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Behavior of toxicologically relevant silver compounds and barium sulfate particles in complex media : from synthesis to biological investigations
This work comprises the synthesis procedures of toxicologically relevant particles, and their behavior in complex biological media in terms of stability and dissolution, with specific biological responses to the synthesized materials. Two toxicologically relevant particulate systems were studied. As a model of a biologically active material, silver compounds, especially silver nanoparticles, were investigated. In contrast, barium sulfate was examined as a bioinert system. The formation of silver nanoparticles during the reduction with glucose in the presence of poly(vinylpyrrolidone) as a capping agent was followed for more than 3000 min. It was shown that spherical silver nanoparticles were formed first, but in later stages, an increasing fraction of nano-triangles and also a few nanorods developed. By electron microscopy, the growth of spherical and trigonal nanoparticles with time was described. From X-ray powder diffractometry, it was concluded that the domain size in the spherical nanoparticles increased proportionally to the particle diameter and was always about ¼ of the diameter, indicating that twinned seeds were formed very early in the process and then simply grew by extending their size. The lattice constant of the nanoparticles was systematically increased in comparison to microcrystalline silver, but did not change as a function of particle diameter. The behavior of silver ions was investigated in biologically relevant concentrations in different media, from physiological salt solution over phosphate-buffered saline solution to protein-containing cell culture media. The results showed that silver ions that were initially present were bound as silver chloride due to the presence of chloride. Only in the absence of chloride, glucose was able to reduce Ag+ to Ag0. The precipitation of silver phosphate was never observed in any case. It was concluded that the predominant silver species in biological media is dispersed nanoscopic silver chloride, surrounded by a protein corona which prevents the growth of crystals and leads to colloidal stabilization. Using bacteria (S. aureus) and cell culture experiments (human mesenchymal stem cells; T-cells; monocytes), it was possible to show that a toxic effect occurred at the same silver concentration of ionic silver and synthetically prepared silver chloride nanoparticles. Furthermore, the dissolution of silver nanoparticles and the solubility products of silver chloride and silver nitrate were quantified in protein-containing cell culture media. Barium sulfate micro-, sub-micro-, and nanoparticles were synthesized for evaluation of inflammatory potential. The synthesis of fluorescent labeled BaSO4 particles with a narrow size distribution was achieved. The resultant powders were uniform in form, chemical composition and surface modification by the fluorescent dye. All of the particles were colloidally dispersed in cell culture medium with serum. From the cellular uptake studies on rat alveolar macrophages, it was concluded that a significant difference occurred in the case of barium sulfate microparticles for suspended and adherent cell fractions. This was explained by the sedimentation of synthesized particles over time, and confirmed by stability studies of dispersed particles in biological medium by dynamic light scattering. However, in a particle-induced cell migration assay for evaluation of the inflammation potential of the system, no dose- and size-dependency was found.
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