Cellulose nanocrystal composite photonic film, preparation method and application thereof

Abstract

The application belongs to the field of functional materials and chemical sensing technology, and discloses cellulose nanocrystal composite photonic film, a preparation method and application. The application ingeniously regulates the interaction between the composite material and different polar solvents by introducing Pluronic F127 and cellulose nanocrystal. The obtained film is stable in structure in pure methanol, and has bright initial structural color. Once contacting with water-methanol, due to the strong interaction between water molecules and hydrophilic segments in F127 and the change of solvent polarity, the structure pitch of the film photonic crystal and the reflection wavelength are changed significantly, thereby causing the significant color transition from the initial color to orange / red or to colorless which can be distinguished by naked eyes. The "switching type" color change greatly improves the intuitiveness and accuracy of detection, and is convenient for non-professionals to judge on site.

Description

Technical Field

[0001] This invention relates to the fields of functional materials and chemical sensing technology, and in particular to cellulose nanocrystal composite photonic thin films, their preparation methods, and applications. Background Technology

[0002] In the field of energy and safety regulation, methanol adulteration with water is used by criminals as a low-cost method of counterfeiting to produce so-called environmentally friendly oil or biodiesel, which is then sold as fuel for stoves in restaurants, canteens, and other establishments. This illegal fuel not only has a low calorific value and corrodes appliances, but its vapor is also flammable, explosive, and toxic, posing serious safety and fire hazards. Currently, standard laboratory methods for detecting water content in methanol include Karl Fischer titration and gas chromatography. While these methods are accurate, they rely on large equipment and specialized operations, resulting in long testing cycles and high costs, making it difficult to meet the urgent needs of rapid on-site screening by market regulators, initial on-site inspections by grassroots law enforcement personnel, and simple self-identification by end users. Therefore, there is an urgent need to develop a rapid, intuitive, instrument-free, and low-cost on-site detection method for methanol adulteration with water.

[0003] Photonic crystal materials, due to their unique optical modulation properties, have shown great potential in the field of sensing and detection. Among them, cellulose nanocrystals (CNC), as a naturally derived nanomaterial, can form photonic crystal thin films with chiral nematic phase structures through evaporation-induced self-assembly. These films possess distinct structural colors, and their optical signals (such as reflection peak wavelengths) are highly sensitive to environmental stimuli (such as humidity and solvents), thus being considered a promising visual sensing material. Based on the optical response mechanism, CNC photonic crystal thin films have potential applications in solvent identification: when exposed to polar solvents such as water and methanol, their internal hydrogen bond network recombines, causing a reversible change in the film's color.

[0004] However, pure CNC films still have significant limitations in methanol-water detection scenarios. On the one hand, pure CNC films are extremely sensitive to water, and under high water content conditions, they are prone to excessive swelling or even structural collapse, leading to uncontrollable color responses. On the other hand, their response to organic solvents such as methanol is relatively weak, with insignificant color changes. Therefore, when using pure CNC films to distinguish between pure methanol, pure water, and their mixtures, it is difficult to achieve high-contrast, high-sensitivity water adulteration identification in a methanol matrix, failing to meet the requirements of intuitive and accurate results for on-site screening.

[0005] To improve the performance of CNC films, research has attempted to modify them by adding functional components such as polymers. However, for the specific scenario of methanol water content detection, existing CNC-based composite systems still have the following shortcomings: the materials are difficult to maintain a stable structural ground state in a methanol environment, and they do not produce sensitive, rapid, and visually perceptible color transitions in response to trace or even medium-to-high water content incorporation. Their response often presents as a continuous and slow color change, resulting in low detection contrast. Summary of the Invention

[0006] This invention aims to solve at least one of the above-mentioned technical problems, and provides a cellulose nanocrystal composite photonic thin film, its preparation method, and its applications. The CNC-based thin film provided by this invention exhibits structural stability in methanol and displays a clear and rapid visual response to water incorporation.

[0007] Compared with the prior art, the present invention has the following beneficial effects: This invention cleverly modulates the interaction between the composite material and solvents of different polarities by introducing Pluronic F127 and CNC composite. The resulting film exhibits a slight red-shift to green / yellow color in pure methanol; upon contact with water-doped methanol, the strong interaction between water molecules and the hydrophilic segments in F127, along with the change in solvent polarity, leads to a significant change in the structural pitch reflection wavelength of the film's photonic crystal, resulting in a visually perceptible color transition from red to orange / red or colorless. This "on / off" color change (rather than a continuous, subtle hue shift) greatly improves the intuitiveness and accuracy of detection, facilitating on-site judgment by non-professionals.

[0008] Compared to the drawback of pure CNC films, which are prone to excessive swelling and cracking in methanol with high water content, the F127 composite film of this invention maintains good structural integrity in methanol environments and can stably detect methanol adulterated with water from low to high concentrations, thus broadening the scope of applicable detection. The introduction of F127 introduces flexible segments into the rigid CNC network, giving the composite film good flexibility and ductility. It can be bent and folded without easily breaking, making it easy to cut, carry, and operate on-site.

[0009] The main raw material of this invention, microcrystalline cellulose, is widely available, renewable, and low in cost. The preparation process is based on an aqueous system, with mild conditions and low energy consumption. The prepared film can be cut into a large number of test strips, resulting in extremely low cost per test, making it very suitable for large-scale promotion and single-use.

[0010] The detection process of this invention requires no complex instruments or power supply; it only requires contacting the thin film with the test liquid for 10-20 seconds, and a qualitative judgment can be made by observing the color change with the naked eye. This fully meets the needs of on-site, rapid, and real-time screening, greatly reducing the detection threshold and operational difficulty, and is suitable for market supervision, grassroots law enforcement, and daily self-inspection by end users.

[0011] This invention is the first to successfully apply the stimulus-response characteristics of CNC photonic crystal materials to the specific public safety and market supervision problem of "methanol adulteration with water," providing a field solution that cannot be matched by traditional laboratory methods. It has direct and important practical significance for combating illegal fuels and protecting people's lives and property. Attached Figure Description

[0012] Figure 1 The images show the transmission electron microscope (TEM) observation results of cellulose nanocrystals in Test Example 1; (a) is a TEM observation image; (b) is a magnified view of (a). Figure 2 The following is a statistical diagram of the size of cellulose nanocrystals in Test Example 1; (a) shows the length statistics, and (b) shows the width statistics. Figure 3 The PDI and Zeta potentials of the cellulose nanocrystal solution and the composite membrane solution diluent in Test Example 2 are shown in (a) for PDI and (b) for Zeta potential. CNC represents 0.01% cellulose nanocrystal solution. CF5, CF10, CF15 and CF20 correspond to the diluents of the composite membrane solutions in Examples 1 to 4, respectively. Figure 4 The following are the FTIR test results in Test Example 3; where (a) is the FTIR test result of CNC film and CNC-F127 film; and (b) is the FTIR test result of F127. Figure 5 The figures show the XRD diffraction, grain size, and interplanar spacing test results of the CNC thin film and CNC-F127 thin film in Test Example 3; (a) shows the XRD diffraction test results, (b) shows the interplanar spacing test results, and (c) shows the grain size test results. Figure 6 SEM images of the surface and cross-sectional morphology of the CNC film and CNC-F127 film in Test Example 3; Figure 7 The graph shows the UV-Vis spectra of the CNC film and the CNC-F127 film in Test Example 3. Figure 8 The results of the theoretical pitch and calculated pitch of the CNC film and CNC-F127 film in Test Example 3 are shown in the figure. Figure 9 The optical image measurement results of the CNC thin film and CNC-F127 thin film in Test Example 3 are shown. Figure 10 (a) shows the CIE colorimetric test results of the CNC film and CNC-F127 film in Test Example 3; (b) is a magnified view of (a). Figure 11The test results of the mechanical properties of the CNC film and CNC-F127 film in Test Example 3 are shown in the figure; where (a) is the tensile strength and elongation at break, (b) is the Young's modulus, and (c) is the bending, curling and folding test results of each film. Figure 12 The following figures show the humidity response test results of the CNC film and CNC-F127 film in Test Example 3: (a) UV-Vis spectrum of CNC film under different relative humidity conditions; (b) UV-Vis spectrum of CF5 film under different relative humidity conditions; (c) UV-Vis spectrum of CF10 film under different relative humidity conditions; (d) UV-Vis spectrum of CF15 film under different relative humidity conditions; (e) UV-Vis spectrum of CF20 film under different relative humidity conditions; and (f) Test results showing the change in the position of the maximum peak in the UV-Vis spectrum of figures (a)-(e). Figure 13 (a) shows the CIE test results of the humidity response of the CNC film and the CNC-F127 film in Test Example 3; (b) is a magnified view of (a). Figure 14 The graph shows the moisture absorption rate test results of the CNC film and CNC-F127 film in Test Example 3; Figure 15 The results of the humidity cycling test in Test Example 3 are shown in (a) and (b). Figure 16 The graph shows the test results of CNC film and CNC-F127 film with water doping in test example 4 (0-15% v / v); Figure 17 The graph shows the color change test results of the CF20 film in Test Example 4 within 20 seconds in 20%, 25%, and 50% v / v water-doped methanol solutions. Detailed Implementation

[0013] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0014] The first embodiment of the present invention provides a cellulose nanocrystal composite photonic thin film comprising cellulose nanocrystals and Pluronic F127, wherein the mass ratio of Pluronic F127 to cellulose nanocrystals is 0.05 to 7:20, and the thin film has a chiral nematic photonic crystal structure.

[0015] Cellulose nanocrystals (CNCs) are nanoscale rod-shaped crystals obtained from natural cellulose raw materials through acid hydrolysis and other treatments. Their length typically ranges from tens to hundreds of nanometers, and their width from several nanometers to tens of nanometers, exhibiting a high aspect ratio. The CNC surface possesses negatively charged groups such as sulfate ester groups (originating from sulfonation during acid hydrolysis), enabling stable dispersion in the aqueous phase. The high aspect ratio and surface charge of CNCs are the intrinsic basis for their spontaneous arrangement into a chiral nematic phase structure under evaporation-induced conditions.

[0016] The CNC can be obtained from microcrystalline cellulose through conventional processes such as acid hydrolysis, centrifugation, washing, dialysis, and drying. As long as CNCs with uniform size, good dispersibility, and self-assembly capability can be obtained, those skilled in the art can prepare them using any conventional method known in the art, and are not limited to the acid hydrolysis conditions listed in the specific embodiments of this application.

[0017] Pluronic F127, or F127 for short, is an amphiphilic triblock copolymer with a chemical structure of polyoxyethylene-polypropylene-polyoxyethylene (PEO-PPO-PEO). The PEO segments at both ends are hydrophilic, while the PPO segments in the middle are hydrophobic. In the composite system of this invention, F127 does not simply function as an inert filler, but rather exerts a dual regulatory effect through its amphiphilic structure: on the one hand, its hydrophilic PEO segments extend in water to form a hydration layer, improving the stability of the CNC dispersion system and reducing the polydispersity index through steric hindrance; on the other hand, its hydrophobic PPO segments can adsorb onto hydrophobic regions or defects on the CNC surface through hydrophobic interactions, partially covering the charged points on the CNC surface, thereby regulating the surface charge density (Zeta potential) of the CNC. This dual regulatory effect alters the balance of interaction forces between CNC particles, providing a structural basis for the increase in pitch during subsequent self-assembly.

[0018] In the composite photonic thin film, the mass ratio of Pluronic F127 to cellulose nanocrystals is 0.05 to 7:20 (i.e., the mass of F127 is 5% to 35% of the mass of CNC). This mass ratio is one of the key parameters for achieving the specific optical response of this invention.

[0019] When the F127 content is too low (below the above range), its regulatory effect on the CNC dispersion system and self-assembled structure is insufficient, and the color contrast and response sensitivity of the composite film in the methanol-water system are difficult to reach ideal levels. When the F127 content is too high (above the above range), excessive hydrophobic PPO segments will form a relatively dense physical cross-linking network inside the film, generating a strong diffusion barrier effect, inhibiting the penetration and uniform distribution of solvent molecules, which in turn leads to a decrease in response sensitivity and an excessive reduction in the mechanical strength of the film. Therefore, controlling the mass ratio within the range of 0.05 to 7:20 can achieve a balance between structural stability, solvent response sensitivity, and mechanical flexibility. In some preferred embodiments, the mass ratio of F127 to cellulose nanocrystals is 1 to 4:20, within which the film performance is optimal.

[0020] The thin film possesses a chiral nematic photonic crystal structure. A chiral nematic structure refers to a structure in which CNC rod-shaped particles, during evaporation-induced self-assembly, arrange themselves layer by layer along a specific helical axis, forming a periodic layered helical structure similar to cholesteric liquid crystals. The orientation of adjacent CNC layers rotates at a fixed angle, resulting in a helical stack. When the periodic pitch of this structure is on the same order of magnitude as the wavelength of visible light, it can produce Bragg reflection of specific wavelengths of light, thus giving the thin film a bright structural color (such as blue, green, or orange).

[0021] In the composite film of this invention, F127 molecules act as space fillers inserted between CNC layers. Through hydrogen bonding with hydroxyl groups on the CNC surface via their ether bonds, they effectively expand and regulate the inherent helical assembly period (pitch) of the CNC. With increasing F127 content, the film pitch increases accordingly, resulting in a redshift of the Bragg reflection wavelength, macroscopically manifested as a shift in film color from blue / green to yellow / green hues. This tunable structural color provides an optical basis for subsequent solvent response detection.

[0022] The second embodiment of the present invention provides a method for preparing a cellulose nanocrystal composite photonic thin film. Pluronic F127 and cellulose nanocrystals are dispersed in water at a mass ratio of 0.05 to 7:20 to obtain a composite film liquid. The composite film liquid is then evaporated and induced to self-assemble into a film to obtain a composite thin film with a chiral nematic photonic crystal structure.

[0023] The cellulose nanocrystals are obtained from microcrystalline cellulose through acid hydrolysis, centrifugation, washing, dialysis, and drying. The purpose of this step is to break down the amorphous regions in the microcrystalline cellulose, releasing rod-shaped nanocrystals with high crystallinity and uniform size, while introducing negatively charged groups on their surface to ensure the stability of aqueous dispersion.

[0024] Acid hydrolysis is a conventional method in the art for preparing CNC from microcrystalline cellulose. Those skilled in the art can select appropriate types, concentrations, temperatures, and times according to actual needs; the lower the acid concentration and temperature, the longer the reaction time. In one specific embodiment of the invention, a 60% sulfuric acid solution is used, stirred at a constant temperature of 45°C for 70 minutes. This condition ensures sufficient acid hydrolysis while avoiding incomplete hydrolysis due to too short a stirring time or degradation of the nanocrystals due to too long a stirring time. After the reaction is complete, a large amount of ultrapure water (e.g., ten times the volume of the mixture) is added to dilute and terminate the reaction, and the mixture is allowed to settle to allow for complete separation of the hydrolysis products.

[0025] It should be noted that the above-mentioned acid hydrolysis conditions are only preferred embodiments of the present invention, and not the only way to achieve the technical effects of the present invention. Those skilled in the art can adjust the acid concentration (e.g., 45% to 65%), temperature (e.g., 40°C to 50°C), and time (e.g., 60 min to 100 min) within conventional ranges according to the source of raw materials and equipment conditions, as long as a light blue CNC solution with a high aspect ratio and good dispersibility can be obtained.

[0026] Centrifugation and washing are conventional solid-liquid separation and purification methods in the art, aimed at removing residual acid and soluble impurities. In one specific embodiment of the present invention, the settled milky white solution is centrifuged at 24,000 rpm for 20 minutes, the solid precipitate is collected, and repeatedly washed with ultrapure water and centrifuged again until the solution has a neutral pH and is a transparent pale blue color. The centrifugation speed, time, and number of washes can be adjusted according to the actual separation effect. Those skilled in the art can use other conventional centrifugation parameters to achieve the same purification purpose.

[0027] Dialysis is a conventional purification method in this field for further removing trace impurities and residual small molecules. In one specific embodiment of the present invention, a dialysis bag with a molecular weight cutoff of 12-14 kDa is used, and dialysis is performed in ultrapure water for 7 days, with the ultrapure water being changed 1-2 times daily during this period. The molecular weight cutoff of the dialysis bag and the dialysis time can be adjusted according to the impurity removal requirements. Those skilled in the art can select dialysis bags in the range of 10-15 kDa, and the dialysis time can also be appropriately extended or shortened according to the actual situation.

[0028] After dialysis, the CNC dispersion is sonicated (e.g., for 30 minutes) to ensure uniform dispersion of the cellulose nanocrystals and prevent agglomeration. It is then dried to obtain CNC solid powder. Freeze-drying (lyophilization), a conventional method in the art, can effectively maintain the dispersion of the CNC.

[0029] The CNC solid powder prepared above is redispersed in ultrapure water to prepare a CNC solution with a mass concentration of 0.5% to 7.5%. This concentration range ensures the CNC content required for film formation while maintaining the dispersion with suitable flowability for casting. In a preferred embodiment of the invention, the CNC mass concentration is 3%.

[0030] Pluronic F127 is added to the CNC solution to achieve a mass ratio of F127 to CNC of 0.05 to 7:20. Mixing can be performed using conventional methods in the art, such as vortexing, magnetic stirring, or ultrasonic dispersion. In one specific embodiment of the invention, vortexing for 30 minutes is used to uniformly disperse F127 in the CNC aqueous solution, forming a stable and homogeneous composite film solution.

[0031] The composite film liquid is poured onto a flat substrate and then self-assembled into a film by evaporation-induced self-assembly. Evaporation-induced self-assembly is a conventional method in the art for preparing photonic crystal thin films from CNC dispersions. The principle is that as water slowly evaporates, the concentration of CNC particles gradually increases, the interaction between particles is enhanced, and under the synergistic drive of chiral repulsion and van der Waals attraction, the CNC particles spontaneously align to form a chiral nematic phase structure.

[0032] In one specific embodiment of the present invention, 15 mL of the composite film solution is uniformly poured into a polystyrene petri dish with a diameter of 90 mm, ensuring that the film solution is evenly spread and free of air bubbles. The poured petri dish is then placed in a constant temperature and humidity environment for slow drying. The preferred constant temperature and humidity environment is 25°C and 45% relative humidity, and the preferred drying time is 96 hours. These temperature and humidity conditions can control the rate of water evaporation, allowing CNC and F127 molecules sufficient time for orderly self-assembly, thereby obtaining a composite photonic thin film with a regular internal structure, a smooth surface, and a certain degree of flexibility.

[0033] It should be noted that the film-forming conditions (temperature, humidity, and time) for evaporation-induced self-assembly can be conventionally adjusted according to the actual environment and film thickness requirements. For example, the temperature can be adjusted within the range of 20℃ to 45℃, the relative humidity can be adjusted within the range of 30% to 55%, and the drying time can be extended or shortened accordingly with the amount of film liquid and the ambient humidity. Those skilled in the art will understand that as long as the evaporation rate is moderate, avoiding excessively fast evaporation leading to structural disorder or excessively slow evaporation leading to an excessively long film-forming period, a composite film with a chiral nematic phase structure can be obtained. After drying, the cellulose nanocrystal composite photonic film is obtained by peeling it off from the substrate.

[0034] The third embodiment of the present invention provides the application of cellulose nanocrystal composite photonic thin film in identifying water adulteration in methanol.

[0035] The composite photonic thin film of the present invention exhibits specific solvent response behavior to the methanol-water system. Its response mechanism is based on the recombination of the hydrogen bond network and pitch change inside the film caused by the difference in solvent polarity.

[0036] In pure methanol, although methanol molecules can penetrate into the film to a certain extent, the swelling degree of the film is limited because the network structure formed by the hydrophobic PPO segments of F127 has a certain barrier effect on the diffusion of polar solvents. Its chiral nematic phase pitch remains within a small range, so the film exhibits a stable initial structural color.

[0037] When water is added to methanol, water molecules exhibit a strong affinity for the hydrophilic PEO segments in F127. These water molecules are specifically captured by the PEO segments and enter the film interior, causing significant swelling of the CNC network. Simultaneously, water-mediated hydrogen bonding further expands the pitch of the CNC chiral nematic phase structure. This significant increase in pitch leads to a redshift of the Bragg reflection wavelength towards longer wavelengths, macroscopically manifested as a color transition from blue / green to orange / orange-red or even colorless and transparent. This color transition is visually perceptible and rapid, enabling visual identification of water-added methanol.

[0038] It is worth noting that the response sensitivity and endpoint color of the film may vary depending on the F127 content. An appropriate amount of F127 (such as a low mass ratio within the range of 2–7:20) enhances the capture and network swelling capacity of water molecules through PEO segments, resulting in higher sensitivity. However, as the F127 content increases, the diffusion barrier effect formed by PPO segments gradually strengthens, which may inhibit the deep penetration of water molecules to some extent, leading to a decrease in sensitivity. Therefore, controlling the F127 content within the aforementioned mass ratio range can achieve synergistic optimization between structural stability in methanol environments and response sensitivity to water addition.

[0039] The identification is a visual identification, specifically involving cutting the cellulose nanocrystal composite photonic film into an appropriate size (e.g., a 10mm × 40mm sample) and contacting it with the liquid to be tested. The contact method can be immersing the film in the liquid or dripping the liquid onto the film surface. The liquid to be tested is a methanol fuel sample suspected of being adulterated with water. The contact time is 10–20 seconds. Within this time range, the film and the liquid to be tested undergo a sufficient solvent response, and the color change reaches a stable, observable state. Too short a contact time may result in an insufficient response and an unclear color change; while too long a contact time does not affect the determination, it reduces the efficiency of on-site screening. After contact, the film is removed, and its color change is observed directly under natural light or white light.

[0040] Based on the color change of the film, the following criteria are used for judgment: If the film color remains at its initial color without significant change, i.e., there is no transition to orange / red or discoloration, it indicates that the methanol being tested is not adulterated with water and can be judged as high-purity methanol. If the film color changes significantly, from the initial color through a yellow-green (transitional state) to orange / orange-red, it indicates that the methanol being tested has been adulterated with water; if the film color further fades to colorless and transparent, it indicates that the methanol being tested has a high water content. This judgment method is intuitive and fast, requiring no complex instruments or professional training, and is suitable for rapid on-site screening of illegal stove fuels such as counterfeit "environmentally friendly oil" and "biodiesel".

[0041] The composite photonic thin film exhibits good reversibility and structural stability during multiple solvent response cycles. In methanol-water system detection, the chiral nematic phase structure of the film is largely restored after the swelling-deswelling process, allowing for repeated use. However, in practical field screening applications, considering ease of operation and avoiding cross-contamination, the film is more suitable for use as a disposable test strip. Since the main raw material, microcrystalline cellulose, is widely available, low in cost, and has a simple preparation process, the film can be mass-produced into a large number of test strips, resulting in extremely low cost per test.

[0042] The preparation and properties of cellulose nanocrystal composite photonic films are described in detail below through several examples.

[0043] Purchase information for some of the reagents used in the following examples and comparative examples is as follows: Microcrystalline cellulose powder was purchased from Shanghai Maclean Biochemical Technology Co., Ltd. Pluronic F127, purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.

[0044] The composite films of cellulose nanocrystals and Pluronic F127 are collectively referred to as CNC-F127 films, and are designated as CF5, CF10, CF15, and CF20 according to the different amounts of F127 added, where the numbers represent the mass percentage of F127 in the CNC (e.g., CF5 indicates that the amount of F127 added is 5% of the mass of the CNC). Example 1: Preparation of cellulose nanocrystal / Pluronic F127 composite photonic film (CF5) This embodiment discloses a method for preparing cellulose nanocrystal composite photonic thin films. CNC is prepared using microcrystalline cellulose as a raw material, then composited with F127, and a composite thin film with photonic properties is prepared by evaporation-induced self-assembly. The specific steps are as follows: S1. CNC Fabrication This step is a common preparation step for all subsequent composite films, and the specific operation is as follows: a. Acid hydrolysis reaction: Take 10g of microcrystalline cellulose powder and mix it thoroughly with 85mL of 60% sulfuric acid solution. Place it in a constant temperature environment of 45℃ and stir at 300rpm for 70min. After the acid hydrolysis reaction is completed, immediately add ultrapure water to dilute and terminate the reaction. Let it stand for 2 days to allow the acid hydrolysis products to settle completely. b. Centrifugation and washing: After settling, take the milky white solution that has settled at the bottom and centrifuge it at 24,000 rpm for 20 minutes to achieve solid-liquid separation; collect the solid precipitate after centrifugation and wash it until the pH of the solution reaches neutral and the solution is transparent and light blue. c. Dialysis and drying: The above transparent light blue solution was transferred into a dialysis bag with a molecular weight cutoff of 12-14 kDa and dialyzed in ultrapure water for 7 days to remove trace impurities and residual small molecules from the solution. After dialysis, the solution was ultrasonically dispersed and then freeze-dried to obtain CNC solid powder, which was then sealed and stored for later use.

[0045] S2. Preparation of composite photonic thin films (CF5) a. Preparation of composite membrane solution: Prepare an aqueous solution of cellulose nanocrystals (CNC) with a mass fraction of 3%; uniformly disperse F127 in the CNC aqueous solution, wherein the mass ratio of F127 to CNC is 1:20 (i.e., the amount of F127 added is 5% of the mass of CNC powder), to form a stable composite membrane solution; b. Casting and drying: Take 15 mL of composite film liquid and cast it evenly into a polystyrene petri dish. Dry it slowly for 96 h in a constant temperature and humidity environment of 25℃ and 45% relative humidity. After drying, peel it off from the petri dish to obtain the cellulose nanocrystal / Pluronic F127 composite photonic film, denoted as CF5 film.

[0046] Example 2: Preparation of cellulose nanocrystal / Pluronic F127 composite photonic thin film (CF10) The preparation method of the CF10 thin film in this embodiment is exactly the same as that in Example 1. The only difference is that the mass ratio of F127 to CNC in step S2 is 1:10 (that is, the amount of F127 added is 10% of the mass of CNC). The rest of the preparation method is completely the same, and finally a cellulose nanocrystal / Pluronic F127 composite photonic thin film is obtained, which is denoted as CF10 thin film.

[0047] Example 3: Preparation of cellulose nanocrystal / Pluronic F127 composite photonic thin film (CF15) The preparation method of the CF15 film in this embodiment is exactly the same as that in Example 1. The only difference is that the mass ratio of F127 to CNC in step S2 is 3:20 (that is, the amount of F127 added is 15% of the mass of CNC). The rest of the preparation method is exactly the same, and finally a cellulose nanocrystal / Pluronic F127 composite photonic film is obtained, which is denoted as CF15 film.

[0048] Example 4: Preparation of cellulose nanocrystal / Pluronic F127 composite photonic thin film (CF20) The preparation method of the CF20 film in this embodiment is exactly the same as that in Example 1. The only difference is that the mass ratio of F127 to CNC in step S2 is 1:5 (that is, the amount of F127 added is 20% of the mass of CNC). The rest of the preparation method is exactly the same, and finally a cellulose nanocrystal / Pluronic F127 composite photonic film is obtained, which is denoted as CF20 film.

[0049] Comparative Example 1: Preparation of pure cellulose nanocrystal photonic thin film (CNC film) The preparation method of the pure CNC film in this comparative example is the same as that in Example 1, except that F127 was not added. Specifically, the CNC solid powder prepared in step S1 of Example 1 was used to prepare a 3% (w / w) CNC aqueous solution, which was then directly cast and dried to obtain a pure cellulose nanocrystal photonic film, denoted as the CNC film.

[0050] Test Example 1: Microstructure Characterization of Cellulose Nanocrystals 1.1 Experimental Methods The cellulose nanocrystal solid powder prepared in step S1 of Example 1 was dissolved in ultrapure water to prepare a cellulose nanocrystal solution with a mass fraction of 0.001%, and stirred thoroughly until uniformly dispersed. The solution was dropped onto a 200-mesh copper grid covered with a carbon film and negatively stained with a 3wt% phosphotungstic acid solution for 1 min, and then allowed to dry naturally at room temperature. The morphology of the cellulose nanocrystals was observed using a transmission electron microscope (model JEM-2100, JEOL Ltd., Japan). At least 200 rod-shaped cellulose nanocrystals were selected from each sample, and their dimensions (length, width) were measured, statistically analyzed, and the average length, average width, and aspect ratio were calculated using Nano Measure software.

[0051] 1.2 Experimental Results The transmission electron microscope (TEM) results of cellulose nanocrystals are shown in the figure below. Figure 1 As shown, Figure 1 Image (a) in the image is a transmission electron microscope image. Figure 1 (b) in the middle is Figure 1 Enlarged view of a portion of (a); Figure 2 This is a statistical diagram showing the size results of cellulose nanocrystals. Figure 2 (a) in the figure shows the statistical results of the length of cellulose nanocrystals. Figure 2 (b) in the figure shows the statistical results of the width of cellulose nanocrystals.

[0052] The results showed that rod-shaped cellulose nanocrystals were successfully prepared in Example 1, exhibiting a typical rod-shaped crystal structure. Size statistics indicated ( Figure 2 The cellulose nanocrystals have an average length of 100.21 nm and an average width of 9.90 nm, with an aspect ratio of ~10.1, indicating that the obtained material has uniform size and a suitable aspect ratio. A higher aspect ratio is beneficial for the formation of a dense helical structure in the cellulose nanocrystals, thereby enhancing the periodic alignment of photonic crystals and producing more distinct angle-dependent structural colors. If the size of the cellulose nanocrystals is too large (e.g., reaching the micrometer scale), the aspect ratio will decrease, not only interfering with their ordered helical alignment and blurring the structural colors, but also, in severe cases, even hindering the formation of photonic crystals. Therefore, the cellulose nanocrystals prepared in this invention, with their uniform size distribution and suitable aspect ratio, show good application potential in the optical field.

[0053] Test Example 2: Zeta potential and polydispersity index (PDI) values ​​of cellulose nanocrystal solution and cellulose nanocrystal-Pluronic F127 mixed solution. 2.1 Experimental Methods The cellulose nanocrystal solid powder prepared in step S1 of Example 1 was dissolved in ultrapure water to prepare a cellulose nanocrystal solution with a mass concentration of 0.01%. Simultaneously, the composite membrane solutions prepared in Examples 1 to 4 were diluted to ensure that the mass concentration of cellulose nanocrystals in each diluted solution reached 0.01%, resulting in diluted solutions 1 to 4 (i.e., composite membrane diluted solutions, corresponding to CF5, CF10, CF15, and CF20, respectively). Using a Zetasizer (Zetasizer Nano ZS90, Malvern Instruments Ltd., UK), the Zeta potential and PDI values ​​of the 0.01% cellulose nanocrystal solution and the diluted solutions of each composite membrane solution were measured at a constant temperature of 25°C. To ensure the accuracy and repeatability of the test data, a cyclic measurement mode was used for each sample. The measurement was first repeated 5 times, then repeated 3 times, for a total of 15 measurements (5 × 3) for each sample. The average value of the multiple measurements was taken as the test result of the Zeta potential and PDI value of that sample.

[0054] 2.2 Experimental Results like Figure 3As shown in (a), with the increase of F127 addition (from CNC to CF20), the PDI value of the composite system showed a significant decreasing trend, from about 0.26 for CNC to about 0.18 for CF20. This indicates that the introduction of F127 effectively reduced the polydispersity of the CNC dispersion system, making its particle size distribution more uniform and significantly improving the dispersion stability. This may be because F127, as an amphiphilic block copolymer, has hydrophilic segments (PEO) that extend in water to form a hydration layer, which effectively prevents the CNC particles from approaching and agglomerating through steric hindrance, thereby improving the overall dispersion uniformity.

[0055] like Figure 3 As shown in (b), the Zeta potential of all samples was negative, confirming the negative charge carried by the sulfate groups on the CNC surface. With increasing F127 content, the Zeta potential (negative value) gradually increased, decreasing from approximately -43 mV for CNC to approximately -35 mV for CF20. This indicates that the negative charge on the CNC surface was gradually neutralized by F127. This is because the hydrophobic PPO segments of F127 adsorbed onto the CNC surface, partially covering the negatively charged sites and weakening the surface charge density. F127 is a triblock copolymer (PEO-PPO-PEO), in which the intermediate PPO segments are hydrophobic. When F127 is added to the CNC solution, its hydrophobic PPO segments adsorb onto the hydrophobic regions or defects on the CNC surface through hydrophobic interactions. This adsorption directly covers some of the originally negatively charged sites on the CNC surface, weakening the inherent negative charge of the CNC surface, thus leading to a decrease in the absolute value of the Zeta potential. As the amount of F127 added increased from 0% to 20%, the zeta potential of the composite membrane diluent decreased from approximately -45.3 mV at CNC to approximately -37.7 mV at CF20.

[0056] The above results indicate that F127 plays a dual regulatory role in the CNC system. On the one hand, its hydrophilic segments improve dispersion uniformity (PDI decreases) through steric hindrance; on the other hand, its hydrophobic segments adsorb and neutralize surface charge (Zeta potential decreases). Although the electrostatic repulsion is weakened, the steric stabilization mechanism introduced by F127 becomes key to maintaining the colloidal stability of the system. This suggests that the uniformity and surface properties of the CNC dispersion can be controlled by adjusting the amount of F127 added.

[0057] Test Example 3: Testing the Structure and Physical Properties of Thin Films To comprehensively characterize the structure, optical properties, mechanical properties, and humidity response properties of the CF5, CF10, CF15, and CF20 films prepared in Examples 1 to 4, and the CNC film (pure cellulose nanocrystal film) prepared in Comparative Example 1, the following test methods were used for systematic testing, and the specific operations are as follows: 3.1 Fourier Transform Infrared Spectroscopy (FTIR) Test Infrared spectra of each thin film were measured using a Fourier transform infrared spectrometer (IRPrestige-21, Shimadzu, Japan). The test conditions were: wavenumber range 4000–400 cm⁻¹. -1 4cm resolution -1 The infrared spectra of each thin film were obtained by scanning 32 times in an average scanning mode, which were used to analyze the functional group characteristics and interactions of each component in the thin film.

[0058] Experimental results are as follows Figure 4 As shown in Figure a, CNC films, CF5 films, CF10 films, CF15 films, and CF20 films all retain the typical characteristics of CNC films. β Type I absorption. Approximately 3454 cm³. -1 The broad absorption peak at 2902 cm⁻¹ is attributed to the OH stretching vibration. Compared to the CNC film, the absorption peak of the CNC-F127 series films gradually red-shifts, indicating an enhanced interaction between the hydroxyl groups of CNC and F127. -1 The characteristic absorption peak at 1637 cm⁻¹ represents the CH stretching of the aliphatic saturated chain in glucose; -1 The peak at 1166 cm⁻¹ corresponds to the -OH bending vibration of adsorbed water under the interaction of CNC and water; -1 The peak at 889 cm⁻¹ represents the stretching vibration of the glucose pyran ring unit; -1 The peak at that location represents CNC I β Typical stretching of the β-glycosidic bond; 810 cm -1 The absorption band at 1110 cm⁻¹ is attributed to the symmetric vibrations of the sulfate half-ester (SOC) groups on the CNC surface, confirming the successful acid hydrolysis of microcrystalline cellulose. -1 The absorption bands observed at this location are attributed to the stretching vibrations of the glucose-pyran ring skeleton in the CNC. Although pure F127 at 1110 cm⁻¹... -1 A strong COC stretching vibration peak is shown at this location. Figure 4 (b) However, in the composite photonic thin film spectrum, the intensity of this absorption band did not change significantly with increasing F127 content. This lack of proportional intensity enhancement can be attributed to the overlap between the COC stretching vibration signal of F127 and the ring skeleton vibration signal of CNC, and the relatively weak contribution of F127 in the dominant CNC matrix at the concentrations involved in this study. These results indicate that F127 interacts with CNC through hydrogen bonds, but does not alter the basic chemical structure of CNC.

[0059] 3.2. X-ray diffraction (XRD) test An X-ray diffractometer (Rigaku D / MAX-RB, Rigaku, Tokyo, Japan) equipped with a Cu Kα radiation source was used to perform XRD tests on each thin film. The test parameters were set as follows: tube voltage 40 kV, tube current 50 mA, 2θ scan range 5°–40°, scan rate 2° / min, uniform scanning, and real-time data collection to obtain the XRD patterns of each thin film, which were used to analyze the crystal structure, crystallinity, and crystal plane characteristics of the thin films.

[0060] Experimental results are as follows Figure 5 As shown in (a), the CF5, CF10, CF15, and CF20 films all exhibit CNC I at 2θ of approximately 14.8°, 16.6°, and 22.7°. β The characteristic diffraction peaks of the type structure are attributed to the (1-10), (110) and (200) crystal planes, respectively. Figure 5 (b) The results show that the interplanar spacing values ​​of these crystal planes do not change with increasing F127 content. This invariance, along with the fixed peak positions, provides direct evidence that cellulose I... β The fundamental lattice parameters remained unchanged, and F127 was not embedded in or chemically modified in the core crystalline region of the CNC. However, quantitative analysis of the crystallinity index (CrI) showed a decrease in the long-range order of the material. Compared with the CNC film (CrI=94.52%), the CrI of the CF5, CF10, CF15, and CF20 films decreased from 93.46% to 84.47% with the addition of F127, indicating the introduction of amorphous regions into the CNC matrix. Accompanying this trend, the crystallite size calculated by the Scherrer equation exhibited a significant anisotropic response. Figure 5 (c) The calculation results show that the addition of F127 has an orientation-dependent effect on the crystallite size: the size in the (1-10) crystal plane direction decreases from 6.5 nm (CNC) to 4.1 nm (CF20); the size in the (110) crystal plane (the major axis of CNC) direction increases from 3.0 nm to 7.5 nm; and the size in the (200) crystal plane direction remains basically constant, fluctuating between 6.0 and 6.8 nm. These results indicate that the introduction of F127 induces orientation-dependent reorganization of crystal assembly: it selectively reduces the lateral order along the (1-10) crystal plane while enhancing the structural order along the main (110) axis.

[0061] 3.3. Scanning Electron Microscopy (SEM) Cross-sectional Morphology Analysis First, each thin film was surface-plated with gold (to enhance sample conductivity and prevent charge accumulation during testing). Then, a scanning electron microscope was used to scan the fracture surface of each film with an accelerating beam voltage of 5 kV to obtain cross-sectional morphology images. The obtained morphology images were analyzed using ImageJ software, and the pitch of each film was accurately measured to evaluate the regularity of the ordered structure inside the film.

[0062] Experimental results are as follows Figure 6 As shown, the cross-section of a pure CNC film exhibits a typical, dense, and regularly arranged layered structure. With the introduction of F127, the pore size and number of CF5 to CF20 films gradually increase, the regularity of the layered structure decreases accordingly, the layers gradually separate, and the overall morphology becomes more porous. This indicates that the addition of F127 interferes with the tight self-assembly of CNC, leading to an increase in interlayer spacing.

[0063] 3.4. UV-Vis spectroscopy, CIE colorimetric diagram, and pitch analysis of thin films A UV-Vis spectrophotometer was used to perform UV-Vis full-wavelength scanning tests on each thin film. The test wavelength range was 200–800 nm. The UV-Vis absorption spectra of each thin film were scanned at a constant speed and recorded. At the same time, the CIE color coordinates of each thin film were measured. Combining the spectral data and color coordinates, the optical properties and structural color characteristics of each thin film were systematically analyzed.

[0064] Experimental results are as follows Figure 7 and Figure 10 As shown, the pure CNC film exhibits a strong absorption peak at 400 nm in the ultraviolet spectrum. With the addition of F127, the absorption peak of the composite film (CNC-F127) red-shifts. When the F127 content is 5%, 10%, 15%, and 20%, the absorption peaks of the corresponding films (CF5, CF10, CF15, and CF20) red-shift to 445 nm, 469 nm, 514 nm, and 553 nm, respectively. These results suggest that the introduction of F127 produces a dual effect: firstly, it forms a more loosely layered ordered structure inside the film, effectively reducing light scattering loss and thus improving the film's transmittance; secondly, the increased interlayer spacing caused by this structural change modulates the film's optical refractive behavior, thereby inducing a characteristic red-shift in the absorption spectrum.

[0065] The CIE 1931 chromaticity diagram, plotted based on UV-Vis spectral data, is as follows: Figure 10As shown, the chromaticity coordinates of the pure CNC film are (0.25, 0.24). As the F127 content increases from 0% to 20%, the chromaticity coordinates of the composite film exhibit a regular shift, specifically CF5 (0.26, 0.28), CF10 (0.26, 0.30), CF15 (0.30, 0.34), and CF20 (0.31, 0.36). Their positions on the chromaticity diagram systematically and continuously shift from the yellow-green region towards orange-yellow. This regular evolution of the CIE chromaticity coordinates corresponds perfectly to the redshift phenomenon observed in UV-Vis spectroscopy, directly confirming that the hue and color saturation of the composite film can be precisely controlled by changing the F127 content.

[0066] Based on the above UV-Vis spectroscopy test results, the theoretical pitch (P) of each film was further calculated using the Bragg equation. UV ), and compared with the actual pitch (P) directly measured by scanning electron microscopy. SEM A comparative analysis was conducted to clarify the structural essence of F127's pitch control.

[0067] Experimental results are as follows Figure 8 As shown, with the increase of F127 content, P calculated based on the UV absorption peak position and the Bragg equation... UV The wavelength gradually increased from 255.3 nm (CNC) to 391.2 nm (CF20); P was directly measured by scanning electron microscopy. SEM It also shows a consistent increasing trend, increasing from 258.5nm to 351.3nm (see...). Figure 6 (SEM measurement results). The high degree of agreement between the two trends confirms that the expansion of the chiral nematic phase pitch is the fundamental structural reason for the redshift of the ultraviolet absorption edge. The above results consistently indicate that the introduction of F127 has a dual structural regulation effect: it occupies the gaps between CNC layers as a space filler, and it mediates the hydrogen bonding between the ether bonds on its polyethylene oxide segments and the hydroxyl groups on the CNC surface through hydrogen bonding. This dual effect effectively regulates and expands the inherent helical assembly cycle of the CNC, thereby achieving directional adjustment of the optical properties of the composite material.

[0068] In summary, the above UV-Vis spectroscopy, CIE chromaticity diagram, and pitch analysis results consistently demonstrate that F127 regulates the helical assembly structure of CNC at the nanoscale through its intermolecular interactions, expands its chiral nematic phase pitch, and thus macroscopically achieves the maintenance of the transmittance of the composite film, the redshift of the absorption spectrum, and the continuous tunability of the visual color, completing the effective correlation and precise control from nanostructure to macroscopic optical performance.

[0069] 3.5. Optical Image Measurement Each film was placed under the same natural light conditions and photographed to clearly record its appearance, color, transparency, and other visual characteristics. The optical image observation results were preserved for intuitive analysis of the effects of different amounts of Pluronic F127 on the appearance and optical properties of the films.

[0070] Experimental results are as follows Figure 9 As shown, the optical image of the CNC-F127 film exhibits a distinct iridescent color. With the addition of Pluronic F127, the color of the prepared film gradually changes from a predominantly blue (CNC) iridescent color to a predominantly green (CF20) iridescent color.

[0071] 3.6. Mechanical property testing of thin films Each film was cut into a standard rectangular specimen of 10mm × 50mm to ensure uniform size and no damage. Tensile stress tests were performed on each specimen using a BZ2.5 material testing machine (Zwick, Germany). The initial clamping distance was set to 25mm. Each film sample was tested in parallel 6 times. After removing outliers, the average value was taken as the final mechanical property test result to evaluate the mechanical strength and toughness of different composite films.

[0072] Experimental results are as follows Figure 11 As shown. Stress-strain curve ( Figure 11 (a) shows that the pure CNC film is relatively brittle, with a tensile strength and elongation at break of 33.90 MPa and 0.42%, respectively. As the F127 content increases from 5% to 20%, the tensile strength of the composite film shows a monotonically decreasing trend (CF5: 33.4 MPa, CF10: 31.8 MPa, CF15: 29.4 MPa, CF20: 26.4 MPa), while the elongation at break significantly increases (CF5: 0.46%, CF10: 0.68%, CF15: 0.75%, CF20: 0.91%). Young's modulus results ( Figure 11 (b) further confirms this trend, with the value systematically decreasing from 80.55 MPa for the CNC film to 34.44 MPa for the CF20 film. Significance analysis showed no statistically significant difference in modulus between CNC and CF5, while CF20 showed significant differences compared to all other groups. These quantitative results consistently indicate that the addition of F127 introduces an amorphous phase into the material. Its embedding in the rigid network of CNC reduces the overall crystallinity and rigidity, leading to a decrease in strength and modulus. Simultaneously, as a plasticizer, F127 enables the film to absorb more energy and allow for greater deformation under stress through molecular chain movement, thus significantly improving the material's ductility. Qualitative flexibility tests (bending, curling, and folding) yielded results consistent with this. Figure 11As shown in (c), the pure CNC film breaks upon folding, while the composite films (CF10, CF15, CF20) with an F127 content ≥10% can all be folded completely without breaking. In summary, the introduction of F127, by changing the aggregated structure of CNC, effectively endows the material with excellent flexibility and ductility while moderately sacrificing rigidity and strength, thus achieving a controllable transformation of the mechanical properties of composite materials from brittle to flexible.

[0073] 3.7. Humidity Response Test of Thin Film Each film was placed in a constant temperature and humidity chamber with relative humidity (RH) of 30%, 43%, 57%, 75%, 85%, and 100%, respectively, and equilibrated for 40 minutes under each humidity condition to ensure that the film fully adapted to the ambient humidity and reached a stable state. The optical properties of each film after equilibration were tested using a UV spectrophotometer (Shimadzu, Japan), and the UV-Vis spectra of each film under different relative humidity conditions were recorded to analyze the regulatory effect of humidity on the optical properties of the films.

[0074] Experimental results are as follows Figure 12 As shown, under a fixed humidity condition of 30%RH, the optical properties of the CNC / F127 composite film exhibit a significant dependence on the F127 content: the pure CNC film shows a characteristic absorption peak at 460nm, macroscopically appearing as a deep blue-violet color; as the F127 doping content gradually increases from 5% to 20%, the absorption peak of the composite film undergoes a systematic red shift, successively located at 465nm (CF5 film), 480nm (CF10 film), 550nm (CF15 film), and 560nm (CF20 film), with the corresponding macroscopic color gradually changing from blue-violet to green. The underlying mechanism of this phenomenon is that the incorporation of F127 disrupts the hydrogen bonding between CNC molecules through hydroxyl groups, leading to an increase in the pitch of the chiral nematic phase of CNC, ultimately causing a regular change in optical properties.

[0075] Within a relative humidity range of 30% to 100%, the chromaticity coordinates of all films exhibited a consistent trend, namely, a synchronous rightward shift (Δx>0) and an upward shift (Δy>0). This phenomenon indicates that the perceived color of the films undergoes a synchronous redshift, while the green hue is significantly enhanced. Figure 13The underlying reason lies in the humidity-induced hydrogen bond bridging mediated by water molecules, which further expands the helical pitch of the CNC-F127 photonic thin film, thereby modulating the film's optical properties. Under high humidity conditions of 100% RH, the chromaticity coordinates of the pure CNC film shift to (0.32, 0.35), while those of the CF20 film shift to (0.34, 0.34). Notably, the chromaticity coordinate trajectory of the CF20 film exhibits a more significant expansion, highlighting the dual regulatory effect of film composition (F127 content) and ambient humidity on color output.

[0076] 3.8. Moisture Absorption Rate Test of Thin Film First, each film was precisely weighed, and its initial weight (denoted as W0) was recorded with an accuracy of 0.0001g. Then, each film was placed in a constant temperature and humidity chamber with relative humidity of 30%, 43%, 57%, 75%, 85%, and 100% for water absorption testing. After the films stabilized in the corresponding humidity environment for 30 minutes, they were quickly removed and precisely weighed again, and the weight of the films after water absorption was recorded (denoted as W1). The water absorption rate of each film under different humidity conditions was calculated using the following formula: Water absorption rate = {(W1 - W0) / W0} × 100% Wherein, W1 is the weight of the film after absorbing water (unit: g), and W0 is the initial weight of the film (unit: g); each sample is tested in parallel 3 times under each humidity condition, and the average value is taken as the final water absorption rate result, which is used to evaluate the moisture absorption performance of each film.

[0077] Experimental results are as follows Figure 14 As shown in the figure. The test results show that the water absorption rate of all tested films increases monotonically with increasing ambient humidity, but the increase varies significantly among different films. The water absorption rate of the pure CNC film increases relatively slowly, rising from 0.15% to 22.5% as the relative humidity increases from 30% to 100%. In contrast, the composite films containing F127 exhibit more pronounced hygroscopic properties, especially the CF20 film, whose water absorption rate increases dramatically from 1.89% to 77.1% within the same humidity range, showing the most significant increase. This indicates that the introduction of F127 can significantly enhance the hydrophilicity and hygroscopic capacity of the composite material, and this enhancing effect becomes more pronounced with increasing F127 content. Based on the previously observed changes in the optical properties of the thin film (red shift of the characteristic absorption peak), it can be inferred that the increase in the water absorption rate of the thin film under high humidity conditions is mainly attributed to the penetration of water molecules into the interior of the thin film, which causes the CNC network to swell. This swelling effect further expands the pitch of the CNC chiral nematic phase structure, thereby modulating the optical properties of the thin film.

[0078] 3.9. Humidity response cycling test of the thin film The CF20 film prepared in Example 4 and the CNC film of Comparative Example 1 were selected and cut into 10mm × 40mm samples. These samples were placed in a constant temperature and humidity chamber at 25℃ and 30% relative humidity for 30 minutes to equilibrate. They were then transferred to a constant temperature and humidity chamber at 25℃ and 100% relative humidity for another 30 minutes. This process was recorded as one complete humidity response cycle. The above cycle was repeated for a total of 12 cycles. After the two humidity equilibration stages of each cycle, the samples were immediately subjected to spectral measurements in the wavelength range of 200–800 nm using a UV spectrophotometer. The wavelength of the maximum absorption peak (λmax) was recorded for each test to evaluate the stability and reversibility of the humidity response of the CF20 film.

[0079] Experimental results are as follows Figure 15 As shown, both the CNC film and the CF20 film exhibit excellent reversible response and cycle durability. The CF20 film shows a significant bidirectional difference in humidity response speed: when the relative humidity increases from 30% to 100%, its response time is approximately 10 minutes; while when the relative humidity decreases from 100% to 30%, the response time can be controlled within 30 seconds, demonstrating rapid desorption response performance. Regarding cycle stability, both films exhibit excellent performance. The maximum absorption peak λmax of the CNC film varies by approximately 65 nm, and after 12 humidity cycles, its wavelength change (Δλmax) fluctuation is less than 6 nm, demonstrating good repeatability and stability. Similarly, the maximum absorption peak λmax of the CF20 film varies by approximately 60 nm, and after 12 humidity cycles, not only does the color recovery rate exceed 98%, but the wavelength change (Δλmax) fluctuation is also less than 7 nm, also exhibiting excellent repeatability and stability. Meanwhile, in the aforementioned cyclic tests at 30% and 100% humidity, the CNC and CF20 films exhibited reversible changes between "purple-blue / green" and "green / brownish-red," respectively. These test results clearly demonstrate that the CF20 film prepared in Example 4 possesses excellent cyclic stability and can stably and repeatedly respond to dynamic changes in humidity.

[0080] Test Example 4: Response Performance Test of CNC-F127 Thin Film to Methanol Addition with Water 4.1 Experimental Methods Anhydrous methanol was diluted with ultrapure water to prepare methanol-water solutions with water contents of 0%, 5%, 10%, and 15% (v / v), respectively. In addition, to study the color response kinetics and the endpoint color change under high water content, methanol-water solutions with water contents of 20%, 25%, and 50% (v / v) were prepared as supplementary observation groups. The CF5, CF10, CF15, and CF20 films prepared in Examples 1-4, and the CNC film prepared in Comparative Example 1 (all 10 mm × 40 mm in size) were immersed in the above-mentioned methanol-water solutions with different water contents at room temperature (25 ± 5 °C) for 20 seconds. After removal, the films were immediately subjected to transmission spectroscopy testing using a UV-Vis spectrophotometer (Shimadzu, Agilent Cary 60, etc. models are acceptable), and the color change of the films was simultaneously photographed. The wavelength λ of the maximum absorption peak (transmission peak) was recorded from each UV transmission spectrum. t The wavelength λ of the maximum absorption peak (transmission peak) measured with anhydrous methanol (a methanol-water solution with 0% water content) t0 Using Δλ as the baseline, the relation Δλ = λ is adopted. t −λ t0 Calculate the spectral redshift Δλ corresponding to different water contents in methanol-water solutions. Using Δλ as the y-axis and the water content of the methanol-water solution as the x-axis, perform linear least squares regression fitting on the measured Δλ and the corresponding water content to extract the sensitivity (slope, unit: nm / %H2O) and the coefficient of determination (goodness of fit, R²). 2 This is used to evaluate the spectral quantitative detection performance of the thin film, and then determine the film's response capability for doping water in methanol-water solutions.

[0081] At room temperature, the CF20 film was immersed in solutions of 20%, 25%, and 50% water / methanol, and its color evolution during the methanol immersion process was simultaneously photographed and recorded.

[0082] 4.2 Experimental Results 1. Quantitative Spectral Response and Sensitivity Analysis like Figure 16 As shown, by systematically studying the optical response of CNC-F127 composite films with different F127 contents in methanol and water-doped methanol, the mechanism by which F127, as an amphiphilic block copolymer, precisely regulates the swelling behavior and sensing performance of the film through the synergistic competition of network compatibilization effect and diffusion barrier effect is elucidated.

[0083] In pure methanol, the initial UV absorption peak redshift of the film (reflecting the basic degree of swelling) first increases and then decreases with the F127 content, corresponding to a change in macroscopic color from the bright green of the CNC film, through the yellow of the CF5 film, the bright yellow of the CF10 film, to a slightly deeper yellow of the CF15 and CF20 films. The CF5 film exhibits the largest redshift (56 nm) due to its appropriate amount of PEO segments embedded in the CNC network, enhancing solvent compatibility and penetration, demonstrating a significant network compatibility enhancement effect. With increasing F127 content, its hydrophobic PPO segments form a dense physical cross-linked network, generating a strong diffusion barrier effect, inhibiting solvent penetration, making the redshift of the CF20 film (15 nm) similar to that of the pure CNC film (10 nm), and their colors also tend to converge. This indicates that there exists an optimal F127 addition amount (approximately 5% in this system) that can maximize the initial response sensitivity of the film in pure solvent without significantly hindering diffusion.

[0084] In the water-doped methanol (0–50% water content) system, all CNC / F127 films (CF5–CF20) exhibited good linear optical response (R0). 2 With a sensitivity of approximately 0.98 and a wider detection range, it can stably detect water content of at least 15%; while pure CNC membranes cannot be effectively detected when the water content exceeds 10% due to excessive swelling and rupture of the structure. Water molecules can be specifically captured by the PEO segments of CNC and F127, causing the photonic lattice spacing to expand proportionally with the water content, driving the film color to evolve systematically from the initial hue (green / yellow) through orange, red to a colorless system. It is worth noting that the CF5-CF15 membranes completely fade to colorless when the water content reaches about 15%, while the CF20 membrane reaches a colorless state at a water content of 10%. This is related to its stronger diffusion barrier effect, which inhibits the deep penetration and uniform distribution of water molecules.

[0085] Sensitivity analysis further confirmed this competitive mechanism. The sensitivities of CNC, CF5, CF10, CF15, and CF20 were 12.5, 14.3, 12.2, 9.12, and 8.0 nm / %, respectively. Among them, the CNC film only exhibited a linear response within the 0–10% water content range, with a sensitivity of 12.5 nm / ; when the water content exceeded 10%, the film failed due to excessive swelling and rupture. The CF series films maintained a linear response within the 0–15% (and even higher) range, demonstrating a wider applicable detection range. In other words, an appropriate amount of F127 (CF5) enhances the capture and network swelling capabilities of water molecules through PEO segments, thus improving sensitivity; while excessive F127 (such as CF20) results in a decrease in sensitivity because the diffusion barrier formed by PPO segments dominates, inhibiting the penetration and uniform distribution of water molecules.

[0086] Based on the aforementioned color response dynamics and endpoint color patterns, this study establishes a rapid visual discrimination method. After the film is exposed to the test liquid for approximately 20 seconds, if its endpoint color remains unchanged from its initial color in pure methanol (i.e., no transition to orange / red hues or color loss), it can be determined that the methanol is not adulterated with water. If the endpoint color changes to orange / orange-red or even colorless and transparent, it can be determined that the methanol is adulterated with water. Specifically, an endpoint color remaining in the orange / orange-red hue typically corresponds to a low water content, while a endpoint color fading to colorless and transparent corresponds to a high water content. This method is intuitive, rapid, and the discrimination threshold is related to the F127 content in the film.

[0087] 2. Color Response Dynamics of CF20 Thin Films like Figure 17 As shown, in the methanol-water mixture, the CF20 film exhibits a rapid color response in 20%, 25%, and 50% (v / v) water-methanol mixtures, with the color response completed within 15 seconds and reaching a steady state at 20 seconds. The results indicate that the film color, at all three concentrations, exhibits a rapid transition from initial green, through yellow and orange hues, to a colorless and transparent state, with the entire response reaching a steady state within 20 seconds. Specifically, in the 20% water / methanol solution, the color changes from green to yellowish-green within 3 seconds, to reddish-brown in 10-12 seconds, begins to fade after 15 seconds, and stabilizes as transparent in 20 seconds. In the 25% water / methanol solution, the response rate accelerates, turning orange-yellow in 3 seconds, orange-red in 10 seconds, deep red in 12 seconds, and fading completely in 20 seconds. In the 50% water / methanol solution, the response is the most rapid, reaching saturated orange-yellow in 3 seconds, rapidly transitioning from red-orange to dark red within 5-10 seconds, and fading completely within 15-20 seconds. The higher the methanol concentration, the faster the color transition to orange / red hues and the faster the eventual fading. Furthermore, this rapid orange-yellow response (dominant within 2-5 seconds) exhibits solvent specificity in both speed and final hue. These results confirm that the color change of the CF20 film provides a visually-based, rapid (20 seconds), and concentration-dependent method for solvent identification.

[0088] The embodiments described above are some, but not all, of the embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art through related deductions and substitutions based on the inventive concept, without inventive effort, are within the scope of protection of the present invention.

Claims

1. A cellulose nanocrystal composite photonic thin film, characterized in that, The film comprises cellulose nanocrystals and Pluronic F127, wherein the mass ratio of Pluronic F127 to cellulose nanocrystals is 0.05 to 7:20, and the film has a chiral nematic photonic crystal structure.

2. The cellulose nanocrystal composite photonic thin film as described in claim 1, characterized in that, The cellulose nanocrystals are obtained by acid hydrolysis, centrifugation, washing, dialysis and drying of microcrystalline cellulose.

3. A method for preparing cellulose nanocrystalline composite photonic thin films, characterized in that, Pluronic F127 and cellulose nanocrystals were dispersed in water at a mass ratio of 0.05 to 7:20 to obtain a composite membrane solution. The composite membrane solution was then evaporated and induced to self-assemble into a membrane to obtain a composite thin film with a chiral nematic photonic crystal structure.

4. The preparation method according to claim 3, characterized in that, The cellulose nanocrystals are obtained by acid hydrolysis, centrifugation, washing, dialysis and drying of microcrystalline cellulose.

5. The preparation method according to claim 4, characterized in that, The acid hydrolysis is performed using a sulfuric acid solution with a mass fraction of 45-65%, stirred at 40-50°C for 60-100 minutes.

6. The preparation method according to claim 3, characterized in that, The cellulose nanocrystals have a mass concentration of 0.5% to 7.5% in the aqueous solution.

7. The preparation method according to claim 3, characterized in that, The evaporation-induced self-assembly was carried out in a constant temperature and humidity environment of 20–45°C and 30–55% relative humidity, and dried until a film was formed.

8. The application of the cellulose nanocrystalline composite photonic film as described in claim 1 or 2, or the cellulose nanocrystalline composite photonic film obtained by any of the preparation methods described in claims 3-7, in identifying water adulteration in methanol.

9. The application as described in claim 8, characterized in that, The identification is a visual identification, which includes: contacting the composite photonic film with the liquid to be tested and observing the color change; if the color remains unchanged from its initial color, it indicates that the methanol is not adulterated with water, and if the color changes to orange / orange-red or to colorless, it indicates that the methanol is adulterated with water.

10. The application as described in claim 9, characterized in that, The contact time is 10 to 20 seconds.

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