A preparation method and application of strong acid-resistant lignin nanoparticles

Abstract

The present invention belongs to the technical field of new materials, and in particular, relates to the preparation and application of a strong acid-resistant lignin nanoparticle. First, lignin nanoparticles (LNPs) are prepared by a nanoprecipitation method, and LNPs and β-alanine are mixed in a certain proportion to form an LβNPs complex, and the LβNPs dispersion is used as an aqueous phase, and a vegetable oil is used as an oil phase. The oil phase and the aqueous phase are mixed and homogenized according to a ratio to obtain a lignin-based O / W Pickering emulsion. The present invention uses β-alanine and lignin nanoparticles for the first time. Based on the non-covalent bond between β-alanine and LNPs in a strong acid environment (pH<4), the ratio of β-alanine to LNPs is adjusted to achieve the stable existence of lignin nanoparticles under strong acid conditions; the Pickering emulsion prepared by LβNPs has excellent low pH stability, and can provide effective protection for active ingredients that are unstable in a strong acid environment.

Description

Technical Field

[0001] The invention relates to the technical field of new materials, and specifically provides a preparation method of strong acid-resistant lignin nanoparticles and application thereof. Background Art

[0002] Pickering emulsion is an emulsion stabilized by solid particles adsorbed to the interface between the two phases, and plays an important role in the fields of medicine, food and cosmetics. Pickering emulsion contains an oil phase, an aqueous phase and solid particles adsorbed at the interface between the two phases, wherein the solid particles are of great significance for maintaining the stability of Pickering emulsion. Currently, the nanoparticles used to stabilize Pickering emulsion are divided into inorganic nanoparticles (silicon dioxide, titanium dioxide) and organic nanoparticles (cellulose, starch, protein and lignin). In recent years, many nanoparticles have been developed as stabilizers for Pickering emulsions, but most of these nanoparticles have problems such as complex preparation process, unstable emulsion system and poor biocompatibility. Therefore, there is an urgent need for an efficient and green preparation process to make Pickering emulsions have higher biocompatibility and stability.

[0003] Lignin is a biomass resource second only to cellulose in terms of production in nature. It has a hydrophobic benzene ring structure and hydrophilic phenolic hydroxyl and carboxyl functional groups. Lignin-based nanoparticles are an important way to apply lignin with high added value. Some researchers have used them as carriers of active pharmaceutical ingredients, which have broad application prospects in biomedicine and other fields. However, due to its own heterogeneity and structural complexity, the effect of stabilizing Pickering emulsions by lignin nanoparticles alone is not ideal. Therefore, modification or grafting treatment is used to adjust the wetting behavior of lignin nanoparticles at the phase interface, so that its adsorption at the water-oil interface is enhanced to form a stable Pickering emulsion.

[0004] At present, there are still many deficiencies in the preparation technology of lignin nanoparticles as stabilizers for Pickering emulsions. Wang et al. (Application No.: CN202111540365.X) used natural lignin powder as the raw material, adopted N,N-dimethylformamide with strong toxicity and volatility as the solvent to dissolve lignin, prepared lignin nanoparticles by electrospinning, and used them to stabilize Pickering emulsions. Some researchers (Application No.: 202011639371.6) used a novel green deep eutectic solvent to dissolve lignin, and obtained lignin nanoparticles with uniform particle distribution and excellent antioxidant activity by adding an antisolvent. However, this technology has many process steps and long experimental time. Therefore, it is urgent to develop green and environmentally friendly solvents to dissolve lignin and explore preparation technologies with simple process flows. At the same time, the application environment of the prepared lignin-based nanoparticle Pickering emulsion is mostly neutral or alkaline. In a strong acid (1 < pH < 4) environment, the carboxyl functional groups on the lignin molecules are protonated, resulting in a reduction in the surface charge of the particles and an increase in the van der Waals force, leading to a decrease in the stability of lignin nanoparticles and a reduction in the application range of Pickering emulsions. Summary of the Invention

[0005] Aiming at the deficiencies of the existing technology, the primary object of the present invention is to propose a preparation method and application of strong acid-resistant lignin nanoparticles. Using kraft lignin as the raw material, lignin nanoparticles (LNPs) are prepared by the nanoprecipitation method; subsequently, LNPs and β-alanine are mixed in a certain proportion to form lignin-based nanoparticles (LβNPs) that stably exist in a strong acid environment; then, using the LβNPs dispersion as the aqueous phase and vegetable oil as the oil phase, the oil phase and the aqueous phase are mixed and homogenized according to the ratio to obtain a lignin-based O / W Pickering emulsion. The present invention uses β-alanine to compound with lignin nanoparticles. Based on the non-covalent interaction between β-alanine and LNPs in a strong acid environment (pH < 4), by adjusting the ratio of β-alanine and LNPs, the stable existence of lignin nanoparticles under strong acid conditions is achieved; the Pickering emulsion stabilized by LβNPs has excellent low pH stability and can provide effective protection for active ingredients that are unstable in a strong acid environment. The preparation process of the strong acid-resistant lignin nanoparticles of the present invention is green and environmentally friendly, and the raw materials are easily available, which is conducive to broadening the channels for the high-value utilization of lignin.

[0006] Compared with the prior art, the present application uses the green solvent γ-valerolactone to dissolve lignin, aiming to overcome the toxicity problems of other solvents. Secondly, the prepared lignin nanoparticles are compounded with zwitterionic β-alanine in the present application. By means of the non-covalent bond interaction between β-alanine and lignin nanoparticles under acidic conditions, the stability of the lignin nanoparticle system and Pickering emulsion under acidic conditions (1 < pH < 4) is maintained; it has many advantages such as simple operation and environmental friendliness.

[0007] The specific technical solution of the present invention is as follows:

[0008] A preparation method of strong acid-resistant lignin nanoparticles, the specific steps are as follows:

[0009] (1) At room temperature, weigh 5 mg of lignin and dissolve it in 10 mL of γ-valerolactone aqueous solution to prepare a lignin / γ-valerolactone-aqueous solution; then slowly add 40 mL of deionized water to the lignin / γ-valerolactone-aqueous solution while stirring; after uniform dispersion, dialyze the suspension to remove the organic solvent, and a suspension of lignin nanoparticles (LNPs) is obtained;

[0010] The volume ratio of γ-valerolactone to water in the above γ-valerolactone aqueous solution is 7:3; the above-mentioned lignin is alkali lignin or enzymatically hydrolyzed lignin, and alkali lignin is preferably used;

[0011] Lignin aggregates through π-π interaction in an organic solvent / aqueous solution to obtain a saturated lignin mixed solution; due to the hydrophobicity of lignin itself, when deionized water is added dropwise to the saturated lignin solution, the dissolution performance of the mixed solution for lignin continuously decreases, and lignin gradually aggregates and undergoes layer-by-layer self-assembly. In order to reduce the interfacial tension, it finally precipitates in a spherical structure, and a suspension of lignin nanoparticles (LNPs) is finally obtained. After detection, the average particle size of the obtained LNPs is about 60 nm;

[0012] (2) The β-alanine solution and the LNPs suspension obtained in step (1) are compounded in different proportions, and after shaking evenly at 180 - 250 r / min for 6 - 7 h using a shaker, it is transferred to a dialysis bag and dialyzed for 5 days, and 20 wt% hydrochloric acid is added dropwise to the suspension to adjust its pH value to 1 - 4; a β-alanine / lignin nanoparticle system (LβNPs), that is, a strong acid-resistant lignin nanoparticle system, can be obtained;

[0013] The concentration of the β-alanine solution described above is 1 g / L;

[0014] Preferably, the step (1) comprises: uniformly dispersing lignin in a γ-valerolactone aqueous solution, dripping deionized water into the solution at a rate of 4 mL / min via a peristaltic pump, and stirring at 600-700 r / min for 12-24 h to uniformly disperse the lignin nanoparticles, transferring the suspension into a dialysis bag, and placing the dialysis bag in a bucket containing 3 L of deionized water, dialyzing for 1-3 days, changing the water 2-3 times a day, and after the dialysis is completed, removing the liquid in the dialysis bag to obtain a lignin nanoparticle (LNPs) suspension.

[0015] Preferably, in step (2), the mass ratio of the β-alanine solution to the LNPs suspension is 0.1-1:1;

[0016] Preferably, the molecular weight cutoff of the dialysis bag in the above two steps is 8000-14000Da.

[0017] The pH application range of the strong acid-resistant lignin nanoparticles prepared by the above method is 1-4.

[0018] The strong acid-resistant lignin nanoparticles prepared by the above method have an average particle size of 68 nm, are spherical, and show good stability in a strong acid environment.

[0019] Sodium dodecylbenzene sulfonate (SDBS) was used in the prior art to quaternize lignin, so that lignin has anionic and cationic zwitterions, thereby obtaining the ability to resist strong acid. However, due to the cumbersome preparation process, the lignin modification efficiency is low and it is difficult to be promoted and applied. The inventor of the present application used β-alanine with anionic and cationic zwitterions and lignin compound after a large number of experiments and explorations to prepare a strong acid-resistant lignin nanoparticle system. β-alanine has both acidic and alkaline groups (therefore having amphoteric characteristics), mostly exists as zwitterions in a specific pH environment, and can be used to synthesize calcium pantothenate, synthesize carnosine, etc. in the prior art, and also has certain applications in the fields of medicine, chemical industry, food, environment, etc. However, before the present application, there was no report on its use for nanoparticle modification, so the present application used it in combination with lignin nanoparticles for the first time in the art, filling the gap in the art, so that the final product can tolerate the application scenario of pH 1-4.

[0020] After obtaining the above-mentioned strong acid-resistant lignin nanoparticles, the inventors further provided the following specific application methods:

[0021] A Pickering emulsion prepared by using strong acid-resistant lignin nanoparticles, the specific preparation steps are as follows:

[0022] Take the LβNPs suspension obtained in step (2) as the aqueous phase and the olive oil / water mixture as the oil phase. After mixing the two phases, use an ultrasonic cell disruptor to ultrasonically emulsify at 180 - 200 W for 1 - 5 min to obtain an oil-in-water (O / W) emulsion stabilized by LβNPs.

[0023] The volume ratio of the olive oil / water mixture to the LβNPs suspension is 1:9 or 2:8 or 3:7;

[0024] In the olive oil / water mixture, the volume ratio of olive oil to water is 3:7;

[0025] The Pickering emulsion obtained in the present invention maintains good stability under strong acid conditions. When the mass ratio of β-alanine to LNPs is 1:1 and the volume ratio of the olive oil / water mixture to the LβNPs suspension is 3:7, the emulsion droplet size is the smallest, only 15.8 μm.

[0026] The emulsion can be stably stored at room temperature for 30 days. The emulsion stably exists under acidic conditions (1 < pH < 4); demulsification occurs under alkaline conditions. By alternately adding acid or base, the emulsion can cycle between emulsification and demulsification at least 5 times, and the morphology does not change significantly.

[0027] The main mechanism for the above emulsion to resist strong acid is as follows: in a strong acid environment, the amino group (-NH 2 ) functional group of β-alanine is protonated to form -NH 3 + , making β-alanine positively charged, and through electrostatic adsorption with the negatively charged LNPs at this time, the molecules are intertwined with each other, and finally lignin-based nanoparticles stable in a strong acid environment are formed and used to stabilize the Pickering emulsion.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] (1) The raw material used in the present invention is lignin, which has rich sources, good biocompatibility, and can be recycled and degraded.

[0030] (2) The present invention借助β-alanine with two ionic properties, which shows cationic properties in a strong acid environment (pH < 4). Through electrostatic adsorption, it combines with lignin nanoparticles, making the lignin nanoparticles stable in a strong acid environment and having a long lifespan.

[0031] (3) The present invention can be used in the preparation of pH-responsive Pickering emulsions. By utilizing the electrostatic interaction between β-alanine and lignin nanoparticles in a strong acid environment, the stability of the emulsion in a strong acid environment can be improved, and it has pH responsiveness. By adjusting the pH value, the release of the internal phase drug can be achieved. Description of the Drawings

[0032] Figure 1 TEM images of strong acid-resistant lignin nanoparticles (LβNPs) prepared in Example 2 at different pH values;

[0033] Figure 2 Photos of LNPs (a) obtained in Example 1 and LβNPs (b) obtained in Example 2 at different pH values;

[0034] As can be seen from the figure, in a strong acid environment (pH < 4), the carboxyl groups on the lignin molecules are protonated, the surface charge of the particles decreases, resulting in a decrease in the electrostatic interaction between the particles, and LNPs precipitate. In an environment with pH < 4, for LβNPs, due to the protonation of the -NH 2 group, it is converted to -NH 3 + , and it shows cationic properties. It tightly binds to the negatively charged LNPs at this time through electrostatic adsorption, making the LβNPs suspension in a stable state without precipitation. In an environment with pH > 4, part of the lignin in LNPs and LβNPs dissolves in the system, and the color of the system gradually deepens;

[0035] Figure 3 Practical pictures of Pickering emulsions in Example 3 at different pH values;

[0036] Figure 4 Particle size diagrams, optical microscope diagrams and photos of Pickering emulsions in Example 3 at different pH values;

[0037] As can be seen from the figure, in a system with 1 < pH < 4, the emulsion is in a relatively stable state with an average particle size of 15 μm; as the pH increases, the particle size distribution broadens, the aqueous phase gradually becomes turbid, and the emulsion becomes unstable again; when the pH value is greater than 11, it can be observed through a microscope that there are a large number of emulsion droplets. The smaller droplets spontaneously aggregate on the surface of the larger droplets, and then coalescence occurs between the droplets, ultimately resulting in the rupture of the droplets. Detailed Embodiments

[0038] The following is further described in conjunction with embodiments, which can enable those skilled in the art to more comprehensively understand the present invention, but does not limit the present invention in any way. Unless otherwise specified, the following percentages are by mass percentage; each of the materials is directly purchased from the market, and the inventor will not elaborate further.

[0039] Example 1

[0040] A method for preparing lignin nanoparticles, the specific steps are as follows:

[0041] At room temperature, weigh 5 mg of alkali lignin and dissolve it in 10 mL of γ-valerolactone / water solution (V:V = 7:3) to prepare a lignin / γ-valerolactone / water solution. Stir at a speed of 700 r / min, and add deionized water to the solution at a speed of 4 mL / min through a peristaltic pump. When the volume of the added deionized water is 40 mL, stop the device. Continuously stir at 600 - 700 r / min with the help of a magnetic stirrer for 12 h, transfer the suspension to a dialysis bag, and place the dialysis bag in a bucket containing 3 L of deionized water for dialysis for 24 h, changing the water twice in the middle, to obtain LNPs lignin nanoparticles prepared from the γ-valerolactone-water system.

[0042] Add 20 wt% hydrochloric acid or 20 wt% sodium hydroxide solution to the LNPs suspension respectively to adjust its pH value, and study the morphology and properties of LNPs at different pH values. The experimental results are as Figure 2 shown in

[0043] The results show that the prepared LNPs have a uniform size distribution, are spherical, and the average particle size is about 60 nm. In a strong acid environment (pH < 4), obvious precipitation of LNPs occurs. When 4 < pH < 9, as the pH value increases, its particle size gradually increases and the particle size distribution curve broadens. When pH > 9, part of the lignin precipitates and the color of the system gradually deepens.

[0044] Example 2

[0045] This example prepares a strong acid-resistant lignin nanoparticle, and the specific steps are as follows:

[0046] At room temperature, prepare a 1 g / L β-alanine solution, mix β-alanine and the LNPs suspension prepared in Example 1 according to a mass ratio of 1.0:1.0, and shake well with a shaker at 200 r / min for 6 h to obtain a uniformly dispersed mixed suspension of β-alanine and LNPs;

[0047] Transfer the suspension into a dialysis bag, and place the dialysis bag in a bucket containing 3 L of deionized water. Dialyze for 5 days, changing the water 3 times a day, to obtain a suspension of LNPs complexed with β-alanine, namely the strong acid-resistant lignin nanoparticles (LβNPs) system. Dropwise add 20 wt% hydrochloric acid or 20 wt% sodium hydroxide solution to the suspension to adjust its pH value, and study the morphology and properties of LβNPs at different pH values.

[0048] The LβNPs prepared in this example were observed by transmission electron microscopy, and the results are as Figure 1 shown. When the mass ratio of β-alanine / LNPs was 1.0:1.0, the obtained LβNPs had the smallest size, which was 68 nm. LβNPs could stably exist under high-temperature conditions. In a strong acid environment (1 < pH < 4), no precipitation occurred in the LNPs complexed with β-alanine. When 4 < pH < 8, as the pH value increased, its particle size increased and the particle size distribution curve widened. When pH > 8, part of the lignin precipitated, and the color of the whole system gradually deepened ( Figure 2 b).

[0049] Example 3

[0050] The strong acid-resistant lignin nanoparticles obtained in Example 2 can be used to prepare pH-responsive Pickering emulsions. The specific preparation steps are as follows:

[0051] Take 10 mL of an olive oil / water mixture (volume ratio 3:7), add the LβNPs suspension. Using the olive oil / water mixture as the oil phase and the LβNPs suspension as the water phase, with the volume ratio of the olive oil / water mixture to the LβNPs system being 3:7, use a ultrasonic cell disruptor to ultrasonicate for 60 s at 180 W to prepare a Pickering emulsion. Then emulsify for another 60 s (180 W) using the ultrasonic cell disruptor. The process of the emulsion going through stability-destruction-stability is one cycle, and measure the emulsification of the emulsion after 5 cycles.

[0052] The particle size diagrams, optical microscope diagrams and photos of the Pickering emulsions at different pH values ( Figure 3 and 4As shown in the figure, the Pickering emulsion prepared by this embodiment is stable in a strong acid environment. By adjusting the pH value, LβNPs can be switched between stability / destruction. After 5 cycles, the particle size of the emulsion has not changed significantly, and the emulsion is still in a stable state. In this process, under acidic conditions (pH<4), the amino group of β-alanine is protonated, and the positive charge increases. Due to the enhanced electrostatic attraction with the negatively charged LNPs, the system shrinks and the emulsion stability is improved; under alkaline conditions, the carboxyl group of β-alanine is deprotonated to make it negatively charged. Due to the mutual repulsion of the same charges, the polymer network expands, the stability is reduced, the lignin aggregates and precipitates, and the emulsion breaks. It can be seen that LβNPs are pH stimuli responsive and can be used as smart drug controlled release materials.

[0053] The formation mechanism of LβNPs is through the electrostatic adsorption of LNPs and β-alanine. In an environment with pH < 4, β-alanine is positively charged and LNPs are negatively charged. Due to the electrostatic adsorption, the two are tightly combined to form LβNPs.

[0054] Through Examples 1 and 2, it can be found that the stability of the β-alanine compounded lignin nanoparticles prepared by the present invention in a strong acid environment is significantly better than that of the LNPs in Example 1. Example 2 Based on the preparation of LNPs in Example 1, β-alanine is introduced into the LNPs for compounding. Under strong acid conditions, the stability is high and there is no precipitation phenomenon (such as Figure 2 As shown), it shows the importance of Example 2 for the development of strong acid-resistant lignin nanoparticles.

[0055] The above is a detailed description of the implementation methods of the present invention. However, the present invention is not limited to the above content, and any modification, replacement, combination and other replacement methods made to the present invention based on the knowledge of the relevant professional field are all within the technical scope of the present invention.

Claims

1. A method for preparing strong acid-resistant lignin nanoparticles, characterized in that: The following steps are involved: (1) At room temperature, 5 mg of lignin was weighed and dissolved in 10 mL of γ-valerolactone aqueous solution to prepare a lignin / γ-valerolactone aqueous solution; then 40 mL of deionized water was slowly added dropwise to the lignin / γ-valerolactone aqueous solution while stirring; after the suspension was evenly dispersed, the suspension was dialyzed to remove the organic solvent to obtain a lignin nanoparticle suspension, which is a LNPs suspension; The volume ratio of γ-valerolactone to water in the γ-valerolactone aqueous solution is 7:3; the lignin is alkali lignin or enzymatic lignin; (2) Compounding the β-alanine solution and the LNPs suspension obtained in step (1) in different proportions, shaking for 6-7 h on a shaker at 180-250 r / min to mix evenly, transferring the mixture to a dialysis bag and dialyzing for 5 days, and adding 20 wt % hydrochloric acid dropwise to the suspension to adjust the pH value to 1-4; thus, a β-alanine / lignin nanoparticle compound system, i.e., a strong acid-resistant lignin nanoparticle suspension, is obtained. The strong acid-resistant lignin nanoparticle suspension is an LβNPs suspension; The concentration of the β-alanine solution is 1 g / L.

2. The preparation method according to claim 1, characterized in that: In step (1), the lignin is alkali lignin.

3. The preparation method according to claim 1, characterized in that: In step (1), lignin is uniformly dispersed in a γ-valerolactone aqueous solution, and deionized water is added to the solution at a rate of 4 mL / min via a peristaltic pump, while stirring at 600-700 r / min for 12-24 h to uniformly disperse the lignin nanoparticles. The suspension is transferred to a dialysis bag, and the dialysis bag is placed in a bucket containing 3 L of deionized water. The dialysis is performed for 1-3 days, with the water being changed 2-3 times a day. After the dialysis is completed, the liquid in the dialysis bag is removed to obtain a lignin nanoparticle suspension.

4. The preparation method according to claim 1, characterized in that: In step (2), the mass ratio of the β-alanine solution to the LNPs suspension is 0.1-1:

1.

5. The preparation method according to claim 1, characterized in that: The molecular weight cut-off of the dialysis bags in steps (1) and (2) is 8000-14000 Da.

6. The preparation method according to claim 1, characterized in that: The pH application range of the strong acid-resistant lignin nanoparticles obtained in step (2) is 1-4.

7. Use of strong acid-resistant lignin nanoparticles obtained by the preparation method according to any one of claims 1 to 6 in the preparation of Pickering emulsion.

8. The application according to claim 7, characterized in that: The specific preparation steps are as follows: Take the LβNPs suspension as the water phase and the olive oil / water mixture as the oil phase, use an ultrasonic cell disruptor to sonicate for 1-5 min at 180-200 W to obtain a Pickering emulsion, which is an oil-in-water emulsion; The volume ratio of the olive oil / water mixture to the LβNPs suspension is 1:9 or 2:8 or 3:7; The volume ratio of olive oil to water in the olive oil / water mixture is 3:7.

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