History
Originally, the term Janus particle was coined 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. Although the term "Janus particles" was not yet used, Lee and coworkers reported the first 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.
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