Interface-Directed Self-Assembly of Gold Nanoparticles and

Article pubs.acs.org/Langmuir

Interface-Directed Self-Assembly of Gold Nanoparticles and Fabrication of Hybrid Hollow Capsules by Interfacial Cross-Linking Polymerization Jia Tian,† Liang Yuan,† Mingming Zhang,‡ Fan Zheng,† Qingqing Xiong,‡ and Hanying Zhao*,† †

Key Laboratory of Functional Polymer Materials, Ministry of Education, Department of Chemistry, Nankai University, Tianjin 300071, China ‡ Tianjin Key Laboratory of Biomedical Materials, Institute of Biomedical Engineering, Chinese Academy of Medical Sciences & Peking Union Medical College, Tianjin 300192, China S Supporting Information *

ABSTRACT: Amphiphilic gold nanoparticles (AuNPs) were produced at liquid− liquid interface via ligand exchange between hydrophilic AuNPs and disulfidecontaining polymer chains. By using oil droplets as templates, hybrid hollow capsules with AuNPs on the surfaces were obtained after interfacial cross-linking polymerization. The volume ratio of toluene to water exerts an important effect on the size of capsules. The average size of the capsules increases with the volume ratio. Transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM) were used to characterize the hollow structures. In this research, not only one-component but also multicomponent hollow capsules were prepared by copolymerization of acrylamide and hybrid AuNPs at liquid−liquid interface. Because of the improvement in hydrophilicity of the hollow capsules, the average size of multicomponent capsules is bigger than one-component ones in aqueous solution.

■

INTRODUCTION The synthesis of hollow capsules has attracted significant attention in chemistry and materials science communities over the past decade.1−3 The interior void of a hollow structure can serve as a nanoreactor for chemical reactions or an extremely small container for encapsulating and control release of active materials such as drugs, proteins, enzymes, or DNA. Hollow capsules with properties of low density, high surface-to-volume ratio, and low refractive index find a wide range of applications,4 such as lithium batteries,5 drug delivery,6 cell imaging,7 catalyst support,8 nanoengineering,9 and chemical sensing.10 The synthetic strategies for hollow structures are divided into hard templating,11,12 soft templating,4 sacrificial templating,13 and template-free method.14 Soft templating method is an efficient approach to the fabrication of hollow capsules. Emulsions, including oil-in-water (O/W) and water-in-oil (W/O) types, are commonly used as soft templates.15,16 Shell materials were deposited executively around the liquid−liquid interface, and hollow structures were built up.17 In these years, more and more researches were focused on the synthesis of hollow capsules with solid particles on the surfaces. The key step in the fabrication of such structures is the deposition of solid particles around liquid droplets. Solid particles in an emulsion are able to self-assemble onto liquid− liquid or liquid−gas interfaces, and the emulsion is called Pickering emulsion.18 Similar to low-molecular surfactants, after being dispersed in a mixture of oil and water, the solid particles © 2012 American Chemical Society

have a strong tendency to locate at the interface between oil and water so that the total interfacial energy is reduced upon replacing part of the oil−water interface by oil-particle and water−particle interfaces.19 The self-assembly of solid particles at liquid−liquid interfaces has been a powerful approach to the synthesis of hollow capsules.20 As shown in eq 1, in a given Pickering emulsion system, a decrease of the interfacial energy is directly related to the size of the particles as follows:21 E0 − E1 = ΔE1 = −

πr 2 ·[γ − (γP/W − γP/O)]2 γO/W O/W

(1)

where r is the radius of the solid particles and γo/w, γp/w, and γp/o represent interfacial tensions between oil and water, particle and water, and particle and oil. The reduction of the interfacial energy depends on r2, and in comparison to large particles, the assembly structure formed by smaller nanoparticles is less stable. So the fabrication of the hollow capsules based on solid particles with sizes as small as a couple of nanometers is a big challenge. Our previous researches demonstrated that the hydrophilic surfaces of gold nanoparticles (AuNPs) could be partly modified by hydrophobic polymer chains, and amphiphilic hybrid AuNPs with sizes of around 5 nm could be prepared.22,23 The amphiphilic AuNPs are able to undergo Received: April 9, 2012 Revised: May 21, 2012 Published: May 23, 2012 9365

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371

Langmuir

Article

Scheme 1. Preparation of Hybrid AuNPs Hollow Capsulesa

a

Amphiphilic AuNPs were formed in situ at the interface of toluene and water via ligand exchange, and one-component and multicomponent hollow capsules were prepared by interfacial cross-linking (co)polymerization.

AuNPs on the surfaces but also multicomponent capsules with hybrid AuNPs and poly(acrylamide) (PAM) on the surfaces were prepared. The structure of the polymer and the outline for the preparation of the hollow capsules are presented in Scheme 1.

directed self-assembly onto the liquid−liquid interface, which enhances the stability of the interface.24 Herein, a new versatile method for fabrication of hybrid hollow capsules is presented. Hydrophilic citrate-stabilized AuNPs were dispersed in water, and a hydrophobic polymer with a disulfide group at the midpoint and methacrylate groups on the repeating units were dissolved in toluene. An emulsion was obtained upon mixing of the two solutions under stirring. Amphiphilic reactive AuNPs were obtained at the liquid−liquid interface after ligand exchange between disulfide containing polymer and citrate on AuNPs. This process is very similar to the in situ production of compatibilizers at the polymer interface in reactive polymer blends.25 Hollow capsules with AuNPs on the surfaces were synthesized by interfacial cross-linking polymerization of methacrylate. In this strategy, amphiphilic AuNPs produced in situ at liquid−liquid interface were used as surfactants, and oil droplets were used as templates. The amphiphilic nanoparticles can be called a reactive hybrid AuNP surfactant. In this research, not only one-component hollow capsules with hybrid

■

EXPERIMENTAL METHODS

Materials. 2,2-Azoisobutyronitrile (AIBN, Guo Yao Chemical Company, 98%) was purified by recrystallization from ethanol. 4,4Azobis(4-cyanopentanoic acid) (ABCPA, 97%), purchased from Aldrich, was purified by recrystallization from methanol and dried in vacuum at room temperature. HAuCl4·4H2O (Tianjin Chemical Reagent Company), sodium borohydride (NaBH4, Guo Yao Chemical Company, 96%), and trisodium citrate dehydrate (Tianjin Chemical Reagent Company, 99%) were used as received. 2-Hydroxyethyl methacrylate (HEMA, 99.5%) was purchased from Tianjin Institute of Chemical Agents. It was purified by washing an aqueous solution of monomer with hexane to remove ethylene glycol dimethacrylate, salting the monomer out of the aqueous phase by adding NaCl, drying over MgSO4, and distilling under reduced pressure. 2-Hydroxyethyl 9366

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371

Langmuir

Article

disulfide (90%) and 2-bromoisobutyryl bromide (97%) were purchased from Alfa and used as received. Acrylamide (AM, 98.5%) was purchased from Linghai Chem. Co and purified by recrystallization from chloroform. Synthesis of 2-Bromoisobutyrate Ethyl Disulfide. A solution of 2-bromoisobutyryl bromide (4.12 mL, 33.3 mmol) was added dropwise at 0 °C into a mixture of 2-hydroxyethyl disulfide (1.30 mL, 11.1 mmol), triethylamine (4.70 mL, 34.5 mmol), and THF (60.0 mL), and the solution was stirred at 25 °C for 40 h. After filtration, THF was removed by rotating evaporation, and the crude product was redissolved in CH2Cl2 and washed with 2% HCl aqueous solution, NaHCO3 solution, and distilled water, respectively. The organic phase was collected and dried over MgSO4. The ester was further purified with silica column chromatography (eluent, a mixture of petroleum ether and ethyl acetate with a volume ratio of 20:1). The yield of 2bromoisobutyrate ethyl disulfide was about 72%. The chemical structure and 1H NMR spectrum of 2-bromoisobutyrate ethyl disulfide are shown in Figure S1, Supporting Information. 1 H NMR, δ (400 MHz, CDCl3, TMS, ppm): 4.46 (−COOCH2−, t, 4H), 2.96 (−CH2−S−, t, 4H), 1.94 (−CO−C(CH3)2−Br, s, 12H). Atom Transfer Radical Polymerization (ATRP) of 2-Hydroxyethyl Methacrylate Initiated by 2-Bromoisobutyrate Ethyl Disulfide. Poly(2-hydroxyethyl methacrylate) (PHEMA) was prepared by ATRP initiated by 2-bromoisobutyrate ethyl disulfide. Degassed HEMA monomer (3.50 mL, 28.9 mmol), Bpy (0.31 g, 1.98 mmol), CuBr (0.14 g, 0.96 mmol), and methanol (4.0 mL) were added to a dry Schlenk flask. After three freeze−pump−thaw cycles, ATRP initiator 2-bromoisobutyrate ethyl disulfide (0.22 g, 0.48 mmol) was introduced into the flask by using a degassed syringe to initiate the polymerization. The polymerization was conducted at room temperature for 3 h. Copper ions were removed from the polymer solution with silica column chromatography. The monomer conversion was about 56%. 1H NMR spectrum of PHEMA was shown in Figure S2, Supporting Information. On the basis of the 1H NMR result, the degree of polymerization of PHEMA is determined to be 100. 1 H NMR, δ (400 MHz, DMSO-d6, TMS, ppm): 4.83 (−OH, 100H), 3.92 (−CH2−CH2−OH, 200H), 3.56 (−CH2−CH2−OH, 200H), 2.95 (−CH2−S−, 4H). Synthesis of Poly(ethylene Glycol Dimethacrylate) with Disulfide Group at the Mid Point (DS-PEGDMA). The synthesis of DS-PEGDMA was illustrated in Scheme S1, Supporting Information. The detailed steps were described as follows. PHEMA (0.500 g, 3.85 mmol, −OH groups), triethylamine (2.00 mL, 14.4 mmol), and phenothiazine (20 mg, 0.10 mmol) were mixed in dry DMF in a round-bottom flask equipped with a magnetic stirrer. Methacryloyl chloride (1.20 mL, 12.8 mmol) was added dropwise at 0 °C, and the reaction mixture was stirred at room temperature for 36 h. After filtration, the polymer solution was reduced by rotary evaporation. Dichloromethane was added to the polymer solution, and the solution was washed with distilled water. The crude product was purified by precipitating in methanol. After filtration, the polymer was dried under vacuum until a constant weight was reached. In this research, the polymer was referred to as DS-PEGDMA. 1H NMR spectrum and GPC curve of DS-PEGDMA are shown in Figure S3, Supporting Information. 1 H NMR, δ (400 MHz, CDCl3, TMS, ppm): 6.12(CH2 C(CH3)−, 100H), 5.58 (CH2C(CH3)−, 100H), 4.39 (−CH2 C(CH3)−COO−CH2−CH2−COO−, 200H), 4.15 (−CH2C(CH3)−COO−CH2−CH2−COO−, 200H), 2.94 (−CH2−S−, 4H). Preparation of Hollow Capsules. HAuCl4·4H2O (60 mg, 0.15 mmol) and trisodium citrate dehydrate (44 mg, 0.15 mmol) were dissolved in 200 mL of doubly distilled water. After one hour stirring, ice sodium borohydride aqueous solution (3.0 mL, 0.90 mmol) was added, and the solution immediately turned red, which indicated the formation of AuNPs. DS-PEGDMA (5.0 mg) was dissolved in 5.0 mL of toluene, and then, 0.33 mL of the polymer solution was added into 10 mL of aqueous solution of AuNPs in a Schlenk flask. An O/W emulsion with amphiphilic AuNPs at the interface of toluene and water was obtained under stirring.

A typical interfacial cross-linking polymerization was described as follows. AIBN (5 mg) was dissolved in 0.1 mL of toluene and added into the above O/W emulsion. After bubbling argon gas for 15 min, the cross-linking polymerization was conducted at 60 °C for 24 h. In order to synthesize multicomponent hollow capsules with hybrid AuNPs and PAM on the surfaces, 5.0 mg of AIBN dissolved in 0.10 mL of toluene and 100 mg of AM dissolved in 0.5 mL of distilled water were added into an O/W emulsion with amphiphilic AuNPs at liquid−liquid interface. After bubbling argon gas for 15 min, the interfacial cross-linking copolymerization of hybrid AuNPs and AM was conducted at 60 °C for 24 h. Characterization. The apparent molecular weight and molecular weight distribution of the polymer were determined on a GPC equipped with a Hitachi L-2130 HPLC pump, Hitachi L-2350 column oven operated at 40 °C, three Varian PL columns with 5−600 K, 500− 30 K, and 100−10 K molecular ranges, and a Hitachi L-2490 refractive index detector. THF was used as eluent at a flow rate of 1.0 mL/min. Molecular weights were calibrated on PS standards. Transmission electron microscopy (TEM) observations were carried out on a Tecnai G2 20 S-TWIN electron microscope equipped with a Model 794 CCD camera. TEM specimens were prepared by dipping copper grids into solutions and dried in air. UV−vis absorption spectra were recorded on a Shimadzu UV-2450 spectrophotometer using a quartz cell of 1 cm path length. Dynamic light scattering (DLS) measurements were conducted on a Zetasizer Nano ZS from Malvern Instruments equipped with a 10 mW HeNe laser at a wavelength of 633 nm. The results were analyzed in CONTIN mode. Atomic force microscopy (AFM) images were collected on a Nanoscope IV atomic force microscope (Digital Instruments Inc.). The microscope was operated in tapping mode using Si cantilevers with a resonance frequency of 320 kHz. The voltage was between 2 and 3 V, and a tip radius was less than 10 nm. A drive amplitude of 1.2 V and a scan rate of 1.0 Hz were used. Scanning electron microscopy (SEM) observations were conducted on a SHIMADZU SS-550 scanning electron microscope. The goldcoating samples were used in the measurements. Steady-state fluorescence spectra were recorded on a Shimadzu RF-5301PC fluorescence spectrophotometer. The excitation and emission slits were both set at 5 nm. The excitation wavelength of the emission spectra was set at 339 nm.

■

RESULTS AND DISCUSSION The characterizations of DS-PEGDMA and its precursors can be found in the Supporting Information (Figure S1−S3). The structure of the polymer was shown in Scheme 1. The double bonds on the polymer chains can be used in interfacial crosslinking (co)polymerization. On the basis of gel permeation chromatography (GPC) results, the apparent molecular weight and the molecular weight distribution of DS-PEGDMA are 17 kg mol−1 and 1.22, respectively. Figure S4 in the Supporting Information shows a TEM image of citrate-stabilized AuNPs. The average diameter of AuNPs is about 5 nm. In order to prepare an O/W emulsion, toluene solution of DS-PEGDMA was added into aqueous dispersion of AuNPs under stirring. Amphiphilic hybrid AuNPs were prepared in situ at liquid− liquid interface after ligand exchange and the interfacial tension between toluene and water was reduced. Figure 1a−d shows optical microscope images of toluene droplets dispersed in water, where citrate-stabilized AuNPs were previously dispersed in aqueous phase and different amounts of polymer were dissolved in toluene. In an emulsion without polymer in the toluene phase, large toluene droplets with a size distribution from 20 to 210 μm can be observed (Figure 1a). When the molar ratio of DS-PEGDMA to AuNPs reaches 1:1, 3:1, and 10:1, the toluene droplets sizes are in the ranges of 7−130, 6− 25, and 6−8 μm. The decrease in the toluene droplet size is attributed to the self-assembly of the amphiphilic AuNPs at liquid−liquid interface, and the reduction of the interfacial 9367

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371

Langmuir

Article

self-assemble to liquid−liquid interfaces, so randomly distributed AuNPs are observed in the image. Figure 2b,c represents two TEM images of an emulsion with DS-PEGDMA dissolved in toluene droplets. The TEM images at different magnification indicate that AuNPs aggregate together after evaporation of the solvents and no individual nanoparticles are observed outside of the droplet structure, which confirms the self-assembly of amphiphilic reactive AuNPs at liquid−liquid interface. In order to prepare hollow capsules, DS-PEGDMA and AIBN, a free-radical initiator, were dissolved in toluene, and the solution was added into aqueous solution of AuNPs. In the emulsion, the volume ratio of toluene to water was kept at 1:30. The cross-linking polymerization of the reactive hybrid AuNPs was conducted at the interface of toluene and water at an elevated temperature. Figure 3a,b shows two TEM images of the spherical hollow capsules at different magnifications. In the images, no individual AuNPs are observed outside of the structures. The surfaces of the hollow structures are composed

Figure 1. Optical microscope images of toluene droplets dispersed in water, where citrate-stabilized AuNPs were previously dispersed in aqueous phases and different amounts of DS-PEGDMA were dissolved in toluene. The photographs were obtained at different molar ratios of DS-PEGDMA to AuNPs: (a) 0, (b) 1:1, (c) 3:1, and (d) 10:1. The volume ratio of toluene to water is about 1:10.

tension between toluene and water. Figure S5, Supporting Information, is a photograph showing a mixture of toluene and water with a volume ratio of 1:4, where AuNPs and polymer were previously dispersed (or dissolved) in liquid phases. In the photograph, violet toluene droplets are observed under stirring. The observation of the violet toluene droplets is attributed to the self-assembly of amphiphilic AuNPs at liquid−liquid interface. Figure 2a shows a TEM image of an emulsion without DSPEGDMA in the toluene phase. Hydrophilic AuNPs can not

Figure 3. (a,b) TEM images of hollow capsules with AuNPs on the surfaces. In the preparation of hollow capsules, the volume ratio of water to toluene is 30:1. (c) A TEM image of hollow capsules prepared in an emulsion with a volume ratio of water to toluene at 15:1. (d) UV−vis spectra of citrate-stabilized AuNPs in water (curve a), one-component hollow capsules (curve b), and multicomponent hollow capsules (curve c) with AuNPs on the surface prepared via interfacial cross-linking polymerization. (e) TEM image of hollow capsules after being redispersed in THF.

Figure 2. TEM images of dried emulsions with dispersed toluene droplets in aqueous phase, where citrate-stabilized AuNPs were previously dispersed. The TEM images were obtained at different molar ratios of DS-PEGDMA to AuNP: (a) 0 and (b,c) 10:1. 9368

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371

Langmuir

Article

of hydrophobic cross-linked DS-PEGDMA and hydrophilic AuNPs. The volume ratio of toluene to water plays an important role in determining the average size of the dispersed oil droplets and the hollow capsules. Figure 3c shows a TEM image of hollow capsules prepared at a volume ratio of 1:15. In comparison to the capsules prepared in an emulsion with a volume ratio of 1:30, the average size of the capsules increases from 110 to 240 nm, which indicates that the average size of the capsules increases with the volume ratio of toluene to water. The aggregation of AuNPs leads to a red shift of plasmon absorption because of the electronic coupling interaction between neighboring nanoparticles. The plasmon absorption band of citrate-stabilized AuNPs in aqueous solution appeared with a maximum absorbance at 514 nm (curve a in Figure 3d); however, the maximum absorbance of AuNPs in the hollow capsules red-shifted to 530 nm (curve b in Figure 3d) because of the segregation of AuNPs on the surfaces of hollow capsules. In order to demonstrate the cross-linking structure of the hollow capsules, the capsules were redispersed in THF, a good solvent for DS-PEGDMA. The TEM result indicated that the hollow structure was still maintained in THF (Figure 3e), which confirmed the cross-linking structure of the hollow capsules. A SEM image of the hollow capsules is shown in Figure 4a. This image indicates that some of the large capsules in the dry

Figure 5. (a) TEM image of hollow capsules prepared via interfacial cross-linking polymerization initiated by sodium salt of 4,4-azobis(4cyanopentanoic acid). (b) Magnified TEM image of a hollow capsule showing the details of the structure.

Copolymerization is one of the most frequently used methods in the preparation of multicomponent polymeric materials. In this research, copolymerization of AM and hybrid AuNPs was conducted, and multicomponent hollow capsules with AuNPs and PAM on the surfaces were prepared. PAM is a water-soluble polymer, and the introduction of PAM into the surfaces of the hollow capsules will improve the hydrophilicity of the structures significantly. In order to prepare multicomponent hollow capsules, AM monomer was dissolved in an aqueous solution of AuNPs, and AIBN and DS-PEGDMA were dissolved in toluene, the cross-linking copolymerization of AM, and reactive hybrid AuNPs was conducted at liquid−liquid interface. Figure 6a,b shows two TEM images of multicomponent hollow capsules at two different magnifications. In the TEM images, white domains on the hollow capsules are observed. Careful observation of the TEM image at high magnification indicates that the AuNPs’ density in a white domain is much lower than deep domains. PAM is not miscible

Figure 4. SEM (a) and AFM (b) images of hollow capsules.

state are deformed, while the smaller ones stay intact. The deformation of the capsules is related to the composition, capsule size, and membrane thickness.26 In comparison to hollow capsules with small size, it is easier for large ones to deform. In this system, some large deformed capsules are in coexistence with small spherical ones. An AFM image of the hollow capsules is shown in Figure 4b. In the image, the depth of a valley on a deformed capsule is determined to be about 40 nm. The cross-linking polymerization of the reactive hybrid AuNP surfactants was conducted at the liquid−liquid interface, so both hydrophobic and hydrophilic free-radical initiators can be used to initiate the polymerization. Besides AIBN, sodium salt of ABCPA, a water-soluble free-radical initiator, was also used to initiate the interfacial cross-linking polymerization. The decomposition of the water-soluble initiator was in the aqueous phase, and only free radicals diffusing to the liquid−liquid interface were able to initiate the polymerization of the reactive hybrid AuNPs. TEM results indicated that spherical hollow structures were produced after polymerization (Figure 5), which further confirmed that the cross-linking polymerization of the hybrid AuNPs was at the interface of toluene and water. Figure 5b shows a magnified TEM image of a specific hollow structure, where a deformed hollow capsule in the dry state is identified.

Figure 6. (a,b) TEM images of multicomponent hollow capsules with hybrid AuNPs and poly(acrylamide) on the surfaces. (c) Dynamic light scattering curves of one-component and multicomponent hollow capsules. (d) Fluorescence emission spectra of pyrene in water (spectrum a), in one-component (spectrum b) and multicomponent hollow capsules (spectrum c). Excitation wavelength was set at 339 nm. 9369

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371

Langmuir

Article

(co)polymerization of the AuNPs one-component and multicomponent hollow capsules were produced. The approach reported in this article is a general method, and many different hybrid hollow capsules can be prepared based on this strategy. It is expected that the hybrid hollow capsules with hydrophobic interior voids and hydrophilic inorganic nanoparticles on the surfaces can find applications in catalysis, controlled release, cell imaging, and gene therapy.

with DS-PEGDMA grafted to the AuNPs, and there occurs nanosized phase separation on the surfaces of the hollow capsules. The white domains represent PAM-rich phases on the hollow capsules, and the deep domains represent AuNPs-rich phases. Curve c in Figure 3d indicates that the maximum absorbance of AuNPs in the multicomponent hollow capsules red-shifted to 526 nm, and the plasmon absorption band becomes broader. DLS curves of one-component and multicomponent hollow capsules are shown in Figure 6c. The average hydrodynamic diameter of the one-component hollow capsules is about 160 nm; however, after introduction of PAM into the structures, the average hydrodynamic diameter of the structures increases to about 490 nm. Because PAM is a water-soluble polymer, after copolymerization of the hybrid AuNPs with AM monomer, the hydrophilicity of the multicomponent capsules is improved, and the average hydrodynamic diameter of the structures in aqueous solution is increased. Pyrene was widely used as a fluorescent probe to determine the polarity of the microenvironments. The intensity ratio of the first to the third vibronic band (I1/I3) of pyrene monomer emission is very sensitive to the polarity of the microenvironment, which was also used to determine the changes of the polarity of the hollow capsules. The value of I1/I3 ranges from about 1.9 in polar solvents to about 0.6 in hydrocarbons. Figure 6d shows fluorescence emission spectra of pyrene in water (curve a), and in aqueous dispersions of one-component and multicomponent hollow capsules (curve b,c). In water, the value of I1/I3 is 1.80, while it decreases to 1.08 in one-component hollow capsules and 1.24 in multicomponent hollow capsules. The lower I1/I3 value of pyrene in aqueous dispersions of one-component capsules suggested the existence of hydrophobic DS-PEGDMA phases on the surfaces of the structures. In comparison to onecomponent hollow structures, the higher value of I1/I3 of pyrene in multicomponent hollow capsules indicated the improvement of the hydrophilicity of the hollow capsules after introduction of PAM into the structures. The stability of the hybrid hollow capsules in aqueous solution is important for the application of the structures. In this research, the effect of storage on the structures was investigated. Parts a and b in Figure S6 in the Supporting Information represent TEM images of one-component and multicomponent hollow capsules after one year storage. It can be found that the structures did not change even after one year of storage. Part c in Figure S6, Supporting Information, shows a photograph of one-component and multicomponent hollow capsules in aqueous solution after one year storage. Homogeneous solutions were obtained after sonication. Part d in Figure S6, Supporting Information, shows UV−vis spectra of citrate-stabilized AuNPs (curve a), freshly prepared onecomponent hollow capsules (curve b) and hollow structures after one year storage (curve c). In comparison to freshly prepared hollow structures, the maximum absorbance of the structures after one year storage red-shifted from 530 to 533 nm. The red-shift of the UV−vis spectrum indicates the aggregation of AuNPs on the hollow structures.

■

ASSOCIATED CONTENT

S Supporting Information *

1

H NMR, GPC elution chromatograms, TEM images, and calculations. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel: 86-022-2349-8703. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This project was supported by the National Natural Science Foundation of China (NSFC) under contract 21174073 and Science and Technology Committee of Tianjin under contract 10JCYBJC01900.

■

REFERENCES

(1) Giersig, M.; Ung, T.; Liz-Marzan, L. M.; Mulvaney, P. Direct Observation of Chemical Reactions in Silica-Coated Gold and Silver Nanoparticles. Adv. Mater. 1997, 9, 570−575. (2) Walsh, D.; Mann, S. Fabrication of Hollow Porous Shells of Calcium Carbonate from Self-Organizing Media. Nature 1995, 377, 320−323. (3) Zhang, Q.; Wang, W.; Goebl, J.; Yin, Y. Self-Templated Synthesis of Hollow Nanostructures. Nano Today 2009, 4, 494−507. (4) Lou, X. W.; Archer, L. A.; Yang, Z. Hollow Micro-/ Nanostructures: Synthesis and Applications. Adv. Mater. 2008, 20, 3987−4019. (5) Lou, X. W.; Wang, Y.; Yuan, C.; Lee, J. Y.; Archer, L. A. Template-free Synthesis of SnO(2) Hollow Nanostructures with High Lithium Storage Capacity. Adv. Mater. 2006, 18, 2325−2329. (6) Zhu, Y. F.; Shi, J. L.; Shen, W. H.; Dong, X. P.; Feng, J. W.; Ruan, M. L.; Li, Y. S. Stimuli-Responsive Controlled Drug Release From a Hollow Mesoporous Silica Sphere/Polyelectrolyte Multilayer CoreShell Structure. Angew. Chem., Int. Ed. 2005, 44, 5083−5087. (7) Chen, J.; Saeki, F.; Wiley, B. J.; Cang, H.; Cobb, M. J.; Li, Z. Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X. D.; Xia, Y. Gold Nanocages: Bioconjugation and Their Potential Use As Optical Imaging Contrast Agents. Nano Lett. 2005, 5, 473−477. (8) Ikeda, S.; Ishino, S.; Harada, T.; Okamoto, N.; Sakata, T.; Mori, H.; Kuwabata, S.; Torimoto, T.; Matsumura, M. Ligand-Free Platinum Nanoparticles Encapsulated in a Hollow Porous Carbon Shell As a Highly Active Heterogeneous Hydrogenation Catalyst. Angew. Chem., Int. Ed. 2006, 45, 7063−7066. (9) Caruso, F. Nanoengineering of Particle Surfaces. Adv. Mater. 2001, 13, 11−22. (10) Yu, H.; Yu, J.; Liu, S.; Mann, S. Template-Free Hydrothermal Synthesis of CuO/Cu2O Composite Hollow Microspheres. Chem. Mater. 2007, 19, 4327−4334. (11) Caruso, F.; Caruso, R. A.; Mohwald, H. Nanoengineering of Inorganic and Hybrid Hollow Spheres by Colloidal Templating. Science 1998, 282, 1111−1114. (12) (a) Caruso, F.; Spasova, M.; Susha, A.; Giersig, M.; Caruso, R. A. Magnetic Nanocomposite Narticles and Hollow Spheres Constructed by a Sequential Layering Approach. Chem. Mater. 2001, 13,

■

CONCLUSIONS In summary, a new method for the preparation of hybrid hollow capsules based on interface-directed self-assembly of AuNPs and interfacial cross-linking polymerization was reported. Amphiphilic reactive AuNPs were produced in situ at the liquid−liquid interface, and after interfacial cross-linking, 9370

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371

Langmuir

Article

Nanoparticles at Water/Oil Interfaces. Angew. Chem., Int. Ed. 2004, 43, 5639−5642. (c) Wang, D.; Duan, H.; Mohwald, H. The Water/Oil Interface: The Emerging Horizon for Self-Assembly of Nanoparticles. Soft Matter 2005, 1, 412−416. (25) (a) Jeon, H. K.; Kim, J. K. The Effect of the Amount of in Situ Formed Copolymers on the Final Morphology of Reactive Polymer Blends with an in Situ Compatibilizer. Macromolecules 1998, 31, 9273−9280. (b) Sigalov, G. M.; Ibuki, J.; Chiba, T.; Inoue, T. Method of Effective Ellipses for Digital Image Analysis of Size, Shape, Orientation, and Interparticle Distances in Polymer Blends: Application to a Study of Polyamide 6/Polysulfone Reactive Blending. Macromolecules 1997, 30, 7759−7767. (26) Annemie, R.; Laurence, C.; Zhang, Z.; David, H. The Compressive Deformation of Multicomponent Microcapsules: Influence of Size, Membrane Thickness, and Compression Speed. J. Biomater. Sci., Polym. Ed. 2001, 12, 157−170.

109−116. (b) Imhof, A. Preparation and Characterization of TitaniaCoated Polystyrene Spheres and Hollow Titania Shells. Langmuir 2001, 17, 3579−3585. (13) (a) Sun, Y.; Mayers, B. T.; Xia, Y. Template-Engaged Replacement Reaction: A One-Step Approach to the Large-Scale Synthesis of Metal Nanostructures with Hollow Interiors. Nano Lett. 2002, 2, 481−485. (b) Peng, S.; Sun, S. Synthesis and Characterization of Monodisperse Hollow Fe3O4 Nanoparticles. Angew. Chem., Int. Ed. 2007, 46, 4155−4158. (14) Hu, Y.; Ge, J.; Sun, Y.; Zhang, T.; Yin, Y. A Self-Templated Approach to TiO2 Microcapsules. Nano Lett. 2007, 7, 1832−1836. (15) (a) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. Synthesis of Submicrometer-Sized Hollow Silver Spheres in Mixed Polymer− Surfactant Solutions. Adv. Mater. 2002, 14, 1499−1502. (b) Li, Y.; Li, X.; Li, Y.; Liu, H.; Wang, S.; Gan, H.; Li, J.; Wang, N.; He, X.; Zhu, D. Controlled Self-Assembly Behavior of an Amphiphilic Bisporphyrin− Bipyridinium−Palladium Complex: From Multibilayer Vesicles to Hollow Capsules. Angew. Chem., Int. Ed. 2006, 45, 3639−3643. (16) (a) McDonald, C. J.; Bouck, K. J.; Chaput, A. B.; Stevens, C. J. Emulsion Polymerization of Voided Particles by Encapsulation of a Nonsolvent. Macromolecules 2000, 33, 1593−1605. (b) Buchold, D. H.; Feldmann, C. Nanoscale Gamma-AIO(OH) Hollow Spheres: Synthesis and Container-Type Functionality. Nano Lett. 2007, 7, 3489−3492. (17) Zoldesi, C. I.; Imhof, A. Synthesis of Monodisperse Colloidal Spheres, Capsules, and Microballoons by Emulsion Templating. Adv. Mater. 2005, 17, 924−928. (18) Pickering, S. U. Emulsions. J. Chem. Soc. 1907, 91, 2001−2021. (19) (a) Velev, O. D.; Furusawa, K.; Nagayama, K. Assembly of Latex Particles by Using Emulsion Droplets as Templates. 1. Microstructured Hollow Spheres. Langmuir 1996, 12, 2374−2384. (b) Velev, O. D.; Furusawa, K.; Nagayama, K. Assembly of Latex Particles by Using Emulsion Droplets as Templates. 2. Ball-Like and Composite Aggregates. Langmuir 1996, 12, 2385−2391. (c) Binks, B. P.; Lumsdon, S. O. Catastrophic Phase Inversion of Water-in-Oil Emulsions Stabilized by Hydrophobic Silica. Langmuir 2000, 16, 2539−2547. (20) (a) Dinsmore, A. D.; Hsu, M. F.; Nikolaides, M. G.; Marquez, M.; Bausch, A. R.; Weitz, D. A. Colloidosomes: Selectively Permeable Capsules Composed of Colloidal Particles. Science 2002, 298, 1006− 1009. (b) Samanta, B.; Patra, D.; Subramani, C.; Ofir, Y.; Yesilbag, G.; Sanyal, A.; Rotello, V. M. Stable Magnetic Colloidosomes via Click Mediated Crosslinking of Nanoparticles at Water−Oil Interfaces. Small 2009, 5, 685−688. (c) Niu, Z.; He, J.; Russell, T. P.; Wang, Q. Synthesis of Nano/Microstructures at Fluid Interfaces. Angew. Chem., Int. Ed. 2010, 49, 10052−10066. (21) (a) Pieranski, P. Two-Dimensional Interfacial Colloidal Crystals. Phys. Rev. Lett. 1980, 45, 569−572. (b) Böker, A.; He, J.; Emrick, T.; Russell, T. P. Self-Assembly of Nanoparticles at Interfaces. Soft Matter 2007, 10, 1231−1248. (22) (a) Tian, J.; Jin, J.; Zheng, F.; Zhao, H. Self-Assembly of Gold Nanoparticles and Polystyrene: A Highly Versatile Approach to the Preparation of Colloidal Particles with Polystyrene Cores and Gold Nanoparticle Coronae. Langmuir 2010, 26, 8762−8768. (b) Tian, J.; Zheng, F.; Zhao, H. Nanoparticles with Fe3O4-Nanoparticle Cores and Gold-Nanoparticle Coronae Prepared by Self-Assembly Approach. J. Phys. Chem. C 2011, 115, 3304−3312. (23) (a) Zhang, X.; Liu, L.; Tian, J.; Zhang, J.; Zhao, H. Copolymers of Styrene and Gold Nanoparticles. Chem. Commun. 2008, 6549− 6551. (b) Zhang, X.; Yang, Y.; Tian, J.; Zhao, H. Vesicles Fabricated by Hybrid Nanoparticles. Chem. Commun. 2009, 3807−3809. (c) Tian, J.; Zheng, F.; Duan, Q.; Zhao, H. Self-Assembly of Polystyrene with Pendant Hydrophilic Gold Nanoparticles: The Influence of the Hydrophilicity of the Hybrid Polymers. J. Mater. Chem. 2011, 21, 16928−16934. (24) (a) Duan, H.; Wang, D.; Sobal, N. S.; Giersig, M.; Kurth, D. G.; Mohwald, H. Magnetic Colloidosomes Derived from Nanoparticle Interfacial Self-Assembly. Nano Lett. 2005, 5, 949−952. (b) Duan, H.; Wang, D.; Kurth, D. K.; Mohwald, H. Directing Self-Assembly of 9371

dx.doi.org/10.1021/la301453n | Langmuir 2012, 28, 9365−9371