The invention relates to charged single-wall carbon nanotubes and their use in sensing and monitoring devices. The charged single-wall carbon nanotubes, have been found to have spectral shifts in certain regions of the electromagnetic spectrum. The charged single wall nanotubes are very sensitive to environmental perturbations and the nanotube's optical properties will be affected by these perturbations. Accordingly, the charged single wall carbon nanotubes can be used as sensors for a wide variety of applications, such as salt concentrations and pH, signal generators, measuring length of DNA molecules, as well as optical tags for biological detection and mapping of malignant cell activity. Optimal sensor devices are achieved in the present invention when the charged single-wall carbon nanotube carries a linear charge density close to that of DNA. The invention further describes an optical pH sensor comprising at least one charged carbon nanotube in solution.