The presently disclosed subject matter relates to a microfluidic device for measuring an amount of a molecular species in a sample. The device comprises a body and an electrode assembly, the electrode assembly comprising at least one electrode coupled to the body. The device also includes a gas permeable membrane disposed on at least one electrode and a detector for measuring a current at each of the electrodes. Further, the device comprises a channel defined between a substantially planar substrate and planar cover glass. The presently disclosed subject matter also relates to methods of making the microfluidic device.