Janus discs

We describe the synthesis and the solution properties of sheet- and disclike Janus particles, containing an inner crosslinked polybutadiene (PB) layer and two different outer sides of polystyrene (PS) and poly(tert-butyl methacrylate) (PtBMA). The structures formed upon adsorption of the flat Janus particles onto solid substrates as well as in THF solution are investigated. The Janus discs are obtained in a template-assisted synthetic pathway followed by sonication. Selectively crosslinking the lamellar PB domains in a well-ordered lamellar microphase-separated bulk morphology of PS-block-PB-block-PtBMA (SBT) block terpolymers leads to the conservation of the compartmentalization of the two outer blocks. Sonication of the crosslinked block terpolymer templates renders soluble sheet- and disclike Janus particles, the size of which can be tuned from the micrometer range down to the nanometer scale. Small-angle X-ray scattering, transmission electron microscopy, dynamic light scattering, scanning force microscopy, and scanning electron microscopy are used to characterize the template-assisted synthetic process and the solution properties. Cryogenic transmission electron microscopy in THF and TEM of particles, embedded into a photo-crosslinkable silicon oil, indicate a supramolecular aggregation behavior of the Janus discs in concentrated solutions. Pendant drop tensiometry demonstrates that Janus sheets and discs can be used to stabilize liquid-liquid interfaces, rendering these materials interesting for future applications.     

Janus balance of amphiphilic colloidal particles

We introduce the notion of "Janus balance" (J), defined as the dimensionless ratio of work to transfer an amphiphilic colloidal particle (a "Janus particle") from the oil-water interface into the oil phase, normalized by the work needed to move it into the water phase. The J value can be calculated simply from the interfacial contact angle and the geometry of Janus particles, without the need to know the interfacial energy. It is demonstrated that Janus particles of the same chemical composition but different geometries will have the highest adsorption energy when J = 1. Even for particles of homogeneous chemical makeup, the Janus balance concept can be applied when considering the contact angle hysteresis in desorbing the particle from equilibrium into the water or oil phase. The Janus balance concept may enable predictions of how a Janus particle behaves with respect to efficiency and function as a solid surfactant, as the Janus balance of solid surfactants is the analog of the classical hydrophile-lipophile balance of small surfactant molecules.     

Janus Composite Nanotubes

We propose a facile method to achieve paramagnetic Janus nanotubes with two compositions compartmentalized onto the interior and exterior surfaces, respectively. A sulfonated polydivinylbenzene (PDVB) nanotube is prepared by simple sulfonation of the exterior surface of a PDVB nanotube. Silica@FeOOH dual layers are sequentially grown onto the sulfonated PDVB nanotube surface. The composite nanotubes become paramagnetic after calcination and can be broken into shorter pieces under vigorous ultrasonication. After selective modification of the interior and exterior surfaces of the paramagnetic nanotubes, the nanotube shell becomes Janus in wettability. Desired hydrophobic species can be selectively captured inside the cavity. The paramagnetic Janus composite nanotubes can align into parallel chains under a magnetic field, which is self‐disassembled upon removal of the magnetic field.     

Janus粒子制备研究*

具有不对称双面结构的Janus粒子以其独特性能,在乳液稳定、光学、生物传感、药物输送、电子学等领域具有潜在的应用前景。本文就近年来Janus粒子制备技术的研究进展进行了总结,详细地介绍了Janus粒子主要制备方法,包括微流体合成、拓扑选择表面改性、模板导向自组装、可控相分离及可控表面成核,并指出了各种Janus粒子制备技术存在的问题及其发展方向,认为基于可控相分离及表面成核的合成方法成本较低,产率较大,有可能得到更为广泛的应用。     

Sonochemical synthesis of magnetic Janus nanoparticles

The sonochemical synthesis of nanosized surface-dissymmetrical (Janus) particles is described. The Janus particles were composed of silica and polystyrene, with the polystyrene portion loaded with nanosized magnetite particles. It is shown that the Janus particles can be used to form kinetically stable oil-in-water emulsions that can be spontaneously broken on application of an external magnetic field. The one-pot synthetic process used to prepare the Janus particles has several advantages over other conventional methods of producing such particles.     

Janus cylinders at liquid-liquid interfaces

We describe the first study on the self-assembly behavior of Janus cylinders at liquid/liquid interfaces. The Janus cylinders are characterized by a phase separation along the major axis into two hemicylinders of different wettability. The pendant drop technique and microscopic imaging were used to characterize the adsorption behavior and self-assembly of Janus cylinders at perfluorinated oil/dioxane and perfluorinated oil/dimethyl sulfoxide interfaces. According to the evolution of the interfacial tension and a series of TEM images taken during the cylinder adsorption, we will specify the characteristics of early to late stages of the Janus cylinder adsorption at a liquid-liquid interface and discuss the effect of Janus cylinder length and their concentration. We also establish that the broken symmetry of the corona leads to significantly higher interfacial activity as compared to homogeneous core-shell cylinders. The adsorption is characterized by three different adsorption stages: first, free diffusion to the interface, followed by continuous adsorption of cylinders including ordering and domain formation and, finally, additional packing with a rearrangement of domains and formation of a loose multilayer system.     

Polymer/metal segmental Janus nanoparticles

Polymer/metal composite segmental Janus nanoparticles(NPs) are synthesized by sequential growth againstpoly(4-vinylpyridine)(P4VP) crosslinked cP4VP-PS Janus NPs. A Janus cluster of poly(4-vinylpyridine)-block-polystyrene(P4VPb-PS) diblock copolymer is self-organized after absorption onto a silica patchy sphere via hydrogen bonding. Selective crosslinking of P4VP leads to the formation of robust cP4VP-PS Janus NPs. Within the cP4VP domain, functional species such as metals are preferentially grown by in situ reduction. Other thiol-capped polymers, for example, thiol-cappedpoly(Nisopropylacrylamide)(PNIPAM-SH), can be conjugated onto the opposite side to form polymer/metal triple segmental Janus NPs. The hyperthermia effect of Au NP of PNIPAM-Au@cP4VP-PS by near infrared(NIR) irradiation can trigger a fast transition from amphiphilic to hydrophobic of the Janus NPs at low surrounding temperature. De-stabilization ofthe emulsion is NIR triggered although the system temperature is below LCST(~32 °C).     

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.     

Responsive Janus Cage Reactor

A Janus silica cage was synthesized by selectively grafting an ionic liquid (IL) and poly‐N‐isopropylacrylamide (PNIPAM) (lower critical solution temperature (LCST)≈32 °C) onto the exterior and interior sides of the mesoporous SiO₂ shell. The paramagnetic core inside the cavity is responsible for magnetic collection. The PW₁₂O₄₀^3? anion is further conjugated onto the IL side by anion exchange. The Janus cage acts as a thermal‐responsive reactor for catalytic oxidization of dibenzothiophene (DBT) in the presence of H₂O₂. The sulfide in the model oil can be completely decomposed at 25 °C, whilst the oxidative products are more dissoluble in water and preferentially captured inside the Janus cage. The Janus cage reactor could be regenerated at high temperature above 32 °C after releasing the products.     

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.     

Superamphiphilic Janus fabric

Janus fabrics with superamphiphilicity were fabricated via electrospinning of polyacrylonitrile (PAN). PAN nanofibrous mats were formed on an aluminum foil substrate and then thermally treated to cause hydrolysis. An identical PAN solution was subsequently electrospun onto the hydrolyzed PAN layer, followed by peeling off of the bicomposite film from the collector substrate to produce a free-standing Janus fabric. On one side, the electrospun PAN mat exhibited superhydrophobic properties, with a water contact angle of 151.2°, whereas the initially superhydrophobic PAN sheet on the opposite side of the fabric was converted to a superhydrophilic surface (water contact angle of 0°) through hydrolysis of the surface functional groups induced by the thermal treatment. The resulting Janus fabrics exhibited both superhydrophobicity, repelling water on the one side, and superhydrophilicity, absorbing water on the other side. The organic solvent resistance of the PAN nanofibrous sheets was remarkably improved by incorporation of a tetraethyl orthosilicate. This facile and simple technique introduces a new route for the design and development of functional smart, robust fabrics from an inexpensive, commercially available polymer.     

Thermosensitive hollow Janus dumbbells

Thermosensitive hollow Janus dumbbells, consisting of two partially fused hollow poly (N-isopropylacrylamide) (PNIPAM) spheres, were prepared using dumbbell-shaped microgels as templates. One sphere has a shell completely made of PNIPAM while the other one has a hybrid shell, which is a poly(styrene-co-3-(trimethoxysilyl)propyl methacrylate) layer covered by PNIPAM. The morphology of hollow Janus dumbbells is fully characterized by cryo- and transmission electron microscopy, scanning force microscopy, and dynamic light scattering. Transmission electron microscopy demonstrates that the particles have a very narrow size distribution. The analysis by depolarized dynamic light scattering showed that the hollow Janus dumbbells exhibit a thermosensitive behavior comparable to the dumbbell-shaped microgels before the removal of the core.     

Janus particles at liquid-liquid interfaces

Following recent theoretical predictions, we report on the first experiments on the interfacial activity of so-called Janus nanoparticles (i.e., bifacial particles consisting of a gold and an iron oxide moiety). Using pendant drop tensiometry, we show that the amphiphilicity derived from the Janus character of the particles leads to a significantly higher interfacial activity compared to that of the respective homogeneous particles of the same size. The self-assembly of Janus particles at the hexane-water interface results in a significant decrease in the interfacial tension. Furthermore, we demonstrate control over the interfacial activity by tuning the particles' amphiphilicity via ligand-exchange reactions.     

Janus复合材料调控界面

Janus材料集成不同组成/功能于一体,具有明确的空间分区特征,是一类特殊的高分子复合功能材料.有机高分子链提供亲水/亲油及响应特性,无机组成提供丰富的光、电、磁、热等功能性.两亲性的Janus材料在高效稳定界面同时,还能赋予界面功能性并可在外场作用下实现操控.本文重点总结了不同结构和功能的Janus材料在稳定界面和调控界面的近期主要进展,包括聚集行为、固体乳化剂、界面增容、界面催化、功能涂层、细胞诊断与治疗等方面.     

Adhesion force studies of Janus nanoparticles

Janus nanoparticles represent a unique nanoscale analogue to the conventional surfactant molecules, exhibiting hydrophobic characters on one side and hydrophilic characters on the other. Yet, direct visualization of the asymmetric surface structures of the particles remains a challenge. In this paper, we used a simple technique based on AFM adhesion force measurements to examine the two distinctly different hemispheres of the Janus particles at the molecular level. Experimentally, the Janus nanoparticles were prepared by ligand exchange reactions at the air-water interface. The particles were then immobilized onto a substrate surface with the particle orientation controlled by the chemical functionalization of the substrate surface, and an AFM adhesion force was employed to measure the interactions between the tip of a bare silicon probe and the Janus nanoparticles. It was found that when the hydrophilic side of the particles was exposed, the adhesion force was substantially greater than that with the hydrophobic side exposed, as the silicon probes typically exhibit hydrophilic properties. These studies provide further confirmation of the amphiphilic nature of the Janus nanoparticles.     

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.     

Microfluidic synthesis of multifunctional Janus particles for biomedical applications

Multifunctional Janus particles have a variety of applications in a wide range of fields. However, to achieve many of these applications, high-throughput, low-cost techniques are needed to synthesize these particles with precise control of the various structural/physical/chemical properties. Microfluidics provides a unique platform to fabricate Janus particles using carefully controlled liquid flow in microfluidic channels to form Janus droplets and various types of solidification methods to solidify them into Janus particles. In this Focus article, we summarize the most recent representative works on Janus particle fabrication in microfluidics. The applications of Janus particles in biomedical areas are emphasized. We believe that microfluidics-enabled multifunctional Janus particles could resolve multiple prevalent issues in biomedicine (e.g., disease monitoring at an early stage, high-throughput bioassays, therapeutic delivery) if persistent effort and collaboration are devoted to this direction.     

pH responsive Janus polymeric nanosheets

pH responsive polymeric Janus nanosheets with poly(maleic acid) moiety and crosslinked PS onto the corresponding sides have been synthesized by free radical polymerization. The Janus nanosheets can serve as solid emulsifier to stabilize an oil/water emulsion, whose stability is easily triggered by changing pH across pKa of the poly(maleic acid).     

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.     

Pickering乳液模板法制备Janus粒子

本文以SiO₂粒子稳定的水包油(O/W)型Pickering乳液作为模板, 在乳液连续相进行SI-ATRP, 将聚合物刷接枝到SiO₂粒子外半表面, 破乳得到半修饰的Janus粒子.