What exactly is a Janus disk? Well, with a quick internet search you can easily find several references to Janus, the Roman god of doorways, gates, and beginnings (hence the word January for the first month of the year), but a picture search is actually most revealing. Usually Janus is shown with two different faces that look in opposite directions; one represents the sun and the other symbolizes the moon. Interesting--but what does this have to do with chemistry??
Well after that brief review of Roman mythology, one can easily imagine that a Janus particle is composed of two fused hemispheres of different materials--similar to the bust of Janus pictured above. Depending on their actual shape, Janus particles are placed into three categories: spheres, disks and cylinders. Several potential applications of these two-sided particles have been envisioned. For instance, in solar cells two very different types of molecules (donors and acceptors) must work together and convert light into electron movement; thus, using Janus particles within light harvesting devices might increase solar cell efficiencies. One could also imagine a Janus-scaffold as a drug delivery system; half of the disk might target cancer cells, while the other end would deliver a cytotoxic drug.
Synthesis of Janus structures is a daunting task and only a few examples of non-spherical Janus particles exist in the literature; thus, when I came across this article in JACS today, it caught my attention. Researchers at the University of Bayreuth in Germany have recently succeeded in producing Janus disks utilizing a template-assisted synthesis. Polymers made of polystyrene-block-polybutadiene-block-poly(tert-butyl methacrylate) were self-assembled and then treated with either AIBN or S2Cl2 to crosslink the inner polybutadiene layer; this step preserves the orientation of the polystyrene and poly(tert-butyl methacrylate). Finally, after sonication the Janus disks are obtained in their final form; size of the disks is tunable and ranges from the micro- to nanometer scale. As Janus structures have also been proposed to have potential as surfactants, the effect of these Janus disks on the interfacial tension of liquid-liquid interfaces was studied as well. Compared to their un-crosslinked starting materials, the Janus disks have a remarkable ability to decrease interfacial tension, and therefore future technological applications might include the stabilization of emulsions or encapsulation of molecules.
Picture taken from http://dx.doi.org/10.1021/ja068153v