Metal- and Anion-Binding Supramolecular Gels

Metal Cross-Linked Supramolecular Gel Noodles: Structural Insights and Antibacterial Assessment

Achieving precise control over gelator alignment and morphology is crucial for crafting tailored materials and supramolecular structures with distinct properties. We successfully aligned the self-assembled micelles formed by a functionalized dipeptide 2NapFF into long 1-D "gel noodles" by cross-linking with divalent metal chlorides. We identify the most effective cross-linker for alignment, enhancing mechanical stability, and imparting functional properties. Our study shows that Group 2 metal ions are particularly suited for creating mechanically robust yet flexible gel noodles because of their ionic and nondirectional bonding with carboxylate groups. In contrast, the covalent nature and high directional bonds of d-block metal ions with carboxylates tend to disrupt the self-assembly of 2NapFF. Furthermore, the 2NapFF-Cu noodles demonstrated selective antibacterial activity, indicating that the potent antibacterial property of the copper(II) ion is preserved within the cross-linked system. By merging insights into molecular alignment, gel extrusion processing, and integrating specific functionalities, we illustrate how the versatility of dipeptide-based gels can be utilized in creating next-generation soft materials.

Conductive Gels for Energy Storage, Conversion, and Generation: Materials Design Strategies, Properties, and Applications

Gel-based materials have garnered significant interest in recent years, primarily due to their remarkable structural flexibility, ease of modulation, and cost-effective synthesis methodologies. Specifically, polymer-based conductive gels, characterized by their unique conjugated structures incorporating both localized sigma and pi bonds, have emerged as materials of choice for a wide range of applications. These gels demonstrate an exceptional integration of solid and liquid phases within a three-dimensional matrix, further enhanced by the incorporation of conductive nanofillers. This unique composition endows them with a versatility that finds application across a diverse array of fields, including wearable energy devices, health monitoring systems, robotics, and devices designed for interactive human-body integration. The multifunctional nature of gel materials is evidenced by their inherent stretchability, self-healing capabilities, and conductivity (both ionic and electrical), alongside their multidimensional properties. However, the integration of these multidimensional properties into a single gel material, tailored to meet specific mechanical and chemical requirements across various applications, presents a significant challenge. This review aims to shed light on the current advancements in gel materials, with a particular focus on their application in various devices. Additionally, it critically assesses the limitations inherent in current material design strategies and proposes potential avenues for future research, particularly in the realm of conductive gels for energy applications.

Supramolecular Gels Based on C3-Symmetric Amides: Application in Anion-Sensing and Removal of Dyes from Water

We modified C3-symmetric benzene-1,3,5-tris-amide (BTA) by introducing flexible linkers in order to generate an N-centered BTA (N-BTA) molecule. The N-BTA compound formed gels in alcohols and aqueous mixtures of high-polar solvents. Rheological studies showed that the DMSO/water (1:1, v/v) gels were mechanically stronger compared to other gels, and a similar trend was observed for thermal stability. Powder X-ray analysis of the xerogel obtained from various aqueous gels revealed that the packing modes of the gelators in these systems were similar. The stimuli-responsive properties of the N-BTA towards sodium/potassium salts indicated that the gel network collapsed in the presence of more nucleophilic anions such as cyanide, fluoride, and chloride salts at the MGC, but the gel network was intact when in contact with nitrate, sulphate, acetate, bromide, and iodide salts, indicating the anion-responsive properties of N-BTA gels. Anion-induced gel formation was observed for less nucleophilic anions below the MGC of N-BTA. The ability of N-BTA gels to act as an adsorbent for hazardous anionic and cationic dyes in water was evaluated. The results indicated that the ethanolic gels of N-BTA successfully absorbed methylene blue and methyl orange dyes from water. This work demonstrates the potential of the N-BTA gelator to act as a stimuli-responsive material and a promising candidate for water purification.

Critical Conditions Regulating the Gelation in Macroionic Cluster Solutions

The critical gelation conditions observed in dilute aqueous solutions of multiple nanoscale uranyl peroxide molecular clusters are reported, in the presence of multivalent cations. This gelation is dominantly driven by counterion-mediated attraction. The gelation areas in the corresponding phase diagrams all appear in similar locations, with a characteristic triangle shape outlining three critical boundary conditions, corresponding to the critical cluster concentration, cation/cluster ratio, and the degree of counterion association with increasing cluster concentration. These interesting phrasal observations reveal general conditions for gelation driven by electrostatic interactions in hydrophilic macroionic solutions.

Silver Dendritic Gels with Luminescence and Aggregation-Induced Emission Effect

This work reports on a novel family of silver metallogels based on discrete coordination complexes. Structurally, they consist of dendrimers containing a trinuclear silver metallacycle at the core, with the general formula [M(μ-pz)]3, and poly(benzyl)ether branched structures with different numbers or terminal alkoxy chains at the periphery. These silver metallodendrimers are able to gel low-polarity solvents such as dodecane or cyclohexane, giving rise to luminescent organogels at room temperature with the property of aggregation-induced emission (AIE). This property means that in solution or the sol state, they are weak emitters, but in the gel state, luminescence is considerably increased. In this particular case, they exhibit blue luminescence. Two different dendritic scaffolds have been studied, finding significant differences in solubility, gel formation and dependence of luminescence on temperature. The results show that properly tailored silver gelators can show luminescence in the gel state.

An Encyclopedic Compendium on Chemosensing Supramolecular Metal-Organic Gels

Chemosensing, an interdisciplinary scientific domain, plays a pivotal role ranging from environmental monitoring to healthcare diagnostics and (inter)national security. Metal-organic gels (MOGs) are recognized for their stability, selectivity, and responsiveness, making them valuable for chemosensing applications. Researchers have explored the development of MOGs based on different metal ions and ligands, allowing for tailored properties and sensitivities, and have even demonstrated their applications as portable sensors such as paper-based test strips for practical use. Herein, several studies related to MOGs development and their applications in the chemosensing field via UV-visible or luminance along with electrochemical approach are presented. These papers explored MOGs as versatile materials with their use in sensing bio or environmental analytes. This review provides a foundational understanding of key concepts, methodologies, and recent advancements in this field, fostering the scientific community.

Differentiating aliphatic and aromatic alcohols using triazine-based supramolecular organogelators: end group-specific selective gelation with chain length of alcohols

Supramolecular gels have an extensive range of potential applications, out of which stimuli-responsive materials are a topic of contemporary research. Gels being kinetically entrapped materials can be tuned to different forms using external chemical stimuli. In this context, three different triazine gelators, each containing a unique end group, were examined for gelation in various solvent systems. Nevertheless, the gelation was limited to only alcoholic solvents, suggesting that the hydrogen bonds between the gelating solvent and gelator play a crucial role in gelation. Further, it was found that these gelators could gelate only with aliphatic alcohols, which could be degelled easily using aromatic alcohols. The three gelators exhibited distinct gelation of aliphatic alcohols based on their end groups. The gelator with the polar-aromatic end group (C5H4N) was found to gelate with lighter alcohols, whereas that with the nonpolar aromatic end group (C6H5) was found to prefer higher alcohols. The MGC and Tgel values were also found to depend on the alkyl chain length/branching of the alcohols. The crystal structure of one of the gelators provides insights into the model structure of the gels. Cyclohexanol was the only solvent that could produce gels with all three of the as-synthesised gelators. The process of degelation by aromatic alcohols was monitored at different points of the disassembly process by rheological and morphological measurements to understand the extent of controlled degelation. These gels have great potential for use in controlled drug delivery and chemical sensing, among other areas.

Photo-responsive metal-organic gels of rigid phenylene-1,3-di-substituted angular dienes with metal halides: gel-to-gel transformations triggered by [2 + 2] polymerization

Herein, the first report on gel-to-gel transformations via [2 + 2] photopolymerization in MOGs of metal halides and rigid dienes is presented. The MOGs and their xerogels show exceptional ability to undergo [2 + 2] polymerisation upon UV irradiation. Gel-to-gel transformations are very rare as the post-modification of gelators weakens the gel and transforms it to a sol. Such transformations change the molecular assemblies into gels with altered mechanical and chemical properties. These phenomena pave the way to synthesize new MOGs with improved rigidity that cannot be synthesized otherwise.

Supramolecular gels - a panorama of low-molecular-weight gelators from ancient origins to next-generation technologies

Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.

Antibacterial Hydrogel as a Self-Drug-Delivery System Derived from Zn(II)-bis-imidazole/NSAID-Based Organic-Inorganic Hybrids

A skin wound is prone to bacterial infection and growth. An antibacterial topical hydrogel that can act as a self-drug-delivery (SDD) system is reported here. Two bidentate ligands (L2/L1) derived from imidazole/benzimidazole derivatives when reacted with Zn(NO3)2 and a series of nonsteroidal-anti-inflammatory drugs (NSAIDs) produced crystalline products, which were characterized by single-crystal X-ray diffraction (SXRD). Simple mixing of the ingredients of the crystalline products (stoichiometry guided by the corresponding crystal structure) produced an aqueous gel (DMSO/water) when the bidentate ligand was water-insoluble L2, whereas water-soluble L1 readily produced hydrogels under similar conditions. Dynamic rheology and scanning electron microscopy (SEM) were employed to characterize the gels. Zone inhibition diameters, minimum inhibitory concentration (MIC), minimum bactericidal concentration (MBC), and hemolysis data suggested that among the hydrogelators, L1MEC derived from L1, meclofenac and Zn(NO3)2, was found to be the best against the Gram-negative bacteria Escherichia coli. The corresponding hydrogel L1MEC_HG and a piece of a dried cloth bandage coated with the hydrogel also showed appreciable activity against E. coli. The antibacterial property of L1MEC_HG against E. coli, thus demonstrated, is relevant in developing an antibacterial SDD system because E. coli is reported to be present in infected wounds.

Calix[4]arene-based Supramolecular Gels for Mercury Ion Removal in Water

A calix[4]arene-based gelator 1, with lower-rim mono triazolylpyridine group, capable of spontaneous self-assembly into microspheres in different ethanol/H2 O mixtures, is synthesized. The concentration-dependent 1 H NMR spectra and X-ray single-crystal structure of 1 provided evidence for self-assembly of gelator 1 via cooperative interactions of intermolecular noncovalent forces. Furthermore, metallogels by self-assembly of 1 was found to exhibit remarkable selectivity toward Hg2+ ions. 1 H NMR spectra support that Hg2+ ion was bound to the nitrogen atoms of two coordination sites of 1, which composed of triazole and pyridine. Moreover, the results of field emission scanning electron microscopy and rheology experiments indicated that Hg2+ ions not only enhanced the gelling ability of gelator 1 in ethanol but also led to morphological change of its self-assembly through metal-ligand interactions. Finally, the in situ gelation, triggered by mixing a gelator solution of 1 in ethanol with water samples such as deionized (DI), tap, and lake water, leads to the effective removal of Hg(II) from a water sample which reduced from 400 to 1.6 ppm.

Metallosupramolecular polymers: current status and future prospects

Metallo-supramolecular polymers have gained increasing attention and witnessed continuous development as a vibrant new research interest in the domain of soft materials. These nonconventional polymers have found widespread application in materials and biology owing to their well-defined and diversified topologies and the distinct dynamic nature of the metallosupramolecular interactions against various stimuli. Because of the intriguing redox, photonic, electronic, and magnetic properties, these stimuli-responsive supramolecular structures have attracted considerable interest for optoelectronic device fabrication. However, it still remains challenging to develop stimuli responsive systems with offbeat applications. Furthermore, achieving spatiotemporal control remains elusive with thermoresponsive and sono-responsive metallosupramolecular polymers, which encounter the disadvantage of poor precision control. Additionally, controlling the morphology of these soft materials on the mesoscale, both in solution and on substrates, has many challenges. In this review, we discuss the recent developments and future directions for the construction of stimuli responsive metallosupramolecular systems targeting practical applications. Furthermore, we discuss the synthetic methodologies that have been used to regulate the mesoscale morphology of these materials, such as coordination modulation and pseudomorphic replication. Finally, we briefly cover the burgeoning field of programmed synthesis of metallosupramolecular polymers, emphasizing techniques, such as living polymerization and chemical fuel-driven transiently active systems, which we believe will be the major research directions in the future.

Thermoreversible Gelation with Supramolecularly Polymerized Cross-Link Junctions

Structure and reversibility of cross-link junctions play pivotal roles in determining the nature of thermoreversible gelation and dynamic mechanical properties of the produced polymer networks. We attempt to theoretically explore new types of sol-gel transitions with mechanical sharpness by allowing cross-links to grow without upper bound. We consider thermoreversible gelation of the primary molecules R{Af} carrying the number f of low molecular weight functional groups (gelators) A. Gelators A are assumed to form supramolecular assemblies. Some examples are: telechelic polymers (f=2) carrying π-π stacking benzene derivatives at their both ends, and trifunctional star molecules (f=3) bearing multiple hydrogen-bonding gelators. The sol-gel transition of the primary molecules becomes sharper with the cooperativity parameter of the stepwise linear growth of the cross-links. There is a polymerization transition (crossover without singularity) of the junctions in the postgel region after the gel point is passed. If the gelator A tends to form supramolecular rings competitively with linear chains, there is another phase transition in the deep postgel region where the average molecular weight of the rings becomes infinite (Bose-Einstein condensation of rings). As a typical example of binary cross-links where gelators A and B form mixed junctions, we specifically consider metal-coordinated binding of ligands A by metal ions B. Two types of multi-nuclear supramolecular complexes are studied: (i) linear stacking (ladder) of the sandwich A2B units, and (ii) linear train of egg-box A4B units. To find the strategy towards experimental realization of supramolecular cross-links, the average molecular weight, the gel fraction, the average length of the cross-link junctions are numerically calculated for all of these models as functions of the functionality f, the concentration of the solute molecules, and the temperature. Potential candidates for the realization of these new types of thermoreversible gelation are discussed.

Toughening CO2 -Derived Copolymer Elastomers Through Ionomer Networking

Utilizing carbon dioxide (CO2 ) to make polycarbonates through the ring-opening copolymerization (ROCOP) of CO2 and epoxides valorizes and recycles CO2 and reduces pollution in polymer manufacturing. Recent developments in catalysis provide access to polycarbonates with well-defined structures and allow for copolymerization with biomass-derived monomers; however, the resulting material properties are underinvestigated. Here, new types of CO2 -derived thermoplastic elastomers (TPEs) are described together with a generally applicable method to augment tensile mechanical strength and Young's modulus without requiring material re-design. These TPEs combine high glass transition temperature (Tg ) amorphous blocks comprising CO2 -derived poly(carbonates) (A-block), with low Tg poly(ε-decalactone), from castor oil, (B-block) in ABA structures. The poly(carbonate) blocks are selectively functionalized with metal-carboxylates where the metals are Na(I), Mg(II), Ca(II), Zn(II) and Al(III). The colorless polymers, featuring <1 wt% metal, show tunable thermal (Tg ), and mechanical (elongation at break, elasticity, creep-resistance) properties. The best elastomers show >50-fold higher Young's modulus and 21-times greater tensile strength, without compromise to elastic recovery, compared with the starting block polymers. They have wide operating temperatures (-20 to 200 °C), high creep-resistance and yet remain recyclable. In the future, these materials may substitute high-volume petrochemical elastomers and be utilized in high-growth fields like medicine, robotics, and electronics.

Supramolecular Arrangement and Rheological Properties of Bisamide Gels

We report a systematic study of the gelation behavior of nBA gelators in xylene, with odd and even n-methylene spacers between the amide groups (n = 5-10) and 17 carbons at each end. The melting temperatures (Tm0) of nBA gels are obtained from fitting our DSCN(T) model to the experimental DSC data. The found Tm0 of nBA gels is about 35 °C lower than Tm0 of the pure nBA gelators. This is reasonably well explained by a simple model combining theories of Flory-Huggins and Gibbs free energy of melting (FHM model). We attribute this depression to an increase in entropy upon melting of the gel due to mixing with the solvent. The odd-even alternation in Tm0 of nBA gels, which was also found for the nBA gelators, indicates that the solid structures inside the gels are somewhat similar. This was studied using XRD: similar 00l reflections were found in the XRD patterns of all nBA gels and their nBA gelators. For even nBA gels, the same reflections in the 19-25° (2θ) region confirm that the sheetlike supramolecular structure of the gels is analogous to the lamellar structure of the solid gelators. For odd nBA gels, a slight difference in the reflections around 20-25° (2θ) implies a somewhat different side-by-side packing of odd nBA gels compared to the solid state. This variation is found for all the odd gels, and indeed, they show distinctly different morphologies compared to the even nBA gels. The possible effect of this on the rheological properties is discussed using some inspiration from the Halpin-Tsai model for composites where nBA gels are considered to be analogous to composite materials. The change of the storage modulus (G') with the shape factor of woven fibers and sheets in nBA gels (20 wt %) indicates that a rheological odd-even effect might indeed be present.

An injectable supramolecular hydrogel as a self-drug-delivery system for local chemoimmunotherapy against melanoma

Skin-cancer melanoma caused 57k death in 2020. Some of the available therapies are: topical application of a gel loaded with an anti-skin cancer drug and intravenous injection of immune cytokines; however, both the approaches have drawbacks such as inefficient internalization of the drug in cancer cells and a short half-life with severe side effects, respectively. Interestingly, we observed for the first time that a subcutaneously implanted hydrogel designed and synthesized by coordinating NSAIDs and 5-AP with Zn(II) can effectively combat melanoma cell (B16-F10)-induced tumors in C57BL/6 mice. Both in vitro and in vivo results show that it can effectively reduce PGE2 expression, consequently upregulating IFN-γ and IL-12 that eventually engage M1-macrophages for activating T cells (CD8+), triggering apoptosis. This unique all-in-one self-drug-delivery approach, wherein the hydrogel implant is made from the drug molecules itself providing both chemotherapy and immunotherapy in combating deadly melanoma, highlights the supramolecular chemistry-based bottom-up approach in cancer therapy.

The Role of Functional Groups in Tuning the Self-Assembly Modes and Physical Properties of Multicomponent Gels

We have analyzed the nature and role of functional groups on the self-assembly modes and the physical properties of multicomponent gels with structurally similar individual components. The gelation properties of individual and mixed enantiomeric compounds of biphenyl bis-(amides) of alanine (BPA) or phenylalanine (BPP) methyl ester were analyzed in various solvent/solvent mixtures. Multicomponent gels were formed by mixing the enantiomeric BPP compounds at a lower concentration, but a higher concentration was required for mixed alanine-based BPA gels. The comparison of the mechanical strength of the individual and mixed BPP compounds indicated that the mixed BPP gels displayed enhanced mechanical strength (∼2-fold increase) in p-xylene, but a weaker gel was observed in DMSO/water. However, a reverse trend was observed for BPA gels, indicating the role of functional groups in the gel network formation. X-ray diffraction analysis of the gelator and the xerogels in the solid state confirmed the formation of co-assembled networks in mixed enantiomeric gels. The stability of the gels towards anions was evaluated by analyzing the anion induced stimuli-responsive properties. These results indicate the effective modeling of the functional groups of the individual components could lead to multicomponent gels with tunable properties.

Gel transformation as a general strategy for fabrication of highly porous multiscale MOF architectures

The structure and chemistry of metal-organic frameworks or MOFs dictate their properties and functionalities. However, their architecture and form are essential for facilitating the transport of molecules, the flow of electrons, the conduction of heat, the transmission of light, and the propagation of force, which are vital in many applications. This work explores the transformation of inorganic gels into MOFs as a general strategy to construct complex porous MOF architectures at nano, micro, and millimeter length scales. MOFs can be induced to form along three different pathways governed by gel dissolution, MOF nucleation, and crystallization kinetics. Slow gel dissolution, rapid nucleation, and moderate crystal growth result in a pseudomorphic transformation (pathway 1) that preserves the original network structure and pores, while a comparably faster crystallization displays significant localized structural changes but still preserves network interconnectivity (pathway 2). MOF exfoliates from the gel surface during rapid dissolution, thus inducing nucleation in the pore liquid leading to a dense assembly of percolated MOF particles (pathway 3). Thus, the prepared MOF 3D objects and architectures can be fabricated with superb mechanical strength (>98.7 MPa), excellent permeability (>3.4 × 10^-10 m²), and large surface area (1100 m² g^-1) and mesopore volumes (1.1 cm³ g^-1).

Beyond Sol-Gel: Molecular Gels with Different Transitions

The existence of sol–gel transitions is one of the most manifest properties of molecular gels. These transitions reflect their nature since they correspond to the association or dissociation of low weight molecules through non-covalent interactions to form the network constitutive of the gel. Most described molecular gels undergo only one gel-to-sol transition upon heating, and the reverse sol-to-gel transition upon cooling. It has been long observed that different conditions of formation could lead to gels with different morphologies, and that gels can undergo a transition from gel to crystals. However, more recent publications report molecular gels which exhibit additional transitions, for instance gel-to-gel transitions. This review surveys the molecular gels for which, in addition to sol–gel transitions, transitions of different nature have been reported: gel-to-gel transitions, gel-to-crystal transition, liquid–liquid phase separations, eutectic transformations, and synereses.

Progress in Research and Application of Metal-Organic Gels: A Review

In recent years, metal-organic gels (MOGs) have attracted much attention due to their hierarchical porous structure, large specific surface area, and good surface modifiability. Compared with MOFs, the synthesis conditions of MOGs are gentler and more stable. At present, MOGs are widely used in the fields of catalysis, adsorption, energy storage, electrochromic devices, sensing, analysis, and detection. In this paper, literature metrology and knowledge graph visualization analysis are adopted to analyze and summarize the literature data in the field of MOGs. The visualization maps of the temporal distribution, spatial distribution, authors and institutions' distribution, influence of highly cited literature and journals, keyword clustering, and research trends are helpful to clearly grasp the content and development trend of MOG materials research, point out the future research direction for scholars, and promote the practical application of MOGs. At the same time, the paper reviews the research and application progress of MOGs in recent years by combining keyword clustering, time lines, and emergence maps, and looks forward to their challenges, future development trend, and application prospects.

Controllable chiral inversion via thioether bond-activated J- and H-aggregation transformation

Herein, thioether bonds in the peripheral groups of chiral dendrimer that could activate the transformation between J- and H-aggregation in π-systems and caused controllable chiral inversion are described. Furthermore, owing to the close-knit per-aggregation of dendrimers impeding the transfer of inherent molecular chirality, the inversed chirality could be memorized and transferred from oligomers to nano-helices during self-assembly.

Amidoamine Oxide Surfactants as Low-Molecular-Weight Hydrogelators: Effect of Methylene Chain Length on Aggregate Structure and Rheological Behavior

Rheology control is an important issue in many industrial products such as cosmetics and paints. Recently, low-molecular-weight compounds have attracted considerable attention as thickeners/gelators for various solvents; however, there is still a significant need for molecular design guidelines for industrial applications. Amidoamine oxides (AAOs), which are long-chain alkylamine oxides with three amide groups, are surfactants that act as hydrogelators. Here, we show the relationship between the length of methylene chains at four different locations of AAOs, the aggregate structure, the gelation temperature T_gel, and the viscoelasticity of the formed hydrogels. As seen from the results of electron microscopic observations, the aggregate structure (ribbon-like or rod-like) can be controlled by changing the length of methylene chain in the hydrophobic part, the length of methylene chain between the amide and amine oxide groups, and the lengths of methylene chains between amide groups. Furthermore, hydrogels consisting of rod-like aggregates showed significantly higher viscoelasticity than those consisting of ribbon-like aggregates. In other words, it was shown that the gel viscoelasticity could be controlled by changing the methylene chain lengths at four different locations of the AAO.

Recent advances in the development of europium(III) and terbium(III)-based luminescent supramolecular metallogels

In the recent past, special attention has been paid to the development of metallogels as novel luminescent materials from rationally designed gelators with lanthanide ions, especially europium (Eu(III)) and terbium (Tb(III)) metal ions. Lanthanide (Ln(III)) based metallogels possess various useful properties with an extensive range of applications in the field of advanced materials, and electronic and bio-technologies. Lanthanide ions in coordination with appropriate sensitizer ligands can reproduce metal-based optical, redox, and electronic properties in soft gel materials. The optical properties of the luminescent Ln(III) based metallogels can be tuned over the complete visible spectrum (400-750 nm) including the generation of white light by mixing both Eu(III) and Tb(III) with the ligand in various stoichiometric ratios. Additionally, the dynamic nature of the lanthanide-ligand (Ln-N) coordination bond allows the Ln(III) based metallogels to respond to various external stimuli. Luminescent self-healing supramolecular gels using organic ligands as 'hosts' and Ln(III) ions as 'guests' are also a current topic of research interest. In this review, we discuss and summarize some selected recent examples of newly developed luminescent Eu(III) and Tb(III) based supramolecular metallogels with potential applications in the fields of optoelectronic devices, stimuli responsiveness, self-healing, luminescent films, and sensors.

Recent Advances in Stimuli-Responsive Metallogels

Recently, stimuli-responsive supramolecular gels have received significant attention because their properties can be modulated through external stimuli such as heat, light, electricity, magnetic fields, mechanical stress, pH, ions, chemicals and enzymes. Among these gels, stimuli-responsive supramolecular metallogels have shown promising applications in material science because of their fascinating redox, optical, electronic and magnetic properties. In this review, research progress on stimuli-responsive supramolecular metallogels in recent years is systematically summarized. According to external stimulus sources, stimuli-responsive supramolecular metallogels, including chemical, physical and multiple stimuli-responsive metallogels, are discussed separately. Moreover, challenges, suggestions and opportunities regarding the development of novel stimuli-responsive metallogels are presented. We believe the knowledge and inspiration gained from this review will deepen the current understanding of stimuli-responsive smart metallogels and encourage more scientists to provide valuable contributions to this topic in the coming decades.

Cellulose Nanocrystal Gels with Tunable Mechanical Properties from Hybrid Thermal Strategies

Gels are useful materials for drug delivery, wound dressings, tissue engineering, and 3D printing. These various applications require gels with different mechanical properties that can be easily tuned, also preferably excluding the use of chemical additives, which can be toxic or harmful to the body or environment. Here, we report a novel strategy to synthesize cellulose nanocrystal (CNC) gels with tunable mechanical properties. Sequential freeze-thaw cycling and hydrothermal treatments were applied to CNC suspensions in different orders to give a series of pristine CNC hydrogels. Freeze-drying of the hydrogels also afforded a series of lightweight CNC aerogels. The mechanical properties of the hydrogels and aerogels were studied by rheological measurements and compression strength tests, respectively. Specifically, the complex modulus of CNC hydrogels ranged from 160 to 32,000 Pa among eight different hydrogels, while Young's modulus of CNC aerogels was tuned from 0.114 to 3.98 MPa across five different aerogels. The microstructures of aerogels were also investigated by scanning electron microscopy and X-ray microtomography, which revealed remarkable differences between the materials. Solvent sorption-desorption tests showed that the reinforced networks have excellent stability over the basic CNC aerogels in ethanol, demonstrating a material enhancement from the preparation strategies we developed. Thermal conductivity and thermal stability for these materials were also investigated, and it was found that the lowest thermal conductivity was 0.030 W/m K, and all of the aerogels are generally stable below 280 °C. These characteristics also expand the potential applications of this family of CNC gels to lightweight supporting materials and thermal insulators.

Real-time Observation of Macroscopic Helical Morphologies under Optical Microscope: A Curious Case of π-π Stacking Driven Molecular Self-assembly of an Organic Gelator Devoid of Hydrogen Bonding

Supramolecular assemblies such as tubules/helix/double helix/helical tape etc. are usually submicron objects preventing direct observation under optical microscope. Chiral-pure form of these assemblies is important for potential applications. Herein, we report a rare phenomenon wherein a DMSO gel of a simple terpyridine derivative [(4-CNPhe)4PyTerp] produced macroscopic helical morphologies (μm length scale) which could be observed under optical microscope, formation of which could be monitored by optical videography, stable enough to withstand acidic vapour, robust enough to display reversible gel↔sol in response to acidic and ammonia vapour and sturdy enough to be maneuvered with a needle. These properties appeared to be unique to the title compound as the other related derivatives failed to display such assembly structures. SXRD and MD simulation studies suggested that weak interactions (π-π stacking) played a crucial role in the self-assembly process.

Metallogels: a novel approach for the nanostructuration of single-chain magnets

In this study we demonstrate that single-chain magnets (SCMs) can be assembled in gel phase and transferred intact on surface. We take advantage of a family of SCMs based on Tb^III ions and nitronyl-nitroxides radicals functionalized with short alkyl chains known to form crystalline supramolecular nanotubes interacting with heptane acting as crystallizing solvent. When the radicals are functionalized with long aliphatic chains a robust gel is formed with similar structural and functional properties respect to its crystalline parent. Indeed, a small-angle X-ray scattering (SAXS) study unambiguously demonstrates that the gel is made of supramolecular nanotubes: the high stability of the gel allows the determination from SAXS data of precise nanotube metrics such as diameter, helical pitch and monoclinic cell of the folded 2D crystal lattice along the tube direction. Additionally, static and dynamic magnetic investigations show the persistence of the SCM behavior in the metallogel. Last, on-surface gelation provides thick films as well as sub-monolayer deposits of supramolecular nanotubes on surface as evidenced by atomic force microscopy (AFM) observations. This paves the road toward magnetic materials and devices made of SCMs profiting of their isolation on surface as individual chains.

Stimuli-Responsive Properties of Supramolecular Gels Based on Pyridyl-N-oxide Amides

The nature of functional groups and their relative position and orientation play an important role in tuning the gelation properties of stimuli-responsive supramolecular gels. In this work, we synthesized and characterized mono-/bis-pyridyl-N-oxide compounds of N-(4-pyridyl)nicotinamide (L₁-L₃). The gelation properties of these N-oxide compounds were compared with the reported isomeric counterpart mono-/bis-pyridyl-N-oxide compounds of N-(4-pyridyl)isonicotinamide. Hydrogels obtained with L₁ and L₃ were thermally and mechanically more stable than the corresponding isomeric counterparts. The surface morphology of the xerogels of di-N-oxides (L₃ and diNO) obtained from the water was studied using scanning electron microscopy (SEM), which revealed that the relative position of N-oxide moieties did not have a prominent effect on the gel morphology. The solid-state structural analysis was performed using single-crystal X-ray diffraction to understand the key mechanism in gel formation. The versatile nature of N-oxide moieties makes these gels highly responsive toward an external stimulus, and the stimuli-responsive behavior of the gels in water and aqueous mixtures was studied in the presence of various salts. We studied the effect of various salts on the gelation behavior of the hydrogels, and the results indicated that the salts could induce gelation in L₁ and L₃ below the minimum gelator concentration of the gelators. The mechanical properties were evaluated by rheological experiments, indicating that the modified compounds displayed enhanced gel strength in most cases. Interestingly, cadmium chloride formed supergelator at a very low concentration (0.7 wt% of L₃), and robust hydrogels were obtained at higher concentrations of L₃. These results show that the relative position of N-oxide moieties is crucial for the effective interaction of the gelator with salts/ions resulting in LMWGs with tunable properties.

State of the Art of Hydrogel Wound Dressings Developed by Ionizing Radiation

The development of an ideal hydrogel wound dressing with excellent characteristics is currently a significant demand in wound therapy. The ideal hydrogel wound dressing must provide a moist environment between the wound and the dressing, promote wound healing, absorb excess exudate and toxins, be completely sterile, and not adhere to the wound. The evolution and current status of research on hydrogel wound dressings obtained exclusively through production by ionizing radiation are discussed in this paper review, along with the preparation methods, properties, standard characterization techniques, and their applications in wound dressing. First, we described the methods for synthesizing hydrogel wound dressings with ionizing radiation. Then, standard methods of characterization of hydrogel wound dressings such as gel fraction, swelling degree, sol-gel analysis, rheological properties, morphology, moisture retention capability, and water vapor transmission rate have been investigated. In the end, specific attention was paid to the drug release, antibacterial performance, and cytotoxicity of hydrogels. Moreover, the application of hydrogel in regenerative medicine as wound healing dressing was covered.

Modulation of hydrogel networks by metal ions

Self-assembling hydrogels are receiving great attention for both biomedical and technological applications. Self-assembly of protein/peptides as well as organic molecules is commonly induced in response to external triggers such as changes of temperature, concentration, or pH. An interesting strategy to modulate the morphology and mechanical properties of the gels implies the use of metal ions, where coordination bonds regulate the dynamic cross-linking in the construction of hydrogels, and coordination geometries, catalytic, and redox properties of metal ions play crucial roles. This review aims to discuss recent insights into the supramolecular assembly of hydrogels involving metal ions, with a focus on self-assembling peptides, as well as applications of metallogels in biomedical fields including tissue engineering, sensing, wound healing, and drug delivery.

Structural insights into self-assembly of a slow-evolving and mechanically robust supramolecular gel via time-resolved small-angle neutron scattering

The supramolecular assembly process is a widespread phenomenon found in both synthetically engineered and naturally occurring systems, such as colloids, liquid crystals and micelles. However, a basic understanding of the evolution of self-assembly processes over time remains elusive, primarily owing to the fast kinetics involved in these processes and the complex nature of the various non-covalent interactions operating simultaneously. With the help of a slow-evolving supramolecular gel derived from a urea-based gelator, we aim to capture the different stages of the self-assembly process commencing from nucleation. In particular, we are able to study the self-assembly in real time using time-resolved small-angle neutron scattering (SANS) at length scales ranging from approximately 30 Å to 250 Å. Systems with and without sonication are compared simultaneously, to follow the different kinetic paths involved in these two cases. Time-dependent NMR, morphological and rheological studies act complementarily to the SANS data at sub-micron and bulk length scales. A hollow columnar formation comprising of gelator monomers arranged radially along the long axis of the fiber and solvent in the core is detected at the very early stage of the self-assembly process. While sonication promotes uniform growth of fibers and fiber entanglement, the absence of such a stimulus helps extensive bundle formation at a later stage and at the microscopic domain, making the gel system mechanically robust. The results of the present work provide a thorough understanding of the self-assembly process and reveal a path for fine-tuning such growth processes for applications such as the cosmetics industry, 3D printing ink development and paint industry.

Recent advances on small molecular gels: formation mechanism and their application in pharmaceutical fields

INTRODUCTION

As an essential complement to chemically cross-linked macromolecular gels, drug delivery systems based on small molecular gels formed under the driving forces of non-covalent interactions are attracting considerable research interest due to their potential advantages of high structural functionality, lower biological toxicity, reversible stimulus-response, and so on.

AREA COVERED

The present review summarizes recent advances in small molecular gels and provides their updates as a comprehensive overview in terms of gelation mechanism, gel properties, and physicochemical characterizations. In particular, this manuscript reviews the effects of drug-based small molecular gels on the drug development and their potential applications in the pharmaceutical fields.

EXPERT OPINION

Small molecular-based gel systems, constructed by inactive compounds or active pharmaceutical ingredients, have been extensively studied as carriers for drug delivery in pharmaceutical field, such as oral formulations, injectable formulations, and transdermal formulations. However, the construction of such gel systems yet faces several challenges such as rational and efficient design of functional gelators and the great occasionality of drug-based gel formation. Thus, a deeper understanding of the gelation mechanism and its relationship with gel properties will be conducive to the construction of small molecular gels systems and their future application.

Oral Metal-Organic Gel Protected Whole-Cell Biosensor for in Situ Monitoring Nitrosamines in the Gastrointestinal Tract

Nitrite, a type of food additive, has been proved convertible to genotoxic nitrosamines in the gastrointestinal tract by intestinal flora. There is no appropriate method for in situ detection of nitrosamines. Herein, plasmid-introduced Saccharomyces cerevisiae, which can respond to nitrosamine-induced DNA damage and activate pMAG1-based DNA damage repair (DDR), was designed as whole-cell biosensors (WCBs) for monitoring the in situ generated nitrosamines by a reporter gene expressing enhanced green fluorescent protein (EGFP). In order to protect the validity of WCBs (pMAG1 yeast) from the gastric acid environment, a type of metal-organic gel (MOG), coordinated by Fe³⁺ and 2,2'-thiodiacetic acid (TDA), was prepared to embed the WCBs. The MOG(Fe-TDA) is gastric acid resistant and can deliver the pMAG1 yeast to the gut without compromising the performance of pMAG1 yeast to detect in situ generated nitrosamines. The genotoxicity of nitrosamines converted from nitrite was successfully detected in the gastrointestinal tract of mice.

Anion-Coordination-Driven Assembly: From Discrete Supramolecular Self-Assemblies to Functional Soft Materials

Anion templated assembly of supramolecular systems has been extensively explored in previous reports, whereas anions serve only as an auxiliary and spectator role. With the development of anion coordination chemistry in recent years, anion coordination-driven assembly (ACDA) has emerged as a new strategy for the construction of supramolecular self-assemblies. Anions are proved to exist as the main actors in the construction of supramolecular architectures, i. e., serve as the coordination center. This Review will focus on the recent progress in anion-coordination-driven assembly of discrete supramolecular architectures, such as helicates, polyhedrons and polygons, and the various applications of 'aniono'-systems. At the end of this Review, we highlight current challenges and opportunities for future research of anion-coordination-driven self-assembly.

Anion-responsive self-assembled hydrogels of a phenylalanine-TREN conjugate allow sequential release of propranolol and doxorubicin

Stimuli-responsive self-assembled and supramolecular hydrogels derived from peptide amphiphiles have opened exciting new avenues in biomedicine and drug delivery. Herein, we screened a series of phenylalanine-amphiphiles possessing polyamine and oxyethylene appendages for their self-assembly and anion-responsiveness and found that the tris(aminoethyl)amine (TREN) containing amphiphile NapF-TREN formed injectable hydrogels that could be disrupted upon the addition of stoichiometric amounts of tetrahedral monovalent anions such as H₂PO₄^- and HSO₄^-, while the addition of other anions such as Cl^-, HPO₄^2-, CO₃^2-, HCO₃^- or SO₄^2- did not affect the gel stability. The anion-gelator interaction was investigated by ¹H and ³¹P NMR spectroscopy as well as by Isothermal Titration Calorimetry (ITC). These studies confirmed a 1 : 1 stoichiometry and revealed negative enthalpy and negative entropy for the binding of H₂PO₄^- with NapF-TREN. Microscopic investigations by TEM, AFM, and SAXS revealed that H₂PO₄^- anions induced a nanofiber-to-nanoglobule morphological change in the aqueous self-assemblies of NapF-TREN. However, upon ageing the samples, slow reformation of the nanofibers was also observed, reflecting the reversibility of the anion-gelator interaction. The anion- and pH-responsive nature of the NapF-TREN hydrogels was exploited to program sequential release of entrapped drugs propranolol and doxorubicin.

Dendritic organic molecular gel coating with molecular shape selectivity and its application in selective separation by liquid chromatography

Dendritic organic molecular gels are a promising class of three-dimensional network compounds. Here, we have synthesized a new type of dendritic organic molecular gel stationary phase (SiO₂-G3) by using benzyl alcohol as raw material and dimethyl 5-hydroxyisophthalate as growth unit to synthesize a third-generation organic molecular gel G3, which grafted onto the silica surface by cyanogen chloride (CC). The developed stationary phase not only exhibits high molecular shape selectivity but also has a RPLC/HILIC/IEC mixed-mode characteristic for HPLC due to the ordered structure, the multiple strong π-π stacking interactions and the introduction of a hydrophilic triazine fraction during the grafting process. Compared with a commercial C₁₈ column, the developed column exhibited flexible selectivity, enhanced separation performance and excellent separation of monosubstituted benzene, polycyclic aromatic hydrocarbons (PAHs), positional isomers, nucleosides and nucleobases, benzoic acid and aniline compounds. In addition, the new column provided baseline separation of polycyclic aromatic hydrocarbon contaminants in Yellow River water, verifying its potential for application in the analysis of real samples.

PEG-Infiltrated Polyoxometalate Frameworks with Flexible Form-Factors

We present the synthesis of metal oxide frameworks composed of the Preyssler anion, [NaP₅W₃₀O₁₁₀]^14-, bridged with transition-metal cations and infiltrated with polyethylene glycol. The frameworks can be dissolved in water to form freestanding rigid or flexible films or gels. Powder X-ray diffraction shows that all form-factors maintain the short-range order of the original crystals. Raman spectroscopy reveals that, similar to hydrogels, the macroscopic mechanical properties of these composites are dependent on the water content and the extent of hydrogen-bonding within the water network. The understanding gained from these studies facilitates solution-phase processing of polyoxometalate frameworks into flexible form factors.

Anion-Induced Reversible Actuation of Squaramide-Crosslinked Polymer Gels

Supramolecular anion binding to squaramide crosslinkers in poly(N,N-dimethylacrylamide) gel networks enhances swelling and allows reversible chemically driven actuation. The volume swelling ratio of the gels is shown to depend on both the type of anion and its concentration. ¹H NMR and UV-vis titrations with the squaramide crosslinkers reveal a relationship between anion binding affinity and the concentration-dependent swelling behavior. Gel swelling is shown to be reversible, and by embedding a solid support into rod-shaped gels, soft actuators are fabricated that undergo forward and backward bending motion in response to changing anion concentration. The swelling and bending process, which is accompanied by intense green coloration of the gel, is achieved by using only low amounts of crosslinker. This macroscopic actuation achieved by anion binding to specific molecular entities in the polymer network will open new opportunities in the field of chemically responsive materials.

In situ conversion of a MOG to a crystalline MOF: a case study on solvent-dependent gelation and crystallization

Herein, we report in situ transformation of a metal-organic gel (MOG) to a crystalline metal-organic framework (MOF) and solvent-dependent gelation/crystallization via solvothermal reactions of a tetracarboxylic acid, namely 4,4'-dinitro-2,2',6,6'-tetracarboxybiphenyl, and ZnSO₄. The results provide structural insights into MOGs at the molecular level and also help in the synthesis of crystalline MOFs that are otherwise difficult to obtain.

Functional supramolecular gels based on poly(benzyl ether) dendrons and dendrimers

Supramolecular gels, as a fascinating and useful class of soft materials, constructed from low-molecular-weight gelators via noncovalent interactions have attracted increasing attention in the past few decades. Dendrimers and dendrons are highly branched and monodisperse macromolecules with a well-defined three-dimensional architecture and multiple surface functionalities. In recent years, poly(benzyl ether) dendrimers and dendrons are found to be powerful candidates for constructing gel phase materials in organic or aqueous media due to the advantages of capability of forming multiple noncovalent interactions and significant steric impact. In this Feature Article, we provide a comprehensive overview of recent progress in supramolecular gels involving poly(benzyl ether) dendritic molecules. Firstly, we outline the molecular design strategies of dendritic gelators with an emphasis on the discussion of their gelating units and position in molecular structures. Subsequently, we discuss the potential applications of dendritic gels in light harvesting, stimuli responsive materials, sensors and environmental remediation. In addition, the potential challenges and future perspectives of poly(benzyl ether) dendritic gels have also been discussed. It is hoped that this feature article will attract increasing attention and provide some valuable insights for the future design and evolution of supramolecular gels.

Preparation of a multifunctional organogel and its electrochemical properties

A facile methodology to fabricate a highly elastic organogel for supercapacitors is demonstrated. A stable polymer organogel was obtained in DMSO by a simple esterification reaction. This organogel showed high mechanical performance, flexibility, high elasticity, luminous performance and conductivity, as well as high potential values for application in the energy sector.