Molecular Springs: Integration of Complex Dynamic Architectures into Functional Devices

Molecular/supramolecular springs are artificial nanoscale objects possessing well‐defined structures and tunable physicochemical properties. Like a macroscopic spring, supramolecular springs are capable of switching their nanoscale conformation as a response to external stimuli by undergoing mechanical spring‐like motions. This dynamic action offers intriguing opportunities for engineering molecular nanomachines by translating the stimuli‐responsive nanoscopic motions into macroscopic work. These nanoscopic objects are reversible dynamic multifunctional architectures which can express a variety of novel properties and behave as adaptive nanoscopic systems. In this Minireview, we focus on the design and structure–property relationships of supramolecular springs and their (self‐)assembly as a prerequisite towards the generation of novel dynamic materials featuring controlled movements to be readily integrated into macroscopic devices for applications in sensing, robotics, and the internet of things.     

Co‐assembly of Patchy Polymeric Micelles and Protein Molecules

The development in the synthesis and self‐assembly of patchy nanoparticles has resulted in the creation of complex hierarchical structures. Co‐assembly of polymeric nanoparticles and protein molecules combines the advantages of polymeric materials and biomolecules, and will produce new functional materials. Co‐assembly of positively charged patchy micelles and negatively charged bovine serum albumin (BSA) molecules is investigated. The patchy micelles, which were synthesized using block copolymer brushes as templates, leads to co‐assembly with protein molecules into vesicular structures. The average size of the assembled structures can be controlled by the molar ratio of BSA to patchy micelles. The assembled structures are dissociated in the presence of trypsin. The protein–polymer hybrid vesicles could find potential applications in medicine.     

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.     

CdSe Nanoplatelets: Living Polymers

Colloidal CdSe nanoplatelets are considered to be excellent candidates for many applications in nanotechnology. One of the current challenges is to self‐assemble these colloidal quantum wells into large ordered structures to control their collective optical properties. We describe a simple and robust procedure to achieve controlled face‐to‐face self‐assembly of CdSe nanoplatelets into micron‐long polymer‐like threads made of up to ～1000 particles. These structures are formed by addition of oleic acid to a stable colloidal dispersion of platelets, followed by slow drying and re‐dispersion. We could control the average length of the CdSe nanoplatelet threads by varying the amount of added oleic acid. These 1‐dimensional structures are flexible and feature a “living polymer” character because threads of a given length can be further grown through the addition of supplementary nanoplatelets at their reactive ends.     

Reversible Assembly and Disassembly of Receptor‐Decorated Gold Nanoparticles Controlled by Ion Recognition

The controlled assembly of randomly dispersed colloidal particles can provide access to materials with advanced optical and electronic properties while providing fundamental insights into self‐assembly processes in nature and nanotechnology. Typically, self‐assembled nanoparticles are prepared by exploiting electrostatic interactions, lithographic techniques, and covalently linked molecular scaffolds. This results in static morphologies that cannot be disassembled easily. On the other hand, having access to systems that can be assembled or disassembled in a controlled manner could allow for in‐depth understanding of the nanoparticles as well as rational control over the morphology and fundamental properties of the resulting constructs. If the changes in aggregation are induced by a specific external chemical stimulus, it could also permit the development of new chemosensors. Here we demonstrate the reversible assembly and disassembly of gold nanoparticles achieved by modulating the noncovalent interactions between surface‐bound calix[4]pyrroles and added bis‐imidazolium cations. We also demonstrate the use of these nanoparticles in the selective sensing of anions.     

Controlled particle placement through convective and capillary assembly

A wide variety of methods are now available for the synthesis of colloidal particle having controlled shapes, structures, and dimensions. One of the main challenges in the development of devices that utilize micro- and nanoparticles is still particle placement and integration on surfaces. Required are engineering approaches to control the assembly of these building blocks at accurate positions and at high yield. Here, we investigate two complementary methods to create particle assemblies ranging from full layers to sparse arrays of single particles starting from colloidal suspensions of gold and polystyrene particles. Convective assembly was performed on hydrophilic substrates to create crystalline mono- or multilayers using the convective flow of nanoparticles induced by the evaporation of solvent at the three-phase contact line of a solution. On hydrophobic surfaces, capillary assembly was investigated to create sparse arrays and complex three-dimensional structures using capillary forces to trap and organize particles in the recessed regions of a template. In both methods, the hydrodynamic drag exerted on the particle in the suspension plays a key role in the assembly process. We demonstrate for the first time that the velocity and direction of particles in the suspension can be controlled to perform assembly or disassembly of particles. This is achieved by setting the temperature of the colloidal suspension above or below the dew point. The influence of other parameters, such as substrate velocity, wetting properties, and pattern geometry, is also investigated. For the particular case of capillary assembly, we propose a mechanism that takes into account the relative influences of these parameters on the motion of particles and that describes the influence of temperature on the assembly efficiency.     

Janus Nanoparticles: Preparation,Characterization, and Applications

In chemical functionalization of colloidal particles, the functional moieties are generally distributed rather homogeneously on the particle surface. Recently, a variety of synthetic protocols have been developed in which particle functionalization may be carried out in a spatially controlled fashion, leading to the production of structurally asymmetrical particles. Janus particles represent the first example in which the two hemispheres exhibit distinctly different chemical and physical properties, which is analogous to the dual‐faced Roman god, Janus. Whereas a variety of methods have been reported for the preparation of (sub)micron‐sized polymeric Janus particles, it has remained challenging for the synthesis and (unambiguous) structural characterization of much smaller nanometer‐sized Janus particles. Herein, several leading methods for the preparation of nanometer‐sized Janus particles are discussed and the important properties and applications of these Janus nanoparticles in electrochemistry, sensing, and catalysis are highlighted. Some perspectives on research into functional patchy nanoparticles are also given.     

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.     

Nanoparticulate Functional Materials

Nanoparticulate functional materials offer manifold perspectives for the increasing miniaturization and complexity of technical developments. Nanoparticles also make a major contribution to utilization of materials that is sparing of natural resources. Besides these obvious aspects, however, the importance of nanoparticles is due to their fundamentally novel properties and functions. These include photonic crystals and efficient luminophors, single particles and thin films for electronic storage media and switching elements, magnetic fluids and highly selective catalysts, a wide variety of possibilities for surface treatments, novel materials and concepts for energy conversion and storage, contrast agents for molecular biology and medical diagnosis, and fundamentally novel forms and structures of materials, such as nanocontainers and supercrystals. Creating high‐quality nanoparticles requires that numerous parameters, involving the particle core and surface, colloidal properties, and particle deposition, are taken into consideration during synthesis of the material. An appropriate characterization and evaluation of the properties requires the incorporation of a wide range of expertise from widely differing areas. These circumstances are what challenges and appeals to the nanoscientist.     

Anatase–silica composite aerogels: a nanoparticle-based approach

Herein we present the synthesis of anatase–silica aerogels based on the controlled gelation of preformed nanoparticle mixtures. The monolithic aerogels with macroscopic dimensions show large specific surface areas, and high and uniform porosities. The major advantage of such a particle-based approach is the great flexibility in pre-defining the compositional and structural features of the final aerogels before the gelation process by fine-tuning the properties of the titania and silica building blocks (e.g., size, composition and crystallinity) and their relative ratio in the dispersion. Specific surface functionalization enables control over the interaction between the nanoparticles and thus over their distribution in the aerogel. Positively charged titania nanoparticles are co-assembled with negatively charged Stoeber particles, resulting in a binary aerogel with a crystalline anatase and amorphous silica framework directly after supercritical drying without any calcination step. Titania–silica aerogels combine the photocatalytic activity of the anatase nanoparticles with the extensive silica chemistry available for silica surface functionalization.     

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.     

Synthesis of Metallic Nanoparticles Using Closed‐Shell Structures as Templates

The synthesis and study of metallic nanoparticles are of continued and significant interest, with applications in materials science, catalysis, and medicine. The properties of metallic nanoparticles depend strongly on their particle size, shape, and interparticle distances. It is therefore desirable for the synthesis of metallic nanoparticles to be controlled for specific shapes and sizes. The rapid development in this research area has attracted intense interest from researchers with diverse expertise, and numerous methods towards the synthesis of monodisperse nanoparticles have been reported. In this Focus Review, we provide an overview of recent progress in the development of the template synthesis of metallic nanoparticles using closed‐shell structures, including biological molecules/assemblies and cage molecules.     

Controlled synthesis and self-assembly of highly monodisperse Ag and Ag(2)S nanocrystals

A facile method to control the synthesis and self‐assembly of monodisperse Ag and Ag₂S nanocrystals with a narrow‐size distribution is described. Uniform Ag nanoparticles of less than 4 nm were obtained by thermolysis of Ag–oleate complexes in the presence of oleic acid and dodecylamine, and monodisperse Ag nanoparticles of less than 10 nm were also prepared in one step by using dodecylamine and oleic acid as capping agents. Moreover, the surface‐enhanced Raman scattering (SERS) properties of the Ag substrates have also been investigated. It is worth mentioning that these Ag nanoparticles and assemblies show great differences in the SERS activities of Rhodamine B dye. In addition, the superlattices of Ag₂S nanocrystals were synthesized with Ag–oleate complexes, alkanethiol, and sulfur as the reactants. The resulting highly monodisperse nanocrystals can easily self‐assemble into interesting superstructures in the solution phase without any additional assembly steps. This method may be extended to the size‐controlled preparation and assembly of many other noble‐metal and transition‐metal chalcogenide nanoparticles. These results will aid the study of the physicochemical properties of the superlattice assemblies and construction of functional macroscopic architectures or devices.     

Semiconductor Gas Sensors: Dry Synthesis and Application

Since the development of the first chemoresistive metal oxide based gas sensors, transducers with innovative properties have been prepared by a variety of wet‐ and dry‐deposition methods. Among these, direct assembly of nanostructured films from the gas phase promises simple fabrication and control and with the appropriate synthesis and deposition methods nm to μm thick films, can be prepared. Dense structures are achieved by tuning chemical or vapor deposition methods whereas particulate films are obtained by deposition of airborne, mono‐ or polydisperse, aggregated or agglomerated nanoparticles. Innovative materials in non‐equilibrium or sub‐stoichiometric states are captured by rapid cooling during their synthesis. This Review presents some of the most common chemical and vapor‐deposition methods for the synthesis of semiconductor metal oxide based detectors for chemical gas sensors. In addition, the synthesis of highly porous films by novel aerosol methods is discussed. A direct comparison of structural and chemical properties with sensing performance is given.     

Carbon Nitride Aerogels for the Photoredox Conversion of Water

Aerogel structures have attracted increasing research interest in energy storage and conversion owing to their unique structural features, and a variety of materials have been engineered into aerogels, including carbon‐based materials, metal oxides, linear polymers and even metal chalcogenides. However, manufacture of aerogels from nitride‐based materials, particularly the emerging light‐weight carbon nitride (CN) semiconductors is rarely reported. Here, we develop a facile method based on self‐assembly to produce self‐supported CN aerogels, without using any cross‐linking agents. The combination of large surface area, incorporated functional groups and three‐dimensional (3D) network structure, endows the resulting freestanding aerogels with high photocatalytic activity for hydrogen evolution and H₂O₂ production under visible light irradiation. This work presents a simple colloid chemistry strategy to construct 3D CN aerogel networks that shows great potential for solar‐to‐chemical energy conversion by artificial photosynthesis.     

Synthesis of nano/microstructures at fluid interfaces

The generation of novel multifunctional materials with hierarchical ordering is a major focus of current materials science and engineering. For such endeavors, fluid interfaces, such as air-liquid and liquid-liquid interfaces, offer ideal platforms where nanoparticles or colloidal particles can accumulate and self-assemble. Different assembly processes and reactions have been performed at fluid interfaces to generate hierarchical structures, including two-dimensional crystalline films, colloidosomes, raspberry-like core-shell structures, and Janus particles, which lead to broad applications in drug delivery and controlled release, nanoelectronics, sensors, food supplements, and cosmetics.     

Self‐Templated Assembly of AuI/AgI‐Thiolate Sheets with Central Holes

Composite crystalline sheets of Au^I/Ag^I‐thiolate with central holes are achieved by co‐assembly of Ag^I‐thiolate and Au^I‐thiolate in one‐pot without sacrificial template. Both Ag^I‐thiolate and Au^I‐thiolate can separately assemble to lamellar sheets with similar structures, which makes their co‐assembly possible, while the differences in their assembly pathways make the co‐assembly processes highly dynamic and complex. First, a core@shell structure with Ag^I‐thiolate at the core was formed upon the mixing of the two, then the core@shell structure transformed to a hole@shell structure by dissociation of the core. Finally, some instable hole@shell structures further dissociated and grew on stable ones to generate holed Au^I/Ag^I‐thiolate composite sheets, in which the two components neither have severe phase separation nor blend uniformly at atomic level. By tuning the feeding ratios, the average diameter of the holes can be controlled. Therefore, the work demonstrates the advantage of co‐assembly technique in obtaining complex structurers. The holed sheets can further assemble to porous macroscopic materials and transform to composite metal nanoparticles by pyrolysis.     

单分散中空介孔结构的盐模板合成及形貌调控

中空介孔结构因具有丰富的内部空间以及多孔渗透性外壳等优势,在催化、能源储存与转化及生物医药等领域得到了广泛应用.然而,目前仍然缺少高效、简便且绿色的合成中空介孔结构的方法.本文以柠檬酸钠胶体颗粒作为模板,通过十六烷基三甲基溴化氨(Cetyltrimethylammonium bromide, CTAB)胶束与正硅酸四乙酯(Tetraethyl orthosilicate, TEOS)的水解低聚物在胶体颗粒表面进行界面共组装,直接生长介孔二氧化硅壳层;然后通过简便的醇洗和水洗分别除去CTAB胶束和柠檬酸钠胶体颗粒后,得到中空介孔结构.进一步研究表明,负电荷的柠檬酸钠胶体颗粒与CTAB胶束之间的静电相互作用是诱导氧化硅低聚物在颗粒表面进行交联组装的关键.基于此,通过控制生长时间实现了对中空介孔结构形貌和壳层厚度的精确调控.所得中空介孔二氧化硅纳米球可以显著增强物质的扩散传输,是理想的催化剂载体,负载金纳米颗粒后可以高效催化4-硝基苯酚的还原反应.研究结果为中空介孔材料的绿色简便合成提供了思路.     

Structural properties of solid lipid based colloidal drug delivery systems

For about 20 years nanoparticles based on solid lipids have been under investigation as drug carrier systems. They can be prepared from a broad variety of lipid matrix materials including glycerides, fatty acids and waxes and are stabilized by physiologically compatible surfactants. Although the matrix lipids principally retain their material properties when dispersed into the colloidal state there are various peculiarities that have to be observed when dealing with such systems. In particular, the crystallization behavior and the polymorphic transitions are altered in the nanoparticulate systems. These properties as well as the particle shape and structure may be affected by the type of surfactants used for stabilization. Also incorporated drugs can modify the structural characteristics of the nanoparticles. Interactions between the individual particles may lead to alterations of the macroscopic behavior of the dispersions, especially of their rheological properties. Such structural parameters can influence the drug carrier properties of the dispersions.     

Temperature and Composition Dependent Optical Properties of CdSe/CdS Dot/Rod-Based Aerogel Networks

Employing nanocrystals (NCs) as building blocks of porous aerogel network structures allows the conversion of NC materials into macroscopic solid structures while conserving their unique nanoscopic properties. Understanding the interplay of the network formation and its influence on these properties like size-dependent emission is a key to apply techniques for the fabrication of novel nanocrystal aerogels. In this work, CdSe/CdS dot/rod NCs possessing two different CdSe core sizes were synthesized and converted into porous aerogel network structures. Temperature-dependent steady-state and time-resolved photoluminescence measurements were performed to expand the understanding of the optical and electronic properties of these network structures generated from these two different building blocks and correlate their optical with the structural properties. These investigations reveal the influence of network formation and aerogel production on the network-forming nanocrystals. Based on the two investigated NC building blocks and their aerogel networks, mixed network structures with various ratios of the two building blocks were produced and likewise optically characterized. Since the different building blocks show diverse optical response, this technique presents a straightforward way to color-tune the resulting networks simply by choosing the building block ratio in connection with their quantum yield.