Investigation of trimethylacetic acid adsorption on stoichiometric and oxygen-deficient CeO2(111) surfaces.

We studied the interactions between the carboxylate anchoring group from trimethylacetic acid (TMAA) and CeO2(111) surfaces as a function of oxygen stoichiometry using in situ X-ray photoelectron spectroscopy (XPS). The stoichiometric CeO2(111) surface was obtained by annealing the thin film under 2.0 × 10(-5) Torr of oxygen at ∼550 °C for 30 min. In order to reduce the CeO2(111) surface, the thin film was annealed under ∼5.0 × 10(-10) Torr vacuum conditions at 550 °C, 650 °C, 750 °C and 850 °C for 30 min to progressively increase the oxygen defect concentration on the surface. The saturated TMAA coverage on the CeO2(111) surface determined from XPS elemental composition is found to increase with increasing oxygen defect concentration. This is attributed to the increase of under-coordinated cerium sites on the surface with the increase in the oxygen defect concentrations. XPS results were in agreement with periodic density functional theory (DFT) calculations and indicate a stronger binding between the carboxylate group from TMAA and the oxygen deficient CeO2-δ(111) surface through dissociative adsorption.

Enzyme-free detection of hydrogen peroxide from cerium oxide nanoparticles immobilized on poly(4-vinylpyridine) self-assembled monolayers.

A simple enzyme-free spectrophotometric detection of hydrogen peroxide is demonstrated based on its colorimetric reaction with oxygen deficient cerium oxide nanoparticles (CNPs). This colorimetric sensitivity of CNPs towards H2O2 increases significantly with decreasing crystallite size due to an increase in the surface area as well as the concentration of Ce3+ on the surface. The origin of this colorimetric reaction was studied using DFT that suggests the adsorption of peroxide and oxygen molecules on ceria nanoparticles creates new states in the electronic structure leading to transitions absorbing in the visible region of the electromagnetic spectrum. For detection, a single layer of nanoparticles was immobilized on transparent microscopic glass slides using self-assembled monolayers (SAMs) of poly(4-vinylpyridine) (PVP). Cluster-free and uniform immobilization of nanoparticles was confirmed from atomic force microscopy (AFM) and helium ion microscopy (HIM). UV-Visible absorption measurements showed a concentration dependent increase in absorbance from immobilized CNPs that were exposed to increasing concentrations (10-400 µM) of hydrogen peroxide. The immobilized CNPs can be baked at 80 °C after initial use to regenerate the sensor for reuse. The development of a direct, reusable, enzyme-free and dye-free peroxide sensing technology is possible and can be immediately applied in various areas, including biomedicine and national security.