A lignin-based multicolor fluorescent carbon quantum dot paint as well as a preparation method and application thereof

Abstract

This invention provides a lignin-based multicolor fluorescent carbon quantum dot coating, its preparation method, and its application. First, a nitrogen or sulfur source is added to lignin to prepare a precursor solution. Then, a hydrothermal reaction is carried out to obtain a carbon quantum dot solution. Next, a film-forming agent solution and additives are added to the carbon quantum dot solution, and after thorough mixing, a CQDs-PVA coating is obtained. After defoaming, the lignin-based multicolor fluorescent carbon quantum dot coating is obtained. The coating obtained by this invention exhibits excellent and stable luminescent properties. Through the formation of a hydrogen bond network between PVA and the functional groups on the surface of carbon quantum dots, the ACQ effect is effectively suppressed. The fluorescence quantum yield is ≥42%, the fluorescence intensity retention rate after 1000 hours of UV aging is ≥82%, and the multicolor emission wavelength covers 420-650 nm with convenient control. The coating has strong adhesion to wood substrates, excellent mechanical properties, and good weather and water resistance, meeting the requirements for long-term use. It can be applied to high-end decoration, brand anti-counterfeiting, intelligent packaging, and other fields, providing technical support for the functional upgrading of the wood processing industry.

Description

Technical Field

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

[0002] The application of fluorescent coatings in the wood processing field can endow engineered wood panels with decorative and functional properties, especially in information encoding, pattern hiding and rendering, etc., which are of great value. Traditional fluorescent coatings rely on rare earth complexes or organic synthetic dyes, which have drawbacks such as unsustainable raw materials, high pollution in the preparation process, poor luminescence stability, poor mechanical properties of the coating, and complex multicolor control. Specifically, rare earth element mining is energy-intensive and polluting, and petroleum-based dyes violate the principles of green chemistry; the synthesis process uses toxic organic solvents, generating "three wastes" (waste gas, wastewater, and solid waste); fluorescent molecules are prone to agglomeration in the polymer matrix, causing aggregated fluorescence quenching (ACQ) effect; the coating has insufficient adhesion to the wood substrate and poor weather resistance; multicolor emission requires cumbersome compounding or multi-layer coating, which is complex and has poor reproducibility.

[0003] Biomass-derived carbon quantum dots have become a research hotspot due to their advantages such as renewable raw materials, green preparation, and tunable optical properties. Lignin, a byproduct of the papermaking industry, is the second most abundant natural organic polymer in the world, with an annual recycling volume of billions of tons, but its high added value potential has not been fully explored. Lignin possesses a three-dimensional aromatic polymer structure rich in functional groups such as benzene rings, hydroxyl groups, and methoxy groups, providing an ideal carbon source for the preparation of carbon quantum dots. Existing lignin-based carbon dot coatings suffer from problems such as low fluorescence quantum yield, insufficient convenience of multicolor control, and need to improve weather resistance, which limit their industrial application. Therefore, developing a new system of environmentally friendly, efficient, and high-performance lignin-based multicolor fluorescent carbon quantum dot coatings has become an urgent technical challenge to be solved. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a lignin-based multicolor fluorescent carbon quantum dot coating with stable luminescence performance, excellent overall coating performance, and convenient multicolor control, along with its preparation method and applications. The lignin-based multicolor fluorescent carbon quantum dot coating of this invention uses green and sustainable raw materials, employs an environmentally friendly and efficient preparation process, and has expanded application scenarios, realizing the high-value utilization of lignin and the green upgrading of the wood processing industry.

[0005] To solve the above-mentioned technical problems, the present invention provides the following technical solution: A method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating includes the following steps: (1) Preparation of precursor solution Take lignin, add a nitrogen source or a sulfur source, add water and stir the solution thoroughly to obtain a precursor solution; (2) Synthesis of carbon quantum dot solution The precursor solution described in step (1) was placed under hydrothermal conditions for reaction, then cooled to room temperature, and successively centrifuged, filtered and dialyzed to obtain a carbon quantum dot solution. (3) Preparation of film-forming agent aqueous solution Add the film-forming agent to hot water and dissolve it completely under stirring to obtain an aqueous solution of the film-forming agent; (4) Coating compounding and defoaming After the film-forming agent aqueous solution in step (3) is cooled down, the carbon quantum dot solution in step (2) is added, and then the additives are added dropwise under stirring conditions to obtain CQDs-PVA coating. After standing and defoaming, lignin-based multicolor fluorescent carbon quantum dot coating is obtained.

[0006] The preparation process described in this invention is environmentally friendly and efficient. It adopts a "one-pot" hydrothermal synthesis (carbon quantum dot solution, film-forming agent solution, and additives are synthesized in one pot under hydrothermal conditions). Water is used as the solvent throughout the process, and no toxic organic solvents are used. The process has high integration, stable yield, and is easy to industrialize.

[0007] In step (1), the lignin is commercially available, with a purity ≥95% and a molecular weight of 5000-10000 Da; The nitrogen or sulfur source is one of 7-hydroxycoumarin, riboflavin, sodium fluorescein, rhodamine 6G, rhodamine B, or sulfonylrhodamine B.

[0008] In step (2), the hydrothermal conditions are a constant temperature reaction at 160℃ for 10 hours; The centrifugation conditions were 10,000 rpm / min for 15 min; The filtration process involves removing residue by vacuum filtration through a 0.22μm filter membrane. The dialysis process involves transferring the filtrate into a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzing for 48 hours, replacing the deionized water every 6 hours.

[0009] In step (3), the film-forming agent may be one or a mixture of several of polyvinyl alcohol (degree of hydrolysis of 87-89%), polyethylene glycol, polyvinylpyrrolidone, or aqueous polyurethane, and the concentration of the aqueous solution of the film-forming agent is 10wt%-16wt%. The hot water is 85℃ hot water; Add the film-forming agent to the hot water in three portions, with each portion added every 5 minutes, and stir at 300 rpm until completely dissolved.

[0010] In step (4), the aqueous solution of the film-forming agent is cooled to 40°C, and then a carbon quantum dot solution is added; The additives are polydimethylsiloxane and glycerol.

[0011] In step (4), the mass ratio of the film-forming agent aqueous solution, carbon quantum dot solution, polydimethylsiloxane and glycerol is 60-70:10-15:10-15:5-10; The stirring conditions were 400 r / min for 15 min; The viscosity of the coating at 25°C is 500-800 mPa·s.

[0012] The method described yields a lignin-based multicolor fluorescent carbon quantum dot coating.

[0013] The method of using the lignin-based multicolor fluorescent carbon quantum dot coating includes the following steps: (S1) Take the substrate for pretreatment, specifically: use 400-grit sandpaper to sand the surface of the artificial board to remove sawdust and dust; wipe the surface with a clean cloth dipped in anhydrous ethanol and let it air dry naturally; (S2) Apply the coating evenly to the substrate surface in a "horizontal first, then vertical" manner, and control the wet film thickness to 50-80μm by using a wet film comb; then dry at 60℃ for 1h for preliminary curing, and repeat the coating after cooling; finally dry at 70℃ for 1.5h and allow to cool naturally to obtain the lignin-based multicolor fluorescent carbon quantum dot coating.

[0014] The "horizontal first, then vertical" method refers to the method in which the coating is first applied horizontally and then vertically on the substrate surface.

[0015] The dry film thickness of the coating is 30-50 μm.

[0016] Testing revealed that the lignin-based multicolor fluorescent carbon quantum dot coating of this invention exhibits excellent performance indicators, specifically: Carbon quantum dot performance: fluorescence quantum yield ≥42% (integrating sphere method test), emission wavelength range 420-650nm, continuous ultraviolet irradiation (365nm, 100mW / cm²) 2 After 1000h, the fluorescence intensity retention rate is ≥82%; the particle size is 10-20nm, and the PDI is ≤0.2 (DLS test); the surface contains functional groups such as -COOH and -NH2 (XPS or FT-IR verification).

[0017] Coating fluorescence performance: Haze ≤5% under natural light (GB / T 2410-2008), exhibits vivid multicolor fluorescence under 365nm ultraviolet light excitation, fluorescence intensity is ≥3 times higher than pure PVA coating; fluorescence intensity retention rate ≥80% after 1000h of ultraviolet aging (QUV aging chamber), no yellowing.

[0018] Coating mechanical properties: Cross-cut adhesion ≤ Grade 1 (GB / T 9286-1998), pencil hardness ≥ 2H (GB / T6739-2022), no cracking when bending a Φ5mm cylinder (GB / T 1731-1993), fluorescence intensity retention rate ≥ 90% after 500 cycles of Martindale abrasion test.

[0019] The application of the lignin-based multicolor fluorescent carbon quantum dot coating in high-end furniture decoration, brand anti-counterfeiting, or smart packaging.

[0020] High-end furniture decoration: Fluorescent patterns are formed on the surface of artificial boards through screen printing process, which are matched with LED ultraviolet light source. The fluorescence life is ≥5000h, which is suitable for indoor ambient lighting decoration. Brand anti-counterfeiting: The anti-counterfeiting label can be developed and encrypted under 365nm ultraviolet light (10cm distance) for ≤3s. The fluorescent pattern will not decay after 1 year of storage at room temperature. Smart packaging: used for encrypted information storage, it enables product traceability and information anti-counterfeiting through multi-color fluorescent coding, and is compatible with industrialized packaging and printing production.

[0021] Compared with the prior art, the present invention has the following beneficial effects: This invention provides a method for preparing lignin-based multicolor fluorescent carbon quantum dot coatings. First, a nitrogen or sulfur source is added to lignin to prepare a precursor solution. Then, a hydrothermal reaction is carried out to obtain a carbon quantum dot solution. Next, a film-forming agent solution and additives are added to the carbon quantum dot solution, and after thorough mixing, a CQDs-PVA coating is obtained. After defoaming, the lignin-based multicolor fluorescent carbon quantum dot coating is obtained. Firstly, the method of this invention uses green and sustainable raw materials, employing lignin, a byproduct of the papermaking industry, as the carbon source, achieving high-value utilization of waste. The bio-based content is ≥30%, and the raw material cost is ≤80 yuan / kg, far lower than traditional rare-earth-based and organic dye coatings. Secondly, the preparation process of this invention is environmentally friendly and efficient, using a "one-pot" method. The hydrothermal synthesis process uses water as the solvent throughout, eliminating the use of toxic organic solvents. It features high process integration, stable yield, and ease of industrial production. The resulting coating exhibits excellent and stable luminescent properties. Through the formation of a hydrogen bond network between PVA and carbon quantum dot surface functional groups, the ACQ effect is effectively suppressed, resulting in a fluorescence quantum yield ≥42%. After 1000 hours of UV aging, the fluorescence intensity retention rate is ≥82%, and the multicolor emission wavelength covers 420-650nm with convenient control. The coating exhibits strong adhesion to wood substrates (≤1 grade), excellent mechanical properties (hardness ≥2H, good flexibility, strong wear resistance), and good weather and water resistance, meeting long-term use requirements. It is applicable to high-end decoration, brand anti-counterfeiting, and intelligent packaging, providing technical support for the functional upgrading of the wood processing industry. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 The images show the excitation-emission fluorescence spectra of the lignin-based multicolor fluorescent carbon quantum dots (B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs and DR-CQDs) obtained in this invention; where (a)-(f) are respectively the results prepared using lignin as the carbon source and 7-hydroxycoumarin, riboflavin, sodium fluorescein, rhodamine 6G, rhodamine B, and sulfonylrhodamine B as dopants, and are named B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs and DR-CQDs respectively. Figure 2 The images shown are high-resolution transmission electron microscopy (TEM) images and particle size distribution diagrams of the lignin-based multicolor fluorescent carbon quantum dots (B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs and DR-CQDs) obtained in this invention; where (a)-(f) are respectively prepared using lignin as the carbon source and 7-hydroxycoumarin, riboflavin, sodium fluorescein, rhodamine 6G, rhodamine B, and sulfonylrhodamine B as dopants, and are named B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs and DR-CQDs; Figure 3 The images show the infrared spectra of the lignin-based multicolor fluorescent carbon quantum dots (B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs and DR-CQDs) obtained in this invention; where (a)-(f) are respectively prepared using lignin as the carbon source and 7-hydroxycoumarin, riboflavin, sodium fluorescein, rhodamine 6G, rhodamine B, and sulfonylrhodamine B as dopants, and are named B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs and DR-CQDs; Figure 4 Images of six lignin-based carbon quantum dots and lignin-based fluorescent coatings under different wavelengths of light are shown; (a) images of the six lignin-based carbon quantum dots under sunlight, 254 nm and 365 nm ultraviolet light; (b) images of the six lignin-based fluorescent coatings under sunlight, 254 nm and 365 nm ultraviolet light. Figure 5The images show the effects of six lignin-based fluorescent coatings applied to wood chips (square and round) under sunlight, 254 nm and 365 nm ultraviolet light (comparison of full coating and half coating effects). Figure 6 The images show the effects of applying six lignin-based fluorescent coatings to wooden eggs and different fruit-shaped wooden objects (apples, pears, mushrooms, and acorns); (a) an image of the six lignin-based fluorescent coatings applied to wooden eggs; and (b) an image of the six lignin-based fluorescent coatings applied to fruit-shaped wooden objects. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be described in detail below. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other implementation methods obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0025] The above technical solutions are described in detail below with reference to specific embodiments. Unless otherwise specified, the reagents involved in the specific embodiments of the present invention are all commercially available products and can be purchased through commercial channels.

[0026] Example 1 This embodiment provides a method for preparing a lignin-based blue fluorescent carbon quantum dot coating, comprising the following steps: (1) Preparation of precursor solution Add 0.01g of lignin (commercially available, product model JYS1545, derived from papermaking black liquor, purity ≥95%, molecular weight range 5000-10000Da) and 0.03g of 7-hydroxycoumarin to a clean 250mL beaker; then add 20mL of deionized water to the beaker and stir at room temperature for 30min to completely dissolve the solid and obtain a homogeneous precursor solution.

[0027] (2) Hydrothermal synthesis and purification of carbon quantum dots (CQDs) The precursor solution was transferred to a 100 mL polytetrafluoroethylene-lined hydrothermal reactor and reacted at 160 °C for 10 h. After naturally cooling to room temperature, it was centrifuged at 10,000 rpm for 15 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 48 h, with the deionized water replaced every 6 h, to obtain a pure blue fluorescent carbon quantum dot solution.

[0028] (3) Preparation of PVA aqueous solution Heat 50 mL of deionized water to 85 °C, and add 8 g of PVA1799 (87-89% degree of hydrolysis) in three portions, with each portion added at 5 min intervals. Stir at 300 rpm until completely dissolved to obtain a 16 wt% PVA aqueous solution.

[0029] (4) Coating compounding and defoaming Cool the PVA aqueous solution to 40℃, add 10mL of carbon quantum dot solution, then add 0.2mL of polydimethylsiloxane (viscosity 500cps) and 2mL of glycerol (purity ≥99.5%). Stir at 400r / min for 15min to obtain a uniform CQDs-PVA coating. Let stand for 5min to defoam, and control the viscosity of the coating at 25℃ to 500-800mPa·s (tested according to GB / T1723-1993).

[0030] (5) Substrate pretreatment Use 400-grit sandpaper to smooth the surface of the engineered wood, removing sawdust and dust; wipe the surface with a clean cloth dampened with anhydrous ethanol, and let it air dry naturally.

[0031] (6) Coating and curing Apply the coating evenly to the surface of the engineered wood using a wool brush in a "horizontal then vertical" pattern. Control the wet film thickness to 50-80μm using a wet film comb. Allow it to dry at 60℃ for 1 hour for initial curing. After cooling, repeat the coating once more. Finally, dry at 70℃ for 1.5 hours and allow it to cool naturally to obtain a lignin-based blue fluorescent carbon quantum dot coating with a dry film thickness of 30-50μm.

[0032] Performance test results: carbon quantum dot emission wavelength 450±5nm, fluorescence quantum yield 42.2%; coating adhesion grade 1, pencil hardness 2H, fluorescence intensity retention rate 83.5% after 1000h UV aging, fluorescence intensity change 8.2% after 72h immersion in distilled water.

[0033] Example 2 This embodiment provides a method for preparing a lignin-based blue-green fluorescent carbon quantum dot coating. The only difference from Example 1 is that in step (1), riboflavin is selected as the nitrogen / sulfur source, and the parameters of the other steps are the same as those in Example 1.

[0034] Example 3 This embodiment provides a method for preparing a lignin-based green fluorescent carbon quantum dot coating. The only difference from Example 1 is that in step (1), sodium fluorescein is used as the nitrogen / sulfur source, and the parameters of the other steps are the same as those in Example 1.

[0035] Example 4 This embodiment provides a method for preparing a lignin-based orange fluorescent carbon quantum dot coating. The only difference from Example 1 is that in step (1), Rhodamine 6G is selected as the nitrogen / sulfur source, and the parameters of the other steps are the same as those in Example 1.

[0036] Example 5 This embodiment provides a method for preparing a lignin-based red fluorescent carbon quantum dot coating. The only difference from Example 1 is that in step (1), Rhodamine B is selected as the nitrogen / sulfur source, and the parameters of the other steps are the same as those in Example 1.

[0037] Example 6 This embodiment provides a method for preparing a lignin-based deep red fluorescent carbon quantum dot coating. The only difference from Example 1 is that in step (1), sulfonyl rhodamine B is selected as the nitrogen / sulfur source, and the parameters of the other steps are the same as those in Example 1.

[0038] Performance test results: carbon quantum dot emission wavelength 620±5nm, fluorescence quantum yield 42.5%; coating adhesion grade 1, pencil hardness 2H, fluorescence intensity retention rate 82.3% after 1000h UV aging, fluorescence intensity change 7.8% after 72h immersion in distilled water.

[0039] Comparative Example 1 The only difference between this comparative example and Example 1 is that polydimethylsiloxane is not added in step (4), while the rest of the steps are exactly the same as in Example 1.

[0040] Performance test results: After immersion in distilled water for 72 hours, the fluorescence intensity of the coating changed by 23.6%, and the water resistance decreased significantly, indicating that the addition of polydimethylsiloxane additive in the method of this invention plays an important role in improving the water resistance of the coating.

[0041] Comparative Example 2 The only difference between this comparative example and Example 1 is that 0.01g of glucose was used to replace lignin in step (1), while the rest of the steps are exactly the same as in Example 1.

[0042] Performance test results: The fluorescence quantum yield of carbon quantum dots was 7.5%, which was 34.7% lower than that of Example 1, demonstrating the advantage of lignin as a carbon source in the method of the present invention.

[0043] Comparative Example 3 The only difference between this comparative example and Example 1 is that steps (1) and (2) are different; the rest of the steps are exactly the same as in Example 1.

[0044] The specific steps (1) and (2) of this comparative example are as follows: (1) Add 0.01g of lignin to a clean 250mL beaker, then add 20mL of deionized water to the beaker and stir at room temperature for 30min to obtain a solution. (2) Transfer the above solution to a 100mL polytetrafluoroethylene-lined hydrothermal reactor and react at 180℃ for 8h. After naturally cooling to room temperature, add 0.03g of 7-hydroxycoumarin and then heat to 180℃ for 8h. Centrifuge at 10000 rpm / min for 15 min, and remove residue by filtration through a 0.22 μm filter membrane; transfer the filtrate into a dialysis bag with a molecular weight cutoff of 1000 Da and dialyze for 48 h, changing the deionized water every 6 h to obtain a carbon quantum dot solution.

[0045] Performance test results: The carbon quantum dot yield decreased by 32.7% compared with Example 1, and the process complexity increased. Therefore, the "one-pot" method of the present invention can simplify the process while improving the carbon quantum dot yield.

[0046] Comparative Example 4 This comparative example was prepared using the method described in existing literature (CN107033888A), and the specific steps are as follows: A certain amount of herbal medicine residue was heated under reflux with 10% NaOH solution for 2-3 hours, then cooled to room temperature. The residue was washed with water until neutral and then dried in a constant temperature drying oven. The dried residue and solid NaOH were placed in a high-pressure reactor lined with polytetrafluoroethylene at a mass ratio of 1:1 and reacted in a constant temperature drying oven at 200-300℃ for about 5 hours. After cooling to room temperature, the reactor was opened, and the black solution was poured out. The solution was filtered to remove the residue, yielding a black alkaline carbon nanoparticle aqueous solution. The supernatant was retained after centrifugation. The hydroxyl carbon dots were obtained by rotary evaporation of anhydrous ethanol.

[0047] Performance test results: The fluorescence quantum yield of carbon quantum dots decreased by 10% compared with Example 1, proving that the present invention has a high efficiency in converting absorbed light energy into emitted fluorescence.

[0048] Experimental Example 1 1. The lignin-based multicolor fluorescent carbon quantum dots obtained in this invention were characterized by spectral analysis, as follows.

[0049] Figure 1 The images show the excitation-emission fluorescence spectra of the lignin-based multicolor fluorescent carbon quantum dots (B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs, and DR-CQDs) obtained in Examples 1-6 of this invention; Figure 1As shown, six carbon quantum dot samples (a)-(f) exhibited the strongest fluorescence emission at 455 nm, 515 nm, 545 nm, 598 nm, 614 nm, and 645 nm under 365 nm excitation. These six samples (a)-(f) are B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs, and DR-CQDs, respectively. The emission region covers the blue to deep red light band, indicating a wide range of visible light emission modulation capabilities. With the change of sample type, the maximum emission peak position gradually redshifts, indicating significant differences in optical energy level structure, surface state distribution, and electronic transition processes among different carbon quantum dots. This result demonstrates that the tunable design of the emission color of carbon quantum dots can be effectively achieved by controlling the raw material composition or preparation conditions.

[0050] Figure 2 These are high-resolution transmission electron microscopy images and particle size distribution diagrams of the lignin-based multicolor fluorescent carbon quantum dots obtained in Examples 1-6 of this invention; Figure 2 As shown, the six carbon quantum dot samples (a)-(f) all exhibit approximately spherical nanoparticle morphologies with good dispersion, indicating good particle integrity and low agglomeration tendency. High-resolution transmission electron microscopy (HRTEM) images reveal clear particle boundaries and a relatively uniform overall size distribution, suggesting good controllability in the synthesis process of different samples. The insets in the upper right corner of each image show the statistical results of the particle size distribution for the corresponding samples, showing that the average particle sizes of the six carbon quantum dots are approximately 3.5±0.9 nm, 3.6±0.7 nm, 3.2±0.8 nm, 3.8±0.6 nm, 3.6±1.2 nm, and 3.5±0.9 nm, respectively. The particle sizes are mainly concentrated in the 2-5 nm range, with narrow distribution peaks, further demonstrating the small and relatively uniform size characteristics of the samples. The scale bar for all images is 20 nm.

[0051] Figure 3 The infrared spectra of the lignin-based multicolor fluorescent carbon quantum dots obtained in Examples 1-6 of this invention are shown below; Figure 3 It can be seen that the infrared spectra of the lignin-based multicolor fluorescent carbon quantum dots (a)-(f) obtained in Examples 1-6 generally show a consistent trend, all within the range of 3400 cm⁻¹. -1 The presence of broad absorption peaks on the left and right corresponds to O–H and N–H stretching vibrations, indicating the abundance of oxygen- and nitrogen-containing functional groups such as hydroxyl and amino groups on the sample surface; at 1720 cm⁻¹... -1 The characteristic peaks appearing nearby can be attributed to C=O stretching vibrations, indicating that the carbonyl structure is retained on the surface of the carbon quantum dots; simultaneously, at 1600-1610 cm⁻¹... -1Vibrational peaks of the aromatic ring C=C skeleton can be observed nearby, indicating that the sample still contains aromatic structural units derived from lignin. Furthermore, at 1500-1380 cm⁻¹... -1 The absorption peaks within the region can be attributed to C–N stretching vibrations and N–H bending vibrations, while the absorption peaks at 1200–1050 cm⁻¹ can be attributed to these vibrations. -1 The nearby absorption peaks mainly correspond to C–O and C–O–C stretching vibrations, further indicating that lignin molecules underwent partial cleavage, recombination, and surface oxidation / nitrogen doping during carbonization, forming a carbon quantum dot structure with abundant surface functional groups. These results demonstrate that the lignin-based multicolor fluorescent carbon quantum dots prepared in different embodiments share similarities in chemical composition and surface structure, providing a structural basis for their excellent fluorescence performance.

[0052] Figure 4 Images of six lignin-based carbon quantum dots and lignin-based fluorescent coatings under different wavelengths of light; (a) images of the six lignin-based carbon quantum dots under sunlight, 254 nm, and 365 nm ultraviolet light; (b) images of the six lignin-based fluorescent coatings under sunlight, 254 nm, and 365 nm ultraviolet light; from Figure 4 It can be seen that the six lignin-based carbon quantum dots (B-CQDs, C-CQDs, G-CQDs, O-CQDs, R-CQDs, and DR-CQDs) and their prepared fluorescent coatings all exhibited varying degrees of intrinsic color under sunlight. However, under 254 nm and 365 nm ultraviolet light irradiation, they all showed significant fluorescence emission, with differences in the emitted colors among different samples, exhibiting visible fluorescence ranging from blue, green, yellow, orange to red. This indicates that lignin-based carbon quantum dots possess good photoluminescence properties and can effectively impart excellent fluorescence response characteristics to coatings, suggesting their potential application value in fields such as anti-counterfeiting, information encryption, and functional coatings.

[0053] Figure 5 Images showing the effects of six lignin-based fluorescent coatings applied to wood chips (square and round) under sunlight, 254 nm, and 365 nm ultraviolet light (comparison of full and half coating effects); from Figure 5As can be seen, after being coated onto the wood chip surface, the six lignin-based fluorescent coatings exhibited their respective intrinsic colors under sunlight, with a clear color difference between fully coated and partially coated samples, indicating that the coatings have good visual distinguishing effect. Under 254 nm and 365 nm ultraviolet light irradiation, all coatings showed significant fluorescence emission, with clear and bright colors. There were significant differences in fluorescence color between different samples, and a sharp contrast was formed between the coated and uncoated areas in terms of luminescence effect. Especially in the partially coated samples, the fluorescence difference between the coated area and the bare wood chip area was more intuitive, indicating that the lignin-based fluorescent coating has good fluorescence response performance and strong visual recognition ability, providing a basis for its application in anti-counterfeiting labels, information encryption, and functional coatings.

[0054] Figure 6 Images show the effects of applying six lignin-based fluorescent coatings to wooden eggs and various fruit-shaped wooden containers (apples, pears, mushrooms, and acorns); (a) an image of the six lignin-based fluorescent coatings applied to wooden eggs; (b) an image of the six lignin-based fluorescent coatings applied to fruit-shaped wooden containers. Figure 6 It can be seen that the six lignin-based fluorescent coatings can adhere uniformly to the surfaces of wooden eggs and various fruit-shaped wooden products, and exhibit characteristic colors corresponding to their respective samples under sunlight, indicating that the coatings have good film-forming properties and designability. Under 254 nm and 365 nm ultraviolet light irradiation, all coatings showed obvious and stable fluorescence emission, with different samples displaying a variety of visible luminescent colors such as blue, green, yellow, orange, and red. The luminescence effect was clear and highly contrasting, indicating that the lignin-based fluorescent coatings have excellent fluorescence response and strong visual recognition. Meanwhile, the luminescence performance of the coatings remained consistent on different wooden substrates, indicating good substrate adaptability and application versatility. These results further suggest that this type of lignin-based fluorescent coating has potential application value in wooden decoration, anti-counterfeiting of handicrafts, and functional surface treatment.

[0055] 2. The performance of the lignin-based multicolor fluorescent carbon quantum dots obtained in Example 1 of the present invention was tested, as shown in Tables 1 and 2.

[0056] Table 1 - Performance Comparison of the Lignin-Based Multicolor Fluorescent Carbon Quantum Dots of the Present Invention and Traditional Fluorescent Coatings Table 2 - Comparison of the lignin-based multicolor fluorescent carbon quantum dots of the present invention with existing biomass carbon dot coatings (Comparative Example 4) In summary, this invention provides a method for preparing a lignin-based multicolor fluorescent carbon quantum dot coating. First, a nitrogen or sulfur source is added to lignin to prepare a precursor solution. Then, a hydrothermal reaction is carried out to obtain a carbon quantum dot solution. Next, a film-forming agent solution and additives are added to the carbon quantum dot solution, and after thorough mixing, a CQDs-PVA coating is obtained. After defoaming, the lignin-based multicolor fluorescent carbon quantum dot coating is obtained. Firstly, this invention uses green and sustainable raw materials, employing lignin, a byproduct of the papermaking industry, as the carbon source, achieving high-value utilization of waste. The bio-based content is ≥30%, and the raw material cost is ≤80 yuan / kg, far lower than traditional rare-earth-based and organic dye coatings. Secondly, the preparation process described in this invention is environmentally friendly and efficient, using a "one-pot" method. The hydrothermal synthesis process uses water as the solvent throughout, eliminating the use of toxic organic solvents. It features high process integration, stable yield, and ease of industrial production. The resulting coating exhibits excellent and stable luminescent properties. Through the formation of a hydrogen bond network between PVA and carbon quantum dot surface functional groups, the ACQ effect is effectively suppressed, resulting in a fluorescence quantum yield ≥42%. After 1000 hours of UV aging, the fluorescence intensity retention rate is ≥82%, and the multicolor emission wavelength covers 420-650nm with convenient control. The coating exhibits strong adhesion to wood substrates (≤1 grade), excellent mechanical properties (hardness ≥2H, good flexibility, strong wear resistance), and good weather and water resistance, meeting long-term use requirements. It is applicable to high-end decoration, brand anti-counterfeiting, and intelligent packaging, providing technical support for the functional upgrading of the wood processing industry.

[0057] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a lignin-based multi-color fluorescent carbon quantum dots paint, characterized in that, Includes the following steps: (1) Preparation of precursor solution Take lignin, add a nitrogen source or a sulfur source, add water and stir the solution thoroughly to obtain a precursor solution; (2) Synthesis of carbon quantum dot solution The precursor solution described in step (1) was placed under hydrothermal conditions for reaction, then cooled to room temperature, and successively centrifuged, filtered and dialyzed to obtain a carbon quantum dot solution. (3) Preparation of film-forming agent aqueous solution Add the film-forming agent to hot water and dissolve it completely under stirring to obtain an aqueous solution of the film-forming agent; (4) Coating compounding and defoaming After the film-forming agent aqueous solution in step (3) is cooled down, the carbon quantum dot solution in step (2) is added, and then the additives are added dropwise under stirring conditions to obtain CQDs-PVA coating. After standing and defoaming, lignin-based multicolor fluorescent carbon quantum dot coating is obtained.

2. The method for preparing a lignin-based multi-color fluorescent carbon quantum dots paint according to claim 1, characterized in that, In step (1), the molecular weight of the lignin is 5000-10000 Da; The nitrogen or sulfur source is one of 7-hydroxycoumarin, riboflavin, sodium fluorescein, rhodamine 6G, rhodamine B, or sulfonylrhodamine B.

3. The method for preparing lignin-based multicolor fluorescent carbon quantum dot coating according to claim 1, characterized in that, In step (2), the hydrothermal conditions are a constant temperature reaction at 160℃ for 10 hours; The centrifugation conditions were 10,000 rpm / min for 15 min; The filtration process involves removing residue by vacuum filtration through a 0.22μm filter membrane. The dialysis process involves transferring the filtrate into a dialysis bag with a molecular weight cutoff of 1000 Da and dialyzing for 48 hours, replacing the deionized water every 6 hours.

4. The method for preparing lignin-based multicolor fluorescent carbon quantum dot coating according to claim 2, characterized in that, In step (3), the film-forming agent is one of polyvinyl alcohol, polyethylene glycol, polyvinylpyrrolidone, or aqueous polyurethane, and the concentration range of the aqueous solution of the film-forming agent is 10wt%-16wt%. The hot water is at 85℃. Add the film-forming agent to the hot water in three portions, with each portion added every 5 minutes, and stir at 300 rpm until completely dissolved.

5. The method for preparing lignin-based multicolor fluorescent carbon quantum dot coating according to claim 1, characterized in that, In step (4), the aqueous solution of the film-forming agent is cooled to 40°C, and then a carbon quantum dot solution is added; The additives are polydimethylsiloxane and glycerol.

6. The method for preparing lignin-based multicolor fluorescent carbon quantum dot coating according to claim 5, characterized in that, In step (4), the mass ratio of the film-forming agent aqueous solution, carbon quantum dot solution, polydimethylsiloxane and glycerol is 60-70:10-15:10-15:5-10; The stirring conditions were 400 r / min for 15 min; The viscosity of the coating at 25°C is 500-800 mPa·s.

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

8. The method of using the lignin-based multicolor fluorescent carbon quantum dot coating according to claim 7, characterized in that, Includes the following steps: (S1) Take the substrate for pretreatment, specifically: use 400-grit sandpaper to sand the surface of the artificial board to remove sawdust and dust; wipe the surface with a clean cloth dipped in anhydrous ethanol and let it air dry naturally; (S2) The coating is applied evenly to the substrate surface in a horizontal-then-vertical manner, and the wet film thickness is controlled to be 50-80μm by a wet film comb; then it is dried at 60℃ for 1h for preliminary curing, and after cooling, it is applied again; finally, it is dried at 70℃ for 1.5h and then cooled naturally to obtain the lignin-based multicolor fluorescent carbon quantum dot coating.

9. The method of using the lignin-based multicolor fluorescent carbon quantum dot coating according to claim 8, characterized in that, The dry film thickness of the coating is 30-50 μm.

10. The application of the lignin-based multicolor fluorescent carbon quantum dot coating prepared by the method according to any one of claims 1-6, characterized in that, The application is any one of high-end furniture decoration, brand anti-counterfeiting, or smart packaging.

Images