The antimicrobial properties of chitosan have been enhanced by loading chitosan with various metals. Among all antimicrobial metals, silver possesses great toxicity against a wide range of microorganisms. Nanocomposites based on silver nanoparticles (SNPs) have been used as antimicrobial films for food packaging . However, the toxicology of SNPs has still remained unknown. Additionally, SNPs can be absorbed into the bloodstream via different routes of administration, leading to deposition of silver in many organs, including the liver and spleen, and potentially can damage the organ. Previous researches have shown that different surface stabilizers have distinct impacts on SNPs cytotoxicity. Chitosan, because of its good biocompatibility and antibacterial properties, is often employed as the active ingredient of topical wound materials in combination with SNPs . Chitosan is also used as a stabilizer instead of chemical reducing agents for protecting SNPs from agglomeration .
In the present study, chitosan was employed for producing SCNC in which sodium borohydride and sodium hydroxide were used as the reducing agent for silver ions to produce SCNC-AAAS and SCNC-ASHS, respectively. They revealed cytotoxic effects, dependent to the dose and time, on both HT-29 colon cancer cells and normal Vero cells.
Palem et al. reported a 5-7% cytotoxicity on normal 3T3 fibroblasts and cancer HeLa cells in the presence of SCNC . Their results were in accordance with our findings. In the present study, a toxicity in both normal and cancer cells was observed in the presence of SCNC.
SCNC is reported to have anti-cancer effect on A549 lung cancer cells, with IC50 of 29.35 µg/ml . This study also showed that SCNC-AAAS and SCNC-ASHS with IC50 of 4.40 and 11.54 µg/ml possessed anti-cancer effects on HT-29 cells, respectively. It is indicated that Ag-doped chitosan-poly vinyl alcohol nanocomposites impact more on human liver cancer (HEPG2) cells with IC50 of 43.7µg/ml than breast cancer (MCF7) cells with IC50 of 52.5 µg/ml . This result is in accordance with our findings.
Tyliszczak et al. stated that chitosan-based hydrogels modified with SNPs produced by sodium borohydride in concentrations of 25, 50, 75 and 100 (wt%), showed no toxic effect on dermis cells BJ (CRL-2522TM) . Wang et al. reported that silver immobilized in the sliver nanoparticle-doped chitosan composite films, shows a significant influence on the cell adhesion and subsequent proliferation of human umbilical vein endothelial cells . Jena et al. reported that chitosan-coated silver nanoparticles, using chitosan as stabilizing and reducing agent, showed no significant cytotoxic or DNA damage on the macrophages at the bactericidal dose . The less toxic effects of SCNC in former studies was likely due to the type of cells. It appears that the reason of non-toxicity of SCNC in the mentioned studies compared to present study is the difference in the type of investigated cells so that normal dermis cells, umbilical vein endothelial cells and macrophages exerted more resistance to SCNC compared normal kidney epithelial cells and they did not undergo cytotoxicity.
SCNC size is a considerable aspect of different results, which can be varied from the less than 10 nm to more than 100 nm, in our study. It seems that larger size (100 nm) of SCNC causes more cell biological consequences in comparison with smaller particles (10 nm) [9, 31]
Another influential factor is type of synthesis procedure of SCNC. In the current study, chitosan was alternatively used as a silver ion reducing agent instead of sodium borohydride, used in previous studies. Jena et al. showed that the same-size particles above 100 nm, were not toxic to the macrophage . Because of the cytotoxicity of our nanoparticles on the normal cells, the application of them is not recommended in food coatings. Our synthetic nano-particles were highly toxic on the cancerous cells, thus they could be used in treating cancers.