A nanofluidic electrode assembly for detecting a redox active substance secreted by a cell in a liquid sample, the nanofluidic electrode assembly comprising: (a) a nanofluidic channel disposed in a substrate; (b) an inlet and an outlet port in fluid connection with the nanofluidic channel, the inlet and outlet ports configured to add and/or remove liquid from the liquid sample to or from the nanofluidic channel; (c) a first working electrode (310, 510) disposed in the nanofluidic channel, wherein the first working electrode is suitable for applying a potential sufficient to oxidize the redox active substance; (d) a second working electrode (340, 520) disposed in the nanofluidic channel, wherein the second working electrode is suitable for applying a potential sufficient to reduce the redox active substance; (e) a reference electrode disposed in the nanofluidic channel; and (f) a matrix comprising a chemical agent that stimulates secretion of the redox active substance by the cell, wherein the matrix is capable of releasing the chemical agent, and wherein the matrix is disposed in one or more of the following sites: in the nanofluidic channel; in the inlet and/or outlet port; on at least a portion of a surface of the first working electrode; on at least a portion of a surface of the second working electrode; on at least a portion of a surface of the reference electrode; and on at least a portion of a surface of the substrate, wherein the surface of the substrate is adjacent to the inlet and/or outlet port; wherein the concentration of the redox active substance is detected as current flow through the working electrodes. (a) a nanofluidic channel disposed in a substrate; (b) an inlet and an outlet port in fluid connection with the nanofluidic channel, the inlet and outlet ports configured to add and/or remove liquid from the liquid sample to or from the nanofluidic channel; (c) a first working electrode (310, 510) disposed in the nanofluidic channel, wherein the first working electrode is suitable for applying a potential sufficient to oxidize the redox active substance; (d) a second working electrode (340, 520) disposed in the nanofluidic channel, wherein the second working electrode is suitable for applying a potential sufficient to reduce the redox active substance; (e) a reference electrode disposed in the nanofluidic channel; and (f) a matrix comprising a chemical agent that stimulates secretion of the redox active substance by the cell, wherein the matrix is capable of releasing the chemical agent, and wherein the matrix is disposed in one or more of the following sites: in the nanofluidic channel; in the inlet and/or outlet port; on at least a portion of a surface of the first working electrode; on at least a portion of a surface of the second working electrode; on at least a portion of a surface of the reference electrode; and on at least a portion of a surface of the substrate, wherein the surface of the substrate is adjacent to the inlet and/or outlet port; wherein the concentration of the redox active substance is detected as current flow through the working electrodes. in the nanofluidic channel; in the inlet and/or outlet port; on at least a portion of a surface of the first working electrode; on at least a portion of a surface of the second working electrode; on at least a portion of a surface of the reference electrode; and on at least a portion of a surface of the substrate, wherein the surface of the substrate is adjacent to the inlet and/or outlet port;