Dispersible Plasmonic Doped Metal Oxide Nanocrystal Sensors that Optically Track Redox Reactions in Aqueous Media with Single‐Electron Sensitivity

Electron transfer in complex aqueous systems can be observed remotely with single‐electron sensitivity using locally dispersed nanostructures conferred with electronic charge concentration‐dependent plasmonic properties. When introduced to a system out of redox equilibrium, tin‐doped indium oxide nanocrystals undergo rapid multielectron transfer until redox equilibrium is reached; this modulates their free carrier concentration and plasmonic optical properties in the spectrally isolated near‐infrared. This capability is harnessed here to noninvasively track, model, and quantify electron transfer events reversibly for organic, inorganic, biogenic, and even living cells.