Synthesis and reactivity of fluorescent rare-earth doped cerium oxide nanomaterials

Abstract

Nanomaterials based on cerium (IV) oxide, or CeO2, have been investigated due to interesting chemistry from the readily accessible Ce3+/Ce4+ transition. This transition is accompanied by a gain or loss in CeO2 oxygen and/or electrons. As a result, CeO2 has demonstrated reversible antioxidant activity and enzyme mimetic activity, and has also been used in oxygen transport, photocatalysis, and small molecule oxidation applications. Doping CeO2 with lower valent ions creates oxygen vacancies and increases the Ce3+/Ce4+ ratio within the lattice. The dopant may also contribute properties such as fluorescence or magnetism to compliment the CeO2 chemistry. This work presents the synthesis of several europium doped-CeO2 (Eu-CeO2) morphologies, the investigation into parameters controlling their size, morphology, and elemental composition. Select applications highlighting differences in the materials synthesized were also investigated.^Eu-CeO2 nanorods and nanocubes were synthesized using a hydrothermal procedure. The nanorods were also annealed to enhance their Eu3+ fluorescence. An electrospinning and annealing procedure was used to synthesize nanowires. The Eu:Ce ratio for all the rods, cubes, and wires were controlled by changing the Eu3+ precursor concentration. Manipulation of the precursor concentrations provided minor dimension control of the rods, while changing of Ce3+ precursor and hydrothermal reaction time influenced the nanocube morphology. Finally, Eu-CeO2 nanotubes have been synthesized by layered deposition of Ce(OH)3 on a sacrificial ZnO nanowire array. The hydroxide was oxidized to Eu-CeO2 and the ZnO core etched to produce the nanotube morphology. The nanotube dimensions were controlled by the ZnO core, but low yield demands further optimization prior to use in the intended drug delivery applications.^One synthesized, the rods, cubes, and wires were analyzed for Eu3+-contributed fluorescence. While the cubes and wires fluoresced as synthesized, the nanorods required annealing above 500 °C to fluoresce. Nanocubes were also synthesized with neodymium and ytterbium instead of europium. These cubes fluoresce at 800-1000nm, which is useful in bioimaging applications. The four materials (rods, annealed rods, cubes, and wires) were also compared for their effect on the transformation of di-hydroxyphenylalanine (L-Dopa) to eumelanin. Previous CeO2 nanoparticles have captured catechol-containing molecules by oxidation and conjugation with the -OH groups. Our Eu-CeO2 materials suppressed eumelanin-associated fluorescence by up to 60% when added to L-Dopa early in its transformation to eumelanin. The four morphologies exhibited very different activities, with higher Ce3+ concentrations and smaller crystalline domains associated with enhanced suppression of eumelanin synthesis.