Developing a Gel-Based Sensor Using Crystal Morphology Prediction

Fast and Selective Luminescent Sensing by Langmuir-Schaeffer Films Based on Controlled Assembly of Perylene Bisimide Modified with A Cyclometalated AuIII Complex

Condensed films of functional luminophores dominated by the magnitude and dimensionality of the intermolecular interactions play important roles in sensing performance. However, controlling the molecular assembly and regulating photophysical properties remain challenging. In this study, a new luminophore, ortho-PBI-Au, was synthesized by anchoring a cyclometalated alkynyl-gold(III) unit at the ortho-position of perylene bisimide. An unprecedented T-type packing model driven by weak Au-π interaction and Au-H bonds was observed, laying foundation for striking properties of the luminophore. Controlled assembly of ortho-PBI-Au at the air-water interface, realized using the classical Langmuir-Schaeffer technique, afforded the obtained luminescent films with different packing structures. With an optimized film, sensitive, selective, and rapid detection of a hazardous new psychoactive substance, phenylethylamine (PEA), was achieved. The detection limit, response time, and recovery time were <4 ppb, <1 s, and <5 s, respectively, surpassing the performance of the PEA sensors known thus far. The relationship between the characters of films and the sensing performance was systematically examined by grey relational analysis (GRA). The present study suggests that designing novel molecular aggregation with definite adlayer structure is a crucial strategy to enhance the sensing performance, which could be favorable for the film-based fluorescent sensors.

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

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

Acid⋅⋅⋅Amide Supramolecular Synthon for Tuning Amino Acid-Based Hydrogels' Properties

Supramolecular hydrogels formed by the self-assembly of N-Fmoc-l-phenylalanine derivatives are gaining relevance for several applications in the materials and biomedical fields. In the challenging attempt to predict or tune their properties, we selected Fmoc-pentafluorophenylalanine (1) as a model efficient gelator, and studied its self-assembly in the presence of benzamide (2), a non-gelator able to form strong hydrogen bonds with the amino acid carboxylic group. Equimolar mixtures of 1 and 2 in organic solvents afforded a 1 : 1 co-crystal thanks to the formation of an acid⋅⋅⋅amide heterodimeric supramolecular synthon. The same synthon occurred in the transparent gels formed by mixing the two components in 1 : 1 ratio in aqueous media, as revealed by structural, spectroscopic, and thermal characterizations performed on both the co-crystal powder and the lyophilized hydrogel. These findings revealed the possibility of modulating the properties of amino acid-based hydrogels by involving the gelator in the formation of a co-crystal. Such a crystal engineering-based approach is shown also to be useful for the time-delayed release of suitable bioactive molecules, when involved as hydrogel coformers.

Regulating and Predicting the Polyhedral Crystal Morphology in Spirofluorene Molecular Systems

Crystallization of organic steric molecules often leads to multiple polyhedral crystal morphologies. However, the relationships among the molecular structure, supramolecular interaction, aggregation mode and crystal morphology are still unclear. In this work, we elaborate two model crystals formed by spiro[fluorene-9,9'-xanthene] (SFX) and spiro[cyclopenta[1,2-b : 5,4-b']dipyridine-5,9'-xanthene] (SDAFX) to demonstrate the feasibility of morphology prediction by periodic bond chain (PBC) theory based on interaction energy (IE) values in terms of single point energy. With non-directional van der Waals forces, only one PBC direction is found in SFX crystal, leading to the irregular 1D rod-like structure. Compared with SFX, the extra N heteroatoms in SDAFX can bring additional hydrogen bonds and some other interactions into the bulky molecular skeletons, inducing 3-dimensionally oriented PBCs to form the explicit F-face network in SDAFX which leads to the final octahedral structure. A simple and accurate method has been provided to quantify PBC vector on the supramolecular level in the organic molecular system, and the PBC theory has also been further demonstrated and developed in the morphology prediction of organic spiro-molecules.

A novel nonreversible heat-induced low-molecular-weight gel based on naturally-occurring self-assembled fupenzic acid for tumor therapy

Herein, a nonreversible heat-induced supramolecular gel based on natural products was reported for the first time. This natural triterpenoid, fupenzic acid (FA), isolated from the roots of Rosa laevigata, was discovered to be capable of forming supramolecular gel spontaneously in 50 % ethanol-water solution induced by heating. Distinguished from the common thermosensitive gels, the FA-gel showed a distinctive nonreversible phase transition from the liquid to gel state upon heating. In this work, the entire gelation process of FA-gel induced by heating was recorded digitally by microrheology monitor. And a unique heat-induced gelation mechanism based on self-assembled FA has been proposed by using various experimental methods and molecular dynamics (MD) simulation. Its excellent injectability and stability were also demonstrated. Furthermore, the FA-gel had been evaluated to exhibit better anti-tumor activity and higher biosafety comparing with its equivalent free-drug, which opened up a new possibility to reinforce antitumor efficacy by using natural product gelator originated from traditional Chinese medicine (TCM) without any complicated chemical modifications.

Charge-Transfer Complex Doped Photothermal Hydrogels for Discriminating Circularly Polarized Near-Infrared Light

Hydrogels behave as potential candidates to investigate circularly polarized light (CP)-matter interaction, which however suffer from small sensitivity towards circular polarization. Here we report a general protocol to build hydrogels from π-conjugated amino acids with coassembled charge-transfer (CT) complexes, covering a wide scope of donors and acceptors, which were incorporated into stable hydrogel matrices. CT complexes formed block coassemblies with gelators, induced the emergence of macroscopic chiral helices, where efficient chirality transfer occurs to realize tunable Cotton effects from visible light to NIR-I region depending on the structures of CT pairs. The hybrid hydrogels showed tunable photothermal performances with excellent heating-cooling cycling durability. Circularly polarized NIR light selectively triggered gel-solution phase transition at different timescales. Left- and right-CP illumination generates up to 2.5 folds difference in gel collapse time that allows for direct discrimination by naked eyes.

Computational Tools to Rationalize and Predict the Self-Assembly Behavior of Supramolecular Gels

Supramolecular gels form a class of soft materials that has been heavily explored by the chemical community in the past 20 years. While a multitude of experimental techniques has demonstrated its usefulness when characterizing these materials, the potential value of computational techniques has received much less attention. This review aims to provide a complete overview of studies that employ computational tools to obtain a better fundamental understanding of the self-assembly behavior of supramolecular gels or to accelerate their development by means of prediction. As such, we hope to stimulate researchers to consider using computational tools when investigating these intriguing materials. In the concluding remarks, we address future challenges faced by the field and formulate our vision on how computational methods could help overcoming them.

Organogelators derived from the bisphenol A scaffold

Bisphenol A, a common precursor molecule used in the preparation of some polymers, was investigated as a possible scaffold for the design and synthesis of small-molecule gelators.

Gel Formed by Self-Assembly of a Urea-Modified Monopyrrolotetrathiafulvalene Derivative Displays Multi-Stimuli Responsiveness and Absorption of Rhodamine B

A new monopyrrolotetrathiafulvalene-based derivative containing a urea group was designed, synthesized and thoroughly characterized. It proved to be a non-gelator in a single solvent even when heated or sonicated. However, it could self-assemble in a CHCl₃ (CH₂ Cl₂ )/n-hexane mixture to form a thermo-responsive supramolecular organogel. SEM, FT-IR spectroscopy, UV/Vis absorption spectroscopy, and SAXS revealed that in the organogel system, the gelators self-assembled into supramolecular networks with a J-type aggregation mode under the joint effect of π-π stacking, intermolecular hydrogen-bonding, and van der Waals forces. Interestingly, the gel phase was shown to undergo reversible gel-sol transformation induced by Fe³⁺ -Vitamin C (Vc), trifluoroacetic acid-triethylamine (TFA-TEA) and picric acid (PA)-NaOH. In particular, in the presence of picric acid, the experimental results proposed that charge transfer occurred from the electron-donor gelator to the electron-acceptor picric acid due to the possibility of complex formation. Furthermore, the formed organogel could behave as the matrix for encapsulating cationic fluorescent dye from wastewater, and the adsorption efficiency was directly proportional to the concentration of the gelator.

Hydrogelation with a water-insoluble organogelator - surfactant mediated gelation (SMG)

The low-molecular-weight gelator (LMG) 12-hydroxyoctadecanoic acid (12-HOA) is insoluble in water, but can be solubilized in surfactant micelles. We therefore solubilized 12-HOA at 80 °C in an aqueous solution of cetyltrimethylammonium bromide (CTAB) containing spherical micelles. On cooling this system down to room temperature, a hydrogel is obtained. We will refer to this process as "surfactant-mediated gelation" (SMG). The hydrogels were formed at a lower 12-HOA concentration when sodium salicylate (NaSal) was added to the CTAB system, which induced the formation of wormlike micelles. Hydrogels obtained by SMG from spherical and wormlike micelles are referred to as gelled micellar phases (GMs) and gelled wormlike micellar phases (GWLMs), respectively. Optical microscopy and transmission electron microscopy (TEM) showed that 12-HOA forms self-assembled fibrillar networks (SAFiNs) in both GMs and GWLMs. The sol-gel transition temperature, T_sol-gel, of the GWLM samples was higher than that of the GM samples. Dynamic rheological measurements revealed gel properties (G' > G'' at all angular frequencies) for both gels; however, a higher viscoelasticity was observed for the GWLM samples, which in turn, was reflected in the higher T_sol-gel. Small- and wide-angle X-ray scattering (SWAXS) showed that micelles and gel fibers coexist in the GM and GWLM samples. Our study demonstrates the gelation of aqueous micellar solutions with water-insoluble LMGs.

Intermolecular Interactions in Functional Crystalline Materials: From Data to Knowledge

Intermolecular interactions of organic, inorganic, and organometallic compounds are the key to many composition–structure and structure–property networks. In this review, some of these relations and the tools developed by the Cambridge Crystallographic Data Center (CCDC) to analyze them and design solid forms with desired properties are described. The potential of studies supported by the Cambridge Structural Database (CSD)-Materials tools for investigation of dynamic processes in crystals, for analysis of biologically active, high energy, optical, (electro)conductive, and other functional crystalline materials, and for the prediction of novel solid forms (polymorphs, co-crystals, solvates) are discussed. Besides, some unusual applications, the potential for further development and limitations of the CCDC software are reported.

Hydrogen bonded co-assembly of aromatic amino acids and bipyridines that serves as a sacrificial template in superstructure formation

Design and fabrication of superstructures are intriguing yet challenging tasks, which require delicate operations at micro/nanoscale such as template-directed seeding or etching processes. In this study, we prepared integrated one dimensional (1D) microrods from co-assembled N-terminated aromatic amino acids and bipyridines that could serve as sacrificial templates for micro-superstructure formation. Organic polar solvents were utilized for generating a co-assembly that showed selectivity to both molecular topology of building blocks and solvent environments via thermodynamic and kinetic manners. The addition of specific transition metal ions would extract bipyridines from crystalline microrods, leading to well-aligned engraved motifs along the 1D direction as well as the emergence of ordered packed nanostructures on microrod surfaces. Responsive to types of metal ions, diverse superstructures such as etched sculptures and surface-encapsulated heterojunctions of metal-bipyridine coordination polymers were constructed. This study offers a proof-of-concept exploration in the rational design of 1D crystalline micro-superstructures via non-covalent complexation towards potential applications in electrical and optical applications.

A Million Crystal Structures: The Whole Is Greater than the Sum of Its Parts

The founding in 1965 of what is now called the Cambridge Structural Database (CSD) has reaped dividends in numerous and diverse areas of chemical research. Each of the million or so crystal structures in the database was solved for its own particular reason, but collected together, the structures can be reused to address a multitude of new problems. In this Review, which is focused mainly on the last 10 years, we chronicle the contribution of the CSD to research into molecular geometries, molecular interactions, and molecular assemblies and demonstrate its value in the design of biologically active molecules and the solid forms in which they are delivered. Its potential in other commercially relevant areas is described, including gas storage and delivery, thin films, and (opto)electronics. The CSD also aids the solution of new crystal structures. Because no scientific instrument is without shortcomings, the limitations of CSD research are assessed. We emphasize the importance of maintaining database quality: notwithstanding the arrival of big data and machine learning, it remains perilous to ignore the principle of garbage in, garbage out. Finally, we explain why the CSD must evolve with the world around it to ensure it remains fit for purpose in the years ahead.

Fluorescent Metallacage-Core Supramolecular Polymer Gel Formed by Orthogonal Metal Coordination and Host-Guest Interactions

Herein, we report the preparation of a multifunctional metallacage-core supramolecular gel by orthogonal metal coordination and host-guest interactions. A tetragonal prismatic cage with four appended 21-crown-7 (21C7) moieties in its pillar parts was first prepared via the metal-coordination-driven self-assembly of cis-Pt(PEt₃)₂(OTf)₂, tetraphenylethene (TPE)-based sodium benzoate ligands and linear dipyridyl ligands. Further addition of a bisammonium linker to the cage delivered a supramolecular polymer network via the host-guest interactions between the 21C7 moieties and ammonium salts, which formed a supramolecular gel at relatively higher concentrations. Due to the incorporation of a TPE derivative as the fluorophore, the gel shows emission properties. Multiple stimuli responsiveness and good self-healing properties were also observed because of the dynamic metal coordination and host-guest interactions used to stabilize the whole network structure. Moreover, the storage and loss moduli of the gel are 10-fold those of the gel without the metallacage cores, indicating that the rigid metallacage plays a significant role in enhancing the stiffness of the gel. The studies described herein not only enrich the functionalization of fluorescent metallacages via elegant ligand design but also provide a way to prepare stimuli-responsive and self-healing supramolecular gels as robust and smart materials.

Development of New D,L-Methionine-based Gelators

D,L-Methionine was chosen as a starting material for the preparation of a new gelator N-10-undecenoyl-D,L-methionylaminooctadecane (DL-Met-R₁₈). Three oligo (dimethylsiloxane)-containing gelators, DL-Met-R₁₈/Si₃, DL-Met-R₁₈/Si_7-8, and DL-Met-R₁₈/Si_14-15, were also prepared from DL-Met-R₁₈ by hydrosilylation reactions. Their gelation abilities were evaluated on the basis of the minimum gel concentration using nine solvents. Compound DL-Met-R₁₈ was able to gelate liquid paraffin and silicone oil, but it crystallized in most solvents. However, DL-Met-R₁₈/Si_7-8 resulted to be the best gelator, gelling eight solvents at low concentrations. The results of gelation tests demonstrated that the ability to form stable gels decreases in the following order: DL-Met-R₁₈/Si_7-8 ≈ DL-Met-R₁₈/Si_14-15 > DL-Met-R₁₈/Si₃ >> DL-Met-R₁₈. The aspects and thermal stabilities of the gels were investigated using three-component mixtures of solvents composed of hexadecyl 2-ethylhexanoate, liquid paraffin, and decamethylcyclopentasiloxane (66 combinations). DL-Met-R₁₈/Si₃, DL-Met-R₁₈/Si_7-8, and DL-Met-R₁₈/Si_14-15 could form gels with all these mixed solvent combinations; particularly, DL-Met-R₁₈/Si_7-8 gave rise to transparent or translucent gels. FT-IR spectra suggested that the formation of hydrogen bonds between the NH and C=O groups of the amides is one of driving forces involved in the gelation process. Aggregates comprising three-dimensional networks were studied by transmission electron microscopy. Moreover, the viscoelastic behavior of the gels was investigated by rheology measurements.

Determination of the packing model of a supramolecular nanofiber via mass-per-length measurement and de novo simulation

Herein, we report an example of using scanning transmission electron microscopy to determine the mass-per-length of supramolecular nanofibers. Together with the measurement of the diameter of nanofibers via transmission electron microscopy, we could estimate the packing density of assembling molecules along the nanofibers. A parallel unbiased de novo simulation screens and reveals the most plausible molecular packing pattern of small molecules along the supramolecular nanofibers. Remarkably, the simulated packing patterns and density correlate well with the experimental measurements. Unexpectedly, the naphthalene groups are likely facing outward, creating a hydrophobic surface, which is driven by the geometry of the hydrogelator molecule. Overall, this study establishes a complementary method to determine molecular arrangement along the supramolecular nanofibers, which is potentially useful for the guidance of rational design of biomaterials based on self-assembly.

Natural product gelators and a general method for obtaining them from organisms

Since the late 1980s, low molecular weight gelators (LMWGs) based on different classes of natural products have been reported. Until 2011, pure natural LMWGs (i.e., natural product gelators, NPGs) were not found. However, today only five NPGs are reported. We think that this may be due to the lack of awareness about the importance of NPGs and suitable methods to discover new NPGs. Here we illustrate the potential significance of NPGs, and present a general and efficient method for obtaining NPGs from organisms, which provides specific and important guidance to researchers for easy discovery of new NPGs from organisms in the future. Using this method, we screened a total of 64 kinds of organisms (including plants, animals and fungi), and 6 extracts with a gelation ability were tracked and isolated to yield six new NPGs. These new NPGs include new types of NPGs such as tricyclic triterpenes (1) and tetracyclic triterpenes (2), and new classes of NPGs such as steroids (4 and 5) and glycosides (6), which greatly expand the class of NPGs in the LMWG field.

A G4·K+ hydrogel made from 5′-hydrazinoguanosine for remediation of α,β-unsaturated carbonyls

A G₄·KCl hydrogel with a nucleophilic 5′ sidechain absorbs α,β-unsaturated carbonyls via formation of cyclic adducts.

Two solvent-induced porous hydrogen-bonded organic frameworks: solvent effects on structures and functionalities

Two solvent-induced porous hydrogen-bonded organic frameworks have been obtained, and their synthesis, crystal structures, gas sorption behaviours and fluorescence sensing applications have been systematically investigated to elucidate the solvent effects on the structures and functionalities of HOFs.

Molecular Insights into Gelation of Di-Fmoc-l-Lysine in Organic Solvent-Water Mixtures

Despite significant interest in molecular gels due to their intriguing structure formation through self-assembly and their stimuli-responsive behavior, our understanding of the gel formation mechanism of a low-molecular-weight gelator (LMWG) is incomplete. Here, we report a combined experimental and computational study on a LMWG, di-Fmoc-l-lysine, that has two aromatic moieties and multiple hydrogen bond donors and acceptors. Gelation in various organic solvent-water mixtures was obtained through the solvent-triggered technique. We show that an approach based on approximate cohesive energy density derived from density functional theory (DFT) calculations can capture the experimental solubility trend of LMWGs in different organic solvents. Furthermore, DFT calculations indicate parallel and helical structures to be the preferred structural motifs for gelator dimers. We believe that these motifs can potentially lead to fiber formation as observed with microscopy. Our work provides a relatively simple yet effective approach to quantify interactions between solvents and complex gelators that can help rationalize solubility and gelation behavior.

Understanding Pathway Complexity of Organic Micro/Nanofiber Growth in Hydrogen-Bonded Coassembly of Aromatic Amino Acids

Rational engineering of one-dimensional (1D) self-assembled aggregates to produce desired materials for versatile functions remains a challenge. In this work, we report the noncovalent modulation of 1D aggregates at the micro/nanoscale using a coassembly protocol. Aromatic amino acids were employed as the model building blocks, and melamine (Mm) behaves as a modulator to form coassembly arrays with aromatic amino acids selectively. The selective self-assembly behavior between aromatic amino acids and Mm allows distinguishing and detecting Mm and aromatic amino acids from their analogues in macroscopic and microscopic scales. Dimensions and sizes of fibrous aggregates prepared from different amino acids show two opposite pathways from pristine assemblies to coassemblies induced by the addition of Mm. This pathway complexity could be controlled by the molecular conformation determined by α-positioned substituents. The developed hypothesis presents an excellent expansibility to other substrates, which may guide us to rationally design and screen 1D materials with different dimensions and sizes including the production of high-quality self-standing hydrogels.

Controlling the network type in self-assembled dipeptide hydrogels

We show that the same low molecular weight gelator can form gels using three different methods. Gels were formed from a high pH solution either by adding a salt or by adding an acid; gels were also formed by adding water to a solution of the gelator in an organic solvent. The mechanical properties for the gels formed by the different methods are different from one another. We link this to the network type that is formed, as well as the fibrous structures that are formed. The salt-triggered gels show a significant number of fibres that tend to align. The acid-triggered gels contain many thin fibres, which form an entangled network. The solvent-triggered gels show the presence of spherulitic domains. We show that it is tractable to vary the trigger mechanism for an established, robust gelator to prepare gels with targeted properties as opposed to synthesising new gelators.

Supramolecular materials

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.

Rapid self-healing and anion selectivity in metallosupramolecular gels assisted by fluorine–fluorine interactions

Metal complexes from perfluoroalkylamide terpyridine self-assemble into anion selective gels, which manifest self-healing and thermal rearrangement in aqueous dimethyl sulfoxide.

d-Serine enzymatic metabolism induced formation of a powder-remoldable PAAM–CS hydrogel

The metabolism of d-serine by d-amino acid oxidase was developed to induce radical polymerization and formation of a powder-remoldable PAAM–CS hydrogel.

A new rod-shaped BODIPY-acetylene molecule for solution-processed semiconducting microribbons in n-channel organic field-effect transistors

A novel solution-processable BODIPY-based small molecule (BDY-PhAc-BDY) yields highly-crystalline, one-dimensional (1-D) microribbon semiconductors for organic field-effect transistors (OFETs).

Hierarchical Self-Assembly of Luminescent Tartrate-Bridged Chiral Binuclear Tb(III) Complexes in Ethanol

A new family of supramolecular metalloamphiphiles carrying two metal centers is developed. They are formed by bridging two coordinatively unsaturated lipophilic Tb³⁺ complexes (TbL⁺) with chiral dicarboxylate anions. The formation of bridging coordination bonds is confirmed using UV spectroscopy, induced circular dichroism (ICD), increased luminescence intensity of TbL⁺, and electrospray ionization mass spectrometry (ESIMS) analysis. These supramolecular metalloamphiphiles hierarchically self-assemble in ethanol to give luminescent nanospheres, as observed using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The two hydroxyl groups introduced in the bridging ligands of [TbL]₂(d-/l-tartrate) significantly promote self-assembly by increasing coherent forces via intermolecular hydrogen bonding. The observed self-assembly in ethanol also merits mention because such polar alcoholic media have been unfavorable for conventional molecular self-assemblies. The present approach offers a new molecular design strategy for composable metalloamphiphiles.