Controlled Architecture of Hybrid Polymer Nanocapsules with Tunable

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Controlled architecture of hybrid polymer nanocapsules with tunable morphologies by manipulating surface-initiated ARGET ATRP from hydrothermally-modified polydopamine Zhen Zeng, Mingfen Wen, Gang Ye, Xiaomei Huo, Fengcheng Wu, Zhe Wang, Jiajun Yan, Yuexiang Lu, Jing Chen, and Krzysztof Matyjaszewski Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b04319 • Publication Date (Web): 20 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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Chemistry of Materials

Controlled architecture of hybrid polymer nanocapsules with tunable morphologies by manipulating surface-initiated ARGET ATRP from hydrothermally-modified polydopamine Zhen Zeng†, Mingfen Wen†, Gang Ye*†‡, Xiaomei Huo†, Fengcheng Wu†, Zhe Wang†, Jiajun Yan§, Krzysztof Matyjaszewski*§, Yuexiang Lu†‡, Jing Chen†‡ † Collaborative Innovation Center of Advanced Nuclear Energy Technology, Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing, 100084, China ‡ Beijing Key Lab of Radioactive Waste Treatment, Tsinghua University, Beijing 100084, China §Department of Chemistry, Carnegie Mellon University, 4400 Fifth Avenue, Pittsburgh, Pennsylvania 15213, United States ABSTRACT: This study presents a facile hydrothermal approach in ‘Tris’ buffer solution for fabricating polydopamine (PDA) nanostructures with different morphologies (core-shell, yolk-shell, and hollow spheres), while concurrently tuning their stability, permeability, and reactivity. Structural and morphological transformation of PDA induced by hydrothermal treatments was elaborated. Surface-initiated growth of poly(N-isopropylacrylamide) (PNIPAM), a thermally responsive polymer, was then achieved from the hydrothermally-treated PDA by using the activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP). Interestingly, inward- and outward-growth of polymer chains from PDA hollow spheres could be manipulated dependent on the solvation effect and hydrothermally-induced regulation of the pore network in PDA structure. Hybrid PDA/PNIPAM nanocapsules with tunable morphologies could be obtained. This study provided new insights into PDA’s structural evolution under hydrothermal treatments, and developed a promising strategy by combining the bio-inspired PDA chemistry with ARGET ATRP, an environmentally friendly and easily operated polymerization technique, to prepare stimuli-responsive nanocapsules which would be potentially used in biological and biomedical areas.

INTRODUCTION Developing methods for fabricating functional nanocapsules with well-defined structure, controlled morphology, and tailored responsive behavior to external stimuli has drawn significant attention due to their great potential for a range of biomedical applications.1-5 The past decade witnessed the encouraging development of the mussel-inspired polydopamine (PDA), pioneered by Messersmith et al. as a universal surface coating technique.6-8 Then it was extensively exploited as interfaces and building blocks for the architectural design of various functional nanostructures.9-13 Because of the intriguing properties including non-surface specific adhesion, postfunctionalization accessibility, and biocompatibility, PDA provides an ideal platform for fabricating nanocapsules potentially used as stimuli-responsive drug-release systems.14-17 Among the various established strategies, colloidal template synthesis is quite straightforward and versatile, offering a precise control over the size and uniformity of the nanocapsules.18-21 To functionalize the PDA nanocapsules, a promising strategy is grafting of functional polymers from the PDA encapsulated nanoparticles

using atom transfer radical polymerization (ATRP),22-24 which allows the generation of dense polymer brushes with predetermined molecular weight (MW) and controlled architecture,25, 26 followed by selective removal of the sacrificial templates.27, 28 However, traditional ATRP is sensitive to air and impurities in the system, and requires the use of a large amount of transition metal catalyst.29, 30 The residual transition metal catalyst in the products would impede their applications in biomedical and biological areas.25, 31 As a milestone of the development of ATRP techniques, the activators regenerated by electron transfer atom transfer radical polymerization (ARGET ATRP), first reported in Matyjaszewski’s group,29, 32 provides a continuous controlled polymerization with significant reduction of the amount of copper catalyst (down to ~ 10 ppm) due to a constant regeneration of the Cu(I) activator species by using environmentally acceptable reducing agents, which compensate for any loss of Cu(I) by termination.33 In a ARGET ATRP system, the use of reducing agents allows the starting of polymerization with the oxidatively stable Cu(II) species, and improves the tolerance against air and some other radical traps. Besides, ARGET ATRP can be

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even implemented in aqueous media.34, 35 In recent years, surface-initiated ARGET ATRP has been developed as en enabling tool for engineering the structure and properties of polymer/inorganic and polymer/organic interfaces.24 Very recently, the marriage between ARGET ATRP and PDA chemistry has shown promise for developing functional materials and/or interfaces for biological and environmental applications.36, 37 On the other hand, despite increasing number of reports on PDA-based nanostructures found in the literature, the self-polymerization mechanism of dopamine and the exact structure of PDA remains under discussion.38-40 To manipulate the self-polymerization behavior of dopamine, researchers have identified a series of controlling factors by use of oxidants,41, 42 reducing agents,43 UV light,44 atmosphere plasma,45, 46 etc.47 But only limited evidences about the structural transformation of dopamine during the polymerization were obtained.48-50 Rational design of PDA-based functional nanocapsules relies on the establishment of a clear framework for the structure-property-function relationship of PDA.51 In present study, a facile hydrothermal approach was developed for fabricating PDA hollow spheres without extra operations for template removal, while tuning their structural stability, permeability and reactivity. Controlled growth of poly(N-isopropylacrylamide) (PNIPAM) brushes was performed from the initiator-anchored PDA hollow spheres using the ARGET ATRP method. Interestingly, oriented growth of polymer brushes, either mainly inside the cavity or from the external surfaces of PDA hollow spheres, was observed dependent on the hydrothermal treatments under different temperatures. Morphological and structural investigations about the PDA hollow spheres and the obtained hybrid nanocapsules were carried out. This study is expected to shed light on the structural evolution of PDA under hydrothermal treatments. Moreover, the established strategy by integrating surface-initiated ARGET ATRP with PDA chemistry would provide a more favorable way for fabricating bio-friendly nanomaterials.

Experimental Materials. Tris (hydroxymethyl) aminomethane (Tris) (>99%), N-isopropylacrylamide (NIPAM) (99%), dopamine hydrochloride (DA) (98%), 2-bromoisobutyryl bromide (BiBB) (98%), triethylamine (TEA) (98%), Lascorbic acid (98%), copper-(II) chloride (97%), N,Ndimethylformamide (DMF) (99.7%, ACS/HPLC certified), ethanol (99.9%, ACS/HPLC certified), and methanol (99.9%, ACS/HPLC certified) were supplied by J&K Scientific Co., Ltd. Tris[2-(dimethylamino)ethyl]amine (Me6TREN) (98%) and ethyl 2-bromo-2methylpropionate (EBiB) were purchased from SigmaAldrich. Monodisperse SiO2 particles with diameter centered at 120 nm were synthesized following a previously reported method.52 The morphology was examined under TEM (Fig. S1). Synthesis of SiO2@PDA and hydrothermal treatment. As-synthesized SiO2 particles (0.4 g) were pretreated by washing in ethanol and deionized (DI) water,

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respectively, followed by the dispersion in 300 mL Tris buffer solution (10 mM, pH 8.5). After ultrasonication for 3 min, 0.4 g dopamine hydrochloride dissolved in 10 mL DI water was added dropwise into the mixture under mechanical stirring (300 rpm). The coating process lasted for 20 h at room temperature. Then, the black mixture was transferred into a Teflon-lined autoclave. Hydrothermal treatments were carried out in oven under different temperatures (80 oC, 140 oC, and 160 oC) for 24 h. The obtained dark suspension or precipitation was collected by a centrifuge at 5000 rpm for 5 min, washed by ethanol and DI water respectively, and finally dried in atmosphere. Besides, for discussion about the composition and surface chemistry of PDA, pristine PDA nanoparticles were prepared using the same therapy and were hydrothermally treated in Tris buffer under corresponding temperatures. Anchoring of ATRP initiator. Hydrothermally treated SiO2@PDA (0.1 g) was dispersed in 30 mL DMF under magnetic stirring, followed by the addition of 1.0 mL TEA (7.2 mmol). After cooling the mixture below 5 oC in an ice bath, 0.9 mL BiBB (7.2 mmol) dissolved in 10 mL DMF was added dropwise under argon protection. The temperature of the reaction system was gradually raised to room temperature. After 24 h, dark brown products were separated by centrifugation, washed by ethanol repeatedly, and dried under vacuum at 50 oC. ARGET ATRP growth of PNIPAM. The aboveobtained BiBB-anchored sample (50 mg) was dispersed in 10 mL methanol/DI water (V/V=2/1) mixture solution in a 50-mL Schlenk flask equipped with magnetic stir bar. After ultrasonication for 3 min, NIPAM monomer (0.9 g, 8 mmol), copper-(II) chloride (0.001 g, 7 μmol), and Me6TREN (10 μL, 35 μmol) were introduced to the mixture. The reaction system was sealed and vacuum-argon inflation cycle was operated for at least five times. A solution of reducing agent was prepared by dissolving Lascorbic acid (20 mg, 0.11 mmol) in 10 mL methanol/DI water (V/V=2/1). Under argon atmosphere, the L-ascorbic acid solution was continuously injected into the reaction medium using a syringe pump at the rate of 1 μL/min. The products were collected after 24 h by centrifugation, washed with DMF and ethanol in turn for several times, and dried under vacuum at 50 oC. For estimating the molecular weight (MW) and polydispersity of the polymers, A sacrificial initiator EBiB (10 μL, 67 μmol) was introduced to the above reaction system. Free PNIPAM polymer was precipitated and separated by dissolving in THF and re-precipitation in ethanol. The polymer was dried at 50 oC under vacuum overnight before used for MW measurements. Characterizations. Transmission electron microscopy (TEM) images were recorded by use of H-7700 microscope with an accelerating voltage of 120 kV. Power X-ray diffraction (XRD) patterns were obtained on a Rigaku D/max-2400 X-ray diffractometer with Cu Kα radiation. The surface chemistry and composition analysis of the samples were examined by a 250XI X-ray photoelectron spectroscopy (XPS) spectrometer equipped with a mono Al Kα X-ray source (1361 eV). Elemental analysis of C, H, and N was performed on an Elementar Vario EL III. Spe-


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cific surface areas were evaluated by the Brunauer − Emmett−Teller (BET) method, and the Barrett−Joyner− Halenda (BJH) method was employed for the analysis of the pore size distribution. Number-average molecular weights (Mn) and polydispersity index (PDI) was determined by a gel permeation chromatograph (GPC) (GPC20A, Shimadzu Corp.), using THF as eluent at a flow rate of 1.0 mL/min at 40 °C. A series of low-polydispersity polystyrene (PS) standards were employed for the GPC calibration. Thermogravimetric analysis (TGA) was conducted on a TA Instruments SDT Q600 with a heating rate of 10 °C/min from 30 °C to 1000 °C. Samples weighing ~ 10 mg were heated in N2 flow (100 mL/min). Differential scanning calorimetry (DSC) was carried out using DSC Q2000 ((TA instruments Inc.). Samples were placed in sealed aluminum pans. The coolant was liquid nitrogen. The samples were scanned at 20 °C/min from 20 °C to 400°C. Static water contact angles were measured at ambient temperature using the sessile drop method and image analysis of the drop file on a contact angle system (DSA30, Kruss GmbH).

Results and discussion To fabricate functional nanocapsules, one of the wellestablished approaches is colloidal template synthesis followed by selective removal of the core particles using acid or base etching. Monodisperse SiO2 nanoparticles with diameter of ~ 120 nm (Fig. S1) were prepared and used as templates. By controlling the oxidative selfpolymerization of dopamine in Tris buffer solution (pH=8.5), uniform PDA encapsulation layers were deposited on the surfaces of the SiO2 nanoparticles to form core-shell structure (Fig. 1(a)). The synthetic strategy here employed a facile hydrothermal treatment after the coating process, without separating the products, to improve the physicochemical properties of the PDA shell. Simultaneously, taking advantage of the weak basic environment in Tris buffer, the SiO2 cores could be removed, directly forming PDA hollow nanospheres. Morphology of the products obtained at different hydrothermal temperatures was recorded by TEM. Fig. 1(b) shows that, the SiO2 cores were partially dissolved by 80 o C hydrothermal treatment for 24 h, resulting in welldefined yolk-shell structures. Control experiment in DI water (Fig. S2) confirmed that the basic environment of Tris buffer was necessary for hydrothermal dissolving of the SiO2 cores. Raising the hydrothermal temperature to 140 oC and 200 oC, PDA hollow nanospheres were obtained with complete dissolution of the SiO2 cores (Fig. 1 (c) & (d)). It appears that the hydrothermally treated products show relatively smoother external surfaces and decreased thickness of the PDA encapsulation layers in comparison with the as-synthesized core-shell SiO2@PDA nanoparticles. In the following description, hPDA-X (X=80, 140, 200) refers to the hydrothermally-treated PDA hollow spheres obtained at corresponding hydrothermal temperatures.

Figure 1. TEM images of core-shell SiO2@PDA nanoparticles (a) and the counterparts after hydrothermal treatment at 80 o o o C (b), 140 C (c), and 200 C (d) for 24 h.

Composition and structure of hPDA-X (X=80, 140, 200) were examined by detailed analysis of the XPS spectra. High resolution C 1s and N 1s regions with deconvolution analysis were presented in Fig. 2. The chemical species were identified with their contents summarized in Table 1. The C 1s region was fit with three main components assigned to C-C/C=C (284.5 eV), C-N/C-O (285.8 eV), and C=O/C=N (288.3 eV). The peak profiles corresponding to the C 1s components basically remained constant under hydrothermal treatment, with the dominant peaks attributed to C-C/C=C and C-N/C-O species. A significant amount of C=O species was assigned to the tautomers of catechol units.39 The N 1s region was fit with three peaks corresponding to primary (-NH2, 401.0 eV), secondary (NH-, 400.0 eV), and tertiary/aromatic (=N-, 398.9 eV) amine functionalities. It has been accepted that a part of primary amine groups exist in the building blocks of PDA.53 The secondary amine is associated with the cyclized intermediates generated during the oxidative polymerization which also exist in the final PDA. And, the tertiary amine belongs to the tautomeric species of the intermediates 5,6-dihydroxyindole and 5,6indolequinone.54 Compare to the pristine PDA, the hydrothermally-treated counterparts show decreased amount of primary amine, suggesting that hydrothermal treatment would promote the conversion of dopamine and primary amine-containing species into cyclized intermediates and tautomers (secondary and tertiary amine species). Besides, the O 1s region was fit with two peaks assigned to O-C and O=C species, as shown in Fig. S3. And, it is noted that the N/C molar ratio of the samples by XPS analysis remained relatively constant, with a slight decrease tendency under higher temperature hydrothermal treatment (Table 1). This is in accordance with the results obtained by elemental analysis (Table S1).



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Figure 2. High resolution XPS spectra of C 1s and N 1s regions for PDA and the counterparts after hydrothermal treatments at 80 oC, 140 oC, and 200 oC, respectively. Table 1. XPS analysis of chemical species and content of PDA nanoparticles and their hydrothermal treated counterparts. C 1s (%) Samples

N 1s (%) N/C molar ratio

C-C/C=C

C-N/C-O

C=O/C=N

=N-

-NH-

-NH2

284.5 eV

285.8 eV

288.3 eV

398.9 eV

400.0 eV

401. 0 eV

PDA

45.8

43.5

11.7

17.4

51.5

31.1

0.12

hPDA-80

48.1

41.6

11.3

19.6

55.2

25.2

0.12

hPDA-140

51.2

32.9

15.9

24.2

50.0

25.8

0.12

hPDA-200

45.9

46.2

7.9

26.1

50.8

23.1

0.11

Surface and pore properties of PDA and the hydrothermally treated products were examined by N2 adsorption-desorption measurements. It can be seen in Fig. S4 that none of the samples showed typical isotherm of porous materials with uniform micro- or meso-pores, suggesting the heterogeneous porosity in the structure of PDA. The BET specific surface area, pore size and volume were calculated, as shown in Table 2. The PDA nanoparticles after hydrothermal treatment exhibited an evidently increased specific surface area and pore volume. It is speculated that hydrothermal treatment can regulate the structure of PDA and improve the porosity by promoting conversion of monomers and oligomers into cyclized tautomers.48-50 But both small-angle and wide-angle XRD measurements showed similar spectra for PDA and hPDA-X (X=80, 140, 200) (Fig. 3). No well-defined diffraction peaks could be identified except a broad peak around 22o, corresponding to a d-spacing 0.44 nm of layered carbon structures of PDA.55, 56 So, it is concluded that hydrothermally treated PDA is mainly amorphous with some short-range ordering and graphite-like structure which is similar to that of carbonized PDA.50 However, the surface and pore properties of PDA could not be further improved at higher hydrothermal treatment temperature beyond 140 oC. The hPDA-200 sample showed relatively decreased specific surface area and porosity than that of hPDA-140.

Figure 3. Small-angle (a) and wide-angle (b) XRD patterns of PDA and the counterparts after hydrothermal treatments at 80 oC, 140 oC, and 200 oC, respectively.


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Figure 4 Morphologies after initiator anchoring reaction in DMF for SiO2@PDA (a), hPDA-80 (b), hPDA-140 (c), and hPDA-200 (d), and, morphologies with PNIPAM growth in ARGET ATRP system for SiO2@PDA (e), hPDA-80 (f), hPDA140 (g), and hPDA-200 (h). Table 2. BET specific surface area, pore volume and size of PDA and the counterparts after hydrothermal treatments at 80 oC, 140 oC, and 200 oC, respectively. Samples

Specific surface area (m2/g)

Pore volume (cm3/g)

Pore size (nm)

PDA

15.6

0.03

3.1

hPDA-80

219.7

0.29

3.1

hPDA-140

500.7

0.52

3.1

hPDA-200

487.7

0.44

3.1

Due to the well-defined structure and preservation of abundant reactive groups, the PDA hollow nanospheres obtained by hydrothermal treatment can be an ideal platform for fabricating functional nanocapsules. Surfaceinitiated growth of PNIPAM, a thermal sensitive polymer, was performed from the surfaces of PDA by using ARGET ATRP. Firstly, 2-bromoisobutyryl bromide (BiBB) initiators were introduced via a nucleophilic substitution reaction with the primary amino groups in PDA. It is noteworthy that the pristine PDA, due to the existence of a large number of oligomers with non-covalent interactions,10 was not quite stable in polar organic solvent. The PDA encapsulation layer was damaged evidently after 24 h reaction in DMF (Fig. 4(a)). Even for the yolk-shell structured SiO2@PDA after 80 oC hydrothermal treatment, deformation of the PDA shell was observed (Fig. 4(b)). In comparison, the mechanical strength of PDA was improved by thermal treatment under higher temperature. Either hPDA-140 (Fig. 4(c)) or hPDA-200 (Fig. 4(d)) showed well-preserved hollow structure. On the other hand, a concerned issue is the reactivity of PDA after hydrothermal treatment, which is associated with the capacity for accommodating the initiator molecules. Previous XPS analysis revealed that significant amount of primary amine functionalities existed in PDA, which

would provide abundant reactive sites for the anchoring of BiBB. After the reaction, XPS analysis of Br species (Fig. S5) showed that the initiators were successfully introduced to the surfaces of PDA and the hydrothermally treated samples, suggesting that moderate hydrothermal treatment did not compromise the reactivity of PDA. Then, ARGET ATRP was employed for generating PNIPAM brushes from the BiBB-anchored PDA surfaces. Hybrid PDA/PNIPAM nanocapsules with different morphologies were obtained, dependent on the hydrothermal treatments. For the untreated SiO2@PDA samples, with the conformal PDA shells partially damaged when introducing the initiators, near-spherical polymer layers with relatively loose encapsulation were generated (Fig. 4(e)). Surface-initiated polymerization from BiBB-anchored hPDA-80 with deformed yolk-shell structure resulted in irregular polymer growth with interparticle fusion (Fig. 4(f)). In either case, hybrid PDA/PNIPAM nanocapsules could be obtained after further removal of the SiO2 cores by HF etching. Because the PDA framework was not rigid to support the polymer layer, the resultant hybrid nanocapsules showed a red blood cell-like morphology (Fig. S6). For hPDA-140, interestingly, the PNIPAM chains mainly exhibited an inward-growth behavior in the ARGET ATRP system. The inner void of the hPDA-140 hollow spheres was fully occupied by the polymer chains (Fig. 4(g)). Only limited PNIPAM chains grew from the external surfaces. Supportive evidences were also obtained by TEM imaging of the intermediate products of PNIPAM growth from hPDA-140 at different polymerization time, showing the gradual filling of inner void by inward-growth of polymer chains (Fig. 5). On the contrary, the PNIPAM chains were only grown from the external surfaces of the hPDA-200 hollow spheres, forming a double-layered shell while preserving the cavity (Fig. 4(h)). The inset shows that the thickness of exterior PNIPAM



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Figure 5. TEM images of intermediates products of PNIPAM growth from hPDA-140 hollow spheres with polymerization time of 2 h (a), 6 h (b), 12 h (c), and 20 h (d).

Figure 6. Water contact angles of PDA (a) and the hydrothermally-treated counterparts (b-d) at different temperatures. The PDA layers were deposited on polycarbonate membranes. layer is about 20 nm. The hybrid PDA/PNIPAM nanocapsules could be well-dispersed and stabilized in water. By using ethyl 2-bromo-2-methylpropionate (EBiB) as a sacrificial initiator, the number-average molecular weight (Mn) of the PNIPAM chains was estimated to be ~ 11,730 with a dispersity Mw/Mn=1.29.57, 58 The distinct inward- and outward-growth behaviors of polymer chain from the hydrothermally-treated PDA hollow nanospheres, i.e., hPDA-140 and hPDA-200, should be attributed to the solvation effect in the polymerization system and structural transformation of PDA under hydrothermal treatment. Firstly, it was found in our study that the hydrophilicity of PDA could be tuned by hydrothermal treatment in Tris buffer. The PDA deposition layer after 140 oC hydrothermal treatment showed smaller water contact angle than other conditions (Fig. 6). Meanwhile, methanol is a poor solvent for PNIPAM and is usually employed for the precipitation of PNIPAM chains. Thus, to maintain a minimum interfacial energy, it is more preferable for the PNIPAM chains to grow inward

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Figure 7. TGA curves of PDA nanoparticles and the counterparts after hydrothermal treatment at different temperatures (left Y-axis), and DTG plots of PDA nanoparticles (right Y-axis). from the inner surfaces of PDA nanocapsules which can provide a localized hydrophilic environment.9 On the other hand, the prerequisite of polymer growth inside the cavity of the PDA nanocapsules is the permeability to allow the monomer transfer across the PDA shell. Apparently, hPDA-140 was permeable for NIPAM monomer to transport inside and grow from the inner surfaces. It is also noted that the inner void of yolk-shell structured hPDA-80 was filled by PNIPAM chains grown inside during the polymerization. In comparison, the permeability of PDA seemed to be reduced by hydrothermal treatment at higher temperature, leading to the growth of PNIPAM chains only from the external surfaces of hPDA-200. Thermogravimetric analysis (TGA) was used to investigate the structural transformation of PDA and hPDA-X (X=80, 140, 200) for examining the thermal stability. The TGA curves in Fig. 7 show similar profiles with multistep weight loss because of the chemical heterogeneity of PDA and the presence of different volatile species.59 By contrast, less weight loss was found for the samples with hydrothermal treatment at higher temperatures. This indicates the hydrothermal treatment can be an effective way to improve the thermal stability of PDA. In addition, the derivative thermogravimetric analysis (DTG) plots of PDA (Fig. 7) show that the weight loss can be roughly divided into three stages. The first stage of weight loss occurs around 100 oC which corresponds to the water evaporation, and the third stage till 800 oC is attributed to the carbonization of PDA. Endothermic effect in both these processes is shown in the differential scanning calorimetry (DSC) curve (Fig. S7). It is interesting to observe the second stage from 150 oC to 250 oC, with relatively slow weight loss and non-significant heat effect. The stage reflects the conversion of intermediates in PDA structure, such as cyclization, isomerization, crosslinking, etc,60 corresponding to the structural transformation of PDA under hydrothermal treatment. Here, a hypothesis is proposed to explain the reduced permeability of hPDA-200 (or the enhanced compact stacking of the PDA building blocks), as compared to hPDA-140. During the hydrothermal


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treatment at 200 oC, structural transformation of PDA occurs due to substantial conversion of intermediates, which reassembles the building blocks to form more compact stacking, and alter the connectivity of the pore network to retard the diffusion of molecules and ions across the PDA layer. Therefore, the PNIPAM chains can only grow from the external surface of hPDA-200. Regarding the decreased permeability of PDA after hydrothermal treatment, evidence is also provided by the etching test of PDA encapsulated magnetite particles in 0.1 mol/L HNO3 solutions. It can be seen in Fig. S8 that leaching rate of Fe is much slower for the 200 oC hydrothermally-treated sample, suggesting that the mass transfer across the PDA was retarded after hydrothermal treatment.

Conclusion This study presented a facile hydrothermal approach to tailor the structure, morphology, and properties of PDA, which served as a favorable platform for building hybrid nanocapsules via surface-initiated polymer grafting. Controlled growth of thermosensitive PNIPAM chains from the hydrothermally-treated PDA was achieved by using ARGET ATRP. Hybrid nanocapsules with different morphologies were obtained dependent on the hydrothermal conditions. The inward- and outward-growth of polymer from PDA hollow spheres revealed new insights into the structural transformation of PDA induced by hydrothermal treatment, and may inspire the fabrication of stimuliresponsive capsules with different controlled release behaviors. In addition, the integration of ARGET ATRP technique with biocompatible PDA nanostructures provided a versatile tool to develop nanomaterials for broadened biological and biomedical applications.

SUPPORTING INFORMATION TEM images, high-resolution XPS spectra of O 1s regions, N2 adsorption-desorption isotherms, elemental analysis, XPS analysis of surface Br content, HNO3 treatment experiments, DSC curves. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected]. (G. Ye) *E-mail: [email protected]. (K. Matyjaszewski)

ACKNOWLEDGMENT This study was supported by the Changjiang Scholars and Innovative Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), National Natural Science Foundation of China under Project 51673109 and 51473087. KM acknowledged support for the Department of Energy (ER45998).



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Controlled Architecture of Hybrid Polymer Nanocapsules with Tunable

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