Yip Hang Ng

Mechanisms of Antibacterial Activity of MgO: Non‐ROS Mediated Toxicity of MgO Nanoparticles Towards Escherichia coli

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.

Antibacterial activity of ZnO nanoparticles with a modified surface under ambient illumination

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.

Effect of ZnO Nanoparticle Properties on Dye-Sensitized Solar Cell Performance

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.

ZnO nanorod/GaN light-emitting diodes: The origin of yellow and violet emission bands under reverse and forward bias

ZnO nanorods have been prepared by electrodeposition under identical conditions on various p-GaN-based thin film structures. The devices exhibited lighting up under both forward and reverse biases, but the turn-on voltage and the emission color were strongly dependent on the p-GaN-based structure used. The origin of different luminescence peaks under forward and reverse bias has been studied by comparing the devices with and without ZnO and by photoluminescence and cathodoluminescence spectroscopy. We found that both yellow-orange emission under reverse bias and violet emission under forward bias, which are commonly attributed to ZnO, actually originate from the p-GaN substrate and/or surface/interface defects. While the absolute brightness of devices without InGaN multiple quantum wells was low, high brightness with luminance exceeding 10 000 cd/m2 and tunable emission (from orange at 2.1 V to blue at 2.7 V, with nearly white emission with Commission internationale de l’eclairage (CIE) coordinates (0.30,...

Toxicity of CeO2 nanoparticles - the effect of nanoparticle properties.

Conflicting reports on the toxicity of CeO2 nanomaterials have been published in recent years, with some studies finding CeO2 nanoparticles to be toxic, while others found it to have protective effects against oxidative stress. To investigate the possible reasons for this, we have performed a comprehensive study on the physical and chemical properties of nanosized CeO2 from three different suppliers as well as CeO2 synthesized by us, and tested their toxicity. For toxicity tests, we have studied the effects of CeO2 nanoparticles on a Gram-negative bacterium Escherichia coli in the dark, under ambient and UV illuminations. We have also performed toxicity tests on the marine diatom Skeletonema costatum under ambient and UV illuminations. We found that the CeO2 nanoparticle samples exhibited significantly different toxicity, which could likely be attributed to the differences in interactions with cells, and possibly to differences in nanoparticle compositions. Our results also suggest that toxicity tests on bacteria may not be suitable for predicting the ecotoxicity of nanomaterials. The relationship between the toxicity and physicochemical properties of the nanoparticles is explicitly discussed in the light of the current results.

TiO2 anode materials for lithium-ion batteries with different morphology and additives

Electrochemical performances of different TiO2 nanostructures, TiO2/CNT composite and TiO2 with titanium isopropoxide (TTIP) treatment anode were investigated. For different TiO2 nanostructures, we investigated vertically aligned TiO2 nanotubes on Ti foil and TiO2 nanotube-powders fabricated by rapid breakdown anodization technique. The morphology of the prepared samples was characterized by scanning probe microscopy (SEM). The electrochemical lithium storage abilities were studied by galvanostatic method. In addition, carbon nanotubes (CNT) additives and solution treatment process of TiO2 anode were investigated, and the results show that the additives and treatment could enhance the cycling performance of the TiO2 anode on lithium ion batteries.

Influence of defects in ZnO nanomaterials on the performance of dye-sensitized solar cell and photocatalytic activity

ZnO as a wide band gap semiconductor is of significant interest for various applications, including dye-sensitized solar cell (DSSC) and photocatalytic degradation of organic pollutants. For DSSC, although the performance of ZnO-based devices is generally inferior to TiO2-based ones, it is still of interest due to its high electron mobility. While the relationship between the material and the device performance are complicated, many studies have been focused on morphologies and surface area of the nanomaterials. The studies of the effect of the material properties such as the types and concentrations of native defects on the DSSC performance have been scarce. For photocatalytic degradation of pollutants, many reports showed ZnO has a higher or similar efficiency compared to the commonly used TiO2. Reports have also pointed out the important role of native defects of ZnO in its photocatalytic activity. Nevertheless, the effect of the type and location of the defects has been contradictory in the literature indicating that there is a complex relationship. Therefore, we will discuss the effect of ZnO native defects on the dye adsorption, charge transport and hence the DSSC performance. We will also discuss their influence on reactive oxygen species (ROS) generation and photocatalytic dye degradation. As photoluminescence (PL) is a common methodology in studying native defects of ZnO, the relationship between PL, DSSC performance and photocatalytic properties will also be investigated. Preliminary results showed a higher overall PL intensity would result in a better device performance and higher photocatalytic activities.

Effect of electrical properties, transmittance, and morphology of ITO electrode on polymer solar cells characteristics

We investigated the influence of ITO properties on the performance of bulk heterojunction solar cells. The morphology, electrical and optical properties of ITO electrodes were characterized. The power conversion efficiency of cells made on different ITO substrates varied significantly from 2.3% to 3.1%. It was found that for a higher sheet resistance ITO substrate, the sheet resistance was the dominant parameter affecting the performance of the based solar cells, while for the lower ( below 20 ohm/square) sheet resistance, transmittance in the region where polymer strongly absorbs and the surface roughness of the substrate had significant effect on the solar cells performance.