The word Janus has its origin in the Roman god Janus. In Roman mythology this was the god of the gates, doors, doorways, beginning and endings; who is depicted with a double-faced head, each looking in opposite directions. Thus, in nature and materials science, asymmetric particles with two incompatible sides are called Janus particles. The most common type of Janus particle is a sphere commonly with one polar half, and other nonpolar half. The first report of Janus particles was made by Cho et al. [1], who presented asymmetric poly (styrene)/poly (methyl methacrylate) lattices from seeded emulsion polymerization. Four years later, Casagrande et al. [2] described the preparation of Janus beads. They prepared glass spherical particles, with one hydrophilic hemisphere, and the other hydrophobic. In 1991, in his Nobel Prize lecture De Gennes [3], advanced the popularity of Janus particles, in which the coined the term “Janus grains”. He was one of the first scientists to use the term Janus for the description of particles whose surfaces are different from a chemical point of view. Since then, research into asymmetric particles has boomed. Wurm & Kilbinger [4] give a complete review of different mechanisms of Janus particle formation.
Janus particles offer a number of potential applications in electronic displays, sensors, imaging, probes, gene delivery, two-phase stabilizers, and nanoparticle self-propulsion. However, realizing these applications requires precise control over the position and orientation of the particles, something which has until now eluded scientists. Erb et al. [5] reported a new type of spherical Janus (half and dot) that can be manipulated by a combination of optical and magnetic fields.
Currently the application of Janus particles to mimic the behavior of living microorganisms is one of the most exciting challenges in nanotechnology. Particular attention is given to Janus particles with a chemical reaction on one half of the surface. These particles move autonomously by their asymmetric chemical reaction without the need for external power, and operate according to concentration gradients across their surface or in their interfacial region, which in turn results in propulsion. In 2005 Golestanian and coworkers [6] proposed a simple model for the propulsion of a molecular machine by asymmetric distribution of reaction products. They considered a spherical particle (colloid or vesicle) of radius R that has a single enzymatic side located on its surface at a fixed position. In the presence of a reactive substrate in a non-equilibrium state, the enzyme promotes the reaction rate in its vicinity and produces a dynamic and asymmetric distribution of product particles that exert osmotic or interfacial forces and hence propel the sphere in a fixed direction [6]. Those work inspired Howse et al. [7] to synthesize Janus particles to verify the proposed self-diffusiophoretic propulsion mechanisms. They characterized experimentally the motion of an artificial microscale swimmer that uses a chemical reaction catalyzed on its own surface to achieve autonomous propulsion. The particles consisted of a thin layer of platinum deposited on one side of spherical polystyrene beads with diameter of 1.62 mm. The platinum catalyzes the reduction of hydrogen peroxide to oxygen and water, which produces more molecules of reaction product than consumed fuel. Since this experimental study, the analysis used to predict the self-diffusiophoretic swimmer has been extended.
A more general model for Janus particle has been developed by Cordova & Brady [8]. This model is called the osmotic motor, which is a colloidal particle immersed in a dispersion of “bath” particles. A non-equilibrium concentration of bath particles induced by a surface chemical reaction creates an osmotic pressure imbalance on the motor causing it to move. Alternative mechanisms have also been proposed to explain the motion of a silica analogue of the Janus particle described above. In this case a bubble propulsion model based on catalyzed hydrogen peroxide decomposition and momentum exchange via O2 bubbles detaching from the catalytic surface is proposed to explain the autonomous motion of catalytic nanomotors [9]. This model was supported by trends in the silica Janus particle swimming velocity as surface tension was varied using a surfactant. Finally Ye & Carrol [10] developed a chemically – powered asymmetric system of catalytic Janus particles derived from silica microspheres capped with two metals (gold, platinum and nickel). These particles undergo driven motion when placed in a solution of chemical fuel. More details regarding the propulsion of asymmetric catalytic particles has been given by Ebbens et al. [11], who describe recent development in self-propelling nano- and micro-scale swimming devices. The objective of the present post is to show something about Janus particles. Specifically, catalytic Janus particles because of their importance in the world of self propelled nanoparticles, opening up broad possibilities for nanotechnology and device applications.
[1] I. Cho and K. W. Lee. J. Appl. Polym. Sci. 1985, 30, 1903-1926.
[2] C. Casagrande, P. Fabre, E. Raphael and M. Veyssié. Europhys. Lett. 1989, 9, 251-255.
[3] De Gennes, P. G. College of France. 1991.
[4] F. Wurm and A. F. M. Kilbinger. Angew. Chem. Int. Ed. 2009, 48, 8412-8421
[5] R. M. Erb, N. J. Jennes, R. L. Clark and B. B. Yellen. Adv. Mater. 2009, 21, 4825-4829.
[6] R. Golestanian, T. B. Liverpool and A. Adjari. Phys. Rev. Lett. 2005, 94, 220801.
[7] J. R. Howse, R. A. L. Jones, A. J. Ryan, T. Gough, R. Vafabakhsh and R. Golestanian.Phys. Rev. Lett. 2007, 99, 048102.
[8] U. M. Córdova-Figueroa and J. F. Brady. Phys. Rev. Lett. 2008, 100, 158303.
[9] J. G. Gibbs and Y.-P. Zhao. Appl. Phys. Lett. 2009, 94, 163104.
[10] S. Ye and L. Carroll. The 82nd Colloid & Surface Science Symposium. 2008
[11] S. J. Ebbens and J. R. Howse. Eur. Phys. J. E. 2010, 6, 726-738.
Author: Glenn C. Vidal-Urquiza