Dopamine Polymerization in Liquid Marbles: A General Route to

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Dopamine Polymerization in Liquid Marbles: A General Route to Janus Particles Synthesis Yifeng Sheng, Guanqing Sun, and To Ngai Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00525 • Publication Date (Web): 10 Mar 2016 Downloaded from http://pubs.acs.org on March 10, 2016

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Dopamine Polymerization in Liquid Marbles: A General Route to Janus Particles Synthesis Yifeng Sheng,a Guanqing Suna and To Ngaia,b,* a

Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong. b

Shenzhen Municipal Key Laboratory of Chemical Synthesis of Medicinal Organic Molecules, Shenzhen Research Institute, The Chinese University of Hong Kong; Shenzhen, 518057, P. R. China.

Abstract: Coating the liquid with a particle shell not only renders the droplet superhydrophobic, but also isolates a well-confined micro-environment for miniaturized chemical processes. Previously, we have demonstrated that particles at the liquid marble interface would experience different environments at the air/water interface, which provide an ideal platform for the siteselective modification of superhydrophobic particles. However, the needs of a special chemical reaction limit their potential use for the fabrication of Janus particles with various properties. Herein we combine the employment of liquid marbles as microreactors with the remarkable adhesive ability of polydopamine to develop a general route to the synthesis of Janus particles from micron-sized superhydrophobic particles. We demonstrate that dopamine polymerization and deposition inside liquid marbles could be used for the selective surface modification of micro-sized silica particles, resulting in the formation of Janus particles. Moreover, it is possible to manipulate the Janus balance of the particles via the addition of surfactants and/or organic solvents to tune the interfacial energy. More importantly, owing to the many functional groups in polydopamine, we show that versatile strategies could be introduced to use these partially polydopamine-coated silica particles as platforms for further modification, including nanoparticle immobilization, metal ion chelating and reduction, as well as chemical reactions.

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Given the flexibility in the choice of cores and the modification strategies, this developed method is distinctive in its high universality, good controllability and great practicability. Key words: Polydopamine, Liquid marble, Janus particle, Surface modification

* To whom correspondence should be addressed. E-mail: [email protected]


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Introduction Janus particles, which were first coined by Casagrande et al. in 1989 and reiterated by de Gennes,1,

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are special particles whose surfaces have at least two distinct compositions,

architectures and/or polarities.3-8 With asymmetric natures, Janus particles would particularly exhibit dramatically different properties, behaviors and functions, making them one of the most appealing fields in both theoretical researches and practical applications. To meet the diversified requirements, different strategies have been developed for the preparation of well-designed and functionalized Janus particles with tunable morphology, shape and composition, including selfassembly, phase separation, microfluidics, and so on.3-5 Taking consideration of both practicability and economy, one admiring strategy is the application of mask-protecting assisted site-selective modification through protect-and-release process.3-5 Although early endeavors of this method showed high flexibility in the choice of solid particles and medium, as well as excellent applicability of various masking and modification strategies, employing planar (twodimensional, 2D) substrates/interfaces as protecting templates made it suffering greatly from the yields which in turn limiting their uses. To address these limitations, significant progress has evolved with the change of templates from planar substrates (2D) to the high internal interface systems (three-dimensional, 3D).3-5 In 2004, Gu et al. first reported the preparation of silver magnetic heterodimers by selectively nucleating silver metal, Ag(0), onto iron oxide (Fe3O4) nanoparticles at the water/oil interface in emulsion.9 Later, Granick group showed that particles at the Pickering emulsion interface could be fixed via oil-phase solidification for the further selected modification of the exposed moieties,10, 11 and the Janus balance would be controlled via addition of surfactants.6, 12 Thereafter, rapid expansion has achieved in the aspect of different materials, systems, immobilization methods and modification strategies.3, 4 However, successful


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stabilization of Pickering emulsions greatly depends on the particles’ wettability towards both oil and water,13,

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making it a great problem for the fabrication of Janus particles from

superhydrophobic and/or large particles which are not suitable as the emulsion stabilizers. Dry waters or liquid marbles are an emerging class of material under extensive investigation of which the air/liquid interface could serve as a platform to hold superhydrophobic particle in the interfacial energy well.15,

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Liquid droplets can be easily coated with a thin layer of solid

particles to make them superhydrophobic and/or superoleophobic and non-adhesive to solid surfaces and thus facilitates small amount of volume liquid transportation without leakage.17 Various aspects of liquid marbles, such as principles, properties and applications have been investigated in recent years.18-21 Coating the liquid with a particle shell not only renders the droplet superhydrophobic and/or superoleophobic, but also isolates a small-volume system for various potential applications, such as storage,22 sensor,23, 24 accelerometers,25, 26 and so on.18 With great advantages related to a well-confined microenvironment, a versatile platform for different reactions and a large specific interfacial area, liquid marbles have also gained increasing attention in the field of miniaturized chemical processes. For example, Dorvee et al. first showed that precipitation reaction could be carried out in liquid marbles, along with realtime spectroscopic analysis.24 Thereafter, Xue et al. reported that magnetic liquid marbles could be used as smart miniature reactors for various chemical reactions, including luminescence reaction, polymerization, and neutralization.27 And some other groups have demonstrated that the permeability of the porous particle shells coating liquid marbles renders their application for gas sensing,28 interfacial polymerization,29 CO2 capture,30 etc.18 Furthermore, liquid marbles as microreactors for Daniell cell,31 blood typing,32 synthesis,33 and other applications have been reported.


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Different from the previous work which merely using the encapsulating particle shells acted as inert protecting layers, Carter et al.,34, 35 demonstrated that catalytic particles adsorbed at the air/water interface could accelerate the chemical reaction inside the liquids, which proved that the particle shells could also serve as participators in the whole miniature reactions. Thus, similar to the Pickering emulsion, particles at the phase boundaries would experience different environments at the air/water interface,36 which provide an ideal platform for the site-selective modification in liquid phase. Recently, we have demonstrated that the classic silver mirror reaction inside liquid marbles could be used for the selective surface modification of micro-sized silica particle, resulting in the formation of Janus particles.37 However, the needs of a special chemical reaction limit their potential use for the fabrication of Janus particles with various properties, which is also a pending problem in the traditional Janus particle preparation, since most existing methods have to be restricted to one or one class of substrates. To overcome these shortcomings, herein, we combine the employment of liquid marbles as microreactors with the remarkable adhesive ability of polydopamine to develop a general route to the synthesis of Janus particles from micron-sized superhydrophobic particles. In 2007, polydopamine was first used as an efficient building block for spontaneous deposition of thin films onto nearly all inorganic and organic surfaces,38 which makes it one general method of surface modification and functionalization regardless of the natures and morphologies of the substrates.39 Similar to the Pickering emulsion methods, our method could be conducted under a mild environment with high efficiency, and no expensive equipment is needed. In addition to its extraordinarily robust adhesion, our results show that the certain functional groups left in the polymer further render it available for diverse post-reactions, such as nanoparticle adsorption,


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metal ion chelating and redox, substitution reaction, etc., indicating the potential of further modification of polydopamine layers to obtain desired asymmetric properties. Materials and methods Materials 10 µm silica particles were purchased from Suzhou Nano&Micro Technology Company. Calcium carbonate nanoparticles (20 nm, 99%) were from DKnano and polylactic acid particles (Mw = 300000, 99%) were from Daigang-Biomaterials. Dopamine hydrochloride (99%) was purchased from IL USA, Tris base (>99.9%), hydrochloric acid (HCl, 37%), titanium(IV) oxide (TiO2) nanoparticle (< 150 nm, 33-37 wt% in water), carbon nanopowder (< 500 nm, 99.95%), fluorescein 5(6)-isothiocyanate (FITC, > 90%), oligo(ethylene glycol) methyl ether methacrylate (OEGMA, average Mn = 475, 95%), 2-bromoisobutyryl bromide (98%), copper(II) chloride (CuCl2, >99.99%), 2,2’-dipyridyl (>99%), and ascorbic acid were purchased from Sigma-Aldrich. Polystyrene particles (0.21 µm, 4.1% in water; 2 µm, 8.0% in water) were from Invitrogen. Silver nitrate (AgNO3, 99.80%) was purchased from UNI-Chem, and ammonia solution (NH3·H2O, 25%) was purchased from BDH Chemicals. A series of alcohols, acetone and trimethylamine(99.86%) were from Fisher. All chemicals were used without further purification. Deionized water was used to prepare all the solutions. Surface modification of 10 µm silica particles Silica particles (SiO2) (0.5 g) were first dispersed to toluene (50 mL), and then dichlorodimethylsilane (0.5 g) was added. After 24 h reaction under dry N2, hydrophobic silica


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particles were separated from toluene by centrifugation, and then washed by methanol and ethanol, and finally freeze-dried under vacuum. Preparation of liquid marbles A typical procedure of preparing the Janus particles via liquid marble is shown in Figure 1A. First, dopamine hydrochloride aqueous solution (4 mg/mL) was placed on a bed of 10 µm superhydrophobic silica particles, and then isometric Tris solution (pH = 8.5, 20 mmol/L) was injected.39 After rolling these droplets on the particle beds, sufficiently stable liquid marbles would be fabricated. In a basic environment inside the liquid marbles, dopamine monomers spontaneously polymerize into polydopamine (PD) and deposit onto the selected moieties of the silica particles, resulting in PD/SiO2 Janus particles. Separation and purification of polydopamine/silica (PD/SiO2) Janus particles After 12 h reaction, liquid marbles were firstly transferred to 95% ethanol, leading to the fallenoff of the PD/SiO2 Janus particles from the air/water interface. The particle suspension was then naturally precipitated for several minutes, followed by removal of the supernatants containing the excess reactants and polydopamine. After that, the precipitates of Janus particles were repeatedly washed by 95% ethanol and absolute ethanol respectively, and finally freeze-dried. After purification, the PD/SiO2 Janus particles could be modified once again both physically and chemically, such as nanoparticle adsorption, metal ion chelating and redox, as well as substitution reaction to achieve further functionalized Janus particles as shown in Figure 1B. Nanoparticle immobilization on PD/SiO2 particles via Method 1


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As shown in the top route of Figure 1B, purified PD/SiO2 Janus particles (0.05 g) were first distributed in ethanol (5 mL), and then nanoparticle suspension (200 µL) were added. After stirring for 12 h, the particle suspension was filtered through a medium filter paper with pore diameters ranging from 5 to 10 µm. The particles remained on the filter paper were repeatedly washed by 95% ethanol and absolute ethanol respectively, and finally freeze-dried. Nanoparticle immobilization on PD/SiO2 particles via Method 2 As shown in the bottom route of Figure 1B, liquid marble microreactors were first purified by extraction of the reaction solution and injection of deionized water, and this process was repeated for three times (Figure S1 at Supporting Information). After purification, 50 µL particle suspension was injected, and then the liquid marble microreactors were left standing for 12 h in an airtight space. The liquid marbles were then transferred to 95% ethanol, and the mixture was filtered through a medium filter paper with pore diameters ranging from 5 to 10 µm. The particles remained on the filter paper were washed by 95% ethanol and absolute ethanol respectively, and finally freeze-dried. Ag(I) chelating and reduction on PD/SiO2 particles via Method 1 Purified PD/SiO2 Janus particles (0.05 g) were first distributed in the Ag(I) solution for 12 h reaction under stirring, and then the particle suspension was filtered through a medium filter paper with pore diameters ranging from 5 to 10 µm (top route in Figure 1B). The particles remained on the filter paper were washed by 95% ethanol and absolute ethanol orderly, and finally freeze-dried. Ag(I) chelating and reduction on PD/SiO2 particles via Method 2


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Liquid marble microreactors were first purified by extraction of the reaction solution and injection of deionized water, and this process was repeated for three times. After purification, Ag(I) solution (50 µL) was injected, and then the microreactors were left standing for 12 h in an airtight space (bottom route of Figure 1B). The Ag-PD/SiO2 particles were then purified and finally freeze-dried. Grafting of FITC dye on PD/SiO2 particles via Method 1 FITC (2 mg) was dissolved in 95% ethanol (5 mL), and then purified PD/SiO2 Janus particles (0.05 g) were added. After stirring for 24 h, the precipitates of Janus particles were washed by absolute ethanol via static precipitation process until no fluorescence could be detected in the supernatants (top route in Figure 1B). Finally, the particles were freeze-dried. Grafting of POEGMA on PD/SiO2 particles via Method 1 PD/SiO2 Janus particles (0.05 g) in CH2Cl2 (10 mL) in a flask were first degassed by purging with dry N2 for 5 min. Then, 2-bromoisobutyryl bromide (0.5 mL) and triethylamine (0.5 mL) were added under dry N2. After stirring under dry N2 at room temperature for 6 h, the BrPD/SiO2 Janus particles were washed by CH2Cl2, acetone, and methanol, orderly via static precipitation process. And then, the Janus particles were re-dispersed in 8 mL methanol/water mixture (v/v = 1:1). Subsequently, a methanol/water solution (8 mL, v/v = 1:1) containing a water soluble oligo(ethylene glycol) methyl methacrylate (OEGMA) (1.9 g), CuCl2 (17mg), 2,2’dipyridyl (10 mg) was added to this suspension and was then degassed by bubbling dry N2 for 15 minutes. Ascorbic acid (0.017 g) aqueous solution (0.8 mL) was first degassed by purging with dry N2 for 1 min and then injected with a syringe, while degassing continued, giving a dark brown solution. After 24 h reaction, the precipitates of Janus particles were washed by methanol


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and ethanol each three times via static precipitation process. Finally, the particles were redispersed in absolute ethanol. Characterization The digital photographs and optical microscopy images were collected by a digital camera (Canon 650D, Japan) and an optical microscope (Leica DM LFSA, German) respectively, and analyzed with Image J. The SEM images were conducted with Quanta 400 F (FEI Company, USA) operated at 10 kV. The fluorescence image of the FITC-PD/SiO2 particle was taken by a laser scanning confocal microscope (Nikon C1Si, Japan).

Figure 1. (A) The general synthesis procedure of multi-functional Janus particles via dopamine polymerization in liquid marbles: stable liquid marbles are first fabricated and subsequently act


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as microreactors for the polymerization of dopamine. The polymerized polydopamine (PD) then selectively coated the interfacial silica particles in the aqueous phase, resulting in PD/SiO2 Janus particles. The PD/SiO2 Janus particles could be purified and modified once again both physically and chemically to achieve further functionalized Janus particles. (B) Illustration of the transformation of PD/SiO2 Janus particles into further modified Janus particles via two different methods. In the top route, Method 1, liquid marbles were firstly transferred to ethanol, leading to the fallen-off of the PD/SiO2 Janus particles from the air/water interface. The PD/SiO2 Janus particles then could be further modified. In the bottom route, Method 2, liquid marbles were first purified by extraction of the reaction solution and injection of deionized water. After purification, second chemical solution was injected, and then the liquid marble microreactors were left for reaction in an airtight space. The further functionalized Janus particles were separated and purified.

Results and discussion Figure 2 shows that after rolling dopamine solution on the superhydrophobic silica particle beds, sufficiently stable liquid marbles stabilized by a monolayer of silica particles would be fabricated and subsequently act as microreactors for the polymerization of dopamine (Figure 2A). Immediately, the color of the liquid marbles would change from colorless white to pale brown, and finally turn to dark brown or black with passing time, indicating the formation of polydopamine inside the liquid marbles (Figure 3A, see also Figure S2 in the Supporting Information). The static properties of the liquid marbles stabilized by silica particles were also investigated by gradually increasing the volume of the inner dopamine solution and the


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parameters were summarized in Table 1. The gravity of liquid marble will press it into more disc-like shapes. When the inner volume is 20 µL, the shape of the resulting liquid marble is like spherical. However, with increasing of the inner solution volumes, gravity gradually overcomes surface forces and the liquid marble becomes disc-like shape.

Figure 2. (A) Microscopic image of the particle layer at the liquid marble interface. (B) Static contact angles of different liquid marbles fabricated using dopamine aqueous solution (pH = 8.5). Size unit: µL. Scale bar: 50 µm in (A) and 2 mm in (B).

Table 1. The parameters of liquid marbles with different volumes Size (µL)

100

80

60

50

40

30

20

d (mm)

7.0

6.3

5.7

5.2

4.8

4.3

3.8

l (mm)

5.2

4.5

4.0

3.9

3.1

2.6

2.8

h (mm)

3.8

3.8

3.5

3.3

3.3

3.2

2.3

Figure 3B and 3C show the SEM images of the purified particles after 12 h reaction inside the liquid marbles. A significant difference between different moieties of the silica particles (Figure 3B and 3C) was appeared, indicating the successful deposition of thin adherent polydopamine films onto the solid surface and the formation of Janus particles, PD/SiO2. Further results showed


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that parts of the Janus particles gained a much rougher surface with big particulates, which would be resulted from the adhesion of polydopamine nanoparticles by self-polymerization (Figure 3D).40 It has been thought that rotation of the particles at the interface would make it difficult to form Janus structures, but due to the asymmetric deposition of polydopamine, along with the porous surface of the silica particles, the colloid rotation would be greatly hampered and finally lead to the Janus particle formation.37, 41 Since many previous studies have demonstrated its strong adhesion to virtually all types of surfaces, these microreactors would be suitable for depositing polydopamine films to various shells, making it a common method to fabricate Janus particles from various particles.39, 42, 43 It is worth mentioning that in the Pickering emulsion method, the Janus balance of the obtained particles greatly depends on their equilibriumpenetration distance at the liquid/liquid interface, which can be ascribed to surface energies of the system.12, 44 Similar to the Pickering emulsion method, when particles adsorb to the air/liquid interface in liquid marble, the protrusion depth that particles penetrate into the liquid phase is mainly dealt with their surface energies. Thus, it is possible to control the protrusion depth of the particles at the water phase through the addition of surfactants, salts and/or organic solvents to the liquid marbles, resulting in the change of Janus balance at the particle surface.12 For instance, adding 0.013 g/L Tween 80 would increase their contact angle from 150° to 190° (Figure 3E), while adding 25% (v/v) ethanol would increase their contact angle to 250° (Figure 3F). Relied on the natural phase boundaries at high internal interface systems, this method, along with the Pickering emulsion method, both require no expensive equipment and show high efficiency. But different from the Pickering emulsion method, liquid marble method offers excellence for the fabrication of Janus particles from superhydrophobic particles and/or large-gauge particles which are not appropriate emulsion stabilizers. Further, since air acts as the inert protector, the


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separation of oil phase and water phase could be omitted, while demulsifiers could be introduced to the whole system. With a clever choose the liquid phase, this also attributes to modify particles which are sensitive to the organic solvent or water.

Figure 3. (A) Photographic images of the liquid marble microreactor over time. And SEM images of (B) PD/SiO2 Janus particles; (C) PD/SiO2 Janus particles; (D) PD/SiO2 Janus particles with polydopamine particulates; (E) PD/SiO2 Janus particles synthesized from microreactors containing Tween 80; (F) PD/SiO2 Janus particles synthesized from microreactors containing ethanol. Scale bar: 2 mm in (A), 20 µm in (B) and 5 µm in (C) to (F).

The silica particles provided an indispensable substrate for polydopamine deposition, in turn the thin films rendered the inert silica particles high versatility. Since the particles after


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modification would keep their superhydrophobicity, which behaved in the maintenance of liquid marble shape, the further modification of these PD/SiO2 particles could be carried out in two different ways (Figure 1B). The first method (Method 1, top route in Figure 1B) was based on the easy motion of the liquid marbles.18 Transporting the liquid marbles into ethanol would result in the structural damage of them, and then the separation and purification of the Janus particles. Re-dispersing these purified particles in secondary reaction solutions would lead to the formation of new Janus particles. In contrast, the other way (Method 2, the bottom route in Figure 1B, see also Figure S1 in the Supporting Information) was to use micro-pump techniques with capillary devices based on liquid marble’s ability of entry/exit of reactants and products.27, 45 Washing the liquid marbles via repeatedly replacing the inner solution with deionized water would also result in purified PD/SiO2 particles, and subsequently injecting the reactant solution would further modify them. Both of these methods have their suitability and advantages, and to better illustrate it, three different modifications were carried out.


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Figure 4. SEM images of (A) and (B) PS-PD/SiO2 Janus particles synthesized from Method 1; (C) and (D) PS-PD/SiO2 Janus particles synthesized from Method 2. Scale bar: 5 µm.

Taking inspiration from mussel adhesion, a common strategy for PD/SiO2 particle modification is the robust immobilization of nano-/micro-particles through the immersion of Janus particles into colloid suspensions. Immersed in water or alcohol, the active amino acid composition would make the polymer films as bio-glue for anchoring various particles.39 As shown in Figure 4, 0.21 µm polystyrene (PS) spheres could be trapped by the PD/SiO2 particles after 12 h treatment via both methods, as a result of the robust and strong adhesion property of the thin polydopamine films. However, the achieved loading density of PS particles via Method 1 (Figure 4A and Figure 4B) was much higher than that via Method 2 (Figure 4C and 4D),


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reflecting that full immersion of the PD/SiO2 particles would attribute to adsorption of PS particles, as a result of the higher density of surrounded PS particles, the longer contacting time and the repeated-contact with the free PS particles. Compared to Method 1, most of the PD/SiO2 particles on the liquid marble shells would only have a short contact with the PS particles due to their self-sedimentation in the suspension. Owing to the sedimentation, PS particles would be concentrated at the bottom of microreactors, which reduced their effective concentration in solution and further led to the uneven distribution of PS particles on different Janus particles. Also, since the gravity has greater influence on larger particles, Method 2 should not be applicable for the adsorption of micro-sized particles, which would be still attainable via Method 1 (Figure S3 in the Supporting Information). Although Method 1 rendered high efficiency, the full immersion of the PD/SiO2 particles would inevitably contaminate the bare SiO2 parts of the Janus particles, due to the existence of free PS particles and the fallen polydopamine colloids in the solution. In contrast, modification via Method 2 would provide the obtained particles with a better appearance, because the bare parts of the Janus particles were protected by the inert air throughout. Since the adhesion has no limit with respect to the nature of materials, these methods would also be applicable for other colloids, such as carbon nanoparticles (Figure 5A), TiO2 nanoparticles (Figure 5B), regardless of their compositions, sizes and properties.


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Figure 5. SEM images of (A) Carbon-PD/SiO2 and (B) TiO2-PD/SiO2 Janus particles synthesized from Method 1. Scale bar: 5 µm.

In addition to the particle immobilization, another appealing feature of polydopamine is its strong binding to various metal ions through the many functional groups in polydopamine. Apart from metal ion chelation, certain amounts of remaining catechol groups which are able to release electrons further render the thin film reducibility to reduce noble metal ions (Ag+, Au3+, etc.) into metal clusters under mild environments.39, 46 The ability of polydopamine to behaving as both a ligand and a reducer makes it serve as an active substrate for the fabrication of metal/polymer hybrid films on Janus particles. As shown in Figure 6, treating the PD/SiO2 particles with silver nitrate or Tollens’ reagent would achieve the growth of Ag-PD hybrid nano-architectures on the surfaces of the PD/SiO2 particles. Based on the previous studies, the functional groups in polydopamine would first serve as anchors and reducing agents of the silver/diamminesilver(I) ions, and then the obtained Ag(0) binding at the N-ligand and O-ligand would act as the nuclei for the further growth of Ag nano-clusters via the continuous reduction of more


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silver/diamminesilver(I) ions around them. Compared to diamminesilver(I), using Ag+ as oxidant would attribute to the formation of star-studded shells with massive silver colloids, which have been used as the template to build a two-tier roughness mimicking the morphology of lotus leaf.42 In addition to the cations, the morphologies of the silver colloids could also be easily controlled by changing the concentration of the oxidant, which was reflected in that the density of the Ag nanoparticles was augmented as the concentration of silver/diamminesilver(I) ions was increased (Figure 6). Since small molecules/ions could go free motion in the solution, there is no obvious difference in loading efficiency between these two methods, while Method 2 still demonstrated better maintenance of the Janus structures (see also Figure S4 in the Supporting Information).

Figure 6. SEM images of Ag-PD/SiO2 Janus particles synthesized via Method 1 from (A) 0.004 mol/L diamminesilver(I) solution; (B) 0.004 mol/L silver nitrate solution; (C) 0.04 mol/L diamminesilver(I) solution; via Method 2 from (D) 0.004 mol/L diamminesilver(I) solution; (E) 0.004 mol/L silver nitrate solution; (F) 0.04 mol/L diamminesilver(I) solution. Scale bar: 5 µm.


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Figure 7. (A) Confocal image of FITC-PD/SiO2 particles; (B) SEM image of POEGMAPD/SiO2 particles. Scale bar: 50 µm in (A) and 10 µm in (B). Polydopamine also supports a variety of chemical reactions with small molecules for the creation of functional organic layers. The abundant groups in polydopamine are able to carry out various reactions, such as substitution, addition, oxidation, etc., with a wide range of molecules including dyes, polymers, and many other molecules.39 With high reactivity, grafting could be easily achieved via the blending of polydopamine and the reactants. As shown in Figure 7A, immersion of the PD/SiO2 particles into FITC ethanol solution would label the particle with luminous fluorescence via the addition reaction with free amino groups in polydopamine (including the physical adsorption). Besides, immersion of the PD/SiO2 particles in 2bromoisobutyryl bromide solution with trimethylamine would lead to the formation of alkyl halide-functionalized (Br-PD) membranes, which could be subsequently used to carry out surface-initiated atom transfer radical polymerization (ATRP) from these particles (BrPD/SiO2).47, 48 The SEM image (Figure 7B) clearly shows that dense POEGMA layers formed on the particles after 24 h polymerization in methanol/water solution. Since the bromide groups concentrated on the polydopamine-modified moieties, the density of the polymer brushes had


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great difference between different parts of the silica particles. Because these hydrophobic particles could not enable to stabilize the organic liquid marbles, Method 2 could not be applied to the secondary chemical reactions inside organic solvent. Conclusions In summary, liquid marbles can be employed as microreactors to carry out dopamine polymerization at the air/water interface, and the asymmetric deposition of the polydopamine films could in turn lead to the formation of Janus particles, which would be greatly complementary to the Pickering emulsion methods. Owing to the many functional groups in polydopamine, versatile strategies could thus be introduced to use these partially coated PD/SiO2 particles as platforms for further modification, including particle immobilization, metal ion chelating and reduction, and chemical reactions. Given the flexibility in the choice of cores and the modification strategies, this two-step method is distinctive in its high universality, good controllability and great practicability. And with a clever design of the layered structures, tremendous applications could be achieved in physical, chemical, and biological fields. ASSOCIATED CONTENT Supporting Information. Additional experimental details; side view of different liquid marbles at different reaction stages; illustration of Method 2; SEM images of PS(2 µm)-PD/SiO2 particles, and Ag-PD hybrid films. This material is available free of charge via the Internet at http://pubs.acs.org.


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Acknowledgement The financial support of this work by the National Nature Science Foundation of China (B040606, 21374091) and the Shenzhen Science and Technology Innovation Committee for the Municipal Key Laboratory Scheme (ZDSY20130401150914965) is gratefully acknowledged.


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For Table of Contents Only


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Dopamine Polymerization in Liquid Marbles: A General Route to

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