Jacqueline Merikhi

Adhesion of Colloidal ZnO Particles on ZnS-Type Phosphor Surfaces

Chemical and physical aspects of the adhesion of colloidal ZnO particles (d(50)=81 nm) on the surface of ZnS-type phosphors have been studied. Here, the green-emitting phosphor ZnS:Cu,Al,Cu (d(50)=5.0 µm) applied in TV screens was chosen as model compound. The ZnS material was pretreated in various ways (H(2)O, HCl, H(2)O(2)) and reacted thereafter with a suspension containing colloidal ZnO particles. Analytical investigations (SEM, ESCA) have shown that the adhesion of colloidal ZnO particles is strongly affected by the degree of hydrolysis of the ZnS surface. Electroacoustic investigations (ESA) prove that both types of surfaces, hydrolyzed ZnS as well as colloidal ZnO, are positively charged. Even so, adhesion of ZnO particles is encouraged very much under these conditions, indicating that secondary attractive forces (electrostatic interaction, chemical bonding) determine the amount of colloidal ZnO adhered on a ZnS-type phosphor. Copyright 2000 Academic Press.

High emission current density microwave-plasma-grown carbon nanotube arrays by postdepositional radio-frequency oxygen plasma treatment

Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.

Synthesis and characterization of rod-like Y2O3 and Y2O3:Eu3+

Y2O3 rods 100 to 200 nm in diameter and 10 to 20 μm in length are accessible via polyol-mediated synthesis of a precursor material with similar shape. By heating of Y(CH3COO)3 · xH2O and a defined amount of water at 190°C in diethylene glycol, the rod-like precursor material is formed. Infrared spectroscopy (IR), differential thermal analysis (DTA) and thermal gravimetry (TG) evidence that this precursor material still contains acetate. However, the precursor material can be transformed to Y2O3 by sintering at 600°C without destruction of the rod-like shape. According to X-ray powder diffraction analysis, the rods are well crystallized. They can be assumed to be with [100] orientation. By doping with Eu3+ (5 mol%), red emitting phosphor rods can be realized. With optical spectroscopy the typical line emission of Eu3+ is observed. Diffuse reflectance of Y2O3:Eu3+ rods is determined to be higher than 95% in the visible. While exciting at 254 nm (Hg-discharge), a quantum efficiency of 38.5% is proven for the prepared Y2O3:Eu3+ rods.

Development of a multilevel approach for the evaluation of nanomaterials’ toxicity

AIM To develop a multilevel approach that includes different toxicity tests and gene-expression studies for toxicity evaluation of engineered nanomaterials developed for biomedical applications. MATERIALS & METHODS K-562, MCF-7 and U-937 human-derived cell lines were used as models for in vitro toxicity tests. These tests included viability assays (3-[4,5-dimethylthiazol-2-yl]-5-[3-carboxymethoxyphenyl]-2-[4-sulfophenyl]-2H-tetrazolium [MTS] assay); evaluation of apoptosis/necrosis by propidium iodide staining and DNA laddering assay; evaluation of mitochondrial toxicity (5,5´,6,6´-tetrachloro-1,1´,3,3´-tetraethyl-benzimidazolcarbocyanine iodide [JC-1] assay); transmission electron microscopy analysis and gene expression analysis by DNA microarray. For in vivo toxicity evaluation, Swiss mice were used for monitoring acute or chronic effects. Two superparamagnetic contrast agents approved for human use (Resovist and Primovist) and two new lanthanide-based luminescent nanoparticles were tested. RESULTS & DISCUSSION The nanomaterials approved for human use did not show significant toxicities in our assays. Toxicity studies performed on lanthanide-based nanoparticles (EDTA120 and EDTA120D) complexed with the chelating agent EDTA revealed that these nanomaterials induced necrosis in U-937 and K-562 cells while no toxicity was observed in MCF-7 cells. Moreover, no in vivo effects have been observed. The comparative analysis of the nanomaterials and their separated components showed that the toxicity in U-937 and K-562 cells was mainly due to the presence of EDTA. CONCLUSION The multilevel approach proved to be useful for nanomaterial toxicity characterization. In particular, for the lanthanide-based nanoparticles tested in this work, the EDTA was identified as the main cause of the toxicity in vitro, suggesting a possible applicability of these nanoparticle suspensions for in vivo optical imaging.

Near-infrared luminescent nanomaterials for in-vivo optical imaging

Optical imaging using unspecific contrast agents as well as targeted and disease-specific agents play a vital role in preclinical research. Moreover, optical imaging is on the verge of establishing itself as a clinically relevant imaging modality. Also in-vitro diagnostical procedures rely to a large degree on optical labels to report disease-specific events. Materials that fulfill the basic requirements of this market are being used today, with cyanine dyes and semiconductor quantum dots being excellent examples. Other materials are being tested in laboratories throughout the world. Design rules suitable to develop new optical labels for in-vivo near-infrared optical imaging procedures have been formulated by us, and we have developed synthesis routes that lead to nano particles with small diameter, narrow size distribution, high quantum yield, and with stable surfaces required for bioconjugation to disease-specific ligands.

Homogeneous coatings of nanosized Fe2O3 particles on Y2O2S:Eu

The red emitting phosphor Y2O2S:Eu commonly applied in colour television tubes was pigmented with a thin homogeneous layer of nanosized Fe2O3 particles based on a novel coating strategy. In a first step the phosphor was covered with nanosized Fe3O4 particles which themselves were prepared by reduction of an Fe(III)-salt with hydrazine. Thereafter, Fe3O4 was converted to Fe2O3 by heating to 450°C in air. Surface composition and body colour of the corresponding phosphor samples were investigated applying ESCA and measuring diffuse reflectance. The size of the Fe2O3 particles as well as the homogeneity of the resulting coating were studied by SEM. Furthermore, the adhesion of Fe2O3 particles on the phosphor surface was examined.