This invention provides a sensing device comprising an electrode comprising a noble metal layer, on which layer is located a biological material having nitroreductase activity. This invention further provides a method of detecting nitro group containing compounds, the method comprising the steps of: (a) providing a sensing device of the first aspect of the invention and a reference electrode; (b) applying a potential between the electrodes; (c) measuring the current; (d) contacting the sensing device with a sample of substrate material to be tested; and (e) measuring the current change.