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Janus particles are special types of
/ref> This unique surface of Janus particles allows two different types of chemistry to occur on the same particle. The simplest case of a Janus particle is achieved by dividing the particle into two distinct parts, each of them either made of a different material, or bearing different functional groups. For example, a Janus particle may have one half of its surface composed of hydrophile groups and the other half hydrophobe groups, the particles might have two surfaces of different color, fluorescence, or magnetic properties. This gives these particles unique properties related to their asymmetric structure and/or functionalization.
The term was first used in a real-world scientific context by C. Casagrande et al. in 1988Casagrande C., Veyssie M., C. R. Acad. Sci. (Paris), 306 11, 1423, 1988. to describe spherical glass particles with one of the hemispheres hydrophilic and the other hydrophobic. In that work, the amphiphilic beads were synthesized by protecting one hemisphere with varnish and chemically treating the other hemisphere with a silane reagent. This method resulted in a particle with equal hydrophilic and hydrophobic areas. In 1991, Pierre-Gilles de Gennes mentioned the term "Janus" particle in his Nobel Prize lecture. Janus particles are named after the two faced Roman god Janus because these particles may be said to have "two faces" since they possess two distinct types of properties. de Gennes pushed for the advancement of Janus particles by pointing out these "Janus grains" have the unique property of densely self-assembling at liquid–liquid interfaces, while allowing material transport to occur through the gaps between the solid amphiphile particles.
In 1976 Nick Sheridon of Xerox Corporation patented a Twisting Ball Panel Display, where he refers to a "plurality of particles which have an electrical anisotropy." United States Patent 4,126,854 Sheridon 21 November 1978 ---- Twisting ball panel display Although the term "Janus particles" was not yet used, Lee and coworkers also reported particles matching this description in 1985. They introduced asymmetric polystyrene/polymethylmethacrylate lattices from seeded emulsion polymerization. One year later, Casagrande and Veyssie reported the synthesis of glass beads that were made hydrophobic on only one hemisphere using octadecyl trichlorosilane, while the other hemisphere was protected with a cellulose varnish. The glass beads were studied for their potential to stabilize emulsification processes. Then several years later, Binks and Fletcher investigated the wetting of Janus beads at the interface between oil and water. They concluded Janus particles are both surface-active and amphiphilic, whereas homogeneous particles are only surface-active. Twenty years later, a plethora of Janus particles of different sizes, shapes and properties, with applications in textile, chemical sensors, stabilization of emulsions, and magnetic field imaging have been reported. Variety of janus particles in sizes 10 μm to 53 μm in diameter are currently commercially available from Cospheric, who holds a patent on Hemispherical Coating Method for Microelements. United States Patent 8,501,272 Lipovetskaya, et al. 6 August 2013 ----Hemispherical coating method for micro-elements
The phase interface method involves trapping homogeneous nanoparticles at the interface of two immiscible phases. These methods typically involve the liquid–liquid and liquid–solid interfaces, but a gas–liquid interface method has been described.
The liquid–liquid interface method is best exemplified by Gu et al., who made an emulsion from water and an oil and added nanoparticles of magnetite. The magnetite nanoparticles aggregated at the interface of the water-oil mixture, forming a Pickering emulsion. Then, silver nitrate was added to the mixture, resulting in the deposition of silver nanoparticles on the surface of the magnetite nanoparticles. These Janus nanoparticles were then functionalized by the addition of various ligands with specific affinity for either the iron or silver. This method can also use gold or iron-platinum instead of magnetite.
A similar method is the gas–liquid interface method developed by Pradhan et al. In this method, hydrophobic alkane thiolate gold nanoparticles were placed in water, causing the formation of a monolayer of the hydrophobic gold nanoparticles on the surface. Air pressure was then increased, forcing the hydrophobic layer to be pushed into the water, decreasing the contact angle. When the contact angle was at the desired level, a hydrophilic thiol, 3-mercaptopropane-1,2-diol, was added to the water, causing the hydrophilic thiol to competitively replace the hydrophobic thiols, resulting in the formation of amphiphilic Janus nanoparticles.
The liquid–liquid and gas–liquid interface methods do have an issue where the nanoparticles can rotate in solution, causing the deposition of silver on more than one face. A liquid–liquid/liquid–solid hybrid interface method was first introduced by Granick et al. as a solution to this liquid–liquid method problem. In this method, molten paraffin wax was substituted for the oil, and silica nanoparticles for the magnetite. When the solution was cooled, the wax solidified, trapping half of each silica nanoparticle in the wax surface, leaving the other half of the silica exposed. The water was then filtered off and the wax-trapped silica nanoparticles were then exposed to a methanol solution containing (amino- propyl)triethoxysilane, which reacted with the exposed silica surfaces of the nanoparticles. The methanol solution was then filtered off and the wax was dissolved with chloroform, freeing the newly made Janus particles. Liu et al. reported the synthesis of acorn- and mushroom-shaped silica–aminopropyl–trimethoxysilane Janus nanoparticles using the hybrid liquid–liquid/liquid–solid method developed by Granick et al. They exposed homogenous aminopropyl-trimethoxysilane functionalized silica nanoparticles embedded in wax to an ammonium fluoride solution, which etched away the exposed surface. The liquid–liquid/liquid–solid hybrid method also has some drawbacks; when exposed to the second solvent for functionalization, some of the nanoparticles may be released from the wax, resulting in homogenous instead of Janus nanoparticles. This can partially be corrected by using waxes with higher melting points or performing functionalization at lower temperatures. However, these modifications still result in significant loss. Cui et al. designed a more enduring mask made of polydimethylsiloxane (PDMS) polymer film to create a liquid–liquid/liquid–solid interface. The exposed-to-be-modified portion of particle surface can be adjusted by controlling the PDMS curing temperature and time, thus the embedment depth of the particles. The advantage of this fabrication method is that PDMS is inert and enduring in many wet chemistry solutions, and various metal or oxides or alloys such as silver, gold, nickel, titania can modify the exposed surface. Granick et al., in another paper, demonstrated a possible fix by using a liquid–liquid/gas–solid phase hybrid method by first immobilizing silica nanoparticles in paraffin wax using the previously discussed liquid–solid phase interface method, and then filtering off the water. The resulting immobilized nanoparticles were then exposed to silanol vapor produced by bubbling nitrogen or argon gas through liquid silanol, causing the formation of a hydrophilic face. The wax was then dissolved in chloroform, releasing the Janus nanoparticles.
An example of a more traditional liquid–solid technique has been described by Sardar et al. by beginning with the immobilization of gold nanoparticles on a silanized glass surface. Then the glass surface was exposed to 11-mercapto-1-undecanol, which bonded to the exposed hemispheres of the gold nanoparticles. The nanoparticles were then removed from the slide using ethanol containing 16-mercaptohexadecanoic acid, which functionalized the previously masked of the nanoparticles.
Typical organic phase separation methods use cojetting of polymers to produce Janus nanoparticles. This technique is exemplified by the work of Yoshid et al. to produce Janus nanoparticles where one hemisphere has affinity for human cells, while the other hemisphere has no affinity for human cells. This was achieved by cojetting polyacrylamide/poly(acrylic acid) copolymers which have no affinity for human cells with Biotinylation polyacrylamide/poly(acrylic acid) copolymers, which when exposed to streptavidin-modified antibodies, obtain an affinity for human cells.
The inorganic phase separation methods are diverse and vary greatly depending on the application. The most common method uses the growth of a crystal of one inorganic substance on or from another inorganic nanoparticle. A unique method has been developed by Gu et al., where iron-platinum nanoparticles were coated with sulfur reacted with cadmium acetylacetonate, trioctyl, and hexadecane-1,2-diol at 100 °C to produce nanoparticles with an iron-platinum core and an amorphous cadmium-sulfur shell. The mixture was then heated to 280 °C, resulting in a phase transition and a partial eruption of the Fe-Pt from the core, creating a pure Fe-Pt sphere attached to the CdS-coated nanoparticle. A new method of synthesizing inorganic Janus nanoparticles by phase separation has recently been developed by Zhao and Gao. In this method, they explored the use of the common homogeneous nanoparticle synthetic method of flame synthesis. They found when a methanol solution containing ferric triacetylacetonate and tetraethylorthosilicate was burned, the iron and silicon components formed an intermixed solid, which undergoes phase separation when heated to approximately 1100 °C to produce maghemite-Silicon dioxide Janus nanoparticles. Additionally, they found it was possible to modify the silica after producing the Janus nanoparticles, making it hydrophobic by reacting it with oleylamine.
In an aqueous solutions, two kinds of biphasic particles can be distinguished. The first type are particles which are truly amphiphilic and possess one hydrophobic and one hydrophilic side. The second type has two water-soluble, yet chemically distinct, sides. To illustrate the first case, extensive studies have been carried out with spherical Janus particles composed of one hemisphere of water-soluble PMAA and another side of water-insoluble polystyrene. In these studies, the Janus particles were found to aggregate on two hierarchical levels. The first type of self-assembled aggregates look like small clusters, similar to what is found for the case of Janus particles in an organic solution. The second type is noticeably larger than the first and has been termed 'super micelles'. Unfortunately, the structure of the is unknown so far; however, they may be similar to liposome.
For the second case of Janus particles which contain two distinct, but still water-soluble sides, the work of Granick's group provides some insight. Their research deals with the clustering of dipolar (), micronsized Janus particles, whose two sides are both fully water-soluble. Zwitterionic Janus particles do not behave like classical dipoles, since their size is much larger than the distance at which electrostatic attractions are strongly felt. The study of zwitterionic Janus particles once again demonstrates their ability to form defined clusters. However, this particular type of Janus particle prefers to aggregate into larger clusters since this is more energetically favorable because each cluster carries a macroscopic dipole which allows the aggregation of already-formed clusters into larger assemblies. Compared to aggregates formed through van der Waals interactions for homogenous particles, the shapes of the zwitterionic Janus nanoclusters are different and the Janus clusters are less dense and more asymmetric.
In 2007, the amphiphilic nature of the Janus nanoparticles was examined by measuring the adhesion force between the atomic force microscopy (AFM) tip and the particle surface. The stronger interactions between the hydrophilic AFM tip and the hydrophilic side of the Janus nanoparticles were reflected by a greater adhesion force. The Janus nanoparticles were dropcast onto both hydrophobically and hydrophilically modified substrates. The hydrophobic hemisphere of the Janus particles was exposed when a hydrophilic substrate surface was used, resulting in disparities in adhesion force measurements. Thus, the Janus nanoparticles adopted a conformation that maximized the interactions with the substrate surface.
The nature of amphiphilic Janus nanoparticles to orient themselves spontaneously at the interface between oil and water has been well known. This behavior allows considering amphiphilic Janus nanoparticles as analogues of molecular surfactants for the stabilization of emulsions. In 2005, spherical silica particles with amphiphilic properties were prepared by partial modification of the external surface with an alkylsilane agent. These particles form spherical assemblies encapsulating water-immiscible organic compounds in aqueous media by facing their hydrophobic alkylsilylated side to the inner organic phase and their hydrophilic side to the outer aqueous phase, thus stabilizing oil droplets in water. In 2009, hydrophile surface of silica particles was made partially hydrophobic by adsorbing cetyltrimethylammonium bromide. These amphiphilic nanoparticles spontaneously assembled at the water-dichloromethane interface. In 2010, Janus particles composed from silica and polystyrene, with the polystyrene portion loaded with nanosized magnetite particles, were used to form kinetically stable oil-in-water emulsions that can be spontaneously broken on application of an external magnetic field. Such Janus materials will find applications in magnetically controlled optical switches and other related areas. The first real applications of Janus nanoparticles were in polymer synthesis. In 2008, spherical amphiphilic Janus nanoparticles, having one polystyrene and one poly(methyl methacrylate) side, were shown to be effective as compatibilizing agents of multigram scale compatibilization of two immiscible polymer blends, polystyrene and poly(methyl methacrylate). The Janus nanoparticles oriented themselves at the interface of the two polymer phases, even under high temperature and shear conditions, allowing the formation of much smaller domains of poly(methyl methacrylate) in a polystyrene phase. The performance of the Janus nanoparticles as compatibilizing agents was significantly superior to other state-of-the-art compatibilizers, such as linear block .
In 2013, based on the computer simulation results it has been shown that self-propelled Janus particles can be used for direct demonstration of the non-equilibrium phenomenon, ratchet effect. Ratcheting of Janus particles can be orders of magnitude stronger than for ordinary thermal potential ratchets and thus easily experimentally accessible. In particular, autonomous pumping of a large mixture of passive particles can be induced by just adding a small fraction of Janus particles.
In 2011, silica-coated Janus nanoparticles, composed of silver oxide and iron oxide (Fe2O3), were prepared in one step with scalable flame aerosol technology. These hybrid plasmonic-magnetic nanoparticles bear properties that are applicable in bioimaging, targeted drug delivery, in vivo diagnosis, and therapy. The purpose of the nanothin Silicon dioxide shell was to reduce the release of toxic silver+ ions from the nanoparticle surface to live cells. As a result, these hybrid nanoparticles showed no cyctotoxicity during bioimaging and remained stable in suspension with no signs of agglomeration or settling, thus enabling these nanoparticles as biocompatible multifunctional probes for bioimaging. Next, by labeling their surfaces and selectively binding them on the membrane of live-tagged Raji and HeLa cells, this demonstrated the nanoparticles as and their detection under dark-field illumination was achieved. These new hybrid Janus nanoparticles overcame the individual limitations of Fe2O3 (poor particle stability in suspension) and of silver (toxicity) nanoparticles, while retaining the desired magnetic properties of Fe2O3 and the plasmonic optical properties of silver.
Graphene Janus particles have been used in experimental sodium-ion batteries to increase energy density. One side provides interaction sites while the other provides inter-layer separation. Energy density reached 337 mAh/g.
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