Lignin-based multicolor fluorescent carbon quantum dot paint, and preparation method and application thereof

Abstract

The application discloses lignin-based multicolor fluorescent carbon quantum dot paint and a preparation method and application thereof, and belongs to the technical field of coating materials. The preparation method specifically comprises the following steps: (1) preparation of a precursor solution; (2) solvothermal synthesis and purification of carbon quantum dots; (3) preparation of an aqueous film-forming agent solution; and (4) coating compound and defoaming. The application forms a dense hydrogen bond network through the film-forming agent and the surface functional groups of the carbon quantum dots, and effectively inhibits the aggregation-induced quenching (ACQ) effect. The obtained lignin-based multicolor fluorescent carbon quantum dot paint has a fluorescent quantum yield of greater than or equal to 18%, a multicolor emission wavelength range of 455-585 nm, and a fluorescent intensity retention rate of greater than or equal to 82% after ultraviolet accelerated aging for 1000 hours; the paint coating has a grid method adhesion of less than or equal to 1 grade, a pencil hardness of greater than or equal to 2H, and excellent weather resistance and water resistance. The lignin-based multicolor fluorescent carbon quantum dot paint can be applied to high-end decoration, brand anti-counterfeiting and intelligent packaging fields, and provides technical support for high-value utilization of biomass and functional upgrading of wood processing industry.

Description

Technical Field

[0001] This invention relates to the field of coating materials technology, and more specifically to a lignin-based multicolor fluorescent carbon quantum dot coating, its preparation method, and its application. Background Technology

[0002] Fluorescent coatings are special functional coatings that absorb light energy under ultraviolet light and convert it into visible light radiation. Their luminescence characteristics are determined by the fluorescent pigments they contain. Based on the different fluorescent pigments, they are mainly divided into two categories: inorganic fluorescent coatings and organic fluorescent coatings. Inorganic fluorescent coatings use metal oxides or sulfides (such as Y₂O₃S, ZnS, etc.) as pigments. They have excellent weather resistance, heat resistance, and chemical resistance, and good outdoor durability. However, they have high density and coarse particles, requiring high viscosity formulation to prevent sedimentation, and must be diluted with a thinner or water before application. Organic fluorescent coatings mostly use acrylic resins, polyurethane resins, etc., as base materials, relying on organic fluorescent pigments to achieve luminescence. They have bright colors and a brightness approximately three times that of ordinary paint, but are generally only suitable for indoor environments and have poor durability.

[0003] In recent years, the application of fluorescent materials in the field of intelligent sensing has attracted much attention. For example, patent CN112876835A discloses a multicolor fluorescent polymer film with environmental humidity response, its preparation method, and its application. This film can achieve a wide range of fluorescence changes from blue (450nm) to yellow (547nm) under different relative humidity (RH) environments, exhibiting multicolor fluorescence of blue→blue-green→yellow-green→yellow, with high discrimination, enabling visual monitoring of environmental humidity. Another patent CN106756825A discloses a wavelength-tunable fluorescent coating, its preparation method, and its application. By controlling the boron doping amount (1.5-4.5 at.%), the maximum emission wavelength is gradually shifted from 650nm to 410nm, achieving color modulation of red→green→white→blue. In addition, crystal form modulation strategies have also been used in the development of multicolor fluorescent materials, such as Ca2SiO4:Ce 3+ Al 3+ The system can emit blue light (435nm), green light (510nm), and yellow light (560nm) under different crystal forms.

[0004] However, the aforementioned existing technologies still have the following significant problems that cannot be ignored:

[0005] (1) The emission wavelength variation range is narrow and the visual recognition is insufficient. The maximum variation range of emission wavelength of traditional humidity-responsive fluorescent materials is usually no more than 50nm, and the color change is weak, making it difficult to achieve direct observation and judgment with the naked eye, which seriously limits its practical value in the field of visual sensing.

[0006] (2) It is difficult to achieve both multicolor response and reversibility. Although some fluorescent coatings can achieve multicolor emission, they often rely on high-temperature annealing (such as 900-1100℃) or complex co-sputtering processes (silicon target power 120-150W, boron target power 30-120W, chamber pressure 1-5Pa), which are demanding in preparation conditions, have poor industrial compatibility, and make it difficult to simultaneously meet the requirements of response reversibility and sensitivity.

[0007] (3) Fluorescent pigments in coating systems are prone to precipitation and have poor workability. Inorganic fluorescent coatings must be diluted before application due to their high pigment density; organic fluorescent coatings can be applied directly, but they have insufficient weather resistance and poor dispersion stability of fluorescent pigments in the base material, which can easily lead to problems such as fluorescence quenching and color drift after long-term use.

[0008] (4) Single function and lack of integrated "sensing-display" capability. Most existing fluorescent coatings are passive light-emitting materials, lacking active response and visualization feedback functions for external environmental parameters (such as humidity, temperature, pollutant concentration, etc.), which makes it difficult to meet the urgent needs of intelligent monitoring.

[0009] Therefore, how to develop a high-performance lignin-based multicolor fluorescent carbon quantum dot coating is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0010] In view of this, the purpose of the present invention is to provide a lignin-based multicolor fluorescent carbon quantum dot coating, its preparation method and application, so as to overcome the shortcomings of the prior art.

[0011] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating specifically includes the following steps: (1) Preparation of precursor solution The main carbon source and co-carbon source are added to an organic solvent and stirred to dissolve, thus obtaining a precursor solution. The main carbon source is alkali lignin; The co-carbon source is puerarin, gallocatechin gallate (EGCG), or curcumin; (2) Solvent thermal synthesis and purification of carbon quantum dots The precursor solution was subjected to a solvothermal reaction, and after cooling, it was centrifuged, filtered and dialyzed to obtain a carbon quantum dot solution for later use. (3) Preparation of film-forming agent aqueous solution Add the film-forming agent to water, stir to dissolve, and obtain an aqueous solution of the film-forming agent; (4) Coating compounding and defoaming Add the carbon quantum dot solution to the film-forming agent aqueous solution, add the additives dropwise under stirring, and let stand to defoam, thus obtaining the lignin-based multicolor fluorescent carbon quantum dot coating.

[0012] In this invention, the choice of different co-carbon sources determines the final fluorescence color of the carbon quantum dots: puerarin → blue fluorescence (B-CQDs), EGCG → green fluorescence (G-CQDs), curcumin → yellow fluorescence (Y-CQDs). This is the core design of this invention: "one-pot, one-way directional synthesis of carbon quantum dots of any target color using the same solvothermal process platform."

[0013] It is important to emphasize that existing lignin-based multicolor carbon quantum dot systems require two main challenges: firstly, they necessitate the design of multiple precursor blends (such as multi-step regulation using different elemental dopants); and secondly, they often require mixing or stepwise modification within the multicolor system, making the adjustment process complex. In contrast, this invention only requires changing one stoichiometric small molecule component in a simple binary system of "main carbon source-co-carbon source," allowing for the production of three independent luminescent carbon quantum dots with extremely wide visible wavelength coverage under almost identical solvothermal conditions. This truly achieves a controllable "one-pot" preparation process, with simple technology, minimal batch-to-batch variation, and suitability for large-scale production.

[0014] Furthermore, in step (1) above, the organic solvent is ethanol; the ratio of alkali lignin, puerarin and ethanol is 0.1 g:0.0125 g:25 mL; the ratio of alkali lignin, epigallocatechin gallate and ethanol is 0.1 g:0.0143 g:25 mL; the ratio of alkali lignin, curcumin and ethanol is 0.1 g:0.0167 g:25 mL; the stirring temperature is room temperature and the time is 30 min.

[0015] Furthermore, in step (2) above, the equipment for the solvothermal reaction is a polytetrafluoroethylene-lined hydrothermal reactor, the temperature is 180℃, and the time is 8 h; the centrifugation speed is 80000 rpm, and the time is 30 min; the filtration is by suction filtration through a 0.22 μm filter membrane; the dialysis uses a dialysis bag with a molecular weight cutoff of 1000 Da, the time is 72 h, and the deionized water is replaced every 8 h.

[0016] Furthermore, in step (3) above, the film-forming agent is at least one of polyvinyl alcohol (PVA, degree of hydrolysis 87%-89%), polyethylene glycol, polyvinylpyrrolidone and waterborne polyurethane; the mass concentration of the aqueous solution of the film-forming agent is 10 wt%-16 wt%; the water temperature is 75℃; and the stirring speed is 400 rpm.

[0017] Furthermore, in step (4) above, the temperature of the film-forming agent aqueous solution is 40℃; the additives are polydimethylsiloxane (viscosity 500 cps, defoaming / hydrophobic regulator) and glycerol (purity ≥99.5%, plasticizer); the volume ratio of carbon quantum dot solution, film-forming agent aqueous solution, polydimethylsiloxane and glycerol is (10-15):(60-70):(0.1-1):(1-5), and the viscosity of the coating at 25℃ is controlled to be 500-800 mPa·s; the stirring speed is 400 rpm and the time is 15 min; the standing time is 5 min.

[0018] This invention also claims protection for a lignin-based multicolor fluorescent carbon quantum dot coating prepared by the above-described method.

[0019] This invention also claims protection for the application of a lignin-based multicolor fluorescent carbon quantum dot coating prepared by the above method in high-end furniture decoration, brand anti-counterfeiting, or smart packaging.

[0020] A method for preparing a board containing the lignin-based multicolor fluorescent carbon quantum dot coating obtained by the above preparation method specifically includes the following steps: applying the lignin-based multicolor fluorescent carbon quantum dot coating to the surface of a pretreated wood substrate, pre-curing, cooling and then applying the coating again, followed by curing, to form a lignin-based multicolor fluorescent carbon quantum dot coating film, which is then obtained.

[0021] Furthermore, the aforementioned wood-based material is an engineered wood product; the pretreatment specifically involves: sanding it smooth with 400-grit sandpaper, wiping it with anhydrous ethanol, and then letting it air dry.

[0022] Furthermore, the above coating process specifically involves applying the coating evenly in a "horizontal first, then vertical" manner, and controlling the wet film thickness to be 50-80 μm using a wet film comb.

[0023] Furthermore, the pre-curing temperature is 60°C and the time is 1 hour.

[0024] Furthermore, the post-curing temperature is 75°C and the time is 1.5 h.

[0025] Furthermore, the thickness of the aforementioned lignin-based multicolor fluorescent carbon quantum dot coating film is 30-50 μm.

[0026] As can be seen from the above technical solution, compared with the prior art, the beneficial effects of the present invention are as follows: 1. Excellent and stable fluorescence properties By forming a dense hydrogen bond network with the carboxyl, hydroxyl, and amino functional groups on the surface of carbon quantum dots through a film-forming agent, the aggregation fluorescence quenching (ACQ) effect is effectively suppressed. The resulting lignin-based multicolor fluorescent carbon quantum dot coating has carbon quantum dot particle size of 10-20 nm, PDI ≤ 0.2; fluorescence quantum yield ≥ 18% (tested by integrating sphere method); multicolor emission wavelength range of 455-585 nm, covering blue, green, and yellow light; fluorescence intensity retention ≥ 85% after continuous UV irradiation (365 nm, 100 mW / cm²) for 100 h; fluorescence intensity retention ≥ 82% after UV accelerated aging for 1000 h, with no yellowing of the coating film; haze of the coating film under natural light ≤ 5% (GB / T2410-2008), and exhibits vivid multicolor fluorescence under 365 nm UV excitation, with fluorescence intensity ≥ 3 times higher than that of pure PVA coating.

[0027] 2. Excellent overall coating performance The adhesion to wood substrates by cross-cut test is ≤ Grade 1 (GB / T 9286-1998), the pencil hardness is ≥ 2H (GB / T 6739-2022), and there is no cracking when bending a Φ5 mm cylinder (GB / T 1731-1993). The fluorescence intensity retention rate after 500 cycles of Martindale abrasion test is ≥ 90%. The fluorescence intensity change rate after immersion in distilled water for 72 h is ≤ 8.2%. The weather resistance and water resistance meet the requirements for long-term use.

[0028] 3. The "one-pot" process is green and efficient. Using alkali lignin, a byproduct of the papermaking industry, as the main carbon source, this method achieves high-value utilization of waste. The bio-based content is ≥30%, and the raw material cost is ≤80 yuan / kg, which is much lower than that of traditional rare earth-based (approximately 520 yuan / kg) and organic dyes and coatings (approximately 210 yuan / kg). The solvothermal reaction uses ethanol as the solvent throughout, without the use of toxic organic solvents or the addition of heavy metals (heavy metal content ≤3.2mg / kg). The process has high integration, stable yield, good controllability, and is easy to scale up industrially.

[0029] 4. Multi-color, convenient and adjustable By simply changing the type of a single organic small molecule dopant, three types of high-purity fluorescent carbon quantum dots were prepared in a one-pot method under the same solvothermal conditions, from B-CQDs to Y-CQDs. The emission peaks can cover 455 nm (blue), 545 nm (green), and 585 nm (yellow). No complicated compounding, multi-layer coating, or post-treatment modification and multi-step surface group chemical reactions are required. The color purity is excellent and the reproducibility is good.

[0030] 5. Applicable to multiple scenarios It can be widely used in high-end furniture decoration (matched with LED ultraviolet light source, fluorescence life ≥5000 h), brand anti-counterfeiting (365 nm ultraviolet lamp excitation ≤3 s development and encryption of information, no decay of fluorescence pattern after 1 year of storage at room temperature), and smart packaging (multi-color fluorescent coding realizes product traceability and information anti-counterfeiting), providing practical and feasible technical solutions for the functional upgrading of the wood processing industry and the high-value utilization of biomass.

[0031] In summary, this invention discloses a lignin-based multicolor fluorescent carbon quantum dot coating with high stability, adjustable multiple colors, green processing, and excellent mechanical properties, as well as its preparation method and application. The ACQ effect is effectively suppressed through the design of a "polymer-carbon quantum dot" hydrogen bond network structure. Wide color gamut directional emission from blue to deep red light is achieved through the selection of dopants. The adhesion, hardness, weather resistance, and water resistance of the coating are optimized through the composite of film-forming agents and additives. This provides technical support for the functional upgrading of the wood processing industry and high-end anti-counterfeiting applications. Attached Figure Description

[0032] Figure 1 The following are excitation-emission fluorescence spectra of fluorescent carbon quantum dots in Examples 1-3; Figure 2 These are high-resolution transmission electron microscopy (HRTEM) images of fluorescent carbon quantum dots from Examples 1-3. Figure 3 The particle size distribution diagrams are for the fluorescent carbon quantum dots in Examples 1-3. Figure 4 Fourier transform infrared (FT-IR) spectra of fluorescent carbon quantum dots in Examples 1-3; Figure 5 The images show the effects of fluorescent carbon quantum dot coatings applied to wood chips (circular) in Examples 1-3 under sunlight, 254 nm and 365 nm ultraviolet light. Detailed Implementation

[0033] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0034] In the following examples, the alkali lignin was extracted from black liquor used in papermaking, with a purity of ≥95% and a molecular weight range of 5000-10000 Da.

[0035] Example 1 The preparation method of lignin-based blue fluorescent (B-CQDs) carbon quantum dot coatings specifically includes the following steps: (1) Preparation of precursor solution Add 0.1 g of alkali lignin and 0.0125 g of puerarin to a 250 mL beaker, then add 25 mL of ethanol, and stir at room temperature for 30 min until completely dissolved to obtain a homogeneous precursor solution. (2) Solvent thermal synthesis and purification of carbon quantum dots The precursor solution was transferred to a 100 mL polytetrafluoroethylene-lined hydrothermal reactor and reacted at 180 °C for 8 h. After natural cooling, it was centrifuged at 80,000 rpm for 30 min and filtered through a 0.22 μm filter membrane to remove the residue. The filtrate was transferred to a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed for 72 h, with deionized water replaced every 8 h to obtain a blue fluorescent carbon quantum dot (B-CQDs) solution. (3) Preparation of PVA aqueous solution Heat 50 mL of deionized water to 75°C, and add 8 g of PVA 1788 (87%-89% degree of alcoholysis) in three portions, with each portion added at 5 min intervals. Stir at 400 rpm until completely dissolved to obtain a 16 wt% PVA aqueous solution. (4) Coating compounding and defoaming Cool the PVA aqueous solution to 40℃, add 10 mL of blue fluorescent carbon quantum dot (B-CQDs) solution, add 0.2 mL of polydimethylsiloxane (viscosity 500 cps) and 2 mL of glycerol (purity 99.5%), stir at 400 rpm for 15 min, let stand for 5 min to defoam, and you will get lignin-based blue fluorescent (B-CQDs) carbon quantum dot coating.

[0036] The preparation method of the board containing the above-mentioned lignin-based blue fluorescent (B-CQDs) carbon quantum dot coating specifically includes the following steps: (1) Substrate pretreatment Sand the surface of the engineered wood panel with 400-grit sandpaper to remove sawdust and dust. Wipe the surface with a clean cloth dampened with anhydrous ethanol and let it air dry. (2) Coating and curing The coating was evenly applied to the surface of the engineered wood panel using a "horizontal first, then vertical" method, with the wet film thickness controlled to 60 μm using a wet film comb. After initial curing at 60℃ for 1 h, the coating was repeated after cooling. The panel was then dried at 70℃ for 1.5 h and allowed to cool naturally to form a lignin-based blue fluorescent (B-CQDs) carbon quantum dot coating film with a dry film thickness of 40 μm.

[0037] Example 2 The preparation method of lignin-based green fluorescent (G-CQDs) carbon quantum dot coating differs from that of Example 1 only in that 0.0143 g EGCG is used instead of 0.0125 g puerarin as a co-carbon source in step (1), while the remaining steps and parameters are exactly the same as in Example 1.

[0038] A lignin-based green fluorescent (G-CQDs) carbon quantum dot coating film was prepared using the method for preparing the board in Example 1.

[0039] Example 3 The preparation method of lignin-based yellow fluorescent (Y-CQDs) carbon quantum dot coating differs from that of Example 1 only in that 0.0167 g curcumin is used instead of 0.0125 g puerarin as a co-carbon source in step (1), while the remaining steps and parameters are exactly the same as those in Example 1.

[0040] A lignin-based yellow fluorescent (Y-CQDs) carbon quantum dot coating film was prepared using the method for preparing the board in Example 1.

[0041] Comparative Example 1 The preparation method of carbon quantum dot coating is different from that of Example 1 only in that polydimethylsiloxane is not added in step (4), and the remaining steps and parameters are exactly the same as those of Example 1.

[0042] A carbon quantum dot coating film was prepared using the method for preparing the substrate in Example 1.

[0043] Comparative Example 2 The preparation method of carbon quantum dot coating is different from that of Example 1 only in that 0.1 g of glucose is used instead of alkali lignin in step (1), and the rest of the steps and parameters are exactly the same as those of Example 1.

[0044] A carbon quantum dot coating film was prepared using the method for preparing the substrate in Example 1.

[0045] Comparative Example 3 The preparation method of carbon quantum dot coating differs from that of Example 1 only in that the addition of puerarin is changed to after the solvothermal reaction; the remaining steps are basically the same as in Example 1. Specifically, a carbon quantum dot solution is first obtained by solvothermal reaction of alkali lignin alone (160°C, 8 h). After cooling, 0.0125 g of puerarin is added, and the temperature is raised to 160°C again for 8 h. The remaining purification and compounding steps and parameters are the same as in Example 1.

[0046] A carbon quantum dot coating film was prepared using the method for preparing the substrate in Example 1.

[0047] Comparative Example 4 The preparation method of carbon quantum dot coating differs from that of Example 1 only in that: the carbon quantum dot solution is prepared by the alkaline-thermal method described in existing literature CN107033888A: a certain amount of Chinese herbal medicine residue is heated under reflux with 10% NaOH solution for 3 hours and then cooled to room temperature. The residue is washed with water until neutral and then dried in a constant temperature drying oven. The dried residue and solid NaOH are placed in a high-pressure reactor lined with polytetrafluoroethylene at a mass ratio of 1:1 and reacted at 300°C for 5 hours in a constant temperature drying oven. After cooling to room temperature, the reactor is opened and the black solution is poured out. After natural cooling, it is centrifuged at 10,000 rpm for 15 minutes and filtered through a 0.22 μm filter membrane to remove the residue. The filtrate is transferred to a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzed for 48 hours, with deionized water replaced every 6 hours to obtain an aqueous solution of carbon quantum dots. The remaining steps and parameters are exactly the same as in Example 1.

[0048] A carbon quantum dot coating film was prepared using the method for preparing the substrate in Example 1.

[0049] Performance testing 1. Examples and Comparative Examples The fluorescent carbon quantum dot coatings prepared in Examples 1-3 were tested for carbon quantum dot emission wavelength, fluorescence quantum yield, coating adhesion by cross-cut test, pencil hardness, bending of a Φ5 mm cylinder, fluorescence intensity retention rate after 500 Martindale abrasion tests, fluorescence intensity retention rate after 1000 h of UV aging, fluorescence intensity change rate after 72 h of distilled water immersion, and fluorescence intensity retention rate after 100 h of continuous 365 nm UV irradiation.

[0050] The method for determining the emission wavelength of carbon quantum dots is as follows: a fluorescence spectrophotometer (Hitachi F-7000) is used, the excitation wavelength is set to 365 nm, the emission spectrum is scanned (scanning range 400-750 nm, slit 5 nm), and the position of the maximum emission peak is recorded.

[0051] The fluorescence quantum yield was determined by the integrating sphere method, using a fluorescence spectrometer (Edinburgh FLS1000) equipped with an integrating sphere accessory, with a barium sulfate white plate as a reference, and the absorption and emission spectra of the sample and the reference were measured under 365 nm excitation.

[0052] The method for determining the adhesion of coatings by cross-cut test is as follows: According to GB / T 9286-1998 standard, use a cross-cut tester to draw a 1 mm × 1 mm grid (10 × 10 grids) on the coating surface, stick it with special tape (peel force about 10 N / 25 mm) and then peel it off vertically. The coating is rated according to the area of ​​peeling off (0 is the best and 5 is the worst).

[0053] The method for determining pencil hardness (GB / T 6739-2022) is as follows: According to the GB / T 6739-2022 standard, pencils of different hardness (Mitsubishi UNI series) are used, with the tip of the lead exposed by 3 mm and ground flat. The coating surface is scratched at a 45° angle with a load of 750 g. The hardness of the hardest pencil that does not break the coating is expressed.

[0054] The method for determining the bending of a Φ5 mm cylinder is as follows: According to the GB / T 1731-1993 standard, the coated test plate (size ≥50 mm × 20 mm) is bent 180° around a cylindrical shaft with a diameter of 5 mm, and the coating is observed to see if cracking or peeling occurs. The result is "no cracking" or "qualified".

[0055] The method for determining the fluorescence intensity retention rate after 500 cycles of Martindale abrasion test is as follows: Referring to GB / T 21196.2-2007, a Martindale abrasion tester is used with a loading pressure of 9 kPa and wool felt as the friction medium. After 500 cycles of reciprocating rubbing, the fluorescence intensity of the coating (excitation at 365 nm, emission peak) is measured, and the retention rate is calculated: Retention rate (%) = I 初始 / I 磨损后 ×100%.

[0056] The method for determining the fluorescence intensity retention rate after 1000 h of UV aging is as follows: Following GB / T 16422.3-2014, a UVA-340 lamp was used with an irradiance of 0.89 W / (m²·nm)@340 nm, under cyclic conditions (4 h of illumination, 4 h of condensation, and temperatures of 60 / 50℃). After a cumulative aging period of 1000 h, the fluorescence intensity was measured, and the retention rate was calculated: Retention rate (%) = I 初始 / I 老化后 ×100%.

[0057] The method for determining the fluorescence intensity change rate after immersion in distilled water for 72 h is as follows: The coated test plate (50 mm × 50 mm) is completely immersed in distilled water and left to stand at room temperature (25℃) for 72 h. After removal, the surface moisture is blotted dry with filter paper, and the fluorescence intensity is immediately measured. The change rate is calculated as follows: Change rate (%) = I 初始 / (I 浸泡后 -I 初始 ) × 100%.

[0058] The method for determining the fluorescence intensity retention rate after continuous 365 nm ultraviolet irradiation for 100 h was as follows: A 365 nm LED light source (irradiance 100 mW / cm², calibrated by an optical power meter) was used to vertically irradiate the coating surface at room temperature. After continuous irradiation for 100 h, the fluorescence intensity was measured, and the retention rate was calculated: Retention rate (%) = I 初始 / I 照射后 ×100%.

[0059] The results are shown in Table 1.

[0060] Table 1 Performance test results of fluorescent carbon quantum dot coatings in Examples 1-3

[0061] As shown in Table 1, the three parallel samples in Examples 1-3 exhibited blue (455 nm), green (545 nm), and yellow (585 nm) emission under 365 nm UV excitation, respectively, with yields ranging from 35% to 42%. The batch repeatability was good (RSD ≤ 5.3%), and the fluorescence quantum yield was ≥ 18%. The coatings had adhesion ≤ 1 grade and hardness ≥ 2H under the same film-forming process.

[0062] Furthermore, the performance test results of Comparative Example 1 (comparison without polydimethylsiloxane additive) showed that the fluorescence intensity change rate of the coating after immersion in distilled water for 72 hours was 23.6%, indicating a significant decrease in water resistance. This demonstrates that the addition of polydimethylsiloxane plays an important role in improving the hydrophobicity and water resistance of the coating.

[0063] Performance test results of Comparative Example 2 (comparison of alkali lignin carbon source): The fluorescence quantum yield of carbon quantum dots was only 7.5%, which was about 60.9% lower than that of Example 1. This demonstrates that the unique aromatic three-dimensional polymer structure of alkali lignin plays a key role in achieving efficient fluorescence emission.

[0064] Performance test results of Comparative Example 3 (non-one-pot doping comparison): The carbon quantum dot yield decreased by approximately 32.7% compared to Example 1, and the fluorescence quantum yield decreased by approximately 26%. This indicates that puerarin participates in carbon core formation and surface modification during the hydrothermal carbonization stage of alkali lignin, thus enabling effective optical control.

[0065] Performance test results of Comparative Example 4 (existing technology comparison: CN107033888A alkaline thermal method): The fluorescence quantum yield of carbon quantum dots decreased by approximately 10% compared to Example 1; the fluorescence intensity retention rate decreased by ≥17% after 1000 h of UV aging; and the adhesion of the coating was grade 2, with a hardness below 2H. This confirms the significant improvement of the present invention in terms of quantum efficiency and long-term stability achieved through a hydrothermal system and the "one-pot synergistic carbonization of alkali lignin with organic small molecule doping" strategy.

[0066] The above experiments demonstrate that this invention establishes a complete technical solution that "by controlling only a single organic doped small molecule, lignin-based multicolor fluorescent carbon quantum dots of any target color can be directionally synthesized in a hydrothermal platform, and then rapidly converted into highly stable, high-performance, and tunable fluorescent coatings." This solution is fully applicable to the production of multicolor fluorescent high-end decorative, anti-counterfeiting, or smart packaging film products, and significantly simplifies the bottlenecks of the existing multi-step modification and post-mixing processes for lignin-based multicolor carbon quantum dots.

[0067] 2. This invention and existing technology Examples 1-3 (carbon quantum dot coatings), traditional rare earth-based fluorescent coatings, traditional organic dye-based fluorescent coatings, and Comparative Example 4 (carbon quantum dot coatings, existing biomass carbon quantum dot coatings) were used to test their fluorescence quantum yield, carbon quantum dot emission wavelength, fluorescence intensity retention rate after 1000 hours of UV aging, coating adhesion by cross-cut test, pencil hardness, fluorescence intensity change rate after 72 hours of immersion in distilled water, raw material cost, heavy metal content, and bio-based content.

[0068] The method for determining the heavy metal content is as follows: referring to GB / T 30647-2014 "Determination of Total Content of Harmful Elements in Coatings", after the coating is prepared into a dry film, all organic substances are removed by dry ashing. After dissolution, filtration and volume adjustment, the content of the element to be tested is determined by inductively coupled plasma optical emission spectrometry (ICP-OES).

[0069] The method for determining bio-based content is as follows: Referring to ASTM D6866, "Standard Test Method for Determination of Bio-based Content in Solid, Liquid and Gas Samples by Radiocarbon Analysis," the bio-based content in the sample is determined by accelerator mass spectrometry (AMS). 14 The carbon isotope content is used to distinguish between bio-based carbon and fossil-based carbon, and the percentage of bio-based carbon in total organic carbon is calculated.

[0070] The results are shown in Table 2.

[0071] Table 2 Performance test results of fluorescent coatings of the present invention and prior art

[0072] As shown in Table 2, the fluorescent carbon quantum dot solution obtained in this invention exhibits a fluorescence quantum yield ≥19.2%, a fluorescence intensity retention rate ≥83.5% after 1000 h of UV aging, excellent coating adhesion (≤1 grade), pencil hardness ≥2H, and superior water resistance (fluorescence change rate ≤8.2% after 72 h). Its overall performance is significantly superior to traditional rare-earth-based, organic dye-based, and existing biomass carbon quantum dot coatings. Furthermore, this invention uses water as the solvent throughout the process, has a raw material cost ≤80 RMB / kg, a bio-based content ≥30%, and a heavy metal content ≤3.2 mg / kg, achieving a harmonious balance between green chemistry and high performance.

[0073] 3. Characterization of fluorescent carbon quantum dots Figure 1 Images (a)-(c) show the excitation-emission fluorescence spectra of fluorescent carbon quantum dots prepared by co-carbonization of alkali lignin with puerarin, EGCG, and curcumin, as described in Examples 1-3. Figure 1It can be seen that all three exhibit obvious emission peaks under 365 nm excitation, indicating that the obtained carbon quantum dots possess strong blue / green / yellow light emission characteristics. Among them, Figure 1 (a) B-CQDs show a blue emission peak at 455 nm; Figure 1 (b) G-CQDs exhibit a green emission peak at 545 nm; Figure 1 (c) Y-CQDs exhibit a yellow emission peak at 585 nm. The emission peak position red-shifts with the difference in molecular structure of the co-carbon source, indicating that the co-carbon source participates in regulating the surface states and electronic structure of the carbon dots. The solution in the inset is lighter in color under sunlight, but emits blue, green, and yellow fluorescence under 365 nm ultraviolet light, further verifying the successful preparation of carbon quantum dots and their excellent fluorescence response characteristics.

[0074] Figure 2 Images (a)-(c) are high-resolution transmission electron microscopy (HRTEM) images of fluorescent carbon quantum dots prepared by co-carbonization of alkali lignin with puerarin, EGCG, and curcumin in Examples 1-3, respectively. Figure 2 As can be seen, all three samples exhibited well-dispersed near-spherical nanoparticles with relatively uniform particle size and no obvious agglomeration, indicating that the fluorescent carbon quantum dots possess good water dispersibility and high preparation uniformity. Clear lattice fringes can be observed in the magnified images, indicating that the samples have a certain graphitized structure and good crystallinity. Among them, Figure 2 The lattice spacing in (a) is approximately 0.22 nm. Figure 2 In the middle (b), the wavelength is approximately 0.23 nm. Figure 2 The (c) value is approximately 0.22 nm, which is consistent with the (100) / (002) related crystal plane characteristics of graphitic carbon. The fluorescent carbon quantum dots prepared by different co-carbon sources show little difference in size and lattice spacing, indicating that alkali lignin and different natural phenolic co-carbon sources can successfully construct fluorescent carbon quantum dots with stable structures and clear crystal states.

[0075] Figure 3 Images (a)-(c) show the particle size distribution of fluorescent carbon quantum dots prepared by co-carbonization of alkali lignin with puerarin, EGCG, and curcumin in Examples 1-3, respectively. Figure 3 It can be seen that the particle size of the three samples is mainly concentrated in the range of 2-5 nm, exhibiting an approximately normal distribution, indicating that the carbon dots are small in size and relatively uniformly distributed, possessing typical quantum dot characteristics. Among them, Figure 3 The particle size of (a)B-CDs is mainly concentrated at about 3-4 nm, with a moderate average particle size; Figure 3 The particle size distribution of G-CDs in (b) is slightly smaller, with the peak value located at about 3 nm, indicating that its nucleation is more complete and the particle size is better controlled. Figure 3The particle size distribution of (c) Y-CDs is relatively wide, mainly concentrated in the 3-4 nm range, with some particles reaching over 5 nm. Overall, all three types of fluorescent carbon quantum dots exhibit good size uniformity and narrow particle size distribution, indicating that the synergistic carbonization of alkali lignin with different co-carbon sources can effectively regulate the size formation process of fluorescent carbon quantum dots.

[0076] Figure 4 Images (a)-(c) show the Fourier transform infrared (FT-IR) spectra of fluorescent carbon quantum dots prepared by co-carbonization of alkali lignin with puerarin, EGCG, and curcumin, as described in Examples 1-3. Figure 4 It can be seen that all three groups of samples are within the range of 3430-3400 cm. -1 The appearance of a broad peak at approximately 1710-1720 cm⁻¹ corresponds to the stretching vibrations of OH and CH, indicating that the surface of fluorescent carbon quantum dots is rich in oxygen-containing functional groups such as hydroxyl groups, which is beneficial for improving hydrophilicity and dispersibility. -1 A distinct absorption peak is present at all locations, attributed to the C=O stretching vibration, indicating that the sample retains carboxyl, ester, or quinone structures. (1600-1620 cm⁻¹) -1 The nearby characteristic peaks can be attributed to the vibration of the aromatic ring C=C or sp² carbon skeleton, indicating that all three fluorescent carbon quantum dots have a certain conjugated graphitization structure. Figure 4 1030cm in (a) -1 1120 cm -1 and 1380 cm -1 The peaks correspond to the CO, COC and CN vibrations, respectively, indicating that puerarin participation resulted in the formation of relatively abundant oxygen- and nitrogen-containing functional groups; Figure 4 (b) in 2920 -1 2850 cm -1 A CH stretching peak appears at 1330. -1 1050cm -1 There is significant CO absorption at this location; Figure 4 (c) in 1270 -1 1140 -1 and 810 cm -1 The peaks appearing at the specified locations are related to the out-of-plane bending vibrations of CO, COC, and aromatic CH derived from lignin or curcumin, respectively. Overall, different co-carbon sources successfully modulated the surface chemical structure of fluorescent carbon quantum dots, providing a foundation for their subsequent fluorescence properties and coating applications.

[0077] Figure 5 The image shows the appearance and fluorescence response of three fluorescent carbon quantum dot coatings prepared by co-carbonization of alkali lignin with puerarin, EGCG, and curcumin on the surface of circular wood chips, as illustrated in Examples 1-3. Figure 5Under sunlight, all three coatings exhibited uniform coverage, displaying colors of light beige, golden yellow, and yellow, respectively, indicating good film-forming properties and relatively uniform dispersion. Under 254 nm ultraviolet light irradiation, all three samples showed significant luminescence. The puerarin-based fluorescent carbon quantum dot coating exhibited blue-violet fluorescence, the EGCG-based coating showed strong green fluorescence, and the curcumin-based coating emitted bright yellow fluorescence, indicating that different co-carbon sources significantly modulated the luminescence behavior of the fluorescent carbon quantum dots. Under 365 nm ultraviolet light, the three coatings also showed clearly visible fluorescence differences: the puerarin-based coating emitted blue-white light, the EGCG-based coating maintained green fluorescence, and the curcumin-based coating emitted a bright yellow fluorescence with high brightness. These results demonstrate that the prepared fluorescent carbon quantum dot coatings possess excellent photoluminescence properties, can be successfully applied to wood chip surface coating, and impart visual fluorescent characteristics to the material.

[0078] In summary, this invention utilizes alkali lignin, a byproduct of the papermaking industry, as the main carbon source, and naturally occurring phenolic compounds or their derivatives (puerarin, EGCG, curcumin) as co-carbon sources. Under the same solvothermal conditions, three high-purity fluorescent carbon quantum dots (B-CQDs, G-CQDs, and Y-CQDs) with emission wavelengths covering 455-585 nm were prepared in a one-pot process. Further compounding with polyvinyl alcohol (PVA) film-forming agents, the dense hydrogen bond network formed by PVA and the functional groups (-COOH, -NH2, -OH) on the carbon quantum dot surface effectively suppressed the aggregation fluorescence quenching (ACQ) effect, significantly improving solid-state luminescence efficiency and stability.

[0079] The core advantage of this invention lies in the synergistic design of "one-pot" solvothermal synthesis and "polymer-carbon quantum dot hydrogen bond network", which achieves a balance between high fluorescence quantum yield (≥19.2%), wide color gamut (three colors), ultra-high aging resistance (1000h retention rate >83%) and excellent mechanical properties (adhesion grade 1, hardness 2H). At the same time, it significantly reduces raw material costs (≤80 yuan / kg) and environmental impact, reflecting the organic unity of green chemistry and high performance.

[0080] This invention provides practical multi-color fluorescent coating materials for high-end furniture decoration, brand anti-counterfeiting, and smart packaging, and also provides important technical support for the high-value utilization of lignin waste and the green functional upgrading of the wood processing industry.

[0081] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating, characterized in that, Specifically, the following steps are included: (1) Preparation of precursor solution The main carbon source and co-carbon source are added to an organic solvent and stirred to dissolve, thus obtaining a precursor solution. The main carbon source is alkali lignin; The cocarbon source is puerarin, epigallocatechin gallate, or curcumin. (2) Solvent thermal synthesis and purification of carbon quantum dots The precursor solution was subjected to a solvothermal reaction, and after cooling, it was centrifuged, filtered and dialyzed to obtain a carbon quantum dot solution for later use. (3) Preparation of film-forming agent aqueous solution Add the film-forming agent to water, stir to dissolve, and obtain an aqueous solution of the film-forming agent; (4) Coating compounding and defoaming The carbon quantum dot solution is added to the film-forming agent aqueous solution, and the additives are added dropwise under stirring. After standing and defoaming, the lignin-based multicolor fluorescent carbon quantum dot coating is obtained.

2. The method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating according to claim 1, characterized in that, In step (1), the organic solvent is ethanol; the ratio of alkali lignin, puerarin and ethanol is 0.1 g:0.0125 g:25 mL; the ratio of alkali lignin, epigallocatechin gallate and ethanol is 0.1 g:0.0143 g:25 mL; the ratio of alkali lignin, curcumin and ethanol is 0.1 g:0.0167 g:25 mL; the stirring temperature is room temperature and the stirring time is 30 min.

3. The method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating according to claim 1, characterized in that, In step (2), the equipment for the solvothermal reaction is a polytetrafluoroethylene-lined hydrothermal reactor, the temperature is 180℃, and the time is 8 h; the centrifugation speed is 80000 rpm, and the time is 30 min; the filtration is by vacuum filtration through a 0.22 μm filter membrane; the dialysis uses a dialysis bag with a molecular weight cutoff of 1000 Da, the time is 72 h, and the deionized water is replaced every 8 h.

4. The method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating according to claim 1, characterized in that, In step (3), the film-forming agent is at least one of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone and waterborne polyurethane; the mass concentration of the aqueous solution of the film-forming agent is 10 wt%-16 wt%; the temperature of the water is 75℃; and the stirring speed is 400 rpm.

5. The method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating according to claim 1, characterized in that, In step (4), the temperature of the film-forming agent aqueous solution is 40°C; the auxiliary agent is polydimethylsiloxane and glycerol; the volume ratio of the carbon quantum dot solution, the film-forming agent aqueous solution, polydimethylsiloxane and glycerol is (10-15):(60-70):(0.1-1):(1-5); the stirring speed is 400 rpm and the stirring time is 15 min; the standing time is 5 min.

6. A lignin-based multicolor fluorescent carbon quantum dot coating prepared by the preparation method according to any one of claims 1-5.

7. The application of a lignin-based multicolor fluorescent carbon quantum dot coating prepared by any one of claims 1-5 in high-end furniture decoration, brand anti-counterfeiting, or smart packaging.

8. A method for preparing a board containing a lignin-based multicolor fluorescent carbon quantum dot coating obtained by any one of the preparation methods described in claims 1-5, characterized in that, Specifically, the following steps are included: The lignin-based multicolor fluorescent carbon quantum dot coating is applied to the surface of a pretreated wood substrate, pre-cured, cooled, and then applied again for curing to form a lignin-based multicolor fluorescent carbon quantum dot coating film.

9. The method for preparing a sheet metal according to claim 1, characterized in that, The wood substrate is an engineered wood product; the pretreatment specifically involves: sanding it smooth with 400-grit sandpaper and wiping it dry with anhydrous ethanol; the coating specifically involves: applying the coating evenly in a "horizontal first, then vertical" manner, and controlling the wet film thickness to be 50-80 μm using a wet film comb.

10. The method for preparing a sheet metal according to claim 1, characterized in that, The pre-curing temperature is 60℃ and the time is 1 h; the post-curing temperature is 75℃ and the time is 1.5 h; the thickness of the lignin-based multicolor fluorescent carbon quantum dot coating film is 30-50 μm.

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