Janus particles

{{Short description|Type of nanoparticle or microparticle}} {{Use dmy dates|date=October 2014}} thumb|200px|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. '''Janus particles''' are special types of nanoparticles or microparticles whose surfaces have two or more distinct physical properties.<ref>{{Cite journal | doi = 10.1002/anie.201001451 | volume = 50 | issue = 2 | pages = 360–388 | title = Colloidal Assembly: The Road from Particles to Colloidal Molecules and Crystals | journal = Angewandte Chemie International Edition | date = 10 January 2011 | last1 = Li | first1 = Fan | last2 = Josephson | first2 = David P. | last3 = Stein | first3 = Andreas | pmid=21038335 | bibcode = 2011ACIE...50..360L }}</ref><ref>Janus Particle Synthesis, Self-Assembly and Applications, Editors: Shan Jiang, Steve Granick, Royal Society of Chemistry, Cambridge 2013, https://pubs.rsc.org/en/content/ebook/978-1-84973-510-0</ref> This unique surface of Janus particles allows two different types of chemistry 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.<ref name="Nano today review 2011" /> For example, a Janus particle may have one half of its surface composed of hydrophilic groups and the other half hydrophobic groups,<ref>{{Cite journal | doi = 10.1063/1.3177238 | volume = 62 | issue = 7 | pages = 68–69 | title = Janus particles | journal = Physics Today | year = 2009 |bibcode = 2009PhT....62g..68G | last1 = Granick | first1 = Steve | last2 = Jiang | first2 = Shan | last3 = Chen | first3 = Qian }}</ref> the particles might have two surfaces of different color,<ref>{{Cite web|url=https://www.cospheric.com/rotation_orientation_electrophoretic_particles/electomagnetic_field.htm|title=Rotation and Orientation of Dual-Functionalized Electrophoretic Microspheres in Electromagnetic Field|website=www.cospheric.com|access-date=2019-04-30}}</ref> fluorescence, or magnetic properties.<ref>{{Cite web|url=https://www.cospheric.com/retroreflective_particles_spheres.htm|title=Retroreflective Microspheres, Metal-Coated Glass Particles, Microbeads, Spherical Glass Powder - Principles and Operation|website=www.cospheric.com|access-date=2019-04-30}}</ref> This gives these particles unique properties related to their asymmetric structure and/or functionalization.<ref>{{cite journal|last=Walther|first=Andreas|author2=Müller, Axel|title=Janus Particles: Synthesis, Self-Assembly, Physical Properties, and Applications|journal=Chemical Reviews|volume=113|issue=7|pages=5194–261|year=2013|doi=10.1021/cr300089t|pmid=23557169}}</ref> Janus particles are so-named in reference to the two-faced Roman god Janus, as they may be said to—similarly—have "two faces".<ref name = "deGennes">{{Cite journal | doi = 10.1002/anie.199208421 | volume = 31 | issue = 7 | pages = 842–845 | last = de Gennes | first = Pierre-Gilles | title = Soft Matter (Nobel Lecture) | journal = Angewandte Chemie International Edition in English | date = 1992 }}</ref>

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==History== The term "Janus Particle" was coined by author Leonard Wibberley in his [https://books.google.com/books?id=1sBLAAAAMAAJ&q=Janus+particle 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<ref>Casagrande C., Veyssie M., C. R. Acad. Sci. (Paris), 306 11, 1423, 1988.</ref> 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.<ref name="casagrande 1989">{{cite journal|author1=Casagrande. C. |author2=Fabre P. |author3=Veyssie M. |author4=Raphael E. |title="Janus Beads": Realization and Behaviour at Water/Oil Interfaces|journal=Europhysics Letters|volume=9|issue=3|pages=251–255|doi=10.1209/0295-5075/9/3/011|year=1989|bibcode = 1989EL......9..251C |s2cid=250749951 }}</ref> In 1991, Pierre-Gilles de Gennes mentioned the term "Janus grains" in his Nobel lecture;<ref name = "deGennes"/> de Gennes advocated for the utility and scientific interest of Janus particles by pointing out that 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 particles.<ref>{{Cite journal |volume=71 |issue=4 |pages=833–836 |last=de Gennes |first=Pierre-Gilles |title=Nanoparticles and Dendrimers: Hopes and Illusions |journal=Croatica Chemica Acta |access-date=2011-10-04 |date=15 July 1997 |url=http://www.stkpula.hr/ccacaa/co984.html |archive-url=https://web.archive.org/web/20120425074255/http://www.stkpula.hr/ccacaa/co984.html |archive-date=25 April 2012 }}</ref>

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."<ref>'''United States Patent''' '''4,126,854''' ''''''''Sheridon'''''''' '''21 November 1978''' ----'''''Twisting ball''''' panel display</ref> Although the term "Janus particles" was not yet used, Lee and coworkers also reported particles matching this description in 1985.<ref>{{Cite journal | doi = 10.1002/app.1985.070300510 | volume = 30 | issue = 5 | pages = 1903–1926 | journal = Journal of Applied Polymer Science | date = 1985 | title = Morphology of latex particles formed by poly(methyl methacrylate)-seeded emulsion polymerization of styrene | last1 = Cho | first1 = Iwhan | last2 = Lee | first2 = Kyung-Woo | bibcode = 1985JAPS...30.1903C }}</ref> They introduced asymmetric polystyrene/polymethylmethacrylate lattices from seeded emulsion polymerization. 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.<ref name="casagrande 1989" /> The glass beads were studied for their potential to stabilize emulsification processes. Then several years later, Binks and Fletcher investigated the wettability of Janus beads at the interface between oil and water.<ref>{{Cite journal | doi = 10.1021/la0103315 | volume = 17 | issue = 16 | pages = 4708–4710 | last = Binks | first = B. P. |author2=Fletcher, P. D. I. | title = Particles Adsorbed at the Oil-Water Interface: A Theoretical Comparison between Spheres of Uniform Wettability and Janus Particles | journal = Langmuir | date = 5 October 2011 }}</ref> They concluded Janus particles are both surface-active and amphiphilic, whereas homogeneous particles are only surface-active. Twenty years later, a plethora of Janus particles of different sizes, shapes and properties, with applications in textile,<ref name="applications textile">{{Cite journal | doi = 10.1021/am200033u | pmid = 21366338 | volume = 3 | issue = 4 | pages = 1216–1220 | title = Water-Repellent Textile via Decorating Fibers with Amphiphilic Janus Particles | journal = ACS Appl. Mater. Interfaces | date = 25 September 2011 | last1 = Synytska | first1 = Alla | last2 = Khanum | first2 = Rina | last3 = Ionov | first3 = Leonid | last4 = Cherif | first4 = Chokri | last5 = Bellmann | first5 = C. | bibcode = 2011AAMI....3.1216S }}</ref> sensors,<ref name="applications sensor">{{Cite journal | doi = 10.1002/adma.200901971 | pmid = 25377943 | volume = 21 | issue = 48 | pages = 4920–4925 | journal = Advanced Materials | date = 2009 | title = Structurally Controlled Bio-hybrid Materials Based on Unidirectional Association of Anisotropic Microparticles with Human Endothelial Cells | last1 = Yoshida | first1 = Mutsumi | last2 = Roh | first2 = Kyung-Ho | last3 = Mandal | first3 = Suparna | last4 = Bhaskar | first4 = Srijanani | last5 = Lim | first5 = Dongwoo | last6 = Nandivada | first6 = Himabindu | last7 = Deng | first7 = Xiaopei | last8 = Lahann | first8 = Joerg | bibcode = 2009AdM....21.4920Y | hdl = 2027.42/64554 | s2cid = 205234179 | hdl-access = free }}</ref> stabilization of emulsions,<ref name="applications emulsion">{{Cite journal | doi = 10.1021/nn800108y | pmid = 19206334 | volume = 2 | issue = 6 | pages = 1167–1178 | title = Engineering Nanostructured Polymer Blends with Controlled Nanoparticle Location using Janus Particles | journal = ACS Nano | date = 25 September 2011 | last1 = Walther | first1 = Andreas | last2 = Matussek | first2 = Kerstin | last3 = Müller | first3 = Axel H. E. }}</ref> and magnetic field imaging<ref name="applications magnetic field imaging">{{Cite journal | doi = 10.1021/ja102489q | pmid = 20459132 | volume = 132 | issue = 21 | pages = 7234–7237 | title = Nanocomposites with Spatially Separated Functionalities for Combined Imaging and Magnetolytic Therapy | journal = J. Am. Chem. Soc. | date = 25 September 2011 | last1 = Hu | first1 = Shang-Hsiu | last2 = Gao | first2 = Xiaohu | pmc=2907143 }}</ref> have been reported. Variety of janus particles in sizes 10&nbsp;μm to 53&nbsp;μm in diameter are currently commercially available from Cospheric,<ref>{{Cite web|url=https://www.cospheric.com/janus_particles_manufacture/custom_half-shell_coatings_microspheres.htm|title=Custom Janus Particles - Bichromal and Bipolar Microspheres - Half-Magnetic Spheres - Partial Coating on Microparticles|website=www.cospheric.com|access-date=2019-04-30}}</ref> who holds a patent on Hemispherical Coating Method for Microelements.<ref>'''United States Patent''' '''8,501,272''' '''Lipovetskaya, et al.''' '''6 August 2013''' ----Hemispherical coating method for micro-elements</ref>

== 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.<ref name="Nano today review 2011" />

===Masking=== thumb|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. thumb|alt=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.<ref>{{cite journal | last1=Honegger | first1=T. | last2=Lecarme | first2=O. | last3=Berton | first3=K. | last4=Peyrade | first4=D. | title=Rotation speed control of Janus particles by dielectrophoresis in a microfluidic channel | journal=Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena | publisher=American Vacuum Society | volume=28 | issue=6 | year=2010 | issn=2166-2746 | doi=10.1116/1.3502670 | pages=C6I14–C6I19 | bibcode=2010JVSTB..28I..14H }}</ref> Masking was one of the first techniques developed for the synthesis of Janus nanoparticles.<ref name="Walther Soft Matter">{{cite journal|last=Walther|first=Andreas|author2=Müller, Axel H. E.|title=Janus particles|journal=Soft Matter|date=1 January 2008|volume=4|issue=4|pages=663–668|doi=10.1039/b718131k|pmid=32907169|bibcode = 2008SMat....4..663W }}</ref> This technique was developed by simply taking synthesis techniques of larger Janus particles and scaling down to the nanoscale.<ref name="Walther Soft Matter" /><ref name="Perro Journal of Materials Chemistry 2005">{{cite journal|last=Perro|first=Adeline|author2=Reculusa, Stéphane, Ravaine, Serge, Bourgeat-Lami, Elodie, Duguet, Etienne|title=Design and synthesis of Janus micro- and nanoparticles|journal=Journal of Materials Chemistry|date=1 January 2005|volume=15|issue=35–36|page=3745|doi=10.1039/b505099e}}</ref><ref name="Lu JACS 2003">{{cite journal|last=Lu|first=Yu|author2=Xiong, Hui, Jiang, Xuchuan, Xia, Younan, Prentiss, Mara, Whitesides, George M.|title=Asymmetric Dimers Can Be Formed by Dewetting Half-Shells of Gold Deposited on the Surfaces of Spherical Oxide Colloids|journal=Journal of the American Chemical Society|date=1 October 2003|volume=125|issue=42|pages=12724–12725|doi=10.1021/ja0373014|pmid=14558817|bibcode=2003JAChS.12512724L |citeseerx=10.1.1.650.6058}}</ref> 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<ref name="He Langmuir">{{cite journal|last=He|first=Zhenping|author2=Kretzschmar, Ilona |title=Template-Assisted Fabrication of Patchy Particles with Uniform Patches|journal=Langmuir|date=18 June 2012|volume=28|issue=26|pages=9915–9|doi=10.1021/la3017563 |pmid=22708736}}</ref><ref>{{cite journal|last=He|first=Zhenping|author2=Kretzschmar, Ilona|title=Template-Assisted GLAD: Approach to Single and Multipatch Patchy Particles with Controlled Patch Shape|journal=Langmuir|date=6 December 2013 |volume=29|issue=51|pages=15755–61|doi=10.1021/la404592z|pmid=24313824}}</ref> and a technique where the nanoparticle is suspended at the interface of two phases. However, only the phase separation technique scales well to the nanoscale.<ref name="Jiang Advanced Materials 2010">{{cite journal|last=Jiang|first=Shan|author2=Chen, Qian, Tripathy, Mukta, Luijten, Erik, Schweizer, Kenneth S., Granick, Steve|title=Janus Particle Synthesis and Assembly|journal=Advanced Materials|date=27 January 2010|volume=22|issue=10|pages=1060–1071|doi=10.1002/adma.200904094|pmid=20401930|bibcode=2010AdM....22.1060J |osti=1875526 |s2cid=7999750 }}</ref>

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.<ref name="Jiang Langmuir 2008">{{cite journal|last=Jiang|first=Shan|author2=Schultz, Mitchell J. |author3=Chen, Qian |author4=Moore, Jeffrey S. |author5=Granick, Steve |title=Solvent-Free Synthesis of Janus Colloidal Particles|journal=Langmuir|date=16 September 2008|volume=24|issue=18|pages=10073–10077|doi=10.1021/la800895g|pmid=18715019|osti=1876383 }}</ref><ref name="Pradhan Advanced Functional Materials 2007">{{cite journal|last=Pradhan|first=S.|author2=Xu, L. |author3=Chen, S. |title=Janus Nanoparticles by Interfacial Engineering|journal=Advanced Functional Materials|date=24 September 2007|volume=17|issue=14|pages=2385–2392|doi=10.1002/adfm.200601034|s2cid=11591797 }}</ref>

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. The magnetite nanoparticles aggregated at the interface of the water-oil mixture, forming a Pickering emulsion. Then, silver nitrate 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.<ref name="Gu JACS 2005">{{cite journal|last=Gu|first=Hongwei|author2=Yang, Zhimou, Gao, Jinhao, Chang, C. K., Xu, Bing|title=Heterodimers of Nanoparticles: Formation at a Liquid−Liquid Interface and Particle-Specific Surface Modification by Functional Molecules|journal=Journal of the American Chemical Society|date=1 January 2005|volume=127|issue=1|pages=34–35|doi=10.1021/ja045220h|pmid=15631435 |bibcode=2005JAChS.127...34G }}</ref> This method can also use gold or iron-platinum instead of magnetite.<ref name="Nano today review 2011" />

A similar method is the gas–liquid interface method developed by Pradhan ''et al.'' In this method, hydrophobic alkane thiolate gold nanoparticles were placed in water, causing the formation of a monolayer of the hydrophobic gold nanoparticles on the surface. Air pressure was then increased, forcing the hydrophobic layer to be pushed into the water, decreasing the contact angle. 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.<ref name="Pradhan Advanced Functional Materials 2007" />

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.<ref name="Hong Langmuir 2006">{{cite journal|last=Hong|first=Liang|author2=Jiang, Shan, Granick, Steve|title=Simple Method to Produce Janus Colloidal Particles in Large Quantity|journal=Langmuir|date=1 November 2006|volume=22|issue=23|pages=9495–9499|doi=10.1021/la062716z|pmid=17073470}}</ref> 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 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 solution was then filtered off and the wax was dissolved with chloroform, freeing the newly made Janus particles. Liu ''et al.'' reported the synthesis of acorn- and mushroom-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 nanoparticles embedded in wax to an ammonium fluoride solution, which etched away the exposed surface. The liquid–liquid/liquid–solid hybrid method also has some drawbacks; when exposed to the second solvent 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 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.<ref>{{cite journal|last=Cui|first=Jing-Qin|author2=Kretzschmar, Ilona|title=Surface anisotropic polystyrene spheres by electroless deposition|journal=Langmuir|date=29 August 2006|volume=22|issue=20|pages=8281–8284|doi=10.1021/la061742u|pmid=16981737}}</ref> 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 vapor produced by bubbling nitrogen or argon gas through liquid silanol, causing the formation of a hydrophilic face. The wax was then dissolved in chloroform, releasing the Janus nanoparticles.<ref name="Jiang Langmuir 2008" />

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 containing 16-mercaptohexadecanoic acid, which functionalized the previously masked hemispheres of the nanoparticles.<ref name="Sardar JACS 2007">{{cite journal|last=Sardar|first=Rajesh|author2=Heap, Tyler B. |author3=Shumaker-Parry, Jennifer S. |title=Versatile Solid Phase Synthesis of Gold Nanoparticle Dimers Using an Asymmetric Functionalization Approach|journal=Journal of the American Chemical Society|date=1 May 2007|volume=129|issue=17|pages=5356–5357|doi=10.1021/ja070933w|pmid=17425320 |bibcode=2007JAChS.129.5356S }}</ref>

===Self-assembly===

====Block copolymers==== thumb|Schematic representation of the synthesis of Janus nanoparticles using the block copolymer self-assembly method This method uses the well-studied methods of producing block copolymers with well-defined geometries and compositions across a large variety of substrates.<ref name="Nano today review 2011">{{cite journal|last=Lattuada|first=Marco|author2=Hatton, T. Alan|title=Synthesis, properties and applications of Janus nanoparticles|journal=Nano Today|date=1 June 2011|volume=6|issue=3|pages=286–308|doi=10.1016/j.nantod.2011.04.008}}</ref><ref name="Kim Physical Review Letters 2009">{{cite journal|last=Kim|first=Jaeup|author2=Matsen, Mark|title=Positioning Janus Nanoparticles in Block Copolymer Scaffolds|journal=Physical Review Letters|date=1 February 2009|volume=102|issue=7|article-number=078303|doi=10.1103/PhysRevLett.102.078303|pmid=19257718|bibcode=2009PhRvL.102g8303K|url=https://scholarworks.unist.ac.kr/handle/201301/8500|url-access=subscription}}</ref> 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 polybutadiene. The polystyrene and polymethylacrylate formed alternating layers in between which polybutadiene sat in nanosized spheres. 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.<ref name="Erhardt macromolecules 2001">{{cite journal|last=Erhardt|first=Rainer|author2=Böker, Alexander, Zettl, Heiko, Kaya, Håkon, Pyckhout-Hintzen, Wim, Krausch, Georg, Abetz, Volker, Müller, Axel H. E.|title=Janus Micelles|journal=Macromolecules|date=1 February 2001|volume=34|issue=4|pages=1069–1075|doi=10.1021/ma000670p|bibcode = 2001MaMol..34.1069E |url=http://juser.fz-juelich.de/record/34163/files/4042.pdf}}</ref> 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.<ref name="Nano today review 2011" /><ref name="wolf macromolecules 2011">{{cite journal|last=Wolf|first=Andrea|author2=Walther, Andreas, Müller, Axel H. E.|title=Janus Triad: Three Types of Nonspherical, Nanoscale Janus Particles from One Single Triblock Terpolymer|journal=Macromolecules|volume=44|issue=23|date=3 November 2011|page=111103075619002|doi=10.1021/ma2020408|bibcode = 2011MaMol..44.9221W }}</ref>

====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.<ref name="Nano today review 2011" /><ref name="Vilain J mat chem 2007">{{cite journal|last=Vilain|first=Claire|author2=Goettmann, Frédéric, Moores, Audrey, Le Floch, Pascal, Sanchez, Clément|title=Study of metal nanoparticles stabilised by mixed ligand shell: a striking blue shift of the surface-plasmon band evidencing the formation of Janus nanoparticles|journal=Journal of Materials Chemistry|date=1 January 2007|volume=17|issue=33|page=3509|doi=10.1039/b706613a|s2cid=98355020}}</ref> 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.<ref name="Vilain J mat chem 2007" /> 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.<ref name="Jakobs J mat chem 2008">{{cite journal|last=Jakobs|first=Robert T. M.|author2=van Herrikhuyzen, Jeroen, Gielen, Jeroen C., Christianen, Peter C. M., Meskers, Stefan C. J., Schenning, Albertus P. H. J.|title=Self-assembly of amphiphilic gold nanoparticles decorated with a mixed shell of oligo(p-phenylene vinylene)s and ethyleneoxide ligands|journal=Journal of Materials Chemistry|date=1 January 2008|volume=18|issue=29|page=3438|doi=10.1039/b803935f|hdl=2066/72609|hdl-access=free}}</ref> The synthesis demonstrated was quite simple and only involved two steps. First gold nanoparticles capped with tetra-n-octylammonium 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 angles of water on the surface of a monolayer 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.<ref name="Jakobs J mat chem 2008" /> This result highlights that the ligand choice is extremely important and any changes may result in incomplete phase separation.<ref name="Nano today review 2011" /><ref name="Jakobs J mat chem 2008" />

===Phase separation=== thumb|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. 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, substances.<ref name="Nano today review 2011" />

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, 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-modified antibodies, obtain an affinity for human cells.<ref name="applications sensor" />

The inorganic phase separation methods are diverse and vary greatly depending on the application.<ref name="Nano today review 2011" /> The most common method uses the growth of a crystal of one inorganic substance on or from another inorganic nanoparticle.<ref name="Nano today review 2011" /><ref name="Gu JACS 2004">{{cite journal|last=Gu|first=Hongwei|author2=Zheng, Rongkun, Zhang, XiXiang, Xu, Bing|title=Facile One-Pot Synthesis of Bifunctional Heterodimers of Nanoparticles: A Conjugate of Quantum Dot and Magnetic Nanoparticles|journal=Journal of the American Chemical Society|date=1 May 2004|volume=126|issue=18|pages=5664–5665|doi=10.1021/ja0496423|pmid=15125648 |bibcode=2004JAChS.126.5664G }}</ref> A unique method has been developed by Gu ''et al.'', where iron-platinum nanoparticles were coated with sulfur reacted with cadmium acetylacetonate, trioctylphosphineoxide, and hexadecane-1,2-diol at 100&nbsp;°C to produce nanoparticles with an iron-platinum core and an amorphous cadmium-sulfur shell. The mixture was then heated to 280&nbsp;°C, resulting in a phase transition and a partial eruption of the Fe-Pt from the core, creating a pure Fe-Pt sphere attached to the CdS-coated nanoparticle.<ref name="Gu JACS 2004" /> 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 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&nbsp;°C to produce maghemite-silica 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.<ref name="Zhao Advanced Materials 2009">{{cite journal|last=Zhao|first=Nan|author2=Gao, Mingyuan|title=Magnetic Janus Particles Prepared by a Flame Synthetic Approach: Synthesis, Characterizations and Properties|journal=Advanced Materials|date=12 January 2009|volume=21|issue=2|pages=184–187|doi=10.1002/adma.200800570|bibcode=2009AdM....21..184Z |s2cid=136731328 }}</ref>

== Properties and applications == {{microbial and microbot movement|microbot}}

===Task-specific Janus materials=== The term "task-specific Janus materials" refers to the non-emulsifying roles of Janus particles.<ref>{{Cite journal | doi = 10.1002/anie.202206403 | volume = 61 | article-number = e202206403 | author = M. Vafaeezadeh, W. R. Thiel | title = Task-Specific Janus Materials in Heterogeneous Catalysis | journal = Angew. Chem. Int. Ed. | year = 2022 | issue = 39 | pmid = 35670287 | pmc = 9804448 | bibcode = 2022ACIE...61E6403V | s2cid = 249488989 }}</ref>

===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 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. 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.<ref name="Walther Soft Matter"/> 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.<ref name="Walther Soft Matter"/>

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 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 supermicelles is unknown so far; however, they may be similar to multilamellar vesicles.<ref name="Walther Soft Matter"/>

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.<ref name="Nano Letters">{{cite journal|last=Hong|first=Liang|author2=Angelo Cacciuto |author3=Erik Luijten |author4=Steve Granick |title=Clusters of Charged Janus Spheres|journal=Nano Letters|year=2006|volume=6|issue=11|pages=2510–2514|doi=10.1021/nl061857i|pmid=17090082|bibcode = 2006NanoL...6.2510H |citeseerx=10.1.1.79.7546}}</ref> 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. 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.<ref name="Walther Soft Matter"/>

===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).<ref name="Nano today review 2011" /> 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.<ref name="Nano today review 2011" />

===Reversibility of cluster formation and control of cluster size===

Control of cluster size 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.<ref name="Nano today review 2011" /> 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.<ref>{{Cite journal | doi = 10.1021/la060693i | pmid = 16732643 | volume = 22 | issue = 12 | pages = 5227–5229 | title = Janus Particles at Liquid-Liquid Interfaces | journal = Langmuir | year = 2006 | last1 = Glaser | first1 = N | last2 = Adams | first2 = D. J. | last3 = Böker | first3 = A | last4 = Krausch | first4 = G }}</ref> 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 adhesion force between the atomic force microscopy (AFM) tip and the particle surface.<ref>{{Cite journal | doi = 10.1021/la700774g | pmid = 17595125 | volume = 23 | issue = 16 | pages = 8544–8548 | last = Xu | first = Li-Ping |author2=Sulolit Pradhan |author3=Shaowei Chen | title = Adhesion Force Studies of Janus Nanoparticles | journal = Langmuir | year = 2007 }}</ref> The stronger interactions between the hydrophilic AFM tip and the hydrophilic side of the Janus nanoparticles were reflected by a greater adhesion 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.<ref>{{Cite journal | doi = 10.1021/la991081j | volume = 16 | issue = 6 | pages = 2539–2547 | last = Binks | first = B. P. |author2=S. O. Lumsdon | title = Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica | journal = Langmuir | year = 2000 }}</ref><ref>{{Cite journal | doi = 10.1126/science.1074868 | pmid = 12411700 | volume = 298 | issue = 5595 | pages = 1006–1009 | last = Dinsmore | first = A. D. |author2=Ming F. Hsu |author3=M. G. Nikolaides |author4=Manuel Marquez |author5=A. R. Bausch |author6=D. A. Weitz | title = Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles | journal = Science | date = 1 November 2002 |bibcode = 2002Sci...298.1006D | citeseerx = 10.1.1.476.7703 | s2cid = 2333453 }}</ref><ref>{{Cite journal | doi = 10.1016/S0001-8686(02)00069-6 | volume = 100–102 | pages = 503–546 | last = Aveyard | first = Robert |author2=Bernard P Binks |author3=John H Clint | title = Emulsions stabilised solely by colloidal particles | journal = Advances in Colloid and Interface Science | date = 28 February 2003 }}</ref> 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.<ref>{{Cite journal | doi = 10.1021/ja043581r | pmid = 15853333 | volume = 127 | issue = 17 | pages = 6271–6275 | last = Takahara | first = Yoshiko K. |author2=Shigeru Ikeda |author3=Satoru Ishino |author4=Koji Tachi |author5=Keita Ikeue |author6=Takao Sakata |author7=Toshiaki Hasegawa |author8=Hirotaro Mori |author9=Michio Matsumura |author10=Bunsho Ohtani | title = Asymmetrically Modified Silica Particles: A Simple Particulate Surfactant for Stabilization of Oil Droplets in Water | journal = J. Am. Chem. Soc. | year = 2005 | bibcode = 2005JAChS.127.6271T }}</ref> In 2009, hydrophilic surface of silica particles was made partially hydrophobic by adsorbing cetyltrimethylammonium bromide. These amphiphilic nanoparticles spontaneously assembled at the water-dichloromethane interface.<ref>{{Cite journal | doi = 10.1016/j.colsurfa.2008.08.027 | volume = 332 | issue = 1 | pages = 57–62 | journal = Colloids and Surfaces A: Physicochemical and Engineering Aspects | date = 2009 | title = Production of large quantities of "Janus" nanoparticles using wax-in-water emulsions | last1 = Perro | first1 = Adeline | last2 = Meunier | first2 = Fabrice | last3 = Schmitt | first3 = Véronique | last4 = Ravaine | first4 = Serge }}</ref> In 2010, Janus particles composed from silica and polystyrene, with the polystyrene portion loaded with nanosized magnetite particles, were used to form kinetically stable oil-in-water emulsions that can be spontaneously broken on application of an external magnetic field.<ref>{{Cite journal | doi = 10.1021/la104284v | pmid = 21133341 | volume = 27 | issue = 1 | pages = 30–33 | last = Teo | first = Boon M. |author2=Su Kyung Suh | author3-link = T. Alan Hatton |author3=T. Alan Hatton |author4=Muthupandian Ashokkumar |author5=Franz Grieser | title = Sonochemical Synthesis of Magnetic Janus Nanoparticles | journal = Langmuir | year = 2010 }}</ref> 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 synthesis. In 2008, spherical amphiphilic Janus nanoparticles, having one polystyrene and one poly(methyl methacrylate) side, were shown to be effective as compatibilizing agents of multigram scale compatibilization of two immiscible polymer blends, polystyrene and poly(methyl methacrylate).<ref name="applications emulsion" /> 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 copolymers.

===Stabilizers in emulsions===

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.<ref>{{Cite journal | doi = 10.1002/anie.200703224 | pmid = 18069717 | volume = 47 | issue = 4 | pages = 711–714 | title = Emulsion Polymerization Using Janus Particles as Stabilizers | journal = Angewandte Chemie International Edition | date = 11 January 2008 | last1 = Walther | first1 = Andreas | last2 = Hoffmann | first2 = Martin | last3 = Müller | first3 = Axel H. E. | bibcode = 2008ACIE...47..711W }}</ref> 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.<ref>{{Cite journal | doi = 10.1016/j.molliq.2020.113735 | volume = 315 | article-number = 113735 | author = M. Vafaeezadeh, W. R. Thiel | title = Janus interphase catalysts for interfacial organic reactions | journal = J. Mol. Liq. | year = 2020 | s2cid = 225004256 }}</ref>

===Catalyst in hydrogen peroxide decomposition===

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).<ref>{{Cite journal |last1=Howse |first1=Jonathan R. |last2=Jones |first2=Richard A. L. |last3=Ryan |first3=Anthony J. |last4=Gough |first4=Tim |last5=Vafabakhsh |first5=Reza |last6=Golestanian |first6=Ramin |date=2007-07-27 |title=Self-Motile Colloidal Particles: From Directed Propulsion to Random Walk |url=https://link.aps.org/doi/10.1103/PhysRevLett.99.048102 |journal=Physical Review Letters |volume=99 |issue=4 |article-number=048102 |doi=10.1103/PhysRevLett.99.048102|pmid=17678409 |arxiv=0706.4406 |bibcode=2007PhRvL..99d8102H }}</ref><ref name="applications catalysis">{{Cite journal | doi = 10.1002/smll.200901976 | pmid = 20108240 | volume = 6 | issue = 4 | pages = 565–572 | last = Valadares | first = Leonardo F | author2 = Yu-Guo Tao, Nicole S Zacharia, Vladimir Kitaev, Fernando Galembeck, Raymond Kapral, Geoffrey A Ozin | title = Catalytic Nanomotors: Self-Propelled Sphere Dimers | journal = Small | date = 22 February 2010 | bibcode = 2010Small...6..565V }}</ref> The platinum particle catalyzes the surface chemical reaction: 2H₂O₂ → O₂ + 2H₂O. 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. The motion of the spherical Janus nano-particles was found to agree with the predictions of theoretical findings. Ultimately, catalytic nano-motors 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.

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. 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.<ref name="applications demonstrating Noise-induced non-equilibrium phenomena">{{Cite journal | doi = 10.1103/PhysRevLett.110.268301 | pmid = 23848928 | volume = 110 | issue = 26 | article-number = 268301 | last = Ghosh | first = Pulak K | author2 = Misko, Vyacheslav R; Marchesoni, F; Nori, F | title = Self-Propelled Janus Particles in a Ratchet: Numerical Simulations | journal = Physical Review Letters | date = 24 June 2013 |arxiv = 1307.0090 |bibcode = 2013PhRvL.110z8301G | s2cid = 2990747 }}</ref>

===Water-repellent fibers===

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.<ref name="applications textile" /> 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&nbsp;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 cells was reported.<ref name="applications sensor" /> 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.<ref>{{Cite journal | doi = 10.1002/smll.200901604 | pmid = 20108232 | volume = 6 | issue = 4 | pages = 503–507 | last = Wu | first = Liz Y |author2=Benjamin M Ross |author3=SoonGweon Hong |author4=Luke P Lee | title = Bioinspired Nanocorals with Decoupled Cellular Targeting and Sensing Functionality | journal = Small | date = 22 February 2010 | bibcode = 2010Small...6..503W }}</ref> 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.

====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.<ref name="applications magnetic field imaging" /> 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.

In 2011, silica-coated Janus nanoparticles, composed of silver oxide and iron oxide (Fe₂O₃), were prepared in one step with scalable flame aerosol technology.<ref>{{Cite journal | doi = 10.1021/cm200399t | pmid = 23729990 | volume = 23 | issue = 7 | pages = 1985–1992 | last = Sotiriou | first = Georgios A. | author2 = Ann M. Hirt, Pierre-Yves Lozach, Alexandra Teleki, Frank Krumeich, Sotiris E. Pratsinis | title = Hybrid, Silica-Coated, Janus-Like Plasmonic-Magnetic Nanoparticles | journal = Chem. Mater. | year = 2011 | pmc = 3667481 }}</ref> 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.<ref>{{Cite journal | doi = 10.1002/adma.200502431 | volume = 18 | issue = 9 | pages = 1152–1156 | last = Takasi | first = Nisisako | author2 = T. Torii, T. Takahashi, Y. Takizawa | title = Synthesis of Monodisperse Bicolored Janus Particles with Electrical Anisotropy Using a Microfluidic Co-Flow System | journal = Adv. Mater. | year = 2006 | bibcode = 2006AdM....18.1152N | s2cid = 137260731 }}</ref> 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.

Graphene Janus particles have been used in experimental sodium-ion batteries to increase energy density. One side provides interaction sites while the other provides inter-layer separation. Energy density reached 337 mAh/g.<ref>{{Cite web|last=Lavars|first=Nick|date=2021-08-26|title=Two-faced graphene offers sodium-ion battery a tenfold boost in capacity|url=https://newatlas.com/energy/janus-graphene-sodium-battery-capacity/|access-date=2021-08-26|website=New Atlas|language=en-US}}</ref>

== See also == * Microswimmer * Active Brownian particle

== References == {{reflist}}

==External links== *[http://janus-particles.com/en/ An innovative process for their versatile large-scale synthesis] {{Webarchive|url=https://web.archive.org/web/20160304030939/http://janus-particles.com/en/ |date=4 March 2016 }}, Groupe NanoSytèmes Analytiques *[http://www.rsc.org/Shop/books/2012/9781849734233.asp Book: Janus Particle Synthesis, Self-assembly and Applications], RSC Smart Materials *[http://ptonline.aip.org/journals/doc/PHTOAD-ft/vol_62/iss_7/68_1.shtml?type=RSS Janus particles]{{Dead link|date=January 2020 |bot=InternetArchiveBot |fix-attempted=yes }}, Physics Today *[http://www.eurekalert.org/pub_releases/2008-02/ncsu-pa022708.php '2-faced' particles act like tiny submarines] {{Webarchive|url=https://web.archive.org/web/20080304140947/http://www.eurekalert.org/pub_releases/2008-02/ncsu-pa022708.php |date=4 March 2008 }}, EurekAlert! *[http://www.physorg.com/news6811.html Nano World: Two-faced Janus nanoparticles], PhysOrg.com

Category:Nanoparticles by morphology