Inorganic nanoparticles synthesized from biological precursors for environmental remediation
Abstract
Size control is a key factor in determining the ability of nanoparticles to act as catalysts, especially when quantum confinement effects are an important consideration. In this work, ferritin, an iron sequestration protein, was used as a constrained reaction vessel for the synthesis of nanoparticles having well-defined sizes, compositions and morphologies. Horse spleen ferritin, which has a shell that is roughly spherical with a 12 nm outer diameter and an 8 nm internal diameter, was used to prepare ferrihydrite particles of 2--3 nm and 5--6 nm, whilst Listeria innocua ferritin-like protein, which has an inner cavity of diameter 6 nm, was used to prepare 2--3 nm cobalt oxide particles. Removing the protein shell by UV-ozone treatment revealed the metal oxide nanoparticles, and subsequent reduction in an atmosphere of hydrogen at elevated temperatures produced the metallic nanoparticles. Atomic force microscopy was used as a tool for the characterization of these nanostructures and provided a range of information regarding surface topography and surface texture. X-ray photoelectron spectroscopy was used to provide important information about the environment of the atoms probed, such as changes in the electron density around atoms, an indication of changes in oxidation states. The surface reactivity of the nanoparticles was investigated as a function of size. The objective of this study was to establish the size-reactivity relationships for the oxidation of sulfur dioxide over the iron and cobalt oxide nanoparticles produced within ferritin. Attenuated total reflectance Fourier-transform infrared spectroscopy was used to study the reactions of these probe molecules on different sizes of iron oxide and cobalt oxide nanoparticles at room temperature, with and without the presence of oxygen. This technique was sufficiently sensitive to allow in-situ analysis at room temperature and atmospheric pressure and led to several results about the size-dependent sulfation reactions occurring on the surfaces of these nanoparticles including the formation of inner-sphere versus outer sphere sulfate complexes. Scanning tunneling spectroscopy was used to determine the gap states of the metal oxide nanoparticles. The use of ferritin for the remediation of toxic metals in the environment was also investigated. Ferritin, which contains a semiconductor core, was used as a photocatalyst for facilitating the reduction of aqueous hexavalent chromium (Cr6+) to the trivalent species. The ferritin protein without the semiconductor Fe(O)OH core (i.e., apoferritin) was inactive toward Cr6+ reduction under our experimental conditions, suggesting that the Fe(O)OH provided the active catalytic sites in the redox chemistry.
- Publication:
-
Ph.D. Thesis
- Pub Date:
- November 2005
- Bibcode:
- 2005PhDT........86H