![Janus nanoparticle](https://upload.wikimedia.org/wikipedia/commons/3/3b/Janus_nanoparticle.svg)
Janus particles are special types of
nanoparticle
A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...
s or microparticles whose surfaces have two or more distinct
physical properties
A physical property is any property that is measurable, whose value describes a state of a physical system. The changes in the physical properties of a system can be used to describe its changes between momentary states. Physical properties are o ...
. This unique surface of Janus particles allows two different types of
chemistry
Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds made of atoms, molecules and ions ...
to occur on the same particle. The simplest case of a Janus particle is achieved by dividing the particle into two distinct parts, each of them either made of a different material, or bearing different functional groups.
For example, a Janus particle may have one-half of its surface composed of
hydrophilic
A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press.
In contrast, hydrophobes are no ...
groups and the other half
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
groups, the particles might have two surfaces of different color, fluorescence, or magnetic properties. This gives these particles unique properties related to their asymmetric structure and/or functionalization.
History
The term "Janus Particle" was coined by author
Leonard Wibberley
Leonard Patrick O'Connor Wibberley (9 April 1915 – 22 November 1983), who also published under the name Patrick O'Connor, among others, was an Irish author who spent most of his life in the United States. Wibberley, who published more than 100 ...
in hi
1962 novelThe Mouse on the Moon
''The Mouse on the Moon'' is a 1963 British comedy film, the sequel to ''The Mouse That Roared''. It is an adaptation of the 1962 novel ''The Mouse on the Moon'' by Irish author Leonard Wibberley, and was directed by Richard Lester. In it, the p ...
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 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
Pierre-Gilles de Gennes (; 24 October 1932 – 18 May 2007) was a French physicist and the Nobel Prize laureate in physics in 1991.
Education and early life
He was born in Paris, France, and was home-schooled to the age of 12. By the age of ...
mentioned the term "Janus" particle in his
Nobel lecture. Janus particles are named after the two faced Roman god
Janus
In ancient Roman religion and myth, Janus ( ; la, Ianvs ) is the god of beginnings, gates, transitions, time, duality, doorways, passages, frames, and endings. He is usually depicted as having two faces. The month of January is named for Janu ...
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
An amphiphile (from the Greek αμφις amphis, both, and φιλíα philia, love, friendship), or amphipath, is a chemical compound possessing both hydrophilic (''water-loving'', polar) and lipophilic (''fat-loving'') properties. Such a compoun ...
particles.
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." Although the term "Janus particles" was not yet used, Lee and coworkers also reported particles matching this description in 1985. They introduced asymmetric
polystyrene
Polystyrene (PS) is a synthetic polymer made from monomers of the aromatic hydrocarbon styrene. Polystyrene can be solid or foamed. General-purpose polystyrene is clear, hard, and brittle. It is an inexpensive resin per unit weight. It is a ...
/
polymethylmethacrylate
Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylite, ...
lattices from seeded
emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
. 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
Wetting is the ability of a liquid to maintain contact with a solid surface, resulting from intermolecular interactions when the two are brought together. This happens in presence of a gaseous phase or another liquid phase not miscible with th ...
of Janus beads at the interface between oil and water. They concluded Janus particles are both surface-active and amphiphilic, whereas
homogeneous
Homogeneity and heterogeneity are concepts often used in the sciences and statistics relating to the uniformity of a substance or organism. A material or image that is homogeneous is uniform in composition or character (i.e. color, shape, siz ...
particles are only surface-active. Twenty years later, a plethora of Janus particles of different sizes, shapes and properties, with applications in textile,
sensors
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
,
stabilization of
emulsions
An emulsion is a mixture of two or more liquids that are normally immiscible (unmixable or unblendable) owing to liquid-liquid phase separation. Emulsions are part of a more general class of two-phase systems of matter called colloids. Althoug ...
,
and
magnetic field imaging
Magnetic Field Imaging (MFI) is a non-invasive and side-effect-free cardiac diagnostic method. In more recent technology, magnetocardiography (MCG) has become the clinically predominant application for recording the heart's magnetic signals. tha ...
have been reported. Variety of janus particles in sizes 10µm to 53µm in diameter are currently commercially available from Cospheric, who holds a patent on Hemispherical Coating Method for Microelements.
Synthesis
The synthesis of Janus nanoparticles requires the ability to selectively create each side of a nanometer-sized particle with different chemical properties in a cost-effective and reliable way that produces the particle of interest in high yield. Initially, this was a difficult task, but within the last 10 years, methods have been refined to make it easier. Currently, three major methods are used in the synthesis of Janus nanoparticles.
Masking
![Masking janus particles](https://upload.wikimedia.org/wikipedia/commons/2/26/Masking_janus_particles.svg)
Masking was one of the first techniques developed for the synthesis of Janus nanoparticles.
This technique was developed by simply taking synthesis techniques of larger Janus particles and scaling down to the nanoscale.
Masking, as the name suggests, involves the protection of one side of a nanoparticle followed by the modification of the unprotected side and the removal of the protection. Two masking techniques are common to produce Janus particles,
evaporative deposition
Evaporation is a common method of thin-film deposition. The source material is evaporated in a vacuum. The vacuum allows vapor particles to travel directly to the target object (substrate), where they condense back to a solid state. Evaporation ...
and a technique where the nanoparticle is suspended at the
interface
Interface or interfacing may refer to:
Academic journals
* ''Interface'' (journal), by the Electrochemical Society
* ''Interface, Journal of Applied Linguistics'', now merged with ''ITL International Journal of Applied Linguistics''
* '' Inte ...
of two phases. However, only the phase separation technique scales well to the nanoscale.
The phase interface method involves trapping homogeneous nanoparticles at the interface of two immiscible phases. These methods typically involve the liquid–liquid and liquid–solid interfaces, but a gas–liquid interface method has been described.
The liquid–liquid interface method is best exemplified by Gu ''et al.'', who made an emulsion from water and an oil and added nanoparticles of
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
. The magnetite nanoparticles aggregated at the interface of the water-oil mixture, forming a
Pickering emulsion
A Pickering emulsion is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the water and oil phases. Typically, the emulsions are either water-in-oil or oil-in-water emulsions, b ...
. Then,
silver nitrate
Silver nitrate is an inorganic compound with chemical formula . It is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides. It was once called ''lunar caustic' ...
was added to the mixture, resulting in the deposition of silver nanoparticles on the surface of the magnetite nanoparticles. These Janus nanoparticles were then functionalized by the addition of various ligands with specific affinity for either the iron or silver.
This method can also use gold or iron-platinum instead of magnetite.
A similar method is the gas–liquid interface method developed by Pradhan ''et al.'' In this method, hydrophobic
alkane
In organic chemistry, an alkane, or paraffin (a historical trivial name that also has other meanings), is an acyclic saturated hydrocarbon. In other words, an alkane consists of hydrogen and carbon atoms arranged in a tree structure in which ...
thiolate
In organic chemistry, a thiol (; ), or thiol derivative, is any organosulfur compound of the form , where R represents an alkyl or other organic substituent. The functional group itself is referred to as either a thiol group or a sulfhydryl grou ...
gold nanoparticles were placed in water, causing the formation of a
monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials.
...
of the hydrophobic gold nanoparticles on the surface. Air pressure was then increased, forcing the
hydrophobic
In chemistry, hydrophobicity is the physical property of a molecule that is seemingly repelled from a mass of water (known as a hydrophobe). In contrast, hydrophiles are attracted to water.
Hydrophobic molecules tend to be nonpolar and, th ...
layer to be pushed into the water, decreasing the
contact angle
The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
. When the contact angle was at the desired level, a hydrophilic thiol, 3-mercaptopropane-1,2-diol, was added to the water, causing the hydrophilic thiol to competitively replace the hydrophobic thiols, resulting in the formation of amphiphilic Janus nanoparticles.
The liquid–liquid and gas–liquid interface methods do have an issue where the nanoparticles can rotate in solution, causing the deposition of silver on more than one face.
A liquid–liquid/liquid–solid hybrid interface method was first introduced by Granick ''et al.'' as a solution to this liquid–liquid method problem. In this method, molten
paraffin wax
Paraffin wax (or petroleum wax) is a soft colorless solid derived from petroleum, coal, or oil shale that consists of a mixture of hydrocarbon molecules containing between 20 and 40 carbon atoms. It is solid at room temperature and begins to m ...
was substituted for the oil, and silica nanoparticles for the magnetite. When the solution was cooled, the wax solidified, trapping half of each silica nanoparticle in the wax surface, leaving the other half of the silica exposed. The water was then filtered off and the wax-trapped silica nanoparticles were then exposed to a methanol solution containing (amino-
propyl)triethoxysilane, which reacted with the exposed silica surfaces of the nanoparticles. The
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
solution was then filtered off and the wax was dissolved with
chloroform
Chloroform, or trichloromethane, is an organic compound with chemical formula, formula Carbon, CHydrogen, HChlorine, Cl3 and a common organic solvent. It is a colorless, strong-smelling, dense liquid produced on a large scale as a precursor to ...
, freeing the newly made Janus particles. Liu ''et al.'' reported the synthesis of
acorn
The acorn, or oaknut, is the nut of the oaks and their close relatives (genera ''Quercus'' and '' Lithocarpus'', in the family Fagaceae). It usually contains one seed (occasionally
two seeds), enclosed in a tough, leathery shell, and borne ...
- and
mushroom
A mushroom or toadstool is the fleshy, spore-bearing fruiting body of a fungus, typically produced above ground, on soil, or on its food source. ''Toadstool'' generally denotes one poisonous to humans.
The standard for the name "mushroom" is t ...
-shaped silica–aminopropyl–trimethoxysilane Janus nanoparticles using the hybrid liquid–liquid/liquid–solid method developed by Granick ''et al.'' They exposed homogenous aminopropyl-trimethoxysilane functionalized
silica
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
nanoparticles embedded in wax to an
ammonium fluoride
Ammonium fluoride is the inorganic compound with the formula NH4F. It crystallizes as small colourless prisms, having a sharp saline taste, and is highly soluble in water. Like all fluoride salts, it is moderately toxic in both acute and chronic o ...
solution, which etched away the exposed surface. The liquid–liquid/liquid–solid hybrid method also has some drawbacks; when exposed to the second
solvent
A solvent (s) (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for ...
for functionalization, some of the nanoparticles may be released from the wax, resulting in homogenous instead of Janus nanoparticles. This can partially be corrected by using waxes with higher
melting points
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depend ...
or performing functionalization at lower temperatures. However, these modifications still result in significant loss. Cui et al. designed a more enduring mask made of polydimethylsiloxane (PDMS) polymer film to create a liquid–liquid/liquid–solid interface. The exposed-to-be-modified portion of particle surface can be adjusted by controlling the PDMS curing temperature and time, thus the embedment depth of the particles. 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. Granick ''et al.'', in another paper, demonstrated a possible fix by using a liquid–liquid/gas–solid phase hybrid method by first
immobilizing silica nanoparticles in paraffin wax using the previously discussed liquid–solid phase interface method, and then filtering off the water. The resulting immobilized nanoparticles were then exposed to
silanol
A silanol is a functional group in silicon chemistry with the connectivity Si–O–H. It is related to the hydroxy functional group (C–O–H) found in all alcohols. Silanols are often invoked as intermediates in organosilicon c ...
vapor produced by bubbling
nitrogen
Nitrogen is the chemical element with the symbol N and atomic number 7. Nitrogen is a nonmetal and the lightest member of group 15 of the periodic table, often called the pnictogens. It is a common element in the universe, estimated at se ...
or
argon
Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
gas through liquid silanol, causing the formation of a hydrophilic face. The wax was then dissolved in chloroform, releasing the Janus nanoparticles.
An example of a more traditional liquid–solid technique has been described by Sardar ''et al.'' by beginning with the immobilization of gold nanoparticles on a silanized glass surface. Then the glass surface was exposed to 11-mercapto-1-undecanol, which bonded to the exposed hemispheres of the gold nanoparticles. The nanoparticles were then removed from the slide using
ethanol
Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an Alcohol (chemistry), alcohol with the chemical formula . Its formula can be also written as or (an ethyl ...
containing 16-mercaptohexadecanoic acid, which functionalized the previously masked
hemisphere
Hemisphere refers to:
* A half of a sphere
As half of the Earth
* A hemisphere of Earth
** Northern Hemisphere
** Southern Hemisphere
** Eastern Hemisphere
** Western Hemisphere
** Land and water hemispheres
* A half of the (geocentric) celestia ...
s of the nanoparticles.
Self-assembly
Block copolymers
![Triblock copolymer janus particles](https://upload.wikimedia.org/wikipedia/commons/2/2e/Triblock_copolymer_janus_particles.svg)
This method uses the well-studied methods of producing
block copolymers
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
with well-defined geometries and compositions across a large variety of substrates.
Synthesis of Janus particles by self-assembly via block copolymers was first described in 2001 by Erhardt ''et al.'' They produced a triblock polymer from
polymethylacrylate, polystyrene and low-
molecular-weight
The molecular mass (''m'') is the mass of a given molecule: it is measured in daltons (Da or u). Different molecules of the same compound may have different molecular masses because they contain different isotopes of an element. The related quanti ...
polybutadiene
Polybutadiene utadiene rubber BRis a synthetic rubber. Polybutadiene rubber is a polymer formed from the polymerization of the monomer 1,3-butadiene. Polybutadiene has a high resistance to wear and is used especially in the manufacture of tir ...
. The polystyrene and polymethylacrylate formed alternating layers in between which polybutadiene sat in nanosized spheres. The blocks were then
cross-linked
In chemistry and biology a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...
and dissolved in
THF
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water- miscible organic liquid with low viscosity. It is ...
, and after several washing steps, yielded spherical Janus particles with polystyrene on one face and polymethylacrylate on the other, with a polybutadiene core.
The production of Janus
spheres
The Synchronized Position Hold Engage and Reorient Experimental Satellite (SPHERES) are a series of miniaturized satellites developed by MIT's Space Systems Laboratory for NASA and US Military, to be used as a low-risk, extensible test bed for the ...
,
cylinders
A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base.
A cylinder may also be defined as an infini ...
, sheets, and
ribbons
A ribbon or riband is a thin band of material, typically cloth but also plastic or sometimes metal, used primarily as decorative binding and tying. Cloth ribbons are made of natural materials such as silk, cotton, and jute and of synthetic mater ...
is possible using this method by adjusting the
molecular weight
A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
s of the blocks in the initial polymer and also the degree of cross-linking.
Competitive adsorption
The key aspect of competitive absorption involves two substrates that phase-separate due to one or more opposite physical or chemical properties. When these substrates are mixed with a nanoparticle, typically gold, they maintain their separation and form two faces.
A good example of this technique has been demonstrated by Vilain ''et al.'', where
phosphinine-coated gold nanoparticles were exposed to long-chain thiols, resulting in substitution of the phosphinine ligands in a phase-separated manner to produce Janus nanoparticles. Phase separation was proven by showing the
thiols formed one locally pure domain on the nanoparticle using
FT-IR.
Jakobs ''et al.'' demonstrated a major issue with the competitive adsorption method when they attempted to synthesize amphiphilic gold Janus nanoparticles using the competitive adsorption of hydrophobic and hydrophilic
thiols.
The synthesis demonstrated was quite simple and only involved two steps. First gold nanoparticles capped with tetra-n-octyl
ammonium
The ammonium cation is a positively-charged polyatomic ion with the chemical formula or . It is formed by the protonation of ammonia (). Ammonium is also a general name for positively charged or protonated substituted amines and quaternary a ...
bromide were produced. Then the capping agent was removed followed by the addition of various ratios of hydrophilic disulfide functionalized
ethylene oxide and hydrophobic disulfide functionalized oligo(p-phenylenevinylene). They then attempted to prove that phase separation on the particle surface occurred by comparing the
contact angle
The contact angle is the angle, conventionally measured through the liquid, where a liquid–vapor interface meets a solid surface. It quantifies the wettability of a solid surface by a liquid via the Young equation. A given system of solid, liq ...
s of water on the surface of a
monolayer A monolayer is a single, closely packed layer of atoms, molecules, or cells. In some cases it is referred to as a self-assembled monolayer. Monolayers of layered crystals like graphene and molybdenum disulfide are generally called 2D materials.
...
of the Janus particles with nanoparticles made with only the hydrophobic or hydrophobic ligands. Instead the results of this experiment showed that while there was some phase separation, it was not complete.
This result highlights that the ligand choice is extremely important and any changes may result in incomplete phase separation.
Phase separation
![Phase separation janus particles](https://upload.wikimedia.org/wikipedia/commons/9/98/Phase_separation_janus_particles.svg)
This method involves the mixing of two or more incompatible substances which then separate into their own domains while still part of a single nanoparticle. These methods can involve the production of Janus nanoparticles of two
inorganic, as well as two
organic
Organic may refer to:
* Organic, of or relating to an organism, a living entity
* Organic, of or relating to an anatomical organ
Chemistry
* Organic matter, matter that has come from a once-living organism, is capable of decay or is the product ...
, substances.
Typical organic phase separation methods use cojetting of polymers to produce Janus nanoparticles. This technique is exemplified by the work of Yoshid ''et al.'' to produce Janus nanoparticles where one hemisphere has affinity for human
cells
Cell most often refers to:
* Cell (biology), the functional basic unit of life
Cell may also refer to:
Locations
* Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...
, while the other hemisphere has no affinity for human cells. This was achieved by cojetting polyacrylamide/poly(acrylic acid) copolymers which have no affinity for human cells with
biotinylated polyacrylamide/poly(acrylic acid) copolymers, which when exposed to
streptavidin
Streptavidin is a 66.0 (tetramer) kDa protein purified from the bacterium '' Streptomyces avidinii''. Streptavidin homo-tetramers have an extraordinarily high affinity for biotin (also known as vitamin B7 or vitamin H). With a dissociation co ...
-modified
antibodies
An antibody (Ab), also known as an immunoglobulin (Ig), is a large, Y-shaped protein used by the immune system to identify and neutralize foreign objects such as pathogenic bacteria and viruses. The antibody recognizes a unique molecule of the ...
, obtain an affinity for human cells.
The inorganic phase separation methods are diverse and vary greatly depending on the application.
The most common method uses the growth of a
crystal
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
of one inorganic substance on or from another inorganic nanoparticle.
A unique method has been developed by Gu ''et al.'', where iron-
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
nanoparticles were coated with
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
reacted with
cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...
acetylacetonate, trioctyl
phosphine
Phosphine (IUPAC name: phosphane) is a colorless, flammable, highly toxic compound with the chemical formula , classed as a pnictogen hydride. Pure phosphine is odorless, but technical grade samples have a highly unpleasant odor like rotting ...
oxide, and hexadecane-1,2-
diol
A diol is a chemical compound containing two hydroxyl groups ( groups). An aliphatic diol is also called a glycol. This pairing of functional groups is pervasive, and many subcategories have been identified.
The most common industrial diol is e ...
at 100 °C to produce nanoparticles with an iron-platinum core and an
amorphous
In condensed matter physics and materials science, an amorphous solid (or non-crystalline solid, glassy solid) is a solid that lacks the long-range order that is characteristic of a crystal.
Etymology
The term comes from the Greek ''a'' ("wi ...
cadmium-sulfur shell. The mixture was then heated to 280 °C, resulting in a
phase transition
In chemistry, thermodynamics, and other related fields, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic states of ...
and a partial eruption of the Fe-Pt from the core, creating a pure Fe-Pt sphere attached to the CdS-coated nanoparticle.
A new method of synthesizing inorganic Janus nanoparticles by phase separation has recently been developed by Zhao and Gao. In this method, they explored the use of the common homogeneous nanoparticle synthetic method of flame synthesis. They found when a
methanol
Methanol (also called methyl alcohol and wood spirit, amongst other names) is an organic chemical and the simplest aliphatic alcohol, with the formula C H3 O H (a methyl group linked to a hydroxyl group, often abbreviated as MeOH). It is a ...
solution containing
ferric triacetylacetonate and
tetraethylorthosilicate
Tetraethyl orthosilicate, formally named tetraethoxysilane (TEOS), ethyl silicate is the organic chemical compound with the formula Si(OC2H5)4. TEOS is a colorless liquid. It degrades in water. TEOS is the of orthosilicic acid, Si(OH)4. It ...
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
Silicon dioxide, also known as silica, is an oxide of silicon with the chemical formula , most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one ...
Janus nanoparticles. Additionally, they found it was possible to modify the silica after producing the Janus nanoparticles, making it hydrophobic by reacting it with
oleylamine
Oleylamine is an organic compound with a molecular formula C18H35NH2. It is an unsaturated fatty amine related to the fatty acid oleic acid. The pure compound is a clear and colorless liquid. Commercially available oleylamine reagents vary in colo ...
.
Properties and applications
Task-specific Janus materials
The term "task-specific Janus materials" refers to the non-emulsifying roles of Janus particles.
Self-assembly behavior of Janus nanoparticles
Janus particles' two or more distinct faces give them special properties in solution. In particular, they have been observed to
self-assemble
Self-assembly is a process in which a disordered system of pre-existing components forms an organized structure or pattern as a consequence of specific, local interactions among the components themselves, without external direction. When the ...
in a specific way in
aqueous or organic solutions. 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
Tetrahydrofuran (THF), or oxolane, is an organic compound with the formula (CH2)4O. The compound is classified as heterocyclic compound, specifically a cyclic ether. It is a colorless, water-miscible organic liquid with low viscosity. It is ma ...
. 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.
These particular Janus particles form aggregates in organic solvents considering that both sides of these particles are soluble in the organic solvent. It appears that the slight selectivity of the solvent is able to induce self-assembly of the particles into discrete clusters of Janus particles. This type of aggregation does not occur for either standard block copolymers nor for homogeneous particles and thus is a feature specific to Janus particles.
In an aqueous solutions, two kinds of
biphasic particles can be distinguished. The first type are particles which are truly amphiphilic and possess one hydrophobic and one hydrophilic side. The second type has two water-soluble, yet chemically distinct, sides. 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. In these studies, the Janus particles were found to aggregate on two
hierarchical
A hierarchy (from Greek: , from , 'president of sacred rites') is an arrangement of items (objects, names, values, categories, etc.) that are represented as being "above", "below", or "at the same level as" one another. Hierarchy is an important ...
levels. 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. The second type is noticeably larger than the first and has been termed 'super micelles'. Unfortunately, the structure of the
supermicelle
Supermicelle is a hierarchical micelle structure (supramolecular assembly) where individual components are also micelles. Supermicelles are formed via Top-down and bottom-up design#Nanotechnology, bottom-up chemical approaches, such as self- ...
s is unknown so far; however, they may be similar to
multilamellar vesicles.
For the second case of Janus particles which contain two distinct, but still water-soluble sides, the work of Granick's group provides some insight. Their research deals with the clustering of dipolar (
zwitterionic), micronsized Janus particles, whose two sides are both fully water-soluble.
Zwitterionic Janus particles do not behave like classical
dipoles
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...
, since their size is much larger than the distance at which electrostatic attractions are strongly felt. The study of zwitterionic Janus particles once again demonstrates their ability to form defined clusters. However, this particular type of Janus particle prefers to aggregate into larger clusters since this is more energetically favorable because each cluster carries a
macroscopic dipole which allows the aggregation of already-formed clusters into larger assemblies. Compared to aggregates formed through
van der Waals interactions for homogenous particles, the shapes of the zwitterionic Janus nanoclusters are different and the Janus clusters are less dense and more asymmetric.
Self-assembly modification using pH
The self-assembly of certain types of Janus particles may be controlled by modifying the
pH of their solution. Lattuada'' et al.'' prepared nanoparticles with one side coated with a pH-responsive polymer (polyacrylic acid, PAA) and the other with either a positively charged polymer (poly dimethylamino ethyl methacrylate, PDMAEMA), a negatively charged, pH-insensitive polymer, or a temperature-responsive polymer (poly-N-isopropyl acrylamide, PNIPAm).
In changing the pH of their solution, they noticed a change in the clustering of their Janus nanoparticles. At very high pH values, where PDMAEMA is uncharged while PAA is highly charged, the Janus nanoparticles were very stable in solution. However, below a pH of 4, when PAA is uncharged and PDMAEMA is positively charged, they formed finite clusters. At intermediate pH values, they found that the Janus nanoparticles were unstable due to
dipolar interaction
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system i ...
between the positively and negatively charged hemispheres.
Reversibility of cluster formation and control of cluster size
Control of cluster size for in the
aggregation of Janus nanoparticles has also been demonstrated. Lattuada ''et al.'' achieved control of the cluster size of Janus particles with one face PAA and the other either PDMAEMA or PNIPAm by mixing small amounts of these Janus nanoparticles with PAA-coated particles.
One unique feature of these clusters was stable particles could be recovered reversibly when high pH conditions were restored. Furthermore, Janus nanoparticles functionalized with PNIPAm showed controlled and reversible aggregation could be achieved by increasing the temperature above the lower critical
solubility
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubil ...
temperature of PNIPAm.
Amphiphilic properties
A significant characteristic of Janus nanoparticles is the capability of having both hydrophilic and hydrophobic parts. Many research groups have investigated the surface activities of nanoparticles with amphiphilic properties.
In 2006, Janus nanoparticles, made from
gold
Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
and
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
s, were compared with their homogeneous counterparts by measuring the ability of the particles to reduce the interfacial tension between water and
n-hexane. Experimental results indicated Janus nanoparticles are considerably more surface-active than homogeneous particles of comparable size and chemical nature. Furthermore, increasing the amphiphilic character of the particles can increase the interfacial activity. The ability of Janus nanoparticles to lower interfacial tension between water and
n-hexane confirmed previous theoretical predictions on their ability to stabilize
Pickering emulsion
A Pickering emulsion is an emulsion that is stabilized by solid particles (for example colloidal silica) which adsorb onto the interface between the water and oil phases. Typically, the emulsions are either water-in-oil or oil-in-water emulsions, b ...
s.
In 2007, the amphiphilic nature of the Janus nanoparticles was examined by measuring the
adhesion
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another).
The forces that cause adhesion and cohesion can be ...
force between the
atomic force microscopy
Atomic force microscopy (AFM) or scanning force microscopy (SFM) is a very-high-resolution type of scanning probe microscopy (SPM), with demonstrated resolution on the order of fractions of a nanometer, more than 1000 times better than the op ...
(AFM) tip and the particle surface. The stronger interactions between the hydrophilic
AFM tip and the hydrophilic side of the Janus nanoparticles were reflected by a greater
adhesion
Adhesion is the tendency of dissimilar particles or surfaces to cling to one another ( cohesion refers to the tendency of similar or identical particles/surfaces to cling to one another).
The forces that cause adhesion and cohesion can be ...
force. The Janus nanoparticles were dropcast onto both hydrophobically and hydrophilically modified substrates. The hydrophobic hemisphere of the Janus particles was exposed when a hydrophilic substrate surface was used, resulting in disparities in adhesion force measurements. Thus, the Janus nanoparticles adopted a conformation that maximized the interactions with the substrate surface.
The nature of amphiphilic Janus nanoparticles to orient themselves spontaneously at the interface between oil and water has been well known. This behavior allows considering amphiphilic Janus nanoparticles as analogues of molecular surfactants for the stabilization of emulsions. In 2005, spherical silica particles with amphiphilic properties were prepared by partial modification of the external surface with an alkylsilane agent. These particles form spherical assemblies encapsulating water-immiscible organic compounds in aqueous media by facing their hydrophobic alkylsilylated side to the inner organic phase and their hydrophilic side to the outer aqueous phase, thus stabilizing oil droplets in water. In 2009,
hydrophilic
A hydrophile is a molecule or other molecular entity that is attracted to water molecules and tends to be dissolved by water.Liddell, H.G. & Scott, R. (1940). ''A Greek-English Lexicon'' Oxford: Clarendon Press.
In contrast, hydrophobes are no ...
surface of silica particles was made partially hydrophobic by adsorbing
cetyltrimethylammonium bromide
Cetrimonium bromide ( C16H33)N(CH3)3r; cetyltrimethylammonium bromide; hexadecyltrimethylammonium bromide; CTAB) is a quaternary ammonium surfactant.
It is one of the components of the topical antiseptic cetrimide. The cetrimonium (hexadecyltrim ...
. These amphiphilic nanoparticles spontaneously assembled at the water-
dichloromethane
Dichloromethane (DCM or methylene chloride, methylene bichloride) is an organochlorine compound with the formula . This colorless, volatile liquid with a chloroform-like, sweet odour is widely used as a solvent. Although it is not miscible with ...
interface. In 2010, Janus particles composed from silica and polystyrene, with the polystyrene portion loaded with nanosized
magnetite
Magnetite is a mineral and one of the main iron ores, with the chemical formula Fe2+Fe3+2O4. It is one of the oxides of iron, and is ferrimagnetic; it is attracted to a magnet and can be magnetized to become a permanent magnet itself. With the ...
particles, were used to form kinetically stable oil-in-water emulsions that can be spontaneously broken on application of an external magnetic field. Such Janus materials will find applications in magnetically controlled optical switches and other related areas.
The first real applications of Janus nanoparticles were in
polymer
A polymer (; Greek '' poly-'', "many" + ''-mer'', "part")
is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits. Due to their broad spectrum of properties, both synthetic a ...
synthesis. In 2008, spherical amphiphilic Janus nanoparticles, having one polystyrene and one
poly(methyl methacrylate)
Poly(methyl methacrylate) (PMMA) belongs to a group of materials called engineering plastics. It is a transparent thermoplastic. PMMA is also known as acrylic, acrylic glass, as well as by the trade names and brands Crylux, Plexiglas, Acrylite, ...
side, were shown to be effective as compatibilizing agents of multigram scale compatibilization of two immiscible polymer blends, polystyrene and poly(methyl methacrylate).
The Janus nanoparticles oriented themselves at the interface of the two polymer phases, even under high temperature and shear conditions, allowing the formation of much smaller domains of poly(methyl methacrylate) in a polystyrene phase. The performance of the Janus nanoparticles as compatibilizing agents was significantly superior to other state-of-the-art compatibilizers, such as linear block
copolymer
In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are some ...
s.
Stabilizers in emulsions
A similar application of Janus nanoparticles as stabilizers was shown in emulsion
polymerization
In polymer chemistry, polymerization (American English), or polymerisation (British English), is a process of reacting monomer, monomer molecules together in a chemical reaction to form polymer chains or three-dimensional networks. There are ...
. In 2008, spherical amphiphilic Janus nanoparticles were applied for the first time to the emulsion polymerization of
styrene and n-butyl acrylate. The polymerization did not require additives or miniemulsion polymerization techniques, as do other Pickering emulsion polymerizations. Also, by applying Janus nanoparticles, the emulsion polymerization produced very well-controlled particle sizes with low polydispersities.
Janus interphase catalyst
Janus interphase catalyst is a new generation of heterogeneous catalysts, which is capable to do organic reactions on the interface of two phases via the formation of Pickering emulsion.
Catalyst in hydrogen peroxide decomposition
In 2010, spherical silica Janus nanoparticles with one side coated with
platinum
Platinum is a chemical element with the symbol Pt and atomic number 78. It is a dense, malleable, ductile, highly unreactive, precious, silverish-white transition metal. Its name originates from Spanish , a diminutive of "silver".
Platinu ...
were used for the first time to catalyze the decomposition of
hydrogen peroxide
Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
(H
2O
2).
The platinum particle catalyzes the surface chemical reaction: 2H
2O
2 → O
2 + 2H
2O. The decomposition of hydrogen peroxide created Janus catalytic nanomotors, the motion of which was analyzed experimentally and theoretically using computer simulations. The motion of the spherical Janus nanoparticles was found to agree with the predictions of computed simulations. Ultimately, catalytic nanomotors have practical applications in delivering chemical payloads in
microfluidic
Microfluidics refers to the behavior, precise control, and manipulation of fluids that are geometrically constrained to a small scale (typically sub-millimeter) at which surface forces dominate volumetric forces. It is a multidisciplinary field tha ...
chips, eliminating pollution in aquatic media, removing toxic chemicals within biological systems, and performing medical procedures.
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 ratchet effect is an instance of the restrained ability of human processes to be reversed once a specific thing has happened, analogous with the mechanical ratchet that holds the spring tight as a clock is wound up. It is related to the phenom ...
. Ratcheting of Janus particles can be orders of magnitude stronger than for ordinary thermal potential ratchets and thus easily experimentally accessible. In particular, autonomous pumping of a large mixture of passive particles can be induced by just adding a small fraction of Janus particles.
Water-repellent fibers
In 2011, Janus nanoparticles were shown to be applicable in textiles. Water-repellent fibers can be prepared by coating
polyethylene terephthalate
Polyethylene terephthalate (or poly(ethylene terephthalate), PET, PETE, or the obsolete PETP or PET-P), is the most common thermoplastic polymer resin of the polyester family and is used in fibres for clothing, containers for liquids and foods ...
fabric with amphiphilic spherical Janus nanoparticles.
The Janus particles bind with the hydrophilic reactive side of the textile surface, while the hydrophobic side is exposed to the environment, thus providing the water-repellent behavior. 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.
Applications in biological sciences
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. Inherently Janus nanoparticles play a crucial role in such applications. In 2009, a new type of bio-hybrid material composed of Janus nanoparticles with spatially controlled affinity towards human
endothelial
The endothelium is a single layer of squamous endothelial cells that line the interior surface of blood vessels and lymphatic vessels. The endothelium forms an interface between circulating blood or lymph in the lumen and the rest of the vessel ...
cells was reported.
These nanoparticles were synthesized by selective surface modification with one hemisphere exhibiting high binding affinity for human endothelial cells and the other hemisphere being resistant towards cell binding. The Janus nanoparticles were fabricated via electrohydrodynamic jetting of two polymer liquid solutions. When incubated with human endothelial cells, these Janus nanoparticles exhibited expected behavior, where one face binds toward human endothelial cells, while the other face was not bonding. These Janus nanoparticles not only bound to the top of the human endothelial cells, but also associated all around the perimeter of cells forming a single particle lining. The biocompatibility between the Janus nanoparticles and cells was excellent. The concept is to eventually design probes based on Janus nanoparticles to attain directional information about cell-particle interactions.
Nanocorals
In 2010, a new type of cellular probe synthesized from Janus nanoparticles called a nanocoral, combining cellular specific targeting and biomolecular sensing, was presented. Nanocoral is composed of polystyrene and gold hemispheres. The polystyrene hemisphere of the nanocoral was selectively functionalized with antibodies to target receptors of specific cells. This was demonstrated by functionalizing the polystyrene region with antibodies that specifically attached to breast cancer cells. The gold region of the nanocoral surface was used for detecting and imaging. Thus, the targeting and sensing mechanisms were decoupled and could be separately engineered for a particular experiment. Additionally, the polystyrene region may also be used as a carrier for drugs and other chemicals by surface hydrophobic
adsorption
Adsorption is the adhesion of atoms, ions or molecules from a gas, liquid or dissolved solid to a surface. This process creates a film of the ''adsorbate'' on the surface of the ''adsorbent''. This process differs from absorption, in which a f ...
or encapsulation, making the nanocoral a possible multifunctional
nanosensor
Nanosensors are nanoscale devices that measure physical quantities and convert these to signals that can be detected and analyzed. There are several ways proposed today to make nanosensors; these include top-down lithography, bottom-up assembly, ...
.
Imaging and magnetolytic therapy
Also in 2010, Janus nanoparticles synthesized from hydrophobic magnetic nanoparticles on one side and
poly(styrene-block-allyl alcohol) on the other side were used for imaging and magnetolytic therapy.
The magnetic side of the Janus nanoparticles responded well to external magnetic stimuli. The nanoparticles were quickly attached to the cell surfaces using a magnetic field. Magnetolytic therapy was achieved through magnetic field-modulated cell membrane damage. First, the nanoparticles were brought close in contact with the tumor cells, and then a spinning magnetic field was applied. After 15 minutes, the majority of the tumor cells were killed. Magnetic Janus nanoparticles could serve as the basis for potential applications in medicine and electronics. Quick responses to external magnetic fields could become an effective approach for targeted imaging, therapy ''
in vitro
''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called "test-tube experiments", these studies in biology an ...
'' and ''
in vivo
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
'', and cancer treatment. Similarly, a quick response to magnetic fields is also desirable to fabricate smart displays, opening new opportunities in electronics and
spintronics
Spintronics (a portmanteau meaning spin transport electronics), also known as spin electronics, is the study of the intrinsic spin of the electron and its associated magnetic moment, in addition to its fundamental electronic charge, in solid-sta ...
.
In 2011, silica-coated Janus nanoparticles, composed of
silver oxide and
iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. Several iron oxides are recognized. All are black magnetic solids. Often they are non-stoichiometric. Oxyhydroxides are a related class of compounds, perhaps the best known of whic ...
(Fe
2O
3), were prepared in one step with scalable flame aerosol technology. These hybrid plasmonic-magnetic nanoparticles bear properties that are applicable in bioimaging, targeted drug delivery, ''in vivo'' diagnosis, and therapy. The purpose of the nanothin
SiO2 shell was to reduce the release of toxic
Ag+ ions from the nanoparticle surface to live cells. As a result, these hybrid nanoparticles showed no cyctotoxicity during bioimaging and remained stable in suspension with no signs of agglomeration or settling, thus enabling these nanoparticles as biocompatible multifunctional probes for bioimaging. 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. These new hybrid Janus nanoparticles overcame the individual limitations of
Fe2O3 (poor particle stability in suspension) and of
Ag (toxicity) nanoparticles, while retaining the desired magnetic properties of
Fe2O3 and the plasmonic optical properties of
Ag.
Applications in electronics
The potential application of Janus particles was first demonstrated by Nisisako ''et al.'', who made use of the electrical
anisotropy of Janus particles filled with white and black
pigments in both hemispheres. These particles were used to make switchable screens by placing a thin layer of these spheres between two
electrodes. Upon changing the applied
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
, the particles orient their black sides to the
anode
An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
and their white sides to the
cathode
A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in whi ...
. Thus the orientation and the color of the display can be changed by simply reversing the electric field. With this method, it may be possible to make very thin and environmentally friendly displays.
Graphene
Graphene () is an allotrope of carbon consisting of a single layer of atoms arranged in a hexagonal lattice nanostructure. Janus particles have been used in experimental
Sodium-ion battery, sodium-ion batteries to increase
energy density
In physics, energy density is the amount of energy stored in a given system or region of space per unit volume. It is sometimes confused with energy per unit mass which is properly called specific energy or .
Often only the ''useful'' or extract ...
. One side provides interaction sites while the other provides inter-layer separation. Energy density reached 337 mAh/g.
See also
*
Microswimmer
*
Active Brownian particle An active Brownian particle (ABP) is a model of self-propelled motion in a dissipative environment. It is a nonequilibrium generalization of a Brownian particle.
The self-propulsion results from a force that acts on the particle's center of mas ...
References
External links
An innovative process for their versatile large-scale synthesis Groupe NanoSytèmes Analytiques
Book: Janus Particle Synthesis, Self-assembly and Applications RSC Smart Materials
Janus particles{Dead link, date=January 2020 , bot=InternetArchiveBot , fix-attempted=yes ,
Physics Today
''Physics Today'' is the membership magazine of the American Institute of Physics. First published in May 1948, it is issued on a monthly schedule, and is provided to the members of ten physics societies, including the American Physical Society. I ...
'2-faced' particles act like tiny submarines EurekAlert!
PhysOrg.com
Nanoparticles by morphology