Photo-thermal responsive color-changing hydrogel and preparation method thereof

Abstract

The application provides a photo-thermal response color-changing hydrogel and a preparation method. The photo-thermal response color-changing hydrogel comprises a hydrogel matrix, titanium dioxide nanoparticles dispersed in the hydrogel matrix, and a viologen derivative dispersed in the hydrogel matrix. The photo-thermal response color-changing hydrogel can realize a significant photochromic effect under ultraviolet light irradiation, and can also realize limited thermochromism under heating conditions. The color-changing and color-fading processes of the color-changing hydrogel are affected by environmental humidity and atmosphere, and the color-changing hydrogel has adjustability and good stability.

Description

Technical Field

[0001] This application relates to the field of functional thin film materials, specifically to a photothermal responsive color-changing hydrogel and its preparation method. Background Technology

[0002] Intelligent responsive color-changing materials have enormous application potential in fields such as imaging display, information storage, intelligent sensing, and optical control. Electrochromism, photochromism, and thermochromism are three representative stimulus-response modes. Traditional research has often treated them as independent systems, leading to limitations in approaches to common challenges such as response speed, stability, and functional adaptability, making it difficult to meet the needs of integrating multiple color-changing modes. Achieving integrated intelligent color-changing materials with multi-field synergistic responses provides a new path to overcome existing technological bottlenecks.

[0003] Currently, there are few reports on organic materials that can simultaneously integrate multiple color-changing functions. Therefore, systematically exploring the intrinsic physical relationship and universal mechanism between photochromism and thermochromism is of great significance. By precisely controlling the carrier transfer, transport, and redox processes within organic color-changing systems, efficient coupling and controllable switching of color-changing behavior under multiple stimuli can be achieved, promoting the upgrade from single response to multifunctional synergistic response.

[0004] Further exploration of the intrinsic mapping relationship between photochromic and thermochromic mechanisms, and the revelation of the structure-activity relationship of multi-stimulus responses, can provide theoretical support for the design of intelligent response materials with high response speed, high stability, and multifunctional integration, and promote the development of intelligent optical materials towards multi-mode and intelligent directions. Summary of the Invention

[0005] In view of the above problems, this application proposes a photothermal responsive color-changing hydrogel and its preparation method, aiming to realize the use of classic electrochromic materials for photochromism and thermochromism, and solve the problem of multifunctional integration of smart responsive color-changing materials in high-end applications.

[0006] According to a first aspect of this application, a photothermal responsive color-changing hydrogel is provided, comprising: a hydrogel matrix; titanium dioxide nanoparticles dispersed in the hydrogel matrix; and a viologen derivative dispersed in the hydrogel matrix.

[0007] According to embodiments of this application, the hydrogel matrix is ​​formed from polyvinylpyrrolidone and sodium carboxymethyl cellulose.

[0008] According to embodiments of this application, the viologen derivative is at least one of ethyl viologen, benzyl viologen, and 1,1'-bis(2-carboxyethyl) viologen.

[0009] According to an embodiment of this application, the average particle size of the titanium dioxide nanoparticles is 20 nm.

[0010] According to a second aspect of this application, a method for preparing a photothermal-responsive color-changing hydrogel is provided, comprising: preparing a mixed gel solution, wherein the mixed gel solution comprises titanium dioxide nanoparticles, viologen derivatives, polyvinylpyrrolidone, and sodium carboxymethyl cellulose; spin-coating the mixed gel solution onto a substrate to form the mixed gel solution on the substrate; and annealing the formed mixed gel solution to obtain the photothermal-responsive color-changing hydrogel.

[0011] According to an embodiment of this application, the steps for preparing the mixed gel solution include: dissolving titanium dioxide nanoparticles in water and subjecting them to ultrasonic treatment to obtain a titanium dioxide dispersion; mixing the titanium dioxide dispersion with an aqueous solution of viologen derivative to obtain a mixed solution of titanium dioxide and viologen derivative; and adding polyvinylpyrrolidone and sodium carboxymethyl cellulose to the mixed solution of titanium dioxide and viologen derivative, and subjecting it to heating and stirring to obtain the mixed gel solution.

[0012] According to the embodiments of this application, the concentration of titanium dioxide in the titanium dioxide dispersion is 23 mg / ml-29 mg / ml; the concentration of viologen derivative in the aqueous solution of the viologen derivative is 3.0 mmol / L-3.6 mmol / L.

[0013] According to the embodiments of this application, the amount of polyvinylpyrrolidone added is 10 mg / ml; the amount of sodium carboxymethyl cellulose added is 8.33 mg / ml.

[0014] According to an embodiment of this application, the spin coating speed for spin coating the mixed gel solution onto the substrate is 400 rpm-600 rpm, and the spin coating time is 25 s-35 s.

[0015] According to an embodiment of this application, the annealing temperature for annealing the molded mixed gel solution is 35°C-45°C; the annealing time is 5 min-15 min.

[0016] The photothermal responsive color-changing hydrogel and its preparation method provided in this application have at least the following beneficial effects:

[0017] (1) This photothermal responsive color-changing hydrogel is based on the reduction reaction mechanism of viologen molecules. By selecting different viologen derivatives and using different energy stimulation methods, multiple color switching can be achieved. Under ultraviolet light irradiation, titanium dioxide nanoparticles absorb photon energy to generate photogenerated electrons, which are injected into viologen molecules to reduce them to free radical cations (V). •+The photochromic material initially appears purple; under heating conditions, thermal energy lowers the interfacial electron migration barrier, activating a weak electron transfer process that also reduces viologen, but due to the slower rate and lower concentration of free radical generation, it appears blue. Photochromism and thermochromism share a similar electron transfer mechanism but differ in their kinetic processes, thus endowing the material with differentiated dual-mode response capabilities.

[0018] (2) The photothermal responsive color-changing hydrogel uses a three-dimensional network structure formed by polyvinylpyrrolidone and sodium carboxymethyl cellulose as the hydrogel matrix. This network structure not only provides a close interfacial contact between titanium dioxide and viologen molecules, effectively shortening the electron transport path and inhibiting interfacial carrier recombination, but also promotes photogenerated charge separation by using water molecules bound in the network as hole trapping agents. Attached Figure Description

[0019] The above-mentioned contents, other objects, features and advantages of this application will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0020] Figure 1 A flowchart illustrating a method for preparing a photothermal responsive color-changing hydrogel according to an embodiment of this application is shown.

[0021] Figure 2 The illustration shows the transmittance and color changes of photothermal responsive color-changing hydrogels with different viologen derivatives according to embodiments of this application before and after light exposure;

[0022] Figure 3 This schematic diagram illustrates the energy level matching of photothermal responsive color-changing hydrogels of different viologen derivatives according to embodiments of this application.

[0023] Figure 4 The illustration shows a schematic diagram of the photo / thermal color change of photothermal responsive color-changing hydrogels of different viologen derivatives according to embodiments of this application. Detailed Implementation

[0024] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0025] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of the stated features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0026] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0027] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0028] This application provides a photothermal responsive color-changing hydrogel, comprising a hydrogel matrix, titanium dioxide nanoparticles, and viologen derivatives. The titanium dioxide nanoparticles and viologen derivatives are dispersed within the hydrogel matrix.

[0029] In this embodiment, the color-changing principle of the photothermal responsive color-changing hydrogel is based on the reversible reduction reaction of viologen molecules. Viologen molecules (V²⁺) are reduced to free radical cations (V²⁺) after gaining electrons. • (⁺) This free radical species exhibits characteristic absorption in the visible light region, resulting in color changes. Depending on the mode of energy stimulation, the source and transfer pathway of electrons differ, specifically categorizing it into two modes: photochromism and thermochromism.

[0030] Under ultraviolet light irradiation, titanium dioxide nanoparticles, acting as photosensitive semiconductors, absorb photon energy, causing electrons in the valence band (VB) to transition to the conduction band (CB), generating photogenerated electron-hole pairs. Since the conduction band edge energy level of titanium dioxide (approximately -0.3V vs. NHE) is higher than the lowest unoccupied molecular orbital (LUMO) energy level of viologen molecules (approximately -0.4V to -0.5V), photogenerated electrons can thermodynamically and spontaneously inject into viologen molecules from the titanium dioxide conduction band, reducing them to free radical cations (V0, V0, V0, V0). • (⁺), while holes remain in the titanium dioxide valence band. The formation of this free radical cation is the direct cause of the color change in the system, manifested as purple coloration.

[0031] In some embodiments, the hydrogel matrix is ​​formed from polyvinylpyrrolidone and sodium carboxymethyl cellulose. The three-dimensional porous network structure formed by the hydrogel matrix (polyvinylpyrrolidone and sodium carboxymethyl cellulose) allows for close contact between titanium dioxide nanoparticles and viologen molecules, effectively shortening the electron transport path and suppressing interfacial carrier recombination. Especially under an N2 atmosphere, the quenching effect of O2 on free radicals is effectively eliminated. Under high humidity conditions, the carrier transport performance of the hydrogel system is improved, and the molecular microenvironment is more conducive to the transfer of photogenerated electrons to viologen molecules. This, in turn, improves photoreduction efficiency, resulting in a deeper and more uniform coloring effect. Furthermore, the large number of water molecules bound in the hydrogel network can act as hole traps, reacting with photogenerated holes in the titanium dioxide valence band (h⁺ + H₂O → •OH + H⁺), promoting effective separation of photogenerated charges, further improving photoreduction efficiency and color depth.

[0032] Under heating conditions (e.g., 80℃), the thermal energy is insufficient to directly excite titanium dioxide to generate photogenerated electrons. Instead, it lowers the interfacial electron migration energy barrier, activating the weak electron transfer process present in the system. This also achieves the injection of electrons into viologen molecules, reducing viologen to free radical cations (V). • (⁺), producing a color change.

[0033] Unlike photochromism, thermochromism involves a slower electron generation rate and a lower free radical concentration, resulting in a different blue coloring effect. This difference reflects the fundamental difference in the kinetics of the two excitation methods: photochromism relies on the rapid generation and high concentration injection of photogenerated carriers, while thermochromism relies on the reduction of the electron transfer barrier by thermal energy and slow electron transfer.

[0034] Both photochromism and thermochromism are based on the same core mechanism, namely the reduction reaction of viologen molecules (V²⁺ → V). • (⁺), but there are significant differences in electron generation rate, free radical concentration, and color depth. This "mechanism consistency" and "kinetic difference" is the basis for realizing photothermal dual-response differentiated color development, and also provides the possibility for further control of color-changing performance.

[0035] Optionally, the viologen derivative is at least one of ethyl viologen, benzyl viologen, and 1,1'-bis(2-carboxyethyl) viologen. The LUMO energy levels of different viologen derivatives vary slightly, but are all lower than the conduction band energy level of titanium dioxide, satisfying the thermodynamic conditions for electron injection. Differences in side-chain structure may affect the stability, electron transfer rate, and color development efficiency of the viologen radical, thus providing additional degrees of freedom for color modulation.

[0036] The color-changing and fading process of the photothermal responsive color-changing hydrogel in this application embodiment is affected by environmental humidity and atmosphere, and has adjustability and good stability. It can be applied to smart windows, visual photothermal sensors and solar ultraviolet detection fields.

[0037] In some embodiments, the average particle size of the titanium dioxide nanoparticles is 20 nm.

[0038] Figure 1 A flowchart illustrating a method for preparing a photothermal responsive color-changing hydrogel according to an embodiment of this application is shown.

[0039] like Figure 1 As shown, this application discloses a method for preparing a photothermal responsive color-changing hydrogel, comprising steps S1 to S3.

[0040] S1, Prepare a mixed gel solution containing titanium dioxide nanoparticles, viologen derivatives, polyvinylpyrrolidone, and sodium carboxymethyl cellulose.

[0041] S1 includes S11 to S13.

[0042] S11, titanium dioxide nanoparticles are dissolved in water and subjected to ultrasonic treatment to obtain a titanium dioxide dispersion.

[0043] Commercial TiO2 nanoparticles with an average particle size of 20 nm were dissolved in deionized water and ultrasonically treated for 20 min to obtain a TiO2 dispersion.

[0044] The concentration of titanium dioxide in the titanium dioxide dispersion is 23 mg / ml-29 mg / ml. In this example, the concentration of the TiO2 dispersion is 26 mg / ml.

[0045] S12, mix the titanium dioxide dispersion with the aqueous solution of viologen derivative to obtain a mixed solution of titanium dioxide and viologen derivative.

[0046] The concentration of viologen derivatives in aqueous solutions ranges from 3.0 mmol / L to 3.6 mmol / L.

[0047] In this embodiment, 10 ml of a 3.3 mmol / L viologen derivative aqueous solution was mixed with the above TiO2 dispersion at a volume ratio of 1:1 and stirred until homogeneous to obtain a mixed solution of TiO2 and viologen derivative. The viologen derivative is selected from at least one of ethyl viologen, benzyl viologen, or 1,1'-bis(2-carboxyethyl) viologen.

[0048] S13, polyvinylpyrrolidone and sodium carboxymethyl cellulose are added to a mixed solution of titanium dioxide and viologen derivative, and heated and stirred to obtain a mixed gel solution.

[0049] Polyvinylpyrrolidone and sodium carboxymethyl cellulose were added to the above mixed solution of TiO2 and viologen derivative, and stirred at 60°C for 40 min, and then stirred at 65°C for 3 h to obtain a homogeneous mixed gel solution.

[0050] The dosage of polyvinylpyrrolidone is 100 mg per 10 ml of solution; the dosage of sodium carboxymethyl cellulose is 83.3 mg per 10 ml of solution.

[0051] S2, spin-coating the mixed gel solution onto the substrate to form the mixed gel solution on the substrate.

[0052] In this embodiment, a quartz sheet with dimensions of 15mm × 15mm × 1mm was selected as the substrate. The substrate was sequentially ultrasonically cleaned with diluted detergent, deionized water, acetone, and isopropanol for 20 minutes each, and then placed in ethanol for later use. Before use, the quartz substrate was removed from the ethanol, dried with dry nitrogen gas, and then subjected to UV-ozone treatment for 15 minutes. The mixed gel solution was dropped onto the treated quartz substrate and placed in a spin coater. The spin coater was used to spin-coat the mixed gel solution onto the substrate at a speed of 400-600 rpm for 25-35 seconds, allowing the mixed gel solution to form on the substrate.

[0053] S3, the molded mixed gel solution is annealed to obtain a photothermal responsive color-changing hydrogel.

[0054] When the substrate is a quartz substrate, the process parameters for annealing the sample to be processed include: annealing temperature of 40℃±5℃; and annealing time of 10min±5min.

[0055] In this embodiment of the invention, when the substrate is a petri dish, the sample to be treated is annealed. The process parameters for annealing the sample to be treated include: annealing temperature of 40℃±5℃; and annealing time of 4h±1h.

[0056] Based on the above-mentioned color-changing principle, the photothermal responsive color-changing hydrogel provided in this application realizes the photothermal dual-response differentiated color display function, which will be described in detail below through specific embodiments.

[0057] In the embodiments of this application, three photothermal responsive color-changing hydrogels of different viologen derivatives are provided, referred to as sample A (ethyl viologen), sample B (benzyl viologen), and sample C (1,1'-bis(2-carboxyethyl) viologen).

[0058] Figure 2 The illustration shows the transmittance and color changes of photothermal responsive color-changing hydrogels of different viologen derivatives according to embodiments of this application before and after light exposure.

[0059] like Figure 2As shown, the wavelength range of the illumination is 300nm-800nm. Figure 2 a, b, and c in the figures show that three different viologen derivatives (ethyl viologen, benzyl viologen, and 1,1'-bis(2-carboxyethyl) viologen) all exhibit varying degrees of transmittance reduction in the visible light band (400nm-700nm), resulting in a purple coloration. This indicates that the photothermal responsive color-changing hydrogel prepared in the embodiments of this application has good photochromic properties.

[0060] Figure 3 The diagram illustrates the energy level matching of photothermal responsive color-changing hydrogels of different viologen derivatives according to embodiments of this application.

[0061] like Figure 3 As shown, the conduction band edge energy level of titanium dioxide is higher than the lowest unoccupied molecular orbital energy level of the three viologen derivatives. Therefore, photogenerated electrons can be thermodynamically and spontaneously injected into viologen molecules from the conduction band of titanium dioxide, providing an energy level basis for photochromism.

[0062] Figure 4 The illustration shows a schematic diagram of the photo / thermal color change of photothermal responsive color-changing hydrogels of different viologen derivatives according to embodiments of this application.

[0063] like Figure 4 As shown, photothermal responsive color-changing hydrogels of three different viologen derivatives were prepared using a solution casting method. Approximately 1.5 mL of the mixed solution was poured into a petri dish, evenly distributed, and then annealed at 40 °C for 4-5 hours to obtain the viologen-TiO2@PVP composite hydrogel in the petri dish. Figure 4 The photochromic (PC) comparison experiment shown on the left demonstrates that after irradiating the three viologen derivatives-TiO2@PVP composite hydrogels with 365nm ultraviolet light for 5 minutes, a purple photochromic phenomenon can be observed. Figure 4 The thermochromic (TC) comparison experiment shown on the right shows that when the three viologen derivatives-TiO2@PVP composite hydrogels were heated at 80 °C for 10 min, a blue thermochromic phenomenon could be observed.

[0064] Figure 4 The main principle of the experiment shown is that this difference can be attributed primarily to the different electron generation rates and free radical concentration distributions of the two excitation methods. During photochromism, TiO2, under ultraviolet light excitation, can rapidly generate a high concentration of electrons and directionally transfer them to viologen molecules, resulting in a higher concentration of V in the system. •+The higher concentration of photochromism results in a deeper and more typical purple hue. Thermochromism, on the other hand, relies on thermal energy to lower the interfacial electron migration barrier and activate a weaker electron transfer process, leading to a slower rate and lower concentration of free radicals, resulting in a relatively lighter blue color. This demonstrates that while photochromism and thermochromism share similarities in electron transfer, their macroscopic optical responses are significantly influenced by differences in excitation methods and kinetic processes.

[0065] The photothermal responsive color-changing hydrogel provided in this application has significant application prospects in the fields of intelligent sensing and anti-counterfeiting encryption. Utilizing the differentiated response characteristics of the same material—rapidly displaying purple under ultraviolet light and transforming into blue under heating—multimodal information display and encrypted identification can be achieved. This ability to produce significantly different optical responses to light and heat provides a material basis for developing reusable novel intelligent windows, visual sensors, and high-security anti-counterfeiting labels. Its modulated electron transfer paths and kinetic differences hold promise for achieving precise and visualized dual signal output in scenarios such as intelligent packaging, information security, and environmental monitoring.

[0066] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

[0067] The embodiments of this application have been described above. However, these embodiments are merely illustrative and not intended to limit the scope of this application. Although various embodiments have been described above, this does not mean that the measures in the various embodiments cannot be used advantageously in combination. Without departing from the scope of this application, those skilled in the art can make various substitutions and modifications, all of which should fall within the scope of this application.

Claims

1. A photothermal responsive color-changing hydrogel, characterized in that, include: Hydrogel matrix; Titanium dioxide nanoparticles are dispersed in the hydrogel matrix; as well as Violet derivatives are dispersed in the hydrogel matrix.

2. The photothermal responsive color-changing hydrogel according to claim 1, characterized in that, The hydrogel matrix is ​​formed from polyvinylpyrrolidone and sodium carboxymethyl cellulose.

3. The photothermal responsive color-changing hydrogel according to claim 1, characterized in that, The viologen derivative is at least one of ethyl viologen, benzyl viologen, and 1,1'-bis(2-carboxyethyl) viologen.

4. The photothermal responsive color-changing hydrogel according to claim 1, characterized in that, The average particle size of the titanium dioxide nanoparticles is 20 nm.

5. A method for preparing a photothermal-responsive color-changing hydrogel, characterized in that, include: A mixed gel solution was prepared, wherein the mixed gel solution contained titanium dioxide nanoparticles, viologen derivatives, polyvinylpyrrolidone, and sodium carboxymethyl cellulose; The mixed gel solution is spin-coated onto a substrate to form the mixed gel solution on the substrate; The molded mixed gel solution is annealed to obtain the photothermal responsive color-changing hydrogel.

6. The preparation method according to claim 5, characterized in that, The steps for preparing the mixed gel solution include: Titanium dioxide nanoparticles were dissolved in water and subjected to ultrasonic treatment to obtain a titanium dioxide dispersion. The titanium dioxide dispersion was mixed with an aqueous solution of viologen derivative to obtain a mixed solution of titanium dioxide and viologen derivative; and Polyvinylpyrrolidone and sodium carboxymethyl cellulose were added to the mixed solution of titanium dioxide and viologen derivative, and the mixture was heated and stirred to obtain the mixed gel solution.

7. The preparation method according to claim 6, characterized in that, The concentration of titanium dioxide in the titanium dioxide dispersion is 23 mg / ml-29 mg / ml; the concentration of viologen derivative in the aqueous solution is 3.0 mmol / L-3.6 mmol / L.

8. The preparation method according to claim 5, characterized in that, The amount of polyvinylpyrrolidone added is 100 mg per 10 ml of solution; the amount of sodium carboxymethyl cellulose added is 83.3 mg per 10 ml of solution.

9. The preparation method according to claim 8, characterized in that, The mixed gel solution was spin-coated onto the substrate at a spin speed of 400 rpm-600 rpm for 25 s-35 s.

10. The preparation method according to claim 5, characterized in that, The annealing temperature for the shaped mixed gel solution is 35℃-45℃; the annealing time is 5min-15min.

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