Trapping Iron Oxide into Hollow Gold Nanoparticles
© Huang et al. 2010
Received: 22 July 2010
Accepted: 9 September 2010
Published: 28 September 2010
Synthesis of the core/shell-structured Fe3O4/Au nanoparticles by trapping Fe3O4 inside hollow Au nanoparticles is described. The produced composite nanoparticles are strongly magnetic with their surface plasmon resonance peaks in the near infrared region (wavelength from 700 to 800 nm), combining desirable magnetic and plasmonic properties into one nanoparticle. They are particularly suitable for in vivo diagnostic and therapeutic applications. The intact Au surface provides convenient anchorage sites for attachment of targeting molecules, and the particles can be activated by both near infrared lights and magnetic fields. As more and more hollow nanoparticles become available, this synthetic method would find general applications in the fabrication of core–shell multifunctional nanostructures.
Gold nanoparticles (AuNPs) and superparamagnetic iron oxide nanoparticles (SPIONs) have been subjects of intensive research in the last decade [1, 2]. They are generally considered as biocompatible and are of great interest for diagnostic imaging and therapeutic applications. SPIONs are currently being used as magnetic resonance imaging contrast agents in clinic. Heating effect of SPIONs in an alternating magnetic field has been extensively explored for potential hyperthermia treatment of cancer . Biomedical applications of AuNPs originate from their surface plasmon resonance (SPR) effect, a strong enhancement of absorption and scattering of light in resonant with the SPR frequency, which has been utilized for photothermal ablation treatment and optical imaging. To tune the SPR wavelength to the near infrared (NIR) region that is commonly regarded as a 'clear window' for deep tissue penetration of light, various types of Au nanoparticles such as nanorods , nanoprisms , nanoshells [6, 7], and nanocages [8, 9] have been developed and investigated.
In recent years, combining SPIONs with Au to form a composite multifunctional nanoparticles has attracted considerable attention [10–14]. To date, the effort has been mostly limited on coating iron oxide particles with a thin layer of Au, where the Au shell not only provides convenient anchorage sites for functionalization of biomolecules through the well-established Au-thiol conjugation procedure but also protects SPIONs from dissolution and aggregation. However, by such an approach, it is difficult, if not impossible, to tune the SPR wavelengths to the NIR region. Reported core/shell particles usually have their SPR in the visible light range (from 500 to 600 nm), which limits their optical functions for in vivo applications. Here, we report an approach to construct iron-oxide/Au core/shell nanoparticles by trapping iron oxide nanoparticles into hollow AuNPs. Such nanoparticles are magnetic with their SPR peaks in the NIR region (wavelength from 700 to 800 nm).
Synthesis of Porous Hollow Au Nanoparticles
As a result, an Au shell forms around the hydrogen bubble. Metal Au evolves from clusters, particles to porous networks, forming PHAuNPs, which adhere to the inner wall surface of the pores inside both (bottom and top) membranes. The PHAuNPs-loaded top membrane was ready for the next step of loading Fe3O4 nanoparticles into these PHAuNPs after being washed by passing deionized water through the membrane several times.
Loading Fe3O4 Nanoparticles into PHAuNPs
The formation of Fe3O4 nanoparticles via alkaline precipitation was conducted by following a previously reported procedure . Briefly, anhydrous 5.2 g FeCl3 (0.032 mol) and 2 g FeCl2 (0.016 mol) were mixed in 25 ml of DI water containing 0.85 ml of 12.1 N HCl under vigorous stirring. This aqueous solution flew through the PHAuNP-loaded AAO membrane using a vacuum filtration setup, which guarantees all PHAuNPs were wetted with the solution. The membrane was removed from filtration setup and immersed into the solution for additional 30 min. The wet membrane was then transferred into 5 ml of 0.5% NH4OH and was allowed to sit for 20 min. The color change to yellow–orange indicates the precipitation of iron oxide particles. After the precipitation, the iron oxide nanoparticles (~10 nm) formed within the membrane were washed away by flowing DI water through the membrane using the vacuum filtration setup. The Fe3O4/PHAuNPs core/shell nanoparticles were released into water after the dissolution of membrane using 2 M NaOH solution. The particles were cleaned by several cycles of dispersion in DI water followed by centrifugation.
Characterization of Fe3O4/PHAuNPs Core/Shell Nanoparticles
Samples for TEM were made by simply dipping a copper grid into the diluted nanoparticle water suspension. TEM micrographs were taken using a Hitachi H9500 HR-TEM. Absorption spectra of the particle water suspensions were measured using a Perkin-Elmer Lambda 19 UV/VIS/NIR Spectrometer. Hysteresis loop of dried particle powder was measured using a vibrating sample magnetometer.
Results and Discussions
As shown in Figure 4a, the particles can be dragged toward a permanent magnet, unequivocally indicating the magnetic characteristics of the Au nanoparticles. Hysteresis loop of dried particle powder is shown in Figure 4c. Since the Fe3O4 nanoparticles synthesized using the above-mentioned method are normally smaller than 20 nm, we expect to see a typical superparamagnetic behavior: zero remanence, zero coercivity, and a large saturation field. The small hysteresis shown in the measurement may reflect the presence of some large Fe3O4 nanoparticles (>30 nm) inside PHAuNPs. Given the size of the hollow space (>50 nm) and the thickness of the porous shell (25 nm), the inward diffusion of OH- ions may be partially obstructed, resulting in a much slower nucleation rate. As such, the inside particles could grow large. The measured high saturation field is in consistence with the superparamagnetic characteristic. This suggests a mixture of superparamagnetic and ferromagnetic nanoparticles. Ferromagnetic nanoparticles are usually undesirable for bioapplications because of their agglomeration caused by magnetic attraction. However, for iron oxide nanoparticles-loaded PHAuNPs, the thick Au shell can effectively separate them far apart to avoid such magnetic aggregation.
We have shown that the core/shell-structured Fe3O4/Au nanoparticles can be synthesized by trapping Fe3O4 nanoparticles inside hollow Au nanoparticles. Because the resulted composite nanoparticles combine the desirable magnetic and plasmonic properties into one nanoentity, they are particularly suitable for in vivo diagnostic and therapeutic applications, where the Au surface provides anchorage sites for attachment of functional molecules and the particles can be activated by both NIR light and magnetic field. As more and more hollow nanoparticles become available, we believe that this synthetic method would find general applications in the fabrication of core–shell multifunctional nanostructures.
This work was supported by the National Science Foundation (ECCS-0901849), the Texas Higher Education Coordinating Board Norman Hackerman Advanced Research Program, and the USAMRMC Prostate Cancer Research Program (W81XWH-05-1-0592). We thank the Characterization Center for Materials and Biology (CCMB) at University of Texas at Arlington for providing financial and technical support for the electron microscopic characterization of the nanoparticles.
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