All in the Packaging: Structural and Electronic Effects of Nanoconfinement on Metal Oxide Nanoparticles

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 biointeractive 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.