A sensor apparatus is disclosed for sensing substances, species, substrates, etc. using oxidase. The apparatus includes a pair of potentiostat operating electrode systems. In one embodiment each electrode system includes respective working, counter and reference electrodes, and in another embodiment each electrode system includes a respective working electrode and the counter and reference electrodes are shared by both electrode systems. The oxidase material is immobilized on the working electrode of one of the electrode systems. A method also is disclosed for concentration sensing using oxidases.