Silver nanoparticles - From the synthesis to the biological application

The main topic of this doctoral thesis was the synthesis and characterization of defined silver nanoparticles (Ag NPs). These particles were synthesized with different sizes and modified surface chemistry. Two core sizes were synthesized. One, very small particle (~2 nm core) with hydrophilic ligands at the surface and second, bigger particles (~4.2 nm core) with different kinds of coatings were prepared. Additionally, the potential of these particles were tested for their use in biological systems. For this, they had to be stable in aqueous media. To assess this, their stability against different salt concentration was measured. Another important point, especially for silver nanoparticles, was its cytotoxicity. The cytotoxicity of the silver particles is a very important research topic due to the antimicrobial effect observed in silver NPs. Here, the toxicity of the different particles was measured in two different cell lines. The very small particles (~2 nm), so called nanoclusters (NCs), were prepared in two steps. After the reduction of the silver precursor first an etching step and afterwards a ligand exchange reaction were done. This ligand exchange also allowed the particles to be stable in aqueous media. During the whole synthesis the size and the distribution of the particles changed. At the beginning the particles showed a broad distribution and sizes up to almost 30 nm. After the etching the distribution decreased a lot and the sizes shrunk to 5 nm. The final core size of about 2 nm with a narrow distribution was reached after the ligand exchange being the particles stable in an aqueous suspension. A big advantage of these nanoclusters was that they showed fluorescence in the red and did not need to be labeled with an additional dye for in vitro experiments. The second type of particles showed a core size of around 4.2 nm and was synthesized in a simple reduction reaction. For this reaction first a precursor of the stabilizing ligand had to be synthesized. After the synthesis the particles were stable as a gray powder and showed a hydrophobic surface. To get the particles into aqueous phase two different methods were used. First a ligand exchange reaction with a hydrophilic ligand and second the coating of the particles with an amphiphilic polymer. One advantage of the coating process was the easy modification of the surface afterwards. This was shown by the modification of the particles with a dye and/or polyethylene glycol (PEG) chains. A further advantage was that the toxicity of the particles was highly reduced by this process. This reduced toxicity was on one hand due to the increased stability of the particles and on the other hand due to the reduced uptake of the particles when their surface was saturated with PEG. An increase of the stability against sodium chloride could also been shown for commercial gold particles using the same coating process. The commercial gold nanoparticles were stabilized by citrate molecules at the beginning and so first a ligand exchange including a phase transfer to the organic phase had to be done to reach the same surface like the silver nanoparticles. Nevertheless, the Ag NPs showed a cytotoxic effect, which was due to the release of Ag(I) ions. This release was measured under different pH conditions. Under neutral pH values neither the Ag NPs stabilized by hydrophilic ligand molecules nor the once stabilized by the polymer coating showed a release of more than 0.1% of ions up to 14 days. Under acidic conditions (pH 3) all the particles showed a release of about 1% of ions already after 7 days. In comparison of the total amount of silver with a silver salt the concentration of silver ions from the particles are low but they are “more toxic” because of the better uptake respectively their release inside the cell. To summarize it can be said that a defined synthesis and modification of different Ag NPs could be done but although they showed a very high stability they never lost their cytotoxicity