Preparation method of pure chitosan nanofiber

Abstract

This invention provides a method for preparing pure chitosan nanofibers, belonging to the field of polymer fibers. The method first involves dissolving chitosan in a dilute acid aqueous solution to obtain a chitosan solution. A dilute alkali aqueous solution is then added dropwise to the chitosan solution to obtain a chitosan hydrogel. The chitosan hydrogel is then washed with deionized water to remove salt, centrifuged, and dissolved in hexafluoroisopropanol to prepare a spinning solution. Finally, pure chitosan nanofibers are prepared using electrospinning technology. This invention is simple to operate, has a short preparation process, requires no additional expensive reagents, produces a pure product, and is less prone to introducing other impurities. It yields fine-diameter, low-cost, green, and biodegradable pure chitosan nanofibers, which have broad application prospects in biomedicine, adsorbents, tissue engineering, membrane materials, and new energy materials.

Description

Technical Field

[0001] This invention relates to the field of polymer fibers, and more specifically to a method for preparing pure chitosan nanofibers. Background Technology

[0002] Chitosan is a polysaccharide extracted from the shells of crustaceans, and it is the second most abundant natural polysaccharide after cellulose. Chitosan is a deacetylated derivative of chitosan, readily soluble in dilute acids but insoluble in water, alkalis, and most organic solvents. Chitosan possesses various physiological functions, including biodegradability, biocompatibility, non-toxicity, antibacterial properties, anticancer effects, lipid-lowering effects, and immune enhancement. It is widely used in food additives, textiles, agriculture, environmental protection, beauty and health care, cosmetics, antibacterial agents, medical fibers, medical dressings, artificial tissue materials, drug sustained-release materials, gene transduction vectors, biomedical applications, absorbable medical materials, tissue engineering carrier materials, medical and drug development, and many other fields and daily chemical industries.

[0003] Electrospinning technology, by applying a high-voltage electric field, can spin fibers and fiber web structures, three-dimensional materials with diameters ranging from tens to hundreds of nanometers. The nanoscale fibers prepared have the characteristics of large specific surface area and controllable diameter and morphology, and are widely used in textile engineering, biomedicine, agricultural protection and environmental engineering.

[0004] Given the unique properties of chitosan and the advantages of electrospun nanostructures, researchers both domestically and internationally have conducted extensive research in this area. Chitosan is insoluble in both organic solvents and water. It is often dissolved in dilute acids such as acetic acid and trifluoroacetic acid, and then electrospun together with other components. However, due to chitosan's polycationic properties and the presence of numerous hydrogen bonds between its molecules, the solution becomes positively charged after dissolving in dilute acids. This results in intermolecular repulsion due to the electric field, leading to unstable jetting and making it difficult to prepare pure chitosan nanofibers using electrospinning technology. (Akbari-alavijeh S, Shaddel R, Jafari S. Encapsulation of food bioactives and nutraceuticals by various chitosan-basednanocarriers. Food Hydrocolloids, 2020, (105), 105774; M. Yang, R. Sun, Y.Feng, et al. Preparation, characterization and kinetics study of chitosan / PVAelectrospun nanofiber membranes for the adsorption of dye from water. Journal of Polymer Engineering, 2019, 39(5):459-471).

[0005] Currently, the main methods for preparing chitosan nanofibers are phase separation and electrospinning.

[0006] Phase separation is generally achieved by preparing pure chitosan nanofibers using liquid nitrogen freeze-drying. Tu Mei et al. prepared pure chitosan nanofibers using freeze-drying (Tu Mei et al., A Chitosan Nanofiber and Its Preparation Method and Application, CN101736438 B, 2012-02-15), but the freeze-drying process is time-consuming and requires the use of liquid nitrogen, which increases costs.

[0007] When using electrospinning technology, one method is to add spinning aids to the spinning solution. Spinning aids include crosslinking agents, surfactants, and other polymer materials. Adding crosslinking agents utilizes the amino groups on chitosan molecules to form chemical crosslinks with the aldehyde groups on the crosslinking agent molecules (Jebreil M, Khatib M, Seddigheh Fazli B, et al. Evaluation of wound-healing efficiency of a functional Chitosan / Aloe vera hydrogel on theimprovement of re-epithelialization in full thickness wound model of rat. Journal of Tissue Viability, 2022, 31(4): 649-656). However, chemical crosslinking agents are not easily removed from the system, and residual crosslinking agents will affect the performance of pure chitosan nanofibers. Adding surfactants can increase the conductivity of the solution and reduce its surface tension, thus preparing pure chitosan nanofibers (Vrieze SD, Camp TV, Nelvig A, et al. The effect of temperature and humidity on electrospinning. Journal of Materials Science, 2009 (44):1357-1362; C. Li, T. Lou, Y. Xu, et al. Fabrication of pure chitosan nanofibrous membranes as effective absorbent fordye removal. International Journal of Biological Macromolecules, 2018, 106:768-774; Xiong Ping, Preparation of multi-scale chitosan nanofiber membranes and their adsorption and filtration performance, Tianjin University of Technology, 2021).One method involves blending chitosan with other polymers to form nanofibers, followed by dissolving these polymers in a solvent to obtain pure chitosan nanofibers (Fujita, Satoshi Nishimoto, Shohei, et al. Method for producing hydrogel fiber, and hydrogel fiber produced by method. JP2016216861A, 2016-12-22; Hu Junli et al., A chitosan-based nanofiber and its preparation method, CN113445155B, 2022-11-08). However, this method is lengthy and carries the risk of secondary pollution, limiting its application. More importantly, the dilute acid solvent for chitosan cannot completely evaporate during spinning, leaving some residue in the chitosan nanofibers. This causes the chitosan nanofibers to swell and dissolve in water, making them unusable in aqueous environments. Another approach is to use non-acidic chitosan solvents, such as ionic liquids, to increase the spinnability of chitosan. Adding ionic liquids utilizes the electrostatic interaction between polyvalent anions and protonated amino groups on chitosan molecules, thereby cross-linking them into nanofibers (Zhuang et al., A method for preparing pure chitosan ultrafine fibers, CN 110295401 A, 2019-10-01). However, the introduction of ionic liquids is not only complex and time-consuming, but the ionic liquids themselves are also expensive, resulting in high costs.

[0008] Kondo Hideo et al. dissolved collagen in an aqueous solution of hexafluoroisopropanol to obtain a collagen spinning solution, and then prepared micron-sized collagen fibers using electrospinning technology (Kondo Hideo, Kuwata Minoru, NiwaHideo et al. Method for producing collagen fiber using electrospinning and collagen fiber produced by the method. JP2007138364A, 2007-06-07). Chitosan is insoluble in organic solvents and water, as well as in solutions containing water and hexafluoroisopropanol in any ratio, making it impossible to prepare a transparent and uniform spinning solution.

[0009] The inventors believe that the insolubility of chitosan in any ratio of water and hexafluoroisopropanol in an aqueous solution may be due to its crystallinity and the large number of hydrogen bonds between its molecules. This invention first prepares chitosan into a hydrogel to reduce its crystallinity and hydrogen bonding, allowing the hydrogel to dissolve in hexafluoroisopropanol. Thorough washing of the chitosan hydrogel removes salts, ensuring the hexafluoroisopropanol solution is non-ionic. Furthermore, the spinning solution prepared by dissolving chitosan in dilute acid carries a positive charge due to the protonation of the chitosan amino groups. Thorough washing of the chitosan hydrogel and the avoidance of using dilute acid in the preparation of the chitosan spinning solution both eliminate the interference of the chitosan spinning solution's charge on the jet during spinning, thus enabling the preparation of pure chitosan nanofibers using electrospinning technology.

[0010] The purpose of this invention is to provide a simple method for preparing pure chitosan nanofibers that does not require the removal of salts or the addition of any spinning aids. The process is simple, low-cost, and can yield nanofibers with diameters between 250.0 and 900.0 nm. Summary of the Invention

[0011] This invention includes the following steps:

[0012] Step 1: Dissolve chitosan powder in a dilute acid aqueous solution and stir to obtain a transparent and uniform chitosan solution;

[0013] Step 2: Add a dilute alkaline aqueous solution dropwise into the chitosan solution obtained in Step 1 to prepare a chitosan hydrogel. The solution is neutral.

[0014] Step 3: Wash the chitosan hydrogel with deionized water to remove salt until the conductivity of the washing solution is the same as that of the deionized water, and then centrifuge the washed chitosan hydrogel.

[0015] Step 4: Dissolve the chitosan hydrogel obtained in Step 3 in hexafluoroisopropanol and stir until fully dissolved and the solution is uniform and transparent to obtain chitosan spinning solution;

[0016] Step 5: Using high-voltage electrospinning, the chitosan spinning solution obtained in Step 4 is used to prepare pre-shaped pure chitosan nanofibers under certain conditions.

[0017] Step 6: Dry the pre-shaped pure chitosan nanofibers obtained in Step 5 completely under certain conditions to obtain pure chitosan nanofibers.

[0018] In the method for preparing pure chitosan nanofibers as described above, preferably, the dilute acid aqueous solution in step one directly affects the subsequent preparation of chitosan hydrogel. The acid is acetic acid, formic acid, lactic acid, hydrochloric acid, nitric acid, sulfuric acid, trifluoroacetic acid, or phosphoric acid, and the concentration of the dilute acid solution is 0.1% to 30.0%.

[0019] In the method for preparing pure chitosan nanofibers as described above, preferably, the dilute alkaline aqueous solution in step one directly affects the subsequent preparation of chitosan hydrogel. The alkaline solution is sodium hydroxide, potassium hydroxide, calcium hydroxide, sodium carbonate, sodium bicarbonate, or ammonia, and the concentration of the dilute alkaline solution is 0.1% to 30.0%.

[0020] In the method for preparing pure chitosan nanofibers as described above, preferably, the concentration of the pure chitosan spinning solution in step four is 5.0% to 30.0%. If the concentration of the spinning solution is too low, the surface tension characteristics of the solution required for electrospinning may be lost, and fibers cannot be formed; if the concentration of the spinning solution is too high, the solution is prone to solidify into a gel, making it impossible to fiberize chitosan.

[0021] In the method for preparing pure chitosan nanofibers as described above, preferably, the electrospinning voltage in step five is 5.0~25.0kV, the receiving distance of the flat plate receiver is 5.0~20.0cm, the spinning solution flow rate is 0.1~5.0ml / h, the needle outer diameter is 0.64~1.65mm, the inner diameter is 0.25~1.37mm, and the spinning temperature is 15.0℃~35.0℃. When the DC voltage is less than 5.0kV, the chitosan solution is difficult to split into fine charged mist; if it exceeds 25.0kV, the splitting force of the charged mist is too strong, and the chitosan solution is difficult to fall in a fibrous state.

[0022] The method for preparing pure chitosan nanofibers as described above, wherein preferably, the drying method selected in step six is ​​vacuum drying, the drying pressure is -1 kPa, and the drying temperature is 40.0℃~80.0℃, to obtain the pure chitosan nanofibers with a fiber diameter between 250.0 nm and 900.0 nm.

[0023] This invention provides a method for preparing pure chitosan nanofibers. The method first prepares a dilute acid solution of chitosan and a chitosan hydrogel. The chitosan hydrogel, after being washed and desalted, is dissolved in hexafluoroisopropanol to obtain a chitosan spinning solution. Then, the chitosan spinning solution is used to prepare pre-shaped pure chitosan nanofibers by electrospinning technology. Finally, the pure chitosan nanofibers are obtained by drying. Attached Figure Description

[0024] Figure 1 This is a 1,000x magnified scanning electron microscope image of the pure chitosan nanofibers mentioned in Example 4 of this invention.

[0025] Figure 2 This is a scanning electron microscope image of the pure chitosan nanofibers mentioned in Example 4 of the present invention, magnified 5,000 times.

[0026] Figure 3This is a 5,000x scanning electron microscope image of the pure chitosan nanofibers prepared by incomplete water washing and desalination as mentioned in Example 2 of the present invention. Detailed Implementation

[0027] The present invention will be further illustrated by specific examples below, but the present invention is not limited thereto.

[0028] Example 1: Using chitosan with a weight-average molecular weight of 50,000 and a degree of deacetylation of 70% as raw material, pure chitosan spinning solution was prepared by dissolving it in an aqueous acetic acid solution, but pure chitosan nanofibers could not be obtained.

[0029] (1) Weigh 0.20g of chitosan powder with a weight average molecular weight of 50,000 and a degree of deacetylation of 70%, dissolve it in 30.0ml of acetic acid aqueous solution with a concentration of 5.0%, and stir until completely dissolved to obtain pure chitosan spinning solution;

[0030] (2) Using high-voltage electrospinning technology, the obtained chitosan solution was loaded into a 10ml syringe and the syringe was installed in the micro-propulsion slot of the high-voltage electrospinning device. Below the needle was a flat receiver with aluminum foil. The spinning voltage was 18.0kV, the receiving distance was 10.0cm, the outer diameter of the needle was 0.6mm, the inner diameter was 0.3mm, the spinning solution flow rate was 2.0ml / h, and the spinning temperature was room temperature. Pure chitosan spinning solution dripped and could not be used to prepare nanofibers.

[0031] Example 2: Chitosan with a weight-average molecular weight of 50,000 and a degree of deacetylation of 70% was used as raw material to prepare chitosan hydrogel. Without washing and desalting with water, pure chitosan spinning solution was obtained by dissolving it in hexafluoroisopropanol. Pure chitosan electrospun nanofibers could not be prepared.

[0032] (1) Weigh 0.20g of chitosan powder with a weight average molecular weight of 50,000 and a degree of deacetylation of 70%, dissolve it in 30.0ml of 5.0% acetic acid aqueous solution, stir until completely dissolved, and obtain chitosan solution;

[0033] (2) Take 10 ml of 10.0% sodium hydroxide solution and add it dropwise into the chitosan solution. Stir to obtain chitosan hydrogel and centrifuge at 4000 rpm for 30 min.

[0034] (3) Weigh 1.0g of the centrifuged chitosan hydrogel and dissolve it in 9.0ml of hexafluoroisopropanol. Stir thoroughly until completely dissolved to obtain pure chitosan spinning solution.

[0035] (4) Using high-voltage electrospinning technology and spinning conditions as in Example 1, nanofibers could not be prepared.

[0036] Example 3: Chitosan hydrogel was prepared using chitosan with a weight-average molecular weight of 50,000 and a degree of deacetylation of 70%. After washing with water to remove salt, it was dissolved in N,N-dimethylformamide, but pure chitosan spinning solution could not be obtained.

[0037] (1) Weigh 0.20g of chitosan powder with a weight average molecular weight of 50,000 and a degree of deacetylation of 70%, dissolve it in 30.0ml of 5.0% acetic acid aqueous solution, stir until completely dissolved, and obtain chitosan solution;

[0038] (2) Take 10 ml of 10.0% sodium hydroxide solution and add it dropwise into the chitosan solution. Stir to obtain chitosan hydrogel and centrifuge at 4000 rpm for 30 min.

[0039] (3) The obtained chitosan hydrogel was washed with deionized water to remove salt until the conductivity of the washing solution was the same as that of the deionized water, and then centrifuged at 4000 rpm for 30 min.

[0040] (4) Weigh 1.0g of the centrifuged chitosan hydrogel and dissolve it in 9.0ml of N,N-dimethylformamide. It does not dissolve, and pure chitosan spinning solution cannot be obtained.

[0041] Example 4: Chitosan with a weight-average molecular weight of 50,000 and a degree of deacetylation of 70% was used as raw material to prepare chitosan hydrogel. After washing and desalting with water, the hydrogel was dissolved in hexafluoroisopropanol to obtain pure chitosan spinning solution, and pure chitosan electrospun nanofibers were prepared.

[0042] (1) Weigh 0.20g of chitosan powder with a weight average molecular weight of 50,000 and a degree of deacetylation of 70%, dissolve it in 30.0ml of 5.0% acetic acid aqueous solution, stir until fully dissolved, and obtain chitosan solution;

[0043] (2) Take 10 ml of 10.0% sodium hydroxide solution and add it dropwise into the chitosan solution, and stir to obtain chitosan hydrogel;

[0044] (3) The obtained chitosan hydrogel was washed with deionized water to remove salt until the conductivity of the washing solution was the same as that of the deionized water, and then centrifuged at 4000 rpm for 30 min.

[0045] (4) Weigh 1.0g of the centrifuged chitosan hydrogel and dissolve it in 9.0ml of hexafluoroisopropanol. Stir thoroughly until completely dissolved to obtain pure chitosan spinning solution.

[0046] (5) High-voltage electrospinning technology was used, and the spinning conditions were the same as in Example 1, to obtain the predetermined pure chitosan nanofibers;

[0047] (6) The obtained pre-shaped pure chitosan nanofibers were dried in a vacuum drying oven with a vacuum pressure of -1.0 kPa, a drying temperature of 60°C, and a drying time of 10 h to obtain pure chitosan nanofibers.

[0048] The above figures illustrate preferred embodiments of the present invention. However, the embodiments of the present invention are not limited to the above examples. Any changes made in accordance with the concept of the present invention, or equivalent embodiments modified to have equivalent variations, shall be within the protection scope of the present invention as long as they do not exceed the spirit covered by the specification and figures.

Claims

1. A method for preparing pure chitosan nanofibers, characterized in that, Includes the following steps: Step 1: Dissolve chitosan in an aqueous solution of acetic acid with a concentration of 0.1%~30.0% to obtain a transparent and homogeneous chitosan solution; Step 2: Add a 0.1%~30.0% sodium hydroxide aqueous solution to the chitosan solution obtained in Step 1 to obtain chitosan hydrogel; Step 3: Wash the chitosan hydrogel obtained in Step 2 with deionized water to remove salt, and then centrifuge. Step 4: Dissolve the chitosan hydrogel obtained in Step 3 in hexafluoroisopropanol to obtain pure chitosan spinning solution; Step 5: Electrospin the pure chitosan spinning solution obtained in Step 4, and then vacuum dry it under a pressure of -1 kPa and a temperature of 40.0~80.0℃ to obtain pure chitosan nanofibers.

2. The method for preparing pure chitosan nanofibers according to claim 1, characterized in that, In step four, the concentration of the pure chitosan spinning solution is 0.1% to 30.0%.

3. Pure chitosan nanofibers obtained by any one of the preparation methods of claims 1 to 2.

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