An environmental responsive supramolecular hydrogel based on amino acid ionic liquid structure

Abstract

The application discloses an amino acid ionic liquid-based supramolecular hydrogel, which is prepared by self-assembly at room temperature after a gel factor is dissolved by heating. n mim] + , 12<=n<=18); and the anion comprises L-alanine (L-Ala), L-phenylalanine (L-Phe) or L-proline (L-Pro). The gel formation process is simple and does not depend on polymerization reaction; compared with the prior art, the gel formation does not need freezing, and the gel can be self-aggregated below the phase transition temperature. The physical state embodies the dual responsiveness to temperature and pH, that is, when the temperature and pH change, the system can reversibly convert between the solution state and the gel state. Meanwhile, the hydrogel has good electrocatalytic performance, and has the application potential of being developed into a special material for scaleable production.

Description

Technical Field

[0001] This invention discloses a supramolecular hydrogel and its preparation method. Specifically, it is a hydrogel formed based on an amino acid-type ionic liquid gelling factor, belonging to the field of new materials. Background Technology

[0002] Supramolecular structures are complex, organized aggregates of two or more small molecules bound together by intermolecular forces (π-π interactions, electrostatic interactions, hydrogen bonds, van der Waals forces, and other non-covalent bonds). They are widely used in molecular recognition, coordination chemistry, organocatalysis, drug delivery, and drug carriers (Organic Chemistry, 2020, 40, 380, 39-50). Supramolecular gels, as typical representatives of ordered supramolecular structures, have always been a research hotspot in materials science and other fields. Unlike polymeric gels, supramolecular gels are generally spatial network structures formed by the self-assembly of gelling factors through non-covalent interactions; they possess advantages such as responsiveness, reversibility, tunability, and easy degradation. Common gelling factor structures include aliphatic, amino acid, peptide, carbohydrate, steroid, nucleotide base, and organometallic complexes (Acta Physico-Chimica Sinica, 2016, 32, 227-238). Among them, amino acid-type supramolecular gels exhibit good biocompatibility and have great potential for practical applications.

[0003] Ionic liquids are molten salts composed of anions and cations, possessing excellent properties such as low volatility, non-flammability, high conductivity, and a wide potential window (Journal of Molecular Liquids, 2020, 297, 112038). Based on the different cation structures, they can be classified into imidazole type, quaternary ammonium type, quaternary phosphorus type, piperidine type, pyrrolidine type, pyridine type, and morpholine type, etc. (Journal of Chemical Reviews, 2020, 2, 169-181), among which imidazole type ionic liquids are the most common and have the widest application.

[0004] Currently, hydrogels based on ionic liquids are mainly polyionic liquid hydrogels (CN108707209A, 2018; CN111333767A, 2020), or hydrogels prepared using ionic liquids as polymerization media (Applied Chemical Industry, 2019, 048, 2577-2584). The synthesis processes of these two types of ionic liquid hydrogels are relatively cumbersome and complex. This patent designs the structure of the amino acid gelling factor to be ionic liquid type, simplifying the material preparation steps while endowing the target product with the dual properties of ionic liquid and supramolecular gel.

[0005] This patent synthesizes an amino acid ionic liquid gel factor through a three-step method and prepares a supramolecular gel using a heating-cooling method. The gel factor is characterized by a cation consisting of one of the following N-methylimidazoles substituted with 12, 14, 16, or 18 carbon atoms; and an anion consisting of one of L-alanine, L-phenylalanine, or L-proline. The preparation method is simple, requiring no freezing, and is convenient while saving energy and reagent consumption, making it suitable for large-scale production. In terms of performance, all six gels exhibit good stability; after three months at room temperature, the gel state remained unchanged. Furthermore, the gel demonstrates dual response to temperature and pH and certain electrocatalytic properties. Due to its reversible morphological changes under external environmental stimuli, it has potential applications in separation, drug delivery, electrocatalysis, and electroanalysis. Summary of the Invention

[0006] The purpose of this invention is to provide an amino acid ionic liquid-type supramolecular hydrogel and its preparation method. This gel combines the functions of supramolecular materials and ionic liquids, exhibits dual sensitivity to temperature and pH, and also possesses certain electrochemical properties. This invention is achieved through the following steps:

[0007] 1. N-methylimidazolium bromide [C n Synthesis of [mim]Br: N-methylimidazole was mixed with equimolar amounts of bromododecane, bromotetradecane, bromohexadecane, and bromooctadecane, respectively, and stirred under reflux at 80 °C for 4 h under N2 protection. After the reaction was completed, the mixture was washed with ethyl acetate and recrystallized from acetonitrile to obtain white crystals, which were dried to obtain [C]. n mim]Br;

[0008] 2. Synthesis of [C] via anion exchange n mim]OH: The [C] synthesized in step (1) n [mim]Br is pre-dissolved in methanol and then flows through a styrene-type 201*7 strong base ion exchange resin column for anion exchange; the effluent is collected and tested with AgNO3 solution, and the absence of AgBr white precipitate indicates that the anion exchange is complete;

[0009] 3. Amino acid-based gelling factor [C] n Synthesis and characterization of [mim]R: A certain amount of L-Phe, L-Ala, and L-Pro were weighed and dissolved in methanol, and then the [C] obtained in step (2) was synthesized. n [mim]OH was added dropwise to methanol solutions of the three amino acids, and the mixture was reacted at room temperature for 12 hours, then placed in an ice-water mixture. After unreacted amino acids completely precipitated, they were filtered off, and the methanol was recovered from the filtrate to obtain the final product. The complete synthetic route and a schematic diagram of gel formation are shown below. Figure 1As shown. Further, infrared and hydrogen nuclear magnetic resonance spectroscopy were used to confirm its structure. [C] 12 Taking [mim]L-Phe as an example, its IR spectrum is as follows: Figure 2 As shown: 3000~2800cm -1 This is the absorption peak for the CH stretching vibration of long-chain alkane, at 1600 cm⁻¹. -1 The absorption peak is the conjugate C=C stretching vibration, 1650–1450 cm⁻¹. -1 The four-fold absorption peaks are due to the vibration of the benzene ring skeleton, at 750 and 690 cm⁻¹. -1 Characteristic peaks of bending vibrations of monosubstituted benzene rings are observed, and the characteristic absorption peaks of carboxylate salts overlap with those of the benzene ring skeleton. NMR spectra are as follows: Figure 3 As shown, 1 H NMR (400MHz, CDCl3) δ9.31 (s, 1H), 7.43 (d, J = 1.5Hz, 1H), 7.17~6.90 (m, 1H), 4.29 (s, 10H), 4.08 ( dd,J=9.6,5.1Hz,2H),3.87(d,J=3.4Hz,3H),1.71(s,2H),1.38~1.12(m,18H),0.99~0.70(m,3H);

[0010] 4. Preparation of supramolecular hydrogels by heating-cooling method: Weigh a certain amount of the gelling agent described in step (3), mix it with UP water, stir at 40-65℃ until just dissolved, then cool and stand at room temperature for 2-20 min, and observe whether gelation has occurred using fluidity as an indicator. The results show that [C 12 mim]L-Phe、[C 14 mim]L-Phe、[C 16 mim]L-Phe、[C 18 mim]L-Phe、[C 18 mim]L-Ala and [C 18 Both mim]L-Pro can form supramolecular hydrogels;

[0011] 5. Characterization of supramolecular gels: The phase transition temperature (T0) of the hydrogel in step (4) was determined by the falling ball method. gel-sol The minimum gel concentration (MGC) of the gel and the mass fraction of gel factor (wt%) corresponding to the MGC were determined by the concentration gradient method.

[0012] 6. Observation of supramolecular gel self-assembly process and thermogravimetric analysis of gel based on electron microscopy: A drop of hot gel solution was taken onto a glass slide using a capillary tube, and the morphological changes of the gel during the gradual cooling process were observed using an electron microscope. Simultaneously, a thermogravimetric analyzer was used for characterization: temperature range: 30–800℃, heating rate 20℃ / min, N2 atmosphere;

[0013] 7. The electrochemical properties of six gels were characterized using cyclic voltammetry (CV) on an electrochemical workstation: 5 mM [K3Fe(CN)6] / [K4Fe(CN)6] (containing 0.1 M KCl solution); the working electrode was a gel-modified glassy carbon electrode; the counter electrode was a platinum sheet electrode; and the reference electrode was a silver chloride electrode. The voltage range was -0.2 to 0.5 V, the scan rate was 0.05 V / s, and the number of cycles was 5. Attached Figure Description

[0014] Figure 1 The synthesis route of the gelling agent (a) and the schematic diagram of gel formation (b) are shown.

[0015] Figure 2 For [C] 12 mim]Br and gelling factor [C 12 Infrared spectrum of L-Phe (KBr plate);

[0016] Figure 3 For [C] 12 mim]Br(a) and gelling factor [C 12 mim]L-Phe(b) 1 H NMR spectrum (400MHz, CDCl3);

[0017] Figure 4 For [C] 12 The appearance of the L-Phe supramolecular gel (a) and its self-assembly under an electron microscope (b);

[0018] Figure 5 For [C] 12 Thermogravimetric curve of [mim]L-Phe supramolecular gel;

[0019] Figure 6 The CV curve of the gel in 5 mM [K3Fe(CN)6] / [K4Fe(CN)6] (containing 0.1 M KCl solution) is shown. Detailed Implementation

[0020] The technical solution of the present invention is further illustrated below with specific implementation examples. It should be noted that the following examples should not be construed as limiting the scope of protection of the present invention. Any non-essential improvements and adjustments made to the present invention based on the above description still fall within the scope of protection of the present invention.

[0021] Implementation Example 1: [C 12 [mim]L-Phe gel prepared by heating-cooling method

[0022] Weigh 1.2g [C12 Add 3 mL of UP water to the [mm]L-Phe solution and stir at 60°C to fully dissolve the gelling agent to form a homogeneous and transparent solution with a concentration of 0.4 g / mL. Then, place the solution at room temperature to cool naturally. After 15 minutes, invert the sample vial. If there is no flow, a gel has formed.

[0023] Implementation Example 2: [C 14 [mim]L-Phe gel prepared by heating-cooling method

[0024] Weigh 1.2g [C 14 Add 3 mL of UP water to the [mm]L-Phe solution and stir at 60°C to fully dissolve the gelling agent to form a homogeneous and transparent solution with a concentration of 0.4 g / mL. Then, allow the solution to cool naturally at room temperature. After 15 minutes, invert the sample vial. If there is no flow, it has formed a gel.

[0025] Implementation Example 3: [C 16 [mim]L-Phe gel prepared by heating-cooling method

[0026] Weigh 1.2g [C 16 Add 3 mL of UP water to the [mm]L-Phe solution and stir at 60°C to fully dissolve the gelling agent to form a homogeneous and transparent solution with a concentration of 0.4 g / mL. Then, allow the solution to cool naturally at room temperature. After 15 minutes, invert the sample vial. If there is no flow, it has formed a gel.

[0027] Implementation Example 4: [C 18 [mim]L-Phe gel prepared by heating-cooling method

[0028] Weigh 1.2g [C 18 Add 4 mL of UP water to the [mm]L-Phe solution and stir at 60°C to fully dissolve the gelling agent, forming a homogeneous and transparent solution with a concentration of 0.3 g / mL. Then, allow the solution to cool naturally at room temperature. After 15 minutes, invert the sample vial. If there is no flow, a gel has formed.

[0029] Implementation Example 5: [C 18 [mim]L-Ala gel was prepared by heating-cooling method

[0030] Weigh 1.2g [C 18 Add 4 mL of UP water to the L-Ala solution and stir at 60°C to fully dissolve the gelling agent, forming a homogeneous and transparent solution with a concentration of 0.3 g / mL. Then, allow the solution to cool naturally at room temperature. After 15 minutes, invert the sample vial. If there is no flow, a gel has formed.

[0031] Implementation Example 6: [C 18mim]L-Pro gel was prepared using a heating-cooling method

[0032] Weigh 1.2g [C 18 Add 4 mL of UP water to the L-Pro sample vial and stir at 60°C to fully dissolve the gelling agent and form a uniform, transparent solution with a concentration of 0.3 g / mL. Then, allow the solution to cool naturally at room temperature. After 15 minutes, invert the sample vial. If there is no flow, a gel has formed.

[0033] Implementation Example 7: [C n pH responsiveness of mim]L-Phe gel

[0034] [C] n The gel formed by [m]L-Phe dissolved in UP water, and the pH of the solution was adjusted by adding 1.2 mol / L hydrochloric acid aqueous solution and 1.2 mol / L sodium hydroxide aqueous solution. The sample was allowed to cool naturally at room temperature, and after 15 min, the sample bottle was inverted to examine the gelation performance using flowability as an indicator. The results showed that when pH>10, [C n [mim]L-Phe is in solution state; when pH<10, [C n [mim]L-Phe is in a gel state at room temperature.

[0035] Implementation Example 8: [C 18 pH responsiveness of mim]L-Ala gel

[0036] [C] 18 The gel formed by [m]L-Ala dissolved in UP water, and the pH of the solution was adjusted by adding 1.2 mol / L hydrochloric acid aqueous solution and 1.2 mol / L sodium hydroxide aqueous solution. The sample was allowed to cool naturally at room temperature, and after 15 min, the sample bottle was inverted to examine the gelation performance using flowability as an indicator. The results showed that [C] 18 [mim]L-Ala ionic liquid is in a gel state between pH 3 and 10; it is in a solution state when pH < 3 or > 10.

[0037] Implementation Example 9: [C 18 pH responsiveness of mim]L-Pro gel

[0038] [C] 18 The gel formed by [mim]L-Pro dissolved in UP water, and the pH of the solution was adjusted by adding 1.2 mol / L hydrochloric acid aqueous solution and 1.2 mol / L sodium hydroxide aqueous solution. The sample vials were allowed to cool naturally at room temperature. After 15 min, the sample vials were inverted, and the gelation performance was evaluated using flowability as an indicator. The results showed that [C 18[mim]L-Pro ionic liquid is in a gel state between pH 1 and 10; it is in a solution state when pH > 10.

[0039] Implementation Example 10: Six Gel Phase Transition Temperatures T gel-sol Measurement

[0040] Take any of the above hydrogels, place a glass sphere with a diameter of 5 mm and a weight of 0.2 g on the surface of the gel, and slowly heat it in a water bath to gradually raise the temperature of the gel. The temperature at which the sphere just begins to fall is the phase transition temperature of the hydrogel. The results are shown in Table 1.

[0041] Implementation Example 11: Determination of Minimum Gel Concentration (MGC) and Mass Fraction (wt%) of Six Types of Gels

[0042] The above-mentioned gelling agent was dissolved in UP water at 60℃ at a ratio of 1:1 (g / mL), and the solution was divided into several test tubes by volume. Then, UP water was added to each test tube in increasing volumes to dilute the solution, forming a series of gelling agent solutions with a concentration gradient. The test tubes were then allowed to cool naturally at room temperature. After 15 minutes, the test tubes were inverted, and the lowest concentration at which the gel formed was the MGC. The MGC and wt% data of the six gels are shown in Table 1. Table 1. Comparison of three index parameters for supramolecular hydrogels of six amino acid ionic liquids

[0043] Implementation Example 12: Observation of the self-assembly process of supramolecular gels using electron microscopy

[0044] With [C 12 Taking the [mim]L-Phe gelling factor as an example, after heating and dissolving the supramolecular gel formed by it, a drop of the hot gel solution was transferred onto a glass slide using a capillary tube, and the self-aggregation morphology of the gel during the cooling process was recorded using an electron microscope. The results are as follows: Figure 4 As shown.

[0045] Implementation Example 13: Thermogravimetric Analysis of Supramolecular Gels

[0046] With [C 12 Taking supramolecular gels formed by the [mim]L-Phe gelling factor as an example, the thermogravimetric analysis temperature range was 30–800℃, the heating rate was 20℃ / min, and the atmosphere was N2. The results obtained under these conditions are as follows: Figure 5As shown, the thermal weight loss of the gel mainly consists of three stages: the first stage is at 50–150℃, with a weight loss of 59%, which is the evaporation of water in the gel; the second stage is at 210–310℃, with a weight loss of 22.5%, during which the gel factor structure undergoes partial decomposition; the third stage is at 310–350℃, with a weight loss of 4.6%, which is the further decomposition of the gel factor structure; above 350℃, the mass of the residue tends to stabilize.

[0047] Implementation Example 14: Electrochemical Performance Characterization of Supramolecular Gels

[0048] The electrochemical properties of the gels were determined using cyclic voltammetry on an electrochemical workstation. A three-electrode system was used, consisting of a glassy carbon electrode (working electrode), Ag-AgCl (reference electrode), and a platinum sheet electrode (counter electrode). The electrolyte consisted of a solution containing 5 mM potassium ferrocyanide, 5 mM potassium ferrocyanide, and 0.1 M potassium chloride. Before use, the glassy carbon electrode was sonicated three times, polished successively with 0.3 mm and 0.05 μm Al₂O₃ powder, and then sonicated three more times. The electrode surface was then dried with N₂, and 2 μL of the six gel solutions were loaded onto the electrode surface. The test parameters were as follows: voltage range -0.2 to 0.5 V, scan rate 0.05 V / s, and 5 cycles. The results are as follows: Figure 6 As shown, the electrocatalytic performance is in the following order: [C] 14 mim]L-Phe gel>[C 12 mim]L-Phe gel>[C 16 mim]L-Phe gel>[C 18 mim]L-Ala gel>[C 18 mim]L-Pro gel>[C 18 mim]L-Phe gel.

Claims

1. An amino acid ionic liquid supramolecular hydrogel, characterized in that, The structure of the gelling factor is: [C 12 mim]L-Phe、[C 14 mim]L-Phe、[C 16 mim]L-Phe、[C 18 mim]L-Phe、[C 18 mim]L-Ala or [C 18 mim]L-Pro, the structural formula is as follows: 。 2. The amino acid ionic liquid supramolecular hydrogel as described in claim 1, characterized in that, The gelling agent was prepared into an aqueous solution with a concentration of 0.1~3M.

3. The amino acid ionic liquid supramolecular hydrogel as described in claim 1, characterized in that, The self-assembly temperature of the gel factor is -10℃ to 43℃.

4. The amino acid ionic liquid supramolecular hydrogel as described in claim 1, characterized in that, The gel completes self-assembly in 1-30 minutes below the phase transition temperature.

5. The amino acid ionic liquid supramolecular hydrogel as described in claim 1, characterized in that, The aqueous solutions of the six gelling agents all formed white gel solids below their respective phase transition temperatures, became clear and transparent solutions above their phase transition temperatures, and reverted to an ordered gel structure when cooled again below their phase transition temperatures, demonstrating temperature responsiveness.

6. The amino acid ionic liquid supramolecular hydrogel as described in claim 1, characterized in that, All six hydrogels exhibit pH responsiveness; that is, by adjusting the solution pH ∈ [1, 10], the system can reversibly switch between solution and gel states.

7. The amino acid ionic liquid supramolecular hydrogel as described in claim 1, characterized in that, Supramolecular hydrogels possess certain electrocatalytic properties, and these properties are characterized by [C]. 14 mim]L-Phe gel>[C 12 mim]L-Phe gel>[C 16 mim]L-Phe gel>[C 18 mim]L-Ala gel>[C 18 mim]L-Pro gel>[C 18 mim]L-Phe gel.

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