Huang et al. Nanoscale Res Lett 2011, 6:43 http://www.nanoscalereslett.com/content/6/1/43 Page 4 of 5 Figure 3 a EDS spectrum of one single particle, showing the coexistence of Au and Fe. b SAED pattern from three particles, showing a superposition of Au and Fe3O4 lattices. 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 characteris- tic. This suggests a mixture of superparamagnetic and ferromagnetic nanoparticles. Ferromagnetic nanoparti- cles 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. Figure 4 The plasmonic and magnetic properties of the Fe3O4- loaded PHAuNPs. a Appearance of a bottle of particle water suspension. The particles can be dragged toward a permanent magnet. b Absorption spectrum of the particle water suspension, showing a broad peak centering at 750 nm. c Hysteresis loop of dried particle powder, showing that the suspension consists of a mixture of superparamagnetic and ferromagnetic nanoparticles. Conclusion 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 desir- able magnetic and plasmonic properties into one nanoen- tity, they are particularly suitable for in vivo diagnostic and therapeutic applications, where the Au surface pro- vides anchorage sites for attachment of functional mole- cules and the particles can be activated by both NIR light and magnetic field. As more and more hollow nanoparti- cles become available, we believe that this synthetic method would find general applications in the fabrication of core–shell multifunctional nanostructures. Acknowledgements 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. Author details 1Department of Materials Science and Engineering, University of Texas at Arlington, Arlington, TX 76019, USA. 2Department of Radiology and Advanced Imaging Research Center, University of Texas Southwestern Medical Center at Dallas, Dallas, TX 75390, USA. Received: 22 July 2010 Accepted: 9 September 2010 Published: 28 September 2010 References 1. Boisselier E, Astruc D: Chem Soc Rev 2009, 38:1759-1782. 2. Duguet E, Vasseur S, Mornet S, Devoisselle JM: Nanomedicine 2006, 1:157-168. 3. Hergt R, Dutz S, Muller R, Zeisberger M: J Phys Condes Matter 2006, 18: S2919-S2934. 4. Jana NR, Gearheart L, Murphy CJ: J Phys Chem B 2001, 105:4065-4067. 5. Millstone JE, Park S, Shuford KL, Qin LD, Schatz GC, Mirkin CA: J Am Chem Soc 2005, 127:5312-5313.