Reversible Morphological Transformation between Polymer Nanocapsules and Thin Films through Dynamic Covalent Self-Assembly

Exploiting the Structural Metamorphosis of Polymers to 'Wrap' Micron-Sized Spherical Objects

There is growing interest in developing methods to 'wrap' nano- and micron-sized biological objects within films that may offer protection, enhance their stability or improve performance. We describe the successful 'wrapping' of lectin-decorated microspheres, which serve as appealing model micron-sized objects, within cross-linked polymer film. This approach utilizes polymer chains able to undergo a structural metamorphosis, from being intramolecularly cross-linked to intermolecularly cross-linked, a process that is triggered by polymer concentration upon the particle surface. Experiments demonstrate that both complementary molecular recognition and the dynamic covalent nature of the crosslinker are required for successful 'wrapping' to occur. This work is significant as it suggests that nano- and micron-sized biological objects such as virus-like particles, bacteria or mammalian cells-all of which may benefit from additional environmental protection or stabilization in emerging applications-may also be 'wrapped' by this approach.

Hierarchical Self-Assembly of Poly-Pseudorotaxanes into Artificial Microtubules

Hierarchical self-assembly of building blocks over multiple length scales is ubiquitous in living organisms. Microtubules are one of the principal cellular components formed by hierarchical self-assembly of nanometer-sized tubulin heterodimers into protofilaments, which then associate to form micron-length-scale, multi-stranded tubes. This peculiar biological process is now mimicked with a fully synthetic molecule, which forms a 1:1 host-guest complex with cucurbit[7]uril as a globular building block, and then polymerizes into linear poly-pseudorotaxanes that associate laterally with each other in a self-shape-complementary manner to form a tubular structure with a length over tens of micrometers. Molecular dynamic simulations suggest that the tubular assembly consists of eight poly-pseudorotaxanes that wind together to form a 4.5 nm wide multi-stranded tubule.

Exfoliating a CdII -Purine Framework: Conversion of Nanosheets-to-Nanofibers and Studies of Elastic and Capacitive Properties

Layered bulk crystals are amenable to exfoliation to yield 2D nanosheets through isolation and intercalation processes, which could be further converted to 1D nanoscale structures. The latter inherit gross morphological and physical properties associated with the precursor structures. Herein, we report three purine-based crystal structures 1, 2, and 3, where 3 is obtained by a single-crystal-to-single-crystal transformation from 2 and is a conformational polymorph of 1. Next, we describe the sonication-assisted liquid exfoliation of 1, a Cd^II -purine coordination framework, into nanosheets and nanofibers in a solvent-dependent process. The exfoliation was carefully studied at low temperatures to ascertain this unique conversion. This work also features the determination of the Young's modulus and surface potential of the bioinspired Cd^II -based nanostructures by using amplitude modulation-frequency modulation atomic force microscopy and Kelvin probe force microscopy, respectively, revealing their interesting elastic and capacitive properties for their possible use in electronics and energy devices. Electron impedance spectroscopy measurements further established a higher value of capacitance for the exfoliated Cd^II framework as compared to the ligand alone.

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.

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

This paper reports an interesting type of self-assembly systems based on dynamic covalent bonds. The systems are pH-responsible and reversible, which could be utilized for controlling the morphology transformation of the assemblies. In alkaline conditions, the amine group of 11-aminoundecanoic acid (AUA) can connect with the aldehyde group of benzaldehyde (BA) or 1-naphthaledhyde (NA) by dynamic covalent bond to form a small organic building block accompanied by the morphological transformation from vesicles to fibers. When pH is lowered to a neutral value, the dynamic covalent bonds (imine bonds) can be hydrolyzed, leading to the dissociation of fibers and appearance of spherical aggregates. The transformation was confirmed reversible as fibers appeared again when the pH was changed back to alkaline value. In addition, a reversibly controlled gel was designed based on the nanofiber formation. NaCl, which is capable of greatly enhance the nanofiber density and cross-linking, was used to induce the growth of free-standing gel from free-flowing nanofiber system, and the resultant gel was proven to be pH-reversible.