New biosensor measures extracellular hydrogen peroxide levels with nanometer-resolution Several processes in the human body are regulated by biochemical reactions involving hydrogen peroxide (H 2O 2). 2O 2 is generally toxic because of its oxidant character. The latter means that it converts (oxidizes) biochemical molecules like proteins and DNA. The oxidizing property of H 2O 2 is of potential therapeutic relevance for cancer, though: deliberately causing tumor cells to increase their H 2O 2 concentration would be a way to destroy them. In light of this, but also for monitoring pathologies associated with H 2O 2 overproduction, it is crucial to have a means to reliably quantify hydrogen peroxide concentrations in the extracellular environment.