The reliability and limits of the MTT reduction assay for carbon nanotubes–cell interaction Original Research Article

The unique physical and chemical properties of carbon nanotubes (CNTs) suggest enormous potential for many areas of research and application. The urgent need for toxicological studies on CNTs has arisen from the steadily increasing production volume. 2-(4,5-dimethyl-2-thiazolyl)-3,5-diphenyl-2H-tetrazolium bromide (MTT) assay is among the most versatile and popular assays used for in vitro toxicology. For particulate samples like CNTs the applicability and limits of various tests including the MTT assay have to be defined. Two MTT assays were applied to study the effect of single-walled CNTs (SWCNTs) on A549 cells. Both assays revealed significant and comparable loss of cell functionality that was confirmed by DNA assay. However, using a cell-free system, we have found that polyoxyethylene sorbitan monooleate-suspended SWCNTs interfere less with MTT assay than sodium dodecyl sulfate-suspended SWCNTs. Moreover, depending on the purification procedure of SWCNTs, they were able to convert MTT into its MTT-formazan insoluble form in the absence of any living system. In summary, a careful validation of MTT assay procedures is therefore suggested in experiments where CNTs are one of the constituents, to avoid a potential bias in concluding results of cytotoxicity studies.