Analytical Chemistry experiments are essential to establish this threshold for interac- tion prior to the use of such probes for assessment of cell viability and oxidative stress responses following exposure. Where such interference is detected, alternative test systems should be used and may even require those that do not rely on quantitating colorimetric or ﬂuorometric changes. A number of studies can be found in the literature which have used colorimetric and ﬂuoro- metric assays, but do not indicate whether the possibility of test system/NM interactions have been controlled for. Thus, in some of these cases, it is possible that results may be misleading due to interactions between the NM and the selected test system, thereby confounding interpretation. Test system/NM interac- tions may represent a source for some of the conﬂicting observa- tions in the current literature, in reports assessing apparently the same material but with diﬀerent experimental systems. Addition- ally, it is important to note that even subtle diﬀerences in oxida- tion state of the metal oxide NP is suﬃcient to result in major diﬀerences in their ability to interfere with ﬂuorometric dyes. Thus, until we develop a more comprehensive understanding of the parameters that inﬂuence such interactions, test system validation assessments are necessary on a NM-by-NM basis. ’ AUTHOR INFORMATION Corresponding Author *E-mail: S.H.Doak@swansea.ac.uk. Tel: þ44 (0)1792 295388. ’ ACKNOWLEDGMENT S.M.G. and N.S. are joint ﬁrst authors for this work. This work is supported by funds from the Medical Research Council and the Research Councils UK (S.H.D. Academic Fellowship). The authors would like to thank Prof. Rik Brydson and Dr Andy Brown for access to the Institute for Materials Research, Uni- versity of Leeds, TEM facility via the Leeds EPSRC Nanoscience and Nanotechnology Research Equipment Facility (9EP/ F056311/1). 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