Huang et al. Nanoscale Res Lett 2011, 6:43 http://www.nanoscalereslett.com/content/6/1/43 N A N O E X P R E S S Trapping Iron Oxide into Hollow Gold Nanoparticles Chienwen Huang1, Jiechao Jiang1, Chivarat Muangphat1, Xiankai Sun2, Yaowu Hao1* Open Access Abstract 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. Introduction Gold nanoparticles (AuNPs) and superparamagnetic iron oxide nanoparticles (SPIONs) have been subjects of intensive research in the last decade [1,2]. They are gen- erally considered as biocompatible and are of great interest for diagnostic imaging and therapeutic applica- tions. 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 [3]. 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 treat- ment 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 [4], nanoprisms [5], 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 * Correspondence: yhao@uta.edu 1Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, TX 76019, USA. Full list of author information is available at the end of the article a thin layer of Au, where the Au shell not only provides convenient anchorage sites for functionalization of bio- molecules through the well-established Au-thiol conju- gation 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 func- tions for in vivo applications. Here, we report an approach to construct iron-oxide/Au core/shell nano- particles by trapping iron oxide nanoparticles into hol- low AuNPs. Such nanoparticles are magnetic with their SPR peaks in the NIR region (wavelength from 700 to 800 nm). Experimental Methods Synthesis of Porous Hollow Au Nanoparticles Synthesis of porous hollow Au nanoparticles (PHAuNPs) was detailed in the reference [15]. The key processes are listed here. The synthesis process was conducted inside a typical three-electrode electrodeposition cell with Ag/AgCl electrode in 3 M NaCl solution as the reference [Uep = 0.250 V vs. standard hydrogen electrode (SHE)] and a platinum mesh as the counter electrode. A stack of two anodic alumina filtration membranes (from What- man Corp.) with the pore diameter of about 300 nm and thickness of 60 μm were used as templates. A 700-nm- thick film of Cu was sputter-deposited to block the pores © 2010 Huang et al. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.