Janus nanoparticles

Anisotropy and Dissymmetry Silica Nanotubes and Janus Nanoparticles... 

The synthetic route for making the Janus nanoparticles first consists in the emulsion polymerization of styrene in the presence of silica nanoparticles surface-modified by polymerizable groups. Snowman-like hybrid nanostructures are thus obtained with 85% yield in which the... 

Figure 1.28 Schematic representation of the consecutive stages for the fabrication of silica Janus nanoparticles. (Reproduced from ref. 54, with permission.)...

Fig. 5 Left Consecutive stages involved in the formation of Janus nanopaiticles through a protection-deprotection route. Right TEM image of Janus nanoparticles. Reproduced from [38]...

Fig. 1 TEM images of Au MnO Janus nanoparticles, where tuning the reaction conditions can yield (a) flower-like assemblies and heteronanoparticles of different sizes (b) 7 25nm, (c) 7 20nm and (d) 4(325 nm for Au MnO domains. Reproduced with permission from ref. 15. Copyright 2014 American Chemical Society. ...

Figure 5.45 Schematic illustration of the processes for the preparation of MSMs in DMF (A) and in water (B), and the intramicelle complexation to form Janus nanoparticles (C). (D) TEM image of the aggregates (scale bar is 100 nm).

Recently, nanoparticles with different surface chemistries have been widely used in Pickering emulsion field. A series of works to study the interfacial behaviors of nanoparticles adsorbed at the water-oil interface have been conducted by using DPD simulations (Luu and Striolo, 2014 Luu et al, 2013a, 2013b). The structural and dynamic properties of spherical homogenous and Janus nanoparticles with different surface compositions accumulated at the water—oil interface were investigated (Fig. 35). It is found that the surface density plays an important role in the a jregation state of nanoparticles. 

However, the shape of nanoparticles also plays an important role in the interfacial properties. It was found that the orientation of ellipsoidal Janus nanoparticles at oil—water interface was influenced by two nanoparticle aspect ratios, namely, the amount of polar with respect to nonpolar groups and the interactions between the nanoparticle surface groups and aqueous and nonaqueous solvents. Most importantly, when the nanoparticles lay with their longer axis parallel to the water—oil interface, the interface tension will be reduced with nanoparticles with sufficiently high surface coverage. Besides, the equilibrium behavior of ellipsoidal Janus nanoparticles adsorbed at spherical water—oil interface was also investigated by DPD simulations. It is found that several phenomena that happened on planar water—oil interface were not observed, demonstrating that the curvature of interface can also influence the state of nanoparticles adsorbed at water—oil interface (Fig. 36). 

Luu X-C, Striolo A Ellipsoidal Janus nanoparticles assembled at spherical oil/water interfaces,/Phys Chem B 118 13737-13743, 2014. 

Walther [111] was among the first to study and report the effect of the incorporation of Janus nanoparticles on the morphology of PS/PMMA blend. The organic Janus nanoparticles were composed of a crosslinked polybutadiene core grafted with two PS and PMMA half-sphere (Figure 11.8). 

Kim JU, Matsen MW. Positioning Janus nanoparticles in block copolymer scaffolds. Phys Rev Lett 2009 102(7) 078303. ... 

Fig. 20 Synthesis of Janus nanoparticles using a combination of polymer single-crystal templating with SI-ATRP. Reprinted with permission from Zhou et al. [205]...

Keywords Block copolymers ABC triblock copolymers Janus micelles Cylinder brushes Core-shell nanoparticles Graft copolymers Micelles Vesicles Copolyampholytes Polyelectrolyte block copolymers Aggregation... 

Janus micelles are non-centrosymmetric, surface-compartmentalized nanoparticles, in which a cross-linked core is surrounded by two different corona hemispheres. Their intrinsic amphiphilicity leads to the collapse of one hemisphere in a selective solvent, followed by self-assembly into higher ordered superstructures. Recently, the synthesis of such structures was achieved by crosslinking of the center block of ABC triblock copolymers in the bulk state, using a morphology where the B block forms spheres between lamellae of the A and C blocks [95, 96]. In solution, Janus micelles with polystyrene (PS) and poly(methyl methacrylate) (PMMA) half-coronas around a crosslinked polybutadiene (PB) core aggregate to larger entities with a sharp size distribution, which can be considered as supermicelles (Fig. 20). They coexist with single Janus micelles (unimers) both in THF solution and on silicon and water surfaces [95, 97]. 

Finally, it is shown that non-linear amphiphilic structures show different aggregation behavior as compared to block copolymers. Graft copolymers with non-polar backbone polyelectrolyte side chains have a smaller tendency to form micelles than their block copolymer analogs which is attributed to the more facile stabilization of unimers by the sidechains. In contrast, unimolecular micelles are the only possibility for core-shell nanoparticles. Janus micelles, on the other hand, form unique non-centrosymmetrical micelles that have a strong tendency to form centrosymmetrical supermicelles. 

Keywords Block copolymer/nanoparticle mixtures Interfacial assembly Nanoparticles Janus particles... 

As discussed by Binks and Lumsdon, amphiphilic Janus particles can exhibit an interfacial activity several times higher than simple homogeneous particles [54], Janus particles combine the amphiphilic character of surfactants and the physical properties of nanoparticles, which opens new opportunities in emerging areas of nanotechnology and emulsion stabilization. 

Fig. 10 TEM images of the nanoparticles (a) Janus particles consisting of gold (darker spheres) and iron oxide (brighter spheres)-, (b) homogeneous iron oxide particles (c) gold particles. Scale bars 25 nm. Reprinted with permission from Langmuir [68], Copyright (2006) American Chemical Society...

A special case of nanoparticle self-assembly is the Janus particle. It was shown that Janus particles are considerably more active than homogeneous particles of comparable size and chemical nature and that the interfacial activity can be increased by increasing the amphiphilic character of the particles. Thus, the Janus particles show a significant advantage in the stabilization of emulsions and foams over homogeneous particles as they unify the Pickering concept and the amphiphilicity of a simple surfactant. 

Perro A et al (2005) Design and synthesis of Janus micro- and nanoparticles. J Mater Chem 15(35-36) 3745-3760... 

Walther A, Matussek K, Muller AHE (2008) Engineering nanostructured polymer blends with controlled nanoparticle location using Janus particles. Acs Nano 2(6) 1167-1178... 

An interesting molecular approach using the hydrophobic effect to assemble gold nanoparticles was taken by Zubarev and coworkers who attached V-shaped (twin-tailed) amphiphilic polystyrene- Zock-poly(ethylene oxide) with a central carboxylic acid moiety (which binds to the gold nanoparticle), effectively giving biphasic, Janus-type characteristics [39]. Self-assembly led to wormlike aggregates (see Fig. 6). 

Recently, Miller and Cacciuto explored the self-assembly of spherical amphiphilic particles using molecular dynamics simulations [46]. They found that, as well as spherical micellar-type structures and wormlike strings, also bilayers and faceted polyhedra were possible as supracolloidal structures. Whitelam and Bon [47] used computer simulations to investigate the self-assembly of Janus-like peanut-shaped nanoparticles and found phases of clusters, bilayers, and non-spherical and spherical micelles, in accordance with a packing parameter that is used conventionally and in analogy to predict the assembled structures for molecular surfactants. They also found faceted polyhedra, a structure not predicted by the packing parameter (see Fig. 8). In both studies, faceted polyhedra and bilayers coexist, a phenomenon that is still unexplained. 

Muller and coworkers prepared disc-like polymer Janus particles from assembled films of the triblock copolymer SBM and, after hydrolysis of the ester groups into methacrylic acid units, used these as Pickering stabilizer in the soap-free emulsion polymerization of styrene and butyl acrylate [111]. Armes and coworkers described the synthesis of PMMA/siUca nanocomposite particles in aqueous alcoholic media using silica nanoparticles as stabilizer [112], extending this method to operate in water with a glycerol-modified silica sol [113, 114]. Sacanna showed that methacryloxypropyltrimethoxysilane [115] in the presence of nanosized silica led to spontaneous emulsification in water, which upon a two-step polymerization procedure afforded armored particles with an outer shell of PMMA [116]. Bon and coworkers demonstrated the preparation of armored hybrid polymer latex particles via emulsion polymerization of methyl methacrylate and ethyl methacrylate stabilized by unmodified silica nanoparticles (Ludox TM O) [117]. Performance of an additional conventional seeded emulsion polymerization step provided a straightforward route to more complex multilayered nanocomposite polymer colloids (see Fig. 14). 

As we are interested in reversible Janus micelles, i.e. non-centrosymmetric nanoparticles with compartmentalised shells (Fig. 1), complex coacervate core micelles are a rather natural choice. As described in the previous section, electrostatic interaction is a rather weak driving force as compared to hydrophobic interaction. C3Ms may thus form under full thermodynamic control. Although ABC triblock copolymers in selective solvents (poor solvent for B good solvent for both A and C) may also yield Janus micelles, they most frequently aggregate into micelles with a quenched rather than a dynamic nature, such that the aggregation number is fixed and no reversible association/dissociation is observed (on experimental time scales). 

The use of acrylic acid not only led to a functionalization of nanoparticles, but also was important as a structure-directing monomer for the formation of nanocapsules. In this case, the hydrophilic groups of the acrylic acid [30] or methacrylic acid [31] resulted in the formation of a nanocapsule structure, instead of Janus-like or even separate nanoparticles. The copolymerization of the functional n-methylol acrylamide with vinyl acetate was found to follow (in batch miniemulsion) the Mayo-Lewis equation, despite huge differences in the solubility of the monomers in the aqueous continuous phase [32]. A functionality of fluori-nated particles could be easily introduced by copolymerizing fluoroalkylacrylates with protonated monomers, such as acrylic acid and methacryloxyethyltrimethyl ammonium chloride [33]. 

Similarly, Lan et al. [7] developed a one-step microfluidic method for fabricating nanoparticle-coated patchy particles. A coaxial microfluidic device was employed to produce Janus droplets composed of curable phase and non-curable phase. The results showed that nanoparticles were dispersed either in the continuous fluid or the non-curable phase fluid. The nanoparticles (30 nm or 300-500 nm) were adsorbed onto the interface between these phases, and the curable phase was solidified by UV-irradiated polymerization. Thus, the patchy microparticles asymmetrically coated by nanoparticles were synthesized. They also employed Si02, TS-1, and fluorescent polystyrene nanoparticles as the coating materials to demonstrate the validity of the method. The microfluidic approach exhibited excellent controllability in morphology, monodispersity, and size for the nanocomposites. The morphology of the particles could be controlled from less than a hemisphere to a sphere by adjusting the flow rate ratio of the two dispersed phases. The method can be applied to other nanoparticles with specific surface properties. 

Amphiphilic Janus micelles were prepared using PSt-PBd-PMMA terpolymers synthesized by sequential anionic polymerization and cross-linking of the PBd spherical meso-phase in solid state. After solubilization and alkaline hydrolysis of PMMA to PMAA non-centrosymmetric compartmentalized micellar nanoparticles consisting of cross-linked PBd core and PSt, PMMA hemispheres were prepared and evaluated. ... 

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