Modulating the Catalytic Activity of Cerium Oxide Nanoparticles with the Anion of the Precursor Salt.

In this work, we tested our hypothesis that surface chemistry and antioxidant properties of cerium nanoparticles (CNPs) are affected by presence of counterions. We first employed various precursor cerium (III) (Ce(III)) salts with different counterions (acetate, nitrate, chloride, sulfate) to synthesize CNPs following the same wet chemical methodology. Electron spin resonance (ESR) studies provided evidence for the formation of radicals from counterions (e.g., NO3•2- from reduction of NO3- in CNPs synthesized from Ce(III) nitrate). Physicochemical properties of these CNPs, e.g., dispersion stability, hydrodynamic size, signature surface chemistry, SOD-mimetic activity, and oxidation potentials were found to be significantly affected by the anions of the precursor salts. CNPs synthesized from Ce(III) nitrate and Ce(III) chloride exhibited higher extent of SOD-mimetic activities. Therefore, these CNPs were studied extensively employing in-situ UV-Visible spectroelectrochemistry and changing the counterion concentrations affected the oxidation potentials of these CNPs. Thus, the physicochemical and antioxidant properties of CNPs can be modulated by anions of the precursor. Furthermore, our ESR studies present evidence of the formation of guanine cation radical (G•+) in 5'-dGMP via UV-photoionization at 77 K in the presence of CNPs synthesized from Ce(III) nitrate and chloride and CNPs act as the scavenger of radiation-produced electrons.

The change in antioxidant properties of dextran-coated redox active nanoparticles due to synergetic photoreduction-oxidation.

Nanoparticles have proven to be novel material with resourceful applications in the field of nanomedicine. Cerium oxide nanoparticles (CNPs) coated with dextran (Dex-CNPs) have been shown to exhibit anticancer properties which is attributed to the change in oxidation states mediated at the oxygen vacancies on the surface of CNPs. In this study, the extreme sensitivity of Dex-CNPs to visible light is demonstrated using room light with a clear indication of synergetic phenomenon of photoreduction of CNPs in the presence of dextran which undergoes simultaneous oxidation. The phenomenon was further confirmed through a systematic time-based expedited study using a high intensity visible light source. The physiochemical changes of Dex-CNPs such as dispersion stability, pH, surface chemistry, antioxidant property, cytotoxicity and the surrounding microenvironment of Dex-CNPs were significantly altered on exposure to visible light, thereby affecting the biological response. Given the significance of nanoparticles which are widely researched nanomaterials, in different fields of nanotechnology and biomedicine, this study demonstrates the significant changes in physiochemical properties of Dex-CNPs with light. The photoreduction of Dex-CNPs affects its bifunctional applications in cancer therapy and thereby this study puts forward the necessity to preserve and sustain their properties through proper storage.

Understanding the adsorption interface of polyelectrolyte coating on redox active nanoparticles using soft particle electrokinetics and its biological activity.

The application of cerium oxide nanoparticles (CNPs) for therapeutic purposes requires a stable dispersion of nanoparticles in a biological environment. The objective of this study is to tailor the properties of polyelectrolyte coated CNPs as a function of molecular weight to achieve a stable and catalytic active dispersion. The coating of CNPs with polyacrylic acid (PAA) has increased the dispersion stability of CNPs and enhanced the catalytic ability. The stability of PAA coating was analyzed using the change in the Gibbs free energy computed by the Langmuir adsorption model. The adsorption isotherms were determined using soft particle electrokinetics which overcomes the challenges presented by other techniques. The change in Gibbs free energy was highest for CNPs coated with PAA of 250 kg/mol indicating the most stable coating. The change in free energy for PAA of 100 kg/mol coated CNPs was 85% lower than the PAA of 250 kg/mol coated CNPs. This significant difference is caused by the strong adsorption of PAA of 100 kg/mol on CNPs. Catalytic activity of PAA-CNPs is assessed by the catalase enzymatic mimetic activity of nanoparticles. The catalase activity was higher for PAA coated CNPs as compared to bare CNPs which indicated preferential adsorption of hydrogen peroxide induced by coating. This indicates that the catalase activity is also affected by the structure of the coating layer.

Cellular interaction and toxicity depend on physicochemical properties and surface modification of redox-active nanomaterials.

The study of the chemical and biological properties of CeO2 nanoparticles (CNPs) has expanded recently due to its therapeutic potential, and the methods used to synthesize these materials are diverse. Moreover, conflicting reports exist regarding the toxicity of CNPs. To help resolve these discrepancies, we must first determine whether CNPs made by different methods are similar or different in their physicochemical and catalytic properties. In this paper, we have synthesized several forms of CNPs using identical precursors through a wet chemical process but using different oxidizer/reducer; H2O2 (CNP1), NH4OH (CNP2), or hexamethylenetetramine (HMT-CNP1). Physicochemical properties of these CNPs were extensively studied and found to be different depending on the preparation methods. Unlike CNP1 and CNP2, HMT-CNP1 was readily taken into endothelial cells and the aggregation can be visualized using light microscopy. Exposure to HMT-CNP1 also reduced cell viability at a 10-fold lower concentration than CNP1 or CNP2. Surprisingly, exposure to HMT-CNP1 led to substantial decreases in ATP levels. Mechanistic studies revealed that HMT-CNP1 exhibited substantial ATPase (phosphatase) activity. Though CNP2 also exhibits ATPase activity, CNP1 lacked ATPase activity. The difference in catalytic (ATPase) activity of different CNPs preparation may be due to differences in their morphology and oxygen extraction energy. These results suggest that the combination of increased uptake and ATPase activity of HMT-CNP1 may underlie the biomechanism of the toxicity of this preparation of CNPs and may suggest that ATPase activity should be considered when synthesizing CNPs for use in biomedical applications.

Laser irradiated nano-architectured undoped tin oxide arrays: mechanism of ultrasensitive room temperature hydrogen sensing.

Undoped nanostructured tin oxide (SnO(2)) arrays were prepared on oxidized Si substrates by nanosecond pulsed laser interference irradiation for hydrogen gas sensing applications. Scanning electron microscopy (SEM), in combination with Atomic Force Microscopy (AFM), showed that the SnO(2) surface consisted of periodic features of ∼130 nm width, ∼228 nm spacing, an average height of ∼8 nm along the periodicity and tens of microns length. The SnO(2) nanostructured arrays and precursor thin films were tested by cyclic exposure under dynamic conditions of hydrogen in the concentration range of 300-9000 ppm. The observed electrical response of SnO(2) towards hydrogen at low concentrations and room temperature drastically improved in the nanostructured array as compared to the thin film. The results suggest that this method to fabricate SnO(2) nanostructured arrays has the potential to produce nanodevices that have ultra-low detection limits, and fast response and recovery times, which are suited for practical hydrogen sensing applications.