Effects of surface-modifying ligands on the colloidal stability of ZnO nanoparticle dispersions in in vitro cytotoxicity test media

The extrinsic physicochemical properties of nanoparticles (NPs), such as hydrodynamic size, surface charge, surface functional group, and colloidal stabilities, in toxicity testing media are known to have a significant influence on in vitro toxicity assessments. Therefore, interpretation of nanotoxicity test results should be based on reliable characterization of the NPs' extrinsic properties in actual toxicity testing media. Here, we present a set of physicochemical characterization results for commercially available ZnO NPs, including core diameter, hydrodynamic diameter, surface charges, and colloidal stabilities, in two in vitro toxicity testing media (Roswell Park Memorial Institute [RPMI] and Dulbecco's Modified Eagle's Medium [DMEM]), as well as simple cell viability assay results for selected ZnO NPs. Four commercially available and manufactured ZnO NPs, with different core sizes, were used in this study, and their surface charge was modified with five different surface coating materials (sodium citrate, tris(2-aminoethyl)amine, poly(acrylic acid), poly(allylamine hydrochloride), and poly-L-lysine hydrochloride). The results showed that ZnO NPs were better dispersed in cell culture media via surface modification with positively or negatively charged molecules. Moreover, in the presence of fetal bovine serum (FBS) in RPMI and DMEM media, ZnO NPs were found even better dispersed for a longer period (at least 48 hours). For the HeLa cells exposed to ZnO NPs in DMEM media without FBS, surface charge-dependent cytotoxicity trends were observed, while these trends were not observed for those cells cultured in FBS-containing media. This confirmed the important roles of surface-modifying compounds and of surface charge on the resultant cytotoxicities of NPs.