073) and a trend of cytotoxicity of SiNP-2 in A549 cells ( Fig. 4A) which contrasted with the pattern of effects in J774A.1 cells ( Fig. 4B), in which SiNP-1 and SiNP-2 were both cytotoxic. Contrasts in potencies were also observed among the CNTs, with CNT-1 and CNT-3 being relatively non-cytotoxic

by CTB assay, while CNT-2 and CNT-4 were clearly cytotoxic in both cell lines. The apparent higher cytotoxicity of CNT-4 by comparison to CNT-2 (decreased rate of reduction of resazurin to resorufin) is attributable in part to its chemical interference in the assay, probably through re-oxidation of resorufin or hyper-reduction of resorufin to non-fluorescent hydroresorufin. The magnitude of this interference can be assessed easily in Target Selective Inhibitor Library ic50 an acellular assay, either by correcting dose by dose,

or by fitting data in our potency model Forskolin (βINT; Table 1). Once corrected for βINT, potency of CNT-4 was more comparable to that of CNT-2, notably in A549 cells. In the present report, we describe the potential for interaction of the CNTs with both single-wall CNTs and multi-wall CNTs with the resazurin assay in two cell lines, A549 lung epithelial cells and J774A.1 murine macrophages. Our results indicate that all CNTs tested caused physical/optical interference with the fluorescence quantitation of reduced resazurin (resorufin) when wells were read directly with the test material (CNTs) present. This physical quench was particularly intense for the CNTs and for other carbonaceous materials such as carbon black and diesel emission particles (data not shown), and less pronounced for TiO2, SiO2 and SiNPs. As indicated by Oostingh et al. (2011), this type of interference is expected for highly optically dense materials such as CNTs, preventing the transmitted/emitted light from reaching the detector, or physically adsorbing the assay dye. Casey et al. (2007) have observed a total quenching of fluorescence and a complete loss of the pink color of the reduced dye resorufin, at concentrations as low as 0.08 mg/mL

of single-wall CNTs. Similarly, Monteiro-Riviere et al. (2009) have shown fluorescence quenching in the form of decreased Immune system fluorescence of HEK cell-reduced resazurin in the presence of single-wall CNTs (>0.1 mg/mL) and carbon black. Other NMs such as quantum dots and C60 fullerene did not interact with the resazurin fluorimetric assays. In addition to the optical interference, here we detected some chemical quenching of fluorescence by CNT-2 and CNT-4 particles, accompanied by visually observed decrease in pink color intensity. The decrease of fluorescence signal may result from the re-oxidation of resorufin (pink) back to non-fluorescent resazurin (blue) in the presence of CNTs (Monteiro-Riviere et al., 2009), or from hyper-reduction of resorufin (pink) to a the non-fluorescent hydroresorufin (colorless), a phenomenon described before (O’Brien et al.