Facile Stimuli-Responsive Transformation of Vesicle to Nanofiber to Supramolecular Gel via ω-Amino Acid-Based Dynamic Covalent Chemistry

Dynamic covalent surfactants and their uses in the development of smart materials

Dynamic covalent chemistry, which leverages the dynamic nature of reversible covalent bonds controlled by the conditions of reaction equilibrium, has demonstrated great potential in diverse applications related to both the stability of covalent bonds and the possibility of exchanging building blocks, imparting to the systems the possibility of "error checking" and "proof-reading". By incorporating dynamic covalent bonds into surfactant molecular architectures, combinatorial libraries of surfactants with bespoke functionalities can be readily fabricated through a facile strategy, with minimum effort in organic synthesis. Consequently, a multidisciplinary field of research involving the creation and application of dynamic covalent surfactants has recently emerged, which has aroused great attention in surfactant and colloid science, supramolecular chemistry, self-assembly, smart materials, drug delivery, and nanotechnology. This review reports results in this field published over recent years, discusses the possibilities presented by dynamic covalent surfactants and their applications in developing smart self-assembled materials, and outlines some future perspectives.

Vesicles, fibres, films and crystals: A low-molecular-weight-gelator [Au(6-thioguanosine)2]Cl which exhibits a co-operative anion-induced transition from vesicles to a fibrous metallo-hydrogel

We describe a simple coordination compound of Au(I) and 6-thioguanosine, [Au(6-tGH)2]Cl, that has a rich self-assembly chemistry. In aqueous solution, the discrete complex assembles into a supramolecular fibre and forms a luminescent hydrogel at concentrations above about 1 mM. Below this concentration, the macromolecular structure is a vesicle. Through appropriate control of the solvent polarity, the gel can be turned into a lamellar film or crystallised. The molecular structure of [Au(6-tGH)2]Cl was determined using single crystal X-ray diffraction, which showed bis-6-thioguanosine linearly coordinated through the thione moiety to a central Au(I) ion. In the vesicles, the photoluminescence spectrum shows a broad, weak band at 550 nm owing to aurophilic interactions. Co-operative self-assembly from vesicle to fibre is made possible through halogen hydrogen bonding interactions and the aurophilic interactions are lost, resulting in a strong photoluminescence band at 490 nm with vibronic structure typical of an intraligand transition. The vesicle-fibre transition is also revealed by a large increase of ellipticity in the circular dichroism spectrum with a prominent peak near 390 nm owing to the helical structure of the fibres. Atomic force microscopy shows that at the same time as fibres form, the sample gels. Imaging near the vesicle-fibre transition shows that the fibres form between vesicles and a mechanism for the transition based on vesicle collisions is proposed.

Surface supramolecular assemblies tailored by chemical/physical and synergistic stimuli: a scanning tunneling microscopy study

Supramolecular self-assemblies formed by various non-covalent interactions can produce diverse functional networks on solid surfaces. These networks have recently attracted much interest from both fundamental and application points of view. Unlike covalent organic frameworks (COFs), the properties of the assemblies differ from each other depending on the constituent motifs. These various motifs may find diverse applications such as in crystal engineering, surface modification, and molecular electronics. Significantly, these interactions between/among the molecular tectonics are relatively weak and reversible, which makes them responsive to external stimuli. Moreover, for a liquid-solid-interface environment, the dynamic processes are amenable to in situ observation using scanning tunneling microscopy (STM). In the literature, most review articles focus on supramolecular self-assembly interactions. This review summarizes the recent literature in which stimulation sources, including chemical, physical, and their combined stimuli, cooperatively tailor supramolecular assemblies on surfaces. The appropriate design and synthesis of functional molecules that can be integrated on different surfaces permits the use of nanostructured materials and devices for bottom-up nanotechnology. Finally, we discuss synergic effect on materials science.

Self-Healable Covalently Adaptable Networks Based on Disulfide Exchange

Introducing dynamic covalent bonding into thermoset polymers has received considerable attention because they can repair or recover when damaged, thereby minimizing waste and extending the service life of thermoset polymers. However, most of the yielded dynamic covalent bonds require an extra catalyst, high temperature and high-pressure conditions to trigger their self-healing properties. Herein, we report on a catalyst-free bis-dynamic covalent polymer network containing vinylogous urethane and disulfide bonds. It is revealed that the introduction of disulfide bonds significantly reduces the activation energy (reduced from 94 kJ/mol to 51 kJ/mol) of the polymer system for exchanging and promotes the self-healing efficiency (with a high efficiency of 86.92% after being heated at 100 °C for 20 h) of the material. More importantly, the mechanical properties of the healed materials are comparable to those of the initial ones due to the special bis-dynamic covalent polymer network. These results suggest that the bis-dynamic covalent polymer network made of disulfide and inter-vinyl ester bonds opens a new strategy for developing high-performance vitrimer polymers.

Stimuli-responsive and core cross-linked micelles developed by NiCCo-PISA of helical poly(aryl isocyanide)s

We report the synthesis of redox- and pH-sensitive block copolymer micelles that contain chiral cores composed of helical poly(aryl isocyanide)s. Pentafluorophenyl (PFP) ester-containing micelles synthesised via nickel-catalysed coordination polymerisation-induced self-assembly (NiCCo-PISA) of helical poly(aryl isocyanide) amphiphilic diblock copolymers are modified post-polymerisation with various diamines to introduce cross-links and/or achieve stimulus-sensitive nanostructures. The successful introduction of the diamines is confirmed by Fourier-transform infrared spectroscopy (FT-IR), while the stabilisation effect of the cross-linking is explored by dynamic light scattering (DLS). The retention of the helicity of the core-forming polymer block is verified by circular dichroism (CD) spectroscopy and the stimuli-responsiveness of the nanoparticles towards a reducing agent (l-glutathione, GSH) and pH is evaluated by following the change in the size of the nanoparticles by DLS. These stimuli-responsive nanoparticles could find use in applications such as drug delivery, nanosensors or biological imaging.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.

Oxidation-Induced Breakage of the Imine Bond and Aggregate Transition in a Se-Containing Dynamic Covalent Surfactant

Controlling the dynamic imine bonds upon a novel trigger except for pH and temperature is still a significant challenge. Here, a Se-containing imine-based dynamic covalent surfactant (HOBAB-BSeEA) was developed for the first time by mixing two precursors in situ: an asymmetric double-chain cationic surfactant bearing a formyl group at the terminal of one hydrophobic tail and a Se-containing amine (2-(benzylselanyl)ethan-1-amine) in order to confirm the effect of redox on the imine bonds. The imine bond in HOBAB-BSeEA can be regulated not only upon changing the pH as well as other common imine-based surfactants but also by oxidation. The conversion efficiency of imine bonds is closely related with the degree of oxidation and pH. Complete oxidation can decrease the conversion efficiency from ∼87 to 48%, which is comparable to the result of changing the pH from 10.0 to 7.0. With the formation and breaking of imine bonds, the surfactant can be reversibly switched between symmetric and asymmetric structures, accompanied by a morphological transition from vesicles to spherical micelles. Although oxidation cannot demolish all imine bonds, it can completely convert vesicles to spherical micelles, which is mainly ascribed to an increase in the polarity of the micellar microenvironment stemming from the oxidation of Se. However, this transition can only be achieved by reducing the pH to 5.0 instead of 7.0. Nile red loaded in HOBAB-BSeEA vesicles can be quickly, controllably, and step-by-step released upon oxidation stimulus but not pH. Understanding the mechanism of oxidation-induced breakage of imine bonds and disruption of vesicles would be useful in designing redox-responsive imine-based carriers that can unload cargoes according to the level of the local reactive oxygen species.

Control of seed formation allows two distinct self-sorting patterns of supramolecular nanofibers

Self-sorting double network hydrogels comprising orthogonal supramolecular nanofibers have attracted attention as artificially-regulated multi-component systems. Regulation of network patterns of self-sorted nanofibers is considered as a key for potential applications such as optoelectronics, but still challenging owing to a lack of useful methods to prepare and analyze the network patterns. Herein, we describe the selective construction of two distinct self-sorting network patterns, interpenetrated and parallel, by controlling the kinetics of seed formation with dynamic covalent oxime chemistry. Confocal imaging reveals the interpenetrated self-sorting network was formed upon addition of O-benzylhydroxylamine to a benzaldehyde-tethered peptide-type hydrogelator in the presence of lipid-type nanofibers. We also succeed in construction of a parallel self-sorting network through deceleration of seed formation using a slow oxime exchange reaction. Through careful observation, the formation of peptide-type seeds and nanofibers is shown to predominantly occur on the surface of the lipid-type nanofibers via highly dynamic and thermally-fluctuated processes.

Regulation of self-sorted nanofiber network patterns in double network hydrogels comprising supramolecular nanofibers is considered as key for potential applications. Here, the authors describe a selective construction of two distinct self-sorting network patterns, by controlling the kinetics of seed formation with dynamic covalent chemistry.

Multiple-responsive supramolecular vesicle based on azobenzene-cyclodextrin host-guest interaction

Multiple-responsive supramolecular vesicles have been successfully fabricated by the complexation between β-cyclodextrin (β-CD) and a pH/photo dual-responsive amphiphile 4-(4-(hexyloxy)phenylazo)benzoate sodium (HPB) with azobenzene and carboxylate groups. When mixing β-CD with HPB to reach a host/guest molar ratio of 1 : 1, the azobenzene group of HPB could be spontaneously included by β-CD molecules. Then, the formed inclusion complexes (HPB@β-CD) could self-assemble into vesicles, which was driven by the hydrophobic interaction of the alkyl chain of HPB and the hydrogen bonds between neighboring β-CDs. The reversible assembly/disassembly of the vesicles could be simply regulated under UV or visible light irradiation. The reversible phase transformation between vesicles and microbelts could also be realized by adjusting the pH values of the sample. Adding both competitive guest molecules (1-adamantane carboxylic acid sodium (ADA)) and α-amylase would result in the phase transformation from vesicles to micelles. Moreover, the vesicles would be destroyed when β-CD was continuously added until the ratio of host/guest reached 2 : 1. Such an interesting quintuple-responsive vesicle system reported here not only has potential applications in various fields such as controlled release or drug delivery, but also provides a reference for the design and construction of multiple responsive systems.

A quintuple-responsive vesicle system was successfully fabricated by simply mixing HPB with an equal amount of β-CD.

pH-Responsive Dipeptide-Based Dynamic Covalent Chemistry Systems Whose Products and Self-Assemblies Depend on the Structure of Isomeric Aromatic Dialdehydes

Facile control over preparation of organic building blocks and self-assembled aggregations to construct the desired materials remains challenges. This article reports selective dynamic covalent bonds formation and the corresponding self-assembly behaviors by using a dipeptide, glycylglycine (GlyGly), reacting with isomeric aromatic dialdehydes o-phthalaldehyde (OPA), p-phthalaldehyde (PPA), and m-phthalaldehyde (MPA) to demonstrate diversified aggregation forms caused by structure topology variations. Under alkaline condition, the aldehyde groups of phthalaldehydes can be connected with the amino groups of GlyGly by imine bonds as the dynamic chemical bonds. Owing to the fact that formation and dissociation of the imine bonds were reversibly pH-responsive, the reactions and aggregates assembled by their products were also reversibly controlled by changing pH. Three products, including two-armed product (OPGG, in which two GlyGly molecules were connected with one OPA molecule), single-armed product (PPG, in which only one GlyGly molecule was connected with a PPA molecule), and a mixture product (MPGG and MPG), as well as their different self-assembly behaviors, were obtained from OPA/GlyGly, PPA/GlyGly, and MPA/GlyGly systems, respectively, at the same condition of pH 8.6 in 90% methanol aqueous solution. However, for OPA/GlyGly system, another different type of product with benzopyrrole structure (OPG) was obtained by nucleophilic substitution via mixing OPA and GlyGly in water, which generated organic nanoparticles. Based on the results above, we conjectured the differences in dynamic covalent bond formation and supramolecular assembly clearly were influenced by the structure topologies of phthalaldehydes (OPA, PPA, and MPA). The experimental phenomenon verified the hypothesis as well, which may guide us to realize facile construction of selective reaction products and intelligent reversibly responsive materials with diverse morphologies and functions.

Impact of "half-crown/two carbonyl"-Ca2+ metal ion interactions of a low molecular weight gelator (LMWG) on its fiber to nanosphere morphology transformation with a gel-to-sol phase transition

We report here a smart functional low molecular weight gelator (LMWG) L, containing an unusual metal ion coordination site, i.e. "half-crown/two carbonyl". The gelator L shows excellent gelation behavior with typical fibrillar morphology in acetonitrile, methanol and ethanol media. Upon Ca2+ ion binding with its "half-crown/two carbonyl" coordination site, the acetonitrile gel of L exhibits a fiber to nanosphere morphology transformation along with a gel-to-sol phase transition as confirmed by microscopic investigation and by direct naked eye visualization, respectively. The mechanism involved in this morphology transformation and gel-to-sol phase transition process was studied thoroughly with the help of computational calculations and various spectroscopic experiments and discussed.

Coacervation of dynamic covalent surfactants with polyacrylamides: properties and applications

Dynamic covalent surfactants have been prepared from a mixture of 4-formyl-N,N,N-trimethylbenzenaminium iodide (FBA) with heptylamine (C7A) or octylamine (C8A) in alkaline aqueous solutions. The reversible pH-dependent nature of the imine bond is characterized by 1H NMR and fluorescence analysis. The dynamic covalent surfactants self-assemble into micelles under alkaline conditions and exhibit coacervation with 10% hydrolyzed polyacrylamide (PAM) over a wide concentration range. The coacervate phase with a network structure was found to effectively extract the anionic dye Conge Red (CR). When the solution is adjusted to acidity, the imine bond is hydrolyzed, leading to the transition of the coacervates into a homogeneous and clear solution, and the precipitation of CR into purple-black solids due to the protonation of sulfonic groups. Thus, the extraction and release of CR molecules are realized with this dynamic covalent surfactant/PAM system. Moreover the initial components, FBA, amine, and PAM, can be easily regenerated with hydrochloric acid. This method shows potential applications in wastewater treatment.

A tetraphenylethylene-based acylhydrazone gel for selective luminescence sensing

A supramolecular gel based on dynamic covalent acylhydrazone bonding for selective and sensitive Cu²⁺ and subsequent CN⁻ detection has been reported.

A ferrocene-based organogel with multi-stimuli properties and applications in naked-eye recognition of F− and Al3+

Ferrocene-based gelator F-6/chloroform gel exhibited multi-stimuli responsiveness, through which ‘naked eye’ recognition of both Al³⁺ and F⁻ was realized.