Huang et al. Nanoscale Res Lett 2011, 6:43 http://www.nanoscalereslett.com/content/6/1/43 Page 3 of 5 Figure 1 a and b HRTEM micrographs of PHAuNPs, showing the hollow core and the porous shell with pore size about 2–3 nm. change. For core/shell nanoparticles, it is well known that the SPR wavelength is dependent on the refractive indices of medium, shell and core. Changing core mate- rial usually causes a shift of the SPR wavelength. How- ever, PHAuNPs have a relatively thick shell (>20 nm). Figure 2 TEM micrographs of PHAuNPs before a and after b loading iron oxide nanoparticles. Through a three-dimensional finite difference time domain (FDTD) simulation (using a commercial soft- ware from Lumerical Inc), we have proved that at this thickness, the red-shifts of SPR peaks are mainly caused by their surface roughness, and the hollow nature of these particles plays only a minor role [17]. The simula- tion results show that SPR peaks for hollow particles are only slightly red-shifted compared to solid particles with the same outer diameter (100 nm). For particles with a roughness of 5 nm, SPR peak shifts to longer wave- length (~630 nm). As the roughness increases to 8 nm which is the average grain size in the shell, a much greater red-shift (to 720 nm) is observed. This rough- ness effect is due to the strong interaction of electric fields from adjacent bumps on the surface, similar to the plasmonic properties of the aggregates of several nanoparticles. The simulated results are in good agree- ment with experimental results. This unique SPR tuning mechanism makes it possible to maintain the optical properties of PHAuNPs even after the loading of iron oxide. 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 superparamag- netic 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-