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.

Rapid Semiconducting Supramolecular Mg(II)-Metallohydrogel: Exploring Its Potential in Nonvolatile Resistive Switching Applications and Antiseptic Wound Healing Properties

An effective strategy was employed for the rapid development of a supramolecular metallohydrogel of Mg(II) ion (i.e., Mg@PEHA) using pentaethylenehexamine (PEHA) as a low-molecular-weight gelator in aqueous medium under ambient conditions. The mechanical stability of the synthesized Mg@PEHA metallohydrogel was characterized by using rheological analysis, which showed its robustness across different angular frequencies and oscillator stress levels. The metallohydrogel exhibited excellent thixotropic behavior, which signifies that Mg@PEHA has a self-healing nature. Field emission scanning electron microscopy and transmission electron microscopy images were utilized to explore the rectangular pebble-like hierarchical network of the Mg@PEHA metallohydrogel. Elemental mapping through energy-dispersive X-ray spectroscopy analysis confirmed the presence of primary chemical constituents in the metallohydrogel. Fourier transform infrared spectroscopy spectroscopy provided insights into the possible formation strategy of the metallohydrogel. In this work, Schottky diode structures in a metal-semiconductor-metal geometry based on a magnesium(II) metallohydrogel (Mg@PEHA) were constructed, and the charge transport behavior was observed. Additionally, a resistive random access memory (RRAM) device was developed using Mg@PEHA, which displayed bipolar resistive switching behavior at room temperature. The researchers investigated the switching mechanism, which involved the formation or rupture of conduction filaments, to gain insights into the resistive switching process. The RRAM device demonstrated excellent performance with a high ON/OFF ratio of approximately 100 and remarkable endurance of over 5000 switching cycles. RRAM devices exhibit good endurance, meaning they can endure a large number of read and write cycles without significant degradation in performance. RRAM devices have shown promising reliability in terms of long-term performance and stability, making them suitable for critical applications that require reliable memory solutions. Significant inhibitory activity against the drug-resistant Klebsiella pneumonia strain and its biofilm formation ability was demonstrated by Mg@PEHA. The minimum inhibitory concentration value of the metallohydrogel was determined to be 3 mg/mL when it was dissolved in 1% DMSO. To study the antibiofilm activity, an MTT assay was performed, revealing that biofilm inhibition (60%) commenced at 1 mg/mL of Mg@PEHA when dissolved in 1% DMSO. Moreover, in the mouse excisional wound model, Mg@PEHA played a crucial role in preventing postoperative wound infections and promoting wound healing.

Solvent-Driven Variations of Third-Order Nonlinear Thermo-Optical Features of Glutaric Acid-Directed Self-Healing Supramolecular Ni(II) Metallogels

The generation of solvent-directed self-healing supramolecular Ni(II) metallogels of glutaric acid (i.e., Ni-Glu-DMF and Ni-Glu-DMSO) is described in this article. Polar aprotic solvents like N,N'-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) are separately entrapped into the Ni(II)-acetate salt and glutaric acid-mediated networks to attain the semisolid flexible scaffolds. The gel nature of the fabricated materials is experimentally proven through different rheological tests such as amplitude sweep, frequency sweep, and thixotropic (time sweep) measurements. The self-repairing strategy and load-bearing features of the synthesized metallogel are studied in this work. The different supramolecular noncovalent interactions working within the soft scaffold are clearly explored. The formation strategy and the microstructural features of these synthesized metallogels are scrutinized through a Fourier transform infrared (FT-IR) spectroscopy study and field-emission scanning electron microscopy (FESEM) morphological analyses. The FT-IR spectroscopy observation displays a considerable amount of shifting of the infrared (IR) peaks of the xerogel samples of both the metallogels Ni-Glu-DMF and Ni-Glu-DMSO. The electrospray ionization (ESI)-mass spectroscopy result demonstrates the plausible construction of the metallogel network. In order to examine the nonlinear optical characteristics of the two synthesized self-healing metallogels Ni-Glu-DMSO and Ni-Glu-DMF, Z-scan measurements are carried out with a continuous wave (CW) diode-pumped solid-state (DPSS) laser at 532 nm. The nonlinear refractive index, nonlinear absorption coefficient, thermo-optical coefficient, and third-order susceptibility of these metallogels were evaluated by analyzing the experimental data from the Sheik-Bahae formalism. The nonlinear thermo-optical study reveals that these solvent-dependent metallogels show negative signs of nonlinear refractive index and nonlinear absorption coefficient. The figure of merit calculated for these compounds shows good agreement for their use in nonlinear photonic devices.

Motorized Photomodulator: Making A Non-photoresponsive Supramolecular Gel Switchable by Light

Introducing photo-responsive molecules offers an attractive approach for remote and selective control and dynamic manipulation of material properties. However, it remains highly challenging how to use a minimal amount of photo-responsive units to optically modulate materials that are inherently inert to light irradiation. Here we show the application of a light-driven rotary molecular motor as a "motorized photo-modulator" to endow a typical H-bond-based gel system with the ability to respond to light irradiation and create a reversible sol-gel transition. The key molecular design feature is the introduction of a minimal amount (2 mol %) of molecular motors into the supramolecular network as photo-switchable non-covalent crosslinkers. Advantage is taken of the subtle interplay of the large geometry change during photo-isomerization of the molecular motor guest and the dynamic nature of a supramolecular gel host system. As a result, a tiny amount of molecular motors is enough to switch the mechanical modulus of the entire supramolecular systems. This study proves the concept of designing photo-responsive materials with minimum use of non-covalent light-absorbing units.

Anion-assisted supramolecular polymerization of luminescent organic π-conjugated chromophores in a moderately polar solvent: tunable nanostructures and their corresponding effects on electronic properties

Supramolecular polymers of π-conjugated organic chromophores have emerged as promising candidates in organic electronics because of their dynamic and highly ordered molecular organization. Herein, we demonstrate the formation of luminescent, highly conducting supramolecular polymers of a functionalized naphthalimide π-chromophore-based organic semiconductor in a moderately polar organic solvent (tetrahydrofuran) by overcoming solute-solvent H-bonding via assistance from fluoride anions. The polymerization is exclusively guided by the synergistic effects of cascade H-bonding (F-⋯H-N- of primary amines, followed by -CO⋯H-N- of amides), π-π stacking and hydrophobic interactions. An increasing molar equivalent of anions leads to a morphology transition from 1D nanowires to 2D nanosheets via nanotubes and nanorings, but above a particular threshold of the same anion, depolymerization-mediated disruption of long-range order and formation of non-luminescent spherical particles was observed. Such significant impacts of anions in supramolecular polymerization-depolymerization were utilized in modulating the electronic properties of this naphthalimide-based organic semiconductor.

Effect of Imposing Spatial Constraints on Low Molecular Weight Gels

We outline the effect of imposing spatial constraints during gelation on hydrogels formed by dipeptide-based low molecular weight gelators. The gels were formed via either a solvent switch or a change in pH and formed in different sized vessels to produce gels of different thickness while maintaining the same volume. The different methods of gelation led to gels with different underlying microstructure. Confocal microscopy was used to visualize the resulting microstructures, while the corresponding mechanical properties were probed via cavitation rheology. We show that solvent-switch-triggered gels are sensitive to imposed spatial constraints, in both altered microstructure and mechanical properties, while their pH-triggered equivalents are not. These results are significant because it is often necessary to form gels of different thicknesses for different analytical techniques. Also, gels of different thicknesses are utilized between various applications of these materials. Our data show that it is important to consider the spatial constraints imposed in these situations.

Alkaline cations dramatically control molecular hydrogelation by an amino acid-derived anionic amphiphile

Understanding the factors that control the formation of (supra)molecular hydrogels permits a rational tuning of their properties and represents a primary challenge for developing smart biocompatible soft materials. Hydrogel formation by molecular amphiphilic anions at millimolar concentrations is counterintuitive, considering the solubility of these species in water. Here we report hydrogel formation by a simple anionic molecular amphiphile and a rationale for the fibrillisation process observed. The studied molecule, DodValSuc, consists of a 12C alkyl chain, an l-valine unit and a terminal succinic acid moiety. Hydrogelation depends to a large degree on the nature and concentration of the alkaline cations present in the medium (Li⁺, Na⁺ or K⁺). As a result, gelation efficiency and properties like thermal stability or rheology are highly tunable using the alkaline cation present or its concentration as variables. A detailed study is reported, which includes the determination of minimum gelation concentration (MGC) by tabletop rheology, critical micelle concentration (CMC) using pyrene as a fluorescent probe, thermal stability (solubility) by ¹H NMR, the morphology of the fibres by transmission electron microscopy (TEM), crystallinity by X-ray diffraction (XRD) and gel strength by oscillatory rheology. Additionally, dynamic light scattering (DLS) was used to evaluate the size of the micelles and permitted monitoring of the fibrillisation process. Altogether, the results are consistent with the formation of micelles that experience head crystallisation and subsequent aggregation into crystalline fibres. The alkaline cations play a crucial role in providing the cement that glues together the gelator molecules, making their concentration a critical parameter for gelation efficiency and properties. Furthermore, the gelation-promoting effects are inversely correlated with the size of the cations so that the highest thermal stability and rheological strength were found for the hydrogels formed in the presence of Li^+.

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.

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

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

Designer Gelators for the Crystallization of a Salt Active Pharmaceutical Ingredient-Mexiletine Hydrochloride

We report an approach to obtain drug-mimetic supramolecular gelators, which are capable of stabilizing metastable polymorphs of the pharmaceutical salt mexiletine hydrochloride, a highly polymorphic antiarrhythmic drug. Solution-phase screening led to the discovery of two new solvated solid forms of mexiletine, a type C 1,2,4-trichlorobenzene tetarto-solvate and a type D nitrobenzene solvate. Various metastable forms were crystallized within the gels under conditions which would not have been possible in solution. Despite typically crystallizing concomitantly with form 1, a pure sample of form 3 was crystallized within a gel of ethyl methyl ketone. Various type A channel solvates were crystallized from gels of toluene and ethyl acetate, in which the contents of the channels varied from those of solution-phase forms. Most strikingly, the high-temperature-stable form 2 was crystallized from a gel in 1,2-dibromoethane: the only known route to access this form at room temperature. These results exemplify the powerful stabilizing effect of drug-mimetic supramolecular gels, which can be exploited in pharmaceutical polymorph screens to access highly metastable or difficult-to-nucleate solid forms.

Understanding the Interaction of Gluconamides and Gluconates with Amino Acids in Hair Care

A hair care mixture formed from a gluconamide derivative and 3-hydroxypropyl ammonium gluconate is known to strengthen hair fibers; however, the mechanism by which the mixture affects hair is unknown. To give insight into the aggregation of the target gluconamide and potential interactions between the gluconate-derived mixture and hair fibers, a range of systems were characterized by X-ray crystallography namely two polymorphic forms of the target gluconamide and three salts of 3-hydroxypropylammonium with sulfuric acid, methane sulfonic acid, and oxalic acid. The gluconamide proves to aggregate and becomes a supramolecular gelator in aniline and benzyl alcohol solution. The resulting gels were characterized by rheology, scanning electron microscopy, proton nuclear magnetic resonance, Fourier transform infrared spectroscopy, and powder X-ray diffraction.

Hybrid Hydrogels from Nongelling Polymers Using a Fibrous Peptide Hydrogelator at Low Concentrations

Nature-made hydrogels typically combine a wide range of multiscale fibers into biological composite networks, which offer an adaptive property. Inspired by nature, we report a facile approach to construct hybrid hydrogels from a range of natural or commercially available synthetic nongelling polymers (e.g., poly(ethylene glycol), poly(acrylic acid), carboxylated cellulose nanocrystal, and sodium alginate) at a concentration as low as 0.53 wt % using a nonionic fibrous peptide hydrogelator. Through simply mixing the peptide hydrogelator with a polymer aqueous solution, stable hybrid hydrogels can be formed with the concentration of hydrogelator at ∼0.05 wt %. The gel strength of the resulting hydrogels can be effectively modulated by the concentration, molecular weight, and terminal group of the polymer. We further demonstrate that the molecular interactions between the peptide hydrogelator and the polymer are very crucial for the formation of hybrid hydrogel, which synergically induce the gelation at considerably low concentrations. A peptide hydrogelator can be easily obtained by aminolysis of alkyl-oilgo(γ-benzyl-l-glutamate) samples. Live/Dead assays indicate low cytotoxicity of the hybrid hydrogel toward HeLa cells. Combining the low-cost, scalable synthesis, and biocompatibility, the prepared peptide hydrogelator presents a potential candidate to expand the scope of polymer hydrogels for biomedical applications and also shows considerable commercial significance.

Adipic acid directed self-healable supramolecular metallogels of Co(II) and Ni(II): intriguing scaffolds for comparative optical-phenomenon in terms of third-order optical non-linearity

Two brilliant outcomes of supramolecular self-assembly directed, low molecular weight organic gelator based self-healable Co(II) and Ni(II) metallogels were achieved. Adipic acid as the low molecular weight organic gelator and dimethylformamide (DMF) solvent are employed for the metallogelation process. Rheological analyses of both gel-scaffolds reveal mechanical toughness as well as visco-elasticity. Thixotropic behaviours of both the gels were scrutinized. Morphological variations due to the presence of two different metal ions with diverse metal-ligand coordinating interactions were established. The mechanistic pathways for forming stable metallogels of Co(II)-adipic acid (Co-AA) and Ni(II)-adipic acid (Ni-AA) were judiciously developed through infrared absorption spectral analysis. The nonlinear optical properties, such as the third-order process, of these synthesized metallogels were scrutinized by means of the Z-scan method at a beam excitation wavelength of 750 nm by a femtosecond laser with different excitation intensities ranging from 64 to 140 GW cm^-2. The third-order nonlinear optical susceptibility (χ⁽³⁾) of the order of 10^-14 esu was obtained from the measured Z-scan data. Both the metallogels exhibit positive nonlinear refraction and reverse saturable (RSA) absorption at high-intensity excitation. Co(II) and Ni(II) metallogels show nonlinear refractive indices (n₂^I) of (3.619 ± 0.146) × 10^-6 cm² GW^-1 and (3.472 ± 0.102) × 10^-6 cm² GW^-1, respectively, and two photon absorption coefficients (β) of (1.503 ± 0.045) × 10^-1 cm GW^-1 and (1.381 ± 0.029) × 10^-1 cm GW^-1 at an excitation intensity of 140 GW cm^-2. We also studied the optical limiting properties with a limiting threshold of 9.57 mJ cm^-2. Therefore, both metallogels can be considered promising materials for photonic devices: for instance, for optical switching and optical limiting.

Domination of H-Bond Interactions in the Solvent-Triggering Gelation Process

Gels prepared with the solvent-triggering method are attractive for their easy and fast preparation; however, the role of solvents in this process remains unclear, which hinders the efficient and accurate control of desired gel properties. In this study, the role of solvents in the solvent-triggering gelation process is studied using 9-fluorenylmethoxycarbonyl (Fmoc)-protected diphenylalanine (Fmoc-FF) as the gelator. Density functional theory (DFT)-based calculations and corresponding wavefunction analyses are conducted to identify the H-bonding interaction sites between the molecules. The calculation results clearly annotate the activating role of DMF and the triggering role of H₂O in the gelation process. The solvation of Fmoc-FF by DMF can activate the H-bonding sites on the peptide chain, showing a conformation reversal and higher electrostatic potentials. Then, the H-bonding between Fmoc-FF and H₂O is facilitated to trigger gelation. The physical Fmoc-FF/DMF/H₂O gels show easily tuned mechanical strengths (G' of 10²-10⁵ Pa), injectable potentials (general yield strain < 100%), and stable recoverability (80-98% within 100 s). The regulation of these properties depends on not only the gelator concentration but also the H-bonding interactions with solvent molecules, which have seldom been studied in detail before. By understanding the effect of solvents, low-molecular-weight gelator-based gels can be designed, prepared, and tuned efficiently for potential applications.

Nucleoside-Derived Low-Molecular-Weight Gelators as a Synthetic Microenvironment for 3D Cell Culture

For the last few decades, many efforts have been made in developing cell culture methods in order to overcome the biological limitations of the conventional two-dimensional culture. This paradigm shift is driven by a large amount of new hydrogel-based systems for three-dimensional culture, among other systems, since they are known to mimic some living tissue properties. One class of hydrogel precursors has received interest in the field of biomaterials, low-molecular-weight gelators (LMWGs). In comparison to polymer gels, LMWG gels are formed by weak interactions upon an external trigger between the molecular subunits, giving them the ability to reverse the gelation, thus showing potential for many applications of practical interest. This study presents the use of the nucleoside derivative subclass of LMWGs, which are glyco-nucleo-bola-amphiphiles, as a proof of concept of a 3D cell culture scaffold. Physicochemical characterization was performed in order to reach the optimal features to fulfill the requirements of the cell culture microenvironment, in terms of the mechanical properties, architecture, molecular diffusion, porosity, and experimental practicality. The retained conditions were tested by culturing glioblastoma cells for over a month. The cell viability, proliferation, and spatial organization showed during the experiments demonstrate the proof of concept of nucleoside-derived LMWGs as a soft 3D cell culture scaffold. One of the hydrogels tested permits cell proliferation and spheroidal organization over the entire culture time. These systems offer many advantages as they consume very few matters within the optimal range of viscoelasticity for cell culture, and the thermoreversibility of these hydrogels permits their use with few instruments. The LMWG-based scaffold for the 3D cell culture presented in this study unlocked the ability to grow spheroids from patient cells to reach personalized therapies by dramatically reducing the variability of the lattice used.

Crystal Engineering of Pharmaceutical Cocrystals in the Discovery and Development of Improved Drugs

The subject of crystal engineering started in the 1970s with the study of topochemical reactions in the solid state. A broad chemical definition of crystal engineering was published in 1989, and the supramolecular synthon concept was proposed in 1995 followed by heterosynthons and their potential applications for the design of pharmaceutical cocrystals in 2004. This review traces the development of supramolecular synthons as robust and recurring hydrogen bond patterns for the design and construction of supramolecular architectures, notably, pharmaceutical cocrystals beginning in the early 2000s to the present time. The ability of a cocrystal between an active pharmaceutical ingredient (API) and a pharmaceutically acceptable coformer to systematically tune the physicochemical properties of a drug (i.e., solubility, permeability, hydration, color, compaction, tableting, bioavailability) without changing its molecular structure is the hallmark of the pharmaceutical cocrystals platform, as a bridge between drug discovery and pharmaceutical development. With the design of cocrystals via heterosynthons and prototype case studies to improve drug solubility in place (2000-2015), the period between 2015 to the present time has witnessed the launch of several salt-cocrystal drugs with improved efficacy and high bioavailability. This review on the design, synthesis, and applications of pharmaceutical cocrystals to afford improved drug products and drug substances will interest researchers in crystal engineering, supramolecular chemistry, medicinal chemistry, process development, and pharmaceutical and materials sciences. The scale-up of drug cocrystals and salts using continuous manufacturing technologies provides high-value pharmaceuticals with economic and environmental benefits.

Triarylamine Enriched Organostannoxane Drums: Synthesis, Optoelectrochemical Properties, Association Studies, and Gelation Behavior

The straightforward synthesis of three organotin clusters endowed with six triarylamine-based moieties is reported herein. The optoelectronic properties of the molecules, as well as their ability to form gels, were investigated. The association ability of the compounds was studied as well by means of variable temperature nuclear magnetic resonance (NMR) and ultraviolet-visible (UV-vis) spectroscopy. The optimization of the geometry of the compounds has been performed and compared to the X-ray diffraction of the crystals. The results obtained through this comparison are useful for the explanation of their different gelation behaviors. In fact, organostannoxane drum 1 exhibits a strong ability to form organized supramolecular structures by means of a number of noncovalent short contacts that finally yield luminescent organogels in aromatic solvents.

Gamma Radiation- and Ultraviolet-Induced Polymerization of Bis(amino acid)fumaramide Gel Assemblies

Controlling the polymerization of supramolecular self-assembly through external stimuli holds great potential for the development of responsive soft materials and manipulation at the nanoscale. Vinyl esters of bis(leu or val)fumaramide (1a and 2a) have been found to be gelators of various organic solvents and were applied in this investigation of the influence of organogelators’ self-assembly on solid-state polymerization induced by gamma and ultraviolet irradiation. Here, we report our investigation into the influences of self-assemblies of bis(amino acid vinyl ester)fumaramides on gamma-ray- and ultraviolet-induced polymerization. The gelator molecules self-assembled by non-covalent interactions, mainly through hydrogen bonds between the amide group (CONH) and the carboxyl group (COO), thus forming a gel network. NMR and FTIR spectroscopy were used to investigate and characterize supramolecular gels. TEM and SEM microscopy were used to investigate the morphology of gels and polymers. Morphology studies showed that the gels contained a filamentous structure of nanometer dimensions that was exhaustive in a three-dimensional network. The prepared derivatives contained reactive alkyl groups suitable for carrying out the polymerization reaction initiated by gamma or ultraviolet radiation in the supramolecular aggregates of selected gels. It was found that the polymerization reaction occurred only in the network of the gel and was dependent on the structure of aggregates or the proximity and orientation of double bonds in the gel network. Polymers were formed by the gels exposure to gamma and ultraviolet radiation in toluene, and water/DMF gels with transcripts of their gel structure into polymers. The polymeric material was able to immobilize various solvents by swelling. Furthermore, methyl esters of bis(leu and val)fumaramide (1b and 2b) were synthesized; these compounds showed no gelling properties, and the crystal structure of the valine derivative 2b was determined.

A multistimulus-responsive self-healable supramolecular copper(ii)-metallogel derived from l-(+) tartaric acid: an efficient Schottky barrier diode

A low molecular weight gelator l-(+) tartaric acid- based self-healing supramolecular Cu(ii)-metallogel offers an electronic device of Schottky barrier diode at room temperature.

A Review on Synthesis Methods of Phyllosilicate- and Graphene-Filled Composite Hydrogels

This review discusses, in brief, the various synthetic methods of two widely-used nanofillers; phyllosilicate and graphene. Both are 2D fillers introduced into hydrogel matrices to achieve mechanical robustness and water uptake behavior. Both the fillers are inserted by physical and chemical gelation methods where most of the chemical gelation, i.e., covalent approaches, results in better physical properties compared to their physical gels. Physical gels occur due to supramolecular assembly, van der Waals interactions, electrostatic interactions, hydrophobic associations, and H-bonding. For chemical gelation, in situ radical triggered gelation mostly occurs.

Self-assembled sonogels formed from 1,4-naphthalenedicarbonyldinicotinic acid hydrazide

Ultrasound-induced gelation of a novel type of gelator, 1,4-naphthalenedicarbonyl- dinicotinic acid hydrazide, is reported. The gelator self-assembled into various architectures in different solvents.

Emerging iongel materials towards applications in energy and bioelectronics

In the past two decades, ionic liquids (ILs) have blossomed as versatile task-specific materials with a unique combination of properties, which can be beneficial for a plethora of different applications. The additional need of incorporating ILs into solid devices led to the development of a new class of ionic soft-solid materials, named here iongels. Nowadays, iongels cover a wide range of materials mostly composed of an IL component immobilized within different matrices such as polymers, inorganic networks, biopolymers or inorganic nanoparticles. This review aims at presenting an integrated perspective on the recent progress and advances in this emerging type of material. We provide an analysis of the main families of iongels and highlight the emerging types of these ionic soft materials offering additional properties, such as thermoresponsiveness, self-healing, mixed ionic/electronic properties, and (photo)luminescence, among others. Next, recent trends in additive manufacturing (3D printing) of iongels are presented. Finally, their new applications in the areas of energy, gas separation and (bio)electronics are detailed and discussed in terms of performance, underpinning it to the structural features and processing of iongel materials.

Skin-like hydrogel devices for wearable sensing, soft robotics and beyond

Skin-like electronics are developing rapidly to realize a variety of applications such as wearable sensing and soft robotics. Hydrogels, as soft biomaterials, have been studied intensively for skin-like electronic utilities due to their unique features such as softness, wetness, biocompatibility and ionic sensing capability. These features could potentially blur the gap between soft biological systems and hard artificial machines. However, the development of skin-like hydrogel devices is still in its infancy and faces challenges including limited functionality, low ambient stability, poor surface adhesion, and relatively high power consumption (as ionic sensors). This review aims to summarize current development of skin-inspired hydrogel devices to address these challenges. We first conduct an overview of hydrogels and existing strategies to increase their toughness and conductivity. Next, we describe current approaches to leverage hydrogel devices with advanced merits including anti-dehydration, anti-freezing, and adhesion. Thereafter, we highlight state-of-the-art skin-like hydrogel devices for applications including wearable electronics, soft robotics, and energy harvesting. Finally, we conclude and outline the future trends.

Smart and Functionalized Development of Nucleic Acid-Based Hydrogels: Assembly Strategies, Recent Advances, and Challenges

Nucleic acid-based hydrogels that integrate intrinsic biological properties of nucleic acids and mechanical behavior of their advanced assemblies are appealing bioanalysis and biomedical studies for the development of new-generation smart biomaterials. It is inseparable from development and incorporation of novel structural and functional units. This review highlights different functional units of nucleic acids, polymers, and novel nanomaterials in the order of structures, properties, and functions, and their assembly strategies for the fabrication of nucleic acid-based hydrogels. Also, recent advances in the design of multifunctional and stimuli-responsive nucleic acid-based hydrogels in bioanalysis and biomedical science are discussed, focusing on the applications of customized hydrogels for emerging directions, including 3D cell cultivation and 3D bioprinting. Finally, the key challenge and future perspectives are outlined.

Porous Colloidal Hydrogels Formed by Coordination-Driven Self-Assembly of Charged Metal-Organic Polyhedra

Introduction of porosity into supramolecular gels endows soft materials with functionalities for molecular encapsulation, release, separation and conversion. Metal-organic polyhedra (MOPs), discrete coordination cages containing an internal cavity, have recently been employed as building blocks to construct polymeric gel networks with potential porosity. However, most of the materials can only be synthesized in organic solvents, and the examples of porous, MOP-based hydrogels are scarce. Here, we demonstrate the fabrication of porous hydrogels based on [Rh₂ (OH-bdc)₂ ]₁₂ , a rhodium-based MOP containing hydroxyl groups on its periphery (OH-bdc=5-hydroxy-1,3-benzenedicarboxylate). By simply deprotonating [Rh₂ (OH-bdc)₂ ]₁₂ with the base NaOH, the supramolecular polymerization between MOPs and organic linkers can be induced in the aqueous solution, leading to the kinetically controllable formation of hydrogels with hierarchical colloidal networks. When heating the deprotonated MOP, Na_x [Rh₂₄ (O-bdc)_x (OH-bdc)_24-x ], to induce gelation, the MOP was found to partially decompose, affecting the mechanical property of the resulting gels. By applying a post-synthetic deprotonation strategy, we show that the deprotonation degree of the MOP can be altered after the gel formation without serious decomposition of the MOPs. Gas sorption measurements confirmed the permanent porosity of the corresponding aerogels obtained from these MOP-based hydrogels, showing potentials for applications in gas sorption and catalysis.

Supramolecular Template-Directed In Situ Click Chemistry: A Bioinspired Approach to Synthesize G-Quadruplex DNA Ligands

The assembly of guanosine and boronic acids produces anionic hydrogels (G-B hydrogels) that mimic the topology of the DNA G-quadruplex. We herein demonstrate an unconventional approach of using the G-B hydrogel as a supramolecular template that assembles the irreversible formation of DNA G-quadruplex-selective 1,4-triazole ligands from a pool of alkyne-azide building blocks. These generated ligands could also stabilize and strengthen the gel assembly.

Magneto-responsive hydrogels by self-assembly of low molecular weight peptides and crosslinking with iron oxide nanoparticles

Hydrogels that respond to non-invasive, external stimuli such as a magnetic field are of exceptional interest for the development of adaptive soft materials. To date magneto tuneable gels are predominantly based on macromolecular building blocks, while comparable low molecular weight systems are rarely found in the literature. Herein, we report a highly efficient peptide-based gelator (Nap GFYE), which can form hydrogels and incorporate Fe3O4 superparamagnetic nanoparticles in the gel matrix. The magnetic nanoparticles act as a physical crosslinker for the self-assembled peptide nanostructures and thus give rise to a fortified hybrid gel with distinctively improved mechanical properties. Furthermore, the particles provide the material with magnetic susceptibility and a gel to sol transition is observed upon application of a weak magnetic field. Magnetization of the inorganic-organic hybrid nanomaterial leads to on-demand release of an incorporated fluorescent dye into the supernatant.

Bolaamphiphile-based supramolecular gels with drugs eliciting membrane effects

Supramolecular chemistry has garnered important interest in recent years toward improving therapeutic efficacy via drug delivery approaches. Although self-assemblies have been deeply investigated, the design of novel drugs leveraging supramolecular chemistry is less known. In this contribution, we show that a Low Molecular Weight Gel (LMWG) can elicit cancer cell apoptosis. This biological effect results from the unique supramolecular properties of a bolaamphiphile-based gelator, which allow for strong interaction with the lipid membrane. This novel supramolecular-drug paradigm opens up new possibilities for therapeutic applications targeting membrane lipids.

Self-Assembly of Nucleoside-Derived Low-Molecular-Weight Gelators: A Thermodynamics and Kinetics Study on Different Length Scales

Biocompatible materials are of paramount importance in numerous fields. Unlike chemically bridge polymer-based hydrogels, low-molecular-weight gelators can form a reversible hydrogel as their structures rely on noncovalent interaction. Although many applications with this type of hydrogel can be envisioned, we still lack their understanding due to the complexity of their self-assembly process and the difficulty in predicting their behaviors (transition temperature, gelation kinetics, the impact of solvent, etc.). In this study, we extend the investigations of a series of nucleoside-derived gelators, which only differ by subtle chemical modifications. Using a multitechnique approach, we determined their thermodynamic and kinetic features on various scale (molecular to macro) in different conditions. Monitored at the supramolecular level by circular dichroism as well as macroscopic scales by rheology and turbidimetry, we found out that the sol-gel and gel-sol transitions are greatly dependent on the concentration and on the mechanisms that are probed. Self-assembly kinetics depends on hydrogel molecules and is modulated by temperature and solvent. This fundamental study provides insight on the impact of some parameters on the gelation process, such as concentration, cooling rate, and the nature of the solvent.

Glucose Binding Drives Reconfiguration of a Dynamic Library of Urea-Containing Metal-Organic Assemblies

A bis-urea-functionalized ditopic subcomponent assembled with 2-formylpyridine and Fe^II , resulting in a dynamic library of metal-organic assemblies: an irregular Fe^II ₄ L₆ structure and three Fe^II ₂ L₃ stereoisomers: left- and right-handed helicates and a meso-structure. This library reconfigured in response to the addition of monosaccharide derivatives, which served as guests for specific library members, and the rate of saccharide mutarotation was also enhanced by the library. The (P) enantiomer of the Fe^II ₂ L₃ helical structure bound β-D-glucose selectively over α-D-glucose. As a consequence, the library collapsed into the (P)-Fe^II ₂ L₃ helicate following glucose addition. The α-D-glucose was likewise transformed into the β-D-anomer during equilibration and binding. Thus, β-D-glucose and (P)-3 amplified each other in the product mixture, as metal-organic and saccharide libraries geared together into a single equilibrating system.

Tuning of Morphology by Chirality in Self-Assembled Structures of Bis(Urea) Amphiphiles in Water

We present the synthesis and self‐assembly of a chiral bis(urea) amphiphile and show that chirality offers a remarkable level of control towards different morphologies. Upon self‐assembly in water, the molecular‐scale chiral information is translated to the mesoscopic level. Both enantiomers of the amphiphile self‐assemble into chiral twisted ribbons with opposite handedness, as supported by Cryo‐TEM and circular dichroism (CD) measurements. The system presents thermo‐responsive aggregation behavior and combined transmittance measurements, temperature‐dependent UV, CD, TEM, and micro‐differential scanning calorimetry (DSC) show that a ribbon‐to‐vesicles transition occurs upon heating. Remarkably, chirality allows easy control of morphology as the self‐assembly into distinct aggregates can be tuned by varying the enantiomeric excess of the amphiphile, giving access to flat sheets, helical ribbons, and twisted ribbons.

Multi-responsive organo- and hydrogelation based on the supramolecular assembly of fluorocarbon- and hydrocarbon-containing hybrid surfactants

HYPOTHESIS

Novel photoresponsive hybrid surfactants, in which a combination of perfluoroalkyl and alkyl chains and cationic head groups are connected via azobenzene moieties, are excellent candidates for assembling low-molecular-weight organogels (LMOGs) with reversibly switchable viscoelasticities triggered by external stimuli.

EXPERIMENTS

The structure-composition-property relationships of gels assembled with the hybrid surfactants were investigated by UV-vis and NMR spectroscopy, SEM, XRD, and rheology.

FINDINGS

Hybrid surfactants containing perfluorohexyl chains with more than six carbons gelled in a variety of organic solvents at concentrations of less than a few percent. In particular, compositions with the perfluorooctyl and somewhat shorter hydrocarbon chains (C1-C4) gelled in both organic solvents and water. The gellable solvent species can be well grouped according to their solubility parameters, suggesting that gelation properties can be predicted from the chemical structure of the surfactant. Mechanical and structural investigations revealed that gel viscoelasticity can be reversibly altered by applying photo, shear, and heat stimuli, which is achieved through the formation and deformation of lamella-like molecular aggregates. The multi-responsive gelation and facile molecular design of the present hybrid surfactants will expand the fields in which fluorinated LMOGs can be applied.

Protonation-Triggered Supramolecular Gel from Macrocyclic Diacetylene: Gelation Behavior, Topochemical Polymerization, and Colorimetric Response

Supramolecular gels originating from the hierarchical self-assembly of low molecular weight organic molecules is a strongly emerging field of advanced material research for the fabrication of soft functional materials. Herein, a novel supramolecular gel was fabricated through the protonation-triggered unidirectional self-assembly of pyridine-attached macrocyclic diacetylene (PyMCDA). Basic nitrogen of a pyridine ring with a strong affinity toward proton transforms the neutral PyMCDA into gelator in its protonated pyridinium salt form (PyMCDA-H⁺), which further evolves to nano-fibrillar networks to yield a supramolecular gel. Under the irradiation of UV light, the white color gel turned to a robust covalently cross-linked blue-phase PDA gel. Interestingly, polymeric PyMCPDA-H⁺ gel exhibits a naked-eye detectable reversible blue-red colorimetric response for alternating acid/base (H₂SO₄/NH₄OH) and colorimetric sensitivity toward selected anions: CH₃COO^-, CN^-, HCOO^-, and CH₃CH₂COO^-. It is with the hope that this work point toward the utility and versatility of macrocyclic PDAs for constructing chromogenic supramolecular gels for their possible use in sensing systems.

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.

The aging effect on the enhancement of thermal stability, mechanical stiffness and fluorescence properties of histidine-appended naphthalenediimide based two-component hydrogels

A histidine attached naphthalenediimide (NDI)-containing amphiphilic molecule (NDIP) self-assembles into nanotubes in aqueous solution at pH 6.6 as revealed by high-resolution transmission electron microscopy studies. This histidine-appended NDI forms a two-component hydrogel in the presence of tartaric acid at a molar ratio of 1 : 2. A morphological transformation was observed from a nanotube structure in the non-gel aggregated state of histidine appended NDI to interconnected cross-linked nanofibers of the two-component hydrogel in the presence of tartaric acid. Interestingly, the gel exhibits an unusual behavior upon aging compared to the fresh gel. It is found that the thermal stability and gel stiffness increase very significantly upon aging. Another important feature noted is that the very weak fluorescence of the fresh gel is transformed into bright greenish fluorescence upon aging. These results suggest that intermolecular interactions among the gelator molecules and tartaric acid in the gel phase slowly increase with time to form a mechanically very stiff and thermally robust gel.