Shape‐Tunable Colloids from Structured Liquid Droplet Templates

Shaping colloidal building units is of paramount importance for their self‐assembly into complex objects. Continuous tuning of colloidal shapes is highly desired for understanding self‐assembly, but it still remains a challenge. Herein, we report a new template strategy for the shape‐tunable synthesis of anisotropic colloids with shapes that can be continuously tuned from discs (oblate spheroids) to spheres to theta shapes to dumbbells. This was realized by creating structured shape‐tunable droplets from patchy colloidal discs and using these droplets as templates. In particular, we found that a controlled dumbbell‐to‐eyeball droplet transformation can be used for the synthesis of eyeball‐shaped colloids. We also demonstrated the droplet transformation pathways and applied the method to the synthesis of colloidal molecules. These colloids provide possibilities for exploring their ordered packing structures, and the method based on the use of structured droplets can be adapted for the synthesis of other functional colloidal particles.     

Self‐Assembly Fractal Microstructures of HPAM/Chromium Acetate and HPAM/Phenolic Aldehyde Colloidal Dispersion Gel

Abstract

HPAM (partially hydrolyzed polyacrylamide)/chromium acetate and HPAM/phenolic aldehyde colloidal dispersion gels (CDGs) were investigated microscopically using atomic force microscope. The results show that the colloidal dispersion gels eventually form self‐assembly branch‐like fractal structures over a scanning range of micrometers. The fractal aggregates of single twigs formed by compact assembly of nanometer particles were observed over a smaller scanning range regardless of the concentration of HPAM and the crosslinking reagent. This indicated an HPAM‐dependence for the formation of the fractal structure and the crosslinking reagent independence of the geometrical morphology of the gel. Also, the results demonstrated that the elastic modulus (G′) of the fractal structure formed by the smaller (nanometer‐sized) colloidal particles was one order of magnitude higher than obtained for the micrometer‐sized particles. The elastic modulus (G′) and the dynamic stability of the gels increased with decreasing particle diameter.     

Colloidal Rings by Site‐Selective Growth on Patchy Colloidal Disc Templates

Anisotropic colloidal building blocks are quite attractive as they enable the self‐assembly towards new materials with designated hierarchical structures. Although many advances have been achieved in colloidal synthetic methodology, synthesis of colloidal rings with low polydispersity and on a large scale remains a challenge. To address this issue we introduce a new site‐selective growth strategy, which relies on using patchy particles. For example, by using patchy discs as templates, silica can selectively be grown on only side surfaces, resulting in formation of silica rings. We demonstrate that shape parameters are tunable and find that these silica rings can be used as secondary template to synthesize other types of rings. This method for synthesizing ring‐like colloids provides possibilities for studying their self‐assembly and associated phase transitions, and this patchy particles template strategy paves a new route for fabricating other new colloidal particles.     

Hierarchical Supramolecular Spinning of Nanofibers in a Microfluidic Channel: Tuning Nanostructures at a Dynamic Interface

One of the fundamental problems in supramolecular chemistry, as well as in material sciences, is how to control the self‐assembly of polymers on the nanometer scale and how to spontaneously organize them towards the macroscopic scale. To overcome this problem, inspired by the self‐assembly systems in nature, which feature the dynamically controlled self‐assembly of biopolymers, we have previously proposed a self‐assembly system that uses a dynamic liquid/liquid interface with dimensions in the micrometer regime, thereby allowing polymers to self‐assemble under precisely controlled nonequilibrium conditions. Herein, we further extend this system to the creation of hierarchical self‐assembled architectures of polysaccharides. A natural polysaccharide, β‐1,3‐glucan (SPG), and water were injected into opposite “legs” of microfluidic devices that had a Y‐shape junction, so that two solvents would gradually mix in the down stem, thereby causing SPG to spontaneously self‐assemble along the flow in a head‐to‐tail fashion, mainly through hydrophobic interactions. In the initial stage, several SPG nanofibers would self‐assemble at the Y‐junction owing to the shearing force, thereby creating oligomers with a three‐way junction point. This unique structure, which could not be created through conventional mixing procedures, has a divergent self‐assembly capability. The dynamic flow allows the oligomers to interact continuously with SPG nanofibers that are fed into the Y‐junction, thus amplifying the nanostructure along the flow to form SPG networks. Consequently, we were able to create stable, centimeter‐length macroscopic polysaccharide strands under the selected flow conditions, which implies that SPG nanofibers were assembled hierarchically in a supramolecular fashion in the dynamic flow. Microscopic observations, including SEM and AFM analysis, revealed the existence of clear hierarchical structures inside the obtained strand. The network structures self‐assembled to form sub‐micrometer‐sized fibers. The long fibers further entangled with each other to give stable micrometer‐sized fibers, which finally assembled to form the macroscopic strands, in which the final stabilities in the macroscopic regime were governed by that of the network structures in the nanometer regime. Thus, we have exploited this new supramolecular system to create hierarchical polymeric architectures under precisely controlled flow conditions, by combining the conventional supramolecular strategy with microfluidic science.     

Reconfigurable Swarms of Nematic Colloids Controlled by Photoactivated Surface Patterns

Different phoretic driving mechanisms have been proposed for the transport of solid or liquid microscopic inclusions in integrated chemical processes. It is now shown that a substrate that was chemically modified with photosensitive self‐assembled monolayers enables the direct control of the assembly and transport of large ensembles of micrometer‐sized particles and drops that were dispersed in a thin layer of anisotropic fluid. This strategy separates particle driving, which was realized by AC electrophoresis, and steering, which was achieved by elastic modulation of the nematic host fluid. Inclusions respond individually or in collective modes following arbitrary reconfigurable paths that were imprinted by irradiation with UV or blue light. Relying solely on generic material properties, the proposed procedure is versatile enough for the development of applications that involve either inanimate or living materials.     

Hierarchical Self‐Assembly of Poly‐Pseudorotaxanes into Artificial Microtubules

Hierarchical self‐assembly of building blocks over multiple length scales is ubiquitous in living organisms. Microtubules are one of the principal cellular components formed by hierarchical self‐assembly of nanometer‐sized tubulin heterodimers into protofilaments, which then associate to form micron‐length‐scale, multi‐stranded tubes. This peculiar biological process is now mimicked with a fully synthetic molecule, which forms a 1:1 host‐guest complex with cucurbit[7]uril as a globular building block, and then polymerizes into linear poly‐pseudorotaxanes that associate laterally with each other in a self‐shape‐complementary manner to form a tubular structure with a length over tens of micrometers. Molecular dynamic simulations suggest that the tubular assembly consists of eight poly‐pseudorotaxanes that wind together to form a 4.5 nm wide multi‐stranded tubule.     

Tuning the Colloidal Crystal Structure of Magnetic Particles by External Field

Manipulation of the self‐assembly of magnetic colloidal particles by an externally applied magnetic field paves a way toward developing novel stimuli responsive photonic structures. Using microradian X‐ray scattering technique we have investigated the different crystal structures exhibited by self‐assembly of core–shell magnetite/silica nanoparticles. An external magnetic field was employed to tune the colloidal crystallization. We find that the equilibrium structure in absence of the field is random hexagonal close‐packed (RHCP) one. External field drives the self‐assembly toward a body‐centered tetragonal (BCT) structure. Our findings are in good agreement with simulation results on the assembly of these particles.     

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.     

Colloidal assembly: the road from particles to colloidal molecules and crystals

Colloidal particles may be considered as building blocks for materials, just like atoms are the bricks of molecules, macromolecules, and crystals. Periodic arrays of colloids (colloidal crystals) have attracted much interest over the last two decades, largely because of their unique photonic properties. The archetype opal structures are based on close-packed arrays of spheres of submicrometer diameter. Interest in structuring materials at this length scale, but with more complex features and ideally by self-assembly processes, has led to much progress in controlling features of both building blocks and assemblies. The necessary ingredients include colloids, colloidal clusters, and colloidal "molecules" which have special shapes and the ability to bind directionally, the control over short-range and long-range interactions, and the capability to place and orientate these bricks. This Review highlights recent experimental and theoretical progress in the assembly of colloids larger than 50 nm.     

Amphiphilic Janus Gold Nanoparticles Prepared by Interface‐Directed Self‐Assembly: Synthesis and Self‐Assembly

Materials with Janus structures are attractive for wide applications in materials science. Although extensive efforts in the synthesis of Janus particles have been reported, the synthesis of sub‐10 nm Janus nanoparticles is still challenging. Herein, the synthesis of Janus gold nanoparticles (AuNPs) based on interface‐directed self‐assembly is reported. Polystyrene (PS) colloidal particles with AuNPs on the surface were prepared by interface‐directed self‐assembly, and the colloidal particles were used as templates for the synthesis of Janus AuNPs. To prepare colloidal particles, thiol‐terminated polystyrene (PS‐SH) was dissolved in toluene and citrate‐stabilized AuNPs were dispersed in aqueous solution. Upon mixing the two solutions, PS‐SH chains were grafted to the surface of AuNPs and amphiphilic AuNPs were formed at the liquid–liquid interface. PS colloidal particles decorated with AuNPs on the surfaces were prepared by adding the emulsion to excess methanol. On the surface, AuNPs were partially embedded in the colloidal particles. The outer regions of the AuNPs were exposed to the solution and were functionalized through the grafting of atom‐transfer radical polymerization (ATRP) initiator. Poly[2‐(dimethamino)ethyl methacrylate] (PDMAEMA) on AuNPs were prepared by surface‐initiated ATRP. After centrifugation and dissolving the colloidal particles in tetrahydrofuran (THF), Janus AuNPs with PS and PDMAEMA on two hemispheres were obtained. In acidic pH, Janus AuNPs are amphiphilic and are able to emulsify oil droplets in water; in basic pH, the Janus AuNPs are hydrophobic. In mixtures of THF/methanol at a volume ratio of 1:5, the Janus AuNPs self‐assemble into bilayer structures with collapsed PS in the interiors and solvated PDMAEMA at the exteriors of the structures.     

Interfacial Activity of Amine‐Functionalized Polyhedral Oligomeric Silsesquioxanes (POSS): A Simple Strategy To Structure Liquids

Amine‐functionalized polyhedral oligomeric silsesquioxane (POSS), the smallest, monodisperse cage‐shaped silica cubic nanoparticle, is exceptionally interfacially active and can form assemblies that jam the toluene/water interface, locking in non‐equilibrium shapes of one liquid phase in another. The packing density of the amine‐functionalized POSS assembly at the water/toluene interface can be tuned by varying the concentration, the pH value, and the degree of POSS functionalization. Functionalized POSS gives a higher interface coverage, and hence a lower interfacial tension, than nanoparticle surfactants formed by interactions between functionalized nanoparticles and polymeric ligands. Hydrogen‐bonded POSS surfactants are more stable at the interface, offering some unique advantages for generating Pickering emulsions over typical micron‐sized colloidal particles and ligand‐stabilized nanoparticle surfactants.     

Evaporation‐Driven Self‐Assembly of Colloidal Silica Dispersion: New Insights on Janus Particles

The evaporation driven self‐assembly of novel colloidal silica Janus particles was evaluated by scanning electron microscopy in comparison to unfunctionalized silica particles. The cyclodextrin‐ and azobenzene‐modified compound was obtained utilizing Pickering emulsion approach, in which the particles were immobilized on solidified wax droplets and subsequently functionalized. Silica particles were modified with 3‐aminopropyl trimethoxysilane and afterward reacted with tosyl‐β‐CD or phenylazo(benzoic acid), respectively. Mesoscopic structures of the colloidal dispersions, as dried films from aqueous solution, have been investigated by scanning electron microscopy and dynamic light scattering. Interestingly, it has been observed that the Janus particles show a significantly different evaporation‐induced assembly than the unmodified particles.     

Template-assisted self-assembly: a practical route to complex aggregates of monodispersed colloids with well-defined sizes, shapes, and structures

This paper describes a strategy that combines physical templating and capillary forces to assemble monodispersed spherical colloids into uniform aggregates with well-controlled sizes, shapes, and structures. When an aqueous dispersion of colloidal particles was allowed to dewet from a solid surface that had been patterned with appropriate relief structures, the particles were trapped by the recessed regions and assembled into aggregates whose structures were determined by the geometric confinement provided by the templates. We have demonstrated the capability and feasibility of this approach by assembling polystyrene beads and silica colloids (> or =150 nm in diameter) into complex aggregates that include polygonal or polyhedral clusters, linear or zigzag chains, and circular rings. We have also been able to generate hybrid aggregates in the shape of HF or H2O molecules that are composed of polymer beads having different diameters, polymer beads labeled with different organic dyes, and a combination of polymeric and inorganic beads. These colloidal aggregates can serve as a useful model system to investigate the hydrodynamic and optical scattering properties of colloidal particles having nonspherical morphologies. They should also find use as the building blocks to generate hierarchically self-assembled systems that may exhibit interesting properties highly valuable to areas ranging from photonics to condensed matter physics.     

Versatile Aerogel Fabrication by Freezing and Subsequent Freeze‐Drying of Colloidal Nanoparticle Solutions

A versatile method to fabricate self‐supported aerogels of nanoparticle (NP) building blocks is presented. This approach is based on freezing colloidal NPs and subsequent freeze drying. This means that the colloidal NPs are directly transferred into dry aerogel‐like monolithic superstructures without previous lyogelation as would be the case for conventional aerogel and cryogel fabrication methods. The assembly process, based on a physical concept, is highly versatile: cryogelation is applicable for noble metal, metal oxide, and semiconductor NPs, and no impact of the surface chemistry or NP shape on the resulting morphology is observed. Under optimized conditions the shape and volume of the liquid equal those of the resulting aerogels. Also, we show that thin and homogeneous films of the material can be obtained. Furthermore, the physical properties of the aerogels are discussed.     

Light‐Driven Self‐Healing of Nanoparticle‐Based Metamolecules

Metamolecules and crystals consisting of nanoscale building blocks offer rich models to study colloidal chemistry, materials science, and photonics. Herein we demonstrate the self‐assembly of colloidal Ag nanoparticles into quasi‐one‐dimensional metamolecules with an intriguing self‐healing ability in a linearly polarized optical field. By investigating the spatial stability of the metamolecules, we found that the origin of self‐healing is the inhomogeneous interparticle electrodynamic interactions enhanced by the formation of unusual nanoparticle dimers, which minimize the free energy of the whole structure. The equilibrium configuration and self‐healing behavior can be further tuned by modifying the electrical double layers surrounding the nanoparticles. Our results reveal a unique route to build self‐healing colloidal structures assembled from simple metal nanoparticles. This approach could potentially lead to reconfigurable plasmonic devices for photonic and sensing applications.     

Playing with Structures at the Nanoscale: Designing Catalysts by Manipulation of Clusters and Nanocrystals as Building Blocks

The purpose of this Concept is to highlight some of the most recent and promising methods for the preparation of tailored catalysts by designing and preparing the component building blocks and by assembling them in a controlled fashion. We want to emphasize how rational design and synthesis of catalysts must be coupled to precise catalytic and structural characterization of the systems in an ideal feedback loop. New catalyst design and preparation techniques, dictated by information about the active sites that the specific application requires, are frequently available. The building blocks for developing these novel catalysts include colloidal methods for the preparation of uniform nanostructures, physical methods for rational assembly of the building blocks (Langmuir–Blodgett, liquid–air self‐assembly), and development of rational interactions between the building blocks for enhanced activity of the assemblies. These methods, which apply techniques normally used in other fields of nanotechnology to catalysis, offer exciting opportunities to help improve currently available catalytic systems in terms of activity, stability and selectivity.     

Small but Smart: Sensitive Microgel Capsules

Microgel capsules are micrometer‐sized particles that consist of a cross‐linked and swollen polymer network complexed with additives. These capsules can be actuated by external stimulation if they are formed from sensitive or supramolecular polymer networks. To make this truly useful, it is crucial to control the microgel size, shape, and loading; this can be achieved by droplet‐based microfluidic templating.     

Preparation of enzyme multilayers on colloids for biocatalysis

The construction of enzyme multilayer films on colloidal particles for biocatalysis is described. The enzyme multilayers were assembled on submicrometer‐sized polystyrene spheres via the alternate adsorption of poly(ethyleneimine) and glucose oxidase using a layer‐by‐layer approach. Microelectrophoresis and single particle light scattering measurements revealed regular and step‐wise assembly of the multilayers on the colloids. The high surface area bio‐multilayer coated particles formed were subsequently utilized in enzymatic catalysis.     

Hooking Together Sigmoidal Monomers into Supramolecular Polymers

Supramolecular polymers show great potential in the development of new materials because of their inherent recyclability and their self‐healing and stimuli‐responsive properties. Supramolecular conductive polymers are generally obtained by the assembly of individual aromatic molecules into columnar arrays that provide an optimal channel for electronic transport. A new approach is reported to prepare supramolecular polymers by hooking together sigmoidal monomers into 1D arrays of π‐stacked anthracene and acridine units, which gives rise to micrometer‐sized fibrils that show pseudoconductivities in line with other conducting materials. This approach paves the way for the design of new supramolecular polymers constituted by acene derivatives with enhanced excitonic and electronic transporting properties.     

Synthesis of a Benzenethiol‐derivatized Porphyrin for Self‐assembly

The synthesis of a benzenethiol‐derivatized porphyrin for flat‐lying self‐assembly on gold substrates is described. Acetyl protected thiol is not stable enough in Pd‐catalyzed reactions. While acrylate derivatives protected thiol group shows good tolerance in Pd‐catalyzed borylations and Suzuki‐Miyaura coupling reactions and no catalyst poisoning was observed.