Robust multi-responsive supramolecular hydrogel based on a mono-component host-guest gelator

A quadruple-stimuli responsive supramolecular hydrogel constructed from a poly(acrylic acid) derivative and β-cyclodextrin dimer

The fabrication of stimulus-responsive supramolecular hydrogels as smart materials has attracted much attention in recent years. However, the multi-stimuli responsiveness often requires complicated chemical synthesis and rational molecular design. Herein, a quadruple-stimuli responsive supramolecular hydrogel was designed through the host-guest interaction between a β-CD dimer and a methoxy-azobenzene (mAzo) and ferrocene (Fc) grafted poly(acrylic acid) derivative, as well as through the electrostatic interaction of negatively charged carboxyl side groups. Owing to the dynamic properties of the host-guest and electrostatic interactions, reversible sol-gel transition can be triggered by various stimuli, including temperature, light irradiations, pH changes and chemical redox reagents. As a result, the release of rhodamine B loaded in the hydrogel can be accelerated by green light irradiation, oxidizing agents and low pH, demonstrating potential applications in biomedical materials.

Extending Conjugation of Linear Cyanostilbene Derivatives via a Pyridine Moiety for Multi-Stimuli-Responsive Fluorescence Organogels

In the design of effective gelators with aggregation-induced emission behavior, amide and cholesterol moieties are generally used to provide multiple driving forces for gelation. In this work, a series of linear cyanostilbene gelators with a pyridine moiety and different lengths of the alkoxyl group, i.e., CSpy-Cn, have been synthesized through nucleophilic substitution and Knoevenagel reaction. The direct connection of pyridine extends the conjugation of the cyanostilbene moiety, while the alkoxyl group can regulate the solubility of the compounds so that the compounds can serve as gelators for common solvents such as acetonitrile, dimethyl sulfoxide, and ethanol at ultra-low concentrations. At the same time, the cyanostilbene group makes the compounds undergo photoisomerization and emit fluorescence under UV light, while the pyridine group can serve as an acid-base responsive group due to easy protonation. The gels can respond to temperature, light, and organic acid/base. The fluorescence intensity and color can reversibly change during the gel-sol transitions. Finally, a thin film based on the CSpy-C8 xerogel has been prepared and utilized as a multi-stimuli-responsive fluorescence display for information storage and anti-counterfeiting.

Balancing Compatibility and Gelability for High-Performance Cholesteric Liquid Crystalline Physical Gels

Liquid crystalline physical gels (LCPGs) have attracted increasing interest because of their mechanical properties and stimulus-response behaviors. However, due to their gelator properties such as thermal stability, gelation capability, and compatibility in liquid crystals, development of LCPGs with high performances still remains a huge challenging task. Herein, four novel gelators ((l)-PH, (d)-PH, (l)-P2H, and (d)-P2H) based on 1,4-benzenedicarboxamide phenylalanine derivatives containing one or two ethylene glycol groups have been designed and synthesized. It is found that the ethylene glycol group plays a significant role in improving the compatibility between the gelator and the liquid crystal. All of the prepared compounds can form stable LCPGs in P0616A. In particular, the storage modulus of LCPG with 9.0 wt % of (l)-PH with one ethylene glycol unit is higher than 10⁶ Pa, which is similar to SmC gels and advantageous over previously reported nematic LCPGs. Furthermore, the prepared gels display a strong Cotton effect with hand-preferred twisted fiber networks and the self-assembled aggregates of (l)-PH can induce P0616A to form a cholesteric fingerprint structure. Thus, these low molecular weight gelators provide a strategy to construct high-performance cholesteric LCPGs for the realization of LC device applications.

Does Supramolecular Gelation Require an External Trigger?

The supramolecular gelation of small molecules is typically preceded by an external stimulus to trigger the self-assembly. The need for this trigger stems from the metastable nature of most supramolecular gels and can limit their applicability. Herein, we present a small urea-based molecule that spontaneously forms a stable hydrogel by simple mixing without the addition of an external trigger. Single particle tracking experiments and observations made from scanning electron microscopy indicated that triggerless gelation occurred in a similar fashion as the archetypical heat-triggered gelation. These results could stimulate the search for other supramolecular hydrogels that can be obtained by simple mixing. Furthermore, the mechanism of the heat-triggered supramolecular gelation was elucidated by a combination of molecular dynamics simulations and quantitative NMR experiments. Surprisingly, hydrogelation seemingly occurs via a stepwise self-assembly in which spherical nanoparticles mature into an entangled fibrillary network.

Configuration-Dependent Liquid Crystal and Gel Behaviors of Tetraphenylethene-Containing Main-Chain Copolyesters

The construction of aggregation-induced emission-active (AIE-active) gelators with liquid crystal properties remains a challenge. Moreover, it is still unclear for the effect of AIE configuration on liquid crystal, gel and AIE behaviors in one system. Herein, two main-chain liquid crystalline copolyester gelators with a single configuration of AIEgen TPE, mesogen biphenyl and pendent amide groups have been synthesized through melt polycondensation. Both copolyesters display smectic phase, while E-P20 possesses a wider temperature range of liquid crystal and a narrower layer distance owing to the more serious nonlinear "defect" of Z-TPE than E-TPE units. In addition, E-P20 and Z-P20 can form AIE-active gels with the minimum gelation concentration (MGC) values of 10 wt% and 4 wt% in ethyl acetate mainly via hydrogen bond between the pendent amide groups, respectively. These AIE-active gels show potential applications in temperature sensor, information storage, and so on. This article is protected by copyright. All rights reserved.

Construction of a pH-sensitive self-assembly in aqueous solutions based on a dansyl-modified β-cyclodextrin

Here we present a pH-responsive self-assembly based on a β-cyclodextrin (β-CD) derivative bearing a dansyl terminus (βCD-C₆-Dns). Vesicular structures were formed over the entire studied pH range (8.5-0.7); however, the molecular configuration and packing within the vesicles were different at different pH values. Intramolecular host-guest complexation occurred mainly between the dansyl group and β-CD at pH values where the dansyl group was not protonated. The alkyl chain also acted as a competitive guest to form host-guest inclusions as confirmed by 2D ¹H NMR measurements. The pH-responsive βCD-C₆-Dns vesicles have potential application prospects in pH-controlled drug release based on the low cytotoxicity of βCD-C₆-Dns.

Construction of a hydroxide responsive C3-symmetric supramolecular gel for controlled release of small molecules

A C3-symmetric acylhydrazone-based low molecular weight gelator (BHTP) bearing three pyridine units was synthesized and it was found to form a stable supramolecular gel in the mixture solvent of DMSO-H2O. The morphology of the gel as observed by FE-SEM showed a dense sheet structure. Hydrogen bonding and π-π stacking between the gelators were determined as the non-covalent interactions for the gelation, which were investigated thoroughly using XRD, UV-Vis, 1H NMR and FT-IR instruments. BHTP could form pH tolerant supramolecular gels in the widest range of pH values from 1 to 11. The DMSO-H2O (v : v = 1 : 1) gel exhibited selective response to OH- over a series of other anions through the color change from a white gel to a yellow solution, and the OH- response mechanism was proved by 1H NMR experiments. In solution, the lowest detection limit of BHTP for OH- was calculated to be as low as 1.62 × 10-7 M via UV-Vis titration experiments. Finally, encapsulation and controlled release of small molecules such as rhodamine B, crystal violet and methyl orange have been successfully carried out, demonstrating the potential for drug delivery application of this C3-symmetric supramolecular gel. This work opens a novel avenue for the preparation of supramolecular gel-based multiple functional smart materials.

Light regulation and long-lived stability of RGB colors in cholesteric liquid crystal physical gels via a mixing strategy

Photo-responsive cholesteric liquid crystals (CLCs) have attracted much attention due to the dynamic tunability of their unique helical superstructure. However, it is still a challenge to endow the mechanical properties and to regulate the reflection colors at the same time. In this work, a simple strategy is developed for the construction of thermo-responsive CLC physical gels via the direct mixing of photo-responsive dopants and a gelator with nematic LCs. The reflection colors of CLCs and the mechanical properties of gels can be independently regulated due to the separation of the photo-responsive chiral group from the gelator. In addition, the CLC reflection colors can be regulated via visible light in the range of RGB with long-lived thermal stability. Finally, the information storage properties of this kind of CLC gel have been investigated.

Multi-stimuli-responsive hydrogels and their medical applications

This review highlights the medical applications of multi-stimuli-responsive hydrogels as self-healing hydrogels, antibacterial materials and drug-delivery systems.

Self-healing supramolecular hydrogels through host-guest interaction between cyclodextrin and carborane

New self-healing hydrogels based on the strong host-guest interaction of carborane (CB) and β-cylcodextrin (CD) were constructed through CB-grafted dextran and β-CD-grafted poly(acrylic acid). The storage modulus of the hydrogels could reach as high as 10 kPa, and the hydrogels exhibited an outstanding self-healing rate in minutes.

Intrinsically Visible Light-Responsive Liquid Crystalline Physical Gels Driven by a Halogen Bond

Photoresponsive physical gels using liquid crystals (LCs) as solvents have attracted great interest owing to their potential applications. But, current investigations mainly focus on UV light, which is not environment-friendly enough. On the other hand, the halogen bond is a novel tool for constructing supramolecular gels because of good hydrophobicity, high directionality, tunable strength, and large size of halogen atoms. Herein, to construct an LC physical gel with both the advantages of a halogen bond and visible light response, azopyridine-containing Azopy-C₁₀ is chosen as a halogen bond acceptor, while 1,2-bis(2,3,5,6-tetrafluoro-4-iodophenyl)diazene is selected both as the halogen bond donor and for the intrinsically visible light response. Such a binary gelator can self-assemble in the anisotropic solvent of nematic LC 5CB to form an LC physical gel. It experiences the gel-to-sol transition by green light irradiation. As the gelator concentration increases, the saturation voltage increases, but the switch-off time decreases. The combination of the halogen bond and controllable visible light-responsive LC physical gel provides the feasibilities of manipulating these smart soft materials.

Wholly Visible-Light-Responsive Host-Guest Supramolecular Gels Based on Methoxy Azobenzene and β-Cyclodextrin Dimers

Much attention has been paid to construct photoresponsive host-guest supramolecular gels; however, red-shifting the responsive wavelength remains a formidable challenge. Here, a wholly visible-light-responsive supramolecular gel was fabricated through the host-guest interaction between a β-cyclodextrin (β-CD) dimer and a tetra-ortho-methoxy-substituted azobenzene (mAzo) dimer (binary gelator) in DMSO/H₂O (V/V = 8/2). The minimum gelation concentration of the low-molecular-weight binary gelator was 6 wt % measured via the tube inversion method. The substituted methoxy groups shifted the responsive wavelengths of trans-mAzo and cis-mAzo to the green and blue light regions, respectively. The host-guest interaction between mAzo and β-CD as the driving force for gelation was confirmed using the ¹H-NMR and 2D ¹H NOESY spectra. The supramolecular gel showed good self-supporting ability with a storage modulus higher than 10⁴ Pa. The release of Rhodamine B loaded in the gel as a model drug could be controlled by green light irradiation. We envisioned the potential applications of the wholly visible-light-responsive supramolecular compounds ranging from biomedical materials to smart materials.

Fabrication of Hydrogels via Host-Guest Polymers as Highly Efficient Organic Dye Adsorbents for Wastewater Treatment

New self-assembled hydrogel materials of poly(vinyl alcohol)/cyclodextrin-modified poly(acrylic acid)/azobenzene-modified poly(acrylic acid) (PVA/PAA-CD/PAA-Azo) were successfully prepared via host-guest interactions and hydrogen bonds. The as-prepared hydrogel materials were characterized by various techniques, including Fourier transform infrared spectroscopy, X-ray diffraction analysis, scanning electron microscopy, ultraviolet spectroscopy, and specific surface area tests. The prepared hydrogels with different concentrations of PVA exhibited different network structures. In addition, ultraviolet (UV) light irradiation and temperature change induce a gel-sol phase transition in the hydrogel materials. The obtained hydrogel materials could be used as good adsorbents for two model organic dye molecules, which was mainly due to electrostatic interactions between methylene blue/rhodamine B (MB/RhB) and the gels in the adsorption process. In particular, the adsorption processes of the as-prepared hydrogel materials conformed to the pseudo-first-order model with a high correlation coefficient, which indicates that gel has a potential application in the field of wastewater purification.

Reversible photo-responsive gel-sol transitions of robust organogels based on an azobenzene-containing main-chain liquid crystalline polymer

Stimuli-responsive supramolecular gels have been widely investigated, but the construction of a liquid crystalline gel with a high mechanical property and reversible photo-response still remains a challenge. This is due to the difficulty of designing gelators with liquid crystal properties and gelation abilities in organic solvents simultaneously. In this study, an azobenzene-containing main-chain polyester (Azo-mLCP) with a pendant amide group was synthesized. The organogel of Azo-mLCP via a hydrogen bond in dioxane possessed reversible thermal- and photo-responsive behaviours. The organogel exhibited a good self-supporting ability when the concentration of the gelator was more than 7.5 wt%. The rapid trans-to-cis isomerization of Azo-mLCP in solution was studied via UV-Vis absorption spectra. In addition, the gel-to-sol transition of the organogel could be triggered efficiently by an incomplete trans-to-cis conversion strategy. This study opens a way for the main-chain liquid crystalline polymers to serve in potential applications in photo-responsive robust actuators, electro-optical devices, and so on.

Gel Formed by Self-Assembly of a Urea-Modified Monopyrrolotetrathiafulvalene Derivative Displays Multi-Stimuli Responsiveness and Absorption of Rhodamine B

A new monopyrrolotetrathiafulvalene-based derivative containing a urea group was designed, synthesized and thoroughly characterized. It proved to be a non-gelator in a single solvent even when heated or sonicated. However, it could self-assemble in a CHCl₃ (CH₂ Cl₂ )/n-hexane mixture to form a thermo-responsive supramolecular organogel. SEM, FT-IR spectroscopy, UV/Vis absorption spectroscopy, and SAXS revealed that in the organogel system, the gelators self-assembled into supramolecular networks with a J-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen-bonding, and van der Waals forces. Interestingly, the gel phase was shown to undergo reversible gel-sol transformation induced by Fe³⁺ -Vitamin C (Vc), trifluoroacetic acid-triethylamine (TFA-TEA) and picric acid (PA)-NaOH. In particular, in the presence of picric acid, the experimental results proposed that charge transfer occurred from the electron-donor gelator to the electron-acceptor picric acid due to the possibility of complex formation. Furthermore, the formed organogel could behave as the matrix for encapsulating cationic fluorescent dye from wastewater, and the adsorption efficiency was directly proportional to the concentration of the gelator.

A tetrathiafulvalene–l-glutamine conjugated derivative as a supramolecular gelator for embedded C60 and absorbed rhodamine B

An l-glutamine-containing tetrathiafulvalene gelator could form charge-transfer complex gels in the presence of C₆₀, and also the native gel exhibited excellent absorption properties for the removal of rhodamine B from aqueous solution.

Visible light-triggered gel-to-sol transition in halogen-bond-based supramolecules

Photoresponsive supramolecular gels have aroused continuous attention because of their extensive applications; however, most studies utilize UV light, which inevitably brings about some health and environmental issues. The halogen bond is an important driving force for constructing supramolecules due to its high directionality, tunable strength, good hydrophobicity, and large size of the halogen atoms. Yet, it still remains a formidable challenge to utilize halogen bonds as a driving force to fabricate a visible light responsive gel. In this work, to fabricate such a gel, azopyridine-containing Azopy-C_n (n = 8, 10, 12) was selected as a halogen bond acceptor, while 1,2-bis(2,3,5,6-tetrafluoro-4-iodophenyl)diazene (BTFIPD) was chosen as both the halogen bond donor and visible light responsive moiety. The visible light response of BTFIPD resulted from the significant separation of n-π* energy levels between trans and cis isomers due to the introduction of an electron-withdrawing group (fluorine) to azobenzene at the ortho-position. Interestingly, the gel exhibited a good gel-to-sol transition behavior upon green light irradiation. At the same time, the morphologies varied from uniform narrow flakes to broad sheets with increasing illumination time. We provide an environmentally-friendly visible light-triggered method to regulate the phase transition of supramolecular materials in applications ranging from energy conversion to information storage.

Facile preparation of self-assembled hydrogels constructed from poly-cyclodextrin and poly-adamantane as highly selective adsorbents for wastewater treatment

Self-assembled hydrogel materials constructed from cyclodextrin polymer (P-CD)/adamantane-modified poly acrylic acid (PAA-Ad) were designed and prepared via host-guest interactions. It was observed that the prepared supramolecular hydrogels had an interconnected three-dimensional porous network. In addition, the obtained hydrogels showed a recovery performance and it was confirmed that the host-guest interactions between β-cyclodextrin and adamantane were the main driving force for the formation of the hydrogels. The mechanical properties of the hydrogels could be adjusted by varying the concentrations of PAA-Ad. In particular, the prepared supramolecular hydrogels exhibited superior performances in water purification. The results demonstrated that the hydrogels possessed different mechanisms in the adsorption of the four typical poisonous organic dye molecules used, including bisphenol A (BPA), 4-aminoazobenzene (N-Azo), methylene blue (MB), and rhodamine B (RhB). The hydrogels mainly adsorbed N-Azo by host-guest interaction and adsorbed BPA by host-guest interaction and hydrogen bond synergy. They also adsorbed MB and RhB by hydrogen bonding and electrostatic interaction.

An injectable self-healing hydrogel-cellulose nanocrystals conjugate with excellent mechanical strength and good biocompatibility

In this work, a novel strategy for the construction of injectable self-healing nanocomposite (NC) hydrogels dominated by reversible boronic ester bonds was demonstrated. Specifically, NC hydrogels were constructed by the solution-mixing of N,N-dimethylacrylamide-stat-3-acrylamidophenylboronicacid statistical copolymers (PDMA-stat-PAPBA) and poly(glycerolmonomethacrylate) (PGMA) chains grafted cellulose nanocrystals (CNC-g-PGMA). Rheology analysis indicated the as-constructed NC hydrogel displayed about 7-fold increase in the storage modulus with a low CNCs loading level of 1.43 wt% in comparison with PGMA/PDMA-stat-PAPBA hydrogel without CNCs. Furthermore, the mechanical strength of the CNC-g-PGMA/PDMA-stat-PAPBA hydrogel was far superior to that of its PGMA/PDMA-stat-PAPBA/CNCs hydrogel counterpart, in which PGMA chains were not covalently grafted on the surfaces of CNCs. Due to reversible boronic ester bonds cross-linking networks, CNC-g-PGMA/PDMA-stat-PAPBA NC hydrogel exhibited excellent self-healing and injectable properties as well as pH/glucose responsive sol-gel transitions. Good biocompatibility was also demonstrated through in vitro cytotoxicity tests.

Construction of a continuously layered structure of h-BN nanosheets in the liquid phase via sonication-induced gelation to achieve low friction and wear

Herein, to endow h-BN nanosheets with gelling ability, a diurea compound was decorated on the h-BN nanosheets via designed adsorption and in situ reaction processes. The prepared h-BN-based gelator, BTO, exhibited excellent dispersibility in non-polar liquid media, and the gelation of BTO dispersions could be readily triggered by ultrasonic treatments. The sol-gel transformation of the system was found to be highly reversible by stirring and sonication. Based on the investigation on the self-assembly behavior of BTO nanosheets in the liquid phase, it was proposed that a continuous and layered structure formed by BTO during sonication was the key factor for the gelling properties of these nanosheets. The viscoelasticity of the sonication-induced gel was studied using a rheometer. Tribological evaluations show that the prepared h-BN nanogel exhibits outstanding lubricating performances, and more importantly, it has been proved that the gel state of the h-BN nanosheets provides superior and more reliable lubricating performances than the corresponding dispersion state under certain conditions; this can be ascribed to the formation of a continuous and uniform structure of modified h-BN nanosheets during gelation. Thus, this study not only clarifies the key role of the assembly structure in the tribological performances of 2D nanomaterials, but also demonstrates the potential of gelation in improving the macroscopic friction reduction and wear resistance of 2D nanomaterials.

Multiple modulations for supramolecular hydrogels of bola-form surfactants bearing rigid and flexible groups

A series of bola-form surfactants with two identical azobenzene ends separated by a flexible chain but different cationic heads were synthesized. These amphiphilic molecules exhibited rich self-assembly properties in aqueous solutions. The physical characterizations demonstrated that the cationic heads showed a decisive influence on both the gelation behavior and the gel strength. The surfactant with a trimethylammonium head group did not form a hydrogel even at a higher concentration, yet it promoted the gelation of pyridinium and methylimidazolium head bearing surfactants, which formed hydrogels but could not boost each other's gelation capability. Besides the heating induced reversible gel-sol transition, the UV light irradiation also triggered the change of gel to sol while the irradiation with visible light led to a reverse process. Through the addition of cyclodextrins, the hydrogels transformed into sols, while the sols reverted to gels accompanied by the addition of competitive guest molecules. The surfactant hydrogels displayed a stimulus-response to both anionic and cationic additives based on different mechanisms, and reverse order of addition of the external guest molecules and host cyclodextrins also resulted in the modulation of the gel-sol transition. By the combination of photo- and chemical-stimuli, multiple responses on the transformations between the gel and sol were realized.

Photo-adaptable shape memory hydrogels based on orthogonal supramolecular interactions

A novel shape memory hydrogel with photo-adaptable permanent shape has been developed on the basis of alginate–Ca²⁺ coordination and the host–guest interaction between α-cyclodextrin and azobenzene.

Stimuli-Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

Supramolecular hydrogels are a class of self-assembled network structures formed via non-covalent interactions of the hydrogelators. These hydrogels capable of responding to external stimuli are considered to be smart materials due to their ability to undergo sol-gel and/or gel-sol transition upon subtle changes in their surroundings. Such stimuli-responsive hydrogels are intriguing biomaterials with applications in tissue engineering, delivery of cells and drugs, modulating tissue environment to promote innate tissue repair, and imaging for medical diagnostics among others. This review summarizes the recent developments in stimuli-responsive supramolecular hydrogels and their potential applications in regenerative medicine. Specifically, various structural aspects of supramolecular hydrogelators involved in self-assembly, the role of external stimuli in tuning/controlling their phase transitions, and how these functions could be harnessed to advance applications in regenerative medicine are focused on. Finally, the key challenges and future prospects for these versatile materials are briefly described.

Chirality-Enabled Liquid Crystalline Physical Gels with High Modulus but Low Driving Voltage

Self-supporting liquid crystalline physical gels with facile electro-optic response are highly desirable, but their development is challenging because both the storage modulus and driving voltage increase simultaneously with gelator loading. Herein, we report liquid crystalline physical gels with high modulus but low driving voltage. This behavior is enabled by chirality transfer from the molecular level to three-dimensional fibrous networks during the self-assembly of 1,4-benzenedicarboxamide phenylalanine derivatives. Interestingly, the critical gel concentration is as low as 0.1 wt %. Our findings open doors to understanding and exploiting the role of chirality in organic gels.

Cyclodextrin-based host–guest supramolecular hydrogel and its application in biomedical fields

CD-based host–guest supramolecular hydrogels and their potential biomedical application.