The dissolution of citrate-stabilized and poly(vinylpyrrolidone)-stabilized silver nanoparticles in water was studied by dialysis for up to 125 days at 5, 25, and 37 °C. The particles slowly dissolve into ions on a time scale of several days. However, in all cases, a limiting value of the released silver was observed, i.e., the particles did not completely dissolve. In some cases, the nanoparticles released up to 90% of their weight. Formal kinetic data were computed. Rate and degree of dissolution depended on the functionalization as well as on the storage temperature. The release of silver led to a considerably increased toxicity of silver nanoparticles which had been stored in dispersion for several weeks toward human mesenchymal stem cells due to the increased concentration of silver ions. Consequently, "aged" (i.e., immersed) silver nanoparticles are much more toxic to cells than freshly prepared silver nanoparticles.

Spherical silver nanoparticles with a diameter of 50 AE 20 nm and stabilized with either poly(N-vinylpyrrolidone) (PVP) or citrate were dispersed in different cell culture media: (i) pure RPMI, (ii) RPMI containing up to 10% of bovine serum albumin (BSA), and (iii) RPMI containing up to 10% of fetal calf serum (FCS). The agglomeration behavior of the nanoparticles was studied with dynamic light scattering and optical microscopy of individually tracked single particles. Whereas strong agglomeration was observed in pure RPMI and in the RPMI-BSA mixture within a few hours, the particles remained well dispersed in RPMI-FCS. In addition, the biological effect of PVP-stabilized silver nanoparticles and of silver ions on human mesenchymal stem cells (hMSCs) was studied in pure RPMI and also in RPMI-BSA and RPMI-FCS mixtures, respectively. Both proteins considerably increased the cell viability in the presence of silver ions and as well as silver nanoparticles, indicating a binding of silver by these proteins.

In summary, the results showed that Ag-NPs exert cytotoxic effects on hMSCs at high concentrations but also induce cell activation (as analyzed by the release of IL-8) at high but nontoxic concentrations of nanosilver.

SummaryPVP-capped silver nanoparticles with a diameter of the metallic core of 70 nm, a hydrodynamic diameter of 120 nm and a zeta potential of −20 mV were prepared and investigated with regard to their biological activity. This review summarizes the physicochemical properties (dissolution, protein adsorption, dispersability) of these nanoparticles and the cellular consequences of the exposure of a broad range of biological test systems to this defined type of silver nanoparticles. Silver nanoparticles dissolve in water in the presence of oxygen. In addition, in biological media (i.e., in the presence of proteins) the surface of silver nanoparticles is rapidly coated by a protein corona that influences their physicochemical and biological properties including cellular uptake. Silver nanoparticles are taken up by cell-type specific endocytosis pathways as demonstrated for hMSC, primary T-cells, primary monocytes, and astrocytes. A visualization of particles inside cells is possible by X-ray microscopy, fluorescence microscopy, and combined FIB/SEM analysis. By staining organelles, their localization inside the cell can be additionally determined. While primary brain astrocytes are shown to be fairly tolerant toward silver nanoparticles, silver nanoparticles induce the formation of DNA double-strand-breaks (DSB) and lead to chromosomal aberrations and sister-chromatid exchanges in Chinese hamster fibroblast cell lines (CHO9, K1, V79B). An exposure of rats to silver nanoparticles in vivo induced a moderate pulmonary toxicity, however, only at rather high concentrations. The same was found in precision-cut lung slices of rats in which silver nanoparticles remained mainly at the tissue surface. In a human 3D triple-cell culture model consisting of three cell types (alveolar epithelial cells, macrophages, and dendritic cells), adverse effects were also only found at high silver concentrations. The silver ions that are released from silver nanoparticles may be harmful to skin with disrupted barrier (e.g., wounds) and induce oxidative stress in skin cells (HaCaT). In conclusion, the data obtained on the effects of this well-defined type of silver nanoparticles on various biological systems clearly demonstrate that cell-type specific properties as well as experimental conditions determine the biocompatibility of and the cellular responses to an exposure with silver nanoparticles.

Silver nanoparticles were prepared by the polyol process, i.e. by the reduction of silver nitrate with ethylene glycol in the presence of polyvinylpyrrolidone, PVP. Thereby, the silver nanoparticles were colloidally stabilized by the polymer. The synthesis of nanoparticles of different size and shape (cubes, rods and spheres) was possible by changing the reaction conditions such as reagent ratio and temperature. The silver nanoparticles were characterized by dynamic light scattering (DLS), zeta-potential measurements, UVspectroscopy, and scanning electron microscopy (SEM). The biological activity of spherical PVP-coated silver nanoparticles (about 100 nm diameter) was tested on human mesenchymal stem cells (hMSC) in comparison with equivalent amounts of silver ions (silver acetate). hMSC were treated with silver concentrations in the range of 50 ng mL -1 to 50 lg mL -1 for 7 days under cell culture conditions. Cytotoxic cell reactions occurred at !2.5 lg Ag mL -1 for nanoparticles and !1 lg Ag mL -1 for silver acetate, indicating a critical role of the silver ions for toxic reactions.Key words: Silver nanoparticles, colloids, biological activity, mesenchymal stem cells.Die Silber-Nanopartikel wurden durch die Reduktion von Silbernitrat durch Ethylenglykol in Gegenwart von Polyvinylpyrrolidon, PVP, hergestellt (Polyol-Prozess). Die Silber-Nanopartikel werden dabei vom Polymer umhüllt und dadurch kolloidal stabilisiert. Die Synthese von Nanopartikeln mit unterschiedlicher Größe und Form (Würfel, Stäbchen, Kugeln) ist durch die Ä nderung der Reaktionsbedingungen (Verhältnis der Reagenzien, Temperatur) möglich. Die Silber-Nanopartikel wurden mit Dynamischer Lichtstreuung (DLS), Zetapotential-Messungen, UV-Spektroskopie und Rasterelektronenmikroskopie (REM) analysiert. Die biologische Aktivität kugelförmiger PVP-beschichteter Silber-Nanopartikel (Durchmesser ca. 100 nm) wurde im Vergleich mit einer äquivalenten Menge Silber-Ionen (Silberacetat) an humanen mesenchymalen Stammzellen (hMSC) getestet. Die hMSCs wurden jeweils mit Silberkonzentrationen im Bereich von 50 ng mL -1 bis 50 lg mL -1 für 7 Tage in Kultur genommen. Zelltoxische Reaktionen ergaben sich bei Silberkonzentrationen !2.5 lg Ag mL -1 für die Nanopartikel und !1 lg Ag mL -1 für Silberacetat, was die kritische Rolle der Silber-Ionen bei toxischen Reaktionen unterstreicht.

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