Method for preparing lignin-based carbon quantum dots by solvent-free one-pot method, prepared lignin-based carbon quantum dots and application of lignin-based carbon quantum dots
By reacting lignin, oxalic acid and lysine at high temperatures without solvent, nitrogen-doped lignin-based carbon quantum dots with high quantum yield and yield, solving the problems of low quantum yield and yield, complex process and environmental protection in the prior art, and realizing a green, environmentally friendly, safe and economical preparation method.
Patent Information
- Application Number
- CN202510357092.7
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- Filing Date
- 2025-03-25
- Publication Date
- 2025-06-27
AI Technical Summary
The existing lignin-based carbon quantum dots have low quantum yields and yields, and the preparation method has problems such as high cost, complex process, harsh reaction conditions, high energy consumption and the use of harmful solvents.
The solvent-free one-pot method was adopted to carry out a heat-induced self-foaming reaction of lignin, oxalic acid and lysine at 160-200°C to form nitrogen-doped lignin-based carbon quantum dots, and the described lignin-based carbon quantum dots were obtained through centrifugation, dialysis purification, filtration, and freeze-drying.
The high quantum yield and yield of lignin-based carbon quantum dots was achieved, with the quantum yield reaching 21.6%, while avoiding the use of harmful solvents, reducing production costs, simplifying the process, and the process is environmentally friendly and safe.
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Abstract
Description
Technical Field
[0001] The present invention belongs to the technical field of carbon quantum dots, and particularly relates to a method for preparing lignin-based carbon quantum dots by a solvent-free one-pot method, the prepared lignin-based carbon quantum dots, and applications thereof. Background Art
[0002] Carbon quantum dots are a new type of fluorescent nanoparticles with a size less than 10 nm, belonging to the category of zero-dimensional carbon nanomaterials. They have good biocompatibility, excellent water solubility and stability, and excellent optical properties. As an important new type of fluorescent nanomaterial emerging in recent years, carbon quantum dots show great application potential in the fields of biological imaging, catalysis, ion detection, and drug carriers. The luminescence mechanism of carbon quantum dots mainly involves quantum size effect and surface defect effect. When carbon quantum dots are excited by energy, their electrons will transition to high energy levels and then return to low energy levels to release photons, generating luminescence. There are mainly two preparation methods for carbon dots: "top-down" and "bottom-up". "Top-down" mainly uses graphene-based materials as carbon sources to exfoliate very small carbon quantum dots through laser etching, arc discharge, and electrochemical oxidation methods. Generally, the carbon quantum dots prepared by the "top-down" method have a highly crystalline carbon core and oxidized functional groups on the surface or edges, and their optical properties mainly depend on the surface / edge structure and the size of the sp 2 Conjugated structural domain. This "top-down" method for preparing carbon quantum dots usually requires extreme reaction conditions, such as strong oxidizing acids, strong bases, and electrochemical etching. The carbon quantum dots prepared by these methods are usually blue fluorescent with a very low fluorescence quantum yield (<10%), and the application effect is not ideal. "Bottom-up" uses organic molecules as raw materials to prepare carbon quantum dots through microwave synthesis, hydrothermal and solvothermal, and chemical oxidation methods. Commonly used organic carbon sources include sugars and their derivatives, carboxylic acids and their derivatives, amines, and biomass. The reaction conditions in the bottom-up method are adjustable and the raw materials are abundant. Therefore, carbon quantum dots with high quantum yields can be synthesized by this method, and the emission wavelength can be adjusted.
[0003] In recent years, lignin-based carbon quantum dots have become a hot topic in the research of carbon quantum dots due to their low price, good biocompatibility, and rich carbon and oxygen-containing functional groups. Lignin is the second most abundant non-petroleum resource globally, with a wide range of sources and the ability to be recycled in nature. From the perspectives of sustainable development and environmental protection, lignin is green and renewable, but most of it exists as by-products in the pulp and paper industries and has not been efficiently utilized, leading to energy waste. In addition, the discharge of paper mill black liquor also causes environmental pollution problems. From the structure of lignin, its high carbon content, aromatic structure, and oxygen-containing functional groups not only facilitate the formation of the carbon core of carbon quantum dots but also provide a favorable basis for doping other heteroatoms to improve fluorescence properties. Jiang et al. (Jiang X, Shi Y, Liu X, et al. Synthesis of nitrogen-doped lignin / DES carbon quantum dots as a fluorescent probe for the detection of Fe 3+ions[J].Polymers,2018,10(11):1282.) Using alkali lignin as the carbon source and deep eutectic solvent (DES) as the solution and nitrogen source, nitrogen-doped lignin carbon quantum dots were prepared at 300 °C, and the quantum yield of the prepared carbon dots was 7.95%. Ding et al. (Ding Z, Li F, Wen J, et al. Gram-scale synthesis of single-crystalline graphene quantum dots derived from lignin biomass[J]. Green chemistry, 2018, 20(6): 1383-1390.) oxidized and cut alkali lignin into lignin fragments by ultrasonic pretreatment with nitric acid, and then synthesized green fluorescent lignin carbon quantum dots by hydrothermal reaction at 180 °C, with a yield of 21%. Shi et al. (Shi Y, Liu X, Wang M, et al. Synthesis of N-doped carbon quantum dots from bio-waste lignin for selective irons detection and cellular imaging[J]. International journal of biological macromolecules, 2019, 128: 537-545.) modified and ammonium lignin by Mannich addition reaction using ethylenediamine, and synthesized amine-rich lignin carbon quantum dots by hydrothermal method for cell imaging. However, the quantum yields and yields of current lignin-based carbon quantum dots are generally low, and the preparation methods mostly use one-pot solvothermal reaction for high-temperature carbonization, such as nitrogen-containing organic solvent ethylenediamine, or the two-step method is to first perform pretreatment such as modification and acidolysis and then perform hydrothermal reaction under high temperature and high pressure. Most of them generally have the disadvantages of high cost, complex process, harsh reaction conditions, high energy consumption, etc., and most of the strongly corrosive acids and organic solvents used will pollute the environment or harm human health. Therefore, it is crucial to find a synthesis method for lignin-based carbon quantum dots that is inexpensive, simple in process, green and non-toxic. Summary of the Invention
[0004] In order to solve the disadvantages and deficiencies of poor fluorescence performance of lignin carbon quantum dots and non-environmental protection of organic solvents for preparation raw materials in the above-mentioned prior art, the primary object of the present invention is to provide a method for preparing lignin-based carbon quantum dots by solvent-free one-pot method.
[0005] Another object of the present invention is to provide a lignin-based carbon quantum dot prepared by the above method.
[0006] Another object of the present invention is to provide an application of the above lignin-based carbon quantum dots.
[0007] The object of the present invention is achieved by the following technical solutions:
[0008] A method for preparing lignin-based carbon quantum dots by a solvent-free one-pot method, comprising the following operating steps:
[0009] Lignin, oxalic acid and lysine are ground together to be fully and uniformly mixed, and a thermal-induced spontaneous foaming reaction is carried out at 160-200 °C. After the reaction is completed, ultrapure water is added for dissolution to obtain a crude product of nitrogen-doped lignin-based carbon quantum dots; the crude product of nitrogen-doped lignin-based carbon quantum dots is centrifuged, purified by dialysis, filtered, and freeze-dried to obtain the lignin-based carbon quantum dots.
[0010] The lignin is industrial alkali lignin.
[0011] The mass ratio of the lignin, oxalic acid and lysine is (1-2):5:8, preferably 1:5:8.
[0012] The grinding time is 2-8 min, preferably 5 min.
[0013] The reaction temperature is preferably 180 °C.
[0014] The reaction time is 1-12 h, preferably 5-8 h, more preferably 6 h.
[0015] The centrifugation speed is 10,000 rpm and the centrifugation time is 5 min.
[0016] The dialysis purification is carried out by dialysis using a dialysis bag with a molecular weight cut-off of 100-1000 Da (preferably 200 Da) for 24-72 h.
[0017] The filtration is carried out using a microporous organic filter membrane with a pore size of 0.1 μm.
[0018] A lignin-based carbon quantum dot prepared by the above method, the average particle size of the lignin-based carbon quantum dot is 3-10 nm; the lignin-based carbon quantum dot has good luminescence properties, presents bright cyan under the irradiation of a 365 nm ultraviolet lamp, and the quantum yield reaches 21.6%.
[0019] The above lignin-based carbon quantum dots have strong wavelength dependence and biocompatibility; they have strong wavelength dependence and can achieve laser emission at different wavelengths. The high photostability and biocompatibility of the carbon dots make them ideal photosensitizers and are suitable for various biological imaging and photodynamic therapy needs.
[0020] The principle of the present invention:
[0021] The present invention has found through research that under the conditions of high temperature and normal pressure without solvent, lignin, oxalic acid and lysine interact with each other. The carbonaceous fragments generated by the depolymerization and carbonization of lignin are further refined and dispersed under the action of bubbles. At the same time, substances such as polymers formed by the polymerization of lysine play a role in stabilizing and encapsulating these carbonaceous fragments, inhibiting their excessive growth and agglomeration, thereby promoting the formation of lignin-based carbon quantum dots with smaller sizes and uniform distribution.
[0022] The present invention has the following advantages and effects compared with the prior art:
[0023] (1) The present invention has developed a simple solvent-free one-pot method for preparing lignin-based carbon quantum dots. Through a spontaneous foaming reaction induced by heat under normal pressure and high temperature, lignin is first cracked into small molecule carbon in a top-down manner, and then carbon quantum dots are synthesized from bottom to top.
[0024] (2) The preparation method of the present invention does not need to be carried out under high pressure conditions, making the preparation method safer. And it does not use acids, strongly alkaline solvents and toxic organic solutions to assist in the generation of carbon quantum dots, and has excellent green environmental protection. The lignin-based carbon quantum dots prepared by the preparation method of the present invention have excellent luminescence properties. Description of the Drawings
[0025] Figure 1 TEM image of the carbon quantum dots prepared for the examples; it can be seen from the figure that the obtained lignin carbon dots are roughly spherical, with obvious lattice patterns, and the diameter size is between 3 - 10 nm.
[0026] Figure 2 Photoluminescence spectra of lignin carbon dots at different reaction temperatures for Examples 1, 2, and 3.
[0027] Figure 3 Photoluminescence spectra of lignin carbon dots at different reaction times for Examples 1, 4, and 5.
[0028] Figure 4 Photoluminescence spectra of the lignin carbon dots of Example 1 at different excitation wavelengths.
[0029] Figure 5 Photoluminescence spectra of Examples 1, 6 - 10 with different reactants under the same reaction conditions.
[0030] Figure 6 Fluorescence actual photo of the carbon quantum dot solution after dialysis purification of Example 1 under a 365 nm ultraviolet lamp. Detailed Embodiments
[0031] The following further illustrates the content of the present invention with specific examples, but it should not be construed as a limitation to the present invention.
[0032] The raw materials in the embodiments and comparative examples can be obtained from commercial sources, specifically:
[0033] Lignin was purchased from Beijing Solebow Technology Co., Ltd.;
[0034] Lysine, purchased from Aladdin Reagents;
[0035] Oxalic acid, purchased from Aladdin Reagents;
[0036] Dialysis bag, LiGe brand, molecular weight cutoff 200 Dalton, radius 22mm.
[0037] The raw materials in the examples and comparative examples can be obtained from commercial sources. Unless otherwise specified, the reagents, methods and equipment used in the present invention are conventional reagents, methods and equipment in the art.
[0038] Example 1
[0039] Put 0.3g of alkali lignin, 1.5g of lysine and 2.4g of oxalic acid into a mortar, grind for 5 minutes to fully mix, then put into an open flask and perform a heat-induced self-foaming reaction at 180°C for 6 hours; after the reaction, add 100mL of ultrapure water to dissolve, and stir magnetically for 30 minutes to 60 minutes to obtain a crude nitrogen-doped lignin-based carbon quantum dot product; centrifuge the crude product at a speed of 10,000rpm for 5 minutes, take the supernatant and place it in a 200Da dialysis bag for dialysis purification for 48 hours, and after the dialysis, use a 0.1 micron microporous filter paper for vacuum filtration to obtain the liquid part, and freeze-dry it to obtain a lignin-based carbon quantum dot solid powder, which is recorded as OKLCD-180-6. Use a transmission electron microscope to perform electron microscope scanning on the lignin-based carbon quantum dots prepared in this example, and obtain Figure 1 From the figure, we can see that the diameter of lignin carbon quantum dots is between 3-10nm, and has good dispersibility. Figure 6 As shown, the prepared carbon quantum dots exhibit bright green color under 365nm ultraviolet light, and the fluorescence quantum yield is measured to be 21.6% and the yield is 64.8%.
[0040] Embodiments 2 to 10
[0041] The reaction temperature in Example 2 was 200° C., and the other preparation processes were consistent with Example 1, to obtain a lignin-based carbon quantum dot solid powder, denoted as OKLCD-200-6, whose fluorescence quantum yield was measured to be 7.5% and the yield was 4.2%.
[0042] The reaction temperature in Example 3 was 160° C., and the other preparation processes were consistent with Example 1, to obtain a lignin-based carbon quantum dot solid powder, denoted as OKLCD-160-6, whose fluorescence quantum yield was measured to be 5.5% and the yield was 39.4%.
[0043] In Example 4, the reaction time was 12 h, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as OKLCD-180-12. Its fluorescence quantum yield was measured to be 9.3%, and the yield was 10.5%.
[0044] In Example 5, the reaction time was 1 h, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as OKLCD-180-1. Its fluorescence quantum yield was measured to be 2.4%.
[0045] In Example 6, the reactant contained only alkali lignin, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as LCD-180-6. Its fluorescence quantum yield was measured to be 1.5%.
[0046] In Example 7, the reactant contained only lysine, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as KCD-180-6. Its fluorescence quantum yield was measured to be 6.0%.
[0047] In Example 8, the reactants contained only lysine and oxalic acid, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as OKCD-180-6. Its fluorescence quantum yield was measured to be 2.3%.
[0048] In Example 9, the reactants contained only alkali lignin and oxalic acid, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as OLCD-180-6. Its fluorescence quantum yield was measured to be 1.9%.
[0049] In Example 10, the reactants contained only alkali lignin and lysine, and the other preparation processes were the same as those in Example 1. A lignin-based carbon quantum dot solid powder was obtained, denoted as KLCD-180-6. Its fluorescence quantum yield was measured to be 7.2%, and the yield was 4.1%.
[0050] The specific parameter indicators are shown in Table 1.
[0051] Table 1 Parameter indicators of the preparation methods in Examples 1-10
[0052]
[0053]
[0054] Performance testing
[0055] (1) Morphology of lignin-based carbon quantum dots
[0056] The lignin-based carbon quantum dots prepared in Example 1 were subjected to TEM testing by transmission electron microscopy, and the TEM image is shown inFigure 1 as shown in (a-d).
[0057] From the test results, it can be seen that the lignin carbon dots have good dispersibility, are roughly spherical in shape, have a size between 3 - 10 nm, and have obvious lattice patterns.
[0058] (2) Fluorescent properties of carbon quantum dots
[0059] Take 0.2 ml of the liquid part after vacuum filtration of Examples 1 - 10 respectively, that is, the carbon quantum dot sample solution, place it in a 20 ml glass sample bottle, add 20 ml of ultrapure water for dilution, and adjust the absorbance of the diluted sample solution at the optimal excitation wavelength to be less than 0.1 for photoluminescence (PL) spectroscopy testing. Select an excitation light of 340 - 460 nm and a slit width of 3 nm, and detect the luminescence intensity between 355 - 800 nm.
[0060] According to the fluorescence test results of Examples 1 - 3 Figure 2 it can be seen that as the reaction temperature increases, the fluorescence intensity of the generated lignin first increases and then decreases. When the reaction temperature is low, the activity of lignin molecules is low, the molecular chains are not broken sufficiently, it is difficult to form carbon nuclei small enough and further grow into carbon quantum dots, resulting in a small amount of carbon dots generated and a low yield. Too high a temperature will make the reaction too violent, which may lead to excessive pyrolysis of lignin, generating a large amount of small molecule gases and by-products such as tar, reducing the effective carbon source for forming carbon quantum dots, and causing the yield of carbon quantum dots to decrease. Therefore, a reaction temperature of 180 °C is appropriate to promote the polycondensation reaction to proceed smoothly. These fragments can effectively crosslink and agglomerate to form carbon quantum dot nuclei, and further grow into complete carbon quantum dots, so that the prepared carbon quantum dots have stronger fluorescence properties and a higher yield.
[0061] According to the fluorescence test results of Example 1 combined with Examples 4 - 5 Figure 3 , the fluorescence intensity and quantum yield of the carbon quantum dots first increase and then decrease with the reaction time. This may be because the reaction time is short and the reaction is not sufficient, so that stable carbon dots cannot be generated and the fluorescence properties are weak; too long a hydrothermal reaction time causes excessive carbonization on the surface of the carbon dots, reducing the fluorescence properties and the yield relatively. When the reaction time is 6 h, the time is appropriate, the surface state and internal structure of the carbon dots reach a better balance, the conjugated system and the surface functional groups are reasonably distributed, and good fluorescence properties and a relatively high fluorescence quantum yield can be shown.
[0062] From the fluorescence spectra of Example 1 at different excitation wavelengths Figure 4 , it can be concluded that the carbon quantum dots have strong wavelength dependence and good application potential. Using a 365 nm ultraviolet lamp to irradiate the carbon dot solution after dialysis of Example 1 from the bottom, Figure 6 a bright blue - green color appears, and the optical properties are good.
[0063] Obtained according to the fluorescence test results of Example 1 in combination with Examples 6-10 Figure 5 , from Figure 5 It can be seen that the thermally induced spontaneous foaming cross-linking reaction of lignin, lysine and oxalic acid in this system can greatly improve the fluorescence performance and yield of carbon quantum dots prepared with lignin as the carbon source.
[0064] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited by the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications made without departing from the spirit and principle of the present invention shall be equivalent replacement methods and are all included in the protection scope of the present invention.
Claims
1. A method for preparing lignin-based carbon quantum dots by a solvent-free one-pot method, characterized in that The steps include: The lignin, oxalic acid and lysine are ground together to be fully mixed, and a heat-induced self-foaming reaction is carried out at 160-200° C. After the reaction is completed, ultrapure water is added to dissolve to obtain a crude nitrogen-doped lignin-based carbon quantum dot product; the crude nitrogen-doped lignin-based carbon quantum dot product is centrifuged, dialyzed, filtered, and freeze-dried to obtain the lignin-based carbon quantum dots.
2. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 1, characterized in that: The lignin is industrial alkali lignin.
3. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 1, characterized in that: The mass ratio of the lignin, oxalic acid and lysine is (1-2):5:
8.
4. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 3, characterized in that: The mass ratio of the lignin, oxalic acid and lysine is 1:5:
8.
5. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 1, characterized in that: The grinding time is 2-8 minutes; the reaction time is 1-12 hours; the centrifugal speed is 10000 rpm, and the centrifugal time is 5 minutes; the dialysis purification is performed using a 100-1000Da dialysis bag for 24-72 hours.
6. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 5, characterized in that: The grinding time is 5 minutes; the reaction time is 5 to 8 hours; and the dialysis purification is performed using a 200Da dialysis bag.
7. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 1, characterized in that: The reaction temperature is 180° C. and the reaction time is 6 h.
8. The method for preparing lignin-based carbon quantum dots by a solvent-free one-pot process according to claim 1, characterized in that: The filtration is performed using a microporous organic filter membrane with a pore size of 0.1 μm.
9. A lignin-based carbon quantum dot prepared by the method according to any one of claims 1 to 8, characterized in that: The average particle size of the lignin-based carbon quantum dots is 3 to 10 nm; the lignin-based carbon quantum dots have good luminescence performance, showing a bright cyan color under 365 nm ultraviolet light, and the quantum yield reaches 21.6%.
10. Application of the lignin-based carbon quantum dots according to claim 9 in biological imaging and photodynamic therapy.