Article pubs.acs.org/cm
Selectively Initiated Ship-In-A-Bottle Assembly of Yolk−Shell Nanostructures Sergey N. Shmakov, Ying Jia, and Eugene Pinkhassik* Department of Chemistry, Saint Louis University, 3501 Laclede Avenue, Saint Louis, Missouri 63103, United States S Supporting Information *
ABSTRACT: Yolk−shell nanostructures, or nanorattles, are created by growing metal nanoparticles exclusively inside hollow porous polymer nanocapsules. Metal ions enter the nanocapsules through size-selective nanopores. Synthesis of metal nanoparticles is initiated by an agent entrapped in nanocapsules. Tannic acid, β-cyclodextrin, and polyether dendrimer were used as sacrificial molecules for initiation and growth of gold nanoparticles. If needed, initiator molecules can be fragmented by acid hydrolysis and removed from nanocapsules. Variations in reaction conditions yield encapsulated nanoparticles with different size and shape. Further functionalization of nanorattles forms encapsulated core−shell nanoparticles. KEYWORDS: yolk−shell nanostructures, nanorattles, nanocapsules, core−shell nanoparticles, directed assembly
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rates were identical for free and entrapped dyes, suggesting that the transport through the shells of nanocapsules was not the rate-limiting step.10 These shells offering fast and selective permeability are especially suitable for encapsulating metal NPs. We reported the synthesis of nanorattles by simultaneous formation of nanocapsules and entrapped NPs.5c Similar yolk− shell nanostructures can be conceivably prepared by entrapment of NPs during the fabrication of nanocapsules. The drawback of these methods is that nonentrapped NPs or metal ions do not become part of the hybrid nanostructure, complicating the preparation and separation procedures and resulting in unnecessary losses. It is essential for the progress in the field to develop an efficient and economical method for the synthesis of nanorattles. Here we show that metal NPs can be formed exclusively inside prefabricated hollow nanocapsules. Our strategy is to use an initiator entrapped in nanocapsules. In this approach, the initiator cannot escape from the capsules but metal ions can enter the capsule (Scheme 1). The formation of NPs happens only inside the nanocapsule. The initiator can be removed from the capsules or left coentrapped depending on the application. Entrapped metal NPs remain encapsulated. They can be modified further, either by increasing their size or by forming core−shell NPs inside nanocapsules. This general approach offers a convenient and economical synthesis of yolk−shell nanostructures containing metal NPs formed exclusively inside hollow porous nanocapsules. An important benefit of this method is that it is compatible with previously reported
etal nanoparticles (NPs) attracted tremendous interest due to their applications in catalysis,1 imaging,2 sensing,3 and many other areas.4 Yolk−shell objects, or nanorattles, are hybrid nanostructures, in which NPs are entrapped in a hollow capsule.5 Recent reports described several types of these structures with different compositions of shells and entrapped nanoparticles.6 Nanorattles can be particularly useful for practical applications of metal NPs.7 Indeed, use of metal NPs with a naked surface has led to catalysts with higher activity and improved analytical tools;8 however, NPs require capping agents to prevent aggregation and sintering, lowering the benefits of highly active surface of NPs. In yolk−shell structures, semipermeable shells of nanocapsules may provide a natural barrier against aggregation of NPs while allowing communication with substrates, analytes, etc. This approach may permit forming and using NPs without capping agents or other stabilizers, exposing their highly active naked surface. To obtain full benefits of yolk−shell or nanorattle structures, we need to combine the methods for controlling the permeability of the nanocapsule shells with an efficient strategy for the synthesis of encapsulated NPs. We and others have previously shown vesicle-templated synthesis of polymer nanocapsules.9 The permeability of the nanocapsule shell can be controlled by imprinting uniform molecular size nanopores.9c,10 For example, using glucose pentaacetate and glucose pentabenzoate as pore-forming templates, we created pores with approximate diameters of 0.8 and 1.2 nm, respectively.9c Nanometer-thin polymer shells enable ultrafast transport of ions and small molecules while retaining molecules larger than the pore size.9a−c,11 In a study of encapsulated pH-sensitive dyes, the protonation and deprotonation reactions occurred on millisecond time scale, and the © 2013 American Chemical Society
Received: October 18, 2013 Revised: December 16, 2013 Published: December 16, 2013 1126
dx.doi.org/10.1021/cm403442k | Chem. Mater. 2014, 26, 1126−1132
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Article
the sample) for 60 min. Methanol (10 mL) was added, and the precipitate was washed 3−5 times with methanol and then 2 times with water and 3 times with acetonitrile over a period of 24 h. (B) Acrylic NCs: Liposome-Templated Method. Nanocapsules with acrylic monomers were prepared according to procedure A. The following concentrations were used: t-BMA (43 μL, 0.193 mmol), BMA (42 μL, 0.199 mmol), EGDMA (32 μL, 0.17 mmol), DMPA (3 mg, 0.01 mmol), and 0.4 mL of a solution of DMPC (160 mg, 0.236 mmol) in CHCl3. After polymerization methanol (10 mL) was added, the precipitate was washed 3−5 times with methanol and then 5−6 times with water over a period of 24 h. (C) Synthesis of Styrene NCs: Vesicle-Templated Method. Nanocapsules were prepared using a previously described procedure.14 Two monomers solutions containing (S1) cetyltrimethylammonium tosylate (CTAT) and (S2) sodium dodecylbenzenesulfonate (SDBS) were prepared. Solution S1: t-BuSt (105 μL, 0.57 mmol), DVB (81 μL, 0.57 mmol), DMPA, (3 mg, 0.01 mmol), and CTAT (200 mg, 0.438 mmol) were added to a test tube, followed by addition of 20 mL of deionized water. Solution S2: t-BuSt (105 μL, 0.57 mmol), DVB (81 μL, 0.57 mmol), DMPA, (3 mg, 0.01 mmol), and SDBS (200 mg, 0.57 mmol) were added to a test tube, followed by addition of 20 mL of deionized water. Solutions were kept at 35−40 °C for 15 min and then shaken or briefly sonicated to give homogeneous dispersion. Then solutions were quickly mixed up in desired proportion. In a typical experiment solutions were mixed in the ratio S1:S2 = 2:8 (i.e., 2 mL of S1 and 8 mL of S2), shaken several times, and then incubated undisturbed for 30 min at 35 °C. The resulting suspension containing vesicles of different sizes was extruded 4−5 times at 35 °C through a track-etched polyester membrane with 200 or 400 nm pore size. Polymerization and washing were done as described in procedure A. (D) Acrylic NCs: Vesicle-Templated Method. Nanocapsules with acrylic monomers were prepared according to procedure C. Solution S1: t-BMA (64 μL, 0.4 mmol), BMA (64 μL, 0.4 mmol), EGDMA (64 μL, 0.34 mmol), DMPA, (3 mg, 0.01 mmol), and CTAT (200 mg, 0.438 mmol) were added to a test tube, followed by addition of 20 mL of deionized water. Solution S2: t-BMA (64 μL, 0.4 mmol), BMA (64 μL, 0.4 mmol), EGDMA (64 μL, 0.34 mmol), DMPA, (3 mg, 0.01 mmol), and SDBS (200 mg, 0.57 mmol) were added to a test tube, followed by addition of 20 mL of deionized water. Polymerization and washing were done as described in procedure B. Cyclodextrin-Containing Nanocapsules. Polymer nanocapsules with entrapped β-cyclodextrin or hydroxypropyl β-cyclodextrin were prepared following the same procedures described above for synthesis of polymer nanocapsules except for the hydration step where 10 mM aqueous solution of the corresponding β-cyclodextrin was used instead of deionized water. TA-Containing Nanocapsules. Nanocapsules with entrapped tannic acid were prepared following the same procedures described above for synthesis of nanocapsules except for the hydration step, where aqueous solution of freshly prepared and neutralized tannic acid of desired concentration was used instead of deionized water. DE-(OH)12-Containing Nanocapsules. Polymer nanocapsules with entrapped DE-(OH)12 were prepared following the same procedures described above for synthesis of polymer nanocapsules except for the hydration step, where 2.5 mM aqueous solution of the dendrimer was used instead of deionized water. Dye Loading and Retention Experiments. Polymer nanocapsules with entrapped tetrasodium-meso-tetra(4-sulfonatophenyl)porphine (TSPPNa) were prepared following the same procedures described above for synthesis of liposome-templated nanocapsules except for the hydration step, where 2.5 mM aqueous solution of TSPPNa was used instead of deionized water. After encapsulation, nanocapsules were thoroughly washed with methanol and finally with water until disappearance of the Soret band at 416 nm in UV/vis spectra of supernatant. Hydrolysis was carried out at pH ∼ 0.5 and 95 °C for 2 h followed by neutralization and recording of the UV spectrum of the supernatant. Synthesis of Gold Nanoparticles inside Nanocapsules with Entrapped CD or DE-(OH)12. A slurry of precipitated nanocapsules (ca. 5 mg of dry material) was transferred to a screw-capped test tube
Scheme 1. Selectively Initiated Synthesis of Gold NPs inside Hollow Porous Nanocapsulesa
a
An entrapped initiator cannot escape from the capsules, but metal ions can enter the capsule freely. In this study, β-cyclodextrin (β-CD), polyol dendrimer (DE-(OH)12), and tannic acid (TA) were used as initiators. After the synthesis, metal NPs remain permanently entrapped inside nanocapsules, while initiators may be removed.
methods for the creation of nanocapsules with controlled permeability. We used initiators containing multiple hydroxyl groups encapsulated in porous hollow nanocapsules to demonstrate site-selective initiation of NP formation. Alcohols can act as a reducing agent in the synthesis of metal NPs.12 Previously, βCD and tannic acid were used as an initiator of the NP growth.13
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EXPERIMENTAL SECTION
General. Chemicals were purchased from Sigma-Aldrich and were used as received, unless noted otherwise. 1,2-Dimyristoyl-sn-glycero-3phosphocholine (DMPC) was purchased from Avanti Polar Lipids, Inc., as a dry powder. tert-Butyl methacrylate (t-BMA), butyl methacrylate (BMA), and tert-butylstyrene (t-BuSt), ethylene glycol dimethacrylate (EGDMA), and p-divinylbenzene (DVB) were passed through an alumina column to remove the inhibitor shortly before the polymerization. HPLC-grade methanol, chloroform, and acetonitrile (Fisher Scientific) were used as received. All glassware was washed with aqua regia prior to synthesis of gold nanoparticles. Synthesis of Polymer Nanocapsules. (A) Synthesis of Styrene NCs: Liposome-Templated Method. Nanocapsules were prepared using a previously described procedure.9a In a typical experiment tBuSt (43 μL, 0.236 mmol), DVB (34 μL, 0.236 mmol), and 2,2dimethoxy-2-phenyl-acetophenone DMPA (1 mg, 3.9 × 10−6 mol) were added to a 0.4 mL solution of DMPC (160 mg, 0.236 mmol) in CHCl3. The CHCl3 was evaporated using a stream of argon to form a lipid/monomer mixture on the wall of a test tube. The film was further dried under vacuum for 5 min to remove traces of CHCl3. The dried film was hydrated with 8 mL of deionized water to give a dispersion of multilamellar vesicles. During the hydration of the lipid/monomer mixture, the test tube was briefly agitated on a Vortexer every 5 min. The suspension was extruded 16 times at 35 °C through a track-etched polyester Nucleopore membrane (Sterlytech) with 0.2 μm pore size using a Lipex stainless steel extruder (Northern Lipids). Oxygen was removed by passing argon through the solution. The sample was irradiated (λ = 254 nm) in a photochemical reactor equipped with a stirrer (10 lamps of 32 W each; 10-cm distance between the lamps and 1127
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Catalytic Hydrogenation of 4-Nitrophenol. A slurry of precipitated nanorattles (ca. 5 mg of dry material with 0.4−0.6% gold content by weight) was transferred to a screw-capped test tube equipped with a stir bar and dispersed in 1 mL of 1.5 mM 4nitrophenol followed by addition of 0.15 mmol of NaBH4. The mixture was stirred at 25 °C for 5 min and analyzed with UV−vis spectroscopy. Methods. Hydrodynamic diameter measurements were performed on a Malvern Zetasizer Nano ZS (Malvern Instruments Ltd., Worcestershire, U.K.). Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) images were acquired on an FEI Inspect F50 microscope. Transmission electron microscopy (TEM) was carried out on FEI Tecnai Spirit microscope. XRD spectra were collected on a Rigaku Ultima IV powder diffractometer using Cu Kα radiation at 40 kV and 44 mA. The diffraction patterns were obtained in the 2θ scan range 30−85° using a 1 s dwell time and a step size of 0.02 degrees. LCMS analyses were done on a Shimadzu Single Quad LCMS-2010EV with ESI modes using a C18 column for separations. A Bruker 400 MHz Broadband NMR spectrometer was used to collect data for all of the prepared compounds. A Shimadzu GC−MS QP2010S was used to collect molecular fragmentation information for MBO. Atomic absorption experiments were carried out on a GBC 908AA instrument equipped with a graphite furnace. UV/vis measurements were performed on Shimadzu UV-1700 and OLIS 245 absorbance and fluorescence spectrophotometer equipped with CLARiTY integrating cavities.
equipped with a stir bar and dispersed in 2 mL of deionized water (for acrylic-type NCs) or acetonitrile (for styrene-type NCs). The test tube was placed in a water bath and stirred at constant preset temperature. Then the desired amount of HAuCl4 was added to the mixture. In a typical experiment, 10 μL of 100 mM HAuCl4 aqueous solution was added to nanocapsules dispersion, and the mixture was agitated for 5 min at 60 °C. Then 10 μL of 1 M NaOH was added, making the solution pH ca. 10−11. After 15 min, the mixture was removed from the bath, and 8 mL of ice-cold water was added. Resulting colored precipitate was washed with water. Longer heating (e.g., 30 min) and further addition of 10 μL of 100 mM HAuCl4 aqueous solution lead to growth of larger gold nanoparticles. To form nonspherical nanoparticles, 10 μL of aqueous 5 mM KBr was added to the reaction mixture described above prior to addition of NaOH solution. Synthesis of Gold Nanoparticles inside Nanocapsules with Entrapped TA. A slurry of precipitated nanocapsules (ca. 5 mg of dry material) was transferred to a screw-capped test tube equipped with a stir bar and dispersed in 2 mL of deionized water. The desired amount of HAuCl4 was then added to the mixture. In a typical experiment, 10 μL of 20 mM HAuCl4 aqueous solution was added to the dispersion of nanocapsules, and the mixture was stirred for 30 min at room temperature. A total of 8 mL of ice-cold methanol was added to the mixture, and the resulting colored precipitate was washed with water. Synthesis of Ag@Au Core−Shell Nanoparticles. Aqueous precipitate of β-CD-containing nanocapsules with gold nanoparticles prepared at a 50 mM concentration of Au3+ was dispersed in 2 mL of deionized water (for acrylic-type NCs) or acetonitrile (for styrene-type NCs). The test tube was placed in a water bath and stirred at 60 °C. Then 10 μL of 50 mM AgNO3 was added to the mixture. After 5 min, 10 μL of 1 M NaOH was added, and the mixture was stirred for 30 min. Then it was removed from the bath, and 8 mL of ice-cold water was added. The resulting colored precipitate was washed with water. Pentaerythrityl Tetrabromide (PE-Br4). Pentaerythritol was treated with p-toluenesulfonyl chloride in pyridine and subsequently with sodium bromide in diethylene glycol according to the procedure in the literature.15 1-Methyl-4(hydroxymethyl)-2,6,7-trioxabicyclo[2.2.2]octane (MBO). Pentaerythritol (13.6 g, 100 mmol), triethyl orthoacetate (16.22 g, 18.3 mL, 100 mmol), pyridine p-toluenesulfonate (0.5 g, 2 mmol), and 100 mL of dioctylphthalate were mixed in a roundbottomed flask (500 mL) equipped with a Dean−Stark trap fitted with a reflux condenser. The reaction mixture was stirred at 140 °C for 2−3 h under nitrogen until quantitative recovery of ethanol (17.5 mL theoretical) is obtained (residual ethanol was removed under vacuum). The trap was replaced with a large condenser, and the mixture was evacuated at
