Engineering Hierarchical Nanostructures by Elastocapillary Self‐Assembly

Surfaces coated with nanoscale filaments such as silicon nanowires and carbon nanotubes are potentially compelling for high‐performance battery and capacitor electrodes, photovoltaics, electrical interconnects, substrates for engineered cell growth, dry adhesives, and other smart materials. However, many of these applications require a wet environment or involve wet processing during their synthesis. The capillary forces introduced by these wet environments can lead to undesirable aggregation of nanoscale filaments, but control of capillary forces can enable manipulation of the filaments into discrete aggregates and novel hierarchical structures. Recent studies suggest that the elastocapillary self‐assembly of nanofilaments can be a versatile and scalable means to build complex and robust surface architectures. To enable a wider understanding and use of elastocapillary self‐assembly as a fabrication technology, we give an overview of the underlying fundamentals and classify typical implementations and surface designs for nanowires, nanotubes, and nanopillars made from a wide variety of materials. Finally, we discuss exemplary applications and future opportunities to realize new engineered surfaces by the elastocapillary self‐assembly of nanofilaments.     

Hierarchical Self‐Assembly of Supramolecular Hydrophobic Metallacycles into Ordered Nanostructures

We describe herein the hierarchical self‐assembly of discrete supramolecular metallacycles into ordered fibers or spherical particles through multiple noncovalent interactions. A new series of well‐defined metallacycles decorated with long alkyl chains were obtained through metal–ligand interactions, which were capable of aggregating into ordered fibroid or spherical nanostructures on the surface, mostly driven by hydrophobic interactions. In‐depth studies indicated that the morphology diversity was originated from the structural information encoded in the metallacycles, including the number of alkyl chains and their spatial orientation. Interestingly, the morphology of the metallacycle aggregates could be tuned by changing the solvent polarity. These findings are of special significance since they provide a simple yet highly controllable approach to prepare ordered and tunable nanostructures from small building blocks by means of hierarchical self‐assembly.     

Access to Different Nanostructures via Self‐Assembly of Thiourea‐Containing PEGylated Amphiphiles

Readily water‐soluble PEGylated amphiphiles containing bis‐thiourea‐based molecular recognition units at the interface of hydrophobic and hydrophilic blocks are developed. Self‐assembly of these amphiphiles is found to be dependent on the exact chemical composition of the hydrophobic component. Elongated, spherical, and disk‐like micelles are formed with the change in hydrophobic group from stearyl (2A), oleyl (2B), and dodecanol (2C), respectively. The length of the rod‐like elongated micelles formed by 2A could be tuned by thermal treatment as well. Synthesis and detailed structural characterization of these amphiphiles by TEM, DSC, synchrotron SAXS techniques are reported. Organic solvent‐free direct aqueous encapsulation of doxorubicin, an anticancer drug into these nanostructures is demonstrated.

     

Local Probe Oxidation of Self‐Assembled Monolayers: Templates for the Assembly of Functional Nanostructures

Surfaces with purposes : The electroinitiated patterning of self‐assembled monolayers enables the fabrication of a variety of complex nanostructures (see picture). The possibilities offered by the introduction of chemical selectivity through the local generation of chemically active groups and subsequent derivatization are reviewed, with a focus on progress in this area of research over the last four years.

     

Site‐Directed,On‐Surface Assembly of DNA Nanostructures

Two‐dimensional DNA lattices have been assembled from DNA double‐crossover (DX) motifs on DNA‐encoded surfaces in a site‐specific manner. The lattices contained two types of single‐stranded protruding arms pointing into opposite directions of the plane. One type of these protruding arms served to anchor the DNA lattice on the solid support through specific hybridization with surface‐bound, complementary capture oligomers. The other type of arms allowed for further attachment of DNA‐tethered probe molecules on the opposite side of the lattices exposed to the solution. Site‐specific lattice assembly and attachment of fluorophore‐labeled oligonucleotides and DNA–protein conjugates was demonstrated using DNA microarrays on flat, transparent mica substrates. Owing to their programmable orientation and addressability over a broad dynamic range from the nanometer to the millimeter length scale, such supramolecular architecture might be used for presenting biomolecules on surfaces, for instance, in biosensor applications.     

Structural Diversity in the Self‐Assembly of Pseudopeptidic Macrocycles

The self‐assembling abilities of several pseudopeptidic macrocycles have been thoroughly studied both in the solid (SEM, TEM, FTIR) and in solution (NMR, UV, CD, FTIR) states. Detailed microscopy revealed large differences in the morphology of the self‐assembling micro/nanostructures depending on the macrocyclic chemical structures. Self‐assembly was triggered by the presence of additional methylene groups or by changing from para to meta geometry of the aromatic phenylene backbone moiety. More interestingly, the nature of the side chain also plays a fundamental role in some of the obtained nanostructures, thus producing structures from long fibers to hollow spheres. These nanostructures were obtained in different solvents and on different surfaces, thus implying that the chemical information for the self‐assembly is contained in the molecular structure. Dilution NMR studies (chemical shift and self‐diffusion rates) suggest the formation of incipient aggregates in solution by a combination of hydrogen‐bonding and π–π interactions, thus implicating amide and aryl groups, respectively. Electronic spectroscopy further supports the π–π interactions because the compounds that lead to fibers show large hypochromic shifts in the UV spectra. Moreover, the fiber‐forming macrocycles also showed a more intense CD signature. The hydrogen‐bonding interactions within the nanostructures were also characterized by attenuated total‐reflectance FTIR spectroscopy, which allowed us to monitor the complete transition from the solution to the dried nanostructure. Overall, we concluded that the self‐assembly of this family of pseudopeptidic macrocycles is dictated by a synergic action of hydrogen‐bonding and π–π interactions. The feasibility and geometrical disposition of these interactions finally render a hierarchical organization, which has been rationalized with a proposal of a model. The understanding of the process at the molecular level has allowed us to prepare hybrid soft materials.     

Bioactive Seed Layer for Surface‐Confined Self‐Assembly of Peptides

The design and control of molecular systems that self‐assemble spontaneously and exclusively at or near an interface represents a real scientific challenge. We present here a new concept, an active seed layer that allows to overcome this challenge. It is based on enzyme‐assisted self‐assembly. An enzyme, alkaline phosphatase, which transforms an original peptide, Fmoc‐FFY(PO₄^2?), into an efficient gelation agent by dephosphorylation, is embedded in a polyelectrolyte multilayer and constitutes the “reaction motor”. A seed layer composed of a polyelectrolyte covalently modified by anchoring hydrogelator peptides constitutes the top of the multilayer. This layer is the nucleation site for the Fmoc‐FFY peptide self‐assembly. When such a film is brought in contact with a Fmoc‐FFY(PO₄^2?) solution, a nanofiber network starts to form almost instantaneously which extents up to several micrometers into the solution after several hours. We demonstrate that the active seed layer allows convenient control over the self‐assembly kinetics and the geometric features of the fiber network simply by changing its peptide density.     

Self‐Assembly of Tyrosine into Controlled Supramolecular Nanostructures

In the context of designing novel amino acid nanostructures, the capacity of tyrosine alone to form well‐ordered structures under different conditions was explored. It was observed that Tyr can self‐assemble into well‐defined morphologies when deposited onto surfaces for transmission electron microscopy, atomic force microscopy, and scanning electron microscopy. The influence of various parameters that can modulate the self‐assembly process, including concentration of the amino acid, aging time, and solvent, was studied. Different supramolecular architectures, including nanoribbons, branched structures, and fern‐like arrangements were also observed.     

Fabrication of Pyrene and Tetraphenylethylene Nanostructures by a Hydrolysis‐Assisted Co‐Assembly

The poly(allylamine hydrochloride)‐g‐pyrene‐tetraphenylethylene (PAH‐g‐Py‐g‐TPE) copolymers with different ratios of Py and TPE are synthesized by grafting 1‐pyrenecarboxaldehyde (Py‐CHO) and tetraphenylethylenecarboxaldehyde (TPE‐CHO) to PAH via a Schiff base reaction in methanol. The PAH‐g‐Py‐g‐TPE forms spherical micelles in water regardless of the ratios of Py and TPE, which can transform into different nanostructures after being incubated in pH 0 and pH 2 solutions, respectively. These nanomaterials including nanoparticles (NPs), nanorods (NRs), nanotubes (NTs) and nanoribbons (NBs) are composed of Py‐CHO and TPE‐CHO with different ratios, and emit fluorescence with colors different from the pure Py NRs and NTs, and TPE NPs.     

Flow‐Enabled Self‐Assembly of Large‐Scale Aligned Nanowires

One‐dimensional nanowires enable the realization of optical and electronic nanodevices that may find applications in energy conversion and storage systems. Herein, large‐scale aligned DNA nanowires were crafted by flow‐enabled self‐assembly (FESA). The highly oriented and continuous DNA nanowires were then capitalized on either as a template to form metallic nanowires by exposing DNA nanowires that had been preloaded with metal salts to an oxygen plasma or as a scaffold to direct the positioning and alignment of metal nanoparticles and nanorods. The FESA strategy is simple and easy to implement and thus a promising new method for the low‐cost synthesis of large‐scale one‐dimensional nanostructures for nanodevices.     

Colloid‐Assisted Self‐Assembly of Robust,Three‐Dimensional Networks of Carbon Nanotubes over Large Areas

Natural materials, such as bone and spider silk, possess remarkable properties as a result of sophisticated nanoscale structuring. They have inspired the design of synthetic materials whose structure at the nanoscale is carefully engineered or where nanoparticles, such as rods or wires, are self‐assembled. Although much work has been done in recent years to create ordered structures using diblock copolymers and template‐assisted assembly, no reports describe highly ordered, three‐dimensional nanotube arrays within a polymeric material. There are only reports of two‐dimensional network structures and structures on micrometer‐size scales. Here, we describe an approach that uses plasticized colloidal particles as a template for the self‐assembly of carbon nanotubes (CNTs) into ordered, three‐dimensional networks. The nanocomposites can be strained by over 200% and still retain high conductivity when relaxed. The method is potentially general and so may find applications in areas such as sensing, photonics, and functional composites.

     

Self‐Assembly Studies of a Chiral Bisurea‐Based Superhydrogelator

A chiral bisurea‐based superhydrogelator that is capable of forming supramolecular hydrogels at concentrations as low as 0.2 mM is reported. This soft material has been characterized by thermal studies, rheology, X‐ray diffraction analysis, transmission electron microscopy (TEM), and by various spectroscopic techniques (electronic and vibrational circular dichroism and by FTIR and Raman spectroscopy). The expression of chirality on the molecular and supramolecular levels has been studied and a clear amplification of its chirality into the achiral analogue has been observed. Furthermore, thermal analysis showed that the hydrogelation of compound 1 has a high response to temperature, which corresponds to an enthalpy‐driven self‐assembly process. These particular thermal characteristics make these materials easy to handle for soft‐application technologies.     

Proton‐Coupled Self‐Assembly of a Porphyrin‐Naphthalenediimide Dyad

The construction of an n–p heterojunction through the self‐assembly of a dyad based on tetraphenylporphyrin (TPP) and 1,4,5,8‐naphthalenedimide (NDI) ( 1 ) is described. Proton transfer from the lysine head group of 1 to the porphyrin ring occurs concomitantly with self‐assembly into 1D nanorods in CHCl₃. TEM and AFM studies showed that the nanorods are formed by the lateral and vertical fusion of multilameller vesicles into networks and hollow ribbons, respectively. These intermediate structures transitioned to nanorods over the course of 4–6 days. Time‐resolved spectroscopy revealed that photoinduced charge separation occurs with rate constants that depend on the nature of the aggregation.