文章編號(hào)1000-5269（2024）06-0019-07

DOI：10.15958/j.cnki.gdxbzrb.2024.06.04

Abstract：

Compared to traditional polymer hydrogels， supramolecular hydrogels exhibits superior reversibility and stimulus response due to the instantaneous and reversible nature of non-covalent bonds. In this paper， we utilized the host-guest exclusion interaction between Decamethylcucurbit［5］uril （Me10Q［5］） and the 2，7-diaminofluorenedihydrochloride（DAF·HCl） to construct a Q［n］-based hydrogel system. The composition， structure， and properties of the hydrogel were compre-hensively characterized using rheometer， nuclear magnetic resonance， scanning electron microscope. This cost-effective and straightforward hydrogel synthesis method paves the way for the scalable production of practical and commercially viable Q［n］-based hydrogels.

Key words：

decamethylcucurbit[5]uril; 2，7-diaminofluorenedihydrochloride; supramolecular hydrogel; host-guest exclusion interaction

CLC number：O641.3

Document code：A

Traditional hydrogels， assembled by water-soluble polymers with cross-linked network structure， have been widely used in fields such as biology and medicine［1-3］. However， in recent years， supramolecular hydrogels composed of simple small molecules have attracted much attention because of their simple preparation and impressive stimulus responsiveness［4-7］. Among the array of macrocyclic compound， cucurbit［n］urils （Q［n］） ［8-10］stand out as a commonly utilized component in supramolecular hydrogel system because of its unique host-guest properties［11-13］.In 2007， KIM reported the first supramolecular hydrogel based on Q［7］，utilizing outer-surface interactions and the hydrogen bonding with port oxygen atoms［14］. In 2010， SCHERMAN et al. proposed to use the ternary complexes of Q［8］ as crosslinkers in polymer systems， introducing the charge transfer-inducing guest to form supramolecular hydrogels［15］. Up to now， hydrogel systems based on Q［5］， Q［6］， Q［7］， and Q［8］ have been successfully developed and reported［16-19］. In this study， a fluorescent hydrogel was engineered through the exclusion interaction between the carbonyl oxygen atom of Decamethylcucurbit［5］uril （Me10Q［5］） and the amine group of the 2，7-diaminofluorene-dihydrochloride （DAF·HCl）. The resulting hydrogel is heat sensitive， which can be converted into the solution upon heating， and then converted into the gel upon cooling.

1Experimental Section

1.1Reagents

Me10Q［5］was made in the laboratory［20］.DAF·HCl was purchased from the laboratory of Aldrich Reagent Company （Shanghai， China）.All reagents are analytical reagent grade. Ultrapure water was used in all experiments.

1.2Instrument

Nano isothermal calorimeter（TA Company， USA）， Agilent 8453 ultraviolet-visible spectrophotometer （Agilent Company， USA）， Fluorescence spectrometer（Agilent Company， USA）， JEOL JNM-ECZ400s MHz nuclear magnetic instrument （Japan Electronics Co.， LTD.）， Thermo Field Mars 40 Rheometer， Freeze dryer， Field emission scanning electron micromirror （SEM， HITACHI S-4800）， Sarteorius-BS110S Electronic Balance， Ultrasonic oscillators， etc.， volumetric bottles， cylinders， beakers and other glass instruments.

1.3Preparation of hydrogels

The Me10Q［5］ （1.5%-2.5%） and DAF·HCl （1%-3%） were separately heated in water until dissolved. The two solutions are shaken and mixed to form the hydrogel. Under ultraviolet irradiation， the Me10Q［5］/DAF·HCl hydrogel emits blue fluorescence， as shown in Fig.1. In addition， the hydrogel can also change its hydrogel state by temperature as shown in the Fig.2， which provides a new direction for the application of this hydrogel.

2Results and Discussion

2.1Mechanical properties

The rheological characterization was performed by a Thermo Fisher Mars 40 rheometer with a diameter of 8 mm parallel plate. The G′ （storage modulus） and G″ （loss modulus） of the hydrogel were monitored by amplitude and frequency scanning. The oscillatory strain test was carried out in the strain range of 01%-100%. The oscillation frequency experiment was carried out in the range of 001-20 Hz.

Rheology， a branch of physics that studies the flow of matter， is particularly relevant to the study of hydrogels， which often exhibit non-Newtonian mechanical behavior. The mechanical properties of hydrogel materials include shear rate， shear strain， and viscoelasticity. The viscoelastic variation of the Me10Q［5］/DAF·HCl hydrogel was characterized by rheology. The frequency dependence of the storage and loss oscillating shear modulus （G′ and G″， respectively） can clearly identify the mechanical properties of the hydrogel. The curves of the hydrogel’s G′， G″， and tan δ with amplitude strain in the linear range are shown in Fig.3（a）. The strain-dependent oscillatory rheology shows a very wide linear viscoelastic region. The hydrogel has a rela-tively stable structure and solid state in the amplitude strain range of 0.01%-30%， with G′ always greater than G″.This observation underscores the resilient and solid nature of the hydrogel， deviating from linearity at around 30% amplitude strain， where G″ surpasses G′ to signify a shift towards viscous flow behavior.Although G′ is already larger than G″， the G′ value is small and the hydrogel is transparent and soft， indicating that the hydrogel is easily affected by external forces.

Fig.3（b） shows the G′， G″， and tan δ of the hydrogel as a function of frequency in the linear range. It is evident that both G′ and G″ exhibit comparable linear and parallel trends， with G′ consistently outper-forming G′ across the entire frequency spectrum.We can make it clear that the G′ and G″ of the hydrogel Under constant amplitude strain， the G′ is always greater than G″， confirming its elastic behavior over its viscous nature. Simul taneously，the approximate values of tan δ for the hydrogel system in the two modes are 01 and 05 respectively， further emp-hasizing its elastic qualities.However， when the frequency is greater than 20 Hz， G′， G″， and tan δ of the hydrogel displayed marked instability， suggesting a relatively soft nature of the material.Furthermore， the viscosity and mechanical properties can be simply changing the ratio of Me10Q［5］ and DAF·HCl or their concentrations in their respective solutions.

Additionally， hydrogels prepared by non-covalent interactions often exhibit frequency dependence due to the limited cross-link lifetime. The frequency scan results that in the angular frequency range of 0.1-100 Hz （as depicted in Fig.3（b））， G′ prevails， suggesting a gel-like structure characterized by enduring cross-linkages.

2.2Optical properties

Prepare a Me10Q［5］ solution with a concentration of 10-3mol/L and a DAF·HCl solution with a concentration of 10-5mol/L using ultrapure water. Subsequently， take 3 mL of the DAF·HCl solution and gradually add the Me10Q［5］ solution to prepare a series of solutions with various substance ratios. Dilute the mixtures to volume with ultrapure water and leave them at room temperature for 20 mins. Finally， use Agilent 8453 UV-visible spectrophotometer to measure its UV absorption spectrum.

The interaction between Me10Q[5] and DAF·HCl was investigated by UV-visible absorption spectroscopy. As shown in Fig.4， as the concentration of Me10Q[5] increases， the absorption intensity of the DAF·HCl decreases at 287 nm while increases at 358 nm.

Prepare the Me10Q[5] solution with a concen-tration of 10-2 mol/L and the DAF·HCl solution with a concentration of 10-4 mol/L using ultrapure water. Then， gradually add DAF·HCl solution to Me10Q[5] solution to prepare a series of solutions with varying substance ratios. Dilute the solutions to volume with ultrapure water and leave them at room temperature for 20 mins. Subsequently， utilizing the Cary Eclipse fluorescence spectrometer with an excitation wavelength of 289 nm， an excitation slit of 5 nm， and an emission slit of 5 nm， measure the fluorescence emission spectra.

The interaction between Me10Q［5］ and DAF·HCl was studied by fluorescence spectrum. As shown in Fig.5， with the increase of Me10Q［5］， the intensity of the absorption wavelength at 426 nm of the DAF·HCl gradually increases and is accompanied by partial blue shift， which indicating the interaction between the two. Therefore， we hypothesized that the fluorescence was enhanced because the Me10Q［5］ reduced the aggregation of the DAF·HCl.

2.3Formation mechanism

The guest molecule DAF·HCl was prepared at a concentration of 10-2mol/L to measure its 1H NMR spectrum. On this basis， a certain amount of Me10Q［5］ was gradually added dropwise， and the 1H NMR titration spectrums were measured. The 1H NMR spectrums were measured by JEOL JNM-ECZ400s MHz NMR spectrometer at 25 ℃ with deuterated water （D2O） serving as the solvent for the determination of 1H NMR spectrometry.

Based on the data rom the 1H NMR titration spectra， insight into the structural characteristics of the interaction between Me10Q［5］ and the guest molecule can be inferred. As shown in the Fig.6， the proton resonances of the three groups in the DAF·HCl undergo significant downfield shifts with the progressive addition of the Me10Q［5］ solution. This shift indicates that the free guest molecules interact with the port of the Me10Q［5］ rather than its inner cavity. Concurrently， the peak intensities of DAF·HCl decreases gradually with the increase of the Me10Q［5］ until it is very weak， indicating the formation of supramolecular polymer.The DAF·HCl guest molecule was prepared at a concent-ration of 10-2 mol/L to measure its 1H NMR spectrum. On this basis， a certain amount of Me10Q［5］ was gradually added dropwise， and the 1H NMR titration spectrums were measured. The 1H NMR spectrums were measured by JEOL JNM-ECZ400s MHz NMR spectrometer at 25 ℃. Deuterated water（D2O） was used as solvent for the determination of 1H NMR spectrometry.

Based on the information of the 1H NMR titration spectrums， the structural characteristics of the inter-action between Me10Q［5］ and the guest molecule can be inferred. As shown in the Fig.6， the protons formants of the three groups in the DAF·HCl moved significantly to the lower field with the continuous increase of the Me10Q［5］ solution. This indicates that the free guest molecules interact with the port of the Me10Q［5］， and do not enter the inner cavity of the Me10Q［5］. At the same time， the characteristic peak height of the DAF·HCl decreases gradually with the increase of the Me10Q［5］until it is very weak， indicating the formation of supramolecular polymer.

The formation of supramolecular assemblies in aqueous solution was detected by dynamic light scattering （DLS）. DAF·HCl and Me10Q［5］were both prepared in ultra-pure water with a concentration of 1.00×10-3 mol/L， diluted， and mixed in a 1∶1 molar ratio， and then analyzedusing dynamic light scatterer. As shown in the Fig.7， the hydrodynamic diameter was about 1 100 nm. These results showed that an aggregation structure was formed between DAF·HCl and Me10Q［5］， which provided indirect evidence for the formation of supramolecular self-assembly structure between the two components.

Slowly add 1.3 mL of 1.0× 10-4 mol/L DAF·HCl solution to the sample pool to prevent blisters. Absorb 250 μL 2.0× 10-3 mol/L Me10Q［5］ solution in the titration needle， set the system temperature at 25 ℃. The Me10Q［5］ solution is titrated every 300 s， 8 μL each time， and 30 times without interruption. Before performing the isothermal titration assay， all the solutions were ultrasonic degassed with an ultrasonic machine. Finally， as shown in the Fig.8， Nano software was used for computer simulation（curve fitting） to obtain the relevant data.According to the analysis of experimental date， the information of the assembly process is as follows： Ka=2.597×104L/mol， ΔG=-25.20 kJ/mol，ΔH=-11.50 kJ/mol，ΔS=45.96 J/mol·K. All values are consistent with the formation of a stable complex in aqueous solution.

The field emission scanning electron microscopy （SEM， HITACHIS-4800） was used to characterize the micromorphology of the hydrogel. The prepared hydrogel was dried in a freeze-dryer for 48 h， and the dried powder was placed on the surface of the conductive adhesive. After gold spraying treatment， the morphology of the hydrogel was characterized. As shown in the Fig.9， the SEM images showed that the hydrogel had a porous cluster structure， which further proved that the hydrogel was formed by the self-assemble of the Me10Q［5］ and the DAF·HCl.

3Conclusion

In summary， this paper uses a simple method to fabricatethe Me10Q［5］/ DAF·HCl supramolecular hydrogel， followed by an investigation of the properties of hydrogel， such as micromorphology， mechanical performance， etc. Me10Q［5］ and DAF·HCl can form supramolecular chains through hydrogen bonding and ion dipole interaction in aqueous medium， leading to the formation of a three-dimensional network hydrogel. Rheological analysis of supramolecular hydrogels revealed the hydrogel’s transparency， softness， and susceptibility to external forces. The SEM image shows a dense 3D cross-linked network， exhibiting a porous cluster structure. Fluorescence spectroscopy， nuclear magnetic resonance， dynamic light scattering and other methods have further shown that the Me10Q［5］ and DAF·HCl form a supramolecular polymer through exclusion interaction. The Q［n］s-based supramolecular hydrogels have broad research prospects. This simple preparation method provides a direction for the development and application expansion of the Q［n］s， and enriches the construction methods of supramolecular hydrogels.

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（責(zé)任編輯：周曉南）

收稿日期：2024-04-12

基金項(xiàng)目：貴州省省級(jí)科技計(jì)劃資助項(xiàng)目（黔科合基礎(chǔ)-ZK[2022]一般049）

作者簡(jiǎn)介：郭漢靈（1994—），男，在讀博士，研究方向：超分子化學(xué)，E-mail：g996934139@163.com.

*通訊作者：高瑞晗，E-mail：gzugao@126.com.

Me10Q［5］/2，7-二氨基芴二鹽酸鹽超分子水凝膠的構(gòu)建與性能研究

郭漢靈" 高瑞晗*" 陶朱

摘要：

超分子水凝膠由于非共價(jià)鍵的瞬時(shí)和可逆性，與傳統(tǒng)聚合物凝膠相比，具有良好的可逆性和刺激響應(yīng)性。利用Me10Q［5］與2，7-二氨基芴二鹽酸鹽之間的端口作用構(gòu)筑了瓜環(huán)基水凝膠體系，采用流變儀、核磁共振、掃描電鏡等多種方法研究了水凝膠的組成、結(jié)構(gòu)和性能。結(jié)果顯示：這種易于合成，低成本的水凝膠制備方法有利于后續(xù)制備更實(shí)用、更具有市場(chǎng)可行性的瓜環(huán)基水凝膠。

關(guān)鍵詞：甲基五元瓜環(huán)；2，7-二氨基芴二鹽酸鹽；超分子水凝膠；主客體端口作用