pubs.acs.org/Langmuir © 2010 American Chemical Society
Light-Triggered Reversible Phase Transfer of Composite Colloids Ying Wu,†,‡ Chengliang Zhang,† Xiaozhong Qu,† Zhengping Liu,*,‡ and Zhenzhong Yang*,† †
State Key Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese Academy of Science, Beijing 100190, China, and ‡College of Chemistry, Beijing Normal University, Beijing 100875, China Received January 30, 2010. Revised Manuscript Received April 13, 2010
Composite colloids were prepared via grafting optically responsive spiropyran polymer brushes onto silica colloids. Similar to spiropyran, the polymer brushes undergo a reversible inversion from a hydrophobic state to a hydrophilic state upon irradiation with UV light (or vice versa by visible light). The composite colloids can thus reversibly transfer between oil and water phases, and this can be remotely triggered using light. At intermediate stages of irradiation, both hydrophobic and hydrophilic components coexist, resulting in the amphiphilic performance of the composite colloids. Such amphiphilic composite colloids can be used as particulate emulsifiers.
Introduction Colloids composed of inorganic or metallic cores are useful in many areas, including optoelectronics,1,2 biotechnology,3-5 diagnostics,6 colloidal crystals,7 sensors,8 etc. The properties and performances of the colloidal system can be altered or enhanced by introducing a corona onto the cores to form the composite colloids. Especially, those composite colloids possessing triggered phase transfer ability between oil and water according to varied physicochemical parameters9,10 have gained more and more attention recently because they will be closely related with drug delivery transportation from plasma into a cell cross the amphiphilic membrane. Research efforts have been directed toward the phase transfer of colloids from an organic phase into an aqueous phase (and vice versa) by different mechanisms including ligand *To whom correspondence should be addressed: e-mail yangzz@ iccas.ac.cn, Fax (þ86) 10-62559373 (Z.Y.); e-mail
[email protected], Tel (þ86) 10-58806896 (Z.L.).
(1) Beecroft, L. L.; Ober, C. K. Chem. Mater. 1997, 9, 1302. (2) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (3) Niemeyer, C. M. Angew. Chem., Int. Ed. 2001, 40, 4128. (4) Hogemann, D.; Josephson, L.; Weissleder, R.; Basilion, J. P. Bioconjugate Chem. 2000, 11, 941. (5) Vestal, C. R.; Zhang, Z. J. J. Am. Chem. Soc. 2002, 124, 14312. (6) Rosi, N. L.; Mirkin, C. A. Chem. Rev. 2005, 105, 1547. (7) Ohno, K.; Morinaga, T.; Takeno, S.; Tsujii, Y.; Fukuda, T. Macromolecules 2006, 39, 1245. (8) Zhu, M. Q.; Wang, L. Q.; Exarhos, G. J.; Li, A. D. Q. J. Am. Chem. Soc. 2004, 126, 2656. (9) Hirai, H.; Aizawa, H.; Shiozaki, H. Chem. Lett. 1992, 21, 1527. (10) Hirai, H.; Aizawa, H. J. Colloid Interface Sci. 1993, 161, 471. (11) Simard, J.; Briggs, C.; Boal, A. K.; Rotello, V. M. Chem. Commun. 2000, 1943. (12) Chan, W. C. W.; Nie, S. M. Science 1998, 281, 2016. (13) Gittins, D. I.; Caruso, F. Angew. Chem., Int. Ed. 2001, 40, 3001. (14) Zhang, T. R.; Ge, J. P.; Hu, Y. X.; Yin, Y. D. Nano Lett. 2007, 7, 3203. (15) Kim, M.; Chen, Y. F.; Liu, Y. C.; Peng, X. G. Adv. Mater. 2005, 17, 1429. (16) Kairdolf, B. A.; Smith, A. M.; Nie, S. M. J. Am. Chem. Soc. 2008, 130, 12866. (17) Fan, H.; Yang, K.; Boye, D. M.; Sigmon, T.; Malloy, K. J.; Xu, H.; Lopez, G. P.; Brinker, C. J. Science 2004, 304, 567. (18) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Eychmu1ller, A.; Weller, H. Nano Lett. 2002, 2, 803. (19) Chen, M.; Ding, W. H.; Kong, Y.; Diao, G. W. Langmuir 2008, 24, 3471. (20) Wang, Y.; Wong, J. F.; Teng, X. W.; Lin, X. Z.; Yang, H. Nano Lett. 2003, 3, 1555. (21) Liu, J.; Alvarez, J.; Ong, W.; Roman, E.; Kaifer, A. E. J. Am. Chem. Soc. 2001, 123, 11148. (22) Yao, H.; Momozawa, O.; Hamatani, T.; Kimura, K. Chem. Mater. 2001, 13, 4692. (23) Mayya, K. S.; Caruso, F. Langmuir 2003, 19, 6987.
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exchange reactions,11-18 host-guest binding,19-21 electrostatic22,23 and covalent24 interactions, addition of phase transfer agents25,26 or stabilizers,27 and encapsulation of the original nanocrystals into a polymer coating.28 It is important to note that these phase transfer mechanisms are all irreversible. In recent years, there has been a growing interest in the reversible phase transfer of colloids.29-34 For example, it was shown that thioglycolic acid (TGA)-capped CdTe nanoparticles can be transferred reversibly between water and chloroform, assisted by a phase transfer agent, primary amine 1-hexadecylamine (HDA), together with varying the pH of the aqueous phase.29 Ferrocene-coated CdSe/ZnS nanoparticles can transfer between chloroform and water upon the reversible formation/ release of inclusion complexes of β-cyclodextrin with the ferrocene units.32 Hybrid silica particles with a thermosensitive polymer shell can reversibly transfer between water and ethyl acetate by changing the temperature.34 However, these reversible phase transfer processes are based mainly on in situ physicochemical stimuli, which cannot be controlled remotely. Here, we propose a routine to remotely control the reversible phase transfer of colloids using light. Spiropyran (SP) is a photochromic compound, whose closed form is soluble in a wide range of organic solvents and has quite low water solubility. Upon UV irradiation (365 nm), the closed SP undergoes a photochemical cleavage of the C-O bond to form a zwitterionic merocyanine (MC). In addition, the color changes from colorless to blue/ purple. The MC form is hydrophilic and soluble in water.35 This isomerization process can be reversed by exposure to visible light (24) McMahon, J. M.; Emory, S. R. Langmuir 2007, 23, 1414. (25) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (26) Edwards, E. W.; Chanana, M.; Wang, D. Y. J. Phys. Chem. C 2008, 112, 15207. (27) Underwood, S.; Mulvaney, P. Langmuir 1994, 10, 3427. (28) Duan, H. W.; Kuang, M.; Wang, D. Y.; Kurth, D. G.; M€ohwald, H. Angew. Chem., Int. Ed. 2005, 44, 1717. (29) Jiang, H. B.; Jia, J. G. J. Mater. Chem. 2008, 18, 344. (30) Yang, Y.; Wang, W.; Li, J. R.; Mu, J.; Rong, H. L. J. Phys. Chem. B 2006, 110, 16867. (31) Zhao, S. Y.; Qiao, R.; Zhang, X. L.; Kang, Y. S. J. Phys. Chem. C 2007, 111, 7875. (32) Dorokhin, D.; Tomczak, N.; Han, M. Y.; Reinhoudt, D. N.; Velders, A. H.; Vancso, G. J. Nano Lett. 2009, 3, 661. (33) Chen, S. H.; Yao, H.; Kimura, K. Langmuir 2001, 17, 733. (34) Li, D. J.; Zhao, B. Langmuir 2007, 23, 2208. (35) Chibisov, A. K.; Gorner, H. J. Photochem. Photobiol. A 1997, 105, 261.
Published on Web 04/26/2010
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Figure 1. SEM and TEM images of (A) the CMPTS-modified silica colloids and (B) the PmSP/silica composite colloids. (C) XPS spectra of three representative samples: (a) silica colloids, (b) CMPTS-modified silica colloids, and (c) PmSP/silica composite colloids. (D) TEM image of PmSP/silica composite colloids with Ag nanoparticles absorbed by reduction in situ. Scheme 1. (A) Synthesis of the PmSP/Silica Composite Colloids by ATRP and Their Resulting Photochromic Behavior; (B) Reversible Polarity Change of the PmSP Polymer by UV (365 nm) and Visible Light (>450 nm) Irradiation
(>450 nm) at room temperature. Initiator-functionalized silica colloids are grafted with a photochromic spiropyran polymer brush by atom-transfer radical polymerization (ATRP). These polymer/silica composite colloids can transfer reversibly between hydrophobic and hydrophilic phases upon irradiation with light of different wavelengths.
Experimental Section Materials. N,N,N0 ,N0 ,N00 -Pentamethyldiethylenetriamine (PMDETA, 98%) and 4-(chloromethyl)phenyltrichlorosilane (CMPTS) were purchased from Aldrich and Lancaster, respectively. Copper(I) bromide (CuBr) was crystallized in glacial acetic acid and washed with ethanol. 10 -(2-Hydroxyethyl)-30 ,30 -dimethyl-6-nitrospiro(2H-1-benzopyran-2,20 -indoline) (SPOH) was a gift from Nankai University. Methacryloyl chloride was distilled under vacuum and stored at 4 °C prior to use. Dichloromethane and triethylamine were dried over CaH2 and distilled before use. Toluene was dried over potassium and distilled before use. DMF was dried over CaH2 and distilled under vacuum. Unless otherwise specified, all other reagents were used without further purification. Langmuir 2010, 26(12), 9442–9448
Figure 2. (A) Absorption spectra of the PmSP/silica composite colloids (PmSP content: 22.4 wt %) dispersed in DMF upon UV (365 nm, 4 W) irradiation for increasing amounts of time. (B) The equilibrium system in (A) irradiated with visible light (>450 nm) for increasing amounts of time. The concentration of the PmSP/ silica composite colloids was 0.1 mg/mL.
Synthesis of Methacrylated Spiropyran (mSP). Monomer mSP was synthesized by esterification of SPOH with methacryloyl chloride.36 SPOH (3 g, 8.5 mmol) and triethylamine (1.03 g, 10.23 mmol) were dissolved in dichloromethane (20 mL) and stirred at 0 °C for 10 min. Methacryloyl chloride (1.08 g, 10.23 mmol) in dichloromethane (10 mL) was added dropwise. After the mixture was stirred at room temperature for 24 h, the solvent was evaporated using a rotary evaporator. The residual product was dissolved in benzene and passed through a silica gel column. After (36) Chung, D. J.; Ito, Y.; Imanishi, Y. J. Appl. Polym. Sci. 1994, 51, 2026.
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Figure 3. Appearance of the PmSP/silica composite colloid dispersions in toluene: (A) before and (B-D) after UV irradiation for 1, 3, and 6 min, respectively; (E) after the dispersion as shown in (D) was irradiated with visible light for 6 min.
Figure 4. Light-triggered reversible transfer of the PmSP/silica composite colloids between oil and water phases: (A) the original mixture; (B-E) UV irradiation for 30 s, 1 min, 4 min, and 7 min, respectively; (F) after the system shown in (E) was agitated slightly; (G, H) after the system shown in (F) was irradiated with visible light for 7 min, before and after agitation, respectively. being dried under vacuum, the mSP 10 -(2-methacryloxyethyl)30 ,30 -dimethyl-6-nitrospiro(2H-1-benzopyran-2,20 -indoline) yellow powder was obtained with 15.7% yield. The chemical structure was confirmed by 1H NMR (400 MHz CDCl3) δ: 1.17 (s, 3H), 1.28 (s, 3H), 1.92 (s, 3H), 3.40-3.60 (m, 2H), 4.30 (t, 2H), 5.57 (s, 1H), 5.88 (d, J = 10.2 Hz, 1H), 6.07 (s, 1H), 6.71 (d, J = 7.6 Hz, 1H), 6.75 (d, J = 8.6 Hz, 1H), 6.89-6.92 (m, 2H), 7.09 (d, J = 7.0 Hz, 1H), 7.21 (d, J = 7.4 Hz, 1H), 8.00-8.03 (m, 2H).
Synthesis of Silica Modified with 4-Chloromethyl (-CH2Cl) Surface. Monodisperse silica colloids were prepared by the
St€ ober method37 and dried at 60 °C for 12 h. The dried colloids (1.2 g, about 400 nm in diameter) were added to 23 mL of anhydrous toluene in a flask. After the suspension was sonicated for 10 min, 2.5 g of CMPTS was added under sonication for 15 min to obtain a homogeneous dispersion. After refluxing the system at 120 °C for 24 h, the CMPTS-modified colloids were washed by centrifugation/sonication with toluene and ethanol.
Synthesis of a Polymer Brush from the Silica Colloid Surface. A polymer brush layer from mSP monomer was synthesized from the functionalized silica colloid surface by ATRP. 50.7 mg of CMPTS-modified silica colloids, 48.5 mg of mSP, and 15 μL of PMDETA were added into 1 mL of DMF (N,N-dimethylformamide). After the mixture was degassed through four freeze-evacuate-thaw cycles, 10 mg of CuBr was added. ATRP was carried out at 80 °C for 24 h in nitrogen. Then the PmSP/silica composite colloids were washed by centrifugation using anhydrous DMF, anhydrous toluene, and anhydrous THF (tetrahydrofuran) until the supernatant was free of mSP. Characterization. Solution 1H NMR spectra were recorded on a Bruker DMX400 spectrometer with CDCl3 as solvent and tetramethylsilane as an internal standard at room temperature. Scanning electron microscopy (SEM) images were obtained using a HITACHI S-4800 instrument operated at an accelerating voltage of 3 kV. The SEM samples were dried under ambient conditions and vacuum sputtered with Pt. Transmission electron (37) St€ober, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62.
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microscopy (TEM) images were obtained using a JEOL-1011 instrument operated at an accelerating voltage of 100 kV. The samples were dispersed into ethanol and dropped onto a carboncoated copper grid. X-ray photoelectron (XPS) spectra were obtained using VG ESCALab 220I-XL X-ray photoelectron spectrometer. FT-IR spectroscopy was performed using a Bruker EQUINOX 55 spectrometer with the samples/KBr pressed pellets. The polymer content of the composite colloids was measured by a thermogravimetric analyzer (Perkin-Elmer TGA 7) in air at a temperature scan rate of 10 °C/min. Gel permeation chromatography (GPC) was performed by a Waters 515 HPLC pump with a Waters 2414 refractive index detector. DMF with 1 g/L LiBr was used as an eluent at a flow rate of 1.0 mL/min at 50 °C. UV-vis absorption spectra were recorded on a UV/vis spectrophotometer (UV-1601PC, Shimadzu, Japan). The powder of PmSP/silica colloids was dispersed in DMF/toluene with the concentration of 0.1 mg/mL under vigorous ultrasonication, and the dispersion was exposed to visible light for 1 h to ensure that the ring-closed mSP group was dominant before UV irradiation. When the sample was alternately irradiated by UV and visible light, the two light sources were held at the same distance from the sample.
Results and Discussion Synthesis of the PmSP/Silica Composite Colloids. As illustrated in Scheme 1A, the PmSP/silica composite colloids were prepared by polymerization of the mSP monomer from the silica colloid surface, where monodisperse silica colloids of about 400 nm in diameter were used. The silica colloids were modified using the silane CMPTS to introduce -CH2Cl groups onto their surface first for initiating ATRP. The colloids remained well dispersed after modification (Figure 1A), and the presence of Cl was confirmed by XPS (Figure 1C-b). Subsequently, from the modified silica colloid surface, mSP was polymerized at 80 °C for 24 h (Figure 1B). The successful grafting of PmSP layer was verified by the XPS. As shown in Figure 1C-c, the Cl content Langmuir 2010, 26(12), 9442–9448
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decreased after polymerization for 24 h, and the new N 1s line appeared. FT-IR spectra provided more detailed information (Figure S1, Supporting Information). The characteristic bands at 2851 and 2921 cm-1 were assigned to the methylene groups and became stronger after polymerization. The presence of methyl groups gave rise to a resonance at 2958 cm-1. The band at 1460 cm-1 corresponded to benzene rings and also became stronger after polymerization. A typical stretching band of the aryl nitro group at 1340 cm-1 was observed. The new band at 1720 cm-1 indicated the presence of the carbonyl groups of PmSP. In order to further confirm the formation of PmSP layer, the PmSP/silica composite colloid dispersion in THF was irradiated under UV light (365 nm, 4 W) for 2 min, and then AgNO3 ethanol solution (10 mM) was added. After stirring for 30 min at room temperature, Agþ was reduced in situ to Ag nanoparticles. The nanoparticles were absorbed onto the colloid surfaces (Figure 1D). As a comparison, no Ag nanoparticles were observed on the CMPTS-modified silica colloid surfaces under the same conditions (Figure S2, Supporting Information). Polymer contents of the composite colloids were measured by TGA in air (Figure S3, Supporting Information). The mass of the silica colloids decreased gradually due to further condensation of silica and leveled off to a plateau at 700 °C (Figure S3a, Supporting Information). The weight gained from CMPTS was estimated to be about 2 wt % (Figure S3b, Supporting Information). The grafted PmSP contents onto the CMPTS-modified silica colloids were dependent on the mSP feeding amounts, for example 3.5 wt % (Figure S3c, Supporting Information), 5.4 wt % (Figure S3d, Supporting Information), 22.4 wt % (Figure S3e, Supporting Information), and 24.6 wt % (Figure S3f, Supporting Information) (by discounting CMPTS). As a result, at the same polymerization time, the polymer grafting contents were proportional to the monomer feeding amounts. The grafted PmSP chains were cleaved from the composite colloids by dissolving the silica using aqueous hydrofluoric acid (HF). Their molecular weights and distributions were characterized by GPC (Figure S4 and Table S1, Supporting Information). Since the PmSP contents had been measured by TGA, the graft density could be calculated (Table S1).38 Graft density dramatically increased with polymer content. At a high grafting density, the geometric restriction of the solid-state microenvironment of the composite colloids was stronger, which influenced the kinetics of isomerization. Photochromic Performance of the PmSP/Silica Composite Colloids. The photochromic reactions of the PmSP/silica composite colloids were demonstrated by UV/vis absorption spectroscopy, using DMF as a dispersion media since both mSP and MC forms were dispersible. The temporal evolutions of the absorption spectra of the composite colloid dispersions are shown in Figure 2. It is apparent from Figure 2A that a strong absorption band around 577 nm appeared upon UV irradiation (365 nm, 4 W), which can be ascribed to the isomerization of the mSP form to MC. Furthermore, at the initial stage, the absorption strength increased remarkably. With prolonged irradiation, the increment became slower. After 6 min, the absorption reached the saturation level, meaning the isomerization reached equilibrium. In addition, the isomerization kinetics were also related to the PmSP grafting density (Figure S5, Supporting Information). At a low grafting density (PmSP content: 5.4 wt %), the isomerization was faster in the initial stage and reached the equilibrium absorption in a shorter time. In contrast, at a high (38) Bartholome, C.; Beyou, E.; Lami, E. B.; Chaumont, P.; Lefebvre, F.; Zydowicz, N. Macromolecules 2005, 38, 1099.
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Figure 5. Absorption spectra of the PmSP/silica composite colloids dispersion (PmSP content: 22.4 wt %) in (A) Figure 4A-D, (B) Figure 4F, and (C) Figure 4H. The dashed line in (A) represents data of the bottom water layer in Figure 4A-D; the solid lines from down to top in (A) correspond to spectra of the top toluene layer in Figure 4A-D acquiring after UV irradiation light irradiation at the times indicated.
grafting density (PmSP content: 22.4 wt %), both the initial isomerization and approach to equilibrium were slower. The equilibrium absorption, however, became stronger with increased grafting density. As shown in Figure 2B, after the equilibrium system was irradiated with visible light (>450 nm), the MC form isomerized back to the mSP form, confirmed by the absorption peak weakening with irradiation time. After a long time, for example 15 min, the system became almost colorless, meaning the mSP form dominated. Similarly, at a low grafting density, the isomerization from the MC form to the mSP form was faster than at a high grafting density (Figure S5B, Supporting Information). The composite colloids of mSP form could be well dispersed in toluene, and the dilute dispersion was transparent (Figure 3A). The nearly colorless appearance indicated that the less polar mSP form dominated without aggregation (Figure S6A, Supporting Information). Upon UV irradiation for 1 min, the dispersion became purple (Figure 3B). With further UV irradiation (3 min), the color became deeper and some sedimentation appeared as indicated by the arrow (Figure 3C). This meant that large aggregates formed, which was verified by the SEM image of the dried dispersion (Figure S6B, Supporting Information). After UV irradiation for 6 min, the aggregates sank to the bottom (marked by the arrow) and the liquid phase became less colored (Figure 3D). This could be explained by the aggregation of the polar colloids in toluene. The aggregates could not be broken under sonication (Figure S6C, Supporting Information). When the sedimentation was irradiated using visible light for 6 min, the DOI: 10.1021/la100458j
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Figure 6. Appearance of (A) water-in-toluene emulsion (toluene/water = 1/1 vol/vol), stabilized with the PmSP/silica composite colloids without UV irradiation; and (B-D) the mixture after UV irradiation while stirring for varied lengths of time, from 15 s to 8 min, and the same systems after standing for 2 and 6 days.
purple color gradually became weaker, and the system became transparent (Figure 3E). Individual colloids rather than aggregates were observed under SEM (Figure S6D, Supporting Information), meaning that the nonpolar colloids could be dispersed in toluene. Similar dispersibility control of colloidal silica in nonpolar solvents by surface treatment with photochromic spirobenzopyran has been reported.39,40 In order to further understand the polarity change of the composite colloids, a similar process was conducted by forming a flat PmSP film on a silicon wafer. Before UV irradiation, the water contact angle was 92°, meaning the film was hydrophobic. After UV irradiation, the water contact angle was 78°, confirming the hydrophilic properties. With visible light irradiation, the water contact angle went back to 91°. This wettability change was reversible. Photo-Triggered Reversible Phase Transfer of PmSP/ Silica Composite Colloids. The light triggered reversible polarity change of the composite colloids can be used in remotely controlling phase transfer. At the beginning, a toluene dispersion containing PmSP/silica composite colloids (PmSP content: 22.4 wt %) and equal volume of water were mixed together at room temperature. As shown in Figure 4A, the mixture separated into two layers. The composite colloids were dispersed in the top oil layer. The top layer was transparent since the solid content was rather low. After the whole system was irradiated with UV light (39) Ueda, M.; Kudo, K.; Ichimura, K. J. Mater. Chem. 1995, 5, 1007. (40) Bell, N. S.; Piech, M. Langmuir 2006, 22, 1420.
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(365 nm) for 30 s, the toluene layer became slightly purple but remained transparent (Figure 4B). With the UV irradiation time extended to 1 min, the top layer gradually became purple and less transparent (Figure 4C). After the dispersion was irradiated for 4 min, the color became deeper and sedimentation appeared (Figure 4D). This was consistent with formation of aggregates among polar colloids in toluene. With prolonged UV irradiation time up to 7 min, large purple aggregates formed and were located at the toluene-water interface (Figure 4E). After slight agitation, the purple layer at the interface disappeared. Meanwhile, the bottom aqueous layer became pink while the top layer became colorless, meaning all the colloids were transferred into the aqueous phase (Figure 4F). When the system was irradiated with visible light while stirring, the aqueous phase became transparent. As seen in Figure 4G, some composite colloids started to exist at the interface. With prolonged visible light irradiation, the interface layer disappeared and the colloids were transferred into the top toluene phase with a light pink color (Figure 4H). After this transfer cycle, seen by comparing Figure 4H with Figure 4A, it was obvious that the color of toluene layer could not be matched, which was caused by the small amount of residual MC in Figure 4H. After UV irradiation followed by a slight agitation, the composite colloids could be transferred to the aqueous phase again. The reversible phase transfer has been demonstrated. Figure 5 shows the corresponding UV/vis absorption spectra of the PmSP/silica composite colloids dispersion in Figure 4. As shown in Figure 5A (compared to Figure 4A-D), a strong absorption band around 560 nm appeared upon UV (365 nm, Langmuir 2010, 26(12), 9442–9448
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Figure 7. Optical microscope images of (A) W/O emulsion stabilized with the PmSP/silica composite colloids without any irradiation (the top layer of Figure 6); (B, C) W/O emulsions after UV irradiation for 15 and 50 s under slight agitation (the top layer of Figure 6); (D-F) reversed O/W emulsions after irradiation by UV light for 1, 3, and 8 min, respectively, with slight agitation (the bottom layer of Figure 6).
4 W) irradiation of the suspension of PmSP/silica composite colloids in toluene, due to the isomerization of the mSP form to the MC form. With prolonged irradiation, the absorption strength increased gradually. From Figure 5B (compared to Figure 4F), when the composite colloids were transferred to the bottom water layer, there was a strong absorption band around 545 nm, and for the top toluene layer, the absorption peak disappeared. In Figure 5C (compared to Figure 4H), when the PmSP/silica composite colloids were transferred back to upper toluene layer by visible light irradiation, there was an obvious absorption around 560 nm left in the upper toluene suspension, which clearly showed that the MC form cannot change back to the initial mSP form completely after this cycle. Otherwise, by the intensity ratio of the absorption maximum at about 545 nm of the composite colloid in the bottom water layer shown in Figure 5B, C, a reversibility of ∼93% can be obtained in one transfer cycle. Furthermore, as shown in Figure 5, UV irradiation generated λmax = 560 nm in toluene but λmax = 545 nm in water, where a significant blue shift in λmax showed markedly more polar MC molecular microenvironment in water. Also, in toluene, in addition to main peak of λmax = 560 nm corresponding to the open MC in the trans form around the double bond,41 two shoulder peaks with λ around 520-550 and 600-640 nm were clearly observed (Figure 5A), which can be ascribed to the molecular H-stacks characterized by antiparallel alignment of alternate MC (41) Kalisky, Y.; Williams, D. J. Macromolecules 1984, 17, 292. (42) Goldburt, E.; Shvartsman, F.; Krongauz, V. Macromolecules 1984, 17, 1225.
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dipoles and the J-stacks with parallel orientation of dipoles, respectively.41-45 In water, however, the absorption curve was broad, and only one main peak with λmax = 545 nm was observed. Figure S7 shows the TEM images of the PmSP/silica composite colloids suspended in the bottom water layer (Figure 4F) and in the top toluene layer (Figure 4H). The composite colloids remained well dispersed regardless of transferring to the bottom water layer or the top toluene layer again. Amphiphilic Performance of the PmSP/Silica Composite Colloids. For the formation of a Pickering emulsion, the capillary forces at interfaces with high surface tensions are the critical factor.46-48 For small particles suspended at the interface, the capillary forces are substantially larger than gravity, and thus they can use their interfacial orientation to minimize the interfacial free energy.49 In the emulsion system, the segregated and ordered composite colloids at the oil-water interface can act as excellent emulsifiers, forming a film around dispersed drops and impeding their coalescence. As a result, because of the uniform size of about 400 nm, our PmSP/silica composite colloids can be used as (43) Goldburt, E.; Krongauz, V. Macromolecules 1986, 19, 246. (44) Eckhardt, H.; Bose, A.; Krongauz, V. A. Polymer 1987, 28, 1959. (45) Piech, M.; Bell, N. S. Macromolecules 2006, 39, 915. (46) Breen, T. L.; Tien, J.; Oliver, S. R. J.; Hadzic, T.; Whitesides, G. M. Science 1999, 284, 948. (47) Gracias, D. H.; Tien, J.; Breen, T. L.; Hsu, C.; Whitesides, G. M. Science 2000, 289, 1170. (48) Choi, I. S.; Bowden, N.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 1754. (49) Huck, W. T. S.; Tien, J.; Whitesides, G. M. J. Am. Chem. Soc. 1998, 120, 8267.
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particle emulsifiers for a Pickering emulsion. Most importantly, the light-responsive amphiphilicity of the PmSP grafted on the colloid surface endows the PmSP/silica composite colloids with this changeable emulsifying capacity. It was observed that the degree of isomerization of PmSP was a function of UV irradiation time, implying that both mSP and MC forms coexisted after a proper irradiation time. This gave rise to the amphiphilic property of the composite colloids. The composite colloids could be made amphiphilic by tuning the MC/mSP ratio with UV light (365 nm). The composite colloids could be well dispersed in both water and oil, making it possible for them to serve as particulate emulsifiers. In the presence of 2 wt % composite colloids, a stable water-in-oil (W/O) emulsion formed without UV irradiation (Figure 6A). Figure 7A shows the optical microscope image, where the W/O emulsion droplets with a spherical shape of 50-200 μm in diameter were obtained. The photographs displayed in Figure 6B illustrate the corresponding photomodulated emulsion inversion behavior. When the whole system was irradiated with UV light (365 nm) for 15 and 50 s, the W/O emulsion type remained, but from their optical microscope images (Figure 7B,C), the number of smaller drops increased as compared with Figure 7A, which meant that the emulsification capability of the colloids became stronger with the UV irradiation. However, after the emulsion was irradiated over 1 min, the emulsions were the O/W. Shown in Figure 7D-F are the corresponding optical microscope images of the obtained O/W emulsions using UV irradiation times of 1, 3, and 8 min, respectively, where the size of O/W emulsion droplets decreased to 10-100 μm. For the longer irradiation times (3 and 8 min), some drops became nonspherical, implying that the emulsification capability of the colloids became poor under these conditions. In an emulsion containing solid material, one of the liquids will probably wet the solid more than the other liquid, resulting in the more poorly wetting liquid becoming the dispersed phase. This means that hydrophilic particles tend to stabilize O/W emulsions whereas hydrophobic particles tend to stabilize W/O emulsions.50,51 At the beginning and after the short UV irradiation time, the PmSP/silica composite colloids were hydrophobic, (50) Binks, B. P.; Lumsdon, S. O. Langmuir 2000, 16, 2539. (51) Binks, B. P.; Rodrigues, J. A. Angew. Chem., Int. Ed. 2005, 44, 441. (52) Binks, B. P.; Rodrigues, J. A. Angew. Chem., Int. Ed. 2007, 46, 5389.
9448 DOI: 10.1021/la100458j
Wu et al.
so the emulsions were W/O. After prolonged UV irradiation time, the composite colloids became hydrophilic, and the O/W emulsions were obtained. Recent work52 has shown that flocculated particles adsorbed on liquid drop interfaces resulted in nonspherical emulsion drops; conversely, the emulsion drops were spherical and discrete when the particles were unaggregated. Before and after brief UV irradiation, the PmSP/silica composite colloids could be dispersed well in toluene, and the W/O drops were spherical. However, these composite colloids formed aggregates upon photoswitching in toluene, and nonspherical O/W drops were obtained. The stabilities of these emulsions in the dark at ambient temperature were studied over time. Parts C and D of Figure 6 show the appearance of the emulsions after standing for 2 days and 6 days, respectively. After 6 days, the W/O emulsions were relatively stable without remarkable coalescence. But for the O/W emulsions, the bottom emulsions became remarkably delaminated, and the color changed from deep pink to less pink. This was consistent with the isomerization back from MC to mSP, which decreased the emulsification capability of the colloids.
Conclusions We have demonstrated the light-triggered phase transfer of composite colloids by grafting a photoresponsive PmSP brush onto silica colloid surfaces. This process is reversible and can be remotely controlled. The composite colloids experience a reversible polarity inversion upon light irradiation. At intermediate irradiation times, both hydrophobic and hydrophilic components coexist, making the composite colloids amphiphilic and capable of serving as particulate emulsifiers. Acknowledgment. We thank financial support by the NSF of China (50733004, 20720102041, and 20128004), Chinese Academy of Sciences, and China Ministry of Science and Technology (KJCX2-YW-H20 and 2006CB605300). We also thank Dr. M. Lackey at Department of Polymer Science and Engineering, University of Massachusetts Amherst, for language polishing. Supporting Information Available: Figures S1-7 and Table S1. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2010, 26(12), 9442–9448
