Huang et al. Nanoscale Res Lett 2011, 6:43 http://www.nanoscalereslett.com/content/6/1/43 of the bottom membrane, and this acted as the working electrode for electrodeposition. A commercial Au sulfite electrodeposition solution (Techni-Gold 25 ES from Technic Inc.) was used as the electrolyte. The pH of the solution was about 7.0, which was changed to 6.0 by add- ing 0.4 M Ni sulfamate solution. A potential of 0.80 V (vs. Ag/AgCl reference) was applied to the working elec- trode using a Princeton Applied Research 273A Potentio- stat/Galvanostat, at which hydrogen evolution occurred. As hydrogen molecules diffused into the pores inside the membranes, hydrogen nanobubbles formed on the inner wall surface of the pores. These hydrogen nanobubbles served as templates. The high concentration of hydrogen molecules in the bubble boundary reduced the Au+ com- plex ion to form Au clusters. Then, these clusters act as a catalyst to trigger the autocatalytical disproportionation reaction: 3Au1+(in sulfite complex) ⎯ ⎯ ⎯ on Au surf+a→ce Au 3+ 2Au 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 mem- brane was ready for the next step of loading Fe3O4 nanoparticles into these PHAuNPs after being washed by passing deionized water through the membrane sev- eral times. Loading Fe3O4 Nanoparticles into PHAuNPs The formation of Fe3O4 nanoparticles via alkaline preci- pitation was conducted by following a previously reported procedure [16]. 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 disper- sion in DI water followed by centrifugation. Page 2 of 5 Characterization of Fe3O4/PHAuNPs Core/Shell Nanoparticles Samples for TEM were made by simply dipping a cop- per grid into the diluted nanoparticle water suspension. TEM micrographs were taken using a Hitachi H9500 HR-TEM. Absorption spectra of the particle water sus- pensions 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 Synthesized PHAuNPs feature a sub-25-nm shell with a 50-nm hollow core. The shell is of porous nature with the pore size about 2–3 nm, as measured in the high- resolution transmission electron microscopy (HRTEM) image shown in Figure 1a. These nanoscale pores in the shell allow ions (Fe2+ and Fe3+) to diffuse into the hol- low space in the core, where precipitation of Fe3O4 takes place upon the addition of OH-. The sizes of pre- cipitated Fe3O4 nanoparticles (5–20 nm) are larger than the pore size, resulting in the trapping of the iron oxide nanoparticles inside the PHAuNPs (Figure 1b). Figure 2 shows TEM analysis before and after loading of iron oxide nanoparticles. After loading, the hollow core of PHAuNPs is occupied by solid substances. Dur- ing the precipitation, Fe3O4 nanoparticles also formed outside of PHAuNPs, but TEM micrographs clearly show that no small iron oxide nanoparticles were attached to the PHAuNP surface. This is in agreement with the common notion that iron oxide usually does not stick to the Au surface [11]. Given the very different sizes of PHAuNPs (~100 nm) and non-trapped Fe3O4 nanoparticles (<20 nm), they can be readily separated using centrifugation. The loading of Fe3O4 to the core of PHAuNPs was confirmed by energy-dispersive X-ray (EDS) analysis of one single particle and the selected area electron diffrac- tion (SAED) pattern from three particles. EDS shows the coexistence of Au and Fe in a single particle (Figure 3a, Cu peak is from the TEM grid). The low intensity of Fe may be due to the shield effect of the thick Au shell. The SAED pattern is a superposition of Au and Fe3O4 lattices (Figure 3b), showing three distinguishable planes of (311), (511), and (731) from Fe3O4. Other Fe3O4 planes overlap with Au planes. Shown in Figure 4a is the appearance of a bottle of particle water suspension. The cyan color indicates that the suspension absorbs red light. The absorption spec- trum is shown in Figure 4b, which has a broad peak centering at 750 nm. This absorption peak corresponds to the SPR wavelength. Compared to PHAuNPs before loading iron oxide, the absorption spectrum shows little