Self-assembly of block copolymers

Block copolymer (BCP) self-assembly has attracted considerable attention for many decades because it can yield ordered structures in a wide range of morphologies, including spheres, cylinders, bicontinuous structures, lamellae, vesicles, and many other complex or hierarchical assemblies. These aggregates provide potential or practical applications in many fields. The present tutorial review introduces the primary principles of BCP self-assembly in bulk and in solution, by describing experiments, theories, accessible morphologies and morphological transitions, factors affecting the morphology, thermodynamics and kinetics, among others. As one specific example at a more advanced level, BCP vesicles (polymersomes) and their potential applications are discussed in some detail.     

Supramolecular assemblies with tunable morphologies from homopolymeric and small organic molecular building blocks

This work demonstrates the formation of micrometer-sized supramolecular assemblies with tunable morphologies using a homopolymer, poly(4-vinylpyridine), and a small organic acid, 5,7-dodecadiynedioic acid, as the building molecules. Three different morphologies (hollow spheres, solid spheres, and rods) were obtained, depending on the molar ratio of the building molecules. It is proposed that hydrogen bonding between P4VP and DCDA and the pi-pi stacking of the diacetylenic moieties are responsible for the formation of these assemblies. Interestingly, ordered hexagonal and lamellar mesostructures were also formed within the microstructure during the co-assembly process. As a result, UV irradiation of the supramolecular assemblies polymerized the diacetylenic moieties, resulting in cross-linked and responsive blue polydiacetylenic assemblies that can change color to red upon external stimuli (e.g., thermal stimuli). This work provides a novel concept of the synthesis of responsive supramolecular assemblies from a homopolymer and small organic molecules.     

纳米粒子的精准组装

纳米材料由于其独特的光、电、磁、力学等性质,成为了构建功能材料与器件的理想基元。实现纳米粒子的精确组装,是探究粒子之间的耦合聚集性质和制备宏观功能器件的基础。但是由于纳米粒子的小尺寸以及在溶液中运动的随机性与复杂性,精准控制纳米粒子组装体的形貌以及在空间中的相对位置仍存在巨大挑战。为了将纳米粒子组装成理想的有序结构,许多控制粒子组装的策略与方法得到发展。本文首先概述了纳米粒子自组装的控制方法与典型形貌,着重分析了影响粒子精准排布的因素与控制方法,并对纳米粒子及其组装体的光学性质与器件应用的最新研究进展进行了讨论,最后对目前纳米粒子精准组装所面临的挑战以及未来发展的方向进行了展望。     

3D Assembly of All‐Inorganic Colloidal Nanocrystals into Gels and Aerogels

We report an efficient approach to assemble a variety of electrostatically stabilized all‐inorganic semiconductor nanocrystals (NCs) by their linking with appropriate ions into multibranched gel networks. These all‐inorganic non‐ordered 3D assemblies benefit from strong interparticle coupling, which facilitates charge transport between the NCs with diverse morphologies, compositions, sizes, and functional capping ligands. Moreover, the resulting dry gels (aerogels) are highly porous monolithic structures, which preserve the quantum confinement of their building blocks. The inorganic semiconductor aerogel made of 4.5 nm CdSe colloidal NCs capped with I^? ions and bridged with Cd²⁺ ions had a large surface area of 146 m² g^?1.     

Anion modules: building blocks of supramolecular assemblies by combination with π-conjugated anion receptors

Dipyrrolyldiketone boron complexes, as π-conjugated acyclic anion receptors, act as building subunits of various assemblies through noncovalent interactions in the form of receptor-anion complexes. Instead of, or in addition to, the modification of receptor structures, the introduction of anion modules as building blocks for the assemblies was found to be useful in forming various soft materials. Gallic carboxylate derivatives 3-n (n = 16, 18, 20), as tetrabutylammonium (TBA) salts, form receptor-anion-module complexes that can be used to fabricate supramolecular assemblies. Combinations of aliphatic anion modules 3-n and receptors 1a,b along with a TBA cation afforded products with mesophases, which were indicated by differential scanning calorimetry and polarized optical microscopy. X-ray diffraction measurements of the solid states and mesophases of 1a·3-n·TBA and 1b·3-n·TBA revealed highly ordered structures including lamellar structures, which could be modulated by the lengths of the alkyl chains of the modules. Functional materials exhibiting electrical conductivity were fabricated by using combinations of anionic building blocks that form assemblies by themselves and π-conjugated acyclic receptors.     

Precise Assembly of Polyoxometalates at Single-cluster Levels

Polyoxometalate (POM) clusters with atomic precision structures are promising candidates construct functional nanomaterials via self-assembly. Non-covalent interactions at molecular levels can govern the self-assembly of POM clusters, for which the precise control of POM-based assemblies can be realized at single-cluster levels. This mini-review focuses on the synthesis and properties of POM-based nanostructures, including amphiphilic POM assemblies and co-assemblies of POM clusters and other subnanometer building blocks. Several synthetic strategies have been developed for rational control of POM-based assemblies in terms of morphologies, compositions and properties. 1D subnanometer POM assemblies demonstrate remarkable enhanced mechanical properties due to the topological interactions between nanowires and surroundings. The in-depth understanding of POM-based assemblies may help in the design of functional nanomaterials in fundamental perspectives and applications.     

Surface-tuned assembly of porphyrin coordination oligomers

Two self-complementary phenanthroline-strapped porphyrins bearing imidazole arms and C 12 or C 18 alkyl chains were synthesized, and their surface self-assembly was investigated by atomic force microscopy (AFM) on mica and highly ordered pyrrolitic graphite (HOPG). Upon zinc(II) complexation, stable porphyrin dimers formed, as confirmed by DOSY (1)H NMR and UV-visible spectroscopy. In solution, the dimers formed J-aggregates. AFM studies of the solutions dip-coated onto mica or drop-casted onto HOPG revealed that the morphologies of the assemblies formed were surface-tuned. On mica, fiber-like assemblies of short stacks of J-aggregates were observed. The strong influence of the mica's epitaxy on the orientation of the fibers suggested a surface-assisted assembly process. On HOPG, interactions between the alkyl chains and the graphite surface resulted in the stabilization and trapping of monomer species followed by their subsequent association into coordination polymers on the surface. Interdigitation of the alkyl chains of separate polymer strands induced lateral association of wires to form islands that grew preferentially upon drop-casting and slow evaporation. Clusters of laterally assembled wires were observed for the more mobile functionalized porphyrins bearing C 12 chains.     

Post-polymerization of preorganized assemblies for creating shape-controlled functional materials

Combination of supramolecular chemistry with molecular recognition has been successfully applied to creating large superstructures with a wide variety of morphologies. Control of shapes and patterns of ordered molecular assemblies in nano and micro scales has attracted considerable interest as promising bottom-up technology. It is known, however, that these molecular assembling superstructures are fragile, reflecting the characteristic of the non-covalent interaction, a driving force operating in these molecular systems. In fact, they easily collapse or change by small perturbation in the environmental conditions. Thus, over the last decade, researchers have been seeking possible methods for the immobilization these superstructures. This critical review focuses on recent advances in in situ post-modification under the influence of the molecular assemblies as templates and polymerization of ordered molecular assemblies such as organogel fibers and crystals to preserve their original superstructures and intensify their mechanical strength.     

Magnetic-field-induced assemblies of cobalt nanoparticles

Under the influence of a 0.05 T magnetic field, 15-nm diameter cobalt nanoparticles covered with surfactants in a colloidal solution assemble into highly constrained linear chains along the direction of the magnetic field. The magnetic-field-induced (MFI) chains become floppy after removal of the field, folding into three-dimensional (3D) coiled structures upon gentle agitation. The 3D structures are broken into smaller units with vigorous agitation. The nanoparticles redisperse into the solvent upon ultrasonic agitation. Optical microscopy and transmission electron microscopy (TEM) are used to characterize the morphologies of the nanoparticle assemblies at various stages of this reversible process. The hysteresis loops and zero-field cooled/field cooled (ZFC/FC) curves reveal the interparticle coupling in the assemblies. MFI assembly provides a powerful tool to manipulate magnetic nanoparticles.     

Phase networks of cross-β peptide assemblies

Recent evidence suggests that simple peptides can access diverse amphiphilic phases, and that these structures underlie the robust and widely distributed assemblies implicated in nearly 40 protein misfolding diseases. Here we exploit a minimal nucleating core of the Aβ peptide of Alzheimer's disease to map its morphologically accessible phases that include stable intermolecular molten particles, fibers, twisted and helical ribbons, and nanotubes. Analyses with both fluorescence lifetime imaging microscopy (FLIM) and transmission electron microscopy provide evidence for liquid-liquid phase separations, similar to the coexisting dilute and dense protein-rich liquid phases so critical for the liquid-solid transition in protein crystallization. We show that the observed particles are critical for transitions to the more ordered cross-β peptide phases, which are prevalent in all amyloid assemblies, and identify specific conditions that arrest assembly at the phase boundaries. We have identified a size dependence of the particles in order to transition to the para-crystalline phase and a width of the cross-β assemblies that defines the transition between twisted fibers and helically coiled ribbons. These experimental results reveal an interconnected network of increasing molecularly ordered cross-β transitions, greatly extending the initial computational models for cross-β assemblies.     

Characterization of 3D DNA Assemblies Using Cryogenic Electron Microscopy

DNA nanotechnology utilizes DNA double strands as building units for self-assembly of DNA nanostructures.The specific base-pairing interaction between DNA molecules is the basis of these assemblies.After decades of development,this technology has been able to construct complex and programmable structures.With the increase in delicate nature and complexity of the synthesized nanostructures,a characterization technology that can observe these structures in three dimensions has become necessary,and developing such a technology is considerably challenging.DNA assemblies have been studied using different characterization methods including atomic force microscopy(AFM),scanning electron microscopy(SEM),and transmission electron microscopy(TEM).However,the three-dimensional(3D)DNA assemblies always collapse locally due to the dehydration during the drying process.Cryogenic electron microscopy(cryo-EM)can overcome the challenge by maintaining three-dimensional morphologies of the cryogenic samples and reconstruct the 3D models from cryogenic samples accordingly by collecting thousands of two-dimensional(2D)projection images,which can restore their original morphologies in solution.Here,we have reviewed several typical cases of 3D DNA-assemblies and highlighted the applications of cryo-EM in characterization of these assemblies.By comparing with some other characterization methods,we have shown how cryo-EM promoted the development of structural characterization in the field of DNA nanotechnology.     

Pillararene‐Based Assemblies: Design Principle,Preparation and Applications

In this review, we highlight recent advancements on pillararene‐based assemblies. The driving forces for the formation of the pillararene‐based assemblies are discussed first. The host–guest interactions are deemed as not only general strategy for constructing assemblies but also essential components for preventing the assemblies from the dissociation. Solvent effect is also important in the assembling process, since it could influence the host–guest interactions and provide solvophobic effect on pillararenes for the assembly. Then, several pillararene‐based assembly architectures are introduced, including pillararene‐based interlocked structures, such as (poly)pseudorotaxanes, (poly)rotaxanes, and daisy chains, classified by their topological structures and synthetic strategy. The morphologies of the supramolecular assemblies are divided into several types, for example, nanospheres, nanotubes and supramolecular polymers. Furthermore, the functions and potential applications are summarized accompanied with related assembly structures. The review not only provides fundamental findings, but also foresights future research directions in the research area of pillararene‐based assemblies.     

Self-assembly of designed oligomeric siloxanes with alkyl chains into silica-based hybrid mesostructures

A novel self-assembly route to ordered silica-organic hybrids using well-defined siloxane oligomers with alkoxy functionality and covalently attached alkyl chains has been investigated. Various hybrid mesostructures were obtained by hydrolysis and polycondensation without the use of any structure-directing agents. The oligomers 1(Cn), having an alkylsilane core and three branched trimethoxysilyl groups, formed highly ordered lamellar phases when n = 14-18, while those with shorter alkyl chains formed cylindrical assemblies, slightly distorted two-dimensional (2D) hexagonal structures (n = 6-10), and a novel 2D monoclinic structure (n = 12). Furthermore, the mixtures of 1(Cn) with different chain lengths yielded well-ordered 2D hexagonal phases, possibly due to the better packing of the precursors. The hybrids consisting of cylindrical assemblies were converted to ordered porous silica with tunable pore sizes upon calcination to remove organic groups. The liquid-state 29Si NMR analysis of the hydrolysis and polycondensation processes of 1(Cn) revealed a unique intramolecular reaction yielding primarily the oligomer with a tetrasiloxane ring which is a new class of amphiphilic molecule having both self-assembling ability and high cross-linking ability. We also found that the mesostructure (lamellar or 2D hexagonal) was strictly controlled by varying the number of siloxane units per alkyl chain. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of oligosiloxane precursors.     

Self-organized microporous structures based on surfactant-encapsulated polyoxometalate complexes

Self-organized microporous structures based on a series of surfactant-encapsulated polyoxometalate complexes (SECs) have been prepared by using ordered condensed droplets as a template. Among these structures, ordered honeycomb structures were obtained and characterized in detail by taking (DODA)(12)H[Eu(SiW(11)O(39))2] (SEC-1) as an example. Optical microscope, atomic force microscopic, and scanning electron microscopic measurements confirmed the formation of three-dimensional microporous structure, in which the top surface shows a highly ordered honeycomb structure. As compared to common solvent-casting films, the corresponding honeycomb films are more hydrophobic and possess more ordered lamellar structures. Both the wettability and the size of SECs exert significant influence on the formation of microporous structures. The proper hydrophobicity of SECs was proposed to be an essential factor for the formation of honeycomb films, and large-sized SECs are favorable for the fabrication of highly ordered honeycomb structures. The conditions for the formation of different surface morphologies have been discussed in terms of the contact angle of SECs at the interface between water and chloroform, and a contact angle slightly greater than 90 degrees is found to be a prerequisite for the formation of honeycomb structures. The results reported in this paper not only help to further comprehend the mechanism of the formation of honeycomb structures, but also provide some guidance for the design of ordered microporous films based on organic/inorganic hybrid materials, exemplified by the organic/nanoparticle complexes.     

Supramolecular Approaches towards Ordered Polymer Materials

Controlled organization of polymer chains into ordered structures is highly important to tune or enhance the properties of the polymeric materials. A supramolecular approach using host–guest chemistry has allowed rational design of chain assemblies with many functional properties. Nanoporous materials with ordered channel structures are particularly useful for attaining precise assemblies of polymer chains through nanoconfinement.     

Supramolecular assemblies of block copolymers as templates for fabrication of nanomaterials

Self-assembled polymeric systems have played an important role as templates for nanofabrication; they offer nanotemplates with different morphologies and tunable sizes, are easily removed after reactions, and could be further modified with different functional groups to enhance the interactions. Among the various self-assembled polymeric systems, block copolymer supramolecular assemblies have received considerable attention because of the inherent processing advantages. These supramolecular assemblies are formed by the non-covalent interactions of one of the blocks of the block copolymer with a low molar-mass additive. Selective extraction of the additive leads to porous membranes or nano-objects which could then be used as templates for nanofabrication leading to a variety of ordered organic/inorganic nanostructures. In this feature article, we present an over-view of the recent developments in this area with a special focus on some examples from our group.     

Influence of interfacial features in the supramolecular assembly of 3,5-dihydroxybenzylalcohol based dendrimers on Si/SiO2 surfaces

Self-organization of 3,5-dihydroxybenzylalcohol (DHBA) based dendrimers of generations 0-3 (G0-G3) on bare and functionalized single crystal silicon (Si/SiO2) surfaces has been examined. The underlying monolayer plays a significant role in the supramolecular assembly leading to ordered structures of DHBA (G0) and generation 1-3 (G1-G3) dendrimers at interfaces. Ordered hyperbranched structures are formed on surfaces containing self-assembled monolayers with complimentary features to the assembling molecules, whereas no such organized assemblies are observed on unfunctionalized surfaces.     

Gold nanoparticle assemblies through hydrogen-bonded supramolecular mediators

The synthesis of spherical gold nanoparticle assemblies with multicomponent double rosette molecular boxes as mediators is presented. These nine-component hydrogen-bonded supramolecular structures held together by 36 hydrogen bonds induce gold nanoparticle assembly. The morphologies of the nanoparticle assemblies can be tuned easily by changing the quantity of the building block chemisorbed on the nanoparticle surface.     

Anodic TiO2 nanotube layers: Why does self-organized growth occur—A mini review

The present review gives an overview of the highlights of more than 10 years of research on synthesis and applications of ordered oxide structures (nanotube layers, hexagonal pore arrangements) that are formed by self-organizing anodization of metals. In particular we address the questions after the critical factors that lead to the spectacular self-ordering during the growth of anodic oxides that finally yield morphologies such as highly ordered TiO₂ nanotube arrays and similar structures. Why are tubes and pores formed—what are the key parameters controlling these processes?     

Self‐Assembly of Chiral Gold Clusters into Crystalline Nanocubes of Exceptional Optical Activity

Self‐assembly of inorganic nanoparticles into ordered structures is of interest in both science and technology because it is expected to generate new properties through collective behavior; however, such nanoparticle assemblies with characteristics distinct from those of individual building blocks are rare. Herein we use atomically precise Au clusters to make ordered assemblies with emerging optical activity. Chiral Au clusters with strong circular dichroism (CD) but free of circularly polarized luminescence (CPL) are synthesized and organized into uniform body‐centered cubic (BCC) packing nanocubes. Once the ordered structure is formed, the CD intensity is significantly enhanced and a remarkable CPL response appears. Both experiment and theory calculation disclose that the CPL originates from restricted intramolecular rotation and the ordered stacking of the chiral stabilizers, which are fastened in the crystalline lattices.