A multiresponsive, shape-persistent, and elastic supramolecular polymer network gel constructed by orthogonal self-assembly

Strong, tough and mechanically self-recoverable poly(vinyl alcohol)/alginate dual-physical double-network hydrogels with large cross-link density contrast

Strong and tough poly(vinyl alcohol) (PVA)/alginate hydrogen-bonded-ionic dual-physical double-network (DN) hydrogels have been successfully prepared by a facile route of a freeze–thaw (25–25–25 °C) cycle followed by concentrated (1.0 mol L⁻¹ of) aqueous-Ca²⁺ immersion of PVA/Na alginate (SA) mixed aqueous solutions. It was found that, at mole ratios of the PVA- to SA repeat units of 20/1 to 80/1, the DN gels likely evolved a semi-interpenetrating polymer network (IPN) morphology of rigid alginate networks dispersed in while interlocking with ductile PVA network to accomplish DN synergy that gave their high strength and toughness, where the high alginate rigidity originated probably from its dense cross-link induced syneresis and dispersion along crosslink-defective voids to result in little internal stress concentration. Tentatively mechanistically, as the 20/1–80/1 DN gels were stretched steadily, their mechanical response was gradually differentiated into distinct synergistic states: the sparsely hydrogen-bonded PVA served as a ductile matrix to bear small fractions of the established stresses at its large elongations; whereas the densely ionically (i.e. Ca²⁺) cross-linked alginate functioned as a rigid skeleton to sustain the remaining larger stresses upon its smaller local strains. Promisingly, this ductile-rigid matrix-skeleton synergistic mechanism of semi-IPN morphology may be universally extended to all A/B DN hydrogels of large A–B rigidity (or cross-link density) contrast, whether the cross-link nature of network(s) A or B is covalent, ionic, hydrogen bonded or van der Waals interacted. The strong and tough DN gels also displayed satisfactory self-recovery of viscoelastic behaviour, in that their Young's modulus and dissipated energy in the uniaxial tensile mode and dynamic storage and loss moduli in the oscillatory shear mode all recovered significantly from non-linear viscoelastic regimes despite different degrees of failure to revert to (quasi)linear viscoelasticity.

Intermediate compositions enable synergised, strong-and-tough dual-physical double-network hydrogels of dispersion-induced rigid, ionic alginate-networks interlocking with global ductile, hydrogen-bonded poly(vinyl alcohol)-network.

Chirality control of multi-stimuli responsive and self-healing supramolecular metallo-hydrogels

The spontaneous formation of supramolecular metallo-hydrogels showing multi stimuli-responsiveness and intrinsic self-healing properties upon the interaction of chiral-histidine with Zn²⁺ ions.

A crown-ether-based moldable supramolecular gel with unusual mechanical properties and controllable electrical conductivity prepared by cation-mediated cross-linking

Supramolecular gels that possess high mechanical properties and unusual electrical conductivity were prepared by incorporating Cs⁺.

Proton triggered circularly polarized luminescence in orthogonal- and co-assemblies of chiral gelators with achiral perylene bisimide

Acid–base exposure switched circularly polarized luminescence was achieved in a coassembled gel which is composed of a chiral gelator and achiral perylene bisimide.

A thermoresponsive supramolecular polymer gel from a heteroditopic calix[4]pyrrole

Anion recognition was utilized on heteroditopic calix[4]pyrrole host–guest systems to obtain a thermoresponsive supramolecular polymer gel.

A dual-responsive cross-linked supramolecular polymer network gel: hierarchical supramolecular self-assembly driven by pillararene-based molecular recognition and metal–ligand interactions

A novel 2,2′-bipyridine-bridged pillar[5]arene dimer was synthesized and further applied for constructing linear supramolecular polymers and supramolecular polymer network gels.

Helicity Inversion of Supramolecular Hydrogels Induced by Achiral Substituents

Probing the supramolecular chirality of assemblies and controlling their handedness are closely related to the origin of chirality at the supramolecular level and the development of smart materials with desired handedness. However, it remains unclear how achiral residues covalently bonded to chiral amino acids can function in the chirality inversion of supramolecular assemblies. Herein, we report macroscopic chirality and dynamic manipulation of chiroptical activity of hydrogels self-assembled from phenylalanine derivatives, together with the inversion of their handedness achieved solely by exchanging achiral substituents between oligo(ethylene glycol) and carboxylic acid groups. This helicity inversion is mainly induced by distinct stacking mode of the self-assembled building blocks, as collectively confirmed by scanning electron microscopy, circular dichroism, crystallography, and molecular dynamics calculations. Through this straightforward approach, we were able to invert the handedness of helical assemblies by merely exchanging achiral substituents at the terminal of chiral gelators. This work not only presents a feasible strategy to achieve the handedness inversion of helical nanostructures for better understanding of chiral self-assembly process in supramolecular chemistry but also facilities the development of smart materials with controllable handedness in materials science.

Self-assembly of water-soluble silver nanoclusters: superstructure formation and morphological evolution

Supramolecular self-assembly, based on non-covalent interactions, has been employed as an efficient approach to obtain various functional materials from nanometer-sized building blocks, in particular, [Ag₆(mna)₆]^6-, mna = mercaptonicotinate (Ag₆-NC). A challenging issue is how to modulate the self-assembly process through regulating the relationship between building blocks and solvents. Herein, we report the controlled self-assembly of hexanuclear silver nanoclusters into robust multilayer vesicles in different solvents, DMSO, CH₃CN, EG and MeOH. Their unique luminescence enables them to be bifunctional probes to sense Fe³⁺ and dl-dithiothreitol (DTT). By protonating the Ag₆-NC to Ag₆-H-NC using hydrochloric acid (HCl), the multilayer vesicles survive in aprotic solvents, DMSO and CH₃CN, but are transformed to nanowires in protic solvents, water, EG and MeOH. Our results demonstrated that the solvent-bridged H-bond plays a key role in the evolution of the morphologies from vesicles to nanowires. Moreover, the nanowires could further hierarchically self-assemble in water into hydrogels with high water content (99.5%), and with remarkable mechanical strength and self-healing properties. This study introduces a robust cluster-based building block in a supramolecular self-assembly system and reveals the significance of aprotic and protic solvents for the modulation of the morphologies of cluster-based aggregates.

Low-Molecular-Weight Supramolecular Ionogel Based on Host-Guest Interaction

Supramolecular ionogels were prepared by self-assembly of small molecules through host-guest interaction between β-cyclodextrin (β-CD) and a room-temperature ionic liquid (IL) 3-(1-methyl-3-imidazolio)propanesulfonate-lithium bis(trifluoromethanesulfonyl)imide (MIPS-LiTFSI) which contains zwitterion MIPS. ¹⁹F NMR and 2D ROESY ¹H NMR have been used to prove that only TFSI^- is involved in the complexation. ¹H NMR, FT-IR, and comparative tests indicated that the electrostatic interaction between imidazole cation and TFSI^- anion and intermolecular hydrogen bonding between three compounds also contribute to the formation of supramolecular ionogel. Ionogels with different gel-sol phase transition temperatures can be obtained by adjusting the molar ratio between β-CD and MIPS-LiTFSI. In addition, the supramolecular ionogels composed of "channel type" structural β-CD have been constructed. The ionogel with high conductivity and low activation energy open a door to new fields for special applications.

A novel thermo-responsive supramolecular organogel based on dual acylhydrazone: fluorescent detection for Al3+ ions

A new dual acylhydrazone-functionalized gelator (L) has been synthesized, which behaves as a thermal-responsive supramolecular organogel (L-gel) in DMSO. This L-gel exhibits very weak fluorescence based on the photoinduced electron transfer (PET) mechanism. The L-gel can recognize Al³⁺ and assemble into an enhanced blue-light-emitting supramolecular metallogel (Al@gel).

Thermosensitive Phase Behavior of Benzo-21-crown-7 and Its Derivatives

For designing water-soluble responsive materials, utilizing crown ethers as main building blocks has been rarely explored in contrast to their linear poly(ethylene glycol) counterparts. In the current study, we report the robust thermoresponsive properties of the benzo-21-crown-7 (B21C7) family with lower critical solution temperature (LCST) and upper critical solution temperature (UCST) behavior. Different substituent groups on the benzene ring exhibit significant effects on water solubility and thermoresponsiveness. B21C7 and its cyano derivative display LCST phenomena, while B21C7-based carboxylic acid derivative presents UCST followed by LCST phase behavior. Supramolecular interactions with KCl provide an additional tuning approach for this crown ether system. These results demonstrate that B21C7s can serve as an easily accessible toolbox to develop new thermosensitive systems and prepare thermally responsive materials.

l-Carnosine-Derived Fmoc-Tripeptides Forming pH-Sensitive and Proteolytically Stable Supramolecular Hydrogels

A series of β-amino acid containing tripeptides has been designed and synthesized in order to develop oligopeptide-based, thermoreversible, pH-sensitive, and proteolytically stable hydrogels. The Fmoc [N-(fluorenyl-9-methoxycarbonyl)]-protected tripeptides were found to produce hydrogels in both pH 7 and 2 buffers at a very low concentration (<0.2% w/v). It has been shown that the Fmoc group plays an important role in the gelation process. Also a dependence of gelation ability on hydrophobicity of the side chain of the Fmoc-protected α-amino acid was observed. The effect of the addition of inorganic salts on the gelation process was investigated as well. Spectroscopic studies indicated formation of J-aggregates through π-π stacking interactions between Fmoc groups in solution as well as in the gel state. In the gel phase, these self-assembling tripeptides form long interconnected nanofibrils leading to the formation of 3-dimensional network structure. The hydrogels were characterized by various techniques, including field emission electron microscopy, transmission electron microscopy, atomic force microscopy, rheology, Fourier transform IR, circular dichroism (CD), and wide-angle X-ray diffraction (WAXD) spectroscopy. The CD studies and WAXD analyses show an antiparallel β-sheet structure in the gel state. l-Phenylalanine and l-tyrosine containing tripeptides formed helical aggregates with handedness opposite to those containing l-valine and l-leucine residues. The mechanical stability of the hydrogels was found to depend on the hydrophobicity of the side chain of the tripeptide as well as on the pH of the solution. Also, the tripeptides exhibit in vitro proteolytic stability against proteinase K enzyme.

Structural water as an essential comonomer in supramolecular polymerization

Although the concept of structural water that is bound inside hydrophobic pockets and helps to stabilize protein structures is well established, water has rarely found a similar role in supramolecular polymers. Water is often used as a solvent for supramolecular polymerization, however without taking the role of a comonomer for the supramolecular polymer structure. We report a low-molecular weight monomer whose supramolecular polymerization is triggered by the incorporation of water. The presence of water molecules as comonomers is essential to the polymerization process. The supramolecular polymeric material exhibits strong adhesion to surfaces, such as glass and paper. It can be used as a water-activated glue, which can be released at higher temperatures and reused many times without losing its performance.

In situ formation of luminescent CdSe QDs in a metallohydrogel: a strategy towards synthesis, isolation, storage and re-dispersion of the QDs

A one step, in situ, room temperature synthesis of yellow luminescent CdSe QDs was achieved in a metallohydrogel derived from a facially amphiphilic bile salt, resulting in a QD-gel hybrid. An ordered self-assembly and homogeneous distribution of the CdSe QDs in the hydrogel network was observed from optical and electron micrographs. The different excited state behavior of the CdSe QDs in the hybrid was revealed for the first time using time resolved spectroscopy. We also describe the successful isolation of the photoluminescent CdSe QDs from the gel followed by their re-dispersion in an organic solvent using suitable capping ligands.

Sequential dynamic structuralisation by in situ production of supramolecular building blocks

We herein report an example of sequential dynamic higher structuralisation from disordered droplets into higher-ordered structures, resulting in the formation of a hydrogel. This is a potential model for understanding complicated biosystems in which lower-ordered structures evolve into higher-ordered structures that interact with specific molecules.

Ultraviolet Patterned Calixarene-Derived Supramolecular Gels and Films with Spatially Resolved Mechanical and Fluorescent Properties

Supramolecular assemblies have in the past been considered mechanically weak and in most cases unable to withstand their own weight. Calixarene-derived networks can, however, provide robust supramolecular gels. Incorporating a photoreactive stilbene moiety, we show that the aggregation state of the material can be tuned by heating and UV exposure in order to control the mechanical as well as the fluorescent properties. Regulating the extent of heating to control the proportion of H-aggregates and J-aggregates and further cross-linking of H-aggregates by control over UV exposure allows for adjustable photopatterning of the fluorescence as well as the material stiffness in the range from ∼100 to 450 kPa. We expect this straightforward supramolecular system will be suitable for advanced prototyping in applications where modulus and shape are important design criteria.

Fluorescent Supramolecular Polymeric Materials

Fluorescent supramolecular polymeric materials are rising stars in the field of fluorescent materials not only because of the inherent optoelectronic properties originating from their chromophores, but also due to the fascinating stimuli-responsiveness and reversibility coming from their noncovalent connections. Especially, these noncovalent connections influence the fluorescence properties of the chromophores because their state of aggregation and energy transfer can be regulated by the assembly-disassembly process. Considering these unique properties, fluorescent supramolecular polymeric materials have facilitated the evolution of new materials useful for applications in fluorescent sensors, probes, as imaging agents in biological systems, light-emitting diodes, and organic electronic devices. In this Review, fluorescent supramolecular polymeric materials are classified depending on the types of main driving forces for supramolecular polymerization, including multiple hydrogen bonding, electrostatic interactions, π-π stacking interactions, metal-coordination, van der Waals interactions and host-guest interactions. Through the summary of the studies about fluorescent supramolecular polymeric materials, the status quo of this research field is assessed. Based on existing challenges, directions for the future development of this field are furnished.

Synthesis of a Cationic Supramolecular Block Copolymer with Covalent and Noncovalent Polymer Blocks for Gene Delivery

The design and fabrication of safe and highly efficient nonviral vectors is the key scientific issue for the achievement of clinical gene therapy. Supramolecular cationic polymers have unique structures and specific functions compared to covalent cationic polymers, such as low cytotoxicity, excellent biodegradability, and smart environmental responsiveness, thereby showing great application prospect for gene therapy. However, supramolecular gene vectors are facile to be degraded under physiological conditions, leading to a significant reduction of gene transfection efficiency. In order to achieve highly efficient gene expression, it is necessary for supramolecular gene vectors being provided with appropriate biostability to overcome various cell obstacles. To this end, a novel cationic supramolecular block copolymer composed of a conventional polymer and a noncovalent polymer was constructed through robust β-cyclodextrin/ferrocene host-guest recognition. The resultant supramolecular block copolymer perfectly combines the advantages of both conventional polymers and supramolecular polymers ranging from structures to functions. This supramolecular copolymer not only has the ability to effectively condense pDNA for enhanced cell uptake, but also releases pDNA inside cancer cells triggered by H₂O₂, which can be utilized as a prospective nonviral delivery vehicle for gene delivery. The block polymer exhibited low cytotoxicity, good biostability, excellent biodegradability, and intelligent responsiveness, ascribing to the dynamic/reversible nature of noncovalent linkages. In vitro studies further illustrated that the supramolecular block polymer exhibited greatly improved gene transfection efficiency in cancer cells. This work offers an alternative platform for the exploitation of smart nonviral vehicles for specific cancer gene therapy in the future.

Fabrication of thermal sensitive folic acid based supramolecular hybrid gels for injectable drug release gels

Thermal sensitive supramolecular hybrid gels for injectable drug release were prepared by adding different amounts of agar into folic acid (FA) gelator. The gelation temperature was modulated in order to form injectable gel with body temperature (37°C). Such kind of folic acid-agar (FAG) hybrid gel makes it possible to use supramolecular gel as injectable drug loaded gels for drug release. FT-IR and UV-vis spectra indicate that agar macromolecules involve in the self-assembly process through intermolecular H-bonding and π-π stacking interactions with FA molecules. The SEM and TEM images demonstrate that the fiber diameter of FAG hybrid gel is about 20nm, much smaller than that of FA gel (40nm). However, FAG hybrid has a denser nano-fibrous network structure than FA gels. Moreover, FAG hybrid gel is endowed with a more ordered network structure and a little better crystallization capability by adding agar. FAG hybrid gel also shows a shear-thinning behavior but the shear viscosity is about 2 times higher than that of FA gel. Compared with FA gel, the storage (G') and loss (G″) moduli of the FAG gel are higher, which implies an enhanced gel strength. At the same time, both FA and FAG gels are facilely affected by some external factors such as acid, base and salts. In acidic or basic conditions, the strength became weak and the gelation temperature (Tg) decreased. While, within certain concentrations, the salt (NaCl) increased the gel strength and Tg. FAG gel suffered lower mass loss and owned better stability in different pH solutions compared with pure FA gel. The release behavior of the FA and injectable FAG gels was investigated by using Rhodamine B as a mimic model drug. FAG hybrid gel shows a long release profile and the release time is 3 times longer than that of FA gel, up to 30h, and the accumulative release amount reaches about 86%. So it is a potential injectable gel for sustained release drug delivery system.

Anion dependent self-assembly of sandwich 13-metal Ni-Ln nanoclusters with a long-chain Schiff base ligand

Two classes of Ni-Ln clusters [Ln₄Ni₃L₃(OAc)₆(NO₃)₃(OH)₃] (Ln = Gd (1) and Tb (2)) and [Ln₆Ni₇L₆(OAc)₁₂(OH)₆](OH)₂ (Ln = Gd (3) and Dy (4)) were prepared using a specifically designed Schiff base ligand built around a flexible (CH₂)₂O(CH₂)₂O(CH₂)₂ chain. 1 and 2 exhibit cone-like structures, while 3 and 4 have nanosized sandwich architectures. The structures were studied by single-crystal X-ray diffraction and TEM, and magnetic properties were investigated.

Dynamic supramolecular self-assembly: hydrogen bonding-induced contraction and extension of functional polymers

A ureido-cytosine-functionalized supramolecular polymer can be manipulated to control nano-scale microstructures and its ability to form long-range order during self-assembly.

Nucleobase-functionalized supramolecular polymer films with tailorable properties and tunable biodegradation rates

Nucleobase-functionalized supramolecular films can directly self-assemble into a non-covalently cross-linked three-dimensional network structure with tuned physical properties and biodegradation rates.

Dual-pH responsive host–guest complexation between a water-soluble pillar[9]arene and a 2,7-diazapyrenium salt

The host–guest complexation between a water-soluble pillar[9]arene and a 2,7-diazapyrenium salt not only can be controlled by the sequential addition of an acid and a base but also can be switched through the sequential addition of a base and an acid.

Probing sub-nano level molecular packing and correlated positron annihilation characteristics of ionic cross-linked chitosan membranes using positron annihilation spectroscopy

Applications of positron annihilation spectroscopy to study the microstructure and correlated positron annihilation characteristics of an ionic cross-linked CS system.

Preparation and investigation of temperature-responsive calix[4]arene-based molecular gels

Higher temperature enhances the strength and the toughness of the gel comprised of kerosene and a tetracholesteryl derivative based on calix[4]arene.

Supramolecular block copolymers for gene delivery: enhancement of transfection efficiency by charge regulation

A charge controlled supramolecular block copolymer exhibits significantly enhanced gene delivery efficacy in cancer cells without sacrificing the biocompatibility.

Multiple noncovalent interaction constructed polymeric supramolecular crystals: recognition of butyl viologen by para-dicyclohexanocucurbit[6]uril and α,α′,δ,δ′-tetramethylcucurbit[6]uril

Binding interactions of butyl viologen (BV⁺) with two kinds of cucurbit[6]uril derivatives were investigated, showing the formation of polymeric supramolecular crystals through multiple noncovalent interactions.