Janus particle
Encyclopedia
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. Since then, and as anticipated by Pierre-Gilles de Gennes
on the occasion of his Nobel lecture, the surface properties of these Janus particles have become an area of great interest for a variety of applications, such as drug delivery
, catalysis
, sensors, stabilization of water-oil emulsions and gas-liquid foams.
These particles are composed of at least two physically (or chemically) distinctive surfaces. The name Janus arises from the double-faced Roman god Janus
.
Janus particles can be grafted with proteins on one side only. Recent work has presented a study on adsorption/covalent coupling
of proteins (fibronectin and fibrinogen) onto 1 µm PS, 1 µm SiO_2 and 250 nm thioled-Au particles. The authors studied adsorption reactions in 3 different biological buffers to minimize the protein adsorption on the particles. Covalent couplings performed in MES buffer allows us to extract the best amount of crosslinker and proteins for our range of values. Finally, they have prepared Janus particles according to the side on which proteins have to be grafted: either thioled-Au/unfunctionalized dielectric JPs for grafting on the Au side, or Au/carboxylate activated dielectric JPs for grafting on the dielectric side. The desired sides were successfully grafted according to the previous protocols. Those new generations of JPs could lead to promising cellular sensor or manipulators when coupled with dielectrophoresis handling.
) of the liquid at the droplet-droplet interface that hinders the drainage of the liquid between coalescing droplets. Additionally, it has been demonstrated that the stability of the Pickering emulsions is enhanced by the utilization of the Janus Particles, due to its highly active amphiphilic surface that increases the adsorption energy of the particle at the liquid-liquid interface.
The contact angle of the Janus particles can be tuned by modifying the hydrophilic:hydrophobic balance of the surface, which determines the type of emulsion formed (oil-in-water or water-in-oil). For example, when the particle's contact angle (measured through the aqueous phase) is lower than 90°, the emulsion type is oil-in-water, while when it is larger than 90° the emulsion obtained is water-in-oil.
A novel type of Janus particles that not only stabilize water/oil emulsions but also catalyze reactions at the interface has been reported. These “Janus catalysts” were prepared by fusing carbon nanotube and inorganic oxide nanoparticles, onto which catalytic species, such as Pd clusters were deposited to catalyze reaction in biphasic systems through a combination of high interface concentration and high interfacial surface area. Janus catalysts can be used to upgrade biofuels, by adding a basic catalyst (e.g. MgO) on the hydrophilic side and thus catalyzing the coupling of small molecules, while removing some oxygenated groups. The growth of the carbon chain and the removal of the oxygenated groups lower the water solubility of the molecules, and so, the desired products become more soluble in the organic phase, facilitating separation.
Pierre-Gilles de Gennes
Pierre-Gilles de Gennes was a French physicist and the Nobel Prize laureate in physics in 1991.-Biography:...
on the occasion of his Nobel lecture, the surface properties of these Janus particles have become an area of great interest for a variety of applications, such as drug delivery
Drug delivery
Drug delivery is the method or process of administering a pharmaceutical compound to achieve a therapeutic effect in humans or animals. Drug delivery technologies modify drug release profile, absorption, distribution and elimination for the benefit of improving product efficacy and safety, as well...
, catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
, sensors, stabilization of water-oil emulsions and gas-liquid foams.
These particles are composed of at least two physically (or chemically) distinctive surfaces. The name Janus arises from the double-faced Roman god Janus
Janus
-General:*Janus , the two-faced Roman god of gates, doors, doorways, beginnings, and endings*Janus , a moon of Saturn*Janus Patera, a shallow volcanic crater on Io, a moon of Jupiter...
.
Janus particles fabrication
The general process to fabricate a large amount 10^3 particles/ml of lifted-off Janus particles is to create a monolayer of particles on a wafer and then to evaporate metal on the whole wafer in a metal beam evaporator. The wafer is then placed in a beaker with 2 ml of DI water and sonicated for 2 h. The microfabrication process is detailed in the Figure.Janus particles can be grafted with proteins on one side only. Recent work has presented a study on adsorption/covalent coupling
of proteins (fibronectin and fibrinogen) onto 1 µm PS, 1 µm SiO_2 and 250 nm thioled-Au particles. The authors studied adsorption reactions in 3 different biological buffers to minimize the protein adsorption on the particles. Covalent couplings performed in MES buffer allows us to extract the best amount of crosslinker and proteins for our range of values. Finally, they have prepared Janus particles according to the side on which proteins have to be grafted: either thioled-Au/unfunctionalized dielectric JPs for grafting on the Au side, or Au/carboxylate activated dielectric JPs for grafting on the dielectric side. The desired sides were successfully grafted according to the previous protocols. Those new generations of JPs could lead to promising cellular sensor or manipulators when coupled with dielectrophoresis handling.
Janus particles and Pickering emulsions
The stabilization of water-oil emulsions by Janus particles is a kinetically-stabilized process characterized by: (1) strong absorption of particles at the liquid-liquid interface that forms a rigid film and prevents coalescence, and (2) the modification of the rheological properties (viscosityViscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
) of the liquid at the droplet-droplet interface that hinders the drainage of the liquid between coalescing droplets. Additionally, it has been demonstrated that the stability of the Pickering emulsions is enhanced by the utilization of the Janus Particles, due to its highly active amphiphilic surface that increases the adsorption energy of the particle at the liquid-liquid interface.
The contact angle of the Janus particles can be tuned by modifying the hydrophilic:hydrophobic balance of the surface, which determines the type of emulsion formed (oil-in-water or water-in-oil). For example, when the particle's contact angle (measured through the aqueous phase) is lower than 90°, the emulsion type is oil-in-water, while when it is larger than 90° the emulsion obtained is water-in-oil.
A novel type of Janus particles that not only stabilize water/oil emulsions but also catalyze reactions at the interface has been reported. These “Janus catalysts” were prepared by fusing carbon nanotube and inorganic oxide nanoparticles, onto which catalytic species, such as Pd clusters were deposited to catalyze reaction in biphasic systems through a combination of high interface concentration and high interfacial surface area. Janus catalysts can be used to upgrade biofuels, by adding a basic catalyst (e.g. MgO) on the hydrophilic side and thus catalyzing the coupling of small molecules, while removing some oxygenated groups. The growth of the carbon chain and the removal of the oxygenated groups lower the water solubility of the molecules, and so, the desired products become more soluble in the organic phase, facilitating separation.
Janus particles handling by dielectrophoresis
Janus particles Au/fluorescent polystyrene are fabricated and their flip/flop rotational effect is studied in a microfluidic channel thanks to dielectrophoresis, providing a new type of local light switch. A method for producing large amounts more than 10^6 particles/ml of Janus particles is first presented. Those particles were then injected in an electromicrofluidic chip and stabilized in the fluid by a dielectrophoretic trap. The spanning frequency of this trap allowed performing a “flip-flop” effect of the Janus particles by recording their fluorescent intensities. Flip Au top side and flop PS top side frequencies are identified. Experiments were performed on the time triggered commutations between flip and flop frequencies to define the capability of each Janus particle to sustain speed control of their flip-flop. ,External links
- Janus particles, Physics TodayPhysics TodayPhysics Today, created in 1948, is the membership journal of the American Institute of Physics. It is provided to 130,000 members of twelve physics societies, including the American Physical Society...
- '2-faced' particles act like tiny submarines, EurekAlert!
- Nano World: Two-faced Janus nanoparticles, PhysOrg.com