The intrinsic near-infrared fluorescence of single-walled carbon nanotubes (SWNT) is sensitive to changes in the nanotubes' environment. Biosensors which exploit this property were used to detect molecular changes from within single cells as well as highly absorptive tissue matrices. Non-covalently bound DNA on the nanotube surface permits nanotube fluorescence in aqueous media and allows uptake into live cells via endocytosis. We employ these non-cytotoxic DNA-SWNT complexes as chemical sensors, detecting DNA structural changes in real-time through a range of biological media.