Janus particles are special types of nanoparticles or microparticles whose surfaces have two or more distinct physical properties.Janus Particle Synthesis, Self-Assembly and Applications, Editors: Shan Jiang, Steve Granick, Royal Society of Chemistry, Cambridge 2013, https://pubs.rsc.org/en/content/ebook/978-1-84973-510-0 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 hydrophilic groups and the other half hydrophobic 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. Janus particles are so-named in reference to the two-faced Roman god Janus, as they may be said to—similarly—have "two faces".
History The term "Janus Particle" was coined by author Leonard Wibberley in his 1962 novel The Mouse on the Moon as a science-fictional device for space travel.
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 grains" in his Nobel lecture; de Gennes advocated for the utility and scientific interest of Janus particles by pointing out that 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 amphiphilic 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 wettability 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, 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
Synthesis The synthesis of Janus nanoparticles requires the ability to selectively create each side of a nanometer-sized particle with different chemical properties in a cost-effective and reliable way that produces the particle of interest in high yield. Initially, this was a difficult task, but within the last 10 years, methods have been refined to make it easier. Currently, three major methods are used in the synthesis of Janus nanoparticles.
Masking
Masking was one of the first techniques developed for the synthesis of Janus nanoparticles. This technique was developed by simply taking synthesis techniques of larger Janus particles and scaling down to the nanoscale. Masking, as the name suggests, involves the protection of one side of a nanoparticle followed by the modification of the unprotected side and the removal of the protection. Two masking techniques are common to produce Janus particles, evaporative deposition and a technique where the nanoparticle is suspended at the interface of two phases. However, only the phase separation technique scales well to the nanoscale.
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 hemispheres of the nanoparticles.
Self-assembly
Block copolymers
This method uses the well-studied methods of producing block copolymers with well-defined geometries and compositions across a large variety of substrates. Synthesis of Janus particles by self-assembly via block copolymers was first described in 2001 by Erhardt et al. They produced a triblock polymer from polymethylacrylate, polystyrene and low-molecular-weight polybutadiene. The polystyrene and polymethylacrylate formed alternating layers in between which polybutadiene sat in nanosized spheres. The blocks were then cross-linked and dissolved in THF, and after several washing steps, yielded spherical Janus particles with polystyrene on one face and polymethylacrylate on the other, with a polybutadiene core. The production of Janus spheres, cylinders, sheets, and ribbons is possible using this method by adjusting the molecular weights of the blocks in the initial polymer and also the degree of cross-linking.
Competitive adsorption The key aspect of competitive absorption involves two substrates that phase-separate due to one or more opposite physical or chemical properties. When these substrates are mixed with a nanoparticle, typically gold, they maintain their separation and form two faces. A good example of this technique has been demonstrated by Vilain et al., where phosphinine-coated gold nanoparticles were exposed to long-chain thiols, resulting in substitution of the phosphinine ligands in a phase-separated manner to produce Janus nanoparticles. Phase separation was proven by showing the thiols formed one locally pure domain on the nanoparticle using FT-IR.