Structural color double-response opal gel as well as preparation method and application thereof

Abstract

The invention discloses structural color double-response opal gel as well as a preparation method and application thereof, and belongs to the technical field of food freshness indication packaging materials. The method specifically comprises the following steps: step 1, carrying out self-assembly on monodisperse nano-microspheres on the surface of a hydrophilic glass slide through a vertical deposition method to obtain an opal template; 2, adding the component A into ultrapure water, stirring, dispersing and activating, sequentially adding the component B and a thermal initiator, uniformly stirring to obtain a polymer monomer solution, adding a cross-linking agent or a catalyst into the polymer monomer solution, and uniformly mixing to obtain a gel pre-polymerized solution; and step 3, adding the gel pre-polymerization liquid into an opal template, and initiating a polymerization reaction. The obtained opal gel is taken down from the hydrophilic glass slide, cleaned and soaked, and the structural color double-response opal gel is obtained. The structural color double-response opal gel is applied to food freshness indication packaging, food freshness indication is achieved, the response sensitivity is high, and the indication effect is good.

Description

Technical Field

[0001] This invention belongs to the technical field of food freshness indicator packaging materials, specifically relating to a structural color dual-response opal gel, its preparation method, and its application. Background Technology

[0002] Responsive opal gel materials are structural color gel materials generated by combining opal templates with stimulus-responsive materials.

[0003] Currently, most responsive opal gel materials have a single response mode and the technology is relatively mature. Existing responsive opal gel materials can be classified according to their response characteristics into: temperature-responsive, humidity-responsive, gas-responsive, solvent-responsive, mechanical force-responsive, and electrical-responsive types, etc. Their technical content is low, and their response sensitivity is low and the effect is poor in the application of food freshness indication, which greatly limits their application and development in the function of food freshness indication.

[0004] Therefore, in the field of food freshness indication packaging, the preparation of freshness indication labels with multi-responsiveness and high sensitivity is one of the future trends. It can effectively improve the sensitivity of food freshness indication, greatly increase the indication effect, and break through the bottlenecks of instability and low sensitivity of traditional chemical colors. It opens up a new way for food freshness indication packaging, reduces food safety risks and lowers costs. Summary of the Invention

[0005] The primary objective of this invention is to provide a method for preparing a structural color dual-responsive opal gel. This method involves preparing a gel prepolymer using amino-rich component A and component B as monomers, filling the gaps between opal template microspheres, and then performing thermally initiated polymerization to prepare a structural color opal gel with dual responses.

[0006] The second objective of this invention is to provide a structural color dual-response opal gel prepared by the above method, which has dual-response properties of aldehyde response and pH response.

[0007] The final objective of this invention is to provide an application of the above-mentioned structural color dual-response opal gel in food freshness indication packaging, achieving food freshness indication with high response sensitivity and good indication effect.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for preparing structural color dual-response opal gel includes the following steps:

[0010] Step 1: Disperse monodisperse nanospheres in anhydrous ethanol or ultrapure water to obtain a nanosphere dispersion emulsion. Vertically insert the treated hydrophilic glass slide into the nanosphere dispersion emulsion and keep it at a constant temperature under vacuum to evaporate the anhydrous ethanol or ultrapure water. Through vertical deposition, the monodisperse nanospheres self-assemble on the surface of the hydrophilic glass slide to obtain an artificial opal template.

[0011] Step 2: Add component A to ultrapure water and stir in a water bath at 40°C for 2 hours to activate it. Once it is completely dissolved in the ultrapure water, add component B and the thermal initiator in sequence, stir evenly, and sonicate to remove bubbles to obtain a polymer monomer solution. Component A includes carboxymethyl chitosan, polylysine, polyethyleneimine, aminated gelatin or aminated PVA, and component B includes acrylamide, N-isopropylacrylamide (NIPAM) or 2-acrylamide-2-methylpropanesulfonic acid (AMPS). Add the crosslinking agent or catalyst to the polymer monomer solution and mix well to obtain a gel prepolymer solution.

[0012] Step 3: Add the gel prepolymer solution from Step 2 into the opal template from Step 1. The gel prepolymer solution will slowly fill the gaps between the microspheres inside the opal template and stand at a certain temperature for a certain period of time to initiate a cross-linking polymerization reaction.

[0013] Step 4: After the cross-linking polymerization reaction in Step 3 is completed, the obtained opal gel is removed from the hydrophilic glass slide, and then it is washed and soaked to remove unreacted monomers and byproducts, thus obtaining the structural color dual-response opal gel.

[0014] The prepared structural color dual-response opal gel exhibits structural color response characteristics to different concentrations of aldehyde gas or solution and different pH values.

[0015] The monodisperse nanospheres in this invention were not removed and remained embedded in the opal gel.

[0016] Furthermore, in step one, the monodisperse nanospheres include silica monodisperse nanospheres, polystyrene microspheres, or polymethyl methacrylate microspheres. The particle size of the monodisperse nanospheres is 50 nm to 100 μm; the mass percentage of monodisperse nanospheres in the nanosphere dispersion emulsion is 0.1% to 10 wt%; the constant temperature is 25 to 80 °C, and the vacuum degree under vacuum conditions is 40 to 100 kPa.

[0017] Preferably, the monodisperse nanospheres in step one are silica monodisperse nanospheres with a particle size of 190~320nm. The mass percentage of monodisperse nanospheres in the nanosphere dispersion emulsion is 0.8~1wt%. The isothermal temperature is 50~60℃, and the vacuum degree is 60~100Kpa.

[0018] Further, in step two, after component A is dissolved in ultrapure water, glycerol is added and the mixture is ultrasonically mixed; the mass ratio of component A (when carboxymethyl chitosan is selected, the degree of carboxymethylation is 80%~95%), component B, and liquid component is 0.1~0.3:2~4:9.7~9.9; the liquid component includes ultrapure water and glycerol; the amount of glycerol added is 0%~50% of the mass fraction of the liquid component; the amount of thermal initiator added is 0.1%~1% of the total mass of the polymer monomer solution. The amount of crosslinking agent added accounts for 2.0%~2.5% of the total mass of component A and component B, and the amount of catalyst added accounts for 0.05~0.2% of the volume of the polymer monomer solution.

[0019] Preferably, the amount of thermal initiator added in step two is 0.1% to 0.5% of the total mass of the polymer monomer solution.

[0020] In step two of this invention, components A and B are used. Components A and B are the key components that cause the opal hydrogel to swell to different degrees in different concentrations of aldehyde gas or solution, thereby displaying different colors.

[0021] Furthermore, in step two, the thermal initiator includes potassium persulfate, ammonium persulfate, sodium persulfate, or sodium bisulfite; the crosslinking agent includes N-N'-methylenebisacrylamide, N,N'-bis(acryloyl)cysteine, polyethylene glycol diacrylate, bis(3-aminopropyl)dimethylsiloxane, or genipin; and the catalyst includes tetramethylethylenediamine, N,N,N',N'-tetramethyl-1,3-propanediamine, triethanolamine, or N,N-dimethylethanolamine.

[0022] Preferably, the thermal initiator is potassium persulfate or ammonium persulfate. The crosslinking agent is N-N'-methylenebisacrylamide or N,N'-bis(acryloyl)cysteine. The catalyst is tetramethylethylenediamine or N,N,N',N'-tetramethyl-1,3-propanediamine.

[0023] Furthermore, in step three, the polymerization reaction is carried out at a temperature of 25~80℃ for 1~24h.

[0024] Preferably, in step three, the mixture is placed in an oven at 50~70℃ to initiate a thermal polymerization reaction.

[0025] Furthermore, the thickness of the structural color dual-response opal gel prepared in step four is 0.1~3 mm, preferably 1~2 mm.

[0026] A structural color dual-responsive opal gel was prepared using the method described above.

[0027] The application of the structural color dual-response opal gel prepared by the above method in indicating food freshness. It is used to indicate the freshness of foods that release aldehyde gases; preferably, the opal gel is used to indicate the freshness of kiwifruit.

[0028] The principle of this invention is as follows:

[0029] (1) Before thermal initiation crosslinking, the gel prepolymer is a highly fluid liquid that can fill the gaps in the opal template microspheres. After thermal initiation crosslinking, the opal template is completely embedded in the gel, thus forming an opal gel with structural color.

[0030] (2) Since the swelling degree of opal gel varies in solutions with different pH values, the colors of the structural color dual-response opal gel pattern and the background also vary in solutions with different pH values.

[0031] (3) In aldehyde gas or aldehyde solution, the amino groups in the structural color dual-response opal gel react with the aldehyde groups in the aldehyde gas or solution to generate imine bonds, which leads to further cross-linking and shrinkage of the opal gel. The microsphere periodic structure inside changes accordingly, causing the overall color of the opal gel to change.

[0032] (4) According to Bragg's law of diffraction, the wavelength of the diffracted light from opal is related to the incident angle and the outgoing angle. Therefore, by changing the viewing angle, the structural color dual-response opal gel sample will also show different colors.

[0033] Therefore, to overcome the instability of existing chemical pigments and the shortcomings and deficiencies of single-response opal gel technology, this invention provides a structural color dual-response opal gel. The dual responsiveness of the structural color opal gel can significantly enhance the information acquisition dimensions. Through a multi-signal synergistic verification mechanism, the reliability of the response signal can be enhanced, and the risk of misjudgment caused by environmental interference can be reduced, thereby improving the indication effect of food freshness. The method for preparing the structural color dual-response opal gel provided by this invention yields a structural color dual-response opal gel with both aldehyde and pH responses.

[0034] The structural color dual-responsive opal gel of this invention has the following characteristics:

[0035] ① Aldehyde responsiveness: Whether in aldehyde solution or aldehyde gas, the amino group in component A will undergo a cross-linking reaction with the aldehyde. As the aldehyde concentration varies, the degree of cross-linking of the opal gel will vary, and the structural color opal gel will exhibit different structural colors, thus indicating the freshness of food.

[0036] ②pH responsiveness: Because the opal gel swells to different degrees at different pH values, the microspheres inside the gel have different periods after swelling, resulting in different wavelengths of diffracted light and thus different colors.

[0037] ③ High water retention: Because the hydroxyl groups of glycerol can form multiple hydrogen bonds with the carboxyl groups in component A, the amino groups in component B, and water molecules, free water is converted into bound water, which improves the water retention performance of opal gel. The water retention rate of opal gel remains above 90% after 30 days.

[0038] ④ Antifreeze properties: Glycerol molecules form strong hydrogen bonds with water molecules, competitively preventing water molecules from arranging themselves in an orderly manner to form ice crystal nuclei, thus maintaining the flexibility and function of opal gel at -20℃.

[0039] Compared with the prior art, the present invention has the following advantages:

[0040] (1) In this invention, monodisperse nanospheres are first self-assembled into an opal template, and then amino-rich components A and B are filled into the gaps between the spheres inside the opal template. Through thermally initiated polymerization, a dual-response opal gel with structural color is prepared.

[0041] (2) The present invention has pH responsiveness because the opal gel swells to different degrees in solutions with different pH values. That is, it exhibits different structural colors in solutions with different pH values. In addition, the degree of crosslinking of the opal gel changes in an aldehyde atmosphere, and the opal lattice constant changes, which also results in a color change effect.

[0042] (3) Due to the addition of glycerol, the present invention has high water retention and antifreeze properties, so that it still has a water retention rate of more than 90% after 30 days, and will not form ice crystals when frozen at -20℃ for 12 hours, thus maintaining the flexibility and structural color response function of the gel.

[0043] (4) Therefore, the structural color dual-response opal gel of the present invention has dual effects of pH response and aldehyde response. The change of structural color intuitively indicates freshness (e.g., red → fresh, blue → spoilage), without the need for instrument reading. Due to the high water retention and freeze resistance of opal gel, the structural color opal gel has a long lifespan, can be applied to a wide temperature range, supports non-contact detection, and the gel can be directly attached to the inner wall of the packaging without migrating harmful substances, thus avoiding food contamination. Attached Figure Description

[0044] Figure 1 Optical photographs and reflectance spectra of the silica opal template and structural color opal gel assembled in Example 1;

[0045] Figure 2 The water retention rate of opal gels with different glycerol ratios is shown in the graph.

[0046] Figure 3 Figures showing the antifreeze properties of opal gels with different glycerol ratios;

[0047] Figure 4 The response spectrum and optical photograph of the structural color dual-response opal gel prepared in Example 1 to trans-2-hexenal;

[0048] Figure 5 The images show the response spectra and optical photographs of the structural color dual-response opal gel prepared in Example 1 to different pH values. Detailed Implementation

[0049] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0050] Unless otherwise specified, the raw materials used in the following examples are commercially available products or are prepared according to conventional methods or references.

[0051] Example 1:

[0052] The method for preparing structural color dual-responsive opal gel provided in this embodiment includes the following steps:

[0053] (I) A method for preparing a silica opal template, comprising the following steps:

[0054] (S1) Opal templates were obtained by self-assembly using the vertical deposition method: commercially available glass slides were cleaned and then immersed in concentrated sulfuric acid / hydrogen peroxide = 7:3 V / V for 24 hours. They were then subjected to hydrophilic treatment by sonication with water and anhydrous ethanol for half an hour each.

[0055] Monodisperse silica nanosphere powder (microsphere particle size of 201 nm) was diluted with anhydrous ethanol to a mass fraction of 1 wt% to obtain a nanosphere dispersion emulsion, which was then poured into a beaker.

[0056] Subsequently, the hydrophilicized glass slide was vertically inserted into the nanosphere dispersion emulsion and fixed. It was then placed in a vacuum drying oven at a temperature of 50°C and a vacuum of 100 kPa for 5 days. After the anhydrous ethanol evaporated, the monodisperse nanospheres self-assembled on the surface of the glass slide to obtain an opal template (also known as a polymer photonic crystal).

[0057] (II) A method for preparing structural color dual-response opal gel, comprising the following steps:

[0058] (S2) Place 0.1g of carboxymethyl chitosan in a beaker, add 6.93mL of ultrapure water and stir to disperse, then add 2.97g (30%) of glycerol and sonicate to mix. When it becomes transparent and uniform, add 3g of acrylamide and 0.04g of thermal initiator potassium persulfate in sequence, and stir to obtain a polymer monomer solution.

[0059] Then, 0.1% of the volume of the catalyst tetramethylethylenediamine was added to the polymer monomer solution, and after mixing evenly, a gel prepolymer was obtained.

[0060] Cut a 1-2mm thick silicone pad to the size of the slide assembled with the opal template. Hole out the center to form a rectangular silicone pad with a hollow center. Cut a 5mm slit from one of the two short sides of the rectangular silicone pad to facilitate the injection of the hydrogel prepolymer solution. Fix the cut silicone pad on top of the slide. Then cover the silicone pad with another clean slide so that the silicone pad is between the two slides. Clamp it in place to form a "sandwich" structure.

[0061] Use a syringe to inject the gel prepolymer into the mold through the small opening in the silicone sheet. Let it stand at room temperature for 5 minutes until the gel prepolymer completely fills the gaps between the spheres inside the opal template.

[0062] The opal template filled with gel prepolymer solution was placed in a 50°C oven to initiate a thermal polymerization reaction. The opal template, assembled from monodisperse nanospheres, was embedded within the hydrogel to obtain the opal gel. The opal gel was then peeled off from the hydrophilic glass slide and rinsed with plenty of ultrapure water to remove unreacted monomers and byproducts, ultimately yielding a structural color biresponsive opal gel.

[0063] The thickness of the resulting structural color dual-response opal gel is determined by the thickness of the silicone pad. The final thickness of the structural color dual-response opal gel is 1 mm.

[0064] The optical photographs and spectra of the structural color dual-response opal gel in this embodiment are as follows: Figure 1 As shown, from Figure 1 As can be seen, the opal template is embedded in the hydrogel to form an opal gel, and the spectrum shows a red shift.

[0065] Optical photographs and spectra of the structural color dual-response opal gel prepared in this embodiment at different concentrations of trans-2-hexenal gas are shown below. Figure 4 As shown, from Figure 4 As can be seen, with the increase of trans-2-hexenal gas concentration, the opal gel gradually blue-shifts, changing from light pink to blue, with obvious color change, exhibiting aldehyde gas responsiveness.

[0066] Therefore, it can be seen that the structural color dual-response opal gel in this invention has aldehyde gas responsiveness. Since the carboxymethyl chitosan and acrylamide in the opal gel are rich in amino groups, the amino groups react with the aldehyde groups in trans-2-hexenal to generate imine bonds, and the hydrogel is further cross-linked. The arrangement of microspheres in the gel changes periodically, and the color changes accordingly. It can be used as a label to indicate the freshness of food.

[0067] Optical photographs and spectra of the structural color dual-responsive opal gel prepared in this embodiment in solutions with different pH values ​​are shown below. Figure 5 As shown, the structural color dual-response opal gel red shifts with increasing pH and blue shifts with decreasing pH.

[0068] Therefore, it can be seen that the structural color dual-responsive opal gel in this invention has pH responsiveness. Since the carboxymethyl chitosan in the opal gel has carboxyl groups, the opal gel shrinks and expands and its structural color changes without the need for a pH solution to cause the carboxyl groups to be protonated or deprotonated.

[0069] Example 2

[0070] Unlike Example 1, in step two, the amount of glycerol added is 0%, 10%, 20%, 40%, and 50% of the mass fraction of the liquid component (ultrapure water + glycerol), respectively, and the addition of the remaining components satisfies the mass ratio of carboxymethyl chitosan, acrylamide, and the liquid component as 0.1:3:9.9. The remaining steps are the same as in Example 1.

[0071] The water retention rates of structural color dual-response opal gels prepared with different glycerol ratios are as follows: Figure 2 As shown, from Figure 2 As can be seen, the water retention rate of opal gel gradually increases with the increase of glycerol content.

[0072] Depend on Figure 3 It can be seen that the structural color dual-response opal gel has antifreeze properties. After being frozen at -20℃ for 12 hours, the opal gel without glycerol has frozen, while the opal gel containing 30% glycerol still maintains its original shape without change, and the structural color is stable.

[0073] Example 3

[0074] The method for preparing structural color dual-responsive opal gel provided in this embodiment includes the following steps:

[0075] (I) A method for preparing a polystyrene opal template, comprising the following steps:

[0076] (S1) Opal templates were obtained by self-assembly using the vertical deposition method: commercially available glass slides were cleaned and then immersed in concentrated sulfuric acid / hydrogen peroxide = 7:3 V / V for 24 hours. They were then subjected to hydrophilic treatment by sonication with water and anhydrous ethanol for half an hour each.

[0077] Polystyrene microsphere powder (microsphere particle size of 217 nm) was diluted with ultrapure water to a mass fraction of 5 wt% to obtain a nanosphere dispersion emulsion, which was then poured into a beaker.

[0078] Subsequently, the hydrophilicized glass slide was vertically inserted into the nanosphere dispersion emulsion and fixed. It was then placed in a vacuum drying oven at a temperature of 60°C and a vacuum of 60 kPa for 3 days. After the ultrapure water evaporated, the monodisperse nanospheres self-assembled on the surface of the glass slide to obtain an opal template (also known as a polymer photonic crystal).

[0079] (II) A method for preparing structural color dual-response opal gel, comprising the following steps:

[0080] (S2) Place 0.1g of polylysine in a beaker, add 6.93mL of ultrapure water and stir to disperse. Then add 2.97g (30%) of glycerol and sonicate to mix. When it becomes transparent and uniform, add 3g of N-isopropylacrylamide and 0.04g of thermal initiator sodium bisulfite in sequence and stir to obtain a polymer monomer solution.

[0081] Subsequently, N-N'-methylenebisacrylamide, at a mass fraction of 2.2% of the total mass of polylysine and N-isopropylacrylamide, was added to the polymer monomer solution and mixed thoroughly to obtain the hydrogel prepolymer solution. A 1-2 mm thick silicone pad was cut to the size of a glass slide assembled with an opal template. A hole was cut in the center to form a rectangular silicone pad with a hollowed-out center. A 5 mm slit was then cut from one of the two short sides of the rectangular silicone pad to facilitate the injection of the hydrogel prepolymer solution. The cut silicone pad was fixed on top of the glass slide, and another clean glass slide was placed on top of the silicone pad, with the silicone pad positioned between the two slides, and clamped in place to form a "sandwich" structure.

[0082] Use a syringe to inject the gel prepolymer into the mold through the small opening in the silicone sheet. Let it stand at room temperature for 5 minutes until the gel prepolymer completely fills the gaps between the spheres inside the opal template.

[0083] The opal template filled with gel prepolymer solution was placed in a 70°C oven to initiate a thermal polymerization reaction. The opal template, assembled from monodisperse nanospheres, was embedded within the hydrogel to obtain an opal hydrogel. The opal hydrogel was then peeled off from the hydrophilic glass slide and rinsed with plenty of ultrapure water to remove unreacted monomers and byproducts, ultimately yielding a structural color biresponsive opal hydrogel.

[0084] The thickness of the resulting structural color dual-responsive opal hydrogel was determined by the thickness of the silicone pad. The final thickness of the hydrogel was 1 mm.

[0085] Example 4

[0086] The method for preparing structural color dual-responsive opal gel provided in this embodiment includes the following steps:

[0087] (I) A method for preparing a polymethyl methacrylate opal template, comprising the following steps:

[0088] (S1) Opal templates were obtained by self-assembly using the vertical deposition method: commercially available glass slides were cleaned and then immersed in concentrated sulfuric acid / hydrogen peroxide = 7:3 V / V for 24 hours. They were then subjected to hydrophilic treatment by sonication with water and anhydrous ethanol for half an hour each.

[0089] Polymethyl methacrylate nanosphere powder (microsphere particle size of 232 nm) was diluted with ultrapure water to a mass fraction of 9 wt% to obtain a nanosphere dispersion emulsion, which was then poured into a beaker.

[0090] Subsequently, the hydrophilicized glass slide was vertically inserted into the nanosphere dispersion emulsion and fixed. It was then placed in a vacuum drying oven at a temperature of 55°C and a vacuum of 80 kPa for 4 days. After the anhydrous ethanol evaporated, the monodisperse nanospheres self-assembled on the surface of the glass slide to obtain an opal template (also known as a polymer photonic crystal).

[0091] (II) A method for preparing structural color dual-response opal gel, comprising the following steps:

[0092] (S2) Place 0.2g of polyethyleneimine in a beaker, add 13.86mL of ultrapure water and stir to disperse. Then add 5.94g (30%) of glycerol and sonicate to mix. When it becomes transparent and uniform, add 6g of 2-acrylamide-2-methylpropanesulfonic acid and 0.08g of thermal initiator ammonium persulfate in sequence and stir to obtain a polymer monomer solution.

[0093] Subsequently, 0.15% of the volume of the catalyst N,N,N',N'-tetramethyl-1,3-propanediamine was added to the polymer monomer solution, and after mixing evenly, a gel prepolymer was obtained.

[0094] Cut a 1-2mm thick silicone pad to the size of the slide assembled with the opal template. Hole out the center to form a rectangular silicone pad with a hollow center. Cut a 5mm slit from one of the two short sides of the rectangular silicone pad to facilitate the injection of the hydrogel prepolymer solution. Fix the cut silicone pad on top of the slide. Then cover the silicone pad with another clean slide so that the silicone pad is between the two slides. Clamp it in place to form a "sandwich" structure.

[0095] Use a syringe to inject the gel prepolymer into the mold through the small opening in the silicone sheet. Let it stand at room temperature for 5 minutes until the gel prepolymer completely fills the gaps between the spheres inside the opal template.

[0096] The opal template filled with gel prepolymer solution was placed in a 60°C oven to initiate a thermal polymerization reaction. The opal template, assembled from monodisperse nanospheres, was embedded within the hydrogel to obtain an opal hydrogel. The opal hydrogel was then peeled off from the hydrophilic glass slide and rinsed with plenty of ultrapure water to remove unreacted monomers and byproducts, ultimately yielding a structural color biresponsive opal hydrogel.

[0097] The thickness of the resulting structural color dual-responsive opal hydrogel was determined by the thickness of the silicone pad. The final thickness of the hydrogel was 1 mm.

[0098] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A method for preparing a structural color dual-responsive opal gel, characterized in that, Includes the following steps: Step 1: Disperse monodisperse nanospheres in anhydrous ethanol or ultrapure water to obtain a nanosphere dispersion emulsion. Insert a hydrophilic glass slide vertically into the nanosphere dispersion emulsion. Under vacuum conditions, evaporate the anhydrous ethanol or ultrapure water at a constant temperature. Through vertical deposition, the monodisperse nanospheres self-assemble on the surface of the hydrophilic glass slide to obtain an opal template. Step 2: Add component A to ultrapure water and stir to disperse and activate it. After it is completely dissolved, add component B and thermal initiator in sequence and stir evenly to obtain a polymer monomer solution. Component A includes carboxymethyl chitosan, polylysine, polyethyleneimine, aminated gelatin or aminated PVA, and component B includes acrylamide, N-isopropylacrylamide or 2-acrylamide-2-methylpropanesulfonic acid. Add crosslinking agent or catalyst to polymer monomer solution and mix evenly to obtain gel prepolymer solution. Step 3: Add the gel prepolymer solution to the opal template and let it stand at a certain temperature for a certain period of time to initiate the polymerization reaction; Step 4: After the polymerization reaction is complete, remove the obtained opal gel from the hydrophilic glass slide, and then wash and soak it to obtain the structural color dual-response opal gel.

2. The preparation method according to claim 1, characterized in that: The prepared structural color dual-response opal gel exhibits structural color response characteristics to different concentrations of aldehyde gas or solution and different pH values.

3. The preparation method according to claim 1, characterized in that: In step one, the monodisperse nanospheres include silica monodisperse nanospheres, polystyrene microspheres, or polymethyl methacrylate microspheres.

4. The preparation method according to claim 1, characterized in that: In step one, the particle size of the monodisperse nanospheres is 50 nm to 100 μm; the mass percentage of monodisperse nanospheres in the nanosphere dispersion emulsion is 0.1 to 10 wt%; the constant temperature is 25 to 80 °C; and the vacuum degree under vacuum conditions is 40 to 100 kPa.

5. The preparation method according to claim 1, characterized in that: In step two, after component A is dissolved in ultrapure water, glycerol is added and the mixture is ultrasonically mixed. The mass ratio of component A, component B, and liquid component is 0.1~0.3:2~4:9.7~9.

9. The liquid component includes ultrapure water and glycerol. The amount of glycerol added is 0%~50% of the mass fraction of the liquid component. The amount of thermal initiator added is 0.1%~1% of the total mass of the polymer monomer solution.

6. The preparation method according to claim 1, characterized in that: The thermal initiator includes potassium persulfate, ammonium persulfate, sodium persulfate, or sodium bisulfite; the crosslinking agent includes N-N'-methylenebisacrylamide, N,N'-bis(acryloyl)cysteine, polyethylene glycol diacrylate, bis(3-aminopropyl)dimethylsiloxane, or genipin; and the catalyst includes tetramethylethylenediamine, N,N,N',N'-tetramethyl-1,3-propanediamine, triethanolamine, or N,N-dimethylethanolamine.

7. The preparation method according to claim 1, characterized in that: In step three, the polymerization reaction is carried out at a temperature of 25~80℃ for 1h~24h.

8. A structural color dual-responsive opal gel, characterized in that: The opal gel was prepared using the preparation method described in any one of claims 1 to 7.

9. The application of an opal gel prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The opal gel is used to indicate the freshness of foods that release aldehyde gases.

10. The application according to claim 9, characterized in that: The opal gel is used to indicate the freshness of kiwifruit.

Images