Colloidosome-based Synthesis of a Multifunctional Nanostructure of Silver

Colloidosome-based Synthesis of a Multifunctional Nanostructure of Silver

(Parte 1 de 2)

DOI: 10.1021/la904067q ALangmuir X, X(X), X–X ©XXXX American Chemical Society

Colloidosome-based Synthesis of a Multifunctional Nanostructure of Silver and Hollow Iron Oxide Nanoparticles

Yue Pan,† Jinhao Gao,‡ Bei Zhang,§ Xixiang Zhang,§, ) and Bing Xu*,†,‡

†Department of Chemistry, Brandeis University, 415 South Street, Waltham, Massachusetts 02453, ‡Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay,


Bay, Hong Kong, China, and ) Advanced Nanofabrication Core Lab, King Abdullah University of Science and

Technology, Thuwal 23955-6900 Kingdom of Saudi Arabia

Received August 28, 2009

Nanoparticles that self-assemble on a liquid-liquid interface serve as the building block for making heterodimeric nanostructures. Specifically, hollow iron oxide nanoparticles within hexane form colloidosomes in the aqueous solution of silver nitrate, and iron oxide exposed to the aqueous phase catalyzes the reduction of silver ions to afford a heterodimer of silver and hollow iron oxide nanoparticles. Transmission electron microscopy, selected area electron diffraction, energy-dispersive X-ray spectrometry, X-ray diffraction, UV-vis spectroscopy, and SQUID were used to characterize the heterodimers. Interestingly, the formation of silver nanoparticles helps the removal of spinglass layer on the hollow iron oxide nanoparticles. This work demonstrates a powerful yet convenient strategy for producing sophisticated, multifunctional nanostructures.


This paper reports a facile synthesis of a sophisticated, multifunctionalnanostructure that consistsofa silvernanoparticleand a hollow iron oxide nanoparticle. The construction of heterodimers of nanoparticles consisting of two inorganic phases provides a powerful approach for tailoring the properties of nanomaterials for a wide range of applications.1 For example, the unique structure of heterodimeric nanoparticles promises many advantages in their applications in physical science and biology.2 After the elegant demonstration of heterodimers of microparticles,3 several research groups have reported the production of heterodimers of nanoparticles with two different inorganic compositions, such as FePt-CdS/CdSe, γ-Fe2O3-CdSe/ZnS, CoPt3-Au heterodimers, and dumbbell- likeAu-Fe3O4heterodimernanoparticles.4,5Wealsoreportedan efficient method to form heterodimeric nanostructures based on the reactions on the colloidosome of iron oxide nanoparticles.6

“Colloidosomes”, a unique biphasic system formed by the selfassembly of nanoparticles at the interface of an organic solvent and water, allow a heterogeneous reaction to take place on the exposed surface of the nanoparticles and to produce the heterodimers of two distinct nanospheres with a simple but relatively precise control.7,8 This procedure not only controls the composi-

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B DOI: 10.1021/la904067q Langmuir X, X(X), X–X

Article Pan et al.

tions of heterodimers of nanoparticles, but also allows functional molecules to be attached on specific parts of the heterodimers.6 Despite its promising potentials, this useful and versatile method remains less explored. Thus, we are interested in studying the scope of the colloidosome-based synthesis for making heterodimeric or hybrid nanostructures and fully characterizing the resulted nanostructures.

Among various nanocrystals, magnetic nanoparticles, especially iron oxide nanoparticles, have attracted broad attention because they promise new applications in the rapidly advancing field of biofunctional nanomaterials.9 Recently, Sun et al. and Alivisatos et al. have shown that Kirkendall effect10 at nanoscale canleadtotheproductionofhollowironoxidenanoparticles,1,12 and their works demonstrated that hollow iron oxide nanoparticles with controlled interior void and thickness of the shell are an important class of nanoporous materials. We have shown that hollow iron oxide nanoparticles can exhibit high relaxivity for MRI enhancement.13 These attractive features of hollow iron oxide nanoparticles make them ideal building blocks of heterodimeric nanostructures for the further exploration and expansion of their functions.

Here, we report the use of the colloidosome approach to synthesize the heterodimers of silver and hollow iron oxide nanoparticles based on the reactions at a liquid-liquid interface. Besides being the first example of heterodimers that contain hollow nanoparticles and further demonstrating the versatility of this method for constructing sophisticated nanostructures at the liquid-liquid interface, these heterodimeric nanostructures could provide a new class of nanomaterials for useful applications. Silver nanoparticles have excellent surface plasma resonance properties and are themselves a Raman enhancer,14 and hollow iron oxide nanoparticles are superparamagnetic at room temperature. Therefore, potentially, the silver part can serve as optical tags and the hollow iron oxide as magnetic resonance imaging (MRI) contrast and hyperthermia therapy agents.

Materials and Methods

General Data. Iron pentacarbonyl (Fe(CO)5), oleylamine (70%), and 1-octadecene (90%) were purchased from Sigma

Aldrich,and silvernitratefromFisherChemical.Allthereactions were carried out at ambient conditions unless otherwise stated.

Synthesis. Hollow iron oxide nanoparticles were synthesized using a reported procedure.13 Typically, oleylamine (0.3 mL) and 1-octadecene (20 mL) were heated at 120 C for 30 min under argon atmosphere before Fe(CO)5 (0.7 mL) was injected into the hotsolution.Then,thesolutionwaskeptat180 Cfor20minand afforded the Fe nanoparticles as the intermediate. Then, the dispersion was moved to ambient atmosphere and heated up to

180 Cw ith an O2 gas flow at a rate of 2 m3/h for 2 h. The Fe nanoparticles were completely oxidized to iron oxide. After the black-brown colored solution was cooled to room temperature, the hollow iron oxide nanoparticles were precipitated by adding isopropanolfollowedbycentrifugation(6000rpm)andwashwith pure ethanol. The hollow nanoparticles were then dispersed in hexane in the presence of oleylamine.

Inatypicalsynthesisofthe heterodimers,thehollowironoxide nanoparticles (2.5 mg) in 2 mL of hexane in the presence of oleylamine was mixed with the solution of silver nitrate (2 mL, 30 mg/mL) in a small vial. Ultrasonic emulsification afforded a stablebrownoil-in-wateremulsionofthetwophases.Themixture was shaken frequently to make the two liquid phases to mix well. After reacting for 5 h (temperature of the vial: ≈40 C), the mixture was precipitated by adding ethanol. The heterodimers were separated by centrifugation (6000 rpm). Then, the heterodimers were dispersed in hexane in the presence of oleylamine. In the final step, the heterodimer nanoparticles were further purified by magnetic harvesting and redispersed in hexane for the further analysis.

Characterization. The nanostructures were characterized by transmission electron microscopes (TEM) (JEOL 2010, 200 kV), high-resolution TEM, and correlative Energy-dispersive X-ray spectrometric (EDX) (JEOL 2010F, 200 kV). The UV-vis absorbance spectra were obtained on a Perkin-Elmer Lambda 900 UV/vis/NIR spectrometer. The magnetic properties of the nanostructures were measured by a superconducting quantum interference device (SQUID) magnetometer.

Results and Discussion

The construction of the heterodimeric nanoparticles involves an initial synthesis of the hollow iron oxide nanoparticles as the seedsandasubsequentreductionofAgNO3inthepresenceofthe seeds. According to the typical synthetic route illustrated in

Scheme1,thefollowingprocesscouldcontributetotheformation of the heterodimers: Ultrasonic agitation and frequent shaking causetheformationofaheterogeneousmicroemulsionoforganic droplet in silvernitrate watersolution. Inthis biphasic system, the hydrophobic hollow iron oxide nanoparticles self-assemble at the water-organic interface7,15 and provide the catalytic sites onto which the Agþ ions can be reduced by oleylamine16 to form silver nanoparticles. In this process, oleylamine serves as the mild reducing agent as well as the surfactant.17 Most likely, the small defects on the surfaces of the hollow iron oxide nanoparticles that are exposed to the aqueous phase catalyze the reduction of the Agþ ions to provide the initial nucleation sites of silver.18 As a result, the heterodimers with gradually grown silver spheres on the surface of the hollow iron oxide form while the reaction progresses.

We used TEM to follow the progress of the synthesis and to characterize the products. TEM image in Figure 1a shows the assynthesized hollow iron oxide nanoparticles with uniform hollow

Scheme 1. Illustration of the Synthetic Steps of the Heterodimers

(10) (a) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai,

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DOI: 10.1021/la904067q CLangmuir X, X(X), X–X

Pan et al. Article spheres.The HRTEM image (Figure1b) ofthe hollowironoxide reveals that they have the overall diameter of 12 nm and an oxide shell of around 3 nm thick. Furthermore, EDP (electron diffraction patterns) analysis (Figure 1a,inset) of the nanoparticles confirms its excellent crystallinity: the diffraction rings of the hollow nanoparticles are consistent with the crystal planes of iron oxide phase ({220}, {311}, {400}, {422,} and {440}).6 Being oxidized by pure O2, the hollow iron oxide nanoparticles likely are maghemite.13

(Parte 1 de 2)