Preparation method and application of a cold white light lignin-based carbon dot composite material with fluorescence / phosphorescence dual emission

Abstract

The present invention provides a method for preparing and applying a cold white light lignin-based carbon dot composite material with dual fluorescence / phosphorescence emission. This method uses alkali lignin as a precursor, first synthesizing lignin-derived carbon dots via a hydrothermal method, then covalently bonding them to a boric acid matrix via high-temperature calcination, resulting in a single-component cold white light lignin-based carbon dot composite material. The raw materials used in this invention are renewable, inexpensive, and readily available. The resulting white light carbon dot material exhibits a high fluorescence quantum yield and excellent stability. It exhibits cold white fluorescence emission under ultraviolet light excitation and green phosphorescence emission upon removal of ultraviolet light. It can be used in the preparation of white light-emitting diodes or for anti-counterfeiting applications.

Description

Technical Field

[0001] The present invention belongs to the technical field of luminescent materials, and in particular relates to a preparation method and application of a cold white light lignin-based carbon dot composite material with fluorescence / phosphorescence dual emission. Background Art

[0002] As global environmental problems become increasingly severe, low-carbon and environmentally friendly materials have become a research hotspot, especially white light emitting diodes (WLEDs), which have attracted the attention of many researchers due to their long life, low energy consumption, small size and high energy efficiency. However, most WLEDs to date are composed of multiple luminescent bodies with different emission colors. The manufacturing process of WLEDs prepared in this way is complex and has poor stability. In addition, some luminescent bodies are expensive, which limits the development and application of WLEDs. Therefore, the development of single-component white luminescent materials with simple preparation, low cost, good stability and no color difference and aging has always been the focus of academic research. Studies have shown that single-component white luminescent materials can be obtained by designing multiple luminescent centers in a single material system. Among the above-mentioned luminescent centers, thanks to the high energy conversion efficiency of the phosphorescent triplet state, the process of preparing single-component WLEDs by combining phosphorescence with fluorescence has attracted the attention of researchers (Wang ZF, et al., Adv Sci. 2020; 7(4): 1902688.). Unfortunately, single-component white light materials with dual fluorescence / phosphorescence emission are quite limited. These materials are generally based on rare earth metals, organometallic complexes, and non-metallic organic compounds. These materials are expensive, have low thermal stability, are complex to prepare, and are cytotoxic. Therefore, the development of low-cost, non-toxic, and stable single-component white light materials is highly desirable.

[0003] Carbon dots (CDs), as an emerging luminescent nanomaterial, offer advantages such as excellent optical properties, good stability, low cost, low toxicity, and simple preparation. In recent years, CDs have been found to exhibit dual fluorescence / phosphorescence emission properties, with a significant Stokes shift between fluorescence and phosphorescence, making them one of the most promising phosphors for WLEDs. Although numerous studies have reported on the preparation of dual-emission white light-emitting carbon dots, the precursors used are mostly derived from non-renewable resources such as petroleum or coal. For example, Li et al. (LiY, et al., Chin Chem Lett. 2023; 34(5): 107794.) proposed a method for preparing a single-component white light emitting material using 3,4,9,10-perylenetetracarboxylic acid diimide and boric acid as raw materials. The method first used the precursor 3,4,9,10-perylenetetracarboxylic acid diimide to prepare carbon dots by a solvent thermal method, and then covalently bound the carbon dots to the boric acid matrix by a constraint effect to prepare a white light carbon dot composite material. Although the carbon dot material prepared by this method has a high fluorescence quantum yield and pure white light emission, its carbon dot precursor is a non-renewable resource and has the disadvantages of complex structure, difficulty in obtaining, and certain biological toxicity, which limits the development of low-cost, simple preparation process, and green environmentally friendly WLEDs. Summary of the Invention

[0004] The purpose of the present invention is to address the bottlenecks of the current technology for preparing fluorescent / phosphorescent dual-emission single-component white-light carbon dot material precursors, such as non-renewable, complex structure, and difficulty in obtaining. Instead, it provides a preparation method and application of a single-component cold white-light carbon dot composite material made from lignin, a natural renewable biomass-based aromatic resource. This method uses alkali lignin as a precursor, first synthesizing lignin-derived carbon dots by a hydrothermal method, and then covalently binding them to a boric acid matrix by high-temperature calcination to obtain a single-component cold white-light lignin-based carbon dot composite material. This raw material is renewable, inexpensive, and easily available. The prepared white-light carbon dot material has a high fluorescence quantum yield and good stability. It exhibits cold white light fluorescence emission under ultraviolet light excitation and produces green phosphorescence emission when the ultraviolet light is removed.

[0005] The purpose of the present invention is achieved through the following solutions:

[0006] A method for preparing a cold white light lignin-based carbon dot composite material with fluorescence / phosphorescence dual emission, the method comprising the following steps:

[0007] Step 1: dissolving alkali lignin in deionized water and vigorously stirring at 85-95°C for 1-2 hours to obtain an alkali lignin solution;

[0008] Wherein, the concentration of the alkali lignin solution is 9.5-10.5 mg / mL;

[0009] The vigorous stirring is 100-300 rpm / min;

[0010] Step 2: The alkaline lignin solution cooled to room temperature in the previous step is transferred to a polytetrafluoroethylene hydrothermal reactor, and an inorganic acid solution is added. The reaction is then sealed and subjected to a constant temperature hydrothermal reaction at 180-220°C for 8-12 hours. The reaction is then filtered through a filter membrane, the pH value is adjusted, and the reaction is purified by dialysis to obtain a lignin-derived carbon dot solution.

[0011] Wherein, the volume ratio of the alkali lignin solution to the inorganic acid is 1:0.03 to 1:0.05;

[0012] The inorganic acid is a nitric acid solution with a mass fraction of 60-70%;

[0013] The filter membrane is a 0.22 μm filter membrane to remove large lignin particles;

[0014] The pH is adjusted to neutral using nitric acid.

[0015] The dialysis is performed using a 200-500Da dialysis bag;

[0016] The concentration of the lignin-derived carbon dot solution was 9.5–10.5 mg / mL;

[0017] Step 3: Add lignin-derived carbon dots and boric acid to deionized water and heat at 70-90°C until the water is completely evaporated. After fully grinding the mixture, transfer it to a crucible and calcine it at 330-370°C for 1-3 hours. After cooling to room temperature, grind it to obtain a cold white light lignin-based carbon dot composite material with fluorescent / phosphorescent dual emission.

[0018] 1-5 g of boric acid and 100-300 μL of lignin-derived carbon dot solution were added to every 40 mL of deionized water.

[0019] The cold white light lignin-based carbon dot composite material prepared by the method is used for the preparation of white light emitting diodes or for anti-counterfeiting.

[0020] The method for preparing a white light emitting diode comprises the following steps:

[0021] The lignin carbon dot composite material and epoxy resin AB glue (A glue: B glue = 1:4) are mixed in a mass ratio of 1:4 and stirred to form a mixture; the mixture is then applied to the center of a UV-LED chip with an emission wavelength of 395 nm, covering the center of the LED chip, and then heated and cured in an oven at 95 to 105° C. for 0.5 to 1.5 hours to obtain a WLEDs device;

[0022] The anti-counterfeiting method comprises the following steps:

[0023] (1) Drawing a characteristic mark on the lignin carbon dot composite material and drawing an interference mark using an interference material, and combining them together to form an anti-counterfeiting mark;

[0024] (2) When identifying, the anti-counterfeiting mark is first excited and irradiated with 365nm ultraviolet light, and white fluorescence emission is shown. After 2 seconds of irradiation, the ultraviolet light is turned off, and the anti-counterfeiting mark first shows green phosphorescence emission. After 5 to 10 seconds, the interference mark information disappears, and only the characteristic mark is displayed, thus realizing characteristic anti-counterfeiting;

[0025] The other preparation steps of the interference material are the same as those of the preparation method of the lignin carbon dot composite material, except that the calcination temperature in step 3 is 400-450°C.

[0026] The beneficial effects of the present invention are:

[0027] (1) The present invention uses natural biomass material - lignin as raw material, and controls the preparation of lignin-based carbon dot composite materials with fluorescence / phosphorescence dual emission properties through a two-step method. The preparation process is simple, the raw materials are renewable, low-cost, easily available, green and environmentally friendly, and have the potential for large-scale production;

[0028] (2) The single-component cold white light carbon dot composite material prepared by the present invention has excellent cold white light emission under ultraviolet light excitation, good stability, and a high fluorescence quantum yield of 35.40%;

[0029] (3) The lignin-based carbon dot phosphorescent composite material prepared by the present invention produces green phosphorescent emission visible to the naked eye for up to 14 seconds when UV irradiation is stopped, has a long phosphorescent lifetime of 1094 ms, and a high phosphorescent quantum yield of 19.87%, which is at the leading level among lignin-based carbon dot phosphorescent materials;

[0030] (4) The lignin-based carbon dot composite material prepared by the present invention has excellent cool white fluorescence and long-life room temperature phosphorescence, which provides a new idea for the application of lignin in optoelectronic devices, advanced anti-counterfeiting and information encryption. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 Transmission electron microscopy images of lignin-based carbon dots;

[0032] Figure 2 is the Fourier transform infrared spectrum of lignin-based carbon dot composite material;

[0033] Figure 3 is the X-ray diffraction pattern of the lignin-based carbon dot composite material;

[0034] Figure 4The fluorescence emission spectrum and digital photo of the lignin-based carbon dot composite material when excited by 395nm ultraviolet light;

[0035] Figure 5 This is a digital photo of phosphorescence of the lignin-based carbon dot composite material before and after irradiation with 365nm ultraviolet light;

[0036] Figure 6 The phosphorescence emission spectrum and phosphorescence emission decay curve of the lignin-based carbon dot composite material when excited by 365nm ultraviolet light;

[0037] Figure 7 This is a photo of the white light-emitting diode prepared in Example 1 after current is passed through it;

[0038] Figure 8 The normalized fluorescence emission spectra of Example 1, Example 2 and Example 3 when excited by 395 nm ultraviolet light are shown;

[0039] Figure 9 The phosphorescence emission spectra of Example 1 and thermally annealed boric acid powder when excited by 365 nm ultraviolet light;

[0040] Figure 10 This is an application example of Example 1 and Comparative Example 2 in the field of anti-counterfeiting. DETAILED DESCRIPTION

[0041] The present invention will be further described below with reference to the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto.

[0042] Example 1:

[0043] 100 mg of alkali lignin and 10 mL of deionized water were weighed and placed in a 50 mL flask. Ultrasonic treatment was performed for 10 min until completely dissolved. The mixture was then placed in a 90 ° C oil bath and vigorously stirred at 200 rpm / min for 1 hour. After cooling to room temperature, the mixed solution was transferred to a polytetrafluoroethylene hydrothermal reactor liner and 400 μL of 65% mass fraction nitric acid solution was added. After sealing, the mixture was hydrothermally reacted in an oven at 200 ° C for 12 hours. After being taken out and allowed to cool naturally, it was filtered through a filter membrane (0.22 μm), the pH value was adjusted to neutral, and dialyzed (200 Da dialysis bag) for purification to obtain a lignin-derived carbon dot solution (10 mg / mL).

[0044] Take 200 μL of the lignin-derived carbon dot solution obtained above, 3 g of boric acid and 40 mL of deionized water and place them in a 100 mL beaker and ultrasonicate for 1 hour until completely dissolved. Then place it in a 90°C oven until the water is completely evaporated. After fully grinding the mixture, transfer it to a crucible and calcine it in a muffle furnace at 350°C for 2 hours. Grind it again to obtain the final product. After testing, the fluorescence quantum yield is 35.40% and the phosphorescence quantum yield is 19.87%.

[0045] Figure 1 This is a transmission electron microscope image of the lignin-based carbon dots used in Example 1 to characterize their morphology using a JEOL JEM-2100F transmission electron microscope at an accelerating voltage of 200 kV. As can be seen from the image, the CDs are quasi-spherical with an average particle size of 3.67 nm.

[0046] Figure 2 The Fourier transform infrared spectra of the lignin-based carbon dot composite material were characterized by a Bruker Vector 22 Fourier transform infrared spectrometer in Example 1. As can be seen from the figure, the carbon dot composite material has the following wavelengths: 3217, 1706, 1458, 1195, 1018 and 925 cm -1 Characteristic absorption peaks are shown at , corresponding to the stretching vibrations of O─H / N─H, C═O, B─O, B─O─H, B─O─C and C─B, respectively, indicating that electron-deficient B is successfully introduced on the surface of carbon dots, which has an electron-withdrawing effect and can effectively suppress non-radiative transitions.

[0047] Figure 3 This is the X-ray diffraction pattern of the lignin-based carbon dot composite material characterized by a Bruker D8 Discover model X-ray diffractometer in Example 1. As can be seen from the figure, boric acid is dehydrated during the calcination process to form amorphous boron trioxide (B2O3). The above results indicate that the lignin-derived carbon dots are successfully embedded in the B2O3 matrix, stabilizing the triplet excited state of the carbon dots and suppressing non-radiative transitions, thereby promoting the generation of phosphorescence.

[0048] Figure 4The fluorescence emission spectrum of the lignin-based carbon dot composite material measured in Example 1 using an Edinburgh Instruments FS920P fluorescence spectrometer under ultraviolet excitation at an optimal excitation wavelength of 395 nm is shown. The figure shows that under ultraviolet excitation, the fluorescence spectrum covers the entire visible spectrum, exhibiting a distinct broad peak pattern and multimodal emission. Further analysis revealed that this broad multimodal emission originates from singlet fluorescence emission from the lignin-derived carbon dots and triplet phosphorescence emission generated after embedding in the B2O3 matrix. The spectral data were used to calculate chromaticity coordinates using the CIE 1931 software, revealing cool white fluorescence emission with Commission Internationale de l'Eclairage (CIE) coordinates of (0.264, 0.331).

[0049] Figure 5 These are digital photos of the lignin-based carbon dot composite material before and after excitation with ultraviolet light at the optimal phosphorescence excitation wavelength of 365nm. After the ultraviolet light excitation is stopped, the phosphorescence visible to the naked eye continues for 14 seconds.

[0050] Figure 6 This is a graph of the phosphorescence emission spectrum of the lignin-based carbon dot composite material measured in Example 1 using an Edinburgh Instruments FS920P fluorescence spectrometer when excited by ultraviolet light at the optimal phosphorescence excitation wavelength of 365 nm. It can be seen from the figure that when the ultraviolet light is removed, the material exhibits a long-lived green phosphorescence emission of 1094 ms.

[0051] Figure 7 This photograph shows the white light-emitting diode (LED) prepared in Example 1 after current flow. The lignin-carbon dot composite material from Example 1 was mixed with epoxy resin AB glue (A glue:B glue = 1:4) in a mass ratio of 1:4, stirring continuously to form a uniform mixture. A portion of the mixture was then applied to the center of a UV-LED chip emitting at a wavelength of 395 nm, completely covering the center of the LED. The mixture was then heat-cured in a 100°C oven for 1 hour to produce a white LED device. As can be seen from the image, at a voltage of 3.0V, the LED emits bright, cool white light.

[0052] Example 2:

[0053] The other steps were the same as in Example 1, except that the amount of lignin-derived carbon dots used was 100 μL. The relevant specific steps are as follows:

[0054] 100 μL (10 mg / mL) lignin-derived carbon dot solution, 3 g boric acid and 40 mL deionized water were placed in a 100 mL beaker and ultrasonicated for 1 h until completely dissolved. The mixture was then placed in an oven at 90°C until the water was completely evaporated. The mixture was fully ground and transferred to a crucible. It was calcined in a muffle furnace at 350°C for 2 h and ground again to obtain the final product, whose fluorescence CIE coordinates were (0.242, 0.305).

[0055] Example 3:

[0056] The other steps were the same as in Example 1, except that the amount of lignin-derived carbon dots used was 300 μL. The relevant specific steps are as follows:

[0057] 300 μL (10 mg / mL) of lignin-derived carbon dot solution, 3 g of boric acid and 40 mL of deionized water were placed in a 100 mL beaker and ultrasonicated for 1 hour until completely dissolved. The mixture was then placed in an oven at 90°C until the water was completely evaporated. The mixture was fully ground and transferred to a crucible, calcined in a muffle furnace at 350°C for 2 hours, and ground again to obtain the final product, whose fluorescence CIE coordinates were (0.275, 0.347).

[0058] Figure 8 The normalized fluorescence emission spectra of Example 1, Example 2 and Example 3 under 395 nm ultraviolet light excitation are shown in FIG. 1 . As can be seen from the figure, the amount of carbon dots used affects the peak width of the fluorescence emission peak, causing a certain degree of change in the CIE coordinates.

[0059] Example 4:

[0060] The other steps are the same as in Example 1, except that the calcination temperature in the muffle furnace is 330° C. The relevant specific steps are as follows:

[0061] 200 μL (10 mg / mL) of lignin-derived carbon dot solution, 3 g of boric acid, and 40 mL of deionized water were placed in a 100 mL beaker and ultrasonicated for 1 h until completely dissolved. The mixture was then placed in an oven at 90°C until the water was completely evaporated. The mixture was fully ground and transferred to a crucible. The mixture was calcined in a muffle furnace at 330°C for 2 h and ground again to obtain the final product. Fluorescence testing showed that the fluorescence performance of this embodiment was close to that of Example 1.

[0062] Example 5:

[0063] The other steps are the same as those in Example 1, except that the calcination temperature in the muffle furnace is 370° C. The relevant specific steps are as follows:

[0064] 200 μL (10 mg / mL) of lignin-derived carbon dot solution, 3 g of boric acid, and 40 mL of deionized water were placed in a 100 mL beaker and ultrasonicated for 1 h until completely dissolved. The mixture was then placed in an oven at 90°C until the water was completely evaporated. The mixture was fully ground and transferred to a crucible. The mixture was calcined in a muffle furnace at 370°C for 2 h and ground again to obtain the final product. Fluorescence testing showed that the fluorescence performance of this embodiment was close to that of Example 1.

[0065] Comparative Example 1:

[0066] The other steps are the same as in Example 1, except that the amount of lignin-derived carbon dots used is 0 μL, i.e., no lignin-derived carbon dots are prepared and no lignin-derived carbon dots are added. The specific steps are as follows:

[0067] 3 g of boric acid and 40 mL of deionized water were placed in a 100 mL beaker and ultrasonicated for 1 h until completely dissolved. The mixture was then placed in a 90°C oven until the water was completely evaporated. The mixture was fully ground and transferred to a crucible. It was calcined in a muffle furnace at 350°C for 2 h to obtain thermally annealed boric acid.

[0068] Figure 9 Figure 2 is the phosphorescence emission spectra of Example 1 and thermally annealed boric acid powder under 365 nm ultraviolet excitation. It can be seen from the figure that the boric acid material after thermal annealing exhibits very weak phosphorescence emission, which indicates that the phosphorescence and its white fluorescence are attributed to the lignin-derived carbon dots rather than boric acid.

[0069] Comparative Example 2:

[0070] The other steps are the same as those in Example 1, except that the calcination temperature in the muffle furnace is 400° C. The corresponding specific steps are as follows:

[0071] Take 200 μL (10 mg / mL) of lignin-derived carbon dot solution, 3 g of boric acid and 40 mL of deionized water and place them in a 100 mL beaker and ultrasonicate for 1 hour until fully dissolved. Then place it in a 90°C oven until the water is completely evaporated. After fully grinding the mixture, transfer it to a crucible and calcine it in a muffle furnace at 400°C for 2 hours. Grind it again to obtain the final product. The test found that its fluorescence CIE coordinates were (0.230, 0.300), and the phosphorescence emission intensity and lifetime became lower. The reason for this phenomenon can be attributed to the excessively high calcination temperature causing excessive carbonization of the lignin-derived carbon dots, destroying the structure of the composite material and thus affecting its optical properties.

[0072] Figure 10The invention relates to an anti-counterfeiting mark application using a combination of the powder of Example 1 having high phosphorescence emission intensity and lifespan and the powder of Comparative Example 2 having low phosphorescence emission intensity and lifespan. Example 1 is placed on the "666" portion of the "888" mold, and this portion of the mold is completely coated to draw the characteristic mark. The remaining portion is drawn with the interference mark of Comparative Example 2, and the two are combined to form an anti-counterfeiting mark. Under 365nm ultraviolet light excitation, the entire anti-counterfeiting mark immediately displays white fluorescence emission of "888". After turning off the 365nm ultraviolet light excitation for 2s, the interference information disappears after 7s due to the different phosphorescence emission intensities, so that only the characteristic mark information "666" is displayed, achieving characteristic anti-counterfeiting.

[0073] Comparative Example 3:

[0074] The other steps are the same as those in Example 1, except that the calcination temperature in the muffle furnace is 300° C. The corresponding specific steps are as follows:

[0075] 200 μL (10 mg / mL) of lignin-derived carbon dot solution, 3 g of boric acid, and 40 mL of deionized water were placed in a 100 mL beaker and ultrasonicated for 1 hour until completely dissolved. The mixture was then placed in a 90°C oven until the water completely evaporated. The mixture was thoroughly ground and transferred to a crucible. It was calcined in a muffle furnace at 300°C for 2 hours and ground again to obtain the final product, whose fluorescence CIE coordinates were (0.224, 0.301). By comparison, it was found that increasing the calcination temperature can broaden the long-wavelength emission center peak, thereby adjusting its CIE coordinates. Therefore, the comparative example has a narrower emission peak due to the lower calcination temperature, reflecting a bluish white light fluorescence emission.

[0076] As can be seen from the above examples and comparative examples, the product of the present invention uses alkali lignin as a precursor, first preparing lignin-derived carbon dots via a hydrothermal method, then embedding them into a B2O3 matrix via high-temperature calcination to produce a cool white light lignin-based carbon dot composite material with dual fluorescence / phosphorescence emission. As shown in Table 1, the fluorescence CIE coordinates and phosphorescence properties can be adjusted by regulating the calcination temperature and the amount of lignin-derived carbon dots. The results demonstrate that the product of the present invention exhibits excellent cool white light fluorescence emission and long-life green phosphorescence, thereby expanding the application of lignin in optoelectronic devices, advanced anti-counterfeiting, and information encryption.

[0077] Table 1 shows the CIE coordinates of the examples and comparative examples;

[0078] sample CIE coordinates Example 1 (0.264，0.331) Example 2 (0.242，0.305) Example 3 (0.275，0.347) Comparative Example 2 (0.230，0.300) Comparative Example 3 (0.224，0.301)

[0079] The embodiments described are preferred implementations of the present invention, but the present invention is not limited to the above implementations. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essence of the present invention are within the scope of protection of the present invention.

[0080] Matters not covered by the present invention are known technologies.

Claims

1. A method for preparing a cold white light lignin-based carbon dot composite material with fluorescent / phosphorescent dual emission, characterized in that the method comprises the following steps: Step 1: Dissolve alkali lignin in deionized water and stir vigorously at 85-95 °C for 1-2 h to obtain an alkali lignin solution; in, The concentration of the alkali lignin solution was 9.5–10.5 mg / mL; Step 2: The alkaline lignin solution cooled to room temperature in the previous step was transferred to a polytetrafluoroethylene hydrothermal reactor, and an inorganic acid solution was added. The reactor was sealed and subjected to a constant temperature hydrothermal reaction at 180-220°C for 8-12 hours. The reaction was then filtered through a filter membrane, the pH value was adjusted, and the reaction was purified by dialysis to obtain a lignin-derived carbon dot solution. Wherein, the volume ratio of the alkali lignin solution to the inorganic acid is 1:0.03~1:0.05; The concentration of the lignin-derived carbon dot solution was 9.5–10.5 mg / mL; Step 3: Add lignin-derived carbon dots and boric acid to deionized water and heat at 70-90°C until the water is completely evaporated. Grind the mixture and transfer it to a crucible. Then calcine it at 330-370°C for 1-3 hours. After cooling to room temperature, grind it to obtain a cold white light lignin-based carbon dot composite material with dual fluorescence / phosphorescence emission. Among them, 1-5 g of boric acid and 100-300 μL of lignin-derived carbon dots solution were added to every 40 mL of deionized water; The inorganic acid is a nitric acid solution with a mass fraction of 60-70%; The filter membrane is a 0.22 μm filter membrane; and the pH is adjusted by using nitric acid to adjust the pH value to neutral.

2. The method for preparing the cold white light lignin-based carbon dot composite material with fluorescence / phosphorescence dual emission according to claim 1, characterized in that The vigorous stirring in step (1) is 100-300 rpm / min.

3. The method for preparing the cold white light lignin-based carbon dot composite material with fluorescence / phosphorescence dual emission according to claim 1, characterized in that The dialysis is performed using a dialysis bag with a capacity of 200 to 500 Da.

4. Application of the cold white light lignin-based carbon dot composite material prepared by the preparation method as claimed in claim 1, characterized in that it is used for the preparation of white light emitting diodes or anti-counterfeiting.

5. The use according to claim 4, wherein the method for preparing the white light emitting diode comprises the following steps: The lignin-based carbon dot composite material and epoxy resin AB glue (A glue: B glue = 1:4) were mixed in a mass ratio of 1:4 and stirred to form a mixture. The mixture was then applied to the center of a UV-LED chip with an emission wavelength of 395 nm, covering the center of the LED chip. The mixture was then heated and cured in an oven at 95-105°C for 0.5-1.5 h to obtain a WLEDs device.

6. The use according to claim 4, wherein the anti-counterfeiting method comprises the following steps: (1) Drawing a characteristic mark on the lignin-based carbon dot composite material and drawing an interference mark using an interference material, and combining them together to form an anti-counterfeiting mark; (2) When identifying, the anti-counterfeiting mark is first excited by 365 nm ultraviolet light, and white fluorescence emission is displayed. After 2 seconds of irradiation, the ultraviolet light is turned off, and the anti-counterfeiting mark first shows green phosphorescence emission. After 5 to 10 seconds, the interference mark information disappears, and only the characteristic mark is displayed, thus achieving characteristic anti-counterfeiting; The other preparation steps of the interference material are the same as those of the preparation method of the lignin carbon dot composite material, except that the calcination temperature in step 3 is 400-450°C.

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