Effect of crystal size and surface functionalization on the cytotoxicity of silicalite-1 nanoparticles

In this report, we describe the synthesis and characterization of nanocrystalline silicalite (the purely siliceous form of the zeolite, ZSM-5) of defined crystal size and surface functionalization and determine the effect on the type and degree of cytotoxicity induced in two distinct model cell lines. The silicalite materials were characterized by powder X-ray diffraction, dynamic light scattering and zeta potential, solid state NMR, thermal gravimetric analysis, and nitrogen adsorption using the BET method to determine specific surface area. The silicalite samples were functionalized with amino, thiol, and carboxy groups and had crystal sizes of approximately 30, 150, and 500 nm. The cytotoxicities of the silicalite samples with different crystal sizes and different surface functional groups were investigated using human embryonic kidney 293 (HEK-293) cells and RAW264.7 macrophage cell lines. We used the lactic dehydrogenase release assay to measure damage to the cell membrane, the caspase 3/7 activity assay to measure key molecules involved in apoptosis, and the Annexin V-propidium iodide staining method to provide visual confirmation of the types of cell death induced. We have shown that the impact of size and surface functionalization of silicalite nanoparticles on cell toxicity and mechanism of cell death is cell type-dependent. Thirty nanometer silicalite nanoparticles were nontoxic in RAW264.7 cells relative to untreated controls but caused necrosis in HEK293 cells. Carboxy-functionalized 500 nm silicalite nanoparticles resulted in apoptosis and necrosis in RAW264.7 cells and predominantly activated apoptosis in HEK293 cells.