Non‐Equilibrium Assembly of Light‐Activated Colloidal Mixtures

The collective phenomena exhibited by artificial active matter systems present novel routes to fabricating out‐of‐equilibrium microscale assemblies. Here, the crystallization of passive silica colloids into well‐controlled 2D assemblies is shown, which is directed by a small number of self‐propelled active colloids. The active colloids are titania–silica Janus particles that are propelled when illuminated by UV light. The strength of the attractive interaction and thus the extent of the assembled clusters can be regulated by the light intensity. A remarkably small number of the active colloids is sufficient to induce the assembly of the dynamic crystals. The approach produces rationally designed colloidal clusters and crystals with controllable sizes, shapes, and symmetries. This multicomponent active matter system offers the possibility of obtaining structures and assemblies that cannot be found in equilibrium systems.     

Pick‐and‐Place Assembly of Single Microtubules

Intracellular transport is affected by the filament network in the densely packed cytoplasm. Biophysical studies focusing on intracellular transport based on microtubule–kinesin system frequently use in vitro motility assays, which are performed either on individual microtubules or on random (or simple) microtubule networks. Assembling intricate networks with high flexibility requires the manipulation of 25 nm diameter microtubules individually, which can be achieved through the use of pick‐and‐place assembly. Although widely used to assemble tiny objects, pick‐and‐place is not a common practice for the manipulation of biological materials. Using the high‐level handling capabilities of microelectromechanical systems (MEMS) technology, tweezers are designed and fabricated to pick and place single microtubule filaments. Repeated picking and placing cycles provide a multilayered and multidirectional microtubule network even for different surface topographies. On‐demand assembly of microtubules forms crossings at desired angles for biophysical studies as well as complex networks that can be used as nanotransport systems.     

Active Self‐Assembly of Train‐Shaped DNA Nanostructures via Catalytic Hairpin Assembly Reactions

Biomolecular self‐assembly is a powerful approach for fabricating supramolecular architectures. Over the past decade, a myriad of biomolecular assemblies, such as self‐assembly proteins, lipids, and DNA nanostructures, have been used in a wide range of applications, from nano‐optics to nanoelectronics and drug delivery. The method of controlling when and where the self‐assembly starts is essential for assembly dynamics and functionalization. Here, train‐shaped DNA nanostructures are actively self‐assembled using DNA tiles as artificial “carriages,” hairpin structures as “couplers,” and initiators of catalytic hairpin assembly (CHA) reactions as “wrenches.” The initiator wrench can selectively open the hairpin couplers to couple the DNA tile carriages with high product yield. As such, DNA nanotrains are actively prepared with two, three, four, or more carriages. Furthermore, by flexibly modifying the carriages with “biotin seats” (biotin‐modified DNA tiles), streptavidin “passengers” are precisely arranged in corresponding seats. The applications of the CHA‐triggered self‐assembly mechanism are also extended for assembling the large DNA origami dimer. With the creation of 1D architectures established, it is thought that this CHA‐triggered self‐assembly mechanism may provide a new element of control for complex autonomous assemblies from a variety of starting materials with specific sites and times.     

Real‐Time Tracking of Superparamagnetic Nanoparticle Self‐Assembly

The spontaneous self‐assembly process of superparamagnetic nanoparticles in a fast‐drying colloidal drop is observed in real time. The grazing‐incidence small‐angle X‐ray scattering (GISAXS) technique is employed for an in situ tracking of the reciprocal space, with a 3 ms delay time between subsequent frames delivered by a new generation of X‐ray cameras. A focused synchrotron beam and sophisticated sample oscillations make it possible to relate the dynamic reciprocal to direct space features and to localize the self‐assembly. In particular, no nanoparticle ordering is found inside the evaporating drop and near‐surface region down to a drop thickness of 90 µm. Scanning through the shrinking drop‐contact line indicates the start of self‐assembly near the drop three‐phase interface, in accord with theoretical predictions. The results obtained have direct implications for establishing the self‐assembly process as a routine technological step in the preparation of new nanostructures.     

Multiscale Plasma‐Catalytic On‐Surface Assembly

Controlled modification of surfaces is one of the key pursuits of the nanoscience and nanotechnology fields, allowing for the fabrication of bespoke materials with targeted functionalities. However, many surface modifications currently require painstakingly precise and/or energy intensive processing to implement, and are thus limited in scope and scale. Here, a concept which can enhance the capacity for control of surfaces is introduced: plasma‐assisted nucleation and self‐assembly at atomic to nanoscales, scalable at atmospheric pressures.     

Interface‐Directed Self‐Assembly of Cell‐Laden Microgels

Cell‐laden hydrogels show great promise for creating engineered tissues. However, a major shortcoming with these systems has been the inability to fabricate structures with controlled micrometer‐scale features on a biologically relevant length scale. In this Full Paper, a rapid method is demonstrated for creating centimeter‐scale, cell‐laden hydrogels through the assembly of shape‐controlled microgels or a liquid–air interface. Cell‐laden microgels of specific shapes are randomly placed on the surface of a high‐density, hydrophobic solution, induced to aggregate and then crosslinked into macroscale tissue‐like structures. The resulting assemblies are cell‐laden hydrogel sheets consisting of tightly packed, ordered microgel units. In addition, a hierarchical approach creates complex multigel building blocks, which are then assembled into tissues with precise spatial control over the cell distribution. The results demonstrate that forces at an air–liquid interface can be used to self‐assemble spatially controllable, cocultured tissue‐like structures.     

Self‐Assembly of Ferromagnetic Organic–Inorganic Perovskite‐Like Films

Perovskite‐based organic–inorganic hybrids hold great potential as active layers in electronics or optoelectronics or as components of biosensors. However, many of these applications require thin films grown with good control over structure and thickness—a major challenge that needs to be addressed. The work presented here is an effort towards this goal and concerns the layer‐by‐layer deposition at ambient conditions of ferromagnetic organic–inorganic hybrids consisting of alternating CuCl₄‐octahedra and organic layers. The Langmuir‐Blodgett technique used to assemble these structures provides intrinsic control over the molecular organization and film thickness down to the molecular level. Magnetic characterization reveals that the coercive field for these thin films is larger than that for solution‐grown layered bulk crystals. The strategy presented here suggests a promising cost effective route to facilitate the excellently controlled growth of sophisticated materials on a wide variety of substrates that have properties relevant for the high density storage media and spintronic devices.     

Self‐Assembly: I‐Motif Nanospheres: Unusual Self‐Assembly of Long Cytosine Strands (Small 8/2011)

The synthesis and characterization of novel DNA structures based on tetraplex cytosine (C) arrangements, known as i‐motifs or i‐tetraplexes, is reported. Atomic force microscopy (AFM) investigation shows that long C‐strands in mild acidic conditions form compact spherically shaped nanostructures. The DNA nanospheres are characterized by a typical uniform shape and narrow height distribution. Electrostatic force microscopy (EFM) measurements performed on the i‐motif spheres clearly show their electrical polarizability. Further investigations by scanning tunneling microscopy (STM) at ultrahigh vacuum reveals that the structures exhibit an average voltage gap of 1.9 eV, which is narrower than the voltage gap previously measured for poly(dG)–poly(dC) molecules in similar conditions.     

Translocation Pathway‐Dependent Assembly of Streptavidin‐ and Antibody‐Binding Filamentous Virus‐Like Particles

Compared to well‐tolerated p3 fusion, the display of fast‐folding proteins fused to the minor capsid p7 and the major capsid p8, as well as in vivo biotinylation of biotin acceptor peptide (AP) fused to p7, are found to be markedly inefficient using the filamentous phage. Here, to overcome such limitations, the effect of translocation pathways, amber mutation, and phage and phagemid display systems on p7 and p8 display of antibody‐binding domains are examined, while comparing the level of in vivo biotinylation of AP fused to p7 or p3. Interestingly, the in vivo biotinylation of AP occurs only in p3 fusion and the fast‐folding antibody‐binding scaffolds fused to p7 and p8 are best displayed via a twin‐arginine translocation pathway in TG1 cells. The lower the expression level of the wild‐type p8 and the smaller the size of the guest protein, the better the display of Z‐domain fused to the recombinant p8. The in vivo biotinylated multifunctional filamentous virus‐like particles can be vertically immobilized on streptavidin (SAV)‐coated microspheres to resemble cellular microvilli‐like structures, which reportedly enhance protein–protein interactions due to dramatically expanded flexible surface area.