Modified lignin with high photo-thermal conversion efficiency and antibacterial property as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of lignin modification treatment, and particularly relates to modified lignin with high photo-thermal conversion efficiency and antibacterial property and a preparation method and application thereof.The preparation method comprises the following steps that a hydrogen bond acceptor and a hydrogen bond donor are mixed and then heated in a stirring state till a solution is clarified, and a eutectic solvent is obtained; mixing lignin with the eutectic solvent, carrying out a co-thermal reaction, and cooling to room temperature to obtain a solid-liquid mixture; adding ultrapure water into the solid-liquid mixture to separate out solute, and then centrifuging and washing to obtain the modified lignin, wherein the lignin comprises at least one of industrial lignin and natural lignin. According to the invention, the eutecticevaporate solvent is utilized, a very simple preparation process is adopted, and the photo-thermal conversion efficiency and the antibacterial property of the lignin are improved on the premise of ensuring the environmental protection property and the functionality of the lignin, so that the obtained modified lignin has a very excellent antibacterial effect.

Description

Technical Field

[0001] This application belongs to the technical field of lignin modification treatment, and specifically relates to modified lignin with high photothermal conversion efficiency and antibacterial properties, and its preparation method and application. Background Art

[0002] Lignin is one of the most abundant natural aromatic polymers in nature, widely existing in plant cell walls, and is the main by-product in the paper industry and biomass refining processes. Lignin can be divided into natural lignin and industrial lignin. Natural lignin refers to the original lignin structure in lignocellulose without any modification. Another type of lignin that is mostly being studied is modified lignin or industrial lignin, which is lignin extracted from biomass or recovered from industrial by-products. The content and composition of lignin in natural biomass raw materials vary depending on the plant source or species. Industrial lignin (such as kraft lignin, sulfonated lignin, and alkali lignin, etc.) is regarded as a green material with great potential due to its wide source, renewable, degradable, and biocompatible characteristics, and can be applied in fields such as functional materials, biomedicine, and environmental protection. However, during the separation and processing of lignin, due to problems such as complex structure, strong intermolecular interactions, and limited active sites, there are certain performance deficiencies in lignin, and physical, chemical, or biological modification methods are generally used to improve the performance of industrial lignin. Physical modification is simple to operate, but the improvement of material functionality is not significant, especially in terms of photothermal response and antibacterial properties; although biological modification is environmentally friendly, it has low efficiency and high cost, making it difficult to achieve large-scale application; chemical modification requires reliance on strong acids, strong bases, or organic solvents, and the reaction conditions are relatively harsh. If the process is not complete, there may also be environmental pollution problems.

[0003] In the field of antibacterial materials, the natural antibacterial property of lignin stems from its phenolic hydroxyl and aromatic ring structures, but due to insufficient exposure of active sites and chemical inertness, its antibacterial effect is far lower than the actual application requirements. In the prior art, although the performance can be enhanced by chemical grafting or compounding with other antibacterial agents, it often relies on toxic reagents or complex processes, and may cause biosafety problems. For example, using highly reactive chemical substances for grafting antibacterial groups results in a risk of residual toxicity; compounding with other materials is prone to defects such as poor interfacial compatibility and insufficient antibacterial persistence.

[0004] Deep eutectic solvents (DESs) are composed of hydrogen bond donors (such as urea and lactic acid) and hydrogen bond acceptors (such as choline chloride). In recent years, due to their green, designable, low-cost, and efficient ability to dissolve biomass, they have become a research hotspot in biomass modification. DESs can achieve structural reorganization by disrupting the intermolecular forces of lignin and introduce functional groups (such as phenolic hydroxyl groups and carboxyl groups) at the same time, thereby activating its potential properties. At present, existing research mainly focuses on the dissolution of lignin by DESs or the optimization of single functions. For example, in the prior art CN112080022A, deep eutectic solvents are used to improve the solubility, dispersibility, and compatibility of lignin with other polymers. In the prior art CN112029115B, deep eutectic solvents are used to treat biomass fiber raw materials to improve the extraction efficiency of lignin. There has been no reported systematic method for using DESs to modify lignin and improve the photothermal conversion efficiency and antibacterial properties of lignin.

[0005] Therefore, a new preparation method is needed to fill the technical gap of DESs in improving the light conversion efficiency and antibacterial properties of lignin. Summary of the Invention

[0006] The problem to be solved by this application is to provide modified lignin with high photothermal conversion efficiency and antibacterial properties, its preparation method and application. Using deep eutectic solvents, with a very simple preparation process, on the premise of ensuring the environmental protection and functionality of lignin, improve the photothermal conversion efficiency and antibacterial properties of lignin, so that the obtained modified lignin has an excellent antibacterial effect.

[0007] To solve the above technical problems, this application adopts the following technical solutions:

[0008] On the one hand, this application provides a method for preparing modified lignin with high photothermal conversion efficiency and antibacterial properties, including the following steps:

[0009] Mix the hydrogen bond acceptor and the hydrogen bond donor and heat them under stirring until the solution becomes clear to obtain a deep eutectic solvent; mix the lignin with the deep eutectic solvent and carry out a co-heating reaction, then cool to room temperature to obtain a solid-liquid mixture; add ultrapure water to the solid-liquid mixture to precipitate the solute, and then carry out centrifugation and washing to obtain modified lignin;

[0010] Wherein, the lignin includes at least one of industrial lignin and natural lignin.

[0011] In the above technical solution, DESs can disrupt the intermolecular forces of lignin, achieve structural reorganization, and introduce functional groups, adding a large number of exposed active sites to lignin, thereby improving the photothermal conversion efficiency and thermal resistance of lignin.

[0012] Further, the industrial lignin includes at least one of alkali lignin, sodium lignosulfonate, and delignified lignin.

[0013] Further, the natural lignin includes at least one of straw lignin and bamboo powder lignin.

[0014] In the above technical solution, the molecule of alkali lignin has designability and has abundant active sites, which is convenient for introducing functional groups.

[0015] Further, the hydrogen bond acceptor includes one of choline chloride and betaine.

[0016] Further, the hydrogen bond donor includes one of lactic acid, citric acid, oxalic acid, and acetic acid.

[0017] In the above technical solution, defining the types of the hydrogen bond acceptor and the hydrogen bond donor can ensure that the prepared DES can not only be used as a solvent but also as a reaction medium to promote the grafting reaction of lignin and introduce more functional groups into lignin.

[0018] Further, the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:0.1 - 10.

[0019] In the above technical solution, defining the molar ratio of the hydrogen bond acceptor to the hydrogen bond donor can ensure that the obtained deep eutectic solvent can effectively modify lignin.

[0020] Further, the solid-liquid ratio of the lignin to the deep eutectic solvent is 1:1 - 10.

[0021] In the above technical solution, defining the solid-liquid ratio of the lignin to the deep eutectic solvent can ensure that lignin is effectively modified.

[0022] Further, the weight ratio of the ultrapure water to the solid-liquid mixture is 1:1 - 10.

[0023] In the above technical solution, defining the ultrapure water and the solid-liquid mixture can ensure the effective precipitation of the modified lignin and improve the purity of the product.

[0024] Further, the heating temperature is 60 - 80 °C.

[0025] In the above technical solution, defining the heating temperature can ensure the sufficient reaction of the hydrogen bond acceptor and the hydrogen bond donor to obtain a deep eutectic solvent.

[0026] Further, the temperature of the co-heating reaction is 120 - 180 °C, and the time of the co-heating reaction is 1 - 3 h.

[0027] In the above technical solution, limiting the temperature of the co-heating reaction can ensure that the deep eutectic solvent effectively modifies lignin.

[0028] On the other hand, the present application provides the modified lignin prepared by the above method.

[0029] On yet another aspect, the present application provides the use of the modified lignin prepared by the above method or the above modified lignin, and the use includes applications in at least one of medicine, agronomy, and composite material development.

[0030] The present application has the following beneficial effects:

[0031] 1. The present application uses a deep eutectic solvent to reorganize and modify the structure of lignin, introducing more functional groups and adding more active sites to lignin, thereby achieving an improvement in the functional effects of lignin.

[0032] 2. The present application first uses a deep eutectic solvent alone to improve the photothermal conversion efficiency and antibacterial effect of lignin, and the effect is very significant. The finally obtained modified lignin has sufficient application potential in medicine, agriculture, and other fields.

[0033] 3. The preparation method of the present application is very simple, the reaction time is very short, and the reaction temperature is not high. It can greatly save energy and shorten the production cycle during industrial production, and is a very valuable lignin modification method. Description of the Drawings

[0034] Figure 1 It is the photothermal conversion effect diagram of the modified lignin prepared in Example 1 and Comparative Example 1 of the present application;

[0035] Figure 2 It is the antibacterial effect diagram of the modified lignin prepared in Example 1 and Comparative Example 1 of the present application under near-infrared light irradiation with different power densities;

[0036] Figure 3 It is the scanning electron microscope image of the modified lignin prepared in Example 1 of the present application after near-infrared light irradiation in an Escherichia coli suspension. Detailed Embodiments

[0037] Next, the technical solutions in some embodiments of the present application will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments provided by the present application, all other embodiments obtained by those of ordinary skill in the art belong to the scope of protection of the present application.

[0038] Unless the context otherwise requires, throughout the specification and claims, the term "comprising" is interpreted as open and inclusive, that is, "including, but not limited to".

[0039] When describing some embodiments, the expressions "at least one of A, B, and C" and "at least one of A, B, or C" may be used, and both have the same meaning, including the following combinations of A, B, and C: only A, only B, only C, the combination of A and B, the combination of A and C, the combination of B and C, and the combination of A, B, and C.

[0040] Example 1

[0041] Prepare modified lignin according to the following steps:

[0042] S11. Stir and heat choline chloride and lactic acid at a molar ratio of 1:0.1 at 60 °C until the solution is clear to obtain a deep eutectic solvent;

[0043] S12. Add 10.0 g of alkali lignin and 10 mL of the deep eutectic solvent to a 200 mL hydrothermal reactor, stir evenly, and co-heat react at 120 °C for 1 h, then cool to room temperature to obtain a solid-liquid mixture;

[0044] S13. Add 500 mL of ultrapure water to 100 g of the solid-liquid mixture, stir at room temperature for the precipitation of solutes, then centrifuge at a speed of 4000 rpm for 10 min, collect the precipitate and wash it three times to obtain the modified lignin.

[0045] Example 2

[0046] Prepare modified lignin according to the following steps:

[0047] S1. Stir and heat choline chloride and citric acid at a molar ratio of 1:5 at 70 °C until the solution is clear to obtain a deep eutectic solvent;

[0048] S12. Add 5.0 g of sodium lignosulfonate, 5.0 g of delignified lignin and 50 mL of the deep eutectic solvent to a 200 mL hydrothermal reactor, stir evenly, and co-heat react at 150 °C for 1 h, then cool to room temperature to obtain a solid-liquid mixture;

[0049] S13. Add 1000 mL of ultrapure water to 100 g of the solid-liquid mixture, stir at room temperature for the precipitation of solutes, then centrifuge at a speed of 4000 rpm for 10 min, collect the precipitate and wash it three times to obtain the modified lignin.

[0050] Example 3

[0051] Prepare modified lignin according to the following steps

[0052] S11. Stir and heat betaine, oxalic acid, and acetic acid at a molar ratio of 1:5:5 at 80 °C until the solution is clear to obtain a deep eutectic solvent;

[0053] S12. Add 5.0 g of straw lignin, 5.0 g of bamboo powder lignin and 100 mL of the eutectic solvent into a 200 mL hydrothermal reactor. After stirring evenly, react at 180 °C for 3 h, and then cool to room temperature to obtain a solid-liquid mixture.

[0054] S13. Add 100 mL of ultrapure water to 100 g of the solid-liquid mixture, stir at room temperature for the precipitation of solutes, then centrifuge at a speed of 4000 rpm for 10 min, collect the precipitate and wash it three times to obtain the modified lignin.

[0055] Comparative Example 1

[0056] The steps for preparing the modified lignin in this comparative example refer to Example 1; the difference between Comparative Example 1 and Example 1 is only that:

[0057] In S2, the temperature of the co-heating reaction is 25 °C.

[0058] Comparative Example 2

[0059] Prepare the modified lignin according to the following steps:

[0060] S21. Add 10.0 g of lignin and 10 mL of water into a 200 mL hydrothermal reactor. After stirring evenly, react at 120 °C for 1 h, and then cool to room temperature to obtain a solid-liquid mixture.

[0061] S22. Add 500 mL of ultrapure water to 100 g of the solid-liquid mixture, stir at room temperature for the precipitation of solutes, then centrifuge at a speed of 4000 rpm for 10 min, collect the precipitate and wash it three times to obtain the modified lignin.

[0062] Experimental Example 1 Antibacterial Effect of Modified Lignin

[0063] Select Escherichia coli (Gram-negative bacteria) and Staphylococcus aureus (Gram-positive bacteria) to evaluate the antibacterial activity of the modified lignin prepared in Examples 1-3 and Comparative Examples 1-2. The specific steps are as follows:

[0064] S31. Pick a single colony of Escherichia coli (or Staphylococcus aureus) on the LB agar plate and inoculate it into 10 mL of LB broth, and culture it in an oscillating incubator at 37 °C at a speed of 180 rpm for 24 h to obtain a bacterial suspension.

[0065] S32. Weigh 25 mg of the modified lignin prepared in Examples 1-3 and Comparative Examples 1-2 respectively and add them to 9.8 mL of sterile physiological saline to make a lignin suspension. Add 200 μL of the bacterial suspension to each lignin suspension, and set up a group that only adds 200 μL of the bacterial suspension as a blank control group, and dilute to 1×106 CFU / mL, and cultured with shaking at a speed of 180 rpm in an incubator at 37 °C for 24 h to obtain a sample solution.

[0066] S33. Take 100 μL of the sample solution of each group and dilute it to 1×10 3 After that, evenly coat it on an LB agar plate. After incubating at 37 °C for 24 h, count the number of colonies on the culture medium to obtain the antibacterial effect data shown in Table 1 and Table 2.

[0067] Table 1 Antibacterial effect of modified lignin against Escherichia coli:

[0068] Group Colony count Dilution factor Antibacterial rate Example 1 0 <![CDATA[10 3 > 99.99％ Example 2 0 <![CDATA[10 3 > 99.99％ Example 3 0 <![CDATA[10 3 > 99.99％ Comparative example 1 985 <![CDATA[10 3 > 54.73％ Comparative example 2 1040 <![CDATA[10 3 > 52.20％ Blank control 2176 <![CDATA[10 3 > /

[0069] Table 2 Antibacterial effect of modified lignin against Staphylococcus aureus:

[0070] Group Colony count Dilution factor Antibacterial rate Example 1 0 <![CDATA[10 3 > 99.99％ Example 2 0 <![CDATA[10 3 > 99.99％ Example 3 0 <![CDATA[10 3 > 99.99％ Comparative example 1 532 <![CDATA[10 3 > 63.38％ Comparative example 2 749 <![CDATA[10 3 > 48.45％ Blank control 1453 <![CDATA[10 3 > /

[0071] As can be seen from Table 1 and Table 2, after the modified lignin prepared in Experimental Examples 1-3 was mixed with the bacterial suspension for 24 h, it had a more obvious antibacterial effect compared with the modified lignin prepared in Comparative Examples 1-2, and the antibacterial rate reached 99.99%. That is to say, the method of this application can successfully improve the antibacterial performance of lignin, and the subsequent experiments will be carried out with the modified lignin prepared in Example 1.

[0072] Experimental Example 2 verifies the photothermal conversion efficiency of modified lignin

[0073] The modified lignin prepared in Example 1 and Comparative Example 1 was tested, and the specific method is as follows:

[0074] S41. Press the modified lignin prepared in Example 1 and Comparative Example 1 into circular tablets respectively to obtain the samples to be tested;

[0075] S42. Select a near-infrared laser at 808 nm and an infrared camera. The distance between the sample to be tested and the near-infrared laser is fixed at 10 cm, and the distance between the sample to be tested and the infrared camera is fixed at 20 cm;

[0076] S43. Irradiate the sample continuously for 3 minutes at power densities of 0.30, 0.42, 0.55, 0.68, and 0.81 W / cm 2 in sequence. After each irradiation is completed, turn off the near-infrared laser for 3 minutes, record the maximum temperature change on the surface of the lignin with an infrared camera, and then perform the irradiation at the next power density. Finally, obtain the photothermal conversion effect diagram as Figure 1 shown.

[0077] According to Figure 1It can be seen that the modified lignin prepared by the method of the present application has a higher peak temperature at different power densities compared to the lignin prepared in Comparative Example 1, indicating that the method of the present application can effectively improve the photothermal conversion efficiency of lignin.

[0078] Experimental Example 3 verifies the photothermal antibacterial effect of modified lignin

[0079] Escherichia coli (Gram-negative bacteria) was selected to evaluate the antibacterial activity of the modified lignin solution. The specific steps are as follows:

[0080] S51. Pick a single colony of Escherichia coli (or Staphylococcus aureus) on the LB agar plate and inoculate it into 10 mL of LB broth, and culture it in a shaking incubator at 37 °C at a speed of 180 rpm for 24 h to obtain an Escherichia coli suspension.

[0081] S52. Weigh 4 portions of 25 mg of the modified lignin prepared in Example 1, respectively add them to sterile physiological saline to make a lignin suspension with a concentration of 2.5 mg / mL, then add 200 μL of the Escherichia coli suspension to each group and dilute it to 1×10 6 CFU / mL, and then irradiate it with a near-infrared laser at 808 nm with a power density of 1.21 W / cm 2 for 5, 10, 15, and 20 min respectively to obtain sample solutions.

[0082] S53. Take 100 μL of each group of sample solutions and dilute them to 1×10 3 After that, evenly coat them on LB agar plates respectively. After incubating at 37 °C for 24 h, the bacterial colony diagrams as Figure 2 shown are obtained.

[0083] According to Figure 2 It can be seen that after adding the modified lignin obtained by the method of the present application to the Escherichia coli suspension, excellent antibacterial effects can be achieved only after 20 min of irradiation, indicating that the modified lignin obtained by the present application can improve the antibacterial effect through photothermal conversion, greatly reducing the time required for antibacterial.

[0084] Experimental Example 4 Observe the changes of bacteria after photothermal antibacterial of modified lignin

[0085] S61. Pick a single colony of Escherichia coli on the LB agar plate and inoculate it into 10 mL of LB broth, and culture it in a shaking incubator at 37 °C at a speed of 180 rpm for 24 h to obtain an Escherichia coli suspension.

[0086] S62. Weigh 25 mg of the modified lignin prepared in Example 1, add it to sterile physiological saline to make a lignin suspension with a concentration of 2.5 mg / mL, then add 200 μL of the Escherichia coli suspension, and dilute it to 1×106 CFU / mL. Using the Escherichia coli suspension without added material as the control group, it was irradiated with a near-infrared laser at 808 nm at a power density of 1.21 W / cm 2 for 20 min respectively. After that, a 750-fold electron microscope scan was performed using ZEISS Sigma 360 in Germany, and the scanning electron microscope image as shown Figure 3 was obtained.

[0087] It can be seen from Figure 3 that after the control group was irradiated with 808 nm near-infrared light for 20 min, Escherichia coli still maintained its typical rod shape. After Escherichia coli treated with added modified lignin was irradiated with 808 nm near-infrared light for 20 min, serious physical damage occurred, indicating that the modified lignin prepared in this application can effectively destroy the morphology of Escherichia coli, thereby achieving the antibacterial effect.

[0088] In summary, the modified lignin obtained by the method of this application has excellent photothermal conversion effect, and the photothermal conversion effect can promote the antibacterial property of the modified lignin. Only by irradiating with near-infrared light for 20 min can a significant antibacterial effect be achieved. It is an excellent antibacterial agent and light absorber, providing a new raw material for medicine, agronomy and composite material development.

[0089] The above are only the preferred embodiments of the present invention. It should be understood that the present invention is not limited to the form disclosed herein, should not be regarded as excluding other embodiments, but can be used in various other combinations, modifications and environments, and can be changed within the scope of the concept described herein through the above teachings or the technology or knowledge in related fields. And any changes and modifications made by those skilled in the art without departing from the spirit and scope of the present invention shall fall within the protection scope of the appended claims of the present invention.

Claims

1. A method for preparing modified lignin with high photothermal conversion efficiency and antibacterial properties, characterized in that, It includes the following steps: Mix a hydrogen bond acceptor and a hydrogen bond donor, and heat them under stirring until the solution becomes clear to obtain a deep eutectic solvent; Mix lignin with the deep eutectic solvent, carry out a co-heating reaction, and then cool to room temperature to obtain a solid-liquid mixture; Add ultrapure water to the solid-liquid mixture to precipitate the solute, and then carry out centrifugation and washing to obtain the modified lignin; Wherein, the lignin includes at least one of industrial lignin and natural lignin.

2. The method according to claim 1, wherein The industrial lignin includes at least one of alkali lignin, sodium lignosulfonate, and delignified lignin; And / or, the natural lignin includes at least one of straw lignin and bamboo powder lignin.

3. The method according to claim 1, characterized in that The hydrogen bond acceptor includes one of choline chloride and betaine.

4. The method according to claim 1, characterized in that The hydrogen bond donor includes one of lactic acid, citric acid, oxalic acid, and acetic acid.

5. The method according to claim 1, characterized in that, The molar ratio of the hydrogen bond acceptor to the hydrogen bond donor is 1:0.1 - 10.

6. The method according to claim 1, wherein The solid-liquid ratio of the lignin to the deep eutectic solvent is 1:1 - 10.

7. The method according to claim 1, characterized in that, The weight ratio of the ultrapure water to the solid-liquid mixture is 1:1 - 10.

8. The method according to claim 1, characterized in that, The heating temperature is 60 - 80 °C; And / or, the co-heating reaction temperature is 120 - 180 °C, and the co-heating reaction time is 1 - 3 h.

9. The modified lignin prepared by the method according to any one of claims 1 - 8.

10. Use of the modified lignin prepared by the method according to any one of claims 1 to 8 or the modified lignin according to claim 9, characterized in that, The application includes applications in at least one of medicine, agriculture, and composite material development.

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