ULTRAFAST TIME-RESOLVED X-RAY ABSORPTION MEASUREMENTS OF THE REDUCTIVE DISSOLUTION OF IRON OXIDE NANOPARTICLES

Reactions that occur on timescales of less than one millionth of a second are central to biogeochemical processes that shape the Earth's surface. In particular, the reduction of Fe(III) is one of the most important chemical changes that takes place in the development of anaerobic soils and sediments, and the reductive dissolution of iron-bearing minerals by microbes plays a critical role in this process. Despite its importance in biogeochemistry, many questions remain about the mechanism of this electron transfer reaction, in part because the speed of the fundamental chemical steps renders them inaccessible to conventional study. Ultrafast time-resolved x-ray spectroscopy is a technique that can overcome this limitation and measure changes in oxidation state and structure occurring during chemical reactions that can be initiated by a fast laser pulse. We use this approach with ~100 ps resolution to monitor the speciation of Fe atoms in maghemite nanoparticles following photo-induced electron transfer from a surface-bound photoactive dye molecule.

Magnetite nanoparticles, synthesized by co-precipitation of Fe(II) and Fe(III) in basic aqueous solution, were oxidized by O₂ to give maghemite particles 2-3 nm in diameter (by XRD). UV/vis absorption and fluorescence spectroscopy showed that the dye, 2,7-dichlorofluorescein, binds strongly to the particle surface and that its emission is quenched fully when bound. Under steady-state photoexcitation, dye-sensitized particles evolved Fe²⁺(aq) over time, indicating successful electron transfer from sensitizer to particle, followed by reductive dissolution. Pump-probe x-ray absorption spectroscopy measurements at the Fe K edge, using a free jet of a deoxygenated aqueous suspension of dye-sensitized maghemite, showed the transient formation of a reduced iron species following laser excitation. No transient species were observed in control experiments using unsensitized nanoparticles. These data represent the first direct real-time observation of the dynamics of ferrous ion formation and subsequent re-oxidation or dissolution in iron oxide.