Devices for electrochemical measurement of a redox active substance secreted by a cell in a liquid sample are provided. The devices include one or more electrodes and a matrix capable of releasing a chemical agent that stimulates secretes of the redox active substance by the cell. In some embodiments, the devices include a nanofluidic electrode assembly in which the one or more electrodes are disposed within a nanofluidic channel. Methods of fabricating the devices and methods of using the devices to detect secretion of a redox active substance by a cell are also provided.