A versatile colloidal Janus platform: surface asymmetry control, functionalization, and applications

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Janus particles, which generally refer to entities with anisotropic structures and properties, were highlighted and brought to colloidal sciences by Pierre-Gilles de Gennes in his Nobel Prize lecture. 1 After tepid exploration over about a decade, the research on colloidal Janus particles (roughly 1 nm to 1 mm in size 2,3 ) has recently received considerable attention. 4 Colloidal Janus particles, a specific case of patchy particles, 5-7 constitute a unique class of colloids, which integrate broken symmetries in chemical and physical properties allowing for multi-functionalization of materials. 4,[8][9][10][11][12][13] They can assemble into novel structures 4,8,11,[13][14][15][16][17][18] leading to a rich portfolio of properties, suitable for a multitude of applications. 8,9,12,[19][20][21][22] In general, monodisperse colloidal Janus particles would be desirable with uniform and tunable functional asymmetries, and for mainstream applications a facile scalable synthesis is needed. 8,9,11,13,18,23 Even if a large variety of synthetic strategies have been developed, e.g., based on self-assembly, seeded emulsion polymerization, and microfluidics, [8][9][10][11][12] it still remains a challenge to produce large quantities of monodisperse colloidal Janus particles with precise control over the anisotropic properties and functions. Here, we introduce a sequential surface modification by which colloidal Janus particles with adjustable surface asymmetries and functions can be prepared using a simple process, even suggesting scalable production. These colloidal Janus particles can be multifunctional, e.g., in wastewater treatment, stabilizing oil-in-water emulsions, selectively decomposing water-miscible organic wastes, and collecting immiscible organics from water.
This approach is illustrated for SiO 2 particles, as schematically elaborated in Fig. 1a. It consists of three steps and integrates surface wetting tuning, 24 Pickering emulsification, 25 and polydopamine (PDA) chemistry. 26 They are conducted in sequence: (1) first, the highly hydrophilic monodisperse SiO 2 colloidal spheres (see Fig. S1, ESI †) are tunably hydrophobized with silane mixtures to systematically increase the contact angle to water (takes ca. 2 h); (2) next, such particles are assembled at the waxwater interfaces by Pickering emulsification and immobilized by solidifying the wax (ca. 1 h); 27 (3) PDA is then introduced onto the partially coated silane-modified SiO 2 particles from the aqueous phase to form Janus particles with tunable sizes of PDA patches opposite to the hydrophobic side (ca. 1 h). The PDA patch size depends on the extent of the hydrophobicity brought by the silane mixtures. The chemical reactivity of PDA then serves as a generic chemically reactive platform to introduce a rich variety of functional materials including a catalytic metal (Ag), magnetic metal oxide (Fe 3 O 4 ) nanoparticles, and fluorescent organic molecules (fluorescein isothiocyanate, FITC). Fig. 1b-e illustrate each step along the approach.
The geometry of the colloidal particles residing at the oilwater interface in Pickering emulsions tightly associates with the surface wetting properties of the particles. 28 The resident geometry of the particles is quantified in terms of the angle 'a', while the wetting properties of the particles are reflected by measuring their sessile drop contact angles (y) with water, as illustrated in Fig. 2a. As such, the hydrophilic particles (y o 901) favour the aqueous phase with a 4 901, while the hydrophobic particles (y 4 901) tend to lean to the oil phase as a o 901. 28 Achieving stable oil-in-water Pickering emulsions (1801 4 a 4 901) is subtle. Therein, the surface wetting properties of the particles should be in-depth controllable for y o 901. 28 As indicated in Fig. S2 (ESI †), neither the pristine SiO 2 particles (sample A1 in Table 1) nor the hydrophobically dichlorodimethylsilane (DCDMS) treated SiO 2 particles (sample A7 in Table 1) stabilize oil-in-water Pickering emulsions. The pristine SiO 2 particles are superhydrophilic (Fig. S2a, ESI †) with a y E 01 (Fig. 2b), favouring fully staying in the aqueous phase (a = 1801), while the DCDMS-treated particles are highly hydrophobic (y = 140.9 AE 5.61, see Fig. 2b) and tend to form water-in-oil Pickering emulsions (Fig. S2b, ESI †). Therefore, silane mixtures comprising both DCDMS and (3-aminopropyl)triethoxysilane (APTES) were assumed to allow the formation of surfaces with tunable wetting properties. Note that APTES is expected to co-react with DCDMS and to introduce hydrophilic -NH 2 and -OH groups (see Fig. S3 and S4a, ESI †).
The concentration ratios of APTES and DCDMS (C APTES /C DCDMS ) are systematically varied and detailed in Table 1 (also see Table S1, ESI †). As expected, APTES assists the formation of hydrophilic surfaces (see sample A2 in Fig. 2b), while DCDMS brings -CH 3 (see Fig. S4a, ESI †), enhancing the hydrophobicity of the surfaces (Fig. 2b). Therefore, the integrated use of both silanes gives rise to surfaces with intermediate wetting properties. X-ray photoelectron spectroscopy (XPS) reveals the co-grafting of APTES and DCDMS, and verifies the role of DCDMS as the hydrophobicity enhancer, as demonstrated in Fig. 2c and Fig. S5 (ESI †). Further results from Fourier-transform infrared spectroscopy (Fig. S4a, ESI †), zeta potential analysis (Fig. S4b, ESI †), size comparison from SEM observation (Fig. S4c, ESI †), and dispersibility measurements (Fig. S4d, ESI †) corroborate those revealed by XPS. More importantly, increasing the C APTES /C DCDMS ratio from 1 : 0 to 1 : 0.636 leads to an increase of y from 12.6 to 73.71, and a decline of a from 145.5 to 671 as indicated in Fig. 2b and d-iii. Correspondingly, the fractional area of the PDA patch vs. the particle total surface decreases from 91.0% to 30.4%, as displayed in Table 1. Clearly, SiO 2 particles are capable of stabilizing oil-in-water Pickering emulsions upon treatment with APTES and DCDMS in the C APTES /C DCDMS range from 1 : 0 to ca. 1 : 0.636 (Fig. 2d-i and  Fig. S6, ESI †), whereupon their resident geometries at the interfaces become C APTES /C DCDMS dependent (Fig. 2d-ii). The particles tend to lean into the wax phase as an increased composition of C APTES /C DCDMS is applied as shown in Fig. 2d-ii. Upon solidifying the wax phase via lowering the temperature, the particles are fixed at the interfaces, allowing subsequent topochemical PDA modification of the patches on the particle surface that are exclusively immersed in the water phase ( Fig. 2d-i, ii and Fig. S6, ESI †). After the removal of the wax with chloroform, a series of SiO 2 /PDA Janus particles are formed as shown in Fig. 2d-iv (also see the relevant characterizations in Fig. S7, ESI †).  Table 1; (c) X-ray photoelectron spectroscopy (XPS) results of samples A1, A2, A5 and A7, see also Fig. S5 (ESI †). The insets highlight the characteristic patterns of N1s in each sample; (d) SEM observations of wax droplets stabilized by silane-modified SiO 2 particles (i, A2-A6), magnified local details (ii), and SiO 2 /PDA Janus particles (iv, A2-A6/PDA), and schematic illustration of particle resident geometries at the wax-water interfaces where a is specifically defined (iii). Table 1 The concentration ratios (C APTES /C DCDMS ) of (3-aminopropyl)triethoxysilane (APTES) and dichlorodimethylsilane (DCDMS), in relation to the detailed recipes in Table S1 (ESI), and the PDA patch areas as a fraction of the total particle surface area (S PDA /S total ) PDA is abundant in catechol and amino groups, allowing for a rich variety of secondary functionalizations. 26,29,30 Here, we demonstrate the generality of the approach based on three types of surface functionalization based on PDA patches, i.e., (1) in situ metallization, 31 (2) nanoparticle adhesion, and (3) molecule grafting. First, the PDA patches facilitate in situ reduction of Ag + ions to Ag nanoparticles (about 10 nm in diameter, see Fig. 3b and Fig. S8, ESI †), resulting in SiO 2 /PDA-Ag Janus particles with variable asymmetrical geometries as shown in Fig. 3a. The formation of SiO 2 /PDA-Ag Janus particles is verified with transmission electron microscopy (TEM, Fig. 3b-i and ii, and  Fig. S8a-c, ESI †), energy-dispersive X-ray spectroscopy (EDX, Fig. S8d, ESI †), elemental mapping (Fig. 3b-iii), XPS (Fig. 3b-iv), and X-ray diffraction (XRD, Fig. 3b-v).
Finally, we demonstrate how Janus particles can be feasible to selectively capture and then decolorize oils from wastewater, facilitating the wastewater remedy by avoiding the combined employment of a number of conventional colloidal materials that only play a single role during the remedy. 34 To simulate the wastewater, water-soluble (4-nitrophenol) and -immiscible oil (red-stained toluene) organics are mixed with water (see Fig. 4a-i). After the employment of SiO 2 /PDA-Ag Janus particles, the oil droplets are surrounded with Janus particles in the formation