Heat‐Induced Morphological Transformation of Supramolecular Nanostructures by Retro‐Diels–Alder Reaction

Controlling the morphology of supramolecular nanostructures in response to external stimuli is an important challenge in the development of functional soft materials. Here we show that a morphological transformation from 2D nanosheets to a network of 1D nanofibers is triggered by heating, which induces molecular conversion of a bolaamphiphile to a hydrogelator by means of a retro‐Diels–Alder reaction, thereby producing a new heat‐set supramolecular hydrogel. We anticipate that our design will be a starting point for more sophisticated supramolecular systems that integrate the thermodynamics of molecular assembly and the kinetics of chemical reactions to create complex supramolecular nanostructures.     

Rational Fabrication of Multiple Dimensional Assemblies from Tryptophan-Based Racemate

Fabricating structural complex assemblies from simple amino acid-based derivatives is attracting great research interests due to their easy accessibility and preparation. However, the morphological regulation of racemates (an equimolar mixture of enantiomers) were largely overlooked. In this work, through rational modulation of kinetic and thermodynamic parameters, we achieved multiple dimensional architectures employing tryptophan-based racemate (RPWM). Upon assembling, 1D bundled nanofibers, 2D lamellar nanostructure and 3D urchin-like microflowers could be obtained depending on the solvents used. The corresponding morphology evolutions were successfully illustrated by changing the enantiomeric excess (ee) value. Moreover, for RPWM, uniform 0D nanospheres were formed in H₂O under 4 °C, which could spontaneously convert into lamella under ambient temperature. Taking advantages of its temperature-responsive phase change behavior, RPWM assemblies exhibited excellent removal efficiency for organic dye RhB, and could be reused for several consecutive cycles without significant changes in its removal performance. Taken together, it's rational to envision that the engineering of racemates assembly pathways can greatly increase the robustness in a wide variety of supramolecular materials and further lead to their blooming versatile applications.     

Strategies for the construction of supramolecular assemblies from poly-NHC ligand precursors

The field of supramolecular assemblies has developed rapidly in the last few decades, thanks in a large part to their diverse applications. These assemblies have been mostly based on Werner-type coordination motifs in which metal centres are coordinated by nitrogen or oxygen donors. Recently, N-heterocyclic carbene(NHC) ligands have been employed as carbon donors not only because of their appealing structures but also due to the extensive applications in catalysis, biomedicine and material science of the resulting assemblies. During the last decade, NHC-based supramolecular assemblies have witnessed rapid growth and extensive application in molecular recognition, luminescent materials and catalysis. For different topological systems, a diverse selection of poly-NHC precursors and synthetic strategies is crucial to precisely control the synthesis of supramolecular architectures. Several synthetic strategies have been developed to synthesise two-dimensional(2D) molecular metallacycles and three-dimensional(3D) metallacages from a wide range of poly-NHC precursors, including a straightforward one-pot strategy,supramolecular transmetalation, stepwise synthesis, an improved one-pot strategy involving self-sorting behaviour of 3D metallacages and a subtle variation strategy of poly-NHC ligand precursors. This review offers a summary of the synthetic strategies applied for the construction of different poly-NHC-based supramolecular assemblies, particularly emphasizes recent progress in the synthesis of large and complex supramolecular assemblies from poly-NHC precursors, and further attention is given to their application in postsynthetic modifications(PSMs), host-guest chemistry, luminescent properties and biomedical applications.     

Dynamic cylindrical assembly of triblock copolymers by a hierarchical process of covalent and supramolecular interactions

We have developed a hierarchical process that combines linear triblock copolymers into concentric globular subunits through strong chemical bonds and is followed by their supramolecular assembly via weak noncovalent interactions to afford one-dimensionally assembled, dynamic cylindrical nanostructures. The molecular brush architecture forces triblock copolymers to adopt intramolecular interactions within confined frameworks and then drives their intermolecular interactions in the mixtures of organic solvent and water. In contrast, the triblock copolymers, when not preconnected into the molecular brush architectures, organize only into globular assemblies.     

Hybrid Soft Nanomaterials Composed of DNA Microspheres and Supramolecular Nanostructures of Semi-artificial Glycopeptides

Aqueous hybrid soft nanomaterials consisting of plural supramolecular architectures with a high degree of segregation (orthogonal coexistence) and precise hierarchy at the nano- and microscales, which are reminiscent of complex biomolecular systems, have attracted increasing attention. Remarkable progress has been witnessed in the construction of DNA nanostructures obtained by rational sequence design and supramolecular nanostructures of peptide derivatives through self-assembly under aqueous conditions. However, orthogonal self-assembly of DNA nanostructures and supramolecular nanostructures of peptide derivatives in a single medium has not yet been explored in detail. In this study, DNA microspheres, which can be obtained from three single-stranded DNAs, and three different supramolecular nanostructures (helical nanofibers, straight nanoribbons, and flowerlike microaggregates) of semi-artificial glycopeptides were simultaneously constructed in a single medium by a simple thermal annealing process, which gives rise to hybrid soft nanomaterials. Fluorescence imaging with selective staining of each supramolecular nanostructure uncovered the orthogonal coexistence of these structures with only marginal impact on their morphology. Additionally, the biostimuli-responsive degradation propensity of each supramolecular architecture is retained, and this may allow the construction of active soft nanomaterials exhibiting intelligent biofunctions.     

9,10-Bis(diphenylmethylene)-9,10-dihydroanthracene-based metal-organic assemblies with aggregation-induced emission for multiple sensing

Developing novel emissive supramolecular assemblies with elegant architectures and tunable performance remains highly desirable yet challenging. Herein, we report the design and synthesis of several 9,10-bis(diphenylmethylene)-9,10-dihydroanthracene-based metal-organic assembles with aggregation-induced emission characteristics. Such assemblies feature intriguing thermochromic and mechanochromic properties, i.e., distinguishable fluorescence responses in terms of emission wavelength and intensity under variable temperatures and pressures. Moreover, these assemblies can serve as excellent fluorescent sensors for the detection of polysaccharide molecules. Due to the differentiated charge type and density, the assembles display distinct sensing mechanisms toward different polysaccharide molecules. This study provides novel perspectives for the synthesis of butterfly-like platinum(II) supramolecular coordination complexes with multistimuli-responsiveness for polysaccharide sensing, which will facilitate the development of stimuli-responsive materials     

Vibrational spectroscopic investigations of ammonia gas sensing mechanism in polypyrrole nanostructures

This paper reports the application of Raman and Fourier transform infrared (FTIR) spectroscopy techniques for the investigation of molecular restructuring of polypyrrole (PPy) nanostructures in ammonia environment. Different types of PPy nanostructures such as nanofibers, nanorods, and nanoparticles were prepared in the presence of different surfactants such as cetyltrimethyl ammonium bromide (CTAB), methyl orange, sodium dodecyl sulfate, and Triton X-100, respectively. The prepared nanostructures were characterized for structural, morphological, and the gas sensing properties. The gas sensing reponse towards ammonia is estimated from change in the surface resistance of the sample. PPy nanofibers prepared in the presence of CTAB have a diameter of ∼63 nm and the gas sensing response of ∼18%, whereas, PPy nanoparticles prepared in the presence of Triton X-100 have a diameter of ∼94 nm and the lowest gas sensing response (6.5%) at 100 ppm level of ammonia. The mechanism of gas sensing has been investigated through vibrational (Raman and FTIR) spectroscopy techniques performed in the presence of analyte (ammonia) gas. The charge compensation via proton transfer process in ammonia environment is found to be main cause for the gas sensing response in the PPy nanostructures.     

Self-assembly of supra-amphiphiles based on dual charge-transfer interactions: from nanosheets to nanofibers

With the elaborate engineering of supra-amphiphiles based on dual charge-transfer interactions, the rational design and programmable transformation of well-defined 1D and 2D nanostructures have been demonstrated. First, H-shaped supra-amphiphiles are successfully obtained on the basis of the directional charge-transfer interactions of naphthalene diimide and naphthalene, which self-assemble in water to form 2D nanosheets. Second, by complexation of the H-shaped supra-amphiphiles with pyrene derivatives, the 2D nanosheets transform into ultralong 1D nanofibers. Therefore, this line of research represents a successful example of supramolecular engineering and has enriched its realm.     

Supramolecular Assemblies with Aggregation-Induced Emission Properties for Sensing and Detection

The fabrication of new supramolecular materials for real-time detection of analytes including ions, organic pollutants, gases, biomolecules, and drugs is of pivotal importance in industrial manufacture, clinical treatment, and environmental remediation. Incorporating fluorescent molecules with distinct aggregation-induced emission (AIE) effects into supramolecular assemblies has received much attention over the past two decades, owing to the remarkable performance of the AIE-active supramolecular materials in sensing and detection. In this minireview, we summarize the recent progress of superior detection systems on the basis of supramolecular assemblies accompanied with AIE features. We envision that this minireview will be helpful and timely for relevant researchers to stimulate new thinking for constructing new AIE-based supramolecular materials with advanced architectures for effective sensing and detection.     

Macroscopic Free‐Standing Hierarchical 3D Architectures Assembled from Silver Nanowires by Ice Templating

As macroscopic three dimensional (3D) architectures show increasing significance, much effort has been devoted to the hierarchical organization of 1D nanomaterials into serviceable macroscopic 3D assemblies. How to assemble 1D nanoscale building blocks into 3D hierarchical architectures is still a challenge. Herein we report a general strategy based on the use of ice as a template for assembling 1D nanostructures with high efficiency and good controllability. Free‐standing macroscopic 3D Ag nanowire (AgNW) assemblies with hierarchical binary‐network architectures are then fabricated from a 1D AgNW suspension for the first time. The microstructure of this 3D AgNW network endows it with electrical conductivity and allows it to be made into stretchable and foldable conductors with high electromechanical stability. These properties should make this kind of macroscopic 3D AgNW architecture and it composites suitable for electronic applications.     

Peptide amphiphile nanofibers with conjugated polydiacetylene backbones in their core

The coupling of electronic and biological functionality through self-assembly is an interesting target in supramolecular chemistry. We report here on a set of diacetylene-derivatized peptide amphiphiles (PAs) that react to form conjugated polydiacetylene backbones following self-assembly into cylindrical nanofibers. The polymerization reaction yields highly conjugated backbones when the peptidic segment of the PAs has a linear, as opposed to a branched, architecture. Given the topotactic nature of the polymerization, these results suggest that a high degree of internal order exists in the supramolecular nanofibers formed by the linear PA. On the basis of microscopy, the formation of a polydiacetylene backbone to covalently connect the beta-sheets that help form the fibers does not disrupt the fiber shape. Interestingly, we observe the appearance of a polydiacetylene (PDA) circular dichroism band at 547 nm in linear PA nanofibers suggesting the conjugated backbone in the core of the nanostructures is twisted. We believe this CD signal is due to chiral induction by the beta-sheets, which are normally twisted in helical fashion. Heating and cooling shows simultaneous changes in beta-sheet and conjugated backbone structure, indicating they are both correlated. At the same time, poor polymerization in nanofibers formed by branched PAs indicates that less internal order exists in these nanostructures and, as expected, then a circular dichroism signal is not observed for the conjugated backbone. The general variety of materials investigated here has the obvious potential to couple electronic properties and in vitro bioactivity. Furthermore, the polymerization of monomers in peptide amphiphile assemblies by a rigid conjugated backbone also leads to mechanical robustness and insolubility, two properties that may be important for the patterning of these materials at the cellular scale.     

Formation and Characterization of Europium Bisphthalocyanine Organic Nanowires by Electrochemical Deposition

Europium bisphthalocyanine (EuPc2) nanowires were prepared by electrochemicaldeposition method. Scanning electron microscopy (SEM) images show the evolution of themorphologies of nanowires obtained under different deposition time (Td). The optical properties ofeuropium bisphthalocyanine films were studied by UV-Vis absorption spectra. The morphology of EuPc2 nanowires could be controlled by changing deposition conditions, which provides a usefulmethod to make organic nanowires.     

Hierarchical supramolecular fullerene architectures with controlled dimensionality

Self-assembly of a fullerene derivative with long alkyl chains in different solvents results in the formation of hierarchically-ordered supramolecular assemblies with well-defined 1, 2 and 3D architectures such as vesicles, fibers, discs and cones, whose fundamental structural sub-unit consists of bilayers.     

Self-Assembly of a Pyridine-Based Amphiphile Complexed with Regioisomeric Dihydroxy Naphthalenes into Supramolecular Nanotubes with Different Inner Diameters

A pyridine-based amphiphile complexed with 1,5-, 1,6-, 2,6-, or 2,7-dihydroxy naphthalene self-assembled in water to form nanotubes with inner diameters of 46, 38, 24, 18, and 11 nm in which the naphthalene molecules formed J-type aggregates. In contrast, the amphiphile complexed with 1,2-, 1,3-, 1,4-, 1,7-, 1,8-, or 2,3-dihydroxy naphthalene formed nanofibers in which the naphthalene molecules formed H-type aggregates. The inner diameter of the nanotubes strongly depended on the regioisomeric dihydroxy naphthalene. UV–vis, fluorescence, infrared spectroscopy, X-ray diffraction analysis, and differential scanning calorimetry showed that nanotubes with smaller inner diameters had weaker intermolecular hydrogen bonds between the tilted amphiphiles complexed with the naphthalene molecules within the membrane walls and showed larger Stokes shifts in the excimer fluorescence of the naphthalene moiety. These findings should be useful not only for fine-tuning the inner diameters of supramolecular nanotubes but also for controlling the aggregation states of functional aromatic molecules to generate nanostructures with useful optical and electronic properties in water.     

Peptide Nanomaterials Designed from Natural Supramolecular Systems

Natural supramolecular assemblies exhibit unique structural and functional properties that have been optimized over the course of evolution. Inspired by these natural systems, various bio‐nanomaterials have been developed using peptides, proteins, and nucleic acids as components. Peptides are attractive building blocks because they enable the important domains of natural protein assemblies to be isolated and optimized while retaining the original structures and functions. Furthermore, the peptide subunits can be conjugated with exogenous molecules such as peptides, proteins, nucleic acids, and metal nanoparticles to generate advanced functions. In this personal account, we summarize recent progress in the construction of peptide‐based nanomaterial designed from natural supramolecular systems, including (1) artificial viral capsids, (2) self‐assembled nanofibers, and (3) protein‐binding motifs. The peptides inspired by nature should provide new design principles for bio‐nanomaterials.     

Formation of supramolecular cavitands on copper electrode surfaces

The adsorption of 1,1'-dibenzyl-4,4'-bipyridinium molecules (dibenzyl-viologen or DBV(2+) for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV(2+) spontaneously forms a highly ordered phase on the chloride c(2 x 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV(2+). This ordered phase can be described by a ( radical 53 x radical 53)R15.9 degrees unit cell relating the organic adlayer to the chloride c(2 x 2) structure underneath or alternatively by a ( radical 106 x radical 106)R29.05 degrees unit cell relating the organic layer to the Cu(1 x 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV(2+). Compared to the copper-chloride interaction, the DBV(2+)-chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV(2+) adlayer is an assembly of four individual DBV(2+) molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the ( radical 53 x radical 53)R15.9 degrees structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.     

Calixarene-encapsulated nanoparticles: self-assembly into functional nanomaterials

Calixarenes are excellent surfactants for enhancing the dispersion and self-assembly of metal nanoparticles into well-defined structures, particularly those with unit length scales in the 10-100 nm size range. Particles within these ensembles are strongly coupled, giving rise to unique collective optical or magnetic properties. The self-assembled nanostructures described in this feature article include 2D arrays of colloidal Au nanoparticles with size-dependent plasmonic responses, and sub-100 nm Co nanoparticle rings with chiral magnetic states. These nanoparticle assemblies may be further developed for applications in chemical sensing based on surface-enhanced Raman scattering (SERS) and as binary elements for nonvolatile memory, respectively.     

Surface engineering of one-dimensional tin oxide nanostructures for chemical sensors

Nanostructured materials are promising candidates for chemical sensors due to their fascinating physicochemical properties. Among various candidates, tin oxide (SnO₂) has been widely explored in gas sensing elements due to its excellent chemical stability, low cost, ease of fabrication and remarkable reproducibility. We are presenting an overview on recent investigations on 1-dimensional (1D) SnO₂ nanostructures for chemical sensing. In particular, we focus on the performance of devices based on surface engineered SnO₂ nanostructures, and on aspects of morphology, size, and functionality. The synthesis and sensing mechanism of highly selective, sensitive and stable 1D nanostructures for use in chemical sensing are discussed first. This is followed by a discussion of the relationship between the surface properties of the SnO₂ layer and the sensor performance from a thermodynamic point of view. Then, the opportunities and recent progress of chemical sensors fabricated from 1D SnO₂ heterogeneous nanostructures are discussed. Finally, we summarize current challenges in terms of improving the performance of chemical (gas) sensors using such nanostructures and suggest potential applications. Contains 101 references.

Metal ion directed self-assembly of sensors for ions, molecules and biomolecules

Recent years have seen a surge of interest in the metal-ion directed construction of discrete molecular assemblies. Once the versatility of this approach for the construction of hitherto inaccessible molecular architectures was demonstrated, work towards fully functional systems rapidly developed. Since these architectures have a wide range of possible applications, this perspective review will focus on one important aspect of this research: the construction of hosts with optical or electrochemical sensing outputs. As this overview will illustrate, research in this area is now producing working examples of sensors for variety of ionic, molecular, and biomolecular guests.     

Two-Dimensional Supramolecular Polymerization of DNA Amphiphiles is Driven by Sequence-Dependent DNA-Chromophore Interactions

Two-dimensional (2D) assemblies of water-soluble block copolymers have been limited by a dearth of systematic studies that relate polymer structure to pathway mechanism and supramolecular morphology. Here, we employ sequence-defined triblock DNA amphiphiles for the supramolecular polymerization of free-standing DNA nanosheets in water. Our systematic modulation of amphiphile sequence shows the alkyl chain core forming a cell membrane-like structure and the distal π-stacking chromophore block folding back to interact with the hydrophilic DNA block on the nanosheet surface. This interaction is crucial to sheet formation, marked by a chiral “signature”, and sensitive to DNA sequence, where nanosheets form with a mixed sequence, but not with a homogeneous poly(thymine) sequence. This work opens the possibility of forming well-ordered, bilayer-like assemblies using a single DNA amphiphile for applications in cell sensing, nucleic acid therapeutic delivery and enzyme arrays.