Published: January 26, 2011 r2011 American Chemical Society 2205 dx.doi.org/10.1021/ja108781w | J. Am. Chem. Soc. 2011, 133, 2205–2217 ARTICLE pubs.acs.org/JACS A Cast-Mold Approach to Iron Oxide and Pt/Iron Oxide Nanocontainers and Nanoparticles with a Reactive Concave Surface Chandramohan George, † Dirk Dorfs, † Giovanni Bertoni, † Andrea Falqui, † Alessandro Genovese, † Teresa Pellegrino, †,‡ Anna Roig, § Alessandra Quarta, ‡ Roberto Comparelli, || M. Lucia Curri, || Roberto Cingolani, † and Liberato Manna* ,† † Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy ‡ Nanoscience Institute of CNR, via Arnesano 16, 73100 Lecce, Italy § Institut de Ci  encia de Materials de Barcelona (ICMAB-CSIC), Campus de la UAB, 08913 Bellaterra, Spain ) CNR, Istituto per i Processi Chimico-Fisici, via Orabona 4, 73100 Bari, Italy b S Supporting Information ABSTRACT: We report the synthesis of various iron oxide nanocontainers and Pt-iron oxide nanoparticles based on a cast-mold approach, starting from nanoparticles having a metal core (either Au or AuPt) and an iron oxide shell. Upon annealing, the particles evolve to asymmetric core-shells and then to heterodimers. If iodine is used to leach Au out of these structures, asymmetric core-shells evolve into “nano- containers”, that is, iron oxide nanoparticles enclosing a cavity accessible through nanometer-sized pores, while het- erodimers evolve into particles with a concave region. When starting from a metal domain made of AuPt, selective leaching of the Au atoms yields the same iron oxide nanoparticle morphologies but now encasing Pt domains (in their concave region or in their cavity). We found that the concave nanoparticles are capable of destabilizing Au nanocrystals of sizes matching that of the concave region. In addition, for the nanocontainers, we propose two different applications: (i) we demonstrate loading of the cavity region of the nanocontainers with the antitumoral drug cis-platin; and (ii) we show that nanocontainers encasing Pt domains can act as recoverable photocatalysts for the reduction of a model dye. 1. INTRODUCTION For many years, iron oxide-based nanostructures have been synthesized and exploited in drug delivery, magnetic resonance imaging, ferrofluids, catalysis, gas sensing, and lithium ion batteries. 1-8 Nowadays, the synthesis of new nanostructures out of this material, in terms of desired morphology, chemical composition, and stability, is a clearly demanding task. On the other hand, hollow colloidal inorganic nanoparticles (NPs) with various morphologies (cage, box, frame, ring, tube, and wire-like) have recently emerged as an interesting class of materials, and also porous and interior hollow nanostructures (especially those based on magnetic oxides) have been synthesized. 9-11 Various approaches have been proposed for the synthesis of hollow inor- ganic NPs, most notably via chemical etching, galvanic replace- ment, via the Kirkendall effect and by using sacrificial temp- lates. 12 Hollow NPs are thermodynamically less stable than their “filled” counterparts, due to their much higher surface to volume ratio, and their inner surface can provide effective sites for chemi- cal reactions that can take place in a region that is “protected” from the external environment, and such hollow nanostructures could act as efficient nanocatalysts, as carriers for drug delivery and as nanocontainers for bioseparation and for removal of contaminants. 13 Recently, Alivisatos et al. have reported the synthesis of hollow iron oxide nanospheres starting from iron NPs, by exploiting the Kirkendall effect, 14 and Sun et al. have shown how leaching of gold from colloidal Au-iron oxide heterodimers yields iron oxide NPs with a concave region. 15 Here, we further elaborate the “cast-mold” procedure of Sun by first synthesizing core-shell NPs of Au-iron oxide and AuPt-iron oxide, and we show that, upon annealing, all these NPs were reshaped to asymmetric core-shells and then to heterodimers. We then used iodine to leach Au atoms out of the NPs. Upon this treatment, Au-iron oxide asymmetric core-shells evolved into iron oxide NPs carrying a cavity that was accessible from the external environ- ment through nanometer-sized pores, while Au-iron oxide asymmetric heterodimers evolved into NPs with a concave region (see Scheme 1a). Hence, the degree of annealing of the initial heterostructures was critical in defining their overall mor- phology, which in turn yielded either concave NPs or nanocon- tainers upon Au leaching. When instead the metal domain was made of an AuPt alloy, selective leaching of the Au atoms led to the same morphologies for the iron oxide NPs as when starting Received: September 29, 2010