[16]
Additionally, the behavior of Janus particle in aqueous multilamellar vesicles.[16]
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.[35] Zwitterionic Janus particles are interesting because they do not behave like classical dipoles, 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.[16]
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).[2] 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 between the positively and negatively charged hemispheres.[2]
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.[2] 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 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 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.[36] 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 emulsions.
In 2007, the amphiphilic nature of the Janus nanoparticles was examined by measuring the
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An innovative process for their versatile large-scale synthesis, Groupe NanoSytèmes Analytiques
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Book:Janus Particle Synthesis, Self-assembly and Applications, RSC Smart Materials
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Janus particles, Physics Today
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'2-faced' particles act like tiny submarines, EurekAlert!
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Nano World: Two-faced Janus nanoparticles, PhysOrg.com
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References
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 106 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.,[50][51]
Janus particles handling by dielectrophoresis
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.[49] 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, the particles orient their black sides to the anode and their white sides to the cathode. 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.
Applications in electronics
In 2011, silica-coated Janus nanoparticles, composed of silver oxide and iron oxide (Fe2O3), were prepared in one step with scalable flame aerosol technology.[48] 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.
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.[14] 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 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.
Imaging and magnetolytic therapy
In 2010, a new type of cellular probe synthesized from Janus nanoparticles called a nanocoral, combining cellular specific targeting and biomolecular sensing, was presented.[47] 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 or encapsulation, making the nanocoral a possible multifunctional nanosensor.
Nanocorals
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 cells was reported.[12] 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.
Applications in biological sciences
In 2011, Janus nanoparticles were shown to be applicable in textiles. Water-repellent fibers can be prepared by coating polyethylene terephthalate fabric with amphiphilic spherical Janus nanoparticles.[11] 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.
Water-repellent fibers
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 very interesting non-equilibrium phenomenon, ratchet effect. 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.[46]
In 2010, spherical silica Janus nanoparticles with one side coated with platinum were used for the first time to catalyze the decomposition of hydrogen peroxide (H2O2).[45] The platinum particle catalyzes the surface chemical reaction: 2H2O2 → O2 + 2H2O. 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 chips, eliminating pollution in aquatic media, removing toxic chemicals within biological systems, and performing medical procedures.
Catalyst in hydrogen peroxide decomposition
A similar application of Janus nanoparticles as stabilizers was shown in emulsion polymerization. In 2008, spherical amphiphilic Janus nanoparticles were applied for the first time to the emulsion polymerization of styrene and n-butyl acrylate.[44] 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.
Stabilizers in emulsions
.
copolymers 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 [13] side, were shown to be effective as compatibilizing agents of multigram scale compatibilization of two immiscible polymer blends, polystyrene and poly(methyl methacrylate).poly(methyl methacrylate) synthesis. In 2008, spherical amphiphilic Janus nanoparticles, having one polystyrene and one polymer Such Janus materials will find applications in magnetically controlled optical switches and other related areas. The first real applications of Janus nanoparticles were in [43] particles, were used to form kinetically stable oil-in-water emulsions that can be spontaneously broken on application of an external magnetic field.magnetite In 2010, Janus particles composed from silica and polystyrene, with the polystyrene portion loaded with nanosized [42] interface.dichloromethane. These amphiphilic nanoparticles spontaneously assembled at the water-cetyltrimethylammonium bromide surface of silica particles was made partially hydrophobic by adsorbing hydrophilic In 2009, [41]
Janus particles' two or more distinct faces give them special properties in solution. In particular, they have been observed to