Preparation method of humidity-responsive cellulose nanocrystal structural color thin film

Abstract

The application discloses a preparation method of a humidity-responsive cellulose nanocrystal structural color thin film and belongs to the field of humidity-responsive color-changing materials. Regenerated cellulose nanocrystals with different length-diameter ratios are prepared by controlling the time of sulfuric acid hydrolysis, and a humidity-responsive structural color thin film with uniformity and different colors is obtained by regulating and controlling self-assembly. The prepared humidity-responsive structural color thin film solves the problem of phase separation caused by color regulation of a conventional polymer and the difficulty in forming a uniform chiral nematic phase structure. The humidity-responsive performance is good in repeatability. The efficient recycling of waste cotton is realized, and the application is conducive to the recycling of waste resources.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of humidity-responsive color-changing materials, and particularly relates to a preparation method of a humidity-responsive cellulose nanocrystal structural color thin film. BACKGROUND

[0002] Cellulose is the most widely known and largest amount of sugar substances in nature, accounting for about 50% of the total carbon content of the plant kingdom, and has excellent mechanical properties, good biocompatibility, easy availability, and low cost. Cellulose nanocrystals prepared by hydrolyzing cellulose with sulfuric acid generally have anisotropic rod shape and a negative charge on the surface, and thus can be well dispersed in water. After evaporation, the colloidal cellulose nanocrystal suspension spontaneously assembles into a solid thin film with a chiral nematic structure and structural color. At the same time, the cellulose nanocrystal thin film retains its left-handed structure during the evaporation process and can reflect left-handed circularly polarized light. Photonic crystals are spatially ordered structures with a lattice parameter comparable to the wavelength of propagating light. Since the periodicity of the chiral nematic helical structure of cellulose nanocrystals matches the wavelength of visible light, the cellulose nanocrystal thin film can also be regarded as a kind of photonic crystal, and thus can be used as a sensor for visual identification.

[0003] Currently, the color of the cellulose nanocrystal thin film can be regulated by introducing additives. Typically, the additives, such as acrylamide, acrylic acid, water-soluble polymers (such as polyvinyl alcohol) and polyethylene glycol, and latex nanoparticles, are introduced into the cellulose nanocrystal suspension. The introduction of the additives into the cellulose nanocrystal suspension can obtain an adjustable pitch of the cellulose nanocrystals, thereby controlling the photonic band gap. The additives combine with the cellulose nanocrystals, participate in self-assembly, and form a chiral nematic structure. The cellulose nanocrystal thin film prepared by introducing the additives can adjust the reflected wavelength in the visible light spectrum, which provides the possibility for preparing a sensitive sensor for visual identification. However, the addition of these additives can cause phase separation, affecting the self-assembly of the chiral nematic structure of the thin film, and causing large cracks in the pitch, which affects the uniformity of the structural color of the thin film. SUMMARY

[0004] [TECHNICAL PROBLEM]

[0005] The present application aims to solve the problem that the introduction of additives into the cellulose nanocrystal suspension can cause phase separation and hinder the uniformity of the cellulose nanocrystal thin film.

[0006] [TECHNICAL SCHEME]

[0007] To solve the above problems, the application provides a preparation method of a humidity-responsive cellulose nanocrystal structural color film, which comprises the following steps:

[0008] The application provides a preparation method of a humidity-responsive cellulose nanocrystal structural color film, and specifically comprises the following steps:

[0009] (1) preparing a cellulose nanocrystal suspension by acid hydrolysis of sulfuric acid on the basis of alkali pretreatment of waste cotton powder, and preparing regenerated cellulose nanocrystal suspensions CNC n with different aspect ratios by controlling the acid hydrolysis time of sulfuric acid;

[0010] (2) mixing the regenerated cellulose nanocrystal suspensions CNC n with different aspect ratios in different proportions, evaporating and inducing self-assembly into a film to obtain a humidity-responsive cellulose nanocrystal structural color film.

[0011] In an embodiment of the application, in step (1), the alkali pretreatment of the waste cotton powder is soaking the waste cotton powder in a NaOH solution with a mass concentration of 3-10% for 8-10 hours.

[0012] In an embodiment of the application, in step (1), the regenerated cellulose nanocrystal suspensions CNC n with different aspect ratios have a mass concentration of 2.0-8.4%.

[0013] In an embodiment of the application, in step (1), the mass concentration of sulfuric acid is 60-65%.

[0014] In an embodiment of the application, in step (1), in the regenerated cellulose nanocrystal suspensions CNC n with different aspect ratios, the aspect ratio of the regenerated cellulose nanocrystals is 15-35.

[0015] In an embodiment of the application, in step (2), the regenerated cellulose nanocrystal suspensions CNC n with different aspect ratios, n represents the acid hydrolysis time, and the hydrolysis time is 30-90 minutes.

[0016] In an embodiment of the application, in step (2), when n is 60 minutes and 30 minutes, the obtained CNC 60 and CNC 30The CNC obtained when n is 70 min and 40 min is prepared by mixing uniformly and evaporating according to a mass ratio of 1:9 to induce self-assembly into a film 70 and the CNC 40 The CNC obtained when n is 80 min and 50 min is prepared by mixing uniformly and evaporating according to a mass ratio of 2:8 to induce self-assembly into a film 80 and the CNC 50 The CNC obtained when n is 90 min and 60 min is prepared by mixing uniformly and evaporating according to a mass ratio of 4:6 to induce self-assembly into a film 90 and the CNC 60 The CNC obtained when n is 90 min and 30 min is prepared by mixing uniformly and evaporating according to a mass ratio of 9:1 to induce self-assembly into a film 90 and the CNC 30 The CNC obtained when n is 90 min and 30 min is prepared by mixing uniformly and evaporating according to a mass ratio of 9:1 to induce self-assembly into a film

[0017] In an embodiment of the present application, in step (2), the time for evaporation-induced self-assembly is 4-6 days.

[0018] The present application provides a humidity-responsive cellulose nanocrystal structural color thin film prepared by the above-mentioned method.

[0019] The present application also provides the above-mentioned humidity-responsive cellulose nanocrystal structural color thin film for use in the fields of environmental detection, medical diagnostic tools, and food processing safety applications.

[0020] [Advantages]

[0021] (1) The present application regenerates cellulose nanocrystals from waste cotton by acid hydrolysis, and prepares a humidity-responsive chiral nematic structural color thin film by evaporation-induced self-assembly. Different aspect ratios of regenerated cellulose nanocrystals are prepared by controlling the time of sulfuric acid hydrolysis, and different aspect ratios of cellulose nanocrystals are used for common self-assembly to obtain humidity-responsive thin films with different colors (blue, blue-green, yellow, and orange);

[0022] (2) When the relative humidity increases from 11% to 97%, the color of the thin film changes from blue to yellow and then to red, and the diffraction peak moves from 494 nm to 621 nm and then to 723 nm. The thin film of the present application has the characteristics of responding to changes in humidity in the environment and changing color, and the humidity-responsive color-changing performance does not show obvious loss within 10 cycles, and the humidity-responsive performance is good in repeatability;

[0023] (3) The cellulose nanocrystals with different aspect ratios as the polymers with excellent compatibility and affinity can not only adjust the structural color of the cellulose nanocrystal film, but also will not affect the uniformity of the film, solving the problem that the introduction of additives in the cellulose nanocrystal suspension will cause phase separation and hinder the uniformity of the cellulose nanocrystal film;

[0024] (4) The present application realizes efficient recycling of waste cotton, which is conducive to the reuse of waste resources. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 Figure (a) is a color change real object diagram of the humidity-responsive color-changing structural color film of the regenerated nanocrystal under different relative humidities in Example 5; Figure (b) is a UV-Vis spectrum diagram; and Figure (c) is a cycle response diagram.

[0026] Figure 2 Figure is a SEM diagram of the cross section of the humidity-responsive color-changing structural color film of the regenerated nanocrystal in Example 5. DETAILED DESCRIPTION

[0027] CNC film real object diagram shooting: place the CNC film on a black background paper, and take a picture from directly above the sample using a camera.

[0028] UV-Vis spectrum analysis (UV-Vis) test method of CNC film: different relative humidities are prepared by using saturated salt solution, and the CNC film is placed in a bottle containing different relative humidities and left for 5 min. The ultraviolet-visible light spectrum analyzer (LAMBDA950) is adjusted to the reflection mode, and the baseline correction is calibrated. Then the CNC film is placed in the ultraviolet-visible light spectrum analyzer for testing, and the ultraviolet-visible light reflection spectrum is obtained.

[0029] Particle size analysis test method: prepare a CNC suspension with a mass fraction of 1% and place it in a cuvette for testing in a particle size analyzer.

[0030] Scanning electron microscope (SEM) test method of CNC film: scanning electron microscope characterization is carried out on a cold field emission scanning electron microscope (Hitachi SU8100) at 5kv. The CNC film sample is brittle fractured with liquid nitrogen, the cross section is pasted upward on the carbon conductive glue on the scanning table, and then coated with gold sputtering to make it conductive before analysis, and the morphology of the CNC film is observed.

[0031] Example 1

[0032] A preparation method of a regenerated nanocrystal humidity-responsive color-changing structural color film, the specific steps are as follows:

[0033] (1) The crushed waste cotton powder is soaked in a 3% mass concentration NaOH solution for 10 h, and washed with distilled water to obtain the alkali pretreated waste cotton powder;

[0034] (2) The alkali pretreated waste cotton powder is added to concentrated sulfuric acid with a mass concentration of 65%, and hydrolyzed by heating and stirring for 60 min, with the temperature controlled at 45°C and the stirring speed at 200 r / min during the hydrolysis. After complete hydrolysis, distilled water is added for dilution, and then centrifugal washing is performed at a speed of 6000 r / min for 8 min to remove residual acid until neutralization, to obtain a cellulose nanocrystal suspension CNC 60 ; The cellulose nanocrystal suspension CNC 60 is mixed with deionized water at a mass ratio of 1:100 to prepare a cellulose nanocrystal solution, which is subjected to ultrasonic treatment at a power of 500 W for 20 min, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 60 with a mass concentration of 2.5%. 60 The length of the cellulose nanocrystal CNC 60 is 163.2±24 nm, the diameter is 6.0±0.2 nm, and the aspect ratio is 27.

[0035] (3) The alkali pretreated waste cotton powder is added to concentrated sulfuric acid with a mass concentration of 65%, and hydrolyzed by heating and stirring for 30 min, with the temperature controlled at 45°C and the stirring speed at 200 r / min during the hydrolysis. After complete hydrolysis, distilled water is added for dilution, and then centrifugal washing is performed at a speed of 6000 r / min for 8 min to remove residual acid until neutralization, to obtain a cellulose nanocrystal suspension CNC 30 ; The cellulose nanocrystal suspension CNC 30 is mixed with deionized water at a mass ratio of 1:100 to prepare a cellulose nanocrystal solution, which is subjected to ultrasonic treatment at a power of 500 W for 20 min, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 30 with a mass concentration of 2.5%. 30 The length of the cellulose nanocrystal CNC 30 is 223.6±18 nm, the diameter is 7.5±0.4 nm, and the aspect ratio is 30.

[0036] (4) The cellulose nanocrystal suspensions CNC 60 and CNC 30 with a mass concentration of 2.5% are mixed uniformly at a mass ratio of 1:9, and then the mixed solution is transferred to a polystyrene petri dish, evaporated and dried at room temperature, and evaporated for 6 days to self-assemble into a film, to obtain a blue humidity-responsive cellulose nanocrystal structural color thin film.

[0037] Example 2

[0038] A preparation method of a regenerated nanocrystal humidity-responsive color-changing structural color film, the specific steps are as follows:

[0039] (1) The crushed waste cotton powder is soaked in a 3% mass concentration NaOH solution for 9h, and then washed with distilled water to obtain an alkali pretreated waste cotton powder;

[0040] (2) The alkali pretreated waste cotton powder is added into a 62% mass concentration concentrated sulfuric acid, and hydrolyzed by heating and stirring for 70min, wherein the temperature is controlled at 50℃ and the stirring speed is 250r / min; after complete hydrolysis, the residual acid is removed by adding distilled water to dilute and then centrifuging and washing at a speed of 5500r / min for 9min until neutral, to obtain a cellulose nanocrystal suspension CNC 70 ; The cellulose nanocrystal suspension CNC 70 is mixed with deionized water at a mass ratio of 1:150 to prepare a cellulose nanocrystal solution, which is subjected to ultrasonic treatment at a power of 500W for 15min, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 70 with a mass concentration of 4%; the length of the cellulose nanocrystal CNC 70 is 143.2±26nm, the diameter is 5.8±0.2nm, and the aspect ratio is 24;

[0041] (3) The alkali pretreated waste cotton powder is added into a 62% mass concentration concentrated sulfuric acid, and hydrolyzed by heating and stirring for 40min, wherein the temperature is controlled at 50℃ and the stirring speed is 250r / min; after complete hydrolysis, the residual acid is removed by adding distilled water to dilute and then centrifuging and washing at a speed of 5500r / min for 9min until neutral, to obtain a cellulose nanocrystal suspension CNC 40 ; The cellulose nanocrystal suspension CNC 40 is mixed with deionized water at a mass ratio of 1:150 to prepare a cellulose nanocrystal solution, which is subjected to ultrasonic treatment at a power of 500W for 15min, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 40 with a mass concentration of 4%; the length of the cellulose nanocrystal CNC 40 is 188.2±23nm, the diameter is 6.5±0.5nm, and the aspect ratio is 29;

[0042] (4) The cellulose nanocrystal suspensions CNC 70 and CNC 40The waste cotton powder is mixed with NaOH solution with a mass ratio of 2:8, and then the mixed solution is transferred to a polystyrene surface dish, evaporated and dried at room temperature, and evaporated for 6 days to self-assemble into a film to obtain a blue-green humidity-responsive cellulose nanocrystal structural color film.

[0043] Example 3

[0044] A method for preparing a regenerated nanocrystal humidity-responsive color-changing structural color film, the specific steps are as follows:

[0045] (1) The crushed waste cotton powder is soaked in a 3% mass concentration NaOH solution for 8 hours, and then washed with distilled water to obtain an alkali pretreated waste cotton powder;

[0046] (2) The alkali pretreated waste cotton powder is added to concentrated sulfuric acid with a mass concentration of 61%, and hydrolyzed by heating and stirring for 80 minutes, with the temperature controlled at 55°C and the stirring speed at 300 r / min during the hydrolysis by heating and stirring; after complete hydrolysis, the residual acid is removed by adding distilled water to dilute and then centrifuging and washing at a speed of 5000 r / min for 8 minutes until neutral, to obtain a cellulose nanocrystal suspension CNC 80 ; The cellulose nanocrystal suspension CNC 80 is mixed with deionized water at a mass ratio of 1:200 to prepare a cellulose nanocrystal solution, which is ultrasonically treated at a power of 1000 W for 15 minutes, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 80 with a mass concentration of 6%; the length of the cellulose nanocrystal CNC 80 is 123.2±31 nm, the diameter is 5.6±0.2 nm, and the aspect ratio is 21;

[0047] (3) The alkali pretreated waste cotton powder is added to concentrated sulfuric acid with a mass concentration of 61%, and hydrolyzed by heating and stirring for 50 minutes, with the temperature controlled at 55°C and the stirring speed at 300 r / min during the hydrolysis by heating and stirring; after complete hydrolysis, the residual acid is removed by adding distilled water to dilute and then centrifuging and washing at a speed of 5000 r / min for 8 minutes until neutral, to obtain a cellulose nanocrystal suspension CNC 50 ; The cellulose nanocrystal suspension CNC 50 is mixed with deionized water at a mass ratio of 1:200 to prepare a cellulose nanocrystal solution, which is ultrasonically treated at a power of 1000 W for 15 minutes, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 50 with a mass concentration of 6%; the length of the cellulose nanocrystal CNC 50 is 177.2±23 nm, the diameter is 6.3±0.5 nm, and the aspect ratio is 28;

[0048] (4) The cellulose nanocrystal suspension CNC 80 and CNC 50 were mixed in a mass ratio of 4:6, and then the mixed solution was transferred to a polystyrene surface dish, evaporated and dried at room temperature, and self-assembled into a film by evaporation for 5 days to obtain a yellow humidity-responsive cellulose nanocrystal structural color film.

[0049] Example 4

[0050] A method for preparing a regenerated nanocrystal humidity-responsive color-changing structural color film, the specific steps being as follows:

[0051] (1) The crushed waste cotton powder was soaked in a 3% mass concentration NaOH solution for 8h, and washed with distilled water to obtain an alkali pretreated waste cotton powder;

[0052] (2) The alkali pretreated waste cotton powder was added to concentrated sulfuric acid with a mass concentration of 60%, and hydrolyzed by heating and stirring for 90min, with the temperature controlled at 60℃ and the stirring speed at 300r / min during the hydrolysis by heating and stirring; after complete hydrolysis, distilled water was added for dilution, followed by centrifugal washing at a speed of 4000r / min for 10min to remove residual acid until neutralization, to obtain a cellulose nanocrystal suspension CNC 90 ; The cellulose nanocrystal suspension CNC 90 was mixed with deionized water in a mass ratio of 1:150 to prepare a cellulose nanocrystal solution, which was subjected to ultrasonic treatment at a power of 1500W for 10min, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 90 with a mass concentration of 8.4%; The cellulose nanocrystal CNC 90 has a length of 100.1±20nm and a diameter of 5.5±0.3nm, with an aspect ratio of 18;

[0053] (3) The alkali pretreated waste cotton powder was added to concentrated sulfuric acid with a mass concentration of 60%, and hydrolyzed by heating and stirring for 50min, with the temperature controlled at 60℃ and the stirring speed at 300r / min during the hydrolysis by heating and stirring; after complete hydrolysis, distilled water was added for dilution, followed by centrifugal washing at a speed of 4000r / min for 10min to remove residual acid until neutralization, to obtain a cellulose nanocrystal suspension CNC 60 ; The cellulose nanocrystal suspension CNC 60 was mixed with deionized water in a mass ratio of 1:150 to prepare a cellulose nanocrystal solution, which was subjected to ultrasonic treatment at a power of 1500W for 10min, and then sequentially washed and concentrated to obtain a cellulose nanocrystal suspension CNC 60 with a mass concentration of 8.4%; The cellulose nanocrystal CNC 60The length of the cellulose nanocrystals CNC is 163.2±24 nm, the diameter is 6.0±0.2 nm, and the aspect ratio is 27;

[0054] (4) The cellulose nanocrystal suspension CNC with a mass concentration of 8.4% is prepared by an evaporation film forming method 90 and CNC 60 are mixed in a mass ratio of 9:1, and then the mixed solution is transferred to a polystyrene surface dish, evaporated and dried at room temperature, and self-assembled into a film by evaporation for 4 days to obtain an orange humidity-responsive cellulose nanocrystal structural color film.

[0055] Example 5

[0056] A method for preparing a regenerated cellulose nanocrystal humidity-responsive color-changing structural color film, the specific steps are as follows:

[0057] (1) The crushed waste cotton powder is soaked in a 3% mass concentration NaOH solution for 8 h, and then washed with distilled water to obtain an alkali pretreated waste cotton powder;

[0058] (2) The alkali pretreated waste cotton powder is added to a 64% mass concentration concentrated sulfuric acid, and hydrolyzed by heating and stirring for 90 min. The temperature is controlled at 45°C and the stirring speed is 300 r / min during the hydrolysis by heating and stirring. After complete hydrolysis, the residual acid is removed by adding distilled water and centrifugal washing at a speed of 5000 r / min for 10 min until neutralization to obtain a cellulose nanocrystal suspension CNC 90 ; The cellulose nanocrystal suspension CNC 90 is mixed with deionized water in a mass ratio of 1:200 to prepare a cellulose nanocrystal solution, which is ultrasonically treated at a power of 1500 W for 10 min, and then washed and concentrated in sequence to obtain a cellulose nanocrystal suspension CNC 90 with a mass concentration of 4.2%; the length of the cellulose nanocrystals CNC 90 is 100.1±20 nm, the diameter is 5.5±0.3 nm, and the aspect ratio is 18;

[0059] (3) The alkali pretreated waste cotton powder is added to a 64% mass concentration concentrated sulfuric acid, and hydrolyzed by heating and stirring for 30 min. The temperature is controlled at 45°C and the stirring speed is 300 r / min during the hydrolysis by heating and stirring. After complete hydrolysis, the residual acid is removed by adding distilled water and centrifugal washing at a speed of 5000 r / min for 10 min until neutralization to obtain a cellulose nanocrystal suspension CNC 30 ; The cellulose nanocrystal suspension CNC 30The cellulose nanocrystals solution was prepared by mixing cellulose nanocrystals CNC 30 with deionized water at a mass ratio of 1:200, and then performing ultrasonic treatment at an ultrasonic power of 1500 W for 10 min. The cellulose nanocrystals CNC 30 were sequentially washed and concentrated to obtain a cellulose nanocrystals suspension CNC 90 with a mass concentration of 4.2%. 30 The length of the cellulose nanocrystals CNC 30 was 223.6±18 nm, the diameter was 7.5±0.4 nm, and the aspect ratio was 30.

[0060] (4) The cellulose nanocrystals suspension CNC 90 with a mass concentration of 4.2% and CNC 30 were mixed at a mass ratio of 8:2, and then the mixed solution was transferred to a polystyrene petri dish. The mixed solution was evaporated and dried at room temperature for 5 days to self-assemble into a film to obtain a blue-green humidity-responsive cellulose nanocrystals structural color film.

[0061] Table 1 CNC size at different acidolysis times

[0062]

[0063] Comparative Example 1

[0064] (1) The crushed waste cotton powder was soaked in a 3% NaOH solution for 8 h, and then washed with distilled water to obtain an alkali pretreated waste cotton powder.

[0065] (2) The alkali pretreated waste cotton powder was added to a 64% concentrated sulfuric acid, and then hydrolyzed by heating and stirring for 30 min. During the hydrolysis by heating and stirring, the temperature was controlled at 45°C, and the stirring speed was 300 r / min. After complete hydrolysis, the residual acid was removed by adding distilled water to dilute the solution, and then centrifuging at a speed of 5000 r / min for 10 min until the solution was neutral. A cellulose nanocrystals suspension CNC 30 was obtained. 30 The cellulose nanocrystals solution was prepared by mixing cellulose nanocrystals CNC 30 with deionized water at a mass ratio of 1:200, and then performing ultrasonic treatment at an ultrasonic power of 1500 W for 10 min. The cellulose nanocrystals CNC 30 were sequentially washed and concentrated to obtain a cellulose nanocrystals suspension CNC 90 with a mass concentration of 4.2%. 30 The length of the cellulose nanocrystals CNC 30 was 223.6±18 nm, the diameter was 7.5±0.4 nm, and the aspect ratio was 30.

[0066] (3) The alkali pretreated waste cotton powder is added to concentrated sulfuric acid with a mass concentration of 64%, and hydrolysis is carried out by heating and stirring for 40 min. During the hydrolysis process, the temperature is controlled at 45°C, and the stirring speed is 300 r / min; after complete hydrolysis, distilled water is added for dilution, and then centrifugal washing is carried out at a speed of 5000 r / min for 10 min to remove residual acid until neutralization, to obtain a cellulose nanocrystal suspension CNC 40 ; The cellulose nanocrystal suspension CNC 40 is mixed with deionized water at a mass ratio of 1:200 to prepare a cellulose nanocrystal solution, which is subjected to ultrasonic treatment at a power of 1500 W for 10 min, and then is washed and concentrated in sequence to obtain a cellulose nanocrystal suspension CNC 40 with a mass concentration of 4.2%; the length of the cellulose nanocrystal CNC 40 is 188.2±23 nm, the diameter is 6.5±0.5 nm, and the aspect ratio is 29;

[0067] (4) The cellulose nanocrystal suspensions CNC 40 and CNC 30 with a mass concentration of 4.2% are mixed uniformly at a mass ratio of 9:1, and then the mixed solution is transferred to a polystyrene surface dish, and is evaporated and dried at room temperature for 5 days to self-assemble into a film to obtain an orange-red cellulose nanocrystal structural color film. The two kinds of cellulose nanocrystals have different color regulation performances on the film, and the obtained film can only be orange-red, and cannot be adjusted to a blue film.

[0068] Figure 1 is a practical picture and UV-Vis spectrum and a cyclic response graph of the humidity-responsive cellulose nanocrystal structural color film of Example 5 under different relative humidities. As can be seen from Figure 1 , when the relative humidity gradually changes from 11% to 97%, the color of the film changes from blue-green to yellow and then to red, and the UV-Vis diffraction peak moves from 494 nm to 621 nm and then to 723 nm. And the humidity-responsive color change performance does not show obvious loss within 10 cycles. This work realizes efficient recycling of waste cotton, which is conducive to the utilization of waste resources and the conservation of primary resources.

[0069] Figure 2 is an SEM graph of the cross section of the regenerated nanocrystal humidity-responsive color-changing structural color film of Example 5, from which it can be observed that the pitch of the film is uniform, and there is no phenomenon of cracks caused by phase separation.

[0070] The above examples are not intended to limit the scope of the present application, nor are the described steps intended to limit the order in which they are performed. Modifications apparent to one skilled in the art in light of the above teachings are intended to fall within the scope of the claims.

Claims

1. A method for producing a humidity-responsive cellulose nanocrystal structural color thin film, characterized by, The method comprises the following steps: (1) The method for preparing cellulose nanocrystal suspension by acid hydrolysis of alkali pretreated waste cotton powder, by controlling the acid hydrolysis time of sulfuric acid, regenerated cellulose nanocrystal suspension CNC with different aspect ratios is prepared n , n represents acid hydrolysis time; (2) Regenerated cellulose nanocrystal suspensions CNC 90 and CNC 30 were mixed in a mass ratio of 8:2, uniformly mixed, and evaporated to induce self-assembly into a film, obtaining a humidity-responsive cellulose nanocrystal structural color thin film, CNC 90 with an aspect ratio of 18, and CNC 30 with an aspect ratio of 30.

2. The method for preparing a humidity-responsive cellulose nanocrystalline structured color film according to claim 1, characterized in that, In step (1), the waste cotton powder is soaked in a 3-10% NaOH solution for 8-10 hours to obtain an alkali pretreated waste cotton powder.

3. The method of claim 1, wherein the method is characterized by: In step (1), the regenerated cellulose nanocrystal suspensions CNC having different aspect ratios n have a mass concentration of 2.0-8.4%.

4. The method for preparing a humidity-responsive cellulose nanocrystalline structured color film according to claim 1, characterized in that, In step (1), the mass concentration of sulfuric acid is 60-65%.

5. The method for preparing a humidity-responsive cellulose nanocrystalline structured color film according to claim 1, characterized in that, In step (1), the acid hydrolysis time is 30-90 minutes.

6. The method for preparing a humidity-responsive cellulose nanocrystalline structured color film according to claim 1, characterized in that, In step (2), the evaporation-induced self-assembly time is 4-6 days.

7. A humidity-responsive cellulose nanocrystal structural color thin film prepared by the method of any one of claims 1-6.

8. Application of the humidity-responsive cellulose nanocrystal structural color thin film of claim 7 in the fields of environmental detection, medical diagnostic tools and food processing safety.

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