Supramolecular materials Amabilino, David B; Smith, David K; Steed, Jonathan W
Chemical Society reviews,
05/2017, Volume:
46, Issue:
9
Journal Article
Peer reviewed
Open access
Molecular material properties depend upon the contacts between and the arrangement of the component parts, and therefore supramolecular chemistry has developed a highly important role in this area. ...This Tutorial Review, after briefly introducing the history of the field, discusses some of the most exciting and inspiring recent achievements, with special focus on soft materials, particularly gels and liquid crystals.
Molecular material properties depend upon the contacts between and the arrangement of the component parts, and therefore supramolecular chemistry has developed a highly important role in this area.
This review explores supramolecular gels as materials for environmental remediation. These soft materials are formed by self-assembling low-molecular-weight building blocks, which can be programmed ...with molecular-scale information by simple organic synthesis. The resulting gels often have nanoscale 'solid-like' networks which are sample-spanning within a 'liquid-like' solvent phase. There is intimate contact between the solvent and the gel nanostructure, which has a very high effective surface area as a result of its dimensions. As such, these materials have the ability to bring a solid-like phase into contact with liquids in an environmental setting. Such materials can therefore remediate unwanted pollutants from the environment including: immobilisation of oil spills, removal of dyes, extraction of heavy metals or toxic anions, and the detection or removal of chemical weapons. Controlling the interactions between the gel nanofibres and pollutants can lead to selective uptake and extraction. Furthermore, if suitably designed, such materials can be recyclable and environmentally benign, while the responsive and tunable nature of the self-assembled network offers significant advantages over other materials solutions to problems caused by pollution in an environmental setting.
Self-assembled gels have nanoscale 'solid-like' networks spanning across a liquid-like phase and are ideally suited for bringing these into intimate contact with polluted solution-phase media in an environmental setting, with the ultimate goal of environmental remediation.
Gel-phase materials are generated when molecular building blocks assemble into nanoscale architectures, usually 'one-dimensional' fibrils, which hierarchically assemble into bundles and subsequently ...form an entangled sample spanning network, capable of preventing the flow of bulk solvent. This tutorial review explores the vital role of chirality in gel formation. In particular, we focus on how fundamental self-assembly processes can translate molecular scale chiral information into nanoscale architectures, and then into the macroscopic behaviour of the gel. Chiral molecular gels have potential applications in nanofabrication and as addressable functional nanomaterials.
Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future. This review provides an overview of these materials, from their use in ...lubrication and personal care in the ancient world, through to next-generation technologies. In academic terms, colloid scientists in the 19th and early 20th centuries first understood such gels as being physically assembled as a result of weak interactions, combining a solid-like network having a degree of crystalline order with a highly mobile liquid-like phase. During the 20th century, industrial scientists began using these materials in new applications in the polymer, oil and food industries. The advent of supramolecular chemistry in the late 20th century, with its focus on non-covalent interactions and controlled self-assembly, saw the horizons for these materials shifted significantly beyond their historic rheological applications, expanding their potential. The ability to tune the LMWG chemical structure, manipulate hierarchical assembly, develop multi-component systems, and introduce new types of responsive and interactive behaviour, has been transformative. Furthermore, the dynamics of these materials are increasingly understood, creating metastable gels and transiently-fueled systems. New approaches to shaping and patterning gels are providing a unique opportunity for more sophisticated uses. These supramolecular advances are increasingly underpinning and informing next-generation applications - from drug delivery and regenerative medicine to environmental remediation and sustainable energy. In summary, this article presents a panorama over the field of supramolecular gels, emphasising how both academic and industrial scientists are building on the past, and engaging new fundamental insights and innovative concepts to open up exciting horizons for their future use.
Supramolecular gels, self-assembled from low-molecular-weight gelators (LMWGs), have a long history and a bright future.
It is likely that nanofabrication will underpin many technologies in the 21st century. Synthetic chemistry is a powerful approach to generate molecular structures that are capable of assembling into ...functional nanoscale architectures. There has been intense interest in self‐assembling low‐molecular‐weight gelators, which has led to a general understanding of gelation based on the self‐assembly of molecular‐scale building blocks in terms of non‐covalent interactions and packing parameters. The gelator molecules generate hierarchical, supramolecular structures that are macroscopically expressed in gel formation. Molecular modification can therefore control nanoscale assembly, a process that ultimately endows specific material function. The combination of supramolecular chemistry, materials science, and biomedicine allows application‐based materials to be developed. Regenerative medicine and tissue engineering using molecular gels as nanostructured scaffolds for the regrowth of nerve cells has been demonstrated in vivo, and the prospect of using self‐assembled fibers as one‐dimensional conductors in gel materials has captured much interest in the field of nanoelectronics.
Starting to gel: Recent developments in self‐assembling low‐molecular‐weight gelators has led to rationally designed materials with increasingly advanced and specialized applications. Recent demonstrations of molecular gels in diverse technological fields are reviewed, including tissue engineering, regenerative medicine, and nanoelectronics.
Multivalency is a powerful strategy for achieving high‐affinity molecular recognition in biological systems. Recently, attention has begun to focus on using self‐assembly rather than covalent ...scaffold synthesis to organize multiple ligands. This approach has a number of advantages, including ease of synthesis/assembly, tunability of nanostructure morphology and ligands, potential to incorporate multiple active units, and the responsive nature of self‐assembly. We suggest that self‐assembled multivalency is a strategy of fundamental importance in the design of synthetic nanosystems to intervene in biological pathways and has potential applications in nanomedicine.
The power of many: The use of self‐assembly to create dynamic multivalency (see scheme) is a powerful strategy, with some significant advantages over the use of static multivalent arrays. It mimics processes which occur naturally within cell membranes, and has a wide range of potential applications, both in nanomaterials science and nanomedicine.
Supramolecular gels assemble via non-covalent interactions between low-molecular-weight gelators (LMWGs). The gels form a solid-like nanoscale network spanning a liquid-like continuous phase, ...translating molecular-scale information into materials performance. However, gels based on LMWGs are often difficult to manipulate, easily destroyed and have poor rheological performance. The recurring image of newly discovered supramolecular gels is that of an inverted vial showing that the gel can support its own weight against gravity. Such images reflect the limitation that these gels simply fill the vessel in which they are made, with limited ability to be shaped. This property prevents supramolecular gels from having the same impact as polymer gels, despite greater synthetic tunability, reversibility and bio/environmental compatibility. In this Review, we evaluate strategies for imposing different shapes onto supramolecular gels and for patterning structures within them. We review fabrication methods including moulding, self-healing, 3D printing, photopatterning, diffusion and surface-mediated patterning. We discuss gelator chemistries amenable to each method, highlighting how a multicomponent approach can aid shaping and structuring. Supramolecular gels with defined shapes, or patterned structures with precisely controlled compositions, have the potential to intervene in applications, such as tissue engineering and nanoscale electronics, as well as opening up new technologies.Supramolecular gels comprise low-molecular weight gelators that assemble by non-covalent interactions. In this Review, a range of fabrication methods, as well as strategies for shaping, structuring and patterning supramolecular gels are discussed.
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.
Bead it! The combination of a supramolecular gel and alginate can be used to enforce spherical core–shell bead structures onto catalytic gels self‐assembled from low‐molecular‐weight gelators. The system is able to remediate PdII and reduce it in situ to yield catalytically active Pd0 nanoparticles and can also catalyse the Suzuki–Miyaura reaction when loaded with these nanoparticles.
We report a two-component acid–amine gelation system which forms instant organogels on simple mixing. We investigate self-assembly using a wide range of different amines and identify the optimum ...amines for gelation to occur. Using NMR and other spectroscopic methods, we unambiguously determine the stoichiometry of the complex responsible for gelation (1:1) and characterize the noncovalent interactions responsible for gelation. Using Kamlet–Taft parameters we gain a detailed understanding of the role of solvent on gelation. Most importantly, we explore the ability of these multicomponent systems to assemble from complex mixtures, and using NMR can determine which components are preferentially taken up into the immobile “solid-like” fiber network and which components remain mobile in the “liquid-like” solvent phase. In this way, we determine that the component selection process is controlled by the two key steps in hierarchical assembly: (i) acid–base complex formation (as predicted by the pK a of the amine) and (ii) gel fiber assembly (as predicted by the T gel value). These parameters therefore enable a predictive understanding of the way in which complex mixtures self-organize and assemble and also how the sorted assemblies disassemble on heating. In a key experiment, we demonstrate that these materials are highly responsive and that a preformed gel, exposed to a new component, evolves, adapts, and heals its composition in response to the thermodynamic preferences of the overall system.
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 3demonstrating 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.
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