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.     

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.     

Feedback Controlled Colloidal Self‐Assembly

Colloidal self‐assembly provides one promising route to fabricate spatially periodic meta‐materials with novel properties important to a number of emerging technologies. However, colloidal assembly is generally initiated via irreversible step‐changes and proceeds along unspecified, non‐equilibrium kinetic pathways with little opportunity to manipulate defects or reconfigure microstructures. Here, a conceptually new approach that enables the use of feedback control to quantitatively and reversibly guide the dynamic evolution of colloid ensembles between disordered fluid and crystalline configurations is demonstrated. The key to this approach is the use of free energy landscape models to inform feedback control laws that close the loop between real‐time sensing (via order parameters) and actuation (via tunable electrical potentials). This approach, which demonstrates controlled assembly to create ordered materials and perform active reconfiguration, is based on chemical physics that suggest it can be generalized to other microscopic processes.     

Direct Transcription of Two‐Dimensional Colloidal Crystal Arrays into Three‐Dimensional Photonic Crystals

A simple protocol for the fabrication of three‐dimensional (3D) photonic crystals in silicon is presented. Surface structuring by nanosphere lithography is merged with a novel silicon etching method to fabricate ordered 3D architectures. The SPRIE method, sequential passivation reactive ion etching, is a one‐step processing protocol relying on sequential passivation and reactive ion etching reactions using C₄F₈ and SF₆ plasma chemistries. The diffusion of fresh reactants and etch product species inside the etched channels is found to play an important role affecting the structural uniformity of the designed structures and the etch rate drift is corrected by adjusting the reaction times. High quality photonic crystals are thus obtained by adding the third dimension to the two‐dimensional (2D) colloidal crystal assemblies through SPRIE. Careful adjustments of both mask design and lateral etch extent balance allow the implementation of even more complex functionalities including photonic crystal slabs and precise defect engineering. 3D photonic crystal lattices exhibiting optical stop‐bands in the infrared spectral region are demonstrated, proving the potential of SPRIE for fast, simple, and large‐scale fabrication of photonic structures.     

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.     

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.     

Controlling the Self‐Assembly of Periodic Defect Patterns in Smectic Liquid Crystal Films with Electric Fields

Large‐area periodic defect patterns are produced in smectic A liquid crystals confined between rigid plate electrodes that impose conflicting parallel and normal anchoring conditions, inducing the formation of topological defects. Highly oriented stripe patterns are created in samples thinner than 2 μm due to self‐assembly of linear defect domains with period smaller than 4 μm, whereas hexagonal lattices of focal conic domains appear for thicker samples. The pattern type (1d/2d) and period can be controlled at the nematic–smectic phase transition by applying an electric field, which confines the defect domains to a thin surface layer with thickness comparable to the nematic coherence length. The pattern morphology persists in the smectic phase even after varying the field or switching it off. Bistable, non‐equilibrium patterns are stabilized by topological constraints of the smectic phase that hinder the rearrangement of defects in response to field variations.     

Electric Field‐Directed Convective Assembly of Ellipsoidal Colloidal Particles to Create Optically and Mechanically Anisotropic Thin Films

A method of simultaneous field‐ and flow‐directed assembly of anisotropic titania (TiO₂) nanoparticle films from a colloidal suspension is presented. Titania particles are oriented by an alternating (ac) electric field as they simultaneously advect towards a drying front due to evaporation of the solvent. At high field frequencies (ν > ～25 kHz) and field strengths (E > 300 V cm^?1), the particles orient with their major axis along the field direction. As the front recedes, a uniform film with thicknesses of 1–10 µm is deposited on the substrate. The films exhibit a large birefringence (Δn ≈ 0.15) and high packing fraction (? = 0.75 ± 0.08), due to the orientation of the particles. When the frequency is lowered, the particle orientation undergoes a parallel–random–perpendicular transition with respect to the field direction. The orientation dependence on field frequency and strength is explained by the polarizability of ellipsoidal particles using an interfacial polarization model. Particle orientation in the films also leads to anisotropic mechanical properties, which are manifested in their cracking patterns. In all, it is demonstrated that the field‐directed assembly of anisotropic particles provides a powerful means for tailoring nanoparticle film properties in situ during the deposition process.     

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.     

Protein Particles Formed by Protein Activation and Spontaneous Self‐Assembly

In this article, a non‐chemical crosslinking method is used to produce pure protein microparticles with an innovative approach, so‐called protein activation spontaneous and self‐assembly (PASS). The fabrication of protein microparticles is based on the idea of using the internal disulfide bridges within protein molecules as molecular linkers to assemble protein molecules into a microparticle form. The assembly process is triggered by an activating reagent–dithiothreitol (DTT), which only involved in the intermediate step without being incorporated into the resulting protein microparticles. Conventional protein microparticle fabrication methods usually involve emulsification process and chemical crosslink reactions using amine reactive reagents such as glutaraldehdye or EDC/NHS. The resulting protein microparticles are usually having various size distributions. Most importantly crosslinking reactions using amine reactive reagents will result in producing protein microparticles with undesired properties such as auto‐fluorescence and high toxicity. In contrast to the conventional methods, our technology provides a simple and robust method to produce highly homogeneous, stable and non‐fluorescence pure protein microparticles under mild conditions at physiological pH and temperature. The protein microparticles are found to be biodegradable, non‐toxic to MDCK cells and with preserved biological activities. Results on the cytotoxcity study and enzyme function demonstrate the potential applications of the protein microparticles in the area of pharmaceutics and analytical chemistry.     

Synthetic Self‐Limiting Structures Engineered with Defective Colloidal Clusters

One of the key challenges in the study of self‐assembly with synthetic particles is how to build finite‐sized constructs that resemble self‐limiting structures such as well‐known proteins and biomolecules found in nature. Inspired by this concept, a novel method for realizing self‐limiting self‐assembly of colloidal clusters by establishing design rules to obtain desired final structures using a bottom‐up assembly approach is presented. The constructs identified in this work will be “locked” in a well‐defined configuration as the structure morphologies will not allow them to grow any further. The approach presented here provides two distinct advantages. First, the self‐limiting characteristics of the resulting constructs are preserved no matter how many building blocks are present within the system. Second, the setup of interparticle interactions reflected by interaction matrices are much simpler compared to finite‐sized structures of simple spherical particles which may require engineering pairwise interactions between as many particle types as the number of particles present in the system. The self‐assembly process as well as phase transformation and kinetics of several intriguing finite‐sized configurations are studied. Possible extensions of this concept to produce sophisticated multi‐phased structures where different types of finite‐sized and large assemblies may be present at once are also discussed.     

Hierarchical Self‐Organization of Nanomaterials into Two‐Dimensional Arrays Using Functional Polymer Scaffold

Fabrication of two and three‐dimensional nanostructures requires the development of new methodologies for the assembly of molecular/macromolecular objects on substrates in predetermined arrangements. Templated self‐assembly approach is a powerful strategy for the creation of materials from assembly of molecular components or nanoparticles. The present study describes the development of a facile, template directed self‐assembly of (metal/organic) nanomaterials into periodic micro‐ and nanostructures. The positioning and the organization of nanomaterials into spatially well‐defined arrays were achieved using an amphiphilic conjugated polymer‐aided, self‐organization process. Arrays of honeycomb patterns formed from conjugated C₁₂PPPOH film with homogenous distribution of metal/organic nanomaterials. Our approach offers a straightforward and inexpensive method of preparation for hybrid thin films without environmentally controlled chambers or sophisticated instruments as compared to multistep micro‐fabrication techniques.     

Built‐In Electric Fields Dramatically Induce Enhancement of Osseointegration

Rapid and effective osseointegration is a great challenge in clinical practice. Endogenous electronegative potentials spontaneously appear on bone defect sites and mediate healing. Thus, bone healing can potentially be stimulated using physiologically relevant electrical signals in implants. However, it is difficult to directly introduce physiologically relevant electric fields in bone tissue. In this study, built‐in electric fields are established between electropositive ferroelectric BiFeO₃ (BFO) nanofilms and electronegative bone defect walls to trigger implant osseointegration and biological healing. Epitaxial growth technique is used to organize the crystal panel at an atomic scale, and ferroelectric polarization of BFO nanofilms matching the amplitude and direction of endogenous electric potentials on bone defect walls is achieved. In the presence of built‐in electric fields, implants with BFO nanofilms with downward polarization (BFO+) show rapid and superior osseointegration in the rat femur. The mechanism of this phenotypic osteogenic behavior is further studied by protein adsorption and stem cell behavior in different time points. BFO+ promotes protein adsorption and mesenchymal stem cell (MSC) attachment, spreading, and osteogenic differentiation. Custom‐designed PCR array examination shows sequentially initiated Ca²⁺ signaling, cell adhesion and spreading, and PI3K‐AKT signaling in MSCs. The results of this study provide a novel strategy for the development of implant surface modification technology.     

Two‐Stage Size Decrease and Enhanced Photoacoustic Performance of Stimuli‐Responsive Polymer‐Gold Nanorod Assembly for Increased Tumor Penetration

A promising theranostic platform for solid tumors would deliver and release anticancer nanomedicine effectively in tumor cells. However, diverse biological barriers, especially related to the tumor microenvironment, impede these theranostic agents from reaching the tumor cell. Herein, a sequential pH and reduction‐responsive polymer and gold nanorod (AuNR) core–shell assembly to overcome these barriers via a two‐stage size decrease and disassembly of the nanoplatform responding to the specified tumor microenvironment are reported. The tumor uptake of the hybrid nanoparticle (NP) is 14.2% ID g^?1, which is two and four times higher than the noneresponsive hybrid NPs and small AuNR@PEG, respectively. After tumor uptake of the hybrid NPs, the disassembled ultrasmall AuNRs coated with a polymer of polymerized reduction‐responsive doxorubicin (DOX) prodrug monomers penetrate into the solid tumor and lead to localized DOX release in the tumor cell. A linear increase in photoacustic (PA) effects from the PA activating polymer on an AuNR cluster surface indicates a critical role of electromagnetic fields in the AuNR assembly, which is consistent with the theoretical calculation results. Furthermore, the hybrid NP can serve as a promising deep‐tissue PA and surface‐enhanced Raman scattering imaging agent for real‐time in vivo investigation of physiological behaviors and deep tumor penetrating nanotherapy effects.     

Ultralight,Soft Polymer Sponges by Self‐Assembly of Short Electrospun Fibers in Colloidal Dispersions

Ultralight polymer sponges are prepared by freeze‐drying of dispersions of short electrospun fibers. In contrast to many other highly porous materials, these sponges show extremely low densities (<3 mg cm^?3) in combination with low specific surface areas. The resulting hierarchical pore structure of the sponges gives basis for soft and reversibly compressible materials and to hydrophobic behavior in combination with excellent uptake for hydrophobic liquids. Owing to their large porosity, cell culturing is successful after hydrophilic modification of the sponges.     

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.     

Kirigami‐Inspired Self‐Assembly of 3D Structures

Self‐assembly of 3D structures presents an attractive and scalable route to realize reconfigurable and functionally capable mesoscale devices without human intervention. A common approach for achieving this is to utilize stimuli‐responsive folding of hinged structures, which requires the integration of different materials and/or geometric arrangements along the hinges. It is demonstrated that the inclusion of Kirigami cuts in planar, hingeless bilayer thin sheets can be used to produce complex 3D shapes in an on‐demand manner. Nonlinear finite element models are developed to elucidate the mechanics of shape morphing in bilayer thin sheets and verify the predictions through swelling experiments of planar, millimeter‐scaled PDMS (polydimethylsiloxane) bilayers in organic solvents. Building upon the mechanistic understandings, The transformation of Kirigami‐cut simple bilayers into 3D shapes such as letters from the Roman alphabet (to make “ADVANCED FUNCTIONAL MATERIALS”) and open/closed polyhedral architectures is experimentally demonstrated. A possible application of the bilayers as tether‐less optical metamaterials with dynamically tunable light transmission and reflection behaviors is also shown. As the proposed mechanistic design principles could be applied to a variety of materials, this research broadly contributes toward the development of smart, tetherless, and reconfigurable multifunctional systems.     

Highly Conductive Nanocomposites with Three‐Dimensional,Compactly Interconnected Graphene Networks via a Self‐Assembly Process

Polymer‐based materials with high electrical conductivity are of considerable interest because of their wide range of applications. The construction of a 3D, compactly interconnected graphene network can offer a huge increase in the electrical conductivity of polymer composites. However, it is still a great challenge to achieve desirable 3D architectures in the polymer matrix. Here, highly conductive polymer nanocomposites with 3D compactly interconnected graphene networks are obtained using a self‐assembly process. Polystyrene (PS) and ethylene vinyl acetate (EVA) are used as polymer matrixes. The obtained PS composite film with 4.8 vol% graphene shows a high electrical conductivity of 1083.3 S/m, which is superior to that of the graphene composite prepared by a solvent mixing method. The electrical conductivity of the composites is closely related to the compact contact between graphene sheets in the 3D structures and the high reduction level of graphene sheets. The obtained EVA composite films with the 3D graphene structure not only show high electrical conductivity but also exhibit high flexibility. Importantly, the method to fabricate 3D graphene structures in polymer matrix is facile, green, low‐cost, and scalable, providing a universal route for the rational design and engineering of highly conductive polymer composites.     

Electric Field Controlled Self‐Assembly of Hierarchically Ordered Membranes

Self‐assembly in the presence of external forces is an adaptive, directed organization of molecular components under nonequilibrium conditions. While forces may be generated as a result of spontaneous interactions among components of a system, intervention with external forces can significantly alter the final outcome of self‐assembly. Superimposing these intrinsic and extrinsic forces provides greater degrees of freedom to control the structure and function of self‐assembling materials. In this work we investigate the role of electric fields during the dynamic self‐assembly of a negatively charged polyelectrolyte and a positively charged peptide amphiphile in water leading to the formation of an ordered membrane. In the absence of electric fields, contact between the two solutions of oppositely charged molecules triggers the growth of closed membranes with vertically oriented fibrils that encapsulate the polyelectrolyte solution. This process of self‐assembly is intrinsically driven by excess osmotic pressure of counterions and the electric field is found to modify the kinetics of membrane formation as well as membrane morphology and properties. Depending on the strength and orientation of the field we observe a significant increase or decrease of up to nearly 100% in membrane thickness, or the controlled rotation of nanofiber growth direction by 90 degrees which leads to a significant increase in mechanical stiffness. These results suggest the possibility of using electric fields to control structure in self‐assembly processes that involve the diffusion of oppositely charged molecules.