Published: August 09, 2011 r2011 American Chemical Society 3921 dx.doi.org/10.1021/cm201295p | Chem. Mater. 2011, 23, 3921–3929 ARTICLE pubs.acs.org/cm All in the Packaging: Structural and Electronic Effects of Nanoconfinement on Metal Oxide Nanoparticles Craig Jolley, †,‡,|| Vanessa Pool, ‡,§ Yves Idzerda, ‡,§ and Trevor Douglas* ,†,‡ † Department of Chemistry & Biochemistry, ‡ Center for Bio-inspired Nanomaterials, and § Department of Physics, Montana State University, Bozeman, Montana 59717, United States b S Supporting Information ’ INTRODUCTION The formation of hard biomaterials such as mollusk shell or vertebrate bone involves intimate interactions between biopoly- mers and nascent inorganic crystals. 15 Soft materials such as proteins have shown a remarkable capacity to direct the growth and influence the final material properties of hard materials. 6 A growing class of bioinspired nanomaterials 7,8 mimics this forma- tion process by using protein nanocages such as virus capsids 912 or ferritins 1319 to encapsulate inorganic nanoparticles. This approach has a number of immediate advantages for nanotech- nology applications: the protein shell can help to prevent aggregation of nanoparticles and can be chemically or genetically modified for in vivo targeting 17,20 or in vitro assembly into hierarchical structures. 2123 Another, more subtle effect of protein-confined nanoparticle synthesis is the potential for dramatic changes in the nanopar- ticle morphology. A simple example is the constraint on crystal- line domain size imposed not only by the size of the protein cage but possibly also by the arrangement of crystal nucleation sites on the cage interior. 24 More dramatic effects are also possible: ferritin forms a 24-meric protein cage with interior ferroxidase sites that can oxidize soluble Fe 2+ to insoluble Fe 3+ , forming an Fe oxide nanoparticle in the cage interior. Under biologically relevant conditions (room temperature, near-ne- tural pH, ambient O 2 ), recombinant ferritin forms a ferrihydrite phase 25 similar to what is observed in native mineralized ferritin. If Fe 2+ is exposed to the same conditions in the absence of ferritin, however, it precipitates to form a lepidocrocite phase. 26 This can be understood in terms of a kinetic trap: ferritin stabilizes the metastable ferrihydrite phase that trans- forms into more-ordered Fe oxide structures under most conditions. In fact, such disorderorder transitions in ferri- tin-templated Fe oxides appear to be feasible only under elevated temperatures requiring the use of ferritins from hyperthermophilic organisms. 19 Kinetic stabilization of amorphous phases is also relevant to the formation of structural biominerals such as teeth and bones. 3,27 In contrast to classical precipitation models in which growing crystals form directly from the addition of solution ions, calcium phosphate and carbonate have been shown to exist as stable nanometer-sized clusters in solution. 28 Amor- phous phases form by aggregation of these nanoclusters, and macroscopically ordered crystals can arise through interaction with an ordered template that establishes crystal phase and orientation. 29 In the case of collagen biomineralization, 3 the amorphous precursor phases are thought to be stabilized by noncollagenous proteins (NCPs), forming negatively charged clusters that are subsequently oriented on the positively charged collagen scaffold and reorganize into a larger-scale crystal. Similar solution-phase molecular clusters have been characterized for transition metals, 30 and ferritin (which also carries a significant Received: May 6, 2011 Revised: July 17, 2011 ABSTRACT: Encapsulation of inorganic nanoparticles within oligomeric protein cages can provide a multivalent approach for the synthesis of biocompatible nanomaterials by combining the nanoparticle-forming catalytic abilities of the cage interior with the biointer- active exterior surface of the cage. Protein cages provide more than simply a passive compartment for nanoparticle formation: protein-templated nanoparticles can exhibit structural and electronic properties that are dramatically different from materials synthesized without protein templating. Mixed Fe/Mn oxides formed under hydrothermal conditions form a structural series ranging from the γ-Fe 2 O 3 (maghemite) to the Mn 3 O 4 (hausmannite) spinel structure as the Mn fraction is increased from 0 to 100%, while similar materials formed inside of human ferritin transition instead from maghemite to a layered Mn oxide structure similar to chalcophanite. The electronic properties of the protein-templated nanoparticles, as determined from soft X-ray absorption spectroscopy, also differ from those of their protein-free counterparts, in agreement with the structural results. Protein-templated synthesis may provide the opportunity for powerful control over nanomaterial properties through nanoconfinement, but the ultimate physical basis for these effects remains to be determined. KEYWORDS: composite materials, ferrites, X-ray spectroscopy, pair distribution functions, biomineralization