Does Drying Affect Gel Networks?

Designing Stimuli-Responsive Supramolecular Gels by Tuning the Non-Covalent Interactions of the Functional Groups

The physical characteristics of a supramolecular gel are greatly influenced by the nature and arrangement of functional groups in the gelator. This work focuses on the impact of the functional groups, specifically the hydroxyl group, on the stimuli-responsive properties of a gel. We used a C3-symmetric benzene-1,3,5-tricarboxamide (BTA) platform, which was attached to the methyl ester of phenylalanine (MPBTA) and tyrosine (MTBTA). The gelation studies revealed that MPBTA gelled in alcohols, non-polar aromatic solvents, and aqueous mixtures (1:1, v/v) of high-polar solvents, whereas MTBTA gelled only in an aqueous mixture of DMF (1:1, v/v). The mechanical and thermal strength of the gels were evaluated by rheological and Tgel studies, and the results indicated that MPBTA gels were stronger than MTBTA gels. The gels were characterized by powder X-ray diffraction and scanning electron microscopy (SEM). The analysis of stimuli-responsive properties revealed that MPBTA gels were intact in the presence of sodium/potassium salts, but the MTBTA gel network was disrupted. These results indicate that the elegant choice of functional groups could be used to tune the constructive or destructive stimuli-responsive behavior of gels. This study highlights the significant role of functional groups in modulating the stimuli-responsive properties of supramolecular gels.

Water-Vapor Responsive Metallo-Peptide Nanofibers

Short peptides are versatile molecules for the construction of supramolecular materials. Most reported peptide materials are hydrophobic, stiff, and show limited response to environmental conditions in the solid-state. Herein, we describe a design strategy for minimalistic supramolecular metallo-peptide nanofibers that, depending on their sequence, change stiffness, or reversibly assemble in the solid-state, in response to changes in relative humidity (RH). We tested a series of histidine (H) containing dipeptides with varying hydrophobicity, XH, where X is G, A, L, Y (glycine, alanine, leucine, and tyrosine). The one-dimensional fiber formation is supported by metal coordination and dynamic H-bonds. Solvent conditions were identified where GH/Zn and AH/Zn formed gels that upon air-drying gave rise to nanofibers. Upon exposure of the nanofiber networks to increasing RH, a reduction in stiffness was observed with GH/Zn fibers reversibly (dis-)assembled at 60-70 % RH driven by a rebalancing of hydrogen bonding interactions between peptides and water. When these metallo-peptide nanofibers were deposited on the surface of polyimide films and exposed to varying RH, peptide/water-vapor interactions in the solid-state mechanically transferred to the polymer film, leading to the rapid and reversible folding-unfolding of the films, thus demonstrating RH-responsive actuation.

Single Crystal to Single Crystal Transformation of Porous Materials Based on Zinc Nodes and Mercaptobenzoic Acid

Porous coordination polymers (PCPs) are an excellent class of porous crystalline materials with tunable properties and intriguing potential applications spanning multiple disciplines. In this work, we report the synthesis and characterization of a PCP (HI-103) based on 4,4'-dithiodibenzoic acid ligand and zinc nitrate with two DMF molecules residing in the porous network. The stability of the porous network was analyzed by heating the compound at 60.0 °C for two days, and the structural analysis revealed a new PCP (HI-104) was formed with one of the DMF molecules, indicating a single-crystal to single-crystal (SCSC) transformation. The solvent molecules were completely removed by extensive drying (HI-103-dry), and the integrity of the porous network was verified by powder X-ray diffraction (PXRD) and thermogravimetric analysis. The reversibility of SCSC transformation was confirmed by treating HI-103-dry with DMF molecules, resulting in HI-103 after five days. The adsorption studies of HI-103-dry with other solvents revealed that SCSC transformation was not observed for DMA and DEA, but some structural changes were observed in the presence of DMSO. The adsorption studies performed in the presence of an equimolar mixture of DMF, DMA, and DMA indicated that HI-103-dry could selectively adsorb DMF molecules from the analogous mixture.

Nanoscale assembly of enantiomeric supramolecular gels driven by the nature of solvents

Understanding the key parameters that control the self-assembly process is critical to predict self-assembly modes in multi-component systems, which will lead to the development of nanofibrous materials with tuneable properties. Enantiomeric amino acid-based low-molecular-weight gelators (LMWGs) were mixed in polar (polar protic) and aromatic apolar (aromatic) solvents and compared to their individual counterparts to probe the effect of solvent polarity on the self-assembly process. Scanning electron microscopy (SEM) reveals that xerogels of individual components display hollow needles in polar protic solvents, while chiral coils are observed in aromatic solvents. In contrast, the multi-component gel displays hollow needle morphologies in both solvents, indicating similar morphologies in polar protic solvents but an entirely different nanostructure for the individual gel networks in aromatic solvents. PXRD experiments performed on the dried gels showed that the nature of the solvents plays a vital role in the co-assembly process of multi-component gels. The self-assembly modes and the gel state structure of the gels are analysed by wide-angle X-ray diffraction (WAXS) and small-angle neutron diffraction (SANS), which reveals that the mixed gel undergoes different co-assembly modes depending on the nature of the solvent systems. This study shows that different co-assembly modes can be achieved for structurally similar components by varying the solvent polarity, demonstrating the importance of solvent choice in the self-assembly process of multi-component gels.

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.

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.

Phenylalanine conjugated supramolecular hydrogels developed from the mafenide and flurbiprofen multidrug for biological applications

A well-known nonsteroidal anti-inflammatory drug (NSAID), flurbiprofen (FLR), was first conjugated individually with two naturally occurring amino acids such as L-phenylalanine (PHE) and L-alanine (ALA). These covalent amidic bioconjugates were further reacted individually with mafenide (a drug for treating burn wounds) and amantadine (an antiviral drug) to develop primary ammonium monocarboxylate (PAM) salts. Interestingly, both the PHE-containing multidrug salts exhibited significant gelation ability with various solvents including biologically potent water or methyl salicylate (MS). The isolated hydrogel (HG) as well as all the organogels obtained from multidrug gelators were extensively characterized by dynamic rheology and rheoreversibility studies. The hydrogel of FLR·PHE·MAF and MS gels of FLR·PHE·AMN/FLR·AMN were also selectively characterized by table-top and FEG-TEM analyses. The temperature-dependent ¹H-NMR spectroscopy of the selected HG further provided insights into the gelation mechanism and the only isolated single-crystal of the weakly diffracted gelator FLR·AMN also revealed the presence of 1D hydrogen-bonded networks. The pure hydrogelator FLR·PHE·MAF salt (which is also an ambidextrous gelator) was found to be promising in both mechanical (rheoreversible) and biological applications and was found to be effective in cytotoxicity, biocompatibility, anti-cancer activity (MTT and cell migration assay), antibacterial response (zone inhibition, turbidity, INT, and resazurin assay) and haemolysis studies.

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.

Double-interpenetrating nanostructured networks of marine polysaccharides possessing properties comparable to synthetic polymers

Sustainable circular economy requires materials that possess a property profile comparable to synthetic polymers and, additionally, processing and sourcing of raw materials that have a small environmental footprint. Here, we present a paradigm for processing marine biopolymers into materials that possess both elastic and plastic behavior within a single system involving a double-interpenetrating polymer network comprising the elastic phase of dynamic physical cross-links and stress-dissipating ionically cross-linked domains. As a proof of principle, films possessing more than twofold higher elastic modulus, ultimate tensile strength, and yield stress than those of polylactic acid were realized by blending two water-soluble marine polysaccharides, namely alginic acid (Alg) with physically cross-linkable carboxylated agarose (CA) followed by ionic cross-linking with a divalent cation. Dried CAAlg films showed homogeneous nano-micro-scale domains, with yield stress and size of the domains scaling inversely with calcium concentration. Through surface activation/cross-linking using calcium, CAAlg films could be further processed using wet bonding to yield laminated structures with interfacial failure loads (13.2 ± 0.81 N) similar to the ultimate loads of unlaminated films (10.09 ± 1.47 N). Toward the engineering of wood-marine biopolymer composites, an array of lines of CAAlg were printed on wood veneers (panels), dried, and then bonded following activation with calcium to yield fully bonded wood two-ply laminate. The system presented herein provides a blueprint for the adoption of marine algae-derived polysaccharides in the development of sustainable high-performance materials.

Wet-spinning multi-component low-molecular-weight gelators to print synergistic soft materials

Two different low-molecular-weight gelators (LMWGs) have been 3D-printed as filaments by wet-spininng. When the two LMWGs are simultaneously wet-spun, the co-assembled hybrid gel filaments combine the individual properties of the two gelators (dynamic pH response and in-situ metal nanoparticle formation) in synergistic ways, creating gel objects with new properties.

Chirality-directed hydrogel assembly and interactions with enantiomers of an active pharmaceutical ingredient

Enantiomers of the low-molecular-weight gelator (LMWG) DBS-CONHNH₂, based on D- or L- 1,3 : 2,4-dibenzylidenesorbitol (DBS), were synthesised. Enantiomeric gels are equivalent, but when mixtures of enantiomers are used, although gels still form, they are weaker than homochiral gels. Nanoscale chirality is lost on adding even a small proportion of the opposite enantiomer - homochiral assembly underpins effective gelation. Enantiomeric gels encapsulate the two enantiomers of anti-inflammatory drug naproxen, with thermal & mechanical differences between diastereomeric systems. We hence demonstrate the importance of chirality in DBS assembly and its interactions with chiral additives.

Supramolecular Hydrogels Developed from Mafenide and Indomethacin as a Plausible Multidrug Self-Delivery System as Antibacterial and Anti-inflammatory Topical Gels

Following a structural rationale, a series of simple organic salts derived from mafenide (a drug for treating burn wounds) and n-alkyl carboxylic acids (Me-(CH₂)_n-COOH; n = 1-3, 10-15) and various nonsteroidal anti-inflammatory drugs (NSAIDs), namely, indomethacin (IND), diclofenac (DIC), meclofenamic acid (MEC), tolfenamic acid (TOL), and flufenamic acid (FLU) (designated as salts 1-14, respectively) were synthesized as potential hydrogelators. Gelation studies revealed that mafenide n-alkyl carboxylates with n = 11-14, i.e., salts 5-8, and the indomethacin salt of mafenide, i.e., salt 10, were hydrogelators. The corresponding hydrogels, namely, 5(HG)-8(HG) and 10(HG), were characterized by table-top and dynamic rheology and high-resolution transmission electron microscopy (HR-TEM). Single-crystal structures of the nongelator salts 1-3 and the gelator salt 10 were determined by X-ray diffraction. The results obtained from various studies, which included the solubility, biostability, biocompatibility (MTT assay), and anti-inflammatory (PGE₂ assay) response of salt 10, the antibacterial response (zone inhibition assay) of salt 10, its components, and 10(HG), and the release of salt 10 in vitro from the corresponding hydrogel bed to the bulk solvent at 37 °C in 24 h, suggested their plausible use in developing multidrug-derived topical hydrogels for self-delivery applications.

Making and Breaking of Gels: Stimuli-Responsive Properties of Bis(Pyridyl-N-oxide Urea) Gelators

The structural modification of existing supramolecular architecture is an efficient strategy to design and synthesize supramolecular gels with tunable and predictable properties. In this work, we have modified bis(pyridyl urea) compounds with different linkers, namely hexylene and butylene, to their corresponding bis(pyridyl-N-oxide urea). The gelation properties of both the parent and the modified compounds were studied, and the results indicated that modification of the 3-pyridyl moieties to the corresponding 3-pyridyl-N-oxides induced hydrogelation. The stability of the parent and modified compounds were evaluated by sol-gel transition temperature (T_gel) and rheological measurements, and single-crystal X-ray diffraction was used to analyze the solid-state interactions of the gelators. The morphologies of the dried gels were analyzed by scanning electron microscopy (SEM), which revealed that the structural modification did not induce any prominent effect on the gel morphology. The stimuli-responsive behavior of these gels in the presence of salts in DMSO/water was evaluated by rheological experiments, which indicated that the modified compounds displayed enhanced gel strength in most cases. However, the gel network collapsed in the presence of the chloride salts of aluminum(III), zinc(II), copper(II), and cadmium(II). The mechanical strength of the parent gels decreased in the presence of salts, indicating that the structural modification resulted in robust gels in most cases. The modified compounds formed gels below minimum gel concentration in the presence of various salts, indicating salt-induced gelation. These results show the making and breaking ability of the gel network in the presence of external stimuli (salts), which explains the potential of using LMWGs based on N-oxide moieties as stimuli-responsive materials.

Double diffusion for the programmable spatiotemporal patterning of multi-domain supramolecular gels

To achieve spatial resolution of a multi-component gel, a double diffusion approach is used which enables the precise programming of self-assembled patterned domains with well-defined shapes and sizes. The low-molecular-weight gelators (LMWGs) used in this study are pH-responsive DBS-CO₂H and thermally-responsive DBS-CONHNH₂ (both based on 1,3:2,4-dibenzylidenesorbitol, DBS). A DBS-CONHNH₂ gel was initially assembled in a tray, and then loaded at carefully-selected positions with either basified DBS-CO₂H (i.e. DBS-carboxylate) or an acid. These soluble components subsequently diffuse through the pre-formed gel matrix, and in the domains when/where they mix, protonation of the DBS-carboxylate induces self-assembly of the DBS-CO₂H network, leading to a patterned gel-in-gel object with well-defined shape and dimensions. Using a strong acid achieves fast gelation kinetics, creating smaller, better-defined macroscale objects but with less nanoscale order. Using a weak acid source with slow kinetics, gives slightly larger objects, but on the nanoscale the DBS-CO₂H network formation is better controlled, giving more homogeneous nanoscale structures and stiffer objects. The patterned objects can be further reinforced by the presence of agarose polymer gelator. The shape of the patterning is programmed by both the shape of the central reservoir and the starting geometry in which the reservoirs are organised, with the balance between factors depending on assembly kinetics, as dictated by the choice of acid. This simple methodology therefore enables programming of patterned gels with spatiotemporal control and emergent patterning characteristics.

To achieve spatial resolution of a multi-component gel, a double diffusion approach is used which enables the precise programming of emergent self-assembled patterned domains with well-defined shapes and sizes.

Two-component supramolecular hydrogel for controlled drug release

A hybrid gel has been developed by combining two supramolecular gelators. Each gelator endows the hybrid gel with its own characteristics. One gelator enables pH-mediated controlled release of the active pharmaceutical ingredient naproxen, while the other new gelator enhances mechanical stability. Self-assembly thus gives multi-functional gels with potential applications.

Spatial and temporal diffusion-control of dynamic multi-domain self-assembled gels

The dynamic assembly of a pH-responsive low-molecular-weight gelator (LMWG) within the pre-formed matrix of a second LMWG has been achieved via diffusion of an acid from a reservoir cut into the gel. Self-assembly of the acid-triggered LMWG as it converts from micellar aggregates at basic pH into gel nanofibers at lower pH values can be both spatially and temporally controlled. The pH-responsive LMWG has an impact on the stiffness of the pre-formed gel in the domains in which it assembles. When low acid concentrations are used, LMWG assembly is transient – after the initial proton diffusion phase, the pH rises and disassembly occurs as the system equilibrates. Re-application of additional acid as ‘fuel’ can then re-assemble the LMWG network. Using glucono-δ-lactone (which slowly hydrolyses to gluconic acid) instead of HCl gives slower, more spatially-restricted assembly, and creates longer-lasting pH gradients within the gel. The presence of an agarose polymer gel network improves the mechanical strength of the gels and appears to slightly enhance the rate of proton diffusion. More sophisticated reservoir shapes can be cut into these more mechanically robust gels, enabling the creation of diffusion waves with different geometries, and hence different patterns of LMWG activation. Multiple reservoirs can be used to create overlapping proton diffusion waves, hence achieving differentiated pH patterns in the gel. Using acid diffusion in this way within gels is an intriguing and powerful way of dynamic patterning. The ability to temporally-evolve spatially-resolved patterns using biocompatible weak acids, and the change in rheological performance of the triggered domains, suggest potential future applications of this strategy in tissue engineering.

The assembly of a pH-sensitive LMWG within a pre-formed network of a second LMWG can be achieved by diffusing acids from pre-cut reservoirs, giving rise to patterned gels in which the rheological properties evolve with spatial and temporal control.

Evaluating the role of a urea-like motif in enhancing the thermal and mechanical strength of supramolecular gels

Enhanced thermal and mechanical strength in semicarbazone gels with a urea-like motif obtained by modifying the hydrogen bonding motif of the hydrazone compound.

Smart Thixotropic Hydrogels by Disulfide-Linked Short Peptides for Effective Three-Dimensional Cell Proliferation

Supramolecular assembly of short peptides is a crucial process and has shown numerous potential applications as biomaterials. In the present work, the hydrogelation process of short peptides containing C-terminal "Lys-Cys" (KC) residues have been studied in detail. The N-terminal capping is found to be essential for effective gelation. Out of 12 peptides we studied, two of them could form hydrogels efficiently: Ac-VVKC-NH₂ and Ac-FFKC-NH₂. In both cases, the monomer-to-dimer formation through disulfide linkages by Cys residues controls the aggregation process. Interestingly, the presence of H₂O₂ facilitated the dimerization and thereby reduced the gelation time but could not impart much effect on the mechanical properties of the gels. Detailed rheological study revealed that both hydrogels are thixotropic in nature. Moreover, they are responsive to glutathione (GSH) due to the presence of disulfide linkages. However, the hydrogel of Ac-FFKC-NH₂ is found to be stronger and more effective for biological applications. The thixotropic nature as well as a model drug release study in response to varying GSH concentration indicates the possible use of the hydrogel as an injectable local drug delivery vehicle. The hydrogel of Ac-FFKC-NH₂ is noncytotoxic in nature. Three-dimensional cell proliferation has been found to be more effective than 2D, as it mimics the in vivo situation more closely if not exactly. In the present study, we have shown that both differentiated RAW macrophages and undifferentiated THP-1 monocytes could proliferate significantly within the 3D matrix of the hydrogel, without depicting any apparent cytotoxicity. Thus, the hydrogel of Ac-FFKC-NH₂ has potential for application in localized drug administration and as a supporting biomaterial to study basic phenomena involving cell behavior.

Role of N-Oxide Moieties in Tuning Supramolecular Gel-State Properties

The role of specific interactions in the self-assembly process of low molecular weight gelators (LMWGs) was studied by altering the nonbonding interactions responsible for gel formation via structural modification of the gelator/nongelator. This was achieved by modifying pyridyl moieties of bis(pyridyl) urea-based hydrogelator (4–BPU) and the isomer (3–BPU) to pyridyl N–oxide compounds (L₁ and L₂, respectively). The modification of the functional groups resulted in the tuning of the gelation properties of the parent gelator, which induced/enhanced the gelation properties. The modified compounds displayed better mechanical and thermal stabilities and the introduction of the N–oxide moieties had a prominent effect on the morphologies of the gel network, which was evident from the scanning electron microscopy (SEM) images. The effect of various interactions due to the introduction of N–oxide moieties in the gel network formation was analyzed by comparing the solid-state interactions of the compounds using single crystal X-ray diffraction and computational studies, which were correlated with the enhanced gelation properties. This study shows the importance of specific nonbonding interactions and the spatial arrangement of the functional groups in the supramolecular gel network formation.

Amino Acid-Containing Phase-Selective Organogelators: A Water-Based Delivery System for Oil Spill Treatment

The simple structural modification of replacing a terminal carboxylic acid with a primary amide group was found to lower the minimum gelation concentration (MGC), by at least an order of magnitude, for a series of N-lauroyl-l-amino acid phase-selective organogelators in decane. The amide-functionalized analogue N-lauroyl-l-alanine-CONH₂ was demonstrated to gel a broad range of solvents from diesel to THF at MGCs of 2.5% w/v or less, as well as to produce gels with a higher thermal stability (ca. 30 °C) and enhanced mechanical properties (5 times increase in complex modulus), compared to the carboxylic acid analogue, N-lauroyl-l-alanine-COOH. These improved properties may be due to the additional hydrogen bonding in the primary amide analogue as revealed by SCXRD. Most significantly for this study, the introduction of the primary amide functionality enabled N-lauroyl-l-alanine-CONH₂ to form a self-assembled fibrillar network in water. The aqueous network could then actively uptake and rapidly gel decane, diesel, and diluted bitumen ("dilbit") with MGCs of 2.5% w/v or less. This aqueous delivery method is advantageous for oil-remediation applications as no harmful carrier solvents are required and the gel can be easily separated from the water, allowing the oil to be recovered and the gelator recycled.

Peripheral groups of polyhedral oligomeric silsesquioxane (POSS) core-based dendrimers: a crucial factor for higher-level supra-architecture building

The role of peripheral groups (PGs) on dendrimers in the spontaneous higher-level organization of hierarchically assembled nanofibers was investigated in a series of POSS-based dendritic gelators (POSS-Lys-X, X: -Boc, -Cbz, -Fmoc, etc.). We demonstrate that the PGs not only affect the gelation ability in solutions, but also the construction of orderly entangled fibrous supramolecular networks, e.g., "loofah-like" networks. Attributed to the PGs (especially the -Boc group) causing a lower cooperative assembly, the steady state with the lowest potential energy of gelators can be easily achieved by the higher ordering of nanofiber entanglement into superstructures. The -Boc group-containing dendrimers show low molar enthalpy and molar entropy of gelation, which help the construction of unique three-dimensional (3D) "loofah-like" superstructures. In contrast, the high cooperative assembly of the dendrimer (-Cbz as the PG) promotes the gelator into a higher enthalpy gelation process, with a constructed normal fibrous network. Hence, the PGs of POSS-based dendrimers act as the crucial factor in controlling the hierarchical self-assembly via a thermodynamics approach. This research presents new perspectives to explicate the relationships between PGs of dendrimers, supra-architectures and gel performances, which further guide the design of functional supramolecular materials via controllable self-assembly.

Sulfonamide as amide isostere for fine-tuning the gelation properties of physical gels

(S)-2-Stearamidopentanedioic acid (C18-Glu) is a known LMW gelator that forms supramolecular gels in a variety of solvents. In this work, we have carried out the isosteric substitution of the amide group by a sulfonamide moiety yielding the new isosteric gelator (S)-2-(octadecylsulfonamido)pentanedioic acid (Sulfo-Glu). The gelation ability and the key properties of the corresponding gels were compared in terms of gelation concentration, gel-to-sol transition temperature, mechanical properties, morphology, and gelation kinetics in several organic solvents and water. This comparison was also extended to (S)-2-(4-hexadecyl-1H-1,2,3-triazol-4-yl)pentanedioic acid (Click-Glu), which also constitutes an isostere of C18-Glu. The stabilizing interactions were explored through computational calculations. In general, Sulfo-Glu enabled the formation of non-toxic gels at lower concentrations, faster, and with higher thermal-mechanical stabilities than those obtained with the other isosteres in most solvents. Furthermore, the amide-sulfonamide isosteric substitution also influenced the morphology of the gel networks as well as the release rate of an embedded antibiotic (vancomycin) leading to antibacterial activity in vitro against Staphylococcus aureus.

Catalytic Gels for a Prebiotically Relevant Asymmetric Aldol Reaction in Water: From Organocatalyst Design to Hydrogel Discovery and Back Again

This paper reports an investigation into organocatalytic hydrogels as prebiotically relevant systems. Gels are interesting prebiotic reaction media, combining heterogeneous and homogeneous characteristics with a structurally organized active "solid-like" catalyst separated from the surrounding environment, yet in intimate contact with the solution phase and readily accessible via "liquid-like" diffusion. A simple self-assembling glutamine amide derivative 1 was initially found to catalyze a model aldol reaction between cyclohexanone and 4-nitrobenzaldehyde, but it did not maintain its gel structure during reaction. In this study, it was observed that compound 1 could react directly with the benzaldehyde to form a hydrogel in situ based on Schiff base 2 as a low-molecular-weight gelator (LMWG). This new dynamic gel is a rare example of a two-component self-assembled LMWG hydrogel and was fully characterized. It was demonstrated that glutamine amide 1 could select an optimal aldehyde component and preferentially assemble from mixtures. In the hunt for an organocatalyst, reductive conditions were applied to the Schiff base to yield secondary amine 3, which is also a highly effective hydrogelator at very low loadings with a high degree of nanoscale order. Most importantly, the hydrogel based on 3 catalyzed the prebiotically relevant aldol dimerization of glycolaldehyde to give threose and erythrose. In buffered conditions, this reaction gave excellent conversions, good diastereoselectivity, and some enantioselectivity. Catalysis using the hydrogel of 3 was much better than that using non-assembled 3-demonstrating a clear benefit of self-assembly. The results suggest that hydrogels offer a potential strategy by which prebiotic reactions can be promoted using simple, prebiotically plausible LMWGs that can selectively self-organize from complex mixtures. Such processes may have been of prebiotic importance.

Novel Approach to Generate a Self-Deliverable Ru(II)-Based Anticancer Agent in the Self-Reacting Confined Gel Space

Finding the most effective method for cancer treatment is one of the thought-provoking tasks. Drug delivery by collapsing of metallogel to the cancer cell is an appealing way out. Cancer cells have an acidic environment due to excessive accumulation of lactic acid. In this work, the novel G5 gelator with a strategically free carboxylic acid arm has been designed and fabricated and characterized by several spectroscopic and microscopic techniques. These experiments suggest the formation of an ordered supramolecular gel with clover-leaf-like morphology. Mechanical properties from rheological measurements suggest the viscoelastic nature of the gel. Furthermore, we have obtained crystals of G5 from the pure dimethyl sulfoxide solution, whereas gelation gets induced by addition of water. This G5 gelator loses its gelation capability once the carboxylate is esterified by layering with methanol, which furnished the crystals of Me-G5' (G5' = G5-H). Further, the G5 gelator is used for the formation of ruthenium metallogel. Interestingly, we obtained the monomeric species [Ru(G5')(η⁶-p-cymene)Cl] [Ru(II)G5] only in confined gel space upon addition of a [Ru₂(η⁶-p-cymene)₂Cl₄] dimer to G5. The Ru(II)G5 metallogel has an inherent anticancer property with an IC₅₀ value of 10.53 μM for the A549 cancer cell line. Treatment of the Ru(II)G5 metallogel by lactic acid for mimicking the acidic environment of the malignant cell results in collapsing of the gel by releasing the ruthenium metal ion. This released ruthenium ion binds with the lactic acid derivative making the gelator G5 free and producing a new compound Ru(II)L, which has also shown the anticancer property. The molecular docking study revealed that the released G5 could interact with a monocarboxylate transporter to disrupt the lactate transport chain, which might induce apoptosis.

Characterisation of hydrogels: Linking the nano to the microscale

Hydrogels are water enriched soft materials widely used for applications as varied as super absorbents, breast implants and contact lenses. Hydrogels have also been designed for smart functional devices including drug delivery, tissue engineering and diagnostics such as blood typing. The hydrogel properties and functionality depend on their crosslinking density, water holding capacity and fibre/polymer composition, strength and internal structure. Determining these parameters and properties are challenging. This review presents the main characterisation methods providing both qualitative and quantitative information of the structures and compositions of hydrogel. The length scale of interest ranges from the nano to the micro scale and the techniques and results are analysed in relationship to the hydrogel macroscopic applications. The characterisation methods examined aim at quantifying swelling, mechanical strength, mesh size, bound and free water content, pore structure, chemical composition, strength of chemical bonds and mechanical strength. These hydrogel parameters enable us to understand the fundamental mechanisms of hydrogel formation, to control their structure and functionality, and to optimize and tailor specific hydrogel properties to engineer particular applications.

In situ aldehyde-modification of self-assembled acyl hydrazide hydrogels and dynamic component selection from complex aldehyde mixtures

Acyl hydrazide functionalised hydrogels can react with aldehydes yielding modified gels with adapted performance, and can dynamically select specific aldehyde components from mixtures.

Discotic Organic Gelators in Ion Sensing, Metallogel Formation, and Bioinspired Catalysis

Two organogelators G2 and G3 with a carboxamide group have been synthesized and characterized with different spectroscopic tools. Dimethylformamide or dimethyl sulfoxide solutions of both the compounds upon the addition of a minute quantity of water show the tendency to form gels. Supramolecular self-assembly for gel formation paves the way for aggregation-induced emission enhancement (AIEE) phenomena for both the gelator molecules. Introduction of metal ions in organogels strengthens the gel property without much affecting the fluorescence behavior. However, the introduction of Ag⁺, Fe²⁺, and Fe³⁺ ions in the G2 organogel separately results in total quenching of AIEE, making it possible to sense that particular cation in the gel state. The G3 organogel shows a similar behavior with the Fe²⁺ ion. Remarkably, other metallogels such as Ni(II)G2 and Co(II)G2 can sense sulfide ion and Cu(II)G2 can sense iodide ion by switching off the fluorescence even in multianalyte conditions. Furthermore, the copper-based metallogel Cu(II)G2 can be utilized as a catalyst and reaction medium for aerobic oxidation of catechol to quinone. To the best of our knowledge, this is the first attempt known so far to utilize a metallogel material for bioinspired catalysis such as catechol oxidation.

Synthesis and supramolecular self-assembly of glutamic acid-based squaramides

We describe the preparation and characterization of two new unsymmetrical squaramide-based organogelators. The synthesis of the compounds was carried out by subsequent amine condensations starting from dimethyl squarate. The design of the gelators involved a squaramide core connected on one side to a long aliphatic chain and on the other side to a glutamic acid residue. The gelator bearing the free carboxylic groups showed a lower gelation capacity than its precursor diester derivative. Some selected gels were further studied by infrared spectroscopy, rheology and electron microscopy. Critical gelation concentrations and gel-to-sol transition temperatures were also determined for each case. In addition, the superior squaramide diester gelator was compared with an analogue triazole-based gelator in terms of critical gelation concentration, gelation kinetics and thermal phase transition.

Increased rate of solvent diffusion in a prototypical supramolecular gel measured on the picosecond timescale

Solvent diffusion in a prototypical supramolecular gel probed by quasi-elastic neutron scattering on the picosecond timescale is faster than that in the respective bulk solvent.