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Functional Janus Particles Modified with Ionic Liquids for Dye Degradation ruotong zhao, xiaotian yu, dayin sun, Liyan Huang, Fuxin Liang, and Zhengping Liu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00090 • Publication Date (Web): 04 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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ACS Applied Nano Materials

Functional Janus Particles Modified with Ionic Liquids for Dye Degradation Ruotong Zhao,a,b Xiaotian Yu,b Dayin Sun,b Liyan Huang,a Fuxin Liang,b* Zhengping Liua* a

BNU Key Lab of Environmentally Friendly and Functional Polymer Materials, Beijing Key

Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China. b

State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese

Academy of Sciences, Beijing 100190, China. KEYWORDS Janus particle, ionic liquid, anion exchange, solid surfactant, dye degradation

ABSTRACT

Snowman-like silica@PDVB/PS Janus particles were fabricated via seeded emulsion polymerization and modified with hydrophilic ionic liquid moieties. The Janus particles can readily be further modified by incorporating new functionalities through simple anion exchange. PW12O403- as a catalytically active species for dye degradation was introduced into the Janus particles and the performance in the catalytic degradation of methyl orange was evaluated.

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INTRODUCTION Janus particles have diverse compositions, structures and properties that are segmented on the two sides of the particle.1 In Janus particles, opposing properties, such as hydrophilic/hydrophobic, polar/nonpolar, anion/cation, exist on one object. Due to their distinct performances, Janus materials have received increasing attention as solid surfactants, optical probes, and self-propelled motors in biomedical fields as well as the food industry.2-15 When hydrophobic and hydrophilic compositions are segmented on opposite sides, the Janus particle is similar to a molecular surfactant and can act as a solid surfactant.16-22 Janus particles combine the advantages of a Pickering emulsion and a molecular surfactant to achieve better stability of the emulsion.23-25 Furthermore, theoretical studies have shown that Janus particles possess more surface activity compared to homogeneous surfactant particles. In comparison with that of homogeneous particles, the desorption energy of the Janus particles increases almost 3-fold due to the enhanced amphiphilicity,23,26,27 as suggested by theoretical studies on the desorption energy of Janus discs and rods at the liquid-liquid interface.28,29 Following the theoretical predictions, it was reported that the Janus particles generally exhibit significantly higher interfacial activities than homogeneous particles of a similar size.30 At the water-oil or air-solid interface, amphiphilic Janus particles tend to spontaneously self-assemble into membrane-like single layers, making them efficient emulsifiers and surfactants in oil-water emulsion32-34 or functional coatings.35-38

Snowman-like Janus particles composed of two subunits with distinctively different properties are interesting due to their adjustable compositions, morphology and water-wettability.20,32,39-41


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When stimulus-response properties are introduced, the Janus balance is actively controlled by external triggers.42-44 It has been reported that the water-wettability of ionic liquids (ILs) modified Janus particles can be easily adjusted by simple anion exchanging processes.45 When opposite sides of Janus nanosheets are modified by ILs, the wettability can be tuned by exchanging anions.45 Furthermore, functions such as catalytic degradation can be introduced to the emulsion interface when poly (ionic liquid)s (PILs) modified Janus nanosheets are used as solid surfactants.46 However, the fabrication process of the Janus nanosheets is complicated and too difficult to be used in industrial production. Currently, snowman-like Janus particles are fabricated via seeded emulsion polymerization.32 The morphology and composition of the Janus particles can be controlled by the amount of the previous monomer. Most importantly, this method can be employed to prepare Janus particles with strict segmentation on a large-scale. On the other hand, when Janus particles are used, there are more intergranular channels than that in Janus nanosheets. This detail is crucial for the transmission of chemistry through the interface. In our previous report, two different polymers or groups were selectively added onto the two different sides of the Janus particles.2,47 In this methodology, two different ionic liquids can be added on the different sides of snowman-like Janus particles. However, if two different ionic liquids are added on opposite sides, the anions will easily exchange in the solvent. Therefore, only one side is modified with the ionic liquid in this work. Based on our previous work, we designed and fabricated ILs modified silica@PDVB/PS Janus particles (Scheme 1). Owing to the simple structure of the ILs, the fabrication was easy, and some experimental steps were saved in comparison to the fabrication with PILs. It is expected that the functional ILs modified snowman-like silica@PDVB/PS Janus particles can be used to catalyze the degradation of methyl orange.


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Scheme 1. Illustration of the synthesis of ionic liquids modified Janus particles and the anion exchange process. Snowman-like silica@PDVB/PS Janus particles were prepared via seeded emulsion polymerization of 3-methacryloxypropyltrimethoxysilane with PDVB/PS hollow spheres as seeds. The imidazole groups were then selectively introduced onto the silica surface. After quaternization of the imidazole groups with 1-chlorobutane, ILs modified Janus particles were obtained. The water-wettability of the particles was tuned by anion exchange of the ILs. Changing the anions of the incorporated ILs from Cl- to PF6- dramatically increased the hydrophobicity of the particles. Replacing Cl- with PW12O403- made the particles highly hydrophilic. PW12O403- anions are efficient catalysts for the catalytic degradation of organic dyes. Thus, the ILs-silica@PDVB/PS Janus particles with PW12O403- as anions became catalytically active solid surfactants. EXPERIMENTAL METHODS Materials. 3-Methacryloxypropyltrimethoxysilane (MPS) was purchased from Alfa Aesar. Dimethyl sulfoxide (DMSO), hydrogen peroxide (H2O2, 30 wt%), ammonia (NH3.H2O, 28 wt%), toluene, ethanol, paraffin (Tm: 52-54 °C), potassium peroxydisulfate (KPS) were purchased from Sinopharm

Chemical

Reagents.

Divinylbenzene

(DVB),

phosphotungstic

acid

(H3PW12O40.12H2O), triethoxy-3-(2-imidazolin-1-yl) propylsilane (IZPES), 1-chlorobutane


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(C4H9Cl), methyl orange, Fe3O4 (10 nm-50 nm), potassium hexafluorophosphate (KPF6), azobisisobutyronitrile (AIBN) and sodium dodecyl sulfate (SDS) were purchased from Aladdin. HP-433 polystyrene hollow spheres were purchased from Rohm & Haas products. The silica@PDVB/PS particles were fabricated as described in the recent literature.32 Synthesis of the imidazoline modified silica@PDVB/PS Janus particles. First, 100 mg of silica@PDVB/PS Janus particles and 50 μL of IZPES were added to 25 mL of ethanol under ultrasonication. The mixture was stirred under stirring at 70 oC for 8 h. After washing 3 times with ethanol and water, separately, the imidazoline modified silica@PDVB/PS Janus particles were obtained. Synthesis of the ILs-silica@PDVB/PS Janus particles with Cl- as anions. A 100 mg aliquot of imidazoline modified silica@PDVB/PS Janus particles and 1 mL of C4H9Cl were added to 40 mL of DMSO. The system was refluxed at 80 oC for 24 h. The ILs-silica@PDVB/PS Janus particles with Cl- as anions were obtained after centrifugation and washing 3 times with DMSO and water, separately. Synthesis of the ILs-silica@PDVB/PS Janus particles with PF6- and PW12O403- as anions. First, 100 mg of KPF6 and 20 mg of the ILs-silica@PDVB/PS Janus particles with Cl- as anions were added to 20 mL of water. The reaction was stirred under stirring at 50 oC for 3 h to initiate the anion exchange. The ILs-silica@PDVB/PS Janus particles with PF6- as anions were obtained after centrifugation and washing 3 times with water. Similarly, the ILs-silica@PDVB/PS Janus particles with PW12O403- as anions were obtained when H3PW12O40.12H2O was used. Degradation of methyl orange (MO). Degradation in toluene-in-water media: 20 mg of ILssilica@PDVB/PS Janus particles with PW12O403- as anions were added to the 2 mL of 50 ppm MO


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aqueous solution and 0.2 mL of toluene was added to the above dispersion to obtain an oil-inwater (O/W) emulsion. After 15μL of H2O2 was added to the emulsion, the mixture was performed at 25 oC for 4 h under magnetic stirring. Degradation in aqueous media: 20 mg of ILssilica@PDVB/PS Janus particles with PW12O403- as anions were added to the 2 mL of 50 ppm MO aqueous solution and 15 μL of H2O2 was added to the dispersion. For comparison, 20 mg of ILssilica@PDVB/PS Janus particles with PW12O403- as anions were added to the 2 mL of a 50 ppm MO aqueous solution without H2O2. The catalytic system was kept at 25 oC for 8 h. The concentration of the MO aqueous solution was monitored by UV-vis. We accurately prepared different concentrations of the MO aqueous solution with an MO content of 10~50 ppm. The MO standard working curve was drawn by taking the concentration of MO as the abscissa and the absorbance as the ordinate. The concentration of the MO aqueous solution during the reaction was calculated from the standard working curve. The degradation rate of MO can be calculated by the following formula: Degradation rate (%) = (Co -C) / Co × 100 %. Co is initial concentration and C is the concentration of MO in the catalytic experiment. The sample was tested every hour, and the concentration was detected by the above method. Recycling of the catalyst. ILs-silica@PDVB/PS Janus particles with PW12O403- as anions were recovered from the degradation reaction mixture by centrifugation and washing 3 times with DMF. Characterization. The morphology of the samples was observed by a scanning electron microscopy (SEM) (HITACHI S-4800 operating at 15 kV) and a transmission electron microscopy (TEM) (JEOL 1011 at 100 kV). The samples were prepared by vacuum sputtering with Pt for the SEM observation. The samples were prepared by dropping the suspension onto copper grids for the TEM observation. The degree of anion exchange was characterized by X-ray photoelectron


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spectroscopy (XPS) (ESCALAB MK II). Fourier transform infrared (FT-IR) spectrometry was performed by a Bruker EQUINOX 55 spectrometer with sample/KBr pressed pellets. Fluorescence microscopy images were observed using a fluorescence inverted microscope from Olympus IX71. The morphology of the emulsion was observed via confocal laser scanning microscopy (CLSM) by an FV1000 confocal fluorescence microscope with an IX-81 inverted base and a PMT detector from Olympus. The zeta potential was measured with a Malvern Nanosizer ZS-90. The UV-vis spectroscopy was measured via a UV spectrophotometer (TU1901). RESULTS AND DISCUSSION Snowman-like silica@PDVB/PS Janus particles were fabricated via emulsion polymerization with the crosslinked PDVB/PS hollow spheres as the seeds. Crosslinked PDVB/PS hollow spheres were fabricated via swelling emulsion polymerization of DVB in the presence of a commercially available PS hollow spheres. The size of the crosslinked PDVB/PS hollow spheres was approximately 430 nm in diameter, and the thickness of their shells was approximately 50 nm (Figure S1). Then, the PDVB/PS hollow spheres were used as seeds for the polymerization of MPS monomers to form the “snowman head” subunits. After the sol-gel process, a silica subunit was formed on the surface of the PDVB/PS hollow spheres to afford the snowman-like silica@PDVB/PS Janus particles (Figure 1a). IZPES was then employed to introduce an imidazole alkoxysilane layer onto the surface of the silica subunit by silanization. The morphology of the silica@PDVB/PS Janus particles did not change significantly after the introduction of the imidazole functional layer, as suggested by SEM (Figure 1b). Since the imidazole modified silica subunits of the Janus particles were positively charged, the negatively charged Fe3O4 particles could be used to label the imidazole moieties to verify the Janus nature of the particles. As revealed by SEM (Figure 1c), Fe3O4 particles were found only on the surface of the silica subunit. As a


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control experiment, the silica@PDVB/PS Janus particles without an imidazole functional layer could not be labeled with Fe3O4 particles (Figure S2). These results suggested that imidazole functional layers were successfully introduced onto the surfaces of the silica subunits of the Janus particle. After quaternization of imidazole groups with 1-chlorobutane, ILs modified Janus particles were obtained (Figure 1d), and a color change of the particles from white to yellow was observed. The anion of the ILs could be broadly tuned by simple anion exchange. The Cl- anions obtained during the quaternization of the imidazole could readily be easily exchanged with other anions such as PF6- and PW12O403- anions. Anion exchange did not cause considerable changes to the overall morphologies of the ILs modified Janus particles (Figure 1e, 1f).

Figure 1. Morphologies of the representative PDVB/PS Janus particles: (a) SEM and inset TEM images of the silica@PDVB/PS Janus particles; (b) SEM and inset TEM images of the imidazoline modified silica@PDVB/PS Janus particles; (c) TEM image of the imidazoline modified silica@PDVB/PS Janus particles after selective labelled with Fe3O4 particles onto the imidazoline subunit; (d) SEM and inset TEM images of ILs-silica@PDVB/PS Janus particles with Cl- as anions; (e) SEM and inset TEM images of ILs-silica@PDVB/PS Janus particles with PF6- as


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anions; (f) SEM and inset TEM images of ILs-silica@PDVB/PS Janus particles with PW12O403as anions. XPS was employed to monitor the element content changes45 (Figure 2, Table S1). The N1s peak at 409 eV was assigned to the imidazole, with a content of approximately 0.9 %, and the Cl content was 0.2 % after quaternization with C4H9Cl. From the XPS result, almost all the Cl- anions on the surface were exchanged with PF6- or PW12O403-. To ensure the completion of the anion exchange, excess KPF6 or H3PW12O40 was added in the experiment. The F content was found to be near 1.1 % and the W content was 2.0 % after anion exchange with PW12O403-, which indicated that the anion exchange process was successful. In addition, the group change was confirmed by measurement of the zeta potential (Figure S3). After the silica subunits were modified with imidazole groups, the zeta potential changed from -27.5 mV to +16.5 mV. Quaternization of the imidazole groups changed the zeta potential from +16.5 mV to +28.3 mV. When the Cl- anion was replaced with PF6-, the zeta potential of the Janus particles was 30.4 mV.


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Figure 2. Representative XPS spectra the Janus particles: (a) imidazoline modified silica@PDVB/PS Janus particles; (b) ILs-silica@PDVB/PS Janus particles with Cl- as anions; (c) ILs-silica@PDVB/PS Janus particles with PF6- as anions; (d) ILs-silica@PDVB/PS Janus particles with PW12O403- as anions. The water-wettability of the ILs modified silica subunits of the Janus particles were readily tuned by anion exchange. ILs bearing Cl- as the counterpart anions rendered the silica subunits highly hydrophilic and the PDVB/PS subunits highly hydrophobic, as demonstrated by the observation that the amphiphilic ILs modified silica@PDVB/PS Janus particles with the Clcounterpart anions were dispersed in water and oil (Figure S4a). After the Cl- anions of the ILs were replaced by PF6- through anion exchange, the ILs modified silica subunits became hydrophobic while the PDVB/PS subunits remained hydrophobic and the Janus particles lost their amphiphilicity, as determined by the observation that the ILs modified silica@PDVB/PS Janus particles with PF6- as the counterpart anions could only be dispersed in oil (Figure S4b). When the Cl- anions of the ILs were replaced with PW12O403-, the silica subunits of the Janus particles remained hydrophilic and the Janus particles were capable of dispersion in both water and oil. (Figure S4c). It was demonstrated that the ILs modified silica@PDVB/PS Janus particles could be employed as solid surfactants to stabilize emulsions (right images in Figure S4a-c). To gain insight into the oil-in-water emulsions process with our amphiphilic Janus particles as solid surfactants, CLMS was employed to probe the microstructure of the emulsion. The oil phase was stained with the coumarin 6, while the Janus particles were labeled with the fluorescein isothiocyanate (FITC) dye. The oil phase and the Janus particles appeared blue and green in the CLSM images, respectively. In the oil (blue)-in-water (black) emulsion structures, most of the Janus particles (green) located in the oil-water interfaces were clearly visible (Figure 3). It could


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be clearly seen that ILs-silica@PDVB/PS Janus particles with Cl- or PW12O403- as counterpart anions (green) served as solid surfactants and were located around the oil droplets (blue) (Figure 3a, 3c). In comparison, the oil droplets formed in the presence of the ILs-silica@PDVB/PS Janus particles with PF6- as anions appeared to be less regular and uniform in size and shape (Figure 3b) because the ILs-silica@PDVB/PS Janus particles with PF6- as anions lost the amphiphilicity. In addition, the emulsion of IL-silica@PDVB/PS Janus particles with Cl- and PW12O403- as anions as solid surfactants can remain stable for several months. To further verify the orientation and location of the Janus particles in the emulsion structures, oil-in-water emulsion experiments were carried out with different ILs-Janus particles as solid surfactants at 70 oC. After cooling, the orientation and location of the Janus particles attached to the solidified paraffin droplets was fixed. It was shown that the hydrophilic ILs-silica subunits of the Janus particles with Cl- or PW12O403- as anions were in the aqueous phase and the hydrophobic PDVB/PS subunits pointed towards the surfaces of the paraffin droplets (Figure 3d, 3f). In contrast, in the presence of ILs-Janus-particles with PF6- as anions, the orientation of the Janus particles was rather random (Figure 3e).


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Figure 3. Emulsion stabilized with different Janus particles as solid surfactants. The CLSM images of emulsions stabilized with different Janus particles: (a) ILs-silica@PDVB/PS Janus particles with Cl- as anions; (b) ILs-silica@PDVB/PS Janus particles with PF6- as anions; (c) ILssilica@PDVB/PS Janus particles with PW12O403- as anions. The SEM images of paraffin droplets stabilized with Janus particles: (d) ILs-silica@PDVB/PS Janus particles with Cl- as anions; (e) ILs-silica@PDVB/PS Janus particles with PF6- as anions; (f) ILs-silica@PDVB/PS Janus particles with PW12O403- as anions. Since PW12O403- anions are known to effectively catalyze the degradation of organic dyes in the presence of H2O2,48 ILs modified silica@PDVB/PS Janus particles with PW12O403- as anions were then evaluated as a potential catalyst in the degradation of organic dyes. MO was selected as a model dye in the evaluation. The Janus particles were dispersed in an aqueous solution containing 50 ppm MO, and toluene was added to the solution (left image in Figure 4a). After addition of H2O2, the degradation proceeded at room temperature with magnetic stirring. Eight hours later, it was observed that the bottom MO aqueous phase became colorless (right image in Figure 4a).


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Toluene-in-water emulsion formed with the ILs-silica@PDVB/PS Janus particles with PW12O403as anions as stabilizers (Figure 4b). During the degradation process, aliquots of the reaction mixture were removed at suitable intervals and analyzed via UV-vis to monitor the change in concentration of MO (curve 1 in Figure 4c). It was found that 91.1 wt% of MO was degraded in 8 h (curve 4 in Figure 4c, and curve 1 in Figure 4d). As a control, the degradation test was carried out in a homogeneous aqueous solution, and it was found that 84.6 wt% of MO was degraded under similar conditions (curve 3 in Figure 4c, and curve 2 in Figure 4d). In the absence of H2O2, only 74.5 wt% of MO was degraded in the homogeneous aqueous solution (curve 2 in Figure 4c, and curve 3 in Figure 4d). It is shown that formation of the emulsion enhances the degradation efficiency of MO. It was hypothesized that the degradation products of MO were hydrophobic compounds, such as monomethylaniline and N,N-dimethylaniline,46 and tended to migrate into the toluene phase from the aqueous phase, which could shift the balance towards the formation of the degradation products and enhance the MO degradation efficiency. To verify this hypothesis, the concentration changes of MO in the aqueous phase (Figure 4e) and the degradation products in the oil phase (Figure 4f) during degradation were monitored with UV-vis. It was found that as the degradation proceeded, the concentration of MO in the aqueous phase decreased, while the degradation products in oil phase increased as expected. The concentration of the MO aqueous solution was calculated from the MO standard working curve (Table S2, Figure S5). Furthermore, the ILs-silica@PDVB/PS Janus particles with PW12O403- as anions were recovered by centrifugation and washing with DMF and water for 3 times, separately. After 10 catalytic recycles, the degradation efficiency was slightly decreased from 91.1% to 88.4% (Figure S6).


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Figure 4. Degradation of water-soluble dye MO. (a) Immiscible toluene and water mixture (left) and the emulsion after degradation in the presence of 15 μL H2O2 (right); (b) Optical microscopy image of the toluene-in-water emulsion; (c) UV-vis spectra of 50 ppm MO aqueous solution (curve 1), MO concentration in the aqueous phase after degradation by ILs-silica@PDVB/PS Janus particles with PW12O403- as anions in water phase without H2O2 (curve 2), MO concentration in the aqueous phase after degradation by ILs-silica@PDVB/PS Janus particles with PW12O403- as anions in water phase with H2O2 (curve 3), MO concentration in the aqueous phase after degradation by ILs-silica@PDVB/PS Janus particles with PW12O403- as anions in the oil-in-water emulsion with H2O2 (curve 4); (d) Degradation kinetics of MO by emulsion system with H2O2 (curve 1), water phase with H2O2 (curve 2) and water phase without H2O2 (curve 3); (e) UV-vis spectra of MO in the aqueous phase after degradation by ILs-silica@PDVB/PS Janus particles with PW12O403- as anions in the oil-in-water emulsion with different degradation time: 0 h (curve 1); 1 h (curve 2); 2 h (curve 3); 3 h (curve 4); 4 h (curve 5). (f) UV-vis spectra of the hydrophobic


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degraded products in oil phase with different degradation time: 0 h (curve 1); 1 h (curve 2); 2 h (curve 3); 3 h (curve 4); 4 h (curve 5). CONCLUSION In summary, a facile and universal method was developed for the synthesis of functional snowman-like ILs modified silica@PDVB/PS Janus particles. The water-wettability of the ILs modified silica subunits was easily tuned between hydrophilic and hydrophobic by anion exchange of the ILs. By choosing the appropriate anions for the ILs, the ILs-Janus particles could become amphiphilic due to the distinctively different water-wettability of its two subunits and can be used as a solid surfactant for emulsion processes. In the emulsion system, the ILs-silica@PDVB/PS Janus particles with PW12O403- as anions degraded MO in the aqueous phase, and the degradation products transferred to the oil phase to improve the degradation efficiency. By simple anion exchange of the ILs, the functional anions were easily introduced into the Janus particles, which provided a facile access to the functionalization of the snowman-like Janus particles.

ASSOCIATED CONTENT Supporting Information. SEM and TEM images of the PDVB/PS hollow spheres; TEM images of the silica@PDVB/PS based Janus particles after selective labeling with Fe3O4 nanoparticles; XPS and Zeta potential measurement of ILs modified Janus particles. Polarizing microscope of emulsion with ILs modified silica@PDVB/PS based Janus particles. Catalyst recycling of the ILs-silica@PDVB/PS Janus particles with PW12O403- as anions.


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AUTHOR INFORMATION Corresponding Author * Email: [email protected]* Email: [email protected] Author Contributions Prof. Zhengping Liu and Prof. Fuxin Liang conceived and designed the experiments, Ruotong Zhao and Prof. Fuxin Liang performed the experiments, Ruotong Zhao and Dayin Sun analyzed the data and prepared for the figures, Dr. Liyan Huang and Xiaotian Yu co-wrote the paper. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by National Natural Science Foundation of China (51622308, 51233007).

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