Anisotropic particles with patchy, multicompartment and Janus architectures: preparation and application

Anisotropic particles, such as patchy, multicompartment and Janus particles, have attracted significant attention in recent years due to their novel morphologies and diverse potential applications. The non-centrosymmetric features of these particles make them a unique class of nano- or micro-colloidal materials. Patchy particles usually have different compositional patches in the corona, whereas multicompartment particles have a multi-phasic anisotropic architecture in the core domain. In contrast, Janus particles, named after the double-faced Roman god, have a strictly biphasic geometry of distinct compositions and properties in the core and/or corona. The term Janus particles, multicompartment particles and patchy particles frequently appears in the literature, however, they are sometimes misused due to their structural similarity. Therefore, in this critical review we classify the key features of these different anisotropic colloidal particles and compare structural properties as well as discuss their preparation and application. This review brings together and highlights the significant advances in the last 2 to 3 years in the fabrication and application of these novel patchy, multicompartment and Janus particles (98 references).     

Fabrication, properties and applications of Janus particles

Although the concept of Janus particles was raised in the early 1990s, the related research has not attracted considerable interest until recently due to the special properties and applications of these colloidal particles as well as the advances in new fabrications. Janus particles can be divided into three categories: polymeric, inorganic, and polymeric-inorganic, and each kind of Janus particles can be spherical, dumbbell-like, half raspberry-like, cylindrical, disk-like, or any of a variety of other shapes. Different Janus particles may share common preparation principles or require specific fabrication processes, and may have different assembly behaviours and properties. This critical review discusses the main fabrication methods of the three kinds of Janus particles, and then highlights the important properties and applications of these Janus particles developed in recent years, and finally proposes some perspectives on the future of Janus particle research and development.     

非对称性Janus粒子的制备与可控组装

古罗马的双面神(Janus)常被用来描述具有两种不同化学结构或性质的不对称粒子, Janus粒子由于自身的特殊性能在药物载体、电子器件和乳液稳定等方面表现出良好的发展势头, 其应用前景日益受到人们的重视. 目前, Janus 粒子作为基本的组装基元受到越来越多的关注, 相关组装方法也被广泛地研究, 包括本体组装、界面组装和外界驱动力调控等, 特别是Janus 粒子的双亲修饰与功能化. 本文综述了现今Janus 粒子制备方法及对其进行修饰组装的最新研究进展, 详细讨论比较了一步合成法、聚合物自组装法和晶种直接生长等方法的特点及差异, 并对一些新型功能Janus粒子的设计及潜在的应用前景进行了展望.     

Janus Nanoparticles: Preparation,Characterization, and Applications

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

双面粒子的制备及应用研究进展

双面粒子是指粒子的两个半球表面具有不同组成或性质的微粒.这种各向异性或不对称的结构使其有很广泛的应用,但也使其制备变得困难.近年来已发展了很多制备方法,本文总结了最近几年双面粒子在制备及应用方面的研究进展.     

Recent advances in microfluidic production of Janus droplets and particles

Microfluidic production of multicompartmental emulsion droplets and particles has received considerable attention of late. In particular, droplets having two physically and chemically distinct segments (so-called Janus droplets) and the anisotropic particles synthesized from these droplets, are becoming increasingly popular because of their novel and promising properties, which make them suitable for use in numerous applications, including for controlled drug release, display devices, and self-assembly. So far, a range of interesting anisotropies have been accorded to Janus droplets and particles via microfluidics; these span from chromatic, magnetic, and hydrophobic–hydrophilic characteristics to selective degradation properties. Here, we summarize and discuss the recent trends related to Janus droplets and particles produced through microfluidic processing. We also review the parallelization technologies being developed for scaling up microfluidic emulsification in the industry.     

Janus particles with a functional gold surface for control of surface plasmon resonance

We report in this study the presence of Janus particles, which are candidates for use with electronic color papers. We used negatively charged polystyrene particles (370 nm) as the core particles, and gold was then sputtered onto their packed monolayer under several conditions. The sputtered particles were next redispersed into the aqueous medium by gentle sonication. Gold nanoparticles localized on one side of the cores could also serve as seeds for subsequent shell growth by electroless gold plating. Through these treatments, a series of well-dispersed Janus particles were obtained with gold nanostructures of different size and shape only on one side. Their dispersions showed different colors originating from the surface plasmon resonance absorption of gold nanoparticles localized on the hemisphere. The particles obtained by this approach have potential applications such as in sensors and electronic color paper.     

Amphiphilic Janus Gold Nanoparticles Prepared by Interface‐Directed Self‐Assembly: Synthesis and Self‐Assembly

Materials with Janus structures are attractive for wide applications in materials science. Although extensive efforts in the synthesis of Janus particles have been reported, the synthesis of sub‐10 nm Janus nanoparticles is still challenging. Herein, the synthesis of Janus gold nanoparticles (AuNPs) based on interface‐directed self‐assembly is reported. Polystyrene (PS) colloidal particles with AuNPs on the surface were prepared by interface‐directed self‐assembly, and the colloidal particles were used as templates for the synthesis of Janus AuNPs. To prepare colloidal particles, thiol‐terminated polystyrene (PS‐SH) was dissolved in toluene and citrate‐stabilized AuNPs were dispersed in aqueous solution. Upon mixing the two solutions, PS‐SH chains were grafted to the surface of AuNPs and amphiphilic AuNPs were formed at the liquid–liquid interface. PS colloidal particles decorated with AuNPs on the surfaces were prepared by adding the emulsion to excess methanol. On the surface, AuNPs were partially embedded in the colloidal particles. The outer regions of the AuNPs were exposed to the solution and were functionalized through the grafting of atom‐transfer radical polymerization (ATRP) initiator. Poly[2‐(dimethamino)ethyl methacrylate] (PDMAEMA) on AuNPs were prepared by surface‐initiated ATRP. After centrifugation and dissolving the colloidal particles in tetrahydrofuran (THF), Janus AuNPs with PS and PDMAEMA on two hemispheres were obtained. In acidic pH, Janus AuNPs are amphiphilic and are able to emulsify oil droplets in water; in basic pH, the Janus AuNPs are hydrophobic. In mixtures of THF/methanol at a volume ratio of 1:5, the Janus AuNPs self‐assemble into bilayer structures with collapsed PS in the interiors and solvated PDMAEMA at the exteriors of the structures.     

Fabrication of a composite colloidal particle with unusual janus structure as a high-performance solid emulsifier

Core-shell particles with cross-linked core and shell were used as seed particles to produce composite Janus particles. It was found that when the shell has distinctly higher cross-linking degree than the core, Janus particles with very unusual structures can be obtained. These particles have two parts, with one part embraced partially or entirely by the other part, adjustable by parameters such as phase ratio or cross-linking degree. On the basis of experimental observations, a possible mechanism for the formation of such unusual Janus particles has been proposed. Janus particles with arms are used to emulsify water-toluene mixtures, forming oil-in-water (O/W) emulsions at very high internal phase content with rather low concentration of particles. Nonspherical emulsion droplets were observed, indicating that these Janus particles are likely to jam at the interface, forming a strong protecting layer to stabilize emulsions.     

One-step fabrication of polymeric Janus nanoparticles for drug delivery

With its unique structure of two compartments, Janus particles can be used for many applications for which monomorphic particles are inadequate, including to be used as a drug delivery system to deliver multiple payloads with widely different solubility. Here we report on a fluidic nanoprecipitation system (FNPS), capable of fabricating biocompatible Janus polymeric nanoparticles comprised of the FDA-approved polymer poly(lactic-co-glycolic acid) (PLGA). The FNPS contains dual inlets, one for each half of the particle, that insert into the precipitation stream. The system provides a one-step approach for production of Janus polymeric particles with submicrometer diameters and is likely amenable to substantial scale-up. To the best of our knowledge, this is the first demonstration of biocompatible Janus nanoparticles that encapsulate a hydrophobic drug (paclitaxel) on one side and a hydrophilic drug (doxorubicin hydrochloride) on the other.     

Electrophoresis of a particle at an arbitrary surface potential and double layer thickness: importance of nonuniformly charged conditions

Recent advances in material science and technology yield not only various kinds of nano- and sub-micro-scaled particles but also particles of various charged conditions such as Janus particles. The characterization of these particles can be challenging because conventional electrophoresis theory is usually based on drastic assumptions that are unable to realistically describe the actual situation. In this study, the influence of the nonuniform charged conditions on the surface of a particle at an arbitrary level of surface potential and double layer thickness on its electrophoretic behavior is investigated for the first time in the literature taking account of the effect of double-layer polarization. Several important results are observed. For instance, for the same averaged surface potential, the mobility of a nonuniformly charged particle is generally smaller than that of a uniformly charged particle, and the difference between the two depends upon the thickness of double layer. This implies that using the conventional electrophoresis theory may result in appreciable deviation, which can be on the order of ca. 20%. In addition, the nonuniform surface charge can yield double vortex in the vicinity of a particle by breaking the symmetric of the flow field, which has potential applications in mixing and/or regulating the medium confined in a submicrometer-sized space, where conventional mixing devices are inapplicable.     

Dielectrophoretic assembly of metallodielectric Janus particles in AC electric fields

"Janus" particles with two hemispheres of different polarizability or charge demonstrate a multitude of interesting effects in external electric fields. We reported earlier how particles with one metallic hemisphere and one dielectric hemisphere self-propel in low-frequency alternating current (AC) electric fields. Here, we demonstrate the assembly of such Janus particles driven by AC electric fields at frequencies above 10 kHz. We investigated the relation between field-induced dielectrophoretic force, field distribution, and structure of the assemblies. The phase space for electric field intensity and frequency was explored for particle concentrations large enough to form a monolayer on a glass surface between two gold electrodes. A rich variety of metallodielectric particle structures and dynamics were uncovered, which are very different from those obtained from directed assembly of plain dielectric or plain conductive particles under the action of fields of similar frequency and intensity. The metallodielectric particles assemble into new types of chain structures, where the metallized halves of neighboring particles align into lanes along the direction of the electric field, while the dielectric halves face in alternating direction. The staggered chains may assemble in various orientations to form different types of two-dimensional metallodielectric crystals. The experimental results on the formation of staggered chains are interpreted by means of numerical simulations of the electric energy of the system. The assembly of Janus metallodielectric particles may find applications in liquid-borne microcircuits and materials with directional electric and heat transfer.     

Controlled assembly of silica microspheres in spherically confined polymer particles by using self-organized precipitation (SORP) method

Assembly of submicron-sized microspheres has received much attention due to its high potential for wide variety of applications. We have developed a preparation method of polymer particles by simple mixing of a poor solvent into the polymer solution followed by evaporation of a good solvent. By using this technique, silica nanoparticles and polymer composite particles were prepared. Preparation of three-dimensional assemblies of silica particles in polymer particles and composite Janus particles are shown.     

Facile, solution-based synthesis of soft, nanoscale janus particles with tunable janus balance

We present a novel, versatile, and simple solution-based routine to produce soft, nanosized Janus particles with tunable structural and physical properties at high volume yield. This process is based on the cross-linking of compartments within precisely defined multicompartment micelles (MCMs), which are themselves formed by the self-assembly of ABC triblock terpolymers. Therein, the C blocks form the stabilizing corona emanating from B compartments, which in turn reside on an A core. Cross-linking of the B compartments allows to permanently fixate the phase-separated state and dissolution in a good solvent for all blocks breaks up the MCMs into single Janus particles. They now consist of a core of cross-linked B blocks and two phase-separated hemispheres of A and C. The process gives access to unprecedented structural features such as tunable core diameter and control over the Janus balance ranging from dominant A side to equal hemispheres to dominant C side. We demonstrate that this simple one-pot approach can be extended to a range of triblock terpolymers with different block lengths and block chemistries to furnish a library of tailor-made Janus particles with widely tunable physical properties. Such a diversity and simplicity has remained unreachable with our previously developed approach using the controlled cross-linking of bulk morphologies. We show that this new synthetic route can be upscaled to a high volume yield of 10 wt %, thereby enabling large-scale applications. We further demonstrate the effect of the Janus balance on colloidal self-assembly. Janus particles with a dominant hydrophobic and a small hydrophilic patch aggregate into large clusters in water, but merely di- or trimerize in chloroform.     

Strictly Biphasic Soft and Hard Janus Structures: Synthesis,Properties, and Applications

Janus structures, named after the ancient two‐faced Roman god Janus, comprise two hemistructures (e.g. hemispheres) with different compositions and functionalities. Much research has been carried out over the past few years on Janus structures because of the intriguing properties and promising potential applications of these unusually shaped materials. This Review discusses recent progress made in the synthesis, properties, and applications of strictly biphasic Janus structures possessing symmetrical structures but made of disparate materials. Depending on the chemical compositions, such biphasic structures can be categorized into soft, hard, and hybrid soft/hard Janus structures of different architectures, including spheres, rodlike, disclike, or any other shape. The main synthetic routes to soft, hard, and hybrid soft/hard Janus structures are summarized and their unique properties and applications are introduced. The perspectives for future research and development are also described.     

Self-assembly of model amphiphilic Janus particles

We apply molecular dynamics simulations to investigate the structure formation of amphiphilic Janus particles in the bulk phase. The Janus particles are modeled as (soft) spheres composed of a hydrophilic and hydrophobic part. Their orientation is described by a vector representing an internal degree of freedom. Investigating energy fluctuations and cluster size distributions, we determine the aggregation line in a temperature-density-diagram, where the reduced temperature is an inverse measure for the anisotropic coupling. Below this aggregation line clusters of various sizes depending on density and reduced temperature are found. For low densities in the range ρ? ≤ 0.3, the cluster size distribution has a broad maximum, indicating simultaneous existence of various cluster sizes between 5 and 10. We find no hint of a condensation transition of these clustered systems. In the case of higher densities (ρ? = 0.5 and 0.6), the cluster size distribution shows an extremely narrow peak at clusters of size 13. In these icosahedrons, the particles are arranged in a closed-packed manner, thereby maximizing the number of bonds. Analyzing the translational mean-square displacement we also observe indications of hindered diffusion due to aggregation.     

两亲性Janus粒子在蜂窝状多孔聚合物膜上的自组装性能

采用具有两亲性的两面体(Janus)粒子实现稳定的粒子界面组装与水滴模板法自组装过程相结合的方法获得了粒子在蜂窝状多孔聚合物薄膜内壁的高效定向修饰.通过与均质粒子组装形貌的对比,证明了Janus粒子因其特殊的界面自组装活性,可以获得高粒子加量条件下的规则多孔结构,解决了使用均质粒子时存在的结构有序性和粒子修饰密度之间的矛盾.而在较低粒子加量的条件下,Janus粒子也展示出与均质粒子极为不同的组装形貌.这一方法的建立,为新型表面功能化材料的制备提供了一个新的思路.     

Solvent-free synthesis of Janus colloidal particles

Taking advantage of the quick and efficient access of vapor to surfaces, a simple, solvent-free method is demonstrated to synthesize Janus colloidal particles in large quantity and with high efficiency. First, at the liquid-liquid interface of emulsified molten wax and water, untreated silica particles adsorb and are frozen in place when the wax solidifies. The exposed surfaces of the immobilized particles are modified chemically by exposure to silane vapor and, in principle, subsequent dissolution of the wax opens up the inner particle surface for further chemical modification. Applying this scheme, this paper describes the production of amphiphilic Janus particles (hydrophobic on one side, hydrophilic on the other) and dipolar Janus particles (positively charged on one side, negatively charged on the other). Janus geometry is confirmed by fluorescence microscopy and flow cytometry. Amphiphilic Janus particles are found to adsorb strongly to the water-oil interface, whereas dipolar particles assemble into chains in the aqueous phase.     

聚苯胺/聚苯乙烯复合Janus颗粒的制备

利用有机溶剂溶胀磺化聚苯乙烯@二氧化钛(SPS@TiO₂)核壳粒子制得二氧化钛/聚苯乙烯(TiO₂/PS)双面神(Janus)颗粒, 并在TiO₂端进行改性, 得到墨绿色的聚苯胺/聚苯乙烯(PANi/PS) Janus颗粒. 用扫描电子显微镜(SEM)、电子能谱(EDS)、元素分析、透射电子显微镜(TEM)、红外光谱(IR)、热重分析(TGA)、固体紫外-可见分析(UV-Vis)和四探针法考察Janus颗粒组成、微结构和Janus性质. 结果表明, Janus颗粒为雪人状结构, PS端的平均粒径为228 nm, PANi端的平均粒径从TiO₂的258 nm增大为295 nm; 并且在EDS谱上可以观察到N元素, 而未观察到Ti元素; 包覆的PANi的质量分数为23.7%. 掺杂后PANi/PS Janus颗粒的导电性能较好, 电导率为0.247 S/cm.     

两亲性Janus粒子的合成及其在Pickering乳液中的应用

Janus粒子由于在光、电、力、磁及表面亲/疏水性等方面表现出各向异性,因此在稳定乳液、生物医药及功能涂层等方面展现出广阔的应用价值.两亲性Janus粒子是指一侧具有亲水性、另一侧具有疏水性的不对称材料,由于同时具有表面活性剂的性质和固体颗粒的效应,在稳定Pickenng乳液方面极具优势.基于此,本文对两亲性Janus...