The iron storage protein ferritin was used to catalyze the photoreduction of aqueous Cr(VI) species to Cr(III). Ferritin is a 24 subunit protein of roughly spherical shape with outer and inner diameters of approximately 12 and 8 nm, respectively. The native mineral core of ferritin is the ferric oxyhydroxide ferrihydrite (Fe(O)OH). Fe(O)OH particles that were used in these experiments ranged from 5 to 7.5 nm in diameter. The ferritin protein without the Fe(O)OH core (i.e., apoferritin) was inactive toward Cr(VI) reduction under our experimental conditions, suggesting that the Fe(O)OH provided the active catalytic sites in the redox chemistry. Experiments using photon band-pass filters suggested that the reaction occurred out of a photoinduced electron−hole pair and the optical band gap for the Fe(O)OH semiconductor was determined to be in the range 2.5−3.5 eV. Comparison of ferritin and protein-free Fe(O)OH mineral nanoparticles indicated that ferritin provided a photocatalyst with significantly more stability to aggregation and the loss of catalytic activity.

Metallic Fe and Co and Fe- and Co-based oxide nanoparticles were prepared by a novel method utilizing the biologically relevant protein ferritin. In particular, iron and cobalt oxyhydroxide nanoparticles were assembled within horse spleen and Listeria innocua derived ferritin, respectively, in the aqueous phase. Ferritin containing either Fe or Co oxide was transferred and dried on a SiO2 support where the protein shell was removed during exposure to a highly oxidizing environment. It was also shown that the metal oxide particles could be reduced to the respective metal by heating in hydrogen. X-ray photoelectron spectroscopy was used to characterize the composition of the particles and atomic force microscopy was used to characterize the size of the nanoparticles. Depending on the Fe or Co loading and/or type of ferritin used, metallic and oxide nanoparticles could be produced within a range of 20-60 A.

Ferrihydrite nanoparticles with nominal sizes of 3 and 6 nm were assembled within ferritin, an iron storage protein. The crystallinity and structure of the nanoparticles (after removal of the protein shell) were evaluated by high-resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). HRTEM showed that amorphous and crystalline nanoparticles were copresent, and the degree of crystallinity improved with increasing size of the particles. The dominant phase of the crystalline nanoparticles was ferrihydrite. Morphology and electronic structure of the nanoparticles were characterized by AFM and STM. Scanning tunneling spectroscopy (STS) measurements suggested that the band gap associated with the 6 nm particles was larger than the band gap associated with the 3 nm particles. Interaction of SO2(g) with the nanoparticles was investigated by attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy, and results were interpreted with the aid of molecular orbital/density functional theory (MO/DFT) frequency calculations. Reaction of SO2(g) with the nanoparticles resulted primarily in SO(3)2- surface species. The concentration of SO3(2-) appeared to be dependent on the ferrihydrite particle size (or differences in structural properties).