Preparation methods of superparamagnetic Fe3O4 nanoclusters and pH-responsive photonic crystal thin films

Abstract

This invention relates to a method for preparing a pH-responsive photonic crystal thin film, comprising synthesizing Fe3O4 magnetic nanoclusters using sodium poly(4-styrenesulfonic acid-copolymer-maleic acid) salt, ferric salt, etc., as raw materials and polyols such as ethylene glycol as solvents via a solvothermal method. The nanoclusters are then surface-modified with low-molecular-weight PAA to obtain Fe3O4@PAA. Fe3O4@PAA is dispersed in a prepolymer solution containing acrylic acid, hydroxyethyl methacrylate, ethylene glycol dimethacrylate, and an initiator, and injected into a film mold cavity with a magnetic field applied to one side. The film is then thermosetting to obtain a pH-responsive photonic crystal thin film. This film exhibits sensitive magnetic field and pH responsiveness, showing significant and visible structural color changes under different magnetic fields or pH values, with vibrant colors and a wide pH response range (3.6–6.2). This invention offers advantages such as stable product performance, high reproducibility, and ease of storage.

Description

Technical Field

[0001] This invention relates to the field of photonic crystal materials technology, specifically to a method for preparing superparamagnetic Fe3O4 nanoclusters and pH-responsive photonic crystal thin films. Background Technology

[0002] Photonic crystals, as an important class of optical materials, are most notably characterized by their photonic bandgap structure and their ability to modulate light waves. When the photonic bandgap shifts significantly, the color change can be directly observed with the naked eye. Building upon this, if photonic crystal structures are fabricated into functional materials responsive to external stimuli, these materials could not only respond to external stimuli but also convert those responses into readable optical signals. Such functional materials with these structures and properties would undoubtedly have broad application prospects.

[0003] Fe3O4 magnetic nanoparticles possess physical properties completely different from macroscopic materials, and these properties are closely related to their composition, size, morphology, and degree of order. Once the size of the magnetic particles reaches the nanoscale, their unique size range gives them superparamagnetism, making them suitable for preparing magneto-controlled photonic crystals. Currently reported methods for preparing Fe3O4 nanoclusters suffer from problems such as complex preparation steps, poor product stability, low yield, and poor dispersibility in organic solvents. Furthermore, currently reported pH-responsive photonic crystals exhibit issues such as dull color, insufficient pH sensitivity, and long response times. Summary of the Invention

[0004] (a) Technical problems to be solved

[0005] In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a method for preparing superparamagnetic Fe3O4 nanoclusters with good product stability, high reproducibility and high yield. Furthermore, the method utilizes the aforementioned superparamagnetic Fe3O4 nanoclusters to combine with pH-sensitive hydrogel to form a photonic crystal film with a wide pH response range. This method has the advantages of simple preparation steps, low cost, bright color, fast pH response time and wide pH response range.

[0006] (II) Technical Solution

[0007] In a first aspect, the present invention provides a method for preparing superparamagnetic Fe3O4 nanoclusters, which has the characteristics of good product stability, high reproducibility and high yield;

[0008] The preparation method is as follows: under ultrasonic vibration and stirring conditions, poly(4-styrenesulfonic acid-copoly-maleic acid) sodium salt PSSMA powder is dissolved in a polyol solvent. After the PSSMA is dissolved, soluble ferric salt is added, and ultrasonic vibration and vigorous stirring are continued. After the solution turns brownish-red, an alkaline salt is added to provide an alkaline environment. The reaction is continued with ultrasonic vibration and vigorous stirring for 10-30 min. The reaction solution is then transferred to a sealed reaction vessel and subjected to a solvothermal reaction at 180-220℃ for 6-14 h. After the reaction is completed, the reaction solution is cooled to room temperature and ultrasonically washed and magnetically separated with ethanol and deionized water, respectively, to obtain superparamagnetic Fe3O4 nanoclusters.

[0009] According to a preferred embodiment of the present invention, the conditions for ultrasonic oscillation and stirring are: ultrasonic frequency ≥ 40 kHz, stirring speed 600-1000 rpm, preferably 800 rpm.

[0010] According to a preferred embodiment of the present invention, in the PSSMA, the molar ratio of 4-styrenesulfonic acid to maleic acid is 3:1 to 1:1, and the average molecular weight Mw of the PSSMA is 15000-25000.

[0011] According to a preferred embodiment of the present invention, the polyol solvent is one or a combination of two or more of ethylene glycol, triethylene glycol and tetraethylene triethylene glycol; the mass-volume ratio of PSSMA to the polyol solvent is 3.75 g: 40-80 mL.

[0012] According to a preferred embodiment of the present invention, the ferric salt is one or a combination of two or more of ferric chloride or its hydrate, ferric nitrate or its hydrate, and ferric sulfate or its hydrate; the mass of PSSMA is 7-15 times the mass of iron in the ferric salt, preferably 9-10 times.

[0013] According to a preferred embodiment of the present invention, the alkaline salt is at least one of sodium citrate, sodium acetate, and sodium carbonate; preferably sodium acetate, wherein the molar amount of sodium acetate is 4-6 times the molar amount of ferric ions in the ferric salt.

[0014] According to a preferred embodiment of the present invention, after adding alkaline salt, the reaction is continued with ultrasonication and vigorous stirring for 20 min, and then a solvothermal reaction is carried out in a sealed reaction vessel at 200°C for 10 h. After the reaction is completed, the superparamagnetic Fe3O4 nanoclusters are obtained by ultrasonic washing with ethanol and deionized water and magnetic separation 2-4 times.

[0015] Secondly, the present invention provides a pH-responsive photonic crystal thin film, which is a thin film formed by combining superparamagnetic Fe3O4 nanoclusters prepared in any of the above embodiments with a pH-sensitive hydrogel.

[0016] Thirdly, the present invention provides a method for preparing a pH-responsive photonic crystal thin film, comprising:

[0017] S1. The superparamagnetic Fe3O4 nanoclusters prepared in any of the above embodiments were modified by coating with polyacrylic acid to obtain Fe3O4@PAA nanoclusters with PAA coating on the surface.

[0018] S2. Add Fe3O4@PAA nanoclusters to a mixed solution containing AA acrylate, HEMA hydroxyethyl methacrylate, and EGDMA ethylene glycol dimethacrylate, stir or sonicate to mix evenly, add polymerization initiator and mix well to obtain prepolymer solution;

[0019] S3. Fill the prepolymer liquid into the pre-prepared film mold cavity, apply a stable magnetic field to one side outside the film mold cavity, and perform thermal curing treatment on the prepolymer liquid. After curing, remove the film, clean it, and obtain the pH-responsive photonic crystal film.

[0020] According to a preferred embodiment of the present invention, in S1, superparamagnetic Fe3O4 nanoclusters are first dispersed in anhydrous ethanol, polyacrylic acid (PAA) is added, and after ultrasonic vibration or stirring, magnetic separation is performed to obtain Fe3O4@PAA nanoclusters; wherein, polyacrylic acid M W =1000-3000; the amount of polyacrylic acid used is 0.4-0.8 times the mass of Fe3O4 nanoclusters.

[0021] Fe3O4@PAA nanoclusters were resuspended in anhydrous ethanol to prepare Fe3O4@PAA nanoclusters / ethanol solution, which can exhibit different structural colors under different magnetic field strengths.

[0022] According to a preferred embodiment of the present invention, in S2, the solvent of the mixed solution is dimethyl sulfoxide. In the mixed solution, AA acrylic acid, HEMA hydroxyethyl methacrylate, and EGDMA ethylene glycol dimethacrylate are mixed in a volume ratio of 16-24:7-10:1 to obtain a reactive monomer composition. Then, the reactive monomer composition is dispersed in dimethyl sulfoxide to obtain the mixed solution. Each 100g of Fe3O4@PAA nanoclusters is mixed with 0.4-0.6L of the reactive monomer composition.

[0023] According to a preferred embodiment of the present invention, in S3, the magnetic field is provided by a magnet, the thickness of the film cavity is 1 mm ± 0.2 mm, the thermal curing treatment is performed at 55-70°C for 8-12 minutes; after curing, the film is removed and rinsed with ethanol and deionized water to obtain the pH-responsive photonic crystal film. Preferably, the pH-responsive photonic crystal film is stored in PBS buffer for later use.

[0024] (III) Beneficial Effects

[0025] (1) The Fe3O4 nanoclusters prepared in this invention are synthesized in one step using a solvothermal method, which has the advantages of simple preparation method and high yield. The morphology of the Fe3O4 nanoclusters was characterized by scanning electron microscopy (SEM), showing that the Fe3O4 nanoclusters are spherical structures with regular structure and uniform size. The size of the nanospheres is 140nm-160nm, and the average diameter is about 150nm.

[0026] Under varying external magnetic fields of different intensities, Fe3O4 nanocluster photonic crystals exhibit different structural colors, and their response speed to magnetic field strength is fast, the colors are bright, and the transitions are obvious.

[0027] (2) In the process of preparing pH-responsive photonic crystal thin films, this invention uses low molecular weight PAA (M W The coating of Fe3O4 nanoclusters with a molecular weight of 1000-3000 (preferably 2000) can prevent further aggregation of the nanoclusters and improve their dispersibility in organic solvents. Low molecular weight PAA readily undergoes rapid cross-linking reactions with other acrylic monomers. Therefore, Fe3O4@PAA nanoclusters are added to a mixed solution containing AA, HEMA, and EGDMA to stably graft the Fe3O4 nanoclusters onto the acrylic-acrylate polymer film, thus obtaining a pH-responsive photonic crystal film.

[0028] (3) In the preparation of superparamagnetic Fe3O4 nanoclusters, the present invention uses PSSMA to complex iron cations, thereby controlling the particle size of the nanocrystals and thus controlling the size of the Fe3O4 nanoclusters. The control conditions are simple and the product morphology is reproducible. PSSMA is an anionic polyelectrolyte containing sulfonate and carboxylic acid anionic groups. These charged groups can coordinate with iron cations in the solution to form Fe3O4 nanoclusters with controllable particle size and self-assembled structure.

[0029] (4) The pH-responsive photonic crystal thin film prepared by the present invention affects the volume change of the photonic crystal thin film under different pH environments, which leads to obvious structural color changes. Moreover, the pH-responsive photonic crystal thin film material is sensitive to pH value, has a wide response range of 3.6-6.2, fast response speed, bright color, and obvious transformation.

[0030] In summary, the preparation process of this invention is simple, requiring no complex equipment, and is a simple, environmentally friendly, and easily scalable method for preparing pH-responsive photonic crystal thin films. Attached Figure Description

[0031] Figure 1The images shown are SEM images of the Fe3O4 nanoclusters prepared in this invention. Image a is a SEM image at 50,000x magnification, and image b is a SEM image at 200,000x magnification.

[0032] Figure 2 Optical color photographs of the Fe3O4@PAA nanoclusters / ethanol dispersion prepared in this invention under different magnetic field strengths; where a→g: magnetic field strength gradually increases; h represents no external magnetic field.

[0033] Figure 3 Photographs showing the optical colors of the pH-responsive photonic crystal thin film prepared for this invention in PBS buffer solutions of different pH values.

[0034] Figure 4 The reflectance spectra of the pH-responsive photonic crystal thin film prepared in this invention at different pH values ​​(pH 3.6 to 6.2). Detailed Implementation

[0035] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0036] This invention provides a method for preparing superparamagnetic Fe3O4 nanoclusters, which features good product stability, high reproducibility, and high yield. The preparation method is as follows:

[0037] Under ultrasonic vibration and stirring conditions, sodium poly(4-styrenesulfonic acid-copolymer-maleic acid) salt PSSMA powder was dissolved in a polyol solvent. After the PSSMA was completely dissolved, soluble ferric salt was added, and ultrasonic vibration and vigorous stirring were continued. After the solution turned brownish-red, an alkaline salt was added to provide an alkaline environment, and the reaction was continued with ultrasonic vibration and vigorous stirring for 10-30 min. The reaction solution was then transferred to a sealed reaction vessel and subjected to a solvothermal reaction at 180-220℃ for 6-14 h. After the reaction was completed, the reaction solution was cooled to room temperature and ultrasonically washed and magnetically separated with ethanol and deionized water, respectively, to obtain superparamagnetic Fe3O4 nanoclusters.

[0038] The conditions for ultrasonic oscillation and stirring are: ultrasonic frequency ≥ 40KHz, stirring speed 600-1000rpm, preferably 800rpm.

[0039] Sodium poly(4-styrenesulfonic acid-copolymer-maleic acid) salt A is an anionic polyelectrolyte containing sulfonate and carboxylic acid anionic groups. These charged groups can coordinate with iron cations in the solution, thereby forming Fe3O4 nanoclusters with controllable particle size and self-assembled structure. It mainly functions to complex iron cations and control the nanocrystal size. The average molecular weight (Mw) of PSSMA is between 15,000 and 25,000, preferably around 20,000. If the molecular weight of PSSMA is too large, the reaction solution will have too high a viscosity, requiring too much ultrasonication or stirring, making it difficult to control the nanocrystal size; if the molecular weight is too small, the complexation and coordination effect on iron ions is poor, and it is also difficult to obtain nano-sized clusters. Therefore, the molecular weight of PSSMA should not be too large or too small. Furthermore, PSSMA cannot be replaced with common polymers such as PVA and PEG. In PSSMA, 4-styrenesulfonic acid and maleic acid are copolymerized in a molar ratio of 3:1 to 1:1.

[0040] The solvent used is a polyol that is liquid at room temperature, such as at least one of ethylene glycol, triethylene glycol, and tetraethylene triethylene glycol. Ethylene glycol serves as both a solvent and a reducing agent in the reaction system, partially reducing ferric ions to ferrous ions to obtain magnetic Fe3O4. To ensure complete dissolution of PSSMA, the mass-to-volume ratio of PSSMA to ethylene glycol is 3.75 g: 40-80 mL, meaning 3.75 g of PSSMA can dissolve in 40-80 mL of ethylene glycol.

[0041] The ferric salt is at least one of ferric chloride or its hydrate, ferric nitrate or its hydrate, and ferric sulfate or its hydrate. To obtain magnetic Fe3O4 nanoclusters of the desired particle size, the amount of ferric salt added must be controlled. Preferably, the mass of PSSMA is 7-15 times the mass of iron, and more preferably 9-10 times.

[0042] The alkaline salt is at least one of sodium citrate, sodium acetate, and sodium carbonate; preferably sodium acetate, with a molar amount of sodium acetate being 4-6 times the molar amount of ferric ions in the ferric salt. Sodium acetate provides an alkaline environment, which is conducive to combining with ferric ions to form Fe(OH)3, which is then reduced by ethylene glycol to form magnetic Fe3O4.

[0043] To prepare a pH-responsive photonic crystal thin film, this invention further combines the superparamagnetic Fe3O4 nanoclusters prepared above with a pH-sensitive hydrogel, so that the product has both magnetic field responsiveness and pH responsiveness, so as to be better applied to fields such as biochemical sensing.

[0044] This invention also provides a method for preparing a pH-responsive photonic crystal thin film, comprising: coating and modifying superparamagnetic Fe3O4 nanoclusters with low molecular weight polyacrylic acid (PAA) to obtain Fe3O4@PAA nanoclusters with PAA coating on the surface; adding the Fe3O4@PAA nanoclusters to a mixed solution (DMSO) containing AA acrylic acid, HEMA methacrylate, and EGDMA; with the addition of an initiator, the PAA coating on the nanoclusters crosslinks with AA, HEMA, EGDMA, etc., to form a prepolymer solution; filling the prepolymer solution into a pre-prepared film mold cavity; applying a stable magnetic field to one side outside the film mold cavity; thermally curing the prepolymer solution; after curing, removing the film, cleaning, and obtaining a pH-responsive photonic crystal thin film.

[0045] Among them, the M of low molecular weight polyacrylic acid PAA W =1000-3000, preferably 2000. Low molecular weight PAA has good solubility and dispersibility, low viscosity after dissolution, and is easy to coat the surface of Fe3O4 nanoclusters to modify the nanoclusters and improve their dispersibility in organic solvents. More importantly, low molecular weight PAA is easy to polymerize with acrylic acid or acrylate monomers such as AA, HEMA, and EGDMA, thereby uniformly and stably fixing the Fe3O4 nanoclusters coated by PAA to various positions of the film material. PAA acts as a stabilizer for Fe3O4 nanoclusters during the film preparation process, and the coating is conducive to the stable existence of nanoclusters and prevents agglomeration. Preferably, the amount of polyacrylic acid PAA is 0.4-0.8 times the mass of Fe3O4 nanoclusters, more preferably 0.5 times.

[0046] In a mixed solution, AA acrylic acid, HEMA hydroxyethyl methacrylate, and EGDMA ethylene glycol dimethacrylate are mixed in a volume ratio of 16-24:7-10:1 to obtain a reactive monomer composition. The reactive monomer composition is then dispersed in dimethyl sulfoxide to prepare the mixed solution. Each 100g of Fe3O4@PAA nanoclusters is mixed with 0.4-0.6L of the reactive monomer composition.

[0047] In preparing pH-responsive photonic crystal films, this invention uses various acrylic acids or acrylates with different functional groups as polymerizing monomers. Under the action of an initiator and curing heating, these monomers polymerize with PAA on the surface of Fe3O4@PAA nanoclusters to form a pH-responsive hydrogel film. This hydrogel film has a high water swelling ratio and contains various functional groups with different characteristics. These functional groups have unique sensitivities to different pH environments, thus producing a pH-sensitive hydrogel film that can capture and release protons in response to changes in external pH, thereby altering the spatial and phase structures of the hydrogel and exhibiting different structural colors under different pH environments. Compared with other responsive gels, the pH-responsive gel exhibits the characteristics of being fast, reversible, and biocompatible.

[0048] A stable magnetic field, which can be provided by a magnet, is applied to one side outside the film cavity, followed by thermosetting. The magnetic field applied by the magnet causes the magnetic Fe3O4@PAA nanoclusters to align regularly along the magnetic field lines. After a thermopolymerization and curing reaction, the ordered structure is fixed in the polymer, resulting in a colored photonic crystal. In actual production, an electromagnetic field can be used instead of a magnet. The prepared pH-responsive photonic crystal film can be stored in PBS buffer for later use.

[0049] To further clarify the technical solution of the present invention, the following description is provided in conjunction with specific embodiments.

[0050] Example 1

[0051] Under ultrasonic oscillation at 40 kHz and stirring at 800 rpm, 3.75 g of sodium poly(4-styrenesulfonic acid-copoly-maleic acid) (PSSMA) powder was dissolved in 60 mL of ethylene glycol. The average Mw of PSSMA was 20,000, and the molar ratio of 4-styrenesulfonic acid to maleic acid was 3:1. After PSSMA completely dissolved to form a clear solution within 20 minutes, 1.89 g of ferric chloride hexahydrate (FeCl3·6H2O) was added. The mixture was then sonicated and vigorously stirred for another 20 minutes until a reddish-brown solution formed. Next, 4.5 g of sodium acetate trihydrate (NaAc·3H2O) was added, followed by continued sonication and vigorous stirring until completely dissolved after another 20 minutes. The solution was cooled to room temperature, transferred to a reaction vessel, and sealed tightly. The vessel was then placed in an electrically heated drying oven and heated to 200°C for a solvothermal reaction for 10 hours. After the reaction was complete, the reaction vessel liner was removed and cooled to room temperature. The internal solution was then ultrasonically washed and magnetically separated three times with ethanol and deionized water, respectively, to obtain Fe3O4 nanoclusters. The product showed a high iron recovery rate of 91.7%.

[0052] The morphology of Fe3O4 nanoclusters was characterized using scanning electron microscopy (SEM). Figure 1The images shown are SEM images of Fe3O4 nanoclusters. Image a is a SEM image magnified 5000 times, and image b is a SEM image magnified 20,000 times. Image a clearly shows several Fe3O4 nanospheres with regular shapes and uniform sizes. Image b shows that these nanospheres are very uniform in size and have a regular spherical shape. The nanospheres have a size of 140nm-160nm and an average diameter of about 150nm. Each nanosphere is composed of several Fe3O4 particles arranged in a cluster to form a regular spherical particle.

[0053] Example 2

[0054] In this embodiment, a pH-responsive photonic crystal thin film was prepared using Fe3O4 nanoclusters prepared in Example 1. The preparation process is as follows:

[0055] (1) Using PAA to coat Fe3O4 nanoclusters

[0056] 50 mg of Fe3O4 nanoclusters were placed in 100 mL of anhydrous ethanol, and 25 mg of polyacrylic acid (PAA, MW = 2000) was added. The mixture was ultrasonically vibrated for 30 min and magnetically separated for 1 h to obtain Fe3O4@PAA nanoclusters.

[0057] The prepared Fe3O4@PAA nanoclusters were added to anhydrous ethanol to obtain Fe3O4@PAA nanoclusters / ethanol solution, and the response to different magnetic fields was tested.

[0058] like Figure 2 As shown, Fe3O4@PAA nanoclusters / ethanol solution were placed in magnetic fields with continuously increasing magnetic field strengths (a→g), and the changes in the optical color of the solution were observed. Figure a shows the optical color of the solution with a magnetic field of 30 mT; Figure b shows the optical color of the solution with a magnetic field of 40 mT; Figure c shows the optical color of the solution with a magnetic field of 50 mT; Figure d shows the optical color of the solution with a magnetic field of 60 mT; Figure e shows the optical color of the solution with a magnetic field of 70 mT; Figure f shows the optical color of the solution with a magnetic field of 80 mT; Figure g shows the optical color of the solution with a magnetic field of 90 mT; and Figure h shows the optical color of the solution without an external magnetic field. From figures a to g, it can be seen that as the magnetic field strength gradually increases, the displayed color gradually transitions from red, orange, yellow, green, cyan, blue, and purple, with bright colors and obvious changes. The color contrast is large at different magnetic field strengths. Figure h shows the inherent color of the Fe3O4 nanoclusters without an external magnetic field, which is brown; clearly distinguishable from the seven colors with applied magnetic fields.

[0059] Therefore, the color change of the Fe3O4 nanoclusters provided by this invention can be directly observed with the naked eye, solving the problems of current photonic crystals such as dull color, insensitive reaction, and long response time.

[0060] (2) Preparation of pH-responsive photonic crystal thin films

[0061] 100 mg of Fe3O4@PAA nanoclusters were added to a mixed solution (solvent: DMSO) containing 0.274 mL of AA acrylic acid, 0.121 mL of HEMA hydroxyethyl methacrylate, and 0.014 mL of EGDMA ethylene glycol dimethacrylate. The mixture was then sonicated until homogeneous. Separately, 0.0228 g of ammonium persulfate (APS) was added to 0.57 mL of dimethyl sulfoxide (DMSO). The APS was dissolved completely by sonication. This solution was then added to the previous mixed solution and sonicated until homogeneous.

[0062] Fabrication of the thin film mold cavity: On a large glass substrate, four 1mm thick, 2cm x 2cm square glass sheets are used as supports. A glass sheet of the same size is placed on top, creating a 1mm high space between the top and bottom glass sheets to fill with prepolymer solution. After adding the prepolymer solution, the entire glass substrate is placed in an electrically heated drying oven. A magnet is placed directly below the glass substrate, and its height and position are adjusted to provide an appropriate magnetic field strength to the prepolymer solution. The oven temperature is set to 60℃, and curing is performed for 10 minutes. The film is then removed and rinsed with ethanol and deionized water to obtain a pH-responsive photonic crystal film. The magnet causes the Fe3O4 nanoclusters in the prepolymer solution to align regularly along the magnetic field lines. After the curing reaction, the ordered structure is fixed in the polymer, resulting in a colored photonic crystal film.

[0063] The photonic crystal film was stored in pH 7.2 PBS buffer for later use. The pH of the PBS buffer was adjusted to achieve the desired buffer concentration. Figure 3 The different pH values ​​shown, such as pH 6.2, 5.6, 5.3, 5.1, 4.6, 4.2, and 3.6, were reacted for 10-15 seconds, and the color change of the photonic crystal film in the buffer solution was observed. Figure 3 As shown, the prepared photonic crystal film exhibits clear structural colors under different pH buffer solutions: red, orange, yellow, yellow-green, light green, dark green, and dark green. The colors are vibrant, with large color contrasts and obvious transitions at different pH levels, which can be directly observed with the naked eye. It is bound to have broad application prospects in fields such as biochemical sensing, and solves the problems of current pH-responsive photonic crystals, such as dull colors, insufficient pH response, and long response time.

[0064] The prepared samples were further placed in buffer solutions with different pH values ​​and ionic strengths of 0.2 M to investigate their pH response and optical properties. Figure 4The image shows the reflectance spectra of the pH-responsive photonic crystal film at different pH values ​​(pH 3.6 to 6.2). The film is red at pH 6.2 with a peak at 640 nm; orange at pH 5.6 with a peak at 600 nm; yellow at pH 5.3 with a peak at 580 nm; green at pH 5.1 with a peak at 570 nm; green at pH 4.6 with a peak at 540 nm; and dark green at pH 3.6 with a peak at 530 nm. As the pH increases, the reflectance peaks of the film redshift. The experiment clearly demonstrates that the pH-responsive photonic crystal film prepared by the method of this invention undergoes volume expansion or contraction when the pH environment is changed, resulting in changes in its internal periodic structure and structural color. It can respond rapidly between pH 3.6 and 6.2, producing different structural colors and exhibiting a wide pH responsiveness. The present invention has the advantages of adjustable product structure, stable performance, high reproducibility, and easy storage and use.

[0065] Comparative Example 1

[0066] In the comparative example, when preparing Fe3O4 nanoclusters, PSSMA in Example 1 was replaced with an equal amount of PEG (polyethylene glycol) with a molecular weight of 20,000. Finally, the experiment could not obtain the desired result. Figure 1 The Fe3O4 nanospheres shown are of regular shape and uniform size.

[0067] Comparative Example 2

[0068] This comparative example describes the preparation of a pH-responsive photonic crystal film. Instead of using PAA to coat Fe3O4@PAA, the crystal film was prepared directly following step (2) of Example 2 using 100 mg of Fe3O4 nanoclusters. Analysis revealed that the Fe3O4 nanoclusters in the prepared photonic crystal film were extremely unevenly distributed, with more concentrated at the film edges. This is mainly due to the poor dispersibility of the Fe3O4 nanoclusters in the organic solvent, causing them to settle at the bottom of the prepolymer solution.

[0069] Comparative Example 3

[0070] This comparative example describes the preparation of a pH-responsive photonic crystal film. The process is the same as in Example 2, except that when coating Fe3O4@PAA with PAA, a PAA with a MW of 6000 was used. The remaining steps are identical to Example 2. Testing revealed poor uniformity of Fe3O4 nanoclusters in the prepared photonic crystal film, with some clusters still agglomerated. This is mainly because the large molecular weight of PAA results in high solution viscosity when coating Fe3O4 nanoclusters, making it difficult to disperse the Fe3O4 nanoclusters uniformly, thus affecting the overall uniformity of Fe3O4 dispersion in the photonic crystal film.

[0071] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a pH-responsive photonic crystal thin film, characterized in that, It includes: S1. Superparamagnetic Fe3O4 nanoclusters were modified by coating them with polyacrylic acid to obtain Fe3O4@PAA nanoclusters coated with PAA. The specific method was as follows: superparamagnetic Fe3O4 nanoclusters were first dispersed in anhydrous ethanol, then polyacrylic acid (PAA) was added. After ultrasonic vibration or stirring, magnetic separation was performed to obtain Fe3O4@PAA nanoclusters. Among them, polyacrylic acid M... W =1000-3000; the amount of polyacrylic acid used is 0.4-0.8 times the mass of Fe3O4 nanoclusters; The preparation method of superparamagnetic Fe3O4 nanoclusters is as follows: Under the conditions of ultrasonic frequency ≥40KHz and stirring speed of 600-1000rpm, sodium poly(4-styrenesulfonic acid-copolymer-maleic acid) powder is dissolved in a polyol solvent. After the PSSMA is completely dissolved, a soluble ferric salt is added, and ultrasonic vibration and vigorous stirring are continued. After observing that the solution turns brownish-red, an alkaline salt is added to provide an alkaline environment, and the reaction is continued with ultrasonic vibration and vigorous stirring for 10-30min. The reaction solution was transferred to a sealed reaction vessel and subjected to a solvothermal reaction at 180-220℃ for 6-14 hours. After the reaction was completed, the reaction solution was cooled to room temperature and ultrasonically washed and magnetically separated with ethanol and deionized water, respectively, to obtain superparamagnetic Fe3O4 nanoclusters with a size of 140nm-160nm. The average molecular weight (Mw) of PSSMA was 15000-25000, and the mass of PSSMA was 7-15 times that of iron in ferric salts. S2. Add Fe3O4@PAA nanoclusters to a mixed solution containing AA acrylate, HEMA hydroxyethyl methacrylate, and EGDMA ethylene glycol dimethacrylate, stir or sonicate to mix evenly, add polymerization initiator and mix well to obtain prepolymer solution; S3. Fill the prepolymer liquid into the pre-prepared film mold cavity, apply a stable magnetic field to one side outside the film mold cavity, and perform thermal curing treatment on the prepolymer liquid. After curing, remove the film, clean it, and obtain the pH-responsive photonic crystal film.

2. The preparation method according to claim 1, characterized in that, In S1, the molar ratio of 4-styrenesulfonic acid to maleic acid in the PSSMA is 3:1 to 1:

1.

3. The preparation method according to claim 1, characterized in that, In S1, the polyol solvent is one or more of ethylene glycol, triethylene glycol and tetraethylene glycol; the mass-volume ratio of PSSMA to the polyol solvent is 3.75g:40-80mL.

4. The preparation method according to claim 1, characterized in that, In S1, the trivalent iron salt is one or a combination of two or more of the following: ferric chloride or its hydrate, ferric nitrate or its hydrate, and ferric sulfate or its hydrate.

5. The preparation method according to claim 1, characterized in that, In S1, the alkaline salt is at least one of sodium citrate, sodium acetate, and sodium carbonate.

6. The method for preparing a pH-responsive photonic crystal thin film according to claim 1, characterized in that, In S2, the solvent of the mixed solution is dimethyl sulfoxide. In the mixed solution, AA acrylic acid, HEMA hydroxyethyl methacrylate, and EGDMA ethylene glycol dimethacrylate are mixed in a volume ratio of 16-24:7-10:1 to obtain a reactive monomer composition. Then, the reactive monomer composition is dispersed in dimethyl sulfoxide to obtain the mixed solution. Each 100g of Fe3O4@PAA nanoclusters is combined with 0.4-0.6L of the reactive monomer composition for a combined reaction.

7. A pH-responsive photonic crystal thin film, which is prepared by the preparation method according to any one of claims 1-6.

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