Janus particles

[7] 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[8] to describe spherical glass particles with one of the hemispheres hydrophilic and the other hydrophobic.

[11] 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.

Twenty years later, a plethora of Janus particles of different sizes, shapes and properties, with applications in textile,[15] sensors,[16] stabilization of emulsions,[17] and magnetic field imaging[18] have been reported.

The advantage of this fabrication method is that PDMS is inert and enduring in many wet chemistry solutions, and various metal or oxides or alloys such as silver, gold, nickel, titania can modify the exposed surface.

The blocks were then cross-linked and dissolved in THF, and after several washing steps, yielded spherical Janus particles with polystyrene on one face and polymethylacrylate on the other, with a polybutadiene core.

[35] The production of Janus spheres, cylinders, sheets, and ribbons is possible using this method by adjusting the molecular weights of the blocks in the initial polymer and also the degree of cross-linking.

They found when a methanol solution containing ferric triacetylacetonate and tetraethylorthosilicate was burned, the iron and silicon components formed an intermixed solid, which undergoes phase separation when heated to approximately 1100 °C to produce maghemite-silica Janus nanoparticles.

In the case of spherical Janus micelles having hemispheres of polystyrene (PS) and poly(methyl methacrylate) (PMMA), aggregation into clusters has been observed in various organic solvents, such as tetrahydrofuran.

Similarly, Janus discs composed of sides of PS and poly(tert-butyl methacrylate) (PtBMA) can undergo back-to-back stacking into superstructures when in an organic solution.

To illustrate the first case, extensive studies have been carried out with spherical Janus particles composed of one hemisphere of water-soluble PMAA and another side of water-insoluble polystyrene.

The first type of self-assembled aggregates look like small clusters, similar to what is found for the case of Janus particles in an organic solution.

[42] Zwitterionic Janus particles do not behave like classical dipoles, since their size is much larger than the distance at which electrostatic attractions are strongly felt.

At intermediate pH values, they found that the Janus nanoparticles were unstable due to dipolar interaction between the positively and negatively charged hemispheres.

In 2006, Janus nanoparticles, made from gold and iron oxides, were compared with their homogeneous counterparts by measuring the ability of the particles to reduce the interfacial tension between water and n-hexane.

[43] Experimental results indicated Janus nanoparticles are considerably more surface-active than homogeneous particles of comparable size and chemical nature.

The hydrophobic hemisphere of the Janus particles was exposed when a hydrophilic substrate surface was used, resulting in disparities in adhesion force measurements.

In 2005, spherical silica particles with amphiphilic properties were prepared by partial modification of the external surface with an alkylsilane agent.

[52] In 2007, spherical polystyrene Janus nano-particles with one side coated with platinum were used for the first time to catalyze the decomposition of hydrogen peroxide (H2O2).

The decomposition of hydrogen peroxide created Janus catalytic nano-motors, the motion of which was analyzed experimentally and theoretically using analytical techniques and computer simulations.

In 2013, based on the computer simulation results it has been shown that self-propelled Janus particles can be used for direct demonstration of the non-equilibrium phenomenon, ratchet effect.

A Janus particle size of 200 nm was found to deposit on the surface of fibers and were very efficient for the design of water-repellent textiles.

The groundbreaking progress in the biological sciences has led to a drive towards custom made materials with precisely designed physical/chemical properties at the nanoscale level.

In 2009, a new type of bio-hybrid material composed of Janus nanoparticles with spatially controlled affinity towards human endothelial cells was reported.

The concept is to eventually design probes based on Janus nanoparticles to attain directional information about cell-particle interactions.

Additionally, the polystyrene region may also be used as a carrier for drugs and other chemicals by surface hydrophobic adsorption or encapsulation, making the nanocoral a possible multifunctional nanosensor.

Quick responses to external magnetic fields could become an effective approach for targeted imaging, therapy in vitro and in vivo, and cancer treatment.

Similarly, a quick response to magnetic fields is also desirable to fabricate smart displays, opening new opportunities in electronics and spintronics.

[57] These hybrid plasmonic-magnetic nanoparticles bear properties that are applicable in bioimaging, targeted drug delivery, in vivo diagnosis, and therapy.

Next, by labeling their surfaces and selectively binding them on the membrane of live-tagged Raji and HeLa cells, this demonstrated the nanoparticles as biomarkers and their detection under dark-field illumination was achieved.

A schematic view of a basic spherical Janus particle with two distinct faces: Sides A and B represent two surfaces with different physical or chemical properties.
Schematic view of the synthesis of Janus nanoparticles via masking. 1) Homogeneous nanoparticles are placed in or on a surface in such a way that only one hemisphere is exposed. 2) The exposed surface is exposed to a chemical 3) which change its properties. 4) The masking agent is then removed, releasing the Janus nanoparticles.
Example of janus nanoparticles fabricated by a masking process
(a) Schematic representation of a masking microfabrication process. After creating a monolayer of fluorescent particles, bilayers of 1:10 Ti/Au are deposited on the top half of particles. The wafer is then placed in a beaker with 2 ml DI water and sonicated for 2 h to resuspend them. (b) SEM micrographs show the three types of JPs fabricated. Scale bar represents 500 nm. [ 21 ]
Schematic representation of the synthesis of Janus nanoparticles using the block copolymer self-assembly method
Scheme of the basic principle of phase separation method of producing Janus nanoparticles: Two incompatible substances (A and B) were mixed to form a nanoparticle. A and B then separate into their own domains while still part of a single nanoparticle.