www.advmat.de www.MaterialsViews.com
RESEARCH NEWS
Rational Design and Synthesis of Janus Composites Fuxin Liang, Chengliang Zhang, and Zhenzhong Yang* absorption energy over thermal energy, Janus particles possess an additional amphiphilic performance, which can be more strongly immobilized on the interface and are more effective to stabilize emulsions.[5] In analogy to the hydrophilelipophile balance (HLB) of surfactants, Janus balance of particles is proposed to describe their amphiphilic performance which is controlled by chemistry, size and shape of the two corresponding sides. Janus balance determines selfassembled structures and stability/type of emulsions.[6–8] Moreover, Janus particles can enhance miscibility of polymer alloys as new compatibilizers.[9] As colloidal molecules, Janus particles can assemble into superstructures.[10] If the two sides are hydrophobic and charged respectively, the Janus colloids can assemble into extended structures in water.[11] Tri-block Janus colloids with two hydrophobic poles and a charged equator band can form amphiphilic kagome lattice after a compromise between electrostatic and hydrophobic interactions (Figure 1a).[12] Colloidal ribbons and rings can subsequently undergo a hierarchical self-assembly.[13] Compared with the thermodynamically driven self-assembly process of molecular surfactants, self-assembly of Janus colloids forming clusters is kinetically driven. Different from surfactants which can fast reach an equilibrium shape, Janus colloids usually present transient isomeric structures with long lifetime before they progressively fuse to form more stable and highly ordered helices.[14] Self-assembly of Janus composite particles is tunable under electric or magnetic fields. Magnetic Janus colloids can show synchronized rotation and oscillation under a magnetic field.[15] Staggered and linear chains of magnetic Janus particles are observed, in which the equatorial plane between the two hemispheres is parallel to the field (Figure 1b).[16–18] In imaging performance, color contrast of the Janus two colored particles is triggered under a magnetic or electric field.[19] Janus particles containing quantum dots and paramagnetic nanoparticles can realize a magneto-driven fluorescent switch and display.[20] Magnetic Janus nanoparticles can recognize and manipulate desired cells to carry out synchronous diagnosis and treatment.[21] A dually functionalized Janus nanocomposite enables to target tumor cells and internalize via the folate receptor. The cells are significantly induced to death by stimuli-induced drug release under acidic condition.[22]
Janus composites with two different components divided on the same object have gained growing interest in many fields, such as solid emulsion stabilizers, sensors, optical probes and self-propellers. Over the past twenty years, various synthesis methods have been developed including Pickering emulsion interfacial modification, block copolymer self-assembly, microfluidics, electro co-jetting, and swelling emulsion polymerization. Anisotropic shape and asymmetric spatial distribution of compositions and functionalities determine their unique performances. Rational design and large scale synthesis of functional Janus materials are crucial for the systematical characterization of performance and exploitation of practical applications.
1. Introduction In 1991, P. G. de Gennes first employed the term of two faceted Roman god “Janus” in his Nobel lecture to describe the characteristics and performance of such particles with two different components districted onto the same surface.[1] In analogy to molecular surfactants, amphiphilic Janus particles were predicated to stack densely at an interface to form a porous monolayer, where small molecules can transport. Over the past twenty years, Janus materials have experienced a fast development. Rational design of the Janus composites with tunable morphology, size and composition is a core concern for their Janus performances. Some representative morphologies are shown in Scheme 1. They have displayed various potential applications as solid surfactants, building blocks towards superstructures, optical detection and display, and self-propelled nanomotors.[2–4] On the other hand, large scale production of Janus materials is significant for their practical applications. In this Research News, after introducing several representative performances and potential applications of Janus materials, recent advances in synthetic methodology will be summarized in more detail. The corresponding challenges will also be discussed.
2. Performance and Applications As solid emulsifiers, Janus particles are amphiphilic and can stabilize emulsions. Different from homogenous solid particles, which can stabilize Pickering emulsions due to a strong Dr. F. Liang, Dr. C. Zhang, Prof. Z. Yang State Key Laboratory of Polymer Physics and Chemistry Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected]
DOI: 10.1002/adma.201305415
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
3. Synthetic Methods In 1989, the first bead (partially modified glass particle) named with “Janus” was obtained by selectively modifying the
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
RESEARCH NEWS
www.MaterialsViews.com
Scheme 1. Different morphologies of Janus materials.
exposed part with hydrophobic octadecyltrichlorosilane while the other part was embedded in a planar matrix.[23] They are amphiphilic and can arrange at an oil/water interface to stabilize emulsions. Since then, Janus materials have experienced rapid development, which have displayed diversified performances and potential applications. Various new synthesis methods have been proposed including Pickering emulsion
Figure 1. Some representative performances and applications of Janus materials. a) Colloidal kagome lattice by the assembly of tri-block Janus colloids with two hydrophobic poles and a charged equator band. Reproduced with permission.[12] Copyright 2011 NPG. b) Self-assembled staggered and linear chains like superstructures from the anisotropic Janus particles. Reproduced with permission.[17] Copyright 2008 NPG.
2
wileyonlinelibrary.com
interfacial synthesis, block copolymer self-assembly, microfluidics, electro co-jetting, and swelling emulsion polymerization during the past twenty years. Ramsden and Pickering discovered that a emulsion could be stabilized with proper solid particles, so called Pickering Emulsion.[24,25] The Pickering emulsion provides larger interfacial synthesis space to greatly increase quantity of Janus materials. Besides Pickering emulsion, traditional emulsion polymerization of monomers against a premade seed particle can large scale produce asymmetric particles by controlling phase separation thermodynamics and kinetics. Methodology is crucial to precisely segment the particles with different chemistries onto desired regions and finely tune morphology and microstructure. Two strategies of partial protection combined selective modification and phase separation of multiple components are usually employed for the segmentation. Recent methodological development is summarized according to the category, and the corresponding challenges are presented therein.
3.1. Partial Protection and Modification Selective modification of the exposed part of a particle while the other part is protected with a planar substrate provides an easy method to fabricate Janus particles. The compositions are distinctly subdivided.[23] The method is straightforward and general. A variety of approaches including electron-beam sputtering, micro-contact printing, and reactive deposition are valid for the selective modification.[26–28] Besides large particles, after nanosized ones for example Au nanoparticles are protected after a favorable tethering onto the thiol- terminated polymer crystallite surface, the functional polymer chains can be covalently grafted onto the contact part achieving Janus nanoparticles.[29] In order to increase the yield, Pickering emulsion droplet interface has been extensively employed to partially protect the particles. They are spontaneously subdivided into two parts and immersed in oil and water phases, respectively. Rotation of particles at a fluidic Pickering emulsion interface will usually cause a failure to selectively modify the desired part. Granick et al. firstly reported an elegant synthesis of Janus silica particles using frozen wax droplet interface rather than a fluid one (Figure 2a).[30] However, excess growth of materials onto the exposed part usually leads agglomeration among the particles. Alternatively, we proposed selectively asymmetric wet-etching the corona group from the exposed part to acquire fresh hydrophilic silica surface, achieving Janus particles. By prolonging etching extent, the particles are evolved into nonspherical while Janus performance is preserved.[31] Onto one side of the Janus particles, polymers can be further grown to derive Janus polymer/inorganic composite particles. In order to restrict rotation of particles at a fluidic interface, we proposed to simultaneously graft polymer brushes onto the two parts of a silica particle (Figure 2b).[32] At the beginning of materials growth, the particles become amphiphilic (Janus) with both hydrophilic and lipophilic groups grown onto the two parts. This can greatly confine rotation of the particles at the interface ensuring a further modification. Since the absorption energy is proportional to particle size square, the Pickering emulsion interface synthesis is valid for large particles above
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
www.advmat.de www.MaterialsViews.com
RESEARCH NEWS Figure 2. Some representative synthetic methods. a) Pickering emulsion interfacial synthesis of Janus particles. Reproduced with permission.[30] Copyright 2006 ACS. b) Pickering emulsion interfacial biphasic grafting toward Janus colloids. Reproduced with permission.[32] c) Self-assembly of block copolymers for polymeric Janus discs. Reproduced with permission.[35] Copyright 2007 ACS. d) Seeded emulsion synthesis of Janus particles with distinct composition compartmentalization. Reproduced with permission.[43] Copyright 2010 ACS. e) Janus hollow spheres by emulsion interfacial self-organized sol-gel process. Reproduced with permission.[46] Copyright 2011 RSC. f) Electrohydrodynamic co-jetting for bicompartmental Janus particles. Reproduced with permission.[56]
the sub-micrometer level. The absorption energy of nanometersized ones is comparable with thermal energy, the interface will be wrinkled and covered with multiple layers of particles. The synthesis of nanosized Janus particles at Pickering emulsion interface is therefore hampered.
3.2. Phase Separation The knowledge about phase separation leading domains in multiple composites can be employed in combination with other micro-processing methods to achieve Janus materials. It is flexible to tune their composition, shape and size. Amphiphilic block copolymers are inherently Janus with different segments. Selfassembly into nanoscale supramolecular structures in the form such as micelle, cylinder and lamella, provides a wealthy of material reservoir to derive Janus materials with tunable shape such as particle, rod and disk after partial dis-assembling the supramolecular structures (Figure 2c).[33–35] Selective crosslinking one domain is usually required in order to retain integrity of the Janus materials. Co-assembly of different block copolymers can lead phase separation at the emulsion interface, selective fixation of the dispersed domains can give Janus particles.[36] Janus particles can also be prepared from multi-component systems in a confined space such as an emulsion droplet. Phase separation forming asymmetric Janus particles is easily induced by many parameters including polymerization, solvent evaporation and temperature or external fields. Emulsion
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
polymerization of polymer particles can give many unique morphologies including sandwich-, acorn- and octopus-like shapes, which can be thermodynamically predicted according to interfacial tensions.[37] Moreover, large quantity is ensured, which is important for practical applications. As early as in 1985, polystyrene (PS)/polymethylmethacrylate (PMMA) dumbbell like non-spherical particles were synthesized by emulsion polymerization.[38] Non-spherical Janus particles with tunable shape from hemisphere to disc are achieved from deformable polymer particles that are in situ formed by polymerization in a dispersed droplet. The particles can immigrate towards the emulsion interface due to Pickering effect. Another hydrophilic polymer chain can be grafted onto the exposed part facing the aqueous phase. An interfacial tension mismatch at the triple phase contact line plays a key role in shaping the particles by elongation.[39] If the resultant polymer is immiscible with the seed, a polymer lobe will protrude from the seed particle.[40] Similarly, anisotropic particles consisting of two lobes are prepared when using two immiscible polymer pairs.[41] The non-spherical shape is significantly influenced by phase separation kinetics, which can be accelerated by crosslinking thus high internal stress.[42] It is noted that the phase separation is usually incomplete especially in a highly viscoelastic system, components are not distinctly segregated. The anisotropic particles are not strictly Janus in chemistry. We have presented a facile approach to large scale produce sub-micrometer anisotropic Janus polymer/inorganic composite particles by seeded emulsion
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de www.MaterialsViews.com
RESEARCH NEWS
materials. A liquid drop at the nozzle tip is distorted into a polymerization against a crosslinked polyacrylonitrile (PAN) narrow Taylor cone under an electrical field, which is further polymer hollow particle (Figure 2d).[43] Although the freshly stretched forming a nanometer thick polymer after solvent formed PS lobe contains no seed component, the seed PAN evaporation (Figure 2f).[56] Biphasic polymeric Janus partiparticle should contain a minority of PS. After a selective modification of the PAN part of the anisotropic particles to introduce cles are prepared by controlling competitive diffusion of two functional groups for example polyacrylic acid, inorganic spepolymer solutions and solidification during solvent evaporacies are preferentially grown thereby. The particles are strictly tion, and the compartment number of the Janus particles is Janus both in shape and chemistry. In a dispersion, polymerizatunable.[57,58] The shape is also tunable from spherical, cylintion induced de-wetting onto a seed silica particle surface can drical, to fibers relying on the stream break-up process in the give anisotropic silica/PS composite particles.[44] After selecelectrohydrodynamic co-jetting.[59] Janus particles with tunable tively etching the thin polymer layer from the opposite pole of size are obtained by microtoming the Janus long fibers.[60] the polymer lobe, fresh hydrophilic silica surface is exposed. The composite particles are strictly Janus in chemistry besides anisotropic shape. Recently, snowman-like Janus particles with 4. The Other Janus Materials distinct segregation in composition are simply derived from a core-shell particle by dynamic protruding the core material Janus nanosheets deserve more attention due to their highly forming a lobe onto the shell.[45] High solid content is guarananisotropic shape and asymmetric chemistry. Compared with Janus particles, the rotation of Janus sheets at an interface is teed if the protrusion is carried out while avoiding coalescence greatly restricted because the planar configuration facilitates among the particles in the presence of excess emulsifiers. their favorable orientation.[61,62] Janus sheets as solid surfactants The concept of Janus can be extended to hollow spheres achieving Janus cages. We realize that an emulsion interface can make emulsions more stable. Although polymeric Janus is Janus with both hydrophilic exterior and oleophilic intedisks are flexible to encapsulate desired species (Figure 3a), rior surfaces, which can act as a temporal template to prepare they are easily swollen, and thus deformed, in the presence of Janus shell after duplication. Hydrophilic and lipophilic spesolvents to lose their original shape.[63] Silica Janus nanosheets cies in the dispersed droplets are self-organize at the interface are fabricated by multi-step etching a wafer substrate, which forming a silica shell by a sol-gel process, which can spontacan manipulate liquid droplets in the form of marbles.[64] 1 nm neously face towards the external aqueous and the internal oil thick Janus nanosheets with different chemical groups onto phases, respectively. Janus hollow spheres are achieved, mimtwo sides are fabricated by self-assembling biphenyl monicking micelles but robust. Hydrophobic organic components olayer on a TEM grid.[65] Recently, selective modification of can be selectively captured inside the cavity from their aqueous graphene while another side is partially protected with PMMA surroundings (Figure 2e).[46] Selective grafting responsive polyallows to achieve Janus graphene.[66] In order to increase yield mers for example thermal responsive poly(N-isopropylacrylaof Janus nanosheets, we have recently proposed a simple way mide) (PNIPAM) onto the interior surface, the Janus hollow to produce inorganic Janus nanosheets by crushing the correspheres can load oil at high temperature. At lower temperasponding Janus hollow spheres.[62] The shell thickness is tun[ 47 ] ture, the cavity becomes hydrophilic and the oil is released. able from several nanometers, therefore mechanic strenth is Janus micro-reactors are further derived after incorporating catalysts such as photocatalytic nanoparticles P25. Decomposition of organic pollutants is completed at a local higher concentration after being collected from their aqueous surroundings.[48] Microfluidics approach is also efficient to create multi-component droplets by passing through multiple channels. Janus particles with complex structures are thus derived after solidification either by polymerization or solvent evaporation. Characteristic dimension of the Janus particles is mainly determined by the channel size, which is usually micrometers or larger.[49] By adjusting the channels contour, volume fraction of different components and shape are tunable from hemisphere, rod to disk.[50–52] Functional inorganic nanoparticles are easily introduced into the polymer phase forming functional Janus composite particles which Figure 3. Some representative miscellaneous Janus materials. a) Janus nanosheets from Janus are optical and electromagnetic active.[53–55] hollow spheres and their Janus performance as solid surfactants. Reproduced with permisElectrohydrodynamic co-jetting is another sion.[62] b) Self-assembly of polymeric Janus cylinders at a liquid/liquid interface. Reproduced effective method to synthesize Janus with permission.[74] Copyright 2011 ACS.
4
wileyonlinelibrary.com
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
www.advmat.de www.MaterialsViews.com
Method 2D/3D modification Self-assembly Emulsion polymerization Microfluidics
Composition
Size
Morphology
Quantity
Organic, Inorganic, Inorganic–Organic
0.01–90 μm
Particle, Disc/sheet, Hollow sphere
Large (3D) Low (2D)
Organic, Inorganic–Organic
RESEARCH NEWS
Rational Design and Synthesis of Janus Composites Fuxin Liang, Chengliang Zhang, and Zhenzhong Yang* absorption energy over thermal energy, Janus particles possess an additional amphiphilic performance, which can be more strongly immobilized on the interface and are more effective to stabilize emulsions.[5] In analogy to the hydrophilelipophile balance (HLB) of surfactants, Janus balance of particles is proposed to describe their amphiphilic performance which is controlled by chemistry, size and shape of the two corresponding sides. Janus balance determines selfassembled structures and stability/type of emulsions.[6–8] Moreover, Janus particles can enhance miscibility of polymer alloys as new compatibilizers.[9] As colloidal molecules, Janus particles can assemble into superstructures.[10] If the two sides are hydrophobic and charged respectively, the Janus colloids can assemble into extended structures in water.[11] Tri-block Janus colloids with two hydrophobic poles and a charged equator band can form amphiphilic kagome lattice after a compromise between electrostatic and hydrophobic interactions (Figure 1a).[12] Colloidal ribbons and rings can subsequently undergo a hierarchical self-assembly.[13] Compared with the thermodynamically driven self-assembly process of molecular surfactants, self-assembly of Janus colloids forming clusters is kinetically driven. Different from surfactants which can fast reach an equilibrium shape, Janus colloids usually present transient isomeric structures with long lifetime before they progressively fuse to form more stable and highly ordered helices.[14] Self-assembly of Janus composite particles is tunable under electric or magnetic fields. Magnetic Janus colloids can show synchronized rotation and oscillation under a magnetic field.[15] Staggered and linear chains of magnetic Janus particles are observed, in which the equatorial plane between the two hemispheres is parallel to the field (Figure 1b).[16–18] In imaging performance, color contrast of the Janus two colored particles is triggered under a magnetic or electric field.[19] Janus particles containing quantum dots and paramagnetic nanoparticles can realize a magneto-driven fluorescent switch and display.[20] Magnetic Janus nanoparticles can recognize and manipulate desired cells to carry out synchronous diagnosis and treatment.[21] A dually functionalized Janus nanocomposite enables to target tumor cells and internalize via the folate receptor. The cells are significantly induced to death by stimuli-induced drug release under acidic condition.[22]
Janus composites with two different components divided on the same object have gained growing interest in many fields, such as solid emulsion stabilizers, sensors, optical probes and self-propellers. Over the past twenty years, various synthesis methods have been developed including Pickering emulsion interfacial modification, block copolymer self-assembly, microfluidics, electro co-jetting, and swelling emulsion polymerization. Anisotropic shape and asymmetric spatial distribution of compositions and functionalities determine their unique performances. Rational design and large scale synthesis of functional Janus materials are crucial for the systematical characterization of performance and exploitation of practical applications.
1. Introduction In 1991, P. G. de Gennes first employed the term of two faceted Roman god “Janus” in his Nobel lecture to describe the characteristics and performance of such particles with two different components districted onto the same surface.[1] In analogy to molecular surfactants, amphiphilic Janus particles were predicated to stack densely at an interface to form a porous monolayer, where small molecules can transport. Over the past twenty years, Janus materials have experienced a fast development. Rational design of the Janus composites with tunable morphology, size and composition is a core concern for their Janus performances. Some representative morphologies are shown in Scheme 1. They have displayed various potential applications as solid surfactants, building blocks towards superstructures, optical detection and display, and self-propelled nanomotors.[2–4] On the other hand, large scale production of Janus materials is significant for their practical applications. In this Research News, after introducing several representative performances and potential applications of Janus materials, recent advances in synthetic methodology will be summarized in more detail. The corresponding challenges will also be discussed.
2. Performance and Applications As solid emulsifiers, Janus particles are amphiphilic and can stabilize emulsions. Different from homogenous solid particles, which can stabilize Pickering emulsions due to a strong Dr. F. Liang, Dr. C. Zhang, Prof. Z. Yang State Key Laboratory of Polymer Physics and Chemistry Institute of Chemistry Chinese Academy of Sciences Beijing 100190, China E-mail: [email protected]
DOI: 10.1002/adma.201305415
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
3. Synthetic Methods In 1989, the first bead (partially modified glass particle) named with “Janus” was obtained by selectively modifying the
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
1
www.advmat.de
RESEARCH NEWS
www.MaterialsViews.com
Scheme 1. Different morphologies of Janus materials.
exposed part with hydrophobic octadecyltrichlorosilane while the other part was embedded in a planar matrix.[23] They are amphiphilic and can arrange at an oil/water interface to stabilize emulsions. Since then, Janus materials have experienced rapid development, which have displayed diversified performances and potential applications. Various new synthesis methods have been proposed including Pickering emulsion
Figure 1. Some representative performances and applications of Janus materials. a) Colloidal kagome lattice by the assembly of tri-block Janus colloids with two hydrophobic poles and a charged equator band. Reproduced with permission.[12] Copyright 2011 NPG. b) Self-assembled staggered and linear chains like superstructures from the anisotropic Janus particles. Reproduced with permission.[17] Copyright 2008 NPG.
2
wileyonlinelibrary.com
interfacial synthesis, block copolymer self-assembly, microfluidics, electro co-jetting, and swelling emulsion polymerization during the past twenty years. Ramsden and Pickering discovered that a emulsion could be stabilized with proper solid particles, so called Pickering Emulsion.[24,25] The Pickering emulsion provides larger interfacial synthesis space to greatly increase quantity of Janus materials. Besides Pickering emulsion, traditional emulsion polymerization of monomers against a premade seed particle can large scale produce asymmetric particles by controlling phase separation thermodynamics and kinetics. Methodology is crucial to precisely segment the particles with different chemistries onto desired regions and finely tune morphology and microstructure. Two strategies of partial protection combined selective modification and phase separation of multiple components are usually employed for the segmentation. Recent methodological development is summarized according to the category, and the corresponding challenges are presented therein.
3.1. Partial Protection and Modification Selective modification of the exposed part of a particle while the other part is protected with a planar substrate provides an easy method to fabricate Janus particles. The compositions are distinctly subdivided.[23] The method is straightforward and general. A variety of approaches including electron-beam sputtering, micro-contact printing, and reactive deposition are valid for the selective modification.[26–28] Besides large particles, after nanosized ones for example Au nanoparticles are protected after a favorable tethering onto the thiol- terminated polymer crystallite surface, the functional polymer chains can be covalently grafted onto the contact part achieving Janus nanoparticles.[29] In order to increase the yield, Pickering emulsion droplet interface has been extensively employed to partially protect the particles. They are spontaneously subdivided into two parts and immersed in oil and water phases, respectively. Rotation of particles at a fluidic Pickering emulsion interface will usually cause a failure to selectively modify the desired part. Granick et al. firstly reported an elegant synthesis of Janus silica particles using frozen wax droplet interface rather than a fluid one (Figure 2a).[30] However, excess growth of materials onto the exposed part usually leads agglomeration among the particles. Alternatively, we proposed selectively asymmetric wet-etching the corona group from the exposed part to acquire fresh hydrophilic silica surface, achieving Janus particles. By prolonging etching extent, the particles are evolved into nonspherical while Janus performance is preserved.[31] Onto one side of the Janus particles, polymers can be further grown to derive Janus polymer/inorganic composite particles. In order to restrict rotation of particles at a fluidic interface, we proposed to simultaneously graft polymer brushes onto the two parts of a silica particle (Figure 2b).[32] At the beginning of materials growth, the particles become amphiphilic (Janus) with both hydrophilic and lipophilic groups grown onto the two parts. This can greatly confine rotation of the particles at the interface ensuring a further modification. Since the absorption energy is proportional to particle size square, the Pickering emulsion interface synthesis is valid for large particles above
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
www.advmat.de www.MaterialsViews.com
RESEARCH NEWS Figure 2. Some representative synthetic methods. a) Pickering emulsion interfacial synthesis of Janus particles. Reproduced with permission.[30] Copyright 2006 ACS. b) Pickering emulsion interfacial biphasic grafting toward Janus colloids. Reproduced with permission.[32] c) Self-assembly of block copolymers for polymeric Janus discs. Reproduced with permission.[35] Copyright 2007 ACS. d) Seeded emulsion synthesis of Janus particles with distinct composition compartmentalization. Reproduced with permission.[43] Copyright 2010 ACS. e) Janus hollow spheres by emulsion interfacial self-organized sol-gel process. Reproduced with permission.[46] Copyright 2011 RSC. f) Electrohydrodynamic co-jetting for bicompartmental Janus particles. Reproduced with permission.[56]
the sub-micrometer level. The absorption energy of nanometersized ones is comparable with thermal energy, the interface will be wrinkled and covered with multiple layers of particles. The synthesis of nanosized Janus particles at Pickering emulsion interface is therefore hampered.
3.2. Phase Separation The knowledge about phase separation leading domains in multiple composites can be employed in combination with other micro-processing methods to achieve Janus materials. It is flexible to tune their composition, shape and size. Amphiphilic block copolymers are inherently Janus with different segments. Selfassembly into nanoscale supramolecular structures in the form such as micelle, cylinder and lamella, provides a wealthy of material reservoir to derive Janus materials with tunable shape such as particle, rod and disk after partial dis-assembling the supramolecular structures (Figure 2c).[33–35] Selective crosslinking one domain is usually required in order to retain integrity of the Janus materials. Co-assembly of different block copolymers can lead phase separation at the emulsion interface, selective fixation of the dispersed domains can give Janus particles.[36] Janus particles can also be prepared from multi-component systems in a confined space such as an emulsion droplet. Phase separation forming asymmetric Janus particles is easily induced by many parameters including polymerization, solvent evaporation and temperature or external fields. Emulsion
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
polymerization of polymer particles can give many unique morphologies including sandwich-, acorn- and octopus-like shapes, which can be thermodynamically predicted according to interfacial tensions.[37] Moreover, large quantity is ensured, which is important for practical applications. As early as in 1985, polystyrene (PS)/polymethylmethacrylate (PMMA) dumbbell like non-spherical particles were synthesized by emulsion polymerization.[38] Non-spherical Janus particles with tunable shape from hemisphere to disc are achieved from deformable polymer particles that are in situ formed by polymerization in a dispersed droplet. The particles can immigrate towards the emulsion interface due to Pickering effect. Another hydrophilic polymer chain can be grafted onto the exposed part facing the aqueous phase. An interfacial tension mismatch at the triple phase contact line plays a key role in shaping the particles by elongation.[39] If the resultant polymer is immiscible with the seed, a polymer lobe will protrude from the seed particle.[40] Similarly, anisotropic particles consisting of two lobes are prepared when using two immiscible polymer pairs.[41] The non-spherical shape is significantly influenced by phase separation kinetics, which can be accelerated by crosslinking thus high internal stress.[42] It is noted that the phase separation is usually incomplete especially in a highly viscoelastic system, components are not distinctly segregated. The anisotropic particles are not strictly Janus in chemistry. We have presented a facile approach to large scale produce sub-micrometer anisotropic Janus polymer/inorganic composite particles by seeded emulsion
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
3
www.advmat.de www.MaterialsViews.com
RESEARCH NEWS
materials. A liquid drop at the nozzle tip is distorted into a polymerization against a crosslinked polyacrylonitrile (PAN) narrow Taylor cone under an electrical field, which is further polymer hollow particle (Figure 2d).[43] Although the freshly stretched forming a nanometer thick polymer after solvent formed PS lobe contains no seed component, the seed PAN evaporation (Figure 2f).[56] Biphasic polymeric Janus partiparticle should contain a minority of PS. After a selective modification of the PAN part of the anisotropic particles to introduce cles are prepared by controlling competitive diffusion of two functional groups for example polyacrylic acid, inorganic spepolymer solutions and solidification during solvent evaporacies are preferentially grown thereby. The particles are strictly tion, and the compartment number of the Janus particles is Janus both in shape and chemistry. In a dispersion, polymerizatunable.[57,58] The shape is also tunable from spherical, cylintion induced de-wetting onto a seed silica particle surface can drical, to fibers relying on the stream break-up process in the give anisotropic silica/PS composite particles.[44] After selecelectrohydrodynamic co-jetting.[59] Janus particles with tunable tively etching the thin polymer layer from the opposite pole of size are obtained by microtoming the Janus long fibers.[60] the polymer lobe, fresh hydrophilic silica surface is exposed. The composite particles are strictly Janus in chemistry besides anisotropic shape. Recently, snowman-like Janus particles with 4. The Other Janus Materials distinct segregation in composition are simply derived from a core-shell particle by dynamic protruding the core material Janus nanosheets deserve more attention due to their highly forming a lobe onto the shell.[45] High solid content is guarananisotropic shape and asymmetric chemistry. Compared with Janus particles, the rotation of Janus sheets at an interface is teed if the protrusion is carried out while avoiding coalescence greatly restricted because the planar configuration facilitates among the particles in the presence of excess emulsifiers. their favorable orientation.[61,62] Janus sheets as solid surfactants The concept of Janus can be extended to hollow spheres achieving Janus cages. We realize that an emulsion interface can make emulsions more stable. Although polymeric Janus is Janus with both hydrophilic exterior and oleophilic intedisks are flexible to encapsulate desired species (Figure 3a), rior surfaces, which can act as a temporal template to prepare they are easily swollen, and thus deformed, in the presence of Janus shell after duplication. Hydrophilic and lipophilic spesolvents to lose their original shape.[63] Silica Janus nanosheets cies in the dispersed droplets are self-organize at the interface are fabricated by multi-step etching a wafer substrate, which forming a silica shell by a sol-gel process, which can spontacan manipulate liquid droplets in the form of marbles.[64] 1 nm neously face towards the external aqueous and the internal oil thick Janus nanosheets with different chemical groups onto phases, respectively. Janus hollow spheres are achieved, mimtwo sides are fabricated by self-assembling biphenyl monicking micelles but robust. Hydrophobic organic components olayer on a TEM grid.[65] Recently, selective modification of can be selectively captured inside the cavity from their aqueous graphene while another side is partially protected with PMMA surroundings (Figure 2e).[46] Selective grafting responsive polyallows to achieve Janus graphene.[66] In order to increase yield mers for example thermal responsive poly(N-isopropylacrylaof Janus nanosheets, we have recently proposed a simple way mide) (PNIPAM) onto the interior surface, the Janus hollow to produce inorganic Janus nanosheets by crushing the correspheres can load oil at high temperature. At lower temperasponding Janus hollow spheres.[62] The shell thickness is tun[ 47 ] ture, the cavity becomes hydrophilic and the oil is released. able from several nanometers, therefore mechanic strenth is Janus micro-reactors are further derived after incorporating catalysts such as photocatalytic nanoparticles P25. Decomposition of organic pollutants is completed at a local higher concentration after being collected from their aqueous surroundings.[48] Microfluidics approach is also efficient to create multi-component droplets by passing through multiple channels. Janus particles with complex structures are thus derived after solidification either by polymerization or solvent evaporation. Characteristic dimension of the Janus particles is mainly determined by the channel size, which is usually micrometers or larger.[49] By adjusting the channels contour, volume fraction of different components and shape are tunable from hemisphere, rod to disk.[50–52] Functional inorganic nanoparticles are easily introduced into the polymer phase forming functional Janus composite particles which Figure 3. Some representative miscellaneous Janus materials. a) Janus nanosheets from Janus are optical and electromagnetic active.[53–55] hollow spheres and their Janus performance as solid surfactants. Reproduced with permisElectrohydrodynamic co-jetting is another sion.[62] b) Self-assembly of polymeric Janus cylinders at a liquid/liquid interface. Reproduced effective method to synthesize Janus with permission.[74] Copyright 2011 ACS.
4
wileyonlinelibrary.com
© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2014, DOI: 10.1002/adma.201305415
www.advmat.de www.MaterialsViews.com
Method 2D/3D modification Self-assembly Emulsion polymerization Microfluidics
Composition
Size
Morphology
Quantity
Organic, Inorganic, Inorganic–Organic
0.01–90 μm
Particle, Disc/sheet, Hollow sphere
Large (3D) Low (2D)
Organic, Inorganic–Organic
