Recent Advances in Supramolecular Gels and Catalysis

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.

Nanoarchitectonics from Atom to Life

Functional materials with rational organization cannot be directly created only by nanotechnology-related top-down approaches. For this purpose, a novel research paradigm next to nanotechnology has to be established to create functional materials on the basis of deep nanotechnology knowledge. This task can be assigned to an emerging concept, nanoarchitectonics. In the nanoarchitectonics approaches, functional materials were architected through combination of atom/molecular manipulation, organic chemical synthesis, self-assembly and related spontaneous processes, field-applied assembly, micro/nano fabrications, and bio-related processes. In this short review article, nanoarchitectonics-related approaches on materials fabrications and functions are exemplified from atom-scale to living creature level. Based on their features, unsolved problems for future developments of the nanoarchitectonics concept are finally discussed.

Self-Assembling Supramolecular Hybrid Hydrogel Beads

With the goal of imposing shape and structure on supramolecular gels, we combine a low-molecular-weight gelator (LMWG) with the polymer gelator (PG) calcium alginate in a hybrid hydrogel. By imposing thermal and temporal control of the orthogonal gelation methods, the system either forms an extended interpenetrating network or core-shell-structured gel beads-a rare example of a supramolecular gel formulated inside discrete gel spheres. The self-assembled LMWG retains its unique properties within the beads, such as remediating PdII and reducing it in situ to yield catalytically active Pd0 nanoparticles. A single PdNP-loaded gel bead can catalyse the Suzuki-Miyaura reaction, constituting a simple and easy-to-use reaction-dosing form. These uniquely shaped and structured LMWG-filled gel beads are a versatile platform technology with great potential in a range of applications.

The effect of vicinal di-halo substituents on the organogelling properties of aromatic supramolecular gelators and their application as soft templates

Vicinal di-halo substituents have a determinant effect on the supramolecular self-assembly and properties of aromatic physical gelators with application as soft templates.

First-generation shaped gel reactors based on photo-patterned hybrid hydrogels

This paper reports the development of first-generation photo-patterned ring-shaped gel reactors that catalyse the hydrolysis of para-nitrophenol phosphate using a phosphatase enzyme.

Bioinspired catalysis and bromoperoxidase like activity of a multistimuli-responsive supramolecular metallogel: supramolecular assembly triggered by pi–pi stacking and hydrogen bonding interactions

We report the synthesis and characterization of a new self-assembled VO₂-L metallogel. Multi-responsive properties of the gel were also studied and can be used for sensing OH⁻ anions. Bromoperoxidase-like activity of VO₂-L metallogel for oxidative bromination reaction was also reported.

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.

Synthesis and Characterization of Hybrid Glycolipids as Functional Organogelators and Hydrogelators

Carbohydrate-based low-molecular-weight gelators are useful and versatile compounds for the preparation of soft materials. Using N-acetyl-d-glucosamine as the starting material, we synthesized and characterized 15 glycolipids containing an amide with different ester functional groups. These include aliphatic derivatives with varying chain lengths and aromatic derivatives. Most of the hybrid amide-esters have molecular weights less than 500 D. These glycolipids were found to be effective gelators for several organic solvents, water, and aqueous solutions. Two efficient hydrogelators were also obtained at low concentrations. A few representative gels were characterized using optical microscopy, atomic force microscopy, and rheology to obtain information on their morphology and gel stability. Three gelators were also used to encapsulate naproxen sodium and toluidine blue. The sustained release of the drug from the gel to the aqueous phase was monitored by UV-vis spectroscopy. These gelators have structural flexibility that can be stimuli responsive. The esters can be hydrolyzed and several gels were converted to solutions under basic conditions. These rationally designed gelators could be utilized as stimuli-responsive smart materials with controlled release properties.

Selective Cu(ii) sensing by a versatile AIE cyanostilbene-based gel system

Cyanostilbene-based derivatives 1-3 were designed, synthesized and fully characterized. By screening their gelating abilities, we observed that the subtle difference in the position of the pyridine nitrogen greatly affected the resulting fluorescence and gelation properties. Notably, 1 was found to be a versatile ambidextrous gelator capable of forming organo-, hydro-, and Cu(ii) specific metallogels. Furthermore, a rare organogel-to-metallogel transformation at room temperature was also observed upon exposure of the 1-DMSO/H₂O gel to aqueous Cu(ii). This process, accompanied by colour and fluorescence changes, provides an effective strategy for the preparation of novel sensing soft materials.

Multi-stimuli-responsive hydrogels of gluconamide-tailored anthracene

In this work, two amphiphilic gluconamide-tailored anthracene gelators 1 and 2 have been synthesized, and found to form stable hydrogels with fibril structures. The stimuli-responsive behaviors of hydrogel 1 and 2 were investigated thoroughly by temperature-dependent 1H NMR, UV-Vis, rheometry, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The results showed that hydrogel 1 exhibited multiple responsive behaviours upon exposure to stimuli including temperature, anions, light, electron-deficient chemicals and external stress; conversely, hydrogel 2 showed a distinct responsive phenomenon attributed to a subtle structural difference in the linker. This work demonstrates that gluconamide-tailored anthracene gelators could be a potential soft material and highlights the importance of a precisely designed structure.

Weak Links To Differentiate Weak Bonds: Size-Selective Response of π-Conjugated Macrocycle Gels to Ammonium Ions

Molecular-level host-guest interactions can drive gel-to-sol phase transitions of the bulk material. Using supramolecular gels constructed from π-conjugated aza-crown macrocycles, we have investigated the effects of guest chemical structures on the kinetics of gel disassembly. While ammonium ions bind only weakly to the individual macrocycles in solution, gel-to-sol transitions of self-assembled macrocycles occur readily under ambient conditions. This net signal amplification process was monitored conveniently by time-dependent spectroscopic studies to reveal a straightforward correlation between the response rate and shape/size of the guest species. Well-designed weak links thus respond to subtle differences in weak bonds and translate them into visually discernible macroscopic signaling events.

Multi-functional organic gelator derived from phenyllactic acid for phenol removal and oil recovery

Supramolecular gels, a fascinating class of soft materials, are of great interest for their wide applications. In this work, a series of organic gelators derived from phenyllactic acid were prepared, and their gelation properties were further investigated. It was found that the gelator 1e bearing a hydrazine moiety could congeal 17 kinds of common organic liquids (polar and non-polar) efficiently. Meanwhile, the morphological structures and dominant factors of the gel were examined by scanning electron microscope (SEM), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), concentration and temperature-dependent ¹H NMR. Crucially, the gelator displayed outstanding performances in toxic phenol removal and spilled oil and petroleum products recovery. Moreover, it also displayed a satisfactory recyclability, which will greatly promote its application in practice. These impressive results will provide a novel avenue for the water treatment and the development of functional supramolecular gel materials.

The Supramolecular Organogel Formed by Self-Assembly of Ursolic Acid Appended with Aromatic Rings

Ursolic acid (UA) as a natural ursane-triterpenoid has rich pharmacological activities. We have found that it possesses aggregation properties and could self-assemble into organogels. Based on the aggregation property of ursolic acid in suitable solvents, its derivative appended with aromatic rings by amide groups was synthesized. The property of self-assembly into organogel was studied in this paper. The results revealed that this derivative could form supramolecular gel in halogenated benzene and also gelate chloroform in the presence of toluene or p-xylene. By Fourier-transform infrared spectra (FT-IR) and variable temperature proton nuclear magnetic resonance (¹H NMR), it was proved that intermolecular hydrogen bonding and π⁻π stacking interaction were the primary driving forces for the aggregation to form organogel.

Supramolecular Gel as the Template for Catalysis, Inorganic Superstructure, and Pharmaceutical Crystallization

A supramolecular gel is a fascinating combination of flexibility and orderliness. While the supramolecular nature of crosslinking contributes towards the adaptivity and the reversibility of the system, orderliness at the molecular level amplifies the functional output and induces extraordinary selectivity into the system. Therefore, use of supramolecular gels as the soft template is an emerging area of research, which includes but not limited to catalysis of a chemical or a photochemical process, transcription of gel property to a substrate, or even controlling the nucleation of drug molecules. This review aims to highlight the template effect of supramolecular gels in the above-mentioned areas relevant to novel fundamental chemistry, technology, and healthcare.

Supramolecular Chemistry and Self-Organization: A Veritable Playground for Catalysis

The directed assembly of molecular building blocks into discrete supermolecules or extended supramolecular networks through noncovalent intermolecular interactions is an ongoing challenge in chemistry. This challenge may be overcome by establishing a hierarchy of intermolecular interactions that, in turn, may facilitate the edification of supramolecular assemblies. As noncovalent interactions can be used to accelerate the reaction rates and/or to increase their selectivity, the development of efficient and practical catalytic systems, using supramolecular chemistry, has been achieved during the last few decades. However, between discrete and extended supramolecular assemblies, the newly developed “colloidal tectonics” concept allows us to link the molecular and macroscopic scales through the structured engineering of colloidal structures that can be applied to the design of predictable, versatile, and switchable catalytic systems. The main cutting-edge strategies involving supramolecular chemistry and self-organization in catalysis will be discussed and compared in this review.

Design and gelation behaviors of cholesterol-based derivatives as organogelators: an investigation of the correlation between molecular structures and gelation behaviors

The carboxylic group and the number of cholesteryl moieties have an important influence on gelation behaviors.

Post-modified porphyrin imine gels with improved chemical stability and efficient heterogeneous activity in CO2 transformation

Gel catalysts have been developed based on dynamic covalent chemistry and post-modification methods for improved chemical stability and catalytic activity.

Shape induced sorting via rim-to-rim complementarity in the formation of pillar[5, 6]arene-based supramolecular organogels

The first two-component rim-to-rim pillar[6]arene-based supramolecular organogels were prepared. Shape complementarity was found to be an important determinant in the formation of such gels which also show shape-induced sorting in their formation.

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.

Discrete and Polymeric Self-Assembled Palladium(II) Complexes as Supramolecular Gelators

Supramolecular gels prepared from low-molecular-weight gelators have been extensively explored. However, the exploitation of discrete or polymeric metal complexes as gelators is a relatively recent trend. The synthesis of self-assembled coordination complexes from palladium(II) and selected ligands is well established, but the potential of these complexes as gelators is a less explored treasure. Herein we focus on the gelation abilities of some self-assembled palladium(II) complexes and the resulting unique properties. First, discrete complexes with PdL, PdL₂ , Pd₂ L, Pd₂ L₂ , Pd₂ L₄ , and Pd₃ L₆ compositions are discussed. Second, gelation behavior promoted by coordination-polymer-like gelators formed in situ is explored. These gel samples have been employed in catalysis and the uptake of organic and dye molecules from the solution and gas phases. It is concluded that untapped unique properties can be realized by further exploration of designer palladium(II) complexes.

Palladium-scavenging self-assembled hybrid hydrogels - reusable highly-active green catalysts for Suzuki-Miyaura cross-coupling reactions

A hybrid hydrogel based on 1,3:2,4-dibenzylidene sorbitol (DBS) modified with acyl hydrazides combined with agarose was used for in situ reduction and binding of palladium from aqueous mixtures without the need for an external reducing agent. Palladium uptake was monitored and the formation of Pd nanoparticles (PdNPs) trapped within the gel and located close to the nanofibres was confirmed. This gel effectively scavenges palladium from solution to concentrations < 0.04 ppm - well below the recommended limits for pharmaceutical products. The resulting hybrid hydrogel with embedded PdNPs was used as a catalyst for Suzuki-Miyaura cross-coupling reactions. The gel network stabilises PdNPs, preventing aggregation/leaching and giving excellent catalytic lifetimes. The gel acts as a simple reaction dosing form, being simply added to reactions performed in green solvents in air. Once reactions are complete, the gel can be simply removed, recycled and reused (>10 times). Reactions were purified by simple washing protocols, and leaching of Pd from the gels is limited (<1 ppm). The gels were also used in flow-through mode, giving efficient, rapid reactions, with easy work-up. These catalytic gels combine advantages of homogeneous and heterogeneous catalysts - they are solvent compatible with the reaction taking place in a solution-like environment, while the solid-like gel network enables catalyst recycling. In summary, these hydrogels scavenge 'waste' palladium and convert it into gel 'wealth' capable of efficient, environmentally-friendly Suzuki-Miyaura catalysis.

Down-regulating Proteolysis to Enhance Anticancer Activity of Peptide Nanofibers

Nanofibers of short peptides are emerging as a promising type of agents for inhibiting cancer cells. But the proteolysis of peptides decreases the anticancer efficacy of the peptide nanofibers. Here we show that decreasing the activity of proteasomes enhance the activity of peptide nanofibers for inhibiting cancer cells. Based on the structure of galactin-3, we designed a heptapeptide, which self-assembles to form nanofibers. The nanofibers of the heptapeptide exhibit moderate cytotoxicity to three representative cancer cell lines (HeLa, MCF-7, and HepG2), largely due to the proteolysis of the peptides. Using a clinically approved proteasome inhibitor, bortezomib, to treat the cancer cells significantly decreases the proteolysis of the peptides and enhances the activity of the peptide nanofibers for inhibiting the cancer cells. This work illustrates a promising approach for enhancing the anticancer efficacy of peptide nanofibers by modulating intracellular protein degradation machinery, as well as provides insights for understanding the cytotoxicity of aberrant protein or peptide aggregates in complicated cellular environment.