An emerging concept in cell signaling is the natural role of reactive oxygen species such as hydrogen peroxide (H2O2) as beneficial messengers in redox signaling pathways. The nature of H2O2 signaling is confounded, however, by difficulties in tracking it in living systems, both spatially and temporally, at low concns. Here, the authors develop an array of fluorescent single-walled carbon nanotubes that can selectively record, in real time, the discrete, stochastic quenching events that occur as H2O2 mols. are emitted from individual human epidermal carcinoma cells stimulated by epidermal growth factor. We show math. that such arrays can distinguish between mols. originating locally on the cell membrane from other contributions. The authors find that epidermal growth factor induces 2 nmol H2O2 locally over a period of 50 min. This platform promises a new approach to understanding the signaling of reactive oxygen species at the cellular level. [on SciFinder(R)]