There are two possibilities. Either that nano agent decrease the mitochondrion activity (MTT measures the activity of mitochondrial enzyme) or your nano agent just decrease the proliferation rate. If so cells do not proliferate, their number is constant, so you do not observe the increase of activity of mitochondrial enzyme. The easiest and cheapest way to determine whether the cells are dying is to stain them with Hoechst and PI - then count apoptotic cells (apoptotic bodies, increase fluorescence of Hoechst) and dead cells (PI positive). Other method isannexin and FITC on flow cytometry, or measurments the activity of effector caspases 3/7.

I agree with Agnieszka Bojko. You could also try Neutral Red and LDH assays to test cytotoxicity.

What I suggest is to use another colorimetric assay. Sometimes your nano agent can reduce your mtt dye and give you faules results. Personal opinion MTT for nano work is not ideal, neutral red, SRB and BRDU assays are much better than the MTT.

Hope this would help