Layer‐by‐Layer Assembly: Recent Progress from Layered Assemblies to Layered Nanoarchitectonics

As an emerging concept for the development of new materials with nanoscale features, nanoarchitectonics has received significant recent attention. Among the various approaches that have been developed in this area, the fixed‐direction construction of functional materials, such as layered fabrication, offers a helpful starting point to demonstrate the huge potential of nanoarchitectonics. In particular, the combination of nanoarchitectonics with layer‐by‐layer (LbL) assembly and a large degree of freedom in component availability and technical applicability would offer significant benefits to the fabrication of functional materials. In this Minireview, recent progress in LbL assembly is briefly summarized. After introducing the basics of LbL assembly, recent advances in LbL research are discussed, categorized according to physical, chemical, and biological innovations, along with the fabrication of hierarchical structures. Examples of LbL assemblies with graphene oxide are also described to demonstrate the broad applicability of LbL assembly, even with a fixed material.     

Nanoarchitectonics from Atom to Life

Functional materials with rational organization cannot be directly created only by nanotechnology‐related top‐down approaches. For this purpose, a novel research paradigm next to nanotechnology has to be established to create functional materials on the basis of deep nanotechnology knowledge. This task can be assigned to an emerging concept, nanoarchitectonics. In the nanoarchitectonics approaches, functional materials are architected through combination of atom/molecular manipulation, organic chemical synthesis, self‐assembly and related spontaneous processes, field‐applied assembly, micro/nano fabrications, and bio‐related processes. In this short review article, nanoarchitectonics‐related approaches on materials fabrications and functions are exemplified from atom‐scale to living creature level. Based on their features, unsolved problems for future developments of the nanoarchitectonics concept are finally discussed.     

Nanoarchitectonic‐Based Material Platforms for Environmental and Bioprocessing Applications

The challenges of pollution, environmental science, and energy consumption have become global issues of broad societal importance. In order to address these challenges, novel functional systems and advanced materials are needed to achieve high efficiency, low emission, and environmentally friendly performance. A promising approach involves nanostructure‐level controls of functional material design through a novel concept, nanoarchitectonics. In this account article, we summarize nanoarchitectonic approaches to create nanoscale platform structures that are potentially useful for environmentally green and bioprocessing applications. The introduced platforms are roughly classified into (i) membrane platforms and (ii) nanostructured platforms. The examples are discussed together with the relevant chemical processes, environmental sensing, bio‐related interaction analyses, materials for environmental remediation, non‐precious metal catalysts, and facile separation for biomedical uses.     

Oriented Gold Nanorod Arrays: Self‐Assembly and Optoelectronic Applications

Self‐assembly of anisotropic plasmonic nanomaterials into ordered superstructures has become popular in nanoscience because of their unique anisotropic optical and electronic properties. Gold nanorods (GNRs) are a well‐defined functional building block for fabrication of these superstructures. They possess important anisotropic plasmonic characteristics that result from strong local electric field and are responsive to visible and near‐IR light. There are recent examples of assembling the GNRs into ordered arrays or superstructures through processes such as solvent evaporation and interfacial assembly. In this Minireview, recent progress in the development of the self‐assembled GNR arrays is described, with focus on the formation of oriented GNR arrays on substrates. Key driving forces are discussed, and different strategies and self‐assembly processes of forming oriented GNR arrays are presented. The applications of the oriented GNR arrays in optoelectronic devices are also overviewed, especially surface enhanced Raman scattering (SERS).     

Access to Metastable Gel States Using Seeded Self‐Assembly of Low‐Molecular‐Weight Gelators

Here we report on how metastable supramolecular gels can be formed through seeded self‐assembly of multicomponent gelators. Hydrazone‐based gelators decorated with non‐ionic and anionic groups are formed in situ from hydrazide and aldehyde building blocks, and lead through multiple self‐sorting processes to the formation of heterogeneous gels approaching thermodynamic equilibrium. Interestingly, the addition of seeds composing of oligomers of gelators bypasses the self‐sorting processes and accelerates the self‐assembly along a kinetically favored pathway, resulting in homogeneous gels of which the network morphologies and gel stiffness are markedly different from the thermodynamically more stable gel products. Importantly, over time, these metastable homogeneous gel networks are capable of converting into the thermodynamically more stable state. This seeding‐driven formation of out‐of‐equilibrium supramolecular structures is expected to serve as a simple approach towards functional materials with pathway‐dependent properties.     

Programmable Supra‐Assembly of a DNA Surface Adapter for Tunable Chiral Directional Self‐Assembly of Gold Nanorods

An important challenge in molecular assembly and hierarchical molecular engineering is to control and program the directional self‐assembly into chiral structures. Here, we present a versatile DNA surface adapter that can programmably self‐assemble into various chiral supramolecular architectures, thereby regulating the chiral directional “bonding” of gold nanorods decorated by the surface adapter. Distinct optical chirality relevant to the ensemble conformation is demonstrated from the assembled novel stair‐like and coil‐like gold nanorod chiral metastructures, which is strongly affected by the spatial arrangement of neighboring nanorod pair. Our strategy provides new avenues for fabrication of tunable optical metamaterials by manipulating the directional self‐assembly of nanoparticles using programmable surface adapters.     

Emulsion‐Assisted Polymerization‐Induced Hierarchical Self‐Assembly of Giant Sea Urchin‐like Aggregates on a Large Scale

Hierarchical solution self‐assembly has become an important biomimetic method to prepare highly complex and multifunctional supramolecular structures. However, despite great progress, it is still highly challenging to prepare hierarchical self‐assemblies on a large scale because the self‐assembly processes are generally performed at high dilution. Now, an emulsion‐assisted polymerization‐induced self‐assembly (EAPISA) method with the advantages of in situ self‐assembly, scalable preparation, and facile functionalization was used to prepare hierarchical multiscale sea urchin‐like aggregates (SUAs). The obtained SUAs from amphiphilic alternating copolymers have a micrometer‐sized rattan ball‐like capsule (RBC) acting as the hollow core body and radiating nanotubes tens of micrometers in length as the hollow spines. They can capture model proteins effectively at an ultra‐low concentration (ca. 10 nm ) after functionalization with amino groups through click copolymerization.     

Regulating Higher‐Order Organization through the Synergy of Two Self‐Sorted Assemblies

The extracellular matrix (ECM) is the natural fibrous scaffold that regulates cell behavior in a hierarchical manner. By mimicking the dynamic and reciprocal interactions between ECM and cells, higher‐order molecular self‐assembly (SA), mediated through the dynamic growth of scaffold‐like nanostructures assembled by different molecular components, was developed. Designed and synthesized were two self‐sorted coumarin‐based gelators, a peptide molecule and a benzoate molecule, which self‐assemble into nanofibers and nanobelts, respectively, with different dynamic profiles. Upon the dynamic growth of the fibrous scaffold assembled from peptide gelators, nanobelts assembled from benzoate gelators transform into a layer‐by‐layer nanosheet, reaching ninefold increase in height. By using light and an enzyme, the spatial–temporal growth of the scaffold can be modified, leading to in situ height regulation of the higher‐order architecture.     

Self‐Assembly of Bolaamphiphilic Molecules

The current buzzword in science and technology is self‐assembly and molecular self‐assembly is one of the most prominent fields as far as research in chemical and biological sciences is concerned. Generally, self‐assembly of molecules occurs through weak non‐covalent interactions like hydrogen bonding, π–π stacking, hydrophobic effects, etc. Inspired by many natural systems consisting of self‐assembled structures, scientists have been trying to understand their formation and mimic such processes in the laboratory to create functional “smart” materials, which respond to temperature, light, pH, electromagnetic field, mechanical stress, and/or chemical stimuli. These responses are usually manifested as remarkable changes from the molecular (e. g., conformational state, hierarchical order) to the macroscopic level (e. g., shape, surface properties). Many molecules such as peptides, viruses, and surfactants are known to self‐assemble into different structures. Among them, glycolipids are the new entries in the area of molecules that are being investigated for their self‐assembly characteristics. Among the different classes of glycolipids like rhamnolipids and trehalose lipids, owing to their biological preparations and their structural novelty, sophorolipids (SLs) are evoking greater interest among researchers. Sophorolipids are a class of asymmetric bolas bearing COOH groups at one end and sophorose (dimeric glucose linked by an unusual β(1→2) linkage). The extreme membrane stability of Archaea, attributed to the membrane‐spanning bolas (tetraether glycolipids), has inspired chemists to unravel the molecular designs that underpin the self‐assembly of bolaamphiphilic molecules. Apart from these self‐assembled structures, bolaamphiphiles find applications in many fields such as drug delivery, membrane mimicking, siRNA therapies, etc. The first part of this Personal Account presents some possible self‐assembled structures of bolaamphiphiles and their mechanism of formation. The later part covers our work on one of the typical bolaamphiphiles known as sophorolipids.     

Molecular Pom Poms from Self‐Assembling α,γ‐Cyclic Peptides

The hierarchical self‐assembly properties of a dimer‐forming cyclic peptide that bears a nicotinic acid moiety to form molecular pom‐pom‐like structures are described. This dimeric assembly self organizes into spherical structures that can encapsulate small organic molecules owing to its porosity and it can also facilitate metal deposition on its surface directed by the pyridine moiety.     

Multivalency by Self‐Assembly: Binding of Concanavalin A to Metallosupramolecular Architectures Decorated with Multiple Carbohydrate Groups

Multiplication of functional units through self‐assembly is a powerful way to new properties and functions. In particular, self‐organization of components decorated with recognition groups leads to multivalent entities, amenable to strong and selective binding with multivalent targets, such as protein receptors. Here we describe an efficient, supramolecular, one‐pot valency multiplication process proceeding through self‐organization of monovalent components into well‐defined, grid‐shaped [2×2] tetranuclear complexes bearing eight sugar residues for multivalent interaction with the tetrameric lectin, concanavalin A (Con A). The grids are stable in water under physiological pH at a relatively high concentration, but dissociate readily at slightly more acidic pH or upon dilution below a certain threshold, in a type of on–off behavior. The carbohydrate‐decorated grids interact strongly and selectively with Con A forming triply supramolecular bio‐hybrid polymeric networks, which lead to a highly specific phase‐separation and quasi‐quantitative precipitation of Con A out of solution. Dramatic effects of valency number on agglutination properties were demonstrated by comparison of grids with divalent carbohydrates of covalent and non‐covalent (L ‐shaped, mononuclear zinc complex) scaffolds. The results presented here provide prototypical illustration of the power of multivalency generation by self‐assembly leading to defined arrays of functional groups and binding patterns.     

Hierarchical Self‐Assembly of Supramolecular Spintronic Modules into 1D‐ and 2D‐Architectures with Emergence of Magnetic Properties

Hierarchical self‐assembly of complex supramolecular architectures allows for the emergence of novel properties at each level of complexity. The reaction of the ligand components A and B with Fe^II cations generates the [2×2] grid‐type functional building modules 1 and 2 , presenting spin‐transition properties and preorganizing an array of coordination sites that sets the stage for a second assembly step. Indeed, binding of La^III ions to 1 and of Ag^I ions to 2 leads to a 1D columnar superstructure 3 and to a wall‐like 2D layer 4 , respectively, with concomitant modulation of the magnetic properties of 1 and 2 . Thus, to each of the two levels of structural complexity generated by the two sequential self‐assembly steps corresponds the emergence of novel functional features.     

Controlling the Structure and Length of Self‐Synthesizing Supramolecular Polymers through Nucleated Growth and Disassembly

Directing self‐assembly processes out‐of‐equilibrium to yield kinetically trapped materials with well‐defined dimensions remains a considerable challenge. Kinetically controlled assembly of self‐synthesizing peptide‐functionalized macrocycles through a nucleation–growth mechanism is reported. Spontaneous fiber formation in this system is effectively shut down as most of the material is diverted into metastable non‐assembling trimeric and tetrameric macrocycles. However, upon adding seeds to this mixture, well‐defined fibers with controllable lengths and narrow polydispersities are obtained. This seeded growth strategy also allows access to supramolecular triblock copolymers. The resulting noncovalent assemblies can be further stabilized through covalent capture. Taken together, these results show that self‐synthesizing materials, through their interplay between dynamic covalent bonds and noncovalent interactions, are uniquely suited for out‐of‐equilibrium self‐assembly.     

Oligopeptide‐Assisted Self‐Assembly of Oligothiophenes: Co‐Assembly and Chirality Transfer

The biomolecule‐assisted self‐assembly of semiconductive molecules has been developed recently for the formation of potential bio‐based functional materials. Oligopeptide‐assisted self‐assembly of oligothiophene through weak intermolecular interactions was investigated; specifically the self‐assembly and chirality‐transfer behavior of achiral oligothiophenes in the presence of an oligopeptide with a strong tendency to form β‐sheets. Two kinds of oligothiophenes without (QT) or with (QTDA) carboxylic groups were selected to explore the effect of the end functional group on self‐assembly and chirality transfer. In both cases, organogels were formed. However, the assembly behavior of QT was quite different from that of QTDA. It was found that QT formed an organogel with the oligopeptide and co‐assembled into chiral nanostructures. Conversely, although QTDA also formed a gel with the oligopeptide, it has a strong tendency to self‐assemble independently. However, during the formation of the xerogel, the chirality of the oligopeptide can also be transferred to the QTDA assemblies. Different assembly models were proposed to explain the assembly behavior.     

Dynamic Formation of Hybrid Peptidic Capsules by Chiral Self‐Sorting and Self‐Assembly

Owing to their versatility and biocompatibility, peptide‐based self‐assembled structures constitute valuable targets for complex functional designs. It is now shown that artificial capsules based on β‐barrel binding motifs can be obtained by means of dynamic covalent chemistry (DCC) and self‐assembly. Short peptides (up to tetrapeptides) are reversibly attached to resorcinarene scaffolds. Peptidic capsules are thus selectively formed in either a heterochiral or a homochiral way by simultaneous and spontaneous processes, involving chiral sorting, tautomerization, diastereoselective induction of inherent chirality, and chiral self‐assembly. Self‐assembly is shown to direct the regioselectivity of reversible chemical reactions. It is also responsible for shifting the tautomeric equilibrium for one of the homochiral capsules. Two different tautomers (keto‐enamine hemisphere and enol‐imine hemisphere) are observed in this capsule, allowing the structure to adapt for self‐assembly.     

Self‐Assembly of Aniline Oligomers

A great number of nano/microscaled morphologies have recently been prepared during the oxidation of aniline. At the early stage of oxidation, aniline oligomers are obtained, often in spectacular morphologies depending on reaction conditions. Herein, the flower‐like hierarchical architectures assembled from aniline oligomers by a template‐free method are reported. Their formation process is ascribed to the self‐assembly of oligoanilines through non‐covalent interactions, such as hydrogen bonding, hydrophobic forces, and π–π stacking. The model of directional growth is offered to explain the formation of petal‐like objects and, subsequently, flowers. In order to investigate the chemical structure of the oligomers, a series of characterizations have been carried out, such as matrix‐assisted laser desorption ionization, time‐of‐flight mass spectrometry, gas chromatography coupled with mass spectrometry analysis, X‐ray diffraction, and UV/Vis, Fourier‐transform infrared, and Raman spectroscopies. Based on the results of characterization methods, a formation mechanism for aniline oligomers and their self‐assembly is proposed.     

G‐Quartet‐Based Nanostructure for Mimicking Light‐Harvesting Antenna

Artificial light‐harvesting systems have received great attention for use in photosynthetic and optoelectronic devices. Herein, a system involving G‐quartet‐based hierarchical nanofibers generated from the self‐assembly of guanosine 5′‐monophosphate (GMP) and a two‐step Förster resonance energy transfer (FRET) is presented that mimics natural light‐harvesting antenna. This solid‐state property offers advantages for future device fabrication. The generation of photocurrent under visible light shows it has potential for use as a nanoscale photoelectric device. The work will be beneficial for the development of light‐harvesting systems by the self‐assembly of supramolecular nanostructures.     

Bio‐inspired Design and Additive Manufacturing of Soft Materials,Machines, Robots,and Haptic Interfaces

Soft materials possess several distinctive characteristics, such as controllable deformation, infinite degrees of freedom, and self‐assembly, which make them promising candidates for building soft machines, robots, and haptic interfaces. In this Review, we give an overview of recent advances in these areas, with an emphasis on two specific topics: bio‐inspired design and additive manufacturing. Biology is an abundant source of inspiration for functional materials and systems that mimic the function or mechanism of biological tissues, agents, and behaviors. Additive manufacturing has enabled the fabrication of materials and structures prevalent in biology, thereby leading to more‐capable soft robots and machines. We believe that bio‐inspired design and additive manufacturing have been, and will continue to be, important tools for the design of soft robots.     

Self‐Assembly of Hierarchical Nanostructures from Dopamine and Polyoxometalate for Oral Drug Delivery

The preparation of 3D hierarchical nanostructures by a simple and versatile strategy of self‐assembly of dopamine (DA) and phosphotungstic acid (PTA) is described. The size and morphology of the hierarchical nanostructures could be simply controlled by varying the ratio of the two components, their concentrations, and the pH of the initial Tris‐HCl solution. The self‐assembly of the flowerlike microspheres has been found to involve a two‐stage growth process. Moreover, use of the hierarchical nanostructures as a possible carrier for an anticancer drug in chemotherapy has been explored. The nanostructures showed an intriguing pH‐dependent release behavior, making them promising for applications in biomedical science.     

Local‐Curvature‐Controlled Non‐Epitaxial Growth of Hierarchical Nanostructures

Site‐selective growth on non‐spherical seeds provides an indispensable route to hierarchical complex nanostructures that are interesting for diverse applications. However, this has only been achieved through epitaxial growth, which is restricted to crystalline materials with similar crystal structures and physicochemical properties. A non‐epitaxial growth strategy is reported for hierarchical nanostructures, where site‐selective growth is controlled by the curvature of non‐spherical seeds. This strategy is effective for site‐selective growth of silica nanorods from non‐spherical seeds of different shapes and materials, such as α‐Fe₂O₃, NaYF₄, and ZnO. This growth strategy is not limited by the stringent requirements of epitaxy and is thus a versatile general method suitable for the preparation of hierarchical nanostructures with controlled morphologies and compositions to open up a verity of applications in self‐assembly, nanorobotics, catalysis, electronics, and biotechnology.