cutoff of the dialysis bag was 10-100 kDa, and the dialysis time was 20-28 h; the freeze-drying temperature was -30 to -50℃, and the drying time was 18-36 h.

[0014] Preferably, in step (3), the mass fraction of polyvinyl alcohol is 5-15%, the amount of citric acid added is 5-15% of the mass of polyvinyl alcohol, the amount of graphitic carbon nitride added is 10-30% of the mass of TBs-CQDs, and the mass-volume ratio of TBs-CQDs carbon quantum dots to deionized water is 1-5 mg / mL; 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; the vacuum drying temperature is 30-50℃, and the drying time is 20-30 h; the temperature for thermal esterification curing is 100-140℃, and the treatment time is 1-3 h.

[0015] The present invention also provides a theabrownin carbon quantum dot preservation film with both monitoring and antibacterial functions, which is prepared by the aforementioned method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial functions.

[0016] The present invention also provides the application of the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects in fruit preservation.

[0017] Specifically, the application method is as follows: wrap fresh fruit with a theabrownin carbon quantum dot preservation film containing 0.1-0.3% of its mass, which has both monitoring and antibacterial effects, irradiate it under 808 nm near-infrared light for 5-15 minutes, and then store it at room temperature;

[0018] When observed under ultraviolet light, the plastic wrap shows a bright blue fluorescence when the fruit is fresh. As the fruit rots and releases volatile gases, the fluorescence quenches and dims.

[0019] The principle of this invention: The degree of conjugation of the precursor can be directly "inherited" and determines the band structure of the synthesized CQDs, 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, this invention uses theaflavins as a precursor to prepare TBs-CQDs via a one-step hydrothermal method. Further, using electrospinning technology, TB-CQDs, graphitic carbon nitride, food-grade citric acid, and polyvinyl alcohol are composited. The spatial confinement effect of a high-voltage electrostatic field promotes the tight binding of two-dimensional graphitic carbon nitride with zero-dimensional TBs-CQDs, further accelerating the separation of photogenerated electron-hole pairs. Simultaneously, subsequent high-temperature treatment causes 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 TB-CQDs, constructing a stable three-dimensional cross-linked network. This film not only solves the problems of high-humidity leakage and stabilization of TB-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 triple synergistic antibacterial mechanism of "chemical-photodynamic-photothermal" 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 (ammonia, putrescine, cadaverine, etc.), 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.

[0020] Using 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. This results in a more complete sp² hybrid carbon network and narrower band gap in the carbon quantum dots (TBs-CQDs). These structural characteristics enhance near-infrared light absorption, promote efficient separation of photogenerated carriers, generate reactive oxygen species (ROS) through electronic transitions, and improve photodynamic effects. 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.

[0021] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0022] (1) The carbon quantum dots prepared by the present invention inhibit bacteria through a chemical-photodynamic-photothermal synergistic mechanism, and significantly improve the ROS yield under the combined action of two-dimensional graphitic carbon nitride and zero-dimensional TBs-CQDs, achieving highly efficient sterilization of Escherichia coli and Staphylococcus aureus with an inhibition rate of up to 99%.

[0023] (2) The electrospun antibacterial nanofiber membrane prepared by this invention has excellent density, film-forming properties, and extensibility. Furthermore, 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 spoilage and deterioration of the fruit.

[0024] (3) The preservation film prepared by the present invention has the function of microenvironment self-response intelligent indication. It can directly and non-destructively monitor the freshness of fruit by observing the quenching and darkening of fluorescence under ultraviolet light.

[0025] (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

[0026] Figure 1 Transmission electron micrographs of tea polyphenol carbon dots (TP-CQDs) (AB) and theabrownin carbon dots (TBs-CQDs) (CD).

[0027] Figure 2 The total XPS measurement spectrum and high-resolution C1s XPS spectrum of TP-CQDs (AB) and TBs-CQDs (CD) are shown.

[0028] Figure 3The UV-Vis absorption spectra of TP-CQDs(A) and TBs-CQDs(C); the PL emission spectra of TP-CQDs(B) and TBs-CQDs(D).

[0029] Figure 4 CV curves for TP-CQDs (A) and TBs-CQDs (B).

[0030] Figure 5 The graph shows the ROS degradation of DCFH-DA generated by TP-CQDs (A) and TBs-CQDs (B) under an 808 nm light source.

[0031] Figure 6 To use near-infrared laser (808 nm, 2 W / cm) 2 Temperature variation curves over time for TP-CQDs(A) and TBs-CQDs(B) at different concentrations under irradiation.

[0032] Figure 7 The bactericidal effects of TP-CQDs (A) and TBs-CQDs (B) at 500 μg / mL on Staphylococcus aureus and Escherichia coli under NIR irradiation and without NIR irradiation were evaluated.

[0033] Figure 8 This is an electron microscope image of a polyvinyl alcohol electrospun nanofiber membrane.

[0034] Figure 9 Electron micrograph of a theabrownin-based carbon quantum dot preservation film that combines monitoring and antibacterial properties.

[0035] Figure 10 A comparison of the effects of different fiber membranes on fruit preservation. Weight loss rates of strawberries (A) and bananas (B).

[0036] Figure 11 Photofluorescence spectra and corresponding fluorescence images of fruits at different freshness levels using a theabrownin-based carbon quantum dot preservation film that combines monitoring and antibacterial functions. Detailed Implementation

[0037] The present invention is further described below through specific embodiments, but the scope of protection of the present invention is not limited thereto.

[0038] Example 1

[0039] (1) 1 kg of tea leaves were crushed into 100 mesh fine powder, mixed with 15 L of distilled water, and 1 million U of laccase was added. The mixture was reacted at 40 °C for 8 h, and then the temperature was raised to 90 °C to end the reaction. The mixture was cooled to 25 °C, 60 L of ethanol was added, and the mixture was centrifuged at 6000 rpm for 15 min. The supernatant was dried at 50 °C and 0.01 Pa for 16 h to obtain 360 g of theabrownin.

[0040] (2) Weigh 100 g of theaflavins, dissolve them in 6 L of deionized water, sonicate at 1500 W for 30 min, and then place them in a high-temperature reactor. Heat to 170 °C and maintain for 12 h, then cool naturally to room temperature; centrifuge at 8000 r / min for 30 min, and filter the supernatant using a 0.18 μm pore size aqueous phase membrane; dialyze the filtrate using a dialysis bag with a molecular weight cutoff of 10 kDa for 28 h; freeze-dry the dialyzed solution at -30 °C for 36 h. 27 g of brown carbon quantum dot powder was obtained, which is theaflavins-based (TBs-CQDs) carbon quantum dots.

[0041] (3) Prepare a 5% polyvinyl alcohol aqueous solution by mixing 5g of polyvinyl alcohol with 100mL of deionized water. Then add 0.75g of citric acid, 30mg of graphitic carbon nitride, and 100mg of TBs-CQDs carbon quantum dots to prepare a carbon quantum dot concentration of 1mg / mL for electrostatic spraying. Apply a voltage of 16 kV, a propulsion speed of 0.4 mL / h, and a receiving distance of 10cm for electrospinning. Collect the spun fibers and vacuum dry them at 30℃ for 30 h. Then heat-cur the spun film at 100℃ for 3 h to obtain 4.6g of tea brown carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0042] 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 each fruit was gently wrapped with 0.1 g of a theaflavin carbon quantum dot plastic wrap that has both monitoring and antibacterial properties. The wrapped fruit was then irradiated under an 808 nm lamp for 15 min and stored at room temperature. During storage, the plastic wrap exhibited bright blue fluorescence under ultraviolet light. After 15 days, as the fruit spoiled and released volatile gases, the fluorescence quenched and dimmed.

[0043] Example 2

[0044] (1) 1 kg of tea leaves were crushed into fine powder of 200 mesh, mixed with 30 L of distilled water, and 2 million U of laccase was added. The mixture was reacted at 60 °C for 4 h, and then the temperature was raised to 100 °C to end the reaction. The mixture was cooled to 35 °C, 90 L of ethanol was added, and the mixture was centrifuged at 4000 rpm for 30 min. The supernatant was dried at 60 °C and 0.2 Pa for 8 h to obtain 385 g of theabrownin.

[0045] (2) Weigh 100 g of theaflavins, dissolve them in 3 L of deionized water, sonicate at 2500 W for 10 min, and then place them in a high-temperature reactor. Heat to 190 °C and maintain for 8 h, then cool naturally to room temperature; centrifuge at 12000 r / min for 15 min, and filter the supernatant using a 0.24 μm pore size aqueous phase membrane; dialyze the filtrate using a dialysis bag with a molecular weight cutoff of 100 kDa for 20 h; freeze-dry the dialyzed solution at -50 °C for 18 h. 22 g of brown carbon quantum dot powder is obtained, which is theaflavins-based (TBs-CQDs) carbon quantum dots.

[0046] (3) 15g of polyvinyl alcohol was prepared into a 15% polyvinyl alcohol aqueous solution in 100mL of deionized water. Then, 0.75g of citric acid, 30mg of graphitic carbon nitride, and 300mg of TBs-CQDs carbon quantum dots were added to prepare a carbon quantum dot concentration of 3mg / mL electrostatic spray precursor fluid. Electrospinning was performed with an applied voltage of 20kV, a propulsion speed of 0.8mL / h, and a receiving distance of 20cm. The spun fibers were collected and vacuum dried at 50℃ for 20h. The spun film was then thermally esterified and cured at 120℃ for 2h to obtain 14.7g of tea brown carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0047] 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 each fruit was gently wrapped with 0.3 g of a theaflavin carbon quantum dot plastic wrap that has both monitoring and antibacterial properties. The wrapped fruit was then irradiated under 808 nm near-infrared light for 5 min 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.

[0048] Example 3

[0049] (1) 1 kg of tea leaves were crushed into fine powder of 150 mesh, mixed with 20 L of distilled water, and 1.5 million U of laccase was added. The mixture was reacted at 50 °C for 6 h, and then the temperature was raised to 95 °C to end the reaction. The mixture was cooled to 30 °C, 70 L of ethanol was added, and the mixture was centrifuged at 5000 rpm for 20 min. The supernatant was dried at 55 °C and 0.1 Pa for 10 h to obtain 370 g of theabrownin.

[0050] (2) Weigh 100 g of theaflavins, dissolve them in 4 L of deionized water, sonicate at 2000 W for 20 min, and then place them in a high-temperature reactor. Heat to 180℃ and maintain for 10 h, then cool naturally to room temperature; centrifuge at 10000 r / min for 20 min, and filter the supernatant using a 0.20 μm pore size aqueous phase membrane; dialyze the filtrate using a dialysis bag with a molecular weight cutoff of 50 kDa for 24 h; freeze-dry the dialyzed solution at -40℃ for 30 h. 25 g of brown carbon quantum dot powder is obtained, which is theaflavins-based (TBs-CQDs) carbon quantum dots.

[0051] (3) Prepare a 10% polyvinyl alcohol aqueous solution by mixing 10g of polyvinyl alcohol with 100mL of deionized water. Then add 1.0g of citric acid, 40mg of graphitic carbon nitride, and 200mg of TBs-CQDs carbon quantum dots to prepare a carbon quantum dot concentration of 2mg / mL for electrostatic spraying. Electrospinning is performed with 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 9.6g of tea brown carbon quantum dot preservation film with both monitoring and antibacterial effects.

[0052] 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. Each fruit was then gently wrapped with 0.2 g of a theaflavin-containing carbon quantum dot plastic wrap, which has both monitoring and antibacterial properties. The wrapped fruit was then irradiated under 808 nm near-infrared light for 10 min 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.

[0053] Comparative Example 1

[0054] Compared to Example 1, with other conditions unchanged, the prepared electrospun membrane did not contain added tea brown carbon quantum dots, citric acid, or graphitic carbon nitride, yielding 4.5 g of pure polyvinyl alcohol electrospun membrane (PVA). Fruit preservation results are as follows... Figure 10 As shown, compared to the naturally placed blank control group, it has virtually no preservation effect.

[0055] Comparative Example 2

[0056] Compared to Example 1, other conditions remained unchanged, but laccase was not added in step (1), resulting in 174g of tea polyphenols. Subsequent steps used the tea polyphenol preparation method with carbon quantum dots (TP-CQDs) to obtain 4.6g of tea polyphenol carbon quantum dot electrospun film (TP-CQDs / C3N4 / PVA). Fruit preservation results are as follows... Figure 10As shown, the electrospun film also has a preservation effect, but the tea brown carbon quantum dot preservation film (TBs-CQDs / C3N4 / 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.

[0057] Comparative Example 3

[0058] Compared to Example 1, other conditions remained unchanged, but citric acid and graphitic carbon nitride were not added in step (3), resulting in 4.4 g of TBs-CQDs electrospun film (TBs-CQDs / PVA). Its fruit preservation effect was similar to that of Comparative Example 2.

[0059] Comparative Example 4

[0060] Compared to Example 1, other conditions remained unchanged, but TBs-CQDs carbon quantum dots were not added in step (3), resulting in 4.2g of graphitic carbon nitride electrospun film (C3N4 / PVA), which had a poor fruit preservation effect.

[0061] Material performance testing:

[0062] The embodiments of the present invention are characterized and analyzed. The following are the characterization results and specific analyses of the embodiments and comparative examples.

[0063] (I) Transmission electron micrographs of tea polyphenol carbon dots (TP-CQDs) and theabrownin carbon dots (TBs-CQDs).

[0064] The morphology of carbon quantum dots was characterized using a JEM-2100 transmission electron microscope from JOEL Corporation, Japan.

[0065] Figure 1 Figures (AD) show 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 (B) and (D) 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 and 0.22 nm, respectively, indicating that the lattice structures of TP-CQDs and TBs-CQDs are not the same.

[0066] (II) Total XPS Spectra and High-Resolution C1s XPS Spectra of TP-CQDs and TBs-CQDs

[0067] Figure 2The image (AD) shows that the total XPS measured spectra of the two CQDs consist of C1s and O1s peaks, indicating 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, 286.3, and 288.9 eV, indicating the presence of C=C / CC, CO, and C=O, respectively. This suggests that both TP-CQDs and TBs-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.

[0068] (III) UV-Vis absorption spectra of TP-CQDs and TBs-CQDs; PL emission spectra of TP-CQDs and TBs-CQDs.

[0069] The UV-Vis absorption spectra of the two CQDs exhibit the same characteristics, such as Figure 3 As shown in (A) and (C), the absorption peak at around 210 nm corresponds to the π-π bond in the sp² hybrid structure of the carbon nucleus (C=C bond). ∗ 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, with continuous absorption across a wide 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 emission wavelengths of 400 and 470 nm, respectively, indicating that the π-conjugated system of tea polyphenols significantly increases after polymerization into theaflavins, leading to a redshift in fluorescence. Furthermore, calculations revealed relative fluorescence quantum yields of TP-CQDs and TBs-CQDs of 0.99% and 0.62%, respectively. These low 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.

[0070] (iv) CV curves of TP-CQDs and TBs-CQDs

[0071] Figure 4In Figures (A) and (B), the CV curves of TP-CQDs and TBs-CQDs are shown. The band gaps of TP-CQDs and TBs-CQDs are 1.796 eV and 0.987 eV, respectively, showing a gradual decrease in band gap. This indicates that carbon quantum dots with higher conjugation levels have lower band gaps. This is because highly graphitized carbon quantum dots have higher crystallinity, resulting in a more complete conjugated system formed by sp² hybridized carbon atoms. This structure allows electrons to move more freely within the conjugated system, thus reducing the band gap. Furthermore, a narrower band gap implies a lower photon energy threshold, making carbon quantum dots more susceptible to near-infrared excitation and more effectively converting absorbed light energy into heat energy.

[0072] (V) Irradiation degradation of TP-CQDs and TBs-CQDs under an 808 nm light source in DCFH-DA diagram

[0073] 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.

[0074] from Figure 5 The test results in (A) and (B) show that both CQDs have the ability to generate reactive oxygen species (ROS) under 808 nm near-infrared laser irradiation. Under laser irradiation from 0 to 300 s, the fluorescence intensity of TBs-CQDs increased from 80 to 1050, an increase of 970, while the fluorescence intensity of TP-CQDs only increased from 180 to 290, an increase of 110, indicating that the ROS generation capacity of TBs-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.

[0075] (vi) In near-infrared laser (808 nm, 2 W / cm) 2 Temperature variation curves over time for different concentrations of (A) TP-CQDs and (B) TBs-CQDs under irradiation.

[0076] 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. 2Under different concentrations (0.5 to 2.0 mg / mL), the temperature change over time for each CQD was recorded.

[0077] Figure 6 Images (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, TBs-CQDs show a more significant temperature rise, reaching 78℃ after 10 minutes of irradiation at 2 mg / mL, while TP-CQDs only reach 57.5℃. This indicates that the thermal effect of TBs-CQDs is more significant. Because it is a localized thermal effect targeting bacteria, controlling the irradiation time will not affect fruit quality during use.

[0078] (vii) The bactericidal effects of 500 μg / mL TP-CQDs and TBs-CQDs on Staphylococcus aureus and Escherichia coli under NIR irradiation and without NIR irradiation.

[0079] 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.

[0080] The results are as follows Figure 7 As shown, without the addition of CQDs, there was no significant difference in colony survival rate between the near-infrared laser irradiation group (808 nm, 2 W / cm², 15 min) and the dark group, indicating that simple 808 nm 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 their inherent chemical antibacterial effect. Under 808 nm laser activation (2 W / cm², 15 min), the synergistic antibacterial efficacy of CQDs against Staphylococcus aureus was significantly enhanced. In particular, with increasing conjugation, the photothermal conversion efficiency and ROS yield of CQDs increased, thus exhibiting a superior synergistic antibacterial effect.

[0081] (VIII) Electron micrographs of electrospun nanofiber membranes

[0082] Figure 8-9The figures show the polyvinyl alcohol electrospun fiber membrane prepared in Comparative Example 1 and the tea brown carbon quantum dot preservation film (TBs-CQDs / C3N4 / 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 tea brown 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 TBs-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 the 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.

[0083] (ix) The effect of electrospun nanofiber membranes on photodynamic antibacterial fruit preservation and their fluorescence spectra on fruits (bananas) at different freshness levels.

[0084] 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 PVA 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 / C3N4 / PVA 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. Its 28% mass loss rate for strawberries is 6 days, which is 39.5% longer than the 4.3 days of the control group, and its 9% mass loss rate for bananas is 5.5 days, which is 37.5% longer than the 4 days of the control group. The TBs-CQDs film in Comparative Example 3 has a similar effect to Comparative Example 2, while the C3N4 / PVA film in Comparative Example 4 has a poor preservation effect. Its 28% mass loss rate for strawberries is 5 days, which is 16.3% longer than the 4.3 days of the control group, and its 9% mass loss rate for bananas is 4.5 days, which is 12.5% ​​longer than the 4 days of the control group. In comparison, the theabrownin carbon quantum dot preservation film (TBs-CQDs / C3N4 / PVA), which combines monitoring and antibacterial effects, exhibited the most significant preservation effect. For strawberries, the preservation time was 9 days with a 28% weight loss, a 109.3% extension compared to the control group's 4.3 days. For bananas, the preservation time was 9 days with a 9% weight loss, a 125% extension compared to the control group's 4 days. This is because the theabrownin carbon quantum dots have a higher degree of conjugation than tea polyphenols and exhibit synergistic effects with citric acid and graphitic carbon nitride, resulting in better photothermal synergistic bactericidal effects and a more pronounced preservation effect on fruits.

[0085] Strawberries and bananas coated with TBs-CQDs / C3N4 / PVA membranes were observed after being left at room temperature for 5, 10, and 15 days. No obvious changes were observed in their appearance. However, when placed under a UV lamp, the TBs-CQDs / C3N4 / PVA membrane gradually changed from its original bright blue fluorescence to dark gray. Figure 11 As can be seen, as bananas rot, amine gases in the fruit specifically bind to functional groups such as carboxyl groups on the surface of carbon dots, resulting in a significant decrease in the fluorescence intensity of the carbon dots. Furthermore, as shown in the inset, the decrease in fluorescence intensity enables effective monitoring of fruit freshness that is visible to the naked eye.

[0086] Those skilled in the art will readily understand that the above description is merely an embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a theabrownin-carbon quantum dot preservation film with both monitoring and antibacterial effects, characterized in that, Includes the following steps: (1) Preparation of theabrownin: Tea leaves are crushed into fine powder, mixed with distilled water, laccase is added to react, the reaction is stopped by heating, cooled to room temperature, ethanol is added to precipitate, centrifuged, and the supernatant is dried under reduced pressure to obtain theabrownin; (2) Add theabrownin to the deionized water solution, disperse it by ultrasonication, place it in a sealed reaction vessel, heat it to react, cool it naturally to room temperature, centrifuge it, filter the supernatant through a microporous membrane, dialyze the filtrate and freeze dry it to obtain theabrownin-based carbon quantum dots, namely TBs-CQDs carbon quantum dots; (3) Preparation of electrospun nanofiber membrane: Polyvinyl alcohol was prepared into a polyvinyl alcohol solution with deionized water, and citric acid, graphitic carbon nitride and TBs-CQDs carbon quantum dots were added and mixed to form a carbon quantum dot electrostatic spraying precursor liquid. After electrostatic spraying, the sample was collected, vacuum dried and thermally esterified to obtain a tea brown carbon quantum dot preservation film with both monitoring and antibacterial effects.

2. The method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, In step (3), the mass fraction of polyvinyl alcohol is 5-15%, the amount of citric acid added is 5-15% of the mass of polyvinyl alcohol; the amount of graphitic carbon nitride added is 10-30% of the mass of TBs-CQDs; and the mass-volume ratio of TBs-CQDs carbon quantum dots to deionized water is 1-5 mg / mL.

3. The method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, In step (3), the applied voltage for electrospinning is 16~20 kV, the feed rate is 0.4~0.8 mL / h, and the receiving distance is 10~20 cm.

4. The method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, In step (3), the temperature for thermal esterification crosslinking is 100~140 ℃ and the treatment time is 1~3 h.

5. The method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, In step (3), the vacuum drying temperature is 30~50℃ and the drying time is 20~30 h.

6. The method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, In step (1), the tea leaves are pulverized into fine powder of 100-200 mesh, the distilled water is 15-30 times the mass of the tea leaves, the ratio of laccase added to the mass of tea leaves is 1000-2000U:1g, the reaction temperature is 40-60℃, the reaction time is 4-8h, the reaction is stopped when the temperature is raised to 90-100℃, and then cooled to 25-35℃. The amount of ethanol added is 3-4 times the volume of distilled water, the centrifugation speed is 4000-6000rpm, the centrifugation time is 15-30min, and the vacuum drying is carried out at 50-60℃, 0.01-0.2Pa, and the drying time is 8-16h.

7. The method for preparing the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects according to claim 1, characterized in that, In step (2), the volume-to-mass ratio of deionized water to theaflavins is 30-60 mL: 1 g; the ultrasonic power is 1500-2500 W, and the ultrasonic time is 10-30 min; the temperature is heated to 170-190℃ and held for 8-12 h; the centrifugation speed is 8000-12000 r / min, and the centrifugation time is 15-30 min; the pore size of the aqueous phase membrane used for filtration is 0.18-0.24 μm; the molecular weight cutoff of the dialysis bag is 10-100 kDa, and the dialysis time is 20-28 h; the freeze-drying temperature is -30 to -50℃, and the drying time is 18-36 h.

8. A theabrownin-based carbon quantum dot preservation film with both monitoring and antibacterial properties, characterized in that: It is prepared by the method for preparing the tea brown carbon quantum dot preservation film with both monitoring and antibacterial effects as described in any one of claims 1 to 7.

9. The application of the theabrownin carbon quantum dot preservation film with both monitoring and antibacterial effects as described in claim 8 in fruit preservation.

10. The application according to claim 9, characterized in that, The specific method is as follows: Wrap fresh fruit in a plastic wrap containing 0.1-0.3% of the weight of theabrownin carbon quantum dot, which has both monitoring and antibacterial effects, place it under 808 nm near-infrared light for 5-15 minutes, and then store it at room temperature; When observed under ultraviolet light, the plastic wrap shows a bright blue fluorescence when the fruit is fresh. As the fruit rots and releases volatile gases, the fluorescence quenches and dims.

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