Hierarchical Assembly of Amphiphilic POSS-Cyclodextrin Molecules

Aug 14, 2014 - Triple stimuli-responsive supramolecular assemblies based on host-guest inclusion complexation between β-cyclodextrin and azobenzene...
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Hierarchical Assembly of Amphiphilic POSS-Cyclodextrin Molecules and Azobenzene End-Capped Polymers Jinze Li,†,§ Zheng Zhou,†,§ Li Ma,†,§ Guangxin Chen,*,†,‡ and Qifang Li*,†,‡,§ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Key Laboratory on Preparation and Processing of Novel Polymer Materials of Beijing, Beijing University of Chemical Technology, Beijing 100029, China § College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China ‡

S Supporting Information *

ABSTRACT: Stimuli-responsive polymers have been widely studied because of their potential use in nanocarriers and nanocontainers. In this study, a smart multistimuli responsive system was prepared through a light controlled supramolecular assembly. Mono cyclodextrin substituted isobutyl polyhedral oligomeric silsesquioxane (mCPOSS), an amphiphilic molecule, was synthesized by copper(I)-catalyzed azide−alkyne cycloaddition (CuAAC), while azobenzene end-capped poly(ethylene glycol)-b-poly(2-(dimethylamino)ethyl methacrylate) copolymer (PEG-b-PDMAEMA-azo, PPA), a biocompatible pH/temperature-responsive macromolecule, was synthesized by atom transfer radical polymerization and CuAAC. The selfassembly process of PPA and mCPOSS in aqueous solution was as followed: first, mCPOSS self-assembled into a nanosphere in aqueous solution because of its amphipathic property; then, the trans-azo end groups of the PPA interacted with the cyclodextrin cavities on the nanosphere, and complex micelles were formed by the supramolecular assembly between PPA and mCPOSS. In addition, the morphology of the micelles could be adjusted by the ratio of PPA and mCPOSS, the formation and the dissociation of the micelles could be controlled by visible and ultraviolet light, and the size of the micelles could be tuned by pH.

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INTRODUCTION Supramolecular chemistry based on noncovalent bonds has become a powerful means to construct amphiphiles in a simple, dynamic way. The complex formed between cyclodextrin (CD) and azobenzene (AZO) is a typical supramolecular assembly system;1−3 the formation and the dissociation of the complex can be controlled by light because of the reversible isomerization of azobenzene under UV and visible light.4,5 Thus, this system has been widely used in building molecular shuttles, motors, machines, surfactants, ion channels, hydrogels, and so on.6−13 Stimuli-responsive polymers have attracted great interest recently and have potential biological applications.14−17 Multistimuli responsive polymers that are sensitive to two or more stimuli are materials of emerging interest. These materials commonly adopt a variety of morphologies that could be controlled by stimuli. Moreover, multifaceted responsiveness could greatly enhance the versatility of these materials in a variety of applications, such as in mimicking biological processes18,19 or enriching the molecular toolbox.20 However, to our knowledge, the reported multistimuli responsive polymers are generally synthesized by the covalent attachment of polymeric segments. Polyhedral oligomeric silsesquioxane (POSS) is the smallest well-defined cage-like silica nanoparticle, and it contains a steady and rigid cubic silica core surrounded by eight tunable © 2014 American Chemical Society

substituent groups. The substituent groups provide good reactivity to POSS, and allow it to function as self-assembly nanobuilding blocks that can form function materials. On the other hand, POSS is a nanoparticle that is nontoxic, biocompatible, chemically inert, mechanically stable, and thus usable as a biomedical material. The self-assembly of POSSbased amphiphilic polymers in solution has been widely studied. Hydrophobic POSS molecules have strong aggregating ability in water. Even with one POSS molecule at the end of the polymer chain,21 it still allows effective control of the motion of the chain and induces self-assembled molecular aggregates with controlled nanometer size.22 The position of POSS in a polymer can be at one side,21,22 two sides,23 or in the center.24 Meanwhile, POSS can be either a single nanoparticle25 or a polymer segment26 in the amphiphilic polymer. The POSSbased amphiphilic polymers can form micelles, complex micelles, vesicles, and other organized shapes.27−30 Zhou and co-workers31 reported a supramolecular self-assembly method rather than covalent attachment to build POSS-based amphiphiles. However, the POSS-based amphiphilic assembly could not respond to external stimuli. Received: May 27, 2014 Revised: July 25, 2014 Published: August 14, 2014 5739

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Scheme 1. Synthetic Route of mCPOSS

In this study, we first connected the hydrophilic CD to the hydrophobic POSS to prepare an amphiphilic molecule, mono cyclodextrin substituted isobutyl polyhedral oligomeric silsesquioxane (mCPOSS). We subsequently designed a stimuliresponsive polymer, azobenzene end-capped poly(ethylene glycol)-b-poly(2-(dimethylamino)ethyl methacrylate) copolymer (PEG-b-PDMAEMA-azo, PPA). PEG32,33 is a hydrophilic, biocompatible polymer, and PDMAEMA34−36 is a pH/ temperature-responsive polymer. By combining the advantages of PEG and PDMAEMA, we obtained a water-soluble, biocompatible stimuli-responsive smart polymer. The twocomponent system, PPA and mCPOSS, could construct aggregates by supramolecular assembly between CD and AZO. The size of these aggregates could be tuned by pH, while a photoswitchable azobenzene group induced the formation or dissociation of the aggregates.

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carbodiimide hydrochloride (EDCI; 97%, J&K Chemical Ltd.) were all used without further purification. Tetrahydrofuran (THF) was distilled from a purple sodium ketyl solution. Dimethylformamide (DMF) was dried over calcium hydride (CaH2) and distilled under reduced pressure immediately before use. All other solvents were purchased from Beijing Chemical Reagent Factory and used without further purification. Synthesis of Mono Cyclodextrin Substituted Isobutyl Polyhedral Oligomeric Silsesquioxane by Copper(I)-Catalyzed Azide−Alkyne Cycloaddition (CuAAC). The synthetic route is outlined in Scheme 1. Mono-6-deoxy-6-azido-β-cyclodextrin (β-CDN3) provided an azide group and N-propiolamidopropylisobutyl polyhedral oligomeric silsesquioxane (mPPOSS) provided an alkyne group. β-CD-N3 was prepared from mono-(6-O-(p-tolylsulfonyl))-β-cyclodextrin and sodium azide (see the Supporting Information). mPPOSS was prepared from mAPOSS and propiolic acid. The procedure was as followed: In a three-neck flask protected by a N2 atmosphere, the mAPOSS (0.87 g, 1.0 mmol), EDCI (0.23 g, 1.2 mmol), and DMPA (0.024 g, 0.2 mmol) were mixed with 10 mL of THF. Propiolic acid (0.084 g, 1.2 mmol) with 5 mL of THF was then added dropwise. The reaction mixture was left to stir at room temperature for 24 h, and the solvent was thereafter evaporated. The residue was dispersed in 30 mL of water and extracted with 20 mL of ethyl acetate three times. The combined organic layer was concentrated to 2 mL, which was subsequently separated with petroleum ether/ethyl acetate (v/v = 2/1) using silica gel chromatography. The final product was dried in a vacuum oven at 60 °C, yielding a white solid (0.80 g, yield: 86%). IR (KBr, cm−1): 3308 (CCH), 2950 (CH), 2113 (CC), 1106 (SiOSi). 1 H NMR (CDCl3, ppm): 5.88 (m, 1H, NH), 3.22−3.37 (q, 2H, NCH2), 2.76 (s, 1H, CCH), 1.77−1.95 (m, 7H, CH),

EXPERIMENTAL SECTION

Materials. AminopropylIsobutyl POSS (mAPOSS; Hybrid Plastics) was obtained. 2-(Dimethylamino)ethyl methacrylate (DMAEMA; 98%, Alfa Aesar) was dried over calcium hydride (CaH2) and distilled under reduced pressure immediately before use. Propiolic acid (95%, Aldrich), monomethoxy poly ethylene glycol (MPEG; Mn = 750, PDI ≤ 1.1, Aldrich), sodium azide (NaN3; 99%, Alfa Aesar), 2bromoisobutyryl bromide (BIBB; 98%, Alfa Aesar), copper(I) chloride (CuCl; 99.999%, Alfa Aesar), cuprous bromide (CuBr; 99.99%, Alfa Aesar), N,N,N,N′,N′-pentamethyldiethylenetriamine (PMDETA; 97%, Alfa Aesar), 1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA; 97%, J&K Chemical Ltd.), 4-(dimethylamino)pyridine (DMAP; 99%, J&K Chemical Ltd.), and N-(3-(dimethylamino)propyl)-N′-ethyl5740

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Scheme 2. Synthetic Route of PEG-b-PDMAEMA-AZO

1.58−1.70 (m, 2H, CH2), 0.89−1.00 (m, 42H, CH3), 0.55−0.65 (m, 16H, SiCH2). 13C NMR (CDCl3, ppm): 152.00 (CO), 77.47 (COCCH), 72.83 (COCCH), 42.02 (CH2NH), 25.48− 25.95 (SiCH 2 CH(CH 3 ) 2 ), 23.67−24.04 (SiCH 2  CH(CH3)2), 22.68 (SiCH2CH2), 22.30−22.60 (SiCH2 CH(CH3)2), 9.36 (SiCH2CH2). Synthesis of mCPOSS. To a Schlenk flask equipped with a magnetic stir bar, β-CD-N3 (1.2 g, 1.0 mmol), mPPOSS (1.1 g, 1.2 mmol), and CuBr (0.14 g, 1.1 mmol) were added. After one brief freeze−pump− thaw cycle, PMDETA (0.25 mL, 1.2 mmol), DMF (8 mL), and THF (2 mL) were injected under an atmosphere of nitrogen. The flask was carefully degassed by three freeze−pump−thaw cycles, sealed under a vacuum, and then left to stir at room temperature for 24 h. The reaction mixture was then exposed to air and precipitated into an excess of acetone/H2O (2:1 v/v). The final product was recovered by suction filtration and dried in a vacuum oven, yielding a white solid (1.8 g, yield: 87%). 1H NMR (DMSOD6, ppm): 8.44−8.66 (m, 1H, NH), 8.45 (s, 1H, CCHN), 5.55−6.02 (m, 14H, OH-2,3), 5.00−5.15 (m, 1H, OH-6), 4.68−4.98 (m, 7H, H-1), 4.43−4.61 (m, 5H, OH-6), 4.27−4.38 (t, 1H, H-6′), 4.01−4.15 (t, 1H, H-6′), 3.49− 3.92 (m, 28H, H-3, 5, 6), 3.08−3.49 (m, 16H, H-2, 4, NCH2, overlaps with HOD), 1.71−1.92 (m, 7H, CH), 1.48−1.68 (m, 2H, CH2), 0.81−1.08 (m, 42H, CH3), 0.48−0.70 (t, 16H, SiCH2). 13C NMR (DMSOD6, ppm): 159.80 (CO), 142.70 (Cquat triazole), 127.18 (CH triazole), 101.07−102.40 (C1), 80.76−83.23 (C4), 69.23−73.56 (C2, C3, C5), 66.99 (C6′), 58.76−60.47 (C6), 40.40 (CH2NH), 25.15−25.47 (SiCH2CH(CH3)2), 23.30−23.55 (SiCH2CH(CH3)2), 22.51 (SiCH2CH2), 21.72−22.06 (SiCH2CH(CH3)2), 8.99 (SiCH2CH2). 29Si NMR (DMSOD6, ppm): −66.95, −67.40, −67.71. ESI-MS calculated for C76H140N4O47Si8, 2086.61, [M + H]+: 2087.2, [M + Na]+: 2109.2. Self-Assembly of mCPOSS in Aqueous Solution. A typical preparation procedure was as follows: mCPOSS (4 mg) was dissolved in 10 mL of dimethyl sulfoxide and added dropwise in 70 mL of constantly strring deionized water. After 2 h, the mixture was dialyzed against deionized water (MWCO = 1000) for 3 days. Then, the final solution was adjusted to 100 mL, and the concentration was 2 × 10−5 M. Synthesis of Poly(ethylene glycol)-b-Poly(2(dimethylamino)ethyl methacrylate) with Azobenzene Ends (PEG-b-PDMAEMA-AZO, PPA) by Atom Transfer Radical Polymerization (ATRP) and CuAAC. The synthetic route is

outlined in Scheme 2. ATRP initiator PEG-Br was prepared by a reaction between monomethoxy poly ethylene glycol (MPEG) and BIBB. With this PEG-Br macroinitiator, the PEG-PDMAEMA-Br polymer was prepared by ATRP. The Br end group of the polymer was then substituted by an azide group. Finally, the targeted product PPA was prepared by CuAAC between the PEG-PDMAEMA-N3 polymer and 1-phenyl-2-(4-(prop-2-ynyloxy)phenyl)diazene (see the Supporting Information). Synthesis of PEG-Br. A typical reaction procedure was as follows: THF (10 mL) containing BIBB (0.28 g, 1.2 mmol) was added to cold (0 °C) triethylamine (0.15 g, 1.5 mmol) containing MPEG (5.0 g, 1.0 mmol) dissolved in THF. The mixture was magnetically stirred for 1 h at 0 °C and then for another 10 h at room temperature. After filtration, the filtrate was concentrated by a rotary evaporator. The residue was then dissolved in 30 mL of CH2Cl2 and washed using 20 mL of alkaline water, 20 mL of acidic water, and 20 mL of brine in sequence. After that, the organic layer was concentrated to obtain the solid. The final product was dried in a vacuum oven, yielding a white solid (4.2 g, yield: 80%). Synthesis of PEG-PDMAEMA-Br. PEG-PDMAEMA-Br was synthesized by atom transfer radical polymerization (ATRP) using PEG-Br as an initiator. A typical polymerization procedure was as follows: a 50 mL flask, connected to a standard Schlenk line system with highly pure nitrogen, was charged with PEG-Br (0.55 g, 0.11 mmol) and CuCl (0.012 g, 0.12 mmol). Three freeze−pump−thaw nitrogen cycles were performed to remove oxygen from the system. Then, DMAEMA (5.0 g, 32 mmol), HMTETA (0.029 g, 0.13 mmol), and THF (5 mL) were injected into the flask with a syringe, followed by three freeze−pump− thaw cycles to remove oxygen from the solution. The flask was sealed under a nitrogen atmosphere and kept in an oil bath at 60 °C. The polymerization was carried out for 6 h and terminated by letting oxygen into the system. The reaction mixture was then allowed to pass through a basic alumina column; the filtrate was concentrated in a rotary evaporator and poured into cold n-hexane to precipitate the polymer. Finally, the polymer was dried in a vacuum oven for 12 h (4.7 g, yield: 84%). Synthesis of PEG-PDMAEMA-N3. A typical reaction procedure was as follows: PEG-PDMAEMA-Br (4.7 g, 0.078 mmol) was suspended in 10 mL of DMF. After heating to 60 °C, NaN3 (0.025 g, 0.39 mmol) was added. The reaction mixture was stirred at 60 °C for 72 h. The reaction mixture was then allowed to pass through a basic alumina column; the filtrate was concentrated in a rotary evaporator and 5741

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poured into cold n-hexane to precipitate the polymer. Finally, the polymer was dried in a vacuum oven for 12 h (4.3 g, yield: 92%). Synthesis of PEG-b-PDMAEMA-AZO. A typical polymerization procedure was as follows: To a Schlenk flask with a magnetic stir bar, PEG-PDMAEMA-N3 (3.0 g, 0.05 mmol), 1-phenyl-2-(4-(prop-2ynyloxy)phenyl)diazene (0.0236 g, 0.10 mmol), and CuBr (0.0079 g, 0.055 mmol) were added. After one brief freeze−pump−thaw cycle, PMDETA (13 μL, 0.060 mmol) and DMF (5 mL) were injected under a nitrogen atmosphere. The flask was carefully degassed using three freeze−pump−thaw cycles and sealed under a vacuum, and the mixture was then stirred at room temperature for 24 h. The reaction mixture was then allowed to pass through a basic alumina column; the filtrate was concentrated in a rotary evaporator and poured into cold nhexane to precipitate the polymer. The final product was recovered by suction filtration and dried in a vacuum oven, yielding a yellow solid (2.7 g, yield: 88%). Self-Assembly of PPA in Aqueous Solution. All samples were obtained by directly dissolving the polymers in deionized water. The solutions were left to stir for at least for 24 h to ensure that the system reached equilibrium. Self-Assembly of PPA and mCPOSS in Aqueous Solution. All the samples were obtained by directly mixing mCPOSS aqueous solutions and PPA aqueous solutions. The mixtures were left to stir at least for 24 h to ensure the formation of supramolecular complexes. Characterization. 1H NMR, 13C NMR, and 29Si NMR measurements were carried out on a Bruker AV400 spectrometer at room temperature with CDCl3 or DMSO−D6 as a solvent. FT-IR measurements were performed on a Bruker Tensor-27 Fourier transform infrared spectrometer using the KBr disk method. The relative molecular weight and molecular weight distribution of polymers were determined with a Waters 515-2410 gel permeation chromatography (GPC) instrument equipped with a Styragel HT6EHT5-HT3 chromotographic column following a guard column and a differential refractive index detector. The sample solution was filtered with a 0.45 μm syringe filter prior to injection. The measurements were carried out at 35 °C, and THF was used as the eluent at a flow rate of 1.0 mL/min. The system was calibrated with polystyrene standards. Ultrahigh performance liquid chromatography-mass spectrometry (UPLC-MS) was performed on an ACQUITY UltraPerformance LC (Waters, ACQUITY UPLC) equipped with an ACQUITY TQ detector. A constant spray voltage of 3.00 kV was used. The critical micellization concentration (cmc) was investigated using the fluorescence probe method. Fluorescence spectra were recorded on a Varian Cary Eclipse fluorescence spectrofluorometer, and pyrene was used as a hydrophobic fluorescent probe. A predetermined amount of pyrene in acetone was transferred into separate vials, and the acetone was allowed to evaporate. A series of aqueous PPA at different concentrations were added to the vials; the final concentration of pyrene was 6 × 10−7 M in each vial. These aqueous solutions were allowed to equilibrate overnight at ambient temperature. Excitation was carried out at 335 nm, and emission spectra were recorded ranging from 350 to 500 nm. The excitation and emission bandwidths were set at 5 and 10 nm, respectively. The ratios of the peak intensities at 384 and 373 nm (I384/I373) of the excitation spectra were analyzed as a function of polymer concentration. UV−vis absorption measurements were carried out on a Shimadzu 1800 UV− visible spectrophotometer. High-resolution transmission electron microscopy (HRTEM) images were taken on a JEM 3010 instrument operating at an acceleration voltage of 200 kV. A drop of 10−5 M sample aqueous solution was directly dropped onto a copper grid (200 mesh) coated with carbon film, and the sample was allowed to dry at room temperature. Dynamic light scattering (DLS) measurements were carried out on a Brookhaven Nanoparticle size Analyzer 90Plus. The measurements were performed at a fixed scattering angle of 90° at room temperature. All samples had a concentration of 10−5 M and were filtered using a 0.45 μm Millipore filter before experimentation. In order to investigate photoresponse, UV irradiation was applied by a UV lamp (8 W) with an emission maximum at 365 nm for the DLS and TEM samples.

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RESULTS AND DISCUSSION Synthesis of mCPOSS. The mPPOSS was prepared by DCC coupling between mAPOSS and propiolic acid, and the final product mCPOSS was prepared by CuAAC between βCD-N3 and mPPOSS. mPPOSS was characterized by 1H and 13 C NMR (Figures 1b and 2) and FT-IR spectroscopy (Figure

Figure 1. 1H NMR spectrum of (a) β-CD-N3, (b) mPPOSS, and (c) mCPOSS.

Figure 2. 13C NMR spectrum of mPPOSS.

3c). The appearance of the peak at 2.76 ppm in 1H NMR, the peak at 77.47 and 72.83 ppm in 13C NMR, and the peak at 2113 and 3308 cm−1 in FT-IR indicated that an alkynyl group substituted POSS was successfully prepared. The mCPOSS was characterized by 1H, 13C, and 29Si NMR (Figures 1c, 4, and 5), FT-IR spectroscopy (Figure 3d), and ESI-MS. Upon the reaction of PEG-PDMAEMA-N3 and mCPOSS, the peak at 2.76 ppm shifted to 8.45 ppm in 1H NMR, the peaks at 77.47 and 72.83 ppm shifted to 142.70 and 127.18 ppm in 13C NMR, and peaks around 2100 cm−1 in FT-IR disappeared; these indicated that the azide and the alkyne formed a five-membered heterocycle, connecting CD to POSS. Three different Si signals were shown in 29Si NMR, which indicated that mCPOSS was a monosubstituted POSS. The molecular weight of mCPOSS was 5742

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Information) fitted the calculated value, proving the successful preparation of mCPOSS. Synthesis of the Polymer PPA. The structures of PEG-Br and PEG-PDMAEMA-Br were characterized by 1H NMR (Figure 6a and b), PEG-PDMAEMA-N3 was characterized by

Figure 3. FT-IR spectra of (a) CD-N3, (b) mAPOSS, (c) mPPOSS, and (d) mCPOSS.

Figure 6. 1H NMR spectrum of (a) PEG-Br, (b) PEG-PDMAEMA-Br, and (c) PPA.

Figure 4. 13C NMR spectrum of mCPOSS.

Figure 7. FT-IR spectra of (a) PEG-PDMAEMA-N3 and (b) PPA.

FT-IR (Figure 7a), and PPA was characterized by 1H NMR and FT-IR (Figures 6c and 7b). The ratio of the peak at 1.95 ppm and the peak at 3.64 ppm in Figure 6a was 6:64, which indicated the successful preparation of PEG-Br macroinitiator. The peaks at 0.88, 1.05, 1.81, 1.90, 2.27, 2.55, and 4.05 ppm in Figure 6b were the H signals of PDMAEMA and the peak at 3.64 ppm was the H signal of PEG, which indicated that the PEG-Br macroinitiator initiated the polymerization of PDMAEMA. The appearance of an azide characteristic peak around 2100 cm−1 in FT-IR (Figure 7a) indicated the successful azido reaction between PEG-PDMAEMA-Br and NaN 3. The disappearance of the same peak at 2100 cm−1 (Figure 7b) and the new peaks between 7 and 8 ppm (Figure 6c), corresponding to the H signals of azobenzene, indicated that the goal product PPA was successfully obtained.

Figure 5. 29Si NMR spectrum of mCPOSS.

calculated using a molecular formula. The molecular ion peak of [M + H]+ and [M + Na]+ in ESI-MS (Figure S1, Supporting 5743

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determine the cmc value of PPA. Pyrene is a common probe used to monitor micropolarity, since the intensity ratio of the third to the first vibronic peaks (I3/I1) in the pyrene fluorescence spectrum is sensitive to polarity; the I3/I1 ratio becomes larger in less polar media.37 The intensity ratio I3/I1 versus log C (C is the concentration of PPA aqueous solution) is shown in Figure 9, and the cmc value of PPA in aqueous

The number-average molecular weight (Mn) of PEGPDMAEMA and PPA was characterized using GPC (Figure S2, Supporting Information) and 1H NMR (Figure 6b and c) to prove whether or not there was successful polymerization. GPC results showed that the Mn of PPA was 7300 g/mol with a PDI value of 1.18. The molar ratio of PEG:PDMAEMA:AZO was calculated by 1H NMR (Figure S4, Supporting Information). When we regard PPA as PEGm-PDMAEMAn-AZOo, the theoretic integration of peaks a2, b5, and c3 is 4m, 6n, and 4o. The measurement integration of peaks a2, b5, and c3 is 68, 388, and 4, which resulted in a ratio of m:n:o of 17:65:1. The actual Mn, which could be calculated by the molar ratio of PEG:PDMAEMA:AZO, was 11000 g/mol. Every PPA product was shown to have one azo end rather than a bromo or azide substituent. Excess azobenzene was added to make sure that the azide group was completely consumed. FT-IR showed the disappearance of the peak at 2100 cm−1, and 1H NMR showed that the mole ratio of the glycol unit and azo end was 17:1. Both the synthesis method and measurement results indicated that the product PPA had a 100% degree of end-functionalization. Self-Assembly of PPA and mCPOSS in Aqueous Solution. mCPOSS could form nanospheres in aqueous media because of its amphiphilic structure, comprising a hydrophilic CD and a hydrophobic POSS. The morphology and the size of the mCPOSS nanosphere were characterized by HRTEM. The result in Figure 8 revealed that mCPOSS aggregated to form 10 nm nanospheres.

Figure 9. Intensity ratio (I384/I373) as a function of concentration of PPA.

solution was determined to be approximately 0.153 mg/mL. To avoid self-aggregation, the concentration of PPA was maintained at 0.12 mg/mL (10−5 M). PPA and mCPOSS self-assembled into micelles or complex micelles in water, and their size and morphology could be adjusted by the feed ratio of the PPA and mCPOSS, light irradiation, and pH value. The self-assembly process was studied using UV−vis spectra, DLS, and TEM. The PPA contains an AZO end and can form supramolecular complexes with β-CD in aqueous solution. The inclusion of PPA with β-CD was proven by UV−vis spectra. The UV−vis spectra of PPA solution and the mixture of PPA and mCPOSS are shown in Figure 10. The peak at 345 nm was a characteristic

Figure 8. HRTEM images of self-assembled nanospheres obtained from 0.04 mg/mL mCPOSS aqueous solution. Bar = 100 nm.

Figure 10. UV−vis spectra of PPA (10−5 M) in the presence of β-CD (0.5, 1, 2, 5, 10 × 10−5 M from a to f).

PPA is a block copolymer composed of a hydrophilic PEG block, a pH/temperature responsive PDMAEMA block, and a photoisomerized azobenzene end group. Because of the poor hydrophilicity of PDMAEMA in neutral water at room temperature, PPA could form aggregates in aqueous solution. The critical micellization concentration (cmc) value can reflect the aggregation ability of the molecule. Amphiphilic molecules will aggregate when the concentration of the solution is above the cmc value. We used the fluorescence probe method to

absorption peak of the AZO unit, and it gradually decreased as the molar ratio of the CD to AZO units increased from 0.5 to 10. This suggests that AZO enters into the highly hydrophobic and electron dense CD cavity.37 Besides, DLS measurements showed that the size of the inclusion complex (Figure 11b) was larger than that of mCPOSS nanospheres (Figure 11a), further supporting the successful supramolecular assembly between PPA and mCPOSS. 5744

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interacted with the cis-azo isomers. This is also why not all CDs connected to the POSS core interacted with AZO when the ratio of PPA:mCPOSS was 1:1. As a result, some mCPOSS nanospheres remained free (without inclusion) while other nanospheres interacting with AZO end-capped polymers formed complex micelles with a multicore structure by rearrangement of mCPOSS nanospheres: when the AZO end-capped polymers interacted with mCPOSS, the aggregation degree of mCPOSS decreased due to the hydrophilicity of PPA; in other words, a nanosphere formed by mCPOSS (as shown in Figure 8) was taken apart by the hydrophilic polymers, forming multiple cores that scattered in the complex micelle. Also, in this process, the AZO end-capped polymers were randomly interacted with mCPOSS and each micelle had an unequal amount of polymer chains, which made the size distribution of the micelles wide. As the ratio of PPA:mCPOSS increased, all the CDs connected to the POSS core interacted with the AZO end-capped polymers and each micelle had an equal number of polymer chains so that the size of the micelles became uniform. Thus, the size and the morphology of the sample can be adjusted by the ratio of PPA:mCPOSS. The formation and dissociation of the micelles are controlled by visible and ultraviolet light. As we discussed above, the original sample formed micelles under visible light irradiation. When the sample was irradiated by ultraviolet light (sample U), the trans-azo groups gradually transformed into cis-azo and were excluded from the CDs, resulting in a dissociation of the micelles. The size of the sample U decreased substantially after a 10 min irradiation with UV light (Figure 11d) and nanospheres with a diameter of 10 nm start forming again, the same as free mCPOSS nanospheres (Figure 13c and d). The DLS and the TEM results indicated that the complex micelles formed by PPA and mCPOSS dissociated under UV irradiation; the mCPOSS reformed nanospheres and the PPA polymers redissolved in the system. The reason is that the inclusion constant of cis-azo and β-CD is much smaller than that of trans-azo and β-CD, so the cis-azo tended to exclude from the β-CD. Also, some irregular aggregates were observed in Figure 13c and d and they were within the transition state of the dissociating micelles. In addition, the formation and the dissociation of the micelles could be replicated by controlling the wavelength of the light and this was proven by periodic change to the UV−visible spectra of the sample (Figure 14). When the sample was under visible light irradiation, an absorption peak at 345 nm, characteristic of trans-azo, was observed; when the same sample was irradiated using UV light, the peak blue-shifted and weakened, and a new peak around 430 nm appeared, indicating the appearance of cis-azo. A repeating trans−cis photoisomerization led to the cyclic inclusion and exclusion of PPA and mCPOSS. In addition, the size of the micelles formed by PPA and mCPOSS could respond to the pH value of the aqueous solution due to the pH sensitivity of PDMAEMA. The pH value of the solution was adjusted by 1 M HCl and 1 M NaOH. When the pH value was adjusted to 3, the PDMAEMA chains were protonated, enhancing the hydrophilicity of the polymer and thus causing the size of the micelles to increase (Figures 11c and 13e and f). When the pH value was adjusted from 3 to 9, the size of the micelles decreased due to the deprotonation of the PDMAEMA chains. The cyclic size change of the micelles was observed by repeatedly modifying the pH value, as shown in Figure 15.

Figure 11. Hydrodynamic diameters (Dh) of the self-assembled aggregates of mCPOSS (a) and host−guest inclusion complexes (b− d) in 10−5 M aqueous solution: (b) original sample; (c) the sample obtained at pH 3.0; (d) the sample obtained under UV light irradiation for 10 min.

The original sample (named sample O) was prepared by directly mixing the aqueous solutions of PPA (10−5 M) and mCPOSS at room temperature with the visible light irradiation. The size and morphology of the sample O can be adjusted by the feed ratio of the PPA and mCPOSS. DLS results revealed that the particle size and the particle size distribution of the sample O decreased as the ratio of PPA:mCPOSS increased from 1:1 to 5:1 (Figure 12); the size of sample O would remain

Figure 12. Change in the hydrodynamic diameters (Dh) of the selfassembled aggregates under different ratios of PPA:mCPOSS.

constant when the ratio of PPA:mCPOSS was larger than 5:1. TEM images revealed that, when the ratio of PPA:mCPOSS was 1:1 (Figure 13a), both small nanospheres and complex micelles containing several cores existed in the sample O, and that the size of the complex micelles was nonuniform; when the ratio of PPA:mCPOSS increased to 5:1 (Figure 13b), the size of the micelles became uniform and no obvious core was observed inside the micelles. DLS and TEM results indicated that PPA and mCPOSS formed complex micelles and the morphology of the micelles could be adjusted by the ratio of PPA and mCPOSS. The reason that the micelles and the complex micelles formed in sample O under different PPA:mCPOSS ratio is due to the fact that the trans-azo unit at the end of the polymer was hydrophobic and became surrounded by the hydrophilic polymer chain in aqueous solution; meanwhile, the trans−cis isomerization of azobenzene was in dynamic equilibrium,4 so that cis-azo isomers always existed and mCPOSS hardly ever 5745

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Figure 13. HRTEM images of PPA and mCPOSS assembly at a concentration of 10−5 M aqueous solution (calculated by PPA) under different conditions: the original sample obtained under visible light irradiation in neutral water (a, b), the sample obtained under UV light irradiation (c, d), and the sample obtained at pH 3.0 (e, f); the ratio of PPA:mCPOSS is 1:1 for parts a, c, and e, and the ratio of PPA:mCPOSS is 5:1 for parts b, d, and f. Bar = 100 nm in parts a, c, and e, and bar = 50 nm in parts b, d, and f.

Figure 14. Periodical change of UV−visible spectroscopy under the control of light for self-assembly aggregates.

The proposed assembling process of PPA and mCPOSS in aqueous solution was outlined in Scheme 3.

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CONCLUSION mCPOSS was synthesized by CuAAC, and PPA was synthesized by ATRP and CuAAC. Amphiphilic mCPOSS first self-assembled into a nanosphere with the POSS core surrounded by CDs. Then, the CD at the surface of the nanosphere interacted with the AZO at the end of the PPA to form complex micelles. As the ratio of PPA:mCPOSS increased from 1:1 to 5:1, most of the CD at the surface of the nanosphere interacted with PPA; complex micelles converted to uniform micelles. The formation and dissociation of micelles

Figure 15. Periodical change of hydrodynamic diameters (Dh) under the control of pH value for self-assembly aggregates.

could be controlled by visible and ultraviolet light, while the size of the micelles changed in response to changes in pH value. Micelles with controllable size and morphology may have potential applications in nanocontainer and nanocarrier fields. 5746

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Scheme 3. Proposed Self-Assembly Process of PPA and mCPOSS in Aqueous Solution

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis of mono-6-deoxy-6-(p-tolylsulfonyl)-β-cyclodextrin (β-CD-OTs), mono-6-deoxy-6-azido-β-cyclodextrin (β-CDN3), and 1-phenyl-2-(4-(prop-2-ynyloxy)phenyl)diazene, ESIMS results of mCPOSS, GPC results of PPA, and 1H NMR of mCPOSS and PPA. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Phone/Fax: 86-10-64421693. E-mail: [email protected]. cn (G.C.). *Phone/Fax: 86-10-64421693. E-mail: qfl[email protected] (Q.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 51273017), Polymer Chemistry and Physics, Beijing Municipal Education Commission (BMEC, No. XK100100640), for financial support.

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