Cytotoxicity of Au compounds during the logarithmic growth phase of four cell lines. A) HeLa cells were seeded at 2000 cells/well and grown for 3 days into the logarithmic growth phase. Au compounds were added for 48 h and MTT tests were performed as detailed in the Experimental Section. The logarithmic curve fits of tabulated MTT readings are shown. Each data point represents the mean±standard error (SE) of sample triplicates. B) Note that the IC50 values of Au1.4MS were lowest across all cell lines and that Au compounds of smaller or larger size were progressively less cytotoxic, which suggests a stringent size dependency of cytotoxicity. All concentrations relate to the amount of gold detected by atomic-absorption spectroscopy (AAS) in the authentic samples after performing the cytotoxicity test. This procedure ruled out the possibility that cluster synthesis contaminants or dilution errors may have caused erroneous results.