High-stability fluorescent hydrogel based on hydrogen bond anchoring mechanism and preparation method thereof

Abstract

The invention discloses a preparation method of high-stability fluorescent hydrogel based on a hydrogen bond anchoring mechanism. The method comprises the following steps: firstly, mixing polyvinyl alcohol and N-methyl pyrrolidone, heating, stirring and dissolving to obtain a PVA solution; after bubbles in the solution are removed, the solution is poured into a mold for freezing-gelation, and PVA organic gel is obtained; then, the PVA organic gel is immersed in deionized water for water replacement, and pure PVA hydrogel is obtained; and finally, printing and pressing the dyed fiber paper on the surface of the hydrogel to obtain the fluorescent hydrogel. The hydrogen bonds in the fluorescent hydrogel have strong interaction, so that the resolution ratio of the surface pattern of the hydrogel (can reach the micron level) is remarkably improved, the interface stability of the hydrogel is enhanced, and a foundation is laid for the application of the hydrogel in the fields of biosensing, optical display, flexible electronics and the like.

Description

Technical Field

[0001] The present invention belongs to the technical field of polymer materials, and particularly relates to a highly stable fluorescent hydrogel based on a hydrogen bond anchoring mechanism and a preparation method thereof. Background Art

[0002] Hydrogels are three-dimensional network structure materials formed by hydrophilic polymers through physical or chemical cross-linking. Due to their high water content, good biocompatibility, and adjustable mechanical properties, they have received extensive attention in the fields of biomedicine, optics, and electronics. In recent years, as a new type of functional material, fluorescent hydrogels have shown great application potential in the fields of bioimaging, optical sensing, and information storage due to their unique optical properties and patterning characteristics. However, traditional fluorescent hydrogels still face the following challenges in practical applications: 1) The fluorescent dyes in traditional fluorescent hydrogels are prone to photobleaching or leakage, resulting in a significant decrease in fluorescence intensity over time, which limits their long-term use; 2) In the patterning process of existing fluorescent hydrogels, dye diffusion is likely to occur, leading to blurred pattern boundaries and making it difficult to achieve high-precision patterning; 3) In a complex environment, the interfacial stability of fluorescent hydrogels is poor, and structural damage or functional failure is likely to occur.

[0003] To solve the above problems, researchers have tried to improve the performance of fluorescent hydrogels through methods such as chemical cross-linking, nanocomposite, or surface modification. However, these methods are often complex in process, high in cost, and difficult to simultaneously consider fluorescence stability, patterning resolution, and interfacial stability. Therefore, developing a highly stable fluorescent hydrogel based on a simple and efficient mechanism has important scientific significance and application value. Summary of the Invention

[0004] The purpose of the present invention is to provide a highly stable fluorescent hydrogel based on a hydrogen bond anchoring mechanism and a preparation method thereof, aiming to solve the technical problems of easy migration of dyes, poor interfacial stability, and low pattern resolution in traditional fluorescent hydrogels. The present invention first constructs a high-density hydrogen bond network, and then uses a contact printing method to anchor various fluorescent dyes in a polyvinyl alcohol (PVA) hydrogel matrix, obtaining a fluorescent hydrogel with high-precision patterning and excellent interfacial stability, realizing the uniform distribution and stable fixation of fluorescent dyes in the hydrogel matrix. The strong hydrogen bond interaction in this fluorescent hydrogel not only significantly improves the resolution of the surface pattern of the hydrogel (up to the micron level), but also enhances its interfacial stability, laying a foundation for its application in the fields of biosensing, optical display, and flexible electronics.

[0005] The preparation method of the highly stable fluorescent hydrogel based on the hydrogen bond anchoring mechanism is as follows: Mix polyvinyl alcohol and N-methylpyrrolidone, heat and stir to dissolve to obtain a PVA solution; after removing the bubbles in the solution, pour it into a mold for freeze-gelation to obtain a PVA organogel; then immerse the PVA organogel in deionized water for water replacement to obtain a pure PVA hydrogel; finally, press the dyed fiber paper on the surface of the hydrogel to obtain a fluorescent hydrogel.

[0006] The concentration of the PVA solution is 2-30 wt%.

[0007] The mass fraction of the dye in the dyed fiber paper is 0.01-5 wt%. The dye is BBU.

[0008] The heating temperature is 30-160 °C and the time is 1-24 hours.

[0009] The application of the above-prepared fluorescent hydrogel in the preparation of sensors, optical display devices or flexible electronic devices.

[0010] The beneficial effects of the present invention are as follows: 1. Ultra-stable fluorescence: Based on the multiple hydrogen bond anchoring between PVA and BBU, the prepared fluorescent hydrogel can still maintain a fixed shape and high fluorescence brightness after 50 days of immersion. In contrast, the FS and RhB patterns on the surface of the PVA hydrogel quickly blur within 11 hours, and at the same time, the fluorescence brightness significantly decreases. The BBU patterns on the surfaces of PHEA and PAM hydrogels can only be maintained for dozens of hours and a few hours respectively.

[0011] 2. Strong anti-interference ability: After being immersed in a RhB competitive solution for 14 days, the BBU pattern on the surface of the PVA hydrogel still maintains its original boundary and thickness. This indicates that the BBU molecules are firmly fixed in the PVA hydrogel matrix and will not be replaced by RhB in the aqueous phase; 3. High-precision patterning: During the contact printing process, the fiber texture and 90-degree angle formed by the BBU and PVA hydrogel system are clearly visible. In contrast, the fiber texture cannot be seen in the fluorescence images of the FS and RhB patterns, and the corners are blurred into a circle. Description of the Drawings

[0012] Figure 1 : Schematic diagram of the fluorescence staining of the fiber paper in Example 1; Figure 2 : Schematic diagram of the preparation of the fluorescent hydrogel in Example 1; Figure 3 : UV absorption spectra of the BBU, FS, and RhB (a-c) fluorescent dyes in Test 1, and their standard curves at the maximum absorption peaks (e-g); Figure 4: Fluorescence spectra of aqueous solutions of different concentrations of BBU, FS, and RhB (a - c) in Test 1; Figure 5 : Normalized fluorescence spectra of aqueous solutions of different concentrations of BBU, FS, and RhB (a - c) in Test 1; Figure 6 : Photographs of fiber papers dyed with different dyes under sunlight and ultraviolet light irradiation in Test 2; Figure 7 : 2D fluorescence images of fiber papers dyed with different dyes BBU, FS, and RhB (a - c) in Test 2; Figure 8 : Fluorescence spectra and normalized fluorescence spectra of three selected regions of fiber papers dyed with BBU, FS, and RhB (a - c) in Test 2 (d - f); Figure 9 : 3D fluorescence images of fiber papers dyed with different dyes (a - c) BBU, (d - f) FS, (g - i) RhB in Test 2, including top view, side view, and corner view; Figure 10 : Scanning electron microscope images of fiber papers before and after adsorbing BBU, FS, and RhB in Test 2; Figure 11 : Fluorescence images of BBU patterns printed on the surface of PVA hydrogel in Test 3, center (a 1-2 ), corner (a 3-4 ), (b) Stability of BBU patterns on the surface of PVA hydrogel at different soaking times; Figure 12 : Fluorescence images of FS patterns printed on the surface of PVA hydrogel in Test 3, center (a1), corner (a2), (b) Stability of FS patterns on the surface of PVA hydrogel at different soaking times; Figure 13 : Fluorescence images of RhB patterns printed on the surface of PVA hydrogel in Test 3, center (a1), corner (a2), (b) Stability of RhB patterns on the surface of PVA hydrogel at different soaking times; Figure 14 : Stability of BBU patterns on the surface of PHEA (a), PAM (b) hydrogels at different soaking times in Test 3; Figure 15 : Anti - interference experiment of BBU fluorescent strips printed on the surface of PVA hydrogel in Test 3; Figure 16 : Ultraviolet absorption spectra of solutions after soaking PVA (a), PAM (b), PHEA (c) hydrogels printed with BBU test strips for different times in Test 3, (d) Change curves of absorbance of solutions after soaking three types of hydrogels for different times; Figure 17: (a) UV-Vis spectra of the BBU (0.015 wt%) and PVA mixed system, (b) 1 1H NMR spectra of the PVA (2 wt%) and BBU mixed system in Test 3. Detailed implementation mode

[0013] The following elaborates on the preferred embodiments of the present invention in conjunction with the attached drawings, so that those skilled in the art can more clearly understand the advantages and features of the present invention, thereby making a more clear and definite definition of the protection scope of the present invention. Example 1

[0014] Preparation of fluorescently stained fiber paper: Immerse the fiber paper completely in an aqueous solution of a fluorescent dye (stilbene-type fluorescent dye BBU, sodium fluorescein FS, or rhodamine B (RhB)) with a mass fraction of 0.01 - 5 wt%. After soaking for a period of time, lay the wet fiber paper flat on a glass slide and fix both ends with clips to keep it taut. Subsequently, dry it at 25 - 100 °C to finally obtain a flat fiber paper marked with different fluorescent dyes. Figure 1 The figure shows a schematic diagram of the fluorescent staining of the fiber paper.

[0015] Synthesis of hydrogels: (1) Synthesis of PVA hydrogel: In a 100 mL two-necked flask equipped with a reflux condenser, add PVA and NMP to it respectively. Heat the mixed solution to 40 °C and stir magnetically for 20 h to fully dissolve PVA to obtain a PVA solution. After the solution becomes uniformly viscous, remove the bubbles in it. Subsequently, pour the degassed solution into a polytetrafluoroethylene mold and place the mold in an environment of -20 °C for 10 h. Through the freeze-gelation process, a PVA organogel is successfully prepared. Immerse the prepared PVA organogel completely in deionized water and stir continuously for 12 h. During the stirring process, NMP in the PVA organogel is gradually completely replaced by water, and finally a pure PVA hydrogel is obtained.

[0016] (2) Synthesis of PAM and PHEA hydrogels: First, dissolve 6 g of monomers (acrylamide or 2-hydroxyethyl acrylate), 0.3 g of crosslinking agent (N,N'-methylenebisacrylamide), and 0.2 g of initiator (2-ketoglutaric acid) in 13.5 g of deionized water to prepare a prepolymer solution for PHEA and PAM hydrogels. Then, uniformly mix the prepolymer solution and pour it into a glass mold. Place it in a UV curing device and cure it with UV light for 2 minutes under the conditions of an irradiation intensity of 80% and a wavelength of 365 nm to finally obtain poly(2-hydroxyethyl acrylate) PHEA and polyacrylamide PAM hydrogels.

[0017] Synthesis of fluorescent hydrogel: After drying the surface moisture of the prepared hydrogel, it was laid flat on a square glass slide. Then, the rectangular fluorescent fiber paper was placed in the middle of the hydrogel, and a glass slide was covered on top of the hydrogel. Press the glass slide evenly and firmly, then lift the glass slide and remove the paper to obtain a hydrogel with patterned indentations. The hydrogel with patterned indentations was placed in ultrapure water and allowed to stand. After a period of time, the patterned indentations would gradually recover until they were flush with the hydrogel surface, as Figure 2 shown is the specific preparation schematic diagram of the fluorescent hydrogel.

[0018] Test 1: Optical property analysis of fluorescent dyes Figure 3 Using ultrapure water as a solvent, a stock solution was prepared and diluted to obtain a series of solutions with concentrations of 1 - 20 μM. UV–vis absorption spectra were measured for them. The results showed that the fluorescent brightener BBU exhibited three characteristic absorption peaks at 210 nm, 276 nm, and 348 nm. Among them, both 210 nm and 276 nm were characteristic absorption bands of aromatic rings; the maximum absorption value of fluorescein sodium FS was at 488 nm, and the solution was yellow-green at this time; the maximum absorption value of RhB was at 554 nm. In the concentration range of 10 - 20 μM, when the concentration gradually increased, the peak shapes of the characteristic absorption peaks of BBU, FS, and RhB did not change. At the same time, the concentration-absorbance scatter plots showed that as the concentration increased, the absorbance of the three at the characteristic absorption peaks increased linearly.

[0019] Fluorescence spectral analysis ( Figure 4 ) showed that the fluorescence intensities of BBU (10 μM - 9 mM), FS (20 μM - 500 μM), and RhB (20 μM - 500 μM) all showed a trend of first increasing and then decreasing with increasing concentration, indicating that all three had a critical aggregation concentration (CAC). In addition, the normalized fluorescence spectra ( Figure 5 ) showed that as the concentration increased, the maximum emission wavelengths of the three dyes all underwent significant red shifts (BBU red shifted by about 40 nm, FS red shifted by about 20 nm, and RhB red shifted by about 15 nm). The above fluorescence behaviors were highly consistent with the typical aggregation-caused quenching (ACQ) mechanism, that is, dye molecules formed non-radiative decay channels due to π-π stacking or hydrophobic association at high concentrations, resulting in a decrease in fluorescence efficiency.

[0020] Test 2: Structural characterization of dyed fiber paper As Figure 6As shown, the fiber paper treated with dye immersion shows significant color differences under two light conditions. When irradiated with white light, the BBU-stained sample remains the base white, while the FS- and RhB-stained samples show yellow and magenta colors, respectively; under ultraviolet excitation, the BBU-, FS-, and RhB-stained samples show high-contrast blue, green, and red fluorescence in sequence. This phenomenon confirms that the dye molecules have successfully imparted light-responsive colorimetric properties to the material by specifically binding to the fiber matrix.

[0021] Based on the 2D fluorescence images ( Figure 7 , BBU / FS / RhB-stained fiber paper) and the top, side, and partial views of the 3D fluorescence images ( Figure 9 a-i), it can be confirmed that the three dyes are uniformly adsorbed on each fiber in the entire polymer matrix. Further, fluorescence spectral analysis of the Figure 7 selected regions was performed by a confocal laser scanning microscope (CLSM) to study the dispersion state of these dyes adsorbed on the fibers ( Figure 8 a-c). The results show that although the fluorescence intensities in different regions fluctuate due to differences in local dye adsorption amounts, their normalized fluorescence spectra highly overlap ( Figure 8 d-f), demonstrating that the dye molecules adsorbed on the fiber surface are in a homogeneous aggregated state. It is worth noting that the maximum emission wavelengths of the dyes adsorbed on the fibers (BBU: 438 nm, FS: 523 nm, RhB: 581 nm) are in good agreement with the emission wavelengths corresponding to the peak fluorescence intensities in the solution state (BBU: 434 nm, FS: 521 nm, RhB: 585 nm) (offset < 5 nm). This spectral consistency indicates that there is no significant aggregation-induced spectral shift of the dye molecules adsorbed on the fiber surface, and their dispersion state is similar to the monodisperse state when the fluorescence intensity is maximized in the solution, thus verifying the monodisperse and stable adsorption characteristics of the dyes in the fiber matrix.

[0022] Figure 10 The states of the original fiber paper and the fiber papers adsorbed with BBU, FS, and RhB are shown. According to the scanning electron microscope images, the microscopic morphology of the fibers remains unchanged before and after the immersion treatment, indicating the physical adsorption process of BBU, FS, and RhB.

[0023] Test 3: Optical Property and Stability Analysis of Fluorescent Hydrogels Figure 11 - 13CLSM fluorescence images revealed the microscopic details of BBU, FS, and RhB patterns on the surface of PVA hydrogels. During the contact printing process, the fiber texture and 90-degree angles formed by BBU and the PVA hydrogel system were clearly visible. In contrast, the fiber texture could not be seen in the fluorescence images of FS and RhB patterns, and the corners blurred into circles. These imaging data indicate that BBU molecules are stably immobilized in the PVA hydrogel matrix, and the stability of BBU shows significant matrix dependence, while FS and RhB molecules can freely move into the external aqueous environment or the internal hydrogel network. In the stability test, as Figure 11 shown in Figure 12 b, the BBU pattern on the surface of the PVA hydrogel could still maintain its fixed shape and high fluorescence brightness after being immersed in ultrapure water for 50 days. In contrast, the FS and RhB patterns on the surface of the PVA hydrogel quickly blurred within 11 hours, and at the same time, the fluorescence brightness decreased significantly ( Figure 13 b).

[0024] In addition, when printing BBU patterns on the surfaces of different types of hydrogels, there were also significant differences in the stability of the patterns. The BBU patterns printed on the surfaces of PHEA and PAM hydrogels only maintained their stability for dozens of hours and several hours respectively ( Figure 14 ). Comparing Figure 11 b and Figure 14 results showed that BBU molecules were easily desorbed from the surfaces of PHEA and PAM hydrogels into the external aqueous environment or the internal hydrogel network. Based on the above observations, it can be concluded that only the BBU pattern printed on the surface of the PVA hydrogel can maintain long-term stability.

[0025] To further verify this conclusion, Figure 15 an anti-interference experiment was demonstrated in which a PVA hydrogel with a BBU pattern on its surface was immersed in an RhB aqueous solution. The results showed that even after being immersed in the RhB solution for two weeks, the BBU pattern printed on the surface of the PVA hydrogel still maintained its original boundary and thickness. This phenomenon indicates that BBU molecules are firmly immobilized in the PVA hydrogel matrix and cannot be replaced by RhB in the aqueous phase, thus demonstrating that the BBU pattern on the surface of the PVA hydrogel has stronger stability.

[0026] Figure 16 The diffusion behavior of BBU dye molecules on the surfaces of different types of hydrogels was demonstrated. PVA, PAM, and PHEA hydrogels printed with BBU test strips were completely immersed in the same volume of aqueous solution. Each time, 2 mL of the solution was taken to measure its absorbance. After the test, the solution was poured back to maintain a constant volume of the system, and the hydrogels continued to be immersed. Subsequently, the absorbance of the solution at different times was measured, and a change curve of the absorbance of the solution after the three types of hydrogels were immersed for different times was plotted with the immersion time as the abscissa and the absorbance as the ordinate. FromFigure 16 As shown in a-c, with the increase of soaking time, the absorbance at the characteristic peaks gradually increases, indicating an upward trend in the desorption amount of BBU on the three hydrogels. Further observation shows that Figure 16 as shown in d, with the increase of soaking time, the desorption amount of BBU from the three hydrogels all gradually increases, but there are differences: the desorption amount of PAM hydrogel is the highest, followed by PHEA hydrogel, and the desorption amount of PVA hydrogel is the lowest. This result indicates that PVA hydrogel has the strongest adsorption ability for BBU dye molecules, further verifying the strong binding force between BBU dye molecules and PVA hydrogel.

[0027] Figure 17 Using ultraviolet absorption spectroscopy and 1 1H NMR spectroscopy confirmed the existence of covalent hydrogen bonds between BBU and the PVA polymer chain. First, when using ultraviolet absorption spectroscopy to confirm the existence of covalent hydrogen bonds between BBU and the PVA polymer chain, the mass concentration of BBU in the mixed solution system was fixed at 0.015%, and the changes in the ultraviolet absorption spectrum were studied when the mass concentration of PVA increased from 0.015% to 0.75%. As Figure 17 shown in a, the absorption peaks observed at approximately 208 nm, 275 nm, and 350 nm are generally considered to be the ultraviolet absorption characteristics of BBU. With the increase of PVA concentration, the absorption peak near 350 nm redshifts. This spectral change may be due to the formation of intermolecular hydrogen bonds between the dense hydroxyl groups in the PVA structure and the large number of hydrogen bond binding sites in the BBU structure. The hydroxyl group, as an electron donor, increases the electron density of BBU, thereby reducing the energy level difference between the ground state and the excited state and shifting the absorption peak towards the long-wavelength direction.

[0028] In addition, using 1 1H NMR spectrum confirmed the existence of hydrogen bonds between BBU and the PVA polymer chain. In the experiment, the mass concentration of PVA was fixed at 2%, and the mass concentration of BBU was increased to 5 - 30% to enhance the strength of the hydrogen bond interaction between BBU and PVA. As Figure 17 shown in b, in the 1 1H NMR spectrum of pure PVA, the chemical shifts at 1.55 ppm and 3.95 ppm are attributed to the protons on the carbon of the -CH2- group and the protons on the carbon of the -CHOH- group, respectively. With the gradual increase of the BBU content in the system, -CH 2-The protons on the carbon of the group shifted from 1.55 ppm to 1.20 ppm, while the protons on the carbon of the -CHOH- group shifted from 3.95 ppm to 3.54 ppm. The change in chemical shift indicates that groups such as secondary and tertiary amines in BBU form hydrogen bonds with the hydroxyl groups of PVA, thereby shielding some of the hydrogen bonds formed between the hydroxyl groups of different PVA chains. The electron-withdrawing ability of these nitrogen-containing groups is relatively weak, resulting in an increase in the electron cloud density of the protons on the -CH2- group and the -CHOH- group carbon, thus shifting the chemical shift to a higher field.

Claims

1. A method for preparing a highly stable fluorescent hydrogel based on a hydrogen bond anchoring mechanism, characterized in that, The specific operation of the preparation method is as follows: Mix polyvinyl alcohol and N-methylpyrrolidone and heat with stirring to dissolve, obtaining a PVA solution; after removing the bubbles in the solution, pour it into a mold for freezing-gelation to obtain a PVA organic gel; then immerse the PVA organic gel in deionized water for water displacement to obtain a pure PVA hydrogel; finally, press the dyed fiber paper on the surface of the hydrogel to obtain a fluorescent hydrogel.

2. The preparation method according to claim 1, characterized in that, The concentration of the PVA solution is 2-30 wt%.

3. The preparation method according to claim 1, characterized in that, The mass fraction of the dye in the dyed fiber paper is 0.01-5 wt%.

4. The preparation method according to claim 3, wherein The dye is BBU.

5. The preparation method according to claim 1, wherein The heating temperature is 30-160 °C and the time is 1-24 hours.

6. Application of the fluorescent hydrogel prepared by the method according to any one of claims 1-5 in the preparation of sensors, optical display devices or flexible electronic devices.

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