The toxicity of metal oxide nanomaterials and their antimicrobial activity is attracting increasing attention. Among these materials, MgO is particularly interesting as a low cost, environmentally-friendly material. The toxicity of MgO, similar to other metal oxide nanomaterials, is commonly attributed to the production of reactive oxygen species (ROS). We investigated the toxicity of three different MgO nanoparticle samples, and clearly demonstrated robust toxicity towards Escherichia coli bacterial cells in the absence of ROS production for two MgO nanoparticle samples. Proteomics data also clearly demonstrate the absence of oxidative stress and indicate that the primary mechanism of cell death is related to the cell membrane damage, which does not appear to be due to lipid peroxidation.

In various practical applications, nanomaterials typically have functionalized surfaces. Yet, the studies of toxicity and antibacterial activity of functionalized nanoparticles are scarce. We investigated the effect of surface modifications on antibacterial activity of ZnO under ambient illumination, and we found that nanoparticles coated with different surface modifying reagents could exhibit higher or lower toxicity compared to bare ZnO, depending on the surface modifying reagent used. Different surface modifying reagent molecules resulted in differences in the release of Zn(2+) ions and the production of reactive oxygen species (ROS). However, the antibacterial activity did not correlate with the ROS levels or the Zn(2+) ion release. One of the surface-modified ZnO samples exhibited significantly lower Zn(2+) ion release while at the same time exhibiting improved antibacterial activity. In all cases, damage of the cell wall membranes and/or changes in the membrane permeability have been observed, together with the changes in ATR-FTIR spectra indicating differences in protein conformation. Mechanisms of antibacterial activity are discussed.

Using valence band and Si 2p core level photoelectron spectroscopy, it is shown that the short range order in amorphous silicon oxynitride ͑a-SiO x N y ͒ is governed by the Mott rule. According to this rule, each Si atom is coordinated by four O and/or N atoms, each O atom (as in SiO 2) is coordinated by two Si atoms, and each N atom (as in Si 3 N 4) is coordinated by three Si atoms. The nature of the removal of Si-Si bonds (hole traps) at the interface of SiO 2 ͞Si by nitridation and the origin of Si-Si bond creation near the top surface of gate oxynitride in metal-oxide-semiconductor devices are understood for the first time by the Mott rule. [S0031-9007(98)06731-3]

We have investigated the effect of ZnO nanoparticle properties on the dye-sensitized solar cell performance. Nanoparticles with different sizes and optical properties were considered. We found that there is a complex relationship between native defects, dye adsorption, charge transport and solar cell performance. The presence of a high concentration of nonradiative defects was found to be detrimental to photovoltaic performance, whereas for radiative defects, samples displaying orange-red defect emission exhibited better performance compared to samples with green defect emission (when the samples had similar emission intensities). Detailed discussion of the nanoparticle properties and their relationship with dye adsorption, electron injection, electron lifetime, electron transport time, and solar cell performance is given.

No abstract