Hybrid Janus Particles: Challenges & Opportunities towards the

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Review

Hybrid Janus Particles: Challenges & Opportunities towards the Design of Active Functional Interfaces and Surfaces Alina Kirillova, Claudia Marschelke, and Alla Synytska ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b17709 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 5, 2019

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ACS Applied Materials & Interfaces

Hybrid Janus Particles: Challenges & Opportunities towards the Design of Active Functional Interfaces and Surfaces Alina Kirillova,a Claudia Marschelke,b,c and Alla Synytskab,c* a

Department of Mechanical Engineering and Materials Science, Edmund T. Pratt Jr. School of Engineering, Duke University, Durham, NC 27708 b

Leibniz-Institut für Polymerforschung Dresden e.V., Hohe Str. 6, 01069 Dresden, Germany c

Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, 01062 Dresden, Germany

Keywords: Janus particles, active surfaces, active interfaces, multifunctional coatings, materials based on Janus particles

Abstract

Janus particles are a unique class of multi-functional patchy particles, combining two dissimilar chemical or physical functionalities at their opposite sides. The asymmetry characteristic for Janus particles allows them to self-assemble into sophisticated structures and materials not

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attainable by their homogeneous counterparts. Significant breakthroughs have recently been made in the synthesis of Janus particles and the understanding of their assembly. Nevertheless, the advancement of their applications is still a challenging field. In the present review article, we highlight recent developments in the use of Janus particles as building blocks for functional materials. We provide a brief introduction into the synthetic strategies for the fabrication of JPs, their properties and assembly, outlining the existing challenges. The focus of this review article is placed on the applications of Janus particles for active interfaces and surfaces. Active functional interfaces are created owing to the stabilization efficiency of Janus particles combined with their capability for interface structuring/functionalizing. On the other hand, Janus particles can be employed as building blocks to fabricate active functional surfaces with controlled chemical and topographical heterogeneity. Ultimately, we will provide implications for the rational design of multi-functional materials based on Janus particles.


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Outline 1.

Introduction ......................................................................................................................................... 4 1.1.

Janus particles ............................................................................................................................... 4

1.2.

Overview of the Janus particle field ............................................................................................. 6

1.2.1.

Synthesis .................................................................................................................................... 6

1.2.2.

Self-assembly........................................................................................................................... 10

1.2.3.

Applications ............................................................................................................................ 12

1.3. 2.

Challenges ................................................................................................................................... 14

Active Functional Interfaces ............................................................................................................ 17 2.1. Janus particle wettability and interfacial activity ............................................................................. 17 2.2. Functional emulsion droplets ........................................................................................................... 20 2.2.1. Optically active droplets ............................................................................................................... 20 2.2.2. Liquid marbles ............................................................................................................................. 23 2.3. Displays............................................................................................................................................ 26 2.4. Catalysis ........................................................................................................................................... 32

3.

Active Functional Surfaces ............................................................................................................... 40 3.1. Biological applications..................................................................................................................... 41 3.1.1. Bio-functional substrates .............................................................................................................. 41 3.1.2. Imaging ......................................................................................................................................... 46 3.1.3. Targeted therapy ........................................................................................................................... 50 3.2. Textiles............................................................................................................................................ 53 3.3. Active substrates .............................................................................................................................. 57 3.3.1. Photocatalytic substrates ............................................................................................................. 57 3.3.2. Surface enhanced Raman spectroscopy (SERS) substrates .......................................................... 57 3.3.3. Optically active substrates ........................................................................................................... 59 3.4. Functional coatings .......................................................................................................................... 60 3.4.1. Superhydrophobic coatings........................................................................................................... 60 3.4.2. Anti-icing coatings ........................................................................................................................ 65 3.4.3. Anti-fouling coatings ..................................................................................................................... 68

4.

Conclusions and Outlook.................................................................................................................. 73 References .............................................................................................................................................. 77


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1. Introduction 1.1. Janus particles Asymmetry is a frequently occurring phenomenon in nature, covering all levels of organization from individual molecules to entire organisms (Figure 1 a). On the molecular level, asymmetric amino acids and amphiphilic lipids serve as building blocks that self-organize into proteins and membranes, respectively. In synthetic chemistry, asymmetric molecules can self-assemble into micelles, vesicles, and complex bulk phases.1

Colloidal particles can be envisioned as the next generation of smart building blocks for advanced materials. Their sizes typically range from several nanometers to a few micrometers in at least one dimension.2 Such particles can impart materials with rich functionality, including enhanced optical properties, mechanical behavior, conductivity, etc. As a result of recent developments in the particle synthesis, a huge variety of colloidal building blocks has emerged, having different sizes, shapes, compositions, as well as functionalities. 3 Asymmetric patchy particles have recently attracted significant attention due to their multi-functional anisotropic character and properties that contrast those of isotropic particles.4

Janus particles (JPs) represent an unparalleled group of patchy particles, combining two dissimilar functionalities at their opposite sides (Figure 1 b). They were named after the twofaced god Janus, the god of beginnings and transitions in Roman mythology. The term “Janus beads” first appeared in 1988, when Casagrande et al. was describing the behavior of amphiphilic beads at oil/water interfaces.5 Promising potential of JPs was later addressed by Pierre-Gilles de Gennes in his Nobel lecture.6 The unique asymmetry of JPs offers an


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opportunity to access sophisticated assembled structures that might not be possible to achieve with symmetric particles. In the past two decades, research on JPs has flourished, resulting in the development of new synthetic routes to fabricate JPs and a better understanding of their properties and self-assembly.1 Consequently, significant attention has been focused on the application possibilities for JPs, which will be the main topic of this review article.

Figure 1. Janus particle concept: (a) asymmetry in nature - an example of a half-green/halforange fish (own photograph), and (b) hairy and partly/not hairy Janus particles synthesized up to date (own schematics).

1.2. Overview of the Janus particle field 1.2.1. Synthesis The development of new synthetic pathways lead to the creation of JPs with various functions, shapes, architectures, and sizes.1 Nevertheless, successful scalable fabrication of JPs remains an important issue in the field. JPs can be classified into three major groups:

soft (organic,


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polymer-based), hard (inorganic, metallic), and hybrid organic/inorganic JPs. Since JPs are a subcategory of patchy particles, we will consider particles that have one patch (distinct from the other part of the particle) as Janus structures in this review article. It is worth to mention that although the majority of JPs tend to have spherical shapes, other geometries of JPs have also been successfully attained. For example, Janus nanosheets are an attractive class of JPs due to their high aspect ratio, large adsorption energy, and highly confined rotation at interfaces.7-8 In this section, we provide a very brief overview of the synthetic principles for the fabrication of JPs, their self-assembly, and potential applications. For more information on these and other aspects, we would kindly refer the reader to further excellent review papers covering various elements of the topic of JPs: comprehensive general discussions on the synthesis, assembly, and applications of JPs,1,

9-11

strictly biphasic soft and hard Janus structures,12 stimuli-responsive

JPs,13 metallic Janus and patchy particles,14 patchy colloids in general and their self-assembly,4 Janus micro- and nano-motors,15 and the use of JPs for various aspects of biological applications.16-18 The main focus of this review paper, however, will be placed on the application of Janus particles as unique asymmetric building blocks for active interfaces and surfaces and the advancement of these fields of application. Soft Janus particles: Soft, organic JPs are the largest group of JPs. They include JPs with sizes ranging from single molecules to micrometers. Some of the known examples of soft JPs are Janus dendrimers,19-23 cylindrical polymer brushes,24 Janus-type heteroarm star copolymers,25 biological and biomimetic JPs,26-31 self-assembled Janus polymer micelles,32-41 sand microscale polymeric JPs.42-52 The smallest JPs are usually fabricated using classical organic and polymer synthesis,19-25 whereas biomimetic JPs can be produced via protein engineering or controlled masking and modification.26-30 Other strategies for the synthesis of polymeric Janus micelles are


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block copolymer self-assembly in solution (Figure 2 a),32-35 and self-assembly of block terpolymer bulk structures.36-41 (Sub-) micrometer-sized soft JPs can be synthesized using various straightforward approaches, such as phase separation in confined volumes,42-44,

52-53

seeded emulsion polymerization,45-46,

and

microfluidics.49-51,

57-59

54-55

electrohydrodynamic co-jetting,47-48,

In particular, as demonstrated by Müller et al.,

56

polystyrene-block-

polybutadiene-block-poly(methyl methacrylate) (SBM) triblock terpolymers can self-assemble into multicompartment micelles (MCMs) with different architectures depending on the block volume fractions and solvents (Figure 2 a).35 The phase-separated state within the MCMs is fixed by selective UV crosslinking of the polybutadiene (PB) compartments. Subsequent redispersion in a good solvent for polystyrene (PS) and poly(methyl methacrylate) (PMMA) breaks up the MCMs and single SBM JPs are released. In terms of microfluidics, Kumacheva et al. demonstrated fabrication of JPs in a microfluidic flow-focusing device.49 Two liquid monomers, each containing a photoinitiator, are supplied to two central channels of the microfluidic device. An aqueous solution of sodium dodecylsulfate (SDS) is injected to the side channels. At the exit from the central channels, the monomers form a two-liquid thread that is forced through a narrow orifice. Under the action of shear produced by the continuous phase, the thread breaks up to release Janus droplets. Hard Janus particles: Hard, inorganic JPs are usually micrometer-sized metal oxide particles (silica, titanium dioxide) partly coated with metals or inorganic nanoparticles (NPs), or bimetallic Janus NPs. The most popular approach to prepare microscale hard JPs is based on temporary masking one side of the particles using an immobilization template. The exposed particle surfaces are then modified (e.g. metal or NP deposition). This can occur either using particle monolayers on planar substrates (Figure 2 b),60-69 or in Pickering emulsions.70-73 For


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example, Janus micromotors were recently fabricated, consisting of a black TiO2 microsphere asymmetrically coated with a thin Au layer (Figure 2 b).69 For this purpose, a suspension of black TiO2 microspheres was dispensed onto a water-immersed Si wafer. The wafer was then vertically positioned, slowly removed from water and dried, producing a monolayer of particles. The monolayer was then half-coated with an Au layer by means of evaporation, yielding asymmetrically modified JPs. On the other hand, inorganic Janus NPs, typically containing Au or Fe3O4 NPs, can be generated via selective modification of metal NPs,74-75 or synthesis of bimetallic dumbbell-shaped NPs.76-77


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Figure 2. Synthetic strategies for the fabrication of (A) soft polymeric, (B) hard inorganic, and (C) hybrid Janus particles. (A) Soft JP synthesis via self-assembly of triblock terpolymers into MCMs, subsequent crosslinking of the compartments, and redispersion to release JPs.(B) Schematic illustration showing the synthesis of Janus micromotors, composed of a black TiO2 microsphere asymmetrically coated with a thin Au layer. (C) Schematic diagram showing the


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synthetic steps for the preparation of hybrid JPs with a single magnetic patch via seeded emulsion polymerization. These particles may be viewed more as patchy particles than JPs, but in this review, we consider asymmetrically modified particles with one patch as Janus structures. Reproduced with permission from refs

35, 69, 78

. Copyright 2012, 2017 American Chemical

Society.

Hybrid Janus particles: Hybrid, organic/inorganic JPs are a particularly attractive group of JPs, because they are multi-functional owing to their hybrid composition. Moreover, stimuliresponsive polymers can be combined with the mechanical stability of inorganic cores. Hence, one of the most frequently used strategies is to selectively modify an inorganic core with one or two dissimilar polymer shells. The same synthetic approaches mentioned for soft and hard JPs can be adapted to fabricate hybrid JPs. These include

phase separation in miniemulsions,79

seeded emulsion polymerization (Figure 2 c),78, 80-81 microfluidics,82 and selective modification of particle monolayers on planar substrates,83 or in Pickering emulsions.84-88 In addition, controlled/living radical polymerization techniques can be utilized to fabricate hybrid JPs by simultaneous grafting of polymer brushes onto the opposite sides of the core particles.85-87, 89-90 For example, hybrid hairy JPs of different sizes can be synthesized using a combination of “grafting from” and “grafting to” approaches (Figure 3).86, 90 Here, modified silica particles are partially immersed in wax during the preparation of wax colloidosomes in a Pickering emulsion. The exposed part of the particles is then modified with an initiator for polymerization. Particles are released by melting the wax, and the “grafting from” approach is used to grow the first polymer shell on one side of the particles via living radical polymerization. The second polymer chains are anchored to the opposite side of the particles using the “grafting to” approach.


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Figure 3. Schematic illustration of the fabrication technique yielding hybrid hairy JPs using “grafting from”/“grafting to” approaches. Reproduced with permission from ref

86

. Copyright

2008 American Chemical Society.

1.2.2. Self-assembly The understanding and directing of the colloidal particle self-assembly processes is essential for the fabrication of novel materials, where advanced functions can be achieved that are not inherent to the individual components. JPs are a particularly attractive class of particles in this regard, as their unique anisotropic features allow them to assemble into complex structures.

1, 13


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Hence, a large number of experimental and theoretical studies was dedicated to the self-assembly of JPs, including nanoscale JPs (Figure 4a),22, dipolar72,

86, 97-100

36-37, 54, 91

amphiphilic (Figure 4b)92-96 or

Janus colloids, and JPs in external electric (Figure 4c)101-104 or magnetic

(Figure 4d)78, 105-110 fields.

Figure 4. Self-assembly of Janus particles. (a) Suprastructures obtained from the coassembly of JPs with different phase separation degrees. These snowman-shaped JPs may be classified as patchy particles rather than JPs, but in this review, we consider asymmetric particles with one modified patch as Janus structures. (b) The formation of a self-assembled JP monolayer on the water/air interface and views of the opposite sides of the amphiphilic monolayer.(c) JP assembly


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in electric fields: phase diagram as a function ofelectric field intensity versus field frequency.(d) JP assembly in magnetic fields: images and corresponding models (of microtubes parallel to the precession axis formed via synchronized self-assembly using magnetic Janus colloids. Reproduced with permission from refs

54, 95, 101, 107

. Copyright 2008, 2018 American Chemical

Society, 2012 Nature Publishing Group, 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

1.2.3. Applications One of the most appealing features of JPs is their enhanced surface activity when compared to homogeneous particles. In this regard, JPs are ideal candidates for the stabilization of various interfaces, such as emulsions (Figure 5 a),43, 111-116 or polymer blends (Figure 5 b).117-122 Aside from pure stabilization, JPs offer the opportunity for interface functionalizing and surface structuring. These application fields will the thoroughly discussed in the next sections. For example, Due to their anisotropic features, individual JPs used as modulated optical nanoprobes,149-151 for targeting tumor cells (Figure 5 c)152 and other biological applications,16, 59, 153-159

or as self-propelled micro-/nanomotors that could serve as micromachines for various on-

demand operations (Figure 5 d).15, 160-170


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Figure 5. Applications of Janus particles. (a) Nanofluid of graphene-based amphiphilic Janus nanosheets for oil recovery: behaviors of nanosheets in oil/brine system with increasing hydrodynamic power. (b) Schematic illustrating the mediation of JP surface chemistry for the compatibilization of polystyrene/polyisoprene (PS/PI) blends. (c) Janus mesoporous silica NPs for dual targeting of tumor cells and mitochondria.(d) Self-propelled activated carbon/Pt Janus micromotors for water purification. Reproduced with permission from refs

112, 122, 152, 163

.

Copyright 2017, 2018 American Chemical Society, 2015 Wiley-VCH Verlag GmbH & Co. KGaA, 2016 National Academy of Sciences USA.


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1.3. Challenges Janus particles (Figure 6 a) are an engaging and constantly growing field of research (Figure 6 b). However, despite the large number of contributions on the topic of JPs in general (Figure 6 b), there are almost 10 times less contributions on the applications of JPs (Figure 6 c). This clearly demonstrates that the field of JP applications has to be advanced. The major challenge to be overcome in order to do this is the scalable production of JPs. Another important challenge is the limited number of application possibilities addressed so far.

Figure 6. Overview of the topic of Janus particles and their applications. (a) SEM image and schematic illustration of a synthesized hairy Janus particle representing the progress made up to date (own work); (b-c) number of publications published each year over the last 20 years on the topic of Janus particles in general (b) and applications of JPs (c). Source: Web of Science (data up to 11.12.2018). 171


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The enhanced interfacial activity of JPs compared to homogeneous particles allows them to adsorb to interfaces more efficiently, opening their application pathway to the stabilization of emulsions, foams, and polymer blends. In general, almost half of the studies dedicated to the applications of JPs are about interfaces. Apart from purely stabilizing the interfaces, JPs may be used to structure and functionalize them, creating active functional interfaces. In this review article, we use the term active to define materials that are very engaged in certain activities and processes. We define the term functional as having a certain function/feature and specific application. For instance, the inner and outer sides of JP-stabilized interfaces can be furnished with active compounds, catalysts, or they can feature different optical appearances Apart from interfaces, surfaces can be structured with JPs to impart them with heterogeneity and possible synergy between the anisotropic features of JPs. Active functional surfaces are another promising application field for JPs, which can result in their technological use. For example, multifunctionality can be afforded to coatings, advancing them to critical features, such as anti-fouling or anti-icing properties. Correspondingly, these two directions could significantly broaden the possible application opportunities of JPs. In the next sections, we will discuss the specific applications of JPs for active interfaces and surfaces (Figure 7). We have grouped the individual applications into several categories. For example, in terms of active interfaces, JPs can be applied for functional emulsion droplets, displays, and catalysis. In terms of active surfaces, JPs can be used for various biological applications, textiles, active substrates, and functional coatings.


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Figure 7. Janus particles and their application opportunities as building blocks for active interfaces and surfaces.

2. Active Functional Interfaces

2.1. Janus particle wettability and interfacial activity The capability of solid particles to adsorb at gas-liquid or liquid-liquid interfaces can be used to apply these particles for the stabilization of foams and emulsions, respectively. Pickering emulsions are an example of a two-phase system consisting of oil and water, where the interface is stabilized by solid particles (e.g. colloidal silica) that bind to the interface via physical interactions.172


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Janus particles possess anisotropic wettability and amphiphilicity, making them similar toto low molecular weight surfactants or block copolymers. Therefore, they can be used to stabilize Pickering emulsions during longer periods of time and under experimentally more stressful conditions (temperature changes or shear effects) than homogeneous particles.173 JPs can exhibit high interfacial activity regardless of their amphiphilicity degree, simply due to the spatial separation of the different wettability regions.174 It is critical to investigate the interfacial activity of JPs to apply them for stabilization of interfaces. Binks and Fletcher demonstrated a theoretical comparison between the desorption energies of particles with uniform wettability and JPs adsorbed at an oil-water interface demonstrated by.173 Their calculations showed that the particle amphiphilicity, which can be tuned by varying the size of the different wettability regions or their wettability contrast, influences the strength of particle adsorption to an interface. Increasing the amphiphilicity of the particles can lead to a 3-fold increase in surface activity for average contact angles around 90°. Unlike homogeneous particles, JPs were shown to retain strong surface activity for average contact angles approaching either 0 or 180°. The authors, therefor, expected that JPs with either low or high average contact angles will prove to be efficient emulsion stabilizers. In the recent years, a growing number of contributions appeared, demonstrating theoretical aspects and simulations of JPs at interfaces. Their contrast to homogeneous particles was repeatedly highlighted.174-177 Apart from planar interfaces, adsorption to curved interfaces was also studied.178-179 Other investigation parameters included the influence of JP shape on their surface activity,180 the influence of their Janus ratio,181 and the behavior of magnetic JPs at interfaces.182 Experimental observations of JP interfacial activity are mainly based on the classical Langmuir balance, or the pendant drop tensiometry methods.174 In general, the aspects


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that influence the interfacial activity of JPs are their composition, shape, morphology and chemistry, spatial distribution of the dissimilar domains, as well as colloidal stability.174 Corresponding to the theoretical predictions and experimental models showing the improved surface activity of JPs, many studies have appeared focused on the use of JPs for the stabilization of mostly oil-water interfaces (Figure 8 a, c).43, 79, 87, 111, 183-188 Another appealing area for the interfacial use of JPs is compatibilization of polymer blends. Emerging studies have discussed advantageous effects of JP compatibilization (Figure 8 c, d).117-119, 121, 189-192


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Figure 8. Applications of Janus particles for the stabilization of emulsions (a, c) and polymer blends (b, d). (a) Stabilization of water-oil emulsions with native kaolinite particles and coreshell kaolinite-based JPs, showing the stability of the JP-stabilized emulsion after 7 days and the smaller droplet sizes.(b): Processing of polymer blends using JPs as compatibilizers: schematic illustration and transmission electron microscopy (TEM) images of the compatibilized blend at different magnifications. (c) Fluorescence optical microscopy images of Pickering emulsions stabilized with shape-controlled biphasic JPs with different volume ratios of PS to poly(2vinylpyridine) (P2VP) spheres at various pH conditions. (d) Schematic illustration, scanning electron microscopy (SEM) and TEM images of JPs at the interface, compatibilizing polymer blends. Reproduced with permission from refs

87, 116, 119, 190

. Copyright 2014, 2017 American

Chemical Society, 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

As mentioned in the previous section, it may be highly beneficial to combine the enhanced interfacial activity of JPs with their multi-functionality to create active functional interfaces. An overview of the various application possibilities of JPs for active interfaces is summarized in Table 1. In general, selective orientation of JPs at interfaces and decreased rotational diffusion allow to modify the inner and outer sides of emulsion droplets, leading to droplets with specific functions. Other large areas of JP application for active interfaces include the electronic paper display technology and catalysis. These applications will be discussed further in the next sections. The general principles behind designing JPs for active functional surfaces are: (1) JPs have to possess high wettability contrast between their distinct compartments to provide better emulsion


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stabilization; (2) JPs have to offer the possibility to immobilize functional moieties into their structure for subsequent functionalization of the interface; (3) JPs have to possess an appropriate structure for a certain application (i.e. in displays, JPs have to be spherical but they don’t have to be chemically anisotropic, color anisotropy would be sufficient).

Table 1. Applications of JPs for active interfaces

Functional emulsion droplets

Application

Optically active droplets

Liquid marbles

Electrical actuation

Displays

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Magnetic actuation

JP description Size: microparticles (micrometer-sized) Shape: nanoplatelets Composition: smart-dust Si-based JPs, containing an alkylated hydrophobic green mirror and an oxidized hydrophilic red mirror Size: microparticles Shape: nanowires Composition: carbon nanotube (CNT)-Au hybrid nanowires with hydrophobic CNT tails and hydrophilic metal nanowire heads Size: microparticles Shape: hexagonal tiles Composition: Si/SiO2, selectively functionalized with Au Size: microparticles Shape: spheres Composition: porous crosslinked ethoxylated trimethylolpropane triacrylate (ETPTA) resin-based particles partially coated with sulfur hexafluoride Size: microparticles Shape: spheres Composition: monodisperse silica or α-Fe2O3 NPs in crosslinked ETPTA resin selectively treated with fluoroalkylsilane to make hydrophobic patches Size: microparticles Shape: spheres Composition: polymerized isobornyl acrylate with pigments of carbon black and titanium oxide in the opposite hemispheres Size: microparticles Shape: spheres Composition: organized silica particles in ETPTA resin with carbon black pigments in one hemisphere Size: microparticles Shape: spheres Composition: ETPTA-based, containing Sudan black B, with Fe, Cr, silica and titania deposited onto one hemisphere Size: microparticles Shape: spheres Composition: poly(methyl methacrylate-co-2-hydroxyethyl methacrylate)/cadmium acrylate ionomer (poly(MMA-co-HEMA)/Cd(AA)2 with Fe3O4 NPs in one hemisphere and poly(MMA-co-HEMA)/CdS quantum dots in the other hemisphere Size: microparticles Shape: spheres

Ref. 123

193

124

67

194

125

127

128

195

196


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Composition: photopolymerized trimethylolpropane ethoxylate triacrylate with αFe2O3 NPs in one hemisphere and organized PS microspheres in the other hemisphere

Dual photonic band gap

Temperature -opticsmagnetism triple response

Electrical and magnetic actuation

Capsules

Stabilization of foams for catalysis

Size: microparticles Shape: spheres Composition: organized polystyrene (PS) colloidal microspheres in one hemisphere, photopolymerized trimethylolpropane triacrylate with monodispersed SiO 2 or Fe3O4 NPs in the other hemisphere

Size: microparticles Shape: spheres Composition: Cd2+-loaded PS colloidal photonic crystals, N-isopropylacrylamide (NIPAm), and Fe3O4 NPs were used as building blocks for the fabrication of diverse JPs Size: microparticles Shape: spheres Composition: carbon black and TiO2 particles dispersed in acrylic monomers (mixture of isobornyl acrylate and 1,6-hexanediol diacrylate) with superparamagnetic NPs and a negative charge control agent added into the black hemisphere Size: submicrometer Shape: spheres Composition: silica particles modified with (3-aminopropyl) triethoxysilane (APTES) on one side and octadecyl trichlorosilane (ODTS) on the other side Size: single molecules Shape: tadpole-like Composition: single chain polymer NPs consisting of a linear PMMA “tail” and a crosslinked poly(4-vinylpyridine) “head” Size: nanoparticles Shape: irregular Composition: palladium selectively deposited onto carbon nanotube-inorganic oxide (silica or MgO) hybrid NPs Size: nanoparticles Shape: irregular Composition: silica particles selectively modified with APTES, Pd selectively deposited onto the APTES-functionalized hydrophobic side

Catalysis

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Interfacial catalysis

Size: submicrometer Shape: spheres Composition: hybrid hairy JPs with a silica particle core and two polymer shells at the opposite sides: polyacrylic acid (PAA) and PS, Au and Ag NPs immobilized into the PAA shell

Size: microparticles Shape: dimers Composition: two-compartmental PS particle where one non-crosslinked compartment encapsulates the catalysts (Oil Red O, palladium(II) acetate, or Wilkinson catalysts) and the other compartment consists of a highly cross-linked

197

198

199

200

201

202

203

131

130


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polymer Size: submicrometer Shape: snowman-like Composition: silica lobe and poly(divinylbenzene)/PS lobe with Au NPs loaded into one or both Size: submicrometer Shape: dumbbell-shaped Composition: mesoporous carbon-organosilica dumbbell-shaped JPs Size: microparticles Shape: ellipsoids Composition: hydrophilic poly(styrene-co-vinyl alcohol) bulb and hydrophobic poly(tetradecyl acrylate) bulb with Fe2O3 NPs patched on the hydrophilic bulb, Pd and Ag NPs patched on the entire particle surface

133

134

135

2.2. Functional emulsion droplets 2.2.1. Optically active droplets Sailor and Link demonstrated the concept of Janus particle-loaded emulsion droplets that have different characteristics at their inner vs. outer sides.123 The synthesized porous Si smart-dust particles comprised a hydrophobic green mirror and a hydrophilic red mirror on their opposite sides. The hydrophobic side resulted from thermal hydrosilylation of the initial film with dodecane, while the hydrophilic side resulted from the oxide layer after etching of the second side of the film (Figure 9 a). Smaller JPs were obtained by sonicating and fracturing of the modified film. Selective orientation of the particles at an organic liquid-water interface according to their polarity could be shown, where the alkylated hydrophobic side was oriented toward the organic phase, and the oxidized hydrophilic side was oriented toward the water phase. This resulted in an emulsion droplet colored red on the outside and green on the inside (Figure 9 a). Sensing could be accomplished when the liquid infusing into the porous smart-dust particles from the interface induced predictable shifts in the optical spectra of both green and red mirrors.


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Figure 9. Optically active emulsion droplets stabilized by JPs. (a) Schematic illustration of the synthesis of smart-dust JPs (left) and an optical image of their assembly at the interface of a dichloromethane droplet in water (right). (b) SEM images and schematics of the carbon nanotube (CNT)-Au hybrid nanowires and optical images of the photoinduced surface modification of the hybrid nanowire assembly using UV irradiation. (c) Self-assembled liquid mirror based on JP micromirror organization: schematic illustration micrograph and optical image of the micromirror assembly at the oil-water interface, and frame captures of an acoustically excited


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liquid mirror. Reproduced with permission from refs

123-124, 193

. Copyright 2003 National

Academy of Sciences USA, 2008, 2009 American Chemical Society. Another example of an optically active emulsion droplet furnished with JPs was demonstrated by Ajayan et al., who fabricated hybrid inorganic multi-segmented nanowires with hydrophobic carbon nanotube (CNT) tails and hydrophilic metal nanowire heads.193 This allowed assembly and manipulation of oil-water emulsion droplets in solution (Figure 9 b). The thiolfunctionalized hydrophobic Au surface was facing the interior of a dichloromethane (DCM) droplet, making the black CNTs point to the surrounding water. (Figure 9 b).Upon photoinduced cleavage of the thiol, the hydrophobic-hydrophilic balance between the Au and CNT segments could be changed, flipping the CNT orientation and yielding a golden droplet surface (Figure 9 b). These responsive assembled droplets are envisioned to be used in smart delivery systems. As demonstrated by Krupenkin et al., hexagonal Si/SiO2 micromirrors (Janus tiles) selectively functionalized with gold could be assembled on the surface of an oil droplet to yield a concave liquid mirror (Figure 9 c).124 The hydrophobic gold-functionalized sides of the JPs were facing the oil phase. The obtained liquid mirror was mechanically robust and retained its integrity even at high levels of vibrational excitation of the interface (Figure 9 c). It is envisioned to be used for new optofluidic devices, such as a 3D projector, where a light source can scan an image onto a moving, nonplanar focal surface.


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2.2.2. Liquid marbles Flexible barriers and liquid marbles are another appealing type of application for JPs in terms of active interfaces. As demonstrated by Kim et al., a highly flexible and impregnable waterrepelling interface can be created via self-assembly of JPs with superhydrophobic and hydrophilic sides.67The JPs were based on porous ethoxylated trimethylolpropane triacrylate (ETPTA) resin particles, which were made anisotropic by applying reactive ion etching (RIE) with sulfur hexafluoride. Such JPs can form a hexagonal array at the air-water interface, preventing droplet coalescence and letting water droplets freely float on top of the formed barrier. The barrier could maintain its integrity even under dynamic disturbance. The same JPs could also be used to fabricate liquid marbles, where a water droplet was completely covered with JPs facing towards water with their hydrophilic sides. The marbles were stable enough to be handled by tweezers. The developed JP-loaded interfaces may have potential for the use in semipermeable membranes, buoys for water floating micromachines, and superhydrophobic coatings.

Using a similar approach, Zhao et al. demonstrated the fabrication of JPs with multiple properties, such as a wettability contrast, specific arrangement of structural colors, and magnetic properties

(Figure 10).194 To create particles with the desired surface morphology, Janus

Pickering emulsions were generated in microfluidic devices and used as templates for the final particles. Colloidal crystal solutions served as the dispersed oil phases for the emulsions. These solutions were prepared by dispersing monodisperse silica or α-Fe2O3 NPs in an ETPTA resin. To impart the particles with additional amphiphilicity, they were surface-treated (Figure 10 a), which resulted in a . hydrophilic plasma-treated colored surface and a hydrophobic


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fluoroalkylsilane-modified magnetic surface (Figure 10 b). These JPs are able to form a monolayer at the air-water interface, forming a flexible barrier that prevented water droplet coalescence (Figure 10 c). In addition, liquid marbles were fabricated by coating water droplets with JPs. The marbles could be handled with tweezers and put onto a hydrophilic glass substrate or water surface (Figure 10 c). In addition, they could be manipulated using a magnetic field (Figure 10 d). These newly developed JPs are believed to have potential in constructing intelligent interfacial objects.


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Figure 10. Multifunctional JPs for liquid marbles and droplet manipulation. (a) Schematic illustration of the procedure for the surface modification of JPs; (b) photographs of the synthesized JPs; (c) photographs of water droplets sitting on a monolayer of JPs, and JP-coated liquid marbles placed on a glass slide or on a water surface ; (d) forced coalescence of liquid marbles under magnetic force. Reproduced with permission from ref

194

. Copyright 2013

American Chemical Society.

2.3. Displays E-paper displays offer several unique and desirable properties in the display technology, including light weight, flexibility, high visibility, and low energy consumption. They can be electrophoretic, electrochromic, electrowetting-type, cholesteric liquid crystal-type, etc.199 Janus particles have found their use in a twisting ball display, which is based on black and white JPs having electrically anisotropic features dispersed in a transparent silicone elastomer sheet, which is sandwiched between two parallel electrodes. Each JP is in a liquid-filled cavity and is able to rotate in response to an electrical field. Such JP-based displays can be considered as active functional interfaces. Due to the dissimilar electrical properties of the opposite hemispheres, JPs can rotate with either their white or black side towards the viewing window. The first twisted ball display, Gyricon, was developed in the 1970s.204 Improvements have been since then, optimizing the JP quality and display performance.205-206 As demonstrated by Nisisako et al., monodisperse polymer-based bicolored JPs could be produced using a microfluidic co-flow system by thermal polymerization of the generated Janus droplets.125 The developed JPs had an asymmetric charge distribution and pigments of carbon black and white titanium oxide in their opposite hemispheres (Figure 11 a). Electrical actuation


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was shown on a display model, where a monolayer of JPs was placed between electrodes. Depending on the electric field actuation, JPs oriented their black hemispheres to the negatively charged panel and vice versa. Reversing the electric field gradient made the particles flip their sides (Figure 11 a).

An optofluidic approach was used by Yang et al. to fabricate electro-responsive photonic Janus balls possessing black and structural color regions (Figure 11 b).127 Suspensions containing silica particles in an ETPTA resin with or without the addition of carbon black particles were used for the synthesis of JPs. The resulting JPs had a black hemisphere due to carbon black particles and a structural colored hemisphere due to the organization of silica particles. Electrical actuation of JPs was demonstrated using a twisting ball display (Figure 11 b). The developed photonic Janus balls are envisioned to be used as color pigments in reflection-mode displays.

Aside from electrical actuation, magnetic actuation can also be used to manipulate JPs (Figure 11 c).128 Kim et al. demonstrated a method to create photonic Janus microspheres with a gradient of structural color and magnetic anisotropy, which enables them to display multiple structural colors depending on their orientation (Figure 11 c).128 These JPs were fabricated from ETPTA containing a black dye using a microfluidics approach, and dielectric materials were then selectively deposited onto the particle surface. Microsphere orientation could be controlled using an external magnetic field, making the structural color from the 1D photonic structure tunable. A monolayer of JPs was used to observe the color change in a macroscopic view (Figure 11 c).


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Such photonic JPs with a color gradient could potentially be useful as active structural color pigments for outdoor signboards or wallpaper displays operated in reflection mode.

Figure 11. JPs for displays. (a) Schematic illustration and optical micrograph of bicolored JPs and their electrical actuation. (b) Schematic illustration and optical micrographs of the photonic


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Janus balls and images of the JPs embedded in a flexible matrix oriented with their green or black side up. (c) Schematic illustration and optical micrographs showing alignment and corresponding color of photonic JPs under external magnetic field and photographs showing the color change of the JP array depending on their orientation. Reproduced with permission from refs 125, 127-128. Copyright 2006, 2008, 2017 Wiley-VCH Verlag GmbH & Co. KGaA.

Chen et al. recently demonstrated a magnetically switchable display based on magneticfluorescent Janus supraballs.195 They were synthesized using a microfluidic approach based on ionomers, and contained superparamagnetic Fe3O4 NPs in one hemisphere and quantum dots (QDs) in the other hemisphere (Figure 12 a). Free-writing could be achieved on a rotating JP panel under an external magnetic field (Figure 12 a). In a later contribution by the same group, a triphase microfluidic-directed self-assembly approach was used to construct colloidal photonic crystal Janus supraparticles with controllable shape.196 Encapsulation of magnetic NPs created JPs with superparamagnetism and a photonic bandgap in two distinct hemispheres. These multifunctional JPs exhibited “Dark” and “Light” switchable behaviors under an external magnetic field and could be processed into rewritable and color-tunable photonic patterns. In another contribution, Chen et al. used the microfluidic approach to synthesize JPs with a slightly different composition, possessing dual photonic band gaps.197 By using polystyrene (PS), SiO2 and Fe3O4 monodispersed colloidal particles as building blocks, PS/SiO2 and PS/Fe3O4 JPs were fabricated. A dual-color PS/Fe3O4 photonic JP-based display pattern was developed, which responded to strong and weak visible light, showing potential for application as a panel display (Figure 12 b). Moreover, thermal sensitivity could be introduced to the Janus supraballs, resulting in JPs with temperature-magnetism-optics triple responses (Figure 12 c).198 In order to


31


accomplish

this

triple

response,

Cd2+-loaded

PS

colloidal

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photonic

crystals,

N-

isopropylacrylamide (NIPAm), and magnetic Fe3O4 NPs were used as building blocks in the microfluidics-based assembly process. Besides the magnetic response, the pad constructed with JPs could also display based on temperature response (Figure 12 c).


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Figure 12. Janus supraballs for displays. (a) Schematic illustration of a microfluidic system for the synthesis of JPs and of a fluorescent switch based onJPs, controlled by varying the direction of the magnetic field . (b) Photographs of PS/Fe3O4 JPs with their PS hemispheres facing up under weak light intensity (red) and Fe3O4 hemispheres facing up under strong light intensity (green). (c) I-pad prepared from multifunctional JPs showing a temperature and a reactioninduced response. Reproduced with permission from refs

195, 197-198

. Copyright 2011 Wiley-VCH

Verlag GmbH & Co. KGaA, 2014 Royal Society of Chemistry, 2015 American Chemical Society.

A twisting ball display that combined both electrical and magnetic actuation was developed by Komazaki et al.199 Bicolored JPs were fabricated via microfluidics. For this purpose, black and white monomers were prepared by dispersing carbon black and TiO2 particles in acrylic monomers. Superparamagnetic NPs and a negative charge control agent were added into the black monomer for dual-driving. A dual-driven twisting ball display was shown, exhibiting electric color control and allowing handwriting using a magnet. Such displays could be applicable for e-writers and electronic whiteboards.

2.4. Catalysis There are several ways how JPs can be used as building blocks for catalytically active interfaces. For instance, enhanced interfacial activity of JPs can be utilized for stabilization of Pickering emulsions towards interfacial catalysis. Catalytically active compounds can be included into the different emulsion phases, thus creating active barriers separating the phases.


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JPs can also serve as phase transfer vehicles, delivering catalysts to a specific phase. On the other hand, catalytic compounds can be embedded in the JPs, making the interface itself catalytically active. As demonstrated by Huang and Qi et al., amphiphilic silica JPs stabilizing emulsion droplets can be used for encapsulation of enzyme molecules for catalytic purposes in organic media.200 Submicrometer-sized silica JPs were prepared using selective modification of silica particles with silanes in a Pickering emulsion. The resulting JPs had different wettability at their opposite sides and self-assembled into capsules at the oil/water interface. A model lipase enzyme was encapsulated into the JP capsules during their formation. The catalytic performance of lipase was evaluated according to the esterification of 1-hexanol with hexanoic acid. It was found that the specific activity of the encapsulated enzymes was more than 5.6 times higher than that of free enzymes in a biphasic system. Using a different approach, Gao et al. showed that Janus polymer NPs, consisting of a linear poly(methyl methacrylate) (PMMA) “tail” and a crosslinked poly(4-vinylpyridine) (P4VP) “head” could be used to stabilize water in oil high internal phase emulsions (HIPEs) with styrene and divinylbenzene as the oil phase.201 These HIPEs could be converted to polyHIPE foams by polymerizing the oil phase. JPs were thus left embedded in the foams. When the foams were modified with Pd, they could be applied for heterogeneous Suzuki-Miyaura carbon-carbon coupling reactions between iodobenzene and benzeneboronic acid. The foams generated high catalytic activity and demonstrated good recyclability. Another strategy for the use of JPs for active interfaces is based on selective immobilization of catalytic compounds onto the JPs and their subsequent use for stabilization of emulsions while catalyzing reactions that occur at the interface. One of the major advantages of this technique is


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the possibility to recover JPs after the reaction, thus allowing the catalysts to be recycled and used multiple times. In addition, JPs can improve the conversion rates. This strategy was first realized on asymmetrically modified hybrid NPs made by Resasco et al.202 They demonstrated the ability of these solid hybrid catalysts to catalyze reactions at the water-oil interface. The hybrid catalysts were produced by selectively depositing palladium onto carbon nanotubeinorganic oxide (silica or MgO) hybrid NPs. In a following contribution from the same group, they developed a Janus-type catalyst, which consisted of nanoscale silica JPs with a wettability contrast and Pd particles selectively attached to their hydrophobic silane-modified side.203 Phaseselectivity was demonstrated by performing hydrogenation of aldehydes with different solubilities in the organic and aqueous phases. When the Pd catalyst was immobilized selectively onto the hydrophobic side of JPs, a 100% conversion of benzaldehyde was achieved in the oil phase, while the conversion of glutaraldehyde in the water phase decreased to 2%.A different type of Janus catalyst for interfacial catalysis was developed by Synytska et al. 131 The synthesized submicrometer-sized hybrid hairy JPs were composed of an inorganic silica core and two types of polymer shells at the opposite sides of the core: hydrophilic polyacrylic acid (PAA) and hydrophobic PS (Figure 13 a). These JPs were synthesized according to the approach demonstrated in Figure 3. Catalytic silver or gold NPs were selectively immobilized directly into the PAA polymer shell (Figure 13 b-d). The polymer shells provided several important characteristics: (1) high wettability contrast between the two polymers allowed for better emulsion stabilization; (2) hairy polymer shell architecture provided a better distribution of the NPs in the polymer because of its swelling; (3) stimuli-responsiveness may enable inducing or blocking of the catalytic activity for on-demand reactions. JP catalysts were shown to efficiently stabilize emulsions with responsiveness to pH values (Figure 13 e). Interfacial catalysis was


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demonstrated by preparing an emulsion stabilized by JPs, containing the Eosin Y dye in the water phase (Figure 13 f), and performing successful catalytic reduction of the dye (Figure 13 g). The catalysts could be successfully recovered after the reaction by centrifugation (Figure 13 h).


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Figure 13. Hybrid hairy JPs for interfacial catalysis. (a) Schematic illustration of the selective NP immobilization; representative TEM (b, c) and cryo-TEM (d) images of the JPs with selectively immobilized Ag NPs into the PAA shell; (e) photographs and optical micrographs of the emulsions prepared with JP catalysts at different pH values. (f-h) Reduction of Eosin Y in a water-oil emulsion stabilized by the JP catalysts: (f) photographs of the emulsion stabilized by the JPs, the same emulsion during the catalytic reaction, and re-stabilized emulsion after the reaction; (h) emulsion from (f) after centrifugation; (g) schematic illustration of the Eosin Y reduction. Reproduced with permission from ref

131

. Copyright 2015 American Chemical

Society.

JPs were also demonstrated to be used as bicompartmental phase transfer vehicles that could encapsulate catalytic molecules, then accumulate at oil-water interfaces, release the catalysts towards the oil phase, and perform hydrogenation reactions of unsaturated oil (Figure 14 a).130 The synthesized JPs were polystyrene dimers, in which the non-crosslinked compartment encapsulated the catalysts, and the other compartment consisted of a highly crosslinked polymer (Figure 14 a). As the non-crosslinked lobe swelled and eventually dissolved in oil, the encapsulated catalysts were released (Figure 14 a). Two Janus-type catalysts were synthesized by Liu and Liang et al. via selective modification and of one or both compartments of snowman-like JPs and their further functionalization with metal NPs 133 The catalytic performance of the JP catalysts both in homogeneous and interfacial reaction systems was investigated using the reduction of a nitro-compound as a model reaction. It was demonstrated that the Janus-type catalysts exhibited more efficient catalytic activity at the emulsion interface than at the oil-water biphasic interface due to the exposed Au NPs on


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snowman-like JPs that offer high accessibility to reactants. Another type of dumbbell-shaped JPs for biphasic interface catalysis was developed by Yang and Liu et al. (Figure 14 b).134 Anisotropic dumbbell-shaped mesoporous carbon-organosilica JPs with asymmetric wettability were synthesized using

one-step compartmentalized growth of a mesoporous organosilica

sphere attached to a mesoporous resorcinol-formaldehyde sphere (Figure 14 b). It was demonstrated that the JP catalyst could assemble at the oil-water interface, exhibiting more than three-fold increase in the catalytic efficiency compared to a Pt-loaded carbon sphere catalyst in aqueous hydrogenation reactions (Figure 14 b). In a recent contribution, Cho et al. introduced a Janus colloid surfactant catalyst platform exhibiting catalytic activity and magnetic responsiveness in Pickering emulsion microreactors (Figure 14 c).135 The immobilization of catalytic Pd and Ag NPs onto amphiphilic JPs permitted the control of catalytic activity for the organic reactions occurring in emulsion droplets. Through model organic reactions, including oxidation, amination, and reduction, it was shown that the JP catalysts enhance the reaction kinetics and product yields (Figure 14 c). Moreover, when Fe2O3 NPs were selectively immobilized, the Pickering emulsion droplets showed magnetic responsiveness, leading to the recovery of products and recycling of the JP catalysts.


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Figure 14. JPs for interfacial catalysis. (a) Schematic illustration of bicompartmental vehicles for the delivery of catalysts to the oil phase and photographs of oil-phase delivery of the Wilkinson’s catalyst and hydrogenation reaction (b) Schematic illustration of the JP synthesis, the kinetic profile for nitrobenzene reductions, and conversions of five different substrates in the catalytic reaction. (c) Schematic illustration of the oxidation reaction in an emulsion microreactor, conversion kinetics with various concentrations of Pd NPs on JPs and reaction temperatures, microscopic image of Pickering emulsion droplets, recovery of the emulsion droplets, and recyclability of JP catalysts after the reaction.Reproduced with permission from refs

130, 134-135

.

Copyright 2014 American Chemical Society, 2017 Wiley-VCH Verlag GmbH & Co. KGaA, 2018 Royal Society of Chemistry.

3. Active Functional Surfaces Active functional surfaces are an attractive class of JP applications as the JPs offer the possibility of surface structuring and impart the surfaces with multi-functionality. Heterogeneous surfaces based on JPs may exhibit synergistic effects of the two functions combined in single JPs. JPs can be employed to structure either non-planar substrates, such as cells or textiles, or planar substrates, leading to functional coatings. An overview of the application opportunities for JPs with regard to active functional surfaces is provided in Table 2.The general principles behind designing JPs for active functional surfaces are: (1) in certain applications (biological applications, textiles), JPs have to possess sufficient chemical heterogeneity to be able to chemically attach to surfaces with their specific sides; (2) JPs have to be robust to stay on the


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surface once immobilized; (3) JPs have to possess a sufficient level of functionality to provide advantages over homogeneous particles.

Table 2. Applications of JPs for active surfaces Application

Biofunctional substrates

Biological applications

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


JP description Size: microparticles Shape: spherical Composition: Bicompartmental polymeric JPs based on polyacrylamide/PAA copolymers, modified with polyethylene glycol (PEG) and selectively modified with streptavidin Size: submicrometer Shape: spheres Composition: streptavidin-coated magnetic iron oxide NPs, partially coated with glucose oxidase Size: submicrometer Shape: strawberry-like Composition: silica particles with light-responsive hydrophobic spiropyran-based polymers selectively grafted onto the rough particle side and hydrophilic imidazoline groups immobilized onto the opposite flat side Size: microparticles Shape: spheres Composition: transparent silica particles selectively coated with Al and Au layers

Size: microparticles Shape: spheres Composition: silica particles selectively coated with Al and Au layers, the Aumodified side was then decorated with an amine-reactive self-assembled monolayer and modified with streptavidin Size: nanoparticles Shape: dumbbell-like Composition: Au-Fe3O4 dumbbell-like JPs with epidermal growth factor receptor antibody linked to the Fe3O4 part, and the Au part protected with PEG

Imaging

Targeted therapy

Size: submicrometer Shape: spheres Composition: JPs based on an amphiphilic block copolymer, poly(styrene-block-allyl alcohol), loaded with a fluorescent dye and with magnetic NPs assembled in one hemisphere Size: nanoparticles Shape: snowman-like Composition: Heterodimers containing an Au domain and a MnO domain, where the metal oxide component is coated with a thin silica layer Size: nanoparticles Shape: octopus-like Composition: Au@PAA JPs with a mesoporous silica shell on the PAA side and Au branches grown on the other Au side, subsequently modified with methoxy-PEG-thiol and lactobionic acid on the opposite domains

Ref. 140

144

146

143

147

141

142

207

208


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Size: nanoparticles Shape: spheres Composition: spherical polydopamine/mesoporous calcium phosphate hollow JPs, functionalized with indocyanine green and methoxy-PEG-thiol on the polydopamine domains, and an anti-cancer drug was incorporated into the cavities of calcium phosphate sides Size: nanoparticles Shape: snowman-like Composition: Au/Fe3O4@C JPs selectively functionalized with amino-PEG-thiol and folic acid on the Au domains, while the other Fe3O4@C sides with mesoporous structure served as a drug delivery vehicle for doxorubicin Size: submicrometer Shape: spheres Composition: silica particles selectively modified with APTES and octadecyltrichlorosilane on their opposite sides Textiles

Size: microparticles Shape: spheres Composition: Ag–SiO2 JPs having Ag NPs deposited on one half of SiO2 particles, while the other half was functionalized for attachment to the textile surface Size: microparticles Shape: spheres Composition: silica particles with nano-titania deposited onto one hemisphere

Active substrates

Photocatal ytic substrates Surface enhanced Raman spectrosco py (SERS) substrates

Optically active substrates

Functional coatings

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Superhydr ophobic coatings

Size: nanoparticles Shape: snowman-like Composition: Au-TiO2 Janus NPs having an Au and a TiO2 NP parts Size: submicrometer Shape: spheres Composition: Ag-SiO2 JPs with Ag NPs deposited onto one hemisphere with amine functionalization on the other one

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145

148

136

209

210

211

212

Size: microparticles Shape: flower-shaped Composition: physically anisotropic JPs based on a porphyrin derivative of 5-(4(ethylcarboxypropoxy)phenyl)-10,15,20-tri(naphthyl)porphyrin

213

Size: nanoparticles Shape: cubic-shaped Composition: monodisperse CsPbX3/SiO2 (X = Cl, Br, I) and CsPbBr3/Ta2O5 Janus NPs

214

Size: microparticles Shape: spheres Composition: hybrid hairy JPs composed of a silica core along with one or two distinct polymer shells on the opposite sides of the core (PS, poly(tert-butyl acrylate), poly(2vinylpiridine))

137


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Anti-icing coatings

Antifouling coatings

Size: submicrometer Shape: cone-like Composition: PS-based cone-like JPs with their flat hydrophilic side aminofunctionalized

215

Size: submicrometer Shape: strawberry-like Composition: silica particle with a rough hydrophobic PS-modified side and an imidazoline-modified hydrophilic side

216

Size: microparticles Shape: spheres Composition: hybrid hairy JPs composed of a silica core along with PEG and PDMS polymer shells grafted at the opposite sides of the core

138

Size: microparticles Shape: spheres and platelets Composition: hybrid hairy JPs composed of a silica core or a kaolinite nanoclay core along with PEG and PDMS polymer shells grafted at the opposite sides of the cores

139

3.1. Biological applications 3.1.1. Bio-functional substrates As demonstrated by Lahann et al., JPs with spatially-controlled affinity towards human endothelial cells can be used to develop a microstructured bio-hybrid material.140 For this purpose, polymeric JPs were fabricated using electrohydrodynamic co-jetting and selectively modified to attain two biologically distinguishable hemispheres. One of the hemispheres one exhibited high binding affinity for human endothelial cells and the other hemisphere was made resistant towards cell binding. As a result, JPs not only bound to the surface of the cells with one of their hemispheres, but also organized all around the perimeter of the cells forming a single particle lining. Another type of JP-based bio-functional substrate was developed by de la Rica et al. using enzyme-coated Janus NPs that selectively bind cell receptors as a function of the concentration


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of glucose (Figure 15 a).144

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In this approach, JPs partially covered with glucose oxidase

enzymes establish interactions with cell membranes and move in the presence of the enzyme substrate glucose, which generates a mechanical force that disrupts biomolecular interactions Controlling the number of JPs bound to cell membranes as a function of the concentration of metabolites could be useful for designing medicines and bioimaging probes that selectively target cells as a function of biomolecular cues in their microenvironment. Light-responsive JP-based coatings for cell capture and release were developed by Liang and Wang et al. (Figure 15 b).146 Strawberry-like silica JPs were synthesized using the sol-gel process. One side of the silica JPs possessed nanoscale roughness, while the other side was flat. Spiropyran-containing polymer brushes were grafted onto the rough side of JPs, and the flat side was modified with imidazoline groups. The light-responsive polymer brush-modified rough sides of the JPs oriented towards the air when the JPs self-organized into a layer on the surface of an epoxy resin substrate (Figure 15 b). The imidazoline groups reacted with the epoxy groups in the epoxy resin to form a robust coating. The coating could be reversibly triggered between hydrophobic and hydrophilic by UV and visible-light irradiation (Figure 15 b). When the hydrophobic ring-closed spiropyran form was dominant, HeLa cells could be effectively captured onto the coating. After UV light irradiation, the ring-closed spiropyran form changed to the hydrophilic ring-opened zwitterionic merocyanine form, and the captured cells were released (Figure 15 b). Yoon et al. fabricated retroreflective JPs and showed their applicability as nonspectroscopic optical immunosensing probes (Figure 15 c).143 JPs were fabricated by selectively coating metals onto the surfaces of silica particles. The retroreflection signals from JPs were distinctively recognized as shining dots, which could be counted using a digital camera setup. Using the


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developed retroreflective immunosensing system, cardiac troponin I, a specific biomarker of acute myocardial infarction, was detected with high sensitivity (Figure 15 c). The features of the retroreflective immunosensing platform may be applied for various point-of-care-testing applications. In a later contribution, Yoon et al. reported an optical sensing platform for mercury ions (Hg2+) in water based on the integration of Hg2+-mediated thymine-thymine stabilization, a biotinylated stem-loop DNA probe, and a streptavidin-modified retroreflective JP.147 Using the developed system, a highly selective and sensitive measurement of Hg2+ was accomplished with a limit of detection of 0.027 nM.147


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Figure 15. JPs for bio-active surfaces. (a) Enzyme-coated JPs that selectively bind cell receptors as a function of the concentration of glucose: schematic illustration of the method, fluorescence microscopy images of fixated cells after incubation with fluorescent JPs, and mean fluorescence intensity of individual cells as a function of the concentration of glucose. (b) Light-responsive


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JP-based coatings for cell capture and release: schematic illustration of cell capture and release on the coatings before and after UV light irradiation; schematic illustration of the partially shielded coating with captured cells under UV light irradiation; fluorescence micrographs of cells on the coating after regional cell release; SEM image of the coating and time dependence of water contact angle on the coatings induced by UV light irradiation. (c) Retroreflective JPs as a nonspectroscopic optical immunosensing probes: schematic illustration of a typical light path in retroreflection; results of a retroreflective immunoassay and dose-response curve for the number of SMJPs in the square-patterned gold immunosensing zone. Reproduced with permission from ref

143-144, 146

. Copyright 2016, 2017 American Chemical Society, 2017 Royal Society of

Chemistry.

3.1.2. Imaging Optically or magnetically active JPs can be used for biological imaging applications owing to the possibility of their selective binding to surfaces. Sun et al. demonstrated the use of dumbbelllike Au-Fe3O4 Janus NPs for simultaneous magnetic and optical detection due to the plasmonic Au unit and the magnetic Fe3O4 unit.141 The magnetic part of JPs was modified with the epidermal growth factor receptor antibody, which was linked to the surface through PEG and dopamine, while the plasmonic part was protected by PEG-based polymer. JPs were able to bind to the cells and magnetic resonance imaging analysis showed that cells labeled with JPs shorten the T2 relaxation of the surrounding water molecules, whereas the Au part of JPs allowed obtaining reflection images with a scanning confocal microscope.


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A combined imaging and magnetolytic therapy approach using JPs was demonstrated by Hu and Gao.142 The fabricated JPs were based on an amphiphilic block copolymer modified with a fluorescent dye in one hemisphere and superparamagnetic NPs assembled in the other hemisphere. The orientation of JPs could be controlled making them promising for optical imaging applications (Figure 16 a). Moreover, magnetolytic therapy was demonstrated via magnetic field-modulated cell membrane damage (Figure 16 a). JPs were able to attach to the cell surface, and when a spinning magnetic field was applied, the majority of tumor cells were killed due to the disruption of cell membranes.


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Figure 16. JPs for bio-imaging. (a) JPs

for combined imaging and magnetolytic therapy:

schematic illustration of JP orientation control with magnetic fields; TEM images of the JPs; fluorescence images of the JPs with different orientations; simultaneous imaging and treatment of cancer cells (b) JPs for selective dual functionalization and imaging: schematic illustration


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and TEM image of a single JP; images of cells co-incubated with JPs; two-photon images of the same sample, excited with a two-photon laser. Reproduced with permission from refs

142, 207

.

Copyright 2010, 2014 American Chemical Society.

A different kind of JPs applicable for optical and magnetic detection was demonstrated by Tremel et al.207 Monodisperse Au@MnO JPs were fabricated using a seed-mediated nucleation and growth approach. The MnO part was then coated with a thin silica layer, leaving the metal domain unmodified (Figure 16 b). The developed JPs were shown to be superparamagnetic and two-photon active, allowing simultaneous magnetic and optical detection (Figure 16 b).

3.1.3. Targeted therapy Li et al. fabricated octopus-type Janus NPs for synergistic actively-targeted and chemophotothermal therapy (Figure 17).208 The JPs were composed of Au@PAA and were used as templates to preferentially grow a mesoporous silica shell and Au branches separately modified with methoxy-poly(ethylene glycol)-thiol (PEG) to improve their stability, and lactobionic acid (LA) for tumor-specific targeting (Figure 17). The obtained octopus-type JPs possessed pH and NIR dual-responsive release properties. It was shown that the JPs could be utilized as a multifunctional nanoplatform for in vitro and in vivo actively-targeted and chemo-photothermal cancer therapy (Figure 17). In another contribution, polydopamine/mesoporous calcium phosphate hollow Janus NPs were developed for imaging-guided chemo-photothermal synergistic therapy.145 The JPs were further selectively functionalized with indocyanine green and methoxy-PEG-thiol on polydopamine domains to achieve better photoacoustic imaging


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capability and stability, while the other calcium phosphate sides with hollow cavities served as storage spaces and passages for the anti-cancer drug. The resultant JPs possessed excellent biocompatibility, competent drug loading capability, high photothermal conversion efficiency, strong NIR absorbance, and pH/NIR dual-responsive properties. Li and Wang et al. further extended their JP library by introducing Au/Fe3O4@C JNPs, which were further selectively functionalized with NH2-PEG-SH and folic acid on the exposed Au domains to achieve high contrast

for

X-ray computed tomography (CT) imaging,

excellent

stability,

good

biocompatibility, as well as cancer cell-specific targeting, making them intriguing nanoplatforms for dual-modal CT and MR imaging-guided actively targeted chemo-photothermal synergistic cancer therapy.148

Figure 17. JPs for chemo-photothermal therapy. Fabrication of octopus-type JPs with pH and NIR light dual-stimuli responsive properties for actively-targeted and chemo-photothermal


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cancer therapy in vitro and in vivo. Reproduced with permission from ref 208. Copyright 2016 Wiley-VCH Verlag GmbH & Co. KGaA.

3.2. Textiles Durability and water-repellent properties can be imparted to textiles through their surface modification. A textile modification strategy was recently demonstrated by Synytska et al., which was based on the chemical immobilization of amphiphilic JPs

136

JPs were fabricated by

preparing a wax-water Pickering emulsion stabilized by silica particles. The exposed side of the particles was modified with moderately hydrophilic APTES, while the opposite side was modified with hydrophobic octadecyltrichlorosilane (OTS) after dissolution of the wax. The textile surface was modified with poly(glycidyl methacrylate) (PGMA) prior to JP immobilization, which provided reactive epoxy groups on the textile surface. JPs were chemically bound to the textile surface with their amino-modified sides, while their hydrophobic sides were facing the surroundings (Figure 18 a). Regardless of the JP size, the immobilization of JPs resulted in a hydrophobic textile surface (Figure 18 b). In the case of smaller JPs, higher concentration of particles lead to a very low tilt angle, which resulted in the sliding of the droplet off the textile surface (Figure 18 c). Hence, water-repellent behavior could be observed on the textiles modified with sufficiently high amounts of smaller JPs.


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Figure 18. JPs for water-repelling textiles. (a) Schematic illustration of the immobilization of the JPs onto the textile surface. (b) Wetting properties of the textiles modified with JPs.(c) Advancing water contact angles and tilt angles of water droplets on the textiles with grafted large and small JPs. Reproduced with permission from ref

136

. Copyright 2011 American Chemical

Society.

Another strategy for functional modification of fabric using JPs was proposed by Jassal and Agrawal et al.209-210 In one approach, they synthesized Ag–SiO2 JPs with varying functionalities, i.e. amine, thiol, and epoxy, on the exposed surface of SiO2 particles and explored their antimicrobial activity.209 Ag–SiO2 JPs could interact with cotton fabric and provide durable functionality. Epoxy-functionalized cotton fabrics treated with JPs were found to have uniform distribution of JPs with high antimicrobial activity and high durability to washing. In another approach, TiO2–SiO2 JPs were synthesized and applied on cotton fabric.210 The photocatalytic performance along with the effect on mechanical properties were investigated in comparison to commercially available TiO2 NPs. Unlike commercial NP-treated fabrics, JP-treated cotton showed higher wash durability, higher activity at lower concentration and at neutral pH, and could retain its mechanical properties even after long UV exposure.

3.3. Active substrates 3.3.1. Photocatalytic substrates Li and Jia et al. recently demonstrated the fabrication of Au-TiO2 JP arrays for photocatalytic applications (Figure 19 a).211 The photocatalytic activity of the prepared JP-based arrays was


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tested using photocatalytic degradation of methylene blue and compared to that of the TiO2 NPs and Au-TiO2 composite NPs, which served as a reference (Figure 19 a). It was shown that JP arrays exhibited a better catalytic performance than the control samples in the decomposition of methylene blue when illuminated by UV light. The increased photocatalytic activity of Au-TiO2 JPs was attributed to the Au-TiO2 heterointerfaces.

3.3.2. Surface enhanced Raman spectroscopy (SERS) substrates Macroscopic SERS substrates with high activity were fabricated by Panwar et al. via attaching Ag-SiO2 JPs with Ag NPs on a cellulosic film (Figure 19 b).212 The synthesized Ag-SiO2 JPs had in-situ deposited Ag NPs on one side and amine functionalization on the opposite side, which was utilized for their fixation as a monolayer on a cellulosic substrate (Figure 19 b). Raman spectrum of Rhodamine B (RhB) showed an enhancement factor of ∼5×104 for dye tagged on both JPs and M-SERS (Figure 19 b). In comparison, Ag NPs with almost the same diameter did not show enhancement of Raman signals even after aggregation. The enhancement is attributed to in-situ reduction of the silver precursor on the SiO2 surface leading to the deposition of Ag NPs and generation of a large number of fixed hot-spots in a controlled manner. JPs and M-SERS with ∼3 nm sized Ag NPs can find application as both SERS-active and catalytic materials.


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Figure 19. JPs for active substrates. (a) JP arrays for photocatalytic applications: schematic illustration of the Au-TiO2 Janus NP array production; SEM image of the JP array; change in the methylene blue concentration as a function of irradiation time, and curves showing the use ofAuTiO2 JP arrays as a catalyst. (b) JP-based SERS macroscopic substrates: schematic illustration of the mechanism of attachment of Ag-SiO2 JPs on a cellulosic film; SEM micrographs of the films; SERS spectra of Rhodamine B and RhB-Janus at different concentrations of RhB. (c) JPbased optically active surfaces: SEM images of the flower-shaped JPs; large-area flower-shaped JP assembly into a layer with controlled arrangement and its optical properties. Reproduced with permission from ref 211-213. Copyright 2011, 2017 Elsevier, 2015 Royal Society of Chemistry.


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3.3.3. Optically active substrates Flower-shaped porphyrin JPs were recently developed by Wang et al.213 The physically anisotropic flower-shaped JPs were fabricated from a porphyrin derivative based on a two-step water droplet condensation process. The resulting JPs possessed a smooth back side and a concave front side (Figure 19 c). Controlled orientation of the JPs on a surface could be achieved with their concave side facing the suspension phase and their flat side facing the air phase (Figure 19 c). The as-prepared flower-shaped colloidal crystals were demonstrated to produce a multi-wavelength optical signal coming from the hierarchical structures, which may be utilized in the creation of novel optical materials. Using a different strategy, Hu et al. demonstrated monodisperse CsPbX3/SiO2 (X = Cl, Br, I) and CsPbBr3/Ta2O5 JNPs prepared by combining a water-triggered transformation process and a sol-gel method.214 The CsPbBr3/SiO2 nanocrystals exhibited a photoluminescence quantum yield of 80% and a lifetime of 19.8 ns. The product showed dramatically improved stability against destruction by air, water, and light irradiation. The advantageous features of the JPs were further highlighted in practical applications by using them as the green light source for the fabrication of a prototype white light emitting diode and demonstrating a wide color gamut covering up to 138% of the National Television System Committee standard.

3.4. Functional coatings 3.4.1. Superhydrophobic coatings Coatings having superhydrophobic properties could be particularly interesting for the engineering of new materials with self-cleaning features. Synytska et al. recently demonstrated


57


the use of JPs for ultrahydrophobic coatings.

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Controlled aggregation of JPs into hierarchical

structures in dispersions was used. The aggregates subsequently formed structured rough layers after deposition on a substrate, and the layers possessed ultra-hydrophobic properties (Figure 20 a). JPs were synthesized according to the procedure illustrated in Figure 3, and were composed of a silica particle core along with either one or two grafted polymer shells on the opposite hemispheres of the particles. It was shown that the wetting properties of the certain JP layers approached an almost ultra-hydrophobic behavior (both advancing and receding water contact angles were higher than 140°), which appeared from the multi-scale roughness of the layers ranging from the nanometer scale up to the dimension of several hundreds of microns(Figure 20 b).

Finally,

JPs

with

a

hydrophilic

P2VP

side

and

a

highly

hydrophobic

poly(3,3,4,4,5,5,6,6,7,7,8,8,9,9,10,10,10-heptadecafluorodecyl methacrylate) (PHDFMA) side were fabricated and deposited onto P2VP-modified substrates, yielding an ultra-hydrophobic surface (Figure 20 c, the advancing and receding water contact angles were higher than 150°).


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Figure 20. Hybrid hairy JPs for ultra-hydrophobic surfaces. (a) Schematic illustration of the surface design. (b) Illustration of the multi-level roughness hierarchy in the JP layers; microscopy images of the JP-based layers. (c) Topography of the JPs on a P2VP substrate as well as snapshots of static and receding water droplets on the layer formed by PHDFMA-P2VP-


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JPs. Reproduced with permission from ref 137. Copyright 2011 Wiley-VCH Verlag GmbH & Co. KGaA.

In a later contribution by Liang and Yang et al., cone-like JPs were fabricated and used for coatings with tunable wettability that could be changed from highly adhesive to water to superhydrophobic by altering the size distribution of JPs (Figure 21 a).215 After spraying a dispersion of the modified PS/titania cone-like JPs onto a surface, they self-organized into a layer upon drying, where the amino groups on the flat sides of the particles were covalently bound to the epoxy resin, while the hydrophobic cone sides were facing the air. The same group demonstrated the fabrication of superhydrophobic coatings based on a different kind of strawberry-like JPs (Figure 21 b).216 They were synthesized using a self-organized sol-gel process at a patchy emulsion interface. The flat side of the particles contained silanol groups, and the imidazolin group could be introduced through modification with a silane. A thin layer of liquid epoxy resin was coated onto the substrates, on which aqueous dispersion of the JPs were dried. This resulted in JPs self-organizing into an ordered layer on the substrate (Figure 21 b). The hydrophobic side was facing the air, while the hydrophilic side was facing the epoxy resin layer, binding the particles covalently to the surface. As a result of the nanoscale roughness on the

hydrophobic

sides

of

the

JPs,

the

coating

exhibited

superhydrophobic

performance(Figure 21 b).


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Figure 21. JPs for superhydrophobic surfaces. (a) Schematic illustration of a coating based on cone-like JPs; side-view SEM image of the coating and an image of a water droplet contour on the coating with a contact angle of 165 ± 2° and a tilt angle ∼6°. (b) Schematic illustration of a coating based on strawberry-like JPs; SEM image of the JPs; side-view SEM image of the JPbased coating; image of a water droplet contour on the coating. Reproduced with permission from refs 215-216. Copyright 2015 American Chemical Society, 2015 Nature Publishing Group.

3.4.2. Anti-icing coatings Coatings with anti-icing properties are of critical importance for various industries, including the aircraft and automotive industries. The development of such coatings could lead to


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substantial reduction of costs and a lower number of emergency situations. A new approach for the fabrication of anti-icing surfaces based on JPs was recently demonstrated by Synytska et al.138 Robust heterogeneous surfaces were produced based on JPs that were composed of a silica core

along

with

poly(dimethylsiloxane)

poly(poly(ethylene polymer

shells

glycol) at

the

methyl opposite

ether sides

methacrylate) of

the

core

and (SiO2-

P(PEGMA)/PDMS-JP; Figure 22 a).138 JPs were synthesized using a combination of “grafting from” and “grafting to” approaches, as shown in Figure 3. Coatings were fabricated using a simple solvent casting method via depositing and subsequent drying of particle suspensions on silicon wafers that were pre-modified with PGMA Random JP orientation in the layer could be observed (Figure 22 a). It was shown that there was a 55%: 45% distribution of the JPs exposing their hydrophobic or hydrophilic side to the surrounding, respectively. Anisotropic wetting properties of JP-based layers were observed (Figure 22 b).


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Figure 22. Hybrid hairy JPs for anti-icing surfaces. (a) Representative layer prepared with P(PEGMA)/PDMS JPs: chemical formulas of the polymers and a false color SEM image. (b) Representative cryo-SEM images of frozen water droplets on the different surfaces. (c) Representative optical microscopy images of the JP-based surface during icing: native surface, surface after the water droplet condensation, growth of large liquid clusters and their solidification, freezing of the condensed water droplets and formation of dry bands around large dendrites, and thawing of ice after the test. (d) Ice adhesion measurement results.. Reproduced with permission from ref 138. Copyright 2016 American Chemical Society.

The anti-icing performance of the heterogeneous JP-based surfaces was investigated using different techniques. First, the JP-based surface was investigated during icing on a Peltier


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element under an optical microscope (Figure 22 c), where during a gradual temperature decrease, water droplets condensed from the air and froze. It was shown that, instead of forming small ice crystals that slowly grow, large crystals were formed on the JP-based surfaces due to heterogeneous nucleation (Figure 22 c). This resulted in substantial ice-free areas of the surface and subsequent water-free areas during the ice thawing (Figure 22 c). Furthermore, ice adhesion measurements were performed (Figure 22 d) and it was found that the adhesion of ice to the heterogeneous amphiphilic JP-based surfaces is lower than that corresponding to the homogeneously decorated hydrophilic or hydrophobic particle-based surfaces. Therefore, the employment of JPs may be highly beneficial for the fabrication of heterogeneous anti-icing coatings.

3.4.3. Anti-fouling coatings Marine biofouling is a critical technological problem, which may lead to substantially increased fuel consumption in the case of ship hull biofouling, and the subsequent dramatic increase in the annual worldwide costs. Hence, there is a strong demand for new anti-fouling strategies. The adhesion of bacteria to a surface is influenced by the properties of the surface, such as its chemistry, energy, mechanical properties, topography, as well as the environmental conditions in general. A new approach for the design of anti-fouling coatings based on multifunctional JPs was recently demonstrated by Synytska et al.139 The utilized hybrid hairy JPs were composed

of either a spherical silica, or a platelet-like kaolinite core with hydrophilic

P(PEGMA) and hydrophobic PDMS (or P(PDMSMA)) polymer shells grafted at the opposite sides of the core. Spherical hybrid hairy JPs were synthesized according to the scheme illustrated in Figure 3 and a procedure described earlier.86On the other hand, platelet-like JPs were


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synthesized according to a previously reported approach, which involves one-step simultaneous grafting of both polymers from the opposite sides of inorganic kaolinite core particles.87 Robust structured surfaces could be prepared with both kinds of JPs via simple solvent casting using wafers pre-modified with PGMA (Figure 23 a).139 The orientation of spherical JPs in the layers was random (Figure 23 a), and it could be concluded that both spherical and platelet-like JPbased surfaces revealed an approx. 1:1 distribution of hydrophilic to hydrophobic moieties.


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Figure 23. Hybrid hairy JPs for anti-fouling surfaces. (a) Representative SEM images of coatings based on spherical and platelet-like JPs; false color SEM image of the JP-based layer


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(b) Representative false color SEM images of the reference native wafers, spherical JP-based surfaces, and platelet-like JP-based surfaces after the biofilm formation assay under static or dynamic conditions. (c) Antifouling and fouling-release performance of the control and JP-based surfaces under static or dynamic conditions. Reproduced with permission from ref 139. Copyright 2016 American Chemical Society.

The anti-fouling performance of the designed surfaces was tested by seeding a marine bacteria Cobetia marina in growth medium onto different samples and incubating them for 24 hours under static, low shear stress, or dynamic, high shear stress conditions. Samples with the adhered bacteria were observed in the SEM (Figure 23 b). It was found that a much higher number of bacteria adhered to the control surfaces compared to the JP-based surfaces (Figure 23 b) under both static and dynamic conditions. The number of attached bacteria was quantified using a chemoluminescence assay (Figure 23 c). It was demonstrated that less bacteria adhered to all the tested surfaces under dynamic conditions if compared to the static ones, and that the JP-based surfaces, independently of their geometry, successfully reduced bacterial adhesion under both static and dynamic conditions compared to the control flat and particle-based surfaces (Figure 23 c). In summary, it was demonstrated that, compared to the native control surfaces, chemically heterogeneous surfaces made of JPs allowed significant lowering of bacterial adhesion. Moreover, JP-based surfaces were shown to be very robust, which is critical for their future application. Aside from spherical JPs, kaolinite-based JPs represented a system with potential a for scalable production due to the synthetic procedure with a reduced number of steps. This could potentially result in the fabrication of large-area coatings.


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4. Conclusions and Outlook The field of Janus particles has flourished in the past decade with the development of new synthetic strategies for the production of JPs and a deeper understanding of their properties and assembly. Based on the growing number of contributions about JPs, it is evident that there is a need for the development of innovative kinds of asymmetric building blocks for the creating of novel multi-functional materials. Despite the numerous approaches for the fabrication of JPs with various sizes, shapes, and compositions, the major challenge in the field remains the production scalability of JPs, which is critical for their application on an industrial scale. The limitations related to scalability of the JP synthesis arise mainly due to the sophisticated synthetic strategies that are too expensive for a large-scale production. Another limitation is the time-consuming process for the preparation of many types of JPs. Cost- and time-effective production of JPs is therefore of critical importance. On the other hand, purification procedures for many kinds of JPs may also be an issue due to an increased use of solvents. As with all multi-step synthetic procedures, there are also losses in the final yield of JPs along the way. All of these challenged have to be considered to be able to synthesize JPs on an industrially relevant scale. The self-assembly of JPs has also been extensively investigated, leading to the creation of more and more sophisticated assembled architectures. Nevertheless, there is still a need for more fundamental studies to attain novel structures and to translate the mechanisms of JP assembly to the creation of functional materials. However, the most essential field in the area of JPs is their application for the rational design of materials. Considering the number of contributions published each year on the topic of Janus particles in general, the number of contributions regarding their applications is by far smaller. This certainly pinpoints an important challenge.


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New application possibilities have to be explored in order to employ the tremendous potential inherent to JPs. There are two major principles for the application of JPs: they can be either applied as single particles, or as building blocks for larger materials. The most thriving state-ofthe-art application fields for single JPs are biological/biomedical applications and self-propelled Janus micromotors that could be used as advanced micromachines. On the other hand, in the second strategy, active functional interfaces and surfaces might be one of the most promising areas of research. Enormous demand exists for the development of new materials with certain functionalities. For instance, surfaces and interfaces with controlled adhesion and wettability in dry, wet, underwater, cold, etc. conditions are critical to fabricate novel materials, coatings, or membranes, with anti-fouling, anti-icing, or self-cleaning properties. Currently, significant attention is drawn to heterogeneous surfaces, where a combination of advantageous properties of several components in one surface may lead to synergistic effects and enhanced features of the developed material. Multi-functional surfaces and interfaces offer attractive capabilities for technological applications, potentially allowing the reduction of costs, fuel consumption, and even the number of emergency situations in certain applications. Herein, the robustness and durability of the developed surfaces is essential for their long-term operation.

JPs may present an ideal solution towards the development of multi-functional active surfaces and interfaces.217 They can be utilized to construct materials with controlled chemical and topographical heterogeneity as well as wettability and adhesion. As shown in this review article, promising examples of the use of JPs for active functional interfaces and surfaces have appeared


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in the recent years.. However, more investigations are needed in order to understand the complex effects of the JP assembly on the final surface/interface functions. In terms of single JPs, aside from organic polymer-based or inorganic metal/metal oxide-based JPs, a blend of these physical properties can be used for the creation of hybrid JPs, where the mechanical stability would be provided by the inorganic core, while the functionality and stimuli-responsiveness would be introduced by the polymeric shells. Up to now, such hybrid hairy JPs were utilized to produce functional active surfaces with controlled wettability towards ultra-hydrophobic surfaces, anti-icing and anti-fouling coatings. Due to the robust design of the single building blocks, also the developed structured surfaces possess mechanical stability and enhanced robustness, while allowing an easy and scalable production via simple solvent casting or spraying. Ultimately, despite the fact that JPs already offer many feasible solutions for the rational design of active multi-functional materials, there are several challenges to be overcome for the future technological applications, such as the production of materials at reasonably low time and price expenses, and the enhancement of their durability towards long-term operation. However, if these challenges could be addressed, it would open completely new pathways for the production of advanced materials. Smart materials are the future. Janus particles, being a class of smart building blocks, could lead to materials that were not yet accessed before. For example, surfaces and interfaces possessing several distinct functions would be responsive to different stimuli, thus making it possible to alter their properties on demand. This could be particularly useful in technological fields involving catalysis, displays, functional coatings, and biomedical applications.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (A.S.) Tel.: +49 351 4658 475 Fax: +49 351 4658 474

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Funding Sources C.M. and A.S. acknowledge DFG (Grant SY 125/4-1) and AiF (18195BR) for funding.

References Walther, A.; Müller, A. H. E. Janus Particles: Synthesis, Self-Assembly, Physical 1. Properties, and Applications. Chem. Rev. 2013, 113 (7), 5194-5261. Mao, Z.; Xu, H.; Wang, D. Molecular Mimetic Self-Assembly of Colloidal Particles. 2. Adv. Funct. Mater. 2010, 20 (7), 1053-1074. 3. Glotzer, S. C.; Solomon, M. J. Anisotropy of Building Blocks and Their Assembly into Complex Structures. Nat. Mater. 2007, 6 (7), 557-562. 4. Yi, G. R.; Pine, D. J.; Sacanna, S. Recent Progress on Patchy Colloids and their SelfAssembly. J. Phys. Condens. Matter 2013, 25 (19), 193101. 5. Casagrande, C.; Fabre, P.; Raphaël, E.; Veyssié, M. "Janus Beads": Realization and Behaviour at Water/Oil Interfaces. EPL 1989, 9 (3), 251-255. 6. de Gennes, P.-G. Soft Matter (Nobel Lecture). Angew. Chem. Int. Ed. 1992, 31 (7), 842845.


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