Hairy Hybrid Nanorattles of Platinum Nanoclusters with Dual

Article pubs.acs.org/Macromolecules

Hairy Hybrid Nanorattles of Platinum Nanoclusters with DualResponsive Polymer Shells for Confined Nanocatalysis Xue Li,†,‡ Tao Cai,*,† and En-Tang Kang*,‡ †

Key Laboratory of Biomedical Polymers of Ministry of Education, College of Chemistry and Molecular Science, Wuhan University, Wuhan, Hubei 430072, P. R. China ‡ Department of Chemical & Biomolecular Engineering, National University of Singapore, Kent Ridge, Singapore 119260 S Supporting Information *

ABSTRACT: Nanoparticles of transition metals, particularly noble metals, have been widely used in nanocatalysis. However, enhancing their stability and reusability during catalytic reactions has been a challenge that has limited the full use of the benefits associated with their miniature size. The encapsulation of noble metal nanocores as guest species into the hollow polymeric shells has emerged with the promise to solve this problem, which usually arises from a combined effect of the properties from their inorganic and polymeric components. In the present study, template-assisted synthesis of monodispersed hairy hybrid nanorattles, consisting of a movable platinum nanocluster and a hairy temperature- and pHresponsive polymer shell (Pt@air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] HHNs; PMAA: poly(methacrylic acid); PPMA: poly(propargyl methacrylate); βCD: β-cyclodextrin; PVCL: poly(N-vinylcaprolactam)), was carried out and preformed as nanocatalyst. The novelty of this approach lies in the use of click chemistry and supramolecular assembly (referred to as “grafting to” approaches) to assist the creation of a protective and stimuli-responsive polymer shell to promote efficient mass transfer to encapsulated metal nanoparticles. The polymer shell not only acts as a physical barrier that prevents the coalescence of Pt nanocores but also provides a void space where organic transformation occurs on the surface of the ligand-free Pt nanocluster in a controlled manner. The as-synthesized HHNs were found to perform as a robust and reusable heterogeneous catalyst for catalytic reactions. One may find the present study a general and effective way for the synthesis of monodispersed hollow nanomaterials in a controllable and green manner.

1. INTRODUCTION

stimuli-responsive properties, to the resulting nanocomposites.19 The conventional fabrications of inorganic−polymer nanocomposites, though very useful, offer a fairly limited opportunity for molecular engineering of polymer shells through a facile and effective way. The click chemistry concept has been raised by Sharpless in 2001 as modular and highly efficient conjugation methods of coupling specific chemical moieties.20 Copper(I)-catalyzed alkyne−azide click (CuAAC) reaction is commonly identified as a quintessential example of click chemistry. Because of its high selectivity, quantitative yields, short reaction time, mild reaction conditions, and high fidelity in the presence of most functional groups, CuAAC reaction has demonstrated its versatility for fabricating nanocomposites.21−25 To the best of our knowledge, the click coupling techniques have yet to be fully explored for the fabrication of inorganic−polymer hybrid nanocomposites. The use of click reactions would lead to fruitful approaches when combined with controlled/living radical polymerizations.26

Nanostructures of noble metals have been recognized as promising catalysts due to their high surface-to-volume ratio and high catalytic selectivity.1−7 However, low stability and poor reusability of nanostructured materials seriously limit their practical applications as nanocatalysis.8−11 Sintering or selfagglomeration of nanoparticles (NPs) often occurs during the catalytic reaction and consequently leads to the undesirable reduction of the active surface area. As a special class of hollow structure, the encapsulation of inorganic NPs as guest species into the hollow polymeric shells, termed as “rattle type” or “yolk−shell” nanocomposites, has recently attracted considerable interest due to their outstanding properties, which usually arise from a combined and/or synergistic effect of the properties of their inorganic and polymer components.12−14 This emerging area of nanotechnology has potential applications in confined nanocatalysis, controlled release, optics, electrons, and energy storage, owing to their low density and miniature size, large surface area, multifunctionality, and good loading capacity.15−18 Not only can polymeric shells prevent the inner nanocores from self-aggregation, they can also impart various functionalities, such as hydrophilicity and © XXXX American Chemical Society

Received: May 5, 2016 Revised: June 30, 2016

A

DOI: 10.1021/acs.macromol.6b00945 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules

Scheme 1. Schematic Illustration of the Fabrication of Hairy Pt@Air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] Hybrid Nanorattles by Combined Sol−Gel Reaction, Distillation−Precipitation Polymerization, Alkyne−Azide Click Reaction, Supramolecular Assembly and HF Etchinga

a

PMAA = poly(methacrylic acid); PPMA = poly(propargyl methacrylate); βCD = β-cyclodextrin; PVCL = poly(N-vinylcaprolactam).

acid), PPMA = poly(propargyl methacrylate), βCD = βcyclodextrin, PVCL = poly(N-vinylcaprolactam)). The process for the synthesis of HHNs, consisting of five major steps, is shown in Scheme 1.

On the other hand, supramolecular assembly has also been employed as an alternative approach to prepare hybrid nanomaterials anchoring polymer chains to NP surfaces.27−29 β-Cyclodextrin (βCD) is a cyclic oligosaccharide composed of seven glucopyranose units with a hydrophilic external cavity and hydrophobic internal surface. This particular structure allows it to selectively accommodate a variety of molecules as guests (e.g., adamantine). In addition to direct polymerization of functional monomers, supramolecular assembly of prefabricated polymers through noncovalent bond interaction has been globally recognized for its potential to prepare specific functional polymer brushes that are inaccessible by the direct polymerization methods. Precision engineering of the surface properties can be well controlled owing to the thoroughly characterized tethered macromolecules prior to conjugation. Herein, we report a versatile template-assisted strategy for the preparation of monodispersed rattle-type hybrid nanocomposites, encapsulating a movable Pt nanocluster in the hollow cavity of a pH and temperature-responsive polymer brushes decorated polymer shell (Pt@air@P[MAA-co-(PMAclick-βCD-guest-PVCL)] HHNs, PMAA = poly(methacrylic

2. EXPERIMENTAL SECTION 2.1. Materials. Chloroplatinic acid hexahydrate (H2PtCl6·6H2O, ACS reagent), trisodium citrate dihydrate (Na3Cit·2H2O, ≥99%), citric acid monohydrate (H3Cit·H2O, ≥99%), sodium borohydride (NaBH4, ≥99%), L-ascorbic acid (AA, ≥99%), polyvinylpyrrolidone (PVP, Mw = 10 000 g/mol), tetraethyl orthosilicate (TEOS, 98%), [3(methacryloyloxy)propyl]trimethoxysilane (MPS, 98%), N,N,N′,N′,N″-pentamethyldiethylenetriamine (PMDETA, 99%), and ethylenediaminetetraacetic acid (EDTA, ≥99%) were obtained from Sigma-Aldrich Chem. Co. and were used as received without further purification. Divinylbenzene (DVB, Sigma-Aldrich, containing 80% divinylbenzene isomers) was washed with 5% aqueous sodium hydroxide and water and then dried over anhydrous magnesium sulfate. The monomers propargyl methacrylate (PMA, Alfa Aesar, 98%) and methacrylic acid (MAA, Sigma-Aldrich, 99%) were passed through an inhibitor removal column prior to being stored under an argon atmosphere at −10 °C. Copper(I) bromide (CuBr, SigmaAldrich, 99%) was purified by stirring in acetic acid for 4 h, followed by B


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Macromolecules washing thoroughly with ethanol and diethyl ether before being stored under an argon atmosphere. Ammonium hydroxide solution (Merck, 13.2 M), acetonitrile (Merck, HPLC grade), hydrofluoric acid (HF, Riedel-de Häen, 48 wt %), and N,N-dimethylformamide (DMF, Merck, reagent grade) were used as received without further purification. Mono(6-azido-6-desoxy)-β-cyclodextrin (N3-βCD) was prepared according to procedures described in the literature.30−32 2.2. Synthesis of the Pt@SiO2-MPS Core−Shell Nanoparticles via Sol−Gel Reaction. All glassware used in the following procedures was cleaned in a bath of freshly prepared aqua regia (HCl:HNO3, 3:1, v/v) and rinsed thoroughly with ultrapure water. Monodispersed Pt seed NPs of 5 nm in diameter were prepared according to Brown et al.33 Briefly, 11 mL of aqueous solution of Na3Cit·2H2O (134.75 mg, 0.458 mmol) and H3Cit·H2O (6.02 mg, 0.0287 mmol) were added to 500 mL of aqueous solution of H2PtCl6·6H2O (72 mg, 0.139 mmol) under reflux conditions. After the water continues to boil for 1 min, about 5.5 mL of a freshly prepared aqueous solution of NaBH4 (4.4 mg, 0.116 mmol), Na3Cit·2H2O (67.4 mg, 0.229 mmol), and H3Cit· H2O (3 mg, 0.0143 mmol) was quickly injected into the reaction mixture under vigorous stirring. A color change from yellow to dark brown indicated the formation of Pt seed NPs. The reaction was allowed to proceed under reflux for another 10 min. The Pt seed solution was cooled down to ambient temperature prior to being stored at 0 °C. The Pt nanoclusters (Pt NCs) of 12, 16, 21, and 28 nm in diameter were synthesized by the reduction of H2PtCl6·6H2O with L-ascorbic acid (AA) from initial volume ratio of Pt seed solution of 12, 8, 4, and 1 mL (per 30 mL reaction solution), respectively.34 Briefly, 10 mL of aqueous solution of Na3Cit·2H2O (18.375 mg, 0.0625 mmol) was added to 10 mL of aqueous solution of H2PtCl6·6H2O (27.97 mg, 0.054 mmol) under stirring. The corresponding Pt seed solution and double distilled water were then added to adjust the total solution to 90 mL. About 1.5 mL of a freshly prepared aqueous solution of AA (18.75 mg, 0.107 mmol) was injected into the reaction mixture under vigorous stirring. The reaction flask was then increased to 100 °C with a heating rate of 10 °C/min. The reaction was allowed to proceed under reflux for another 1 h. About 20 mg of PVP (Mw = 10 000 g/ mol) was added to the mixture to stabilize the Pt NCs as the reaction mixture was cooled down to ambient temperature. The mixture was allowed to stand at room temperature for 24 h under magnetic stirring, allowing PVP to attach to Pt NCs, which were then separated from solution by centrifuging at 11 000 rpm for 20 min and redispersed in 20 mL of water. The Pt@SiO2-MPS core−shell NPs were prepared according to the modified Stöber process.16,17,35,36 The as-prepared aqueous solution of Pt-2 NCs was first mixed with 100 mL of ethanol and 2.5 mL of ammonium hydroxide solution (13.2 M). The reaction mixture was placed in an ultrasonic water bath for 30 min prior to the addition of 0.1 mL (0.45 mmol) of TEOS. The reaction mixture was stirred for 6 h at room temperature to allow for the coating of silica on Pt NCs. About 0.15 mL of MPS (0.63 mmol) was then added, and the reaction mixture was stirred for another 18 h to allow for the modification of the silica surface with MPS. After the reaction, the Pt@SiO2-MPS NPs were washed three times with a 1:1 (v/v) mixture of ethanol and water. The purified Pt@SiO2-MPS NPs were dried in a vacuum oven at room temperature overnight. The Pt@SiO2-MPS core−shell NPs of 23, 28, and 33 nm in shell thickness were synthesized via the sol−gel process using initial TEOS feed volumes of 0.1, 0.15, and 0.2 mL. 2.3. Synthesis of the Pt@SiO2@P[MAA-co-PMA] Core− Double Shell Nanoparticles via Distillation−Precipitation Polymerization. The Pt@SiO2@P[MAA-co-PMA] core−double shell NPs were prepared by distillation−precipitation polymerization (DPP) of methacrylic acid (MAA), propargyl methacrylate (PMA), and divinylbenzene (DVB) in acetonitrile. Briefly, about 40 mg of Pt@ SiO2-MPS template NPs was first dispersed into 30 mL of acetonitrile with aid of sonication for 30 min in a 100 mL round-bottom flask equipped with a reflux condenser. A mixture of MAA (120 μL, 1.42 mmol), PMA (60 μL, 0.48 mmol), DVB (a cross-linking agent, 56 μL, 0.41 mmol), and AIBN (4 mg, 0.024 mmol) was then introduced into the flask. The polymerization reaction was allowed to proceed for 2 h

under reflux conditions. The Pt@SiO2@P[MAA-co-PMA] core− double shell NPs were collected by centrifugation and purified by extraction three times with THF, acetone, and ethanol to remove the unreacted monomers and oligomers. The purified Pt@SiO2@P[MAAco-PMA] NPs were dried in a vacuum oven at room temperature until a constant weight was obtained. The P[MAA-co-PMA] outer shells of 8, 14, and 18 nm in thickness were tuned by the MAA (PMA) feed concentrations of 47.3 mM (16 mM), 59.2 mM (20 mM), and 71 mM (24 mM), respectively (Table 1).

Table 1. Size, Size Distribution, and Shell Thickness of the Platinum Nanoclusters, Platinum@Silica Core−Shell, and Platinum@Silica@Polymer Core−Double Shell Nanoparticles sample Pt-1 Pt-2 Pt-3 Pt-4 Pt@SiO2-MPS-1c Pt@SiO2-MPS-2c Pt@SiO2-MPS-3c Pt@SiO2@P[MAA-coPMA]-1d Pt@SiO2@P[MAA-coPMA]-2d Pt@SiO2@P[MAA-coPMA]-3d Pt@SiO2@P[MAA-co-(PMAclick-βCD)] Pt@SiO2@P[MAA-co-(PMAclick-βCD-guest-PVCL)]-1e Pt@SiO2@P[MAA-co-(PMAclick-βCD-guest-PVCL)]-2e Pt@SiO2@P[MAA-co-(PMAclick-βCD-guest-PVCL)]-3e

shell thicknessb (nm)

CVa (%)

1.08 1.06 1.05 1.07 1.06 1.06 1.07 1.06

23 28 33 23 + 8

18 23 19 21 10 11 12 10

95

1.04

23 + 14

11

98

102

1.04

23 + 18

8

85

90

1.06

23 + 12

8

91

95

1.04

23 + 15

7

97

102

1.05

23 + 18

8

105

109

1.04

23 + 22

6

Dna (nm)

Dwa (nm)

PDIa

12 16 21 28 62 71 82 80

13 17 22 30 66 75 88 85

90

a

Dn is the number-average diameter, Dw is the weight-average diameter, PDI is the polydispersity index, and CV is the coefficient of variation or the ratio of standard deviation to the mean (see Experimental Section). bThe shell thickness of the core−shell and core−double shell was determined from the TEM images. cThe Pt@ SiO2-MPS core−shell nanoparticles were prepared using the Pt-2 nanoclusters as seeds. dThe Pt@SiO2@P[MAA-co-PMA] core−double shell nanoparticles were prepared using the Pt@SiO2-MPS-1 nanoparticles as seeds. eThe hairy Pt@SiO2@P[MAA-co-(PMA-click-βCDguest-PVCL)] core−double shell nanoparticles were prepared using the Pt@SiO2@P[MAA-co-PMA]-1 nanoparticles as seeds. 2.4. Decoration of the Pt@SiO2@P[MAA-co-PMA] Core− Double Shell Nanoparticles with N3-βCD via Alkyne−Azide Click Reaction. About 20 mg of Pt@SiO2@P[MAA-co-PMA]-1 core−double shell NPs was dispersed into 20 mL of DMF, and the mixture was sonicated for 20 min. N3-βCD (116 mg, 0.1 mmol) and PMDETA (10.5 μL, 0.05 mmol) were then added into the reaction mixture under vigorous stirring. The reaction mixture was degassed with argon for 20 min. Then, CuBr (7.2 mg, 0.05 mmol) was added into the reaction mixture to induce the surface CuAAC reaction. The flask was sealed under an argon atmosphere, and the reaction was allowed to proceed at 50 °C for 12 h. The Pt@SiO2@P[MAA-co(PMA-click-βCD)] core−double shell NPs were purified by extraction three times with DMF and ethanol to remove the bound reagents. After the final centrifugation, the NPs were rinsed with a solution of sodium salt of EDTA and a water/ethanol (1/1, v/v) mixture to remove the copper catalyst. The purified Pt@SiO2@P[MAA-co-(PMAclick-βCD)] NPs were dried in a vacuum oven at room temperature until a constant weight was obtained. C


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Figure 1. TEM and FESEM images of the (a) 16 nm Pt-2, (b) 21 nm Pt-3, (c) 28 nm Pt-4 nanoclusters, (d, e) 62 nm Pt@SiO2-MPS-1, and (f) 71 nm Pt@SiO2-MPS-2 nanoparticles of Table 1. The inset (e′) shows the TEM image of the Pt@SiO2-MPS nanoparticles synthesized without the addition of PVP. The respective scale bars for (a−f) are 50, 50, 50, 100, 100, and 100 nm. 2.5. Supramolecular Assembly between the Pt@SiO2@ P[MAA-co-(PMA-click-βCD)] Core−Double Shell Nanoparticles and Ada-PVCL Guests. The adamantyl-terminated poly(N-vinylcaprolactam) (Ada-PVCL) guests were prepared according to the methods reported in the literature with slight modification.37−39 Their synthesis and characterization are described in the Supporting Information. About 20 mg of the Pt@SiO2@P[MAA-co-(PMA-click-βCD)] core−double shell NPs and Ada-PVCL (Mn,GPC = 22 500 g/mol, 225 mg, 0.01 mmol) were introduced into 15 mL of doubly distilled water. The mixture was sonicated for 30 min and stirred vigorously at room temperature for 48 h. The hairy Pt@SiO2@P[MAA-co-(PMAclick-βCD-guest-PVCL)] core−double shell NPs were purified by extraction three times with doubly distilled water and ethanol to remove the unreacted polymers. The purified Pt@SiO2@P[MAA-co(PMA-click-βCD-guest-PVCL)] NPs were dried in a vacuum oven at room temperature overnight. The PVCL brushes decorated P[MAAco-(PMA-click-βCD)] shells of 15, 18 and 22 nm in thickness were tuned by Ada-PVCL guests with different molecular weight (Mn,GPC) of 12 200, 22 500, and 34 600 g/mol, respectively. 2.6. Preparation of the Hairy Pt@Air@P[MAA-co-(PMA-clickβCD-guest-PVCL)] Hybrid Nanorattles by HF Etching. The Pt@ air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] HHNs were prepared by selective removal of the silica inner shell from the hairy Pt@SiO2@ P[MAA-co-(PMA-click-βCD-guest-PVCL)] hybrid NPs by hydrofluoric acid (HF) etching. About 20 mg of Pt@SiO2@P[MAA-co-(PMA-clickβCD-guest-PVCL)] NPs was stirred in 10 mL of 24 wt % HF at room

temperature for 12 h. The excess HF and SiF4 were extracted from the HHNs by five centrifugation−redispersion cycles in ethanol and water. The resulting HHNs were dialyzed in doubly distilled water for 1 week. Finally the Pt@air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] HHNs were recovered by freeze-drying. To investigate the mechanical stability of the HHNs, the as-synthesized HHNs were immersed in HCl solution of pH 2 for 48 h and then NaOH solution of pH 12 for 48 h. Finally, the HHNs were collected by centrifugation at 8000 rpm in an Eppendorf 5810 centrifuge. 2.7. Catalytic Reduction of 4-Nitrophenol in the Hairy Pt@ Air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] Hybrid Nanorattles. For the confined catalytic reduction of 4-nitrophenol in the nanorattles, the Pt@air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] HHNs suspension (0.5 mL, 10 mg/mL) in deionized water was added to 1 mL of 4-nitrophenol aqueous solution (0.15 mM) in a quartz cuvette. The mixture was stirred vigorously for 20 min at room temperature. About 1.5 mL of a freshly prepared aqueous solution of NaBH4 (0.01 M) was quickly injected into the reaction mixture under vigorous stirring. The progress of the catalytic reaction was monitored by UV−vis absorption at the wavelength of 400 nm at an interval of 10 min. The yellow solution turned colorless with the progress of the catalytic reaction. After each run, the catalyst was recovered by centrifugation and reused directly for the next cycle. This procedure was repeated ten times. 2.8. Materials Characterization. Fourier transform infrared (FTIR) spectroscopy analysis was carried out on a Bio-Rad FTS 135 Fourier transform infrared spectrophotometer, and the diffuse D


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Macromolecules reflectance spectra were scanned over the range of 400−4000 cm−1. Transmission electron microscopy (TEM) images were obtained on a JEOL JEM-2100F field emission TEM. Field-emission scanning electron microscopy (FESEM) images were obtained on a JEOL JSM-6700 SEM. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Kratos AXIS Ultra HSA spectrometer equipped with a monochromatized Al Kα X-ray source (1468.6 eV photons). The dynamic laser scattering (DLS) measurements were performed on a Brookhaven 90 plus laser light scattering spectrometer at the scattering angle θ = 90°. The hydrodynamic diameter of NPs was obtained by averaging the values from five measurements. The polydispersity index (PDI) of the NPs was calculated from the following statistical formulas:40 k

PDI = Dw /Dn , Dn =

k

∑ niDi /∑ ni , i=1

i=1

k

Dw =

amphiphilic polymer, the poly(vinylpyrrolidone) (PVP) stabilizer can absorb onto Pt NC surfaces, making the affinity of Pt NC surface for silica coating sufficiently high so that no coupling agent is required during the subsequent synthesis of Pt@SiO2-MPS core−shell NPs.43,44 Without PVP stabilizer, some silica coating to Pt NC surfaces unavoidably aggregated to multiple cores as shown in Figure 1e′. The silica coating prepared in the presence of MPS acts not only as the starting anchor for the second shell but also as the sacrificial template for the hollow structure.16,17,44 The respective field-emission scanning electron microscopy (FESEM) and TEM images of the so-obtained Pt@SiO2-MPS core−shell NPs with two different SiO2-MPS shell thickness of about 23 and 28 nm are shown in Figures 1d, 1e, and 1f, respectively. For each type of core−shell NPs, a narrowly distributed and smooth silica shell of lower image contrast surrounding the Pt NC is readily discernible. The thickness of silica shell encapsulating the metal NC can be controlled by using different TEOS feed volume. The size, size distribution, and shell thickness of as-synthesized Pt@SiO2-MPS core−shell NPs are summarized in Table 1. The Fourier transform infrared (FTIR) spectrum of the Pt@ SiO2-MPS-1 NPs in Table 1 is shown in Figure 2a. The

k

∑ niDi 4 /∑ niDi 3 i=1

i=1

where Dn is the number-average diameter, Dw is the weight-average diameter, Di is the diameter of each NP, and n is the number of NPs. In each case, about 100 NPs in the TEM or FESEM images were used for the analysis. Coefficient of variation (CV), defined as the ratio of standard deviation (δ) to the mean (Dn) (CV = δ/Dn), was used to estimate the error in NP size.

3. RESULTS AND DISCUSSION Procedures for the synthesis of monodispersed “rattle-type” hollow nanoparticles (NPs), consisting of a movable platinum nanocluster (Pt NC), a cross-linked polymer shell, and surface inclusion complexation of temperature-responsive polymer brushes are illustrated in Scheme 1. The first step involves coating of Pt NCs with a uniform silica shell with vinyl groups on the surface, via the sol−gel reaction of tetraethyl orthosilicate (TEOS) and 3-(trimethoxysilyl)propyl methacrylate (MPS),41 to produce the Pt@SiO2-MPS core−shell NPs. Subsequent distillation−precipitation copolymerization (DPP) of methacrylic acid (MAA) and propargyl methacrylate (PMA) in the presence of a cross-linking agent, divinylbenzene (DVB), from the Pt@SiO2-MPS core−shell templates produces the Pt@SiO2@P[MAA-co-PMA] core−double shell NPs with pendant alkyne groups on the surface were carried out. The alkyne groups on the exterior surface of the Pt@SiO2@P[MAAco-PMA] core−double shell NPs serve as coupling sites for the CuAAC reaction of mono(6-azido-6-desoxy)-β-cyclodextrin (N3-βCD) to generate the Pt@SiO2@P[MAA-co-(PMA-clickβCD)] NPs. Supramolecular assembly of the adamantylterminated poly(N-vinylcaprolactam) (Ada-PVCL) chains with the surface-tethered βCD molecules on the Pt@SiO2@ P[MAA-co-(PMA-click-βCD)] NPs generates the hairy Pt@ SiO2@P[MAA-co-(PMA-click-βCD-guest-PVCL)] core−double shell NPs. The silica inner shell in the hairy Pt@SiO2@P[MAAco-(PMA-click-βCD-guest-PVCL)] core−double shell NPs can be selectively removed by hydrofluoric acid (HF) etching to produce the hairy Pt@air@P[MAA-co-(PMA-click-βCD-guestPVCL)] hybrid nanorattles (HHNs). 3.1. Preparation of the Pt Nanoclusters and Pt@SiO2MPS Core−Shell Nanoparticles. The Pt NCs were synthesized by the reduction of H2PtCl6·6H2O with L-ascorbic acid (AA) for further deposition of Pt on the seed NPs.35 The seed-mediated growth method leads to the formation of NCs of monodispersed size and shape and allows facile secondary silica coating. Figures 1a, 1b, and 1c show the transmission electron microscopy (TEM) images of the as-synthesized Pt NCs with average diameters of 16, 21, and 28 nm, respectively. Silica layer coating was subsequently introduced onto Pt NC surfaces to stabilize and functionalize Pt NCs. As an

Figure 2. FTIR spectra of the (a) Pt@SiO2-MPS-1, (b) Pt@SiO2@ P[MAA-co-PMA]-1 core−double shell, (c) Pt@SiO2@P[MAA-co(PMA-click-βCD)] core−double shell, (d) hairy Pt@SiO2@P[MAAco-(PMA-click-βCD-guest-PVCL)]-2 core−double shell nanoparticles, and (e) hairy Pt@air@P[MAA-co-(PMA-click-βCD-guest-PVCL)]-2 hybrid nanorattles.

absorption bands at 1098 and 1632 cm−1 are assigned respectively to the asymmetric stretching vibration of Si−O− Si bonds and stretching vibration of the vinyl groups of Pt@ SiO2-MPS-1 NPs.44,45 The vinyl groups on the surface of Pt@ SiO2-MPS core−shell NPs, introduced by the organosilicon coupling agent (MPS), will serve as initiation and anchoring sites for subsequent DPP for the fabrication of core−double shell NPs. The X-ray photoelectron spectroscopy (XPS) widescan, C 1s, and Pt 4f spectra of the Pt@SiO2-MPS-1 template NPs of Table 1 are shown in Figures 3a, 3e, and 3e′. The Pt 4f signal at the binding energy (BE) of ∼74 eV is not discernible in the XPS wide-scan and Pt 4f core-level spectra of the Pt@ SiO2-MPS-1 NPs (Figures 3a and 3e′), consistent with the fact that the SiO2-MPS shell thickness (∼23 nm) is greater than the probing depth of the XPS technique (∼8 nm in an organic E


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Figure 3. XPS wide-scan, C 1s, N 1s, Si 2p, and Pt 4f core-level spectra of the (a, e, e′) Pt@SiO2-MPS-1, (b, f, f′) Pt@SiO2@P[MAA-co-PMA]-1 core−double shell, (c, g, g′) Pt@SiO2@P[MAA-co-(PMA-click-βCD)] core−double shell, and (d, h, h′) hairy Pt@SiO2@P[MAA-co-(PMA-clickβCD-guest-PVCL)]-2 core−double shell nanoparticles.

matrix46). The C 1s core-level spectrum (Figure 3e) can be curve-fitted into three peak components with BEs at about 284.6, 286.2, and 288.5 eV, attributable to the respective C−C/ C−H/C−Si, C−O, and OC−O species of the SiO2-MPS layer coated on the Pt NCs surface.44,47 3.2. Preparation of the Pt@SiO2@P[MAA-co-PMA] and Pt@SiO 2 @P[MAA-co-(PMA-click-βCD)] Core−Double Shell Nanoparticles. In this work, DPP of MAA and PMA was carried out in the presence of DVB in acetonitrile, using the Pt@SiO2-MPS-1 core−shell NPs in Table 1 as templates, to produce the core−double shell NPs with a cross-linked P[MAA-co-PMA] outer shell. DPP, the surfactant-free heterogeneous polymerization technique, has been developed to prepare rattle-type hollow nanostructures and other various uniform and neat polymer NPs.18 The choice of MAA monomers in DPP based on the fact that hydrogen bonding between the polyacid chains can serve as a noncovalent linkage for the formation of uniform pH-responsive polymer shell.18 The pendant alkyne groups in the PMA units allow postfunctionalization via CuAAC reaction in a “grafting to” approach. The TEM image of Figure 4a reveals an outer polymer shell of low contrast encapsulating a dense Pt@SiO2MPS inorganic core−shell NP of differential contrast, giving rise to a distinctive core−double shell hybrid nanostructure. The thickness of cross-linked P[MAA-co-PMA] outer shell of the core−double shell NPs was determined to be about 8 nm, by comparing the mean sizes of Pt@SiO2-MPS-1 and Pt@ SiO2@P[MAA-co-PMA]-1 NPs. The cross-linked P[MAA-coPMA] shell of different thickness can be obtained via adjustment of the MAA and PMA feed concentrations during DPP. The size, size distribution, and shell thickness of the resultant Pt@SiO2@P[MAA-co-PMA] NPs are summarized in Table 1. The FTIR spectrum of Pt@SiO2@P[MAA-co-PMA]-1 NPs shows a strong absorption band at 1730 cm−1, which is attributable to the ester and carboxylic acid groups from the cross-linked P[MAA-co-PMA] outer shell, as shown in Figure

2b.44,45 XPS was also used to identify the changes in chemical composition of the outer shell of the NPs. The barely discernible Si 2p signal in the XPS wide-scan and Si 2p corelevel spectra of the Pt@SiO2@P[MAA-co-PMA]-1 core− double shell NPs in Figures 3b and 3f′ is consistent with the fact that the P[MAA-co-PMA] outer layer thickness of about 8 nm, which is comparable to the probing depth of the XPS technique (∼8 nm in an organic matrix46). The C 1s core-level spectrum of the Pt@SiO2@P[MAA-co-PMA]-1 NPs in Figure 3f can be curve-fitted with three peak components having BEs at about 284.6, 286.2, and 288.6 eV, attributable to the C−C/ C−H, C−O, and OC−O species, respectively.44,47 The increase in intensity of the OC−O specie and the decrease in intensity of the C−O specie in Figure 3f are consistent with the formation of a cross-linked P[MAA-co-PMA] outer shell. Cyclodextrins (CDs) form inclusion complexes by acting as “host” to hydrophobic “guest” molecules that reside in their toroids’ hydrophobic interior, meanwhile imparting the molecules with good water solubility due to toroids’ hydrophilic outsides. For example, adamantane forms inclusion complexes with βCD molecules with high association constants on the order of 105.48,49 The surface-initiated CuAAC reaction of mono-(6-azido-6-desoxy)-β-cyclodextrin (N3-βCD) on the Pt@SiO2@P[MAA-co-PMA] core−double shell NPs led to the formation of βCD molecules covalently attached onto to surface of the NPs.50 The Pt@SiO2@P[MAA-co-PMA-clickβCD]-1 NPs were characterized by FTIR spectroscopy (Figure 2c). The complete disappearance of alkyne stretching vibration absorption at 2120 cm−1 indicates all the alkyne groups have undergone the CuAAC reactions.45,50 Figures 3c, 3g, and 3g′ show the XPS wide-scan, C 1s, and N 1s core-level spectra of the Pt@SiO2@P[MAA-co-(PMA-click-βCD)] core−double shell NPs. In comparison to the XPS wide-scan spectrum of Pt@SiO2@P[MAA-co-PMA]-1 NPs (Figure 3b), the appearance of N 1s signal at the BEs of ∼400 eV in the wide-scan spectrum of the Pt@SiO2@P[MAA-co-(PMA-click-βCD)] core−double shell NPs (Figure 3c) is consistent with the F


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Figure 4. TEM and FESEM images of the (a) 80 nm Pt@SiO2@P[MAA-co-PMA]-1 core−double shell, (b, c, d) 97 nm hairy Pt@SiO2@P[MAA-co(PMA-click-βCD-guest-PVCL)]-2 core−double shell nanoparticles, and (e, f) hairy Pt@air@P[MAA-co-(PMA-click-βCD-guest-PVCL)]-2 hybrid nanorattles after removal of the silica core by HF etching. The respective scale bars for (a−f) are 100, 100, 200, 50, 200, and 50 nm.

successful formation of surface-tethered βCD molecules on the NPs.47,50 The C 1s core-level spectrum of the NPs can be curve-fitted into five peak components with BEs at about 284.6, 285.7, 286.2, 287.6, and 288.6 eV, attributable to the C−H/C− C, C−N, C−O, O−C−O, and OC−O species, respectively.47,50 The N 1s core-level spectrum of the NPs can be curve-fitted into two peak components with BEs at 398.4 and 399.7 eV and with an area ratio of about 2:1, attributable to the imine nitrogen (=N−) and amine nitrogen (−N−) atoms in the triazole ring, respectively.47,50 3.3. Preparation of the Hairy Pt@SiO2@P[MAA-co(PMA-click-βCD-guest-PVCL)] Core−Double Shell Nanoparticles and Pt@Air@P[MAA-co-(PMA-click-βCD-guestPVCL)] Hybrid Nanorattles. PVCL is one of most popular temperature-responsive polymers, which possesses reversible phase transition behavior in an aqueous solution and a lower critical solution temperature (LCST) of 32−40 °C.51−53 Its good biocompatibility makes PVCL popular in biomedical and environmental applications. It is of great importance to control over the molecular weight and molecular weight distribution for PVCL, as the temperature-responsive property of this polymer relies on the molecular weights (Table S1).39,51−53 Controlled radical polymerizations of less activated monomer N-vinylcaprolactam (VCL) with low molecular weight distribution

were only realized by reversible addition−fragmentation chain transfer (RAFT) polymerization using O-alkyl xanthate and N,N-dialkyl dithiocarbomate as the chain transfer agents (CTAs) through the interaction of lone pairs of electrons in oxygen and nitrogen atom.53,54 Surface-initiated RAFT polymerization of VCL monomer from the material surfaces remains a great challenge owing to the difficulty of employing CTAs to the surfaces.53,54 In addition to direct surface-initiated polymerization of functional monomers, supramolecular assembly of prefabricated polymers through noncovalent bond interaction has been globally recognized for its potential to prepare specific functional polymer brushes that are inaccessible by the direct polymerization methods. To this end, the prominent adamantyl moiety was incorporated to the CTA (Ada-CTA) in the synthesis of PVCL polymer due to its high complexation constant with βCD molecules of up to 105. Narrowly dispersed adamantyl-terminated poly(N-vinylcaprolactam) (Ada-PVCL) was prepared via xanthate-mediated RAFT polymerization of VCL monomers at 80 °C in 1,4dioxane, using Ada-CTA as the CTA and AIBN as the initiator. The Ada-PVCL homopolymers with different chain lengths were synthesized by varying the molar feed ratio of VCL monomer to Ada-CTA. The gel permeation chromatography (GPC) analysis results of Ada-PVCL are shown in Figure S1. G


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deformation of the polymer shell was observed, suggesting that the cross-linked P[MAA-co-PMA] shell is stable and rigid enough to sustain the cavity upon removal of the silica inner shell. Moreover, the as-synthesized HHNs can be dispersed in both organic solvents (such as ethanol, THF, and DMF) and aqueous medium at room temperature, due to the simultaneous presence of an amphiphilic P[MAA-co-PMA] shell and hydrophilic PVCL brushes. The good stability and dispersity in both organic solvents and aqueous media allow further device fabrication of these HHNs via solution-processing. The so-obtained HHNs are of great interest not only for their unique morphology but also for their stimuli-responsive properties.18 The PMAA polymer shell and PVCL brushes have endowed the HHNs with pH and temperature-responsive properties. The hydrodynamic diameter (Dh) of the HHNs was characterized by dynamic laser scattering (DLS). As shown in Figure 5a, at the constant solution pH of 7, the Dh of the

With the increase in concentration of VCL monomers, the number-average molecular weight (Mn,GPC) of Ada-PVCL homopolymers increases from 1.22 × 104 to 3.46 × 104 g/ mol, with the total number of VCL repeat units increases correspondingly from 85 to 246 (Table S1). The polydispersity index (PDI) of Ada-PVCL homopolymers remains less than 1.2, indicating that xanthate-mediated RAFT polymerization of VCL in the presence of Ada-CTA is well-controlled. The Ada-PVCL guests were assembled onto the surface of Pt@SiO2@P[MAA-co-(PMA-click-βCD)] core−double shell NPs by fitting the adamantane groups inside the cavities of the surface-tethered βCD molecules (Scheme 1). The morphology of Pt@SiO2@P[MAA-co-(PMA-click-βCD-guestPVCL)] core−double shell NPs was revealed by the FESEM and TEM images in Figures 4b, 4c, and 4d. The surface inclusion complexation of Ada-PVCL brushes on the core− double shell NPs produces a hairy surface, as suggested by the increase in shell thickness and coarse surface structure of the NPs in the TEM images (Figures 4c and 4d). The average diameter of the Pt@SiO2@P[MAA-co-(PMA-click-βCD-guestPVCL)]-2 NPs, as determined from the TEM images, increases from 80 nm of that of Pt@SiO2@P[MAA-co-(PMA-clickβCD)] NPs to about 97 nm. The hairy Pt@SiO2@P[MAA-co-(PMA-click-βCD-guestPVCL)] core−double shell NPs were characterized by FTIR spectroscopy (Figure 2d). The adsorption peak at 1640 cm−1 is associated with the characteristic amide stretching vibration of the PVCL brushes.39,45 The inclusion complexation of PVCL brushes on the P[MAA-co-(PMA-click-βCD)] outer shell has caused a significant increase in the intensity of the N 1s signal in the XPS wide-scan spectrum of the hairy Pt@SiO2@P[MAAco-(PMA-click-βCD-guest-PVCL)] core−double shell NPs (Figure 3d). The N 1s core-level spectrum (Figure 3h′) is dominated by the peak component with BE at about 399.7 eV, attributed to the C−N species of the PVCL chains. The same result can also be deduced from the change in the C 1s corelevel line shapes of the hairy Pt@SiO2@P[MAA-co-(PMA-clickβCD-guest-PVCL)] core−double shell NPs. The two carbon species associated with the P[MAA-co-(PMA-click-βCD)] shell, viz. C−O and OC−O with respective BE’s at 286.2 and 288.6 eV, have disappeared completely. The spectrum can be curve-fitted into three peak components with BEs at about 284.6, 285.7, and 287.4 eV, attributable to the C−H/C−C, C− N, and OC−N species, respectively, of the assembled PVCL guest polymer chains.41,47 The [C−N]:[OC−N] peak component area ratio of about 2:1 is in good agreement with the theoretical ratio based on the chemical structure of PVCL. Removal of the silica sacrificial layer of the hairy Pt@SiO2@ P[MAA-co-(PMA-click-βCD-guest-PVCL)] core−double shell NPs by HF etching leads to the formation of Pt@air@P[MAAco-(PMA-click-βCD-guest-PVCL)] HHNs. The HHNs were characterized by FTIR spectroscopy (Figure 2e). The disappearance of Si−O−Si strong stretching vibration absorption at 1098 cm−1 indicates that the SiO2 inner layer have been completely removed by HF etching.45 The mechanical stability of the HHNs is one of the most important criteria governing their practical applications. The rattle-type nanostructure was retained, upon exposure to various organic solvents, an acid medium of pH 2 for 48 h, a base medium of pH 12 for 48 h, and high centrifugation force of 10 000 rpm, as shown by the TEM images of the stressed NPs in Figures 4e and 4f. The TEM images clearly reveal the rattle-type nanostructure with a movable metal NC encapsulated in a hollow polymer shell. No

Figure 5. Hydrodynamic diameters (Dh) of the hairy Pt@air@ P[MAA-co-(PMA-click-βCD-guest-PVCL)]-2 hybrid nanorattles in aqueous media of 25 and 50 °C at solution pH of (a) 7 and (b) 2.

HHNs decreases from about 220 nm to about 170 nm as the temperature of the aqueous medium increases from 25 to 50 °C. This change in NP sizes is consistent with the fact that the guest PVCL brushes exhibit a LCST behavior in aqueous medium. As the temperature of the medium is raised to above the LCST of about 32 °C, the PVCL brushes in the HHNs associate hydrophobically on the P[MAA-co-(PMA-click-βCD)] shell to decrease the effective Dh of the HHNs. At a fixed temperature of 25 °C, the Dh of the HHNs decreases from 220 nm at pH = 7 to 180 nm at pH = 2, as shown in Figures 5a and 5b. This change is due to the Donnan osmotic swelling of the H


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Macromolecules PMAA shell (pKa ≈ 4.2) as the carboxylic acid groups are ionized at high pH values. Clearly, the pH/temperature tunable HHNs enable controlled release of encapsulated guest molecules by means of size separation. Inorganic−polymer nanocomposites have been considered as nanoreactor systems for organic reactions.55,56 The advantages of HHNs with the exterior surface decorated by hydrophilic PVCL brushes lie in their protective capability and enhanced dispersity in an aqueous medium. The present Pt@air@ P[MAA-co-(PMA-click-βCD-guest-PVCL)] HHNs were employed as a nanoreactor system for the model reaction of 4nitrophenol (4-NP) to 4-aminophenol (4-AP) to illustrate their potential applications. Upon addition of NaBH4 into the reaction mixture, catalytic reduction was initiated by the Pt NC in the confined cavity. The reduction kinetics was monitored by time-dependent changes in UV−vis absorption spectra of the 4NP reactant in the reaction mixture in Figure 6a. The

considered as a pseudo-first-order reaction. As expected, a linear relationship was obtained between −ln[Ct/C0] and reaction time t in Figure 6b. Thus, the apparent reaction rate constant kapp was estimated to be 7.9 × 10−4 s−1, which is comparable to most of previously reported cases.55,56 No catalytic reduction of 4-NP was observed in the absence of HHNs. The conversion is higher than 90% after 60 min or as high as 95% when the reaction time is extended to 120 min. In addition, the catalytic HHNs can be easily recovered by centrifugation and reused for 10 successive cycles without significant loss of reactivity (yield loss within 2% after ten successive cycles, inset of Figure 6b). Thus, the synthesized HHNs can serve as a confined nanoreactor system for catalytic reactions. When the Pt@air@P[MAA-co-(PMA-click-βCD-guestPVCL)] HHNs with temperature-responsive PVCL brushes were used as the nanocatalysts for 4-NP reduction, the influence of reaction temperature on the kapp does not follow a typical Arrhenius-type dependence on temperature, which can be attributed to the change in conformation of the PVCL brushes on the polymer shells (Figure 7).55,56 At reaction

Figure 7. Influence of reaction temperature on the pseudo-first-order rate constant kapp (open circles) measured in the presence of hairy Pt@air@P[MAA-co-(PMA-click-βCD-guest-PVCL)] hybrid nanorattles compared with the corresponding hydrodynamic diameters Dh (solid squares).

temperature below the LCST (≤30 °C), the PVCL brushes are hydrophilic and assume an extended conformation on the polymer shells and become less hindered to the diffusion of reactants through the shells. As a result, the kapp is found to increase with the increase of reaction temperature as expected from Arrhenius law. However, when the reaction temperature approximates the LCST (30−35 °C), the PVCL brushes associate hydrophobically and adopt a compact conformation on the polymer shells, resulting in the decrease of diffusion of reactants and hence the kapp. This discrepancy probably results from the decrease of diffusion rate is not compensated by the Arrhenius-like increase in the kapp with reaction temperature. Further increase in reaction temperature (>35 °C) will lead to a gradual increase in the kapp as the diffusion of reactants will no longer be affected by the fully collapsed PVCL brushes. Thus, the temperature-responsive PVCL brushes on the Pt@air@ P[MAA-co-(PMA-click-βCD-guest-PVCL)] HHNs can act as nanogates to adjust the diffusion of reactants and the corresponding kapp toward the catalytic reaction. The cavity of the Pt@air@P[MAA-co-(PMA-click-βCD-guestPVCL)] HHNs can be controlled by varying the size of SiO2

Figure 6. (a) UV−vis absorption spectra recorded at different reaction times during the catalytic reduction of 4-NP using the hairy Pt@air@ P[MAA-co-(PMA-click-βCD-guest-PVCL)] hybrid nanorattles as nanocatalyst. (b) Plot of −ln(Ct/C0) versus reaction time for the aforementioned reaction. Inset: average conversion of 4-NP in ten successive cycles from catalytic reduction with the hairy Pt@air@ P[MAA-co-(PMA-click-βCD-guest-PVCL)] hybrid nanorattles.

characteristic absorption peak of 4-NP would shift from 317 to 400 nm with the addition of NaBH4 due to the formation of 4-nitrophenolate ions. As the catalytic reaction proceeds, the characteristic absorption peak of the reactant (4-NP) at 400 nm decreases with a concomitant increase in the absorption peak of the product (4-AP) at 295−300 nm. During the reaction process, the concentration of NaBH4 was greatly larger than that of 4-NP, so the reduction of 4-NP to 4-AP can be I


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Synthesis and Efficient Synergetic Catalysis. J. Mater. Chem. A 2016, 4, 3278−3286. (2) Ren, F.; Lu, H.; Liu, H.; Wang, Z.; Wu, Y.; Li, Y. Surface LigandMediated Isolated Growth of Pt on Pd Nanocubes for Enhanced Hydrogen Evolution Activity. J. Mater. Chem. A 2015, 3, 23660− 23663. (3) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H.; Snyder, J.; Li, D.; Herron, J.; Mavrikakis, M.; Chi, M.; More, K.; Li, Y.; Markovic, N.; Somorjai, G.; Yang, P.; Stamenkovic, V. Highly Crystalline Multimetallic Nanoframes with Three-Dimensional Electrocatalytic Surfaces. Science 2014, 343, 1339−1343. (4) Zhang, L.; Roling, L. T.; Wang, X.; Vara, M.; Chi, M. F.; Liu, J. Y.; Choi, S.; Park, J.; Herron, J. A.; Xie, Z. X.; Mavrikakis, M.; Xia, Y. N. Platinum-Based Nanocages with Subnanometer-Thick Walls and WellDefined, Controllable Facets. Science 2015, 349, 412−416. (5) Huang, X.; Zhao, Z.; Cao, L.; Chen, Y.; Zhu, E.; Lin, Z.; Li, M.; Yan, A.; Zettl, A.; Wang, Y. M.; Duan, X.; Mueller, T.; Huang, Y. HighPerformance Transition Metal-Doped Pt3Ni Octahedra for Oxygen Reduction Reaction. Science 2015, 348, 1230−1234. (6) Zhang, Y.; Cui, X.; Shi, F.; Deng, Y. Nano-Gold Catalysis in Fine Chemical Synthesis. Chem. Rev. 2012, 112, 2467−2505. (7) Dai, L.; Mo, S.; Qin, Q.; Zhao, X.; Zheng, N. Carbon MonoxideAssisted Synthesis of Ultrathin PtCu3 Alloy Wavy Nanowires and Their Enhanced Electrocatalysis. Small 2016, 12, 1572−1577. (8) Kang, Y. J.; Ye, X. C.; Murray, C. B. Size- and Shape-Selective Synthesis of Metal Nanocrystals and Nanowires Using CO as A Reducing Agent. Angew. Chem., Int. Ed. 2010, 49, 6156−6159. (9) Schrader, I.; Warneke, J.; Backenkohler, J.; Kunz, S. Functionalization of Platinum Nanoparticles with L-Proline: Simultaneous Enhancements of Catalytic Activity and Selectivity. J. Am. Chem. Soc. 2015, 137, 905−912. (10) Wang, L.; Yamauchi, Y. Block Copolymer Mediated Synthesis of Dendritic Platinum Nanoparticles. J. Am. Chem. Soc. 2009, 131, 9152− 9153. (11) Chang, L. Y.; Barnard, A. S.; Gontard, L. C.; Dunin-Borkowski, R. E. Resolving the Structure of Active Sites on Platinum Catalytic Nanoparticles. Nano Lett. 2010, 10, 3073−3076. (12) Chaudhuri, R. G.; Paria, S. Core/Shell Nanoparticles: Classes, Properties, Synthesis Mechanisms, Characterization, and Applications. Chem. Rev. 2012, 112, 2373−2433. (13) Lou, X. W.; Archer, L. A.; Yang, Z. C. Hollow Micro-/ Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987−4019. (14) Li, G. L.; Tai, C. A.; Neoh, K. G.; Kang, E. T.; Yang, X. L. Hybrid Nanorattles of Metal Core and Stimuli-Responsive Polymer Shell for Confined Catalytic Reactions. Polym. Chem. 2011, 2, 1368− 1374. (15) Li, G. L.; Xu, L. Q.; Neoh, K. G.; Kang, E. T. Hairy Hybrid Microrattles of Metal Nanocore with Functional Polymer Shell and Brushes. Macromolecules 2011, 44, 2365−2370. (16) Cai, T.; Zhang, B.; Chen, Y.; Wang, C.; Zhu, C. X.; Neoh, K. G.; Kang, E. T. Preparation and Unique Electrical Behaviors of Monodispersed Hybrid Nanorattles of Metal Nanocores with Hairy Electroactive Polymer Shells. Chem. - Eur. J. 2014, 20, 2723−2731. (17) Zhang, B.; Cai, T.; Li, S.; Zhang, X.; Chen, Y.; Neoh, K. G.; Kang, E. T.; Wang, C. Yolk-Shell Nanorattles Encapsulating A Movable Au Nanocore in Electroactive Polyaniline Shells for Flexible Memory Device. J. Mater. Chem. C 2014, 2, 5189−5197. (18) Li, G. L.; Mohwald, H.; Shchukin, D. G. Precipitation Polymerization for Fabrication of Complex Core-Shell Hybrid Particles and Hollow Structures. Chem. Soc. Rev. 2013, 42, 3628−3646. (19) Zhang, J.; Zhang, M.; Tang, K.; Verpoort, F.; Sun, T. PolymerBased Stimuli-Responsive Recyclable Catalytic Systems for Organic Synthesis. Small 2014, 10, 32−46. (20) Kolb, H. C.; Finn, M. G.; Sharpless, K. B. Click Chemistry: Diverse Chemical Function from A Few Good Reactions. Angew. Chem., Int. Ed. 2001, 40, 2004−2021. (21) Dam, H. H.; Caruso, F. Modular Click Assembly of Degradable Capsules Using Polyrotaxanes. ACS Nano 2012, 6, 4686−4693.

intertemplate layer during the sol−gel process. The thickness of P[MAA-co-(PMA-click-βCD)] shell and the length of PVCL brushes can also be regulated through the simple adjustment of initial MAA and PMA monomer concentrations and molecular weight of the PVCL chains, respectively. Moreover, the Pt@ SiO2@P[MAA-co-PMA] NPs are produced by surface-initiated DPP in a “grafting from” process. The click grafting of βCD and inclusion complexation of Ada-PVCL brushes to the surface of Pt@SiO2@P[MAA-co-PMA] NPs, on the other hand, is a “grafting to” process. The present work has thus illustrated the versatility of combination of the “grafting from” and “grafting to” processes in the construction of multifunctional inorganic−polymer HHNs.

4. CONCLUSIONS Monodispersed Pt@air@P[MAA-co-(PMA-click-βCD-guestPVCL)] HHNs, comprised of a Pt NC in a hollow crosslinked P[MAA-co-(PMA-click-βCD] shell decorated with inclusion complexes of temperature-responsive PVCL brushes, have been synthesized by selective etching of the inorganic silica inner shell of hairy Pt@SiO2@P[MAA-co-(PMA-clickβCD-guest-PVCL)] core−double shell NPs. The latter NPs were prepared a priori by combined sol−gel reaction of TEOS in the presence of MPS, DPP of MAA, PMA, and DVB, CuAAC reaction of N3-βCD, and supramolecular assembly of Ada-PVCL. In addition to the well-defined and stable yolk− shell nanostructure, the Pt@air@P[MAA-co-(PMA-click-βCDguest-PVCL)] HHNs also exhibited pH- and temperatureresponsive properties. More significantly, the catalytic reduction of 4-NP by NaBH4 in aqueous solution reveals that the assynthesized Pt@air@P[MAA-co-(PMA-click-βCD-guestPVCL)] HHNs have high catalytic activity and good reusability. The versatile synthesis protocol for the HHNs in the present study opens up the possibility of designing a broad range of nanostructures to allow the mimicking of more complex macromolecular architecture for applications in multifunctional and stimuli-responsive delivery systems and nanocatalysis in a controllable and green manner.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b00945. Experimental details on the synthesis and characterization of Ada-PVCL homopolymers from RAFT polymerization (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail [email protected] (T.C.). *E-mail [email protected] (E.T.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research is supported by the National Natural Science Foundation of China (Grant No. 51503155). REFERENCES

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