Are you functionalizing commercial colloidal silver? In that case I would recommend stirring them in aqueous NaBH4 solution with excess lipoic acid. Alternatively, you can reduce Ag+ directly with NaBH4 in the presence of lipoic acid. As far as determining the necessary cytotoxic dose, I suggest starting with a colon cancer cell line and treating the cells with varying concentrations of your nanoparticles. Then you can determine the cell viability by MTT assay.