Hydrophobic Nanocontainers for Stimulus-Selective Release in

Article pubs.acs.org/Macromolecules

Hydrophobic Nanocontainers for Stimulus-Selective Release in Aqueous Environments Roland H. Staff,† Markus Gallei,*,‡ Katharina Landfester,† and Daniel Crespy*,† †

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany Ernst-Berl-Institute for Chemical Engineering and Macromolecular Science, Technische Universität Darmstadt, Alarich-Weiss-Strasse 4, D-64287 Darmstadt, Germany

‡

S Supporting Information *

ABSTRACT: The preparation of nanocontainers with a hydrophilic core from water-in-oil emulsions and their subsequent transfer to aqueous medium is crucial because it enables the efficient encapsulation of hydrophilic payloads in large quantities. However, major challenges are associated with their synthesis including low colloidal stability, leakage of encapsulated payloads due to osmotic pressure, and a demanding transfer of the nanocontainers from apolar to aqueous media. We present here a general approach for the synthesis of polymer nanocontainers that are colloidally stable, not sensitive to osmotic pressure, and responsive to environmental stimuli that trigger release of the nanocontainer contents. Additionally, the nanocontainers can selectively deliver one or two different payloads upon oxidation and changes of pH or temperature. Our approach uniquely enables the synthesis of nanocontainers for applications in which aqueous environments are desired or inevitable.

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INTRODUCTION

Mimicking selective transport of ions and molecules through a biobased membrane with its highly sophisticated channel functions triggered by external stimuli has sparked much interest among researchers over decades. Selectivity is one important factor for building complexity in natural systems.1,2 It refers not only to the ability of proteins or nucleic acids to interact selectively with other molecules, but also to the exchange of comparably simple compounds, which are also essential for survival, such as water3,4 or ions.5,6 Synthetic hollow nanoparticles−also referred as nanocontainers−that are filled with specific payloads are not only promising model systems for achieving a better fundamental understanding of gated transport but they also have the potential for a manifold of applications. Indeed, the controlled, selective, and independent release of different payloads upon the application of different stimuli,7−10 is of high relevance for, e.g., biomedicine,11−18 anticorrosion,19,20 light emission,21,22 and water purification.23 In these cases, the payload needs to be delivered in aqueous environment from colloidal nanocontainers that are stable. Moreover, in the case of the simultaneous encapsulation of several payloads, the release of different payloads has to be case selective; i.e., one payload is released upon one specific stimulus while the other payload is released upon another stimulus. An ideal nanocontainer for a controllable nanoscale delivery is therefore a highly complex system that should display features present in nature. Thus, in a perfect scenario, the payload is simultaneously encapsulated and again selectively delivered in aqueous environment from stable nanocontainers upon one specific stimulus while the other payload is released upon another stimulus (Figure 1). © 2014 American Chemical Society

Figure 1. Simplified schematics of the nanocontainers. The nanocontainers display a stimulus-selective release of two different types of payloads (marked as cones and cubes) upon different stimuli.

The quest toward smart materials able to mimic one or several features of natural systems24−27 has led to the concept of multicompartmentalization in polymeric systems.28 Block copolymers, i.e., polymers consisting of two or more homogeneous polymer fragments that are covalently connected, feature the intrinsic capability to undergo microphase separation yielding fascinating structures in the bulk or selective solvents. Encapsulation of small polymer objects has already been carried out in a larger polymer entity.29−34 For instance, multicompartmentalized micelles were synthesized from triblock copolymers35,36 and their fascinating self-assembled structures studied in water or organic solvents.37,38 Furthermore, in first attempts the selective release of two different payloads out of a block copolymer micelle possessing three compartments with payloads separately distributed in two of these compartments has been studied.39 Compared to those important findings based on self-assembled multiblock copolymer structures in selective solvents, the expansion to the preparation of dense nanocapsules provides several Received: June 14, 2014 Revised: July 10, 2014 Published: July 18, 2014 4876

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(Wyatt Technology, Santa Barbara, CA) was used. The Astra version 4.73 (Wyatt Technology, Santa Barbara, CA) was used for data acquisition and the evaluation of the light-scattering experiments. The light scattering instrument was calibrated using pure toluene, assuming a Rayleigh ratio of 9.78 × 10−6 cm−1 at 690 nm. An injection volume of 118 μL, a sample concentration of 1 to 2 g L−1, a column temperature of 35 °C, and a THF flow rate of 1 mL min−1 were applied. In all release experiments, the fluorescence intensity at the emission wavelength of 624 nm of the treated dispersion was measured on a Tecan Plate Reader Infinite M1000 at an excitation wavelength of 520 nm. Droplets of 3 μL of the dispersions were placed on small silica platelets for scanning electron microscopy and on copper grids for TEM measurements. Both sample types were sputtered with carbon on a BALZERS BAE250 for 5 s to prevent beam damage during the transmission and scanning electron microscopy measurements. TEM observations were carried out on a JEOL 1400 at a voltage of 120 kV and images were taken with a GATAN Ultrascan 1000 CCD-camera. SEM images were carried out on a Zeiss 1530 Gemini Leo at 0.4 kV. Additional TEM images of the PS-bPDMAEMA bulk samples were performed on a Zeiss EM10 with an operating voltage of 60 kV. The block copolymer was diluted in methylene chloride and the solvent was allowed to evaporate at room temperature. Thin films were heated at 150 °C in vacuum for 24 h. Ultrathin sections of the block copolymer film were cut into slices of 50−80 nm by using an ultramicrotome Ultracut UTC (Leica) equipped with a diamond knife. TEM images were recorded with a slow-scan CCD camera TRS (Tröndle). The hydrodynamic diameter of the nanocapsules was measured on the dispersions with a Nicomp 380 Submicron Particle Sizer (PSS-Nicomp) at an angle of 90° (DLS) for 300 s. NMR spectra were recorded on a Bruker DRX 500 NMR operating at 500 MHz for the determination of the molecular weights. The stimuli-selective release from the block copolymer nanocapsules was measured on a Bruker DRX 300 spectrometer working at 300 MHz. Exemplary Synthesis of PS-b-PDMAEMA. PS-b-PDMAEMA was prepared by sequential anionic polymerization applying a similar protocol as reported by Schacher et al.46 Therefore, styrene is polymerized in cyclohexane (CH) at room temperature by initiation with sec-butyl lithium (sec-BuLi) followed by treatment with 1,1′diphenylene ethylene (DPE) (1.1 equiv concerning the living chain ends) inside a glovebox. After the addition of 10 equiv of lithium chloride (LiCl) dissolved in neat THF and cooling the reaction mixture to −80 °C by using a Coldwell apparatus, the DMAEMA monomer which is freshly distilled from trioctyl aluminum is added quickly. After 2 h of reaction time, the polymerization is terminated by the addition of degassed methanol. The polymer was poured into a 10fold excess of methanol, filtrated, and dried in vacuo. Preparation of the Nanocapsules. A 50 mg sample of the respective polymer and 50 mg of hexadecane are dissolved in 2.5 g of dichloromethane containing 1 mg of Nile-red. The organic phase is added to 20 g of an aqueous solution with concentrations of 0.05 mg mL−1 and 0.1 mmol L−1 of cetyltrimethylammonium chloride (CTMA-Cl) and NaOH, respectively. The mixture is stirred at 1250 rpm for 1 h in a closed glass vial to obtain a macro emulsion. The emulsion is then subjected to ultrasonication under ice-cooling for 2 min in a 30 s pulse/10 s pause regimen (Branson W450-D sonifier with a 1/2 in. tip). Afterward, the dichloromethane is evaporated at room temperature while stirring at 500 rpm for 16 h. The dispersions are then dialyzed against their continuous phases for 2 days to remove the nonencapsulated dye. General Procedure for Release Experiments. For the release experiments with dimethyldodecylamine (DDA) and diphenyl disulfide (DPDS), water is replaced by deuterated water and 50 mg of DDA is used instead of hexadecane or 20 mg of DPDS is dissolved together with hexadecane and the corresponding polymer. In order to elucidate the pH-responsiveness of the nanocapsules, equal volumes of the dispersion and aqueous solutions containing CTMA-Cl (0.05 mg mL−1) and hydrochloric acid (HCl) at different concentrations (0.00001 to 1 mol L−1) are mixed and stirred for 24 h. To verify and measure the response of nanocapsules containing PVFc to

advantages: First, the preparation of such responsive nanocontainers can be produced by convenient miniemulsion polymerization protocols in large scales and second, the obtained capsules are intrinsically more stable due to the presence of the continuous polymeric shell. Polymer or inorganic nanocapsules for the encapsulation and delivery of payloads in aqueous solutions are usually prepared from inverse emulsions, i.e., water-in-oil emulsions with a hydrophobic “oil” as continuous phase, and subsequently transferred to an aqueous continuous phase. However, this approach suffers from severe drawbacks. First of all, inverse emulsions are generally less stable than direct emulsions because they cannot be electrostatically stabilized. Second, the demanding transfer of the nanocapsules from the oil to the water phase is accompanied by a loss of nanocapsules. Finally, leakage of encapsulated payloads from the nanocapsules can occur due to the osmotic pressure applied to the nanocapsules membrane. In the present study we introduce a new concept for the preparation of nanocapsules based on different types of stimuliresponsive polymers, namely (i) polyvinylferrocene-b-poly(2vinylpyridine) (PVFc-b-P2VP) featuring both redox- and pHresponsiveness and (ii) polystyrene-b-poly(N,N-dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) feasible for pH and temperature response. The novel stimuli-responsive nanocapsules feature remarkable colloidal stability, they do not suffer from burst due to osmotic pressure, and they allow for the selective release of two different molecules from their reservoirs. The selective release of simultaneously encapsulated payloads out of the interior of the nanocapsules is studied upon triggering by various external stimuli (pH, redox reagents, temperature). We expect manifold applications for herein presented functional polymer-based nanocarrier systems, e.g., in fields of sensing, drug delivery or self-healing.

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EXPERIMENTAL SECTION

Reagents. Dichloromethane (Fisher, 99.99%), hexadecane (HD, Acros, 99.8%), Nile-red (Sigma-Aldrich), cetyltrimethylammonium chloride (CTMA-Cl, Acros, 99%) hydrogen peroxide (H2O2, SigmaAldrich, 35%), deuterated water (D2O, Sigma-Aldrich, 99.9%), dimethyldodecylamine (DDA, Fluka, 99.9%), and diphenyl disulfide (DPDS, Sigma-Aldrich, 99%) were used as received. Sodium hydroxide and hydrochloric acid solutions were prepared by diluting stock solutions (both VWR, 1 mol·L−1) to the desired value. Distilled water was used for the preparation of all solutions. All other solvents and reagents were purchased from Fisher Scientific and used as received, unless otherwise stated. Tetrahydrofuran (THF) and cyclohexane were distilled from sodium/benzophenone under reduced pressure (cryo-transfer) prior to the addition of 1,1-diphenylethylene (DPE) and n-BuLi as well as a second cryo-transfer. Styrene and N,Ndimethylaminoethyl methacrylate (DMAEMA) were purified by 2-fold distillation over calcium hydride (CaH2). In the case of DMAEMA, trioctyl aluminum (25 wt % in hexane) was subsequently added dropwise until a pale yellow color appeared. Prior to use in anionic polymerization protocols, the monomers were freshly distilled from these solutions. The synthesis of PVFc-b-P2VP was previously reported.53 Instrumentation. All syntheses were carried out under an atmosphere of nitrogen using Schlenk techniques or a glovebox equipped with a Coldwell apparatus. Standard SEC was performed with THF as the mobile phase (flow rate 1 mL min−1) on a SDV column set from PSS, Mainz (SDV 1000, SDV 105, SDV 106) at 30 °C. The calibration was carried out using PS standards from PSS (Mainz). For the SEC-MALLS experiments, a system composed of a Waters 515 pump (Waters, Milford, CT), a TSP AS100 autosampler, a Waters column oven, a Waters 486 UV-detector operating at 254 nm, a Waters 410 RI-detector, and a DAWN DSP light scattering detector 4877

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Figure 2. Syntheses schemes for the sequential anionic polymerization of (i) vinylferrocene (VFc) and 2-vinylpyridine for formation of PVFc-bP2VP53 (blue box) and (ii) styrene with N,N-dimethylaminoethyl methycrylate (DMAEMA) yielding PS-b-PDMAEMA.

Table 1. Molecular Weights of the Block Copolymers and Reference Homopolymers Used in This Study Mn [kg mol−1]

polymer PVFc14-b-P2VP204

a

218

218

PS74-b-PDMAEMA1185

85

PS homopolymer PVFc homopolymera PDMAEMA P2VP

74 199 11.9 25 25.8

Mw [kg mol−1] 232

77 207 13.8 − 25.9

PDI

comment

stimuli

1.06

PVFc: SEC-MALLSa block: SEC vs PS PS: SEC vs PS block: 1H NMR PS precursor SEC vs PS SEC vs PS SEC MALLS 1 H NMR SEC MALLS

pH, Ox

1.03 1.04 1.15 − 1.01

pH, T − − Ox pH, T pH

Values for PVFc homopolymers were determined by using SEC-MALLS. The refractive index increment dn/dc was determined to 0.186.

copolymers were designed to meet these criteria. The first block copolymer is composed of two independently responsive segments, i.e., a redox-responsive polyvinylferrocene (PVFc) and a pH-responsive poly(2-vinylpyridine) (P2VP) block. The syntheses of the responsive diblock copolymers was carried out by anionic polymerization as given in Figure 2 and the Experimental Section. Their molecular weights, the method of molecular weight determination, and the stimuli applied are provided and summarized in Table 1. Both blocks alternatively play the role of the structural unit or functional unit depending on the applied external stimulus. The second model block copolymer is composed of an inert block of polystyrene (PS) as structural material and a dual-responsive block of poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) sensitive to both pH and temperature.40 The first copolymer possesses two blocks producing two independently outputs upon triggering whereas the output is a function of both stimuli for the second copolymer. The overall molecular weight of the PVFc-b-P2VP and PS-b-PDMAEMA prepared by anionic polymerization were selected to present PVFc and PDMAEMA as minor block components (Table 1).

oxidation, equal volumes of dispersion and freshly prepared hydrogen peroxide solution containing CTMA-Cl (0.05 mg mL−1) are mixed and stirred for 24 h. The response of PDMAEMA blocks to temperature is measured by heating the dispersions with temperature-controlled oil baths at different temperatures and treating the dispersions with hydrochloric acid as described above. Control samples of the dispersions are also diluted with their continuous phase without addition of acid or oxidant.

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RESULTS AND DISCUSSION A. Block Copolymer Synthesis and Characterization. The nanoscale objects presented here possess two compartments. The first compartment is a reservoir for two different payloads while the second compartment is a stimuli-responsive shell that acts as membrane for the selective release of these payloads on demand (Figure 1). To achieve this functional nanocontainer, we employed a dual-responsive shell in combination with switchable payloads. Furthermore, the shell was designed to simultaneously fulfill two requirements: To be selectively triggered by two different stimuli and to keep its structural integrity while being selectively triggered; i.e., the shell material contains a structural material unit and a functional material unit. Two hydrophobic model block 4878

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The bulk morphology of PS-b-PDMAEMA displayed PDMAEMA spheres in a PS matrix as shown in Figure S1 (Supporting Information) while the morphology of PVFc-bP2VP is expected to be PVFc spheres in a P2VP matrix. However, in the latter case the bulk morphology could not be observed owing to the weak tendency of PVFc-containing block copolymers to undergo microphase separation.53−56 The response of the nanocapsules with a continuous polymer shell is dictated by the four possible states of the shell: nontriggered, triggered by one stimulus or the other, and triggered by both stimuli. As a consequence, four release modes are expected: no release, two selective releases, and a nonselective release by simultaneous activation of both stimuli. B. Stimuli-Responsive Nanocapsule Preparation and Characterization. The aforementioned block copolymers were used to prepare nanocapsules, i.e., core−shell spherical structures with a liquid core. Such structures are easily formed by the solvent evaporation process from miniemulsion droplets.41 This method was found to be suitable for the synthesis of nanocapsules from functional block copolymers, followed by the encapsulation of a large variety of chemical payloads.42 As originally reported by Torza et al., the key point for the successful formation of nanocapsules via the emulsionsolvent evaporation process is a careful balance of the interfacial tensions of the materials constituting the core, the shell and the solvent(s).41,57 In our case, the block copolymers PVFc-b-P2VP or PS-b-PDMAEMA were used as shell-forming material in this process. For this purpose, corresponding block copolymers were separately dissolved in a mixture of a good and a bad solvent for the two blocks of the copolymers, the good solvent being used in significantly larger amounts. The good solvent has a low-boiling point while the bad solvent features a high boiling point. The hydrophobic liquid was then emulsified in a basic aqueous solution of surfactant by ultrasonication to form a miniemulsion. The low-boiling point solvent was evaporated and the polymers precipitated at the droplets surface and formed a shell around the liquid core consisting of the highboiling point oil. The obtained nanocapsules displayed hydrodynamic diameters around 250 ± 110 nm. It is known that the relatively broad size distribution of the nanocapsules is inherent to the process used and not the result of droplet coalescence or aggregation.43 Core−shell structures were identified by electron microscopy (see Figure 3) with “holes” appearing in the inner side of the capsules created by the evaporation of the liquid core in the vacuum of the microscope. For comparison, the same procedure was used for corresponding homopolymers of the block copolymers. However, with the exception of P2VP, no capsules were formed under the described experimental conditions (see Figure S2). The responsivity of the nanocapsules was verified by applying stimuli to the nanocapsules dispersions. Upon acidification of the dispersions of both block copolymers, an increase in the hydrodynamic diameter of the capsules was detected (see Figure S3) due to the gradual protonation and subsequent swelling of the pH-responsive blocks in water. As expected, the increase was more pronounced for PVFc-b-P2VP than for PS-bPDMAEMA as the relative amount of pH-responsive groups was lower in the latter copolymer (13 wt % PDMAEMA against 93 wt % P2VP). The swelling of the PVFc-b-P2VP nanocapsules was also observed after their oxidation with an increase of capsules size from 277 to 354 nm. Remarkably, in all cases the colloidal dispersions remained stable. The nonresponsive

Figure 3. Nanocontainers display hollow nanocapsules morphology. SEM (a, b) and TEM (c, d) micrographs of the nanocapsules prepared with PVFc-b-P2VP (a, c) and with PS-b-PDMAEMA (c, d). In the upper right-hand corner, a capsule with two payloads is schematically depicted.

blocks ensured the structural integrity of the nanocapsules by preventing dissolution of the copolymer into the aqueous phase. The changes in the hydrodynamic diameter were accompanied by changes in the topography of the nanocapsules surface. Therefore, changes of the topography were detected by electron microscopy, which revealed small outgrowths on the surface of the block copolymer nanocapsules (Figure 4). Pronounced differences between PVFc-b-P2VP and PS-bPDMAEMA were visible. Whereas the acidified PVFc-b-P2VP nanocapsules formed film-like structures upon drying in which the capsular morphology was difficult to be detected, the PS-bPDMAEMA surface only displayed surface roughening while preserving the capsule identity. This difference was attributed to the different amounts of stimuli-responsive blocks in each block copolymer. C. Stimuli-Responsive Release Out of the Interior of the Nanocontainers. The stimuli-induced release of the nanocapsules payload was studied by encapsulating a fluorescent dye (Nile-red). Here, we made use of the fact that the fluorescence of Nile-red decreases significantly in polar media.44,45 Therefore, the opening of the capsules in an aqueous medium can be monitored by fluorescence spectroscopy without separating the nanocapsules from the aqueous phase. The capsule dispersions were dialyzed before applying the stimuli−pH change, redox reagent and temperature−in order to remove any residual nonencapsulated dye. Dispersions of both types of capsules were acidified to different pH values and change of the fluorescence intensity was followed. The PVFc-b-P2VP and PS-b-PDMAEMA nanocapsules displayed a pH-dependent release (Figure 5). Please note the inversed labeled y-axis for the fluorescence which indicates the decrease of fluorescence intensity due to exposure of the dye to water. At basic pH, there was hardly any observable release whereas the release reached 40% at intermediate pH-values (5−7). Upon further decreasing the pH to 2 and below, there was no dependence of the release on the pH anymore, i.e., the maximum release of Nile-red was achieved. 4879

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Figure 4. SEM micrographs of the PVFc-b-P2VP (a, b) and PS-b-PDMAEMA nanocapsules after their triggering by a lowering of the pH value (a, c) and by oxidation (b).

Figure 5. Nanocontainers can release a dye upon different stimuli. Release profiles of the dye for the nanocontainers of PVFc-b-P2VP (a) and PS-bPDMAEMA (b) upon acidification.

Figure 6. Release profiles of the dye for the nanocontainers consisting of PVFc-b-P2VP at different pH values upon oxidation (a) and PS-bPDMAEMA at different temperatures (b).

intensity was detected at basic or neutral pH, which indicates a release of Nile-red (Figure 6a). Therefore, the nanocapsules can be selectively addressed by a specific stimulus (pH or oxidation) to yield an output in the form of a swelling and a release. Upon further acidification of the dispersion, the release was more pronounced because of the protonation and subsequent swelling of the P2VP blocks as simplified schematized in Figure S5. In the case of PS-bPDMAEMA copolymer, PDMAEMA as block segment is responsive to both pH and temperature changes and therefore the temperature responsiveness were verified at various pH values. No effect of the temperature on the release or absence of release was observed in basic and very acidic pH solutions,

For comparison, control samples were synthesized with PS, P2VP, PDMAEMA, and PVFc homopolymers and acidified. As expected, the control samples of P2VP and PDMAEMA showed a release profile similar to the block copolymer ones (Figure S4), whereas the release from the PS control sample was negligible. Aggregation and sedimentation was observed upon the acidification of PVFc control sample so that no release study could be performed. The response of the dispersions of PVFc-b-P2VP capsules was investigated further by oxidizing them by adding H2O2 at various concentrations to the dispersions (see Experimental Section). Remarkably, a significant drop of the fluorescence 4880

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Figure 7. Schematics of the release behavior and swelling of PS-b-PDMAEMA capsules. At high pH, there is no release independently of the temperature, whereas release occurs only below the LCST at intermediate pH. At low pH, the release behavior is again independent of the temperature.

Figure 8. 1H NMR spectra of the dispersions of untreated and treated aqueous dispersions of the stimuli-responsive nanocontainers. The release of DDA occurred upon a pH-change and not upon oxidation (a) while the release of DPDS occurred upon oxidation and not upon a pH-change (b).

can be easily monitored by 1H NMR spectroscopy because the signal of the methyl group protons changes from ∼2.10 ppm in the deprotonated state to ∼2.85 ppm in the protonated state. Second, diphenyl disulfide (DPDS) was selected as model for redox-switchable payloads because the hydrophobic sulfide can be converted into a water-soluble sulfoxide or sulfone upon oxidation.47 Therefore, the release could be monitored directly by 1H NMR by using deuterated water instead of water as continuous phase. As shown in Figure 8a by the appearance of a signal at ∼2.85 ppm in the NMR spectrum of the nanocontainers treated with an acidic solution, DDA could be selectively released upon protonation of the capsules whereas it remained in the capsules upon triggering by oxidation. Similarly, DPDS was selectively released from the capsules upon oxidation but not upon acidification (Figure 8b). This means that a selective release from nanocapsules containing two encapsulated payloads was achieved by the combination of a dual-responsive shell and the presence of two encapsulated switchable payloads that can be selectively addressed by the same stimuli that trigger the shell. The simplified concept is depicted as a matrix in Figure 9. Remarkably, an inert polymer shell encapsulating switchable molecules did not yield any release because of the lack of contact between the chemical stimuli and the encapsulated payloads. Furthermore, a dual-responsive shell with nonswitchable molecules yielded a release of the encapsulated payloads that is not selective. The concept can be used in biomedicine as for instance it was shown that the dual delivery

i.e., the lower critical solution temperature (LCST) behavior was quenched under these conditions (Figure 6b). However, the release is suppressed at intermediate pH-values upon heating the dispersions above 75 °C. This is attributed to the PDMAEMA-chains that are not able to swell; hence no release is possible as schematically shown in Figure 7. The increasing of fluorescence intensity at higher temperatures and intermediate pH value indicates that the environment of the dye (Nile-red) was changed back to a hydrophobic state. Very similar observations were reported by other authors who investigated the flow of water through PS-b-PDMAEMA porous membranes at elevated temperatures under different pH-conditions.46 D. Investigating the Specific Release of Different Payloads out of the Nanocapsules. In order to achieve a system for which selective release of different payloads can be realized upon independent triggering by different stimuli, switchable molecules were introduced in the core of the nanocapsules consisting of PVFc-b-P2VP and PS-b-PDMAEMA. Indeed, encapsulated hydrophobic molecules cannot be released by activation of the nanocontainers with a stimulus if they are not switchable. Both types of stimuli-responsive nanocapsules containing two different payloads in the interior of the capsule were subjected to external triggers, i.e., change of pH value, oxidation reagents, and temperature. First, dimethyldodecylamine (DDA) was chosen as a model for pH-switchable payloads because it is hydrophobic at basic pH and water-soluble upon protonation. The state of the molecule 4881

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ASSOCIATED CONTENT

S Supporting Information *

Additional TEM image of PS-b-PDMAEMA, REM images for homopolymer capsules, DLS measurements upon acidification, release experiments and scheme for the redox-responsiveness (Figure S1−S5). This material is available free of charge via the Internet at http://pubs.acs.org/.

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AUTHOR INFORMATION

Corresponding Authors

*(M.G.) E-mail: [email protected]. *(D.C.) E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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Figure 9. Simplified matrix summarizing the stimulus-selective release of two payloads symbolized by cones and cubes in dependence on the stimuli used as external trigger.

ACKNOWLEDGMENTS We thank Mischa Bonn for the careful reading of the manuscript and his very useful comments. We acknowledge the scholarship from the Fonds der Chemischen Industrie (FCI) attributed to R.H.S. M.G. thanks Christian Schmidt for support with TEM measurements and the Landesoffensive zur Entwicklung Wissenschaftlisch-Ö konomischer Exzellenz (LOEWE) of the State of Hesse through research initiative Soft Control for ongoing financial support.

of growth factors was beneficial compared to the delivery of only one growth factor.48 Compared to systems previously reported for which a dual-response yields a release of payloads simultaneously,48 or with the same stimuli,49 or for which the systems are triggered by two different stimuli but from two different types of platforms,50 our system provides an independent selective release from the same platform.

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REFERENCES

(1) Arnold, F. H. Nature 2001, 409, 253. (2) Walsh, C. Nature 2001, 409, 226. (3) Preston, G. M.; Carroll, T. P.; Guggino, W. B.; Agre, P. Science 1992, 256, 385. (4) Murata, K.; Mitsuoka, K.; Hirai, T.; Walz, T.; Agre, P.; Heymann, J. B.; Engel, A.; Fujiyoshi, Y. Nature 2000, 407, 599. (5) Gouaux, E.; MacKinnon, R. Science 2005, 310, 1461. (6) Caterina, M. J.; Schumacher, M. A.; Tominaga, M.; Rosen, T. A.; Levine, J. D.; Julius, D. Nature 1997, 389, 816. (7) Cohen Stuart, M. A.; Huck, W. T. S.; Genzer, J.; Muller, M.; Ober, C.; Stamm, M.; Sukhorukov, G. B.; Szleifer, I.; Tsukruk, V. V.; Urban, M.; Winnik, F.; Zauscher, S.; Luzinov, I.; Minko, S. Nat. Mater. 2010, 9, 101. (8) Ma, Y. J.; Dong, W. F.; Hempenius, M. A.; Möhwald, H.; Vancso, G. J. Nat. Mater. 2006, 5, 724. (9) Power-Billard, K. N.; Spontak, R. J.; Manners, I. Angew. Chem., Int. Ed. 2004, 43, 1260. (10) Ma, N.; Li, Y.; Xu, H.; Wang, Z.; Zhang, X. J. Am. Chem. Soc. 2010, 132, 442. (11) Bae, Y.; Fukushima, S.; Harada, A.; Kataoka, K. Angew. Chem., Int. Ed. 2003, 42, 4640. (12) Haag, R. Angew. Chem., Int. Ed. 2004, 43, 278. (13) Alarcon, C. D. H.; Pennadam, S.; Alexander, C. Chem. Soc. Rev. 2005, 34, 276. (14) Yoshida, M.; Langer, R.; Lendlein, A.; Lahann, J. Polym. Rev. 2006, 46, 347. (15) Tauk, L.; Schroder, A. P.; Decher, G.; Giuseppone, N. Nat. Chem. 2009, 1, 649. (16) Hendricksson, G. R.; Smith, M. H.; South, A. B.; Lyon, L. A. Adv. Funct. Mater. 2010, 20, 1697. (17) Kim, J.; Yoon, J.; Hayward, R. C. Nat. Mater. 2010, 9, 159. (18) Mura, S.; Nicolas, J.; Couvreur, P. Nat. Mater. 2013, 12, 991. (19) Shchukin, D. G.; Möhwald, H. Science 2013, 341, 1458. (20) Lv, L. P.; Zhao, Y.; Vilbrandt, N.; Gallei, M.; Vimalanandan, A.; Rohwerder, M.; Landfester, K.; Crespy, D. J. Am. Chem. Soc. 2013, 135, 14198. (21) Beck, J. B.; Rowan, S. J. J. Am. Chem. Soc. 2003, 125, 13922. (22) Sagara, Y.; Kato, T. Nat. Chem. 2009, 1, 605. (23) Kota, A. K.; Kwon, G.; Choi, W.; Mabry, J. M.; Tuteja, A. Nature Commun. 2012, 3, 1025.

CONCLUSION

In conclusion, dual stimuli-responsive nanocapsules with a selectively addressable release of encapsulated payloads were prepared by the solvent-evaporation process from miniemulsion droplets. On the contrary to previously reported examples, the payloads were not covalently linked to the nanoparticles.51,52 The cores were mainly composed of a hydrophobic liquid and the shells were addressable by two different stimuli and based on functional block copolymers with either two different responsive blocks or one dual responsive block. The selective release of simultaneously encapsulated payloads in the nanocapsules was achieved by combining the dual responsive shell with a core containing payloads that switched their hydrophilicity upon triggering by various stimuli. The concept was demonstrated here for pH, temperature, and redox-responsive systems. This novel concept of stimuliresponsive selective release of payloads from hydrophobic platforms in water represents a significant advance in the synthesis of nanoparticles and nanocapsules for applications desired in water such as drug-delivery. Indeed, the drawbacks associated with the preparation of nanocapsules with hydrophilic core from inverse emulsions, i.e., low colloidal stability, leakage of encapsulated payloads due to osmotic pressure, and demanding transfer from oil to aqueous continuous phases, are completely avoided with this new strategy. This study will pave the way to addressable and highly functional polymer-based nanocarrier systems with unprecedented properties that are stable and can be employed in water. Applications are fore seen in a variety of disciplines, in particular in biomedicine: for instance, it has been demonstrated that the dual delivery of growth factors is beneficial compared to the delivery of only one growth factor.48 4882

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dx.doi.org/10.1021/ma501233y | Macromolecules 2014, 47, 4876−4883