Janus particles

Janus particles are special types of nanoparticles or microparticles whose surfaces have two or more distinct physical properties. 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. 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 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 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 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.' 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 10um to 53um in diameter are currently commericially available from Cospheric, who holds a patent on Hemispherical Coating Method for Microelements . 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 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.