Dual Redox Responsive Coassemblies of Diselenide-Containing

May 1, 2014 - Lu Wang, Wei Cao, Yu Yi, and Huaping Xu*. Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, ...
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Dual Redox Responsive Coassemblies of Diselenide-Containing Block Copolymers and Polymer Lipids Lu Wang, Wei Cao, Yu Yi, and Huaping Xu* Key Laboratory of Organic Optoelectronics and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: A general approach is reported to fabricate a stimuli responsive system via coassembly of diselenide-containing block copolymers with polymer lipids, which integrates the stimuli-responsiveness of diselenide chemistry and the biocompatibility of polymer lipids. By using dynamic light scattering, transmission electron microscopy, and zeta potential analyzer, coassembly behavior of these two kinds of polymers and responsiveness of coassemblies have been investigated. These coassemblies can exhibit redox-responsiveness inheriting from the diselenide-containing block copolymers. In the presence of low concentration of hydrogen peroxide or glutathione, the coassemblies can be disrupted.

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INTRODUCTION Living organisms’ survival in nature should mostly be ascribed to their ability to adapt to changes of the external environment.1 In order to further study behaviors of life, it is of great importance to develop novel smart biomaterials and relevant nanotechnology. Stimuli-responsive polymers are important biomaterials which can respond to the external stimuli of environments and change their properties, such as solubility, conformation, adhesion to different molecules, and amphiphilicity, and thus transforming chemical and/or biochemical signals to optical, thermal, electrical, and/or mechanical signals.2−8 Therefore, these materials have attracted great interest in recent years and are widely used,9 including for drug delivery,10−13 gene carriers,14,15 biotechnology applications,16 smart gels,17 biosensors,18 nanodevices,19 and so on. The development of a stimuli responsive system, however, still encounters many challenges. On the one hand, the cellular microenvironment could change in a complicated and delicate way, demanding the responsive polymers be multireponsive.20,21 On the other hand, the concentration of chemicals delivering stimuli in living organisms can be as low as millimolar or micromolar, demanding the responsive polymers be sensitive enough. In this regard, redox-responsive polymers deserve special attention,22 since many diseases are closely related to redox-homeostasis of the cellular environment.23−26 Liposomes and lipid nanoparticles are widely utilized as molecule vehicles27−30 on account of their excellent biocompatibility and biodegradability. Liposomes are usually vesicles formed by lipid bilayers, while lipid nanoparticles are composed of lipids without bilayers. Although common lipids for drug delivery are small molecules, recently some kinds of polymer lipids start to appear. Distearoyl-sn-glycero-3-phos© 2014 American Chemical Society

phoethanolamine-n-[methoxy-(poly(ethylene glycol))-2000] (PEG-PE) is one of the polymer lipids that can be well utilized to encapsulate and deliver drugs.31−34 In addition, vesicles composed of polymer lipids are more stable than those of traditional lipids. In spite of these merits of polymer lipids, they still have some shortcomings. For example, they cannot respond to external stimuli, which is vital for selective or target drug delivery in the physiological environment. To push forward the development of controlled drug delivery system, it is of great significance to combine biocompatibility and biodegradability of polymer lipids with stimuli-responsiveness of other kinds of polymers. As we know, coassembly, a general approach which can combine both merits of the materials, has already been used to produce some functional structures, such as cation-permeable currents,35 mesoporous polymer−silica and carbon−silica nanocomposites,36 polyferrocenylsilane block polyelectrolytes,37 and so on. To this end, by coassembling the amphiphilic polymer lipids with responsive polymers, both of their advantages can be integrated to the coassembly materials. As for stimuli-responsive polymers, selenium-containing polymers are one of the appropriate candidates which are far less studied compared with sulfur-containing polymers. First examples of selenium-containing polymers could be dated back to 1960s and 1970s.38 However, relevant reports were quite few due to their poor solubility and low stability. Our research group managed to synthesize selenium-containing polymers of different topology structures, such as main-chain seleniumReceived: March 19, 2014 Revised: April 28, 2014 Published: May 1, 2014 5628

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Scheme 1. Redox Responsive Coassemblies Formed by Diselenide-Containing Block Copolymers and Polymer Lipids

containing polymers,39−41 side-chain selenium-containing polymers,42 dendrimers,43 and hyperbranched polymers.44 In addition, their application in enzyme mimics and controlled drug carriers were also investigated.45 One of the most representative cases is an amphiphilic diselenide-containing block copolymer PEG-PUSeSe-PEG, bearing several diselenidecontaining bonds and polyurethane blocks flanked by poly(ethylene glycol) as terminal blocks. This kind of copolymers can self-assemble into spherical micelles in aqueous solution, exhibiting dual redox responsiveness. Such aggregates can disassemble and release the encapsulated molecules upon the addition of oxidants or reductants.46 Furthermore, the diselenide-containing block copolymer assemblies are endowed with the characteristic γ-ray responsiveness as well, which shows great potential for the combination of chemotherapy and radiotherapy.47,48 In this study, we present a general approach to fabricate a stimuli responsive system via coassembly of diselenidecontaining block copolymers with polymer lipids, which can take advantages of both the responsiveness of diselenide chemistry and the biocompatibility of polymer lipids. Through adding a small portion of diselenide-containing block copolymers into polymer lipids, the coassembly system can exhibit ultrasensitive responsiveness to both oxidants (H2O2) and reductants (GSH) (as shown in Scheme 1).

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Instruments. The 1H NMR spectra were recorded on a JEOL JNM-ECA 400 (400M) spectrometer at 25 °C. Gel permeation chromatography (GPC) measurement was analyzed via a Waters 515 apparatus by using polystyrene as a standard and N,N- dimethylformamine (DMF) as an eluent. Fluorescence was measured by using a Hitachi F-7000 spectrofluorometer, and fluorescence images were taken on an Olympus BX 51 apparatus. The size distributions, zeta potential, and count rate measurement were performed on a Malvern 3000HS Zetasizer by using a monochromatic coherent He−Ne laser (633 nm) as the light source. The scattered light was detected at an angle of 90°. Transmission electron microscopy (TEM) images were taken from an H-7650B microscope, and the accelerating voltage was 80 kV. All the TEM samples were prepared by drop-coating the solution on the carbon-coated copper grid and stained by 1.5% uranyl acetate before observation. FTIR spectra were analyzed via a Bruker IFS-66v/S FTIR spectrometer using KBr plates as the substrates. Fluorescence was measured by using a Hitachi F-7000 spectrofluorometer. Synthesis of the Diselenide-Containing Block Copolymer PSe-1900. At first, 0.22 g (0.45 mmol) of 11,11′-diselanediylbis(undecan-1-ol) (denoted as DSeOH) and 0.077 g (0.44 mmol) of 2,2′-(piperazine-1,4-diyl) diethanol were dissolved in 15 mL of anhydrous THF solution. Then the flask was sealed with a rubber plug. After that, the flask was degassed by N2 for 5 min. Under N2 flow, a solution of 132 μL (0.93 mmol) of TDI in 1 mL of anhydrous THF was injected to the flask. The system was transferred into an oil bath at 50 °C in order to react for 24 h with stirring. Then, 0.10 g (0.051 mmol) of PEG monomethyl was dissolved in 2 mL of anhydrous THF and injected into the flask under N2 flow, and the reaction was carried out for another 24 h. After the reaction, the solution was added into 200 mL of diethyl ether dropwise, followed by filtration. Additionally, the filtration residue was purified by redissolving in THF, precipitating with diethyl ether, filtering, and drying under vacuum. The obtained product was a yellow powder (yield 65%). Fabrication of Different Mass Ratios of Coassemblies. To fabricate 10:1 coassemblies, 0.4 mg of PSe-1900 and 4.0 mg of PEGPE were first dissolved in 1 mL of DMF solution for about 10 min to be fully mixed. Then the solution was dispersed in 40.0 mL of water under sonication drop by drop. After that, the solution of coassemblies can be investigated by dynamic light scattering (DLS). Fabrication of 4:1 and 1:1 coassemblies was similar, as shown in Table 1. Measurement of Normalized Count Rate in Responsiveness Experiment. In the H2O2 and GSH responsiveness experiments, a

EXPERIMENTAL SECTION

Materials. 1,2-Distearoyl-sn-glycero-3-phosphoethanolamine-n[methoxy-(poly(ethylene glycol))-2000] (ammonium salt) (PEG-PE, Mw = 2805.54) was purchased from Avanti Polar Lipids, Alabaster, AL. Poly(ethylene glycol) (PEG) monomethyl ether (Mw = 1900) (analytical reagent, AR), glutathione (reduced) (GSH) (≥98.0%), and Pluronic L121 were purchased from Sigma-Aldrich. Curcumin (98.0%) was purchased from J&K Scientific Ltd. 11-Bromoundecanol (AR) was purchased from Aladdin Chemical Reagent Co. Ltd. Selenium powder (AR), sodium borohydride (AR), 2,4-toluenediisocyanate (TDI) (AR), 30% H2O2 (AR), and other organic solvents and chemicals used in this work were analytical grade products from Beijing Chemical Reagent Company. PEG was dried in vacuum at 60 °C for 4 h before employ. Tetrahydrofuran (AR; THF) was dried using Sodium Type A (4A) molecular sieves to remove moisture. 5629

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the middle block of PSe-1900 so that there will be electrostatic interactions between PEG-PE and PSe-1900. The similar structures and electrostatic interactions between them are of great significance for their coassembly behaviors. Thus, diselenide-containing 11,11′-diselanediylbis(undecan-1-ol) and positively charged 2,2′-(piperazine-1,4-diyl) diethanol were utilized as the monomers to copolymerize with slight excess of TDI undergoing stepwise polymerization in a solution of THF. Then PEG monomethyl ether (Mw = 1900) was used to terminate the active ends to yield the diselenide-containing block copolymers (see Scheme 2). The product of the polymerization is a diselenide-containing block copolymer. Its molecular weight Mn was 2.8 × 104 g mol−1, which was estimated from 1H NMR spectrum (Figure S1, Supporting Information). The Mw value of PSe-1900 was also measured by using GPC in a DMF solution (Mw = 4.4 × 104 g mol−1, Mn = 1.5 × 104 g mol−1, Mw/Mn = 2.92). It is reasonable that the molecular weight of the block copolymer detected by GPC is different since it exhibits different conformations compared with the polystyrene reference. Self-Assembly Properties of PEG-PE and PSe-1900. Owing to their amphiphilic nature, PEG-PE and PSe-1900 both can self-assemble in aqueous solution. The critical micelle concentration (CMC) of PEG-PE was determined by fluorescent probe using pyrene as the probe.49 The results indicated that the CMC was 2 × 10−2 mg mL−1. By employing DLS, the size distribution of the aggregates was approximately 5 nm (Figure S2). Similarly, the critical aggregate concentration (CAC) of PSe-1900 was determined to be 2 × 10−4 mg mL−1. The size distribution of the aggregates was 122 nm, as shown in Figure S3a. TEM images showed that PSe-1900 self-assembled into spherical micelles in aqueous solution, which was consistent with results of DLS (Figure S3b). Preparation of Coassemblies of PEG-PE and PSe1900. On account of the hydrophobic interaction and electrostatic interactions between PEG-PE and PSe-1900, coassemblies were fabricated successfully. The DLS measurements and TEM images of different coassemblies with various mass ratio are shown in Figure 1a, b (PEG-PE and PSe-1900 at a mass ratio of 10:1), Figure S4a, b (a mass ratio of 4:1), and Figure S4c, d (a mass ratio of 1:1). The concentration of both polymers should be low enough to avoid phase separation (see Table 2). In order to determine whether they had coassembled, size distribution of the aggregates was measured by DLS, which turned out to be approximately 51 nm and was between that of PEG-PE and PSe-1900 themselves. The results indicated that a new kind of aggregates were generated in the solution. Size distribution of the aggregates in TEM images also agreed with the results in DLS. Similarly, size distribution of the aggregates of 4:1 and 1:1 samples was also determined by DLS (Table 3). TEM images agreed with these results as well. Because the TEM samples were stained by 1.5% uranyl acetate, different contents of uranium remained on the particles are also capable

Table 1. Fabrication of Different Mass Ratios of Coassemblies mass ratios of coassemblies 10:1 4:1 1:1

quantities of quantities of volume of PSe-1900 (mg) PEG-PE (mg) DMF (mL) 0.4 0.4 0.4

4.0 1.6 0.4

volume of pure water (mL)

1.0 1.0 1.0

40.0 40.0 40.0

solution of 30% H2O2 or powder of GSH was added in the solution of coassemblies to achieve different concentration of H2O2 or GSH, as shown in Table 2. Then the change of count rate of the solution was measured to investigate the redox responsiveness of the coassemblies. For easier presentation, normalized count rate of a specific point was calculated by the formula shown below:

normalized count rate of a specific point count rate of the specific point = × 100% count rate of the initial solution In Vitro Release Experiment and Cytotoxicity Studies. Curcumin release experiment of 10:1 coassemblies in the presence of H2O2 was taken. Amounts of 4.0 mg of PEG-PE, 0.4 mg of PSe1900, and 1.0 mg of curcumin were dissolved in 1 mL of DMF for 10 min until fully mixed. Then the solution was dispersed in 40.0 mL of PBS buffer under sonication drop by drop. Then 30% H2O2 was added in the solution to a final concentration of 0.01% or 0.1%. The samples were shaken at 37 °C. Because curcumin is insoluble in water, the released curcumin will precipitate and the fluorescence will decrease. To monitor the curcumin released, fluorescence of the supernatant was measured and the excitation wavelength was 372 nm. The cytotoxicity of different concentration of 10:1, 4:1 and 1:1 coassemblies was evaluated by the MTT assay. L-02 cells were incubated in DMEM, which contains 100 μg/mL streptomycin and 10% fetal bovine serum (FBS) with 100 U/mL penicillin at 37 °C under a humidified atmosphere with 5% CO2. The cells seeded in culture plates were attached for 24 h, with a density of 1 × 105 cells/ mL (1 × 104 cells/mL). Then effect of these coassemblies on viability was identified by the MTT assay. Cells were incubated in the presence of coassemblies which containing 0, 0.0001, 0.001, and 0.1 mg/mL PSe-1900. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) solution was added in each well to a final concentration of 0.5 mg/mL, and cells were returned to the incubator for another 4 h. Then the medium was removed and 100 μL of DMSO was added to each well to dissolve the metabolite of MTT, formazan crystals. After fully mixing, optical density at 490 nm was detected using a microplate reader.

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RESULTS AND DISCUSSION Preparation of PSe-1900. A polymer lipid named 1,2distearoyl-sn-glycero-3-phosphoethanolamine-n-[methoxy(poly(ethylene glycol))-2000] (PEG-PE) and diselenidecontaining block copolymer PSe-1900 were employed to coassemble in this study. PEG-PE is commercially available. The molecular weight of the PEG block in both PEG-PE and PSe-1900 is about 1900. Since the PEG-PE bears negative charged phosphate groups, positive charges are introduced in Table 2. H2O2 and GSH Responsiveness Experiments

volume of 30% H2O2 (μL) or quantities of GSH (mg) H2O2

0.1% (v/v) 0.01% (v/v)

GSH

0.5 mM 0.05 mM

40.0 4.0

volume of the solution of coassemblies (10:1, 4:1 or 1:1) (mL) 12.0 12.0

1.842 0.1842

12.0 12.0 5630

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Scheme 2. Synthetic Routes for Diselenide-Containing Block Copolymer PSe-1900

adjusting the mass ratio of PEG-PE and PSe-1900. To further understand the relation between the size and the sample ratio, zeta potential analysis was used (Figure 1c). The results indicated that, with the gradual addition of PSe-1900, the absolute value of zeta potential decreased. More positively charged PSe-1900 coassembled in the aggregates lead to a more positive zeta potential value. All these data suggested that PEGPE and PSe-1900 had successfully coassembled in aqueous environment. According to a previous work, fluorescent molecule will generate singlet oxygen in the presence of light,41 which makes the system too complicated. Therefore, we did not employ fluorescent marker in this system. Oxidation Responsiveness of Coassemblies. Coassemblies of PEG-PE and PSe-1900 exhibited good responsiveness to hydrogen peroxide (H2O2). Count rates of the coassemblies were determined to reflect the sensitivity of the response. The extent of decrease of the count rate indicated the amount of the disassembled coassemblies. Thus, the more the count rate decreases, the more sensitive the coassemblies are. As shown in Figure 2a−c, all these coassemblies responded smoothly to 0.1% H2O2 (v/v). After 3 h oxidation by 0.1% H2O2, with count rates of 10:1, 4:1, and 1:1, coassemblies decreased to 32%, 10%, and 14% of the initial value, respectively. But to 0.01% H2O2 (v/v), only 4:1 coassemblies showed responsiveness, and the count rate of them decreased to 63% of the initial value. Also, 10:1 and 1:1 coassemblies showed no responsiveness to 0.01% H2O2. We suppose the reason was that, in the system of 10:1 coassemblies, the content of PSe-1900 was much less than that of the 4:1 system, so the 10:1 coassemblies were not sensitive enough to respond to 0.01% H2O2. Distinct from the 10:1 system, the content of PSe-1900 in the 1:1 system was more than that in the 4:1 system, so the content of H2O2 is not so much to oxidize all of the PSe-1900 molecules and disrupt the coassemblies. Thus, all the coassemblies showed good responsiveness to 0.1% H2O2, and 4:1 coassemblies showed responsiveness to 0.01% H2O2. Reduction Responsiveness of Coassemblies. Besides responsiveness to H 2 O 2 , coassemblies also had good responsiveness to GSH. Similarly, responsiveness of different mass ratios of coassemblies was investigated. As shown in Figure 2d−f, all these coassemblies responded well to 0.5 mM GSH and even to 0.05 mM GSH. In more specific terms, after 5 h reduction by 0.5 mM GSH, the count rate of the 10:1, 4:1, and 1:1 coassemblies decreased to 35%, 50%, and 53% of the initial value, respectively. And after 5 h reduction by 0.05 mM GSH, the value decreased to 44%, 65%, and 62%, respectively.

Figure 1. Coassembly behaviors of PEG-PE and PSe-1900. (a) Size distribution plots of 10:1 coassemblies. (b) TEM image of 10:1 coassemblies, stained by uranyl acetate. (c) Comparison of zeta potential of coassemblies with different mass ratio of PEG-PE and PSe1900.

to influence the darkness of them. All of these systems had generated coassemblies instead of self-assemblies of PEG-PE or PSe-1900, and different-sized coassemblies were fabricated by 5631

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Table 3. Size Distributions of Self- and Coassemblies of PSe-1900 and PEG-PE size distributions

self-assemblies of PEG-PE

10:1 coassemblies

4:1 coassemblies

1:1 coassemblies

self-assemblies of PSe-1900

5 nm

51 nm

79 nm

59 nm

122 nm

Figure 2. Responsiveness of coassemblies to different concentration of H2O2 or GSH of PEG-PE and PSe-1900, of which the mass ratio was (a) 10:1, H2O2; (b) 4:1, H2O2; (c) 1:1, H2O2; (d) 10:1, GSH; (e) 4:1, GSH; and (f) 1:1, GSH.

Figure 3. XPS analysis (a) and FTIR spectra (b) of the product of 10:1 coassemblies after oxidation of 0.1% H2O2. After oxidation of 12 h, (a) the Se 3d binding energy increased from 55.7 to 59.0 eV, while (b) there was a new peak of the vibration of Se−O at about 887 cm−1 in the FTIR spectrum, indicating the oxidation product was selenite.

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Figure 4. XPS analysis (a) and FTIR spectra (b) of the product of 10:1 coassemblies after reduction of 0.5 mM GSH. After reduction of 12 h, (a) the Se 3d binding energy decreased from 55.7 to 55.0 eV, while (b) the vibration of C−Se shifted from 529 to 534 cm−1.

Figure 5. Responsiveness of 10:1 coassemblies of PEG-PE and PEG-PUSeSe-PEG to H2O2 or GSH. Concentrations were (a) 0.1% H2O2 and (b) 1.0 mM GSH.

these data, the diselenide structure in PSe-1900 had been reduced, and the product might have a structure of Se−S. Similarly, the amphiphilic property was broken after reduction, which led to the disaggregation of the coassemblies. In Vitro Release Experiment and Cytotoxicity Studies. The release profile of curcumin in the presence of H2O2 could be achieved by the coassemblies. Curcumin is a kind of anticancer drug, which is certificated as useful for different types of carcinoma.54−59 The release results showed that, in the presence of 0.1% H2O2, about 45% of loaded curcumin was released after 4 h (Figure S5). However, in the presence of PBS or 0.01% H2O2, the less than 15% curcumin was released. The results were in agreement with the responsiveness experiment. The biocompatibility of these coassemblies was tested. An in vitro cytotoxicity study with different concentrations of coassemblies was conducted. As shown in Figure S6, cell viability of all these experimental groups was more than 90%. These results confirmed that the biocompatibility of these coassemblies was good for further research. Coassembly as a General Approach to Fabricate Responsive Systems. The coassembly methodology may be utilized as a general approach to fabricate stimuli responsive systems. In order to ensure this viewpoint, PEG-PE and PEGPUSeSe-PEG were also employed to coassemble. PEGPUSeSe-PEG had diselenide in the middle block and poly(ethylene glycol) in each terminal block; each PEG block had a molecular weight of about 1900, which has a similar structure as PSe-1900 without the positive piperazine groups and has been employed in previous work46 (structure of PEGPUSeSe-PEG were shown in Figure S7). Self-assembly

It is reported that the intracellular GSH concentration is around 10 mM and is significantly higher than that of extracellular environments (∼2 μM).50,51 The concentration of GSH in this system is much lower than the reported intracellular value, suggesting that this system could be suitable for further physiological investigation. Characterization of the Product after Oxidation. In order to determine the product after oxidation, X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR spectra) were employed. Based on the results of XPS, the Se 3d binding energy increased from 55.7 to 59.0 eV after oxidation. According to the XPS handbook52 and NIST X-ray Photoelectron Spectroscopy Database, the increased binding energy indicated that the oxidation product of selenium might be seleninic acid (Se−O). In addition, results of FTIR showed a new peak at about 887 cm−1, which presented the vibration of Se−O. According to the literature,53 such results further confirmed the existence of Se− O (Figure 3). Therefore, the hydrophobic diselenide groups were changed to hydrophilic seleninic acid after oxidation, which broke the amphiphilic property of PSe-1900 and further caused the disaggregation of the coassemblies. Characterization of the Product after Reduction. Moreover, the product after reduction was also analyzed. As we expected, the Se 3d binding energy decreased from 55.7 to 55.0 eV after reduction by GSH (Figure 4). The decreased binding energy indicated that the reduction product might have a structure of Se−S, as sulfur is a relatively electron-rich element. Furthermore, results of FTIR showed that the vibration of C−Se shifts from 529 to 534 cm−1. Considering 5633

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properties of PEG-PUSeSe-PEG were investigated at first. The size distributions of the aggregates was about 51 nm, as determined by DLS. TEM images showed that PEG-PUSeSePEG will self-assemble into spherical aggregates, of which the size corresponded to data measured by DLS (Figure S8). By mixing PEG-PE and PEG-PUSeSe-PEG with a mass ratio of 10:1, a new kind of aggregates was obtained. The size of these aggregates was 28 nm, between the size of self-assemblies of PEG-PE and PEG-PUSeSe-PEG (Figure S9). Moreover, the zeta potential of them was measured (Figure S10). The results showed the zeta potential of 10:1 coassemblies was just between that of PEG-PE and PEG-PUSeSe-PEG. Successful coassembly between them further indicated that the hydrophobic interaction was powerful enough to stabilize the coassemblies. All these data indicated that the two molecules had successfully coassembled in the solution. Afterward, the redox responsiveness of the coassemblies was determined by measuring the change of their count rate (Figure 5). After oxidation by 0.1% H2O2 for 2 h, count rate of the coassemblies reduced to 28% of the initial value. Then after reduction by 1.0 mM GSH for 3 h, count rate of the coassemblies was reduced to 67% of the initial value. These data suggested that 10:1 coassemblies of PEG-PE and PEG-PUSeSe-PEG also exhibited sensitive redox responsiveness. Therefore, coassembly can be utilized as a general approach to prepare promising responsive systems. Driving Force for Coassembly. Interactions between PSe1900 and PEG-PE contain hydrophobic interaction, electrostatic interaction and hydrogen bond between the amide groups of them. It is certificated that the most vital interaction is hydrophobic interaction. On the one hand, PEG-PUSeSe-PEG and PEG-PE were capable to coassemble, which indicated the electrostatic interaction was not essential. On the other hand, we also employed PEG-PUSeSe-PEG and Pluronic L121 to coassemble. Pluronic L121 is a commercial available PEG-PPGPEG block copolymer with 68 oxypropylene units in the middle and 5 oxyethylene units in each end block. There is only hydrophobic interaction between PEG-PUSeSe-PEG and Pluronic L121, and they still coassemble successfully. As Figure S11 shows, the size of self-assemblies of L121 was about 220 nm, and the size of coassemblies was about 44 nm. In this case, the most important interaction in the coassembly process is hydrophobic interaction. Then the relationships between size distributions with mass ratios of the coassemblies were determined at the same time. The results show nonmonotonic trend between mass ratios with size distribution (Table S1). We assume that the positive charges in PSe-1900 will lead to a loose core structure in the assemblies. When adding negative charged PEG-PE, a small amount of negative charges will neutralize the positive charges, which might lead to a tighter core. However, too much PEG-PE might result in a loose core again, even influence the distribution of selenium of the core, because of the PEG chain of PEG-PE. Therefore, it is possible to observe an extreme point in the size of coassemblies. In addition, the density of the peripheral PEG chain and the distribution of selenium of the core will further influence the responsiveness of the coassemblies.

lipid PEG-PE. Using coassembly, we obtained a system which could respond to low concentration of oxidants or reductants. By using DLS and TEM, the self-assembly and coassembly behaviors were observed and determined. It was indicated that, in the presence of 0.1% H2O2 or 0.05 mM GSH, most of the coassemblies would be disrupted. We also showed that coassembly of PEG-PUSeSe-PEG with PEG-PE was possible, and the driving force of coassembly was mostly due to the hydrophobic interaction. Therefore, it is anticipated that the coassembly of responsive polymers and polymer lipids can be utilized as a general approach to fabricate responsive systems. By encapsulating drugs or other molecules, these systems may also be utilized in controlled drug delivery toward some hopeful targets.

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR of PSe-1900, self-assemble properties of PEG-PE and PSe-1900, size distributions and TEM images of 4:1 and 1:1 coassemblies, and results of control experiments. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported financially supported by the National Basic Research Program of China (2013CB834502), the Foundation for Innovative Research Groups of the National Natural Science Foundation of China (21121004), Tsinghua University Initiative Scientific Research Program (2012Z02131), the NSFC-DFG joint grant (TRR 61) and The Importation and Development of High-Caliber Talents Project of Beijing Municipal Institutions. The authors acknowledge Prof. Xi Zhang (Tsinghua University) for his stimulative suggestion.

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REFERENCES

(1) Staddon, J. E. R. Adaptive Behavior and Learning; Cambridge University Press: Cambridge, 1983. (2) Zhang, Y.; Jiang, M. New Approaches to Stimuli-Responsive Polymeric Micelles and Hollow Spheres. Acta Polym. Sin. 2005, 5, 650−654. (3) Matsukuma, D.; Yamamoto, K.; Aoyagi, T. Stimuli-Responsive Properties of N-Isopropylacrylamide-Based Ultrathin Hydrogel Films Prepared by Photo-Cross-Linking. Langmuir 2006, 22, 5911−5915. (4) Wang, Y.; Xu, H.; Zhang, X. Tuning the Amphiphilicity of Building Blocks: Controlled Self-Assembly and Disassembly for Functional Supramolecular Materials. Adv. Mater. 2009, 21, 2849− 2864. (5) Stuart, M. A. C.; Huck, W. T. S.; Genzer, J.; Müller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Emerging Applications of Stimuli-Responsive Polymer Materials. Nat. Mater. 2010, 9, 101− 113. (6) Ma, X.; Ashaduzzaman, M.; Kunitake, M.; Crombez, R.; Texter, J.; Slater, L.; Mourey, T. Stimuli Responsive Poly(1-[11-acryloylundecyl]-3-methyl-imidazolium bromide): Dewetting and Nanoparticle Condensation Phenomena. Langmuir 2011, 27, 7148−7157.

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CONCLUSION We have successfully fabricated a responsive system by coassembling two different kinds of polymers, including diselenide-containing block copolymer PSe-1900 and polymer 5634

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