A sensing element comprises a transistor having a gate electrode, a source electrode, a gate electrode and a semiconductor nanostructure connecting between the source and the gate electrodes. The semiconductor nanostructure is modified by a functional moiety covalently attached thereto. A voltage source is connected to the gate electrode. A controller controls a gate voltage applied by the voltage source to the gate electrode such as to reverse a redox reaction occurring when the moiety contacts a redox reactive agent.