Salinity-dependent toxicities of zinc oxide nanoparticles to the marine diatom Thalassiosira pseudonana.

This study comprehensively investigated the influences of salinity, exposure concentration and time on the aggregate size, surface charge and dissolution of zinc oxide nanoparticles (ZnO-NPs; 20nm) in seawater, and examined the interacting effect of salinity and waterborne exposure of ZnO-NPs on the marine diatom Thalassiosira pseudonana for 96h. We found that aggregate sizes of ZnO-NPs significantly increased with increasing salinity, but generally decreased with increasing exposure concentration. Ion release decreased with increasing salinity, whereas the surface charge of the particles was not affected by salinity. The increased aggregate size and decreased ion release with increasing salinity, and consequently lower concentration of bioavailable zinc ions, resulted in decreased toxicity of ZnO-NPs at higher salinity in general in terms of growth inhibition (IC50) and chlorophyll fluorescence (EC50 - ФPo and EC50 - Ф2). However, IC50s and EC50s of ZnO-NPs were smaller than those of Zn(2+) (from ZnO-NPs ultrafiltrate and ZnCl2), indicating that dissolved Zn(2+) can only partially explain the toxicity of ZnO-NPs. SEM images showed that ZnO-NPs attached on the diatom frustule surface, suggesting that the interaction between the nanoparticles and the cell surface may acerbate the toxicity of ZnO-NPs. Our results linked the physicochemical characteristics of ZnO-NPs in seawater with their toxicities to the marine diatom and highlighted the importance of salinity as an influential environmental factor governing the aggregation, dissolution and the toxicity of ZnO-NPs.

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.

Recent progress in hydrothermal synthesis of zinc oxide nanomaterials.

Hydrothermal synthesis is of considerable interest due to its low cost, simplicity and relatively low growth temperature (typically below 200 °C). Since the synthesis is performed in aqueous solutions (no organic solvents), it can also be safe and environmentally friendly (depending on precursor chemicals). Consequently, it has been a subject of intense research in recent years. In this article, we review recent progress in hydrothermal synthesis of zinc oxide nanomaterials, with focus on practical relevance for a variety of applications.

Toxicities of nano zinc oxide to five marine organisms: influences of aggregate size and ion solubility.

Nano zinc oxide (nZnO) is increasingly used in sunscreen products, with high potential of being released directly into marine environments. This study primarily aimed to characterize the aggregate size and solubility of nZnO and bulk ZnO, and to assess their toxicities towards five selected marine organisms. Chemical characterization showed that nZnO formed larger aggregates in seawater than ZnO, while nZnO had a higher solubility in seawater (3.7 mg L(-1)) than that of ZnO (1.6 mg L(-1)). Acute tests were conducted using the marine diatoms Skeletonema costatum and Thalassiosia pseudonana, the crustaceans Tigriopus japonicus and Elasmopus rapax, and the medaka fish Oryzias melastigma. In general, nZnO was more toxic towards algae than ZnO, but relatively less toxic towards crustaceans and fish. The toxicity of nZnO could be mainly attributed to dissolved Zn(2+) ions. Furthermore, molecular biomarkers including superoxide dismutase (SOD), metallothionein (MT) and heat shock protein 70 (HSP70) were employed to assess the sublethal toxicities of the test chemicals to O. melastigma. Although SOD and MT expressions were not significantly increased in nZnO-treated medaka compared to the controls, exposure to ZnO caused a significant up-regulation of SOD and MT. HSP70 was increased two to fourfold in all treatments indicating that there were probably other forms of stress in additional to oxidative stress such as cellular injury.

GaN/ZnO nanorod light emitting diodes with different emission spectra.

Light emitting diodes (LEDs) consisting of p-GaN epitaxial films and n-ZnO nanorods have been fabricated and characterized. The rectifying behavior and emission spectra were strongly dependent on the electronic properties of both GaN film and ZnO nanorods. Light emission under both forward and reverse bias was obtained in all cases, and emission spectra could be changed by annealing the ZnO nanorods. The emission spectra could be further tuned by using a GaN LED epiwafer as a substrate. Both forward and backward diode behavior has been observed and the emission spectra were significantly affected by both the properties of the GaN substrate and the annealing conditions for the ZnO nanorods.

The influence of source material composition on morphology and optical properties of ZnO nanostructures.

We investigated the influence of the composition of the source materials on the morphology and optical properties of ZnO nanostructures. The source materials consisted of a mixture of ZnO and carbon, or ZnO, carbon, and another metal oxide (In2O3, MnO2, or V2O5). The addition of a different metal oxide to the source materials is a commonly used method to achieve doping and/or alteration of the morphology of ZnO nanostructures. For each metal oxide additive, we investigated the influence of different forms of carbon (graphite, carbon nanofibers, and single wall carbon nanotubes). Obtained nanostructures were studied using scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, selected area electron diffraction, and photoluminescence. The morphology and the optical properties of the obtained nanostructures were strongly dependent on the source material composition. Possible reasons for observed differences are discussed.

Defects in ZnO nanorods prepared by a hydrothermal method.

ZnO nanorod arrays were fabricated using a hydrothermal method. The nanorods were studied by scanning electron microscopy, photoluminescence (PL), time-resolved PL, X-ray photoelectron spectroscopy, and positron annihilation spectroscopy before and after annealing in different environments and at different temperatures. Annealing atmosphere and temperature had significant effects on the PL spectrum, while in all cases the positron diffusion length and PL decay times were increased. We found that, while the defect emission can be significantly reduced by annealing at 200 degrees C, the rods still have large defect concentrations as confirmed by their low positron diffusion length and short PL decay time constants.

Metal phthalocyanine nanoribbons and nanowires.

Nanoribbons and nanowires of different metal phthalocyanines (copper, nickel, iron, cobalt, and zinc), as well as copper hexadecafluorophthalocyanine (F(16)CuPc), have been grown by organic vapor-phase deposition. Their properties, as a function of substrate type, source-to-substrate distance, and substrate temperature, were studied by scanning electron microscopy, transmission electron microscopy, X-ray diffraction, and absorption measurements. The size and morphology of the nanostructures were found to be mainly determined by the substrate temperature. The crystal structure was dependent on the substrate temperature as well. At substrate temperatures below 200 degrees C, in addition to straight nanoribbons, twisted nanoribbons were found for all investigated materials except F(16)CuPc, which formed helical nanoribbons upon exposure to an electron beam. The formation of different nanostructures (nanoribbons, twisted nanoribbons, and helical nanoribbons) is discussed.

Tailoring and modifications of a ZnO nanostructure surface by the layer-by-layer deposition technique.

Different ZnO nanostructures have been modified using the layer-by-layer polyelectrolyte deposition process. The polymer multilayers were deposited on free standing ZnO tetrapods, ZnO tetrapods on a substrate and ZnO nanorod arrays. In addition, attachment of metallic (Au) nanoparticles to the ZnO nanostructure surface using layer-by-layer deposition was demonstrated. The properties of the ZnO nanostructures with modified surfaces were investigated by electron microscopy, absorption and photoluminescence measurements. A linear increase in polymer thickness with the number of polymer multilayers was confirmed by absorption and transmission electron microscopy. The technique can be readily extended to different nanoparticles and different morphologies of ZnO.

Improved method for determination of optical constants of organic thin films from reflection and transmission measurements.

A new technique for determining the optical properties of organic thin films is presented. A detailed evaluation of the accuracy of the determined optical constants has been performed, and the best combination of measured values yielding the smallest errors in the index of refraction for realistic experimental uncertainties has been found. The proposed method utilizes the fact that optical constants are smooth continuous functions, which reduces the possibility of encountering multiple solutions. The method consists of two steps. In the first step the optical constants at all wavelengths and the film thickness are determined. In the second step the thickness and the imaginary part of the index of refraction are kept fixed while we reevaluate the real part of the index of refraction by using a different objective function with improved sensitivity to the refractive index. After verifying that the proposed method is capable of an accurate estimation of optical constants, we determine the index of refraction data of vanadyl-phthalocyanine in the visible spectral range.

Modeling the temperature dependence of the index of refraction of liquid water in the visible and the near-ultraviolet ranges by a genetic algorithm.

A simple formula describing the dependence of the index of refraction of water on wavelength in the visible and the near-UV ranges and at temperature from 0 degrees C to 100 degrees C is given. Parameters of the formula were determined by minimization of discrepancies between calculated and experimental data by use of an elite genetic algorithm with adaptive mutations. This algorithm was devised with a particular application in mind, the determination of model parameters. Its superiority over the simple genetic algorithm in locating the global minimum was demonstrated on a family of multiminima test functions for as many as 100 variables.

Optical properties of metallic films for vertical-cavity optoelectronic devices.

We present models for the optical functions of 11 metals used as mirrors and contacts in optoelectronic and optical devices: noble metals (Ag, Au, Cu), aluminum, beryllium, and transition metals (Cr, Ni, Pd, Pt, Ti, W). We used two simple phenomenological models, the Lorentz-Drude (LD) and the Brendel-Bormann (BB), to interpret both the free-electron and the interband parts of the dielectric response of metals in a wide spectral range from 0.1 to 6 eV. Our results show that the BB model was needed to describe appropriately the interband absorption in noble metals, while for Al, Be, and the transition metals both models exhibit good agreement with the experimental data. A comparison with measurements on surface normal structures confirmed that the reflectance and the phase change on reflection from semiconductor-metal interfaces (including the case of metallic multilayers) can be accurately described by use of the proposed models for the optical functions of metallic films and the matrix method for multilayer calculations.

Modeling the index of refraction of insulating solids with a modified lorentz oscillator model.

A modification of the Lorentz oscillator model for optical constants is proposed in an effort to achieve better agreement with experimental data while keeping the calculation simple. Improvement in agreement between theoretical and experimental data obtained with a variable line shape (frequency-dependent damping constant) over a wide spectral range is demonstrated through modeling the index of refraction of Si(3)N(4) (1-24 eV), SiO (0.15-25 eV) and amorphous and crystalline SiO(2) (0.15-25 eV). Model parameters are estimated by acceptance-probability-controlled simulated annealing. Excellent agreement between the modified model and the experimental data is obtained for both real and imaginary parts of the index of refraction.