Reactive oxygen species, specifically hydrogen peroxide (H2O2), activate signal transduction pathways during angiogenesis and therefore play an important role in physiol. development as well as various pathophysiologies. Herein, the authors utilize a near-IR fluorescent single-walled carbon nanotube (SWNT) sensor array to measure the single-mol. efflux of H2O2 from human umbilical vein endothelial cells (HUVEC) in response to angiogenic stimulation. Two angiogenic agents were investigated: the pro-angiogenic cytokine, vascular endothelial growth factor A (VEGF-A) and the recently identified inorg. pro-angiogenic factor, europium(III) hydroxide in nanorod form. The nanosensor array consists of a SWNT embedded within a collagen matrix that exhibits high selectivity and sensitivity to single mols. of H2O2. A calibration from 12.5 to 400 nM quantifies the prodn. of H2O2 at nanomolar concn. in HUVEC with 1 s temporal and 300 nm spatial resolns. The authors find that the prodn. of H2O2 following VEGF stimulation is elevated outside of HUVEC, but not for stimulation via nanorods, while increased generation is obsd. in the cytoplasm for both cases, suggesting two distinct signaling pathways. [on SciFinder(R)]