Devices and methods are provided for determining the concentration of a reduced form of a redox species. For example, a device can include a working electrode and a counter electrode spaced by a predetermined distance so that reaction produces from the counter electrode arrive at the working electrode. An electric potential difference can be applied between the electrodes, and the potential of the working electrode can be selected such that the rate of electro-oxidation of the reduced form of the species is diffusion controlled. Current as a function of time can be determined, the magnitude of the steady state current can be estimated, and a value indicative of the diffusion coefficient and/or of the concentration of the reduced form of the species can be obtained from the change in current with time and the magnitude of the steady state current. Other embodiments of apparatuses, devices, and methods are also provided.