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.     

Rapid Fabrication of Two‐ and Three‐Dimensional Colloidal Crystal Films via Confined Convective Assembly

We have developed a self‐assembly method for fabricating well‐ordered two‐dimensional (2D) and three‐dimensional (3D) colloidal crystal films. With a minute amount of a polystyrene colloidal suspension and without any special equipment, the proposed method can be used to rapidly deposit high‐quality colloidal crystal films over a large surface area. By controlling the lift‐up rate of the substrate, we modulate the meniscus thinning rate, which determines whether the colloidal particles are assembled into two or three dimensions. The proposed method can be used to fabricate not only monolayered colloidal crystals with colloidal particles of various sizes, but also multilayered colloidal crystals. In addition, the method enables us to fabricate binary colloidal crystals by consecutively depositing large and small particles.     

Multicolor Printing Using Electric‐Field‐Responsive and Photocurable Photonic Crystals

Efficient and large scale printing of photonic crystal patterns with multicolor, multigrayscale, and fine resolution is highly desired due to its application in smart prints, sensors, and photonic devices. Here, an electric‐field‐assisted multicolor printing is reported based on electrically responsive and photocurable colloidal photonic crystal, which is prepared by supersaturation‐induced self‐assembly of SiO₂ particles in the mixture of propylene carbonate (PC) and trimethylolpropane ethoxylate triacrylate (ETPTA). This colloidal crystal suspension, named as E‐ink, has tunable structural color, controllable grayscale, and instantly fixable characteristics at the same time because the SiO₂/ETPTA‐PC photonic crystal has metastable and reversible assembly as well as polymerizable features. Lithographical printing with photomask and maskless pixel printing techniques are developed respectively to efficiently prepare multicolor and high‐resolution photonic patterns using a single‐component E‐ink.     

Multifaceted and Nanobored Particle Arrays Sculpted Using Colloidal Lithography

A novel method of fabricating multifaceted and nanobored particle arrays via colloidal lithography using colloidal‐crystal layers as masks for anisotropic reactive‐ion etching (RIE) is reported. The shape of the sculpted particles is dependent on the crystal orientation relative to the etchant flow, the number of colloidal layers, the RIE conditions, and the matrix (or mask) structure in colloidal lithography. Arrays of non‐spherical particles with sculpted shapes, which to date could not otherwise be produced, are fabricated using a tilted anisotropic RIE process and the layer‐by‐layer growth of a colloidal mask. These non‐spherical particles and their ordered arrays can be used for antireflection surfaces, biosensors, and nanopatterning masks, as well as non‐spherical building blocks for novel colloidal crystals. In addition, polymeric particles with patterned holes of controlled depths obtained by the present method can be applied to the fabrication of functional composite particles.     

垂直沉积法在GaAs衬底上制备有序SiO2胶体晶体

报道了一种利用直径为286nm的单分散SiO2胶体颗粒制备胶体晶体的方法。乙醇悬浮中的SiO2颗粒通过毛细作用力在垂直插入其中的GaAs衬底表面自组装成胶体晶体。扫描电子显微镜（SEM）和紫外-可见分光光度计对胶体晶体的形貌和光学特性进行了表征。结果显示,所得到的胶体晶体膜具有较好的三维有序结构。分析了退火对样品光子带隙的影响。     

Fabrication of Photonic Microbricks via Crack Engineering of Colloidal Crystals

Evaporation‐induced self‐assembly of colloidal particles is one of the most versatile fabrication routes to obtain large‐area colloidal crystals; however, the formation of uncontrolled “drying cracks” due to gradual solvent evaporation represents a significant challenge of this process. While several methods are reported to minimize crack formation during evaporation‐induced colloidal assembly, here an approach is reported to take advantage of the crack formation as a patterning tool to fabricate microscopic photonic structures with controlled sizes and geometries. This is achieved through a mechanistic understanding of the fracture behavior of three different types of opal structures, namely, direct opals (colloidal crystals with no matrix material), compound opals (colloidal crystals with matrix material), and inverse opals (matrix material templated by a sacrificial colloidal crystal). This work explains why, while direct and inverse opals tend to fracture along the expected {111} planes, the compound opals exhibit a different cracking behavior along the nonclose‐packed {110} planes, which is facilitated by the formation of cleavage‐like fracture surfaces. The discovered principles are utilized to fabricate photonic microbricks by programming the crack initiation at specific locations and by guiding propagation along predefined orientations during the self‐assembly process, resulting in photonic microbricks with controlled sizes and geometries.     

Engineering of Complex Macroporous Materials Through Controlled Electrodeposition in Colloidal Superstructures

Macroporous materials with a sophisticated architecture are obtained by electrochemical deposition of gold or polypyrrole in colloidal‐crystal templates. The Langmuir–Blodgett technique enables assembly of sub‐micrometric silica‐particle monolayers on conductive gold substrates, thus leading to colloidal superstructures with an unprecedented control of their features at the single‐bead‐layer level. This allows the integration of deliberate planar defects or the elaboration of well‐defined gradients in terms of sphere size. Controlled infiltration using electrochemical deposition preserves the architecture of the original templates and leads to inverse opals with well‐defined pore structures after the removal of the inorganic particles.     

Direct Writing of Three‐Dimensional Polymer Scaffolds Using Colloidal Gels

Polymer scaffolds intended to provide a substrate for cell attachment and proliferation benefit if the geometric architecture, mechanical properties, and surface chemistry are controllable within the range applicable for the target tissue. Such scaffolds may be made bioinductive through the inclusion of surface proteins and release of growth factors. Furthermore, the polymer support may be formed of biodegradable polymers for use as tissue‐engineering scaffolds. In this study, a new scaffold‐fabrication technique based on the direct writing of polymer colloidal‐gel‐based inks is described. The colloidal approach allows for the modular design of inks where the structure and composition of the colloidal particles, surface adsorbed molecules, and dissolved species may be easily controlled. Polyacrylate latex particles are formulated into colloidal gels by using a thermoreversible gel‐forming poly(ethylene oxide)–poly(propylene oxide) block‐copolymer adsorbed layer. The resulting colloidal gels are laced with the model protein bovine serum albumin (BSA) either dissolved in the solvent phase of the ink or dispersed in chitosan nanoparticles as a second colloid. Ink development and rheological characterization are presented along with demonstration of assembly of mesoporous scaffolds. After assembly and drying of the scaffold structure, the drug‐release kinetics are measured upon re‐exposure to an aqueous environment. Protein activity appears to be unaffected by the processing route of these scaffolds. Finally, the assembly of heterogeneous scaffolds is demonstrated to illustrate the possibilities for staged or heterogeneous drug release. This approach to scaffold fabrication offers a new route for scaffold assembly from water‐insoluble polymers while allowing the inclusion of sensitive biomolecules without risk of denaturation.     

Transferable Crack‐Free Colloidal Crystals on an Elastomeric Matrix with Surface Relief

Crack‐free three‐dimensional (3D) colloidal silica crystals are fabricated on an elastomeric polydimethylsiloxane (PDMS) stamp via the lift‐up method. A surface relief structure is fabricated on the PDMS substrate to enable the formation of colloidal crystal assemblies that cannot be achieved on a plane PDMS substrate owing to the hydrophobic nature of its surface. Four samples of uniform silica particles having different sizes are prepared for colloidal crystal assembly on PDMS substrates with various relief patterns. This strategy not only provides a means for the assembly of crack‐free colloidal crystals on a soft hydrophobic surface via the lift‐up method but enables the transfer of the crack‐free colloidal crystals onto a curved surface.     

Molecular Mimetic Self‐Assembly of Colloidal Particles

This article presents an overview of the current progress in molecular mimetic self‐assembly of colloidal particles. Firstly, the recent study of colloidal particles at interfaces is highlighted, underlining the mesoscopic mimicry of the surface activity of amphiphilic molecules using colloidal particles. Secondly, various strategies developed thus far to impart colloidal particles with anisotropy in terms of chemical composition, surface chemistry and particle morphology, which are regarded as mesoscopic atoms and molecules, are reviewed. Thirdly, an overview of the current theoretical and experimental results of using the rules of molecular synthesis and self‐assembly to direct self‐assembly of colloidal particles is presented. Finally, the experimental challenges associated with molecular mimetic self‐assembly of colloidal particles are outlined, giving a rather conservative conclusion of the status quo of this new research field with a very optimistic outlook.     

Interfacial Self‐Assembly of Amphiphilic Dual Temperature Responsive Actuating Janus Particles

Amphiphilic Janus particles feature the combination of two different functional materials in one single colloid, as well as the possibility of self‐assembly at interfaces into complex superstructures. In this article, the self‐assembly of dual temperature responsive amphiphilic Janus particles at liquid–liquid interfaces and their subsequent conversion into an actuating layer‐shaped surface are presented. These microparticles are produced in a capillaries based continuous flow microfluidic device by photoinitiated radical polymerization. The hydrophobic part of the Janus particles contains a liquid crystalline elastomer (LCE), which performs a strong actuation up to 95% during the nematic–isotropic phase transition. The other side consists of a p(NIPAAm) hydrogel, which features volumetric expansions up to 280% below the lower critical solution temperature. A multistep molding process is developed to uniformly align the Janus particles at a toluene/water boundary surface and to embed the particles into a hydrogel matrix. A particle covered hydrogel layer is obtained, which features a collective actuation of the rod‐like LCE parts on the surface and a bundling of the resulting forces during the phase transition.     

Contact‐Killing Polyelectrolyte Microcapsules Based on Chitosan Derivatives

Polyelectrolyte‐multilayer microcapsules are made by layer‐by‐layer (LbL) assembly of oppositely charged polyelectrolytes onto sacrificial colloidal particles, followed by core removal. In this paper, contact‐killing polyelectrolyte microcapsules are prepared based solely on polysaccharides. To this end, water‐soluble quaternized chitosan (QCHI) with varying degrees of substitution (DS) and hyaluronic acid (HA) are assembled into thin films. The quaternary ammonium groups are selectively grafted on the primary amine group of chitosan by exploiting its reaction with glycidyltrimethylammonium chloride (GTMAC) under homogeneous aqueous acidic conditions. The morphology of the capsules is closely dependent on the DS of the quaternized chitosan derivatives, which suggests differences in their complexation with HA. The DS is also a key parameter to control the antibacterial activity of QCHI against Escherichia Coli (E. coli). Thus, capsules containing the QCHI derivative with the highest DS are shown to be the most efficient to kill E. coli while retaining their biocompatibility toward myoblast cells, which suggests their potential as drug carriers able to combat bacterial infections.     

Enhanced Optical Properties and Opaline Self‐Assembly of PPV Encapsulated in Mesoporous Silica Spheres

A new poly(p‐phenylenevinylene) (PPV) composite material has been developed by the incorporation of insoluble PPV polymer chains in the pores of monodisperse mesoporous silica spheres through an ion‐exchange and in situ polymerization method. The polymer distribution within the resultant colloidal particles is characterized by electron microscopy, energy dispersive X‐ray microanalysis, powder X‐ray diffraction, and nitrogen adsorption. It was found that the polymer was selectively incorporated into the mesopores of the silica host and was well distributed throughout the body of the particles. This confinement of the polymer influences the optical properties of the composite; these were examined by UV–vis and fluorescence spectroscopy and time‐correlated single‐photon counting. The results show a material that exhibits an extremely high fluorescence quantum yield (approaching 85%), and an improved resistance to oxidative photobleaching compared to PPV. These enhanced optical properties are further complemented by the overall processability of the colloidal material. In marked contrast to the insolubility of PPV, the material can be processed as a stable colloidal dispersion, and the individual composite spheres can be self‐assembled into opaline films using the vertical deposition method. The bandgap of the opal can be engineered to overlap with the emission band of the polymer, which has significant ramifications for lasing.     

Site‐Selective Self‐Assembly of Colloidal Photonic Crystals

A scalable method for site‐selective, directed self‐assembly of colloidal opals on topologically patterned substrates is presented. Here, such substrate contains optical waveguides which couple to the colloidal crystal. The site‐selectivity is achieved by a capillary network, whereas the self‐assembly process is based on controlled solvent evaporation. In the deposition process, a suspension of colloidal microspheres is dispensed on the substrate and driven into the desired crystallization sites by capillary flow. The method has been applied to realize colloidal crystals from monodisperse dielectric spheres with diameters ranging from 290 to 890 nm. The method can be implemented in an industrial wafer‐scale process.     

Dimer‐Based Three‐Dimensional Photonic Crystals

The self‐assembly of polystyrene dimer‐ and spherocylinder‐shaped colloids is achieved via controlled drying on glass and silicon substrates. 3D monoclinic colloidal crystal structures are determined from scanning electron microscopy images of sections prepared using focused ion‐beam (FIB) milling. Full photonic bandgaps between the eighth and ninth bands are found for a systematic range of colloidal dimer shapes explored with respect to the degree of constituent lobe fusion and radius ratio. The pseudogap between bands 2 and 3 for spherocylinder‐based monoclinic crystals is also probed using normal incidence reflection spectroscopy.     

sol-gel法制备多孔纳米SiO2薄膜

以 NH_3·H_2O 为催化剂,用正硅酸乙酯(TEOS)溶胶–凝胶工艺制备纳米多孔二氧化硅薄膜。强碱催化使二氧化硅胶粒溶解度增大并增大了体系的离子强度;丙三醇的加入,与水解中间体结合,有效抑制了二氧化硅溶胶胶粒的长大;PVA(聚乙烯醇)对二氧化硅胶粒有强的吸附作用,使胶粒聚联成大的网络结构,增加了成膜性能。通过改变反应物的剂量,调节添加剂的用量,可以制得折射率 1.18~1.42,对应孔率 60.50%~7.75%并且孔径小于 100 nm 的纳米多孔二氧化硅薄膜,。单层薄膜厚度在 3 μm 以内精确可控。     

Vapor Sorption and Electrical Response of Au‐Nanoparticle– Dendrimer Composites

Films comprising Au nanoparticles and polyphenylene dendrimers (first and second generation) are deposited onto transducer substrates via layer‐by‐layer self‐assembly and characterized by atomic force microscopy and X‐ray photoelectron spectroscopy. Their sorption behavior is studied by measuring the uptake of solvents from the vapor phase with quartz crystal microbalances (QCMs). The resistance of the films is simultaneously monitored. Both sensor types, QCMs and chemiresistors, give qualitatively very similar response isotherms that are consistent with a combination of Henry‐ and Langmuir‐type sorption processes. The sorption‐induced increase in relative differential resistance scales linearly with the amount of analyte accumulated in the films. This result is in general agreement with an activated tunneling process for charge transport, if little swelling and only small changes in the permittivity of the film occur during analyte sorption (a first‐order approximation). The relative sensitivity of the films to different solvents decreases in the order toluene ≈ tetrachloroethylene > 1‐propanol ? water. Films containing the larger second‐generation dendrimers show higher sensitivity than films containing first‐generation dendrimers.     

Three‐Dimensionally Ordered Macroporous Polymeric Materials by Colloidal Crystal Templating for Reversible CO2 Capture

The design and preparation of porous materials with controlled structures and functionalities is crucial to a variety of absorption‐ or separation‐relevant applications, including CO₂ capture. Here, novel functional polymeric materials with three‐dimensionally ordered macroporous (3DOM) structures are prepared by using colloidal crystals as templates using relatively simple, rapid, and inexpensive approaches. These ordered structures are used for the reversible CO₂ capture from ambient air by humidity swing. Typically, the colloidal crystal template is synthesized from polymer latex particles of poly(methyl methacrylate) (PMMA) or polystyrene (PS). To maintain the functionality of the material, it is important to prevent the porous structure collapsing, which can occur by the hydrolysis of the ester bonds in conventional crosslinkers under basic conditions. This hydrolysis can be prevented by using a water‐soluble crosslinker containing two quaternary ammonium moieties, which can be used to prepare stable porous crosslinked polymers with the monomer (vinylbenzyl)trimethylammonium chloride (VBTMACl) and using a PMMA‐based colloidal crystal template. The hydroxide‐containing monomer and dicationic crosslinker are synthesized from their chloride precursors, avoiding the ion‐exchange step which causes shrinkage of the pores. An analysis of different methods for infiltrating the monomer solution into the colloidal crystal template shows that infiltration using capillary forces leads to fewer defects than infiltration under a partial vacuum. In addition, functional macroporous films with micrometer thickness are prepared from a template of PS‐based colloidal crystals in a thin film. In general, the colloidal crystal templated materials showed improved CO₂ absorption/desorption rates and swing sizes compared to a commercially available material with similar functional groups. This work could easily be extended to create a new generation of ordered macroporous polymeric materials with tunable functionalities for other applications.     

Two‐Dimensional Assembly of Symmetric Colloidal Dimers under Electric Fields

Like atoms and molecules with directional interactions, anisotropic particles could potentially assemble into a much wider range of crystalline arrays and meso‐structures than spherical particles with isotropic interactions. In this paper, the electric‐field directed assembly of geometrically anisotropic particles–colloidal dimers is studied. Rich phase behavior and different assembly regimes are found, primarily arising from the broken radial symmetry in particles. The orientations of individual dimers depend on the frequency of the electric field, the ramping direction of frequency, and the salt concentration. The competition and balance between the hydrodynamic, electric, and Brownian torques determine the orientation of individual particles, while the competition between the electrohydrodynamic force and dipolar interaction determines the aggregation of aligned particles at a given experimental condition. The field distribution near the electrode is critical to understand the orientation and assembly behavior of colloidal dimers on a conducting substrate. This study also demonstrates the effectiveness, the reversibility, and potential opportunity of applying electric field to control the orientation and direct the assembly of non‐spherical particles. In particular, two dimensional close‐packed crystals of perpendicularly aligned dimers are obtained, which shows promise in fabricating 3D photonic crystals based on dimer‐like colloids and field‐directed display.     

Inside Front Cover: Fine Control of the Wettability Transition Temperature of Colloidal‐Crystal Films: From Superhydrophilic to Superhydrophobic (Adv. Funct. Mater. 2/2007)

A facile strategy for finely controlling the wettability transition temperature of colloidal‐crystal films from superhydrophilic to superhydrophobic is demonstrated by Song and co‐workers on p. 219. The films are assembled from poly(styrene‐n‐butyl acrylate–acrylic acid) latex spheres. The wettability transition temperature of the films is tuned by adjusting the n‐butyl acrylate/styrene balance. This approach offers flexibile fabrication of colloidal crystals with tunable wettability, and can be further extended to general materials. A facile strategy for finely controlling the wettability transition temperature of colloidal‐crystal films from superhydrophilic (water contact angle, CA, 0°) to superhydrophobic (water CA, 150.5°) is demonstrated. The colloidal‐crystal films are assembled from poly(styrene‐n‐butyl acrylate–acrylic acid) amphiphilic latex spheres. The wettability transition temperature of the films can be well tuned by adjusting the n‐butyl acrylate/styrene balance of the latex spheres. Superhydrophobic films are achieved when assembled at 90, 80, 70, 60, 40, or even 20 °C. This approach offers the flexibility of fabricating colloidal crystals with desired and tunable wettability, and can be further extended to general materials, opening up new perspectives in controlling the wettability behavior by chemical composition.