Acute Toxicity of Silver Free and Encapsulated in Nanosized Zeolite for Eukaryotic Cells

The potential toxicity of encapsulated silver in EMT-type nanosized zeolites on prokaryotic cells, human tumor cell lines from various origins, and primary cultures of neurons and astrocytes was investigated. Silver in cationic form (Ag⁺) was encapsulated in EMT-type nanosized zeolites via an ion exchange process (Ag⁺-EMT) and compared with the reduced silver (Ag⁰) in the zeolite (Ag⁰-EMT). As reference samples for the toxicity measurements, pure EMT-type zeolite and silver perchlorate were used. Cells were exposed to silver-containing zeolites (50, 100, and 400 μg/mL) for 24 and 48 h. After exposure to Ag⁺-EMT (50 μg/mL) for 24 h, a loss in cell viability independent of the cell type was observed, ranging from -34.37 ± 23.90% for astrocytes to -99.5 ± 0.24% for U87-MG cells. These results were comparable with the toxicity for silver perchlorate. The Ag⁰-EMT sample showed lower toxicity on human cell lines in comparison to that of Ag⁺-EMT. A decrease in cell viability, i.e., -73.46 ± 20.78% and -62.3 ± 17.96% for U87-MG and HEK 293 cells, respectively, under exposure only to high concentration of Ag⁰-EMT (400 μg/mL) for 24 h was measured. However, the Ag⁰-EMT was as toxic as the Ag⁺-EMT for astrocytes and neurons (-97.95 ± 3.31% and -100 ± 1.11%, respectively, after exposure to 50 μg/mL for 24 h). No decrease in cell viability exposed to pure EMT zeolite was found. The results demonstrate the severe toxicity of silver cations, either free or encapsulated, in comparison to reduced silver encapsulated in zeolite nanocrystals. Therefore, silver cations, either free or encapsulated, must be used with great caution regarding their toxicity on eukaryotic cells.