Magnetic-Directed Assembly from Janus Building Blocks to Multiplex Molecular-Analogue Photonic Crystal Structures

Anisotropic Microcarriers: Fabrication Strategies and Biomedical Applications

Anisotropic microcarriers (AMs) have attracted increasing attention. Although significant efforts have been made to explore AMs with various morphologies, their full potential is yet to be realized, as most studies have primarily focused on materials or fabrication methods. A thorough analysis of the interactional and interdependent relationships between these factors is required, along with proposed countermeasures tailored for researchers from various backgrounds. These countermeasures include specific fabrication strategies for various morphologies and guidelines for selecting the most suitable AM for certain biomedical applications. In this review, a comprehensive summary of AMs, ranging from their fabrication methods to biomedical applications, based on the past two decades of research, is provided. The fabrication of various morphologies is investigated using different strategies and their corresponding biomedical applications. By systematically examining these morphology-dependent effects, a better utilization of AMs with diverse morphologies can be achieved and clear strategies for breakthroughs in the biomedical field are established. Additionally, certain challenges are identified, new frontiers are opened, and promising and exciting opportunities are provided for fabricating functional AMs with broad implications across various fields that must be addressed in biomaterials and biotechnology.

Inverse Opal Torus-Shaped Photonic Microobjects with Superior Stimulus-Responsive Properties to Their Spherical Equivalents

Because of their many uses and simplicity of manufacture, colloidal photonic microobjects made from droplet templates have been the subject of extensive research. Owing to the low interfacial energy, the majority are spherical, however nonspherical forms, such as torus-shaped photonic microobjects (TSPMs) have also been seen. Although there have been reports of TSPMs based on various colloidal building blocks, their usual lack of stimulus-responsive qualities restricts their potential uses. In this work, hydrogel-based inverse opal TSPMs (IO-TSPMs) that are sensitive to alcohol and pH are created. IO-TSPMs that react more quickly than spherical ones are produced by first creating opal-structured TSPMs, then infiltrating monomers, polymerizing under UV light, and etching, to visibly illustrate the difference. Unlike spherical structures, which only provide unidirectional stimulus propagation, torus structures allow stimulation to bidirectionally propagate from both the inner and outer borders. Potential applications for these IO-TSPMs include biomimetic materials, quick diagnostic and inspection tools, and building blocks for innovative patterns.

Facile Access to High Solid Content Monodispersed Microspheres via Dual-Component Surfactants Regulation toward High-Performance Colloidal Photonic Crystals

Monodispersed microspheres play a major role in optical science and engineering, providing ideal building blocks for structural color materials. However, the method toward high solid content (HSC) monodispersed microspheres has remained a key hurdle. Herein, a facile access to harvest monodispersed microspheres based on the emulsion polymerization mechanism is demonstrated, where anionic and nonionic surfactants are employed to achieve the electrostatic and steric dual-stabilization balance in a synergistic manner. Monodispersed poly(styrene-butyl acrylate-methacrylic acid) colloidal latex with 55 wt% HSC is achieved, which shows an enhanced self-assembly efficiency of 280% compared with the low solid content (10 wt%) latex. In addition, Ag-coated colloidal photonic crystal (Ag@CPC) coating with near-zero refractive index is achieved, presenting the characteristics of metamaterials. And an 11-fold photoluminescence emission enhancement of CdSe@ZnS quantum dots is realized by the Ag@CPC metamaterial coating. Taking advantage of high assembly efficiency, easily large-scale film-forming of the 55 wt% HSC microspheres latex, robust Ag@CPC metamaterial coatings could be easily produced for passive cooling. The coating demonstrates excellent thermal insulation performance with theoretical cooling power of 30.4 W m-2, providing practical significance for scalable CPC architecture coatings in passive cooling.

Magnetic photonic crystals for biomedical applications

Magnetic photonic crystals (PhCs), as a representative responsive structural color material, have attracted increasing research focus due to merits such as brilliant refraction colors, instant responsiveness, and excellent manipuility, thus having been widely applied for color displaying, three-dimensional printing, sensing, and so on. Featured with traits such as contactless manner, flexible orientations, and adjustable intensity of external magnetism, magnetic PhCs have shown great superiority especially in the field of biomedical applications such as bioimaging and auxiliary clinical diagnosis. In this review, we summarize the current advancements of magnetic PhCs. We first introduce the fundamental principles and typical characteristics of PhCs. Afterward, we present several typical self-assembly strategies with their frontiers in practical applications. Finally, we analyze the current situations of magnetic PhCs and put forward the prospective challenges and future development directions.

Photo-and thermo-regulation by photonic crystals for extended longevity of C. elegans

Methods allowing light energy to be modulated in a controllable fashion are potentially important for finding the correlation between light-related environmental factors and aging-related lifespan. Here, we report photo- and thermo-regulation based on photonic crystals (PCs) for extended longevity of C. elegans. We show that PCs can function as a regulator of visible spectrum to tune photonic energy received by C.elegans. We provide direct evidence that lifespan depends on photonic energy, and the use of PCs reflecting blue light (440-537nm) gives 8.3% increasement in lifespan. We demonstrate that the exposure to modulated light alleviates photo-oxidative stress and unfolded-protein response. We realize reflective passive cooling temperature using PCs, and favorable low temperature could be created for worms to extend lifespan. This work offers a new path based on PCs to resist negative effects light and temperature for longevity, provides an available platform for studying the role of light in aging.

Electric, magnetic, and shear field-directed assembly of inorganic nanoparticles

Ordered assemblies of inorganic nanoparticles (NPs) have shown tremendous potential for wide applications due to their unique collective properties, which differ from those of individual NPs. Various assembly methods, such as external field-directed assembly, interfacial assembly, template assembly, biomolecular recognition-mediated assembly, confined assembly, and others, have been employed to generate ordered inorganic NP assemblies with hierarchical structures. Among them, the external field-directed assembly method is particularly fascinating, as it can remotely assemble NPs into well-ordered superstructures. Moreover, external fields (e.g., electric, magnetic, and shear fields) can introduce a local and/or global field intensity gradient, resulting in an additional force on NPs to drive their rotation and/or translation. Therefore, the external field-directed assembly of NPs becomes a robust method to fabricate well-defined functional materials with the desired optical, electronic, and magnetic properties, which have various applications in catalysis, sensing, disease diagnosis, energy conversion/storage, photonics, nano-floating-gate memory, and others. In this review, the effects of an electric field, magnetic field, and shear field on the organization of inorganic NPs are highlighted. The methods for controlling the well-ordered organization of inorganic NPs at different scales and their advantages are reviewed. Finally, future challenges and perspectives in this field are discussed.

Mechanical Janus Structures by Soft-Hard Material Integration

Engineering Janus structures that possess anisotropic features in functions have attracted growing attention for a wide range of applications in sensors, catalysis, and biomedicine, and are yet usually designed at the nanoscale with distinct physical or chemical functionalities in their opposite sides. Inspired by the seamless integration of soft and hard materials in biological structures, here a mechanical Janus structure composed of soft and hard materials with a dramatic difference in mechanical properties at an additively manufacturable macroscale is presented. In the combination of extensive experimental, theoretical, and computational studies, the design principle of soft-hard materials integrated mechanical Janus structures is established and their unique rotation mechanism is addressed. The systematic studies of assembling the Janus structure units into superstructures with well-ordered organizations by programming the local rotations are further shown, providing a direct route of designing superstructures by leveraging mechanical Janus structures with unique soft-hard material integration. Applications are conducted to demonstrate the features and functionalities of assembled superstructures with local ordered organizations in regulating and filtering acoustic wave propagations, thereby providing exemplification applications of mechanical Janus design in functional structures and devices.

Preparation of Janus Droplets and Hydrogels with Controllable Morphologies by an Aqueous Two-Phase System on the Superamphiphobic Surface

Janus particles, having the property integration of each component, have attracted increasing attention due to their considerable potential in the field of material engineering applications. However, organic solvents or sophisticated equipment during the fabrication processes is generally inevitable. Here, we report a facile route to prepare Janus droplets and hydrogels via aqueous two-phase systems (ATPS). Simply merging two polymers, i.e., polyethylene glycol (PEG) and dextran (DEX), as aqueous droplets on a superamphiphobic surface leads to phase separation, provided that their concentrations exceed the threshold in the mixed aqueous droplets, thus generating a Janus structure. Various morphologies of such Janus droplets can be well controlled by manipulating the locations of these two polymers' concentration on the phase diagram, and the evolution of the mixed droplets are deterministic on the basis of the kinetics of their phase separation and the degree of hydrophobicity of the substrate. Introducing monomers and/or nanoparticles, further, into a certain phase of the ATPS droplet followed by photopolymerizing enables Janus hydrogel particles with diverse functionalities to be obtained. The ease and green techniques with which the Janus balance and curvature between two phases of the Janus droplet can be finely tuned point to new directions in designing Janus particles and hold great promises in biological engineering.

Janus Photonic Microspheres with Bridged Lamellar Structures via Droplet-Confined Block Copolymer Co-Assembly

Artificial self-assembly systems typically exhibit limited capability in creating nature-inspired complex materials with advanced functionalities. Here, an effective co-assembly strategy is demonstrated for the facile creation of complex photonic structures with intriguing light reflections. Two different lipophilic and amphiphilic bottlebrush block copolymers (BCPs) are placed within shrinking droplets to enable a cooperative working mechanism of microphase segregation and organized spontaneous emulsification, respectively. Layer assemblies of the lipophilic BCP and uniform water nanodroplets stabilized by the bottlebrush surfactant are both generated, and co-assembled into a bridged lamellar structure with the alternating arrangement of layers and closely packed nanodroplet arrays. Janus microspheres with diverse dual optical characteristics are successfully fabricated, and reflected wavelengths of light are highly tunable simply by changing the formulation or molecular weight of BCP.

Arrested Coalescence of Ionic Liquid Droplets: A Facile Strategy for Spatially Organized Multicompartment Assemblies

Multicompartment assemblies attract much attention for their wide applications. However, the fabrication of multicompartment assemblies usually requires elaborately designed building blocks and careful controlling. The emergence of droplet networks has provided a facile way to construct multiple droplet architectures, which can further be converted to multicompartment assemblies. Herein, the bind motif-free building blocks are presented, which consist of the hydrophobic Tf₂ N^- -based ionic liquid (IL) dissolving LiTf₂ N salt, that can conjugate via arrested coalescence in confined-space templates to form IL droplet networks. Subsequent ultraviolent polymerization generates robust free-standing multicompartment assemblies. The conjugation of building blocks relies not on the peripheral bind motif but on the interfacial instability-induced arrested coalescence, avoiding tedious surface modification and assembly process. By tuning structures of templates and building blocks, multicompartment assemblies with 0D, 1D, 2D, and 3D structures are prepared in a facile and high-throughput way. Importantly, the bottom-up construction enables modular control over the compositions and spatial positions of individual building blocks. Combining with the excellent solvency of ILs, this system can serve as a general platform towards versatile multicompartment architectures. As demonstrations, by tailoring the chambers the multicompartment assemblies can spatiotemporally sense and report the chemical cues and perform various modes of motion.

Photonic Crystal of Polystyrene Nanomembrane: Signal Amplification and Low Triggered Potential Electrochemiluminescence for Tetracycline Detection

In this work, a low triggered potential electrochemiluminescence strategy based on gold-filled photonic crystals (GPCs) electrodes composed of photonic crystals self-assembled with polystyrene spheres and gold nanoparticles embedded in the gaps of the photonic crystals was proposed. The GPCs electrodes served as the detection platform to bind antigen, and Ru(bpy)₃²⁺-COOH as a luminophore was labeled on the antibody (Ab). Then, Ru(bpy)₃²⁺-COOH/Ab was connected to the immobilized antigen on the surface of the photonic crystals by the immunoreaction to avoid direct contact with the gold nanoparticles surface. ECL emission can only be initiated by electrochemical oxidation of tripropylamine (TPrA) since Ru(bpy)₃²⁺-COOH cannot be oxidized directly on the electrode surface. The TPrA^·+ and TPrA^· radicals generated by the oxidation of TPrA can spread to the vicinity of Ru(bpy)₃²⁺-COOH over a short distance and react with the Ru(bpy)₃²⁺-COOH, eventually producing ECL emission. The potential of ECL emission caused by TPrA oxidation was about 300 mV lower than that caused by Ru(bpy)₃²⁺-COOH oxidation because the oxidation potential of TPrA (0.95 V vs SCE) was lower than Ru(bpy)₃²⁺-COOH (1.25 V vs SCE). Furthermore, the photonic crystals nanomembrane has the capability to enhance electrochemiluminescence. Thereafter, tetracycline antibiotic as a model compound was successfully detected via competitive immunoassay on GPCs electrodes with a detection limit of 0.075 pg/mL (S/N = 3), which has broad application prospects in the field of analysis and detection.

Kinetic Control of Length and Morphology of Segmented Polymeric Nanofibers in Microfluidic Chips

In this work, we report an approach to prepare segmented polymer nanofibers (SPNFs) composed of rodlike subunits by kinetically controlled self-assembly of polystyrene-b-poly(4-vinylpyridine)-based supramolecules in microfluidic chips. The length and morphology of the SPNFs could be effectively adjusted by changing the total flow rate (V_total) and the molar ratio (x) of 4-vinylpyridine (4VP) unit to a hydrogen-bonding molecule, 3-n-pentadecyphenol. Moreover, the subunits of SPNFs could transform from short rods to spheres when the interfacial tension between PS core and solvent increased. On the contrary, the SPNFs elongated along the major axis when the interfacial tension decreased. This work not only offers mechanism insights into the hierarchical self-assembly of block copolymer-based supramolecules but also provides a versatile and effective method for kinetically controlling the hierarchical structures of assemblies.

Robust hydrophobic veova10-based colloidal photonic crystals towards fluorescence enhancement of quantum dots

Hydrophobic photonic crystals (PCs) has been increasingly appreciated as a promising functional material due to their distinct surface characteristic of structural color and hydrophobicity. However, it remains a challenge to fabricate hydrophobic PCs via a one-step process. Inspired by the development of high-performance waterborne coatings, we propose an easy-to-perform and high-efficiency strategy to construct hydrophobic building blocks (diameter of 221, 247, 276 and 305 nm), where the umbelli-form hydrophobic long chain (veova10 C_n > 9) was loaded onto polystyrene (PS) colloidal particles in situ. Taking advantage of the hydrophobic driving force between the colloidal particles, large-scale colloidal photonic crystals (CPCs) film with crack-free morphology was obtained efficiently. The derived CPCs exhibit robust mechanical stability, prominent hydrophobicity and excellent optical properties. In addition, the colloidal latex holds great potential toward PCs coatings on a variety of substrates (glass, plastic and steel) with excellent adhesiveness. Furthermore, we contrive to construct angle-independent structural color films and supraballs, and explore their application in quantum dots (QDs) fluorescence enhancement, which achieved an enhancement effect by more than eight times. From the standpoint of practical applications, we achieved the flexible high-brightness wearable light-emitting diode (LED) displays. This work will lay a foundation for the development of high-efficiency PCs building blocks, and indicate the direction for the meaningful application of CPCs.

Customized Chiral Colloids

This contribution describes a synthetic strategy for the fabrication of multicomponent colloidal "molecules" with controllable complex morphologies and compositionally distinct lobes. Using 3-(trimethoxysilyl)propyl methacrylate (TPM) as the building block, the methodology enables a scalable bulk synthesis of customized chiral colloidal particles with geometric and compositional chirality by a sequential seeded growth method. The synthetic protocol presents a versatile platform for constructing colloidal molecules with multiple components having customized shapes and functionalities, with the potential to impact the design of chromatic patchy particles, colloidal swimmers, and chiral optical materials, as well as informing programmable assembly.

Three-dimensional ordered macroporous magnetic photonic crystal microspheres for enrichment and detection of mycotoxins (I): Droplet-based microfluidic self-assembly synthesis

Ordered porous materials are attracting enormous attention due to their uniform pore structures, particularly the magnetic photonic crystal microspheres (PCMs) which not only possess unique photonic crystal structure but also can achieve separation easily based on magnet. Here, a two-phase microfluidic self-assembly synthetic system was established simply and employed for the preparation of three dimensional PCMs (3DPCMs) by using the emulsion droplet approach. One phase (dispersed phase) was an aqueous emulsion containing Fe₃O₄, silica (SiO₂) and polystyrene (PS) nanoparticles; another phase (continuous phase) was pure silicone oil. The droplets were formed by introducing the dispersed phase into the continuous phase through a tee valve. By heating the droplets, the water would evaporate and the nanoparticles would finally assemble into solid microspheres, which could be changed into macroporous 3DPCMs after removal of the PS nanoparticles by calcination. The contents and particle sizes of Fe₃O₄, SiO₂ and PS nanoparticles in the dispersed phase were investigated in detail and optimized to prepare macroporous magnetic 3DPCMs with high quality. The morphologies, surface crystal structure, magnetic property, particle size distribution, specific surface area and pore size of the macroporous magnetic 3DPCMs were characterized. The expected 3DPCM displayed regular and uniform photonic crystal structure, narrow particle size distribution and strong magnetic property. The macroporous magnetic 3DPCMs grafted with vomitoxin (DON)-antibodies could be applied for selective enrichment of DON in real samples.

Photonic crystal enhanced gold-silver nanoclusters fluorescent sensor for Hg2+ ion

Luminescent nanoclusters (NCs) have attracted much attention because of their good photostability and low toxicity, however, the low quantum yield is still a deficiency, and many increasing efforts are being devoted to enhance the luminescence intensity of NCs. In this paper, a method of enhancing the fluorescent signal of gold-silver nanoclusters (AuAgNCs) by photonic crystals (PhCs) was proposed. The fluorescent intensity of AuAgNCs on PhCs can be enhanced 8.0-fold in comparison to the control sample without PhCs. Furthermore, a novel fluorescence sensor of AuAgNCs based on PhCs is used for the sensitive and selective detection of Hg²⁺ ion in the aqueous solution, the detection limit is 0.35 nM due to the PhCs enhancement effect for the fluorescence. This proposed method may not only develop a highly sensitive method for determination of Hg²⁺ ion, but also expand the application of AuAgNCs in ultra-trace analysis.

Controlled Group Motion of Anisotropic Janus Droplets Prepared by One-Step Vortex Mixing

In living systems, highly efficient biological micromotors are fascinating and crucial to the maintenance and regulation of normal functions. Inspired by this, solid colloid motors with controlled movements were recently developed for diverse applications. However, to meet the requirements of more elaborate functionalities, the development of droplet-based micromotors, which feature with appealing advantages such as deformability, encapsulation capability, and biocompatibility, is demanding. Herein, responsive Janus droplets with intrinsic magnetic anisotropy were fabricated, taking advantage of the traditional one-step vortex mixing that guarantees large-scale production. Furthermore, the size range of the droplets can be easily extended continuously from hundreds of micrometers down to tens of nanometers. What is more appealing, directed in situ group motions that include alignment, rotation, and transfer of the Janus droplets prepared were successfully realized and precisely controlled by using an external magnetic field. These collective motions induced excellent performances in pollutant adsorption and separation, switchable conductivities, and the size grading. Such scalable, simple, and controllable strategy can expand the application of Janus emulsions to complicated fields of microreactors, microsensors, and environmental regulation.

Creation of Nonspherical Microparticles through Osmosis-Driven Arrested Coalescence of Microfluidic Emulsions

Droplet-based microfluidics enable the production of emulsions and microparticles with spherical shapes, but the high-throughput fabrication of nonspherical emulsions and microparticles still remains challenging because interfacial tension plays a dominant role during preparation. Herein, ionic liquids (ILs) containing salts, which possess sufficient osmotic pressure to realize water transport and phase separation, are introduced as inner cores of oil-in-oil-in-water double emulsions and it is shown that nonspherical emulsions can be constructed by osmosis-driven arrested coalescence of inner cores. Subsequently, ultraviolet polymerization of the nonspherical emulsions leads to nonspherical microparticles. By tailoring the number, composition, and size of inner cores as well as coalescence time, a variety of nonspherical shapes such as dumbbell, rod, spindle, snowman, tumbler, three-pointed star, triangle, and scalene triangle are created. Importantly, benefitting from excellent solvency of ILs, this system can serve as a general platform to produce nonspherical microparticles made from different materials. Moreover, by controlling the osmotic pressure, programmed coalescence of inner cores in double emulsions is realizable, which indicates the potential to build microreactors. Thus, a simple and high-throughput strategy to create nonspherical microparticles with arrested coalescence shapes is developed for the first time and can be further used to construct novel materials and microreactors.

Microfluidics-Assisted Assembly of Injectable Photonic Hydrogels toward Reflective Cooling

Development of fast curing and easy modeling of colloidal photonic crystals is highly desirable for various applications. Here, a novel type of injectable photonic hydrogel (IPH) is proposed to achieve self-healable structural color by integrating microfluidics-derived photonic supraballs with supramolecular hydrogels. The supramolecular hydrogel is engineered via incorporating β-cyclodextrin/poly(2-hydroxypropyl acrylate-co-N-vinylimidazole) (CD/poly(HPA-co-VI)) with methacrylated gelatin (GelMA), and serves as a scaffold for colloidal crystal arrays. The photonic supraballs derived from the microfluidics techniques, exhibit excellent compatibility with the hydrogel scaffolds, leading to enhanced assembly efficiency. By virtue of hydrogen bonds and host-guest interactions, a series of self-healable photonic hydrogels (linear, planar, and spiral assemblies) can be facilely assembled. It is demonstrated that the spherical symmetry of the photonic supraballs endows them with identical optical responses independent of viewing angles. In addition, by taking the advantage of angle independent spectrum characteristics, the IPH presents beneficial effects in reflective cooling, which can achieve up to 17.4 °C in passive solar reflective cooling. The strategy represents an easy-to-perform platform for the construction of IPH, providing novel insights into macroscopic self-assembly toward thermal management applications.

MOF-Based Photonic Crystal Film toward Separation of Organic Dyes

Metal-organic framework (MOF)-directed photonic structure materials have inspired great attention for extended and enhanced functions. However, the direct construction of photonic crystals (PCs) with MOF particles as building blocks still remains a challenge. Herein, we designed and synthesized monodisperse polyamidoamine (PAMAM) dendrimer-modified zeolitic imidazolate framework (ZIF-8) particles (PAMAM@ZIF-8) via a postsynthetic method, rendering ZIF-8 with hydrophilicity. It was found that the PAMAM@ZIF-8 particles could directly assemble into a uniform photonic structure and effectively suppressed the coffee-ring effect, forming homogeneous PC films with different structural colors. A PC pattern with angle-dependent colors was also achieved, which might have potential applications in the field of anticounterfeiting printing. More importantly, by taking advantages of a membrane separation-assisted assembly process, a colorful and robust PC film was accomplished on the surface of reduced graphene oxide (rGO). The hierarchal PAMAM@ZIF-8/rGO film demonstrates a superior separation ability toward organic dye solutions, which enriches the function of PC materials. This work gives a new insight into the fabrication of MOF-based functional PC materials, which will extend the application of PCs in the high selective and effective separation field.

Janus dimers from tunable phase separation and reactivity ratios

Janus dimers, as a typical species of anisotropic material, are useful for both theoretical simulations and practical applications.

Ratiometric sensors with selective fluorescence enhancement effects based on photonic crystals for the determination of acetylcholinesterase and its inhibitor

Ratiometric fluorescent sensors are powerful tools for quantitative analyses.

Spherical Colloidal Photonic Crystals with Selected Lattice Plane Exposure and Enhanced Color Saturation for Dynamic Optical Displays

While structural color materials have nonfading properties and contribute significantly to the sustainable development of pigments or dyes, they are plagued by low color saturation and limited color tunability. Here, we describe a new type of spherical colloidal photonic crystals (CPCs) prepared by a droplet-based microfluidic strategy, featuring enhanced color saturation and tunable structural colors. Methyl viologen (MV) functionalized SiO₂ colloids were synthesized and used for the preparation of CPCs in microdroplets. Because of the absorption of incoherently scattered light by MV, the ratio of peak-to-background amplitude in the reflectance spectra of CPCs is increased, leading to brilliant structural color with enhanced saturation. The lattice plane exposure of spherical CPCs depends on the refractive index contrast between the filling medium and SiO₂ building blocks, and this offers an alternative way to tune the structural color in a spherical CPC. Accordingly, a dynamic optical display was constructed, providing valuable insights to the future development of structural color-based sensors, surface coatings, or displays.

Self-assembly of colloids based on microfluidics

Self-assembly of colloids provides a powerful way for the construction of complex multi-scale materials. Microfluidic techniques possess great potential to precisely control the assembly of micro- and nano-scale building blocks via the rational design of various microfluidic environments. In this review, we first discuss the self-assembly of colloids without templates by using the laminar microfluidic technique. The self-assembly of colloids based on a droplet as a template was subsequently summarized and discussed via droplet microfluidic technique. Moreover, the evaporation-driven self-assembly of colloids in microfluidic channels has been discussed and analysed. Finally, the representative applications in this field have been pointed out. The aim of this review is to summarize the state-of-art on the self-assembly of colloids based on various microfluidic techniques, exhibit their representative applications, and point out the current challenges in this field, hoping to inspire and guide future work.

Hydrophobic Poly(tert-butyl acrylate) Photonic Crystals towards Robust Energy-Saving Performance

Photonic crystals (PCs) have been widely applied in optical, energy, and biological fields owing to their periodic crystal structure. However, the major challenges are easy cracking and poor structural color, seriously hindering their practical applications. Now, hydrophobic poly(tert-butyl acrylate) (P(t-BA)) PCs have been developed with relatively lower glass transition temperature (T_g ), large crack-free area, excellent hydrophobic properties, and brilliant structure color. This method based on hydrophobic groups (tertiary butyl groups) provides a reference for designing new kinds of PCs via the monomers with relatively lower T_g . Moreover, the P(t-BA) PCs film were applied as the photoluminescence (PL) enhanced film to enhance the PL intensity of CdSe@ZnS QDs by 10-fold in a liquid-crystal display (LCD) device. The new-type hydrophobic force assembled PCs may open an innovative avenue toward new-generation energy-saving devices.

Rapid preparation of auto-healing gels with actuating behaviour

Gels with multiple stimuli-responsive actuating behaviour have shown great potential in many applications. Nevertheless, facile approaches to rapidly preparing gel actuators are still highly needed, and obtaining gels possessing both actuating and auto-healing capabilities remains a challenge. Herein, we report the rapid preparation of gel actuators with a self-healing ability. Dual-component gels, composed of poly(BA-co-VI-co-AM) (G-1) and poly(BA-co-AA-co-AM/β-CD) (G-2) (BA = butyl acrylate, VI = N-vinyl imidazole, AM = acrylamide, AA = acrylic acid, β-CD = β-cyclodextrin), are prepared within 10 minutes (min) via biphase frontal polymerization (FP). Both G-1 and G-2 gels show excellent intrinsic self-healing properties based on hydrogen bonds, with healing efficiencies of 91% and 97%, respectively; self-healing between G-1 and G-2 also occurs due to hydrogen bonding and host-guest interactions. Moreover, dual-component gels, in terms of G-1 and G-2 bilayer gel flowers and strips, heterogeneous healed bilayer gel strips, and microfluidic-directed bilayer gel microsphere ensembles, all show actuating behaviour in acidic, alkaline and organic solutions, with actuation degrees up to 96% in 5 min. The actuation mechanism is also proposed. This work might provide new insights into fast synthesis of self-healing dual-component gels towards application in the actuator field.

Separation-cooperated assembly of liquid photonic crystals from polydisperse particles

Easy and cost-effective production of high-quality photonic crystals (PCs) remains challenging but attractive, not just because they are a type of gemstone but more for their scientific applications (e.g., serving as lossless waveguides, visual sensors, novel pigments and novel separation media). Herein presented is a separation-cooperated assembly (SCA) strategy able to organize cheap polydisperse particles into PCs. Its feasibility was validated through sink-induced SCA of poorly disperse (size variation up to 56%) particles into iridescent liquid PCs in 3 days or more. Strikingly, with a sharp photonic band gap down to 10 nm (ca. 1/7 of the reported 66 nm), the liquid PCs are able to cyclically recover their iridescent color in ca 20 s after agitation, and keep their structural order after dryness, making them practicable to write and paint directly. Also significant is that SCA yielded uniform particles with size variation down to 0.7%. It is thus an easy way to isolate homogeneous particles from disperse ones.

Gold-loaded microspheres via carbosilane-thioether dendrimers as stabilizers and their performance in layer-controllable photonic crystals

Gold nanoparticle (Au NP) incorporated photonic crystals (PCs) have been extensively studied due to the intricate interplay between the surface plasmon resonance of Au NPs and the periodic nanostructure of PCs. Herein, we successfully synthesized Au NP decorated poly(styrene-co-(generation 3 carbosilane-thioether vinyl-terminated dendrimer)) (P(st-co-G3Vi)) microspheres via in situ reduction of Au ions based on the strong coordination between the Au ions and the sulfur atom in G3Vi dendrimers. These composite Au-doped microspheres demonstrate a bumpy surface topography, which gives rise to a higher hydrophobicity and could effectively suppress the formation of an ubiquitous coffee-ring during the drying process of a colloidal suspension. More importantly, layer-controllable PCs were constructed with Au-doped microspheres by combining the Langmuir-Blodgett method with a layer-by-layer stacking strategy. By manipulating the stacking layers and diameters of microspheres, multifarious PCs with different photonic band gaps and reflection intensities were obtained, which can serve as an effective substrate for amplified quantum dot fluorescence. Further investigation reveals that fluorescence could be significantly pronounced by five-layer PCs. This work offers a facile and reproducible strategy to prepare Au NP incorporated PCs by in situ synthesis of Au NPs within dendrimer-functionalized microspheres, resulting in an enhancement of quantum dot fluorescence, which will lead to promising applications in energy-saving optoelectronic devices.

Interfacial Emulsification: An Emerging Monodisperse Droplet Generation Method for Microreactors and Bioanalysis

The generation of uniform droplets has been extensively investigated owing to its profound potentials both in scientific research and engineering applications. Although various methods have been put forward to expand this area, new innovations are still needed to improve the technical convenience and save instrumental cost. In this feature article, we highlight an interfacial emulsification technique that we developed in the past several years. This technique serves as a platform for preparing uniform droplets that are formed on the air-liquid interface of the continuous phase based on interfacial shearing. Three specific aspects of interfacial emulsification are reviewed, including its basic design and principle, the preparation of droplets with controllable size and adjustable components, and practical applications of the method in bioanalysis, microreactors, and particle synthesis. Compared to other droplet generation methods, several attractive advantages and perspectives for further development have been summarized.

Lyophilic but Nonwettable Organosilane-Polymerized Carbon Dots Inverse Opals with Closed-Cell Structure

This paper presents a unique lyophilic but nonwettable property of organosilane-polymerized carbon dots inverse opals photonic crystals (SiCDPCs) with closed-cell structure. Little stopband shift was observed for the SiCDPCs when being immersed into the solvents such as isopropanol, olive oil, DMSO, hexane, silicone oil, ethanediol, etc. but keeping lyophilic property. This could be attributed to the combined effect of closed-cell structure and the unique chemical composition of SiCDPCs. Furthermore, more than 30 kinds of organic solvents had been investigated, it was found that there were two kinds of factors that affected the stopband shift upon solvent's immersing; one was the polarity of solvent, and the other one was the viscosity of solvent. That is, mainly nonpolar or high viscosity solvents showed lyophilic but nonwettable property. The distinct solvent-responsive behaviors of the SiCDPCs toward polar/nonpolar solvents had been utilized for the fabrication of 2D/3D pattern. Additionally, the as-prepared SiCDPCs showed improved optical limiting property, excellent low-temperature resistance, and abrasion tolerant property. It is of great importance for the development of multifunctional novel coating materials and creation of novel optical devices.

Janus particles self-assembled from a small organic atypical asymmetric gemini surfactant

A series of atypical asymmetric gemini surfactants with an amphiphilic carbonate group (-O-CO-O-) have been prepared. Some of these compounds could self-assemble in water into gourd-shaped Janus particles (JPs). Initial results suggested that the formation of JPs was highly likely to be related to their atypical gemini surfactant structure. To our knowledge, this is the first report on JPs that are self-assembled from a single kind of small organic molecule. We believe that our results will be utilized in many fields.

Magnetic-Patchy Janus Colloid Surfactants for Reversible Recovery of Pickering Emulsions

We present a straightforward and robust method for the synthesis of Janus colloid surfactants with distinct amphiphilicity and magnetic responsiveness. To this end, hydroxyl-functionalized amphiphilic Janus microparticles (JMPs) are synthesized by seeded monomer swelling and subsequent photopolymerization. By incorporating controlled amounts of hydroxyl groups on poly(styrene-co-vinyl alcohol) seed particles, we adjust the interfacial tension between the seed polymer and the poly(tetradecyl acrylate) secondary polymer (γ₁₃). From theoretical and experimental observations, we verify that when γ₁₃ is tuned to ∼8.5 mN/m in a medium with controlled solvency, which corresponds to a 0.6 volume fraction of ethanol in water, the particles bicompartmentalize to form oval or ellipsoidal JMPs with controllable bulb dimensions. We also show that bulb site-specific patching of magnetic nanoparticles (NPs) can be achieved using the electrostatic interaction between the polyethylenimine-coated bulb surface and the polyvinylpyrrolidone-stabilized Fe₂O₃ NPs. Finally, we demonstrate that our magnetic-patchy JMPs can assemble at the oil-water interface, enabling magnetic-responsive reversible recovery of Pickering emulsions.

Interfacially active polydopamine for nanoparticle stabilized nanocapsules in a one-pot assembly strategy toward efficient drug delivery

Polydopamine (PDA) nanoparticle stabilized nanocapsules possess great potential for drug delivery via the non-endocytotic pathway.

Multichannel Dynamic Interfacial Printing: An Alternative Multicomponent Droplet Generation Technique for Lab in a Drop

Generation of uniform emulsion droplets mixed with multiple components is one of the key issues in the field of lab in a drop. Traditionally, droplet microfluidic chips are often served as the prime choice while designing and fabricating microfluidic chips always rely on skilled technician and specialized equipment, severely restricting its wide accessibility. In this work, an alternative technique, called multichannel dynamic interfacial printing (MC-DIP), was proposed for multicomponent droplet generation. The MC-DIP device was designed modularly and could be set up manually without any microfabrication process, exhibiting full accessibility for freshmen after a brief training. This new technique owns advantages in the generation of droplets with predictable sizes and composites. Quantitative experiments of measuring minimum inhibitory concentration (MIC) value via mixing microbes and antibiotics into droplet were conducted to proving its application potential for lab in a drop. Further research on a clinical pathogenic strain revealed that this technique could be potentially applied in the clinical laboratory for antibiotic susceptibility testing.

Oriented Enzyme Immobilization at the Oil/Water Interface Enhances Catalytic Activity and Recyclability in a Pickering Emulsion

Enzyme-loaded water-in-oil Pickering emulsion is a promising system for biphasic catalytic reactions. In this paper, we report on oriented enzyme immobilization at the oil/water interface in a Pickering emulsion, in which CHO-Janus silica nanoparticles (CHO-JNPs) are utilized as a stabilizer of the emulsion and support for the enzyme to enhance both catalytic activity and recyclability. The catalytic performance of this immobilized enzyme (lipase from Candida sp.) was evaluated by esterification of hexanoic acid and 1-hexanol in a water/heptane biphasic medium. The results show that the specific catalytic activity of the immobilized enzyme (33.2 U mL^-1) was 6.5 and 1.4 times higher than that of free enzyme (5.1 U mL^-1) and encapsulated enzyme in the liquid core (23.3 U mL^-1), respectively. Moreover, the immobilized enzyme demonstrated good stability and recyclability, retaining 75% of its activity after 9 cycles. We expect that oriented enzyme immobilization at the oil/water interface will be an important strategy for enhancing catalytic performance in Pickering emulsions.

Emerging Droplet Microfluidics

Droplet microfluidics generates and manipulates discrete droplets through immiscible multiphase flows inside microchannels. Due to its remarkable advantages, droplet microfluidics bears significant value in an extremely wide range of area. In this review, we provide a comprehensive and in-depth insight into droplet microfluidics, covering fundamental research from microfluidic chip fabrication and droplet generation to the applications of droplets in bio(chemical) analysis and materials generation. The purpose of this review is to convey the fundamentals of droplet microfluidics, a critical analysis on its current status and challenges, and opinions on its future development. We believe this review will promote communications among biology, chemistry, physics, and materials science.

Magnetoresponsive Photonic Microspheres with Structural Color Gradient

Photonic Janus particles are created by alternately sputtering silica and titania on microspheres in order to obtain a structural color gradient. In addition, the microspheres are rendered magnetoresponsive. The Janus microspheres with optical and magnetic anisotropy enable on-demand control over orientation and structural color through manipulation of an external magnetic field, thereby being useful as active color pigments for reflection-mode displays.