Fig. 8. Single-molecule detection of H₂O₂ by stepwise quenching of individual SWNTs.

a Near-infrared image of SWNTs under A431 cells. b Single-molecule tracking (green) performed by measuring the fluorescence of single sensors and monitoring step-changes in fluorescence, which can be fitted to a hidden Markov algorithm (red) to identify single binding and release events. Adapted from ref. ¹¹⁷ with permission. c A single nanotube (bottom) consists of multiple regions, where each state is either quenched or unquenched. The nanotube fluorescence intensity varies over time upon single-quencher molecular binding events. When unquenched, the fluorescence state at a particular region of the SWNT results in a nanotube fluorescence intensity value. d The adsorption of quenching H₂O₂ molecule results in a single-state quenching event, and a stepwise decrease in overall SWNT intensity. e Adsorption of another quenching molecule leads to a further decrease in overall intensity. f Desorption of H₂O₂ from the SWNT surface recovers the nanotube fluorescence with a stepwise increase in intensity.