Layer‐by‐Layer Growth of Multicomponent Colloidal Crystals Over Large Areas

Self‐assembly of different sized colloidal particles into multicomponent crystals results in novel material properties compared to the properties of the individual components alone. The formation of binary and, for the first time, ternary colloidal crystals through a simple and inexpensive confined‐area evaporation‐induced layer‐by‐layer (LBL) assembly method is reported. The proposed method produces high quality multicomponent colloidal crystal films over a broad range of particle size‐ratios and large surface areas (cm²) from silica/polystyrene colloidal suspensions of low concentration. By adjusting the size‐ratio and concentration of the colloidal particles, complex crystals of tunable stoichiometries are fabricated and their structural characteristics are further confirmed with reported crystal analogues. In addition, complex structures form as a result of the interplay of the template layer effect, the surface forces exerted by the meniscus of the drying liquid, the space filling principle, and entropic forces. Thus, this LBL approach is a versatile way to grow colloidal crystals with binary, ternary, or more complex structures.     

Brilliant Pitaya‐Type Silica Colloids with Central–Radial and High‐Density Quantum Dots Incorporation for Ultrasensitive Fluorescence Immunoassays

Creating secondary nanostructures from fundamental building blocks with simultaneous high loading capacity and well‐controlled size/uniformity, is highly desired for nanoscale synergism and integration of functional units. Here a novel strategy is reported for hydrophobic quantum dots (QDs) assembley with porous templates, to form pitaya‐type fluorescent silica colloids with densely packed and intact QDs throughout the silica matrix. The mercapto‐terminated dendritic silica spheres with highly accessible central–radial pores and metal‐affinity interior surface, are adopted as a powerful absorbent host for direct immobilization of QDs from organic phase with high loading capacity. The alkylsilane mediated silica encapsulation prevents QDs' optical degradation induced by ligand exchange and favors the homogeneous silica shell formation. These multiple QD embedded silica spheres exhibit good compatibility for different colored QDs with well‐preserved fluorescence, high colloidal/optical stability, and versatile surface functionality. It is demonstrated that after integration with a lateral flow strip platform, these silica colloids provide an ultrasensitive, specific, and robust immunoassay for C‐reaction protein in clinical samples as promising fluorescent reporters.     

A Shape Memory Acrylamide/DNA Hydrogel Exhibiting Switchable Dual pH‐Responsiveness

Shape memory acrylamide/DNA hydrogels include two different crosslinkers as stabilizing elements. The triggered dissociation of one of the crosslinking elements transforms the shaped hydrogel into an arbitrarily shaped (or shapeless) quasi‐liquid state. The remaining crosslinking element, present in the quasi‐liquid, provides an internal memory that restores the original shaped hydrogel upon the stimulus‐triggered regeneration of the second crosslinking element. Two pH‐sensitive shape memory hydrogels, forming Hoogsten‐type triplex DNA structures, are described. In one system, the shaped hydrogel is stabilized at pH = 7.0 by two different duplex crosslinkers, and the transition of the hydrogel into the shapeless quasi‐liquid proceeds at pH = 5.0 by separating one of the crosslinking units into a protonated cytosine–guanine–cytosine (C–G·C⁺ ) triplex. The second shaped hydrogel is stabilized at pH = 7.0, by cooperative duplex and thymine–adenine–thymine triplex (T–A·T) bridges. At pH = 10.0, the triplex units separate, leading to the dissociation of the hydrogel into the quasi‐liquid state. The cyclic, pH‐stimulated transitions of the two systems between shaped hydrogels and shapeless states are demonstrated. Integrating the two hydrogels into a shaped “two‐arrowhead” hybrid structure allows the pH‐stimulated cyclic transitions of addressable domains of the hybrid between shaped and quasi‐liquid states.     

High‐Tensile Strength,Composite Bijels through Microfluidic Twisting

Rope making is a millennia old technique to collectively assemble numerous weak filaments into flexible and high tensile strength bundles. However, delicate soft matter fibers lack the robustness to be twisted into bundles by means of mechanical rope making tools. Here, weak microfibers with tensile strengths of a few kilopascals are combined into ropes via microfluidic twisting. This is demonstrated for recently introduced fibers made of bicontinuous interfacially jammed emulsion gels (bijels). Bijels show promising applications in use as membranes, microreactors, energy and healthcare materials, but their low tensile strength make reinforcement strategies imperative. Hydrodynamic twisting allows to produce continuous bijel fiber bundles of controllable architecture. Modelling the fluid flow field reveals the bundle geometry dependence on a subtle force balance composed of rotational and translational shear stresses. Moreover, combining multiple bijel fibers of different compositions enables the introduction of polymeric support fibers to raise the tensile strength to tens of megapascals, while simultaneously preserving the liquid like properties of the bijel fibers for transport applications. Hydrodynamic twisting shows potentials to enable the combination of a wide range of materials resulting in composites with features greater than the sum of their parts.     

Thermal and Electrical Properties of Nanocomposites Based on Self‐Assembled Pristine Graphene

Thermodynamically‐driven exfoliation and self‐assembly of pristine graphene sheets is shown to provide thermally and electrically functional polymer composites. The spreading of graphene sheets at a high energy liquid/liquid interface is driven by lowering the overall energy of the system, and provides for the formation of water‐in‐oil emulsions stabilized by overlapping graphene sheets. Polymerization of the oil phase, followed by removal of the dispersed water phase, produces inexpensive and porous composite foams. Contact between the graphene‐stabilized water droplets provides a pathway for electrical and thermal transport through the composite. Unlike other graphene foams, the graphite used to synthesize these composites is natural flake material, with no oxidation, reduction, sonication, high temperature thermal treatment, addition of surfactants, or high shear mixing required. The result is an inexpensive, low‐density material that exhibits Joule heating and displays increasing electrical conductivity with decreasing thermal conductivity.     

A New Understanding of Au‐Assisted Growth of III–V Semiconductor Nanowires

Semiconductor nanowires of III–V materials have generated much interest in recent years. However, the growth mechanisms by which these structures form are not well understood. The so‐called vapor–liquid–solid (VLS) mechanism has often been proposed for III–V systems, with a chemically inert, liquid metal particle (typically Au) acting as a physical catalyst. We assert here that Au is, in fact, not inert with respect to the semiconductor material but rather interacts with it to form a variety of intermetallic compounds. Moreover, we suggest that III–V nanowire growth can best be understood if the metallic particle is not a liquid, but a solid‐phase solution or compound containing Au and the group III material. The four materials GaP, GaAs, InP, and InAs will be considered, and growth behavior related to their particular temperature‐dependent interaction with Au.     

Mesoporous Anatase Titania Hollow Nanostructures though Silica‐Protected Calcination

The crystallization of nanometer‐scale materials during high‐temperature calcination can be controlled by a thin layer of surface coating. Here, a novel silica‐protected calcination process for preparing mesoporous hollow TiO₂ nanostructures with a high surface area and a controllable crystallinity is presented. This method involves the preparation of uniform silica colloidal templates, sequential deposition of TiO₂ and then SiO₂ layers through sol–gel processes, calcination to transform amorphous TiO₂ to crystalline anatase, and finally etching of the inner and outer silica to produce mesoporous anatase TiO₂ shells. The silica‐protected calcination step allows crystallization of the amorphous TiO₂ layer into anatase nanocrystals, while simultaneously limiting the growth of anatase grains to within several nanometers, eventually producing mesoporous anatase shells with a high surface area (～311 m² g^?1) and good water dispersibility upon chemical etching of the silica. When used as photocatalysts for the degradation of Rhodamine B under UV irradiation, the as‐synthesized mesoporous anatase shells show significantly enhanced photocatalytic activity with greater enhancement for samples calcined at higher temperatures thanks to their improved crystallinity.     

Robust,Self‐Healing Superamphiphobic Fabrics Prepared by Two‐Step Coating of Fluoro‐Containing Polymer,Fluoroalkyl Silane,and Modified Silica Nanoparticles

A robust, superamphiphobic fabric with a novel self‐healing ability to autorepair from chemical damage is prepared by a two‐step wet‐chemistry coating technique using an easily available material system consisting of poly(vinylidene fluoride‐co‐hexafluoropropylene), fluoroalkyl silane, and modified silica nanoparticles. The coated fabrics can withstand at least 600 cycles of standard laundry and 8000 cycles of abrasion without apparently changing the superamphiphobicity. The coating is also very stable to strong acid/base, ozone, and boiling treatments. After being damaged chemically, the coating can restore its super liquid‐repellent properties by a short‐time heating treatment or room temperature ageing. This simple but novel and effective coating system may be useful for the development of robust protective clothing for various applications.     

Mechanisms for High‐Performance and Non‐Local Photoisomerization Gratings in a Sol–Gel Material

Novel azo‐containing sol–gel films exhibiting outstanding properties for optical applications via nonlocal photoisomerization gratings were reported recently, although the underlying mechanisms were not well understood, especially regarding the unexpected non‐local effect. Here, this photoisomerizable sol–gel material is characterized in‐depth, analyzing the design and fabrication strategy, and discussing the aspects that enable the efficient photoresponse, with focus on the holographic recording. The material consists of an azochromophore‐rich silica matrix containing glycidoxypropyl groups, which provide increased flexibility and internal free volume for improved dye photoresponse. The matrix characteristics allow a novel procedure for fabrication of thick optical films, in which chromophore aggregation is ruptured by thermal annealing while keeping the material centrosymmetry (beneficial for high hologram contrast). The molecular photo‐orientation promotes alignment of microscopic domains in a cooperative motion, not reported previously in sol–gel materials. This collective mechanism enhances the material response and explains some intriguing features of photoisomerization gratings. In particular, there is evidence that spatially shifted domains are related to the grating nonlocal nature. Different recording (write–erase–write) procedures that distinctly affect the photoalignment at both molecular and microscopic level are studied. The holographic performance drastically changes, which can be selectively exploited for either long‐term or dynamic holography.     

Polymerization‐Induced Colloidal Assembly and Photonic Crystal Multilayer for Coding and Decoding

Photonic crystal (PC) films are prepared by precipitation of colloidal crystal seeds in supersaturated solution of particles, followed by crystal growth and structure fixing with photo‐polymerization. As the liquid monomer becomes a solid matrix, the highly concentrated particles are forced to precipitate into colloidal microcrystals in short time, and ‘polymerization‐induced colloidal assembly’ (PICA) is shown to be the major driving force to form colloidal crystals. PICA is intrinsically different from evaporation‐induced colloidal assembly, because the seed formation and crystal growth are separated into two independent steps, which makes the synthesis more flexible, controllable, and efficient. The PICA process is capable of quickly producing PC films with an ultra‐narrow bandgap, tunable thickness, and large size. Based on these characteristics and the blocking effect of the outer PC layer to the reflection signal of inner layer, a coding–decoding system is developed in which the film's composition and stacking sequence can be identified by its distinctive reflection spectrum.     

Engineering Inorganic Nanoemulsions/Nanoliposomes by Fluoride‐Silica Chemistry for Efficient Delivery/Co‐Delivery of Hydrophobic Agents

A novel drug‐formulation protocol is developed to solve the delivery problem of hydrophobic drug molecules by using inorganic mesoporous silica nanocapsules (IMNCs) as an alternative to traditional organic emulsions and liposomes while preserving the advantages of inorganic materials. The unique structures of IMNCs are engineered by a novel fluoride‐silica chemistry based on a structural difference‐based selective etching strategy. The prepared IMNCs combine the functions of organic nanoemulsions or nanoliposomes with the properties of inorganic materials. Various spherical nanostructures can be fabricated simply by varying the synthetic parameters. The drug loading amount of a typical highly hydrophobic anticancer drug‐camptothecin (CPT) in IMNCs reaches as high as 35.1 wt%. The intracellular release of CPT from carriers is demonstrated in situ. In addition, IMNCs can play the role of organic nanoliposome (multivesicular liposome) in co‐encapsulating and co‐delivering hydrophobic (CPT) and hydrophilic (doxorubicin, DOX) anticancer drugs simultaneously. The co‐delivery of multi‐drugs in the same carrier and the intracellular release of the drug combinations enables a drug delivery system with efficient enhanced chemotherapeutic effect for DOX‐resistant MCF‐7/ADR cancer cells. The special IMNCs‐based “inorganic nanoemulsion”, as a proof‐of‐concept, can also be employed successfully to encapsulate and deliver biocompatible hydrophobic perfluorohexane (PFH) molecules for high intensity focused ultrasound (HIFU) synergistic therapy ex vivo and in vivo. Based on this novel design strategy, a wide range of inorganic material systems with similar “inorganic nanoemulsion or nanoliposome” functions will be developed to satisfy varied clinical requirements.     

Biocatalytic Nanoscale Coatings Through Biomimetic Layer‐by‐Layer Mineralization

Recent insight into the molecular mechanisms of biological mineral formation (biomineralization) has enabled biomimetic approaches for the synthesis of functional organic‐inorganic hybrid materials under mild reaction conditions. Here we describe a novel method for enzyme immobilization in thin (nanoscale) conformal mineral coatings using biomimetic layer‐by‐layer (LbL) mineralization. The method utilizes a multifunctional molecule comprised of a naturally‐occurring peptide, protamine (PA), covalently bound to the redox enzyme Glucose oxidase (GOx). PA mimics the mineralizing properties of biomolecules involved in silica biomineralization in diatoms, and its covalent attachment to GOx does not interfere with the catalytic activity. Highly efficient and stable incorporation of this modified enzyme (GOx‐PA) into nanoscale layers (～5–7 nm thickness) of Ti‐O and Si‐O is accomplished during protamine‐enabled LbL mineralization on silica spheres. Depending on the layer location of the enzyme and the type of mineral (silica or titania) within which the enzyme is incorporated, the resulting multilayer biocatalytic hybrid materials exhibit between 20–100% of the activity of the free enzyme in solution. Analyses of kinetic properties (V_max, K_M) of the immobilized enzyme, coupled with characterization of physical properties of the mineral‐bearing layers (thickness, porosity, pore size distribution), indicates that the catalytic activities of the synthesized hybrid nanoscale coatings are largely determined by substrate diffusion rather than enzyme functionality. The GOx‐PA immobilized in these nanoscale layers is substantially stabilized against heat‐induced denaturation and largely protected from proteolytic attack. The method for enzyme immobilization described here enables, for the first time, the high yield immobilization and stabilization of enzymes within continuous, conformal, and nanoscale coatings through biomimetic LbL mineralization. This approach will likely be applicable to a wide variety of surfaces and functional biomolecules. The ability to synthesize thin (nanoscale) conformal enzyme‐loaded layers is of interest for numerous applications, including enzyme‐based biofuel cells and biosensors.     

Excitons and Electron–Hole Liquid State in 2D γ‐Phase Group‐IV Monochalcogenides

Different dispersion near the electronic band edge of a semiconductor can have great influence on its transport, thermoelectric, and optical properties. Using first‐principles calculations, it is demonstrated that a new phase of group‐IV monochalcogenides (γ‐MX, M = Ge, Sn; X = S, Se, or Te) can be stabilized in monolayer limit. γ‐MXs are shown to possess a unique band dispersion—that is, camel's back like structure—in the top valence band. The band nesting effect near the camel's back region induces a large excitonic absorbance and significantly different exciton behaviors from other 2D materials. Importantly, the small effective mass and the indirect characteristics of lowest‐energy exciton render it advantageous for the generation of electron–hole liquid state. After careful evaluation of the electron–hole dissociation temperature and the Mott critical density, it is predicted that a high‐temperature exciton gas to electron–hole liquid phase transition can be achieved in these materials with a low excitation power density. The findings open up new opportunities for both the fundamental research on exciton physics and design of excitonic devices based on 2D materials with distinct band dispersion.     

Silica‐Based,Organically Modified Host Material for Waveguide Structuring by Two‐Photon‐Induced Photopolymerization

The three‐dimensional fabrication of optical waveguides has gained increasing interest in recent years to establish interconnections between electrical components on a very small scale where copper circuits encounter severe limitations. In this work the application of optically clear, organically modified porous silica monoliths and thin films as a host material for polymeric waveguides to be inscribed into the solid host structure by two‐photon‐induced photopolymerization is investigated. Porosity is generated using a lyotropic liquid crystalline surfactant/solvent system as a template for the solid silica material obtained by a sol–gel transition of a liquid precursor. In order to reduce the brittleness of the purely inorganic material, organic–inorganic co‐precursor molecules that contain poly(ethylene glycol) chains are synthesized and added to the mixture, which successfully suppresses macroscopic cracking and leads to flexible thin films. The structure of the thus‐obtained porous organic–inorganic hybrid material is investigated by atomic force microscopy. It is shown that the modified material is suitable for infiltration with photocurable monomers and functional polymeric waveguides can be inscribed by selective two‐photon‐induced photopolymerization.     

Large‐Area Ordered Quantum‐Dot Monolayers via Phase Separation During Spin‐Casting

We investigate a new method for forming large‐area (> cm²) ordered monolayers of colloidal nanocrystal quantum dots (QDs). The QD thin films are formed in a single step by spin‐casting a mixed solution of aromatic organic materials and aliphatically capped QDs. The two different materials phase separate during solvent drying, and for a predefined set of conditions the QDs can assemble into hexagonally close‐packed crystalline domains. We demonstrate the robustness and flexibility of this phase‐separation process, as well as how the properties of the resulting films can be controlled in a precise and repeatable manner. Solution concentration, solvent ratio, QD size distribution, and QD aspect ratio affect the morphology of the cast thin‐film structure. Controlling all of these factors allows the creation of colloidal‐crystal domains that are square micrometers in size, containing tens of thousands of individual nanocrystals per grain. Such fabrication of large‐area, engineered layers of nanoscale materials brings the beneficial properties of inorganic QDs into the realm of nanotechnology. For example, this technique has already enabled significant improvements in the performance of QD light‐emitting devices.     

Electrically Directed On‐Chip Reversible Patterning of Two‐Dimensional Tunable Colloidal Structures

In this work, we report a versatile approach to two‐dimensional colloidal patterning based on the lateral assembly of colloidal particles by an alternating electric field (AEF). Under the AEF, the lithographically templated electrodes provide an effective way to reversibly and rapidly assemble colloidal particles into some desirable patterns. By controlling the AEF and the electrode pattern geometry, various colloidal patterns with tunable lattice spacing and even with binary lattice spacing have been formed. Particularly, we demonstrate that well‐defined linear defects can be embedded inside the colloidal crystals, whereas the unwanted existing defects can be controllably relaxed by this patterning process. This novel patterning technique is amenable to both large scale on‐chip patterning and micro‐structural control with single‐particle resolution on a time scale of seconds. Furthermore, it introduces a new class of colloidal structures with the properties that can be finely tuned, reversibly switched, or permanently fixed, opening a new way for the engineering of novel materials and devices at micro levels.     

Polymer Light‐Emitting Electrochemical Cells: The Formation and Effects of Doping‐Induced Micro Shorts

A planar surface cell structure has been utilized to investigate a polymer light‐emitting electrochemical cell (LEC), consisting of an active‐material mixture of poly[2‐methoxy‐5‐(2′‐ethylhexyloxy)‐1,4‐phenylenevinylene] (MEH‐PPV) and an ionic liquid, tetra‐n‐butylammonium trifluoromethanesulfonate, contacted by Au electrodes. It is shown that diffuse and needle‐shaped doping fronts originate from the electrodes (p‐type from the positive electrode and n‐type from the negative electrode) during the charging process at a temperature (T) of 393 K. After a turn‐on time, a significant portion of the doping fronts makes contact close to the negative electrode to form a light‐emitting p–i–n junction, but at some points p‐type doping protrudes all the way to the negative electrode to form what are effectively micro shorts. It is shown that the consequences of such doping‐induced micro shorts during a subsequent stabilized “frozen junction” operation at T = 200 K are drastic: the current‐rectification ratio (RR) is low and the quantum efficiency (QE) is far from optimum. Both RR and QE increase significantly during long‐term operation under frozen‐junction conditions, demonstrating that the lifetime of the doping‐induced micro shorts is limited even under such stabilized conditions. Still, it is clear that it is critical for the optimization and further development of LECs to find new types of active material and electrode combinations that allow for formation of a continuous p–i–n junction in the center of the interelectrode gap.     

Colloidal Suspensions of Nanometer‐Sized Mesoporous Silica

Nanometer‐sized surfactant‐templated materials are prepared in the form of stable suspensions of colloidal mesoporous silica (CMS) consisting of discrete, nonaggregated particles with dimensions smaller than 200 nm. A high‐yield synthesis procedure is reported based on a cationic surfactant and low water content that additionally enables the adjustment of the size range of the individual particles between 50 and 100 nm. Particularly, the use of the base triethanolamine (TEA) and the specific reaction conditions result in long‐lived suspensions. Dynamic light scattering reveals narrow particle size distributions in these suspensions. Smooth spherical particles with pores growing from the center to the periphery are observed by using transmission electron microscopy, suggesting a seed‐growth mechanism. The template molecules could be extracted from the nanoscale mesoporous particles via sonication in acidic media. The resulting nanoparticles give rise to type IV adsorption isotherms revealing typical mesopores and additional textural porosity. High surface areas of over 1000 m² g^–1 and large pore volumes of up to 1 mL g^–1 are obtained for these extracted samples.     

Hybrid Materials from Ultrahigh‐Inorganic‐Content Mineral Plastic Hydrogels: Arbitrarily Shapeable,Strong, and Tough

Natural mineralized structural materials such as nacre and bone possess a unique hierarchical structure comprising both hard and soft phases, which can achieve the perfect balance between mechanical strength and shape controllability. Nevertheless, it remains a great challenge to control the complex and predesigned shapes of artificial organic–inorganic hybrid materials at ambient conditions. Inspired by the plasticity of polymer‐induced liquid precursor phases that can penetrate and solidify in porous organic frameworks for biomineral formation, here a mineral plastic hydrogel is shown with ultrahigh silica content (≈95 wt%) that can be similarly hybridized into a porous delignified wood scaffold, and the resultant composite hydrogels can be manually made into arbitrary shapes. Subsequent air drying well preserves the designed shapes and produces fire‐retardant, ultrastrong, and tough structural organic–inorganic hybrids. The proposed mineral plastic hydrogel strategy opens an easy and eco‐friendly way for fabricating bioinspired structural materials that compromise both precise shape control and high mechanical strength.