Synthesis of nonspherical superparamagnetic particles: in situ coprecipitation of magnetic nanoparticles in microgels prepared by stop-flow lithography

We present the synthesis of nonspherical magnetic microparticles with multiple functionalities, shapes, and chemistries. Particle synthesis was performed in two steps: polymeric microparticles functionalized homogenously with carboxyl groups were generated using stop-flow lithography, and then in situ coprecipitation was used to grow magnetic nanoparticles at these carboxyl sites. With successive growth of magnetic nanoparticles, we obtained polymeric particles with saturation magnetizations of up to 42 emu/g microparticle. The growth in the magnetic nanoparticle mean size and polydispersity was determined from the magnetization curves obtained following each growth cycle; nanoparticle sizes were limited by the physical constraint of the effective mesh within the hosting gel microparticle. Particles with spatially segregated domains of varying magnetic properties (e.g., Janus particles, particles with step changes in magnetite concentration, etc.) can be synthesized readily using this approach.     

Enhanced stability of Janus nanoparticles by covalent cross-linking of surface ligands

A mercapto derivative of diacetylene was used as the hydrophilic ligand to prepare Janus nanoparticles by using hydrophobic hexanethiolate-protected gold (AuC6, diameter 5 nm) nanoparticles as the starting materials. The amphiphilic surface characters of the Janus nanoparticles were verified by contact angle measurements, as compared to those of the bulk-exchange counterparts where the two types of ligands were distributed rather homogeneously on the nanoparticle surface. Dynamic light scattering studies showed that the Janus nanoparticles formed stable superstructures in various solvent media that were significantly larger than those by the bulk-exchange counterparts. This was ascribed to the amphiphilic characters of the Janus nanoparticles that rendered the particles to behave analogously to conventional surfactant molecules. Notably, because of the close proximity of the diacetylene moieties on the Janus nanoparticle surface, exposure to UV irradiation led to effective covalent cross-linking between the diacetylene moieties of neighboring ligands, as manifested in UV-vis and fluorescence measurements where the emission characteristics of dimers and trimers of diacetylene were rather well-defined, in addition to the monomeric emission. In contrast, for bulk-exchange nanoparticles, no trimer emission could be identified, and the intensity of dimer emission was markedly lower (though the intensity increased with increasing diacetylene coverage on the particle surface) under the otherwise identical experimental conditions. This is largely because the diacetylene ligands were distributed on the entire particle surface, and it was difficult to find a large number of ligands situated closely so that the stringent topochemical principles for the polymerization of diacetylene derivatives could be met. Importantly, the cross-linked Janus nanoparticles were found to exhibit marked enhancement of the structural integrity, which was attributable to the impeded surface diffusion of the thiol ligands on the nanoparticle surface, as manifested in fluorescence measurements of aged nanoparticles.     

Preparation and controlled self-assembly of Janus magnetic nanoparticles

Janus magnetic nanoparticles (~20 nm) were prepared by grafting either polystyrene sodium sulfonate (PSSNa) or polydimethylamino ethylmethacrylate (PDMAEMA) to the exposed surfaces of negatively charged poly(acrylic acid) (PAA)-coated magnetite nanoparticles adsorbed onto positively charged silica beads. Individually dispersed Janus nanoparticles were obtained by repulsion from the beads on reversal of the silica surface charge when the solution pH was increased. Controlled aggregation of the Janus nanoparticles was observed at low pH values, with the formation of stable clusters of approximately 2-4 times the initial size of the particles. Cluster formation was reversed, and individually dispersed nanoparticles recovered, by restoring the pH to high values. At intermediate pH values, PSSNa Janus nanoparticles showed moderate clustering, while PDMAEMA Janus nanoparticles aggregated uncontrollably due to dipolar interactions. The size of the stable clusters could be controlled by increasing the molecular weight of the grafted polymer, or by decreasing the magnetic nanoparticle surface availability for grafting, both of which yielded larger cluster sizes. The addition of small amounts of PAA-coated magnetic nanoparticles to the Janus nanoparticle suspension resulted in a further increase in the final cluster size. Monte Carlo simulation results compared favorably with experimental observations and showed the formation of small, elongated clusters similar in structure to those observed in cryo-TEM images.     

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.     

Light‐induced self‐thermophoresis of Janus spheroidal nanoparticles

A theoretical framework is provided for determining the self‐thermophoretic velocity of a light irradiated spheroidal Janus nanoparticle consisting of symmetric dielectric and perfectly conducting semi‐spheroids. The analysis is based on solving the linearized Joule heating problem due to uniform laser irradiance and on explicitly determining the temperature fields inside and outside the particle. We employ the thermoelectric (Peltier‐ Seebeck) methodology to find the surface self ‐ induced temperature gradient and the related slip velocity which determines the autonomous phoretic (self ‐ propulsion) mobility of the Janus particle. Simplified explicit expressions for the self ‐ thermophoretic velocities of spheroidal (prolate and oblate) Janus particles in terms of their aspect ratios are found and few practical limiting cases (i.e., sphere, disk and needle) are also discussed.     

Remote Control of T Cell Activation Using Magnetic Janus Particles

We report a strategy for using magnetic Janus microparticles to control the stimulation of T cell signaling with single‐cell precision. To achieve this, we designed Janus particles that are magnetically responsive on one hemisphere and stimulatory to T cells on the other side. By manipulating the rotation and locomotion of Janus particles under an external magnetic field, we could control the orientation of the particle–cell recognition and thereby the initiation of T cell activation. This study demonstrates a step towards employing anisotropic material properties of Janus particles to control single‐cell activities without the need of complex magnetic manipulation devices.     

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.     

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.     

配体链包覆纳米粒子的表面相分离及自组装的模拟研究

通过耗散粒子动力学方法,模拟了二元配体链包覆的纳米粒子表面的相分离行为,并与现有的模拟和实验体系进行对比.研究结果印证了相分离驱动力是配体链错位所导致的构象熵的结论.进一步以相分离得到的Janus和三嵌段Janus结构纳米粒子作为构筑单元,研究了其在选择性溶剂中的自组装行为.结果表明,Janus粒子易自组装成为双层囊泡结构,而三嵌段Janus粒子则更易形成单层囊泡结构.对于从配体链包覆的纳米粒子出发,设计具有特殊功能的囊泡提供了理论支持.     

Amphiphilic silica nanoparticles at the decane-water interface: insights from atomistic simulations

The properties of 3 nm-diameter silica nanoparticles with different surface chemistry were systematically investigated at the decane-water interface using molecular dynamics simulations. Our results show that the decane-water interfacial tension is not much influenced by the presence of the nanoparticles. The three-phase contact angle increases with nanoparticle surface hydrophobicity. Contact angles observed for the nanoparticles at 300 and at 350 K differ very little. The contact angle of the nanoparticle with randomly dispersed hydrophobic groups is smaller than that observed in Janus nanoparticles of equal overall surface chemistry composition. The energy necessary to desorb Janus nanoparticles from the interface is usually higher than that required to desorb the corresponding homogeneous nanoparticles. Desorption from the interface into the aqueous phase is preferred over that into the organic phase for all except one of the nanoparticles considered. Structural and dynamic properties including nanoparticle rotational relaxation, solvent density profiles, and solvent residence autocorrelation functions near the nanoparticles are also presented. The data are useful for designing Pickering emulsions.     

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.     

Preparation of various Janus composite particles with two components differently combined

Janus composite particles with a combination of organic and inorganic substances were synthesized by soap-free emulsion polymerization in which an amphoteric initiator of 2,2′-azobis[N-(2-carboxyethyl)-2-2-methylpropionamidine] (VA-057) was employed to introduce a polystyrene (PSt) lobe onto silica cores surface-modified with 3-methacryloxypropyltrimethoxy silane (MPTMS). Thermogravimetric analysis and X-ray photoelectron spectroscopy were used to characterize the surface-modified silica particles and showed that a small amount of MPTMS introduced onto the surface of silica particles could successfully prepare SiO₂–PSt Janus particles. The oxide part of SiO₂–PSt Janus particles obtained with the polymerization was further surface-modified with 3-aminopropyltriethoxysilane (APS) to introduce positively charged amino groups on the silica surface. The silica surface modified with APS was covered with gold by electroless deposition in which a gold precursor of auric chloride was reduced with ascorbic acid in the presence of polyvinylpyrrolidone. The electroless deposition of gold successfully produced Janus particles with a combination of gold and PSt surfaces. Furthermore, dissolution of the polymer component of the Au–PSt Janus particles in tetrahydrofuran led to another Janus type of particles with an inorganic combination of Au and SiO₂.     

Controlling the geometry (Janus balance) of amphiphilic colloidal particles

A simple, generalizable method is described to produce Janus colloidal particles in large quantity with control over their respective hydrophobic and hydrophilic areas (Janus balance) in large quantity. To this end, charged particles adsorb onto the liquid-liquid interface of emulsions of molten wax and water in the presence of surfactants of opposite charge, whose concentration modifies how deeply particles penetrate the oil-water interface, and subsequent surface chemical modification of the resulting colloidosomes is performed after lowering temperature to solidify the wax. Silica particles modified in this way using different amounts of didodecyldimethylammonium bromide (DDAB) display contact angles that vary controllably between 37 degrees and 75 degrees. Janus balance also varies but over a more limited range with control of pH, salt concentration, or the presence of nonionic surfactant (Tween 20 or ethanol). Purity, Janus balance, and colloidosome structure are evaluated by a combination of fluorescence microscopy, flow cytometry, and scanning electron microscopy (SEM). The three-phase contact angle is obtained by observing SEM images of voids left by particles escaped from the surface. Colloidosomes made in the presence of DDAB are markedly improved with respect to the hexagonal close packing, which helps increase the efficiency of the method. Gram-sized quantities of particles are synthesized.     

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.     

Selective surface chemistry using alumina nanoparticles generated from block copolymers

Developing orthogonal surface chemistry techniques that perform at the nanoscale is key to achieving precise control over molecular patterning on surfaces. We report the formation and selective functionalization of alumina nanoparticle arrays generated from block copolymer templates. This new material provides an alternative to gold for orthogonal surface chemistry at the nanometer scale. Atomic force microscopy and X-ray photoelectron spectroscopy confirm these particles show excellent selectivity over silica for phosphonic and carboxylic acid adsorption. As this is the first reported synthesis of alumina nanoparticles from block copolymer templates, characterizations via Fourier transform infrared spectroscopy, Auger electron spectroscopy, and transmission electron microscopy are presented. Reproducible formation of alumina nanoparticles was dependent on a counterintuitive synthetic step wherein a small amount of water is added to an anhydrous toluene solution of block copolymer and aluminum chloride. The oxidation environment of the aluminum in these particles, as measured by Auger electron spectroscopy, is similar to that of native aluminum oxide and alumina grown by atomic layer deposition. This discovery expands the library of available surface chemistries for nanoscale molecular patterning.     

Membrane emulsification and solvent pervaporation processes for the continuous synthesis of functional magnetic and Janus nanobeads

We discuss the integration of membrane emulsification and pervaporation processes for the continuous production of functional materials, such as silica-encapsulated magnetite nanoparticle clusters and asymmetric Janus nanoparticles, by the emulsion droplet solvent evaporation method, which has traditionally been performed in small-scale batch systems. An organic solvent containing primary magnetite nanoparticles (～10 nm) coated with oleic acid was dispersed in a continuous aqueous phase by membrane emulsification, which enabled the consistent production of nanoparticle-laden solvent droplets of well-controlled size with narrow size distributions. The solvent was removed from the emulsion by pervaporation. Prior to complete solvent removal, the nanoparticle packing density within the clusters was a function of the residence time in the pervaporation unit. The final clusters formed, ～100-300 nm in size, exhibited the same superparamagnetic behavior as the primary nanoparticles, and were stable in aqueous media with a zeta potential of -70 mV at neutral pH. A facile method was used to coat the nanoclusters with a silica shell, providing sites for surface functionalization with a range of organic ligands. The nanoparticles and clusters were analyzed by a variety of techniques, including TGA, DLS, TEM, EDS, and SQUID. The effects of various parameters, such as the membrane dimensions and flow rate through the unit, on the mass transport rates were elucidated through a parametric modeling study. The applicability of the methods to the production of polymeric beads and more complex particles was demonstrated; to create Janus structures, organic polymer solutions were dispersed as droplets in continuous aqueous phases, and the solvent was subsequently evaporated. The Janus particles consisted either of polymeric cores with magnetite nanoparticles clustered as islands on their surfaces, or of two phase-separated polymers, each constituting half of any given polymeric particle.     

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.     

Novel fabrication of Janus particles from the surfaces of electrospun polymer fibers

A novel synthetic approach for the efficient fabrication of Janus silica particles was demonstrated by embedment of zero-dimensional colloids on one-dimensional polymer fiber surfaces, followed by the surface modification on the exposed silica hemispheres. Electrospinning of poly(methyl methacrylate) and poly(4-vinyl pyridine) blends produced polymer fibers with high specific surface area and desired surface hydrophilicities. Fiber compositions determined the colloid adsorption density and uniformity. The colloid embedding resulted from the polymer softening was manipulated by the isothermal heat treatment. Subsequent silianization completed the amino functionalities on hemispherical surfaces of embedded silica colloids. Janus particles with uniform asymmetric chemical features were further labeled with gold nanoparticles before their recovery from fiber substrates. Fabrication of Janus particles, including colloid adsorption, temperature-driven embedding, and hemispherical surface modification, were investigated and are discussed.     

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...     

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.