Controlled Architecture of Hybrid Polymer Nanocapsules with Tunable

Article Cite This: Chem. Mater. 2017, 29, 10212−10219

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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,§ Krzysztof Matyjaszewski,*,§ Yuexiang Lu,†,‡ and 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 S Supporting Information *

ABSTRACT: This study presents a facile hydrothermal approach in a “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 is described in detail. Surfaceinitiated 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 depending on the solvation effect and hydrothermally induced regulation of the pore network in the PDA structure. Hybrid PDA/PNIPAM nanocapsules with tunable morphologies could be obtained. This study provides new insights into PDA’s structural evolution under hydrothermal treatments and develops a promising strategy by combining the bioinspired PDA chemistry with ARGET ATRP, an environmentally friendly and easily operated polymerization technique, to prepare stimulus-responsive nanocapsules that would be potentially used in biological and biomedical areas.

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(ATRP),22−24 which allows the generation of dense polymer brushes with a 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 a significant reduction in 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

INTRODUCTION Developing methods for fabricating functional nanocapsules with well-defined structure, controlled morphology, and tailored responsive behavior to external stimuli has drawn significant attention because of 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 its intriguing properties, including non-surface specific adhesion, postfunctionalization accessibility, and biocompatibility, PDA provides an ideal platform for fabricating nanocapsules potentially used as stimulus-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 © 2017 American Chemical Society

Received: October 13, 2017 Revised: November 15, 2017 Published: November 20, 2017 10212

DOI: 10.1021/acs.chemmater.7b04319 Chem. Mater. 2017, 29, 10212−10219

Article

Chemistry of Materials termination.33 In a ARGET ATRP system, the use of reducing agents allows polymerization to start with the oxidatively stable Cu(II) species and improves the tolerance against air and some other radical traps. In addition, ARGET ATRP can be even implemented in aqueous media.34,35 In recent years, surfaceinitiated 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 the increasing number of reports on PDA-based nanostructures found in the literature, the selfpolymerization mechanism of dopamine and the exact structure of PDA remain under discussion.38−40 To manipulate the selfpolymerization behavior of dopamine, researchers have identified a series of controlling factors by use of oxidants,41,42 reducing agents,43 ultraviolet light,44 atmosphere plasma,45,46 etc.,47 but only limited evidence about the structural transformation of dopamine during the polymerization has been 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 this 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(Nisopropylacrylamide) (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 depending on the hydrothermal treatments at different temperatures. Morphological and structural investigations of the PDA hollow spheres and the obtained hybrid nanocapsules were performed. This study is expected to shed light on the structural evolution of PDA under hydrothermal treatments. Moreover, the established strategy of integration of surface-initiated ARGET ATRP with PDA chemistry would provide a more favorable way for fabricating biofriendly nanomaterials.

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suspension or precipitate was collected by centrifugation at 5000 rpm for 5 min, washed with ethanol and DI water, and finally dried in the atmosphere. In addition, for discussion of the composition and surface chemistry of PDA, pristine PDA nanoparticles were prepared using the same therapy and were hydrothermally treated in Tris buffer at rhe corresponding temperatures. Anchoring of the ATRP Initiator. Hydrothermally treated SiO2@ PDA (0.1 g) was dispersed in 30 mL of DMF under magnetic stirring, followed by the addition of 1.0 mL of TEA (7.2 mmol). After the mixture had been cooled to 99%), Nisopropylacrylamide (NIPAM, 99%), dopamine hydrochloride (DA, 98%), 2-bromoisobutyryl bromide (BiBB, 98%), triethylamine (TEA, 98%), L-ascorbic 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 2bromo-2-methylpropionate (EBiB) were purchased from SigmaAldrich. Monodisperse SiO2 particles with a diameter centered at 120 nm were synthesized following a previously reported method.52 The morphology was examined via transmission electron microscopy (TEM) (Figure S1). Synthesis of SiO2@PDA and Hydrothermal Treatment. Assynthesized SiO2 particles (0.4 g) were pretreated by being washed in ethanol and deionized (DI) water, followed by dispersion in 300 mL of a Tris buffer solution (10 mM, pH 8.5). After ultrasonication for 3 min, 0.4 g of dopamine hydrochloride dissolved in 10 mL of DI water was added dropwise to 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 performed in an oven at different temperatures (80, 140, and 160 °C) for 24 h. The obtained dark

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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 a diameter of ∼120 nm (Figure S1) were prepared and used as templates. Via the control of the oxidative self-polymerization of dopamine in 10213


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°C, PDA hollow nanospheres were obtained with complete dissolution of the SiO2 cores (Figure 1c,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 SiO 2@PDA nanoparticles. In the following description, hPDA-X (X = 80, 140, or 200) refers to the hydrothermally treated PDA hollow spheres obtained at corresponding hydrothermal temperatures. The composition and structure of hPDA-X (X = 80, 140, or 200) were examined by detailed analysis of the XPS spectra. High-resolution C 1s and N 1s regions with deconvolution analysis are presented in Figure 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) species. The peak profiles corresponding to the C 1s components basically remained constant during 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 some 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 that also exist in the final PDA, and the tertiary amine belongs to the tautomeric species of the intermediates 5,6-dihydroxyindole and 5,6-indolequinone.54 Compared to the pristine PDA, the hydrothermally treated counterparts show decreased amounts 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). In addition, the O 1s region was fit with two peaks assigned to O− C and OC species, as shown in Figure S3. It is noted that the N/C molar ratio of the samples determined by XPS analysis remained relatively constant, with a slight tendency to decrease during higher-temperature hydrothermal treatment (Table 1). This is in accordance with the results obtained by elemental analysis (Table S1).

a Tris buffer solution (pH 8.5), uniform PDA encapsulation layers were deposited on the surfaces of the SiO2 nanoparticles to form a core−shell structure (Figure 1a). The synthetic

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

strategy 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, we could remove the SiO2 cores, directly forming PDA hollow nanospheres. The morphology of the products obtained at different hydrothermal temperatures was recorded by TEM. Figure 1b shows that the SiO2 cores were partially dissolved by an 80 °C hydrothermal treatment for 24 h, resulting in well-defined yolk−shell structures. A control experiment in DI water (Figure S2) confirmed that the basic environment of Tris buffer was necessary for hydrothermal dissolving of the SiO2 cores. With an increase in the hydrothermal temperature to 140 and 200

Figure 2. High-resolution XPS spectra of C 1s and N 1s regions for PDA and the counterparts after hydrothermal treatments at 80, 140, and 200 °C. 10214


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Table 1. XPS Analysis of the Chemical Species and Content of PDA Nanoparticles and Their Hydrothermally Treated Counterparts C 1s (%)

N 1s (%)

sample

C−C/CC, 284.5 eV

C−N/C−O, 285.8 eV

CO/CN, 288.3 eV

N−, 398.9 eV

−NH−, 400.0 eV

−NH2, 401.0 eV

N/C molar ratio

PDA hPDA-80 hPDA-140 hPDA-200

45.8 48.1 51.2 45.9

43.5 41.6 32.9 46.2

11.7 11.3 15.9 7.9

17.4 19.6 24.2 26.1

51.5 55.2 50.0 50.8

31.1 25.2 25.8 23.1

0.12 0.12 0.12 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 Figure S4 that none of the samples showed the typical isotherm of porous materials with uniform micro- or mesopores, suggesting 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 Table 2. BET Specific Surface Areas, Pore Volumes, and Sizes of PDA and Their Counterparts after Hydrothermal Treatment at 80, 140, and 200 °C sample

specific surface area (m2/g)

pore volume (cm3/g)

pore size (nm)

PDA hPDA-80 hPDA-140 hPDA-200

15.6 219.7 500.7 487.7

0.03 0.29 0.52 0.44

3.1 3.1 3.1 3.1

exhibited an evidently increased specific surface area and pore volume. There has been speculation that hydrothermal treatment can regulate the structure of PDA and improve the porosity by promoting the conversion of monomers and oligomers into cyclized tautomers.48−50 Both small-angle and wide-angle XRD measurements showed similar spectra for PDA and hPDA-X (X = 80, 140, or 200) (Figure 3). No well-defined diffraction peaks could be identified except a broad peak around 22°, corresponding to a d spacing of 0.44 nm of layered carbon structures of PDA.55,56 Therefore, it is concluded that hydrothermally treated PDA is mainly amorphous with some short-range ordering and graphite-like structure that is similar to that of carbonized PDA.50 However, the surface and pore properties of PDA could not be further improved at hydrothermal treatment temperatures of >140 °C. The hPDA-200 sample showed a relatively decreased specific surface area and porosity compared to those of hPDA-140. Because of 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. Surface-initiated growth of PNIPAM, a thermally sensitive polymer, was performed from the surfaces of PDA by using ARGET ATRP. First, 2bromoisobutyryl bromide (BiBB) initiators were introduced via a nucleophilic substitution reaction with the primary amino groups in PDA. It is noteworthy that the pristine PDA, because of the existence of a large number of oligomers with noncovalent interactions,10 was not quite stable in a polar organic solvent. The PDA encapsulation layer was obviously damaged after reaction for 24 h in DMF (Figure 4a). Even for yolk−shell structure SiO2@PDA after 80 °C hydrothermal treatment, deformation of the PDA shell was observed (Figure 4b). In comparison, the mechanical strength of PDA was

Figure 3. (a) Small-angle and (b) wide-angle XRD patterns of PDA and their counterparts after hydrothermal treatment at 80, 140, and 200 °C.

improved by thermal treatment at higher temperatures. Either hPDA-140 (Figure 4c) or hPDA-200 (Figure 4d) showed a well-preserved hollow structure. On the other hand, one issue is the reactivity of PDA after hydrothermal treatment, which is associated with the capacity to accommodate the initiator molecules. Previous XPS analysis revealed that a 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 (Figure S5) showed that the initiators were successfully introduced onto 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 to generate PNIPAM brushes from the BiBB-anchored PDA surfaces. Hybrid PDA/ PNIPAM nanocapsules with different morphologies were obtained, depending on the hydrothermal treatments. For the untreated SiO2@PDA samples, with the conformal PDA shells partially damaged upon introduction of the initiators, nearly spherical polymer layers with relatively loose encapsulation were generated (Figure 4e). Surface-initiated polymerization from BiBB-anchored hPDA-80 with a deformed yolk−shell 10215


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Figure 4. Morphologies after the initiator anchoring reaction in DMF for (a) SiO2@PDA, (b) hPDA-80, (c) hPDA-140, and (d) hPDA-200 and morphologies with PNIPAM growth in the ARGET ATRP system for (e) SiO2@PDA, (f) hPDA-80, (g) hPDA-140, and (h) hPDA-200.

structure resulted in irregular polymer growth with interparticle fusion (Figure 4f). 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 sufficiently rigid to support the polymer layer, the resultant hybrid nanocapsules showed a red blood cell-like morphology (Figure 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 (Figure 4g). Only limited PNIPAM chains grew from the external surfaces. Supportive evidence was also obtained via TEM imaging of the intermediate products of PNIPAM growth from hPDA-140 at different polymerization times, showing the gradual filling of the inner void by the inward growth of polymer chains (Figure 5). On the contrary, the PNIPAM chains were grown from only the external surfaces of the hPDA-200 hollow spheres, forming a double-layer shell while preserving the cavity (Figure 4h). The inset shows that the thickness of the exterior PNIPAM layer is ∼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 ∼11730 with a dispersity Mw/Mn of 1.29.57,58 The distinct inward- and outward-growth behaviors of the 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. First, 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 °C hydrothermal treatment showed a water contact angle smaller than those under other conditions (Figure 6). Meanwhile, methanol is a poor solvent for PNIPAM and is usually employed for the precipitation of PNIPAM chains.

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

Thus, to maintain a minimum interfacial energy, it is more preferable for the PNIPAM chains to grow inward from the inner surfaces of PDA nanocapsules, which can provide a localized hydrophilic environment.9 On the other hand, a 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 the NIPAM monomer to transport inside and grow from the inner surfaces. It is also noted that the inner void of yolk−shell structure 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 temperatures, leading to the growth of PNIPAM chains only from the external surfaces of hPDA-200. 10216


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explain the reduced permeability of hPDA-200 (or the enhanced compact stacking of the PDA building blocks), as compared to that of hPDA-140. During the hydrothermal treatment at 200 °C, structural transformation of PDA occurs due to substantial conversion of intermediates, which reassembles the building blocks to form a more compact stacking and alters the connectivity of the pore network to retard the diffusion of molecules and ions across the PDA layer. Therefore, the PNIPAM chains can grow from only the external surface of hPDA-200. With regard to 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 Figure S8 that the rate of leaching of Fe is much slower for the 200 °C hydrothermally treated sample, suggesting that the mass transfer across the PDA was retarded after hydrothermal treatment.

Figure 6. (a) Water contact angles of PDA and (b−d) their hydrothermally treated counterparts at different temperatures. The PDA layers were deposited on polycarbonate membranes.

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CONCLUSION This study presents a facile hydrothermal approach to tailoring 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 depending on the hydrothermal conditions. The inward and outward growth of the polymer from PDA hollow spheres revealed new insights into the structural transformation of PDA induced by hydrothermal treatment and may inspire the fabrication of stimulus-responsive capsules with different controlled-release behaviors. In addition, the integration of the ARGET ATRP technique with biocompatible PDA nanostructures provided a versatile tool for developing nanomaterials for broadened biological and biomedical applications.

Thermogravimetric analysis (TGA) was used to investigate the structural transformation of PDA and hPDA-X (X = 80, 140, or 200) for examining thermal stability. The TGA curves in Figure 7 show similar profiles with multistep weight loss

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ASSOCIATED CONTENT

S Supporting Information *

Figure 7. TGA curves of PDA nanoparticles and their counterparts after hydrothermal treatment at different temperatures (left Y-axis) and DTG plots of PDA nanoparticles (right Y-axis).

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04319. 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, and DSC curves (PDF)

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 (Figure 7) show that the weight loss can be roughly divided into three stages. The first stage of weight loss occurs around 100 °C, which corresponds to water evaporation, and the third stage until 800 °C is attributed to the carbonization of PDA. An endothermic effect in both processes is shown in the DSC curve (Figure S7). It is interesting to observe the second stage from 150 to 250 °C, with relatively slow weight loss and a nonsignificant heat effect. The stage reflects the conversion of intermediates in the PDA structure, such as cyclization, isomerization, cross-linking, etc.,60 corresponding to the structural transformation of PDA under hydrothermal treatment. Here, a hypothesis is proposed to

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Gang Ye: 0000-0002-7066-940X Jiajun Yan: 0000-0003-3286-3268 Krzysztof Matyjaszewski: 0000-0003-1960-3402 Yuexiang Lu: 0000-0003-2755-7733 Notes

The authors declare no competing financial interest. 10217


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ACKNOWLEDGMENTS This study was supported by the Changjiang Scholars and Innovative Research Team in University (IRT13026), the National Science Fund for Distinguished Young Scholars (51425403), and the National Natural Science Foundation of China under Projects 51673109 and 51473087. K.M. acknowledges support from the U.S. Department of Energy (ER45998).

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